Third World Congress on Oxidation Catalysis

Studies in Surface Science and Catalysis 110 3rd WORLD CONGRESS ON OXIDATION CATALYSIS

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S t u d i e s in S u r f a c e S c i e n c e a n d C a t a l y s i s Advisory Editors: B. Delmon and J.T. Yates

Vol. 110

3rd WORLD CONGRESS ON OXIDATION CATALYSIS Proceedings of the 3rd World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997

Editors R.K. Grasselli

University of Delaware, Newark, DE, U.S.A.

S.T. O y a m a

Virginia Polytechnic Institute, Blacksburg, VA, U.S.A.

A.M. Gaffney

ARCO Chemical Company, Newton Square, PA, U.S.A. J.E. Lyons

Sun Corporation, Marcus Hook, PA, U.S.A.

1997 ELSEVIER Amsterdam - - Lausanne m New York--- Oxford m Shannon m Singapore m Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

ISBN 0-444-82772-2 91997 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. 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. This book is printed on acid-free paper. Printed in The Netherlands

3rd World Congress on Oxidation Catalysis - Atom Efficient Catalytic Oxidations for Global Technologies

PREFACE

The 3rd World Congress on Oxidation Catalysis has its roots in the European Workshop on Selective Oxidation held in Louvain, Belgium in 1985. Out of this workshop grew the 1st World Congress held in Rimini, Italy in 1989, the 2nd in Benalmadena, Spain in 1992, and the 3rd being held now in 1997 in San Diego, California, USA. Out of the small core of dedicated and enthusiastic scientists assembled in Louvain in 1985, grew now a broad base of scientists and technologists from academia, industry and government laboratories who are fervently pursuing the subject of oxidation catalysis and are eager and willing to exchange their findings at the current meeting. The overall theme of the 3rd World Congress is "Atom Efficient Catalytic Oxidations for Global Technologies". We chose this theme to stimulate the participants to report their findings with an emphasis on conserving valuable material in their catalytic transformations, as well as conserving energy, and that in an environmentally responsible manner. Progress towards this stated goal is substantial as evidenced by the tremendous response of our community in their participation of quality publications compiled in these Proceedings of the Congress. The subjects presented span a wide range of oxidation reactions and catalysts. These include the currently important area of lower alkane oxidation to the corresponding olefins, unsaturated aldehydes, acids and nitriles. In this manner, the abundant and less expensive alkanes replace the less abundant and more expensive olefins as starting materials for industrially important intermediates and chemicals. In the oxidative activation of methane the emphasis is shifting towards the use of extremely short contact times and newer more rugged catalysts. In the area of olefin oxidations, of particular note are the high efficiency epoxidation of propylene, and new detailed mechanistic insights into the oxidation of ct,~-unsaturated aldehydes to the corresponding unsaturated acids. Substantial progress is reported in the area of the selective oxidation and ammoxidation of substituted aromatics and heteroaromatics. These include higher yields of desired products, new and more durable catalysts, as well as a reduction of undesirable byproducts. The use of oxidation catalysis to produce fine chemicals is experiencing an explosive growth. A plethora of novel approaches are presented which include shape selective epoxidations. Oxidation in confined structures is coming out of its infancy and the use of TS-'I as catalyst is becoming a standard. New approaches are being presented invoking the shape selective character of the nano-environment of peroxytungstates anchored on selected supports. The important areas of combustion, engineering and environmental aspects of oxidation catalysis, as well as their theoretical, computational and modeling approaches round off the program. Not to be overlooked is perhaps the most ambitious, the subject of structure selectivity/activity correlations, an area always worthy of further attention and in depth study. A noble effort thereof has been put forward and is reported here. We are coming ever closer to the ultimate goal of a rational approach to catalyst design and synthesis. There is still ample room for further efforts towards this goal, but the foundations are being formed for a bright and rewarding future of rationally predictive oxidation catalysis. The five featured lectures and seven plenary lectures constitute the general background and overview of the subject matter at hand. The 104 contributed papers and 13 poster manuscripts, summarized in this compendium, probe new avenues to achieve catalytically efficient oxidation reactions for the future needs of mankind in a global

environment. A large number of countries responded to this challenge by their representatives giving oral presentations or posters, and in particular by supplying the written scientific documents contained in this volume. Our sincere thanks go out to all of the contributors. The countries participating in the Congress and contributing to the Proceedings reported here made the 3rd World Congress on Oxidation Catalysis a truly international event, they are: Argentina, Azerbaijan, Belgium, Brazil, Bulgaria, Canada, China, Czech Republic, Finland, France, Germany, Greece, India, Iran, Ireland, Israel, Italy, Japan, Korea, Latvia, Netherlands, New Zealand, Poland, Portugal, Russia, Saudi Arabia, Singapore, Slovenia, South Africa, Spain, Sweden, Taiwan, Thailand, Ukraine, United Kingdom, and United States. We conclude on the basis of the foregoing, that the future of oxidation catalysis is secure and has never been brighter than at this juncture. We are confident, and the Proceedings support this view, that many new and improved selective oxidation processes and catalysts will be discovered and commercialized over the next decade, and that our knowledge towards a rational design of selective oxidation catalysts is within our grasp. With this optimism we look confidently towards the future and to a successful 4th World Congress on Oxidation Catalysis in the next millennium, in the year 2001. Thank you all for partaking in our Congress and for working in an exciting and promising area of catalysis. May success come your way in abundance in the coming years.

Robert K. Grasselli S. Ted Oyama Anne M. Gaffney James E. Lyons

vii

TABLE OF CONTENTS

Preface R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons Featured Lectures F-1

Molecular Mechanism of Heterogeneous Oxidation-Organic and Sofid State Chemists' Views J. Haber

F-2

The Multifunctional Properties of Heterogeneous Catalysts, Active and Selective in the Oxidation of Light Paraffins F. Cavani and F. Trifirb

19

Selective Oxidation of Hydrocarbons Catalyzed by Heteropoly Compounds M. Misono, N. Mizuno, K. Inumaru, G. Koyano and X. H. Lu

35

The Future of Industrial Oxidation Catalysis Spurred by Fundamental Advances B. Delmon

43

F-3

F-4

Plenary Lectures P-1

P-2

P-3

P-4

Molecular Approach to Active Sites on Metallic Oxides for Partial Oxidation Reactions J.C. V6drine

61

In Situ Electrochemically Controlled Promotion of Complete and Partial Oxidation Catalysts C.G. Vayenas and S.I. Bebelis

77

Reductive and Oxidative Activation of Oxygen for Selective Oxygenation of Hydrocarbons K. Otsuka

93

The Selective Oxidation of Methanol: A Comparison of the Mode of Action of Metal and Oxide Catalysts D. Herein, H. Werner, Th. SchedeI-Niedrig, Th. Neisius, A. Nagy, S. Bernd and R. Schl5gl 103

viii

P-5

P-6

P-7

Gold as a Low-Temperature Oxidation Catalyst: Factors Controlling Activity and Selectivity M. Haruta

123

The Selective Epoxidation of Non-Allylic Olefins over Supported Silver Catalysts J.R. Monnier

135

Redox Molecular Sieves as Heterogeneous Catalysts for Liquid Phase Oxidations R.A. Sheldon

151

PartA Structure Selectivity/Activity Correlation A-1

Synergistic Effects in Mulficomponent Catalysts for Selective Oxidation P. Courtine and E. Bordes

177

A-2

Synergetic Effects Promoted by in Operandi Surface Reconstructions of Oxides E.M. Gaigneaux, J. Naud, P. Ruiz and B. Delmon

185

Further Study on the Synergetic Effects between MoO3 And Sn02 E.M. Gaigneaux, S.R.G. Carrazan, L. Ghenne, A. Moulard, U. Roland, P. Ruiz and B. Delmon

197

The Nature of the Active/Selective Phase in VPO Catalysts and the Kinetics of n-Butane Oxidation D. Dowell and J.T. Gleaves

199

Understanding the Microstructural Transformation Mechanism which Takes Place During the Activation of Vanadium Phosphorus Oxide Catalysts G.J. Hutchings, A. Burrows, S. Sajip, C.J. Kiely, K.E. Bere, J.C. Volta, A. Tuel and M. Abon

209

Structural and Catalytic Aspects of Some Nasicon - Based Mixed Metal Phosphates P.A. Agaskar, R.K. Grasselli, D.J. Buttrey and B. White

219

Selective Reactivity of Oxygen Adatoms on Mo(112) for Methanol Oxidation K. Fukui, K. Motoda and Y. Iwasawa

227

A-3

A-4

A-5

A-6

A-7

A-8

A-9

A-10

Mechanistic Studies of Alkane Partial Oxidation Reactions on Nickel Oxide by Modern Surface Science Techniques N.R. Gleason and F. Zaera

235

Structure and Catalysis of LixNi2_x02 Oxide Systems for Oxidative Coupling of Methane T. Miyazaki, T. Doi, T. Miyamae and I. Matsuura

245

Reaction Induced Spreading of Metal Oxides: in situ Raman Spectroscopic Studies During Oxidation Reactions Y. Cai, C-B. Wang and I.E. Wachs

255

A-11

Temperature Programmed Desorpfion of Ethylene and Acetaldehyde on Uranium Oxides. Evidence of Furan Formation from Ethylene 265 H. Madhavaram and H. Idriss

A-12

Active Sites of Vanadium-Molybdenum-Containing Catalyst for Allyl Alcohol Oxidation: ESR Study in situ O.V. Krylov, N.T. Tai and B.V. Rozentuller

275

Lower Alkane Oxidation

A-13

Oxidative Dehydrogenation of Ethane over Vanadium and Niobium Oxides Supported Catalysts P. Ciambelli, L. Lisi, G. Ruoppolo, G. Russo and J.C. Volta

285

A-14

Partial Oxidation of Ethane over Monolayers of Vanadium Oxide. Effect of the Support and Surface Coverage. M.A. BaSares, X. Gao, J.L.G. Fierro and I.E. Wachs 295

A-15

The Ethane Oxidative Chlorination Process and Efficient Catalyst for It M.R. Flid, 1.1. Kurlyandskaya, Yu.A. Treger and T.D. Guzhnovskaya

305

A-16

Oxidative Conversion of LPG to Olefins with Mixed Oxide Catalysts: Surface Chemistry and Reactions Network M.V. Landau, M.L. Kaliya, A. Gutman, L.O. Kogan, M. Herskowitz and P.F. van den Oosterkamp

315

Free Radicals as Intermediates in Oxidative Transformations of Lower Alkanes M.Yu. Sinev, L.Ya. Margolis, V.Yu. Bychkov and V.N. Korchak

327

A-17

A-18

Alternative Methods to Prepare and Modify Vanadium-Phosphorus Catalysts for Selective Oxidation of Hydrocarbons V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Kharlamov, I.V. Bacherikova and L.V. Bogutskaya

337

Active Species and Working Mechanism of Silica Supported MoO3 and V205 Catalysts in the Selective Oxidation of Light Alkanes A. Parmaliana, F. Arena, F. Frusteri, G. Martra, S. Coluccia and V. Sokolovskii

347

Mechanistic Aspects of Propane Oxidation over Ni-Co-Molybdate Catalysts D.L. Stern and R.K. Grasselli

357

Oxidative Dehydrogenation of Propane by Non-Stoichiometric Nickel Molybdates D. Levin and J.Y. Ying

367

Selective Oxidation of Propane into Oxygenated Compounds over Promoted Nickel-Molybdenum Catalysts J. Barrault, C. Batiot, L. Magaud and M. Ganne

375

Oxidative Dehydrogenation of Propane on CeNixOy (0 ~ x <_ 1) Mixed Oxides Hydrogen Acceptors L. Jalowiecki-Duhamel, A. Ponchel and Y. Barbaux

383

The Role of Adsorption in the Oxidation of a,[J-unsaturated Aldehydes on Mo-V-Oxide Based Catalysts B. Stein, C. Weimer and J. Gaube

393

A-25

A New Catalyst for Propane Ammoxidation: The Sn/V/Sb Mixed Oxide S. Albonetti, G. Blanchard, P.Burattin, S. Masetti and F. Trifir6

403

A-26

Formation of Active Phases in the Sb-V-, AI-Sb-V-, and AI-Sb-V-W-Oxide Systems for Propane Ammoxidation J. Nilsson, A.R. Landa-C&novas, S. Hansen and A. Andersson 413

A-27

Influence of Antimony Content in the Iron Antimony Oxide Catalyst and Reaction Conditions on the (Amm)Oxidation of Propene and Propane E. van Steen, G. Kuwert, A. Naidoo and M. Williams

423

Catalytic Selective Oxidation of C2-C4Alkanes over Reduced Heteropolymolybdates W. Li and W. Ueda

433

A-19

A-20

A-21

A-22

A-23

A-24

A-28

A-29

A-30

The Role of Metal Oxides as Promoters of V205A/-AI20~ Catalysts in the Oxidative Dehydrogenation of Propane J.M. L6pez Nieto, R. Coenraads, A. Dejoz and M.I. Vazquez

443

Alkane Oxidation over Bulk and Silica-Supported VO(H2PO4)2-Derived Catalysts G.K. Bethke, D. Wang, J.M.C. Bueno, M.C. Kung and H.H. Kung

453

A-31

The Nature of the Active Site of the (VO)2P207 Catalyst: An Investigation of the Chemical Composition and Dynamics of the Catalyst Surface B. Kubias, F. Richter, H. Papp, A. Krepel and A. Kretschmer 461

A-32

Partial Oxidation of C5 Hydrocarbons to Phthalic and Maleic Anhydrides over Suboxides of Vanadia: Use of Dicyclopentadiene as a Probe Molecule U.S. Ozkan, G. Karakas, B.T. Schilf and S.S. Ang 471

A-33

Role of Homogeneous Reactions in the Control of the Selectivity to Maleic and Phthalic Anhydrides in the Oxidation of n-Pentane Z. Sobalik, P. Ruiz and B. Delmon

481

A-34

Catalytic Oxidation of Alkanes at Millisecond Contact Times L.D. Schmidt and C.T. Goralski, Jr.

491

A-35

Catalytic Oxidative Dehydrogenation of Isobutane in a Pd Membrane Reactor T.M. Raybold and M.C. Huff

501

Fine Chemicals and Pharmaceuticals

A-36

A-37

A-38

Chemoselective Catalytic Oxidation of Polyols with Dioxygen on Gold Supported Catalysts L. Prati and M. Rossi

509

Promoting Effects of Bismuth in Carbon-Supported Bimetallic Pd-Bi Catalysts for the Selective Oxidation of Glucose to Gluconic Acid M. Wenkin, C. Renard, P. Ruiz, B. Delmon and M. Devillers

517

Oxidative Dehydrogenation of Glycofic Acid to Glyoxylic Acid over Fe-P-O Catalyst M. Ai and K. Ohdan

527

xii

A-39

Shape Selective Epoxidation of Crotyl Alcohol with H202 in the Presence of TS-1 G.J. Hutchings, P.G. Firth, D.F. Lee, P. McMorn, D. Bethell, P.C. Bulman Page, F. King and F. Hancock 535

A-40

Epoxidation of Tertiary Allylic Alcohols and Subsequent Isomerization of Tertiary Epoxy-Alcohols: A Comparison of some Catalytic Systems for Demanding Ketonization Processes J.-M. Br6geault, C. Lepetit, F. Ziani-Derdar, O. Mohammedi, L. Salles and A. Deloffre 545

A-41

Metal-Catalyzed Oxidations with Alkyl Hydroperoxides: A Comparison between tert-Butyl Hydroperoxide and Pinane Hydroperoxide H.E.B. Lempers and R.A. Sheldon

557

On the Way to Redox-Molecular Sieves as Multifuncfional Solid Catalysts for the One-Step Conversion of Olefins to Aldehydes or Ketones M. van Klaveren and R.A. Sheldon

567

Liquid-Phase Oxidation of Cyclohexane to Adipic Acid Catalysed by Cobalt Containing ~-zeofites I. Belkhir, A. Germain, F. Fajula and E. Fache

577

A-42

A-43

A-44

A-45 A-46

Nitrogen Oxides Catalyzed Selective Oxidation by Oxygen in the Liquid Phase A.B. Levina, S.S. Chornaja, I.A. Grigorjeva, O.N. Sergejeva and S.R. Trusov 585 Oxidative Coupling of Isobutene in a Two Step Process

H. Hiltner and G. Emig

593

Sofid Solutions for Cleaning up Chemical Processes using Hydrogen Peroxide S.L. Wilson and C.W. Jones

603

Engineering and Environmental Applications A-47 A-48

Catalytic Wet Air Oxidation of Wastewaters

J.C. B6ziat, M. Besson, P. Gallezot, S. Juif and S. Dur~cu

Catalytic Partial Oxidation of Methanol: H2 Production for Fuel Cells

L. Alejo, R. Lago, M.A. Pefia and J.L.G. Fierro

615 623

xiii

A-49

Catalytic Liquid-Phase Phenol Oxidation over Metal Oxides and Molecular Sieves. Reaction Kinetics and Mechanism A. Pintar, G. Ber~i(~, J. Batista and J. Levec 633

A-50

Ammonia Oxidation over CuO/Ti02 Catalyst: Selectivity and Mechanistic Study G. Bagnasco, G. Peluso, G. Russo, M. Turco, G. Busca and G. Ramis 643

A-51

Metalloporphyrin-Catalysed Oxidation of Azonaphthol Dyes: The Mechanism of Oxidative Bleaching by Oxoiron (IV) Porphyrins in Aqueous Solution G. Hodges, J.R. Lindsay Smith and J. Oakes

A-52

653

VOC's Abatement: Photocatalytic Oxidation of Toluene in Vapour Phase on Anatase Ti02 Catalyst V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano, M. Pantaleone and M. Schiavello 663 PartB Methane Activation

B-1

Oxidation Processes on Stoichiometric and Nonstoichiometric Hydroxyapatites H. Hayashi, H. Kanai, Y. Matsumura, S.Sugiyama and J.B. Moffat

673

B-2

Oxidative Coupling of Methane in Sofid Oxide Fuels Cells Guo Xiu-Mei, Kus Hidajat and Chi-Bun Ching

683

B-3

Partial Oxidation of Methane to Synthesis Gas in a Fast Flow Membrane Reactor M. Alibrando and E.E. Wolf 693

B-4

Sustainable Ni/Ba TiO3 Catalysts for Partial Oxidation of Methane to Synthesis Gas R. Shiozaki, A.G. Andersen, T. Hayakawa, S. Hamakawa, K. Suzuki, M. Shimizu and K. Takehira

B-5

701

Synthesis of Early Transition Metal Carbides and their Application for the Reforming of Methane to Synthesis Gas A.P.E. York, J.B. Claridge, C. Marquez-Alvarez, A.J. Brungs, S.C. Tsang and M.L.H. Green 711

xiv

B-6

Partial Oxidation of Methane to Synthesis Gas using LnCoO3 Perovskites as Catalyst Precursors 721 R. Lago, G. Bini, M.A. PeSa and J.L.G. Fierro

B-7

Performance of Catalytic Properties of Reagent Catalyst in the Processes such as Methane Oxidative Coupling and Hydrogen Production by Methane Conversion M.I. Levinbuk, N.Y. Usachev, M.L. Pavlov, A.U. Loginov, L.V. Surkova, E.M. Savin, V.K. Smirnov and I.V. Ivkova 731

Combustion B-8

The Effect of the PbO Loading in the Oxidative Coupling of Methane over PbO/Si02 Catalysts H.J. Lugo, N. Teran, L. Villasmil, G. Castillo and D.M. Finol 737

B-9

Catalytic Combustion of Ethane over High Surface Area Lnl_xKxMnOa (Ln=La, Nd) Perovskites: The Effect of Potassium Substitution Y.Ng Lee, F. Sapi~a, E. Martinez, J.V. Folgado and V. Cortes Corber&n 747

B-10

Effect of Redox Treatment on Methane Oxidation over Binary Catalyst Yu.P. Tulenin, M.Yu. Sinev, V.V. Savkin and V.N. Korchak

757

B-11

Catalytic Combustion of Methane: Activation and Characterization of Pd/AI203 M.G. Carneiro da Rocha and Roger Frety

767

B-12

Activity of Manganese Dioxides towards VOC Total Oxidation in Relation with their Crystallographic Characteristics C. Lahousse, A. Bernier, E. Gaigneaux, P. Ruiz, P. Grange, B. Delmon 777

Catalyst Preparation B-13

Understanding the Surface Chemistry for Supported Vanadium Oxide Systems Modified with Phosphorus Oxide at Hydrocarbons Oxidation V.A. Zazhigalov, L.V. Bogutskaya, L.V. Lyashenko and I.V. Bacherikova 787

B-14

Effects of Cesium Doping on the Kinetics and Mechanism of the n-Butane Oxidative Dehydrogenation over Nickel Molybate Catalysts L.M. Madeira and M.F. Portela 797

B-15

A Comparison of Iron Molybdate Catalysts for Methanol Oxidation Prepared by Coprecipitation and New Sol-Gel Method A.P. Vieira Soares, M. Farinha Portela and A. Kiennemann

807

XV

B-16

B-17

B-18

Oxidation Catalysts Prepared by Mechanically and Thermally Induced Spreading of Sb203 and V205 on Ti02 U.A. Schubert, J. Spengler, R.K. Grasselli, B. Pillep, P. Behrens and H. KnOzinger

817

The Effect of Preparation Parameters on the BET Surface Area of Zr02 Powder YuanYang Wang, YanZhen Fan, YuHan Sun and SongYing Chen

829

Preparation of VOHPO4-O.5H20 and (VO)2P207 and their Catalytic Performance for Maleic Anhydride Synthesis T. Miyake and T. Doi

835

Alternate Oxidants

B-19 B-20

B-21

B-22

B-23

Hydroxylation of Benzene on ZSM5 Type Catalysts M. H~fele, A.Reitzmann, E. Klemm and G. Emig

847

Direct Hydroxylation of Benzene to Phenol by Nitrous Oxide

A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov

857

Rapid Catalytic Oxygenation of Hydrocarbons with Perhalogenated Ruthenium Porphyrin Complexes J.T. Groves, K.V. Shalyaev, M. Bonchio and T. Carofiglio

865

Ethanol Oxidation Using Ozone over Supported Manganese Oxide Catalysts: An in situ Laser Raman Study Wei Li and S.T. Oyama

873

Generation of Singlet Oxygen from the Catalytic System H20/Ca(OH)2 and Appfications to the Selective Oxidation of Unsaturated Compounds J.M. Aubry and V. Nardello

883

Oxidation of Olefins and Aromatics

B-25

B-26

Toluene Gas Phase Oxidation to Benzaldehyde and Phenol over V-containing Micro- and Mesoporous Materials G. Centi, F. Fazzini and L. Canesson and A. Tuel

893

A Novel Selective Oxidation Catalyst: Ultrafine Complex Molybdenum Based Oxide Particles Y. Fan, W. Kuang, W. Zhang and Yi Chen

903

xvi

Liquid Phase Oxidation of Alkylaromatic Hydrocarbons over Titanium Silicalites G.N. Vayssilov, Z. Popova, S. Bratinova and A. Tuel

909

Coupled Vanadyl Centres in Vanadium Phosphorus Oxide Catalysts: Essential Structural Units for Effective Catalytic Performance in the Ammoxidation of Methylaromatics A. Br0ckner, A. Martin, B. LOcke and F.K. Hannour

919

B-29

Ammoxidation of Xylenes- Kinetics and Selectivity K. Beschmann, S. Fuchs and T. Hahn

929

B-30

Vanadium-Titanium Oxide System in B-Picoline Oxidation E.M. Al'kaeva, T.V. Andrushkevich, G.A. Zenkovets, G.N. Kryukova, S.V. Tsybulya and E.B. Burgina

939

Selective Alkene Epoxidation by Molecular Oxygen in the Presence of Aldehyde and Different Type Catalysts Containing Cobalt O.A. Kholdeeva, I.V. Khavrutskii, V.N. Romannikov, A.V. Tkachev and K.I. Zamaraev

947

Epoxidation of Olefins over Thermally Stable Polyimide-Supported Mo(VI) Complexes J.H. Ahn, J.C. Kim, S.K. Ihm and D.C. Sherrington

957

Selective Partial Oxidation of Propylene to Propylene Oxide on Au/Ti-MCM Catalysts in the Presence of Hydrogen and Oxygen Y.A. Kalvachev, T. Hayashi, S. Tsubota and M. Haruta

965

B-27

B-28

B-31

B-32

B-33

B-34

Immobilization of Triazacyclononane-type Metal Complexes on Inorganic Supports via Covalent Linking: Spectroscopy and Catalytic Activity in Olefin Oxidation Y.V. Subba Rao, D.E. DeVos, B. Wouters, P.J. Grobet and P.A. Jacobs 973

B-35

Simultaneous Determination of Reaction Kinetics and Oxygen Activity during Selective Oxidation of an Aldehyde over an Oxidic Multicomponent Catalyst M. Estenfelder and H.-G. Lintz

981

On the Mechanism of the Selective Oxy-Dehydrogenation of n-Butenes to 1,3-Butadiene on Magnesium Ferrite: an FT-IR Study E. Finocchio, G. Busca, G. Ramis and V. Lorenzelli

989

B-36

xvii

Oxidation in Confined Structures B-37

Cyclohexene Oxidation Catalyzed by Titanium Modified Hexagonal Y Type Zeofites

K.J. Balkus, Jr., A.K. Khanmamedova and J. Shi B-38 B-39

Oxidations Catalyzed by Zeofite Ti-UTD-1

K.J. Balkus, Jr. and A.K. Khanmamedova

Zeofite Titanium Beta: A Selective Catalyst in the Meerwein-PonndorfVerley-Oppenauer Reactions J.C. van der Waal, P.J. Kunkeler, K. Tan and H. van Bekkum

B-40

Selective Oxidation of Cyclohexane over Rare Earth Exchanged Zeofite Y E.L. Pires, M. Wallau and U. Schuchardt

B-41

Rationally Designed Oxidation Catalysts: Functionalized Metalloporphyrins Encapsulated in Transition Metal-Doped Mesoporous Silica Lei Zhang, Tao Sun and J.Y. Ying

B-42 B-43

Catalytic Oxidations with Biomimetic Vanadium Systems I.W.C.E. Arends, M. Pellizon Birelli and R.A. Sheldon

1015

1025

1029 1031

1043

Catalytic Oxidations by in situ Generated Peroxotungsten Complexes Immobilized on Layered Double Hydroxides (LDH): Relation Between Catalytic Properties and Peroxotungstate Micro-Environment B.F. Sels, D.E. DeVos and P.A. Jacobs

B-45

1007

Highly Selective Photochemical and Dark Oxidation of Hydrocarbons by 02 in Zeofites H. Frei

B-44

999

Homobimetallic Heptadentate Coordinated Iron Complexes in Montmorillonites as Methane MonoOxygenase Mimics P.P. Knops-Gerrits, S. Dick, A. Weiss, M. Genet, P.Rouxhet, X.Y. Li

and P.A. Jacobs

1051

1061

Theoretical, Computational and Modeling Studies B-46

Modeling the Transient CO Oxidation over Platinum

T.A. Nijhuis, M. Makkee, A.D. van Langeveld and J.A. Moulijn

1071

xviii

B-47

Dioxygen Activation with Sterically Hindered Tris(pyrazolyl)borate Cobalt Complexes K.H. Theopold, O.M. Reinaud, D. Doren and R. Konecny

1081

Designing Industrial Redox Catalysts for Selective Autoxidations of Hydrocarbons - A New Paradigm D. Masilamani

1089

B-49

Selectivity of Active Sites on Oxide Catalysts C. Batiot, F.E. Cassidy, A.M. Doyle and B.K. Hodnett

1097

B-50

A Novel Computer-Aided Technique for the Development of Catalysts for Propane Ammoxidation to Acrylonitrile X.-Q. Wu, Q.-X. Zhang, Q.-L. Dai, Z.-Y. Hou and D.-W. Lu

1107

Catalysts by Rational Design: Prediction and Confirmation of the Properties of the Co/Ce/Br Liquid-Phase Autoxidation Catalyst Based on the Kinetic Similarity to the Co/Mn/Br Catalyst R.K. Gipe and W. Partenheimer

1117

B-48

B-51

PartC Additional Oxidation Studies

C-1

The Kinetics of the Partial Oxidation of Methane to Formaldehyde over a Silica-Supported Vanadia Catalyst A.W. Sexton and B.K. Hodnett 1129

C-2

Catalytic Destruction of Volatile Organic Compounds on Platinum/Zeolite A. O'Malley and B.K. Hodnett 1137

C-3

High Temperature Propane Oxidation to Reducing Gas over Promoted Ni/MgO Catalysts. Role of Impregnation Condition and Promoter on Properties of Catalysts M.V. Stankovi6 and N.N. Jovanovi6 1145

C-4

Structural Sensitivity of the Oxidation Reactions Catalyzed by Dispersed Transition Metal Oxides: Role of Defect Structure V.A. Sadykov, S.F. Tikhov, S.V. Tsybulya, G.N. Kryukova, S.A. Veniaminov,V.N. Kolmiichuk, N.N. Bulgakov, L.A. Isupova, E.A. Paukshtis, V.I. Zaikovskii, G.N. Kustova and L.B.Burgina 1155

C-5

Oxidation of Cyclohexane using Polymer Bound Ru(lll) Complexes as Catalysts J. John, M.K. Dalai and R.N. Ram

1165

xix

C-6

Photoimmobilized Catalysts for Low-Temperature Oxidation of Olefins L.V. Lyashenko, V.M. Belousov, E.V. Kashuba

1175

C-7

Selective Oxidation Catalysis over Heteropoly Acid Supported on Polymer In Kyu Song, Jong Koog Lee, Gyo Ik Park and Wha Young Lee

1183

A Study of V205-K2S04-Si02 Catalysts for Catalytic Vapor-Phase Oxidation of Toluene to Benzaldehyde A.O. Rocha Jr., A.L. Chagas, L.S.V.S. Sufi6, M.F.S. Lopes and J.A.F.R. Pereira

1193

C-8

C-9

OyH2 Oxidation of Hydrocarbons on the Catalysts Prepared from Pd(ll) Complexes with Heteropolytungstates N.I. Kuznetsova, L.I. Kuznetsova, L.G. Detusheva, V.A. Likholobov, 1203 M.A. Fedotov, S.V. Koscheev and E.B. Burgina

C-10

Oxidation of n-Pentane to Phthalic or Maleic Anhydride: The Role of the VPO Catalyst Structural Disorder Z. Sobalik, S.Gonzalez, P. Ruiz and B. Delmon

1213

C-11

Electrochemical Oxidation of Propene using a Membrane Reactor with Sofid Electrolyte S. Hamakawa, T. Hayakawa, K. Suzuki, M. Shimizu and K. Takehira 1223

C-12

Vanadium Pentoxide Catalytic Membrane Reactor for Partial Oxidation of 1-Butene Sangjin Moon, Tayoon Kim, Seungdoo Park, Jihoon Jung and Sukin Hong 1231

C-13

Peroxidase Oxidation of Phenol by Catalase Immobilized on Carbon Materials E. Horozova, N. Dimcheva and Z. Jordanova

1239

Author Index

1245

This Page Intentionally Left Blank

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

Molecular mechanism of heterogeneous oxidation - organic and solid state chemists' views Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland Based on the discussion of the state-of-the-art in heterogeneous oxidation of hydrocarbons this paper reviews the most important but yet unanswered questions. It is emphasized that the oxide surface is in dynamic interaction with the gas phase. The role of electrophilic oxygen is illustrated and the scarcity of information on its formation at the oxide surface with point and extended defects is stressed. The molecular mechanisms of C-H bond activation are discussed and a conclusion is substantiated that both parts of the cleaved C-H bond bind to surface oxide ions with injection of electrons into the conductivity band of the oxide. The importance of the transfer of electrons between the oxide and the hydrocarbon+oxygen gas mixture as a redox system is illustrated. The lack of data on the role of coordination of surface cations, of the presence of defects and of the degree of surface hydroxylation in catalytic oxidation is underlined. The transition metal oxide monolayer catalysts are discussed and the question is raised concerning the mechanism, by which the support influences the behaviour of the monolayer. Also the mechanism of the addition of nucleophilic oxygen to the hydrocarbon molecule remains unanswered, although a hypothesis has been advanced that is favoured when crystallographic shear mechanism operates. A number of questions may be raised as to the mechanism of the operation of an oxide monolayer catalyst are isolated metal-oxygen polyhedra playing the role of active sites, or is the presence of oligomeric species required? Where are these sites located and how are their properties influenced by the type of crystal plane and the presence of defects? General conclusions concerning the mechanism of heterogeneous catalytic oxidation are discussed. 1.

INTRODUCTION

Oxygen is one of the most interesting elements playing a fundamental role in catalysis, because on the one hand it is a component of the most widely used type of catalysts - oxides, and on the other hand it is the reactant in one of the most important types of catalytic reactions - oxidation. The attack of oxygen on the hydrocarbon molecular is the easiest route to functionalize this molecule, and selective oxidation processes, in which hydrocarbon molecules are oxygenated to form alcohols, aldehydes or acids are the basis of the modern petrochemical

industry. They may be divided into vapour or liquid phase reactions, which are catalyzed by solid oxide catalysts and are carried out as heterogeneous catalytic processes and reactions in the liquid phase, catalyzed by transition metal complexes or by enzymes, which are commonly realized as homogeneous catalytic processes. In all these processes the essence of the catalytic act is the same and consists in the interaction of reacting hydrocarbon molecules with a group of atoms playing the role of an active site of the catalyst. In the case of heterogeneous catalytic processes carried out in the presence of solid oxide catalysts the atoms of active sites are either constituents of the oxide lattice or are supported on its surface forming part of the solid. When discussing the mechanism of heterogeneous catalytic oxidation of hydrocarbons, the organic chemists (1) usually consider only the fate of the reacting molecule, trying to unravel the reaction network in terms of consecutive oxidative additions and reductive eliminations and molecular rearrangements. The analysis is based on the consideration of possible transition states formed as a result of rearrangement of electron pairs due to exchange of electrons with an active site of the catalyst, which serves as an acceptor or a donor of electrons. The observed influence of different substituents in the organic molecule on the reaction rate and selectivity helps in elucidation of the transition state. When radical reactions are involved, one electron exchange with the catalyst serves as chain initiation. Little attention is paid to the nature and structure of the active site and the role of its environment. The solid state chemist approaches the problem in a different way(2). His main interest focus on the phase composition of the solid, type of crystal planes exposed, presence of additives and impurities, oxidation states of the cations and their changes in the course of the reaction, type of defects in the oxide lattice, etc. Correlation is sought between these parameters and the activity and selectivity of the oxide system in the given reaction, but little attention is usually paid to the type of interactions between the hydrocarbon molecule and the surface and to the possible transition states. When these two approaches are integrated, several general conclusions may be formulated, but also a number of important yet unanswered questions emerge. 2. DYNAMIC

INTERACTIONS

AT OXIDE SURFACES

On the formation of the oxide surface dangling bonds and the dangling bonds charge density appears in an energetically unfavourable conformation. Therefore, a driving force is generated at the surface to redistribute this charge density so as to create an insulating surface (pair up the dangling bond electrons and open a gap between the occupied and unoccupied surface states). This can be done structurally by rearranging the surface atoms, electronically through strong electron correlation effects or by forming new bonds at the surface of the oxide between surface atoms and the adsorbate molecules (3). Experimental evidence (4-6) and results of quantum chemical calculations (7-9) indicate that the oxide surface is in dynamic interaction with reactants of the catalytic reaction. The interaction of an adsorbing molecule with the surface not only produces the changes of the structure

of the adsorbate, but induces also reconstruction of the oxide surface. The surface adapts itself to the requirements of the reaction, generating a more facile pathway for a concerted rearrangement of electrons and nuclei. Ample experimental evidence (10,11) shows that the same is true for metal surfaces. Reconstruction of the surface of heterogeneous catalysts under the influence of a reaction mixture demonstrates the validity of Newton's third law in heterogeneous catalysis (12). Thus, the heterogeneous catalytic system should be considered not as a twophase, but rather as a three phase system, composed of the gas phase reactants, the solid catalyst and the surface region built of adsorbed molecules interacting with the surface layer of the solid (13). The reacting molecules and the atoms of the solid surface constitute one quantum chemical system. The state of this surface region may be modified either by changing the parameters of the gas phase or by altering the parameters of the solid. Therefore, to obtain information about the mechanism of a heterogeneous catalytic reaction, "in situ" studies of all parameters are essential. In every oxidation reaction two reactants take part: the oxygen molecule and the hydrocarbon molecule to be oxidized. Molecular oxygen may be activated in different ways (14-16): by excitation to the singlet state or by transfer of electrons from the catalyst to the oxygen molecule to form molecular or atomic ion radicals 02or O-. All these forms are strongly electrophilic reactants. They may abstract hydrogen from the hydrocarbon molecule with the formation of alkyl radicals, which may start a chain reaction. Under mild conditions, e.g. in the case of the reaction in the liquid state the chain reaction leads to the formation of alcohols or carbonyl compounds. Under harsh conditions of heterogeneous reactions radical chain leads usually to total oxidation so that in oxidative coupling of methane the problem consists in finding reaction conditions, in which two methyl radicals combine before they can be attacked by oxygen (17). In reactions with olefins or aromatics the electrophilic oxygen species attack the regions of highest electron density - the ~bond system (18). Peroxo- and super oxo-complexes are formed, which decompose by C-C bond cleavage to give oxygenated fragments or undergo combustion. These reactions may be classified as electrophilic oxidations (19). The second route of heterogeneous oxidation starts with the activation of the hydrocarbon molecule by abstraction of hydrogen from a given carbon atom, which becomes prone to nucleophilic addition of the oxide ion O2-. It should be emphasized that the latter has no oxidizing properties, but is a nucleophilic reactant. The consecutive steps of hydrogen abstraction and oxygen addition may be then repeated to obtain selectively more oxygenated molecules. These reactions are classified as nucleophilic oxidation (19). The role of oxidizing agents in these steps of the reaction sequence is played by cations of the catalyst lattice. After the nucleophilic addition of the lattice. After the nucleophilic addition of the lattice oxygen ion the oxygenated product is desorbed, leaving a vacancy at the catalyst surface. 3. INTERACTION OF OXIDES WITH GAS PHASE OXYGEN Little is known about the population of the surfaces of transition metal oxides

with electrophilic oxygen species. Transition metal oxides are non-stoichiometric compounds, their composition depending on the equilibrium between the lattice and its constituents in the gas phase. Changes in oxygen pressure cause changes in stoichiometry of the oxide, which may be accommodated by the crystal lattice in two ways: either by generation of point defects or by alteration of the mode of linkage between the oxide coordination polyhedra, resulting in the formation of extended defects (crystallographic shear). When non-stoichiometry is introduced by the presence of point defects, a series of equilibria is established at the surface on the pathway of lattice oxygen from the bulk to the gas phase or in the reversed process (Figure 1).

02

-

0

2-

~

02

-

~-

02

0 2- M(n-1)+'~ 'Mn+ 0 2- Mn+ 0 2- Mn+ 0 2- [~ ~0

Mn + 0 2- Mn +

2-

0 2-..

Mn+ Mn+ 0 2- M n+ 0 2- M n+ 0 2- M n+

0 2- Mn+ 0 2- M(n-1)+[[[]

Mn

0 2- Mn+ 0 2-

Mn+ 0 2-

Figure 1. Oxygen equilibria at the surface of an oxide When the temperature increases, the equilibrium shifts in the direction of higher dissociation pressure of the oxide and the surface becomes more and more populated with electrophilic oxygen species. When used as catalysts in oxidation of hydrocarbons, such oxides may show high selectivity to partial oxidation products at low temperatures, wherein the surface coverage with transient electrophilic oxygen forms is low. Under such conditions the conversion is very low. On raising the temperature the selectivity to partial oxidation products rapidly decreases, whereas the conversion in total oxidation increases, becoming the predominant reaction

pathway (4). It should be borne in mind that in the case of those oxides, in which the transition metal cation is not in its highest oxidation state, chemisorption of oxygen takes place at low temperatures, electrons being transfered from the oxide to adsorbed oxygen molecules with formation of higher valent cations and electrophilic oxygen species. This may be followed by surface reconstruction and formation of a monolayer of higher valent oxide at the surface of the lower valent one (4). Practically no experimental data exist on the mechanism of the dissociation of transition metal oxides. Little information can be found on the temperature dependence of the equilibrium oxygen pressure, and oxygen adsorption isobars are in most cases unknown. One should also bear in mind that doping of the oxide by altervalent ions may strongly influence its equilibrium oxygen pressure (20), that could permit a control of the activity of surface electrophilic species and hence the increase of selectivity. Different behaviour is shown by oxides, in which the change of stoichiometry is accommodated by the formation of shear structures. Because there is no vacancy formation on extraction of nucleophilic oxygen ion from the oxide surface due to its addition to the hydrocarbon molecule and desorption of the oxygenated product, but instead a shear plane is formed or an existing one grows (21,22), only nucleophilic oxygen species remain exposed at the surface. Very scarce data indicate that these oxides, in which the metal cation is in its highest oxidation state, do not adsorb oxygen. On raising the temperature the activity increases with growing mobility of oxygen, but the selectivity to partial oxidation products remains very high because of the absence of electrophilic oxygen. 4. ACTIVATION OF THE C-H BOND

The process of oxidation of a hydrocarbon molecule must begin with the activation of the C-H bond. Realization of this first elementary step is particularly challenging for physical chemists and chemical engineers because it must be achieved in the presence of many constraints. Namely, the C-H bonds in the initial reactant are usually stronger than those in the intermediate products, which makes these intermediates prone to rapid further oxidation and renders the C-H bond activation rate determining. Very little information on the molecular mechanism of the C-H bond activation at oxide surfaces can be found in the literature, and practically nothing is known about the activation of this bond in alkanes. Coordination of aikanes to transition metals has been observed both in liquid and in the gas phase (23), but the complexes are unstable at higher temperatures and their existence has been detected by matrix isolation technique. The alkane molecule can be coordinated to the transition metal through one, two or three hydrogen atoms forming C-H...M o--complexes, or it can be coordinated through its C-H bond forming with the electrophilic metal atom a two-electron, three-center bond, as in the triangular species H3+ formed from H2 and H+ in the gas phase. The interpretation was borrowed from the idea that carbonium ions such as C3HF+have non-classical bridged structures with two electron three center bonds (24). When the metal is capable of back donating

electrons to the antibonding C-H orbital, oxidative addition of the alkane to the metal may take place. Many ~-complexes are in equilibrium with their oxidative addition products (25). A growing body of evidence supports the intermediacy of transition metal alkane complexes in solution, C-H activation and reductive elimination. I r, Rh and W complexes may be mentioned. Reactions in the gas phase of neutral transition metal atoms (Pd, Pt) and ions (first row transition metal cations M+) with different alkanes and alkenes have been demonstrated and the metal-alkane complexes have been implicated as intermediates in the dehydrogenation. A question must be raised as to whether this type of mechanism could operate in the activation of hydrocarbon molecules at the surface of oxide catalysts. Very scarce experimental data seem to indicate that alkanes are adsorbed at oxide surfaces weakly and only at low temperatures. Stronger is the adsorption of olefins, which form surface ~-complexes with coordinately unsaturated metal ions characterized by Lewis acid properties. Activation of the CH bond is usually the rate determining step and the reactions are first order in respect to hydrocarbon indicating that the non-dissociative adsorption is absent. At the surface of oxides of transition metal ions in the highest oxidation state, which are usually components of oxidation catalysts, the mechanism of back-donation weakening the C-H bond cannot operate, and indeed pure MoO3 or WO3 are inactive in the conversion of hydrocarbons. They are, however, known to perform the addition of nucleophilic oxygen efficiently once these molecules have been activated. Some other oxide component must be introduced, e.g. Bi203, SnO2 etc. to activate the hydrocarbon molecule and render the catalyst both active and selective (18, 21, 67). In processes of heterogeneous catalytic oxidation, heterolytic C-H bond cleavage on the acid-base pair of sites is usually considered. Two possibilities are taken into account: - abstraction of hydrogen in the form of a proton and formation of a carboanion fragment. ~/ C-(T ----- H+~ i ~ M+n O-2 abstraction of hydrogen in the form of a hydride ion and formation of a positively

charged hydrocarbon fragment ~ !

M+n

0-2

Three mechanisms of homolytic bond rupture are also possible, two with the generation of radicals: - when the transition metal ion has electrophilic properties, and a vacant coordination site, a two-electron three-center bond between the C-H (~ bond and

the metal can be formed, and when the metal contains non-bonding d-electrons which can be back-donated, oxidative addition takes place: C .... H ~' 9 d

Mn+

.~ "~'

"

C M(n+2)+

when an easily reducible cation is present at the surface with basic properties, C-H bond cleavage may take place with the formation of an alkyl radical a proton with simultaneous reduction of the metal cation. C ~

H+ M+n

when atomic or molecular oxygen radicals are formed at the surface, they may abstract the hydrogen atom generating alkyl radical: C'~

~-H

o: Such a mechanism is postulated to operate in the activation of methane at high temperatures in the process of the oxidative coupling (65). Catalysts which are both active and selective for the oxidative coupling of methane may be classified as strongly basic metal oxides. Substitution of Iower-valent cations in their lattice generates oxygen vacancies, which constitute electron acceptor levels and are responsible for the appearance of electron holes in the valence band. These holes diffuse to the surface, because the lone pair orbitals of surface oxide ions are the HOMOs of the oxide and their energy levels form the top of the valence band. Localization of a hole on such lone pair orbital is equivalent to the formation of a surface O- species. On superacidic catalysts the formation of a carbenium ion is suggested to be the first step of the C-H bond activation R\C / R

/i\

H H+H O The data on hydrogen/deuterium exchange in paraffins on oxides indicated that a negatively charged intermediate is formed (26,27). Also the studies of adsorption of ethane and propane on oxide catalysts by I R spectroscopy showed that heterolytic dissociation of the C-H bond takes place with the formation of negatively

charged alkyl fragment (28). Indirect evidence of the abstraction of a proton as the first step in oxidation of hydrocarbons comes from experiments, in which catalytic activity was measured on series of catalysts with different acid-base properties. It has been found that the rates of oxidative dehydrodimerization of propene to 1,5hexadiene and the rates of its oxidation to acrolein increased with decreasing binding energy of O ls electrons and with decreasing difference between binding energy and Auger kinetic energy (29) of SnO2-based catalysts, which were due to the increasing negative charge on the oxygen ions i.e. an increasing basicity. Similar conclusions were derived (30) from studies of the oxidation of propene on SnO2 impregnated with acidic (P205) and basic (Na20) oxides. Optimum basicity was found to exist, reflecting the requirement that the oxide efficiently abstracts protons from hydrocarbon molecules in the first step of the reaction and releases the protons easily in the dehydroxylation step. Recently a distinct correlation was found (31) to exist between the rate of the oxidation of butane on vanadyl pyrophosphate catalysts, doped with alkali and alkaline earth cations, and the negative charge on oxide ions as determined by XPS. On the other hand no experimental evidence exists, except of the case of zinc oxide (32), of the formation of hydride ions in the course of adsorption of hydrocarbons on oxides. The activation of the C-H bond in CH4 has been analyzed on the basis of quantum-chemical calculations using the ASDED-MO (Atom Superposition and Electron Delocatization Molecular Orbital) approach (66). It has been concluded that on oxide catalysts the oxidative addition of the C-H bond to the metal cation requires a very high activation energy because the antibonding orbital, which must accept two electrons, lies very high. When a hole appears in this orbital, the reaction becomes more facile. In the series of consecutive elementary steps of the selective oxidation of a hydrocarbon molecule, the negatively charged alkyl, formed after abstraction of a proton, must become bonded to a surface oxide ion of the catalyst to form the alkoxy-species, generally accepted, to be next intermediates. Their appearance at the surface of oxide catalysts in the course of the selective oxidation of hydrocarbons has been proved experimentally by many techniques, e.g. in-situ IR and Raman spectroscopies (33). No attention was paid to date to the fact that a molecule approaching the surface of an oxide enters into the region of a strong electrostatic field stretching above this surface. Indeed, ab initio HF calculations carried out for V209and V209H8clusters, modelling the surface of V205 catalysts, showed (34) that a strong negative electrostatic field is surrounding the cluster. It may be thus hypothesized that in the oxidation of hydrocarbons at oxide surfaces the activation of the C-H bond starts with the polarization of the molecule by this electrostatic field. Semiempirical quantum-chemical calculations (35) of the interactions developing on approach of a propene molecule to the V10031H12 cluster taken as a model of V205 catalysts showed that at first a bond is formed between the carbon atom of propene and the bridging oxygen atom of the cluster. This is followed by abstraction of a proton with simultaneous injection of electrons into the empty d-levels of the cluster. A general conclusion may be advanced that in the activation of a C-H bond at the surface of a

transition metal oxide with semiconducting properties both parts of the cleaved C-H bond become attached to surface oxide ions. The proton forms an OH group, the hydrocarbon fragment forms an alkoxy group. Simultaneously the two electrons of this bond are injected into the conductivity band of the solid. This process may be represented by a scheme:

-•C+• 0-2

,,,

H+,,

M+

0-2

i

The alkoxy group then loses a second hydrogen and desorbs as an aldehyde or ketone. This series of transformations forms the first part of the Mars-van Krevelen redox mechanism, which is then followed by adsorption of oxygen from the gas phase, transfer of electrons from the solid to adsorbed oxygen molecule and incorporation of oxygen ions into the lattice of the oxide, which completes the redox cycle. The cycle involves two adsorbed redox couples: RH + O21/2 02 -!- 2e-

~ -~

R-O-+ H+ + 2e0 2-

of which the first injects electrons into the oxide, the second one extracts them from the oxide. The conditions of the electron transfer across the gas/solid interface are seldom considered in catalysis, but they are discussed in detail in electrochemistry of semiconductors and the same rules must be valid in the case of a catalytic reaction. The injection of electrons from an adsorbed redox pair into a semiconductor can take place spontaneously only if the redox potential of this pair is situated above the Fermi level and above the bottom of the conductivity band and the extraction of electrons from the solid - when the redox potential is located below the Fermi level and below the top of the valence band (Figure 2).

10

OROCARBON...... . I . . . .

CATALYTIC OXIDATION OF FIYDROCARBON MOLECULE CAN PROCEED

~

OXYGEN O~YGEN.~~~~,,.~'~~ REDOX

CATALYTIC oXIDATION OF HYDROCARBON MOLECULE CANNOT PROCEED, BECAUSE THE MOLECULE IS NOT ACTIVATED

]

CATALYTIC OXIDATION OF H Y D R O C A R B O N MOLECULE CANNOT PROCCED BECAUSE THE CATALYST IS NOT REOXIDIZED

Figure 2: The transfer of electrons across the adsorbate/oxide interface in the course of the heterogeneous catalytic oxidation of a hydrocarbon molecule by gas phase oxygen. The probability of these processes is a function of the density of states in the conductivity and valence band respectively at the potentials corresponding to the redox potential of the adsorbed species. In the electronic theory of catalysis in the later fifties only general thermodynamic rules were considered, but the conditions of the electron transfer were never applied to analyze the behaviour of different oxide catalysts in the oxidation of hydrocarbons. No quantitative data are available to make such analysis, although recently an attempt was made to interpret the changes of the rate of ethanol oxidation in terms of the density of states (36). The relative positions of the energy bands in the solid and the redox potential of the reacting molecule may be adjusted by: a) formation of one or more oxide/oxide interfaces with such values of the contact potentials that the conductivity band will

]! shift to the optimum position, b) doping of the oxide with altervalent ions, which will shift the Fermi-level, c) generation of surface defects, which will create a broad distribution of surface electronic states participating in the exchange of electrons with the reacting adsorbed molecules (37). 5. STRUCTURE SENSITIVITY AND THE ROLE OF DEFECTS

In the analysis of the mechanism of homogeneous catalytic reactions catalyzed by organometallic complexes, the number of vacant coordination sites at the transition metal cation is taken into account, required for the reactants to be coordinated (38). In the heterogeneous catalytic reaction, taking place at the surface of the solid the number of vacant coordination sites can vary either because of the location of the cation at different crystal faces or as the result of the generation of lattice defects. No information is available as to the role of either of these factors. Ample experimental evidence indicates that the habit of crystallites of the catalyst has a profound influence on the activity and selectivity of the reaction, which results in the appearance of structure sensitivity of the selective oxidation reactions at oxide catalysts (39). However, little is known about the origin of this phenomenon and about the differences of the structure of active sites present at various crystal planes. Even less is known about the role of defects present at the surface of an oxide, in determining the catalytic properties. Only recently studies of the properties of (100) surface of a monocrystal of NiO revealed that an ideal surface is chemically inert and the reactivity of the system increases only if defects are introduced in the surface (40). At such a surface, dissociation of molecular water to form hydroxyl groups is observed in contrast to an ideal surface which is inactive in water dissociation. 6. OXIDE

MONOLAYER

CATALYSTS

In recent years considerable attention has been paid to oxide monolayer catalysts, obtained by deposition of a transition metal oxide in submonolayer up to few monolayers coverage on the surface of another oxide of a main group or transition metal playing the role of a support. In the submonolayer range, isolated transition metal-oxygen polyhedra are anchored at the surface of the oxide support. Recent investigations showed (41) that under ambient conditions the surface molecular structures of transition metal oxy-ions are directly related to the surface pH at point of zero charge of the aqueous film on the oxide supports and can be predicted from the corresponding aqueous transition metal ions chemistry (42). Depending on the type of the system and the method of preparation transition metal ions condensation to form clusters, cover completely the support as a surface layer or form small crystallites of a second phase (43). The minority oxide (supported phase) can accumulate entirely at the surface of another oxide (support) when the temperature of annealing is low enough and the miscibility as well as chemical affinity of the two oxides are very limited. In the case of miscibility of the two oxide phases they may also be incorporated into to outermost surface layer of the support (surface framework) or diffuse into the bulk and form a solid solution at a high

12 enough temperature. In the case of chemical affinity of the two oxides new surface or bulk compounds may be synthesized. In the case of preparation by impregnation from the solution the oxide surface may play the role of solvating agent, counter ion or a macroligand (43) as illustrated in Figure 3.

I I

TRANSITION METAL ION IN SOLUTION

TRANSITION METAL ION

1 ISOLATED I OXO SPECIES I

, , I

/

~

g

.-o o-. i

,,% ~

I

SOLVE,NT

/

J

\~ o~)

~

OXIDE SURFACE PLAYS THE

/

(,,-~,,'F~I

",,. 2- 7,-J /

COUNTER ION

\

OF

ROLE

/

J

77

/

~

L,

/

/P

/

IN BIDIMENTIONAL POLYOXO METALATE CLUSTER

I

TRANSITION I METAL ION IN GAS PHASE I I~/L I

I

J

~

[]

<, <-, Y

/

LIGAND

/ / / / J

.,Y,.

,

O-M-O

J

I

IN OXIDE CRYSTALLITE

.,

,

I

J

1

/

FRAMEWORK

-o-' -o-~

o

o o o I

.,

J ~

o

Q

i

1

I

I

~'-,~-'~

J

.

~SOLI'~SOLUTION

J / /

6-~','9~ o

<,% ~' o- o

J

o,~o o;%

1

I

J

..

i

/

~

l

I

/ ./

/

Figure 3. Deposition of transition metal ions by impregnation from their aqueous solution on the oxide surface and their further transformations. As the solid support is a fairly rigid macroligand, mono- or polydentate, it distorts the environment of the transition metal ion and generates new active sites exhibiting specific properties. On increasing the loading of the support with the active phase, islands of the monolayer may at first be formed and three dimensional clusters of the active phase may grow either simultaneously or after completion of the monolayer. It should be borne in mind that the active phase may not be uniformly distributed over the whole surface of the crystallites of the support, but may be

]3 preferentially deposited on certain crystal planes only, giving rise to the phenomenon of structure-sensitivity of impregnation [44]. Properties of the active phase may be in different ways affected by supporting on different crystal planes. It has been shown in recent years that an equilibrium between hydrated (Fig. 3, form III) and dehydrated (Fig. 3, form IV) metal-oxygen polyhedra is established at the surface of a support depending on the water vapour pressure, the hydrationdehydration process being reversible [44]. It may be anticipated that the two types of species would have different catalytic properties, these properties being thus strongly dependent on the water vapour pressure. Moreover, the change of the water vapour pressure shifts the equilibrium of surface hydroxylation, which changes the ratio of Bronsted-to-Lewis acid sites. As yet, little is known about the influence of water on the behavior of the oxide catalysts, although in many industrial oxidation processes water vapour is introduced into the stream of reactants to improve selectivity. Further studies are required to unravel the mechanism of the influence of water on the surface properties of oxides. 7.

WETTING OF OXIDE SURFACE BY OTHER OXIDES

At higher temperatures thermal spreading may occur resulting in the formation of an oxide monolayer [46-48]. Spontaneous spreading of one oxide over the surface of another oxide is the manifestation of the wetting of one solid by another solid due to the operation of the forces of surface tension. It occurs when the free energy of adhesion of the mobile phase to the support is greater than the free energy of cohesion of the mobile phase and continues until the formation of a thermodynamically stable overlayer is completed, characterized by a surface free energy equal to that of the bulk of the active oxide phase. The rate of spreading is limited by surface diffusion described by a parabolic rate equation and depends strongly on the crystallinity of the support and type of the atmosphere [49]. When the monolayer is deposited on a support which is not wetted, coalescence of the monolayer into crystallites of the deposited transition metal oxide takes place on annealing. As the free energies of cohesion and adhesion depend on the oxidation state of the oxide monolayer, reduction and reoxidation entail strong changes of the degree of dispersion. It should be also remembered that the surface free energy is very sensitive to the presence of additives (impurities). Therefore wetting of the surface of the support by the active phase may be controlled by the introduction of appropriate additives, which either remain at the surface or diffuce into the subsurface layer. Unfortunately, practically no experimental data are available on the surface free energy of oxides and their dependence on the type and concentration of defects. A simplified method to calculate the surface free energy on the basis of the ionic model of solids has been proposed some time ago [50]. In recent years attempts are being undertaken to introduce the clusters of transition metal oxides into frameworks, which could control the spacial arrangement of molecules reacting at the active sites provided by the oxide. To this end pillared clays are synthesized, in which either the pillars are built of the active transition metal oxide phase, or this phase id deposited onto the pillars, composed of an inert

14 material. Introduction of clusters of transition metal oxides into the zeolite framework has also been reported. Such "ship-in-the-bottle" catalysts are now the subject of considerable interest. 8.

NUCLEOPHILIC

ADDITION OF OXYGEN

The fundamental question, which has not been answered yet, concerns the properties of the oxide required to perform the nucleophilic addition of oxygen to the hydrocarbon molecule. In the course of the desorption of the oxygenated hydrocarbon molecule as a product of the reaction, an oxygen atom becomes extracted from the surface of the oxide catalyst, leaving behind an oxygen vacancy. Simultaneously, hydrogen atoms abstracted from the hydrocarbon molecules and present at the surface in the form of hydroxyl groups, must be removed by dehydroxylation, which also generated surface oxygen vacancies. It is a well known fact that all active and selective catalysts for partial oxidation of hydrocarbons are based on group V, VI or VII transition metal oxides. These oxides show a strong tendency to eliminate the vacancies by the formation of shear planes, which are nucleated at the surface with the simultaneous release of oxygen by the crystal. A hypothesis was advanced [51-53] that this tendency is a driving force facilitating the desorption of the oxygenated product. An easier and efficient route is thus provided for the addition of a nucleophilic lattice oxygen into the hydrocarbon molecule. It should be however borne in mind that crystallographic shear may not be the only pathway of nucleophilic insertion of oxygen into the hydrocarbon molecule. A number of oxide systems are known, which show good activity and high selectivity in selective oxidation, but do not undergo crystallographic shear. One can mention heteropolyacids and their salts [54] and the important class of oxide monolayer catalysts. With respect to the monolayer catalysts an important question has been raised as to the nature of active sites at the surface of such catalysts. As already mentioned, depending on the surface coverage of the support, the monolayer is composed of isolated MOx polyhedra, regions of a monolayer composed of clusters of condensed polyhedra or three dimensional crystallites of the active phase. Moreover, transition metal ions of the monolayer diffuse into the subsurface region forming a solid solution. Experiments show that part of the monolayer can be dissolved in appropriate solvents, but part is strongly bound [55-57]. In spite of ample experimental data there is no agreement as to which is the nature of the soluble part or the structure of active sites, responsible for the catalytic reaction. If the activity were related to the presence of loosely bound clusters of condensed MOx polyhedra, as indicated by some experiments [58], one could imagine a local rearrangement from corner-linked to edge-linked set of MOx polyhedra, resembling the nucleation of a shear plane, to be responsible for the nucleophilic addition of oxygen. Experiments show that on vandium oxides supported on titania, butane is oxidized to maleic anhydride, whereas on vanadium oxides supported on magnesia it undergoes oxidative dehydrogenation to butene. The mechanism, by which the support changes the properties of vanadium oxides so dramatically remains a fascinating yet unaswered question.

].5 As the phenomenon of crystallographic shear appears in transition metal oxides with anisotropic lattices, pronounced structure-sensitivity of catalytic properties is observed and the habit of crystallites of the catalyst may have strong influence on the selectivity of the reaction [39,59-61]. Multiple examples of the dependence of catalytic properties on the type of exposed crystal plane have been described in literature, but the only attempt to explain this phenomenon in terms of the molecular structure of different crystal planes was undertaken in the crystallogrphic model of active sites [62]. The question awaits more dedicated experiments and deeper theoretical analysis. The first quantum-chemical approach addressing this question has been quite recently published [63]. 9.

CONCLUSIONS

Quantum chemical calculations have already permitted some general conclusions concerning the mechanism of heterogeneous catalytic oxidation [64]. In electrophilic oxidation the type of product formed depends on the direction of the approach and mutual orientation of the hydrocarbon molecule and oxygen, and on the mode of oxygen activation. In nucleophilic oxidation by surface lattice oxygen the site of the hydrocarbon molecule attacked and hence the reaction pathway followed is critically dependent on the structure of the active site and the orientation of the molecule approaching the surface. The desired chemo- and regio-selectivity could be thus achieved by imposing a molecular field appropriately directing the incoming molecule. The incorporation of the molecular recognition function will add the reaction specificity. The ultimate goal of the science of catalysis is the accumulation of knowledge, necessary for molecular design of catalytic systems tailored to the needs of any imaginable catalytic reaction. Answers to the questions raised in this paper will make this goal possible, and hence will change the paradigm of catalysis. REFERENCES

1. G.A. Olah, A. Molnar, Hydrocarbon Chemistry, John Wiley Sons Inc., New York 1995. 2. Solid State Chemistry in Catalysis, eds R.K. Grasselli, J.F. Brazdil, ACS Symp.Series 279, American Chemical Society, Washington, 1985 3. J.P. LaFemina, Surf.Sci.Rep. 16 (1992) 133; Critical Rev.Surf.Chem.3 (1994) 297 4. J.Haber, Materials Sci. Forum 25 (1988) 17 5. J.Haber, Proc. 4th Inern Conf. Chemistry and Uses of Molybdenum, Golden CO 1982, eds H.F.Brry. P.C.W.Mitchell, Climax Molybdenum Co, AnnArbor 1982, p.395 6. H.J.Freund.B.Dillmann, O.Seiferth, G.Klivenyi, M.Bender, D.Ehrlich, I.Hermmerich,D. Cappus, Catal. Today 32 (1996) 1. 7. J. Haber. M. Witko, Catal. Today 23 (1995) 311.

16 8. M. Witko, R.Tokarz, J.Haber, Catal.Today in print. 9. M.J. Gillan, L.N.Kantorovich,P.S.D. Lindan, Current Opinion in Solid State Mat. Sci., 1 (1996) 820 10. M.A. vanHove, G.A. Samorjai, Surf.Sci. 299/300 (1994) 487 11. G.A. Samorjai, Catal.Lett 9 (1991) 311, Surf.Sci. 299/300 (1994) 849 12. K.l.Zamaraiev, Topics in Catal.3 (1996) 1. 13. J. Haber, in "Perspectives in Catalysis", eds J.M. Thomas, K.l.Zamaraiev, Blackwell Scientific Publ., Oxford 1992, p.371 14. M. Che, A.J. Tench. Adv. Catal. 31 (1982) 78;32 (1983) 1. 15. Z.Sojka, Catal. Rev.Sci. Eng. 37 (1995) 461. 16. J. Haber, T. Mlodnicka, J. Molec. Catal. 74 (1992) 131 17. J.Lundsford, Stud.Surf.Sci.Catal., vol 75, Elsevier, Amsterdam 1993, p.103 18. J. Haber, in Proc. 8th Intern.Congr.Catalysis,Berlin 1984, Verlag ChemieDechema 1984, vol.1 Plenary Lectures, p.85 19. J.Haber, in "Solid State Chemistry in Catalysis", eds.R.K.Grasselli, J.F.Brazdill, ACS Symp.Series 279, American Chemical Society, Washington 1985, p.3. 20. F.A.Kroger, The Chemistry of Imperfect Crystals, 2nd Ed., North Holland Publ. Co, Amsterdam 1973. 21. J.Haber, in "Surface Properties and Catalysis by Non-metals", eds. J.P.Bonnelle, B. Delmon, E. Derouane, D.Reidel 1983, p.l. 22. J. Haber, J. Janas, M. Schiavello, R.J.D. Tilley, J.Catal. 82 (1983) 395 23. C. Hall, R.N.Perutz, Chem.Rev. 96 (1996) 3125 24. G.A.Olah, G.U.S. Prakash, R.E. Williams,L.D. Field, K.Wade, Hypercarbon Chemistry, J. Wiley Sons Ltd, New York 1987. 25. R.H. Crabtree, Chem.Rev.95 (1995) 987 26. R.L.Burwell, A. Littlewood, M.Cardew, C.T.H. Stoddart, J. Am.Chem.Soc. 82 (1960) 6272 27. P.J.Robertson, M.S.Scurrell, C.Kemball, J.Chem.Soc. Chem.Comm 20 (1973) 799. 28. V.D.Sokolovskii, S.M. Aliev,O.V. Buyevskaya, A.A. Davydov, Catal.Today 4 (1989) 292 29. E.A. Manedov, Kinet.Katal. (russ)25(1984)868 30. T.Seiyarna, M.Ehashira, M.Iwarnoto, in "Some Theoretical Problems of Catalysis" eds T.Kwan, G.K. Boreskov, K.Tamaru. Univ. Of Tokyo Press, Tokyo 1973, p.35 31. V.Zazhigalov, J.Haber, J.Stoch, V.Batcherikova, Appl. Catal.A:General 134 (1996) 225. 32. R.J.Kokes in Proc.5th lntern.Congr.Catalysis, MiamiBeach FL 1972, ed J.W. Hightower, North Holland/American Elsevier, New York 1973,p.A.1 33. F.Finocchio, G.Busca,V.Lorenzelli,R.J.Viley,J.Catal. 151 (1995) 204 34. M.Witko, Catal.Today 32 (1996) 89 35. J.Haber, M.Witko, R.Tokarz, in print 36. W.Zhang, SA.Desikan, S.T.Oyama, J. Phys.Chem. 99 (1995) 14468 37. I.Manassidis, J. Goniakowski, L.N. Kantoroich, M.J. Gillan, Surf.Sci. 339 (1995) 258 38. R.A.Sheldon, J.K.Kochi, Metal-Catalyzed Oxidations of Organic Compounds,

17 Academic Press, New York 1981 39. J.Haber, Stud.Surf.Sci.Catal.Vol.48 Elsevier, Amsterdam 1989, p447. 40. M.Baumer, D. Cappus, H. Kuhlenbeck, H.J. Freundm G. Wilhemi, A. Brodde H. Neddermeyer, Surf. Sci. 252 (1991) 116. 41. G. Dep. I.E. Wachs, J. Phys. Chem. 95 (1991) 5889 42. I.E. Wachs, G. Deo, D. S. Kim, M. A. Vuurman, H. Hu, Stud. Surf.Sci. Catal., vol.75A, Elsevier, Amsterdam 1993, p. 543. 43. M. Che, L. Bonneviot, Pure Appl. Chem. 60 (1988) 1369 44. K. Bruckman, J. Haber, T. Wiltowski, J. Catal. 106 (1987) 188 45. G. Deo, I.E. Wachs, J. Haber, Critical Rev. Surf.Chem. 4 (1994) 141 46. J. Haber, T.Machej, T.Czeppe, Surf. Sci 151 (1985) 301. 47. H. Knozinger, E. Taglauer, Specialist Period. Reports "Catalysis" vol. 10, The Royal Soc. Chem., London 1993, p.l. 48. J. Haber, Pure Appl.Chem. 56 (1984) 1663 49. J.Haber, T. Machej, E.Serwicka, I.E.Wachs, Catal.Lett. 32 (1995) 101 50. J. Ziolkowski, Surf.Sci. 209 (1989) 536 51. F.S.Stone, J. Solid State Chem. 12 (1975) 271. 52. J. Haber, in Proc 34rd Intern.Conf.Chemistry and Uses of Molybdenum, eds. H.F. Barry, P.C.H. Mitchell, Ann Arbor, MI 1979, Climax Molybdenum, Ann Arbor 1979, p. 114. 53. E.Broclawik, J.Haber, J.Catal. 72 (1981) 379 54. M. Missono, T.Okuhara, N.Mizuno, Adv.Catal. 41 (1996) 113. 55. G.Centi, E.Giamello, D.Pinelli, F.Trifiro, J.Catal. 130 (1991) 220;ibid.238

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

19

The multifunctional properties of heterogeneous catalysts, active and selective in the oxidation of light paraffins F. Cavani and F. Trifir6 Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy This review analyzes the properties which are necessary for heterogeneous catalysts to promote the oxyfunctionalization of light paraffins to valuable chemicals. Three catalytic systems are discussed: i) vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n-butane to maleic anhydride, which is here also examined for reactions aimed at the transformation of other hydrocarbons; ii) Keggin-type heteropolycompounds, which are claimed for the oxidation of propane and isobutane, whose composition can be tuned in order to direct the reaction either to the formation of olefins or to the formation of oxygenated compounds; iii) futile-based mixed oxides, where rutile can act as the matrix for hosting transition metal ions or favour the dispersion of other metal oxides, thus promoting the different role of the various elements in the formation of acrylonitrile from propane.

Introduction In recent years a significant interest has been directed towards the study of new catalytic materials able to promote the transformation of alkanes to valuable chemicals by means of selective oxidation (1-8). This interest arises from the possibility of replacing the currently used raw materials, mostly olefins and aromatics, with cheaper and more environmentally friendly organic substrates. The scientific challenge of the activation and selective functionalization of a saturated molecule thus is coupled with the economic and environmental benefit which would arise from the exploitation of natural gas components. These efforts have successfully turned into the commercial application of the process for maleic anhydride synthesis from n-butane, which is replacing the process from benzene, and which is still the only example of a gas-phase industrial oxidation of a paraffin for the synthesis of an intermediate for the petrochemical industry. This has been made possible by the elucidation of the unique properties of vanadyl pyrophosphate, (VO)2P207, which is able to promote the selective transformation of n-butane (9-13). On the other hand, in the last few decades several companies have claimed the development of catalytic systems and processes for the oxyfunctionalization of light paraffins. Table 1 summarizes the various reactions which are possible starting from light alkanes. These processes are in various stages of industrial application applied industrially. Some have been developed but not applied, and others are still at the research stage. In more recent years, a proliferation of papers and patents devoted to the study of catalysts for the oxidative dehydrogenation of paraffins to the corresponding olefins is a clear indication of the interest of industry in these processes, as an alternative to the energy-intensive endothermic steam cracking or dehydrogenation of natural gas or oil components or fractions (5,14-17). It is possible to summarize the important aspects which need to be studied in the development of a catalytic material for the oxyfunctionalization of a paraffin by the following questions:

20 1) What is the mechanism of activation of a paraffin, or what is the best way to activate a paraffin and subsequently transform the intermediate to the desired product ? Which oxygen species is needed to favour this oxidative and selective conversion ? 2) Does the mechanism of oxygen insertion into the activated hydrocarbon involve an intermediate step of formation of an olefin ? Is this step necessary for achieving the highest selectivity to the desired product, or do alternative and more selective pathways exist, which do not involve the formation of olefinic species ? 3) To what extent does the stability of the product affect the selectivity at reaction conditions 9 4) Does the presence of heterogeneously-initiated, gas-phase reactions contribute to the reaction mechanism ?

Table 1. Industrial processes and processes under study or development for the oxyfunctionalization of light paraffins (C 1-C6) in the petrochemical industry. Raw Product Phase Stage of development material Methane Chloromethanes Gas, heterog. Industrial Methane Methanol Gas, het./hom. Pilot plant Methane Syngas Gas, het./hom. Research Methane Ethylene Gas, het./hom. Pilot plant Ethane 1,2 Dichloroethane, Vinyl Gas, heterog. Pilot plant chloride Ethane Acetic acid Gas, heterog. Research Ethane Ethylene Gas, het./hom. Research Propane Acrylic acid Gas or liquid Research Propane Propyl alcohol Liquid, het. or hom. Research Propane Acrylonitrile Gas, heterog. Demonstrative plant Propane Propylene Gas, heterog. Research n-Butane Acetic acid Liquid, homog. Industrial n-Butane Maleic anhydride Gas, heterog. Industrial n-Butane Butadiene Gas, heterog. Industrial, abandoned Isobutane Methacrylic acid Gas, heterog. Pilot plant Isobutane Isobutene Gas, heterog. Research Isobutane t-Butyl alcohol Liquid, het. or hom. Research n-Pentane Phthalic anhydride Gas, heterog. Research Cyclohexane Cyclohexanol, one Liquid, homog. Industrial Cyclohexane Cyclohexanone Liquid, het. or hom. Research Het./hom. indicates the likely presence of a mechanism initiated on the catalyst surface and transferred to the gas phase.

Some of these questions, amongst others, still need to be answered; however, the following points can be considered as definite ones: 1) The very different reactivity of methane and ethane with respect to the heavier paraffins requires completely different classes of catalysts as well as different reaction conditions which are needed for the reactant activation and transformation. Completely different conditions and catalysts imply different mechanisms. With methane and ethane (the

21

transformation of which is catalyzed by alkali and alkaline earth oxides), the contribution of homogeneous radical reactions is fundamental, while it is less important for higher paraffins and transition metal oxide-based catalysts. 2) The generally accepted rule that the olefin is more reactive than the corresponding paraffin is not always true. This is the case for ethylene and also in some cases for propylene. The presence of these olefins may substantially lower the selectivity to oxygenated compounds, in cases where the latter are the preferred compounds. 3) The most selective reactions are those which lead to the formation of stable oxygenated compounds, such as anhydrides (this is the case of the n-butane and n-pentane oxidation). On the contrary, products such as the unsaturated acids and aldehydes are unstable under reaction conditions which are necessary for the activation of the paraffin; and hence, low selectivities are observed. 4) Finally, it is clear that the many aspects related to the successful development of a new process for the oxidation of a light paraffin necessarily involve several disciplines and expertise, to join together the physico-chemical, material, kinetic and technological aspects. From the materials point of view, it is now clear that there are no "simple" catalysts which can be used for the partial oxidation of a paraffin. All of the best systems are either constituted by several different components which cooperate in achieving the final product, or made of multicomponent structures which perform the different functions for selective conversion. These different functions seem, therefore, to be a necessary feature for the activation of the C-H bond in the saturated substrate, and for the multi-electron oxidation and oxygen insertion onto the latter. It is more or less clear that the cooperation of acidic and suitable oxidizing properties is a necessary condition. However, this is not sufficient, and best catalytic performances are obtained only when these properties are structured in such a way to allow the final product to be quickly reached, and rapidly desorbed before further oxidative transformations may lower selectivity. The aim of this review is to describe the reactivity of three catalytic systems which have been widely studied in recent years for the oxidative transformation of light paraffins: i) vanadyl pyrophosphate, which is the industrial catalyst for the oxidation of n-butane, but has also been claimed to be selective in the oxidation of n-pentane to maleic and phthalic anhydrides (18-22), ii) heteropolycompounds, which are currently being studied for the oxidation of isobutane and propane to the corresponding unsaturated acids (methacrylic acid and acrylic acid) (5,23-29), and whose composition can be tuned to change the acidic and oxidizing properties; and iii) futile-based mixed oxides, which can act as the matrix to host various metal components, and which have been claimed as optimal catalysts for the ammoxidation of propane to acrylonitrile (15,30-33).

The oxidation of alkanes on V/P/O catalysts Table 2 reports the maximum yield to the oxygenated products from each paraffin which has been rePorted in the literature over V/P/O-based catalysts. In all cases the catalyst is mainly constituted by vanadyl pyrophosphate, which however, may contain either metal dopants or other vanadium phosphates. The selectivity to the product of partial oxidation is a function of the structure of the reactant. From n-butane and n-pentane the selectivity to maleic anhydride and to maleic plus phthalic anhydrides, respectively, is high, while from ethane the prevailing products are either ethylene or carbon oxides (depending on the reaction conditions); acetic acid is formed in rather low amounts. From propane very low amounts of acrylic acid are formed, and carbon oxides prevail. These differences can be attributed to the formation of very stable products

22 (the anhydrides) from C 4 and from C 5 paraffins, while acrylic acid (from propane) undergoes consecutive reactions of oxidative degradation. The low selectivity to acetic acid from ethane is attributed to the low specificity of the active sites in this reaction, rather than to the instability of the product, since acetic acid can be regarded as a stable compound.

Table 2. Maximum ~,ields to the desired products in the oxidation of light alkanes.

Reactant

Catalyst comp.

Desired product

Max. yield, tool.% (T, ~

Ethane Propane Propane n-Butane

V-P-O/TiO 2 V-P-Te-O V-P-O V-P-O (+Zn, Li, Mo, Zr) V-P-Co-O V-P-O

Acetic acid Acrylic acid Acrylonitrile Maleic anhydride

1.2 (350) 8 (400) 6 (550) 56-60 (380-420)

34 35 36 5

Methacrylic acid, Methacrolein Phthalic anh., Maleic anh.

4.5, 3.1 (280) 23, 22 (350)

37 18

Isobutane n-Pentane

Ref.

It has been proposed that the transformation of n-butane to maleic anhydride involves the following steps at the adsorbed state: 1) n-butane + 1/2 0 2 --~ butenes + H20 (oxidative dehydrogenation) 2) butenes + 1/2 0 2 ~ butadiene + H20 (allylic H-abstraction) 3) butadiene + 1/2 0 2 ~ 2,5-dihydrofuran(1,4 oxygen insertion) 4) 2,5-dihydrofuran + 2 0 2 ~ maleic anhydride + 2 H20 (allylic O-insertion, possibly via y-but-2-enoic lactone) or, 4a) 2,5-dihydrofuran + 1/2 0 2 --~ furan + H20 (allylic H-abstraction) 5a) furan + 3/2 0 2 ~ maleic anhydride + H20 (electrophilic oxygen insertion) Other proposed mechanisms involve either a direct attack of 0 2- species at the 1,4 C atoms of n-butane (38), or an allylic oxidation of an olefinic-like C 4 intermediate to crotonaldehyde, followed by internal cyclization and oxidation (39). A dienic intermediate has also been proposed by Grasselli et al. (40). In any case, the reaction patterns proposed evidence the need for different kinds of active sites on the surface of the vanadyl pyrophosphate which are able to perform with high selectivity each step in the reaction pathway. The polyfunctional nature of the vanadyl pyrophosphate is clearly evidenced by the different classes of reaction which can be catalyzed by this material, as summarized in Table 3. Vanadyl pyrophosphate can catalyze most of these transformations with good selectivity. This also indicates that the multiple steps proposed for the mechanism of n-butane oxidation to maleic anhydride can effectively occur on vanadyl pyrophosphate. The multifunctional properties of the vanadyl pyrophosphate can be summarized as follows: 1) This system possesses sites able to perform the oxidative dehydrogenation of paraffins. This is demonstrated by the formation of benzene with high specificity from cyclohexane, as well as by the formation of olefins and diolefins from n-paraffins, of aromatic compounds

23 from decaline, and of cycloolefins from cycloparaffins. However, this is not the optimal system for the synthesis of light olefins from paraffins.

Table 3. Classes of reaction catalyzed b~' the vanadyl pyrophosphate Reactant Product Reaction type isobutyric acid methacrylic acid oxidehydrogenation cyclohexane benzene succinic anhydride maleic anhydride hexahydrophthalic anhydride phthalic anhydride paraffin olefin olefin diolefin allylic oxidation (Habstraction or O-insertion) 2,5-dihydrofuran furan tetrahydrophthalic anhydride phthalic anhydride benzene maleic anhydride electrophilic oxygeninsertion butadiene furan naphthalene naphthoquinone furan maleic anhydride

2) It possesses centres able to perform the allylic oxidation with high specificity. This is the reason why vanadyl pyrophosphate does not yield olefins with high selectivity in the oxidation of paraffins. In fact the desorption of intermediate olefins is not rapid, since they are quickly transformed to the oxygenated products. Only when molecular oxygen is absent (and the catalyst possesses a low number of O-insertion sites), can the olefin be saved from consecutive transformations, and a good selectivity to the olefin can be achieved. 3) It possesses centres able to insert oxygen into electron-rich substrates. 4) It possesses acid centres. Acidity is recognized to play important roles in the activation of the paraffin, in the desorption of acid products, and in accelerating specific transformations over reactive intermediates (41). 5) It may favour bimolecular condensation reactions which are not acid-catalyzed, but which are accelerated by the proper geometry of sites at which the molecules are adsorbed. This property likely plays an important role in the mechanism of phthalic anhydride formation from n-pentane (19). 6) The oxidation state of vanadium under reaction conditions is a very important parameter in the control of the process selectivity; a certain number of oxidized vanadium sites is necessary to transform the intermediate olefins to the oxygenated products (11,42,43). On the other hand, an overoxidized surface may be responsible for the further oxidative degradation of the desired products. When olefins are used as the reactant, the interaction of the electronrich organic substrate with the vanadium ions leads to a reduction of the latter, and hence to a lower availability of O-insertion sites. The average oxidation state of vanadium under reaction conditions is determined by the nature of the V-P-O phases which have formed during the catalyst activation and ageing (i.e., during the thermal treatment), and by the operative reaction conditions. These properties, however, are not sufficient to lead to the high selectivity obtained in the formation of maleic anhydride from n-butane, and of maleic and phthalic anhydrides from n-

24 pentane. The multifunctionality must be accompanied by other properties, which allow the multi-step transformation of the paraffin to be carried out i) without permitting desorption of any intermediate to occur; and ii) through the proper sequence of dehydrogenation and oxygen insertion reactions. One main characteristics of n-butane oxidation is the substantial absence of by-products of partial oxidation other than maleic anhydride (apart from a low amount of phthalic anhydride). This means that once the alkane has been adsorbed and transformed to the first intermediate species, the latter has to be quickly transformed up to the final stable product. If this requirement is not met, the adsorbed olefinic-like intermediate may desorb. This leads to a lower selectivity to the final desired product, because the olefin may be readsorbed on nonspecific oxidizing sites yielding other undesired products (aldehydes or acids), which can also be precursors for the formation of carbon oxides. Therefore, a rapid transformation of the adsorbed intermediates to oxidized products is necessary in order to obtain high selectivity to the desired product. In order to guarantee this selective pathway, the catalyst surface must provide the required arrangement of specific oxidizing sites: the different functional properties must be arranged so as to provide an ensemble of sites (or, alternatively, sites with multifunctional properties) able to allow the reaction pathway from alkane adsorption and activation up to its transformation to the final product to be completed. On the contrary, when vanadyl pyrophosphate is used to catalyze the transformation of butenes and pentenes, the catalyst deactivates easily and the performance is very poor (11). This occurs because the acidity of the catalyst is responsible for side reactions when olefins are directly used as the feedstock. In this case, in fact, due to its high nucleophilicity, the olefin may easily interact with different types of sites, other than those able to transform it directly to the final anhydrides. Therefore, the surface acidity of the vanadyl pyrophosphate, which seems to be a necessary property for the transformation of n-butane, is a negative feature when the corresponding olefin is the reactant. For the same reason, the synthesis of acrylic acid from propylene must be carried out in two separate reactors, one for the oxidation of propylene to acrolein and one for the oxidation of the aldehyde to acrylic acid. This is due to the fact that the requirements needed for the two steps make the two reactions incompatible. Acidity is needed in the second step, to favour the desorption of acrylic acid and save it from unselective consecutive reaction, while on the other hand, acidity is detrimental for the first reaction, because it favours the transformation of propylene to undesired products. Therefore, the development of a process for the one-step transformation of propane to acrylic acid will be possible when a catalyst is developed which possesses active sites able to perform quickly the complete transformation of adsorbed propane to the acrylic acid, the latter being the only product which finally desorbs into the gas phase. Accordingly, best performances in the oxidation of propane to acrylic acid have been reported to be obtained on heteropolyoxomolybdates (26), which are known to couple tuneable acid and redox properties. In this case, acid properties may facilitate the desorption of acrylic acid. A further requirement for the high selectivity to maleic anhydride from n-butane is the need for a correct sequence of oxidehydrogenation and oxygen insertion reactions. In the oxidation of n-butane the olefinic-like intermediate must be quickly oxidehydrogenated to an adsorbed dienic-like compound in order to favour the selective pathway towards maleic anhydride. In fact, this reaction may occur concurrently with the oxidation of allylic carbon atoms, with formation of aldehydes and acids which can also be precursors of carbon oxides. Thus, the selectivity to maleic anhydride depends on the relative rates of hydrogen abstraction and oxygen insertion. This property can be considered as typical of the vanadyl pyrophosphate; for instance, in the case of the V/Mo/O system the rate of oxygen insertion

25 (represented by the reaction of benzene oxidation to maleic anhydride) is very high in comparison to that of n-butane oxidation. In fact, the V/Mo/O system is selective in benzene oxidation but not in n-butane oxidation. The combination of acidic and oxidizing properties of vanadyl pyrophosphate makes several different transformations possible over paraffins, as illustrated by the scheme in Figure 1 for the reactions which may occur on n-pentane. The relative contribution of the different pathways (i.e., cyclization of intermediate olefin or dienic compound vs. Oinsertion, or dimerization vs. cyclization) is a function of the nature of the reactant and of the availability of surface oxidizing centres or of sites which can favour the dimerization or condensation reactions.

~

I ....

0 aollxYlaClion / ....

COx

,~ allyllc ----b .o.,,.,,o.

/J'~'

X

cox

/

" ~ 1....7 6 0

allylic

\ electrophilic

I

allylic I

~____rtzatlon

.

l,,o.,..,,on o,,.*

~

COx * / z

Figure 1. Possible reactions which may occur on paraffins catalyzed by vanadyl pyrophosphate, as exemplified for n-pentane transformation.

The oxidation of alkanes on heteropolycompounds Heteropolyacids and their salts have been studied as catalysts for oxidation both in the liquid and in the gas phase of several organic saturated and unsaturated substrates (44-50). The main features of these systems, which make them suitable for application as heterogeneous catalysts, can be summarized as follows (51-53): 1) Relatively high thermal structural stability; the Keggin anion in heteropolyacids begins to decompose at temperatures close to 250-300~ Salification leads to a remarkable improvement in the stability, allowing operations up to 400-450~ to be carried out. Salts can be prepared by ion-exchange in aqueous solutions of proton form of the heteropolyacid, or by direct precipitation of the insoluble salt. In some cases, such as the cesium salts of 12molybdophosphoric or tungstophosphoric acids, the structure is stable up to 550~ 2) High intrinsic acidity which arises form the presence of a highly delocalized anion charge and of mobile protons; in both aqueous and organic media the acidity is even higher than that of some mineral acids. The Broensted acidity can be controlled by partial neutralization of the protons.

26 3) Easy reducibility and reoxidizability by means of molecular oxygen. This allows them to be used as catalysts for liquid-phase or gas-phase multi-electron oxidations. The redox properties of these materials can be affected properly by modifying the anionic composition (for instance, by substitution of some Mo 6+ cations by V5+), or the cationic composition. Several different cations can be introduced in the structure, i.e. alkali and alkaline earth metals, ammonium, divalent and trivalent metals such as Cu 2+, Co 2+, VO 2+. Much interest is related to the use of HPC's as catalysts for the oxidative functionalization of light paraffins, since the multifunctional properties of these systems may be of utility for the activation of saturated organic substrates. On the other hand, the high temperatures which are usually necessary to activate the paraffins may be deleterious for the structural stability of HPC's, since the destruction of the primary structure leads to a loss of the unique properties of the compound and to a decrease in catalytic performance. Therefore, it is necessary to use stable salts, which, on the other hand, are less reactive. This seems to leave little room for the design of a catalyst suitable for the activation of paraffins in the gas phase. However, different possibilities exist to resolve these problems: 1) Operation in the liquid phase at high pressure and moderate temperature; this is the case of the liquid-phase hydroxylation of alkanes (54). 2) Use supported HPCs, in order to obtain better spreading of the active phases (i.e., increase the specific surface area of the active compound); a problem may be the strong chemical interaction that develops between the support and the HPC, which can lead to the destruction of the compound itself. Less reactive supports are therefore needed, such as silica. 3) Improve at the same time the structural stability of the HPC and its oxidation potential, by using stable salts and by adding transition metal ions which are known to enhance the oxidation potential of the oxometal, such as low-valence early transition metal ions in the cationic position of the HPC, or vanadium as an additional oxometal. 4) In gas phase oxidations usually water is also added to the stream. This is supposed to guarantee stable catalytic performance and higher activity, likely because the presence of water can favour the surface reconstruction of the heteropolyacid even under conditions at which it would usually decompose. In addition, water may favour the desorption of the products, saving them from unselective consecutive combustion. In recent years heteropolycompounds have been studied for the oxidation of propane to acrylic acid and of isobutane to methacrylic acid. Rohm & Haas Company was the first in 1981 to claim the one-step oxidation of isobutane to methacrolein and methacrylic acid (55). Even though no reference is made to heteropolycompounds, the claimed catalyst compositions correspond to Keggin-type structures. In the patents later issued by Sumitomo (56,57) an important role was claimed to be played by vanadium (in an anionic position), by cesium (in a cationic position), as well as by an excess of phosphorus with respect to the stoichiometric composition. These catalysts gave selectivities to methacrylic acid plus methacrolein close to 70 %, with isobutane conversions in the 10 to 13 % range. Besides carbon oxides, acetic acid was the main by-product. Asahi also has patented Keggin-type heteropolycompounds containing vanadium, copper and alkali metals (27). The necessity for the use of salts instead of acids was also pointed out, in order to increase the catalyst stability. The use of a catalyst suitable for fluid-bed operation was also claimed, so as to allow continuous transport of catalyst from the reaction to the regeneration vessel, and vice versa. Also, alternate feeding of isobutane and of oxygen into the catalytic bed allowed higher selectivities to be obtained. Fundamental features of the claimed heteropolycompounds were i) a partial degree of reduction, and ii) the cubic crystalline structure. This was achieved by calcining the ammonium salt of H3PMo12040 in a nitrogen atmosphere. The heteropolycompounds characterized by the cubic structure, and

27 by a partial degree of reduction (also achieved by in situ treatment with isobutene at 450~ led to superior activity and selectivity to methacrylic acid. The importance of a partial molybdenum reduction has been also recently pointed out by Mizuno et al. (26). The high variability in the compositions of heteropolycompounds makes them potentially useful for the oxidation of different organic substrates, or for addressing the transformation of one reactant into different kinds of products. One example is given by the comparison of the reactivity of Keggin-type molybdenum-based compounds against tungsten-based compounds (58-61). Figures 2 and 3 show the distribution of the products obtained in the oxidation of isobutane at the following conditions: 26% isobutane, 12 % oxygen, 12% water, balance helium, with (NH4)3PMo12040 and with (NH4)3PW12040 catalysts, respectively. The former compound yields methacrolein and methacrylic acid with an overall selectivity around 60%; the main by-products are carbon oxides and acetic acid, with minor amounts of maleic anhydride and of acrylic acid. On the contrary, the W-containing compound exclusively yields isobutene as the product of partial oxidation. The olefin is obtained with a selectivity which is close to 60%. 50

60 methacrylic acid

40

o~ =" 30 0

oxyge

~,

co

L

r 20 -

0

~

o

methacrolein - 20

v

acetic acid

10-

0 320

.>

carbon oxides

>

~

~ 340

n

r

e

360

G)

u~

0 380

400

temperature, ~

Figure 2. Oxidation of isobutane over (NH4)3PMo12040; reaction conditions" residence time 3.6 s, feed composition 26% isobutane, 12% water, 13% oxygen, rest helium.

The very different catalytic behavior originates from the different reducibility of the Mo 6+containing HPC with respect to the W o+ -containing compounds. The Keggin anion in the former compound easily undergoes 2-electron reduction, furnishing nucleophilic 0 2- species which can be incorporated into the activated organic substrate. The PW120403- Keggin anion is much less reducible under the same conditions, and can only perform hydrogen abstraction with water formation. This difference arises for the greater electronegativity of the Mo 6+ and the lower Mo-O bond strength. Only in the case of ethane oxidation, are the Mo-containing heteropolycompounds active and selective catalysts in the formation of ethylene, since this molecule does not possess allylic carbon atoms for O-insertion (62-64). The two different mechanisms which have been proposed for the oxidation of isobutane over i) Mo-containing Keggin-type compounds and ii) W-containing Keggin-type and Wells-

28

Dawson-type heteropolycompounds are illustrated in Figure 4 (24,58). In both cases the first step consists of the formation of an alkoxy species, which is the first reaction intermediate. 4

80 carbon oxides

- 70

_

- 60 isobutane conversion

- 50

o~

0

m 2 -

- 4 0 .~-

G) > tO 0

0

-30

;obutene

u)

- 20

1-

"10 m e t h a c r y l i c acid + m e t h a c r o l e i n + a c e t i c acid

0 330

'~

="

340

"

350

-

360

temperature,

0

370

380

~

Figure 3. Oxidation of isobutane over (NH4)3PW 12040; conditions as in Figure 2. CH3 H3C. /10H3

o.'',.

OH3 H3C~.CI/0H3

," - ~

.

<~..o\

o,.,..

~o.~

GIH3 2Hc~C=CH 2 "---* ~ / H H

o,o.

o,

\ Mo I / O \ 1 Mo / O\ M Io ? H3 , '' H-~c-IC =CH2

/o

--,

~

CH3 H3C-~-CI----OH3 + .H

\

.....

0-%

CH3 2HC_ ~._r oH2 -H./" " "

o.,.o

o CH3 H I - - . ~ c_~C=CH 2 H ---.~ ~ H.~""I

o

Mo./i~

-

O O Vl~/fO...~v,

o

CH3 1 H~c~-C=CH 2

-

I /O. I ~Mo Mo methacrylic acid

s,~, methacrolein T"~ j -H~.c.~C=CH~

:" o/.',..;--,.I ,,--'

' '<.1 / o \ Mo

H3C~_> ~ H3 / - - C H

"'H

.~,Mo ..~.I 1 Mo.8~ Mo

o/

? H3 c f C =CH2 ~ 0~1-1~ /

"/ H~",,-, ~ oI ' , o .~ ,.,I 'i' k_.9. \ M ' 0 " " Mo O\Mo'O"~o

"

'~

Mo

H3C /H H3C .H H3C~C..~C-~.._ H H3C.._NCI~_H C~ H H / H H O *'--" O I O ,V I O ' v, v ~vfO..... ~v W" t ~ W .

'~+1/2 O2 O vI II - O ~ IV W f "~W H3C H /C-- C\~ 4 o i .. + ,, vwIt ~ O~ w v H,C H H20

/ H H3C H,C>C I ( - H . o/ H 6"

,vdV~O...~v

Figure 4. Proposed mechanisms for isobutane oxidation on Mo- and W-containing heteropolycompounds

29 However, with the former catalyst the mechanism is essentially an ionic one, with evolution to a dioxyalkylidene species which can yield either methacrolein, or a carboxylate species, precursor of methacrylic acid. No isobutene is detected among the reaction products since the dioxyalkylidene species is strongly bound to the surface. The overall process involves the transfer of 8 electrons from the organic substrate to the catalyst per molecule of methacrylic acid produced. In the case of W-containing heteropolycompounds the mechanism is essentially a radical one (analogous to that proposed for the radical-like chemistry exhbited by W-containing heteropolycompounds in liquid phase oxidations (46)). In the absence of centres able to insert 0 2- species, the radical alkoxy intermediate converts to isobutene. The process only involves 2 electrons. o~ 12 "~

~

0 ffJ

8

L.

r> cO o

6 4'

e-

=

,I1 0 9"-

2 (

0

0.25

I

I

I

I

I

0.5

0.75

1

1.25

1.5

number of iron atom per Keggin unit

Figure 5. Catalytic performance of KI (NH4)2PMol2040/FexO1.5x in the oxidation of isobutane.

A role can also be played by the cations. Catalysts were prepared which contained iron ions, with the composition, KI(NH4)2PMol2040/FexO1.5x (23-25). Iron was found to substitute for ammonium in cationic positions, possibly in the form of partially hydrolyzed cations, i.e. [Fe(OH)3_x] x+. Figure 5 shows that increasing amounts of iron led to a progressive increase in the isobutane conversion and yield to methacrolein plus methacrylic acid, while the selectivity was negatively affected by the dopant. It is known that the nature of the cation may affect the redox properties of the molybdenum in the primary structure, and this might explain the progressive increase in catalytic activity. However, the addition of iron also was found to increase the Broensted and Lewis acidity of the compound; this acidity might be involved in the rate-determining step of the reaction, facilitating the polarization of the C-H bond.

The ammoxidation of propane over rutile-based mixed oxides The synthesis of acrylonitrile from propane, as an alternative route to the industrial process which employs the olefin as the raw material, is carried out on catalysts which are based on vanadium and antimony mixed oxides. The catalyst contains a large excess of antimony with respect to the stoichiometric requirement for the formation of VSbO 4, and the

30 excess of antimony oxide is claimed to remarkably promote the selectivity to acrylonitrile (15,31-33). This catalytic system, as well as systems based on MoN/Te/Nb mixed oxides which have been developed by Mitsubishi (65), also represent an example of catalyst characterized by multifunctional properties. The rutile structure is the matrix to host vanadium ions as solid solutions, while the antimony oxide is present as a dispersed microcrystalline oxide. Vanadium is the component which is more active in paraffin conversion, while the high selectivity to the desired product is due to the presence of dispersed, separate phase, antimony oxide. In this case the design of the catalytic system is aimed to accomplish the following transformations: C3H 8 + 1/2 0 2 --~ C3H 6 + H20 C3H 6 + NH 3 + 3/2 0 2 ~ C3H3N + 3 H20 Sohio issued several patents claiming catalysts based on vanadium, antimony and some promoters which are able to ammoxidize propane with a completely heterogeneous mechanism (66). These catalysts can be considered intrinsically multifunctional since both dehydrogenation and nitrogen insertion functions are present (67,68). The main problem with this type of catalyst is the low rate of the subsequent ammoxidation of intermediate propylene. Indeed, propylene is always present as a by-product. The mechanism for acrylonitrile formation via intermediate propylene is demonstrated by the results given in Figure 6, which compares the oxidation and the ammoxidation of propane over a V/Sb/O mixed oxide catalyst (69). The conversion of propane and the selectivity to the main products are reported (acrolein was formed only in traces).

~6

t~ ~D m .m

s

0

tO2 0

product Figure 6. Ammoxidation of propane over V/Sb/O catalyst. Reaction conditions" temperature 430~ residence time 2s, propane 25 mol.%, ammonia 10 mol.%, oxygen 20 mol.%.

The rate of conversion of propane is practically the same in the presence and in the absence of ammonia. The oxidation yields propylene and carbon oxides, which are the prevailing products. However, when ammonia is added to the feedstock, the yield to propylene remains unchanged, while the yield to carbon oxides is remarkably decreased in favour of the formation of acrylonitrile. This suggests that in the absence of ammonia the propylene is oxidized to a compound or to an intermediate which under these conditions is burnt to carbon oxides. The addition of ammonia allows this intermediate compound (which might be acrolein or an allyl radical species) to be quickly transformed to the stable

31 acrylonitrile, thus saving it from the unselective subsequent combustion. In any case, the presence of residual propylene suggests that the catalyst is not very efficient in its transformation, likely because of the absence of O or N-insertion sites in proximity to the centres responsible for the formation of the allylic intermediate. A more efficient catalyst will be one where the different functions are organized into arrays of active centres, able to quickly transform the propylene and avoid its desorption, in the same way as it occurs in the oxidation of n-butane over vanadyl pyrophosphate (which does not yield olefins or diolefins at all), and for the oxidation of isobutane to methacrylic acid over Mo-heteropolycompounds (which do not yield isobutene). An important concept, first developed by Callahan and Grasselli (70) is the "siteisolation" theory, which requires that the active oxygen species is present in isolated regions on the catalyst surface, in order to obtain high selectivity to the desired product. In this case, the presence of diluted V sites provides centres for paraffin activation, while bulk vanadium oxide is responsible for side undesired combustion reactions (32).

C o n c l u s i o n s

The main topics which have been emphasized in this review are the following: - the role of vanadium, of molybdenum and of tungsten in some heterogeneous catalysts which are active and selective in the mild oxidation of light paraffins; - the role of the stability of the products; - the role of the multifunctionality of the catalysts; - the role of non-desorption of reaction intermediates; - the role of the relative rates of the reactions of dehydrogenation and of O-insertion; - the role of acidity and basicity of the catalysts; - the role of isolation of active sites. Three catalytic systems active and selective in the (atom)oxidation of saturated organic substrates have been discussed, and the properties which lead to their superior catalytic performance have been examined in relationship to the mechanism of paraffin activation and transformation to the desired products. Common features for these systems are: 1) The necessity of multifunctional properties, arising either i) from the presence of different components, having a specific role in the reaction mechanism but intimately interacting with each other in a complex array of surface centres, or ii) from the presence of monophasic structures characterized by different functions. 2) A weak interaction of the desired product with the catalyst surface, which favours its desorption into the gas phase. The stability of the product, moreover, must be high enough to allow operation to be carried out under conditions which are necessary for the paraffin activation. Properties which are specific for the compounds examined, and which make them rather unique for some peculiar reactions of paraffin oxidation are: 1) The availability of surface arrays of active centres able to cooperate in such a way as to avoid the desorption of olefin intermediates, and to allow the rapid transformation of the latter to the final oxygenated compounds. This is a property of the vanadyl pyrophosphate and of heteropolycompounds; for these compounds the "intrinsic multifunctionality" arises from the molecular-type organization of the structure into well-defined moieties, each one characterized by specific properties. 2) The specificity in the synthesis of anhydrides from n-butane and n-pentane, typical of the vanadyl pyrophosphate, which likely arises from its property of electrophilic O-insertion onto

32 dienic-like intermediates. The heterocyclic compound is then oxidized to the very stable anhydrides. On the contrary, low selectivity to unsaturated acids are obtained in the oxidation of propane and of isobutane; this may be due to the fact that this catalyst does not allow a rapid desorption of these intrinsically unstable products, and thus favours the occurrence of consecutive combustion reactions. One possible hypothesis is the formation of stable esters by reaction between the acids and the surface P-OH groups. 3) The specificity of Mo-heteropolycompounds in the synthesis of unsaturated acids starting from paraffins which can not yield heterocyclic compounds (i.e. propane and isobutane). This may arise from the very strong acidity which is typical of these compounds (even when they are not in a protonic form) which favours the desorption of the organic acids, sparing them from consecutive reactions. These compounds also yield maleic anhydride from nbutane and n-pentane (but with remarkably lower selectivity than the vanadyl pyrophosphate), but they lack the surface properties which are necessary for the formation of phthalic anhydride from n-pentane. 4) The unique possibility of tuning the redox properties (and the acidic properties as well) of heteropolycompounds by chancing the composition of the anion, or by modifying the cationic composition. Changes in composition have dramatic effects in the nature of the products obtained in paraffin oxidation, i.e. oxygenated compounds vs. olefins. 5) The rutile structure is characterized by a very elastic lattice, able to guest foreign metal ions, as well as domains or dispersed amorphous or microcrystalline phases. It is thus possible to build a multifunctionality by properly choosing the nature and amount of the various components which can be added to the rutile matrix. The most studied application for rutile-based materials is the transformation of propane to acrylonitrile, because this structure makes it possible to couple oxidehydrogenating properties with O- or N-insertion characteristics. This is an example of how the use of nanosized technologies for catalyst preparation may substantially enhance the interaction between different phases, favouring phenomena at the interface and promoting the system multifunctionality (71).

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

Selective oxidation compounds

of

hydrocarbons

35

catalyzed

by

heteropoly

Makoto Misono, Noritaka Mizuno, Kei Inumaru, Gaku Koyano, and Xin-Hong Lu Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Selected characteristic features of heteropoly catalysts for the selective oxidation of hydrocarbons are described based on recent studies from our laboratory as well as from other groups. 1. C O R R E L A T I O N PERFORMANCE

BETWEEN

REDOX

PROPERTIES

AND

CATALYTIC

Fundamental correlations between redox properties and catalytic activity have successfully been established for the hydrogen form and alkali salts of 12molybdophosphoric acid [1]. Provided that the contributions of surface- and bulk-type catalysis are properly taken into account, good monotonic relationships are obtained between the catalytic activity for oxidation and the reducibility (or the oxidizing power) of the catalyst. The rate of oxidation of aldehydes, a surface-type reaction, correlates linearly with the surface reducibility of the catalyst, and the rate of oxidative dehydrogenation of cyclohexene, a bulk-type reaction, with the bulk reducibility [2].

~ a

1

~

2

~ ~0

m

t'~

3 0

N

~m ~o

1 2

L

0

.

|

2O

I

I

I

I

!

4O 6O 80 N MAA Yield/% Figure 1. Effect of V and Cs contents on the yield of methacrylic acid (MAA) at 350~ over H3+xPMo1~.-xVxO40and Cs2.75H0.2~+xPMo12_xVxO40catalysts.

36 The quantitative agreement between the rates of catalytic oxidation observed experimentally and those predicted from the reduction and oxidation rates of the catalysts measured independently demonstrated that the catalytic oxidation proceeded by redox cycles of the catalysts, that is, the redox mechanism or Marsvan Krevelen mechanism [3]. However, attempts to find similar relationships for mixed-metal heteropoly compounds such as molybdovanadophosphates have not been successful. This has been due to the low thermal stability of these compounds. For example, PMollVO4o and PMoloV204o decomposed to PM012040 and VOx above 200~ [4]. We attempted to stabilize the heteropolyanions by forming their cesium salts. Although the possibility of slight decomposition could not be excluded, high yields were obtained for the conversion of isobutyric acid to methacrylic acid (MAA) as shown in Fig. 1 [5]. A crystalline vanadium phosphorous oxide may be regarded in a broad sense to be a heteropoly compound. By applying e x s i t u and in s i t u spectroscopies (Raman spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), electron diffraction (ED)), we found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase of VOPO4 under reaction conditions in the oxidation of n-butane to maleic anhydride [6]. For example, the in s i t u Raman spectra measured at steady state flow reaction conditions showed that the surface changed reversibly between (VO)2P207 and X1 phase depending on the butane/oxygen ratio in the feed [7]. Correspondingly, the catalytic selectivity varied reversibly (Table 1). Table 1 Changes in the selectivity to maleic anhydride over (VO)2P207 with the partial pressure of n-butane Partial pressure Conversion a/% Selectivity/% Phase b of C4Hlo/% 1.5 c 53.8 52.4 P 0.75 c 52.0 48.8 P 0.25 c 56.8 23.4 P (+ X) 0.25 d 56.0 22.8 P +X 0.75 d 51.3 44.1 P 1.5 d 45.5 61.7 P The flow rate of the feed was adjusted at each step to obtain approximately the same conversion, b Determined by in situ Raman. P: (VO)2P207 and X: XI. r Partial pressure of butane was decreased stepwise from 1.5% (17%, partial pressure of 02 in parentheses) to 0.75 (18.5)% and then 0.25 (19.5)% after N2 treatment of the catalyst at 500~ The balance was N2. ~ Partial pressure of butane (02 content) was increased stepwise from 0.25% (19.5%, partial pressure of 02) to 0.75 (18.5)% and 1.5 (17)% after treatment in air at 460~ ,

,

,

,

37 2. S E L E C T I V E OXIDATION OF ALKANES There have been several a t t e m p t s to obtain oxygenated products from lower alkanes (C2 - C5) by using heteropoly catalysts. It has been reported t h a t the hydrogen form of H3PMo12040 catalyzes the oxidation of lower alkanes to aldehydes and carboxylic acids [8] and t h a t the substitution of V 5§ for Mo 6+ modified the catalytic activity and selectivity [1, 9 - 12]. By optimizing the q u a n t i t y and type of constituent elements of heteropolyanions and counter cations, fairly good yields were obtained for the oxidation of isobutane [13 - 17]. Recently, it was found t h a t acidic cesium salts of Keggin-type heteropolymolybdates can efficiently catalyze the oxidation of isobutane to methacrylic acid with molecular oxygen. The optimal contents of Cs and V were 2.5 and 1, respectively, and the addition of Ni enhanced the catalytic activity even further will be discussed below [13- 16]. The results for CsxH3-xPMo1204o catalysts are shown in Table 2 [13]. The highest conversion was observed around x = 2.5 - 2.85. The main products were methacrylic acid (MAA), methacrolein (MAL) and acetic acid (AcOH). The substitution of Cs for H in H3PMo12040 resulted in a great e n h a n c e m e n t of the MAA production and the yield reached a m a x i m u m at x = 2.5. The sum of the yields of MAA and MAL on Cs2.sHo.sPMo1204o reached 5.1%. The catalytic properties of Cs2.sHo.~PMo~2040 changed by the addition of transition metal ions [16]. The addition of Ni, Mn, or Fe increased the yields of MAA and MAL. In the case of Ni, the yields of MAA and MAL reached 6.5 and 1.5%, respectively. In contrast, Co, Cu, Hg, Pt, and Pd decreased the yields. The results for Cs2.sNioosHo.34+xPMo12-xVxO4o catalysts are shown in Table 3 [16]. The conversions were 10 - 15%. The highest selectivity to MAA was also observed at x = 1. It follows t h a t the substitution of V 5+ for Mo 6+ in Cs2.sNio.0sHo.34PMo~204o resulted in the e n h a n c e m e n t of MAA production and the yield reached a m a x i m u m at x = 1. Table 2 Oxidation of isobutane over CsxH3-xPMo12040 at 340~ a x Surface Conv. Rate Selectivity 5/% area /% /10 .5 mol /m 2 g-1 min -1 m 2 MAA MAL AcOH CO

Sum of

yields of MAA+ C02 MAL/% 0 1.1 7 1.34 4 18 8 44 26 1.5 1 2.1 6 0.60 23 17 10 32 18 2.4 2 5.9 11 0.39 34 10 7 29 21 4.8 2.5 9.5 16 0.36 24 7 7 41 21 5.1 2.85 46.0 17 0.08 5 10 5 44 37 2.4 3c 46.0 8 0.04 0 10 6 32 35 0.8 a Isobutane, 17 vol%; 02, 33 vol%; N2, balance; catalyst, 1.0 g; total flow rate, ca. 30 cm 3 min-1, b Calculated on C4 (isobutane)-basis. c The selectivity to acetone was 17%.

38 Table 3 Oxidation of isobutane catalyzed by Cs2.sNio.osHo.sa+~PMol2-xV~O4o at 320~ a X Conv./% Selectivity 5/% MAA MAL AcOH CO CO2 0 10 27 12 5 30 26 1 15 36 9 6 25 24 2 13 28 8 6 25 33 3 12 10 8 9 35 38 a,b Experimental conditions. See Table 2. Thus, the Keggin-type heteropolymolybdates such as Cse.5Ni0.08Hl.s4PMollVO40 fairly selectively catalyze the oxidation of isobutane into methacrolein and methacrylic acid with molecular oxygen. At 340~ the yield of methacrylic acid reached 9.0%. The 9.0% yield of A A was greater than the highest value of 6.2% reported in the patent literature at similar steady-state conditions [10]. Figure 2 shows a good correlation between the rates of oxidation of isobutane and non-catalytic reduction of catalysts by CO. The correlation noted in Fig. 2 indicates that the catalytic activity is controlled by the oxidizing ability of catalysts. It has been suggested for the case of CsxHs-~PMo1204o catalysts that the factors controlling the catalytic activity are the o 2 oxidizing ability and the O protonic acidity of catalysts [16]. r 9

C s2.5Mn+o.08H 1.5-0.08nPMo I 1VO 40

(M = Ni z+, Fe 3+) also catalyzed the oxidation of propane and ethane [18- 20]. Here, the rate and role of vanadium is of concern [21]. It is interesting that the reduced heteropoly compounds showed higher selectivity to methacrylic acid for the oxidation of isobutane [10, 13, 15, 22, 23]. Ueda et al. applied reduced 12-molybdophosphoric acid to the oxidation of propane and obtained 50% selectivity to acrylic acid and acrolein at 12% conversion [22, 23].

O

O

H

~

Cs2.85~

o~ r

0

S~Cs2 / ~ - - - Cs3

1 2 3 Rate of reduction by CO /10 .6 mol min-1 m-2 Figure 2 Correlation between the rates of catalytic oxidation of isobutane and those of non-catalytic reduction of catalysts by CO. Csx and H represent CsxHs-~PMoy204o and HsPMo12040, respectively.

39 3. SELECTIVE HYDROXYLATION OF BENZENE Various oxidants such as dioxygen, hydrogen peroxide, and alkyl hydroperoxides have been applied for the oxidation of hydrocarbons in the homogeneous liquid phase catalyzed by heteropoly catalysts [1, 24]. Below are presented results on the selective hydroxylation of benzene to phenol with H202 catalyzed by Keggin-type heteropolyanions. It is known t h a t Fenton or related reagents also catalyze this reaction [25]. A research group that includes one of the present authors has previously reported that the reaction proceeded selectively by using H202 and vanadium-substituted heteropolymolybdates or tungstates [26]. The present study is an extension of this earlier study. Recently the same reaction was attempted by using wellcharacterized K salts of vanadium-substituted heteropolytungstates [27]. The selectivity based on benzene was high, but the yield based on H202 was not given. Heteropoly compounds obtained commercially, H3+~PMo12-~V~O40 (x = 0 - 4), were purified by extraction with ether and subsequent recrystrallization. They are abbreviated as PMol2-xVx hereafter. Their IR spectra agreed with those reported in the literature [27]. 51V-NMR spectrum of PMoloV2 in aqueous solution showed that PMoloV2 was a mixture of three to four positional isomers of PMol0V2 and contained PMol~V at less t h a n 30% level. For comparison, NaVOa, V203, V204, and V205 were used. In the case of V2Ox, sulfuric acid was added in order to dissolve the catalyst. This addition resulted in improved yields. The reaction was usually carried out at 20 - 70~ in a four neck flask, by adding dropwise 10 ml of 0.08 M H202 aqueous solution (0.8 mmol) into a mixture of benzene 10 ml and water 15 ml. Catalyst (0.05 - 0.3 mmol) was dissolved in the water phase before the reaction. Hydroxylation of benzene took place in the water phase and a majority of phenol formed was transferred to the benzene phase. The concentration of water and benzene phases were analyzed periodically by liquid chromatography with o-cresol as a standard. The evolution of oxygen gas by the decomposition of H202 was measured volumetrically. The yield of phenol on the basis of H202 consumed tended to increase in parallel with the catalytic activity of each catalyst for the H202 decomposition in the absence of benzene. The sudden introduction of H202 into the reaction system caused the evolution of a significant amount of oxygen. After a short induction period the yield of phenol increased rapidly with time. When the concentration of H202 in the reaction system was kept low by adding dropwise very slowly a diluted H202 solution, the yield of phenol increased remarkably, the unproductive decomposition of H202 being suppressed. The selectivity to phenol was almost 100 % on the basis of benzene and reached above 90 % on the basis of H202. Typical results thus obtained at 65~ are summarized in Table 4. The yields are in the order of PMol0V2 > PMo9V3 > PMosV4 > PMo~V >> PMol2. NaVO3 and V2Ox showed modest performance. The turnover based on the catalyst was about 3 in the case of PMoloV2. It was confirmed that, upon the addition of H202

40 Table 4 Yields of phenol from the oxidation of benzene by hydrogen peroxide and vanadium compounds Yield of Selectivity of Catalyst Selectivity of Catalyst Yield of phenol/% phenol/% phenol/% phenol/% (H202 (benzene (benzene (H202 basis) basis) basis) basis) 65~ 65~ 45~ VO(C5H702)2 46.6 PMo1204o 0 0 92.1 NaVO3 23.8 PMo11VO4o 9.0 0 100 56.0 V205 (I-12804) 52.5 PMoloV204o 92.6 69.2 100 81.6 V204 (H2SO4) 21.8 PMooV304o 90.1 90.7 100 V203 (a2so4) 13.5 PMosV404o 68.1 73.6 100 Catalyst, 0.3 mmol; H20, 15 ml; C6H6, 10 ml. 10 ml of 0.08 M H202 was added dropwise very slowly. Reaction time: 1.5 h. after the reaction, the oxidation proceeded again at a similar rate. Therefore, there was little deactivation of catalyst. The optimum pH range for the phenol yield was 2 to 3 for PMoloV2 and PMo9V3, and 1 to 2 for PMollV and PMosV4. According to the UV-vis spectra, PMoIoV2 and PMogV3 were stable in the pH ranges of 2 - 3 and 1.5 - 3.5, respectively. The temperature dependencies are shown in Fig. 3. The optimum reaction temperature and time appear to depend on the polyanion species. 100 The IR spectra of the reaction solutions showed that 80the structures of the Keggin polyanions remained unchanged 60 r during the reaction. When the concentration of H202 was 40 O increased, however, new b a n d s CD .t-.4 appeared in the 500 - 600 cm -1 20 region possibly due to peroxo I I species. It seems that active 0 "AI " 10 20 30 40 50 60 70 80 peroxo species are formed by the Temperature/~ reaction of vanadiumFigure 3. Temperature dependencies of the substituted polyanions and activity for the oxidation of benzene with H2Oe or that other active oxygen hydrogen peroxide for 1.5h reaction time. species are derived from the Catalyst, 0.3 retool; H20, 15 ml; C6H6, 10 ml. peroxo species. Either or both 10 ml of 0.08 M H202 was added dropwise species are probably active for very slowly. the hydroxylation of benzene. A; H4PMollVO4o, A; H5PMoloV2040, The species tend to deactivate O; H6PMogV304o, 0; H:PMosV404o, by reaction between them (e. g., ["l; H3PM012040, X; V205 (H2804) dimerization), as indicated by

41 the fact that the unproductive decomposition of H202 to oxygen and water became dominant when their concentration was high. The induction period observed when H202 was added suddenly probably corresponds to the period for the formation of the active species. Thus, by choosing appropriate vanadium-substituted heteropolymolybdates and keeping the concentration of H202 low, efficient hydroxylation of benzene to phenol was achieved. The highest yield based on H202 was 93%, where the selectivity with respect to benzene consumed was 100%. 4. CONCLUSION Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield from isobutyric acid was successfully enhanced by the stabilization of the vanadiumsubstituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (VO)2P207 was reversibly oxidized to the X1 (5) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12molybdates in solution showed 93% conversion on H202 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. REFERENCES

1. 2.

T. Okuhara, N. Mizuno, and M. Misono, Advan. Catal., 41 (1996) 113. M. Misono, N. Mizuno, H. Mori, K. Y. Lee, and T. Okuhara, Stud. Surf. Sci. Catal., 67 (1991)87. 3. N. Mizuno, T. Watanabe, and M. Misono, J. Phys. Chem., 94 (1990) 890. 4. E. Cadot, C. Marchal, M. Fournier, A. Teze, and G. Herve, in Polyoxometalates: From Platonic Solids to Anti-retroviral Activity (Eds. M. T. Pope and A. Muller), Kluwer, Dordrecht, 1994, p 315. 5. K.Y. Lee, S. Oishi, H. Igarashi, and M. Misono, Catal. Today, 33 (1997) 183. 6. G. Koyano, F. Yamaguchi, T. Okuhara, and M. Misono, Catal. Lett., 41 (1996) 149. 7. G. Koyano, T. Saito, and M. Misono, Chem. Lett., (1997) in press. 8. H. Krieger and L. S. Kirch, Rhom and Haas Co., Eur. Patent No. 0010902 (1979). 9. G. Centi, M. Burttini, and F. Trifiro, Appl. Catal., 32 (1987) 353; J. B. Moffat, ibid., 146 (1996) 65. 10. H. Imai, T. Yamaguchi, and M. Sugiyama, Asahi Chemical Industry Co., JP 145249 (1988). 11. K. Nagai, Y. Nagaoka, H. Sato, and M. Osu, Sumitomo Chemical Co., JP 106839 (1991).

42 12. G. Centi, J. L. Nieto, C. Iapalucci, K. Bruckman, and E. M. Serwicka, Appl. Catal., 46 (1989) 197; F. Cavani, E. Etienne, M. Favaro, A. Galli, F. Trifiro, and G. Hecquet, Catal. Lett., 32 (1995) 215. 13. N. Mizuno, M. Tateishi, and M. Iwamoto, J. Chem. Soc. Chem. Commun., (1994) 1411. 14. N. Mizuno, M. Tateishi, and M. Iwamoto, Appl. Catal. A: General, 118 (1994) L1. 15. N. Mizuno, W. Han, T. Kudo, and M. Iwamoto, Stud. Surf. Sci. Catal., 101 (1996) 1001. 16. N. Mizuno, M. Tateishi, and M. Iwamoto, J. Catal., 163 (1996) 87. 17. F. Cavani, E. Etinne, M. Favaro, A. Gall, F. Trifiro, and G. Hecquet, Catal. Lett., 32 (1995) 215. 18. N. Mizuno, M. Tateishi, and M. Iwamoto, Appl. Catal. A: General, 128 (1995) L165. 19. N. Mizuno, D. J. Suh, W. Han, T. Kudo, and M. Iwamoto, J. Mol. Catal. A: Chemical, in press. 20. N. Mizuno, W. Han, and T. Kudo, Chem. Lett., (1996) 1121. 21. R. Bayer, C. Marchal, F. X. Liu, A. Teze, and G. Herve, J. Mol. Catal. A: Chemical, 110 (1996) 65. 22. W. Ueda, Y. Suzuki, W. Lee, and S. Imaoka, Stud Surf. Sci. Catal., 101 (1996) 1065. 23. W. Ueda and Y. Suzuki, Chem. Lett., (1995) 541. 24. C. L. Hill and C. M. P-M. Charta, Coord. Chem. Rev., 143 (1995) 407. 25. E. g., S. Tamagaki, M. Sasaki, and W. Takagi, Bull. Chem. Soc. Jpn., 62 (1989) 153; S. Ito, A. Mitarai, K. Hikino, M. Hirama, and K. Sasaki, J. Org. Chem., 57 (1992)6937. 26. R. D. Huang, X. H. Lu, B. J. Zhang, and E. B. Wang, Chinese Chem. Lett., 4 (1993) 319. 27. K. Nomiya, H. Yanagibayashi, C. Nozaki, K. Kondoh, E. Hiramatsu, and Y. Shimizu, J. Mol. Catal., 114 (1996) 181.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

The Future Fundamental

of I n d u s t r i a l Advances

Oxidation

43

Catalysis

Spurred

by

B. Delmon Universit~ catholique de Louvain, Unit~ de Catalyse et Chimie des Mat~riaux Divis~s, Place Croix du Sud, 2/17, B-1348 Louvain-la-Neuve, Belgium.

This short review is a response to the invitation of the Organizers to present the state of Industrial Oxidation Catalysis. This is an attempt to sketch a bird's eye picture about fundamental aspects as well as applied perspectives, of catalysts, catalytic mechanisms, reactors and industrial processes. First, a glimpse at the present situation is given, covering both i n d u s t r i a l developments and f u n d a m e n t a l concepts, with particular emphasis on describing trends. The message is essentially that completely new catalytic materials are appearing, new mechanisms in all their complexity are being elucidated and new kinds of reactors and types of unit operations are being designed, all thanks to an exceptionally close cooperation between specialists in different fields. Catalytic oxidation is breaking new ground in the field of catalysis with the development of new concepts and industrial achievements. In view of the limited size allowed for the paper, only very few references will be cited compared to the richness of literature, and many important aspects will not be mentioned. 1. INTRODUCTION The objective of the present contribution is to shed light on how new industrial applications of oxidation catalysis could develop in the future, with account taken of foreseeable fundamental developments. Many excellent review papers and talks have described the new challenges industry is faced with when catalytic oxidation is considered. Equally excellent review papers and lectures have dealt with the scientific aspects of catalytic oxidation. The aim here is not to summarize these articles. It is said t h a t industrial innovation results either from a "push" or a "pull" action. The scientific advances constitute the "push", and the industrial challenges the "pull". Our wish is to provide insight on the way the push-pull process could operate in the next, let us say, 5 or 10 years, in the field of catalytic oxidation. Some scientists, like Professor Y. Moro-Oka, are of the opinion that the science of catalysis, in general, is driven b y applications (strong "pull"). Industry, looking at the situation from the other side, indirectly suggests the same trend, in particular by saying that concepts are lacking (suggesting that the "push" is weak) [1]. However, given the number of novel catalysts, novel reactors and novel processes in the field of oxidation, it would rather seem that neither "push" nor

44 "pull" is determining, but the combination of both. If this is true, it would be useful to attempt to discern the components of both the push and pull actions. By doing so, we may discover that oxidation catalysis, both industrial and fundamental, is in the process of evolving considerably differently from other fields of catalysis. C o n s i d e r i n g i n d u s t r i a l challenges in oxidation, especially the transformation of light alkanes to useful intermediates, the incredible variety of formulations giving equivalent performances suggests that old concepts developed in allylic oxidation are of limited use to guide the search for active catalysts in an effective way [2]. Conversely, new fundamental data open promising perspectives, and new concepts for reactor design lead to surprisingly high performances. This is due to the fact that new academic laboratories in close contact with industry (namely aware of the "pull") are taking radically new approaches in the search for new active catalysts, new reaction conditions, new types of reactors and new types of operation. Research in Academia and Industry is recognising t h a t investigation should be comprehensive, involving many different approaches to solutions. Recent spectacular laboratory results obtained in methane coupling or oxidative dehydrogenations and elegant technical innovations like the moving bed reactor in butane oxidation demonstrate that considerable progress can be made if a multifaceted approach is taken. These are the points we wish to outline shortly in the following pages. 2. P R E S E N T SITUATION IN OXH~ATION CATALYSIS Catalytic oxidation is a strategic part of catalytic industry. Selective oxidation, oxidative dehydrogenation and ammoxidation, represent about 11% in value of the catalysts consumed by the chemical industry. This does not include auto exhaust catalysts, also active in oxidation, which account for 30% in value of catalyst sales (not very far from the 36% share in value of catalysts used by the whole chemical industry). The growth of selective oxidation catalysts is presently about 11% per year, twice as large as the average (for auto exhaust: 13% world-wide, with large differences according to continents). The catalytic elimination of volatile organic compounds (VOC) or organic matter in water by oxidation still represents less than approximately 10% of the total value for selective oxidation catalysts. Auto exhaust purification, the oxidations of SO2 or NH3 for the manufacture of sulfuric acid or nitric acid respectively, and oxychlorination are in the list of the 20 major processes of the chemical industry; formaldehyde and ethylene oxide manufacture are not far down the list. These few figures summarize both the diversity of the catalytic reactions involving oxygen and their industrial importance. With the permission of the author [3], we borrow here data (Table 1) which indicates the production capacity of the major industrial processes using oxygen for functionalizing hydrocarbons. The production of acetic acid should be added to the list, although 60% of its 6.1 million t/year total world capacity (to reach 67% in the next future) is due to the Monsanto process (methanol carbonylation) [4]. Only the rest (2.4 million t/year) is produced by oxidation of butane or other alkanes or acetaldehyde or, for a small proportion, by the Showa Denko process (oxidation of ethylene).

45 Table I. Production capacities. Million tons per year (Mt/year) Japan North America Europe Formaldehyde (pure) 2.00 4.00 0.70 Acrylonitrile 1.40 1.60 0.60 Maleic anhydride 0.21 0.20 0.10 Phthalic anhydride 0.46 1.30 0.30 Acrylic acid 0.71 0.65 0.42 Methyl methacrylate 0.68 0.70 0.46 Ethylene oxide 4.20 2.50 0.75 G. Hecquet, Plenary Lecture, EUROPACAT II, Maastricht, 1995.

World 6.70 4.50 0.65 3.30 2.00 2.20 11.00

Two facts are striking, when considering this table. The first is that, except for maleic anhydride (MAA), which is now produced from butane, and part of acetic acid (in the liquid oxidation route), the feedstocks used in the other processes are olefins, aromatics or molecules already containing oxygen. The ammoxidation of propane is in an advanced state of development but, on the whole, processes based on paraffins are not yet strongly emerging even though their development constitutes a major "pull". Paraffins are, of course, desirable because they are cheaper feedstocks. Besides, based on present knowledge, it seems that in spite of leading to more extensive oxidation to CO2 and H20, their use would be cleaner, environmentally speaking, because less partially oxidized by-products would be produced. A notable exception is the production of methanol or formaldehyde, where the energy wasting route through synthesis gas will remain more attractive for many years. The second fact, linked to the first, is that a much lower yield in valuable products is obtained when paraffins are used instead of olefins or aromatics. The success of the butane-based MAA process occurred at the cost of a dramatic drop of molar selectivity (at most 65-67% compared to 75-77% when starting from benzene) and a drop of productivity by nearly 20% [3]. Compared to the benzene route, butane oxidation gives molar yields per pass of only about 55% instead of 75%. A third r e m a r k can be made when considering the catalysts used in reactions of light paraffins, butane excepted. It is surprising to discover how large the variety of catalysts which have been claimed to activate light alkanes is. This could be attributed to the fact that we are only at the first stages of progress towards really selective systems, and no catalyst has yet emerged as potentially the best. A notable exception is vanadium phosphate: this is the only performing catalyst for butane oxidation. The a r g u m e n t t h a t a very special surface structure is needed for the concerted oxidation mechanism of butane and easy desorption of the product (a pair of oppositely oriented square pyramides) could explain the exception that vanadium phosphate represents. But acrylonitrile, in the ammoxidation of propane, is also very stable and nevertheless many catalytic formulations seem to give similar results. The diversity of formulations is therefore a fact we must take into account in future investigations concerning selective reactions of alkanes with oxygen. As an example of such a diversity, Table 2 lists some of the catalyst f o r m u l a t i o n s claimed to give good r e s u l t s in p r o p a n e oxidative dehydrogenation. As it is only aimed at underlining the variety of formulations proposed for a single reaction, the table is only sketchy, in the sense that the reaction conditions are not reported. The references and a few more details are

45 given in another article [2]. Various magnesium vanadates have been the object of many studies, but other systems seem to have comparable performances (systems based on cerium, niobium, or vanadium, molybdates and noble metals on monoliths used with very short contact time). Table II. Propane oxidative dehydrogenation Reactant Product Catalyst Conversion Yield Selectivity % % % (oxygen not indicated except if defined phases) Propane Propene Nb based catalysts 7 85 Propane Propene VMg, VMg+Ag, 10 84, 86.9 Electrochemical pumping of oxygen Propane Propene VMg and chloride of 23.1 Cu +, Li +, Ag+, Cd2+ Propane Propene noble metals (Pt,Pd) 100 65 (total Ethene on ceramic foam olefins) monoliths at short contact time, 5 ms Propane Propene 19 60 Co0.95MoO4 Propane Propene V/Mg= 2/1 23 46 2/2 23 59 23 49 2/3 Propane Propene 25 60 VMgfrio2 Propane Propene 20 12.5 62 NiMoO4 43 14.8 34.5 Propane Propene 41.3 33.5 81.1 CeO2/2CeF3/Cs20 Propane Propene 40.3 66.2 FeV-supported Nd203 Propane~ Butane I Alkenes Hexane J Propane Propane Propane Propane

Propene Propene Propene Propene Acrylonitrile

Propane

Propene

Propane

Propene

Propane

Propene

Propane Propane

Propene Propene

Vanadate catalysts V-Fe-Nd-A1 VMg CeO2/CeF3 (NH3)3PO4 + In(NO3)3 + Vanadyl phosphate NiMoOx (x=number determined by Ni or Mo valency) A1203-supported Pt/Cs/Sm MgV206 (50% V205+MgO calcined at 610 ~ CoMoO4/SiO2 NaOH/Na3VO4/A1

50 40.3 10 53.4 12 29

50 66.2

26.7 65

36.7 35 36.7 18.1 91 71

4.1 20.9

16.6

77.9 79.8

47 No new formulations seem to have been proposed for propane oxidative dehydrogenation since 1993, the year of the work cited above, and a literature computer survey indicates only 7 articles which deal with mechanisms, one mentioning catalysts containing Bi, Mo, W, V and Ti [5], and 2 r a t h e r general patents. The literature is not richer for the oxidation of propane to acrolein, with mention of the formulation Ag0.ol Bi0.85 V0.54 Mo0.45 04.0, giving a selectivity to acrolein of 32% at a propane conversion of 52% [6], quite comparable to results obtained in 1991 by other authors [7]. Similar remarks can be made for other oxidation reactions. Considering all the recent work on the various selective reactions of light alkanes with oxygen, three remarks must be made: ~ In spite of the "pull" (functionalization of light alkanes), the intensity of academic and industrial research seems modest. The effort perhaps started too short a time ago to have produced patents or articles. 9 Several lines have been followed, but almost all were inspired by former research r a t h e r t h a n by new approaches. It is striking t h a t the proposed catalysts are similar to those tested in the other selective reactions of alkanes with oxygen, principally, oxidative coupling of methane or oxidation of butane to maleic anhydride. Many of them also have compositions comparable to those of catalysts used for the reactions of olefins with oxygen (molybdates or antimonates) or for dehydrogenations in the absence of oxygen (chromium containing catalysts). Because of the success of vanadyl phosphate in butane oxidation, there is a tendency to focus on vanadium containing catalysts. It is not sure, however, that the data available justify this preference, or suggest the exclusion of other formulation. The "push" in this respect seems rather weak. 9 On the other hand, the reaction of methane, ethane, propane, isobutane and pentanes with oxygen described until now are poorly selective at high, or even moderate conversions. The low stability of the products compared to maleic anhydride may be the explanation. But it could also be argued that the reaction with paraffins is more complex than with olefins. In the case of vanadium phosphate in the oxidation of butane, the idea seems to emerge t h a t a special configuration (two adjacent oppositely oriented flat pyramids) is critical for the smooth r u n n i n g of the concerted process leading to MAA [2,8]. This configuration could permit the adsorption of butane in a correct configuration and the successive intervention of the seven oxygen atoms needed in the reaction of a single butane molecule. If such a special configuration of the active site is essential for selective oxidation of alkanes, it could be hoped to find specific s t r u c t u r e s p a r t i c u l a r l y active and selective in other cases. The requirements are perhaps less stringent, however, because a smaller number of oxygen atoms is required than in butane oxidation. The final r e m a r k of this sketchy section should be made cautiously. It seems that surprisingly, light alkane activation, although being heralded as a major area of future development, appears to benefit only from moderate "push" and moderate "pull". Catalytic combustion and the complete oxidation of pollutants seem areas with much more activity presently. But the "pull" by environmental concerns is strong in those cases.

48 3. 'N-IE ,PUSI-r' EXERTED BY CATALYTIC SCIENCE Keeping in line with the objective of this short article, it is useful to analyse the components of the "push" exerted by catalytic science to innovation in selective oxidation. Ideas and concepts have progressively evolved since the time when the oxidation of olefins and butane oxidation to MAA developed industrially. It is useful to examine critically the potential of these ideas and concepts if we wish to discover new catalysts or improve substantially those mentioned in the literature (such as those of Table 2). We summarize here comments made previously [2,9]. It should be remarked incidentally that, according to a literature survey made at the time of writing this paper, only 5 review papers on selective oxidation have been written since 1993. Except a specific one dealing with bismuth molybdate catalysts, we cite all of them in this article. Role of traditional parameters Doping with minute amounts of elements that authors suppose (often without proof) inserted in an oxide lattice or spread atomically on the surface is in principle a good approach for modifying catalysts. It should, however, be emphasized that it has seldom been verified that the doping elements were really incorporated in the host oxide and, if so, did not spontaneously segregate out. In almost all the cases (unfortunately surprisingly rare) where the check has been made, such a segregation was shown to occur. This suggests that the concepts underlying the addition of dopants (e.g. control of valency of active elements, change in the rate of the reduction-oxidation process at the surface, change of "acidity") may not offer a relevant interpretation of the effects observed. More precisely, they do not seem to do so when used in the way they were in the past. This may suggest that reliable lines for further improvements were missing. It is a pity that ideas which are in principle very well grounded could lead only to uncertain conclusions and modest advances for lack of properly planned experiments. Microanalysis allows the determination of composition at the nanometer level. One can expect that accurate analysis of phases in active catalysts in the state they are during use or quenched to room temperature after use, correlated with activity and selectivity measurements, will lead to substantial progress in the future. Supports are presumed either to be inert (e.g. SiO2), or to permit atomic dispersion or formation of a monolayer (e.g. anatase when V205 is the active phase). Hence the idea that epitaxial layers could be the active and selective species. This view is in principle correct, but incomplete. What is still more disappointing is that the existence of the supposed atomic dispersion or monolayer structure has been verified only in a very limited number of cases, and possibly using sometimes inadequate tools. For example, strong doubts begin to be raised concerning the atomic dispersion of V205 in the form of a monolayer on anatase. The most likely structure seems to be that of severallayer thick islets of a suboxide of vanadium (V6013) [10,11]. These islets would be stabilized by epitaxy, thus permitting the suboxide to lose and reincorporate oxygen during the catalytic cycle without irreversible transformation to a different structure. This is in line with a view which is beginning to emerge, namely that the emphasis should be laid on the catalyst in the state it is while it works during the catalytic cycle. Systematic examination of used catalysts could have avoided misleading interpretations. In particular, this could have

49 avoided to attribute a catalytic behaviour to monolayer formation in cases where transformation to islets occurs in the reaction conditions. Epitaxy may reasonably be mentioned for the VOx/anatase system. But there are other cases where epitaxy is doubtful. There is no detectable evidence of Sb204 decorating epitaxially FeSb204. If the picture of oppositely oriented flat pyramids as active sites for butane oxidation is correct, the hypothesis attributing a crucial role to an epitaxy between various vanadium phosphates may lose credibility. On the whole, the parameters which have been generally considered in the last 25 years are certainly relevant in principle, but the conclusions or speculations concerning their real intervention in given catalysts deserve careful verification using modern physico-chemical techniques. The minimum precaution is to check whether the feature mentioned (e.g. doping or epitaxy) survives after the catalyst has worked at realistic catalytic conditions. Are there n e w lines for the search of n e w catalysts ? Several facts suggest new directions for research aimed at the discovery of new catalyst formulations or structures, and new routes for catalyst improvement. Two critical reviews of the role of important parameters in selective oxidation have been presented recently. One of them focuses on factors which have perhaps not attracted sufficient attention, like mode of adsorbate bonding and site isolation, together with new views on more traditional parameters [12]. On the basis of this and the other review [2], we wish to show that the real picture is substantially different from the traditional one. The essential reason is that most academic studies in the past implicitly assumed that the solids submitted to characterization as freshly prepared were already in the catalytically active form. A real and credible "push" is gaining force now. It rests on the extensive characterization of catalysts after they have worked catalytically or during their work (a still better approach). This "push" is constituted of lines of work inspired by newly discovered facts. These are the n a t u r e of the real phases active catalytically, their texture (e.g. the crystallographic phases really exposed to reactants) and the relation between these phases. Three lines of work, among others, can be cited. One is typically represented by the work dealing with the changes in structure and texture of VPO catalysts during activation in the presence of the reacting mixture (butane or propane with 02), by J.C. Volta et al. [13,14]. R.A. Overbeek and J.W. Geus took a complementary approach [15,16]. Another line is that of P.L. Gai-Boyes concerning the defect structure of catalytically active phases during catalysis [17]. The third line deals with the cooperation between phases. One important objective is thus becoming clearer, namely synthesising the phase or phases which are really active during catalysis. Nowadays, this objective can be reached, because we have the tools to identify these phases. This "push" will gain strength in the future, when the impressive inventiveness of catalysis scientists for finding new preparation methods will be directed to the synthesis of these phases. A more extensive characterization of catalysts progressively suggested that the key to activity and selectivity might be the existence of s e v e r a l phases in catalysts. As regards to this aspect, the ideas have been progressing thanks to O.V. Krylov, I. Matsuura, Y. Moro-Oka and U.S. Ozkan. The group of the present author focused on it. In this paper, citations and illustrations will

5o come from other groups. Breiter et al. [18] demonstrated that, at industrial conditions, a phase inactive in the oxidation of acrolein to acrylic acid, namely NiSb206 (or CoSb206) considerably increased the performance of an already optimized catalyst of formula Mol2V3Wl.2Cu2.2Ox. They attributed this effect to the oxygen-donor properties of NiSb206 or the other compounds they used, namely Co85206, CuSb206 and Sb204. These properties were in line with those indicated by the remote control concept [19]. Another set of experiments concerns the reaction of lower alkanes. Those reported in fig. 1 show that MgMoO4 and MoO3 exhibit a cooperative effect in the oxidative dehydrogenation of propane. The result is completely in line with the oxygen donor-acceptor scale resulting from the remote control theory [19]. ~

10

0 100 ",~

~

500~ '

~

.

.

5

5

0

~

(b

500~ 550 ~

#, 90

l

0.5

Moo3 /I.9.oo

1

. .oo3 j

0

10 Propone conversion %

Fig. 1. Oxidative dehydrogenation of propene on MgMoO4-MoO3 mechanical mixtures (C3H8: O2:He=1:1:25) MoO3/(MgMoO3+MoO3): O 5 wt% 9 20 wt% (after M.C. Abello, M.F. GSmez, L.E. Cadus, Actas, XV Simposio Iberoamericano de Cat~lisis, C6rdoba, Argentina, (1996), pp. 233-238.) Last but not least, the discovery of titanium silicalite and its activity in an increasing number of new oxidation reactions not only opens promising prospects of future discoveries with respect to reactions, but also suggests that materials with new catalytic properties will be discovered. Their distinctive features should logically be a control of coordination of the active atom or atoms that zeolites permit more easily than other structures, and shape selectively. Two recent papers review the potential of zeolites as oxidation catalysts [21,22] and very stimulaing comments are made in a third one concerning the oxidation (mainly epoxidation) of large molecules [23]. We mentioned 3 promising lines in this section, namely: catalysts "as they work", catalysts in which remote control is optimised, and zeolites. The first two lines take cognisance of the fact that catalysis is a dynamic process in which the solid, especially in the case of oxidation, cyclically undergoes extensive superficial changes, and its surface is the theater of a continuous

51 movement and transformation of adsorbed species. This suggests t h a t the above advances and others not cited here offer a vast potential for catalyst improvement in selective oxidation. Composition of the gas phase When comparing test conditions in most investigations carried out by Academia with those used in Industry, striking differences may appear. In several processes, for example, steam is injected together with the hydrocarbon and oxygen, but very few university researches included the influence of steam. Practically each time reactions with and without steam have been compared, results turned out to be different. Possible explanations may be: 9 role as a simple diluent (including a possible diminution of the surface concentration of reactants); 9 influence on the thermodynamics, an explanation sometimes quoted for the simple catalytic dehydrogenation of ethylbenzene to styrene in the absence of oxygen (although it is not clear why steam could achieve more favorable thermodynamic conditions); 9 more efficient heat transfer; 9 t u n i n g of catalyst surface acidity; this explanation seems relevant in principle, because other parameters do bring about measurable modifications of the number of BrSnsted sites on the surface of catalysts; water molecules could very easily do the same [19]; 9 other more complicated reactions of water with the catalyst surface [24]; 9 modification of homogeneous oxidation reactions: when comparing the homogeneous oxidative dehydrogenation of propene in the absence and presence of steam, it was found in the second case t h a t a relative increase in the selectivity to propene by more than 25% occurred at low conversions and remained substantial at high conversions [2]. Feed composition does not only concern steam. Industrially, additives are often constantly added to the feed. A reason frequently advanced is t h a t the catalyst could in this way be replenished of elements it loses continuously. This suggests an area of research which should have fascinated Academia, because the use of such additives helps control the degree of doping during the whole life of the catalysts. In this case, the restriction concerning doping expressed above is not valid any more, because the doping elements are fed continuously rather than just introduced in the fresh catalyst. A still more exciting line of research based on modifications of feed composition is the addition of hydrogen in catalytic oxidation. This seems absurd at first sight: one speculates that hydrogen would react immediately with oxygen just to form water! The experiments reported in fig. 2 give surprising results, namely the unexpected catalytic transformation of propene to propene oxide, and the selective oxidation of propane and isobutane respectively to acetone and t-butanol (25,26]. The catalyst is strange (Aufl~i02) and the yields very low. This result and a few other ones mentioned in the literature, however, trigger interesting questions. The role of hydrogen cannot be just to react with oxygen to produce water, because the amount is too small. Is it to produce heat locally on the surface of the catalyst, or to create new surface intermediates? If we accept the view that spillover species can play a role (remote control), a speculation along the following line can also be proposed. In selective oxidation, the remote control operates so t h a t spiUover

52 oxygen keeps catalysts more oxidised than in the state normally resulting from the balance between surface reduction and oxidation during the catalytic cycle. It is believed, and this rightly at least in butane oxidation on VPO catalysts, that the surface must be kept relatively reduced in alkane oxidation, in order to catalyse the initial activation steps. The speculation could therefore be that hydrogen (spillover hydrogen?) could impose a more favorable reduced state to the catalyst. A still stranger reaction involving both air and hydrogen, namely the oxidation of benzene to phenol, will be the object of more comments near the end of this article. A recent review paper stresses in a more general way the importance of the reactive atmosphere in selective oxidation [27]. 0.20 C3

E

{b

E -

1.0

-

0.5

"~

0.15 propylene oxide

~3

oc

0.10

co 2

0.05

acetone co 2

propene (SO~

acetone propane (80~

t-butanol isobutane (80~

0

Fig. 2. Oxidation of propene, propane and isobutane over Aufs in the presence of hydrogen. Feed: H2:O2:hydrocarbon:Ar = 1:1:1:7; flow rate 2000 ml h-l; catalyst AudiO2 (1 wt% Au), 0.5 g). Yields are calculated in mol% on the basis of the starting hydrocarbon (after T. Hayashi, M. Haruta, Shokubai, 37 (1995) 75). Intervention of homogeneous gas phase reactions In this section, we again select the case of light alkanes functionalization in our attempt to discuss fruitful lines of research. It is well known t h a t gasphase reactions play an important role in methane oxidative coupling. This is expected, as this occurs at very high temperature. But this is a general phenomenon. Contrary to the case of olefins, homogeneous catalytic oxidations of alkanes with more than 3 carbon atoms proceed at temperatures similar to those of the catalytic reaction and these are relatively low. This probably has

53 led to m i s i n t e r p r e t a t i o n of data in certain cases, namely a t t r i b u t i n g homogeneous reactions to catalysis. A very interesting contribution in this respect is that of Burch and Crabb, who investigated in detail the role of homogeneous and heterogeneous reactions in the oxidative dehydrogenation of propane [28]. The reaction temperature for the catalyzed reaction was about 130 ~ lower, but differences with the homogeneous reaction depended on the oxygen/hydrocarbon ratio. The unexpected result is that there are similar conversion vs. selectivity relationships for both the heterogeneous reaction and the homogeneous one with most catalysts. Even the best catalysts are not better than no catalyst at all at the higher temperatures. This could seem pessimistic, but obviously does not rule out t h a t new catalysts could give a decisive advantage to catalyzed reactions. Incidentally [2], the homogeneous reaction in the presence of steam seemed to exceed the performances of the catalysts tested in the work cited above [28]. Burch and Crabb noted in the abstract of their paper: "A combination of homogeneous and heterogeneous contributions to the oxidative dehydrogenation reaction may provide a means of obtaining higher yields in propene". This prediction seems to be supported by at least one result concerning a very special catalytic system which may combine homogeneous and heterogeneous processes. This is constituted of lithium hydroxide/lithium iodide melts, which give considerably higher propene yields at higher propane conversion than either homogeneous reactions or reactions catalyzed by solid catalysts [29]. It has been mentioned that both maleic and phthalic anhydrides can be produced in the catalytic oxidation of n-pentane in the presence of VPO catalysts. With n-pentane, however, the homogeneous reaction begins to be significant above 300~ a fact the consequence of which has perhaps been underestimated. By using reactors with empty spaces of different volumes (lengths), it is possible to detect the influence of both the heterogeneous and homogeneous reactions. The non-selective homogeneous reaction increases npentane conversion, but the surprising finding is that the maleic/phthalic anhydride selectivity varies substantially, from 1.8 with as little empty space as possible to over 4 with substantial empty space [2]. This demonstrates that the homogeneous reaction, which begins to play an important role in the oxidation of n-pentane in the range of temperature where catalysts like VPO are active (around 350-400 ~ leads to a modification of selectivity. Conclusion concerning the role of catalytic science Concluding this section, it seems that research on oxidation catalysts and oxidation reactions provides some push to innovation, through (i) the high level of activity in the synthesis of new structures and development of new approaches to catalyst fabrication, (ii) a better knowledge of the state of catalysts during their use, (iii) the role of surface mobility, spillover and remote control, (iv) the use of additives in the feed and (v) a better understanding of the contribution of homogeneous gas phase reactions. 4. 22-IE"PUSH" EXERTED BY THE USE OF NEW TYPES OF REACTORS AND NEW CHEMICAL ENGINEERING CONCEP2~ According to the philosophy of this overview, we shall comment on some of the facets of present science and technology in catalytic oxidation. Because of

54 the importance of light alkane oxidation as a "pulling factor", we shall again consider principally this group of reactions when examining the role of chemical engineering. One must recognise that the headlines concerning selective oxidation of light alkanes in the last years did not concern spectacular advances in catalytic science, in the narrow sense, but rather resulted from the use of reactors which had not been used traditionally in catalytic oxidation. The impressive yield in olefins (65%) using a monolith reactor at very short contact times (5 ms) is not approached by any other result mentioned in Table 2. This result, due to Huff and Schmidt [30] demonstrates that employing a type of reactor not used previously in selective oxidation, but rather in complete oxidation, and very short residence times can lead to promising prospects. It can also be underlined that the active catalytic species used in this work are regarded as among those most active in complete oxidation (platinum in particular). This constitutes an outstanding case where the type of reactor is much more important than the intrinsic selectivity of the catalyst for reaching high selectivities. But recent results also show that the chemical engineering approach as a whole has a key role to play in the development of catalytic oxidation. To illustrate this, the example of methane oxidative coupling is considered. In spite of a wide recognition of the importance of homogeneous reactions, an overwhelming fraction of research has been directed to the discovery of new catalysts (perhaps over 90%) with only a very small number of investigations trying to take these homogeneous phenomena into account. The progress has been deceivingly modest. This has led respected industrial scientists to discourage further research. It was right to recognize t h a t the results published were very far from being economically attractive. But, instead of discouraging research, these scientists should have spurred research and, at the same time, stressed that this should be conducted on different lines. One such possibility is clearly to design reactors taking into account the specificity of homogeneous-heterogeneous reactions (perhaps inspired by the ultrafast monolith reactor of Huff and Schmidt). But considering the whole apparatus or the whole future chemical plant can also be very fruitful, even without changing the reactors. Two recent results convincingly demonstrate that integrating recycle and separation features with a catalytic reactor leads to impressive yield. Tonkovich et al. reached a 50% yield in C2 hydrocarbons in the oxidative coupling of methane. They used a moving bed reactor, thus permitting some sort of a chromatographic separation [31]. The problem remained of the high reactivity of ethylene compared to CH4. One way to deal with this was to recognize that the reaction of methane to ethylene can be extremely selective at very low methane conversions. Considering this, Vayenas et al. achieved an ethylene yield of 85% (calculated on the carbon contained in CH4) [32]. The key was alternating the highly selective adsorption of ethylene, ethane and CO2 on a 5A molecular sieve with periodical release. Conversion was kept very low, and the non-reacted methane recycled. A striking fact was that the catalyst did not belong to the group giving the best performance in simple flow reactors. Besides silver on which oxygen was "pumped" electrolytically, the authors used a Sm203(20%)-CaO(l%)-Ag(79%) catalyst simply contacted with molecular oxygen from the gas phase (both systems gave approximately the same results). Not only did the reactor concept offer new perspectives, but the results proved (as the result of Huff and Schmidt

55 suggested) that a key to success is to adapt catalysts to reactors and operation conditions and vice versa. Another conspicuous illustration of this remark is the success of the recirculating fluidized bed reactor developed by DuPont for butane oxidation (using the so-called "riser" reactor). The catalyst is now more a "carrier" of selective oxygen t h a n a real catalyst. Its characteristics, particularly resistance to attrition combined with easy access of the reactants to particles situated inside the silica shell, resulted from an adaptation to the new type of reactor. The reactor itself necessitated innovative developments for taking advantage of the "oxygen carrier" properties of the catalyst, and to overcome a drop of productivity in MAA per pass. The development of the reactor is a real achievement considering that, contrary to those using classical fluidized beds, this kind of recirculated bed reactor has not been developed very much in industry (except in platinum catalyzed reforming). Another conspicuous innovation is the ultrafast oxidation reactor developed by BASF for the oxidation of isoprenol (2 methyl-butl-ene-4 ol.) to isoprenal for the synthesis of citral:

02

~~...~.CHO

This is a continuous flow reactor operating at 500~ with a residence time of the order of 1 millisecond. The yield in the extremely sensitive isoprenol product is 95% [33]. Other innovative developments have appeared, pertaining to both the process and the reactors. The SMARTsm reactor design derived from the Styro-Plus process of UOP is the key to a more efficient route from ethylebenzene to styrene by non oxidative dehydrogenation. Hydrogen is formed in a first dehydrogenation reactor. The SMARTsm reactor combines 2 catalysts: (i) a platinum oxidation catalyst that reacts the hydrogen produced in the first reactor with oxygen to generate heat (ii) a dehydrogenation catalyst. This innovation permits a large increase in productivity by retrofitting of existing radial flow reactors [34]. Also considering complete oxidation, an interesting tendency appears in catalytic combustion, which can be optimised by using 2 (or several) monoliths in series [35]. Other new concepts are certainly developing, for various simple catalytic oxidations as well as for complex successions of catalytic oxidation and/or catalytic dehydrogenation (new routes to methacrylic acid). Catalytic combustion is the object of many efforts (as the ones of Catalytica Associates and Osaka Gas Co.). The idea of reactors integrating several functions (for example, reaction and separation, as in catalytic membranes; or oxidation coupled with an energy generator, e.g. a turbine) is attracting increased attention. For example, the recently commercialized combined cycle coal gasification technology offers unprecedent efficiency in the transformation of coal to electricity. It is still difficult to discern the driving forces in the development of these new reactors. Three of them are perhaps dominating: (i) control of the homogeneous-heterogeneous processes (methane oxidative coupling, monoliths, catalytic combustion) (ii) full efficient use of the unusual potential of selective oxidation catalysts, which, after all, are reduction-oxidation "reactants" (riser reactor)

56 (iii) integration of the catalytic step in a more complicated network of functions to be carried out by a system (several reactions in a same reactor; reaction and separation; combustion and production of energy). 5. PERSPECTIVES Specialists of chemical engineering and catalysis scientists are certainly ready to agree that a good process needs a good catalyst. The conclusion of the previous paragraphs was that the "push" concerning catalysts is perhaps not strong enough. On the other hand, one discerns a strong "push" in the invention of new reactors or new process designs. It seems likely that the "push", namely the innovation thanks to fundamental concepts, in selective oxidation will essentially be exerted by a close cooperation between catalytic science and chemical engineering in the next years. This necessity of close cooperation clearly appears when considering the recent successes. On the whole, oxidation catalysis conveys a positive image of dynamism and inventiveness. Improvements of already existing oxidation processes are continuously made (in MAA manufacture, with the riser reactor by DuPont, or in oxychlorination, by Montecatini Technologie and ICI). In addition, and still more clearly demonstrating the dynamism of industrial catalytic oxidation, completely new catalysts are discovered, especially with the titanium silicalite which permits the synthesis of hydroquinone from phenol, selective epoxidations, oxidations of alcohols to aldehydes, and the manufacture of cyclohexanoneoxime. New processes have appeared in the last ten years, in addition to the now well established routes from isobutene to methylmethacrylate (Asahi Chemical Ind.). These are: 9 oxidation of methylal to formaldehyde: 02 CH30-CH2-OCH3

~ 3HCHO + H20 300 - 400 ~ FexMoyOz

9 very selective gas phase oxidation of ethylene to acetic acid (Showa Denko). 9 oxidation of 4-methoxy toluene (p-methyl anisole) to paramethoxybenzaldehyde over a catalyst containing vanadium and alkali metals, around 400~ (Nippon Shokubai and Kagaku Kogyo). 9 direct access to methyl isocyanate from methylformamide by oxidative dehydrogenation over a silver catalyst (DuPont). 9 direct oxidation of benzene to phenol catalyzed by a solid catalyst (0.5% Pt/20% V205/Si02) under pressure at 200~ at unusual conditions which will be mentioned further in this section (Tosoh Corp.). It is striking t h a t many new processes get integrated in a new manufacturing chain, as the oxidations of methyl methacrylate were. Other examples are: 9 oxidation of butane to maleic anhydride and tetrahydrofuran (DuPont)

57 acetoxylations, as a route to 1,4 butane-diol and tetrahydrofuran on Pd catalysts containing additives (Te, etc.) (Mitsubishi Kasei): butadiene + 2AcOH + 1/202 -~ CH3COOCH2-CH=CH-CH2OOCCH3+H20 It is remarkable that this reaction and the direct hydroxylation of benzene cited above occur in a 3-phase system: gas-liquid-solid. The oxidation of benzene constitutes an extraordinary case in that the liquid is a mixture of benzene and acetic acid and the gas phase contains both air and hydrogen! This suggests the formidable problems that chemical engineering had to solve to develop these processes. The spectacular results obtained in the most extensively investigated reaction, namely the oxidative coupling of methane, the development of ultrafast catalytic reactors and catalytic combustors and the above examples constitute clear indications that we are still at the beginning of a strongly innovative research period in catalytic oxidation and, in particular, concerning the functionalization of light alkanes. This is a clear indication that the discoveries triggering these innovations will have their roots in all the relevant scientific fields, heterogeneous catalysts as well as homogeneous reactions, discovery of new catalyst formulations as well as new chemical engineering concepts. 6. ACKNOWLEDGMEN'I~ Many colleagues assisted the author in the preparation of this article, by giving suggestions and comments and sending documents. I wish to acknowledge their help here: J. Bongaarts, J.F. Brazdil, M.G. Clerici, G. Hecquet, H.H. Kung, J.J. Lerou, P.G. Menon, S.T. Oyama and P. Ruiz. 7. REFERENCES 1. J. Rabo during his plenary lecture, in "New Frontiers in Catalysis" (L. Guczi, F. Solymosi, P. T~t6nyi, eds.), Akad~miai Kiad6, Budapest, 1993, pp. 1-29. 2. B. Delmon, P. Ruiz, S.R.G. Carraz~n, S. Korili, M.A. Vicente Rodriguez, Z. Sobalik, in "Catalysts in Petroleum Refining and Petrochemical Industries 1995" (M. Absi-Halabi, J. Beshara, H. Qabazard, A. Stanislaus, eds.), Elsevier, Amsterdam, 1996, pp. 1-25. 3. G. Hecquet, Plenary Lecture, 2nd European Congress on Catalysis, EUROPACAT II, Sept. 3-8, 1995, Maastricht (The Netherlands), 1995. 4. Chem. Eng. News, 1996, July 1, p. 7. 5. J. Barrault, L. Magaud, M. Ganne, M. Tournoux, in "New Developments in Selective Oxidation II" (V. Cortes Corber~n, S. Vic Bell6n, eds), Elsevier, Amsterdam, 1994, pp. 305-314. 6. J. Li, W. Song, B.-Sh. Dou, Yongyong Huaxue, 13 (1996), 88. 7. Y.C. Kim, W. Ueda, Y. Moro-Oka, Appl. Catal., 70 (1991), 175. 8. J.R. Ebner, M.R. Thompson, Catal. Today, 16 (1993), 51 (see also in ref. 2).

58 0

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31. 32. 33.

J.T. Wrobleski, B. Delmon, H.H. Kung, Y. Moro-Oka, J.L. Cihonski, "Catalyst Modification: Selective Partial Oxidation", Catalytica Study nr. 4190CM, Catalytica, Mountain View, CA (1991). T. Machej, M. Remy, P. Ruiz, B. Delmon, J. Chem. Soc. Faraday Trans., 1990, 86, 715. T. Machej, M. Remy, P. Ruiz, B. Delmon, J. Chem. Soc. Faraday Trans., 1990, 86, 723. S.T Oyama, in "Heterogeneous Hydrocarbon Oxidation" (B.W. Warren, S.T. Oyama, eds.), ACS Symp. Ser. 638, Am. Chem. Soc., Washington (D.C.), 1996, pp. 2-19. C.J. Kiely, A. Burrows, S. Sajip, G.J. Hutchings, M.-T. Sananes, A. Tuel, J.C. Volta, J. Catal., 162 (1996) 31. J.C. Volta, Catal. Today, 32 (1996) 29. Eur. Pat. EP942011177.6, ass. to Engelhard De Meern. R.A. Overbeek, PhD Thesis, Univ. Utrecht, 1996. P.L. Gai-Boyes, Catal.Rev.-Sci.Eng., 34 (1992) 1. S. Breiter, M. Estenfelder, H.-G. Lintz, A. Trenten, H. Hibst, Appl. Catal., 134 (1996), 81. L.T. Weng, B. Delmon, Appl. Catal. A, 1992, 81, 141. M.C. Abello, M.F. GSmez, L.E. Cadus, Actas, XV Simposio Iberoamericano de Cat~lisis, CSrdoba (Argentina), Sept. 16-20, 1996 (E.R. Herrero, O. Anunziata, C. Perez, eds.), Unica Ed., CSrdoba, 1996, pp. 233238. E. Hoeft, K.H. Kosslick, R. Fricke, H.-J. Hamann, J. Prakt.Chem./Chem. Ztg., 338 (1996) 1. P. Ratnasamy, R. Kumar, in "Zeolites: a Refined Tool for Designing Catalytic Sites" (L. Bonneviot, S. Kaliaguine, eds.), Elsevier, Amsterdam, 1995, pp. 367-376. A. Baiker, in " l l t h International Congress on Catalysis - 40th Anniversary" (J.W. Hightower, W.N. Delgass, E. Iglesia, A.T. Bell, eds), Elsevier, Amsterdam, 1996, pp 51-61. J.-M. Jehng, G. Deo, B.M. Weckhuysen, I.E. Wachs, J. Mol. Cat., 100 (1996) 41. T. Hayashi, M. Haruta, Shokubai, 37 (1995) 75. M. Haruta, Catal. Today, in press. E.A. Mamedov, Appl. Catal. 116 (1994) 49. R. Burch, E.M. Crabs, Appl. Catal. 100 (1993) 111. I.M. Dahl, K. Grande, K.-J. Jens, E. Rytter,/~. Slagtern, Appl. Catal., 77 (1991) 163. M. Huff, L.D. Schmidt, J. Catal. 149 (1994) 127. A.L. Tonkovich, R.W. Carr, R. Aris, Science, 262 (1993) 221. Y. Jiang, I.V. Yentekakis, C.G. Vayenas, Science, 264 (1994) 1563. W.F. HSlderich, in "New Frontiers in Catalysis" (L. Guczi, F. Solymosi, P. T~t~nyi, eds.), Elsevier, Amsterdam, 1993, p. 127-163.

59 34. T. Imai, R.R. Herber, G.J Thompson, D.J. Ward, AIChE National Meeting, New Orleans (LA), March 1988. 35. M.F.M. Zinkels, S.G. J~ir~is, P.G. Menon, in "Structured Reactors and Catalysts" (J.A. Moulijn, A. Cybulski, eds.), M. Dekker, New York, 1997.

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

61

Molecular approach to active sites on metallic oxides for partial oxidation reactions Jacques C. Vrdrine Institut de Recherches sur la Catalyse, CNRS, UPR 5401 associ6e /t l'Universit6 Claude Bernard, Lyon I, 2 avenue A. Einstein, F-69626 Villeurbanne, France

Abstract Through a variety of examples, it is shown that the catalytic oxidation reactions, which operate via a Mars and van Krevelen mechanism, imply ensembles of atoms containing variable valence cations i.e. Lewis acid and redox sites and Lewis bases such as 02% O H or PO 43- able to act in the different elementary steps. The structure and the size of these ensembles, defined as <~, determine their reactional specificity. The examples chosen show that the <<designs >~of such ensembles are strongly dependent on the nature of the material, on its morphology (structure sensitivity as for MOO3, vanadyl phosphates), and on its structure (e.g. vanadyl phosphates, iron phosphates, heteropolyoxometallates, titanosilicates). It is also possible to build such ensembles upon dispersion of an active oxide on a relatively inert support (e.g. MoOx/SiO2, VOx/TiO2, NbOx/SiO2, TiOx/SiO2, etc).

Keywords: Molecular design; Active sites; Partial oxidation; Metallic oxides 1. INTRODUCTION Since the original view in Catalysis of an active site as single atoms various concepts have been developed in particular that of structure sensitivity. In partial oxidation reactions the Mars and van Krevelen mechanism proposed in 1953 is usually occurring. It involves both a redox function of the solid catalyst surface and oxygen insertion into the reagent molecule from lattice oxygen ions. The reaction is rather demanding since it involves for the substrate H atom abstraction, O atom insertion and electron transfer. This occurs on given active sites which should then obligatorily be of a certain size from several atoms up to a complete crystalline face or even a given phase. In the late seventies, early eighties a concept of cristalline faces being active for a partial oxidation reaction has been proposed by J.C. Volta for MoO3 single crystals (1). Many other examples have been further published with the same approach and show clearly that for an oxidation reaction much more than one surface atom is necessary to describe the active site. A new concept has then been developed [2, 3] considering an ensemble of surface atoms insuring the complex reaction mechanism to take place at the catalyst surface. Such a

62 molecular approach concept considers several cations and oxygen ions as constituting the active site. It is known that oxygen species on an oxide surface may exhibit different properties being considered as monoatomic species with a more or less electrophilic or nucleophilic character (O', O 2, ...) or diatomic species (O2", peroxo, ...). The more or less electrophilic character of the oxygen species is important since it may favor the deprotonation of the hydrocarbon molecule (nucleophilic attack) or the allylic attack (weak polarisation of M = O bond) and the direct attack of a double bond or of an aromatic ring (electrophilic attack). Electrophilic oxygen species correspond to O', superoxo (M-O-O) or peroxo (02") while nucleophilic species are rather metal oxo (M = O). The electrophilic attack occurs in the region of high electron density of the reagent molecule as at the YI bond. In such reaction when lattice ions are concerned an anionic vacancy is created and should be replenished by anion transfer process via lattice oxygen anions. It was then suggested that metal oxide materials exhibiting extended defects as shear plane structures should be favorable for such reactions since they exhibit free valency space and thus could favor a redox mechanism to occur during the catalytic reaction. Such a transformation should obviously be reversible and should then correspond to very localized crystallographic modifications like in topotactic type transformations. This also explains why non perfectly crystallized surfaces are more active than well crystallized one. The presence of local defects like anionic vacancies or coordinatively unsaturated cations plays certainly a role which is difficult to characterize due to the lack of ordering necessary in mainly physical techniques to be detectable, in particular in the case of X ray diffraction technique. Majority of the catalysts correspond to metallic oxides with V or Mo as one of the key elements but also cations of variable oxidation states as Fe3+/Fe2+, Cr6+/Cr3+, Cu2+/Cu+, Sb3+/Sb5+, etc. Some metals (mainly Ag for ethylene epoxidation), noble metals (as Pt, Pd), zeolites (titanosilicalite TS-1 from ENI for phenol oxidation) and heteropolyoxometallates (e.g. H4PMOlIVO40for isobutene oxidation to methacrolein) may also be used. Several examples have been chosen in this presentation to illustrate how such a concept may be valid in oxidation reactions and how it can be determining for the choice of the metallic single or mixed oxide and/or for the choice of its preparation procedure. 2. GENERAL REACTIONS

FEATURES

OF OXIDATION CATALYSTS AND OXIDATION

2.1. Oxidation catalysts The oxidation catalysts are usually mixed oxides which operate according to the redox process suggested by Mars and van Krevelen. According to this mechanism the substrate is oxidized by the solid and not directly by molecular oxygen of the gaseous phase. The r61e of dioxygen is to regenerate or to maintain the oxidized state of the catalyst. The oxygen atom(s) introduced into the substrate (or giving H20 for oxidative dehydrogenation reactions) stems from the lattice. This mechanism involves the presence of two types of distinct active species: an active cationic species which oxidises the substrate and another species active for dioxygen reduction. An adequate structure of the material should also facilitate both electrons and oxygen species transfer.

63

2.2. Oxygen species The oxygen atom incorporated into the substrate stems from the lattice and is at -2 oxidation state. Its replacement by molecular oxygen necessitates electrons according to: 02 + 4e--) 202. . This process has its own kinetics related to the reactivity of the sites with oxygen, their concentration, the efficiency of electron transfer, the partial pressure of oxygen, etc.. Usually, it is much faster than the oxidation of the substrate i.e. it is generally admitted that the rate determining state is the substrate activation. The homolytic fragmentation of a C-H bond in the coordination sphere of the acceptor metal ion may occur via a transfer of the hydrogen to the oxygen ion at -2 oxidation state. This is a concerted action with homolytic breaking of metal - oxygen bond which transfers one electron to the metal. Without any hypothesis about the nature of the metal-oxygen bond one can write the reaction with formation of a 7t-alkyl complex (as usually admitted) or a 8-alkyl complex. Depending on the nature, oxidation state of the metal ion and its environment (coordination structure), the metal-oxygen bonds may be more or less polarized and therefore the oxygen ion may exhibit electrophilic or nucleophilic properties. One may distinguish three extreme cases: 8 + 88- 8 + a: M = O (nucleophilic) --~ 02. b: M = O --->Oand c: M = O (electrophilic) --+ Q I Each case corresponds to specific properties. Case a (nucleophilic character) will intervene in activation of a C-H bond in ot of the double bond or of an aromatic ring; Case b (weakly polarized) will favor concerted homolytic - type reaction as for allylic dehydrogenation of olefins. Case c (electrophilic character) allows one a direct attack of a double bond (oxidative breaking). Let's take some examples which will be considered for some of them in more details later m

CH3wCH:CH2 propene

+ 2(O2-)

~

CH3~CH--CH2

+ NH3 + 3 (O2-)

OHC--CH:CHz acrolein

+

H20 + 4e-

NC--CH:CHz + 3 H20 + 6eacrylonitrile

propene

O O

CH3--CH2-CH2--CH 3 + 7 (O2-)

isobutyric acid

4H20 + 14e-

O maleic anhydride

butane

CH3N ,O /CH--C + CH3 OH

+

02-

CH2%c--c ,O CH/

+

"OH

methacrylic acid

H20

+

2e-

64 It clearly appears that a single and isolated metallic ion site cannot take into account all the necessary transformations involved in the reaction since several steps as replenishing of oxygen anion vacancies, H atoms extraction and electrons transfer are concerned. For instance n-butane oxidation reaction to maleic anhydride necessitates 7 lattice oxide ions, 8 hydrogen atoms abstraction from the substrate, 3 oxygen atoms insertion and 14 electrons transfer! 3. STRUCTURE SENSITIVITY OF OXIDATION REACTIONS ON OXIDES Such a concept has been introduced by M. Boudart on metals. It has been introduced for oxides in the late seventies by J.C. Volta et al [ 1], or early 80'ies by J.M. Tatibouet and J.E. Germain [4], J. Haber et al [5], etc and it is widely accepted at present. For instance in the work by J.C. Volta et al, it was shown that single crystal type samples of MoO 3 exhibiting different relative amounts of the different faces (010) basal, (100) side and (101) and (101) apical, exhibit different activities and selectivities in the oxidation of propene to acrolein and COx [6]. The originality of the work was to synthesize crystals of various shapes by epitaxial growth via oxyhydrolysis of MoC15 inserted between the layers of graphite. Table 1 summarizes the main results obtained for propene, but-1-ene and isobutene oxidation on MoO3 crystals . It clearly appears that for propene oxidation the (100) side face is selective for acrolein formation and the (010) for total oxidation. At variance for isobutene oxidation the side face gives both methacrolein and COx while the basal plane has low activity for acetone formation. Such specificity depends on the hydrocarbon molecule. It may thus be proposed that the stereochemistry of the hydrocarbon molecule and that of the oxide face play a determining rSle. Table 1 Structure sensitivity of the different faces of MoO3 crystals in olefin oxidation at 380~ (from ref 6).

Reactant Olefin Propene But-l-ene Isobutene

Products Acrolein CO, CO2 Butadiene CO, CO2 Methacrolein acetone CO, CO2

Basal (010) 0.06 1

Relative Selectivity per face Side Apical (100) (101), (101) 2.3 0.7 0 0

3

9.3

2

1 0 0.06 0

0 0.6 0 1

0 0.1 0.06 0

A more precise analysis and characterization of the MoO3 crystallites shape have shown that in fact the better plane for propene oxidation to acrolein corresponds to the (lk0) plane as schematised in fig. 1 [7]. It is then suggested that the propene activation (H abstraction) into the rt-allyl intermediate occurs on the side (100) plane while the O atom insertion occurs on the (010) basal plane. As a matter of fact it is worth noting that due to the layered structure of MOO3, the lattice oxide ions are much more labile in the (0k0) plane than in the others. Recently such a structure sensitivity was also derived for the partial oxidation of methane to formaldehyde 18].

65

(loo)

[010]

Q

@o13

~

1oo]

",(120) ',,

, ,,

Fig. 1. Cross section view of otMoO3 (100) and (120) planes (projection of the lattice on the (001) plane) (from [7]). 4. VANADYL P Y R O P H O S P H A T E [9] Such a catalyst is well known for the oxidation of n-butane into maleic anhydride. The preparation necessitates the formation of VOPO4, 0.5H20 as a precursor synthesized in an aqueous or better in an organic medium and its activation in a flow of 1 to 2% butane in air at the reaction temperature (ca 380~ Here also, the preparation and the activation of the samples appeared to be particularly crucial in order to obtain a well performing catalyst. In all cases, whatever the catalysts being good or exceptionally good, the (VO)2P207 phase (V 4+ cations) as a main constituent was detected by X Ray Diffraction and by in situ Laser Raman spectroscopy [10] in addition to small amounts of some VOPO4 phases (V 5+ cations) as otii, 13, Y or 8. Moreover VOPO4 pure phases were observed to be active and selective although to a lesser extent. It turned out that the presence of some V 5+ cations on the V 4+ catalyst surface of (VO)2P207 was necessary although an excess was observed to be detrimental. Moreover the catalyst surface was observed to be richer in P than the bulk by a ratio of about 2. The (100) face of (VO)2P207 corresponding to edge sharing dimers of VO6 octahedra bonded to the following chain by PO4 tetrahedra was shown to be the active and selective face [ 11]. One has one oxygen of V = O bond pointing away from the surface and the second one pointing downward in the form of a dimer as schematized in fig. 2. In a recent paper Grasselli et al [12] have proposed a mechanism with activated 02. (peroxo) species on unsaturated V ion at the surface and have proposed that the active site is composed of an ensemble of four dimers isolated one from the other by excess of phosphate species as schematised in fig. 2. Note that if such an assumption is true it is hard to understand how such activated oxygen species may be stable at high temperature and to understand the moving bed technology recently developed by the Dupont researchers [13]. In the later case one can expect surface oxygen species to be consumed and thus the VS+/V4+ ratio value to decrease with the reaction time. However the concept of four dimers as an active site seems reasonable and coherent with the scheme proposed by Ziolkowski [14] and Bordes [15] and with the presence of excess P species on the surface.

66

,' ",

iiiiol 1..i l~176 ',," ,"~',, ',P..' ,", ', .'

6;,

",L'

.",

....

', ,,"

',,~,,' ,,';',, ",,Y .,~,, ,,K, ,,;,, 'X' ,~, %; .,';',,

..".."....'..",,....,..--7.... '.;:;..;-"..,'.:--)'--:~.;-

t::)ol.L'L.,ioi;')oi411.... i i oi ..

iit~

"

I

1oi

,,

,L,

0

o~11..o~

1o

I -- o~V.o~bo

0 s/p'~ 0 o

.---.

o

o..,,.o

0

0

Fig. 2. Schematic representation of the surface structure of one polytype of (VO)2P207. The arrows represent the possible pathways for facile exchange of surface bound oxygen, either monoatomic or diatomic, between the active sites. The <<site-isolation >> due to the diffusion barrier created by the pyrophosphate groups is clearly shown by these arrows (from [ 12]). 5. IRON PHOSPHATES AND HYDROXYPHOSPHATES [16-19] Such catalysts appeared to be potentially important catalysts for the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA). This reaction is important as a first step to form methyl methacrylate monomer used for plexiglass or altuglass formation by polymerisation. The industrial type catalyst contains iron hydroxyphosphate of uncertain nature and Cs or NH4 as additive and unfortunately necessitates a large amount of water in the feed (namely 10 to 12 mol. H20 per tool. of IBA) to remain stable with time on steam. This makes this process difficult to be develop ,ed industrially. Taking into account the ph ase diagramme Fe203-FeO-P205 it could be possible to select and study several phases wt~ch contain Fe 2 , Fe 3+ or both catio,,s able to insure the redox mechanism necessary for the reaction to take place as it was shown to proceed via a Mars and van Krevelen mechanism [20]. It was also shown that the active phase was a phosphate with iron ion at a two oxidation states +2 and +3, ctFe3(P207)2, composed of trimers of face sharing FeO6 octahedra (as schematized in fig. 3a) separated one from the other by PO4 tetrahedra. The active sites were shown to correspond to a group of two trimers facing each other on their respective layers. The r61e of redox couple and of hydroxylation is shown below:

H20 02 Fe23+Fe2+(P207)2 ~---~ Fe23+Fe 2+ (PO3OH)4 > Fe2+x3+Fel-x2+(PO3OH)4-x(PO4)x 400oC The way the octahedra are connected to each other is an important parameter since it has been shown that the other polymorphic form of Fe3(P207)2 namely J3Fe3(P207)2 was poorly active and selective. In this phase, the central iron atom is in fact occupying a FeO6 trigonal prism (fig. 3b). A study of many different iron hydroxyphosphates exhibiting clusters of face sharing FeO6 octahedra of different sizes has been carried out. It turned out that all catalysts were belonging after catalytic testing to the same solid solution of the type Fe3+4.~Fe2+3x(PO4)3(OH)3-

67

3xO3xwith 0_<x _<1. Depending upon their composition these phases contain clusters of different sizes ranging from dimers (Fe4(PO4)3(OH)3) to continuous chains (J3Fe2(PO4)O) of FeO6 octahedra. The results showed that all samples were active and selective for the reaction whatever the different sizes of the octahedra arrangements but the optimum activity and selectivity were observed for the phase Fe3(PO4)z(OH)2 called barbosalite and containing limited size clusters, namely trimers as schematized in fig. 3a. The added water plays a role as for the active phase of the industrial catalyst in stabilizing the hydroxylated catalysts and the redox couple is the same but implies O/OH instead of PO4/POaOH couple. a

b ;3o

,

Fig. 3. Arrangements of FeO6 octahedra in trimeric clusters isolated one from the other by PzO7 groups: (a) otFe3(P2Ov)z; (b) 13Fe3(P2OT)2; grey circles, FEZ+;black circles, Fe 3+. In a theoretical study using extended Hfickel molecular orbital calculation it has been shown that for iron octahedra assembled as dimers [21], trimers [22] or larger clusters there may occur fast electron exchange between Fe z+ and Fe 3+ cations. This occurs in the case of trimers but neither for dimers nor for trimers in 13Fe3(P207)2 (fig. 4). This fast electron exchange between iron cations which very probably favors the redox mechanism and thus the catalytic properties, should be limited to the closest neighbours to give the most performant catalysts [23 ]. Such clusters of FeO6 octahedra also exist in other inorganic compounds. For instance they exist in ilvaite (CaFe3+Fe2+2Si2OvOOH) where silicate layers replace phosphate anions and FeO6 octahedra form ribbons. It is interesting to note that such a material is active and selective for the reaction although to a lesser extent than the previous hydroxyphosphates [24]. The lower catalytic behaviour may be due to the presence of Ca 2+ cations in the structure and/or to the silicate counter anion whose basicity in the sense of Pearson is different from that of the phosphate anion and/or the infinite size ofFeO6 octahedra clusters. The main idea one has to keep in mind from this study is that inorganic clusters of iron octahedra with iron at two oxidation states are active for the reaction studied and that one has to consider the active sites as these clusters, preferentially as ensemble of two trimers although clusters of other sizes (dimers, tetramers, pentamers...) are also active and selective but to a lesser extent. The iron oxidation state is changing during the reaction in a similar way as the

68 V4+/V5+ redox couple on VPO catalysts (vide supra w This again shows that metallic oxides have to be considered with a dynamical view during the oxidation reaction.

X=2

X=3

X>3

Fig. 4. Electron exchange between iron cations in (FeO6)n octahedra clusters calculated by extended Huckel molecular orbital theory (from [21] and [22]. 6. SUPPORTED OXIDE CATALYSTS Supports are very often used in catalysis for several reasons, namely: i. Dispersion of the active phase in order to increase the surface to volume ratio since heterogeneous catalysis is occuring at the solid surface. ii. Heat transfer: this is a particularly important aspect in oxidation reactions because of the high exothermicity involved. This holds particularly true at industrial scale since the hot spot problem is very crucial and should be monitored with precision within a few degrees to avoid local overactivity and subsequently overheating with presumably irreversible phase transformation of the catalyst. iii. Attrition this aspect is of main importance for fluidized or solid transported beds. One very usually uses silica or carborandum as a binder or as a coating to limitate attrition or to facilitate heat transfer respectively. iv. Formation of new catalytic sites this point will be emphasised below in some examples. One will see that well dispersed species of limited size could be formed and exhibit peculiar catalytic properties. v. Modification of the active phase properties due to its chemical interaction with the support, including epitaxial induced modifications. In some cases the chemical effect of the support will be determining for catalytic properties. For example several oxides (V205, MOO3, RezO7, Cr/O3...) deposited on several oxide supports (SiO2, TiO2, AIzO3) were shown by I. Wachs et al [25] to exhibit very different catalytic properties in methanol oxidation reactions as summarized in table 2.

69

Table 2 Turn over number values in (S)"1 for methanol reaction at 230~ deposited on several supports (from ref. [25]). Oxide support Supported oxide V205 MoO3 CrO3 SiOz 2 39 160 A1203 20 2 Nb205 700 32 58 TiO2 1800 310 300 ZrO2 2300 92 1300 6.1. M o l y b d e n u m

on lwt % metallic oxide

Re207 20 12 1200 170

oxide s u p p o r t e d on silica

Different procedures may be used to prepare such samples as impregnation of silica with a molybdate salt or grafting molybdenum chloride or molybdenum based organo metallic compounds as Mo carbonyls on the hydroxyl groups of silica or at last solid-solid reaction between MoO3 and SiO2 at temperatures near or above 500~ A parameter important for the impregnation method is the pH of the molybdate solution. As a matter of fact the following equilibrium has been well established: MO70246" + 4H20 6-~ 7 M0042 + 8H +. The monomeric tetrahedral MoO42 species is favored at high pH and vice versa for the polymeric heptamolybdate anion. In a study of ammonium heptamolybdate impregnated silica [26], the molybdenum loading was varied up to about 20 wt%. The hydroxyl groups of the silica were observed by infrared and/or UV vis-NIR spectroscopies to decrease with Mo loading and to disappear at 7 wt% Mo loading while two UV bands were appearing at 245 and 340 nm. The former band may be assigned to tetrahedral MoO 24 monomeric species and the latter one to octahedral polymeric (polymolybdate) species. The former band was observed to increase in intensity proportionally to Mo loading at low Mo content and to saturate at roughly 3 wt% Mo. The latter band was observed to appear near 2 wt% Mo, to increase then proportionally to Mo content up to 7 wt% Mo and then to decrease slowly with Mo loading. The formation of the former species was described by: \ / /Si x I

2

-SiI

O

OH

+ MOO]- +2H +

--)

/. O

\ Mo "/ / \ /0 \0 / Si \

+ HzO

Catalytic properties were studied for two reactions namely isopropanol conversion and propene partial oxidation. The first reaction is a test reaction which allows to characterize acidic, basic or redox properties of a catalyst. One gets dehydration to propene or diisopropylether for acid catalyst, acetone for basic catalyst in absence of air and acetone and water for redox type catalyst in presence of air. The experimental results at 100~ clearly show that at low Mo loadings acidic features are favored while redox features are favored at higher loadings. This indicates that monomeric MOO]- species are acidic (presumably as in silicomolybdic acid) while polymeric species exhibit redox properties.

70 The second reaction studied is propene oxidation at 380~ Weak activity was observed for low Mo loading. At higher Mo loading, propanal was the major product while acrolein was also observed. At very high Mo loadings and for MoO3 one gets almost exclusively acrolein. Propanal is known to stem from propene by an electrophilic attack while acrolein corresponds rather to a nucleophilic attack. These results indicate that monomeric MoO~- species does not oxidize propene in our conditions while polymeric (polymolybdate) species exhibit redox properties, with O 2 species being rather electrophilic. Molybdenum oxide exhibits redox properties with 02. species being rather nucleophilic. One can thus realize how the size of the active site is important in oxidation catalysis. This is typical of structure sensitive reaction. 6.2. Vanadium oxide on TiO2 support

Such a catalyst is well known for several reactions, such as o-xylene oxidation to phthalic anhydride and selective catalytic reduction (SCR) of NO by ammonia. The anatase form of TiO2 appears to be better than the rutile form. Such catalysts with 1 and 8 wt% V2Oflanatase was prepared by Rh6ne-Poulenc (S ~ 10 m2 g-l) for an exercise of characterization by 25 different european laboratories. All results are assembled in one issue of Catalysis Today published in May 1994, vol. 20 n~ Surface vanadium species were observed to exist in three different forms: monomeric VO43 species, polymeric vanadate species and V205 crystallites [27], the relative amount of which depended on initial wt % V2OJanatase and on the subsequent selective dissolution treatment. The following conclusions could be drawn considering that a polyvanadate species occupies a ca circular zone of a diameter of 0.38 nm per V atom (i.e. 0.165 nm2) and an isolated monovanadate species 0.66 nm per V atom (i.e. 0.43 nm2): i. Monomeric VO43 species exhibit acidic character with OH groups (Br6nsted acidity) and result in propene formation for isopropanol conversion and in total oxidation for o-xylene oxidation. Maximum coverage equals 0.43 wt% V205. ii. Polyvanadate surface species exhibit redox properties, namely give rise to acetone for isopropanol conversion and phthalic anhydride for o-xylene oxidation. iii V205 crystallites exhibit low activity for o-xylene conversion and high selectivity in total oxidation. Here too the size of the VOx species is of primary importance for the catalytic behaviour. 6.3. Niobium oxide species on silica support [28]

Deposition of NbOx species of different sizes on a silica support has been performed starting from organo metallic complexes as Nb(~I3-C3Hs)4 for a monomer, [Nb(qs-CsHs)H-ta(~5, ~1C5i_h)]2 for a dimer and Nb(OC2Hs)5 for a monolayer. All species were characterized by EXAFS, XANES, FT-I~ Raman, XPS and ESR techniques. Catalytic performances for conversion of ethanol are summarized in table 3. The structure of the species was well characterized particularly the NbO, NbNb, NbSi bonds lengths and coordination numbers. The catalytic data show clearly that dehydrogenation occurs majoritarily on monomeric species and dehydration on monolayer (bidimensional) structure for ethene (intra molecular reaction) and at last on dimeric species for both intra and inter molecular dehydration reaction.

71 Table 3 Catalytic performances of Nb monomeric, dimeric or monolayered species NbOx on SiO2 support for dehydrogenation (---> AA) and dehydration (intra ---> E, inter ~ DE) of ethanol (from ref. 29). Initial rate mmol. min~ g(Nb) ~ Selectivity % Catalyst Total AA E + DE AA E DE 1.25 1.2 0.049 96.1 2.8 1.1 monomer* 0.18 0.004 0.176 dimer* 2.1 24.2 73.7 0.11 0.001 0.106 monolayer** 0.9 99.1 0.0 0.17 0.052 0.118 impregnated* 30.5 20.2 49.3 0.0026 0.0004 0.0022 14.8 46.9 38.3 Nb205 bulk** AA acetaldehyde, E Ethene, DE Diethylether Reaction: Ethanol 3.1 kPa *at 523 K **at 573 K 6.4. Various oxides deposited on different oxide supports The idea is here to determine how a support may modify the catalytic properties of the different oxide species (as inorganic clusters of different size). It has been described above how the size of the deposited oxide species (monomefic, polymeric, bulk-type) results in different catalytic properties. Vanadium oxide has been deposited on several supports as silica, alumina, titania but also zirconia, niobia, zirconium hydroxyphosphates, etc. Its reductibility was studied by reduction with hydrogen at 400~ It was observed that V on TiO2 and yA1203 was reduced rather fast and one reached a consumption such as an O/V atomic ratio near to 1 for TiO2 and 0.65 for A1203 [30]. At variance for silica the reduction was much slower and an O/V ratio of 0.57 was obtained. Such differences were interpreted by the authors as due to different species on the surface: mainly monomefic VO43 species for TiO2, dimefic species V2074"for y.AI203 [30] and V205 crystallites for silica [31 ]. The size of the polyvanadate species in solution corresponds to V3093, V40124"with tetrahedrally coordinated V at pH = 7 and 4.5 respectively and V100286 or VI0028H 5 with octahedrally coordinated V at pH = 2.5. Deposition of such polyvanadates of different sizes was performed carefully on yA1203 support and their initial structures despicted above were shown to remain stable even after calcination in flowing air at 500~ and to be only partly modified after catalytic reaction of oxidative dehydrogenation of propane in the 350 to 450~ temperature range. Moreover in such cases, the selectivity towards propene was observed to be the same at the same conversion level, indicating that in this size domain of polyvanadate species the catalytic selectivity was not changed, only the activity was observed to increase with vanadium loading [32]. In a study of VO 2§ and Cr 3§ cations exchanged or impregnated on the surface of zirconium hydrogenophosphates and compared to vanadyl pyrophosphate and chromium (III) phosphate, the best catalytic properties for ethane to ethylene oxidative dehydrogenation reaction at 500~ were observed when the material present either continuous chains of vanadyl or CrO6 octahedra as in (VO)EP207and et CrPO4 respectively or chains of limited size [33]. By comparison with bulk oxides exhibiting similar environment, it was suggested as for supported VOx and CrOx species on zirconium hydrogenophosphates that the catal~ic performances were related to the stronger basic character ofPO4 3" anions with respect to O anions.

72 7. VMGO CATALYSTS [34-42] Such a system was interesting to consider since it is basically composed of an acidic (V205) and a basic (MgO) oxides. One may then expect either to have well defined phases of given catalytic properties or to have such phases deposited on a basic support (MgO). Three phases are well known namely the orthovanadate Mg3V2Os, the pyrovanadate Mg2V207 and the metavanadate MgV206. The first phase exhibits isolated VO4 tetrahedra (separated by MgO6 octahedra), the second one has comer sharing VO4 tetrahedra and the third one has an octahedral structure. The pyrovanadate phase was found [36] to be the more selective phase at equal conversion with respect to the other two phases for oxidative dehydrogenation of propane at 500~ At variance the Mg3V208 phase (exhibiting isolated VO4 entities separated by octahedral MgO6 entities) was found by H. Kung et al [35] to be more efficient for propene formation in propane oxidative dehydrogenation than the other two phases. A. Pantazidis and C. Mirodatos [37] found recently for the same reaction that propane selectivity was about the same (65 to 75 %) within a large range in V205 content (10 to 80 V205 wt %) and at similar conversion levels. A. Corma et al [36] concluded that tetrahedral isolated VO4 species are more selective for oxidative dehydrogenation of Ca and nC4 alkanes. It then appears than contradictory results have been obtained resulting obviously in contradictory interpretations concerning the role of atom arrangements. The discrepancies between authors may arise from differences between samples, supposedly similar, and from different catalytic reaction conditions particularly the alkane to oxygen ratio values. Taking the samples prepared in our group we have tried to go deeper in the characterization of the catalysts. Reducibly and electrical conductivity measurements [39] have shown that the MgEV207 phase was more easily reduced than the other two while anionic vacancies were created according to: C3H8 q- O2"surf > C3H6 (g) + H20 + [-7 + 2e. Moreover, basicity of surface oxygen ions acts on the ability to create anionic vacancies and thus on the catalytic activity. Moreover if the sample contains more MgO than the above stoichiometries for pure phases one may imagine that its basicity may also play an additional r61e. As a matter of fact propene as a base will be more easily eliminated from the surface of a basic catalyst. The acid-base features of the catalysts were studied by the reaction of isopropanol conversion to propene (acidic feature) and acetone (basic feature) under N2 in the feed and the redox features by the reaction under air in the feed. It was observed at 230~ (table 4 from ref 40) that the pyrovanadate sample was much more basic than the other two pure phases and that excess MgO with respect to crystallized phase stoichiometry induced even more basic character (table 4). As the olefins and to a lesser extent the alkanes are basic one may expect the desorption to be favored by surface basic sites. In other words oxidative dehydrogenation of alkanes is expected to be easier on surface exhibiting basic properties. As a matter of fact the results given in table 5 from ref. 41 show that Mg2V207 which is more basic as shown in table 4 is more selective for olefins in propane conversion and to a lesser extent for n-butane and isobutane oxidation reactions than the other two phases. Such a feature is even more pronounced for the samples with excess MgO at least for propane oxidation, samples which were also shown to present higher basicity (table 4).

73 Table 4 Catalytic data for isopropanol conversion under nitrogen (A) or air (B) in the flow at 230~ alter activation of the sample at 230~ for lh30 under N2 flow, values taken (from ref 40). S t2g"1

Samples

V205 (wt%)

n

MgVzO6 Mg2V207 Mg3V208 MgO 40VMgO" 60VMgO" V205

82.1 70.1 6O.8 0 38 58.5 100

0.1 1.7 0.9 1 J,2 43 19 2.1

Conversion % 7.5 1.5

2.2 0.2 1.2 3.6 64

m= 100 mg; flow rate 18.6 cm 3. min"l 9 Pi= 2.0 x 103 Pa

A Propene %

B

Acetone Conversion Propene % % %

90 43 69 0 14 24 94

10 57 31 100 86 76 6

8

1.7 2.8 0.2 1.7 4.3 4.8

90 40 74 0 17 24 75

Acetone % 10 60 26 100 80 76 25

" mixture ofMgO, Mg3V208 and otMgzV207 as determined by XRD analysis.

Table 5 Catalytic data for oxidative dehydrogenation of propane (A) n-butane (B) and isobutane (C) at 540~ (from ref 41).

Samples MgV206 Mg2V207 Mg3V208 40VMgO 60VMgO

Conversion % 7.4 6.9 8.3 8 8

A Propene" % 15 53 6 65 65

B

COx' % 53 28 94 35 35

Conversion % 10 11

15 15c 42 c

C Dehydr. Conversion i butene % products b 33 14 19 40 9 40 13 10 20 20 23 33 6 13 30

flow rate = 50 cm 3min1, P(C3--)=2 x 103pa, C3 or C4: air = 2:98 "" balance to 100 % corresponds to oxygenates as ethanal, acrolein, propanal and acetic acid. b. dehydrogenates correspond to butenes (major) (but l ene and but 2 ene) and butadiene (minor). c taken at 450~ instead of 540~ because of its high conversion level due to its high surface area. It follows that one may conclude that basic properties as well as atomic arrangements at molecular level play an important r61e in alkane oxidative dehydrogenation reactions. Such a conclusion could also be reached from the study of VMgO catalysts [42] for oxidative dehydrogenation of several alkanes as ethane, propane and butane under similar conditions (see e.g. fig. 5 in ref. 42).

74 8 CONCLUSION Some general conclusions may be drawn from this general presentation: i. Oxidation reactions in gas-phase heterogeneous catalysis usually proceed via Mars and van Krevelen mechanism i.e. involve lattice oxygen ions. Such ions exhibit an electrophilic or a nucleophilic character and therefore present different catalytic properties. As a matter of fact the electrophilic oxygen has been suggested to interact with a double bond or an aromatic ring and the nucleophilic oxygen to interact with a C-H bond in ~ of the double bond or of an aromatic ring. ii. Oxidation reactions are structure sensitive and therefore greatly depend on the local and surface structure of the oxide catalysts. A peculiar fitting between stereochemistry of the solid surface and of the reactant molecule(s) is to be obtained to get the best catalyst. Parameters such as reducibility and reoxidability features of the oxides are very important for catalytic reactions. iii. Active sites for oxidation reactions appear to be molecular "inorganic ensembles" of metallic oxide atoms whose size greatly influences the catalytic properties [2, 3, 43]. In some examples the number of atoms constituting the active sites could be established. For instance double trimers of face sharing FeO6 octahedra are particularly active and selective for oxidative dehydrogenation of isobutyric acid to methacrylic acid on iron hydroxyphosphates; ensembles of four dimers of VO6 octahedra are suggested to be the active sites for butane oxidation to maleic anhydride in vanadium pyrophosphate catalysts. Usually monomeric species as MoO~or VO43" exhibit acidic features and then total oxidation properties. At variance low size polymeric species exhibit better selectivity for many partial oxidation reactions than large size species or bulk-type oxide. It is thus obvious that the size of these r inorganic molecular ensembles )) as active sites is important depending on the reaction considered and the support used. The characterization of such ~ clusters )~ laying on the surface is rather difficult and the above statements for supported oxide catalysts are only qualitative. To better characterize such clusters, techniques as XANES (sensitive to local symmetry and oxidation state of transition metal cations), EXAFS (sensitive to the coordination number and nearby elements distances) and radial electron distribution fRED) of X ray diffraction peaks (sensitive to the nearby elements distances) have to be used. Already data have been obtained for MoO3 and V205 deposited on SiO2, Al203 or TiO2 support and for NbOx species on SiO2 (28, 29). This holds true only if one has one type of cluster species to analyse. The reader interested may read a chapter devoted to XANES and EXAFS techniques by B. Moraweck in the book by B. Imelik et J.C. Vrdrine (1988 and 1994) [44, 45]. iv. Many other examples may have been given. For instance heteropolyoxometallates as H~Mo~VO40 material or zeolite materials as titanosilicalite constitute other materials where the isolated or more or less condensed active species results in very different catalytic behaviour. The case of the titanosilicalite from ENI Co is typical since isolated Ti species are active for phenol oxidation to catechol and anthraquinone while condensed Ti species results in H202 decomposition. v. Oxidation catalysts have to be considered with a dynamical view under reaction conditions. This is related to the Mars and van Krevelen mechanism which involves a redox mechanism and also to the mobility of the oxide lattice. This dynamical phenomenon results in the wetting effect observed under catalytic reaction conditions for multicomponent and supported oxide catalysts [46, 47]. It follows that for many catalysts a certain time on stream is necessary before the catalyst reaches its steady state. It is frequent that in an industrial plant a

75 steady state is reached only after one or two hundreds hours, the catalysts lasting several years before having to be replaced. For simple catalysts as doped vanadyl pyrophosphates used for butane oxidation to maleic anhydride the fight size of the active sites (e.g. tetramers of vanadyl dimers) is monitored by the reactants in catalytic reaction conditions leading to the fight VS+/V4+ ratio, by the preparation procedure to change the material morphology (the (100) face of (VO)2P207 being developed), and by the adequate addition of additive elements which regulate the site size and VS+/V4+ ion ratio. The view of an oxidation catalyst as dynamical under catalytic reaction conditions is essential for our understanding of its functioning. REFERENCES

[1] J.C. Volta, W. Desquesnes, B. Moraweck and G. Coudurier, React. Kinet. Catal. Lett.,12 (1979) 241; J.C. Volta and J.L. Portefaix, Appl. Catal., 18 (1985) 1. [2] J.C. V6drine, J.M.M. Millet and J.C. Volta, Catal. Today, 32 (1996) 115. [3] J.C. V6drine, G. Coudurier and J.M.M. Millet, Catal. Today, 33 (1997) 3. [4] J.M. Tatibouet and J.E. Germain, C.R. Acad. Sci., 290 (1980) 321; J. Catal., 72 (1981) 37. [5] J. Haber, in R.A. Sheldon and R.A. van Santen, Editors, Catalytic oxidation, Principles and Applications, World Scientific, Singapore, 1995, p. 17. [6] J.M. Tatibouet, J.E. Germain and J.C. Volta, J. Catal., 82 (1983) 240. [7] M. Abon, B. Mingot, J. Massardier and J.C. Volta in <<Structure-activity and selectivity relationships in heterogeneous catalysis )), R.K. Grasselli and A.W. Sleight (Ed.), Stud. Surf. Sci. and Catal., Elsevier, Amsterdam, 67 (1991) 67. [8] R.M. Smith and U.S. Ozkan, J. Catal., 141 (1993) 124. [9] G. Centi, Editor, Vanadyl Pyrophosphate Catalyst, Catal. Today, 16(1) (1993). [10] G. Koyano, T. Okuhara and M. Misono, Catal. Lett., 32 (1995) 205; G.J. Hutchings, A. Desmartin Chomel, R. Olier and J.C. Volta, Nature, 368 (1994) 41. [ 11] F. Trifiro and F. Cavani, Chem. Technol., April (1994) 18. [12] P.A. Agashar, L. de Caul and R.K. Grasselli, Catal. Lett., 23 (1994) 339. [ 13] G.S. Patience and P.L. Millo, Stud. Surf. Sci. Catal., 82 (1994) 1. [14] J. Ziolkowski, J. Catal., 100 (1986) 45. [ 15] E. Bordes, Stud. Surf. Sci. Catal., 67 (1991) 21. [16] J.M.M. Millet, J.C. V6drine and G. Hecquet in <
76 [25] I. Wachs, G. Deo, M.A. Vuurman, H. Hu, D.S. Kim and J.M. Jehng, J. Mol. Catal., 82 (1993) 443. [26] T.C. Liu, M. Forissier, G. Coudurier and J.C. V6drine, J. Chem. Soc., Faraday Trans. I, 85 (1989) 1607. [27] J.C. V6drine, Editor, Eurocat oxide, Catal. Today, 20(1) (1994). [28] Y. Iwasawa, Catal. Today 18 (1993) 21-72. [29] Y. Iwasawa in ~( l lth International congress on catalysis-40th anniversary )), J.W. Hightower et al (Ed), Stud. in Surf. Sci. and Catal., ser., Elsevier, Amsterdam, 101 (1996)21. [30] J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 102 (1986) 52. [31] J. Le Bars, J.C. V6drine, A. Auroux, S. Trautmann and M. Baems, Appl. Catal. A: Gen., 88 (1992) 179. [32] J.G. Eon, R. Olier and J.C. Volta, J. Catal., 145 (1994) 318. [33] M. Loukah, G. Coudurier, J.C. V6drine and M. Ziyad, Microporous Mater., 4 (1995) 345. [34] M.A. Chaar, D. Patel, M.C. Kung and H.H. Kung, J. Catal., 105 (1987) 483. [35] M.A. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463. [36] D. Siew Hew Sam, V. Soenen and J.C. Volta, J. Catal., 123 (1990)417. [37] A. Pantazidis and C. Mirodatos in ~(Heterogeneous hydrocarbon oxidation )), S.T. Oyama and B.K. Warren, Ed., ACS Sympos., ser., Washington, 638 (1996) 207. [38] A. Corma, J.M. Lopez Nieto, N. Paredes, A. Dejoz and I. Vazquez, in (( New Developments in Selective Oxidation II )), V. Cortes Corberan and S. Vic Bellon (Ed.), Stud. in Surf. Sci. and Catal. ser., Elsevier, Amsterdam 82 (1994) 113. [39] A. Guerrero Ruiz, I. Rodriguez Ramos, J.L.G. Freno, V. Soenen, J.M. Herrmann and J.C. Volta in ~(New Develpments in Selective Oxidation by Heterogeneous Catalysis ~), P. Ruiz and B. Delmon (Ed.), Stud. in Surf. Sci. and Catal. ser., Elsevier, Amsterdam, 72 (1992) 203. [40] A. Ouquour, Doctoral Thesis N ~ 105-91, University C. Bernard, Lyon, 1991. [41] V. Soenen Lebeau, Doctoral Thesis N ~ 173-91, University C. Bernard, Lyon, 1991. [42] P. Concepci6n, A. Galli, J.M. L6pez Nieto, A. Dejoz and M.I. Vazquez, Topics in Catalysis 3 (1996) 451. [43] J.C. V6drine in ~( Catalytic Oxidation, Principles and Applications )~, R.A. Sheldon and R.A. van Santen (Ed.) World Scientific, Singapore, 1995, pp. 53-78. [44] B. Moraweck in ~(Les Techniques Physiques d'Etude des Catalyseurs )~, B. Imelik et J.C. V6drine (Ed.), Technip, Paris, 1988, pp. 587-598. [45] B. Moraweck in ~ Catalyst Characterization, Physical Techniques for Solid Materials )), B. Imelik and J.C. V6drine (Ed.), Plenum, New York, 1994, pp. 377-416. [46] J.M.M. Millet, H. Ponceblanc, G. Coudurier, J.M. Herrmann and J.C. V6drine, J. Catal. 142(1993) 381. [47] H. Ponceblanc, J.M.M. Millet, G. Coudurier and J.C. V6drine in ~ Catalytic Selective Oxidation )), S.T. Oyama and J.W. Hightower (Ed.), ACS Sympos. ser., Washington, 523 (1993) 262.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

In situ e l e c t r o c h e m i c a l l y oxidation catalysts

controlled promotion

77

of c o m p l e t e

and p a r t i a l

Constantinos G. Vayenas and Symeon I. Bebelis Department of Chemical Engineering University of Patras GR-26500 Patras, Greece

Oxidation catalysis on metal catalysts can be affected significantly by electrochemical means. The catalytic activity and selectivity of metal films interfaced with solid electrolytes can be varied in situ in a very pronounced and reversible manner via electrical potential application (typically +1-2 V) between the catalyst film and a counter electrode also in contact with the electrolyte. The effect is particularly pronounced for complete and partial oxidation reactions. Catalytic rates can be varied by up to a factor of 200. The induced steady-state change in catalytic rate is up to five orders of magnitude larger than the steady-state rate of electrochemically supplied ionic species from the electrolyte onto the catalyst surface. This novel effect of Electrochemical Promotion or non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) has been studied in over 45 catalytic reactions on Pt, Rh, Pd, Ag, Ni and IrO 2 catalyst-electrodes using a variety of solid electrolytes (O 2-, F-, Na § H § conductors) and more recently mixed ionic-electronic conductors and aqueous alkaline solutions. In addition to possible technological applications, this new effect allows for a systematic study of the role of promoters in heterogeneous catalysis. In this paper the main features of electrochemical promotion are summarized with emphasis on complete and partial oxidation reactions and the origin of the effect is discussed on the basis of work function measurements, recent surface spectroscopic investigations and ab initio quantum mechanical studies. 1. INFRODUCTION Controlled modification of the activity and selectivity of complete and partial oxidation reactions on metals is a long-sought goal in oxidation catalysis, [ 1]. Metal alloying [2], metalsupport interactions [3] and the use of promoters [4] have been investigated extensively for this purpose. The use of electrochemistry for this purpose, i.e. to activate and precisely tune heterogeneous catalytic processes is a new development [5-15] of considerable theoretical and practical interest [ 16-18]. The electrochemically induced catalytic rate enhancement can be several orders of magnitude higher than that anticipated from Faraday's Law [5-15]. Furthermore the product selectivity of catalytic processes can be affected in a pronounced and reversible manner [9,10,14,15]. Most studies in this area have focused on oxidation reactions using solid electrolyte cells [5-15]. Recent work, however, has shown that similar phenomena occur in aqueous electrochemistry [ 19]. The experimental setup is quite simple:

78 The gaseous reactants (e.g. C2H4 plus 02) are cofed over the working electrode of a solid electrolyte cell: Catalyst working ] solid electrolyte [ counter auxiliary gaseous reactants electrode [(e.g. ZrO2-Y203) electrode gas (e.g.C2H4+O2) (e.g. Pt,Rh,Ag,IrO2) (e.g. Au) (e.g. 02) The working electrode serves simultaneously as an electrode and as the catalyst for the catalytic reaction under study, e.g., C2H 4 or H 2 oxidation. The auxiliary gas can be the reactive gas mixture itself in the, so called, "single-pellet" design [9,10]. Upon varying the potential of the working electrode/catalyst it is found that not only the electrocatalytic (net charge-transfer) reaction rate is affected, as anticipated from Faraday's Law, but also the catalytic (no net-charge transfer) reaction rate changes in a very pronounced, controlled and reversible manner. The increase in the catalytic rate can be up to a factor of 100 higher than the open-circuit catalytic rate and up to 3x105 times larger than the change in the electrocatalytic rate, e.g., each 02- supplied to the catalyst electrode can cause the catalytic reaction of up to 3x105 chemisorbed oxygen atoms [9,14]. This novel effect has been termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect [5-15])or electrochemical promotion [16] or in situ controlled promotion [20]. Its importance in catalysis and electrochemistry has been discussed by Haber [ 18], Pritchard [16] and Bockris [17], respectively. In addition to the group which first reported this new phenomenon [5-7], the groups of Lambert [12], Hailer [10], Sobyanin [8], Comninellis [ 13], Pacchioni [21] and Stoukides [11] have also made important contributions in this area, which has been reviewed recently [14,15]. In this review the main phenomenological features of NEMCA for oxidation reactions are briefly surveyed and the origin of the effect is discussed in the light of recent kinetic, surface spectroscopic and quantum mechanical investigations. 2. EXPERIMENTAL The experimental setup for kinetic electrochemical promotion studies is shown schematically in Fig. 1a. The electrically conductive working catalyst electrode, usually in the form of a porous film 3-20 gm in thickness with a roughness factor 3 to 500 [9,14,15] is deposited on the surface of a ceramic solid electrolyte (e.g. Y203-stabilized-ZrO 2 (YSZ), an O 2- conductor, Na-13"-AI203, a Na § conductor, CaZr0.9In0.103-a, a H § conductor [22], or TiO 2, a mixed electronic-ionic conductor [23]). Catalyst, counter and reference electrode preparation and characterization details have been presented in detail elsewhere [9,14] together with the gas analysis system for on-line monitoring of the rates of catalytic reactions via gas chromatography, mass spectrometry and infrared spectroscopy. The superficial surface area of the metal working electrode-catalyst is typically 1-2 cm 2 as measured via reactive titration of oxygen with CO or C2H4 [9,14] or via reactive titration of CO with O2 [9,14]. The catalyst-electrode is exposed to the reactive gas mixture (e.g. C2H4+O2) in a continuous flow gradientless reactor (CSTR). The counter and reference electrodes are usually exposed to air when using the "fuel cell" type design and to the reactive gas mixture itself when using the "single-pellet" type design. In the latter case the counter and

79 reference electrodes must be catalytically inert (e.g. Au) while the reference electrode is only a monitoring (pseudoreference) electrode [24]. A galvanostat or potentiostat is used to apply constant currents between the catalyst and counter electrode or constant potentials, VWR, between the catalyst-working electrode (W) and the reference (R) electrode. In this way ions (02- in the case of YSZ, Na + in the case of Na-I3"-AI20 3, H + in the case of CaZr0.9In0.103_ ~) are supplied from (or to) the solid electrolyte to (or from) the catalyst electrode surface. The current, I, is defined positive when anions are supplied to or cations removed from the catalyst surface. As shown under Results and Discussion there is concrete evidence obtained from several techniques, including work function measurements, cyclic voltammetry, temperature programmed desorption (TPD), Xray photoelectron-spectroscopy (XPS) and scanning tunneling microscopy (STM) that these ions (accompanied by their compensating (screening) charge in the metal thus forming surface dipoles) migrate (back-spillover in catalytic terminology) onto the gas-exposed, i.e. catalytically active, catalyst electrode surface. Consequently the electrolyte acts as an electrically activated catalyst support and establishes an "effective electrochemical double layer" on the gas-exposed, i.e. catalytically active, electrode surface. The experimental setups and procedures for using XPS [25] (Fig. l b), TPD [26] and cyclic voltammetry [27] under ultra-high-vacuum (uhv) conditions and work function measurements [7,27], cyclic voltammetry [26,28] and STM [29] under atmospheric pressure conditions to investigate the origin of electrochemical promotion, have been described in detail recently [25-29]. X-Ray Source

/

~=,o

cou

II

ER

Phoroel ec trQn Energy Analyzer

I

CATALYST

I

??

R~FE~RENCE

~~-~7 --J

v~" cE Q

,

Figure 1. Schematic of the experimental setup for electrochemical promotion studies using the fuel-cell type design (a) and for using x-ray photoelectron spectroscopy (XPS) (b) to investigate the catalyst-electrode surface; G-P: Galvanostat-Potentiostat; WE: Working electrode, RE: Reference electrode, CE: Counter Electrode (adapted from refs. [6], [25]). 3. RESULTS AND DISCUSSION 3.1. Catalytic oxidation rate modification A typical transient NEMCA experiment, carried out in the setup depicted in Fig. la, is

80 shown in Fig. 2. The solid electrolyte is YSZ. The catalytic reaction is the complete oxidation of C2H 4 on Pt [6]. The Pt catalyst film has a gas exposed surface area corresponding to N=4.2x10 .9 mol Pt and is exposed to Po2--4.6 kPa, PC2H4=0.36 kPa, T=370~

in the

gradientless continuous flow CSTR reactor of Fig. la. Initially (t<0) the electrical circuit is open (I=0) and the open-circuit catalytic rate, r o, is 1.5xl 0 .8 mol O/s. The corresponding turnover frequency (TOF), i.e., oxygen atoms reacting with C2H4 per surface Pt site per s is 3.57 s-1. At t=0 a galvanostat is used to apply a constant current between the catalyst and the counter electrode. In this way oxygen ions, O 2-, are supplied to the catalyst-gas-solid electrolyte three-phase boundaries (tpb) at a rate I/2F=5.2xl 0 -12 mol O/s. The catalytic rate starts increasing (Fig. 2) and within 25 min gradually reaches a value r=4.0-10 -7 mol O/s which is 26 times larger than r o. The new TOF is 95.2 s -1. The increase in catalytic rate Ar=rro=3.85xl 0 -7 mol O/s is 74,000 times larger than I/2F, which is the maximum rate increase anticipated from Faraday's Law. Thus each 0 2- ion supplied to the Pt catalyst causes at steady-state 74,000 oxygen atoms chemisorbed on the Pt surface to react with C2H4 and form CO 2 and H20. For this reason this novel effect has been termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect).

50 t i = 0

30--

0!

~-'-"I = + l ~ t A

40-

C)

20--

I=O ;t... 800 r 0 = 1.5xlO s mol O / s 'i Ar = 38.5x10 -s mol O / s I / 2 F = 5.2x10 -12 mol O / s

~ o~, =26 0

-~

30

---" ~

I ~-

_...

74000 -

-4oo~

Ji

~2

J i

10--

-o

lO-

0 "-I

I

0

I 0

"--T-20 Time, min

40

I:, 110

1 130

Figure 2. Electrochemical promotion: Catalytic rate and catalyst potential response to step changes in applied current during C2H 4 oxidation on Pt [6]. T=370~ Po2=4.6 kPa, PC2H4=0.36 kPa. The experimental (x) and computed (2FN/I) rate relaxation time constants are shown on the figure. The steady-state rate increase Ar is 74,000 times higher than the steady-state rate of supply of 0 2- to the catalyst-electrode (A=74,000); (adapted from ref. [6]).

81 The Faradaic efficiency, or enhancement factor, A, is defined [6,9] from: A =Ar/(I/2F)

(1)

In the experiment of Fig. 2 the maximum A value is 74,000. A reaction exhibits the NEMCA effect when IAI > 1. Depending on the observed sign of A, catalytic reactions are termed electrophobic (A> 1) or electrophilic (A<-I); A values ranging from 3xl 05 [6,9,14,15] and down to -5•

[9,14,15] have been measured. Relatively safe predictions about the

order of magnitude of A can be made as discussed below. A second important parameter is the rate enhancement ratio, p, defined [6,9] from p =r/ro

(2)

In the experiment of Fig. 2, the maximum p value is 26; p values up to 100 [30] or even higher [ 12] and down to zero [31-33] have been obtained. The NEMCA rate relaxation time constant, x, is defined [9,14] as the time required for the catalytic rate increase to reach 63% of its final steady-state value in galvanostatic transient experiments, such as the one depicted in Fig. 2. As shown in this Figure, "c is of the order of 2FN/I. This is a general observation in electrochemical promotion studies utilizing YSZ: x = 2FN/I

(3)

The parameter 2FN/I equals the time required to form a monolayer of O on a surface with N adsorption sites, when O is supplied as 02. at a rate I/2F as is the case here. This provided the first, kinetic, evidence that NEMCA is due to the electrochemically controlled migration (backspillover) of an oxidic species from the solid electrolyte onto the gas-exposed catalytically active catalyst surface [6,9]. As shown below via XPS, TPD and cyclic voltammetry this electrochemically supplied oxygen forms a new strongly bonded ionic adsorption state on the Pt surface and is far less reactive than normally chemisorbed oxygen formed via gas phase adsorption. It severely modifies (increases) the catalyst surface work function [7,27], forces normally chemisorbed oxygen into a weakly bonded adsorption state [26] and acts as a sacrificial promoter by reacting with C2H4 (or other oxidizable molecules) at a rate which is A times slower than the weakly bonded atomic oxygen [26,30]. Figure 2 also shows this point: At steady-state the rate, rc, of consumption of the promoting 0 2- species via reaction with C2H4, has to equal its rate of formation I/2F. Consequently, since A=Ar/(I/2F) and Ar=r, it follows A=r/rc=TOF/TOF c where TOF is the turnover frequency of the catalytic reaction in the NEMCA-promoted state and TOF c is the turnover frequency of the reaction of the promoting oxygen species with ethylene. It thus follows for the experiment of Fig. 2 that TOFc=TOF/A= 1.3• 10 -3 s. This implies that that average lifetime of the promoting species on the catalyst surface is TOFcl=770 s in excellent qualitative agreement with the catalytic rate relaxation time constant upon current interruption (Fig. 2). This observation provides strong support for the oxygen backspillover mechanism of electrochemical promotion.

82

Q

100 -

A

9 9

"

" 250

T

ate

80

200

o

T=370~

," 60 ,~ c "" 40 "

D

Regular Open Circuit Rate

20

150

er ~ r.r.,

100

~ ol> g: ~" ~

PC2H4=0"65kPa

00_ ~..,~.-c~c_,5 ' r I0~ po 2/Pc2H4

n

15' o,

50

2u~0

Figure 3. Effect of gaseous composition on the open-circuit (unpromoted) catalytic rate of C2H4 oxidation on Pt/YSZ and on the electrochemically promoted catalytic rate when the Pt catalyst film is maintained at VWR = 1V (adapted from ref. [6]).

Figure 3 shows the steady-state effect of constant positive potential application VWR=(=+ 1V) on the rate of C2H4 oxidation on P t ~ S Z as a function of the PO2fPC2H4 ratio. The promotion is much more pronounced under oxidizing gaseous compositions, where a sixty-fold enhancement in catalytic rate and turnover frequency is obtained. The stronger electrochemical enhancement under oxidizing conditions is a general observation in NEMCA studies of oxidation reactions [ 14,15] and is due to the fact that the promoting ionic oxygen species forms, upon 02- supply, only after the coverage of normally adsorbed oxygen is near completion [25,26]. An example of using Na-lY'-AI203, a Na § conductor, as the solid electrolyte to induce NEMCA is shown in Fig. 4. The figure depicts the effect of catalyst potential VWR,

,v,

2O

s

Figure 4. Electrochemical promotion of Pt for CO

t.....

"

"

oxidation using Na-13"-AleO3: Effect of P c o , catalyst potential and corresponding linearized sodium coverage on the rate of CO oxidation at

-0.4

~. "'Q'

---.v.. ~ o.oz -"'i 0

5

T = 3 5 0 ~ and Po2=6 kPa (reprinted with permission from ref. [20]).

83 corresponding coulometrically measured [20,32] Na coverage ONa, and Pco on the rate of CO oxidation on Pt/Na-[Y'-A1203 at 350~ [20]. When the Pt surface is Na-free, the rate goes through a sharp maximum with respect to Pco, indicative of competitive adsorption of CO and O (Langmuir-Hinshelwood type kinetics). For high Pco values the surface coverage of O is very low and thus the catalytic rate is also low. Increasing Na coverage (via negative current or potential application) under these conditions causes a 6-fold enhancement in the rate of CO oxidation due to Na-assisted enhanced O chemisorption. The promotion index of Na, PNa, defined from: PNa=(Ar/ro)/AONa

(4)

takes values up to 200 under these conditions. Higher Na coverages (Fig. 4) poison the rate due to the formation of surface Na-CO complexes [20].

3.2. Selectivity modification: Ethylene epoxidation Electrochemical promotion can be used to modify significantly the product selectivity, of catalytic oxidation reactions. An example is presented in Fig. 5 which shows the effect of catalyst potential and corresponding work function change on the selectivity to ethylene oxide (Fig. 5a) and acetaldehyde (Fig. 5b) of ethylene oxidation on A g ~ S Z at various levels of gas phase chlorinated hydrocarbon moderators [31] (The third, undesirable, product is CO2). As shown in the Figure a 500 mV decrease in catalyst potential causes the Ag surface to change from selective (up to 70%) ethylene oxide production to selective (up~to 55%) acetaldehyde production. The same study [31 ] has shown that the total rate of ethylene oxidation varies by a factor of 200 upon varying the catalyst potential. (Fig. 6)

80

A(e@), eV

-0.4

,

,

-0.z

,

',-

0.0

,

,

0.z

,-

A(e@), eV 8o,-q.4

,

-q.~.

o.o

0,.2

l

8.5% 0 2, 7.8%C , H 4 o O'x T=270~ P=500~cPa 50 ~ ~ k~ ~ 0.0 ppm C2t-I,C12 [~ ~, 9 0.4ppm L ~ \ [] 0.8 ppm 40j" / % 9 1.2 ppm I ~ x~lm 1.6 ppm

5O 9 r~

30

~ ~ , . / ~

I0

/~

o600

~-4oo

T__'52~/0Ooc~8_~

iiii m

a

(b)

z0

A 1.2 ppm [] 1.6 ppm @ 2.0 ppm

-~oo

V~ , mV

6

-~

",s

u

-

-z~o, mY

'

a

Figure 5. Effect of catalyst potential and work function and of the gas-phase addition of various levels of 1,2-C2H4C12 on the selectivity to ethylene oxide (a) and acetaldehyde (b) of ethylene oxidation on Ag supported on YSZ (reprinted with permission from ref. [31]).

84

-0.4

-0.2

0.0

0.2

[

10 1

m

0

M

o G

1-

O

I

7

T=260~C, P = 5 O O k P a

//

a.5~

l~

0.1

~/-/ ~w/" ,'~

-700

o~, 9.8~

9 CO,_

-500

~

I

t

-300 V~ , rnV

I

C,_H,

* CJI40 [] C H a C H O

I

-0.1

I

-i00

T

0.01 !00

Figure 6. Effect of Ag/TSZ catalyst potential and work function on the rates of formation of ethylene oxide (A), acetaldehyde (~) and CO2 (O); T = 260 ~ ; P=500 kPa, Po2 = 17.5 kPa; PCzH4=49 kPa.(reprinted with permission from ref. [31])

Figure 7 refers to ethylene epoxidation on a Ag film deposited on [Y'-A1203 [24] and shows the effect of catalyst potential, VWR, and partial pressure of gas phase chlorinated hydrocarbon moderator on the selectivity to ethylene oxide. For VWR--0 and VWR=-0.4 V the Na coverage on Ag is nil and 0.04, respectively. As shown in the Figure, there is an optimal combination of VWR(0Na) and PCzH4CI 2 leading to a selectivity of ethylene oxide of 88%. This is one of the highest selectivity values reported for the epoxidation of ethylene. Figure 7 provides an example of how in situ controlled promotion can be used for a systematic investigation of the role of promoters in technologically important partial oxidation reactions. Sm~x=88~

90ES

r~ 7S

70

t0 ~s J'(""~ 1"J~~~ PP'~ G ~ ~.a ~ . . r "

~c/2

Figure 7. Effect of catalyst potential and gas-phase 1,2C2H4C12 partial pressure on _ / ~ , ~ 3 the selectivity of ethylene / ' ~ epoxidation on Ag/]Y'-A1203. ~ ~ ~ (reprinted with permission ~:~ ~t ,~,~' from ref. [24])

85 3.3. Aqueous electrolyte NEMCA: An example of NEMCA in an aqueous alkaline solution is shown in Fig. 8. The catalytic reaction is the oxidation of H 2 by gaseous 0 2 on finely dispersed Pt supported on graphite [19]. Similar results have been obtained with Pt black supported on a Teflon frit [34]. The figure shows the effect of positive and then negative current application on the rates of consumption of hydrogen (rH2) and oxygen (ro=2ro2) and on the catalyst-electrode potential. It can be shown easily that at steady-state rH2-ro=I/2F [19,34], as also confirmed by experiment (Fig. 8). The induced increase in the catalytic rate is clearly non-Faradaic with A values up to 7.2 and 9 values up to 3.5. Similarly to the case of solid electrolyte induced NEMCA, a detailed kinetic study [34] has shown that increasing catalyst potential weakens the Pt=O chemisorptive bond and weakens the Pt-H chemisorptive bond. This is due to the electron donor and electron acceptor character of chemisorbed hydrogen and oxygen, respectively. !t

Figure 8. Electrochemical

tO

t ~

-

-

~

~

t

promotion of H2 oxidation on Pt in contact with 0.1 M KOH solution ] [19]. Transient effect of .8 applied positive and negative current on the ~ rates of consumption of 4 _~ H2 (rH2) and ~ ( r o = 2 r 0 2 ) and on Pt

!

9 8 7

,, . . . . 4

"

' 6

/--'4,, / - - '~ ~ ' ~ - - - - - - . 4

,'

t,~ ~

~

":~"23y/I(2F ) ~.[ 0 -t L !

I

" "~"'-

t.2

.... t / . ' ~ O " %

1

~ - .....

[r t t , l i ~ , ,. , I . : , ~ : i't~ . . . .

0 ~ I 2 5 7 9 11 1.3 15 25 2FN/7 t, rain

electrode;Vs.Catalyst_electrodea reversiblepH2 =0.75potentialkpa,Ha

_ rt'0.4

20

P~176 total gas flowrate 280 cm3min -1 at STP(adapted from ref. I19]).

3.4. Electrochemical kinetics and the magnitude of A: Table 1 provides a list of the catalytic oxidation reactions studied so far from the view point of electrochemical promotion and of the measured A, P and Pi values. As shown in this table measured IAI values range from 1 to 3x10 $. It has been shown both theoretically [9] and experimentally [9,14] that the order of magnitude of the absolute value IAI of the Faradaic efficiency A can be estimated for any reaction, catalyst and solid electrolyte from the following approximate expression:

= ro/(I0/2F)

(5)

where r o is the open-circuit catalytic rate and I0 is the exchange current of the catalyst-solid electrolyte interface. The latter can be obtained from standard current-overpotential (Tafel)

86 plots [9,14]. Equation (5) indicates that in order to obtain a non-Faradaic rate enhancement (IAI>I) it is necessary for the intrinsic catalytic rate, r o, be higher than the intrinsic electrocatalytic rate I0/2F.

Table 1 Catalytic oxidation reactions investigated in electrochemical promotion studies (Adapted from Table 3 in ref. [ 14] which provides specific references to each reaction) I. Electrophobic reactions ~/~e~)>0; ~r/bVwR>0; ~r/~I>0; A>0 Reactants

Products

Cata- Electrolyte lyst (Promoting ion)

C2H4, 02 C2H4, 02 C2H4, 02 C2H4, O2 C2H6, 02 C3H6, O2 CH4, O2 CH4, O2 CH4, 02 CO, 02 CO, 02 CO, 02 CH3OH, 02 C2H4, 02 CO,O2 H2, 02 H2, 02 CO, 02 C2H4, 02

CO2 Pt CO2 Rh CO2 IrO2 C2H40, CO2 Ag CO2 Pt C3H60, CO2 Ag CO2 Pt CO2 Pt CO2,C2H4,C2H6 Ag CO2 Pt CO2 Pd CO2 Ag H2CO, CO2 Pt CO2 Pt CO2 Pt H20 Pt H20 Pt CO2 Pt CO2 Pt

YSZ (O2-) YSZ (O2-) YSZ (Oz-) YSZ (O2-) YSZ (02-) YSZ (O2-) YSZ (O2-) YSZ (O2-) YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) ~"-A1203(Na+) 13"-A1203(Na+) Nafion (H§ KOH-H20 (OH-) CaF2(F-) T102(T1Ox,O-) 9

9

+

2

T(oC)

A

P

Pi

260-450 250-400 350-400 320-470 270-500 320-420 600-750 590 650-850 300-550 400-550 350-450 300-500 180-300 300-450 25 25-50 500-700 450-600

3.105 5.104 200 300 300 300 5 50 5 2.103 103 20 104 5.104 105 20 20 200 5.103

55 90 6 30* 20 2* 70 3 30* 3 2 5 4* 0.25 0.3 6 6 2.5 20

55 90 5 30 20 1 7 3 30 2 1 4 3 -30 -30 5 5 1.5 20

-100 -3-103 -500 -60 -104 -3 -1.2 -105

7 6 6 3 15" 3 8* 8

250

II. Electrophilic reactions ~9r/3(e~)<0; ~r/~VwR<0; ~/~)I<0; A<0 C2H6, 02 C3H6, 02 CO, 02 CO, 02 CH3OH, 02 CH4, 02 CH4, 02 CO, 02

CO2 Pt CO2 Pt CO2 Pt CO2 Au H2CO, CO2 Pt CO2 Au C2Hn,C2H6,CO2 Ag CO2 Pt

YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) YSZ (02-) 13"-A1203(Na+)

* Promotion-induced change in product selectivity

270-500 350-480 300-550 450-600 300-550 700-750 700-750 300-450

87 3.5. Work function modification An important step in the understanding of the origin of NEMCA was the realization that solid electrolyte cells with metal electrodes are both work function probes and work function controllers for the gas-exposed surfaces of their electrodes [7,27]. Both theory [9,14] and experiments via the Kelvin probe (vibrating capacitor) technique [7,27] and more recently via UPS [35] have shown that: eV~

= e~-e~R

(6)

eAVwR =A(e(I~v)

(7)

and

where eC~w is the catalyst surface work function and e(I)R is the work function of the reference electrode surface. Equation (6) provides an interesting additional physical meaning to the EMF of solid electrolyte cells, in addition to its well-known Nernstian meaning [9,14]. Equation (7) is equally important as it shows that the work function of the gas-exposed, i.e., catalytically active, surface of solid electrolyte cell electrodes can be varied and controlled via current or potential application. Positive currents increase e~ and negative currents decrease it. Physically this variation is brought about at the molecular level by the spillover of ions from or to the electrolyte to or from the gas-exposed catalyst surface [ 14]. 3.6. XPS studies The use of X-ray photoelectron spectroscopy (XPS) has provided conclusive evidence that spillover/backspillover phenomena are real and that electr, ochemically controlled backspillover of oxide ions, O ~, is the origin of electrochemical promotion. The first XPS investigation of Ag electrodes on YSZ under electrochemical 02- pumping conditions was published in 1983 [36] and provided strong evidence for the creation of backspillover oxide ions on Ag (O l s at 529.2 eV) upon positive current application. These results were confirmed by G6pel and coworkers who used XPS, UPS and EELS to study Ag/YSZ catalyst surfaces under electrochemical bias conditions [35]. A similar detailed XPS study of Pt films interfaced with YSZ [25] has shown conclusively that: I. Backspillover oxide ions (Ols at 528.8 eV) are generated on the gas-exposed Pt electrode surface upon positive current application (peak ~5in Fig. 9 top). II. Normally chemisorbed atomic oxygen (Ols at 530. 2 eV) also forms upon positive current application (peak 7 in Fig. 9 top). The maximum coverages of the ], and ~5states are comparable and of the order of 0.5 each. III. Oxidic backspillover oxygen (5-state) is significantly less reactive with the reducing (H 2 and CO) ultra high vacuum background than normally chemisorbed atomic oxygen. These observations provide a direct explanation of electrochemical promotion when using O2--conducting solid electrolytes [25]. The use of XPS has also confirmed recently that electrochemically controlled Na backspillover is the origin of electrochemical promotion when using Na+-conducting solid electrolytes such as ~"-A1203 [12,37,38].

88

534

Eb , eV 530 528

532

I

I

1

526

t

653K 0.1 kPa

-

1

--

~-5

800

_ oo

,/

.-/0 -/5

52/,

I

-

6

!

-7OO

t

I

i

i

~

"400

!

0

~R,

mV

;o

4O lO0 2OO SO0 4O0 600

8OO I ,

400

Figure 9. XPS (top) and linear potential sweep voltammographic (bottom) investigation of oxygen adsorbed on Pt films deposited on YSZ following positive overpotential application. Top: O 1s photoelectron spectrum of oxygen adsorbed on a Pt electrode supported on YSZ under UHV conditions after applying a constant overpotential AVwR=1.2 V, corresponding to a steady-state current I=40~tA for 15 min at 673 K (ref. [25]). The same O ls spectrum is maintained after turning off the potentiostat and rapidly cooling to 400K (ref. [25]). The ),-state is normally chemisorbed atomic oxygen (Eb=530.2 eV) and the 8state is backspillover oxidic oxygen (Eb=528.8 eV). Bottom: Linear potential sweep voltammogram obtained at T=653 K and Po2=0.1 kPa on a Pt electrode supported on YSZ showing the effect of holding time t H at VWR=300 mV on the reduction of the ),- and ~5 -states of adsorbed oxygen; sweep rate: 30 mV/s (ref. [39]).

3.7. Cyclic voltammetric studies The two types of electrochemically formed chemisorbed oxygen on Pt films interfaced with YSZ are also clearly manifest via solid state linear potential sweep voltammetry (Fig. 9 bottom, Ref. [39]): The first oxygen\ reduction peak corresponds to normally chemisorbed oxygen (y-state) and the second reduction peak which appears only after prolonged positive current application [39] corresponds to the 8-state of oxygen, i.e. backspillover oxidic oxygen, which is significantly less reactive than the ),-state. 3.8. Temperature p r o g r a m n ~ desorpfion The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to NEMCA conditions is also manifest clearly by temperature-programmed-desorption (TPD) [26] as shown in Fig. 10. The strongly bonded backspillover oxygen species (peak desorption temperature Tp=750-780 K) displaces the normal chemisorption state of atomic oxygen obtained via gas phase adsorption (Tp-740 K) to a significantly more weakly bonded state (Tp=680 K). The pronounced rate enhancement in NEMCA studies of catalytic

89 oxidations with positive potentials (electrophobic behaviour) is due to the very fast oxidative action of this weakly bonded oxygen. The strongly bonded backspillover anionic oxygen is significantly (A times) less reactive and acts as a sacrificial promoter. 20 Pt-WE ... Au-CE

f~

16

~12

E

Z

8

0 50O

600

700 T,K

800

900

Figure 10. Oxygen TPD spectra after gaseous oxygen adsorption at 673 K and Po2=4.x10 -6 Ton" for 1800 s (7.2 kL) followed by electrochemical 0 2- supply (I=151.tA) for various time periods t/s comparable to 2FN/I (2570 s). Gaseous oxygen supply creates a single adsorption state (Tp=740K), but additional electrochemical oxygen supply creates two adsorption states. The weakly bonded state is highly reactive [26], while the strongly bonded (backspillover) state (Tp=750-780 K) acts as a sacrificial promoter for catalytic oxidations [26]. (Adapted from ref. [26]).

3.9. Scanning tunneling microscopy (STM) The very clear demonstration of ion backspillover as the cause of NEMCA when using Na-~"-A1203 as the solid electrolyte was recently obtained via atmospheric pressure scanning tunneling microscopy (STM) [29]. APt monocrystal with an exposed Pt(111) surface was interfaced with a Na-~"-A1203 component using a Pt paste electrode along the perimeter [29]. Negative current application was found to cause Na backspillover on the Pt(111) surface forming at low surface coverage (<0.01) a Pt(111)-(12xl2)-Na adlattice on the previously existing Pt(111)-(2x2)-O adlattice. Positive current application was found to totally remove the Na adlattice leaving the Pt(111)-(2x2)-O adlattice intact [29].

90 This study, in addition to explaining NEMCA with Na+-conducting solid electrolytes, provided the first STM confirmation of spillover/backspillover phenomena.

3.10. Ab initio quantum-mechanical calculations A very frequent feature of electrochemical promotion studies is the observed linear variation of catalytic activation energies with varying catalyst work function [9,14,15]. It had been proposed that this is due to a linear variation in chemisorptive bond strengths with catalyst work function [9,14], a proposition recently supported by TPD studies for oxygen chemisorption on Pt/YSZ [26]. A recent first-principles investigation of electrochemical promotion using cluster models of oxygen adsorbed on Cu and Pt metal surfaces with coadsorbed positive and negative ions or point charges has confirmed the experimentally observed linear dependence of the O bond strength on metal work function [21 ]. This linear relationship was also found at the first-order perturbation theory level by taking into account only the purely electrostatic interaction between the field induced by the ions and the polar metal-oxygen bond. This suggests that the observed pronounced variation in oxygen desorption energy is largely due to electrostatic effects [21]. In addition to providing a direct explanation for the effect of electrochemical promotion, this study provides a theoretical methodology for the investigation of the role of promoters in heterogeneous catalysis.

4. CONCLUSIONS Electrochemistry can be used to affect oxidation catalysis on metals and metallic oxides [13] in a very pronounced and reversible manner. The observed promotional phenomena are due to an electrochemically driven and controlled backspillover of ionic species on the catalyst surface. These species, which in some cases cannot form via gas phase adsorption, alter the catalyst work function and affect the binding strengths of chemisorbed reactants and intermediates in a pronounced and theoretically predictable manner. This electrochemically controlled variation in the binding strength of adsorbates causes the observed pronounced modification in catalytic activity and selectivity. The ability of solid electrolytes to act as reversible promoter donors to influence oxidation catalysis is of considerable theoretical and, potentially, practical interest.

Acknowledgement: We thank the JOULE CEC programme, and the EPET and PENED programmes of the Hellenic Secretariat of Research and Technology for financial support.

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

93

Reductive and oxidative activation of oxygen for selective o x y g e n a t i o n of h y d r o c a r b o n s Kiyoshi Otsuka Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan

A simple method for reductive activation of dioxygen by applying electrochemical O 2 - H 2 cell reactions is proposed for the oxygenation of alkanes and aromatics at cathodes. The active oxygen species for most of the cathodes with and without metal compounds was suggested to be OH radicals. However, the active oxygen species in the case of a SmC13/graphite cathode was not an OH radical. The electrolysis of water formed oxidatively activated O 2-, which caused the epoxidation of various alkenes including propene to propene oxide on a PtO 2 phase for a Pt black anode. A catalytic system composed from Eu-salts, carboxylic acids and zinc powder caused the partial oxidations of light alkanes (CH4, C2H6, C3Hs) at room temperature and atmospheric pressure. In this system 02 was reductively activated by Eu(II) which was generated from Eu(III) through reduction by zinc powder. The active oxygen formed in this manner showed a strong electrophilicity as well as a radical character.

1. I N T R O D U C T I O N Partial oxidations of alkenes to epoxides, light alkanes (CH4, C2H6, C~Hs) to their alcohols and aldehydes or hydroxylation of aromatics in one-step using 02 as the oxidant are the most attractive but difficult targets in the field of selective oxidation catalysis. Monooxygenase or its mimic systems are often applied for these oxygenations under mild conditions, using a reductant such as ascorbic acid, NADPH, NaBH 4 or zinc for reductive activation of oxygen making possible the monooxygenations [1-6]. Although the mechanisms for the activation of oxygen are complicated and the active oxygen species suggested are quite different, the reductive activation of O2 in acidic media and its suggested reaction may be represented by equations 1 and 2. 02 + 2H + + 2e-----O* + H 2 0 RH + O* --~ R OH

(1) (2)

94 I n this scheme e- is provided by the reductant added to or present in the system and O* is the active oxygen species responsible for the oxygenations of hydrocarbons. The attempt to reductively activate O2 using the cheapest reductant H 2 [7-11] is attractive but careful handling is required for the explosive O2-H 2 gas mixture. Moreover, the deep reduction of O2 to H20 must be controlled. 1 . 1 R e d u c t i v e a c t i v a t i o n o f 0 2 by O2-H 2 c e l l Recently, we have reported a simple method for the reductive activation of 0 2 by applying an electrochemical O2-H 2 cell reaction for the oxygenation of alkanes and aromatics at the cathode [12-14]. With an acidic electrolyte in the O2-H z cell, the stoichiometric anode and cathode reactions are written simply as follows, (Anode) (Cathode)

H 2 ---, 2H* + 2 e 1/2 0 2 "[" 2H* + 2 e ---~ H 2 0

(3) (4)

Although the elementary steps for the cathode reaction must be complicated and dependent on the nature of the electrocatalyst, the reduction of O2 at the cathode may be written schematically as follows. O2

e-

~-

02-

e-

~- O222HO" MO2H

e-

~

023-

e-

~- 2H20

(5)

HO; H20 M-O; H20

If the reduced oxygen intermediates including the protonated ones in parentheses have a finite lifetime in the presence of a suitable catalyst (M), these reduced oxygen species might be able to attack the hydrocarbons in the cathode compartment, resulting in their oxygenation during the O2-H z cell reactions. In fact, we succeeded in the oxygenation of alkanes and the hydroxylation of aromatics on the basis of this concept [12-14]. 1 . 2 O x i d a t i v e a c t i v a t i o n o f H20 by In contrast to the reductive activation described above, the reverse reaction, should oxidatively activate the oxygen follows. electrolysis H20 H2+ O** RH + O** -~ ROH

electrolysis of 0 2 during the O2-H 2 cell reaction i.e., the electrolysis of acidic water of water at the anode (equation 6).

(6) (7)

The so-formed oxidatively activated oxygen O** can be expected to also activate hydrocarbons producing oxygenated products under mild reaction conditions. In fact, the epoxidation of olefins proceeded selectively by the oxygen generated at the anode during water electrolysis at room temperature as will be demonstrated later.

95

1.3 Design of catalytic systems based on the zinc-air battery The zinc-air battery (ZnIKOH]Air) is a commercialized, small b u t t o n - s h a p e d battery that uses KOH as an electrolyte. If we use a q u e o u s acid s o l u t i o n as an electrolyte, zinc is oxidized into zinc salt (equation 8) at the anode and o x y g e n is r e d u c e d into water at the cathode (equation 4). (Anode)

(8)

Zn + 2 H A --~ ZnA 2 + 2H + + 2 e

Similar to the concept d e s c r i b e d for the O2-H 2 cell system, the r e d u c t i v e activation of o x y g e n can be expected at the cathode in the p r e s e n c e of a suitable catalyst. On the basis of this zinc-air battery model, we have d e s i g n e d a n u m b e r of catalytic s y s t e m s from mixtures of zinc p o w d e r , c a r b o x y l i c acid and v a r i o u s metal chlorides for o x y g e n a t i o n s of alkanes and alkenes. In these catalytic s y s t e m s , zinc p o w d e r w o r k s as the reductant as well as the electron c o n d u c t i n g medium. The c a r b o x y l i c acid w o r k s as a p r o t o n - c o n d u c t i n g m e d i u m . O x y g e n is reductively activated on the metal cations by p r o t o n s from the c a r b o x y l i c acid and electrons from zinc p o w d e r .

2. E X P E R I M E N T A L

2.1

Oxygenation

0 2 Vent in

an

~~HT~ Vent O 2 - H 2 cell reactor The O2-H 2 cell reactor and the principle of the method for the o x y g e n a t i o n of hydroc a r b o n s are d e m o n s t r a t e d in F i g u r e 1. A detailed description of the cell setup has been -, H2 given elsewhere [ 13]. A --~ S ' ~ ~""J silica-wool disk ( t h i c k n e s s /"<x:~" I / ]~'\~ ~Anode 2.0 mm and diameter 26 ram) [ Cathode HaPO4 aq impregnated with aqueous Spinbar H3PO 4 (1M, 1 ml) as an electrolyte separates the Figure 1. Reductive activation of dioxygen using O2-H 2 a n o d e and cathode compartH 2 cell reaction for oxygenation of hydrocarbons. ments. The anode was prepared by a h o t - p r e s s m e t h o d from a mixture of graphite, Pt-black and Teflon p o w d e r . The cathodes w e r e p r e p a r e d from a mixture of carbon w h i s k e r s with v a r i o u s metal oxides or metal salts. The superficial s u r f a c e area of both electrodes was 3.1 cm 2. The o x y g e n a t i o n of b e n z e n e was carried out in the f o l l o w i n g s t a n d a r d manner: (i) O x y g e n (101 kPa) was b u b b l e d into the liquid s u b s t r a t e s (40 ml) in the c a t h o d e c o m p a r t m e n t , (ii) h y d r o g e n (49 kPa) and water v a p o r (4 kPa, to keep the electrolyte wet) w e r e p a s s e d t h r o u g h the anode c o m p a r t m e n t , (iii) the reaction was initiated by s h o r t i n g the circuit at 300 K and c o n t i n u e d for 3 h. In the case of o x y g e n a t i o n of light alkanes (CH4, C2H6, C3H8) , a gas mixture

/Ioo- 9

22~[ n'~l~

:.

96 of alkanes (50 kPa) and oxygen (51 kPa) were passed in the cathode compartment. Such an explosive gas mixture must be handled carefully in the reactor barricaded by thick (2 cm) acrylic plates. The oxidation efficiency (O.E.) for the formation of oxygenates was defined as follows, where F=96500 C. amount of the oxygenates(mol) (O.E.)=

x100%

(9)

charge passed(C)/2F 2 . 2 Oxygenation by H20Ar or electrolysis Propylene The reactor for H20electrolysis used in the epoxidation of alkenes is shown schematically in e Figure 2. The anode was prepared from metal blacks (70 mg) mixed with Teflon powder by the hot-press - R-CH=CH-R' method. The cathode was I-hO/Ar prepared from a mixture of Pt ~R.CH.CH.R? ( black, graphite and Teflon \O/ ~. powder. Propylene was bubbled into CH2C12 (40 ml used as a solvent) in the "T--'~-"~node H3[~O4aq. cathode anode compartment. In the magnetic case of 1-hexene epoxidation, spinbar 10 ml of the alkene was Figure 2. Schematic diagram of the reactor and dissolved in 30 ml of the principle of the epoxidation of alkenes duCH2CI 2. Argon (98 kPa) and ring electrolysis of water. water vapor (4 kPa) were passed through the cathode compartment to prevent the electrolyte from drying out as well as to remove hydrogen during the electrolysis. The oxidations of propylene and 1-hexene were performed under the following conditions unless otherwise stated" T=303 K, P(C3H6)=101 kPa, applied v o l t a g e = l . 7 V and reaction time=2 h. The oxidation efficiency for the formation of the oxygenates concerned was calculated according to equation 9.

J;LI

_

2.3 Design of catalytic systems for oxygenation The standard procedurc for thc oxygcnation of hydrocarbons using the catalytic systems dcsigncd from thc zinc-air battery was as follows. Mctal salts wcrc dissolvcd in a stirred solution of CH3COOH (or CF3COOH ) and CHzC12 in a three-necked flask with a reflux condcnser. Then a substratc and zinc powder were added to the solution. Thc oxygcnation of the substratc was started by stirring thc mixturc undcr a strcam of 02 (101 kPa) at 313 K, and the rcaction was continucd for 1 h.

97 3. R E S U L T S

AND DISCUSSION

3 . 1 P a r t i a l o x i d a t i o n d u r i n g O2-H 2 r e a c t i o n s . Effective cathode catalysts for partial oxidation 50 of hydrocarbons by the ~] cyclohexanone method in Figure 1 were 1 cyclohexanol looked for using cyclo40

hexane as a model substrate. The metal salts to ~" be tested as electro~ 30 catalysts were added to ,~ graphite by impregnation ~ 20 from aqueous solutions of their metal chlorides and were dried at 373 K. The 10 reaction products were cyclohexanol and cyclohexanone and no CO 2 was 0 Gr. Mg Sr Sc La Pr Sm Gd Dy Er Yb produced. The active caFe Ca Ba Y Ce Nd Eu Tb HoTm Lu thode catalysts contained Figure 3. Product yield in the oxidation of cyclohexane rare earth metal cations. for the cathodes containing various metal chlorides. Among them, S m 3+ was T=298K, reaction time=20h, most active for the partial Cathode: Metal chloride (25pmol)/graphite (70mg) oxidation of cyclohexane cyclohexane (40ml), P(O2)=101kPa, (Figure 3). The integrated turnover number for the Anode: Pt-black/graphite, P(H2)=98kPa , P(H20)=3.3kPa. Sm 3+ exceeded 10 after six successive reactions. The SmC13/graphite cathode was also effective for oxygenations of other alkanes (n-hexane and adamantane) and hydroxylations of aromatics (benzene, toluene and naphthalene) at room temperature during O2-H z cell reactions. The treatment of graphite in oxidizing agents such as potassiumpermanganate, boiling HNO 3 before the impregnation of SmC13 enhanced the catalytic activity of SmC13/graphite. This favorable effect of oxidation pretreatment of the host graphite suggests that surface functional groups such as -COOH, -OH or -CHO are required for coordinating the active S m 3§ ions. The addition of H202 during reaction under standard experimental conditions did not enhance the oxygenation of cyclohexane, suggesting that H202, which should have been generated at the cathode during the O2-H 2 cell reaction, could not be the active oxygen species in the case of rare earth metal-embedded cathodes. Comparison of the results of toluene oxidation using SmC13/graphite with those using Fenton's reagent indicated that the active oxygen species O ' g e n e r a t e d on the SmC13/graphite was quite different from the OH radical. The O* s h o w e d more electrophilic character than the OH radical.

98 Cyclic-voltammetry studies of the SmC13-doped glassy carbon in the presence and absence of cyclohexane suggested that a hydroperoxy radical generated on Sm 3+ was directly or 50 r indirectly responsible for oxygenation of cyclohexane. 40 The h y d r o p e r o x y radicals o could be further activated on ~ 30 Sm 3+ giving a strongly perturbed h y d r o p e r o x y species ~ 2o Sm(3-~)+(O2H) ~§ which had electrophilic character and 0 ~9 lO could activate the C-H bond of alkanes. 0 The use of carbon ~ 0 whiskers instead of graphite CH4 C2H6 C3H8 Alkanes as the host cathode material markedly improved the Figure 4. Oxidation of light alkanes during O2-H2 cell rate of oxygenation of reactions 9 cyclohexane and benzene, Cathode: Carbon whisker; P(alkane)= 51, P(O2)=50kPa. especially for cathodes Anode: Pt-black/graphite; P(H2)=50, P(H20)=4 , containing iron compounds P(He)=47kPa. and Pd black [15]. The 1 , acetaldehyde for ethane and acetone for propane. incorporation of iron I--1 , carbon dioxide. compounds together with Pd black showed a m a r k e d synergism for the formation of oxygenates. In this case, hydroxyl radicals were suggested to be the active oxygen species responsible for the h y d r o x y l a t i o n of benzene and oxygenation of cyclohexane [15]. The method of Figure 1 without solvent was also effective for the oxidation of methane, ethane and propane at room temperature (Figure 4). The carbon whisker cathode without additives was the most effective for the partial oxidation of propane. The selectivity to useful oxygenates (acetone) in this case exceeded 65% on the basis of the propane reacted [16].

3 . 2 E p o x i d a t i o n of aikenes during H20 e l e c t r o l y s i s Materials effective as anode catalysts for epoxidation of 1-hexene by the method in Figure 2 were screened. Among various metal oxides, metal salts and metal blacks tested, the most active and selective anode catalyst for the formation of 1,2-epoxyhexane was Pt black (Table 1). The oxidation efficiency for the formation of epoxide defined by equation 9 was about 26% and its selectivity was 66%. Pt black samples obtained from different producers or prepared in this work showed quite low electrocatalytic activity. However, the calcination of these inactive Pt blacks in air at 673 K substantially enhanced the catalytic activities of these samples. XPS studies on various Pt black samples suggested that a PtO 2 phase was associated with the active oxygen for the epoxidation.

99 Table 1. Epoxidation of 1-hexene on various anodes of noble metals Amount of product / mmol 9m -2 Anode

~O

Others

C.P./ 104 C 9m -2

Total O.E. /%

Epoxide O.E. /%

SEpo. /%

Pt-black

1066

394

170

80.8

42.4

25.5

65.6

Pd-black

43

24

25

118.6

1.8

0.7

47.3

Rh-black

3

3

21

142.1

0.6

0

9.8

Ru-powder

2

1

6

1.9

9.4

2.3

30.8

Au-powder

1

2

4

1.7

9.3

1.0

12.5

Reaction conditions: T-303K, time 2h, applied voltage 1.7V, H3PO4 1.0M. Anode: noble metals, P(Ar)=101kPa; Cathode: Pt-black/graphite, P(Ar)=98. P(HzO)=3kPa. The Pt black sample active for the epoxidation of 1-hexene and 2hexenes was also tested in the oxidation of propylene. Figure 5 shows the results of p r o p y l e n e oxidation as a function of the applied voltage across the cell. The oxidation of propylene was initiated at an applied voltage higher than ca. 1.1 V. The formation of p r o p y l e n e oxide and acetone were remarkably enhanced at an applied voltage >1.1 V. The m a x i m u m oxidation efficiency for the propylene oxide was 25% and the selectivity to propylene oxide was 53% at 1.7 V. These results indicate that the epoxidation of propylene by the oxidative activation of H20 proceeds with fairly good current efficiency and selectivity on the Pt black anode [17].

100

600 i

O 400

.,.,~ ~3

5o ~

r

O 9,,,-i .,..a

~" 200 (D

~

o 0 2.0O

rJ3 ,,,,o

o

1.0-

0

0

0.0

1

2

3

Applied voltage / V Figure 5. Partial oxidation of propylene during electrolysis of water as functions of applied voltage. Standard reaction conditions: T=303K, reaction time=2h, concentration of H3POg=I.0M. Anode: Pt black, P(C3H6)=101kPa. Cathode: Pt black/graphite, P(H20)=3, P(Ar)=98kPa. V , O.E. for propylene oxide and acetone; II, O.E. for propylene oxide; 0 , propylene oxide; ~ , acetone; [-7, COa;/X, acetic acid; X, propionic acid 9

100 The product ratio in cisand trans-2,3epoxyhexanes observed in the epoxidation of cis- and trans-2-hexenes indicated the retention of the cis-trans configuration of the starting 2hexenes. This observation demonstrates that the rotation around the C(2)-C(3) axis of the activated complex is strictly prohibited. Kinetic results and tests using propylene oxide and acetone as the s tar tin g s u bs trate suggest the reaction mechanism indicated in Figure 6 [17]. 3.3

Design

of catalytic

H

H

XO"

(1)

_ O _pt4_+O _pt4_+O _

~ -2H+, -2eo*

(2)

A

"......O*

(3)

: ..

J

~9O ,

O - P'r

(4)

- P'4+ ~

~,H,,~,O,(6)

.O'~*"'a~O, (8)

+

+

(5) O- Pt4-+O-Pt4+

(7)

O- Pt4-+0- Pt4+

4+

COl (9)

"~OX~H(10)

O-Pt4+-O- Pt4+

Figure 6. Reaction mechanism for oxidation of propylene. systems

based

on m i c r o c e l l

models.

Based on the zinc-air battery, a new catalytic system designed was composed of metal chlorides (MClx), carboxylic acids and zinc powder for oxygenation of alkenes to epoxides and partial oxidation of light alkanes (CH 4, CzH 6 and C3H8) into alcohols and aldehydes. These catalytic systems are likely to activate oxygen as schematically illustrated in Figure 7, where CF3COOH is used as a proton-conducting medium. Oxygen is reductively activated on the metal cations by protons from CF3COOH and electrons from zinc. Among the many metal salts tested in this work, Eu Salts "Microcell Catalyst" (EuC13, Eu2(CO3)3, Eu(AcO)3 ROH, RO RH Eu(C104)3, and Eu(NO3)3) 2 CF3COOH ~/// showed catalytic activity for the oxygenation of i + H20 hydrocarbons at room (CF3COO_)2Zn Mn+ temperature [18-20]. The /-~ 2 H + " - - ~ \ results for the oxidation of " ~ " C1 C1 (cathode) cyclohexane over various metal chlorides and Eu-salts 2eare compared in Figure 8. Zn The mixture of Eu saltsFigure 7. Reductive activation of dioxygen for light CH3COOH-Zn powder with alkane oxidation. CH2CI z as a solvent catalyzed the epoxidation of alkenes (hexenes, butenes, propene)

/

~

101 Yield based on cyclohexane / % 1

2

i

I

~ LaC13 ! CeCI3 " ! PrC13 NdCI3 SmC13

CyOH

CyO EuC13

" 1 GdCI3 1 TbC13 ~ DyC13 HoCI3 ErC13 "1 TmC13 YbCI3 LuCI3

Eu(NO3)3 Eu(CIO4)3 Eu2(CO3)3

"! Eu2(SO4)3

Eu(AcO)3

" FeCI3 " ! CuCI2 "1 PdCI2

0

5 TON based on catalyst / h -1

10

Figure 8. Partial oxidation of cyclohexane using various metal chlorideCH3COOH-Zn powder systems. Reaction conditions: T=313K, rection time=lh, catalyst=30gmol, Cy-C6H12=2ml, O2=101kPa, CH3COOH=2ml, Zn powder=l.0g, CH2C12=2ml. and partial oxidation of alkanes (adamantane, cyclohexane, n-hexane, propane). The mixture of Eu salts-CF3COOH-Zn powder in the absence of solvent catalyzed the oxidation of methane and ethane into their alcohols with turnover numbers of 4 and 8, respectively, in one hour at room temperature [20]. The important role of CF3COOH is to stabilize the alcohols formed by converting them into their esters. The reductively activated oxygen generated in this catalytic system showed strong electrophilicy as well as radical character [21]. If the catalytic system of Eu salts-CH3COOH (or CF3COOH)-Zn powder is deprived of one of its components, the partial oxidation of alkanes and epoxidation of alkenes occur very slowly or not at all. These observations suggest that the concept of designing catalytic systems based on the microcell model in Figure 7 may be justified.

102 REFERENCES

1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18. 19. 20. 21.

J . T . Groves, T. E. Nemo and R. S. Myers, J. Am. Chem. Soc., 101 (1979) 1032. Ortiz de Montellano (ed.), "Cytochrome P-450, Structure, Mechanism and Biochemistry", Plenum Press, New York, 1986. F. Montanari and L. Casella (eds.), "Metalloporphyrins Catalyzed Oxidations", Kluwer Acad. Pub., Dordrecht, (1994). D . H . R . Barton, M. J. Gastiger and W. B. Motherwell, J. Chem. Soc., Chem. Commun., ( 1 9 8 3 ) 4 1 ; D. H. R. Barton et al., J. Chem. Soc., Perkin Trans. I, ( 1 9 8 6 ) 9 4 7 . N. Kitajima, H. Fukui and Y. Moro-oka, J. Chem. Soc., Chem. Commun., (1988) 485. H. Dalton and J. Green, J. Biol. Chem., 264 (1989) 17698; J. Colby, K. I.Stirling and H. Dalton, Biochem. J., 165 (1977) 395. I. Tabushi and A. Yazaki, J. Am. Chem. Soc., 103 (1981) 1771. N. Herron and C. A. Tolman, J. Am. Chem. Soc., 109 (1987) 2837. A. Kunai, T. Wani, Y. Uehara, F. Iwasaki, Y. Kuroda, S. Ito and K. Sasaki, Bull. Chem. Soc. Jpn., 62 (1989) 2613. A. Sato, T. Miyake and T. Saito, Shokubai (Catalyst), 34 (1992) 132. Y e W a n g and K. Otsuka, J. Catal., 155 (1995) 256. K. Otsuka, I. Yamanaka and K. Hosokawa, Nature, 345 (1990) 697. I. Yamanaka, and K. Otsuka, J. Chem. Soc., Faraday Trans., 89 (1993) 1791. I. Yamanaka, and K. Otsuka, J. Chem. Soc., Faraday Trans., 90 (1994) 451. K. Otsuka, M. Kunieda and H. Yamagata, J. Electrochem. Soc., 139 (1992) 2381; K. Otsuka, M. Kunieda and I. Yamanaka, Stud. Surf. Sci. Catal., 82 (1994) 703. Q. Zhang and K. Otsuka, Chem. Lett., No 4, (1997) 363. K. Otsuka, T. Ushiyama, I. Yamanaka and K. Ebitani, J. Catal., 157 (1995) 450. I. Yamanaka, K. Nakagaki and K. Otsuka, J. Chem. Soc., Chem. Commun., (1995) 1185. I. Yamanaka, T. Akimoto, K. Nakagaki and K. Otsuka, J. Mol. Catal. A., 110 (1996) 119. I. Yamanaka, M. Soma and K. Otsuka, J. Chem. Soc., Chem. Commun., (1995) 2235. I. Yamanaka, K. Nakagaki, T. Akimoto and K. Otsuka, J. Chem. Soc., Perkin Trans. H, ( 1 9 9 6 ) 2 5 1 1 .

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

103

T h e S e l e c t i v e O x i d a t i o n of M e t h a n o l : A C o m p a r i s o n of t h e M o d e of A c t i o n of M e t a l a n d O x i d e C a t a l y s t s D. Herein, H. Werner, Th. Schedel-Niedrig, Th. Neisius, A. Nagy, S. Bernd, R. SchlSgl Fritz Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin i

1. A b s t r a c t The reactivity of methanol towards oxygenated silver and copper substrates and towards the molecular oxide H4PVMoiiO40 has been investigated by a variety of insitu techniques. The intention was to find the chemical origin of the selective action of atomic oxygen and to discuss the influence of the metal species on the oxygen reactivity. The exceptionally high reaction temperature over silver was traced back to the difficulty of forming one active oxygen species. Surface spectroscopies found evidence for three chemically inequivalent atomic oxygen species in the metal-oxygen systems in analogy to the three oxygen forms in the molecular oxide characterized by a single crystal structure analysis. The large body of surface science experiments in these systems has contributed to our understanding of the reaction possibilities but needs to be treated with reservation when a ,,high pressure" reaction mechanism is considered as these experiments describe only part of the system in the static limit of low chemical potential of the gas phase.

1. I n t r o d u c t i o n The selective oxidation of methanol to formaldehyde is a technologically relevant [1,2] reaction carried out over metallic silver or iron-molybdate catalysts. The reaction is also suitable as a model reaction to understand the chemical principles of modifying the oxidizing potential for dissociated oxygen on various surfaces [3,4,5,6,7]. The methanol molecule is a special organic substrate as it contains only hydrogen atoms in alpha position to the functional group. Chemically speaking

The work was supported by the Bundesministerium ftir Bildung und Forschungthrough its catalysis programme. Special supportand helpful discussionscamefromthe BASFAG, Ludwigshafen.

104 methanol is more a derivative of the water molecule r a t h e r t h a n an organic molecule with a functional group. The acidity of the methyl protons is thus exceptionally high allowing their easy activation in selective oxidation. All larger molecules contain additional hydrogen atoms much less acidic than those of methanol and these are much more difficult to activate for elimination or substitution. Methanol is thus a poor model molecule when specific acidity or delicate chemisorption properties of e.g. olefin functions are of interest. The surface chemistry of methanol [5,8,9] and its co-adsorption systems with oxygen over single crystal surfaces have been reviewed extensively [3,10]. Mechanistic studies on single crystalline metal oxide surfaces under well-defined conditions are much more scarce. The purpose of the present paper is to present a cumulative view on methanol oxidation under ,,high pressure" reaction conditions over a variety of surfaces. Experiments probing the geometric surface structure, its electronic structure and its reactivity will be presented which complement the significant body of ultrahigh vacuum (UHV)-oriented studies. Insitu experiments in which the surface was exposed to a flow of methanol-oxygen mixtures for prolonged times will be presented. The property of interest was either determined directly during the experiment or measured after a transfer into UHV. In contrast to m a n y preceding studies the attention is focused on the state of the active surface and not on the observation or indirect identification of short living reaction intermediates which are very difficult to characterize under insitu conditions. In this way the dimension of the material gap will be reduced between well-defined single crystals of metals (single crystals of relevant oxides of size and defect concentrations as applied for element metals do not exist as yet) and technical catalysts.

2.The Activation of Molecular Oxygen Molecular oxygen is a difficult-to activate chemical species and requires the presence of an electron donating catalytic surface such as a metal or a partly reduced oxide. The sticking coefficient of oxygen is usually low (about 10 .2) on clean metals and on fully oxidized oxides but rises to values of about 10 -2 on oxygen pre-covered metals [11,12] or partly reduced [13] substrates. A general reaction pathway [8] is depicted in Scheme 1. From a molecular precursor [14] (not shown) oxygen reacts to the peroxo state at which the oxygen-oxygen bonding is still present in a weak form. This state is extremely active in oxidation [15,16] but usually unselective. It occurs on silver [17] at about 300 K. At higher temperatures the peroxo species dissociates completely and forms atomic oxygen which can be either present as surface-adsorbed (a) species or as surface-intercalated (7) species [18]. The overall sticking coefficient into these selectively reacting species is low as the likely processes for depleting the precursor states are desorption or bulk-dissolution [12]. The chemical bonding [19] of the two species to a metal substrate is significantly different and can be characterized as O 2p-metal sp hybrid for (a)

105 and O 2p-metal dsp hybrid for (7) oxygen. On silver the (7) state is the most stable form of oxygen complex and can be formed either directly, or peroxo by conversion from the (a) state or by segregation from bulk-dissolved oxygen. The chemical reactivity of the two surface species is quite different + and can be characterized as oxidizing for the more weakly held (a) species and as basic (,,proton activating") for the strongly bound (7) species. Evidence for these assignments comes from a suite of spectroscopic experiments [7,17,18,20,21,22,23]. oxidizing basic Figure 1 shows the thermal desorption spectroscopic (TDS) BijerrumBr~ electrophilic j / ~ responses of a polycrystalline electrolytic silver catalyst [24] from the three species (a), (7) and bulk dissolved with their respective T+ desorption temperatures and line profiles. The superimposed conversion curve (atmospheric Scheme 1" Products of the activation of molecular pressure, methanol : oxygen ratio x = oxygen on metals. 1.0, SV 80.000 h -1, methanol to water ratio 1"1, reaction time 130 h) illustrates that under technical reaction conditions the reaction is driven by (7) and by bulk-dissolved oxygen converting during segregation into (a). The fact that the conversion curve TDS 100 .-. alpha exhibits at 50% the plateau in 10 g exact agreement with the :5 8o ~ changeover from bulk 8 m eAg- poly Q~ dissolved to (7) oxygen is seen 60 E 6 as qualitative indication (see 0 r also Figure 7) for the change 0 4 40 .~ in the dominant reaction III g pathway from oxidative 20 ~ dehydrogenation to dehydrogenation together with 400 500 600 700 800 900 the dominating surface Temperature (K) species. Figure 1" Deconvoluted TDS responses from three different Figure 2 exemplifies the oxygen species on silver spectroscopic fingerprints for

I

A

106 the different bonding schemes mentioned above. The method of near edge X-ray absorption spectroscopy (NEXAFS) at low photon energies requires a complex UHV system with a reaction cell AgxO O K-edge high-pressure ....o 4 allowing treatments at 10 mbar oxygen for 300 s t o 3600 s on an -o .>.. - 3 Ag (111) single crystal between 600 K and 900 K in order to 0 ~2 prepare the relevant species. NEXAFS [19] can be used as a highly surface-sensitive tool with about 80 % of the information 011 arising from less than 2 nm depth 520 530 540 550 560 570 of the sample. The unique Photon Energy (eV) resolving power of the method for the details of the hybridisation is Figure 2: NEXAFS data of three atomic oxygen evident. The sharp first structure species on silver is the n* resonance arising from transitions from O ls into unoccupied d-sp hybridised states exhibiting a crystal-field splitting fine structure. The broader features above 536 eV arise from the o* ' transitions from O ls into unoccupied sp r states. The significant differences in chemical bonding between oxygen atoms ~ 2 2o and substrate are much more clearly seen than in the more conventional X-ray 0 photoelectron spectroscopy (XPS) in which ((z) and (?) give rise to a chemical shift of s about 1.5 eV without disclosing, however, 6 ~ ~ / ~ ~ the origin of the chemical shift. The absence of multiple n* resonances and of any polarization effects on well ordered Ag surfaces (data not shown, see [19]) prove . ~ 1 together with Raman spectra [25,18] without any doubt the atomic nature of all three species. The peroxo species existing 40o 600 soo at low temperatures and being not Temperature(K) relevant to selective methanol oxidation is difficult to prepare in a spectroscopically Figure 3: Isotopic exchange TDS with 18 02 pure form as its reactivity leads always to of fully oxidized HPA at 1 K/s heating rate. interference with carbonate [17,8] H2CO was obtained in a TPRS run. formation. Observation of reactive oxygen species on oxides is significantly more difficult. Figure 3 shows an isotope exchange experiment with the molecular oxide HnPVMollO40-14 H20, a heteropoly acid (HPA) [26,27] and oxygen 18 in the gas ,

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107 phase. The sample stored molecular oxygen together with the crystal water which exchanged readily at low temperatures due to the acidic environment. Attempts to chemisorb 18 02 onto the HPA surface after dehydration were unsuccessful even at atmospheric pressure. In an atmosphere of 18 Oz it was possible, however, to observe a dynamic exchange between gas phase and lattice oxygen as could be seen from the increase of the scrambled oxygen partial pressure with temperature. At 685 K the structure began to collapse giving rise to an increased scrambling rate and at slightly higher temperatures to oxygen evolution from the sample bulk. The correlation with the formaldehyde formation in an atmosphere of methanol is useful for an assignment of oxygen reactivity. A sustained production of formaldehyde was observed only after the constitutional water was lost lasting up to the temperature at which the breakdown of the structure finally gave rise to an increased (but not lasting) activity. The data show that the HPA is only active in this reaction in its dehydrated state and that the useful temperature window at which steady state oxygen activation from the gas phase is observed is limited to the interval between full dehydration and beginning decomposition. The activation of molecular oxygen is likely to be a complex process requiring a significant amount of activation energy to overcome the 493 kJ/mole bond energy in molecular oxygen which can be moderated by the ease at which electrons are transferred from the catalyst to the chemisorbed oxygen molecule. As the whole process needs to be reversible it is of course not possible to use strong reductants such as early transition metals or alkalis as these materials will not be reducible by the organic substrate. Rather inert metals such as silver and gold are a good choice as well as oxides with multiply valent states (defects).

3. Selective Oxidation Pathways The term ,,selective oxidation" can be defined as a set of reactions in which the formal oxidation state of the organic substrate is increased but not to the level of t h a t of CO or CO2. Selective oxidation is then not only the addition of an oxygen atom to the organic molecule , the abstraction of hydrogen via dehydrogenation and oxidative dehydrogenation (or frequently oxidehydation) is also enclosed in ,,selective oxidation". The dehydrogenation is an endothermic process due to the high energy content of C-H bonds without a suitable compensation by the exothermic water formation. For this reason there is a high tendency of the hydrogen product to react irreversibly with the metal-oxygen complex of the active site and to irreversibly damage it. Dehydrogenations can thus only occur with very stable catalysts and under conditions where the reductive chemical potential is modified by the presence of e.g. a large excess of molecular water [28](e.g. in styrene formation over iron oxide). Different redox reactivities with atomic oxygen are the consequence of differing oxygen-substrate interactions. The (a) species is either transferred to the substrate (oxidation) and/or to hydrogen (oxidehydrogenation). The (~,) species reacts as a basic center and is strongly attached to the catalyst which needs not to be a metal such as silver or

108 platinum [11,12] but may well be a partly reduced metal ion in a sub-oxide catalyst [29].

4 . O x i d a t i o n of M e t h a n o l Methanol can be transformed into formaldehyde in principally two different ways known as dehydrogenation and oxidative dehydrogenation. The main side products are hydrogen and water respectively. A cumulation of the mechanistic considerations drawn from model experiments on metal surfaces [3,6,30] is given in Scheme 2. The catalyst surface containing pre-adsorbed oxygen of either ((z) or ('1I) type transforms methanol into methoxy (1). This can react along three main pathways. Reaction (2) is the simple oxidative dehydrogenation leading to formaldehyde and water. Reaction (3) occurs with (~) oxygen and is a thermodynamically unfavorable dehydrogenation path. At long residence times and under excess (a) oxygen a series of complex reactions occurs which begins with the surface oxidation of methoxy (4) to formate. The labile formate + will either decompose into carbon dioxide and water (5) or even into carbon dioxide and hydrogen in the absence of sufficient chemisorbed oxygen. The desorption of formic acid (6) or the formation of dimethyl ether or dimethyl ketone are alternative side reactions. The relative values of the relevant rate constants will control the selectivity to the side reactions although they rarely dominate the overall conversion. The reaction pathways denoted by the intermediates (2) and (4) have been subject to extensive fundamental studies [3,6,31,32] whereas the dehydrogenation pathway was rarely found in such studies. The obvious reason for this problem Scheme 2: Reaction pathways of methanol on metals in the presence of different surface oxygen species. For the is that (?) oxygen cannot be generated from the gas phase numbers se text. under conditions accessible in UHV systems [20]. Only after prolonged use of a single crystal the bulk-dissolved

CH3OH

(1)~O--HH O--CH H

~ HH~1 methoxy HC

(2

H--O HC--O--~

H H

i

H

(3)

(4) 0

(5~0

formate I H--d-- I

/~'~

HC. (6) IH-o

109 species (unintentionally prepared) will segregate to the surface during heat t r e a t m e n t s and create some (7) oxygen. The reactions described in Scheme 2 require the existence of metal sites besides chemisorbed oxygen. The existence of active sites containing both metallic and oxygen centers was imaged directly in atomic resolution on silver saturated with (7) oxygen. An insitu (111) faceted grain of electrolytic silver was used for the image shown in Figure 4. Within the hexagonal a r r a y of silver atoms (normal interatomic distance of 0.285 nm, white) oxygen atoms are intercalated in the top atomic layer of the catalyst. The large contrast arises from the profound change in local electronic work function [19] on going from metallic to anionic atom sites. The acidity of intermediate OH groups formed on metal Figure 4: STM image of Ag (11 l) with (7) catalysts is assumed to be insufficient to oxygen. The interatomic distance of silver interfere with the elementary steps of measured along the white line is 0.285 nm. oxidative dehydrogenation. Oxide surfaces are significantly more For the ,,hole" contrast see text. complex in the range of reactivity of their t e r m i n a t i n g atoms. On HPA strongly acidic OH groups with pka values of mineral acids exist [33]. Surface hydroxyl groups on nominally pure oxides are also known to react acidic with a wide distribution of pka values. ,,Metal" centers a r e , in contrast to frequent colloquial designations, not existent on most catalytic oxides. Cation sites from regular metal-oxygen polyhedra are also difficult to access by adsorbates due to the inherently large size of oxygen ions relative to all cations. Defects in closed-packed surfaces and incompletely coordinated oxide polyhedra (e.g. octahedra) are, however, accessible for chemisorption of oxygen containing adsorbates such as methoxy. These sites are Lewis acids in contrast to metal sites on the metal-oxygen catalysts which are all in contact with the conduction band and hence Lewis bases. A variety of defect types is conceivable and has also been observed experimentally on binary and on complex oxide surfaces [34]. Figure 5 shows an atomically resolved STM image of a FeaO4 (111) single crystalline surface to give an impression about the complexity of a defective oxide surface. The oxygen atoms of 0.26 nm a p p a r e n t size form a regular a r r a y of a hexagonal closed packing as expected from the bulk structure. A hexagonal Moiree pattern indicates slight lattice mismatch between surface and bulk of the oxide film [35]. Point defects of various geometry and the modified local electron density on the adjacent oxygen atoms (lighter contrast) can clearly be seen. A fundamental problem for experimental [36,37] and theoretical [38] studies is our still very limited knowledge about geometry and defect disposition [34] on most oxide surfaces even when they are considered as

110 single crystals by diffraction techniques. The view that oxides contain two types of oxygen sites termed frequently as ,,terminal" (7) and ,,bridging" ((z) is certainly not incorrect [39] but maybe oversimplifying. As a consequence of the oxygen species distribution and the considerable acidity of oxide surfaces alternative reaction pathway for oxi-dehydrogenations have to be taken into account. The cleavage of the C-O bond in methanol becomes now a feasible reaction. The strong acidity allows further dehydration and the formation of dimethyl ether and of the acetal of formaldehyde as a prominent side reactions at low conversions [37, 40]. The kinetics of the oxidative dehydrogenation may exhibit four rate determining processes involving the formation of the methoxy intermediate, the cleavage of the C-O bond, the activation of the methyl hydrogen combined with the desorption of the formaldehyde product or the re-oxidation of the active site under evolution of water. It is the overwhelming conjecture [32] that for alcohol dehydrogenations the methyl activation step should be rate-limiting and neither the re-oxidation [40,41] nor the formation of alkoxy [42,43,44] control the overall kinetics. The methyl protons should be quite acidic considering the bonding situation in methanol and hence should be activated by a basic oxygen site [44]. This is in conflict with the observation [41] that basic oxide surfaces tend to totally oxidize the alcohol (multiple proton abstraction) and that acidic oxides exhibit a correlation between acidity and formaldehyde production. On suitably acidic surfaces the reaction mechanism may be dominated by the electrophilic action of protons on the methanol leading to an ,,embedded" methoxy which is easily activated at the methyl positions. On HPA catalysts this reaction path was claimed to be dominating [27] as long as the catalyst was sufficiently acidic. For the dehydrogenation of the methyl proton a radically different view was developed recently from theoretical Figure 5" STM image of Fe304 (111). The surface is considerations of HPA oxygen-terminated with each oxygen atom being resolved catalysts [38]. A direct 9LEED and ISS were used to verify the termination. ,,metal"-hydrogen bonding interaction between the proton

111 and one Mo 6§ center was claimed from an orbital analysis of an extended Hfickel calculation of transition geometries of this reaction step. It will be necessary to spectroscopically assess the covalency of the ,,metal"-oxygen interaction at the active sites in order to support this chemically surprising interpretation. The complexity of the methanol-oxide surface interaction is responsible for the still inconsistent picture about this reaction which may well proceed via several p a t h w a y s on the same bulk oxide. The reaction scenario in Scheme 3 is a reflection of the site complexity depicted in Figure 5 allowing for a wide spectrum of chemical reactivity of ,,metal"-oxygen groups. This spectrum is directly related Mo to the local geometric structure (,,bond I + CH3OH distance") and it is thus not surprising that a very pronounced structure H3CO sensitivity of the selective oxidation of Ogasphase O--..~ ~--~O methanol was found [36,37,45]. The silver-oxygen system restructures under reaction conditions into a uniformly active surface regardless of its initial O~ljO ~ O~ /O Mo Mo + CH30 H orientation [46,47] and acts as a structure-insensitive catalyst. - H2CO t + Olattice Y " H20 In the following we will investigate H H tO---.. ~ O t the methanol oxidation process over H Mo ] H silver, copper and heteropoly Mo molybdates in order to identify the + CH3OH occurrence of the possible reaction l H2CO pathways from Schemes 2 and 3 on l~3COH polycrystalline surfaces and at atmospheric pressure. The main O~ jO Mo Mo emphasis in these experiments will be on the source of active oxygen. This focus was chosen to better u n d e r s t a n d Scheme 3" Reaction pathways of methanol the involvement of bulk and sub-surface [10,48] chemistry in selective oxidation oxidation over early transition metal oxides. catalysis. Such an u n d e r s t a n d i n g is required when the catalytic performance is compared between series of chemically different systems which are chosen to investigate only one surface property such as acidity. These experiments will naturally only cover a small selection of the problems discussed with the reaction Schemes.

5. A n , , I n t e r m e d i a t e "

Experiment

The surface science studies of methanol oxidation over metals were carried out u n d e r conditions where no (y) oxygen was present on the surfaces. A t e m p e r a t u r e - p r o g r a m m e d reaction experiment was performed [49] in order to

112

prove that the reaction paths (2) and (3) can coexist on a surface. Electrolytic silver was loaded with molecular oxygen at 900 K and 10 mbar for 10 min. The generation of two species of atomic oxygen ((z) and (7) was confirmed by a TDS scan. After reloading the sample was heated in methanol vapor. The results are collected in Figure 6. The TDS trace exemplifies the well separated responses from (a) and bulk-dissolved oxygen (670 K) and from (y) oxygen at 990 K. In the TPRS traces the formaldehyde generation coincides exactly with the desorption peaks for the two oxygen species. This confirms the reactivity of both species and shows that the reaction is of the Langmuir-Hinshelwood type as the formaldehyde knows the bonding of the - ~ IIII" oxygen involved. The trace for carbon dioxide shows a parallel selectivity I ~ I I ~ I with partial oxidation for the (a) state II ~ iI but no total oxidation from the (7) state. 02 I A "'~" /"% This allows to assign the (a) state as the species for oxidative H2 dehydrogenation and the (y) state as the center specific for dehydrogenation. This assignment is supported by the m/e 18 trace exhibiting no intensity at the high temperature formaldehyde signal. The two sharp desorption peaks in the m/e 44 trace above 400 K arise from I I t i 400 600 800 1000 methoxy and carbonate (formate) Temperature[K] species which hardly produce Figure 6: TDS (dashed) and TPRS (full) of formaldehyde. This finding strongly oxygen/methanol on electrolytic silver. Heating underlines the notion that methoxy and rate: 1.3 K/s formate prepared in the surface science experiment as long-lived species [50] are different from methoxy and formate existing as reaction intermediates in the steady state conversion situation. It is speculated that minor differences in the chemical bonding of the chemically identical ad-species decide over their character as stable spectator species or unstable intermediate structures. Vibrational spectroscopic data [44] of methoxy and the analysis of the homologue ethoxy system [51] strongly support this view.

6. E x p e r i m e n t s w i t h Silver and C o p p e r The existence of the two reaction pathways to formaldehyde shown in Scheme 2 is documented in principal with the data from Figure 4. At steady state and at atmospheric pressure the contribution of the two reaction channels needs to be shown independently. In Figure 7 a section through the reaction parameter space of electrolytic silver under isothermal conditions is shown. The molar

113 stoichiometry was varied from oxygen-rich (in the explosion limit!) to methanolrich and the effect on conversion and selectivity was monitored. It is significant t h a t the optimum formaldehyde production was found at slightly methanol-rich conditions precluding the exclusive [2] contribution of oxidative dehydrogenation to the practical conversion. About 50 % of the formaldehyde production is observed at strongly understoichiometric feed compositions indicating t h a t dehydrogenation contributes significantly to the total activity. The selectivity to total oxidation is "-. "~ strongly coupled with 6O the excess of oxygen in the gas phase indicating ',/ \ w , t h a t the total oxidation II / I \ 10 . 20 o 40 of methoxy via formate is a rapid process when f, the surface is oxygen20 covered. The follow-up ,,............................ reaction to formic acid .............. i...................................... m ~I I I (see Scheme3) occurs not 50 1O0 150 200 250 in parallel with the Ratio CH30H : 0 2 (%) production of the Figure 7: Conversion over electrolytic silver at atmosphereic precursor formaldehyde pressure. A reactor holding 5.0 g catalyst in a 6 mm high bed was indicating that the used. The catalyst load was 10 g/h cm2. All data in mole % oxidation of reactive methoxy to formate is a unlikely process and that formic acid m a y be produced from the strongly adsorbed form of methoxy which occurs under conditions of lean reactive surface oxygen. From this only moderately reactive methoxy a finite selectivity leads to carbon dioxide in full accord with Scheme 2 as indicated by the increase of the CO2 production at very methanol-rich compositions. A quantification of the contribution of dehydrogenation to the total activity can be obtained from determination of the hydrogen gas in the tail gas. This value is s o m e w h a t uncertain, as reaction (5) from Scheme 2 will positively contribute and the consecutive hydrogen + oxygen reaction to water will negatively contribute. The data in the inset of Figure 7 indicate the contribution of several effects to the hydrogen yield. The change in slope at 873 K coincides with the occurrence of the (y) oxygen species (see Figure 4). The high t e m p e r a t u r e branch is attributed to the dehydrogenation with (~,) oxygen (reaction 3 in Scheme 2). The low t e m p e r a t u r e branch with low total conversion and significant evolution of CO2 contains significant contributions from formate decomposition to the hydrogen production. Evidence for the presence of several intermediates with significantly different residence times on the surface of silver was gained from a non-steady state conversion experiment. Electrolytic silver after 200 h time on stream was loaded at 870 K with pure oxygen [25,24]. After purging with nitrogen a stream of methanol was injected in the nitrogen stream from time 0 to 100 s and the

114 e v o l u t i o n of products was monitored every 40 s. The normalized response curves in Figure 8 indicate t h a t metallic silver cannot only store oxygen but also activated methanol in CO2 significant quantities X - " 6.4 T= 680 K ~. . . . HOHO~ on its surface. This is unexpected in the light of the failure to analyze the bonding I state of adsorbed methanol on silver at ! ! high temperatures ! I [17] with surface analytical tools where ! molecular methanol was found to desorb I I I 1 above ca. 200 K. The 32 33 34 35 product curves show Time (min) t h a t the Figure 9: Rate oscillations over copper chips. The methanol : oxygen oxygen-covered ratio was 6.4. Set temperature 680 K. SV 250 h~. surface is highly active for total oxidation. With progressing removal of the surface oxygen species [19] the selective partial oxidation wins with most likely a sizable contribution from dehydrogenation which does not require a stoichiometric a m o u n t of oxygen and can thus produce formaldehyde in a 100 situation lean in ~ ~ , ~ k H2CO surface oxygen (after ,, 80 ca. 300 s in Figure 8). The reaction product formic acid / / \ / \ \ sToK 60 from methoxy-toformate oxidation 40 does not form in parallel with the 20 abundance of methoxy (formaldehyde) nor C with the abundance of 0 1O0 200 300 400 500 Time (s) oxidizing surface oxygen. It occurs as final product from an Figure 8: Response of a 100 s pulse of pure methanol over oxygeni n t e r m e d i a t e residing predosed electrolytic silver. SV: 82000 h~. The response function of the non-reacting system (with SiC) was smaller than the CO2 profile, long on the surface as re-adsorption of m e t h a n o l is highly unlikely 300 s after the end of the pulse in a s t r e a m i n g system. The methoxy intermediate is, however, a precursor to this state as its

',: Ii

]

•

115 population vanishes with the removal of the methoxy species responsible for gas phase formaldehyde. The copper catalyst [6,30] offers an additional way of analyzing the contribution of multiple pathways to the practical reaction by studying the regime of oscillatory behavior. For a wide range of compositions temperaturecontrolled rate oscillations [52] can be observed in the methanol partial oxidation reaction. The origin of this unstable activity is the thermally driven interchange from an inactive bare copper metal surface to an oxygen-covered state which is triggered by oxygen being dissolved in the bulk [53]. This state leads easily to a binary oxide which is only active for total 1.5 oxidation. The rate """~ T = 620 K Cu-L3 oscillations for hydrogen, _ ~ O-K o,s,,o SS'* '~t ? 1.4 formaldehyde and CO2 are presented in high temporal resolution in 1.3 j," C u L III '~~~/ Figure 9. The contribution of several . ,,, 1.2 sources to the hydrogen production can clearly be I1 ~ ~ 1.1 seen. In the beginning of each~ oscillation period "I" 1, I I I I hydrogen and CO2 evolve 0 0.1 0.2 0.3 0.4 0.5 from decomposition of CHsOH- Pressure (mbar) formate. The selective Figure 10: Integrated normalized intensities of the first resonances oxidation via oxidative of Cu L II and 0 K for copper foil treated insitu with a mixture of dehydrogenation and via methanol in oxygen gas. dehydrogenation gradually wins over the total oxidation indicating the gradual modification of the copper-oxygen chemical bonding. This interpretation of the oscillations requires again the existence of chemically inequivalent oxygen species present at the copper surface in atomic form. Using X-ray absorption spectroscopy it was possible to substantiate the dependence of the chemical bonding between oxygen and copper on the chemical potential of the gas phase. Figure 10 shows the systematic increase in the spectral weight of the ~* resonance with decreasing oxidation potential of the gas phase (modified by the addition of methanol to the oxygen atmosphere at 50 mbar). The simultaneous change of the Cu L II white line indicates the gradual transition in bonding character from strongly interacting with rehybridised states (maximum in Cu sub-oxide, higher than in Cu II oxide) to non-bonding atomic-like with a weak O 2p-Cu 4s-p interaction. The reaction conditions imply that this state is responsible for the oxidative dehydrogenation. .

.

.

.

.

=(',,

y

OK

~,

116

7. Heteropoly Acids The discussion so far has shown that several surface oxygen species coexist with a bulk-dissolved species in metallic catalysts. The HPA as molecular analogue of complex oxide catalysts possesses three types of oxygen species namely surface terminating oxygen, surface bridging oxygen and bulk bridging oxygen. The latter species are possibly similar to bulk mobile anionic species that replenish defective oxygen sites where an organic substrate has removed an atom from a surface bridge or terminating site [54,55,56]. Aspects of scientific dispute [57,58,59,60,61] are the actual structure of the HPA anion in its active state which may be an opened Keggin cage (,,lacunary form"), the amount of water present under reaction conditions and the location of the excess electronic charges during a catalytic cycle. It has to be mentioned that the structural definition of the reacting HPA is difficult [62,63,64,58] despite its ,,molecular" character. The catalytically most relevant partially cation-exchanged derivatives [65,27,66] of the free acid forms are difficult to structurally assess as the apparently simple cubic X-ray diffraction pattern needs a detailed evaluation to disclose the real structure. For example four different models have been proposed for partly cation exchanged derivatives of H4PVMollO40. Here we concentrate on the free-acid form to avoid any interference with these structural problems. Under reaction conditions for methanol selective oxidation the HPA is nominally in its anhydrous form [63,64,67] which is metastable with respect to irreversible phase separation under prolonged catalytic reaction. Pulse experiments with varying delay times are suitable to follow the reactivity pattern. A single pulse response with a weakly oxidizing feed is presented in Figure 11. The most prominent effect is the strong capacity of the HPA to store the reaction water from the oxidative dehydrogenation reaction. Under reaction conditions the catalyst contains water of insufficient abundance, however, to

117 transform the anhydrous structure into the nominal 2-hydrate form [63,64]. The selective oxidation of methanol can proceed without gas-phase oxygen as seen from the pulse profile for formaldehyde in Figure 11. The dehydration to dimethyl ether occurs only in parallel to the presence of oxygen from the gas phase indicating a change in the surface acidity when the lattice oxygen reservoir is depleted. A series of 100 consecutive pulses was insufficient to consume all lattice oxygen available to the reaction at 573 K revealing the participation of deep volumes of the catalyst which should consist of a molecular crystal. Under reaction conditions the compacted crystal structure of the anhydride seems to allow oxygen transport to the surface. A series of pure oxygen pulses shown in Figure 12 allowed the replenishment of the lattice-oxygen reservoir. In addition, the total oxidation of strongly chemisorbed intermediates was indicated by the formation of a small amount of CO2. A measurable amount of formaldehyde was found only with the very first oxygen pulse indicating t h a t methoxy represents only a small proportion of the organic surface coverage. A significant abundance of water (see Figure 12) was liberated during the oxygen regeneration indicating that the catalyst was able to store hydrogen. This shows that also on the oxide catalyst a significant activity for dehydrogenation occurs which is not detected in steady-state measurements due to the consecutive hydrogen+oxygen reaction. A series of methanol pulses showed t h a t the catalyst converted it with lattice oxygen into formaldehyde and t h a t in a slow process some methanol was stored together with the reaction water. No liberation of the stored methanol with the gradually beginning water desorption was observed. The methanol seemed to be strongly adsorbed and to be converted into reaction products. All these findings show t h a t the bulk of the HPA crystal is actively involved in the catalytic action as it can store water, polar molecules, hydrogen and oxygen. Insitu UV-VIS spectroscopy [57] is a suitable tool to follow geometric structural integrity and electronic structural modification as function of reaction parameters. Figure 13

118 s u m m a r i z e s some results. The spectra exhibit two high energy m a x i m a at 250 and 310 nm characteristic of oligo-molybdate and the intact Keggin-ion (disappears upon opening of the cage structure). In its pristine (yellow) state the catalyst shows no other absorption. Upon h e a t i n g in oxygen or u n d e r inert conditions the loss of hydration w a t e r occurs (see inset) which is accompanied w i t h a slight electronic reduction as seen from peaks at 680 nm and at 1330 nm. These w e a k structures are due to intervalence charge t r a n s f e r bands from oxov a n a d i u m structures and do not occur with vanadium-free HPA. The inset shows the unexpected correlation of reduction and dehydration t r a n s f o r m i n g the HPA into an active state with the partially reduced electronic structure which is also reflected in the broadening of the Mo-O charger transfer bands. Cooling in w a t e r vapor restores fully the initial state indicating the reversibility of the slight reduction. After 723 K in oxygen and w a t e r vapor, the reduction is irreversible as seen by the change of the 310 nm absorption indicating a partial b r e a k d o w n of the Keggin ion. U n d e r m e t h a n o l vapor and conversion to formaldehyde the s a m e spectrum as u n d e r He at elevated t e m p e r a t u r e s can be seen. The intensity of the peaks depends critically on the methanol to oxygen ratio indicating t h a t this intensity can be used to monitor the degree of 1.2 .~ ~ . ~ . ~ bulk reduction of the ~'/~ catalyst or the 1

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600

800 1000 1200 Wavelength (mn)

1400

1600

1800

involvement of lattice oxygen. Figure 14

reports such an experiment in w h i c h the methanol-tooxygen ratio was varied from 10:1 to 1:2 without a significant change in reactivity. The 680 nm

absorption scales

very well with the Figure 13: Insitu UV-VIS spectra of HPA under various conditions, modified oxygen The inset compares the water TDS (thin) with UV absorption at 1330 abundance in the gas nm (thick) revealing reduction during dehydration, phase and is fully reversible in its intensity upon re-oxidation. The time scale for reversible reaction is, however, longer (6000 s) t h a n the waiting times in Figure 14. A slow diffusion controlled process of anion migration through a lattice is a likely explanation for the response characteristics observed. In s u m m a r y , the d a t a confirm the involvement of the bulk of the HPA crystals in the catalytic activity. They support the conjecture t h a t the v a n a d i u m acts as local sink for electrons from the catalytic cycle. The slow and highly activated oxygen m i g r a t i o n plays a practical role in catalytic conversion which is very m u c h more selective with lattice oxygen t h a n with gas phase oxygen. W a t e r is

119

beneficial at least in helping to maintain the structural integrity. At high levels of hydration which are structurally relevant, water suppresses the catalytic activity in this reaction. Thermal load alone leads to a slight reduction of the catalyst which is seen as beneficial for the function as it will enhance the sticking coefficient of molecular oxygen.

7. C o n c l u s i o n s The experiments have shown that mechanistic aspects discussed in the surface-science literature of methanol oxidation are of relevance under highpressure high-temperature conditions. Surface science has provided the techniques and fundamental insight into species present on metal surfaces. The remote reaction conditions do not allow the assignment of the stable intermediates which were spectroscopically characterized as the reaction intermediates. Experiments at conditions closer to practical conversion have revealed a novel oxygen species and cast doubt on the relevance of a stable methoxy species for catalysis. Several species of atomic chemisorbed I A I B I C I B I A oxygen were 0,2identified with surface science tools 0.15 and assigned to specific functions. n" ~" 0.1 These species occur on all catalysts investigated. Their 0.05 -02/MeOH structural disposition is, however, quite 0 20 40 60 80 1 O0 120 140 different in the Time (min) materials studied. The most relevant Figure 14: Evolution of the UV absorption for reduced V centers with different insitu reaction conditions. From times A to C the oxygen to problem for surface science remains to methanol ratio was reduced produce structural details of terminating and defective oxide surfaces which would be needed for meaningful theoretical approaches to the problem of selective oxidation. The key to understand selective oxidation is to characterize the chemical bonding of various atomic-oxygen species interacting with metal ,,ions" in quite distinct ways. Several examples of characterizations were discussed in this text. The degree of d-state interaction of the oxygen 2 p states decides over the covalent or anionic bonding character which translates into a basic or oxidizing function. In the presence of hydrogen the metal-to-oxygen bonding determines further the acidity of hydroxyl groups which can open additional reaction channels by electrophilic activation of the C-O bond. In all three catalyst systems ,

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120 investigated, the bulk plays an active and catalytically relevant role by providing reservoirs of oxygen atoms which control the type and abundance of surface oxygen species.. The reaction intermediates methoxy and formate seem to occur in two forms each. One is very active and carries the reaction under steady-state conditions. The other is more a spectator species with increased binding energy to the substrate which can, however, undergo complex side reactions and can contribute to the selectivity spectrum of the overall process. The external reaction conditions affect sensitively the co-operation of the dehydrogenation and oxidative dehydrogenation pathways leaving room for the speculation to devise a technical process for undiluted formaldehyde production at reasonable conversions.

References 1 G. Reuss, W. Disteldorf, O. Grler A. Hilt, Formaldehyde, in: Encyclopedia of Industrial Chemistry, Vol. Al1., Verlag Chemie 1988, S. 619-651. 2 H. Sperber, Chemie-Ing.-Techn. 41 1969, S. 962-966. 3 M. A. Barteau R. J. Madix, The Surface Reactivity of Silver: Oxidation Reactions, in: The Chemical Physics of Solid Surf and Heterogeneous Catal. D. A. King B. P. Woodruff, Ed., Vol./4, Elsevier 1982, S. 95-142. 4 M. A. Barteau, M. Bowker R. J. Madix, Surf Sci. 94 1980, S. 303-322. 5 M. A. Barteau R. J. Madix, Surf Sci. 97 1980, S. 101-110. 6 I. E. Wachs R. J. Madix, J. Catal. 53 1978, S. 208-227. 7 L. Lefferts, J. G. van Ommen J. R. H. Ross, Appl. Catal. 23 1986, S. 385-402. 8 C. T. Campbell, Surf Sci. 157 1985, S. 43-60. 9 M. Bowker, M. A. Barteau R. J. Madix, Surf. Sci. 92 1980, S. 528-548. 10 G. R. Meima, L. M. Knijf, A. J. van Dillen, J. W. Geus, J. E. Bongaarts, F. R. van Buren K. Delcour, Catal. Today 1 1987, S. 117-131. 11 R. J. Madix M. A. Barteau, Surf Sci. 97 1980, S. 101-110. 12 B. A. Sexton, G. B. Fisher J. L. Gland, Surf Sci. 95 1980, S. 587-602. 13 T. Seiyama, Surface Reactivity of Oxide Materials in Oxidation-Reduction Environment, in: Surface and Near-Surface Chemistry of Oxide Materials, Materials Science Monographs L.-C. Dufour J. Nowotny, Ed., vol 47, Elsevier, Amsterdam 1988, S. 189-215. 14 X. Bao, J. Deng S. Dong, Surf. Sci. 163 1985, S. 444-456. 15 V. I. Bukhtiyarov, A. I. Boronin, I. P. Prosvirin V. I. Savchenko, J. Catal. 150 1994, S. 268-273. 16 V. I. Bukhtiyarov, A. I. Boronin O. A. Baschenko, Surf Rev. Lett. 1 1994 Nr. 4, S. 577-579. 17 C. Rehren, M. Muhler, X. Bao, R. SchlSgl G. Ertl, Zeitschrift f Phys. Chemie 174 1991, S. 11-52. 18 X. Bao, M. Muhler, B. Pettinger, R. SchlSgl G. Ertl, Catal. Lett. 22 1993, S. 215-225. 19 X. Bao, M. Muhler, Th. Schedel-Niedrig R. SchlSgl, Phys. Rev. B 54 1996 Nr. 3, S. 2249-2262.

121 20 C. Rehren, G. Isaak, R. SchlSgl G. Ertl, Catal. Lett. 11 1993, S. 253. 21 H. Schubert, U. Tegtmeyer, D. Herein, X. Bao, M. Muhler R. SchlSgl, Catal. Lett. 33 1995 Nr. 3-4, S. 305-319. 22 G. Rovida, F. Pratesi, M. Maglietta E. Ferroni, Surf. Sci. 43 1974, S. 230-256. 23 K. C. Prince, G. Paolucci A. M. Bradshaw, Surf. Sci. 175 1986, S. 101-122. 24 D. Herein, A. Nagy, H. Schubert, G. Weinberg, E. Kitzelmann R. SchlSgl, Z. Phys. Chem. 197 1996, S. 67-96. 25 X. Bao, B. Pettinger, G. Ertl R. SchlSgl, Ber. Bunsenges. 97 1993, S. 322-325; Phys. Chemie. 26 T. Komaya M. Misono, Chem. Lett. 1983, S. 1177-1180. 27 J.-M. Tatibouet, M. Che, E. Serwicka, J. Haber K. Brfickmann, J. Catal. 139 1993, S. 455-467. 28 C. Gleitzer, Solid State Ionics 38 1990, S. 133-141. 29 A. Ferretti L. E. Firment, Surf. Sci. 129 1983, S. 155-176. 30 A. F. Carley, A. W. Owens, M. K. Rajumon M. W. Roberts, Catal. Lett. 37 1996, S. 79-87. 31 , Surface Reactions, in: Springer Series in Surface Sciences R. J. Madix, Ed., Vol. 34, Springer 1994, S. 1-282. 32 S. T. Oyama, Heterogeneous Hydrocarbon Oxidation, in: ACS Symposium Series B. K. Warren S. T. Oyama, Ed., Vol. 638, American Chemical Society 1996. 33 A. Bielanski, A. Cichowlas, D. Kostrzewa A. Malecka, Z. Phys. Chem. NF. 167 1990, S. 93-103. 34 P. L. Gai-Boyes, Catal. Rev.-Sci. Eng. 34 1992 Nr. 1-2, S. 1-54. 35 M. Ritter, H. Over W. WeiB, Surf. Sci. 371 1997, S. 245. 36 J.M. Tatibouet, J.E. Germain, J.C. Volta, J. Catal. 82 1983, S. 240-244. 37 J.M. Tatibouet, J.E. Germain, J. Catal. 72 1981, S. 375-378. 38 R. S. Weber, J. Phys. Chem. 98 1994, S. 2999-3005. 39 S. T. Oyama, W. L. Holstein W. Zhang, Catal. Lett. 39 1996, S. 67-71. 40 F. Lazzerin, G. Liberti, G. Lanzavecchia N. Pernicone, J. Catal. 14 1969, S. 293-302. 41 M. A], J. Catal. 54 1978, S. 426-435. 42 A. Desikan, S. T. Oyama W. Zhang, J. Phys. Chem. 99 1995 Nr. 39, S. 1446814476. 43 S. Ted Oyama W. Zhang, J. Phys. Chem. 1996 100, S. 10759-10767. 44 R. Miranda, C. O. Bennett J. S. Chung, J. Chem. Soc., Faraday Trans. 1 81 1985, S. 19-36. 45 J. E. Germain, Structure-Sensitive Catalytic Reactions on Oxide Surfaces, in: Adsorption and Catalysis on Oxide Surfaces G. C. Bond M. Che, Ed., Vol. 21, Elsevier Science Publishers, Amsterdam 1985, S. 355-368. 46 X. Bao, G. Lehmpfuhl, G. Weinberg, R. SchlSgl G. Ertl, J. Chem. Soc. Faraday Trans. 88 1992 Nr. 6, S. 865-872. 47 X. Bao, J. V. Barth, G. Lehmpfuhl, R. Schuster, Y. Uchida, R. SchlSgl G. Ertl, Surf. Sci. 284 1993, S. 14-22. 48, in: Surface and Near-Surface Chemistry of Oxide Materials, Materials Science Monographs L.-C. Dufour J. Nowotny, Ed., Vol. 47, Elsevier, Amsterdam 1998.

122 49 H. Schubert, U. Tegtmeyer R. SchlSgl, Catal. Lett. 28 1994 Nr. 2-4, S. 383-395. 50 M. Bowker, S. Poulston, R. A. Bennett A. H. Jones, Catal. Lett. 43 1997, S. 267-271. 51 W. Zhang S. T. Oyama, J. Am. Chem. Soc. 1996 118, S. 7173-7177. 52 D. Herein, G. Schulz, U. Wild, R. SchlSgl H. Werner, Catal. Lett. submitted 1997. 53 M. Lenglet, K. Kartouni, J. Machefert, J. M. Claude, P. Steinmetz, E. Beauprez, J. Heinrich, N. Celati, Mater. Res. Bull. 30 1995 Nr. 4, S. 393-403. 54 Oxygen in Catalysis J. Haber A. Bielanski, Ed., Marcel Dekker, Inc., New York 1991, S. 1-467. 55 K. Brfickmann, M. Che J. Haber J. M. Tatibouet, Catal. Lett. 22 1994, S. 241255. 56 G. Ya. Popova, T. V. Andrushkevich, V. M. Bondareva I. I. Zakharov, Kinet. Catal. 35 1994, S. 80-83. 57 M. Fournier, C. Louis, M. Che, P. Chaquin D. Masure, J. Catal. 119 1989, S. 400-414. 58 M. Fournier, A. Aouissi C. Rocchiccioli-Deltcheff, J. Chem. Soc., Chem. Commun. 1994, S. 307-308. 59 B. Herzog, M. Wohlers R. SchlSgl, Microchimica Acta 14 1997, S. 703-704. 60 V. M. Bondareva, T. V. Andrushkevich, R. I. Maksimovskaya, L. M. Plyasova, A. V. Ziborov, G. S. Litvak L. G. Detusheva, Kinet. Catal. 35 1994, S. 114-119. 61 E. Cadot, C. Marchal, M. Fournier, A. T~z~ G. Herv~, Role of Vanadium in Oxidation Catalysis by Heteropolyanions, in: Polyoxometalates M. T. Pope A. Mfiller, Ed., Kluwer Academic Publisher 1994, S. 315-326. 62 H. T. Evans jr. M. T. Pope, Inorg. Chem. 23 1984, S. 501-504. 63 Th. Ilkenhans, B. Herzog, Th. Braun R. SchlSgl, J. Catal. 153 1995 Nr. 2, S. 275-292. 64 B. Herzog, T. Ilkenhans R. SchlSgl, Fresenius J. Anal. Chemie 349 1994, S. 247-249. 65 N. Essayem, G. Coudurier, M. Fournier J. C. V~drine, Catal. Lett. 34 1995, S. 223-235. 66 Y. Toyozawa, N. Yamazoe, T. Seiyama K. Eguchi, J. Catal. 83 1983, S. 32-41. 67 B. Herzog, W. Bensch, Th. Ilkenhans, R. SchlSgl N. Deutsch, Catal. Lett. 20 1993, S. 203-219.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama,A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

123

Gold as a l o w - t e m p e r a t u r e o x i d a t i o n c a t a l y s t : f a c t o r s c o n t r o l l i n g activity and selectivity M. Haruta Osaka National Research Institute, AIST Midorigaoka 1-8-31, Ikeda, Osaka 563, Japan

The catalytic activity and selectivity of gold for oxidation can be widely tuned by control of three major factors ; selection of type of metal oxide supports, size of the gold particles, and control of the contact structure of the gold particles with the supports. These factors are markedly influenced by the preparation method. Examples are presented which show surprisingly high activities for CO oxidation at temperatures as low as -77~ and excellent selectivities to propylene oxide in the partial oxidation of propylene. It is found that there is a critical particle size of gold around 1-2 nm where the catalytic nature of the supported gold changes dramatically.

1. I N T R O D U C T I O N

Gold has attracted little attention as a catalyst because of its inert character and low melting point (1063~ which causes difficulties in depositing gold on supports with high dispersion. In the past, gold catalysts were considered not to be competitive with other noble metal catalysts in terms of activity [1, 2]. However, we have found remarkably high catalytic activity in the lowtemperature oxidation of CO with some supported gold catalysts prepared by coprecipitation [3, 4]. Since this work catalysis by gold has received growing attention and is currently investigated by many research groups [5]. The catalytic properties of supported gold catalysts appear to change dramatically when different metal oxide supports or different preparation methods are used. From this standpoint, supported gold catalysts behave differently from platinum-group metal catalysts. This paper attempts to summarize major factors which determine catalytic activity and selectivity in supported gold catalysts.

124

2. EXPERIMENTAL 2.1. Preparation of highly dispersed gold catalysts The catalytic properties of Au depend markedly on the preparation method, because it can bring about a large difference in the size of Au particles and the interaction with supports. Until now six methods, including solid-, liquid-, and vapor-phase techniques, have proven effective for preparing active gold catalysts. Amorphous alloys of gold produced by arc-melting can be transformed to highly dispersed gold particles that interact strongly with the oxides formed from the alloy counter metals during catalytic reaction or through oxidizing treatment. A good example is presented by Shibata, et al. for an Au-Zr alloy used to prepare AufZrO2 [6]. Coprecipitation [4] is useful for the preparation of powder catalysts, especially in the case of Au/a-Fe203, Au/CoaO4, Au/NiO, and Au/Be(OH)2, which are extraordinarily active for the low-temperature oxidation of CO. Active AufPiO2 catalysts can also be prepared by coprecipitation, but only in the presence of Mg citrate [7]. Deposition-precipitation [8] is effective for depositing gold with high dispersion on MgO, TiO2, and A1203. This method is applicable to any form of support including beads, honeycombs, and thin films. In principle, support materials are required to have high specific surface areas, most desirably, larger than 50 m2/g. Since gold hydroxide does not deposit at low pH, this method is not applicable to acidic metal oxides having low points of zero charge, for example, SiO2. The surface reaction of [Au(PPh3)](NO3) with the OH groups of freshly prepared metal hydroxides has recently been reported by the group of Iwasawa [9]. This process is especially useful for the preparation of Au/MnOx, which is active for CO oxidation in air. Chemical vapor deposition of organic gold complexes, typically, dimethylgold (m) ~-diketone, is applicable to the widest range of metal oxides [10]. This technique can be easily used with high surface area materials. It can also be used to deposit Au as nanoparticles even on acidic metal oxides, including M CM41 [11]. Co-sputtering [12] is used for preparing clean thin films which are applicable as gas sensors. A target composed of an Au plate and a metal oxide disk is sputtered in 0.4 Pa of oxygen to deposit on a glass substrate heated at 250~ Oblique deposition on a rotating substrate produces columnarstructured porous thin films.

2.2. Catalytic activity measurements Catalytic activity measurements were carried out in a fixed bed quartz reactor of inner diameter 6 ram. The catalyst (usually 200 rag, 125-212 gm fraction) was placed between two glass wool plugs with a bed length of 10-20 ram. For the oxidation of CO and H2, a standard gas containing 1 vol% CO or H2 in air was dried in a silica gel column cooled down to 0~ or -77~ and passed through the catalyst bed at a space velocity of 20,000 h-lml/g-cat.. For the partial

125 oxidation of propylene a mixture of propylene, oxygen, hydrogen, and argon, most frequently, in a volume ratio 1:1:1:7, was passed through the catalyst bed without drying at a space velocity of 2000 h-lml/g-cat.. 2.3. Catalyst characterization Characterization of the catalysts was made using high-resolution transmission electron microscopy (TEM) (Hitachi H-9000), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and Fourier transform-infrared spectroscopy (FT-IR).

3. RESULTS 3.1. CO oxidation Gold supported on metal oxides and hydroxides is active for CO oxidation at temperatures below 0~ irrespective of the type of metal oxide support when gold is deposited as nanoparticles. Table I shows three groups of gold catalysts which are classified in terms of stability, activity, and requirement as the size of gold particles for activity.

Table 1 Classification of supported gold catalysts for low temperature CO oxidation. Group Stability and Support Preparation Size Activity Methods Requirement less stable but A the most active Be(OH)e, Mg(OH)e CP, DP Au clusters at -77 ~C hydroxides D<= lnm most stable and TiO2, Fe203, Co304, NiO active at -77~ semiconducting & reducible CP, DP Au particles metal oxides D<10nm less stable or A1203, SiO2, ZrO2 less active insulating & non-reducible DP, CVD, CP Au particles at -77~ metal oxides D<10nm DP: deposition precipitation, CP: coprecipitation, CVD: chemical vapor deposition B

Gold supported on Be(OH)2 [13] and Mg(OH)2 [14] (group A) exhibits surprisingly high activity even at-77~ but only when gold remains as clusters of size about 1 nm and the supports are in the form of hydroxides. The second group (group B) is represented by semiconducting and reducible metal oxide supports [4, 15]. They are thermally more stable, but have catalytic activities at -77~ a little inferior to those of group A. The third group (group C)

126 comprises gold supported on insulating and non-reducible metal oxides [10, 11]. It is often difficult to deposit gold with high dispersion on these metal oxides, especially on SiO2, however, once gold is deposited as nanoparticles with diameters smaller t h a n 10 nm catalytic activity is observed at temperatures even below-50~ It appears that the gold catalysts of this group are either less stable during exposure to air (Au/SiO2) or less active at low temperatures below -50 ~C (Au/A1203). Table 2 shows the optimum calcination conditions to prepare active AtffMg(OH)2 catalysts [14]. When gold is atomically dispersed over Mg(OH)2 having no Au-Au coordination or gold is deposited on MgO as metallic particles havingAu-Au coordination numbers close to that of bulk gold, it is inactive[16].

Table 2 Catalytic activity, XRD and EXAFS analyses of Au/Mg(OH)2 as a function of calcination temperature. Calc. Temp. Catalytic Activity XRD Au*** ~C CO(rain)* H2(~C)** Coord. No. 200 250 280 300 400

13 113 Mg(OH)2 2000< 67 Mg(OH)2 2.8 720 88 Mg(OH)2 2.4 0 200< Au/MgO 10.4 0 200< Au/MgO 12.0 * Period of 100% conversion at -70 ~C ** Temperature for 50% conversion *** Coordination number of Au-Au bonding (2.73-2.83A) Feed gas: lvol% CO or H2 in air, 2x104 h -1. ml/g-cat.

Debye function analysis (DFA) obtained through computer simulations of XRD patterns for gold species in Au/Mg(OH)2 calcined at 280~ in air gives the probable size and structure distributions of Au for a fresh sample and an aged sample which has lost activity over a period of three to four months (Figure 1) [17]. It is suggested that gold clusters of 13 and 55 atoms with icosahedral and face centered cubic cuboctahedral structures found when Mg is in hydroxide form are responsible for the low temperature CO oxidation. These results may present an interesting correlation with the reactivity changes of Au clusters with different atoms and charges in the gas phase [18]. The coordination number of icosahedral and cuboctahedral clusters of 13 atoms is five and is larger than the values obtained from EXAFS measurements for the samples calcined at 250~ and 280 ~ This discrepancy suggests that smaller gold clusters composed of less than 10 atoms may also exist in fresh active catalysts.

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Figure 1. Mass fractions of clusters corresponding to (a) the DFA of freshly prepared (1 week old) Au/Mg(OH)2 and (b) aged Au/Mg(OH)2 (3 months). The solid line gives the corresponding non-discrete distribution of mass. The influence of preparation methods on the catalytic properties of supported gold is markedly large, whereas it is marginally small on supported Pt [19]. In Table 3 are listed the mean particle diameters of Pt and Au, rate of CO oxidation at 27~ turnover frequency (TOF) based on surface exposed metal atoms calculated from the mean particle diameter and actual metal loading, and apparent activation energy. TOFs for AudiO2 prepared by depositionprecipitation are larger by about one order of magnitude than those for Pt catalysts and by about 4 order of magnitude than those for AufFiO2 prepared by photochemical deposition. The apparent activation energies also provide an interesting contrast: for all Pt catalysts and A u catalysts prepared by impregnation and photochemical deposition they are in the range of 50-60 kJ/mol, while for Au catalysts prepared by deposition-precipitation they are

128 around 20 kJ/mol.

Table 3 CO oxidation over P t f r i 0 2 and Aufrio2 Metal P r e p a r a t i o n Loading Size Method wt% nm DP 1.0 1.3 IMP 1.0 1.4 Pt FD 0.9 2.4 DP 0.7 3.1 tt 1.8 2.7 Au IMP 1.0 ? FD 1.0 4.6

Rate at 27 ~C mol.s-l.g i 1.4x10 -7 1.9x10 -7 2.4x10 -s 6.9x10 -7 5.5x10 -6 1.7x10 -1~ 1.5x10 -10

TOF at 27 ~C s -1 2.7x10 -3 3.8x10 -3 9.2x10 -3 3.4x10 -2 1.2x10 -1 .... 9.6x10 -6

Ea kJ/mol 49 60 53 19 18 58 56

DP: deposition-precipitation, IMP: impregnation, FD: photochemical deposition

3.2. Partial oxidation of propylene Table 4 shows t h at, in the presence of both oxygen and hydrogen, propylene is converted to propylene oxide(PO) with selectivities above 90% over Aufrio2 [20]. Carbon dioxide is a m a i n by-product. Over P d f r i o 2 and PtfriO2, in contrast, propane is m a i n l y produced. It should also be noted t h a t AufriO2 prepared by impre g n a t i o n is poorly active and produces mainly CO2 at higher t e m p e r a t u r e s . The oxidation of propylene in the absence of hydrogen occurs only at t e m p e r a t u r e s above 250~ to produce a l m o s t exclusively CO2, while the oxidation of hydrogen is r e t a r d e d by the presence of propylene. This implies t h a t the presence of hydrogen not only enhances the oxidation of propylene but

Table 4 Reaction of propylene with oxygen and hydrogen over Au/, Pd/, P t f r i 0 2 catalysts Catalyst Metal T Conversion, Selectivity, % PO PO STY a % yield wt% ~C C~I6 H2 PO acetone C~Is C02 % retool/h/g-cat Aufr i o 2 b 1 50 1.1 3.2 >99 1.09 0.18 Aufrio2 c 1 80 0.2 8.9 <10 >70 0.00 0.00 Pdfrio2 c 1 25 57.1 97.7 - 0.4 98 1 0.00 0.00 Ptfri02 c 1 25 12.1 86.6 2 92 6 0.00 0.00 Feed gas: propylene/~e/Ar=lO/lO/lO/70 (vol%), flow rate: 2,000 ml/hr, catalysts: 0.5g. a space time yield, b p r e p a r e d by deposition-precipitation, cprepared by impregnation.

129 also leads to the selective partial oxidation to the epoxide. Dioxygen may be transformed, probably at the perimeter interface of TiO2, through reduction with hydrogen to an active species, possibly hydroperoxo species, that can selectively oxidize propylene adsorbed on the surface of the Au particles. It is surprising that the reaction pathway switches from oxidation to hydrogenation at specific Au metal loadings. Figure 2 shows that while partial oxidation occurs at Au loadings above 0.2 wt%, below this amount the reaction switches to hydrogenation to produce propane[21]. Hydrogenation takes place with higher turnover frequencies at temperatures below 100~ and the selectivity to propane reaches 100 %. This result appears consistent with the result reported by Naito, et al. for Au/SiOe catalyst, where using very low Au loadings, it was found that the hydrogenation of olefin was accelerated by the presence of oxygen [22]. Although the number of gold particles that could be observed by TEM was not large for the catalysts with Au loadings smaller than 0.1 wt%, the main difference between the 0.4 wt% and 0.1 wt% samples appear to be whether the mean particle diameter of Au is above or below 2.0 nm (Figure 3). Taken together these results indicate that gold particles larger than 2.0 nm are primarily responsible for the formation of propylene oxide, whereas those below

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130 2.0 nm catalyze the formation of propane. It is also probable t h a t the presence of Au clusters, smaller t h a n 1.0 nm, which could not be observed by TEM, may contribute to hydrogenation. In order to improve the conversion of propylene while m a i n t a i n i n g selectivity to propylene oxide above 90%, TiO2 or Ti cations were dispersed on high-surface area supports. This i s to obtain a high density of active sites which would be, nevertheless, sufficiently separated from each other. It was expected t h a t higher conversions of propylene and the depression of successive oxidation of propylene oxide to CO2 would result. Both TiO2 deposited on SiO2 (specific surface area, 310 m2/g) and Ti-MCM supports for Au gave slightly improved conversions but with different time-on-stream behavior. TiO2 on SiO2 showed a decreasing conversion with time while Ti-MCM showed gradually increasing conversion reaching a steady s t a t e value after l h [23]. It is likely that the reaction pathways are different for Au supported on TiOJSiO2 t h a n for Au supported on Ti-MCM.

3.3. Spectroscopic

investigations

FT-IR spectroscopy clearly shows that the reactants, CO and propylene, are moderately adsorbed on the surface of Au particles. The presence of water has either no effect or an enhancing effect on the adsorption. The two absorption bands for carbonyl on the Au surface (2106-2118 cm-1 and 2090-2110 cm-1) indicate that there are the neutral and the positively charged surface of Au [24]. The Au atoms at the perimeter interface with the support (TiO2) are probably positively charged and can activate oxygen molecules. The TPD spectra in Figure 4 show that the deposition of Au on TiO2 appreciably increases the intensity of the desorption peak of oxygen. The desorption temperature at around 250~ suggests t h a t the surface oxygen species is weakly adsorbed on the surface. The desorption spectra of CO from AufriO2 show t h a t the amount of CO taken up is increased by Au deposition on TiO2 and t h a t the desorption always takes place as CO2 even at 50~ P r e t r e a t m e n t of the catalyst sample in a stream of He instead of 02 decreases the amount of CO t a k e n up and causes the desorption to take place as CO in the lower temperature region. These results indicate t h a t the uptake of CO involves oxygen adsorbed on the surface of Aufrio2.

4. DISCUSSION The experimental results presented above clearly show t h a t there are three important controlling factors which determine the catalytic properties of gold supported on metal oxides for oxidation. The first is the selection of suitable m e t a l oxide supports. In the complete oxidation (combustion) of CO and nitrogen-containing organic compounds like trimethylamine gold catalysts are

131

Introd. CO pulse

(c)

m I

(d)

CO

(e)

0 Q 0 I=

02 p r e - t r e a t . (b)

x4 I

0

.,

100

I

I

200

300

400

Temperature, ~

Figure 4. TPD for Au/TiO2 (Au loading: 3.3 wt%, DAu: 3.5nm) and TiO2. Pretreatment in O2: (a) Au/TiO2 and (b) TiO2. Pretreatment in 02 or He followed by exposure to CO pulses up to full uptake: (c) Au/TiO2 in 02, (d) Au/TiO2 in He, and (e) TiO2 in 02. Heating rate: 10 ~C/min, rate of flow: 30 ml/min, sample weight: 200mg.

more active than Pt-group metal catalysts. For the combustion of trimethylamine, iron-based metal oxide supports gave the most active gold catalysts. Since iron oxides adsorb trimethylamine strongly, this suggests that metal oxides having a stronger affinity for the reactants provide higher catalytic activity. For CO oxidation, when gold is deposited as nanoparticles, almost all metal oxides can provide activity at temperatures below 0 ~C. In a previous paper [4] we reported that only select metal oxides, the oxides of 3d transition metals of group VIII (Fe, Co, Ni), gave catalytic activity at -77 ~C. However, this was only for the case where gold catalysts were prepared by coprecipitation. Since then, we have succeeded in depositing gold on SiO2 as nanoparticles and have found similar catalytic activities for CO oxidation at temperatures below 0~ Because a simple mechanical mixture of gold nanoparticles with diameter of 5

132 nm in colloidal dispersion and TiO2powder exhibits poor activity [25] and Au deposited on SiO2 exhibits lower activity when calcined at 300 ~C t h a n at 400 ~C [11], a strong interaction between gold particles and the metal oxide support appears to be necessary in the genesis of the low-temperature activity for CO oxidation. A second factor important for activity is the minimization of the size of gold particles. In general, the diameter of gold particles should be smaller than 10 nm for CO oxidation over Au supported on metal oxides. It is very interesting t h a t for CO oxidation over Au/Be(OH)2 and Au/Mg(OH)2 extraordinarily high catalytic activity is observed only when gold clusters of 13 to 55 atoms with icosahedral and fcc cuboctahedral structures are deposited on these metal hydroxides. This requirement as the size and structure of gold needs further investigation. The reaction of propylene with oxygen and hydrogen over Aufrio2 also indicate the existence of a critical size for gold, in this case, at around 2 nm. When gold particles are larger than 2 nm propylene oxide is selectively formed, while, when they are smaller than 2 nm only propane is formed. The results also indicate that hydrogen molecules can be dissociated on the surface of gold in the presence of oxygen. Since propane was not obtained from propylene and hydrogen in the absence of oxygen, oxygen might modify such small gold particles rendering them electron deficient so as to alow them behave like Pd and Pt. The third factor necessary to obtain high activity is to control the interaction of the gold particles with the metal oxide supports. The remarkably large influence of preparation methods on the catalytic properties of supported gold is observed for the oxidation of CO and H2. Over unsupported gold powder and gold deposited on SiO2 by impregnation, the oxidation of hydrogen takes place at lower temperatures than the oxidation of CO, whereas the opposite is the case over gold deposited on metal oxides (including SiO2) by deposition-precipitation and chemical vapor deposition. The turnover frequency of Aufrio2 prepared by deposition-precipitation for CO oxidation is larger by 4 orders of magnitude than that of Aufrio2 prepared by photo-deposition. These differences may arise from different contact structures at the periphery of the gold particles.. While poorly active gold catalysts are composed of spherical gold particles weakly interacting with the metal oxide supports, highly active gold catalysts are composed of hemispherical gold particles attached to the supports on their flat planes. This contact structure presents a longer distance around the perimeter interface of the gold particles. It is likely that the perimeter junction contains the sites for the activation of oxygen for CO oxidation and for the epoxidation of propylene.

133 5. CONCLUSIONS Gold supported on metal oxides and hydroxides has been prepared by several different methods and tested for CO oxidation and partial oxidation of propylene. The conclusions obtained are as follows: 1) The method of preparation has a considerable effect on the catalytic properties of supported gold. Coprecipitation, deposition-precipitation, chemical vapor deposition methods are especially effective for depositing gold as nanoparticles with diameters smaller than 5 nm and with strong interaction with the supports. 2) Over highly dispersed gold catalysts, CO oxidation can take place even at -77 ~C, and propylene oxide can be selectively produced at temperatures around 100 ~C. 3) For CO oxidation over gold supported on metal oxides, the catalytic activity is almost insensitive as to the type of metal oxide support but strongly depends on the strength of interaction with the supports. For the reaction of propylene with oxygen and hydrogen, only titanium-based oxide supports lead to the selective production of propylene oxide. 4) The size effect is dramatic in CO oxidation over Au/Be(OH)2 and Au/Mg(OH)2 and in the reaction of propylene with oxygen and hydrogen over Aufrio2 catalysts. In the latter case, a critical diameter of gold particles is 2 nm; above this value propylene oxide is selectively obtained, while below it propane is favored. 5) The catalytic nature of gold can be widely tuned by the three factors; type of support, size of the gold particles, and the structure of the contact interface between gold and the support.

REFERENCES

1. I.E. Wachs, Gold Bull., 16 (1983) 98. 2. J. Schwank, Gold Bull., 16 (1983) 103, and 18 (1985) 2. 3. M. Haruta, T. Kobayashi, H. Sano, and N. Yamada, Chem. Lett., (1987) 405. 4. M. Haruta, N. Yamada, T. Kobayashi, and S. Iijima, J. Catal., 115 (1989) 301. 5. M. Haruta, Catal. Today, 36 (1996) 153. 6. M. Shibata, N. Kawata, T. Masumoto, and H. Kimura, Chem. Lett., (1985) 1605. 7. M. Haruta, T. Kobayashi, S. Tsubota, and Y. Nakahara, Jap. Pat. 1778730 (1993). 8. S. Tsubota, D. A. H. Cunningham, Y. Bando, and M. Haruta, Preparation of Catalysts VI, G. Poncelet, et al. (Eds.) Elsevier, Amsterdam, pp.227-

134 235, 1995. 9. Y. Yuan, K. Asakura, H. Wan, K. Tsai, and Y. Iwasawa, Chem. Lett., (1996) 755 and Catal. Lett., 42 (1996) 15. 10. M. Okumura, K. Tanaka, A. Ueda, and M. Haruta, Solid State Ionics, 95 (1997) 143. 11. M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, and M. Haruta, Catal. Lett., submitted. 12. T. Kobayashi, M. Haruta, S. Tsubota, and H. Sano, Sensors and Actuators, B1 (1990) 222. 13. M. Haruta, T. Kobayashi, S. Tsubota, and Y. Nakahara, Chem. Express, 3 (1988) 159. 14. S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, and Y. Nakahara, Preparation of Catalysts VI, G. Poncelet, et al. (Eds.), Elsevier, Amsterdam, pp. 695-704, 1991. 15. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet, and B. Delmon, J. Catal., 144 (1993) 175. 16. D.A.H. Cunningham, W. Vogel, H. Kageyama, S. Tsubota, and M. Haruta, submitted to J. Catal.. 17. W. Vogel, D.A.H. Cunningham, K. Tanaka, and M. Haruta, Catal. Lett., 40 (1996) 175. 18. D.M. Cox, R. Brickman, K. Creegan, and A. Kaldor, Atoms, Molecules and Clusters, 19 (1991) 353. 19. G.R. Bamwenda, S. Tsubota, T. Nakamura, and M. Haruta, Catal. Lett., 44 (1997) 83. 20. T. Hayashi, K. Tanaka, and M. Haruta, Symp. Heterogeneous Hydrocarbon Oxidation, 211th ACS Meeting, New Orleans, March 1996, pp. 71-74. 21. K. Tanaka, T. Hayashi, and M. Haruta, Interf. Sci. Mater. Interconnection, Proc. JIMIS-8, Jpn. Inst. Metals, pp.547-550, 1996. 22. S. Naito and M. Tanimoto, J. Chem. Soc. Chem. Commun., (1988) 832. 23. Y.A. Kalvachev, T. Hayashi, S. Tsubota, and M. Haruta, Proc. 3rd World Congr. Oxid. Catal., September 21-26, 1997, San Diego. 24. F. Boccuzzi, A. Chiorino, S. Tsubota, and M. Haruta, J. Phys. Chem., 100 (1996) 3625. 25. S. Tsubota, T. Nakamura, and M. Haruta, Catal. Lett., submitted.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

135

The Selective Epoxidation of Non-Allylic Olefins Over Supported Silver Catalysts John R. Monnier Chemicals Research Division, Research Laboratories, Eastman Chemical Company, P.O. Box 1972, Kingsport, TN, 37662, USA.

ABSTRACT The epoxidation of non-allylic, or kinetically-hindered, olefins can be carried out using supported silver catalysts. While epoxidation does occur for unpromoted catalysts, the strength of olefin epoxide adsorption leads to low activity and selectivity, as well as irreversible catalyst fouling. The additon of certain alkali metal salts, such as CsCI, lowers the desorption energy of the olefin epoxide, permitting dramatic increases in activity, selectivity, and catalyst lifetime. In the case of butadiene, the addition of an optimum level of CsCI increases activity and selectivity from approximately 1% butadiene conversion and 50% selectivity for epoxybutene to 15% conversion and 95% selectivity, respectively. Epoxidation of butadiene occurs by addition of dissociatively-adsorbed oxygen to one of the localized C=C bonds to form epoxybutene. The addition of oxygen across the terminal carbon atoms does not occur to any measurable extent. The direct participation of molecular oxygen can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism. One site, which is unpromoted, serves as the site for butadiene adsorption, while the second site, which is promoted, functions as the site for dissociative oxygen adsorption and epoxybutene formation. Epoxybutene and derivatives represent the beginning of several new families of chemicals that were either not available, or were too expensive, to be considered for large-scale, or even fine chemical, production. More than one hundred chemicals have been prepared so far; several of these are in commercial production at the semiworks scale. 1. INTRODUCTION The gas phase epoxidation of ethylene using molecular oxygen to produce ethylene oxide is one of the most successful examples of heterogeneous catalysis to date. In 1995, over 7.62 billion lb. of ethylene oxide were produced in the US alone(I). In fact, ethylene oxide accounts for 40-50% of the total value of organic chemicals produced by heterogeneously-catalyzed oxidation(2). Thus, while the importance of olefin epoxidation is apparent, efforts to selectively epoxidize higher olefins to their corresponding epoxides using molecular oxygen have been unsuccessful. Many explanations have been proposed for the lack of success in this important area, although the most obvious reason is the reactivity of allylic C-H bonds, which are present in the higher olefins that have been tested. The bond dissociation energy of the allylic C-H bond in propylene is 77 kcal/mole, while the vinylic C-H bond in ethylene is 112 kcal/mole (3). Thus, abstraction of one of the

136

allylic C-H bonds in propylene becomes energetically favorable relative to electrophilic addition of oxygen across the C=C double bond. Once abstraction of hydrogen occurs, formation of an epoxide is precluded. Because of the higher bond strengths of the C-H bonds in ethylene, oxygen addition is the energeticallypreferred route. Previous attempts at higher olefin epoxidation have largely ignored the kinetic reactivity of allylic C-H bonds, but focused on modifying the desirable and unique catalytic properties of silver (the ability to activate oxygen so that it electrophilically adds across a C=C double bond) by promotion or selection of reaction conditions to hopefully suppress allylic hydrogen abstraction while not affecting the addition of oxygen to the C=C double bond. In almost all cases, the only effect has been to poison the silver surface with no substantial improvement in selectivity to the olefin epoxide. In this study we show that it is possible to selectively epoxidize higher olefins to their corresponding epoxides if the olefins do not contain reactive allylic hydrogen atoms. Many olefins, including styrene, substituted butadienes, and norbornene have been selectively epoxidized. Because of the usefulness of 3,4-epoxy-l-butene as a new chemical intermediate, most of the data and discussion will involve the selective epoxidation of butadiene. The catalyst composition and overall kinetics of the reaction will be discussed in some detail. 2. EXPERIMENTAL METHODS

Catalyst preparation has been discussed earlier (4,5) in greater detail. The methods of preparation are typical of those used for supported silver catalysts. In the cases where AgNO3 was used as the silver source, water was used as the solvent. The promoter salts in the appropriate amounts were added to the AgNO3 solution before impregnation and reduction of the AgNO3. The volume of the aqueous salt solution was typically in slight excess of the volume required for incipient wetness. If necessary, excess liquid was decanted before tumble drying at approximately 60-70~ The dried precursor was reduced in a flowing H2/N2 stream at temperatures up to 400~ The various combinations of reduction conditions (gas composition, space velocity, temperature ranges, etc.) have been previously discussed (6). Unless otherwise stated, the silver weight Ioadings for all the catalysts ranged from 12-15%. Promoter Ioadings were varied from a few ppmw to several thousand ppmw. In addition to AgNO3 as the silver source, in some cases silver oxalate, Ag2C204, was used as the silver source. In these cases, a solution of ethylenediamine and water was used to dissolve Ag2C204. Promoter salts were added to the Ag2C204 solution before impregnation and calcination. These catalysts were activated by thermal reduction in flowing air at temperatures up to 300~ The supports used for the various compositions were low surface area, fused alpha-AI203 carriers typically used for supported silver catalysts. Surface areas ranged from 0.2-1.0 m/gm with pore volumes between 0.4-0.6 cc/gm; median pore diameters ranged from 1-10 microns. Most catalysts were prepared on shaped carriers, which were sieved to give a narrow range of granules before evaluation. All catalysts were evaluated at steady-state conditions in a one atmosphere flow reactor using gas flows of He, C4H6, and 02, which were delivered by mass flow controllers. For the kinetic dependency studies, CO2 was also added by a mass flow controller, while different levels of epoxybutene and H20 were introduced from a helium-swept vapor-liquid saturator. In a similar manner, furan, epoxybutene, 2,5-dihydrofuran and crotonaldehyde were added to the feed stream

137

during investigation of the reaction mechanism. Analyses of both the feed and product gas streams were made using in-line gas sampling. Comparison of the feed and product streams permitted accurate mass balances at all reaction conditions. The gas chromatograph employed both thermal conductivity and flame ionization detectors hooked in series to give quantitative analyses of all gaseous products. 3. RESULTS AND DISCUSSION Table 1 Comparison of olefin epoxidation reactions over unpromoted 5% Ag/A120 3. Reaction conditions: T = 250~

catalyst weight = 2~

grams, and flow rate = 50 SCCM of

He/olefin/O 2 = 3/1/1.

Reaction

% Select

CH2=CH2+ 02 EO CO2

53 47

CH2=CHCH3+ 02 CO2 PO CH2=CHCHO

92 0 8

CH2=CHCH2CH3+ 02 CO2 BO CH2=CHCH=CH2

91 0 9

CH3CH=CHCH3+ 02 CO2 2-BO Other

98 0 2

CH2=CHCH=CH2+ 02 CO2 EpBT M Oxirane

25 75

% Conv. 12

3~

2.8

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/AI203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an

138

unpromoted Ag catalyst is shown in Figure 1. In addition to the epoxybutene, other oxygenated products are formed. Crotonaldehyde, or 2-butenal, is the expected product from the acid-catalyzed isomerization of epoxybutene. The existence of furan and 2,5-dihydrofuran suggests that 1,4-electrophilic addition of oxygen across the ends of butadiene may be one of the reaction pathways. Allylic oxidation of 2,5dihydrofuran is the likely source of the furan. The existence of acrolein, or propenal, indicates that hydrogenolysis of some C4-intermediate has also occurred. In fact, formaldehyde (detected by the TC detector hooked in series below the FI detector) was usually formed in a 1:1 molar ratio along with acrolein. The mechanistic scheme summarized in Figure 2 provides three different reaction pathways to account for all the products. In this mechanistic scheme, 1,2electrophilic addition to either of the localized C=C bonds forms epoxybutene, while 1,4-electrophilic addition across the ends of butadiene forms 2,5-dihydrofuran. This is the pathway favored by Madix (7), who analyzed the thermal desorption products of butadiene chemisorbed on oxygen-precovered Ag(110) surfaces. Madix concluded that oxygen preferentially adds to the ends of adsorbed butadiene to form 2,5-dihydrofuran, which then undergoes oxidation to form furan. In agreement with the results of Madix, Jorgensen (8), using extended Huckel calculations for butadiene interaction on oxygen-precovered Ag(110) surfaces, calculated that addition of oxygen at the 1,4-position is energetically favored over addition at the 1,2-position. The mechanism of Madix and calculations of Jorgensen do not provide any reasonable explanation for the formation of the epoxybutene seen in Figure 1. Thermodynamically, the formation of epoxybutene from 2,5-dihydrofuran is unfavorable by approximately 19.5 kcai/mole. The third mechanistic pathway is direct hydrogenolysis of an unspecified C4H6-O2 type of intermediate to form 1:1 amounts of acrolein and formaldehyde.

~

~O

1,4-electrophilic~-~

fast

o

~ §

2 electiop "ic l addition~ ~3, r- ~ Ag / '

C-C scission HCHO

H ~"""~i"~Oerization H~-+ ~isom ~Combustion O2~" CO2/H20 -.O

Figure 1. Gas chromatogram for unpromoted silver Figure 2. Mechanistic scheme for butadiene epoxidation. The catalyst. See Table 1 for reaction conditions~ arrow widths approximate relative rates of reaction.

139

In order to determine the relative roles of each of these pathways during butadiene epoxidation, different reaction products were added to the feed stream during the epoxidation reaction. These results, summarized in Table 2, indicate that epoxybutene undergoes conversion to form virtually all of the non-selective products seen in Figure 1. In fact, the relative amounts of the conversion products resulting from epoxybutene addition are in reasonable agreement with those displayed in the gas chromatogram of Figure 1. The epoxybutene addition data also revealed only 55% accountability of the epoxybutene which was added to the feed stream. Further, during the period of time that epoxybutene was being added the conversion of butadiene was greatly suppressed, indicating some type of kinetic inhibition by epoxybutene. After removal of epoxybutene from the feed, much of the activity was slowly restored. These results suggested that epoxybutene was the preferred product and that the strong adsorption of epoxybutene resulted in (1), rearrangement to 2,5-dihydrofuran (with subsequent oxidation to furan), (2), isomerization to crotonaldehyde, (3), hydrogenolysis to acrolein, (4), irreversible adsorption leading to fouling of the catalyst, thus giving poor mass accountability, and (5), strong, reversible adsorption resulting in a kinetic inhibition effect. The failure of 2,5-dihydrofuran to produce any epoxybutene also indicated that 1,4addition of butadiene to oxygen was at best only a minor reaction pathway. The data did imply, however, that lowering the desorption energy of epoxybutene from the Ag surface would be critical in improving the overall performance for butadiene epoxidation. Table 2 Decomposition products from the addition of intermediates to feedstream during reaction condition at 250~

Decomposition products are expressed

as percentage of additive introduced into feed. Feed composition is He/C4H6/O2/additive = 3:1:1:0.003. Feed Additive Furan

Crotonaldehyde

2,5-DHF

Epoxybutene

Furan

N/A

0

75

4

Acrolein

0

0

0

4

2,5-DHF

0

0

N/A

0

Epoxybutene

0

0

0

N/A

Crotonaldehyde

0

N/A

6

25

CO2

0

4

16

14

% Conv. of

0

4

24

46

100

100

97

55

Additive % Accountability of Additive

140

GC chromatogram for unpromoted catalyst C4H 6 Conv: 0.8% EpB TM Oxirane Select: 45% (CO2 = 4%) Other = 51%

GC chromatogram for CsCl-promoted catalyst C4H 6 Conv: 13% EpB TM Oxirane Select: 92% (CO2 = 8%)

1 ~__ _11 1

Figure 3. Comparison of gas chromatograms for unpromoted and CsCl-promoted, Ag/AI203 catalysts at similar reaction conditions.

The gas chromatograms in Figure 3 show the effects of CsCI promotion on a Ag catalyst. Both the selectivity and activity are dramatically increased by CsCI promotion. The only detected reaction products were epoxybutene, CO2, and H20. Further, no decline in activity was detected over an eight hour reaction period. This type of promoter effect is not usually seen for most catalyst promoters. Typically, selectivity is increased at the expense of activity. In the case of butadiene epoxidation, however, both selectivity and activity are strongly dependent upon the strength of adsorption of the epoxide. These results are consistent with the hypothesis that the CsCI promoter lowers the desorption energy of epoxybutene, which increases both the selectivity to epoxybutene and the turnover rate for epoxybutene formation. Silver catalysts promoted by Cs salts are also claimed to increase the selectivity to ethylene oxide (9,10). Arguably, the Cs salt promoters have the same effect for ethylene oxide formation as for epoxybutene formation, but because the rate determining step is not linked with desorption of the epoxide, the observed effect of Cs salt promoters is limited to higher selectivity, since lower desorption energy of ethylene oxide decreases the rate of ethylene oxide combustion without affecting the turnover rate. The curves in Figure 4 show the effects of promoter loading vs. activity (% C4H6 conversion) for three different families of CsCI-promoted, silver catalysts. Each curve is for a different support or method of catalyst preparation. These curve shapes are the typical "volcano" curves often seen for promoted catalysts. The optimum level of promoter represents the balance between under-promotion (not all sites are promoted) and over-promotion (surface is poisoned by excess promoter concentration). In order to assess whether a catalyst optimized for epoxybutene formation was active and selective for ethylene oxide formation as well as whether a state of the art ethylene oxide catalyst (11) was active and selective for epoxybutene production, the two different catalysts were evaluated and the data summarized in Table 3. The ethylene oxide catalyst showed excellent performance (even at one atmosphere pressure) for the formation of ethylene oxide, yet was virtually inactive

141

20 Cs added as CsC1 15 -

-

,o'"

-

:

~o,/~u U

'~

/ /!

% C4H 6 Conversion 10

-

.o

i

.. /

\

='

.;

,, |

1

I

PPM Cs Per Gram of Figure 4. The effect of Cs loading on catalytic activities for three different families of catalysts. The selectivities to EpB TM oxirane are 93 - 95% at maximum activities.

Table 3 Comparison of catalysts optimized for ethylene and budadiene epoxidation. EO Catalyst CH2=CH2 + 02 Conversion EO Selectivity

Unpromoted

Promoted

N/A N/A

11.3 88.4

N/A N/A

0.1 75

CH2=CH-CH=CH 2 + 02 Conversion EpB TM Oxirane Seiectivity EpB TM Oxirane Catalyst CH2=CH2 + 02 Conversion EO Selectivity

Unpromoted

Promoted

12 47

0.3 90

2.8 75

21 96

CH2=CH-CH=CH 2 + 02 Conversion EpB TM Oxirane Selectivity

142

Table 4

Epoxidation of other olefins using CsCl-promoted, Ag/AI20 3 catalysts. Reaction

+02

,0oc

r~'-

Conversion

(%)

95

19

85

21

225~ r-~'~ CO2,H20

0

100

250~

95

1.5

92

43

36

4

~N~ +02 245~

+02

Molar

Select. (%)

CH3 ~;~

,•

+02

,~0

+02 225~~.= O ~ ~ ' +02

210~

O~

for the formation of epoxybutene. Likewise, the catalyst promoted for optimum epoxy butene formation gave excellent yields to epoxybutene, but was almost inactive for ethylene oxide formation. This direct comparison of these two catalysts indicate the compositions are quite different, not altogether surprising since the rate determining steps for the two reactions are different; these results clearly indicate the impact of differences in kinetics on promoter requirements for otherwise similar reactions. As stated earlier, it should be possible to epoxidize non-allylic olefins other than butadiene, or even olefins with allylic hydrogen atoms, as long as the allylic hydrogens are kinetically non-reactive. The olefins in Table 4 indicate this is indeed the case. In all cases, the catalysts were promoted with CsCI; the unpromoted catalysts were either inactive or exhibited very low and transient activities. The data for the epoxidation of styrene and 4-vinylpyridine are discussed in greater detail in an earlier patent (12). The epoxidation of styrene over silver surfaces has also been observed by Blum (13) and Hawker, Lambert et al (14), although the catalysts evaluated in Table 4 are much more active and selective than those described by Blum. The transient temperature programmed reaction spectroscopy (TPRS)

143

results described by Hawker for oxygen precovered Ag(111) surfaces also support the formation of styrene oxide; styrene adsorbed on oxygen-precovered Ag(111) at 110~ resulted in the appearance of styrene oxide at a temperature of approximately 500~ Modification of the Ag(111) surface with 0.33ML of CI atoms increased the selectivity from approximately 60% to 93% styrene oxide. It is not possible to assess long term operation from the TPRS results, since there was only one reaction turnover. From our earlier work with epoxidation of butadiene, it is not possible to assess longer term activity based only on initial rates, especially when the formation of olefin epoxide is desorption-limited. Interestingly, Table 4 indicates that the presence of the para -CH3 group in 4-vinyltoluene results in combustion of this otefin to CO2/H20; the GC column used in these experiments was not capable of detecting any small amounts of other oxidation side products. Clearly, there was no formation of the epoxide product. The -CH3 group is both allylic and benzylic to the aromatic ring and is highly reactive towards C-H bond breaking during oxidation. Thus, as in the case of propylene and 1-butene, the existence of any reactive, allylic C-H bond on a substrate olefin will result in extensive combustion in the presence of 02 and a Ag surface. Table 4 also indicates that norbornene can be easily and selectively epoxidized to norbornene oxide over a CsCI-promoted, Ag/AI203 catalyst, in agreement with the transient TPRS data from Madix (15), who observed that norbornene oxide was formed at 310~ when coadsorbed norbornene and atomic oxygen were heated from 120 to 700~ on an unpromoted Ag(110) surface. On the other hand, Cant et al (16) reported that during continuous oxidation of norbornene by molecular 02 over an unpromoted Ag sponge catalyst in a single-pass flow reactor only benzene was formed as the partial oxidation product. No norbornene oxide was detected; Cant also concluded that norbornene oxide was not an intermediate in the formation of benzene. Our data, taken from continuous flow experiments, suggest that unpromoted silver catalysts do not produce any measurable norbornene oxide (most likely because the norbornene oxide initially formed does not desorb from the silver surface), but that CsCI-promoted, silver catalysts are active and selective for norbornene oxide. Comparison of these different studies indicate the difficulties in extrapolating from transient ultra-high vacuum studies to continuous flow experiments, especially when the kinetics of the catalytic reaction may involve product desorption as the slow step. What is interesting is that norbornene does contain a bridgehead C-H bond that is allylic to the C=C bond. However, either due to the puckered nature of the norbornene molecule, which projects the allylic C-H bond of adsorbed norbornene away from the Ag-O surface to give lower reactivity or due to the high strain energy of the resultant pi-allylic structure if C-H bond breaking does occur, the selectivity to norbornene oxide indicates that bridgehead C-H bond breaking does not take place. In contrast, epoxidation of bicyclo[2,2,2]oct-2-ene is only 36% selective to the desired epoxide. The addition of the an additional-CH2-group in the bridging position results in a dramatic decrease in selectivity to the epoxide. The extra -CH2group decreases the puckered geometry of bicyclo[2,2,2]oct-2-ene in comparison to norbornene, which results in the bridgehead C-H bond for bicyclo[2,2,2]oct-2-ene being projected 5-10 ~ more over the Ag-O surface than in the case of norbornene. It is not possible to state whether the more favorable C-H bond angle or the lower strain energy of the allylic structure (after C-H bond rupture) is responsible for the lower selectivity of bicyclo[2,2,2]oct-2-ene. However, comparison of these two bicyclic olefins indicate that relatively small changes in structure can have dramatic effects on selectivities to corresponding epoxides. Prevention of allylic C-H bond breaking is critical for epoxide formation.

144

Table 5 Reaction conditions during kinetic experiments. 1. Catalyst A. CsCl-promoted, Ag/AI20 3 B. 1.00 grams sieved to 0.0788-0.125" diameter 2.

Reaction Conditions A. Temperature: I90~ B. Flow Rate: 300 SCCM to give GHSV = 30,000 hr- 1. (1; = 0.12 sec) C. V= 8.6 cm/sec D. DCE = 1.0 ppm E. For 0 2 dependency, C4H 6 = 0.17 atm E For C4H 6 dependency, 0 2 = 0.17 atm G. For EpB TM oxirane and CO 2 dependencies, C4H 6 and 0 2 = 0.17 atm H. For Arrhenius plots, feed = diluent/C4H6/O 2 = 4:1 :l

3. Reactor Alumium-clad reactor to minimize thermal gradients

As stated earlier, the product distribution during butadiene epoxidation over an unpromoted catalyst indicated that epoxybutene was strongly bound to the Ag surface and that the CsCI promoter lowered the desorption energy of epoxybutene. These observations should be reflected in the steady-state kinetics of the reaction. The data summarized in Table 5 list the steady-state reaction conditions used to determine the reaction orders for the reactants C4H6 and 02 as well the reaction products epoxybutene, CO2, and H20. In all these experiments differential conversions of C4H6 and 02 were maintained and the data fitted to the typical power rate law expression for epoxybutene formation

eepox.vbutene - keca,6 02 eco2X pEps en20 z Application of the power rate law method is straightforward for those reactions in which the reaction products do not inhibit the rate of product formation. In the case of butadiene epoxidation, the partial pressures of epoxybutene, CO2, and H20 inhibit the rate of additional epoxybutene formation. In these experiments, the effects of CO2 and H20 on the rate of epoxybutene formation can be neglected since the molar selectivity to epoxybutene was typically 98-99%, which gave very low and relatively constant amouts of CO2 and H20 in the gas stream. However, the epoxybutene formed in the reactor required the normalization of the rate of epoxybutene formation to account for the inhibition effect of epoxybutene. Finally, one ppm of 1,2-dichloroethane (DCE) was added to the feed stream to maintain constant activity for the 3-4 week period of time over which the kinetic experiments were conducted. The kinetic plots, which are summarized in Figure 5, reveal many different and interesting dependencies. The 02 dependency is first order over the entire 02 pressure range that was investigated, while butadiene is zero order until very low partial pressures of butadiene are encountered, at which point there is a transition to first order. The pressure dependencies for both epoxybutene and CO2 exhibit transitions from fractional negative orders (-0.4 for epoxybutene and -0.5 for CO2) to full negative first reaction orders. The reaction order for H20 is-0.15 order over the full range of H20 partial pressures. These kinetic dependencies provide very clear strategies from both catalyst design and process control standpoints to maximize the formation of epoxybutene.

145 Ln Rate EpB T M Oxirane Formation 0.50 m

0.06

9

~I~

1

~

9

D

9

H20

-0.38 -0.82 -1.26 -1.70

EpB T M Oxirane

=2.14 --2.58 --

CO 2

-3.02 -3.46 --3.90 -9.20

I

-8.33

I

-7.46

I

-6.59

I

-5.72

I

-4.85

I

-3.98

I

-3.11

I

-2.24

I

-1.37

-0.50

Ln Pressure

Figure 5. Kinetic map for epoxybutene formation using conditions described in Table 5. The combination of (1), transition in negative reaction orders from fractional negative to full negative first order for both epoxybutene and CO2 pressures, (2), a positive first order in 02 pressure, and (3), a simultaneous zero order in butadiene pressure is best explained by assuming a dual-site mechanism for epoxybutene formation. One site is associated with unpromoted Ag, while the second site is associated with a Cs-promoted Ag site. The unpromoted site serves as the site for butadiene adsorption, while the promoted site functions as the site for dissociative oxygen adsorption which is incorporated into adsorbed C4H6 to form epoxybutene. Thus, the sites for butadiene adsorption are different from the sites for 02 adsorption and each site is 1/2 order in 02 pressure. Our results agree with those of Lambert (14) and Madix(15) in the type of active oxygen for selective epoxidation. If we apply the "6/7" rule (see Sachtler (17) for explanation) typically cited as evidence for the role of molecular 02 in selective epoxidation of ethylene for the case of butadiene epoxidation, we would not expect selectivity for epoxybutene to exceed "11/12", or 91.7%. In fact, selectivities of 93-96% are typically seen at all reaction conditions. Selectivities of 97-98% are observed at differential conditions and lower reaction temperatures. Therefore, based only upon the observed selectivities to epoxybutene, dissociatively-adsorbed oxygen is clearly the active oxygen in butadiene epoxidation. Further, the kinetic model, which has been derived from the kinetic plots in Figure 5 has been used to very satisfactorily fit a wide variety of reaction data from several different reactor formats, assumes dissociatively-adsorbed oxygen at both promoted and unpromoted Ag sites. The oxygen incorporated into epoxybutene is dissociatively-adsorbed oxygen, not molecular oxygen. Finally, consistent with the earlier statement that promoters lower the desorption energy of epoxybutene from the Ag surface, the Arrhenius plots for both

146

Ln RateEpB Formation 4.00 2.40 0.80 -0.80 -2.40

Promoted

m

- .

.

m

-4.00i -5.40 -7.20 --

'~~

Unpromoted

E[app] = 40.7kcal/mole

-8.80t -10.40 I I I ~ t -12.00 1.90 1.94 1.99 2.03 2.08 2.12 2.17 2.21 2.26 2.30 2.35 1000/'1"(~ Figure 6. Arrhenius plots for EpB TM oxirane formation. Conversions were differential in both cases and feed compositions were diluent/C4H6/O 2 =4/1/1.

CsCI-promoted and unpromoted Ag catalysts are shown in Figure 6. The apparent activation energy for epoxybutene formation is >14 kcal/mole lower than that for the unpromoted Ag catalyst. Epoxybutene is a very versatile and reactive chemical intermediate that has not been prepared before in large, or even reasonable, quantities. Currently, it is available only from chemical supply houses for prices > $10/gram. The molecule is highly functional, with each carbon atom chemically distinct. It even contains an asymmetric carbon atom at the C3-position. Epoxybutene has two separate and reactive functionalities, a C=C bond and an epoxide group. Each of these groups has rich chemistry, both from a polymer viewpoint and a chemical feedstock perspective. The histogram plot in Figure 7 indicates many different structural isomers are thermodynamically accessible from epoxybutene. In fact, the derivative tree in Figure 8 outlines some of the more than 100 compounds that have been prepared from epoxybutene. Key to this tree of chemical compounds is the thermodynamically favored transition of the family of C4H60 structural isomers from epoxybutene to 2,5-dihydrofuran (by 19 kcal/mole) to 2,3-dihydrofuran (by 7 kcal/mole). Even the formation of cyclopropylcarboxyaldehyde (CPCA) becomes thermodynamically favorable enough at elevated temperature, so that high conversions and high selectivities are observed at temperatures above 350~ In fact, this transition from epoxybutene to CPCA is being commercially practiced to provide a rich variety of chemicals to be used in the pharmaceutical and agricultural sectors (18,19). The various classes of products outlined in Figure 8 include addition of oxygen-containing and nitrogen-containing nucleophiles to epoxybutene to yield an almost endless variety of hydroxy ethers and amino alcohols, respectively, having an extremely wide range of chemical properties as well as boiling points and solvating properties. Addition of H20 to epoxybutene gives both 1,4- and 1,2butenediols, providing a novel means of formation for some interesting glycols.

147

Kcal/Mole 40 30

29.7

20

,04//~o 4.75

-20

t

.,3., ~.,.o-----r~-1s.1 1

I~-cH~

-40

/~,/CHO

1

-21.5

-25.6

Compound

Figure 7. Thermodynamic enthalpies of formation of epoxybutene and related isomers. Enthalpies calculated from CHETAH (Chemical Thermodynamic and Energy Release Evaluation Program) at 25~ and ideal gas conditions.

HO~

O

R

t

OR

OH

t,/

CH ~:~OH

t

X

t

''

RHN~

O

H

1 Figure 8. Abbreviated EpB TM oxirane derivative tree. Compounds in boxes are currently being produced or have been demonstrated in pilot plant operation.

148

Addition reactions to the C=C bond include hydrogenation to give epoxybutane, providing an indirect, yet efficient way to prepare this important epoxide (when coupled with hydrogenation) using molecular 02. Halogenation with CI2 and Br2 form the corresponding dihaloepoxybutanes, which are possible components in flame retardents. Another interesting addition reaction is the selective addition of CO2 to form the highly versatile monomer, vinyl ethylene carbonate, which is similar to ethylene carbonate. From 2,5-dihydrofuran, the most obvious derivative is tetrahydrofuran (THF), formed by the hydrogenation of the C=C bond. The C=C bond is also reactive for hydroformylation and olefin metathesis to add additonal functionality and structure. As stated earlier, the next intermediate, 2,3-dihydrofuran (2,3-DHF) serves as the gateway to CPCA family of chemicals. Hydration of 2,3-DHF produces 2-hydroxytetrahydrofuran, which can be readily converted to gamma-butyrolactone, pyrrolidinone (and N-substituted pyrrolidinones). Finally hydrogenation of hydroxytetrahydrofuran yields 1,4-butanediol in high yields. Yet another demonstration of the versatility of epoxybutene comes from the asymmetric center, which has been converted into a number of four-carbon chiral synthons, such as 3-butene-l,2-diol and various derivatives, in >99% enantiomeric purity (20,21). 4. CONCLUSIONS

The epoxidation of non-allylic olefins, or olefins containly kinetically-hindered allylic olefins, using promoted silver catalysts has been demonstrated. The epoxidation of butadiene to form epoxybutene marks the first example of an olefin other than ethylene to be selectively epoxidized at steady state and commerciallyrelevant conditions using gas phase oxygen and heterogeneous silver catalysts. Epoxidation of higher, non-allylic olefins does occur without the use of an alkali metal salt promoter. However, the olefin epoxide is strongly adsorbed to the silver surface and undergoes a number of side reactions as well as surface fouling. The addition of a promoter, such as CsCI, apparently lowers the desorption energy of the olefin epoxide from the surface, permitting both selectivity and activity to dramatically increase. In the case of butadiene, the addition of an optimum level of CsCI promoter to the silver catalyst increases selectivity and activity from about 50% selectivity and 1% conversion to approximately 95% selectivity and 15% conversion, respectively. Catalyst lifetime also increases from less than 3-4 hours to commercially-relevant periods of time. Epoxidation of butadiene occurs by electrophilic addition of dissociativelyadsorbed oxygen to one of the localized C=C bonds to form the epoxide. The addition of oxygen across the terminal carbon atoms to form 2,5-dihydrofuran does not occur to any measurable extent. When it is observed for unpromoted catalysts, 2,5-dihydrofuran is formed from the isomerization of strongly-bound epoxybutene. The direct participation of molecular oxygen addition to the C=C bond can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism for butadiene epoxidation. One site, which is unpromoted, functions as the butadiene adsorption site, while the second site, which is promoted, serves as the site for dissociative oxygen adsorption and epoxybutene formation. Under normal reaction conditions, the reaction is zero-order in butadiene pressure and first-order in oxygen pressure (each site is 1/2 order in oxygen pressure). Because of the kinetic inhibition effect of epoxybutene, the reaction at high conversion is negative first order in epoxybutene.

149

Finally, epoxybutene and derivatives represent the beginning of several new families of chemicals that were previously either not available or simply too expensive to be considered for large-scale industrial, or even fine chemical, application. More than one hundred chemicals have been synthesized from epoxybutene, and many more are currently being synthesized and evaluated for a wide variety of applications.

ACKNOWLEDGEMENTS

The author acknowledges the efforts of Peter Muehlbauer and George Oltean for assistance in catalyst preparation and reaction kinetics, David Hitch in reaction engineering, and Jerome Stavinoha and Windell Watkins for many meaningful discussions regarding development of this technology. Steve Godleski and Steve Falling, among many, were instrumental in much of the derivative work involving epoxybutene as a new organic intermediate. REFERENCES ,

2. 3. .

5. 6. 7. ,

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Chem. & Eng. News, April 8, 1996, p. 1. R. K. Grasselli and J. D. Burrington, Adv. Catal. 30, 133 (1981). J. A. Dean, "Lange's Handbook of Chemistry," p. 4.25, McGraw-Hill, Inc., New York, 1992. J. R. Monnier and P. J. Muehlbauer, U.S. Patent No. 4,897,498 (1990). J. R. Monnier and P. J. Muehlbauer, U.S. Patent No. 4,990,773 (1990). J. R. Monnier and P. J. Muehlbauer, U.S. Patent No. 5,081,096 (1992). J. T. Roberts, A. J. Capote, and R. J. Madix, J. Am. Chem. Soc. 113,9848 (1991). B. Schiott and K. A. Jorgenson, J. Phys. Chem. 97, 10738 (1993). A. M. Lauritzen, U.S. Patent No. 4,833,261 (1989). M. M. Bhasin, P. C. EIIgen, and C. D. Hendrix, U.S. Patent No. 4,916,243 (1990). Commercial ethylene oxide catalyst graciously supplied for purposes of comparison. J. R. Monnier and P.J. Muehlbauer, U.S. Patent No. 5,145,968 (1992). P. R. Blum, U.S. Patent No. 4,894,467 (1990). S. Hawker, C. Mukoid, J. S. Badyal, and R.M. Lambert, Surf. Sci. 219, L615 (1989). J. T. Roberts and R. J. Madix, J. Am. Chem. Soc. 110, 8540 (1988). N. W. Cant, E. M. Kennedy, and N. J. Ossipoff, Catal. Lett. 9, 133 (1991). W. M. H. Sachtler, C. Backx, and R. A. Van Santen, Catal. Rev.-Sci. Eng. 23, 127 (1981). Chem. & Eng. News, August 21, 1995, p. 7. D. Denton, S. N. Falling, J. R. Monnier, J. L. Stavinoha, Jr., and W. C. Watkins, Chimica Oggi, May 1996, p. 17. N. W. Boaz and R. L. Zimmerman, Tetrahedron: Asymmetry, 5, 153 (1994). N. W. Boaz, U.S. Patent No. 5,445,963 (1995).

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

151

R e d o x M o l e c u l a r Sieves as Heterogeneous Catalysts for Liquid Phase Oxidations R.A. Sheldon Laboratory for Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 B L Delft, The Netherlands 1. INTRODUCTION Catalytic oxidation is widely used for the conversion of petroleum-derived hydrocarbons to commodity chemicals [1 ]. Moreover, in fine chemicals manufacture there is increasing environmental pressure to replace traditional stoichiometric oxidations with inorganic reagents such as dichromate and permanganate, with cleaner, catalytic alternatives which do not generate excessive amounts of inorganic salts as byproducts [2]. Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. However, solid catalysts have several advantages compared to their homogeneous counterparts, such as ease of recovery and recycling and amenability to continuous processing. Moreover, siteisolation of discrete redox metal centres in inorganic matrices can lead to oxidation catalysts with unique activities and selectivities by virtue of the fact that oligomeriTation of active oxometal (M = O) species to inactive ~t-oxo complexes is circumvented. One approach to designing stable solid catalysts with unique activities is to incorporate redox metal ions or complexes into the framework or cavities of zeolites and related molecular sieves. So-called redox molecular sieves [3, 4], unlike conventional supported catalysts, possess a regular microenvironment with homogeneous internal structures consisting of uniform, well-defined cavities and channels. Confinement of the redox active site in these channels and/or cavities can endow the catalyst with unique activity as a result of strong electrostatic interactions between acidic and basic sites on the internal surface and the substrate or reaction intermediate analogous to interactions with acidic carboxyl and basic amino groups of amino acid residues in the active site of (redox) enzymes. Indeed, the analogy with enzymes can be taken even further: f'me-tuning of the size and hydrophobicity of the redox cavity (see later) can provide unique, tailor-made catalysts by influencing which molecules have ready access to the active site, on the basis of their size and/or hydrophobic/hydrophilic character. Hence, application of the terms 'mineral enzymes' and zeozymes to such catalysts is appropriate [5]. Up until the late seventies attempts to develop redox molecular sieves were mainly limited to the ion-exchange approach (see later). This situation changed dramatically with the discovery, by Enichem scientists in 1983 [6, 7], of the unique activity of titanium silicalite-1 (TS-1) as a catalyst for oxidations with 30% aqueous hydrogen peroxide. Following the success ofTS-1, interest in the development, and application in organic synthesis, of redox molecular sieves has increased exponentially and has been the subject of several recent reviews [8-11 ]. It has even provoked a revival of interest in another approach to producing redox molecular sieves: the so-called ship-in-a-bottle method [ 12-15]. Why use a redox molecular sieve? Although the major motivation stems from the expectation of producing a catalyst with unique activity we note that, in many cases, a stable,

152 recyclable solid catalyst exhibiting the same activity/selectivity as its homogeneous counterpart would be a useful objective. Thus, even trace amounts of 'heavy metals' in aqueous effluent are undesirable which means that catalysts should be recyclable. A good case in point is chromium: the 'chromofobia' which is currently in vogue imposes an essentially zero emission constraint on this metal. 2. OXIDATION MECHANISMS A conditio sine qua non for understanding oxidations of organic substrates with 02, H202 and RO2H catalyzed by redox molecular sieves is a thorough appreciation of the elementary mechanisms of oxidations in the liquid phase [ 1]. One can safely assume that the elementary steps involved in liquid phase oxidations (see Figure 1) do not change fundamentally when the metal catalyst is located in a molecular sieve; only that the relative rates of these steps may change substantially.

Free r a d i c a l c h a i n a u t o x i d a t i o n initiation

RH

diff.controiled

R. + 0 2 RO 2 9+ RH ROaH + M n§ RO2H + M(n-1)+ ROe + RH RO 9+ RO2H 2 RO 2 9 \ 2 CHO 2 9 /

rate iim. slow

(1)

RO 2

(2)

ROaH + Re

(3)

~- RO 2, + M(n+)+ + H +

fast fast fast

termination

R,

=

(4)

RO, + Mn'IOH

(5)

ROH + R,

(6)

ROH + RO 2 9

(7)

2 ROe + 0 2

(8)

~HOH + ~ C = O + ' 0 2 / /

(9)

Catalytic oxygen transfer RO2H + S

M

~--

ROH + SO

(10)

M+SO

(11)

2M--O

(12)

Mars-van Krevelen mechanism M~O+S 2M+O

2

Figure 1. Oxidation mechanisms

153 One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily undergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). From the viewpoint of selectivity this ubiquitous competing free radical chain autoxidation is often, but not always, something to be avoided: In principle, it can be circumvented by employing H202 or RO2H as the terminal oxidant in an oxygen tranfer process (Reaction 10). Particularly in the context of fine chemicals such reagents can be economically viable. However, as noted above, variable valence metals can also be expected to catalyze the homolytic decomposition of RO2H and H202 via the so-called Haber-Weiss mechanism (Reactions 4 and 5; R = alkyl or H). Moreover, subsequent free radical chain decomposition of RO2H via reactions 7 and 8 leads to the formation of dioxygen which returns the reaction to an autoxidation manifold (Reactions 1-3). In the case of secondary alkylperoxy radicals, termination via the Russell mechanism [16] can even lead to the formation of singlet dioxygen (Reaction 9). Obviously competition between reactions 6 and 7 will depend on the relative concentrations of substrate (RH) and oxidant (R'O2H). In many cases both homolytic and heterolytic pathways afford the same products, e.g. alcohols and ketones from hydrocarbons, which means that results have to be interpreted with care. Certain elementary tests for homolytic pathways need to be performed, e.g. inhibition by a radical scavenger such as Ionol indicates a free radical chain mechanism and loss of yield on flushing with an inert gas suggests the intermediacy of dioxygen in reactions with H20~ or RO2H. More sophisticated 'reality tests' can also be performed to demonstrate the intermediacy of alkoxy radicals in oxidations with RO,H [17]. The 'holy grail' of oxidation chemistry is the design of catalytic systems capable of mediating oxygen transfer from dioxygen, without the need for a sacrificial reductant, i.e. a Mars-van Krevelen mechanism [18] in the liquid phase. Indeed, the confinement of substrate molecules in the micropores of molecular sieves might be expected to create quasi gas phase conditions conductive to such a mechanism at the expense of free radical chain autoxidation (we note, however, that a mechanism involving two metal centres as shown in eq. 12 would be difficult to achieve in a molecular sieve). The active oxidant in catalytic oxygen transfer processes may be an oxometal (M = O) or a peroxymetal (MOOR) species (Figure 2) [19]. It will be readily appreciated that catalytic oxygen transfer may be considered as a special case of the Mars-van Krevelen mechanism. Peroxometal mechanisms are favourable when the metal in its highest oxidation is both a Lewis acid and a weak oxidant;e.g, early transition metal ions with do configurations (Mo v~, W vl and Tirv). The oxidation state of the metal ion does not change during the catalytic cycle; catalysis being due to the Lewis acid character of the metal ion. Strong (one-electron) oxidants, exemplified by later and/or first row transition elements such as Crv~, Mn m, Co tu and Fe I", favour oxometal pathways and/or Haber-Weiss type homolytic decomposition of RO2H. Vanadium(V), being a strong Lewis acid and a reasonably strong (one-electron) oxidant exhibits all three types of activity.

154

( RO,,H

ROM n*

....=

-ROH

v

M.*

=

MoVI

b)

M(n*2)+

-

Zr w

(a)1

\O/

I

[

Ti w

S

M" * OR

RO

V v

WVX

ROMn* + SO

0 =

V v ' C r vl , F e v , R u vx

M .§

Co m

C r vI

Mn m

Vv

Fe m'

E~

1.82

1.48

1.51

1.0

0.771

I I

I

1

TiIV

MoVX

WVl

0.08

0.2

0.03

Figure 2. Peroxometal (a) vs oxometal Co) pathways in oxygen transfer Hence, the oxometal pathway is more complicated than one might assume from Figure 2. Conversion of peroxometal to oxometal species can involve either homolytic or heterolytic cleavage of the O-O bond (Figure 3). If'leak~e' of RO. (or HO- in the case of H202) occurs, the reaction is transferred to an autoxidation manifold (Figure l). Competition between homolytic and heterolytic pathways for oxometal formation will be influenced by many factors, e.g. ligands surrounding the metal, solvent, etc. In short, catalytic oxidations with 02, H202 and RO2H are, from a mechanistic viewpoint, intricately interwoven, including various homolytic and heterolytic pathways. Examples of oxidative transformations involving heterolytic, pemxometal pathways are olefin epoxidation, sulfoxidation and oxidations of various nitrogen compounds. In contrast, allylic and benzylic oxidations and (cyclo)alkane oxidations are typical of oxometal and/or free radical autoxidation pathways, which are difficult to distinguish. Alcohol oxidations may involve peroxometal or oxometal pathways. There are few cases, e.g. stereoselective olefin epoxidation with TiW/RO2H (R ffi H or alkyl) must involve a heterolytic, peroxometal pathway, which are unambiguous.

155

M.~...01~0 R hemolysis= I!(,.~)+.O 1 diffusion....=RO, =

recombination Mn+(~ b,,'f~'ORheterolysis /?~") ~,,~-o/ = 0 OR Figure 3. Hemolysis vs heterolysis ofperoxomelal complexes

3. REDOX MOLECULAR SIEVES: GENERAL CONSIDEI~TIONS 3.1. Structures and composition of molecular sieves Zeolitcs and zeotypes are crystalline oxides comprising comer slmfing TO4 tetrahedra (T = Si, AI, P, etc.) and consisting of a regular pore system with diameters of molecular dimensions, hence the term molecular sieve. Zeolites refers to altmfinosilicates (T = Si and AI) and zeotypes to molecular sieves having analogous sm~tures but with a different elemental composition, e.g. aluminophospha~s (AIP0's; T = AI and P) and siticoaluminophosphates (SAPO's; T = Si, AI and P). Molecular sieves are categorized on the basis of their pore diameters into small pore (< 4 A~, medium pore (4-6 A), large pore (6-8 A), extralarge pore (8-14 A) and mesoporous (15-100 A). The pore system may be one, two or three dimensional. This can be important in the context of catalysis as a few obstructions in a one dimensional pore system would seriously impede access to a large proportion ofthe catalytic sites while in two or three dimensional pore systems alternative diffusion paths can be found. A molecular sieve having a particular topology is described by a mnemonic three letter code [201. The AFI structure (7.3 ~), for example, is one dimensional while the FAU structure (7.4 A) consists of three orthogonal channel sytems (7.4 A) intersecting in larger cavities (13 A), so-called supercages, and molecules can travel in all three directions. Other selected examples are given in Table 1.

156 Table 1 Pore dimensions and dimensionalities of molecular sieves Pore size Small Medium

Structure type LTA

Trival name

Pore Ringdiameter [/~] size

Zeolite A

4.1

8

3 3 3

MFI MEL AEL

ZSM- 5, TS-I ZSM-11, TS-2 AIPO4-11

5.3 x 5.5 5.3 x 5.4 3.9 x 6.3

10 10 10

MOR BEA FAU

6.5 x 7.0 7.6 x 6.4 7.4

12 12 12

7.4 x 6.5

12

2

LTL AFI

Mordenite Zeolite beta Faujasite Zeolite X or Y Hexagonal faujasite Linde type L AIPO4-5

7.1 7.3

12 12

1 1

VFI CLO

VPI-5 Cloverite

12.1 13.2

MCM-41

ca. 40

Large EMT

i

Extra large Mesoporous

Dimensionality

'

18 20

1

I I

1

3 1

The framework of molecular sieves is not completely rigid and incoming molecules are able to induce slight structural changes. Hence, ca. 10% should be added to the pore diameters given in Table 1 to obtain the limiting sizes of molecules having access to the pores. Their well-defined pore systems combined with their capacity for at least small substrate-induced structural changes enable molecular sieves to recognize, discriminate and organize molecules with a precision of < 1 A [21 ]. This capability to organize and discriminate molecules with high precision endows them with shape selective properties [22], analogous to enzymes. Hence, also by analogy with enzymes one would expect the highest activity to be observed with the best fit, i.e. when the dimensions of the substrate are comparable to those of the micropores. The recently discovered mesoporous (alumino)silicates, e.g. MCM-41, consist of a regular array of uniform one dimensional pores with diameters in the range 15-100 A and have properties intermediate between those of amorphous SiO2 and A1203 and microporous sieves [23]. This has considerably extended the size of molecules that can be adsorbed: the immobilization of enzymes in MCM-41 has even been achieved [24]. 3.2 A c i d i t y a n d h y d r o p h o b i e i t y

In zeolites the different valencies of Si (tetravalent) and AI (tdvalent) produce an overall negative charge for each A1 atom which is balanced by an alkali (alkaline earth) cation. Exchange of these cations by protons affords strong Brvnsted acids, comparable in strength to sulfuric acid. The acid strength increases with decreasing Al content, while the number of acid sites decreases. Substitution of tfivalent aluminium in the zeolite framework by tetravalent

157 species, e.g. Si or Ti produces (metallo)silicalites with an electrically neutral, hydrophobic framework. Likewise, substitution of silicon by phosphorus produces the electrically neutral, hydrophilic aluminophosphates (AIPO's) or acidic SAPO' s.

3.3 Types of redox molecular sieve We can distinguish three types ofredox molecular sieve on the basis of the method of synthesis: (a) ion exchange, Co) framework substitution and (c) encapsulation. As noted above, the negative charges of zeolite and SAPO frameworks are compensated by exchangeable cations, so that redox cations can be introduced by direct ion exchange. However, the high mobility of the exchangeable cations results in a marked propensity for leaching. Moreover, ion exchange is not applicable to neutral molecular sieves such as AIPO's and silicalites. Framework substitution of A1, Si or P by a redox metal ion leads, in general to more stable redox molecular sieves (Figure 4). So-caUed isomorphous substitution, in which the metal ion is coordinated tetrahedraUy by four oxygen atoms should be possible when the r~io~/roxys~ ratio is between 0.225 and 0.414 [25]. It should be noted, however, that the oxidation state of the metal and, hence, structure and charge of the framework, may change substantially when an as-synthesized material is calcined. For example, Cr-substituted sieves generally contain Crm in the as-synthesirzd material but on calcination this is transformed to Cr vl. Since the latter contains two extra-framework Cr = 0 bonds it can only be anchored to a surface defect site rather than isomorphously substitutexl. By the same token, as-synthesized CrmAPO contains a neutral frwnework (Crm replaces AIm) while in the calcined CrVIAPOthe fraraework contains one Brznsted acid site per Crv~. Hydrophobicity

I

205 !

I AI.

I I

ALPOs

I

M-APO

T

I !

SAPOs

Redox MetaI,M

! s ~)2

I [

Y

Fe

Ti

Zr

III

IV

IV

III

IV

IV

If

ZEOLITES

l

Ti-beta

V

IV I IV IV I

V

Cr

9

ii T

TI-ZSM-5

Sn I I

']

m

i

J IM-SILICALITES I

M-ZEOL

-TAPSO -CrAPSO

-VAPO -CrAPO -CoAPO

As-synthesized Calcined

~

Mn

Co

III

II

II

VI

III

III

TS-1 VS-1 CrS-1

I

Figure 4. Types of redox molecular sieves and oxidation states of the metal ions

158 Another approach to creating novel redox active molecular sieves involves the encapsulation of transition metal complexes in intrazeolitc space: the so-called ship-in-abottle method [ 12-15]. Encapsulated metal complexes should, in principle, ideally combine the advantages of homogeneous and heterogeneous catalysis. Molecular sieves containing supercages, e.g. FAU and EMT (hexagonal faujasite) are ideal for encapsulation as substrate molecules have ready access via the micropores (7.4 A) to the metal complex which is trapped in the supercages (13 A) An advantage of encapsulation is that it allows for the synthesis of redox molecular sieves containing elements that are too large to be incorporated by isomorphous substitution (see earlier). 3.4 Choice of solvent Redox molecular sieves have one important advantage over other heterogeneous catalysts: it is possible to influence which substrate molecules approach the active site by a suitable choice of molecular sieve and solvent. The molecular sieve can be viewed as a second solvent which extracts the substrate molecules from the bulk solvent. Which molecules are selectively extracted depends on the size and hydrophobicity of the micropores and of the substrate. Highly siliceous molecular sieves, such as silicalite-1, are hydrophobic and will selectively adsorb apolar hydrocarbon substrates. This is especially important in hydrocarbon oxidations where the primary products - alcohols, aldehydes and ketones - are polar molecules and are more susceptible to oxidation than the hydrocarbon substrates. Hence, by using a redox molecular sieve it is, in principle, possible to obtain much higher selectivities to primary oxidation products than with analogous homogeneous catalysts. The selective oxidation of alkanes with H202 in methanol solvent, in the presence of TS-1 is, presumably, a manifestation of this effect. Catalytic oxidation with H202 in homogeneous solution are generally strongly inhibited by the water present in the H202 or formed in the reaction. Hence, another important advantage of hydrophobic redox molecular sieves, such as TS-1, is that they are not deactivated by water owing to the preferential adsorption of the hydrocarbon substrate and H202. In general, a hydrophilic solvent, e.g. acetone or methanol, is used to create a single liquid phase with aq. H202 although it has been claimed that this is not necessary [26]. Hydrophilicity of redox molecular sieves increases with increasing aluminium content. Hence, high-alumina zeolites, AIPO's and SAPO's are strongly hydrophilic and selectively adsorb hydrophilic substrates. In this case a hydrophobic solvent should be used to facilitate the adsorption of substrates. Furthermore, it should be noted that the incorporation of AI in silicalites or Si in AIPO's generates Brensted acid sites which may catalyze undesirable sidereactions (see later).

4. SYNTHESIS AND CHARACTERIZATION 4.1 Framework-substituted molecular sieves The so-called hydrothermal synthesis of molecular sieves involves allowing an aqueous gel, containing a source of the framework building elements (AI, Si, P) and a structuredirecting agent (template; usually an amine or a tetraalkylammonium salt) to crystallize in an autoclave, under autogenous pressure, at temperatures ranging from 80 to 200 ~ [27]. Crystallization times can vary from several hours to weeks. Redox molecular sieves are similarly prepared by adding a source of the redox metal ion to the synthesis gel. The as-

159 synthesized material is calcined at ca. 500 ~ to remove the template. As noted above, this can lead to oxidation of the redox metal ion to a higher valence state. In the synthesis of silica-based materials a mineralizer (OH, F) is required to regulate the dissolution and condensation process, i.e. synthesis is generally carried out at high pH. In contrast, (redox) aluminophosphates are crystallized from gels prepared by mixing an alumina slurry with a solution of the redox metal ion in aq. H3PO4 and the template, i.e. synthesis occurs at low pH. Titanium-substituted silica-based molecular sieves, in particular TS-1 (MFI), have been the most intensively studied [6, 7, 9]. This generally involves controlled hydrolysis of a mixture of Si(OEt)4 and Ti(OEt)4 in the presence of the template, the tetrapropylammonium cation in the case of TS-1. Many workers have experienced problems in TS-1 synthesis and the various pitfalls haven been reviewed [9]. Small amounts ofNa § or K § orginating from commercial samples of the template suffice to prevent the substitution of Ti into the framework. Similarly, the presence of F leads to the formation of octahedral extra framework titanium. Following the success of TS-1 a variety of Ti-substituted molecular sieves were prepared by hydrothermal synthesis (Table 2) [28-32]. Furthermore, various redox metals, e.g. V, Cr, Mn, Fe, Co, Cu, Zr, and Sn, have been reportedly incorporated into silicalites, zeolites, A1PO' s and SAPO's [8-11 ] and the list is still increasing. Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCI4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts.

Table 2 Titanium-substituted molecular sieves Material

Template

Pore size (A)

Ref.

Ti-silicalite- 1 (TS- 1)

Pr4NOH

5.3 x 5.5

6,7

Ti-silicalite-2 (TS-2)

Bu4NOH

5.3 x 5.4

28

Ti-ZSM-48

H2N(CH2)sNH2

5.4 x5.1

29

Ti-beta

Et4NOH

7.6 x 6.4

30

Ti-MOR

none

7.0x6.7

31

Ti-APSO-5

C6HIINH2

7.3 x 7.3

32

Ti-MCM-41

CI6H33(CH3)3NOH

ca. 40

33

Ti-HMS

Cl2H25NH2

ca. 40

33

160 A veritable arsenal of techniques has been mobilized to provide information regarding the structure of redox molecular sieves [9-14]. X-Ray powder diffraction (XRD) provides an immediate check for crystallinity and structural type. X-Ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge spectroscopy (XANES) give further insights into coordination geometry and bond lengths. Infrared and Raman spectroscopy have been used to identify characteristic features, e.g. the 960 cm -~ bond attributed to the Si-OTi stretching vibration in TS-1 [9]. Diffuse reflectance UV-Vis spectroscopy (DREAS) and EPR provide useful information regarding the oxidation state of the metal. Other techniques that are regularly applied are MAS-NMR, X-ray photoelectron spectroscopy (XPS) and scanning and transmission electron microscopy (SEM and TEM). Finally, BET surface area measurements and adsorption experiments are indispensible for checking the structural integrity of the molecular sieve. 4.2. Molecular sieve-encapsulated metal complexes Three different approaches are used to achieve encapsulation: a) Intrazeolite complexation (flexible ligand method) b) Intrazeolite ligand synthesis c) Metal complexes as templates for zeolite synthesis In the first method the metal complex is assembled in the zeolite cavities by allowing the metal-exchanged zeolite to react with ligands that are small enough to access the micropores. The metal complex, once formed, is too large to diffuse out. For example, bis- or trisbipyridyl complexes of FeII, Ru n, Mn H, Co t~and Cun have been encapsulated in zeolite Y (FAU) [ 12-15, 36], Metal-Salen and related Schiff's base complexes have been similarly encapsulated in faujasites [12-15, 37, 38]. However, in this case there is virtually no difference in kinetic diameter between the complex and the free ligand and metal-Salen complexes are readily leached by protic solvents, such as ethanol [ 12]. In the second method the ligand itself, constructed by intrazeolite synthesis, is too large to exit the supercages via the micropores. Most examples of this type pertain to FAUencapsulated metallophthalocyanines, first reported by Romanovsky and coworkers in 1977 [39]. They are prepared by first introducing the metal into the zeolite and then adding 1,2dicyanobenzene, which reacts at elevated temperatures to form the metallophthalocyanine in the supercages. Different methods have been used to introduce the metal ion: via ion exchange or as a metal carbonyl or metallocene [12]. In the former two cases the phthalocyanine ligands are largely metallated but many uncomplexed metal ions are also present. In the rnetallocene method, in contrast, there are no uncomplexed metal ions present but a large proportion of the encapsulated phthalocyanine ligands are metal-free. By using substituted 1,2-dicyanobenzenes encapsulated analogs of substituted metallophthalocyanines can be prepared [ 12]. In the template method the zeolite is allowed to crystallize around the metal complex which is assumed to act as a structure directing agent, i.e. the bottle is built around the ship. This allows for the encapsulation of well-defined complexes without contamination by the free ligand or uncomplexed metal ions (see above). The method is restricted to metal complexes that are stable under the relatively harsh conditions of temperature and pH involved in hydrothermal synthesis. Balkus and coworkers [14, 40, 41] used this approach for the encapsulation of metallophthalocyanines in faujasite. However, in order to fit into the faujasite supercages the phthalocyanine ligands are strongly deformed and Jacobs has

161 expressed some doubt [ 12] regarding the structure-directing capability of a template that requires initial deformation. The characterization of zeolite-encapsulated complexes is by no means simple and the same battery of techniques (see earlier) has been brought to bear [14] as with frameworksubstituted sieves. 5. CATALYTIC OXIDATIONS- FRAMEWORK-SUBSTITUTED MOLECULAR SIEVES 5.1. Ti, Zr, Sn and V Titanium(IV) silicalite (TS- 1), the first example of a framework-substituted redox molecular sieve, catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen peroxide under mild conditions (see Figure 5). Examples include phenol hydroxylation [42], olefin epoxidation [43], cyclohexanone ammoximation with N113/I-I202 [44], secondary alcohols to ketones [42], primary amines to oximes [45], secondary amines to hydroxylamines [46], sulfides to sulfoxides [47] and alkane oxygenation [48]. The remarkable reactivity of TS-1 is believed to be largely due to site-isolation of tetrahedral titanium(IV) in a hydrophobic environment. The latter ensures that hydrophobie substrates will be adsorbed from a reaction medium containing large amounts of water.

OH

O

NOH

11

1 : 1 o:p

~'

R~/ "- C H 2

L~

o NH a

R2CHOH + ~.

[ :

/R,S

I~-1

+

~ J

R2NH ~ , " =_ R2NOH

~RR'CHNH, RR'CHOH

RzSO

RR'C-'-NOH RR'C~O

Figure 5. TS-1 catalyzed oxidations with H20~

162

The TS- I catalyzedhydroxylationof phenol to a I:I mixture of catcchol and hydroquinonc has been commercialized by Enichem. Similarly,the arnmoximation of cyclohcxanone is being developed commercially as a low-saltalternativeto the conventional process for the production of cyclohexanone oxime, the raw materialfor nylon-6. The reaction involves initialTS-I catalyzedoxidationof NHa by H20~ to give NH2OH. The factthat bulky kctones such as cyclododecanone undergo ammoximation is consistentwith subsequent reaction of N H ~ O H with the ketone substratetaking place outsidethe molecular sieve.The method has been used [49] for the conversion of p-hydroxyacctophenonc to the corresponding oxime which is the precursorof the analgesicparacetamol (Reaction 13). NHCOCH

NOH

1

NH3/H202

(13)

TS-I OH

OH

OH

TS-1 is a particularly active catalyst for olefin epoxidation [43], even unreactive olefins such as allyl chloride being smoothly epoxidized at temperatures close to ambient. Relative reactivities of olefin substrates are completely different to those observed in analogous homogeneous systems. Owing to the steric restrictions of the micropores of TS-1 only straight-chain olefins are smoothly epoxidized. Cyclohexene is completely unreactive (see Table 3). Similarly, in contrast to homogeneous titanium catalysts, TS-1 shows no enhanced reactivity towards allylic alcohols indicating that there is no coordination through the OH group. Table 3 TS-1 catalyzed epoxidations with H202a Olefin

T (~

conv. (%)

Epoxide sel. (%)'

H202

Propene

40

72

90

94

l-Pentene

25

60

94

91

1-Hexene

25

70

88

90

Cyclohexene

25

90

10

n.d.

Allyl chloride

45

30

98

92

Allyl alcohol

45

35

81

72

.

_

t (min)

.

.

.

.

.

_

"MeOH solvem; olefin~202 molar ratio = 5; data taken from M.G. Clerici and P. Ingallina, J. Catal., 140 (1993) 71.

163 The solvent of choice is methanol which gives higher rates than aprotic solvents [43]. This is attributed to the formation of a titanium(IV)-hydroperoxo comples (I) in which coordination of the alcohol promotes oxygen transfer to the olefin (Figure 6). Coordination of the alcohol becomes increasingly difficult with i n ~ g steric bulk, consistent with the observed decrease in rate methanol > ethanol > tert-butanol.

sio

\

SiO --Ti--OSi / sio

SiO

R

SiO __XTiJ~H SiO

/C~~C~'~H

H202

SiO HOSi

\ ~- S i O - - T i

OH

ROH

sio/ \o /

c--c

sio \

/o\ c--c

=- SiO - - / T i - SiO

OR

Figure 6. Mechanism of TS-I catalyzed epoxidation Similar heterolytic mechanisms can be envisaged for other nucleophilic substrates, e.g. ammonia, amines, sulfides, phenols, alcohols. With alkanes or aromatic hydrocarbons, on the other hand, homolytic mechanisms, with possible involvement of HO. radicals, would seem more likely. A titanium(IV)-silicalite-2 (TS-2) catalyst having the MEL topology gives similar reactions to TS-I [50]. However, the scope of TS-1 and TS-2 catalyzed oxidations is limited to the relatively small molecules which can access the micropores (5.6 x 5.3 A and 5.3 x 5.4 A, respectively). This stimulated several groups to investigate the incorporation of titanium into larger pore sieves. Thus, Corma and coworkers [30] s u ~ e d in incorporating titanium in zeolite beta. The resulting titanium~silicoaluminate, Ti-BEA, catalyzed the oxidation of larger substrates such as cyclohexene and cyclododecane [51]. However, owing to the Br~rnsted acidity of Ti-BEA, the major products of olefm oxidation were glycols and glycol monomethyl ethers resulting fTom ring opening of the epoxide by H20 or MeOH, respectively. We subsequently showed [52] that ring opening could be suppressed by simply neutralizing the Brvnsted acid sites by ion exchange with alkali metal ions (see Table 4). Recently, aluminium-free titanium-substituted beta was synthesized, using a different template, and shown to catalyze the epoxidation of olefms with H20,, albeit with some ringopening [53].Ti-BEA also catalyzes epoxidations with TBHP [54], in contrast to TS-1 which cannot accommodate the transition state for epoxidation with the bulky TBHP. Ti-APSO-5, which also contains Brznsted acid sites afforded the diol as the main product in the oxidation of cyclohexene with H202 while with TBHP the epoxide was formed in 79% selectivity [55].

164

Table 4 Titanium-catalyzed epoxidations Olefin

Catalyst

Oxidant

, Conv. ]

Res

Selectivity (%)

(%) /

m

1=Hexenea

m

H202

TS-I Ti-beta

Cyclohexene

i

TS- 1 Ti-beta

,,....

98 80

H202 i

.,

,

epoxide

glycol(ether)

96 .12

4 88

nn

,,

:

l

m

,

,,=,l,

l

<5

100

-

H202

80

-

100

.

,

el

,,,

51

H202 ~

,=

51

|

i

1-Octene"

Li-TS-I Ti-beta Li-Ti-beta Na-Ti-beta

M H202 H202 H202 m

1-Octene b

Ti-beta Li-Ti-beta

Cyclohexene ~

TAPSO-5 TAPSO-5

m

,,

TBHP TBI-IP m

"MeOH; 40-60~

b

85 48 31 22

H202

,

98 O 87 84 9

m

47 38 |

0 97 5 6 ,,,,,, ,,

38 100

39 0

0 79

79 12

i !

,

'54 i

,|

H202 TBHP

,|

!

55

CF3CH2OH; 90~ c Me:CO/70 o.

In some cases the presence of both redox and Breasted acid sites can be an asset as it can provide the possibility of perfom~g bifunctional catalysis. For example, Ti-BEA and TiMCM-41 (see later) catalyzed the epoxidation of linalool with TBHP, with in-situ acidcatalyzed rean~gement to a mixture of cyclic ethers (Reaction 14) [56].

OH TBHP

OH v

CHaCN;80~

-I-

H

Catalyst

Furan/pyran

Ti-BEA Ti-MCM-41

1.62

0.89

(14)

165 The titanium-substituted mesoporous zeolite, Ti-MCM-41 has recently been synthesized and shown to catalyze oxidations of bulky substrates with H~O2or TBHP [57], e.g. reactions 15 and 16.

TBHP

Ti-MCM-41 CHzCI z " 4 0 ~ "5h

_

(15)

Conv. 30% Sol.

OH

90%

O

30% H20~, Bu~/But

(16)

Ti-MCM-41 o Conv. 20% Sel.

98%

I n short, the development of T S - 1 and related titanium-substituted molecular sieves has opened up a whole new area of selective oxidation chemistry with H202 and TBHP. For Some bulk chemicals, e.g. propylene oxide, the relatively high price of H=Ozcould result in unfavourable economics. This led Enichem workers [58] to investigate the possibility of insitu generation of H202. They showed that TS-1 was an effective catalyst for the epoxidation of propylene with H~O~ generated in-situ by autoxidation of a dihydroanthraquinone (Reaction 17). The antiuaquinone coproduct is hydrogenated in a separate step (Reaction 18) resulting in an overall two-step process for the epoxidation ofpropylene with H2 and 05.

OH

§247

Ts.,

+

+ H~.O

(17)

OH

OH

O + H2

O

PdlC

(18) OH

The success of titanium-substituted molecular sieves stimulated the investigation of

166 other metal-substituted silicalites, e.g. zirconium [59] and tin silicalites [60] with the MFI structure have been synthesized. They were shown to catalyze the hydroxylation of phenol with H202, albeit with lower activities and selectivities than TS-1. The incorporation of vanadium in silicalites and aluminophosphates has also been widely investigated [9-12]. The as-synthesized materials contain vanadium(~) which is converted to the pentavalent state on calcination. The resulting materials appear to contain vanadium(V) attached to the framework at defect sites [9 ]. V-MFI and V-MEL silicalites have been shown to catalyze the hydroxylation of aromatics and alkanes with H202. However, H202 efficiencies are generally low and, owing to the relatively high redox potential of the V v / v iv couple, homolytic mechanisms, involving intermediatehydroxyl radicals,probably predominate, as is observed in homogeneous vanadium systems [61].V-APO-5 was found to catalyze epoxidations and benzylic oxidationswith T B H P [62].However, more recent studies have revealed thatcatalysisis probably due to leached vanadium [63]. 5.2. Co, Cr, Mn and Fe Typical one=electron oxidants such as Mnm, Com, and Fem do not generally react via peroxometal pathways and one would not expect the same type of reactivity as that observed with titanium. ,4 priori one might expect free radical chain autoxidation pathways analogous to homogeneously catalyzed oxidations with these metals [1]. This appears to be borne out in practice, e.g. CoAPO-5 catalyzes the autoxidation of cyclohexane in acetic acid [64]. More careful examination revealed that cobalt is leached by the acetic acid and that the observed catalysis was homogeneous [65]. In contrast, it was claimed [65] that, in the absence of acid, the integrity of CoAPO and CrAPO (see later) is preserved in the autoxidafion of cyclohexane at 130 ~ However, it is worth noting that adipic acid is formed in these reactions which can also be expected to leach the cobalt (or chromium) at conversions higher than a few percent. Chromium-substituted molecular sieves contain Crv~after calcination, which might be expected to give reactions typical of oxometal pathways with ROzH, as is observed in solution [66]. Indeed, this proved to be the case: we found that CrAPO-5 effectively catalyzes benzylic [67], allylic [68] and alkane [67] oxidations, oxidation of secondary alcohols to ketones [69] and the selective decomposition of secondary alkyl hydroperoxides to the corresponding ketones [70]. The stoichiometric oxidants are either TBHP or 02 (Figure 7). Similar reactions are observed with CrAPO-11 and chromium silicalites with MFI or MEL structure [71].

R~,C--O

R2CHO2H R2C--O O~=R,CHOH

ArCOR ArCH,R~Oz

Figure 7. Selective oxidations catalyzed by CrAPO-5

167 For example, a series of alkylaromatics afforded the corresponding aralkyl ketones in high selectivities with TBHP in the presence of CrAPO-5 (3 m %) at 80 ~ (Table 5). TBHP could be replaced by Oz but this required neutralization of Brensted acid sites on the CrAPO5, by ion-exchange, in order to avoid acid-catalyzed decomposition of the benzylic hydroperoxide to the corresponding phenol, which inhibits the autoxidation. The addition of a small amount of TBHP to initiate the reaction also had a beneficial effect.

Table 5 CrAPO-5 catalyzed oxidations of alkylaromatics with TBHP [67] Conv. (%)a

ArCOR sel. (%)a

Ethylbenzene

70

90

p-Ethyltoluene

68

97

n-Propylbezene

59

93

n-Butylbenzene

59

92

Diphenylmethane

50

94

3-Ethylpyridine

43

80

ArCH2R

"Based on substrate.

The oxidation of hydrocarbons and alcohols with TBHP is presumed to involve oxidation of the substrate by oxochromium(VI) followed by reoxidation of Cr TM by TBHP, i.e. an oxometal pathway. When 02 is the oxidant the substrate undergoes initial chromiumcatalyzed autoxidation to the corresponding hydroperoxide. The latter undergoes catalytic decomposition and/or functions as the oxidant. The observation that the bulky triphenylmethyl hydroperoxide, which cannot be accommodated in the micropores, gave no reaction was construed as evidence for the reactions taking place in the micropores. This subsequently proved to be a misinterpretation of the results (see later). Manganese- and iron-substituted aluminophosphates have also been synthesized but they have proven to be particularly unreactive oxidation catalysts [72]. This lack of redox activity may be a result of the stable environment of isomorphously substituted Mn m and Fe tII. Interest in iron-substituted molecular sieves is derived from he fact that many redox enzymes contain iron in their active site. Hence, with this element one is obliged to adopt the alternative approach of encapsulation. 6. CATALYTIC OXIDATIONS: ENCAPSULATED COMPLEXES Pioneering studies of zeolite-encapsulated iron phthalocyanine (FePc) complexes were performed by Herron [73] who coined the term ship-in-a-bottle complex. He studied the 'oxidation of alkanes with iodosylbenzene catalyzed by FePc encapsulated in zeolites Na-X

168 and Na-Y. Although rather interesting (shape) selectivities were observed, reminiscent of heme-containing enzymes, activities were poor (ca. 6 turnovers) which was attributed to pore blockage by iodoxybenzene. After laying dormant for a number of years, Jacobs and coworkers re-initiated these studies using TBHP as the oxidant [11-13]. They found that FePc-Y, containing 1 FePc per 77 supercages, gave turnovers as high as 6000, compared to 25 with homogeneous FePc, in the oxidation of n-octane with TBHP [74]. It was important that the catalyst be prepared by the metallocene method (see earlier) in order to avoid the presence of uncomplexed iron. The catalyst eventually deactivates, presumably owing to ligand degradation (the TBHP should also be added slowly to minimize degradation). More recently, this was extended to the use of EMT [75] the hexagonal polymorph of faujasite and the 18membered ring aluminophosphate VPI-5 [74, 76] as the host. A further increase in catalytic activity/stability was obtained by substitution of the phthalocyanine periphery with electronwithdrawing nitro groups [77]. Balkus and coworkers [ 14, 78] synthesized a Na-X encapsulated ruthenium perfluorophthalocyanine complex (Ru-Fl6Pc-X) and showed it to be an active catalyst for the room temperature oxidation of cyclohexane with TBHP (Table 6). The catalyst showed no sign of deactivation, in contrast to the iron analogue.

Table 6 Ru phthalocyanine-catalyzed oxidation of cyclohexane with TBHP [78] Catalyst

Time (h)

Conv. (%)

Selectivity (%) ketone

alcohol

TBHP efficiency (%)

TOF (h-!)

RuPc

5

47

72

27

30

7.5

RuFi6Pc

24

83

78

22

48

15

RuFI6Pc-X

192

86

98

97

125

Another recent development is the use of faujasite-encapsulated manganese(II) bipyridyl complexes as catalysts for the oxidation of cyclohexene to adipic acid using 30% aqueous H202 [79]. The reaction proceeds via initial epoxidation followed by ring opening to the diol and oxidative cleavage. The turnover frequency was 15 h ~ and H202 efficiency 47-62% at cyclohexene conversions of 9-17%. Other recent variations on this theme include the encapsulation of VO(bipy)22+ [37], VO Salen [38] and iron(II) N,N-bis-(2-pyridinecarboxamide) [80] complexes in zeolite Y. The vanadium complexes were used as catalysts for the oxidation of cyclohexene or cyclohexane with TBHP. Oxidation of the latter, to a mixture of cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide, presumably involves hydroxyl radicals by analogy with homogeneous systems [61 ]. Moreover, the question of leaching still has to be addressed in these systems. The encapsulated iron complex catalyzed the oxidation of cyclohexane with H202, probably via a homolytic Haber-Weiss-type mechanism.

169 More recently, these systems have been elevated to a new level of biomimetic sophistication, namely by embedding the zeolite-encapsulated complex in a polydimethylsiloxane membrane their performance could be improved even further [81 ]. The hydrophobic membrane mimics the phospholipid membrane in which cytochrome P-450 resides and acts as an interface between the two immiscible phases (cyclohexane and aqueous 70% TBHP). Based on these recent developments ship-in-a-bottle catalysts appear to hold much promise, extending even to the design of chiral 'ships' for enantioselective oxidation. 7. THE QUESTION OF LEACHING An important question regarding the observed catalytic activity of divers redox molecular sieves is the following: is the catalyst truly heterogeneous or is the observed activity due to leached metal ions? A more than cursary examination of the literature reveals that in most cases an adequate verification of true heterogeneity has not been performed. One point which has become clear from our own work is that the standard recycling of the catalyst without significant loss of activity can by no means be construed as evidence of heterogeneity (see later). Another important question to ask is: is it important if trace amounts of metal ion are leached from the catalyst? Obviously if virtuaUy all of the observed catalysis is due to leached metal ion then it would be simpler to carry out the reaction with a homogeneous catalyst. On the other hand if, as is the case with titanium(IV), the metal ion shows little activity in homogeneous solution, and is not considered environmentally hazardous, then leaching of trace amounts may be less of a problem. We recently performed a detailed investigation of the stability of chromium-substituted molecular sieves [82]. As a model reaction we chose the allylic oxidation of a-pinene with TBHP (Reaction 19) which gives verbenone in high selectivity.

I TBHPICrCatalyst PhCI, 80 ~ "-'-

(19)

Experiments were performed in which the catalyst (CrAPO;5, CrAPO-11 and CrS-1) was filtered, at the reaction temperature (80 *C), after 30 minutes, which corresponded to 30-40% conversion. Filtration at the reaction temperature is important in order to avoid possible readsorption of leached metal ion on cooling. When the filtrate was allowed to react further at 80 ~ the reaction continued at roughly the same rate. Further insights into the mechanism of catalysis were obtained from the allylic oxidation of valencene (I) which is too bulky to enter

170 the pores of CrAPO-5. When TBHP was the oxidant smooth oxidation was observed. In contrast, when the bulky triphenylmethyl hydroperoxide (H) was used, essentially no reaction was observed.

OH

I

II

These results can be rationalized by assuming that chromium is leached by reaction with RO2H, which occurs with TBHP but is impossible with the bulky (II) which cannot access the micropores. This was confirmed by performing an experiment in which CrAPO-5 was stirred with a solution of TBHP. After filtration the filtrate exhibited the same activity as was observed with the CrAPO-5 present. When the CrAPO-5 was initially stirred with the substrate, filtered and TBHP added to the filtrate no catalysis was observed. Having established that the observed catalysis was due to leached chromium we quantitatively determined the amount of chromium in the filtrates [83] which was found to vary with the catalyst used. The lowest amount of leaching was observed with CrAPO-5 and corresponded to 0.3% of the chromum present in the catalyst, i.e. 0.3% of 0.88%. In the above mentioned oxidations this corresponds to a substrate/catalyst ratio (S/C) in solution of 17000. Hence, we performed an oxidation of g-pinene with TBHP and homogeneous chromium(VI) (pyridiniumdichromate) at S/C = 17000 and observed essentially the same reaction rate as with CrAPO-5. An important lesson learnt from this work is that with some metals, e.g. chromium, minute amounts of leached metal ions, in our ease 0.3% of the available chromium, can account for the observed catalysis. This means that the catalyg could, for example, be recycled ten or even a hunderd times while still observing the same activity. Hence, we conclude that, in the absence of unambiguous evidence to the contrary, many literature claims for heterogeneous catalysis by redox molecular sieves are, to say the least, questionable. Indeed, solubilization by reaction wih RO2H appears to be widespread phenomenon. For example, van Hooff and coworkers [63] observed this with VAPO catalysts and Schuchardt and coworkers similarly observed leaching with V, Cr, Mn, Fe and Co-substituted MCM-41

[841.

There seems little doubt that the remarkable activity of TS-1 is heterogeneous in nature. We note, however, that small amounts of leaching to give catalytically inactive species cannot be excluded, especially in the presence of NH3 or carboxylic acids. Similarly, the enhanced reactivity and stability of zeolite-encapsulated metallophthalocyanines, compared to homogeneous counterparts suggests that the catalysis is truly heterogeneous, although it would be comforting to see unambiguous verification. The same cannot be said, however, of

171 other encapsulated complexes, e.g. we have recently shown that vanadium-Schiff's base complexes encapsulated in zeolite Y undergo facile leaching by reaction with TBHP [85]. 8. FUTURE PROSPECTS The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H202 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically useful systems. In this context the development of chiral ship-ina-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation 'mineral enzyme'. 9. REFERENCES

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

177

Synergistic Effects in M u l t i c o m p o n e n t Catalysts for Selective O x i d a t i o n P. Courtine and E. Bordes, D6partement de G6nie Chimique, Universit6 B.P. 20529, 60205 Compibgne Cedex, France.

de Technologie

de Compi~gne,

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled: (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. One important fact is that synergistic effects are also known in solid state chemistry, in the absence of reactive atmosphere. For a two-phases AOx/BOy sytem, a physical property of one component BOy, e.g., the temperature at which an allotropic transition proceeds, can be modified by the presence of AOx. In the examples of catalytic synergy we have studied in the past, we have also observed such a "solid state synergy" in inert atmosphere, which cannot be explained by anything else than cooperative transformation due to coherent interfaces. Therefore, the question to answer is: what is the common explanation to these catalytic and non catalytic phenomena, observed as well with oxide/oxide (or chloride/chloride) as with metal/oxide? The oldest and first example in selective oxidation is V205-TiO2(anatase), catalyst of o-xylene oxidation to phthalic anhydride. Nearly in the same time, three teams in Poland, Great-Britain and France [4-10] observed the catalytic synergy and studied also the

178 non-catalytic phenomenon by thermal analysis and X-ray diffraction (XRD). Vrjux and Courtine proposed an explanation by using the concept of coherent interfaces [9], already known to apply in other fields like semiconductivity. This concept, proposed earlier by Ubbelohde [ 11] to account for hysteresis and other non-equilibrium phenomena in other kinds of systems, had soon after been used in the field of reactivity of solids when progress in electron microscopy allowed to evidence extended defects and microdomains formed during the reduction of titanium, vanadium and niobium oxides [ 12]. Since that time, we have made several experiments to check the validity of the model [13-23], which has been extended to other systems than those based on ReO3-type oxides [1, 24-28]. The present paper aims at considering briefly the methodology we used and the main examples we studied. We shall also consider the remote control model proposed by Delmon et al. [29, 30] to interpret the same kind of synergistic effects for similar systems, in order to see whether or not the two models are conciliable and on what points of view. 1. THE CONCEPT OF COHERENT INTERFACES It is well-known in thermodynamics that, when two phases are in physical contact, the surface and interface energies barriers are so high that each phase behaves as if it was alone and keeps its own properties. However, when the crystal structures are closely related, the crystallographic misfit can be very low, and the interface is said to be coherent [ 11, 12]. The atoms close to the boundaries are strained, so that their potential reactivity is greater than when they are inside the pure phases, and the diffusion of various specie (atoms, ions, electrons) across the interface is favored because the interfacial energy barrier is strongly lowered. At short range order (say, 1-10 nm), that is when considering the crystal framework, microdomains of one phase inside (and/or at the surface of) the other can be formed. This is a way to account for non-stoichiometry and solid-solid cooperative reactions, but also for the generation of new active centres in catalytic reactions [9, 15, 28, 31-33]. The turn-over frequency or the specific activity is therefore expected to increase. Microdomains can be formed during the transient state and be kinetically, but not thermodynamically, stable during the steady state. However, other conditions are needed to observe selectivity enhancement, because the additional active sites must be also selective. Several examples of the formation of coherent microdomains have been given usingin situ High Resolution Transmission Electron Microscopy (HRTEM), in the case of V205 or MoO3 reduction as well as during catalytic reactions by Gai [34]. A low crystallographic misfit (few percents) is to be considered as a pre-requisite condition to observe synergy. Finally, it is well conceivable that, since synergistic effects depend strongly on the way the interfaces are "prepared", factors such as the method of preparation and activation of the catalyst (supported or

179 multicomponent), the operating catalytic conditions, the steady or transient state, etc., are preponderant. 2. E X A M P L E S

With an AOx/BOy powder system, and apart the formation of a third phase ABOz in conditions where normally it should not exist, either only one phase reacts and the other acts as an external boundary surface, or both are transformed. Obviously, the observed situation depends on the operating conditions and on the chemical reactivity (and physical properties) of the phase(s), but also on surface characteristics. 2.1. One phase reacts while in contact with the other.

In the first case, one (or more) crystallite face exposed by AOx in a convenient AOx/MOy system may share coherently a face of MOy, provided its surface plasticity is large enough, that is, if the constituents are labile enough to accommodate the misfit. The model of coherent interfaces was applied to "bulk" V205 supported on TiO2 (anatase) to account for the synergistic effect evidenced in the seventies [4-10]. Since that date, the demonstration has been extended to other supports structurally related to anatase [14, 19]. The degree of "plasticity" is related to the Tamman temperature which amounts ca. 450 K for V205, well below the usual temperature of reaction. Several names have been given to this phenomenon, from "premelting" by Ubbelohde [8], to sintering, contamination, wetting and thermal spreading. The latter terms were introduced mostly for monolayer supported catalysts [35, 36], for which the concept of coherence does not seem to be appropriate. However, if the structure of the monolayer does not depend on the specific oxide support MOy (M = Ti, Nb, Zr, A1, Si), its reactivity does since the ease of reduction of VOx depends on the strength of the V-O-support bonds [37, 38]. We have been able to oxidize ethane to acetic acid using VPOx/TiO2 and VOx/TiO2 catalysts at low temperature (200-250~

[21, 22], but when

zirconia or silica were used, no acetic acid was found. The majority of examples of catalytic synergy in selective oxidation and related reactions belongs to this first category and involves ReO3-type oxides and related oxysalts, including rutile structures and compounds containing antimony (VSbO4, Sb204). V205 and vanadiumcontaining catalyts are supported on titania or other structurally related supports, or mixed with Mo and/0r Nb as in the more recent examples of oxidation of ethane to acetic acid [21] or propane dehydrogenation [39]. When TiO2 anatase was used as a support for other oxides (MOO3) or oxysalts like, e.g., CoMoO4, the partial conversion of TiO2 anatase to rutile was observed in non catalytic conditions and n-butane was oxidized to maleic anhydride on the three-phases CoMoO4/MoO3/TiO2 system [20].

180 Metallic molybdates MMoO4 (M= Mn, Fe, Co, Ni, Cd, etc.) are more active and/or selective when MoO3 is present in such an amount so as to be detected by XRD. Several examples have been given by Ozkan et al. [40-42] for oxidative dehydrogenation and oxidation reactions. We studied the system N i M o O ~ o O 3 which is one of the catalysts for propane to propene. The best stoichiometry is Mo/Ni = 1.26/1, and, to observe the highest conversion, the catalyst must be prepared by a special method which leads to a maximum of contacts between NiMoO4 and MoO3 during the calcination [23]. Structural relationships between bismuth molybdates have soon be suspected [38], and complex formulae involving several metallic molybdates have allowed to increase the yield in acrylonitrile or acrolein obtained by (amm)oxidation of propylene [44]. The only way to account for this necessary complexity was to consider that coherent interfaces exist between the main phases because they have close structural relationships [15, 31-33]. The system has been recently investigated by Ponceblanc et al., who measured the change of electronic conductivity according to the number of phases simultaneously present [45]. In other examples, extensively studied by Delmon et al., Sb204 was used with MoO3 for isobutene oxidation to methacrolein [29, 30], and for the dehydration of N-ethyl formamide [46, 47]. Antimony is one of the elements frequently found in selective oxidation catalysts, as in the pionneering work on uranium antimony oxides for ammoxidation of propene [48], and more recently in ammoxidation of propane on V-Sb-A1 system [49]. Courtine et al., have shown that some effects, like the drastic increase of activity in Ziegler-Natta polymerization of ethylene when the catalyst is supported by MgC12 [ 1, 24], or the stabilization of platinum catalysts for abatement of automotive pollutants when the alumina is doped with rare earth oxides [25-27], could also be interpreted by using the same concept. In the first example, the active solid solution TiC13-A1C13 is supported by MgC12. These second generation of catalysts are prepared by ball-milling of the two components in inert atmosphere, and their activity has been increased by (at least) a 103 factor. The optimum depends on the time of ball-milling, and is related to a definite degree of disorder in the stacking planes of TiC13-A1C13crystallites. Both parameters and framework of the hexagonal platelets of MgC12 and of TiC13-A1C13 are indeed very close [1, 24]. Obviously, the mechanical energy brought during ball-milling favors the interactions between the platelets, and the resulting supported catalyst could be said "contaminated" by the support. Ball-milling is now a method to increase the activity of VPO catalysts for n-butane oxidation to maleic anhydride [50], as well as of perovskites for NOx abatement [51]. In the second example, it has been shown that the thermal stabilization of the ~5-alumina used as the washcoat on cordierite is achieved by means of the nucleation of a cubic LnA103 (Ln = La, Pr,'Nd, Ce) perovskite structure. Microdomains of LnA103 have been evidenced by HRTEM. Moreover,

181 the trend to sintering shown usually by platinum crystallites would be lowered because the matching between the [011] zone axis of Pt and the [110] of 8-alumina is very good [25-27]. 2.2. The two phases are transformed (cooperative reactions) The second case, is, to our opinion, as important as the first, because it is a way to demonstrate that the catalytic atmosphere is not necessary for the existence of interfacial coherence [9, 15, 17]. Heating the systems under nitrogen leads generally to transformations which would not proceed for components alone. The most important experiment concerned the transformation of anatase into ruffle in the presence of V205 by heating under nitrogen, V205 being simultaneously reduced. Then the idea was that, if the framework of TiO2 (anatase) was responsible for reduction of V205, similar frameworks like those of MNbO4 (M= A1, Ga) as well as of TiO2(B), would lead to the same effect [14, 19]. The reverse (rutilization of anatase in the presence of MOO3, VOPO4, etc.) was also observed [17]. Similarly, the transition c~ ~ ~-CoMoO4 is delayed in the presence of MoO3/TiO2 (Mo/Co = 1.9:1), the result being an increase of the activity/selectivity of the mixed system for butanemaleic anhydride. On the contrary, the transition a ~ 13-NiMoO4 is advanced in the presence of MoO3 (Mo/Ni = 1.26:1) in catalysts prepared by the fight way for propane-propylene, which is correlated to a stronger stabilization of MoO3 since it does not sublimate easily [23]. The formation of a third phase from the two first precursors can also be responsible for the stabilization of a support, as in the case of the three-way catalysts Pt/LnA103/8-A1203 just evoked. Other examples can be found in the ternary V-Nb-Mo-O system for ethane oxidation to acetic acid: one composition (Mo/V/Nb= 0.73/0.18/0.09) gives the best catalytic results, which has been interpreted as due to the formation of definite microdomains of (V, Nb) solid solutions of Mo18052 and Mo5014-type, embedded in a matrix of MoO3 [21, 22]. In such a way, the active sites would be more isolated than in pure MOO3, and consecutive oxidation of acetic acid or C2H4 to CO2 could be avoided. According to the considered system, the coherence of interfaces can lead to two opposite effects: the surface lattice of the catalyst being maintained in a metastable state, the result is an increase of activity (and/or selectivity) compared to the unsupported catalyst, which can be accompanied by a stabilization on time (and on stream). A last example is indeed provided by the case of H4PMollVO40 supported on SiO2 doped with potassium for the oxidative dehydrogenation of isobutyric acid to methacrylic acid [52, 53]. The easy adaptation of the amount of water and of protons between the Keggin units according to the degree of hydration and to the kind of reaction results in a remarkable "elasticity" of the heteropolyacid lattice allowing its anchoring on suitable supports. The formation of layer(s) of KxHl-xPMo12040type on the surface of K-SiO2 (cristoballite) would be responsible for the anchoring of the active H4PMol 1VO40 phase of cubic symmetry.

182 3. CONCLUSION In all the cases studied above, the common point is the existence of coherent interfaces due to the closely related structures of the two (or more) components. Very recently, a computer modelling study of the interfaces between V205 and TiO2 has given support to this model by considering the morphology of the two components, and showed also the differences in the case of a monolayer of V205/TiO2 [54, 55]. The atoms on the boundaries are strained, so that their activity is greater than when they are inside the pure phases, and the lowering of surface energy allows various specie to cross the interface. The coherence does not depend on the reactants surrounding the crystallites, since cooperative reactions can proceed in non reactive atmosphere. Microdomains of one phase inside the other can be formed during the transient state and be kinetically, but not thermodynamically, stable during the steady state. These microdomains are also a way to account for the necessity of site isolation proposed by Callahan and Grasselli to avoid total oxidation [56]. Finally, in the case of mixed oxides as, e.g., Bi-Fe-Mo-O [31, 32] or Mo-V-Nb-O [21], in which a second redox couple (Fe3+/Fe 2+ or V5+/V 4+ respectively) is supposed to modify the Mo6+/Mo5+potential as compared to that in the single phase (bismuth molybdate or MoO3 respectively), the lowering of the surface energies which allows the transfer of electrons through the interface is the only way to account for the mutual and beneficial influence of the two redox couples. Therefore, the generation of new active centres which accelerate the reaction, as proposed by Delmon [29, 30], can be understood as the result of the coherence of interfaces. The same idea is valid for oxygen ions which are supposed to be able to cross the interface if an oxygen vacancy is available on the other side. Though hydrogen, a far smaller atom, is well known to spill over metals and oxides, any anionic oxygen vacancy present on the surface of a metallic oxide will act as a trap for 0 2- (or other ionic forms). When applying the Crystallochemical Model of Active Sites (CMAS) to vanadyl pyrophosphate [57], thermodynamic considerations showed that hydrogen could indeed spill over the (100) (VO)2P207 surface (2 to 3 sites) but that for oxygen, a kind of hopping model (known for electrons), or afortiori oxygen spill over, was unlikely.

Acknowledgements:

Part of this work has been made in the frame of the EC-HCM

programme EBRXCHCT-93-0261.

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184 33. R.G. Teller, M.R. Antonio, J.F. Brazdil and R.K. Grasselli, J. Solid State Chem., 64 (1986) 249. 34. P. Gai-Boyes, Catal. Rev.-Sci. Eng., 34 (1992) 1. 35. J. Haber, New Developments in Selective Oxidation by Heterogeneous Catalysis, P. Ruiz and B. Delmon (eds.), Stud. Surf. Sci. Catal., 72 (1992) 279. 36. H. Kn6zinger and E. Taglauer, Catalysis, 10 (1993) 1. 37. I.E. Wachs, R.V. Saleh, S.S. Chan and C.C. Cherzick, Appl. Catal. 15 (1985) 339. 38. G. Deo and I.E. Wachs, J. Catal. 146 (1994) 323. 39. R.H.H. Smits, K. Seshan, J.R.H. Ross, L.C.A. van den Otelaar, J.H. Helwegen, M.R. Anantharaman and H.H. Brongersma, J. Catal., 157 (1995) 584. 40. U. Ozkan and G.L. Schrader, J. Catal., 95 (1985) 120, 137 and 147; ibid Appl. Catal., 23 (1986) 327. 41. U. Ozkan, R.C. Gill and M. Smith, J. Catal., 116 (1989) 171. 42. U.S. Ozkan, M.R. Smith and S.A. Drsicoll, J. Catal., 123 (1990) 173. 43. J.L. Callahan, R.K. Grasselli, E.C. Milberger and H.A. Strecker, Ind. Eng. Chem. Prod.Res. Develop., 9 (1970) 134. 44. W. Gerhartz ed., Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., vol A (1985) 153. 45. H. Ponceblanc, G. Coudurier, J.M. Herrmann and J. Vedrine, J. Catal., 142 (1993) 373. 46. P. Ruiz, B. Zhou, M. Rely, T. Machej, B. Doumain, F. Aoun and B. Delmon, Catal. Today, 1 (1987) 181. 47. B. Zhou, E. Sham, T. Machej, P. Bertrand, P. Ruiz and B. Delmon, J. Catal. 132 (1991) 157, 183 and 200. 48. R.K. Grasselli, J.D. Burrington and J.F. Brazdil, J. Chem. Soc., Farad. Disc., 72 (1982) 203. 49. G. Centi, E. Foresti and F. Guarnieri, New Developments in Selective Oxidation II, V. Cortbs-Corberan and S. Vic Bellon (eds), Stud. Surf. Sci. Catal., 82 (1994) 281. 50. V.A. Zazhigalov, J. Haber, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, Appl. Catal. A, 135 (1996) 155. 51. T. Hibino, K. Suzuki, K. Ushiki, Y. Kuwahara, M. Mizuno, Appl. Catal. A, 145 (1996) 297. 52. C. Desquilles, M.J. Bartoli, E. Bordes, G. Hecquet and P. Courtine, Erd61 Erdgas Kohle, 3 (1993) 130. 53. Richard-Tessier, E. Bordes, P. Courtine, l lth Int. Congress on Catalysis, Baltimore, July 1-7, 1996, Preprint p. 247. 54. D.C. Sayle, D.H. Gray, A.L. Rohl, C.R.A. Catlow, J.H. Harding, M.A. Perrin and P. Nortier, J. Mater. Chem., 6 (1996) 653. 55. D.C. Sayle, C.R.A. Catlow, M.A. Perrin and P. Nortier, J. Phys. Chem., 100 (1996) 8940. 56. J.L. Callahan and R.K. Grasselli, AIChE. J., 9 (1963) 755. 57. J. Zio~kowski, E. Bordes and P. Courtine, J. Catal., 122 (1990) 126; ibid., Stud. Surf. Sci. Catal., 55 ( 1991) 625.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

Synergetic oxides

effects promoted

by

in operandi s u r f a c e

185

reconstructions

of

Eric M. Gaigneauxa, c, J. N a u d b, P. Ruiz a and B. Delmon a a Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain - Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium. b Laboratoire de G6ologie et de Min6ralogie, Universit6 catholique de L o u v a i n Place L. Pasteur 3, B-1348 Louvain-la-Neuve, Belgium. c "Aspirant" fellow for the Fonds National pour la Recherche Scientifique (F.N.R.S.) of Belgium. Synergetic effects are observed in the selective oxidation of isobutene to methacrolein w h e n catalysts are prepared by mechanically mixing MoO3 with BiPO4. Both higher conversion and selectivity to methacrolein are observed. Catalyst characterization was carried out before a n d after catalytic tests. Scanning electron microscopy revealed that, when mixed with BiPO4, the (010) faces of the MoO3 crystals, known to be non selective, reconstructed to more active and selective (100) faces d u r i n g the test. No effect was observed with p u r e MoO3 in the same conditions. In agreement with previous results, there was no indication of mutual contamination. The results lead to conclude that the two effects, namely the increase in activity and selectivity as well as the reconstruction, are linked and are both due to a remote control mechanism. This remote control is due to the action of spillover oxygen "Oso" generated by BiPO4 (the "donor") on MoO3 (behavin@ as an Oso "acceptor"). This confirms the interpretation given to identical results with mixtures of MoO3 and ~-Sb204 (which, as BiPO4, is k n o w n to be an Oso donor) and reinforces the conclusion that the origin of the synergy is not mutual contamination. As different donors have the same effects, the conclusion is that the synergy is due to spillover oxygen and not to the chemical nature of the phase mixed with MOO3. 1. I N T R O D U C T I O N Synergetic effects between oxide phases are very frequent in the selective oxidation of hydrocarbons. Multiphasic catalysts have performances superior to the sum of those of the constituting phases used alone. The main phenomena observed are an increase of the conversion of the hydrocarbon, an I m p r o v e m e n t of the selectivity for the desired partially oxygenated products and an increase of the lifetime and resistance to deactivation in severe operating conditions. Different hypotheses could in principle explain this cooperation (or these synergetic effects): formation of more active mixed phases or solid solutions, m u t u a l surface contamination, support effects, existence of structurally coherent phase boundaries (epitaxy), etc. However, only the remote control mechanism ("RCM") is relevant when the synergetic effects are obtained with physical mixtures of simple crystalline phases with no chemical reaction nor contamination of the phases even after long reaction times [1,2]. In the particular case of oxidation, this "RCM" theory considers that one of the phases (called the "donor") is able to activate molecular oxygen into a

186 highly active mobile species, called "spillover oxygen" ("Oso"). This phase migrates onto the surface of the other phase (the "acceptor")with which it reacts. This reaction brings about the creation of new selective sites a n d / o r the regeneration of sites deactivated during the normal redox cycles of the oxidation mechanism (these sites are usually slightly reduced). The remote control mechanism (rather than other possible hypotheses) has been shown to explain the synergetic effects observed between various phases (transition metal oxides, mixed metal oxides, etc) in various oxidation reactions (true selective oxidation, oxidative dehydrogenation, oxygenassisted dehydration) on various hydrocarbons (light alkanes, olefi-ns, amides, etc) in a wide range of operating temperatures (from 423 K to 723 K) [1,3-6]. The theory has also been strongly s u p p o r t e d b v many more fundamental arguments includin~ proofs of the migration of labellec~ 18Oso and accurate kinetic modelling [7-10]. ~' In the past years, many independent investigators have shown strong evidences of the existence of oxygen spillover and of the positive role played by this species in improving the performance of catalysts. In the latter case, these authors often alluded to a "spillover effect" rather than a "remote control". These two different terms nevertheless reflect the same concept [11-15]. The general purpose of the present work is to investigate new experimental lines to further clarify the mechanisms underlying the effects of the RCM and to better understand the role of Oso at the surface of the "acceptor phases" to improve their performances. In this context, the synergetic effects between MoO3 and (zSb204, prepared separately and physically mixed afterwards, have been extensively investigated in the selective oxidation ofisobutene to methacrolein. These effects corresponded to an increase of the isobutene conversion and of the selectivity for methacrolein at the expense of the total oxidation products. Strong arguments showed that the cooperation was caused by a remote control through the migration of Oso, with oc-Sb204 as "donorphase" and MoO3 as "acceptor phase" [16]. A special series of experiments was performed with MoO3 samples mixed with oc-Sb204, which were mainly developing the (010) basal face. In agreement with the wellknown "structural specificities" of MoO3 (the (010) crystalIographic faces of MoO3 are non selective), these special MoO3 were weakly active and totally non selective when used alone [17-20]. But when mixed with oc-Sb204, the synergetic effects observed both for the conversion and the selectivity to methacrolein, were the most dramatic ever detected. These suggested that selective sites had really been created in. operandi (during the reaction) at the surface of MOO3. Scanning electron mzcroscopy investigation of the MoO3 crystallites after the reaction in the presence of 0c-Sb204 revealed a reconstruction. The edges between the (010) faces and the (100) lateral faces, which were sharp in the fresh samples, had developed at the micrometric scale a facetted structure composed of a succession of steps with the vertical walls oriented as (100) [21,22]. The phenomenon was also observed at the nanometric scale using atomic force microscopy on macroscopic MoO3 monocrystals [23]. The reconstruction was not observed in either sets of experiments when MoO3 was used in the absence of 0c-Sb204. This is because Oso, produced by 0c-Sb204, reacts with the surface of MoO3 and favors the coordinations of surface Mo atoms typical of the (100) faces. In the in operandi conditions, this leads progressively to the reconstruction of the crystallites from (010) faces to (100) steps. The enhanced performance of the mixtures supposes the creation of new selective sites that are attributed to the creation of more of these (100) steps which are known to be more active and more selective than (010) faces (another aspect of the "structural specificities" mentioned in the literature). As this only occurred in the presence of ~Sb204, the picture was consistent with the occurrence of the synergetic effects for the mixtures and fitted perfectly with the atomic scale model proposed in the RCM for the creation of selechve sites under the influence of Oso [3,4]. The objective of the present communication is to further support this picture. If the model proposed above for the role of Oso in the creation of new selective sites

187 at the surface, of MoO3 is.correct, it can be speculated, that the same effects, namely synergetlc effects and simultaneous reconstructxons of the crystals, should be observed whatever the origin of the Oso, namely whatever the "Oso donor phase" with which MoO3 is mixed. Following this line, a series of experiments parallel to the ones summarized above was performed with another "donor phase", switching from the MoO3 + 0~-Sb204 system to the MoO3 + BiPO4 system. BiPO4 has been shown to act as an "Oso donor phase" in many other oxidation reactions over multiphasic catalysts [16,24-27]. Moreover, extensive experiments have discarded the possibility of mutual contamination between MoO3 andBiPO4 [25]. The oxidation of ~sobutene was carried out in the same conditions as with c~-Sb204 and, similarly, the catalysts were characterized by scanning electron microscopy, X-ray diffraction and specific area measurements before and after the catalytic test.

2. EXPERIMENT 2.1. Catalyst preparation (010) oriented molybdenum trioxide. MoO3 crystallites developing preferentially the (010) faces were synthesized by recrystallisation of an isotropic molybdenum trioxide commercial powder (BDH Chemicals, 99.5+%) in a flow of pure 0 2 at 873 K during 12 hours. The position of the X-ray diffraction (XRD) peaks of the obtained yellowish solid fitted perfectly those of the JCPDS molybdite phase standard [28]. The anisotropy of the crystallites was determined by scanning electron microscopy (SEM) and checked by XRD, according to the method proposed by Ozkan et al. (Y. IhO0 / ~ IOkO = 0.005, where ~ IhO0 and ~ IOkO are the sums of the intensities of the peaks corresponding respectively to the reflections of the (h00) and the (0k0) series of crystallographic planes) ~18]. The SBET area was 0.81 m2.g -1. Bismuth plwsphate. A 0.05 M aqueous solution of Bi(NO3)3.5 H 2 0 (previously complexed with mannitol, mannitol / Bi3+ = 3 / 1 molar) was precipitated at room temperature using a 0.05 M aqueous solution of (NH4)2HPO4. The quantity of h y dro g eno..phosphate, used. was. determined considerin g a com p lete stoichiometric reaction with bismuth ions (1 B13+ for 1 PO43-). After washing with distilled water and subsequent lyophilisation, BiPO4 was obtained as a fine colloidal powder. The sample was thereafter calcined at 773 K in air during 20 hours. XRD pattern of the obtained solid corresponded to the JCPDS bismuth phosphate standard [28]. SBET area was 6.39 m2.g -1. Mechanical mixture. MoO3 (2.184 g i;,e. 1.77 m 2) were mixed in 250 ml of npentane with BiPO4 (0.316 g i.e. 2.01 m z) and physically interdispersed using ultrasounds during 10 minutes. No further mechanical agitation was made in order not to damage the MoO3 crystallites. Pentane was then removed by vacuum evaporation at room temperature before the mixture was dried at 353 K during 20 hours. The success of the mterdispersion of the two phases was checked by SEM. To rigorously compare the activity otthe mixture with that of the pure oxides, each pure oxide was submitted to exactly the same "mixing" procedure before being catalytically tested. Figure 1 shows SEM micrographs of the pure oriented MoO3 and the pure BiPO4 h a v i n g undergone the "mechanical mixture" procedure, and the mechanical mixture of these two. 2.2. Catalytic activity measurement The catalytic activity measurements were performed in a fixed bed reactor at 693 K. The gas tlow composition was isobutene 7 0 2 / He = 1 / 2 / 7 (vol.) with a total flow of 30 ml.min -1. The masses of catalysts used were so that the area developed by each phase (when present) in the reactor was identical when tested alone and in2the m!xture : namely, 150 mg of BiPO4 (i.e. 0.716 m2), 1039 mg of MoO3 d.e.U./3u m ) for the ~sts with the pure phases and 1189 mg ofdnechanical mixture (i.e. 150 mg or 0.716 m z of BiPO4 mixed with 1039 mg or 0.730 m z of MOO3). In order

188

Figure 1. SEM micrographs of the pure oriented MoO3 (left) having undergone the "mechanical mixture" procedure, and the mechanical mixture of the t w o - BiPO4 corresponds to the small white rod-shaped particles (right). not to perturb, the orientation, of the MoO3 crystals, the catalysts were used as powders without being pressed into pellets. The volume of the catalytic bed was kept constant for all the tests by diluting the catalysts in small glass balls previously checked to be inactive. The heating of the reactor was realized in the same flow as during the reaction at a carefully controlled rate of 7.5 K.min -1. The activity was measured during 3 hours, after which the catalysts were cooled down in the reactant stream at 7.5 K.min -1. Untransformed isobutene and the products of the reaction were analyzed at the reactor oulet by on-line chromatography. The catalytic activity was expressed in terms of conversion of isobutene (%C), yields (%Y) and selectivities (%S) in the different products, calculated as shown in equations la, b and c. Conversion of isobutene : %C = moles of isobutene transf~

of isobutene injected * 100

(la)

Yield in the product X (containing c atoms of carbon) : (lb) %Yx = (moles of X formed * ~ / m o l e of isobutene injected * 4) * 100 Selectivity for the product X" (lc) %SX = %Yx/~%C* 100 The synergetic effects between the oxide phases in the mechanical mixture were evaluated b y comparing the observed activities with theoretical values calculated assuming that no cooperation occurred, namely that the two phases in the mixture were behaving as if they were alone in the reactor. Equations 2a, b, c show how these theoretical values (noted with superscript ' th") were estimated.

189 Theoretical conversion of isobutene 9 %C th = %C obtained with BiPO4 + %C obtained with MoO3

(2a)

Theoretical yield in the product X" (2b) %yth,~ = %Yx obtained with BiPO4 + %Yx obtained with MoO3 Theoretical selectivity for the product X" (2c) %S~ = %Y ~ / / %cth 2.3. Characterization All the catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific area measurement (SBET) before and after the catalytic tests. XRD was achieved in the continuous symmetric analysis mode on a Kristalloflex Siemens D5000 diffractometer using the Kc~l radiation of Cu (~=1.5406A) for 20 angles going from 10 d e g to 80 deg. The scan rate was 0.4 deg.min -I (step size = 0.04 deg, step time = 6s). An additional high resolution XRD analysis from 10 to 60 de;~ (stev size = 0.016 de~, step time = 20 s, scan rate : 0.048 deg.min -1) was performec~ on the used mixture ~'or detecting an eventual very small amount of a contamination phase formed during catalysis. SEM was performed on a Hitachi S-570 microscope using a 15kV accelerating voltage. During the analysis, attention was focused on the detection of any modification in size, morphology and orientation of the crystals for both oxides during the catalytic work. All the SEM micrographs shown are representative of the whole of the samples, checking for the occurrence of the presented features in m a n y different places throughout the samples. Specific area measurement was made on a Micromeritics ASAP 2000 device. The analysis was based on the adsorption and desorption isotherms of Kr at the liquid nitrogen temperature. Specific areas were calculated according to the B.E.T. equation. Theoretical SBET values were calculated both for the fresh and the used mechanical mixtures on the basis of the specific areas developed by the pure oxides submitted to the same treatment and of the massic composition o f the mixture. 3. RESULTS

3.1. Catalytic activity measurement Table 1 summarizes the catalytic activity measurements for both pure phases and the mechanical mixture. The pure MoO3 presented a low conversion of isobutene, with a weak selectivity for methacrolein. The activity of the pure BiPO4 was similar to that of the molybdenum trioxide, but with a slightly higher conversion and selectivity for methacrolein. Important synergetic effects were detected for the mechanical mixture: (i) both the conversion ofisobutene and the yield and selectivity for methacrolein were higher than the theoretically calculated values (assuming the absence of cooperation between the phases, see equations 2a, b and c), (ii)the selectivity for CO2 was lower than the values expected if the phases had been behaving completely individually, (iii) propenol (which was not formed on any of the pure phases) was produced in small amounts on the mechanical mixture.

190 Table 1. Observed and theoretical values of the conversion of isobutene (%C), yields in methacrolein and propenol (%Ymeth and %Yprop), and selectivities in methacrolein and propenoI(%Smeth and %Sprop). Thedretical values (figures in parenthesis) have been calculated according tdeqhations 2a, b and c (assuming the absence of cooperation). Catalyst Pure MoO3 Mech. Mixt.

%C 5.13 25.87 (20.12) 14.99

Pure BiPO4

%Ymeth 0.34 6.01 (2.12) 1.78

%Yprop 0 0.41 (0) 0

%YcO2 3.94 10.71 (12.75) 8.81

%Smeth 6.59 23.24 (10.51) 11.85

%Sprop 0 1.59 (0) 0

%Sco2 76.86 41.39 (63.37) 58.75

3.2. Characterization results

3.2.1. X-ray diffraction The X-ray diffraction patterns obtained for the fresh pure phases submitted to the "mechanical mixture" procedure did not present any modification with respect to those obtained with the freshly synthesized samples. Similarly, for the fresh mechanical mixture, all the reflections corresponded perfectly, for the position and for the intensity ratios, to those of the constituting pure phases. No additional peaks were detected. After the catalytic tests, the comparison of the patterns of both used pure phases with the corres.,ponding fresh ones did not. reveal any modification. A similar perfect superposmon of the patterns was obtained for the fresh and used mixtures: (i) the MoO3 reflections presented unmodified ~ IhO0 / ~ IOkO ratios, (ii) all the detected reflections were assigned to either MoO3 or BiPO4 (this even if considering shoulders of intense peaks and small peaks with intensity in the range of the noise level- noise level = 0.2% of the intensity of the main peak) [28]. For the all the catalysts, no sign of amorphisation was observed. Figure 2 shows the XRD patterns obtained with the fresh and the used mechanical mixture.

ca B _

_

.

J _ _

J

ol.-.I

'

0

'

'

I

20

'

'

'

I

'

'

'

I

40 60 2 T H E T A A n g l e s (deg)

'

'

|

I

80

Figure 2. XRD patterns (lower resolution analysis) of the mechanical mixture MoO3 + BiPO4 before (A) and after (B) the catalytic reaction. For further exploring the possible occurence of a crystallographic modification in the mechanical mixture during the reaction, in particular the possible formation of mixed Bi-Mo-O or Bi-Mo-P-O phases, a systematical critical investigation was carried out on the high resolution XRD pattern (noise level = 0.1% of the intensity of the

191 main peak) of the used mixture. First, table 2 shows that it was possible to assign all the peaks detected (even the less intense ones) to either MoO3 or BiPO4. Table 2. XRD p e a k s d e t e c t e d for the u s e d m e c h a n i c a l m i x t u r e (high r e s o l u t i o n analysis 9scan rate = 0.048 d e g . m i n -1) - letters in r e g a r d of each p e a k r e p r e s e n t the p h a s e to which it is assigned" B = BiPO4, M = MOO3.

ilW.+~mIT~ 6.9334 5.1961 4.7701 4.6616 4.1630 4.0870 3.8519 3.8093 3.5072 3.4673 3.2812 3.2627

36.1 0.4 0.6 1.5 2.0 0.9 1.4 19.4 4.7 100 4.1 25.2

M B B B B B M M B B+M B M

3.1426 3.0688 3.0087 2.9644 2.9378 2.8656 2.7023 2.6534 2.5987 2.5293 2.4422 2.4355

0.8 4.4 2.5 0.9 0.7 2.4 3.0 5.5 1.5 3.0 0.9 1.0

B B M B B B M M B+M M B B

~lIIV:,/FII~ 2.3284 5.5 2.3113 78.8 2.2712 6.3 2.1742 1.4 2.1508 1.2 2.1314 1.8 2.1148 1.5 1.9812 3.2 1.9593 6.6 1.9312 0.7 1.8851 0.9 1.8661 0.9

B+M M M B B B+M B M B+M B B B

~lIIV'.ll/IVII~ 1.8493 3.6 1.8221 15 1.7957 0.9 1.7702 1.8 1.7597 1.2 1.7564 1.6 1.7343 3.9 1.6938 1.6 1.6636 2.1 1.6313 2.0 1.5974 5.0 1.5650 5.7

B+M M B M M B M M M M M B

Second, the s t a n d a r d peak files of all the phases involving Bi, Mo and O or Bi, Mo, P and O (which w e call "mixed phases") available in a regularly u p d a t e d JCPDS data b a n k w e r e c o m p a r e d with the m e a s u r e d p a t t e r n of the u s e d m l x t u r e [28]. Table 3 lists the s t a n d a r d reflection m a i n lines of these s t a n d a r d s that w e r e missing on the p a t t e r n of the tested mixture. A peak was considered as missing w h e n (i) if present, it w o u l d not h a v e o v e r l a p p e d with a line of MoO3 or of BiPO4, (ii) w h e n for the considered d-spacing, the m e a s u r e d pattern only presented a b a c k g r o u n d signal, (iii) a n d w h e n the closest m e a s u r e d peak p r e s e n t e d a shift of at least 0.2 deg f r o m the considered s t a n d a r d line. A c c o u n t taken of these requirements, no s t a n d a r d line of the m i x e d phases (as d e f i n e d above) was detectable. The conclusion is that, on the basis of the actual JCPDS files, the two sets of information, (i) the one from the high resolution XRD analysis, and (ii) the perfect s u p e r p o s i t i o n of the l o w e r resolution p a t t e r n s for the fresh and the used mixture, allow to discard the eventuality that a m u t u a l crystalline contamination b e t w e e n MoO3 and BiPO4 has f o r m e d in operandi in the mixture [28]. 3.2.2. Specific area m e a s u r e m e n t s Table 4 s h o w s the o b s e r v e d and theoretical SBET values for the catalysts before and after the catalytic work. The p u r e phases having u n d e r g o n e the "mixture" p r o c e d u r e p r e s e n t e d a lower SBET than the freshly p r e p a r e d ones. This was very likely d u e to a flocculation of the grains d u r i n g the low t e m p e r a t u r e drying after the p e n t a n e evaporation. O n the other hand, for all the catalysts, a slight increase of the SBET w a s o b s e r v e d after the catalytic reaction, p r o b a b l y c o r r e s p o n d i n g to deflocculation and p e r h a p s an attrition p h e n o m e n o n . For the mixture, h o w e v e r , the difference b e t w e e n the o b s e r v e d and the theoretical values always r e m a i n e d similar (in the range of the precision of the analysis).

192 Table 3. d-spacing (~) of the missing non-overlapping standard reflection lines for the Bi-Mo-O and Bi-Mo-O-P phases satisfying the detection criteria (see text). Phase Bi2MoO6 Bi4MoO9 Bi2MoO6 Bi2MoO6 Bi2Mo3012 q-Bi2MoO6 ~/'-Bi2MoO6 Bi20MoO33 Bi2xMo1-xO3 Bi2Mo3012 Bi6Mo2015 Bi2MoO9 ~/'-Bi2MoO6 [3-Bi2Mo209 3Bi203.2Mo03 7Bi203.MoO3 Bi0.27Mo205 Bi4MoO9 Bi6MoO12 Bi38Mo7078 Bi12Mo0.12018.4+x Bi7.9Mo0.1012.15 Bi3.64Mo0.3606.55 Bi9PMo12052

JCPDS N ~ 07-0401 12-0149 18-0243 21-0102 21-0103 22-0112 22-0113 23-1031 23-1032 23-1033 26-0216 31-0196 33-0208 33-0209 34-1250 34-1270 35-1491 36-0115 36-0116 38-0249 43-0196 43-0443 43-0446 30-0193

d-spacing (/k) 2.73, 2.68, 2.47,1.92,1.65,1.37,1.26, 1.25,1.22,1.21.... 2.83, 1.70, 1.41 4.30, 2.79, 2.00 8.09, 2.75, 2.74,1.94,1.65 7.89, 6.29, 4.90, 4..57, 3.62, 3.59, 3.27, 3.19, 2.80, 2.00.... 5.62, 2.79, 2.06 3.20, 2.80, 2.48,1.65 2.88, 2.75, 1.61 2.90, 2.79, 2.07, 1.68, 1.64 4.92, 3.60, 3.18, 2.76, 1.65, 1.54 6.04, 5.91, 5.71, 2.90, 2.68, 2.39, 2.07 2.83, 2.72 5.66, 2.80, 2.00 6.65, 5.95, 3.21, 3.20, 2.81 2.82, 2.73, 2.01,1.70 2.75 3.33, 2.04 2.90, 2.83, 2.77, 2.03, 1.70 2.80, 1.70 2.82, 2.80,1.70 2.74, 2.19,1.71 3.19, 2.74 2.82,1.70 4.86, 4.55, 3.59r 3.18, 2.88r 2.77~2.49, 2.00, 1.99r....

Table 4. SBET values (m2.g -1) of the catalysts before and after catalytic reaction. Values in parenthesis are the theoretical values calculated on the basis on the composition of the mixture and the observed values of the c o r r e s p o n d i n g constituting pure phases. Catalysts Pure MoO3 Mechanical Mixture Pure BiPO4

Before 0.7026 1.2699 4.7747

(1.2141)

After 0.7241 1.4491 5.2239

(1.2919)

3.2.3. Scanning electron microscopy The comparison of the SEM micrographs (magnification up to 30,000) of the pure MoO3 a n d the p u r e BiPO4 before and after the catalytic reaction did not show any modification o f the samples. Size and m o r p h o l o g y of the crystallites were unchanged. For the pure MOO3, in particular, the edges between the (010) and the (100) faces of the crystallites were totally identical in the fresh and the used samples, presenting a sharp intersection. On the other hand, for the mechanical mixture, important modifications occurred during catalytic reaction. While the BiPO4 crystallites remained unchanged, the MoO3 crystallites exhibited a morphology reflecting reconstruction. The edges between the (010) and the (100) faces acquired a facetted structure. Instead of a single intersection border (as in the fresh sample), these edges were composed o f a succession of small parallel steps oriented in the [001] direction of the crystals, namely with the vertical walls indexed as (100) faces. These special features were never observed in the fresh sample nor in MoO3 catalytically tested in the absence of

193 BiPO4. Figure 3 shows close views of the edges of a MoO3 crystallites before and after reconstruction in the presence of BiPO4, namely before and after catalytic test in mixture with BiPO4.

Figure 3. SEM micrographs of the edges between (010) and (100) faces of MoO3 crystallites in the mixture with BiPO4, before catalysis (left) and after catalysis (right). Arrows indicate the in operandi reconstructed features. 4. DISCUSSION The results of the catalytic tests clearly indicate the existence of a cooperation between MoO3 and BiPO4. The fact that synergetic effects were observed both for the conversion of isobutene and for the selectivity to methacrolein suggests that there has been creation of new selective sites during the reaction. It could first be argued that this creation of new selective sites is due to the formation of a more active mixed phase through a reaction between MoO3 and BiPO4. This hypothesis can be discarded, as high resolution XRD gives no indication of the presence of any of the 23 bismuth molybdate phases or of Bi9PMo12052 (the only Bi-P-Mo-O mixed phase reported in JCPDS files), even in very small amounts, in the used mechanical mixture [28]. The SEM investigations d i d not suggest the formation of crystallites or domains that could not be assigned to MoO3 or BiPO4 in the mixture after the test. In the same line, it could be also argued that the higher activity obtained with the mixture could be due to a sort of contamination that could not be detected by XRD, namely amorphous phase, solid solution or surface contamination (formation of active monolayer). Authors have reported the existence of such phenomena when starting from MoO3 and Bi203 (either from mixtures of these two phases, or from one phase i m p r e g n a t e d with the other) [29,30]. To our knowledge, no such possibilities have ever been reported w h e n starting from MoO3 and BiPO4. Moreover, exhaustive characterizations of the surface (including ISS) of mechanical mixtures of MoO3 with BiPO4 used in a similar reaction, namely the oxygen-assisted dehydration of N-ethyI-formamide, have definitely shown the absence of any mutual contamination [25].

194 Another possible . .explanation . of .the synert~eetic effects is alp hysical change of. the catalyst particles during the preparation of mxxture, or cturmg the catalytic reaction [31]. This could be a decrease of the size of the crystallites or a deflocculation phenomenon of the aggregates of crystallites, both corresponding to an increase of the amount of catalytic sites exposed. The tendency of the catalysts to present higher SBET values after the tests could be an argument in favor of thx's hypothesis. However, even if such phenomena could not be discarded, the observed SBET values for the mixture and those calculated on the basis of the massic composition of the mixture and of the SBET values measured for the pure oxides submitted to the same treaments actually remained similar. This implies that, even if some attrition or deflocculation occurred, the intensities of these phenomena were identical for the mixture and for the pure phases reacted alone. Each phase in the mixture should then behave as if it were aIone in the reactor without being influenced by the other one. This is not the case. Consequently, neither of these two phenomena could explain the enhanced performances of the mixture with respect to the pure phases (especially in selectivity, which is increased by a factor > 2). On the other hand, there is a conspicuous reconstruction of the MoO3 crystallites, namely the formation of more (100) steps at the edges with the (010) faces. The (100) crystallographic faces are selective for the partial oxidation [17-20]. It can then be concluded that the creation of selective sxtes as deduced from the catalytic activity measurements corresponds to the creation of more (100) selective faces by reconstruction of non selective (010) faces. Since the phenomenon only occurred when MoO3 was tested in the presence of BiPO4, this accounts perfectly for the observed synergetic effects. Recently, Smith and Rohrer indicated that some modifications of the catalytic performances of MoO3 could be correlated with the appearance of (100) steps on the (010) faces. But the difference with thepresent study is that they artificially triggered the reconstruction by drastically reducing MoO3 crystals with hydrogen instead of observing the phenomenon in a real catalytic process [32]. At this stage of the investigation, the types of newly formed sites responsible for the propenol formation are still not unc~erstood. Nevertheless, the correlation between reconstruction and synergetic effects observed in the system MoO3 + BiPO4 is perfectly identical to that observed in the system MoO3 + 0~-Sb204 [21,22]. This constitutes a further argument for discarding the contamination to explain the synergy. It is very unlikely that two different systems composed of different elements and phases, which would have brought about contaminations with different compositions, would present similar catalytic performances and similar morphologicalfeatures. As both BiPO4 and 0~-Sb204 are "spillover oxygen donors", it is much more logical to consider that the reconstruction was triggered by the reaction of Oso (flowing from BiPO4 or from,,~-Sb204) with the (010) faces of MOO3. Oso would favor the 'selective coordination of Mo atoms or groups of atoms with different coordinations typical of the (100) faces. This probably concerns groups of 4 Mo atoms permitting the concerted elimination o f 2 hydrogen atoms and the insertion of one oxygen atom into the hydrocarbon molecule. The formation of this selective structure would take place at the expense of the non selective one typical of (010) (namely an arrangement of Mo atoms, all identical in coordination, exhibiting one Mo=O bond and linked together by bridging oxygens). The following mechanism by which Oso influences the surface of MoO3 crystallites (reacts with it) for triggering the observed reconstruction can be proposed. It has been shown that the selective sites for partial oxidation at the surface of MoO3 were constituted of pairs of fully oxidized MOO6- octaedra resenting the "corner-sharing" structure (2 octaedra linked by 1 corner). After aving transferred an oxygen atom to the hydrocarbon, the resulting reduced structure reorganizes in an "edge-sharing" structure (2 octaedra linked by 1 edge). When the catalytic reaction is carried out in the absence of Oso, the concentration of

~

195 edge-sharing octaedra, pairs becomes progressively, higher and these edge-sharin, g structures start forming aggregates, leading to the nucleation of non selective shearstructures tfiat are difficult to reverse. The role of Oso is then, by reoxidizing the edge-sharing structures, to stop the growth of the rows of edge-sharing octaedra and to maintain these rows below the critical nucleus size corresponding to the formation of the shear-structures. In other words, Oso will continuously maintain a rapid dynamic swing between corner- and edge-sharing octaedra, so favorin, g frequent and rapid catalytic cycles...However,. at each. cy.cle, the reduced ed g...e-sharlng structure can eventually be reoxldlzed in a configuration different to the lmtlal one (a different crystallographic orientation). Hence, by increasing the frequency of the cycles, Oso also increases the probability to trigger a modification of the orientation of the pairs of octaedra. At the micrometric scale, this phenomenon leads, cycle after cycle, to the facetting of the large basal (010) faces of MoO3 crystals to (100) lateral steps. This modification of the surface must thus be considered as a dynamic phenomenon, namely a progressive process occurring during the succession of the reductionreoxidation cycles that are typical of the oxidation mechanism in steady state conditions [21-23]. 5. CONCLUSION When MoO3 was catalytically reacted in the presence of a spillover oxygen donor phase, namely BiPO4, the (010) faces of the crystallites got reconstructed to (100) steps. This corresponds to the creation of more selective sites (on (100) faces) at the expense of non selective ones (of (010) faces). This does not occur when MoO3 is tested alone. The phenomenon then accounts perfectly for the important synergetic effects observed between the two phases in the oxidation of isobutene to methacrolein. These results are completely identical to the ones observed when MoO3 was used in mixture with 0~-Sb204, another oxygen spillover donor. The observation of the identical phenomenon in two catalytic systems of different compositions, but each one presenting MoO3 (Oso acceptor) mixed with an Oso donor, strongly supports the relevance of this hypothesis. Even if corresponding to a different approach, the mechanism proposed here is intimately connected with the theoretical model corresponding to the remote control concept explaining how Oso maintains (or restores) the activity of MoO3 by keeping its surface covered by more selective corner-sharing pairs of octaedra at the expense of non selective edgesharing ones [3,4]. ACKNOWLEDGMENTS

Authors aregrateful to the Fonds National de la Recherche Scientifique for the fellowship awarded to Eric M. Gaigneaux. REFERENCES

1. 2. 3. 4. 5. 6.

L.T. Weng and B. Delmon. Appl. Catal. A : General, 81 (1992) 141. S. Breiter, M. Estenfelder, H.G. Lintz, A. Tenten and H. Hibst. Appl. Catal. A : General, 134 (1996) 81. B. Delmon. Surface Review and Letters, 2 N ~ 1 (1995) 25. B. Delmon. Heterogenous Chemistry Letters, 1 (1994) 219. E.M. Gaigneaux, D. Herla, P. Tsiakaras, U. Roland, P. Ruiz and B. Delmon. in : Heterogeneous Hydrocarbon Oxidation, ACS Symposium Series, Vol. 638, Eds. B.K. Warren and S.T. Oyama (USA, Washington, 1996) p. 330. E.M. Gaigneaux, P. Tsiakaras, D. Herla, L. Ghenne, P. Ruiz and B. Delmon. American Chemical Society Annual Meeting, Symposium on "Catalysis and

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Photocatalysis on Metal Oxides", Chicago, IL, USA, August 21-25, 1995. (accepted for publication in Catalysis Today, Eds. U.S. Ozkan, under press). L.T. Weng, P. Ruiz, B. Delmon and D. Duprez. J. Mol. Catal., 52 (1989) 349. D. Martin, P. Kaur, D. Duprez, E. Gaigneaux, P. Ruiz and B. Delmon. Catal. Today, Vol. 32 N ~ 1-4 (1996) 329. a) G. Mestl, P. Ruiz, B. Delmon and H. Kn6zinger. J. Phys. Chem., 98 (1994) 11269. b) G. Mestl, P. Ruiz, B. Delmon and H. Kn6zinger. J. Phys. Chem., 98 (1994) 11276. c) G. Mestl, P. Ruiz, B. Delmon and H. Kn6zinger. J. Phys. Chem., 98 (1994) 11283. D. Vande Putte, S. Hoornaerts, F.C. Thyrion, P. Ruiz, B. Delmon. Catal. Today, Vol 32 N ~ 1-4 (1996) 255. T. Otsubo, H. Miura, Y. Morikawa and T. Shirakaki. J. CataI., 36 (1975) 240. W.C. Conner, G.M. Pajonk and S.J. Teichner. Adv. Catal., 34 (1986) 1. G.E. Batley and A. Ekstr6m. J. Catal., 34 (1974) 36. D. Maret, G.M. Pajonk and S.J. Teichner. in Catalysis on the Energy Scene, Elsevier Science Edition (Amsterdam, The Netherlands)(1984), 347. in New Aspects of Spillover Effects in Catalysis (T. Inui et al, Editors), Elsevier Science Publishers (Amsterdam, The Netherlands) (1993) : (a) K. Fujimoto. p9, (b) S.J. Teichner. p27, (c) G.M. Pajonk. p85, (d) Y. Moro-Oka. p95. L.T. Weng, S.Y. Ma, P. Ruiz and B. Delmon. J. Mol. Catal., 61 (1990) 99. J.M. Tatibouet, J.E. Germain. J. Catal., 72 (1981) 375. R.A. Hernandez and U.S. Ozkan. Ind. Eng. Chem. Res., 29 (1990) 1454. K. Briickman, R. Grabowski, J. Haber, A. Mazurkiewicz, J. Sloczyniski and T. Wiltowski. J. Catal., 104 (1987) 71. a) B. Mingot, N. Floquet, O. Bertrand, M. Treilleux, J.J. Heizmann, J. Massardier and M. Abon. J. Catal., 118 (1989) 424. b) M. Abon, J. Massardier, B. Mingot, J.C. Volta, N. Floquet and O. Bertrand. J. Catal., 134 (1992) 542. E.M. Gaigneaux, P. Ruiz and B. Delmon. Catal. Today, Vol. 32 N ~ 1-4 (1996) 37. E.M. Gaigneaux, P. Ruiz and B. Delmon. 11th International Congress on Catalysis. Baltimore, MA, USA. June 30 - July 5 (1996). E.M. Gaigneaux, P. Ruiz, E.E. Wolf and B. Delmon. 6th Iketani Conference. International Symposium on Surface Nano-Control of Environmental Catalysts and Related Materials. Tokyo, Japan. Nov. 25- 27 (1996).(sumitted for publication in Appl. Surf. Sc.). L.T. Weng, P. Ruiz and B. Delmon. in New Developments in Selective Oxidation by Heterogeneous Catalysis (Eds. P. Ruiz and B. Delmon), Studies in Surface Science andCatalysis, Vol. 72 (1992) 399. a) J.M.D. Tascon, P. Grange and B. Delmon. J. Catal., 97 (1986) 287. b) J.M.D. Tascon, P. Bertrand, M. Genet and B. Delmon. J. Catal., 97 (1986) 300. c) J.M.D. Tascon, M.M. Mestdagh and B. Delmon. J. Catal., 97 (1986) 312. F.Y. Qiu, L.T. Weng, P. Ruiz and B. Delmon. Appl. Catal., 47 (1989) 115. F.Y. Qiu, L.T. Weng, E. Sham, P. Ruiz and B. Delmon. Appl. Catal., 51 (1989) 235. @ 1996 JCPDS- International Centre for Diffraction Data. K. Brtickman, J. Haber and T. Wiltowski. J. Catal., 106 (1987) 188. N. Arora, G. Deo, I.E. Wachs and A. M. Hirt. J. Catal., 159 (1996) 1. G. Mestl, B. Herzog, R. SchI6gI and H. Kn6zinger. Langmuir, 11 (1995) 3027. R.L. Smith and G.S. Rohrer. J. Catal., 163 (1996) 12.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

197

F u r t h e r S t u d y o n the S y n e r g e t i c E f f e c t s b e t w e e n M o O 3 a n d S n O 2 E.M. Gaigneaux 1, S.R.G. Carraz~n, L. Ghenne, A. Moulard, U. Roland 2, P. Ruiz and B. Delmon Unit~ de Catalyse et Chimie des MatEriaux Divis6s, Universitd Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium 1 Aspirant of Fonds National de la Recherche Scientifique, Belgium 2Instituto de Qui'mica, Facultad de Quimica, Universidad de Salamanca, Spain

In an earlier investigation, synergistic catalytic effects were observed between SnO 2 and MoO 3 in the dehydration - dehydrogenation of 2-butanol to butene and methyl-ethyl ketone at low temperature (190~

Synergy was explained on the basis of a remote control

mechanism involving the migration of spillover oxygen, "Oso".

SnO 2 was deemed the

"donor of Oso" and MoO 3 the "acceptor of Oso". A main concern voiced about this explanation was the probability of mutual contamination of the two phases during catalytic reaction which could account for the observed enhanced catalytic performance. The current study deals with experiments designed to investigate specially prepared compositions with "mutual contamination" between SnO 2 and MoO 3. The conclusion is, that the contamination, in any form, if formed in the mixture, cannot account for the synergistic effects. Additional experiments conducted with the same catalysts in the selective oxidation of isobutene at high temperatures (380 - 420~ also exhibit a catalytic synergism between SnO 2 and MoO 3. In this case, SnO 2 becomes highly active, which triggers its continuous reduction during the reaction. To restore its high oxidation level, SnO 2 pumps lattice oxygen from MoO 3. Consequently, MoO 3 becomes reduced. A further supply of Oso can prevent this reduction phenomenon. Finally, experiments using TPD of NH 3 show that Oso increases the acidity of MoO 3, consistent with an increase of butene production from 2-butanol when mixtures of SnO 2 and MoO 3 are used as the catalyst.

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

199

T h e N a t u r e of the A c t i v e / S e l e c t i v e Phase in V P O C a t a l y s t s and the Kinetics of n - B u t a n e O x i d a t i o n D. Dowell, and J.T. Gleaves, Washington University Department of Chemical Engineering, One Brookings Drive, Campus Box 1198 St. Louis, Missouri 63130 Y. Schuurman, Centre National De Recherche Scientifique, Institut de Recherche sur la Catalyse, Delegation Regionale Rhone-Alpes, Secteur Vallee du Rhone, 2 avenue Albert Einstein, BP 1335, 69626 Villeurbanne Cedex, France The reaction of n-butane with "oxygen-treated" (VO)2P207 based catalysts has been investigated using high speed transient response techniques and Raman spectroscopy. Results indicate that two types of active oxygen are present on the VPO surface after oxidation. One type is associated with the production of CO2 and the other with the production of MA and CO2. Results also indicate that significant amounts of active oxygen can be stored in the VPO lattice, but Raman spectroscopic data indicates that the stored oxygen is not associated with a crystalline VOPO4 phase. 1. Introduction

The unique properties of VPO catalysts have motivated a large number of research studies, and there is strong interest in gaining a fundamental understanding of how the VPO system functions. Although the VPO has been investigated extensively a number of important features are still not well understood. This paper presents new results from high speed transient response experiments and Raman spectroscopic studies that provide insights into one of the key unresolved issues, namely, the nature of the active-selective phase. It is well established [1-3] that vanadyl pyrophosphate (VO)2P207 is an essential component of the most selective VPO catalysts. For example, structural and chemical characterization studies of "reactor equilibrated" VPO catalysts indicate that the predominate crb'stalline phase is vanadyl pyrophosphate (VO)2P207 [1-3], that the bulk P/V ratio is close to 1.0, and that the average vanadium oxidation state is close to +4.0 [35]. A number of studies [2,5] have indicated that alkane oxidation primarily involves oxygen adspecies adsorbed at vanadium surface sites, and relatively little bulk lattice oxygen. It is also generally agreed that surface vS+ species play a role in the n-butane reaction [4-10]. Recent studies using Raman spectroscopy, 31p_NMR, and other physical characterization techniques indicate that, in addition to surface species, V 5+ phases may be involved in catalyst operation [4-8,11,12,13]. Consistent with this idea is the fact that in the presence of oxygen (VO)2P207 can be readily transformed into different V5+ phases such as ~-VOPO4, t~-VOPO4, Y-VOPO4, and 5-VOPO4 [6]. The amount of V5+ depends on the oxygen partial pressure and the temperature. Consequently, under reaction conditions, small amounts of V5+ phases may be present at the catalyst surface [12]. Recent TAP-2 transient response studies have shown that (VO)2P207 can readily absorb oxygen into its lattice, and then efficiently channel it to the active catalytic site. It is not clear however how this oxygen is stored in the lattice. It is also well known that pure V OPO4 phases do not perform as well as "reactor equilibrated" (VO)2P207 for n-butane oxidation [6]. This fact indicates that a VOPO4 surface by itself cannot be the active phase. Consequently, a number of workers have suggested that the VPO active site is situated at a (VO)2P207/VOPO4 interface [5,7,8]. The purpose of this paper is to examine how exposure of a "reactor equilibrated" catalyst to different gas phase oxygen treatments influences the selectivity and rate of n-

200 butane conversion, and to determine if the reaction selectivity is associated with the formation of VOP04 phases.

2. Experimental 2.1. Catalyst Preparation. "Reactor-equilibrated" VPO catalysts were prepared by a nonaqueous procedure detailed in previous papers [ 14], and operated at steady-state conditions (1.5% n-butane, 15 psig reactor inlet pressure and 2000 GHSV) for approximately 3000 hours. Under steady-state conditions the catalyst gave selectivities to MA of approximately 66% at 78% conversion. XRD analysis of the reactorequilibrated samples showed that they were monophasic (VO)2P2OT. Chemical analysis gave a P/V ratio of 1.01 and vanadium oxidation state of 4.02. The samples had a BET surface area of 16.5 m2/gm. 2.2. TAP-2 Reactor System. Steady-state reaction studies, transient response experiments, and oxygen activation experiments were performed with a TAP-2 multifunctional reactor system. Details of the TAP-2 system have been presented in previous publications [5,14]. The system is comprised of 1) a high-throughput, liquid nitrogen trapped, ultra-high vacuum system, 2) a microreactor-oven assembly with temperature controller, 3) a heatable gas manifold with five input ports containing four pulse valves, and one manual bleed valve, 5) a valve control module for actuating the pulse valves, 6) a gas blending station for preparing reactant mixtures from gases and liquids, 7) a pressure transducer oven, 8) a quadrupole mass spectrometer (QMS), 9) a Pentium PC computer based control and data acquisition system, and 10) a slide valve assembly with heated exhaust line. Reactant gases can be introduced into the microreactor as steady flows or transient inputs with pulse widths (FWHH) of = 250/~sec. Reaction products are analyzed in realtime using a quadrupole mass spectrometer. Pulses from separate valves can be introduced as sets of pulses of predetermined length or in a pump-probe format alternating between two valves. For operation at atmospheric pressures the vacuum system is isolated from the microreactor using a slide valve. The slide valve contains an adjustable leak valve that controls the amount of reactor effluent that enters the vacuum system. The portion of the effluent that does not escape through the leak valve exits through an external vent that contains an adjustable pressure regulator. When the reactor is operated at atmospheric pressures the mass spectral data can be collected in a standard mass intensity versus mass number format. When the reactor is operated at vacuum pressures the slide valve is retracted, and the microreactor vents directly into the vacuum system. In this case transient response data are collected one mass peak at time in a mass intensity versus time format. The high pumping speed of the vacuum system, and the near proximity of the quadrupole to the microreactor insure that pulses measured by the quadrupole reflect the true microreactor transient response. 2.3. Collection of Raman Spectra. Raman spectra were obtained using a modified SPEX double grating spectrometer, and a Coherent Innova 90 argon ion (At+) laser operating at 514.5 nanometers (nm). The spectrometer was operated with a PCbased data acquisition and control system that was developed in-house. Reactor equilibrated VPO samples produced detectable Raman scattering using 30-60 mw of light (measured at the sample) and a scan rate of .02 nm/s. Catalyst samples from reaction studies were removed from the TAP-2 microreactor, immediately placed "as is" in a sealed quartz tube under one atmosphere of air. The quartz tube was rotated at 500-1000 rpm during data acquisition to prevent excess heating of the sample, and to give a spectra indicative of the average composition.

201 In a typical experiment the Raman signal was averaged for 10-20 scans to increase the signal to noise ratio. Experiments were performed to determine if any sample degradation was caused by exposure to the laser light, and none was observed for laser intensities up to 150 mw. Physical mixtures of reactor equilibrated VPO and different VOPO4 phases were used to gauge the detection limit of the system. Physical mixtures containing 4% or more of a VOPO4 phase produced peaks characteristic of the phase. 2.4. Reaction Procedures. All reaction studies were performed with reactorequilibrated catalyst samples. A standard catalyst charge weighed between 100 and 125 milligrams with an average particle diameter of 275 microns. The catalyst bed was centered between two beds of quartz particles with the same average particle diameter as the catalyst. The quartz beds were used to thermally insulates the catalyst and reduce the axial temperature gradient (< 5~ across the catalyst bed. Steady-state flow experiments were performed at 673 K and = 1 atmosphere pressure using a gas blend of 88% Argon, 10% oxygen and 2% n-butane. Molar flow rates were set to insure turbulent flow [14]. The reactor effluent was monitored by leaking a small amount into the TAP-2 vacuum system and collecting the mass spectrum. Oxygen uptake experiments were performed at constant pressure( = 1 atmosphere) and constant temperature in the TAP-2 microreactor. In a typical flow oxidation a reactor equilibrated catalyst was exposed to a flow of pure oxygen for a period of one hour. Oxidation temperatures ranged from 683 K to 803 K. After a period of one hour, the reactor was evacuated and the temperature was lowered to reaction temperature (653 K). Transient response experiments were performed immediately after oxidation treatments or steady-state reaction studies without exposing the catalyst to ambient conditions. All pulse response experiments using n-butane were performed with a gas blend containing a mixture of 78% n-butane and 12% argon. Typical pulse intensifies were in the range of 1014 molecules per pulse. The argon pulse response was determined to be independent of the pulse intensity. Under these conditions gas transport through the microreactor can be described by Knudsen diffusion. The argon, CO2, n-butane, and maleic anhydride responses were collected at m/e values of 40, 44, 58, and 98 respectively. From continuous flow experiments using pure reagents it was determined that the QMS signals at m/e = 40, 58, and 98 are unique to argon, n-butane and maleic anhydride respectively. The area of the transient responses at these mass numbers is directly proportional to the amount of each species. The experimentally observed CO2 response contained contributions from n-butane and maleic anhydride. To obtain the true CO2 response these contributions were subtracted out. Nbutane conversion and the relative selectivities to maleic anhydride and CO2 were obtained by measuring the areas of their respective response curves. 3. Results

Figure 1 shows a set of transient responses curves for a) n-butane (m/e = 58), b) CO2 (m/e = 44), and c) maleic anhydride (m/e = 98) when a 3.5/1 C4H10/Ar mixture is pulsed over an oxygen-treated catalyst sample maintained at 653 K. The oxygen treated catalyst was prepared by oxidizing a reactor equilibrated catalyst sample at atmospheric pressure for 1 hour at 723 K. Subsequently, the catalyst was exposed to a series of 10000 equally intense pulses of the C4HI0/Ar mixture. The labeled curves (Ox) in Figures la-c were acquired after 1000 pulses had been injected into the catalyst bed. The unlabeled curves were acquired after 8000 pulses had been injected. In going from the 1000th pulse to the 8000th pulse the n-butane response increases = 2.7 times and becomes broader. These changes indicate that 2.7 times more n-butane passes through the reactor unreacted, or that the n-butane conversion decreases 2.7 times. Conversely, the MA and CO2 responses decrease and become broader. The decrease in the response is due mainly to a change in the adsorption properties of the VPO surface as oxygen is removed. The MA

202 pulse width increases because MA is adsorbed more strongly on a V 4+ surface than a on V 5+ surface [141. Thus, the changes in size and shape of the response curves reflect a change in the kinetic state of the VPO catalyst.

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203 Both samples were prepared from the same batch of reactor equilibrated VPO, but one was oxidized for 1 hour at 723 K, and the other for 1 hour at 798 K. The responses indicate the relative n-butane conversion (2a) and MA production (2b) for the same sample size, and the same n-butane pulse intensity. The n-butane response is ~ 2 times smaller for the 723 K sample indicating that conversion is 2 times higher. The MA response is ~ 10 times larger on the 723 K sample indicating that MA production is 10 times higher, and the MA selectivity is 5 times higher. The height normalized MA responses displayed in the inset in Figure 2b show that the shape of MA response curve changes very little in comparison to the change in conversion. This change should be compared with the shape change shown in Figure lc. 1

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Figure 2a. N-butane responses over two oxygen-treated VPO samples, b. MA responses over the same two samples. Both samples were prepared from the same VPO, but one was oxygen-treated at 723 K and other at 798 K. Figure 3 displays Raman spectra of the samples used to produce the response curves presented in Figure 2 taken after exposure to ~, 30 n-butane pulse. The spectra (3a) of the sample oxidized at 723 K is characteristic of (VO)2P207, and exhibits no features indicative of crystalline vS+ compounds. The spectra (3b) of the sample oxidized at 798 K displays bands characteristic of (VO)2P207, and crystalline VOPO4 phases. In general, V 5+ bands were detected when the oxidation temperature exceeded 773 K. Figures 4a-g' display plots of the areas of the MA, CO2, and n-butane transient responses versus pulse number when a 3.5/1 C4I-I10/Ar mixture is pulsed over seven different oxygen-treated catalyst samples at 653 K. The seven catalysts were prepared from the same batch of reactor equilibrated VPO, by oxidizing each sample at a different temperature, and 1 atmosphere of oxygen for 1 hour. The seven oxidation temperatures, corresponding to the seven sets of MA, CO2, and n-butane data, are in order, 683, 703, 723, 7 4 3 , 7 6 3 , 7 8 3 , and 803 K. The sets of three curves reflect the changes in MA and CO2 production, and in n-butane conversion with pulse number. The MA curves (Figures 2a,b,c,d,e) of catalyst samples oxidized at or below 763 K display a pronounced

204 maximum that occurs between 500 and 1500 pulses of n-butane. The rise in the MA production coincides with a rapid decrease in the CO2 production (Figures 2a',b',c',d',e'). Figures 5a-f plot the product of the MA and n-butane pulse areas (i.e., the relative MA selectivity) versus the n-butane pulse number. Taken together, these plots show that the MA selectivity depends on the pulse number, and the oxygen treatment temperature

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Previous TAP-2 studies [5,14] have shown that reactor-equilibrated VPO readily adsorbs oxygen at reaction temperatures, and that the oxidation rate increases with temperature. Conversely, when an oxygen-treated catalyst is heated in vacuum it evolves

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208 were exposed to extended pulse reductions with n-butane, the phases were not depleted. These results indicate that these phases are not involved in the catalytic process. The series of curves displayed in Figures 4 and 5 show that during the initial pulse reduction of an oxygen-treated catalyst, MA production increases while n-butane conversion and CO2 production decrease. The CO2 curve is characterized by a slope change that occurs when the MA curve reaches a maximum. This behavior indicates that the production of CO2, prior to the MA maximum, involves a different type of oxygen species than is involved in the formation of MA. This result supports theoretical studies that indicate that more than one type of oxygen adspecies is present on an oxygen covered VPO surface [91. The amount of oxygen consumed during the CO2 decrease and the MA increase can be estimated from the n-butane conversion. Assuming that on average, each converted n-butane molecule consumes 10 oxygen atoms, the total number of oxygen atoms consumed per pulse is = 1015. In the most selective oxygen-treated sample (Tox = 723 K) the MA response reaches a maximum after = 1000 pulses or 1018 oxygen atoms have been consumed. In a typical 100 mg sample there are = 1018 surface vanadium atoms [5] so that the number of oxygen atoms consumed is approximately equal to number of surface vanadium atoms. The total amount of oxygen consumed during the entire reduction process is = 1019 atoms taking into account the decrease in n-butane conversion. Storage of this amount of oxygen by the conversion of V4+ species into V5+ species would require the transformation of ~- 10 monolayers of V 4+ species, and would change the VPO oxidation state from 4.02 to 4.12. This increase is consistent with previous chemical analyses of oxygen-treated VPO samples [5]. These results indicate that significant quantities of oxygen can be stored in the VPO lattice and used in the selective conversion of n-butane. The stored oxygen does not however, appear to be associated with a crystalline VOPO4 phase. The financial support provided by the National Science Foundation Grant Number CTS-9322829, Huntsman Chemical Company, and Amoco Foundation is gratefully acknowledged. The authors would also like to acknowledge Professor Gregory Yablonsky for many fruitful discussions. References

1. G. Centi, F. Trifiro, J. Ebner, V. Franchetti, Chem Rev., 88 (1988) 55. 2. J. Ebner, J. Gleaves, In Oxygen Complexes and Oxygen Activation by Transition Metals, A. Martell and D. Sawyer Eds., Plenum Pub.:(1988) 273. 3. G. Centi, Catalysis Today, 16 (1993) 5, and references therein. 4. Y. Zhang, R, Sneeded, J. Volta, Catalysis Today, 16 (1993) 39. 5. Y. Schuurman, J. Gleaves, J. Ebner, M. Mummey, In New Developments in Selective Oxidation II, V. Corberan and S. Vic Bellon Eds., Elsevier Pub.: Amsterdam (1994) 203. 6 Y. Zhang, M. Forissier, R. Sneeden, J. Vedrine, J. Volta, J. Catalysis, 145 (1994) 256. 7. M. Abon, K. Bere, A. Tuel, P. Delichere, J. Catalysis, 156 (1995) 28. 8. Y. Zhang, M. Forissier, J. Vedrine, J Volta, J. Catalysis, 145 (1994) 267. 9. P. Agaskar, L. Decaul, R. Grasselli, Catalysis Letters, 23 (1994) 339. 10. S. Albonetti, F. Cavani, F. Trifiro, P. Venturoli, G. Calestani, M. Lopez Granados, J. Fierro, J. Catalysis, 160 (1996) 52. 11. G. Hutchings, A. Desmartin-Chomel, R. Olier, J. Volta, Nature, 368 (1994) 41. 12. R. Overbeek, M. Versluijs-Helder, P. Wamnga, E. Bosma, J. Geus, In New Developments in Selective Oxidation II, V. Corberan and S. Vic Bellon Eds., Elsevier PUb.: Amsterdam (1994) 183. 13. B. Abdelouahab, R. Olier, N. Guilhaume, F. Lefebvre, and J.C. Volta, J. Catalysis, 134 (1992) 151. 14. Y. Schuurman, and J. T. Gleaves, Ind. & Eng. Chem. Research, 33 (1994) 2935.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

209

Understanding the microstructural transformation mechanism which takes place during the activation of vanadium phosphorus oxide catalysts G r a h a m J. Hutchings a, Andrew Burrows b, Sujata Sajip b, Christopher J. Kiely a'b, Kossi E Bere c, J e a n - C l a u d e Volta c, Alain Tuel c and Michel Abon c aLeverhulme Centre for Innovative Catalysis, D e p a r t m e n t of Chemistry, University of Liverpool, Liverpool, L69 3BX, United Kingdom. bDepartment of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, United Kingdom. CInstitut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626, Villeurbanne Cedex, France.

S t r u c t u r a l characterisation studies on an undoped set of VPO samples have allowed us to follow the structural evolution of the catalyst during the activation procedure. The initial VOHPO4.0.5H20 precursor has a platelike morphology with an [001] surface normal. As the transformation proceeds a direct topotactic t r a n s f o r m a t i o n of the [001] VOHPO4.0.5H20 to [100] (VO)2P207 occurs at the periphery of the platelet. In the interior of the platelet, a more complex indirect transformation sequence occurs. D a r k field imaging experiments show t h a t regions exist where the VOHPO4.0.5H20 precursor t r a n s f o r m s epitaxially into [100] 5-VOPO4. As the activation time increases, the domains of 5-VOPO4, which are embedded in a disordered matrix, s h r i n k and further t r a n s f o r m to the final (VO)2P207 phase. An a t t e m p t has been made to correlate m e a s u r e d catalytic performance data with the catalyst microstructure at various stages of the transformation process. It is found t h a t there is a distinct parallel between improving catalytic performance and a decrease in the a m o u n t of V 5§ phases present. F u r t h e r m o r e , we show t h a t when Co is added to the catalyst as a promoter, it is initially homogeneously distributed throughout the hemihydrate platelet. As activation proceeds, however, the Co is seen to have limited solubility in the (VO)2P207 phase and preferentially segregates into, as well as structurally stabilising, the disordered matrix material. 1. I N T R O D U C T I O N

V a n a d i u m phosphate (VPO) compounds are important industrial catalysts used in the conversion of n-butane to maleic anhydride[l]. Although it is now generally accepted t h a t the reaction requires the presence of both V S§ and V 4§ cations in close proximity, the precise n a t u r e of the active site in this catalyst is still a m a t t e r for

210 speculation [2,3]. The catalysts are produced by activating a hemihydrate compound, VOHPOa.0.5H20, in an n-butane/air mixture at about 400~ for an extended time period. The resultant catalyst often consists of a complex mixture of vanadium phosphorus oxide phases {i.e. (VO)2P2OT, aI-VOPO4, ~II-VOPO4, T-VOPO4, 5-VOPO4 and VO(POa)2}. Some researchers [4] favour a single compound, (VO)2P2OT, to be the sole active phase and have indicated that the presence of other phases may be due to incomplete activation. However, in-situ Raman experiments by Hutchings et al [5] suggest t h a t specific combinations of some V 5§ phases (aii and 5-VOPO4) and a V 4§ phase ((VO)2P207) phase are a necessary requirement if the catalyst is to simultaneously exhibit good activity and selectivity. The picture is complicated further by evidence of 'disordered phases' or 'poorly crystallized pyrophosphate' material in VPO catalysts prepared in organic media, which is the preferred method for the preparation of industrial catalysts [6]. Such disordered phases are also likely candidates for regions that exhibit V 5§ and V 4§ ions in close proximity. The relative proportions of the phases present depends on a number of conditions such as activation temperature, activation time and precise composition of the activation atmosphere [7]. The phase distribution and morphology of the activated material also seem to depend critically on the precise preparation route used to form the hemihydrate precursor [8] and whether or not the material has been promoted by a dopant such as Fe or Co. The details of the solid state transformation processes taking place during catalyst activation need to be understood before an optimum set of transformation parameters can be defined. In this paper we present the results of a combined transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) study of the activation of an undoped and Co doped VOHPO4.0.5H20 precursor prepared in an organic medium.

2. EXPERIMENTAL 2.1 Catalyst Preparation The precursor was prepared by adding V205 (11.8g) to isobutanol (250ml). HaPO4 (16.49g, 85%) was then introduced and the whole mixture was refluxed for 16h. The light blue suspension was then separated from the organic solution by filtration and washed with isobutanol (200ml) and ethanol (150ml, 100%). The resulting solid was refluxed in water (9ml/g solid), filtered hot and dried in air (ll0~ 16h). The XRD pattern of the precursor showed that a well crystallised VOHPO4.0.5H20 phase was obtained. Starting with the same powdered precursor in a standard laboratory microreactor, four separate experiments were carried out using the same reaction mixture (n-C4Hlo/O~Ie : 1.6/18/80.4) and flow conditions (2.4 1.h1 with VSHV=1500hI). Note t h a t the composition of the gas mixture was chosen to be similar to t h a t used to activate industrial VPO catalysts. The temperature was ramped up from room t e m p e r a t u r e to 400~ at a constant rate of 0.5~ -1. Although the initial t r e a t m e n t was identical for all four experiments, the time on stream at 400~ was systematically varied ; namely 0.1h., 8h., 84h. and 132h. The four 'activated' catalysts are denoted VPO-0.1, VPO-8, VPO-84 and VPO-132 respectively. The catalysts were quenched after these different activation times by cooling the reactor rapidly with reactants being present. Reactor products during each of the four activation runs were analysed using on-line chromatography. Carbon mass balances were typically 98-102% for all data cited.

211 A set of three Co-doped precursors (with 1, 2 and 5 wt% Co) were prepared by dissolving the required amount of cobalt acetylacetonate in isobutanol prior to the operation of refluxing with isobutanol and 85% H3PO 4. The subsequent filtration, washing and doping procedures were identical to that employed for the undoped precursor. These doped catalysts were then activated for 25h at 400~ under the same reaction mixture and flow conditions as described previously. 2.2. C a t a l y s t C h a r a c t e r i s a t i o n A combination of physical techniques were employed to characterise the catalyst microstructure. XRD analysis was performed using a SIEMENS D500 diffractometer operating with a CuK~ source. BET surface area measurements using nitrogen adsorption at liquid nitrogen temperatures were also carried out. Samples suitable for transmission electron microscopy analysis were prepared by dispersing the catalyst powder onto a lacey carbon film supported on a copper mesh grid. TEM observations were made in a JEOL 2000EX high resolution electron microscope operating at 200kV. 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 s a t i o n of the h e m i h y d r a t e p h a s e The XRD pattern of the undoped hemihydrate precursor was characteristic of well crystallised VOHPO4.0.5H20 as shown in figure l(a). When observed in the TEM each individual hemihydrate crystallite exhibited a characteristic rhomboid platelike morphology as shown in Figure l(b). The lateral dimensions along the major and minor axes of the rhomboid were typically about 2~m by ll~m respectively. The platelet thickness was usually in the 0.03 to 0.1~m range. Selected area diffraction patterns taken normal to one of these platelet confirmed that the major and minor axes of the rhomboid correspond to the [100] and [010] directions of the VOHPO4.0.5H20 crystal structure respectively.

Figure 1. (a) XRD pattern and (b) bright field image of the hemihydrate material. All the Co-doped catalyst precursors had a platelike morphology very similar to the undoped material. In addition, their XRD spectra were essentially identical to that

212 shown in Figure l(a) for the undoped VOHPO4.0.5H20. The chemical composition of the doped materials was monitored using energy dispersive X,ray (EDX) analysis, which confirmed t h a t the Co was homogeneously distributed throughout the h e m i h y d r a t e platelet.

3.2. Catalytic performance measurements The catalytic performance data for the four undoped samples activated for different time periods are summarized in Table 1. The n-butane conversion increases from 22 to 65% with increasing activation time. We also observe a decrease in the selectivity for CO and CO2. It is particularly noticeable t h a t the selectivity to CO drops off very rapidly in the first 10 hours. When the variation in surface areas is t a k e n into account we find t h a t the intrinsic activity, VMA, for maleic anhydride production increases steadily with time on s t r e a m up to 84 hours and then tends to level off as the catalyst becomes stabilized. Table 1 Catalytic performance data for the undoped VPO materials Selectivity (%) Time

Intrinsic

BET Area

on Stream (h)

Conv(%)

MA

CO 2

CO

Activity (10 -s mol cm -2)

(m2gi)

VPO-0.1

0.1

10.5

22

34

47

17

0.43

VPO-8

8

7.6

26

48

36

15

0.99

VPO-84

84

14.8

55

66

20

12

1.48

VPO-132

132

19.4

65

69

19

11

1.39

Catalyst

The catalytic performance of the Co-doped catalysts was compared to t h a t of an undoped VPO catalyst activated for the same time period. The results of our analysis are presented in Table 2, where it is clear t h a t the addition of Co in all cases has a beneficial effect on both the selectivity to maleic anhydride production and the specific activity of the VPO catalyst. The most significant improvement, however, was noted for the catalyst with the lowest cobalt loading. Table 2 Catalytic performance data for the Co-doped VPO materials (25h on stream) Catalyst

BET Area (mZg_l)

Conv (%)

Selectivity to MA (%)

Specific Activity (mol MA/mZh 1)

VPO

13.0

16

50.3

0.43

VPO: 1at%Co

16.3

36

71.4

0.99

VPO: 2at%Co

10.0

10

63.0

1.48

VPO: 5at%Co

10.0

10

62.4

1.39

213 3.3. S t r u c t u r a l e v o l u t i o n of t h e u n d o p e d c a t a l y s t a c t i v a t e d in b u t a n e / a i r XRD spectra for the VPO-O.1, VPO-8, VPO-84 and VPO-132 activated catalysts are presented in Figure 2. The materials can all be classed as 'poorly crystalline', although the peaks in the spectra for VPO-84 and VPO-132 do gradually become a little sharper. A comparison of the spectra in figure 2 with those obtained from crystalline standards of (VO)2PzO7 and the various V O P O 4 phases [9], does clearly show the presence of (VO)2P207 as characterised by the 200, 024 and 032 reflections at 23 ~ 28.45 ~ and 29.94 ~ respectively. Any peaks from VOPO4 phases, if present, are hidden in the background noise of the spectra. The XRD spectra therefore indicate t h a t the amount and degree of crystallinity of (VO)2P20 v present are steadily increasing with activation time. 150

150

(a)

100

100

50

50

0

,

5

=~15o

,

.

,

,

,

,

,

,

(c)

,

5

10 15 20 25 30 35 40 45 50

1"0 1'5 20 2} 3'0 3'5 4"0 4"5 5'0

(b)

.,.

150 100 100 50

50 ,

5

,

,

,

,

,

,

,

,

0

9

,

,

,

,

9

,

,

,

10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50 20/degrees

Figure 2. XRD patterns (a) VPO-0.1, (b) VPO-8, (c) VPO-84 and (d) VPO-132. Low magnification transmission electron micrographs of typical crystallites from the VPO-0.1, VPO-8, VPO-84 and VPO-132 samples are shown in figures 3(a), (b), (c) and (d) respectively. It is clear from this sequence of micrographs t h a t the characteristic rhomboid platelet morphology of the hemihydrate is retained to varying extents in all the specimens. The VPO-0.1 sample (fig.3(a)) shows a number of subtle changes from the crystalline hemihydrate platelet. Firstly a number of circular features, which are probably internal voids, have appeared. Secondly, isolated patches of the platelet show very characteristic fissures which seem to be crystallographic in origin because they tend to align along the minor axis of the rhomboid platelet. As the activation time increases to 8 hours (VPO-8; Fig.3(b)) the density of the circular void features has increased markedly and a distinct dark fringe about 25nm thick is beginning to form along the periphery of the platelet. The fringe appears darker t h a n the centre of the platelet due to diffraction contrast, implying that well crystallized material is nucleating at the platelet rim. The material in the interior of the platelet is much more disordered in character and is very sensitive to electron beam damage. As activation progresses further (VPO-84; Fig.3(c)) the crystalline fringe at the periphery is now continuous and has coarsened to a thickness of 50-70 nm.

214 a

b

......

!!:i:~i!~i:~i!~i:i'ii:il~ i:!,~:i~~,:

Figure 3. Bright field transmission electron micrographs of platelet morphologies in (a) VPO-0.1, (b) VPO-8, (c) VPO-84 and (d) VPO-132 activated catalysts. Detailed higher magnification studies confirmed that this periphery comprised small crystallites of (VO)2P2OT. In addition, large holes in the interior of the platelet of up to 300nm in diameter are now apparent. The residual interior material is still rather disordered and beam sensitive. Finally, the VPO-132 sample in figure 3(d) shows the 'end-state' where in addition to the well crystallized rim, the material in the interior of what remains of the platelet appears more crystalline in character. Selected area diffraction patterns obtained from the VPO-0.1 sample [7] confirmed the coexistence of VOHPO4.0.5H20, (VO)2P207 and 5-VOPO4. Furthermore these studies showed that the orientations of all three phases are epitaxially related. For instance, the epitaxial orientation relationship between VOHPO4.0.5H20 and (VO)2P207 is; [001]hemi // [100] pyr~ and [010] hemi// [010] pyr~ This orientation relationship has been reported previously [9]

215 in terms of the topotactic transformation that can occur between VOHPO4.0.5H20 and (VO)2P207. Our work however [7] also demonstrates a previously unreported epitaxial relationship between VOHPO4.0.5H20 and ~-VOPO 4 in which [001] hemi// [100] delta and [010]hemi // [001] delta

Figure 4. (a) Bright field image showing a typical platelet from the VPO 0.1 sample. Corresponding dark field micrographs taken in (b) the gpyr~ reflection of (VO)2P207 and (c) the gdelta=022 reflection of 5-VOPO 4. 5-VOPO 4 has been shown [7] to be suffer severe beam damage, and to overcome this we have carried some low dose dark field imaging experiments on the VPO-0.1 sample in an attempt to locate this phase spatially. Figure 4(a) shows a bright field image from a typical platelet from the VPO-0.1 sample. The corresponding dark field image shown in Figure 4(b) was taken in the 024 (VO)2P207 reflection, in which a thin peripheral fringe of (VO)zP207 crystallites is clearly seen. If the experiment is

216 repeated using the 022 5-VOPO 4 reflection then occasional crystalline patches about 100-200nm in diameter can be seen in the interior of the platelet (fig 4(c)). Furthermore, these domains of crystalline 5-VOPQ phase nearly always seem to be associated with the regions of the platelet showing crystallographic fissure features. Dark field imaging using a reflection associated with the hemihydrate phase just gives rise to a very diffuse image of the entire interior of the platelet. In summary our dark field imaging experiments suggest that after 0.1 hours on stream, crystalline [001] oriented pyrophosphate is just beginning to form at the rim of the platelet, whereas the interior seems to consist of very disordered hemihydrate phase in which discrete domains of 5-VOPO 4 have nucleated. These domains of 5-VOPO 4 and the residual hemihydrate material subsequently appear to progressively convert to the pyrophosphate phase with increasing time on stream. Simultaneously the pyrophosphate phase which forms epitaxially at the edge of the platelet gradually coarsens and thickens.

Figure 5. (a) Typical morphology observed in the 1, 2 and 5at% Co-doped VPO catalysts. EDX spectra obtained from the disordered interior material (b) and a blocky (VO)2PzOv crystallite at the platelet edge (c).

217 The major morphology observed in the 1, 2 and 5at% Co containing catalysts is shown in figure 5(a). The interior of the platelet after 25h of activation is largely disordered and exhibits large circular perforations. However, the blocky (VO)2P207 crystallites not only appear at the rim, but are also seen to decorate the flat platelet surfaces. EDX spectra obtained from the disordered interior material and a blocky (VO)2P207 crystallite at the platelet edge are shown in Figure 5(b) and (c) respectively. It is clear that the (VO)2P207 crystallites do not contain Co (at least at the 0.1at% detectability limit of the technique) which suggests that the dopant is preferentially segregating into the disordered phase. Observations on Co-doped samples which have been activated for extended time periods indicate that a rather large proportion of disordered phase is still retained even after 150h of activation. Hence, it seems that the dissolved Co may have a tendency to structurally stabilise the disordered phase. EDX characterisation of the 2 and 5at% Co-doped VPO samples showed similar concentrations of Co in the disordered regions of the platelet to that observed in the lat%Co-doped VPO sample. In the former of the two materials, the excess Co was found to be present as a secondary phase of vanadium doped cobalt phosphate. This suggests that there is a limited solubility of Co in the disordered VPO material and explains why increasing the Co loading does not necessarily lead to improved catalytic performance.

4. C o n c l u d i n g c o m m e n t s Under the particular butane/air transformation conditions we have used, there are two routes by which the undoped hemihydrate phase can convert to the pyrophosphate phase. At the edge of the platelet a direct topotactic transformation between VOHPO4.0.5H20 and (VO)2P207 occurs. In the interior of the platelet the transformation between the two phases can occur indirectly via an intermediate 5-VOPO 4 phase. As these two types of transformation can apparently occur side by side, it is highly likely that small changes in the transformation conditions or the precursor morphology may critically affect the proportion of direct to indirect transformation occurring. This may go some way to explaining why catalyst samples that have been prepared via different routes and which have not been fully equilibrated can show considerably different relative proportions of crystalline VOPO4, crystalline (VO)2P207 and disordered mixed V4+-V5+ phases. A further key feature which emerges from our study is that as the time on stream increases, the domains of crystalline 8-VOPO 4 gradually reduce to the pyrophosphate phase. We have not as yet been able to image this second stage in the transformation to determine whether or not a topotactic change between 8 - V O P O 4 and (VO)2P207 occurs. It is interesting to note that 31p NMR MAS and Raman evidence exists [9,11] to suggest that the 8 - V O P O 4 phase may in some circumstances convert to (VO)2P207 via a n ( z i i - V O P O 4 intermediate. Our current results do however demonstrate a distinct parallel between improving catalytic performances (n-butane conversion, activity and selectivity for MA production) and a decrease in the amount of V ~§ present. It seems likely that in fully equilibrated catalysts the V~+-V4§ redox couples that are needed for maleic anhydride production are either associated with V 5§ sites existing on the surface of crystalline (VO)2P207 or with the V5+-V4+ centres present in the persistent 'disordered pyrophosphate' phase. Our observations on Co-containing materials show that although the dopant is homogeneously distributed in the hemihydrate precursor phase, it has only a very

218 limited solubility in crystalline (VO)2P2OT. This in turn leads to a Co enrichment in, and structural stabilisation of, the "disordered" VPO component of the activated catalyst. The optimum 1at% Co loading level appears to correspond roughly to the composition at which all the Co can be incorporated in the disordered phase without leading to secondary cobalt phosphate formation. 5. A C K N O W L E D G E M E N T S

This work has been funded under an EEC BRITE-EURAM research programme (contract number BRPR-CT95-0046). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

G.Centi, (Ed), "Forum on vanadyl pyrophosphate catalysts", Catalysis Today, 16, (1994), Elsevier, Amsterdam. P.LGai and K.Kourtakis, Science, 267, (1995), 661. G.Centi, Catal.Today, 16, (1994), 1. J.R.Ebner and M.R.Thompson, Catal.Today, 18, (1994), 51. G.J.Hutchings, A.Desmartin-Chomel, R.Olier and J.C.Volta, Nature, 368, (1994), 41. M.T.Sananes, A.Tuel, G.J.Hutchings and J.C.Volta, J.Catal., 148, (1994), 395. C.J.Kiely, A.Burrows, G.J. Hutchings, K.E. Bere, J.C. Volta, A. Tuel and M. Abon, Discuss. Faraday Soc. No. 105 (1997) in press. C.J.Kiely, A.Burrows, S.Sajip, G.J.Hutchings, M.T.Sananes, A.Tuel and J.C.Volta, J.Catal, 162 (1996) 31. F.Benabdelouahab, R.Olier, N.Guilhaume, F.Lefevre and J.C.Volta, J.Catal., 134, (1992), 151. E.Bordes, Catal.Today, 1, (1977), 499. M.Abon, K.E.Bere, A.Tuel and P.Delichere, J.Catal, 38, (1988), 83.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

Structural and Catalytic A s p e c t s o f Phosphates

Some NASICON-

219

Based Mixed Metal

P.A. Agaskar a, R.K. Grasselli a'b, D.J. Buttrey b and B. White b Central Research Laboratory, Mobil Research and Development Corporation, PO Box 1025, Princeton, NJ 08543-1025, USA *

a

b Department of Chemical Engineering, University of Delaware, Newark, DE 19716-3116, USA

Some binary and ternary metal phosphates, e.g. NbVP3012, NbTiP3012, NbTil_xVxP3012, SbVP3012, SbTiP30~2 and SbTi~_xVxP3Ol2, possessing empty NASICON structures were synthesized, structurally characterized by x-ray powder diffraction, and a phase having the nominal composition of NbTi0.vY0.25P3012 tested for the oxidation of n-butane. The main aim of the investigation was to prepare compositions which would have structurally isolated vanadium moieties, thereby facilitating the partial oxidation of paraffins to oxidation products involving less than 14 electrons. Although encouraging results were obtained by synthesizing novel compositions with structurally defined topologies, the isolation of vanadium sites was insufficient in the Nb-Ti-V-P-oxide phase, which was catalytically investigated, to obtain the desired, anticipated results. The composition produced maleic anhydride (max. yield 30.6% at 57.8 conversion) as the sole non-COx partial oxidation product. At this juncture it is believed that extraneous, V- rich amorphous overlayers observed by TEM are partially responsible for the catalytic results obtained. Future studies are underway to further refine the syntheses methods and isostructural, key catalytic element substitutions, to achieve the desired goals of rational catalyst design and thereby influencing the reaction channels of oxidation reactions.

1. INTRODUCTION The rational design of catalysts has been a desired aim of catalyst researchers for a long time. Our current attempt at this goal centers on the partial oxidation of paraffins, and entails the incorporation of key catalytic elements into a structural framework which by its very nature would favor structural isolation of such catalytic functionalities. It is well known by now, that vanadium is one of the key elements for the oxidative activation of paraffins [1-5]. It is also well known, that structural isolation of catalytic moieties is desirable to achieve selectivity to useful oxidized products, thereby preventing overoxidation to waste products, CO and CO2 [6-8].

* Former address

220 With this background in mind, we chose vanadium as our paraffin activating element and the NASICON structure as the framework into which the vanadium and other catalytic moieties would be incorporated. The reaction to be studied was chosen to be the oxidation of n-butane. The NASICON structure was chosen because it can be readily synthesized, is thermally very stable, and can accommodate a large fraction of vacancies and cation substitutions [9-12]. In addition, this structure possesses two features which should be important for the catalyst design as envisioned above. First, it is a phosphate and hence expected, owing to its acidic nature, to stabilize the lower oxidation states of transition metals, e.g., V4+; second, owing to its structure, layered octahedral metal centers with variable valence are separated from each other by redox inactive tetrahedral phosphate groups, i.e., the structure provides for isolation of descrete layers. With these design parameters in mind, we synthesized a number of vanadium containing NASICONS, characterized them and checked the catalytic properties of some of them for the oxidation of n-butane.

2. EXPERIMENTAL

2.1 Synthesis and Characterization of Compositions a) Preparation of NASICON Phases. The phases were prepared by coprecipitation as described by Agaskar and Grasselli [13]. The following compositions were prepared in this manner: NbVP3012, NbTiP3Ot2, NbTi0.sV0.sP3Ol2, NbTi0.75V0.25P3O12, SbVP3012, SbTiP3012, SbTiP3012 and SbTi0.sV0.sP3012. The detailed description of the preparation of one of these phases is given below. b) Preparation of Nb Ti0.75V0.25P3012. This composition was prepared by coprecipitation as follows [13]: Ti2(C204)3 x 10H20 (0.015 mole) was mixed with V205 (0.005 mole) and Nb(C204)5 (0.04 mole) to which 2 5 0 ml distilled water was added and the mixture stirred and heated, until a blue-green slurry was obtained. To this slurry, 85 wt% H3PO 4 (0.12 mole) was added with additional distilled water (150 ml), and the mixture evaporated to dryness. The dry residue was ground, placed into a quartz crucible, heated to 450 ~ and held at that temperature for 10 hours. The calcined mixture was reground and calcined at 900 ~ for 2 hours. c) Surface Area Measurements. The BET surface area of the Nb-Ti-V-P-oxide catalyst described above was measured by a Micromeritics ASAP 2400 N2 physisorption apparatus. The surface area of the sample as synthesized measured 16.9 mZ/g. d) X-Ray Diffraction Measurements. XRD measurements of the Nb-Ti-V-P-oxide catalyst were performed at room temperature on a Rigaku instrument, using Cu Ka radiation (~=1.54178 A~ scanning conditions of 0.04 ~ step size at 2 s/step, automatic baseline correction, 11 point binomial smoothing and data accumulation from 2 to 52 ~ 20 . The indexing was performed using LaB6 (NIST SRM 660) as an internal standard. e) Magnetic Susceptibility Measurements. The magnetic susceptibility measurements were performed with a Johnson Matthey Magnetic Susceptibility Balance. The balance utilizes the Evans method for measuring susceptibilities [14]. The measurements reported here do not include diamagnetic corrections.

221

2.2 Catalyst Evaluation The Nb-Ti-V-P-O-catalyst was evaluated in a fixed bed microreactor unit described elsewhere [ 15], modified in such a manner, that all reactor effluent lines were heated to -200 C to prevent condensation of products. The product stream was analyzed by an on-line Varian Model 3600 GC for oxygenates and C5+ hydrocarbons and a Carle refinary gas analyzer for the lower hydrocarbons and fixed gases. The composition of the feed was adjusted to contain about 1.5% by volume n-butane in air, and hence below the lower explosion limit of this mixture. The butane and oxygen conversions are reported as the mole percentages of each in the feed, converted to products. The yields of carbon containing products are defined as the mole percentages of n-butane converted to the respective products. The selectivity of each product is defined as the ratio of the yield of each product divided by butane conversion. The carbon, hydrogen and oxygen balances are 100 +/- 5 %, and all results are normalized on a no loss carbon basis. Blank runs over acid-washed quartz chips showed no reaction under conditions similar to those made with the catalyst.

3. RESULTS and DISCUSSION 3.1 Structural Properties of Compositions The synthesized compositions were characterized by X-ray diffraction, and the results are summarized in Tables 1 and 2. Table 1 X-ray diffraction powder patterns of Nb-based mixed metal phosphates having NASICON structures

NbVP3012 d 6.185 4.431 4.311 3.714 3.087 2.793 2.508 1.968 1.935 1.856

NbTiP3012

NbTio.sVo.sP3012

I/Io

d

I/Io

d

I/Io

50 100 55 84 79 45 20 13 13 12

6.14 4.41 4.28 3.693 3.071 2.781 2.494 1.961 1.925 1.847

56 86 59 100 67 59 20 16 16 15

6.109 4.599 4.291 3.685 3.063 2.784 2.481 2.058 1.929 1.831

31 18 10 63 53 29 32 5 9 5

NbTi0.75V0.25P3012 d

I/Io

6.129 24 4.587 3 4.300 100 3.685 50 3.063 49 2.784 20 2.485 29 1.931 14 1.908 8 1.850 3

222 Table 2 X-ray diffraction powder patterns of Sb-based mixed metal phosphates having NASICON structures

SbVP3012 d 6.006 4.364 4.143 3.611 3.008 2.751 2.436 2.392 2.186 2.108 2.074 2.006 1.933 1.875 1.807

SbTiP3012

SbTio.sVo.sP3012

I/Io

d

d

50 1O0 59 53 54 44 25 21 4 7 4 5 6 17 14

6.01 4.36 4.16 3.615 3.013 2.748 2.442 2.405 2.186 2.108 2.101 2.010 1.934 1.876 1.810

I/Io 58 1O0 58 79 72 59 25 22 4 7 7 8 16 17 17

6.026 4.364 4.162 3.619 3.013 2.751 2.442 2.401 2.186 2.108 2.083 2.010 1.935 1.879 1.811

I/Io 59 1O0 53 62 59 51 25 19 3 6 2 5 9 19 12

It is apparent from the X-ray diffraction patterns presented (Tables 1 and 2) that the structures of the mixed metal phosphates synthesized here possess the empty NASICON structure. A single crystal of nominal composition SbVP3OI2with sufficient size for four-circle X-ray diffraction study was isolated. The unit cell is" a = 8.287, b = 8.287, c = 22.086, c~ = 90.00, 13 = 90.00, 3, = 120.00. The fractional coordinates are" Sb/V (1): xyz = 0.3333, 0.6667, 0.3100; Sb/V (2)" xyz = 0.6667, 0.3333, 0.1867; P" x y z = 0.2856, 0.2862, 0.2501; O (1)" xyz = 0.4511, 0.2523, 2355; O (2): xyz = 0.3579, 0.4973, 0.2554; O (3): xyz = 0.1349; 0.4786, 0.3579; O (4): xyz = 0.5209, 0.1172, 0.1373. Structural refinement reveals two interesting observations: First, the c-glide operation present in the parent NASICON structure ( space group R 3c ) is absent here ( space group R 3 ), which implies that neighboring octahedral layers are crystallographically unrelated. Second, the occupancies of the octahedral sites within the two distinct layers are indistinguishable from a random 50%V / 50%Sb mixture. The ramifications of this observation in the context of our catalytic anticipations need to be further contemplated and influenced, if need be, to achieve the desired catalytic goals. Most of the NASICON phases which we synthesized are relatively pure. However, some do contain small amounts (i.e., < 5%) of extraneous impurities, which have to be further determined and eliminated in future investigations. It is to be anticipated, that such impurities m

223 might deletariously affect the catalytic properties under investigation, and in particular influence the realization of the catalytic concepts which we proposed for this study and which we tried to verify here. One of the less contaminated phases was NbTio.vsVo.25P3012, although not entirely free of impurity phases, which was chosen for the catalytic investigation of nbutane oxidation. The results are presented below. Magnetic susceptibility measurements of two of the synthesized compositions strongly suggest, that the majority oxidation state of vanadium in these NASICON phases is V 4+. The respective measured values are 1.57 BM for NbVP3012 and 2.13 BM for SbVP3012. Additional measurements, including XPS are necessary to more exactly define the oxidation state of the vanadium.

3.2 Structural and Catalytic Properties of NbTi0.7sV0.2sP3012 The composition of NbTi075V025P3012 exhibits an XRD pattern characteristic of a NASICON structure (Table 1 and Figure 1) and has a surface area of 16.9 m 2/g. Its structural features lie intermediate between those of NbTiP3OI2 and NbVP3012 NASICONS, as might be expected. The catalytic properties of this phase were investigated for the oxidation of n-butane and are summarized in Table 3 and Figure 2. The results reveal that this composition possesses catalytic properties, activates n-butane under oxidizing conditions and converts it to maleic anhydride and waste products, CO and CO2. The sole non-COx product is maleic anhydride.

'

I

~

10.0

'

'"

'

I

20.0

'

'

'

'

I

'

30.0

Degrees 20

'

'

'

I'

'

40.0

'

'

'

I'

'

50.0

Figure 1. XRD pattern ofNbTio.75Vo 25P30~2 exhibiting a NASICON structure

224 Table 1: Oxidation of n-butane over NbTi0.75V0.2sP3012 (V/P - 1/12) Experimental Conditions Temperature (~ Feed Flow Rate (cc/min) n-C4~ %) O2/n-C4~ ratio) WHSV (per hour) Contact Time (see) Conversion Oxygen n-C4 ~ Selectivity Maleic Anhydride CO CO2 Yield Maleic Anhydride CO CO2 Balances C H O

450 142.7 2 11.9 0.09 2.4

500 142.7 2 11.9 0.09 2.4

450 71.2 1 15.1 0.04 4.9

500 71.2 1 15.1 0.04 4.9

2.9 8.6

9.1 26.2

6.2 24.0

22.3 57.8

71.1 20.2 8.8

57.5 33.9 8.6

66.6 23.3 9.3

52.9 37.3 12.4

6.1 1.7 0.8

15.0 8.9 2.3

16.0 5.6 2.2

30.6 21.6 7.2

99.7 99.7 98.4

100.9 100.9 100.0

100.1 100.1 99.9

99.3 99.3 103.3

As is customary for redox reactions, the yield of the useful product (maleic anhydride) rises with (n-butane) conversion, while selectivity to maleic anhydride declines with conversion. Under the conditions studied, the highest maleic anhydride yield of 30.6%, at a conversion of 57.8%, was obtained at 500 ~ using a feed composition of 1 n-C4/15 O2 / 84 N2, a contact time of 4.9 sec, and a WHSV of 0.04/hr. Under comparable conversion conditions we obtained with the most celebrated, n-butane to maleic anhydride catalyst (VO)zP207 [16], a maleic anhydride yield of 38.5%. We conclude therefore, that the Nb-Ti-V-P-oxide catalyst investigated is relatively active for the oxidation of paraffins, since it has a V/P ratio of only 1/12 as compared to a 1/1 ratio for (VO)zP207. Of course, we had hoped that the V in the NASICON structure would be sufficiently site isolated to yield products less oxidized than maleic anhydride from n-butane. However, unfortunately that does not appear to be the case. One explanation for this might be that there are still too many adjacent V atoms, i.e., (V-O-V)n moieties, where n > 0. Nonetheless, the NASICON structure provides for some desired V site isolation, however, apparently not complete and hence not sufficient to achieve our desired catalytic goal. Another observed fact is, that the Nb-Ti-V-P-oxide under investigation shows an amorphous overlayer via TEM which is enriched in vanadium. The (V/P)surface > (g/P)particle" One can reason that at the temperature of 900 ~ required to obtain the NASICON structure, the more

225 40

/ / / / / / /

O

E

30

LIJ r'n

.

ICl >...

/

"l"-9 20 Z <

/// /

(.) m

U.I .._1

9

<

1~

l.l..

10

/ / /

O

c3 _J LLI

>-

0

"1"

'

0

I

10

' 2 ; ' 3 ; ' 4 1 0 n-C,

o

' 5 ; ' 6 0

CONVERSION

Figure 2. Maleic anhydride yield vs n-butane conversion with NbTi0.75V0.25P30~2 as catalyst volatile vanadium preferentially migrates to the surface. In order to prevent such surface enrichment, it will be necessary in the future to control the synthesis environment. It might be possible to slow down the observed surface enrichment of vanadium by controlling the partial pressure of oxygen over the solid sample. It is reasoned, that under mildly reducing conditions (e.g., nitrogen, containing only residual oxygen, typically in the range -4 < log pO2 (atm) < -3), the in situ formation of a lower valent vanadium will slow down, or possibly prevent the undesirable vanadium to-the-surface migration, since the lower valent vanadium species are much less likely to migrate than the highly oxidized vanadium species. Such studies are currently underway in our laboratory. Additionally, it will be necessary to test our NASICON phases under milder reaction conditions; particularly at lower temperatures and greater hydrocabon dilution. Such conditions would be more conducive to yield less oxidized useful intermediates than those employed in this study. 4. CONCLUSIONS Vanadium containing NASICON compositions were synthesized, structurally characterized, and a composition of the empirical formula NbTio.75Vo.25P30~2 tested for the catalytic oxidation of n-butane. The study was undertaken with the premise to rationally engineer compositions which by choice of key catalytic elements and their placement in a chosen structure, might influence the reaction channel of given oxidation reactions. It was reasoned, that placing vanadium, a known paraffin activating element into a NASICON structure might

226 result in sufficient site isolation of vanadium, so as to lead to solids which might catalyze the oxidation of paraffins in a controlled way, giving partial oxidation products involving only a few electrons. While partial oxidation of n-butane occurred over NbTio.75Vo.25 P3012 leading to maleic anhydride as the sole partial oxidation product, the 14 electron oxidation was not exactly planned. Two explanations for the latter occurrence are advanced in the paper and possible remedies to channel the oxidation reaction into a less aggressive oxidation path are given. The latter include a focused approach to the synthesis of ternary and quaternary NASICON systems under controlled conditions, leading to expected site isolated small vanadium clusters, with completely isolated vanadium centers as an upper limit of site isolation in the supporting framework, and thus lower intermediate oxidation products such as furan from C4 hydrocarbons. Studies are currently under way in our laboratories to explore the hypotheses advanced here.

REFERENCES

1.a.A.T. Guttmann, R.K. Grasselli, J.F. Brazdil and D.D. Suresh, US Patent No. 4 746 641 (1988). b. R. Catani, G. Centi, F. Trifiro and R.K. Grasselli, Ind. Eng. Chem. Res. 31 (1992) 107. c. A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati and F. Trifiro, Appl. Catal. A, 113 (1994) 43. 2.a.M.C. Kung and H.H. Kung, J. Catal., 134 (1992) 668. b. A. Corma, J.M. Nieto Lopez and N. Paredes, J. Catal. 144 (1993) 425. 3.a. Y-C. Kim, W. Ueda and Y. Moro-oka, Catal. Today, 13 (1992) 673). b. J.P. Bartek, A.M. Ebner and J.F. Brazdil, US Patent No. 5 198 580 (1993). 4.a.M. Ai, J. Catal., 101 (1986) 389. b. M. Ai, Catal. Today, 12 (1992) 679. 5.a.J.N. Michaels, D.L. Stern and R.K. Grasselli, Catal. Lett. 42 (1996) 135; 139. b. D.L. Stern, J.N. Michaels L. DeCaul and R.K. Grasselli, Appl. Catal. (1997) in press. 6. J.L. Callahan and R.K. Grasselli, AIChE J, 9 (1963) 755. 7. R.K. Grasselli and D.D. Suresh, J. Catal. 25 (1972) 273. 8. J. Nilsson, A.R. Lana-Canovas, S. Hansen and A. Andersson, J. Catal. 160 (1996) 224. 9. P. Hagenmuller, "Solid Electrolytes" Acad. Press, New York, W. van Gool (ed.), (1978). 10. A. E1 Jazouli, etal., C.R. Acad. Sc., Paris, t. 300, Serie II, 11, (1985) 493. 11. G.V.S. Rao, U.V. Varadaraju, K.A. Thomas and B. Sivashankar, J. Solid State Chem. 70 (1987) 101. 12. A. Sereghini, etal., J. Chem. Soc., Farad. Trans., 87 (1991) 2487. 13. P.A. Agaskar and R.K. Grasselli, US Patent No. 5 354 722 (1994). 14. D.F. Evans, Physics E. Sci. Instr., 7 (1974) 247. 15. D.L. Stern and R.K. Grasselli, J. Catal. 167 (1997) in press. 16.a.G. Centi, F. Trifiro and V.M. Franchetti, Chem. Rev. 88 (1988) 55. b. G. Centi, Catal. Today 16 (1993) 1. c. P.A. Agaskar and R.K. Grasselli, Catal. Lett. 23 (1994) 339.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

227

S e l e c t i v e R e a c t i v i t y of O x y g e n A d a t o m s on M o ( 1 1 2 ) for M e t h a n o l O x i d a t i o n Ken-ichi Fukui, Katsuya Motoda, and Yasuhiro Iwasawa Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

Abstract

The selective oxidation of methanol on a Mo(112) surface was investigated by temperatureprogrammed reaction (TPR) and catalytic reaction in a constant flow condition of CH3OH and O2 (10-6-10-5 Pa). Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to characterize the surface structure and the amount of elements on the surface. It has been found that formaldehyde (HzCO) was a major product during TPR of methanol on a Mo(112)-p(1 • surface (0o=1.0), while CH4, H2, C(a), and O(a) were the products at lower coverages of preadsorbed oxygen. Besides, this reaction proceeded without formation of H 2 0 and was considered to be a simple dehydrogenation (CH30(a)--> HzCO (g)+ 1/2H2(g)). Excess oxygen adatoms on Mo(112)-p(1• which were not incorporated into the p(1• structure, enhanced the selectivity to formaldehyde from 50 % to 90 % and lowered the activation energy of the methanol oxidation. Such oxygen adatoms were more reactive than the oxygen atoms of the p( 1 • structure and reacted with the methoxy species to form H20 by the oxidative dehydrogenation mechanism (CH30(a) + 1/20(a)~ H2CO(g) + 1/2 H20(g)). In a constant flow of methanol, the reaction proceeded several cycles but was deactivated by C(a) accumulated on the surface. The selective oxidation of methanol in flow conditions of CH3OH and 02 successfully proceeded on the Mo(112)-p(1• surface without deactivation.

1. I n t r o d u c t i o n

Control of reaction paths on catalyst surfaces by optimizing the structure and electronic properties is a key issue to be solved in surface science. Iron/molybdenum oxides are used as industrial catalysts for methanol oxidation to form formaldehyde selectively. The iron /molybdenum oxide catalyst consists of Fez(MoO4)3 and MOO3, and shows kinetics and selectivity similar to those of MoO3 for methanol oxidation [1]. It suggests that Mo-O sites play an important role in the reaction. MoO3 has a layered structure along a (010) plane, but the (010) surface is not reactive because it has no unsaturated Mo site [1]. On Mo metal surfaces such as (100) [2,3] and (112) [4], major products in methanol reactions were H2 and CO. Therefore, we considered that partial oxidation of Mo sites is needed for the selective oxidation of methanol. We have reported that methanol reaction pathways on Mo(112) could

228

van der Waals snhere \

oxygen adatom \ \ [1 TO]

oxygen atom

ii, ,!ii

!,, s,,

I

......

, il I

....

iiii,.

....

ti!i!,-

It

p(1 x2)-O

I

0 adatom + p(1 x2)-O

Figure 1. Models for oxygen-modified Mo(112) surfaces. be controlled by modification of the surface by oxygen atoms [4-6]. Formaldehyde (H2CO) was a major product during temperature-programmed reaction (TPR) of methanol on a Mo(l12)-p(l• surface (0o=1.0), while CH4, HE, C(a), and O(a) were the products on surfaces with lower coverages of preadsorbed oxygen. Besides, the reaction on Mo(112)p(1• proceeded without formation of H20 (CH30(a)--~ HECO(g)+ 1/2HE(g)). We have suggested that formaldehyde was formed due to suppression of C-O bonds of methoxy intermediates by selective blocking of the second-layer Mo atoms with the oxygen atoms. Oxygen modification of metal surfaces have been examined on methanol reactions. On some metal surfaces such as Cu(110) [7], Cu(111) [8], Cu(100) [9,10], Ag(110) [11], Ru(001) [12], Rh(111) [13], and Fe(100) [14], oxygen atoms enhanced the formation of methoxy intermediate by extracting the hydroxyl hydrogen of methanol to form OH(a). Another effect of oxygen modification was to stabilize the methoxy species as observed on Ni(110) [15], Mo(100) [3], and W(112) [16]. On Fe(100) surface, the stabilization of methoxy by oxygen atoms resulted in a change of selectivity [ 14,17]. The Mo(112) surface has a ridge-and-trough structure, where the top layer Mo atoms form close-packed atomic rows along the [ 111] direction separated by 0.445 nm from each other and oxygen atoms are expected to occupy quasi-3-fold sites composed of one second-layer and two first-layer Mo atoms [18]. Left-half of Figure 1 shows a model of the p(1• surface (0o= 1.0) which was proposed on the basis of LEED patterns and CO titration experiments [ 19]. Every second Mo row is coordinated by oxygen atoms on both sides(Mo2c), while the other Mo rows are not directly coordinated (MONc). This structure preserves adsorption sites on MoNc for admolecules such as CO, methanol, and ammonia. Selective blocking of the second-layer Mo atoms by oxygen atoms suppressed bondbreaking of C-O or N-H, resulting in m

CH30 [4-6] or NHx (x~2) [20] species persisting on the surface up to 500 K. In this study, we show that excess oxygen adatoms on Mo(112)-p(1• which are not incorporated into the p(1 • structure, enhance the selectivity to formaldehyde and lower the activation energy of the methanol oxidation. The selective oxidation of methanol in a flow of CH3OH and 0 2 successfully proceeds on Mo(112)-p(1• without deactivation.

229

2. Experimental The experiments were performed in an ultrahigh vacuum chamber which was equipped with a low energy electron diffraction (LEED) optics, which was also used for Auger electron spectroscopy (AES), and a quadrupole mass spectrometer (QMS) for TPR. A Mo(112) sample was cleaned by cycles of At+ sputtering and annealing to 1300 K. The sample could be cooled to 150 K by liq.N2 and resistively heated at linear sweep rates of 0.5-15 K/s. The cleanliness and surface order were checked by AES and LEED. The clean surface exhibited a sharp and well-contrasted p(1 • 1) LEED pattern, indicating that the surface preserves the bulk truncated structure. The p(1 • structure (0o-=1.0) was prepared by exposing the clean surface to 2 L (1 L=1.33• Pa.s) of O2 at 300 K and subsequent annealing to 600 K [19]. The oxygen coverage was monitored by AES and LEED patterns.

3. Results and Discussion 3.1. Temperature-programmed reactions (TPR) TPR spectra from the Mo(112)-p(1 • surface exposed to methanol at 200 K (Figure 2A) showed simultaneous desorption of HzCO, CI--I4, CO, and H2 at 560 K. We showed that the composition of the species remaining on the surface above 400 K was C:O:H = 1:1:3 and that the hydroxyl hydrogen recombinatively desorbed as H2 below 400 K [4]. We considered, therefore, that the intermediate was methoxy species as supposed on other metal surfaces [2,3, 7,8,11-13,15,17,21-23] or on MoO3 [1,24]. Methoxy species were also observed on oxygen-modified Mo(110) by X-ray photoelectron spectroscopy (XPS) [25] and on oxygenmodified Mo(100) by high-resolution electron energy loss spectroscopy (HREELS) [3]. The A, r~

I

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,

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I

' .,

,

'

~--.,.,~_

_~.._,..~

I

16 amu (CH4)

-

~

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I

'

i

I

'

I

16 amu (CH4) 28 amu

(CO)

2 ainu (H2) r

._~ O9u~ ~

32 amu (CH3OH)

~;

30 amu (H2CO)

200

400

600

T/K

800

1000

18 amu (H20) x2 30 ainu (H2CO) 200

,

I

400

,

I

,

600

T/K

I

800

,

I

1000

Figure 2. TPR spectra after exposing (A) a Mo(ll2)-p(lx2)-O surface and (B) a Mo(112)p(l•

surface with 0.15 ML of oxygen adatoms to 4 L of CH3OH at 200 K. The heating

rate was 5 K/s.

230 Table 1 The product distribution in TPR for the methanol reaction around 560 K on the Mo(112) surfaces modified with oxygen. Product s peci es

Y ield / ML p( 1x 2)-O

Oxygen adatoms (00' = 0.15)

surface (00 = 1.0)

+ p(1 x2)-O surface (00 = 1.0)

H2(g )

0.10

0.01

H2CO(g )

0.09

0.06

H20(g ) CO(g)

0 0.02

0.04 << 0.01

CH4(g )

0.04

<< 0.01

C(a)

0.03

0

O(a)

0.07

0 a)

a) 00, after TPR was 0.11.

adsorption sites of methoxy species are considered to be the MoNc rows in Figure 1 because of considerable steric blocking at the MoEc sites. For the methoxy decomposition on Mo( 112)p(1• selectivity to HECO was 50 % among C-containing products (Table 1). As noted above, the reaction was a simple dehydrogenation of methanol because no desorption of H 20 was observed at any temperature. Some of the methoxy species decomposed to C(a) (17 %) and O(a), and recombinative desorption of CO was observed at 800 K. Except for this recombinative desorption, the reaction products did not include the oxygen atoms in the p(1• structure. Therefore, the oxygen atoms worked as modifiers on the surface. It must be noted again that formation of H2CO was not observed on oxygen-modified Mo(112) surfaces with lower oxygen coverages [4]. The blocking of the second-layer Mo atoms, which are considered to be effective to dissociate C-O bonds [26,27], by oxygen atoms was needed to change the selectivity. Stabilization of methoxy species and enhanced selectivity to HECO by preadsorbed oxygen atoms were also reported on oxygen-modified Fe(100) surfaces [ 14,17]. Further adsorption of oxygen on the p(1 • surface at 300 K preserved relatively sharp subspots of p(1• while a little increase of background intensity was observed. The maximum coverage of the oxygen adatoms by exposure to 02 at 300 K was 0.5 ML. Therefore we assume that the oxygen adatoms adsorb on the MoNc rows of p(1 • structure as shown in the right half of Figure 1. Then the adsorption sites of the oxygen adatoms are competing with those of methoxy species. Figure 2B shows TPR spectra of methanol with oxygen adatoms(0o'=0.15). The oxygen adatoms were adsorbed on Mo(112)-p(1• at 300 K and then the surface was exposed to methanol at 200 K. The spectra show that oxygen adatoms (0o'=0.15) on p( 1• changed the selectivity; formation of CH4 and CO was suppressed and H2CO was formed with nearly 90 % selectivity at 565 K. Recombinative desorption peak of CO from C(a) and O(a) at 800 K was not observed. These results indicate that C-O bonds of methoxy intermediates are further stabilized by coadsorbed oxygen adatoms. It is to be noted

231 that desorption of H20 was observed around 565 K by reaction of oxygen adatoms. The reaction, therefore, changed to the oxidative dehydrogenation (CH3OH + 1/2 O2---" HzCO + H20 ) which is usually observed on oxide surfaces such as MoO3 or Fez(MoO4)3 [1,24]. We confirmed by separate experiments using NH3 as a reactant that oxygen adatoms reacted to form H20 at 520 K [28], while oxygen atoms incorporated in the p( 1 x2)-O structure reacted at 650 K [20]. Heating the p( 1• surface with oxygen adatoms over 803 K caused destruction of the p(l• structure and gave a distorted p(1 • LEED pattern. Once the p(1• structure was broken, oxygen adatoms became less reactive and H20 was not formed below 600 K any more [28]. TPR spectra of methanol on the p( 1 • surface showed that formation of CH4, CO, and C(a) increased compared to no preannealing, and selectivity to H 2CO became similar to that on the p( 1• surface without oxygen adatoms. When deuterated methanol was used as a reactant, the simultaneous desorption of TPR products at 560-565 K in Figures 2A and 2B shifted to higher temperature. It suggests that the rate-determining step of the reaction is cleavage of the C-H bond of methoxy species in both cases. We measured the peak temperature Tm of the reaction as a function of heating rate (2-15 K/s) and calculated the activation energy Ea for the reaction, assuming that the rate of the reaction-limited desorption is of the first order with respect to the coverage of methoxy on the surfaces. The activation energy for the reaction of CH30(a) on Mo(112)-p(1 • without oxygen adatoms was 177+_5 kJ mol-1 The value for CD30(a) increased by ca.10 kJ mol-1, which corresponds to the difference in the zero point energy of C-H and C-D bond. Coadsorption of oxygen adatoms lowered the activation energy by 30-40 kJ mol-1 both for CH30(a) and CD30(a). It suggests that the hydrogen atom is extracted by the oxygen adatoms themselves or by the MONc atoms in the first layer Mo, which are influenced by adsorption of the oxygen adatoms.

3.2. Catalytic reactions in flow conditions We also examined catalytic reactions on Mo(112)-p(1 • in constant flow conditions of CH3OH and O2 (10-6-10-5 Pa). We measured the amount of reaction products by a temperature-jump method as shown in Figure 3, because some mass fragments of CH3OH overlapped with reaction products and background level of each species tended to rise during the gas flow. The sample temperature was kept at 450 K and then CH3OH and O2 were admitted to the chamber by two variable leak valves while monitoring the partial pressure of each gas. After the partial pressures reached desired values, the sample temperature was jumped to the reaction temperature, kept for several minutes, and decreased to 450 K. No reaction occurred at 450 K, therefore, the area of a mass signal over the base line, which is bound between the signals at 450 K, corresponds to the amount of a product from the surface. The temperature jump was repeated and data were accumulated. Yields of products for typical conditions were summarized in Table 2. The amounts of C(a) and O(a) were measured by AES after stopping the reaction gas dose and evacuating the system. When only CH3OH was supplied to the Mo(112)-p(1 • surface above 560 K, the reaction products were HzCO, CH4, CO, H2, H 2 0 and a negligible amount of C2I-I6. The reaction rate gradually decreased at any temperature (560-800 K) and pressure (10-6-10- 5 Pa) examined. No reply to a temperature jump was observed after 870 s in the case of TR=560 K and PCmOH=2.1• Pa. But in any case, more than 1 ML of methanol reacted on the

232

'

I

'

I

'

I

'

I

'

N,' 700 460 16 amu (OH4) 28 amu (CO)

e-

x:i L_ t~ t~

e18 ainu (H20)

(/3 F

"

30 amu (H2CO)

z;

0

,

I

40

,

!

80

i

I

120

I

I

160

,

200

Time / s Figure 3. QMS response in a temperature-jump measurement during the catalytic reaction of methanol on Mo( 112)-p(1 x2)-O at Pcmoi4=2.1 x 105 Pa and Po2=6.5x 10-6 Pa.

Table 2 Yields of products during the catalytic reactions of methanol on Mo( 112)-p(1x2)-O. PCH3OH

PO2

/ Pa

/ Pa

TR / K Time / s

Yield / ML H2CO(g) CH4(g ) CO(g) H2(g ) H20(g ) C(a) O(a)

2.1x10-5

560

870

2.0

1.0

0.2

2.5

2.0

1.05

< 0.05

-6

560

1580

8.5

1.2

0.5

2.8

8.2

0.55

0.45

8.1• -6 1.6x10 -5 2.1x10-5

560 700

1900 a) 550

5.4 2.7

0 0.9

0.3 0.3

0.2 3.0

5.6 2.1

0.10 1.10

0.30 < 0.05

2.1x10 -5 6.5x10 -6

700

1790 a)

16.7

0

1.2

0

19.3

0.05

0.15

2.1x10 "5 6.5•

a) The reaction has not been deactivated and proceeded further in the condition.

233 surface, which means that the reaction proceeded catalytically. As show in Table 2, the selectivity to H2CO was 47 % at 560 K, which is close to the value obtained by TPR of methanol on the p(1• surface without the oxygen adatoms (Table 1). After the reaction, p(1• subspots of LEED were not observed due to a considerable increase in the background intensity. The results of AES showed that the surface was covered with C(a) (0r Therefore, the reaction ceased due to accumulation of C(a) by nonselective decomposition of CH3OH. A remarkable difference between TPR and the constant-flow reaction was formation of H20. As noted above, the oxygen adatoms on the MoNr sites reacted with H(a) to form H20 at 520 K if H(a) was supplied sufficiently. Once O(a) is formed by decomposition of CH30(a), it can react to form H20 with H(a) above 520 K. But during TPR, the reaction is initiated by the C-H bond scission of the methoxy species and cleavage of the C-O bond may occur later, which may result in insufficient supply of H(a) to O(a). As expected by the results of TPR in Figure 2, deactivation of the methanol dehydrogenation in the flow conditions was suppressed by providing oxygen adatoms on the Mo(l12)p(1• surface. Providing 6.5• 10-6 Pa of O2 at 560 K, the amount of methanol reacted on the surface was over twice as many as that without oxygen supply and the selectivity to H2CO increased from 47 % to 79 %. But nonselective decomposition of methanol to C(a) also occurred in this condition and the reaction eventually stopped. At a higher ratio of 02 to CH3OH ( PCH3OI-I=8.lx 10-6 Pa, Po2 = 1.6• 10-5 Pa), the selectivity to H2CO increased to 93 % and the reaction seemed to proceed catalytically without significant deactivation. Thus, the oxygen adatoms retain the C-O bond of methoxy on the p(1 • surface and the selective formation of H2CO can be achieved. The condition listed at the bottom of Table 2 showed the best result in our experiments. Much more than 10 ML of H2CO was formed with a selectivity of 93 %. Note that these yields were the minimum values because no deactivation was observed even after 3200 s. Relatively sharp subspots of p(l• were observed and accumulation of C(a) was not observed by AES. In this condition, the reaction rate had the first order relation with PCmOH and had nearly 0th order relation with Po2. The activation energy for the reaction in a flow condition (PcmoH=8.4• Pa) was estimated by QMS response at the initial temperature-jump as a function of the reaction temperature. Addition of oxygen (Po2=2.6• 10-5 Pa) lowered the activation energy by 15 kJ tool-1. Thus, the role of oxygen adatoms to enhance the selectivity to H2CO and to decrease the activation energy of the reaction was confirmed similarly to the TPR experiments.

4.

Conclusions

The results presented here indicate that the methanol reaction path is dramatically sensitive to the coverage and arrangement of the oxygen adatoms and the surface metal structure. On Mo(112)-p(1• methoxy species were stabilized by the oxygen modifiers in the p(1 • phase, which selectively blocked the Mo atoms with high coordination. This type of modification contributed to the formation of H2CO on the surface, but with 50 % selectivity. The coadsorbed oxygen adatoms enhanced the selectivity to H2CO over 90 % and the reaction proceeded catalytically in the flow conditions on the Mo(112)-p(1• surface without significant deactivation. Besides, the oxygen adatoms decreased the activation energy for the selective oxidation. Thus, the present results suggest that we can control the reaction path by

234 designing the reaction field with the two types of coadsorbed oxygens, modifier atoms and adatoms.

Acknowledgement This work has been supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST).

References 1. U. Chowdhry, A. Ferretti, L. E. Firment, C. J. Machiels, F. Ohuchi, A. W. Sleight and R.H. Staley, Appl. Surf. Sci. 19 (1984) 360. 2. E. I. Ko and R. J. Madix, Surf. Sci. 112 (1981) 373. 3. S. L. Miles, S. L. Bernasek and J. L. Gland, J. Phys. Chem. 87 (1983) 1626. 4. K. Fukui, T. Aruga and Y. I wasawa, Surf. Sci. 295 (1993) 160. 5. T. Aruga, K. Fukui and Y. I wasawa, J. Am. Chem. Soc. 114 (1992) 4911. 6. T. Aruga, K. Fukui and Y. Iwasawa, in: Catalytic Selective Oxidation, Vol.523, eds. S. T. Oyama and J.W.Hightower (American Chemical Society, 1993) p. 110. 7. I. E. Wachs and R. J. Madix, J. Catal. 53 (1978) 208. 8. J. N. Russell, Jr., S. M. Gates and J. T. Yates, Jr., Surf. Sci. 163 (1985) 516. 9. B. A. Sexton, Surf. Sci. 88 (1979) 299. 10. R. Ryberg, Phys. Rev. B 31 (1985) 2545. 11. I. E. Wachs and R. J. Madix, Surf. Sci. 76 (1978) 531. 12. J. Hrbek, R. De paola and F. M. Hoffmann, Surf. Sci. 166 (1986) 361. 13. F. Solymosi, T. I. Tarn6czi and Berk6,A, J. Phys. Chem. 88 (1984) 6170. 14. J.-P. Lu, M. Albert, S. L. Bernasek and D. J. Dwyer, Surf. Sci. 239 (1990) 49. 15. S. R. Bare, J. A. Stroscio and W. Ho, Surf. Sci. 155 (1985) L281. 16. J. B. Benziger and R. E. Preston, J. Phys. Chem. 89 (1985) 5002. 17. J.-P. Lu, M. Albert and S. L. Bernasek, Surf. Sci. 218 (1989) 1. 18. H. Bu, O. Grizzi, M. Shi and J. W. Rabalais, Phys. Rev. B 40 (1989) 10147. 19. K. Fukui, T. Aruga and Y. Iwasawa, Surf. Sci. 281 (1993) 241. 20. T. Aruga, K. Tateno, K. Fukui and Y. Iwasawa, Surf. Sci. 324 (1995) 17. 21. F. M. Leibsle, S. M. Francis, R. Davis, N. Xiang, S. Haq and M. Bowker, Phys. Rev. Lett. 72 (1994) 2569. 22. F. M. Leibsle, S. M. Francis, S. Haq and M. Bowker, Surf. Sci. 318 (1994) 46. 23. S. M. Francis, F. M. Leibsle, S. Haq, N. Xiang and M. Bowker, Surf. Sci. 315 (1994) 284. 24. W. E. Farneth, F. Ohuchi, R. H. Staley, U. Chowdhry and A. W. Sleight, J. Phys. Chem. 89 (1985) 2493. 25. J. G. Serafin and C. M. Friend, J. Am. Chem. Soc. 111 (1989) 8967. 26. L. M. Falicov and G. A. Somorjai, Proc. Nat. Acad. Sci. USA 82 (1985) 2207. 27. J. Wang and R. I. Masel, J. Catal. 126 (1990) 519. 28. K. Fukui, K. Motoda and Y. Iwasawa, to be published.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

235

M e c h a n i s t i c studies of alkane partial oxidation reactions on nickel oxide by m o d e m surface science t e c h n i q u e s # Nancy R. Gleason~ and Francisco Zaera* Department of Chemistry, University of California, Riverside, CA 92521, USA.

The reactions of alkyl iodides, precursors to alkyl surface moieties, with adsorbed oxygen have been studied on Ni(100) by using temperature-programmed desorption, x-ray photoelectron spectroscopy, and ion scattering spectroscopy. The oxygen coverage was varied from submonolayer values to exposures which lead to surface oxidation. The reaction of 2-propyl iodide on surfaces completely covered with oxygen was found to lead to the complete oxidation of the alkyl iodide, but for low oxygen precoverages some acetone production was observed as well. The mechanism for the formation of acetone from 2-propyl iodide was investigated, and the reactions of other alkyl groups on both oxygen- and hydroxide-covered surfaces were also studied. 1. INTRODUCTION The partial oxidation of alkanes is an important industrial reaction for the manufacturing of oxygenated hydrocarbons such as alcohols, aldehydes, and ketones, which in turn are the precursors for the synthesis of higher molecular-weight hydrocarbons. A variety of metal- and metal oxide-based catalysts can be used to oxidize saturated hydrocarbons, but the challenge to successfully producing oxygenated products is in stopping the reaction before total oxidation occurs [ 1]: while the partial oxidation of hydrocarbons is thermodynamically feasible, complete oxidation to carbon dioxide and water is far more energetically favorable, so high yields for partial oxidation products can only be achieved by controlling the kinetics of both pathways. Much research in both the catalytic and surface science communities has been aimed at determining the conditions which favor partial over total oxidation, i.e., at finding ways to selectively control the reaction products [2]. It is generally accepted that the initial rate-limiting step in the partial oxidation of alkanes is the initial scission of a C-H bond to generate alkyl species on the catalyst surface [3, 4]. Alkanes have very low sticking coefficients, and therefore require the high temperature and pressure conditions used in industrial catalytic processes for their activation. Given that these conditions are difficult (if not impossible) to emulate under the ultra-high vacuum environment (UHV) normally used in surface science studies, the C-H activation step needs to be bypassed there in order to reach reasonable coverages of alkyl species on metal surfaces. One approach to achieve this is via the adsorption and decomposition of the corresponding alkyl iodides, since the C-I bonds can be readily activated to yield significant concentrations of the desired alkyl surface species [5-8]. A variety of alkyl moieties can be produced on # Funded by a grant from the Department of Energy, Basic Energy Sciences. Present address: Department of Chemistry, Canisius College, Buffalo, NY 14208, USA. * Corresponding author.

236 surfaces this way, the selection being only limited by the availability of the corresponding alkyl iodide precursors. In the present study the reaction of 2-propyl iodide with oxygen was investigated on Ni(100) surfaces. A variety of products were observed to desorb from the O/Ni(100) system, andthe selectivity among them was found to depend strongly on the coverage of oxygen. For low oxygen coverages acetone is produced in addition to hydrogen, propane, and propene, but at oxygen surface concentrations close to monolayer saturation (0.50 ML) neither acetone nor hydrogen are detected, and the amounts of propane and propene produced from the iodide are substantially reduced. Furthermore, high oxygen exposures lead to oxidation of the nickel surface [9], at which point no hydrocarbons desorb at all; only the products associated with total oxidation, namely, CO, CO2, and H20, are observed. It was also determined that the C-I bond-scission occurs in a temperature range similar to that on the clean metal, between 120180 K, suggesting that 2-propyl fragments are created on the surface at low temperatures, and that the presence of adsorbed oxygen does not alter the kinetics of that dissociation step. These results are quite different to those from the O/Rh(111) system [ 10, 11 ], indicating that a different reaction mechanism operates on nickel. In particular, we propose that the conversion to 2-propyl iodide involves the formation of a 2-propoxide species, which then yields acetone via a rate-limiting 13-hydride elimination step. 2. E X P E R I M E N T A L All experiments were done in a stainless-steel ultra-high-vacuum chamber with a base pressure of lx 10-10 Torr equipped to do temperature-programmed desorption (TPD), X-ray photoelectron (XPS), and ion-scattering (ISS) spectroscopies [12, 13]. TPD spectra were obtained by simultaneously monitoring the mass spectrometer signal of up to 15 masses with an interfaced computer while heating the sample at a rate of 10 K/s. XPS spectra were taken by using an A1 anode and a hemispherical electron-energy analyzer with an overall energy resolution of about 1.2 eV full width at half maximum. The binding energy scale was calibrated against the Pt 4f7/2 and Cu 2p3/2 core levels. ISS spectra were obtained using the same hemispherical energy analyzer as for XPS but with the voltage biases reversed to detect ions rather than electrons, and with a scattering geometry such that the angle between the ion source and the analyzer was 115 ~ [ 14]. A 1-2 ~A 500 eV He+ ion beam was focused to a spot size of about 2 mm diameter on the crystal, and the kinetic energy of the scattered ions was monitored with an interfaced computer. Control experiments on oxygen-covered Ni(100) surfaces, prepared by dosing 3.0 L of 02 at 300 K, showed that sputtering by the 500 eV He+ beam was insignificant [ 15]. Finally, because of the potential problems with the quantitative use of ISS, several methods were used to calibrate the ISS signal, and the conclusion was reached that work function changes due to the various adsorbates (which ultimately affect the neutralization probabilities of the outgoing ions) are negligible in this system and do not influence the quantitative analysis of the data reported here in any significant fashion [ 16]. The nickel (100) single crystal was cut, oriented, and polished using standard procedures, and mounted on a manipulator by spot-welding it to tantalum support wires in contact with a liquid nitrogen reservoir; the sample could be resistively heated to 1200 K and cooled rapidly back to 90 K with this arrangement. The surface temperature was monitored by a chromel-alumel thermocouple spot-welded to the edge of the crystal. Surface cleaning was done by cycles of Ar+ ion bombardment and annealing to 1200 K as well as oxygen treatments (to remove atomic carbon) until no impurities were detected by XPS or ISS. The 2-propyl iodide was obtained from Alfa Products (98% purity), protected from light, and subjected to several freeze-pump-thaw cycles before using; its purity was routinely checked by mass spectrometry. Compressed oxygen (99.999%), argon (99.999%), and hydrogen (99.999%) gases were obtained from Matheson and used as supplied. All gas exposures were

237

done by backfilling of the vacuum chamber via leak valves, and are reported in Langmuirs (1 L = lx 10-6 Torr-s), not corrected for efficiency differences in the ionization gauge. For the studies on the reaction of the alkyl iodides with O/Ni(100), the surface was prepared as follows: (1) Different amounts of molecular oxygen were dosed on the surface at 300 K, a temperature at which oxygen dissociates. (2) The sample was cooled below 100 K. (3) In the case of OH covered surfaces, the hydroxide groups were prepared by then dosing water. (4) The 2-propyl iodide was adsorbed. A constant 4.0 L exposure of the iodide was used for the TPD studies as a function of oxygen coverage. (5) For the XPS and ISS annealing studies, the sample was finally heated rapidly to the indicated temperature, at a rate of nearly 10 K/s in order to reproduce the surface conditions that exist during the TPD experiments, and immediately allowed to cool to below 100 K to freeze the relevant intermediates. 3. R E S U L T S AND DISCUSSION

3.0L

02+ xL 2-C3H71/Ni(100)

1 --

~"T?",

I:1. o

: 0Ni

ISS

E. = 5 0 0 e V H e +

0

2 4 2-C3H71 Exp. I L

Fig. 1. Ion scattering spectra (ISS) as a function of 2-propyl iodide exposure on O/Ni(100) surfaces prepared by adsorption of 3.0 L of 02 at 300 K. The inset shows the ISS peak areas normalized to the signal of the Ni and O peaks for the surface with 0.0 L 2-C3H7I. These data highlight the fact that the alkyl iodides prefer to adsorb on the metal sites.

llJ I•J

A ILl Z

0.1

~^ ~ 100

2.0 4.0

~ I

i

200

300

'

i

400

Kinetic Energy / eV

'

500

The nature of the adsorption sites for 2-propyl iodide on the O/Ni(100) surfaces were probed by ISS first. A set of experiments were done as a function of 2-propyl iodide exposure for given fixed oxygen coverages in order to probe the location of the binding sites for the iodide -- either Ni or O sites. Figure 1 shows the 2-propyl iodide coverage-dependent ISS data obtained for a fixed 3.0 L pre-exposure to oxygen. The top trace, labelled 0.0 L, corresponds to the oxygen-dosed surface before any 2-propyl iodide adsorption, and is the spectrum to which all other traces are compared and normalized. The peaks at 245 and 410 eV kinetic energies correspond to O and Ni, respectively, as determined by using standard elastic collision theory and by taking into account the geometry of our system. Upon exposures of the oxygen-dosed surface to small amounts of 2-C3H7I, the Ni signal decreases, while the O signal remains almost constant. Specifically, over 80% of the Ni peak disappears after a 1.0 L alkyl iodide exposure, but the oxygen peak still retains over 60% of its initial intensity at that point. Nevertheless, most of the oxygen signal is lost after a 2.0 L 2-C3H7I dose, and no Ni or O peaks are seen after a 4.0 L exposure. The coverage dependence of the ISS signals is better illustrated in the inset, in which plots of the normalized Ni and O ISS peak intensities are displayed versus the alkyl iodide exposure. This figure highlights the selective titration of the nickel sites for low 2-propyl iodide exposures, which means that the alkyl iodide binds

238 preferentially to nickel sites. It is important to note that the exposure at which the oxygen signal starts to be significantly attenuated, around 2.0 L of 2-propyl iodide, is the same required to detect any acetone by TPD (see below). The adsorption of 2-propyl iodide is molecular at 100 K regardless of the initial oxygen coverage. This is clearly demonstrated by the I 3d XPS data obtained for 2-propyl iodide adsorbed on both clean and oxygen-precovered Ni(100) surfaces [17]. However, heating the 02 + 2-C3H7I-dosed surfaces between 120 and 180 K induces the dissociation of the C-I bond, as also determined by XPS. We propose that this bond-activation step occurs on the Ni sites, because: (1) Ni is the preferred adsorption site; (2) the temperature range for the C-I bond-cleavage on the oxygen-covered surfaces is the same as that on the clean surface; and (3) as the number of nickel sites decreases with increasing 00, the amount of 2propyl iodide that dissociates decreases. We also assume that the majority species that form at low temperatures upon the breaking of the C-I bond are 2-propyl fragments attached to nickel atoms. 2-Propyl Iodide on O/Ni(100) T P D Effect of O x y g e n on Product Selectivity u)

Clean Ni(100)

3.0 L O2/Ni(100) Submonolayer

40.0 L 02/Ni(100 ) 3 ML NiO

Metal Function

Partial Oxidation

Total Oxidation

,1-1 =m

t~

(U

!_

!_

Water

I

'

100

'

,) '

I

'

300

'

~Hydrogen '

I

'

500

'

9

I

'

700

'

Hydrogen '

100

'

'

I

'

'

'

I

'

9

'

I

'

300 500 700 Temperature / K

'

I

'

100

'

'

I

'

300

'

'

I

'

500

'

'

I

'

'

700

Fig. 2. Temperature-programmeddesorption (TPD) spectra from 4.0 L of 2-C3H71 adsorbed on Ni(100) surfaces predosed with various amounts of oxygen. Three regimes are observed for this system: (1) that for the clean nickel, where only the hydrogenation-dehydrogenationsteps typical of transition metals are seen (left); (2) that for nickel oxide, where there is little reactivity, and where only complete oxidation is observed (right); and (3) that for an intermediateoxygen surface coverage, where some partial oxidation is manifestedby the appearance of a TPD peak for acetone around 350 K (center). The reaction of the resulting 2-propyl groups with oxygen on Ni(100) was characterized as a function of oxygen coverage by temperature-programmed desorption (TPD). A variety of species were found to desorb from this surface, namely, hydrogen, water, carbon monoxide, propene, propane, carbon dioxide, acetone, and the original hydrocarbon molecule. The distribution of reaction products was found to be strongly dependent on 00, as shown in Figure 2, which summarizes the TPD profiles for the main desorbing species after three different oxygen predoses. It can be seen there that the thermal chemistry of the

239 resulting 2-propyl species on the oxygen-treated Ni(100) follows three distinct pathways depending on the nature of the oxygenated surface. On the one end, the clean nickel behaves in the same way as many other metals, that is, it promotes both 13-hydride and reductive elimination steps to yield propene and propane, respectively [7, 13, 18]. At the other extreme, NiO films as thin as 1-2 ML thick passivate the metal and induce total oxidation to CO, CO2, and H20. It is at the intermediate coverages obtained after doses of less than 10.0 L of 02 (which yield atomic oxygen coverages below approximately 80% of monolayer saturation [9]) where the most interesting chemistry is seen, because a small amount of partial oxidation to acetone is detected. Figure 3 compares the molecular (left) and acetone (right) TPD data for the reaction of 3.0 L of oxygen (approximately 0.30 ML of O atoms, or 60% of monolayer saturation) with varying amounts of 2-propyl iodide on Ni(100). A 0.5 L exposure of 2-propyl iodide leads to the desorption of hydrogen, propene and propane, but not acetone, and results in TPD traces quite similar to those obtained from the same 2-C3H7I dose on the clean surface. The onset of acetone formation is seen as a small peak around 350 K only after a 2.0 L alkyl halide dose, and the molecular desorption data shows that monolayer saturation of 2-propyl iodide on this surface occurs between 2.0 and 4.0 L. Notice in particular that the 2.0 L mark corresponds to the point at which all the nickel sites become occupied (see Figure 1). This suggests that, in order for acetone to be produced, a particular surface ensemble is required with the 2-propyl groups adsorbed next to oxygen atoms [ 19-21 ].

II

3.0 L 0 2 -I- X L 2-C3H71/Ni(100 ) T P D

. m

Molecular Desorption

:3

zi

Acetone (xl0 scale)

!._

t~ 61

L._

~t

u~ 61 In

13. t~

2"C3H71 Exposure(x) / L

\

J 2.0

t._

13.

0.5 I

'

100

'

'

I

'

300

'

'

I

'

500

'

'

I

'

700

'

I

'

100

'

'

I

'

300

Temperature / K

'

'

I

'

500

'

'

I

'

700

'

Fig. 3. Molecular (left) and acetone (right) TPD spectra from 0.5, 2.0, and 4.0 L of 2-propyl iodide adsorbed on Ni(100) pretreated with a fixed 3.0 L oxygen dose at 300 K. The results from this figure indicate that acetone production starts at the point where the nickel adsorption sites become saturated, suggesting that the proximity of alkyl and oxygen groups on the surface is a requisite for

partial oxidation reactions. Several pieces of evidence point to the fact that the next step in the reaction that leads to the formation of acetone is the insertion of an oxygen atom into the metal-carbon bond. Figure 4 shows three of them. The left frame displays ISS data obtained after annealing the 2propyl iodide + oxygen system to different temperatures. The main result from this work is highlighted in the inset, which indicates that the O ISS peak never recovers the initial intensity seen before the alkyl halide dose. This suggests that at temperatures slightly above 200 K some of the alkyl groups that remain on the surface migrate to sites on top of the

240

chemisorbed oxygen atoms, blocking the latter from the incoming probing ions. The two other frames of Figure 4 provide XPS and TPD evidence for the similar behavior of 2-propyl iodide and 2-propanol on oxygen-covered Ni(100) surfaces; since propanol is known to produce 2-propoxide groups at low temperatures [22], the same intermediate is inferred to be involved in the case of the alkyl halide. In particular, the C ls XPS traces present in both cases the small shoulder around 285.7 eV binding energy most likely associated with the carbon atom adjacent to the oxygen, and both acetone TPD traces display similar peaks around 320 K. Notice also that molecular acetone desorbs at much lower temperatures from these surfaces, which means that acetone detection in the TPD experiments with the iodo alkane is reaction (not desorption) limited. E v i d e n c e for P r o p o x i d e F o r m a t i o n from 2-Propyl Iodide on O/Ni(100) S u r f a c e P r e p a r e d by D o s i n g 3.0 L 0 2 at 3 0 0 K

ISS 1

"1

C l s XPS

half-covered

Still

i\

|

(middlecarbon in propoxide)

'=/"ee-e'------e

n

o =-./ . . . . 100 TI K

i

~1

I

300

,

, ~ . 4.o L 2-%H~1

2001 15Ol

.......

200

/~

7L~I 4.o L2-C~H,, 4L.~_~I heatedto 170 K

~.~~--~'x ,

320 K

5.0 L 2-C3H7OH ::' heated to 200 K ,:

, 700

I ~ , I.~-~--~~J'

i

Acetone TPD

,oo !

i

400

.

Kinetic Energy / eV

i

500

!"

'1

280

,

,

'

9 ' ,

i

285

.

.

.

.

i

,

290 100

Binding Energy I eV

,'

,

i

,

300

,

,

i

500

9

,

,

i

,

700

Temperature I K

,

9

900

Fig. 4. Left: ISS annealing data for 4.0 L of 2-propyl iodide adsorbed on Ni(100) predosed with 3.0 L of 02 at 300 K. The inset indicates how the oxygen ISS signal never reaches its original value from before alkyl halide adsorption, suggesting that some of the oxygen atoms become covered by alkyl groups. Center: C ls XPS data for 2-propanol (top) and 2-propyl iodide (bottom), adsorbed on oxygen-precovered Ni(100) surfaces and then annealed to induce 2-propoxide formation. The alkoxide intermediate is identified by the signal at 285.7 eV binding energy that corresponds to the middle carbon atom. Right: acetone TPD spectra from O-precovered surfaces dosed with acetone, propene, 2-propanol, and 2-propyl iodide. The similar behavior seen in the latter two cases suggest that 2-propanol and 2-propyl moieties react via a common intermediate. Also, the absence of any acetone desorption from adsorbed propene discards such a species as a possible intermediate in the partial oxidation of propyl groups in this case.

Lastly, 2-propoxide intermediates undergo a 13-hydride elimination step above 300 K to yield the final acetone product. The selectivity of this step is best illustrated by the acetone TPD traces shown in Figure 5, which were obtained with the partially labelled CD3CHICD3 isotopomer of 2-propyl iodide. The only acetone detected in these experiments is the perdeutero species, which means that the hydrogen-removing step is regiospecific and involves only the secondary middle hydrogen. These experiments also confirm that propene is not involved in the production of acetone in this system. The data presented so far suggest that a few requirements need to be met in order for partial oxidation reactions to take place on oxides, namely: (1) There is a need for metal

241 atoms to be exposed on the surface. This in fact was a requisite here only because the Ni sites are the ones that facilitate the dissociation of the C-I bond in the alkyl halides (a reaction that is not relevant for the oxidation of alkanes), but they may also be necessary to induce the initial C-H bond-activation in alkanes. (2) The migration of alkyl groups attached to metal atoms seems to be somewhat limited, which means that they need to form next to oxygen atoms for the partial oxidation process to proceed. (3) The partial oxidation reaction involves alkoxide intermediates, and therefore may be favored by anything that stabilizes such species on the surface. (4) The rate-limiting step appears to be the last 13-hydride elimination, so it is important to facilitate the fast desorption of the ketone (or aldehyde) products, because otherwise they may decompose on the surface immediately after their formation.

4.0 L CD3CHICD 3 on O/Ni(100), TPD Surface prepared by dosing 3.0 L 02 at 300 K CD3Ncll/CD3 ~ ........

-1/2H2

Ni

~

O

>300 K

Ni

Acetone

Propoxide

i

100

'

'

'

i

Fig. 5. Acetone TPD traces for the main isotopomers expected from the reaction of CD3CHICD3 with oxygen on Ni(100) surfaces. The exclusive formation of perdeutero acetone in this case indicates the high selectivity towards a 13-hydride elimination step from the 2-propoxide intermediate, and rules out a mechanism where an initial 13-hydrideelimination from 2propyl groups on Ni sites is followed by oxygen incorporation.

CD3Nf/C~

'

'

'

i

'

'

'

!

,

300 500 700 Temperature / K

It should be noted that the experiments reported here were carried out on oxygendosed nickel surfaces, not on nickel oxide. Nevertheless, it appears that the conclusions enumerated above may be applicable to substoichiometric oxide surfaces as well [19-21]. The possibility of extrapolating our surface science studies to more realistic systems is supported by the data in Figure 6, which shows TPD data illustrating the ability of different surfaces towards acetone production from propyl moieties. As mentioned before, the clean nickel surface only yields propane and propene; oxygen atoms are needed to produce oxygenated products (bottom trace). The second trace from the bottom reproduces the acetone desorption profile shown in Figure 2 for surfaces covered with oxygen submonolayers, which are active systems for partial oxidation processes. Next up there is another flat trace, that obtained from a stoichiometric nickel oxide film: no acetone is produced there because of the inert character of that surface. It is the last (top) trace the one that shows the promise of our studies. The NiO film was annealed in this case to high (>600 K) temperatures prior to alkyl halide dosing in order to induce the diffusion of some oxygen atoms into the bulk and the formation of a substoichiometric oxide top layer. The TPD data proves that such a surface is still somewhat active towards acetone formation. This appears to imply that even subsurface oxygen may be capable of migrating to the surface in order to insert itself into the Ni-C bond during the 2-propoxide formation step. One final observation is worth reporting here, that is, that concerning the role of OH surface groups in the partial oxidation process. Figure 7 provides TPD traces for the respective desorption of acetaldehyde and acetone from ethyl and 2-propyl iodides adsorbed

242

on oxygen- (left) and hydroxide- (right) covered nickel surfaces. Two important observations can be highlighted from these data: (1) The formation of acetone from 2-propyl groups is enhanced by the presence of OH groups on the surface, to the point of yielding TPD traces similar to those seen for the case of 2-propanol (compare with Figure 4); and (2) Some acetaldehyde is produced during the oxidation of ethyl groups if hydroxide species are present on the surface. In contrast, no aldehydes were detected in TPD experiments with methyl, ethyl, 1-propyl, or 1-butyl iodides on purely oxygen-precovered nickel surfaces, perhaps because the resulting alkoxides decompose on the surface before yielding the desired products. The mechanism by which the OH groups enhance the ability of the surface to induce partial oxidation reactions is still under investigation.

Effect of Oxygen Environment on Partial Oxidation Selectivity Acetone TPD from 2-C3H71 on O/Ni(100) 355 K

Fig. 6.Acetone TPD for 2-propyl iodide adsorbed on Ni(100) surfaces after different oxygen treatments. The bottom and third traces indicate that neither the clean nickel metal nor stoichiometric NiO films are capable of inducing the partial oxidation of 2propyl groups to acetone. In contrast, the second and four traces show that substoichiometric nickel oxides are able to promote such a reaction even if the oxygen atoms are in the subsurface region.

Subsurface

x8

Oxide 55 K

Clean metal I

100

'

'

'

I

'

'

300

'

I

500

'

'

'

I

700 Temperature / K

'

'

'

I

900

4. CONCLUSIONS It was shown here that, under the right conditions, substoichiometric nickel oxides may be capable of catalyzing partial oxidation reactions. A number of criteria were identified for establishing the activity of this pathway, namely: (1) Nickel atoms need to be exposed in order to promote the initial alkane activation and to stabilize the resulting alkyl surface groups. (2) Oxygen atoms need to be present in the proximity of the alkyl groups in order for the insertion step that leads to alkoxide formation to take place. This oxygen could be located in the immediate subsurface region. (3) Alkenes appear to not be direct intermediates in the conversion of alkyl groups to aldehydes or ketones. This is an interesting observation, because alkenes can indeed be converted to such products catalytically [23]. It could be suggested that perhaps alkenes convert to alkyl groups on the surface before undergoing oxygen incorporation. (4) Surface hydroxide groups appear to enhance the partial oxidation pathway, either because they facilitate the formation of alkoxides, or because they help in the limiting H-abstraction step. (5) The formation of ketones seems to be easier than the production of aldehydes. This is somewhat encouraging, because secondary C-H bonds are weaker than primary ones and therefore easier to break; alkane activation is likely to yield branched alkyl surface moieties.

243 | !

Effect of OH Surface Groups on Selectivity for Partial Oxidation TPD on O/Ni(100) 3 L 0 2 dose at 300 K

TPD on O+OH/Ni(100) 0.5 L 0 2 4- 2.0 L H20

~

360 K

355 K ,tv~'~

acetone from 3.0 L 2-C H_,l

/ -/ J

acetaldehyde from 2.0 L C2H51A 228 K 100

300

500

700

900100

300

500

700

900

Temperature / K Fig. 7. TPD traces from alkyl iodides adsorbed on oxygen- (left) and hydroxide- (right) precovered Ni(100) surfaces. The bottom traces correspond to the formation of acetaldehyde from ethyl iodide, while those on top display the desorption of acetone from 2-propyl iodide conversion. The enhancing power of OH surface groups towards partial oxidation pathways is indicated by two observations from these data: (1) the yield for acetone increases to the point of resembling that seen with 2-propanol; and (2) some acetaldehyde is detected as well. It is at the present time unclear if the OH groups favor the formation of alkoxide intermediates or the subsequent [3-hydride elimination step.

A s c h e m a t i c representation of the m e c h a n i s m p r o p o s e d here for the partial oxidation of 2-propyl iodide on o x y g e n - c o v e r e d Ni(100) surfaces is p r o v i d e d as S c h e m e 1 below. W e believe that s o m e of these ideas m a y be applicable not only to other alkyl groups, but also to other oxide surfaces and to more realistic catalytic conditions. CH3CH2CH3 (g) CH3CH2CH3(g) +

CH3~HCH3 I O

I

CH3CH=CH2(g) 225 K xO CnH

CH3CH=CH2 ~)

/CH3CHCI-~

/ /-

.o

200 K

CH3"rH / CH3

Migration

-150-200K

O

CH3 13'--H

~

>400

H2(g) + C O ~NI~

CH3 C ~)

> 3 0 0 K ' - ~

CH3x /CH3 C (g) 3SSK--

Propoxide

l Hydroxyl proton abstraction CH3CHCH3 O

I

OH

Scheme 1. Proposed mechanism for the partial oxidation of 2-propyl surface species on O/Ni(100) at low oxygen coverages. A similar mechanism is likely to operate during the catalytic conversion of alkanes on substoichiometric nickel oxides.

244 REFERENCES

[1] [21 [3] [41 [51 [6] [71 [81 [9] [10] [11] [12] [13] [ 14] [15] [16] [ 17] [18] [19] [20] [21] [22] [23]

R. J. Madix and J. T. Roberts, in Surface Reactions, Eds. R. J. Madix (Springer-Verlag, Berlin, 1994) pp. 2. J. H. Lunsford, Angew. Chem. Int. Ed. Engl. 34 (1995) 970. G. A. Somorjai, Catal. Rev.-Sci. Eng. 23 (1981) 189. R. Pitchai and K. Klier, Catal. Rev.-Sci. Eng. 28 (1986) 13. F. Zaera, Acc. Chem. Res. 25 (1992) 260. X.-L. Zhou, X.-L. Zhu and J. M. White, Acc. Chem. Res. 23 (1990) 327. F. Zaera, Chem. Rev. 95 (1995) 2651. B. E. Bent, Chem. Rev. 96 (1996) 1361. C. R. Brundle, in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3A (Chemisorption Systems), Eds. D. A. King and D. P. Woodruff (Elsevier, Amsterdam, 1990) pp. 132. C.W.J. Bol and C. M. Friend, J. Phys. Chem. 99 (1995) 11930. C.W.J. Bol and C. M. Friend, J. Am. Chem. Soc. 117 (1995) 11572. F. Zaera, Surf. Sci. 219 (1989) 453. S. Tjandra and F. Zaera, J. Am. Chem. Soc. 117 (1995) 9749. F. Zaera, Langmuir 7 (1991) 1188. N.R. Gleason and F. Zaera, J. Catal. (1997) in press. N.R. Gleason and F. Zaera, Surf. Sci. (1997) in press. N. R. Gleason and F. Zaera, 211 th American Chemical Society National Meeting, New Orleans, 1996, Paper No. COLL. S. Tjandra and F. Zaera, Langmuir 10 (1994) 2640. J. L. Callahan and R. K. Grasselli, AIChe J. 9 (1963) 755. R. K. Grasselli and J. D. Burrington, Adv. Catal. 30 (1981) 133. J. Nilsson, A. R. Lana-Canovas, S. Hansen and A. Anderson, J. Catal. 160 (1996) 244. X. Xu and C. M. Friend, Surf. Sci. 260 (1992) 14. H.H. Kung, Adv. Catal. 40 (1994) 1.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

245

S t r u c t u r e and Catalysis of LixNi2_xO2 Oxide Systems for Oxidative Coupling of M e t h a n e T. l~liyazaki a, T. Doi b, T. M i y a m a e c, and I. M a t s u u r a a a D e p a r t m e n t of C h e m i s t r y , F a c u l t y of Science, T o y a m a University, Gofuku, T o y a m a 930, J a p a n byokkaichi R e s e a r c h L a b o r a t o r y , Tosoh Corp., Yokkaichi 510, J a p a n CInstitute for M o l e c u l a r Science, Myodaiji, Okazaki 444, J a p a n

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 ( X P S and U P S ) studies have b e e n applied to t h e investigation of active oxygen species of solid solution series LixNi2_ • w i t h wide range of 0<x_< 1 for u n d e r s t a n d i n g t h e origin of high a c t i v i t y and C2selectivity of t h e oxidative coupling of m e t h a n e . F r o m the Comparison of these s p e c t r a l features, the surface oxygen of Li0.3Nil.702 w i t h a cubic s t r u c t u r e was assigned to 0 2 - species, while t h a t of LiNiO2 w i t h a h e x a g o n a l s t r u c t u r e was a t t r i b u t e d to O- species a s s o c i a t e d w i t h t h e c a t a l y t i c a c t i v i t y for the oxidative coupling of m e t h a n e . F r o m the r e l a t i o n s h i p b e t w e e n these s t r u c t u r e s a n d the c a t a l y t i c activity, it was inferred t h a t t h e a n i s o t r o p i c layered s u p e r s t r u c t u r e of LiNiO2 largely affected the f o r m a t i o n of the active oxygen species for the oxidative coupling reaction.

1. I N T R O D U C T I O N C a t a l y s t s used for t h e oxidative coupling of m e t h a n e ( O C M ) have b e e n widely i n v e s t i g a t e d for t h e chemical utilization of n a t u r a l gas. T h e role of t h e c a t a l y s t is p r i m a r i l y to provide a r e a c t i o n p a t h w a y which facilitates the a b s t r a c t i o n of one h y d r o g e n from m e t h a n e w i t h surface oxygen species while s u p p r e s s i n g deep oxidative reactions. Since Keller and B h a s i n i n v e s t i g a t e d t h e possibility of s y n t h e s i z i n g C 2 - h y d r o c m ' b o n s by t h e oxidation of m e t h a n e using an oxide c a t a l y s t , m a n y different c a t a l y t i c s y s t e m s have b e e n e x a m ined [1]. Interestingly, all r e p o r t e d selective c a t a l y s t s t o w a r d C 2 - h y d r o c a r b o n s in m e t h a n e conversion contain an alkali c o m p o n e n t . One of the m o s t active and selective c a t a l y s t is m a g n e s i u m oxide d o p e d w i t h alkali m e t a l ions such as Li 4. A l i t h i u m - d o p e d m a g n e s i u m oxide, L i / M g O , has b e e n extensively s t u d i e d as a c a t a l y s t for the O C M reaction. Lunsford et al. p r o p o s e d t h a t the active centers of L i / M g O c a t a l y s t is [Li ~O-] species on the c a t a l y s t surface [2]. O t s u k a et al. also r e p o r t e d t h e s t u d y of alkali-doped t r a n s i t i o n m e t a l

246 oxides [3]. This r e p o r t p o i n t e d out t h a t LiNiO2 contained reducible comp o n e n t s h a d good efficiency w i t h this reaction b a s e d on a redox m e c h a n i s m involving lattice oxygen anions. Gaffney et al. s u g g e s t e d t h a t N a p r o m o t e d P r 6 O l l c a t a l y s t forms N a 2 0 2 ( 0 2 2 - , peroxide anion) as an active species by a redox reaction w i t h P r 6 O l l [4]. T h e selectivity a n d a c t i v i t y of the O C M r e a c t i o n is m a i n l y d e t e r m i n e d by the n a t u r e of surface oxygen species, which is affected by t h e electrical p r o p e r t i e s of the m e t a l oxides used as c a t a l y s t . T h u s , a d e t a i l e d s t u d y of t h e oxygen species on the c a t a l y s t surface is essential for u n d e r s t a n d i n g t h e origin of t h e catalysis. Accordingly, t h e p u r p o s e of this investigation is to m a k e out t h e r e l a t i o n s h i p b e t w e e n t h e a c t i v i t y of the O C I ~ a n d surface oxygen species of LiNiO2 c a t a l y s t s y s t e m . An u n d e r s t a n d ing of t h e s e c a t a l y s t p r o p e r t i e s and t h e i r c a t a l y t i c p e r f o r m a n c e would m o s t p r o b a b l y provide a f u n d a m e n t a l knowledge for designing c a t a l y s t s to increase C2-selectivity. T h e s t r u c t u r e of LiNiO2 are classified into the hexagonal t y p e , l i t h i u m a n d nickel ions in LiNiO2 a l t e r n a t e l y c o n f i g u r a t e d on the (111) planes of rock-salt type. F r o m a different point of view, electrochemically, LiNiO2 has a t t r a c t e d m u c h a t t e n t i o n due to t h e c a p a b i l i t y of i n t e r c a l a t i o n w i t h alkali m e t a l s a n d a p p l i c a t i o n to l i t h i u m - b a s e d b a t t e r i e s [5]. In this p a p e r , we r e p o r t a s t u d y of t h e LixNi2_xO2 s y s t e m c a t a l y s t s w i t h rock-salt s t r u c t u r e . C a t a l y s t s were c h a r a c t e r i z e d by X R D a n d X P S and t e s t e d in t h e oxidative coupling of m e t h a n e in order to e l u c i d a t e the different s t r u c t u r e and surface species.

2. E X P E R I M E N T A L T h e l i t h i u m nickelate(III) c a t a l y s t used in the e x p e r i m e n t s was p r e p a r e d by a solid-state reaction of an i n t i m a t e m i x t u r e of LiNO3 and Ni(OH)2 . T h e L i - i m p r e g n a t e d Ni(OH)2 was calcined in air at 873K for 2h a n d s u b s e q u e n t l y at 1073K for 6h. T h e s y n t h e s i z e d p o w d e r were identified by m e a n s of powder X - r a y diffraction ( X R D , S h i m a d z u XD-3A) [6]. T h e O C M r e a c t i o n was carried out in a fixed-bed r e a c t o r using 0.3g of the c a t a l y s t a n d was monit o r e d by gas c h r o m a t o g r a p h y . T h e flow r a t e s and t h e m i x e d ratio of gas, for all e x p e r i m e n t s , were u n d e r the a t m o s p h e r i c p r e s s u r e as follows 95 0 m l / m i n ; CH4:O2:He----2:l:3, respectively. T h e electronic d e n s i t y of LiMO2 ( M - - N i , Co, Fe, a n d In) a n d n o n s t o i c h i o m e t r i c LixNi2_xO2 (0_<x_
247 the Fermi edge of gold films e v a p o r a t e d in situ.

3. R E S U L T S

AND

DISCUSSION

3.1 C r y s t a l s t r u c t u r e of LixNie_xO2 X-ray diffraction p a t t e r n s of the solid solution series LixNi2_xO2 are shown in F i g u r e 1. FI'om the p o w d e r X-ray diffraction p a t t e r n s , the s t r u c t u r e of L i x N i 2 _ x O 2 ( 0 < x < l ) could be s t r u c t u r a l l y classified into some groups, t h o u g h the basic s t r u c t u r e of" these c o m p o u n d s consists of the rock-salt type. T h e solid solution series LixNi2_xO2 ( 0 < x < 0 . 6 ) , Lio.3Nil ~O2 and Lio.6Nil.402 are shown in F i g . l - ( a ) and F i g . l - ( b ) , have a typical cubic t y p e s t r u c t u r e analogous to the s t r u c t u r e of NiO. On the o t h e r hand, the X R D p a t t e r n s of LiNiO2 (Fig. l-e) can be seen by the a p p e m ' a n c e of the s u p e r l a t t i c e peaks at t w o - t h e t a values of 18.33 ~ (003), 36.2 ~ (101), 48.5 ~ (105), and 58.2 ~ (107) and (009) in a h e x a g o n a l setting. C o m p a r e d with these X R D p a t t e r n s , LixNi2_• for the range of 0 . 6 < x < 1 , Li0.rNil.302, Li0.9Nil.lO2 are shown in Fig.l-(c) and F i g . l (d), m a y be an incomplete hexagonal type. F r o m these results, the lattice s t r u c t u r e t r a n s f o r m e d from a cubic t y p e to a hexagonal t y p e as lithium cont e n t was increasingly doped. T h e r e is considerable evidence to show t h a t the t r a n s f o r m a t i o n should occur at x--0.6 to be shown in F i g . l - ( b ) . T h e ionic configuration of the solid solution series LixNi2_xO2 are w r i t t e n by the formula Lix(Ni 3. )x(Ni 2. )2_x02 . NiO c o r r e s p o n d s to LixNi2_xO2 ( x ~ 0 ) is classified into

{003)

(107), (009) /

(006), (102)

I {lO4)

(e) ('~

I

I (1o8)

l('os)

I.t

,,.A_._...~L--~ . . . . . . .

(d)

o..

]

(,,o)

9

A

,

b 1.

(b)

g~

w

l { .....

(a) .

dl

10

A

I(220)

( I L _ ~ (200) ,

i

20

,

I.

30

9

.I

.,

40

I

50

,

l

60

9

J

70

2theta / deg. F i g u r e 1. X-ray diffraction p a t t e r n s of the solid solution LixNi2_xO2. (a) Lio.3Nil.rO2; (b) Lio.6Nil.402; (c)Lio.TNil.302; (d) Lio.9Nil.102" (e) LiNiO2.

248 a cubic s t r u c t u r e in w h i c h nickel cations a n d o x y g e n anions are c o n f i g u r a t e d on t h e (111) planes of t h e rock-salt t y p e . C r y s t a l s t r u c t u r e of L i N i 0 2 a n d LixNi2_xO2 solid solution are s h o w n in F i g u r e 2. NiO has a rock-salt t y p e s t r u c t u r e in which Ni cations are c o o r d i n a t e d in t h e s i x - c o o r d i n a t e l o c a t i o n f o r m e d in t h e cubic closest p a c k i n g (ccp) of oxygen ions. As for LixNi2 xO2 solid s o l u t i o n in w h i c h Li ions are dissolved into NiO, t h e solid solution w i t h t h e r a n g e of x<.0.6 has a cubic s t r u c t u r e , t h a t is a d i s o r d e r e d rock-salt s t r u c . . . .

(c)

0

0

o Ni

9 Li

9 Ni or Li

F i g u r e 2. T h e s t r u c t u r e of t h e rock-salt s t r u c t u r e . (a) C u b i c s t r u c t u r e , (b) I n c o m p l e t e h e x a g o n a l s t r u c t u r e a n d (c) H e x a g o n a l s t r u c t u r e . 573K 773K

2931

1093K

Lio.7NiuO 2

. 893K [293Kl "~ I- I 11033K

II

" ~

--~

i

9

l0

il II

[[ f[I

II03K

.

l

20

I

Lil.oNil.oO2

9

I

30 40 2 theta / deg.

i

i

50

60

=-

ar

.

9

10

.

I

20

9

~

!

9

a

30 40 2 theta / deg.

50

60

F i g u r e 3. T h e t e m p e r a t u r e d e p e n d e n c e on X R D p a t t e r n s of Lio.rNil.302 a n d LiNiO2 c a t a l y s t s . t u r e , this c r y s t a l p h a s e is r e p l a c e d by one in which t h e Li a n d Ni c a t i o n s are s e g r e g a t e d on a l t e r n a t e (111) planes a n d are r a n d o m l y d i s t r i b u t e d over t h e available o c t a h e d r a l m e t a l sites of t h e rock-salt s t r u c t u r e ( s e e Fig. 2-a). On t h e o t h e r h a n d , solid s o l u t i o n s w i t h t h e r a n g e of 0 . 6 < x < l have a h e x a g o n a l

249 layered s t r u c t u r e w i t h t h e oxygen layers ((111) direction of t h e rock-salt t y p e s t r u c t u r e ) m i d w a y b e t w e e n cation layers and these cations are a r r a n g e d in a l t e r n a t i n g layers of p u r e Ni a n d m i x e d layers of r a n d o m l y d i s t r i b u t e d Li and Ni cations (see Fig. 2-b). T h e s t r u c t u r e of LiNiO2 has a h e x a g o n a l s y s t e m (~-LiFeO2 t y p e s t r u c t u r e in which M 3~ (Ni) cation layers and Li cation layers are a l t e r n a t e l y a n d r e g u l a r l y a r r a n g e d to oxygen ion layers (see Fig. 2-c). T h e t e m p e r a t u r e d e p e n d e n c e of X R D p a t t e r n s of Li0.rNil.302 a n d LiNiO2 are shown in F i g u r e 3. T h e i n c o m p l e t e h e x a g o n a l s t r u c t u r e was t r a n s f o r m e d to t h e cubic lattice at 973K~ while t h e s t r u c t u r e of LiNiO2 was held on t h e h e x a g o n a l t y p e above the t e m p e r a t u r e at 1073K. 3.2 C2-selectivity of the OCM reaction T h e results of m e t h a n e conversion and the selectivity of C 2 - h y d r o c a r b o n s are c o m p a r e d in Table 1. LixNi2_xO2 c a t a l y s t s w i t h different l i t h i u m cont e n t for t h e range of 0 < x < l . 0 were p r e s e n t e d . It is k n o w n t h a t the selective O C M reaction g e n e r a l l y p r o c e e d s only at t e m p e r a t u r e above ca. 900K. T h e intrinsic r e q u i r e m e n t of such high t e m p e r a t u r e for t h e OCIVI r e a c t i o n is still not fully u n d e r s t o o d . T h e c a t a l y t i c r u n s were carried out u n d e r conditions in which t h e C2-selectivity was achieved at the m a x i m u m . F i g u r e 4 shows a plot of the selectivity of C 2 - h y d r o c a r b o n s as functions of l i t h i u m c o n t e n t s x in Li• T h e f o r m a t i o n of C 2 - h y d r o c a r b o n s is initially observed at t h e l i t h i u m c o n t e n t of x - 0.2. F r o m these results~ it is found t h a t the solid solutions of LixNi2_xO2 w i t h x > 0 . 6 having t h e h e x a g o n a l s t r u c t u r e b e c o m e a c a t a l y s t which has high selectivity to t h e O C M reaction. However~ even in the Table 1 C a t a l y t i c a c t i v i t y of t h e oxidative coupling of m e t h a n e over LixNi2_xO2. Catalyst NiO Li0.3 Ni 1.rO2 Li0.6 Ni 1.4 02 Li0.rNil.3 02 Li0.9Nil.lO~ LiNiO2

React. Temp. (K)

CH4- Conv. (%)

C2-Select. (%)

Structure

993 1053 993 1053 993 1053 993 1053 993 1053

21.4 33.8 31.7 33.6 33.3 34.1 37.2 36.1 32.2 42.5

32.0 27.3 40.0 25.4 54.1 20.7 51.4 25.1 53.7 63.6

Cubic C u b ic Cubic Cubic Cubic Hexagonal Cubic Hexagonal Cubic Hexagonal Hexagonal

case of these selective catalysts~ its selectivity d r o p s a b r u p t l y at a high t e m p e r a t u r e at 1053K as shown in Table 1. T h e e x p e r i m e n t a l result over Li0 7Nil.302 is shown Figure 5. T h e s t r u c t u r e of LixNi2_xO2 ( 0 . 7 < x < 0 . 9 ) t r a n s f o r m e d from

250

---Hexagonal type --~

Cubic type

v

70

I!!:: :::i(

60 k,

iii l i ! )...e...~--o--'~ 'i ,ii,i[l

50

! !

9-. 40 ..~ -~ 3O rfl ',.,, ~ 20

,

.

,

,I

.......

:~-~*~_~:

l

a. ~**

~ ~i~,!

D

,

i~i:: ::!

,

l:L::::i

10 00

0.0

0.2

0.4 0.6 X (LixNi2_xO2)

Figure 4. The relationship between LixNi2_xO2 ( 0 < x < l ) catalyst 9 100

A

0.8

C2-selectivity

1.0

and the structure of

O2-conv. O..._o

80

t=,

A

60 .~

C2-select.

.,~

~9

4020

yo / O / o / CH4-conv.

O

.._...O 900

950

1000

1050

Temperature / K

Figure 5. The temperature d e p e n d e n c e catalyst.

o f methane oxidation o v e r L i 0 . ? N i 1 . 3 0 2

251 a hexagonal t y p e to a cubic t y p e at higher t e m p e r a t u r e t h a n 973K. It should be noted t h a t the C2-selectivity may be correlated to the crystal s t r u c t u r e of these catalysts. The catalytic activity of the O C M reaction should be predictable by the n a t u r e of the oxygen species on the catalyst surface. In addition, the electronic states of the surface oxygen could be largely related to the bulk s t r u c t u r e in the crystal.

3.3 A c t i v e sites for t h e O C M r e a c t i o n A typical p h o t o e l e c t r o n spectroscopy e x p e r i m e n t was conducted as follows for u n d e r s t a n d i n g of the catalyst surface. The characterization of the surface oxygen species on these oxides were investigated using XPS and UPS. Figure 6 displays representative typical X P S spectra in the O ls region for Lil.0Nil.002 (Fig. 6-a), Li0.3Ni1.702 (Fig. 6-b) and Ni304 (Fig. 6-c). The O ls spectra in Fig. 6 reveal the presence of at least two different types of the near-surface oxygen species on these catalysts. Two structures, denoted by the characters A and B, were observed at 529.4 and 531.3 eV in the XPS O ls s p e c t r a of Ni304, and at 529.2 and 531.4 eV in those of Li0.3Nil.702. The peak A located at 529.3+0.1 eV could be a t t r i b u t e d to 0 2 - species which the catalytic n a t u r e must be nonactive for the O C M reaction [8]. It seems reasonable to u n d e r s t a n d t h a t the main existence of the 0 2 - species on Ni304 oxides are nonselective for the O C M reaction. A n o t h e r O ls peaks (peak B) at higher binding energy in these oxides may be associated with the catalytic

A

Ols

~

(c)

(b)

m

(a) I

538

9

I

536

,

I

,

I

,

I

.

I

,

i - - ,

530 528 526 Binding Energy / eV

534

532

I

524

,

I

522

Figure 6. X P S O ls spectra of LiNiO2 system. (a) LiNiO2; (b) Lio3Nil.rO2; (c) Ni3Oa.

252 activity for the O C M reaction. T h e peak intensity ratio A / B of the Li0.3Nil.rO2 was given to be 36/64 in the last column on Table 2, and the oxygen species of the peak B is reasonably existence on the Li0.3Ni1.rO2 catalyst surface. B u t this catalyst was low activity for the O C M reaction as described previous section. In t h e case of LiNiO2 with the active and high C2-selectivity catalyst for the O C M reaction, a peak B' in the XPS s p e c t r u m of LiNiO2 was observed at Table 2 The O ls level of the binding energy of LixNi2_xO2 catalysts. Crystal

Structure

Ni304 Li0.3Nil.rO2 Li0.vNil.302 LiNiO2

Cubic Hexagonal Hexagonal

Ols (eV) Peak A 529.4 529.2 529.1 529.2

Ols (eV) Peak B 531.3 531.4 531.6 531.9

Intensity ratio A/B 63/37 36/64 21//79 11//89

531.9 eV to higher binding energy level of the peak B and largely related to the selective O C M reaction. The peak B' was also observed at 531.8 eV in the X P S s p e c t r u m of LiCoO2 which exhibit high .activity for the O C M reaction [9]. The slight difference in the binding energy between the peak B and the peak B' m a y be often a b e t t e r indication of the n a t u r e of oxygen species r a t h e r t h a n the absolute value of the binding energy. Thus, the a p p e a r a n c e of the oxygen species is a t t r i b u t a b l e to the high C2-selectivity for the OC]~I reaction. C

r~

m

(b)

(a) "

20

I

15

"

"

1'0

I

5

"

'

EF=()

Binding energy / eV Figure 7. U P S s p e c t r a of the solid solution LixNi2_xO2. (a) Lio.3Nil.rO2; (b) LiNiO2.

253 T h e results of X P S m e a s u r e m e n t for t h e solid solution series LixNi2_xO2 are s u m m a r i z e d in Table 2. T h e peaks B of Li07Ni1302 w i t h a i n c o m p l e t e hexagonal s t r u c t u r e was also observed at 531.6 eV to slight higher b i n d i n g energy. T h e i m p o r t a n t point to note t h a t the crystal s t r u c t u r e of these c a t a l y s t also largely affected to t h e electronic states of the surface oxygen. Figure 7 shows U P S s p e c t r a of Li0.3Ni1702 and LiNiO2. These s p e c t r a were m e a s u r e d at t h e incident hp of 40 eV w i t h reference to EF as t h e zero on the e n e r g y scale. Several structures~ d e n o t e d by the c h a r a c t e r s A-C~ were observed at 2.0, 6.0, and 10.5 eV in those of LiNiO2 (Fig. 6-a)~ a n d at Eb ---- 2.2, 6.0 and 11.0 eV in t h e U P S s p e c t r a of Li0.3Nil.702 (Fig. 6-b), T h e s t r u c t u r e located at Eb -- 2.0~2.2 eV can be assigned the 3d b a n d of nickel oxide~ a n d the b r o a d peak at 6.0 eV in the valence b a n d can be assigned to the O 2- species associated w i t h NiO [10]. It was suggested from these results t h a t the bulk s t r u c t u r e largely affected t h e surface oxygen species which play an i m p o r t a n t role for the selective of t h e O C M reaction. Figure 8 shows the location of oxygen ions, Ni and Li cations viewed t w o - d i m e n s i o n a l l y along the vertical direction of the cubic closest packing of oxygen ions of LixNi2_xO2 series. As l i t h i u m c o n t e n t was increasingly d o p e d , t h e lattice s t r u c t u r e t r a n s f o r m e d from a cubic t y p e (Fig. 8-a) to a hexagonal t y p e (Fig. 8-c) t h r o u g h a interm e d i a t e t y p e (Fig. 8-b) b e t w e e n these s t r u c t u r e . T h e oxygen species in the cubic lattice is placed in a h o m o g e n e o u s charge d e n s i t y w i t h a r a n d o m mixed Li ~~ Ni 2 ~ and Ni 3~ . On the o t h e r hand, the oxygen species in a hexagonal lattice is placed in a anisotropic charge density b e t w e e n m o n o c a t i o n Li ~ layer and t r i c a t i o n Ni 3~- layer. It is inferred from the relationship b e t w e e n t h e i r s t r u c t u r e of LixNi2_xO2 a n d t h e selectivity of the O C M t h a t Li + and Ni 3~ cations t w o - d i m e n s i o n a l l y configurated as shown in Fig. 8-c is the i m p o r t a n t

0

0

o

Ni

9

Li

9

Ni

or

Li

Figure 8. Active oxygen species of the solid solution series LixNi2_xO2. (a) Cubic type; ( b ) I n c o m p l e t e hexagonal type; (c) H e x a g o n a l type. role for the oxidative reaction. T h e f o r m a t i o n of active O- species occurs in t h e lattice oxygen and on t h e catalyst surface. It is p r o p o s e d t h a t an active O - species of the c a t a l y s t surface e x t r a c t s a h y d r o g e n from m e t h a n e , and accordingly t h e coupling of m e t h y l radicals take place.

254

CONCLUSIONS In this work, we present the results for LixNi2_xO2 ( 0 < x < l ) catalysts of the O C M reaction. In order to u n d e r s t a n d the surface catalytic active species, the characterization of the oxygen species of LixNi2_xO2 oxides is examined by the combined approach of XPS and UPS. From the X P S O ls and U P S spectral analysis, the main oxygen species of LixNi2_xO2 (x<0.6) with a cubic s t r u c t u r e can be assigned to 0 2 - , while those of Lil.0Nil.002 with a hexagonal s t r u c t u r e can be a t t r i b u t e d to catalytic active O- species for the O C M reaction. These results indicate t h a t the electronic structure and the catalytic activity for the O C M reaction strongly depends on crystal s t r u c t u r e of LixNi2_xO2 catalyst systems.

ACKNOWLEDGMENT The a u t h o r s acknowledge the staffs of the U V S O R Facility and IMS for their helpful advice and technical support.

References [1] G. E. Keller and M. M . Bhasin, J. Catal., r3 (~9s2) 9. [2] T. Ito J. X. Wang, C. H. Lin, and J. H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062. [3] M. H a t a n o and K. Otsuka, Shokubai, 29 (1987) 46. [4] A. M. Gaffney, C. A. Jones, J. J. Leonard, and J. A. Sofranko, J. Catal., 114 (1988) 422. [5] K. Mizushima, P. C. Jones, P. J. Wiseman, and J. B. Goodenough, Mater. Res. Bull., 15 (1980) 783. [6] J. R. Dahn, U. von Sacken, ics, 44 (1990) 87.

and C. A. Michal,

Solid States Ion-

[7] K. Seki in: Optical techniques to characterize polymer ed. H. Baessler (Elsevier, A m s t e r d a m , 1989) pp.l15-180.

system,

[8] C. L. Padro, W. E. Grasso, G. T. Baronetti, A. A. Cstro and O. A. Scelza, " N e w Developments in Selective Oxidation", 82 (1994) 411. [9] T. Miyazaki, T. Doi, M. Kato, T. Miyake, and I. M a t u u r a , manuscript submitted. [10] M. S. Hegde and M. Ayyoob, Surf. Sci. Lett., 173 (1986) L635.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

255

R e a c t i o n i n d u c e d spreading o f metal oxides: in situ R a m a n spectroscopic studies during o x i d a t i o n reactions Y. Cai +*, C.-B. Wang + and I. E. Wachs + Zettlemoyer Center for Surface Studies and Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA 1. INTRODUCTION Metal oxides, particularly oxides of vanadium and molybdenum, are widely used as catalysts in numerous industrial applications (oxidation, oxidative dehydrogenation, dehydrogenation, olefin metathesis, olefin polymerization, selective catalytic reduction (SCR) and hydrodesulfurization (HDS)/hydrodenitrogenation (HDN)). In many of the applications, these metal oxides are supported on a high surface area metal oxide substrate, such a s A1203 or TiO2, to form an active surface metal oxide species (two-dimensional metal oxide overlayer). These supported metal oxide catalysts are typically prepared by impregnation of the corresponding soluble metal oxide salts, followed by drying and calcination at elevated temperatures (400-600~ There have been recent reports in the literature that an alternative route to the preparation of such supported metal oxide catalysts involves solid-state reactions, thermal spreading or spontaneous dispersion, of physical mixtures of the pure metal oxides at temperatures of 400-500~ [1-6]. However, no studies have examined the direct influence of the reaction environment on the spreading of metal oxides. In the present investigation, a new phenomenon of reaction induced spreading of crystalline MoO3 and V205 on oxide supports is observed at temperatures much lower than that required for thermal spreading via solid-state reactions, 200-250~ vs. 400-500~ 2. E X P E R I M E N T A L

The M o O 3 and V205 crystalline powders were obtained from Aldrich. The T i O 2 support (P-25; 55 m2/g) was purchased from Degussa and the SnO2 support (3.7 m2/g) from Aldrich. The physical mixtures of the binary oxides were prepared by combining an appropriate amount of MoO3 (or V205) with 5.0 g of TiO 2 (or SnO2) and 150 ml of pentane (Aldrich) in a beaker, and vibrating the mixtures for 15 minutes in an ultrasonic bath. After evaporation of pentane, the samples were dried for 16 hours at 100 ~ in air. No further treatments were performed.

*Current address: United Catalysts Inc., 1227 South 12th Street, Louisville, KY 40210. +Financial support of the Division of Basic Energy Sciences of the Department of Energy, grant DEFG02-93ER 14350, is gratefully acknowledged.

256 Thermal spreading was studied by treating the catalysts in an oven at a constant temperature for 1 h. Both loose powder catalysts and samples pressed into self-supporting wafers were investigated in the current studies. After the thermal treatments, Raman spectra of the catalysts were recorded in an ambient environment or in a controlled oxygen atmosphere. In the latter case, the samples were heated in oxygen (Linde Gas, ultra-high purity) in a dehydration cell for 45 minutes at 300~ and then cooled to about 50~ to obtain the Raman spectra. Additional details about the Raman spectrometer can be found elsewhere [5]. Reaction induced spreading during alcohol oxidation (methanol, ethanol and 2-butanol) was also investigated with both loose powder catalysts and samples pressed into self-supporting wafers. The self-supporting wafers were investigated with an in situ Raman spectrometer system that allowed monitoring of the changes during the reaction. A 100-200 mg sample disc was placed in the sample holder, which is mounted into a ceramic shaft rotating at 1500 rpm (see reference [7] for additional experimental details). The catalysts were initially heated in an O2/He =16/84 stream at 230~ for 30 minutes before recording the reference spectrum. A reaction gas mixture of CHaOH/OJHe - 4/16/80 was subsequently introduced into the in situ cell at a flow rate of 100 ml(STP)/min and the in situ Raman spectra were collected as a function of time. At the end of the reaction experiment, the methanol was removed from the gas stream and the catalyst was reoxidized in the O2/He stream. Alcohol oxidation over the loose powder catalysts were conducted in a fixed bed reactor at 230~ and atmospheric pressure. The details of the reactor system was previously described elsewhere [8]. The catalysts were pretreated in a flow of O2/He for 15 min prior to oxidation reaction. A reactant stream of C H a O H / O E / H e = 6/13/81 with a total flow rate of 100 ml/min was used for methanol oxidation. For ethanol and 2-butanol oxidation, a gaseous mixture of OR/He (13/81; ml/min) containing saturated ethanol or 2-butanol vapor at ambient temperature was introduced into the reactor. Analyses of reactants and products were carried out by an on-line Hewlett Packard 5890B GC. The spent catalysts were also characterized by Raman spectrometer. 3. RESULTS

3.1. Thermal spreading The ambient Raman spectra of a catalyst pellet containing a 4% MoO3/TiO 2 physical mixture after being exposed to different thermal treatments in a furnace are shown in Figure 1. The starting physical mixture only exhibited the Raman bands of crystalline MoO 3 (strong bands at about 814 and 990 cm -1) and the TiO 2 support (strong bands below 700 cm-1). After the one-hour 400 ~ thermal treatments, in both dry air and wet air, the Raman bands of crystalline M o O 3 predominate and only a trace of hydrated surface molybdenum oxide species is observed (Raman band at 956 cm-1). The Raman band of the surface molybdenum oxide species slightly increased as the temperature of the thermal treatments increased to 500~ Comparison of the initial Raman spectrum with the spectrum of the 500 ~ thermally treated catalyst pellet showed that most of the molybdenum oxide was still present as M o O 3 crystals and that only a small amount of surface molybdenum oxide species was present. In contrast to the thermal treatments with the catalyst pellet, almost complete dispersion of crystalline M o O 3 w a s observed after a 500~ treatment with the 4% MoOa/TiO 2 catalyst in the loose powder form. Thermal treatment of the loose powder at 400~ also showed a significant disperion of crystalline M o O 3 o n the titania support. This demonstrates that strong mass transfer limitations exist when the catalyst is in the form of a pellet, but not in the form of the loose powder. However, the design of the current in

257 situ Raman cell requires that the catalyst be in the form of a pellet and, consequently, the thermally treated pellet is the appropriate reference for the in situ Raman studies. Analogous thermal treatments, in dry as well as wet air, with a catalyst pellet and catalyst powder consisting of a 4% V2OJTiO 2 physical mixture revealed that no thermal dispersion of V205 occurred for these samples. Thus, thermal treatments of physical mixtures of MoO3/TiO2 and VzOs/TiO 2 at 400-500~ for 1 hour (1) result in the formation of surface molybdena species, (2) do not result in the formation of surface vanadia species and (3) the concentrations of surface species are significantly enhanced for catalysts in loose powder form compared to catalysts in pellet form.

Thermal Spreading of 4% MoO3/TiO z

814

Physical Mixture in Pellet Form (1 h)

A

I/) e=

990

s <

956

(d)

>i

w ~= C

=.,=.

(c)

o.,=.

(b) (a) 1100

1000

900

800

Raman Shift (cm "1)

Figure 1. Ambient Raman spectra of physical mixture of 4% MoO3/TiO2 catalyst pellet after different thermal treatments: (a) dry air (400~ 1 h), (b) wet air (400~ lh), (c) dry air (450~ 1 h) and (d) dry air (500~ 1 h).

258

4% MoOs+Ti02, Physical Mixture (230~

988 A

C: ::)

-969

s

,< w C q)

->

m

, i

814

q)

m

n,

x0.25

11 O0

1000

900

800

Raman Shift (crn "1)

Figure 2. In situ Raman spectra of physical mixture of 4% MoO3/TiO 2 catalyst pellet during methanol oxidation at 230~ 9(a) before methanol oxidation, (b) 20 min, (c) 1 h, (d) 3 h, (e) 5 h, (f) after reaction, oxidation of catalyst for 30 min and (g) after reaction, oxidation of catalyst for lh. 3.2. M e t h a n o l Oxidation

The in situ Raman spectra of a catalyst pellet consisting of a 4% MoOa/TiO2 physical mixture are shown during methanol oxidation at 230~ in Figure 2. The starting sample, Fig. 2a, only possesses the strong Raman bands of crystalline M o O 3 at 814 and 988 cm ~. Upon exposure to methanol oxidation conditions, Fig. 2b-e, the sharp Raman bands due to crystalline MoO 3 slowly diminish with reaction time and a new broad Raman band at 969 cm ~ is formed. The in situ Raman band at 969 cm ~ has previously been assigned to a surface molybdenum oxide coordinated methoxy, CHBO, species [9]. Upon switching to a flowing oxygen stream in the absence of methanol, Fig. 2f and g, the Raman band at 969 cm -~ shifted to about 990 cm -~ reflecting the decomposition of the surface methoxy-molybdate complex to a dehydrated surface molybdenum oxide species [9]. Simultaneously, there was also an increase in the Raman bands of crystalline MoO3 due to the oxidation of the partially reduced M o O 3 particles during the methanol oxidation reaction. Raising the reaction temperature to 300~ for about an hour, figure

259 not shown, resulted in the complete disappearance of the crystalline MoO3 Raman bands and only the appearance of the Raman bands associated with the surface molybdenum oxide species. Reoxidation of the sample at 300~ also resulted in the appearance of weak crystalline MoO3 Raman bands revealing that some residual reduced crystallites still remained and that higher temperature treatments are required to completely disperse the MoO3 crystals on the titania support. Essentially the same MoO3 dispersion behavior was observed during methanol oxidation with catalyst pellets consisting of 0.5-1% MoO3/SnO2 physical mixtures. Thus, the above in situ Raman studies clearly demonstrate that reaction induced spreading of crystalline MoO3 readily occurs over oxide supports during methanol oxidation at very mild temperatures, 230 ~C.

4% V2Os/TiO2,Physical Mixture

-791

(230~

-1022

A

C

E',

(e)

-E

<

>,1 rW

(c)

C (b) al

990

11 O0

1000

900

800

Raman Shift (cm "1)

Figure 3. In situ Raman spectra of physical mixture of 4% V2Os/TiO 2 catalyst pellet during methanol oxidation at 230~ 9(a) before reaction, (b) 30 min, (c) 1 h, (d) 3 h, (e) after reaction, oxidation of catalyst for 30 min, (f) after reaction, oxidation of catalyst for 1 h.

260

100

80

60 eCm~

r~

~,. 40

2O

I

0

50

'

I

100

'

Time (min)

I

150

'

200

Figure 4. Oxidation of alcohols over catalysts in loose powder form at 230 ~ as a function of reaction time: (a) methanol oxidation over 4% MoO3/YiO 2physical mixture, (b) methanol oxidation over 4% VzOs/TiO 2 physical mixture, (c) ethanol oxidation over 4% MoO3/TiO2 physical mixture and (d) 2-butanol oxidation over 4% MoO3/TiO 2 physical mixture. The in situ Raman spectra of a catalyst pellet consisting of a 4% V2Os/TiO2 physical mixture are shown during methanol oxidation at 230~ in Figure 3. The starting sample, Figure 3a, only exhibits the Raman bands of crystalline V205 at about 990 cm -1 and the titania support at about 790 cm 1. Exposure of the vanadia-titania catalyst to methanol oxidation at 230~ Figures 3b-d, completely removes the Raman bands of the V205 crystals and no new bands due to surface vanadia species are observed. The complete absence of any vanadia Raman bands suggests that the vanadia component of the catalyst was reduced (reduced vanadia gives rise to very weak Raman bands). Reoxidation of the 4% VzOs/TiO2 physical mixture catalyst pellet resulted in the appearance of a new Raman band at 1022 cm 1 associated with surface vanadia species [ 10,11 ] and the complete absence of crystalline V205 particles (no sharp Raman band at about 990 cm-1). Thus, the above in situ Raman studies clearly demonstrate that reaction induced spreading of crystalline V205 readily occurs over oxide supports during methanol oxidation at very mild temperatures, 230 ~ The catalytic behavior of the above physical mixtures, in loose powder form, were also investigated during methanol oxidation in a fixed-bed reactor as shown in Figure 4a and b. The methanol conversion over 4% MoO3/TiO2 continuously increased from about 8 to 16% during the first three hours of reaction. The corresponding methanol oxidation studies over the 4% VzOs/TiO 2 catalyst were more dramatic: the methanol conversion continuously increased from about 18 to 37% with reaction time during the first 110 minutes and then exhibited a sharp jump

261 to 100% methanol conversion at approximately 140 minutes. The jump in methanol conversion was accompanied by an increase in the temperature of the catalyst bed, approximately 244 ~C, due to the exothermic heat of reaction. Ambient Raman analysis of the spent catalysts revealed that both crystalline MoO 3 and V205 became almost completely dispersed during the methanol oxidation studies. Additional studies in an oxygen-free methanol environment further demonstrated that the dispersion of the crystalline oxides was not related to the presence of gas phase oxygen. Thus, the increase in methanol conversion as a function of time over MoO3/TiO 2 and V2OJTiO2 physical mixtures is directly related to the transformation of crystalline MoO3 and V205 into surface molybdena and vanadia species, respectively. 3.3 Ethanol and 2-butanol oxidation

The influence of oxidation reaction environments involving higher alcohols, ethanol and 2butanol, upon the catalytic behavior and dispersion of M o O 3 o n a TiO2 support was also examined. The catalytic behavior during ethanol and 2-butanol oxidation are shown in Figure 4c and d as a function of reaction time in a fixed-bed reactor containing the catalyst in loose powder form. The higher alcohols were more active than methanol and their conversions increased continuously with reaction time. In the case of ethanol oxidation, some blue Mo deposits were observed on the walls of the reactor exit due to the formation of volatile Mo species. Ambient Raman analysis of the spent catalysts revealed the presence of significant amounts of crystalline M o O 3 as well as the presence of some surface molybdena species. However, the concentrations of the surface molybdena species were much lower than that found after methanol oxidation and the surface molybdena species concentration was greater after ethanol oxidation than 2-butanol oxidation. Thus, these experiments reveal that the dispersion of crystalline MoO 3 particles on TiO2 supports during oxidation of alcohols follows the trend: methanol > ethanol > 2-butanol. 4. DISCUSSION The thermal spreading of metal oxides over oxide supports has been intensively investigated over the past decade and much is currently known about this process [ 1,5]. The driving force for the thermal spreading of metal oxides is related to the lower surface free energy of crystalline oxides such as V205 and MoO3 compared to crystalline oxide supports such as TiO2, S n O 2, A1203, etc. This process is analogous to the wetting of one solid by another induced by the forces of surface tension in order to lower the surface free energy of the system [2]. The low Tamman temperatures o f V 2 0 5 and M o O 3 (345 and 397.5~ respectively) are responsible for the efficient spreading of these metal oxides at temperatures of 400-500~ Furthermore, the spreading kinetics of the metal oxides are (1) dependent on the structure and morphology of the oxide support, (2) enhanced over well-developed crystal planes and (3) dependent on the specific gaseous environment (oxidizing vs. reducing or wet vs. dry) [5]. Under oxidizing conditions and elevated temperatures, the spreading of crystalline V205 and M o O 3 is initiated spontaneously at the metal oxide-support interface and subsequent migration occurs by surface diffusion of the metal oxides via vacancies or unoccupied sites in the two-dimensional metal oxide overlayer. Amorphous metal oxide phases are suggested as a transient form between the crystalline metal oxides and the two-dimensional metal oxide overlayers. Moisture enhances the surface diffusion of the metal oxides [1 ]. Under mildly reducing conditions, the spreading of crystalline metal oxides is significantly retarded due to the much higher Tamman temperatures of the

262 corresponding reduced crystalline metal oxides [1,5]. The present thermal treatment experiments in air revealed that extensive dispersion of MoO3 occurred at 400~ and essentially complete dispersion took place at 500~ for the loose powder physical mixture of 4% MoO3/TiO2. In contrast, very little dispersion was observed for comparable thermal treatments for the loose powder physical mixture of 4% VzOs/TiO 2. The observation that the kinetics of MoO 3 disperion are faster than the kinetics of V205 dispersion were also previously observed [12]. The lack of V205 dispersion by the thermal treatments is somewhat surprising and may be related to the structure and morphology of the titania support employed in the present investigation. The form of the physically mixed metal oxide was also found to significantly affect the dispersion kinetics due to the presence of significant mass transfer limitations in the catalyst pellet relative to the loose powder. The presence of mass transfer limitations in catalyst pellets or wafers typically employed for Raman and IR studies is welldocumented in the literature [ 13]. The present studies demonstrated that significant dispersion of M o O 3 o r V2 05 o n a titania support could not be achieved at temperatures of 500 ~ with the physically mixed oxides in the form of a catalyst pellet. Thus, dispersion of M o O 3 and V205 on oxide supports at much lower temperatures for physically mixed catalysts in the form of pellets can not be due to thermal spreading and must occur by another mechanism. The in situ Raman studies clearly demonstrate that spreading of MoO3 and V205 over different oxide supports in the form of catalyst pellets readily occurred during methanol oxidation at temperatures as low as 230~ Such low temperatures, which are below the Tamman temperatures of these oxides and the temperatures required for thermal spreading in the catalyst pellet (above 500~ implies that thermal spreading is not involved in the spreading mechanism taking place during methanol oxidation. This suggests that a strong interaction between the gas phase components and the crystalline metal oxide phases may be occurring. Formaldehyde is the major selective oxidation reaction product and is known to interact very weakly with metal oxides such as molybdates and vanadates, and adsorbed formaldehyde is readily displaced by the presence of moisture and methanol [14,15]. Moisture interacts strongly with molybdates [ 14,15] and vanadates [8], but the thermal spreading experiments did not result in significant dispersion of the crystalline metal oxides in the physically mixed catalyst pellet. The interaction of carbon dioxide with molybdates and vanadates is extremely weak and adsorption is usually not even observed at room temperature [16,17]. In contrast to these gaseous components, the interaction of methanol with molybdates and vanadates is very strong and is much stronger than moisture since adsorption of methanol can displace adsorbed moisture [14,15]. Furthermore, methanol oxidation over crystalline M o O 3 and V205 results in the deposition of molybdena and vanadia at the exit of the reactor, which possesses lower temperatures [ 18]. This observation suggests that methanol is able to strongly complex with Mo and V present in crystalline MoO3 and V205 to form volatile Mo(OCH3) n and V(OCH3)n complexes. The alkoxy complexes of vanadia and molybdena are well known and are liquids at room temperature possessing high vapor pressures. Thus, the low temperature dispersion of metal oxides over oxide supports during methanol oxidation is due to the formation of volatile metal-methoxy complexes that result in vapor phase transport of the oxides. The dispersion mechanism may also occur by surface diffusion of the metal-methoxy complex, but no such information is currently available. The absence of Mo and V deposits at the reactor exit during methanol oxidation suggests that either surface diffusion or readsorption of the volatile M-alkoxides is also taking place. In summary, a new phenomenon of reaction induced spreading of crystalline metal oxides on oxide supports is observed in the present investigation at temperatures much lower than that required for thermal spreading via

263 solid-state reactions, 200-250~ vs. 400-500~ Thermal spreading depends on the Tamman temperature of the crystalline metal oxide phases and reduced metal oxide phases possess very high Tamman temperatures which significantly retard migration [1,5]. However, essentially complete dispersion ofV205 on TiO2 was observed during methanol oxidation even though the in situ Raman spectra revealed that the vanadia was reduced under the reaction conditions (see Fig. 3). Essentially complete dispersion of M o O 3 o n TiO2 was also observed after treatment of the catalyst in an oxygen-free methanol environment. Thus, the oxidation state of the metal oxide does not appear to influence the kinetics of reaction induced spreading of crystalline metal oxides. Reaction induced spreading of MoO3 on oxide supports during oxidation of higher alcohols is significantly reduced relative to methanol oxidation (methanol > ethanol > 2-butanol). The reduced migration kinetics is most probably related to the stability and reactivity of the various alcohols. The rate determining step during the oxidation of alcohols to their corresponding aldehydes or ketones involves breaking the alpha C-H bond of the alkoxides (the carbon bonded to the alkoxy oxygen), and the stability of this bond is related to the number of additional carbon atoms coordinated to the alpha carbon: stability decreases with increasing number of carbon atoms coordinated to the alpha carbon [ 15,19]. Thus, the methoxy complex is more stable than the ethoxy complex, and the 2-propoxy complex is the least stable among these alkoxy complexes. The greater stability of the Mo-methoxy complex most probably is responsible for the greater volatility and spreading observed during methanol oxidation. The current findings that reaction induced spreading of metal oxides on oxide supports can occur during oxidation reactions at very low temperatures have important implications for commercial applications as well as fundamental studies. The oxidation of methanol to formaldehyde is industrially conducted with F e 2 ( M o O 4 ) 3 . M o O 3 catalysts that contain excess MOO3. The strong interaction between methanol and the MoO 3 component results in the stripping of the molybdena from the catalyst and its deposition as MoO3 crystalline needles at the bottom of the reactor where the temperatures are somewhat cooler. This volatilization phenomenon is responsible for catalyst deactivation (loss of activity and selectivity) and pressure build-up in such commercial reactors [20]. The opposite behavior is observed during methanol oxidation over MoO3/SiO 2 catalysts at 230~ The strong interaction of methanol with Mo and the weak interaction between surface molybdena species and the silica support results in agglomeration and crystallization of the surface molybdena species to beta-MoO3 particles during methanol oxidation [21 ]. A very important consequence of reaction induced spreading of metal oxides during alcohol oxidation is that the catalyst preparation method of many supported metal oxide systems is not critical since the same surface metal oxide species will form during reaction (especially methanol oxidation) [ 12,21 ]. Furthermore, the possibility that reaction induced spreading occurs during oxidation reactions over catalysts composed of physical mixtures needs to be very carefully investigated in such systems before other mechanisms are proposed to account for observed reactivity patterns [22]. 5. CONCLUSIONS A new phenomenon of reaction induced spreading of crystalline M o O 3 and V205 on oxide supports is observed during methanol oxidation at temperatures much lower than that required for thermal spreading via solid-state reactions, 230~ vs. 400-500~ The migration of the metal oxides appears to proceed by the formation of volatile M-(OCH3) . complexes and is not

264 influenced by the oxidation state of the metal oxide (both oxidized and reduced metal oxides are readily dispersed). The kinetics of reaction induced spreading of metal oxides during alcohol oxidation is much slower for higher alcohols because of the low stability of the corresponding M-alkoxides compared with the more stable M-methoxides. REFERENCES

1. 2. 3. 4. 5. 6.

H. Knoezinger and E. Taglauer, Catalysis, 10 (1993) 1. J. Haber, T. Machej and T. Czeppe, Surf. Sci., 151 (1985) 301. D. Honicke and J. Xu, J. Phys. Chem., 92 (1988) 4699. Y. Xie and T. Tang, adv. Catal., 37 (1990) 1. J. Haber, T. Machej, E. M. Serwicka and I. E. Wachs, Catal. Lett., 32 (1995) 101. F. Cavani, G. Centi, E. Foresti, F. Trifiro and G. Busca, J. Chem. Soc., Faraday Trans., 1, 84 (1988) 237. 7. J.-M. Jehng, H. Hu, X. Gao and I. E. Wachs, Catal. Today, 28 (1996) 335. 8. G. Deo and I. E. Wachs, J. Catal., 146 (1994) 323. 9. H. Hu and I. E. Wachs, J. Phys. Chem., 99 (1995) 10911. 10. M. A. Vuurman, I. E. Wachs and A. M. Hirt, J. Phys. Chem., 95 (1991) 9928. 11. G. Went, S. T. Oyama and A. T. Bell, J. Phys. Chem., 94 (1990) 4240. 12. T. Machej, J. Haber, A. M. Turek and I. E. Wachs, Appl. Catal., 70 (1991) 115. 13. Y. Cai and I. E. Wachs, to be published. 14. W.-H. Cheng, J. Catal., 158 (1996) 477. 15. W. Holstein and C. J. Machiels, J. Catal., 162 (1996) 118. 16. K. Segawa and W. K. Hall, J. Catal., 77 (1982) 221. 17. A. M. Turek, I. E. Wachs and E. DeCanio, J. Phys. Chem., 96 (1992) 5000. 18. G. Deo, H. Hu and I. E. Wachs, to be published. 19. W. E. Farneth, R. H. Staley and A. W. Sleight, J. Am. Chem. Soc., 108 (1986) 2327. 20. R. Pearce and W. R. Patterson, Catalysis and Chemical Processes (Wiley, New York, 1981) p. 263. 21. M. Banares, H. Hu and I. E. Wachs, J. Catal., 150 (1994) 407. 22. P. Ruiz and B. Delmon, Catal. Today, 3 (1988) 199.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

Temperature

Programmed

Desorption of Ethylene

265

and A c e t a l d e h y d e

on

U r a n i u m Oxides. E v i d e n c e o f F u r a n F o r m a t i o n from Ethylene. H. Madhavaram and H. Idriss Materials Chemistry, Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand The reactions of acetaldehyde and ethylene have been investigated on the surfaces of UO 2 and UO3 by temperature programmed desorption (TPD). On UO 2 two molecules of acetaldehyde undergo reductive coupling to C4 olefins. This is due to the fluorite structure of UO 2, which can accommodate large numbers of excess oxygen, up to UO2.25. The vacant surface oxygen of UO 2 were titrated by N20 adsorption and were equal to 1.86 x 1019 molecules/g, representing an apparent surface area of vacant oxygen sites of 1.9 m2/g. On the other hand, ethylene-TPD on [3 UO3 indicated the desorption of acetaldehyde (490 K). In addition, an unexpected product was also observed. This product was identified as furan (C4H40, m/e 68, 39) which desorbed at ca. 550 K with a carbon selectivity of ca. 40 %. Furan formation from ethylene on UO 3 requiring the formation of one carbon-carbon bond and of one carbon-oxygen bond, is most likely accompanied by oxygen depletion from the UO 3 surfaces and subsequent reduction of U cations into lower oxidation states. The observation of furan from ethylene shows that one may obtain oxygenated products with a high carbon number from ethylene (a relatively abundant feed stock) via one single step. 1. INTRODUCTION Oxidation-Reduction reactions very often track the cation oxidation states of oxide materials [1 ]. Changing the oxidation state of a given cation is accompanied by structural change (such as from a rutile or anatase TiO 2 to corundum Ti203 or from orthorhombic V205 to rutile VO 2, i.e., changing of the coordination numbers of metal cations [2]). Another way of changing the oxidation states of cations is by creation of surface defects, where the surface looses its ordered structure [1, 2]. In the case of oxide materials several factors affect oxygen depletion (or in other words reduction of cations) the most important are the mass difference between the cation and the anion, the bond energy, and the formation of ordered or semi ordered cluster defects. While the mass difference is essentially important in the case of reduction via particle bombardment (see Sigmund theory [3]), bond energy and surface structure are most likely the dominant factors during

266 chemical reduction. The investigation of the effects of changing the structures and oxidation states of oxide materials is crucial to the understanding of their catalytic properties. The uranium oxides system is a good candidate for this investigation due to its presence in different stable and metastable structures - the main product of oxidations of uranium metal are UO2, U407, U308, U409, and UO 3 - as well as the presence of a wide range of oxidation states (from +2 to +6) [4]. The main reason for this wide range of oxidation states in U oxide (and the early actinides in general) is relativistic effects [5], which is simply a mass correction for the core electrons that lead to greater shielding of the higher orbitals, or in other words a decrease in the ionization potential and work function. Another important feature of some phases of U oxides is their possibility of accommodating large numbers of interstitial oxygen atoms in clusters, [2:2:2] clusters [6], without changing the crystal structure, such as UO 2 to UO2+x (x up to ca. 0.25); U cations in proximity of these clusters may have higher oxidation states than +4. There is also another implication to the ease of removing electrons form the outer shells, one can change the surface from one electronic state to another by reduction or by oxidation. Recently we observed that H2-reduction as + +4 well as Ar -sputtering of U308 resulted in surface cations exclusively in a U oxidation state [7]. This net change in oxidation state is unlike what one observes on early transition metal oxides such as Ti [8] and V [9] oxides. In addition, and most important, U oxides are known as good catalysts (or as components of catalysts) for serval industrial reactions such as olefins and paraffins ammoxidation [10-13], hydrocarbons dehydrogenation [14], and very recently for total oxidation of chlorinated compounds [15]. It has also been observed that U308 is active in C-C bond formation reactions such as the formation of isobutene from acetone [7]. Despite these technological importance fundamental understanding of the reactivity of U oxides surfaces towards organic reactions is lacking due to the very few amounts of work interested in this system, the most complicate oxide known [16]. This work is devoted to the understanding of the oxidation of CH2=CH 2 on UO 3 and the reduction of CH3CHO on UO 2 by temperature programmed desorption (TPD). Surface and bulk characteristics were investigated by X-ray Photoelectron Spectroscopy (XPS) and Xray Diffraction (XRD) as well as by N20 adsorption.

2. EXPERIMENTAL

TPD at atmospheric pressure was performed using a fixed-bed reactor interfaced to a high vacuum chamber equipped with a Spectra Vision quadrupole mass spectrometer (base pressure ca. 1 x 10.7 ton). The mass spectrometer is multiplexed with an IBM PC which is equipped with a programme (RGA for windows) that allows the monitoring of 12 masses simultaneously at a cycling rate of ca. 5 s. Catalysts were loaded into the reactor and heated under dry air (or hydrogen) for 2 hours at 800 K (or 10 hours, in the case of hydrogen reduction) prior to reaction. After cooling to room temperature (under H2 or air), the carrier gas was displaced by He (ultra pure) before adsorption of acetaldehyde or of ethylene. Acetaldehyde was placed in a saturator at room temperature. In order to obtain surface

267 saturation, dosing of acetaldehyde was performed while monitoring its m/e 29. A decrease in the signal (due to adsorption) followed by signal restoration is indicative of surface saturation (in ca. 2 minutes). In the case of ethylene dosing was obtained upon exposure for 5 minutes. The catalyst was then purged with He for ca. one hour at room temperature in order to remove traces of the reactant in the TPD line as well as weakly adsorbed molecules on the surface of the catalyst. The gas flow was introduced into the chamber through an -1 interface which consists of a leak valve differentially pumped to 10 torr during operating -6 conditions. A constant pressure of ca. 5 x 10 torr was maintained during all TPD runs. During the purging the m/e 29 (the highest m/e of acetaldehyde) or m/e 27 (for ethylene) were monitored and the TPD started when no change in this m/e signal was observed (after ca. one hour of purging time). The ramping rate during TPD was kept fixed at 15 K/min. The fragmentation pattems of each product were checked in order to identify unambiguously the reaction products by the method described previously [17]. This involved: (a) the separation of the desorption peaks into different domains of temperatures, (b) the analysis of the fragmentation pattern of each product separately, (c) starting from the most intense fragment for each product (m/e 29 for acetaldehyde, for example) and subtracting the corresponding amount of its fragmentation until the majority of the signals were accounted for. XPS was performed using a KRATOS XSAM-800 model with a base -9 pressure of ca. 10 torr. U (4f), O(ls), C(ls) and Ar(ls) (in the case of the Ar-ion sputtered samples) regions were scanned each run. Unreduced samples were loaded into the system without further treatment. Ex situ reduced samples (using H 2 at the same conditions as for TPD) were exposed to air (although, under oxygen free N 2 flow) for about 30 to 60 s, at room temperature, before introduction into the XPS chamber. Ar-ion sputtering was m /

performed using a direct beam KRATOS ion gun at a pressure of ca. 5 x 10 torr. Mg Ko~ radiation was used at 170 W. Collection of spectra were performed at a pass energy of 38 eV. Sample charging up to 5 eV occurred under X-ray irradiation. Binding energies were calibrated with respect to the signal of adventitious carbon (binding energy at 284.7 eV). No charging was observed with UO 3 samples. XRD data were collected using a Phillips 1130 generator, and a Phillips 1050 goniometer. XR radiation was achieved using a Cu tube (broad focus) (Kc~; X = 1.514 A) at 44 kV and 20 mA. N20 titration was performed in a pulse reactor. Pulses of N20 were introduced into the reactor at 480 K. A thermal Detector at the end of a Porapack Q column allowed the monitoring of N2 and N20 peaks. The absence of formation of N 2 accompanied by total restoration of N20 signal was indicative of total titration. This took about 4 pulses of l ml each (1 atm.) per 1 g of UO 3. UO 3 was prepared from a uranium nitrate solution by precipitation with NH 3 at pH 9. After filtration and drying at 373 K over night the powder was calcined at 673 K for 5 hours. XRD indicated a pure 13UO 3. Polysrystalline U30 8 (from BDH No. 26216) was used as received.

268 3. RESULTS 3.1. Surface and bulk characterisation of UO 2 and UO 3.

XRD spectra of UO3, U308 and H2-reduced UO 3 are presented in Figure 1. H2-reduction of UO 3 at 800 K for 10 hours resulted in transformation of the monoclinic phase of [3 UO3 into the orthorhombic fluorite structure of UO 2, although some orthorhombic ~ U308 is also present. Similar results were observed from H2-reduction of U308 [7], with complete transformation of o~U308 to UO 2, however. Table 1 Titration of oxygen vacancies by N20 adsorption on unreduced UO 3 and H2-reduced UO 3 (UO2). Reactor temperature 480 K, BET surface area of UO 3 = 33 m2/g, 1 g of catalyst, reduction temperature 773 K, 16 hours. Pulse number

N 2 (molecules)

N20 (molecules)

H2-reduced UO 3 (UO2) (1 g) 1 1.37 1019

1.32 1019

2

0.33 1019

2.4 1019

3

0.14 1019

2.6 1019

4

0.02 1019

2.75 1019

5

negligible

2.78 1019

total

1.86 10

19

Unreduced UO 3 (1 g) 1

no reaction

2.78 1019

XP spectra of UO2, U308, and UO 3 were analysed elsewhere [7]. A brief description is +

given here. Figure 2 (adapted from ref. 7) shows the XPS U4f region of UO 3, Ar -sputtered UO 3 and H 2 reduced U308. Three important points need indication. First, XPS U4f7/2 and U4f5/2 of UO 3 (spectra a) are higher in binding energy than those in spectra b and c. Second, spectrum a does not contain satellites while both spectra b and c contain satellites + at 386.5 and 397.5. Third, the XPS U4f peak positions of Ar -sputtered UO 3 as well as of H2-reduced U30 8 are those of UO 2 and UO2+ x respectively (see Table 1 in ref. 7), clearly indicating that one can shift the cation oxidation state from one position to the other (from +6 in UO 3 to +4 in UO2). This is unlike early transition metal oxides where, although they + are sensitive to H 2- or Ar - reduction, the resultant surfaces still contain considerable

I

XRD H2 - reduced U 0 3

'3O8

Ar+-sputtered UO, El

a,

u

El

L

U

d\x,j H,-reduced U,08

I

I

I

I

20

40

60

80

28 Figure 1. XRD of U03, U30s, and H2 - reduced U03 (mainly U02).

WOJ

400

395

390 385 380 375 Binding Energy (eV)

Figure 2. X P S of U03, Art -sputtered UO3 (U02) and H2 - reduced U30x (UOz,x)

270 amounts of stoichiometric phases. This unique characteristics of U oxides affect its chemical reactivity (see below), particularly with regard to oxidation-reduction reactions. The pulse method of N20 was investigated on UO 3 and H2-reduced UO 3 (UO2) (Table 1). This method is successful for titration of the surface area of Cu ~ and Ag o catalysts [ 18] and we wanted to try it to titrate oxygen vacancies instead of using oxygen in order to avoid formation of multilayers of dioxygen on the surface. N20 dissociated on UO 2 but not on UO 3 (Table 1).The dissociation reaction is activated, below ca. 425 K no dissociation occurred. A temperature of 480 K was observed as optimum were N20 dissociated non catalytically (catalytic decomposition occurred at ca. 525 K and above). From Table 1 one may estimate the total surface area of potential vacant sites which may abstract oxygen 2 19 from oxygenated compounds. Assuming that one m contains 1 x 10 atoms, N20 titration data indicated a surface of ca. 1.9 m2/g, or about 6 % of the total BET surface is composed of oxygen vacancies. 3.2. Acetaldehyde-TPD on UO 2.

Figure 3 and Table 2 present the desorption products during TPD after acetaldehyde adsorption on UO 2 (H2-reduced UO3). Table 2 Carbon yield and carbon selectivity of products formed during TPD after acetaldehyde adsorption at room temperature on UO 2 Product

Desorption Temperature (K) Acetaldehyde (m/e 29) 390 Propane (m/e 39) 610 Butadiene (m/e 54) 540 butene (m/e 56) 673 Ethanol (m/e 45) 415 CO 2 (m/e 44)

730

Carbon Yield (100%) 65.9 12.2 6.3 0.9 0.7 14.0

Carbon Selectivity (100 %) 35.8 18.5 2.6 2.0 41

Serval reactions occurred evidenced by a complex desorption products. First, acetaldehyde (m/e 29, 15, 44) desorbed at 390 K followed by traces of ethanol at 415 K (2 % of carbon selectivity, table 2). Three other products were observed. Butadiene and butene desorbed at 540 and 673 K respectively with a combined carbon selectivity of 21.1%. This reaction pathway follows a reductive coupling mechanism which has been observed previously on the surfaces of TiO 2 single crystal and powder [19-21]. The formation of C4 olefins is a clear example of the capacity of UO 2 surfaces to abstract large amounts of oxygen from surface carbonyls, via pinacolates [ 19], as follow

271

2 CH3CHO + 2 U

+4

- Vint.vac.

)

CH3CH=CHCH 3 + 2 U

+4+x - Oint.

Vint.vac.: interstitial oxygen vacancy, Oint.: interstitial oxygen. 3.3. E t h y l e n e - T P D on U O 3

Figure 4 and Table 3 show the desorption products during ethylene-TPD on ~ UO 3. Table 3 Carbon yield and carbon selectivity of products formed during TPD after ethylene adsorption at room temperature on UO 3 Product

Desorption Temperature (K) Ethylene (m/e 28, 27) 400-700 Acetaldehyde (m/e 29) 480 Furan (m/e 68, 39) 550 CO 2 (m/e 44)

above 800

H20 (m/e 18)

ca. 500

Carbon Yield (100%) 85.7 8.3 6.0

Carbon Selectivity (100%) 58 42

not calculated

In addition to ethylene desorption in a large temperature domain, acetaldehyde was clearly observed evidenced by its m/e 15, 29 and 44 (Table 3). The formation of acetaldehyde from ethylene indicates the facile removal of surface oxygen on UO 3 and shows its high reactivity towards oxidation of olefins. It is important to note that during TPD there is no regeneration of surface sites (in contrast to a steady state oxidation reaction with oxygen). This reaction requires a subsequent reduction of surface cations as follow

CH2=CH 2 + U

+6

-O

~

CH3CHO + U

+4

+ VO

(480K)

Vo: surface oxygen vacancy In addition, another important product was observed, furan (C4H40, m/e 68 and 39) at 550 K with comparable yield to acetaldehyde (42 % carbon selectivity). Thus, furan formation indicated that U surfaces are also active for C-C bond formation in their oxidised form, in addition of being an active C-O bond formation catalyst. The key route to this reaction is the formation of C4 olefin (most likely butadiene) which in its turn reacts with the surface oxygen to give furan as follow

CH2=CH 2 + CH2=CH 2 + O 1

CH2=CH-CH=CH 2

+ H20

cthylene/UO

acetaldehyde/U02

G

0 0-

-I--,

E4 k

0

v1 Q)

ct1i;inol x 40 0

pl-opane x 5

Q)

k k

0

0

5 300 400 500 600 700 800

300 400 500 600 700 800

273 CH2=CH-CH-CH 2 + 201

C4H40 (furan) + H20

(550 K)

Ol: lattice oxygen

Tow further points are worth mentioning. Firstly, XRD of the used ]3 UO3 (after TPD) indicated a mixed phase materials composed mainly of [3 UO3 and o~ U308. TPD of ethylene on this used UO3 (which have been regenerated under a dry air flow at 473 K for 90 minutes) showed a furan yield very similar to that on pure 13UO3 [22]. This result (which is under further investigation) may indicate that o~ U308 is also active towards this oxidative coupling reaction. It is important to mention that U308 contains substantial amounts of U +6 cations (together with U +4 or U +5 cations [7]). Secondly, in order to understand the reaction mechanism, TPD after butadiene adsorption at room temperature on ]3 UO3 was also investigated. Furan was clearly observed together with maleic anhydride [22]. This last point reinforces the above reaction mechanism.

4. CONCLUSIONS The oxidation of ethylene has been investigated on polycrystalline 13 UO 3 surfaces. Two oxygen containing products were observed: acetaldehyde, and furan. Furan desorption which requires a C-C bond formation, most likely is formed via dimerization of two adsorbed ethylene molecules followed by cyclization with available surface oxygen. Both the formation of acetaldehyde and furan from ethylene on UO 3 are clear examples of the ease of removing oxygen atoms from UO 3 surfaces. The reduction of acetaldehyde was also investigated on UO 2. Two molecules of acetaldehyde couple together to make a symmetric olefin: butene (which undergoes further dehydrogenation to butadiene). This is similar to what has been observed on TiO 2 and CeO 2 surfaces before [ 19-21, 23 ]. These complex chemical pathways indicate the richness of the U oxides system and open routes to further investigations.

References 1. M.A. Barteau, Chem. Rev., 96 (1996) 1413 and references therein. 2. V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides, 1994, Cambridge University Press, and references therein. 3. P. Sigmund, Sputtering by Ion Bombardment: Theoretical Concepts. Topics in Applied Physiscs, 47 (1981) 9. 4. C.A. Colmenars, Prog. Solid State Chem., 9 (1975) 139. 5. M. Pepper and B.E. Bursten, Chem. Rev., 91 (1991) 271. 6. R.J.D. Tilley, Defect Crystal Chemistry, Blakie, Glasgow and London, 1986.

274 7. H. Madhavaram, P. Buckanan and H. Idriss, J. Vac. Sci. Technol. A, 1997, in press. 8. H. Idriss and M.A. Barteau, Catal. Lett., 26 (1994) 123. 9. H. Poelman, L. Fiermans, J. Vennik and G. Dalmai, Surf. Sci., 275 (1992) 351. 10. K.M. Taylor, US Patent No. 3,670,006 (1972). 11. R.K. Grasselli and R.C. Miller, US patent No. 4010188 (1977) 12. R.K. Grasselli and D.D. Suresh, J. Catal. 25 (1972) 273. 13. D.D. Suresh, M.J. Seely, J.F. Brazdil and R.K. Grasselli, US Patent No. 4855275 (1989) 14. J.M. Hermann, J. Disdier, F.G. Freira and M.F. Portela, J. Chem. Soc. Farad. Trans., 91 (1995) 2343. 15. G.J. Hutchings, C.S. Heneghan, I.D. Hudson and S.H. Taylor, Nature, 384 (1996) 341. 16. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 1972, Wiley, New York, third edition. 17. H. Idriss, K.S. Kim and M.A. Barteau, J. Catal., 139 (1993) 119. 18. M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, 1984, Princeton University Press. 19. H. Idriss, K.G. Pierce and M.A. Barteau, J. Am. Chem. Soc. 116 (1994) 3063. 20. H. Idriss, K.G. Pierce and M.A. Barteau, J. Am. Chem. Soc. 113 (1991) 715. 21. J.E. Rekoske and M.A. Barteau, Ind. Eng. Chem. Res., 34 (1995) 2931. 22. H. Madhavaram and H. Idriss, work in progress. 23. H. Idriss, C. Diagne, J.P. Hindermann, A. Kiennemann and M.A. Barteau, J. Catal., 155 (1995)219.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

275

Active Sites of Vanadium-Molybdenum-Containing Catalyst for Allyl Alcohol Oxidation: ESR S t u d y i n s i t u . O.V~Krviov. N~.uen Tien Tai. B.V.Rozentuiler N.N.Seme~ov Ln.stitute o! ( . h . m l c a ! Physics, Russian A c a d e m y of Sciences, ul.Kosygina 4, Moscow, 117884, Russia Allyl alc.r)hol oxidation into acrolein on the rhombic phase of molybu,:J~uJJ~ oxide in,-,u_ified with v a n a d i u m oxide has been studied by the Kne~nou ,Kl~leLic ,1 . , aim, , t,y: ESR of V . I + ions in sit'u. It was showl,, that active sites for ti~is reaction are V 'r ions situated in the bulk of the catalyst. ~w n e a r its s~rface, but n~:.,t at the surface. Fast diffusion r)f ,?iectro_ns and a more slower diffusion of oxygen ions b e t w e e n the s,~rface and t;he bulk occur d u r i n g the reaction. AI .

.

.

.

.

.

.

.

.

.

.

!1

u.

' 'roduction. A widespread opinion exists about the m e c h a n i s m s of oxidative catalytic reactions, that active sites for these reactions are surface coordinatively u n s a t u r a t e d transition metal ions. which can be r e d u c e d and reoxidized dllring the reaction. O u r studies of sevel-a] oxidation reactions [1,2] by .tt',S.R .in situ have shown t h a t the active sites can be situated not on!v at the surface, but in the bulk of the eata!y~t. We havc~ studied CO oxidation over the paraelectric phase of BaTiO3. , which contained about 0.01% Mn z+ ions as a n a t u r a l impurity. It t u r n e d .... J. ~u,o,L, 1 . _ ~. ~~ A~2+ ,_,u~., at 400-~t~0 K the active sites for CO oxidation are ..,,., ions. .Al(,r~g with adsorption and catalytic m e a s u r e m e n t s , we have proved using in situ ESR s ! ~ c t r a studies of Mn 2+ ions t h a t t h e y are situated in the bulk of BaTif) 3. It was found out that CO adsorption proceeds in exact accordance with the stoicbiometric equation, w h e r e C)~2- is a c,]rface c,x,,~en it,-' '~, Mr, 4+ + CO + O~'~- ---~ Mn ~+ 4- CO2,

(1)

i.e. adsorption of one CQ molecule results in a p p e a r a n c e of one Mn 2+ ion. On the contrary, 02 adsorption decreased the i n t e n s i t y of the ESR spo.otrum in accordance with the s t n i e h i n m e t r y

276 a~,~ ":,-,rf-'-! ~

~: u+_"__> 2Mn~ +-+ zuo.+"-' -, _ ,,

:.~,

tzl

i o. t:tvo Mn zl- inn~ d+sapp~.ar on a d s o r p t i o n of o n e nxyg~.n mnloe~iP. (In e x p o s u r e to t h e 2CO4-O z s t m e b i o m e t r i e m i x t u r e ~.he c a t a l y t i c r e a c t i o n is pr.,..-.eeeding t h r e u g h t h e r e d o x m e c h a n i s m f o l l o w s E q u a t i o n s (1) a n d {2). The 99 ..... - ~+.- o f m , , , . ~ , , . ~ o u r c o x i d a t i o n ( E q u a t i o n 2) is a l m o s t 2 ordcr~" of iiia,~iii|,tid~, N* u , , '~ J " t h a n t,ia~ ' ' oI~ t h e r e d u c t i o n ( E q u a t i o n l ) . In tht ~, 9 "........ " ' - "

.

.

established. cal,

.

. . . . . . .

.

The

.

.

lOW

reaction

proceeds

lu

predominantly

"

t2.oliCuIJtI

over

the

" "~t 'L.IUII

i8

oxidized

a J v s++. t.

~|'ho.~o invo.~tigntions wPro. e o n t i n ~ e d in m+r stud.les of a iiyi aieohoi ]r,+,:~ .'.~er,,l,~in m , e r t b e h e x a g o n a l pb_a~e of MoO a m o d i f i e d b y ! w ! % of Vz() 5 [2 ~]. T h e reacti~;n "" "~ '~

~,,,II~O11

:' .--~ \

+' '-'

.

Lot

ii2,k)

p r o c e e d s at 400-470 K w i t h i O0% s e l e c t i v i t y . P r a c t i c a l l y all t h e V r h,t~s rneam+t'o+d w i t h ~ S R i n si~.u w ~ r e in t h e b u J k of the, e a t a j v s t . VorT.r!atjoD of o~.e V 4.b i o n d u r i n g r e d u e t i o n of t h e catalyst. ]_s a c c o m p a n i e d b y t h e disapl.x~aranee of o n e allyl alcohol m o l e c u l e ,

. ,-, . + .'--:.i.,*~ . -,** ....§

+ [ ]~ + C : + 1 1 4 0

(+~,~

+ 119.O

it w a s s h o w n [3] t h a t V 4+ ions o b s e r v e d in E S R s o e e t r a a r e u n i f o r m l y c i i s t r i b u t e d o v e r t h e wiaole b u l k of h e x a g o n a i MoO~. (Ipym,~ito. offoet~ havo. bo~n o b s e r v e d on o x i d a t i o n of t h e c a t a l y s t b y ~,y.ygen. T h i s p r o , _ ' e s : ~ e.9....n+ be d e s c r i b e d b y f h e s t o i e h i o m e t r i e e q t m t i n n : q'~r4-t._t_

I 1

.l.. , q

., ~'tI1)',~-t--

/3-~

tV,~

AL 435 K Lhe t'aLe or Lhe s t a t i o n a r y c a t a l y t i c r e a c t i o n is proport"I O I l ~ l ' i:o t h e c,.Jneentration of v a n a d i u m ions a n d to t h e o x y g e n p r e s s u r e in th~ mixture

. . . .

k.;z.i '

J

,~ .... /

*~, i.e., at ~,~e- t e m p e r a ~++...4 at t h e t e m p e r a t u r e s ,,,+s,,~' 1.{~+1...... +l , ~ n.- a~-nn ~ - 6 0 0 ~r l;ui~'~ ot" at+ai,ioij~ti- s ~ t M y t i c oxidMiui,, t h ~ h e x a g o n M p h a s e of ~+ ***vO,,~ i~ [l'Hill

o I slt~n:~e.u

inl, o

the

more

s~ame, +

~

1

rhotnbie

phase 1

[6]. Thi~-

puper

is

277

devoted to the investigation of the mechanism of allyl alcohol oxidation into acrolein on the rhombic phase of MoO~ modified with V20~.

Experimental Samples of V-Mo-oxide catalysts have been prepared by mixing (NI-I4)6MoTOz4 and NH4VO.3, drying for 4 hours at 470 K, and calcination at 550-80~2 K Catalysts with 0.2, 0.5, 12, ,~.0, and 3.0 wt% of VzO5 were prepared. Their phase coml~)sition has been d e t e r m i n e d on the in~ t r u m e n t DRON-2 with Fe-K..~-emission. ESR s t ~ c t r a have been registered with the spectrometer EPR-V constructed in the Institute of Chemical Physics and equipped with an a t t a c h m e n t for the t e m p e r a t u r e regulation of the ampol!le with the s'ample directly :in the resonator of the spectrometer. Accuracy of the . is 0.2 ~ Calibration of the spectrometer has been t e m p e r a t u r e ~e"ulation, .~. Clone with the help of solutions of stable nitroxyl radicMs in benzene. g-values and IIFS constants were d e t e r m i n e d by comparison with a Mn24-/MgO standmxl. The absolute error in the d e t e r m i n a t i o n of the spin concentration by means of double integration is '_*50,%, the relative one is +2%. The error in g-value is 0.001. and that in ItFS constants is 0.1 Gs. The V4+ ions concentration was d e t e r m i n e d via double integration using the second parallel component of the ESR signal. A flow microcatalytic set-up has been combined with the ESR-spectrometer. Catalyst samples were placed into a flow- reactor which was at the same time an ampoule for the ESR studies. Gas mixtures He+O~, He+CsHsOII. and tIe§ have been prepared with the help of special 4- and 6-way valves. Gas analysis have been performed on the Carbowax column of the gas chromatograph, the length of the column was 1 m, and its t e m p e r a t u r e was 300 K. The catalytic reaction was studied at 380-540 K directly in the heated resonator of the ESR-spectrometer, at higher t e m p e r a t u r e s it was studied outside of the resonator. '

}

.

.

9

.

.

Results and discussion. Paramagnetic centres in V-Mo-oxide catalyst. Each of the observed ESR signals consists of 24 ItFS component~ due to interaction of an unpaired electron with the v a n a d i u m nucleus (s!V I=7/2. p=99.7o~). Their dependence on t h e , t e m p e r a t u r e of calcination of 2%V2Os/MoO 3 is shown in Fig.l The si.~nal tt (gx=gy=l.95r gz=l.908) corresponds to V4+ in the hexagonal phase of MoO~, the signals A, B, and C

278

~} 3"V.,(reI.uni tz )

Figure 1. De~mndence of the intencity of ESR signa]s observed in V-MoO~ catalyst on the heating temperature.

,K 593 623 673 723 873 q23 correspond to V4+ in the rhombic phase. It is seen that the rhombic phase only exists at temperatures higher than 600 K. This result was also confirmed by the XRD study. The signals A (gx=1.976, gy=1.974, gz=l.921) and B (gx=1.974, gy=l.970, g~.=1.928) are characterized by additional tIFS, which is typical for the interaction between unpaired electron of vanadyl and the nitrogen nucleus l'14N. I=l, p=99.63%). A calculation of the orbitals with an unt~aired electron [7] shows that the A signal corresl~mds to the V imide complex, where the NH group occupies the tx.,sition of one of the oxygen ligands around vanadyl. The B-signal a p p e a ~ at a higher t e m w r a t u r e (700 K). A calculation shows, that this signal corresponds to the interaction of the unpaired electron of vanadyl with a NO molecule. Both, the A and B complexes, are formed during preparation of the samples from arr lmonium molybdates and vanadates. The signal C (gx=l.971, gy=1.969, gz=1.872) s essentially different. It is fi~rmed at high temtmratures (800-900 K). Every component of its additional ftFS consists of one intensive line in the centre and 6 lines of equal intensity, separated by equal distances. The intensities of these lin,::s are in the ratio of 100:5.1. The appearance of this signal can be explained by the formation of non-stoichiometric phases in MoOa, the so called Magnelli phases [81, where the MoOs octahedrons are connected by planes and edges, but not by apexes. Such a non-stoichiometry contracts distances between the metal cations. Interaction of an tlnpaived electron of V4~ of the first octahedron with the molybdenum nucleus ('~,97Mo, I=5/2, p=25.18 %) of the adjacent octahedron connected by an edge with the fi.,~'t one gives additional HFS. The signals A, B, and C were observed at all concentrations of VzOa. Fig.2 shows the dependence of the n u m b e r of V 4+ ions and of the ESR line width on the VaO~ content. The maximal concentration of V 4+ ions

279

al:ter the catalyst reduction by allyl alcohol has been obsex"qed for 2% o[ V205. The line width increases monotonically with increasing percent of V~.O5 The V 4t ions are distributed uniformly and separately in the catalyst bulk with the increase of VzO 5 up to 2%. At higher V205 concentrations the signal intensity increases and the lines are broadened, because of strong mutual interaction of the V 4+ ions. At higher VzO 5 concentrations clusters of V 4+ ions are formed, and the n u m b e r of V 4+ ions observed in ESR diminishes. ,......

'~ ,3

<3

%

Figure 2. Dependence of V 4+ ions n u m b e r in V-MoO 3 catalyst and ESR line width on V20 5 content.

4O

o%

z

Io

Vo05 (wt %)

Reduction of rhombic MoO 3 modified with VzO 5 by allyl alcohol. The reduction kinetics of the rhombic MoOs with 2 wt% of VzO5 ha~ been studied at 387-437 K and at 1.2-2.8% of alcohol in the gas flow. Fig.3 shows kinetic curves of the V 4+ ions increase in the catalyst bulk arid of the decrease of acroiein formation t~te. These cur'ves are described by the first order equation. Acrolein and traces of water are appear in the gas phase. No significant decrease of the oxidation degree of the main MoOs matrix was observed, ~'~ rC4H~.O (10 ~6 motQc/9.=) . . . .

[V4+](10 TM ions/g.s)

~.~ Figure 3, Kinetic curves of the change of V 4+ ions ~$ u urnber and of aerolein formation rate; [C~H5OI{] = 7,1x l 0 I7 molec./cm s,

280 A q u a n t i t a t i v e comparison of these two processes, i.e., aclx~lein formation and V 4+ ions reduction shows t h a t 1.2-1.3 CstI,~O molecules are formed [~.r one V 44 ion produced. It indicates t~ssibly some reduction of Mo sites. The effective rate constants of rhombic phase reduction are shown in Table 1. The results of the s t u d y of hexagonal Table 1. Effective rate constants [C3IIsOII= 6.7xI017 m o l e c / c m 'z. MoO~ phase

T. K

of

V-MoOs-catalyst

387

397

417

437

Ris ......... i~e;iiid2);~2~.......(ii~ Hexagonal k~.r:c.,10-as -t 0.3

(I.6 0.4

1.1 1.0

1.9 2.4

reduction:

E a, k c a l / m o l e 11 16

V-MoO~ redl~ction for" similar conditions [3] are shown in the same tublu. 'Phes,a. dut~ indicttte a r a t h e r small influence of the MoOa structure, on the rate of its reduction. The formation of one acroiein molecule per one reduced V ion shows t h a t the charge compensation of V 5+ in MoO 3 takes place at the u c_am pJex formation. Some deviations from this expence of the v~75+-~' ratio can be explained by aerolein formation on o t h e r eentres. Thus, the catalyst reduction by allyl alcohol is described by the Equation (4). This process consists of at least of two steps" the surface one (7), which proceeds with pm-tieipation of the surface oxygen ion, and bulk step ('8), w h e n the charge carriers diffuse to the bulk and reduce the V 5+ ions: V.~5+O~" + C,~HsOH -~ Vs4+[ ]so + C.~tt40 + H20 V b5+O b- -~, Vsa+Os -

(7) (8)

The process (P,) represents diffusion of vacancies from the surface to the bulk und oxygen filling up of vacancies up at. the surface. We propose, as in [2], Thst the charge compensation of V 5+ in MoOs takes place at the expense of the formation of V5+O" ejmplex. Such complex should be paramagnetic, but it was not frecorde by ESR.. Special properties of r :vel~ not important for f u r t h e r discusision. The complex V4+[ ]0 is probably located in the i m m e d i a t e proximity to s t r u c t u r e s of the crystallographic shift, or Magnelly phases.

281

rteoxidation of the reduced rhombic phase of V-MoO~ by oxygen. Fig.4 shows the V "i+ decrease kinetics during reoxidation of the reduced rhombic V-MoO3 by oxygen. Reoxidation proceeds in accordance with b~quation (5), but the vacancy [ ]0 remains at the surface. The diffusion V~,+O - § V,+[ ]bo __> V~+[ ]o + Vs+Ob-

(9)

i~, slow. No products of this reaction were registered by the chromatographic analysis. A,l analysis of these results by the method of affine transformations showed that at temperatures higher than 520 K all the kinetic curves belong to the same family and can be described by a hype.rbolic equation No/[V 4+] - keff.t + 1, where N O is the initial n u m b e r of V 4+ ions in the sample. The values of " and the ,'elevant data for the hexagonal phase of V-MoOs [4] are h'eff. given in Table 2. These data show that the hexagonal phase reoxidation proceeds much faster. For the hexagonal phase similar values of kerr. are observed at tempera.tllres approximately by 100-1200 lower.

Table 2. Rate constants of V-MoO3. reoxidation;[O2]=2.7:• MoO~phase

T, K

426

460

486

528

554

570

lt~ molec/cm 3. 593

i~i~oml~ic........k~f~ii-O:~-s~ ........................................................................ 1.0 1.7 2.6 3.8 Hexagonal keffl0 -3 s -I 1.2 3.6 7.7

A comparison of Tables 1 and 2 shows that the kinetics of V 5+ ions reductions are identical in both, rhombic and hexagonal phases of MoO~, but the reoxidation of the rhombic phase is much slower.. This result probably explains the loss of activity of the catalyst induced by overheating during its preparation. In the hexagonal phase there are numerous cavities throughout the whole crystal.. Their diameter is about 0.5 nm, i.e. they can provide oxygen diffusion and, therefore, fast reoxidation. Such cavities are absent in the rhombic phase. From the other side:, no such channels are: necessary for the reduction of V-MoO.~, because they are too small for organic molecules. The reduction apparerently proceeds at the expence of electron migration from the. surface t

282

T ' 9 "

~

"

~90l;

~

; 03

~

~.

fO

r L.

2We "--~,--,..~ 0

t

t~

20

,,

I

40

0

~.

? 97~ ~ZoY ,.

9t (m;.) "~-

Q

'

e

4

GO

Figure 4. Kinetic curves of the change of V 4+ ions n u m b e r during catalyst reoxidation; [021 = 2 . 7.~10-~ molec./cm~.

,

i

* v2o 5(wt ~;)

Figure 5. Dependence of the reduction and reoxidation rate on V~O5 content in the rhombic phase of MOO3.

surface to the bulk. Thus, the increased catalytic activity of the hexagonal V-MoO s is explained by the easiness of catalyst reoxidation. Fig,5 shows rate constants dependence for reduction and reoxidation of r hombic V-MoO~ on the VzO 5 content. Both curves have a m a x i m u m at 2.0 wt% V~Os. These regularities correlate with changes of the ESR spectra (Fig,2)' the active sites are isolated v a n a d i u m ions. But with increase of V205 content higher than 2% pail~ed ions or clusters are formed. This leads to the loss of catalytic activity.

Stationary oxidation of allyl alcohol on the rhombic phase of V-MoO 3. Fig,6 shows the kinetic curves for the acrolein formation rate on the rhombic V-MoO s and for the change of V 4+ ions content in the

'z'c . 3

o t10*%oto~-~'g-- )

4~

~0

a,0

Oil

II

9

",~

-""--'4

0

Figure 6. Kinetics of the change of V 4+ ions n u m b e r in the rhombic phase of V-MoO 3 and the rate at 525 K; [C3HsOH] = 5.5x I 017 me]co./cm s. of acrolein formation

283

C3HsOH+O2 reaction mixture at 525 K. It is seen that the stationary reaction rate increases with increasing oxygen partial pressure in the mi• The acrolein formation rate is at any m o m e n t unambiguosly related to the n u m b e r of V4+ions in the catalyst. Such a regularity can be explained by the participation of the bulk V 4+ ions in the catalytic process. The kinetics of allyl alcohol oxidation on the rhombic phase of the V-MoO3 catalyst can be described by the equation

rc3mo =

N,:k ~d.[ C~H~O H] k ox.[02]

( 11 )

k~.~.a.lC.sHsCH] + ko~.[Oz] Table 3. Rate constants of the reduction and oxidation of the rhombic phase of V-MoO~ catalyst at 525 K; [C3H5OH] = 5.5• molec./cm :3. V20 5 content (wt. %)

0.5

1.0

2.0

3.0

kt..ed. ( 10 "21 cm~s "~)

from the experiment on reduction calculated by eq.(11)

0.25 0.29

0.40 0.52

0.58 0.96

0.90 0.90

k (10-2!.cm3s -I)

from the experiment on reoxidation calculated by eq.(ll)

2.0 2.3

3.8 3.0

5.6 4.6

5.4 4.1

o

.

~

.

.

This Equation does not differ from the usual Mars-Van Krevelen redox equation. The rate constants of the separate steps of oxidation and reduction from Equation (11) are listed in Table 3. They are compared in the same Table with the rate constants determined separately from the experiments on reduction and reoxidation. The coincidence b e t w e e n the calculated and experimental rate constants confirms the proposed redox mechanism of allyl alcohol oxidation over the rhombic phase ,:ff V-MoO 8 catalyst. Conclusion. A graphic scheme of allyl alcohol oxidation is represented in Fig.7. The ESR study of this reaction in situ has shown that the active sites

284

Alcohol O z- O z- O z- O~.- O z- 0 2Mo Mo Mo Mo V 5+ O z- O 2- O2-O 2- O ~- Oz--

-,

O 2- O z- 0 2- 0 2- 0 2- 0 2Mo Mo Mo Mo V 4+ 0 2-[ ] O 2- 0 2-O- 0 2-

-O

--~

Acrolein + w a t e r 0 2- [ ] O ~-- O~- O~- O~.Mo Mo Mo Mo V 4+ O 2- O 2- O ~- O ~- O- O ~-

-->

0 2- 0 2- 0 2- 0 2- O z- 0 2Mo Mo Mo Mo V4+ 0 2- 0 2- 0 2- 0 2-[ ] O-

Figure 7. The scheme of allyl alcohol interaction with the lattice of VMoOs catalyst. of the V-MoO 8 catalyst are V 4+ ions, which are situated mainly in the bulk of the catalyst, or near the surface. These ions supply electrons for the oxygen activation. Th~ second step of the redox catalytic process, reduction, proceeds via alcohol interaction with V 5+ ions at the surface. Thus, a continuous electron exchange between the catalyst surface and the bulk takes place during the reaction. This electron exchange is accompanied by oxygen ions (or vacancies) exchange in the case of hexagonal phase of V-MoO s. This catalyst contain~ r a t h e r broad cavities which allow fast oxygen diffusion. But the rhombic phase has a more dense lattice and oxygen diffusion in this phase is slower. This slow oxygen diffusion also explains the s o m e w h a t smaller catalytic activity of the rhombic phase of V-MoOs, References.

1. B.V.Rozentuller, K.N.Spiridonov, O.V.Krylov, Kinetika i Kataliz, 22 (1981) 797 2. M.A.Makarova, B.V.Rozentuller, O.V.Kry]ov, Kinetika i Kataliz, 28 (1987) 1143 3. M.A.Makarova, B.V.Rozentuller, O.V.Krylov, Kinetika i Kataliz, 28 (1987) 1395 4. M.A.Makarova, B.V.Rozentuller, O.V.Krylov. K i n e t i k a i Kataliz, 29 (1988) 872 5. M.A.Makarova, B.V.Rozentuller, O.V.Krylov, Kinetika i Kataliz, 20 (1988) 876 6. D.P.Shashkin, M.Yu.Kutyrev, P.A.Shiryaev. Proc. 5-th Intern. Symp. on Heterogeneous Catalysis (Varna, Bulgary) V.1 (1988) 177 7. D~Kivelson, S.-K~Lee, J.Chem.Phys, 41 (1964) 1806 8. L.C.Dufour, O.Bertrand. N~Floquet, Surface Sci., 147 (1984) 396

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

285

O x i d a t i v e d e h y d r o g e n a t i o n of e t h a n e over v a n a d i u m a n d n i o b i u m oxides s u p p o r t e d catalysts P. Ciambelli a, L. Lisi b, G. Ruoppolo ~ G. Russo o and J. C. Volta d. a Dipartimento di Ingegneria Chimica e Alimentare, Universith di Salerno, 84084 Fisciano (SA), Italy. Istituto di Ricerche sulla Combustione, CNR, Napoli, Italy. oDipartimento di Ingegneria Chimica, Universith "Federico II", Napoli, Italy. d Institut de Recherches sur la Catalyse, CNRS, Villeurbanne, France.

Ethane oxidehydrogenation has been investigated over niobium and vanadium oxides supported on high surface area TiO2. The vanadia-titania catalysts are very active but with low selectivity due to their high reducibility. The selectivity to ethylene is enhanced by the presence of niobium. By changing the order of addition of vanadia and niobia to the support, catalysts with slightly different redox and acid properties are obtained. At low vanadium loading, supporting the two oxides at the same time results in the best catalytic performances, while at high loading a two steps impregnation gives the best results.

1. I N T R O D U C T I O N Oxidative dehydrogenation (ODH) of light paraffins could be an alternative route to produce high purity olefins compared to conventional methods such as dehydrogenation and cracking [1]. Paraffin dehydrogenation in the presence of oxygen is thermodynamically favoured due to water formation, however the selectivity to olefins is generally poor because of the low reactivity of the alkane compared to the formed olefin [2]. In the last decade attention has been paid to ethane ODH due to the interest in transforming an abundant component of natural gas into a more valuable product. It has been shown that catalysts based on Li and Mg mixed oxides, active in the reaction of methane coupling, are also active in ethane ODH [3-4]. A similar behavior has been also found for various rare earth oxides [5]. Catalysts based on transition metal oxides are the principal materials investigated in alkane ODH. Ethane is oxidatively dehydrogenated to ethylene with high conversion and selectivity over bulk metal oxide catalysts containing

286 Mo, V and Nb. It was proposed that the role of Nb20~ is to enhance the intrinsic activity of Mo and V and to improve the selectivity by inhibiting the oxidation of ethane to carbon oxides [6, 7]. The catalytic properties of pure V20~ are inadequate [8], whereas crystalline Nb20~ is highly selective in propane ODH [9]. Supporting niobia on a-A1203 does not result in better performance, whereas niobia supported vanadia is more active than pure niobia, maintaining the high selectivity [ 10]. Moreover, the effect of supporting vanadium oxide on either silica or alumina on the catalytic properties of V20~ in alkane ODH [2, 11-13] has been investigated. VgOdTiO9 catalysts have been widely used in selective oxidation of hydrocarbons. A new application of ethane ODH has been reported for a combination of vanadium and molybdenum as phosphates on TiO9 [14]. In this work, the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase TiO2 in ethane ODH have been investigated. Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components inducing respectively redox and acid properties, together with the effect of the preparation conditions on the catalytic performances have been studied. 2. E X P E R I M E N T A L

2.1. Catalyst preparation The support was pure anatase TiO2 (surface area 125 m 2 g-l) supplied by hoxide Specialties. Binary and ternary catalysts, containing respectively only supported vanadium or niobium oxide or both oxides, were prepared by wet impregnation with vanadium metavanadate (BDH Laboratory Supplies) and niobium ammonia complex (Companhia Brasileira de Metalurgia e Minera~fio) aqueous solutions. After impregnation the materials were dried at 110~ and calcined at 550~ in flowing air. The ternary catalysts were prepared either by using both precursor salts of vanadium and niobium oxides at the same time or by changing the order of addition of the two components to TiO2. Before the addition of the second component, the catalyst containing vanadium or niobium oxide was dried and calcined. These operations were repeated after the second impregnation. The supported catalysts will be denoted as xVyNb/Ti, x being the V20~ and y the Nb205 nominal content (weight percentage). Furthermore, the ternary catalysts prepared in two steps will be denoted as (I) ff vanadium oxide is deposited on the support in the first step and as (II) if niobium oxide is added in the first step. Binary and ternary catalysts with vanadium content of 1 and 6 wt% V20~ and binary and ternary catalyst with niobium content of 6 wt% Nb~O~ were prepared.

2.2. Physico-Chemical Characterization XRD analysis was performed with a PW 1710 Philips diffractometer. The BET surface areas were determined by N2 adsorption at 77K with a Carlo Erba 1900 Sorptomatic.

287 Temperature Programmed Desorption (TPD) of NH3 and Temperature Programmed Reduction (TPR) with H2 were performed using a Micromeritics TPD/TPR 2900 analyzer equipped with a TCD and coupled with a Hiden HPR 20 mass spectrometer. The samples were preheated in flowing air at 550~ for 2 h. In TPR analyses a 5% HdAr mixture (25 cm~/min) was used to reduce the sample by heating 10~ up to 670~ In NH3 TPD analyses, after saturating the sample with a 2% NH3/He mixture, ammonia was desorbed by heating 10~ up to 650~ in flowing He (25 cm~/min). In situ Raman spectra were recorded with a DILOR OMARS 89 spectrophotometer equipped with an intensified photodiode array detector. The emission line at 514.5 nm from Ar § ion laser (Spectra Physics, Model 164) was used for excitation. The power of the incident beam on the sample was 36 mW. Before the acquisition of the spectrum the sample was heated up to 400~ (2~ and kept at this temperature for 12h to obtain a complete dehydration of the surface. After cooling down to 300~ the spectra were recorded. The same procedure was used to acquire Raman signals of the pure TiO9 support. The final spectra of the catalysts were obtained by subtracting the TiO~ contribution.

2.3. Catalytic activity tests Catalytic activity tests were carried out with a fixed bed quartz microreactor at atmospheric pressure. The catalyst (particle size = 300-400~m) was placed on the porous septum of the reactor. In order to limit the occurrence of homogeneous reactions a-A120~ particles were loaded before the catalytic bed and the reactor diameter was reduced after the catalytic bed. The temperature was monitored by a type K thermocouple located in the catalyst bed. The reactor outlet gas was analyzed with a H a r t m a n n & Braun URAS 10 E electrochemical/IR continuos photometer for 02, CO and CO2 and with a Hewlett Packard 5890A gaschromatograph equipped with a flame ionization detector for C2H6 and C2Ha. Water produced by the reaction was kept by a silica gel trap in order to avoid condensation in the cold parts of the apparatus. The contact time ranged from 0.001 to 1 g s cm ~ depending on the different activity of the catalysts. The reaction temperature was 550~ The feed composition was 2% 02 and 4% C2H6 in helium. Carbon balance was always closed to within + 2%.

3. R E S U L T S AND D I S C U S S I O N By assuming a monolayer capacity of 15.3 wt% V20~ [14] and of 17.5 wt% Nb205 [15] with reference to the surface area of the support and considering that the surface coverage in the ternary catalysts is the sum of V20~ and Nb20~ single coverages, it can be concluded that the theoretical monolayer capacity was never exceeded even in the 6V6Nb/Ti catalyst. However, traces of T or TT Nb205 phases [9] were detected in this sample by XRD suggesting that some aggregation occurs when the coverage approaches the monolayer. No peaks due to TiO2 rutile phase were observed in the XRD spectra of the catalysts indicating that the calcination temperature is low enough with respect to the phase transition from anatase.

288 For all the catalysts a quite negligible loss of surface area with respect to TiO2 was observed, the values ranging between 111 and 105 m2g1.

3.1. TPR e x p e r i m e n t s The results of TPR experiments are reported in Table 1. For all the samples no significant effects of H2 reduction were observed below 330~ Similar results were obtained by Topsoe et al. [ 16] on vanadia-titania catalysts. The TiO2 support and the 6Nb/Ti catalyst undergo a very poor reduction compared to that occurring when vanadium is present in the catalyst. The TiO2 support shows a tailed peak with maximum at 554~ while the addition of niobium oxide results in the appearance of a new peak at 609~ For both V/Ti catalysts the onset and the peak temperature of reduction are lower than those of pure TiO2. The reduction of the support is still clearly detected as a distinct peak when the V20~ content is 1 wt%, whereas it is completely hidden by the more intense reduction peak of vanadium oxide at the larger V20~ content. The value of V/I-I2 ratio of about 1 suggests the reduction of vanadium from a +5 to a +3 average oxidation state probably occurring in a single step. Values of V/H2 approaching 1 were also found by Went et al. [17] for vanadia-titania catalysts and by Blasco et al. [18] for V20~ on different supports. The presence of niobium in the ternary catalysts results in shifting the peak temperatures of H2 uptake to higher values, if compared to those of the corresponding Vfl~ catalysts. The temperature shift is greater at low than at high vanadium loading, indicating a different effect of niobium on the redox properties of the ternary catalysts. Moreover, the shift is smaller for the samples impregnated in two steps with respect to the single step impregnation. The values of V/H2 are somewhat affected by the presence of niobium, but it is d~f-ficult to quantify the contribution due to the reduction of the support, especially at low vanadium loading. 3.2. TPD experiments. No ammonia oxidation products were detected during the TPD experiments, except for a very small amount of N2 observed at T>500~ with the vanadium containing catalysts, likely due to reaction with catalyst surface oxygen. This is in agreement with the easy reducibility of vanadia-titania samples found in TPR experiments. Lietti and Forzatti [19] observed ammonia oxidation at high temperature on V20~/TiO2 but on pure TiO2. In Figure 1 NH~ TPD profiles are reported. The tailed TPD peak with maximum at 190~ indicates that ammonia is desorbed from the TiO2 surface in a wide range of temperatures (100-650~ due to the presence of different adsorbed species as also found in FT-IR studies [20]. The addition of V20~ or Nb205 to TiO2 results in the modification of the ammonia desorption profile of the support, strongly depending on the metal oxide. Niobium oxide causes a slight decrease of NH~ adsorption with respect to titania only in the range 180-350~ At lower and higher temperature the two profiles are totally superimposed suggesting that the nature of acid sites on TiO2 and 6Nb/Ti catalyst is the same, probably of Lewis type as proposed by Pittman and Bell [21].

289

r,.) o 1.5-

~

o ~ l . 0

oo

-1.5

, , 6V6Nb/Ti [\/ . 6V/Ti

1V6Nb/Ti

o

,,/1V/Ti

o

9

-

1

-

1.0

~"

oo

o

o

=9 0.5 -

-

a,

0.5

o

=

o

o

r~

o'3

~ 0 . 0

-

Z

0

I

I

I

200

400

600

0

I

I

[

200

400

600

-0.0

Z

Temperature (~

Temperature (~

Figure 1. NHa TPD of TiO2, 1V/Ti, 1V6Nb/Ti, 6Nb/Ti, 6V/Ti and 6V6Nb/Ti.

Table 1 H2 u p t a k e and peak t e m p e r a t u r e in TPR experiments and NHs desorbed in TPD experiments. Tmax V/H2 ratio NHa desorbed Catalyst H2 uptake (10 .4 mol -g-') (~ (~mol .m 2) , TiO2 0.6 554 3.5 1V/Ti

1.1

496

1.0

3.3

1V6Nb/Ti

1.3

539

0.8

3.7

1V6Nb/Ti (I)

1.6

521

0.7

3.7

1V6Nb/Ti (II)

1.3

523

0.8

3.8

6V/Ti

6.1

516

1.1

2.7

6V6Nb/Ti

6.5

539

1.0

3.3

6V6Nb/Ti (I)

5.6

532

1.2

3.5

6V6Nb/Ti (II)

5.6

526

1.2

3.8

6Nb/Ti

1.1

609

3.4

Differently, the change of the NHa TPD profile of Ti02 due to the presence of v a n a d i u m suggests t h a t medium and high temperature sites of the support are preferentially covered with lower acid strength sites. This is indicated by the appearence of a sharp peak with m a x i m u m in the range 110-155~ shown by all

290 the vanadium containing samples and by the absence of any signal of NH3 at T > 500~ for the high vanadium content catalysts. It was reported [16] that at low loading vanadia interacts preferentially with the most basic hych'oxyl groups present on the titania surface. At high vanadium loading, most of Ti-OH groups are replaced by new Bronsted acid sites which give rise to a NH~ band whose intensity increases with the vanadium content. This result is in agreement with the substitution of strong Lewis acid sites of TiO2 with weaker Bronsted acid sites due to V-OH groups observed in the vanadium containing samples. The TPD profiles of ternary catalysts are also different from those of binary catalysts, showing that the presence of niobium results in a different distribution of acid sites, especially at low vanadium loading. However, both xV6Nb/Ti (I) and (II) give rise to a lower temperature signal. It is noteworthing that the presence of niobium oxide enhances the acidity of the bynary catalysts, more strongly at high vanadium loading. The amount of desorbed NH3 from the ternary catalysts is slightly affected by the preparation method, especially at high vanadium loading (Table 1).

3.3. Laser-Raman spectroscopy. In Figure 2 Laser-Raman spectra of 6Nb, 1V/Ti, 6V/Ti, 1V6Nb/Ti and 6V6Nb/Ti catalysts are reported. The spectrum of 6Nb/Ti shows a narrow peak at ca. 990 cm 1 which can be attributed to the double bond Nb=O of both tetrahedral and octahedral NbOx species [21]. A broad band, with lower intensity and centred at ca. 920 cm -1, probably due to Nb-O-Nb bridges in octahedrally coordinated species, is also present [21]. 6V/Ti catalyst shows a narrow peak at 1030 cm 1 due to the double bond V=O and a broad band at 915-920 cm 1 attributed to V-O-V bridges in polycondensed species [17]. The band at 915-920 cm -1 is absent in the spectrum of 1V/Ti sample, where tetrahedral isolated species prevail. The spectrum of 6V6Nb/Ti catalyst shows no remarkable difference from that of 6V/Ti; moreover there is no evidence of a contribution at 990 cm 1 related to Nb=O bonds. However, in the spectrum of 1V6Nb/Ti catalyst a broad signal at ca. 990 cm -1 indicates the presence of Nb=O bond. Its intensity is negligible if compared to the corresponding signal in the spectrum of 6Nb/Ti. The disappearence of the Nb=O signal in the ternary catalysts could suggest an interaction between the two oxide phases. The formation of V-O-Nb-O-V bridges can be suggested, or the grafting of vanadium onto niobium oxide phase can be hypothesized.

3.4 Catalytic activity tests The possible occurrence of homogeneous reactions was tested by performing experiments in the absence of catalyst under the same reaction conditions of the catalytic tests. No ethane conversion was observed up to 700~ In the activity tests the oxygen conversion was kept always <100% and the temperature increase, due to the exothermal reactions, was negligible. All the catalysts produce C2H4, CO and CO2.

291

6NbtTi

1VITi 6V/Ti - - _

.

_

~

~ 12~)o

11()0

1000

6V6Nb/Ti

901)

80'0

Wavenumber (cm -1) Figure 2. Laser-Raman spectra of 6Nb, 1V/Ti, 6V/Ti, 1V6Nb/Ti and 6V6Nb/Ti catalysts.

In Table 2 the results of catalytic tests are reported for ethane conversions of 10-15%. It is noteworthing that the support is able to activate the reaction with a quite good selectivity to ethylene (64%). The addition of niobium oxide improves the selectivity slightly depressing the original activity of TiO2. On the other hand, the addition of vanadium oxide strongly enhances the activity of TiO2 in the whole range of compositions investigated as shown by the much lower contact time required to obtain the same conversion levels. At the higher vanadium loading a decreased selectivity to ethylene (37%) is observed. In the ternary catalysts the presence of niobium increases the selectivity to ethylene only at low vanadium content, maintaining a comparable activity with respect to 1V/Ti. At higher vanadium content both activity and selectivity to ethylene are only slightly affected by the presence of niobium. In Figure 3 selectivity to ethylene is reported as a function of ethane conversion over TiO2, 6Nb/Ti, xV/Ti and xV6Nb/Ti catalysts. The most selective catalyst is 6Nb/Ti; the high selectivity to C2H4 exhibited by TiO2 is lowered by the increasing addition of vanadia. Moreover, the addition of niobium oxide to 1V/Ti has a promoting effect on the selectivity to ethylene, whereas the addition to 6V/Ti does not result in any marked change of selectivity.

292 Table 2 Results of catalytic activity tests of ethane ODH (T-550_~ Catalyst

W/F

X02

(g s cm -3) (%)

Xc2H6

SC2H4

(%)

(%) .

.

.

.

.

.

.

.

Sco

8c02

(%)

(%)

.

.

.

.

.

TiO2

0.34

24

11

64

33

3

6Nb/Ti

0.60

20

12

73

25

8

1V/Ti

0.22

45

13

44

46

10

1V6Nb/Ti

0.14

24

10

56

41

3

1V6Nb/~ (I)

0.30

33

12

51

44

5

1V6Nb/Ti (II)

0.20

35

12

53

45

2

6V/Ti

0.01

35

12

37

60

3

6V6Nb/Ti

0.01

42

12

35

61

4

6V6Nb/Ti (I)

0.01

41

12

37

59

4

6V6Nb/Ti ~II~

0.01

42

14

39

.... 59

2

It must be remembered that the results of catalyst characterization give evidence for a different effect of niobium depending on the vanadium content. In fact (Table 1) at low vanadium content the redox properties are much more affected by the presence of niobium with respect to the catalysts with high vanadium content. In contrast, a lower effect on the acidic properties has been found at low vanadium with respect to high vanadium content. The Raman spectrum of 1V6Nb/Ti seems to indicate that Nb interacts with V which is dispersed as isolated tetrahedral species at low loading. In contrast, negligible effects on the catalytic performance are caused by the addition of niobium to 6V/Ti, in agreement with the smaller effect observed with respect to the redox properties. These results should indicate that the effect of niobium on the catalytic properties of vanadia-titania catalysts depends on the nature of the VOx surface species. In Figure 4 the effect of the preparation method on the selectivity to ethylene is shown. The selectivity of 1V6Nb/Ti is only slightly higher than that of the corresponding samples prepared in two steps, confirming the results of Table 2. On the contrary, at high vanadium loading the catalysts of type (I) and (II) are more selective than 6V6Nb/Ti, but less selective than the 1V6Nb/Ti series. This supports the hypothesis that the nature of vanadium oxide species is a key factor to provide high selectivity to ethylene in ethane ODH.

293

Figure 3. C2H4 selectivity as a function of C2H6 conversion at 550~ 6Nb/Ti (0), 1V/Ti (A), 6V/Ti (El), 1V6Nb/Ti (A) and 6V6Nb/Ti (u).

on TiO2 (o),

Figure 4. C2H4 selectivity as a function of C2H~ conversion at 550~ on 1V6Nb/Ti (o), 1V6Nb/Ti (I) (E]),IV6Nb/Ti (II) (A), 6V6Nb/Ti (o), 6V6Nb/Ti (I) (u) and 6V6Nb/Ti (II) (A).

294 3. CONCLUSIONS Catalytic performances of VOx/TiO2 systems in ethane ODH are improved by the addition of niobium. When TiO2 is coimpregnated by vanadium and niobium oxides, the presence of niobium enhances the selectivity to ethylene at low vanadium content, whereas it slightly depresses the activity without enhancing the selectivity at high vanadium content. This should be due to the effect of niobium on vanadium reducibility, especially affected at low vanadium content. The interaction between the two supported oxides can be modified by changing the preparation technique. By reversing the order of addition of vanadium and niobium at high V20~ loading, catalysts having better performances can be obtained, while at low loading no effect has been found. This difference is likely due to the different nature of vanadium oxide supported species. The catalytic performances are also changed as a result of catalyst acidity modifications induced by the presence of niobium. REFERENCES

1. 2. 3 4. 5 6. 7. 8 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

F. Cavani and F. Trifir6, Catal. Today., 24 (1995) 307. E.A. Mamedov and V. Cort6s Corber~n, Appl. Catal., 127 (1995) 1. E. Morales and J.H. Lunsford, J. Catal., 118 (1989) 255. R. Burch and S.C. Tsang, Appl. Catal., 65 (1990) 259. E.M. Kennedy and N.W. Cant, Appl. Catal., 75 (1991) 321. K. Tanabe and S. Okazaki, Appl. Catal. A: General, 133 (1995) 191. R. Butch and R. Swarnakar, Appl. Catal., 70 (1991) 129. J. Le Bars, J.C. V6drine, A. Auroux, S. Trautmann and M. Baerns, Appl. Catal. A: General, 88 (1992) 179. R.H.H. Smits, K.Seshan and J.R.H. Ross, Studies Surf. Sci. Catal., 72 (1992) 221. R.H.H. Smits, K. Seshan, H. Leemreize and J.R.H. Ross, Catal. Today, 16 (1993) 513. J. Le Bars, A. Auroux, M. Forissier and J.C. V6drine, J. Catal., 162 (1996) 250. A. Erdohely and F. Solymosi, J. Catal., 123 (1990) 31. J.G. Eon, R. Olier and J.C. Volta, J. Catal., 145 (1994) 318. M.Roy, M. Gubelmann-Bonneau, H. Ponceblanc and J.C. Volta, Catal. Lett., 42 (1996) 93. J.-M. Jehng and I.E Wachs, J. Phys. Chem., 95 (1991) 7373. N.-Y. Topsoe, H. Topsoe and J.A. Dumesic, J. Catal., 151 (1995) 226. G.T. Went, L.-J. Leu and A.T. Bell, J. Catal., 134 (1992) 479. T. Blasco, J.M. L6pez-Nieto, A. Dejoz and M.I. V~squez, J. Catal., 157 (1995) 271. L. Lietti and P. Forzatti, J. Catal., 147 (1994) 241. G. Ramis, G. Busca, V. Lorenzelli and P. Forzatti, Appl. Catal., 64 (1990) 243. R.M. Pittman and A.T. Bell, Catal. Lett., 24 (1994) 1.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

P a r t i a l O x i d a t i o n of E t h a n e over M o n o l a y e r s of V a n a d i u m Effect of t h e S u p p o r t a n d S u r f a c e C o v e r a g e .

295

Oxide.

Miguel A. Bafiares a, Xingtao Gaob, Jos~ L. G. Fierro a, and Israel E. Wachs b aInstituto de Cat~lisis y Petroleoquimica, CSIC. Campus Cantoblanco, E-28049, Spain bZettlemoyer Center for Surface Studies, Departments of Chemistry and Chemical Engineering, Lehigh University, Bethlehem, PA-18015, USA

The nature of supported oxides and of the support plays a critical role in the partial oxidation of hydrocarbons since the support is not only providing a high surface area, but also dispersing the oxide. The interaction between the metal oxide overlayer and the underlying support similarly determines the performance of the catalyst, which may also be affected by the exposed sites of the support. To fully understand these effects, a series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. The monolayer coverage was determined by Raman spectroscopy and X -ray photoelectron spectroscopy. The activity of the supported vanadium oxide catalysts is determined by the specific support and surface vanadia coverage.

1. INTRODUCTION Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective catalytic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane.

296

2. EXPERIMENTAL 2.1. S y n t h e s i s The oxide supports employed in the present study were: SiO2 (Cabot), A1203 (Engelhard), CeO2 (Engelhard), TiO2 (Degussa), ZrO2 (Degussa), and Nb205 (Niobium Products Co.). All supports were pretreated at 773 K overnight. The silica support was also treated with water vapor, this support will be referred to as SiO2-H20. The catalysts were prepared by incipient wetness impregnation with V-isopropanol in a glove box with nitrogen flow. The impregnated samples were kept at room temperature overnight in the glove box. Then, the samples were dried at 393 K for 1 h and at 573 K another 1 h with nitrogen flow. Finally, the samples were calcined at 573 K for 1 h and 723 K for 2 h in flowing oxygen. The catalysts were prepared with several vanadium oxide loading ranging from very low surface vanadium coverage to the presence of crystalline V205. The catalysts are referred to as "xVS", where "x ~ represents the weight percent of V205 and "S" stands for the element of the specific oxide support. 2.2. Characterization The vanadium content in the catalysts was determined with a PerkinElmer Mod. 3030 Atomic Absorption Spectrometer. The surface areas of the catalysts and of the corresponding supports were determined by nitrogen adsorption/desorption isotherms on a Micromeritics Mod. 2000 ASAP. X Ray photoelectron (XPS) spectra were acquired with a Fisons ESCALAB 200R electron spectrometer equipped with a hemispherical electron analyzer and an MgKa X-ray source (h.v = 1253.6 eV) powered at 120 watts. A PDP 11/05 computer from DEC was used for collecting and analyzing the spectra. The samples were placed in small copper cylinders and mounted in a transfer rod placed in the pretreatment chamber of the instrument. The base pressure in the ion-pumped analysis chamber was maintained below 5 x 10-9 Torr during data acquisition. The spectra were collected for 30 to 100 min at a pass energy of 10 eV (1 eV = 1.602 x 10 -19 J), which is typical of high resolution conditions. The intensities were estimated by calculating the integral of each peak after smoothing and subtraction of the "S-shaped" background and fitting the experimental curve to a combination of Gaussian and Lorentzian distributions, the G/L proportion of which varied in the range 5-27%. All binding energies (BE) were referenced to the support cation, giving values with an accuracy of + 0.2 eV. The molecular structures of the surface vanadium oxide species on the different supports were examined with Raman spectroscopy. The Raman spectrometer system possessed a Spectra-Physics Ar + laser (model 2020-05) tuned to the exciting line at 514.5 nm. The radiation intensity at the samples was varied from 10 to 70 mW. The scattered radiation was passed through a Spex Triplemate spectrometer (Model 1877) coupled to a Princeton Applied Research OMA III optical multichannel analyzer (Model 1463) with an intensified photo diode array cooled to 233 K. Slit widths ranged from 60 to 550]~m. The overall resolution was better than 2 cm -1. For the in situ Raman spectra of dehydrated samples, a pressed wafer was placed into a stationary sample holder that was installed in an in situ cell. Spectra were recorded in flowing oxygen at room temperature after the samples were dehydrated in flowing oxygen at 573 K.

297 2.3. E t h a n e o x i d a t i o n The catalysts (20 mg) were tested for the partial oxidation of ethane with oxygen at atmospheric pressure in the temperature range 760-880 K. The reactor consisted of a quartz tube of 6 mm o.d. (4 mm i.d.), where no void volume was permitted to avoid homogenous reaction from the gas phase. The O2/C2H6 molar ratio was 2 and He/O2 molar ratio was 4. The gas feed was controlled by means of mass flow controllers (Brooks). The total flow range was 15-60 mL/min. The reactor effluent was analyzed by an on-line Hewlett-Packard Gas Chromatograph 5890 Series-II fitted with a thermal conductivity detector. Chromosorb 107 and Molecular Sieve 5A packed columns were used with a column isolation analysis system. The TOF (turnover frequencies, number of ethane molecules converted per surface vanadia species per second) were calculated assuming that all the supported vanadium oxide is active, in agreement with the absence of tridimensional aggregates of vanadium oxide (100 % dispersion). 3. R E S U L T S 3.1. C h a r a c t e r i z a t i o n . The oxide supports used have the surface areas reported in Table 1 and a wide range of vanadium oxide loading on the supports has been prepared. The values presented in Table 1 correspond to monolayer coverage of vanadium oxide. The monolayer coverage can not be determined by theoretical calculation based on the coverage per VOx unit (2), since the dispersion requires an interaction with the support and the monolayer coverage does not only rely on the surface area of the support, but also on its chemical nature. The monolayer coverage of surface vanadium oxide has been determined by Raman spectroscopy and XPS. The low monolayer coverage for the silica support is due to the low surface density of hydroxyl groups. The highly dispersed surface vanadium oxide species are characterized by a Raman band at ca. 1030 cm -1 (Table 1), characteristic of a highly dispersed surface vanadium oxide species. At higher vanadia loadings, crystalline V205 species dominate (strong Raman band at 994 cm-1), and present a significantly different spectrum. The supported vanadium oxide sample with the highest vanadium oxide loading before the onset of crystalline vanadia corresponds to the monolayer catalyst. A parallel characterization has also been performed by XPS since the V/Support atomic ratio determined by XPS is very

Table 1 Characterization of the supports and of the monolaser catalysts Support Surface Area Catalyst %V205 Surface Area Vatoms V=O band (cm -1) BET (m2/g) (wt %) BET (m2/g) per nm 2 1039 SiO2 337 12VSi 11,0 247 2,1 1039 SiO2-H20 332 12VSi-H20 11,7 254 2,3 1026 A1203 222 25VA1 29,9 169 13,4 1028 CeO2 36 4VCe 4,8 23 7,6 1030 TiO2 45 6VTi 6,7 47 9,4 1030 ZrO2 34 4VZr 3,0 31 8,2 1031 Nl~O5 57 5VNb 6,1 35 6,1 1036 5TiSi 280 10V5TiSi ca. 10 . . . .

298

0,4

225, A

o

0,3

I

~A

200

9 VA1

I

mi D

, A VTi A', ,' monolayer

'N

0,2

,

A1

,

!

I |

r

175

~

150~,,

~

152

ca

48

!

0,1

9

l 0 _~' 9149 ' 0 20

% V205

I I I I I I I I

$ !

B

l

!

o

monolayer

~

II

i.v l

I

A VTi

, monolayer 4A I

l

44 40

I

0

I

I

20

40

% V20 5

Figure 1. XPS V/Support atomic ratio (A) and Surface area (BET) oxide loading of the representative series VTi and VA1.

vs.

vanadium

sensitive to the dispersion degree of the surface vanadium oxide species. Figure 1 presents the XPS V/Support atomic ratio determined for dehydrated samples on two representative series (v205friO2 and V205/A1203). The V/Support atomic ratio increases linearly with the vanadium oxide loading and then levels off. This plateau corresponds to the formation of tridimensional aggregates of vanadium oxide, which Raman spectroscopy identifies as crystalline V205. The addition of vanadium oxide to the support continuously decreases the surface area. Close to the monolayer coverage, the surface vanadium oxide species show some polymerization (3,4) as evidenced by the Raman features observed at 920, 800, 600, and 550 cm -1 (8). 3.2. Ethane

oxidation

The oxidation of ethane over the supported vanadium oxide catalysts yields ethylene, CO, CO2, and minor amounts of acetaldehyde and formaldehyde. Methane production was not observed. The initial ethane conversions and TOF's for the catalysts with monolayer coverage of surface vanadium oxide are presented in Figure 2. There are significant differences in the activity of the different monolayer catalysts, where vanadium loading corresponds to the monolayer coverage. The treatment with water appears to increase the ethane conversion on silica-supported vanadium oxide monolayer catalysts. The TiO2 and ZrO2 supports result in the highest TOF. The catalysts with high activity deactivate also after few hours on stream, and the non-selective oxidation products are dominant. The selectivity-conversion plots are presented in Figure 3. Formaldehyde is observed at low conversion for all the catalysts but acetaldehyde can only be observed at low conversions on silica-supported catalysts.

299 0,10

8O 60 0

*,'=4

~J

005 ~

~" 40 o

~

9 20 0,00

0 12VSi- 12VSi H20

4VCe

5VNb

25VA1 4VZr

6VTi

Figure 2. Conversion of ethane, absolute (columns) and T.O.F. numbers (circles) of the catalyst. Total flow 30 mL/min. Reaction temperature 823 K. W = 20 mg. O2/C2H6 = 2 molar and He/O2 = 4 molar. The most active catalysts (6VTi, 4VZr, and 25VA1) show very high selectivity to deep oxidation, mainly CO, which increases at the expense of ethylene with ethane conversion. CO2 is also present and its selectivity increases with conversion too. The catalyst 4VCe and 5VNb catalysts show an important trend to non-selective oxidation at low conversion levels. The selectivity to ethylene decreases markedly with conversion for 4VCe, and this trend is much smoother for 5VNb. The TOF values for the alumina series evidences, that at the surface monolayer coverage of vanadium oxide, the vanadia sites behave differently: nonselective oxidation is dominant and the TOF of ethane and ethylene decreases (Figure 4). The continuous increase in oxygen TOF corresponds to the lower selectivity at vanadium oxide monolayer coverage on alumina. The titanium oxide supported vanadia, on the contrary, shows a more selective activity than alumina. The TOF's of ethylene are higher at submonolayer coverage (1VTi and 3VTi) and decrease at vanadia monolayer coverage. The TOF profiles show increase of CO and CO2 with vanadia coverage but a significant decrease is observed for ethylene. These trends suggest a change in the environment of the vanadia sites as the surface coverage increases. The high activity of titania-supported vanadium oxide and the selectivity of silica-supported vanadia suggests that a ternary catalyst (V205-TiO2-SiO2) may possess positive characteristics. Vanadium oxide tends to preferentially coordinate to titania sites for the titania-silica supports (9). For this reason, the titania-silica support has been prepared with a highly dispersed titanium oxide surface species, strongly interacting with silica support, as determined by Raman spectroscopy and XPS (10). The activity of vanadium oxide on a highly dispersed titanium oxide surface species on silica is compared with the monolayers of

300 80

80

60

1 VSi

60

40

40

20

20

~"

~

9

0 80 60

A

9

-

=

0 80

~

25VA1

40

60

4VCe

4O

0 80

'

0

'

~'--F

m

....

mm

40

40

c~ 20

20

0 8O

0

I

20

40

9 CO A CO2

20

o C2H4 20

I

!

40

60

40

6O

80

% C2H6 Conversion

5VNb

0

I

4VZr

0

0

'

60

6VTi

60

'

80

x

60 ~J

.

O HCHO n CH3CHO

80

% C2H 6 Conversion Figure 3. Selectivity conversion plots for the monolayers of vanadium oxide on the different supports. Reaction conditions: Total flow 30 mL/min. Reaction temperature 740-883 K. W = 20 mg. O2/C2H6 = 2 molar and He/O2 = 4 molar. vanadium oxide on silica and on titania in Table 2. As previously mentioned, the 6VTi catalyst is the most active system (highest TOF), the temperature to reach 15 % conversion of ethane is very low (750 K), but CO is the major oxidation product. The activities of the 12VSi and 10V5TiSi catalysts are similar but the selectivity to ethylene is higher for 10V5TiSi, and CO2 is not produced for this catalyst.

301

Figure 4. TOF numbers for the oxidation of ethane. Reaction conditions as in Figure 2.

Table 2 Selectivity of vanadium monolaser on silica and titania and titania-silica Temp. Conversion % Selectivity Catalyst (K) (mole%) CO CO2 C2H4 HCHO CH3CHO 25VA1 730 15.0 53.2 2.2 44.1 0.2 0.0 4VZr 794 15.0 55.4 5.4 39.1 0.1 0.0 4VCe 891 15.0 54.6 10.0 35.3 0.2 0.0 5VNb 908 15.0 51.1 8.9 39.7 0.3 0.0 12VSi-H20 838 15.0 32.8 7.6 54.6 2.9 2.1 12VSi 847 15.0 31.7 8.3 55.9 2.9 1.1 6VTi 750 15.0 60.5 5.6 33.8 0.1 0.0 10V5TiSi 843 15.0 41.3 0.0 58.2 0.4 0.0 Reaction conditions as in Figure 2.

302 4. D I S C U S S I O N

The dehydrated surface vanadium oxide species on the different supports possess the same structure as previous studies have already shown by Raman and 51V-NMR studies (3,11-13). Surface vanadium oxide species are present as isolated VO units containing one terminal V=O bond and three bridging V-OSupport bonds. Polymeric surface vanadium oxides are also present and their concentration increases with surface vanadia coverage. These species have one terminal V=O bond and possess bridging V-O-V and V-O-Support bonds. The linear increase of the XPS Vfri atomic ratio is consistent with the dispersed nature of the surface vanadium oxide species at low coverage. The alumina supported series, exhibits a lower V/A1 atomic XPS ratio at low vanadium oxide loadings, due to the higher surface area and porosity of the alumina support. The V/A1 atomic XPS ratio is also linear with vanadia loading and supports the highly dispersed nature of surface vanadium oxide species on alumina at coverages below the monolayer. Water treatments of the silica support did not show appreciable differences between the two silica supported vanadium oxide catalysts since both catalysts perform very similar during the oxidation of ethane. Only higher activity is observed for the 12VSi-H20. The selectivity conversion trends suggests that ethane is initially oxidized to ethylene and that ethylene is further oxidized to CO. CO2 could be a primary product, since its selectivity at zero conversion limit does no appear to be zero. The most important differences observed in catalytic behavior results from an interaction of the support with the active phase. For the same reaction conditions, the TOF's differ by more than an order of magnitude for the different catalysts. The changes in TOF's do not correspond with the changes in the terminal V=O Raman bands. A similar result has also been observed for the oxidation of methanol (14) and butane (8). Consequently, the active oxygen must be the bridging oxygen. At monolayer coverage, both V-O-V and V-O-Support bonds are present. Both may play a role in the reaction. The more reducible oxide titania and zirconia yield the most active catalysts (higher TOF's). Ceria is also a reducible oxide, which makes the supported vanadium oxide species more reducible than on alumina or silica, like titania and ceria (1,15), but TOF's on 4VCe are very low. Acidic supports, alumina and niobia, show some moderate activity and the non-acidic, non-reducible silica yields the lowest TOF's. Consequently, the activity of supported vanadium oxide for the oxidation of ethane follows the trends TiO2 N ZrO2 > A1203 > ND205 > SiO2. The low activity of 4VCe and 5VNb catalysts, despite their reducibility (4VCe) and acidity (5VNb) may be due to structural transformations by reaction of vanadia with the underlying oxide at the high temperatures required for ethane oxidation. Concerning selectivity, the more reducible oxide support systems show a high selectivity to deep oxidation (CO). 4VCe shows high selectivity to CO at low ethane conversion. The acidic supports, alumina and niobia, also yield CO as the main oxidation products. Only silica-supported vanadium oxide shows higher selectivites for ethylene. Acetaldehyde and formaldehyde are also produced son 12VSi and 12VSi-H20. The relevance of V-O-V bonds can be evaluated for the performance of V205/A1203 and V205friO2 at different surface coverages. Alumina supported vanadium oxide shows increasing TOF numbers for oxygen, CO and CO2 as

303 vanadium oxide loading increases up to monolayer coverage. At monolayer coverage, where the (V-O-V) / (V-O-Support) ratio is expected to be highest, the TOF's of ethane and ethylene decrease, but TOF of oxygen, CO and CO2 increase. This could be indicative of the higher reducibility of surface polymeric vanadium oxide species with respect to isolated surface vanadium oxide species (4,8), which appears to lead to a less active and selective catalyst. A similar trend is observed for VTi series: at monolayer coverage, ethane and ethylene TOF numbers decrease. For the titania-supported vanadium oxide catalysts, the TOF's for oxygen, CO and CO2 do not increase at vanadia monolayer coverage as in the case of the VA1 series. On the contrary, they decrease slightly, but isolated surface vanadium oxide species on titania are more reducible than isolated surface vanadium oxide species on alumina. This may account for the higher TOF(oxygen)/TOF(ethane) ratio observed on the VTi series. This ratio becomes closer for VTi and VA1 series at monolayer coverage, where both series are expected to show s higher reducibility of the surface vanadium oxide species. The ternary V205/TiO2-SiO2 catalyst shows interesting structural and catalytic properties. Surface vanadium oxide species preferentially coordinate to titania sites in the TiO2/SiO2 supports (8). However, the use of a titania-silica support prepared so that titanium oxide is highly dispersed and strongly interacting with silica support results in titania with different characteristics to pure titania. The titania-silica support used here has 20% of the titanium atoms in tetrahedral coordination as determined by XPS and no crystalline aggregates of titania are formed, as determined by Raman spectroscopy (10). The V=O mode observed for the dehydrated 10V5TiSi catalyst is at 1036 cm -1, much closer to that of silica-supported vanadium oxide than to that of titanium-supported vanadium oxide (Table 1). The surface vanadium oxide species are isolated (100 % dispersion) and must also have a different coordination environment (probably anchored on both, titania and silica sites) that yields an activity similar to that on 12VSi but more selective, since no CO2 is formed and the selectivity of ethylene increases. The lower selectivity of oxygen -containing products suggest that vanadia species on the highly dispersed titania-on-silica supports may be less reducible than on the pure constituting oxide supports. 5. C O N C L U S I O N S The surface vanadium oxide species on silica, water-treated silica, alumina, ceria, titania, zirconia, niobia and titania-silica have been characterized and studied for the selective oxidation of ethane. The terminal V=O bond does not appear to be directly involved in the reaction (no correlation with TOF). However, the bridging V-O-V or V-O-Support bonds appear to critical for the oxidation of ethane. The nature of the V-OSupport bond is determined by the specific support. Bonding to a reducible support metal ion yields active catalysts (e.g. 6VTi and 4VZr). Acidic supports show some activity, but much lower than the reducible ones. The silica support is not reducible and does not possess acidic sites and shows the lowest TOF numbers. However, silica-supported vanadium oxide catalysts possess the highest selectivity. The very low activity of 4VCe and 5VNb could originate from a reaction of vanadia with the underlying support. The surface coverage increases

304 the (V-O-V) / (V-O-Support) ratio. Polymeric surface vanadium oxide species are more reducible than isolated surface vanadium oxide species in the presence of butane (15). If we assume a similar trend of reducibility with ethane than with butane, if turns out that more reducible surface vanadium oxide species are less active and selective. This effect is more evident for the VA1 series than for the VTi series, since the isolated surface vanadium oxide species on alumina are much less reducible than on titania. All the catalysts that show higher reducibility, either due to its interaction with the support or due to its surface polymerisation show lower selectivity. The surface vanadium oxide species have a different environment for 10V5TiSi catalyst, which yields an activity similar to that of 12VSi but is more selective. Further research is going on to fully understand the environments of vanadia sites in this catalyst.

ACKNOWLEDGEMENTS This research has been partially funded by the Fundaci6n Caja de Madrid (Spain).

REFERENCES "

.

3. "

5. 6. .

"

.

10. 11. 12. 13. 14. 15.

E. A. Mamedov, and C. Cortes Corberfin, Appl Catal A : General, 127, 1 (1995) G. Bond, and S. Flamerz Tahir, Appl. Catal., 1 (1991) G. Deo, I. E. Wachs, and J. Haber, Critical Reviews in Surface Chemistry 4 (3/4), 141 (1994) I. E. Wachs, and B. M. Wechkhuysen, Appl. Catal. in press (1997) S. T. Oyama, and G. A. Somorjai J. Phys. Chem., 94, 5022 (1990) J. Le Bars, J. C. Vedrine, and A. Auroux, S. Trautmann, and M. Baerns, Appl. Catal tk" General 88, 179 (1992) M. Merzouki, B. Taouk, L. Tessier, E. Bordes, and P. Courtine, in "New Frontiers in Catalysis" (Guczi et al., Eds.), p. 753. Elsevier, Amsterdam, 1993 I. E. Wachs, J.-M. Jehng, G. Deo, B. M. Weckhuysen, V. V. Guliants and J. B. Benziger, Catal. Today, 32, 47 (1996) J. -Mirn Jehng, and I. E. Wachs, Catal. Letter, 13, 9 (1992) X. Gao, M. A. Bafiares, J. L. G. Fierro. and I. E. Wachs, unpublished results G. Busca, Mater. Chem. Phys., 19, 157 (1988) H. Eckerdt, and I. E. Wachs, J. Phys. Chem., 93, 6796 (1989) J. Hanuza, B. Jezowska-Trzebiatowska and W. Oganowski, J. Mol. Catal., 29, 109 (1985) G. Deo, and I. E. Wachs, J. Catal., 146,323 (1994) J. Haber, A. Kozlowska, and R. Kozlowski, J. Catal, 102, 52 (1986)

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

305

The ethane o x i d a t i v e c h l o r i n a t i o n process and efficient catalyst for it M.R. Flid, I.I. Kurlyandskaya, Yu.A. Treger and T.D. Guzhnovskaya Scientific Research Institute "Syntez", 2 ,Ugreshskaya str., P.O. Box 56, Moscow, 109432 Russia Formation of the mixed cement-containing systems within the range of low copper concentrations with addition of alkali metal dopants as well as catalytical properties of these systems in the ethane oxidative chlorination process have been investigated. Based on the obtained data the efficient and stable copper-cement catalyst has been worked out. This catalyst will assist in the development of a new technology of the vinyl chloride production from ethane. The basic parameters of the ethane oxychlorination process have been determined : at 623-673K, time-on-stream 3-5s and reactant ratio of C2H6: HCI: :02 = 1:2:1 the conversion of ethane is more than 90% and the total selectivity to ethylene and vinyl chloride is 85-90%.

1.1ntroduction The gas-phase catalytic process for oxidative chlorination of ethane to vinyl chloride according to overall equation C2H6 + HC1 + 02 = C2H3C1 + 2H20,

(I)

proceeds in two consecutive kinetically independent reactions: (1) the oxidation of hydrogen chloride to chlorine and (2) the chlorination of ethane. This process is promising for developing a rational technology of vinyl chloride production, because ethane utilized in it is a cheap hydrocarbon raw material [ 1,2 ]. The process is conducted at high temperatures, and ethane converts to vinyl chloride due to a combination of consecutive and parallel radical-chain and heterogeneously catalyzed reactions: oxidation, chlorination, and dehydrochlorination. The contributions of homogeneous and heterogeneous reactions to the overall rate of chlorinated hydrocarbon conversion depends on the temperature ranges at which the reaction proceeds. The process as the whole may be represented by the following schematic diagram [3]: CO + C02

C2H6 - -

t

t

C2H5CI~2H4CI2 C2H4 ~

C2H3C1

t

~2H3C13

t

~2H2C14

--~ C2H2C12 --~ C2HC13

CO + CO2

t

~2HCl5 --~ C2C14

--

C2C16 (II)

306 from which it follows that the major products of ethane oxidative chlorination are ethyl chloride, 1,2- and 1,1-dichloroethanes, 1,1,2-trichloroethane, chloroethylenes, and carbon oxides as the products of deep oxidation. At relatively low temperatures (623m723K), the reaction mixture consists mainly of chloroorganic saturated compounds [3-6]. The situation changes dramatically with raising the temperature. Figure 1 demonstrates the effect of the temperature on the oxidative chlorination of ethane over the well-known conventional salt CuC12--KC1/silica gel copper-containing catalyst. 80

1

70

.

'

~

-

"

60,

-~

3

~,9 50

L 0

,'.o

40

2

30 "0

o

5

20 10

4

"'

0 723

773

823

Temperature,K

Figure 1. The effect of temperature on the ethane oxidative chlorination process (silica gel as the support, copper content of 6.0 wt %, potassium content of 4.0 wt %, reactant ratio C2H6 : HC1 : O2= 1 : 1 : 1, x = 3 s). 1 is the conversion of ethane; 2 is the yield of oxidation chlorination products; 3.4, and 5 are the yields of ethylene, deep oxidation products, and vinyl chloride, respectively ( x is time- on-stream ). Thus, in the presence of traditional catalytic systems, the yield of vinyl chloride to converted ethane does not exceed 35%. The total yield of vinyl chloride and ethylene ranges up to 80%. It was shown [3,5,6] for saturated compounds ethane, ethyl chloride, 1,2dichloroethane, and 1,1,1-trichloroethane that the observed conversion rates are satisfactory described by the equation r, = k,. Pi" Pci2 0"5

(1)

The observed rate constant in equation (1) in this case decreases in the order C2H6 > > C2H5C1 > C2H4C12. The activation energies for the transformations of saturated (130 kJ/mol) and unsaturated compounds (40--90 kJ/mol) differ dramatically; as a consequence, the yield of chloroalkenes increases with temperature. Oxidative chlorination of ethane gives rise to considerable amounts of carbon oxides. The overall rate of these side reactions is described by the empirical equation rco + c02 = ki. pi" Po_~"Pcl2~

(2)

Unsaturated compounds make the dominant contribution to formation of carbon oxides. Whereas the introduction of one chlorine atom into ethylene molecule results in a 7m 8-fold increase in the observed rate constant of deep oxidation, the further increase of chlorine content in molecule diminishes the oxidation of chloroalkenes.

307 It is essential that the reactions of saturated compounds exhibit zero orders with respect to both oxygen and hydrogen chloride and proceed kinetically independently of one another. For the unsaturated compounds, the conversion rates represent complex functions of the reaction mixture composition. Under the conditions when the reaction exhibits zero order with respect to hydrogen chloride, the kinetics of unsaturated compounds oxidative chlorination is described by the equation: 2ko2 po2" ki'pi ri

(3)

--

2ko2 "po2+ L-k,.p, where index i relates to unsaturated compounds. The process of ethane oxidative chlorination imposes heavy demands on the catalysts. The conventional salt supported catalysts are composed of Cu, K, Ca, Mn, Co, Fe, Mg, and other metal chlorides containing various additives; these salts are precipitated on alumina, zeolites, silica gel, and other supports. Catalytic systems that represent solid solutions of iron cations in the lattice of the o~-A1203 and a-Cr203 phases doped with cations, such as K, Ba, Ce, and Ag are also known [7]. The activity of the known catalytic systems and, especially, their selectivity to vinyl chloride are insufficient. In addition, the known catalytic systems tend to rapid deactivation because of gumming and carbonization of their surfaces. The main problem that determines the possibility for industrial utilization of the process is the creation of highly efficient, stable, and selective catalytic systems performing at relatively low temperatures. This problem was alleviated due to the development of a new generation of heterogeneous catalysts based on high-alumina cements and intended for the synthesis of chloroorganic compounds, l These catalysts fortunately combine the properties required in industry and genetically intrinsic to cements thermal stability, high mechanical strength, and basicity of the surface, which prevents its carbonization with the possibility of imparting the system special properties desired in a particular process [8]. The mechanism of the ethane oxidative chlorination process is distinguished by the fact that the catalyst accelerates primarily the reactions of hydrogen chloride oxidation and dichloroethane dehydrochlorination. This necessitates the modeling of cement catalytic system with the surface carrying active sites capable of catalyzing both reactions mentioned. The analysis of the known and our own experimental data indicated that the properties required may be offered by a copper-containing cement-based catalytic system modified with alkali metals. In this catalyst, copper-containing active sites catalyze the oxidation of hydrogen chloride, whereas the activity of the catalyst in the dehydrochlorination reaction is determined by the acid--base surface properties, which are inherent to cements with different phase compositions. The development of this catalytic system made it necessary to investigate the formation process of mixed cement systems within the range of low copper concentrations and with addition of alkali dopants and determination of the correlation between properties of the obtained catalytical systems and their activity in the ethane oxychlorination process.

I 'Fhe catalysts based on high-alumina cements were developed in collaboration with Prof. V.I. Yakerson (Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia) and Prof. E.Z. Golozman (Institute of Nitrogen Industry, Novomoskovsk, Russia).

308

2.Experimental. The catalysts were prepared by chemical mixing of high-alumina cements ( technical calcium aluminate- talyum) or cement-based supports (calcium aluminates with the developed surface area and various CaO/A1203 ratio - galyumin or galyumin C) [9] with the sources of copper and alkali metals in water--ammonia or ammonia ---carbonate media; the mixing was followed by the drying and thermal treatment of the samples obtained. A comprehensive study on the formation of cement catalytic systems was performed by X-ray diffraction, thermal analysis, electronic diffuse-reflectance spectroscopy and IRspectroscopy. Table 1 presents characteristics of some of the investigated catalytic systems. Table 1 Characteristics of copper--cement catalysts No

Sample

Support

Preparation

Phase composition

Ssoec,

conditions

without thermal with thermal m2/g treatment treatment at 673K 1 CuO-K20- Galyumin 348K water-gibbsite, C3AH6, KC1, CaCO3, CuO, 130 CaO-A1203 ammonium CaCO3, CHA, CuO, ,/-A1203,C12A7 solution KC1

2 CuO-Cs20- Galyumin 348K CaO-Al203

C

water-- CsC1, CaCO3, CuO, CsC1, CuO, ammonium C3AH6 CaCO3(calcite), solution C12A7, CaCO3 (aragonite)

15

Kinetic measurements were made at 623 - 773K using a circulatory flow installation. Reactions were studied in the fixed bed catalyst. Time - on- stream was varied within the range 1,5 - 10s at a reactant ratio of C2H6: HCI: O 2 = 1:1 +3,3, : 1 +1,4. Air was used as a source of oxygen. The grains of the catalyst were 0,25 - 0,5 mm in size. The gas was fed at a volumetric flow rate of 600 h - ~ . The catalytic systems were preactivated with a hydrogen chloride nitrogen mixture at 573-623 K. The analyses were based on the chemical methods (determination of hydrogen chloride and chlorine) and the gas chromatography.

3.Discussion and results. The stage of chemical mixing of the catalysts preparation involves the hydration of cements with forming C3AH6 (C is CaO, A is A1203, and H is H20), gibbsite, and calcium carboaluminate as well as the exchange processes with forming CaCO3 and copper hydroxoaluminate (CHA). The depth and the rate of hydration as well as the distinctions in the exchange processes are determined by the type of cement-containing agent. The stage of thermal treatment involves the formation of C12A7, ,/-alumina, and solid solution of aluminum and copper oxides, which is followed by the precipitation of the excess of highly dispersed copper oxide and by the formation of copper aluminate spinels with various degrees of disorder.

309 Thus, copper-containing phases can occur both as free oxide and as the forms bound with the matrix of the support; the concentrations of bound forms increase with temperature and with the duration of chemical mixing. The estimation of the depth of interaction revealed that not only the implantation of the Cu 2. ions into the matrix lattice with forming isolated ions is possible, but the formation of small surface clusters (CuO)• with highly covalent Cu--O bonds. The distribution of catalytically active component between the free oxide, clusters, and ions implanted into the matrix lattice depends both on the conditions of formation and on the composition of the catalytic system as well as on the type of cement-containing agent. As it was shown in [ 10], cement-containing matrix exerts a strong modifying effect on the active copper-containing sites. At equal concentrations of the active component, the activity of copper-containing sites incorporated into the copper--cement catalyst is higher than that of the supported salt catalysts. When the concentration of copper and surface concentration of copper-containing sites are decreased, specific catalytic activity of coppercontaining centers sharply increases. So, at an extended specific surface of the copper-cement catalyst, high catalytic activity to the oxidation of hydrogen chloride can be accomplished even at a low concentration of active copper-containing component provided that the latter is bound with the matrix of the support. The surface area of cement catalysts, which carries aluminum- and calcium-containing oxide fragments, exhibits pronounced acid--base properties. These properties can manifest itself as a catalytic activity to the reactions of dehydrochlorination, which proceed via the formation of donor--acceptor complexes between the substrate and acid or base sites at the catalyst surface. The existence of different calcium aluminate phases in the aluminum-calcium catalysts was proved by diffuse-reflectance IR spectroscopy. The presence of these phases is responsible for the complex structure of the catalyst surface. At the surfaces of these catalysts, calcium ions with lower coordination numbers can occur together with the ions octahedrally surrounded by oxygen anions. These ions can act as balance cations in the structure of C12A7, being responsible for the existence of specific terminal hydroxyls and Lewis acid sites bound to calcium. At the surfaces of galyumins, bridging hydroxyls exhibiting somewhat stronger acid properties are present along with terminal hydroxyl groups. The hydration of galyumin surface can supposedly be attended with the weakening of the A1--O--M bond (M = A1 and Ca) resulting in the appearance of additional strong adsorption sites[8]. The enrichment of surface layer in galyumin C with Ca )-+ ions at the increase of CaO/ AL203 ratio is essential for reducing the yield of deep oxidation products and preventing the carbonization of the surface. The data on the state of copper-containing phases and acid--base properties of active sites occurring at the surface of mixed cement systems, which were presented above, enable us to conclude that these catalysts can be employed in the oxidative chlorination of ethane. It ~is known that the chlorination of ethane with chlorine formed in the oxidation of hydrogen chloride proceeds by a heterogeneous--homogeneous mechanism [3]. This is why the efficiency of cement catalysts was studied separately by the examples of Deacon reaction and dichloroethane dehydrochlorination reaction. It was found that for galyumin-based cement system, the variation of copper content within 8--25% (in terms of CuO) virtually does not affect the rate of chlorine formation. For the oxidation of HC1, the rate constant is 1.2.10 -3 mol HC1/g cat.h. This value is comparable with the rate constant of HC1 oxidation in the presence of copper-containing salt catalysts. The

310 introduction of potassium chloride into a copper--cement system results in a 1.5-fold rise of the rate constant for the HC1 oxidation. Thus, the activity of copper--cement catalysts in Deacon reaction is comparable with that of commonly used salt catalysts. Systematic investigations on the performance of cement-containing catalytic systems with various chemical and phase compositions in the reaction of 1,2-dichloroethane dehydrochlorination with forming vinyl chloride C2H4CI 2 -4

C2H3C1 +

HC1

(III)

revealed that the catalytic activity of these catalysts in the process under consideration is high. At the constant composition of the reaction mixture, the maximum reaction rate was accomplished with using a cement system whose specific surface is 130 m2/g. Thus, at 623K and time-on-stream of 3.8 s, the reaction rate was 0.42--0.46 mol of vinyl chloride per litre.hour. This value is more than two times higher than the reaction rate accomplished with using a well-known supported salt catalyst CsC1--SiO2. It was also shown that the presence of copper in cement catalytic systems does not affect the activity of the catalyst in the dehydrochlorination reaction (see Fig. 2).

70 60

"6 ,..

0

50 40

P,

~

~ L

30

C 0

0

20

0

[

523

.

.

.

.

.

r

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

573

' ....

623

" ........

673

Temperature,K

Fig. 2. The conversion of 1,2-dichloroethane in the dehydrochlorination reaction at various catalysts as a function of temperature, z = 8 s; 1- galyumin (Sspec = 130 m2/g), 2- galyumin with a dopant of copper (8 wt % in terms of CuO); 3- CsC1/silica gel. Thus, cement-containing systems provide the conversion of dichloroethane to be increased to more than 70% even at 673K. An important positive factor is that vinyl chloride molecule is stable at this temperature. At 673K, the side reaction of vinyl chloride dehydrochlorination with forming acetylene proceeds slowly, acetylene does not form, and the reaction is not complicated by the formation of a number of by-products, for example, of perchloroethylene. Thus, the above-made supposition about bifunctional character of copper--cement catalytic systems was confirmed in the investigations of their activity in the above-mentioned reactions.

31l The oxidative chlorination of ethane as a whole was studied by using of the cementcontaining catalysts with a specific surface of 130 m2/g (sample 1) and 15 m2/g (sample 2). Copper concentration was kept constant and equal to 8 wt % in terms of CuO (see Table 1). It was found during the investigations that when the temperature was raised from 623 to 773 K, the conversion of ethane somewhat increased, and sample 1 exhibited better activity in comparison with that of sample 2. At the moderate temperatures (623--673K), an extended specific surface of sample 1 was favorable for increasing the yield of target unsaturated compounds: ethylene and vinyl chloride. The further temperature increase led to a decrease in the process selectivity because of a noticeable increase in the yield of deep oxidation products, CO• The effect is more pronounced for sample 1 (see Fig. 3). 100 F

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

...,,-

90

-,--I'"

.

.

.

.

I-'-

""

""

-6

.~

__,a

-----4

80 7O

.__

..... -1 b

......._-

I

6o 5O

t-

40

o

(..)

30 20 ~C

1~ L 0 ,"F:

623

-.

-

--T

,

673

'"'-

. w

.

723

.

.

.

.

_

.

.~C

._~

773

Temperature,K

Fig. 3. The conversion of ethane and the yields of reaction products for catalysts 1 and 2 as functions of temperature. Time-on-stream of 3 s; 1; the reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1;-- - catalyst i ; catalyst 2; a is the conversion of ethane, b is the total yield of ethylene and vinyl chloride to converted ethane, c is the yield of deep oxidation products COx. The dependences shown in Fig. 3 reveal that employing a catalyst with a larger specific surface area with rising temperature would, probably, lead to the deep oxidation of vinyl chloride and, to a lesser extent, of ethylene, resulting in a decrease in the total yield of ethylene and vinyl chloride. A certain increase in the overall yield of CO• products, which was observed for catalyst 2, is accompanied with an increase in the total yield of ethylene and vinyl chloride. This suggests that saturated chlorinated h y d r o c a r b o n s - ethyl chloride and 1,2-dichloroethane m are oxidized predominantly and that the rate of oxidation is lower rate compared to that of the dehydrochlorination of these compounds. Thus, the decrease in specific surface of the catalyst involves a noticeable drop of the yield of deep oxidation products, whereas the yields of vinyl chloride and ethylene remain high. We see little reason in the further cut of the specific surface, because the rate of catalytic dehydrochlorination therewith decreases.

312 The results obtained circumstantially testify that the dehydrochlorination and oxidation reactions proceed at different active sites. It is likely that the oxidation of chlorinated hydrocarbons proceeds at the copper-containing sites. This agrees with the data we obtained in the oxidative chlorination of ethylene [ 11 ]. Taking into account the fact that the value of specific surface is a crucial factor in the choice of catalyst, the further investigations we conducted with using catalyst 2. Both the time-on-stream and the reactant ratio are important chemical engineering parameters affecting the characteristics of the process. It was found that the increase in the time-on-stream at T = 673K can improve both the conversion of ethane and the yield of ethylene. The total yield of chloroorganic products therewith decreases, but the concentration of vinyl chloride passes through a maximum. We also observed an increase in the yield of deep oxidation products COx (see Table 2). Table 2 The effect of time-on-stream on the oxidative chlorination of ethane Catalyst- 8 wt % CuO/cement; T = 673K; reactant ratio C2H6 : HC1 : O2 = 1 : 2 : 1. No.

~, s

Reactant conversion, % C2H6

HC1

O2

Yields scaled to converted ethane, % C2H4C12

C2H3C1

C2H4

COx

1

1.5

80.6

36.0

91.2

25.0

34.1

34.6

2.6

2

3.2

87.5

31.7

89.5

19.4

36.8

40.1

3.2

3

5.6

89.1

30.4

88.6

11.3

38.2

43.5

4.0

4

7.9

90.9

30.0

87.9

10.1

35.6

46.8

6.5

5

10.0

92.7

29.1

86.0

8.2

33.0

47.6

9.7

We can suppose on the strength of the data listed in Table 2 that at the short times-onstream, the major contribution to the formation of deep oxidation products is made by saturated chlorinated hydrocarbons: 1,2 dichloroethane and ethyl chloride. On increasing timeon-stream to more than 6 s, we observed a sharp increase in the yield of deep oxidation products together with the decrease in the yield of vinyl chloride. It is likely that at the longer times-on-stream, the rate of deep oxidation of vinyl chloride would increase and become higher than the rate of dichloroethane dehydrochlorination. Taking into account this fact, we believe that the optimum time-on-stream assuring the best total yield of ethylene and vinyl chloride would be 3--5 s. It was shown in the investigations that the ratio of initial reactants also essentially affects the process. It was found that the excess of hydrogen chloride is favorable for improving the selectivity of the process with reducing the yield of deep oxidation products. At 673K and the reactant ratio of C2H6 : HC1 = 1 : 1, the yield of COx ranges from 6 to 7%; at the reactant ratio of C2H6 : HC1 = 1 : 2, the corresponding yield is 3-----4% (see Table 2). A positive factor is that the carbonization of the catalyst therewith decreases. On the other hand, the increase in the excess of HC1 to ethane up to 3 : 1 involves the decrease in the yield of unsaturated hydrocarbons due to the inhibition of the dehydrochlorination of 1,2dichloroethane and ethyl chloride with hydrogen chloride. The excess of oxygen increases the conversion of ethane mainly due to its oxidation: the yield of carbon oxides increases by 1.8-2 times. Thus, the optimum reactant ratio to provide the best yields of the target products is C2H6 : HC1 : O2 = 1 : 2 : 1.

313 Perfect stability of copper-containing cement catalysts in the oxidative chlorination of ethane was confirmed by their performance for 1500 hours without any decrease in the catalytic activity.

4.Conclusions The results obtained substantiate that the utilization of copper---cement catalysts offers promise for the synthesis of vinyl chloride from ethane at law temperatures in a single step. The proposed efficient and stable copper-cement catalyst will assist in the development of a new technology for the production of vinyl chloride from ethane. This technology is lowwaste and balanced in raw materials with meeting modem requirements of ecological safety. It would be appropriate to conduct the process of vinyl chloride production from ethane, hydrogen chloride, and oxygen in a fixed bed of copper---cement catalyst modified with alkali metals, for example, at 623--673K, time-on-stream of 3--5 s, and reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1. Under these conditions, the conversion of ethane is more than 90%, and the total selectivity to ethylene and vinyl chloride is 85-90% at the yield of deep oxidation products COx no more than 3--4%.

REFERENCES

1. Yu.A. Treger, V.N. Rozanov, M.R. Flid, L.M.Kartaschov, Usp. Khim., 57,No 4(1988) 577 2. H.Rigel, H.D.Schindler, M.C.Sze. Chem.Engng.Progr.,.69, Nol0, (1973) 89 3. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 19, No 6 (1978) 527. 4. A.J.Magistro, P.P.Nicholas, R.T.Carrol, J.Organ. Chem., 34 (1969) 271 5. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 20, No 1 (1979) 129. 6. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 24, No 3 (1983) 633. 7. M.M. Mallikarjunan and S. Zahed Hussain, J. Sci., Ind. Res., 42 (1983) 209. 8. V.I. Yakerson, E.Z. Golosman. React. Kinet. Catal. Let., 55, No2 (1995) 455 9. V.I. Yakerson, E.Z. Golosman. Scientific Bases for the Preparation of Heterogeneous Catalysts. VI Intern. Symp.Preprint. 3 Poster Session II Louvain-la-Neuve (Belgium), (1994) 105 10. I.I. Kurlyandskaya, I.G. Solomonik, E.D.Glazunova, E.A.Boevskaya, Yu.M.Bakshi, E.Z. Golosman,V.I. Yakerson, Khim. Prom-st, Moscow, No. 6 (1996) 368. 11. M.R. Flid, I.I. Kurlyandskaya, I.G. Solomonik, M.V.Babotina, Khim. Prom-st, Moscow, No. 6 (1996) 364.

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

315

Oxidative Conversion of LPG to olefins with Mixed Oxide catalysts: Surface Chemistry and Reactions Network M.V.Landau a, M.L.Kaliya a, A.Gutman a, L.O.Kogan a, M.Herskowitz a and P.F. van den Oosterkamp b aBlechner Center for Industrial Catalysis and Process Development, Ben-Gurion University of the Negev POB 653, Beer-Sheva 84105, Israel Tel. (972-7)-6472141, Fax.(972-7)-6472902 bKinetics Technology International (KTI) B.V., POB 86, 2700 AB Zoetermeer,The Netherlands,Tel.31 (79)-3531453, Fax.31 (79)-3513561 The catalyic performance of three mixed oxide catalytic systems V-Mo-, V-Mg and RE-LiHalogen (RLH) in LPG oxidative conversion was measured at different O2/LPG ratios, temperatures and WHSV. At high LPG conversions V-Mo-based catalysts yielded low olefins selectivity and high LPG combustion (CB), V-Mg - medium olefins selectivity by oxidative dehydrogenation (ODH) route and medium LPG CB selectivity, while RLH catalysts displayed high olefins selectivity by ODH and cracking (CR) routes at low CB. TP-reaction experiments and the effects of oxygen partial pressure on catalytic performance indicated a dynamic interaction of surface oxygen in the ODH, CB and CR routes. ESCA and TPD measurements detected three types of surface oxygen with different nucleophility and bonding strength. Their distribution correlated with LPG conversion selectivities. A correlation between catalysts acidity, the surface exposed metal cations concentration and the productivity by the CR route was derived. The surface basicity was also significant in olefins productivity by the ODH and CR routes. The selectivity of LPG oxidative reactions were attributed to different intermediates formed on the surface as a result of interaction of C3-C4 paraffins with oxygen atoms of different nucleophility. Both the redox balance of surface metal cations and the acidity-basicity balance are proposed to be significant. 1. I N T R O D U C T I O N Catalytic oxidative conversion of low paraffins into olefins, a potential alternative to steam cracking, is one of the attractive optiopns that could decrease the process temperature, minimize the coke deposition at the reactors walls and increase the olefins productivity. Various catalytic processes for oxidative production of ethylene, propylene and butylenes have been published. A review of the published results measured with individual C2-C4 paraffins [1] allowed to select three most efficient oxide catalyst systems for the study: V-Mo- [2], V-Mg- [3] and Mg-RE-LiHalogen (Mg-RLH) [4]. Comparison of their performance in LPG oxidation showed that V-Mocatalyzed mainly the full paraffins CB, V-Mg- displayed average olefins selectivity producing a large amount of butadiene while the RLH - containing oxide systems showed the highest olefins selectivity at high LPG conversions producing substantial amounts of C2-C3 olefins by CR and ODH routes [ 1] The purpose of this work was to study the states of surface oxygen and relate them to the catalytic performance of selected catalysts: V-Mo, V-Mg and RLH.

2. EXPERIMENTAL

Preparation of Catalysts. V-Mo-catalysts were prepared according to procedure described in [2]. Ammonium metavanadate and paramolybdate were dissolved separately at 70~ third solution containing all the other metal components in form of nitrate salts was mixed with the first two evaporated by mixing. The catalyst material was crushed, sieved, dried at 120~ and calcined at 350oc for 5 h. V-Mg samples were prepared by mixing the MgO obtained by decomposition of Mg(NO3)2 or Mg(OH)2 (with addition of SiO2 or TiO2 powders in some cases) with water solution of ammonium metavanadate (containing metal nitrates in some cases), evaporation the suspension to dryness, dried at 120~ and calcined at 550~ for 6 h. The RLH- catalysts were prepared via an aqueous slurry containing LiNO3, NI-I4-halogen salt, Dy-oxide and the second

316

metal oxide (MgO,Ce-oxide or transition metal oxide). The water was evaporated, the paste dried at 130~ resulting solid was crushed,sieved and calcined at 500oc for 2h and at 750oc for 16h. Catalysts testing. A tubular titanium reactor 17 mm ID and 250 mm length supplied with the central thermowell was designed to test the catalysts over wide range of temperature and various feed compositions. Hydrocarbons - 25wt.% n-C4H10- 25wt.% i-C4H10- 50wt.%C3H8 (LPG artificial mixture) or its components, oxygen and nitrogen were fed separately by mass flow controllers (Brooks Instrument) and mixed in preheater at 450oc. The reactor was inserted into Carbolate tubulat oven, uniformly heated over a length about 50 mm. 1-5 g catalyst diluted with quartz pellets at 1:3 ratio was loaded between layers of quartz pellets. Axial temperature gradient in the catalyst layer during the tests was less than 5~ Homogeneous LPG oxidation in titanium reactor filled with quartz pellets at temperatures lower than 600oc was less than 5 wt.% conversion. The analysis of the reaction products excluding water was performed on line with GC HP-5890 that contained four columns - 45/60 Molecular Sieve 13X, 10 ft x 1/8"; 50 m x 0.53 mm Plot A1203; 80/100 Hysep Q 4 ft x 1/8" and 1 ft x 1/8", with internal switching valves and two detectors TCD and FID controlled by ChemStation analytical software. Selectivity was defined as wt of olefins in product divided by the wt of converted LPG feed. Catalysts characterizations. The catalysts composition was measured by energy -dispersive Xray (EDAX) - JEM-35, JEOL Co., link system AN-1000, Si-Li detector. The surface area was determined using BET method (ASTM 3663-84). Phase composition was measured by XRD in conventional, automated Philips PW 1050/70 diffractometer equipped with a long, fine focus Cu anode tube, 40 kW, 28 mA, a scintillation detector and a diffracted beam monochromator. The phase identification was carried out according to JCPDS-ICDD powder diffraction cards. PHI 549 SAM/AES/XPS apparatus with double CMA and Mg Ka X-ray source has been used for X-ray Photoelectron Spectroscopy (XPS) measurements of the catalysts. After recording general survey spectra, high resolution scans were taken at pass energy (25 eV) for the O ls peaks. The spectral components of O signals were found by fitting a sum of single component lines to the experimental data by means of non-linear least-square c.urve fitting to Gauss-Lorentz shape function using software provided by instruments manufacturer for peaks deconvolution. Care was taken to protect the calcined fresh samples from the contact with atmosphere by pressing them into 10 mm disks and transfering to the ESCA analytical chamber. The quantitative distribution of oxygen atoms with different O ls characteristics as well as total atomic surface concentrations of oxygen were calculated by conversion the peak areas into atomic compositions taking in account the sensitivity factors of all detected elements. Binding energies were referenced to the carbon ls line at 284.5 eV. The TPD and TP-reaction measurements were carried out in AMI-100 Catalyst Characterization System (Zeton-Altamira) equipped with quadrupol mass-spectrometer (Ametek1000). 3. RESULTS AND D I S C U S S I O N

3.1. Phenomenological description of observed catalytic effects Table 1 presents the olefins selectivitiy and productivity measured catalysts at about 30% LPG conversion. The measurements were temperature and O2/LPG ratio, keeping the LPG conversion constant by olefins selectivity is determined by a few basic components increasing in

with all the tested oxide carried out at constant varying the WHSV. The following sequence:

V-Mo- (5.1-8.4%) < V-Mg- (39.2-55.0%) < RLH (67.0-79.0%). The nature of promoters or components in RLH catalysts affected mainly the olefins productivity. The significance of different reaction routes is apparent in Table 2 that compares the CR and CB selectivities measured with selected representatives of the three catalyst groups: V-Mo-Nb-SbCa (Cat.A), 0.07V2Os-Mg(Cat.B) and Mg-Dy-Li-C1 (Cat.C). It also includes the results obtained with a catalyst that yielded a higher olefins productivity where the RLH composition was supported on a transition metal oxide (TM-RE-Li-C1, Cat.D). LPG was almost fully combusted on the V-Mo-catalyst. V-Mg-catalyst converted LPG mainly by ODH and CB routes with about equal efficiency. RLH catalysts enhance the ODH and CR routes with relatively low CB. Table 3 comoares the catalvtic oerformance of M~-Dv-Li-C1 catalyst in oxidation of individual LPG

317

components. All the hydrocarbons were converted mostly by ODH and CR routes, with CR selectivity increasing in the sequence: propane < n-butane < i-butane, so that the contribution of cracking products to total olefins yield was 55-65%. Figure 1 presents the olefins selectivity as a function of LPG conversion. Such plots are commonly used for comparison of low paraffins oxidation catalysts [5,6]. The V-based catalysts showed strong decrease in olefins selectivity with increasing conversion ( more expressed with V-Mo-) normally found with ODH catalysts [5.6], while the selectivity of RLH catalysts was almost independent on LPG conversion. Table 1 Compositions and performance of the tested catalyst belonging to the three selected groups Catalyst group

V-Mo

V-Mg

Catalyst composition

S.A., m2/g

0.09V205 0.74MOO3 0.02Nb205 0.02Sb203 0.13CaO 0.05V205 0.83MOO3 0.12CaO 0.17V205 0.83MOO3

Phase composition Olefins Olefins sel.*),% product. g/gCat., h

14

Sb204, Nb205, 8.4 SbNbO4 [Mo4011]O 5.1 MoO3,[Mo4011]O, 7.5 VMoO14

6 10

0.07V205 0.93MgO 0.07V205 0.93MgO 0.05V20 5 0.79MgO 0.16SIO2

60 100 90

0.05V205 0.94MGO 0.006TIO2 0.004Cr203 0.07V205 0.88MGO 0.05Li20 0.06V205 0.79MGO 0.05Li20 0.1C1

55

MgO, Mg3V208 MgO, Mg3V208 MgO, Mg3V208, Mg2SiO4 MgO, Mg3V208

57 52

Mg-RLH 0.8MgO 0.09Li20 0.002Dy203 0.1C1 0.7MgO 0.09Li20 0.002Ce203 0.21C1 0.39MgO 0.43Ce203 0.003Dy203 0.08Li20 0.1 C1 0.88MgO0.01Li20 0.001Dy203 0.1I 0.82MgO 0.1Li20 0.004Dy203 0.08Br 0.77MgO 0.09Li20 0.005Dy203 0.14F *) T = 585~

V-Mo catalysts T = 500~

0.03 0.028 0.027

44.9 44.3 43.5

0.15 0.15 0.14

39.2

0.13

MgO, Mg3V208 --

55.0 54.5

0.18 0.18

20 18 19

MgO,LiDyO2,Li20 -MgO, CeO2

77.3 79.0 78.5

0.08 0.1 0.1

-15 20

MgO,LiDyO2,Dy203 MgO, DyOBr,Dy203 MgO,LiDyO2,Li20

82.0 70.0 77.3

0.25 0.16 0.02

O2/LPG = 1; LPG conversion -- 30%

Table 2 Performance of selected representatives of the three catalyst groups in oxidation of LPG *) Catalyst

A

B d

a

b

C c

d

a

b

D

a

b

c

c

d

a

b

c

d

8.4

91.6

-- 0.03 44.9 55.1 3.1 0.15 77.3 22.0 39.00.08 74.7 28.0 36.2 1.03

*) Testing conditions as in Table 1; LPG conversion -30%; a-olefins selectivity,%, b -combustion selectivity,%, c -cracking selectivity (C1+C2),%, d - olefins productivity, g/g Cat.h A scheme of LPG reactions is proposed in Fig.2 to show the possible low paraffins transformations according to main three routes. It is based on measured products distributions and

318 90

.....

e

9 O'ql~

r

:

9 4~

9

4P,

41,~

9

;>

O r

,,, 30

0

.

.

.

.

I~

I-

0

A

I. . . . . . .

20

40 LPG c o n v e r t ; I o n ,

A

,.A

~.. 60

%

Figure 1. Olefins selectivity vs. LPG conversion plots for all the testcd catalysts

"~

C2H6

CzH4~

C3H 8 ~ - - . E . _ ~ - C " H ? ~

9 i-C4Hzo

~

8. ~ ,.,,

~

n-C4HI0 . ~ ' " 7.

~ *oa

~.

~

, O ~ , ,...~6 ~._,0.

~.~" .n__,_

.

tg.1

~

L ,-,-

7-- co~_

n-C4H~ ~~ cis,trans-C4H8 t6. H24-O2 - - ~ H20 ~7.CH4 + O 2 ~ CO + H20 ~ i-C4H8 l,.C H 4+O 2 ~ CO 2 + H 2

Cn_xH2(n.x),CH4,Cn_xH2(n.x)+2,H20

CRI 1.3,6,8,15,19 ODH CnH2n+2 CB

CO

,.~ ,o2

r.- CnH2n,H20

2,4,7,14 5,9,10,11,12,13,16,17,18

CO,CO/,H20,Hz Figure 2. Reactions network in oxidative conversion of LPG

8O

319

Table 3 Performance of Mg-RLH catalyst C in oxidation of LPG components *) Paraffin

n-Butane

i-butane

a

b

c

d

a

b

72.2

26.2

50.3

0.13

76.1

20.1

c

propane d

a

58.4 0.19

78.2

b

c

d

21.5

60.1

0.18

*) Testing conditions as in Table 1" Hydrocarbons conversion -- 30%. a - olefins selectivity,%, b combustion selectivity,%, c - cracking selectivity,%, d - olefins productivity, g/g Cat.h kinetic studies. The molar amount of hydrogen detected in products was higher than the amount of olefins could produce without a change in the number of carbon atoms while the amount of consumed oxygen was lower than needed for combustion of hydrogen stochiometrically. Therefore reactions like 5, 10, 13 and 19 in Fig.2 were included in the reactions network assuming production of hydrogen as a result of partial combustion. 3.2. Surface oxygen role

in

oxidative conversion of light

alkanes

Lattice oxygen in metal oxides reacted in catalytic cycles is replenished by reoxidation [7-9]. The effect of O2/LPG ratio on the catalytic performance of three selected catalysts shown in Fig.3 indicates that oxygen from gas phase is a reactant in all the three routes of catalytic conversion. Molecular oxygen could react with adsorbed hydrocarbons or oxygen bonding and activation at the catalysts surface could be nesessary.

90

/ ?

60

Catalyst A

~

80

3

90

atalyst B

1

2 ~

3

60

40

3O

30

0 0

0.5

1

1.5

2

OxygenlLPG molar rallo

2.5

--

0

I

0.5

--I .....

1

t-

1.5

OxygenlLPG molar ratio

t---

2

----t--

0

I

0.4

0.8

1.2

OxygenlLPG molar rallo

Figure 3. Effect of O2/LPG ratio on performance of selected catalysts in LPG oxidation at 585~ 1-LPG conversion, 2 - olefins selectivity, 3 - oxygen conversion, 4 - cracking selectivity Three consecutive runs of n-C4H 10-TP-reaction experiments were carried out with selected catalysts A,B,C and D. 25 cm3/min mixture 9%.vol. n-C4H10-He was fed to the reactor of the AMI-100 Catalysts Characterization System containing 3 g catalyst after heating to 200oc in He flow. Then the temperature was gradually increased at 5OC/min up to 600oc (Cat.A), 750oc (Cat.B,C) and 800oc (Cat.D). After reaching the required temperature, the gas flow was switched to He, catalysts were purged for 1 h,cooled to 200oc. Then the procedure of the first run was repeated. Before the third run, performed at the same conditions, the catalysts were reoxidized in 5%vol Oa-He flow at 550oc for 2 hours with subsequent cooling to 200oc. During the n-C4H10-TP-reaction runs the concentration of n-C4H10 in effluent gas as well as

320

concentrations of C4H8 (ODH product), C2H4,CH4 (CR products), CO2, H20 and H2 (CB productg) were monitored by MS. TP-reaction spectra for butane consumption (similar in shape for all catalysts) is shown in Fig.4a. It could be divided into three parts reflecting different catalysts performance as the temperature increases: I - no butane consumption at low temperature, II increasing butane consumption by fresh and reoxidized catalyst and no consumption with reduced catalyst, III- increasing butane consumption in all the runs that could be a result of other reaction routes (e.g. homogeneous reactions with oxygen evoluted by oxides decomposition). In the second series of TP-reaction experiments, the temperature during butane flow was changed in a ramp mode: it was increased in the same way as in previous series up to value a little higher than it corresponded to the end of the part II and kept constant for 1 hour. In this case (Fig.4b) the butane concentration spectra with fresh and reoxidized catalysts showed a minimum as a result of gradual conversion of surface oxygen while the reduced catalyst did not display defined peaks. The concentrations of all the other compounds monitored by MS displayed a -

(a)

F~gure 4. n-C4H10-TP-reaction spectra recorded with Mg.RLHcatalyst B

321

maximum over the same time pcricxt. The normalized MS peaks intensities lot butane, butylene, ethylene, methane, water, carlx~n dioxide and hydrogen measured at maximum butane consumption lot catalysts A,B,C and D are presented in Fig.5. The peaks normalization was done separately for every experiment, so their relative intensities shown in Fig.5 for different catalysts could not be compared. In all cases the products distribution with fresh and reduced catalysts were close to those measurcd in steady-state experiments, excluding high CO2 evolution with the fresh RLH catalysts. Reducing the V-Mg and RLH catalysts in butane flow almost fully depressed their ODH and CB activity shifting the products distribution in the direction of CR and dehydrogenation while the V-Mo- catalyst in reduced form produced the same CB products as fresh and reoxidized form with lower efficiency. These results are evident for the need for adsorbed oxygen species in the reaction cycles producing products according to the three main conversion routes detected in steady state experiments. Then the differences in performance of the three selected catalysts groups in LPG o~dation is probably caused by different states and concentrations of the surface lattice oxygen atoms. It is widely accepted 18-101 that the ability, of surface oxygens Os to react with hydrocarbons and the type of reaction depend on the distribution of Os among the different species: O2(gas) ~ O2(ads) w," O2-(ads) ~-*'20-(~s) w-~'202-(lattice). The performance of the most strongly bonded lattice oxygen that could be removed at high temperatures by the reaction with hydrocarbons in catalytic cycles is governed by their nucleophilicity being directly related to the effective negative charge and bonding strength [8-11]. Those characteristics together with surface concentrations of different oxygen forms for selected catalysts A-D were measured by TPD and ESCA.

Fi~zure 5. n-C4Hlo-TP-reaction products distribution with catalysts A-D at butane consumption

322

3.3. Surface chemistry characterizations The TPD experiments were carried out with 3 g catalysts A,B,C and D in He flow 25 cm3/min monitoring by MS the evolution of 02, CO2 and H20 over a temperature range 200-800oc ( for VMo- catalyst 200-600oc), heating at 5~ The results presented in Table 4 showed that only V-Mo- catalyst contains comparatively weakly bonded oxygen that could be partially desorbed at the temperatures used in steady-state catalytic tests. The oxygen bonding strength corresponding to Table 4 He-TPD of fresh catalyst Catalysts

Desorbed species:

02

A B C D

H20

CO 2

a

b

c

a

b

c

a

b

c

>600 680 705 ND

>20 40 50 ND

480 560 650 ND

ND 700 720 480

ND 500 80 8

ND 510 640 420

ND 710 720 580

ND 30 300 90

ND 580 600 550

a - Temperature of peaks maximum,~ : b - normalized MS peaks intensity c - Temperature of initial product desorption, oC; ND - not detected the temperatures of initial oxygen desorption and its maxima, increased in catalysts sequence: A
A

B

C

D

3 1 8 220 100 22500

1 ND ND 2 1 100

1 ND ND ND ND 30

1 ND ND 1 ND 130

323

absent in reoxidized catalysts (Fig.5). The hydroxyls are formed during the reaction of butane with fresh or reoxidized catalysts. The amount of evoluted water in butane-TP-reaction experiments was comparable with the amounts of other products (Fig.5). After switching the butane flow to He substantial amount of water was desorbed at the purging stage at a much higher concentration compared to the other products (Table 5). Taking into account that no water evolution was detected at the purging stage as well as during butane-TP-reaction with reduced catalysts (except V-Mo- ) it appears that at least part of surface lattice oxygen reacts with hydrocarbons forming hydroxyl groups that are removed at the purging stage as a result of nonreductive dehydroxylation. The ESCA measurements of the O ls electrons BE carried out with fresh catalysts A-D detected one, two or three bands in RFE-spectra depending on catalysts origin corresponding to the O ls electrons BE range 528.4-529.3 eV (OI), 529.9-530.3 eV (OII) and 531.0-531.8 eV (OIII). The values of total oxygen surface concentrations, O ls electrons BE corresponding to different oxygen states and the relative amounts of those oxygen species exposed at the catalysts surface are shown in Table 6. Decreasing the O ls electrons BE in the sequence OIII >OII>OI reflects increasing of electron density or effective negative charge on oxygen atoms. This corresponds to increasing of oxygen atoms nucleophility (basicity or ability for proton abstraction from hydrocarbon molecule). Other observations showed that: Ar sputtering with increased duration removes from the RFES spectra of RLH catalysts the peaks corresponding to OIII species leaving the OI species unchanged; in case of Vcontaining catalysts Ar sputtering does not affect the shape of the spectra and the O ls characteristic was very close to 530.0 eV observed with pure V205. The ESCA measurements with separate individual oxides, hydroxides and carbonates of the elements building the catalysts compositions showed that the O ls electrons BE values of OIII oxygen species correspond to carbonates, hydroxyls, magnesia or lithia. It could be concluded that OIII oxygen atoms with low nucleophility being included mainly in subsurface species cannot play significant role in catalytic cycles. According to ESCA measurements carried out with vanadium oxide, consistent with the data presented in [ 12], the O ls characteristic of OII oxygen species is very close to the V==O doubly bonded oxygen atoms exposed at the surface of (010) planes of V205 crystals. The oxygen species with O ls characteristic of OI do not exist at the surface of individual main components of catalysts A,B vanadia, molybdena, or exist in small concentration at the surface of magnesia or TM. They -

-

-

Table 6 Characteristics of surface oxygen species in selected catalysts according to ESCA Catalysts A Oxygen surface concentration, % at. 80 Metal cations surface concentration, % at. 20 O Is characteristics of oxygen species,eV: OI -OII 530.3 OIII -Normalized oxygen species concentrations: OI 0 OII 100 OIII 0 1- (OII/total) 0

B 70 30

C 55 48

D 36 55

529.2 530.3 531.8

529.3 -531.0

528.8 530.2 531.5

47 40 13 0.6

48 0 52 1

58 20 22 0.8

form mainly as a result of interaction of those components with additives: V-Mg, Mg-RLH or TM-RLH.This is illustrated in Table 7 for Mg-RLH catalyst. From the results of catalytic tests in butane oxidation it is evident that existence of all the components is essential for the performance of this catalytic system. In case of Mg-RLH system magnesia displayed two RFES peaks of O III oxygen at 530,5 and 531.5 eV corresponding to MgO and Mg(OH) 2 in agreement with [13,14] and less than 10% of oxygen in form of O I (529.7 eV) which could be attributed to cationic

324

vacancies at the MgO surface. Introduction of lithia creates additional O I oxygen species (Table 7) while subsequent introduction of CI and Dy strongly shifts the distribution of surface oxygens into OI direction with increasing the effective negative charge at those atoms. The formation of highly nucleophilic oxygens is a result of changes in coordination and chemical bonds polarity of lattice oxygen atoms caused, for example, by formation of new phases like Mg3V208 where oxygen ions became bridged between V and Mg ions [15], substituting Li into MgO lattice [ 16] or formation of LiC1 crystals covered by thin lithia layer[17]. Decreasing the total oxygen surface concentration in the catalysts sequence A --> D ( Table 6) expose more metal cations and chlorine that behave as electron acceptors. Thus increasing the amount of highly nucleophilic oxygen atoms in the same row as electron donors should be accompanied by substantial changes in catalysts acidity-basicity. Those characteristics were measured by NH3- and CO2-TPD after saturation the catalysts samples with corresponding gases at 40oc. The results are shown in Table 8. V-Mo- catalyst displayed the lowest acidity corresponding to the lowest metal cations concentration but about 50% of the acid sites were strong desorbing ammonia at >250~ The other catalysts contain few strong acid sites but the total acidity strongly increased in the sequence B
Catalysts

529.7 MgO 529.2 0.94MgO-Li20 . . . . 0.994MgO-Dy203 -0.93MgO-0.006Dy203-Li20 529.2 0.87MgO-0.06Li20-C1 0.86MgO-0.006Dy203529.3 0.06Li20-C1 *) T= 550~

--530.3 ---

530.5; 531.5 530.5; 531,5 531.2 531.3 530.6;531.5 531.0

11.4 4.5 25.0 5.2 18.0

55 74 56 70 69

38.0

70

WHSV = 0.33 h -1, O2/C4H10 = 1

Table 8 Acis-base characteristics of selected catalysts Catalysts Acidity: total, ~M NH 3/g - > 250oC/total Basicity: total, l.tM CO 2/g >250oC/total -

-

-

A

B

C

D

30 0.5

81 0.002

140 0.04

340 0.03

0.5 0

7 0.3

4 0.4

4 0.6

to the absence of highly nucleophilic oxygen species. The basicity of other catalysts was about one order of magnitude higher: V-Mg and Mg-RLH catalysts displayed about equal distribution between strong and weak basic sites while at the surface of catalyst D the relative amount of strong basic sites was more than twice higher. It corresponds to apperance of highly nucleophilic oxygen species and increasing their nucleophility from C to D (Table 6).

325

3.4. R o l e of d i f f e r e n t

s u r f a c e s p e c i e s in catalytic cycles

Comparison the surface characteristics of selected representatives of the three catalysts groups with their catalytic performance in LPG oxidation show: i - at temperatures less than 600oc all three LPG oxidative conversion cycles - ODH, CR and CB, are controlled by interaction of hydrocarbons with surface lattice oxygen atoms OI and OII, that form surface OH-groups being removed by dehydroxylation before reoxidation, as indicated from the results of TP-reaction and TPD experiments discussed in the part 3.2. ii - combination of OII oxygen species with low nucleophilicity (basicity) bonded to easy reducible metal cations (V,Mo) with acid sites leads to CB increasing with increased acid sites strength; it was indicated by direct correlation between olefins selectivity measured with catalysts A-D (Table 2) and parameter [ 1-OII/Ototal] (Table 6)reflecting decrease of CB selectivity with decrease of the fraction of OII in all the surface oxygen atoms and furthermore by substantial increase of the strong acid sites concentration from V-Mg to V- Mo (Table 8). iii - combination of OI oxygen species with high nucleophility (basicity) bonded to hardly reducible cations (Mg,RE) with weak acid sites leads to ODH and CR increasing with increased basicity of OI atoms, as indicated by comparing changes in the fraction of OI (Table 6) and their basic strength (Table 8) from catalyst A to catalyst D with CR and olefins selectivities of those catalysts presented in Table 2. iiii - the efficiency of CR conversion route increases with increased weak acidity of the catalyst as indicated from the direct correlation between CR productivity of A-D catalysts that could be easily estimated from the data of Table 2 and surface concentrations of metal cations and chlorine given in Table 6. Based on this information two different modes of paraffins activation are assumed, leading to CB or ODH-CR products depending on catalysts surface chemistry that are consistent with generally accepted models [8-11]. V-Mo- catalyst containing strong acid (electron-acceptor) sites, easy reducible cations andweak nucleophilic (proton-acceptor) oxygen atoms could adsorb the hydrocarbon molecule as a result of hydride-ion abstraction by acid sites. Reduction of metal cation with splitting of one of metal-oxygen bonds and stabilizing the proton and carbanion in form of OH and alkoxy species: CnH2n+2 ?-

O H!1

/z,,

CnH2n+ 1 +

OH OCnH2n 1 (n-l)+ M e - O_ MIe (m--l; . _

0

I] m+

n+Me 0 lvle !

! !

| i

!

RLH catalysts do not contain strong acid sites and easy reducible metal cations but have strongly nucleophilic (proton-acceptor) oxygen atoms and weak acid sites. The hydrocarbon molecule could be adsorbed as a result of proton abstraction by strongly nucleophilic lattice oxygen without splitting the metal-oxygen bond and stabilization of proton and carbcation in form of OH and alkyl species: CnH2n+2 O~ CnH2n+ I O - M e (n'l)+ - 0 - M e n+ - 0 ~ ! i

! i

CnH2n+l O - M e (n-l)+ i i

H O - M e n+ - 0 | i

The subsequent transformations of alkoxy radicals containing strong C-O bonds at the surface of V-Mo- catalyst with weakly bonded oxygen atoms yields preferentially formation of full CB products with some hydrogen evolution, while alkyl radicals stabilized on acid sites at the surface of RLH catalysts as a result of C-H bonds polarization in the strong field of metal-

326

oxygen ion pairs should be preferentially transformed to olefins as a result of further hydrogen abstraction (ODH) or CR. The fraction of CR products in olefins depends on catalysts acidity increasing the lifetime of alkyl radicals on the catalyst surface.The V-Mg- catalyst contained the both types of surface oxygen OI and OII in about equal amounts (Table 6) displaying average acidity and basicity (Table 8) and including easy reducible (V) as well as hardly reducible (Mg) metal cations. As a sequence it showed an average catalytic performance. In both cases the catalytic reaction cycle became closed as a result of dehydroxyation of catalysts surface and further oxygen adsorption-insertion in the oxide lattice that in case of V- or V-Mo-containing catalysts is accompanied by increasing of metals oxidation extent. In addition to further reacting of alkyl and alkoxy intermediates at the catalysts surface with dynamic lattice oxygen they could be desorbed into gas phase and react there homogeneously with gas oxygen as it was demonstrated in [ 13] for V-Mg-catalyst. Testing the RLH catalysts in fixed-bed reactor with void fraction of catalysts layer varied from 28 to 43% showed that this route became significant at temperatures higher than 590oc but no substantial changes in products distribution were observed. 4.

SUMMARY

The RLH-based catalysts display high olefins selectivity at high LPG conversions producing olefins by oxydative dehydrogenation and oxidative cracking. The last charactristics allow them to produce ethylene from LPG that is the main product of steam cracking. Supporting the RLH system at different carriers affects mostly the catalysts productivity. The RLH-based catalysts display about 50% olefins yield with productivity per reaction volume close to steam cracking. The high selectivity of RLH-catalysts to olefins is a result of a definite combination of surface oxygen state, oxygen / metal cations ratio, redox properties of metal cations and acidity-basicity balance. Further studies are needed in order to understand the role of the support and the proper functioning of RE-Alkali-Halogen systems in oxidation of low paraffins. REFERENSES

1. M.V.Landau, M.L.Kaliya, M.Herskowitz, P.F.van den Oosterkamp and P.S.G.Bocqu6, CHEMTECH, 26, No.2 (1996) 24. 2. J.H.McCain, US Patent No. 4 524 236 (1985). 3. H.H.Kung and M.A.Chaar, US Patent No. 4 777 319 (1988). 4. C.J.Conway, D.J.Wang and J.H.Lunsford, Appl.Catal., 79 (1991) L 1. 5. F.Cavani and F.Trifiro, Catal.Today, 24 (1995) 307. 6. S.Albonetti, F.Cavani and F.Trifiro, Catal.Rev.-Sci.Eng., (1996), 413. 7. P.Mars and D.W.van Krevelen, Chem.Eng.Sci.(Special Suppl.), 3 (1954) 41. 8. A.Bielanski and J.Haber, "Oxygen in Catalysis", Marcel Dekker, Ink., New York, 1991. 9. V.D.Sokolovskii, Catal.Rev.-Sci.Eng., 32, No. l&2 (1990) 1. 10. G.Centi, F.Trifiro, J.R.Ebner and V.M.Franchetti,Chem.Rev., 88 (1988) 55. 11. H.H.Kung, Ind.Eng.Chem.Prod.Res.Dev., 25 (1986) 171. 12. J.Zi61kowski and J.Janas, J.Catal., 81 (1983) 298. 13. X.D.Peng, D.A.Richards and P.C.Stair, J.Catal., 121 (1990) 99. 14. J.C.Fuggle, L.M.Watson and D.J.Fabian, Surf.Sci.,49 (1975) 61. 15. H.H.Kung, Adv. in Catal., 40 (1994) 1. 16. T.Ito, J.X.Wang, C.H.Lin and J.H.Lunsford, J.Amer.Chem.Soc., 107 (1985) 5062. 17. D.Wang, M.P.Rosynek and J.H.Lunsford, J.Catal., 151 (1995) 155.

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

327

Free Radicals as I n t e r m e d i a t e s in Oxidative T r a n s f o r m a t i o n s of L o w e r Alkanes. M. Yu. Sinev, L. Ya. Margolis, V. Yu. Bychkov, and V. N. Korchak Semenov Institute of Chemical Physics, Russian Academy of Sciences 4 Kosygin street, Moscow 117334, Russia Catalytic oxidative transformations of lower alkanes attract the attention as possible ways to transfer these substances into more suitable chemicals - olefins and oxygenates (alcohols, aldehydes, acids, etc.) - and to involve them into the industrial use as raw materials for chemical and petrochemical synthesis. However, the yields of desirable products reached up to date are not sufficiently high. The progress in the studies of intrinsic mechanism of catalytic partial oxidation of lower alkanes is not sustainable either. We believe that these two facts are correlated and that the analysis we performed in the present work can brighten up some important details of the mechanism of catalytic oxidation of lower alkanes*. Experimental facts and theoretical concepts existing in the literature indicate that the formation of free radicals plays an important role in a number of catalytic oxidation reactions [1-5]. In the present paper we analyze the contribution of free radicals to several oxidative transformations of lower alkanes over oxide catalysts. Based on the thermochemical data and on the results of kinetic simulations it is shown that the observed reaction kinetics and product compositions in the mentioned above processes are determined by a set of interdependent heterogeneous and homogeneous reactions of free radicals, i.e. they should not be considered as "spectators" taking part in side reactions, but as principal intermediates causing the main features of lower alkanes oxidation and design of catalysts. 1. ACTIVATION OF ALKANE MOLECULES Alkane molecules do not have any specific "reactive centers", like functional groups or multiple bonds. This means that their activation can be carried out only by the bond dissociation or charge transfer processes. It is evident that the more energetically favorable is the first step of activation, the higher is the probability of its contribution to the overall reaction. In other words, using the value of the energy expenditures in different elementary steps ( Eex = AH - Est, where AH is the overall enthalpy change and Est is the energy of stabilization of the activated molecule or its fragment on the catalyst surface) one can estimate which one of them is more feasible. Such an analysis may be performed on the basis of the available thermochemical data (see, for example, [6]). The possible processes and corresponding values of Eex which we have considered are given below: * This study was carried out under the financial support of the Russian Foundation for Basic Research (research grant No. 96-03-32440)

328 (i) homolytic dissociation of C-H bonds accompanied by the formation of surface OH-group and free radicals [O] + RI-I => [OH] + R

(1)

Eex = DR_H where [O] - strong oxidizing surface center having a high affinity to the hydrogen atom; D i - energy of corresponding bond dissociation; (ii) heterolytic C-H bond dissociation with a proton abstraction on a strong basic center [ 0 2-] + RH => [ 0 2- ... H +] + R-

(2)

Eex = DR_H + IH - IRwhere I i - ionization potentials of corresponding particles; (iii) heterolytic C-H bond dissociation with a hydride-ion abstraction on a Lewis acidic site [M n+] + RH => [M n+ ... H-] + R +

(3)

E e x - DR_H + I R - I H(iv) ionization of alkane molecule [h+] + RI-I => [h+ ... e-] + RH +

(4)

Eex = IRH where [h+]

- hole center;

[h+ ... e-] - trapped electron. It is easy to demonstrate that the sign and the magnitude of energy changes in all these processes depend on the compensation of Eex by binding the fragments of the activated molecule to the surface centers. The values of Eex given in Table 1 show that the energy which has to be compensated is minimal for the process (1). On the other hand, one may assume that the energy of the O-H bond formed in this process is comparable to Eex, i.e. the second fragment (free radical R) should not be bound to the surface in order to compensate the energy expenditure. This assumption is in a good agreement with calorimetric measurements, according to which in the case of oxide catalysts active in oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane [7,8], as well as in total oxidation of alkanes [9] the O-H bond strength ranges from 250 to 470 kJ mo1-1. On the contrary, in the case of heterolytic C-H bond dissociation the energy expenditure is so large that its compensation requires the binding of both fragments, which must occur in a

329

Table 1 Energy expenditure on the activation of lower alkane molecules Energy expenditure ( kJ mol "1 )

Molecule

reaction (1)

reaction (2)

reaction (3)

reaction (4) .

CH4

431

1630

1308

1250

C2H6

410

1615

1183

1120

C3H8

398

1609

1162

1078

n-C4HI0

393

1605

1154

1037

iso-C4H10

389

1601

1120

1016

single reaction step. This requires the presence of paired centers with specific configurations and energy relations. Such a mechanism including the simultaneous abstraction of H + and H ions from n-butane as a first step of maleic anhydride formation was suggested by Trifiro et al. [ 10] to explain the unique properties of V-P-O catalysts. One may assume, however, that due to steric restrictions the smaller an alkane molecule, the lower is the probability of such a synchronic mechanism. The process (4) seems to be improbable because it requires the existence of the hole centers with an electron affinity comparable with ionization potentials of alkane molecules. The above analysis shows that the formation of free radicals in the interaction of alkane molecules with the surface of oxides may prove to be energetically preferable as compared to any other mechanisms of their activation. Furthermore, this process requires only one type of single active centers and it proceeds in a single step. The combination of these factors may render this process the most favorable. This conclusion is experimentally confirmed by the correlation between the concentration of strong oxidative sites and the catalytic properties of the oxide catalysts (see, for instance, [ 11,12]); the Polanyi-type relation between the activation energy for the oxidative dehydrogenation of alkanes in their interaction with oxides and the energy of O-H bonds formed simultaneously [13-15]. The difference in the reactivities of C1 - C4 alkanes is mainly caused by the difference in the Eex values. Estimations based on the data of Table 1 show that the difference in rate constants at 700 - 1000 K between methane and butanes over the same catalyst can exceed 103. The H-atom affinity in the case of efficient catalysts for methane activation should be the highest. As a result, if the O-H bond strength is high enough to compensate the energy expenditure in the reaction (1), the process of active sites regeneration (reoxidation) becomes more impeded and the difference in the optimal reaction temperatures for different alkanes can reach 100 K or more. ff the catalytic oxidation of alkane molecule starts with the formation of a free radical on the surface of an active catalyst particle and its escape to the gas phase, the complete reaction network includes both homogeneous and heterogeneous steps of the transformation of primary (CnH2n+l) and secondary radicals. Since all these processes are sufficient for the formation of the final products, the analysis of the influence of different factors on the -

-

330 selectivity of a complex heterogeneous-homogeneous process can be carried out by considering the elementary reactions of free radicals. 2. ELEMENTARY REACTIONS OF FREE RADICALS 2.1 H o m o g e n e o u s

reactions

The main types of primary gas-phase transformations of free radicals formed in the reaction (1) are - recombination CnH2n+l + CnH2n+l (+ M)=> C2nH4n+2 (+ M*)

(5)

- H-transfer and elimination (ifn >__2) CnH2n+l + CnH2n+l => CnH2n+2 + CnH2n

(6)

or CnH2n+l + 0 2 => CnH2n + HO2

(7)

or

CnI-I2n+l => CnH2n + H

(8)

- oxygen molecule capture CnH2n+l + 0 2 <=> CnH2n+102

(9)

- oxidation Cnn2n+l + 0 2 => CnH2nO + O H

(lO)

or CnH2n+l + 0 2 => CnH2n+lO + O

(ll)

Reactions (5)-(8) and (10) lead to the formation of stable molecules (hydrocarbons and aldehydes). Subsequent reactions of peroxy- (CnH2n+lO2) and oxy-radicals (CnH2n+lO) formed in reactions (9) and (11) lead to the formation of oxygenates (alcohols, aldehydes, etc.), carbon oxides, and/or olefins. The fractions of radicals transformed into different fmal products depend on the reaction conditions (temperature, oxygen pressure) and on the number of carbon atoms in the alkane molecule. For example, the stability of peroxy radicals decreases with increase of the number of carbon atoms in the alkyl fragment, that is why the probability of total oxidation via their subsequent transformations decreases from methane to

331 butanes. The higher temperature also decreases this probability, due to the shift of the equilibrium (9) towards alkyl radicals, increasing the fraction of radicals transformed into the products of coupling and dehydrogenation. However, if the temperature increases beyond some certain value, the fraction of oxygen containing products increases again because of more sufficient contribution of reactions (10) and (11). In particular, the low efficiency of reactions of metyl radicals with 02 molecules likely causes the existence of a temperature "window" for the OCM process at 900-1100 K. The development of chains in the gas phase leads to the acceleration of the secondary radicals formation, as well as to the additional conversion of the initial reactants and to the shifts of product selectivities. 2.2. H e t e r o g e n e o u s r e a c t i o n s

As we have mentioned above, if the catalyst pores are sufficiently narrow, i.e. the species diffuse through them in Knudsen or transitional re~mes, the contribution of heterogeneous reactions of free radicals to the overall reaction rate and selectivity may be predominant. The main types of elementary reactions between radicals and surface sites proposed elsewhere [ 15] are the following: - H-atom transfer, for example [O] + CnH2n+l => [OH] + CnH2n

(12)

- O-atom transfer, for example [O] + CnH2n+l => [ ] + CnH2n+~O

(13)

- radical capture [0] + CnH2n+~ => [OCnH2n+q

(14)

Let us consider the possible role of these reactions in the formation of final products. If n > 2, the successive reactions (1) and (12) lead to the formation of the desirable product in the case of oxidative dehydrogenation processes. The possible contribution of alkoxy radicals to the formation of reaction products is already mentioned above. In this section we should emphasize that the relative probability of reactions (13) and (14) depends on the properties of the catalyst (oxygen binding energy E[ol) and on the reaction temperature: the higher the temperature and the lower the E[o], the more probable is the reaction (13). In this case one may expect an increase of selectivity of partial oxidation to oxygenates. The fate of radicals captured by the surface sites with the formation of the alkoxy groups depends on the number of carbon atoms in the alkane molecule, as well as on the properties of the catalyst surface. According to the data obtained by Aika and Lunsford with the use of IR spectroscopy and TPD [ 16], in the case of MgO (an oxide with very high E[ol) the methoxy groups decompose forming CO and H2, but in the case of higher alkoxides the formation of corresponding olefins takes place.

332 Taking into account the whole set of homogeneous and heterogeneous reactions, one may conclude that depending on the target product the requirements to the catalyst and to the reaction conditions should be different: if we wish to increase the yield of oxidative dehydrogenation products, we have to increase the temperature and to use the catalysts with higher E/ol. The rigidity of these requirements increases as the number of carbon atoms in alkane molecule decreases due to the increasing strength of C-H bonds and stability of peroxy-radicals. On the contrary, the lower the temperature and oxygen binding energy Eiol, the higher is the probability of the oxygenate production. We have to notice, however, that these requirements are contradictory if the olefm is an intermediate for the further formation of oxygenates. In this case it is substantial that the catalyst contains the active sites of different types: one of them (strongly-bound oxygen with high H-atom affinity) is responsible for the formation of free radicals, and the second one (with lower Etol) supplies O-atoms for the insertion into the organic molecule. The efficiency of oxides containing vanadium, molybdenum, tungsten, and similar cations as catalysts for partial oxidation of hydrocarbons to oxygenates is likely due to the division of functions between oxygen species of different types (terminal M n+ = O and bridge Mn+-O-M n+) which these cations are able to form. An analogous co-operation of oxygen species is likely to take place in the case of multicomponent catalysts: if one oxide phase actively produces free radicals which react subsequently with weakly-bound oxygen species on the surface of another component, the total rate of the final product formation will be higher as compared to that measured over each individual oxide. This explanation is alternative (or complementary) to the so-called "oxygen remote-control mechanism" discussed by Weng and Delmon [17], according to which a synergy in catalytic action may be caused by the transfer of active oxygen species between two oxide phases with different donor-acceptor properties. 3. SIMULATIONS OF SURFACE-ASSISTED FREE-RADICAL PROCESS The preliminary analysis of elementary reactions of free radicals in the presence of an active catalyst demonstrates that the heterogeneous generation of primary radicals initiates the homogeneous processes. In their turn, both primary and secondary free radicals affect reciprocally on the surface active sites. A kinetic model which considers the heterogeneous and homogeneous transformations as interdependent and presumes that all the particles (stable molecules and free radicals) present in the gas phase oxidation undergo both homogeneous transformations and interactions with active sites of the catalyst surface was discussed elsewhere [18]. This model was previously used to simulate the OCM reaction in a quasihomogeneous system. Taking into account that the subsequent fate of the radicals formed in the reaction (1) depends, in the general case, on the relation between the number of collisions with other particles in the gas phase and with the surface and also on the nature, concentration and reactivity of the surface centers, we utilized the approach proposed in [18] to simulate the heterogeneous-homogeneous oxidation of methane in combination with mass-transfer in the gas-solid system. The reaction space was considered as a gas volume of a varied thickness (L) exposed to a flat surface with a varied concentration of active sites (C). The results of simulations of the reaction accompanied by the one-dimensional masstransfer directed normally to the surface are given in Fig. 1-2. If C = 0, a self-acceleration

333

d[CH4]/dt, -

100

C

nmol/s.

5"10

-

-

16

m-2

5 " 1 0 15 hi-2

10-

5 " 1 0 14

-2

m

i

0.1

I

I I

I I

I

I

i

i

I

1 I

-3

t

I

-2

I

I I

I

I 1 I

I

I

i

-1

I

I

0

I I

I

I

I

I

I

i

I

log t (s.)

Figure 1. Methane conversion rate as the fimction of time at different concentrations of active sites on the surface (1000 K, 1 atm., CH4 902 = 10 91, L - 1xl0 -4 cm)

W(s)/W(tot)

log(lO 4 to.t) gas r e a c t i o n -4

0.8

-3

0.6 -2 0.4>

0

o _

i

i

i 4

0

-6

-5

-4

-3

-2

-1

"-

0 log L ( c m )

io

Figure 2. Effect of gas volume thickness on the fraction of the rate of heterogeneous reaction in total conversion and on the time of 1 0 % oxygen conversion ( 1 0 0 0 K , 1 atm., CH4 9 0 2 1 0 " 1, C = 5 x 1 0 1 6 m 2)

334 typical for chain reactions with branching was reproduced. At C = 5x1016 m -2 a kinetic behavior of that kind disappears, and the process becomes "linear", i.e. its rate reaches the maximum at t = 0 and then declines due to the consumption of the reactants. At intermediate C values the gradual transformation of kinetic behavior from a self-acceleration type to a "linear" type takes place. The effect of the gas volume thickness on the contribution of the surface reaction to the overall kinetics is presented in Fig.2. At L = 10 nm the time of 10% conversion characterizing the rate of reaction is -104 times less than in the case of a homogeneous gas reaction and the fraction of the rate of heterogeneous reaction Ws in the total conversion rate Wtot is nearly 1. At increasing thickness of the gas volume the fraction of the heterogeneous reaction and the rate of overall process both decline. However, even at L = 1 cm, the reaction occurring in the gas volume still experiences the influence of the surface taking part in the radical reactions. Taking into account that the specific surface areas of the oxide catalysts usually used for partial oxidation range between-~1 and 20 m2g1, the characteristic size of solid crystallites is -~10-5-104 cm and the role of heterogeneous reactions of radical species in the catalyst pores is likely predominant. According to the results of kinetic simulations, if the specific surface area of the catalyst is more than-~1 m2g-1, the most of radicals formed in the reaction (1) undergo the reverse transformation into the initial alkane molecules: [OH] + CnH2n+l--> [O] + CnH2n+2

(-1)

This means that, although the surface of pores is much larger than the outer surface of the grains, the contribution of the latter to the formation of the final products can be sufficient due to the lower probability of the reaction (-1) for the radicals formed outside the pores. The grain size which usually ranges f r o m - 0.1 mm to few centimeters, makes it possible for the homogeneous chain reaction to develop in the free volume of the catalyst bed (see Fig.2). Recently we observed the effect which supports the conclusion about the substantial role of the radical reaction outside of the catalyst grains. When a very efficient OCM oxide catalyst (10% Nd/MgO) was placed into the reactor together with an inactive metal filament (Ni-based alloy) the sharp increase of conversion accompanied by the selectivity shift from oxidative coupling to the formation of CO and 1-12 was observed [19]. Since the metal component has a low activity also with respect to ethane oxidation, this behavior is not due to successive oxidation or decomposition of C2 hydrocarbons on the metal surface, but should be attributed to the reactions of methane oxidation intermediates. Almost total disappearance of ethane (which is a product of CI-I3 radicals recombination) and acceleration of the apparent reaction rate by the addition of an "inert" material indicate that the efficiency of methane oxidative transformations can be substantially increased if the radicals have a chance to react outside the zone where they formed and the role of reaction (-1) decreases. Although the second (metal) surface is not active enough to conduct the reaction of saturated hydrocarbon molecules (methane and ethane), the radicals generated by the oxide can react further on the metal surface. As a result, the fraction of the products formed from methane activated in the reaction (1) increases, and the formation of the final reaction mixture of different composition takes place.

335 4. CONCLUSIONS 1. The most energetically favorable process of lower alkanes activation over oxide catalysts is a homolytic C-H bond dissociation with the formation of free radicals. The difference in energy expenditures for the formation of free alkyl radicals cause the difference in reactivities between C1-C4 alkanes. 2. The main factors determining the efficiency of different oxides as catalysts for lower alkanes oxidation are the H-atom affinity of strong oxidizing surface sites and the oxygen binding energy. These thermochemical factors cause the rates and directions of free-radical reactions and, as a result, the catalytic activity and selectivity to certain products. 3. The total rate of reaction and the selectivity to different products (olefins, oxygenates, carbon oxides) depend on relative efficiencies of different transformations of free radicals in the gas phase and in the heterogeneous steps, as well as on the transport phenomena. REFERENCES

1. 2. 3. 4. 5.

V.M. Polyakov, Usp. Khim., 17 (1948) 351. D. J. Driscoll, K. D. Campbell, and J. H. Lunsford, Adv. Catal., 35 (1987) 139. J.H. Lunsford, Langmuir, 5 (1989) 12. T.A. Garibyan and L. Ya. Margolis, Catal. Rev., Sci. Eng., 31 (1989-1990) 35. M. Yu. Sinev, L. Ya. Margolis and V. N. Korchak, Usp. Khim. (Russ. Chem. Rev.), 64 (1995) 373. 6. V.N. Kondratiev (ed.), Chemical Bond Dissociation Energies, Ionization Potentials and Electron Affinities, Handbook, Moscow, Nauka, 1962 (in Russian). 7. V. Yu. Bychkov, M. Yu. Sinev, V. N. Korchak, E. L. Aptekar' and O. V. Krylov, Russ. Kinet. Catal., 30 (1989) 1137. 8. M. Yu. Sinev, V. Yu. Bychkov, V. N. Korchak, and O. V. Krylov, Catal. Today, 6 (1990) 543. 9. V. Yu. Bychkov, M. Yu. Sinev, Z. T. Fattakhova, and V. N. Korchak, Russ. Kinet.Catal., 37 (1996) 366. 10. G. Centi, F. Trifiro, J. R. Ebner, and V. M. Franchetti, Chem. Rev., 88 (1988) 55. 11. D. J. DriscoU, W. Martir, J.-X. Wang, and J. H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062. 12. M. Yu. Sinev, V. Yu. Bychkov, Yu. P. Tulenin, B. V. Rozentuller, and A. M. Rajput, 9th Soviet- Japanese Seminar on Catalysis, Novosibirsk, Nauka, 1990, p. 75. 13. A. A. Bobyshev, V. A. Radtsig, Russ. Chem. Physics, 7 (1988) 950. 14. A. A. Bobyshev, V. A. Radtsig, Russ. Kinet. Catal., 29 (1989) 638. 15. M. Yu. Sinev, Catal. Today, 13 (1992) 561. 16. K.-I. Aika and J. H. Lunsford, J. Phys. Chem., 81 (1977) 1393. 17. L. T. Weng and B. Delmon, Appl. Catal., 81 (1992) 141. 18. M. Yu. Sinev, Catal. Today, 24 (1995) 389. 19. Yu. P. Tulenin, M. Yu. Sinev, and V. N. Korchak, 1 lth Int. Congress on Catalysis, June 30 - July 5, 1996, Baltimore, ML, USA, Programme and Book of Abstracts, P-275.

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

337

Alternative m e t h o d s to prepare and m o d i f y v a n a d i u m - p h o s p h o r u s catalysts for selective oxidation o f h y d r o c a r b o n s . V.A.Zazhigalovla, J.Haberlb, J.Stochlb, A.i.Kharlamov 2, i.V.Bacherikovala and L.V.Bogutskaya Ia Ukrainian-Polish Laboratory of Catalysis: a) Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Pr.Nauki 31, Kyjiv-22, 252022 Ukraine b) Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek, Krakow, 30-239 Poland 2 Institute for Materials Science Problems, National Academy of Sciences of Ukraine, Kryjanovski 3, Kyjiv, 252680, Ukraine Among numerous compounds formed in vanadium-phosphorus-oxide system, vanadyl pyrophosphate is known to be an efficient catalyst for C4-C5 paraffins partial oxidation [1 ]. Typical process of its synthesis can be represented by a following scheme: ROH, Reductant V 2 0 5 -k- H3PO4

> V O H P O 4 . 0 . 5 H 2 0 . . . . . 5> ( V O ) 2 P 2 0 7

It has been established that the properties of vanadyl pyrophospate are strongly dependent on its biography, i.e. the preparation method, presence of overstoichiometric phosphorus and additives [1-4]. Therefore, considerable effort of the researchers was directed to optimization of the synthesis technique and in recent studies also, non-traditional methods for the catalysts preparation were considered [5-10]. It has been shown by us [9,10] that mechanochemical treatment is a perspective method to modify the properties of the precursor VOHPOa.0.5H20 and thus, to influence the catalyst prehistory. The present paper deals with the possibilities of mechanochemical and barothermal treatments applied at different stages of the catalyst synthesis: the initial reactants, the precursor and the final catalyst. 1. E X P E R I M E N T A L

V205 (purefor analysis) and H3PO4 (pure) were used as initial reagents. The synthesis of the precursor of VPO catalysts was carried out according to the procedure described in [11 ], starting from V205 and H3PO4 in butanol medium in the presence of organic reducing agent. The solid product, after filtration, was heated stepwise up to 300 ~ in vacuum (total time was 60 h). The activation* of the precursor i.e. its transformation into the (VO)2P207 phase was performed in a reactor, just before the catalytic test, with a gas reaction mixture consisting of 1.7% Call10 in air. The activation was carried for 72 h at the temperature gradually rising up to 440~ Witht the exception of samples activated by means of mechanochemistry

338 Mechanochemical treatment was applied at different stages of the synthesis described by scheme 1: to the starting reagent V205, precursor VOHPO4.0.5H20, final product (VO)2P207 and mixtures of the powder VPBiO precursor + La203. It was carried out in a planetary mill at 3,000 rpm. La203 was prepared prior to the milling, by decomposition of La2(CO3)3.xH20 (Aldrich) in an inert gas flow. The solids for the treatment were either suspended in ethanol or water or used without any dispersant (dry milling). Barothermal synthesis and treatment were carried out in a stainless steel autoclave lined with internal Teflon glass (V - 20 cm3). The "barothermal" procedure included its both well known variants named hydrothermal synthesis (in the presence of water) and organothermal one (in the presence of organic compounds) as well as synthesis without any solvent. For the synthesis the powders of starting compounds and phosphoric acid with/without solvent were loaded into the Teflon glass. For modification of VPBiO, precursor grains (D = 5 mm, L = 5-6 mm) were placed in the glass mold in the reactor, and the modificator was located in the space between the glass and autoclave walls. Different temperatures and times of treatment were applied in these experiments. The method of the barothermal treatment was described in details in [ 12]. Phase composition of the samples was analyzed using DRON-3M X-ray diffractometer with Cu Ka radiation. The specific surface area (SsA) of the samples was measured by BET method on Gasochrom-1. Thermal analysis was carried out with the thermoanalytical instrument Derivatograph Q-1500 D (system F.Paulik-J.Paulik-L.Erdey) in helium atmosphere at a heating rate of 10 K/min. The surface composition was examined using VG ESCA-3 X-ray photoelectron spectrometer (A1 Kal.2). The spectra were calibrated against C ls (284.8 eV) line as the standard in the binding energy determination. Jeol-100 CX transmission electron microscope and Nanoscope scanning tunneling microscope were used for the investigation of morphology. Details of the measurements and data processing are given in [ 11 ]. Catalytic properties of the synthesized samples after activation were examined in the hydrocarbon-air reaction mixture in reactions of the oxidation of: i) n-butane (1.7 vol. % in air) to maleic anhydride, ii) butene-2 (1.6 vol. % in air) to maleic anhydride, iii) n-pentane (1.2 vol. % in air) to maleic and phthalic anhydrides, and iv) propane (1.8 vol. % in air) to acrylic acid. Catalytic tests were performed in the flow system with GC control of the reaction products. 2. RESULTS AND DISCUSSION

2.1. Mechanochemistry 2.1.1. Mechanochemical modification of the initial reagents for synthesis of VPO catalyst To prepare VPO precursor (P/V = 1.15) samples of the V205 reagent untreated (V205-R) and after wet (in ethanol, V205-E) and dry (V2Os-D) milling were used. Some properties of these solids are given in Table 1. It has been established that mechanochemical treatment increases the specific surface area of V205 and produces V 4+ ions. The latter phenomenon is indicated by an appearance of the low-energy contribution in the XPS spectrum of V 2p electrons. The STM study [13,14] showed that after the mechanochemical treatment in ethanol the change of V205 texture took place due to an anisotropic plastic shift deformation along the planes parallel to (001). This leads to an increase of the relative exposure

339

Table 1. Properties of V2_Qs_before and after its mechanochemical treatment Sample

V205-R V2Os-D V2Os-E

SSA

Treatment Medium Time, min.

m2/g

Air Ethanol

3.8 13.8 8.8

XRD R*

12 30

1.33 0.85 4.33

W*

3.5 7.5 3.5

XPS Binding energy O 1s V2p(1) V2p(2)

v(1)/

531.0 531.1 530.6

0 0.09 0.37

516.3 516.1

517.8 517.6 517.6

V(2) .

* R - the ratio of I(001)/I(~10) indexes intensity, W - WHPM - width at the half of the (001) reflection of the latter over the surface of V205 (see Table 1) while an average size of the particles remains almost unchanged [13]. On the contrary, dry milling of V205 by chaotic destruction of the crystals produced smaller particles, which is reflected by the increase of the XRD peaks width (Table 1). The degree of surface reduction of vanadium pentoxide (given by the content ratio V(1)/V(2) in Table 1) was much higher after the treatment in ethanol as compared to dry milling. No special features of the synthesis of VPO-D from VzOs-D were observed as compared to traditional VPO-R compound synthesis, both lasting 12-16 h. But synthesis of VPO-E precursor using VzOs-E proceeded at a much higher rate and in the presence of organic reducing agent was completed in 1h (sample VPO-E 1). Another preparation using V2Os-E was carried out without reducing agent and with smaller amount of the solvent (sample VPO-E2). In this case the time needed for the full formation of the precursor phase was about 2.5-3.0 h. Some properties of the prepared VPO precursors are listed in Table 2. Table 2 Properties of VPO precursors prepared from V2Q5 treated mechonochemically Sample

SsA

DTA

XRD*

Tendo Texo I0.570/I0.329 VPO-R VPO-D VPO-E1 VPO-E2

mZ/g

~

~

20.2 19.5 4.6 12.1

448 435 468 448

500 485 505 495

/I0.293 75/46/100 100/40/90 73/45/100 100/35/76

XPS _Binding energy, eV O 1S V 2p P 2P

(P/V)s

532.3 532.2 532.4 532.3

2.08 2.00 3.30 2.05

517.5 517.4 517.5 517.5

133.6 133.6 133.7 133.7

*Intensity ratio for reflections at d = 0.570, 0.329 and 0.293 nm It follows from the data in Tab.2, that reduction of the V205 particles size results in VPOD precursor in increased intensity of the 0.570 nm peak attributed to the exposure of (001) plane containing the vanadyl groups. Also some decrease of the temperatures, at which the amorphous (Tendo)and the crystal (Texo) phases are formed in the course of vanadyl pyrophosphate preparation, was observed (see [10, 15] for details on phase transformations). The change of V205 texture during its mechanochemical treatment in ethanol leads to the synthesis of VPO-E 1 precursor with low specific surface area and unchanged texture. It can be

340 assumed that freshly formed microcrystalls of the precursor, during the fast synthesis, rapidly grow into agglomerates. This sample shows also an increased temperature of amorphization and of crystallization during the formation of vanadyl pyrophosphate and the increased surface P/V ratio. Modification of the conditions of synthesis, consisting in some deceleration of the process, allows the preparation of the sample (VPO-E2) with larger specific surface area. Moreover, it has favourable morphology with high exposure of (001) crystallographic plane. Table 3 shows the catalytic properties of these samples. One can see from the data, that all catalysts synthesized on the basis of the mechanochemically treated V205 show an increased selectivity towards maleic anhydride and higher specific rate of n-butane and npentane oxidation as compared to those obtained in traditional synthesis. The best effect in the improvement of selectivity can be reached by increase of the relative exposure of (001) plane at the VOHPO4.0.5.H20 surface which is known to be transformed into (200) plane of (VO)2PzO7. The low paraffins conversion over VPO-E samples at the given reaction conditions can be directly connected with their low specific surface area. The comparison made between samples VPO-E1 and VPO-E2 shows that the precursor synthesis using VzOs-E needs to be optimized in order to improve the catalytic performance. Nevertheless, the results clearly demonstrate that mechanochemistry is obviously a promising method for the pretreatment of initial reagents in order to synthesize efficient VPO catalysts of paraffins oxidation. Table 3 Properties of VPO catalysts prepared using mechanochemically treated V205 in reactions of paraffins oxidation. Sample

n-Butane oxidation X, % SMA,% W. 104

n-Pentane oxidation X,% SMA,% SPA,% W. 104

VPO-R VPO-D VPO-E1 VPO-E2

73 79 25 63

52 59 17 43

58.5 68.5 60.0 69.0

1.13 1.45 1.60 1.59

21 35 33 30

12 10 4 6

0.92 1.17 1.32 1.29

Note: X - paraffin conversion, SMA, SpA- selectivity to maleic and phthalic anhydride, respectively, W - specific rate of the paraffin oxidation, mol/h m 2. For n-butane T = 425 ~ GHSV = 3000 h -1 and for n-pentane T = 320 ~ GHSV = 1800 h -~ 2.1.2. Mechanochemical modification of the VPO precursor Previously we showed [ 13,16] that the nature of dispersant had a quite remarkable effect on the properties of VPBiO precursor which was subjected to a short-time mechanochemical treatment (up to 10 rain.) and the best result was obtained when ethanol was used. Here we will describe the influence of the time of treatment on the properties of VPO precursor (P/V = 1.07). As can be seen from Table 4, the longer is the mechanochemical treatment (up to 20 min) the higher are both specific surface area of the precursor and relative intensity of its (001) crystallographic plane reflection. The latter observation can be explained by anisotropic plastic deformation of the crystals arising from their layered structure. The observed increase of the surface P/V ratio is in a good agreement with the mechanism of the VOHPO4.0.5H20 phase transformation [ 15].

341 Table 4. Properties of VPO precursor after its mechanochemical treatment in ethanol Sample Time SsA treatm.,

min

mZ/g

XRD

I0.570/ I0.293

XPS Binding energy, eV O 1s V 2p P2p

n-butane oxidation* (P/V)s

SSA1

X

S

VPO 6.0 74/100 531.7 517.5 133.9 1.43 9.4 62 61 VPO10 10 8.8 95/100 531.6 517.4 133.8 1.62 12.3 68 65 VPO20 20 14.2 100/78 531.8 517.4 133.7 1.80 17.2 73 70 VPO30 30 8.0 ** 532.1 517.3 133.7 1.92 8.5 77 74 VPO60 60 6.4 *** 532.2 517.3 133.7 1.84 6.5 70 68 *T=440~ GHSV=3200h 1, S S A 1 - specific surface area after catalysis (m2/g), X - n-butane conversion (%), S- selectivity to maleic anhydride (%); the samples VPO30 and VPO60 did not need activation prior to catalysis, **Amorphization of the sample, very weak peaks at d = 0.328, 0.305 and 0.285 nm ***All reflections ofvanadyl pyrophosphate are present, the most intense one is at d = 0.313 nm Continuation of the mechanochemical treatment leads to amorphization of the sample followed by the formation of vanadyl pyrophosphate phase. It should be however noted that even after 60 min. of the treatment this compound is not well crystallized and the specific surface area of the final catalyst is much smaller than that of a sample obtained by in situ activation of a precursor after 20 min of its mechanochemical treatment. It follows from the results presented in Table 4, that the samples obtained by mechanochemical treatment of the precursor become more active in the reaction of n-butane partial oxidation so that the hydrocarbon conversion and selectivity to maleic aL!hydride increase. The sample converted into vanadyl pyrophosphate by means of the mechanochemical treatment turned out to be more efficient than that activated with the reaction mixture. The most interesting is the sample after 30 min. treatment which is "half-activated" and consists of the amorphous phase. The active component forming directly in the catalytic mixture without long activation procedure gives rise to the most active and selective catalyst for n-butane oxidation.

2.1.3. Mechanochemical promotion of VPBiO precursor Recently [ 13, 16] we have shown the possibility of efficient promotion of VPO precursor with bismuth compounds. The present paper reports new results on promotion of VPBiO precursor with lanthanum compounds (previously a similar catalyst was shown to be active in tetrahydrofuran formation [17]). Table 5 compares the properties of traditionally prepared VPBiLaO sample (by simultaneous introduction of bismuth and lanthanum additives in the course of the synthesis of VPO precursor) with that (VPBiO-La-M) produced by the mechanochemical treatment of VPBiO precursor and lanthanum oxide powders. The treatment in the latter case was carried out for 10 min. in ethanol medium. It can be seen that introduction of lanthanum by both methods leads to an increase of the catalytic activity. At the same time, its introduction in the course of the synthesis of VPBiO precursor causes a decrease of the selectivity to partial oxidation products in both investigated reactions: oxidation of butane and propane. This negative effect on selectivity was also observed when the catalyst was prepared by means of mechanochemistry but to a much lesser degree. As a result, the latter catalysts were more efficient and produced higher yield of the desired product (see Table 5 data). It can be noted that for the traditional sample a higher value of V 2p-electrons binding energy is observed suggesting that the oxidation degree of vanadium can be in this case higher than in the traditional samples.

342 Table 5 Properties of VPO precursor with additives bismuth and lanthanum. Sample*

VPBiO VPBiLaO3 VPBiLaO5 VPBiO-La3M VPBiO-La5M

XPS Binding energy, eV** P/V Bi/V La/V O 1 s V 2p P 2p

Oxidation n-Butane*** Propane**** X SMA Y X SAg Y

531.5 531.8 531.7 531.6 531.5

48 56 61 55 58

517.4 517.9 517.9 517.5 517.5

133.7 1.58 0.12 134.0 1.61 0.17 0.027 133.9 1.88 0.14 0.063 133.8 1.73 0.09 0.023 133.6 1.95 0.08 0.058

69 61 54 68 66

33 34 33 37 38

40 49 60 47 59

32 21 17 30 28

13 10 10 14 16

*Number in the sample name represents the atomic ratio (La/V).100 ** B.E. Bi 4f = 159.9 160.2 eV, La 3d = 836.5-836.7 eV *** T = 420 ~ GHSV = 3000 h "l, X - n-butane conversion (%), SMA - selectivity to maleic anhydride (%), Y - yield of maleic anhydride (mol. %); **** T = 435 ~ GHSV = 1500 h -1, X - propane conversion, %, SAA - selectivity to acrylic acid, %, Y - yield of acrylic acid, mol. %

2.1.4. Mechanochemical modification of vanadyl pyrophosphate Vanadyl pyrophosphate was prepared by heating the VPO precursor in an inert gas flow at 550 ~ for 24 h. Its characteristics are listed in Table 6. One can see from TSM pictures (Figure 1a) that the catalyst prepared in this way is composed of quite large aggregates including crystals of geometrically-regular shape. Mechanochemical treatment disintegrates them to produce much smaller particles of different shape (Figures 1 b,c,d). It is noteworthy, that in the case of the treatment of (VO)2P207, there is no dependence of the morphology on the dispersant nature at variance with the case of the VPO precursor treated similarly [13]. An increase of the intensity of the reflection at d = 0.387 nm corresponding to (200) plane of vanadyl pyrophosphate was observed by XRD to be the only structural change occurring at the treatment in ethanol. As a result, the catalyst after treatment shows an increase of both activity and selectivity in n-butane partial oxidation. The specific rate of the hydrocarbon conversion decreases after the treatment which is believed to be due to non-proportional growth of the number of active centers and the specific surface area. Table 6 The properties of vanadyl pyrophosphate after mechanochemical treatment Treatment

SSA

Solvent Time, min. m2/g Water Ethanol

10 10 10

* T = 440 ~

4.6 9.2 5.4 7.1

XRD

I0.387/ I0.313 82/100 95/100 92/100 100/73

XPS Binding energy, eV O lS V 2p P 2p 531.5 531.6 531.8 .

517.5 517.4 517.5 . .

133.8 133.6 133.8 .

n-Butane oxidation* SMA, W-10 4 %

P/V

X, %

1.22 1.34 1.42

68 76 69 77

GHSV = 1500 h l , W - rate of n-butane oxidation, mol/h.m 2

60 58 62 69

1.44 0.86 1.19 1.08

343

a.

b.

Figure 1. Transmission Scanning Microscope pictures of vanadyl phyrophosphate, a) initial, and after mechanochemical treatment: b) in water, c) in ethanol and d) dry milling. 2.2. Barothermal synthesis 2.2.1. Barothermal synthesis of VPO catalysts Soon after publications of J.Johnson and A.Jacobson [18,19] hydrothermal synthesis has begun to be applied to different vanadium phosphates synthesis. In the present work an attempt has been undertaken to use organothermal (with n-butanol addition) synthesis and that without any solvent. It has been established that in hydrothermal synthesis starting from V205 and H3PO4 it is possible "depending on the synthesis temperature and duration" to obtain VOPO.2H20 and VOPO4.H20. When VO2 is used the mixture of vanadium (IV) and (V) compounds such as 13VOPO4 and VOHPO4.0.5H20 can be formed. Their catalytic performance in n-butane and butene-2 oxidation (better for samples HS-1) is worse than that of the catalysts prepared by other methods.

344 Organothermal synthesis with the use V205 leads to the formation of [3-VOPO4 (OS-1). The latter compound shows low activity in paraffin oxidation, but is a quite efficient catalyst for olefin oxidation (Table 7). When VO2 was used for organothermal synthesis, an unknown compound was obtained at low temperature and/or short time of the synthesis. Continuation of the synthesis led to the formation of VOHPO4.0.5H20 (OS-2). In the synthesis using V205 in solvent-free conditions, 13-VOPO4 was found to be the only product (AS-1) but quite a high temperature and long time were needed for the reaction to be completed. In the case of the use of VO2, a new compound was formed. Table 7 Properties of the barothermally synthesized samples Butene-2 oxidation*** SMA,% W 104

n-Butane oxidation** W 104

Sample*

X, % SMA,% TS-1 TS-2 HS-1 OS-1 OS-2 AS-1

68 63 35 75 28

60 60 20 64 21

X, o~

1.44 1.11 1.02 1.76 0.48

76 78 74 71 70

71 55 69 51 72

3.22 2.86 3.31 3.14 3.06

* T S - 1 , -2 - catalysts prepared following traditional methods for n-butane and butene-2 oxidation, respectively **T = 440 ~ GHSV = 1500 h -l', ***T = 380 ~ GHSV = 3600 h -1', X hydrocarbon conversion, SMA - selectivity to maleic anhydride, W - rate of hydrocarbon oxidation, mol/h.m 2 2.2.2. Barothermal

modification

of VPBiO

precursor

The VPBiO precursor treatment was performed with n-butanol and phosphoric acid vapours as the perspective media for the treatment of VPO [ 12]. The obtained results, sortie of which are listed in Table 8, show that the higher is the temperature and the longer is the treat ment the lower becomes the specific surface area. At the same time, it should be noted that Table 8 An influence of VPBiO precursor barothermal treatment on its properties Sample*

VPBiO VPBiOnbl VPBiOnb2 VPBiOnb3 VPBiOpal VPBiOpa2 VPBiOpa3

Treatment Time T, h ~ -

6 10 6 14 10 6

-

250 250 300 200 250 300

SSA

XRD I0570/ m Z / g I0293 12.5 11.2 10.0 9.3 12.0 10.2 8.7

100/98 100/78 100/76 100/39 100/76 100/49 100/45

XPS Binding energy, eV O ls V 2p P-2p Bi 4f 531.5 531.7 531.6 531.4 531.1 531.4 531.3

517.4 517.5 517.6 517.1 517.1 517.3 517.3

133.6 133.9 134.0 133.5 133.4 133.5 133.4

159.9 160.2 160.1 159.7 159.7 159.8 159.7

*nb, pa- treatment with n-butanol and phosphoric acid, respectively

(P/V)s (Bi/V)s 1.47 1.87 2.08 2.07 2.23 2.21 1.98

0.08 0.12 0.10 0.10 0.10 0.11 0.11

345 such treatment leads to some change of the structure of VPBiO precursor as it can be seen from XRD results. An increase of the relative intensity of the reflection at d = 0.570 nm attributed to crystallographic plane having vanadyl groups is namely observed. Enrichment with phosphorus is found at the surface of the particles after treatment. Similar effect could be expected at the treatment with phosphoric acid but looks unusual in the case of the treatment with alcohol. The catalytic properties of the samples are presented in Table 9. The data indicate that the barothermal treatment favours an increase of the selectivity in paraffins oxidation. Moreover, the treatment in n-butanol also leads to the growth of catalytic activity of the samples. In the case of the treatment with phosphoric acid vapours, the catalytic activity remains almost unchanged, due to effect of water steam as described in [ 12]. Table 9 Catalytic properties VPBiO precursor after barothermal treatment Sample

SSA 1

m2/g VPBiO VPBinbl VPBinb2 VPBinb3 VPBipal VPBipa2 VPBipa3

14.6 12.0 9.8 9.0 11.6 9.7 8.5

n-Butane oxidation* X, % SMA,~ 49 50 57 57 48 50 45

68 72 75 77 73 81 76

n-Pentane oxidation** X,% SMA,% SPhA,% 67 -

32 -

17 -

61 65 65

39 42 36

14 18 21

SSA1 - specific surface area after catalysis * T = 400 ~ GHSV = 2400 h -1 *** T = 420 ~ GHSV = 1500 h -1

Propane oxidation*** X, o~ SAA,o~ 26 28 29 31

48 55 59 58

-

GHSV = 3000 h -1 **T = 350 ~

Concluding, it should be emphasized that mechanochemical and barothermal methods have been shown to be advantageous as alternative technologies for preparation and modification of VPO catalysts for partial oxidation of saturated hydrocarbons. ACKNOWLEDGMENT This study was supported in a part by ISF (Grants UBI000 and UBI200) and in a part by Scientific Research Committee (Poland) Grant No 3T09A 08010. The authors thank Prof. V.G.Iljin, Dr. G.A.Komashko and V.E.Yaremenko for assistance in some experimental work.

REFERENCES 1. 2. 3.

G. Centi (editor), Vanadyl Pyrophosphate Catalysts, Catal. Today, 16 (1993) 1-147. G.J. Hutchings, Appl. Catal., 72 (1991) 1. F. Cavani and F. Trifiro, Preparation of Catalysts VI, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 1.

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V.A. Zazhigalov, V.M. Belousov, G.A. Komashko, A.I. Pyatnitskaya, Yu.N. Merkureva, A.L. Poznyakevich, J. Stoch and J. Haber, Proc. 9th Int. Congr. Catal., Chem. Inst. of Canada, Ottawa, 4 (1988) 1546. 5. P.F. Miquel and J.L. Katz, Preparation of Catalysts VI, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 207. 6. P.M. Michalakos, H.E. Bellis, P. Brusky, H.H. Kung, H.Q. Li, W.R. Moser, W. Partenheimer and L.C. Satek, Ind. Eng. Chem. Res., 34 (1995) 1994. 7. V.V. Guliants, J.B. Benziger and S. Sundaresan, J.Catal., 156 (1995) 298. 8. P.F. Miquel, E. Bordes and J.L. Katz, J.Solid State Chem, 124 (1996) 95. 9. L. Bogutskaya, V. Zazhigalov, M. Misono and T. Okuhara, Japan-FSU Catal. Seminar'94, Catal. Sci and Techn. 21 Century Life, Tsukuba, Japan, (1994) 202. 10. V.A. Zazhigalov, J. Haber, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, Appl. Catal. A, 135 (1996) 155. 11. V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Pyatnitskaya, G.A. Komashko and V.M. Belousov, Appl. Catal. A, 96 (1993) 135. 12. V.A. Zazhigalov, I.V. Bacherikova, V.E. Yaremenko, I.M. Astrelin and J. Stoch, Teoret. Eksperim. Chem., 31 (1995) 206. 13. V.A. Zazhigalov, J. Haber, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, 1 l th Int. Congr. on Catal. - 40th Anniversary, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 101B (1996) 1039. 14. V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Kharlamov, L.V. Bogutskaya, I.V. Bacherikova and A. Kowal, Solid State Ionics (in press). 15. C.C. Torardi, Z.G. Li and H.S. Horowitz, J.Solid State Chem., 119 (1995) 349. 16. J. Haber, V.A. Zazhigalov, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, Catal. Today (in press). 17. V.A. Zazhigalov, J. Haber, J. Stoch, G.A. Komashko, Ai. Pyatnitskaya and I.V. Bacherikova, New Develop. Select. Oxid. II, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 82 (1994) 265. 18. J.W. Johnson, A. Jacobson, J.F. Brody and S.M. Rich, Inorg. Chem., 21 (1982) 3820. J.W. Johnson and A. Jacobson, Angew. Chem., 95 (1983) 442. D

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

Active species and working mechanism of silica supported MoO3 and catalysts in the selective oxidation of light alkanes

347

V205

A. Parmaliana% F. Arena% F. Frusteri b, G. Martra c, S. Coluccia r and V. Sokolovskii d aDipartimento di Chimica Industriale, Universitb, di Messina, Salita Sperone 29, 98166 S. Agata (Messina), Italy blstituto CNR-TAE, Salita S. Lucia 39, 98126 S. Lucia (Messina), Italy CDipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universit/l di Torino, Via P. Giuria 7, 10125 Torino, Italy aDepartment of Chemistry, University ofWitwatersrand, Johannesburgh, P.O. Box 106, South Africa The catalytic performance of a series of silica supported (2-7 wt%) MoO3 and (2-20 wt%) V205 catalysts has been comparatively evaluated in both the partial oxidation of methane to formaldehyde (MPO) and the oxidative dehydrogenation of propane to propylene (POD) in the range 500-800~ and 500-525 ~ respectively. V205 acts as a promoter of the reactivity of the SiO2 for both MPO and POD, while MoO3 plays a generally negative effect on the catalytic functionality of the SiO2 surface acting however as a promoter in the MPO at T>650~ A direct relationship between the density of reduced sites of low-medium loaded silica based oxide catalysts and reaction rate in both MPO and POD strongly suggests the occurrence of a concerted reaction mechanism involving the activation of gas-phase 02 on the reduced sites of the catalyst surface. The nature of the active sites has been defined on the basis of a complete characterization of the surface and redox features of MoO3/SiOz and V205/SIO2catalysts. 1. INTRODUCTION The disclosure of the mechanism of a catalytic reaction leads to the identification of the active sites being then the basis for the design of more effective catalytic systems. Generally, a mechanistic model is adequate for describing the pathway of a class of reactions on a class of catalysts, however this rule is not completely valid for the partial oxidation of light alkanes on bulk and/or supported oxide catalysts [ 1]. Since the excellent performance of supported MoO3 and V205 based catalysts in the selective oxidation of light (C~-C3) alkanes, during the last decade a considerable research effort has been directed to ascertain the working mechanism of such oxide systems as well as the nature of the active sites and the origin of the oxygen involved in the formation of reaction products. However, no definitive conclusions have been still provided about these issues and therefore a much deeper investigation of the reaction dynamic mechanism and the correlation with the nature of the surface is necessary [2]. Several factors, such as the nature of the support, the oxide loading and the reaction conditions control the formation, the coordination and the stabilization as well as the catalytic action of the

348 various surface oxide species. On this account, we have evaluated the catalytic behavior of silica supported MoO3 and V205 systems in both the selective oxidation of methane to formaldehyde (MPO) [3,4] and the oxidative dehydrogenation of propane to propylene (POD) [5] disclosing that V205 acts as a promoter of the reactivity of the SiO2 surface while the action of MoO3 strictly depends upon the kind of the silica support [3-5]. The aim of this paper is to provide a correlation between the catalytic pattern of differently loaded silica supported MoO3 and V205 catalysts in MPO and POD reactions with their surface and redox features in order to highlight the nature of the active surface species in the selective oxidation of light alkanes. 2. EXPERIMENTAL 2.1. Catalysts

Differently loaded (2-7 wt%) MoO3/SiO2 and (2-50 wt%) V205/SIO2 catalysts were prepared by incipient wetness impregnation of a "precipitated" silica (PS) support (Si 4-5P Grade, Akzo Product; S.A.BET, 400 m2.g~) according to the procedure described elsewhere [3]. The list of the catalysts along with their composition and BET surface area values are reported in Table 1. Table 1 List of catalysts Code Chemical composition

(wt %) PS VPS 2 VPS 5 VPS 10 VPS 20 VPS 50 V205 MPS 2 NIPS 4 NIPS 7 MoO3

SiO2 2.0% V205/SIO2 5.3% V205/5iO2 10.1% V205/5i02 20.8% V2OJSi02 50.8% V2OJSi02 V205 2.0% MoO3/SiO2 4% MoOa/SiO2 7% MoO3/SiO2 MoO3

S.A.BET

(m~.~-') 400 260 230 200 190 160 5 300 190 75 2

2.2. Catalytic measurements

Catalytic measurements in MPO were performed by Temperature Programmed Reaction (TPR) tests [4] using a conventional flow apparatus and a linear quartz microreactor connected on line with a Thermolab (Fisons Instruments) Quadrupole Mass Spectrometer (QMS) for continuous scanning of the reaction stream. TPR tests were run in the T range 400-800~ by using 0.05 g of catalyst, a heating rate (13) of 10~ "1 and a reaction mixture He/CH4/O2 in the molar ratio 7:2:1 flowing at 50 STP cma.min-1. Catalytic measurements in POD have been performed at atmospheric pressure in the range 500-525~ using 0.25 g of catalyst sample diluted with same sized SiC (1/5, l/vol) and a reaction mixture in the molar composition CaH8:O2:N2:He=2:l:l:6 flowing at the rate of 100 STP cmS-min'l[5]. All the tests have been carried out at GHSV of 1,700 h"1 (STP m3C3H89m'aeat-h'l).

349

2.3. Catalyst characterization Temperature Programmed Reduction (H2-TPR) measurements were performed using a linear quartz gradientless microreactor and a 6% H2/Ar mixture flowing at 60 STP cm3 rain~ according to the procedure described elsewhere [6]. High Temperature Oxygen Chemisorption (HTOC) measurements were performed in a pulse mode according to the procedure described in detail elsewhere [6]. Reaction Temperature Oxygen Chemisorption (RTOC) measurements in the range 500650~ were performed in a pulse mode after treatment of the samples in the C3HffO2/He or CH4/Oz/He reaction mixture. 02 pulses (Vp,l,o, 4-10.8 mol 02) were injected into the carrier gas until saturation of the sample was attained, the density of reduced sites (p, 1016 sites.gc,t~) being calculated by assuming a chemisorption stoichiometry 02:reduced site = 1:2 [3,5]. Diffuse Reflectance UV-Vis DR UV-Vis spectra of differently loaded V2OflSiO2 samples, calcined in situ in 02 at 600~ were obtained by a Perkin Elmer Lambda 19 spectrophotometer, equipped with an integrating sphere. 3. RESULTS and DISCUSSION

3.1. Catalytic activity Methane partial oxidation (MPO). The catalytic activity of differently loaded MPS and VPS samples in the range 500-800~ expressed in terms of normalized specific surface activity, NSSA (NSSA=SS&/SSAps, where SSAi and SSAps are the specific surface activity of the catalyst i and bare PS support, respectively), is compared in Figure 1A. 1.0

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350 The HCHO selectivity as a function of CH4 conversion in the range 500-800~ for MPS and VPS catalysts is shown in Figure lB. For MPS 2 and MPS 4 catalysts, the trend of HCHO selectivity with CH4 conversion (Fig. 1B) is similar to that of the unloaded PS, whereas for the MPS 7 sample a significant improvement in HCHO selectivity at conversion levels lower than 3% is observed. By contrast, at the same level of CH4 conversion, a progressive decline in HCHO selectivity with V205 loading occurs on VPS catalysts. Medium loaded MPS 7 and highly loaded VPS 20 samples are the most and the least selective systems respectively. Oxidative dehydrogenation of propane (POD). The predominant products in the POD over silica based oxide catalysts in the range 500-525~ were propylene and carbon oxides. Ethylene and acetaldehyde along with a considerable amount of acrolein and traces of propionaldehdye are formed on PS [5]. The addition of MoO3 and V205 to the SiO2 support implies a higher selectivity to propylene and correspondingly a lower production of COx, a slight cracking activity and a significant decrease in the amount of oxygenates. Table 2 shows a detailed comparison of the activity of bare and differently loaded MPS and VPS catalysts in terms of propane conversion, propylene selectivity, reaction rate and C3I-I6 productivity values. The catalytic functionality of the SiO2 surface is strongly promoted by the addition of V205, while it is depressed by MOO3, the extent of this effect rising with the MoO3 loading [9]. Table 2 Activity of bare and promoted silica catalysts in the oxidative dehydro[genation of propane Catalyst T C3Hs conv. C3I-I6 s e l . Reaction rate STYc3H6 (%) (].tmol.gc.tl.s "1) (g.kgc.t'l.h "1) (~ (%) PS 5OO 0.9 37 0.49 27.5 525 1.9 37 1.04 57.7 MPS 2 500 0.8 62 0.44 41.4 525 1.3 58 0.71 62.5 MPS 4 5OO 0.7 69 0.39 40.8 525 1.4 59 0.75 67.1 MPS 7 500 0.2 80 0.09 6.5 0.4 525 85 0.18 11.6 VPS 5 500 7.8 60 4.25 386.4 525 13.3 55 7.25 602.8 VPS 10 5O0 9.8 51 5.34 413.0 525 177 41 9.65 600.0 VPS 20 500 7.8 27 4.25 174.0

3.2. Redox properties of MoO3/SiO2 and V2Os/SiO2 catalysts under reaction conditions In previous papers we disclosed a direct relationship between catalytic activity in MPO of low-medium loaded silica based oxide catalysts and oxygen uptake under steady state reaction conditions pointing out that such property governs the catalytic behavior of MPO catalysts [3,7,8]. Then, in order to find out whether such a relationship is also valid for POD reaction, the density of reduced sites (p) of various silica based MoO3 and V205 catalysts in both MPO and POD has been evaluated and related with their catalytic activity. The direct relationships between reaction rate in MPO and POD and p, shown in Figure 2A, well account for the opposite effects exerted by MoO3 and V205 on the activity of the bare PS carrier in both MPO [3,4,6-8] and POD [5]. Indeed, V205 effectively promotes the activity of bare PS, and allows

351 for the stabilization of a higher density of reduced sites owing to its easier "reducibility" [6] whereas MOO3, being essentially unreducible under reaction conditions [6], depresses the activity of the PS carrier because of a negative physical effect linked to the partial coverage of the silica surface own active sites [3,6,8]. However, though the above relationships (Fig. 2A) explain the origin of the catalytic action of low-medium loaded (___5wt%) MPS and VPS systems, they do not account for the activity of highly loaded (>__10wt%) VPS catalysts in MPO. 15

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Indeed, the activity of VPS catalysts reaches the maximum for VPS 5 sample, while the density of reduced sites increases steadily up to a loading of 20% (VPS 20) resulting in the peculiar volcano-shaped relationship between P and reaction rate shown in Figure 2B. This singular behavior is to be related with the capability of highly loaded V2Os/SiOz systems to "loose" constitutional oxygen [6] under reaction conditions which, however, is ineffective towards the selective oxidation of light alkanes [5,8,9]. 3.3. Surface structures of MoO3/SiO2 and V2Os/SiO2 catalysts

The H2-TPR profiles of differently loaded MPS and VPS catalysts in the range 200-1200~ compared with those of bulk MoO3 and V205 respectively, are shown in Figure 3A and B, while the values of oxygen uptake (HTOC) and oxide dispersion (O/Me) are listed in Table 3. A wide and convoluted band of H2 consumption starting (To, ,~d) at T ranging between 435 (MPS 7) and 486~ (MPS 2) and spanning a T range of 500-600~ accounts for the stoichiometric reduction of MoO3 to Mo Oin all MPS catalysts. The spectrum of the low loaded MPS 2 catalyst features a low rate of H2 consumption up to ca. 800~ thereafter the reduction rate increases sharply giving rise to a main reduction peak with maximum at 934~ (Fig. 3A, a). The increase in the MoO3 loading from 2 to 4 wt% (MPS 4) causes a marked shift of To,red to lower T (436~ and a concomitant enhancement of the reduction kinetics at lower T (<800~ giving rise to a convoluted reduction profile with two unresolved peaks with maxima at 588 and 765~ respectively (Fig. 3A, b).

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A further increase in the MoO3 loading strongly enhances the reduction rate of the system at T<800~ (Fig. 3A, c), even if the To,~d value (435~ of the MPS 7 catalyst is equal to that of the MPS 4 sample (436~ Table 3 HTOC characterization data of MPS and VPS catalysts Catalyst 02 uptake (l.tmol.g.t 1) PS 0.5 MPS 2 6.1 MPS 4 66.4 MPS 7 111.5 MoO3 20.7 VPS 2 68.0 VPS 5 186.9 VPS 10 262.1 VPS 20 328.3 VPS 50 182.3 V20s 42.8

O/Me (%) 8.8 47.8 45.9 0.6 61.9 64.5 46.6 28.7 6.5 0.8

The reduction pattern consists of two sharp peaks with resolved maxima at 574 and 703~ respectively, along with a shoulder of Hz consumption in the range 800-1050~ Because of a poor dispersion (Table 3), the reduction of bulk MoO3 (Fig. 3A, d) starts at T considerably higher (To,,od, 532~ than those found for supported MPS systems [10], resulting in two

353 overlapped peaks, with maxima at 763 and 840~ which account for the step-wise (MoVL--~MorV---~Mo~ reduction of MoO3 to Mo ~ The 02 uptake of MPS catalysts increases monotonically with the loading from 6.1 (MPS 2) to 111.5 i.tmol-g1 (MPS 7). The oxide dispersion (O/Me) results very low for the MPS 2 catalyst (8.8%), while it suddenly increases for MPS 4 sample (47.8%) keeping unchanged (45.9%) at higher MoO3 loading (MPS 7). The broad band of H2 consumption featuring the reduction pattern of MPS catalysts in comparison to bulk MoO3 is diagnostic of a strong metal oxide-support interaction which markedly depresses the reduction of MoO3 promoter [10]. By using a least-square fitting program, the spectra of MPS catalysts have been resolved into the contribution of three similar discrete Gaussian-shaped peaks, assuming that the first two peaks (i.e., MI and M2) monitor the step-wise reduction (MoVL-~MorV-->Mo~ of "MOO3 crystallites" (Mc) while the third one (M3) refers to the one-step reduction (MoVI---~Mo~ of"Isolated molybdates" species (Ira) [10]. The fitting parameters of the deconvoluted TPR profiles of MPS 2, MPS 4 and MPS 7 samples (Figures not reported here), expressed as peak maximum position (M~), full width at half maximum (FWHM0, percentage peak area (Ai, %) and concentration of Mc and Im species are reported in Table 4. Such data signal that the intensity of the first two peaks, (M0 and (ME) respectively, rises monotonically with MoO3 loading; whilst the highest temperature peak (M3), monitoring the reduction oflm species, follows an opposite decreasing trend [ 10]. Table 4 Fitting parameters of TPR spectra and percentage of Mc and Im species in differently loaded MPS catalysts Sample Mc Im Mc Im M3 FWHM3 A3 Concentration M1 FWHMI A1 M2 FWHM2 A2

(oc) (oc) (%) (~

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On the whole, the surface composition of the differently loaded MPS catalysts provides evidence of the influence of the loading on the surface structures of MoO3/SiO2 catalysts, allowing also to explain the unusual trend of chemisorption data (Table 3). Indeed, the prevailing stabilization of"hardly reducible" Im species accounts for the small oxygen uptake and quite low dispersion value in MPS 2 sample (Table 3). At MoO3 loadings higher than 2 wt%, the increasing formation of Mc (Table 4) enhances the reducibility of the system as well as the chemisorption capability and dispersion values (Table 3). However, also for the MPS 4 sample the dispersion degree calculated from HTOC data (Table 3) would be underestimated, being really much higher than that of MPS 7 catalyst [10]. On the basis of the above considerations, it can be inferred that three types of "surface sites" contribute to the reactivity of MoO3/SiO2 catalysts in the selective oxidation of light alkanes: the siloxane bridges of the silica surface [6,7]; the Mo-O-Mo bridging functionalities and Mo=O terminal bonds of Mc [6]. Then, the catalytic pattern of MoO3/SiO2 system in MPO and POD depends on both concentration and activity of various surface sites. The H2-TPR profiles of differently loaded VPS catalysts in the range 200-1100~ shown in Figure 3B, account for the stoichiometric reduction of V 5+ to V 3+ in both VPS and bulk V2Os systems. The H2-TPR pattern of the low VPS 2 catalyst (Fig. 3B, a) entails a very sharp

354 reduction peak with the maximum (TM0 at 55 I~ slightly asymmetric on the high temperature side due to the presence of a shoulder of H2 consumption zeroing at T~850~ The To,~d is equal to 360~ being the lowest found in the series. An increase in the V205 loading to 5 wt% (VPS 5) does not substantially affect the reduction pattern of the system neither in terms of To,red (364~ nor in peak shape (Fig. 4B, b) even if the TM~ is slightly displaced to higher T (561~ More evident changes in the reduction pattern of the V2OJSiO2 system (Fig. 3B, c) occur at higher V205 loading (VPS 10), since the To,roavalue further shifts to higher T (372~ while a broadening Of TM~ peak along with a further rise Of TM~ value (571~ are recorded. Moreover, two new smaller peaks, bearing resolved maxima at 638 (TM2) and 730~ (TM3), are observable (Fig. 3B, d). At a loading of 20 wt% (VPS 20), the TM~ peak decreases sharply in intensity assuming an asymmetric shape, while its maximum further shifts to higher T (TM~=584~ TM2 and TM3 peaks rise in intensity, the former becoming the predominant one. Bulk V205 (Fig. 3B, f) displays two very sharp, partially overlapping, reduction peaks centered at 649~ (TM2) and 716~ (TM3) respectively and a third broader peak at higher T with maximum at 939~ (TM4). The relative intensity of such three peaks accounts for the following sequential reduction path" V2Os~VtO~3~VO2~V203. The oxygen uptake of VPS catalysts increases monotonically with the loading attaining the maximum value (~328 l.tmol.g"~) for the VPS 20 sample. The oxide dispersion reaches the maximum value (60-62%) for low-medium loaded (2-5 wt%) catalysts, thereafter at loadings higher than 5% it decreases to 46.6 and 28.7% for VPS 10 and VPS 20 samples respectively. The energy of the oxygen~vanadium charge-transfer absorption band, which is correlated with the minimum diffuse reflectance, is strongly influenced by the number of ligands surrounding the central vanadium ion [ 11] and thus it provides insights into the influence of the loading on the coordination of V S+ ions in VPS samples. DR-UV-Vis spectra of differently loaded VPS catalysts, shown in Figure 4, signal that the loading markedly affects the structure of supported V s+ ions.

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355 keeping unchanged the Td coordination of V ions 6 [ 11]. The DR-UV-Vis spectral features of the VPS 5 sample, showing a predominance of the band at ca. 34,000 cm~, suggest that the concentration of the oligomeric structures has increased with loading, even if monomeric forms are still present (Fig. 4b). The spectral pattern of VPS 10 system is characteristic of more complex structures with a higher degree of"nuclearity" (Fig. 4c) though the Td coordination of V ions is still preserved [ 11]. Finally, the bands at ca. 25,000 and 21,500 cm~, attributable to bidimensional patches of pentacoordinated vanadium ions (Fig. 4d) and tridimensional V2Os crystallites (Fig. 4e) [ 11], with V 5+ in octahedral coordination (Oh), feature the spectra of the highly loaded VPS 20 and VPS 50 catalysts. Oxygen chemisorption data match with the above spectroscopic findings, since the maximum oxide dispersion, found for low loaded VPS 2 and VPS 5 catalysts, really corresponds to the maximum development of mono/oligomeric Td structures. Thus, the TPR spectra of differently loaded VPS catalysts indicate that the reduction pattern of V205/SIO2 depends upon the nuclearity of surface V species; in fact a rise in the extent of agglomeration of V 5+ ions causes a change in their coordination symmetry [ 11] rendering them less reducible. In fact, the very sharp reduction peak featuring the TPR spectra of low-medium (2-5.3 wt%) VPS catalysts signals mostly the presence of an easily reducible "surface V205 species" characterized by a rather uniform interaction strength with the PS support. For V205 loadings higher than 5 wt%, the concomitant shift of To,tea and TM~ to higher T, the increasing intensity of TM2-TM3peaks, characteristic of the reduction of bulk V205 (Fig. 3B), and the lowering in oxide dispersion (Table 3) altogether provide unambiguous evidences of the incipient nucleation of VO43 units into Td oligomeric and polymeric structures, like "polyvanadates" (Pv), and V205 clusters (Vc). Pv and Vc exhibit a lower reducibility than Iv species because of the change in V 5+ symmetry from Td to square-pyramidal (Sp) and octahedral coordination (Oh) respectively [6,11 ]. 4. SUMMARY The direct relationship between reaction rate and p (Fig. 2A) indicates that the selective oxidation of light alkanes on low loaded (0-5 wt%) MPS and VPS catalysts proceeds mainly via the surface mechanism [4-8]. The addition of V205 to SiOz implies a simultaneous increase in P and reactivity whilst MoO3 negatively affects both 9 and reaction rate[8]. Indeed, on increasing the MoO3 loading, the decrease in BET S.A. (Table 1) and the increasing coverage of the silica surface concur to lower the concentration of active sites of the SiOz surface causing, mainly at T<_650~ a regular decrease in SSA of MPS systems with respect to PS (Fig. 1A). Yet, a corresponding increase in HCHO selectivity leads to a featureless trend of the Yield at 600~ with the O2 uptake as shown in Figure 5A. At higher T, the capability of MoO3 promoter to interact with CH4 molecules [6] improves the SSA of MPS catalysts with respect to PS (Fig. 1A). Then, the increasing oxygen uptake (Table 3), paralleling the higher concentration of Mc (Table 4), accounts for the promoting effect of MoO3 loading on both SSA and Yield of MPS catalysts in MPO at T>650~ [7]. This is well supported by the straight relationship between Yield to HCHO at 800~ and 02 uptake, shown in Figure 5A. The negative effect of MoO3 on the functionality of PS surface also in POD results in a fiat trend of the Yield to C3H6 at 500~ vs. the 02 uptake (Fig. 5A). Such results likely reflect the particular functionality of"Mo=O '' terminal bonds of MoO3 crystallites towards the formation of selective oxidation products [6,12].

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(~ " 5'0 100 1,50 (3 2'0 4'0 ' 6'0 ' 02 uptake (nmol.mcat "2) V205 dispersion (%) Figure 5. Yield to HCHO (11, 0 ) and to C 3 H 6 (rl)ofMPS (A) and VPS (B) catalysts in MPO and POD reactions vs. O2 uptake and V205 dispersion respectively. The sudden growth in activity along with the moderate increase in p (Fig. 2B) observed for the VPS 5 catalyst suggest that at medium loading vanadia exerts a positive influence on the functionality of the PS support because of the stabilization of dispersed surface species enabling the formation of very active surface reduced sites [7]. Namely, on the basis of characterization data we infer that the highest performance of low-medium loaded (2-10 wt%) VPS catalysts stems from Iv species which ensure the formation of high extents of "active" oxygen species [5,6,8,13]. The role of Iv species in the formation of selective oxidation products is strongly supported by the exponential-like relationships between the specific oxide productivity to HCHO and C3H6 in MPO and POD respectively and V2Os dispersion, shown in Figure 5B. By contrast, the less effective redox mechanism becomes predominant for highly loaded (>5 wt%) VPS catalysts and bulk V system [8] as confirmed by the volcano-shaped relationship between reaction rate and p (Fig. 2B). Then, on highly loaded VPS catalysts, the high density of reduced sites favors a rapid incorporation of activated oxygen species into oxide lattice [8] allowing the occurrence of a classical redox mechanism which is less effective and selective than the surface concerted mechanism [6-8]. REFERENCES 1. V.D. Sokolovskii, Catal. Rev.-Sci. Eng., 32 (1990) 1 2. E.A. Mamedov and V. Cort6s Corberb.n, Appl. Catal. A, 127 (1995) 1 3. A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena and N. Giordano, J. Catal., 148 (1994) 514 4. F.Arena, F. Frusteri, A. Parmaliana and N. Giordano, Appl. Catal. A, 125 (1995) 39 5. A. Parmaliana, V. Sokolovskii, F. Arena, F. Frusteri and D. Miceli, Catal. Lea., 40 (1996) 105 6. F. Arena, N. Giordano and A. Parmaliana, J. Catal., 167 (1997) 66 7. A. Parmaliana and F. Arena, J. Catal., 167 (1997) 57 8. A. Parmaliana, F. Arena, V. Sokolovskii, F. Frusteri and N. Giordano, Catal. Today, 28 (1996) 363 9. M. Puglisi, F. Arena, F. Frusteri, V. Sokolovskii, A. Parmaliana, Catal. Lea., 41 (1996) 41 10. F. Arena and A. Parmaliana, J. Phys. Chem., 100 (1996) 19995 11. M. Schraml-Marth, A. Wokaun, M Pohl and H.-L. Krauss, J. Chem. Soc., Faraday Trans., 87 (1991)2635 12. M.R. Smith and U. S. Ozkan, J. Catal., 141(1993)124 13. L. Owens and H. H. Kung, J. Catal., 144(1993)202

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.

357

M e c h a n i s t i c Aspects of P r o p a n e O x i d a t i o n over N i - C o - M o l y b d a t e Catalysts David L.Stern a and Robert K. Grasselli b aMobil Technology Company, Strategic Research Center, 600 Billingsport Road, Paulsboro, NJ 08066-0480, USA bUniversity of Delaware, Department of Chemical Engineering, Newark, DE 19716-3116, USA

Mechanistic aspects of propane oxidation over Nio.5Coo.5MoO4 supported on SiO2 reveal, that the homolytic C-H bond breaking of the methylene hydrogen abstraction is the rate limiting step of the reaction, involving most probably an electron rich oxygen of a molybdenum located adjacent to the catalyst's cation (A---O-Mo), with the second hydrogen abstraction involving an oxygen of an adjacent molybdenum to the moiety of the first abstraction (A---O-Mo-O-Mo-O), where Ni comprises a more efficient A atom than Co, and where the transition state is relatively symmetric in nature. The reaction is first order in propane and zero order in oxygen, consistent with a Mars-van-Krevelen mechanism involving lattice oxygen of the catalyst. Because this catalyst produces propylene as the exclusive first formed product from propane at low conversions, and since propane and propylene compete for the metal oxide catalytic sites with similar effectiveness, the named paraffin activating catalyst or a modification therefrom show promise, when combined with known multicomponent Bi-molybdates efficient in olefin oxidation, for the direct conversion of propane to acrylic acid or in presence of ammonia to acrylonitrile.

1. INTRODUCTION Functionalization of paraffins to the corresponding olefins, dienes, anhydrides, unsaturated acids and nitriles is of industrial interest [1 ] since paraffins are much more abundant and less expensive than the corresponding olefins. For this reason research interests have recently shifted away from olefins and towards paraffins as starting feeds for the production of petrochemicals and industrial intermediates. Oxydehydrogenation of paraffins is a viable approach towards this goal. The most studied systems for oxidative propane upgrading are vanadium [2], vanadiumantimony [3], vanadium-molybdenum [4], and vanadium-phosphorus [5] based catalysts. Another family of light paraffin oxidation catalysts are molybdenum based systems, e.g. nickel-molybdates [6], cobalt-molybdates [7] and various metal-molybdates [8-9]. Recently, we investigated binary molybdates of the formula AMoO4 where A = Ni, Co, Mg, Mn, and/or Zn; and some ternary Ni-Co-molybdates promoted with P, Bi, Fe, Cr, V, Ce, K or Cs [10-11 ]. A good representative of these systems is the composition Nio.5Coo 5M004 which was recently selected for an in depth kinetic study [12] and whose mechanistic aspects are now further illuminated here.

358 2. EXPERIMENTAL The Ni-Co-molybdate catalysts were prepared by refluxing the metal nitrates with ammonium heptamolybdate in the presence of silica sol (Ludox AS-40). After drying, pulverizing and sizing, they were air calcined for four hours in air at 290 C ~ and for four hours at 600 C ~ The catalyst contained 80 wt% active phase and 20 wt% silica support. Additional data on catalyst preparation, catalyst evaluation, and analytical methodology were described already earlier [ 10-12].

3. RESULTS AND DISCUSSION The system Ni0.sCoo.sMoO4 is effective for the oxidative activation of n-butane, i-butane, propane and ethane, and the conversion to their respective olefins (Figure 1). The selectivity to useful products is similar for propane and n-butane, but is lower for ethane. Methane is rather refractive, and only trace conversions to formaldehyde were achieved under the conditions studied. As is typical of oxidation catalysts, the selectivity to useful products declines with conversion, as seen for the conversion of propane to propylene (Figure 2) and the conversion of propylene to acrolein (Figure 3), respectively. From these results, Figure 1. Oxidation of Light Alkanes. measured at low conversions under C1 at 770~ C~ C~ at 560~ n-C~ at 500~ essentially differential reactor Feed: 15 Hydrocarbon / 1502 / 70 N2. conditions, it is apparent that propylene is the sole primary product of propane oxidation over this catalyst, since extrapolation to zero propane conversion results in 100 percent propylene selectivity, while the conversion to COx (i.e. CO and CO2) waste products at zero propane conversion extrapolates to zero COx selectivity.

1 ~

~

~

2

,9 "'-, 4"',~ 9

~ H I

I

3~

5 .....

0

~

~1110H 0 ./" i

~,

i

,,ss S

CO, CO 2

Scheme 1. Reaction Network of Propane Oxidation

359 When propylene is used as the feed, acrolein is observed as the major primary oxidation product. These are significant mechanistic observations, since it follows therefrom that all higher oxidized products, when starting with propane as feed must derive from the first formed propylene or a subsequently formed intermediate. A reaction network (Scheme 1) consistent with these observations portends that propane is first oxidized to propylene, which further oxidizes to acrolein, which in turn can be further oxidized to acrylic acid, but more preferentially to COx waste products.

Figure 2. Selectivity vs. Propane Conversion [12].

Figure 3. Selectivity vs. Propylene Conversion [12].

In a separate study, selectivities to useful products at higher conversions were also investigated, for propane and propylene oxidation, respectively (Figure 4). It was observed, as is common for sequential reactions, that propylene yield from propane, and acrolein yield from propylene, go over a maximum as hydrocarbon conversion is increased. The maximum propylene yield from propane oxidation is 16% at 34 propane conversion, while the maximum yield of acrolein from propylene is 6% at 9% propylene conversion. By assuming as a first approximation that direct oxidation of propylene to COx is negligible, consistent with the observations above at low conversions, and that only insignificant amounts of acrylic acid are formed, also consistent with our experiments, then a simplified scheme of series reactions can be written

as follows: A (propane)

kl k2 k3 ---> B (propylene) ---> C (acrolein) ---> D (COx)

Reactions 1 and 2 are both first order in hydrocarbon disappearance as will be seen below. By taking into account the above described maximum yields and utilizing the equations from [13], the relative reaction rates can be calculated as being kz]kl = 3.5, k3/k2 = 13 and k3/kl = 46. From these values it is apparent that high selectivities to propylene can be achieved only at relatively low propane conversions using Ni0.sCo0.sMoO4 as catalyst. Conversely, it might be possible to achieve high yields of oxygenates such as acrolein/acrylic acid or nitriles such as acrylonitrile, by combining the studied catalyst or a modification therefrom with a known

360 multicomponent olefin conversion catalyst as a physical mixture or a conglomerate in a single reactor.

Figure 4. Propylene Yields from Propane Oxidation and Acrolein Yields from Propylene Oxidation vs. Conversion [12].

Reaction orders for propylene and acrolein formation were studied by investigating the rate of propylene formation from propane as a function of oxygen concentration (Figure 5a) and as a function of propane concentration (Figure 5b). The results reveal, that the conversion of propane is zero order in oxygen and first order in propane. This is consistent with a Mars van Krevelen mechanism [14], whereby lattice oxygen of the catalyst is the oxidizing agent, and with methylene hydrogen abstraction being the rate limiting step in the activation of the paraffin. A similar dependence is found for acrolein formation from propylene [ 12]. Reaction orders for COx formation from propane are 1/2 order in oxygen and first order in propane [12] . For propylene oxidation they are 1/2 order in oxygen (Figure 6a), but exhibit a Langmuir-Hinshelwood dependence on propylene (Figure 6b).The mechanistic implication of these data is that at low propane conversions, and hence low propylene concentrations in the gas phase over the catalyst, the in situ first formed propylene product readily desorbes from the catalyst surface, and that the readsorbed propylene converts to acrolein, which is readily converted to COx waste

Figure 5. Reaction Orders for Propylene Formation from products. At higher conversions of Propane; a. Oxygen Dependence, b. Propane Dependence propane, under conditions where the [12]. concentration of propylene is rather high, the acrolein formed therefrom appears to react with another catalytic center, possibly a highly acidic site (Mo-O-H § or a reduced site ( M o O ) leading to waste products, or it reacts without desorption with an

361 adjacent site giving waste products. This explanation is consistent with the observed Langmuir-Hinshelwood dependence for waste formation from propylene. The studied catalyst exhibits a significant primary isotope effect for both propane disappearance (kn/ko = 1.7) with theoretical maximum being 1.95) and propylene disappearance (kH/kD = 1 . 9 ) and a theoretical maximum of 2.05). These large, observed kinetic isotope effects are consistent with a homolytic C-H bond breaking as the rate determining step, and a relatively symmetric transition state; hence, methylene hydrogen abstraction for propane activation and t~-hydrogen abstraction for propylene activation. Competition experiments of propane and propylene [ 12] reveal that propane and propylene compete with similar effectiveness for the catalytic Figure 6. Reaction Orders of CO2 Formation from metal oxide sites, albeit as expected Propylene; a. Oxygen Dependence, b. Propylene propylene is favored by a factor of 2.3. Dependence [12]. Since the operating temperature is rather high, the results also imply that the thermal contribution to the respective C-H bond breaking is significant, diminishing the customary importance of the o~-hydrogen bond weakening in propylene due to the rt-bond interaction of the olefin with the catalyst surface.

4. MECHANISTIC MODEL Under the differential reaction conditions used in this study [ 12], the concentrations of all products in the gas phase are small and therefore their respective surface coverages are small. Under these constraints, the following kinetic expressions apply for the partial oxidation of propane to propylene, and propylene to acrolein, respectively: rc3- = k c3= xc3 ~ (02) ~

(1)

racr. = kacr. xc3

(2)

(02) ~

The deep oxidation rates of propylene to COx ( i.e. CO2 as well as CO ) are described by the rate expression:

362

rcox =

112

kcox_._~Xc3 = xo~ ( 1 + Kc3-xc3-)

(3)

Where XC3 o, XC3=, and x02 are the mole fractions of propane, propylene, and oxygen in the gas phase, kc3--, kacr. and kcox are the rate constants of propylene, acrolein, and COx production, Kc3-- is a propylene adsorption constant, and (02) denotes the zero order oxygen dependence. From our study reported here, from our earlier studies of divalent metal molybdates [ 1011 ], and oxidation literature in general [ 1], we can postulate a reaction mechanism (Scheme 2) which takes all of these factors into account and is consistent with them. Accordingly, methylene bond breaking is the rate determining step in the activation of propane and it proceeds by interaction with the NiA .--(~ OI Co-molybdate catalyst surface. The site /MOo/MO responsible for the methylene abstraction is most likely the electron rich oxygen of a molybdenum atom adjacent to the A atom o" El .o o / I I I (A...O-Mo) on the surface of the AMoO4 /M~176 ~ /M~176 ~ catalyst. One could reason, that the activating sites are comprised of an A (~+~)--O.-MoS+-moiety; i.e., Ni3§ 5§ in Ni HO OH HO OH I I I I -molybdate; where O. denotes the partial ~M~176 ~ radical character of this specific oxygen. And the surface concentration of such paraffin activating sites is anticipated to be relatively small, yet sufficient to catalyze the Scheme 2. Proposed Mechanism of Propane reaction in question. As a consequence, Oxidation over Ni0.sCo0.sMoOx,where A= Ni,Co. Ni...O-Mo is more active than Co...O-Mo, which is much more active than Zn...O-Mo for the activation of paraffins [11]. The second hydrogen abstraction from the so formed propyl radical is performed by a Mo-O moiety adjacent to the activating A...Mo-O site, i.e., (A...Mo-O-Mo-O), as depicted in Scheme 2. Propylene desorbs, leaving a hydroxylated site, which after dehydration forms a reduced surface site, which in turn is reoxidized by lattice oxygen of the catalyst. The depleted lattice oxygen is then replenished by gaseous oxygen, thus completing the redox cycle of the catalytic process. Waste products are formed from propane on this catalyst primarily via the second formed acrolein. It is postulated that the acrolein, once formed, readily readsorbs on the catalyst surface, and presumably interacts with a second site which might be either a highly acidic surface site (Mo-O- H +) or a reduced surface site ( Mo I-i ) or simply by interaction, before desorption, with an adjacent overactive surface site. This scenario is particularly strongly implied by the observed Langmuir-Hinshelwood dependence of waste formation from propylene. In order to minimize waste formation and maximize useful product yields, several opportunities reveal themselves from the observations gained in our studies and those gleaned

363 from oxidation catalysis in general, ff propylene is to be the desired end product, then operation at low conversions using a Ni-Co-molybdate catalyst is possible. An improvement would be operation at very short contact times and particularly by employing a transferline reactor in the absence of gaseous oxygen, using external regeneration of the catalyst. Another potential improvement would be to use a redox promoted Ni-Co-MoOx system [10-11], because with such systems the reaction could be carried out at lower temperatures, and hence improved selectively. / 0\

/0/

NH3

Olefin , NH3 n ,s\ ctivating Site

-~ [] M1

/ [ ~ " ~2 ~ R e ~ 1 7 6 [1 # M2 J

Reducedsite

! HzC~-CHCN

3M [ ]

3/2 02 J

~ -~ //NH M2

..-O"

" M1

[o12-

~

1/2o2

x~.~.~ H20

/\

~ M1

JM~176

NH O" [] I I // / M ~ 1 7 6 ~ M2

AmmoxidatiOnsite H20" ~

H~

.o-

. oI

.,Mo _Mo

.~H HzC.-" ~""Z' " ~H2 ,

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

.

.

.

.

.

.

.

.

.

.

.

O

- d 2 ' HO

Paraffin Activating Site

~

I

O I I / M~ O/M~

oI

J

jMO~oIMO ~

.

M1 M2 AllylicSurface Complex

Olefin Activation (where M]=Bi, M2=Mo)

Paraffin Activation (where A=Ni, Co)

Scheme 3. Proposed Mechanism of Propane Ammoxidation using a Paraffin Activating Catalyst, e.g.,NiaCObMcMoOx; and a Multicomponent Mixed Metal Molybdate Olefin Ammoxidation Catalyst, e.g., CSaKbNicMgdMeBifSbgMohOx, where M = Ce, Cr, or Fe [ 15]. Still another scenario, and one which we feel has particular promise, is starting with propane to aim for and isolate a useful product more stable than propylene or acrolein. To achieve this [15], it might be possible to combine the paraffin activation prowess of the systems discussed here and earlier [10-12], with the olefin conversion efficiency of known multicomponent molybdate catalysts [16] to produce more stable compounds such as unsaturated acids (e.g., acrylic acid) or nitriles (e.g., acrylonitrile). The two catalysts could be commingled as a physical mixture in a single reactor, or sequenced and commingled in a single catalyst composition. This idea is presented in Scheme 3 for the production of acrylonitrile from propane. In this scheme the propane is activated on a Ni-Co-molybdate catalyst, preferably one doped with redox elements such as Ce or Cr, producing propylene as the major intermediate. The so produced propylene reacts then further with the olefin conversion catalyst, which is selected from one of the well known alkali doped Bi- molybdate multicomponent catalysts, preferably one containing a redox element such as Ce , e.g. CSaKbNicMgdCeeBifSbgMohOx. In the presence of ammonia, the in situ produced propylene is

364 immediately converted to acrylonitrile, a useful product of commerce, and relatively stable under the conditions of the reaction. It is important in this processing scheme that the two catalysts are chemically compatible (therefore both are selected from the molybdate family), and that they are temperature matched. The latter implies that compositions must be found which operate at a compromise optimum temperature. For this reason the paraffin activating catalyst is redox doped with say Ce or Cr, to lower its operating temperature, enhance its activity and improve its selectivity towards the production of propylene. Similarly, the olefin activating catalyst is doped with Ce instead of the customary Fe, in order to increase its operating temperature in the direction of the operating temperature of the paraffin activating catalyst. It is of course also imperative, that the two catalysts be in intimate contact with each other. This is achieved by commingling the two compositions in a single extrudate or coprecipitate, or by commingling physical mixtures of the two catalyst powders (small particle size) in a single reactor. Additional experimental work is needed to optimize the choice of the named catalyst compositions, and to optimize the reaction and reactor conditions.

5. CONCLUSIONS Ni-Co-Molybdates are viable oxidation catalysts for the activation of light paraffins such as propane and butanes to produce the respective olefins. Maximum yields are in the range of 16% at about 80% selectivity. The catalysts activate methylene C-H bonds, abstracting the hydrogen of the substrate in the rate limiting step of the reaction. With propane as feed, propylene is the only first formed product, and all higher oxidized products ensue in subsequent steps, after the propylene has been formed. Acrolein is formed from the in situ produced propylene, and acrolein is the main intermediate leading to waste products CO and CO2. The first hydrogen abstraction is deemed to involve the oxygen of the Mo adjacent to the A metal of the catalyst, i.e. A---O-Mo, with the reactivity order of the A metal being Ni > Co >> Zn. The second hydrogen of the so formed propyl radical is deemed to occur by an oxygen of a Mo located adjacent to the first abstracting site, i.e. A---O-Mo-O-Mo-O. Propylene is desorbed from the catalyst surface, and after loss of a water molecule the so formed reduced catalyst site is reoxidized through lattice oxygen to is original state, and the depleted lattice oxygen is in turn replenished by gaseous oxygen, which completes the redox catalytic cycle. The large primary kinetic isotope effects observed for propane and propylene oxidation over these catalysts suggest homolytic C-H bond breaking as the rate limiting step, and a relatively symmetric transition state. Propane and propylene compete with similar effectiveness for the metal oxide catalytic sites, with propylene favored by a factor of 2.3. The mechanistic results of this and earlier studies [ 12] suggest that in addition to producing propylene from propane, the described paraffin activating catalysts could be combined with known olefin oxidation catalysts in a single reactor to produce industrially desirable value added products which are more stable than propylene under the reaction conditions employed; such as acrylic acid or acrylonitrile.

365 ACKNOWLEDGEMENTS

The authors thank Professor D. J. Buttrey, University of Delaware, for fruitful discussions of structure-mechanism relationships, and to Mr. B. Leyer, University of Munich, for the artwork.

REFERENCES

1.a.R.K. Grasselli and J.D. Burrington, Adv. Catal., 30 (1981) 133 b. R.K. Grasselli, J. Chem. Ed., 63 (1989) 216. 2.a.M.C. Kung and H.H. Kung, J. Catal., 134 (1992) 668. b. A. Corma, J.M. Nieto Lopez and N. Paredes, ibid, 144 (1993) 425. 3.a.A.T. Guttmann, R. K. Grasselli, J.F. Brazdil and D.D. Suresh, US Patent No. 4 746 641 (1988). b. R. Catani, G. Centi, F. Trifiro and R.K. Grasselli, Ind. Eng. Chem. Res. 31 (1992) 107. c. A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati and F. Trifiro, Appl. Catal. A, 113 (1994) 43. 4.a. Y-C. Kim, W. Ueda and Y. Moro-oka, Catal. Today, 13 (1992) 673. b. J.P. Bartek, A.M. Ebner and J.F. Brazdil, US Patent No. 5 198 580 (1993). 5.a.M. Ai, Catal. Today, 12 (1992) 679. b. M. Ai, J. Catal.,101 (1986) 389. 6.a.C. Mazzocchia, E. Tempesti and C. Aboumard, US Patent No. 5 086 032 (1992). b. C. Mazzocchia, C. Aboumard, C. Diagne, E. Tempesti, J.M. Herrmann and G. Thomas, Catal. Lett., 10 (1991) 181. 7.a.H.F. Hardman, US Patent No. 4 131 631 (1978). b. H.F. Hardman, US Patent No. 4 255 284 (1981). 8. F. Cavani and F. Trifiro, Catal. Today, 24 (1995) 307. 9. Y.S. Yoon, N. Fujikawa, N. Ueda. Y. Moro-oka and K.W. Lee, ibid, 24 (1995) 327. 10. D.L. Stern and R.K. Grasselli, 211 Nat. Mtg., ACS, Petr. Chem. Pre-Prints, (1996) 172. 11. D.L. Stern and R.K. Grasselli," Propane Oxydehydrogenation over Molybdate Catalysts" J. Catal., (1997) in press. 12. D.L. Stern and R.K. Grasselli, "Reaction Network and Kinetics of Propane Oxydehydrogenation over Nickel Cobalt Molybdate" J. Catal., (1997) inpress. 13. A.A. Frost and R.G. Pearson, "Kinetics and Mechanism" John Wiley & Sons, (1953). 14. P. Mars and D.W. van Krevelen, Chem. Eng. Sci. Suppl. 3 (1954) 41. 15. R.K. Grasselli, Handbook of Heterogeneous Catalysis, G. Ertl, H. Knoezinger and J. Weitkamp (eds), B (1997) 4.6.7. 16.a.R.K. Grasselli, D.D. Suresh and H.F. Hardman, US Patent Nos. 4 139 552 (1979), 4 162 234 (1979), 4 190 608 (1980), 4 778 930 (1988). b. R.K. Grasselli and H.F. Hardman, US Patent No.4 503 001 (1985). c. R.K. Grasselli, Appl. Catal., 15 (1985) 127. e. D.D. Suresh, M.S. Friedrich and M.J. Seely, US Patent No. 5 212 137 (1993).

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

367

Oxidative Dehydrogenation of Propane by Non-Stoichiometric Nickel Molybdates* Doron Levin and Jackie Y. Ying* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 A series of catalysts represented by the formula Nil+6Mol_~/304, where - 1/5 _<6 _< 1/3, were investigated for activity towards oxidative dehydrogenation of propane. The catalysts were synthesized from layered ammonium nickel molybdate materials having a general formula (NH4)HzANi3_xO(OH)(MoO4) 2, where 0 _< x _< 3/2. The location of the nickel atoms on a crystallographic site of variable occupancy allowed for the careful manipulation of chemical composition of these precursors. The nickel/molybdenum ratio of the precursor was inherited by the catalyst, enabling the synthesis of non-stoichiometric molybdates. These catalysts were tested for the catalytic oxidative dehydrogenation of propane to elucidate the effect of the extent of nonstoichiometry (6) on catalytic performance. The activities of the catalysts are highly sensitive to the catalyst composition and increase with decreasing Ni content. Selectivity towards propene is independent of the Ni content when the Ni/Mo ratio is less than unity, but decreases linearly with increasing amounts of Ni when Ni/Mo > 1. 1. I N T R O D U C T I O N Transition metal molybdates have been examined as multicomponent catalytic systems for a number of reactions. They are well known to be catalytically active for partial oxidation reactions, particularly for the selective oxidations of lower alkanes. There is currently increased industrial interest in the oxidative dehydrogenation (ODH) of paraffins to olefins [1], due to several limitations in thermal and catalytic dehydrogenation processes. The thermodynamics of thermal light paraffin dehydrogenation are such that under conditions of low temperature and high pressure, the equilibrium favors the paraffin. Catalytic dehydrogenation at elevated temperatures leads to undesirable side reactions such as cracking and coking of the catalyst. In addition, paraffin dehydrogenation is strongly endothermic, making the process extremely energy intensive. To supply the heat of reaction, hydrogen oxidation can be coupled with dehydrogenation. The addition of oxygen to the feed, however, exposes the synthesized olefins to oxidation conditions that could result in the formation of economically-useless carbon oxides. Desired catalysts for oxidative dehydrogenation of lower alkanes need to be sufficiently active for hydrogen atom

* This work was supported by the National Science Foundation (CTS-9257223, CTS-9411901) $ To whom correspondence should be addressed

368 abstraction from a C-H bond, yet operate at temperatures that minimize oxygenation of the desired products. Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMoO4, have attracted the greatest interest. NiMoO 4 presents two polymorphic phases at atmospheric pressure: a low temperature a phase, and a high temperature [3 phase [2,7]. Both phases are monoclinic with space group C2/m. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the phase and distorted tetrahedral in the [3 phase. The [3 phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temperature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the 13phase being approximately three times more selective in butene formation than the o~phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. The structure of one such precursor has recently been solved [9]. This precursor, having a nickel to molybdenum molar ratio equal to one, forms a stoichiometric NiMoO4 upon calcination at 550 ~ The solution of the crystal structure showed the chemical formula of the precursor to be (NHn)HNi2(OH)2(MoO4) 2. A detailed investigation into the structure of this material showed it to be a member of a solid solution series of (NHa)H2xNi3_xO(OH)(MoO4) 2, where 0 _<x _<3/2 [9]. The non-stoichiometry arises from variable occupancy of the octahedral site accommodating the nickel atoms. The nickel atoms are situated at a crystallographic site having a Wyckoff designation 9(e) and it has been shown that the occupancy of that site can vary from V2to 1 [9]. The molybdenum atoms have been determined to fully occupy site 6(c), thereby allowing the Ni/Mo ratio to vary from 0.75 to 1.5. The variation in the Ni content is made possible by the ability of the structure to accommodate protons required for charge balancing. The accommodation of the nickel on a crystallographic site of variable occupancy allows for the careful manipulation of chemical composition of these precursors. This structure is also not limited to this class of nickel molybdates, but can be synthesized with a variety of elements such as Co, Zn, Mg, and Cu, either alone or in combination with others [ 10]. These materials form a class of layered transition metal molybdates, termed LTM. It is these properties of this LTM precursor that make it so versatile for the preparation of engineered catalytic phases. Since the properties of a catalyst will be dependent on the precursor from which it is derived, the ability to systematically manipulate the chemical composition of the LTM precursor allowed for a detailed examination of the structure/property relationship for these materials.

2. EXPERIMENTAL 2.1. Catalyst Preparation The ammonium nickel molybdate precursors were prepared by chemical precipitation [9]. Ammonium heptamolybdate ((NH4)6MOTOz4.4H20) and nickel nitrate (Ni(NO3)2.6H20) were used

369 to prepare a solution containing Ni and Mo in a molar ratio between 0.75 and 1.5. The addition of concentrated ammonium hydroxide (28.8% NH3) precipitated a green solid that dissolved in an excess of ammonia to give a deep blue solution. This solution was heated with constant stirring for four hours, leading to the formation of a pale green precipitate. The products were isolated by vacuum filtration, washed with deionized water, and dried overnight at 110 ~ and atmospheric pressure. The catalysts of the form Ni~+sMOl_6/304, as indicated in Table 1, were prepared by in situ calcination of the LTM precursors at 550 ~ in flowing He for 1 hour. Table 1 Catalysts Derived from (NH4)Hz~Ni.~_,O(OH)(MoO4)z Precursors x Ni/Mo Precursor Formula Catalyst Formula Ratio 0.0 1.5 (NH4)Ni30(OH)(MoO4) 2 Nil.333Moo.8890 4 0.5 1.25 (NH4)Ni2.5 (OH)z(MoO4)2 Nil.|v6Moo.9410 4 1.0 1.0 (NH4)HNiz(OH)z(MoO4) 2 NiMoO4 1.25 0.875 (NHa)H l.sNi 1.75(OH)z(MoO4)2 Ni0.903MOl.03204 1.5 0.75 (NH4)HzNil.5(OH)z(MoO4) 2 Ni0.g0Mo~.o670 4

0.333 0.176 0.000 -0.097 -0.200

2.2. Catalytic Activity Measurements The catalytic activity of the nickel molybdates [ 11 ] was tested in a 1/4 inch quartz flowthrough tubular reactor operated at atmospheric pressure. The reactor was contained within an electrically heated tube furnace. The temperature of the reactor was controlled according to the temperature of the gases at the base of the catalyst bed. The composition and flow rate of the gas feed mixture was measured using MKS mass flow controllers calibrated for each specific gas. Certified gas mixtures with Grade 5 helium (99.999%) as the balance gas were used throughout. The reactor effluent was analyzed by gas chromatography using a Perkin Elmer Autosystem GC equipped with a TCD detector. A two-column setup incorporating a 15-foot Poropak QS column and a 6-foot Carbosphere column was used with a 10-port sampling valve to fully separate the gases. This setup resulted in good separation of the individual gases, allowing for closure of the carbon balance to within + 5%. Conversion has been defined as the percentage conversion of propane into all possible products. Selectivity has been defined as the number of moles of propane converted into a specific product divided by the total number of moles of propane converted into all products, expressed as a percentage. The yield of propene has been defined as the product of the propane conversion and the selectivity towards propene, and is expressed as a percentage.

3. RESULTS AND DISCUSSION The series of nickel molybdates tested for oxidative dehydrogenation of propane produced a product spectrum limited to propene, carbon dioxide, carbon monoxide, and water. No cracking

370 products such as ethane, or oxygenated products such as aldehydes were present in the exhaust from the reactor.

3.1. Effect of Non-Stoichiometry The yield of propene attainable with nickel molybdates of the form Nil+6Mol_6/304, where -1/5 _< 6 _< 1/3, is shown in Figure 1. These data were collected at 500 ~ with a feed flowrate of 30 ml/min composed of 5 mol% O2, 5 mol% C3H8, and the balance being He. The data of Figure 1 show an increase in the yield of propene with a decrease in the nickel content of the materials. The material with the highest Ni/Mo ratio of 1.5 is a better catalyst for combustion than oxidative dehydrogenation, with approximately 60% of the product spectrum being carbon oxides. The material with the lowest Ni/Mo ratio of 0.75 has the highest activity under the reaction conditions, leading to the highest propene yield.

Figure 1. Propene yield as a function of Ni/Mo ratio.

Figure 2. Propene selectivity at 20% propane conversion.

The series of nickel molybdate catalysts show a decrease in the selectivity towards propene with an increase in conversion. Figure 2 shows the selectivity towards propene at a propane conversion level of 20%. As the Ni/Mo ratio increases above 1, the selectivity towards propene decreases almost linearly as carbon oxides become the dominant products. It is interesting to note, however, that the selectivity towards propene is essentially independent of the Ni/Mo ratio at values less than and equal to 1. This suggests that it is the increase in activity with decreasing Ni/Mo values that accounts for the increase in yield noted above.

3.2. Effect of Oxygen:Propane Ratio To further improve the yield of propene attainable with the non-stoichiometric nickel molybdates, the oxygen:propane molar ratio was varied from a 1:1 ratio to an oxygen-rich 2.5:1 value. The results of this study are shown in Figure 3. These data were collected at 550 ~ with

371 a feed flowrate of 70 ml/min composed of 1 mol% C3H8, an 0 2 concentration set by the ratio under investigation, and the balance being He. In an analogous set of results to the data shown in Figure 1 collected at a lower space velocity, the series of nickel molybdates showed an increase in propene yield with a decrease in the Ni/Mo ratio. These data also show that the propene yield increases slightly as the oxygen:propane ratio increases, but with the increase becoming less significant as the Ni/Mo ratio decreases.

Figure 3. Propene yield as a function of

Figure 4. Propane conversion as a function

Ni/Mo ratio and 02:C3H 8 ratio.

of Ni/Mo ratio and 02:C3H 8 ratio.

The effect of increasing the oxygen:propane ratio is more evident when examining the propane conversion. As shown in Figure 4, the propane conversion increase with increasing oxygen concentrations is significant for all Ni/Mo ratios. However, as the Ni/Mo ratio decreases, the higher conversion resulting from a higher oxygen concentration does not lead to higher propene yields due to a shift in selectivity towards carbon oxides.

3.3. Structure/Catalytic Property Relationship Physical characterization of the phase present under catalytic reaction conditions has shown a single [3-phase of Nil+6Mo~_6/304 is present at 550 ~ following calcination of the (NH4)HzxNi3_xO(OH)(MoO4)2 precursors [12]. Analysis of the defect chemistry of these nonstoichiometric nickel molybdates identified majority point defects, leading to a correlation between the electrical conductivity and the defect structure [ 12]. This correlation is summarized below. For nickel molybdates having a Ni/Mo ratio greater than one, corresponding to an excess of NiO, the structure is proposed to have interstitial Ni atoms as the major defects that are compensated for by the presence of Mo vacancies. Given this excess of Ni atoms, it was proposed that the majority carrier arises from the oxidation of Ni u to Ni m, represented by the equation,

372 1

2 Nixi + ~ O 2 ( g ) -

2 NiNi + 0 o + Vffi

(1)

This formation of Ni In leads to the formation of electron-acceptor levels close to the valence band, leading to hole conductivity (p-type semiconductor.) For nickel molybdates having a Ni/Mo ratio less than one, corresponding to an excess of M o O 3, the structure is proposed to have Ni vacancies as the major defects that are compensated for by either interstitial Mo atoms, or Mo atoms occupying Ni sites. Given this excess of Mo atoms, it was proposed that the majority carrier arises from the reduction of Mo vx to Mo v, represented by the equation 2MOMo + 0 o

1

=,

~O2(g )

/

+

V 0

+

Z

2MoMo

(2)

This formation of MoV leads to the formation of electron-donating levels close to the conduction band, leading to electron conductivity (n-type semiconductor.) It has been reported in the literature that adsorbed oxygen species transform at the surface of an oxide according to the general scheme: 2-

02(ads)-*

O•ads ) ~ O(ads ) --' O(lattice )

(3)

in which they are gradually becoming richer in electrons [ 13]. Transition metal oxides containing cations that are capable of increasing their degree of oxidation and thereby supplying adsorbed oxygen molecules or atoms with electrons (i.e. p-type materials) tend to form electron-rich species such as O- and O 2-. Transition metal oxides that are n-type in nature have a small concentration of donor centers capable of transmitting electrons to the adsorbed oxygen and tend to form oxygen species that are less rich in electrons, such as O 2 ions. Focusing on the nickel molybdate having a Ni/Mo ratio of 1.5 (Nil.333Mo0.88904) , it can be seen that both the high number of Ni atoms capable of oxidation and the low number of Mo atoms capable of reduction leads to a high number of electrons available for the formation of electron-rich species. It is therefore proposed that the dominant surface oxygen species on this nickel molybdate is O-. Turning to the nickel molybdate having a Ni/Mo ratio of 0.75 (Ni0.80Mo~.06704), it can be seen that both the low number of Ni atoms capable of oxidation and the high number of Mo atoms capable of reduction leads to a low number of electrons available for the formation of electron-rich species. It is therefore proposed that the dominant surface oxygen species on this nickel molybdate is 02-. With these two nickel molybdates forming the bounds on the range of Ni/Mo ratios possible, it can therefore be proposed that as the Ni/Mo ratio is varied through this range, the

373 nature of the surface oxygen species changes with increasing amounts of electron-rich species as the Ni/Mo ratio increases. It must, however, be mentioned that this would only be true with 02 as the oxidant. It has been shown that conversion decreases with an increase in the Ni/Mo ratio (Figure 4). This can be attributed to a decrease in the reactivity of the surface oxygen species as the nature of the surface oxygen coverage shifts from an O2-majority to an O- majority. The catalytic data suggests that the O2- form of adsorbed oxygen is more reactive than O- for H atom abstraction from propane. When O2 is used as the oxidant, the amount of electrons available for donation to adsorbed oxygen molecules decreases as the Ni/Mo ratio decreases, resulting in a more reactive oxygen surface coverage that leads to higher propane conversion. 4. CONCLUSIONS The series of non-stoichiometric ammonium nickel molybdate precursors having the general formula (NH4)HzxNi3_xO(OH)(MoO4)2, where 0 _< x _< 3/2, is an excellent family of materials for the preparation of transition metal molybdate catalysts having well-controlled stoichiometry. This flexibility allowed for a systematic study into the relationship between catalytic performance and catalyst structure. This class of LTM precursors encompassing a Ni/Mo ratio of 0.75 to 1.5 forms pure ~-Nil+6Mol_6/304 on calcination at reaction conditions. These nickel molybdates are catalytically active for the oxidative dehydrogenation of propane into propene, with the propene yield being maximized by minimizing the Ni/Mo ratio.

REFERENCES

.

10. 11. 12. 13.

Cavani, F., and Trifirb, F., Catalysis Today 24, 307 (1995). Mazzocchia, C., Aboumrad, C., Diagne, C., Tempesti, E., Herrmann, J.M., and Thomas, G., Catal. Lett. 10, 181 (1991). Mazzocchia, C., Anouchinsky, R., Kaddouri, A., Sautel, M., and Thomas, G., J. Therm. Anal. 40, 1253 (1993). Mazzocchia, C., Kaddouri, A., Anouchinsky, R., Sautel, M., and Thomas, G., Solid State Ionics 63, 731 (1993). Minow, G., Schnabel, K., and Ohlmann, G., React. Kinet. Catal. Lett. 22, 389 (1983). Yoon, Y. S., Fujikawa, N., Ueda, W., and Moro-oka, Y., Chem. Lett. 1635 (1994). Mazzocchia, C., Di Renzo, F., Aboumrad, Ch., and Thomas, G., Solid State Ionics 32, 228 (1989). Martin-Aranda, R. M., Portela, M. F., Madeira, L. M., Freire, F., and Oliveira, M., Appl. Catal. A 127, 201 (1995). Levin, D., Soled, S. L., and Ying, J. Y., Inorg. Chem. 35, 4191 (1996). Levin, D., Soled, S. L., and Ying, J. Y., Chem. Mater. 8, 836 (1996). Levin, D., and Ying, J. Y., J. Catal., to be submitted. Levin, D., and Ying, J. Y., J. Electroceram., to be submitted. Bielmiski, A., and Haber, J., Catal. Rev. - Sci. Eng. 19, 1 (1979).

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3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

375

Selective oxidation of propane into oxygenated compounds over promoted nickel-molybdenum catalysts J. Barrault a, C. Batiot a, L. Magaud a and M. Ganne b a Laboratoire de Catalyse en Chimie Organique, ESIP, UMR CNRS 6503 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France

b Laboratoire de Chimie des Solides, Institut des Materiaux de Nantes 2 rue de la Houssiniere, 44072 Nantes cedex 03, France Partial oxidation of propane was investigated in the presence of molybdenum oxide based catalysts. We have shown the existence of a synergetic effect between the two phases (~NiMoO4 and ~MoO3 . Indeed activity and selectivity towards acetic acid and acrylic acid were maximal with a ratio t~MoO3 / (otNiMoO4 + t~MoO3 ) close to 0.25. These results could be explained by an interaction and a mutual covering of the two phases. The addition of bismuth to these mixed systems led to a total or a partial inhibition in the production of acetic acid and an increase in the formation of acrolein and acrylic acid. 1. INTRODUCTION The selective oxidation of light alkanes has attracted much attention because it represents a route to obtain more valuable organic compounds from low cost saturated hydrocarbons. Numerous works have been carried out in the last decade on the oxidative coupling of methane (OCM) and the selective oxidation of the other light paraffins (ethane, propane, isobutane). The oxidative dehydrogenation of propane to propene and its selective oxidation to acrolein and acrylic acid were studied over various solids. With catalysts containing molybdenum Morooka et coll. claimed that a selectivity to acrolein of over 60% for a 13% conversion of propane in the presence of a silver doped bismuth vanadomolybdate [1,2]. Mazzochia et coll. proposed that the propane oxidative dehydrogenation (P.O.D.) occurred mainly over a 13NiMoO4 phase (containing tetrahedral molybdenum centers) [3]. Previously these authors found a synergetic effect on acrylic acid selectivity by adding MoO3 to otNiMoO4 [4]. Schrader et coll. also showed that nickel molybdenum catalysts contained mainly a mixture of otNiMoO4 and t~MoO3 [5,6], and that the catalytic properties resulted of <<specific interactions )) between the two phases. More recently Y.S. Yoon et coll. proposed that the P.O.D. involved surface molybdenum oxide clusters supported on metal molybdate matrix when Using cobalt and magnesium molybdates [7]. In our laboratory we showed that Bi-Mo (W,V, Ti) Aurivillius or Sillen phases and bismuth-molybdenum oxides supported on titania were more selective for the P.O.D. to propene [8,9]. In this field our objective was to improve the activity of molybdenum based materials either for the P.O.D. to propene or the selective oxidation of propane to acrolein and acrylic acid. For these reasons we have focused on the catalytic performances of NiMoO4-MoO3 phases doped with bismuth.

376 2. SYNTHESIS AND CHARACTERIZATION OF MATERIALS 2.1 Synthesis of materials and reaction procedure Nickel - molybdenum catalysts were prepared by coprecipitation to obtain a significant surface area ( 5-50 m2gl). The preparation procedure was an optimisation of the procedure proposed by Mazzocchia [3,4] and Schrader [5]. 13NiMoO4 was obtained after a thermal treatment of ct phase at 700~ in oxygen for 1 h. Nevertheless this ot<----~13 transition was reversible and occurred below 300~ [3]. In order to compare all the (Ni-Mo) samples, we used a molar ratio RA = ot M o O 3 / (r M o O 3 + ~ NiMoO4) Bismuth compounds were added to some of the previous samples. These modifications consisted of an impregnation of bismuth salt or of a precipitation of bismuth hydroxide over the [ nickel-molybdenum] solids. Prior to the reaction lg of catalyst was activated under helium at 500~ The reactions were performed in a flow reactor at a temperature of 375-425~ under atmospheric pressure. The feed gas consisted of a 60 vol % propane, 20 vol% 02 and 20 vol % He or H20. The total flow rate was 1.5 l.h 1. The outlet gases were analysed by FID and TCD gas chromatography with Cpsil5, BP 21, Porapack R and Porapack Q columns.

2.2 Characterization of materials All the X R analysis of nickel-molybdenum catalysts with 0
Chemical Analysis (%)

BET (m2.g"1)

Formula

XRD

Mo

Ni

0.01

44.95

26.50

44

Ni0.99MoOx otNiMoO4 (t~ ~MoO3)

0.25

44.90

20.65

29

N i 0 . 7 5 M o O x otNiMoO4, r

3

0.48

49.54

15.74

21

N i 0 . 5 2 M o O x otNiMoO4, r

3

0.83

56.66

5.74

11

N i 0 . 1 7 M o O x otNiMoO4, ~MoO 3

1

-

-

4

t~MoO 3

All the XRD diagrams showed the presence of the monoclinic otNiMoO4 phase and/or the orthorhombic ~MoO3 phase. There were no mixed ~) phases before and after the catalytic test. The formation however of a solid solution undetected by XRD cannot be excluded. The sample with a formula <) was composed of a mixture of two phases after calcination at 550~ and after the catalytic test. After activation at 700~ and the catalytic test the intensity of different peaks ((0k0) with k=2n) of ~ M o O 3 was considerably

377 modified, which indicates a preferential orientation of the (010) phases, which correspond to the reticular planes perpendicular to the b axis of ct M o O 3 . Tables 2 and 3 give the results of the XPS analysis obtained for NiMoO4 and NiMoO4-MoO3 samples (RA=0.83) before and after the catalytic test. Table 2 XPS analysis of NiMoO4 catalysts (1) XPS Analysisat atom. %

(2)

(3)

(1)

eV

atom. %

atom. %

eV

Bulk Analysis ( atom. %)

Mo 3d5/2

31.67

2.20

34.13

32.51

1.40

16.62

Ni

7.96

3.20

6.88

6.65

1.70

16.38

O ls

60.37

2.30

58.98

61.85

1.70

67.00

Mo/Ni+Mo

0.80

-

0.83

0.85

-

0.50

BET(m2.g -1)

44

-

31

20

2p3/2

Table 3 XPS analysis of

MoO 3 - NiMoO 4

catalysts, RA=0.83

(1)

(2)

(3)

(1)

eV

Bulk Analysis

XPS Analysis

atom. %

eV

% atom.

eV.

Mo 3d5/2

29.35

1.80

33.00

1.40

21.72

Ni

3.44

2.60

2.70

2.50

3.75

O ls

67.21

2.30

64.30

1.80

74.53

Mo/Ni+Mo

0.89

-

0.92

-

0.85

BET(m2.g -1)

12

-

10

-

2p3/2

atom. % :surface composition eV : half height width (1) sample after calcination at 550~ (2) sample after thermal treatment at 500~ (or) and catalytic test (3) sample after thermal treatment at 700~ (13) and catalytic test

3

378 We can see that the surface composition is very different from the bulk composition. After calcination and the catalytic test, XPS peaks are thinner and the molybdenum surface content increases. The oxidation degrees of molybdenum (Mo 3d5/2 = 236.7_+0.1 eV, Mo 3d3/2 = 233.6 _+0.1 eV) and the oxidation degrees of nickel (Ni 2p3/2 =856.7_+0.1 eV, Ni 2pl/2 =875.0 _+0.1 eV ) are not modified by the calcination and the catalytic test. The solids however have been in contact with air before the analysis. The significant differences between energy levels Ni2p of NiMoO4 and ofNiO (Ni 2p3/2 =854.4 eV) show clearly that there is no NiO in the samples.

3. RESULTS AND DISCUSSION 3.l.Nickel-molybdenum catalysts 3.1.1. Effect of MoO 3 on the catalytic properties of ctNiMoO4 We studied the influence of MoO 3 on the catalytic properties of aNiMoO4. The catalytic test was carried out 375~ with 0
Table 4 Propane oxidation over [ctMoO3 - otNiMoO4 ] catalysts RA

Conv. (%)

Selectivity (%)

C3H8

02

C3H6

COx

O2/C

Eoxy.

etha.

acro.

0

5.3

48

45

48

2.43

3

2

2.2

0.06

5.1

46

46

48

1.65

3

0.8

0.8

0.4

0.25

11.8

82

35

35

0.92

30

0.7

1.6

13.9

13.0

0.48

10.0

77

40

40

0.86

20

0.8

1.7

8.1

8.2

0.83

6.3

46

48

36

0.67

15

0.8

1.8

6.0

5.8

1.00

0.5

5

75

24

0.52

1

C3Hs/O2/I-Ie " 60/20/20 (%vol. CNTP), F = 1.5 1.h-1, m = 1.0 g

0.6

acet.

acry.

379 3.1.2 Effect of a thermal treatment at 700~ The catalytic properties of t M o O 3 - NiMoO4 ] catalysts after a thermal treatment at 700~ under an oxygen atmosphere (RA'= otMoO 3/(cxMoO 3 + 13NiMoO4 )) are reported in Table 5. Compared to the results reported in Table 4, this activation procedure causes a decrease in the propane conversion especially when RA'> 0.06, which could be related to a decrease of the surface area. These changes are particularly significant for RA'= 0.25 and 0.83 because the acid formation is completly suppressed. ESCA and XRD characterizations carried out after the catalytic experiments reveal a surface enrichment in molybdenum with a specific (010) orientation of crystal faces of MoO 3. The cristalline structure of the solids however is not very much modified so that the most significant factor of the partial covering of NiMoO4 with MoO 3 could be the thermal treatment. Table 5 Propane oxidation over (MoO3-NiMoO4) activated at 700~ under oxygen RA'

BET (m2.g-1)

Conversion (%)

Selectivity (%)

(1)

(2)

C3H8

0 2

C3I-I6

COx

Eoxy.

13NiMoO4

44

23

5.9

32

67

24

6

0.06

42

19

6.3

33

67

20

12

0.25

30

6

1.0

3

90

6

2

0.83

10

3

1.2

4

89

9

1

~MoO3 (3)

4

4

0.5

5

76

24

e

RA '= ~MoO 3 /(~MoO 3 + [3NiMoO4), C3Hg/O2/He : 60/20/20 (%vol. CNTP), T=375~ F = 1.5 l.h-1, m = 1.0 g (1) and (2) before and after the catalytic test (3) activation at 550~ (under helium)

3.2 Bismuth-nickel-molybdenum catalysts The [cxMoO3-cxNiMoO4 ] catalysts with a ratio RA=0.83 or 0.25 were chosen as references. The effect of bismuth content (precipitation) and of the water were studied. 3.2.1 Effect of bismuth content over a Ni0.17MoOx catalyst The results of Table 6 showed that the addition of a very small amount of bismuth increases significantly the selectivity towards acrolein and acrylic acid with no change in the propane conversion. The propene formation and the acetic acid production decreased at the same time, which is quite an important result of the effect of bismuth on the reaction scheme.

380 Table 6 Effect of bismuth on the propane conversion over [r Catalysts

BET 2

m .g

-1

catalysts, RA=0.83

Conv (%)

C3H8 02

Selectivity (%) C3I-I6 COx

O2/C

Zoxy etha. acro. acet. acry.

Ni0.17MoOx(*)

10

10.8

82

42

40

0.64

17

0.6

2.1

5.7

8.4

Bio.o2/Nio.lsMOOx

7

8.8

73

22

48

0.86

28

0.8

8.8

1.6

16.3

Bio.odNio.olsMOOx

8

9.6

82

21

47

0.87

29

1.3

8.4

2.8

16.6

Bio.17/Ni0.16MoOx

6

8.4

69

29

44

0.88

24

1.9

9.0

1.8

10.7

C3Hg/O2/He" 60/20/20 (%vol. CNTP), T = 425~

F = 1.5 l.h-1, m = 1.0 g, (*) T = 400~

3.2.2 Effect of bismuth and water on a [Ni0.75MoOx] catalysts Like in the preceding example adding bismuth to the ['Nio.75MoOx] catalyst increases the selectivity towards acrolein and acrylic acid and decreases the formation of acetic acid and of propene (Table 7). Table 7 Effect of bismuth and water on the propane conversion over a [czMoO3+czNiMoO4] catalyst, RA=0.25 Catalyst

1/VVH

(~

(g.h.11) C3H8 02

/ (h)

Conv (%)

Selectivity (%) C3I-I6 COx

O2/C

X;oxy etha. acro. acet. acry.

Nio.TsMoOx (29 mZ.g-1) 375/3.3

0.66

11.8

82

35

35

0.92

30

0.7

1.6

13.9

13.0

Bio.oz/Nio.7sMoO~(18 m2.g-1) 375/3.5

0.66

8.3

62

26

36

1.05

37

1.0

5.9

6.9

23.1

425/6.1

0.66

13.9

100

23

38

1.22

35

1.5

5.5

6.8

20.5

425/7.3

0.33

14.0

97

24

34

0.98

39

1.3

7.6

5.0

24.5

98

22

28

0.92

47

1.3

6.6

8.2

30.6

Substitution of HzO for He 425/10.8

0.33

14.7

C.~H~/OJX 960/20/20 (%vol. CNTP) with X = He or n 2 0 , m catalyst = 1.0 g

381 At 425~ instead of at 375~ there is an enhancement of the propane conversion but no significant change of the product distribution, except an increase of the CO2/CO ratio. In the selective oxidation of hydrocarbons to oxygenated compounds like acids, water is introduced with the reagents. This additive can create various phenomena in the oxidation : mainly kinetic effects and catalyst modification. If there is no definite evidence concerning the second point, it is well known that the addition of water favors the desorption of oxygenated compounds. In the experiment presented in Table 7 this hypothesis is well in evidence the selectivity towards oxygenated compounds particularly towards acrylic acid increasing from 25% to 31% with no change in the propane conversion. We can also notice that the total oxidation of propene to carbon oxides is inhibited by water. Under the specific conditions used in our experiment, a selectivity of about 50% towards oxygenated compounds, containing mainly acrolein and acrylic acid, was obtained for a propane conversion of about 15% and for complete conversion of oxygen.

4. CONCLUSION In agreement with previous works of Mazzocchia et coll., we have shown that the properties of nickel molybdate catalysts are greatly dependent of the molybdenum coordination and on the M o O 3 content. Propane oxydehydrogenation occurs mainly in the presence of a 13NiMoO4phase containing tetrahedral centers. When the reaction is carried out with [NiMoO4-MoO3] systems, a synergetic effect both for the activity and the selectivity is obtained when the molar ratio : otMoO3/(otMoO3+otNiMoO4) is around 0.25. According to Schrader et coll., a reciprocal coverage of the two phases (o~MoO3, otNiMoO4) can lead to changes of the surface structure. In the same way, the activity decreases after a thermal treatment at 700~ attributed to the covering of ~NiMoO4 with MOO3. A two-way reaction scheme involved for BiMo/TiO2 catalysts is also valid for bulk catalysts ( see scheme ) 02 2 ~

C3Hs

CH3CH(OH)CH3

02 > CH3COCH3

> CH3COOH + COx

3 COx J /

> C3H6

CH2CHCHO

CH2CHCOOH

Acetic acid formed via isopropanol and acetone could involve Mo 5§ species. With the second way, acrolein and acrylic acid are obtained with the participation of M o 6+ species. When the temperature increases the first way of the scheme is favored owing to a surface restructuration of the oxide: Mo+VIo3 which can contain pentacoordinated molybdenum species. The addition of bismuth to these phases decreases slightly the activity, the formation of acetic acid being supressed. When water is added to the reagent stream, the activity does not change, but the desorption of oxygenated compounds is favo