Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 143

SCIENTIFIC BASES FOR THE PREPARATION OF HETEROGENEOUS CATALYSTS

This Page Intentionally Left Blank

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. Delmonand J.T. Yates

Vol. 143

SCIENTIFIC BASES FOR THE PREPARATION OF

HETEROGENEOUS CATALYSTS

Proceedings of the 8t" International Symposium, Louvain-la-Neuve, Belgium,September 9 -12, 2002

Edited by E. G a i g n e a u x * , D.E. D e V o s * * , P. G r a n g e * , P.A. J a c o b s * * , J . A . M a r t e n s * * , P. R u i z * a n d G. P o n c e l e t *

* Universit# Catholique de Louvain, Louvain-la-Neuve, Belgium ** Katholieke Universiteit Leuven, Heverlee (Leuven), Belgium

2002 ELSEVIER Amsterdam - Boston - London - New Y o r k - Oxford - Paris - San Diego San Francisco - Singapore - Sydney - Tokyo

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

9 2002 Elsevier Science B.V. All rights reserved.

This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science via their homepage (http://www.elsevier.com) by selecting 'Customer support' and then 'Permissions'. Alternatively you can send an e-mail to: [email protected], or fax to: (+44) 1865 853333. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+1) (978) 7508400, fax: (+1) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W1P 0LP, UK; phone: (+44) 207 631 5555; fax: (+44) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2002 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for.

British Library Cataloguing in Publication Data A catalogue record from the British Library has been applied for.

ISBN: ISSN:

0444 511784 0167 2991

O The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). n__._

~ . _ ..1 - _

FOREWORD This volume of the Studies in Surface Science and Catalysis series contains the Proceedings of the Eighth International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, held on the campus of the "Universit6 catholique de Louvain" (UCL) in Louvain-la-Neuve, Belgium, on September 9-12, 2002. This symposium is jointly organized by the "Unit6 de catalyse et chimie des mat6riaux divis6s" of the "Universit6 catholique de Louvain", and the "Centrum voor oppervlaktechemie en katalyse" of the "Katholieke universiteit Leuven". The topic of this series of symposia, which was initiated in 1975 and organized at four-year intervals, has invariably been the discussion of the fundamentals behind the unit operations in the preparation of industrially relevant solid catalysts. Heterogeneous catalysis has always been a lively research field, although in the time period covered by these symposia, the emphasis of catalysis research has significantly shifted and spread from traditional applications in petroleum and bulk chemical production to, among others, the synthesis of fine chemicals, agro- and oleo-chemicals, pharmaceuticals and the environmental protection systems. This area, via the automotive applications, is presently dominating the catalyst market. The future for catalysis research seems bright, as it is now generally recognized as an essential element of sustainable development. In parallel with the broadening of the applications, the diversity of catalyst materials is booming, not only with respect to active elements and the sophistication of molecularly engineered active sites, but also with respect to the never ending discovery of new approaches for structuring porous matrices at the different relevant length scales, including the nanometer, the micrometer as well as the millimeter scale. Catalyst preparation techniques and physico-chemical catalyst characterisation tools likewise are subject to continuous developments. The organisers of these symposia, assisted by a Scientific Committee composed of eminent researchers holding an industrial appointment, try to keep trace of the relevant developments in the field and solicit contributions dealing with preparation aspects of relevant new generations of catalyst materials. In the organisation of this Eighth Symposium, the Scientific Committee was faced with the difficult task to select 139 papers (37 oral communications and 102 posters) out of the more than 200 submitted abstracts. When hesitating among excellent contributions, the decisive criterion was always the catalyst preparation aspect of the work. The organisers hope to be

able to offer a valuable platform for discussion of the science behind catalyst preparation. The attendance of over 250 suggests that the effort is appreciated by the scientific community. The organisers are indebted to public and private sponsors without whom the organisation of this symposium would have been financially very difficult. For obvious reasons, the sponsoring Companies and Agencies cannot be acknowledged properly by citing them in the Proceedings. The same holds true for all those who have contributed to the preparation of the meeting, secretaries, technicians, students and postdocs. Fortunately, the organisers are in position to express their appreciation towards the Rector of the UCL, Professor M. Crochet, and the "Service des Auditoires", for allowing the event to be patronized again by the university. Our grateful acknowledgements also go specifically to Ms Marianne Saenen, who has decisively contributed to the success of the symposium. Thankyou very much Marianne I

The Editors

Georges Poncelet has been a driving force of this Symposia series. Given his retirement next year, the moment has come to express our appreciation and gratefulness for his devotion and dedication to these meetings. Georges, it is with a warm heart that we dedicate this volume to you. Eric

Dirk

Johan

Patricio

Paul

P~rre

vii ORGANIZING COMMITTEE Dr. D. DE VOS, Katholieke Universiteit Leuven Dr. E.M. GAIGNEAUX, Universit6 catholique de Louvain Prof. P. GRANGE, Universit6 catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Prof. J. MARTENS, Katholieke Universiteit Leuven Dr. G. PONCELET, Universit6 catholique de Louvain Dr. P. RUIZ, Universit6 catholique de Louvain SCIENTIFIC COMMITTEE Dr. A. ANUNDSKAS, Norsk Hydro, Norway Dr. M.P. ATKINS, BP Amoco, United Kingdom Dr. G. BELLUSSI, EniTecnologie, Italy Dr. J.-L. BOUSQUET, TotalFinaElf, France Dr. J.A. DELGADO, Repsol, Spain Dr. D. DE VOS, KUL, Belgium Dr. E.M. GAIGNEAUX, UCL, Belgium Prof. P. GRANGE, UCL, Belgium Dr. J. GROOTJANS, Atofina Research, Belgium Dr. K. HARTH, BASF, Germany Dr. G. HECQUET, Atofina, France Dr. S.D. JACKSON, Synetix, United Kingdom Prof. P.A. JACOBS, KUL, Belgium Dr. K. JOHANSEN, Haldor Topsoe, Denmark Dr. S. KASZTELAN, Institut Franqais du P6trole, France Dr. J.-P. LANGE, Shell International, The Netherlands Prof. J.A. MARTENS, KUL, Belgium Dr. L.R. MARTENS, Exxon Mobil, Belgium Dr. R. PARTON, DSM Research, The Netherlands Dr. M.A. PEREZ, CEPSA, Spain Dr. G. PONCELET, UCL, Belgium Dr. P. RUIZ, UCL, Belgium Dr. F. SCHMIDT, Sfid Chemie, Germany Dr. J.-P. SCHOEBRECHTS, Solvay, Belgium Dr. M. SCHOONOVER, UOP, USA Dr. C. STOCKER, Sumitomo Deutschland, Germany Dr. M. TWIGG, Johnson Matthey, United Kingdom

This Page Intentionally Left Blank

ix

Contents Foreword Aspects of scale-up of catalyst production Keld Johansen

Quantitative structure-activity relationships in zeolite-based catalysts: influence of framework structure J.L. Casci and M.D. Shannon

17

Cogelation: an effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides B. Heinrichs, S. Lambert, C. Alig, J.P. Pirard, G. Beketov, V. Nehasil and N. Kruse

25

Steam reforming of CH4 over Ni/Mg-A1 catalyst prepared by spc-method from hydrotalcite T. Shishido and K. Takehira

35

Toward a molecular understanding of noble metal catalyst impregnation J.R. Regalbuto, M. Schrier, X. Hao, W.A. Spieker, J.G. Kim, J.T. Miller and A.J. Kropf

45

Support modification of cobalt based slurry phase Fischer-Tropsch catalysts S. Barradas, E.A. Caricato, P.J. van Berge and J. van de Loosdrecht

55

The effects of nature and pretreatment of surface alumina support on the catalytic nickelsilicate membrane formation C. Constantin, V. Parvulescu, A. Bujor, G. Popescu and B.L. Su

67

Supports and catalysts preparation by using metal alkoxides grafting technique E. Santacesaria, A. Sorrentino, M. Di Serio and R. Tesser

77

Combinatorial approaches for speeding up heterogeneous catalyst discovery, optimisation and scaling-up C. Mirodatos

89

High surface area metal oxides from matrix assisted preparation in activated carbons M. Schwickardi, T. Johann, W. Schmidt, O. Busch and E Schiith

93

Effects of the impregnating and drying process factors on mechanical properties of a PCoMo/A1203 hydrotreating catalyst Dongfang Wu and Yongdan Li

101

Influence of CeO2 content on R h / Y i O 2 monolithic catalysts for N20 decomposition S. Suarez, M. Yates, F.J. Gil Llambfas, J.A. Martfn, P. Avila and J. Blanco

111

Pt combustion catalysts prepared from W/O microemulsions J. Rymes, G. Ehret, L. Hilaire and K. Jir6tov6

121

Preparation of stable catalysts for N20 decomposition under industrial conditions S. Alini, E Basile, A. Bologna, T. Montanari and A. Vaccari

131

The Anderson-type heteropolyanions in the synthesis of alumina- and zeolitesupported HDS oxidic precursors E. Payen, G. Plazenet, C. Martin, C. Lamonier, J. Lynch and V. Harl~

141

Sol-gel preparation of pure and silica-dispersed vanadium and niobium catalysts active in oxidative dehydrogenation of propane P. Moggi, G. Predieri, D. Cauzzi, M. Devillers, P. Ruiz, S. Morselli and O. Ligabue

149

Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao and G. V. Baron

159

Preparation of gold-titanosilicate catalysts for vapor-phase propylene epoxidation using H2 and 02 A.K. Sinha, S. Seelan, S. Tsubota and M. Haruta

167

Sol-gel synthesis of colloids and triflates containing hybrid type catalysts A.N. Parvulescu, B.C. Gagea, M. Alifanti, V. Parvulescu and V.L Parvulescu

177

Preparation of zeogrids through interposed stapling and fusion of MFI zeolite type nanoslabs S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, E Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens

185

Large scale synthesis of carbon nanofibers by catalytic decomposition of hydrocarbon L. Pesant, G. Wine, R. Vieira, P. Leroi, N. Keller, C. Pham-Huu and M.J. Ledoux

193

Synthesis and characterization of carbon nanofiber supported ruthenium catalysts M.L. Toebes, E E Prinsloo, J.H. Bitter, A.J. van Dillen and K.P. de Jong

201

Synthesis of high pore volume and specific surface area mesoporous alumina L. Sicard, B. Lebeau, J. Patarin and E Kolenda

209

xi Investigation on acidity of zeolites bound with silica and alumina X. Wu, A. Alkhawaldeh and R.G. Anthony

217

Preparation of BN catalyst supports from molecular precursors. Influence of the precursor on the properties of the BN ceramic J.A. Perdigon-Melon, A. Auroux, J.M. Guil and B. Bonnetot

227

Monitoring of the particle size of MoSx nanoparticles by a new microemulsionbased synthesis K. Marchand, M. Tarret, L. Normand, S. Kasztelan and T. Cseri

239

Transition metal phosphides. Novel hydrodenitrogenation catalysts V. Zuzaniuk, R. Prins, C. Stinner and T. Weber

247

The application of non-hydrothermally prepared stevensites as support for hydrodesulfurization catalysts M. Sychev, R. Prihod'ko, A. Koryabkina, E.J.M. Hensen, J.A.R. van Veen and R.A. van Santen

257

NiMo/HNazY(s)-A1203 catalysts for the hydrodesulfurization of hindered dibenzothiophenes: effect of the preparation method T. Klimova, D. Solis, J. Ramirez and A. L6pez-Agudo

267

Chiral dirhodium catalysts confined in porous hosts H.M. Hultman, M. de Lang, M. Nowotny, I. W. C.E. Arends, U. Hanefeld, R.A. Sheldon and T. Maschmeyer

277

Synthesis and characterization of zeolite encaged enzyme-mimetic copper histidine complexes J.G. Mesu, H.J. Tromp, D. Baute, E.E. van Faassen and B.M. Weckhuysen

287

Strategies for the heterogenization of rhodium complexes on activated carbon J.A. Diaz-Au~ton, L.C. Romdn-Mart[nez, C. Salinas-Mart[nez de Lecea and H. Alper

295

Heterogeneous metathesis initiators M. Mayr, B. Mayr and M.R. Buchmeiser

305

Preparation of physically heterogeneous and chemically homogeneous catalysts on the base of metal complexes immobilized in polymer gels A.A. Efendiev, T.N. Shakhtakhtinsky and N.A. Zeinalov

313

Hydrocracking catalyst to produce high quality diesel fraction R. Galiasso Tailleur

321

xii Thermostable yttria-doped inorganic oxide catalyst supports for high temperature reactions E. Elaloui, R. Begag, B. Pommier and G.M. Pajonk

331

Preparation and characterization of WOx-CeO2 catalysts M. Alifanti, C.M. Visinescu, V.1. Parvulescu, P. Grange and G. Poncelet

337

Preparation of iridium catalysts by deposition precipitation: room temperature oxidation of CO M. Okumura, E. Konishi, S. Ichikawa and T. Akita

345

New approach to preparation and investigation of active sites in sulfated zirconia catalysts for skeletal isomerization of alcanes N.A. Pakhomov, A.S. Ivanova, A.E Bedilo, E.M. Moroz and A.M. Volodin

353

Supported ruthenium carbido-cluster catalysts for the catalytic removal of nitrogen monoxide and sulfur dioxide: the preparation process monitored by sulfur K-edge X-ray absorption near-edge structure Y. Izumi, T. Minato, K.-L Aika, A. Ishiguro, T. Nakajima and Y. Wakatsuki

361

Catalytic transformation of dichloromethane over Y and X zeolites L. Pinard, J. Mijoin, R. Lapeyrolerie, P. Magnoux and M. Guisnet

369

Preparation of new solid super-acid catalyst, titanium sulfate supported on zirconia and its acid catalytic properties J.R. Sohn, E.H. Park and J.G. Kim

377

Superacid WOx/ZrO2 catalysts for isomerization of n-hexane and for nitration of benzene V.V. Brei, O.V. Melezhyk, S.V. Prudius, M.M. Levchuk and K.I. Patryliak

387

Preparation of copper-oxide catalyst systems for hydrogenation Y. Sakata, N. Kouda, Y. Sakata and H. Imamura

397

Application of experimental design for NOx reduction by Pd-Cu catalysts M. Rebollar, M. Yates and M.A. Valenzuela

407

Marked difference of catalytic behavior by preparation methods in CH4 reforming with CO2 over Mo2C and WC catalysts S. Naito, M. Tsuji, Y. Sakamoto and T. Miyao

415

Synthesis and properties of new catalytic systems based on zirconium dioxide and pentasils for process of NOx selective catalytic reduction by hydrocarbons V.L. Struzhko, S.N. Orlyk, T.V.Myroniuk and V.G. Ilyin

425

xiii Preparation of chitosan based catalysts for several reactions of liquid phase hydrogenation V. Isaeva, A. Ivanov, L. Kozlova and V. Sharf

435

Preparation of Mo/A1203 sulfide catalysts modified by Ir nanoparticles J. Cinibulk and Z. Vit

443

Peptization mechanisms of boehmite used as precursors for catalysts D. Fauchadour, E Kolenda, L. Rouleau, L. Barr~ and L. Normand

453

Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on physicochemical and catalytic properties: application for chlororganics destruction R. Lopez-Fonseca, J.L Guti~rrez-Ortiz, B. de Rivas, S. Cibrian and J.R. Gonz61ez-Velasco

463

Preparation of SiO2 modified SnO2 and ZrO2 with novel thermal stability Y.-Z Zhu, J.-Y. Wei, L. Zeng, X.-D. Zhao, W. Lin and Y.-C. Xie

471

Control of the textural properties of cesium 12-molybdophosphate-based supports S. Paul V. Dubromez, L. Zair, M. Fournier and D. Vanhove

481

MnOx/CeO2-ZrO2 and MnOx/WO3-TiO2 catalysts for the total oxidation of methane and chlorinated hydrocarbons E. Kantzer, D. D6bber, D. Kiessling and G. Wendt

489

Catalytic behaviour of Rh-supported catalysts on lamellar and zeolitic structures by anchoring of organometallic compound C. Blanco, R. Ruiz, C. Pesquera and E Gonzalez

499

The use of sol-gel technique to prepare the TiO2-A1203 binary system over a wide range of Ti-A1 ratios A.Yu. Stakheev, G.N. Baeva, N.S. Telegina, L V. Mishin, T.R. Brueva, G.L Kapustin and L.M. Kustov

509

Catalytic performance in the complete acetone oxidation of manganese and cobalt oxides supported on alumina and silica A. Gil, S.A. Korili, M.A. Vicente and L.M. Gandia

517

Unsupported and supported manganese oxides used in the catalytic combustion of methyl-ethyl-ketone L.M. Gandia, S.A. Korili and A. Gil

527

Ni/Hfl zeolite catalysts prepared by the deposition-precipitation method R. Nares, J. Ramirez, A. Guttierrez-Alejandre, R. Cuevas, C. Louis and T. Klimova

537

xiv Sol-gel A1203 structure modification by Ti and Zr addition. A NMR study J. Escobar, J.A. de Los Reyes and T. Viveros

547

Promotion of Ru/ZrO2 catalysts by platinum A.M. Serrano-S6nchez, F. Blas-Sudrez, P. Steltenpohl, M.P. Gonzdlez-Marcos, J.A. Gonzdlez-Marcos, and J.R. Gonzdlez-Velasco

555

Catalysts based on RhMo6 heteropolymetalates. Bulk and supported preparation and characterisation C.I. Cabello, I.L. Botto, M. Mu~oz and H.J. Thomas

565

Metallosilicate mesoporous catalysts prepared by incorporation of transition metals in the MCM-41 molecular sieves and their catalytic activity in selective oxidation of aromatics (styrene and benzene) V. Parvulescu and B.L. Su

575

Controlled surface modification of alumina-supported Mo or Co-Mo sulfides by surface organometallic chemistry J.-S. Choi, C. Petit-Clair and D. Uzio

585

Novel one step synthesis of cobalt (II) phtalocyanine-hydrotalcite catalysts for mercaptan oxidation in light oil sweetening I. Chatti, A. Ghorbel and J.M. Colin

595

Structural and catalytic properties of Zr-Ce-Pr-O xerogels S. Rossignol, C. Descorme, C. Kappenstein and D. Duprez

601

Influence of the precursor (nature and amount) on the morphology of MoO3 crystallites supported on silica D. Navez, C. Weinberg, G. Mestl, P. Ruiz and E.M. Gaigneaux

609

Single step synthesis of metal catalysts supported on porous carbon with controlled texture N. Job, E Ferauche, R. Pirard and J.P. Pirard

619

A g - S i Q and Cu/SiO2 cogelled xerogel catalysts for benzene combustion and 2-butanol dehydrogenation S. Lambert, N. Tcherkassova, C. Cellier, E Ferauche, B. Heinrichs, P. Grange and J.P. Pirard

627

Preparation of zeolite catalysts for dehydrogenation and isomerization of n-butane M. Inaba, K. Murata, M. Saito, I. Takahara, N. Mimura, H. Hamada and Y. Kurata

637

XV

The application of well-dispersed nickel nanoparticles inside the mesopores of MCM-41 by use of a nickel citrate chelate as precursor D.J. Lensveld, J.G. Mesu, A.J. van Dillen and K.P. de Jong

647

Preparation of Ce-Zr-O composites by a polymerized complex method T.G. Kuznetsova, V.A. Sadykov, E.M. Moroz, S.N. Trukhan, E.A. Paukshtis, V.N. Kolomiichuk, E.B. Burgina, V.L Zaikovskii, M.A. Fedotov, V.V. Lunin and E. Kemnitz

659

Sol-gel routes for the preparation of heterogeneous catalyst based on Ru, Rh, Pd supported metals P. Moggi, S. Morselli and G. Predieri

669

Development of novel heterogeneous catalysts for oxidative reactions: preparation and performance of Co-Nx catalysts in partial oxidation of toluene and n-butane M.L. Kaliya, S.B. Kogan and M. Herskowitz

679

Synthesis and modification of basic mesoporous materials for the selective etherification of glycerol J.-M. Clacens, Y. Pouilloux and J. Barrault

687

Carbon nanotubes: a highly selective support for the C=C bond hydrogenation reaction J.-P. Tessonier, L. Pesant, C. Pham-Huu, G. Ehret and M.J. Ledoux

697

Raman studies of the templated synthesis of zeolites P.P.H.J.M. Knops-Gerrits and M. Cuypers

705

Templateless synthesis of catalysts with narrow mesoporous distribution N. Yao, G. Xiong, S. Sheng, M. He and K.L. Yeung

715

Control of pore structures of titanias and titania/aluminas using complexing agents M. Toba, S. Niwa, N. Kijima and Y. Yoshimura

723

Tungstophosphoric acid immobilized in polyvinyl alcohol hydrogel beads as heterogeneous catalyst L.R. Pizzio, C. C6ceres and M.N. Blanco

731

Functionalized SiMCM-41 as support for heteropolyacid based catalysts L.R. Pizzio, A. Kikot, E. Basaldella, P. Vazquez, C. Cdceres and M.N. Blanco

739

Influence of the preparation method on the surface properties and activity of alumina-supported gallium oxide catalysts A. Petre, B. Bonnetot, A. Gervasini and A. Auroux

747

xvi Preparation and properties of bimetallic Ru-Sn sol-gel catalysts: the influence of catalyst reduction J. Hajek, N. Kumar, H. Karhu, L. Cerveny, J. Vayrynen, T. Salmi and D. Yu. Murzin

757

A new insight into molybdate/boehmite interaction D. Minoux, F. Diehl, P Euzen, J.-P Jolivet and E. Payen

767

Controlled coating of high surface area silica with titania overlayers by atomic layer deposition J. Keriinen, E. Iiskola, C. Guimon, A. Auroux and L. Niinist6

777

Concept of the synthesis of novel platinum catalysts for selective hydrogenation of unsaturated carbonyl compounds J. Kijenski and P. Winiarek

787

Storage and supply of hydrogen mediated by iron oxide: modification of iron oxides S. Takenaka, C. Yamada, T. Kaburagi and K. Otsuka

795

Catalytic activity of bulk and supported sulfated zirconia I.J. Dijs, L.W. Jenneskens and J.W. Geus

803

New one-step synthesis of superacid sulfated zirconia L. Zanibelli, A. Carati, C. Flego and R. Millini

813

Elaboration and characterization of a realistic Phillips model catalyst for ethylene polymerisation P.G. Di Croce, E Aubriet, P. Bertrand, P. Rouxhet and P. Grange

823

Preparation of new basic mesoporous silica catalysts by ammonia grafting H. Yoshida, Y. Inaki, Y. Kajita, K. Ito and T. Hattori

837

Titania-silica catalysts prepared by sol-gel method for photoepoxidation of propene with molecular oxygen C. Murata, H. Yoshida and T. Hattori

845

Preparation of large surface area MnOx-ZrO2 for sorptive NOx removal M. Machida, M. Uto and T. Kijima

855

Preparation of CuOx-TiO2 nano-composite photocatalysts from intercalated layer structure M. Machida, S. Nagasaki and T. Kijima

863

Vanadia-doped titanium pillared clay: preparation, characterization and SCR activity of NO by ammonia L. Khalfallah Boudali, A. Ghorbel, P. Grange and S.M. Jung

873

xvii Advanced preparation by sol-gel method of the encapsulated Pd/A1203 catalysts for methane combustion S. Fessi, A. Ghorbel, A. Rives and R. Hubaut

881

Non-ionic surfactant templated synthesis of mesoporous silica in the presence of platinum salts M.A. Aramendfa, V. Borau, C. Jimdnez, J.M. Marinas, EJ. Romero and EJ. Urbano

891

Synthesis and acid-base properties of catalysts based on magnesium and sodium-magnesium mixed phosphates M.A. Aramendfa, V. Borau, C. Jimdnez, J.M. Marinas, R. Rolddn, EJ. Romero and EJ. Urbano

899

Preparation of Pd-Ce/ZrO2 catalysts for methane oxidation L.S. Escand6n, S. Ord6ftez, E V. Dfez and H. Sastre

907

The effect of cerium introduction on vanadium-USY catalysts C. Ramos Moreira, M. Schmal and M.M. Pereira

915

Rh-Co mordenite catalysts for the selective reduction of NO by methane C.E. Quincoces, M. Incolla, A. De Ambrosio and M.G. Gonzdlez

925

Surface characterization of WO3-TiO2/A1203 catalysts and reactivity on selective catalytic reaction of NO by NH3 S. Egues, N.S. de Resende and M. Schmal

933

Catalytic materials for the synthesis of hydrofluorocarbons P. Cuzzato, V. Giammetta, R. Trabace and E Trifiro

941

Preparation, characterization and reactivity in m-cresol methylation of new heterogeneous materials having basic properties E Cavani, C. Felloni, D. Scagliarini, A. Tubertini, C. Flego and C. Perego

953

The effect of glycols in the organic preparation of V/P mixed oxide catalyst for the oxidation of n-butane to maleic anhydride S. Albonetti, E Cavani, S. Ligi, E Pierelli, E Trifiro, E Ghelfi and G. Mazzoni

963

Synthesis and characterization of nanostructured M02C on carbon material by carbothermal hydrogen reduction C. Liang, Z Wei, Q. Xin and C. Li

975

Carbon composite-based catalysts: new perspectives for the low-temperature H2S removal J.-M. Nhut, R. ~eira, N. Keller, C. Pham-Huu, W. Boll and M.J. Ledoux

983

xviii Active carbon surface oxidation to optimize the support functionality and metallic dispersion of a Pd/C catalyst V. Dubois, Y. Dal and G. Jannes

993

n-Butane isomerization over Al-promoted sulfated zirconias. Influence of the sulfate content J.A. Moreno and G. Poncelet

1003

Influence of preparation procedure on physical and catalytic properties of carbon-supported Pd-Au catalysts P. Canton, E Menegazzo, M. Signoretto, E Pinna, P. Riello, A. Bedetti and N. Pernicone

1011

Preparation of mesoporous highly dispersed Pd-Pt catalysts for deep hydrodesulfurization X. Xu, P. Waller, E. Crezee, Z. Shan, E Kapteijn and J.A. Moulijn

1019

Preparation of highly ordered CMI-1 and wormhole-like DWM mesoporous silica catalyst supports using C16(EO)10 as surfactant A. L~onard, J.L. Blin and B.L. Su

1027

Synthesis and characteriation of nanostructured mesoporous zirconia catalyst supports using non-ionic surfactants as templating agents J.L. Blin, L. Gigot, A. L~onard and B.L. Su

1035

Effect of preparation parameters on the catalytic activities of sulfated ZrO2SiO2 catalysts obtained by sol-gel process R. Akkari and A. Ghorbel

1045

Non-aggressive way for preparation of zirconium sulfate pillared clay using zirconium acetate developing high sulfur thermal stability over 830~ S. Ben Chaabene, L. Bergaoui, A. Ghorbel and J.E Lambert

1053

Influence of the preparation conditions on the structure of the active phase and catalytic properties of Ni-Co-molybdate propane oxydehydrogenation catalysts M.M. Barsan, A. Maione and E C. Thyrion

1063

Oxidized diamond as a new catalyst support T. Suzuki, K. Nakagawa, N.-O. Ikenaga and T. Ando

1073

Preparation of catalytic membranes, micro-capsules and fabrics active in immobilized Fenton chemistry J. Fernandez, V. Nadtochenko, A. Bozzi, T. Yuranova and J. Kiwi

1081

Preparation of vanadium-based catalysts for selective catalytic reduction of nitrogen oxides using titania supports chemically modified with organosilanes H. Kominami, M. Itonaga, A. Shinonaga, K. Kagawa, S. Konishi and Y. Kera

1089

xix Design, preparation and testing of effective FeOx/SiO2 catalysts in methane to formaldehyde selective oxidation E Arena, F. Frusteri, L. Spadaro, A. Venuto and A. Parmaliana

1097

New Fe-Mo-Ti mixed oxides prepared via the sol-gel method: comparison of the textural properties with solids obtained by impregnation S.R.G. Carrazan, C. Martin, C. M. Pedrero and J. Saunders

1107

Index

1115

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Aspects of scale-up of catalyst production Keld Johansen Research & Development Division, Haldor Topsoe A/S Nymollevej 55, DK-2800 Lyngby, Denmark, Tel: +45-45272491, E-mail: [email protected]

1. S C O P E OF C A T A L Y S T P R O D U C T I O N Catalyst production has a significant influence on the economy, since 80-90% of the chemicals used in a modern society are exposed to a catalyst. The value of the US catalyst market was 2.2 billion $ in 2000 [1]. A number of specialised catalyst companies in the world, many global, produce and supply a large number of products to the industry. It is said that products corresponding to 10% of the GNP of the industrialised countries are dependent on the availability of catalyst. Catalyst plants produce quantities of 0.1100 t/day of each product dependant on the type. Thus, transfer of new products and implementation in catalyst plants are important technological disciplines for each catalyst company. 2. N A T U R E OF T H E SCALE-UP P R O B L E M Before the decision is taken on transferring a new recipe from research and development departments to an existing catalyst plant or a new investment, considerable work has already been carried out - many test samples have been prepared and activitytested. All candidates for a new product have one thing in common: in the starting phase they have been prepared and selected from the laboratory processes.

2.1. Description of laboratory prepared samples Laboratory or bench scale prepared catalyst samples for screening are typically made in gram scale (10-50 g). The catalysts can be prepared in many ways depending on the type, but steps for three commonly used preparation routes are shown below in Fig. 1 and, for each step, examples of the typical laboratory equipment are given.

Io Typical co-precipitated catalyst manufacture Preparation step Typical equipment 1. Dissolution of agents Beaker with stirrer 2. Precipitation Pump + beaker with stirrer 3. Ageing Electric heating, thermostatic bath 4. Filtration Buchner funnel 5. Washing Demineralised water on Buchner funnel 6. Drying Drying cabinet 7. Calcination Muffle furnace 8. Lubricant aid addition Powder mixer Single station excenter press 9. Tabletting Electrically heated muffle furnace 10. Calcination Small reactor with H/N2 once through 11. Activation

II. Typical impregnated catalyst carrier process Typical equipment Preparation step 1. Forming a support (from another route) Heated muffle furnace 2. Activation of support Beaker with stirrer (pure chemicals) 3. Dissolution of impregnation liquid(s) Net in a beaker 4. Impregnation Drying cabinet 5. Drying Muffle furnace 6. Decomposition 7. Re-impregnation back to 4. Reactor sulphidation with H2/N2 once 8. Activation through III. Typical process for mixed/compounded catalyst Typical equipment Preparation step From flasks 1. Powders Beaker with stirrer 2. Dissolution of active metals Beaker with stirrer 3. Dissolution of extrusion aids Laboratory kneader 4. Mixing Laboratory piston extruder 5. Extrusion Drying cabinet 6. Drying Muffle furnace 7. Calcination/decomposing Laboratory sieve 8. Sieving Small reactor with H2/N2 9. Activation through Fig. 1. Three examples of commonly used preparation methods

etc. once

The laboratory equipment used is normally characterised by: Small dimensions with short mass and heat transport distances - High energy intensity per volume for stirrer mixers and kneaders - Pure chemicals Small layers in muffle furnaces with low temperature gradients, but long hea cycle H2 activation with low pH20 Drying with low but undefined pH20 Precipitation within small volume dimension Filtration without respect of agglomerate size Washing without respect of time or leakage of particles or ions - Tabletting/forming of non-representative granules - Generated heat during processes easily dissipated to cooling surface -

-

-

-

-

-

-

A modern catalyst laboratory will analyse and describe in detail all intermediates final catalysts from the above three manufacturing routes by means of the metho&, follows: main chemical elements and trace elements, phases (if not amorphous), 1: distribution and BET or selective surface area. A more complete list of commonly u methods is given in Table 1 below: Table 1. Physical and chemical characterisation methods C h e m i c a l

Main chemical elements Trace elements Oxidation state Element distribution

ICP, AAS, XRF and electron micro probe analysis AAS, ICP Electron micro probe analysis, EDS in SEM, ED5 in TEM

P h y s i c a l

Surface area Pore volume- total Pore size distribution Phases, crystallite size Surface composition Surface properties Particle size distribution Structure and texture Thermogravimetry Specially for finished product form Abrasion resistance for granules Attrition resistance for fluid cat and powders Crushing strength

BET (N2, Ar, K), or specific area, chemisorption of H2,CO, N20 Water absorption-Hg intrusion He, or Hg intrusion, N2 adsorption XRD, TEM, Raman Spectroscopy XPS, SIMS IR, microcalorimetry, chemisorption/desorption (TPD,TPR) LLD, SAXS, sieves, TEM, SEM TEM, SEM, optical microscopy TGA, DTA, dilatometer

Attrition loss Texture analyser

However, all the above methods in Table 1 cannot give a scientifically exhaustive description of the intermediates nor of the final catalyst. Amorphous phases often obtained from precipitation cannot be characterised sufficiently (how many kinds of amorphous phases exist?) Furthermore, the final catalyst granule is formed from agglomerates of crystallites. Both crystallites (forming primary agglomerates) and the agglomerates have their own particle size distribution and binding properties. Particle size distribution of primary and secondary agglomerates controls the final pore size distribution. The pore size distribution and particle strength will have significant influence on the final performance of the product in the reactor. Even considering the methods listed in Table 1, there is no method or combined methods today that can give a full description of the crystalliteagglomerate multi-parameter system. To further illustrate the problem, it should be mentioned that even if the overall chemical composition is the same for two different manufacturing routes, the pore size distribution is most probably different. Thus, it is not possible to characterise an intermediate or final catalyst so you can be sure to have the same catalyst without preparing it in the same reproducible way.

2.2 Catalyst manufacturing- unit operations The catalyst plant is operating in ton scale (typically 1-100 t/day) with processes and equipment completely different from bench scale as sketched in Fig. 1, even if the preparation steps are the same. In the open literature, description of catalyst manufacturing processes and equipment is sparse. The reference list contains important monographs and papers [2-24]. The patent literature gives some information, but catalyst manufacturing technologies are often not patented but kept secret. The single step in manufacturing is called a unit operation and can be performed by several types of equipment. Table 2 shows most of the unit operations used and examples of equipment for each. Most of the typical equipment will have more different time constants, heat transfer, flow patterns, temperature profiles etc. than bench scale equipment and, therefore, the final catalyst will achieve other properties.

2.3 Optimal combinations For every commercial catalyst an optimal combination of unit operation sequence exists for the manufacture of that specific catalyst and there will for each unit operation exist preferential process equipment, i.e. fluid bed calciner for calcination. The sequence of unit operations with the special selection of process equipment and all process parameters forms the know-how for manufacturing a catalyst product of large commercial value. But know-how does not mean that you always know why the desired properties are obtained due to the insufficient scientific characterisation of the catalyst material as described above under 2.1. Even small adjustments of the process can change strength, pore size distribution, bulk density, crystallite size etc. of the product and, thus, harm the performance in the industrial reactor. It has normally been costly and time-consuming to reach the final recipe and, therefore, all catalyst companies want to keep it secret. I f a single unit operation is changed it will often influence the optimisation of most of the other unit operations, and much of the development will have to be redone.

Table 2. List of unit operations with typical equipment Unit operation Typical equipment Tanks with stirrer 1. Dissolution Pumps, specially designed reactors and stirrers 2. Precipitation Temperature-regulated tanks, autoclaves 3. Ageing and maturation, gel formation Belt filter 4. Filtration Drum filter Centrifuge Filter press Belt filter 5. Washing Drum filter Centrifuge Belt conveyor furnace 6. Drying Spray drying Fluid bed drying Rotary kiln Vacuum dryer Z-mixer 7. Wet mixing (kneading) Double screw mixer Nauta mixer 8. Dry mixing Double cone mixer Ribbon blender Jet mill 9. Grinding Roller mill Universal mill Pearl mill Screen 10. Sieving Tabletting 11. Forming Extrusion Granulation Spray drying Corrugation Belt conveyor furnace 12. Calcination Rotary kiln Shaft furnaces Chamber and muffle furnace Tunnel furnace Fluid bed Pore filling- incipient wetness 13. Impregnation Immersion in liquid Controlled chemisorption See under calcination 14. Decomposition Electrical hearth 15. Fusion Prereduction reactor 16. Activation Fluid bed, chamber and muffle furnace 17. Cooling and annealing Washcoater, dragee pan 18. Coating Tanks with stirrer 19. Leaching Tanks with stirrer, kneader 20. Reslurrying . .

2.3.1 Examples of combinations A process scheme example of a precipitated catalyst is given below in Fig. 2. Metal salts

Bases

Dissolution of raw materials I

[

Precipitation

I

I

Ageing

I

I

Filtration

I

Washing

Recycle_= -=

Re-slurrying

]

l

Spray drying

I

Rotary kiln calcination

I

Powder mixing

I

Tabletting

I

I Lubricants i -i [

Salt Salt

}=

H20 ~.

I Conveyor belt calcination I

Recycle I I

Sieving

.]

Finished product

I

Fig. 2. The scheme for production of an extruded and impregnated catalyst can be as shown in Fig. 3. Raw materials Recycled

Additives

I~ Mixing of raw materials

-I

Extrusion

I r

Impregnation ~ liquid I

Recycle

[i E

Drying Calcination Impregnation Drying Calcination Sieving Finished product

Fig. 3.

H20

HaO HaO }= H20 + gases

3. P E R F O R M I N G SCALE-UP E X P E R I M E N T S When the unit operation processes and equipment have such a major influence on the final catalyst properties and performance, it would be logical and desirable to perform the catalyst test preparations in the catalyst plant process equipment or in a pilot production plant in order to avoid using development time on "wrongly prepared" samples. However, there must be at least some selection of recipes. The scale-up factor from laboratory to plant is approximately 1,000,000 corresponding to going from g/day to tons/day. A pilot manufacturing plant will typically produce 1-50 kg/day. Then the scale-up factors will be: - From laboratory to pilot: 1000 - From pilot to plant: 1000

3.1. Plant experiments Catalyst plants are large investments in a number of production lines. Some of the lines are dedicated to one product, others are multipurpose lines where various products can be made. The advantages of performing experiments in the plant with the realistic processes and equipment in tons/day scale must be compared to the drawbacks as follows: a. Expensive raw material, energy and labour. b. Expensive disposal/recycling of unusable product. c. Lack of flexibility of production lines. d. Production lines are booked and lost contribution will also be costly. The large economic consequences and lack of flexibility will dictate to perform most of the manufacture development work in a pilot scale, where the cost of experiments is 1001000 times lower, and only to use the plant lines for the last process adjustments.

3.2. Pilot production plants Catalyst pilot production plants have process and equipment facilities that are similar to equipment used in the large scale plant as listed in Table 2, but at the kg capacity scale typically 10-50 kg/day. Equipment in a catalyst pilot production is to a great extent industrial equipment downscaled by a factor 50-1000. It is necessary that equipment can easily be moved, so flexible production lines of new combinations of unit operations can be arranged. A modern pilot plant will contain most of the equipment listed in Table 2. 4. P H Y S I O C H E M I C A L D E S C R I P T I O N OF UNIT O P E R A T I O N S

4.1. Methods of study of unit operations The methods of characterisation of intermediates and final product coming from each unit operation are of course the same as given in Table 1. However, if the unit operation process itself is to be analysed and understood more profoundly in situ or in line, analytical equipment must be installed and mathematical models for heat and mass transport must be set up.

4.2. Examples of unit operation studies 4.2 91. Precipitation High surface activated alumina carrier is important for a large number of catalysts. The precipitation can take place from a number of aluminium salts: AI(NO3)3, A12(SO4)3, A1C13 and NaAIO2 with bases or acids as NaOH, NH3, KOH, HNO3, HC1 and H2SO4. Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles 9 Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. The potential formed types of aluminium compounds present in the solution are numerous (25). The first phases formed are far from equilibrium and, as a consequence of Ostwald's rule of stages, transform within minutes and hours. This illustrates the importance of control of history 9The precipitation methods and parameters in combination with carefully controlled ageing determine agglomerate size distribution, agglomerate packing and agglomerate strength. The precipitation may be controlled by pH and mass flows, but the study of the particle size distribution of amorphous particles and agglomerates within milliseconds is difficult, since only few relevant methods as SAXS and SANS exist. As an example of SAXS measurements, crystallite size distribution of the long dimension of precipitated pseudoboehmite is given for two sets of precipitation conditions (Fig. 4). The samples are taken out during the precipitation.

9 "~

0.05

.4,,.a

'~ 9

0.04

N r~

0.03

C,J

0.02

03 0.01

i/ ,/

0.00 i

20

'

410

'

Particle diameter (nm)

Fig. 4.

!

60

4.2.2. Calcination Controlled calcination of alumina carrier extrudates or tablets is important for obtaining the desired pore volume, BET area and pore size distribution. All these parameters are influenced by the partial pressure of water vapour at a given time and temperature. As an example a dried alumina carrier is used, which should be calcined to the highest possible surface area, but the temperature must reach at least 500~ and the use of a rotary kiln with countercurrent flow of air is considered. The kiln is heated indirectly. From test calcinations of small samples in controlled atmosphere we know the influence of the water vapour pressure. It reduces the surface area and it is quite clear that the water vapour pressure in the kiln must be kept at the lowest possible values. This means drying the air fed to the kiln, if possible, and maximising the ratio between the air flow/carrier flow. The generation of water from calcining the alumina can be found by a simple thermo-gravimetric analysis: The profile of the water vapour pressure in a countercurrent rotary kiln follows the thermogravimetric profile - with adjusted scales and minimum values equal to the water vapour pressure of the feed gas - therefore, it can be expected to reach a high surface area, if the water vapour generated in the first part of the kiln is diluted to a sufficiently low partial pressure. If this is tried disappointing results will probably be obtained due to the diffusion resistance: a. The carrier in the kiln forms a bed of a certain thickness. The carrier is heated by a number of processes: heat transfer from the gas sweeping over the bed surface and from the kiln wall. Therefore, all the carrier at the bottom of the bed, as well, will generate water vapour at a given time. The water vapour can only escape the bed by diffusion or when the bed "folds over". To get an idea of the influence hereof, a one-dimensional equation for diffusion from a bed where all particles generate gas can be solved [26]:

(<-Po) I= H In

dp* ( 1 - r

r

Plotting this for the high water generation rate, r, of 0.0001 g/g*sec for two bed thicknesses of 2 and 5 cm gives the following bed profiles of realistic values for the bed density (dp*(1 - e)) - for p0 (the water vapour pressure in the kiln gas) = 0 atm (Fig. 5), where it can be seen that most of the bed - potentially- will have a water vapour pressure of close to 1 atm as long as water is generated (at high rates) from the carrier. This problem can either be solved by running the kiln with a low loading, bed height = 1-2 pellet diameters, or by another design where (dry) air is passed through the bed, and this has the potential for changing the situation much as seen in: b. where we will solve the diffusion of an internally generated gas (i.e. with a net flow out of the pellet) in a cylindrical pellet:

10 DeUPtRgT[ln ~ ) ( PPs) t - ] = R2-4 x2 dp*r

Calculating for a porosity of the carrier O to 0,67 and a tortuosity (1:) of 1.6, the effective diffusion coefficient becomes 0.19 and 0.38 cm2/sec, respectively. The calculations are still made for a high water generation rate of 0.0001 g/g*sec and using a dp of 1.3 g/ml. The calculation is made for two diameters of the cylindrical carriers 1 and 2 mm and shown in Fig. 6. Bed profiles at 6wt%/10min po = 0 atm

N: d.

"

\\

0,6

i"

0,0

0

,

'

'

0,5

1

1,5

2

2,5

3

3,5

4

4,5

"

400~

cm

--"

400~

cm

-'-"

600~

cm

-- 600~

cm

5

X, r

Fig. 5 Fig. 6 shows that the diffusion out of small pores in the carriers is sufficiently fast to allow for obtaining low water vapour pressures internally at even high rates of water vapour generation. In conclusion, the above analysis shows that in order to obtain a dry calcination, even with industrially high heating rates and high loading, the crucial point is to design the calcination process so that the evolved gas will only have to diffuse short distances. 4.2.3. Forming of carrier Extrusion of a carrier from a paste is very dependent on the particle size distribution and strength of agglomerates. The small pores (below 1000 ,~) of the finished process are not influenced by the mixing and extrusion processes. The physical mixing does not alter the crystallites or primary particles (they are only attacked by chemicals), but larger pores are definitely influenced by the mixing and physical processes. Different types of extruders have a varying degree of energy deposition in the paste and the large agglomerates will be degraded to some extent. Therefore, the choice of extruder type and mixing time is

11 important for every catalyst development, since the result can vary a lot concerning pore volume and pore distribution. An example is given in Fig. 7. Pellet profiles at 6wt%ll0min Ps = 0 atm 0,0035

0,0030

0,0025

"

0,0020

"

400~

"'400~ "~"

0,0015

"--

0,0010

0,0005

" =" =='= =' "= ='== ,,, m=,= =',

"~ :,,,,~ ,,, 0,0000

. 0,01

.

. 0,02

. 0,03

0,04

r

.

0,05

0,06

.

. 0,07

. 0,08

0,09

x, cm

Fig. 6. Pore volume depending of choice of extruder and mixing time for a soft paste with same formulation/dry matter

Fig. 7

0,1

mm mm

600=K,1 mm --600~

mm

12 5. STABLE P R O D U C T I O N

5.1. Identification of critical catalyst process parameters A stable production with constant quality is important since the product must have reproducible performance and lifetime in the customer's reactor as previous deliveries. Thus, it is important to identify the critical parameters of the manufacturing process parameters that have large influence on the product quality and that should be fixed within a narrow interval. Examples could be: a. pH and mixing mass flows during precipitation for obtaining washable slurry and right density of agglomerates b. time and temperature during ageing and re-crystallisation for obtaining right phases and surface area c. temperature and pH20 during calcination of a carrier for obtaining right surface area of pore distribution. 5.1.1. Critical process parameters from successful home work A successful implementation of a manufacturing recipe in the plant is dependent on these critical parameters, and corresponding control intervals are found in the pilot production experiments. Identification of critical parameters and their allowed intervals is often time-consuming. 5.1.2. Critical process parameters from troubleshooting Even with a thorough test programme in pilot production facilities, it still happens that the recipe does not give the expected product quality in large-scale plant. Then a costly investigation must take place in the plant itself and the development team will be under heavy pressure in solving the problem. Maybe a critical parameter is overseen. Sometimes is it only a minor adjustment of a parameter or small physical changes of the equipment that make the difference.

5.2 Stable production and quality assurance Today, customers expect catalyst production to be certified by a quality system, i.e. ISO 9000, ISO 14000 or QS 9000. Thus, identification of the critical parameters is necessary for obtaining a reliable QA system. 6. C O S T - E F F E C T I V E AND W A S T E - F R E E M A N U F A C T U R I N G

6.1. Economic pressure on the process development To this point, only the aspects concerning the technical success of the scale-up process have been treated, but it is evident that the final production process will only be successful if the catalyst can be produced in a cost-effective way. Therefore, it is important to select from the first development work the manufacturing routes that can give the lowest cost and concentrate the R&D effort accordingly in the future development and test period. The raw material cost normally constitutes 60-80% of the direct cost and, therefore, the selection of raw material is an important task. The initial experiments are perhaps made on analytical

13 to select raw materials after the first enrichments of the ore. The price of raw material is tightly connected to the impurity level and a balance must be found. Some impurities such as sulphur, chlorine and alkaline metals can be harmful to the final catalyst, thus, a smart process to clean out the impurities must be found or a more expensive quality must be purchased. Furthermore, in part 2.3 we have seen that there can be several optimal technical unit operation combinations, but one will have lower direct cost than the other. A general rule is to keep the manufacturing process as simple as possible. 6.2. The waste problem A technical and cost-effective manufacturing process has now been developed and proved to give a very competitive product but the process work is not yet finished. An optimal catalyst manufacturing process must be able to handle any waste. For precipitation the cheapest and mostly used salts of the metals are nitrates, sulphates and chlorides. The cheap acids HNO3, H2SO4 and HCl are used as acid precipitation agents, and NaOH, KOH, Na2CO3 and NH3 are used as basic agents for the cheap bases. When a precipitated catalyst is made the corresponding counter-ions form salts as NaNO3, Na2SO4, NaCl, (NH4)2SO4 and NHaC1, which must end somewhere! Some of these salts have zero value or may be of a negative value, since the plant may not be allowed to discharge them to the sewage system, river or fjord. Anyway, any heavy metal particles slipping through filtration and washing must be removed before the discharge. A challenge is to develop processes where the counter-ions form a product (salt) that has a positive value and is sellable, i.e. NH4NO3. Nitrates, carbonates, hydroxides, and NH4 have the advantage that they do not leave impurities in the product after decomposition in calcinations, but they can form harmful gaseous emissions in chimneys as NH3, NO,,, NH4NO3, NH4NO2 and SO,,. Absorption and recycling of these compounds are normally necessary today. Particles from dusty processes must be collected in bag filters recycled by introducing the material in a proper place in the process. Recycle of used catalyst, material out of specifications and sieving scrap may be re-dissolved and recycled in the process. Today, much development work is concerned with closed loops in the manufacturing process layout. 6.3. Investment

A newly developed manufacturing process may be implemented in an existing production line if it is not fully booked and an investment is saved. However, if there is no free capacity or if the new product requires new unit operations and processes a new plant needs to be built. The simpler manufacturing process the cheaper investment, but the investment part of today's closed loops due to environmental concerns, often constitute 30% of the total investment. The capacity of a new plant must be decided. A larger capacity is more expensive, although it is not a linear relationship. The investment cost is seen as a capital cost in the cost calculus of a new product, and it is dependent on the capacity utilisation. Therefore, the decision of a planned capacity will be taken in close cooperation with the company's marketing people.

14 7. FINAL C O N F I R M A T I O N 7.1. Long time end user confirmation problem A typical lifetime of commercial catalysts is 1-10 years. Thus, for a new catalyst product, the final confirmation of the product performance in industry cannot take place until many years after the product has been produced even if accelerated ageing tests have been carried out. 7.2. Success rate

Development and implementation of a production method of a new product as described above is a total optimisation with many constraints, and only a few development programs come so far. Thus, the success rate from idea to product is rather low, and each success is quite an achievement for the development teams and project managers. 8. CONCLUSIONS Scale-up of catalyst manufacturing processes is a multidisciplinary teamwork where science, empirical experiments and knowledge of equipment technology are integrated. Production cost and environmental issues set tight limits on possible manufacturing solutions. A successful work is not frequent, and it can take some years before you really know you have a technological as well as an economical success. REFERENCES

1. 2. 3. 4.

Chemical week September 12 (2001): "New markets Drive Growth for Catalysts". A.B. Stiles and T.A. Koch, "Catalyst Manufacture", (1995), Dekker. J.F. Le Page et al., "Catalyse de contact" (1998), Technip. N. Pernicone and F. Traina, "Commercial Catalyst Preparation" (1984) in Applied Industrial Catalysts, Vol. 3, Academic Press. 5. C.N. Satterfield, "Heterogeneous Catalysis in Practice", p. 68 (1980) Mc Graw-Hill. 6. M.S. Spencer, in "Catalyst Handbook" 2 nd edition, p. 34, (1989), Wolfe Publishers. 7. M.V. Twigg, "The Catalyst: Preparation, Properties and Behaviour in Use" in Catalysis and Chemical Processes, Editors Pearce and Patterson, Leonard Hill. 8. G.W. Higginson, "Making Catalysts - An Overview" Chem. Eng. September 30, (1974), 98. 9. E.F. Sanders and E.J. Schlossmacher, "Catalyst Scale-up-Pitfall or Payoff?" (1983) in Applied Industrial Catalysis, Vol. 1, Academic Press. 10. W. Pletch, "Granulate Dry Particulate Solids by Compaction and Retain Key Powder Particle Properties", Chemical Engineering Progress, April (1997), 24. l l . W . Pletch, "Successfully Use Agglomeration for Size Enlargement", Chemical Engineering Progress, April (1996), 29. 12. J.W. Fulton, "Making the Catalyst", Chem. Eng., Jul 7 (1986). 13. F. Traina and N. Pernicone, "Preparation Techniques and their influence on the Properties of the Solid Catalysts", Chim. Ind., 52 (1970) 1. 14. A.H. Thomas and C.P. Brundrett, "Catalyst Development: Lab to Commercial Scale" Chem. Eng. Progr., June (1980) 41.

15 15. J. Petro, "Preparation of Catalysts" in Contact Catalysis(Szabo and Kallo Eds.), Elsevier, Vol. 2 (1976) 13. 16. I.P. Muchlenov et al., "Tehchnologie der Katalysatoren" (1976), VEB Leipzig. 17. S.P.S. Andrew, "The black art of designing and making catalysts", Chemtech March (1979). 18. G. Berrebi and P. Bernusset, "Making Industrial Catalysts", (1976) in ref. 20, vol. 1. 19. S.P.S. Andrew, "Heterogeneous Catalyst Preparation, The Fabrication of Microstructure", (1976) in below ref. 20, Vol. 1. 20. B. Delmon et al. (Eds.), "Preparation of Catalysts", Vol. 1-Vol. 7, 1976-1998 Elsevier. 21. X. Xu and J.A. Moulijn, "Catalyst Preparation and Characterisation", in Structured Catalysts and Reactors, A. Cybulsky and J.A. Moulijn (Eds.), Marcel Dekker (1998) p. 599. 22. A. Cybulsky and J.A. Moulijn, "Monoliths in Heterogeneous Catalysis", Catal. Rev.Sci. Eng. 36 (2) (1994) 179. 23. T. Beecroft and A.W. Miller, "Preparation and Characterisation of Solid Catalysts", Repts. Progr. Appl. Chem. 55 (1970) 385. 24. T.A. Nihuis et al., "Preparation of Monolithic Catalysts", Catal. Rev.-Sci. Eng., 43(4) (2001) 345. 25. J. Dohrup, PhD study: "Krystallisation af aluminiumhydroxyd til katalysatorb~erere", Copenhagen, 1997. 26. B. Donnis, Haldor Topsoe A/S: Non-published results.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

17

Quantitative structure-activity relationships in zeolite-based catalysts: influence of framework structure J. L. Casci a and M.D. Shannon b a

Synetix, RT&E Department, PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK

b ICI Measurement Science Group, PO Box 90, Wilton Centre, Middlesbrough, TS90 8JE, U.K. This paper will explore the links between framework structure and catalyst activity in toluene disproportionation (TDP) catalysis. The catalyst system that will be described is an intergrowth of framework types EUO and NES[1]: designated NU-85 [2]. The paper will describe the crystallisation of a series of intergrowth "members", their characterisation by powder x-ray diffraction and (Transmission) Electron Microscopy and their use in TDP. It will be shown that activity in TDP, as measured by the temperature required to achieve a particular conversion, is directly proportional to specific features in the x-ray powder patterns that are related to the presence of increasing levels of the intergrowth structure. 1. I N T R O D U C T I O N Quantitative structure-activity or structure-performance (property) relationships (QSAR and QSPR respectively) are of increasing interest in a wide diversity of technology areas. With some notable exceptions, most QSA(P)R studies associated with heterogeneous catalysis are restricted to zeolite-based catalysts. The reasoning is simplethe well defined 3-D structures, of molecular-sieve zeolites, are a reasonable representation of the catalyst "surface", hence bulk characterisation provides information on the catalyst. It is for a similar reason that the majority of modelling studies involves zeolites and zeotypes. In zeolite-based catalysts a number of factors can influence performance (activity, selectivity, life). These have been reviewed recently by Venuto [3]. In a previous study we have explored the impact of surface composition [4] (SiOa/AI203 ratio) on catalytic performance, in TDP, and this paper aims to extend this study by describing the effect of framework structure in the same reaction. TDP is an industrially important reaction [4,5], it provides a means of "upgrading" toluene to more valuable benzene and (ortho-, meta-, para-) xylene: key intermediates in the formation of nylon and polyethyleneterephthalate (PET) respectively. While the study is specific in that it deals with a particular reaction, and structure type, there are general lessons to be learned about the linkage between preparation,

18 characterisation and catalyst performance. In addition this paper follows a natural progression from our previous paper on TDP catalysis using EUO type zeolites [6]. 2. EXPERIMENTAL Samples of zeolite NU-85 were prepared by crystallisation from reaction mixtures of molar composition: 60 SiO2 - x A1203 - 10 Na20 - 10 Hex Br2 - 3000 H20 where Hex Br2 is hexamethonium bromide, the organic template: hexane-l,6bis(trimethylammonium) bromide. Raw materials, gel make-up and autoclave type were as described previously [2,5]. The amount of alumina, x in the above molar composition, crystallisation temperature and time will be described later. Active catalysts were prepared by calcining the as-made materials, in air, at 450~ for 24 hours followed by 550~ for 24 hours. The resulting, template-flee zeolites were ion-exchanged with 1M HC1 (single exchange for either 2 or 4 hours) using 10 cm -3 acid per g zeolite. The resulting materials were filtered, washed and dried (110~ 16 hours) then compacted into pellets that were broken down and sieved to give a 425-1000 microns fraction. l g of the sieved fraction was placed in a 4mm internal diameter stainless steel reactor and calcined at 500~ in air for 16 hours, at atmospheric pressure. The air was replaced by nitrogen and the reactor cooled to 350~ Hydrogen was then passed through the reactor and the pressure raised to 2069 kPa. The hydrogen flow rate was set at 1728 cm -3 per hour (measured at atmospheric pressure). After 1 hour, toluene was introduced into the hydrogen stream at a rate of 1.9 cm -3 of liquid per hour. The mole ratio of hydrogen to toluene was 4:1 and the weight of toluene per unit weight of solid was 1.64. The reaction was continued for 7 days during which the temperature was increased stepwise in order to maintain 47% conversion of toluene: the final temperature was taken as a measure of catalyst activity. Materials were characterised by powder xrd (Philips APD 1700; Cu K(x radiation employing an automatic theta-compensating divergence slit; peak/line intensities were based on height) and EM (lattice image and electron diffraction data were obtained using either a Philips EM400T operating at 120KeV or a Philips CM30ST operating at 300KeV; the former has a "point resolution" of 0.37nm and the latter 0.20nm; electron dose was controlled to minimise beam damage to the zeolite crystals under observation: damage is not responsible for the structures shown/described here). 3.

RESULTS AND DISCUSSION

The starting point for this work was the preparation of zeolite EU-1 [7], that is, crystallisation from a reaction mixture of (molar) composition: 60 SiO2 - x A1203 - 10 Na20 - 10 Hex Br2 - 3000 H20 This material had been shown to have good activity and selectivity in TDP and consequently a more detailed study of its preparation was initiated. Part of this, extensive, study examined materials with different SiO2/A1203 ratios (different values of x in the above composition). For preparations carded out under similar conditions

19 (temperature, reagent sources, method, agitation etc) it was observed that subtle changes occurred in the powder xrd patterns as the A1 content was increased: deviations from the standard pattern for EU-1. These changes are indicated in Fig. 1 which shows diffractograms for a typical sample of EU-1 and a diffractogram for a sample containing these "subtle changes": this material was termed NU-85.

1 4 0 0 0 -_

13000 12000 11000--

-_

-

-

_ _

10000

9000

9

--

000

ooo !

Ii

.~ 7000 6000

_

4000 3000

2000 1000 0

i

3

i

]

i

~

i

i

i

10

i

i

~ i

I

i

I

I

t

I

20

i

[

i

i

I

i

i

[

i

I

i

i

I

I

I

t

i

i

30

t

I

40

i

i

i

i

I

i

i

i

I

I

50

i

[

I

]

I

I

t

i

i

l

60

Degrees Two Theta Fig. 1 XRD patterns for EU-1 (upper) and NU-85 (lower) with affected lines indicated by arrows Highlighted, by arrows, in Figure l b are three lines that are affected by this increasing A1 content (decreasing SiO2/A1203 ratio). Overall, it can be seen that there is good correspondence between the patterns with the most apparent differences being alterations (reductions) in intensity of certain reflections - marked by arrows in Fig. 1. Of particular note are the lines at (ca.) 1.03nm and 0.372nm. More detailed examination shows that the 1.03 and 0.386nm lines are also selectively broadened and shifted to higher d-spacings (lower angles.) Table 1 shows details of a series of preparations with SiO2/A1203 ratios from 60/1 to 32.5/1. All five samples were crystallised at the same temperature (160~ using similar reagents etc, although it may be observed that there are some significant differences in crystallisation time. Also included in Table 1 are ratios of two of the xrd lines highlighted by arrows in Fig.1 above - the lines at 1.03 and 0.372nm. In each case the line intensity has been taken as a ratio to an adjacent line: such that the line at 1.03nm is taken as a ratio to the line at 1.11 and the line at 0.384nm is taken as a ratio to the line at 0.372nm. This method has been employed rather than the more

20 conventional intensity relative to the strongest line (ca. 0.434nm in the patterns) to allow for different diffractometer geometries: specifically the use of fixed slit Vs divergent slit geometries which can significantly alter the (relative) intensities of the low angle reflections. Table 1 Products obtained from reaction mixtures of composition: 60 SiO2 - x A1203 - 10 Na20 - Hex Br2 - 3000 H20 carried out at 160~ 1 or 2 litre autoclaves Sample

I III II IV V

x

SiO2/A1203 ratio

1.000 1.500 1.714 1.791 1.846

60/1 40/1 35/1 33.5/1 32.5/1

Crystallisation Ratio of xrd lines t/h 1.03/1.11 0.384/0.372 142 240 264 263 312

0.6 0.6 0.5 0.4 0.4

1.3 1.3 1.1 1.0 0.9

Impurities

None None None < 1% ANA* < 1% ANA

* ANA was identified from the xrd pattern and quantified by a calibration study in which ANA as doped, at different levels, into a pure EU-1 sample. Also included in Table 1 (final column) is an indication of product purity. The key points to note are: (a) In such a study where relative intensities are being used it is essential to ensure that these changes in relative intensity are not caused by the presence of impurities. (b) Detailed analysis of the NU-85 samples was undertaken and it was observed that the many samples co-crystallised with analcime (ANA) and a detailed calibration study was undertaken to assess the level of the impurities. (c) The presence of ANA (particularly at these low levels) has no effect on the relative intensities described above. From Table 1 it can be observed that there is a downward trend in the relative intensities with decreasing SiO2/A1203 ratios - this is highlighted in Fig. 2 which contains a plot of xrd relative intensity against SiO2/A1203 ratio. The sharp decline in relative intensity at SIO2/A1203 ratios (reaction mixture) below about 35/1 is evident. The detailed explanation of the significance of this ratio encompasses a description of the yield on SiO2 (and A1203 ) [7] and some knowledge of the maximum A1 content of the EUO unit cell [8] (based on structure and template content) - such a description is beyond the scope of this paper and will be described elsewhere. A more extensive study showed that this reduction of intensity in the EU-1 xrd pattern (or formation of NU-85) was a complex interplay between SIO2/A1203 ratio, crystallisation temperature and silica source, such that, a family of curves (xrd line intensity against SIO2/A1203 ratio) could be obtained for each temperature using

21 particular raw materials. However, each member of this series, as defined by their xrd line intensities has specific properties and it is these properties which will now be discussed. 1.4 -~

0.3~/0.372

O ~ ,...~ .,,.a

r/3 (D o...~

t

0.6-

1.03/1.11

0.2

. . . .

20

I

. . . .

I

. . . .

I

. . . .

3~iO2/A12034o0f Reaction ~r

I

70

60

Fig. 2. Plot of relative xrd line intensities as a function of SiO2/A1203 ratio. 0.384nm line relative to 0.372nm; 1.03nm line relative to 1.1 lnm Table 2 contains details of some NU-85 samples together with their activities in TDP (as measured by the temperature required for 47% conversion, after 7 days on line - see experimental section above). Table 2 Nu-85 samples: preparative details, xrd line intensities and activity in TDP catalysis SiO2/A1203" ratio

SiO2 source

60/1.714"** 60/1.714"** 60/2.18'* 60/2.18 60/1.714"**

Cabosil Cabosil Syton X30 Syton X30 Cabosil

Crystallisation Temp./~ 160 180 160 160 150

xrd line ratio 0.384/0.372nm 0.9 1.1 0.6 0.7 1.0

TDP Activity Temp/~ 382 409 359 366 390

* Reaction mixtures of composition: 60 SiO2-x A1203-10Na20-10 Hex Br2-3000 H20 ** 2 litre autoclave *** 19 litre autoclave

22 These materials, as described in Table 2, were prepared at different compositions, crystallisation temperatures and autoclave types: conditions which give rise to unique NU-85 materials in terms of the xrd line intensity 9 When the xrd line intensity (a measure of their NU-85 "character") is plotted against TDP activity - see Fig. 3 - a straight-line results with a correlation coefficient (R2) of 0.96.

1.2-

r r

1.1-

oO r

1-

9. 0.9r,r

= 0.80.70.60.5 350

i

i

i

i

i

i

360

370

380

390

400

410

TDP activity: temperature (~

420

required for 47%

Fig.3. Plot of xrd line intensity (ratio of 0.384/0.372nm) against TDP activity (temperature required for 47% conversion) To understand the underlying reason behind the "reduced" xrd line intensity in NU-85 (compared to EU-1) a detailed structural study was carried out with the most critical information being obtained from a combination of lattice imaging and electron diffraction. It was from this data that the structure of NU-85 was discerned: it was an intergrowth of EU-1 and NU-87 (structure types EUO and NES). In general, lattice images were preferred to electron diffraction patterns as a method of determining the structure of NU-85 because this gave direct visual evidence for the intergrowths since there is a 1:1 correspondence between the structure of a crystallite and the lattice image, when properly recorded. Such a lattice image is shown in Fig. 4. This figure shows a crystal that contains discreet bands of fringes with spacings of ca. 2.02 and ca.l.25nm parallel to one another. The 2.02nm bands are EU-1 and the 1.25nm bands belong to NU-87. In this crystal there are three bands of zeolite NU-87 separated by two bands of zeolite EU-1 - as marked in the Figure.

23

Fig. 4. Lattice Image of NU-85: with fringes associated with EU-1 (2.02nm) and NU-87 (1.25nm) marked. The use of this technique to estimate the proportion of NU-87 in the (different) samples of NU-85 is fraught with difficulties, in particular the need to examine a sufficient number of crystals by lattice imaging to ensure that the results obtained are representative of the whole sample. This is particularly important where a sample is believed to contain a significant amount of either (or both) of the two parent zeolites, EU-1 and NU-87, together with the intergrowth. It is also important for NU85 samples that are close to the end-member materials, such as those containing large amounts e.g. 95% by volume, of EU-1. A further complication is that as the proportion of intergrown NU-87 in NU-85 falls there will be an increasing proportion of pure EU-1 crystallites in any sample. This is for two related reasons. Firstly the crystallites are of finite size, typically 20-100 nm, which means that most crystallites will contain only a few ten's of 2.02nm layers of EU-1. This coupled with the observation that the average band width of NU-87 in intergrown crystals appears to be relatively insensitive to total NU-87 content means that a small number of crystallites will contain a significant fraction of intergrown NU-87 in EU-1. The corollary is that a sample that contains 1% by volume NU-87 as an intergrowth in EU-1 may contain only 1 in 20 crystallites in an orientation that shows any intergrowth. It can then be necessary to examine more than 100 crystallites in the appropriate orientation to characterise the sample by lattice imaging. Even within these limitations sufficient work has been carried out on selected samples to support the view that the reduction in (relative) intensity in the xrd patterns is linearly related to an increase in the NU-87 content of the intergrowth. Thus the linear increase in activity (in TDP) shown in Figure 3 is consequence of increasing amounts of NU-87 in the samples. As may be evident, the work presented here was part of a much larger study and Fig. 3 was shown to apply to over 30 samples. The

24 excellent correlation observed was used, within our laboratories, to guide further development and scale-up, such that xrd was used routinely to predict catalytic activity thereby saving considerable time and effort. To this point the temperature required for 47% conversion has been described as a measure of catalyst "activity". However, since this was taken after 7 days on-line it is evident that it also contains information on catalyst lifetime (or deactivation). NU87 is a 2-D channel system while EU-1 contains uni-dimenional channels. Hence it may be expected that there would be differences in deactivation rates. A separate, but related, topic concerns selectivity. The TDP reaction also generates light gas and C9+ aromatics ("heavy ends"). As with "activity" it may be expected that the difference in pore structure would be reflected in differences in the selectivity. Indeed there is evidence for differences in the xylenes and benzene balance in NU-85 samples (compared to EU-1). Both topics (deactivation and selectivity) are complex and beyond the scope of this contribution and will be addressed in a separate paper. 4.

CONCLUSIONS

An example of a Quantitative-Structure-Activity-Relationship has been found for a material designated NU-85. Under certain combinations of temperature and SIO2/A1203 ratio a range of materials with subtle, but significantly different, xrd patterns were obtained. These materials (designated NU-85) had different activities in TDP catalysis and it was possible to relate the xrd pattern intensities, firstly, with their activity in TDP and then, secondl~y, with the presence of increasing amounts of the NU-87 structure intergrowing with EU-1. ACKNOWLEDGEMENTS The authors gratefully acknowledge the contributions of Dr Ivan Lake (TDP catalysis) and Mr. S Huntley (zeolite synthesis/activation) and thank Synetix for permission to publish this paper. REFERENCES

1. W.M. Meier, D. H. Olson and L. B. McCusker, Atlas of Zeolite Structure Types, Fourth Revised edition, 1996. 2. J.L. Casci, I.J.S. Lake and M.D. Shannon, Eur. Pat. 462745 (1991). 3. P.B. Venuto, Stud. Surf. Sci. Catal., 105 (1997) 811. 4. J.L. Casci and A. Stewart, Proc. 12 th Int. Catalysis Congress, Stud. Surf. Sci. and Cat., 130 (2000) 851. 5. A. Azzouz, V. Hulea, B. Zaoui, M. Attou and E. Dumitriu, J. Soc. Alger. Chim., 3(1993)38. 6. J.L. Casci, B.M. Lowe and T.V. Whittam, Proc. Vlth Int. Zeolite Conf., Butterworths (1984) 894. 7. B.M. Lowe, Zeolites, 3 (1983) 123. 8. J.L. Casci, Stud. Surf. Sci. Catal., 84 (1994) 133-140.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

25

Cogelation: an effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides B. Helnnchs a' , S. Lambert a, C. Ali6, .,a J.-P. Pirard a, G. Beketov b, V. Nehasil b, and N. Kruse b aLaboratoire de G6nie Chimique, B6a, Universit6 de Li6ge, B-4000 Li6ge, Belgium bChimie Physique des Mat6riaux (Catalyse- Tribologie), Universit6 Libre de Bruxelles, C.P. 243, Campus Plaine, B-1050 Bruxelles, Belgium Pd/SiO2, Ag/SiO2, Cu/SiO2 and Pd-Ag/SiO2 catalysts have been prepared by cogelation, that is concomitant hydrolysis and condensation of a complex containing both the active metal cation and hydrolysable methoxy groups -OCH3 with Si(OC2H5)4 (TEOS). The resulting materials contain metal nanocrystallites trapped inside microporous primary silica particles, as a result of the role that the complex plays as nucleation agent of SiO2 particle formation. The primary SiO2 particles are arranged in aggregates such that a highly porous catalyst texture results. This makes the catalysts sinter-proof at high temperature and allows a very easy diffusion of reactants. High activity and selectivity have been found for various reactions. X-ray electron spectroscopy has been applied along with ionic sputtering techniques to demonstrate that metal nanocrystallites are encapsulated in the primary SiO2 particles. This is particularly true for Pd/SiO~. 1. INTRODUCTION As compared to traditional ways of solid catalyst preparation frequently involving twostep procedures [1, 2], sol-gel chemistry allows to do so in one single step. For example, rather than dispersing the active metal component on a previously synthesized support, both metal dispersion and support synthesis can be realized simultaneously. Several authors prepared metal supported catalysts by the sol-gel route and demonstrated high dispersion on gels of finely controlled texture. Most frequently, the metal of interest is introduced in the initial solution (whose main components are for example aluminum tri-sec-butoxide (ATB) or tetraethoxysilane (TEOS) and water in alcohol) in the form of a salt (e.g. H2PtC16, PdC12, Pd(CH3CO2)2, RuC13, etc.) [3-5]. During the past ten years, a particularly interesting method to homogeneously disperse nanometer-sized metal particles in a silica gel has been developed that consists in using modified alkoxides of the type (RO)3Si-X-A [6-10]. In such molecules, a functional organic group A, able to form a chelate with a cation of a metal like palladium, platinum, nickel, silver, copper, etc., is connected to the hydrolysable alkoxide E-mail: [email protected]

26 moiety (RO)3Si- via an inert and hydrolytically stable spacer X. The concomitant hydrolysis and condensation of such molecules with a network-forming reagent such as Si(OC2H5)4 (TEOS), that is their cogelation, result in materials in which the catalytic metal is anchored to the SiO2 matrix. This is illustrated in Fig. 1 where X - propyl and A = diamine. The aim of the present paper is to study the mechanisms of this cogelation and to show that it leads to catalysts with remarkable structure and properties. ! OR [ RO--S~--OR

I

OR' [ +

!

/CH2--- C~2

R'O -- Si -- CH2-- CH2-- CH2-- HN"

OR

1

OR'

"-,M~/

.NH 2

+ H20

-Ro~

- R'OH

O

I

-- o - - s i / N / ~ / N / N

I

M 'v

0

I

--Si--

Fig. 1. Anchoring of the catalytic species M to the SiO2 matrix.

I

2. E X P E R I M E N T A L 2.1. Catalyst preparation Five series of catalysts have been prepared: aerogels Pd/SiO2, xerogels Pd/SiO2, xerogels Ag/SiO2, xerogels Cu/SiO2 and xerogels Pd-Ag/SiO2 (bimetallic catalysts). For each given sample, A or X denote whether the initial wet gel was aerogel-dried under supercritical conditions or xerogel-dried under vacuum, respectively. Metal weight percentages follow the elemental symbol of the metal and are sometimes higher than those expected from synthesis operating variables because of a loss, during gel drying, of unreacted tetraethoxysilane (TEOS), which is the precursor of the silica support. The general synthesis procedure was as follows. For monometallic catalysts, metal precursor (Pd(II) acetylacetonate, Ag(I) acetate or Cu(II) acetate) and 3-(2-aminoethyl)aminopropyltrimethoxysilane, H2NCH2CH2NH(CH2)3-Si(OCH3)3 (EDAS), as ligand are mixed together in half the total volume of ethanol. The slurry is stirred at room temperature until a clear solution (the color of which is characteristic of the metal complex formed) is obtained (about half an hour). After addition of TEOS, a solution of aqueous 0.18N NH3 in the remaining half volume of ethanol is added to the mixture under vigorous stirring. The vessel is then tightly closed and heated to 70~ for 3 days (gelation and aging). For bimetallic catalysts Pd-Ag/SiO2, Pd(II) acetylacetonate and EDAS are mixed in a quart of the volume of ethanol, Ag(I) acetate and 3-(aminopropyl)triethoxysilane, HEN(CHE)aSi(OC2Hs)3(AES), are mixed in another quart of the total volume of ethanol and those two solutions are mixed together after formation of Pd and Ag complexes. Note that, for one particular Ag/SiO2 catalyst (XAgl.7), Ag(I) acetate was mixed with AES instead of EDAS. Nevertheless, in this preparation the addition of EDAS (same amount than AES) after the introduction of all reactants was still necessary to obtain gelation of the solution. After aging, a first series of Pd/SiO2 catalysts were dried under supercritical conditions at 327~ and 12 MPa to produce aerogels (series A). The other gels (Pd/SiO2, Ag/SiO2, Cu/SiO2 and Pd-Ag/SiO2) were dried under vacuum at 150~ for 72 h to produce xerogels (series X). All xerogels

27 were subsequently calcined under air at 400~ during 12 h and reduced under H2 at 350~ (400~ for Cu) during 3 h.

2.2. Catalyst characterization and catalytic experiments The size of metallic particles was examined by transmission electron microscopy (TEM). For Pd/SiO2 samples, the Pd dispersion was additionally calculated from CO chemisorption. The bulk composition of bimetallic particles in Pd-Ag/SiO2 samples was examined by X-ray diffraction (XRD). Texture was examined by N2 adsorption at 77 K, Hg porosimetry, and TEM. Elemental distributions of Ag and Pd in the silica framework were additionally studied by X-ray photoelectron spectroscopy (XPS) along with ionic sputtering. Pd/SiO2 aerogels were tested in H2 combustion, Pd/SiO2 xerogels in C2H4 hydrogenation, Ag/SiO2 and Cu/SiO~ in C6H6 combustion and Pd-Ag/SiO2 in selective hydrodechlorination of 1,2dict'doroethane into ethylene. More details about preparation, characterization and catalytic test can be found in [8, 9, 11-14]. 3. RESULTS Fig. 2 shows a structural model of the cogelled catalysts suggested on account of TEM results. Except for XAgl.7, mono- or bimetallic nanocrystallites (Pd, Ag, Cu or Pd-Ag) with diameters between 2 and 4 nm are located inside the primary silica particles. The narrow size distribution of these latter silica particles reveals mean values generally ranging between 10 and 50 nm depending on the sample. Most frequently, larger metal crystallites (10 to 30 nm) are also observed to be deposited on the outer surface of the primary SiO2 particles. The number density of such larger metallic particles is higher for Ag/SiO2 and Cu/SiO2 than for Pd/SiO2. Primary SiO2 particles are arranged in aggregates constituting Fig. 2. Structural model of cogelled the catalyst pellet. In the case of XAgl.7 prepared catalysts suggested from TEM (M using both AES and EDAS, the structure is = metal, A= alloy), different" silver crystallites exhibit a broad range of sizes (1 to 20 nm) and no individual and welldefined primary SiO2 particles are observed. The texture of the cogelled catalysts was examined by N2 adsorption and Hg porosimetry. Except for XAgl.7, all samples exhibit a similar and very broad pore size distribution. An example is given in Fig. 3 for the case of XPdI.9Ag3.7. Cumulative distributions over the complete pore size range were obtained by applying a combination of various methods within their respective validity domains and by summing up the porous volumes corresponding to these domains [9, 16].

28 In the micropore domain, the catalysts exhibit a very narrow pore size distribution centered around a mean o value of about 0.8 nm that corresponds micro E to the steep volume increase followed by a plateau. In the range of meso- and 0.| " macropores, all samples exhibit a broad distribution starting at about 2 nm and 0.01 extending up to several hundred nanometers. Catalyst XAgl.7 has a continuous pore size distribution from 0.001 the smallest micropores (0.7 nm) to the 0.1 1 10 100 1000 largest macropores (2000 nm). It is particularly interesting to note that the Pore width (nm) pore volurnes, measured by Hg Fig. 3. Pore size distribution. porosimetry, of xerogel catalysts, which are between 1 and 7 cm3/g, are almost in the same order of magnitude as the pore volumes of Pd/SiO2 aerogels (9 to 25 cm3/g) or other aerogels described in the literature [15]. For this reason, those materials are called 'low-density xerogels'. The reason for the high porosity of cogelled xerogels has been examined by Ali6 et al. [ 16]. One Pd/SiO2 sample (XPd3.3: only EDAS as ligand) and two Ag/SiO2 samples (XAgl.5: only EDAS as ligand; XAgl.7: AES + EDAS) were subjected to a combined XPS and SSIMS analysis. We limit the presentation of results largely on the XPS evidence (before and after Ar ion sputtering), which was obtained by inspecting the Pd and Ag 3d doublets along with the background on the high-energy side (low kinetic energy side). Figs. 4 and 5 demonstrate that the spectral features remain essentially unchanged when reducing the calcined catalysts. No noticeable change is seen in either line positions or background, both qualitatively and quantitatively. This may be taken as an indication that both Pd and Ag could be in the reduced (metallic) form after calcination of the catalysts. Comparison of the 3d spectra of Pd/SiO2 with those of Pd metal (foil) yet shows significant differences. Apart from a shift in the binding energies of the characteristic lines (that is, at least partially, due to electrostatic charging) the background tail structure measured for Pd/SiO2 is different from Pd foil. This difference suggests energy losses of Pd 3d electrons to take place while travelling through the SiO~ matrix. The correctness of this suggestion is demonstrated by comparing the Si 2p background features of SiO2 ('model line' in Fig. 4, appropriately scaled to the 3d spectral region) with those of Pd 3d in Pd/SiO2. In fact, it turns out that while the electron energy loss spectrum (EELS) in SiO~ is qualitatively similar to the one in Pd/SiO2, intensities are remarkably different. Note that the tails in the EELS of the SiO2 matrix are mainly associated with the occurrence of interband transitions and plasmons, the latter peaking by about 22 eV above the main lines (zl, z2). e~0

10-

m

F

29

.....

Model 3d line

Model 3d line

3400 1400 4 3200 1200

3

3000

1000

I

2

2800

2600

800

2400

1 600

~o

~

~o

Eb [eV]

;o

~o

Fig. 4. Pd 3d line of XPd3.3" (1) calcined, (2) reduced, (3) sputtered lh, (4) sputtered 2h, (5) Pd foil.

,

440

.

420

,

.

400

,

380

.

360

,

340

Eb[eV]

Fig. 5. Ag 3d line" (1) XAgl.7 calcined, (2) XAgl.5 calcined, (3) XAgl.5 reduced, (4) XAgl.5 sputtered lh.

Turning now to the Ag 3d spectral features (Fig. 5) it is evident that the background tail structure coincides fairly well with the 2p Si model line. A considerable difference in the morphological structure of Pd/SiO2 and Ag/SiO2 can be seen from the above XPS results. While the strong background tail on the high-energy side of the 3d lines pleads for a model in which Pd particles are encapsulated in the SiO2 matrix, the close similarity of the background intensities in Ag 3d and Si 2p is in line with a homogeneous in-depth distribution of Ag metal in SiO2. To provide further support for the 'encapsulation model' of Pd/SiO2 catalysts prepared via EDAS complexation, SSIMS measurements were performed. Although not shown explicitly in this paper, we mention that in the early stages of the 'static' sputtering, Pd ion species were hardly discernible. Only after prolonged sputtering did these species gain in intensity. Clearly, sputtering must have an influence on the XPS background features as well. This is demonstrated in Figs. 4 and 5 for both Pd/SiO: and Ag/SiO2 catalysts. Clearly, while prolonged sputtering causes considerable changes in the background intensity on the high-energy side of 3d Pd no such changes are seen in the respective spectral region of 3d Ag. Silica particle sizes of Pd/SiO2 catalysts can be estimated on the basis of the simple model of photoelectron attenuation. Forming the intensity ratios of the 2d Pd lines before and after sputtering allows us to estimate values of 2.5 to 3.5 nm for the SiO2 particle radii. As to the catalytic performance of the catalysts, we mention that all of them have been proved to be active for the various catalytic reactions mentioned in paragraph 2.2. In

.

30 particular, Pd-Ag/SiO2 cogelled catalysts are highly selective in C2H4 during C1CH2-CH2C1 hydrodechlorination which makes them attractive from an industrial point of view [9,12,17]. 4. DISCUSSION

In cogelled Pd/SiO2, Ag/SiO2 (except for XAgl.7), Cu/SiO2 and Pd-Ag/SiO2 catalysts, TEM micro graphs show many small metal particles to be located inside primary silica particles (see Fig. 2). It has been suggested that such encapsulation could be the result of a nucleation process initiated by the ligand, in our case EDAS (H2NCH2CHzNH(CH2)3Si(OCH3)3) [8,9], used for metal complexation in the sol-gel process. Ali6 et al. [ 16, 18, 19] have examined this hypothesis of nucleation by EDAS in xerogels without metal. Due to the hydrolysable functions of EDAS, Si-O-Si bonds can form all around this additive. Furthermore, the fact that gelation occurs in half an hour in the presence of EDAS, whereas it would take days without EDAS under the same conditions, indicates that EDAS reacts faster than TEOS. This is not surprising on account of the presence of methoxy groups in EDAS as compared to ethoxy groups in TEOS. Thus, in cogelation, silica particles are formed with a hydrolyzed EDAS core (containing the active metal) and a shell mainly made of hydrolyzed TEOS. In order to assess the validity of the nucleation hypothesis, the relation between the SiO2 particle volume and the TEOS and EDAS concentrations has been examined. Expressing that the volume occupied by the silica skeleton is proportional to the sum of TEOS and EDAS amounts which have reacted to form the gel, one obtains: Np Vp ( 1 - ) - C~ ([TEOS] + [EDAS]) (1) where Np is the number of SiO2 particles in the gel, Vp is the volume of one SiO2 particle (nm3), and is its void fraction, C~ is a constant, [TEOS] and [EDAS] are the concentrations of TEOS and EDAS (mol/1). The hypothesis of nucleation by EDAS leads to: Np = C2 [EDAS] (2) where C2 is a constant (1/C2 is proportional to the number of EDAS molecules in one nucleus). Combining (1) and (2) and expressing Vp as a function of dp (diameter of SiO2 particles, nm), one obtains: d 3 = C3 ([TEOS] + [EDAS]) / ( ( 1 - ) [EDAS]) (3) where C3 = 6C1 / ( C2). d o was measured by TEM for all samples including xerogels without metal studied in [ 16, 18, 19] and the void fraction of silica particles, , was derived from texture analysis. Results are shown in Figure 6. One observes that relation (3) represents experimental data in a satisfactory manner. This result, combined with TEM observations, reinforces the hypothesis of nucleation by EDAS and of metal crystallites to be encapsulated in the primary SiO2 particles. Both XPS and SSIMS results obtained with Pd/SiO2 (XPd3.3), are in complete agreement with this.

31 70 ~

On the other hand, different completely

|

60 t 50

9Pd/SiO2, aerogels 9 Pd/SiO2, xerogels

= 40 30 '1 "@ 20 I

9 Ag/SiO2, xerogels 9 Cu/SiO2, xerogels o SiO2 without metal, xerogels __--.

10 0 0

~ ~ ~ ,200 400 600 800 1000 ([TEOS] + [EDAS]) / ((1-) [EDAS])

Fig. 6. SiO2 particle nucleation by EDAS.

results have been obtained with sample XAgl.5 in which EDAS is used as ligand. In that sample, a homogeneous in-depth distribution of Ag metal in SiO2 is found. This could be in ~ agreement with the fact 1200 that, according to TEM analysis, more active metal is located outside silica particles in

Ag/SiO2 (and Cu/SiO2) catalysts than in Pd/SiO2 catalysts. That difference could be explained by the fact that EDAS forms more stable complexes with Pd than with Ag. Note that in xerogel Y~A.gl.7, no nucleation is indicated by TEM. In this sample, AES (containing ethoxy groups) rather than EDAS (containing methoxy groups) was used. EDAS was only added after the introduction of all other reactants so as to obtain the gelation of the solution. For this catalyst, XPS and SSIMS also detected a homogeneous in-depth distribution of Ag metal in SiO2. A very important concern about cogelled catalysts is the accessibility of the active metal which is located inside the silica particles. However, in the case of Pd/SiO2 aerogels and xerogels, the sizes of Pd crystallites obtained by TEM analysis are in agreement with those determined by CO chemisorption. This result demonstrates metal crystallites to be accessible for CO (and possibly leading to structural rearrangements of Pd particles) or other molecules of similar size. This conclusion is reinforced by the activity found for all cogelled catalysts in various catalytic reactions. The complete pore size distribution (Fig. 3) combined with TEM images (Fig. 2) suggests that the narrow micropore size distribution centered around 0.8 nm is located inside the primary SiO2 particles (10 dp 50 nm) and that the continuous meso- and macropore size distribution is caused by voids between these particles and between aggregates of these particles. It has been shown that such a structure allows an easy diffusion of reactants from the external surface of the catalyst pellet to the active metal crystallites located inside SiO2 particles without any di~sional limitations [13]. Moreover, because they are larger than the micropores of the silica particles in which they are located, the highly dispersed metal crystallites in cogelled catalysts are trapped and probably more stable in against physical and chemical processes. In fact, these crystallites are sinter-proof during high temperature activation and reaction.

32 5. CONCLUSIONS Cogelation, comprising concomitant hydrolysis and condensation of a complex containing both the active metal cation and hydrolysable methoxy groups-OCH3 with a network-forming reagent such as Si(OC2H5)4 (TEOS), allows to produce catalysts in which metal nanocrystaUites are trapped inside microporous primary SiO2 particles. The latter are arranged in larger aggregates thus constituting the macroscopic catalyst pellet. The localization of metal crystallites inside the SiO2 particles is a result of the role played by the ligand of the metal cation during nucleation and formation of silica particles. This encapsulation makes metal crystallites sinter-proof during operation at high temperatures. Moreover, even simply dried under vacuum, the resulting xerogels catalysts have a porosity comparable to the one of aerogels dried in supercritical conditions. This high porosity combined with a hierarchical structure - SiO2 particle, aggregate, macroscopic pellet allows a very easy diffusion of reactants from the external surface of the catalyst pellet to the active metal crystallites located inside SiO~ particles without any diffusional limitations. Monometallic as well as bimetallic cogelled catalysts have been shown to be active and selective in various catalytic reactions. In some cases, their performance is much higher as compared to more conventional catalysts. ACKNOWLEDGMENTS

The Rrgion Wallonne, the F.R.I.A. (S.L.), the F.N.R.S. and the Communaut6 Fran~aise de Belgique are gratefully acknowledged for financial support. REFERENCES

1. K. Foger, in 'Catalysis: Science and Technology', J.R. Anderson and M. Boudart (eds.), Vol. 6, p. 227, Springer-Verlag, Berlin, 1984. 2. M. Che, O. Clause and Ch. Marcilly, in 'Handbook of Heterogeneous Catalysis', G. Ertl, H. KnSzinger and J. Weitkamp (eds.), Vol. 1, p. 191, Wiley-VCH, Weinheim, 1997. 3. K. Balakrishnan and R.D. Gonzalez, J. Catal., 144 (1993) 395. 4. J.N. Armor, E.J. Carlson and P.M. Zambri, Appl. Catal., 19 (1985) 339. 5. T. Lopez, P. Bosch, M. Asomoza and R. Gomez, J. Catal., 133 (1992) 247. 6. B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater., 3 (1991) 559. 7. A. Kaiser, C. Gtirsm_ann and U. Schubert, J. Sol-Gel Sci. Technol., 8 (1997) 795. 8. B. Heinrichs, F. Noville and J.-P. Pirard, J. Catal., 170 (1997) 366. 9. B. Heinrichs, P. Delhez, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 172 (1997) 322. 10. A.I. Serykh, O.P. Tchachenko, V.Y. Borovkov, V.B. Kazansky, K.M. Minachev, C. Hippe, N.I. Jaeger and G. Schulz-Ekloff, Phys. Chem. Chem. Phys., 2 (2000) 2667. 11. B. Heinrichs, F. Noville, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 192 (2000) 108. 12. B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 200 (2001) 309. 13. B. Heinrichs, J.-P. Pirard and J.-P. Schoebrechts, AIChE J., 47 (2001) 1866. 14. S. Lambert, N. Tcherkassova, C. Cellier, F. Ferauche, B. Heinrichs, P. Grange and J.-P. Pirard, published in the present volume of Stud. Surf. Sci. Catal. (2002). 15. G.M. Pajonk, Appl. fatal., 72 (1991) 217.

33 16. C. Ali6, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 246 (1999) 216. 17. P. Delhez, B. Heinrichs, J.-P. Pirard and J.-P. Schoebrechts, Process for the Preparation of a Catalyst and its Use for the Conversion of Chloroalkanes into Alkenes Containing Less Chlorine, US Patent No. 6 072 096 (2000). 18. C. Ali6, F. Ferauche, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 289 (2001) 88. 19. C. Ali6, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 285 (2001) 135.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

35

Steam reforming of CH4 o v e r N i / M g - A I c a t a l y s t p r e p a r e d b y spc-method from hydrotalcite T. Shishido and K. Takehira* Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-hiroshima, Hiroshima 739-8527, Japan Ni-supported catalyst was prepared by solid phase crystallization

(spc-) method starting

from Mg-AI hydrotalcite (HT) anionic clay as the precursor and tested for the steam reforming of CH4. The HT precursor was prepared by co-precipitation method from metal nitrates, thermally decomposed and finally reduced to form Ni-supported catalyst,

i.e., spc-Ni/Mg-Al. Ni z+ can well replace the Mg 2+ sites in the HT precursor, followed by the formation of Ni-Mg-O solid solution, resulting in the formation of stable and highly dispersed Ni metal particles on the catalyst, spc-Ni/Mg-A1 catalyst showed higher activity

(imp-) method. Moreover, this catalyst showed higher activity as well as higher stability than a commercial Ni/a-Alz03 catalyst during the than those prepared by the impregnation

reaction for 600h. 1. INTRODUCTION The conversion of hydrocarbon to hydrogen will play an important role in 21st century, especially, for providing hydrogen for polymer electrolyte fuel cell (PEFC).

Currently,

steam reforming of hydrocarbons, especially of CHa (1), is the largest and generally the most economical way to make H2. Alternative industrial chemical approach includes

2CH4

CH4

+

H20

=

CO

+

H20

+

1/202

=

+

3H2 2CO

+

(1) 5H2

(2)

oxidative reforming of CH4 (2). However, the Hz production for PEFC requires enormously high efficiency taking account that the reformer should be compact in the FC system not only on-board of vehicle but also in the stationary FC system. A sufficient

36 amount of H2 should be continuously produced in a small reformer and fed to the PEFC and, therefore, the reforming catalyst must work under far higher space velocity compared to those for methanol or Fisher-Tropsch synthesis. A new concept of the catalyst preparation is required. We have proposed the highly dispersed

metal-supported

catalyst,

spc-method for starting

preparing the stable and

from the crystalline

precursor

containing active metal species in the structure [1]. In this paper, we report the preparation of

spc-Ni/Mg-Al catalyst

from Mg-A1 hydrotalcite like anionic clays as the precursor and

its successful use in the steam reforming of CH4.

2. EXPERIMENTAL 2.1. Catalysts preparation The catalyst,

spc-Ni/Mg-Al, was

clay as the precursor. replaced by Ni 2+.

prepared starting from the hydrotalcite (HT) anionic

Mg3AI-HT was used as the precursor, in which a part of Mg 2+ was

The HT precursor was prepared by co-precipitation from nitrates of Ni,

Mg and Al, thermally decomposed to mixed oxides at 1123K for 5h, and reduced to form supported Ni catalysts and

(spc-Ni/Mg-A1) at

imp-Ni/Al203 catalysts

imp-Ni/Mg-Al,imp-Ni/MgO impregnation (imp-) method, cz- and

1073K for 0.5h.

were prepared by the

7-A1203 (Kanto and JRC-ALO-4) and MgO "smoke" powder (JRC-MgO-4) were used as the support and impregnated in an aqueous solution of nickel nitrate. In the case of

imp-Ni/Mg-Al, Mg-Al

mixed oxide (MgaAl) was prepared by calcining Mg3AI-HT at

1123K for 5h and used as the support.

2.2. Steam reforming of CH4 The catalytic reactions were carried out at atmospheric pressure in a conventional flow reactor using a U-shaped quartz tube (O 10 mm) with a fixed bed catalyst. was diluted with quartz beads.

The catalyst

CH4 and H20 were mixed with N2 in the ratio of 1/2/2.

The flow rate of CH4 and N2 was controlled with a mass flow controller (STEC SEC-400 Mark3).

Distilled water was fed into the reactor with a liquid pump (Shimadzu

LC-10ADvp) through a vaporizer. The space velocity changed from 6.0 x 104 to 3.0 x 105 ml h-lg-cat -1. The products were analyzed by three on-line TCD gas chromatographs with Porapak-Q and Molecular Sieve 5A columns.

37 2.3 Characterization of the catalysts The metal compositions in the HT precursors were determined by ICP analyses

using

OPTIMA3000.

Perkin Surface

Elmer area

of

ICP the

:

3.0 kcps

catalyst was measured by the BET method using N2 at 77 K with a BEL

]

Japan BELSORP18. X-ray diffraction (XRD) was performed on an X-ray

j

~

_

tO

o

diffractometer (Rigaku RINT2550VHF) using Cu Ka radiation (L=1.5405,~). Transmission

electron

(a)

tO

microscopy o

(TEM) observation was carried out on JEOL JEM3000E The dispersion of Ni on the catalysts was measured by the H2 pulse method at room temperature. One hundred mg of catalyst was first reduced at 1073 K for 0.5h in 20 vol% H2 in N2 stream and then used for the measurement.

o

I

20

~

9

o

(c)

o

I

I

I

40

60

80

2 0 / degree Fig. 1. XRD patterns of

spc-Nio.25/Mg2.ys-Al,

(a) before calcination, (b) calcined at 1123 K and (c) reduced at 1073 K; (O) MgO, NiO,

3. RESULTS AND DISCUSSION

(O) Ni, ([]) hydrotalcite

3.1. Structure of the Ni-supported catalysts The atomic ratio of Ni/Mg/AI in HT precursors determined by ICP analysis were very close to that calculated from the amount of reagents, regardless of the content of Ni. sequential result of XRD analyses of

A

spc-Ni/Mg-Al during the preparation is shown in

Fig. 1. The Ni-containing precursor after drying at 393 K (Fig. la) showed the diffraction lines of well-crystallized HT phase (JCPDS 22-700) alone, suggesting that Ni2§ is incorporated in the Mg 2+ site and dispersed uniformly in brucite- (magnesium hydroxide-) like layer of the HT structure. After calcination at 1123K, HT like layered structure collapsed to MgO (JCPDS 45-948) and NiO (JCPDS 47-1049) as seen in the strong lines overlapping each other (Fig. lb), suggesting a formation of solid solution between NiO and MgO. Very weak lines due to spinel phase (MgAl204: JCPDS 33-853) were also observed, indicating that the spinel phase was formed during the calcination. After the

38 reduction at 1073K, weak lines of Ni metal appeared, indicating that a part of NiO was reduced to Ni metal and segregated from Ni-Mg-O cubic lattice during the reduction. Fig. 2a shows the XRD pattern of Mg-AI mixed oxide obtained by calcining Mg-AI HT at 1123K.

~ 3.0kcps

Strong lines of MgO

i

o

were observed in the Mg-AI mixed oxide, but the value of the lattice parameter a (0.420 nm) was smaller than that of pure

o

9

MgO (0.421 nm), suggesting the formation

0

1

0

of a Ni-Mg-O solid solution as well as the presence of AI3+ ion in the Ni-Mg-O cubic lattice as reported by several researchers [2].

o

.'1=I (D

o

Sato et al. [3] also reported that the oxide residue obtained on thermal decomposition

9

of the Mg-A1 HT below 1373K comprises a defect rock salt phase,

-

Mgl-xAlzx/3~bx/30.

Interestingly, upon impregnating the Mg-A1 mixed oxide in an aqueous solution of nickel nitrate, the lines of the HT layered structure appeared instead of decreasing lines of MgO, indicating that a significant reconstruction of the HT structure took place in the sample during the impregnation (Fig. 2b). During the impregnation procedure, the HT like

o o

I

20

I

I

40 60 2 0 /degree

i

80

Fig.2 XRD patterns of imp-6.3wt% Ni/Mg-AI. (a) before calcination, (b) calcined at 1123K and (c) reduced at 1073K; (O) MgO, NiO, (O) MgAlzO4 (0) Ni, (m) hydrotalcite

layered structure might be regenerated by the rehydration of MgO to brucite layer as frequently observed [3-8]. Rajamathi et al. [4] proposed two possible mechanisms for the regeneration of the HT like structure: (i) the insertion of protons/hydroxyl ions at the defect sites of the oxide or (ii) a dissolution-reprecipitation mechanism. They concluded that the reconstruction of HT phase occurs via a dissolution-reprecipitation mechanism based on the following facts: (i) the magnesium hydroxide has relatively high solubility in aqueous solutions, (ii) the defect rock salt phase is not essential to carry out the reconstruction and (iii) the defect rock salt phase causes a rapid reconstruction of the parent hydroxide, followed by the formation of layered HT like structure. This reconstruction via a dissolution-reprecipitation mechanism may induce a replacement of a substantial part of Mg z+ sites with Ni z+, and Ni ions may be dispersed in octahedral sites in

39 brucite layers of the hydrotalcite phase. It is likely that, during the preparation, the

imp-Ni/Mg-Al are similar to that of spc-Ni/Mg-A1. Actually, after the calcination at 1073 K, the XRD pattern of imp-Ni/Mg-Al (Fig. 2c)

circumstances around Ni species in

showed the presence of Ni-Mg-O solid solution and MgAI204. Even after the calcination at 1073K, the BET surface areas of

spc-Ni/Mg-Al catalysts were >100 m 2 g-l, regardless the

Ni content. After the reduction at 1073 K, the lines due to Ni metal appeared in addition to those of MgO phase (Fig. 2d), indicating that a part of NiO was reduced and segregated from the MgO cubic lattice during the reduction.

Nonetheless such circumstances, the

imp-Ni/Mg-Al was still stronger than that in spc-Ni/Mg-Al, indicating that the particle size of Ni metal on imp-Ni/Mg-A1 is larger than that on spc-Ni/Mg-A1. intensity of the lines of Ni metal in

3.2. Catalytic activity of the Ni-supported catalysts We

have

reported

spc-Ni/Mg-Al showed

that

the

100

higher

90

activity for both partial oxidation [9] and

COz

reforming

[10]

of

CH4

80 o

70

compared to the Ni catalysts prepared

imp-method, such as imp-Ni/Alz03, imp-Ni/MgO and imp-Ni/Mg-A1. Even at high space velocity, spc-Ni/Mg-A1 by

showed

no

decline

in

the

CH4

conversion. This may be due to the formation of highly dispersed stable Ni metals on

and

spc-Ni/Mg-Al.

o

60504030 0.0

I

I

1.0

2.0

3.0

GHSV / ml h -1 g-cat -1

Fig. 3 shows the CH4 conversion

Fig. 3. Effect of space velocity on steam reforming over both spc- and of CH4 imp-catalysts in the steam reforming of Reaction temp.: 1073K, CH4/HzO/Nz=I/2/2, Ni: CH4 at the ratio of steam to carbon of 6.3 wt%; (0) spc-Nio.z5/Mgz.75-A1, (A) obtained

2.0 at 1073K when the space velocity

imp-Ni//T-Al203 (e) imp-Ni/Mg-Al imp-Ni/MgO, (.) imp-Ni/a-Al203

(11)

was increased. The CH4 conversion calculated from thermodynamic equilibrium was above 99%. The Ni content in

spc-Nio.zs/Mgz.75-Al was equal to that in imp-6.3wt% Ni-supported catalysts. Evidently, spc-Ni/Mg-Al showed the highest activity among the catalyst tested including

40

imp-Ni-supported catalysts

loaded on various supports at the space velocity of 6.0 x 10 4 ml

h -1 g-cat -l. As the space velocity increased, CH4 conversion significantly decreased on the

imp-Ni/MgO and imp-Ni/c~-Al203. In the

imp-Ni/Mg-Al and imp-Ni/7-Al203, CH4 the space velocity, spc-Ni/Mg-Al showed

cases of

conversion gradually decreased with increasing

the highest activity even at high space velocity of 3.0 x 105 ml h -1 g-cat -a. It is clearly demonstrated that the activity of spc-Ni/Mg-Al was higher than

imp-Ni/Mg-Al at the

same

Ni loading. This strongly suggests that the dispersion of Ni metal is important for the catalytic activity. When ~/-A1203 (JRC-ALO-4) was used as the support, spinel structure NiAlzO4 formed on the catalyst surface, probably resulting in the high dispersion of Ni. Nonetheless such preferable effect expected on

imp-6.3wt%Nihl-Alz03, its

activity was

still lower than spc-Nio.zs/Mgz.75-Al. When

spc-Nio.s/Mga.5-A1was

tested for the steam reforming of CH4, a high and stable

activity was observed for 600 h of the reaction even at the steam to carbon ratio of 1.5. Life test of the catalysts was carried out at 1013K and under SV=2,500h -1. During the life test, the activity was checked under the more severe conditions,

i.e.,

at the lower

temperature of 933K at the higher SV of 10,000 h -1. A commercial Ni/a-AlzO3 catalyst showed a gradual decrease in the activity during the

100

reaction for 600 h. Thus, it

80

was clearly demonstrated that

the

activity

spc-Ni/Mg-A1 was stable

and

of

showed

4

20

for 600h. Stability

supported

40

no

decline during the reaction

3.3

60

high and

of

the

0

I 10

Ni catalysts

spc-Nio.s/Mga.s-Al

was

tested for the steam reforming of CH4 at the steam to carbon ratio of 1.0 and the space velocity of 2.4x105 ml h -1 g-cat -1. The low

I I 20 30 Time on stream / h

I 40

50

Fig. 4. Stability of Ni-supported catalysts at 1073K 5 1 1 CH4/HzO/Nz=I/1/2, GHSV=2.4xl0 ml h- g-catNi: 12.6 wt%; (0) spc-Nio.5/Mg2.s-Al, (A) imp -Ni//~,-Al203 (o) imp-Ni/Mg-Al (~) imp-Ni/MgO, (.)

imp-Ni/oc-Al203

ratio of steam to carbon caused a lowering of equilibrium CH4 conversion to around 92% at 1073K. The other catalysts prepared by

imp-method were

also tested and the results are

41 shown in Fig. 4. imp-12.6wt%Ni/a-A1203 showed a low activity at the beginning of the reaction and moreover a drastic decline in the activity during 50 h of the reaction. The activities of both imp-12.6wt%Ni/MgO and imp-12.6wt%Ni/Mg3-A1 were also lower than that of spc-Nio.5/Mgz.5-Al and decreased gradually during the time course. The best one among the imp-catalysts was imp-12.6wt%Ni/~,-AIzO3, but its activity still decreased along the time for 50 h even though the activity was enough high at the beginning of the reaction. This may be partly due to the severe conditions of the low steam carbon ratio of 1.0.

It is

thus demonstrated that spc-Ni/Mg-A1 was enough sustainable in the steam reforming of CH4 compared to the imp-catalysts.

3.4. Dispersion of Ni metal on the catalysts The adsorption of Hz on the supported Ni catalyst was measured by the H2 pulse method and the dispersion of Ni metal on the catalysts was calculated (Table 1). The amount of H2 adsorbed increased in the order of imp-Ni/a-Al203 < imp-Ni/MgO < imp-Ni/Mg-Al < imp-Ni/'y-Al203 < spc-Ni/Mg-Al, and inevitably the dispersion of Ni metal on the catalyst was calculated in the same order. Interestingly, the dispersion of Ni metal obtained from H2 adsorption was higher on spc-Ni/Mg-Al than on the corresponding

imp-Ni/Mg-Al. TEM observation also showed that spc-Ni/Mg-Al afforded rather sharp distribution of small size of Ni metal, while imp-Ni/Mg-A1 showed a broad distribution of relatively large size of Ni metal. In the spc-preparation starting from HT as the precursors, Ni species might be distributed homogeneously, i.e., not only on the surface but also in the bulk of the catalyst. Such a distribution may cause a decreasing value in the calculation of Ni dispersion. It is noteworthy that spc-Ni/Mg-Al showed higher dispersion than

imp-Ni/Mg-Al nonetheless such an unfavorable circumstance. During the reduction of Ni species in the catalysts, a substantial part of Ni may migrate from the oxide structure to the surface and may form the ultra-highly dispersed and stable Ni metal particles. The high activity observed on imp-Ni/Mg-Al may be due to the fact that a part of Ni ions was incorporated in the HT phase during the preparation, resulting in the formation of stable and highly dispersed Ni metals. This phenomenon is exclusively due to the reconstruction of the Mg-Al HT structure and seems to be important in the catalyst preparation.

42 Table 1 Dispersion of Ni metal on the catalyst a) Catalyst Hz uptake

spc-Nio.5/Mgz.5-A1 spc-Nio.zs/Mgz.75-A1 imp- 12.6wt%Ni/Mg3.0-Al imp-6.3wt%Ni/Mg3.o-A1 imp-6.3wt%Ni/MgO imp-6.3wt%Ni/a-Alz03 imp-6.3wt%Ni/~l-AlzO3

Ni dispersion

/ pmol g-cat -1

/%

76.4

7.1

38.4

7.2

72.5

6.8

17.5

3.3

7.1

1.3

15.7

2.9

36.2

6.8

a) reduced at 1073K for 0.Sh

4. CONCLUSION

spc-Ni/Mg-A1 showed higher activity than imp-catalysts and, moreover, higher stability than the commercial Ni/a-Alz03 catalyst during the reaction for 600h. During the preparation of spc-Ni/Mg-Al, Ni 2+ replaces well the Mg 2+ sites in the HT precursor, followed by the formation of Ni-Mg-O solid solution after the calcination, resulting in the formation of stable and highly dispersed Ni metal particles on the catalyst after the reduction. The high and stable activity observed on spc-Ni/Mg-A1 is due to the stable and highly dispersed Ni metal particles. ACKNOWLEDGEMENT A part of this work was supported by the Hiroshima Prefectural Industrial Technology Promotion Organization. We thank Dr. P. Wang in Japan Science and Technology Corporation and Dr. D. Shouro in Hiroshima Prefectural Institute of Industrial Science and Technology for TEM observation and their helpful discussions. REFERENCES

1. a) T. Hayakawa, H. Harihara, A. G. Andersen, A. E E. York, K. Suzuki, H. Yasuda and K.Takehira, Angew. Chem., Int. Ed. Engl., 35 (1996) 192.

b) K. Takehira, T. Shishido, M.

Kondo, R. Furukawa, E. Tanabe, K. Ito, S. Hamakawa and T. Hayakawa, Stud. Surf. Sci. Catal., 130 (2000) 3525.

43 2. a) D. C. Puxley, I. J. Kitchener, C. Komodroms and N. D. Parkins, Stud. Surf. Sci. Catal., 16 (1983) 237. b) J. H. Ross, In: Special. Periodical Reports, Vol 7, eds. G. C. Bond and G. Webb (Royal Society of Chemistry, London,1985) pp.1-45 and references therein. 3. T. Sato, K. Kato, T. Endo and M. Shimada, React. Solids, 2 (1986) 253. 4. M. Rajamathi, G. D. Nataraja, S. Ananthamurthy and E V. Kamath, J. Mater. Chem., 10 (2000) 2754. 5. s. Miyata, Clays Clay Miner., 28 (1980) 50. 6. N. S. Puttaswamy and P. V. Kamath, J. Mater. Chem., 7 (1997) 1941. 7. W. T. Richle, Chemtech, January (1986) 58. 8. F. Cavani, F. Torifiro and A. Vaccari, Catal. Today, 11 (1991) 173. 9. T. Shishido, M. Sukenobu, H. Morioka, M. Kondo, Y. Wang, K. Takaki and K. Takehira, Appl. Catal. A: General, 223 (2002) 35. 10. T. Shishido, M. Sukenobu, H. Morioka, R. Furukawa, H. Shirahase and K. Takehira, Catal. Lett., 73(2001) 21.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

45

Toward a molecular understanding of noble m e t a l c a t a l y s t impregnation J.R. Regalbuto 1, M. Schrier l, X. Hao 1, W.A. Spieker ~, J.G. Kim 2, J.T. Miller 3, and A.J. Kropf 4 1Dept. of Chem. Eng., U. of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607 2Soonchunhyang University, Chunan, Korea 3Bp Research Center,E-IF, 1510 W. Warrenville Rd., Naperville, IL 60563 4Argonne National Laboratory, CMT, 9700 Cass Ave., Argonne, IL 60439 1. INTRODUCTION There have been many recent advances in the understanding of noble metal catalyst impregnation, dating from the landmark postulation of Brunelle that the adsorption of noble metal complexes onto common oxides supports was essentially coulombic in nature [1]. The hydroxyl groups which populate oxide surfaces become protonated and so positively charged or deprotonated and negatively charged below a characteristic pH value. This pH, at which the surface is neutral, is termed the point of zero charge (PZC). This surface chemistry is depicted in the left side of Fig. 1. Brunelle cited many instances in which oxides placed in solutions at pHs below their PZC would adsorb anions such as hexachloroplatinate [PtCI6] -2, while at[ pH values above their PZC would adsorb cations such as tetraamonium platinate (TAP), (NH3)4Pt] 2+ [1]. This electrostatic mechanism was semi-quantitatively developed by another landmark work, that of Contescu and Vass [2], who studied the adsorption of [PdC14]-2 and [(NH3)4Pd] +2 over alumina at low and high pH, respectively. Much more work on the experimental verification and quantification of this "physical" adsorption process has been conducted in our own laboratory for the chloroplatinic acid (HzPtCI6, or CPA)/alumina system [3,4]. The Revised Physical Adsorption (RPA) model [4], with which all known sets of Pt/alumina adsorption data can be satisfactorily simulated with no adjustable parameters, is a result of these efforts. The basis of the RPA model is the a priori calculation of adsorption equilibrium constants, seen as the center regime of Fig. 1. Most recently, the mechanism of adsorption has been probed at the molecular scale using techniques such as 195Pt NMR and EXAFS to study the coordination chemistry of dissolved and adsorbed complexes arising from CPA [5,6, and references within]. This is the chemistry depicted in the right-hand portion of Fig. 1.

46 This paper will present some of our recent results in the latter two areas, that is, the application of the RPA model to several systems beyond CPA/alumina, and EXAFS characterization of dissolved and adsorbed Pt and Au complexes. 2. EXTENSION OF THE RPA M O D E L TO TAP/SIO2 The RPA model can predict, to a reasonable degree, the adsorption of the aqueous phase metal complexes arising from chloroplatinic acid (CPA) onto alumina as occurs during Oxide surface

!

Oxide-water interface

I

I

K~I

OH2 +

PZC

A

I

~

PtC15(H20)- + C1-

PtC15(H20)- + H20

~ K~5 I

I

PtC16-2 + H20

Kay3

O"

surface charging and pH shift

Bulk liquid solution I

K.,~2

OH

I

I

PtC15(H20)-

PtC15(H20)-2 + H+

PtCI4(HzO)2

PtC14(OH)(H20) + H+

PtCI4(OHXH20)I

adsorption equilibrium constants

PtC14(H20)2+ C1-

PtC14(OH)2-2 + H+

I

i

liquid phase speciation

Fig. 1. Three regimes of fundamental studies of catalyst impregnation. catalyst impregnation [3,4]. The model consists of a non-Nernstian description of the charging of surface hydroxyl groups, a proton balance to account for the shifts in pH caused by the oxide buffeting effect [7], and an adsorption equilibrium constant based solely on the coulombic free energy of adsorption [4]. The model has been mainly applied to CPA adsorption over alumina at low pH, in which a negatively charged Pt complex adsorbs over a positively charged oxide surface. In this section we present the results for the complementary case; cationic Pt tetraamine complexes adsorbing at high pH over a negatively charged silica surface. 2.1 TAP/SiO2 Experimental Adsorption surveys were conducted in which the uptake of TAP was recorded as a function ofpH at constant initial metal concentration. Metal concentrations were measured by ICP in solutions before and after contact with silica. One hour contact time was found to be

47 sufficient for attainment of adsorption equilibrium. Three fumed and two precipitated silicas were employed; their properties are given in Table 1. The precipitated silicas contained substantial amounts of Na impurities, which typically rendered their PZCs above 6.0. After copious washing with deionized water, the PZCs came down to within the range exhibited by the essentially pure fumed silicas. Table 1. Properties of silica samples supplier name Cabosil L90 Cabosil M-7d Cabosil EH-5 Degussa Vn-3s Degussa Fk-300

type fumed fumed fumed precipitated precipitated

surface area 90 200 380 175 300

PZC 4.0 3.7 3.6 3.9 3.6

2.2 TAP/SiO2 results and discussion The adsorption results for tetraamonium platinate are shown in Fig. 2. The PZC of all pure samples was 3.8 _ 0.2, and so one model curve can represent all samples. Specific surface areas varied between 90 and 380 m2/gm; to account for this, different masses of silicas were added to the solutions such that the same surface loading, or (m 2 oxide)/(liter of solution), was attained. The value used, 30,000 mZ/liter, is close to incipient wetness (pore filling) in some of the samples. Two concentrations were employed; the higher, 2400 ppm TAP, corresponds to about one monolayer of TAP assuming a maximum adsorption density of 0.8 ~mol/m 2 [8]. The lower concentration, 600 ppm, corresponds to about 1A of a monolayer of TAP. The model simulates the data to a reasonable degree, without any adjustment of parameters. It would then appear that TAP adsorption over many types of silica is very similar and can be adequately described by the RPA model. The practicality of these "wet impregnation" (WI) catalysts was demonstrated by another set of experiments in which the final Pt dispersion of catalysts prepared by this method were compared to catalysts prepared by incipient wetness or "dry impregnation" (DI). The results are shown in Table 2. Utilizing wet impregnation with a pH only slightly above the PZC of silica, only a small amount of Pt is adsorbed, but is well dispersed after calcination (Table 2, row 1). Higher metal loadings are obtained by adsorbing at higher pH, and the dispersion is excellent after calcination. The poor dispersion of the DI preparations can be explained by the oxide buffering effect [7]. Even when starting with a moderately basic solution in dry impregnation, the silica surface is insignificantly charged and the solution pH will approach the PZC of silica. Platinum complexes are not strongly adsorbed and sinter during calcination.

48 3000

~

2500

o

2000

9 Eh-5

2400 ppm

9 Vn3s 9

0

0

0

L90 9 M7d

1500

x

/ x

1000

~

500

4

6

x

.

=

1

600 p p m

8

Fk30t

"---'--Mode

10

12

14

&

Eh-5

o

Vn-3,,

o

L90

u

M7d

+

Fk30

--" "-Mode

pH final

Fig. 2. Uptake - pH curves for adsorption of TAP over various types of silicas. The amount of silica was adjusted to 30,000 m2/liter by adding appropriate masses of silicas with different specific surfaces areas. Table 2. Effect of method of preparation on Pt dispersion on silica* %Pt Method pH base 0.25 WI 5.0 none 1.06 WI 9.0 NHnOH 1.63 WI 9.5 KOH 1.50 DI 5.0 (initial) none 2.0 DI 9.0 (initial) NI-hOH * Dispersion determinedby 1-12chemisorption. Sampleswere calcinedat 250~ 3. " I M P R E G N A T I O N CARBON

ENGINEERING"

H/Pt 0.87 1.28 1.26 0.29 0.33

OF CPA AND TAP ON ACTIVATED

An exciting application suggested by our accrued understanding of impregnation, at least for adsorption systems that follow an electrostatic mechanism, is the potential to alter the adsorptive properties of the support by changing the support PZC. We attempted this type of control with ion-doped silica and alumina and failed [9]. The RPA model predicts that a K § or Na + silica with a PZC of 8 should behave like alumina in the low pH range and adsorb high amounts of CPA, and conversely CI doped alumina with a PZC 4 should behave like silica, and adsorb high amounts of TAP in the basic regime. In actual practice the doped oxides behaved no differently than the pure oxides. We have most recently found that the dopant redissolves in the pH range of interest [10]; that is, it does not appear that the PZCs of silica and alumina can be irreversibly changed with ion doping.

49 On the other hand, we have achieved success with activated carbon. When treated with successively more rigorous oxidative pretreatments, the PZC of carbon can drop from about 9 to below 3. In this section we will show that a carbon with a PZC of 9 behaves like alumina and that with a PZC of 3 behaves like silica for the uptake of CPA and TAP as a function ofpH. 3.1 "Impregnation Engineering" of CPA and TAP on activated carbon Three types of activated carbon with different oxidative pretreatments and different PZCs are shown in Table 3. Adsorption of cationic and anionic Pt is measured as a function of pH, using constant metal concentration. As before, the extent of uptake is determined by ICP analysis of the liquid phase before and after contact with the support.

Table 3. Activated Carbon Samples Activated Carbon

SurfaceArea (mZ/g)

Pretreatment

PZC

Norit CA1

1400

3.0

Darco KB-B

1500

Norit SX ULTRA

1200

Chemically activated by the phosphoric acid process By chemical activation of hardwood Acid washed steam activated carbon

5.6 9.0

3.2 CPA and TAP/carbon results and discussion 200 180 .. 160

a~

b~

I

"

...'.. § ~

~=140

< 120 o-o 100

9SX,PZC=9.4,1Hr

I , O,,Hr I--,-KB B.PZC=S.8.1H

; ./&~

~ 8o

~9

0

1

2

3

4

5

"'i

6 pH

7

8

.....

9

i. 4

10 11 12

9~,pzc3.o

vl=" 120

I /

A SX,PZC9.4

= 100

w

~ 60 40 20 0

9KB-B,PZC 5.6

14o

\

I /

,.

model for CAl, faetot0.51 t /

80 60

\

,~',$

9

mode,.rKB-B.fa,or0.55/ ,//~..:i~ - - modelforSX.factor0.94

~, 40 20

1 0

2

4

I

6 pH 8

I~

/ i 10

;'. ~, "~."

12

14

Fig. 3. Uptake - pH curves for adsorption of a) CPA and b) TAP over carbons of various PZCs. The amount of carbon was adjusted in all cases to 2000 m2/liter. The uptake of CPA and TAP over these three materials is shown in Fig. 3; the carbon with the PZC of 3 behaves similarly to silica, in that it takes up high amounts of TAP (Fig. 3b)

50 as does silica (Fig. 2), but little CPA (Fig. 3a). The carbon with the PZC of 9 behaves similarly to alumina, adsorbing little TAP at high pH (Fig. 3b) but much CPA at low pH (Fig. 3a). The PZC 5.6 carbon exhibits adsorption behavior in between the other two materials. The uptake over silica can be modeled with the RPA model with an additional constraint that only a certain fraction of the highly porous (and very narrow pore size) material is accessible to the Pt complex. The uptake of CPA onto the PZC 3.0 carbon, while low, is not negligible and is higher than predicted by the RPA model. We are in the process of a minor revision of the model to reflect the more complex hydroxyl group chemistry on the carbon surface. 4. EXAFS C H A R A C T E R I Z A T I O N OF D I S S O L V E D AND A D S O R B E D Pt AND Au COMPLEXES The high sensitivity of the Advanced Photon Source (APS) at Argonne National Laboratory has permitted the study of dilute solutions (200 ppm and above) of the noble metal complexes HzPtCI6 and NaAuCI4. The changes in coordination chemistry of these materials upon adsorption have been investigated. This molecular characterization permits a more detailed study of the adsorption mechanism 4.1 EXAFS experimental EXAFS measurements were performed at the MRCAT undulator beam-line equipped with a double-crystal Si (111) monochromator with resolution of better than 4 eV at 11.5 keV (Pt L3 edge). Spectra of the metal solutions contained in plastic cuvettes were taken in fluorescence mode and those of solids as pressed powders in transmission mode. Phase-shift and backscattering amplitudes were obtained from various solid reference compounds. Details of the experimental and fitting procedures can be found in [6]. 4.2 EXAFS results and discussion 9

I

""'

I

"

I

"

0.03

I 0.02

0

1

2

3

4

radial coordinate R [A]

Fig. 4. Magnitude of the Fourier Transform for Pt coordination sphere: solid-CPA at pH= 1.5 HCI (kZ: Ak = 3.0- 10.7, Ar = 1.0-2.7; fit: 6.0 Pt-CI); dashed-CPA at pH=2.7 (kZ: Ak = 3.0-10.0, Ar = 1.0-2.4; fit 2.8 Pt-CI and 3.2 Pt-O); dotted-CPA at pH=12.5 (kZ: Ak = 3.0-8.6, Ar = 1.02.4; 1.8 Pt-C1 and 4.2 Pt-O)

51 In dilute CPA solutions the degree of hydrolysis is much greater than has been reported in the literature for higher concentrations. For instance, the species immediately formed by dissolving 200 ppm CPA in water, which results in a pH of 2.70, is [PtCI3(H20)3] +1, and upon 24 hrs aging becomes [PtCI2(OH)2(H20)z] ~ at pH 2.40 [6]. Fourier transforms of EXAFS spectra of 200 ppm Pt solutions at various pH values are shown in figure 4. The spectrum of CPA in HCI at a pH of 1.5 is characteristic of [PtCI6]-2 while at a pH of 12 in NaOH, the average Pt coordination is 1.8 Pt-CI and 4.2 Pt-O bonds. The spectrum for fresh CPA at its natural pH (about 2.7) indicates the chlorine coordination between the two. A surprising result was obtained when the adsorption of the chloroaquohydroxo complexes was compared to hexachloroplatinate [PtCl6] -z, which at the same dilution can be obtained with excess chloride [6]. These samples are shown in Table 4. Surface loading (amount of oxide) was varied for three target Pt loadings of 1.0, 4.8 and 7.2 wt %. One sample of each pair was prepared without NaCI, the other with. Table 4 summarizes the important experimental parameters of these samples and the measured chlorine coordination numbers (the balance of the six-fold coordination is oxygen and is not listed in the Table). Upon adsorption, both types of complexes lose cfiloride (much more for the hexachloroplatinate complexes) and in fact converge to an identical, low chlorine content of about 2.0 ligands. The loss of CI and gain of O during adsorption has been taken as an indication of adsorption via surface ligand exchange [5]. An alternate explanation, and one that is consistent with a completely physical mechanism of adsorption, is that the adsorbing complexes respond to the local environment of the surface, where the CI- concentration would be lower and the pH higher than the bulk. Low chloride is consistent with our prior measurements, which indicated CI- is not adsorbed over alumina in this pH range [3]. Secondly, the rise in pH is consistent with electric double layer theory and is due to the equality of electrochemical potentials between bulk and adsorbed protons. Table 4. Chlorine coordination number as a function of added NaCI and surface loading. Sample

Target wt% Pt

A 1 A2 A3 A4 A5 A6

1 1 4.8 4.8 7.2 7.2

Surface Loading NaCI [m2/i] [mol/I] 5000 5000 1000 1000 650 650

--0.1 --0.1 --0.01

Initial pH

CN CI liquid

Final pH

CN CI solid

Actual wt% Pt

2.63 2.60 2.59 2.54 2.50 2.55

2.7 6 2.7 6 2.7 (6)*

4.34 5.59 2.84 3.43 2.81 2.87

1.6 1.5 2.1 1.9 2.1 2.2

1.0 0.7 3.7 1.9 4.1 3.2

*estimated from [6], not measured The effect of increased surface pH is seen most clearly in the EXAFS analysis of dissolved and adsorbed complexes arising from tetrachloroaurate, [AuCI4]-. In Fig. 5, the AuCl coordination number is plotted versus pH (the coordination is always 4 and the balance of ligands are O-containing and are not shown). Unlike Pt(1V) complexes [6], the Au(II)

52 solutions show little dependence on Au or excess C1-concentration. The Au-CI coordination numbers for the adsorbed samples, if plotted versus the equilibrium bulk pH at which the sample was prepared, fall below the species in solution. If they are plotted versus the pH at the adsorbed layer [11], however, the chloride coordination numbers of the adsorbed complexes overlap with the coordination of the liquid phase species. Once again, this is to say that the adsorbing complexes speciate as if in the liquid solution, but at the local conditions of the adsorption plane. These results provide a molecular-scale refinement of RPA model; ligand exchange with the oxide surface, a feature of chemical adsorption mechanisms [5], needs not be invoked.

A

3.5

l=

A

A,

," 2.5 .9 Itl

._

@500 ppm AuCI4-

3

Z

"O

[] 200 ppm AuCI4-

i~ I Am

A

2

[] 200 ppm AuCI4-, 50x CIAadsorbed Au at bulk pH

A

A adsorbed Au at surface pH

O

o 1.5 o m o

:~

1

AA

0.5 0

,

0

2

9

4

,

6

JhA

A

~

8

10

12

14

pH (bulk or surface)

Fig. 5. Au-CI coordination numbers of dissolved and adsorbed complexes stemming from [AuCI4]-

REFERENCES 1. J.P. Brunell, Pure and Appl. Chem., 50 (1978) 1211. 2. C. Contescu and M.I. Vass, Appl. Catal. 33 (1987) 259. 3. J.R. Regalbuto, A. Navada, S. Shadid, M.L. Bricker and Q. Chen, J. Catal., 184 (1999) 335. 4. W.A. Spieker and J.R. Regalbuto, Chem. Engg Sci. 56 (2001) 1. 5. B. Shelimov, J.-F. Lambert, M. Che and B. Didillon, J. Mol. Catal. A: Chem., 158 (2000) 91. 6. W.A. Spieker, J. Liu, J.T. Miller, A.J. Kropf and J.R. Regalbuto, App. Catal. A: General, in press. 7. J. Park and J.R. Regalbuto, J. Colloid Interf. Sc., 175 (1995) 239. 8. N. Santhanam, TA.. Conforti, W.A. Spieker and J.R. Regalbuto, Catal. Today 21 (1994) 141.

53 9. W.A. Spieker and J.R. Regalbuto, Stud. Surf. Sci. Catal. 130 (2000) 203. 10. J. Komh, W.S. Spieker and J.R. Regalbuto, manuscript submitted 11. W.A. Spieker, X. Hao, J. Liu, J.T. Miller, A.J. Kropf and J.R. Regalbuto, manuscript submitted.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

55

Support modification of cobalt based slurry phase Fischer-Tropsch catalysts S. Barradas, E.A. Caricato, P.J. van Berge, J. van de Loosdrecht Heterogeneous Catalysis Research, Sasol Technology (Pty) Ltd, PO Box 1, Sasolburg 9570, South Africa

1. I N T R O D U C T I O N In the Fischer-Tropsch slurry phase conversion of synthesis gas into distillates, the catalytic metal of choice is cobalt [1,2]. As cobalt is relatively more expensive than iron as an alternative, maximum and optimum metal utilisation is a prerequisite, an objective that can be effected through the application of pre-shaped support materials. The commercial supports of choice appear to be alumina, titania and silica. The Fischer-Tropsch synthesis part of the Gas-To-Liquids (GTL) process is preferably performed in a slurry phase bubble column reactor [1], a system favouring cobalt based catalysts due to its gradientless nature [1,3]. The churning nature of the three-phase bubble column reactor, however, exerts significant mechanical stress on the suspended catalyst, placing a high premium on its mechanical integrity. In addition to this demand of sufficient mechanical integrity, it is also important to ensure proper anchoring of the Fischer-Tropsch synthesis active phase (i.e. metallic cobalt). Insufficient anchoring of these metallic crystallites could result in the washing out of cobalt rich ultra-fine particulates from the porous pre-shaped support material during commercial slurry phase Fischer-Tropsch synthesis [4]. These cobalt rich ultra-fine particulates (less than 1 ~tm in diameter) are extremely difficult to remove from the produced waxy product. Not only may the cobalt rich ultra fine particulates poison some of the downstream processes (e.g. the catalytic hydroprocessing of waxy product to gas-oil), but may also contribute to a loss in the specific catalytic productivity, thus compromising the requirement of extended Fischer-Tropsch synthesis activities. Support modification has been reported earlier in the open literature [5,6,7,8,9]. Zirconia modification of silica supports was used to prevent the formation of unreducible cobalt-silicates [5]. Zr, Ce, Hf, or U modification of titania supports was reported to prevent the formation of cobalt-titanates during regeneration [6]. To increase the porosity of titania supports, they were modified with small amounts of binders, e.g. silica, alumina or zirconia [7]. Lanthanum oxide promotion of alumina was reported to be beneficial for improved production of products with higher boiling points [8], and zirconia modification of alumina supports was carried out to decrease the interaction of cobalt with alumina [9]. All these modified supports were either used for fixed bed cobalt based Fischer-Tropsch synthesis catalysts or they were used for slurry phase cobalt catalysts, but not tested under realistic Fischer-Tropsch synthesis conditions in large scale slurry bed reactors.

56 It is well known that alumina, titania [10,11,12] and magnesium oxide [13,14] dissolve in acidic aqueous solutions and even at pH values close to the isoelectric point [15,16]. In this study, it will be shown that these support surfaces were modified with promoters to increase the inertness thereof to acidic/aqueous environments, and not to stabilise the support against sintering and loss in surface area at high temperatures [17,18]. This paper will deal with the modification of alumina and titania supports for cobalt based slurry phase Fischer-Tropsch catalysts to ensure the successful operation of slurry phase bubble column reactors on commercial scale.

2. EXPERIMENTAL 2.1.

Catalyst preparation

Two alumina-supported catalysts were prepared by means of a two-step slurry phase impregnation method, using an aqueous cobalt nitrate solution [4,19,20,21]. A high-purity spray-dried Puralox SCCa 2/150 (trademark) alumina support, in the form of spherical particles, obtainable t~om Sasol Chemic GmbH of Uberseering 40, 22297, Hamburg, Germany, was used in all alumina-supported catalysts. The alumina had a surface area of 150 mE/g and a pore volume of 0.5 ml/g as determined by the B.E.T. method. After impregnation and vacuum drying, the dried intermediate was calcined in air at 250~ for 6 hours. The calcined catalyst was reduced at 425~ in pure hydrogen using a heating rate of l~ Platinum was incorporated as a reduction promoter. The composition of these catalysts were for catalyst A: 30gCo/0.075gPt/100gA1203 and catalyst B: 30gCo/0.075gPt/1.5gSi/100gAl203.

2.2.

Support modification

Silica support modification in the case of catalyst B was performed as reported earlier by Beguin et al. [22], prior to the catalyst preparation, by means of a non-aqueous slurry phase impregnation step using Tetra Ethoxy Ortho Silicate (TEOS) as precursor. TEOS dissolved in dry ethanol was used as impregnation solution. The alumina powder was added and after impregnation, the slurry was dried at 60~ under vacuum for 6 hours. The dried modified support was calcined in air at 500~ for 2 hours [4]. The silica level was found to be 2.5 Si atoms/nm2 fresh support. Zirconium support modification of alumina was performed by adding zirconium, in the form of zirconium isopropoxide, under an inert atmosphere to isopropanol. Alumina was added to this solution, and the mixture stirred at 60 ~ for 1 hour. The solvent was removed under a vacuum of 3kPa (a) with a jacket temperature of 95 ~ The resultant modified support was subsequently calcined at 600 ~ for 2 hours to obtain a protected modified catalyst support. The amount of precursor was found to be 0.1 Zr atoms/nm2 fresh support. Particulate titanium dioxide (Degussa P25 (trademark)) support was calcined at 650 ~ for 16 hours, spray dried and classified to 75-150 micron. The support had a rutile content of 80% and a surface area of 27mE/g.

57

2.3.

Dissolution behaviour The dissolution behaviour of the supports were characterised by monitoring the conductivity of a support in a model acidic aqueous environment. The conductivity was monitored by a Metrohm 712 conductometer with a platinum electrode. The support was added to an acidic aqueous medium with a pH of 2 at 25~ The conductivity was measured continuously for 30 hours with stirring at 25~ and a constant pH 2. The pH was maintained at 2 with a 10% HNO3 solution by continuous and automatic titration with a Metrohm 718 STAT Titrino fitted With a Ag, AgC1 glass and reference electrode. 2.4.

Fischer-Tropsch synthesis tests Laboratory Fischer-Tropsch synthesis tests were performed in a slurry-phase Constant S_tirred Tank Reactor. The pre-reduced catalyst (20-30 g) was suspended in ca 300 ml molten Fischer-Tropsch wax. Realistic Fischer-Tropsch conditions were employed, i.e." 220 ~ 20 bar; commercial synthesis gas feed: 50 vol% H2, 25 vol% CO and 25 vol% inerts; synthesis gas conversion levels in excess of 50%. Use was made of the ampoule sampling technique as the selected on-line synthesis performance monitoring method [23]. Larger scale Fischer-Tropsch synthesis runs were performed in a pilot plant slug-flow slurry reactor using 3-Skg catalyst as well as in a slurry phase bubble column demonstration unit using 500-1500kg catalyst. The reaction conditions were similar to those in the laboratory CSTR runs. The reactor wax production varied between 5 and 30kg per day for the pilot plant runs and up to 60 bbl/day for the demonstration unit. On-line catalyst samples were taken for particle size distribution measurements and _Scanning Electron Microscope analyses. 2.5.

Characterisation techniques Particle size distribution measurements were performed by means of a Leeds & Northrup Microtrac Particle Size Analyser, equipped with a laser beam to determine the particle sizes. A water suspension was prepared by adding ca 2g of the sample and a few drops of a suitable detergent. The sample was ultrasonically treated for 1 minute before the analysis was performed. An average of three rtms was used to calculate the final particle size distribution. SEM analyses were performed on a Cambridge 360 Scanning Electron Microscope at an accelerating voltage of 3.0 kV in the SE mode. T_ransmission Electron Microscopic analyses were performed on a Phih'ps CM200 with a LaB6 electron source at the University of the Witwatersrand, South Africa. High Resolution SEM analyses were performed on a JEOL 6000F ultra high resolution Field Emission Gun Scanning Electron Microscope at the Unit for Electron Microscopy at the University of Pretoria, South Africa. 3. RESULTS AND DISCUSSION

3.1.

Pilot Plant synthesis tests The unmodified almnina-supported cobalt catalyst (catalyst A) was tested in the pilot plant slug-flow reactor under realistic Fischer-Tropsch conditions. The produced wax was

58 secondary filtered over a 2 micron Whatman no 42 filter and analysed for cobalt content by ICP. Initially, clean waxy products were produced (i.e. cobalt free) as depicted in Fig. 1.

400 -

E O.

9

300

r

=

200

0

o

100 0 o y

0

10

9

1

9

20

s 30

t 40

Time (days)

Fig. 1. Cobalt content in secondary filtered wax during pilot plant scale Fischer-Tropsch synthesis runs, using catalyst A: 30gCo/0.075gPt/100gA1203 (run F102).

After about 10 days on stream, however, the secondary filtered wax started discolouring and high levels of cobalt contamination were observed (refer: Fig. 1). This is not desired, as cobalt loss from the reactor decreases catalytic activity, and contamination of the produced waxy products may poison the downstream processes, e.g. the catalytic hydroprocessing of the waxy products to gas-oil. It also increases the cost of the commercial GTL plant due to the increased difficulty in removing the ultra fine contaminants from the produced waxy products. To ascertain the composition of these ultra fine cobalt contaminants, the produced waxy products were then secondary filtered through a 0.1 [am alumina membrane filter and the retained residue was analysed by XRD. The XRD analysis indicated that the residue contained well-crystallised Co304, and hardly any alumina. The ultra fine particles were found to be cobalt rich. During these extensive Fischer-Tropsch synthesis runs in both Pilot Plant and Demonstration Unit scale reactors, the mechanical integrity of the catalyst was monitored by means of Particle Size Distribution (PSD) measurements on extracted catalyst samples. The average particle size did not change even after 8 months of operation, and was qualitatively demonstrated by SEM analyses (refer: Fig. 2). Catalyst break-up is thus not an explanation for the observed contamination of secondary filtered wax with cobalt rich ultra fine particulates.

59

Fig. 2. (a) A SEM image of the alumina-supported cobalt catalyst A after a FischerTropsch synthesis test run in the demonstration unit. (b) Relative mean PSD values of the supported catalyst (catalyst B) after an 8 months test tun on the Pilot Plant slug-flow reactor.

3.2. Characterisation of freshly prepared cobalt supported alumina catalyst and the pure alumina support. The freshly prepared catalyst A was studied by high resolution FESEM (refer: Fig. 3).

Fig. 3. High resolution FESEM image of a fresh alumina-supported cobalt catalyst A 30gCo/100gA1203 Small white crystals were observed on the surface of the particles. A linescan elemental analysis was performed on a small crystal and was found to be pure boehmite (A10(OH)). The alumina used in this study has been calcined at 750 ~ and XRD analysis has shown that there was no boehmite present in the fresh support. It was reported earlier that alumina supports could (partially) dissolve in aqueous solutions [24], even at pH values close to the isoelectric point [15]. Partial dissolution of the alumina used during the preparation of catalysts A was experimentally confirmed during the aforesaid model dissolution test (refer: Fig. 4).

60 0.60 Unmodified alumina

o o.

" 0.40

~

-

_J

% ~, 0.20

1.5 g Si/AI203

o~

E

0.00

I

0

10

I

I

20

30

40

Time(h)

Fig. 4. The dissolution behaviour of an alumina support (used to prepare catalyst A) and a silica modified support (used to prepare catalyst B) It can be seen that rapid dissolution occurs within the first hour, followed by a more gradual dissolution that proceeds continuously (i.e. about 30 hours). A mechanism of metal oxide dissolution is reported in literature [24] (equations 1 and 2).

~Al/ /

H 0

~Al/

~O /

~A1 ~ ~O / H

-OH2

+ 3 H+(aq)

~AI /

__~

/

~OH

H2 O~ /

3+

I-I2 o ,~

.OH2 Al/

NO/ H

(1)

NOH 2

3+ .OH2 AI/ ~OH2

.OH2 in water

,~ slow

~AI "/ /

~OH

+ 21-120+ A13+(aq)

(2)

The abstraction of an aluminium ion from the bulk proceeds via 3 stepwise protonations of the surface hydroxyl groups. Alumina, therefore, dissolved in an acidic aqueous solution, similar to the environment during the aqueous phase cobalt nitrate impregnation step. Five hydroxyl groups have been identified [25] that could be responsible for the dissolution of alumina. These hydroxyl groups vary in acidity, therefore protonation of the most negatively charged would be protonated first, and were the most likely candidates to initialise dissolution. The abstraction of aluminium ions into solution via the mechanism shown above would be more rapid than for the more acidic hydroxyl groups. Therefore, the dissolution rate in the start of the dissolution test should be higher.

61 3.2.

The identification of an amorphous layer on the catalyst surface

An amorphous layer was observed on the surface of the particles (refer: Fig. 5) as analysed by TEM. The possible origin of this amorphous layer can be described by the following hypothesis: During the aqueous phase cobalt nitrate catalyst preparation step, the alumina support dissolves and the dissolved aluminium ions can precipitate either as boehmite or may, in combination with cobalt ions, precipitate as a cobalt aluminium hydrotalcite-like layer [16], producing a physically amorphous layer, uniformly covering the surface of the bulk support material [15]. It was, however, not possible to characterize the observed layer. It is hypothesised that during the aqueous phase impregnation and drying of the slurry, cobalt nitrate was precipitated on the amorphous cobalt-containing layer. The presence of this amorphous layer, prior to the actual impregnation process, resulted in an inferior anchoring of the eventually catalytically active cobalt metal. During the extended testing of the cobalt catalyst in a slurry phase bubble column reactor at Fischer-Tropsch synthesis conditions, the cobalt crystallites dislodged, thus causing the contamination of the produced waxy products, as observed in Fig. 1.

Fig. 5: TEM image of a fresh alumina-supported cobalt catalyst A: 30gCo/100gm1203 3.4.

Prevention of alumina dissolution

The dissolution behaviour of the alumina support during the slurry phase impregnation steps of the catalyst preparation procedure was considered to be the cause of the high levels of cobalt-rich ultra fine particulates in secondary filtered wax. To prevent dissolution of the alumina support during the slurry phase impregnation step of the catalyst preparation procedure, a support modification procedure was developed to prevent acidic attack of the hydroxyl groups on the surface of the support. A modifier was chosen which had to conform to the following requirements: i) Lower the dissolution rate of the modified support in an aqueous/acidic solution ii) The catalyst activity should not be compromised. iii) The porosity should not be decreased to such a point that the maximum cobalt loading on the alumina support, i.e. 30g Co/100g A1203, would not be achieved [4]. iv) No shift in catalyst selectivity, i.e. an increase in methane selectivity.

62 It was therefore necessary to prevem proton attack on the surface hydroxyl groups by either modifying the hydroxyl groups with a alkoxide silica modification agent (Terta exthoxy silane TEOS) or modifying the surface to form a stable spinel structure. The silica modification agent, TEOS, was added in such a manner that monolayer coverage was achieved. A monolayer will have the smallest impact and have the highest probability to pass the four requirements listed above. The support modification step was performed as a slurry impregnation using TEOS dissolved in dry ethanol as impregnation solution [4]. During support modification, TEOS reacted with the hydroxyl groups of the alumina surface. The three remaining ethoxy groups could be decomposed during a calcination step at 500~ in air, but was not compulsory. The support can also be modified by other methods with TEOS, e.g. chemical vapour deposition [26,27,28]. A maximum of 2.5 Si atoms/nm/ support was added during one impregnation step due to the bulkiness of the organic groups of the silicon alkoxide. The ethoxy groups reacted with surface hydroxyl groups and during decomposition all the remaining ethoxy groups on the TEOS molecule react with the surface hydroxyl groups, virtually coveting the total surface with a monolayer. The dissolution behaviour of the silica modified alumina was measured (refer: Fig 4), and was found to be significantly inhibited. It is believed that the silicon increases the inertness of the alumina support in an acidic/aqueous solution. The support geometry is important with respect to Fischer-Tropsch synthesis performance (i.e. activity and selectivity) of the resultant catalyst based thereon. The modified alumina support after silica modification did indeed produce a catalyst with the desired Fischer-Tropsch synthesis characteristics [4]. To verify the influence this monolayer silica had on catalyst activity and selectivity, the intrinsic Fischer-Tropsch synthesis activity as well as the selectivity of the supported cobalt catalysts studied in this paper (i.e. catalyst A and B), were determined at realistic Fischer-Tropsch synthesis conditions: (refer: Table 1). Table 1. Fischer-Tropsch synthesis data for catalysts A and B, as tested under realistic reactor conditions (refer: Experimental). Catalyst B Catalyst A 30Co/0.075Pt/1.5Si/100AI203 30Co/0.075 Pt/100A1203 Run number 1755 98F 15 20 Time on line (h) 75 76 Syngas conversion (%) 6.8 x 10-6 7.1 x 10-6 FT reaction rate (mol/s) 0.34 0.36 Productivity (gHC/gcat/la) 4 7 CH4 selectivity (%C-atom) From Table 1, it can be concluded that silicon modification of alumina did not have any substantial effect on the Fischer-Tropsch synthesis activity nor did it substantially influence the selectivity of the silica modified catalyst B. The silica modified supported cobalt catalyst, Catalyst B, were tested in a Pilot Plant slurry bubble column reactor under realistic Fischer-Tropsch synthesis conditions and it

63 was demonstrated that silica modification of the alumina support successfully eliminated the undesired phenomenon of ultra-fine cobalt rich contamination of filtered reactor wax. 400 3,50

9 Catalyst A

3OO

9Catalyst B

i i

i

ta.

~200

8 ~50

J

lOO 50 0

r

o

liakr1#~

"

".3/.: .

~.

10

m

20

~. i-

30

. -

mt -i

9

9

40

i

50

9

El J

_ --

Fig. 6. Cobalt content in secondary filtered wax during Pilot Plant scale Fischer-Tropsch synthesis rtms, using catalyst A: 30gCo/O.O75gPt/ 100g A1203 (run F102) and catalyst B: 30 Co/0.075 Pt/1.5g Si/100g A1203 (l'un F 117).

_, --I

60

70

8O

Time (days)

added to the pure support to form spinels with A1, i.e. aIIal2IIIo4, which are known [4,29,30] to exhibit decreased solubilities in comparison to alumina. After impregnation of the support with the modifier in an organic solvent, the modified support is calcined at high temperatures to create a protective surface layer of the desired spinel, i.e. aIIal2IIIo4 . In Fig. 7, an example of zirconia modification is shown. 0.6

0.5 1:: O 0. 0.4 0.

Unmodified alumina

o~

E 0.3 0.1 Zr atoms/nm 2 support

0.2 E 0.1

0.0

~'

I

I

I

0

10

20

30

40

Time(h)

Fig. 7. The dissolution behaviour of an unmodified alumina support (used to prepare catalyst A) and a zirconia modified support. The dissolution of titania supports was also investigated. Titania also (partially) dissolves in aqueous solutions and silica modification, using TEOS, effectively inhibited support dissolution [4].

64 4.

CONCLUSIONS

It was demonstrated that the production of clean waxy products (i.e. free of any cobalt contamination) during large scale slurry phase Fischer-Tropsch synthesis runs, was successfully effected with cobalt catalysts that were prepared on modified supports (i.e. supports displaying inhibited dissolution behaviour in aqueous environments). As an example, the silicon modification of alumina supports was discussed in detail. REFERENCES

1. P.J. van Berge, S. Barradas, J. van de Loosdrecht and J.L. Visagie, Erd61Erdgas Kohle, 117:3 (2001), 138. 2. Eisberg and R.A. Fiato, Studies in Surface Science and Catalysis, 199 (1998), 961. 3. P.J. van Berge and R.C. Everson, Studies in Surface Science and Catalysis, Vol. 107 (1997), 207. 4. P.J. van Berge, J. van de Loosdrecht, E.A. Caricato and S. Barradas, EP 1058580,2000. 5. Hoek, J.K. Minderhout, and P.W. Lednor, EP Patent No.110449 (1983). 6. C.H. Mauldin, S.M. Davis, and K.B. Arcuri, US Patent No. 4,663,305 (1987). 7. C.H. Mauldin, and K.L. Riley, US Patent No. 4,992,406 (1991). 8. S. Eri, J.G. Goodwin Jr., G. Marcelin and T. Riis, US Patent No. 4,880,763 (1989). 9. F. Rohr, A. Holmen, K.K. Barbo, P. Warloe and E.A. Blekkan, Studies in Surface Science and Catalysis, Vol 119 (1998) 107. 10. E. Bright and D.W.Readey, J. Am. Ceram. Soc., 70112] (1987) 900. 11. W.H. Casey, M.J. Cart and R.G Graham, Geochim. Cosmochim. Acta, 52 (1988) 1545. 12. M.L. Machesky, D.A. Palmer and D.J. Wesolowski, Letter to Geochim. Cosmochim. Acta, 58 (1994) 5627. 13. A.K. Datye, A.D. Logan and K.J. Blankenburg, Ultramicroscopy, 34 (1990) 47. 14. J.A. Mejias, A.J. Berry, K. Refson and D.G. Fraser, Chem. Phys. Lett., 314 (1999) 558. 15. J.-B. d'Espinose de la Caillerie and O. Clause, Studies in Surface Science and Catalysis, Vol. 101 (part B) (1996) 1321. 16. J.-B. d'Espinose de la Caillerie, C. Bobin, B. Rebours and O. Clause, Prep. of Cat. VI (eds G. Poncelet et al., Elsevier Science B.V.) (1995) 169. 17. A.I. ~[~li,A.E. Aksoylu and Z.I. Onsan, Turk.J.Chem., 22 (1998) 253. 18. T. Horiuchi, Y. Teshima, T. Osaki, T. Sugiyama, K. Suzuki and T. Mori, Catal. Lett., 62 (1999) 107. 19. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H. Bolder, US Patent No. 5,733,839 (1998). 20. P.J. van Berge, J. van de Loosdrecht, E.A. Caricato, S. Barradas and B.H. Sigwebela, WO 00/20116 (2000). 21. P.J. van Berge, J. van de Loosdrecht and J.L. Visagie, WO 01/39882. 22. B. Benguin, E. Garbowski and M. Primet, J. Catal, 127 (1991) 595. 23. E. Bright and D.W.Readey, J. Am. Ceram. Soc., 70112] (1987) 900. 24. G. Furrer and W. Stumm, Geochim. Cosmochim Acta, 50 (1986) 1847. 25. H. Kn6zinger and P. Ratnasamy, Catal. R e v . - Sci. Eng. 1711] (1978) 31. 26. T. Jin and J.M. White, Surf. Interf. Anal., 11 (1988) 517.

65 27. 28. 29. 30.

N. Katada and M. Niwa, Chemical Vapour Deposition, 2[4] (1996) 125. S. Sato, M. Toita, Y.-Q. Yu, T. Sodesawa and F. Nozaki, Chem. Lett., (1987) 1535. P.J. van Berge, J. van de Loosdrecht andS. Barradas, ZA patent no. ZA 2001/6213. P.J. van Berge, J. van de Loosdrecht and S. Barradas, WO 02/07883, 2002.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

67

The effects of nature and pretreatment of surface alumina support on the catalytic nickelsilicate membrane formation C. Constantin l'z, V. Pfirvulescu 1'3#, A. Bujor z, G. Popescu z and B.L. Su 1. 1-Laboratoire de Chimie des Mat~riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-50 Namur, Belgium Z-Research Center for Molecular Materials and Membranes, Spl. Independentei 206, Bucharest, Romania 3-Institute of Physical Chemistry, Spl. Independentei 202, Bucharest, Romania. Catalytic nickelsilicate membranes were synthesized by in-situ techniques on pretreated a- and ~,- alumina supports. The resulting films and bulk materials have been characterized by XRD, Nz adsorption-desorption, SEM, TEM and FTIR spectroscopy. Their permeation performances of gases and liquids and catalytic activity in the selective oxidation of styrene with HzOz have been evaluated. The formation, morphology, permeation and catalytic properties of the membranes have been correlated with nature of alumina supports, their surface pretreatment and conditions of the hydrothermal synthesis. The results show a strong effect of ~,-alumina surface on the morphology and properties of the membranes. 1. I N T R O D U C T I O N The new application fields of membranes in fuel cells and in catalytic membrane reactors expect new membrane materials. The recent researches are focused on three types of inorganic membranes: zeolite, microporous membranes obtained by sol-gel, and dense metallic and perovskite membranes [1-4]. Zeolitic membranes attract a particular attention due to their resistance to high temperatures, shape selectivity and sorption affinities [4-9]. However, the synthesis of zeolite films on various substrates is very sensitive to the chemical and structural properties of the support surface, the conditions of preparation and the substrate position in the reactor [10]. The films prepared by ex-situ coating or in-situ crystal growth methods contain often the intercrystal porosity, the molecular sieve effect of zeolite is thus greatly reduced. Furthermore, the adhesion between the crystals and the supports remains still a great problem [11]. The successful applications of zeolite membranes in the transformation and permeation of the small molecules and the properties of mesoporous silicates, designated as M41S, explain the actual interest in the synthesis of mesoporous molecular sieve membranes. Incorporation of the transition metals into the #: SSTC and DGRE-DRI-RW (Belgium) research fellow Corresponding author ([email protected])

68 MCM-41 network conducts to the active catalytic materials in the oxidation reactions with hydrogen peroxide [12-16]. In this paper we present our recent results on the formation of nickelsilicate membranes on alumina substrates and the effects of alumina surface treatment on the film morphology, permeation and catalytic performances. 2. E X P E R I M E N T A L

2.1. Synthesis Nickelsilicate membranes were hydrothermally synthesized on the porous a or ~, alumina supports. The chemical modification of the supports was achieved by acid and base treatments, deposition of silica and adsorption of sodium silicate or surfactant molecules. Mineral acid as hydrochloric and hydrogen peroxide were used for cleaning support materials. ~,-Alumina support disk (0 =28.5 mm and 3 mm in thickness, 0.5 vm in diameter of the pores and 186 m2/g in surface area) and a-alumina tube (12.3 mm o.d. and 8 mm i.d., 0.2 t.tm in diameter of the pores and 14.9 mZ/g in surface area) were cleaned in an ultrasonic bath containing a HCI acidified solution mixture of 2-propanol, ethanol and water (volume ratio of 1:2:2). To investigate the effects of surface functional groups on the membrane formation, the supports were wetted during 0.Sh or 12h, respectively, with an aqueous solution of a single component such as NaOH, NH4OH, TMAOH and sodium silicate or a mixture of NH4OH+TMAOH, NH4OH+CTMABr and TMAOH+H202. After this pretreatment, the supports were dipped or hydrothermally treated in the gel. The gel with the molar composition of 1.00 SiO2:0.04 Ni2+: 0.48 CTMABr: 0.28 N a 2 0 : 2 . 7 0 TMAOH: 196.00 H20 was obtained from sodium silicate (25.5-28.5% silica), cethyltrimethylammonium bromide (CTMABr), tetramethylammonium hydroxide (25 wt % TMAOH in water), Ni(CH3COO)z-4H20, and H2SO4 and aged in air for 2 days under ambient conditions. The films, deposited only on the top face of the support, or in the interior of the tube, were washed with distilled water. Crystals that were not, or weakly, bonded to the supports were removed by ultrasonic treatment in water for 1 h. The samples were dried in air at 100 ~ overnight and calcined at 550 ~ for 8 h. 2.2. Characterization The supports and obtained membranes were characterized by XRD (Philips PW 170 diffractometer), N2 adsorption-desorption (Tristar, Micromeritics), SEM (Philips XL-20 microscope), TEM (Philips Tecnai microscope), FTIR (Spectrum 2000, Perkin Elmer) and porometry (Colter porometer) techniques. Permeation of 02 through nickelsilicate membranes was carried out in a WickeKallenbach cell at 298K to evaluate the quality of the synthesized membrane. The membranes were tested in a catalytic reactor for the oxidation of styrene with hydrogen peroxide (30% wt) at 323K and molar ratio of styrene/ acetonitrile was 1/7.2. The oxidation products were analyzed using a Carlo Erba gas chromatograph with a stainless steel column containing OV-101 connected to a FID detector. The amount of the H202 was quantitatively analyzed by conventional iodometry. The used catalysts were dried at 373K for 8 h and then used for recycling experiments. Permeation measurements of the H202 and styrene were carried out under the reactor conditions.

69 3. RESULTS AND DISCUSSION The synthesis of the MCM-41-type metallosilicate membranes, using in-situ techniques, has shown a significant effect of the chemistry of the surface support. Nickelsilicate membranes were obtained by two different methods of the hydrothermal treatment. First, the support was treated into the gel and in the second case, the support was coated by dipping and treated in vapor atmosphere. Formation of the heterophase at the gel-substrate interface depends on the substrate surface properties and the vapor atmosphere. Those explain the effects of the support nature, pretreatment of the substrate and conditions of the hydrothermal treatment (time, vapor atmosphere) on the structure, morphology, uniformity and thickness of the films. 3.1 Effects of the support nature on the structure and morphology of the membranes Pretreatment of the alumina supports can clean the surface and change its reactivity. y-Alumina surface after pretreatment under the basic conditions contains a high concentration of the O- and OH functional groups. A high concentration of the cations (NH4 +, TMA § Na + and CTMA +) can be adsorbed by these very active surfaces. Interaction of the synthesis gel with these adsorbed cations leads to the nucleation, the growth and the aggregation of the micro- or sub-microsized globular species and the formation of the multilayer film on support surface when ~,-alumina was used as support. The morphology can be quite different if the pretreatment was different (Fig. 1). Both morphologies were very often encountered in metallic ions modified mesoporous MCM-41 silicas [15, 16]. Therefore, from the morphology point of view, it is very possible that the films obtained contain a hexagonal MCM-41 structure. The TEM and XRD techniques whose results will be discussed in the following section will supply us more information. It is observed that the membrane obtained on the y-alumina disk pretreated with a mixture of TMAOH and H202 was quite homogeneous (Fig. l a). While the treatment with NaOH or TMAOH conducts to a non-uniform composition with a different percent of the zeolite crystals in mesoporous membranes. Contrary of these, no significant effect of pretreatment on the the formation of membranes was noted when a-alumina support was used since membranes with similar morphology, thickness and structure were obtained both on pretreated or untreated a-alumina support (Figs. 2 and 3).

Fig. 1. SEM images of the membranes synthesized on 3,-alumina disk support pretreated in an aqueous solution of TMAOH+H2Oz+H20 (a) and NH4OH (b)

70

Fig. 2. SEM images of the membranes synthesized on a- alumina tubular support untreated (a) and pretreated in an aqueous solution of NH4OH (b) and NaOH+H202 (c) The same morphology was observed on all the membranes obtained on the a-alumina (Figs. 2 and 3). Nickelsilicate films obtained on a-Al203 have a typical morphology and structure for MCM-41 materials (Figs. 2 and 3), but the thickness is higher compared to the membranes synthesized on ~,-alumina disk. A difference in the morphology between outer and inner sides of the film was observed for all the membranes (Fig. 3a). It has to noted that the particle size of outer layers is much smaller than that of inner layers. Typical features in these images (Figs. 2 and 3a) are the small size of the grains on the substrate surface and their relatively homogeneous distribution on the surface.

Fig. 3. SEM images of the sectional view of nickelsilicate film synthesized on a- alumina pretrated in an aqueous solution of TMAOH+H202 (a) and of nickelsilicate powder obtained from the gel in autoclave (b)

71

Fig. 4. SEM images of the nickelsilicate membranes synthesized on 7-alumina pretreated during 2 h with an aqueous solution of TMAOH (a) and 12 h with TMAOH+H202 (b) and 12 h with NH4OH (c)

properties of the 3.2. Effect of the pretreatment of alumina support on the membranes Membranes synthesized on ?- A1203 have various morphology (Figs. 1, 3a and 4). The typical results obtained are shown in Table 1 together with synthesis conditions. The materials obtained are amorphous, MFI type zeolite or MSU mesoporous materials with a disordered wormhole like structure, depending on the support pretreatment and synthesis conditions. The membranes obtained after adsorption of sodium silicate have a non-uniform composition with a high percent of the amorphous phase. Pretreatment with NHnOH solution conducts to membranes with low uniformity. The zeolite or amorphous nickelsilicate with very small particles form these membranes. The ordered mesoporous nickelsilicate membranes were obtained on ~,-alumina treated with an aqueous solution of TMAOH+H202. The formation mechanism of mesoporous nickelsilicate membrane on the alumina substrate is somehow different compared with that of bulk materials. TEM images (Fig.5) and X-ray diffraction diagrams of both the membrane synthesized on ~,-alumina support pretreated with the mixture of TMAOH+H202 and the nickelsilicate powder obtained in the same autoclave (Fig. 6) exhibit a mesoporous structure with a hexagonal channel array of pore system. Anyway, comparing with the bulk nickelsilicate powder, the membrane material has a less resolved XRD patterns. As we discussed in the previous section that the morphology of the particles forming the membranes and that of powders obtained from the gel in autoclaves are identical, we can conclude on the basis of TEM, SEM and XRD results that the membranes obtained contain really an ordered mesoporous MCM-41 structure.

72

Fig. 5. T E M image of the nickelsilicate film

Fig. 6. XRD patterns of the nickelsilicate materials

Table 1. Preparation of the nickelsilicate membranes on 7-A1203 under various synthesis conditions and characteristics of their porous structure Run N ~ Support treated by a Membrane Atmosphere Product SBZT, aqueous solution of growth in autoclave Film P o w d e r (a) m2/g method 14 NH4OH immersion Gel + + 884 29 NaOH immersion Gel no + 13 NaOH,+HzOz immersion Gel + + 542 9 T M A O H + HzOz immersion Gel + + 920 A3 TMAOH dipping NH3 no 22 T M A O H + H202 dipping NH3 * 284 23 T M A O H + H z O z dipping DDA +,* 33 Na silicate immersion Gel no + A2 NHnOH dipping NH3 *,+ 480 29 NaOH dipping Vapor of the * gel 25 TMAOH+H202 dipping H20 no 31 TMAOH dipping NH3 * 28 T M A O H + H202 immersion Gel + + 920 +: Ni-MCM-41, *: MFI zeolite (a) : powder obtained from the gel in autoclaves The N2 adsorption-desorption profiles and the porosimetry measurements confirmed the presence of mesopores in the nickelsilicate membranes obtained by immersion, into the gel, of the alumina support treated with a solution of T M A O H + H z O z + H 2 0 . The average pore diameter was varied between 2.4 and 4.2 nm. Table 2 lists some permeation

73 experimental results of the gases and liquids through the nickelsilicate membrane. The permeation values are influenced by the preparation conditions (pretreatment of alumina support, ex. membranes 9 and 14 and time of the same treatment, ex. membranes 9 and 28). The evolution of the permeation evaluation with pressure on the surface membrane did not show the presence of any cracks on the surface of these membranes, indicating the high quality of these membranes. Table 2. Single gas and liquid permeability at 298 K for some nickelsilicate membranes Membrane Po2 x 102, PN2 x 102, PH2o2 x 102, cm3/cm2.min.atm cm3/cm2.min.atm cm3/cmZ.min 9 1.9 4.2 3.21

5

20

Ps_ty. x 10 2, cm3/cm2.min 0.94

14

4.8

9.2

8.24

2.82

28

3.5

6.7

5.47

1.46

...........

+9 +

-

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

14

.~15 Fig. 7. Variation of styrene conversion with time in membrane reactor

~ 5 o

The mesoporous nickelsilicate membranes obtained are active and very selective to benzaldehyde and condensation products in the oxidation of styrene with hydrogen peroxide. The variation of the synthesis parameters and the pretreatment of the supports affects the permeation (Table 2) and the conversion of styrene (Fig. 7). In comparison with the conventional static reactor with a control of the H202 feed, the conversion of hydrocarbon on membranes after 12 h reaction was lower, but the efficiency of the H202 and selectivity to benzaldehyde and condensation products are higher. A variation of the pressure in the feed room favors the control of the rate of the oxidation. 3.3. Variation of the membranes properties with the synthesis parameters The parameters of interest are the film thickness, surface relief and their temporal evolution. These parameters depend substantially on the substrate surface properties and

74 pre-treatment. The dependences of the film thickness and morphology on the experimental conditions are discussed for two types of nickel-silicate films obtained by hydrothermal treatment in gel or vapour atmosphere. Ni-MCM-41 materials were obtained by immersion of the support into nickelsilicate gel or dip coating of a layer of synthesis gel. Hydrothermal treatment of the coated gel layer in atmosphere of dodecylamine (DDA) or ammonia leads to the zeolite or amorphous materials. From Table 1 and Fig. 8 it can be seen the influence of the atmosphere of the hydrothermal treatment on the resulted materials. The large difference in the surface morphology was observed. Thickness and uniformity of the membranes can be controlled by the time of the pretreatment (Fig. 4 a and b and Fig. 9) and the hydrothermal treatment. Results of our study show a significant effect of time of the pretreatment on the morphology, thickness and uniformity of the membrane surface.

Fig. 8. SEM images of nickelsilicate membranes synthesized on 7-A1203 pretreated for lh with an aqueous solution of TMAOH. The membranes were obtained by dip coating of a layer of gel which was subjected to hydrothermal treatment in the vapor (a) and by immersion of treated support in the gel (b) SEM images of the membranes obtained by immersion of the support into nickelsilicate gel and hydrothermal treatment of the gel during 1, 2, 3 or 5 days show the formation of membrane only after 2 days. Except the membrane synthesized on 7-A1203, pretreated with TMAOH+H202+H20 mixture. However, the uniformity of the membrane obtained after 2 days of the hydrothermal treatment is not very good. Thickness of the membrane synthesized during 5 days is very high with a bad uniformity of the surface. Xray diffraction diagrams of the aggregates formed on the surface of these membranes show a typical MCM-41 materials.

75

Fig. 9. SEM images of nickelsilicate membranes synthesized on 7-AIzO3 pretreated for 30 min (a) and 2 h with a solution of TMAOH+H20 by immersion of the support into the gel 4. CONCLUSIONS The membranes obtained are insensitive to the pretreatment of c~-alumina support. While the structure and morphology of the nickelsilicate membranes synthesized on yalumina support are significantly modified by the synthesis conditions and the pretreatment of the substrate. Nickelsilicate membranes are active in oxidation reaction of styrene but their permeability and the catalytic activity are significantly modified by the synthesis conditions and the pretreatment of the support. The higher permeability for the gases and liquids and the higher conversion for the membranes synthesized on y-alumina supports pretreated during 12 h with TMAOH+HzOz+HzO mixture were obtained. ACKNOWLEDGEMENTS This work was performed within the frame of PAI-IUAP and a bilateral scientific cooperation between the R~gion Wallonne of Belgium and Romania. The VP and CC thanks the SSTC (Federal scientific, technological and cultural office of Prime Minister, Belgium) and DGRE-DRI of R6gion Wallonne, Belgium, for the research scholarships. REFERENCES

1. C. Lange, S. Storck, B. Tesche and W.F. Maier, J. Catal, 175 (1998) 280. 2. V. P~rvulescu, V.I. P~rvulescu, G. Popescu, A. Julbe, C. Guizard and L. Cot, Catal. Today, 25 (1995) 385. 3. S.J. Xu and W.J. Thomson, AIChE Journal, 473 (1997) 2731. 4. U. Illgen, R. Sch~ifer, M. Noack, P. K61sch, A. K/ihnle and J. Caro, Catal. Commun., 2 (2001) 339. 5. J. Caro, M. Noak, P. K61sch and R. Sch~ifer, Microporous Mesoporous Mater., 38 (2000) 3.

76 6. N. van der Puil, F.M. Dautzenberg, H. van Bekkun and J.C. Jansen, Microporous Mesoporous Mater., 27 (1999) 95. 7. Y. Uang, S. R. Chaudhuri and A. Sarkar, Chem. Mater., 8 (1996) 473. 8. M. Matsukata, N. Nishiyama and K. Ueyama, J. Chem. Soc., Chem. Commun., (1994) 339. 9. M.C. Lovallo and M. Tsapatsis, AIChE Journal, 42 (1996) 3020. 10. V. Valtchev, S. Mintova and L. Konstantinov, Zeolites, 15 (0995) 679. 11. Y. Yan, S.R. Chaudhurin and A. Sarkar, Chem. Mater., 8 (1996) 473. 12. M. Stockenhuber, R. W. Joyner, J.M. Dixon, M.J. Hudson and G. Grubert, Microporous Mesoporous Mat., 44-45 (2001) 367. 13. S. Biz and M.L. Occelli, Catal. Rev.Sci. Eng., 40 (1998) 329. 14. T. Blasco, A. Corma, M.T. Navarro and J.P. Pariente, J. Catal. 156 (1995) 65. 15. V. P~rvulescu, C. Dascalescu and B.L. Su, Stud. Surf. Sci. Catal. 135 (2001) 4772 16. V. Parvulescu and B.L. Su, Catal. Today, 69 (2001) 315.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

77

Supports and catalysts preparation by using metal alkoxides grafting technique E. Santacesaria (*), A. Sorrentino, M. Di Serio, R.Tesser Dipartimento di Chimica, Universit~ degli Studi di Napoli Federico II, Complesso di Monte S.Angelo via CINTHIA 80126 Napoli (Italy) E-mail [email protected]

Preparation methods for some acid and redox catalysts and some supports, obtained by grafting alkoxides on the surface of oxides rich of hydroxyls, are described. Acid and redox properties of the prepared catalysts have been determined and discussed. Catalysts have been evaluated in suitable test reactions and the most relevant performances will be reported. 1. I N T R O D U C T I O N The commercial availability of the metal alkoxides has strongly encouraged the use of these interesting compounds for producing new ceramic materials and thin coating films by the innovative techniques of sol-gel and chemical vapour deposition (CVD) [1]. Alkoxides are also largely used for the preparation of catalysts and/or catalytic supports, normally obtained by precipitation or co-precipitation as a consequence of hydrolysis [2]. Some papers have recently dealt with the preparation of catalysts obtained by grafting metal alkoxides on the surface hydroxyls of different carriers. A group of papers, for example, describes grafting of vanadyl alkoxide on oxides such as ZrO2, TiO2, SiO2 and TiO2-SiO2 [3-6]. Others describe the grafting of titanium alkoxides on silica [7-9], leading to a TiOzcoated material with a very high dispersion, which is useful as a catalytic support for vanadia. Few papers, at last, deal with grafting alkoxides with the aim of modifying the acidity of a surface [ 10-15]. Metal alkoxides grafting technique can usefully be used for changing the acid-base and/or the redox properties of the surface of an oxide rich in hydroxyls. To this purpose, oxides surfaces can be modified by grafting metal alkoxides, in different ways as, for example: (i) with low surface coverage to obtain, after steaming and calcination, a high dispersion of the corresponding oxide or isolated acid or redox sites of particular strength, (ii) with a monolayer, by completely altering the nature of the original surface in terms of its hydroxyls density, acidity or redox properties, (iii) with a multilayer, by repeating the grafting operation several times together with the treatments of steaming and calcination after each grafting step, to obtain a solid with a chemically different surface, but retaining To whom correspondence must be send. Work supported by MIUR (PRIN 2000 Funds)

78 the original mechanical and structural properties. It is then possible to modify the available alkoxide: (a) by changing, for example, the alkoxide group with another one having a longer alkyl chain for obtaining a compound more soluble in apolar solvents; (b) by reacting with an electron withdrawing compound such as, for example, a mineral acid in order to increase the Lewis acidity of the metal; (c) by reacting with a stoichiometric amount of water to obtain a partial condensation of the alkoxide molecules before grafting; (d) by preparing, always before grafting, a two metals heterometallic alkoxide. Alkoxide reactions with the hydroxyls of an oxide surface can be performed by contacting the surface with a solution of the alkoxide in the parent alcohol or in another solvent, more or less polar. The role of the solvent can be very important for the anchorage efficiency. The alkoxide concentration to be used can be choisen by knowing the surface hydroxyl density, the prevailing stoichiometry and the alkoxide-surface reactivity. In the present work, the methods of preparation of catalysts and supports, by grafting alkoxides on the most common commercial supports, will be described. Silica, alumina and titania, used as original supports were commercially available, while silica coated with TiOz, used as a support, has been obtained by grafting in three successive steps titanium alkoxide on silica. All the prepared catalysts and supports have been characterized by using different techniques and test reactions. In particular, we observed, in many cases, completely different properties and behaviour with respect to the corresponding catalysts prepared with the traditional impregnation technique [16]. The performances of the catalysts prepared by grafting were evaluated by measuring their activities and selectivities in adequate test reactions such as: methanol dehydration, skeletal isomerization of 1-butene to isobutene and alkane isomerizazion and cracking. The redox properties of vanadium based catalysts have been tested using three different reactions: the SCR (Selective Catalytic Reduction) of NO with NH3, the oxidative dehydrogenation (ODH) of ethanol to acetaldehyde, and the ODH of propane to propene. Very promising results have been obtained in some of the mentioned reactions as a consequence of the peculiarity of the catalysts obtained with the alkoxide grafting technique. In this paper the best results obtained will be described and interpreted on the basis of some of the results of the catalysts characterization. 2. E X P E R I M E N T A L S E C T I O N

2.1. Catalysts and supports preparation methods Catalysts have normally been prepared by contacting a given support with an alkoxide solution, generally kept under inert gaseous atmosphere, at temperature that can be room or solvent boiling temperature, according to the observed reactivity. Apolar solvents are largely preferred giving place to higher grafting yields, provided that the alkoxides are soluble in those solvents. Often, the parent alcohol is used as a solvent, but in this case it must be considered that the following equilibrium occurs: Surface .... OH + Me (OR), ~

Surface--- O--- Me (OR)n-1 + ROH

(1)

rendering the reaction much less favoured in the presence of an excess of the alcohol. However, this could be an advantage when a high dispersion of the catalyst is required. The concentration of alkoxide in the contacting solutions must be decided on the basis of

79 the amount of the element that we want to anchor on the support, but taking into account the yields obtainable, depending on the equilibrium (1) and/or on the reactivities of the involved reacting species. By assuming a conventional stoichiometry of 1 alkoxide molecule for 1 hydroxyl on the surface, we can roughly estimate the amount of alkoxide that is enough for obtaining a monolayer coating. After a grafting experiment, we can determine the yield of the reaction by measuring the amount of residual alkoxide in the solution. After grafting, the solid obtained is normally washed with the used solvent and then submitted to a steam treatment, at 150-190~ for eliminating, by hydrolysis, the residual alkoxide groups from the surface. The solid is then calcined at about 500~ In order to obtain a multilayer coating, the described procedure of grafting, washing, steaming and calcination is repeated several more times. In Tables 1 and 2, the most representative catalysts and supports prepared by using the grafting technique, together with the conditions adopted for preparation and the obtained yields, are reported. Table 1 List of the acid catalysts prepared with the grafting yields. Grafting have been made at room temperature for Si(OEt)4 at the solvent boiling point for Zr(OR)4 Metal initial Anchored Supported Precursor/solvent/support Acronym amount metal oxide (mmol/g) (mmol/g) (% wt.) Z1B 0.08 0.01 0.1 Zr(n-but)4/butanol/Al203 Z2B 0.61 0.02 0.2 Zr(n-but)4/butanol/AlzO3 Z3B 1.26 0.08 1.0 Zr(n-but)4/butanoffAlzO3 Z4B 2.17 0.14 1.7 Zr(n-but)4/butanol/Al203 Z1T 0.61 0.10 1.2 Zr(OCsH17)4/toluene/A1203 Z2T 1.26 0.14 1.7 Zr (OC8H17)4/toluene/AlzO3 ZTSS 2.73 0.52 Zr (OCsH17)2SO4/toluene/SiO2(Aldrich) ATSS 4.33 1.83 AI (i-OPr)SO4/toluene/SiOz(Aldrich) AS1 1.05 0.07 0.4 Si(OEt)4/ethanol/Al203 Si (OEt)4/ethanol/Al203 AS2 2.13 0.14 0.9 Si (OEt)Jeth an ol/A1203 AS3 4.10 0.28 1.7 Si(OEt)4/no-solvent/Al203 ASM 11.0 0.50 3.0 Si(OEt)4/no-solvent/ASM (Two times) ASM3 11.0 0.71 4.3 Since catalysts prepared by grafting are often very dispersed, the stoichiometry of the reaction between an alkoxide and the hydroxyls of the surface can be responsible for the change of the hydroxyls density giving place to respectively: an increase of the density for 1 to 1 stoichiometry, a contraction for a 3 hydroxyls for 1 alkoxide, while 2 to 1 stoichiometry gives invariance. Moreover, the overall stoichiometry can change with the concentration of the alkoxide solutions and this brings, in practice, to an average behaviour falling between 1:1 and 2: 1. The use of an apolar solvent largely favours grafting reactions. Therefore, alkoxides can be modified for obtaining more soluble compounds. It is possible to exchange, for example, one or more alkoxide groups by reacting the alkoxide with fatty alcohols: Me(OR)n + x R ' O H ~

Me(OR)n_x(OR')• + x ROH

80 Table 2 List of the redox catalysts prepared and related support with the grafting yields. Grafting have been made at room temperature for VO(i-OPr)3 and by refluxing at the boiling point of the solvent for Ti(OR)4. Grafting of VO(i-OPr)3 is quantitative in n-hexane. Acronym Metal initial Anchored Supported Precursor/solvent/support amount metal oxide (mmol/g) (mmol/g) (% wt.) Ti (i-OPr)4/toluene/SiO2 (Aldrich) ATS1 2.27 1.5 12.6 Ti (i-OPr)4/toluene/SiO2(Aldrich) ATS2 2.27 2.2 18.4 Ti (i-OPr)4/toluene/SiO2(Aldrich) ATSM 2.27 3.3 27.7 Ti (i-OPr)n/toluene/SiO2(Grace) GTS1 0.95 0.7 5.9 Ti (i-OPr)n/toluene/SiO2(Grace) GTS2 0.95 1.15 9.7 GTSM 0.95 1.41 11.3 Ti (i-OPr)n/toluene/SiOz (Grace) VO(i-OPr)3/n-hexane/ATSM AVTS1 0.32 0.32 2.9 AVTS2 0.71 0.71 6.4 VO(i-OPr)3/n-hexane/ATSM VO(i-OPr)3/n-hexane/ATSM AVTS3 1.17 1.17 10.6 VO(i-OPr)3/n-hexane/GTSM GVTS1 0.09 0.09 0.8 VO(i-OPr)3/isopropanol/GTSM GVpTS 0.09 0.06 0.5 VO(i-OPr)3Hydrol/isopropanol/GTSM GVhTS 0.10 0.9 VO(i-OPr)3nydroR+4Ti(i-OPr)4/ G(VhT4)S 0.09 0.8 Isopropanol/SiO2 Shifted alcohol, having low boiling point, is continuously removed by distillation thus favouring the reaction (see catalysts Z1T, Z2T of Table 1). Another possibility of modifying the grafting precursor is the reaction in a suitable solvent with an acid, for example sulphuric acid, having a strong electron-withdrawing effect on the metal. We obtain sulphated complexes of the type Me(OR)n_2SO4 that can be grafted giving place to very strong acid catalysts (see catalysts ATSS, ZTSS of Table 1). It is possible, at last, to favour the condensation of the alkoxides molecules, before grafting, by reacting them with stoichiometric amounts of water, in the presence of traces of HCI for promoting the condensation reaction by partial hydrolysis. The condensation reaction can be made by using only one type of alkoxide or, alternatively, by mixing two different alkoxides that can react with each other, giving place to a bimetallic alkoxide. However, other procedures can be followed for obtaining hetero-metallic alkoxides [17] to be grafted. Partial hydrolysis has been used by us for obtaining different dispersions of V205 catalysts. The preparation of hetero-metallic alkoxides opens a new perspective in the preparation of catalysts with the grafting technique, and we used this technique for preparing a vanadiumtitanium alkoxide that has been directly grafted on silica. 2.2 Catalysts characterization Three main aspects of catalysts characterizations, related to the changes observed as a consequence of grafting, that will be considered here and discussed are: (1) the change of the specific surface area and/or of the hydroxyls density; (2) the change of the acid-base properties; (3) the change of the redox properties. The changes obtained in the specific surface area and/or of the hydroxyls density, as a consequence of grafting alkoxides and of the following other described treatments, are not dramatic. Grafting is normally accompanied by a relatively small decrease of the specific surface area, as it can be

81 appreciated from the data reported in Tables 3 and 4. This behaviour can be explained by assuming that alkoxides, which are normally aggregates of 2-4 molecules, can occlude by grafting narrower pores. There are some exceptions to the decrease of the specific surface area or to the hydroxyls densities probably due to the preeminence of the 1 to 1 stoichiometry. However, in any case, it is very important to observe that the specific surface area remains very high when compared with that of the pure supported oxides (see for example the difference between pure TiOz anatase and TiOz anchored on silica after 1, 2 or 3 grafting steps reported in Table 4. The acidity has been characterized by using essentially two different techniques: (1) potentiometric titration of the acid sites with the determination of the ZPC (Zero Point Charge) or IEP (Isoelectric Point) and pKal, pKaz (Surface intrinsic ionization constants in water); (2) TPD (Thermal Programmed Desorption) of adsorbed organic bases such as pyridine and dimethyl pyridine to discriminate between Bronsted and Lewis acidic sites. Table 3 Properties of acid catalysts obtained by grafting Acronym ZPC OH pkal pkaz or (mmol/g) support weak

SiOz(Aldrich) ~'-AlzO3 ATSM Z1B Z2B Z3B Z4B Z1T Z2T AS1 AS2 AS3 ASM ASM3

2.2 7.5 2.7 7.5 7.6 7.7 7.9 7.8 7.9 7.4 7.1 6.6 5.6 5.2

2.2 1.05 2.0 1.20 1.32 1.44 1.34 1.42 1.38 1.01 0.95 0.82 0.97 0.91

0.6 5.7 1.1 . . 6.0 . 6.2 5.3 4.9 4.1 3.4 3.2

3.5 9.3 4.6 . . 10.1 . 10.3 9.1 8.8 8.1 7.6 7.5

. .

10 12 16 . 9 14 . . 12 16 .

Acids sites distributions by TPD Pyridine 2-6 Dimethylpyridine (~tmol/g) (~tmol/g) medium

strong

weak

74 84 155

3 35 46

10 2 7 .

.

. .

71 59 . .

.

. 80 . 80

.

.

. 43 37 . . . 19

. . . .

9 .

.

. .

3 3 . .

. .

medium

. .

4 . 3

. . . .

strong

59 18 114

3 9 39

16 13

7 5

24

7

30

6

The main acid-base properties of some of the prepared catalysts are summarized in Table 3. However, as it will be seen also, the catalytic activities in different test reactions give information about the surface acidity characteristics. Redox properties of the catalysts can be determined, on the contrary, by submitting them to TPR (Thermal Programmed Reduction) with hydrogen (see for example Fig.l). Reduced catalysts can be reoxidized with pulses of oxygen so determining also the catalyst dispersion. These determinations have been made for vanadium based catalysts. It is very interesting to observe, for example, that clusters of vanadia of different sizes, corresponding to different dispersion indexes, can show very different redox properties, as it can be seen in Table 4 and Fig. 1.

82

Table 4 Properties of redox catalysts and related supports. Acronym V205 % Specific Pore Pores distributions I Surface volume Area (cm3/g) d< 20 @ 20 100 @ (mZ/g) SiO2 Aldrich SiO2 Grace TiO2 ATS1 ATSM GTS1 GTS2 GTSM AVTS1 AVTS2 AVTS3 GVTS1 GVpTS GVhTS G(VhT4)S

2.9 6.4 10.6 0.8 0.5 0.9 0.8

450 282 125 372 355 237 267 299 283 261 284 249 214 314 335

0,0020

0.72 1.02 0.38 0.50 0.51 0.23 0.26 0.27 0.46 0.41 0.48 0.29 0.24 0.26 -

17 6 15 27 7 5 4 37 22 43 -

82 97 84 83 32 92 94 95 37 28 38 -

1 3 10 _

2

41 1 1 1 26 50 19

0.13 0.19 0.11 0.73 1.00 0.62 0.45

600

GVTS1 (a) GV.TS (b)

5OO

> 0,0015 E

o O

4o0

(,~ 0,0010N--

300

"~ 0,0005-

200 E 100

0,00000

20

40

60

80

100

120

time (minutes)

Fig. 1. A n e x a m p l e o f T P R GVhTS

obtained for the catalysts:

Dispersion Oads/Vsupp

(a) C a t a l y s t G V T S 1 ,

(b)

83 2.3 Most significant catalyst performances obtained in test reactions 2.3.1 Acid catalyzed reactions All the catalysts prepared with acid properties listed in Tables 1 and 3 have been submitted to the most suitable test reaction, chosen from: (i) dehydration of methanol, that is a reaction promoted by acid sites of medium strength; (ii) skeletal isomerization of 1butene to isobutene, promoted by sites of relatively weak acidity; (iii) isomerization and cracking of n-hexane, a reaction promoted only by very strong acid sites. Relevant results of both activities and selectivities have been achieved in all the mentioned reactions by using acidic catalysts prepared by grafting. In particular, methanol dehydration is strongly promoted over zirconium oxide supported on ~,-alumina. The presence of only 0.08 mmoles of Zr/(g of alumina) increased by a factor of 2 the reaction rate of 4 mmol/g h obtained on pure alumina, at 180~ The synergistic effect reaches a maximum value for the mentioned content of ZrO2 on the surface, but the activities always remain greater than on pure alumina. Silicated alumina prepared by grafting silicon tetraethoxyde (TEOS) on ~,-alumina has recently been proposed [10] as a catalyst useful for the isomerization of 1-butene to isobutene, a reaction of great industrial relevance for the production of MTBE (methyl tertbutyl ether), i.e., the most important oxygenated compound used as additive in lead-free gasoline. This reaction is very useful as a reaction test, because the first step of the reaction, that is the double bond shifting giving 2-butene (both cis and trans), is promoted by waek acid sites, while the subsequent skeletal isomerization to isobutene requires more acidic sites. However, if the acidity is too high, other undesired reactions occur such as dimerization to octene, cracking, and coke formation, the latter being followed by catalyst deactivation. The activities and selectivities shown by silicated alumina of the ASM and ASM3 types are always higher and less affected by deactivation than the ones shown by respectively alumina, silica alumina and silicated alumina of the type AS1-3, as it can be 100

%

80.

co

i

60,

o O 20

0

S 0

28-

U

20"

b

~ 4o ----- NO AVTS2 ~ NO3ArnVTSg2natedcat.

y 0-| 150

9i 200

- - o - - NO impregnated cat.

2.4o 3~0 3.4o 4~0 4.4o s~o Temperature ~

Fig. 2. Conversion from butene to isobutene, at 430~ 5 ml/min, N2 50 ml/min, Cat=2g) over catalysts AS2, ASM and ASM3 compared with alumina [ 14].

36

t

l~

.

.

12-

e

o

9

i

.

i

.

.

-"

i

.

1

AS4 ASM

AS2 9

e

n

i

~, Alumina

50

100

150

200

250

300

time (minutes)

350

4.00

Fig. 3. Comparison of the performances of a grafted and of an impregnated catalyst containing the same amount of V2Os in the SCR of NOn with NH3 [16]. (700 ppm of NH3 and of NO, 60 N1/h He, 0.3 g of cat.)

84 appreciated in the examples reported in Fig. 2. In particular, we observed a great improvement of both the activities and selectivities when the preparation of the catalyst occurs by contacting directly the support with the silicon ethoxide TEOS in the absence of any solvent. The use of ethanol, as a solvent, has a detrimental effect, as it can be seen in Fig. 2. From experimental evidence, sylicated alumina catalysts show many islands of silica on the surface of alumina [14]. When the grafting procedure is repeated seeral times, TEOS only reacts with the hydroxyls of the already supported silica, increasing the height of the silica islands. Consequently, catalyst ASM3 prepared repeating the grafting 3 times shows activities and selectivities that are comparable with those of ASM obtained by grafting in one single step (see Fig. 2). This also means that the active sites are probably located at the borderline between silica islands and not covered alumina surface [14]. The n-hexane isomerization reaction, always accompanied by cracking, is promoted only by very strong acid sites as the ones obtained by reacting some alkoxides directly with 100% sulfuric acid. Catalysts prepared and tested in the mentioned reaction are reported in Table 1 together with the conditions adopted in the preparation. Contrary to expectation, sulfated alumina alkoxides grafted on silica gave better results than sulfated zirconia alkoxides [13] especially after the regeneration in air at 400-500~ This is probably due to the instability of the sulfated zirconia prepared in this way, at the relatively high regeneration temperature. However, the activities and selectivities obtained in the reaction by using the two mentioned catalysts are compared in Table 5. Table 5. Product distibution in n-hexane isomerization and cracking, at 250~ (Space velocity- 90 cm3/(h'gcat) ) on sulphated catalysts ATSS and ZTSS of Table 1. Catalyst Cony. % C1 C2 C3 C4 C5 2-MP 3-MP MCP I

ATSS

1.6

-

4.0

16.3

15.1

7.3

29.1

14.3

ZTSS

1.0

-

-

30.1

26.3

11.1

22.5

10.0

l

l

13.9

2.3.2 Redox catalyzed reactions All the redox catalysts prepared and tested are vanadium based catalysts. On the basis of the information largely reported in the literature, the performances of these catalysts are greatly increased when V205 is supported on TiOz. But TiO2 supports, generally in the form of anatase, have a relatively low surface area susceptible to decrease by sintering. For this reason, different attempts have been made for preparing supports of silica impregnated with TiO2, or alternatively of silica coated with TiOz by grafting Ti alkoxide [7-9]. We devoted some efforts [15] in preparing and characterizing supports of silica coated with a multilayer of titania by repeating several times the grafting procedure. The obtained results are reported in Tables 2 and 4. It is interesting to observe that the amount of titania supported after three grafting steps is different according to the hydroxyl density of silica, changing according to the type of silica and the conditions of calcination. Supports prepared in this way have optimal chemical and physical properties with high and stable specific surface area and interact very well with vanadyl tri-isopropoxide in the preparation of vanadia catalysts by grafting. Many different vanadia catalysts have been prepared with

85 the aim to evaluate the effect of the vanadium load and dispersion on the redox properties. For this purpose, we attempted to increase the molecular size of the alkoxide species to be grafted on TiO2/SiO2, by submitting to partial hydrolysis vanadyl tri-isopropoxide dissolved in isopropanol. Other experiments have been made by submitting a mixture of vanadyl tri-isopropoxide and titanium isopropoxide to partial hydrolysis, then grafting the obtained product directly on SiO2. As vanadyl tri-isopropoxide readily reacts with water, it gives place first to autocondensation and then reacts with titanium isopropoxide giving place to a bimetallic alkoxide. The different catalysts prepared are summarized in Tables 2 and 4. These catalysts have been tested in different reactions, such as: (1) selective reduction of NOx (SCR) with NH3 [17]; (2) oxidative dehydrogenation (ODH) of ethanol to acetaldehyde [18]; (3) ODH of propane to propene [19]. In these reactions, relevant results have been obtained. In particular, in the SCR of NOx with NH3, vanadium pentoxide catalysts prepared by grafting on TiO2/SiOz supports resulted in much more active and selective catalysts than the corresponding ones obtained by impregnation of metavanadate, both containing 6 % by weight of VzO5 on the same support. This can be appreciated in Fig. 3. It is important to point out that for these catalysts, the redox as well as the acid properties of the catalysts are important, because of the strong basic character of one of the reagents. Therefore, for this reaction, morphological aspects related to the size of vanadium pentoxide clusters seem to be less important, even if no experiments have been made for clarifying this point. On the contrary, many experiments have been made on the other ODH reactions by changing, for example, vanadium loading from 3 % to 10 % by weight, or by changing the vanadium dispersion for the same loading (about 1% by weight). These last catalysts have been prepared by following different already described techniques based on the partial hydrolysis of vanadyl tri-isopropoxide alone grafted on TiOz/SiOz support, or alternatively treated with titanium isopropoxide and grafted, in this case, directly on SIO2. In Fig 4 the catalysts activities in ODH of ethanol to acetaldehyde for two different vanadium loadings of 0.8 and 6 % by weight, are compared together with the ones of catalysts prepared by controlling the extent of vanadium alkoxide hydrolysis [18]. As it can be seen, the selectivities for this reaction are insensitive to the type of catalyst and are correlated only with the extent of ethanol conversion. The activities, on the contrary, are affected both by the amount of titanium loading and the preparation method but, from the characteristics reported in Tables 2 and 4, we can conclude qualitatively that the activity is related only to the available surface of VzO5 for all the catalysts, except for G(VhT4)S which is much less active. Probably, the active sites necessary for this reaction, occurring at very low temperature, are not abundant on this catalyst while, as will be seen later, the same catalyst has better performances in ODH of propane. This means that by changing the preparation method, it is possible to change the type of sites available on the surface and, then, that those sites involved in ODH of ethanol, are reasonably different than those promoting ODH of propane. Concerning ODH of propane, Fig. 5 shows the propane conversions and selectivities to propene obtained at 500~ for, respectively, (i) catalysts having the same vanadium loading (0.8-0.9 % by weight) but different dispersions due to the different methods adopted for the preparation and (ii) catalysts with different vanadium loadings (from 3 to 10 % by weight) [20]. The three catalysts with the same vanadium loading gave comparable conversions but different selectivities, and catalyst G(VhT4)S, the worst for ODH of ethanol, was the best for ODH of propane.

86 The three catalysts with different vanadium loadings gave different activities and selectivities and the catalyst with 6.4% by weight of VzO5 was the most active and selective. A similar trend has been also obtained in the runs performed at 400~ These results demonstrate the sensitivity of ODH of propane to the structure of the vanadium clusters, but it seems that other factors must also be considered, such as the acidity of these clusters and the effect of titanium that, as we have demonstrated, can be amorphous, in this case, and not necessarily present as a homogeneous surface but simply surrounding vanadium oxide sites. 100

Conversion 1,~176 1 U . . ~ 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 ,

0,9 m' # ' ' 0,8

r

.... t.

,

,

i

,

i

.

,

,

i

,

.

I

,

9 t . . . . . -*'--L~_... ..........

0,7

o,1

80

.%,~

.

,

.~

,

.

,

--. ,--T-:--.,

/"~' ,

.

,

.

[-----] propane conversion selectivity to propene

70 60 0,7

.-~ ---9 0,5 "6 (~ 0,4 "~ 0,6

/

,

80

0,8

9

. ~ . m

9O

'q ,0 0,9

"Z"

._O 0,6 m 0,5: (1)> 0,4- - - ' - - G ( V " T 4 ) S I ('I - - , - - GV,TS ] o O0,3 l 0,2 ---"~-0,0

,

.

,

.

,

90 100 110 120 130 140 150 160 170

.

o~

50 40

30

0,3 O')

20

0,2

10

0,1

0

0,0 8O

GVTS1 GVhTS G(VhT4)SAVTS1 AVTS2 AV-;'S3 Catalyst

Temperature (~

Fig. 4. Conversions and selectivities for ODH of ethanol on different catalysts [18].(1.1 cm3/h of liquid ethanol, 7.7 cm3/min of 02, 22 cm3/min of He, 0.5 g of catalyst.

Fig. 5. Activities and selectivities at 500~ for propane to propene (6%02/29% propane plus N2,flow rate 10-80 cm 3 ,0.03g cat. For the first 3 cat.; 5% O2/10% propane and flow rate 50cm3/min for the other 3)

REFERENCES

1. L.L. Hench and J.K. West, Chem. Rev., 90 (1990) 33. 2. S. Klein, S. Thorimbert and W.F. Mayer, J. Catal. 163 (1996) 476. 3. J. Kijenski, A. Baiker, M. Glinski, P. Dollenmeier and A. Wokaun, J. Catal., 101 (1986) 1. 4. M. Schraml-Marth, A. Wokaun and A. Baiker, J. Catal., 124 (1990) 86. 5. U. Scharf, M. Schraml-Marth, A. Wokaun and A. Baiker, J. Chem. Soc. Faraday Trans., 87 (1991) 3299. 6. M.A. Reiche, E. Ortelli and A. Baiker, Appl. Catal. B: Environ., 23 (1999) 187. 7. A. Fernandez, J. Leyer, A.R. Gonzalez-Elipe, G. Munuera and H. Kn6zinger, J. Catal., 112 (1988) 489. 8. S. Srinivasan, A.K. Dayte, M.H. Smith and C.H.F. Peden, J. Catal., 145 (1994) 565. 9. R. Castillo, B. Koch, P. Ruiz and B. Delmon, J. Catal., 161 (1996) 524.

87 10. F. Buonomo, V. Fattore and B. Notari, US Patents, 4,013,589 (1977), 4,013,590 (1977) assigned to Snamprogetti SpA. 11. B.P. Nielsen, J.H. Onuferko and B.C. Gates, Ind. Eng. Chem Fund., 25 (1986) 337. 12. M. Stocker, T.Riis and H. Hagen, Acta Chem. Scandinavia B, 40 (1986) 200. 13. P. Iengo, M. Di Serio, A. Sorrentino, V. Solinas and E. Santacesaria, Appl. Catal. A: Gen., 167 (1998) 85. 14. P. Iengo, M. Di Serio, V. Solinas, D. Gazzoli, G. Salvio and E. Santacesaria, Appl. Catal. A: Gen., 170 (1998) 225. 15. P.Iengo, G. Aprile, M. Di Serio, D. Gazzoli, E. Santacesaria, Appl. Catal. A: General 170 (1999) 97-109 16. A. Sorrentino, S. Rega, D. Sannino, A. Magliano, P. Ciambelli, E. Santacesaria; Appl. Catal. A: General 209 (2001) 45-57 17. K.C. Caulton, L.G. Hubert-Pfalzgraf; Chem. Rev. 90 (1990) 969-995 18. E. Santacesaria, A Sorrentino, M. Di Serio, R. Tesser; Submitted for publication 19. R. Monaci, E. Rombi, V. Solinas, A. Sorrentino, E. Santacesaria, G. Colon; Appl. Catal. A: General 214 (2001) 203-212 20. A. Comite, A. Sorrentino, G. Capannelli, M. Di Serio, E. Santacesaria; Submitted for publication

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

89

Combinatorial approaches for speeding up heterogeneous catalyst discovery, optimisation and scaling-up Claude Mirodatos Institut de Recherches sur mirodatos @catalyse.univ-lyon 1 .fr

la

Catalyse

-

CNRS,

Villeurbanne-

France,

Over the past five years, combinatorial chemistry applied to heterogeneous catalysis has been dealt with in more and more articles, reviews and patents (Fig. 1). This methodology remains very controversial, however. Today, within universities as well as within public and private research centres, attitudes toward combinatorial methods run the gamut from fascination to scepticism (or even outright rejection). The debate usually originates from a misunderstanding of the strategies at hand. As such, "combinatorial catalysis" is too often mistaken for a random, undisciplined mixing of various chemicals. On the contrary, the combinatorial approach embodies conventional catalysis, micro mechanics, robotics, analytical methodology and information technology.

Fig 1. Number of publications devoted to combinatorial heterogeneous catalysis. Publication numbers for 2001 are estimated over the first 6 months. The SciFinder (CA) search was performed by entering "combinatorial" and "heterogeneous catalysis" Industry essentially seeks to use the combinatorial approach in order to accelerate the discovery of new materials and reduce time-to-market, and this is generally well accepted. The role of academia, however, remains a matter of debate. Some of the most frequently asked questions are:

90 -

Is combinatorial catalysis an accelerated conventional process f o r catalyst preparation or a new methodology? Does academic combinatorial research aim only at discovering entirely new materials ? Are the formulas discovered by combinatorial approach easily scalable to industrial catalysts? Are creativity and fundamental knowledge still required ofscientists?

This presentation aims to clarify the debate. In a first part, the application of combinatorial chemistry to heterogeneous catalysis is analysed in terms of current strategies and perspectives on the industrial and academic levels. Potential methodologies for academic research laboratories are proposed with emphasis on both theoretical and practical considerations. As a case study, the European consortium "COMBICAT .... Catalyst Design and Optimisation by Fast Combinatorial Procedures" is presented focusing on the chosen strategy [1]. That consortium gathers research institutions with widespread basic knowledge on catalyst development, experienced SME's as specialists for development of chemical research software and high-tech robotics hardware and large catalyst production companies as well as catalyst end users (engineering entities) of the European chemical industry. In a second and main part, various aspects of the running research will be presented: - analysis of the combinatorial approach to heterogeneous catalysis, - strategies and technologies for secondary screening, preparation and testing of catalyst libraries : development of hard and software tools adapted to case studies, - scaling up of "hits" or "leads" discovered via the combinatorial loop to catalysts which have to be validated by conventional methods for further industrial developments. These aspects will be illustrated through the case study of catalyst development for the water gas shift reaction : - initial steps of manual, then robotic preparation of egg-shell formulae, optimisation via a sequence of combinatorial cycles : synthesis of a generation / high throughput testing / data analysis for preparing the further generation, and so -

-

-

-

on,

-

- scaling up of the egg-shell powdered materials to catalyst deposition over a monolith structure to fulfil the operating conditions of the targeted industrial application (GHSV, pressure drop, thermal stability, etc).

As a general conclusion, the importance of robotics with respect to scientific creativity is likely overestimated in the HT approach and the discovery of new materials is likely to come essentially from ideas and strategies combining synthesis, screening, scaling up and further optimisation via data mining. Combinatorial catalysis is not a new field in science, but an interdisciplinary topic involving many different research communities. Its

91 success should rely on combining scientist creativity and advanced technology, leading both to discoveries and to a better understanding of catalysis [2-4]. ACKNOWLEDGEMENTS I. Vauthey, L. Baumes, L. Villegas, A.S. Quiney, S. P. Teh, P. Denton and D. Farrusseng are thanked for having produced most of the presented results and the EU programme "COMBICAT" for granting that study. REFERENCES

1. website of COMBICAT programme: www.ec-combicat.org 2. A. Holzwarth, P. Denton, H. Zanthoff and C. Mirodatos, Catal. Today, 2441 (2001) 1. 3. D. Farrusseng, L. Baumes, I. Vauthey, C. Hayaud, P.Denton, C. Mirodatos, Proceedings of the NATO-ASI Conference, July 16-27/2001, Vilamoura, Portugal, Kluwer, 2002 (in press). 4. D. Farruseng, L. Baumes and C. Mirodatos, Data Management for Combinatorial Heterogeneous Catalysis: Methodology and Development of Advanced Tools, Kluwer, 2002 (in press).

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

93

High surface area metal oxides from matrix assisted preparation in activated carbons M. Schwickardi, T. Johann, W. Schmidt, O. Busch, F. Schfith Max-Planck-Institut f/ir Kohlenforschung, Mfilheim a.d. Ruhr, Germany High surface area oxides are attractive materials for numerous applications in catalysis and sorption [1]. There are many techniques to manually prepare these materials, such as precipitation, sol-gel pathways, templating routes and so on [2,3,4,5]. We have developed a novel versatile route which offers a simple and straightforward manner to prepare a great variety of different oxides with even higher surface areas. This method avoids filtering steps and handling of suspensions which enables simple pipette robotic systems to prepare these materials. The method is suitable for the preparation of defined phases, such as spinels or perowskites, but also for the synthesis of amorphous or multiphase mixed metal oxides and can easily be parallelized. 1. I N T R O D U C T I O N High surface area binary and multinary metal oxides are interesting materials which show a high potential for several catalytic applications. The different oxides can either be used as catalyst or as a catalyst support due to their high surface areas. This could lead to new metal oxide supported catalysts with interesting catalyst/metal oxide interactions and therefore potentially new interesting properties. The conventional synthesis of high surface area metal oxide materials is normally performed either by precipitation, sol-gel reactions, or templating routes. Most of these methods require either the handling of large amounts of liquids or suspensions, complex reaction sequences, and/or expensive precursors. For the synthesis of fine chemicals, expensive precursors are not important. For large scale industrial applications, however, where several tons of catalyst may be used, it is of great advantage to avoid expensive precursors or noble metals, and any substance that is potentially dangerous. Because of the complicated preparation, a broad investigation of these metal oxides by high throughput techniques was not possible due to lack of automation. Further, the surface areas of materials resulting from the conventional preparation were sometimes not sufficiently high to be considered for demanding catalytic applications. Recently, we developed a novel preparation method for high surface area metal oxides, which is easy to perform, which does not contain any flammable or explosive compound, which requires no large amounts of liquids, which is applicable for the preparation of binary as well as multinary oxides with high specific surface areas, and which is therefore easily applicable for high throughput screening techniques.

94 2.

PREPARATION

The basic idea of the method is the formation of the oxides by a route in which a supporting material provides confined spaces to restrict the particle size of the oxides formed. This leads to the desired oxide and the supporting material as an exo-template which has to be removed after the internal formation of the metal oxide. Therefore suitable materials had to be found which provided sufficient pore volume for high oxide yield and suitable pore sizes to be impregnated by different metal containing precursor solutions. The support should ideally be removable by gentle procedures which only removes the support without losing the structural information of the templating material by sintering of the oxide or other processes which could harm the small particles and reduce the surface area.

We examined several different materials with respect to the mentioned abilities. For MgO which was used as an oxide model, different supporting materials are listed in Table 1. As can be seen, even corn starch and other well known materials provide good abilities in producing high surface area metal oxides after thermal treatment. However best results were achieved by using certain activated carbons (Table 2). Table 1 Different supporting materials for the formation of MgO and the surface areas of the resulting MgO. MgO-surface area (BET) Impregnated material _ 9m2/g Corn starch 146 m2/g Rice starch 61 m2/g Cellulose-Powder 98 m2/g Agarose 126 m2/g Gelatine-Powder 83 m2/g Polyvinylpyrrolidone 76 m2/g Table 2 Different activated carbon supports for the formation of MgO. Activated carbon BET-surface area ' Yield (g MgO / g Carbon) _ 9m2/g Aldrich: Darco| KB-B 193 m2/g 0.33 Fluka 05120 171 m2/g 0.30 Fluka 03866 57 m2/g 0.045 , ,

....

Further experiments were performed by using Fluka 05120 or Darco| activated carbon. As the TEM pictures show, small particles are formed which can be detected before and after calcinations (Fig. 1). The concentration of the precursor solution and even the thermal treatment before the calcination have a significant influence on the material due to the pre-formation of the precursor particles within the support. We indeed found differences in the surface area

95 which were related to the drying process of the wet support and the concentration of the precursor solution as can be seen in Table 3.

Fig. 1. Activated carbon impregnated with precursor solution of NiAl204.

Fig. 2. NiA1204 aiter calcination at 800 ~

Table 3 Influence of different concentrations of Mg(NO3)2 precursor solution on the BET surface area. ' c0neentration . . . . . . . . BET-surface a r e a Yieid (g Mg() / g'cari~oni..........

.

saturated saturated solution diluted by factor of 2 saturated solution diluted by factor of 5 .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

198 m2/g 168 m2/g 102 m2/g

0.36 0.19 0.07 ,

For some compounds like titanium and zirconium, alkoholates are the best compounds, but for most of the metals we found that very good results can be achieved by using highly concentrated nitrate solutions. This is very important because most of the metal species are available as nitrate compounds, which are normally quite stable and can easily be handled in air. Experiments with halogenides and acetates showed a decreased surface area compared to the nitrates. As can be seen in Table 3, the surface area is higher for higher concentrated metal salt solutions. In some cases, like iron oxides, even molten nitrate compounds can be used and successfully turned into high surface area oxides. The thermal treatment should be as gentle as possible to avoid sintering processes which occur at high temperatures. In air the activated carbon normally requires temperatures above 600 ~ for full combustion. However, we found a strong catalytic effect of most of the metal precursors which decreases the maximum temperature of the full combustion down to 500 ~ for selected compounds. Some examples are shown in Fig. 3. We further found differences in the maximum temperature which were related to

96 the treatment of the impregnated material before calcination. These results are outlined in Fig. 3 as well.

TG [%] 100-

~"a,..,. ............ ., ~ Activated carbon ' ~ . " k , . . . Mg(NO3) " ,, 80__ \ ~'., ........ 9............. ", Cr(NO3)3 dried 60. ":,_., " L-;..--- _.... ""'..,.... ~ _ Water 40Cr(NO3)3 " ' ~ ' " ~ : . ~ . ""~-.:~. N 'x 20-Cu(N~O3)2 TM .'7",,~~~~.2.2.1.L..Z.~...~.:._..~...2.._..._..~..._..:

i

100

230

3~)0

' 400

l

' 500 600 Temperature [~

' 700

' 800

' 900

1000'

Fig. 3. Thermogravimetric measurement of activated carbon impregnated with different precursor solutions. A further advantage of nitrates as precursors is their decomposition at relatively low temperatures. We found that this decomposition always took place before the combustion of the activated carbon (Fig. 3). We further found an increase in surface area if the combustion is performed slowly under reduced oxygen partial pressure. This could either be due to uncontrolled combustion which leads to local overheating and therefore local sintering processes or due to combustion of the templating carbon matrix before the oxide is finally formed. For metals which show a strong effect on carbon combustion and therefore a violent reaction which reduces the surface area when p2erforming the calcination under air, such as chromimn, we achieved surface areas of 155 m / g when performing part of the synthesis under argon. The impregnated carbon was first heated up to certain temperatures where the nitrates are decomposed and afterwards the partial pressure of oxygen in the feed was slowly raised up to atmospheric conditions. 3. PARALELLIZATION To use the activated carbon procedure in high throughput catalyst screening we designed a 77 well plate with quartz vials and set up a pipette robot with concentrated metal nitrate solutions (Fig. 4). A computer based algorithm which automatically produces random metal composition has been developed which directly controls the robot. Subsequently, the plate can be transferred to an oven for calcinations. This enables the production of 77 new

97 materials in less than one day which is basically limited by the cooling period of the calcination oven.

Fig. 4. Set up of the pipette robot for automated preparation of activated carbon based metal oxides. 4. MATERIALS

After calcinations at 450-500 ~ the materials basically consist of small crystalline particles. These sometimes show very broad reflections in XRD due to the small particle size which is below 5 nm at low temperatures. However, XRD patterns show increasing particle size with increasing temperatures. This is consistent with decreasing values in sorption measurements due to sintering processes. The synthesis of pure spinel or perovskite phases (Table 4) could be performed by mixing the desired molar ratios of the materials and applying temperatures above 700 ~ Table 4 Selected metal oxides prepared by the activated carbon route. For further information see experimental section. ........... Composition Phase ....... 13ET'surfa'ce ar'ea [m~g'''I]. . . . . MgO MgO 200 Fe203 maghemite/hematite 120 Cr203 eskolaite 160 TiO2 anatase/rutile 200 ZnCr204 spinel 70 CoA1204 spinel 130 NiAl204 spinel 240 LaFeO3 (perovskite) ~ , 50

98 To find optimal conditions for the preparation of the spinel phases, various calcination temperatures and times have been tested. Some examples are listed in Table 5. Table 5 Surface areas and calcinations temperatures of CoCr204 and NiA1204. CoCr204 Calcination procedure 500 ~ 600 ~ 45 min 600 ~ 2 h 700 ~ 800 ~ min 800 ~ 1 h

Niml204 surface area (BET) 126 m2/g 111 m2/g-107 m2/g . . . . . 89 m2/g 77 m2/g 68 m2/g ,,,

Calcination procedure 700 ~ 800 ~ 1 min 800 ~ 15 min 800 ~ h

,......

" .

800 ~ .

.

.

.

.

.

.

Surface area (BET) 236 m2/g 234 m2/g 220 m2/g 192 m2/g

5h .

.

.

.

165 m2/g .

.

.

.

.

.

.

.

.

XRD analyses of mixed oxide samples showed that spinel structures or mixtures of spinel phases and binary oxides occurred at a calcination temperature of 500 ~ However, at such low temperatures an amorphous character of part of the sample is very likely. The crystalline part of the samples consists of small particles of about 5 nm in size according to TEM studies. These oxides showed interesting catalytic activities in CO-oxidation and are currently a topic of intense investigation.

5. EXPERIMENTAL 5.1. General procedure The activated carbon was treated in two different ways. Method A: The activated carbon was dried at 100~ / 0.1 mbar and stored under Argon. 1.7 - 1.8 g were impregnated with 7 ml of a saturated aqueous metal nitrate solution. The mixture was magnetically stirred for 20 min. The mushy dispersion was filtered by applying vacuum and using a commercially available filter paper. The residual solid was mechanically squeezed and dried for 30 min at 80 ~ in an oven. Some metal nitrates like iron and chromium show nitrous oxide evolution. Method B: The activated carbon was directly used for the synthesis. 1.87 g of the carbon were drop by drop impregnated with 4 ml of a saturated metal nitrate solution. The mixture was manually kneaded. The crumbly mixture was then pyrolyzed in an oven.

5.2. Preparation of selected metal oxides MgO: Activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated magnesium nitrate solution (method B). The impregnated carbon was calcined without being dried (1 h at 500 ~ rate 2 ~ / min).

99 Product: white, 0.36 g / g carbon XRD: broad MgO P2attem N2-sorption: 201 m / g (BET) Fe203: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Fe(NO3)3 solution (method A) and dried at 80 ~ for 1 h. During drying, nitrous oxide is produced. The impregnated carbon was then calcined (1 h at 450 ~ rate 2.4 ~ / min) and taken out of the oven while hot. Product: reddish brown, 0.29 g / g carbon XRD: Fe203 maghemite + hematite pattem N2-sorption: 123 m2/g (BET) Cr203: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Cr(NO3)3 solution (method A) and dried at 80 ~ for 1 h. During drying, nitrous oxide is produced. To maintain the structural properties of the activated carbon the sample was heated under protective atmosphere up to 450 ~ at a rate of 3 ~ / min and kept there for 30 min. The sample was then slowly calcined within 1 h with slowly increasing air ratio. Product: green, 0.31 g / g carbon XRD: Cr203 escolaite, syn pattern N/-sorption: 156 m~/g (BET) TiO2:1.76 g of vacuum dried activated carbon (Fluka 05120) was impregnated under Argon by 3.0 ml of titania(IV)-butylate (Aldrich) and exposed to air. It was calcined for 1 h at 450 ~ (rate 3 ~ / min) and taken out ofthe oven while hot. Product: 0.45 g / g carbon XRD: TiO2 anatase, syn pattern N2-sorption: 200 m2/g (BET) ZnCr204". Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Zn(NO3)E/Cr(NO3)3 (Zn 9Cr, 1 : 2) solution (method A) and dried at 80 ~ for 1 h. The impregnated carbon was then calcined (15 min at 800 ~ rate 3 ~ / min) and taken out of the oven while hot. Product: grey green, 0.33 g / g carbon XRD: spinel pattern N2-sorption: 72 m2/g (BET)

CoCr204: Dried activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated Co(NO3)ffCr(NO3)3 (Co:Cr; 1:2) solution (method A) and calcined without drying. The impregnated carbon was then calcined (1 h at 500 ~ rate 3 ~ / rain) and taken out of the oven while hot. Product: 0.35 g / g carbon XRD: spinel pattern N2-sorption: 126 m2/g (BET)

100

NiA1204: Dried activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated Ni(NO3)2/AI(NO3)3 (Ni:Al; 1:2) solution (method A) and calcined without drying. The impregnated carbon was then calcined (1 h at 700 ~ taken out of the oven while hot. XRD: spinel pattern Nz-sorption: 236 m2/g (BET)

rate 3 ~ / min) and

LaFeO3: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated

La(NO3)3/Fe(NO3)3 (La:Fe; 1:1) solution (method A) and dried at 80 ~ for 1 h. The impregnated carbon was then calcined (20 min at 700 ~ the oven while hot. Product: brown XRD: perovskite pattern Nz-sorption: 49 m2/g (BET)

rate 3 ~ / min) and taken out of

5.3. Parallel preparation The activated carbon used for impregnation was Fluka 05120 and Darco| KB-B, 100 supplied by Aldrich. Metal compounds were hydrates of nitrate salts supplied by Fluka. The precursor solutions were all saturated. Catalyst preparation was automatically performed using a commercial pipette system (Gilson XL 232) which distributed the metal salt solution to quartz vials filled with certain amounts of activated carbon. The carbon was transferred into the vials by using fixed volumes which were equal to 230 mg (+ 10 mg) in weight. We used a total amount of 450 ml of mixed precursor solution. The composition of the precursor solution was determined by a random algorithm which first determines whether a precursor is chosen as an ingredient or not, and afterwards which quantity of the chosen precursor contributes to the total composition. The last step is a calculation for the pipette robot which directly gives volumetric values for the robot. The impregnated samples were then calcined on a 77 well plate covered with gauze to avoid oxide particles from being expelled during carbon combustion. Samples were dried at 90 ~ overnignt and slowly heated to 500 ~ within 3 h. After remaining at 500 ~ for 2 h, which led to fully combustion of the carbon, they were allowed to cool down. REFERENCES 1. A. Beretta and P. Forzatti, J. Catal., 200 (2001) 45. 2. F. Schiith, in: Handbook of Porous Solids, F. Schfith, K. Sing, J. Weitkamp (Eds.), Wiley-VCH, Weinheim, 2002, in print. 3. T.J. Barton et al., Chem. Mater., 11 (1999) 2633. 4. M.A. Valenzuela et al., Appl. Catal. A:General, 148 (1997) 315. 5. C. Otero Ardan et al., Mater. Lett., 39 (1999) 22.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

101

Effects of the impregnating and drying process factors on the mechanical properties of a PCoMo/AI203 hydrotreating catalyst D. Wu, and Y. Li* Department of Catalysis Science and Technology and State Key Lab on C1 Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China Mathematical models for the mechanical properties of a PCoMo/AlaO3 hydrotreating catalyst in the impregnating and drying processes are developed with a response surface methodology. A central composite design is performed to study collectively the effects of process factors on the mean strength, Weibull modulus, and pellet density of the catalyst. Analysis of variance reveals that the models developed are adequate. The validity of the models is verified by experimental data. Results show that the impregnating and drying process factors have great effects on the mechanical properties and pellet density, and that there exists a great potential for increasing the mechanical reliability at low density. 1. I N T R O D U C T I O N The mechanical strength of solid catalysts is one of the key parameters for the efficient performance of a fixed bed converter [1]. Solid catalysts are typical brittle materials, and their mechanical failure is due to brittle fracture arising from a sudden catastrophic growth of a critical flaw under tensile stress induced in the catalyst bulk phase [2, 3]. Variations of size, shape and orientation of the flaws, e.g. pores, defects, dislocations and discontinuations, result in a large scattering range of the strength data. It has been proposed that the mechanical strength data of solid catalysts can be described well by Weibull

Pf (L) = l - e x p ( - / 3 L m )

(1)

distribution [2-4], where Pf is the probability of failure, L the load at failure, m Weibull modulus, [3 a size parameter. The higher the Weibull modulus, the smaller is the scattering range of the strength data. During catalyst production, various process factors have effects on the material properties and microstructures of catalyst particles, and hence have effects on the Corresponding author. E-mail: [email protected] (Y. D. Li).

102 mechanical properties. In the literature, several authors have studied the effects of some factors for several catalysts [5-7]. In previous publications from this group, the mechanical strength of a high-temperature water-gas shift catalyst was optimized in the calcination and reduction processes [8, 9]. In this work, the factors in the impregnating and drying of a PCoMo/AI203 hydrotreating catalyst were examined. A response surface methodology (RSM), an empirical statistical modeling technique [10, 11], was used to develop the mathematical models relating the process factors to the mechanical properties. 2. E X P E R I M E N T A L

2.1. Impregnating and drying The PCoMo/A1203 catalyst was prepared by a co-impregnation technique using pore filling method. The support was commercial ~-A1203 (3.82 mm cylindrical extrudates). After drying at 200 ~ for 3 h, 30 g of support was co-impregnated with 60 ml of a mixed aqueous solution of ammonium heptamolybdate, cobalt nitrate and phosphoric acid. After impregnation, the solids were left at room temperature for 2 h. The drying in air was carried out in a drying cupboard, with putting in the samples after the temperature reached the preset value. Table 1 Experimental range and levels of the factors Factors

Units

Notations Ti

Range and levels -2 -1 0 20 35 50

Impregnating temperature Impregnating time Drying temperature Drying time

~

+1 65

+2 80

h ~ h

ti To td

4 100 4

10 175 10

12 200 12

6 125 6

8 150 8

2.2. Experimental design Four experimental variables, i.e. impregnating temperature, impregnating time, drying temperature and drying time were selected as controlling factors. Table 1 gives their experimental range and levels, where each factor was coded linearly so that the upper and lower limits were +2 and -2. A central composite rotatable design [10], showed in Table 2 was used, which consists of 31 experiments. The xl, x2, x3 and x4 are coded notations of the above four factors respectively. The results were modeled with a second-order polynomial equation, i.e. each responsey was fitted by a quadratic model, given as follows for four factors. 4 j=l

4

i<j

j=l

103

w h e r e y is the p r e d i c t e d response, b0 the offset term, effect, and

bjj

bj

the linear effect,

b O.the

interaction

the s q u a r e d effect. In this work, the m e a n strength, W e i b u l l m o d u l u s , and

pellet density w e r e o b t a i n e d as the responses of the d es i g n e d e x p e r i m en t . Table 2 Central c o m p o s i t e rotatable design and e x p e r i m e n t al results No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Design matrix

Experimental results

X1

X2

X3

X4

-1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 -2 2 0 0 0 0 0 0 0 0 0 0 0 0 0

-1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1 1 1 1 1 0 0 -2 2 0 0 0 0 0 0 0 0 0 0 0

-1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 0 0 0 0 -2 2 0 0 0 0 0 0 0 0 0

-1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 0 0 0 0 0 0 -2 2 0 0 0 0 0 0 0

~9, g/cm 3

1.72 1.78 1.70 1.72 1.82 1.83 1.74 1.79 1.78 1.81 1.71 1.74 1.78 1.86 1.74 1.72 1.70 1.78 1.76 1.87 1.82 1.74 1.77 1.76 1.70 1.72 1.79 1.76 1.77 1.75 1.77

L-, N 24.9 26.5 39.2 40.6 37.3 41.0 33.6 35.3 30.0 35.0 35.2 34.5 36.1 36.6 34.0 35.2 33.7 28.3 38.6 40.1 36.6 43.6 33.5 31.8 32.3 30.9 31.4 29.1 28.2 28.4 33.5

m

[3

Pf(5)

Pe(10)

4.63 3.74 4.53 4.14 4.32 3.28 4.89 3.90 2.77 2.79 3.33 3.55 3.61 2.86 3.53 3.32 5.01 3.76 3.53 4.17 4.04 3.59 3.70 3.28 3.66 3.37 3.60 4.07 3.63 3.98 3.63

2.25x10 -7 3.26x10 -6 3.96x10 -8 1.45x10 -7 1.06x10 -7 3.54x10 -6 2.22x10 -a 6.37x10 -7 5.82x10 -s 3.57x10 -s 5.02x10 -6 2.42x10 -6 1.61x10 -6 2.46x10 -s 2.75x10 -6 5.15x10 -6 1.43x10 -8 2.39x10 -6 1.71x10 -6 1.39x10 -7 3.20x10 -7 8.86• -7 1.57x10 -6 8.29x10 -6 2.05x10 -6 6.52x10 -6 2.77x10 -6 7.52x10 -7 3.75x10 -6 1.13x10 -6 2.02x10 -6

3.89x10 -4 1.34x10 -3 5.84x10 -s 1.14x10 -4 1.11xl0 -4 6.99x10 -4 5.83x10 -s 3.36x10 -4 5.02x10 -3 3.19x10 -3 1.06x10 -3 7.30x10 -4 5.37x10 -4 2.44x10 -3 8.03x10 -4 1.08• -3 4.55x10 -s 1 . 0 1 x l 0 -3 5.03x10 -4 1.14x10 -4 2.14x10 -4 2.87x10 -4 6.02x10 -4 1.63x10 -3 7.39x10 -4 1.49x10 -3 9.14x10 -4 5.23x10 -4 1.29x10 -3 6.78x10 -4 6.94x10 -4

9.59x10 -3 1.77x10 -2 1.35x10 -3 2.02x10 -3 2.23x10 -3 6.79x10 -3 1.73x10 -3 4.99x10 -3 3.37x10 -2 2.19x10 -2 1.06x10 -2 8.51• .3 6.54x10 -3 1.75x10 -2 9.22x10 3 1.07x10 -2 1.47x10 -3 1.36x10 -2 5.80x10 -3 2.05x10 -3 3.51x10 -3 3.46x10 -3 7.78x10 -3 1.57x10 -z 9.28x10 -3 1.53x10 -z 1.11xl0 2 8.72x10 -3 1.59x10 -2 1.06x10 -z 8.54x10 -3

2.3. Determination of the responses The

mechanical

strength

was

measured

with

a ZQJ-II

strength

tester

described

d e t aile dly e l s e w h e r e [3]. A k n i f e - e d g e cutting strength was adopted, since it is m o r e suitable for the e x t r u d a t e s than the c r u s h i n g s t r e n g t h [3]. For each s a m p l e 30 pellets w e r e

104

tested to obtain representative mechanical properties [12]. Mean strength was determined by arithmetical average of the strength data. Weibull parameters were estimated with linear regression [3]. A scale with an accuracy of 0.01 mm was used to measure the pellet size, and a balance with a precision of 0.1 mg was used for determining the pellet weight. The pellet density was determined with 5 pellets using the weight-volume method. Table 3 Multiple regression results and significance of regression coefficients Term

Xl

b0 bl

xz

b2

X3 X4

b3 b4 X1X2 hi2 XlX3 hi3 XIX4 b14 XzX3 b23 XzX4

X3X4 Xl2 z xz 2 X3 z X4

Mean strength Coef. t 30.8 45.8 -0.529 -1.07 1.09 2.21 1.42 2.87

P 0.000 0.297* 0.037 0.009

-1.12

-1.85

0.077

-2.88

-4.75

0.000

b24 b34 bll bz2

b33 b44

1.99 2.18

4.44 4.85

Weibull modulus Coef. t P 3.71 45.1 0.000 -0.425 -7.03 0.000 0.062 1.03 0.313" 0.095 1.57 0.131 -0.203 -3.36 0.003

0.163

2.20

0.039

-0.122

-1.65

0.113

0.139

2.53

0.019

Pellet density Coef. t 1.75 277.6 0.0095 1.69 0.023 4.11 -0.028 -5.04 0.0098 1.75 -0.011 -1.62

0.014

0.000 0.000 -0.085

-1.56

2.69

P 0.000 0.104 0.000 0.000 0.093 0.119

0.013

0.134

R 0.881 0.887 Hierarchical terms added after backward elimination regression

0.841

Table 4 Analysis of variance (ANOVA) Source Mean strength SS a DFb MSc F P REG a 470.0 7 67.1 11.40.000 RESe 135.2 23 5.88 LF f 110.4 17 PE g 24.88 6 a

6.49 4.15

1.560.302

Weibull modulus SS DF MS F P 7.14 8 0.89 10.2 0.00 1.93 22 0.09

Pellet density SS DF MS F P 0.044 6 0.0073 9.67 0.000 0.018 24 0.0008

1.59 16 0.34 6

0.013 18 0.006 6

0.10r. 77 0.247 0.06

0.0007 0.0009

0.75

0.705

Total 605.2 30 9.07 30 0.062 30 Sum of squares; b Degrees of freedom; c Mean square; a Regression; e Residual; f L a c k o f

f i t ; g P u r e error.

The significance of the coefficients was tested with Student's t-tests [11]. The P-value is used as a tool to check the significance of each coefficient or corresponding model term. Firstly, x2x3, x22 and x32 have the most significant effects on the mean strength (P=0.000),

105 followed by x3 (P<0.01) and then by xz (P<0.05). Secondly, Xl has the largest effect on the Weibull modulus (P=0.000), followed by x4 (P<0.005). The terms of Xl2 and XlX4 are also considerably pronounced (P<0.05). Finally, x2 and x3 make the greater contributions towards the pellet density (P=0.000), followed by x22 (P<0.05). The model adequacy was checked with analysis of variance (ANOVA) [11], as shown in Table 4. Due to replicates at the center point of the design domain, the significance of the lack of fit can also determined. P-values for the lack of fit imply that three models exhibit no significant lack of fit relative to the pure error, even at 80% confidence level (P>0.20). P-values for the regression indicate that all the models are greatly significant (P=0.000). Model reduction, i.e. exclusion of insignificant terms from full model improves model adequacy. Consequently, the models developed can be used to navigate the design space. 3.2. Prediction and confirmation An additional experiment was chosen to verify the validity of the models and to check their predictive ability, with the condition of Xl = xz = x3 - x4 = 0.5, i.e. Ti = 57.5 ~ ti = 9 h, To = 162.5 ~ and td = 9 h. The model-predicted and the experimental values of the responses are shown in Table 5. The observed values are within the 95% prediction interval, and within the 95% confidence interval of the predicted ones. The observed values are in good agreement with the predicted ones; therefore the three models are valid and have satisfactory predictive ability. It is also confirmed that RSM is an effective tool for the mathematical modeling of the mechanical properties of solid catalysts. Table 5 Comparison between experimental and model-predicted values Responses

Experimental Predicted

Mean strength Weibull modulus Pellet density

33.00 3.388 1.760

31.84 3.495 1.761

95% Confidence interval 30.32-33.36 3.30-3.69 1.74-1.78

95% Predictioninterval 26.60-37.08 2.85-4.14 1.70-1.82

4. DISCUSSION 4.1. Statistical properties of the mechanical strength Statistics shows that the knife-edge cutting strength data of all the samples follow the Weibull distribution well. It can be deduced that there is still a single flaw distribution existed in the samples after impregnation and drying. Weibull statistics provides a method for calculating the failure probability of a catalyst pellet under a specific load. Table 2 gives the probabilities at 5 and 10 N/pellet. These probabilities in low loading range can be used as an index to the mechanical reliability of solid catalysts. The values of Pf(5) and Pf(10) in Table 2 show remarkable differences between the samples obtained at different operating conditions. The Pf(5) and Pf(10) of sample 17 with

106 the largest Weibull modulus are 2 and 1 orders of magnitude lower than those of sample 9

r~

l~176t

8.0X10-4

/ I

9

~

~ 6"0X10-41

/11~

4~176

//5;/

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

~85

I "

.80

1+,7+

>,

~- 2.0x10 .4

1.72

00

1.74

1.78 ~

40

0 2 4 6 8 10 The knife-edge cutting strength N/pellet '

1.78

10 O,"~"

/8

irn;;ea20 ~ 1 1 : : ~ j 6 ^~,~,~eo~ o"~t~ng ternn 70 n,, 4 ~',e'e perature, oC ou \~-,, Fig. 2. Response surface and contour plot of the pellet density.

Fig. 1. Weibull distribution curves of the typical samples in low loading range

with the lowest Weibull modulus respectively. However, the two probabilities of sample 22 with the largest mean strength are very close to those of sample 1 with the lowest mean strength. Fig. 1 gives the probability of strength failure of the typical samples in low loading range. These results reveal that the probability of strength failure in low loading range is mainly related to the Weibull modulus. The mean strength shows less effect on the probability of failure than the Weibull modulus. Therefore, the Weibull modulus is a decisive parameter for the mechanical reliability of solid catalysts. The density of the samples shows no obvious relationship with the mean strength and

60

.................................. . . . . . . . . . . ~. . . . . . . . . . . / - .

Z

g

50 "+ . .........

". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

z

. . . . . . . . . . .

_E 40

........

m 31

c-

2u

i

.

". . . . . . . . . . .

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

................ ,,4,..,~---~ ~_.~.~._..-----~ ~

4 5 67 ~ ~ ~ ~ - ~ 4 0

35

~

j//

~i

200 o0 2 80 e 160 x~' 2n ~;~:_~_____

140

~r~

ir~ref .._8 ~ 4 8 / 1 2 0 .,,o~~ -~,latit :1t i ~ l ~ lO0 :.r Urne, h ]z "~ a

9 30

u 30 ~

-- 31~ _ ~ . . . . . . /

~

3 3 ~ ~

200 eO 160 x~" ~

140

~

Irnpre.._,9OatZ zlU 50 " ~ t ) U 37.____.7120 ~.e~"~ ng temp e 70 ^_ 100 .~o~ b

Fig. 3. Response surfaces and contour plots of the mean strength.

oc

+

107 with the Weibull modulus. Sample 17 has the lowest density, while its Weibull modulus is the highest in these samples. This indicates that it is possible to increase the mechanical reliability at low density. 4.2. Effects of the factors As can be seen from Table 2, after impregnation and drying, the Weibull moduli of all m the samples decrease, as compared with the original A1203 support (L = 40.17 N/pellet, m - 7.937 and / 3 - 1 . 3 5 • -13). Except sample 4, 6 and 22, the mean strength of the others decreases too. Thereby the impregnation and drying are unfavorable to the mechanical properties of solid catalysts. The results in Table 2 also show that the process factors have great effects on the mechanical properties and pellet density of the catalyst. To facilitate a straightforward examination of the effects of the factors, three-dimensional response surfaces and their corresponding contour plots were constructed using the models developed, with two factors kept at zero level and changing the other two in the design domain, as illustrated in Figures 2-4. These figures can be used to predict the responses for different values of the factors and to identify the interactions between the factors. The response surfaces in Figs. 2 and 3b are a part of distorted parabolic cylinder, which show a minimum ridge in the experimental domain. The response surface in Figure 3a is an inverted paraboloid (dome), and the corresponding contour plot is elliptical. The stationary point, which is the point at which the slope of the response surface is zero when taken in all directions, on this surface is within the design domain; however, it is a minimum point. The response surfaces in Figs. 4a and 4b have a saddle behavior or minimax nature. The stationary point is not a maximum or a minimum point, but a saddle point. According to these figures along with the models, several remarks can be made. Firstly, as the impregnating time or drying temperature increases, the mean strength decreases at first, and then increases. The mean strength takes a minimum at about t i - 7 h and To -

6 5

....

4

...........

~. . . . . . . . . . . . . . .

ffl

,g~ E

=1 :::3 "1:3 0

....

4.0

,,

_

.-.....,.

,,., ,-,. . . . .

__

s 3.5.........

12

N 3.o

12 1 ......" ~ ^ . /

2 " 5 ~ ~ \

rature oC

80

/ . 3 . 7 : _ . q , q ~ (

/~'~

4.0______..._______5/'8

45 , r n ~ 6

b Fig. 4. Response surfaces and contour plots of the Weibull modulus.

10

~

108 140 ~ In the experimental domain examined, the impregnating temperature has no significant effect on the mean strength. There is no the term of the drying time in the model, thus the factor has no effect on the mean strength. Secondly, the Weibull modulus increases with the increase of the impregnating time and with the decrease of the impregnating temperature. The effect of the drying time on the Weibull modulus is complex, but statistically less significant than the other factors. For the Weibull modulus, the drying temperature has no squared effect and interaction effect with others. The linear effect shows that as the drying temperature increases the Weibull modulus increases. Finally, the pellet density increases with the increase of the impregnating temperature. As the impregnating time increases, the pellet density decreases at first, and then increases. The model for pellet density reveals that the pellet density increases with increasing drying time and with decreasing drying temperature. 5. CONCLUSIONS RSM was used to develop mathematical models for the mechanical properties of a PCoMo/AI203 hydrotreating catalyst in the impregnating and drying processes. A central composite rotatable design was conducted to study simultaneously the effects of process factors on the mean strength, Weibull modulus and pellet density of the catalyst. A model is obtained for each response with backward regression method. The adequacy of the models is checked with the analysis of variance. The validity of the models is verified also by experimental data. Statistics of the mechanical strength reveals that the Weibull modulus is a decisive parameter for the mechanical reliability of the catalyst. Results show that the impregnation and drying are unfavorable to the mechanical properties. The impregnating and drying process factors have great effects on the mechanical properties and pellet density, and there exists a great possibility in increasing the mechanical reliability at low density. It is also confirmed that RSM is a good and useful method in mathematical modeling and factors analysis of the mechanical properties of solid catalysts. REFERENCES 1. S.ES. Andrew, Chem. Eng. Sci., 36 (1981) 1431. 2. Y.D. Li, X.M. Li, et al., Catal. Today, 51 (1999) 73. 3. Y.D. Li, D.E Wu, et al., Powder Technol., 113 (2000) 176. 4. W. Weibull, J. Appl. Mech., 18 (1951) 293. 5. EK. Gupta, D.S. Chabbra and A.C. Sengupta, Fertilizer Technol., 18 (1981) 193. 6. E.L. Furen, D.V. Gernet and T.A. Semenova, Kinet. Katal., 13 (1975) 107. 7. B.M. Fedorov, V.I. Nekhoroshev, et al., Kinet. Katal., 33 (1992) 157. 8. Y.D. Li and L. Chang, Ind. Eng. Chem. Res., 35 (1996) 4050. 9. Y.D. Li, R.J. Wang, et al., Catal. Today, 30 (1996) 49. 10. G.E.E Box, W.G. Hunter and J.S. Hunter, Statistics for experimenters: an introduction

109 to design, data analysis and model building, Wiley, New York, 1978. 11. R.H. Myers and D.C. Montgomery, Response surface methodology: process and product optimization using designed experiments, John Wiley & Sons, New York, 1995. 12. D.E Wu, Y.D. Li, et al., Chem. Eng. Sci., 56 (2001) 7035. 13. J. Piexoto, The American Statistician, 44 (1990) 120.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Influence of C e O 2 decomposition

content

on R h / T i O 2

111

monolithic

catalysts for

N20

S. Su~rez a*, M. Yates a, F.J. Gil Llambias b, J.A. Martin a, P. Avila a, J. Blanco a. a* Instituto de Catfilisis y Petroleoquimica, CSIC, Camino de Valdelatas s/n, 28049 Madrid, Spain, Tel.: 34 91 585 48 80, Fax: 34 91 585 47 60, e-marl: [email protected] b Facultad de Quimica y Biologia USACH, Alameda del Libertador Bernardo O'Higgins, 3363, Chile, Tel.: 2-681 25 75, Fax: 2-681 21 08, e-mail: [email protected] In this study the properties of TiOz-CeO2 samples with a cerium oxide content between 0-15 wt.%, prepared by wet impregnation, were analysed by electrophoretic migration technique, XRD, thermogravimetry, N2 adsorption and mercury intrusion porosimetry. The electrophoretic migration results, showed that the sample with a cerium oxide content around 2 wt.%, present a zero point charge higher than that of the single oxides. Also, a maximum in the BET area, pore volume and XRD intensity of TiOzanatase peaks was observed for this sample. Taking into account these results, Rh/TiOzCeO2 monolithic catalysts with different CeO2 content and silicate as binders, were prepared and tested in N20 decomposition reaction. A maximum in NzO conversion was achieved for catalysts with a CeOz content around 2 wt.% with respect to TiOz. The monolithic catalysts behaviour was related with the properties of TiOz-CeO2 powder samples and it is suggested that the formation of an new Ti-O-Ce phase could be responsible for the different properties of this system. 1. I N T R O D U C T I O N In recent years, NzO has been recognised as a contributor to the destruction of stratospheric ozone and as a greenhouse gas [1]. Combustion processes and chemical production such as adipic and nitric acid are among the major anthropogenic sources of NzO emissions [2]. For the year 2010, this emission should be reduced by around 60-90 % [3]. The catalytic decomposition of N20 to nitrogen and oxygen is the preferable technology for nitrous oxide elimination from stationary sources. For this purpose, although catalysts with different metals (Fe, Co, Ni) have been tested [4-7], those based on Rh have shown excellent results especially in the presence of oxygen and water in the gas mixture [8-10]. Although displaying good catalytic properties even at low temperature, at present there are no catalysts used industrially. Honeycomb catalysts have been extensively employed in industrial clean up processes because they offer attractive cost and operational advantages [11].

112 Thus, the main aim of this work has been to study the effect of CeO2 concentration on TiO2 properties, using powdered samples, to determine the optimum composition of Rh/TiO2-CeO2 monolithic catalysts. The studies carried out with powdered samples were critical to analyse the catalytic behaviour in the N20 decomposition reaction. 2. EXPERIMENTAL

2.1 Sample preparation

In order to determine the effect of CeO2 content on the titania properties, TiO2CeO2 powder samples (0< [CeO2]< 15 wt%) were prepared by incipient impregnation of TiO2 with Ce(NO3)3 and subsequently heat treated at 500~ in air. TiO2-CeO2 monolithic supports were manufactured by extrusion of doughs prepared by kneading TiO2-Ce(NO3)3 mixtures with natural silicates as permanent binders and water, varying the CeO2 content between 0-4 wt%. After heat treatment at 500~ in air, catalysts were prepared by impregnation of the formed monoliths with an aqueous solution of RhC13"H20 and heat treated at 450~ for 4 hours in NH3/N2 gas stream. The final Rh concentration was 0.15 wt%. All monolithic catalysts had the following geometric dimensions: square cell size 6.76 mm 2, wall thickness 1.0 mm, geometric surface area 8.02 cm 2 cm "3, and cell density 7.72 cells cm -2.

2.2 Characterisation techniques The cerium and titanium oxide contents were determined by inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer Optima 3300DV) of dissolution of the ground catalysts in acid solutions. The composition of the powder samples and monolithic supports are shown in Tables l a) and 2 respectively. The isoelectric points (IEP) of the individual materials and the zero point charge (ZPC) of the mixtures as defined by Parks [12], were determined by electrophoretic migration, measuring the zeta potentials as a function of the solution pH, as in previous studies [ 13] using a Zeta-Meter Inc. Instrument model 3.0+. Experiments were determined with 30 mg of approximately 2 ~tm diameter particles, suspended in 300 ml of 10.3 M KC1, adjusting the pH value with 0.2 M KOH and HC1 solutions. Each curve obtained was recorded at least twice to ensure reproducibility. Surface areas were measured by nitrogen ad/desorption with a Carlo Erba Sorptomatic 1800 on samples previously outgassed at 250~ overnight. The pore volume of monolithic catalysts were determined by mercury intrusion porosimetry (MIP) using CE Instruments Pascal 140/240 porosimeters, after drying the samples in an oven at 110~ overnight. The TG-DSC curves were measured on a Netzsch 409 EP Simultaneous Thermal Analysis device, using 20 mg of powdered sample. To analyse the effect of heat treatment of the samples, the experiments were carried out heating in air at a rate of 5 ~ rain -1, from ambient to 1000~ using c~-alumina as a reference. X-ray diffraction (XRD) patterns of ground samples were recorded on a Phillips PW 1710 powder diffractometer, using CuK~ radiation: (~, = 0.154 nm), between 20-80 ~ sampling data every 20 = 0.02 ~ with an accumulation time of 3 s. Wall sections of the monolithic support whith the best catalytic performance was analysed by Electron Probe Microanalysis by Wavelength Dispersion Spectroscopy

113 (EPMA-WDS) obtaining the line profile for Ti, Ce, Mg and Si. The axial crushing strength was measured on monolithic samples with a Chantillon LTCM dynamometer. The composition and properties of powdered samples and monolithic supports are collected in Tables 1 and 2. Table 1 a) Properties ofTiO2, CeO2 and TiCex powder samples Sample [CeO2] [TiO2] BET area ZPC I(XRD)TiO2 I(XRD)Ce02 wt.% wt.% m2 g~ pH 2 =25.0 2 =28.5 TiO2 0.0 100.0 67 6.5 97 TiCel 1.1 98.9 71 6.5 99 TiCe2 1.9 98.1 80 7.0 143 TiCe4 3.7 96.3 81 6.6 75 7 TiCel0 10.1 89.9 81 6.5 53 10 TiCel4 14.5 85.5 76 6.3 60 28 CeO2 4000(2 100.0 0.0 5.8 35 CeO2 500~ 100.0 0.0 2.0 40 CeO2 600~ 100.0 0.0 2.2 60 b) Pore volume of TiCex samples, obtained by N2 ad/desorption Sample Total pore volume* Mesopore volume, cm3 g-1 Average pore diameters cm3 g-1 narrow wide nm TiO2 0.28 0.16 0.12 13.7/17.3 TiCel 0.29 0.17 0.12 11.9/17.2 TiCe2 0.30 0.18 0.12 11.0/16.5 TiCe4 0.29 0.17 0.12 9.6/16.5 TiCel0 0.27 0.16 0.11 8.9/16.3 TiCe14 0.24 0.14 0.09 8.9/14.5 * calculated from volume of gas adsorbed at p/p0 = 0.96 (0-50 nm pore diameter) Table 2 Properties of monolithic supports treated at 500~ hours in air .atmosphere Sample [CeO2]* [TiO2] Pore volume** BET area Axial crushing strength wt.% wt.% cm3 g-1 m 2 g-1 MPa A 0.0 50 0.73 108 9.8 B 2.0 48 0.80 108 12.3 C 4.0 44 0.75 106 15.0 * respect to the TiO2 content. measured by mercury intrusion porosimetry (3 m-7.5 nm pore diameter)

2.3. Activity tests Catalytic activity measurement of Rh/TiO2-CeO2 monolithic catalysts for nitrous oxide decomposition was studied, using a fixed bed continuous tubular reactor, working in an integral regime close to isothermal axial profile. The inlet and outlet N20 concentrations were determined using an ADC Double Beam Lufi Type Infra-red Gas Analyser. The experiments were carried out at P = 120 kPa, T = 400-430~ GHSV(NTP) = 8000 h -l, VL =

114 0.91 m s~ using N20 (500 ppm) and N2 as gas balance. For these experiment monoliths of 5 cells and 17 cm length were used. 3. RESULTS AND DISCUSSION

3.1 TiO2-CeO2 powdered samples Fig. 1 shows the zeta potential v s . pH curves of titania (TiCe0), mixtures of TiO2CeO2 with a cerium oxide content around 2 wt% (TiCe2) and 14 wt% (TiCe14) as well as CeO2 treated at 400~ and 500~ in air. The IEP of titania and CeO2 as well as the ZPC of all TiCex samples are listed in Table 1a). These values were calculated by interpolation, if possible, and extrapolation, when the IEP was beyond the accessible pH range.

30-

2O

-10-20-

-30-

o

pH Fig. 1. Zeta potential as a function of the suspension pH at 25~ for samples of TiCex and CeO2 treated at different temperatures, kneaded in water. (11) TiCe0, (O) TiCez, (0) TiCel4, (-)~) CeO2 at 400~ (I-l) CeO2 at 500~ The IEP of TiO2 was 6.5, in accordance with previous results obtained in our group [ 14]. The CeO2 sample treated at 400~ had an IEP value of 5.8 whereas samples treated at 500~ and 600~ had similar values around 2.0 (Table 1a). These results indicate that IEP of CeO2 samples, markedly depends on the temperature treatment. This effect can be explained taking into account the TG-DSC results obtained for the TiCea4 sample before temperature treatment (Fig. 2). The weight loss observed in the temperature range 30400~ corresponds to the elimination of water and nitrates present in the sample after impregnation. According to the literature [15] also Ce203 was formed. At higher temperature, the TG curve shows a band centered at 700~ associated to an increase in the sample weight. This mass gain has been related to an oxidation process from Ce203 to Ce204. The same effect was observed after heat treatment at 500~ when maintained for 4 hours. Thus, the significant change in the ICP from 5.8 to 2.0 after heat treatment at 400~ hours and 500~ hours, respectively, can be related to the change in oxidation state of cerium (Ce +3 to Ce+4).

115 The zero point charge (ZPC) of mixtures TiO2-CeO2 should follow the equations: ZPC -- IEPce*Xce+ IEPTi* XTi Xce+ XTi "- 1

(1)

(2)

where IEPTi and IEPce are the isoelectric point of titania and ceria respectively, whereas Xce Xvi are the surface mole fraction of each component in the mixture. Thus, if the TiO2 surface was poorly covered by ceria, the ZPC of TiO2-CeO2 sample will be close to the IEP of TiO2 but if well covered, close to that of CeO2. 0

i t-

-15 0

200

400

600

800

101)0

Temperature, ~ Fig. 2. TG-DSC curves of TiCel4 sample previous calcination. Cerium impregnated over the titania surface, treated at 500~ in air atmosphere, could show two different behaviours. The cerium could form a superficial oxide similar to CeO2 (Ce +4) with an isoelectric point near 2.0 or the Ce may have a strong interaction with the TiO2 surface giving rise a new Ti-O-Ce species. The curve for TiO2-CeO2 sample with a cerium oxide content of 1.9 wt. % (Fig. 1), showed a ZPC = 7.0. This value was beyond the IEP of the pure oxides and thus suggested the formation of a new Ti-O-Ce species. The ZPC of TiCex mixtures are collected in Table 1. The data pass through a maximum for TiCe2 sample, and at higher CeO2 content the ZPC decreased towards the IEP of CeO2. The creation of a new species with different properties than the single oxides, have been proposed in the literature for Ni/A1203 and Co/A1203 at low metal content [ 16]. In both of these systems, the presence of NiO and COO3, have been only detected for contents above 2-4 wt%, respectively. The CeO2 crystal phase was first detected for TiCe4 and the peak intensities increased with CeO2 concentration. The data of the peak intensity at 20 = 25.0 (TiO2) and 20 = 28.5 (CeO2) are summarised in Table 1. The data reflect a maximum in titania crystallinity for sample TiCe2. Above this value, the crystallinity related to CeO2 start to increase due to the formation of a cerium oxide phase. On careful examination of TiCe2 XRD (Fig. 3), a shift of the anatase peaks may be detected. This effect could be related

116 with the inclusion of the cerium in the titania structure modifying its characteristic diffraction pattern. The textural nature of the titania and TiCex were characterised by nitrogen ad/desorption isotherms. The specific surface areas are presented in Table l a. None of the materials were found to be microporous from t-plot analyses of the adsorption isotherms. From the desorption curves, the mesopore size distribution was calculated using the BJH method. All of the samples had bimodal mesopore size distributions. The volumes of the narrow and wide mesopores, presented in Table lb), were calculated from the minima between the two distributions. These results indicated, that as the amount of ceria incorporation rose, the bimodal mesopore size distribution became narrower. For the titania sample the mesopores were centred in diameters of approximately 14 and 17 nm. With ceria incorporation both of these diameters were reduced until at the highest ceria content they were approximately 9 and 15 nm. Fig. 3. X-ray diffraction patterns for TiCex and pure CeO2, treated at 500~

i

Ceq

.[

TiCe-14 TiCqo TiC& TiCe~ TiO2

The XRD of TiCex samples and CeO2 are presented in Fig. 3. The TiO2 had the characteristic peaks of TiO2-anatase (ASTM 21-1272). The cerium oxide presented typical peaks at 20 = 28.5 and 33.2 assigned to cerinaite (ASTM 43-1002). The XRD patterns of TiCex samples showed these two phases, depending on the CeO2 content.

2o a'o ;i6 g6 6o 29:6

Analysis of these results showed that the incorporation of 1.0 wt% ceria into the titania support led to a 5% increase in the specific surface area. With incorporation of 1.9-10.1 wt% ceria the surface area was 20% higher than the original titania. But, at 14.5 wt% ceria the surface area began to fall, although still 14% higher than the non-doped support. The mesopore volume rose to a maximum at a ceria content of 1.9 wt% and then steadily fell as the ceria content was further increased. This behaviour suggested, that after the new species had been produced reaching a limit at 1.9% ceria, a pore filling mechanism was more pronounced, causing the reduction in the mesopore volume. Division of the mesopore range into the two fractions (wide and narrow, Table 1b) showed that the volume of the wider mesopore remained practically constant up to 3.7% ceria content, and then fell. However, the narrow mesopore volume increased with up to 1.9% ceria incorporation and then steadily decreased. These results suggested that with ceria incorporation a new species was formed that increased both the surface area and the pore volume. As more ceria was incorporated the surface area was maintained but pore narrowing due to deposition in the mesoporous network led to a reduction in the overall pore volume. Finally, at the highest levels of ceria incorporation the surface area also began to fall due to the formation of CeO2.

117

3.2

Monolithic systems

Taking into account the results with the powdered samples, monolithic supports were manufactured, with CeO2 contents between 0-4 wt%. The silicate binders were necessary for the conformation of these structures [17]. The axial profile and mapping obtained by EPMAWDS microscopy revealed that Ti and Ce concentration deviation over the wall cross-section were parallel and contrary to Si and Mg, principal components of the binders. This result indicated that cerium was selectively deposited on titania particles and not on the silicates. The CeO2 content added to titania and confirmed with ICP spectroscopy, total pore volume and BET area for the three monolithic supports, calcined at 500~ for 4 hours are collated in Table 2. For the monolithic samples the presence of the inorganic binder tended to mask the differences between samples due to its high surface area. However, analysis of the pore size distribution from the nitrogen isotherm of the material with no ceria, showed the characteristic diameters for the titania at 14 and 17 nm and a further diameter at about 48 nm associated with the binders. With ceria incorporation the pores at about 48 nm were unchanged but the narrower pores due to the titania were reduced to 9 and 17 nm then to 9 and 15 nm at 2 and 4% respectively. These changes in the pore size distribution of the monolithic samples suggested that the ceria was associated with the titania and not with the binders, in agreement with the results from EMPA-WDS line profiles. 80

Fig. 4. N20 conversion as a function of the cerium oxide content for Rh-monolithic catalysts at (A) 400~ and (O) 430~ Operation conditions: P = 120 kPa, GHSV (NTP)= 8000 h~, VL= 0.91 m s-1. Feed composition: [N20] = 500 ppm and [N2] = balance.

o~ =- 60 0

>

c

8

0

Z"

40 A j

20

'

0

I

1

'

I

2

'

I

3

'

I

4

[Ce02], wt.%

Finally, the effect of the cerium oxide content on the catalytic activity of rhodium catalysts was analysed for N20 decomposition reaction. The rhodium content was kept constant for all samples. The activity curves are represented in Fig. 4 at 400~ and 430~ Data were acquired after 2 hours of reaction to ensure steady state conditions. The systems present catalytic activity at temperature above 300~ Catalysts "A" prepared without ceria, displayed a good catalytic performance for N20 elimination even at 400~ reaching at 430~ a N20 conversion of 55 %. Nevertheless, a maximum was achieved for catalysts with a CeO2 content of 2 wt% with a conversion of 70%. These behaviour can be related with the result obtained with TiCex powder samples. The proposed new specie (Ti-O-Ce) formed when the cerium oxide content was around 2 wt%, could be responsible for the better performance of

118 the monolithic catalyst "B", compared with the others. This phase could increase the rhodium dispersion on the catalysts, and favour the reduced metal state as has been proposed previously in the literature for other Rh catalysts [18]. The reduction in the catalytic activity observed at CeO2 content above 2.0 wt.% (catalyst "C") can be connected with the detection of cerium oxide crystal phase observed for TiCe4 powder sample (Fig. 3) with approximately the same ceria content. The results indicated that the presence of CeO2 crystals reduce the catalytic activity for the N20 decomposition reaction. 4. CONCLUSIONS The influence of CeO2 content on titania samples prepared by wet impregnation have been studied by different techniques, in order to prepare active monolithic catalysts for N20 decomposition reaction for industrial application. The CeO2 formation depends closely on the temperature treatment of sample and affect strongly the zero point charge. From the results obtained we can conclude that the addition of a ceria concentration around 2 wt.% lead to an new Ti-O-Ce phase, that contribute to the better performance of the Rh/TiO2 monolithic catalysts for the N20 decomposition reaction. The presence of CeO2 crystal phase is responsible for a decrease in the catalytic activity. The catalysts tested in this study are the first monolithic systems developed for N20 abatement. These systems can be a way to reduce the N20 emission for industrial processes with relatively low installation cost and lower perturbation in the production process. ACKNOWLEDGEMENTS

We are grateful for the financial support from CICYT (Spain) Project MAT2000-0080-P4-02, the CYTED program (R.T.V.C.) and CSIC-USACH agreement (Project: 99CL0022). REFERENCES

1. F. Kapteinj, J. Rodriguez-Mirasol, and J.A. Moulijn, Appl. Catal. B., 9 (1996) 25. 2. S. Imamura, J. Tadani, Y. Saito, Y. Okamoto, H. Jindai and C. Kaito. Appl. Catal. A. 201 (2000) 121. 3. G. Centi, A. Galli, B. Montanari, S. Perathoner and A. Vaccari, Catal. Today, 35 (1997) 113. 4. H.C. Zeng and X.Y Pang. Appl. Catal. B., 13 (1997) 113. 5. G. Centi and F. Vanazza. Catal. Today, 53 (1999) 683. 6. R. S. Drago, K. Jurczyk and N. Kob. Appl. Catal. B., 13 (1997) 69. 7. Z.H. Zhu, S. Wang, G.Q. Lu and D.K. Zhang. Catal. Today, 53 (1999) 669. 8. G. Centi, L. DallOlio and S. Perathoner, Appl. Catal. A., 194 (2000) 79. 9. K. Yuzaki, T. Yarimizu, K. Aoyagi, Shin-Ichi Ito and K. Kunimori. Catal. Today, 45 (1998) 129. 10. G.Centi, L. Dall'Olio, S. Perathoner, Catal. Lett., 67 (2000) 107. 11. S. Irandoust and B. Andersson, Catal. Rev. Sci. Eng. 30 (1988) 341. 12. G.A. Parks, Chem. Rev., 65 (1965) 177. 13. F. J. Gil-Llambias and A. M. Escudey Castro, J. Chem. Comm., (1982) 478.

119 14. C. Knapp, F.J. Gil-Llambias, M. Gulppi-cabra, P. Avila and J. Blanco, J. Mater. Chem., 7 (8) (1997) 1641. 15. T. Arii, A. Kishi, M. Ogawa and Y. Sawada, Anal. Sci., 17 (2001) 875. 16. F.J. Gil-Llambias, A.M. Escudey-castro and J. Santos-Blanco, J. Catal. 83 (1983) 225. 17. J. Blanco, P. Avila, M. Yates and A. Bahamonde, Surf. Sci., (1995) 755. 18. F.L. Normand, L. Hilaire, K. Kili and G. Maire, J. Phys. Chem., 92 (1988) 2561.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Pt

121

combustion catalysts prepared from w/o mlcroemulslons

J. Ryme~ a, G. Ehret b, L. Hilaire c and K. Jir~tovfi a aInstitute of Chemical Process Fundamentals, 165 02 Praha 6, Rozvojovfi 135, Czech Republic, Tel. +420 2 20390295, E-mail: [email protected] blPCMS, UMR 7504, CNRS-Universite Louis Pasteur, Strasbourg, France CLMSPC-UMR 7515, CNRS-Universite Louis Pasteur, Strasbourg, France Platinum catalysts were prepared by impregnation of formed supports with reverse microemulsions (water-in-oil) or water containing chloroplatinic acid. Microemulsion catalysts were more active in combustion of toluene in toluene-air mixtures than those prepared classically from water solutions. The size of platinum in classically impregnated catalysts was three times higher than that of the catalysts prepared from microemulsions. In the case of microemulsion preparation method, platinum is located near the pellet surface or its position in the pellet can be optimised. The viscosity of the microemulsion affects the concentration profile of platinum in the catalysts. 1. I N T R O D U C T I O N Microemulsions are thermodynamically stable isotropic dispersions of oil in water (o/w) or of water in oil (w/o) containing domains of nanometer dimensions stabilized by an interracial film of surfactant(s). The most typical oil phases are alkanes, the choice of cyclic or aromatic hydrocarbon being dependent on further application. Extensive study of microemulsions has been stimulated by their great potential for practical applications in different fields [1,2] such as pharmaceuticals, cosmetics, enhanced oil recovery and material science (catalysts, semiconductors, etc.). A variant of the microemulsion method was first applied in the preparation of hydrogenation catalysts [3]. The principle of this and following studies consists in the preparation of solid particles in water droplets of w/o microemulsions. The resulting mixture is used a) after separation as a catalyst [3,4] or b) as an initial suspension for further catalyst preparation [5].

The authors (J. R. and K. J.) thank the Grant Agency of the Academy of Sciences of the Czech Republic for the financial support (Grant No. A4072904/1999).

122 Our approach to the preparation of supported combustion catalysts is different: We applied w/o microemulsion containing water-soluble Pt compound directly on a formed catalytic support of defined shape. As the complete characterisation of catalysts with low concentration of platinum is often difficult, we prepared a set of catalysts with 0.3 wt. % Pt. But in addition, we prepared another set of catalysts with 0.1 wt. % Pt. Such catalysts are especially interesting for combustion of VOC, because the price of the catalysts is not so high. This contribution describes some physical properties of reverse w/o microemulsions, physical-chemical properties of platinum catalysts prepared by classical impregnation from water solutions of HzPtCI6 and from reverse w/o microemulsions comprising chloroplatinic acid and their activity in combustion of volatile organic compounds (VOC). In our study we concentrated on Tween80 (polyoxiethylen(20)sorbitan monoleate), as the surfactant does not comprise any potentially harmful ions (like sodium in AOT or bromide in CTAB) that could affect the activity of platinum in catalytic combustion. 2. E X P E R I M E N T A L

Preparation of microemulsions. Microemulsions were prepared of oil (cyclohexanol, cyclohexane or heptane), of surfactant Tween 80 - polyoxiethylen(20)sorbitan monoleate with M=1309.68 and, for comparison, of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) or CTAB (cetyltrimethyl ammonium bromide) and water. The molar ratio R of water and surfactant moved in the range 2 7 0 - 1110. Catalyst preparation. Chloroplatinic acid, in such amounts as to obtain the desired concentration of Pt in the catalysts, was added to previously prepared w/o microemulsions. Two types of catalytic supports, ~,-alumina in the form of full pellets (diameter 5 mm) and 0-alumina in the form of hollow pellets (diameter 5 mm, hole 2 mm), both manufactured in Chemopetrol Litvinov, Czech Republic, were chosen for catalysts preparation. After calcination at 500 ~ q,-alumina had SBET=166 mZ/g, pore volume 0.459 ml/g, mean pore radius 7.0 nm and water absorption capacity 40 %. After calcination at 900 ~ 0-alumina showed SBET=145 mZ/g, pore volume 0.445 ml/g, mean pore radius 8.1 nm and water absorption capacity 50 %. The catalysts with 0.3 and 0.1 wt. % Pt were prepared by impregnation with HzPtCI6 water solutions (denoted as I) or Pt microemulsions (denoted as M). The catalysts were dried 2 h at 120-160~ and calcined 2 h in air at 550 ~

Characterization of microemulsions The viscosity of microemulsions was measured with a Rheotest 2 (Medingen, Germany) instrument in the range of shear rates 5 - 1800 s -1.

Characterization of the catalysts Surface areas of the supports were determined from desorption isotherms of nitrogen adsorbed at -198 ~ after evacuation at 350 ~ for 5 h on ASAP 2010, Micromeritics, USA. Pore size distributions were obtained from mercury porosimeter working in the range 0.1400 MPa (AutoPore III, Micromeritics, USA). Size of platinum particles. High-resolution transmission electron microscopy (HRTEM) was performed to determine the size of Pt particles supported on alumina and to check the

123 dispersion of the platinum particles. HRTEM studies were performed on a TOPCON EM002B apparatus with a top entry device and operating at 160 kV (resolution 0.5 nm). Distribution of Pt throughout the catalyst pellet. Electron microprobe Jeol JXA-50A equipped with an energy dispersive X-ray spectrometer EDAX TV 9400 was used for the determination of platinum concentration as a function of position in a pellet. Catalytic experiments were done with 75 ml of a catalyst and F/W=10 000 m3/h/kgcat. Combustion of toluene was chosen as a model volatile organic compound and non-steadystate reaction condition (linear increase 3.5 ~ in reaction temperature) was applied in the catalyst activity evaluation. Concentration of toluene in air was 1 g / m 3. Temperatures of gaseous reaction mixture entering and leaving the catalyst layer were measured by thermocouples. Catalytic activities expressed as the inlet temperatures Ts0 or T90, at which 50 or 90 % conversions of toluene were achieved, were taken as a measure of the catalytic activity. 3. RESULTS AND D I S C U S S I O N

3.1. Viscosity of microemulsions W/o microemulsions can be either Newton or non-Newton liquids depending on many factors, such as the type of surfactant, oil, the amount of water etc. In our case of catalyst preparation the viscosity plays an important role and so we decided to characterize the flow properties of microemulsions we have prepared from non-polar oils (cyclohexane, heptane) and polar oil (cyclohexanol). The viscosity of microemulsions prepared from nonpolar oils exhibited the typical behavior of non-Newton liquids: With increasing shear rate the viscosity was sharply decreasing (Fig. 1, left). In contrast with that, the viscosity of microemulsions prepared from polar oil, cyclohexanol, showed the properties of nearly Newton liquids: With increasing shear rate the viscosity practically was not changing (Fig. 1, right). When comparing the viscosity of both microemulsion systems at the same conditions (3,=10 s -1 and 1000 s-l), we obtained data shown in Table 1. The viscosity of microemulsions prepared from cyclohexanol, especially at low shear rates, is substantially lower than those prepared from heptane. In the case of microemulsions prepared from cyclohexanol a variation in water amount did not show great effect on the viscosity value. On the contrary, decreasing the amount of water in microemulsions prepared from heptane lead to microemulsions of very viscous, gel consistence. The stability of such a type of preparation was limited in time. Table 1 Properties of microemulsions obtained at 25 ~ (oil: COL - cyclohexanol, H-heptane) Oil 78.5 COL 85.0 COL 94.5 H 89.3 H 83.8 H 77.8 H

Wt. % Tween80 1,4 0,7 0,4 0,7 1,1 1,5

R Water 20 14,3 5,1 10,0 15,1 20,7

1007 1516 927 1039 998 1003

Viscosity, mPa.s At 1000 s -1 At 3,=10 s -1 16 17,8 13 14 12 660 61 330 36 170 25

124

1:~:)

1~ C~

~ ~

5~ H~O IO%HzO

9 14.3% H20 9 20.0%H20

23

10

O0

'

600

'

1~}0

'

1800

Fig. 1. Dependence of the microemulsions viscosity on shear rate T, S-1 (left-CyclohexaneTween80-water microemulsions with R = 923-1039, r i g h t - Cyclohexanol-Tween80-water microemulsions with R -- 1007-1516 and various amount of water) 3.2. Concentration profile of Pt in the catalysts Adding chloroplatinic acid to the w/o microemulsions prepared resulted in an impregnation solution useful for the preparation of catalysts with 0.3 wt. % Pt. 0-Alumina in the form of hollow pellets is a good example for studying the effect of viscosity on Pt concentration profiles (Fig. 2). In the upper part of the figure concentration profiles of the catalysts prepared from the solution of low-viscous microemulsions (cyclohexanol-Tween80-water) and H2PtCI6, measured in the half of the pellets, are shown. With all catalysts the Pt concentration determined on the outer and inner part of the hollow pellets was almost the same. The lower part of the figure documents much higher concentration of Pt on the outer part of the pellets, when microemulsions of high viscosity (heptane or cyclohexane-Tween80-water) were applied in the catalyst preparation (3-4 times higher than in the case of impregnation by water solution of H2PtC16). Obviously, high viscosity of the impregnation solution does not allow a sufficiently fast penetration into the central part of the hollow pellets. Pt catalysts prepared over full pellets of T-alumina showed very non-uniform Pt concentration not depending on the quality of microemulsion, i.e. whether w/o microemulsions were of low or high viscosity (Fig. 3). Highest Pt concentration on the outer part of the pellet was observed with the catalyst (M33) that was prepared from very viscous w/o microemulsion. A lower viscosity of the impregnation solution did not lead to such steep change in the Pt concentration profile of the pellet (M02): Compared to previously mentioned catalyst, the concentration of platinum on the surface of this pellet was lower, but was detected more deeply inside the pellet. A lower viscosity of the impregnation solution did not hinder entering of the solution into the center of the pellet so much as in the case of high-viscous solutions. An extreme case is the catalyst (I01) prepared from water solution of HzPtCI6. The catalyst showed an almost uniform Pt concentration profile throughout the pellet.

125

4

M05

4.

1-

M02

1. 4.

1-

-o9_-e,~ ~,s o,o..Q3 o& o,e

g

Radial c~rdinate r/R

Fig. 2 Concentration profiles from microemulsions (M24, M21, HzPtCI6 (103) over 0-alumina. The viscosity of impregnation solutions,

of platinum in hollow pellets of the catalysts prepared M21, M19, M17, M25) and from water solution of total concentration of Pt is 0.3 wt. %. Upper part: low lower part: high viscosity

Z5

2,5

i0~ ~/~/~~

.

2

!

1,5

L

1,0

t

E-..9. o,5 o,c

M25

1,5. 1,o 0,5 o,o

.•,•/

0,9

-0,6 -0,3

. . . .

0,0

0,3

0,6

i

0,9

Radial c c ~ m a t e i R Fig. 3 Concentration profiles of platinum in full pellets of catalysts prepared from microemulsions (M05, M02, M33) and from water solution of HzPtC16 over 7-alumina (I01). Total concentration of Pt is 0.3 wt. %.

9

126

3.3. Size of platinum particles Transmission electron microscopy measurements revealed that the typical size of platinum particles in the calcined catalysts differ very much depending on the way the catalyst was prepared. The microemulsion catalysts exhibited similar and relatively small platinum particles with diameter (arithmetic mean) from 5 to 55 nm. In contrast with that, the I02 catalyst impregnated by classical method showed much larger Pt particles. An example of histograms of Pt particles observed in various catalysts is given in Fig. 4. The microemulsion catalysts have narrower particle size distribution located between 4 - 90 nm, while the catalysts impregnated by water solutions of chloroplatinic acid have much broader particle size distribution reaching diameters up to around 300 nm. The platinum particle sizes found in our calcined oxidation catalysts are much larger than those observed [6] in the reduced microemulsion catalysts moving in the range 0.5 3.5 nm. Calcination in air of platinum catalysts including microemulsion catalysts apparently evolves a growth of primary platinum particles by a sintering process. if)

M08

M32

4O 33 29

~O

1'o " ~ / / // // //

O

" 3o " ~

" ~

0

1'o " ~o " 3o " do "

M06

" J0" dO" ~0" ~0" f0" ~0" Platinum particle size, n m

I02

.

.

.

.

.

~XDo

Platinum particle size, nm

Fig. 4 Histograms of platinum particles determined by TEM in the catalysts prepared from microemulsions (0.1% Pt)

3.4. Catalyst activity in toluene combustion Table 2 comprises important details concerning microemulsions preparation, characterisation of calcined catalysts and their activity in combustion of toluene expressed

127 as temperatures T50 and T90. The Pt catalysts (0.1 wt. %) prepared over 0-alumina using w/o microemulsions of lower viscosity (e.g. M22) combusted toluene at much lower temperatures (T50_--150~ than the catalyst (I03) with even higher Pt concentration (0.3 wt.%) prepared by conventional impregnation method from water solution of H2PtCI6 (T50_--183~ Microemulsions of higher viscosity lead to the catalyst (M20) with high concentration of platinum on the outer surface of the pellets, but its catalytic activity was somewhat lower than that of comparable microemulsions catalysts. Substantially lower catalytic activity showed the M32 catalyst prepared from CTAB instead of Tween80. As documented in Table 2, the size of platinum particles in the calcined catalyst is five times smaller, and this parameter could be decisive. Table 2 Survey of microemulsion composition (COL-cyclohexanol, CAN-cyclohexane, Hheptane), concentration of surfactant in microemulsion Csu~f, molar ratio R of water and surfactant, Pt concentration in water droplets, nominal concentration of Pt in the catalysts, size of platinum particles dpt (arithmetic mean), and catalytic activity expressed as Ts0 and T90 No.

Oil

Tween80 wt. %

Water wt. %

1.5 1.5 a 1.5 0

20.7 20.7 20.7 100

Csurf

R

~tmol/1

Pt wt. %

Pt mmol/1

dpt nm

T50 ~

T90 ~

37 40 37 27.4

55 9.4 53.3 46.7

153 208 160 183

167 235 172 202

45.9 45.9 9.8 128.5 138.1

33.3 5.7 107.4 33.8 8.3

156 205 209 141 153

172 228 237 155 167

O-Alumina, hollow pellets M22 M32 M20 I03

COL CAN H

10.8 9.5 8.3 0

1011 279 1011 ~

0.1 0.1 0.1 0.3

y-Alumina, full pellets M06 CAN M08 b CAN I02 M05 CAN M33 H aCTAB, bdried

1.4 18.6 1.4 20.6 0 100 1.4 18.6 1.5 20.6 at 200 ~ only

8.6 8.6 8.8 8.6 0

982 1064 ~ 982 982

0.1 0.1 0.1 0.3 0.3

A set of the catalysts with 0.1 and 0.3 wt. % Pt prepared from microemulsions of similar viscosity over 7-alumina (full pellets) showed a great variety of Ts0 (140 to 205 ~ whereas the catalyst prepared by classical method of impregnation showed the lowest activity of all (T50=209~ Comparing the catalysts of the same Pt concentration, we can discuss the effect of Pt concentration in water droplets: The lowest calculated concentration of chloroplatinic acid was found for the solution used during classical impregnation of the support. Low concentration of chloroplatinic acid in water, i.e. low extent of saturation [7], can lead to the growth of Pt particles, which is documented by TEM measurements. The I02 catalyst with the lowest Pt concentration in the impregnation solution showed the highest size of platinum particles (107 nm), whereas the Pt size of the catalyst from microemulsions moved within limit of 5 - 27 nm.

128 More precise effect of platinum particle size on the activity of catalysts having 0.1 wt% Pt can be seen from Fig. 5. The figure demonstrates that the highest activity of the catalysts was observed with those who had size of platinum particles (arithmetic mean) around 4 0 - 50 nm. This statement is supported by our following observation: The M08

Z20

s 0

~ 180

-

1%

MO6A

.... ~ '

Mgg A~:)" 60' I~0 ' 1~:)' lED C~, I'I1TI

Fig. 5 Dependence of the activity of the catalysts prepared from water solution of chloroplatinic acid and from Pt microemulsions in catalytic combustion of toluene on the size of Pt particles catalyst, only dried, showed very low catalytic activity and had the smallest Pt particles (4.4 nm). After repeating the catalytic test with the same catalyst (i.e. the catalyst was in contact with air at 400 ~ its catalytic activity remarkably increased (T50=160 ~ Evidently, the platinum particles during the catalytic test (calcination in air) increased their size. So we can state that the optimal size of platinum in the supported catalysts intended for combustion of toluene should move within the limits 40 - 50 nm. 4. C O N C L U S I O N Microemulsion impregnation of a catalytic support can be a very good method for preparation of heterogeneous catalysts used in reactions with mass transfer limitation, for which high concentration of active metal in the outer shell of catalyst pellets and narrow metal size distribution is important. REFERENCES

1. M.P. Pileni, Langmuir 13 (1997) 3266. 2. K. L. Mittal and P. Kumar, Handbook of Microemulsion, Science and Technology, Marcel Dekker, New York, 1999.

129 3. M. Boutonnet Kizling, C. Bigey and R. Touroude, Appl. Catal. A: Gen., 135 (1996) L13-L17. 4. A. Claerbout and J. B.Nagy in Preparation of Catalysts V, p. 705. (G. Poncelet, P. A. Jacobs, B. Delmon, Eds.), Elsevier Science Publishers, Amsterdam, 1991. 5. H. H. Ingelsten, K. Berquist, J.-Ch. Beziat, A. Palmquist, M. Skoglundh and K. Holmberg, Europacat-V, 2-7 September 2001, Limerick, Ireland. Abstract 21-O-16. 6. M. Boutonnet, J. Kizling, R. Touroude, G. Maire and P. Stenius, Catal. Lett. 9 (1991) 347. 7. B.H. Robinson, A.N. Khan-Lodhi and T. Towey in Structure and Reactivity in Reverse Mice|les, Elsevier, Amsterdam, 1989, Vol. 65, p. 198.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

131

Preparation of stable catalysts for N20 decomposition under industrial conditions S. Alini a, F. Basile b*, A. Bologna a, T. Montanari b and A. Vaccari b a

Radici Chimica S.p.A. via Fauser, 50, 28100 Novara (Italy)

b Dipartimento di Chimica Industriale e dei Materiali, Universitfi degli Studi di Bologna, Viale Risorgimento, 4, 40136 Bologna (Italy) - E-mail: [email protected] The key parameters in the preparation of active and stable catalysts for N20 decomposition under conditions of industrial interest were examined using mixed oxides obtained form hydrotalcite-type (HT) precursors. The relationship between activity and composition of the catalysts was studied, with attention focused on the nature and amount of metal ions present in the HT precursor (Rh/Mg/A1 or Rh/Co/A1). An activation procedure was developed which makes it possible to prepare catalysts, active and stable even under the severe reaction conditions of industrial interest. The relationship between active phase and composition of the bulk matrix was also investigated. Moreover, the flexible structure of the HT precursor made it possible to investigate the use of other active elements (Pd or La) to develop synergetic effects, without any interactions due to side phase segregation, or lack of homogeneity. 1. I N T R O D U C T I O N Nitrous oxide, N20, exercises significant negative effects on the environment, being a relatively strong greenhouse gas and one of the species contributing to the depletion of ozone in the stratosphere [1-4]. The estimated human contribution to N20 emissions to the atmosphere amounts to 4.7-7.0"106 t/a, about 30-40% of the total emissions including natural sources [2,5]. However, the only emissions that can be reduced in the short term are those from energy-related and chemical processes [6-8], with the reasonable target of achieving about 60-90% reduction by 2010 [5]. Primary chemical sources of N20 are the production of nitric and adipic acids. For example, in the latter case for each molecule of adipic acid one molecule of N20 is produced, yielding high concentration levels of nitrous oxide in the outlet stream. Much effort is currently being made to reduce N20 emissions from industrial adipic acid plants (outlet gas stream: 20-25% N20, 5%02, 1-2% H20, 3% CO2 and N2 the remainder) [6-9], for which the high concentration of pollutant and the presence of 02 and H20 have dramatic effects on catalyst activity and stability. Hydrotalcite-type (HT) compounds, having the general formula [MIIl _xMIIIx(OH) 2]x+ (An-)x/n " mHzO, have been reported as useful precursors of catalysts to be employed under severe reaction conditions [10-12]. Previous works [8,11-17] reported the use of several mixed oxides obtained by calcination of HT precursors in N20 decomposition, although

132 they were mainly tested in diluted gas streams and in the absence of O2 and/or H20. The aim of this study on HT precursors was to shed light on the main parameters involved in the development of active and stable catalysts, able to decompose N20 under conditions of industrial interest. 2. E X P E R I M E N T A L SECTION The catalysts (compositions as in Table 1) were obtained by calcination at 650 ~ for 12 h of HT precursors prepared by coprecipitation at 60~ and constant pH (10.0+0.1). A solution ([Mn]+[M III] = 1M)containing the nitrates of the elements in the right atomic ratio was added to an excess of 1 M solution of Na2CO3 (CO32-/MnI = 3); the pH was maintained constant by dropwise NaOH addition. In the case of La-containing precursors to avoid the precipitation of La2(CO3)3, the synthesis was carried out under N2 flow and using NaNO3 instead of Na2CO3. The precipitates were kept in suspension under stirring at 60~ for 40 min., then filtered and washed until a Na20 content lower that 0.02 wt% was obtained. The precipitates were dried overnight at 90~ and calcined for 12 h at 650~ Table 1. Composition, B.E.T. surface area and phases identified by XRD for the samples dried overnight at 90~ or calcined at 650~ for 12 h. Composition

Atomic ratio

(%)

B.E.T. surface area (m2/g)

XRD phase composition

Dried Calcined Dried Calcined Co/A1 71.0:29.0 108 46 HT SP Rh/Co/AI 0.5:74.5:25.0 116 56 HT SP Rh/Mg/A1 1.0:71.0:28.0 110 156 HT M.O. Rh/Mg/A1 0.5:71.0:28.5 130 136 HT M.O. Rh/Mg/AI 1.0:80.0:19.0 8 122 HT M.O. Rh/Pd/Mg/AI 0.5:0.5:70.0:28.0 94 132 HT M.O. + PdO Rh/Pd/Mg/AI 0.5:1.0:70.0:28.5 98 140 HT M.O. + PdO Rh/La/Mg/AI 1.0:5.0:71.0:23.0 74 109 HT + La(OH)3 M.O + LazC05 HT= hydrotalcite-type phase. M.O. = MgO-type mixed oxide phase. SP = spinel-type phase The samples were characterised by XRD powder analyses using a Philips PW1050/81 diffractometer, equipped with a graphite monochromator in the diffracted beam and controlled by a PWl710 unit 0~ = 0.15418 nm, 40kV, 40mA). A 20 range from 10 ~ to 80 ~ was investigated at a scanning speed of 70~ The lattice constants were determined by least-square refinements, from the well-defined positions of the most intense peaks. The surface area was determined by N2 adsorption using a Carlo Erba Sorpty 1700. The temperature-programmed-reduction (TPR) of the catalysts was analysed in the range 80930~ with a heating rate of 15~ using a ThermoQuest TPDRO 1100, operating with a 50 ml/min flow of a Ha/Ar (4:96 v/v) gas mixture. The structure of the calcined samples was analysed by TEM using a TOPCON EM002K HRTEM and operating at 200

133 kV, after deposition on a carbon grid of a few drops of a suspension of a small amount of powder in ethanol. The catalytic tests were carried out using the samples calcined at 650~ as such or after reduction in a 160ml/min flow of Hz/Nz (5:95 v/v) at 750~ The catalytic tests were carried out in a lab-scale fixed bed reactor, operating at atmospheric pressure, in the range Tove, = 300-500~ and feeding a gas mixture = 20wt.% NzO, 5wt.% Oz, 2.6wt.% HzO, and He the remainder. A GHSV value = 50,000 h -1 was employed, significantly higher than the values reported in the literature (5,000-15,000 h -1) [11-17] to improve the economy of the process.

I :I ~

HT o* La(OH)3 + Mixed oxide ^ La2C05 A

t

+ /'~ A

~.

d

+

u.a.

.

o

o

.

b

.

a

10

20

30

40

50

60

70

~

80

Fig. 1. XRD powder patterns of the samples: (a and c) Rh/Mg/A1 (1.0/71.0/28.0, as atomic ratio %) and (b and d) Rh/La/Mg/A1 (1.0/5.0/71.0/23.0, as atomic ratio %) dried overnight at 90~ (a and b) or calcined for 12h at 650~ (c and d) 3. R E S U L T S AND DISCUSSION The composition of the dried samples determined by quantitative analysis (Table 1) is the same as that calculated on the basis of the composition of the solutions used for the preparation. With the exception of the La-containing sample, the XRD patterns of the dried samples show the presence only of a well crystallised HT phase. The absence of side phases for the Pd-containing sample is in agreement with that previously reported in the literature [18,19], even though the ionic radius of Pd z+ ions in octahedral coordination (0.100 nm) [20] is at the limit of the values suitable to prepare HT compounds [10]. For the Rh/Mg/AI HT phases, the c lattice parameter increases as a function of the Mg/(AI+Rh) atomic ratio, being respectively 23.23(6)* for the 71.0/29.0 ratio and 23,90(4)A for the 80.0/20.0 ratio, due to the decrease in the charge density in the cationic layers and the consequent decrease in the strength of interaction between anionic and cationic layers. Finally, the Rh/La/Mg/A1 sample shows the partial segregation of La(OH)3 (Fig. 1), while

134 the residual fraction of La 3+ ions inside the HT framework is responsible for the shift in the d003 diffraction peak towards lower angles (20= 11~ corresponding to a higher value of the c parameter. The samples calcined at 650~ show different XRD patterns as a function of the composition (Table 1). The M/Mg/AI samples (M= Rh or Pd) exhibit the presence of a cubic MgO-type mixed oxide phase with, furthermore, segregation of PdO for the two Pdcontaining samples. On the other hand, the two Co-containing samples show the presence of a spinel-type phase due to the oxidation of Co 2+ to Co 3+ ions during the calcination step [10,17,21]. Finally, the Rh/La/Mg/AI sample shows the formation of La2COs, together with a cubic MgO-type mixed oxide phase, confirming the high tendency of La 3+ ions to react with CO2 to form carbonates [22,23]. The reducibility of Rh in the cubic MgO-type mixed oxides was studied by TPR analysis. The Rh/Mg/AI (1.0/71.0/28.0 as atomic ratio %) sample shows an unresolved peak starting at 400~ attributable to the reduction of Rh 3+ ions to Rh ~ evidence of the presence of these ions in a crystalline-oxide matrix. Furthermore, the TEM analysis of this latter sample (Fig. 2) shows a homogeneous dispersion of the metallic Rh ~ particles on the Mg/Al-matrix surface, with the particle size distribution in the range between 1-3 nm. The catalysts derived from HT precursors provide information on the role of the type and amount of active metal, activation procedure and composition of the inert matrix, without any interaction due to side phase segregation or lack of homogeneity. Unlike that claimed previously by Fig. 2. TEM print of the reduced Rh/Mg/A1 Swamy and coworkers [13, 14], the catalyst (1.0/71.0/28.0, as atomic ratio %). Co/AI (75.0/25.0, as atomic ratio %) Zoom factor 1000Kx, 200kV. mixed oxide shows a very low activity, due to the severe reaction conditions and, mainly, the presence of water, in agreement with previous data for transition metals supported on cubic mixed oxides derived from HT precursors [15]. The addition of a small amount of Rh [i.e., the Rh/Co/AI (0.5/75.0/24.5, as atomic ratio %) catalyst] improves the catalytic performances significantly, giving higher conversions than those of the analogous Rh/Mg/A1 catalysts. Even so the level of conversion is still not satisfactory. Activation by reduction of the Rh-containing cubic mixed oxides changes the order of activity, since the performances of all the Rh/Mg/AI catalysts improved considerably in terms of both activity and stability (Fig. 3). The complete decomposition of N20 (residual amount lower than 50ppm) is reached using an oven temperature of 350-400~ as a

135 function of the Rh-content. On the contrary the Rh/Co/AI sample after an initial flash of activity, exhibits poorer performances, with only 77% of conversion at 500 ~ and quite poor stability. These data evidence the key role of the solubility of Rh 3§ ions in the cubic mixed oxides derived from HT precursors and the enhancement of activity due to the activation step with formation by reduction of well dispersed Rh particles, stabilised inside the Mg/A1 matrix. On the other hand, the activation of the Rh/Co/Al mixed oxide under H2 gives rise to reduction of Rh 3§ and Co3§ 2§ ions, with consequent destruction of the mixed oxide matrix. The reduced metal particles obtained are very active and produce an initial peak of activity, although the fast reoxidation of Co o and Rh ~ leads to rapid deactivation. Rh/Co/AI samples have been reported to have better performances in the presence of residual Na remaining in trace amounts after the washing step [14], thus the role of the basicity was also investigated for the Rh/Mg/A1 catalysts by varying the Mg/A1 atomic ratio in the HT precursors (1.0/71.0/28.0 and 1.0/80.0/19.0 as atomic ratio %, respectively), but maintaining the same structure. The two samples give comparable results in the catalytic decomposition of N20, evidencing that the basicity is not a key parameter in severe reaction conditions. More important to achieve high performances and stability is the unique interaction between Rh ~ particles and the Mg/Aloxide matrix. The activity and stability are strongly affected by the Fig. 3. Catalytic activity of reduced Rh/Co/Al and presence of significant Rh/Mg/Al catalysts with and without the presence of amounts of H20, since it 2.6wt.% H20 in the feed (oven temperature 450~ adsorbs preferentially on the active sites. Reduced Rh/Co/A1 and Rh/Mg/Al catalysts were tested also feeding 2.6 wt.% H20 in the gas stream [24]. When water is present (Fig. 3) in the feed, the activity of the Rh/Co/Al catalyst decreases 32% ca., while the Rh/Mg/A1 catalyst (1.0/71.0/28.0, as atomic ratio %) shows almost complete conversion (> 99.9%) at 450 ~ and the Rh/Mg/A1 catalyst (0.5/71.0/28.5, as atomic ratio %) reaches the same result only at 500~

136 The Rh/Mg/A1 catalyst (1.0/71.0/28.0, as atomic ratio %) was also tested on a larger scale and using an industrial flue stream. In the absence of H20 no decrease in activity was observed after more than 1,000h of time-on-stream, while in the presence of H20, the activity started to decrease after 400h ca. A careful characterisation carried out before and after reaction, evidences that this loss of activity is attributable to the partial surface reoxidation of the Rh ~ particles, without any structural modification. In agreement with these findings, further reduction of the catalyst recovered the initial activity [24], evidencing the high hydrothermal stability of the catalysts obtained from HT precursors and the reversibility of the deactivation process. Considering the strong exothermicity of the process, the hydrothermal stability is a key factor since H20 is always present. Water not only reduces the catalytic activity, but provides information on the role of the basicity of the support, since the catalyst with a higher Mg/AI ratio deactivates faster (Fig. 4). This behavioUr may be attributed to the Fig. 4. Deactivation with time-on-stream of reduced competition of H20 for the Rh/Mg/A1 catalysts with different Mg/A1 atomic ratios in sites responsible for O2 N20 decomposition in the presence of 2.6wt.% H20 removal from the metal (oven temperature 450~

Fig. 5. Catalytic activity as a function of the oven temperature of reduced Rh/Mg/A1 and Rh/Pd/Mg/A1 catalysts in the presence of 2.6wt.% H20

surface [25], that may be affected by the basicity, i.e., by the Mg/A1 ratio. The presence of a high amount of 02 on the Rh ~ surface induces its faster reoxidation, responsible for the deactivation with timeon- stream. Since the HT structure is flexible and allows the insertion of many different elements during the catalysts preparation, the possible development of

137 synergetic interactions between Rh and other elements was investigated. In particular, Pd has been previously claimed to enhance release of oxygen from the catalyst surface [16,22]. The catalytic tests carried out on the two reduced Rh/Pd/Mg/AI samples show catalytic results worse than those obtained using the reduced Rh/Mg/AI sample (0.5/71.0/28.5, as atomic ratio %) (Fig. 5). It may be hypothesised that the significant segregation of PdO occurring during the calcination step does not allow significant interdispersion of the elements, worsening the catalytic activity. Increasing the amount of Pd produces a further decrease in the activity at low temperature, confirming that the segregation of PdO inhibits the synergetic interaction between Rh ~ and Pd ~ A further attempt to generate synergetic effects between two i00 active elements inside the HT precursors, was carried out by the t - 80 insertion of La 3§ ions inside the HT O framework. The interest in the 60 preparation of La-containing tO u40 materials is .due to the fact that 0 La203 have been claimed as active - - + - Rh/Mg/AI 1171128 f,q Z 20 in N20 decomposition [26]. - , , - Rh/La/Mg/AI i/5/71123 Furthermore, La-containing catalysts obtained from HT 300 3so 4o0 450 500 precursors have already been Temperature (~ proposed as active materials [16,22,23], although the presence of Fig. 6. Catalytic activity as a function of the La 3§ ions inside the HT structure has oven temperature of the reduced Rh/Mg/AI and not been clearly demonstrated. The Rh/La/Mg/AI catalysts in the presence of insertion of La 3§ ions in the HT 2.6wt.% H20 framework is not favoured, due to the large atomic radius of La 3§ ions [20]. Furthermore, since in the preparation of La-containing HT phases it is necessary to avoid the formation of LaaCOs, the preparation was carried out under an N2 atmosphere using nitrates as the anionic species for the interlayer. The catalytic tests show a decrease in activity in comparison to the analogous sample without La (Fig. 6). The formation of La2CO5, which it is not active, partially hinders the active surface of the Rh ~ particles and their interaction with the support. It would be possible to avoid the formation of the carbonate by heating the HT precursors under N2, but this step is useless since consistent amounts of CO2 are always present in the industrial outlet streams and, therefore, the segregation and/or formation of La2CO5 would only be delayed. In conclusion, the preparation of HT precursors did not allow the interdispersion of the different active elements investigated to date and therefore it was not possible to obtain any indications of synergetic effects between Rh- and La- or Pd-containing phases. A

.

I

138 5. C O N C L U S I O N S The catalysts obtained from HT precursors have been claimed as active in the decomposition of N20, but the conditions and feeds used so far are almost ideal and thus do not provide indications for their application in conditions of industrial interest. Therefore, in the present work the decomposition of N20 using catalysts obtained from HT precursors was studied using conditions near to those of industrial interest: a concentrated stream, with high percentages of NzO, O2 and H20. In these conditions, the cubic mixed oxide Rh/Co/A1 is more active than the analogous Rh/Mg/A1, although neither have sufficient activity. The activation of the calcined samples by reduction, gives rise to a significant enhancement of the activity for the Rh/Mg/A1, with a total conversion at 400-450~ while the reduced Rh/Co/A1 suffers faster deactivation. The higher activity may be attributed to the formation of well dispersed Rh ~ particles (< 3 nm), stabilised inside an inert Mg/Al-oxide matrix. The presence of large amounts of H20 (3wt.% ca.) gives rise to deactivation due to the reoxidation of metallic Rh ~ particles, although this process is completely reversible by further heating the catalyst under H2. Catalytic tests carried out on a larger scale, using the outlet stream of an industrial adipic acid plant confirm the high activity of Rh-containing catalysts obtained from HT precursors and their stability with time-on-stream. The study of the Mg/AI ratio (i.e. of the basicity of the oxide matrix) did not evidence significant differences in activity in the absence of H20, while when water is present in the feed, increasing the Mg/A1 ratio increases the deactivation rate probably due to slower desorption of the 02 from the metal surface. The flexible structure of HT was used to promote synergetic effects between Rh and Pd or La. Pure HT precursors were obtained with Pd, although the insertion of Pd in the HT structure was not maintained after calcination, due to segregation of increasing amounts of PdO and corresponding decreases in the catalytic activity. The synthesis of a La-containing HT was carried out under N2 to avoid the formation of carbonates, although the La partially segregates as La(OH)3 due its large ionic radius. Calcination of the La-containing HT gave rise to the formation of La2COs, which decreases the catalytic activity by hindering the Rh ~ particles or their interaction with the Mg/Al-oxide matrix. In conclusion, Rh-containing catalysts obtained from HT precursors by calcination and further reduction are very promising for N20 decomposition under conditions of industrial interest (i.e., concentrated N20 stream, also containing 02 and H20). These catalysts exhibit high conversion and stability with time-on-stream. On the contrary, any attempt to improve their performances by addition of Pd or La was fruitless, due to their segregation during calcination, which does not allow the required interdispersion of the elements to be achieved and inhibits any synergetic interaction.

6. ACKNOWLEDGEMENTS The financial support from Radici Chimica acknowledged.

SpA

(Novara,

Italy)

is gratefully

139 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

H. Rodhe, Science 248 (1990)1217. M.A. Wojtowicz, J.R. Pels and A.J. Moulijn, Fuel Proc. Technol. 34 (1993) 1. J.C. Kramlich and W.P. Linak, Prog. Energy Combust. Sci. 20 (1994) 149. A.R. van Amstel and R.J. Steward, Fert. Res. 37 (1994) 213. C. Kroeze, Sci. Total Environ. 152 (1994) 189. E. De Soete, Rev. Inst. Fr. Petr. 48 (1993) 4123. G. Centi, S. Perathoner and F. Vazzana, CHEMTECH 12 (199) 48. F. Kapteijn, J. Rodriguez-Mirasol and J.A. Moulijn, Appl. Catal. B9 (1996) 25. M. Schmidt, H. Glatzel-Mattheier and H. Sartorius, J. Geophysical Res. 106 (2001) 5507. 10. F. Cavani, F. Trifir5 and A. Vaccari, Catal. Today 11 (1991) 173 and references therein. 11. A. Vaccari, Appl. Clay Sci. 14 (2000) 161 and refereces therein. 12. F. Basile and A. Vaccari, in "Layered Double Hydroxides: Present and Future" (V. Rives, Ed.), Nova Science, New York, 2001, p. 285 and references therein. 13. S. Kannan and C.S. Swamy, Appl. Catal. B3 (1994) 109. 14. C. S. Swamy, S. Kannan, Y. Li, J. N. Armor and T. A. Brymer, Eur. Pat. 640,379A1 (1994). 15. G. Centi, A. Galli, B. Montanari, S. Perathoner and A. Vaccari, Catal. Today 35 (1997) 113. 16. J. P6rez-Ramirez, J. Overeijnder, F. Kapteijn and J. A. Moulijn, Appl. Catal. B23 (1999) 59. 17. M.C. Romfin-Martinez, F. Kapteijn, D. Cazorla-Amor6s, J. P6rez-Ramirez and J.A. Moulijn, App. Catal. A225 (2002) 87. 18. F. Basile, G. Fornasari, M. Gazzano and A. Vaccari, Appl. Clay Sci. 16 (2000) 185. 19. F. Basile, G. Fornasari, M. Gazzano and A. Vaccari Appl. Clay Sci. 18 (2001) 51. 20. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. 21. S. Kannan, S. Velu, V. Ramkumar and C.S. Swamy, J. Mater. Sci. 30 (1995) 1462. 22. J. P6rez-Ramirez, F. Kapteijn and J.A. Moulijn, Catal. Letters 60 (1999) 133. 23. J.N. Armor, T.A. Braymer, T.S. Farris, Y. Li, F.P. Petrocelli, E.L. Weist, S. Kannan and C.S. Swamy, Appl. Catal. B7 (1996) 397. 24. S. Alini, A. Bologna, F. Basile, T. Montanari and A. Vaccari, Eur. Pat. 1,830,354A1 (2001). 25. A.L. Yakolev, G.M. Zhidomirov and R.A. van Santen, Catal. Letters, 75 (2001) 45. 26. X. Zhang, A.B. Wakers and M.A. Vannice, Appl. Catal. B4 (1994) 237.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

141

The Anderson-type heteropolyanions in the s y n t h e s i s of aluminaand zeolite-supported H D S o x i d i c p r e c u r s o r s E. Payen a, G. Plazenet a, C. Martin a, C. Lamonier a, J. Lynch b, V. Harl6 b aLaboratoire de Catalyse de Lille, USTL, Brit. C3, 59655 Villeneuve d'Ascq Cedex, France bInstitut Franqais du P6trole, l&4 Avenue de Bois-Pr6au, 92852 Rueil-Malmaison Cedex, France This study is aimed at using and identifying A1- and Co-containing Anderson-type heteropolyanions in the synthesis of (Co)Mo/alumina and (Co)Mo/zeolite HDS catalysts oxidic precursors. Various techniques such as Raman spectroscopy, NMR and EXAFS are used to show the apparition of the AlMo6024H63- entity upon impregnation of an alumina or a zeolite with an ammonium heptamolybdate solution. The question of the Mo-Co interaction in the promoted precursors is also tackled through the use of the CoMo6024H63entity. 1. I N T R O D U C T I O N With increasing social awareness of environmental risks, the will to reduce toxic emissions has increased and the quality of the fuels becomes a key factor. In particular, the legislation concerning the quantity of sulfur emission is increasingly severe, and limits are expected to go down to 50 ppm in Europe by 2005 and to 10 ppm in the following years. This imposes improvements in catalytic hydrotreatment (HDT) of the petroleum feedstocks in general, and in hydrodesulfurization (HDS) in particular. Catalyst improvement requires a better understanding of the structure and genesis of the active phases consisting of well dispersed molybdenum disulfide on a high specific surface area support. These nanocrystallites are generally promoted by cobalt or nickel atoms. The active phases are obtained through sulfidation of an oxidic precursor generally prepared by impregnation of the support with an aqueous solution of the elements to be deposited. The aim of the present study is to determine firstly the exact nature of the oxidic precursor in order to understand more clearly the formation of the active phases. Concerning the most common support, i. e. alumina, it has long been thought that the entities present in the impregnating solution remained intact upon adsorption on the alumina surface. However, recently, Carrier et al. [1-5] showed that some aluminium atoms of the support are extracted during the equilibrium impregnation to give the Anderson-type aluminomolybdate anion AlMo6Oa4H63- (AIMo6), whereas a Raman spectroscopic study [6,7] of the oxomolybdate phase of a Mo/AlzO3 prepared by incipient wetness impregnation revealed the presence of well dispersed aluminomolybdate entities interacting with the support, these entities being preserved during the drying step. In this paper, we will give further evidence of this phenomenon, and see whether it is possible to

142 impregnate directly the A1M06 entity. As the alternative support -zeolites- contain, in addition to framework atoms, variable amounts of amorphous alumina and AI 3+ ions, we will examine whether the same dissolution-precipitation phenomenon can be extended to them. We will also study the case of the Co-promoted compounds and work on the Mo-Co interaction thanks to the use of the Anderson-type cobaltomolybdate anion CoMo6024H63(CoM06). 2. E X P E R I M E N T A L 2.1. Precursors preparation and nomenclature The samples were prepared by incipient wetness impregnation. An aqueous solution of ammonium heptamolybdate (AHM), A1Mo6 or CoMo6 was prepared, its concentration depending on the desired Mo loading. The support was then impregnated with the appropriate volume of solution, depending on its porous volume. After the maturation step (2 h in ambient air), the sample was dried at 100~ overnight. The samples were analysed in the dried state, since the species formed upon impregnation are at stake here. The samples will be designated accordingly to the starting material used (AHM, A1Mo6, CoMo6...) and the support (A for alumina, Z for HY zeolite), separated with a slash. For instance, AHM/Z will identify the sample prepared by impregnation of a HY zeolite with an ammonium heptamolybdate solution. The porous volumes of both supports were 1 mL/g. The alumina had a specific surface area of 200 mZ/g, and the HY zeolite had bulk and framework Si/AI ratios of respectively 13.6 and 19, as determined respectively by Xray fluorescence and 29Si NMR.

2.2. Physical measurements The Raman spectra of the samples, maintained at room temperature, were recorded using a Raman microprobe (Infinity from Dilor), equipped with a photodiode array detector. The exciting light source was the 532 nm line of a Nd. YAG laser, and the wavenumber accuracy was 2 cm -1. 27A1 MAS NMR (magic angle spinning nuclear magnetic resonance) measurements of the samples were carried out on a Bruker AC400 spectrometer. Mo and Co K-edge EXAFS (Extended X-ray Absorption Fine Structure) measurements were carried out in the Laboratoire pour l'Utilisation du Rayonnement Electromagn6tique (Orsay) at the EXAFS D42 and D44 beamlines using synchrotron radiation from the DCI storage ring running at 1.85 GeV with an average current of 250 mA. At the Mo K-edge, the data were taken in the transmission mode through a double crystal monochromator Ge (400) using two ion chambers as detectors. The time measuring an X-ray absorption spectrum (19900-20900 eV) was about 15 rain, and three spectra for each sample were recorded. Due to their low loadings, some samples had to be analysed at the Co K-edge in the fluorescence mode through a channel cut monochromator Si (111) using a multielement detector. Six spectra for each sample were recorded. The Mo and Co K-edges EXAFS regions of the spectra were extracted and analysed using A. Michalowicz's software packages EXAFS 98 ppc and Round Midnight [8]. The EXAFS spectrum was first transformed from k-space (k 3, Kaiser window 3.6-15 ,~-1 for the Mo K-edge and 2.6-14 ~-1 for the Co K-edge) to R-space to obtain the radial distribution

143 function (RDF). The EXAFS spectrum for one or several co-ordination shells was isolated by inverse Fourier transform of the RDF over the appropriate region and fitted using the single scattering EXAFS equation with amplitude and phase functions calculated by FEFF [9-12] or extracted from experimental reference spectra. The UV (Ultra-Violet)-visible spectroscopy experiments were carried out on a Perkin Elmer spectrometer UV-VIS-PIR Lambda 19. 3. RESULTS AND DISCUSSION 3.1. Synthesis and characterisation of ammonium and exchanged salts of andersontype heteropolyanions 3.1.1. Starting materials (NH4)3AIMo6Oz4H6 was prepared according to Hall [13]: a solution of aluminium nitrate (20 mL, 3.1.10 .3 tool) is added to a boiling ammonium heptamolybdate solution (80 mL, 4.2.10 .3 mol). A white precipitate appears and the still warm solution is filtrated. The precipitate is recrystallised twice in water. (NH4)3CoMo6024H6 was prepared according to Nomiya et al. [14]: 2 g of 30% hydrogen peroxide are added to a cobalt sulfate solution (30 mL, 0.015 mol). This solution is slowly added to a boiling ammonium heptamolybdate solution (260 mL, 0.025 mol). A green precipitate appears, which is recrystallised twice in water.

225

947 898

546 566

CoM~

2 -coMo61

952 225

390

90~

6

........ S ~ k. 315

8

10

12

k (~-1)

14

(b)

C0M0/~

897, 919

i

i

i

i

190

490

790

1090

iii

i

1390 0

W o v e n u m b e r (cr~ 1) Fig. 1. Raman spectra of AIM06, CoM06, AIMo6/A, CoMo6/Z and CoMo/Z. *: lines of the zeolite.

1

2

3

R(A) o

4

5

6

Fig. 2. (a) Mo K-edge EXAFS of AIMo6 and CoMo6. (b) Corresponding Fourier transforms.

144 3.1.2. Characterisations The Raman spectra of the bulk AIMo6 and CoMo6 salts are presented in Fig. 1. They are characteristic of the Anderson heteropolyanions according to literature data [2]. A1Mo6 and CoMo6 differ by a shift of the two most intense lines, corresponding to the symmetric and anti-symmetric stretching modes of M o Q ( t ) groups (where t stands for terminal oxygen) observed respectively at 952 and 902 cm -1 for AIMo6, 947 and 898 cm -1 for CoMo6. Another difference concerns the vibrational mode of the bonds with the central heteroatom: a line at 575 cm -1 is to be seen for A1Mo6, whereas a doublet at 566 and 546 cm -1 is present for CoMo6. Finally, a shift of the deformation mode of the MoO2(t) group is also observed: 390 cm -1 for AIMo6, 380 cm -1 for CoMo6. The Infra-Red (IR) spectra (not reported here) exhibit the 1400 cm -1 band characteristic of the N-H bond, which shows the presence of the ammonium counterions. The Mo K-edge EXAFS spectra and their Fourier transforms are shown in Fig. 2. According to previous FEFF simulations [15-17], the first co-ordination shell (1-2.5 A, not phase corrected) corresponds to three types of oxygen neighbours, and the second one (2.53.5 A, not phase corrected) to the neighbouring molybdenum atoms and the central heteroatom. In Table 1 are presented the fitting parameters of these spectra, giving the numbers of neighbours and their distances. Table 1 Fitting parameters of the Mo K-edge EXAFS spectrum of the ammonium or exchanged salts of AIMo6 and CoMo6 Mo-O (1) Mo-O (2) Mo-O (3) Mo-Mo Mo-AI/Co N 2.0 2.0 2.0 2.2 1.0 AIMo6 Ao (A) 0.02 0.06 0.06 0.06 0.02 R (A) 1.72 1.92 2.34 3.33 3.46 AE (eV) 5.4 5.4 5.4 5.4 5.4 N 2.4 2.0 2.2 2.5 1.3 CoMo6 Ao (A) 0.06 0.08 0.08 0.09 0.05 R (A) 1.71 1.95 2.30 3.32 3.31 AE (eV) 1.1 1.1 1.1 1.1 1.1 3.2. Synthesis of non-promoted oxidic precursors 3.2.1. Zeolite-supported samples The appearance of A1Mo6 is observed upon impregnation of a HY zeolite with an AHM solution. The 27A1 MAS NMR spectra of three AHM/Z samples are presented in Fig. 3 and show the presence of AIMo6 whatever the Mo loading. The Mo K-edge EXAFS spectra show this species to be majority at 3 Mo wt.%. The Raman spectra (Fig. 4) indicate that at the intermediate and high loadings, the parallel apparition of AHM is observed, with the main line at 937 cm -1. The study of other HY zeolites containing more or less extraframework aluminium atoms showed that the precipitation of AHM is dependent on this quantity of non-framework A1 atoms. The first reaction to take place upon impregnation of a zeolite is the dissolution-precipitation phenomenon leading to A1Mo6 from solubilisation of amorphous alumina located outside the zeolitic cavities. When amorphous alumina lacks for the formation of AIMo6, AHM precipitates at the surface of the support [15,17]. Upon

145 calcination, AIMo6 evolves into a surface "Ala(MoO4)3" phase (main line at 1003 cm -1) and a not determined polymolybdate phase (line at 980 cm-1), whereas AHM precipitate leads to crystalline MoO3 (Fig. 5). When the sample is put back to ambient atmosphere, MoO3 remains whereas A1Mo6 is recovered (Fig. 6) and represents the majority component on the 3 Mo wt.% sample, as confirmed by Mo K-edge EXAFS (spectrum not reported here).

60

-~ ~ , , .

937 ......902~j \__AH__;M__/ (8_%) Z

~,]

15

93,~ /952

~n

902 .7/3

~" 11 868,\A

L.._..... ~y' \ AHM/Z(5%) 952

130

ppm Fig. 3. 27A1MAS NMR of AHM/Z at 3, 5 and 8 Mo wt.%.

430

730

1030

1330

Wavenumber (cm1)

Fig. 4. Raman spectra of A H M / Z at and 8 Mo wt.%.

818 995

3, 5

818 995 AHM/Z(8~) 980 \j

1003

~

o

~M_./z_(35! 1006

382 270

470

670

870

1070

1270

Fig. 5" Raman spectra of calcined AHM/Z at 3, 5 and 8 Mo wt.%. *: lines of the glass of the in situ cell. +: lines of the zeolite.

AHM/Z(5%)

980 952.1

827 ~029 x~8 ~ LAI2(MoO4) 3 Wavenumber (cm1)

995

1;o

~;o

go

~;o

~3o

o 11;o

Wavenumber (cmI )

1~;o

Fig. 6: Raman spectra of rehydrated A H M / Z at 3, 5 and 8 Mo wt.%. +: lines of the zeolite.

146 3.2.2. Alumina-supported samples Comparison with 10 Mo wt.% Mo/A has therefore been performed as the formation of this AiMo6 has been shown to occur upon impregnation of an alumina with AHM. Further evidence on this can be gained by 27A1 MAS NMR and Mo K-edge EXAFS experiments. In 27A1 MAS NMR, a shoulder is evidenced on the high chemical shifts side of the line characteristic of the octahedral atoms of the support at 0 ppm (Fig. 7). The corresponding line is clearly evidenced by subtraction of the spectrum of the alumina from the AHM/Ai203 one (Fig. 7). The characteristic peak of A1M06 at 15 ppm(2)appears, confirming the presence of the aluminomolybdate anion as already seen by Raman spectroscopy (7). However, liquid state 27A1NMR experiments carried out on the matured samples (not reported here) showed a difference between zeolite- and alumina-supported samples: the characteristic peak of AIMo6 is sharp for AHM/Z and broad for AHM/A. This tends to indicate a confinement of the entity on the alumina. Raman features also gave evidence of the dispersion of A1M06 in AHM/A and of its precipitated form in AHM/Z. The Mo K-edge EXAFS spectrum of AHM/A (not reported here) presents differences with those of A1M06. The fit realised with FEFF-calculated contributions shows that these are adapted to the spectrum, but leads to parameters that do not exactly match those of reference A1M06. The other techniques showing no evidence of heterogeneity, the distortion of the entity resulting from its dispersion might explain the results A!203 of the fit. Confirmation was sought through impregnation of an aluminomolybdate-containing solution. But the low solubility of the starting material (which limits the loading of the sample at 1.4 Mo wt.%) and the buffer effect of alumina impose the decomposition of this species (stable in the pH range 2-6 [18]). It consequently leads to free and adsorbed tetrahedral entities, as shown by the Raman lines at respectively 897 and 919 cm 1 (Fig. 1). Thus the presence of A1Mo6 at high Mo loadings on AHM/A should be ascribed to the high dispersion and to the interaction with the alumina support. Upon calcination, the presence of a surface aluminium molybdate "A12(MoO4)3" is shown by Raman spectroscopy, but A1Mo6 is recovered upon rehydration of the sample [7]. Fig. 7:27A1 MAS NMR spectra of A1M06, A1203 .........

and AHM/A.

147 3.3. Synthesis of promoted oxidic precursors 3.3.1. Dried CoMo/A and CoMo/Z Like for AHM/A, whatever the starting material used for the preparation (CoMo6 or coimpregnation with AHM and cobalt nitrate), in Z7Al NMR, the characteristic peak of AIMo6 at 15 ppm appears (not reported here). Thus, we conclude that CoMo6 is decomposed upon impregnation and leads to the formation of the aluminomolybdate as for solids prepared by co-impregnation with AHM and cobalt nitrate. This decomposition is also evidenced by UV-visible spectroscopy, with the disappearance of the band at 600 nm characteristic of the Co 3+ ions. The molybdenum atoms react to give A1Mo6, without interacting with the cobalt atoms which go into the surface sites of the alumina. Fig. 1 also shows the Raman spectra of two zeolite-supported Co-promoted oxidic precursors. The one prepared by co-impregnation of ammonium heptamolybdate and cobalt nitrate (CoMo/Z) presents the Raman features of A1Mo6, whereas the one prepared by impregnation with ammonium cobaltomolybdate (CoMo6/Z) presents those of CoMo6. Co K-edge EXAFS spectra (Fig. 8) indicate a Mo-Co contribution for CoMo6/Z, whereas it is absent in CoMo/Z. It appears that upon co-impregnation, the molybdenum atoms react to give A1Mo6, without interacting with the cobalt atoms which go into the cationic sites of the zeolite (19-23). This may explain the absence of promoting effect observed before (24). In opposition to that, the use of a CoMo6 solution leads to the preservation of an entity inside of which molybdenum and cobalt are in an interacting position. 3.3.2. Calcined CoMo/A and CoMo/Z In the case of CoMo/A, which presents the same molybdate entities as in the nonpromoted sample, their behaviour upon calcination is identical, and AIM06 evolves into an "aluminium molybdate" phase, and re-appears upon transfer to air. In the case of CoMo/Z, the calcination generally performed at 500~ to eliminate the ammonium counterions induces a decomposition of the cobaltomolybdate entity. Upon rehydration, the so freed molybdenum atoms associate with aluminium as in the impregnation of AHM to give AIM06. Thus, the calcination-rehydration steps have a levelling effect on the structure of the promoted catalyst: the same entities are finally identified whatever the support or the synthesis route.

(a)

(b) -- CoMo6

CoMo6

-- CoMo6/Z

CoMo6/Z

CoMo/Z

CoMo/Z ,

4.7

&7

8.7

k (A -1)

10,7

12,7

14,7

o

1

2

,

,

3

4

5

R (,&,)

Fig. 8. (a) Co K-edge EXAFS of CoM06, CoMo6/Z and CoMo/Z. (b) Corresponding Fourier transforms.

148 4. C O N C L U S I O N This study enables us to draw a parallel between the behaviours of alumina and HY zeolite upon impregnation with oxomolybdate entities. In some conditions, when the buffer effect of the alumina is avoided, both supports see extraction of aluminium atoms and their inclusion in an Anderson-type heteropolyanion, AIMo6024H63-. This entity is well dispersed on the alumina support whereas it appears as a bulk compound located in the macropores of the HY zeolite. However, it is still unknown whether the formation of this species is to be favoured. For promoted samples, a significant difference exists between both supports, since CoMo6024H63- can be preserved upon impregnation on a zeolite. Yet, the preparation of the catalysts has to be controlled: the final calcination-rehydration steps tend to level the various synthesis and lead to the presence of AIMo6024H63-. REFERENCES 1. X. Carrier, Thesis, Paris, 1998. 2. X. Carrier, J.-F. Lambert and M. Che, J. Am. Chem. Soc., 119 (1997) 10137. 3. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 121 (1998) 311. 4. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 118 (1998) 469. 5. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 130 (2000) 1049. 6. L. Le Bihan, Thesis, Lille, 1997. 7. L. Le Bihan, P. Blanchard, M. Fournier, J. Grimblot and E. Payen, J. Chem. Soc., Faraday Trans., 94 (1998) 937. 8. A. Michalowicz, J. Phys. IV, 7 (1997) 235. 9. A. L. Ankoudinov, Thesis, Washington, 1996. 10. J. J. Rehr, J. Mustre de Leon, S. I. Zabinsky and R. C. Albers, J. Am. Chem. Soc., 113 (1991) 5135. 11. J. J. Rehr, S. I. Zabinsky and R. C. Albers, Phys. Rev. Lett., 69 (1992) 3397. 12. S. I. Zabinsky, J. J. Rehr, A. Ankudinov, R. C. Albers and M. J. Eller, Phys. Rev. B, 52 (1995) 2995. 13. R. D. Hall, J. Am. Chem. Soc., 29 (1907) 692. 14. K. Nomiya, T. Takahashi, T. Shirai and M. Miwa, Polyhedron, 6 (1987) 213. 15. G. Plazenet, Thesis, Lille, 2001. 16. G. Plazenet, E. Payen and J. Lynch, Phys. Chem. Chem. Phys., Submitted. 17. G. Plazenet, E. Payen, J. Lynch and B. Rebours, J. Phys. Chem. B., Submitted. 18. G. A. Tsigdinos, Top. Curr. Chem., 76 (1978) 1. 19. P. W. de Bont, Thesis, Delft, 1998. 20. P. W. de Bont, M. J. Vissenberg, E. Boellaard, V. H. J. de Beer, J. A. R. van Veen, R. A. van Santen and A. M. van der Kraan, J. Phys. Chem. B, 101 (1997) 3072. 21. M. J. Vissenberg, Thesis, Eindhoven, 1999. 22. M. J. Vissenberg, P. W. de Bont, J. W. C. Arnouts, L. J. M. van de Ven, J. W. de Haan, A. M. van der Kraan, V. H. J. de Beer, J. A. R. van Veen and R. A. van Santen, Catal. Lett., 47 (1997) 155. 23. M. J. Vissenberg, P. W. de Bont, E. M. van Oers, R. A. de Haan, E. Boellaard, A. M. van der Kraan, V. H. J. de Beer and R. A. van Santen, Catal. Lett., 40 (1996) 25. 24. P. Leyrit, Thesis, Paris, 1999.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

149

Sol-gel preparation of pure and silica-dispersed vanadium and niobium catalysts active in oxidative dehydrogenation of propane P. Moggia, G. PredierP, D. CauzzP, M. Devillers~, P. Ruiza, S. Morselli~' and O. Ligabuea aDepartment of Organic and Industrial Chemistry, University of Parma, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy t'Department of General and Inorganic Chemistry, Analytical Chemistry, Physical Chemistry, University of Parma, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy cunit6 de Chimie des Mat6riaux Inorganiques et Organiques, Universit6 catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-La-Neuve, Belgium dunit6 de Catalyse et de Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-La-Neuve, Belgium Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbV05, SbVO4 and Nb2Mo3Oll are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500~ propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 1. INTRODUCTION The key point to reach high catalytic performances with multimetallic oxides appears to be the ability to tune the catalyst architecture, by controlling the nature and homogeneity of the active phase and, in the case of multiphasic catalysts, the interdispersion of the various oxide phases. This can be accomplished by implementing adequate preparative procedures such as sol-gel methods. The sol-gel preparation of catalytic materials [1] has intrinsic advantages compared with other methods, because it allows a precise control of the synthetic conditions and hence of factors such as purity, stoichiometry, homogeneity (or Corresponding author. Tel. +39-0521-905464 E-mail: [email protected]

150 controlled heterogeneity) that are all relevant to the catalytic activity. Most sol-gel preparation procedures are carried out in the presence of water, and are based on inorganic polymerisation reactions, for which metal alkoxides are used as starting materials. A macromolecular network is then obtained via hydrolysis and subsequent hydroxyl condensation. Alternatively, the so-called non-hydrolytic sol-gel process is available to overcome disadvantages due to the presence of water. In this process, M-O-M' bonds are obtained by direct reaction between suitable precursors, such as metal chlorides or alkoxides; water is not required for hydrolysis, nor produced by condensation [2]. Sol-gel methods have been found to be particularly powerful to control the intimacy of molecular scale mixing in mixed oxides [3]. Niobia and silica-niobia systems were previously prepared by us by the hydrolytic sol-gel method from Nb(OEt)5 and Si(OMe)4 as precursors, obtaining high surface area mixed-oxide materials in a wide composition range, displaying variable acidities [4]. Since transition metal alkoxides with a d o configuration, such as Ti(IV) or Nb(V), are very reactive towards hydrolysis, pure niobia gels were obtained by introducing a mineral acid and/or a complexing agent such as oxalic acid in the solution of Nb precursor, in order to stabilise the sol and prevent the precipitation of niobic acid. Moreover, to obtain uniform high surface area mixed Nb-Si oxides, suitable combinations of metal precursors were used: the rate of hydrolysis and condensation of the individual precursors must be comparable in order to avoid segregation of single oxides. For this reason, the alternative non-hydrolytic sol-gel method was applied by us for the first time to produce mixed Nb-V and Nb-V-Si oxide systems. These materials were preliminary tested in the propane oxidative dehydrogenation (ODH) reaction, also in the presence of a potential promoter such as Sb204 [5]. The results obtained demonstrated the feasibility of the non-hydrolytic sol-gel route to prepare V-Nb mixed-oxide catalysts. Moreover, it seemed that interesting catalysts for the ODH of propane could be obtained by refining the preparation procedure and using suitable promoters. In this work, pure or silica-dispersed Nb-V systems were prepared by either the hydrolytic or the non-hydrolytic procedures. Moreover, the hydrolytic sol-gel route was extended to the preparation of pure or silica-dispersed Sb-V and Nb-V-M (M = Sb, Mo) oxide systems. The catalytic performances of these mixed-oxide systems in propane oxidative dehydrogenation were investigated.

2. EXPERIMENTAL 2.1. Catalysts preparation 2.1.1. Preparation of Nb-V and Nb-V-Si oxides by the non-hydrolytic sol-gel route NbC15, VO(OiPr)3, SiCh and Si(OEt)4 were the precursors adopted in the nonhydrolytic sol-gel preparations. The syntheses were conducted under inert atmosphere. VO(OiPr)3 and eventually SiCh and/or Si(OEt)4 were added dropwise to stirred suspensions of NbCI5 in hexane. By raising the temperature to 100~ limpid solutions were obtained, giving aiter few hours green gels. The metal precursors were mixed in order to obtain the following atomic ratios: 1-1 for the binary systems Nb-V and l" 1:5 for the ternary systems Nb-V-Si. The obtained xerogels were activated in air at 550~ for 5 h.

151

2.1.2. Preparation of Nb-V, Sb-V and Nb-V-M (M = Si, Mo, Sb) oxides by the hydrolytic sol-gel route Nb(OiPr)5, VO(OiPr)3, Sb(OiPr)3, Mo(OiPr)5 and Si(OEt)4 were the precursors adopted in the hydrolytic preparations. The syntheses were conducted under inert atmosphere by adding mixtures of water and 2-propanol to stirred mixtures of the alkoxides in 2-propanol. Each synthesis led to a stable sol, which turned into gel within few days. The metal precursors were mixed in order to obtain the following atomic ratios: 1:1 for the binary systems Nb-V and Sb-V; 1:1: l, 1:2:1 and 1:1:5, for the ternary systems Nb-V-Mo, Nb-VSb and Nb-V-Si, respectively. All the obtained xerogels were activated in air at 550~ for 5h. 2.2. Catalysts characterization The precursor gels were characterized by TGA and the calcined catalysts by FTIR and Raman spectroscopy, XRD, SEM and surface area determination. FTIR spectra in the range 4000-400 cml were recorded by using a Nicolet 5PC spectrophotometer. BET surface areas of the activated catalysts were measured with a Micromeritics ASAP 2000 analyzer using nitrogen at 77K. TGA analysis was carried out with a Mettler Toledo TGA/SDTA 851 e analyzer. Raman spectra were recorded on a DILOR-JOBIN-YVON-SPEX Olympus DX 40 spectrometer using a He-Ne laser ( = 632.8 nm). Powder XRD patterns were measured on a Siemens D-5000 diffractometer using the Cu-K radiation. SEM images were obtained with a Philips XL30 instrument, at the LAMEL Institute (CNR, Bologna, Italy). 2.3. Catalytic tests Propane oxidation experiments were generally carded out with 0.5g catalyst at atmospheric pressure, in the temperature range 400-500~ at a space velocity of 60 ml.min-1 geat-1. The gas feed composition was 10% C3H8, 10% 02 and 80% He (total flow rate 30 ml/min). Some experiments were also performed at a lower space velocity of 36 ml.min~gca(l, with a reactant mixture corresponding to 17% C3H8, 17% 02 and 66% He (total flow rate 18 ml/min). 3. RESULTS AND DISCUSSION All the obtained xerogels were amorphous solids, characterized by very broad IR bands in the region 1000-600 cm1. TGA analysis of all the prepared xerogels showed two successive weight losses associated with endothermal peaks (evidenced by the differential calorimetric scan at 10~ min~) at about 100~ due to the elimination of residual water and 2-propanol, and 200~ related to the elimination of residual alkoxide ligands for all the prepared xerogels. BET surface area measurements are reported for the calcined catalysts in Table 1.

152 Table 1. BET specific surface areas of the catalysts calcined at 550~ Surface Area (m2/g) Catalysts 1:1 Nb-Vnon-hydrolytic

6.4

1:1 Nb-V hydrolytic

1.3

1:1 Sb-V

31

1:2:1 Nb-V-Sb

7.9

1:1:1 Nb-V-Mo

3.0

1:1:5 Nb-V-Si non-hydrolytic

146

1:1:5 Nb-V-Si hydrolytic

132

The observed BET surface areas are rather low for all the samples, with the only exception of the ternary 1:1:5 Nb-V-Si, both hydrolytic and non-hydrolytic, and the binary 1:1 Sb-V systems. The calcination at 550~ of the 1:1 Nb-V binary system prepared either via the nonhydrolytic method or the hydrolytic one led to the formation of crystalline NbVO5, as evidenced by the XRD pattern reported in Fig. 1, which matches exactly the JCPDS file of NbVO5 (46-0046). The SEM image of the Nb:V 1:1 catalyst after the thermal treatment at 550~ (Fig. 2, for the non-hydrolytic preparation) evidenced a rather uniform distribution of micrometer sized crystallites of the NbVO5 phase.

CO t'4-,

400 350 300 250" 20O150

100-

500

0

I

~ I1,.

,

20

fl.I '

40

2-Thel

I

!

60

80

Fig. 1. XRD pattern of 11 Nb:V after the calcination treatment at 550~ practically pure NbVO5 phase.

1 00 containing the

153

Fig. 2. SEM image of 1"1 Nb:V (non-hydrolytic preparation) after the calcination treatment at 550~ evidencing the uniform distribution of the NbVO5 phase. After the catalytic runs, the NblsV4Os5 phase was recognized in the XRD pattern illustrated in Figure 3. Decomposition of the NbVO5 phase to the Nb-enriched NblsV4055 phase and V205 was also hypothesized from the Raman spectra, in which the characteristic pattern of V205 (146, 290, 404, 480, 527, 702 and 994 cm1) was recognized (Figure 4). However, there was no evidence of crystalline V205 provided by XRD.

o,..~ r~

90 8O 70 60 50 40 30 20 10 0

x

x

i

I

|

. !

!

i

25

50 2-Theta

75

Fig. 3. XRD pattern of 1"1 Nb-V after the catalytic reactions, containing Nb~sV4055.

100

154 5000

4000 3000 2000 I 000

0

0

200

400 600 800 wavenumbers (cm-1)

1000

1 200

Fig. 4. Raman spectrum of l'l Nb:V (non- hydrolytic preparation) after calcination at 550~ In the case of the binary system 1:1 Sb-V, calcination at 550~ led to the formation of

crystalline SbVO4, as evidenced by XRD analysis (Fig. 5). Unlike the 1:1 Nb-V system,

crystalline SbVO4 was maintained even after the catalytic runs. AS far as the ternary system 1:1:1 Nb-V-Mo is concerned, the Nb2M03011 and (V0.07Mo0.93)5O14 phases were detected by XRD after the thermal treatment (Fig. 6). Similarly to the 1:1 Sb-V material, the XRD pattern of the 1:1:1 Nb-V-Mo system was unchanged after the catalytic runs. In the case of the ternary system 1:2:1 Nb:V:Sb, besides SbVO4, the NblsV4055 phase was again evidenced. After the catalytic runs, only traces of NblsV4055 were detected besides SbVO4, indicating some structure modifications. Again, although no evidence of crystalline V205 was given by XRD, the characteristic pattern of V205 was recognized in the Raman spectrum. In the case of the ternary system 1:1:5 Nb-V-Si, prepared either via the non-hydrolytic method or the hydrolytic one, the XRD spectra evidenced a predominantly amorphous structure, but with the presence of the NbVO5 phase. However, the characteristic pattern of V205 was recognized in the Raman spectra. Quite good results were achieved in the propane ODH experiments. In Table 2, propane conversion (Xc3HS), selectivity to propene (Sc3H6), selectivity to carbon dioxide (Sco2) and yield in propene (Yc3H6) are reported. Catalytic data were collected at the reaction temperature of 500~ feed composition: 10% C3H8, 10% 02, 80% He and space velocity of 60 ml minl g-1. The 1:1 Sb-V and 1:1:5 Nb-V-Si systems only were tested at the different space velocity of 100 ml min l g~. Almost all the prepared systems exhibited propane conversions of about 30% and propene selectivities higher than 20%. The most selective catalysts with respect to propene formation were the 1:1 Nb-V prepared via the hydrolytic method and 1:1 Sb-V systems (Sc3H6 ca 40o//0). The 1:1 Sb-V and 1:1:5 Nb-V-Si prepared via the non-hydrolytic method gave the best results in terms of propane conversion and yield in propene. In these two cases, while the higher conversion is in contradiction with

155 the lower amount of catalyst loading, the high yield in propene can be explained by the lower contact time adopted during the catalytic test, which preserves propene from further oxidation to carbon dioxide. Moreover, a minor amount of catalyst favours a better control of the reaction thermal efficiency inside the catalytic bed, reducing phenomena of complete oxidation due to hot spots. 400 350 300 r~

250

= 200 150 100

x

50 ........

0

i

,_

X ~

20

L

,r

X

._/t_ .

40

.

.

.

2-Theta

.

~

60

_

i

80

100

Fig. 5. XRD pattem of 1"1 Sb:V after the calcination treatment at 550~ practically pure SbVO4 phase. 400 350 300 .~ 250 = o 200 150 100 50 0

containing the

A,A !

i

i

i

20

40

60

80

100

2-Th eta

Fig. 6. XRD pattern of 1"1"1 V:Nb:Mo after the calcination treatment at 550~ containing the mixed NbEM03Oll (x) and (Vo.07Moo.93)5014 (0) phases.

156 Table 2 Catalytic results in propane ODH. Temperature 500~ 02 and 80% He. Catalyst loading: 0.5 g. Total flow: 30 Catalysts XC3H8 . . . . . . . . (%) 1:1 Nb-V non-hydrolytic 33

a

Feed composition: 10% C3H8, 10% ml/min

8C3H6

8CO2

YC3H6

(%) 23

(%) 30

(%) 7

1:1 Nb-V hydrolytic

8

37

24

3

1:1 Sb-V a

34

38

39

13

1:2:1 Nb-V-Sb

33

23

30

7

1:1:1 Nb-V-Mo

27

25

20

7

1:1:5 Nb-V-Si non-hydrolytic a

39

30

22

11

1"1"5 Nb-V-Si hydrolytic

31

20

20

6

catalyst loading: 0.3g

The 1:1:5 Nb-V-Si catalyst prepared via the non-hydrolytic method was tested also at the higher temperature of 550~ Propane conversion of 37%, slightly lower than at 500~ and propene selectivity of 51% were obtained. The lower conversion was probably related to some catalyst deactivation; however, propene selectivity and consequently the yield in propene were significantly higher than at 500~ Under these conditions, propene productivity was 0.23 kilograms of propene per kilogram of catalyst per hour, in very good agreement with the best propene productivity data reported in literature for the V/Nb/O catalysts [6]. A set of ODH experiments was carried out at the lower space velocity of 36 ml min"1 g-1 in order to study the effect of contact time on the selectivity behaviour. Acrolein was detected among the reaction products of some catalysts, the highest selectivity value of 8% being achieved with the 1"1"5 Nb-V-Si catalyst prepared via the non-hydrolytic method at 500~ 4. CONCLUSIONS In this work, 1:1 Nb-V, 1:1 Sb-V, 1:2:1 Nb-V-Sb, 1:1:1 Nb-V-Mo and 1:1:5 Nb-V-Si mixed-oxide systems were prepared by applying sol-gel techniques and tested as catalysts in the oxidative dehydrogenation of propane. High metal oxides interdispersions were obtained, as confirmed by the practically pure phases NbVO5 and SbVO4 identified by XRD in the silica-free 1:1 Nb-V and Sb-V catalysts. Several mixed-oxide phases were detected in the 1:2"1 Nb-V-Sb and 1:1-1 Nb-V-Mo catalysts, while the systems containing silica were amorphous and characterized by quite high surface areas. All the prepared systems were active in propane ODH and displayed promising selectivities to propene, particularly the 1:1:5 Nb-V-Si prepared via the non-hydrolytic sol-gel method and the 1:1 Sb-V prepared via the hydrolytic route.

157 ACKNOWLEDGEMENTS

This work was supported by MIUR (Ministero dell'Istruzione, Universit/t e RicercaRome) and the Belgian National Fund for Scientific Research (Brussels). Omar Ligabue was recipient of a Socrates-Erasmus grant for a four-month stay in Louvain-la-Neuve. LAMEL Institute of CNR (Bologna, Italy) is acknowledged for the SEM analyses. REFERENCES

1. C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, San Diego, 1990. 2. J. B. Miller and E. I. Ko, Catal. Today, 35 (1997) 269. 3. R.H. Smiths, K. Seshan and J. H. R. Ross, Stud. Surf. Sci. Catal., 72 (1992) 213. 4. S. Morselli, P. Moggi, D. Cauzzi and G. Predieri, Stud. Surf. Sci. Catal., 118 (1998) 65. 5. F. Barbieri, D. Cauzzi, F. De Smet, M. Devillers, P. Moggi, G. Predieri and P. Ruiz, Catal. Today, 61 (2000) 353. 6. S. Albonetti, F. Cavani and F. Trifir6, Catal. Rev. Sci. Eng., 38 (1996) 413.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

159

Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao, G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium To deposit the nickel catalyst in the filter, two preparation methods, depositionprecipitation of nickel nitrate with urea and conventional impregnation with nickel nitrate, were used and compared. In the deposition-precipitation with urea, experimental parameters such as reaction time and urea/nickel molar ratio were investigated to obtain a high controlled fixation of precursor during the slow decomposition of urea. SEM-EDX characterization showed that the urea method gave a fairly uniform spatial distribution of nickel throughout the filter substrate. On the contrary, with the conventional impregnation method, most of the precursor was deposited on the outer surface of the substrate. Tar cracking over nickel-modified catalytic filter prepared by the urea method was studied with a synthetic biomass gasification gas (free of H2S and dust). Benzene and Naphthalene were used as the model compounds, respectively. It was found that both benzene and naphthalene could be completely converted at 800-900~ over the catalytic filter with 1 wt% nickel. 1. INTRODUCTION Catalytic filters and membranes as multifunctional reactors, coupling a catalytically promoted reaction and a separation allowed by the filter or membrane itself, have gained an increasing attention in recent years due to demand for process efficiency improvement as well as environmental concerns [1-6]. The applications of catalytic filters in hot gas cleaning by the simultaneous removal of solid particles and some pollutants such as tar and ammonia from biomass gasification gas are also attracting public interest [1]. Apparently, the novel idea to combine reaction and separation in a single process unit may lead to a higher energy efficiency but also simplify the entire gas cleaning process with a potential reduction in investment costs and space saving, which is an important factor to make biomass gasification a feasible alternative energy source. Tars are always present in gasification gas as a side product and can easily plug downstream process equipment. Catalytic high-temperature gas cleaning is one of the potential solutions to the operational problems caused by tars. It has been demonstrated that nickel-based catalysts are very efficient in decomposing tars in biomass gasification To whom correspondenceshould be addressed. Email: [email protected]; Tel: +32-2-6293250; Fax: +322-6293248.

160 gas at 900 ~ [7]. Regarding the particulate removal, the ceramic candle filters can be applied to remove particles down to micrometer size at high temperature. So, a catalytic filter for simultaneous removal of solid particles and tar from biomass gasification can be achieved if a nickel catalyst for tar decomposition is placed on the pore walls of the inorganic filters through optimised deposition techniques. The most straightforward method is conventional impregnation with a nickel salt solution into the support body, followed by drying to bring about the deposition of salt inside the pores of the support. However, the drying is critical since the solution migrates and the precursor is deposited mainly where the solvent evaporates. Even when performed carefully, some nonuniformity must always be expected [8]. An improvement can be achieved by depositionprecipitation to immobilize the precursor in the pores of the substrate with a precipitant before the drying step [9]. On the other hand, it is required that the precursor and the precipitant are distributed uniformly throughout the pores before the onset of precipitation. Therefore, urea can serve well as a precipitant because it decomposes slowly at 90~ in an aqueous solution with generation of hydroxyl ions to make the precursor precipitate homogeneously throughout the pores [8, 10]. In this work, catalytic ceramic filters with uniform nickel-distribution were attained by the urea precipitation method, which was compared with the conventional impregnation method. To gain more insight on the urea method, the influence of the reaction time of urea decomposition and urea/nickel molar ratio on the fixation of the precursor on the support prior to drying was investigated. Finally, the catalytic performance of the nickel-modified filter in hot gas cleaning was tested on lab-scale using a simulated biomass gasification gas with benzene and naphthalene as mr model compounds. 2. EXPERIMENTAL 2.1. Preparation and characterisation of catalytic filter Some a-A1203-based filter discs (Schumacher, Germany) were vacuum impregnated with a solution containing appropriate amounts of nickel nitrate and urea. After the excess solution was drained off, the discs were placed in a closed vessel and kept at 90 ~ for a certain period, resulting in precipitation of nickel precursor by the slow hydrolysis of urea in the pores of the discs. After reaction, the filter discs were dried at 110 ~ for a few hours and calcined at 450 ~ for 4 h. Then the nickel-modified ceramic filter discs were obtained. For the conventional impregnation method, other steps and experimental conditions were identical to the preparation procedure with the urea method except for the absence of urea in the impregnation solution. The two-dimensional distribution of nickel throughout the modified filter disc was examined with SEM/DEX (energy dispersive X-ray) on a polished radial cross section of the disc. The analysis was performed on a JSM 6400 (JEOL) equipped with a NORAN Xray analysis system. The K line of nickel was used as the X-ray analysis line. The electron probe worked at an acceleration voltage of 15-25 kV and a current intensity of 3 x 109 A. 2.2. Test of reaction performance For reaction tests, the catalytic filter disc was fixed in the middle of a reactor tube (internal diameter 3 cm and length 50 cm), which was made of dense c~-A1203. The

161 catalytic performance was tested in a laboratory reaction setup, which has been described in detail in a previous publication [ 11 ]. In studies of tar cracking using a separate catalyst bed, two types of tar sources are applied, one directly drawn from a biomass gasifier and the other from model compounds. According to VTTs work [ 12], the tar consists mainly of highly stable compounds such as benzene (60-70 wt %), naphthalene (10-20 wt %), and other polyaromatic hydrocarbons (10-20 wt %), which can amount to 15-20 g of tar/Nm 3 in biomass gasification. So, benzene and naphthalene were used in this work as tar model compounds with a fixed concentration of 15 g/Nm 3 (4300 ppm) for benzene and 5 g/Nm 3 (875 ppm) for naphthalene, respectively. The gas composition used was 50 vol % N2, 12 vol % CO, 10 vol % HE, 11 vol % CO2, 12 vol % H20, 5 vol % CH4, 4300 ppm benzene (or 875 ppm naphthalene), which is a typical composition of the product gas from a biomass fluidised bed gasifier operated with air. The reaction tests were performed under three filtration gas velocities 2.5, 4 and 6 cm/s. All experimental points were monitored for at least 60 min after the reaction reached an apparent steady state at the selected operation condition. 3. RESULTS AND DISCUSSION 3.I. Influence of reaction time and urea/nickel molar ratios Urea decomposes slowly at 90~ in aqueous solution [ 10] according to reaction (1). Consequently, hydroxyl groups are slowly generated, uniformly throughout the pores and the precipitation takes place homogeneously. CO(NH2)2 + H2O ~ 2OH- + 2NH4 + + CO2

(1)

In the deposition-precipitation method with urea, experimental parameters such as the reaction temperature, the reaction time and the concentrations of urea and Ni 2+ in the impregnation solution determine the fixation degree of nickel, which is the amount of nickel precursor fixed by precipitation on the support before drying relative to the maximum amount that can be deposited. At present, there is no general agreement in the literature [9, 13-14] about the choice of these experimental parameters. So, it is necessary to investigate the urea method first, to determine adequate values of reaction time and urea/nickel molar ratio for the impregnation of the ceramic substrates. The study of the influence of the reaction time for urea decomposition and the urea concentration in the urea method was performed on ct-A12O 3 powder rather than a ceramic filter disc since the supply of the ceramic discs was limited. Experimental details have been reported elsewhere [ 11]. To investigate the role of the reaction time, the urea/nickel molar ratio was fixed at a value of 1.3. The results are shown in Fig. 1. The precipitation amount of nickel precursor and the suspension pH both increased with reaction time. The amount of nickel precipitate sharply increased during the first 6 h reaction time and increased slowly after 6 h. This suggested that at least 6 h reaction time was needed to fix a reasonable amount of precursor on the support during the wet stage of the urea method. The nickel precipitate was identified By XRD analysis as Ni3(NOa)2(OH)4 in all cases. The effect of the urea/nickel molar ratio on the precipitation was investigated in a range of ratios from 1 up to 3 for a reaction time of 24 h (see Fig. 2). It was found that the

162 amount of the nickel precipitate hardly increased when the molar urea/nickel ration was varied between 1 and 1.7, while for higher urea/nickel molar ratios, the amount of nickel precipitate surprisingly decreased although the pH continued to increase. This decrease of nickel precipitate is due to the increase of complex-formation of the Ni 2+ ion with NH3. The detail explanation for the behaviour in Fig. 2 can be found in our previous publication [15]. In addition, a maximum fixation degree of nickel precursor precipitated on the support (about 75 %) was obtained when the urea/nickel molar ratio is 1.7 after 24 h reaction. Therefore, such optimum experimental parameters as reaction time of 24 h and urea/nickel moral ratio of 1.7 were used to prepare the nickel-activated catalytic filter. 8

8

6

6

A

4== -o

"r:

2

~

5 r

2

suspension pH

I

4

,

I

8

,

I

,

12

Reaction t i m e

I

16

l

I

2O

,

precipitate

tr ,

l

nickel

--a--- suspension pH

I

24

(hr}

Fig. 1. Precipitation amount of the nickel precursor and the suspension pH as a function of reaction time (urea/nickel molar ratio: 1.3)

0.8

|

1.2

,

|

1.6 Urea/nickel

,

|

2 molar

,

i

2.4

,

|

2.8

,

3.2

ratio

Fig. 2. Precipitation amount of the nickel precursor and the suspension pH as a function of urea/nickel molar ratio (reaction time/24 h)

3.2. Nickel distribution throughout the catalytic filter disc To compare the nickel distribution obtained with the urea method, some catalytic filter discs were prepared by conventional impregnation with a solution containing only nickel nitrate. The distribution of NiO throughout the filter disc was investigated by SEM/DEX. Their Ni element mappings are shown in Fig. 3 (urea method) and Fig. 4 (conventional impregnation), respectively. The X-ray area scanning was made of part of a polished radial cross section of the discs till a depth of about 3.5 mm (vertical direction) in the disc. The white dots represent the EDX-mapping for the element Ni and the black background is from both the pores and the ~-A1203 particles of the filter disc. It appears that nickel displays a fairly uniform spatial distribution throughout the filter disc prepared by the urea method. However, throughout the disc prepared with the conventional impregnation method, most of the NiO is situated in the pores close to the outer surface (top area of Fig. 4), while less NiO is detected near the middle of disc (bottom area of Fig. 4). To increase the NiO loading further, the deposition cycle was performed several successive times on the filter disc under the same conditions previously used. SEM/EDX analyses were performed on a cross-section of the twofold impregnated filter discs. The Ni

163 element mapping showed that the urea method still gave a fairly homogeneous distribution of nickel throughout the disc after two cycles. However, the nickel distribution throughout the disc seemed to be less uniform with the conventional impregnation, since more precipitation occurred in the outer surface of the substrate. Therefore, the urea method is reliable and reproducible to deposit nickel catalyst on the filter with a uniform distribution of nickel.

Fig. 3. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the urea method.

Fig. 4. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the conventional impregnation method.

3.3. Catalytic performance of the nickel-activated filter discs Fig. 5 shows the benzene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. As a reference, the benzene conversions using a blank disc (no catalyst inside) in the same conditions are displayed in the figure. It was found that full conversion of benzene was obtained at typical filtration gas velocities (2.5 and 4 crn/s) and in a temperature range from 750~ to 900~ Even at a higher filtration velocity such as 6 cm/s, a complete conversion is still reached above 800 ~ Fig. 6 shows the naphthalene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. The blank disc was also used as a reference. As shown in Fig. 6, almost complete naphthalene cracking was achieved above 800 ~ with any gas velocity lower than 4 crn/s. Even with a gas velocity of 6 cm/s, the conversion still remains about 97 %. However, below 800 ~ the conversion of naphthalene significantly decreased as the reaction temperature decreased and the gas velocity increased. In addition, benzene was identified as one reaction product of naphthalene cracking at 750 and 800 ~ Table 1 lists the amount of benzene in the outlet gas after reaction over the nickel-modified filter disc. It is evident that for all gas velocities at 750 ~ benzene is present in the outlet gas with a significant amount. Above 800 ~

164 nearly no benzene was found in the outlet gas. Since no benzene formation was detected during the tests with the blank disc, it can be concluded that benzene was derived from the catalytic naphthalene cracking. Apparently, the nickel-activated filter disc displayed an excellent catalytic performance for the decomposition of either benzene or naphthalene as tar model compound in the simulated biomass gasification gas at a typical filtration gas velocity when the reaction temperature was in the range of 800-900 ~ Table 1 Yield of benzene as a reaction product of catalytic naphthalene cracking over a 1 wt % nickel-modified filter disc at different temperatures and gas velocities. Gas velocity Temperature (~ 2.5 cm/s 4 cm/s 6 cm/s 750 800 850 900 a Not detectable.

15.4 % ND a ND ND

15.7 % 2.1 % ND ND

100 A r

i

I wt% Ni

--D-- 2.5 cm/s

- o - - 2.5 cm/s

8

~ 4

4 cm/s

I1 e-

,,

8o

.o

0 >

Z

1 wt%Ni

r

A

ra

i

100 /

12.3 % 1.6 % ND ND

cm/s

--o-- 6 crn/s

--o-- 6 cm/s

40

0

20

0

'

70O

blank dis4z

m

blank dis c

20

75O

80O

850

9O0

95

Fig. 5. Benzene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.

|

700

'

750

'

800

'

850

i

900

950

Fig. 6. Naphthalene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.

4. CONCLUSIONS The urea method can be applied to obtain nickel-modified catalytic filter with a uniform spatial distribution of nickel. The study of urea method showed that a reaction time of at least 6 h for urea decomposition is necessary and that a higher urea/nickel molar ration than 1.7 that led to less fixation of nickel precipitation. A maximum fixation of 75 % of the

165 precursor was found for a reaction time of 24 h and an urea/nickel molar ratio of 1.7. Moreover, the complete conversion of tar model compounds, such as benzene or naphthalene in a simulated sulfur-free and dust-free biomass gasification gas over a nickelmodified filter disc was obtained at 800-900 ~ using typical filtration gas velocities. This demonstrated that it is potential to develop a nickel-activated catalytic filter for simultaneous tar and particle removal from biomass gasification gas. This is encouraging to further develop more improved catalysts by the urea method for the removal of more complex tar mixtures and for the tar removal from real biomass gasification gas. ACKNOWLEDGEMENT

This work was financed by Research in Brussels (grant No. RIB-96/32) and the EU 5th Framework Programme (Contract No. ENK5-2000-00305). REFERENCES

1. A. Cybulski and J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc.: New York, 1998. 2. S.R. Ness, G.E. Dunham, G.F. Weber and D.K. Ludlow, Environ. Prog., 14 (1995) 69. 3. G. Saracco and V. Specchia, Catal. Rev. Sci. Eng., 36 (1994) 305. 4. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 5. G. Saracco and V. Specchia, Chem. Eng. Sci., 55 (2000) 897. 6. J.N. Armor, Appl. Catal. A: General, 49 (1989) 1. 7. A.V. Bridgwater, Appl. Catal. A, 116 (1994) 5. 8. J.T. Richardson, Principles of Catalyst Development, Plenum Press, New York, 1989. 9. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, in B. Delmon, P. Grange, P. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 166. 10. W.H.R. Shaw and J.J. Bordeaux, J. Am. Chem. Soc., 77 (1955) 4729. 11. H. Zhao, D.J. Draelants and G.V. Baron, Ind. Eng. Chem. Res., 39 (2000) 3195. 12. P. Simell, Catalytic hot gas cleaning of gasification gas. Espoo 1997, VTT Publications 330; Technical Research Center of Finland: Finland, 1997. 13. K.B. Mok, J.R.H. Ross and R.M. Sambrook, in: G. Poncelet, P. Grange, P. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdmn, 1983, p. 291. 14. X. Xu, H. Vonk, A. Cybulski and J.A. Moulijn, in G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds.), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 1069. 15. H. Zhao, D.J. Draelants and G.V. Baron, Catal. Today, 56 (2000) 229.

This Page Intentionally Left Blank

Studies in Surface Science andCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

167

Preparation of gold-titanosilicate catalysts for vapor-phase propylene epoxidation using H2 and 02 A.K. Sinhaa, S. Seelan a, S. Tsubota a and M. Haruta b aEnvironmental Catalysis Research Group, Special Division of Green Life Technology, AIST, 1-8-31, Midorigaoka, Ikeda 563-8577, Japan bResearch Institute for Green Technology, AIST, Onogawa 16-1, Tsukuba 305-8569, Japan Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 1. INTRODUCTION Propylene oxide (PO) is an industrially important chemical for the manufacture of polyurethane, unsaturated resins, surfactants and other products. Industrially PO is produced using two processes: Chlorohydrin process and Halcon (hydroperoxide) process [1]. The former process produces environmentally unfriendly chlorinated organic byproducts as well as calcium chloride, while the latter process produces equimolar amounts of co-products and requires heavy capital investment. Extensive efforts are being made since long to develop alternative process for direct gas-phase propylene epoxidation using oxygen [2, 3] because it is a promising method of tremendous industrial significance which can replace currently used environmentally disadvantageous chlorohydrin process and hydroperoxide processes. Recently Enichem utilized TS-1 (MFI) as a catalyst for the epoxidation of propylene in the liquid phase using hydrogen peroxide [4-6]. However, due to the very high production cost of H202 and its handling problems it would be highly desirable to produce H202 in-situ. Toso Co. Ltd. [7] has developed Pd/TS-1 catalyst for the in-situ generation of H202 from H2 and O2. Recently H61derich and coworkers [8] modified the catalyst developed by Toso with Pt and reported that the improved yield of PO could be ascribed to the maintainance of Pd in its +2 oxidation state.

168 Our research work on the catalysis by gold [9-11] has opened a new stage for the direct epoxidation of propylene using Ha and Oz in the vapor phase. In a series of papers we have reported the vapor phase epoxidation of propylene over highly dispersed nanosize Au particles supported on TiO2, TiO2/SiO2 [12, 13] and titanosilicates such as TS-1, TS-2, Ti-[-3, Ti-MCM-41 and Ti-MCM-48 [15-17]. These findings are now being followed by few other researchers [18-20] and companies [21,22]. But the problems for industrial viability still exist due to low PO yields and low H2 efficiency. Efficiency of supported metal catalysts often depends on metal dispersion and metal-support interaction. The presence of well dispersed tetrahedrally coordinated Ti sites and the Au nanoparticles on the support surface is thought to be necessary for epoxidation activity [15-17]. The synthesis of dispersed gold nanoparticles is highly sensitive towards the preparation methods and synthesis conditions. It has been shown that Au particles of critical size < 3.0 nm deposited on to titania or titanosilicate (TS-1, Ti-MCM-41, Ti-MCM-48) supports are essential for propylene epoxidation activity and high selectivity (> 90 %), larger Au particle size resulting in CO2 formation [12, 13]. Modified Deposition-Precipitation (DP) method has been found to be most suitable method for preparing supported gold catalysts with desirable gold particle size. The present work intends to study the role of precipitant, catalyst washing, calcinations and pretreatment in different gas streams, during the preparation of Au catalysts by DP method, onto mesoporous titanosilicates of different porosities and pore structures and its effect on the propylene epoxidation activity and selectivity in order to fine tune the synthesis of gold-titanosilicate catalysts for propylene epoxidation reaction. 2. EXPERIMENTAL Mesoporous Ti-MCM-41 support was prepared by hydrothermal crystallization according to literature procedure [23]. Disordered mesoporous titanosilicate Ti-Meso was synthesized by modified sol-gel method and hydrothermal crystallization (at 100~ 5 days) and using cetyltrimethyl ammonium bromide as template following the procedure similar to that for the synthesis of disordered mesoporous silica [24]. A modified sol-gel method without hydrothermal crystallization was used to prepare mesoporous titanosilicates, TiO-SiO(1) and TiO-SiO(2) [25]. Ti grafting on Ti-MCM-41 support was carried out according to literature procedure [26]. The materials were characterized by XRD (Rigaku R i n t - 2400, Cu-Kc~ radiation, 40 kV, 40 mA), UV-Vis (Photal Otsuka Electronics, MC-2530 UV/VIS light source), and nitrogen adsorption/desorption, BET surface area measurements (Micromeritics ASAP 2010 apparatus). Gold nanoparticles were deposited on the supports by deposition precipitation method [13-15] using aqueous HAuCl4 solution (corresponding to 2-4 wt % Au) and NaOH, NH3, urea, NaHCO3 and Na2CO3 as precipitants (at pH 7-9) followed by calcination in air at 300~ The Au particle size and its distribution was observed by TEM (Hitachi H-9000). The catalytic tests were carried out in a vertical fixed-bed U-shaped quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H6, H2 and Oa diluted with Ar passed over the catalyst (0.15 g) bed at a space velocity of 4000 h-lcm3/g.cat. The temperature was

169 controlled and measured using a glass tube covered Cr-A1 thermocouple located in the center of the catalyst bed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min. in a stream of 10 vol% H2 in Ar, followed by 10 vol% Oz in Ar streams. The feeds and products are analyzed using on-line GCs equipped with TCD (Porapak Q column) and FID (HR-20M column) detectors and auto injector. 3. RESULTS AND DISCUSSION XRD spectra for the various mesoporous titanosilicates are shown in Fig. 1. The spectra of Ti-MCM-41 samples correspond to a regular, well-ordered, hexagonal mesoporous structure. But the titanosilicate Ti-Meso prepared hydrothermally shows a single broad peak in the 20 range 2 . 0 - 3.0~ the titanosilicate TiO-SiO(1) prepared by modified sol-gel method shows a broad peak 20 range 1.0 - 2.0 ~ Presence of a single broad peak implies that these titanosilicates are mesostructured amorphous materials [2425]. Absence of any higher order peaks implies that these mesostructured materials do not have any long-range order. UV-Vis spectra of the titanium containing MCM-41 and disordered mesoporous titanosilicate samples are shown in Fig. 2. The UV-Vis analysis of these samples show a band near 220 nm range due to tetrahedrally coordinated Ti. Generally a shoulder at --330 nm is expected in the spectrum if the sample contains some bulk titania, but such a shoulder could not be observed. Absorption band at 260-270 nm has been generally attributed to the presence of Ti atoms in 5- and 6- fold coordinations, which are most likely generated through hydration of the tetrahedrally coordinated sites [24]. Ti/Ti-MCM-41 and TiO-SiO(2) samples with higher Ti content than that for the other titanosilicate samples studied here show broader UV-Vis bands with some red-shift. Table 1 summarizes the surface properties of the various mesoporous titanosilicate samples. The BET surface areas of these titanosilicates (850-1250 m2g-1) was typical for that shown by mesoporous materials. The BET surface area and BJH average pore diameter is found to decrease after Ti grafting onto Ti-MCM-41 sample. The postsynthesis grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size after titanium grafting. The pore size distribution is also found to become narrower around the average pore diameter. The BET surface areas of disordered mesoporous materials is found to be lower than that of the ordered MCM-41 type materials. BJH average pore diameter of mesoporous titanosilicates TiO-SiO(1) and TiO-SiO(2) prepared by modified sol-gel method is much lower than that of disorderd mesoporous Ti-Meso and ordered Ti-MCM-41 samples crystallized hydrothermally. But all these samples show a narrow pore size distribution. All the samples exhibit isotherms of type IV, typical of mesoporous materials, with a H2 hysteresis loop (Fig. not shown).

170

.,,,.-,.,

5

.i

iO-SiO,,,(2)

.

v

CM-41

(I) o c-

(1)

,

..Q L o

t-"

60 ..Q

t-

<

~_

TiO-SiO(1)

2 theta(deg.) Fig. 1. XRD patterns for the mesoporous titanosilicate materials.

Table 1 Surface properties of mesoporous titanosilicate Catalyst Ti/Si Surface area (mag-1) Ti-MCM-41 0.015 1270.4 Ti/Ti-MCM-41 0.03 1016.6 Ti-Meso 0.015 882.9 TiO-SiO(1) 0.01 891.0 TiO-SiO(2) 0.02 834.4

.

.

.

.

.

.

200 250 300 350 400 450 500 550 Wavelength (nm) Fig. 2. UV-Vis spectra of titanium containing mesoporous silica materials.

support samples. Pore size Pore volume (cm3g-1) (~) 38.4 1.80 29.2 0.87 29.6 0.72 14.1 0.55 18.9 0.86

Table 2 shows the results of influence of the nature of the precipitant used for depositing Au on the Ti-MCM-41 support on the propylene epoxidation reactions over supported Au catalysts. The Au/Ti-MCM-41 catalyst prepared by using NaOH as the precipitant showed the best activity and hydrogen efficiency. Catalyst prepared by using urea as the precipitant shows lower activity and hydrogen efficiencies. Catalysts prepared

171 by using NaHCO3 and Na2CO3 as precipitants show the lowest activities. While catalyst prepared by using NH3 as precipitant shows poor PO selectivity due to more CO2 formation. Among different precipitants for Au deposition, NaOH has found to be the most suitable for getting uniform, well dispersed Au particles and maximum Au loading (from TEM observation and elemental analysis) which could be responsible for the best activity and hydrogen efficiency shown by this catalyst. Table 2. Influence of precipitant on propylene epoxidation activity and selectivity for 2 wt% Au loaded Ti-MCM-41 (TOS = 3 h) Precipitant Conversion (%) of PO selectivity H2 efficiency C3H6 H2 (%)1 (%) Urea 1.3 32.3 90.3 4.0 NaHCO3 0.5 28.2 96.8 1.8 NH3 1.4 32.2 75.1 4.4 Na2CO3 0.8 24.5 95.8 3.3 NaOH 2.5 16 92 15.6 Space velocity, 4000 h-lcmg/gcat; catalyst, 0.15 g; feed, Ar/CaH6/H2/Oz = 70/10/10/10; Reaction temperature, 423 K; TOS = 3 h. 1Propylene Oxide selectivity (mol %). Table 3 shows the results of the influence of catalyst washing and pretreatment on the propylene epoxidation activity and PO selectivity. Using NaOH as precipitant it is found that strong washing of the supported Au precipitate (Ti-MCM-41a, Table 3) results in diminished catalytic activity. The catalytic activity is found to decrease with increasing severity of washing. This can be attributed to the washing out of Au and Na + from the support surface due to comparatively weaker Au-support interaction owing to scant and highly dispersed Ti sites on the support. Partially charged Au nanoparticles are held on the support surface mostly due to electrostatic interaction with charged Ti sites while neutral silica surface cannot interact with the Au nanoparticles. The catalyst prepared from unwashed supported precipitate is found to be the best with uniform sized and well dispersed Au nanoparticles on the support (from TEM observation) and shows the best activity (Ti-MCM-41c). Au and Na analysis for different catalysts shows decreasing Au and Na + concentration for catalysts Ti-MCM-41a-c, with increasing severity of the washing. The role of Na ions in the epoxidation activity is still doubtful and it needs further investigation. The promoting effect of alkali ions for propylene epoxidation has been shown earlier [2, 16]. Catalyst calcinations at around 300~ followed by pretreatment in dilute H2 stream and then dilute O2 streams at 250~ is necessary for getting improved activity. Dried catalyst without any pretreatment shows better Ha efficiency but lower propylene oxide yield (Ti-MCM-41d). Pretreatment in NO stream increase propylene conversion and decreases PO selectivity due to more CO2 formation (TiMCM-41e). Typical TEM images of the Au nanoparticles supported on Ti-MCM-41 and TiOSiO(1) samples are shown in Figs. 3a and 3b, respectively. The TEM pictures for the Au deposited catalysts did not show the presence of any bulk titania phase in the samples. Au nanoparticles were found to be uniformly dispersed on the surface of titanosilicate samples. But the surface of mesoporous TiO-SiO(1) and Ti-Meso supports showed more

172 homogeneous Au dispersion than the Ti-MCM-41 support. The mean size of the gold nanoparticles on the various titanosilicate supports varied between 2.8 nm to 3.5 nm. Table 3. Influence of catalyst washing and pretreatment on propylene epoxidation activity and selectivity for 2 wt% Au loaded mesoporous titanosilicates (TOS = 3 h) Catalyst Catalyst Pretreatment Conversion (%) of PO selectivity C3H6 H2 (%) a support washing method TiMCM-41a strong washing Calcination --+ H2 0.8 14 93 -+ 02 TiMCM-41b medium Calcination --+ Ha 1.9 15 92 washing -~ O2 TiMCM-41c no washing Calcinations --) 2.5 16 92 H2 --) 02 TiMCM-41d no washing No calcinations or 1.8 6 92 pretreatment TiMCM-41e no washing Calcination --~ 3.5 26 77 2%NO Space velocity, 4000 h-lcm3/gcat; catalyst, 0.15 g; feed, Ar/C3H6/H2/O2 - 70/10/10/10; Reaction temperature, 423 K; TOS = 3 h. 1Propylene Oxide selectivity (mol %). -+ represents the next pretreatment step.

Figure 3. TEM images of Au supported on Ti-MCM-41 sample (a) sample (b).

and TiO-SiO(1)

173

100 ~AJA--A

95 A / ~ v >, 4--' >

o

_.r

j

I A f

90

.

A

/

.

/

~

/

=

"

V - ~ - - - V - - - - - - - V

85

cb

0 n

80 75

o~ ,-

v

I

9

i

9

i

9

i

9

i

9

i

9

i

'

6

O (1) >

c

O o

9

V ~ V ~

"~:~. ~ - ~

4 (D

"T" O

co

2

'

I

V ~ y ~

o--e,

60

9

I

'

-\

I

'

I

'

I

'

I

'

I

'

V~

c-'

.o

0")

'-{1) > cO ro

40 O ~ ~

20

. -R

A

9

A

--A

,,

0

'

16o

'

26o

'

ado

'

400

Time(min)

Fig. 4. Influence of the nature of support on propylene conversion, PO selectivity and hydrogen conversion. Au/Ti-MCM-41(II), Au/Ti/Ti-MCM-41(@), Au/Ti-Meso(O), Au/TiO-SiO(1) (A) and Au/TiO-SiO(2) (V)

174 The results of propylene epoxidation at 150~ over Au/Ti-MCM-41, Au/Ti/TiMCM-41, Au/Ti-Meso, Au/TiO-SiO(1) and Au/TiO-SiO(2) catalysts are compared in Fig. 4. The increasing order for propylene conversion after TOS > 150 rain. over Au catalysts supported on various mesoporous titanosilicates is: Au/TiO-SiO(1)< Au/TiMCM-41
175 shows higher propylene conversion, hydrogen efficiency and PO selectivity than Au catalysts supported on ordered Ti-MCM-41 support and disordered mesoporous (Ti-Meso) supports prepared by hydrothermal crystallization. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

S.L. Ainsworth, Chem. Eng. News 9 (1992); M. McCoy, Chem. Eng. News, 19 (2001). K. Murata and Y. Koyozumi, Chem. Commun., (2001) 1356. H. Orzesek, R. P. Schulz, U. Dingerdisses and W. E. Maier, Chem. Eng. Technol., 22 (1999) 8. M.G. Clerici, G. Bellusi and U. Romano, J. Catal., 129 (1991) 159. B. Notari, Catal. Today, 18 (1993) 163. E. Occhiello, Chem. Ind., 761 (1997). A. Sato, T. Miyake and T. Saito, Shokubai (Catalysts), 34 (1992) 132. R. Meiers, U. Dingerdissen and W. F. H61derich, J. Catal., 176 (1998) 376 M. Haruta, Catalysis Surveys of Japan, 1 (1991) 61 and references therein. M. Haruta, Catal. Today, 36 (1997) 123 and references therein. M. Haruta, Stud. Surf. Sci. Catai., 110 (1997) 123 and references therein. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566. Y. A. Kalvachev, T. Hayashi, K. Tanaka and M. Haruta, Stud. Surf. Sci. Catal., 110 (1997) 965. M. Haruta, B. S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Intermed., 24 (1998) 329. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen., 190 (2000) 43. B. S. Uphade, Y. Yamada, T. Nakamura and M. Haruta, Appl. Catal. A: Gen., 215 (2000) 137. T. A. Nijhuis, H. Huizinga, M. Makkee and J. A. Moulijn, Ind. Eng. Chem. Res., 38 (1999) 884. E.E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 191 (2000) 332. G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee and J. A. Moulijn, J. Catal., 201 (1) (2001) 128. R.G. Bowman, H. W. Clark, J. J. Maj and G. E. Hartwell, PCT/US97/l1414, PCT Pub. No. WO 98/00413 (1998). T. Hayashi, M. Wada, M. Haruta and S. Tsubota, Jpn. Pat. Pub. No. H10-244156, PCT Pub. No. WO97/00869, U.S. Patent 5,932, 750 (1999). P.T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., 100 (1996) 17718. Z. Shan, E. Gianotti, J. C. Jansen, J. A. Peters, L. Marchese and T. Maschmeyer, Chem. Eur. J., 7 (2001) 1437. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature, 378 (1995) 159. M.G. Clerici and P. Inagallina, J. Catai., 140 (1993) 71.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

177

Sol-gel synthesis of colloid and triflates containing hybrid type catalysts A.N. P~.rvulescu l, B. C. Gagea 1, M. Alifantil, V. Pfirvulescu 2 and V. I. P~rvulescu 1 1-University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, fax" 401-3320588, Email: v_ [email protected] 2_ Institute of Physical Chemistry of Romanian Academy of Sciences, Splaiul Independentei 172, Bucharest, Romania. Various colloid and triflate derivatives embedded catalysts were prepared using sol-gel procedure. Ru, Pt and Pd-Au stabilized colloids were embedded in various inorganic matrices like silica, zirconia or silica-tantalum mixed oxide. Mono- or bifunctional catalysts resulted in function of the properties of the matrix support. Lanthanum, silver and tert-butyldimethylsilyl-trifluoromethanesulfonate in supramolecular ensembles with an ammonium-quternary salt were embedded using the same technique. The preparative conditions leading to the integrity of the embedded structures and the homogeneity of the resulted materials were investigated. To provide such information the catalysts were characterized using adsorption-desorption isotherms of nitrogen at 77 K, H2chemisorption, 1H, 13C and 29Si solid state MAS-NMR, XRD, TEM, SEM, XPS, TG-DTA, NH3-FTIR. 1. INTRODUCTION Literature is abundant in data concerning homogeneous catalytic performances of colloids and triflates [1-2]. Although these catalysts exhibit very high activity and selectivity in a large number of reactions of practical interest, they are rather non-stable and their recovery from the solution is difficult. Therefore the preparation of hybrid catalysts preserving these species unaltered is of current interest. In the case of colloids, the current methodology involves incorporation of these materials in polymers [3-4]. Covalent attachment of nanometer-scale colloidal particles to organosilane-coated substrates is a flexible approach to formation of macroscopic surfaces with well-defined nanostructure [5]. Variations in substrate (nature of the support), geometry (planar, cylindrical), functional group (-C,-NH2, -PR2, -SH) and particle diameter may lead to a large diversity in properties of these materials. Several studies have also been reported in the last years on the heterogeneization of triflic acid [6] and various triflates by impregnation [7, 8] or polymer incorporation [9, 10], and on selective acidcatalyzed reactions using these materials. The use of the sol-gel technique has been proved very effective in immobilization of various species leading to effective heterogeneous catalysts [ 11 ]. In addition, the sol-gel

178 procedure allows the control of texture (very high surface areas and the possibility to tailor the pore size) and may combine the properties of the immobilized species and the support leading to multifunctional catalysts [12-13]. The incorporation of molecular organic additives [14] or co-hydrolysis of alcoxides with alcoxide-derivatives [15] may provide introduction of active surface functional groups and changes in the catalyst morphology. The aim of this contribution is to present data on the preparation of catalysts containing as embedding species a large family of colloids such as colloids of ruthenium, platinum, or palladium-gold alloys and triflate derivatives such as lanthanum and silver triflate or tert-butyldimethylsilyltrifluoromethanesulfonate (BMSTM). Silica, zirconia and tantalum oxides were used as carrier. All these preparations considered the polymeric solgel route using as starting materials silicon, zirconium or tantalum alcoxides. The main problem raised by these preparations is to adapt the procedure to the solvent compatible with the embedding materials. Both the colloids and the triflate derivatives are soluble in a limited number of solvents, mostly not the alcohols generally used in such preparations. Therefore, the protocol procedure described in this contribution followed a specific sol-gel route for each hybrid catalyst. During these preparations we investigated: i) the formation of the sol by hydrolysis of the alcoxide in the selected solvent; ii) the addition of the stabilized colloid or triflate derivative in the formed sol; iii) the addition of a surfactant in the case of triflates; iv) the gelation, and finally v) the drying and calcining of the materials. 2. E X P E R I M E N T A L

The colloids were prepared using a reported procedure [1], while the triflate derivatives were commercially available. The present study considered the following stabilized colloids: Pt, Ru, and Pd-Au. Ru and Pd-Au were stabilized with N+(CgHlv)4Br (TOAB) and N+(CH3)3C16H33Br(TMCB) type ligands, while for Pt several other commercial surfactants were used (see Table 1). The first condition to provide a homogeneous catalyst is to carry out the sol-gel process in the same solvents as for embedding the species. As a function of the stabilizer ligand nature, the colloids were soluble either in tetrahydrofurane, ethanol, 2-propanol or toluene. Lanthanum and silver triflates were soluble in ethanol, and BMSTM in CC14. Accordingly, the alcoxides were dissolved in the above solvents and the preparation of the sols was adapted to these conditions. The hydrolysis was initiated in the presence of hydrochloric acid. All the hybrid materials precursors are either air or moisture sensitive and therefore the syntheses were carried out under inert atmosphere, namely in argon. The materials were dried under vacuum. The characterisation of the colloids both in the free and in the embedded state was first performed using transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and atomic absorption spectroscopy (AAS). In addition, nitrogen 29, adsorption-desorption curves at 77 K, Hz-chemisorption measurements, solid state SiNMR, XRD, SAXS, XPS, MAS-NMR, NH3-FTIR, and 197AuM6ssbauer spectroscopy were applied. For the embedded triflates, the catalysts were characterized by nitrogen adsorption-desorption isotherms at 77K, TG-DTA, 1H, 13C, and 29Si solid state MASNMR, XRD, TEM, SEM, XPS and, FTIR after adsorption of NH3.

179

3. RESULTS AND DISCUSSIONS 3.1. Sol-gel encapsulation of colloids Ru embedded colloids The colloids as prepared were embedded in a zirconia (samples Ru-Zr-TOAB and Ru-Zr-TMCB) or silica sol-gel matrix (samples Ru-Si-TOAB and Ru-TMCB) using the procedure reported elsewhere [19]. According to this method, the Ru colloids were added in the oxide gelation step. Tetraethoxisilicate (TEOS) was used as the precursor for the silica support. However, since the NR4-stabilized Ru colloids are completely insoluble and even decompose at elevated temperatures in alcoholic solutions, the normal sol-gel procedure must be modified using THF as solvent. The molar composition of the sol was TEOS:THF:H20:HC1 = 1:3.5:4:0.05. The colloid was added as a 4.5 wt.% THF-solution at room temperature, after refluxing. The sol was stirred vigorously at 70 ~ (under reflux) until the gelification was complete (after 2 days). To avoid any decomposition in the presence of air, all steps were carried out under Ar. The resulting gel was dried under vacuum at 110 ~ using a ramp of 0.12 ~ rain ~. For zirconia, the gel was obtained using zirconium propoxide (ZP) as precursor. Hydrolysis of ZP was carried out in the presence of acetic acid and the molar composition of the sol was ZP:THF:HzO:CH3COOH = 1:3.5:4:0.05. Samples with 2 wt. % Ru were prepared. Pt embedded colloids Platinum colloids (3% Pt) were incorporated in SiO2 and SiO2-Ta205 (15% Ta2Os) polymeric networks. TEOS and Ta(OC2Hs)5 (PET), respectively, were used as precursors. Since Pt colloids were soluble in ethanol, the sol composition (molar ratio) was: TEOS:EtOH:H20:HC1 of 1:4.5:4.0.02 for silica, and TEOS:PET:EtOH :H20:HC1 of 1:0.05:10:4.5:0.0.3 for SiO2-Ta205 mixed oxide. For mixed oxide, because of the differences in the hydrolysis rate of TEOS and PET, the silica sol was prepared in the first step. The pre-hydrolysis and condensation step was carried out at 80 ~ for 2h, then the mixture was cooled to room temperature. PET was separately solved in ethanol and added at room temperature to the preformed silica sol. Because of the high instability of PET and colloid against water, the key point in these syntheses is the step when they are added. In the present experiments PET and colloid were introduced at room temperature, under vigorous stirring, in the silica gel resulted by refluxing the silica sol at 80 ~ for 2h. After this the hydrolysis was finished, the rate of the condensation processes became stationary and the cross-linking chains process was dominant. The addition of the tantalum oxide sol to the silica preformed gel led to an important enhancement of the gelification rate. PET can be considered to be a cross-linking agent [17]. The water molecules are then dispersed among the silica species resulting from condensation and have no more influence upon the stability of PET and colloid. The silica gel formed during refluxing in acid conditions had a small condensation degree and this favors a better dispersion of tantalum and colloid and a homogeneous incorporation during polymerization and polycondensation. The control of the gelification rate was achieved by the parameters of the sol-gel process (pH and quantity of the solvent). The gelification was conducted at room temperature. Platinum colloids (see Table 1) were solved in ethanol.

180 Table 1. Pt containing catalysts Catalyst Composition SPt0 SIO2-3% Pt SPtl SIO2-3% Pt SPt2 SIO2-3% Pt SPt3 SIO2-3% Pt SPt4 SPt5 SPt6

SIO2-3% Pt SIO2-3% Pt SIO2-3% Pt

SPt7 TSPt0 TSPtl TSPt2 TSPt3

SIO2-3% Pt SiO2-15%Ta205-3% SiO2-15%Ta205-3% SiO2-15%Ta205-3% SiO2-15%Ta205-3%

TSPt4 TSPt5 TSPt6

SiO2-15%Ta205-3% Pt SiO2-15%Ta205-3% Pt SiO2-15%Ta205-3% Pt

TSPt7

SiO2-15%Ta205-3% Pt

Pt Pt Pt Pt

Stabilizer ligand Polyethyleneglycol-dodecylether 2-Hydroxy-propionic acid ARQUAD 2HT-75 QUAB 342 (3-chloro-2-hydroxypropyldimethyldodecylammonium chloride) REWO AGPVY 3 TWENN 40 (polyoxyethylene (20) sorbitan monopalmitate) N+(CsHly)4Br Polyethyleneglycol-dodecylether 2-Hydroxy-propionic acid ARQUAD 2HT-75 QUAB 342, 3-chloro-2-hydroxypropyldimethyldodecylammonium chloride REWO AGPVY 3 TWENN 40 (polyoxyethylene (20) sorbitan monopalmitate) N+(CsHly)aBr

Pd-Au alloys embedded colloids Embedding of the Pd/Au colloid was carried out using TEOS as precursor for the silica support. However, since the NR4-stabilized Pd/Au colloids are completely insoluble and even decompose at elevated temperatures in alcoholic solutions, the normal sol-gel procedure must be modified using THF as solvent. The molar composition of the sol was TEOS:THF:H20:HC1 = 1:3.5:4:0.05. The colloid was added as a 4.5 wt.% THF-solution at room temperature, after refluxing. The sol was stirred vigorously at 70 ~ (under reflux) until gelification was complete (after 2 days). To avoid any decomposition in the presence of air, all steps were carried out under Ar. The resulting gel was dried at 110 ~ using a ramp of 0.12 ~ min 1 then calcined under air at 450 ~ with a ramp of 0.3 ~ min -1. A few samples were dried by lyophilization. 3.1.1. Properties of the encapsulated colloids The stabilization of the colloids used various surfactants, which generate a protector layer. Incorporation of these entities in the sol-gel matrix resulted in large porous materials with pores developed around the stabilized-colloids. The size of the resulting pores was in concordance with the size of the embedded colloid. Some of these properties are summarized in Table 2. XRD, TEM, and M6ssbauer spectroscopy indicated no change in the colloid structure after entrapment in the sol-gel matrix (Figs.1 and 3). 29Si solid state MAS-NMR spectra gave no indication of a direct colloid-silica bond. Table 2 indicates that the resulting hybrid materials exhibit a rather high surface area corresponding to a large

181 pore texture. H2-chemisorption measurements also indicated large availability of metals to hydrogen. These data led to a model in which after drying the oxide matrix embedded a colloid which preserves its initial structure and remains surrounded by the stabilizer ligand. The catalytic data of these materials in hydrogenation or hydrogenolysis reactions confirm such a model [13,18]. The hybrid catalysts containing Ru and Pd-Au colloids were investigated in selective C=C (styrene), CrlC (hexynol) and allylic (cinnamaldehyde) hydrogenation. Drying the samples by lyophilization resulted in smaller surface areas and hydrogen up-take characteristics. Following this route, catalysts with a large spectrum of properties resulted. Only silica embedded colloid catalysts exhibited excellent hydrogenation properties. Mixing the silica with a second oxide like tantalum led to support acid properties as resulted from NH3-FTIR measurements, which conferred to these catalysts bi-functional properties. Fig. 2 shows in-situ NH3-DRIFT spectra. The adsorption of NH3 is consistent with the formation of a new band at 1460 cm ~ for SPT2, and 1440 cm l for TSPT2 as a contribution of v(H-N-H) in adsorbed NH4 +. The bands at 3240, 3045, and 2840 cm -1 result from the same contribution. The adsorption of NH3 also led to the disappearance of the band due to adsorbed water. These data indicated a Br6nsted acidity only for tantalum containing catalysts. No Lewis acid sites were detected. Table 2. General properties of the investigated hybrid catalysts Support Embedded species/loading Surface area m 2 -1 SiO2(SPt7) Pt colloid/3 wt% metal 76 SiO2 Pd-Au alloy/1 wt% metal 193 SiO2 (TOAB) Ru colloid/2 wt% metal 217 ZrO2 (TOAB) Ru colloid/2 wt% metal 51 Ta205 Pt colloid/3 wt% metal 23 SiO2- Ta205 (TSPt7) Pt colloid/3 wt% metal 198 SiO2 (A1) BMSTM 423 SiO2 (LaT1) La triflate 438 SiO2 (AgT1) Ag triflate 415 a-resulted from H2-chemisorption

Pore size

nm

3.5 3.7 4.7 4.7 3.6 3.5 2.5 2.5 2.5

Metal size a nm 2.8 2.9 4.3 4.3 2.8 2.8

3.1.2. Effect of calcining and reduction of colloids The dried samples were either reduced in flowing hydrogen (30 ml min l ) at 450 ~ using a ramp of 0.12 ~ rain l or previously calcined in air at the same temperature, and then reduced in similar conditions. As 13C solid state MAS-NMR showed during the thermal treatment, the stabiliser ligand was decomposed leading to carbonaceous deposits. The Hz-chemisorption was much suppressed, indicating that these carbonaceous deposits cover the colloid surface (Table 3). The same conclusion results from the textural measurements and catalytic activity data. Both reduced and calcined and reduced catalysts exhibited very small surface areas at least three times smaller than only dried catalysts. These data may allow the conclusion that such hybrid materials are effective catalysts only for reactions resulting at lower temperatures, namely, in conditions in which the organic part remained unaffected.

182

Pt~ ;!

TSPt2 treated ~ ~,.,

I

~

.

,r

~,

A,

~,,',

,

.

~s.~",~

Pt ,~ ~,,,V ~ , / , ~ . / ~ \ v r

~:

SPt2 fresh TSPt2 fresh

"

"~' o ~

~ ~ RT / ' NH3 - - T S P t 2

SPt2 treated

i ,

9

"'~/~i 7~]~' ;~i~ ~"'i ~-~' ~ r~'l ..... t-'""1"--'1 .... I .... 5 10 15 20 25 30 35 40 45 50 55

2 theta Fig. 1. XRD patterns of embedded colloids

-v

i

"

4000 3 5 0 0

,

i

,

1

9

"1

,

3 0 0 0 2500 2 0 0 0

i

'

i

1500

Wavenumbers (cm-1) Fig. 2. NH3-FTIR spectra of embedded colloids

Table 3. Influence of the calcination on Hz-up-take and surface area of colloid embedded catalysts Catalyst Surface area, m 2 g-1 Hz-uptake, cm 3 g-1 dried calcined dried calcined Ru-SiO2-TMCB 242 10 0.11 0.05 Ru-ZrO2-TMCB 38 1.5 0.20 0.07 Pd-Au-SiO2 193 15 0.20 0.06 SPtt 54 1.5 0.20 0.07 TSPtl 198 2.0 0.29 0.06 SPt2 48 3.8 0.20 0.08 TSPt2 138 3.4 0.31 0.09

3.2. Sol-gel encapsulated triflates The immobilization of triflate derivatives followed a different strategy. Two kinds of materials were prepared, one in which the triflate derivative is in a direct interaction with the surfactant leading to supramolecular entities and another in which only the triflate derivative existed. However, these are rather small molecules as compared to colloids, and thus in both cases the resulting pore size corresponded to mesoporous materials. The catalysts embedding BMSTM in a silica sol-gel matrix were prepared as following. Since the silyl triflate derivative is only soluble in carbon tetrachloride and triethylamine, the sol-gel synthesis was carried out in inert atmosphere (Ar), using an adapted route in which the silica sol was obtained by acid hydrolysis of a solution of tetraethoxyorthosilicate (TEOS) (10.4 g) in acidified CC14 (TEOS:CC14 molar ratio of 4, pH 1.5 with HC1 37%). Water was then added to the acidic solution in TEOS:H20 molar ratio of l:10 (samples A1 and A2) or 1:4 (sample A3), and the mixture was refluxed at 70~ for 2h. After cooling the solution at room temperature, the silyl-triflate derivative (0.38 ml) was added under vigorous stirring as a 0.3 M solution in CC14 (samples A1 and A3) or as such (sample A2). Hexadecyltrimethyl-ammonium bromide, as a surfactant, was then introduced (surfactant:TEOS ratio of 0.1) and the gelation was carried out at 90~ for

,

183 two days in a teflon cylinder in an autoclave (samples A2 and A3) or at room temperature for 6 days (sample A1). The resulting gel was dried under vacuum at room temperature for 24 h and then at 100~ for 6 h. Samples without surfactant were prepared as well.

d

r d

tD

o r~ ..(:3

,<

-8

1400

I

0

Veloclty, mm s-I

8

Fig. 3. Comparative M6ssbauer spectra of stabilized pure and SiO2 embedded Pd-Au colloids

1600

1800

2000

2200

Wavenumber, cl~ Fig. 4. NH3-FTIR spectra of embedded lanthanum triflate

The immobilization of lanthanum (LaT) and silver (AgT) triflate was made in the same way, but in ethanol. The samples for which the gelation was carried out at 90 ~ in a teflon cylinder within an autoclave where denoted as LaT2, AgT2, LaT3 and AgT3, or at room temperature for 6 days as LaT1 and AgT1. The immobilization and storage of AgT were carried out in a black chamber to avoid decomposition. The resulting gel was dried under vacuum, first at room temperature for 24 h then at 100 ~ for 6 h. Samples with 15 wt% triflate were thus obtained. The immobilization of BMSTM, LaT and AgT in a silica matrix using the sol-gel technique led to mesoporous materials with a monomodal pore size distribution. The surface area of these materials was rather high (Table 2). Characterization of these materials indicated that the triflate remained essentially unaffected during this process. XPS analysis showed a good dispersion of triflate as inferred from the comparison of the chemical and XPS S:Si ratios and a good integrity as shown from the analysis of the XPS S: F ratios. However, FTIR analysis suggested that in BMSTM, a small part of triflate was hydrolyzed, probably leading to a very small fraction of impregnated triflates. No similar evidence was found for LaT or AgT. FTIR analysis also indicated the existence of both Lewis and Br6nsted acid sites (Fig.4). The surfactant also remained embedded in the silica matrix, more probably incorporated as a supramolecular structure with triflate in the solid mesopores than in the silica network. The catalytic performances of the resulting materials were checked in specific reactions. All these catalysts showed excellent activity and selectivity at near room

184 temperature conditions. The hybrid catalysts containing triflates were tested in reaction of in n-hexane and cyclohexane isomerization [14, 19]. 4. CONCLUSIONS New hybrid catalysts might be obtained by embedding colloids or triflates derivatives and surfactants using the sol-gel technique. The nature of the solvent and pH at which are conducted these syntheses are the most important factors controlling the homogeneity and integrity of the embedded species. The morphology of the catalysts and the accessibility to the embedded species is preserved only in dried catalysts. Calcining led to the stabilizer decomposition, and to either the blocking of the active sites or partial decomposition of the active species. In conclusion, these hybrid catalysts are effective in low temperature reactions. ACKNOWLEDGMENT

The authors gratefully acknowledge the valuable support of Prof. H. B6nnemann from MPI Germany, for kindly supplying stabilized metal colloids and Prof. P. Grange, Belgium, for XPS and DRIFT characterizations. REFERENCES

1. H. B6nnemann, G. Braun, W. Brijoux, R. Brinkmann, A. Schulze Tilling, K. Seevogel and K. Siepen, J. Organometal. Chem., 520 (1996) 143. 3. C. Qian, L. Wang, Tetrahedron, 56 (2000) 7193. 4. T. Sugimoto, Adv. Coll. Interface Sci. 28 (1987) 65. 5. N. Toshima, J. Macromol. Sci. Chem., A27 (1990) 1225. 6. K.C. Grabar, K. J. Allison, B. E. Baker, R. M. Bright, K. R. Brown, R. G. Freeman, A. P. Fox, C. D. Keating, M. D. Musick and M. J. Natan, Langmuir, 12 (1996) 2353. 7. M.G. Clerici, C. Perego, A. De Angelis and L. Montanari, Eur. Patent 638 363, ! 995. 8. N.E. Drysdale and N. Herron, WO Patent 9502625, 1994. 9. K. Wilson, A. Renson and J. H. Clark, Catal. Lett., 61 (1999) 51. 10. S. Kobayashi and S. Nagayama, J. Am. Chem. Soc., 120 (1998) 2985. 11. F. Schager and W. Bonrath, Appl. Catal. A: General, 202 (2000) 117. 12. M. T. Reetz, Advanced Materials, 9 (1997) 943. 13. H. BOnnemann, U. Endruschat, B. Tesche, A. Rufinska, C. W. Lehmann, F. E. Wagner, G. Filoti, V. P~rvulescu and V. I. Pfirvulescu, Eur. J. Inorg. Chem., (2000) 819. 14. A. N. Pfirvulescu, B. C. Gagea, M. Alifanti, V. P~rvulescu, V. I. Pfirvulescu, S. Nae, A. R_zu_, G. Poncelet and P. Grange, J. Catal., 202 (2001) 319. 15. S. W. Carr, L. Courtney and A. C. Sullivan, Chem. Mater., 9 (1997) 1751. 16. Y. Sugahara, T. Inoue and K. Kuroda, J. Mater. Chem., 7 (1997) 53. 17. M. L. van Bommel, T. N. M. Bernards, E.W.J.L. Oomen and A. H. Boonstra, J. NonCryst. Solids, 147& 148 (1992) 80. 18. S. Oprescu, V. Pfirvulescu, A. Petride, M. D. Banciu and V. I. P~rvulescu, J. Mol. Catal., in press. 19. B. C. Gagea, A. N. P~rvulescu, V. I. P~rvulescu, V. Pgrvulescu, P. Grange and G. Poncelet, ARCHIVOC, in press.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

185

Preparation of Zeogrids through interposed stapling and fusion of MFI zeolite type nanoslabs S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, F. Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Ordered porous materials, called Zeogrids, are obtained by aggregation of Silicalite-1 nanoslabs upon addition of a secondary template (cetyltrimethylammonium bromide), followed by calcination. The synthesis leads to a lamellar arrangement and fusion of the slabs and a high pore volume, distributed over ultra-micropores (0.14 mL/g) and supermicropores (0.55 mL/g). The use of a secondary template with shorter alkyl chain (dodecyltrimethylammonium bromide) leads to a similar structure, but results in a lower super-microporosity (0.33 mL/g). The interaction of the secondary template with the nanoslabs and their subsequent aggregation were monitored by DLS. The materials were characterized using SEM, XRD, TGA, N~ adsorption and tracer gas chromatography. I. INTRODUCTION Zeolites are currently manufactured as micron size crystals and compacted into millimeter size pellets for applications as packed beds. In many catalytic and adsorptive applications, mass and heat transfer properties could potentially be improved by structuring the zeolite in a different way. Research in this area led already to significant achievements. Alternatively structured zeolite matter are e.g. delaminated zeolites [1], supported zeolite films and membranes [2], hybrid structures with microporosity in walls of ordered mesoporous materials [3-6] and nanosized zeolites such as those synthesized in confined space [7]. The common property of these alternative zeolites is that at least in one direction, the zeolite framework has a dimension of around a nanometer. Recently, we developed an alternative approach to synthesize nanoscopically arranged zeolite material. The preparation departs from Zeosil nanoslabs having the Silicalite-1 framework type and measuring 1.3 x 4.0 x 4.0 nm3 [8, 9]. The formation of these nanoslabs occurs upon the room temperature hydrolysis of tetraethylorthosilicate (TEOS) in concentrated aqueous solution of tetrapropylammonium hydroxide (TPAOH). We discovered a method to obtain an interposed fusion of these nanoslabs into a new material refered to as Zeogrid [10]. The formation process of this microporous solid involves nanoslab stapling in layers with intercalated surfactant molecules. Removal of the surfactant through calcination causes facial and lateral fusion of the nanoslabs (Fig.l). Empty spaces lett laterally are responsible for the super-microporosity. The ultramicropores are the zeolitic channels inside the fused nanoslabs.

186

Fig.1. Genesis of Silicalite-1 Zeogrid. The rectangles represent the Silicalite-1 nanoslabs. In this work we investigated the formation process of two different types of Zeogrids (ZG) in presence of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB), respectively. 2. E X P E R I M E N T A L

Preparation of ZGCTABand ZGDTAB.Silicalite-1 nanoslab suspension was obtained by hydrolyzing 37.32 g TEOS (Acros, 98%) in 32.13 g aqueous TPAOH solution (Alfa, 40 wt.- %) under continuous stirring. After the hydrolysis recognized by the homogenization of the two liquids, 30.55 g de-ionized water was added and stirring continued for 24 h. To an amount of 20 mL of this suspension, a same volume of ethanol (technical grade) was added. Subsequently, 60 mL of a 12 wt.-% solution of CTAB (Acros, 99%+) in ethanol was added dropwise under stirring (30 mL/h). A white precipitate formed during CTAB addition. The product was left undisturbed for 24 h, whereafter it was recovered by filtration over 8 ~tm pore diameter paper (Whatman 1440 090, grade 40), washed with ethanol on the filter and dried at 60~ Part of the precipitate was then compressed into flakes and the flakes crushed and sieved in order to obtain particles with diameters of 0.250.5 mm. An amount of 1 g of sample was loaded as a packed bed in a quartz tube and subjected to a first calcination step at 400~ under nitrogen gas and a second step at 400~ under oxygen gas. The heating rate was 0.5~ and the gas flow 30 mL/min. A second preparation (ZGDTAB) was done using DTAB as template (Acros, 99%). The template solution was added at once under stirring. No precipitation was observed. A white precipitate formed after lh under quiescent conditions. Characterization of ZGCTABand ZGDTAB. Dynamic light scattering (DLS) was done using an ALV-NIBS High Performance Particle Sizer, calibrated with polystyrene spheres of a diameter of 20 + 1.5 nm (Duke Scientific Co.). The samples were filtered (PTFE filters with 200 nm pore diameter, Alltech). The measurements were done in new polystyrene cells. The particle sizes were corrected for variation of the viscosity. The relative viscosities of the samples were determined using a U-tube Ostwald viscometer (Brand). Scanning electron microscopy (SEM) was performed with a JXA-733 JEOL instrument on gold plated samples. X-ray diffraction (XRD) was performed using a Siemens 5000 diffractometer with a Cu K a source (L = 0.154 nm). Thermogravimetric analysis (TGA) was done in a Setaram TGA 92 balance, at a heating rate of 1~ under 20% oxygen in helium atmosphere. Nitrogen adsorption experiments (-196~ were carried out in an Omnisorp instrument (Coulter), operated in the continuous flow mode. The samples were pretreated during 15 hours at 200~ under vacuum (1 Pa).

187 3. RESULTS AND DISCUSSION

3.1. Formation of the Zeogrids The formation of ZGCTABand ZGDTABis followed by in situ DLS (Fig.2). The formation of a precipitate from clear nanoslab suspension following the usual recipe is immediate upon template addition. Therefore, the amount of CTAB solution is reduced from 60 mL to 20 mL to slow the process and to allow for DLS investigation. The maximum in the particle size distribution in the original suspension, before CTAB addition (0'), is at 2.8 nrn, in agreement with the presence of nanoslabs measuring 1.3 x 4.0 x 4.0 nm3 [9]. Addition of CTAB leads first to an increase in particle size in the nanometer range (15' and 25'), followed by a sudden emergence of larger particles (35'). The nanoslabs first aggregate to particles of ca. 100 nm, whereafier a second population at ca. 1,000 nm appears (40'). The size increase in the nanometer range is ascribed to template adsorption on the surface of the nanoslabs and moderate nanoslab aggregation. The observed evolution of particle sizes suggests the existence of a critical CTAB surface coverage coupled with limited nanoslab aggregation, from which the elementary units rapidly organize into large particles. In the ZGDTABpreparation, the original particle size of 2.8 nm increases to ca. 10 nm within the first 55'. After 65', particles beyond a size of 100 nm are formed. The critical elementary unit is significantly larger in the DTAB system (Fig.2). The differences must originate from the size differences of the cetyl versus dodecyl chain. 0' A

15'

25' 40' 35'

5

CTAB

F_

f

0' A

40'

10 10' 20' 30' 45'

55'

i

100 1

90'

:5

t

I

I

10

100

1000

,,~5'

65'

90'

DTAB

.E I

1

10 size (nm)

i

100 1

i

10

100

1000

size (nm)

Fig.2. Normalized particle size distribution during ZGCTAB(top) and ZGDTAB (bottom) formation as a function of time (indicated near the curves) determined with DLS. Time zero corresponds to the starting nanoslab suspension before template addition.

188 3.2. Characterization of the Zeogrids

SEM reveals the as synthesized ZGCTAB and ZGDTAB to be composed of spherical particles with an average diameter between 2 and 10 micrometer (Fig.3a and b). Many spheres are twinned. Sometimes there is complete fusion of differem spheres. Given the layered nature of Zeogrid (Fig.l), it is assumed that in these spheres, the layers are packed in a concemric manner.

Fig.3. SEM micrographs of as synthesized ZGCTAB(a) and ZGDTAB(b). X-ray diffractograms of as synthesized ZGCTAB (b) and ZGDTAB(a) are shown in Fig.4. Both patterns reveal the presence of a layered compound showing first and second order diffraction. The repeat distance in ZGcT~ and ZGDT~ are 3.1 nm and 3.0 ran, respectively. Bragg type diffraction of crystalline (001) material at wider angles is not observed. It is our experience that with CTAB, the repeat distance is sensitive to slight =l variation in the preparation procedure, and especially to W the stirring conditions. The dC values can vary from 4.0 [10] C to 3.0 nm. After removal of the secondary template by (OOl) 9 calcination and annealing of (002) a the Silicalite-1 nanoslabs, the repeat distance invariably is 5 10 15 20 25 30 35 40 3.0 nm (Fig.l). A

t

b

dllll =m

2theta

(~

Fig.4. X-ray diffraction patterns of as synthesized ZGDTAB(a) and ZGcTAB(b).

189 Thermogravimetric analysis is performed on as synthesized ZGCTAB (Fig.5) and ZGDTAB. The ZGcTA~ sample undergoes 53% weight loss upon heating from ambient temperature to 600~ (Fig.5a). DTG curve reveals three consecutive weight losses (Fig.5b). Weight loss I (4%) is situated below 100~ and is ascribed to water desorption. The further weight losses occur around 175~ (II, 37%) and 275~ (III, 7%). Weight loss IH coincides with a significant exothermic effect according to DTA (Fig.5c). From a -lO previous TG analysis of nanoslabs [9], it is known that the occluded TPA gives rise to ,-. -20 exothermic weight losses at temperatures ~ ' ~ -30 exceeding 240~ Weight loss III thus is (3 I-- -40 attributed to occluded TPA. Weight loss II is attributed to the evacuation of surfactant. -50 The relatively low temperature at which -60 CTMA is evacuated and the absence of appreciable thermal effects can be explained by the co incidence of exothermic condensation of silanol groups r -0,1 and endothermic desorption of the E secondary template. The TG/DTG/DTA ~ -o,2 curves of ZGDTAB (not shown) are similar i-. -0,3 to those of ZGcTAB. The total weight loss is 45%, distributed over weight losses of 5%, 29% and 6%, centered at 40~ 180~ and -0,4 285~ respectively. The main difference between ZGcTAB and ZGDTAB is found in 3 C weight loss II, ascribed to surfactant evacuation (37% for ZGcTAB and 29% for :i 2 ZGDTAB). The similar d-values for ZGcTAB and ZGDT~ observed with XRD (Fig.4) suggest that the higher surfactant content of ZGcT~ compared to ZGDT~ is located 0 between individual nanoslabs composing the layers, resulting in a wider lateral of the nanoslabs in as 0 100 200 300 .400 500 600 spreading synthesized ZGCTAB. t e m p e r a t u r e (*C) i

i

i

i

A

, m

A

i

i

i

i

i

Fig.5. Thermogravimetric analysis (a), differential thermogravimetric analysis (b), and differential thermal analysis (c) of as synthesized ZGcTAB sample. Nitrogen adsorption is performed on calcined ZGcTAB and ZGDTAB, as well as on reference Silicalite-1 with a particle size of about 100 nm. The three adsorption isotherms are shown in Fig.6 (black traces). Both Zeogrids exhibit a type I isotherm with a long, almost horizontal plateau, characteristic of a microporous material with a small specific external surface area (Fig.6a and b). Silicalite-1 also exhibits a type I isotherm with a

190 shorter plateau at a lower nitrogen uptake, followed by an upward deviation owing to capillary condensation in the void spaces in between the crystallites (Fig.6c). The Silicalite-1 isotherm is fitted using both a Dubinin-Radushkevich equation for the ultra-micropore contribution and a BET equation for the meso- and macropore contribution [11]. The Zeogrid curves are fitted using similar procedure, except that the supermicropores are characterized using a modified BET type expression, accounting for a finite number of adsorbed nitrogen layers [ 10]. The fittings are represented in Fig.6 (gray traces) and the results are given in Table 1. The presence of a same ultra-micropore volume in ZGCTAB and ZGDTAB samples reflects the presence of a same quantity of Silicalite-1 nanoslabs per weight of calcined material. It is striking that the super-micropore volume is significantly smaller in ZGDT~ (0.33 mL/g) compared to ZGcT~ (0.55 mL/g). 500

a 400

/

/

"-" 300 _J

/

/

f

f--'-

J

b

E

./

200

100

f

0

i

i

i

i

0,2

0,4

0,6

0,8

1

pipo Fig.6. N2 physisorption isotherm at -196~ of calcined ZGcTAB (a), ZGDTAB (b) and Silicalite-1 (c) (black traces). The respective fitted curves are shown (gray traces).

Table 1. Fitting of experimental N2 adsorption isotherms with a model accounting for ultramicropore filling and multilayer adsorption in super-micropores i

Vultra-mieropore (mL/g)

Vsuper-micropore (mL/g)

Vmeso-,maeropore (mL/g)

Ssuper-micro-,meso-, CBET macropore (m2/g) ........

Silicalite- 1

0.138

-~ 0

ZGCTAB

0.138

0.55

ZGDTAB

0.138

0.33

0.40

147

83

- 0

1224

48

-~0

785

84

191 The super-micropores arise from the interposed fusion of the nanoslabs during the calcination [10]. In the as synthesized ZGCTAB, the nanoslabs are laterally more separated than in the as synthesized ZGDTAB. This wider spreading seems to be maintained after the calcination. The smaller super-micropore volume of ZGDTAB is most likely the result of a smaller diameter of the individual super-micropores. The Zeogrids are loaded in 3 cm long chromatographic columns and evaluated as packing for molecular separations using a pulse chromatographic setup. Chromatograms of a octane/iso-octane mixture injected as pulses at 130~ on ZGCTAB and ZGDTAB columns are shown in Fig.7. On both columns, iso-octane elutes faster than octane. This sequence reflects the a exclusion of iso-octane from the Silicalite-1 type ultra-micropores, present in the nanoslabs building the Zeogrids (Fig.l). A moment analysis of the chromatographic peaks is provided in Table 2. ZGCTAB and ZGDTAB show similar separation b factors a between iso- and linear octane, pointing to the presence and to the integrity of identical ultra-microporous building blocks, being the nanoslabs, used for the preparation of both materials. The mass transfer resistance of ZGcTAB is lower than the one of ZGDTAB, as reflected by ,,,,, ""~, 3 , 4 , 5 the lower 02/2~ 2 value. This is in agreement with narrower super-micropores present in the ZGDTAB 0 1 2 3 4 5 sample compared to ZGCTAB. time (rain) !

Fig.7. Chromatograms of binary mixtures of octane and iso-octane separated on ZGcTAB column (a) and ZGDTAB column (b) at 130~ Table 2. Moment analysis of chromatographic peaks : retention times at peak maximum (tMAX), mean retention times (~t), separation factors (a), standard deviations of chromatographic response peaks (o) and mass transfer resistances (o2/2p z) in pulse chromatographic experiments with a mixture of octane (n-C8) and iso-octane (2,2,4-TMC5) separated on ZGcTAB and ZGDTABcolumn at 130~ i

ZGCTAB tMAX(S) (s)

ZGDTAB

n-C8

2,2,4-TMC5

n-C8

2,2,4-TMC5

1.13

0.67

2.72

1.67

1.04

0.61

2.47

1.53

a (s)

1.70

1.61

o (s)

0.174

0.093

0.529

0.307

o 2 / 2~ 2 (xlO 3)

14

12

23

20

192 4. CONCLUSIONS The formation of Zeogrid starting from a Silicalite-1 nanoslab suspension and using CTAB or DTAB as surfactant proceeds according to a similar mechanism. DLS reveals that the surfactant molecules first adsorb onto the nanoslabs, which undergo limited aggregation. After a critical size is reached, the slabs staple into micrometer size spherical particles according to SEM, having a layered structure according to XRD and a large content of surfactant according to TGA. These three observations can be reconciled by the presence of concentric layers of nanoslabs, intercalated with surfactant. In the nanoslab annealing process upon removal of the surfactant, empty spaces are created. These spaces represent super-micropores. The smaller volume of super-micropores in ZGDTAa, determined with N2 adsorption, and the larger mass transfer resistance in the column packed with ZGDTAa, determined by tracer chromatographic experiments, indicate that the super-micropores are narrower in ZGoTAB compared to ZGcTAB. The more "open" stapling of the nanoslabs in ZGCTAB compared to ZGDTAa originates from a wider lateral spreading of the slabs in the aggregates, resulting from a larger surfactant volume between nanoslabs in the individual layers, as derived from TGA and XRD analyses. ACKNOWLEDGEMENTS

The authors acknowledge the IAP-PAI and the PRODEX office for financial support. P.J.G. acknowledges the Flemish F.W.O. for a position as Research Director. REFERENCES

1. A. Corma, V. Fomes, S.B. Pergher, T.L. Maesen and J.G. Buglass, Nature, 396 (1998) 353. 2. J. Caro, M. Noack, P. Kolsch and R. Schafer, Microporous Mesoporous Mater., 38

(2000) 3. 3. Z. Zhang, Y. Han, F.S. Xiao, S. Qiu, L. Zhu, R. Wang, Y. Yu, Z. Zhang, B. Zou, Y. Wang, H. Sun, D. Zhao and Y. Wei, J. Am. Chem. Soc., 123 (2001) 5014. 4. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao and F.S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 5. Y. Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 6. Y. Liu and T.J. Pinnavaia, Chem. Mater., 14 (2002) 3. 7. I. Schmidt, C. Madsen and C.J.H. Jacobsen, Inorg. Chem., 39 (2000) 2279. 8. C.E.A. Kirschhock, V. Buschmann, S. Kremer, R. Ravishankar, C.J.Y. Houssin, B.L. Mojet, R.A. van Santen, P.J. Grobet, P.A. Jacobs and J.A. Martens, Angew. Chem. Int. Ed., 40 (2001) 2637. 9. R. Ravishankar, C.E.A. Kirschhock, P.P. Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C.E.A. De Schryver, G. Miehe, H. Fuess, B.J. Schoenmn, P.A. Jacobs and J.A. Martens, J. Phys. Chem. B, 103 (1999) 4960. 10.S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, F. Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens, Adv. Funct. Mat., 12 (2002) 286. 11 .M.J. Remy and G. Poncelet, J. Phys. Chem., 99 (1995) 773.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

193

Large scale synthesis of carbon nanofibers by catalytic decomposition of hydrocarbon L. Pesant, G. Win~, R. Vieira, P. Leroi, N. Keller, C. Pham-Huu*, M.J. Ledoux Laboratoire des Mat6riaux, Surfaces et Proc~d~s pour la Catalyse (LMSPC), UMR 7515 du CNRS, ECPM, Universit~ Louis Pasteur, 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 02, France. Tel: +33 3 90 24 26 75, Fax: +33 3 90 24 26 74, * e-mail: [email protected]

Large scale carbon nanofibers with homogeneous diameter, i.e. 50 nm, have been synthesised by catalytic hydrocarbon decomposition over a nickel supported catalyst. The carbon nanofiber yield of several dozen of grams per day has been obtained by optimizing the different synthesis conditions. At medium synthesis temperature the carbon nanofibers only expose prismatic planes on the outer surface of the material providing a high interaction with any deposited metal particle. As an example palladium particles were well dispersed on this surface with an average particle size of around 5 nm. The strong interaction between the metal and the carbon nanofiber surface led to faceted metal particles. This catalyst exhibited a high activity in the hydrogenation of nitrobenzene into aniline under moderate pressure. 1. I N T R O D U C T I O N At the beginning of the 21 st century, it is thought that the improvement of the existing chemical industry will pass through the development of new kinds of catalysts to meet the latest environmental requirements. Since their discovery in 1991 [1], carbon nanotubes and nanofibers have received increasing interest in academic research and for several potential applications [2-4]. It has been reported by different authors that carbon nanofibers could be efficiently used as catalyst support for several catalytic reactions either in gas or in liquid phase [5-7]. In both cases, the carbon nanofiber based catalysts always exhibited higher catalytic performances compared to those observed on equivalent conventional catalysts. Carbon nanofiber materials exhibit several advantages with respect to the conventional support materials: (i) a high external surface area in a nanometric-scale size which significantly improves the mass transfer, especially in a liquid medium, (ii) a strong interaction generated by the exposed prismatic planes towards the active phase, which can induce peculiar catalytic activities and selectivities. However, to be used in an efficient way the carbon nanofibers should be able to be synthesized on a large scale with a uniform diameter and a low price. From the numerous reviews published on the subject its seems

194 that Chemical Vapor Deposition (CVD) is the most efficient method for the production of a large amount of carbon nanostructured materials with high selectivity when compared to the physical methods such as laser ablation or arc-discharge [8-12]. The aim of the present article is to report the large scale (several hundred grams per gram of active phase) synthesis of uniform carbon nanofibers (average diameter ranging between 40 and 60 nm) by the catalytic decomposition of a mixture of ethane and hydrogen over a nickel catalyst supported on carbon nanotubes. To illustrate their catalytic potential, the as-synthesized carbon nanofibers are subsequently used as catalyst support for palladium in the hydrogenation of nitrobenzene in a liquid phase reaction. 2. E X P E R I M E N T A L S E C T I O N

2.1 Catalyst preparation The nickel based catalyst was prepared by immersion of the carbon nanotube support (under stiring) in a small excess of an aqueous solution of nickel nitrate (Ni(NO3)2.6H20, Merck) relatively to the pore volume of the material. The tubes were then impregnated with a volume of benzene corresponding to the tubule internal volume. Due to the high affinity of the benzene with the carbon nanotube the aqueous solution containing nickel salt was immediately pushed outside the tubules. After calcination (air at 300 ~ for 2 h) and reduction (H2 at 400 ~ for 2 h) the carbon nanotubes decorated with nickel metal were obtained. The concentration of nickel measured by ICP-MS was 20 wt.%. The TEM micrograph of the sample reduced under flowing hydrogen at 400~ unambiguously proved that the nickel phase was exclusively located on the outer walls of the carbon nanotubes with a particle mean diameter of about 40-50 nm [12]. It should be noted that despite the low reactivity of the exposed basal planes of the carbon nanotubes the nickel particles were well dispersed without formation of aggregates.

2.2 Carbon nanofiber synthesis The carbon nanofiber (CNF) synthesis was performed at atmospheric pressure at temperatures ranging from 550 to 650 ~ and under various mixtures of ethane and hydrogen. After synthesis the material was cooled to room temperature under the synthesis mixture and then discharged. Due to the high carbon nanofiber yield no post-treament was needed as usually encountered with other preparation methods such as acidic treatments in order to remove the metal catalyst from the final product.

2.3 Characterisation techniques The as-prepared sample was characterised by SEM (Philips) and TEM (Topcon EM002-B with a point-to-point resolution of 0.17 nm). The specific surface area was measured by the BET method using N2 as adsorbant at LN2 (SA-3100 sorptometer). The sample was outgassed under dynamic vacuum at 200 ~ for 2 h before measurement.

195 3. RESULTS AND DISCUSSION 3.1 Yield of carbon nanofibers

The total weight of the resulting sample was significantly increased compared to the starting weight of the metal catalyst, i.e. 17 g of carbon nanofibers were obtained from 0.04 g of the metal catalyst. The cumulated yield of the carbon nanofibers at 650 ~ and after 12 hours of synthesis was > 8000 wt. % (425 g of CNFs per gram of catalyst). The high carbon nanofiber yield observed was attributed to the high external surface area of the carbon nanotube support towards the gaseous reactants which significantly increases the number of nuclei for carbon nanofibers formation. The presence of hydrogen in the feed keeps the metal catalyst surface free and provides adsorption-decomposition sites for the hydrocarbon which, in turn, leads to the production of carbon nanofibers. Hydrogen also rapidly saturates the dangling bonds of the carbonized product during the synthesis. However, the hydrogen concentration should be keep below a certain level in order to avoid carbon gaseification which in turn, leads to a carbon loss [13]. Fig. 1. SEM micrograph of the assynthesized carbon nanofibers after CVD at 600~

3.2 Microstructure and surface area characteristics

The carbon nanofibers formed were extremely homogeneous with a mean diameter of about 50 nm and lengths up to several hundred micrometers (Fig. 1). From statistical SEM observation, it seemed that their diameter did not depend on the synthesis temperature but only on the initial diameters of the nickel catalyst particles [12]. This observation was attributed to the absence of significant sintering of the supported nickel phase when the reaction temperature increased from 550 ~ up to 650 ~ The TEM image presented in Fig. 2a clearly showed that the carbon filaments formed were nanofibers in nature with graphene planes stacked in a fishbone structure along the fiber axis with an angle of 75 o between them. The TEM image (Fig. 2b) indicates that the carbon nanofiber growth occurred in several directions from single nickel catalyst particles (octopus-like) which indicates that during the course of the synthesis the nickel surface underwent faceting with a concomitant formation of several active faces for carbon dissolution and precipitation. Fig. 2c shows the high resolution TEM image of the interface between the nickel particle and the carbon nanofiber growing from it.

196

Fig. 2. (a) TEM image of the carbon nanofibers showing graphene planes stacked in a fishbone structure along the fiber axis. (b) TEM image showing the formation of carbon nanofibers in several directions from a single nickel particle. (c) High resolution TEM image showing the nickel face which was active for carbon nanofibers growth. The carbon nanofibers obtained had a specific surface area measured by N2 adsorption ranging from 100 to more than 250 ma/g depending on the synthesis temperatures. Such a surface area was of the same order as those reported in the literature [5,14,15]. The presence of graphite edges which could act as adsorption sites for nitrogen was proposed to explain this high surface area. Fig. 3 shows the TEM micrographs of the carbon nanofibers as a function of the synthesis temperature at 600 and 700 ~ At low synthesis temperature the carbon nanofiber structure was more disordered, with a non-regular diameter (Fig. 3a). The structure became more ordered when the synthesis temperature was increased to 700 ~ (Fig. 3b). High-resolution TEM micrographs (not shown) also revealed the presence of a nickel particle at some fiber tips which indicated that during the synthesis both tip and base growth mechanisms occurred. 3.3 Nitrobenzene hydrogenation The hydrogenation of nitrobenzene was carried out on palladium supported on the carbon nanofibers prepared according to the preceding method without further purification and compared to a commercial palladium catalyst supported on a high surface area activated charcoal (Aldrich, 970 m2/g).

197

Fig. 3. TEM images of the carbon nanofibers structure as a function of the synthesis temperatures: (a) 600 ~ (b) 700 ~

Fig. 4. TEM micrograph of the palladium (5 wt.%) supported on carbon nanofibres showing the presence of faceted metal particles.

The reaction was performed in a batch reactor in an autoclave under 20 atmospheres of hydrogen and at room temperature. A liquid sampler allowed the progress of the reaction to be followed by product analyses. The products were analysed by gas chromatography in a Varian GC-3400 equipped with a capillary column (PONA) allowing the separation of the different reactant and products. The TEM image of the CNFbased catalyst is presented in Fig. 4. The palladium particles were significantly dispersed on the entire surface of the support with an average particle size of ca. 5 nm. The metal particle was highly faceted which indicates the existence of a strong metal-support interaction between the prismatic graphite planes and the metalparticles.

198 The catalytic activity is reported in Fig. 5 as a function of time. The hydrogenation activity, expressed in terms of conversion, was slightly higher over the palladium supported on carbon nanofibers when compared to that of the commercial catalyst supported on a high surface area activated charcoal (AC). The high hydrogenation activity of the CNF-based catalyst, despite its lower specific surface area when compared to that of the activated charcoal, was attributed to the existence of a strong interaction between the metal and the prismatic planes exposed by the support which in turn, modified the electronic properties and the turnover frequency of the metallic palladium active site. Similar results have also been reported by Baker's group [16] for nickel catalyst supported on a high surface area filamentous graphite during the hydrogenation of linear olefins in gas phase reactions. The authors have attributed this catalytic behaviour to the modification of the nickel morphology, i.e. flat metal particle instead of hemispheric particle, due to the strong interaction with the support surface.

100

o~ v

80-

I

I

I

I

F7 Pd/C m Pd/Aq

C .O m t.b > C O O C N C O

"='9z

60-

40-

20-

0

180

270

360

450

Time on stream (h) Fig. 5. Hydrogenation activity, expressed in terms of conversion, over the Pd/CNFs and Pd/AC catalysts at room temperature and under 20 bar total pressure. 4. C O N C L U S I O N Low surface area carbon nanotubes can be efficiently used as a catalyst support for the large scale synthesis of carbon nanofibers under relatively mild synthetic conditions, i.e. < 650 ~ The material obtained has a relatively high surface area (100-250 mZ/g depending on the synthesis temperature) with a mesoporous distribution. Both high carbon

199 yield and selectivity to the fibers can allow the direct use of these nanofibers as catalyst or catalyst support, without any traditional acidic and sonication purifications. The high external surface area and the high density of carbon edges (low-temperature synthesis) of such materials make them very attractive for use as catalyst supports for liquid phase reactions. The Pd/CNF catalyst displays a catalytic activity as high as that obtained on a commercial catalyst supported on activated charcoal despite the large difference between the two supports surface area, i.e. 100 m2/g for the CNFs instead of 1000 m2/g for the activated charcoal. The high hydrogenation activity observed on the CNF-based catalyst was attributed to the high external surface area of the support and to the peculiar interaction existing between the prismatic planes and the metallic particles. Recently, a significant improvement was introduced via a new synthesis route which allows the possibility for these nanoscopic materials, to be supported on a macroscopic host structure [17]. ACKNOWLEDGEMENTS The authors would like to thank members of the Inorganic Chemistry Department of the Fritz-Haber-Institut in Berlin for performing part of the experimental work and Prof. R. Schl6gl for helpful discussions.

REFERENCES .

2. ,

4. 5. 6. 7.

10. 11.

12. 13. 14. 15.

S. Iijima, Nature, 354 (1991) 56. M.S. Dresselhaus, G. Dresselhaus and P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1995. C. Laurent, E. Flahaut, A. Peigney and A. Rousset, New J. Chem., (1998) 1229. P.M. Ajayan, Chem. Rev., 99 (1999) 1787. C. Park and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 5168. F. Salman, C. Park and R.T.K. Baker, Catal. Today, 53 (1999) 385. C. Pham-Huu, N. Keller, G. Ehret, L. Charbonni6re, R. Ziessel and M.J. Ledoux, J. Mol. Catal. A : Chemical, 170 (2001) 155. C. Pham-Huu, N. Keller, G. Ehret and M.J. Ledoux, J. Catal., 200 (2001) 400. In Carbon Nanotubes : preparation and properties (Ed. T.W. Ebbesen) , CRC, Boca Raton, 1997. P.M. Ajayan, Chem. Mater., 11 (1999) 3862. P.M. Ajayan and O.Z. Zhou, in Carbon Nanotubes. Synthesis, Structure, Properties and Applications, Topics in Applied Physics, Vol. 80 (Eds. M.S. Dresselhaus, G. Dresselhaus and Ph. Avouris), Springer, Heidelberg, 2001, pp.391-425. C. Pham-Huu, N. Keller, V.V. Roddatis, G. Mestl, R. Schl6gl and M.J. Ledoux, Phys. Chem.-Chem. Phys., 4(3) (2002) 514. L. Ci, Y. Li, B. Wie, J. Liang, C. Xu and D. Wu, Carbon, 38 (2000) 1933. K. Hernadi, A. Fonseca, J.B. Nagy, D. Bernaerts and A.A. Lucas, J. Mol. Cat. A: Chemical, 107 (1996) 159. M.-S. Kim, N.M. Rodriguez and R.T.K. Baker, Mater. Res. Symp. Proc., 368 (1995) 99.

200 16. 17.

A. Chambers, T. Nemes, N.M. Rodriguez and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 2251. C. Pham-Huu, R. Vieira, M.J. Ledoux, L. Charbonni~re and R. Ziessel, French Pat. Appl. No. 01-15178, assigned to Sicat, 2001.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

201

Synthesis and characterization of carbon nanofiber supported ruthenium catalysts M.L.Toebes a, F.F. Prinsloob, J.H. Bitter a, A.J. van Dillena and K.P. de Jong a* aUtrecht University, Debye Institute, Department of Inorganic Chemistry and Catalysis, P.O. Box 80083, 3508 TB Utrecht, The Netherlands

bSaso1Technology R&D, P.O. Box

1, Sasolburg 9570, Republic of South Africa

Homogeneous deposition precipitation (HDP) is explored for the preparation of carbon nanofiber supported ruthenium catalysts. First, carbon nanofibers (CNF, 177 m~/g) are oxidized using nitric acid thus activating the graphitic carbon surface. Second, ruthenium (hydr)oxide is deposited homogeneously onto the CNF by hydrolysis of urea at 363K. Electron microscopy and hydrogen chemisorption studies showed that, after reduction in hydrogen, the CNF were readily homogeneously covered with 1-2 nm metal particles. This high dispersion remained almost unchanged upon heating in inert gas up to 973 K. These results clearly demonstrate the applicability of the HDP technique for the preparation ofthermostable CNF-supported metal catalysts. 1. INTRODUCTION For a number of reasons the properties of carbon nanofibers (CNF) as a catalyst support are advantageous. Carbon nanofibers have a relatively large external surface area (100 - 200 m2/g) combined with good accessibility and the absence of micropores. During the growth process the fibers interweave and form mechanically strong agglomerates while the dimensions of the skeins allow simple filtration. However, the chemical inertness of the graphitic carbon fibers brings along a problem: the application and anchoring of the catalytically active phase. However, the surface can be modified by treatment of the CNF in concentrated nitric acid. In this way oxygen-containing surface groups are introduced for the anchoring of the active phase or its precursor and to obtain a more hydrophilic surface necessary for the aqueous solution of the metal precursor [ 1]. The great potential of CNF as catalyst support material is demonstrated by several researchers. Metals like Pt [2], Pd [3,4] and Ni [58] are applied on CNF and tested in various reactions, a.o., selective hydrogenations. Selective hydrogenation reactions are important for the fine chemical industry. Ruthenium is one of the active metals for this reaction. Its performance is sensitive to subtle changes in dispersion and nature of the support. In this study we have applied ruthenium on a CNF support. For CNF supported ruthenium catalysts much higher selectivities (up to 92%) to cinnamylalcohol were found

202 [9] than with ruthenium supported on alumina [10](20-30%) and active carbon [11](3040%). Well-known techniques for the application of active phases on a support are, a.o., impregnation, ion-exchange and homogeneous deposition precipitation (HDP). The limits of impregnation often relate to poor reproducibility and very broad particle size distributions, while with ion-exchange only low loadings of the active phase can be achieved [12]. Hoogenraad et al. [13-15] have used ion-exchange to apply palladium on CNF and they found a metal loading of 3 wt% at most. With HDP using oxidic support materials this method leads to catalysts with both a high loading and a high dispersion. Geus and co-workers [16,17] have extensively studied this method for powdered supports and described the background of HDP. In HDP the active phase or its precursor is deposited onto an existing support by slowly and homogeneously introducing the precipitating agent in such a way that nucleation in the solution itself is avoided. To achieve this, locally high degrees of super saturation must be prevented. Notorious is the use of the hydrolysis of urea to precipitate compounds by a homogeneous increase of the hydroxyl concentration. To obtain a uniform layer of small particles the nucleation homogeneously over the surface of the support must proceed at a lower concentration than in the bulk of the solution [16]. This procedure has been used with success to produce a number of different catalysts. For example, Burattin et al. [18,19] have given a detailed description of the molecular mechanism of the HDP of Ni(II) on silica. They concluded that during HDP surface compounds like 1:1 Ni-phyllosilicates are formed. Our goal was to investigate whether HDP is suitable also to achieve high metal dispersions in case no surface compounds can be formed and the interaction is confined to that of the precursor ions with a relatively small number of surface sites.

2. EXPERIMENTAL 2.1. Catalyst preparation For the growth of CNF a 20 wt% Ni/SiO2 was prepared by homogeneous deposition precipitation (HDP) [ 16]. Prior to the fiber growth the Ni-catalyst was reduced in situ for 2 hours in a flow of 20% H2/Ar (1 bar) at 973 K. Next, the CNFs were grown at 823 K in CO/HE/Ar (= 20/7/73) for 24 hours. The CNFs were refluxed for 1 hour in a 1 M KOH solution in order to remove the silica support. For the activation of the CNFs and the removal of nickel, the CNFs were refluxed in concentrated nitric acid for 2 hours and washed with demi-water. Ruthenium (5 wt%) was deposited on the fibers according to the HDP-method. In literature in the majority of the studies on the preparation of ruthenium-supported catalysts ruthenium chloride was utilized as the precursor. Commercial ruthenium chloride can contain as much as 80% Ru(IV) which is only soluble in aqueous solution at very low pH. We attempted the preparation of CNF supported ruthenium catalysts via HDP using this precursor salt. Catalysts with good dispersions (0.25) and relatively small average particle sizes (3 nm) were obtained. TEM and SEM images of these dried catalysts, however, revealed the presence of large lumps of ruthenium oxide between the CNF, see Fig. 1. For this reason RuNO(NO3)3.nH20 could be a better choice as precursor salt for the HDP.

203 An acidified suspension (pH=0.5) of 5 gram CNF in 250 ml demi-water was heated up to 363 K and urea (1.56 gram) and RuNO(NO3)3.nH20 (0.823 gram, 5 wt%) were added. After 6 hours the loaded CNF was filtered and washed thoroughly with demi-water, dried at 393 K and reduced with Hz at 473 K for 1 hour (heating rate = 5 K/min).

Fig. la. TEM image of Ru/CNF prepared from ruthenium chloride

Fig. lb. SEM back scatter image of Ru/ CNF prepared from ruthenium chloride

2.2. Catalyst characterization The carbon fiber support and the catalysts before