Preparation of Catalysts VII (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 118 PREPARATION OF CATALYSTS VII

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Studies in Surface Science and Catalysis 118 Advisory Editors: B. Delmon and J.T. Yates

PREPARATION OF CATALYSTS VII Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4, 1998 Editors

B. Delmon

Universit~ Catholique de Louvain Unit# Catalyse et Chimie de Mat6riaux Divis6s Louvain-la-Neuve, Belgium

P.A. Jaeobs

Katholieke Universiteff Centrum voor Oppervlaktechemie en Katalyse Heverlee (Leuven), Belgium

R. Maggi

Universit~ Catho/ique de Louvain Unit6 Catalyse et Chimie de Materiaux Divises Louvain-la-Neuve, Belgium

J.A. Martens

Katholieke Universite# Centrum voor Oppervlaktechemie en Katalyse Heverlee (Leuven), Belgium

P. Grange, G. Poncelet

Universit# Catholique de Louvain Unit# Catalyse et Chimie de Mat#riaux Divis#s Louvain-la-Neuve, Belgium

1998 ELSEVIER Amsterdam-- Lausanne-- New York-- Oxford-- Shannon-- Singapore--Tokyo

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

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

ISBN 0-444-50031-6 91998 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. Transferred to digital printing 2005 Printed and bound by Antony Rowe Ltd, Eastbourne

CONTENTS Foreword Organizing Committee and Scientific Committee

XV

xvii

SUPPORTED (NOBLE) METALS

The quantitative representation of heterogeneity in supported-metal catalysts A.S. McLeod, K.Y. Cheah, L.F. Gladden Electromechanical behaviour of quasi-graphitic carbons at formation of supported noble metal catalysts P.A. Simonov, A.V. Romanenko, I.P. Prosvirin, G.N. Kryukova, A.L. Chuvilin, S.V. Bogdanov, E.M. Moroz, V.A. Likholobov

15

Anionic clays as precursors of noble metal based catalysts for methane activation F. Basile, L. Basini, G. Fornasari, M. Gazzano, F. Trifir6, A. Vaccari

31

From colloidal particles to supported catalysts: a comprehensive study of palladium oxide hydrosols deposited on alumina B. Didillon, E. Merlen, T. Pag6s, D. Uzio

41

Nanosize palladium loaded catalytic membrane: preparation and cis-trans selectivity in hydrogenation of sunflower oil O.M. Ilinitch, P.A. Simonov, F.P. Cuperus

55

Preparation of hydrogenation catalysts supported on stainless steel or carbon fabrics J.P. Reymond, D. Dubois, P. Fouilloux 63 Chemical vapor deposition of palladium on silica and alumina supports P. Atanasova, J. Wise, M. Fallbach, T. Kodas, M. Hampden-Smith

73

Preparation of Pd/CeO 2 catalyst for methanol decomposition Y. Usami, K. Kagawa, M. Kawazoe, Y. Matsumura, H. Sakurai, M. Haruta

83

Supported aqueous phase catalysts for the C-C and C-N bond formation S. dos Santos, F. Quignard, D. Sinou, A. Choplin

91

vi

Preparation of microscopic catalysts and colloids for catalytic nitrate and nitrite reduction and their use in a hollowfibre dialyser loop reactor M. H~ihnlein, U. Prtisse, J. Daum, V. Morawsky, M. Kr6ger, M. Schr6der, M. Schnabel, K.-D. Vorlop

99

On the effect of cadmium acetate in the preparation of heterogeneous palladium(O) catalysts G. Mestl, S. Adam, O. Timpe, U. Wild, W. Bensch, R. Schl6gl

109

Preparation of BaAl l2019-supp ~ Pd catalysts for high-T combustion by wet impregnation C. Cristiani, G. Groppi, G. Airoldi, P. Forzatti

119

Characterization of y-Al203 supported Pd-Cu bimetallic catalysts by EXAFS, AES and kinetic measurements A. Pintar, J. Batista, I. Ar6on, A. Kodre 127 Encapsulation of microscopic catalysts in polyvinyl alcohol hydrogel beads U. Prtisse, V. Morawsky, A. Dierich, A. Vaccaro, K.-D. Vorlop

137

A scientific description of Pt adsorption onto alumina J.R. Regalbuto, K. Agashe, A. Navada, M.L. Bricker, Q. Chen

147

Pt/AI203/AI monoliths for the complete oxidation of toluene N. Burgos, M. Paulis, J. Sambeth, J.A. Odriozola, M. Montes

157

Monolithic carbon aerogels for fuel cell electrodes G.M. Pajonk, A. Venkateswara Rao, N. Pinto, F. Ehrburger-Dolle, M. Bellido Gil

167

Carbon coating of ceramic monolithic substrates Th. Vergunst, F. Kapteijn, J.A. Moulijn

175

Physicochemical basesfor the preparation of spinel supported bimetallic platinum catalysts for dehydrogenation of lower paraffins N.A. Pakhomov, R.A. Buyanov, B.P. Zolotovskii

185

Preparation of new type of Sn-Pt/SiO2 catalysts for the hydrogenation of crotonaldehyde J.L. Margitfalvi, I. Borb/tth, A. Tompos

195

lnfluence of the preparation variables on the separative and catalytic properties of ruthenium-silica membranes V. Pftrvulescu, V.I. Pgtrvulescu, C. Niculae, G. Popescu, A. Julbe, C. Guizard, L. Cot

205

vii

Preparation of Ru/carbon-catalysts for ammonia synthesis N.M. Dobrynkin, P.G. Tsyrulnikov, A.S. Noskov, N.B. Shitova, I.A. Polukhina, G.G. Savelieva, V.K. Duplyakin, V.A. Likholobov

213

Preparation of ruthenium based catalysts ultradispersed in a silica matrix F. Di Silvestri, P. Moggi, G. Predieri

219

The interrelation of the preparation method and activity of the Co-Ru/SiO 2 catalysts M. Niemel/a, M. Reinikainen, J. Kiviaho

229

Preparation of the metal complex catalysts immobilized on chitosan for carbonyl compounds transfer hydrogenation V. Isaeva, V. Sharf, N. Nifant'ev, V. Chemetskii, Zh. Dykh

237

Preparation and evaluation of novel hydrous metal oxide (HMO)-supported noble metal catalysts T.J. Gardner, L.I. McLaughlin, L.R. Evans, A.K. Datye

245

Rhodium carbonyl catalysts, immobilized on polymeric supports in the hydroformylation of olefins G.V. Terekhova, N.V. Kolesnichenko, E.D. Alieva, N.I. Truhmanova, A.T. Teleshev, N.A. Markova, E.I. Alekseeva, E.V. Slivinsky, S.M. Loktev, O.Yu. Pesin

255

Novel preparation method for supported metal catalysts using microemulsion. Control of catalyst surface area M. Kishida, T. Hanaoka, H. Hayashi, S. Tashiro, K. Wakabayashi

265

Study of rhenium deposition onto Pt surface with electrochemical methods S. Szab6, I. Bakos

269

Deposition of gold nanoparticles on silica by CVD of gold acetylacetonate M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Haruta

277

Influence of the precursors on the formation of a trimetallic defined structure. Application on Ni catalysts used for syngas obtention H. Provendier, C. Petit, A.C. Roger, A. Kiennemann

285

Characterization of zirconia-supported nickel catalysts prepared by multiple ion-exchange M. Mihaylov, K. Hadjiivanov, N. Abadjieva, D. Klissurski, L. Mintchev 295 New heterogeneous nickel catalysts for enantioselective hydrogenation S.D. Jackson, E. Allan, G. Webb, N.C. Young

305

viii

New preparation method of asymmetrically modified supported nickel catalysts for the enantio-differentiating hydrogenation of methyl acetoacetate T. Osawa, S. Mita, A. Iwai, O. Takayasu, T. Harada, I. Matsuura

313

Feitknecht compound used as the precursor of the catalyst for the catalytic growth of carbon fibers from methane Y. Li, J. Chen, L. Chang, J. Zhao

321

The synthesis of zeolite ZSM-5 on Raney nickel: a novel composite catalyst precursor B. Zong, M. Muhler, G. Ertl

331

UNSUPPORTED (MIXED) OXIDES

Effect of texture of cobalt oxide catalyst on its properties in ammonia oxidation J. Petryk, E. Kolakowska, K. Krawczyk, Z. Kowalczyk

341

Supercritical antisolvent precipitation: a novel technique to produce catalyst precursors. Preparation and characterization of samarium oxide nanoparticles E. Reverchon, G. Delia Porta, D. Sannino, L. Lisi, P. Ciambelli

349

Soft chemical synthesis of mixed metal molybdate oxidation catalysts and their structural relationship to hydrotalcite S. Soled, D. Levin, S. Miseo, J. Ying

359

Preparation of supported S042--ZrO2 for isomerization of n-butane C.R. Vera, C.L. Pieck, K. Shimizu, J.M. Parera

369

Sb V04: the chemistry of preparation F. Cavani, S. Ligi, S. Masetti, F. Trifir6

377

Mechanochemistry in preparation of VPO catalysts for paraffins oxidation V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Kharlamov, I.V. Bacherikova, L.V. Bogutskaya

385

Effect of the preparation procedure and parameters on the physico-chemical properties of higher alcohol synthesis ZnCrO catalysts P.L. Villa, L. Lietti

395

ix

Copper-zinc catalysts. Use of new bimetallic precursors and comparison with coprecipitation method R. Brahrni, C. Kappenstein, J. Cemak, D. Duprez

403

Effect of coprecipitation conditions on the surface area, phase composition and reducibility of CeO2-ZrO2-Y203 materials for automotive three-way catalysts O.A. Kirichenko, G.W. Graham, W. Chun, R.W. McCabe

411

Preparation, extrusion and characterization of perovskJte catalysts H.-G. Lintz, S. Ztthlke

421

New methods to prepare perovskJte-type Lao.8Sro.2CoO 3 catalyst at low temperature Z. Shao, G. Xiong, S. Sheng, H. Chen, L. Li

431

SUPPORTED OXIDES AND MIXED OXIDES

An important principle for catalyst preparation - spontaneous monolayer dispersion of solid compounds onto surfaces of supports Y. Xie, Y. Zhu, B. Zhao, Y. Tang

441

Catalyst granule production in a spouted bed: opportunities for creative catalyst design A.J. Kamphuis, J.R. Walls

451

In situ Raman evidence for a barium solid state phase that is active in nitric oxide decomposition: influence of preparation parameters G. Mestl, S. Xie, M.P. Rosynek, J.H. Lunsford

459

Further studies on ligand-promoted oxide dissolution during supported catalysts preparation: evidence for the formation of aluminoheteropolytungstates in the case of the WOx/u system X. Carrier, J.F. Lambert, M. Che

469

Preparation of heteropolyacid-imbeddedpolymerfilm catalysts by membrane technology and their modified catalysis G.I. Park, S.S. Lim, J.S. Choi, I.K. Song, W.Y. Lee

477

Reactive sputtering as a tool for preparingphotocatalysts D. Dumitriu, A.R. Bally, C. Ballif, V.I. Parvulescu, P.E. Schmid, R. Sanjin6s, F. L6vy

485

Preparation of zirconium oxide particles for catalyst supports by the microemulsion technique. Characterization by X-ray diffraction, BET, SEM-EDX, FT-IR and catalytic tests M. Boutonnet Kizling, F. Regali

495

Influence of the impregnation order of molybdenum and cobalt in carbon supported catalysts for hydrodeoxygenation reactions M. Ferrari, C. Lahousse, A. Centeno, R. Maggi, P. Grange, B. Delmon

505

Influence of phosphorous on the preparation of CoMo/Al203 hydrotreating catalysts R. de Back, F. Croonenberghs, P. Grange

517

Polymer supported metal complex catalysts with memory A.A. Efendiev, T.N. Shakhtakhtinsky

533

Development of a catalytic reactor with annular configuration A. Beretta, P. Baiardi, D. Prina, P. Forzatti

541

Effects of silica and titania supports on the catalytic performance of vanadium-phosphorus-oxide catalysts M. Ruitenbeek, A.J. van Dillen, D.C. Koningsberger, J.W. Geus

549

Design and development of novel copper chromite catalysts (unsupported~supported) with enhanced activity D. Mohan, R. Prasad, K.S. Karki

557

Zeolite modification by in-situformed reactive gas-phase species. Preparation and properties of Mo-containing zeolites A.V. Kucherov, A.A. Slinkin

567

Influence of the preparation methodology on the reactivity and characteristics of Fe-Mo-oxide nanocrystals stabilized inside pentasyl-type zeotites G. Centi, F. Fazzini, J.L.G. Fierro, M. L6pez Granados, R. Sanz, D. Serrano

577

SOL-GEL

Scientific bases for the preparation of oxide supports and catalysts via sol-gel methods O.P. Krivoruchko

593

xi

Techniques for preparation of manganese-substituted lanthanum hexaaluminates A.G. Ersson, E.M. Johansson, S.G. J~irfis

601

Formation of the porous structure of titanium dioxide prepared by a sol-gel method V. Yu. Gavrilov, G.A. Zenkovets, G.N. Kryukova

609

Structure control of SiO2 sol-gels via addition of PEG J. Sun, W. Fan, D. Wu, Y. Sun

617

Sulfated zirconia spheres and microspheres by gel supported precipitation F. Pinna, M. Scarpa, M. Signoretto, G. Strukul, M. Marella, M. Tomaselli, L. Meregalli, R. Scattolin

625

Titania-alumina prepared by sol-gel method: influence of pH and drying on textural and structural properties S.S.X. Chiaro, J.L. Zotin, A.C. Faro Jr.

633

Zirconium-titanium phosphate acid catalysts synthesized by sol-gel techniques 643 N.B. Jackson, S.G. Thoma, S. Kohler, T.M. Nenoff Methane combustion over NiO/BaO/ZrO 2 catalysts Y.-Y. Wang, Y.-B. Gao, Y.-H. Sun, S.-Y. Chen

651

Control preparation of aluminium chromium mixed oxides by sol-gel process L. Baraket, A. Ghorbel 657 V205-SiO2 catalyst prepared by the sol-gel process in the oxidative dehydrogenation of n-butane E.L. Sham, V. Murgia, J.C. Gottifredi, E.M. Farffin-Torres

669

Synthesis, structure and catalytic activity of CuO/TiO2 mixed oxides obtained by alkoxo-methods in CO oxidation M.V. Tsodikov, Ye.A. Trusova, Ye.V. Slivinski, G.G. Hernandez, D.I. Kochubey, V.G. Lipovich, J.A. Navio

679

Co-Nb205/SiO 2 sol-gel catalysts: preparation implications on the texture and acidity of the support and dimension of the metal particle V. Parvulescu, R. Craciun, F. Tiu, S. Coman, P. Grange, V.I. Parvulescu

691

Silica-supported bismuth molybdate catalysts obtained by the sol-gel process from silicon alkoxides D. Cauzzi, M. Deltratti, M. Devillers, G. Predieri, O. Tirions, A. Tiripicchio

699

xii

Pd-Ag/SiO2 sol-gel catalysts designed for selective conversion of chlorinated alkanes into alkenes B. Heinrichs, P. Delhez, J.-P. Schoebrechts, J.-P. Pirard 707 Pore-wall modified metal~ceramic catalytic membranes prepared by the sol-gel method H.-B. Zhao, G.-X. Xiong, G.V. Baron

717

Preparation of catalysts supported on Y-PSZfibers via sol-gel technology M. Marella, M. Tomaselli, J.F. Allain, F. Gerolin

725

Preparation of catalyst matrixes of controlled porous texture from silica and alumina sols J.P. Reymond, G. Dessalces, F. Kolenda

735

SUPPORTS and MODIFIED SUPPORTS

Properties of alumina catalysts prepared from boehmite needles H. Ohkita, S. Kuramoto, T. Mizushima, N. Kakuta

745

The structure of silica particles9Prepared by acid treatment of olivine: 2 a nitrogen physisorption and Si-MAS NMR study D.J. Lieftink, B.G. Dekker, J.W. Geus

755

Preparation and characterization of niobia and silica-niobia systems S. Morselli, P. Moggi, D. Cauzzi, G. Predieri

763

The ultrasonic synthesis of nanostructured metal oxide catalysts S.C. Emerson, C.F. Coote, H. Boote III, J.C. Tufts, R. LaRocque, W.R. Moser 773 On the relations between the rheology of TiO2-based ceramic pastes and the morphological and mechanical properties of the extruded catalysts P. Forzatti, C. Orsenigo, D. Ballardini, F. Berti

787

The study offormation of supports and catalysts based upon Al203/Al cermets S.F. Tikhov, V.A. Sadykov, Yu.A. Potapova, A.N. Salanov, G.N. Kustova, G.S. Litvak, V.I. Zaikovskii, S.V. Tsybulya, S.N. Pavlova, A.S. Ivanova, 797 A.Ya. Rozovskii, G.I. Lin, V.V. Lunin, V.N. Ananyin and V.V. Belyaev Tuning of textural and structural characteristics of Al203-TiO 2 mixed oxide supports T. Klimova, H. Gonzfilez, R. Hemfindez, J. Ramirez

807

xiii

Atomically controlled preparation of silica on alumina M. Lindblad, A. Root

817

Preparation of titania supported on silica via vapor phase grafting method: application of statistical and physico-chemical methods R. Castillo, B. Koch, P. Ruiz, B. Delmon

827

The influence of preparation conditions on the surface area and phase formation of zirconia A. Calafat

837

Preparation of monolithic catalysts by dipcoating X. Xu, J.A. Moulijn

845

(OXY)NITRIDE-CARBIDE

Preparation and characterization of SiC microfibers and Cr3C2 with medium specific surface areafor catalytic applications M.J. Ledoux, N. Keller, C. Pham-Huu, C. Estourn6s, B. Heinrich, H. Lamprell, E.M. Harlin

855

Synthesis of mixed galloaluminophosphate oxynitrides: the influence of nitridation on the "AIGaPON" acido-basic properties S. Delsarte, V. Peltier, Y. Laurent, P. Grange

869

Preparation of aluminovanadate oxynitride catalysts: characterization of a dinitrogen intermediate phase by DRIFTS, XPS, TGA H.M. Wiame, M.A. Centeno, L. Legendre, P. Grange

879

MESOPOROUS STRUCTURES

A novelpreparation of cubic form ofmesoporous aluminosilicate J. Medina-Valtierra, J.A. Montoya, J.A. de los Reyes

889

Mesoporous basic catalysts: comparison with alkaline exchange zeolites (basicity and porosity). Application to the selective etherification of glycerol to polyglycerols J-.M. Clacens, Y. Pouilloux, J. Barrault, C. Linares, M. Goldwasser

895

xiv

Propane dehydrogenation on mesoporous chromium-containing silica catalysts P. Maireles-Torres, M. Alcfintara-Rodriguez, F. P6rez-Reina, E. Rodriguez-Castell6n, P. Olivera-Pastor, A. Jim6nez-L6pez

903

Influence of vanadium and cerium additives in the development of porosity and surface acid catalytic properties of mesoporous aluminophosphates E. Galanos, K. Kolonia, D. Petrakis, M. Hudson, P. Pornonis

911

Group IV phosphate - Cationic surfactant mesostructured materials C. Ferragina, A. De Stefanis, A.A.G. Tomlinson

921

Mesoporous AI-Fe-P-O solids prepared in non-aqueous medium: structure and surface acid catalytic behaviour G. Aptel, D. Petrakis, D. Jones, J. Roziere, P. Pornonis

931

LAYERED STRUCTURES

Synthesis and physicochemical properties of new hydrotalcite-like anionic clays contammg .. Zr 4+ in the layers S. Velu, A. Ramani, V. Ramaswamy, B.M. Chanda, S. Sivasanker

941

Preparation and nano-structure of pillared layered titanates and tantalates for photocatalytic materials M. Machida, J. Yabunaka, H. Taniguchi, T. Kijima

951

Synthesis, characterization and catalytic activity of aluminum pillared synthetic Ti-, Fe- or Zn-substituted smectites M. Lenarda, L. Storaro, A. Talon, R. Ganzerla, V. Lucchini

959

Characterization and catalytic activity of non-hydrothermally synthesized saponite-like materials M. Sychev, R. Prihod'ko

967

Author Index

975

Other volumes in this series

981

XV

FOREWORD

These are the Proceedings of the 7th Symposium on the Scientific Basesfor the Preparation of Heterogeneous Catalysts, initiated in 1975 and on a regular basis jointly organized by the Unit6 de Catalyse et Chimie des Matrriaux Divisrs (UCL) and the Center of Surface Chemistry and Catalysis (KULeuven). Apart from the first symposium, the location of all these events has always been the UCL campus. It is relevant to be reminded of the General Remarks formulated at the occasion of the first symposium: "At the moment the Organizing Committee has decided on the subject, the hope was that this symposium could constitute a discussion of various scientific problems which are involved in the manufacture of real, industrially used, heterogeneous catalysts. Catalysts are solid materials or solid chemical products, possessing a high market value on a weight basis. The intention was to discuss in an international meeting the scientific domains on which the activity of an important, well distinct branch, of industry rests. The hope was that as little catalysis as possible would be mentioned. Indeed, the preparation of a solid material involves mainly solid state chemistry and adhesion phenomena. Since such a solid material has a complicated texture and a well developed surface area, manufacture also implicates colloid chemistry and various interface phenomena in addition to adhesion, diffusion and mobility processes in the solids or at their surface. The technology involved is related to other fields, principally to the manufacture of ceramics, powder technology, surface treatments, technical realization of adhesive joints between metal and/or ceramics, materials technology, and cognate areas. The hope was that the symposium would help to define better the fundamental phenomena involved and the technical similarities. People in charge of the manufacture of catalysts are often catalysis-minded or at least are working in such an environment. Some extra-disciplinary contributions, as well as multi-disciplinary approaches, are thus necessary." With minor changes, these remarks might serve as well as Preface of the present Symposium Proceedings. Indeed, emphasis in all Symposia has been on the scientific aspects of the preparation of new and industrial catalysts, or on new methods of preparation, rather than on the catalytic reactions in which such solids are ultimately used. In the present context, the catalytic event itself has only been considered as another, though often decisive, method of catalyst characterization. The series of Symposia have been backed up by Scientific Committees of which the majority of the members were holding an industrial appointment. These Committees have always been able during a joint meeting, to select those papers that best fitted the scope and specific topics of the Symposium. This has allowed to select papers dealing with preparation aspects of real catalytic systems, sometimes at the expense of excellent contributions on less timely systems. Industry has contributed on a regular basis by presenting communications, although the Organizing Committee sometimes had wished to see a higher number of them. Apparently, the share of Academia in the establishment of the scientific bases for the preparation is (and has been) very significant indeed.

xvi The scientific topics of the 7th Symposium are in line with the general scope of this series of events. Emphasis is on what industry considers as being very timely at the end of the 2nd millennium. On the other hand, the editors have decided to make the Proceedings available before the scientific event itself. For obvious reasons, the sponsoring Companies and Agencies this time cannot be acknowledged properly by citing them in the Proceedings. The same holds true for all those who have contributed to the success of the meeting, such as secretaries staff, students, postdocs, and the Lodging Service of UCL. Fortunately, the organizers are in position to express their appreciation towards the Rector of UCL, Professor M. Crochet, for allowing the event to be patronized again by the University, and Professor B. Delmon, now enjoying retirement, by being scientifically more active than ever.

Bernard, you have been at the origin (amongst many others) of this initiative; you have been continuously inspiring the local organizers and the Scientific Committees. As a tribute to your contribution to catalysis, and more specifically to this series of Symposia, the editors of the Proceedings of the 7th Symposium dare to dedicate this volume to you.

The Editors

xvii

ORGANIZING COMMITTEE President

Prof. B. DELMON, Universit6 Catholique de Louvain Executive Chairmen

Prof. P. GRANGE, Universit6 Catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr. R. MAGGI, Universit6 Catholique de Louvain Prof. J. MARTENS, Katholieke Universiteit Leuven Dr G. PONCELET, Universit6 Catholique de Louvain SCIENTIFIC COMMITTEE Dr. G. BARON, VUB, Belgium Dr. P. COURTY, Institut Frangais du P6trole, France Prof. B. DELMON, UCL, Belgium Prof. P. GRANGE, UCL, Belgium Dr. K. HARTH, BASF, Germany Prof. P.A. JACOBS, KUL, Belgium Dr. K. JOHANSEN, Haldor Topsae, Denmark Prof. F. KING, ICI Katalco, United Kingdom Dr. E. KRUISSINK, DSM Research, The Netherlands Dr. E.G.M. KUIJPERS, Engelhard De Meern BV, The Netherlands Dr. E.-L. LAKOMA_A,Neste Oy, Finland Dr. J.-P. LANGE, Shell International, The Netherlands Dr. F. LUCK, G6n6rale des Eaux, France Dr. R. MAGGI, UCL, Belgium Prof. J.A. MARTENS, KUL, Belgium Dr. G. MATrHYS, Exxon Chem. Int., Belgium Dr. M. NEUBER, Clariant GmgH, Germany Dr. G. NEUENFELDT, Solvay Deutschland, Germany Dr. R. NOELS, Universit6 de Liege, Belgium Dr. K. NOWECK, Condea Chemic GmbH, Germany Dr. C. PEREGO, Eniricerche S.p.A., Italy Dr. J.-P. PIRARD, Universit6 de Li6ge, Belgium Dr. G. PONCELET, UCL, Belgium Dr. S. ROSSINI, Snamprogetti, Italy Dr. P. RUIZ, UCL, Belgium Dr. F. SCHMIDT, Sud Chemie AG, Germany Dr. J.-P. SCHOEBRECHTS, Solvay, Belgium Prof. B.-L. SU, FUNDP, Belgium Dr. M. TOKARZ, Eka Nobel AB, Sweden Dr. M. TWIGG, Johnson Matthey, United Kingdom Dr. A. VAN GIJSEL, UCB, Belgium Prof. E. VANSANT, Universiteit Antwerpen, Belgium Dr. E. ZIRNGIEBL, Bayer, Germany

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91998ElsevierScienceB.V.All rightsreserved. Preparation of CatalystsVII B. Delmonet al., editors.

The quantitative representation of heterogeneity in supported metal catalysts A.S. McLeod, K.Y. Cheah and L.F. Gladden University of Cambridge, Department of Chemical Engineering, Pembroke Street, Cambridge, CB2 3RA, U.K.

A micro-reactor study of ethene hydrogenation, catalysed by a series of silica supported platinum catalysts, has been undertaken and the data obtained compared to numerical simulations of this reaction. Monte carlo simulations have been used to account for the complex kinetic features of this reaction and to relate the observed kinetic behaviour to the structure of each of the catalysts studied.

1. I NTR ODUC T I O N Discrete lattice models of surface reactions, as exemplified by monte carlo or cellular automata algorithms, are potentially a valuable computational tool for the simulation of reactions occurring on heterogeneous surfaces. In contrast to algebraic kinetic models, discrete statistical simulations allow for the structural features of a catalyst surface to be incorporated into kinetic models of heterogeneous catalytic reactions. The incorporation of this information into reaction models is necessary if the kinetic characteristics of heterogeneous catalysts are to be unambiguously related to the structure of the catalyst, and consequently to the catalyst preparation route. Previous work on the kinetics of reactions occurring on homogeneous catalyst surfaces has demonstrated that monte carlo simulation can provide the necessary numerical framework for interpreting apparently anomalous kinetic data [ 1,2]. In this paper we extend previous work on the simulation of reactions occurring on homogeneous surfaces by considering the simulation of catalytic reactions occurring on well characterised heterogeneous surfaces. Specifically, we consider the influence of the metal particle distribution and surface heterogeneity on the kinetics of the ethene hydrogenation reaction. Geometric disorder is accounted for by constructing Voronoi tesselations of the plane to represent a particular distribution of particles dispersed on an inert catalyst support.

To demonstrate the application these models to realistic catalytic systems, we have undertaken a combined experimental and theoretical study of the ethene hydrogenation reaction. A series of Pt/silica catalysts have beeja prepared from inorganic and organic precursor compounds and characterised by XRD, TEM and NMR techniques. The results of the eatalyst characterisation are subsequently used to produce lattices representative of each of the catalysts on which the reaction simulations are conducted. A micro-reactor study of the ethene hydrogenation reaction is then presented and the kinetic data obtained correlated with the results of the catalyst characterisation. Monte carlo simulations of the ethene hydrogenation reaction are then conducted and compared to the micro-reactor data. 2. SIMULATION DETAILS In the monte carlo method the catalyst surface is considered to be composed of a regular square lattice of discrete sites. Each single location of the simulation lattice may represent either the catalyst support or the active metal component. The model neglects the influence of metal-support interactions and assumes the support material to be inert. The catalytically active sites can either be vacant or be occupied by one of the hydrocarbon species or a dissociated hydrogen atom. Once adsorbed onto the catalyst surface, a reactant molecule may desorb back into the bulk gas phase or react with a neighbouring molecule.

000 000 000

000 000 000 &

C2H5*+H*---"~ C2H6 ~ rg (~ (~) (~)

000 000

rb

000 ,-.4~---- @ ~ 0 ~'f @ @ 0 re

C2H4*+H*~ C2H5"

rc

.-~

~'d

H2+2 " ~ : 2 H ra

0 0 0

000 @@0

C 2 H 4 + 2 " ~ C2H4.

Figure 1. The transformations of the lattice in a monte carlo simulation that represent the steps a-g of the Horiuti-Polanyi mechanism. Empty circles represent unoccupied lattice sites. Shaded circles represent O - ethene, O hydrogen and e - ethyl.

The hydrogenation of ethene is assumed to proceed by the sequential hydrogenation of the hydrocarbon, the half hydrogenated ethyl species is assumed to be the sole active intermediate [3]. The reaction mechanism is represented as a series of discrete time steps, each leading to a change in the arrangement of the adsorbed species on the lattice. The steps of the hydrogenation mechanism are represented by the transformations of a small section of the lattice, as shown in Figure 1. Diffusion of the dissociated hydrogen atoms across both the metal particles and the support (due to hydrogen spillover) may also occur. Adsorption of both ethene and hydrogen is assumed to require two adjacent, unoccupied metallic sites. Adsorbed species t h a t form nearest neighbour pairs may react with each other with a rate constant, ra, where a denotes one of the elementary steps of the reaction mechanism. As the strength of the substrate-adsorbate bond is typically much greater t h a n any adsorbate-adsorbate interaction, the rate constants are assumed to be dependent only on the location of the adsorbate on the c a t a l y s t surface. An outline of the time-dependent monte carlo algorithm is now presented, further details of the simulation algorithm are presented elsewhere [4]. The probability, Pr{cL}, of a particular event, c~, occurring with respect to the other possible events is given by Pr{a} - ~ r . "

(1)

An event results in the transformation of the simulation lattice, and can represent a diffusion, adsorption, diffusion, reaction or desorption step. The rate of reaction, ra, is dependent on both the intrinsic rate constant and the number of reactive groups on the surface. In general the reactant distribution will be non-random, and therefore cannot be obtained by the simultaneous solution of a set of mean-field site balance equations. Mlowing for both the spatial variation in the rate constant and the non-random reactant distribution the probability of an event occurring will be Pr{a} =

Ek~n-----~,

(2)

where ka is the rate constant for reaction a and na the number of possible reactions of type oz. If it is assumed that the events occur according to a

F

I'" 'i"|" /

-_.

;

:".,% L

t.

4

(a) (b) Figure 2. Simulation lattices used to represent various dispersions of metal particles (black regions) on a catalyst support (white regions) with the fraction of polygons occupied by the metallic crystallites assigned as (a) 0.10 and (b) 0.25.

Poisson process, then the time increment corresponding to each event, St, will be given by 1

dt = - ~ l n ( y ) , ~ naka

(3)

(2

where 7 is a uniform random number between 0 and 1. Simulations were conducted on 128x128 lattices for approximately 107 time steps, this was found to be a sufficient simulation time for steady state to be obtained. At steady state the rate of reaction was obtained as a turnover number, R, where R_

~Nt

(4)

In equation (4) Ng is the total number of reactions of type g that have occurred up to time t and N is the total number of sites on the catalyst surface, r is the fraction of sites occupied by the metallic component. In order to determine the influence of metal particle size, the distribution of the metallic crystallites over the catalyst support is represented by a series of tesselations of the lattice, these tesselations partition the surface into a number of separate regions. Each of these regions is either assigned as a region of catalyst support, or a region representing a metallic crystallite.

Each polygon is then randomly assigned, with a specified probability, as either an active catalytic region or as an inactive region of the support material. Examples of simulation lattices are shown in Figure 2. By assigning a greater number fraction, ~b, of the polygons as active regions, the average size of the regions increases due to the coalescence of the individual polygons into larger single particles. The characteristic length of the metal particles is determined and converted from arbitrary lattice units to "real" units by assuming the side length of each adsorption site to be 0.39 nm, the lattice constant for platinum.

3. P R E P A R A T I O N AND CHARACTERISATION OF MATERIALS

3.1. C a t a l y s t P r e p a r a t i o n Dispersion of platinum on the silica support was achieved by an impregnation from an aqueous medium and by anchoring of an organic precursor. For all the catalysts considered, the silica support was a porous sol-gel silica (Grace, Type 254), used as supplied by the manufacturer. The catalysts prepared by the aqueous route were prepared by impregnation of hexachloroplatinic acid. The anchored catalysts were prepared by the attachment of platinum acetylacetonate, Pt(acac)2, to the silica support. The catalysts were then calcined in air and reduced at 300~ in a stream of 20% v/v hydrogen in helium [5]. The preparation procedures are summarised in Table 1. Table 1. Summary of catalyst preparation conditions. ,

Catalyst

Pt-A Pt-B Pt-C Pt-D

,

.,,

Metal Loading (wt %) 2.5 2.5 2.5 2.5

,,,,,,.

,m,

.,

Calcination Temp (oc) 30 200 200 400

,,

,,

,,

....

Procedure

Aqueous impregnation Aqueous impregnation Organic anchoring Organic anchoring

3.2. Metal particle size and geometry Information on the geometry and size of the dispersed metal particles was obtained by powder X-ray diffraction and electron microscopy. The average particle diameter for each sample was obtained using a Phillips powder diffactometer with a Cu Ka radiation source ()~ = 0.154 nm ). The particle diameters and the corresponding metal dispersion for each material are shown in Table 2. The dispersions of the catalysts prepared from the organic

precursor were found to be lower than those obtained by the aqueous precursor. This result is contrary to what may be expected given that the organic precursor forms a bond with the hydroxyl groups on the silica surface [6]. Recent studies of the characteristics of platinum catalysts prepared from organic anchoring are, however, in agreement with the results presented above

[7]. Table 2. Mean metal particle diameter and the corresponding dispersion of the Pt/silica catalysts. The particle dispersion has been calculated using the approximate method of Bond [8]. i

i

i

Catalyst

i

Particle diameter

Dispersion

(s Pt-A Pt-B Pt-C Pt-D

50 63 163 187

0.20 0.16 0.061 0.053 i

3.3. D e u t e r i u m NMR A series of deuterium NMR studies have been conducted to determine the mobility and the extent of interaction of an adsorbed hydrocarbon molecule with each catalyst surface. This information on molecular mobility will be used in the monte carlo model to determine the possible influence of molecular surface diffusion on the reaction kinetics. Deuterated benzene was used as a probe molecule in this study as the relaxation behaviour of this molecule on metal oxide surfaces has been considered by a number of previous workers [9]. Fully deuterated benzene (99.5% C6D6 HPLC grade, Alltech) was adsorbed on the catalyst sample subsequent to reduction at 400oc, under vacuum, for 2 hours. Following the model introduced Boddenberg and Beerwerth [9] for the dynamics of sorbate motion on catalyst surfaces, the mobility of the adsorbed molecule is considered to be described by three separate motional correlation times. In addition to the isotropic translational motion of the molecule, interpreted as corresponding to the diffusion of the molecule across the catalyst surface, the simultaneous occurrence of librational and rotational motions are also allowed for. As each separate motion is assumed to be a thermally activated process, pre-exponential factors and activation energies are required to describe the temperature dependence of each motional correlation time. For the translational, rotational, and librational motions the pre-exponential factors and activation energies are denoted by tin, tp, ts and by Era, Ep, Es respectively. The parameter S is an order parameter and

is, therefore, bounded by 0 and 1. The librational order parameter quantifie~ the correlation of successive librational motions, higher values of S indicate a lesser degree of librational freedom. By considering the rotational activation energies for each of the catalysts, it can be concluded that Pt-A and Pt-B differ from Pt-C and Pt-D. In the former pair, the activation energy for the rotational motion is significantly higher than for the latter pair while the librational energy, where significant, is lower. This result suggests a greater sorbate-substrate interaction for catalysts Pt-A and Pt-B than for Pt-C or Pt-D. The rotational pre-exponential factors obtained for Pt-A and Pt-B are significantly higher those obtained for Pt-C and Pt-D. This is indicative of a stronger sorbate-substrate interaction for catalysts Pt-A and Pt-B. It is reasonable to conclude that the preparation route is responsible for the differences between these two pairs of catalysts. Table 3. ALS model parameters obtained for the dynamics of benzene adsorbed on silica. Activation energies are given in kJ mo1-1 and preexponential factors in s-1. Parameters not reliably determined are denoted by an asterisk.

,=

,

.,,,

...

Sample

i

Pt-A Pt-B Pt-C Pt-D ii

i

ill

,

Em

ii

,..,,

,

(_+2)

Ep (kJ mo1-1) (+2)

Es (+5)

17.7 19.6 16.9 17.4

11.1 12.2 7.0 6.5

8.0 -*12.0 11.5 ,i

I I

,

....,

,,.

,

tm tp ts S (xl0-1 ls-1)(xl0-14s-1)(xl0-13s -1) (+10%) (+10%) (+50%) (+0.05) 3.0 1.9 0.77 9.8 II

I II

289 204 17.7 3.0

II

1.0 -*1.9 2.4

0.32 0.32 0.28 0.30 ll;

3.4. 1H MAS a n d CRAMPS NMR The extent to which the surface chemistry of the catalyst support was influenced by the preparation procedure was studied by 1H MAS and CRAMPS NMR. Data were collected using a Bruker MSL-200 spectrometer operating at 200MHz (1H resonance). MAS spectra were obtained by rotation of the sample at 4.5 KHz, CRAMPS spectra were obtained at a spinning frequency of 2.3 KHz. Proton MAS spectra of two catalyst samples prepared from inorganic and organic precursors are shown in Figure 5. The organic and inorganic precursor samples appear nominally identical, with a single broad resonance at 3ppm. The spectra obtained for the supported metal catalysts were also found to be identical to that obtained for a sample of the silica support. A reduction in

190K

1801(

}~,]K

// 40000

20000

0

-20000

-40000

Ez

Figure 3. Deuterium spectra for deuterated benzene adsorbed on Pt-A.

?

o T

L,

,. . . . . . . . ~ .......,..........................~ _ j 4•

-3

5 x 1 0 -'~

l i t (ltK)

...........j

6x 10 -3

4 X I 0 -3

I/T

5 x 10 -3

(i/K)

6 x l O -3

Figure 4. Deuterium spin-lattice and spin-spin relaxation times as a function of reciprocal temperature for catalyst Pt-A. Points correspond to experimental data and solid lines to the ALS model fit.

line broading can be achieved using combined MAS and multiple pulse spectroscopy (CRAMPS). In this study, the BR-24 multiple pulse sequence [9] was used to resolve the broad peak obtained by MAS. The increased resolution obtained by CRAMPS resolved the broad peak into two distinct resonances, a sharp peak at 1.8 ppm and a broad peak at 2.9 ppm. These peaks were assigned to isolated silanols and hydrogen bonded silanol groups respectively [10]. The CRAMPS spectra for both the organic precursor and the inorganic precursor catalysts did not differ significantly. Any differences in the kinetic behaviour between the catalysts prepared from the two precursors are, therefore, unlikely to be due result from differences in the silica surface chemistry of the materials. (b)

q

I!

/

1

MAS

/ j

15

10

5

0

-5

ppm

15

10

.... S

0

-5

ppm

Figure 5. 1H MAS and 1H CRAMPS (BR-24) spectra of Pt/silica catalysts prepared from (a) the inorganic and (b) the organic precursor.

4. M I C R O - R E A C T O R S T U D I E S OF E T H E N E H Y D R O G E N A T I O N

Experimental and numerical investigations of the hydrogenation of ethene have demonstrated the existence of a discontinuity separating two regimes characterised by differing kinetics [4,12]. At low temperature and high hydrocarbon pressure the catalyst surface is saturated with hydrocarbon and the adsorption of hydrogen is the rate limiting step. At higher temperatures the surface hydrogenation reaction is the rate limiting step [13]. Previous experimental work has shown that while the transition was observed for a dispersed silica-supported catalyst, no such transitions were observed either for alumina- or molybdena-supported catalysts, or for the EUROPT-1 Pt/silica catalyst [12]. The existence of the transition for some catalysts but

l0 not for others is particularly surprising as ethene hydrogenation is regarded as the archetypal structure insensitive reaction.

4.1. E x p e r i m e n t a l Kinetic data were obtained using a tubular differential reactor. The catalyst samples (approx. 20mg) were diluted in inert silica before being placed in the reactor to minimise mass and Heat-transfer effec4s. Feed streams of helium (99.9998 % purity, BOC), hydrogen (99.996%, BOC) and ethene (99.97%, SIP analytical Ltd.) were passed through an ox~ygen trap (Oxytrap molecular sieve, Alltech) and a moisture filter (Hydropurge, Alltech) before entering the reactor.

14

O

~ 12~10,.Q

0

~ 8Z

0

8-

~ 1 7 6~

Oo o o oo

0

Z

co

6-

.....

o

0 0 0


~176176

~ 4-

0%

4-

o

~176176 0 0 0

o

2-

0

I

0

'

I

i

I

0 20 40 60 80 100 210 410 610 8'0 Ethene Pressure [Torr] Ethene Pressure [Torr] (a) (b) Figxtre 6. Rate of ethene hydrogenation using the Pt/silica catalysts (a) Pt-A and (b) Pt-B at 283 K and a hydrogen pressure of 100 Torr. 30

12

~'~ 10-

~,25-

- 2o-

8-

o O

O

o

o10-~

5

%

2;

Zl~. >

O

oo

o

oo

~> 4~

~176 ~176176 Oo

O

Oo

000%o00 210 410 610 810 1~0 Ethene Pressure [Torr] (a)

OODO00000 0 0

O(

2t0

410

6 t0

8t0

]

Ethene Pressure [Torr] (b)

Figure 7. Rate of ethene hydrogenation using the Pt/silica catalysts (a) Pt-C and (b) Pt-D at 270 K and a hydrogen pressure of 100 Torr.

ll The reactor consisted of a 1 cm diameter type 304-stainless steel tube, the catalyst bed within the reactor being supported by the use of quartz wool plugs above and below the catalyst. The overall conversion was typically kept below 10%. Feed and product analysis were obtained by gas chromatography. A thermal conductivity detector was used in combination with a Heysep-D (alltech) separation column operated at 60~ From the results presented in Figure 6 and 7 it is clear that while the catalysts prepared from the aqueous precursor exhibit a clear discontinuity, those prepared by the organic route do not. The difference in reaction temperature could not lead to the removal of the discontinuity, as reducing the reaction temperature results in a more pronounced discontinuity [4]. Relating these results to the characterisation study there are two possible explanations for this behaviour. First, that the discontinuity is related to the mobility of the dissociated hydrogen. Deuterium NMR data indicate that the rotational correlation time of a sorbate molecule is greater on catalysts Pt-C and Pt-D, suggesting a stronger substrate-sorbate interaction. The translational correlations times, however, only differed slightly between each of the materials and did not correlate with the preparation procedure. Second, it is possible that the discontinuity is related to the smaller metal particles observed for catalysts Pt-A and Pt-B. Following from the results of the characterisation study, it is likely that it is the particle size distribution that is responsible for the differing kinetic behaviour of the catalysts prepared from the organic precursor and those prepared from the inorganic precursor. This hypothesis is now investigated by conducting monte carlo simulations to determine the likely of particle size and surface diffusion on the kinetics of the ethene hydrogenation reaction. 5. MONTE CARLO SIMULATION OF ETHENE H Y D R O G E N A T I O N In this section, the influence of metal particle geometry and surface diffusivity on the kinetics of the ethene hydrogenation reaction are considered. Results for a series of monte carlo simulations conducted on a number of lattices, representing differing metal particle dispersions, are presented. In Figure 8 turnover numbers are shown as a function of particle size for reaction at 280 K. At low particle sizes the reaction is found to be in the hydrocarbon saturated regime. This is due to a higher probability that strongly adsorbed ethene will displace hydrogen from small metal crystallites than from large crystallites. Once an isolated particle is saturated with hydrogen it is effectively poisoned and inactive. The effect of particle size is, therefore, far more significant that would be predicted from a simple ensemble model [14]. A comparison with the ensemble model, which assumes that a reduction in particle size simply reduces the number of available adsorption sites on a

12 metal crystallite, is shown in figure 8. This result demonstrates t h a t the kinetics are, in part, determined by the geometry of the catalyst surface, irrespective of the surface chemistry. Surface diffusion is found to have no influence on the reaction kinetics. Thus, the small differences in translational activation energy observed by deuterium NMR are unlikely to be significant.

25 20-

.

....a

=

.J t

=

f

IS-

z I0o

~

s0

' '-'-" I '" ' ' 1

....

i'"'

'1''

''1

....

I''""

I''

"'1

''

5

10 15 20 25 30 35 40 M e a n P a r t i c l e Size [nm] Figure 8. Dependence of the turnover number for ethene hydrogenation as a function of m e a n particle size. broken line represents the result obtained by the ensemble model. Reaction conditions are: Temperature = 280K, hydrogen pressure = 100 Torr; ethene pressure 10 Torr. 0

10 3

[] []

1 0 2-I ,., 1 0 I ' ~

O0 -

2~ 1 0 1 0 -I -

1 0 .2-

[]

[]a

10 .3 1

10 100 Ethylene Pressure [Torr]

1000

Figure 9. Influence of surface diffusion on the rate of ethene hydrogenation for a dispersed Pt/silica catalyst (r Symbols correspond to no ~ffusion (o), inter-particle diffusion(A), and both inter- and intra-particle diffusion([]).

13 In figure 9, results are presented for a series of simulations that incorporate hydrogen diffusion. Three cases were considered: (i) no surface diffusion, (ii) intra particle diffusion only, and (iii) both intra and inter particle diffusion (i.e. inclusion of hydrogen spillover). Hydrogen diffusion was simulated by redistributing the dissociated hydrogen atoms randomly after each time step. From these simulations we conclude that, provided the catalyst preparation procedure does not significantly modify the surface chemistry~ then differences in the kinetic characteristics of the catalysts can be attributed-to the differing geometry and size of the dispersed m e t a l particles.

6. C O N C L U S I O N S Deuterium NMR, X-Ray diffraction and electron microscopy studies of a series of Pt/silica catalysts have been used to construct lattice models for the structure of these materials. Proton NMR studies of the catalysts have then been used to determine the relationship between the catalyst preparation procedure and the structure and surface chemistry of each catalyst. Monte carlo simulations of the ethene hydrogenation reaction have subsequently been undertaken in order to relate the kinetic features of the reaction observed in micro-reactor studies to the surface structure of each catalyst surface. We are, therefore, able to relate the results of a monte carlo simulation of a surface reaction, conducted on a lattice representative of each sample, to the preparation procedure used. Proton NMR suggests that the surface chemistry of the catalyst support is not significantly altered by the two preparation procedures. Deuterium NMR studies suggest only slight differences in the mobility of a probe sorbate molecule on the surfaces of catalysts prepared from inorganic and organic precursors. As the particle sizes of the aqueous and organic materials were found-to differ significantly, it-was concluded that the observed differences in the kinetic behaviour between the catalyst samples was related to the geometry and size of the metal particles. Monte carlo simulations supported these conclusions, demonstrating : (i) That surface diffusion limitation did not influence the reaction kinetics to a significant extent. (ii) The influence of metal particle size on the reaction kinetics is far more significant than is predicted from the elementary ensemble model.

14 The quantitative approach to kinetic modelling outlined in this work is necessary if rational approach to catalyst design is to be developed. Further experimental studies of temperature programmed desorption spectroscopy are being undertaken to determine the influence of metal particle heterogeneity and hydrogen spillover on the reaction kinetics. The use of monte carlo simulations in combination with optimisation algorithms capable of determining the optimal catalyst surface, and hence the optimal preparation procedure, for a given reaction is also the subject of ongoing research [ 15].

7. A C K N O W L E D G E M E N T S

A. McLeod would like to thank Peterhouse for the Award of the Rolls-Royce Research fellowship.

8. R E F E R E N C E S

1. Zhdanov, V.P. and Kasemo, B., Surf Sci. Rep. 20, 111 (1994). 2. Nieminen, R.M. and Jansen, A.P.J., Appl. Catal. 160, 99 (1997). 3. Cremer, P.S. and-Somorjai, G_A_,_J. Chem._Soc_ Ea~adcLy Trans. 91~ 3671 (1995). 4. McLeod, A.S. and Gladden, L.F., Catal. Lett. 43, 189 (1997). 5. Jackson, S.D., Willis, J., McLellan, G.D., Webb, G., Keegan, M.B.T., Moyes, R.B., Simpson, S., Wells, P.B_and Whyman, R.,J. Catal. 139, 191_(1993). 6. Yermakov, Y.I., Catal. Rev. Sci. Eng 13, 77 (1976). 7. Phillips, J. and Dumesic, J.A., Appl. Catal. 9, 1 (1984). 8. Bond, G.C., Surf Sci. 156, 966 (1985). 9. Boddenberg, B. and Beerwerth, B., J. Phys. Chem. 93, 1440 (1989). 10. Burum, D.P. and Rhim, W.K., J. Chem. Phys. 71,944 (1979). 11. Bronnimann, C.E., Zeigler, R.C. and Maciel, G.E., J. Am. Chem. Soc. 110, 2023 (1988). 12. Jackson, S.D., McLellan, G.D., Conyers, L., Keegan, M.T.B., Mather, S., Simpson, S., Wells, P.B., Whan, D.A. and Whyman, R., J. Catal. 162, 10 (1996). 13. Vassilakis, D., Barbouth, N. and Oudar, J., J. Chim. Phys. 88, 209 (1991). 14. Martin, G.A., Catal. Rev. Sci. Eng. 30, 519 (1988). 15. McLeod, A.S., Johnston, M.E. and Gladden, L.F., J. Catal. 167, 279 (1997).

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

15

E l e c t r o c h e m i c a l b e h a v i o u r o f q u a s i - g r a p h i t i c c a r b o n s at f o r m a t i o n o f s u p p o r t e d n o b l e m e t a l catalysts P.A. Simonov, A.V. Romanenko, I.P. Prosvirin, G.N. Kryukova, A.L. Chuvilin, S.V. Bogdanov, E.M. Moroz and V.A. Likholobov Boreskov Institute of Catalysis, Prospect Akademika Lavrentieva, 5, Novosibirsk 630090, Russia Regularities for adsorption of HAuC14, H2PdC14 and H2PtC16 on carbon supports as a stage of preparation of supported metal catalysts have been studied. The results obtained have been interpreted in terms of the electrochemical theory of adsorption of electrolytes. Relationship between the nature of adsorption of these precursors and the metal state in the target catalysts has been revealed. 1. I N T R O D U C T I O N Physicochemical processes, which take place during the impregnation of a carbon support with a solution of a metal compound, are known to have a strong impact upon the state of the supported metal. For this reason the nature of these processes is the subject of numerous thorough investigations; electrochemical concepts on the interaction between carbon and solutions of catalyst precursors, along with traditional chemical approach, are used in increasing frequency to explain the observed regularities [ 1]. The electrochemical theory implies that the exchange currents of a graphite-like carbon, similar to those of precious metals (Pt, Au), are negligibly low, therefore the carbons do not reveal characteristic potential jump that corresponds to the thermodynamic equilibrium with the solution. An electric charge is imparted to the surface upon establishing redox equilibria between the solutes or adsorbates (pathl). In particular, it was discovered first [2] that carbon, when interacting with a component of the gas phase, behaves as an ordinary gas electrode; its potential acquires the magnitude and sign depending on the atmosphere composition and pH of the electrolyte solution. For example, the carbon becomes an oxygen electrode in the presence of 02: 2C + H20 + 1/2

02

~

2C *-.-OH-

(1)

An electrical double layer (EDL) formed in this way includes OH- ions, which can be replaced by X- ions from the electrolyte solution [2]: C~.-.OH - + X- ~-...... "~ C * ...X- + OH-

(2)

In a chemically inert atmosphere, carbons are involved in redox interconversion of various ions to generate EDL [3,4]. In the presence of metal cations, e.g. Fe 3+, the process of EDL formation is as follows [4]:

16 C +Fe 3+ + X - ~

C a---X-+Fe 2+,

(3)

a constant concentration o f H + ions in the solution being a characteristic feature of Reaction 3. As a result, carbons assume the potential close to the electrode potential of the redox pair implicated in the interconversions [5]. This electrochemical mechanism of carbon to metal ions interaction is mainly observed at relatively low redox power of the ions: thermodynamically favourable oxidation of carbon to form surface oxides, CO or CO2, does not occur under these conditions [4], probably due to a considerable overvoltage of the carbon corrosion. To the contrary, this purely electron mechanism is not the only possible one in solutions of strong oxidants, there may occur the chemisorptive mechanism of carbon oxidation to the surface oxides, CO or CO2, but this process is always accompanied by a variation in pH of the solution [4]. Hence, the contribution of each of these mechanisms to the overall process can be estimated on the basis of analysis of the products of interaction between the carbon and the electrolyte solution. Being essentially an ensemble of electrical dipoles aligned in a certain manner to the interface, EDL may be also formed by physically adsorbed polar molecules (path 2), or specifically adsorbed electrolytes which contain surfactant ions (path 3), as well as chemisorbed complexes and heteroatoms (path 4). Thus, redox transformations of the electrolyte ions (path 1).must be treated as an alternative route to formation of EDL. How profound will be these transformations is strongly dependent on the efficiency of the other of enumerated processes of EDL formation (paths 2-4). Consideration of the electrocapillary phenomenon shows that the position of zero charge point (PZC) and electric capacitance are the principal parameters characterising the EDL [6] and governing electrochemical adsorption of ions [3]. The quantity of ions adsorbed by path 1 will be determined indeed by the potential (qg) and EDL capacitance of the carbon surface, whereas the occurrence of the other adsorption processes (paths 2-4) will cause a shift of PZC and a decrease in the electric capacitance thus varying the contribution of path 1 to the overall adsorption process. From this viewpoint on the nature of adsorption phenomena, we considered in detail the results of a number of studies, including ours, on adsorption of H2PdCI4 on carbon [7]. It was shown that oxidation of carbon with palladium ions to form surface oxides (path 4), CO or CO2, which is thermodynamically favourable under these conditions, did not occur. Actually, EDL is formed on the carbon surface through chemisorption of PdC12 (path 4) and electrochemical adsorption of C1- anions (path 1) induced by reduction of oxidants such as O2 (Reactions 1 and 2) or Pd 2+ (Reaction 5 as an analogue to Reaction 3). In the last case each carbon particles behaves as an individual short-circuited electric cell (because of the electrical conductivity of the carbon matrix): those fragments of its external surface, which possess the least electronic work function, become a cathode, where the oxidant is reduced until the formation of EDL is completed, whilst the remainder of the carbon surface serves as an anode, where positively charged "holes" (C a) formed after the carbon matrix has lost free electrons interact with anions. All the processes contributing to positive dipole jump of the electric potential on carbon surface were found to be competitive. Characteristic features of this kind of electrochemical reduction of metal ions are: I. Reduction of the ions proceeds preferably at the external surface of carbon support particles due to a higher rate of electron transfer from the carbon bulk compared to the diffusion rate of the ions through pores inward the particles [5,7];

17 II.

Neither surface nor gaseous carbon oxides are formed during the reduction, and anions therewith are adsorbed from the solution in the amounts equivalent to the number of electrons spent by the carbon to reduce the metal [4,7]; III. Concentration of H + remains almost constant in the solution [4,7] (however, pH of the solution may increase upon interaction between carbon and oxygen-containing oxidants according to the same mechanism; see, e.g., Reactions 1 and 2). In the present paper we shall base on the above described electrochemical concept of adsorption of electrolytes to demonstrate i)the effects of the nature of metal ions and the carbon support on the course of redox processes, complex formation, and ion exchange on the carbon surface depending on conditions of achieving the contact between the support and solution (with systems of HAuC14-carbon, H2PdC14-carbon and H2PtC16-carbon as examples), and ii) dependence of the state of a supported metal catalyst on the ratio of contributions of these processes to adsorption of the metal species. 2. EXPERIMENTAL Experimental details are described elsewhere [7,8]. If necessary, they will be specified in the course of discussion on the results obtained. General descriptions of procedures and techniques are given below.

2.1. Carbon supports Carbon supports of the Sibunit family [9], filamentous carbon of the CFC family [10], commercial carbon black PM-105 and various kinds of active carbons (Tables 1 and 2) taken as they are or pre-washed with water and acids in air [7] (see also Table 2) were used.

2.2. Adsorption procedure Adsorption of HAuC14 (H2PdC14, HzPtC14 or H2PtC16) was achieved from its aqueous solutions at 20-25~ in a controlled atmosphere. The following adsorption procedures were used [7]: adsorption from dilute solutions a) on the powdered support in a static reactor under strong stirring (method A) and b) on the granulated support through circulative impregnation (method B), as well as wetness impregnation by spraying of more concentrated solutions onto the both types of the support placed in a rotating drum (method C). Each of the reactors used in methods A and B was equipped with a gas burette which operated as a water-gate too; thus, the adsorption of the compounds was allowed in the atmosphere of various gases (He, N2, 02, air) under control of their volumes. After finishing the adsorption, the carbon was filtered off, flushed out with distilled water, then dried in vacuo at 50~ (residual pressure -~0.1 Torr) and kept in an inert atmosphere (to prevent reoxidation by air of reduced metallic species). In specific cases, pores of a support were filled by water or an electrolyte solution in an appropriate atmosphere (Table 2), after that adsorption of a metal compounds proceeds according to the above described procedure.

2.3. Chemical analysis Concentrations of metal species in solutions before and after adsorption runs were determined photometrically, the solutions being pre-diluted with 3M HC1. pH values of the solutions were measured after equal ionic strengths had been established in these solutions through addition of 1M NaC1 (1:1 vol). Gas evolution during the adsorption was measured volumetrically (see Section 2.2).

18

To determine the total C1- content in the solutions, small amounts of magnesium metal were added to the solutions to precipitate the noble metal [7,8]. As soon as H2 stopped evolving, the precipitate (metal black, Mg(OH)2) was centrifuged, and an aliquot of the solution thus produced was titrated with 0.1N AgNO3 by the Mohr method. To conduct the eluent analysis, a support saturated with various adsorbed metal compounds was washed by various eluents (acetone, solutions of acids and their salts) until an eluate aliquot reveals a negative qualitative reaction for the metal ions with a solution of KI or SnC12+HC1. Then the composition of the eluate was analysed.

2.4. Physicochemical studies X-Ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution electron microscopy (HREM) and electron microprobing (EM) methods were used to study the state of the adsorbed metal compounds and final catalysts. The determined characteristics of the metallic species are: quantities of the species (XPS, XRD), their dispersion (HREM, CO-chemisorption), microcrystallinity and phase composition (XRD) as well as distribution within a support particle (XPS, EM). The other products of interaction between these metal solutions and the carbon surface were analysed using chemical methods (Section 2.3) and XPS. 3. RESULTS AND DISCUSSION

3.1. Adsorption of HAuCI4 Gold was adsorbed from solutions of HAuC14 (-5-10 .4 M) in 1M NaC1 free of air. Fig. 1 shows that adsorption of HAuC14 is a combination of at least two processes which differ by rates of absorption of Au 3+ and relative changes in pH of the solution. The fast adsorption of HAuC14 occurs during the earliest several minutes upon the contact with carbon particles. Elemental analysis, as well as the pertinent XPS and HREM data demonstrate that features I-III, corresponding to the electrochemical mechanism described in Section 1, are characteristic of the fast adsorption. Thus, this type of HAuC14 adsorption of is reduction of ions [AuC14]-to gold metal bypath 1: C + [AuC14]-

- Au~

+ 3C1-/C~ + CI-

(4)

Au ~ species (Eb(AU4f7/2)=84.0eV; d=5-30nm) occupy the external surface of carbon granules, dispersion of Au ~ decreasing as the granule size increases. In the course of the slow adsorption of HAuC14, which is not clearly understood as yet, Au ~ is also formed preferably (with an impurity of AuI), however acidification of the solution and gas evolution are observed. The electrode potential of the [AuC14]-/Au ~ pair in 1M NaC1 being rather high (ca.1 V), there may occur the corrosion of carbon to CO and CO2 [13]. However, the further discussion will be focused on the state of the adsorbed species formed predominantly during the fast adsorption of HAuC14. 3.1.1. The state and reactivity of CI-/C ~ Along with lines of Au ~ lines of adsorbed chlorine with Eb(C1 2p3/2), equal to 197.8 and 201.0 eV (Au/Sibunit) and to 198.1 and 200.3 eV (Au/Norit SGM) are seen in XPS spectra recorded for samples prepared by adsorption of HAuC14 from 1M NaC1 (followed by

19

----i--,- slow fast i adsorption adsorption -'~i ~, ,, 4

200.3

i===~

o cN eq

C1 2p

3.6 C) 9 r/l

3.4 O)

i

e

b a

3.2 0

i

z

HAuC14 a d s o r p t i o n , m m o l / g Figure 1. Dependence of pH of the suspension and time of adsorption on the amount of adsorbed gold. The starting composition of the suspension: 1 dm3 of 4x 10"4M HAuC14 in 1 M NaC1 + 0.1 g of active carbon SKT (SBF.T=1200 m2/g, Vmi=0.51 cm3/g). The dashed line separates the regions of the fast and slow adsorption of HAuC14.

198.1 195 2()0 205 Binding energy (eV)

Figure 2. XPS spectra of chlorine which was chemisorbed by active carbon Norit SGM as a result of a) HAuC14reduction (Reaction 4), b) the action of aqueous HC1 in air (Reactions 1 and 2) and e) chlorination with gaseous C12at 200~

thorough removal of NaC1 with water). The chlorine state with a low Eb(C1 2p3/2)=197.5 198.5 eV is characteristic of metal chlorides [11 ], and the state with a high Eb(C1 2p3/2)=200 201.5 eV of chlorinated organics [11] or carbons [12]. The line C1 2p3/2 with the low Eb appears rather intense (Fig.2a). This fact cannot be explained by the presence of metal (Na, Ca, Fe) chlorides, because they are in too small concentration at the carbon surface. Hence, this state can be assigned to those ionic pairs C e ...C1- (Reaction 4), which are retained after the solvent has been removed. The line C1 2p3/2 with the high Eb is assigned to a more covalence of the C1-C bond. Hence, some of chlorine states in the ionic pair Ce...C1 - can relax towards the states with a higher covalence of the chlorine-carbon bond (like ~ c - c I ). Such a transition from pure coulomb interactions towards chemical ones is rather characteristic of the ions adsorbed within the potential-determining layer of the EDL formed on the surface of a solid being in contact with an electrolyte solution [3,6]. Similar chlorine surface states emerge at active carbons (PN, Norit SGM) upon adsorption of HC1 in air according to Reactions 1 and 2 (Fig.2b), when the so-called "basic oxides" are formed, as well as upon chlorination with gaseous C12 (Fig.2c). The similarity in the nature of C-C1 bonds generated both by Reactions 1 and 2 and by Reaction 4 (or 3) reveals as the ability of some portion of C1- ions to be replaced by anionic complexes of metals (see Sections 3.2.2 and 3.3). Chlorinated carbons also behave as anion exchangers. In particular, C-C1 bonds of these carbons can be hydrolysed (the inverse Reaction 2), that results in acidification of the solution. In principle, this process may also bear some responsibility for acidification of the HAuCI4 solution during the slow adsorption.

20

3.1.2. Influence of physicochemical properties of carbons on reduction of HAuCI4 When immersed in a solution with redox properties, carbon starts behaving as an electrode and acquires a stationary potential independent of the carbon nature [3,5]. Therefore, the amount of metal reduced, e.g., by Reaction 3 or 4 will increase with an increase in the doublelayer capacitance, which depends, in its turn, on the carbon surface crystallochemistry and curvature. For graphite, the differential capacitance of EDL for the edge plane is much higher than that for the basal one [13]. It also increases with an increase in the pore size, especially when going from microporous to mesoporous carbons [14]. On the other hand, chemisorbed heteroatoms which produce a positive dipole jump of potential at the carbon-solution interface induce shift of PZC of carbon towards positive q~ and decrease the electric capacitance [3,5, 13] that will lower the reducing power of carbon. The adsorptive capacity for HAuCI4 was discovered to follow the above deduced dependencies for various carbons (Table 1). Table 1 Adsorption capacity of carbon supports with various physicochemical properties with respect to precious metals. Powdered (<90 ~t) supports, adsorption time" 20 h (HAuC14, H2PdC14), 120 h (H2PtC14, H2PtC16) Carbon sample Corax 3 (graphitized) CFC-2 CFC-1 PM-105 PM- 105 (ox. KMnO4) Eponit 113H Sibunit 4GV-K (Japan)

Textural SBEVl) ABET mZ/g 72 67 85 115 112 100 101 850 610 550 1680 806

properties Vmi Surface cm3/g nature 2) 0.00 basal 0.028 edge steps steps steps 0.15 steps 0.20 steps 0.35 steps

Adsorption capacity ~) HAuC14 HEPdC14 HzPtC14 H2PtCI6 ~tmol/m2 (SBET) 1.41 0.17 0.40 0.13 5.97 1.57 1.58 1.01 2.69 0.89 2.58 0.92 0.86 0.48 2.16 0.71 0.40 2.18 0.80 0.82 0.374) 1.71 0.62 1.40 0.57 0.19

i) The data are obtained by one-point measurements. z) From HREM images: predominant crystallographic orientation at the interface. 3) "Strong"adsorption (the remainder after elution with 3M HC104). 4) For other active carbons: 0.40 (Supersorbon H8-3),--0.57 (Norit RB-1) [25] and--0.41 (Norit ROX) [19] (as calculated by us from the adsorption isoterms).

3.2. Adsorption of H2PdCI4 Two competitive reactions co-exist when dissolved H2PdC14 reacts with carbon: reduction of Pd tI anions to the metallic state (Eb(Pd 3d5/2) =335.4 eV) followed by co-adsorption of C1ions (Eb(C12p3/2) =199.5 eV) near the metal particles [7]: C + [PdC14]2- ~

Pd~

+ 2C1-/C * + 2C1-

(5)

and chemisorption ofPdC12 (Eb(Pd 3d5/2) =336.8-337.4 eV) on carbon surface [7,8,15,16]: C + [PdC14] 2-

-

"- PdC12/C +2C1-

(6)

Side Reactions 1 and 2 can also occur in air. Whatever is the ratio of rates of Reactions 5 and 6, the overall process meets the rules I-III (see Section 1). The C1/Pd ratio for the adsor-

1

bed species is much the same in all cases and averaging 2 (usually, 2.00-2.15). After the adsorption equilibrium has been reached, one can find PdC12 complexes to be evenly distributed over the whole surface of the support. In their turn, Pd ~ particles, named by us as "the former Pd ~ assemble on the external surface of the support particles, even if they are no more than 0.2 m m in size [7]. The pathways for Reactions 5 and 6 are seen to depend on the chemical nature of the carbon surface, its textural characteristics and granularity and, on the other hand, on the supporting procedure and the composition of the catalyst precursor solution (Tables 2 and 3). Table 2 Procedures for the preparation of Sibunit support (Samples 1-17) and active carbon SKS (Sample 18) with adsorbed palladium compounds as precursors of 2%Pd/C catalysts -

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Sample of carbon Deposition of palladium compounds 1~ support ABET, Pre-treatment Method; (pre-filling Time, Fraction, Atmomm m2/g pores material) h sphere 0.09-0.18 455 H20 A 1 helium 0.09-0.18 455 HRO A 1 air 0.09-0.18 455 H20 A 3 air 0.09-0.18 455 H20 A 1 oxygen 0.09-0.18 455 HCI+HF A 1 air 3.0-5.0 455 HCI+HF B 3 air 3.0-5.0 455 HCI+HF B 3 helium H20 B 3 air 3.0-5.0 455 H20 C 0.25 air 3.0-5.0 455 3.0-5.0 455 HCI+HF C 0.25 helium B. (H20) 3 air 3.0-5.0 455 H20 B; (0.5 M HC104) 3 air 3.0-5.0 455 H20 3.0-5.0 455 H20 B; (0.5 M H2SO4) 3 air B; (0.5 M HC1) 3 air 3.0-5.0 455 HzO 1.5-2.0 20 none B; (H20) 6 nitrogen 1.5-2.0 527 none B; (H20) 6 nitrogen 1.5-2.0 527 none B, 100 ~ (H20) 2 air B; (H20) 6 nitrogen 1.0-1.5 1100 none

Total metal content, wt.% 2.00 1.90 1.95 1.99 1.97 1.78 1.80 1.70 1.93 1.95

0.31 2.00 1.98 2.00

~ For procedural details see Section 2.2. 3.2.1. Formation of surface n-complexes of PdCI2 Eluent analysis shows different stability of surface PdC12 complexes obtained by Reaction 6: weak (A1), strong (A2) and very strong (A3) adsorption sites are classified [8]. Only PdC12 occupying A1 sites can be removed by elution with water or acetone, whereas the use of 1-5M MC1 (M=H +, Li +, Na +) as an eluent leads to removal of PdC12 from both A1 and A2 sites. Moreover, PdC12.A2 complexes are stable towards action of 1-3M HC104 or H2SO4 and their salts. PdC12.A3 complexes are not appreciably destroyed under the action of all these eluents. Comparative studies [8,15,16] of a set of carbon samples (graphite, active carbons, carbon

22

Table 3 States of palladium catalyst precursors supported on carbons under various conditions (for the preparation conditions of the samples, see Table 1) and the final catalysts

No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

The state of the catalyst precursors Palladium metal phase PdC12 phase Particle size, CSR l~ size, Content, Particle size, CSW ~size, nm nm w.t.o~ 2) nm nm dn dm Ll11 L200 Illl dn L~ 12 14 12 9.5 0.72 <3 15 21 12 10 0.34 1.9 <3 13 9.0 0.39 <3 19 25 8.5 ~3 0.25 1.6 <3 33 42 21 9.5 0.53 1.6 <3 40 30 0.86 <3 27 24 0.70 <3 62 85 30 20 1.13 <3 12 7.5 0.12 1.8 <3 10 -~3 0.09 <3 24 17 0.35 <3 27 18 0.23 <3 22 17 0.16 <3 25 25 0.15 <3 ~4 0.08 <3 22 14 0.71 <3 28 28 1.05 <3 scales, 102-104 18 25 1.56 <3

Dispersion of thefinal Pd/C Catalyst CO/Pd 0.35 0.30 0.41 0.40 0.38 0.25

1)Coherent X-ray Scattering Regions. 111 diffraction peak of metallic palladium. 3)Reduction of a 0.5 g sample in H2 (30 ml/mm) under the following conditions: 50~ (10 min), 100~ (10 rain), 150~ (60 rain), 250~ (200 min).

2) From the integral intensities of the

blacks, Sibunits and CFC including chemically modified ones) revealed that an A1 site may consist of hexagons on a basal plane fragment of a quasi-graphitic crystallite, whereas an A2 site seems to be a composition of >C=C< fragments of both edge and basal planes, which make up a surface steps (terraces), and A3 sites are localised in micropores. It is most likely that PdClz form z~-complexes with the ligands of the Ax-A3 sites. Studies of HzPdC14 adsorption on oxidized carbons [8,15] and XPS studies of the adsorbed species did not reveal formation of noticeable quantities o f P d n complexes with oxygen-containing groups. The fast formation of n-complexes of PdCl2 suppresses reduction of Pd Ix anions (cf. Samples 7 and 10 or Samples 8 and 9; Table 3). The reason is that adsorbed molecules of PdCI2 are aligned with their Pd z+ ions to the surface that causes a positive surface potential jump. As a result, PZC of carbon is shifted towards the positive potential range, the electron work function increases, and the reducing power of carbon decreases. Thus, the chemisorption of PdClz can be considered as path 4 contributing to the process of the EDL formation.

3.2.2. Reduction of PdClz and ion-exchange properties of the chlorinated sites Reaction 5 should be treated as path 1 of the formation of the EDL. From analysis of our data (most of them are presented in Tables 2 and 3), this process proceeds at the highest

23 efficiency provided that: 1)by-processes capable of inducing a shift of PZC of carbon towards the positive potential range or imparting a free charge to the carbon surface are excluded or diffusion controlled, and 2) adsorption conditions favour maintenance of a higher redox potential of the PdZ+/Pd pair and a higher reducing power of the carbon (Table 4). Table 4 Factors affecting the PdC12:Pd ratio of the adsorbed palladium species high ,~ PdC12:Pd ratio ) low 1. Granulated carbon support 1. Powdered carbon support 2. High temperature 2. Low temperature 3. The absence of the pointed ligands 3. The presence of ligands decreasing ( for example, an excess of HC1) E pa2+/pdo ~ 4. High concentration of H2PdC14 (incipient wetness impregnation or adsorption with intensive agitation) 5. The presence of foreign oxidants, for example,

02, H202, etc. (paths 1, 4) 6. Low electric capacitance of the EDL of carbon surface: low surface area of carbon support - oxidized [5] or chlorinated carbons (path 4) (high content of the acidic surface groups) - pre-adsorption of acids (path 1) -

4. Low concentration of H2PdC14 (adsorption from the excess solvent) 5. The absence of the co-oxidants (inert atmosphere) 6. High electric capacitance of the EDL of carbon surface: - high surface area of carbon heat treated carbons (high content of the basic ones) absence of the pre-adsorption -

-

When the contribution of Reaction 5 to the overall adsorption of H2PdC14 becomes remarkable, another two states are revealed, along with PdClz.A2 and PdClz-A3: the former Pd ~ particles, which can be totally dissolved by 1-5M HC1 in air and anionic complexes of Pd II, which can be repelled from the surface by 1-3M HC104 or LiC104. The content of these complexes increases with an increase in the amount of chlorine bonded directly to the carbon surface irrespectively of the way of generation of such a chlorine state: whether due to chemisorption of HC1 in the presence of 02 (Reactions 1 and 2) or H202, or due to reduction of Pd II ions (Reaction 5), or due to chlorination of the support with gaseous C12. Hence, some portion of C1--/Ce (probably, constituting the ionic pairs C*...C1-) is capable of exchanging C1- ions by anionic complexes ofPd II, i.e.: 2C1-/C ~ + [PdC14]2- --

-

[PdC14]2-/2C e + 2C1-

(7)

Pre-chlorinated supports (C12, 100-400~ are the most effective as to the ion-exchange adsorption of [PdCI4]2- but they do not able to reduce H2PdC14. The sum of amounts of PdCI2 "strongly" bonded with the surface according to Reaction 6 (path 4) and Pd ~ deposited by Reaction 5 (path 1) was found to be nearly the same in all cases, whatever the ratio PdClz:Pd ~ for the adsorbed species may be. Hence, the magnitude of "strong" adsorption of H2PdC14, which is defined as the rest after desorption of the weakly bonded species with HC104, can be used to estimate the reducing power of carbon with respect to palladium ions. The term reducing power of carbon with respect to metal ions

24 means the quantity of the metal which can be reduced by the surface of the support by path 1 provided that the rates of formation of EDL of carbon by the rest paths are negligible. The value of the "strong" adsorption depends on physicochemical properties of carbon [8,16], the dependence being similar to that for adsorption of HAuC14 (Table 1). The authors of ref. [17] used redox titration of various carbons with solutions of Ce TM salts and KMnO4 for estimation of their "reducing power". This method, though more indirect then ours, provides the results, which are in compliance with the data of Table 1.

3.2.3. Effect of adsorbed palladium species on dispersion of final Pd/C catalysts The data on adsorption of H2PdC14 allow the conclusion that there may be two origins of metal particles in the final catalyst. These are re-complexes of PdCl2 (Reaction 6) on one hand and the former Pd ~ (Reaction 5) and the related processes (Reaction 7) which may be responsible for their formation. Upon removal of the solvent the complexes of PdCl2 with A1 and A2 sites dissociate partially, only PdCI2 clusters being formed (ca. 1.8 nm in size [7,15], see also Table 3 and Fig.3a). Their size is only slightly dependent, if at all, on the nature of the carbon support, as well as on the weight content of PdCI2 over a rather wide range (it cannot be excluded, however, that the relationship between the cluster and monomer species of PdC12 changes). At the gas-phase reduction by H2, the surface species of PdC12 give birth to fine Pd particles (Fig.3a). Their size ranges typically between 1 and 5 nm depending on the substructural properties of the support, the dimensions of these particles being only slightly affected by PdCl2 loading or by the presence of the surface oxides [15]. The treatment of the samples with H2 leads to small, if at all, changes of the particles size distribution and the amount of the former Pd ~ despite of the reduction of PdC12 (Fig.3b). Dispersion of its particles is very rough and depends, at least, on the physicochemical properties of carbon, impregnation conditions and inorganic impurities in the support (Table 3). Generally, regularities of formation and growth of particles of the former Pd~ are similar to those of electrochemical deposition of metals [7]. As to the influence on the dispersion of final catalysts for palladium bonded through ion exchange, viz. [PdC1412-/2C*, it is not well understood as yet. These species generated by Reaction 5 seem to be arranged in a close neighbourhood to the former Pd ~ particles, since there is a higher concentration of C1-/Ce around these particles [7]. Hence, their reduction is expected to result in a fast decrease in the catalyst dispersion due to sintering. Probably, this is the reason for an increase of 25 percent of the quantity of x-ray detectable components of the metal upon reduction of Sample 8 (Table 3), which can contain a large amount of [PDC1412/2Ce species (because of the high loading of the former Pd~ whereas this phenomenon does not observed with Samples 2, 4, or 5. Thus, co-existence of two different mechanisms for formation of metal particles results in a bimodal size distribution of the particles in the Pd/C catalysts (Fig.3). Hence, one of the reasons for a decrease in the dispersion of the supported metal is generation of the former Pd ~ In this connection, the fact that the amount of the former Pd ~ among the adsorbed precursors decreases as the surface concentration of the supported metal increases, all other factors being the same (for example, adsorption of H2PdC14 from the excess solvent; cf. Samples 15, 16, and 18 of Table 3), appears of importance for understanding of regularities of the formation of Pd/C catalysts. This may also be the reason for an extremum observed in the plot of Pd/C dispersion versus metal concentration [18]:

25

801 !:1 I

~

(a)

=,,==,

H.-,, / / / / ,,/,.,.,

7O

,-r-~

(b)

//t/

,./t.4

;4r ;///~

/ / / /

);.2; "//Z

;;22 //in

n

. . . .

,/1.~

3O

U.4 //// ////

r ,-///

1

9

7r ( /. ~/ ~

0,0 0,8 1,6 2,4 3,2 4,0

0

SlTt=(nm)

4

""///~

9

i

9

i

9

i

,

i

9

i

'

i

8 12 16 20 24 28 32 a6 40

,

(a) From the very beginning the carbon possesses a high reducing power. Therefore, during preparation of a catalyst with low Pd loading almost all the metal ions supported from the solution will be reduced as early as at the impregnation stage to comparatively coarse particles (Reaction 5), and the dispersion of such a catalyst will be low. (b) As the support surface acquires a charge

Figure 3. Typical histograms of particle size distribution for adsorbed (Reaction 5), the work palladium compounds before (F-t) and after (gA)treatment in H2. Sample 2. function of carbon will Histograms for small and large particles are plotted separatedly, increase, and the rate of Reaction 5 will decrease that will result in an increase in the contribution of Reaction 6 to the overall adsorption of the precursor. Hence, its dispersion will increase with the metal loading. (c) Further, upon reaching high enough loading of the metal, Reaction 6 starts dominating. Therefore, the catalyst dispersion will become independent or only slightly dependent on the metal loading within some range. (d) However, at a very high loading of the supported precursors the proportion of their weakly bonded species will increase that will cause eventually a decrease in the catalyst dispersion. The effects of those factors, which determine the rate ratio of Reactions 5 and 6 (Table 4), are expected to show up as a shift in the maximum position in the said curve. A decrease in the carbon reducing power will result in the shift to the region of lower metal loading but an increase to the region of higher metal loading. 3.3. Adsorption of H2PtCI6 and H2PtCI4 At present there are several concepts regarding the nature of chemical interaction of HzPtC16 and H2PtC14 with the surface of carbons. They are based on: 1) redox mechanism, i.e. oxidation of the support by Pt TM and Pt II ions to gaseous or surface carbon oxides [ 19-23], that is accompanied by deposition of Pt~ and by adsorption of Pt n complexes; 2)acid-base interaction yielding salt-like compounds of HzPtC16 with protonated Lewis bases at the surface; these surface bases may be oxygen-containing groups [24] or C~ sites of basal planes of the carbon crystallites [1,21]; 3)coordination mechanism, i.e. substitution of oxygencontaining groups for CI- ions involved in the coordination spheres of Pt TM and Pt II chloride complexes [25,26], as well as formation of such n-complexes as C~-PtC13- [19] or C~-PtC12 [23] with ligands of the C~ sites. It follows from the stoichiometry of the above mentioned reactions that pH of the solutions is proportional to the degree of these transformations of

26

platinum complexes in the most cases except formation of n-complexes of Pt II. Hence, these are pH-dependent adsorption processes. Nevertheless, the magnitude of platinum adsorption from solutions of HzPtC16 [19] or HzPtC14 [23] was found not to depend on pH of these solutions but decreases with an increase in the concentration of C1- ions, pH of these solutions being practically unchanged during their contact with carbon [20,22]. These facts contradict the conclusions cited in refs. [ 19-21] that gaseous or surface carbon oxides can be formed, but are in agreement with ref. [26], where almost no changes in the surface functionality of the supports were revealed. Besides, an increase in the concentration of the surface oxides produced by oxidation of carbons causes a decrease [19] but not increase (as may be deduced from results of [25,26]) in sorbability of H2PtC16. Thus, the conflicting data obtained in this field of carbon chemistry gave rise to rather dramatic situation. We carried out the eluent analysis of the states of platinum adsorbed on various carbons from solutions of H2PtC16 and ItzPtC14 (using the procedures similar to those described in Section 3.2), as well as the studies of surface transformations of these compounds using elemental analysis and XPS. We discovered that regularities of adsorption of these platinum compounds are, again, caused by electrochemical behaviour of carbons as described below. If a support contacts a solution of HzPtC16 in an inert atmosphere, the starting stage of formation of EDL of the carbon is reduction ofPt TM ions: C + [PtC16]2-

~ [PtC14]2-+ 2C1-/C *

(8)

This reaction is a "platinum" version of Reaction 3. Released H2PtC14, along with HzPtC16, is detected in the solution but then absorbed by the carbon (Fig.4), the former Pt ~ species (Eb(Pt4fv/z)=71.3eV) and n-complexes of PtCI2 with the surface >C=C< fragments (Eb(Pt 4f7/2)=73.8 eV) being generated: C + [PtC14]2C + [PtC14]2- -

-

" Pt~ + 2C1-/C * + 2C1PtC12/C + 2C1-

(9) (10)

Pt H involved in n-complexes can be eluated by 3M LiC1 but not by 3M HC104, while the are stable. When a solution of H2PtC16 is in contact with a granulated active carbon CG-48A, Pt II ions are seen (Fig.4) to be accumulated in small quantities upon completing the adsorption process, whereas with a suspension of a powdered carbon the accumulation is more pronounced and observed from the very beginning. Hence, the proportion of the direct reduction of H2PtC16 to the metal (similar to Reaction 4 for HAuC14) is much higher on carbon granules than on the powder; the data of the eluent analysis also support this conclusion. As a result, platinum, even if its final content is as high as 0.33 mmol/g, is collected within a 0.1-0.3 mm layer on the outside of the support granules (Sample 3 of Fig. 4). Chemically, platinum resembles palladium, and it is not surprising that Reactions 9 and 10 are similar to Reactions 5 and 6. However, unlike Reaction 6, Reaction 10 reaches the equilibrium in rather long time. Hence, this way of formation of EDL of the support in the system H2PtC16-carbon cannot compete well with alternative routes, for example with redox transformations of foreign oxidants, including air oxygen.

former Pt ~ species

27

In an inert atmosphere only small change in pH of the solution against the initial one was observed, and the total C1/Pt ratio for the adsorbed species averaged 4 (typically, 3.8 to 3 4.2). Some decrease in pH and "E (C1/Pt)ads is, probably, caused by an acidity of aquacomplexes of platinum (IV) chlorides and hydrolysis of C1-/C*; these processes 1 may be suppressed by mere addition of NaC1 to the carbon suspension 0,1 0,2 0,3 0,4 o upon completing the adsorption. The Platinum content, nmgl/g increases in pH and (C1/Pt)ads are the phenomena, which explain the Figure 4. Intermediate formation of HzPtC14during adsorption formation of EDL according to the of H2PtCI6 on active carbon CG-48A (A~T=952 m2/g, Vmi=0.445 cm3/g)of different granularity: 1) <0.09 mm, 2) 0.18- "oxygen" (Reaction 1) scenario. For 0.32 mm, 3) 1-2 mm. example, when adsorption of H2PtC16 is allowed with air access, the intermediate H2PtC14 is not detected, and the (C1/Pt)ads ratio goes up to 5.6. The magnitudes of "strong" adsorption of H2PtC16 and H2PtC14 shown in Table 1 are also dependent nanotextural and chemical properties of carbon in the manner predicted in Section 3.1.2 in terms of the electrochemical concept. These data show that among various supports, oxidized and graphitized carbons possess the least propensity to strong chemisorption of platinum ions, that is contradictory to the assumption on formation of strong bonds of surface oxides [25-27] and basal sites [21, 26-28] with the precursors of platinum catalysts. The magnitude of"strong" adsorption of precious meta'l ions being the measure of the reducing power of carbon with respect to these ions (Section 3.2.2), it is obvious that a partial oxidation or graphitization of a support will result in a variation in its redox properties, which determine the ratio of various adsorbed species of the precursor and their distribution through the carbon granules, that will eventually affect the state of the metal in the Pt/C catalysts. A great quantity of C1-/Ce states is generated during adsorption of H2PtC16 that favours ion exchange (similar to Reaction 7). ^

,

2C1-/C * + [PtC16]2- ~ 2C1-/C e + [PtC14]2- ~

1

[PTC1612-/2Ce + 2C1"- [PTC1412-/2Ce + 2C1-

(11) (12)

From the XPS data, these salt-like surface compounds seem to contribute to the intensity of lines with Eb(Pt 4f7/2); the intensities are 74.8 and 72.3 eV, respectively. All the platinum species, including Pt ~ adsorbed by both powered and granulated carbons from the solutions of H2PtC16 or H2PtC14 at room temperature are highly dispersed and x-ray amorphous (see also ref. [22]). Only the adsorption of H2PtC16 on granules of the SKS active carbon from an excess of the solvent at as high temperature as 100~ gave rise to formation of XRD-detected Pt ~ crystallites (ca. 8 nm in size). Hence, the growth rate of particles of the former Pt ~ is much lower of the rate of their nucleation, unlike the phenomenon observed during reduction of HaPdCI4 by carbon [7]. Among the probable reasons may be the kinetic

28 inertness of platinum chloride complexes (carbon is capable of accelerating their transformations) and the high work function for Pt compared to that for carbon and palladium. Additional studies on the reactivity of individual platinum species have shown that there may occur side processes under the conditions of Pt/C preparation, which result in reoxidation of the adsorbed platinum species: Pt ~ + [PtC16]2-+ 2C1~ 2[PtC14]2Pt ~ + 6HC1 + 02 - H2[PtC16] + 2H20 Pt~ + X/202 ~ PtOx/C [PtCI4]2-+ 2HC1 + 1/2Oz " [PtC16]2- + H20

9

1,0 -

0,8 cO

0,6E c.-. = 0,413_

0,2 0,0

O0

03

0',2

o13

o',4

o',~

Platinum loading, mmol/m z

Figure 5. Relationships between platinum dispersion and metal loading for Pt/C catalysts with low (white circles) and high (black circles) content of chemisorbed oxygen: 1) C (carbon black CC-40-220), 2) C (He, 2273K), 3) C(H2) 8N, 4) C(H2) 12N, 5) T (carbon black T-10157), 6) T (He, 2273K), 7) W (active carbon), [25,27]" 8) C (He, 1873K), 9) c (He, 2073K), 10) C (He, 2473K) [28]; 11) C2ox (no.9 oxidizied with 12N H202) , 12) C2oxT (no.ll treated in He at 773K) [21]; 13) A (carbonizied phenolformaldehyde resin), 14) A2 (A oxidizied with 15M HNO3), 15) A4 (A2 treated in N2 at 800K) [26]" 16) C1 and 17) C3 (active carbons) [30]; 18) BAU (active carbon) [29]; 19) V3G (48.9% bum-off) [31]" 20)-22) Sibunit (our data). Platinum content: 0.9-1.1 wt.% for Samples 1-17, 19 and 20; 1.5 wt.% for Sample 21; 3.5 wt.% for Sample 22; 5.0 wt.% for Sample 18.

(13) (14)

(15) (16) Unlike Reaction 13, Reactions 14 and 16 cannot be observed but at high (1-3M) concentrations of HC1. It is of common knowledge that the chemisorption of oxygen (Reaction 15) proceeds effectively over highly dispersed platinum. When heated, platinum oxides can easily oxidize the carbon. This may be the reason for releasing huge amounts of CO and CO2 during temperatureprogrammed desorption observed in [21 ]. 3.3.1 Effect of the adsorbed platinum species on dispersion of final Pt/C catalysts Thus, a variety of paths for

absorption from

strong

of platinum ions solutions, the dependence of these

H2PtC16

paths on redox properties of

both solutions and supports, as well as a potentiality of occurrence of various secondary processes are the reasons for the observed diversity of states of adsorbed precursors of Pt/C catalysts. Similar to the case of palladium catalysts, the quantitative ratio of these precursors will be varied with the metal loading. Hence, the platinum dispersion in the final catalysts is expected to vary in the same manner. In fact, if the data [21, 25-31] on dispersion of Pt/C catalysts prepared by the same methods using various supports of similar granularity are revised based on this concept, the dispersion of these catalysts is seen to go through a maximum as the total surface concentration of the supported precursors increases (Fig.5). The interpretation of this

29 correlation is, in principle, the same as for Pd/C catalysts (Section 3.2.3). At a low surface coverage, the former Pt ~ dominates among the precursors. It is arranged preferably at the external surface of the support particles. For this reason there occurs a rapid sintering of the metal under heating that results in a comparatively low dispersion of the Pt/C catalysts. This is also the reason for a decrease in the dispersion of these catalysts with a decrease in the external surface area of the carbon particles [27]. The ascending branch of the curve (for example, in Fig.5a) corresponds to the gradual reduction of the fraction of theformer Pt ~ and to the increase in the proportion of PtC12 among the adsorbed precursors, the proportion of ncomplexes of PtC12 being, obviously, maximal at the point of the maximum. The descending branch of this curve is caused by the catalyst sintering due to an increase in both total concentration of the precursors and the contribution of ion exchange (Reactions 11 and 12). As to the oxidized carbons as supports, their reducing power with respect to H2PtC16 is lower of that of the starting support. That is why the quantity of the deposited former Pt ~ will be lower. Eventually, this phenomenon will result in a shift of the maximum of the dependence of Pt/C catalyst dispersion towards lower metal loading (Fig.5b). Probably, the observed correlation may be treated as the universal one for powdered supports with heterogeneous surface, since it is little affected by the type of the support but determined predominantly by the method used for chemical modification of its surface. 4. CONCLUSIONS Thus, regularities of adsorption processes, which take place during impregnation of carbon supports with solutions of precious metal salts, may be interpreted in terms of the electrochemical concept of carbon behaviour in electrolyte solutions (see Sections 1 and 3.1.2). The formation of adsorbed catalyst precursors appeared to follow several competing routes, the ratio of which determines the metal state and its distribution through the support granules in the final catalyst. The knowledge on the principles for controlling the ratio is the key to designing supported metal catalysts with the required properties. In principle, the electrochemical behaviour of carbon is not reduced to the impregnation stage only. Probably, in a number of cases it also determines the processes of liquid-phase reduction of the adsorbed precursors. When a reductant is added to the support suspension, those of the precursors, which are localized as ions at the support surface, may assume mobility due to the surface repolarization and will be redistributed, there will occur cathodic processes in the bulk of the support granules and anodic processes outside. The liquid-phase redox processes with carbon as the catalyst may be treated in terms of the same concept. REFERENCES

1. L.R. Radovic and F. Rodr guez-Reinoso, in Chemistry and Physics of Carbon, Vol. 25 (P.A. Thrower, ed.), Marcell Dekker, New York, 1997, p. 243. 2. E.M. Kuchinskii, R.Kh. Burshtein and A.N. Frumkin, Zh. Fiz. Khim., 14 (1940) 441. 3. A.N. Frumkin, Potentials of Zero Charge (Russ. Ed.), USSR, Moskow, 1982. 4. E.S. Matskevich, D.N. Strazhesko and V.E. Goba, Adsorbtsiya i Adsorbenty (Russ. Ed.), 2 (1974) 36. 5. Yu. A. Tarasenko, A. A. Bagreev, V. V. Dudarenko, V.K. Mardanenko, and Yu.A. Solodovnikov, Ukr. Khim. Zh. (Russ. Ed.), 55 (1989) 233.

30 6. A.W. Adamson, Physical Chemistry of Surfaces, U.S.A., Easton, PA, 1963. 7. P.A. Simonov, A.V. Romanenko, et al., Carbon, 35 (1997) 73. 8. P.A. Simonov, V.A. Semikolenov, V.A. Likholobov, A.I. Boronin, and Yu.I. Yermakov, Izv. Akad. Nauk SSSR, Ser. Khim. (Russ. Ed.), No.12 (1988) 2719. 9. Yu.I. Yermakov, V.F. Surovikin, G.V. Plaksin, V.A. Semikolenov, V.A. Likholobov, A.L. Chuvilin and S.V. Bogdanov, React. Kinet. Catal. Lett., 33 (1987) 435. 10. V.A. Likholobov, V.B. Fenelonov, et al., React. Kinet. Catal. Lett., 54 (1995) 381. 11. C.D. Wagner, W.M. Riggs, L.E. Davies, J.F. Moulder and G.E. Muilenberg (eds.), Handbook of X-ray photoelectron Spectroscopy, Eden Prairie, Minnesota, 1978. 12. E. Papirer, R. Lacroix, J.B. Donnet, G.Nanse and P. Fioux, Carbon 33 (1995) 63. 13. K. Kinoshita. Carbon, electrochemical and physicochemical properties, N.Y., 1988. 14. V.S. Vilinskaya, A.A. Korobanov, R.Kh. Burshtein and A.V. Gerasimova, Izv. Akad. Nauk SSSR, Ser. Khim. (Russ. Ed.), No. 5 (1978) 1000. 15. P.A. Simonov, E.M. Moroz, A.L. Chuvilin, V.N. Kolomiichuk, A.L. Boronin and V.A.Likholobov. 6-th Intemat. Symp. Sci. Bases for the Preparation of Heterogen. Catalysts, Sept. 5-8, 1994, Louvain-la-Neuve, Belgium. Preprints, 3 (1994) 201. 16. P.A. Simonov, E.M. Moroz, V.A. Likholobov and G.V. Plaksin, Izv. Akad. Nauk SSSR, Ser. Khim. (Russ. Ed.), No.7 (1990) 1478. 17. D. S. Cameron, S. J. Cooper, I. L. Dodgson, B. Harrison and J. W. Jenkins, Catal. Today, 7 (1990) 113. 18. G.R. Heal and L.L. Mkayula, Carbon, 26 (1988) 815. 19. H. E. van Dam, H. van Bekkum, J. Catal., 131 (1991)335. 20. Von E. Czaran, J. Finster, K.-H. Schnabel. Z. Anorg. Allg. Chem., 443 (1978) 175. 21. F. Coloma, A. Sep lveda-Escribano, J. L. G. Fierro and F. Rodr gues-Reinoso, Langmuir, 10 (1994) 750. 22. R. Fu, H. Zeng, Y. Lu, S.Y. Lai, W.H. Chan and C.F. Ng, Carbon, 33 (1995) 657. 23. V.V. Strelko, Yu. A. Tarasenko, A. A. Bagreev, V.V. Dudarenko and V.K. Mardanenko, Zh. Prikl. Khim. (Russ. Ed.), 65 (1992) 1742. 24. K.T. Kim, J.Sh. Chung, K.H. Lee, Y.G. Kim and J.Y. Sung, Carbon, 30 (1992) 467. 25. C. Prado-Burguete, A. Linares-Solano, F. Rodr guez-Reinoso and C. Salinas-Mart nez de Lecea, J. Catal., 115 (1989) 98. 26. M. C. Rom n-Mart nez, D. Gazorla-Amor s, A. Linares-Solano, C. Salinas-Mart nez de Lecea, H. Yamashita and M. Anpo, Carbon, 33 (1995) 3. 27. C. Prado-Burguete, A. Linares-Solano, F. Rodr guez-Reinoso and C. Salinas-Martinez de Lecea, J. Catal., 128 (1991) 397. 28. F. Coloma, A. Sep lveda-Escribano and F. Rodr gues-Reinoso, J. Catal., 154 (1995) 299. 29. G.M. Plavnik, B. Parlits and M.M. Dubinin, Dokl. AN SSSR, 206 (1972) 399. 30. F. Rodr gues-Reinoso, I. Rodr gues-Ramos, C. Moreno-Castilla, A. Guerrero-Ruiz and J.D. Lopez-G nzalez, J. Catal., 99 (1986) 171. 31. P. Ehrburger, O.P. Mahajan and P.L. Walker, Jr, J. Catal., 43 (1976) 61.

91998 Elsevier Science B.V. All rights reserved.

Preparationof CatalystsVII

31

B. Delmon et al., editors.

Anionic Clays as Precursors of Noble Metal Based Catalysts for Methane Activation F. Basile a, L. Basini b, G. Fornasari ~, M. Gazzano c, F. Trifir6 a and A. Vaccari a ~Dipartimento di Chimica Industriale e Materiali, Viale Risorgimento 4, 40136 Bologna, Italy USnamprogetti Research Lab.s, Via Maritano 26, 20097, San Donato Milanese MI, Italy CCSFM/CNR, Via Selmi 2, 40126 Bologna, Italy Rh, Ru, Ir, Pd and Pt based catalysts were prepared from hydrotalcite-type (HT) anionic clays, and their activity in the catalytic partial oxidation (CPO) or catalytic total oxidation (CTO) of methane was investigated. Pt 2+ were the only ions which did not fully coprecipitate in the HT precursors. The nature of the noble metal ions determined also the amount that did not segregate by calcination at 900~ and remained inside spinel-type phases. In the CPO reaction, the syngas synthesis decreased according to the order Rh > Ru =-_Ir >> Pt > Pd. The best catalytic performances were observed for a 1% Rh content (as atomic ratio) and Rh contents above 1% did not increase the activity, unlike that observed for Ru based catalysts. In the CTO of methane lower lightoff temperatures were observed for Pd, Rh and Ir based catalysts, but good performances at high methane conversion were found only for the Pd based catalyst, while the Rh and, mainly, Ir based catalysts deactivated with time-on-stream, due to oxidation or sintering phenomena. 1. INTRODUCTION Noble metal based materials have found wide application as hydrogenation, total oxidation and, more recently, partial oxidation heterogeneous catalysts [1,2]. Usually, they are prepared using impregnation methods [3], trying to achieve maximum utilization of the active phase in the reaction, due to the high cost of the noble metals. Although these methods ensure the maximum exploitation of the noble metal, it being spread on the catalyst surface, the homogeneity of the system and the reproducibility of the preparation method may be poor as well as the stability of the noble metal in severe reaction conditions [4]. Recently many oxidation reactions have been carried out using these catalysts in severe reaction conditions, giving rise to thermal and/or mechanical stresses. Examples of these reactions are: 1) The catalytic total oxidation (CTO) reaction, carried out at reaction temperatures below 1300~ to reduce the NOx formation and emission [5]. CH4 + 2 02 ~ CO2 + 2 H20

(1)

2) The catalytic partial oxidation (CPO) reaction, carried out at short residence times and high pressure to improve the productivity and reduce the reactor dimensions [6,7]. CH4 +

89 02 =~ CO + 2 H2

(2)

32 The exothermicity of the partial oxidation reaction and that of the total oxidation side-reaction can increase the temperature above 1000~ thus approaching the limits of thermal stability and mechanical strength of the catalysts. The aim of the present work was to develop a general method to prepare catalysts, containing noble metals homogeneously distributed and stabilized inside an inert matrix. Hydrotalcite-type (HT) anionic clays, with formula [M2+I.•215215 rnH20 (where: A is generally carbonate) fulfill these requirements, showing a homogeneous distribution of the cations inside the brucite-type sheets of the layer structure of the precursors and giving rise by calcination and reduction to well dispersed, stable metal particles [8,9]. It is worth noting that active and stable Ni-based catalysts prepared from HT precursors are already widely employed in steam-reforming processes [10]. The thermal evolution of these precursors was investigated to shed light on the location of the noble metal ions in the phases obtained, while their activity and stability was investigated in the catalytic partial oxidation and the total oxidation of methane, focusing attention on the role of the nature and amount of noble metal present.

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

The HT precursors having the general formula M/Mg2+/A13+ (M = e h 3+, Ru 3+, Ir3+, Pd 2+ or Pt 2+ions, compositions as reported in Table 1) were prepared by coprecipitation at pH 10.0. A solution, containing the salts of the elements in the right atomic ratio was added to a solution containing a slight excess of Na2CO3 and the pH was maintained constant by dropwise NaOH additions. The precipitates were maintained in suspension at 60~ for 40 min under stirring, then filtered and washed with distilled water until a Na20 content below 0.02% w/w was obtained. The precipitates were dried overnight at 90~ and calcined at 900~ for 14h. The precursor compositions were determined after dissolution in concentrated HC1 using complexometric methods for aluminum and magnesium [ 11 ], while the noble metal ions were determined spectrophotometrically, after formation of coloured complexes [ 12-15]. XRD powder analyses were carried out using a Philips PW1050/81 diffractometer equipped with a graphite monochromator in the diffracted beam and controlled by a P W 1710 unit ()~ = 0.15418 nm). A 20 range from 10~ to 80 ~ was investigated at a scanning speed of 70 ~ The cell parameters were determined by least squares refinements from the well defined positions of the most intense peaks. The amounts of noble metal oxide segregated were quantified according to Klug and Alexander [ 16], by comparing the I~176 ratio [where I ~ is the intensity of the most intense peak of MxOy (M = Ru4+, Ir3+, Pt2+ or Pd 2+ ions) and I~ is the most intense peak of corundum, determined for a reference mixture M• = 1:1 (w/w) (MxOy purity > 98%, Aldrich Germany)] with the I/Ic ratio for the analyzed samples to which a known amount of corundum had been added (I and Ic as above).The surface areas were determined by N2 adsorption using a Carlo Erba Sorpty model 1700. The catalytic tests were carried out at atmospheric pressure in a quartz microreactor using samples calcined at 900 ~ (particle size 600-800 gm), previously reduced at 750 ~ for 5 h in a 7 L/h flow of an equimolar H2/N2 mixture. The CPO tests were carried out at a contact time of 7.2 ms, using 0.025 g of catalyst, an oven temperature of 500 or 750 ~ and two different gas mixtures: CH4/O2/He = 2:1:4 and 2:1:20 (v/v). The CTO tests were carried out at a contact time of 90 ms, using 0.5 g of catalyst, operating in the 200-750 ~ range, with a programmed

33 Table 1 Composition of the investigated catalysts, their surface area and amount of segregated phases containing noble metal ions as a function of the thermal treatment. Temp.

(~

M/Mg/AI b M

c

Rh5

Rhl

Rh0.04

Ir5

Ru5

Rul

Pd5

Pt4

5/71/24 1/71/28 0.04/71/29 5/71/24 5/71/24 1/71/28 5/66/29 4/67/29 10.9

2.3

0.1

18.0

10:7

2.3

11.0

15.0

Surf. area d 90

95

78

88

51

84

87

74

150

900

91

97

98

104

86

90

46

55

90

HT

HT

HT

HT

HT

HT

HT

HT

c o n t e n t

Phases id. e

--

900 Msegregatedf 900

MgOSpMgOSp 0.0

MgOSp M g O S p M g O S p M g O S p M g O S p M g O S p IrO2 RuO2 RuO2 PdO Pd ~ Pt ~

0.0 ,,

0.0 ,

,

7.1+0.8 9.9+1.0 1.2+0.2 ,,

11.0+2

n.d.

,,

a) TemperatUre of drying or calcination, b) Composition (atomic ratio %). c) Noble metal ion content (wt%). d) Surface area (m2/g). e) Phases identified by XRD analysis" HT = hydrotalcite-type phase; MgO = MgO-type phase; Sp = spinel-type phase, f) Amount of noble metal ions segregated (wt%). temperature of 3~ and isothermal steps each 50~ with a CH4/air (2:98 v/v) gas mixture. The temperature inside the catalytic bed was measured by a chromel-alumel thermocouple, inserted in a quartz wire put in the middle of the catalytic bed. The reaction products were analyzed on-line, after water condensation, using two gascromatographs equipped with HWD and Carbosieve SII columns, with He as the carrier gas for the CH4, 02, CO and CO2 analysis and N2 for the analysis of H2. 3. RESULTS AND DISCUSSION 3.1 Chemical-physical characterization. The XRD powder patterns of the samples dried at 90 ~ showed in all cases the presence only of a well crystallized HT phase (Table 1), in agreement with the amounts and nature of the ions present. In fact, Rh 3+, Ru 3+ and Ir3+ ions can partially substitute for A13+ ions, since in octahedral coordination the values of their ionic radii [17] are in the range required for the synthesis of HT phases [8,9]. On the other hand, Pd 2+ and Pt 2+ ions may substitute for Mg 2+ ions, although both give preferentially square-planar coordinated compounds [18] and the Pd 2+ radius (0.086 nm) is at the limit of the suitable values to prepare HT compounds. However, it must be pointed out that for the Ru5 sample the baseline in the XRD powder pattern was not perfectly straight, suggesting the presence of small amounts of an amorphous side phase [ 19]. The compositions of the dried samples determined by quantitative analysis (Table 1) were always the same as those calculated on the basis of the composition of the solutions used for

34 the preparation, with the exception of the Ptcontaining sample, for which only 80% of the Pt> [] [] ions coprecipitated, while 20% was lost during the n 9 ,,,hi * u P-t4 washing step. This result can be explained considering ' ~ i'll?, 9 .~" o . Fas that the stronger preference of Pt2+ ions for a squareplanar coordination did not I 9 g 9 v favour their insertion in the octahedral sites of the brucite-type sheets, unlike that observed for the Pd 2+ ions, which fully o,__I~2, ~. ,'\ _ A .A RU 5 coprecipitated in the HT phase notwithstanding their larger ionic radius. The / ~ d, RhS surface area values (Table 1) ~-~ 20 4o 6o 24 8o do not show any systematic effect related to the nature Figure 1. XRD powder patterns of the samples richest in noble and amount of noble metal metal ions calcined at 900~ (-~) MgO-type phase; (11) spinelions inside the HT phases; type phase; (V) RuO2; (A) IrO2; (Q) PdO; (O) Pd~ (GI) Pt ~ nevertheless, the low value of the Ir5 sample and the very high value detected for the Pt4 sample should be noted. Calcination at 900~ gave rise in all samples to a structural rearrangement, with formation of MgO and spinel type phases (Fig. 1). Furthermore, with the exception of the Rhcontaining samples, note should be taken of the partial segregation of the noble metal ions as oxide or metal phases, which show generally very high crystallinity (Table 1). In the Pd5 catalyst the amounts of PdO and metallic palladium.(Pd~ the latter obtained from PdO by decomposition at high temperature [20], were roughly 7 and 4 wt%, respectively, approaching the total Pd-content. On the other hand, the presence in the Pt4 sample of only metallic platinum (Pt ~ can be explained considering that the PtO ~" Pt decomposition occurs at 573 ~ ca. [21]. All attempts to calculate the amount of the Pt ~ phase failed, because of difficulties in preparing metallic particles with suitable crystal size and without any preferential orientation, due to the high malleability of the Pt ~ particles during preparation of the mixture with the corundum. More detailed information on the location of the non-segregated noble metal ions may be obtained by the behavior of the lattice parameters of the MgO and spinel type phases. The value of the call parameter of the MgO type phase remained almost constant (in the range of experimental error), regardless of the nature and amount of noble metal ions, with values very similar to that determined for the MgO in a reference Mg/A1 (71:29, at. ratio %) sample. On the contrary, in the Rh-containing samples the cell parameter a of the spinel type phase was greater than that of the reference Mg/A1 spinel and increased linearly with increasing Rh

a.u,

~

t

I

~

~

~

n

I-~

~

~

j

I

~

~

~

~

I

~

j

~

~

I

~

~

~

~

I

u

~

j

35 82

content (Fig. 2), indicating that the Rh 3+ ions were preferentially located in the a(nm) spinel type phase, randomly distributed with the A13+ ions. Also for the Ir and 81,5 Ru-containing samples, the cell parameters of the spinel type phases were greater, confirming that the 81 residual ~.mounts of Ir3+ and Ru 3+ ions |,~. were present inside these phases. However, it must be noted that the values did not appreciably change with --I. . . . . . I. . . . I-I . . . . . . I 80,5 increasing Ru-content (Fig. 2), in 1 2 3 4 5 agreement with-the similar amounts of Noble metal content (atomic ratio %) non-segregated Ru 3+ ions (1-2 wt% Figure 2. Behavior of the lattice parameter of the ca.) in Ru5 and Rul catalysts. The cell spinel-type phase as a function of the noble metal parameter of the spinel type phase of ion content for the Rh- and Ru-containing samples the Pd5 sample showed a value calcined at 900~ analogous to that of the similar phase obtained for the reference Mg/A1 sample, confirming the almost complete segregation of the Pd 2+ ions. On the other hand, for the Pt4 sample the cell parameter of the spinel type phase showed only a slight increase; however, the presence of Pt 2+ ions in this phase cannot be clearly established considering the small variation in the value and the impossibility of determining the amount of segregated Pt ~ already discussed. On the basis of the above results, the different tendencies of the noble metal ions to segregate as side metal or oxide phases can be explained considering the stability of the oxidation state used in the preparation and the preferential structure of each of these ions [18,22]. For the Rh 3+, Ir3+ and Ru 3+ ions, it can be correlated to the stability of the M203 phase, considering that this phase is the most common and stable oxide .for rhodium ions. Iridium ions form either Ir203 and, preferentially, IrQ, while only RuO2 is reported in the literature. 9 On the other hand, the high tendency of Pt 2+ and Pd 2+ ions to segregate as side phases has to be mainly attributed to the preferential coordination of these cations. Finally, it must be pointed out that notwithstanding the severe calcination conditions the surface area values were always high, evidencing the good thermal stability of the samples obtained from HT precursors (Table 1). .-- Rh --u--Ru

3.2 Activity of the catalysts with the highest contents of noble metal in the partial oxidation of methane. Referring to CPO of methane, in recent years, attention has been focused mainly on the reaction carried out at very short contact times (~ - 10.2 - 10.4 s) [2,23-26], which not only allows ~high yields in CO and H2 to be obtained, but leads to the introduction of small reactors and advantageous process conditions. Thus, to compare the catalytic activity of the samples with the highest contents of noble metals the tests were carried out at a contact time of 7.2 ms. Furthermore, it is worth noting that although the CPO reaction is only slightly exothermic, the side total oxidation to CO2 and H20 is strongly exothermic and may increase the temperature of9 the reaction zone significantly [6,27]. As previously reported [26], when operating at high

36

Figure 3. Comparison of the activity in the catalytic partial oxidation of methane of the catalysts with the highest contents of noble metal ions. (A) oven temperature = 500 ~ contact time - 7.2 ms, reaction gas mixture: CH4/Oa/He = 2:1:20 (v/v); (B) oven temperature = 750 ~ contact time = 7.2 ms, reaction gas mixture CH4/Q/He = 2:1:4 (v/v). temperature and feeding a concentrated gas mixture, the increase in temperature due to the exothermicity 9 of the reaction lessens the differences in activity related to the catalyst composition, since all the catalytic reactions approach thermodynamic equilibrium. Thus, to evidence the specific behavior of each noble metal, the catalytic tests were carried out in two typical conditions: i) low oven temperature and feeding a diluted mixture, i.e. at controlled reaction temperatures, obtained by decreasing the heat production and improving heat transport and dispersion; ii) high oven temperature and feeding a more concentrated mixture, i.e. in conditions approaching those of industrial interest. All the catalysts were found to be active in all the investigated conditions, showing total oxygen conversion and temperature inside the catalytic bed higherthan that of the oven, due to the presence of the exothermic reactions, mainly total oxidation reactions. It must be pointed out that according to Dissanayake et al. [28] the temperature on the catalyst surface may be nmch higher than that measured by the thermocouple. In the tests carried out maintaining the oven at 500 ~ and feeding the diluted mixture [CH4/O:/He = 2:1:4 (v/v)], the measured temperatures were in the range 590-630 ~ In these conditions conversion and selectivity moved from the equilibrium values and significant differences among these samples can be observed (Fig. 3a). In fact, the Rh5 sample showed the best catalytic performances, with conversion of methane above 55% and selectivity in CO and H2 higher than 65 and 95%, respectively. Although lower than the previous one, the Ru5 and Ir5 samples also showed significant methane conversions (> 40%), while low values were exhibited by Pt4 and Pd5 samples, with the Pd5 sample showing the highest reaction temperature measured. Furthermore, the ratio among CO and H2 selectivities changed as a function of the noble metal ions present. The Rh-, Ru- and Ir-containing samples showed a selectivity in H2 higher than that in CO, while the opposite occurred for the Pt and Pd based samples. The high values of methane conversion for the Rh, Ru and Ir based catalysts

37

evidence a high activity in the CPO reactions, while the higher selectivity in H2 can be attributed to a low tendency to form OH species, precursors of water on the catalyst surface [23]. On the contrary, Pt and Pd based catalysts are more active in the total oxidation reaction and more easily form surface OH's. The catalytic tests carried out with an oven temperature of 750 ~ and feeding the concentrated mixture [ C H 4 / O z / H e = 2:1:4 (v/v)], showed again an increase in the measured temperatures, which ranged from 860 to 950 ~ The conversion of methane as well as the selectivity in syngas (CO + H2) increased significantly in comparison to the values observed in the previous tests, Figure 4. Activity in the catalytic partial in agreement with the trend of the oxidation of methane as a function of the Ruthermodynamic equilibrium, that at high or Rh-content. Oven temperature = 750 ~ temperature favours syngas production [6]. contact time = 7.2 ms; reaction gas mixture: Even if at high temperature the conversion CH4/O2/He = 2:1:4 (v/v). and selectivities approached the equilibrium values, the increase in syngas production was again more evident for the Rh, Ru and Ir based catalysts (Fig. 3b). In fact, while the conversion of methane for the Pt and Pdcontaining catalysts never exceeded 70%, the values observed with the Rh, Ru and Ir based catalysts approached the thermodynamic equilibrium values, confirming that platinum and palladium are not suitable for the CPO of methane. Rh, Ru and Ir based catalysts not only better promoted the direct CPO reaction in comparison to the total oxidation side-reaction [23], but were also more active in the secondary reactions, such as steam- and CQ-reforming, which require high temperatures and increase syngas production [25]. Furthermore, although -the differences between the catalysts were lessened due to the effect of the temperature, the Rh5 catalyst showed the best performance, probably attributable to a higher dispersion in the inert Mg/A1 matrix due to the absence of segregation phenomena. Finally, note should be taken of the very low stability in the reaction conditions of the Pd5 catalyst, which deactivated in 20 h ca. on-streana, due to carbon deposition on the catalyst surface. 3.3 Role of the Rh and Ru content in the partial oxidation of methane On the basis of the above results, attention was focused on. the Rh and Ru based catalysts (also considering the high cost and limited amounts of k-containing salts available on the market), studying the role of the content of noble metal to evaluate the possibility of reducing its amount and, consequently, the final cost of the catalysts. With this aim, the tests were carried out using reaction conditions approaching those of industrial interest [i.e. oven temperature = 750~ and CH4/O2/He = 2:1:4 (v/v) reaction gas mixture]. All the catalysts investigated were active in the partial oxidation of methane, showing total oxygen conversion and high syngas selectivity (Fig. 4). The good catalytic performances can be9 attributed both to the specific activity of the catalysts and to the high reaction temperatures

38 measured (840-920 ~ with the conversion and selectivity values displaced towards those found at thermodynamic equilibrium. Notwithstanding the high reaction temperatures significant differences in the catalytic performances were observed as a function of the nature and amount of noble metal present. In particular, for the Rh based catalysts it must be noted that a content of 0.04% (as atomic ratio) was not sufficient to achieve catalytic performances of industrial interest, probably because of a low amount of noble metal on the catalyst surface. A maximum of activity was observed for a Rh content of about 1% (as atomic ratio), while further increases did Figure 5. Comparison of the activity in the not produce any significant enhancement. catalytic total oxidation of methane of the On the other hand, the Ru based catalysts catalysts with the highest contents of noble showed poorer performances than those of metal ions. For each catalyst the temperatures the Rh based catalysts, which, however, at which 10% (~1) and 90% ( / ) of methane increased with increasing Ru content. This conversion was reached are reported. Contact last point may be explained by also time = 90 ms; reaction gas mixture: CH4/air = considering the contribution to the catalytic 2:98 (v/v). activity by the segregated RuO2, in agreement with the results of previous catalytic tests carried out using a mixture of RuO2 (10.5 wt%, as metal) and a Mg/A1 (71:29 atomic ratio percent) HT precursor calcined at 900 ~ that evidenced a not negligible activity in the CPO of methane, even if significantly lower than that of the Ru5 catalyst. The above results confirm the high activity and selectivity.in the CPO of methane of the Rh based catalysts, evidencing the possibility of reducing the noble metal content due to the preparation of highly dispersed and stable metal particles related to its higher solubility in the spinel-type phase. It must be pointed out that the performances observed using the catalysts Rhl and Rh5 are comparable with the best results reported in the literature [6,27]. Furthermore, during the catalytic tests no deactivation phenomena were observed since the stabilization inside an inert matrix prevented sintering and/or loss of the metal even in severe reaction conditions. 3.4 Activity of the catalysts with the highest contents of noble metal in the total oxidation of methane The activity of the catalysts in the CTO of methane was evaluated by heating the oven with a programmed temperature increase and determining the temperatures at which 10% and 90% of conversion were achieved, which may be assumed as an index of the reaction lightoff and of the almost complete conversion of methane, respectively [28,29] (Fig. 5). Pd5, Rh5 and Ir5 catalysts showed the lower values of lightoff tempe.rature, which can be explained by considering either the specific activity of palladium in the total oxidation reaction [29-30]

39 or a higher dispersion of the metal particles in the Rh and Ir based catalysts, due to the lower amounts of segregated oxide or metal (Table 1). On the other hand, even though it is known that the activation of methane on Pt based catalysts occurs at a higher temperature than on Pd based catalysts [30], the high lightoff temperature of the Pt4 catalyst must be noted, confirming that the preparation from HT precursors may not be useful for Pt based catalysts. The temperatures at which 90% of conversion occurred followed roughly the same order discussed above, with again high values for the Ru5 and Pt4 catalysts. The only exception was the Ir5 sample, for which almost complete conversion was reached at 610 ~ This difference between the Ir5 aM Rh5 catalysts can be explained considering the significantly lower amount of non-segregated ions in the latter sample (Table 1), and after reduction the non-segregated noble metal in Ir5 did not seem sufficient to obtain a high conversion of methane at low temperature. On the other hand, the Ir5 and Rh5 samples deactivated with time-on-stream, due to sintering and/or oxidation of the dispersed noble metal particles, although with the latter catalyst deactivation occurred at higher conversion values. Finally, it must be noted that the .performances of the Pd5 catalyst were comparable with those of the best Pd-supported catalysts reported in the literature [31] and did not change with time-on-stream even at high temperature, evidencing the stability of this catalyst in an oxidizing atmosphere. 4. Conclusions

HT precursors containing Rh 3+, Ru 3+, Ir3+, Pd 2+ or Pt 2+ ions were successfully prepared by coprecipitation at constant pH, although in the last case a significant loss of Pt 2+ ions was observed, attributable to their high preference for square-planar coordination which hinders their insertion in the brucite-type sheets of the HT structure. The HT precursors gave rise by calcination at 900 ~ mainly to MgO and spinel type phases. The Rh 3+ ions were totally present inside the spinel type phase, while the other ions partially or totally segregated as oxide or metal side phases, evidencing the following order of stability inside the spinel type phase: Rh > Ir > Ru >> Pd = Pt. Furthermore, notwithstanding the severe calcination conditions the surface area values were always high, evidencing good thermal stability of the samples obtained from HT precursors. In the CPO reaction of methane, the following scale in syngas formation was observed: Rh > Ru ~ Ir > > Pt >Pd, this last catalyst showing the highest reaction temperature. Furthermore, Rh, Ir and Ru based catalysts showed higher selectivities in H2 than Pt and Pd based catalysts, attributable to a low tendency to form OH species, precursors of water on the catalyst surface. The role of the noble metal content was evaluated for Rh and Ru based catalysts, evidencing maximum activity for a Rh content of 1% (as atomic ratio), while further increases did not produce any significant enhancement. Ru based catalysts always showed lower syngas productivities, which unlike before increased as the Ru content increased, due to the higher tendency to segregate and the additional contribution'to the activity by the less selective RuO2. Finally, in the CTO reaction of methane, the lower values of lightoff temperature observed for Pd, Rh and Ir based catalysts can be explained considering either the specific activity of palladium or a higher dispersion of the iridium and rhodium particles, due to the lower amounts of segregated oxide or metal. On the other hand, the results obtained with the Pt based catalyst confirmed that the preparation from HT precursors may not be suitable for this metal. The temperature at which almost complete conversion was achieved, confirmed the

40 very good performance of the Pd based catalyst comparable with those of the best Pd supported catalysts, while iridium (mainly) and rhodium deactivated with time-on-stream due to sintering and/or oxidation of the dispersed metal particles. Thus, the preparation of noble metal catalysts from HT precursors although simple, with good reproducibility cannot be of wide application, since segregation phenomena have a negative effect on the dispersion of the noble metal particles in the inert matrix and, consequently, their stability in severe reaction conditions.

6. References 1. "Ullmann's Encyclopedia of Industrial Chemistry" Vol. A5 (B. Elvers, Ed.), VCH, Wheinheim, Basel (1985) p 337. 2. D.A. Hickman and L.D. Schmidt, J. Catal, 138 (1992) 267. 3. S. Y. Lee and R. Aris, Catal. Rev.- Sc. Eng., 27 (1985) 207. 4. M. Komiyama, Catal. Rev.- Sc. Eng., 27 (1985) 341. 5. H. Arai and H. Fukuzawa (Ed.s), Catal. Today, 26 (I995). 6. G.A. Foulds and J.A. Lapszewicz, in Catalysis (J.J. Spivey and S.K. Karval, Ed.s), Vol. 11, The Royal Society of Chemistry, London (1994) p. 413. 7. D.A. Hickman, E.A. Haupfear and Schmidt, Catal. Lett., 17 (1993) 223. 8. F. Cavani, F. Trifir6 and A. Vaccari, Catal Today, 11 (1991) 173. 9. F. Trifir6 and A. Vaccari, ill Comprehensive Supramolecular Chemistry (J,L. Atwood, J.E.D. Davies, D.D. Mc Nicol and F. V6gtle Ed.s), Vol. 7, Pergamon, Oxford (1996) ch.8. 10. 9 J.R. Rostrup-Nielsen, Steam Reforming Catalysts, Teknisg Forlag, Copenaghen (1975). 11. A. Vogel, Textbook of Quantitative Inorganic Analysis, Longman, London (1978). 12. G. Charlot, Chimie Analytique Quantitative, Masson, Paris (1974) p 490. 13. E.C. Cerceo and J.J. Markham, Anal. Chem., 38 (1966) 1426. 14. J.H. Ayes and J.H. Alsop III, Anal. Chem., 31 (1959) 1135. 15. I. Maziekien, L. Erlnanis and T.J. Walsh, Anal. Chem. 32 (1960) 645. 16. D.L. Bish and J.E. Post (Ed.s), Modern Powder Diffraction, Mineralogical Society of America, Washington (1989) p. 107. 17. R. Shannon and C.I. Previtt, Acta Cristallog., B26 (1970) 1076. 18. N.N. Greenwood and A. Earnshow, Chemistry of Elements, Pergamon, Oxford (1989). 19.Basile, L. Basini, G. Fornasari, M. Gazzano, F. Trifir6 and A. Vaccari, J. Chem. Soc., Chem. Comm., (1996) 2435. 20. R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Florida, 1978. 21. J.E. Huheey, Inorganic Chemistry, Harpen Collins, London, 1972. 22. R.J. Farrauto, M.C. Hobson, T. Kennelly and E.M. Waterman, Appl. Catal. A81 (1996) 227. 23. D.A. Hickman and L.D. Schmidt, Science, 259 (1993) 343. 24. V.R. Choudary, A.S. Mamman and S.D. Sansare, Angew. Chem. hat. Ed. (Engl.), 31 (1992) 1189. 25. V.R. Choudary, A.M. Rajput and B. Prabhakar, J. Catal., 139 (1993) 326. 26. F. Basile, L. Basini, M. D' Amore, G. Fornasari, A. Guarinoni, D. Matteuzzi, G. Del Piero, F. Trifir6 and A. Vaccari, J. Catal., in press. 27. S.C. Tsang, J.B. Claridge and M.L.H. Green, Catal. Today, 23 (1995) 3. 28. D.L. Trimm, Appl. Catal., 7 (1983) 249. 29. M. Machida, A. Sato, H. Inoue, K. Eguchi and H. Arai, Catal. Today, 26 (1995) 239. 30. R.F. Hicks, H. Qi, M.L. Young and R.J. Lee, J. Catal., 122 (1990) 280. 31. C.F. Cullis and B.M. Willat, J. Catal, 83 (1983) 239.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

41

F r o m c o l l o i d a l particles to s u p p o r t e d catalysts" a c o m p r e h e n s i v e study of palladium oxide hydrosols deposited on alumina.

B. Didillon, E. Merlen, T. Pag6s, D. Uzio IFP, 1&4 avenue de Bois-Pr6au, 92852 Rueil Malmaison cedex, France

1.

INTRODUCTION

Supported metallic catalysts are usually prepared from aqueous or inorganic solutions containing active phase precursor, which are brought into contact with a carrier. After impregnation and drying, activation procedures (calcination and reduction) are applied in order to confer to the catalyst its final characteristics through transformation of precursor adsorbed metal complexes to metallic particles. A lot of work, in the field of catalysts preparation, have been devoted to these activation procedures. However, these classical methods do not allow good control of the active phase properties (particle size, particle size distribution, bimetallicity...) due to the difficulty of controlling the different steps (germination and growth) governing the transformation from the precursor to the particles during the activation procedure. This difficulty can be circumvented through the preparation of particles of controlled size in solution before their deposition on the support if this last step doesn't affect the particle size. This approach would enable a better control of the catalyst particle characteristics if we are capable to master the preparation of particles in solution as described in the field of colloid chemistry. This approach of catalyst preparation via colloidal suspension and deposition of particles on a porous support has already been reported in literature [ 1-2]. Theses studies have shown the high potential of this kind of preparation method for controlling the parameters related to the particles [3]. However, most of these works deal with organic solutions and colloidal particles are under reduced metallic form. In spite of the obvious advantages of using aqueous solutions instead of organic ones (safety, industrial scaling up...), the synthesis of supported metal catalysts from oxide colloidal suspension has not up to now received wide attention in the field of catalyst preparation.

42 In this paper, we will try to demonstrate the high potential of this type of approach in the preparation of supported palladium catalysts. Finally, an example of remarkable catalytic performances in the special case of phenylacetylene hydrogenation will be given.

2.

PREPARATION

AND CHARACTERIZATION

OF HYDROSOLS

In order to produce palladium oxide colloidal particles, two experimental preparation techniques have been employed: - the neutralisation of an acidic or a basic solution by respectively an alkaline or acidic solution; - the thermohydrolysis of the palladium precursor solution.

2.1 S t u d y o f the starting solutions

The initial palladium nitrate solution has been first characterised to determine the type of complexes present. This solution has a palladium concentration of 5 g/1 in nitric acid (pH=0.8). Firstly, the absence of oxide particles in the starting solution was determined by Xray scattering. The scattered intensity I(q) as a function of scattering angle 20 is rather low and constant. So this solution behaves like a liquid ("real" solution), no solid particles being present. The different molecular species in the solution were identified by UV-visible spectroscopy. Two absorption peaks are observed at L=285 nm and L=390 nm. Because addition of NaNO3 enhanced the second one, it was attributed to free nitrate ions corresponding to the electronic transition from the c to the ~* state in the NO3- ions. The other absorption band at ~=285 nm is assigned to a d-d transition in the aquo complex Pd(H20)4 a+ [4-6]. UV-visible results show the non-complexant behaviour of nitrate ions towards palladium metallic centres, they remain free in the solution. The palladium containing species in the starting solution is then the tetra-aquo complex Pd(H20)42+. Absorbance 1

0.9

2i5

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 250

300

350

400

450

(nm)

Figure 1. UV-Visible spectrum of the initial Pd(NO3)2.

500

550

600

43 The water molecule in the Pd cationic complex has an electric charge of +0.37, calculated as described in ref. [7.1]. This positive charge indicates that the hydroxo ligand has an electrophilic character. As a consequence, condensation of aquo cationic complexes cannot occur spontaneously. The charge calculation is thus in agreement with UV-visible results showing monomolecular stable Pd species in the acidic solution.

2.2. Hydrosols

obtained

by neutralisation

The formation of palladium oxide colloidal particles from acidic palladium nitrate solution may be achieved by adding alkaline solution (NaOH, 0.1 N). The chemistry involved in such a process can be described as follows [7.2]: - nucleophilic attack of hydroxyl ions leading to the substitution of hydroxo ligands and formation of a complex with no electric charge: Pd(OH)2(H20)2; - inorganic polymerisation or condensation by olation or oxolation reactions between these complexes and formation of nuclei; - growth of these nuclei with olation or oxolation reactions. In the case of the complex P d ( O H ) 2 ( H 2 0 ) 2 , for example, the partial charge model predicts condensation because of the nucleophilic character of the hydroxo ligand towards the metallic centre (Table 1). Figure 2 summarises the different mechanism steps leading to the formation of colloidal particles. Pd(HzO)42++ 2 OH- ~ Pd(H20)2(OH)2 + 21-120

OH

OH

\ /

Pd

\o.

+ ~-)

Pd

Pd (~-)

.20/ \

OH

(_2HzO)

OH

\ /

Pd

OH

.20

OHr

/ "~o

+

oH

/

Pd

\

%0

(-2n2o)

~O,n.~O~ aV-

~
~--

OH

/OH

\

__

\ /

Pd

o. /

OH

\ /

Pd

%./\o.

%O

Pd

/

Figure 2. Reaction mechanisms involved during the synthesis of PdO colloidal particles from palladium hydroxo complexes. The Figure 3 presents the titration curve of a palladium nitrate solution [Pd]= 5g/1 by addition of soda [NaOH]=0.1 N. One can clearly distinguish three regions: - from the starting point until addition of about 14 ml of soda: the solution is clear;

44 - between 14 and 24 ml added, a progressive cloudy solution appears corresponding to the formation of colloidal particles (colloidal suspension); - above 24 ml which corresponds to the flocculation point, the solution is heterogeneous medium with segregation of the liquid and solid phases. 12

10I 8L

Clear solution

f

Flocculation

Formation of colloidal particles

pH

0

[

I

I

0

5

10

~

t

I

15

20

'l

I

25

30

V NaOH (ml) Figure 3. Neutralisation curve of a palladium nitrate solution [Pd]- 5g/1 by addition of soda [NaOH]=0.1 N. The preparation of colloidal particles will hence be performed by addition of a well defined amount of soda corresponding to the second intermediate region. The neutralisation process corresponds to simultaneous.titrations of free acidity and palladium complex according to the following reactions: HNO3 + NaOH ~ NaNO3 + H20 (1) Pd(H20)42+ + 2NO3- + 2 NaOH

---)

2 NaNO3 + Pd(OH)2 + 4 H20 (2)

X-ray diffraction diagrams of dried suspensions after neutralisation correspond to palladium oxide identified on the basis of JCPDS data base (Figure 4). Intensity u.a.

o

I ,o

I

I

I

;oj

I

I

I

I

8o

Figure 4. X-ray diffraction pattem of suspension dried after neutralisation by soda.

45 Particle size distributions in two hydrosols determined by a statistical analysis of transmission electron microscopy (Jeol 120 CX) micro graphs are presented Figure 5. These hydrosols have been prepared at different neutralisation levels (pH=2 and 2.8), and these suspensions were diluted at constant pH before deposition on electron microscopy grids. Both particle size distributions are centred on 18-19/k and are extremely sharp. 5o- % number

50 % number

45

pH = 2

45

pH = 2.8

40 35 30 25 i 20 15

D pa =18 A

40 35 30 25 20 15

D Pd =19/k

C~=2.7

10

cy=2.8

10 A

5 o

5 o

3

12 21 30 39 48 57 66 75 84

]

3

12 21

30

39

48

57

66 75

84

PdO Particle size (,~)

PdO Particle size (,~)

Figure 5. PdO particle size distributions from Transmission Electron Microscopy. The reverse operation - i.e. the addition of palladium nitrate solution into soda - can also be used to prepare colloidal suspensions. The Figure 6 presents this type of titration curve ([Pd]=5g/l; [NaOH]=0.2N). 14 $

12

#

r

:

10

,IL

Formation of colloidal particles

8

pH

"~ ,._~

Flocculation

6

Redissolutionand formation of colloidal particles

4 Iu

I 0

I, 5

I 10

I 15

I 20

V Pd(NO3)2 (ml) Figure 6. Neutralisation curve of soda [NaOH]=0.1 N by addition of a palladium nitrate solution [Pd]= 5g/1. Particle size distributions obtained for hydrosols generated in the two colloidal regions at pH=12 and pH=l.8 are again very narrow and both centred on 17-18 A (Figure 7).

46 50 7 % number 45 ~40 ~ 35 30 25 20 15 10 5 0

pH = 12

45 40 35

D vd = 17 A -

3

% number

50

pH = 1.8 Dvd= 18 fit

30 25 20 15 10 5 0

12 21 30 39 48 57 66 75

84

t~= 2.8

3

12 21

PdO particle size (/~)

30

39 48

57 66

75

84

PdO particle size (A)

Figure 7. PdO particle size distributions from Transmission Electron Microscopy. For all these preparations, no variation in average particle size has been observed. In order to explain this result, one must consider the chemical reactions occurring during the preparations. Indeed, although several monomolecular species exist in the pH range (Figure 8), the determination of the partial charge of the hydroxo ligand in the different complexes shows that this ligand is the only one which exhibits the nucleophilic character necessary for the condensation mechanism (see Table 1). In our conditions, anionic species PdH20(OH)3 and Pd(OH)42 are not present because the pH is always below 12. So the two types of neutralisation (addition of soda to nitric acid and the reverse) lead to the same complex - i.e. Pd(H20)2(OH)2 - the only one able to generate nuclei by polycondensation. In the first case (neutralisation of palladium nitrate by soda), different hydrolysed intermediate species can be formed from Pd(H20)42+ during the addition of NaOH (Figure 8) (ex: Pd(HaO)ffOH)+). In the second one, the addition of Pd(H=O)42+ in a NaOH solution produces directly the neutral complex Pd(HaO)2(OH)2. As a consequence, the cloudy solution appears in the initial stage of the experiment with the first drops of soda. Table 1. Partial charge of the hydroxo ligand and metallic centre. . . . . . . . . . Partial charge of hydroxo ligand Partial charge of Pd metal centre

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

~

Pd_(H20)3(0H)_+.... Pd_(H20)2(on)2_ .... 0 -0.19 +0.43 +0.35 . . . . . . . . . . . . . . . . . . . . . .

_.. . . . .

47 % 100 -.N~pd2+ f 90 8O 7o 6O 5o 40

T Pd(OH)2

.

(OH) Pd(OH)3

30

0

'

0

1 2

~

3

4 5

-

i

i

I

I

-

I

6

7 8 9 10 11 12 13 14 pH Figure 8. Stable existing species as a function of pH values from [8]. Moreover, the formation of Pd(H20)2(OH)2 by substitution of aquo groups by hydroxo is very fast (k=-10 x~ mol'ls "1) and as a consequence the concentration of this complex is very high for all the preparations. This fast reaction is responsible of high sursaturation values of the solutions. A high value of sursaturation favours the nucleation rate with respect to the growth rate from a kinetic point of view. This "explosive" nucleation step will govern the final PdO particle size whatever the other operating parameters (pH, ionic force, temperature...). One way to modify the final particle size is to generate a different precursor. For example, Na2PdCI4 aqueous solution contains PdC13(H20) complex as seen by l.W-visible measurements. Neutralisation with soda (Figure 9) leads to the formation of two types of neutral complexes: Pd(H20)2C1OH and Pd(H20)2(OH)2. According to the partial charge model, these two complexes are able to polymerise (~5OH=-0.19 and 8Pd=+0.35 in the Pd(H20)2C1OH complex). The final result indicates that the presence of a chloride ligand modifies the reactivity of the entities and the final palladium oxide particle size is lowered compared to the results obtained with palladium nitrate solution (Figure 10). This effect, moderated by the presence of Pd(H20)2(OH)2, is clearly related to the properties of the Pd(H20)2C1OH complex. Indeed, the reverse neutralisation (addition of Na2PdCh or PdC13(H20) under hydrolysed form, to soda) gives rise only to Pd(H20)2(OH)2 formation due to the instability of PdC13(H20) at high pH values [9]. In these conditions, the consecutive condensation reactions generate palladium oxide particles with the same average sine as in preparations using palladium nitrate precursor.

48 12 10

Flocculation

Formation of colloidal particles

pH

1

I

0

2

..-

I

I

I

I

I

4

6

8

10

12

V NaOH (ml) Figure 9. Neutralisation curve of a palladium chloride solution [Pd]= 5g/1 by addition of soda [NaOH]=0.1 N. 60 7-% number 5O 40

DPd= 15/~ C~= 2.2

30 20 10 0

3

9

15 21 27 33 39 45 51 57 63 69 75 81 PdO particle size (/~)

Figure 10. PdO particle size distributions from transmission electron microscopy.

2.3. Hydrosols

obtained

by thermohydrolysis

Another method can be used to prepare colloidal suspensions of palladium oxide particles: thermohydrolysis. In this case, no addition of soda is needed because heat induces a reaction of water molecules with palladium complexes. In the case of palladium nitrate solution, the chemical reaction can be expressed by the equation (3) [Pd(OH2)4] 1+ + 2 NO3-+ h H20 ~ -[Pd(OH)h(OH2)4_h](2-h~++ 2 NO3- + h H30 + (3) In comparison with neutralisation methods, lower sursaturations are obtained with thermohydrolysis so this procedure is more favourable for particle growth.

49 Figures 11-a and b present the influence of temperature and the initial pH of the solution on the average PdO particle size determined by X-ray diffraction on the solid phase after precipitation. (a)

50 - PdO particle size (A)

120 .PdO particle size (A) 110 loo , , , . 9o 80

45

"r +

--...

40 35

(b) = 100~ T = 80~

70 6o

30 25 20

9 20

I

I

I

I

40

60

80

100

Temperature (~

50 40 30 20

~ ~ . . . . . _ ~ i

1

~

1

1,1

1,2

~ , 1,3

~ 1,4

1,5

pH

Figure 11-a. Evolution of PdO particle size with temperature and pH (a: starting Pd(NO3)2 solution [Pd]=5g/1 and b: [Pd]=lg/1). The PdO particle growth may have two origins: a thermodynamic phenomenon due to the spontaneous evolution to minimise surface energy of the system, or a kinetic control of nucleation and condensation reaction rates. In our case, no modification of the final particle size was observed between lh and 18h heating time at 100~ under reflux. So, the evolution of average particle size is more probably related to a kinetic effect. Indeed, lowering the pH displaces the equilibrated reaction (3) to the left depressing the formation of the neutral precursor Pd(H20):(OH)2 (h=2). Moreover, at low pH values the solubility of the PdO oxide is enhanced, so that the sursaturation of the medium (ratio of the concentration of the solid phase to its solubility) is lowered. The nucleation rate which is greatly dependent on the concentration of the nuclei is then lower: the nuclei are less numerous and may grow by condensation reactions. The effect of the temperature may also be explained in term of kinetic control of the formation of nuclei increasing their solubility and as a consequence lowering the sursaturation of the solution.

3. A P P L I C A T I O N TO THE P R E P A R A T I O N OF SUPPORTED CATALYSTS The colloidal suspensions obtained as described previously by neutralisation or thermohydrolysis procedures have been used to prepare supported Pd catalysts. The support is impregnated with a volume of colloidal suspension corresponding exactly to the porous volume of the sofid (dry impregnation). Various supports have been used, the results reported here have been obtained with fi-A1203 (specific surface area: 130 m2/g, porous volume: 1.24 cm3/g). The palladium loading is about 0.1% weight. After impregnation, the samples are dried overnight at 393 K.

50 Figures 12-a and b show the PdO particle size of catalyst prepared from an acidic hydrosol (neutralisation of palladium nitrate solution by addition of soda, Figure 12-a) or from a basic one (neutralisation of soda by addition of palladium nitrate, Figure 12-b). These samples are respectively called SA 139 and SB 139. One can see that the shape of the histogram is not modified with a very slight increase of the spread of particle sizes when compared to the hydrosol distribution (0"=2-3). The average sizes of PdO particles are unchanged. 50 % number 45 40 35 30 25 20 15 10 5 0,~',~ .... ~', ', ~', [ ', _~ 12 21 30 39 48 .

.

.

.

.

.

.

.

.

.

In

50 -% number 45 40

(a) D pd = 20 A 0"=4.2

(b) D p a = 19/k

35 30 25 20

0"=4

10 I ', [ ', ', ', I I : I I I

57

0

66 75 84

'

3

--

I

!

12 21

ii'

~'

I I I I I I I I ', I I I I ', I I

30 39

48

57

66 75

84

PdO particle size (.&)

PdO particle size (A) Figure 12. PdO particle size distributions for samples prepared from an acidic hydrosol SA 139 (a) and a basic one SB 139 (b). Transmission electron microscopy also indicated an important characteristic of the particles: they are either aggregated or isolated. TEM observations and QELS (Quasi Elastic Light Scattering) measurements showed that PdO oxide particles in the colloidal suspensions are generally aggregated forming globular floccules of 1500 to 2000 A in size (Figure 13). After deposition on the support, we observed that samples prepared from acidic hydrosol are characterised by aggregated particles constituting floccules of 300-1700 A and samples prepared via basic hydrosols contain only isolated particles. These different states of the particles can be controlled via the iso electric point of the solids (PdO particles and support) or adding a dispersing agent in the hydrosol before the impregnation step. Hydrodynamic diameter

3500 3000 2500 2000 1500 1000 500 0 0

I

t

I

I

1

I

Z

2

4

6

8

10

12

14

pH Figure 13. Hydrodynamic diameters of colloidal particles as a function of the hydrosol pH.

51 In the case of the impregnation of the acidic hydrosol (pH=2.8), alumina has a positive charge because of its basic character. Protonation of surface hydroxyl groups is rather slow but significant when compared to the time of preparation. After 200 min, equilibrium is reached at approximately pH=7 (Figure 14-a). The iso electric point (IEP) of the palladium oxide particles has been determined by the method described by Jolivet [7.1] and is equal to 4.5. In these conditions, during the deposition of the acidic PdO sol at pH 2.8, both support and PdO particles are initially positively charged and have a repulsive interaction favouring an isolated state. But the progressive increase of pH due to the important alumina protonation destabilises the PdO particles and modifies the final IEP of the solids. Coagulation occurs leading to the aggregated particles. Impregnating a basic colloidal suspension (pH=12) on alumina does not induce proton liberation, pH is constant (Figure 14-b). The system keeps its initial properties, i.e. negative charges for alumina support and PdO particles. Repulsive interactions are maintained so that the particles deposited on the support are isolated from each other. pH

(a)

pH

10

9 8 7 6 5 4 3 I 2 0

I

[]

10 []

8

9

6

42-

1

0

(b)

12111

I 50

',

I

I.,

I

I

I

1

I

I

100 150 200 250 300 350 400 450 500

0

0

I

I

I.

I

I

I

50

100

150

200

250

300

Time (min) Time (min) Figure 14. Evolution of pH of an acidic suspension (a: pH=2.8) and basic suspension (b: pH= 12) in contact with 5-A1203. The aggregation state of the particles can also be controlled by addition of dispersing agents to the hydrosol. Indeed, a sample shown by TEM observation to contain isolated particles has been prepared by adding sodium-nitrite (NOE/Pd=I molar ratio) to the acidic hydrosol prior to deposition on the support. The oxide particles, stabilised by the formation of Pd-NO2 bonds are in the isolated state in the hydrosol. This bond being stable at the high pH values obtained during deposition on alumina, the isolated state is retained on the final PdO supported catalyst.

4. CATALYTIC EVALUATION The colloidal preparation method enables good control of particle characteristics such as size distribution and aggregation. The advantages of this technique were evaluated in the case of phenylacetylene hydrogenation reaction.

52 The catalytic evaluation was performed in a classical well stirred stainless steel reactor operating in batch mode under the following conditions: total pressure: 10 bar, temperature: 17~ amount of prereduced catalyst: 2g (reduction 1 h at 150~ under hydrogen flow), feed: 14 ml of phenyacetylene in 180 ml of n-heptane. Experimental conditions have been previously determined in order to avoid mass transfer limitations. We did not observe significant modification of the particle size distribution (TEM observations) after the reduction step. In other words the particle size does not vary from the oxide to the metal. This experimental result is in agreement with theoretical modelling which showed that for a given number of palladium atoms, the size of the PdO particle is the same as the Pd metallic particle. For all the catalysts, the hydrogenation mechanism is consistent with an Horiuti-Polanyi mechanism [10]. Both phenylacetylene (PhAc) and styrene (Sty) hydrogenation were zero order with respect to hydrogen and PhAc up to 80 % of conversion in agreement with several studies devoted to acetylenic hydrogenation [ 11]. Rate constants k (mol.s-~.gPd l) have been determined from the slope of the experimental curves representing acetylenic molar concentration versus time and turn over numbers (TON mol.s-~.atom Pd -I) were calculated from the following equation: TON -

kM

D with M the atomic weight of Pd and D the fraction of exposed Pd atoms (dispersion) obtained from CO chemisorption measurements Influence of the particle size distribution

The catalytic results reported in table 2 show a comparison between SB 139 catalyst and a sample (9AC) corresponding to a more conventional preparation procedure using palladium acetylacetonate as a precursor [12]. These two catalysts have nearly the same dispersion (see Table 2), but the colloid based catalyst is 6 times more active for PhAc hydrogenation. The particle size distribution of the two samples are presented Figures 12 b and 15. These histograms show a very narrow distribution of particle size for the SB 139 catalyst and a very broad distribution for the 9AC catalyst. This difference explains the relatively poor activity of the 9AC catalyst which contains small particles (lower than 10/k), having a lower activity for a structure sensitive reaction like acetylenic compound hydrogenation, and large particles (size higher than 30 ,~) which present poor accessibility. Thus, the highest hydrogenation activity is obtained for particle sizes between 10 and 30 ,~. This size distribution is obtained in the case of SB 139 prepared from colloidal method.

53 Table 2. Characteristics and catalytic activity of samples prepared from suspension method (SB 139) and conventional preparation method (AC9).

SB 139 9 AC ,,

,,

"Particle' size (A) (TEN) 19 23

,

'Particle size distribution (TEM) narrow o=4 broad o=8.1 ,

Dispersion % by CO measurements

k PhAC mol.s~.gPd -~

T o N PhAc mol.s~.atom Pd -~

63 66

0.052 0.008

8.8 1.3

,,

50 - % number 40 30

~

D

20 10

!1,

Nun

.....

12

21

Pd

23 *

--

o=8.1 n,l,n,n,n,

30

39

, , , , , , , , , , I I , ,

48

57

66

75

84

PdO particle size (*) Figure 15. Particle size distribution of the 9AC catalyst (prepared from acetylacetonate precursor) obtained by TEM.

Influence of the aggregation state Concerning the aggregation state, the comparison of the catalytic activity of SA 139 and SB 139 are given in Table 3.

Table 3. Characteristics and catalytic activity of samples prepared from colloidal suspensions. Particle TEM size (/k) observation .........(by TEM) ...... SA 139 20 aggregated particles SB 139 19 isolated ............................................................................................................ partic_!es ....

Dispersion % by CO measurements

k PhAc mol.s-l.gPd 1

TON PhAc mol.s-l.atom Pd -~

12

0.033

29.3

63

0.052

8.8

It appears that aggregation of particles lowers the specific activity of the catalyst but TON is significantly higher for the catalyst having particles in an aggregation state (SA139). An infra red study showed that aggregation of the particles occurred mainly through corner

54 and edge atoms. Those low coordination atoms are then blocked to the reactants which are adsorbed on terraces of the particles. Moreover the electronic properties, determined by X ray Photoelectron Spectroscopy (XPS), of aggregated particles corresponds more closely to large particles (>30 A) than 20 A particle size. Finally, aggregation leads to geometric and electronic effects and so to an increase of the TOF because of the removal of the unsaturated atoms having a lower intrinsic hydrogenation activity [3,13].

5. CONCLUSION This study shows how it is possible to prepare supported nanometer scale Pd oxide particles using colloid chemistry in aqueous medium. Well defined systems have been obtained in terms of average particle size, particle size distribution and aggregated or isolated deposited particles. The very high potential of this type of approach is linked to the different chemical reactions involved during neutralisation or thermohydrolysis. This study shows the importance of knowledge of the Pd coordination chemistry in the initial precursor solution in controlling the final properties of the catalysts. Finally, applying low temperature activation, the metallic Pd particles obtained have the same characteristics as the colloidal Pd oxide particles.

ACKNOWLEDGEMENTS The authors thank Professor J.P. Jolivet, University Pierre et Marie Curie, Paris, France, for fruitful discussions.

REFERENCES

1. L.N. Lewis, N. Lewis, J. Am. Chem. Soc., 108 (1986) 743, 7228. 2. H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Jouben, R. Fretzen and B. KoraU, J. Mol. Catal., 74, (1992), 323-333. 3. V. Ponec, G.C. Bond in Catalysis by Metals and Alloys, Studies in Surface Science and Catalysis, 95 (1995), 317. 4. A. Rakai, D. Tessier, F. Bozon-Verduraz, New J. Chem., 16, (1992), 869-875. 5. L.I. Elding, L. Olsson, J. Phys. Chem., 82 (1), (1978), 69-74. 6. P.B. Critchlow, S.D. Robinson, Coord. Chem. Rev., 25, (1978), 69-101. 7.1. J.P. Jolivet, "De la solution h roxyde", CNRS eds (1994), 219-254 7.2. J.P.Jolivet, "De la solution ~t l'oxyde", CNRS eds (1994), 52-64. 8. B.I. Nabivanets, L.V. Kalabina, Russ. J. Of Inorg. Chem., 15 (6), (1970), 818. 9. C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, New York, John Wiley & Sons eds., (1976). 10. Horiuti, J, Nakamura T.Z.Z. Phys. Chem., 11, (1957), 358. 11. D. Duca, L.F. Liotta, G. Deganello, Catal Today 24 (1995) 15-21. 12. J.P. Boitiaux, J. Cosyns, S. Vasudevan, Appl. Catal., 6 (1983) 41. 13. Z. Karpinski, Adv. Catal, 37, (1981) 45-100.

91998 ElsevierScienceB.V. All rights reserved. Preparationof Catalysts VII B. Delmonet al., editors.

55

Nanosize Palladium Loaded Catalytic Membrane" Preparation and Cis-Trans Selectivity in Hydrogenation of Sunflower Oil O.M.Ilinitcha, P.A. Simonov a and F.P.Cuperusb aBoreskov Institute of Catalysis, prospekt Lavrentieva 5, Novosibirsk 630090, Russia bAgrotechnological Research Institute, Bomsesteeg 59, P.O.Box 17, NL-6700 AA Wageningen, The Netherlands

Palladium catalyzed hydrogenation of sunflower oil was investigated in conventional and membrane catalysis modes. The technique of preparation of catalytic membranes containing nanosize Pd particles supported over macroporous polymeric membrane was developed. In the catalytic membrane reactor operating in the regime of forced flow of the reaction mixture through the membrane pores, the yield of trans-isomeric triglycerides was suppressed compared to the conventional slurry reactor with carbon-supported Pd catalyst. 1. INTRODUCTION 1.1. Cis-trans selectivity in vegetable oils hydrogenation: outlines of the problem and perspectives for membrane catalysis Catalytic hydrogenation of edible vegetable oils - the basis for industrial production of margarines and cooking fats - is traditionally performed in industry as slurry-type batch processes with nickel catalysts. In these processes, the target reaction - stepwise hydrogenation of C=C bonds in the polyunsaturated fatty acid moieties (FA) of triglyceride molecules - is accompanied by the side transformations (Fig. 1): Hydrogenation H 3 C ~ COOH

H3CVV~/~/~/~ / H3C~/~__~~/~~COOH Positional isomerisation

COOH

Unsaturated fatty acid N ' ~ (cis-form) H~ C~.~,,--.A ~ C ~H Geometric isormrisation (trans-fonn)

Fig. 1. Scheme of the reactions occurring in the course of vegetable oil hydrogenation

56 FA that are characteristic for every oil differ in quantity of carbon atoms, quantity and position of carbon-carbon double bonds and their isomeric composition (cis- or transconfiguration). The most common for edible oils are the unsaturated fatty acids - linolenic, linoleic, oleic (cis-isomers), and saturated stearic acid. It is generally accepted to identify the fatty acids numerically; in such a representation, linolenic acid is designated as 18:3, linoleic - 18:2, oleic- 18:1 and stearic- 18:0 (here the first figure 18 stands for the quantity of carbon atoms, while the second - for the quantity of double bonds). In view of the negative role trans-isomeric triglycerides are suspected to play in increasing the risk of coronary diseases, their content in the products should be minimized. As a means to suppress cis-trans isomerization, hydrogenation in a catalytic membrane reactor was considered under assumptions that (a) facilitation of hydrogen transfer to the active centers oof the catalysts stimulates the target hydrogenation reaction, while cis-trans isomerization is less (if at all) sensitive to hydrogen, and (b) hydrogen transfer in the pores of a catalyst can be improved through the use of catalytic membranes with the reaction mixture flowing through the pores of the membrane, since forced flow of the reactants in the membrane pores is likely to provide better mass transfer conditions compared to the diffusion-driven flow in the pores of a grainy catalyst (Fig. 2). The catalytic membrane thus becomes the key issue of the problem. Our approach, which has not been explored in this application yet, presumes the use of a macroporous membrane. The expected advantages of such membrane-support compared to the classic membrane catalysis with non-porous foils of palladium and Pdbased alloys where high selectivities have been demonstrated for a number of (de)hydrogenation reactions [1], are related to techno-economic rather than chemical features of the catalytic process. Thus, Fig. 2. Principles of reactants mass compared to the classic approach with Pd transport in a porous catalytic membrane foil catalytic membrane, the porous memand pellets of a catalyst brane can provide much higher surface area of a catalyst in a reactor, hence much higher productivity of a reactor and much lower amount of Pd used. The porous membrane to be used as a support in preparation of the catalytic membrane was to comply with the next conditions: high volume of macropores - to assure low hydraulic resistance to the reactants flow through the pores; highest possible surface a r e a - to accommodate relatively high loading of nanosize palladium particles; low cost. The membrane that satisfied the above conditions was found and tested for preparation of Pd-loaded catalytic membrane. This is a polymeric (nylon-6) microfiltration membrane MIFIL with the next characteristics of the porous structure: mean pore size 0.4 gm, pore volume ca. 0.6 cm3/cm 3, thickness 100 gm.

1.2. Preparation of the catalytic membrane: theoretical approach In preparation of heterogeneous metal catalysts, adsorption of metal precursors over various supports is widely employed to achieve high dispersion of active components

57 Catalytic properties of the metal thus obtained are to a large extent determined by this stage. In turn, regularities of the adsorption process are strongly dependent on chemical composition of the support due to interactions between the metal precursor and surface functional groups of the support. These issues were taken into account to develop a procedure for preparation of the catalytic membrane containing nanosize palladium particles over porous polymeric membrane support. (1) Chemical composition of the membrane: The membrane material (polyamide-6) is composed of the units [-HN(CHz)sCO-]n,which in turn comprise two main functional groups, i.e. - N i t - (amide) and -CH2- (fragments of the aliphatic chain). The latter are relatively inert, with the exception of -CH2- group at a-position of carbonyl, where oxidation is possible. Amphoteric amide groups are generally more reactive; they can act as weak bases or N-H acids. Note that acidity of the amide groups is rather low (similar to that of methanol) to be considered as the potential factor governing interactions between the membrane material and the catalyst precursor. (2) The above analysis implies that in order to be chemisorbed onto the membrane, the precursor of palladium should either possess acidic properties (Br~nsted or Lewis type), or to be a powerful oxidizing agent. The complex acids of H2[Pd~] series satisfy the former condition. Of those, palladium hydrochloric acid HzPdC14 and the salts of PdX2 series (X= OAc, C1 etc.) are most commonly used in practice of catalyst preparation. Fig. 3 illustrates the plausible interactions between H2[PdX4] and surface amide groups of the membrane (M), i.e. formation of the salt-like surface compound (route 1) and that of the surface complex (rome 2). In this scheme, only one step of the interactions is shown; meanwhile, in the absence of steric hindrances Pd 2§ ion is capable of binding with two adjacent surface groups. O H2[PdX4]

HO H[PdX4]-+NHde ~ M

+

HN

M

0 PdX3 NH/ ~ M

+ HX

cn2 Fig. 3. Possible interactions between H2[PdX4] and the surface functional groups of polyamide membrane Analysis of the transformations shown in Fig. 1 reveals that the route 2 leading to the formation of surface palladium complexes is favored by the absence of tightly bound ligands in the internal coordination sphere of Pd 2+ ion. On the other hand, the absence of tightly o o bound ligands increases the redox potential of Pd 2+ (Eed2+/p d =0.91 V, while Epdcl]./pa =0.59

V), which is likely to facilitate reduction of palladium up to the metallic state. (3) Equilibria of the above chemical processes can be shifted towards the specific metal surface compounds not only under influence of pH, nature and concentration of ligands, but

58 the nature of a solvent as well. Transfer of ions from a solution onto the surface of a support changes the character of solvation for both the ions and the surface itself, thus influencing thermodynamics of the adsorption process. Tendencies governing the solvations can be predicted in the frameworks of donor-acceptor (D-A) properties of the solvent [2]. Another factor that apparently influences the stability of surface ionic compounds is the dielectric constant of a medium in the near-surface layer, which is usually different from that in the bulk of the solution; this may as well alter the stability of surface ionic compounds. The ability of the solvent to form solvates [2] with both the membrane and the precursor, as well as the dielectric constant of the solvent (e/~0) were considered in this study as the main factors governing the process of the catalysts precursor adsorption over the membrane.

2. RESULTS AND DISCUSSION

2.1. Preparation of the catalytic membrane In preparation of the catalytic membranes, solutions of H2PdC14, Pd3(OAc)6 and Pd6Cll2 (13-PdC12) prepared according to [3] and listed in Table 1 were used. To determine the adsorption capacity for Pd, the membrane samples were kept for three days at room temperature in solutions containing 12 % of the metal with reference to the sample weight. Pd content in the solution before and after adsorption was determined by UVVIS spectrophotometric analysis (Specord M40, Karl Zeiss - Jena). For the analysis, samples of the solutions were dried, and solid residues dissolved in 3M HC1 were analyzed, absorbance maxima at ~-1 = 21100 cm -1 and )~-~- 35000 cm -~ being the measure of palladium content in the solution. The membrane preparation procedures employed for the different palladium precursors are described below.

H2PdCI4 precursor The experimental data (Table 1) evidence that amount of Pd precursor sorbed by the membrane from aqueous solutions of H2PdC14 is rather high; its adsorption from acetone solutions is nearly two-fold lower. In both cases the membrane turns fragile upon contact with the solutions; this does not allow to employ the H2PdC14 precursor in preparation of the catalytic membrane. Table 1. Adsorption of Pd precursors from various solvents over polyamide-6 membrane Solvent Solvent properties [1] Membrane sorption capacity towards Pd 2§ %. D A ~;/~;0 H2PdC14 Pd3(OAc)6 PdrCI~2 Water 18.0 54.8 81.7 2.3 * * Acetone 17.0 12.5 20.7 1.2 2.5 * Benzene 0.1 8.2 2.3 * 2.9 3.5 * The substance is insoluble in the solvent.

Pd3(OAc)6 and PdrC112 precursors These cluster compounds dissolved in aprotic organic solvents are the most promising starting materials for adsorption of the palladium precursor over the membrane. In this case, the adsorption equilibrium is shifted towards the surface complexes. This results in the

59 complete extraction of palladium ions from the solutions, with the catalytic membranes containing up to 4-5 wt.% Pd. The use of palladium compounds in benzene solutions seems to be the most promising, since they provide the highest loading of the metal and do not damage the membrane. The highest adsorption obtained with the benzene solutions apparently results from weak donor-acceptor properties of benzene (Table 1) and hence labile character of the solvates formed with both the compounds in solutions and the functional groups in the adsorption centers. Besides, the dielectric constant of benzene is close to that of the membrane (ca. 3.5-4.0), while the values of t/t0 for water and acetone are much higher; this is likely to be the second factor to promote adsorption of palladium precursors from benzene solutions. It seems likely that high sorption capacity of the membrane in the case under consideration is due to adsorption of the palladium precursor in its cluster molecular form.

Preparation of the catalytic membrane and characterization of Pd particles formed The catalytic membrane samples containing Pd(OAc)2 and PdC12, with the metal content of 2% wt. were prepared via adsorption from benzene solutions of Pd3(OAc)6 and Pd6Cl12 at room temperature. The dried samples were further reduced in aqueous solutions of KBH4 and N2H4, washed in distilled water and dried in vacuum at room temperature. The samples thus obtained were analyzed by X-ray diffraction, SAXS and CO chemisorption to determine the size of Pd particles formed on the membrane surface. All three techniques in good agreement with each other revealed that the uniform Pd particles of 2.5-3.0 nm in diameter have been formed over the membrane, both precursors producing the similar particles (Table 2). Table 2. Diameter of Pd particles formed over the MIFIL membrane yia adsorption of Pd3(OAc)6 and Pd6Cl12 precursors in benzene solutions Reducing agent Pd particle size, nln Pd3(Oac)6 Pd6Cl12 -KBH4 " ' 2i} 3.0 N2I-I4

-

2.5

Since the use of Pd3(OAc)6 is more attractive from the economical point of view, in the further studies the catalytic membrane prepared from this precursor was employed. 2.2. Catalytic hydrogenation of sunflower oil: conventional vs. membrane catalysis Comparative study of sunflower oil hydrogenation (conventional vs. membrane catalysis) was performed. In conventional catalysis, the reaction was studied in a batch reactor with the supported palladium catalyst Pd/C (2 wt.% Pd over Sibunit active carbon support [4]) prepared at Boreskov Institute of Catalysis. The catalyst particle size was below 0.1 ram, the mean pore radius ca. 40 nm. The reaction was studied at hydrogen pressures up to 0.5 MPa and temperatures 323-373K. Typical concentration profiles for the triglycerides of obtained in the course of reaction carried out in the slurry-type batch reactor are shown in Fig. 4. As can be seen, 18:1-trans FA appear already at relatively low conversion of 18:2 FA, but their concentration remains below that of 18:l-cis until conversion of 18:2 approaches 80-90%. At this time concentration of 18:0 begins to increase rapidly. At full conversion of 18:2, it is only trans-isomeric 18:1 FA that remains in the product.

60 8070

=s O L_

c

50 40

30 c O 20 r 0

20

60

105

150

210

Reaction time, minutes

---0--C18:2 ~018 total X C18:1-trans ,~ 018:0

-*

270

330

C18:1-cis

Fig. 4. Concentration profiles of C18 triglycerides in the course of sunflower oil hydrogenation over Pd/C catalyst. Reaction conditions: batch reactor, T=348K, Pm= 0.35 MPa

In the comparative runs performed at the same conditions in the membrane reactor with the above described catalytic membrane, concentration of 18:1 cis-isomers was found to be higher (at the expense of lower concentration of 18:1 trans-isomers) than that in the batch reactor (Fig.5). This apparently supports our hypothesis that the forced flow of the reaction mixture flowing through the pores of the catalytic membrane improves the intraporous mass transfer. 40

=-,x 30 "- 25 O

.o 20 = 15 eO

o

10

61

*-

52 48 Concentration of 18:2, %

conventional 18:1trans ~ conventional 18:1cis X

33

membrane 18:1trans membrane 18:1cis

Fig. 5. Concentrations of C18:1 vs. concentrations of C18:2 triglycerides in the course of sunflower oil hydrogenation over Pd-containing membrane. Reaction conditions: catalytic membrane reactor, T=348K, Pm= 0.35 MPa

61 It might be expected that the approach of the membrane catalysis explored in this study can be applied for a number of three-phase reactions were internal diffusion limitations often constitute a serious problem in achieving high activities and selectivities

3. CONCUSIONS Palladium catalyzed hydrogenation of sunflower oil was investigated in conventional and membrane catalysis modes. The technique of preparation of catalytic membranes containing nanosize Pd particles supported over macroporous polymeric membrane was developed. In the catalytic membrane reactor operating in the regime of forced flow of the reaction mixture through the membrane pores, the yield of trans-isomeric triglycerides was suppressed compared to the conventional slurry reactor with carbon-supported Pd catalyst. The improvements of selectivity achieved over the catalytic membrane apparently originate from the intensification of the reactants mass transfer in the pores of the membrane.

Acknowledgment Support of this work from the Netherlands Organization for Scientific Research (NWO) under the grant No. 047-010-100-96 is gratefully acknowledged.

REFERENCES 1. V.M.Gryaznov, Hydrogen Permeable Palladium Membrane Catalysts, Platinum Metals Rev., 30 (1986) 68. 2. J.E. Huheey, Inorganic Chemistry: Principles of Structure and Reactivity, 3rd Ed., N.Y., Harper & Row, 1983. 3. A. Yatsimirski and R. Ugo, Inorg. Chem., 1983, 22, 1395-1397. 3. V.F.Surovikin, G.V.Plaksin, V.A.Semikolenov, V.A.Likholobov and I.Yu.Tiunova, US Patent 4 978 649 (1990).

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

63

P r e p a r a t i o n o f h y d r o g e n a t i o n catalysts supported on stainless steel or c a r b o n fabrics. J.P. Reymond *, D. Dubois and P. Fouilloux Laboratoire G6nie des Proc6d6s Catalytiques E.S.C.P.E. Lyon 43 Bd du 11 Novembre F-69616 Villeurbanne Cedex. France In order to apply monolithic reactors to hydrogenation of organic molecules in liquid phase, the ways to obtain deposits of precious metals on bidimensinnal supports have been studied. Use of autocatalytic deposition (plating) method leads to the formation of palladium deposits on stainless steel thin grids and on carbon fabrics. These two kinds of catalytic systems exhibit quite good activity in acetophenone hydrogenation. Keywords : preparation, catalyst, hydrogenation, monolith, palladium. 1. INTRODUCTION Whereas monolitic reactors have several applications in the case of gas-solid reactions, the best known being the treatment of exhaust gas of motor cars, they have few applications with liquid phase reactions except in laboratory studies (1). Nevertheless, in the case of catalytic polyphasic reactions (gas-liquid-solid) a monolithic reactor would present some advantages compared to the usual slurry reactors : better reactant diffusion, better heat transfert, fluid plug flow in the monolite channels, catalyst fixed on the monolith channel walls which lead to eliminate the solid-liquid separation problems at the reactor outflow,.... However, the use of a monolithic reactor with liquid and gas phases implies to solve two critical problems : 1) to obtain a good dispersion of the gas bubbles in the liquid phase 2) to prepare active catalyst particles strongly anchored on the channel walls. In order to apply monolithic reactors to hydrogenation of organic molecules in liquid phase we have studied the ways to obtain deposits of noble metals (Pd, Ru, Pt) on supports which could be transformed into monoliths. Bidimensional supports have been selected : stainless steel grids or sheets, and carbon fabrics. The typical porous alumina deposit (washcoat), encountered in catalytic exhaust pipe of cars, does not offer sufficient resistance to liquid flowsand the catalytic metal must be strongly fixed on the support itself. Amongst the chemical processes from which a metal is deposited from an aqueous solution (immersion or displacement; contact; autocatalytic), the autocatalytic one (electroless * To whom all correspondance should be adressed to present adress : LGPC, CPE, 43 Bd 11 Novembre, 69616 Villeurbanne cedex, France

64 plating) which applies on conductor as well on non-conductor substrates (1,2) has been selected. Autocatalytic plating may be defined as"deposition of a metallic coating by controlled chemical reduction that is catalyzed by the metal or alloy being deposited' (3). The deposition of metal is accomplished by the chemical reduction of metallic ions in an aqueous solution containing a reducing agent. The metal salt and the reducting agent react in the presence of a catalyst. The support material can be itself catalytic or be activated by the use of displacements deposits of catalytic metal (Pd or Sn for example) or contact plating of the substrate with a catalytic metal. After the reaction is initiated on catalyst germs, the deposited metal behaves as the catalyst, ensuring continuous build up of the metal (4). The metal to be deposited must be more noble (more electropositive) than the support. Common applications of this process is the controlled deposition of a continuous protective metal film on a support. This is a redox process, similar to a galvanic process (anodic and cathodic reactions take place in the mechanism), the electron supply being the chemical reductant (5). The present work presents the application of electroless plating to the preparation of hydrogenation palladium catalysts supported on two very different bidimensional supports. 2.EXPERIMENTAL 2.1. Raw materials Two kinds of support have been selected : stainless steel and carbon, both are under the form of fabrics. Stainless steel supports are grids characterized by wire diameter, aperture mesh and steel grade. Several kinds of grids (grade, geometry, wire diameter) have been tested, but they all lead to quite similar results. Thus, in this paper, used grids (Spectra/Mesh) are made of non porous 316L stainless steel wires (diameter: 56 9m) and have square meshes (aperture " 51 9m). The surface area of the grids is their geometric area (-~100 cm2/g). Carbon fabrics, obtained by carbonization (hydrogen consumption) and activation treatment (porosity formation) of viscose fabrics, have a high specific surface area (948 m2/g) and are microporous (dmicropore"1nm, Smicropores=665m/g). 2 Palladium precursor is PdC12 (Janssen Chimica); the reducing agent is sodium hypophosphite (NaH2PO2.H20, Fluka). Others reactants (hydrochloric acid, acetone and cyclohexane) are of high purity. 2.2. Preparation process In the case of steel grids the deposition procedure is the following : four grid pieces (each 3x2 cm; total weight - 0.45g) are used fo each preparation test which takes place in glass stirred reactors. 1-cleaning of grids with acetone in a Kumagawa apparatus (elimination of organic contaminants). 2-activation treatment (elimination of surface metal oxides and generation of roughness) of the support by hydrochloric acid solution (HC1 12 wt%, 60 ~ 20ran). 3-preparation of a PdCl2 solution from dissolution of the metal salt in a diluted hydrochloric acid solution. For each test 44 mg of PdC12 and 25 cm 3 water are used, the amount of added concentrated hydrochloric acid (36 wt%) is varied. 4-deposition of the metal. The steel grids are immersed in a solution containing 90 cm 3 water and the palladium solution. A given volume of a solution of reductant (1 wt% NaH2PO2.H20) is added, with a given rate and tempearture, to the former solution. 5-drying of grids (2 hours in air, at room temperature).

65 After each step, grids are thoroughly rinsed in deionised water. Operating parameters of steps 1, 2 and 5, have weak effects on catalytic properties of catalysts and had the same values, indicated above, in all preparation tests. The conditions of formation of active palladium deposits have been studied in varying operating parameters of steps 3 and 4. The amount of palladium (25 mg) in the deposition bath is calculated to lead to catalysts containing -5 wt% of palladium. With carbon fabrics, four pieces are also used (each 4x4 cm; total weight- 0.45 g). As that will be seen below, step 2 is suppressed and step 1 is not essential. Steps 3, 4 and 5 are similar. 2.3.Catalytic

reaction

The best way to evaluate the quality of a preparation method of a solid is to study its activity in a test reaction, so the catalytic properties of the prepared solids has been studied in the hydrogenation of acetophenone (6, 7). Acetophenone molecule (C8H80) has an aromatic ring with a ketone function. Depending on reaction parameters, interesting selectivity changes can be observed. For exemple, the hydrogenation of acetophenon leads to the hydrogenation of the aromatic cycle on Ru catalysts, to the hydrogenation of the ketone function on Pd or to the hydrogenation of both aromatic cycle and ketone function on Pt catalysts.. Reaction scheme of acetophenone hydrogenation is : - Hydrogenation of aromatic cycle Acetophenone

3H~ Methylcyclohexylketone

H2.~ Cyclohexylethanol

(1)

- Hydrogenation of ketone function: Acetophenone -

H2 .~ Phenylethanol

H2.~ Ethylbenzene

(2)

side reactions

Ethylbenzene Phenylethanol

3H2 3H2

Ethylcyclohexane Cyclohexylethanol

(3) (4)

Catalytic tests take place in a 125 c m 3 stainless steel semi-batch reactor. Reactants are diluted in cyclohexane (45 g) in which hydrogen has a good solubility at reaction temperature. A vigorous stirring (magnet bar) provides a good contact between reactants. During reaction, hydrogen pressure is maintained constant (25MPa) in the reactor. First, the catalytic system, immersed in cyclohexane, is treated under hydrogen (25 MPa) at 393 K (reaction temperature) during 20 minutes. Then, acetophenone (3 g) is introduced in the reactor and this is considered as the start point of the reaction. Reaction conversion is evaluated from hydrogen and acetophenone consumption and from product formation. Hydrogen pressure is recorded by means of a data acquisition system. Reactant and product concentrations are quantified by gas phase chromatography. Catalytic activity is expressed by the initial activity, Ai (mol H2.gpd-lhq), which is given by equation 1:

66

Ai =

ADm x V RxTxAtxm

[1] metal

Where APH2 is the hydrogen pressure (Pa) decreasing observed during the time At (h); V is hydrogen tank volume (cm3); T is hydrogen tank temperature (Kelvin); R, Mariotte-Boyle constant (8.314 Pa.m3.mollK -1) and mmetaI is the mass of catalytic metal (g). 3.RESULTS AND DISCUSSION 3.1. Preparation of palladium deposits on stainless steel grids

The palladium deposition procedure is the following : 25 cm 3 of acid PdC12 solution (1 g Pd/liter) is added to 100 cm 3 of deionised water maintened at 25 ~ The four cleaned stainless steel grids are immersed in this bath. The reducing solution (1 wt% NaH2PO2) is poured in the vessel. Then, bath temperature is increased up to 90~ and maintained 20 min at this value. The temperature is allowed to dercrease and the grids are removed at 25 ~ rinsed with deionised water and left to dry in air during 2 hours. When suitable deposition conditions are combined, it can be observed the following phenomena : after addition of the reducer and during temperature increasing the bath darkens and gaseous bubbles appears. Above 60-70 ~ the solution slowly lightens and becomes limpid. The grids are black, covered with palladium particles. Darkening of the solution corresponds to the reduction of PdC1a and formation of very small palladium particles. Lightening of the bath coresponds to the palladium particles fixation on stainless steel wires. Formation and deposition of palladium particles are simultaneous reactions but formation of particles is faster than deposition, leading to temporary accumulation of particles in the liquid which, consequently, darkens. Influence of several operating parameters of PdCla dissolution and of palladium deposition on properties of catalytic systems has been studied : - hydrochloric acid amount added to 25 cm 3 of deionised water (temperature dissolution is 25 ~ - amount and rate of addition of sodium hypophosphite solution, and bath temperature during hypophosphite pouring. Table 1 Influence of hydrochloric acid content in PdC12 dissolving bath on initial activity and selectivities of Pd/SS catalysts. HC1 content pH Ai Selectivities (%) g/1 mol(H2).g-1.h"1 PE EB AC 4 2.15 72 0 100 10 1.98 72 0 100 40 1.3 26 5 92 80 1.09 16 26 74 PE is Phenylethanol, EB is Ethylbenzene and AC is Acetophenone

:_::::=::=::::=:=:=:::::.__:

. . . . . . . . :. . . . . . . . . . . . . . . . . . . . . . . . . . . ~ : : : : : : : ~ : : : : ~ : : : : : : : : - : : - ~ : : - _ : - - - : ` - : : : : : : : . : : : . - : : . : ~ : - : : : : : : . : . : : - - . : : : : . ~ : . : . : . - : : . : . . . : : ~ : ` .

................ ::~:::-:-_-::_::`.:::::::--:--:-:_:::::::~`:--::::::-.::::::..::::-.-:-:~:~::::::::~::::::::::-::`

..:=.

=======================. . . . . . . . ==========================================================================

67 Hydrochloric acid used to improve PdC12 dissolution in water is present in the deposition bath. As shown in table 1 the largest HC1 amounts correspond to the lowest catalyst activities. For very active catalysts the only product detected in the liquid phase after 30 mn on stream is ethylbenzene, acetophenone and phenylethanol are fully conversed. With low activity catalysts acetophenone and phenylethanol can be detected. Figure 1 presents, in the case of an active catalytic system, the variation of reactants and products concentration in the liquid phase versus hydrogen conversion rate. As shown by the curves of figure 1, the first step of the reaction is the hydrogenation of acetophenone in phenylethanol. The second step is hydrogenolizing of phenylethanol in ethylbenzene.

loo

80" ~

-o-- Acetophenone ~ Phenylethanol

y

A

6O = n

>

r m

u~

40 2O 0 0

20 40 60 80 Hydrogen conversion rate (%)

100

Figure 1 Variations 9 of reactants versus hydrogen conversion rate on Pd deposited on 316L stainless steel grids.

Table 2 depicts the influence of characteristics of deposition baths. A low temperature of hypophosphite addition (see tests 1 and 2) leads to a high activity. Precipitation rate of palladium is low and it can be assumed that palladium particles are small and do not aggregate before deposit on grids.Sodium hypophosphite must be in excess from stoichoimetry of PdC12 reduction reaction (see tests 4, 5 and 6). Hypophosphite addition rate (see test 1, 3 and 4) presents an unexplained optimal value (80 cm3/h).

68 Table 2 Influence of deposition bath characteristics on initial activity and selectivities of Pd/SS catalysts. Test NaH2PO 4 Addition pH T Ai Selectivities (%) amount rate ~ cm 3 cm3/h mol(H2).gl.h "l PE EB AC 1 5.3 80 1.98 25 72 0 100 0 2 5.3 80 1.96 60 6 71 26 5 3 5.3 40 1.89 25 40 0 100 0 4 5.3 99 1.88 25 59 0 100 0 5 4.1 80 1.8 25 7 52 32 16 6 7.5 80 2.05 25 58 4 95 0 'PE is Phenyletfianoil EB is Ethylbenzene and A c is Acetophenone ....... Use of scanning electron microscopy shows (Figure 1) the presence of particles with irregular shape and size (up to 50-60 ~tm) deposited on stainless steel wires.

Figure 2: Scanning electron micrography of Pd/stainless steel grid. X-ray emission analysis have been performed in the electron microscope with an electron microprobe analyser. Figure 3 represents the spectrtun obtained from analysis of a wire area without any deposited aggregate. This analysis reveals the presence of the major constituents of steel (Ni, Fe, Cr) and the presence of palladium and phosphorus.

69

El"

Fe

~

Cr

KeV

Figure 3" X-ray emission analysis spectrum of a grid wire after palladium deposition (no palladium aggregate in the analyzed area) Otherwise, as determined from X-ray emission analysis of an area centered on an aggregate deposited on awire (analysis area smaller than aggregate area), the composition of this aggregate is palladium and phosphorus, as depicted on figure 4.

Pd

P

..... i" ........7 1

2

3

4

KeV

5

6

7

8

9

Figure 4: X-ray emission analysis of palladium aggregate deposited on a grid wire. Table 3 presents quantitative chemical analysis (ICP) of some samples. It points out that sample phosphorus content is higher than phosphorus content of stainless steel 316L (-0.045

wt%).

70 Table 3 Chemical analysis and activity of some Pd/stainless steel catalytic systems. Test QHCI Deposiii0n Palladium Phosphorus Pd/P Ai bath pH content content g wt% wt% mol(H2).gpd'X.h"1 1 2 3 4 5 :::=:===:===:=::

2 1 0.25 0.1 0 :...:..::

::::.:

~ .=

: :=:::::===~:

1.03 1.47 1.98 2.15 2.69 = ==

: .....................................

22.74 26 23.27 15.55 16.6 :

1.48 1.378 0.954 0.722 0.872

15.36 18.87 24.39 21.54 19.04

23 58 83 116 110

. . . . . . . . . .

Thus, palladium deposition process leads to the formation of a Pd-P alloy coating on the whole surface of wires, and to the formation of discrete particles of Pd-P alloy. Typically, electroless plating baths, with sodium hypophosphite as reducer, lead to the formation of Pd-P films (8, 9). When sodium borohydride is the reducer deposited films are Pd-B alloys. Phosphorus codeposition could occur via interaction of hypophosphite ion with atomic hydrogen or with hydride ions or with palladium metallic surface (10). Consequently, the catalytic active phase is Pd-P alloy. At the moment, catalytic effect of phosphorus in Pd-P alloy is not elucidated. From values of table it appears that the highest phosphorus contents lead to the lowest hydrogenation activities. Simultaneously, these low activities are observed for the lowest pH values of palladium deposition baths and these baths correspond to the highest amount of hydrochoric acid used to dissolve PdC12. On other electron micrographies, metal particles (diameter : 10-50 ~tm) have the aspect of cauliflowers and seem to result from the aggregation of small metal crystallites (diameter : 1-3 ~m). In the case of a stable deposition bath, as those described for electroless plating, the solution does not darken, palladium does not precipitate in the liquid phase, but the deposited metal is not catalytically active. A non stable bath darkens, palladium precipitation rate is very fast, palladium particles stay in the solution, aggregates and settle. These small particles can be recovered and introduced in the catalytic reactor, but they do not present any catalytic activity. As shown by our experiments, palladium deposit is catalytically active when palladium is under the form of discrete particles on the support surface. This is achieved with metastable deposition baths in which the slow rate reduction of the palladium salt leads to the formation of small palladium particles in the liquid phase and then deposit on the substrate. A coloration in black of the bath evidences the beginning of the palladium deposition. Then, there is a slow lightening of the bath which tums absolutely limpid at the end of the palladium deposition.

3.2. Preparation of palladium deposits on carbon fabrics Deposition of a metal on a carbonaceous support can be done by means of the wellknown impregantion method. In this case pieces of carbon fabrics are immersed in a solution of a salt of the chosen metal, drained, dried and calcined. Typical activation pretreatment of such catalysts is hydrogen reduction. In this work, the autocatalytic method, described above for stainless steel grids, has been used to deposit palladium on carbon fabrics. The influence of several preparation operating

71 parameters on the activity of catalytic systems (labelled Pd/C.fabrics) in acetophenone hydrogenation has been studied. Table 4 presents the main results. Table 4 Influence of operating parameters on catalytic properties of Pd/carbon fabrics ....... Acetone HC1 (12 wt%, Hc1 amount' used Deposition Ai Cleaning step 60~ treatment for dissolution of bath pH mol.g'~.h"~ PdC12 : 1 2 3 4 5 6

Yes No No No Yes No

Yes Yes No No No Yes

1 1 1 .25 .25 .25

1.24 1.34 1.33 2 1.98 1.97

56 54 44 58 68 56

For all the experiments depicted in table 4, dissolved PdCI2 amount is 44 mg. Sodium hypophosphite amount added to the deposition bath, at 25~ with a 80 cm3h"~ addition rate is 5.3 cm 3, and Pd fixation temperature is 90 ~ Initial cleaning with acetone leading to weakly more active catalysts has been retained as a first step of the process. Hydrocloric acid treatment does not have beneficial effect and as HC1 use presents great disadvantages (toxicity, corrosion) this treatment (step 2) is abandoned in the case of carbon fabrics. It appears that preparation operating parameters leading to most active Pd/C fabrics and Pd/stainless-steel grids are the same. Catalytic properties of Pd/C.fabrics and Pd/SS grids have been compared to that of industrial hydrogenation catalysts (Pd/C Degussa and Pd/C Doduco) in which palladium is supported on powdered active carbon of high surface area (~1000 mZ/g). Results are presented in table 5. Table 5 Comparison of initial activity and selectivities (after 30 mn on stream) of commercial and laboratory Pd/carbon catalysts Catalyst Reduction Ai MCC .......CE PE EB AC (H2, 120~ 5 h) molto g l h l % % % % % Pd/carbon (Degussa) Pd/carbon (Doduco) Pd/s. steel Pd/C. fabrics

52 123 72 68

0 0 0 0

0 0 0 0

99 18 0 0

1 82 100 100

0 0 0 0

Pd/carbon (Degussa) 2 219 2 3 Pd/s. steel 2 48 0 0 Pd/C. fabrics 2 42 0 0 A'C is Acetophenone. PE is Phenylethanol ~ d EB is Ethylbenzene.

76 0 15

24 100 85

0 0 0

....

72 Our catalytic systems appear quite different of industrial catalysts. When catalysts are not prereduced by hydrogen (line 1 to 4), Pd/C fabrics, Pd/SS grids and Pd/CDegussa have the same initial activity while Pd/C Doduco is twice more active. Selectivities at the end of the reaction are more different. Our catalysts lead to ethylbenzene while industrial catalysts have a lighter hydrogenolysis activity producing only phenylethanol (Degussa) or phenylethanol and ethylbenzenze (Doduco). It is observed that a reducing pretreatment (H2, 2h, 120 ~ decreases slightly the initial activity of Pd/C fabrics and Pd/SS catalysts, while initial activity of industrial catalyst (Pd/C Degussa) is strongly enhanced. This is not surprising, because the autocatalytic preparation method leads to the deposition of zerovalent palladium which is catalytically active. On the contrary, impregnation of a support from a salt solution leads to the formation of a surface metal complex which must be decomposed in order to obtain the zerovalent active metal. Nevertheless, Pd/C Degussa has a great hydrogenation activity (formation of phenylethanol) and a weak hydrogenolysis action (conversion of phenylethanol in ethylbenzene). Selectivities observed with our catalysts (Pd/ss grids as well as Pd/C.fabrics) are different, they lead to the formation of ethylbenzenze, their hydrogenolysis action is greater (phenylethanol is fully conversed). This could be due to the presence of phosphorus. 4. CONCLUSION From the results presented it appears possible to deposit catalytically active adherent metal particles right on stainlees steel and carbon fabrics. The catalytic activity of Pd/(carbon fabrics) is interesting but lower than that of commercial Pd/(active carbon). This is probably related to the origin of carbon (synthetic polymeric fabrics), the coconut active carbon of the commercial catalytst, renowned to be the best carbon support for catalysis purposes, improving the dispersion of palladium. Although the support of palladium/(stainless steel grid) has an extremly low surface area and palladium is under the form of large aggregates this catalyst presents a surprisingly high activity. REFERENCES

1. J.R. Kosak, in "Catalysis of Organic Reactions" (M.G. Soros and M.L. Prunier Eds., Marcel Dekker Inc., New-York 1995), 25. 2. H.F. Chang and C.F. Yang, Ind. Eng. Chem. Res., 36 (1997), 2080-2086. 3. ASTM B-374, Definitions of Terms Relating to Electroplating. 4. D. Dukes in "Electroless Plating : Fundamentals and Applications." (G.O. Mallory, J.B. Hadju eds, A.E.S.F., Orlando), 511. 5. H. Ocken, B.G. Pound and D.H. Lister, Thin Solid Films, 171 (1989), 313-322. 6. Barinov, D.V. Mushenko and E.G. Lebeva, Zh. Prikl. Khim., 39 (1966), 2599. 7. Masson, P.Cividino, J.M. Bonnier and P. Fouilloux in "Heterogeneous Catalysis and Fine Chemicals II" (1991 ), 245. 8. S.N. Athavale and M.K. Totlani, Metal Finishing, 87 (1989), 23. 9. R.M. Lukes, Plating, 59 (1964), 969 10. G. Salvago and P.L. Cavalotti, Plating, 59 (1972), 665

91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

73

C h e m i c a l v a p o r deposition of p a l l a d i u m on silica and a l u m i n a supports P. Atanasova, a, c J. Wise, c M. Fallbach, b T. Kodasa, c and M. Hampden-Smithb, c aDepartment of Chemical and Nuclear Engineering, University of New Mexico 209 Farris Engineering Center, Albuquerque, NM 87 131, USA bDepartment of Chemistry, University of New Mexico 103 Clark Hall, Albuquerque, NM 87 131, USA CCenter for Micro-Engineered Materials, University of New Mexico 203 Farris Engineering Center, Albuquerque, NM 87 131, USA Successful deposition of Pd onto silica and alumina high-surface-area supports was demonstrated by two-step CVD process using Pd(hfac)2 as a metal-organic precursor. Controlled deposition was achieved to form variable composition of 0.1 to 0.8 wt.% Pd in the samples. Highly dispersed and uniform Pd metal clusters were deposited by controlling the deposition and reduction temperatures of the CVD process. Higher reduction temperatures favor higher Pd loadings but decrease significantly the metal dispersion. An optimal reduction temperature of 200~ leads to impurities-free Pd metal clusters with high dispersion and uniformity. 1. I N T R O D U C T I O N Catalyst preparation by means of chemical vapor deposition (CVD) can be conducted by vaporizing a suitable precursor and adsorbing it on the support material. 1 Subsequently, as a result of a surface reaction with or without a co-reactant, the adsorbate is transformed to the catalytically active species. The key to controlling the metal's dispersion is the understanding of the relationship between the precursor properties and surface reactivity. The formation of highly dispersed metal clusters can be achieved by controlling the surface concentration and reactivity of the adsorption centers and the deposition parameters such as reactivity of the CVD precursor, precursor partial pressure and deposition temperature. Different approaches for chemical vapor deposition (decomposition) have been applied for catalysts fabrication: the precursor molecule may be decomposed to the final species at the temperature of the adsorption of the precursor or only after additional heating to a higher temperature or reaction with a gaseous co-reactant; these processes have been performed under inert, reductive, or oxidative gas atmospheres. 1 Recent results on the preparation of catalysts by CVD have shown that this approach has several advantages when compared with liquid-phase routes. 1-15 In some cases the traditional steps in catalyst preparation such as impregnation, drying, high temperature calcination and/or reduction, which critically affect catalyst performance, can be eliminated when CVD is used; the reduction and activation of the catalysts can be simultaneously carried out in the reactor before the catalytic run. 2-5 To our knowledge, when CVD has been used for applications relevant to catalysis: preparation of monometallic catalysts - Mo/A12036, Ni/A12037,8 and zeolite-supported Pt catalysts; 3-5 controlled modification of support porosity; 9-11 preparation of multicomponent

74 catalytic systems; 12 it has been carried out either by equilibrium adsorption or by flowing the metal precursor vapors through a heated bed containing a particular support, or in a few cases in a fluidized bed reactor. 1 A number of specific examples exist for the benefits of catalyst preparation by CVD. 1-15. Successful application of CVD involves modification of the support properties by depositing a thin layer of oxide thus changing the surface acidity and/or modifying the pore structure and thereby influencing the catalytic properties.l~ 13 CVD of Pt on molecular sieves 2-4,8 as well as other porous supports 5 has produced excellent aromatization catalysts with much higher catalytic activity and longer life than catalysts prepared by the usual routes of ion-exchange or liquid impregnation. There are several areas of application, such as catalytic reforming, hydrocarbon processing and automotive catalysts, where noble-metal-supported catalysts prepared by conventional liquid-phase routes were applied and their performance thoroughly analyzed. The demand for development of novel catalysts with unique characteristics such as high activity and selectivity has led to a fundamental investigations of new methods (CVD, in particular) for preparation of catalysts with uniform composition and controlled morphology. In the studies of C. Dossi and co-workers2,4,14,15 on the application of CVD for catalyst preparation, the research has been focused on Pt- and Rh-based zeolite-supported catalysts. It has been demonstrated that CVD routes to the preparation of catalysts offer the possibility of formation of an extremely wide variety of catalytic structures and materials; however systematic studies are necessary to clarify the parameters of the CVD processes that lead to the desired type of product. Among noble-metal-supported catalysts, palladium-based mono- and bi-metallic catalysts supported on A1203, SiO2, TiO2, ZrO2 and CeO2 are some of the most active catalysts for hydrogenation of unsaturated hydrocarbons 16 and CO, 17 ethylene oxidation,18,19 hydrogenation of aromatic compounds 2~ as well as for the total oxidation of methane. 24-27 The effort in the present study is focused on CVD of Pd onto high surface area silica and alumina supports, and on developing an understanding between the CVD process parameters (precursor sublimation temperature, deposition temperature, reduction temperature) and the dispersion, composition and uniformity of the deposited metal clusters onto model high-surface-area supports.

2. EXPERIMENTAL 2.1. Preparation and composition of the samples A model non-porous support, high surface area silica (Cab-O-Sil, HS-5 grade, with specific surface area of 325 m2g -1, pretreated in N2 at 200~ was used in the experiments for quantifiable analysis of the parameters influencing surface reaction and estimation of metal cluster dispersion. For the experiments with porous support a ~/-A1203 (Alfa Aesar, specific surface area of 100 m2g-1, pretreated in N2 at 200~ was used. The CVD of Pd(hfac)2 was performed by flowing N2 as a carrier gas over a heated precursor vessel and through a heated fixed bed of support. A fixed amount of 0.05 g Pd(hfac)2, Alfa Aesar, was used and completely sublimed each time to obtain reproducibility between the experiments. The only exception was the preparation of samples I-14, I-16 and 117 when a fixed time of 1 hour for the deposition process was used at different sublimation temperatures. The N2 carrier gas flow was controlled by mass-flow controller (UFC 1000, Unit) and was varied between 5 and 20 cm3min-1. The precursor sublimation temperature was varied between 55~ and 75~ The high-surface-area support (0.5 g) was placed in a quartz tube, fitted with silane-treated glass wool or quartz frit. The deposition temperature was controlled by furnace (Lindberg 55035) and was varied between 25~ and 160~ The Pd(hfac)2 adsorption step was followed by reduction in a flow of H2 (20 cm3min -1) at temperatures between 80~ and 400~ A cold trap was used for condensation of the unreacted

75 precursor. A mechanical pump reduced the total pressure to 100 Torr during all steps of preparation. The experimental setup is presented on Figure 1.

Figure 1. Experimental setup for CVD preparation of Pd/SiO2 and Pd/A1203 samples. Table 1 gives the sample annotation, describes the preparation conditions and the Atomic Absorption Spectroscopy (AAS) data on the Pd composition for the reduced samples. Table 1 Preparation conditions and composition for the CVD prepared Pd/SiO2 and Pd/A1203 samples. Sample/ support

Precursor subl. temp.

Deposition temperature

Reduction temperature

wt.% Pd (AAS)

I- 14/silica I- 17/silica I- 16/silica 1-25/silica 1-26/silica 1-30/silica 1-32/silica 1-34/silica I-35/silica I-3 6/silica 1-29/silica 1-40/silica 1-44/silica 1-37/alurnina 1-38/alumina 1-41/alumina

70~ 55~ 60~ 65~ 65~ 65~ 65~ 65~ 70~ 75~ 75~ 75~ 75~ 75~ 75~ 75~

80~ 80~ 80~ 100~ 100~ 120~ 140~ 160~ 100~ 100~ 100~ 100~ 100~ 80~ 80~ 100~

120~ 120~ 120~ 120~ 120~ 120~ 120~ 120~ 1400C 160~ 200~ 250~ 400~ 120~ 200~ 120~

0.5 3 0.37 0.20 n.a. 0.12 0.16 0.18 0.13 0.13 0.20 0.17 0.38 0.7 6 0.08 0.20 0.13

76

2.2. Sample characterization The specific surface area of the samples was measured by the BET method (nitrogen adsorption at 77K) on an ASAP 2000 (Micromeritics). The infrared (IR) spectra were recorded on a Perkin-Elmer 2000 FTIR spectrometer working with resolution of 4 cm -1 and 10 scans, and using self-supported pellets of about 10 mg Cab-O-Sil pressed onto a 0.3 mm thick tantalum mesh and with low pressure setup. For quantitative estimation the IR band intensities were normalized to the area of the band at 1865 cm -1 related to bulk Si-O-Si bonds. The X-ray Photoelectron Spectra (XPS) were recorded on an AXIS HSi (Kratos Analytical) spectrometer, working in AE = constant mode at a pass energy of 80 eV, using a Mg anode (1253.6 eV energy at 15 kV, 15 mA) in a slot mode. The residual pressure in the analysis chamber was 1.10 -9 Torr (1 Torr - 133.3 Pa). A high resolution spectrum of the Si 2p, C ls, O ls, F ls and Pd 3d energy regions was acquired for each sample. All preparation conditions, recording of the spectra and data processing were identical for all samples. The peak positions are relative to the binding energy of C ls - 284.6 eV. The experimental relative intensities used in comparison to the theoretically calculated ones are evaluated after normalization of the peak areas for the number of scans, time per channel, scan width and number of channels. High Resolution Transmission Electron Microscope (HRTEM JEOL 2010) in combination with Electron Dispersive Spectroscopy (EDS) and GATAN digital micrograph with slow scan CCD camera, was used to monitor the Pd clusters distribution, morphology and uniformity. Palladium content in the reduced samples was analyzed by Atomic Absorption Spectroscopy (AAS) using Perkin-Elmer 303 spectrometer with linear recorder.

3. RESULTS AND DISCUSSION 3.1. Chartacterization of the samples after the deposition step When CVD on a high surface area particulate substrate (porous or non porous) is performed it is important to distinguish the different processes that may occur. The first step relies on the selective reaction of the metal-containing precursor with surface functional groups such as surface hydroxyl groups. If conditions of self-limited adsorption are employed (e.g., by choosing a precursor with high reactivity toward a particular type of surface hydroxyl group), once a single monolayer of precursor is adsorbed, the surface is passivated and no reactions occur until a co-reactant is introduced. The self-limiting approach is the one that has been used in most of the reported depositions of metal oxide layers on porous substrates 1,28 and for preparation of supported metallic catalysts. 1 It is well established that the surface chemistry of silica plays a major role in the adsorption process during CVD of metal-organic precursors. 29 A number of studies 3~ has shown that the concentration of the reactive surface groups on the silica surface is influenced by thermal treatment 31 and the changes can be followed by IR spectroscopy. A series of experiments was performed by varying the pretreatment temperature (25-400~ to analyze the distribution of the surface hydroxyl groups for the Cab-O-Sil, HS-5 silica support and to ensure a reproducibility of the silica surface before Pd(hfac)2 deposition. It was observed that a pretreatment of the support in N2 flow up to 400~ doesn't change the concentration of the isolated hydroxyl groups and steadily decreases the amount of the hydrogen-bonded ones, in accordance with the literature data. 29,30 A pretreatment of 1 h at 200~ was sufficient to remove the physisorbed water (followed by the intensity of the deformational vibration at 1645 cm -1) and to achieve a reproducible normalized intensity of the bonds related to the isolated and hydrogen bonded hydroxyl groups. This pretreatment procedure was applied for all CVD experiments. Successful deposition of Pd(hfac)2 onto silica was demonstrated at deposition temperatures of 80~ to 160~ It should be noted that the deposition temperatures were chosen to be at least 5~ higher than the precursor sublimation temperature and the gas lines between the sublimation

77 and the deposition chamber were heated to avoid condensation of the precursor in the lines on the support. All characteristic bands of Pd(hfac) 2 were observed after the adsorption step on silica support: 3140.5 cm -1 (C=O stretch overtone), 1626.5 cm -1 (C=H stretch), 1604.3 cm -1 (C=O stretch), 1558.9 cm -1 (CO stretch + CH bend), 1532.2 cm -1, 1438.2 cm -1 (C=C stretch + CH bend), 1347.2 cm -1 (CC stretch + CCF3 stretch), 752.8 cm -1 (C-CF3 stretch), 718.9 cm1 (ring deformation), 606.8 cm-1. 29,32,33 An estimation was made of the changes in the relative intensities of the IR bands associated with the isolated (3747.5 cm-1)) and hydrogen bonded (3500-3720 cm -1) hydroxyl groups of silica before and after the deposition step. It demonstrated that Pd(hfac) 2 adsorption occurs on both the isolated and hydrogen-bonded OH groups of silica. After the reduction step (1 h in H2 flow) at temperatures as low as 120~ the FTIR spectra did not reveal presence of hfac ligands. The FTIR data on the prereduced samples have demonstrated that Pd(hfac)2 can be successfully deposited on the silica surfaces in contrast with the findings of Dossi and coauthors 14 that non-crystalline oxides of low reactivity, such as silica cannot be employed for CVD technique. The key in answering the question for the applicability of CVD to a particular support is obviously in the relationship between the precursor properties and surface reactivity rather than in the surface properties by itself. The properties and the structure of the metal-organic precursor are of significant importance as well. The chemical vapor deposition of Pd(hfac)2 on different flat substrates (glass, silicon, copper, tungsten, aluminum and nickel) has been studied in detail.34, 35 It has been found that deposition of Pd involves reaction of Pd(hfac)2 with H2, forming (hfacH) as a byproduct and free of impurities palladium films at deposition temperatures as low as 80~ 35 On the other hand Kenvin et al. 36 investigated the adsorption/desorption behavior of three M(acac)2 compounds, where M - Cu, Pd and Pt, on Cab-O-Sil, M5 silica surface. Comparing the adsorption mechanism for Pd(acac)2 and Cu(acac)2, both with square-planar symmetry and adsorbing intact to the silica surface, Kenvin et al. 36 have proposed that the differences in the adsorption uptake depend on the affinity of the compounds to accept axial donors. It was suggested that Pd(II) and Pt(II) (acac)2 compounds coordinate to the silica surface through hydrogen bonding, while Cu(II) compound coordinates through a donor interaction between the surface oxygen and the copper(II) center. 36 This leads to a limited adsorption of Pd(II) compound and to only 0.49 wt.% Pd loadings after the adsorption step. 36 Even though we did not study in details the adsorption mechanism of Pd(hfac)2 from a gas phase onto the silica substrate, from general considerations we were expecting similar Pd loadings for the prereduced samples, which are well within the range of concentrations applied for supported Pd catalysts.

3.2. Chartacterization of the samples after the reduction step After the deposition step which forms the template for further reactions, the second step in the CVD process for catalysts preparation is when thermal treatment is applied (in the particular case with H2 coreactant) for decomposition of the metal-organic precursor and for formation of metal clusters. During the reduction step in addition to the decomposition of Pd(hfac)2 adsorbed onto the silica surface, other processes such as desorption and surface diffusion of Pd(hfac)2 and/or diffusion of Pd atoms as well as sintering of the Pd metal clusters into larger crystallites may occur. All this processes depend on the reduction temperature and determine the final dispersion and uniformity of the CVD deposited Pd clusters. The overall composition of the CVD prepared samples after the reduction step was analyzed by AAS and is presented in Table 1. Figure 2 presents the data for palladium deposition as a function of the deposition temperature for two sets of samples reduced at fixed temperature, 120~ and 250~ respectively. It demonstrates that the amount of Pd deposited depends both on the deposition and the reduction temperatures. The deposition temperatures above 120~ result in a lowest Pd concentrations, especially at 250~ reduction temperature. The data for reduction

78

temperature of 250~ also suggest that lower deposition temperatures favor higher loadings of Pd in the samples. Fixed Reduction Temperature B - 120~ Figure 2. Dependence of the amount Pd deposited on silica vs. the deposition temperature at two fixed reduction temperatures: 120~ and 250~

0.4

C - 250~

,,,,i~,,,i,,,,i,,~,i~,~,

i,,,~i

~,~i,,~

0.35 o

0.3

g. {D

0.25

.,..~ .,..~ r~

0.2 =

0.15 0.1

. . . .

90

. ....

....

....

.... .

>

....

110 130 150 Deposition Temperature [~

170

Fixed Deposition Temperature of 100~ Figure 3. Dependence of the amount Pd deposited on silica vs. the reduction temperature at f i x e d deposition temperature of 100~

.... i .... i .... I .... i .... t .... ; ....

0.8 0.7]~ 0.6'4

0.5-

O

0.4.,..~

0.30.20.1

, O, , ~ . . . . ~, ~ .............. 100 150 200 250 300 350 400 Reduction Temperature [~

450

Figure 3 presents the AAS data for the amount of Pd deposited on silica as a function of the reduction temperature when the deposition temperature was fixed at 100~ A linear increase of the wt.% of Pd in the reduced samples is observed. This result suggests that once a particular amount of Pd is deposited in the absorption step (related to a certain deposition temperature) higher reduction temperatures favor higher Pd loadings. A possible explanation of that experimental fact is that competing processes may simultaneously occur during the reduction step of preparation such as decomposition and desorption of Pd(hfac)2. Depending on the reduction temperature the balance between these two processes will be different and leads to a

79 different amount of Pd left on the silica surface after the reduction step. Therefore, the AAS data on the amount of Pd deposited on silica demonstrate that by varying the deposition and reduction temperatures of the two-step CVD process Pd/SiO2 samples with variable composition of 0.1 to 0.8 wt.% can be obtained. Highest loadings can be achieved combining low deposition temperatures (100~ and high reduction temperatures (400~ However, it is expected that both parameters will also influence the dispersion of the Pd clusters on the silica surface as well as especially the reduction temperature will have a significant effect on the purity of the deposit. Figure 4. HRTEM picture of Pd clusters formed on Cab-O-Sil, silica at a deposition temperature of 100~ and reduction temperature of 200~

The HRTEM micrograph (Fig. 4) showed that at reduction temperatures 6f 200~ a formation of palladium crystallites with high dispersion is achieved. The average size of palladium crystallites both on silica and alumina supports at these conditions was 5 nm, 95% of the Pd particles with d< 5 nm. Increasing the reduction temperature led to a decreased dispersion of the Pd particles on the silica surface, at 400~ the average particle size being 50 nm. Obviously, the increased reduction temperature is enhancing the surface diffusion and the sintering of the Pd clusters into a larger crystallites. Baerns et al. 1 come to a similar conclusions on the high mobility of Pt clusters when prepared by CVD onto silica support. For catalytic applications the purity of the deposited Pd clusters is of great importance and therefore a information on impurity levels was essential and obtained on the base of XPS characterization of the samples. Table 2 presents the XPS data for the binding energies (BE) for Pd 3d5/2, F Is and some of the C 1s peaks, the atomic concentrations for F and the relative intensities I(Pd 3d)/I (Si 2p) or I(Pd 3d)/I(A12p), for silica and alumina samples, respectively. For all silica supported samples a reduction temperatures of 200~ and higher resulted in F-free samples. For these samples were observed only C ls peaks related to hydrocarbon impurities and identical with these of the silica support but no C ls peaks related to Si-F and CF3 species. The Pd 3d5/2 binding energy for these samples was 335.0 eV which according to the literature data corresponds to Pd(0) oxidation state and which confirmed that reduction temperatures of 200~ and higher result in a F and C impurity-free Pd metal clusters. For the alumina supported samples the 200~ reduction temperature was not sufficient to produce Pd(0) oxidation state; for 1-38 sample a second Pd 3d5/2 peak with binding energy of 337.0 eV was observed which can be related to the presence of Pd(II) oxidation state. For all silica supported samples reduced at temperatures below 200~ in addition to the Pd 3d5/2 peak related to a metal Pd (80% of the total Pd 3d5/2 peak intensity) a second peak appears around 388.8 eV which position only slightly varies for different samples deposited at

80 100~ (Table 2). The Pd 3d5/2 peak position of 388.8 eV is very different from the Pd 3d5/2 binding energy observed for bulk Pd(hfac)2 of 366.3 eV and is very close to the values of 338.0 eV for Pd20 and 338.8 eV for Pd(OAc)2 reported in the literature. A shift toward a higher binding energies can be also expected when a strong electron interaction between the adsorbate and the surface adsorption centers is present. It can be speculated that the second Pd 3d5/2 peak originates from Pd species, product of an incomplete reduction and/or in interaction with the silica surface. For the silica supported samples reduced at temperatures below 200~ F ls peaks at 686.3 eV and 688.1 eV and C is peaks at 292.3 and 289.9 eV were also observed. The C ls peaks at 289.9 eV and 292.3 eV can be related to C-CF3 and CF3 groups, respectively and the F 1s peak at 688.1 eV indicates also presence of CF3 species. All together these data confirm that for F and C -flee deposits of Pd on silica a reduction temperature of 200~ is necessary when a two-step CVD process is applied. Table 2 XPS data for the CVD prepared Pd/SiO2 and Pd/A1203 samples. Sample/ support

Pd 3d5/2 BE [eV]

1-14/silica 1-17/silica 1-16/silica 1-25/silica 1-26/silica 1-30/silica 1-32/silica 1-34/silica 1-35/silica 1-29/silica 1-40/silica 1-44/silica 1-37/alumina 1-38/alumina 1-41/alumina

335.2; 338.2 335.3; 338.3 335.2; 338.3 335.4; 339.1 335.4; 339.1 335.0; 336.9 335.0; 336.8 336.5; 338.8 336.8; 338.8 335.0; . 335.0; . 335.0; . - - ; 336.4 335.0; 337.0 - - ; 336.3

F ls BE [eV] 686.2; 688.6 686.2; 688.1 686.3; 687.9 686.4; 688.1 686.4; 688.1 w ;688.1 1. r. - - ;688.6 w , 688.6 . . . . . . 684.0; 687.5 684.0; 687.5 684.2; 687.6

C ls BE leVI 292.3; 292.7; 292.7; 292.6; 292.6; 292.9; 1. r. 292.9; 293.1; . . .

289.6 289.9 289.9 289.5 289.5 290.4

at.% F

290.4 290.5

0.13 0.47 0.09 --0.07 -0.83 0.09

291.4 291.9 291.9

2.93 7.13 11.73

I(Pd 3d)/ I(Si 2p) 0.026 0.018 0.015 0.013 0.018 0.009 0.012 0.032 0.027 0.010 0.004 0.013 0.014 0.043 0.058

It is interesting to note that when the samples with the same reduction temperature of 120~ but with different adsorption temperatures are compared (samples 1-26, 1-30, 1-32), the ratio between the Pd 3d5/2 peak intensity for Pd(0) at 335.0 eV and Pd(II) oxidation state at 337.0 eV decreases from 80/20 ratio for 100~ adsorption temperature to 50150 ratio for 140~ adsorption temperature. The XPS spectra for sample 1-34 (adsorption temperature of 160~ does not show any peak of metal Pd, instead peaks at 336.5 eV and 338.8 eV appear which can be related to adsorbed Pd(hfac)2 species which were not reduced at temperature of 120~ It can be suggested that the higher the adsorption temperature, the less Pd(hfac)2 species remain adsorbed at the silica surface (AAS data, Fig. 2), however their bonding to the support is stronger and needs higher reduction temperatures to form Pd metal species. The XPS data provided further insight into the dispersion of the Pd on silica and alumina supports by comparing the AAS data for the bulk composition (Pd/Si or Pd/A1 atomic concentrations) and the relative intensities I(Pd 3d)/I(Si 2p) and I(Pd 3d)/I(A1 2p), respectively. According to fundamental XPS studies, 37 the experimental intensities of I(Pd 3d)/I(Si 2p) as a function of the Pd/Si elemental ratio should follow a linear dependence if the deposition is close

81 to monolayer distribution or if the particle size does not vary with the metal loading. A deviation of the experimental values from the theoretical ones for monolayer (1-3 nm particle size) distribution is an indication for formation of larger crystallites. Figure 5 presents the comparison between the theoretical (expected) XPS relative intensities I(Pd 3d)/I(Si 2p) and the experimental ones for series of samples deposited at 100~ and reduced at temperatures varied between 120~ and 400~ Within the experimental error all tree samples reduced at temperatures below 200~ are situated close to the theoretical curve. However, at reduction temperatures of 250 and 400~ the experimental points are deviated significantly which indicates formation of larger Pd crystallites in the lather samples. This result is in agreement with the HRTEM data and it confirms that higher than 200~ reduction temperatures are not favorable for the Pd clusters dispersion. 0.1

Figure 5. XPS relative intensities I(Pd 3d)/I(Si 2p) vs. Pd content expressed as Pd/Si atomic ratio. The solid line on the figure corresponds to the theoretical data for monolayer distribution. Experimental data for samples deposited at 100~ and reduced at different temperatures were compared; 1-26, 129, 1-32, 1-40 and 1-44

I

I

I

I

i

i

I

w

J

0.08t'N

0.06-

m 0.04-

0.02-

A

0

II 0

0.001

~lll

0.002

I

till

0.003

I

A I II

0.004

0.005

Pd/Si atomic ratio

A similar comparison for alumina supported samples leads to a conclusion for high dispersion of Pd clusters. However, the F and C impurity levels for these samples were much higher the for silica supported ones. Obviously, a reduction temperature higher than 200~ should be employed in the two-step CVD process for Pd deposition on alumina. 4. C O N C L U S I O N S Successful deposition of Pd onto silica and alumina high-surface-area supports was demonstrated by two-step CVD process using Pd(hfac)2 as a metal-organic precursor. Controlled deposition was achieved to form variable composition of 0.1 to 0.8 wt.% Pd in the samples. Highly dispersed and uniform Pd metal clusters were deposited by controlling the deposition and reduction temperatures of the CVD process. Higher reduction temperatures favor higher Pd loadings but decrease significantly the metal dispersion. An optimal reduction temperature of 200~ leads to impurities-free Pd metal clusters with high dispersion and uniformity. ACKNOWLEDGMENTS Financial support by CMEM, NSF/University of New Mexico is gratefully acknowledged.

82 REFERENCES

1. M. Baerns, S. Termath and M. Reiche, In Advanced Catalysts and Nanostructured Materials, W. R. Moser (Eds), Academic Press, 1996, pp 479. 2. C. Dossi, R. Psaro, A. Bartsch, E. Brivio, A. Galasco and P. Losi, Catalysis Today, 17 (1993) 527. 3. S.B. Hong, E. Mielczarski and M. E. Davis, J. Catalysis, 134 (1992) 349. 4. C. Dossi, R. Psaro, A. Bartsch, A. Fusi, L. Sordelli, R. Ugo, M. Bellatreccia, R. Zanoni and G. Vlaic, J. Catal., 145 (1994) 377. 5. S.B. Hong, E. Mielczarski and M. E. Davis, J. Catalysis, 134 (1992) 359. 6. J. Goldwasser, S. M. Fang, M. Houalla and W. K. Hall, J. Catalysis, 115 (1989) 34. 7. Y. Tsugo, H. Ooi, M. Yano and Y. Harano, In World Congress III of Chemical Engineering; 1986, pp 310 8. Y. Uemura, Y. Hatate and A. Ikari, J. Chem. Eng. Japan, 22 (1989) 48. 9. N. Katada, H. Ishiguro, K. Muto and M. Niwa, Chem. Vap. Deposition, 1 (1995) 54. 10. M. Niwa, N. Katada and Y. Murakami, J. Phys. Chem., 94 (1990) 6441. 11. M. Niwa, N. Katada and Y. Murakami, J. Catalysis, 134 (1992) 340. 12. G.W. Zajak, C. C. Feng, J. Faber, J. S. Yoo, R. Patel and H. Hochst, J. Catalysis, 151 (1995) 338. 13. M. Niwa, T. Hibino, H. Murata, N. Katada and Y. Mukarami, J. Chem. Soc. Chem. Commun., 289 (1989) 14. C. Dossi, J. Schaefer and W. M. H. Sachtler, J. Mol. Catalysis, 52 (1989) 193. 15. C. Dossi, A. Fusi, S. Recchia, G. Tajana and R. Psaro In ll-th International Congress on Catalysis; Baltimor, USA, 1996, pp 357 16. K.H. Lee, Catal. Lett., 26 (1994) 297. 17. W . M . H . Sachtler, Catal. Lett., 9 (1991) 261. 18. N.W. Cant, J. Catal., 16 (1970) 220. 19. A. Stanislaus and B. H. Cooper, Catal. Rev.-Sci. Eng., 36 (1994) 75. 20. W. Juszczyk, Z. Karpinski, D. Lomot, J. Pielaszek, Z. Paal and A. Y. Stakheev, J. Catalysis, 142 (1993) 617. 21. F. Pinna, M. Selva, M. Signoretto, G. Strukul, F. Boccuzzi, A. Benedetti, P. Canton and G. Fagherazzi, J. Catalysis, 150 (1994) 356. 22. M. Koussathana, D. Vamvouka, H. Economou and X. Verykios, Appl. Catal., 477 (1991) 283. 23. L. Kepinski, M. Worlcyrz and J. Okai, J. Chem. Soc., Faraday Trans., 91 (1995) 507. 24. P.E. Marti, M. Maciejewski and A. Baiker, J. Catalysis, 139 (1993) 494. 25. F.H. Ribeiro, M. Chow and R. A. Dalla Betta, J. Catalysis, 146 (1994) 537. 26. D.O. Simone, T. Kennelly, N. Brungard and R.J. Farrauto, Appl. Catal., 70 (1991) 87. 27. J. Phillips, A. Aurox, G. Bergeret, J. Massardier and A. Renouprez, J. Phys. Chem., 97 (1993) 3565. 28. E.L. Lakomaa, Appl. Surf. Sci., 75 (1994) 185. 29. J. Farkas, M. J. Hampden-Smith and T. T. Kodas, J. Phys. Chem., 98 (1994) 6753. 30. R.K. Iler, The Chemistry of Silica, John Wiley and Sons, New York, 1979 31. A.V. Kiselev and V. I. Lygin, Infrared Spectra of Surface Compounds, John Wiley and Sons, New York, 1975 32. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley and Sons, New York, 1986 33. A.R. Siedle, Journal of Organometallic Chemistry, 208 (1981) 115. 34. W. Lin, B. C. Wiegand, R. G. Nuzzo and G. S. Girolami, J. Am. Chem. Soc., 115 (1993) 11644. 3 5. V. Bhaskaran, M. J. Hampden-Smith and T. T. Kodas, Chemical Vapor Deposition, 3 (1997) 281. 36. J.C. Kenvin, M. G. White and M. B. Mitchell, Langmuir, 7 (1991) 1198. 37. F . P . J . M . Kerkhof and J. A. Moulijn, J. Phys. Chem., 83 (1979) 1612.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

83

P r e p a r a t i o n of P d / C e O 2 C a t a l y s t for M e t h a n o l D e c o m p o s i t i o n Yoshikazu Usami a'*, Kenkichi Kagawa a, Masanobu Kawazoe a, Yasuyuki Matsumura b, Hiroaki Sakurai b and Masatake Haruta b aMechanical Engineering Laboratory, Daikin Industries Ltd., Kanaoka-cho, Sakai, Osaka 591-8511, Japan bOsaka National Research Institute, AIST, Midorigaoka, Ikeda, Osaka 563-8577, Japan Palladium supported on cerium oxide of which surface area is as high as 143 m2g~ has been prepared by coprecipitation with sodium carbonate as a precipitant, and it effectively catalyzes the decomposition of methanol to carbon monoxide and hydrogen at 180-220~ in comparison with that prepared by coprecipitation with sodium hydroxide. The alloy of palladium and cerium whose structure is similar to PdgCe is present in the catalyst. 1. INTRODUCTION

Cerium oxide is often employed as a support of catalytically active metals mainly because of its redox behavior and the electronic property [1]. Palladium supported on cerium oxide catalyzes hydrogenation of carbon oxides [2-7] and strong metalsupport interaction (SMSI) has been reported [1,6-11]. Palladium also catalyzes the methanol decomposition to carbon monoxide and hydrogen as well as nickel or platinum [12-18]. The decomposition, which is endothermic, can be applicable to recovery of the waste-heat less than 200~ from industries [19], while methanol is expected to be an alternative energy carrier in the near future [20,21]. Another application is the methanol fueling system for vehicles (on-board reforming) to increase the fuel efficiency and reduce by-production of formaldehyde [22]. It has been reported that the catalytic activity of palladium is largely affected by its support, suggesting that the interaction between the metal and the support is important [12-16]. We have also found that cationic palladium species generated by the interaction with the support are effective in the reaction [17,18]. The cationic species can be observed in palladium supported on cerium oxide, however, the activity was not so high as palladium supported on zirconium oxide probably because the dispersion of palladium on cerium oxide was lower than on zirconium oxide [18]. In this report we have prepared palladium supported on cerium oxide with high surface area by a coprecipitation method and show that Pd-Ce alloy formed in the catalyst is the active species of the methanol decomposition.

84 2.

EXPERIMENTAL

Palladium catalysts supported on cerium oxide were prepared by coprecipitation from an aqueous mixture of PdC12 (Kishida Chemical Co. Ltd., GR grade) and Ce(NO3) 3 6H20 9 (Kishida, GR) with an sodium carbonate or sodium hydroxide solution. The precipitates were washed with distilled water several times and dried for 30 h in vacuum at room temperature; then, calcined in air for 5 h at 500~ The samples precipitated with sodium carbonate and with sodium hydroxide will be denoted as Pd-SC and Pd-SH, respectively. Catalytic tests were performed with a fLxed bed continuous flow reactor. A catalyst (1.0 g) was sandwiched with quartz wool plugs in a quartz tube reactor which were inert under the reaction conditions. The catalyst was reduced in a stream of 10% hydrogen diluted with argon at 325~ for 1 h; then, methanol diluted with argon (concentration, 20 vol%; total flow, 2.5 dm 3 h ~) was fed at 180-220~ The reactant and products were analyzed with two on-line gas chromatographs (Okura Riken Co. Ltd., type 802 with TCD). An activated carbon column (2 m) was used for the analysis of hydrogen, carbon monoxide, methane, and carbon dioxide, and Porapak T (2 m) was used for methanol, dimethyl ether, and methyl formate. The BET surface areas of the catalysts were determined by nitrogen physisorption with a Quantasorb JR (Yuasa Ionics Co. Ltd.). Powder X-ray diffraction (XRD) patterns of the catalysts were recorded with a Rigaku Rotaflex 20 diffractometer using nickel-filtered CuKa radiation. Mean crystallite sizes were determined from the widths of XRD peaks using the equation of Sherrer [23]. X-ray photoelectron spectra (XPS) were measured with a Shimadzu ESCA750. The sample was argonion sputtered for 0.5 min at 2 kV and 20 mA just before the measurement. Binding energies were determined by reference to the C ls binding energy of 284.6 eV[24]. 3. R E S U L T S 3.1. M e t h a n o l d e c o m p o s i t i o n Methanol was decomposed to carbon monoxide and hydrogen over the palladium catalysts at 180, 200, and 220~ (Figure 1). Small amounts of carbon dioxide were by-produced with the selectivity of ca. 5% at 200~ over 30 and 35 wt% Pd-SC while the selectivities to carbon dioxide over other catalysts were always less t h a n 3%. The formation of carbon dioxide was gradually reduced with an increase in the timeon-stream. No formation of methane was observed. The catalytic activities of 10 and 15 wt% Pd-SH were lower t h a n those of Pd-SC with the same palladium contents. The activity of Pd-SC reached almost plateau at the palladium content of 15 wt% while 30 and 35 wt% Pd-SC produced considerably lower conversions of methanol t h a n those for 15-25 wt% Pd-SC. 3.2. BET s u r f a c e a r e a The BET surface areas of the palladium catalysts taken out from the reactor after the reaction roughly corresponded to the methanol conversions produced (Figure 2). Although the surface areas of 5 wt% Pd-SC was only 44 m2g1, that of 10 wt% Pd-SC was as high as 119 m2g1 and the maximum value of 143 m2g1 was observed with 15

85

150

80

2O

T f,-

60-

O.

E

0 .=.=

-15 ~

~100

L_ 0

-

O O

q,1

"40 0 o

- 10r ,-,ii

-

0

o

..,.~ o c

50-

,--20 -

5

ID

k1.1.1 111 0

0 Pd

I

I

I

10

20

3o

content

/

!

._L

0

4o

wt:~

Figure 1. Methanol decomposition over Pd/CeO 2. Open symbols, Pd-SC; solid symbols, Pd-SH.

3

0

i

i

i

10

20

30

o Z.O

Pd content / wt~

Figure 2. BET surface area and palladium surface area of Pd/CeO 2. Open symbols, Pd-SC ; solid symbols, Pd-SH.

9 -1

wt% Pd-SC. The areas of 10 and 15 wt% Pd-SH were 57 and 64 m'g , respectively, significantly lower t h a n those for 10 and 15 wt% Pd-SC. 3.3. X - r a y d i f f r a c t i o n The typical XRD patterns of the palladium catalysts after the reaction were shown in Figure 3. In the patterns of 30 and 35 wt% Pd-SC a broad peak at 39.4 ~ in 20 was observed while a peak at 45.7 ~ was separated from the peak at 47.2 ~ attributed to cerium oxide using Gaussian function. The broad peak was recorded at 39.1 ~ in the patterns of Pd-SH and another peak was present at 45.5 ~ The peak at 39.1 ~ was also seen in the patterns of 15-25 wt% Pd-SC, but the peak intensity was weak. The peak at 33.0 ~ can be attributred to cerium oxide. The crystallite sizes of cerium oxide and the species responsible for the peak at 39 ~ were estimated from the widths of the XRD peaks, respectively (Table 1). The intensity ratio of these peaks at 39 and 33 ~ (Ied.ce/Iceo2) increased with an increase in the palladium content of Pd-SC. 3.4. X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y Surface analyses by XPS were carried out with the catalysts after the reaction. The typical spectra of Pd 3d were shown in Figure 4. The binding energy of Pd 3d5/2 were 335.4-335.6 eV regardless of the samples, and appreciably higher t h a n that for metallic palladium (Pd black) of 335.0 eV. The widths of the Pd 3d5/2 peaks for 30 and 35 wt% Pd-SC were considerably smaller than those for the other samples (see Table 1). The XPS peaks of O ls and Pd 3p3/2 for the catalysts were deconvoluted

85 Table 1 Physical properties of Pd/CeO 2 prepared by coprecipitation method Catalyst

5 wt% 10 wt% 15 wt% 20 wt% 25 wt% 30 wt% 35 wt% 10 wt% 15 wt%

Crystallite size/nm Intensity ratio c Atomic ratio d CeO2a Pd-Ce b Ipd.Ce/Iceo2 Pd/O Ce/O

Pd-SC Pd-SC Pd-SC Pd-SC Pd-SC Pd-SC Pd-SC Pd-SH Pd-SH

10 7 5 5 5 5 5 9 9

6 5 5 7 7

0 (0.09) 0 (0.18) 0.1(0.29) 0.2(0.40) 0.5(0.54) 0.9(0.69) 1.6(0.87) 0.2(0.18) 0.3(0.29)

0.029 0.057 0.079 0.12 0.12 0.16 0.27 0.033 0.078

0.40 0.45 0.43 0.43 0.33 0.31 0.48 0.37 0.43

FWHM of Pd 3d5t2 / eV 2.9 2.9 3.0 3.0 2.9 2.0 2.1 4.1 3.8

aSize determined from the peak at 33 ~ bSize determined from the peak at 39 ~ CIntensity ratio of the peaks at 39 and 33 ~ In parentheses the atomic ratio of Pd/Ce calculated from the chemical composition was given. aAtomic ratio calculated from the intensities of the XPS peaks for Pds/2 (2.0), Ce 4d (1.29), and O ls (0.63).

30

35

40

45

50

2e / degree Figure 3. XRD of Pd/CeO 2 after the reaction.

into three Gaussian peaks at 529.4-529.6, 531.6, and 533.1-533.4 eV. The typical spectra were shown in Figure 5. The peak of Pd 3pa/2 at 533 eV was separated assuming that the size of the peak is 31% of that of the corresponding peak of Pd

5 wtX Pd-SC

d

15

w

~

.,,,.

f-

r-

r ...,,

_.c

35 wtg Pd-SC

15

350

wtgPd-SH I

345

'"}

340

25

35 540

wt~ Pd-SC

I

335

I

330

Binding energy / eV

325

Figure 4. XPS of Pd 3d for Pd/CeO 2.

I

535

I

530

I

525

Binding energy / eV

Figure 5. XPS of O ls and Pd 3p~/2 for Pd/CeO 2.

3d~/2 [25]. The former two peaks are attributed to O ls of cerium oxide because the peaks were present even in the spectrum of 5 wt% Pd-SC in which the intensity of Pd 3d was weak. Redox behavior of cerium oxide is well known and the two O ls peaks may reflect the complicated electronic state of cerium. No significant change in the spectra of Ce 4d at 107-130 eV was observed with the change in the content of palladium (not shown). The surface concentrations of palladium, oxygen, and cerium were determined from the peak intensities of Pd 3d5/2, Ce 4d, and O ls assuming the atomic sensitivity factors of 2.0, 1.29, and 0.63, respectively (see Table 1)[24]. 4.

DISCUSSION

4.1. F o r m a t i o n of Pd-Ce alloy The XRD peaks attributed to metallic palladium are reported to be present at 40.0 and 46.5 ~ and the lattice parameter, a (cF4), is 0.389 nm [26]. Although the intensities of the broad peaks at 39 and 46 ~ depend on the palladium contents of PdSC, the peak positions are appreciably smaller t h a n those for metallic palladium, suggesting t h a t the palladium species formed is different from palladium metal. The lattice parameter, a (cF4), can be calculated as 0.398 nm from the peaks at 39.1 and 45.5 ~ for 15-25 wt%Pd-SC and 10 and 15 wt% Pd-SH. The peak positions for 30 and 35 wt% Pd-SC are 39.4 and 45.7 ~ and the p a r a m e t e r is 0.395 nm. K~pifiski et al. reported t h a t the alloy of palladium and cerium can be formed in 9 wt% Pd/CeO 2 reduced with hydrogen at 500~ or above, and the XRD peaks are present at 39 and 46~ The peak positions are almost the same as those for the broad peaks observed, strongly suggesting that the broad peaks are attributed to the Pd-Ce alloy.

88 The lattice p a r a m e t e r of PdgCe, a (cF4), is 0.398-0.400 nm [26], indicating t h a t the crystalline structure of the alloy is similar to that of PdgCe but the content of cerium in the alloy m a y be smaller t h a n that of PdgCe. The intensity of the peak at 39 ~ increases with an increase in the palladium content of Pd-SC, but the intensity ratio of Ipd.cJIceo2 is not proportional to the composition ratio of Pd/Ce (see Table 1). The intensity ratios for 5-25 wt% Pd-SC are significantly lower t h a n those expected from the ratios for 30 and 35 wt% Pd-SC, although the crystallite sizes of Pd-Ce alloy in 20 and 25 wt% Pd-SC are similar to those for 30 and 35 wt% Pd-SC whose XPS peak of Pd 3d5/2 can be attributed to the Pd-Ce alloy. The binding energy of Pd 3d5/2 for CeTPd 3 is reported as 336.6 eV, showing t h a t presence of cerium in palladium results in an increase in the binding energy from t h a t of the metal [25]. Since the XPS peaks for the latter two samples are sharper t h a n those for 5-25 wt% Pd-SC (see Table 1), the Pd-Ce alloy in 30 and 35 wt% Pd-SC are supposed to be homogeneous, that is, the alloy in the two catalysts are well crystallized. The binding energy of Pd 3d5/2 for 5-25 wt% Pd-SC is almost the same as that for 30 and 35 wt% Pd-SC, suggesting the interaction between palladium and cerium. However, the lattice parameter is discernibly larger t h a n t h a t for 30 and 35 wt% Pd-SC. Hence, it is supposed that the Pd-Ce alloy in 5-25 wt% Pd-SC is not so crystallized as in 30 and 35 wt% Pd-SC. The poor intensity of the peaks attributed to the alloy suggests that major part of the alloy is present as very small particles. In the case of 15 wt% Pd-SH the XPS peak of Pd 3d5/2 is broad, suggesting presence of small palladium particles containing cerium although the XRD peak at 39.1 ~ shows presence of fairly large particles of Pd-Ce alloy. Since the intensity of the XRD peak at 39.1 ~ for 10wt%Pd-SH is significantly smaller t h a n t h a t for 15wt%Pd-SH, the major part of palladium species are present as small particles which probably contain cerium. 4.2. A c t i v e s p e c i e s o f m e t h a n o l d e c o m p o s i t i o n In the cases of the metal catalysts of which support is cerium oxide, the determination of the metallic surface area is difficult by means of hydrogen or carbon monoxide adsorption because the support also adsorbs and/or consumes the adsorbates. We have estimated the surface area of palladium on the basis of the BET surface area and the surface concentration of palladium determined by XPS, assuming t h a t the atomic cross sections are 0.060 nm 2 for Pd, 0.037 nm 2 for Ce 4§ and 0.050 nm 2 for 02. [18]. That is, the surface area of palladium (Spd) is calculated by the equation, Sed = SBET X aed • %d / (aed • %d + ace • ace "}" CO)' where SBE T is BET surface area, aed and ace are atomic ratios of Pd/O and Ce/O, %d, CCe,and co are the cross sections (see Figure 2). We confirmed t h a t similar metallic surface areas can be obtained by this method (7.2 m2g1) and the hydrogen adsorption (6.0 m2g1) in the case of 15 wt% Pd/ZrO 2 prepared by a coprecipitation method [18]. It was confirmed t h a t the catalytic activity of 15 wt% Pd-SH is discernibly lower than that of 15 wt% Pd/ZrO 2 [18] and the palladium surface area of 15 wt% Pd-SH is calculated as 4.2 m 2 g .1 However, the palladium surface area of 15wt%Pd-SC is 9.6 meg ~ and the activity is significantly larger. The catalytic activity of palladium supported on cerium roughly depends on the metallic surface area as shown in Figure 6 in which the methanol conversion at 180~ is employed as the parameter of the activity.

89 40

25 wt~ o

%

30

20

-

'~20 -

15 wt~ ~

'

30 wt~

A

Wt~

f

35 wt~

10 wt~

o 0

0

I

i

I

5

10

15

"

2O

Pd surface area / m2g -1

Figure 6. Relationship between the palladium surface area and the catalytic activity. Open symbols, Pd-SC; solid symbols, Pd-SH.

However, in the cases of 30 and 35 wt% Pd-SC (square symbols in Figure 6) which contain well crystallized Pd-Ce alloy, the catalytic activities are lower than those expected from the plot for 5-25 wt% Pd-SC (solid curve in Figure 6). Hence, it can be supposed that the surface of well-crystallized Pd-Ce alloy is less active. 5. CONCLUSIONS Palladium supported on cerium oxide with high surface as high as 143 m 2 g-1 can be prepared by the coprecipitation with sodium carbonate as a precipitant. Formation of the alloy of palladium and cerium whose structure is similar to PdgCe takes place by the reduction of the catalyst. The crystallite size of the alloy seems to be very small when the surface area of the catalyst is large. The catalytic activity toward the methanol decomposition to carbon monoxide and hydrogen at 180-220~ is mainly depend on the surface area of the alloy while the alloy with smaller particle size may effective to the catalysis. ACKNOWLEDGMENT A part of authors (Y.U., K.K., and M.K.) gratefully acknowledge the financial support by the New Energy and Technology Development Organization.

REFERENCES 1. A. Trovarelli, Catal. Rev., 38 (1996) 439. 2. C. Sudhakar and M.A. Vannice, J. Catal., 95 (1985) 227.

90 3. 4. 5. 6.

M.D. Mitchell and M.A. Vannice, Ind. Eng. Chem. Fundam., 23 (1984) 88. M.A. Vannice, C. Sudhakar and M. Freeman, J. Catal., 108 (1987) 97. L. Fan and K. Fujimoto, Appl. Catal. A, 106 (1993) L1. F.L. Normand, J. Barrault, R. Breault, L. Hilaire and A. Kiennemann, J. Phys. Chem., 95 (1991) 257. 7. L.Fan and K. Fujimoto, J. Catal., 150 (1994) 217. 8. F.L. Normand, L. Hilaire, K. Kill, G. Krill and G. Maire, J. Phys. Chem., 92 (1988) 2561. 9. C. Binet, A. Jadi and J.-C. Lavalley, J. Chem. Soc. Faraday Trans., 88 (1992) 2079. 10. A. Bensalem and F. Bozon-Verduraz, J. Chem. Soc. Faraday Trans., 91 (1995) 2185. 11. L. K~pifiski, M. Wolcyrz and J. Okal, J. Chem. Soc. Faraday Trans., 91 (1995) 507. 12. H. Niiyama, S. Tamai, J.S. Kim and E. Echigoya, J. Jpn. Petrol. Inst., 24 (1981) 322. 13. Y. Saitoh, S. Ohtsu, Y. Makie, T. Okada and Y. Terumuma, Bul. Chem. Soc. Jpn., 63 (1990) 108. 14. D.T. Wickham, B.W. Logsdon and S.W. Cowley and C.D. Butler, J. Catal., 128 (1991) 198. 15. N. Iwasa, O. Yamamoto, T. Akazawa, S. Ohyama and N. Takezawa, J. Chem. Soc., Chem. Commun., (1991) 1322. 16. N. Fukuhara, S. Sekiguchi, H. Mutoh and T. Igarashi, Kagaku Kogaku Ronbunshu, 21 (1995) 1002. 17. Y. Matsumura, M. Okumura, Y. Usami, K. Kagawa, H. Yamashita, M. Anpo and M.Haruta, Catal. Lett., 44 (1997) 189. 18. Y. Usami, K. Kagawa, M. Kawazoe, Y. Matsumura, H. Sakurai and M. Haruta, Appl.Catal. A, under submission. 19. Reports of Eco-Energy City Project, New Energy and Industrial Technology Development Organization and The Energy Conservation Center, Tokyo, 1997. 20. W. Keim (ed.), Catalysis in C1 Chemistry, Reidel Publishing Company, Dordrecht, 1983. 21. J.M. Fox III, Catal. Rev.-Sci. Eng., 35 (1993) 169. 22. National Research Council, Catalysis Looks to the Future, National Academy Press, Washington, D.C., 1992. 23. H.P. klug and L.E. Alexander, X-ray Diffraction Procedures, John Wiley and Sons, Inc., New York, 1954. 24. C.D. Wagner, W.M. Riggs, L.E. Davis, and J.F. Moulder, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp. Minnesota, 1978. 25. D. Briggs and M.P. Seah (eds.), Practical Surface Analysis, Vol. 1, John Wiley & Sons, New York, 1990. 26. P. Villars and L.D. Calvert, Pearson's Hand book of Crystallographic Data for Intermetallic Phases, Second Edition, Vol.2, The Materials Information Society, Okurahoma, 1991.

1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmon et al., editors.

91

S u p p o r t e d A q u e o u s Phase Catalysts for the C - C and C - N B o n d F o r m a t i o n S. dos Santos a, F. Quignard a, D. Sinou b and A. Choplin a* a Institut de Recherches sur la Catalyse - CNRS, 2 av. A. Einstein, 69 626 Villeurbanne C6dex, France b Laboratoire de Synth~se Asym6trique - CNRS, Universit6 Lyon 1,43 Bd du 11 novembre 1918, 69 622 Villeurbanne Cedex, France

ABSTRACT The catalysts, generated in situ from Pd(OAc)2/5tppts/H20 deposited on the surface of a mesoporous silica are efficient for the reactions of C-C (Trost-Tsuji reaction) and C-N bond formation. Their properties are dependent upon the nature of the nitrile solvent and the water content of the solid. The most performant system has a water content comprised between 30 and 50 % wt.: it is highly selective and stable under the reaction conditions. Recycling tests evidence no detectable Pd leaching, but partial water leaching from the solid into the organic phase; regeneration of the catalyst is thus possible by controlled water addition. 1. INTRODUCTION The immobilization of homogeneous organometallic catalysts is still nowadays an economical and environmental challenge. Different methods were described, among which the most promising are those which maintain the integrity of the active complex (same ligand environment, same degree of oxidation of the metal) so as to preserve its catalytic properties in terms of activity and selectivity [1]. Among them, biphasic catalysis appears as the most efficient one and the systems using water as the solvent of the catalyst are particularly appealing; this technology is already used on an industrial scale [2]. Nevertheless, this approach is not universal; for example in the case of the use of a two phase water/organic system, the insolubility of some substrates in water, the sensitivity of some products and/or of the catalyst towards water under the reaction conditions are severe drawbacks. Some of the constraints could be overcome by supporting the catalyst in a thin film of water on a hydrophilic, high specific surface area solid. This method which leads to the so-called Supported Aqueous Phase Catalysts, SAPC, was tested for various reactions such a ~, hydroformylation of heavy [3-5] and functionalized olefins [6], selective hydrogenation of c~,13 unsaturated aldehydes [7] and enantioselective hydrogenation of 2-(6-methoxy-2'-naphtyl) acrylic acid, a Naproxen precursor [8]. We have recently adapted this methodology to palladium (0) phosphinated complexe.~ and tested the related SAP catalysts for the so-called Trost-Tsuji reaction [9]; we have now * Acknowledgement: Dr Y. Tong and Dr J.P. Dutasta are thanked for their contribution to the NMR studies and many fruitfidl discussions.

92 characterized these catalytic systems, determined the parameters essential to reach high activities and tested the recyclability of the solid. 2. RESULTS 2.1. The catalytic system The precise catalyst precursor used in the biphasic system [10], i. e. Pd(OAc)2/ 5 tppts (tppts: sodium salt of tri(m-sulfophenyl)phosphine) was heterogenized by depositing it on the surface of a mesoporous, high specific surface silica using the incipient wetness impregnation technique with water as the solvent (silica from Grace, r(pores): 12 nm, SBET: 185 m2/g.). All manipulations were performed under strict exclusion of oxygen. The reduction of Pd(II) to Pd(0) by the phosphine is not completed at the time when the catalyst solution is poured on to the silica, as is evidenced by 31p NMR studies. Figure la shows clearly the presence of Pd(OAc)2(tppts)2 (6:29.1 ppm), of Pd(tppts)3 in fast exchange with free tppts (6:10 ppm) and of oxidized tppts (6:34.6 ppm), which results from the reduction of Pd(II) to Pd(0) [11]. After evacuation (10 -4 tort), the solid contains typically 3 % wt. H20 and the palladium content is close to 0.76 % wt. ("dry" solid). More than 80 % of the palladium is in the reduced state on the solid, as determined by 3 Ip MAS NMR data (Fig. lb, c). Catalysts with different water contents were obtained by adding via a syringe and under stirring, controlled amounts of deaerated water to aliquots of the same dry solid sample. The narrowing of the 31p MAS NMR peaks with increasing water content of the solid suggests an increase of the mobility of the complex on the surface of silica, which becomes significant above ca. 20 % wt H20. No specific interactions between the complex and the silica could be detected.

b

__L_ 35

I

1

15

i

c

!

40

I

I

20

I

I

0

I

40

I

I

20

o

I

0 ppm

Figure 1.31p NMR spectrum ofPd(OAc)2/5tppts (a) in D20 ' after 85 min; (b) on silica ("dry" SAP catalyst) (c): same sample as in (b), immediately after introduction of water on the sample. 2.2. The catalyzed reactions

In order to test the properties of the SAP catalysts for allylic substitution type reactions and identify some of the pertinent parameters, we have chosen typically the reaction of (E)' cinnamyl ethyl carbonate (1) with two different nucleophiles, ethylacetoacetate (2a) (C-C bond formation) and lnorpholine (2b) (C-N bond formation) (eq.):

93

P ~ h , ~ ~ j , O C O 2 E t + Null 1

2a-b

Pd(0) Ph ~ N u

+ EtOH + CO2

3a-b /--X

Nu: 3a:-CH(COMe)CO2Et; 3b: --N\

/

O

Acetonitrile and benzonitrile, which differ rater alia by their miscibilities with water, were used as solvents. The SAP catalysts were tested under the following experimental conditions: m(cata): 200 rag, [1]~ 5 10-2 mol.L -1, [1]/[NuH]/[Pd] = 25/30/1. Their performances were compared with those of the related biphasic system under the same conditions, except for the amount of water which was such that nitrile/water= 1/1, to allow for the solubilization of the catalyst. The carbonate conversion and the yield in monoalkylated product were determined by GPC, calibrated against authentic samples of the reactants and the products. 2.2.1 Allylic substitution with ethylacetoacetate For the reaction of (E)-cinnamyl ethyl carbonate (1) with ethylacetoacetate (2a), the behavior of the SAP catalysts is sharply dependent on the nature of the nitrile solvent. Thus, with CH3CN, they show an activity quite similar to that of the homogeneous system (Fig. 2), but they present two main advantages: a greater stability of the active complex and a higher selectivity for the desired monoalkylated product 3a (ca. 100 %). Under homogenous conditions, the formation of metal particles is observed after 6 h at 80 ~ and cinnamyl alcohol is produced with a 50 % selectivity. Interestingly, the SAP catalysts show no significant dependence of their activity on the water content.

Figure 2. Influence of the water content of SAP Pd(OAc)2/5 tppts on its catalytic properties for the allylic alkylation of 1 with 2a in CH3CN , % conversion of 1 (shaded boxes), % yield of 3a (blank boxes), % yield of cinnamyl alcohol (black box), tr = 8 h, T = 80 ~

94

1Cs 90 8O 70

40

2O 10 0

50

I00

150

200

250

t (min)

Figure 3. Influence of the water content of SAP Pd(OAc)2/5 tppts on its catalytic properties for the allylic alkylation of 1 with 2 in PhCN. Water content (%): r3:3.4; 0: 23.1; A: 32.8;==: 41.5; , : 48.7; e: 65.4. T = 80 ~ When the same reaction is performed in PhCN as the solvent (Fig. 3), two remarkable phenomena take place. Firstly, the activity of the SAP catalyst is much higher than on one hand that of the SAP solid in CH3CN and on the other hand that of the same system under biphasic H20/PhCN conditions. Thus, total carbonate conversion is achieved within 1 h in PhCN, while 24 h are needed in CH3CN with the same SAP catalyst (30 % wt. H20 ) (Fig. 2, 3); under biphasic PhCN/H20 conditions, no significant activity of the catalytic system is detected. Secondly, the activity of the SAP catalyst is sharply dependent upon the water content of the solid: it reaches a maximum for water contents comprised between 30 and 50 % wt.

2.2.2. Allylic substitution with morpholine The SAP catalyst Pd(OAc)2/5 tppts/H20/silica shows a very high level of activity for the alkylation of (E)-cinnamyl ethyl carbonate (1) with morpholine; with the most active system, carbonate conversion is completely achieved within 5 rain. As in the case of ethylacetoacetate as the nucleophile, the activity of the SAP catalyst is lower using CH3CN as the solvent, but higher in PhCN than that of the related homogeneous systems. Figure 4 shows the yield of product of alkylation 3b obtained after 10 rain of reaction as a function of the water content of the solid, using CH3CN and PhCN as the solvent respectively. In CH3CN, there is a continuous activity increase up to the level of activity of the related homogeneous system. In PhCN, the activity of the SAP catalyst is always higher than that of the biphasic system and a maximum is reached for water contents comprised between 30 and 50 % wt. Under all conditions, a 100 % selectivity for the desired monoalkylated compound 3b is reached, even under liquid phase conditions: the absence of formation of cinnamyl alcohol could be correlated to the lower temperature needed for these experiments (T = 50

~

95

100 90

"~

80 70

J

,50

[]

4o

[]

:30 20 10 0

i

0

20

40

60

I

80

10o

%1/20

Figure 4. Influence of the water content of the solid SAP on the yield of 3b after t r -- 10 min, T= 50 ~ IZl: in CH3CN, 9 in PhCN. % yield of 3b with the related homogeneous systems (nitrile/water: 1/1): __; PhCN;--- CH3CN. 2.3. Recycling of the catalysts The SAP methodology presents some interest if, i , ter a#a, the catalytic solid can be easily recycled. A first experiment (Fig. 5) shows a considerable drop of the activity of the SAP sample containing 65% wt. H20, in the reaction of alkylation of 1 by morpholine, using CH3CN as the solvent.

100 8O 60

"~ 40 'y,, 20 0

I

0

50

I

"

100

I

150

-

-t

200

t (rain)

Figure 5. Recycling test for solid SAP (0.75 % Pd, 65 % H20 ). yield % of 3 b , under the following conditions: mcata~200 rag, T= 50 ~ solvent CH3CN , rn. first run; 9second run.

96 The reasons for this decrease of the activity may be numerous: loss of Pd, partial degradation of the active complex and/or loss of water from the solid during the reaction and/or during the recycling procedure (which involves washing of the solid with the nitrile). The first cause can be ruled out on the basis of the results from the analysis of the catalytic solution at the end of the test: less than 0.4 % of the Pd present on the solid was extracted. This absence of significant leaching is further confirmed by the inactivity of this same solution towards a new dose of reactants. The leaching of water is the most probable explanation; it was indeed confirmed by an experiment using PhCN as the solvent. Analysis of the supernatant solution shows the absence of detectable amounts of Pd. Nevertheless, a significant drop of activity is observed when the solid is recycled by simple filtration (Table 1, entries 1-3).

Table 1. Recycling experiments for SAP Pd catalysts. entry

% H20 (solid)

run

t r (min)

cony. (%)

% H20 solution

1 2 3 4 5 6 7 8 9

49

1 2 3 4* 1 2 4 5** 6

5 5 10 5 90 90 90 90 90

100 68 45 100 100 92 68 97 68

0.66 0.17 0.17 nd nd nd nd nd nd

2.3

Experimental conditions: m(cata) ~ 300 rag, solvent: PhCN (anhydrous), [carbonate] ~3.1 x 10 -2 mol.L-1; [morpholine]~3.7 x 10 -2 mol.L "1, T = 50 ~ Between two runs, the solid is filtered and washed twice with 6 mL of anhydrous PhCN. *: addition of 256 gL H20. ** addition of 10 gL of H20

Analysis of the solution at the end of the test shows the presence of a water amount superior to that present initially in PhCN (H20 < 0.005%). After three recyclings, the activity drop is such that the solid is now roughly as active as a solid with c a . a 10% water content. Interestingly, its activity is fully restored by addition of a small amount of water (Table 1, entry 4). Similar recycling tests were performed on a "dry" solid (2.3 % wt. H20 ) (Table 1, entries 5-7); as expected the decrease in activity is significantly less important and again addition of a small amount of water to the solid after 4 runs regenerates the activity of the catalyst (Table 1, entr'J 8). 3. DISCUSSION AND CONCLUSIONS Immobilization of the biphasic catalytic system Pd(OAc)2/5 tppts/H20 on the surface of a mesoporous silica leads to an efficient catalyst for the alkylation of allylic substrate, the sc~called Trost-Tsuji reaction. Its efficiency is sharply dependent on the nature of the nitrile used as the solvent. With CH3CN, the activity of the Supported Aqueous Phase Catalyst is lower

97 than that of the related liquid phase homogeneous catalyst: this is probably due to the total miscibility of water and CH3CN under the reaction conditions. Indeed, supporting the catalyst on silica lowers considerably the interphase surface area, which is then equal only to that of silica (185 m2/g) while it is infinite under homogeneous conditions. The SAP methodology is most interesting with PhCN as the solvent. The activity of the SAP catalysts is then greater than that of the related truly biphasic system. This phenomenon can again be explained by differences in the interphase surface areas which is sharply increased for the SAP catalyst via the silica surface which immobilizes the aqueous phase. The activity of the SAP catalysts reaches a maximum when the water content of the solid is comprised between 30 and 50 % H20. This amount is close to that which is necessary to achieve the pore filling of the silica used. Nevertheless this same amount is also optimum when a non porous silica [ 12] is used as support. Above this amount, we observed de visu that numerous problems are encountered: aggregation of the solid particles, yellow colouration of the solution at the end of the catalytic experiment suggesting Pd leaching, shallow solution suggesting water leaching from the solid into the organic phase. Recycling the SAP catalysts is possible, but water leaching from the solid leads to lower performances. We are currently investigating different approaches which would keep the water content as constant as possible: however many parameters have to be determined, including the partition coefficient of water between silica and the nitrile solvent, both with and without the catalytic system, the reactants and the reaction products. Only then, can one safely envisage to use this kind of methodology for industrial applications in continuous flow reactors. References

1.

F.R. Hartley, in Supported Metal Complexes, D. Reidel, Dordrecht, 1985; A. Choplin and F. Quignard, Coord. Chem. Rev. (1998) (m press) and references therein. 2. E. Wiebus and B. Cornils, Chem.-Ing.-Tech., 66 (1994) 916. 3. J.P. Arhancet, M. E. Davis, J. S. Merola and B. E. Hanson, Nature, 339 (1989) 454. 4. I.T. Horvath, Catal. Lett., 6 (1990) 43. 5. I. Gou, B. E. Hanson, I. Toth and M. E. Davis, J. Mol. Catal., 70 (1991) 363. 6. G. Fremy, ; Monflier, E.; Carpentier, J. F. Castanet, Y. and Mortreux, A. J. Catal., 162 (1996) 33 9. 7. E. Fache, C. Mercier, N. Pagnier, B. Despeyroux and P. Panster, J. Mol. Catal., 79 (1993) 117. 8. K.T. Wan, M. E. Davis, J. Catal. 152 (1995) 25. 9. P. Schneider, F. Quignard, A. Choplin and D. Sinou, New J. Chem., 20 (1996) 545. 10. E. Blart, J.-P. Gen6t, M. Sail, M. Savignac and D. Sinou, Tetrahedron, 50 (1994) 505. 11 C. Amatore, E. Carre, A. Jutand and M. A. M'Barki, Organometallics, 14 (1995) 1818. 12. S. dos Santos, F. Quignard, A. Choplin, D. Sinou and J. P. Dutasta, Organometallics, (1998) (in press)

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

99

Preparation of m i c r o s c o p i c catalysts and colloids for catalytic nitrate and nitrite reduction and their use in a hollow fibre dialyser loop reactor M. Hiihnlein, U. Prtil3e, J. Daum, V. Morawsky, M. Kr6ger, M. Schr6der, M. Schnabel and K.-D. Voflop Federal Agricultural Research Centre (FAL) Institute of Technology, Bundesallee 50, 38116 Braunschweig, Germany

Microscopic catalysts and polymer stabilized colloids were tested in catalytic nitrate and nitrite reduction. The preparation conditions for the bimetallic catalysts tested in nitrate reduction strongly influence the catalytic properties. An influence of the stabilizing polymers used for the preparation of the colloidal sols on the catalytic properties was found, too. For continuous flow experiments, both microscopic catalysts and colloids were tested in the hollow fibre dialyser loop reactor. 1.

INTRODUCTION

Microscopic catalysts and colloids often show markedly different catalytic properties to macroscopic catalysts. Their use in conventional continuous flow reactors is difficult because of their small particle size. For example, it is difficult to test the long-term stability of small catalyst particles. The retention of small catalyst particles in the intracapillary volume of a hollow fibre dialyser module is promising because of the small fibre diameter (200 gm), the small membrane thickness (20-50 gm) and the high area/volume ratio (fig. 1). Thus, effective diffusive mass transport is reached. Colloids and catalysts may be easily removed from the fibres after use e.g. for regeneration or for further characterization. In the catalytic nitrite/nitrate reduction, nitrate and/or nitrite are selectively reduced to nitrogen via several intermediates [1-8]. As reducing agent, hydrogen or formic acid can be used [8]. In dependence on the catalysts and the reaction conditions different amounts of the undesired by-product ammonium is formed. For nitrite removal, Pd/A1203 catalysts or colloids are preferably used because of their high selectivity to nitrogen i.e. their small ammonium formation. For nitrate removal an additional activating metal is needed (fig. 2). The catalytic nitrate and/or nitrite reduction are strongly pH-dependent. In general, both selectivity and activity decrease with increasing pH value. During nitrate/nitrite reduction hydroxide ions are formed leading to a pH gradient in macroscopic catalysts. The use of formic acid as reductant for nitrate removal is advantageous for technical reasons (high solubility, easy dosation) [8]. Another important advantage is the in-situ formation of CO2 i.e. HCO3 which neutralises the formed hydroxide ions.

100

Figure 1: Scheme of a hollow fibre dialyser, filled with microscopic catalysts or colloidal sol

2

Figure 2: Scheme of the catalytic nitrate and nitrite reduction

EXPERIMENTAL

2.1

Preparation of colloids and catalysts The preparation of the catalysts was described previously [4, 5]. The colloids were prepared by a method similar to [9]. 4 g of the stabilizing polymer (Poly (N-vinylpyrrolidone) PVP K90 purchased from Bayer Leverkusen, Germany, Poly(vinylalcohol), PVAL Type 17-99, purchased from Beijing Chemical Industry Group, Beijing, China, mixtures of PVP and PVAL, Poly(N-vinylpyrrolidone-vinylalcohol)copolymers P(VP-co-VAL) prepared by hydrolysis of Poly(vinylpyrrolidone-vinylacetatecopolymers), purchased from Bayer Leverkusen, Germany) and 0.94 mmol Pd as Na2PdC14solution were dissolved in a mixture of 100 ml of ethanol and water, each, and refluxed for 45 minutes forming stable dark-brown sols. After cooling, the sols were dialysed to remove unreacted ethanol and ethanal. For TEM observations, a drop of the diluted sols was allowed to evaporate on a copper grid covered by a Poly(vinylformal) film. A Philips EM 300 operated at 80 kV was used. Particle size determinations were performed by using enlarged TEM-photographs. At least 100 particles were measured. The standard deviations were in the range of 20-40% of the particle sizes. 2.2

Reaction conditions If not otherwise mentioned, all experiments were performed in deionized water (conductivity < 5 ~tS/cm). Nitrate and nitrite were used as Na-salts.

2.2.1

Batch experiments The standard reaction conditions were: Atmospheric pressure, initial nitrate or nitrite concentration 100 mg/l, 10~ pH value kept constant by adding 0.2 M HC1 in case of hydrogen as reducing agent and formic acid in case of its use as reductant. In case of hydrogen as reductant, it was bubbled to the reaction suspension with a high flow rate to ensure the maximal saturation of hydrogen at 10~ (1.7 mg H2/1) during the whole experiment. Samples were periodically taken, filtered and analysed as described below. Activities were related to a conversion >98%, the selectivities and ammonium formations refer to the total removal of nitrate and/or nitrite.

101 The reaction conditions are described more detailed in [4, 5, 8]. Because of foam formation caused by their surface activities it was not possible to test the Pd/PVAL colloid and Pd colloids stabilized by PVP-PVAL-mixtures in their free-solform. The tests were performed by filling the sols into the intracapillary volume of a hollow fibre dialyser module and by pumping the reaction solution through the extracapillary volume similar to the continuous flow experiments described in 2.2.2.

2.2.2

Continuous flow experiments

In order to test the long-term abilities of the microscopic catalysts and colloids, continuous flow experiments were performed using the hollow fibre dialyser loop reactor. A scheme of the apparatus is given in fig. 3. The colloids or the catalysts were filled into the intracapillary volume which consisted of the hollow fibres. The retention of the colloids and of the catalysts was absolutely complete. For nitrate removal with bimetallic catalysts, hollow fibre dialyser modules COBE 500 HG, membrane area 1.3 m 2, purchased from Hospal Medizintechnik, Ntirnberg, Germany were used. For nitrite removal with monometallic Pd colloids, hollow fibre dialyser modules FRESENIUS F6, membrane area 1.3 m 2, purchased from Fresenius Medical Care, Bad Homburg, Germany were used because a significant adsorption of Pd colloids on the HEMOPHAN | membrane used in COBE 500 HG dialyser modules was observed. These interactions did not occur between the polysulfon membrane used in F6 modules and the colloidal sols. The catalyst- or colloid-loaded hollow fibre modules were used as loop reactors. The apparatus further consisted of a thermostated 1000-ml glass vessel equipped with an automatic pH-controller and a gas distributor for experiments with hydrogen. The content of the glass vessel was continuously pumped through the extracapillary volume of the hollow fibre module (ca. 60 ml) with a high flow rate (-500 ml/min, retention time - 8 seconds) by a tube pump to minimise secondary mass transfer effects. Samples were periodically taken from the reaction vessel. For experiments with hydrogen as reductant, it was bubbled to the reactor continuously. The pH value was kept constant by adding 0.2 M HC1 or H2SO4 (no effect was observed concerning the nature of the acid). The nitrate- or nitrite-containing solution (inlet concentration 100 rag/l, if not mentioned otherwise) was pumped in the reactor from a storage tank which was saturated to air before use. inlet 0 outlet For experiments with formic (feed) 0 ] acid, a formic acid and nitrate conanalysis~.~ 1t - . |' taining solution was pumped into the .......... ! ~ [~i~-[ .....[.....i--ii............!: ......! reactor (inlet concentrations" 100 mg/1 II I/ III nitrate, formic acid concentration depending on the desired reaction conditions). The pH value was kept ~ . ~ constant by adding 2 M NaOH. Nitrate, formic acid (formate) .... and nitrite concentrations were analy.!. sed by HPLC [3]. Ammonium concentrations were analysed by flow-injecFigure 3: Scheme of the hollow fibre dialyser tion-analysis [10], dinitrogenoxid conloop reactor used for continuous flow expericentrations were analysed by GC [ 11]. ments with hydrogen as reductant

102 3

RESULTS AND DISCUSSION

3.1

The use of bimetallic Pd catalysts for nitrate removal

3.1.1

Screening for nitrate removing catalysts For nitrate reduction a bimetallic catalyst is necessary. The use of a commercial 5% Pd/A1203 catalyst (Aldrich, No. 20,571-0) impregnated with different second metals (1.25%) showed suitable nitrate removal abilities for PdCu, PdZn, Pdln and PdSn catalysts (fig. 4). In order to lower the ammonium formation, the deposition/precipitation method for these catalysts was applied. First, PdCu and PdSn catalysts were prepared on different types of alumina (acidic, neutral, basic) showing that basic aluminas gave the most selective catalysts [5]. The best results were achieved with the basic Alumina HL, purchased from Martinswerke, Bergheim/Erft, Germany, which was calcined 24 hours at 1000~ before use. The use of this support material and the deposition/precipitation method resulted in catalysts significantly more selective than those prepared by wet impregnation (fig. 5). The PdCu, Pdln and PdSn catalysts were characterized further with regard to the ratio of the metals. For nitrate reduction, all catalysts showed best results with weight ratios (Pd:second metal) of 46:1 for the use of hydrogen and formic acid, both (fig. 6 and 7).

250t I

700

250,

activity

[---] ammonium formation

impregnation m e t h o d

depositon/precipitation method ,

-~

_l~ activity ammonium formation

~ 200

35

30

~0 150

~ , ,

"~

1~ ~

~

11o "~ 50

o

II

II

ial

PdAu PdAg PdPb PdFe PdHg PdNi PdCu PdZn PdSn Pdln

-

Figure 4: Activity and ammonium formation of different bimetallic catalysts prepared by impregnation (pH 6, reductant: hydrogen) 300

PdCu

PdZn

PdSn

Pdln

PdZn

PdSn

Pdln -

300

5

e, 9

PdCu

15

Figure 5: Activity and ammonium formation of bimetallic catalysts prepared by different methods (pH 6, reductant: hydrogen)

5

.

~

activity

max. activity 4~

~ ' 240

~Zm180

3 ""

~

180

3~

~12o

2 ~=

,_~120

2~

~240

~

~

i

o

n

~x0

'~ 6o i i i i i 1 0 , ~ ~ ~ ; ; 7i 8i 9~ ~011~3~4,5

,J

0

Pd/In-ratio

Figure 6: Influence of the Pd/In ratio on the activity and ammonium formation (pH 5, reductant: formic acid)

0

2 3 4 5

6 7

w

~

v

I

i

~

i

8 9 t0 11 12 13 14 15

!

0

Pd/In-ratio

Figure 7: Influence of the Pd/In ratio on the activity and ammonium formation (pH 5, reductant: hydrogen)

103

3.1.2

Testing of the catalysts in the hollow fibre dialyser loop reactor

To test the long-term stability and to compare the abilities of formic acid and hydrogen as reductants, the 5%Pd-l.25%Cu/, 5%Pd-l.25%In/ and 5%Pd-l.25%Sn/A1203 catalysts, prepared by the deposition/precipitation method, were tested in the hollow fibre loop reactor in continuous flow experiments. When using a 5%Pd-l.25%Cu/A1203 catalyst, a very high dinitrogenoxid concentration was found in the gas phase above the reaction solution (14-40 times higher than for PdSn and PdIn catalysts). Obviously, the removal activity for this intermediate is lower for PdCu/A1203 than for PdSn/A1203 and Pdln/Al203 (fig. 8). The highest nitrate removal activity was found for the Pdln catalyst, whereas the selectivity was slightly higher for PdSn and PdCu catalysts. Further experiments were performed by the use of the 5%Pd-l.25%Sn/A1203 catalyst. The influence of the reductant was tested under appropriate conditions for the PdSn catalyst (fig. 9 and table 1). With hydrogen as reductant the activity was higher, but the selectivity was rather low, due to an increase of the pH value within the hollow fibres (table 3). The "in-situ-buffering effect" of the formic acid leads to very high selectivities above 90%. The lower activity of formic acid as reductant is in contradiction to batch experiments performed in the above described slurry-batch reactor and may be caused by the lower diffusion coefficient of formic acid compared to hydrogen in aqueous solution. Mass transport through the membrane and through the packed catalyst bed in the hollow fibres plays an important role in the observed overall reaction rate. Further experiments must be carried out to compare the nitrate removal activities for hydrogen and formic acid as reductants.

Figure 8: Activity, selectivity and dinitrogenoxid concentration for different bimetallic catalysts (reaction conditions: c(NO3-) 20 mg/1, c(HCOO-) 30 mg/1, pH 6)

Figure 9: Activity and selectivity in dependence on the reductant (reaction conditions: c(NO3-) ca. 50 rag/l, c(HCOOH or 1-/2) ca. 0,85 mmol/1, pH 6)

Table 1 Activity andselectivity in dependence on the reductant (referring to fig. 9) reductant concentration of concentration of activity reductant [mmol/1] nitrate [mg/1] [mg NO3/(g cat.*h)] formic acid 0.85 ..... 55.2 4.29 hydrogen 0.87 46 7.90 formic acid 1.01 55.8 3.17

selectivity [%] 90 74 95

104 For hydrogen as reductant, the buffering effect was studied further by adding hydrogencarbonate to the feed in the same concentration as the in-situ formed HCO3 during the experiments performed with formic acid (4.4 mmol/1). An increase both in selectivity and activity was found, but the selectivity with formic acid as reductant was not reached. A further increase in HCO3 concentration did not further improve the catalytic properties, which shows that the buffering capacity is high enough to keep the pH value constant even within the hollow fibres. In experiments performed with formic acid as reductant, the addition of hydrogencarbonate did not increase the selectivity indicating a non-increased pH value within the fibres (table 2). The decrease in activity in the presence of HCO3- was due to a little loss in activity during reaction time (see also fig. 10). Table 2 Influence of the addition of hydrogencarbonate as buffering substance to the feed (experimental conditions: pH 6, inlet concentration: 100 mg/1 nitrate; outlet concentration 20 mg/1 nitrate, reductant concentration 0,85 mmol/1) ..... reductant concentration of activity selectivity HCO3-[mmol/1] [mg No3/(g cat.*h)] [%] formic acid ca. 4.4 4.45 94 formic acid ca. 8.8 3.83 95.1 hydrogen 0 10.6 70 hydrogen 4.4 18.2 83.5 hydrogen 8.8 18.6 85.3 hydrogen 17.6 19.3 88 Additional experiments were done in order to study the influence of the nitrate- and formate concentration and the pH value on the nitrate removal activity of the PdSn/AI203 catalyst. After a use of more than 60 days the catalyst was removed from the dialyser module. No metal loss neither for Pd nor for Sn was found by AAS analysis. The selectivity remained stable above 90% during the time. A decrease in activity (ca. 15%) was found at the beginning of the experiments (first 2 days). Afterwards, the activity remained stable over a period of 60 days under various reaction conditions. A further loss in activity was found after experiments at a pH value of 4.5 (fig. 10). Using tap water (conductivity 837 100 gS/cm, chloride concentration 54 mg/1) instead of deionized water led to similar 90 ? ' 8~ results when hydrogen was used as reductant (table 3). With formic acid, a 80 ' - " ~6 :~ significant and partly reversible loss in Z loss in activity after ~4' experiments at pH 4.5 70 ~ activity was observed, which may be caused = by a competitive adsorption of a soluble. In 60 batch experiments a sensitivity especially -activity ---a--- selectivity for chloride ions was found which was less ~176' ;~ ' ~~ ' J~ ' 4; ' 5' ~ ' 6' ~ ' #~ for hydrogen. Further experiments should be time [d] Figure 10: Long-term stability of the PdSn carried out to elucidate this effect. catalyst (reaction conditions: c(NO3-) 50 mg/1, c(HCOO-) ca. 30 mg/1, pH 6)

105 Table 3 Influence of the water salinity for hydrogen as reductant (reaction conditions: inlet concentration 100 mg/l nitrate, pH 6) outlet nitrate activity concentration [mg/1] [mg NO3-/(g cat.*h)] deionized water 47 7.8 tap water 59 .... 8.1

selectivity

[%]

....

3.2

77 85

The use of Pd colloids for nitrite removal

Colloids often show remarkably different catalytic properties than supported catalysts [12]. In general, especially polymer stabilized colloids are very stable and the particles can be produced in a narrow size distribution. Little is known about the nature of the stabilizing effect and the effects of the stabilizers on the catalytic properties of the metal [ 13]. Pd colloids stabilized with PVAL and PVP prepared as described above were tested in the hollow fibre loop reactor in batch experiments at different pH values. For both colloids a decrease in activity was found with increasing pH value, which was also found for supported catalysts (fig. 11). Whereas a significant decrease in selectivity was found for Pd/PVAL colloids at higher pH values, the dependence of the ammonium formation for Pd/PVP colloid on the pH value was less. TEM observation revealed very similar particle diameters for both colloids (Pd/PVP 4.1 nm + 0.86 nm; Pd/PVAL 4.4 nm + 0.89 nm), so a particle size effect was not the reason for the different catalytic properties. When mixtures of PVP and PVAL were used as stabilizing polymer, Pd colloids with similar catalytic properties to Pd/PVP sols were prepared. Similar average particle sizes were reached with the polymer mixtures than with the respective polymers (fig. 12). 10 t-==-- activityPd/PVAL 8~ -- activityPd/PVP /--tr-- ammoniumformationPd/PVAL '~ 6f'--'~176 f~176

~ 0 ,,~

110 ~ 8~ ]6 ~

6 7 pH-value

Figure 11: Activity and ammonium formation of Pd/PVP and Pd/PVAL colloids tested in the hollow fibre loop reactor (batch experiments, reductant: hydrogen)

1 ~ 0.8 ~, ~~0.6

I0 -

activity ammoniumformation A av. particlediameter

,-., 8 =~,~ ~' = o~

~0 PVP-contentof stabilizingpolymer[%] Figure 12: Activity and ammonium formation of Pd/PVP-PVAL colloids in dependence on the PVP content of the stabilizing polymer tested in the hollow fibre loop reactor (batch experiments, pH 6, reductant: hydrogen)

The addition of PVP to a previously prepared Pd/PVAL colloid also led to a decrease in ammonium formation in comparison to the Pd/PVAL sol (fig. 13). It is known that PVP has a stronger protective power than PVAL [14]. These results clearly demonstrate that the specific adsorption of polymers on metal colloids may influence the catalytic properties even if they has been absent in the preparation procedure.

106 The high selectivity and long-term stability of the Pd/PVP colloidal sol was also confirmed in continuous flow experiments using the hollow fibre dialyser loop reactor. No significant loss in activity and selectivity was observed in more than 100 hours (fig. 14). Obviously, the excellent selectivity is at least partly a result of the PVP-Pd interaction. [ 0.8

activity

G

4 tion

!:::J :g o.21 ,

i , i , i , b I0 20 30 400 a d d i t i o n o f P V P to P d / P V A L c o l l o i d [rag P V P / m g Pd]

%

10

+

i

Figure 13: Influence of the addition of PVP to a P d ~ V A L colloid tested in the hollow fibre loop reactor (batch experiments, pH 6, reductant: hydrogen)

~[

2.5

~ ~

~ nitrite concentration -----o--- a m m o n i u m c o n c e n t r a t i o n

2 ~ ~

~ctivity:0,4 mg NO~/(mgPd *h) selectivity: 98-99.1% I ~-------____.__~......~ 0.5

~ : 10[-

++ ea

O0 , I0 l ,

2'00' 3'0

' 40''

~0 . . 60 . . . . . 70 [h]

80

90 ~ I~ + 18

time

Figure 14: Long-term activity and selectivity of the Pd/PVP colloid tested in the hollow fibre loop reactor (continuous flow experiment, inlet concentration 100 mg/1 nitrite, pH 6, reductant: hydrogen)

Colloids with interesting catalytic properties and small average particle size diameters can also be prepared by using P(VP-co-VAL) copolymers (fig. 15 and 16). No ammonium formation was observed at a pH value of 6 for Pd/P(VP-co-VAL) colloids, varying the VP-percentage from 20-100% (fig. 15). At a pH value of 9, a decreasing content of VP led to an increase in ammonium formation: No ammonium formation was found for a Pd/P(80%VP-20%VAL) colloid whereas an ammonium formation of 3.8 mg/1 was found for a Pd/P(46%VP-54%VAL) colloidal sol. For both colloidal sols identical average particle diameters of 3.1 nm + 1.0 nm were determined. Further experiments must be carried out to study the metal-polymer interactions. 0.5

10

~4

:

activity

--c>--

~,

ammonium

~ 8~g .~ 6

forrnation

av. particle size

~2

14

"7. t

2

040

+ 5;

6'o

7'0 []' ~0

VP-content of P(VP-VAL)-polymer

9;

[%]

m+

.5 m,

t~

Figure 15: Activity and ammonium formation of Pd/P(VP-co-VAL) colloids in dependence on the VP content of the stabilizing polymer (colloidal sols, batch experiments, pH 6, reductant: hydrogen)

'-" ~0.4 ~o 0.3

lO

: ---or--

activity ammonium formation av. particle s i z e

oo 8 ~.f 6 '~,~

+ 0+

04'0

'~_

50

60

70

80

VP-content of P(VP-VAL)-polymer

.

90 [%]

Figure 16: Activity and ammonium formation of Pd/P(VP-co-VAL) colloids in dependence on the VP content of the stabilizing polymer (colloidal sols, batch experiments, pH 9, reductant: hydrogen)

107 4

CONCLUSIONS

The preparation of microscopic catalysts and polymer stabilized colloids was optimized with regard to high activities and low ammonium formations during nitrate and nitrite removal. For bimetallic catalysts, the nature of the second metal, the preparation method, the metal ratio and the support material strongly influence the catalytic properties. For Pd colloids used in nitrite reduction an influence of the nature of the polymer used for the stabilization of Pd colloids was found. Both colloids and microscopic catalysts may be easily tested and completely retained in continuous flow experiments using the hollow fibre dialyser loop reactor. For nitrate reduction on bimetallic catalysts the advantageous use of formic acid as reductant regarding to a high selectivity could be demonstrated. ACKNOWLEDGMENTS The authors wants to thank the German Government, Ministry of Education, Research, Science and Technology for financial support (Grant 03D0026A8). REFERENCES

[1] [2] [31 [4] [5] [6]

[7] [8] [9]

[101 [11] [12] [131 [141

K.-D. Vorlop and T. Tacke, Chem.-Ing.-Tech. 61 No. 10 (1989) 836 S. H6rold, K.-D. Vorlop, T. Tacke and M. Sell, Catal. Today 17 (1993) 21 S. H6rold, T. Tacke and K.-D. Vorlop, Environmental Technology 14 (1993) 931 M. H~ihnlein, U. PrtifSe, S. H6rold and K.-D. Vorlop, Chem.-Ing.-Tech. 69 No. 1+2 (1997) 89 U. PrtiBe, S. H6rold and K.-D. Vorlop, Chem.-Ing.-Tech. 69 No. 1+2 (1997) 93 A. Pintar, J. Batista., J. Levee and T. Kajiuchi, Appl. Catalysis B: Environmental 11 (1996) 81 G. Strukul, F. Pinna, M. Marella, L. Meregalli and M. Tomaselli, Cat. Today 27 (1996) 209 U. PrtiBe, M. Kr/3ger and K.-D. Vorlop, Chem.-Ing.-Tech. 69 No. 1+2 (1997) 87 H. Hirai, J. Macromol. Sci.-Chem., A13 No. 5 (1979) 633 S.S. Goyal, D.W. Rains and R.C. Huffaker, Anal. Chem. 60 (1988) 175 L. Htither, T. Willke and K.-D. Vorlop, GIT 42 No.1 (1998) 4 H. Hirai, H. Chawanya and N. Toshima; Macromol. Chem., Rapid Commun. 2 (1981) 99 H. Hirai and N. Toshima, Polymer-attached catalysts, in: Y. Iwasawa (ed.), Tailored metal catalysts, D. Reidel Publishing Company, Dordrecht, Holland, 1986 H. Hirai, Macromol. Chem. Suppl. 14 (1985) 55

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors. On

the

effect

of

cadmium

acetate

109 in the

preparation

of

heterogeneous

p a l l a d i u m ( 0 ) catalysts Gerhard Mestl, Steffi Adam, Olaf Timpe, Ute Wild, W. Bensch and Robert Schl6gl* Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Anorganische Chemie Faradayweg 4-6, D-14195 Berlin (Germany) 1. INTRODUCTION Supported palladium catalysts play an important role in organic synthesis [1,2,3,4]. Palladium acetate is used as precursor in hydrogenation and partial oxidation catalysis of olefins [5,6,7]. Palladium acetate, however, is Tablel poorly solvable in polar solvents leading to Bond lengths [pm] of [Pd(CH3COO)2]3 difficulties in catalyst preparation. Solid and Cd(CH3COO)2.2H20 palladium acetate is a trimer [8,9]. Table 1 Compound Bond Length Referenc summarizes relevant structure parameters of e palladium acetate and cadmium acetate. Pd---Pd 310.5-320.3(1) [8,9] The solubility of palladium acetate in acetic P d O 197.3-201.4(1) acid immediately increases when cadmium Pd-O-C 127.8-133.2(8) acetate is added and clear orange solutions are O-C-O 124.8-128.7(11) obtained. Two reasons may explain this effect: Cd-O 229.7-259.7(3) [11] 1. the solubility may be increased because the Cd-O-C 86.9-122.3(5) additional acetate ions act as isoionic additive (if O-C-O 118.0-122.3(5) so, other metal acetates should show the same effect), or 2. complex chemistry of palladium(II) and cadmium(II) may occur in solution. A crystalline solid could be isolated from stoichiometric Pd acetate/Cd acetate solutions, whereas solutions with Na or K acetate did neither result in an increased solubility nor in complex formation. After recrystallisation from acetic acid, rectangular single crystals were obtained for X-ray structure analysis (Table 2). Details of the structure determination are reported elsewhere [10]. The geometric structure of the complex is shown in Fig. 1. The distance of 280 pm between palladium and cadmium is much smaller than the sum of the van der Waals radii (320 pm). I13Cd NMR measurements (spectra not shown) and XPS (vide infra), however, indicated that cadmium is bonded as a divalent cation in the binuclear complex. A metal-metal bond in the Fig. 1. Crystal structure of [PdCd(CH3CO2)4-CH3CO2H]2 , Cirius 2 plot

complex, is thus excluded. Pd-Cd alloys are well reported in the literature [11,12]. A possible Cd promotor effect may thus

,,,

*This work was financially supported by the Fonds der Chemischen Industrie.

110

arise from alloy formation, Table 2 which could be facilitated by the Crystal and structural data mixed metal complex precursor, crystal system Tricline Selected bond lengths [pm] Alternatively, this promotor space group Pi Cd-02 235.2(2) effect could also be due to an Z 1 Cd-03 231.4(2) electronic or a steric effect of dimensions a= 855.2(1) pm Cd-O3a 229.2(2) Cd. The formation of a Pd-Cd b = 899.0(1) pm Cd-05 230.3(2) complex in the precursor soluc = 1071.2(2) pm Cd-07 240.2(2) tion prior to catalyst preparation, ct = 82.231(6)~ Cd-09 222.5(2) [3= 74.230(5)~ Cd-Pd 280.58(4) thus may have a crucial 7 = 76.429(5)~ Pd-04 200.7(2) influence in the generation of Pd-06 197.8(2) the catalyti-cally active Pd Pd-08 199.7(2) species. Pd-OlO 200.0(2) In order to unravel the effect of the Pd Cd complex formation on the catalytic performance, a model reaction is used to probe the different performances of catalysts prepared from Pd acetate and from the Pd-Cd complex. The activation of the precursor into the active Pd catalyst was characterized in detail.

2. RESULTS AND D I S C U S S I O N 2. 1. Catalytic Data The total oxidation of ethylene was chosen as reaction to characterize the catalysts prepared from the ternary precursor [CdPd(CH3COO)4.CH3COOH]2 in comparison to one from the conventional binary precursor [Pd(CH3COO)2]3.600 mg SiO2 (BET surface area: 159 m2/g) were impregnated with solutions of both compounds (0.5 g of a solution of 291.5 mg [Pd(CH3COO)2]3 or [CdPd(CH3COO)4"CH3COOH]2 in 4.6g glacial acetic acid) and dried in vacuum. The BET surface areas were 133 and 130 m2/g for the dried Pd acetate and PdCd acetate catalysts, respectively. 36.4 mg of the dried Pd catalyst (hourly space velocity (hsv) = 51.5 h ~) and 55 mg of the Pd-Cd/SiO2 sample (hsv = 34.1 h 1) were placed into the centre of a quartz tube (diameter 8 mm) reactor, which was filled with quartz wool above and below the catalyst bed. The catalysts were activated in flowing N2 (200 ml/min) by heating to 425 K with a heating rate of 0.5 K/min. At 428 K, the temperature was kept constant for 30 rain, and the decomposition of the acetate ligands was followed mass spectrometrically (mass 44 e/u) with an Ion-Molecule-Reaction-Mass Spectrometer (IMR-MS) (ATOMIKA, type 1500). Further details are reported elsewhere [13]. At 425 K (250 min time on stream), a flow of C2H4 (25 ml/min; LINDE 99.95%,) was added to the N2 flow. At 428 K and t = 260 min time on stream, a flow of 02 (4 ml/min; L1NDE, 99, 999%) was added, while the sample was heated to 573 K with a rate of 0.5 K/min. The ethylene conversions are discussed in terms of the IMR-MS traces of mass 44 m/e (CO2) (Fig. 2). The formation of H20 was found to exactly follow the CO2 profile, and therefore is not shown. Over both catalysts, the formation of CO2 started at 473 K as Fig. 2A shows. This indicates a similar activity with respect to the light-off temperature. Considering the amount of CO2, both catalysts showed a strongly different behaviour. The CO2 concentration increased in a step-like fashion over the Pd-Cd/SiO2 catalyst, whereas a

111

3

2.5

% a

2

t§I

%

% o

"I.5

e.t t

"l

0

0.5

0.5

,

o

300

400 500 Temperature I K

Pd +rl S i O I

l

590

i,

500 450 Temperature / K

,,:l

400

:1,

400

,

i

450 500 Ternperatur& ! K

,,

550

Fig. 2. IMR-MS spectra of the ethylene oxidation over Pd/SiO 2 and Pd-Cd/SiO2catalysts. A: Formation of CO2 between 300 and 575 K (heating rate 0.5 K/min). B: Formation of CO2 between 575 and 375 K (cooling rate 5 K/min). C: Formation of CO2 between 375 and 575 K (heating rate 5 K/min). The traces are vertically shifted for better visualization. monotonous increase was observed over the Pd/SiO2 catalyst. This observation indicates a different catalyst activity depending on the precursor. After reaching 573 K, the temperature of the reactor was kept constant for 30 min. This resulted in a decrease of CO 2 formation over the Pd-Cd/SiO2 catalyst (Inset Fig 2A) The pure Pd/SiO2 catalyst permanently formed CO2 at a low level. Even after cooling the reactor to 523 K (5 K/min), the Pd/SiO2 catalyst still produced CO2 (Fig 2B): Again, this observation points to a different behaviour of both catalyst materials. The temperature of 523 K was kept constant for 10 min before the reactor was reheated to 573 K. Fig. 2C shows the CO2 formation over both catalysts during this second heating cycle. The Pd-Cd/SiO2 catalyst again formed large amounts of C02, opposite to the Pd/SiO2 sample which remained at its low level of CO2 formation. Again, this different behaviour of the two catalysts indicates a considerable influence of the Cd promotor. In order to understand this Cd promotor effect, a detailed characterization was conducted of the activation process of the supported and unsupported PdCd precursor.

2. 2. Physicochemical Characterization In order to prove the Cd promotor effect on the catalytic performance, it is essential to show how the precursor complex is adsorbed on the substrate. UV-Vis spectroscopy was used to characterise the impregnation with the Pd-Cd precursor (Fig. 3). The absorption maximum was detected at 396 nm for pure palladium acetate in acid solution (Fig 3A) [14]. The UV/Vis spectrum of [CdPd(CH3COO)4.CH3COOH]2in acid solution (pH = 1) showed this maximum absorption at 343 nm (Fig. 3A). After impregnation and drying, an absorption maximum was observed at 340 nm together with a shoulder at 394 nm (Fig. 3B). The shoulder at the same

112

340 nm

All i: 1

Pd-Cd in solution Pd in acid solution

i.

_* PdCd/SiO 2 :=:~ PdSiC~z . . . . . . . Differenoe

2.8

0.8

2.6

0.8

"2.4

"1

0.6 O

-

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-

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<

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i

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~-"~

0.4

0.2

394

nm

\X % 1.6

0I

,,

I

400 600 Wavelength / n m

......

1.4

400

600

Wavelength / n m

Fig. 3 A: UV/VIS spectra of [CdPd(CH3COO)4"CH3COOH] 2 in acetic acid and as crystalline solid. B: UV/Vis spectra of [CdPd(CH3COO)4"CH3COOH] 2 and [Pd(OAc)2]3 supported on SiO 2. Dashed line: difference spectrum of a) minus 20% of b). Spectra normalized for quantitation.

wavelength as the absorption maximum of supported palladium acetate (Fig. 3B) indicated that the complex was not adsorbed entirely intact onto silica. The dotted line in Fig 3B is the difference spectrum of supported Pd-Cd acetate minus that of supported Pd acetate. This estimation shows that the binuclear complex is decomposed upon the interaction with the SiO2 support, either during impregnation or drying. A quantitative estimation under the assumption that the absorption coefficents do not change upon interaction with the support points to 20% decomposition of the Pd-Cd complex. This observation was further corroborated by DRIFTS (spectra not shown). A comparison of the spectra of the pure Pd-Cd complex with that of the adsorbed Pd-Cd complex reveals that the spectrum has completely changed. Especially, the bands 1545, 1454, 1383 and 1246 cm-I were not detected for the supported material, which are characteristic for the Pd-Cd complex. A comparison with the spectra of CH3COOH on SiO2, however, shows some similarity. Presumably, most of the detected spectral features in the spectrum of the adsorbed complex arose from the decomposition product acetic acid. But signal intensity ratios, e.g. the band at 1732 cm -~, are different, and there are two bands in the spectrum of the adsorbed PdCd complex at 1570, and 1297 cm -1 which are absent in the spectrum of pure acetic acid on SiO2. These differences in the DRIFT spectra confirm the DR-UV/Vis result and suggest that the complex at least partially decomposed upon adsorption on SiO2. XPS also could show that the interaction of the complex with SiO2 led to decomposition (Fig. 4). The binding energy of the supported complex was observed at 337.3 eV, as

113 referenced to the aliphatic C l s signal at 285 eV, while the Pd 3d signal of the unsupported material was found at 338.4 eV. This shift in binding energy is attributed to an exchange of acetate ligands against surface oxygen ions (BE Pd 3d5~2 of PdO is 337.5 eV). In addition, the 5/2 to 3/2 Pd 3d intensity ratio of the supported cluster is 1.9, while the unsupported material has a ratio of 1.5 close to the theoretical one. The binding energy shift of 1 eV of the supported relative to the unsupported material points to its decomposition and the formation of oxidic Pd surface species (see Table 3), in line with the DR-UV/Vis and DRIFTS observations. The thermal activation of the Pd-Cd/SiO 2 catalyst in UHV was also characterised by XPS. Complete decomposition to elemental Pd was only observed after treatment at 573 K (Fig. 5),

337.3 eV 3 3 8 . 4 eV. I

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.

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Binding Energy / eV Fig. 4. XP Pd 3d spectra signals of the Pd-Cd complex pure and on SiO2. Spectra are vertically shifted for better visualisation. in strong contrast to the unsupported complex, which was already decomposed at 423 K as shown by XPS (Fig 6B). The much higher decomposition temperature as compared to the unsupported complex also points to decom-position of the Pd-Cd complex during adsorption. It can be concluded that the decomposed, oxidic Pd surface complexes can only be reduced at higher temperatures as compared to the pure Pd-Cd acetate. Consequently, palladium acetate adsorbed on SiO2 is reduced to its metallic state after UHV decomposition at the comparably high temperature of 523 K as shown by XPS (data not shown). The difference in the decomposition temperature of 150 K ob-served for supported Pd-Cd complex relative to the unsupported

1.6

o ,i--,

1.4

c~ o

~9 1.2

c d) r

0.8

0.6 345

340

335

330

Bindine E n e r a v / e V

Fig. 5. XP Pd 3d spectra of SiO2 supported [CdPd(CH3COO)4"CH3COOH]2 during thermal decomposition. Spectra are vertically shifted for better

114 material, thus, is in line with the partial decomposition of the com-plex upon adsorption and drying detected by UV/Vis and DRIFTS. This higher stability toward reduction of the Pd species generated from the Pd-Cd precursor is suggested to have a strong influence on the evolution of the active catalyst material. Especially, par_A 28B.61~v2B5 B a3a.6 eV ticle growth and sintering _ A3,3~ 4 ~ooK ~ //~\~3s. may be prevented during 2.s ~ _ , / ' _ ' ~ ~ k ~ the activation or the cata~73K...~I~ lyric action. This thesis ' - ", ' J_ i~_i ~'1~|' ~ was proven correct by 2 ~a3K__ TEM investigations of 8i~#l~rfi~.akl/~aK ..I,--L t % ', ,, the used catalysts (vide -

~

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I ~t o~ ~ _ _ _ . , p - t l t

t*sv ~sa3K

C

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405.4 eV

[

~04"4

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.

~7~K-~~I~J~-----"-I ~ ~ . ~ IL-"I

,,

i

'

t

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573K_ - "~"~ ~ i~~~, i i. -,-.ms ~o 2es Bind i ng Energy/eV

I'

Mur 11 !

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_[,{,i, !jill

too

2o

57~,~/ I

~ i I

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o

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Ii

Binding Ene~jy" leV

equal intensity, characte-

D '~

I

t~

in detail by XPS. The carbon l s data are shown in Fig. 6A. Up to 423 K, the C ls spectra consisted of two lines of

aK , -,"T ~X 3s0 Binding E ne~j~//eV 34o

~ 4o o~ 523K~~]t~~

The thermal decomposition of the unsupported Pd-Cd complex was further investigated

~,~

* ~6~ 7

_

infra).

, ,l

i.S

........

3oo

,

I

4oo

,

,

I

~m

Temparature I K

Fig.6. XP spectra of the Pd-Cd cluster during decomposition A: Cls spectra; B: Pd 3d spectra; C: Cd 3d spectra. The spectra are vertically shifted for better visualisation. D: ASF-corrected intensities of Pd 3d and Cd 3d signals normalised to the initial intensity as function of T [K].

ristic of methyl carbon at 285 eV and of carboxyl carbon at 288.6 eV, indic tingt ,nte r ty oft e sample by the expected 1:1 intensity ratio. The successive broadening of the C ls, Pd 3d (Table 3) and Cd 3d lines with further increasing temperature to 375 K indicates differential charging effects which arise from the desorption of H20 and acetic acid. This desorption is evidenced by the observed decrease in the FWHM of O l s signal and by the TG experiment (data not shown). After treatment at 423 K, the C l s signal intensity has decreased to

115 80% of the initial intensity, confirming a beginning decomposition. At 473 K, above the temperature (458 K) at which the first weight loss occurred in the TG experiment, the total C l s XP intensity was reduced to about 50% of the initial intensity, and the signals were considerably broadened. The FWHM of all elements, except oxygen decreased at the same time. Additionally the binding energy of the O l s signal shifts to higher energies while the signal intensity decreases. These combined effects are evidence for the decomposition of the Pd-Cd complex. The reduction in the FWHM of all elements except O can be explained by the formation of metallic Pd as shown by the low binding energy of this signal (Fig 6B, Table 3). After treatment at 523 K, complete decomposition has already occurred according to TG, carboxylic carbon (289 eV) of further reduced intensity (40% of the initial intensity) was still detected. The aliphatic the C ls signal at 285 eV, on the other hand, showed an increased intensity indicating a beginning surface segregation. After anneal at 573 K, the aliphatic C ls signal (285 eV) had further increased, while carboxylic species were only a weak shoulder at about 288 eV. Table 3 Characteristics of the Pd 3d XP signals of [CdPd(CH3COO)4"CH3COOH]2 during the thermal treatment Temperature BE Pd 3d5/2 Intensity 5/2 Intensity 3/2 Intensity FWHM [K] eV "'b cps cps Ratio 5/2:3/2 5/2[eV] 300 338.5 1.788e6 1.400e6 1.277 1.87 323 338.5 1.705e6 1.386e6 1.230 2.02 373 338.5 1.648e6 1.372e6 1.201 2.02 423 335.5 1.049e6 1.271e6 0.825 1.95 473 335.8 6.082e5 7.993e5 0.761 1.37 523 336.4 4.184e5 3.713e5 1.127 1.30 573 336.4 2.333e5 2.397e5 0.973 1.23 a) BE of metallic Pd 3d5/2:335.5 eV [15, 16]. b) BE of Pd 3d5/2 in PdO: 337.5 eV The Pd 3d spectra in Fig. 6B, recorded after treatment at 323 K and 373 K, revealed the presence of divalent Pd (Table 3). The tailing of the spectra to lower binding energies must be due to partial decomposition in the X-ray beam. Differential charging cannot account for this effect because a comparable tailing was not observed in any other signal. Loss of hydration water and solvating acetic acid leads to a general broadening of all XP signals except O 1s (as expected) but not to tailing. At 423 K, the main part of Pd is reduced to the element (Table 3). The broadening to higher binding energies evidences that some unreduced Pd remained in the sample. This fact explains why the total C ls signal intensity had not decreased by 50 % after this treatment step, as one would expect from a complete Pd reduction. The Pd binding energy is insensitive to the formation of interstitial carbon which is not a carbide. Thus unfortunately, we cannot directly deduce the presence of interstitial carbon from XPS. At the same time, the Pd 3d signal intensity was reduced to 60% of the initial intensity (Table 3). It is most relevant to see from Fig. 6C that the Cd species was still present in its divalent form at that temperature. The complex separated at 423 K into elemental Pd particles and Cd-acetates, a Pd-Cd alloy formation did not occur. A considerable sharpening of the Pd 3d peak profile was observed after heat treatment at 473 K, together with a further reduction of the Pd 3d intensity to 35% of the initial one (Table 3). A spatial segregation of Pd from the Pd-Cd complex

116 crystals can be deduced from this reduced Pd 3d intensity and the peak profile sharpening. Metallic Pd, thus, seems to segregate to the surface of the decomposing crystals. After treatment at 523 K, the Pd 3d signal intensity further decreased to about 30% to finally show an intensity of 20% after treatment at 573 K (Table 3). During this process, the Pd 3d signal continuously shifted to higher binding energies, and the full widths at half maximum (FWHM) decreased from 2.02 eV to 1.23 eV (Table 3). The intensity reduction and the sharpening of the FWHM is evidence for Pd particle sintering. Interestingly, the 5/2 to 3/2 intensity ratios were considerably different from the theoretical value of 1.6 (see Table 3) and especially low after treatment at 423 and 473 K. This is additional proof for the presence of at least two different Pd species during the first decomposition step. The Cd species remained in its divalent form after treatment at 423 K (Fig. 6C). The Cd acetate was mainly decomposed to elemental Cd (binding energy: 404.4 eV [14]) after annealing at 473 K. The broad shoulder at about 406 eV indicated some remaining, inhomogeneous, amorphous Cd(Oac)2 consistent with the C l s spectrum which showed the presence of carboxylates. After treatment at 523, Cd metal and Cd(OAc)2 was observed as indicated by the signal at 404.5 eV and its shoulder at 405.7 eV. The shoulder cannot be attributed to differential charging because such shoulders were not observed in the signals of the other elements. As evidenced by the sharpening of the shoulder at 405.7 eV, the Cd(OAc)2 species was much better defined after annealing at 523 K. At 573 K, cadmium oxide was formed as indicated by the shift of the Cd 3d signal to higher binding energies, while the high energy shoulder due to Cd(OAc)2 at 405.7 eV was completely lost. The Cd 3d signal intensity was gradually reduced during this whole process, with the strongest intensity reduction from 75% to 30% of the initial intensity after the treatment steps at 523 and 573 K. The aliphatic C ls signal intensity, on the other hand, grew from 45% to 70% of the initial intensity after the same treatment steps. The decrease in the intensities of the O ls and Pd 3d XP signals after these treatments did neither correlate with the decrease of the Cd 3d XP signal intensity, nor inversely correlate with the increase of the aliphatic C ls XP signal intensity. In Figure 6D, the intensities of the Pd 3d and Cd 3d XP signals are displayed after correction by atomic sensitivity factors and normalisation. At temperatures above 375 K, the XP signal intensity of Pd strongly decreased, whereas that of the Cd signal remained at higher intensities. These changing XP intensities are clear evidence that the Pd species sintered into small particles, while Cd remained more homogeneously distributed throughout the sample. The XPS analysis shows that a mixture of carbon, Pd oxide, and Cd oxide was present after pure thermal activation in inert atmosphere. The Pd oxide seems to have been formed from an reduced intermediate Pd species. It may be suggested that, prior to exsolution, interstitial carbon prevented the instantaneous reaction of the freshly reduced Pd particles with oxidising species. As a consequence, it seems appropriate to activate catalysts based upon this complex in a reducing atmosphere of a hydrocarbon. Due to the short metal-metal distances in the Pd-Cd precursor complex, it may be suggested that a Pd-Cd alloy phase as Nowotny and co-workers have reported [15,16] may result from catalyst activation at higher temperatures. Differential scanning calorimetry measurements (data not shown) did not reveal any effect in the decomposition temperature interval compatible with an alloy formation. In addition, no powder X-Ray diffraction evidence (data not shown) was found to support the existence of alloy particles larger than 0.5 nm. Furthermore, from XPS intensity considerations (Fig. 6D), we can conclude that Pd is sintering into particles, while the Cd species remains more homogeneously distributed after

117

Fig. 7. TEM micrographs of (a) Pd/SiO2 and (b) Pd-Cd/SiO2 catalysts after reaction. thermal decomposition. An alloy formation between Pd and Cd under these conditions is, thus, excluded. The observed Cd promotor effect is excluded to arise from alloy formation. TEM micrographs of the used catalysts after ethylene oxidation are displayed in Fig. 7. The metal particles in both catalyst materials differed largely in particle sizes and particle size distribution. The decomposition of the binary palladium acetate precursor and the catalytic reaction generally led to large particles (Fig. 7a). Only very few isolated particles were found as small as 5 nm. Activation and catalytic reaction of the Pd-Cd/SiO2 catalyst resulted in highly dispersed Pd particles (Fig. 7b). The particle sizes varied about 2 nm, which was smaller than the size of the smallest particles formed from the binary palladium acetate precursor. Sintering of Pd species seems to be prevented in case of the catalyst formed from the Pd-Cd precursor because of the higher thermal stability, as proven by XPS. In addition, a particular surface dilution of the Pd species by CdO may occur, which could be deduced from the Pd 3d / Cd3d XP intensity ratios during the thermal decomposition of the pure PdCd complex. Vice versa, the higher dispersion of the Pd species obtained from Pd-Cd complex may support their stability toward reduction, as shown by XPS.

3. CONCLUSION An improved reactivity was observed for the material derived from the Pd-Cd complex relative to Pd catalysts. The formation of CO2 starts at 473 K over both catalysts. The similar light-off temperature, however, does not indicate similar catalytic activity as proven by the much higher amount of CO2 produced over the Pd-Cd catalyst. The catalyst activity is normalized to the mass Pd in the samples, therefore, the different observed activities reflect the different degrees of Pd dispersion, as shown by TEM. Thus, we conclude that the active surface area is decisive for the behaviour in catalysis and that Cd increases the active Pd surface. The reduction of the supported Pd(/I) to a metallic phase takes place after treatment at 573 K as shown by XPS. Keeping the temperature of the reactor at 573 K for 30 minutes results in a massive decrease of CO2 evolution over the Pd-Cd catalyst. This reversible catalyst deactivation is most probably connected with the Pd-Cd decomposition at this temperature. Sintering into very small Pd particles (TEM) occurs during the decomposition/activation phase and therefore to high catalytic

118 activity. The catalyst prepared from pure palladium acetate is also reduced to its metallic state, but sintering during activation leads to the formation of large particles (TEM), and therefore to low catalytic activity. The Pd-Cd catalyst containing the small Pd particles again shows catalytic activity during a second heating to 573 K. Whereas the Pd catalyst containing the large Pd particles remained at its low level. These differences in the catalytic action are attributed to a Cd promotor effect which does not arise form an Pd-Cd alloy formation (XPS), although palladium acetate and cadmium acetate form the precursor complex [CdPd(CH3COO)4"CH3COOH]2 in the impregnating acetic acid solution. This complex formation is actually the major advantage of the Cd addition in catalyst preparation due to the enhancement of the poor solubility of palladium acetate. The crystal structure determination has shown a sixfold coordination of palladium with one unsaturated axial position. For this geometric reason, a direct Pd-surface bond may be formed between the complex and the surface. The suggested formation of a Pd-surface bond may be of importance during the catalyst activation, which leads to the second advantage of the complex precursor, the small size of the catalytically active Pd particles compared to those obtained from palladium acetate, ha addition, sintering seems to be inhibited by a dilution effect of CdO species

(XPS). The metal ions in the Pd-Cd precursor are in their divalent state as shown by XPS and ll3Cd NM~. This excludes a preformed metal-metal bond, which might facilitate the suggested alloy formation. The formation of a Pd-Cd alloy, however, must be excluded (XPS). Metallic Pd which may contain interstitial carbon is suggested to be the active catalyst. In the active catalyst state, cadmium plays the important role of a structural promotor which inhibits sintefing. References [1] [21 [31 [41 [5] [61 [7] [8] [9] [10] [11] [12] [13] [14] [151 [16]

K. Weissermel and H.-J. Arpe, Industrielle Organische Chemie. Bedeutende Vor- und Zwischenprodukte, 4. ed., VCH Weinheim, New York, Basel, Cambridge, Tokyo, (1994) 247-52. T. Tisue and W.J. Downs, Chem. Comm. (1969), 410. A. S. Abu-Sarrah and B. Rieger, Angew. Chem., 108, (1996) 2627. J. Louie and J. F. Hartwig, Angew. Chem., 108, (1996) 2531. W. Schwerdtel, Chemie-Ing.-Techn., 40, (1968) 781-4, Review: R. F. Heck, Org. React., 27, (1982) 345. L. Eberson and L. Gomez-Gonzalez, Acta Chim. Scan&, 27, (1973) 1162, 1249. P. Y. Johnson and J. Q. Wen, J. Org. Chem. (1981), 2767. A. C. Skapski and M .L. Smart, J. Chem. Soc. Chem. Comm. (1970), 658. N. N. Lyalina, S. V. Dargina, A. N. Sobolev, T. M. Buslaeva and I. P. Romm, Koord. Khim., 367, (1993) 57. S. Adam, A. Bauer, O. Timpe, U. Wild, G. Mestl, W. Bensch and R. Schl0gl, submitted to Chemistry, A European Journal. H. Nowotny, A. Stempfl, E. Bauer and H. Bittner, Monath. Chem., 81, (1950) 887;, 81, (1950) 1164;, 82, (1951) 949. M. Hansen, Constitution of Binary Alloys, McGraw-Hill Book Company, New York, (1958). H. Schubert, H. Hertle, T. Rtihle, H. Werner and R. Schl0gl, IMR-MS. Eine Methode zur Analyse komplexe Emmissionsgase, VDI Berichte Nr. 1257, (1996). R. W. Brandon and D. V. Claridge, Chem. Comm. (1968), 677. V. I. Nefedov, Y. V. Salyn, I. I. Moiseev, A. P. Sadovskii, A. S. Berenbljum, A. G. Knizhnik and S. L. Mund, Inorg. Chim. Acta, 35, (1979) L343. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin Elmer, 1979.

91998ElsevierScienceB.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

119

Preparation of BaAll2019-supported Pd catalysts for high-T combustion by wet impregnation C. Cristiani, G. Groppi, G. Airoldi and P. Forzatti CIIC-Dipartimento di Chimica industriale ed Ingegneria Chimica "G. Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32 - 20133 Milano (Italy).

Pd-based catalysts for combustion of CH4 have been prepared by wet impregnation methods using Ba~12019 as support and PdCI2 as metal precursor. It has been found that up to 5% Pd (w/w) can be deposited on the investigated support. Pd adsorption is markedly favoured at high temperature, low acid contents, high Pd initial concentrations and low CI content. The effect of the different parameters are explained assuming that Pd deposition occurs via the hydrolisis of the chloro-aquo-Pd complex on basic sites of the support. The prepared samples show good catalytic activity properties in CH4 combustion.

1. INTRODUCTION Catalytic combustion for gas turbines (GT) applications is a promising method for simultaneous reduction of NOx, CO and unburned hydrocarbon emissions. Its commercial application heavily depends on development of stable and active catalysts able to provide low ignition temperature and stable operation at the extreme conditions (T=1000-1300 ~ imposed by GT [1]. Pd-based systems are the most active catalysts in CH4 combustion [2]. Hexaaluminate-type materials have shown excellent thermal stability related to their peculiar layered structure that is able to suppress thermal sintering [3]. Accordingly the use of hexaaluminates-type materials as high high-T resistant support for noble metal catalysts has been proposed in the literature [4]. Deposition of noble metal is considered a critical step in the preparation of effective catalysts [5]. Particularly wet impregnation is reported to provide higher metal dispersions depending on the operating conditions, such as temperature, pH, metal precursors and concentration, nature of the surface of the support. Impregnation of Pd on 7-A1203 via wet techniques has been widely investigated [5-7]. It has been proposed that Pd deposition occurs, through proper selection of pH, via exchange of anionic or cationic metal complexes and the surface of the support with opposite charge. When chloro-containing metal precursors are used chloride species are strongly adsorbed and they decompose only at high temperature. Catalysts containing such chloride species have shown inferior performances than those obtained with Cl-free catalysts [8]. Accordingly, the use of different Pd-precursors, such as Pd nitrates or acetylacetonate, has been proposed. However, the use of chloride-containing Pd precursors will result in minor cost, since Pd can be directly recovered during refinement. In this work the preparation of Pd/BaA1120~9 supported catalysts via wet impregnation has been studied. The effect of the different parameters governing the wet impregnation reaction,

120 such as concentration of H § Pd 2§ and CI, temperature and time of reaction have been investigated. The activities in CH4 combustion of the samples obtained via the different impregnation techniques have also been compared.

2. EXPERIMENTAL

2.1. Preparation and characterisation. BaA112019 has been prepared by coprecipitation in Water using (NH4)2CO3, as precipitating agent and the nitrates of the components as precursor materials [9]. The final support has been obtained calcining the dried precursor at 1300~ for 10 h. Phase composition have been determined by powders X Ray Diffraction;, surface area has been measured by N2 adsorption; Vp has been determined by water adsorption and Zero Point Charge (ZPC) has been measured by mass titration according to the procedure reported in [101. Wet and dry impregnations have been performed using both PdC12 and Pd(NO3)2 metal precursors. A commercial solution of Pd(NO3)2, (Aldrich 10% Pd and 10% HNO3), and PdC12 salt (Aldrich) have been used as metal precursors. In the case of PdC12 a stock solution has been prepared solving PdC12 in hot water by addition of HCI and diluting to Pd concentrations of about 5x10 -2 M. The ratio HC1/PdCI2=I.5 (mol/mol) has been employed, because the amount of HC1 corresponds to the minimum required to completely dissolve PdC12. The stock solution was stable for a period of few months. Such stability has guaranteed high reproducibility of the results Wet impregnation has been performed in four different steps: a) Conditioning of the support - In order to obtain more reproducible results the support has been conditioned for 1 h in aqueous HC1 solution at stable pH=7 and T = 25~ before being contacted with the solution containing Pd precursor. b) Preparation of the Pd comacting solution- The amount of the stock solution comaining the required amount of Pd for the reaction is diluted with water up to the desired Pd concentration and added, if necessary, with HC1, KOH or KC1 to adjust the H + and C1- content to the desired values. c) Contacting of the support with the Pd solution and ageing. The conditioned support (step (a)) is contacted with the diluted solution prepared in (b). The solution is poured in the slurry of the support, under stirring at a given temperature. The system is aged in closed flask, under refluxing to keep constant the reaction volume. d) Filtering, washing and drying at 110~ - After ageing the suspension is cooled down to r.t. and the solid is separated by filtration. The impregnated material is washed three times with hot water, and dried at 110~ overnight. The amount of Pd deposited on the support by the wet impregnation technique has been evaluated by the difference between initial and residual Pd in the solution upon deposition reaction. Residual Pd has been determined by ponderal analysis according to Treadwell [ 11]. C1- and Ba 2+ contents in the solution obtained upon filtration have been determined by potenziometric titration and atomic absorption respectively. The nominal amount of Pd has been fixed as weight percentage of the metal

121

2.2 Catalytic activity tests Catalytic activity in CH4 combustion have been measured on powder materials with an experimental rig described elsewhere [12~. The following operating conditions have been adopted: quartz microreactor (i.d=0.8 cm), powder catalyst (particle size dp=0.1 mm) diluted with quartz powder (V~at./Vqu~----0.5), feed 1% CH4 in air, GHSV=54.000 (Ncc/g~ath), P=I (atm).

3. RESULTS AND DISCUSSION.

3.1. Structural and morphological properties of the supports. BaAI12019 calcined at 1300~ showed the typical Ba-BIBH-A1203 layered-alumina structure [13], a surface area of 15 mZ/g, and a specific pore volume of 0.9 cm3/g. This material is characterised by a Zero Point Charge (ZPC) of 9.5-10 pH evidencing a basic character of the surface. 3.2. Parameters governing the wet impregnation reaction Temperature of reaction. Results of impregnations performed at room temperature (r.t), and at 70~ using the same contacting solution are summarised in Table 1. It is evident that, complete Pd deposition is obtained when the reaction is performed at 70~ whereas no adsorption occurs at r.t. even for prolonged time of reaction. The results prove that deposition is strongly favoured on increasing the temperature. Accordingly, all the following impregnations have been performed at 70~ Table 1. Influence of reaction temperature. T (~

time (h)

Pd nominal % (w/w)

Pd adsorbed % (w/w)

23

2.5

< 0.01

r.t. 70

5 i

t

,

,,

i,

2.5 , ,,,

,

,

,

2.5 i

i

i,,,

i

Reaction time. The amount of Pd deposited onto the surface at different reaction times is reported in Figure 1. It is evident that most of Pd, about 70% of the total, is adsorbed in the first hour of reaction, but deposition is completed only after 5 h. Accordingly, 5 h has been chosen as standard ageing time. Effect of chloride final concentration. It has been reported in the literature that the presence of large amounts of C1- ions can negatively affect Pd adsorption on v-AlzO3 [6,7,14]. In our preparation technique the presence of chlorides cannot be avoided since they originate from both Pd precursor and HCI required for dissolution. The influence of chloride concentration has been studied varying CI content of the reaction medium by addition of known amount KC1. Molar concentration of free CI in the final solution has been taken as the most relevant variable.

122

I

'"

I'

'

I

2.4

2.0 ~

'

'

' I'

'

I El

..................-D"""

.,..,,..,,..,,.... ,,..,, D " " "

/

1.6 =/

o

Q.. 0.,') "0 "9 o

Q.

/

1.2

i

! /

0.8

I

/ / 0.4 / # 0.0

'

o

'~

'

i

z

"'

'~

a

'

'

'

,

,;,

;

Time (h) Figure 1 - Effect of reaction time: expected Pd amount 2.5% w/w

Such concentration has been calculated as follows: [Cl']f = (total C1- moles - 4 x unadsorbed Pd moles)/Vr

(1)

where: total C1- moles = (2 x PdC12 + HC1 of the stock solution + HCI of conditioning+ KC1) Vr = total reaction volume Eq. 1 has been derived under the following hypotheses: i) a [Pd(C1)4]2" complex forms between unadsorbed Pd and free C1- ions; ii) no chloride species are adsorbed on the solid surface. The latter assumption has been verified in the experiments where quantitative Pd deposition has occurred; in these experiments CI- ions in the final solution account for total initial chlorides. Concerning the first assumption it is difficult to specify unambiguously which chloro-Pd species are present in the solution. However, in presence of excess CI and at the low pH values of the final solution, the presence of major amounts of [Pd(CI)4] 2" complex can be reasonably assumed according to literature indications [7 ]. In Figure 2 the weight percentage of adsorbed Pd is plotted as a function of [Cl]f. It is evident that chloride ions inhibit deposition of Pd. On increasing [Cl]f, the amount of Pd adsorbed on the support decreases following an exponential decay law. Effect o f H + concentration. Preliminary experiments have shown that acid concentration greatly affects deposition of Pd. Accordingly the effect of H + molar concentration in the reaction medium has been investigated.

123 .

.

.i .

2.5

9

9

!

i

9

't

'' , "

"

i

'

I

"

I

:

(

"

I

2.0

g"0

~_ 1.0 0.5

0.0

!

,

. . . .

,

~,

,

,,,,

,

1'2

4

,

,,

1'4

,

,

1'6

,

1'8

[CI-] x 10-3 (M)

Figure 2 - Effect of final molar concentration of chloride ions ([C1-]f) [H+] ~=1.79x104; [Pd>] i=l.2xl0 3, expected Pd amount 2.5% This parameter has been evaluated by indirect calculation according to the formula given below, that neglects contribution of acid hydrolisis of PdC12, of ion exchange of the support and of deposition reaction: [I-I+] F(total HC1 moles - KOH moles)/Vr

(2)

where HC1 moles added in all the different steps except of support conditioning are considered. We note that direct experimental determination of pH does not provide satisfactorily results in view of: i) uncertainty of the measure under actual operating conditions (slurry, hot temperature); ii) strong sensitivity of deposition reaction to this parameter. Known amounts of KOH or HC1 have been added to tune [H+] r. In Figure 3 the weight percentage of adsorbed Pd is plotted as a function of [H+]r. for different Pd initial molar concentration. Index numbers reported in the Figure correspond to [CI]r x 10.3 (M) of each experiment. For all the initial Pd concentrations considered it is evident that Pd deposition is dramatically depressed on increasing [H~r. Effect of initial Pd molar concentration From data reported in Figure 3 it can be derived that as a general trend higher Pd amount are deposited when higher [Pd>]i are used. Deviations from this trend have been observed only in the case of high [Cl]f. This behaviour can likely originate from the negative influence of [Cl]f on Pd deposition already discussed. From all the above results the following considerations can be drawn. Considerable amount of Pd, up to 4-5% (w/w), can be deposited on the investigated support using the wet impregnation technique. Pd adsorption is markedly favoured on increasing temperature, and decreasing acid content. Besides, deposition is promoted by [pd2+]i and depressed by [Cl]f.

124 '

'|

'

I

' ....

!

''

I

'

!

',

3.1 "~_A5.8 3.~ "'-.,

.-.4

,I

'

I

'

I

'

I

"

!

f

---=--- [Pd2+] = 0.8 x 10 -3 (M) i - - " - - [Pd 2+] = 1.7 x 10 -3 (M)

8.~-.

""". .6 . .

.

.

e--- [pd2+l = 3.0 x 10-3 (M) i

v

-0 3 13.. "O L_

o2

2"9=~",.,4.5

"'"..,.,..3.6 9

1

"",-%.

" ' ' - -

....... 3

"""0 1.0

,

I

1.5

,

I

2.0

,

I

2.5

,

I

3.0

......

.,,,,,..,~

2.4 ,

I

3.5

.

I

,

4.0

I

4.5

2.3 ,

I

5.0

,

5.5

6.0

6.5

[H § x 10-3 (M) i

Figure 3 - Effect of initial acid molar concentration [H+]i at different [Pd2+]~. (Index numbers correspond to [C1-]fx 10-3 (M) of each experiment). Data collected in this work can be explained assuming that deposition of Pd occurs via a hydrolisis of Pd-aquo-chloro complex which can be tentatively described according to equation 2: [PdC14_x(H20)x]~-2+x)+ 2 S-OH --~ (SO)2-Pd + 2H + + (4-x) C1- + x 1-/20

(2)

(where S = support) Such hydrolisis likely occurs at the basic surface sites of the support. In fact the hydrolisis of Pd ions in the bulk of the solution can be ruled out. Indeed it has been experimentally found, by titration with KOH of solutions with Pd concentrations ([pd2+]i=0.8-3.0 x 10"3M) equal to those used in the wet impregnation, that Pd(OH)2 precipitates at pH= 3.8-4. These values are considerably higher than those of 2.5-3.0 always measured during the impregnation. Hydrolisis at the support surface can account for the strong inhibition played by high [H+]~ and for the effect of the reaction temperature on Pd deposition. Indeed, hydrolisis reactions are favoured at high acid concentrations and at low temperature of reaction. The presence of a hydrolisis reaction is also consistent with the complete elimination of CI ions from the surface of the support during deposition. Also the promoting effect of Pd and the inhibiting effect of chloride ions can be easily explained on the basis of equilibrium considerations On the contrary, the ionic exchange model generally proposed in the literature for this reaction [5-7] does not account for the strong negative influence of [H§ Indeed, according to this model, the increase of [H+]r results in enhancement of the positive charge of the support

125 surface, thus favouring and not inhibiting the adsorption of the negative charged Pd complexes. From an operative point of view, it can be concluded that to obtain quantitative Pd deposition it is convenient to operate at high temperature (70~ and the lowest [H§ required to avoid hydrolisis of Pd-chloride in the bulk solution. Apparently, higher Pd concentrations can also be used to increase deposition. However, when chloride precursor is used, the increase of [pd2§ is accompanied by an increase of free C1- in the solution and eventually in high [C1-]f. Accordingly, quantitative deposition can hardly be obtained with very concentrated solutions. The leaching effect of the reaction medium on the support has been investigated. BaA112019 has been suspended in a chloridric solution ([HC1] = 0.5-3.0 x 10-3 M) and the suspension has been aged in standard conditions (stirring, refluxing at 70~ for 5 h). After ageing the solid has been separated by filtration and the solution has been analysed by atomic absorption. Losses of 5% and 0.33% of Ba and A1 respectively have been found. The XRD spectra of the solid after the leaching treatment did not evidenced structural modifications of the material being the original Ba-13II3~I-A1203the only phase present. Accordingly, it can be assumed that the reaction conditions do not markedly affect the properties of the original support.

Different supports. Some experiments have been performed using an A1203 sample of phase composition (0 + r with a surface area of 130 m2/g and a ZPC of 7.5-8.0 pH. Also for this support, acid concentration and temperature have been found to be the most important parameters to influence Pd adsorption. However, for this support total elimination of chlorides ions cannot be obtained and residual CI-, corresponding to 10-15% of the total CI content, are always retained by the surface. This behaviour can be related either to the high surface area or to the different nature of the surface of the material.

Catalytic Activity- Activity tests have been performed on Pd/BaAI12019 prepared via wet impregnation using PdC12 as metal precursor, and via dry impregnation using PdC12 and Pd(NO3)2. The impregnated samples have been calcined at 1000~ for 20 h before activity tests. XRD of the calcined samples evidenced the presence of crystalline PdO (mean crystallite dimension-- 250-350 .A) together with the original Ba-13-A1203. Temperatures required to achieve 10% and 90% conversions are reported in Table 2 Table 2. Results of activity tests

Sample

T 10% (~

T 90% (~

PdC12 wet.

310

450

PdC12 dry

330

720

Pd(NO3)2 dry

300

400 ,

,

,

,,,

The sample prepared using the wet impregnation method shows low temperature activity similar to that of the sample prepared via dry impregnation with Pd(NO3)2 and slightly better

126 than that of the sample prepared via dry impregnation with PdC12. At high conversion the same activity ranking has been observed but differences become much more evident. Activity differences can be tentatively associated with the effect of surface chloride species which have been reported to inhibit combustion properties also at very low concentrations. Chlorides are absent when using Pd(NO3) and are eliminated also when deposition of Pd occurs through the hydrolisis mechanism sketched above. On the other hand the effect of different Pd dispersion seems not to be important, being PdO crystallites upon the severe calcination treatment of comparable sizes in the different samples.

4. CONCLUSIONS The following conclusions can be drawn from the results discussed above 1. Quantitative Pd deposition on Ba~12019 up to 5% w/w can be obtained upon 5 h at 70~ using diluted PdC12 solutions with very low acid content. 2. Pd deposition occurs via hydrolisis of Pd-chloro-aquo complexes over basic sites on the support surface. This reaction allows for direct elimination of C1- during the deposition reaction.

Acknowledgements This work has been supported by CNR-Roma- Progetto coordinato "combustione catalitica a fini energetici e ambientali".

REFERENCES 1. H. Arai and H. Fukuzawa, Catalysis Today, 26 (1995) 217. 2. M.F.M. Zwinkels, Catal. Rev. Sci. Eng., 35 (1993) 319. 3. G.Groppi et al. Catalysis, 13 (1997) 85. 4. K Sekizawa et al., J. Catal, 142 (1996) 655. 5. R.D. Gonzalez and H. Miura, Catal. Rev. Sci. Eng., 36 (1994) 145. 6. J.A., Schwarz, Catalysis Today, 15 (1992) 395 7. C. Contescu and M.I. Vass, Applied Catalysis, 33 (1987) 259. 8. D.O. Simone et al., Applied Catalysis, 70 (1991) 87. 9. G. Groppi et al. J. Material Science, 29 (1994) 3441 10. G. Busca et al., Catalysis Letters, 31 (1995) 65 11. F.P. TreadweU, "'Chimica Analitica", Vallardi, VII Ed., Vol. II, 1966 12. G. Groppi et al., Appl. Catal. A: General, 104 (1993) 101 13. G. Groppi et al J. Solid State Chem., 114 (1995) 326 14. R.W. Maatman, Ind. Eng. Chem., 51 (1959) 913

91998 Elsevier ScienceB.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

127

Characterization of y-A1203 supported P d - C u bimetallic catalysts by E X A F S , AES and kinetic measurements Albin Pintara'*, Jurka Batlsta, Iztok Ar6onb and Alojz Kodre c 9

a

aNational Institute of Chemistry, Hajdrihova 19, P.O. Box 3430, SI- 1001 Ljubljana, Slovenia bSchool of Environmental Sciences, P.O. Box 301, SI-5001 Nova Gorica, Slovenia eFaculty of Mathematics and Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia

Pd-fu]y-A1203 bimetallic catalysts were prepared according to different impregnation sequences of ~/-A1203 and characterized by XRD, EXAFS and AES. The catalysts were tested for the selective hydrogenation of aqueous nitrate solutions to nitrogen. The reaction selectivity was found to be dependent on the catalyst preparation procedures, which affect the spatial distribution of metallic copper and palladium phases. The AES examination shows that in the catalyst exhibiting a higher nitrogen production yield, the very first surface sublayers are enriched with palladium atoms. On the other hand, the EXAFS investigation confirms no significant structural differences among catalyst samples synthesized, which results into identical activity for nitrate removal. 1. INTRODUCTION Groundwater pollution by nitrates, which are perhaps the most ubiquitous of all groundwater contaminants, is a widespread problem in many locations in the world. Man-made or mancaused sources of nitrogen introduction into the subsurface environment include agricultural fertilizers, septic tank systems, and animal waste disposal. A number of hydrogeological factors, agricultural practices and land usage influence the concentration of nitrates in groundwater at specific locations. Canter [1] and Sell et al. [2] provide statistical data which represent the extent of drinking water contamination by nitrates in United States and European countries. The data clearly show that for some locations, nitrate concentration in water has been found to be at risky levels up to 200 mg/L. Since this value is excessively higher than the admissible concentration (i.e., 50 mg/L) set by the European Drinking Water Directive, the nitrate content in such streams should be necessarily reduced in order to avoid health risk. One of the most promising processes for removing nitrate both from drinking water streams and industrial effluents is liquid-phase hydrogenation with noble metal catalysts [3]. The reaction obeys a consecutive reaction scheme in which nitrite appears as an intermediate, while nitrogen and "Corresponding author. Fax: (+386-61) 1259244; E-mail: [email protected].

128 ammonia are the final products. To maintain electroneutrality of the aqueous phase, consumed nitrates are replaced by hydroxide ions. Supported Pd-Cu bimetallic catalysts promote nitrate reduction in spite of inadequate selectivity towards nitrogen production [3, 4]. Since the described method is in a stage of development, further kinetic and mechanistic studies with different systems in aqueous solutions are needed. Relatively few investigations have been published concerning the characterization of Pd-Cu bimetallic catalysts. Structure of these solids has so far been studied by means of XANES, CO adsorption and FTIR analyses [5, 6]. Skoda et al. [5] were also using two test reactions in order to characterize the nature of active surface sites, i.e., gas-phase toluene hydrogenation and decomposition of ethanol. It is demonstrated by these authors that the preparation procedure drastically affects the surface properties of Pd-Cu bimetaUics; more or less separated phases, alloys or one of the phases being coated or partially covered by the other one could be formed. The potential of various Pd-Cu/7-AI203 catalysts which had been prepared according to different impregnation sequences of alumina was recently evaluated in the process of liquidphase nitrate hydrogenation [4, 7]. It was tentatively concluded that the nitrate-to-nitrite reduction step is a structure-insensitive reaction, catalyzed only by metal ensembles composed of Pd and Cu atoms. On the other hand, the nitrite ion can be simultaneously reduced on both Pd clusters and Pd-Cu contacts, the former being more selective. Behavior of various Pd-Cu solids was found to be different only in the amounts of accumulated nitrite ions; its higher production results in lower reaction selectivity. However, a detailed correlation between PdCu/7-AI203 preparation procedures and reaction activities has not been studied so far. Correspondingly, the objective of this work was to determine the structure-activity and structure-selectivity relationships of variously synthesized Pd-Cu/7-AI203 catalysts, and subsequently to test these solids in the process of liquid-phase nitrate reduction. The prepared samples were characterized by EXAFS, AES and kinetic measurements.

2. EXPERIMENTAL

2.1. Catalyst preparation Pd-Cu bimetallic catalysts were prepared by impregnation of the powdered alumina support ( 7-A1203 of high purity from Nikki-Universal; NST-3H type; 154 mE/g; average particle diameter: 25 ~tm; pore diameter: 10-25 nm) with aqueous solutions of copper and palladium nitrate. After every alumina impregnation step, the resulting solids were dried at T=423 K. The bimetallic catalysts were designated as CAT-1 to CAT-5; the calculated loadings of Pd and Cu were equal to 5 and 1.5 wt. %, respectively. The monometaUic Pd(5 wt. %)/7-A1203 was labelled as CAT-6, while a physical mixture of the Pd/7-Al203 and powdered copper particles as (CAT-6 + Cu). The catalyst preparation procedures were carried out as follows: CAT-l: impregnation by palladium nitrate, drying (523 K), impregnation by copper nitrate, drying, calcination (3 h, 773 K in air), reduction (1 h, 773 K in H2); CAT-2: impregnation by copper nitrate, drying (523 K), impregnation by palladium nitrate, drying, calcination (3 h, 773 K in air), reduction (1 h, 773 K in H2); CAT-3: impregnation by copper nitrate, drying, calcination (1 h, 773 K in air), impregnation by palladium nitrate, drying, calcination (3 h, 773 K in air), reduction (1 h, 773 K in H2); CAT-4: impregnation by palladium nitrate, drying, reduction (1 h, 773 K in H2), impregnation by copper nitrate, drying, calcination (3 h, 773 K in air), reduction (1 h, 773 K in HE),

129

CAT-5: coimpregnation from the aqueous solution of copper and palladium nitrates, drying, calcination (3 h, 773 K in air), reduction (1 h, 773 K in H2).

2.2. Catalyst characterization Bulk palladium and copper elemental compositions were determined by inductively coupled plasma-atomic emission spectroscopy on a Thermo Jarell Ash instrument. For ICP-AES measurements the sample was fused with KHSO4 and dissolved with a diluted HCI solution. Zeta potential of catalyst suspensions (0.07 wt. % in distilled water) has been measured by means of a laser zee meter (Pen Chem, model 501) at T=293 K and different pH values, which were adjusted by adding 0.1 M HCI or 0.1 M NaOH solutions. The XRD patterns were recorded on a Philips PW 1710 diffractometer with Cu Kot radiation (~,=1.5406 A) in the 20 range of 10 to 95 ~ EXAFS spectra of monometallic and bimetallic catalysts at the K-edge of Pd and Cu were measured at the Roemo II and Exafs II beamlines of HASYLAB at DESY, Hamburg, with resolution of 3 eV and 1.5 eV, respectively. Higher harmonics of the beam were eliminated by a slight detuning of the monochromator crystals. Due to low metal content and low density of the samples, layers of several mm were required for optimal absorption. They were prepared by pressing the powder between the windows of a liquid-absorption cell. AES sputter depth profiles of the catalyst grains were measured in a Perkin-Elmer PHI-545 A instrument. To prevent the charging effect during AES depth profiling, the samples were embedded in a silver paste and fixed on the nickel foil [8]. A static primary electron beam of 8 keV energy, 0.5 ~tA beam current and about 20 j.tm diameter was used. The samples were ionsputtered with two symmetrically inclined beams of 3 keV Ar + ions, rastered over a surface area of about 5 ram- 5 mm at an incidence angle of 47 ~ The sputter rate of about 10 nm/min was calibrated on a Cr/Ni multilayer structure with a known thickness. The catalyst samples were sputtered to a depth of about 1000 rim. The Auger peak-to-peak heights of A1 (1396 eV), Pd (330 eV), Cu (920 eV) and O (510 eV) were registered against the sputtering time. The concentration of elements was calculated on the basis of relative elemental sensitivity factors Snl=0.16, Spd--0.60, Scu=0.20 and So=0.35. The possible matrix and preferential sputtering effects were not corrected and the procedure is considered as a semi-quantitative analysis [9]. However, the samples were investigated at the same analysing parameters and the results are comparable between themselves. The activity and selectivity tests of the liquid-phase hydrogenation of aqueous nitrate solution were performed in an isothermal semi-batch slurry reactor at the operating conditions given in the caption of Fig. 5, for which both intraparticle and interfacial mass-transfer resistances are negligible. The apparatus, the procedure for these measurements, and the analysis (concentrations of nitrate, nitrite and ammonium ions) of the representative aqueous samples are described in detail elsewhere [4, 7].

3. RESULTS AND DISCUSSION The XRD patterns of the prepared catalysts show all the main characteristic peaks of the yA1203 support (Fig. l a). No characteristic reflections for Pd- or Cu-containing phases were recorded. The particle sizes of the Pd and Cu metallic phases in the catalysts were either too small to be detected or in the amorphous phase. However, one can see in Fig. l a that the

130 catalyst preparation procedures mainly influence the peak position at 20 close to 40 ~, which could be attributed to the preferred diffusion of Cu and Pd atoms into the y-AI203 lattice [ I0, 11]. According to the bulk chemical analysis, the catalysts have the following content of deposited metals: Pd=4.8 and Cu=1.45 wt. %. Metal deposition on the y-A1203 support during the preparation of catalysts lowers the specific surface area (BET method) for about 15 %. The isoelectric points of the alumina support and CAT 1-6 samples, obtained from streaming potential measurements, were found to be in the range 8.5-8.8 (Fig. lb). 1.00 o A

,,,..

==

o.rs

,=

30

7-AI203 support ]a

-'"

I

9 CAT-3 CAT-6

..13---.

t a : ~zx

20

.. .... ..

.. 9 >

.Q

T: 293 K

""

Cear_: 0.07 wt. %

-. "..

9

"..."~x.::~..

10

il..

..

.2 "~ ~. o . 9, . "

.~ O.SO

"""0.

"-.k "

&

"".'~

t~

~ 0.25

.... o--- y-AI203 support

e~

-20 -

'~

.... .... 9 CAT-3 .... z~.-. CAT-6

0.00

36

38

40 2| o

42

44

-30

2

i 4

I i 6 8 pH v a l u e , /

, 10

Figure 1. X-ray diffractograms (a) and zeta potential as a function of pH value (b) for various monometallic and bimetallic catalysts tested in this study.

The measured EXAFS spectra of samples CAT 1-6 were analyzed by the UWXAFS code [12, 13]. The shape of the Pd K absorption edge, which was found identical with that of bulk Pd, suggested a model based on the fcc crystal structure of Pd metal When 9 applied on the spectrum of CAT-6, excellent fit was found for the region from 0.2 nm to 0.55 nm in the Fourier transform, using the 8 shortest scattering paths of the lattice. For the lower part of the FT spectrum below 0.2 nm, oxygen atom neighbors at 0.19 nm, presumably from contact layer with the substrate, are introduced. The agreement (Fig. 2a) indicates that clusters with fcc structure of Pd metal are formed 9Their size is estimated by the cutoff in the FT spectrum at approximately 0.55 nm. Specifically, the absence of a prominent peak at 0.7 nm which is found in FT spectra of bulk Pd gives a definite upper limit on the cluster size. A better estimate of the cluster size is obtained from the number of first neighbors (6.4), which is considerably lower than the coordination number 12 of an infinite lattice. A calculation shows that the observed value is reached in a cluster of about 20 atoms in a tentative shape of an octahedron with a diameter of 0.56 nm formed by two layers of nearest neighbors around a central atom. The cluster model is extended to samples with Cu content (CAT 1-5) by adding a layer of Cu atoms on top of one octahedron face of the cluster. Again, the agreement is perfect, as shown in Fig. 2b. The entire set of samples is explained by varying the value of the Cu coordination number and of the auxiliary parameters by less than 10% (Table 1).

131 Table 1 Number of neighbors in the first three coordination shells around Pd samples, obtained from Pd Ls-ed~e and Cu K-edge EXAFS spectra Pd neighbors Neighbor (R) CAT:'I CAT-2 CAT-3 CAT-4 O (2.00 A) 0.97+0.0.9 1.19+0.1 1.39+0.14 1.26+0.15 Cu (2.61)k) 2.9+0.2 2.4+0.2 1.9+0.2 2.3+0.2 Pd (2.73 A) 5.2+0.1 4.7+0.1 4.7+0.2 5.0+0.2 ,,

,,

,.,

,.

,

,,,,

'

,

Nei~;hbor (R) 0 (1.91 A) Cu (2.54 ,~) Pd (2.62 A)

,

,,,

,,,,.

and Cu atoms in CAT 1-6 .. ,. C A T 2 5 1.42+0.12 2.1+0.2 4.6+0.1 .,.

6.3+0.1

.

Cu neighbors CAT-1 CAT 2'5 2.11+0.07 2.22+0.07 1.26+0.08 0.81+0.12 5.20+0.10 5.60+0.10

experiment

experiment . . . . . . model

model

,11 20

CA~-6 1.35+0.08

... C :1

0..,.

6~ "0

i-

4 E I-u.

5

21

1

2

3

4 R,A

5

6

7

0~

,

,

= "

0

1

2

3

V, V , ~ l ~ 4

5

6

7

R,A

Figure 2. Fourier transform magnitude of Pd L3-edge EXAFS spectra measured on samples CAT-6 (a) and CAT-1 (b). At first sight, the EXAFS spectra of the Cu-containing samples at the Cu K-edge seem less revealing (Fig. 3). Due to the smaller content of Cu, they are obscured by noise to a considerably greater extent, so that the identification of farther neighbor shells is less reliable. However, with the same cluster model as above, a well established Cu-Pd correlation is found, and a weak Cu-Cu correlation (Table 1). The number of O neighbors is considerably larger for Cu than for Pd: it can be surmised that Cu atoms are mostly retained on the surface of the cluster. Existence of a mixed oxide, PdxCuyO, found in [ 14], cannot be excluded. The shape of K-edges of both Pd and Cu, however, strongly resembles those of bulk metals, so that zerovalence state is definite for Pd and prevailing for Cu. The presence of another hypothesized Cu compound, a copper-aluminium spinel formed by a reaction with the substrate, is not supported

132 by the data. In brief, EXAFS analysis shows 20 that palladium clusters with a diameter of experiment 0.56 nm are formed in all mono- and . . . . . . model bimetallic catalysts samples. In bimetallic catalysts, the Cu atoms are predominately .~ 15 situated on the surface of the Pd clusters. No significant structural difference is observed '~ for different catalyst preparation conditions. ~ 10 To obtain a relation between microscopic (i.e., structure) and macroscopic (i.e., + reaction selectivity) observations, AES was E employed to determine depth profiles of the ,Iv s elemental composition of CAT 1-5 samples. It turns out from the forthcoming discussion 0 that with regard to the observed catalytic 1 2 3 4 5 6 7 0 properties for the liquid-phase nitrate R,A reduction, the behavior of catalysts tested is identical either to CAT-3 or CAT-4 Pd- Figure 3. Fourier transform magnitude of Cu KCu/7-A1203 samples. Correspondingly, in the edge EXAFS spectra measured on CAT-1. text below only the results of AES examination of these two solids are presented. The depth distribution of constituents is shown in Fig. 4. The surface and the thin layers beneath the surface are composed of aluminium, palladium, copper and oxygen. The AES analysis of the CAT-3 sample (Fig. 4a) revealed copper atoms (0.5 at. %) only in layers located about 300 nm under the surface. The copper concentration then raises to 1.5 at. % (approximately up to 600 nm), and does not change significantly with further increasing depth. It is interesting to note that the palladium concentration simultaneously increases from the surface to the layers up to about 100 nm below the CAT-3 surface. Then the Pd content decreases with increasing depth. To summarize, the depth distribution of Pd and Cu metallic phases in the CAT-3 indicates more Pd in the thin layers under the surface, while in the same depth region Cu was not detected. The averaged AES depth profiles of elements in the CAT-4 sample are shown in Fig. 4b. The lowest palladium concentration and, at the same time,, the highest copper concentration were determined on the surface of the catalyst. In depths of about 10 to 50 nm beneath the surface, the copper content decreases slowly, and then remains almost unchanged in deeper layers. On the other hand, the palladium concentration first slightly increases and then slowly decreases in the same depth region. Its surface concentration (4.0 at. %) is lower than at any analyzed depths. In contrast to the results for the CAT-3 sample, the surface and the thin sub-layer (approximately up to 10 nm) of the CAT-4 are enriched with Cu. To conclude, the obtained AES depth profiles of the elemental composition of both catalysts clearly reflect the influence of preparation conditions on the spatial distribution of copper and palladium metallic phases. Temporal course of the liquid-phase nitrate reduction obtained in an isothermal semi-batch slurry reactor in the presence of various catalysts is shown in Fig. 5. The results demonstrate that differently prepared Pd-Cu/u catalysts, and the physical mixture containing Pd/7A1203 (CAT-6) and metallic copper particles exhibit a very similar activity. In the latter case, the Pd-Cu active sites that promote liquid-phase nitrate reduction were formed in situ by collision

133

.... o - - - A i

a

.'[3"1:3 . . . . . . . . . . . . . O ......

---4~---0 Pd

---- 9

Cu

~.

k

.9. ~ ~

0

50

.r"l

....... rl ........

.... n . . .

13......

"'"--rl .....................

: : : : : - ~ . ..... ~ . . . . . . . . . . . . . . . . .

. . . . . . .

.%

50~

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

6

..... ~....

:"

"'o.

- 4

~

).o"

"'-....

:'

{

"""O

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

3"

o

0 .............

---~---AI ---~--- O

40 -O.. 0 ........ 0 . . . ) . . . . . . . . . . . . . . 0 ..... .(3 ..... " . . . . . . . . . O- . . . . . . . . . . . . . ........

9.............

2"5"

....o---Pd

---~---Cu

9.............

.~ .... -

'A.-A .............

..

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

Jk ............. k .....................

...... A""

41()

0 _= _ = . . : - , '2i 0

60

30

100o

80

;o

i

20

0

Sputter time, min

,o

8'0

~oo 0

Sputter time, min

Figure 4. AES sputter depth profiles of the aluminium, copper, oxygen and palladium in the investigated layers of the (a) CAT-3 and (b) CAT-4 samples. 200: T: 293 K PH2:1.0 bar; Ptot.: 1.0 bar

,,,,,.I

E

150

N: 450 rpm

....,

Ccat.: 1.0 g/L; dp: 25 ~tm

"-~

O =11= L_

=

100

r

0 r

z

=

CAT-1 CAT-2

z~

CAT-3

o

(..

o"

CN%,0:200 mg/L

"=.

so -

0

0

"~149 .=

9 CAT-4 v CAT-5 9 CAT--6 + Cu (1 g/L) I

I,

I

10

20

30 T i m e , min

....

0 i

40

50

60

Figure 5. Concentration of nitrate ions as a function of time obtained in the slurry reactor over various Pd-Cu bimetallic systems.

of particles; however, at the given reaction conditions the nitrate disappearance rate was found to be independent of the collision probability [7]. The nitrite ions concentration vs. time dependencies obtained during the catalytic nitrate reduction are illustrated in Fig. 6. It can be seen that the lowest nitrite amounts are determined for the CAT-3 sample.

134 30

...t.

.

o

9

."e. ". . . . .

,.J O~

E

~

20 _

9

0 ,,==

;

9

E

9

,

~ '

i

. o

o,,"

"

- ;,~

e~

,

'."

.-O

9.

.

o

...-

:~, .,

.

.

i, :

".,

, . .

.

q

v.

V"

e~

o

~

.' ,.

b~

~:

z

,

0

CAT-6+Cu(1

g/L)

.

9

9

~.:=.-;~-::

30

CAT-5

9

: ,,

;::*:'

-

CAT-4

.

+::

":

9

v

",.

...-

.~ .IB . . . . . . . . . _ ::,- ......" ...A ..... . ".~, ,,:: : : ," B . ...i~"" "'~,;r.. ::, ,.:.

10

CAT-1

[] CAT-2 A CAT-3

e'"

...........

,

60

90

120

Time, min

Figure 6. Nitrite ion concentrations as a function of time for the liquid-phase nitrate reduction over the CAT 1-5 and (CAT-6 + Cu) samples. For reaction conditions, see Fig. 5.

12

.~.:.......... ~ ............ ::::::::::::::::::::::::::

... ,-i O1

E ff

8-

. 01 t~ IN ,4,,,i

~.~6-~ 9

1~?

. ................. 9t............

..=============================== ...............................

,, .~'-"

4

o

,~" lb.:

CAT-1

9 CAT-2 zx CAT-3

9 CAT-4

!.:B

z

.v .............

~ ........... = .........................

.v ............

!2, ~

v

CAT-5

9 CAT-6 + Cu (1 g/L) 0,,, 0

I

I

100

200

,

I,

300

,

400

Time, min

Figure 7. Ammonium ion concentration vs. time dependencies obtained during the catalytic hydrogenation of aqueous nitrate solution over the CAT 1-5 and (CAT-6 + Cu) samples. For reaction conditions, see Fig. 5.

135 Appropriate concentrations as a function of time for ammonium ions formed during the nitrate reduction are presented in Fig. 7. The reaction selectivities (defined as the molar percentage of initial nitrate content converted to nitrogen at complete conversion of nitrates and nitrites) of the Pd-Cu systems used in this study are given in Table 2. The minimum concentrations of ammonium ions produced were observed again for the CAT-3 sample. The comparison of ammonium ion concentration vs. time dependencies and the values listed in Table 2 show that the reaction selectivity obtained in the presence of CAT-4 equals to the one evaluated for the physical mixture consisting of CAT-6 and metallic copper particles. This finding and in Fig. 5 shown results allude that dispersion of Pd and Cu atoms on the alumina support has no influence on the observed activity and selectivity. Furthermore, the results of EXAFS examination (Table 1), and the fact that the physical mixture consisting of CAT-6 and metallic copper particles exhibits the same nitrate disappearance rate as it was observed in the presence of CAT 1-5 samples, confirm that the nitrate-to-nitrite reduction step is a structureinsensitive reaction, as proposed already in the previous paper [7].

Table 2 Selectivities of Pd-Cu bimetallic samples for the liquid-phas e nitrate hydrogenation Catalyst Selectivity, mol. % CAT-1 83 CAT-2 90 CAT-3 91 CAT-4 82 CAT-5 88 ..... (CAT-6 + Cu) .......... 8,2

Data illustrated in Figs. 6 and 7 clearly demonstrate that the reaction selectivity is related to the amount of free (i.e., in the aqueous solution accumulated) nitrite. As observed, the catalyst preparation procedure in which the alumina was first impregnated by copper salt followed by the deposition of palladium salt (CAT-2, CAT-3), enhances the nitrogen production yield. The influence of the catalyst synthesis on the distribution of active sites and consequently on the reaction selectivity can be explained by taking into account the results of AES examinations and catalytic tests. As reported above, copper appears only in layers about 300 nm under the surface of CAT-3, which means that a reaction zone for +nitrate reduction (consisting of Pd and Cu phases) is located in the interior of a particle and covered by a layer composed of Pd atoms. Due to the higher adsorption affinity of nitrate ions towards Pd-Cu clusters [7], nitrite once formed is forced to migrate into the aqueous solution. During its counter-diffusion from catalyst pores, the nitrite is selectively transformed to nitrogen in the Pd layer, which results in a lower accumulation of nitrite in the bulk solution. After the nitrate has been consumed (at a reaction time of approximately 50 min), the accumulated nitrite is converted at the Pd-Cu sites nonselectively to ammonium ions [7], which is confirmed by unsymmetrical concentration-time profiles for nitrites in Fig. 6. However, it is obvious that during the in situ generation of Pd-Cu active sites (CAT-6 + Cu), no Pd layer playing the same role as described above is created, which leads to lower nitrogen formation (Fig. 7). Since the behavior the CAT-l, CAT-4 and (CAT-6 + Cu) systems is almost identical also with respect to the maximum amounts of nitrites

136 accumulated in the aqueous phase (Fig. 6), it can be concluded that in the presence of CAT-1 and CAT-4 samples nitrate is reduced mainly on the surface of catalyst particles.

4. CONCLUSIONS Combining the results of AES and EXAFS analyses, the following microscoiSic picture of the catalysts can be constructed: Pd is aggregated in clusters of approximately 0.56 nm diameter into which some of Cu atoms are embedded, as evidenced by Cu-Pd correlations established for elements on both sides. Different mesoscopic distributions of Pd and Cu from AES show that some of Cu atoms are attached elsewhere in the matrix: the EXAFS evidence is provided by the Cu-O correlation, which is almost twice as strong as Pd-O correlation. The preparation procedure has a minor effect on the microscopic state of the final product: only CAT-1 sample differs slightly from the average, while the coordination parameters of samples CAT 2-5 stay practically within the estimated error intervals in Pd EXAFS, and are indistinguishable in Cu EXAFS. Behavior of variously synthesized Pd-Cu bimetallic solids in the process of catalytic liquidphase hydrogenation of aqueous nitrate solutions differs in the amounts of accumulated nitrites and final concentrations of ammonium ions produced. The reaction selectivity is appreciably dependent on the catalyst preparation procedures; the specific spatial distribution of metallic copper and palladium phases on ~'-A1203 in the CAT-2 and CAT-3 samples results in minimal formation of ammonium ions.

REFERENCES

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

L.W. Canter, Nitrates in Groundwater, CRC Press, Boca Raton, 1996. M. Sell, M. Bischoffand D. Bonse, Vom Wasser, 79 (1992) 129. S. H0rold, T. Tacke and K.D. Vorlop, Environ. Tech., 14 (1993) 931. A. Pintar, J. Batista, J. Levec and T. Kajiuchi, Appl. Catal. B:Environmental, 11 (1996) 81. F. Skoda, M.P. Astier, G.M. Pajonk and M. Primet, Catal. Lett., 29 (1994) 159. M. Fern/mdez-Garcia, J.A. Anderson and G.L. Hailer, J. Phys. Chem., 100(40) (1996) 16247. 7. A. Pintar and T. Kajiuchi, Acta Chim. Slovenica, 42(4) (1995) 431. 8. A. Zalar, Mikrochim. Acta, 1 (1980) 435. 9. D. Briggs and M.P. Seah (eds.), Practical Surface Analysis, Vol. 1, Wiley, New York, p. 201, 1990. 10. G. Strukul, F. Pinna, M. Marella, L. Meregalli and M. Tomaselli, Catal. Today, 27 (1996) 209. 11. B.R. Strohmeier, D.E. Leyden, R.S. Field and D.M. Hercules, J. Catalysis, 94 (1985) 514. 12. E.A. Stern, M. Newville, B. Ravel, Y. Yacoby and D. Haskel, Physica B, 208&209 (1995) 117. 13. J.J. Rehr, R.C. Albers and S.I. Zabinsky, Phys. Rev. Lett., 69 (1992) 3397. 14. M. Fern/mdez-Garcia and G.L. Haller, J. Phys. IV France, 7 (1997) 875.

91998 l~lseviertscience B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

137

E n c a p s u l a t i o n of m i c r o s c o p i c catalysts in p o l y v i n y l alcohol h y d r o g e l beads U. Prtisse, V. Morawsky, A. Dierich, A. Vaccaro and K.-D. Vorlop Institute of Technology, Federal Agricultural Research Centre, Bundesallee 50, 38116 Braunschweig, Germany

Well defined microscopic supported mono- and bimetallic catalysts and colloidal metals were prepared and encapsulated in hydrogel beads of polyvinyl alcohol resulting in a well defined macroscopic catalyst. Polyvinyl alcohol hydrogel beads possess high chemical and mechanical stability as well as very good diffusional properties. The encapsulated catalysts showed high activity, selectivity and effectiveness factors for the water phase nitrate and nitrite reduction with hydrogen or formic acid as reducing agent.

1.

INTRODUCTION

The preparation of well defined microscopic catalysts (one or more metals on powdered supports, colloidal metals) is possible but these catalysts are difficult to handle in conventional technical reactors. The preparation of well defined macroscopic catalysts by impregnation of macroscopic supports with metals isn't easy at all, especially for bimetallic catalysts due to different impregnation profiles of the metals. The adsorption of colloidal metals on supports always raises leaching problems even on microscopic supports. Nevertheless, the preparation of well defined macroscopic catalysts is possible by encapsulation of well defined microscopic catalysts in macroscopic, polymeric, gel-type supports (Figure 1). By this enlargement these "supported microscopic catalysts" can be handled in conventional technical reactors. Here, the encapsulation of mono- and bimetallic catalysts supported on powdered alumina or colloidal metals in hydrogel Figure 1. Principle of the encapsulation beads of polyvinyl alcohol and their use for of microscopic catalysts in macrosthe reduction of nitrate and nitrite in water as copic, polymeric, gel-type supports. model reaction are described.

138

2.

CATALYSTS ENCAPSULATED IN POLYVINYL A L C O H O L H Y D R O G E L BEADS

The ability of polyvinyl alcohol (PVAL) to form hydrogels, preferably by freezingthawing, by crosslinking with boric acid or formaldehyde or by complexation with Ti 4§ is already widely used in biotechnology for the encapsulation of biocatalysts such as enzymes or whole cells [ 1-3]. The use of PVAL for the encapsulation of metal catalysts was reported by Prtisse et al. [4].

2.1. Polyvinyl alcohol matrix For the preparation of the PVAL gel beads the freezing-thawing method was applied. For the preparation of gel beads with a sufficient mechanical stability the use of PVAL with a degree of saponification of at least 98 % is necessary as well as degrees of polymerization of about 1400 or higher. We used PVAL | 10-98 and 28-99 purchased by Hoechst. The PVAL solution were made by solving 10.7 % 10-98 or 7 % 28-99 in a hot mixture of water and glycerol or polyethylenglycol (PEG), the mixture having a w/w-ratio of 90:10. Complete saponification to further enhance gel stability was achieved by adding stoichiometric amounts of NaOH to the solution, which afterwards was neutralized with HC1. Either liquid nitrogen, orange oil at -50~ or silicone oil at -20~ were used as a cold gelation medium. After freezing, the beads were slowly thawed after the nitrogen was evaporated or within the oils, respectively, using a Dewar vessel. For the preparation of the PVAL encapsulated catalysts the preformed supported metal catalyst (see 3.1) was simply added to the polymer solution (or generated within in case of colloidal palladium catalysts) before the solution was brought into the cold medium for gelation.

2.2. Bead preparation technology Several general techniques are known for droplet generation like simple dropping (Figure 3, left), electrostatic methods, vibration methods, annular atomizer, spinning disk or spinning nozzle atomizers [5-6]. These techniques are not suitable for the preparation of small beads (< 1 mm) from highly viscous polymer solutions such as PVAL solutions. In order to resolve this problem we recently developed a new technique: the jet cutting method (Figure 3, right, [7]).

Figure 2. Gelation of polyvinyl alcohol by freezing-thawing. The microscopic catalysts are encapsulated in the gel structure.

Figure 3. Preparation of gel beads by the dropping (left) or the jet cutting method (fight), p = pressure.

139 The jet cutting method was also used for the preparation of PVAL encapsulated catalysts. By pressing a supension of the microscopic catalyst in a PVAL solution out of a nozzle a solid liquid jet is formed. By a rotating cutting tool this jet is then cut into uniform cylinders which form beads by falling down into a cold medium where gelation due to freezing-thawing occurs. The microscopic catalysts are encapsulated in the gel structure (Figure 2). The jet cutting method enables the preparation of gel beads in the size range of about 0.3-3 mm in high rates (up to 20 kg per hour and per nozzle for 1 mm beads) even from highly viscous solutions. The beads are uniform in diameter (Figure 4). With a special nozzle the preparation of PVAL encapsulated catalysts having an egg-shell type distribution is possible (Figure 5, see also Figure 1). A catalytically inactive PVAL core is surrounded by a PVAL layer of approximately 200-400 pm containing the microscopic catalyst. This catalyst distribution is favourable in order to avoid strong diffusion limitations.

Figure 4. Typical particle size distribution of PVAL beads prepared by the jet cutting method.

Figure 5. Nozzle system for the preparation of egg-shell catalysts.

2.3. Properties of gel beads The gel beads prepared in either of the cold media possessed high mechanical stability and a very high, rubber-like elasticity. In general, the stability of the beads increases with increasing polymer content of the gel and with increasing content of the polyol used as auxiliary agent for gelation (The beads were washed free of polyols after thawing). The beads do not show any abrasion in tank reactors equipped with magnetic stirrers nor in loop reactors equipped with propeller mixers for at least months. In aqueous solutions they are stable in the examined pH range of 1-13 and up to temperatures of about 60-80~ Higher temperatures lead to the rupture of hydrogen bonds resulting in a polymer solution which may be useful for catalyst recycling. The PVAL gel is a three dimensional network of polymer chains in water. An electron micrograph of the inner sponge-like gel structure is shown in Figure 6, whereas the gel structure at the surface of the beads is much more homogeneous. The water content of the gel is 90 % or even higher. In the gel structure the substances can diffuse in three dimensions only hindered by the polymer chains and the catalyst particles. Therefore, compared with convert-

140 tional porous supports, e.g. silica or alumina, where diffusion is possible only in the pores the effective diffusion coefficient of the matrix is considerably higher for the PVAL gel than for porous supports (see Table 1). Effective matrix diffusion coefficients up to 85 % of Do are possible for PVAL gels [8]. Table 1 Effective matrix diffusion coefficients (Deft, matrix) of nitrate and nitrite in different matrices in percent of the non-hindered diffusion coefficient in solution Do. matrix

mean pore diameter,/k

Deft, matrix, a~ of Do

A1203, Type HL, Martinswerke

150

!3

SiO2, Type XWP 500, Grace

500

22

PVAL hydrogel, 7 % 28-99

no pores

36

Figure 6. Electron micrograph of the inner gel structure of a PVAL bead, right bar = 1 pm. PVAL gel made of 10.7 % 10-98 in water-PEG 1000 (90:10) using orange oil at -50~

3.

CATALYTIC NITRATE AND NITRITE REDUCTION AS MODEL REACTION

Nitrite can be reduced via several intermediates on monometallic catalysts, preferably Pd/A1203, forming nitrogen as main product and ammonium as by-product. For the reduction of nitrate an additional, promoting metal, e.g. Cu, Sn or In, is needed [9-10]. Either hydrogen or formic acid can be used as reducing agent. A reaction scheme of the model reaction is displayed in Figure 7.

3.1. Preparation of microscopic catalysts The supported metal catalysts were prepared using alumina (type HL, Martinswerk, calcined for 24 h at 1000~ before use, particle diameter 2-3 pm, BET-surface 36 m2/g, mean pore diameter 70 ,~) as support. The deposition-precipitation of the metals on the support was

141 achieved by dropping the metal precursors, which were the chlorides for each metal, in an alkaline suspension of the support. Bimetallic catalysts were prepared either by co-deposition of the two metals in case of PdCu and PdIn or subsequent deposition in case of PdSn. After deposition the metals were reduced in the liquid phase using sodium formate at 80~ Before use the catalysts were dried at 70-80~ The palladium content for all catalysts was 5 % whereas the copper and tin content was 1.25 % and the indium content 1%. The colloidal palladium was prepared by hydrogen reduction of the palladium precursor in the PVAL 28-99 solution afterwards used for gelation. Thus, the PVAL serves as agent to stabilize the colloid and as gel matrix [4]. 3.2. Reaction conditions Catalytic tests were made in a 500 ml batch reactor at atmospheric pressure, 10~ and initial nitrate/nitrite concentrations of 100 mg/1. The pH value was kept constant at 5 by adding HC1 in case of hydrogen as reducing agent or formic acid in case of its use as reducing agent. When using hydrogen it was continuously bubbled through the reaction suspension to keep the H2-concentration in the liquid phase constant. The activity is related to a conversion higher than 99.5 %, the selectivity and ammonium formation refer to the total removal of the nitrate/nitrite ions. 3.3. Influence of the pH value When nitrate/nitrite are reduced to nitrogen the stoichiometric amount of hydroxide ions are formed. If ammonium is formed twice as much hydroxide ions are released (Figure 7). The catalysts' activity and selectivity strongly depend on the pH value. For all catalysts ammonium formation increases, and thus selectivity decreases, with rising pH as shown for Pd/A1203 in Figure 8. The activity decreases with rising pH for Pd/A1203 and also PdSn/A1203 (similar to the curve in Figure 8) whereas Pdln/A1203 and Pd/CuAI203 have a maximum activity at pH 6 or pH 8, respectively. 1800 1500

(

paancl.~

NO; ~

~ NO2 ~

OH

N20 "O"~" N2 N2

NO ~ /

I~ 0 reducing agent: H2 or

OH"

" NH;

HCOOH

t~ activity 15

1200 b-

Z E ".,~

E E O .m .4...

900

10

E

5

o E E

E

6O0

300

c

pH value

Figure 7. Reaction scheme of the catalytic nitrate/nitrite reduction.

Figure 8. Activity and ammonium formation for the nitrite reduction with Pd]AI203 at different pH values.

Therefore, due to the release of hydroxide ions a pH gradient will be built up inside macroscopic catalyst particles both leading to a drop in activity and selectivity thus leading to a significant increase in the ammonium amount formed for all macroscopic catalysts. This

142 was first described by Vorlop et al. in [ 11] for nitrite reduction with supported palladium catalysts, later by Prtisse et al. [4] for PVAL encapsulated catalysts. The authors showed that buffering eliminates the negative impact of the pH gradient on activity and selectivity. The use of formic acid as reducing agent should also have a buffering effect [12]. When being decomposed by the catalyst hydrogen species capable of reducing nitrate and its following intermediates together with carbon dioxide are formed, the latter ,,in situ" rising the buffering capacity of the water just at the catalyst surface. This will result in a minor ammonium formation compared with the use of hydrogen.

4.

CATALYTIC TESTS W I T H PVAL ENCAPSULATED CATALYSTS

The catalytic test of the catalysts encapsulated in PVAL gel beads were made according to the reaction conditions described above. Both PVAL 28-99 and 10-98 were used for bead preparation. Neither significant differences in stability or catalytic performance were detected for these materials, nor any abrasion of the gel beads or catalyst leaching during the experiments. Beads > 2 mm were made by simple dropping as displayed in Figure 3 and Figure 5, respectively, for the egg-shell catalyst, smaller beads were made by jet cutting (Figure 3). 4.1. Nitrite reduction with hydrogen The reduction of nitrite was performed with different encapsulated catalysts: Pd/AI203, colloidal palladium and PdCu/A1203. The activity and ammonium formations of some encapsulated monometallic palladium catalysts are given in Table 2. An egg-shell distribution of the catalytic material is favourable concerning a high activity and minor ammonium formation since inside the relatively small egg-shell the pH gradients are not as high as inside a pellet with a homogeneous distribution. Table 2 Activity and ammonium formation of different palladium catalysts encapsulated in PVAL for nitrite reduction with hydrogen [4]. The activity and ammonium formation of the free catalyst are shown in Figure 8. PVAL solution made of 28-99 in water-glycerol, cold medium: N2. distribution

bead diameter, mm

activity*, mg NOz-/(h,g cat.)

ammonium formation, mg

Pd/A1203

homogeneous

4

0.15

9.2

Pd/A1203

egg-shell

4

0.22

2.1

Pd/A1203

homogeneous

3

0.12

6.8

Pd-colloid

homogeneous

3

0.23

4.4

catalyst encapsulated

* Activity is based on g PVAL-catalystwith a palladiumcontent of 0.05 %. Compared with the supported palladium catalyst the colloidal palladium catalyst has a significantly higher activity as well as a lower ammonium formation. This catalyst was also studied in a continuous flow loop reactor. During 14 days of operation no loss in activity nor selectivity was observed suggesting that no leaching of the nanoscaled colloidal particles

143 (mean diameter 10 nm) occured. Probably, this stability against leaching is due to the fact that the PVAL molecules stabilizing the metal particle are also part of the gel matrix. The influence of the bead diameter on activity and selectivity was studied for the nitrite reduction with encapsulated Pd/A1203 catalysts (Table 3) and PdCu/A1203 catalysts (Table 4). The decrease in activity and selectivity of encapsulated Pd/A1203 catalysts with increasing bead diameter can be attributed to the pH gradient which should be lower in gel beads smaller in diameter. Table 3 Activity, ammonium formation and effectiveness factor for nitrite reduction with hydrogen using encapsulated Pd/A1203 catalysts of different sizes. Activity of the free catalyst - 900 mg NO2-/(h,g cat.), selectivity to N2 - 99.9 %. PVAL solution made of 10-98 in water-PEG 400, cold medium: silicone oil. bead diameter, mm

activity*, mg NOzY(h,g cat.)

selectivity to N2, %

effectiveness factor, %

0.6

1.11

97.2

6.2

0.8

1.05

96.9

5.8

1

0.75

95.7

4.2

1.3

0.54

95.4

3.0

2.5

0.30

90.9

1.7

* Activity is based on g PVAL-catalystwith 2 % Pd/A1203catalyst. Table 4 Activity, ammonium formation and effectiveness factor for nitrite reduction with hydrogen using encapsulated PdCu/AI203 catalysts of different sizes. Activity of the free catalyst = 220 mg NOzT(h,g cat.), selectivity to N2 = 99.2 %. PVAL solution made of 10-98 in water-PEG 400, cold medium: silicone oil. bead diameter, mm

activity*, mg NO2-/(h,g cat.)

selectivity to Na, %

effectiveness factor, %

0.4

2.24

99.2

51

1.4

0.94

98.2

21

* Activity is based on g PVAL-catalystwith 2 % PdCu/A1203catalyst. With encapsulated PdCu/A1203 catalysts having bead diameters smaller than 0.4 mm the negative effect of the pH gradient on the selectivity was negligable for nitrite reduction. The higher effectiveness factors of the encapsulated PdCu/A1203 catalyst compared with the Pd/AI203 catalyst results from the lower activity of the free catalyst together with a probably different pH dependance of the PdCu catalyst. 4.2. Nitrate reduction Nitrate reduction was performed with PVAL encapsulated PdCu/A1203, PdIn/A1203 and PdSn/A1203 catalysts.

144 In contrast to the nitrite reduction the activity and selectivity of the nitrate reduction with encapsulated catalysts is affected not only by the diffusion inside the gel beads but also by an interaction between the metals and the PVAL. This interaction leads to a drop in activity and especially in selectivity. Since nitrite reducing catalysts are not affected by this interaction we assumed that the PVAL will interact particularly with the second metals necessarely being present in nitrate reducing catalysts. In case of the nitrite reduction with a PdCu/A1203 catalyst copper of course is present but does not contribute to the selectivity in a positive manner so that no negative interaction should take place in this case. To study this interaction the free, i.e. not encapsulated, nitrate reducing catalysts are compared as prepared with catalysts brought into contact with a PVAL solution used for the preparation of gel beads (Table 6 and 7). After the ,,PVAL impregnation step" the catalysts were separated from the PVAL solution and washed before used for nitrate reduction. Table 6 Activity and ammonium formation for nitrate reduction (hydrogen) with different free bimetallic catalysts. 10-98 was used as PVAL. catalyst

activity, mg NO3-/(h,g cat.) PdCu/A1203

after contact with PVAL

as prepared

ammonium activity, formation, mg/1 mg NO3-/(h,g cat.)

93

3.2

ammonium formation, mg/1

21

8.0

Pdln/A1203

183

1.3

70

4.2

PdSrdA1203

253

1.7

203

5.7

Table 7 Activity and ammonium formation for nitrate reduction (formic acid) with different free bimetallic catalysts. 10-98 was used as PVAL. catalyst

after contact with PVAL

as prepared activity, mg NO3/(h,g cat.)

ammonium formation, mg/1

activity, mg NO3-/(h,g cat.)

ammonium formation, mg/1

PdCu/A1203

75

2.5

17

3.7

PdIn/Al203

260

1.4

80

4.2

PdSrdAI203

437

1.0

40

6.5

For each catalyst and for each of the two reducing agents the extent of the poisoning or better fouling effect of the PVAL is different. For example, the selectivity of the PVAL contacted PdCu/A1203 catalyst is significantly lower when using hydrogen instead of formic acid as reducing agent whereas the activity of the PVAL contacted PdSn/A1203 catalyst ist very poor for formic acid but has only little changed for the use of hydrogen. To minimize the PVAL fouling we tried to protect the metal surface by reversible adsorbing species (nitrite, nitrate, formic acid) before encapsulation. By that significant increases in

145 selectivity were achieved (lowering the ammonium formation about one quarter) whereas only small (almost positive) changes in activity were observed. The negative effect of the pH gradient inside the beads for encapsulated nitrate reducing catalysts is the same as described above for nitrite reducing catalysts. Thus, by reducing the bead diameter higher activity and selectivity can be achieved (Table 8). Table 8 Activity, ammonium formation and effectiveness factor for nitrate reduction with hydrogen using encapsulated PdSn/A1203 catalysts of different sizes. Activity of the free catalyst = 250 mg NO3-/(h,g cat.), selectivity to N2 = 93.8 %. PVAL solution made of 10-98 in water-PEG 400, cold medium: silicone oil. activity*, mg NO3-/(h,g cat.)

selectivity to N2, %

effectiveness factor**, %

0.8

0.9

74.9

18

1.4

0.75

67.9

15

1.8

0.6

63.8

12

bead diameter, mm

* Activity is based on g PVAL-catalystwith 2 % PdSn/AI203catalyst. ** Includes also the deactivation of the bimetallic catalyst. As already stated the use of formic acid as reducing agent should also minimize pH gradients inside the catalyst bead by ,,in situ" buffering. To verify this hypothesis PVAL encapsulated PdSn/AI203 catalysts with a 2 % and 10 % catalyst loading were used for nitrate reduction with hydrogen and with formic acid (Figure 9). The activity of the encapsulated catalyst for nitrate reduction with hydrogen should be higher compared with formic acid (see Table 6 and 7). The effect of the pH gradient should lead to a significant increase in ammonium formation for the PVAL catalyst with 10 % loading compared with the one having a 2 % loading for nitrate reduction with hydrogen, whereas for the use of formic acid no or only minor increase in ammonium formation should occur due to ,,in-situ" buffering. For the same reasons the effectiveness factor should decrease to a much lesser extent for the high loaded encapsulated catalyst in case of formic acid compared with hydrogen as reducing agent. As shown in Figure 9 all these assumptions Figure 9. Nitrate reduction with 2 % and 10 % could be proved right leading to PdSn/AI203 catalyst encapsulated in PVAL hydrothe statement that, indeed, the ,,ingel beads using hydrogen or formic acid as redusitu" buffering of formic acid with cing agent. Bead diameter = 2.5 mm, e-factor = efits benefits for this model reaction fectiveness factor (includes deactivation). occurs.

146 5.

CONCLUSIONS

Microscopic catalysts (powders and colloids) were sucessfully encapsulated in polyvinyl alcohol hydrogel beads and used for nitrate and nitrite reduction in water. The beads can be produced in the range of 0.3-3 mm by the jet cutting method. They were stable, showed no abrasion and no leaching of the microscopic catalyst. High activity and selectivity were obtained for nitrite reduction. Fouling by adsorbed PVAL on the bimetallic catalysts used for nitrate reduction could be reduced by using protective agents during encapsulation. Formic acid as reducing agent was proved to be benefitial for the nitrate reduction due to an ,,in-situ" buffering effect. The aim to prepare a well defined macroscopic catalyst for technical use by encapsulation of preformed well defined microscopic catalysts was achieved. This concept can easily be transferred to other catalytic reactions as well as other types of catalysts.

ACKNOWLEDGMENTS The authors wish to thank the German Government, Ministry of Education, Research, Science and Technology for financial support (Grant 03D0026A8), Hoechst, Frankfurt/Main, Germany, for supplying the polyvinyl alcohol free of charge and Dr. Menge-Hartmann, FALBraunschweig, for her support in electron microscopy.

REFERENCES

(1) (2)

M. Watase, K. Nishinari and M. Nambu, Cryo Letters, 4 (1983) 197. P. Remmers and K.-D. Vorlop, Dechema Biotechnology Conferences, Vol. 5 Part B (1992) 939, VCH Weinheim. (3) V.I. Lozinsky, F.M. Plieva, Proceedings of the International Workshop Bioencapsulation 5, (eds. H. Dautzenberg and D. Poncelet), Potsdam, Germany 1996, 13. (4) U. Prtisse, S. H6rold and K.-D. Vorlop, Chem.-Ing.-Tech., 69 (1997) 100. (5) M. Jekel and K.-D. Vorlop, Proceedings of the International Workshop Bioencapsulation 5, (eds. H. Dautzenberg and D. Poncelet), Potsdam 1996, 1. (6) M.-C. L6vy and D. Poncelet, Biofutur, 4 (1994) 16. (7) U. Prtige, B. Fox, M. Kirchhoff, F. Bruske, J. Breford and K.-D. Vorlop, Chem. Eng. Tech., 21 (1998) 29. (8) U. Prtige, A. Dierich, V. Morawsky, A. Vaccaro, K.-D. Vorlop, F. Fergg and F. Keil, Book of abstracts, Dechema Jahrestagung 1998, (eds. Dechema), Frankfurt, Germany. (9) K.-D. Vorlop and T. Tacke, Chem.-Ing.-Tech., 61 (1989) 836. (10) S. tt6rold, K.-D. Vorlop and T. Tacke, Catal. Today, 17 (1993) 21. (11) K.-D. Vorlop, S. H6rold and K Pohlandt, Chem.-Ing.-Tech., 64 (1992)82. (12) U. Prtisse, M. Kr6ger and K.-D. Vorlop, Chem.-Ing.-Tech., 69 (1997) 87.

91998ElsevierScienceB.V. All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

147

A Scientific Description of Pt Adsorption onto Alumina J. R. Regalbuto a*, K. Agashe a, A. Navada a, M. L. Bricker b, and Q. Chen b aDep.t of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton, Chicago, Illinois 60607 USA bUOP Research Center, 50 E. Algonquin Road, Des Plaines, Illinois 60017 USA ABSTRACT

The physical adsorption theory of James and Healy [1] has been revised with a non-Nernstian treatment of surface potential including a balance to account for proton transfer between the liquid phase and surface [2], and the solvation energy term of Levine [3]. Using model parameters measured independently, six sets of CPA adsorption data over alumina have been simulated to a reasonable degree with. no parameter ad'ustment.} . In addition, experiments using aluminas. . . of different phase, solubility and surface area have been conducted to discriminate b e t w e e n a physical adsorption mechanism and several other prevailing adsorption theories. The experimental results further support tl~e revised physical adsorption (RPA) model. The RPA model is readily applicable to other systems. 1. INTRODUCTION Various models have been proposed to describe the uptake of dissolved metal complexes by oxide surfaces as occurs in catalyst impregnation. These models include "coordination chemistry" models, for hydrotalcite-forming materials such as Ni-A120 3 [4], and are echoed in several studies of Pt-A120 3 I5,61. According to this theory, adsorption of the hexachloroplatinate anion PtC16-z can only Eroceed subsequent to complexation with dissolved aluminum. Quantitative adsorption models pertaining to catalyst impregnation have been employed and developed by several groups [7,8], particularly that of Lycourgtiiotis [8 and references within]. These parameter-laden models are primarily based on chemical" interactions between the metal complexes and oxide surface. Adsorption data can be fit by adjusting a number (perhaps 4-7) of adsorption equilibrium constants. One of the earliest adsorption models was that of James and Healy [1], who proposed that the adsorption of hydrolyzeable metal cations was largely a physical process, capable of being described by an a priori calculation of coulombic and solvation contributions to the adsorption free energy. Qualitative studies of noble metal adsorption onto alumina a n d / o r silica suggest that the principle attraction is electrostatic [9,10,11]. At pH values below the oxide's point of zero charge (PZC), the terminal hydroxyl groups at an oxide surface become protonated and so positively charged and capable of adsorbing anions, and vice versa as pH is raised above the PZC [9]. In practice, the original physical adsorption model [1] contained an adjustable term for "chemical" interactions as a correction for an admitted overestimate of the solvation energy. That the magnitude of this adjustable term was so large may explain the eventual abandonment of this model [12]. * To whom correspondence should be addressed. The support of the UOP Research Center, Des Plaines, Illinois, is greatfully acknowledged.

148 Our work has centered on revising and experimentally verifying the original model of James and Healy [12]. First, the revised physical adsorption (RPA) model contains a more realistic, non-Nernstian calculation of surface potential and includes a proton balance which accounts for oftentimes drastic shifts in solution pH caused by proton transfer to and from the oxide surface [2]. This p H buffering effect of oxides was elegantly demonstrated by the "mass titration experiment of Schwarz [13] a decade ago, but has not been incorporated into an adsorption model until now. A second key is the revised solvat~on free energy term of Levine [3] which yields solvation free energies up to an order of magnitude below those of James and Healy [12]. This term, together with the coulombic term constitute the adsorption free energy. The RPA model contains only one or two mildly sensitive adjustable parameters for oxide charging, and these in principle can be measured independently of adsorption. Our verification of the RPA model for Pt/alumina systems has involved two avenues, one a culling and simulation of all available equilibrium adsorption data from the literature, and second, performing experiments to discern between the mechanisms of the various proposed adsorption theories. Both of these aspects will be presented below. 2. EXPERIMENTAL

Aluminas of differentphase (alpha, gamma, theta, and eta) and surface area (28- 193 m 2 /gin) were used~ and wil~ be !,~entified in figure legends by phase and surface area, for example, 'gamma147 or alpha28'. All of these alumina samples had PZC values in the range 8 - 9. An average value of 8.5 was used for the RPA calculations. In adsorption experiments, the surface loading, or oxide area per liter of solution, was adjusted to the same value (500 m 2/1) ~'or all experiments by adding different masses of oxide. Fifty ml of 200 ppm Pt (1.0 x 10-~ M, pH = 2.5), adjusted to various pH values using HC1 or NaOH, was added to oxide powders previously weighed into polypropylene bottles. The amount of Pt corresponds to about 10% excess of 1.6 ~tmoles P t / m 2, which we believe to be near the maximum Pt coverage [14]. The bottles were placed on a rotary shaker and intermittently sampled for pH and for C1- concentration using a C1- ion specific electrode. At various times 3-4 ml portions of the well mixed suspensions were removed from the bottles and the ~solid was filtered to permit-measurement of Pt and A1 concentrations in the liquid phase by ICP. Adsorption density is calculated as the initial minus final concentration of Pt divided by the surface loading, and is expressed as F = ~tmoles Pt a d s o r b e d / m 2. The different solubility of the various aluminas (alpha is sparingly soluble, gamma is relatively highly soluble in acid solutions) permits an examination of the dependence of Pt adsorption on dissolved alumina. Chloride adsorption, a key component of the chemical adsorption mechanism, has been measured directly. 3. THEORY

The basis for the relatively simple non-Nernstian depiction of the oxide surface is a single site amphoteric model [15]. From the two equilibrium reactions A1-OH2 ~ A1-OH + Hs

(1)

A1-OH ~ A1-O- + Hs

(2)

and the assumption of a Boltzman distribution of the proton concentration (Hs+= [H+]exp(-e~0/kT), the surface charge ry0 (C/m 2) is expressed as a function of surface potential ~0 (mV) and bulk proton concentration [I-I+] as

149

o0 = (6"02FNs

[H§ e x p ( ~ T ~ )/K' - K2 exp(-~T~

1

lO-S ) [H+]exp(-ek~)/K,+l+K2exp(-ek-~-)/[H+]]

(3)

where F is the Faraday constant, e the electron charge, k the Boltzman constant, and N s the hydroxyl group density in O H / n m 2. Instead of supplying values for K 1 and K2, the normally reported parameters are the PZC, which is halfway in between pK 1 and pK2, and ApK, the difference between pK 1 and pK 2. The PZC has the pkiysical significance of being that pH at which o0 = ~0 = 0. A second relation between ~0 and ~0 is the Gouy-Chapman model of the electric double layer: (~0

"-

20kWn0 l[exp/ze~

2kT j - exp/- 2kT

(4)

where z is the electrolyte charge, e is the relative dielectric constant of the solution, e0 the permittivity of vacuum and n o the number of electrolyte ions per unit volume. Finally, a proton balance between the liquid and surface phases, as is commonly employed for potentiometric titration, provides a relation between % and [H+]:

where w is the mass of oxide per volume of solution and s the specific surface area. The Langmuir equation is used to compute adsorption density:

r (Kc)

Fm~ -

/6/

1 +KC

Here onlv one Pt species, PtCla -2, is taken into consideration. The stability constants~reported b~r Sillen anKMartell [16] indicate that this is the dominant species at all pH values below about 6. A vastly different set of constants has been employed by Mang et al. [7]. In this case, undissociated and partially dissociated CPA are predicted to be present at lower pH values. Measurements we have made of the pH of CPA solutions always give a proton concentration twice that of Pt concentration, so we believe dissociation is complete and is more accurately modelled by the constants of Sillen and Martell. In the central pH range zero valent Pt complexes are predicted to form [16]; evidence for Pt precipitation in this range at long adsorption times will be shown later. The maximum adsorption density F'.,,ax is determined by assuming a closed packed layer of adsorbing complexes whic~ retain one hydration sheath, or

:/ /ril ) complex 2n

+ 2 rw) 2 angstr~

"

This seems to be the case for both Pt and Pt chloride complexes, while cationic

150 tetraamine c o m p o u n d s of these noble metals appear to retain two hydration sheaths [14]. The steric limit of 1.6 ~tmole/m 2 equals about 1 Pt complex per nm 2. The free energy of adsorption contains a coulombic term for tile electrostatic attraction, and a re.pulsive solvation term which re.presents the work needed to remove the solvatlon sheaths from the complex as it nears the surface [1]. Thus

K = exp

RT

(8)

The full expressions for these two terms are given in reference [12]. No adjustable AGchern term is used here. Equations 3 - 5 can be used separately to simulate the pH shifts and charging behavior of oxides in acidic and basic solutions and are the basis for a simple, accurate method to measure oxide PZC [2]. For the simulation of adsorption, they can be solved simultaneously with equations 4 - 8 using a common math package such as Mathematica | Equations 3 - 5 are coupled to 4 - 8 through the solution ionic strength. Many studies in the colloid science literature add a swamping electrolyte to solutions in order that ionic strength is kept constant. In catalyst impregnation, however, ionic strength is variable and must be solved for simultaneously. Currently this poses some computational difficulties using Mathematica | and as a consequer~ce theoretical curires presented here have been generated with a minimal number of points and appear somewhat disjointed. 4. RESULTS A N D D I S C U S S I O N

Employing an average PZC of 8.5 for all samples and having fixed Fma x by a steric calculation (equation 6), there are two other parameters in the RPA model These are N s, the hydroxyl group density, and ApK, the difference between pK 1 and pK 2. Tfie hydroxyl density of alumina in aqueous solutions is often taken to be 8 0 H / n m 2 [17]; this value will be used here. Values of ApK can be measured independently of adsorption. We have estimated the ApK values of two different types of gamma alumina by adjusting this parameter to obtain a best of equations 3-5 to mass titration data [18]. Botti aluminas gave the same ApK value of 5.0. This value will be used here. In summary, the full set of parameters employed is: PZC = 8.5, N s = 8 0 H / n m 2, ApK = 5.0, and Fma x = 1.6 ~tmole/m 2. To illustrate the applicability of the RPA model to Pt/alumina systems, all calculations shown in tfiis paper will be made without adjusting this set of parameters. The first set of simulated data is shown in figure 1. Kn6zinger's group recorded Pt uptake, the shift in pH, and the amount of dissolved A1 over a wide range of pH as a 5 x 10-4 M Pt soiution was exposed to 4540 m2/1 of ~,-A1203 surface [7]. Shown in figure la are their data, the curve of their "chemical adsorption model achieved with 7 adjusted equilibrium constants, and the RPA m o d e l The fit of the RPA model appears t o b e somewhat better. Experimental and RPA simulated p H shifts are shown in figure lb. The interpretation of Mang et al. is that the shifts are caused by exchange with surface hydroxyl ligands as for example by [7]: AIOH + [PtC14(OH)2] -2 ~

[PtC14(OH)(A1OH)]- + (OH)-

The good fit of the p H shifts by the RPA model implies that the shifts can be explained purely by proton transfer, as was noted earlier [2] for this data set. Our own Pt u take results at 1 and 24 hours contact time, over a variet of P Y 1 aluminas, is shown in figure 2 along with curves representing the RPA m o d e . In the 1 hour contact time of figure 2a, there is a great deal o f scatter in the pH range 5 - 7, and the experimental plateau is somewhat higher than the calculated

151

a) 0.006

-

0.005 -- 0.004 o

ca O. 003 .d O 002 r..)

.E

0.001

'Ib.l 9 ,

,

dl

.

,

I

,

,

.

,

|

2

,

'

,,I

4

I

i

I,,

I

I

6 Final

~

I

|

'

"

.,.

|

8

i0

3.2

pH

b)

4

/

i .

.

,

,

i .

1

.

.

.

.

I

2

,

,

i

3 Initial

4

5

. . . . . . .

i

6

,

,

,

,

j

7

pH

Figure 1. Pt up takeand p2H shift data from reference [7], 227 m2/gm 7-A120 3 powaer, 4540 rn /1, a) experimental and simulated (RPA in solid lines) Pt adsorption density, b) experimental and simulated (RPA) pH shifts.

152

a) oil D

~.~

~B I

o=

1.5

-

g a m m a 71

a

g a m m a 147

m

.~

9 g a m m a 188 ~9

o

1

---

o~ ~

g a m m a 193 model

0.15 g= 0

1

3

5

7

9

11

pH (1 hour)

b) m

~"

1.5 . . . . . . . . ~ . . _

:._-.; . . . . . . . . . . . . .

alpha 28 o

--,4 O

alpha 89

........

::?-i

.....

alpha 130 e

theta 77

..~

A

0

~05

t .........

\

eta 180

"-~

x

gamma 147

0

0

2

4

6

8

10

12

p H (24 h)

Figure 2. Experimental and simulated Pt uptake over various A1203 powders, 500 rn~/1, a) I hour contact time, b) 24hours contact time.

153 steric maximum. Nevertheless, using the same values of N s and ApK as for figure 1, agreement of the model and data is quite reasonable. Shifts in pH (not shown) are lesser in magnitude since the surface loading (500 m2/1) is almost ten times less. The shifts are accounted for reasonably well by the model. At long adsorption time (figure 2b), the low pH branch of the uptake curve is relatively unchanged, while in the 5 - 8 range, Pt continues to be lost from solution and its depletion is underpredicted by the RPA model. In a separate control experiment of CPA solutions in the absence of oxides [19], over long time periods C1- is released to the solution from PtC16~ and is not balanced by the evolution of H+, which suggests that ligand exchange of C1- with water a n d n o t with OH- occurs. This in turn implies that Pt complexes of-1 and 0 charge form. Precipitates were in fact seen in aged CPA solutions in the mid-pH range. The disappearance of Pt in the h i g h p H branch of figure 2b is then likely due to precipitation. Data for chloride disappearance corresponding to figure 2b is shown in figure 3. This plot is made to explore the "chemical adsorption" postulate that Pt retardation at low pH is due to displacement by C1-. A monolayer of C1corresponds to perhaps 400 ppm of C1-. The change in C1 concentration appears to be close to zero for all sampIes except perhaps for gamma alumina at tile lowest pH. A large amount of scatter is p r e s e n t at the lowest pH due to a high background of CI-, indicated as the dashed curve and on the right hand axis of this figure. It would then appear competition from C1- is not the cause of the downturn in Pt uptake at low p-H. The retardation of Pt adsorption at low pH has been specifically explored and reported elsewhere [20]. The explanation according to the RPA model is that the downturn is due to the 'electric screening' effect which results from high ionic strength. The coulombic attraction of complexes toward the surface is weakened; the surface does not become full of C1- at low pH but in fact becomes empty. The adsorption maximum at pH 3-4 is a balance of high surface potential and low ionic strength. Even while the Pt uptake curves of alpha, theta, and gamma alumina fall on the same curve (figure 2), the solubility of these materials is greatly different. Dissolved aluminum is plotted versus pH in figure 4, and for comparison, dissolved aluminum from a series of control experiments performed with no CPA is also plotted. The concentration of dissolved A1 is nearly the same in the presence or absence of CPA, which suggests that the dissolution and adsorption phenomenon are independent. The second indication is that Pt uptake actually decreases as A1 concentration increases in both figure 1 and 2. The "coordinative chemical" model has been postulated on the basis of a kinetic analysis of Pt and A1 concentrations [5] and a correlation between dissolved A1 and Pt uptake for a series of aluminas [6]. We have noted previously that the correIation of Huang et al. [6] is in fact coincidental, since the both the higher Pt uptake and higher A] concentrations are due to higher specific alumina surface area [14]. In fact, the Pt uptake in the samples of reference 6 all approach the aforementioned steric limit of 1.6 g m o l e / m ~ at high Pt concentration [14]. To further test the applicability of the RPA model, the literature has been searched for all other sets o]~ CPA equilibrium adsorption data. Four other sets of Pt equilibrium adsorption data have been found, dating from 1977 to 1996 [5, 2123]. In all four of these works, however, Pt uptake is studied as a function of Pt concentration, and pH is not measured. As seen from figures 1 and 2, Pt uptake is a strong function of pH and pH shifts can be quite large. The surface loadings employed in these four works are 2200, 30,000, 6100, and 11,800 m2/1 respectively. The value of the Mang et al. study [7], for which pH shifts are shown in figure lb, is 4500 m2/1. Thus, large pH shifts are to be expected in all of these works. Adsorption density- pH curves were produced for each of these data sets by

154 200

1600

E 150

~.

.

\

r'~

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

[i>

o '

.......

.-, E

-

\\

alpha 89 ,_,..,

800

-\ . . . . . . . . . . . . . . . . . . . . . . . . .

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

50

,...,

12oo

.

\

.=_ 100

alpha 28

alpha 130 ~

400

.,~

theta 77

r

"--~

.

0

O

.

.

.

.

.

.

.

i

I

i

_- ....

0

eta 180

-400

gamma 147

L----, Z

-50

i

1

I

I

1.5

I

2

, !,,

J

I

2.5 3 3.5 pH (24 hrs)

i

I

i

4

4.5

Figure 3. Chloride disappearance ([Cl]ini t - [Cl]final) vs. pH for the data of figure 2b.

140 120 100 ~" 80 ID,.

alpha, in CPA

!iiiii iiiiiiiiiiiiiiiiiiiiii ............

60

.

40

\---/

~...~

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

\

theta, in CPA

eta, in CPA =

gamma,

I

in CPA

alpha, no CPA

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

20

iiiii

theta, no CPA

0

eta, no CPA

0

2

4

6

pH (24 h)

8

10

---,-

gamma,noCPA

Figure 4. Dissolved A1 concentration vs. p H for the data of figure 2b, and for CPA free solutions at otherwise similar conditions.

155

1.25

1

1 r

F

p.mollm2

0,75

0.75

p.mollm2 0.5

0,5 0.25

0.25

o

.....

;

~

;

d

+

......

;

pH

)

)

1.5

1.5 1.25

1.25 1 1" gmol/m2

F

0.75

0.75

lamol/m2

0.5

0.5

0,25

0.25

pH

m

pH

Figure 5. Pt uptake from CPA solutions of different concentration over a) 177 m~/g ?-A120 3 pellets, 2200 m~/1 [5], b) 150 mZ/g ?-A120 3 sheres, 30,000 mZ/I[21], c) 245 rn~/g 11-A1203 pellets, 6100 m~/1122], d) 78 m~/g ~/-A120 3 pellets, 12,000 m~/1 [23]. prediction of the final pH using the oxide charging parameters N s = 8 0 H / n m 2, apK = 5, and assuming an initial pH of twice the initial Pt concentration. The results are plotted in figure 5. Agreement is reasonable especially since the oxide charging parameters of N s, ApK, and PZC have not been adjusted. In all four studies, adsorption occurred in ttie low pH range, where no precipitation is expected and the results are generally stable over time (figure 4a vs. 4b). However, aluminum dissolution occurs to a significant degree in this pH range. It occurs at the expense of three protons per A1, A120 3 + 6 H + --~ 2 AI(OH)3 , and is a slow and nonequilibrium process. (For this reason it is not included in the model). Aluminum dissolution might explain why in all cases of figure 5 the theory is shifted to the right of the experimental data. Accounting for A1 dissolution, the actual solution pH vaIues would actually be higfier than predicted by the model. This wouId shift the experimental points to tile right in figure 5, in better agreement with the theory. The two sets with the largest

156 discrepancy, figures 5a and 5c, employed the longest contact times 3-8 [5] and 6 [22] hours, compared to 2 [21] and 1.5 [23] hours for figures 5b and d. The higher discrepancy might then be caused by more A1 dissolution. Besides the simulations reported here, the RPA model has been used with no adjustable parameters for the simulation of Pd adsorption on alumina [12]. In simulations of cation adsorption over silica at high pH, the model was employed with small and in some cases, no chemical energy term [12]. Currently, the applicability of the model to other oxides, other metal complexes, oxide mixtures, and catalyst pellets is being studied. 5. CONCLUSIONS Of the prevailing models, for Pt adsorption onto. alumina,, includin, g coordinative, chemical, and physical adsorption, the revised physical adsorption (RPA) model appears to be the most realistic. A key component of the revised model is the incIusion of a proton balance which accounts for pH shifts when a liquid solution is contacted with an oxide. Numerous sets o f adsorption data have been simulated to a reasonable degree with one set of independently determined parameters. Moreover, experimental work has supported the physical adsorption mechanism as it appears that Pt adsorption is not dependent on A1 dissolution, and that the retardation of Pt adsorption at low pH is not caused by C1- competition. The RPA model is readily applied to other impregnation systems. REFERENCES

1. James, R. O., and Healy, T. W., J. Coll. Interf. Sci. 40, 65 (1972). 2. Park, J., and Regalbuto, J. R., J. Coll. Interf. Sci. 175, 239 (1995). 3. Weise, G. R., James, R. O., and Healy, T. W., Chem Soc. London Farad. Disc. 51, 302 (1971). 4. Paulhiac, J. C., and Clause, O., J. Am. Chem. Soc. 115, 11602 (1993). 5. Santacesaria, E., Carra, S., and Adami, I., Ind. Eng. Chem., Prod. Res. Dev. 16, 41 (1977). 6. Huang, X., Yang, Y., and Zhang, J., Appl. Catal. 40, 291 (1988). 7. Mang, T., et al., Appl. Catal. 106, 239 ~i993). 8. Karakonstantis, L.,~Bourikas, K., and Lycourghiotics, A., J. Catal. 162, 295 (1996). 9. Brunelle, J. P., Pure Appl. Chem. 50, 1211 (1978). 10. Contescu, C., and Vass., M. I., Appl. Catal. 33, 259 (1987). 11. Heise, M. S., and Schwarz, J. A., ~Coll. Interf. Sci. 113, 55 (1986). 12. Agashe, K. B., and Regalbuto, J. R., J. Coll. Interf. Sci. 185, 239 (1997). 13. Noh, J. S, and Schwarz, J. A., J. Coll. Interf. Sci., 139, 139 (1990). 14. Santhanam, N., et al., Catal. Today 21, 141 (1994). 15. Healy, T. W., and White, L. R., Adv. Coll. Interf. Sci. 9, 303 (1978) 16. Sillen, L. G., and Martell, A. E., The Stability Constants of Metal Ion Complexes, Suppl. No. 1, Special Publication No. 25, The Chemical Society, Burlington House, London, 1971. 17. James, R. O., and Parks, G. A., Surf. Coll. Sci. 12, 119 (1982). 18. Park, J. P h . D . dissertation, University of Illinois at Chicago, 1995, and manuscript in preparation. 19. Shadid, S., and Regalbuto, J. R., manuscript in preparation. 20. Shah, A. M., and Regalbuto, J. R., Langmuir 10, 500, (1994). 21. Shyr, Y.-S., and Ernst, W. R., J. Catal. 63, 425 (1980). 22. Wang, J., Zhang, J., and Pang, L., Preparation of Catalysts III, G. Poncelet, P. Grange, and P. A. Jacobs, eds., Elsevier Science Publishers B. V., Amsterdam, 1983. 23. Papageorgiou, P., et al., J. Catal. 158, 439 (1996).

: 91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

157

Pt/A1203/A1 monoliths for the complete oxidation of toluene N. Burgos a, M. Paulis a, J. Sambethb*, J.A. Odriozolab, M. Montes ~ a Grupo de Ingenieria Quirnica, Dto. de Quimica Aplicada, Fac. de C. Quirnicas, UPV/EHU, Apdo 1072, E-20080 San Sebasti~in, Spain,

b Dto. de Q. Inorg~.rtica e Instituto de C. de Materiales, Universidad de Sevilla, Av. A. Vespueio s/n, E-41092 Sevilla, Spain. Preparation of monolithic catalysts for VOCs abatement based on A1203/A1 supports has been studied. The influence of electrochemical anodization variables of an alttmim'um foil (current density, electrolyte concentration and time) on the properties of the A1203 layer formed has been studied. Surface areas of up to 1600 m2 of A1203 per square meter of aluminium foil having narrow pore size distribution of around 20 nm, can be produced under easily controlled experimental conditions. Monoliths made of this cermet material impregnated with platinum show higher activity for the complete oxidation of toluene than conventional Pt/A1203 powders. The presence of sulphate anions arising from the anodization step is proposed to explain the activity difference. 1. INTRODUCTION The growing problem of air pollution by VOCs, including solvents such as toluene, has resulted in an increasing search for suitable methods of control. Catalytic combustion has been found to be one of the best pollutant-control techniques. Catalysts used in environmental applications must possess good attrition resistance and very low pressure-drop, due to the high flows they have to treat. Pelletised powder and monolithic support catalysts are industrially used for complete catalytic oxidation. Pelletised catalysts suffer the disadvantage of needing catalytic and physical properties to be provided by the same material [1]. Monolithic catalysts overcome the above stated necessities by decoupling the physical and the catalytic properties between the monolith matrix and the active phase deposited on it. So the advantages of monolithic catalysts are the very low pressure drop, the high external surface area, the uniformity of the distribution of the flow within the honeycomb matrix to improve the pollutant-active site contact, etc. Ceramic monoliths are most commonly obtained by extrusion, and the cost of largescale production is very low. The manufacture of metallic monoliths is easier, due to the ability to modify channel surface and shape, and cheaper for small series. Furthermore the

*Permanent address: CINDECAU. de la Plata, 47 na 257, 1900La Plata, Argentina.

158 metal structures fabricated from thin foil had cell walls thinner than those made from ceramic material, and this results in a lower pressure drop. Apart from this, properly designed metallic systems might have advantages over the ceramic ones with respect to mechanical strength, resistance to thermal degradation, high geometric surface area per unit volume, weight and ease of handling without damage. However metallic substrates might seem to be at a disadvantage compared to ceramic ones due to the lower adhesion of the coating. This problem is overcome by a combination of suitable chemistry in the coating formulation and the use of metal alloys having the ability to form an adherent and stable alumina surface layer, which acts as a key for the washcoat layer. The need of high temperature resistance in the automotive sector applications has led to the use of refractary steels that present the additional advantage of generating at high temperature, an alumina protective layer. Nevertheless, the use of metallic monoliths for VOCs elimination requires much lower temperature (typically 373-773K) and thus usage of metals like aluminium can be envisaged. In this work, the surface oxidation properties of aluminium have offered a very interesting choice to build up a porous layer of alumina, by a controlled anodization process. This work deals with the different variables of the anodic process, such as the current density, electrolyte concentration and anodization time, to produce a catalytically acceptable support. The aluminium layer was characterised by XPS, SEM and N2 adsorption. The impregnation of the porous support with a noble metal precursor and a catalytic test of complete toluene oxidation are presented. 2. EXPERIMENTAL

The Pt/AI2Oa/A1 monolith preparation process consists of two parts: the preliminary production of the A1203/A1 support by anodization and the subsequent rolling up of alternate fiat and corrugated sheets, followed by the wet impregnation of the active phase. The most important operation variables for the controlled generation of the A1203 layer over the aluminium sheet by anodization are: electrolyte concentration, current density, anodization time and temperature [2]. The choice of the electrolyte can vary the A1203 final properties: surface area, porosity, thickness of the layer, etc. Based on literature data [3] H2SO4 has been chosen as the electrolyte in order to obtain A1203 layers. Anodization has been tested at different H2SO4 (Panreac QP) concentrations. Commercial aluminium foils (thickness: 100 ~tm) provided by INASA, have been used in all the experiments, with a chemical composition given in Table 1 (weight average). Table 1. Weight average composition of the commercial aluminium. Fe

Si

Mg

Mn

Cu

Cr

Pb

Ti i

0.34

0.10 i

0.002

0.005 i

0.002 ,

0.0009

0.001 i

Zn

0.011

A1 ,

i

0.005 i

,,,,

balance i

A BLAUSONIC FA-325 apparatus generated the electrical current, with an output variable voltage between 0 and 30V and a current intensity from 0 to 2.5A.

159 The characterisation of the A1203 generated over alttminium test specimens, of 3xl cm was carried out by the following techniques. Textural was evaluated by N2 adsorption at 77K (MICROMERITICS ASAP 2000). A gravimetric method was used to evaluate the amount of the A1203 layer generated, as the difference in weight between the anodized sheet and that obtained after an oxide dissolution treatment. The chromic-phosphoric solution, used to dissolve the A1203 from the anodized sheet, was an aqueous solution of CrO3 (Panreac) and H3PO4 (85% Probus). The sheet is introduced in this solution and kept for 10-15 minutes at a temperature of 353-373K. Evaluating the amount of A1203 that should have been formed (from the aluminium consumption), the process yield can be obtained from Faraday's law[2]. In order to study the structure and thickness of the A1203 layers, the anodized sheets were observed by Scanning Electron Microscopy (SEM). XPS measurements were carried out in a VG ESCALAB 210, to have the final composition of the anodized sheets. Additionally the catalytic activity for complete oxidation of toluene with Pt loaded monoliths was tested. Aluminium sheets of 26x3 cm were anodized and monoliths were formed. The wet impregnation of the monolith was carried out by introducing the monolith in an aqueous solution of (NH4)2PtC16 (Fluka, Puriss.) during 70 minutes under agitation. The impregnated monolith was then dried at 393K for 2 hours and calcined at 723K for 2 hours. Catalytic tests were carried out in a plug flow reactor. Mass flow controllers were used to prepare the feed mixture. Ar was bubbled through two thermostated and pressurised saturators containing toluene. This stream was additionally diluted in air and passed through the monolith placed on the top of a carborundum bed. The carborundum was used to premix and preheat the toluene flow entering the monolith in order to obtain a homogeneous temperature, measured by a thermocouple passed through the centre hole of the monolith, and placed just at the beginning of it. The reactor was surrounded by a furnace with a temperature controller, which allowed us to generate temperature ramps. The detection of the reaction products was carried out on-line by a direct CO2 detector (SENSOTRANS IR) and a TCD-GC containing a semicapilar column (TR-WAX, 30m). Therefore by increasing the reactor temperature at 2.5K/min., data of conversion for each temperature were obtained. Plotting conversion against temperature, ignition curves are formed, characterised by the Ts0 or the ignition temperature, temperature at which the conversion reaches the 50%. DRIFTS spectra under reaction conditions were obtained by using a controlledtemperature-and-environment diffuse reflectance DRIFTS chamber (SPECTRA TECH 0030102) with ZnSe windows in a Nicolet 510P in~ared spectrometer with KBr optics and DTGS detector [4]. The samples, Pt/A1203/A1 sheet and Pt/A1203 powder used to compare, were subjected to an "in situ" pre-treatment process to stabilise the solids. The catalysts were heated at 573K during 30 minutes in air (65 ml/min.). A mixture of 220 ppm of toluene in air (flow rate=2.5 rnl/min.) was introduced at room temperature. The reaction process was analysed in the temperature range from 298 to 673K. 3. RESULTS AND DISCUSSION

Figure 1 and 2 present the N2 adsorption-desorption isotherms and the pore distribution obtained for the layer of alumina generated, in a typical anodization process. The pore diameter distribution obtained from the desorption isotherm is in the range of mesoporous diameters (2-20 nm) for all the samples prepared in this work, and the surface area is between 20 and 80 m2/g A1203 (Tables 2-4). These textural variables together with the

160 amount of alumina generated per m2 of aluminium sheet (W) allows to calculate the surface area exposed per m2 of aluminium sheet (S: m~ A1203/m2 A1) that is the most important characteristic of the final material. The yield (1]: %) represents the percentage of the electric power consumed to produce the alumina layer. Taking into account that part of the alumina layer produced on the aluminium sheet can be dissolved by the liquid medium, both the ohmic consumption and the alumina dissolution produce a decrease in yield.

40

0.16

3o

~Q o.12

~E 20

'

'

'

' '

' ' ' 1

'

E 0.08 3

..,.=

o >

.e 0.04

-~ lO

O

0.

0 ~ <

0

0

0.2

0.4 0.6 0.8 Rel. Press.

i

i

0

1

Figure 1. Typical N~ adsorption-desorption isotherm of the anodized A1 sheets.

~-~_

~_~. 100 10 Pore diameter (nm)

Figure 2. Typical pore distribution graphic of the anodized A1 sheets.

Tables 2, 3 and 4 show examples of the A1203 layer properties after a survey of experimental conditions (current density, electrolyte concentration and anodization time). In every case, two parameters were kept constant, while exploring the third one. Table 2. Characteristics of the alumina layer as a function of the current density (anodization time, 20 minutes; electrolyte concentration, 1.64 mol/1; without temperature control). Vp S Current W 11 SBET Pore density, (g A1203/m2A1) (%) (m2/gA1203) diameter (cm3/g A1203) (m2/m2A1)

(rim)

(A/din') 7.0 13.2 24.1 29.2 28.4 30.9

0.51 1.03

1.75 2.06 2.26 2.47 lll

l

i

64.4 60.6 65.1 67.2 59.3 59.1

34

9.1

0.08

451

33

13.1

0.11

958

28

14.7

0.10

i

85O

Table 2 presents the modifications resulted in the A1203 layer on changing current density. As the current density increases, the amount of A1203 produced increases until an almost constant value results from 2.06 A/dm2 onward. At higher current densities, the A1203 layer dissolution becomes significam due to both, the reported electroosmosis phenomenon [2], which causes the enlargement of the pores, and also the temperature increment. The increase of both of these processes, the dissolution and the generation of A1203, results in the yield fluctuation not to have a definite trend. The pore volume and surface of alumina

161 produced do not change too much, and so the amount of alumina remaining over the aluminium sheet is the controlling variable for the total surface area exposed per mz of aluminium sheet, that increases sharply until 2.06 A/dm 2, decreasing slightly above this value. Table 3. Characteristics of the alumina layer as a function of the electrolyte concentration (anodization time, 20 minutes; cu~..ent densit)r, 2.06 A(dm2; without teml~erature control). . . . . . Electrolyte W 1] SBET Pore Vp S concentration (g A1203/m2A1) (%) (m2/gA1203) diameter (cm3/g A1203) (m2/m2Al) (mol/1) . . . . . . . (..,ran) . . . . . . . . 0.69 3i.1 741'~2 26 15.2 0.10 793 1.64 26.1 62.4 33 13.1 0.11 856 2.48 25.9 61.9 54 13.6 0.19 1409 Hi

i,,

i

- -

,

,

,,

,,,

,

,,,

,,,

The results obtained when the electrolyte concentration is changed, are presented in Table 3. The increase of the electrolyte concentration produces a decrease in the A1203 generated as well as in the process yield, due to the oxide dissolution caused by the decrease of the pH [5], and the temperature increment. At the same time, as reported elsewhere [2], porosity is increased when the electrolyte concentration is increased, increasing also the surface area of the alumina generated. The decrease in the amount of alumina is largely compensated by the increase in surface area of the alumina and the total surface per m 2 of aluminium increases with electrolyte concentration. Ifanodization time is varied, the results are as in Table 4. It can be observed that when the anodization time is longer, W is more or less constant until 30 minutes, and then it decreases. As it is reported in the literature [3], the rate of A1203 generation decreases as time is increased, due to the low conductivity of thicker layers. As a result, the fraction of the electrical energy transformed to heat increases, producing a temperature rise in the electrolyte, which favours the oxide dissolution process, reflected in the yield decrease and the pore diameter increment. This temperature rise produces a porosity increment too, shown in the SBET and in the Vp data. Therefore, even if the A1203 amount decreases, the surface area per aluminium m 2 increases due to this large rise in the porosity, for times lower than 30 minutes. At anodization times longer than 30 minutes, the decrease in the amount of A1203 prevails and the total surface area per m2 of aluminium decreases. Table 4. Characteristics of the alumina layer as a function of the anodization time (electrolyte concentration , 1.64 mol/1; current densit~r 2.06 A/dm2; without temperature control): Anodization W I] SBET Pore Vp S time (g A12Oa/mEA1) (%) (m2/gA1203) diameter (cm3/gA1203) (m2/mEA1)

(min) .... 20 25 30 40 50 i

(nm)

i

i

24.4 25.4 23.0 15.6 15.0

56.3 46.6 35.3 18.0 13.8

ii

35 51 68 66 73

12.6 16.9 19.9 20.3 28.9

0.11 0.22 0.34 0.34 0.53

861 1300 1573 1037 1096

162 When the anodization process was carried out with longer aluminium sheets needed to prepare the monoliths, important heterogeneities were observed as a function of the depth in the anodization vessel. These heterogeneities must be related to differences in temperature and the corresponding convection flows produced. In order to prevent the heterogeneities and the dissolution phenomena observed when the temperature increases, several experiments of anodization were carried out controlling the temperature of the anodization cell. When 30 and 45 minutes of anodization were used, we observed that the surface area did not decrease after 30 minutes of anodization, as it was observed without temperature control (Table 4). So it was possible to work for a longer time with lower yield decrease, increment of the W, and higher SBET. Taking into account all the previous discussions, the following anodization conditions were chosen: time, 40 minutes; current density, 2.06 A/dm2; electrolyte concentration, 1.64 mol H2SO4 per litre; temperature, 293K. Once the anodization conditions were chosen, fiat and corrugated long sheets were anodized in order to make the monoliths. Table 5 present the result obtained with the standard anodization conditions used to prepare the monoliths. The reproducibility was + 5%. The last column (e: gm) shows the A1203 thickness values as observed by SEM, that are in agreement with the values calculated using specific gravity and porosity data. Table 5. Characteristics of the alumina layer, produced with the anodization conditions used to form monoliths. Time W 1] SBET Pore Vp S e (min) (g A1203/m2A1) (%) (m2/gA1203) diameter (cm3/g AlzO3) (m2/m2A1) (~m) i

.

.

40

.

.

.

41

ii i iii

.

(nm)

51

38

18

0.178

1600

15

Figure 3. SEM micrograph showing the thickness of the formed alumina layer.

163 Figures 3 and 4 show two SEM micrographs, of anodized aluminium sheets. In figure 3 it can be observed the texture difference between the aluminium sheet and the generated A1203 layer. Even though at this magnification the pores cannot be observed, the lines due to the porous structure of the layer can be detected. The thickness of the A1203 layer observed is around 15 ~tm. In Figure 4, a high resolution SEM image, the A1203 layer is viewed from the top, where the porous structure can be clearly observed, with pore diameters between 13 and 20nm.

Figure 4. SEM micrograph showing the porous structure of the formed alumina layer. ARerwards monoliths were handmade, the rolling up of alternate corrugated and flat sheets. The geometrical and textural characteristics of these monoliths (cylinders of 3 cm long and 1.6 cm of diameter), comparable to those reported in the literature for commercial monoliths [1], are: Geometric volume: 6 cma Total exposed surface area: 38-43 m2 Number of cells: 520-540 cell/in2

Surface-to-volume ratio" 5200mE/m3 Empty fraction: 74% Wall thickness: 0.1 nm

The advantages of the catalytic oxidation of toluene over the thermic oxidation are clearly shown in Figure 5. In this figure the ignition curve obtained with one of our monoliths for 225 ppm of toluene is compared with that of the homogeneous incineration at the same conditions (calculated from kinetic data from the literature)[6]. As it can be seen the difference in Ts0 is approximately 450K.

164

_

,

_

~ 0.8

In order to evaluate the catalytic behaviour of our monoliths, several experiments have been carried out, varying the toluene concentration and the space velocity. In Figure 6 the change of the ignition curves obtained for different toluene concentrations maintaining constant the space velocity (5050h "1) are shown. Ts0 increases from 4 2 8 to 483K as the toluene concentration is increased from 112 to 750 pprm

io, I " cata't'c j

o.~-

,,,~,,

~(~0 ......'............... 400 '......... 600'..... ""' ....800 T(K)

[]

' iO00

Figure 5. Comparison between thermic and catalytic data for toluene complete oxidation.

On the other hand ignition curves for different space velocities at constant toluene concentration are presented ha Figure 7. It can be observed that the Ts0 increases from 433 to 463K when the space velocity is varied from 2025 to 10100 h "l. ' ' ' ' I ' ' , '

o .o 0.8

=.o.o

=0.4 = aa, 00.2 I-~I

356

,

.o ~

: I::

tO

o

~

0.6

t~

ff 1 ' r .~ ~ ~,i~

:IEJ

~ ~ " ~6

o "

[]

112ppm 225 ppm

450pprn

• 756 ppm

", I , , , , I , , , t I J ~ ~ ~

4so 500 T (K9

sso

600

Figure 6. Ignition curves for different toluene concentrations. We also compared the catalytic activity of our monoliths with powder Pt/A1203 catalysts. A 1% Pt/A1203 powder catalyst was prepared by incipient wetness of the 100-200 lxm fraction of alumina support (Spheralite SCS250, SBET: 250 m2/g), with a solution of (NH4)2PtCI6 precursor. Figure 8 shows the ignition curves for 225 ppm of toluene, of our monolith and of powder Pt/A1203 catalyst. Both catalytic systems contain 2 mg of Pt, dispersed on 40 m2 of A1203 in the case of the monolith and 50 m2 of A1203 in the case of the Pt/A1203 (200 mg). The monolith presents a Ts0 around 35K lower than the powder

catalyst.

1

0.8

o

:

0.4

-

:/:

o

2025 h "1

~t,#

"

5050 h1

500

550

~o.2~ ~ ' ~ 0

350

400

450

600

T (K) Figure 7. Toluene ignition curves for different space velocities.

.9 0.8 > 0.6

t,.o r

=*, 0.4 ID _= o 0.2

I--

400

440

480

T (K)

520

560

Figure 8. Comparison between Pt/A1203 powder and Pt/A1203/A1 monolith for toluene complete oxidation.

165 In order to explain this difference, both catalytic systems were also studied by DRIFTS under similar reaction conditions. When reactive mixture was passed through both Pt/A1203 and Pt/A1203/A1 catalysts at room temperature, two bands attributed to adsorbed toluene (1603 and 1498 cm"l) were observed (Figures 9 and 10). On Pt/A1203 these bands remained between 303-423K but the intensity of both peaks decreased above 423K (Figure 9). At 633K, two main peaks appeared around 1620-1550 and 1480-1400 crn"~, with an additional peak at 1497 crn"~. The two bands around 1600 crn"l and 1450 crn"l are due to va~(COO') and vs(COO') respectively and the peak at 1497 crn"l is assigned to vas (CH). On Pt/A1203/A1 the toluene bands disappeared at 373K, but a band at 1375 cm"~ was found (Figure 10). Additional experiments employing only A1203/AI showed that the peak at 1375 em1 is due to the interaction between toluene and the A1203/A1 substrate. According to Davydov [7] the band is assignable to 5(CH3). On the monolithic structure, the bands around 1600, 1450 and 1497 crn~ were encountered at 513K. The results indicate that Pt/A1203/A1 monolith is more active catalyst than Pt/A1203 powder for the toluene oxidation. Pt / AI 20 3/AI

Pt / AI203 ,1303 K 1373 K ,.-.~1624

ot ' -

5 ~

,.,,,

2

~

~

O

<

<

513 K 6

17'00 16'oo 15'oo 14bo " laoo Wavenumber (cm-1) Figure 9. DRIFT spectra of toluene oxidation over Pt/A1203 at different temperatures.

1~'0o

..

" 16'oo

1~'oo 14'oo laoo

Wavenumber (cm -1) Figure 10. DRIFT spectra of toluene oxidation over Pt/AlzO3/A1 at different temperatures.

A possible explanation for this higher reactivity can be the presence, as seen by XPS, of significant amounts of SO4z" groups on the monolith surface, arising ~om the anodization step. The acidity created by the sulphate groups can play an important role, whether in the Pt impregnation step or in the reaction itself. 4. CONCLUSIONS Electrochemical oxidation of aluminium foils allows to prepare alumina layers of excellent textural properties to be used as catalytic supports. Easily reproducible experimental

166 conditions allows to produce A1203/A1 cermets with surface areas of up to 1600 m2 per square meter of aluminium foil having narrow pore size distribution of around 20 nm. These materials allow to build up monoliths of similar characteristics to commercial ones made of refractory stainless steels. Pt/A1203/A1 monoliths are very active for the complete oxidation of toluene. Its activity is even higher than that of conventional Pt/A1203 powders, probably due to the presence of sulphate groups arising from the anodization step. REFERENCES

1. A. Cybulski, J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc., New York, 1998. 2. R. Lizarbe Ruiz, Control An6dico, Coloracidn y Sellado del Aluminio in, Teoria y Pr~ictica de la Lucha Contra la Corrosi6n, Jos6 A. Gonz~lez Fem~indez, Graftmad S.A., Madrid, 1984. 3. N.P. Fedotiev, S.P. Grilijes, Anodizado del Ahmaim'o y sus Aleaciones in, Electropulido y Anodizaci6n de Metales, Gustavo Gili S.A., Barcelona. 4. J. Benitez, J. Carrizosa, J.A. Odriozola, J. App. Spectrosc., 47 (1993) 1360. 5. E. Baumgarten, F.O. Geldsetzer, V. Kirchausen-Dusing, J.of Colloidal and Interface Science, 173 (1995) 104-111. 6. A.J. Bounicore and W.T. Davis (Eds.), Air Pollution Enginnering Manual, Van Nostrand Reinhold, New York, 1992. 7. A. Davydov, Infrarred Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides, Wiley, New York, 1990. ACKNOWLEDGEMENTS

Financial support by MEC (CICYT-QUI97-1040-CO3), GV (M.P. fellowship), UPV/EHU (N.B. fellowship) and CONICET (J.S. postdoctoral fellowship) are gratefully acknowledged.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

167

Monolithic c a r b o n aerogels for fuel cell e l e c t r o d e s G2VI. Pajonk ~), A. V e n k a t e s w a r a Rao ~2), N. Pinto ~), F.Ehrburger-Dolle ~3) and M.Bellido Gil ~3~ (~)LACE -UMR 5634 CNRS-Universit~ Claude Bernard Lyon 1, 43 boulevard du 11 Novembre 1918 - 69622 Villeurbanne Cedex, France (2) Airglass Laboratory-Department of Physics-Shivaji University - Kolhapur 416004 -India (8)ICSI-CNRS 15 rue Starcky - 68057 Mulhouse Cedex -France

ABSTRACT Electrical vehicles (Zero Emission Vehicles) represent a very elegant solution in order to decrease pollution in industrialised cities. To meet the pollution requirements, it is envisaged to replace gasoline engines by light fuell cells. They necessitate good electroconductive carbon monolithic electrodes. Monolithic organic resorcinol-formaldehyde aerogels (carbogels)can easily be made using the sol to gel method accompanied by the supercritical drying process using liquid CO~ (critical temperature = 31~ Thereafter, these copolymer monolithic carbogels are pyrolysed in an atmosphere of N2 at 10500C for 3 hours, in order to obtain monolithic carbon carbogels. The carbon carbogels are impregnated by H~PtCh to obtain a weight % of metal equal to 0.44. In this work, the unique solvent used was acetone in each step, - the sol to gel reactions were catalysed by perchloric acid, from the copolymer synthesis up to the final impregnation of the carbon monoliths. The advantages of this new preparation method are compared to the aqueous former one. INTRODUCTION Organic as well as inorganic, simple or composite aerogels based upon the well known sol-gel method,represent an interesting way for the preparation of highly divided catalytic materials, because the supercrtical drying process retain most of the very developed textural properties of the wet gel by avoiding the capillary stress gradients [1]. Around 1988, Pekala et al. [2,3] developed the first organic aerogel from the copolymerisation of formaldehyde (noted F) with resorcinol (noted R) in water with the help of sodium carbonate as catalyst. These searchers succeeded in making monolithic and more or less red transparent

168 organic carbogels which furtherly were pyrolysed at high temperature either in dinitrogen or carbon dioxide. Again black carbon monoliths were obtained after pyrolysis, which exhibited large porosities, good thermal and electrical conductivities [4-6]. Due to the chemistry used, it took no less than two weeks to to get a piece of carbon. In the catalytic literature one cannot find numerous papers describing the use of such carbon aerogels as supports, to the best of our knowledge only two articles relate catalytic results involving carbon aerogels (R-F) impregnated with Pt or Pd for the SCR of NO by NH~ on one hand [7], and another one dealing with a carbon aerogel support for Pt, originating from polyacrylonitrile and tested in the oxygen reduction reaction on the second hand [8]. In this paper, we describe a new method of synthesis of carbon aerogels derived of that of Pekala et al. [2-6], i.e a R-F series polymerised in acetone instead of water, and catalysed by an acid, perchloric acid, instead of a base like sodium carbonate.

EXPERIMENTAL Preparation of the Pt/C carbogels The following summarizes the multistep method used in this piece of work: resorcinol-formaldehyde sol gel in acetone catalysed by perchloric acid at 45~ -~ curing (aging) for 3 days at 45~ drying with supercritical carbon dioxide at 37~ -~ pyrolysis under dinitroge at 1050~ -* impregnation by chloroplatinic acid in acetone at room temperature -* supercritical drying at 37~ again -* calcination with dinitrogen at 450~ and reduction by dihydrogen at 450~ Synthesis of the R-F carbogels First, a solution of resorcinol in acetone (0.294 mold) is placed in a vessel and perchloric acid at 70 % in water (0.0294 mold) is added under stirring and finally formaldehyde at 36,5 % in water (0.588 mold) is added to the first mixture. At this stage it is important to respect the above order of mixing to avoid precipitation or flocculation phenomena. The vessel is placed at 45~ and the gel time is of the order of 75 min, then the gel is left to cure at the same temperature for 3 days before drying. If we note by C the concentration of the c a t a l y s t , this gel corresponds to a R/C value of 10, while the F/R ratio is 2.This last value corresponds to recommended value by Knop and Pilato [9]. The cured gel is placed in an autoclave already at the chosen supercritical temperature of 37~ ie about 5~ above the critical one, with an extra amount of acetone (500 ml), flushed with carbon dioxide, while the pressure attains 80 bar. Static and dynamic period of washing are run under the supercritical conditions for 6-7 hours and thereafter the R-F aerogel is recovered. It is a pinky and poorly

169 translucent solid. Figure 1 shows a schematic of the drying procedure used in this work.

300

- 160

"-250

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hol method\\ \

200

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\120 100

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150

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-~7':-"

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X V

-80

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,

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I

10

15 20 25 30 time (hours)

35

2O

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Figure 1. Supercritical drying mode flow charts "a) with alcohol (high temperature supercritical drying), b) with liquid COs (low temperature supercritical drying). Preparation of the carbon monolith. The copolymer carbogel is placed in an oven under a flow of nitrogen (3.6 1/hr) and gently raised to a temperature of 1050~ where it stays for 3 hours. Once cooled down to room temperature, the material is still under the form of a solid and its colour is black. Preparation of the Pt on carbon aerogel catalyst The carbon aerogel is placed in a vessel with a solution of hexachloroplatinic acid in acetone,in a concentration adjusted to give a final Pt weight content of 0.50. The impregnation operated under stirring lasted 36 hrs, then the solid is replaced in the autoclave to be dried in the same conditions than the copolymer.

170

RESULTS Textural Properties For the copolymer carbogel, the BET surface area, measured with N2 at 77K was found equal to 390 m2/g including a microporous surface,calculated with the ,,t,, method of 90 m2//g. After pyrolysis under dinitrogen, the BET surface area quite doubled, it was now of 745 m2/g, and it developed a microporous surface of about the half i.e 371 m~/g. Thermoporometry measurements made with water, clearly showed t h a t both types of carbogels, the organic and the carbon ones, were not mesoporous at all. They were micro and macro porous only. Figures 2 and 3 show the pore size distributions and the cumulated pore volumes for the non pyrolysed copolymer and the carbon respectively.It is clear that, as demonstrated in figure 3, the pyrolysis treatment resulted in a shift of the macropore volumes towards larger ones with respect to the organic aerogel, without any change of the macropore radii ,centered around 50 nm. Nevertheless, both samples essentially keep the same micro and macro porous textures. The pyrolysis treatment developed a doublefold increase of the macropore volume (V. . . . = 0.121 against 0.2 cma/g respectively). This conclusion can be extended to the contributions of the micropores to the surface areas where a fourfold increase of the microporous surface is recorded (90 against 371 m2/g respectively ).

0.02 O.018 0.016 O.014

i "

~

organic aerogel

~

pyrolysed

-r, 0.012 " 0.01 ~

0.008 0.006 0.004 0.002 0 0

I 100

50 R (nm)

Figure 2. Thermoporogram results on the carbogels

150

171 Due to the interaction of the support with dioxygen, it was not possible to determine the dispersion of the metal with the well known H2-O2 titration method. An attempt to chemisorb dihydrogen was performed and it gave a dispersion of 23 %, figure 4, in very good agreement with the chemisorption of CO at low pco (not shown here), taking into account the weight percentage in platinum given by the chemical analysis i.e 0.44. Its BET surface area was of 531 m~/g including a microporous surface of 230 m2/g as shown by its corresponding t-plot in figure 5.The complete N2 isotherm clearly indicates that the solid is essentially micro-and macro porous (figure 6). The electrical resistance measurements of the copolymer carbogel show that it is an electrical insulator while those of the carbon and Pt-C carbogels give evdence that they were good electricity conductors with electrical conductivities equal to 3.10 -2 and 9.10 .8 S/cm for the carbon and Pt-C carbogels respectively.

0.9

0.8

~

organic aerogel

0.7 ~

~, 0.6

~ ' ~ pyrolysed

0.5

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r

~" 0.3 0.2 0.1 0 0

50

100

R (nm) Figure 3. Cumulated pore volumes on the two carbogels.

150

172

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2

4

6

8

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Pressure (tort) F i g u r e 4. H~ c h e m i s o r p t i o n a t 25~

on t h e Pt-C carbogel c a t a l y s t .

400 -350 300 -

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6

8

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t (nm) F i g u r e 5. t p l o t of t h e p h y s i c a l a d s o r p t i o n of N2 on t h e Pt-C carbogel.

I 12

173

500

400 "7

~'~ 300 & 200

100

0

I

I

I

I

0.2

0.4

0.6

0.8

Pressure (torr) Figure 6. N~ physisorption isotherm of the Pt-C carbogel. DISCUSSION

The new conditions used in this work to synthesize the organic copolymer RF give materials which are different from those obtained by Pekala et al [2-6] because of one major change in the nature of the catalyst, acidic here, which lead to a reaction mechanism beginning by a protonation of a formaldehyde moleclule giving a hydroxymethylene carbonium ion which further adds on the resorcinol molecule.The global reaction is an electrophilic substitution between the two reactants [9] .This R-F copolymer exhibits a particular porous texture showing the presence of only micro-and macro-pores which is retained by the pyrolysed carbon material. The solvent preferred in this synthesis, acetone instead of water, avoid s the tedious and time consuming solvent exchange step necessary in the case of the former work [2-6]. Moreover, the sol to gel and cure steps are performed at 45~ instead of 85~ and lasted 3 to 4 days only, the drying step needed only one day more in order to obtain the carbon carbogel. The impregnation of Pt on a carbon aerogel led to a rather poor metal dispersion, the catalyst did not differ in its electrical propery from its pure carbon aerogel support.

174 Contrary to Pekala's carbon aerogels [10-12] which are quite non macroporous, those depicted in this paper contain a substantial amount of macropores. REFERENCES

1. 2. 3. 4. 5.

G.M.Pajonk, Rev. Chimie Appliqu~e., 24 (1989) 13. R.W. Pekala, J. Mater. Sci., 24 (1989) 3221. R.W. Pekala, Rev. Chimie Appliqu~e, 24 (1989) 33. R.W. Pekala, C.T. Alviso and J.D. LeMay, J. Non Cryst, Sol., 125 (1990) 67. R.W. Pekala, C.T. Alviso, F.M. Kong and S.S. Hulsey, J. Non Cryst. Sol., 145 (1992) 90. 6. R.W. Pekala, C.T. Alviso, J.K. Nielsen, T.D. Tran, G.A.M Reynolds and M.S Dresselhaus, Mat. Res. Soc. Symp. Proc. No. 393 (1995) 413. 7. J.L. Zhao, R.J. Willey and R.W. Pekala, Mater. Res. Full Symp. (1990) Boston. 8. S. Ye, A.K. Vijh and Le.H. Dao, J. Non Cryst. Sol., submitted. 9. A. Knop and L. A. Pilato, Phenolic Resins, Springer Verlag. (1985). 10. H. Tamon, H. Ishizaka, M. Mikami and lY[ Okazaki, Carbon., 35 (1997) 791. 11. Y. Hanzawa, IL Kaneko, R.W. Pekala and M.S. Dresselhaus, Langmuir, 12 (1996) 6167. 12. R.W. Pekala, S.T. Mayer J.L. Kaschmitter and F.M. Kong, Sol-Gel Processing and Applications, Y.A Attia (ed), Plenum Press (1994) 369.

9199~ Elsevier bcience B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

175

C a r b o n c o a t i n g o f ceramic monolithic substrates Th. Vergunst, F. Kapteijn, and J.A. Moulijn Delft University of Technology, Faculty of Chemical Technology and Materials Science, Department of Chemical Engineering, Section Industrial Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands

SUMMARY Carbon coated monolithic supports have been prepared starting from furfuryl alcohol based polymers. Pieces of ceramic monoliths were dipcoated using a partially polymerized mixture. After complete solidification, the polymer was carbonized in order to obtain a carbon coating. It was shown that the texture, coating amount, and elemental comPosition can be varied by changing the composition of the starting polymerization mixture. With a coating based on only furfuryl alcohol a weight increase of 10 wt% could easily be obtained with pores of 0.5 gm and a pore volume of 0.09 ml g-1. The Carbon coating could be activated by partial oxidation, followed by functionalization with NaOC1, exchange with [Pt(NH3)4]2§ and reduction with H2 in order to obtain a carbon coated monolithic catalyst. A platinum content of 3.1 wt% Pt/C/cordierite could be obtained with an average metal particle size of 27 nm.

1. INTRODUCTION Many catalytic processes involve the reaction of a gaseous and liquid reactant on a solid catalyst. This type of reaction is generally employed in either a slurry reactor with suspended catalyst particles of 10-100 gm, or a trickle-bed reactor with catalyst particles of 1.5-6 mm [1]. Slurry reactors are often used in batch processes. The catalyst particle size ensures a high catalyst effectiveness and high mass transfer rates, although continuous agitation is required. Attrition of catalyst particles (and herewith loss of active material) and separation of the particles from the reaction mixture are disadvantages of this reactor-type. For high throughput three-phase processes often trickle-bed reactors are used. The particle size used is a compromise between the allowed pressure drop over the catalyst bed and the reduced catalyst effectiveness. An alternative to these types of reactors is a three-phase monolithic reactor, which shows the combined advantages of both a slurry and a trickle-bed reactor [2]. A monolith (see Figure 1) is a structured

Figure 1: Drawing of a monolithic support.

176 catalyst consisting of many parallel channels separated by thin walls. The catalyst is usually attached to the channel walls as a thin layer. The open structure of the channels (75% open frontal area) results in a very low pressure drop, that makes high throughputs possible, while the thin catalytic layer (-30 gm) shows a high catalyst effectiveness [2]. The choice for a catalyst support material is determined by the proposed reaction and the applied reaction conditions. The support has to supply sufficient surface area to disperse the active phase, has to show a desired interaction with the reactants and products, and has to withstand the applied process conditions, like temperature and pH. In several reactions, both oxidations and hydrogenations, carbon is therefore the preferred catalyst support material [3]. To enable the application of monolithic catalysts in these reactions, a method has been developed to coat ceramic monolithic substrates with a carbon layer. In this method a carbon layer is obtained by converting a polymer coating into carbon. The obtained carbon coated monoliths should form the basis for three-phase hydrogenation catalysts.

2. EXPERIMENTAL The coating method is based on a patent [4] and starts from a mixture of furfuryl alcohol (further called FA, >99%, Aldrich, 30 ml) and pyrrole (>97%, Fluka, 9 ml) as carbon yielding binders, poly (ethylene glycol) monomethylether (further called PEG-750, Aldrich, 15 ml) as a pore-former, and concentrated nitric acid (65%, J.T.Baker, 2 ml) as a polymerization catalyst. This mixture readily polymerizes at ambient conditions. The catalyst was added step-wise during 45 minutes under continuous stirring and cooling. Pieces of cordierite monoliths (2MgO'2A1203"5SiO2, 62cellscm -2) were dipcoated using the partially polymerized mixture. Excess liquid was blown out of the channels with air. Initially four polymers differing in polymer composition were prepared (see Table 1: Starting composition for polymer preparation. Table 1). These polymers were FA* PEG-750* pyrrole used partially for dipcoating of A + monolithic substrates and partially B + + for the preparation of unsupported C + + carbon, which is more easily D + + + characterized than the supported * FA=furfuryl alcohol; PEG-750=poly (ethylene glycol) carbon samples. The polymer was monomethylether. allowed to solidify first at room temperature and finally after 4 hours at 353 K. The polymer was carbonized in flowing argon at 823 K for 2 hours with a heating rate of 10 K min 1. The resulting carbon was activated by partial oxidation in 10% 02 in Ar at 623 K for 5 hours, functionalized with concentrated NaOC1 (15wt%) for 5 min., and exchanged with [Pt(NH3)4]2+ followed by reduction in 10% H2 in Ar at 573 K for 2 hours, according to [5]. The prepared samples were characterized applying several techniques. Thermogravimetrical Analysis (TGA) of the carbonization step was performed on an STA 1500H thermobalance of Polymer Laboratories with 2-4 mg sample diluted with silicon carbide in flowing nitrogen (50 Nml min 1) with a heating rate of 10 K min -t. Elemental analysis was performed by flash-combustion of a sample and analysis of the combustion gases using a

177 Carlo Erba EA1108 elemental analyzer. Mercury porosimetry was performed on a Carlo Erba Porosimeter 2000 after outgassing the samples for 16 hours in vacuum at 423 K. Pores with diameters between 8 nm and 15 gm could be measured. Nitrogen adsorption was measured on a Quantachrome Autosorb-6B with N2 at 77 K after outgassing the samples in vacuum for 16 hours at 423 K; surface areas were calculated using the BET method. Carbon dioxide adsorption was performed on a Quantachrome NOVA 1200 at 273 K with CO2 (99.999%) after outgassing the samples at 423 K in vacuum for 16 hours; pore volumes and surface areas were calculated using the Dubinin-Radushkevich equation [6]. Infrared analysis (DRIFT) was performed with samples diluted in KBr on a Nicolet Magna 550 spectrometer by collection of 256 scans at 8 cm I resolution against a KBr background. The platinum content was determined by ICP-OES after dissolution of the platinum in aqua regia. The platinum metal surface area was determined by volumetric CO-chemisorption on a Quantochrome Autosorb-l-C at 308 K after reduction at 573 K for 2 hours in hydrogen.

3. RESULTS AND DISCUSSION 3.1. The carbon coating The carbonization step has been studied using thermogravimetrical analysis (see Figure 2). During carbonization the polymer is converted into carbon by removal of functional groups and condensation. 1.2 At temperatures around 650 K a 1.0 decrease in weight is observed for all polymers, indicating that ~0.8 carbonization takes place. Also the pore-former is removed at this 0.6 B temperature (see Figure 2), which is in @ 0.4 agreement with literature [7]. Curve A E 0.2 C (Figure 2) shows the carbonization step of the furfuryl alcohol polymer. EG-750 ~" 0.0, When pyrrole is added (curve B) the 350 450 550 650 750 850 weight decrease is less, indicating that Temperature [K] polymer B has a different

i••_r.P

composition. Curve C shows a higher Figure 2: Weight decrease of polymers and poreweight loss, which is caused by the former during carbonization, measured by TGA presence of PEG-750, which is (legend, see Table 1). completely decomposed [7] during carbonization. Curve D shows the combined characteristics of curves A, B, and C. During carbonization, the weight loss of the samples varied between 40 and 70 wt% depending on the composition (see Figure 2). Also the size of the polymer pieces decreased upon carbonization. An average linear shrinkage of 15-25% depending on composition is observed. So with the removal of functional groups, the polymer contracts as well. The amount of coating obtained after carbonization varied with the composition of the starting polymer. The weight increase due to the carbon layer was 12.4 wt% for sample A,

178

4.4 wt% for sample B, 6.5 wt% for sample C, and 10.2 wt% for sample D. The reactivity of the polymerization mixture differs with composition. After a fixed reaction time, the degree of polymerization is therefore not identical for all mixtures. This results in a difference in viscosity of the coating mixture, which influences the thickness of the coating applied. Also the residual mass after carbonization (see Figure 2) affects the final carbon content. Increase of the obtained coating thickness can therefore be achieved by increasing the polymerization time, and herewith the degree of polymerization and viscosity. The elemental composition of the polymers and the carbons was studied by elemental analysis (see Figure 3). The polymer composition changes with the starting materials; when pyrrole (C4HsN) is added the nitrogen content is increased, while addition of ,--, 100 I PEG-750 (CH3-(O-CH2-CH2)n-OH), o~.~ , leads to an increase in the oxygen '~ 8 0 tcontent. After carbonization, the o ".~ carbon content is increased, while "~ o 60 oxygen and hydrogen are decreased. E The nitrogen content of the pyrrole 8 40 containing samples is retained, ~ indicating that pyrrole is incorporated ~ 20 in the polymer structure. This is in E t~ contrast with the PEG-750 containing w 0 A C B D samples who show after carbonization no difference in composition with the Figure 3" Elemental composition of polymers non-PEG-750 containing samples. before (left bar) and after carbonization (right bar).

3.2. Texture analysis In order to use the obtained carbon coated monoliths as catalyst support they should exhibit an adequate surface area and pore structure. For three-phase applications meso-pores of (5-15 nm) are preferred for reasonable rates of diffusion. A high surface area in this pore range is desired in order to locate a large number of accessible active sites. To analyze the texture three methods have been applied; N2-adsorption, CO2-adsorption, and mercury-porosimetry. N2-adsorption was not successful, because none of the samples showed noteworthy adsorption of nitrogen. CQ-adsorption showed better results. Micro-pore Table 2: Micro-pore volume and micro-pore surface area calculated with the DubininRadushkevich equation [6], measured by CQ-adsorption at 273 K. Micro-pore volume Micro-pore surface area carbon carbon carbon carbon carbon carbon only coated coating on only coated coating on monolith monolith monolith monolith [m 2 gcarbon"1] [m 2 gsample-1] [m 2 gcarbon"1] [ml gcarbon-1] [ml gsample-1] [ml gcarbon"I] 510 75 590 A 0.185 0.027 0.218 300 30 680 B 0.109 0.011 0.250 430 35 540 C 0.157 0.013 0.200 130 40 390 D 0.047 0.015 0.147

179 Table 3" Macro-pore volume and macro-pore surface area measured by mercury Macro-pore volume Macro-pore surface carbon carbon carbon carbon carbon only coated coating on only coated monolith monolith monolith -1 [m 2 gcarbon"I] [m2 gsample"1] [ml gcarbon-1] [ml gsamp!e"1] [ml gcarbon,) A n.d.* 0.080 0.65 n.d.* 1.05 B n.d.* 0.019 0.43 n.d.* 0.36 C n.d.* 0.114 1.75 n.d.* 0.85 D 0.011 0.104 1.02 1.9 0.25 0.48 E# 0.130 * n.d. = not determined

intrusion. area carbon coating on monolith [m2 gcarbon"1] 8.5 8.2 13.1 2.5

E # bare cordierite

volume and micro-pore surface area were calculated from the adsorption data using the Dubinin-Radushkevich equation [6]. Table 2 shows the results from CO2-adsorption. Mercury porosimetry was used to determine the macro-pore size distribution. Data from mercury porosimetry are shown in Table 3. The carbon sample prepared with only furfuryl alcohol shows the highest micro-pore volume and because of the high correlation between pore volume and surface area also the highest micro-pore surface area (see Table 2). The high micro-porosity is not unexpected, because furfuryl alcohol based synthetic carbons are used as Carbon Molecular Sieves [8]. The addition of other components to the polymerization mixture leads to a decrease in specific surface area. When coated on a monolithic substrate, a carbon shows a higher micro-pore surface area p e r unit c a r b o n mass. The macro-porosity of only one of the unsupported carbon samples (sample D) was determined (see Table 3). The cumulative pore volume p e r unit carbon mass of carbon D and carbon coated monolith D is shown in Figure 4. This figure shows clearly the difference between a supported and an ~ 1.5 0.015 unsupported carbon. The carbon coated monolith shows macro-pores _~ 1.2 0.012 around 1.5 gm, while the unsupported carbon sample shows pores in the ~= 0.9 0.009 mesoporous range around 15 nm. Also --= o the total pore volume for the carbon > 0.6 0.006 coated sample is two orders of o ~" 0.3 0.003 magnitude higher than that of the unsupported carbon sample. Both the = i , i ,~ i 0 differences in the amounts of microO 0 10 100 1 1000 10000 and macro-pores between the Pore radius [nm] supported and unsupported carbon Figure 4: Cumulative pore volume p e r unit carbon samples can be related to the process mass as a function of pore radius for unsupported of pore formation. During carbon D and the carbon coating of monolith D, carbonization functional groups are measured by mercury porosimetry. removed from the polymer. Because

'Carboncoating"~ n\u~iiiliiied

"--

180 of this removal the polymer will 0.14 cordierite--~ _ ~ contract and small pores are formed. C ... I~ 0.12 However, when the polymer is coated D _....___._~' on a cordierite substrate, the anchoring -~ 0.10 ,___, A ---------~ of the coating to the substrate prevents Ill E 0.08 the coating from shrinkage. The 13 o 0.06 coating will therefore crack so that L_ pores are enlarged and new pores are o c, 0 . 0 4 formed. B. ~. =0.02 Figure 5 shows the cumulative pore O volume p e r unit support mass for the 0 '1 I , i various starting mixtures, together 1 10 100 1000 10000 with bare cordierite. The curves in Pore radius [nm] Figure 5 depend on the coating Figure 5" Cumulative pore volume, p e r unit thickness of the samples; these were support mass as a function of pore radius for both a 12.4 wt% (A), 4.4 wt% (B), 6.5 wt% bare and carbon coated monolithic substrates. (C), and 10.2 wt% (D), respectively. ."E. 0.7. The influence of the pore-former FIA PEG-750 is clear. Almost all pores E 0.6 . _0 laB formed when PEG-750 is added, are .-9_ 0.5 IIC macro-pores, indicating that cracking if} laD is easier when a pore-former is -o 0.4 N [] cordierite present. The addition of pyrrole to the 0.3 polymerization mixture probably o 0.2 results in a stronger polymer, because (D > smaller macro-pores are formed. -.~ 0.1 Because the curves in Figure 5 depend tl1 n," 0 on the coating thickness, the relative 7 14 30 70 140 300 700 1 4 0 0 3 0 0 0 pore size distribution is shown in Pore radius [nm] Figure 6. The samples containing no Figure 6: Relative pore size distribution as a pore-former show the most even function of pore radius for both a bare and carbon distribution of pore sizes, ranging coated monolithic substrates. from macro-pores to meso-pores, while the bare cordierite and the PEG-750 containing samples mainly show macro-pores. None of the samples showed an increase in the pore volume when compared with bare cordierite. The decrease in pore volume is accompanied by shift towards lower pore radii in the pore size distribution. The carbon coating is therefore expected to be located on the inside of the cordierite pores. Not only the pore volume of the carbon support, but also the surface area, which has to disperse the active phase is important. Figure 7 shows therefore the specific surface area distribution p e r unit carbon mass. The sample containing only furfuryl alcohol (A) shows a distribution with a maximum around 30 nm. The addition of pyrrole (B) shift this distribution to lower pore radii, although the total surface area p e r unit carbon mass is approximately the same (see Table 3). Addition of PEG-750 (C) creates a bimodal distribution, with maxima

181 around 1.4 gm and 14 nm and a total surface area 13.1 m 2 g-1. The addition of both pyrrole and PEG-750 (D) still shows the bimodal distribution, but the total surface area has almost disappeared (2.5 m 2 g-l).

6.0'

'c~ 5.0 4.0' oo 3.0'

3.3. Activation 2.0' After carbonization, which removes "~ 1.0 almost all functional groups from the carbon surface, the surface has to be : ~ I N 0.0 14 30 70 140 300 700 1400300( 7 made suitable to apply a metal to it. Pore radius [nm] There are several methods available for activation of a carbon support, but Figure 7: Specific surface area distribution p e r unit we have chosen for a method carbon mass as a function of pore radius for the described by Richard [5], because of various carbon coatings. the obtained dispersion. This method comprises four steps: partial oxidation 1730 1650 1600 of the support, functionalization, Exchanged/ exchange, and reduction as described reduced Functionalized in the section two, EXPERIMENTAL. :3 The activation step has been Oxidized performed with a sample B only. The o* sample used for activation was coated Carbonized 0 with 17.4 wt% carbon, due to the (/) .Q < application of a longer polymerization Polymer B time. The oxidation of the support caused a weight loss of approximately 2000 1800 1600 14~00 15 wt% of carbon and increased the Wave number [cm1] reactivity of the support surface. When Figure 8: Infrared spectra of sample B in the range omitting this oxidation step, nothing of carbonylic absorption [9], at various preparation happened upon functionalization with stages. NaOC1. After partial oxidation numerous small gas bubbles are released from the NaOC1 solution during functionalization, indicating a reaction on the support surface. After prolonged time (over 1 h) the carbon coating of the partially oxidized monolith was completely removed, while the non-oxidized coating was still hardly affected. The functionalized support (for 5 rain.) was exchanged with [Pt(NH3)4]2+ and subsequently reduced. The activation of the support is characterized with infrared analysis. Figure 8 shows the absorption spectra of sample B coated on a monolithic substrate during several preparation stages in the region of carbonylic absorption [9]. Polymer B shows some absorption between 1800-1600 cm l. The absorbance around 1730 cm "l (C=O groups) and the absorbance around 1760 cm l (not assigned) is diminished after carbonization, while the absorbance around 1600 cm 1 (C=C, aromatic rings) is increased. Upon oxidation the C=O absorbance (1730 and i

182 1760cm -1) is regained, proving the formation of exchangeable groups. The same transformations are observed for O-H groups (not shown, in the range of 3200-3600 cm-1). Functionalization of the support with NaOC1 does not create additional functional groups, as can be seen from Figure 8. After exchange and reduction, the absorption around 1730 and 1760 cm l is diminished again and the aromatic ring vibrations are shifted to isolated C=C vibrations (from 1600 cm -1 to 1650 cm-1). The platinum content of the sample was 3.1 wt%, which is very high. Per unit carbon mass the platinum content was even 17.8 wt%. The activation/functionalization step provides sufficient anchoring sites for the [Pt(NH3)4]2+ ions. The high platinum content per unit carbon mass can easily compensate for the low catalyst content per unit reactor volume of coated monolithic catalysts. The platinum metal surface area was calculated to be 0.32 mpt 2 gsupport"1 assuming a stoiciometry of CO:Pt=I. This results in an average metal particle size of 27 nm and a metal dispersion of 4.2%. This low dispersion was expected taking the metal loading into account. An increase in the metal dispersion is currently investigated and can be accomplished according to e.g. Rodriguez-Reinoso [10] by introduction of a passivation step in an inert medium before reduction.

4. CONCLUSIONS Starting from a polymerization mixture which contains among others furfuryl alcohol, pieces of ceramic monoliths can easily be coated with a polymer layer. This layer is converted into carbon by means of carbonization. Depending on the composition of the polymerization mixture, the final elemental composition and the coating thickness varied. A carbon load of 10 wt% could easily be obtained. The texture of the obtained carbon coating depends on the composition of the starting polymerization mixture. All samples showed a good micro-pore pore volume and surface area. The shrinkage of the polymer during carbonization caused the coating of the carbon coated monolithic supports to crack so more and larger pores are formed when compared to the unsupported carbon samples. The addition of a pore-former facilitates the cracking, causing the formation of macro-pores o f - 1 . 5 gm, but also a large surface area in the mesopore range. For liquid phase catalysis, optimization should however lead to a larger pore volume and a larger surface area in the pore size range of 5-15 nm. Combined with an increased coating thickness, this should supply sufficient accessible surface area to disperse the active phase. Turning the carbon coated monolithic support into a catalyst can be performed by activation of the support by partial oxidation followed by functionalization with NaOC1, exchange with [Pt(NH3)4]2+ and reduction in hydrogen. Without the partial oxidation the carbon surface is not reactive and consequently functionalization and exchange are not possible. IR showed that the functionalization step can be omitted because no additional functional groups are created. The application of platinum on this type of catalyst using [Pt(NH3)4]2+ works well. Very high metal loading can be obtained (17.8 wt% per unit carbon mass), so that a reasonable metal loading per unit reactor volume is obtained. The dispersion is relatively low due to the high metal loading.

183 Although further optimization of the preparation procedure is probably possible, the obtained carbon coated monolithic support already show characteristics that make them suitable to be used as catalyst support material in oxidation and hydrogenation reactions that are currently performed in slurry or trickle-bed reactors.

Acknowledgment This project is financed by the Ministry of Economic Affairs of the Netherlands (Innovation Oriented Research Program on Catalysis, project IKA94061). Coming Inc. (Coming, USA) is acknowledged for the supply of ceramic monolithic substrates.

REFERENCES 1.

2. 3.

4. 5.

6.

7. 8.

9. I0.

Irandoust, S., Cybulski, A., and Moulijn, J.A., The use of monolithic catalysts for threephase reactions, in Cybulski, A., and Moulijn, J.A. (eds.), Chem. Ind. (Dekker) 71 (Structured catalysts and reactors), Marcel Dekker, New York, pp. 239-266 (1998). Cybulski, A., and Moulijn, J.A., Monoliths in heterogeneous catalysis, Catal. Rev.-Sci. Eng. 36 (1994), 179-270. Radovic, L.R., and Sudhakar, C., Carbon as a catalyst support, in Marsh, H., Heintz, A., and Rodriguez-Reinoso, F. (eds.), Introduction to carbon technologies, University of Alicante Press, Alicante, Spain, pp. 103-165 (1997). Hucke, E.E., Methods of producing carbonaceous bodies and the products thereof, US 3859421 (1975). Richard, D., and Gallezot, P., Preparation of highly dispersed carbon supported platinum catalysts, in Delmon, B., Grange, P., Jacobs, P.A., and Poncelet, G. (eds.), Stud. Surf Sci. CataL 31 (Preparation of catalysts IV), pp. 71-79 (1987). Byrne, J.F., and Marsh, H., Methods for measuring porosity and surface area, in Patrick, J.W. (ed.), Porosity in carbons: characterization and applications, Edward Arnold, London, pp. 28-45 (1995). Voorhees, K.J., Baugh, S.F., and Stevenson, D.N., Investigation of the thermal degradation ofpoly(ethylene glycol), J. Anal. Appl. Pyrolysis 30 (1994), 47-57. Lafyatis, D.S., Tung, J., and Foley, H.C., Poly (furfuryl alcohol) derived carbon molecular sieves: Dependence of adsorptive properties on carbonization temperature, time and poly (ethylene glycol) additives, Ind. Eng. Chem. Res. 30 (1991), 865-873. Mul, G., Catalytic diesel exhaust purification, PhD-thesis, Delft University of Technology, (1997). Rodriguez-Reinoso, F., Rodriguez-Ramos, I., Moreno-Castilla, C., Guerrero-Ruiz, A., and L6pez-Gonzfilez, J.D., Platinum catalysts supported on activated carbons, J. Catal. 99 (1986), 171-183.

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1998 Elsevier Science B.V.

Preparationof CatalystsVII B. Delmonet al., editors.

185

Physicochemical bases for the preparation of spinel supported bimetallic platinum catalysts for dehydrogenation of lower paraffins. N.A. Pakhomov, R.A. Buyanov and B.P. Zolotovskii Boreskov Institute of Catalysis, Novosibirsk, 630090, Russia

1. INTRODUCTION The demand in lower olefins for the production of high-octane ecologically benign gasoline additives such as MTBE and alkylates has considerably grown, stimulating the interest to the dehydrogenation of lower paraffins. The latest tendency in the improvement of the lower paraffin dehydrogenation technology is the use of bimetallic systems based on supported platinum instead of traditional environmentally harmful Cr~O3-A1203 catalysts [1-4]. The use of high thermally stable zinc- and magnesium-aluminum spinels to support such catalysts opened an absolutely new possibility to perform the dehydrogenation in water vapor, thus significantly increasing the yield of the desired products [2,4]. The present situation in this field is somewhat paradoxical: practically all possible methods for the preparation of such catalysts are already patented, while no satisfactory preparation theories have been developed. In the present communication we make an attempt to fill the gap in this field. Physicochemical bases for the preparation of new generation catalysts for dehydrogenation of C3-C5 paraffins presented in this paper are based on the results of investigations performed lately in our laboratory. For the development of the scientific bases for the preparation of such catalysts, it was necessary to find a complex solution of two interconnected problems: i) development of the method for the synthesis of the spinel support with desired composition and textural characteristics; and ii) development of theoretical and experimental approaches for the synthesis of bimetallic active component with desired composition, structure and dispersity.

2. PREPARATION OF SPINEL SUPPORT WITH DESIRED PROPERTIES

2.1. Determination of the optimal phase and chemical composition of the support The use of synthetic aluminum spinels to support platinum catalysts claimed a solution of several problems not typical to single-component supports. First, it was necessary to explore the dependence of the properties of the supported metal on the nature of the bivalent element of the spinel support. Second, it was necessary to establish the maximum possible contents of aluminum, zinc or magnesium oxides not reacted during the synthesis of the support. As modifying additives have a significant effect on the properties of supported platinum, investigations of single-metal catalysts supported on model Zn-A1 and Mg-A1 supports with different initial ratios of the oxides were performed to answer the above questions. Then the results were adjusted for bimetallic systems. The supports were prepared according to [5].

186 Tablel. Influence of the chemical and phase composition of the supports on the state and catalytic properties of supported platinum in n-butane dehydrogenation.

Support

State of platinum in reduced samples *)

Catalyst Phase,

A1203/MeO mol.

MeO free, wt.% BET,m2/g

1.0

ZnA1204 ZnO=2.0 BET=14

0.9

ZnA1204 ZnO=6.3 BET=14

1.5

Defect Zn-Al spinel ZnO=0.2 BET=39

1.0

MgAI204 MgO=0.8 BET=48 7-A203 BET=170

Deposition method

Pt

D

Pt~/Pt z

wt.%

%

%

sorption impregnation impregnation

0.46

74

0

0.50

50

6

impregnation

0.6

sorption

0.42

impregnation sorption

0.59

100 6 . . . . . . . 0.70 66 24 0.55

88

30

**) Catalytic characteristic

Phase and paticle size nm

Dilution with water vapor +/-

X

S

%

mol.%

0.5-1.5 Pt-Zn 10 Pt-Zn 18 8-PtZn

-

55

88

+

43

69

+

19

79

Pt 8/16

+ /+

49 /23

42 /55

0.5-1.0 + flat particles 2.5

+

37 28

67 31

+

6

14

56 46

55 19

1-1.5

-

+

*) D - platinum dispersity by O 2- chemisorption; Pff/Pt z - ratio of platinum in the oxidized state to all surface platinum measured by O2-H2 titration in the water vapor atmosphere. Phase and paticle size by XRD or TEM **) X- n-butane conversion, S - n-C4Hs+C4H6 selectivity, T- 575 ~ time- 5 min. + C4H1o: H2:H20 = 1 : 0.25 : 10; - CnHt0: H2 = 1 : 0.25

Table 1 shows that relatively high catalytic activity and selectivity could be obtained only on platinum supported on stoichiometric ZnA1204 spinel (initial ratio A1203/ZnO ~ 1). In this case, as it was shown by us earlier [6], preliminary thermal treatment of Pt/ZnA1204 catalysts has to be performed under oxidative conditions as the reductive pretreatment results in their deactivation due to the formation of an inactive alloy 8-PtZn [4]. On the contrary, the reduction of preoxidized samples can lead to the formation of Pt-Zn solid solutions with a face-centered cubic (f.c.c.) arrangement. This results in the dehydrogenation selectivity growth [4,7]. The probability of the formation of solid solutions grows with the increase of the

187 platinum dispersity and the content of free ZnO in the support up to a certain optimal value. At the content of free ZnO > 5-6%, the inactive alloy 8-PtZn is also formed. The negative effect of a large amount of free ZnO ( A12Os/ZnO < 0.9) can be expressed at the preparation stage as well. During the impregnation it dissolves in the acidic H2PtC16 solution to form zinc hydroxoclorides: ZnOHC1 and Zns(OH)8C12 [4]. Their decomposition during the calcination in air favors the formation of low-dispersed platinum particles. Catalysts based on a defect Zn-A1 spinel ( AI203/ZnO > 1.1) are quickly irreversibly deactivated during their work in the water vapor atmosphere due to the sintering of platinum particles. A similar deactivation of Pt/7-AI2Os catalysts is accompanied by the decrease of the alumina specific surface area under the action of water vapor. Platinum supported on MgA1204 has lower activity and selectivity in the dehydrogenation reactions than Pt/ZnA1204 samples. During n-butane dehydrogenation in the water vapor presence, Pt/MgA1204 prepared by the sorption technique exhibits catalytic activity in cracking and deep oxidation rather than dehydrogenation. During the isobutane dehydrogenation in hydrogen, the contact gas mostly consists from skeletal isomerization (6070 tool.%) and cracking (20 mol.%) products. Pt/7-A1203 catalysts have similar properties. However, the state of platinum on MgA1204 is significantly different. It is notable that platinum has a 100% dispersity in the adsorption samples and a high dispersity in impregnation samples. Using XRD, TEM, and O2-H2 titration in the water vapor atmosphere, it has been determined that in reduced Pt/MgA1204 samples platinum exists in the form of ultra-dispersed 0.5-1 nm metal particles, flat particles of anomalous morphology and in the oxidized state. According to the EXAFS data [7], a part of platinum exists in the form of oxide compounds with a spinel structure, e.g. MgPtO~. The origin of such unusual state of platinum on this support requires a separate investigation. Here we would only like to emphasize the fact of the strong interaction of platinum with the support. 100| ,tc

a) ....

2

O

100 ,

3 b)

0

+_.___. 50

2

_

.....

,

60 T " I

600

w

W .

i.

.

.

.

700 Temperature, C

I

800

I 0

-i

.... I

600

.

.

.

.

.I .

.

....

i

700 800 Temperature, C

. . . . . . .

I

900

Fig. 1. Convertion (1), selectivity (2) in n-butane dehydrogenztion and platinum despersity (3) in Pt(0.46%)/ZnA1204 (a) and Pt(1.1%)/MgA1204 (b) catalysts as a function of the calcination temperature. (575 ~ C4H10 H2 9 = 1 -0.25, time-5 min.)

It is of fundamental importance that dispersed Pt particles on the surface of stoichiometric spinels have increased resistance to sintering in the oxidative atmosphere in comparison with 7-A1203. In Fig. 1 it is shown that the catalytic activity of Pt/ZnA1204 and the platinum

188 dispersity practically do not change when the calcination temperature is increased from 580 to 800~ The drop of the platinum dispersity and activity of the Pt/MgA1204 catalyst starts above 700~ On the other hand, it is well known that platinum sintering on alumina during calcination in air begins at 600~ [8] and coincides with the start of the decomposition of the surface aluminum-platinum oxide complex [9]. The high thermal stability of platinum on spinel supports is determined, first of all, by the high thermal stability of the spinel structure and texture. The second cause may be connected with the formation of surface and bulk Pt-Zn or Pt-Mg oxide spinel compounds during the oxidative activation, which have higher stability to the thermal dissociation [ 10] in comparison with the aluminum-platinum complex. Thus, at this stage of the study it is possible to conclude that the best properties of the catalyst can be obtained by using stoichiometric ZnA1204 spinel as the support, the content of free ZnO not exceeding 2-4 wt.%.

2.2. Development of an ecologically benign method for the preparation of the spinel support Our studies allowed us to approach purposefully to the choice of the preparation method of a spinel support with desired characteristics. To optimize the preparation method, it was necessary to obtain a maximum yield of the spinel with an acceptable surface area and high mechanical strength of the support granules. As it could be expected, the highest spinel yields at relatively low calcination temperatures (800~ or short calcination times we managed to obtain using the co-precipitation method. In this case the components react during the co-precipitation to form two crystalline zinc hydroxoaluminates ZnA12(OH)8 and Zn2Al(OH)6.6(NO3)0.4(ZHA), which decompose in the temperature range of 280-450~ to yield a spinel [11]. It is interesting that the components react to form Zn3A12(OH)12 not only in the case of co-precipitation, but also if a suspension of freshly precipitated aluminum hydroxide gel is mixed with zinc oxide or hydroxide. These preparation methods have a significant drawback as they involve large amounts of the sewage and harmful atmospheric emission at the calcination stage. Furthermore, the presence of crystalline ZHA in the precipitates substantially impairs the flow properties of the paste during its molding by extrusion resulting in a sharp drop of the mechanical strength of the samples. A compromise solution was found in the mechanical mixing of the components. A product prepared by the thermochemical activation (TCA) of gibbsite [12] was used as a raw material. We studied the effect of the mixing method and conditions, the size of aggregates of the reacting compounds, the phase composition and morphology of the initial aluminum hydroxide, which are determined by the conditions of the TCA product hydration, on the phase composition, texture and mechanical strength of Zn-A1 spinel supports molded by extrusion and calcined at 1000~ It was found that the dry mixing with a simultaneous grinding in a ball mill does not result in the formation of supports with a high spinel yield despite the high dispersity of the aggregates formed. To intensify the dispersing and homogenization of the components, mechanochemical activation of a mixture of zinc oxide and aluminum hydroxide was performed in a planetary mill. An unexpected result was obtained. The water addition during the preparation of the paste to molding resulted in the loss of the plastic properties of the paste due to the formation of crystalline ZHA, as in the case of gel mixing. The best results were obtained by a combination of the dry and, then, wet mixing in a ball

189 mill. It was found that only the achievement of necessary macro structural properties of the components was not sufficient to prepare a support with optimal structure and mechanical strength. In addition, aluminum hydroxide with definite microstructural characteristics- phase composition and morphology- should be used. Thus, the use of baerite results in the formation of samples with a low spinel yield and pore diameter of 60-1000 nm, and therefore, low mechanical strength. When pseudobohemite is used, the quality of the support is significantly better. The highest strength and spinel yield are obtained with the use of pseudobohemite formed from loosely packed fibrils. Samples prepared from pseudobohemite formed from densely packed fibrils have slightly lower strength. In this case the samples contain not only 50-80 nm mesopores but also macropores. Loosely and densely packed fibrils are formed by hydration of the TCA product in acetic and nitric acid, respectively. The above regularities are generally preserved during the synthesis of Mg-A1 supports, however in this case the calcination has to be performed at significantly higher temperatures. Thus, we have developed a practically waste-free ecologically benign method for the preparation of spinel supports with desired characteristics.

3. PREPARATION OF SUPPORTED BIMETALLIC ACTIVE COMPONENT 3.1. Nature of the active component in supported bimetallic catalysts As the main task of the catalyst preparation theory is the development of approaches to the purposeful synthesis of the active component with desired composition, structure and dispersity, let us briefly discuss the nature of active components in supported bimetallic catalysts. The analysis of literature and our experimental results makes it possible to conclude that alloy particles are active components in supported Pt catalysts modified with easily reducible group I-IV elements (Cu, Zn, In, Sn, Pb) [4,6,9,13-20]. We were one of the first to prove the formation of alloys in Pt-Sn/ZnA1204 catalysts with a real platinum content by direct XRD method [ 14]. Changes in the catalytic properties of bulk and supported alloys are usually explained in the literature by the influence of two factors: geometric and electronic [19]. However, in many papers only the fact of the alloy formation is mentioned during the interpretation of the results obtained, whereas changes in the crystalline structure of the active metal are out of sight. Meanwhile, it is well known [21] that with some elements (Cu, Ir, Re) platinum can form a continuous row of solid solutions with the preservation of the initial f.c.c, arrangement of the crystalline lattice. With other elements (Sn, Zn, Pb, In, etc) the intersolubility is limited, but intermetallic compounds of different structure and composition can be formed. We have earlier determined [4,6,7,14-17] that the highest catalytic activity and selectivity in comparison with pure platinum is observed for solid solutions with the f.c.c, arrangement and the content of the second metal below 25 at.%, and for such intermetallic compounds as Pt3Sn, Pt3Zn, Pt3In, etc. Solid solutions with a higher content of the second element (Pt-Cu) and alloys with other crystalline structures (PtSn, 5-PtZn, PtIn, PtPb, etc. ) have lower zatalytic activity. It should be noted that modifying Sn and In additives not bound into alloys ~articles and present in the catalyst in the form of surface oxides are not inert impurities but suppress acid sites responsible for the coking of the support surface [22]. Therefore, we would like to formulate a general rule of the synthesis of bimetallic dehydrogenation catalysts. The

190 synthetic conditions developed have to provide the formation of dispersed particles of two types on the surface of the support: i) active alloys with a f.c.c, arrangement and platinum excess in comparison with the additive; and ii) surface oxide of the additive blocking the acid sites of the support. 3.2. Factors affecting the formation of supported alloys The formation of alloys deposited on spinel supports depends in a complex manner on a number of interconnected factors, including the preparation conditions. However, the most important factor is the occurrence of parallel reactions of the deposited components with the support and with each other at various stages of the catalyst preparation. Let us discuss the main types of such reactions and analyze their influence on the formation of suppoted active components in bimetallic catalysts. 3.2.1. The deposition stage As this problem has been discussed by us in detail earlier [5], here we shall only attract attention to specific features of the deposition stage during the synthesis of spinel-supported bimetallic catalysts. It has been found that the modifying additives are anchored via the cation exchange of Cu 2§ and Sn2§ with surface Mg 2§ (or Zn2§ and In3+ with A13+. The adsorption of chloroplatinate ions occurs via the anion exchange with OH groups [5]. The adsorption of additives forming complex compounds with platinum (Pt-SnC12 system) from binary solutions is significantly different from that of compounds that do not form them. Despite the apparent simplicity and different anchoring mechanisms of the precursors of platinum and additives, the co-adsorption method has significant restrictions due to the complex mutual influence of the components during the sorption. Therefore, in some cases the catalysts should be prepared by successive sorption of platinum on a preliminary modified support. 3.2.2. State of deposited components in bimetallic catalysts after thermal treatment The modifying additives studied can be divided into two groups by the character of their interaction with oxide supports during the thermal treatment in different environment. Copper additives forming surface and bulk solid solutions or aluminates during the oxidative treatment belong to the first group. Additives forming surface compounds with the support during reduction belong to the second group. Additives of tin, indium, lead and other elements which form oxides in the oxidation state > 2+ during the calcination are typical representatives of the second group. Evidently, the degree of the interaction of the additives with the supports must depend on the crystalline structure and specific surface area of the latter. The interaction degree is the highest for A1203 due to the presence of vacancies in the crystalline lattice, and the lowest for spinels. This imposes rigid restrictions on the choice of the environment for the activation of the catalysts. Preliminary reduction of dried Pt-Sn, Pt-Cu and Pt-In catalysts supported on ZnA1 and Mg-A1 spinels results in the preferential reduction of the additives to the zero-valent state and formation of low-dispersed alloys enriched with the inactive metal, leading the catalyst deactivation. Therefore, to obtain active bimetallic catalysts on spine supports, it is necessary to perform the pretreatment under the oxidative conditions (Table 2). Using ESR, electron diffusion reflectance spectroscopy, TPR and XRD [ 17, 23], we have established that copper supported on spinels can exist in different forms after the calcination

191 in air: 1) as isolated Cu2§ ions in a distorted octahedral oxygen environment; 2) as magnetic associates Cu-O-Cu; 3) as Cu 2§ ions in tetrahedral positions of CuxMgl.xA1204 solid solutions; 4) as CuO phase; and 5) as CuA1204 spinel. The fraction of each state depends on the composition of the starting support and its surface area, the copper loading, the deposition method (adsorption or impregnation) and the calcination temperature. For example, the formation of the CuO phase on the Zn-A1 spinel with BET < 20 m2/g is observed at the Cu loading above 0.5 wt.%. On the Mg-A1 spinel this phase is not detected up to the Cu loading of 2.5-3 wt.%. However, parallel to the interaction of copper and platinum with the support, competitive formation of CuPt306 and/or CUl.xPtxO (x = 0.135-0.355) solid solutions takes place during the calcination of a bimetallic Pt-Cu catalyst [17]. The depth of the copper reaction with platinum depends significantly on the nature of the support, the deposition method and conditions, and the ratio of the components. It is maximal when the catalyst is prepared by the co-impregnation or platinum adsorption on a copper-containing support, and minimal when the catalyst is prepared by the successive deposition of copper on calcined platinum catalyst. Due to the effect of the strong interaction of Pt with the Mg-A1 spinel, much more copper is required to prepare a Pt-Cu oxide phase than in the case of the Zn-A1 spinel, other conditions being equal. On the other hand, the formation of these compounds indicates that the copper addition hinders the interactions of Pt with the Mg-A1 spinel. The stabilization of supported platinum in the form of Pt-Cu oxide compounds at the oxidative activation stage favors the increase of the dispersity of metal particles during the following reduction. The size of the Pt-Cu alloy particles decreases by an order of magnitude with the increase of the Cu/Pt ratio in a Pt-Cu/ZnA1204 catalyst prepared by the coimpregnation method, and is not affected by the successive deposition of copper [17]. The reduction of CuPt306 results in the formation of active Pt-Cu solid solutions with a platinum excess (< 25-30 at.% Cu). Less active solid solutions with the copper excess are formed during the reduction of the Cul.xPtxO phase. As the composition of the oxide phases and alloys formed from them depend on the copper content in the catalyst, there is an extremum in the dependence of the catalytic activity on the Cu/Pt ratio (Fig. 2). It is easy to see that the strong interaction of platinum or copper with the Mg-A1 spinel makes significantly higher copper loadings necessary for the achievement of the activity maximum than in the case of the Zn-A1 spinel. According to the XRD, TPR and M6ssbauer spectroscopy data [4, 16], metal platinum particles and SnO2 are formed in Pt-Sn catalysts prepared by the co-impregnation from solutions of complexes after the calcination in air. The redox reaction of the [Pt(SnC13)2C12]2 decomposition proceeds already during the drying in air. The size of the platinum particles in this case does not decrease as much as for Pt-Cu samples. Let us note that the character of the reduction of Pt-Sn complexes results in a partial loss of the stabilization effect of spinels on the thermal stability of supported dispersed particles. The unique properties of spinels are fully expressed during the oxidative activation of Pt-Sn samples prepared by the adsorption of platinum on a Sn-containing support. Two series-parallel processes take place during the reduction of calcined Pt-Sn catalysts: 1) Sn4§ reduction to Sn2§ with the formation of surface tin aluminate SnA1204; 2) reduction of tin particles contacting with metal platinum to Sn~ with the formation of Pt-Sn alloy particles with the tin content increasing with the increase of the Sn/Pt ratio in the catalyst. Therefore, in the general case an extremum in the dependence of the dehydrogenation activity on the additive/Pt ratio in the catalyst is observed in Pt-Sn systems. As the additives differ by the

192 type of their interaction with the supports, the position of the extremum can be changed significantly for the same support by the change of the nature of the additive (Fig. 2). For the same support and additive, the presence and position of the extremum depend on the deposition method and conditions. For example, for Pt-Sn catalysts the formation of alloys has been shown to be determined by the nature of Pt-Sn complexes present in the impregnation solution [7], whose composition can be varied by changing the solvent. Samples prepared specially from the [Pt(SnC13)2C12]2" complex or from aqueous solutions with such Sn(II)/Pt(IV) ratio where this complex dominates have the highest activity. The reduction of such catalysts results in the preferential formation of the Pt-Sn alloy with the f.c.c. arrangement. The deposition from [Pt(SnC13)5]3" and [PtSnsC120]4 complexes results in the formation of PtSn and PtSn2 alloys. As these two complexes dominate in acetone and alcohol solutions in a wide range of the Sn(II)/Pt(IV) ratios, active catalysts cannot be prepared from such solutions. 70 Pt-Sn/B

Pt-ln/B

6O

"i

Pt-Cu/B

50 40

20

i

i

i

0

2

4

i 6 Me/Pt,

i 8

'

i

i

10

12

atom.

Fig. 2. The optimum Me/Pt ratio (Pt ~ 1 wt.%) in the bimetallic catalysts of n-butane dehydrogenation as a function of the nature of the modifying additive (Me) and the chemical composition of spinel support. (Preparationmethod - co-impregnation,T - 575~ n-C4H10: H2=l : 0.25). A- ZnA1204, B -MgA1204 3.2.3. Effect of the reaction medium on properties of supported alloys Changes in the composition of supported active components continue during the catalytic tests. Dehydrogenation of lower paraffins appeared to be a suitable reaction for the investigation of the manifestations of surface and bulk segregation of alloys as the process can be carried out not only in the reductive medium but also by the dilution of the raw materials with water vapor. In the first case, one should expect the enrichment of the alloy surface with platinum. This seems to be the cause of a significant increase of the activity during the dehydrogenation in hydrogen for alloys of the PtSn type (Table 2). In the presence of water vapor, the surface can be enriched with the inactive metal. Thus, the PtSn alloy completely looses its activity. Probably, such alloys as 5-PtZn, PtIn and PtPb are inactive in the presence of water vapor too. The effect of the reaction medium on the properties of alloys with the f.c.c, arrangement depends on the nature of the modifying element. For example, Pt-Sn solid solution and the Pt3Sn alloy are less sensitive to the influence of water vapor. This makes Pt-Sn a suitable

193

zata!yst for the one-stage dehydrogenation of paraffins to diolefins (Table 2) [4]. The catalytic activity and selectivity of Pt-Cu alloys decrease with the dilution of the raw materials with water vapor. The higher the copper content, the more sensitive the system is to the poisoning effect of water vapor [17]. Catalysts containing Pt-In solid solutions have increased selectivity to cracking and deep oxidation during the dehydrogenation in the presence of water vapor. The decomposition of the alloys in the oxidative environment during the regeneration of the catalysts from carbonaceous deposits can be considered a marginal case of the surface segregation. This segregation is reversible for preoxidized samples [4, 17] and irreversible for prereduced ones (Table 2). Table 2. Influence of thermal pretreatment conditions and reaction mixture compositions on the active component composition and catalytic properties of spinel supported bimetallic catalysts.

Thermal pretreatment medium *)

Catalyst 9

,

Calcination in air

Pt-Sn/h/IgAl204

Calcination in air

Pt Sn/Pt=- 1.8

Pt-Cu/ZnA1204 1.3 wt.% Pt Cu/Pt=I.6

Calcination in air

Pt-In/MgA1204

Calcination in air

*) T- 580 ~ 3.3.

Phase

Particle size nm

. . . . **) .... Catalytic characteristics Water vapor +/+ ,

a) Regeneration in air b) Reduction ***) Reduction ***) a) Reduction ***) b) Regeneration and reduction

1.0 % wt.%

1.0 wt.% Pt I i ~ t =2.5

Active component composition

,

Reduction of H2

Pt-Sn/ZnAI204 0.55 wt.% Pt Srl~t=- 1.5 (atom.)

Thermal retreatment conditions

Reduction ***) Reduction of Ha

PtSn Pt

28 12

Pt3Sn + PtSn Pt-Sn alloy f.c.c. Pt-Sn (f.c.c.) +Pt3Sn Pt-Sn (f.c.c.) +Pt3Sn

12

Ptv0Cu30

,,

+

S

yield

mol.% mol.% 97

2.5

-

23 53

91 96

4.5 3.1

12

+

75

83

11.6

10

+

63 65

87 83

16.1 3.9

63

87

3.6

39 57

63 90

9.4 6.5

63 61

62 90

12.5 3.6

-

10

17

+ -

Pt-In alloy f.c.c.

C4H 6

X % <4 44

+ -

,,

time - 1 hour; **) see Table 1; ***)Reduction during n-butane dehydrogenation.

Conclusion

Thus, we have shown the importance of the support for the formation of supported bimetallic catalysts with desired composition, structure, dispersity and thermal stability.

194 Specific and general properties of spinel supports in this process can be noted in comparison with traditional single-component support. Our results additionally emphasize the variety of supported bi- and multimetal catalysts, and today the possibility of the development of a general preparation theory seems to be very doubtful. At the same time, they reinforce the hope in the discovery of general regularities for a wide class of catalysts. ACKNOWLEDGMENT

This work was supported by Russian Foundation for Basic Research (section of scientific schools) under Grant 96-15-97560. REFERENCES 1. 2. 3. 4.

B.V.Vora and T. Imai, Hydrocarb. Process, 61 (1982) 171. US Patent No. 4 191 846. J. Choudhury, Chem. Eng., 91 (1984) 40. N.A.Pakhomov and R.A.Buyanov, in. Advances in Catalysis, Sci.-Technol., Prasado Rao Editor, Wiley Estenu Limited, New Dehli, (1985) 305 5. N.A.Pakhomov and R.A.Buyanov, in Studies in Surface Science and Catalysis, Preparation of Catalysts VI, G.Poncelet et al. Editors, Elsevier, Amsterdam, 105 (1995) 1101. 6. N.A.Pakhomov, R.A.Buyanov., E.M.Moroz et al., React. Kinet. Catal. Lett. 9 (1978) 257. 7. N.A.Pakhomov and R.A.Buyanov, J. Applied Chem., 70 (1997) 1441 (in Russian). 8. J.P.Boumonville, J.P.Franck and G.Martino, in Studies in Surface Science and Catalysis, Preparation of Catalysts HI, G.Poncelet et al. Editors, Elsevier, Amsterdam, 16 (1983) 81. 9. J.V61ter, in Studies in Surface Science and Catalysis, Catalytic Hydrogenation, L.Cerveny Editor, Elsevier, Amsterdam, 27 (1986) 337. 10. I.S. Shaplygin and B.V. Lazarev, Zh. Neorg. Khim., 22 (1977) 3368 (in Russian). 11. N.A.Pakhomov, O.P. Krivoruchko, L.M.Plysova, S.V.Ketehik, R.A.Buyanov and G.R.Kotelnikov, Kinetika i Kataliz, 25 (1984) 456 (in Russian). 12. B.P.Zolotovskii and Buyanov R.A., in Studies in Surface Science and Catalysis, Preparation of Catalysts VI, G. Poncelet et al. Editors, Elsevier, Amsterdam, 105 (1995) 793. 13. I.H.Anderson, P.J.Conn and S.P.Branderberger, J. Catalysis, 16 (1970) 326. 14. E.M.Moroz, N.A.Pakhomov, G.R.Kotelnikov, R.A.Buaynov and V.A.Patanov, React. Kinet. Catal. Lett., 7 (1978)457. 15. N.A.Pakhomov, R.A.Buaynov, E.N.Yurchenko et al., React. Kinet. Catal. Lett., 14 (1980) 329. 16. N.A.Pakhomov, R.A.Buaynov, E.M.Moroz, E.N.Yurchenko et al., Kinetika i Kataliz, 22 (1981) 488 (in Russian). 17. N.A.Pakhomov, N.A.Zaytceva and E.M.Moroz, Kinetika i Kataliz, 33 (1992) 426 (in Russian). 18. S.R.Srinivasan and B.H.Davis, Platinum Metals Rev., 36 (1992) 151. 19. F.Garin, L.Hilaize and Maize G., in Studies in Surface Science and Catalysis, Catalytic Hydrogenation, L.Cerveny Editor, Elsevier, Amsterdam, 27 (1986) 145. 20. P.Bosch, M.A.Valenzuela, B.Zapata, D.Acosta, G.Aguilar-Rios, C.Maldonado and I.Schifter, J. Mol. Catal., 93 (1994) 67. 21. H.Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 22. V.V.Chesnokov, R.A.Buyanov, N.A.Pakhomov and V.I.Zaikovskii, Kinetika i Kataliz, 31 (1991) 1494 (in Russian).. 23. G.A.Dergaleva, N.A.Pakhomov, V.N.Bendurin and V.F.Anufrienko, Kinetika i Kataliz, 32 (1991 ) 490 (in Russian).

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

195

Preparation of new type of Sn-Pt/SiO2 catalysts for the hydrogenation of crotonaldehyde J.L. Margitfalvi, I. BorbS.th and A. Tompos Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, 1525 Budapest, POB. 17, Hungary

In this paper new aspects of anchoring tin organic moieties onto platinum are described. In the earlier method adsorbed hydrogen was exclusively used for tin anchoring resulting in a monolayer of tin organic moieties at the Pt surface. In the new approach the anchoring is carried out in the presence of large excess of tin tetraethyl and oxygen has been added during the tin anchoring reaction. In the new approach new sites for tin anchoring were generated by oxygen. Experimental evidences were obtained for the formation of Surface Complexes in the Second Layer (SCSL). The formation of SCSL is a multistep process. The Sn-Pt/SiO2 catalysts prepared in this way showed high Sc=o selectivity in the hydrogenation of crotonaldehyde to crotylalcohol. The Sc=o selectivity strongly depended on the Snanch/Pts ratio. Slightly higher Sc=o selectivities were obtained in the gas phase than in the liquid phase hydrogenation (around 80 and 50 %, respectively).

1. INTRODUCTION 1.1. Surface organometailic chemistry of tin tetraalkyls

Controlled Surface Reactions (CSRs) between hydrogen adsorbed on transition metals and tin tetraalkyls are widely used to prepare tin modified supported bimetallic catalysts [1-10]. These catalysts appeared to be active and selective in different selective hydrogenation reactions [6,9-11 ] and in the reactions of fatty acid esters [2,8] and hydrocarbons [3]. The surface chemistry involved in the modification of supported platinum catalysts by tin tetraalkyls can be described by the following equations[ 1,3,4]: PtHads.

+

SnR4

. . . . . . > Pt-SnR(4-x)

+

x ma

(1)

(4-x) RH

(2)

PSC Pt-SnR(4-x)

+

(4-x)/2 H2

. . . . . . >Pt-Sn (SBS)

+

In reaction (1) hydrogen adsorbed on platinum reacts with tin tetraalkyls resulting in the formation of a Primary Surface Complex (PSC). The latter is decomposed in a hydrogen atmosphere with the formation of supported bimetallic species (SBS). In our earlier studies the

196 anchoring and decomposition steps were always separated [3,4], however in other studies [2,6-8] no attempt was done to separate surface reactions (1) and (2). The disadvantage of our approach was the strong limitation of the amount of tin introduced. Upon using tin tetraethyl Sn,,~h/Pts ratio around 0.4 could be achieved [1,3,4]. It is worth for mentioning that in studies where no attempt was done to separate the first and second steps of tin anchoring relatively high Snanch/]VIs ratios (around two) were obtained [2,6-8]. The goal of this work is to demonstrate how the amount of anchored tin can be increased when (i) high concentration of Sn(C2H5)4 is used, (ii) oxygen is added during the tin anchoring process, and (iii) the duration of the tin anchoring reaction is increased. In this study only one experimental parameter, the initial concentration of Sn(C2Hs)4 ([Sn]o/Pts) was changed, but the duration of the anchoring step was substantialy increased compared to earlier studies [3,4]. The prepared catalysts were tested in the hydrogenation of crotonaldehyde both in gas and liquid phases. It is known that tin modified platinum catalysts have much higher selectivities for the hydrogenation of the aldehyde group than the unmodified catalyst [11,12].

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

2.1. Catalysts preparation and modification The main characteristics of the Pt/SiO2 catalyst are as follows: Pt content: 3 % (H/Pt = 0.52), surface area: 302 m2/g, pore volume: 0.95 cm3/g, mean pore diameter: 12 ran. It was prepared by ion-exchange using [Pt(NH3)4]Clz. ARer filtration the catalyst was washed with demineralized water and then dried at 60 ~ and 120 ~ for 3 and 2 hours, respectively. The catalyst was reduced under flowing hydrogen for 4 hours at 300 ~ Prior to the tin anchoring step the catalyst was re-reduced in hydrogen at 300 ~ for 60 minutes followed by cooling in a hydrogen atmosphere to room temperature and purging with argon for 30 minutes. The catalyst was introduced into a glass reactor and was slurried with benzene in an argon atmosphere. Reaction (1) was started by injection of Sn(CzH5)4 and was monitored for 5 hours by determining the amount of C2H6 and C2H4 formed. In the 40th minute 20 cm3/gcat oxygen was introduced onto the reactor (this amount corresponded to O/Pts = 20). After reaction (1) the catalyst was washed four times with benzene and three times with n-hexane at 50 ~ followed by drying in vacuum (at 5 torr) at 50 ~ for one hour. The decomposition of PSC was carried out in a hydrogen atmosphere by Temperature Programmed Reaction (TPR) technique. The products of decomposition were analyzed by GC. The amount of C2 hydrocarbons formed in reactions (1) and (2), (n I, mol/gc.~t and nn, mol/gc.,t, respectively) were exactly determined. The material balance allowed us to calculate the value of x , i. e. the stoichiometry of reaction (1), and the amount of tin anchored. The tin content of the modified catalysts determined by AAS had a good agreement with that of calculated from the overall material balance. 2.2. Hydrogenation reaction The hydrogenation of crotonaldehyde (CA) was studied both in gas and liquid phases. The gas phase reaction was carried out under atmospheric pressure at 80 ~ the liquid phase hydrogenation was performed at 4 bar and 40 ~ Prior to the catalytic reaction the catalysts were preactivated in a hydrogen atmosphere at 300 ~ In the gas phase hydrogenation of CA,

197

due to the pronounced ageing of the catalyst the conventional continuous-flow reactor was used in a periodic mode by introducing the CA - hydrogen mixture in the form of a long pulse. Further details about this technique can be found elsewhere [13]. This method allowed us to obtain reliable kinetic data in the whole conversion range.

3. RESULTS AND DISCUSSIONS 3.1. Catalysts modification Typical kinetic curves of ethane and ethylene formation are shown in Figure 1. These results indicate that the initial concentration of tin tetraethyl strongly affects the overall kinetic pattern of surface reaction (1). The influence of oxygen is also very pronounced, after its introduction instantaneous formation of ethylene is observed. Upon increasing the [Sn]o/Pts ratio the amount of ethylene strongly increases. In the absence of oxygen, when Pt is fully reduced, the formation of ethylene is negligible [4,5]. The formation of ethylene is attributed to the following surface reaction [14]: MO z

+

z Sn(C2H5) 4 ..... >M-[Sn(C2H5)2] z +

2z C2H 4 + z H 2 0

(3)

(M = Pt) In surface reaction (3) the hydrogen of the ethyl group is used to remove the oxygen from the oxidized forms of Pt. It is suggested that in surface reaction (3) coordinatively unsaturated surface organometallic species are formed. In surface reaction (1), when x > 1, the formed surface species, such as -SnR2 and -SnR, are also considered to be coordinatively unsaturated. We have recently showed that the high initial concentration of tin tetraethyl is favourable for the formation of coordinatively unsaturated surface species [ 15]. 1

~~

~" ,,

2o, st

i"~

A

80

80

7o

""

i'o ,

0

C

j-

7o

6O

-'~

==

o ~

==~ so ~ ~8

"-

40 ao

.,~ 8

/

,o

i

100 time,

,

'

200 rain,

/k2 :300

, i

0

,0'

3o

s

~176 E m

,

50

=

~ 4 o

/

10 o

"

/ S

i

2o0

100

time,

10

rain.

300

0

= 1 100

time,

I 200

300

rain.

Figure 1. Time dependence of the formation of ethane and ethylene in the tin anchoring reaction. [Sn]o/Pts: A - 0.48, B - 2.51, C - 8.80. II - ethane, F'I _ ethylene. Reaction temperature: 50 ~ solvent: benzene. The arrow indicates the moment of addition of oxygen. Figures 2 and 3 show the decomposition of surface organometallic moieties. The decomposition pattern strongly depends on the [Sn]o/Pts ratio and the presence or absence of oxygen in the tin anchoring step. In the case of catalysts prepared in the presence of oxygen at low tin loading ([Sn]o/Pts = 0.24) only low temperature TPR peaks, around 15, 40 and 70 ~ were detected. The peak around 110 ~ had only minor contribution to the decomposition.

198

Slight increase in the [Sn]o/Pts ratio ([Sn]o/Pts = 0.48) resulted in further TPD peaks around 110, 140, 170 and 200 ~ However, the latter two peaks had only minor contribution. The increase of the [Sn]o/Pts ratio ([Sn]o/Pts = 4.24) resulted in a strong increase of the intensity of peaks at around 170 and 200 ~ and two additional small peaks around 260 and 300 ~ were also observed. Upon further increase of the [Sn]o/Pts ratio ([Sn]o/Pts = 8.8) no additional new peaks were detected and the minor peaks around 260 and 300 ~ showed no further increase. The decomposition of tin tetraethyl adsorbed onto pure silica resulted in decomposition peaks around 250 and 300 ~ consequently the above two peaks were attributed to the decomposition of tin tetraethyl adsorbed onto the support. 3o m

~

x cn

2o

.,..

B

2s

~

2o

"~

lO

. lO

o 0

100 200 temperature oc

80

300

o

C 60

=:

300

D

100

8O

f

4o

100 :)00 temperature, oC

~

"6

so 40

Ro

o

o 0

100 200 temperature, oG

0

300

leo 2oo temperature, oG

3oo

Figure 2. Decomposition of surface complexes formed in the presence of oxygen. A" [Sn]o/Pts = 0.24 (Snane]a/Pts = 0.22), B" [Sn]o/Pt~ = 0.48 (Snanch/Pts = 0.44), C: [Sn]o/Pts = 4.24 (Snanch/Pts = 1.61), 1): [Sn]o/Pts = 8.8 (Snanch/Pts=2.35). II - m e a s u r e d , !"3 _ fitted. 7O

"Oii 5o

o

4o

A

B so

x

i

50

E 4o 3O '~

2O

~

~0

-~

m

so

u

0 0

100 200 temperature, oC

300

O

100 ;t00 temperature, oC

300

Figure 3. Decomposition of surface complexes formed in the absence of oxygen. A: [Sn]o/Pts = 1.6 (Snanch/Pt, = 0.44), B: [Sn]o/Pts=33.5 (Snanch/Pt~ = 1.6). II - measured, i-1. fitted.

199

Figures 3A and B show the TPR pattern of the decomposition of surface complexes formed in the conventional tin anchoring process, i. e. in the absence of added oxygen. In these experiments the Snanch/Pts ratio was 0.4 and 1.6, respectively. Similar Snanch/Pts ratios were obtained in experiments shown in Figure 2B and 2C. Our results also indicate that in the presence of added oxygen high Snanch/Pts ratios can be obtained at relatively low [Sn]o/Pts ratios. In the absence of oxygen in order to reach Snanch/Pts = 1.6 the [Sn]o/Pts ratio should be 33.5 [ 15], while in the presence of oxygen the above ratio was reached at [Sn]o/Pts = 4.2. Recently TPR peaks above 70 ~ were assigned to new types of surface species, i. e. to Surface Complexes in the Second Layer (SCSL) [ 15]. Results obtained by computer modelling indicated that the monolayer coverage of PSC is accomplished around Snanch/Pt~ = 0.4 [15]. The formation of SCSL was especially favourable at high concentration of tin tetraethyl and at prolonged reaction time. The formation of SCSL both in the absence and presence of oxygen can be written in the following way [15] (reactions (3a) and (3b), respectively): Pt-SnR(4.x)

+

n SnR4

.... >

Pt-{SnR(4.x)-(SnR4)n}

(3a)

SCSL Pt-[Sn(C2H5)2] z

+ m SnR4 .... > Pt-{[Sn(C2H5)2I z - (SnR4)m}

(3b)

SCSL The driving force for the above reactions is the coordinative unsaturation of primary surface complexes and the high initial concentration of tin tetraethyl. Let us discuss TPR results given in Figures 2 and 3. The TPR pattern of surface complexes obtained in the presence or absence of oxygen shows deffinite similarities and substantial differences. When the modification of catalyst was done in the presence of oxygen the TPR peaks around 110 and 140 ~ appeared at relatively low Snanch/Pts ratio (compare Fig. 2B and 3A). This fact indicates that in the series with added oxygen the TPR peaks around 110 and 140 o C can be attributed to tin organic moieties formed in the second layer. It is worth for mentioning that the high temperature TPR peaks observed at high Snanch/Pts ratio are narrower when the modification is carried out in the absence of oxygen than in its absence (compare Figures 2D and 3B). It is important to note that in the presence of oxygen the contribution of the peaks at 250 and 300 ~ into the overall decomposition is negligible. This fact indicates that the presence of oxygen strongly suppresses the undesired tin - support interaction observed in the absence of oxygen at high [Sn]o/Pts ratios. It should be emphasized that at high [Sn]o/Pts ratio substantial amount of PSC has already been formed prior to the addition of oxygen. In the conventional anchoring process the average value of x at monolayer coverage was around 1.5 [ 15]. It means that in PSCs formed prior to the introduction of oxygen the Pt surface is covered by-Sn(C2Hs)3, and -Sn(C2Hs)2 formed in 1:1 ratio. It is suggested that PSCs formed prior to the addition of oxygen strongly hinder the formation of a chemisorbed oxygen layer on the Pt surface. However, the presence of a monolayer of PSC cannot prevent the activation of the molecule of dioxygen at the kink and corner sites. After the activation the formed atomic oxygen migrates to the tin organic moieties. Due to the high affinity of tin towards oxygen the formed coordinatively unsaturated PSCs, i. e. surface species -Sn(C2Hs)2, and -Sn(C2Hs) are

200 immediately oxidized or transformed onto oxygen-containing species. These new species are involved in anchoring of Sn(C2Hs)4 with the formation of SCSL. During this anchoring process, due to presence of the large excess of tin tetraethyl, part of the oxygen-containing species can be reduced by Sn(C2Hs)4. It is suggested that the formation ethylene is related to the above reduction process (see reaction (3), when M = Sn). As emerges from data given in Figures 2A-D the role of oxygen in the tin anchoring process is completely different at low and high initial [Sn]o/Pts. At very low [Sn]o~ts ratios the part of Pt covered by PSC is relatively small. Consequently, oxygen can interact both with the platinum surface and the PSC. The observation that at [Sn]o/Pt~ = 0.24 the ethylene formed after introduction of oxygen is hydrogenated indicates that there still free ensembles on the Pt surface to accommodate ethylene and hydrogenate it to ethane. At high [Sn]o/Pt~ ratio the role of oxygen is to provide new type of anchoring sites for the formation of SCSL. The overall material balance of tin anchoring calculated from the first and second steps of tin anchoring are given in Table 1. As emerges from Table 1. the rate of surface reaction (1), as well as the amount of tin anchored showed a strong dependence of the [Sn]o/Pts, i. e. of the initial concentration of tin tetraethyl. Characteristic behaviour of this new anchoring process is the small value of x, i. e. the small extent of the loss of alkyl group in the formed SCSL. The small value of x indicates that part of the tin organic moieties anchored onto the platinum maintains its original SnR4 stoichiometry. Data given in Table 1 indicate also that at low [Sn]o/Pts ratios ([Sn]o/Pts < 1) the tin anchoring is almost quantitative, i. e. [Sn]o/Pts = Snanch/Pts.

Table 1. Material balance of tin anchoring in the presence of added oxygen. [Sn]o/Pts Wo x 10 .6 " n I x 10-6

b

C2]=[4x 10-6 b n n x 10-6 b

X

Snanch/Pts

0.24

0.09

11.8

3.0

57.2

0.68

0.22

0.48

0.15

29.0

7.6

117.7

0.79

0.44

1.01

0.29

48.3

19.3

283.0

0.58

1.00

2.51

0.39

98.7

32.6

3'59.9

0.86

1.43

'4.24

0.51

118.3

42.9

397.7

0.92

1.61

0.67

141.6

61.6

608.8

2.35

a) initial rate of surface reaction (1) in (mol/gcat x min); b) amount of C2 hydrocarbons formed in reaction (1) and (2) in (mol/gcat).

3.2. Catalytic experiments in the gas phase Steady-state activity and selectivity data obtained in the gas phase hydrogenation of crotonaldehyde (CA) are summarized in Table 2. The behaviour of catalysts in this reaction prior to reaching the steady-state activity has been described recently [11,12]. The improvement of the Sc-o selectivity observed during the time on stream period was attributed to the reaction induced oxidation of tin anchored onto the platinum [11,12].

201 As emerges fiom Table 2 over Pt/SiO2 catalyst the main reaction product is butyraldehyde. The introduction of tin leads to an the overall rate increase and the increase of the selectivity to crotylalcohol. The highest selectivity was observed at Snanch/Pts = 1.6. The activity of this catalyst was ahnost the same as that of the parent Pt/SiO2 catalyst, while its selectivity towards the formation of crotylalcohol was close to 80 %. 0.8 0.6 .J

0

0.4

0.2 0

I

0

0.2

0.4

0.6

Conversion

Figure 3, Gas phase hydrogenation of crotonaldehyde. Effect of the tin content on the crotylalcohol selectivity, x - Pt/SiO2, [Sn]anch/Pts: 9- 0.22, + - 0.44, II - 1.04, A - 1.43, [3 1.61, O -2.35. The dependence of the crotylalcohol selectivities of the conversion is shown in Figure 3. This figure clearly shows the role of tin in the selectivity improvement. These results also demonstrate that high Sc=o selectivities can be maintained up to high conversion levels. Table 2. Gas Phase hydrogenation of crotonaldehyde over Sn-Pt/SiOz catalyst. Effect of the tin content on the steady state activity and selectivity. I-

Catalys'ts; ........ Wilan

ll

Snanch/Pts

. . . . . . . . S. e .l e c. t i.v i.t y - SAL ""

SOL

(~

b

UOL

HC

1

Pt/Si0~ [ l Sn'pt/SiO2,''~

..... 1.68 6 0 5 ,_

95 _ 82

0 5

0 '. . . . . . . . .!_3 _

~ 'Sn-Pt/Sio2, 0.44 " 4 '3; .. 72 4 ' 2,'--Pt'-'iO bn /S 1.00 '" . . . . . . . . i Sn-Pt/SiO211.43

...... ;'853"74

SnPt/SiO2, 1161 . [-n-t/-iO2--S P S , 2.35

. i 8.

. ,,

...... 1 3 6 - ' - 2 2 8 2 0

57

3

33

.. 0 , .

05

_

2 0 4 ,,

3.,7 ,

3.

6.4 ......

3,

. 00. . . . . 7278

21

a initial rate, Wo in lamol x g-1 x s"l measured from the conversion - contact time dependencies and extrapolated to zero conversion, b measured at 5 % conversion. Preactivation temperature --- 300 ~ [C]0 = 0.64 mmol/dm 3, SAL - butyraldehyde, SOL - butylalcohol, UOL crotylalcohol, HC - hydrocarbons.

202

3.3. Catalytic experiments in the liquid phase The time dependence of the conversion measured in the liquid phase hydrogenation of crotonaldehyde over Pt/SiO2 and selected Sn-Pt/SiO2 catalysts is shown in the Figure 4. Figure 4 shows a rather complex kinetic pattern over the parent Pt/SiO2 catalysts. A strong rate acceleration has been observed around 15 % conversion. This pattern completely disappears after introduction of tin. Additional activity and selectivity data are given in Table 2. The formation of crotylalcohol was almost negligible over Pt/SiO2 catalyst. Around 6 % selectivity was observed at 10 % conversion. The results indicate that upon introducing small amount of tin the overall activity of the catalyst strongly increases. Further introduction of tin leads to the decrease of the reaction rate and the increase of the crotylalcohol selectivity. 1

c

0.8

.fl i..,.

> o

0.6

0.4 0.2 0 ..,,.--0

,

,

100

I

200

3O0

time (min)

Figure 4. Liquid phase hydrogenation of crotonaldehyde. Time dependence of the conversion. A - Pt/SiO2, Snanch/Pts: 9- 0.22, x - 1.04, ~ - 1.43.

Table 3. Liquid phase hydrogenation of crotonaldehyde over Sn-Pt/SiO2 catalyst. Effect of the tin content. Catalysts,

Wini a

Snanch/Pts Pt/Si02

SOL

UOL

2.91

85

9

6

13.62

64

9

27

Sn'Pt/sio;, 0.44

11.34

61

8

31

Sn-Pt/SiOa, 1.00

5.24

"50

7

43

Sn-Pt/SiO2, 1.43

2.01

42

5

53

0.56

44

7

49

Selectivity (%) b s'AL ,,,

Sn-Pt/Si02, 0.22 ,,,

,.,

.,,

Sn-Pt/SiO2, 1.61

a) initial rate, Wo in ~tmol x g-l x s"1, measured from the conversion - time dependencies extrapolated to zero conversion, b)measured at 10 % conversion. Preactivation temperature = 300 ~ [C]0 =120 mmol/gc,t, SAL - butyraldehyde, SOL butylalcohol, UOL - crotylalcohol.

203 Figure 5 shows the product selectivities v.s. conversion dependencies obtained over a selected Sn-Pt/SiO2 catalyst, while the dependence of the crotylalcohol selectivity of the tin content is shown in Figure 6. The selectivity - conversion dependencies for butyraldehyde and butylalcohol obeys the general pattern characteristic of parallel- consecutive reactions. However, the selectivity vs. conversion dependencies obtained for crotylalcohol is rather complex. The above complex pattern is maintained at different Snanch/Pts ratios (see Figure 6). The maximum crotylalcohol selectivity was obtained over catalyst with Snanch/Pts = 1.43. 0.8

0.6

0.6 -r

SAL 9 t

mm 9

9

/~,

ASOL

0.5

bO

.....x U O L

0.4

mm

,.m. ,i.m

o:~ 0.3

ta 0.4

u

x

0.2

X

I

X

X

i

Or)

m

+

9

+

9

,

0.2

r

0.1

0 0

i

0.2

!

.....

0.4

Conversion

Figure 5.

(

i

0.6

0.8

0

0.2

0.4

0.6

Conversion

0.8

1

Figure 6.

Figure 5. Liquid phase hydrogenation of crotonaldehyde. Product selectivities v.s. conversion. Catalyst: Snanch/Pts = 0.44. Figure 6. Liquid phase hydrogenation of crotonaldehyde. Selectivity of the formation of crotylalcohol obtained at different Snanch/Pts ratio, x - Pt/SiO2, Snanch/Pts" 9- 0.22, + - 0.44, m - 1.04, A - 1.43, l-i- 1.61. As emerges form Figure 6 over Sn-Pt/SiO2 catalysts at low conversion the dependence of the crotylalcohol selectivity with conversion always shows a definite increase part. At low Snanch/Pts ratios the above increase is followed by a decrease due to the hydrogenation of crotylalcohol to butylalcohol. This kinetic pattern does not obeys the general behaviour of parallel - consecutive reaction scheme characteristic for the hydrogenation of unsaturated aldehydes. The above increase is attributed to the reaction induced formation of active sites involved in the activation of the carbonyl group. Consequently similar phenomena take place during the liquid phase hydrogenation as in the ageing period in the gas phase hydrogenation. However, as far as the temperature of liquid phase hydrogenation is only 40 ~ the rate of formation of catalytically active sites is much lower than in the gas phase. It is probably the reason that catalysts above Snanch/Pts > 1.5 cannot be activated by the reaction itself. The catalysts with Snanch/Pts > 1.5 have very low rate of hydrogenation and the Sc-0 selectivity shows no further improvement.

204 SUMMARY

The modification of the procedure used for tin anchoring resulted in Sn-Pt/SiO2 catalysts with high Snanch/Pts ratio. In this new anchoring process the presence of oxygen provides new type of anchoring sites. Most of the tin anchored in this way is in the second layer atop of the PSC formed. The formation of layers takes place layer by layer. The Sn-Pt/SiO2 catalysts prepared showed high selectivity in the hydrogenation of crotonaldehyde. Both in gas and liquid phase hydrogenation the high Sc=o selectivity was developed during the catalytic reaction. The development of the Sc=o selectivity during the catalytic reaction is a specific feature of the Sn-Pt/SiO2 catalyst.

ACKNOWLEDGMENT The authors acknowledge the research grant from OTKA (No T23322). Special thanks to A.S Belyi (Boreskov Institute of Catalysis, Omsk Branch) for providing the Pt/SiO2 catalyst.

REFERENCES

1.

J. Margitfalvi, M. Hegedfis, S. G6b616s, E. Kern-Thlas, P. Szedlacsek, S. Szab6, and F. Nagy, Proc. 8th Int. Congress on Catalysis, Vol. 4, Berlin (West), 2-6 July 1984, p. 903. 2. Ch. Travers, J. P. Bournonville, and G. Martino, Proc. 8th Int. Congress on Catalysis, Vol. 4, Berlin (West), 2-6 July 1984, p. 891. 3. M. Hegedfis, S. G6b616s, P. Szedlacsek, and J. L. Margitfalvi, in B. Delmon et al. (Editors), Preparation of Catalysts IV, Stud. Surf. Sci. Catal., Vol. 31, Elsevier, Amsterdam, 1987, p. 689. 4. J.L. Margitfalvi, E. T/das, and S. G6b616s, Catal. Today, 6 (1989) 73. 5. J.L. Margitfalvi, S. GSb616s, E. Tfilas, M. Hegedfis, and J. Ryczkowski, in M. G. Scaros, M. L. Prunier (Editors), Chem., Ind., Vol. 62, Marcel Dekker, New York, 1995, p. 557. 6. M. Agnelli, P. Louessard, A. El. Mansour, J. P. Candy, J. P. Bournonville, and J. M. Basset, Catal. Today, 6 (1989)63. 7. B. Didillon, A. El. Mansour, J. P. Candy, J. P. Bournonville and J. M. Basset, in M. Guisnet et al. (Editors) Heterogeneous Catalysis and Fine Chemicals II, Stud. Surf. Sci. Catal, Vol. 59, Elsevier, Amsterdam, 1991, p. 137. 8. J.P. Candy, B. Didillon, E. L. Smith, T. B. Shay and J. M. Basset, J. Mol. Catal., 86 (1994) 179. 9. H.R. Aduriz, P. Bodnariuk, B. Coq, and F. Figueras, J. Catal., 119 (1989) 97. 10. J. L. Margitfalvi, H. P. Jalett, E. T~,las, A. Baiker, and H. U. Blaser, Catal. Lett., 10 (1991) 325. 11. J. L. Margitfalvi, A. Tompos, I. Kolosova, and J. Valyon, J. Catal. (accepted for publication). 12. V. Ponec, Appl. Catal. 149 (1997) 27. 13. J. L Margitfalvi, P. Szedlacsek, P., M. Hegedfis, and F. Nagy, Appl. Catal., 15 (1985) 69. 14. J. L. Margitfalvi, I. Kolosova, E. T~las, S., and S. G/Sb616s, Appl. Catal., 154 (1997) L1. 15 . . . . . Margitfalvi, I Borb/~th, and A. Yompos, (Catal. Today, accepted for publication).

91998 ElsevierScience B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

205

Influence of the Preparation Variables on the Separative and Catalytic Properties of Ruthenium-Silica Membranes V.Pgrvulescu a*, V.I.Pgrvulescub, C.Niculae c, G.Popescu c , A.Julbe d, C.Guizard d and L.Cot d

a-Institute of Physical Chemistry "I. G. Murgulescu", Spl. Independentei 202, Bucharest, Romania b -University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Republicii 13, Bucharest, Romania c . Research Centre for Molecular Materials and Membranes, Spl. Independentei 206, Bucharest, Romania a- Laboratoire des Mat6riaux et Proc6d6s Membranaires (UMR 5635 CNRS-UMII- ENSCM), 8 Rue de l'Ecole Normale, 34 296 Montpellier Cedex 5, France

RuO2/SiO 2 and Ru/SiO 2 catalytic membranes supported on alumina were prepared using the sol-gel method from tetraethoxysilane (TEOS) derived sols in the presence of surfactants (tetraalkylammonium salts) and of RuC13.3H20 in ethylene glycol or ethanol. Sols with adapted rheological properties were deposited on alumina supports by spin coating. Several synthesis parameters such as pH and type of solvent for TEOS hydrolysis, refluxing time, surfactant chain length, Ru concentration, were found to exert a concerted action on both the textural and structural characteristics of the catalytic membrane material. These effects were investigated using XRD, FTIR, UV-Vis diffuse reflectance spectroscopy, N 2 adsorptiondesorption and permeability measurements. The influence of the preparation parameters on the membrane catalytic performance have been evaluated for the oxidation of isopropylic alcohol and for the hydrogenolysis-homologation of 1-butene. 1. INTRODUCTION The performance of catalytically active membranes in reactors usually results of a cooperative effect between the catalytic and separative properties of the membrane materials [1-5]. Separative properties can be achieved in porous membranes by tayloring the membrane porous texture. In this respect, the use of surfactants during the sol-gel synthesis of thin layers revealed attractive [6-8]. This procedure leads to narrow pore size distributions in a controllable range and can also be supposed to help the dispersion of catalytically active species. Both selective and catalytically active membranes, with a good dispersion of the active phase, can be expected when using surfactant additives in the sol-gel process. Catalytically active sol-gel derived porous membranes can be namely obtained by postimpregnation of inert membranes with metal salts [9] or by direct incorporation of metallic salts in the starting sols [9]. The incorporation of active metals in sol-gel derived silica has been already investigated in the literature [9-12]. Results showed that the incorporation of metallic species in the sols leads to a high metal dispersion in the final material. On an other hand, a strong anchoring of several of the metallic particles inside the silica network through

206 non-condensed Si-OH groups, has been evidenced [10]. Because of these specific characteristics, the resulting Metal-SiO 2 catalysts prepared directly via the sol-gel method are more resistant to self-deactivation than those prepared via traditional post-impregnation. The sol-gel process is now commonly used for the synthesis of porous ceramic materials and in particular of oxide layers [ 13-15]. The "polymeric route" of the sol-gel process consists in the preparation of a sol from metallo-organic molecular precursors. The hydrolysiscondensation reactions lead to condensed species forming clusters which grow and lead finally to a "chemical" polymeric gel. In case of membrane synthesis, casting must be carried out at the sol stage and the sol rheological properties have to be adapted to the porous support characteristics. The drying and sintering steps determine the final membrane characteristics. This paper describes the preparation by the sol-gel process and the characteristics of RuO2/SiO 2 and Ru/SiO 2 catalytic membranes supported on alumina. The effects of the sol synthesis parameters (pH and solvent for alkoxide hydrolysis, sol refluxing time, ruthenium species content, surfactant chain length) on the textural and chemical properties of the ruthenium-silica membranes have been investigated using XRD, FTIR, UV-Vis. diffuse reflectance spectroscopy, N 2 adsorption-desorption and permeability measurements. The influence of the preparation parameters on the catalytic performance of the membranes have been evaluated for the oxidation of isopropylic alcohol and the hydrogenolysis-homologation of 1-butene. 2. EXPERIMENTAL The molecular precursor of the silica network was commercial tetraethoxysilane (TEOS- Aldrich). Hydrolysis of the alkoxide diluted in ethylene glycol or ethanol has been performed with water (molar ratio TEOS:H20 = 1:10) in acidic medium (HC1) and under reflux at 80~ Different sols were prepared by varying the pH between 1 and 4 and the refluxing time (tr) from 15 min to 30 min or 1 hour. Ruthenium (RuC13.3H20 solution in ethylene glycol or ethanol) has been introduced after sol refluxing. The catalyst dispersion in the TEOS derived sol and membranes was improved by adding surfactants: tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB) or tetra-buthylammonium bromide (TBAB). The molar ratio surfactant/TEOS varied from 0.1 to 0.3. The Ru content in the final catalysts, determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES), has been varied in the range 1-10 wt.%. The sols with adapted rheological properties (binder/plasticizer) were deposited on a porous alumina flat support by spin coating. After drying, the RuO2/SiO 2 membranes were heat-treated at 873 K in air (heating rate: 0.12 Kmin-~). Ru/SiO 2 membranes were obtained by reducing the same precursor at 683 K under H2 flow (30 ml min" 1). The specific surface area and pore size distribution of RuO2/SiO2 and Ru/SiO 2 powders, prepared in similar conditions as for membranes, have been determined from N2 adsorptiondesorption measurements at 77 K (Micromeritics ASAP 2000). The crystalline ruthenium particles in the amorphous silica matrix have been characterized by XRD (SIEMENS D-5000, 20=0-80 ~ CuKa radiation), FT-IR (Bruker IF88, KBr wafers 15 mg.cm 2, 16 scans, 4 cm -1 resolution) and UV-VIS diffuse reflectance spectroscopy (Varian Cary 17D with integrating sphere). The measurements of 02 and H2 permeabilities and of the effective diffusion coefficients through the catalytic membranes were performed by a stationary method in a Wicke-Kallenbach cell. The catalytic performance of the membranes in reactor configuration have been evaluated for the oxidation of isopropylic alcohol and for the hydrogenolysishomologation of 1-butene. Reactions were carried out in gas phase in the temperature range:

207 313-473 K. The H2 or 02 concentration in the feed and permeate side, as well as the composition of reactants and products were determined by on-line gas chromatography (PYE Unicam). The overall molar flow rate and the gas composition were,~maintained constant throughout the reaction. The transmembrane pressure ranged between 0 and 1.5 atria. In such conditions, membranes acted as active contactors : they ensured both the chemical process activation and a controlled contact between the reactants. 3. R E S U L T S AND D I S C U S S I O N S 3.1. Porous texture and permeability of the membrane materials

The textural characteristics of RuOJSiO 2 membrane materials were found to be influenced by the Ru content. This effect was evidenced by a simple comparison with the characteristics of SiO 2 membrane materials prepared in the same conditions (Table 1). The presence of ruthenium in silica sols prepared without surfactant modified the pore diameters in the final material when the Ru content exceeded a critical concentration (Figure 1). Typically when the Ru content was increased above 2-3%, the mean pore sizes, specific surface area and micropore volume decreased (table 1). The critical Ru concentration value for which textural modifications began to be observed was found to be influenced by four sol synthesis parameters : i) pH, ii) type of solvent, iii) refluxing time and iv) presence of surfactants. The decrease of pore diameter when Ru content increased was slightly enhanced when the pH was increased from 1 to 4. The critical Ru concentration inducing textural modifications was in these cases around l wt%. On an other hand the pore diameters and hysteresis area of the N2 ads-desorption curves have been found to increase with the sol refluxing time. Indeed a significant difference was observed between the materials derived from sols refluxed for 15 rain and 1 h (values in Table 1).

Table 1. Characteristics of the membranes porous structure and mass transfer coefficients Preparation conditions lcm-3sPef O2 .lpa. 1 aBET Porous volume, Pore DefO 2 m2g 1 cm3g "1 size cm2s -1 mo a b c d e total micropores A xl08 xl04 0 1 lh Eg 723 0.3982 0.1129 42 1.06 3.6 1 1 15rain Eg 722 0.3577 0.1081 43 1.25 3.5 1 4 15min Eg 668 0.3492 0.0789 45 1.96 3.6 3 1 15 rain Eg 718 0.3645 0.1031 42 1.16 3.5 3 4 15 rain Eg 635 0.4105 0.0661 36 0.69 2.8 5 1 15rain Et 615 0.3871 0.0521 34 0.45 2.8 5 I 15rain Eg 605 0.3705 0.0441 40 0.76 2.9 5 4 15rain Eg 512 0.3871 0.0297 32 0.29 2.7 5 1 lh Eg 598 0.3891 0.0347 34 0.32 2.7 5 1 15 rain Eg TMAB 430 0.5121 0.0016 46 3.1 6.8 5 1 15 min Eg TEAB 421 0.5872 0.0014 50 3.8 6.9 5 1 15 min Eg TBAB 415 0.7012 0.0009 56 4.2 7.1 a: Ru loading, %; b: pH; c" tr; d: solvent; e: surfactant; Eg: C2H4(OH)2;Et: C2HsOH

208 The presence of surfactant in TEOS derived sols was found to strongly influence the texture of RuO2/SiO 2 membrane materials and to modify the effects of the other synthesis parameters (Ru content, pH, solvent, refluxing time). Such a behavior can be explained by an increased sol homogeneity and by a higher dispersion of the metal in the sols prepared with surfactant. Figure 1 shows that the presence of surfactant strongly decreased the effect of Ru content on the pore size. The use of both surfactant and ethylene glycol even led to a slight increase of pore sizes with the Ru content. On an other hand it appeared that surfactants acted as templating agents as far as materials with increasing pore diameters were obtained by increasing the surfactant chain length from TMAB to TEAB and to TBAB (Table 1). This result is in agreement with the measured 02 and H2 permeabilities through the corresponding RuO2/SiO2 membranes (figure 2). Mass transfer through the membranes (diffusion and permeability) were influenced by both the pore diameters and the transmembrane pressure

(aP).

Figure 1. Influence of the preparation the pore diameter ( ~ - pH=l, C2H4(OH)2; t - pH-1, C2HsOH ; ill- pH=I, TBAB/TEOS=0.1,C2H4(OH)2; ill- pH=4, C2HsOH)

Figure 2. Influence of the surfactants on the permeability (5wt.% Ru, pH=l, C2H4(OH)2, surfactant:TEOS=0.3, AP=20 mm H20) ~ - oxygen; ~ - hydrogen

The reducing thermal treatment of the membranes in H2 led to the formation of Ru/SiO 2 membranes. Compared to RuO2-SiO2, the resulting membrane materials exhibited with very similar characteristics. 3.2. Structural characteristics of the membrane materials The X-ray diffraction patterns of RuO2/SiO 2 membrane materials treated at 873K in air (Figure 3) revealed the characteristic lines of tetragonal RuO 2 on an amorphous SiO 2 background. The critical Ru concentration for which these lines were evidenced varied with the synthesis parameters. The critical Ru concentration low (~1%) when TEOS was hydrolysed in more acidic pH, in ethanol and without surfactant. For samples prepared in ethylene glycol or in the presence of TBAB, the RuO2 diffraction lines were only observed for a Ru content higher than 5wt%. Ethylene glycol or TBAB disturb the formation ofRuO2 cristallites probably

209 by a dispersive effect. In the reduced samples, both the lines of Ru metal and oxide have been detected on the XRD patterns (Figure 3).

A 10

30

50

2O

70

Figure 3. X-ray diffraction patterns of RuO2/SiO2 and Ru/SiO2 catalysts (A: 1 wt.%Ru, CzH4(OH)2; B" 1 wt.%Ru, CzHsOH; C: 10 wt.%Ru, CzH4(OH)2; D: 3 wt.%Ru, C2H4(OH)2 )

c

A

4000 3000 2000 1000 wavenumbers,crn-I

Figure 4. IR spectra of samples after different thermic treatments: 10% Ru, 353 K (A), 10% Ru, 473 K (B), 10% Ru 873 K (C) and 0% Ru, 873 K (D).

200

250

300

wavelength,m=

Figure 5. UV-Vis diffuse-relectance spectra of silica samples with different loading of Ru A: SiO2; B: RuO2(l%Ru)/SiO2; C: RuO2(1%Ru)/SiO2, TBAB/TEOS=0.1 D: RuO2(10%Ru)/SiO 2.

As shown in figure 4, the IR spectra of RuO2/SiO2 samples treated at 353,470 or 873 K showed the typical absorption bands of silica based materials. When temperature was increased the band assigned to the alkoxide C-H bonds (2359 cm"l) disappeared and the intensity of the bands due to adsorbed water and solvent (3418 cm "1) decreased. In the same time the vibration modes of Si-O, Si-OH and O-Si-O (1078 cm "l, 985 cm -l, 457 cm "l and 800 cm-I) were modified. The increase of the Ru content led to a broadening of the band assigned to Si-O bond (1078 cm-~). When the metal concentration increased, one can suppose that OH groups stabilized on the silica surface via the formation of Si-O-Ru bonds.

210 Diffuse-reflectance UV-Vis spectra gave additional information on the ruthenium-silica interactions and on the silica OH group. The spectra of the RuO2/SiO 2 calcined samples showed that the presence of Ru induced a broadening of the 220 nm band and the appearance of two bands at 255 and 360 nm (Figure 5). The bands at 220 and 255 nm were assigned respectively to electronic transitions from the n and o orbitals of OH or C1 to the empty t2g orbital of Ru. They correspond to the formation of Ru-OH bonds and to a hexacoordinated Ru m complex on silica [10]. Deconvoluting these bands showed that the 220nm band broadening resulted from a partial shift towards 230 and 240 nm, which could result from interactions between the Ru species and the silica OH groups (inclusion of OH groups in the Ru complex coordination sphere). Therefore, the shift of this band was attributed to a stabilization of silica surface OH groups through the formation of Si-O-Ru bonds. The broadening of the 220 nm band increased with the Ru content (Figure 5). On an other hand the presence of surfactant in the starting sols was shown to slightly increase the intensity and decrease the width of the 220 nm band. 3.3. Evaluation of the catalytic properties of the membranes The catalytic performances of the prepared membranes were influenced by both their textural characteristics (in terms of selectivity) and by the Ru dispersion (in terms of activity). Figure 6 shows the catalytic activity, expressed as acetone yield, of the RuO2/SiO 2 membranes for the oxidation of isopropylic alcohol. The higher activity was reached for a 5 wt.% Ru content. The catalytic activity was found to depend on the TEOS hydrolysis conditions : the activity decreased when the sol refluxing time or pH increased. On an other hand surfactant addition in the sol was found to increase the membrane activity. This results has to be correlated with a better Ru dispersion in the presence of surfactant. The acetone selectivity decreased in the order TBAB > TPAB > TMAB. Acetone yield was found to increase with the quantity of surfactant only for small AP values. High AP values, such as 50 mmol H20, affected the selectivity.

Figure 6. Catalytic activity of the membranes as a function of preparation variables (AP= 60, T=353 K , ~ - pH=I, tr: 15 rain; I - pH=4,; tr: 15 rain; tB - pH=l, tr: 1 h; ~ - pH=l, TBAB/TEOS=0.3, tr: 15 rain).

Figure 7. Influence of Ru content on the conversion of 1-butene as a function of preparation variables (AP= 20, T=393 K, iN- pH-1, tr: 15 rain; t_~- pH=l, t,: 1 h;

211 The performance of Ru/SiO 2 membranes have been evaluated for the hydrogenolysishomologation of 1-butene. Results demonstrated an increasing conversion when Ru loading or refluxing time increased (Figure 7). In this case the addition of surfactant in the starting sol was found to have a negative effect on the membrane performance and a direct dependence conversion- C5 selectivity was evidenced. The increase of the flux of reactants was also unfavourable because of the to high pore diameter of this type of membrane. 4. CONCLUSION The sol-gel process allowed the synthesis of a homogeneous mixture of silica and ruthenium compounds. Catalytically active RuO2-SiQ and Ru-SiO2 mesoporous membranes have been prepared with a narrow pore size distribution and a good dispersion of the Ru species. The dispersion of Ru species seems to be due to the effect of both surfactant additive in the sol and silica surface OH groups. The textural and structural characteristics of the membranes have been shown to depend on sol synthesis parameters such as pH and type of solvent for TEOS hydrolysis, refluxing time, surfactant chain length, type of solvent and metal concentration. These parameters directly affect the growth of clusters at the sol stage as well as the gels cohesion and homogeneity. The presence of surfactant positively acts on the dispersion of the catalytic species but usually induced an increase in pore sizes. The influence of the preparation parameters on the membrane catalytic performance have been evaluated for the oxidation of isopropylic alcohol and for the hydrogenolysishomologation of 1-butene. Results showed that surfactant addition had a positive effect for the first type of reaction but negatively affected the membrane performance for the second reaction. The performance of a catalytic membrane for a specific reaction are highly dependent on their characteristics (synthesis conditions) even in conditions for which the reactor parameters could be more important. REFERENCES

1. J.N.Armor, Appl.Catal., 49 (1989) 1. 2. A.Pantazidis, J.A.Dalmon and C.Mirodatos, Catal.Today, 25 (1995) 403. 3. A.Julbe, C.Guizard, A.Larbot, L.Cot and A.Giroir-Fendler, J.Membrane Sci., 77 (1993) 137. 4. A.Matsuda, N.Tohge and T.Minami, J.Mat.Sci., 27 (1992) 4189. 5. V.P~rvulescu, V.I.Pfirvulescu, G.Popescu, A.Julbe, C.Guizard and L.Cot, Catal.Today, 25 (1995) 385. 6. T.Dabadie, A.Ayral, C.Guizard, L.Cot and P.Lacan, J.Mater.Chem., 6 (1996) 1789. 7. A.Julbe, C.Balzer, J.M.Barthez, C.Guizard, A.Larbot and L.Cot, J.Sol-Gel Sci.Tech., 4 (1995) 89. 8. A.Ayral, C.Balzer, T.Dabadie, C.Guizard and A.Julbe, Catal.Today, 25 (1995) 219. 9. C.Balzer, A.Julbe, A.Larbot, C.Guizard, L.Cot, J.Peureux, A.Giroir-Fendler, J.A.Dalmon, Proc. ICIM 94 (Inorganic membranes), Y.H. Ma (Ed), Worcester (MA.-USA),1994, pp.629632. 10. T.Lopez, P.Bosch, M.Asomoza and R.Gomez, J.Catal., 133 (1992) 247. 11. C.K. Lambert and R.D.Gonzalez, Microporous Materials, 12 (1997) 179. 12. T.Lopez, L.Herrera, J.Mendez-Vivar, P-Bosch, R.Gomez and R.D.Gonzales, J.NonCryst.Solids, 147&148 (1992) 773.

212 13. C.Guizard, in "Fundamentals of Inorganic Membrane Science and Technology", A.J.Burgraaf and L.Cot (Eds.), Elsevier Science B.V., 1996, p.227. 14. C.J.Brinker and G.Scherer, in "Sol-Gel Science", Academic Press, New York, 1990. 15. L.L.Hech and J.K.West, Chem.Rev., 90 (1990) 33.

91998 Elsevier Science B.V. All rights reserved. Preparationof CatalystsVII B. Delmon et al., editors.

213

Preparation of Ru/carbon - catalysts for a m m o n i a synthesis N.M. Dobrynkin a, P.G. Tsyrulnikov b, A.S. Noskov a, N.B. Shitova b, I.A. Polukhina a,

G.G. Savelieva b, V.K. Duplyakin b, V.A. Likholobov a aBoreskov Institute of Catalysis, Novosibirsk-90, 630090, RUSSIA bOmsk Branch of the Boreskov Institute of Catalysis, Omsk, 644040, RUSSIA Abstract The problems of creation ruthenium-carbon catalysts for ammonia synthesis from initial compounds of ruthenium and alkali metals, accessible and convenient for use, are surveyed in order to prepare the new generation ammonia catalyst for the industrial application. Graphite-like active carbon 'Sibunit' was used as a support for preparation of the catalysts. Potassium and cesium as promoters were introduced through hydroxides. Ruthenium (1I) and (111) complexes with N-containing polydentate organic ligands are used for direct regulation of a dispersity of ruthenium-potassium and ruthenium-cesium systems. The important role at preparation of the active catalysts is stipulated by a nature of inner sphere ligand and even outer sphere of a counter-ion in the ruthenium complexes. It has been found that samples synthesized from Cl-free Ru precursors possess a considerably high activity in the ammonia synthesis at temperatures 573+673 K, pressure 5-50 bar, N2:H2=1:3, than that prepared from Cl-containing compounds. It is assumed that the most active are catalytic centres, containing ruthenium in low degrees of oxidation.

Keywords: Ammonia synthesis; Graphite-like carbon; Ruthenium; Supported catalysts INTRODUCTION The modem design of bimetal catalysts means direct synthesis of their catalytic active centres. It is achieved by integrating atoms of metals in the limits of a molecule on one of the stages of catalyst preparation. The direct solution consists of use of bimetal complexes at depositing active metals, as it was made by Shur and co-authors [1] for reaction of lowtemperature ammonia synthesis. For the catalyst preparation potash salt of ruthenium carbonyl hydride K2[Ru4(CO)13] was used. It is necessary to note that the graphite-like carbon material 'Sibunit' was used as the support in these experiments [2]. The other opportunity of synthesis of the supported bimetal catalysts is formation of intermediate bimetal substances during formation of the catalyst. It is important to have a method for direct regulation of dispersity of the supported metal, which influences greatly it catalytic properties. So, for ruthenium catalyst Ru fr-A1203,

214 prepared from carbonyl cluster Ru3(CO)12,the increase of its dispersity, was shown, results in the growth activity in the ammonia synthesis [3]. In the present work more accessible complexes of Ru (111) and Ru (1I) with polydentate volumetric nitrogen-containing organic ligands were used for preparing of highdispersed Ru-Me/Carbon catalysts. The choice of these compounds as precursors, was based upon the following backgrounds" i) - organic ligands promote multidot interaction of complexes Ru-L with a surface of the carbon support, because the extra bonds L-C are formed in this case. It, in turn, is important for more uniform allocation of active metal on a surface of the support and, as a consequence, increase of a dispersity of metallic Ru; ii) - the large size of organic ligand also promotes partitioning of ruthenium atoms, increasing a dispersity, because in the precursor of the catalyst they are surrounded with a shell from organic ligands. It distances the atoms of ruthenium from each other while depositing, preventing the formation of large crystallites at stages of heat treatment and reduction.

EXPERIMENTAL

Preparation of the catalysts. The catalysts were prepared by consecutive impregnation of the carbon support- Sibunit (granules 1.0-1.2 mm in size, BET surface area up 90 to 320 m2/g) by solutions of a ruthenium complex and hydroxide of metal - modifier (KOH, CsOH). After drying in air at 373 K for 1 h, the catalysts were reduced in dihydrogen flow (573 K, 3 h) and then were activated by special thermal treatment. In some cases, the activation procedure was the same, as was described in [4]. Tests of catalysts activity. Experiments on ammonia synthesis were carried out in flow two-reactor setup at 473+673 K, pressure 5+50 bar, volume flow rate (2+30)o103 h "1, using mixtures (I-I2 + N2) in a ratio of 1:1 or 3: 1. Gas chromatography was used to analyze the starting mixtures and ammonia. Oxygen admixture content was controlled by 'Istok' analyzer and did not exceed 1 ppm. Dispersity of the catalysts was determined by TEM technique, using electronic microscope JEM-100 CX.

RESULTS AND DISCUSSION The obtained results of catalysts activity tests indicate that Ru-Me/carbon catalysts prepared from ruthenium complexes with N-containing polydentate organic ligands, using as precursors of modifiers the hydroxides of alkaline metals, have considerable activity in synthesis of ammonia at 573+673 K, 5+ 50 bar, N2:H2 = 1:3. The important role at preparation of the active catalysts is stipulated by the nature of inner sphere ligands and even outer sphere of counter-ions in the ruthenium complex, especially, if these are haloid-ions. It was revealed that the strong influence on activity of RuMe/carbon -catalyst renders both presence of Cl-ions at an initial ruthenium complex and character of their bonds with central atom. The presence of chlorine in inner sphere of an initial complex leads to practically complete loss of activity (Table 1). Catalysts prepared from the precursor in which there is no direct Ru-C1 bond i.e. containing chlorine ions in

215 outer sphere, have substantial level of catalytic activity. The catalysts prepared from chlorine-free complexes of ruthenium have the highest activity.

Table 1 Influence of a nature of the Ru-precursor on activity of Ru-K/Sibunit catalysts (m R~ = 4.0 wt.%, m K = 4.0 wt.%) The precursor of the catalyst

Character of the Ru-C1 bond

Concentration ofNH3, %

D,

at P=30 bar; T=623 K RuOHC13 [Ru- N

]nClm

[Ru- N ]n(OH)m

inner sphere' s chlorine-ion

0

>100

outer sphere's chlorine-ion

6.2

50+60

no Cl-ion

9.0

15+20

The negative influence of chlorine-ions on catalytic activity can be related with their ability to stabilize the ruthenium in oxidized state, as it was shown for platinum catalysts [5]. This allows to assume, that most active ones are the catalytic centres containing ruthenium in the low degree of oxidizing. Besides, the chlorine-ions influence the dispersity of the catalysts (Table 1). It has been determined also by means of supporting ruthenium from chlorine-free complex that high-dispersity particles of ruthenium the size according to TEM about 15+20 ~. are formed. Introduction of the chlorine - ion into the molecule of the precursor is resulted in decrease of a dispersity of the catalyst. The size of ruthenium particles thus increases to 50+60 ~.. It may be related with increasing of mobility of intermediate products of ruthenium reduction in the presence of Cl-ions leading to aggregation of the Ru atoms in crystallites of bigger size. The similar influence of chlorine-ion upon the dispersity is known for platinum catalysts [6]. The strong influence on a catalytic activity is rendered by the nature and concentration of metal - modifier, as well as process of activation, during which ruthenium and alkaline metal begin to interact. It is shown that the activity passes through a maximum with an increase of the content of potassium in the catalytic system. According to [7], the role of the alkaline modifier means of making boosted concentration of electron density on ruthenium atoms in active centre, which is necessary for activation of dinitrogen. As a result, the growth of activity is observed reaching to maximum at atomic ratio K:Ru = 5 (Fig. 1). The further decrease of activity may be related with the blocking of active centres of the catalyst by excess of the modifier oxide.

216

r

:1::

Z

11109-

i

w

Cn

r

d,.l.,a: oo i

w

t:

0

E E ,=:l:

8

~

7

6-

[]

5"[]

4'

0

I

2

'

I

4

'

I

6

'

I

8

'

I

10

K/Ru

Figure 1. Influence of the potassium content on the activity of 4%Ru-FUSibunit catalysts at P=30 bar; T=623 K; W=3.0o103 h -1. For preparation of active Ru-M/Sibunit catalysts it is important not only to enter the modifier in optimal amount, but also to carry out interaction of both metals uniting them in limits of one particle. This stage is basic at formation of bimetal active centres of the Ru-M catalyst for ammonia synthesis. The mentioned stage is the activation of the catalytic system, is its thermotreatments under special conditions. The catalyst of an optimum composition can't be practically active without stage of preliminary activation. The formation of bimetal centre of Ru-M catalysts proceeds in the best way, using hydroxides as the precursors of the modifier. It is connected with the chemical properties of the ruthenium, which, as is known, interacts with alkali, forming oxygen-containing compounds, at the presence of an oxidizing agent, for example, perruthenates. The heat treatment at the presence of little amount of oxygen can lead to the formation of non-stoichiometric compounds of a type MxRuyOz. It is possible that these compounds can be formed by means of surface oxygen-contained groups of Sibunit. It is necessary to note, that the formation of MxRuyOz can be promoted by presence on a surface of the catalyst metallic ruthenium of high dispersity, having ability to

217 link and to activize molecular oxygen. The subsequent decomposition of MxRuyOz in the process of reductional thermotreatments leads to the formation of active centres containing chemical bond both ruthenium and modifier. The state of ruthenium is predominantly Ru ~ . Besides, the formation of active centres containing herewith, and Ru +n bound with alkaline metal through oxygen is not excluded. The obtained results demonstrate, that the transition from potassium to cesium, as more electropositive modifier, leads to the growth of activity of Ru-M/Sibunit catalyst, as it is known for the system Ru-M based on other supports [8]. The catalysts Ru-Cs/Sibunit are more active (approximately 2 times higher) than potassic analogs when atomic ratio of [Ru]/[M] approaches 2.6. It is caused by the greater ability of cesium to donate electrons onto ruthenium, which is necessary for the activation of molecular nitrogen. Obviously, the geometrical factor caused by the different sizes of compared ions is also important. The recent factor causes different opportunity and depth of introduction of alkaline atoms in to the particles of metal ruthenium, which leads to the formation of active centres distinguished as both their composition and quantity. It is necessary also to note that for investigate catalysts known problem arises once again whether the modifier locally acts, being part of active centre, or occurs 'pump' of electronic density in the ruthenium particle as in whole, which activates centres on its surface. The considerable influence on a dispersity and catalytic activity is rendered by value of Sibunit specific surface area. Above-mentioned value of the average size of metallic ruthenium particles (15+20 .~) is obtained for the sample with S B E T -- 320 m2g-1. At decrease of a specific surface area up to ~ 90 m2g1 the size of particles (Table 2) is increased up to 100+150 A. Table 2 Influence of Sibunit's value specific surface area on activity of catalysts 4.0 wt.%Ru-4.0 wt.% K/Sibunit

SBET, m2g'l ..........

Concentration ofNH3, %

D,

at P=30 bar; T=623 K 320

9,0

15+20

215

5,9

45+55

90

1,0

100+150

The obtained results show that the dispersity of supported ruthenium influence considerably the catalytic activity, which decreases - 9 times at increase of the size of ruthenium particles - 6 times (table 2) and, accordingly - 1.5 times at decrease of the dispersity i n - 3 times (Table 2).

218 CONCLUSION Thus, use of ruthenium complexes with poly-dentate volumetric nitrogen-containing organic ligands as the precursor of the catalyst, and also - graphite-like material Sibunit with the advanced surface, as the support, allows to prepare high-dispersity and active catalysts for low-temperature synthesis of ammonia. REFERENCES 1. V.B. Shur, S.M. Yunusov, V.K.Puri, H. Mahapatra, B. Sen, D.K. Mukhergee, E.S. Kalyuztmaya, I.A. Fokina, V.A.Likholobov and M.E. Vol'pin, Izvest. Akad. Nauk, Ser. Khim., No. 6 (1993) 1452. 2. Yu.I. Ermakov, V.F. Surovikin, G.V. Plaksin, V.A. Semikolenov, V.A. Likholobov, L.V. Chuvilin, and S.V. Bogdanov, React. Kinet. Catal. Lett., No. 32 (1987) 435. 3. Moggi Pietro, Albanezi Giancarlo, Predieri Giovanni and Spoto Giuzehhe, Appl. Catal. A. No. 123 (1995) 45. 4. P.D. Rabina, I.G. Brodskaya, L.D. Kuznetsov, E.A. Farberova, M.A. Golubeva, N.P. Solov'eva, I.M. Pavlov and T.S. Salei, Method of catalyst preparation for ammonia synthesis, USSR Patent No. 1747147 (1992). 5. V.A. Drozdov, E.I. Grigorov, P.E. Kolosov, V.A. Semikolenov, L.V. Chuvilin, and P.G. Tsyrul'nikov, Kinet. Katal. No. 30 (1989) 422. 6. H. Lieske, G. Lietz, H. Spindler and Y. Volter, J. Catal., No. 81 (1983) 8. 7. T. Hikita, Y. Kadowaki, K. Aika and T.Onishi, Shokubai (Catalyst), No. 31 (1989) 429. 8. Sh. Murata and K. Aika, J. Catal., No. 136 (1992) 110.

91998ElsevierScienceB.V.All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

219

P r e p a r a t i o n of r u t h e n i u m b a s e d catalysts u l t r a d i s p e r s e d in a silica matrix F. Di Silvestri a, P. Moggia and G. Predieri b aDepartment of Organic and Industrial Chemistry, University of Parma, Viale delle Scienze, 43100 Parma, Italy bDepartment of General and Inorganic Chemistry, Analytical Chemistry and Physical Chemistry, University of Parma, Viale delle Scienze, 43100 Parma, Italy Among Group VIII metals, Ru shows the highest specific activity and selectivity for the production of long-chain hydrocarbons in the Fischer-Tropsch synthesis (FTS) [1]. Ru-based catalysts are also used for selective hydrogenation of carbonyl groups, carboxylic acids, substituted anilines, aromatic and heterocyclic compounds [2]. Metal/support interactions, metal loading and dispersion notably influence the catalytic activity [3-5]. On the other hand, the choice of the support, the metal precursor and the preparation procedure are also determinant for activity and selectivity control [6-7]. Silica-supported Ru is generally indicated as the most active and stable FTS catalyst, while other systems are less active and more sensitive to deactivation [8]. It has also been stated that metal carbonyl dusters as Ru precursors provide high metal dispersions, using inorganic oxides as supports [9-13]. This work deals with the characterization of Ru/SiO2 systems, prepared by different synthetic procedures. In all cases Ru3(CO)12 or related anionic clusters have been used as Ru precursor. The obtained catalysts have been compared from the point of view of the surface area, metal dispersion and catalytic activity and selectivity in FTS. 1. PREPARATION OF CATALYSTS Several methods for supporting Ru3(CO)12 on silica are reported in literature: (a) Impregnation of dichloromethane [14-15], trichloromethane [16], pentane [17], hexane [18-20] or toluene [21] solutions of Ru3(CO)12, at room temperature, and evaporation to dryness in an inert atmosphere. (b) Impregnation by refluxing benzene [22] or cydohexane [23-24] solutions of Ru3(CO)12. (c) Impregnation of Ru3(CO)12 via vapor phase in an evacuated sealed pyrex cell held at a temperature of 353 K over a period of 2 weeks [25].

220 (d) Impregnation of methanol solution of Na[Ru3H(CO)11], (synthesized by reaction of Ru3(CO)12 with Na[BH4], at room temperature in tetrahydrofuran (THF) [26]), followed by evaporation of the solvent at 333 K [27]. (e) Anchoring Ru3(CO)12 to silica via a pendant thiol, by reaction of Ru3(CO)12 with HS(CH2)3Si(OMe)3 in refluxing benzene and by grafting the resulting product [Ru3H(CO)lo{S(CH2)3Si(OMe)3}] to silica in hexane solution; alternatively, Ru3(CO)12 is reacted in refluxing benzene with the thiolprefunctionalized silica HS(CH2)3SiO3/2.nSiO2 [obtained from silica refluxed in a xylene solution of HS(CH2)3Si(OMe)3][22]. (f) Anchoring Ru3(CO)12 to silica via a pendant amino (or posphino) group, by reaction of Ru3(CO)12 with H2N(CH2)3Si(OEt)3 [or Ph2P(CH2)3Si(OEt)3], in toluene (or hexane) under nitrogen at 333 K. The resulting amino- or phosphinosubstituted carbonyl clusters are tethered to silica suspended in toluene, at room temperature; alternatively, Ru3(CO)12 is reacted in toluene at room temperature with ligand-functionalized SiO2 [obtained from silica refluxed in a toluene solution of H2N(CH2)3Si(OEt)3 or Ph2P(CH2)3Si(OEt)3][28]. (g) Anchoring the duster Na[Ru3H(CO)11] to silica by reaction with ammonium (N+Me3 and-N+Et3) - or pyridinium (N+CsH5)-functionalized silicas suspended in methanol at room temperature [27]. Methods (a), (d), (e) and (f) were also adopted in this work, in order to compare these known procedures with other new or modified ways, which will be described in the following paragraphs.

1.1 Impregnation with Ru3(CO)12 solutions Silica gel (Aldrich-Chemie) (1.16 g), which had been dried in an oven at 773 K for 24 h under a nitrogen stream, was suspended in a solution of Ru3(CO)12 (0.05 g) dissolved in a minimum amount of the following dry solvents: pentane (180 ml), hexane (200 ml), cyclohexane (200 ml), dichloromethane (50 ml), trichloromethane (55 ml), methanol (700 ml) or tetrahydrofuran (60 ml); the stirred suspension was evaporated to dryness at room temperature under a nitrogen stream. In another preparation, silica gel (1.16 g) was preliminarly impregnated with KOH in aqueous solution (11 ml, 0.016 M), stirred for 2 h and evaporated to dryness; the solid was finally dried in an air oven at 773 K for 24 h. The alkalimodified support was then impregnated with a solution of Ru3(CO)12 (0.05 g) in dry hexane (200 ml), following the procedure indicated above. 1.2 Impregnation with [Ru3H(CO)11]" solutions Ru3(CO)12 (0.05 g) was suspended in dry methanol (150 ml) and added with a solution of KOH in methanol (11ml, 0.016 M); the stirred mixture was gently heated until the cluster was completely dissolved, then silica gel (1.16 g) was added to obtain the impregnation. In another preparation, Ru3(CO)12 (0.05 g)was dissolved in dry THF (50 ml), then Na[BH4] (0.015 g)was added under a nitrogen stream and the initial orange solution was constantly stirred until it turned deep red, owing to the formation of the anionic cluster [Ru3H(CO)11]-. Silica gel (1.15 g) was then added to obtain the impregnation by the usual procedure.

221 1.3 Anchoring Ru3(CO)12 on functionalized silica R u3(CO)12 (0.05 g) and HS(CH2)3Si(OMe)3 (0.015 g) were refluxed in dry hexane (200 ml) in a nitrogen stream for 60 min, then silica gel (1.15 g) was suspended in the yellow solution, which was stirred for 2 h, successively cooled to room temperature and finally evaporated to dryness. The same procedure was repeated by using H2N(CH2)3Si(OEt)3 (0.051 g) as reacting ligand. In another preparation, a suspension of silica gel (1.50 g) was refluxed in a xylene (70 ml) solution of HS(CH2)3Si(OMe)3 (0.17 g) for 36 h. The solution was cooled to room temperature, then the solid was filtered off, extracted with diethyl ether for 4 h in a Soxhlet apparatus, and finally dried in a vacuum oven at 323 K for 5 h. The thiol-functionalized silica (1.15 g) was then suspended in a solution of Ru3(CO)12 (0.05 g) in dry hexane (200 ml); the mixture was refluxed for 36 h, cooled and filtered off to separate the solid, which was finally dried in vacuo at room temperature. An original method was followed to prepare the ruthenium cluster anchored to amino-functionalized silica. Ru3(CO)12 (0.05 g) and H2N(CH2)3Si(OEt)3 (0.051 g) were refluxed in dry hexane (200 ml) in a nitrogen stream for 60 min, then tetramethoxysilane (TMOS) (2.91 g) and water (3.48 ml) were added; the mixture was refluxed until a white precipitate was initially formed. The solution was then cooled and finally evaporated to dryness at room temperature. 1.4 Anchoring Ru3(CO)12 by reaction of [Ru3H(CO)11]" on functionalized silica Ru3(CO)12 (0.05 g) was dissolved in dry methanol (100 ml), then Na[BH4] (0.015 g) was added under a nitrogen stream and the initial orange solution was constantly stirred until it turned red. 3-Aminopropyl-functionalized silica gel (Aldrich-Chemie) (1.15 g, containing about 7 mol% of 3-aminopropylsilanogroups) was then added to the anionic cluster solution, which was refluxed for 2 h, then cooled and evaporated to dryness at room temperature. The same procedure was followed starting from Ru3(CO)12 (0.05 g) dissolved in dry methanol (100 ml), after addition of KOH in methanol (11 ml, 0.016 M). Finally, H2N(CH2)3Si(OEt)3 (0.051 g) was refluxed for 5 h in the solution prepared by mixing Ru3(CO)12 (0.05 g)dissolved in dry methanol (100 ml) and KOH in methanol (11 ml, 0.016 M), then silica gel (1.15 g) was added and the suspension was even stirred for 16 h before to evaporate it to dryness at room temperature. 1.5 Sol-gel methods Several preparations of Ru/SiO2 catalysts by the sol-gel process have been described in literature, but only starting from RuC13-3H20 and tetraethoxysilane (TEOS), as Ru and SiO2 precursors respectively; the methods consist in refluxing ethanol-water solutions [29-30], eventually using NH4OH [31-32] or concentrated HC1 [33-34] as catalysts to promote the hydrolysis and condensation reactions. In this w o r k three solutions were prepared by dissolving RuC13.3H20 (0.06 g) in absolute ethanol (6 rnl), then adding TEOS (Aldrich-Chemie, > 98%) (4.1 ml) drop by drop under vigorous stirring. The three solutions were differently added, according to the literature methods described above: (i) only with water (1.30 rnl), (ii) with concentrated NH4OH (0.06 tool) in water (1.30 ml), and (iii) with concentrated HC1 (0.07 tool) in water (1.30 ml). In all cases, the solutions were

222 then refluxed until gelation occurred, i.e. after 24 h, 8 h and 4 h, respectively. The gels were dried in vacuo at 353 K for 16 h, and finally calcined in an oven at 573 K for 5 h, under a nitrogen stream. Furthermore, Ru/SiO2 sol-gel catalysts were prepared starting, for the first time, from Ru3(CO)12 as Ru precursor and tetramethoxysilane (TMOS) as SiO2 precursor, following the synthetic routes described below. (a) Methanol solutions of [Ru3H(CO)11]- (50 ml), with K+ or Na + as countercations, were prepared as above described (par. 1.2), then TMOS (Aldrich-Chemie, > 99%) (2.85 ml) was added dropwise under stirring; after 1 h, distilled water (2.43 ml) was added to both solutions, which were transferred in a vessel suitable for easy evaporation of the solvent and gelation. In three days, dark yellow, highly homogeneous gels were obtained, which were crushed and screened to 40-60 mesh, washed with methanol, dried in v a c u o at room temperature, charged in a microreactor and finally dried at 353 K under a helium stream. In another preparation, after addition of TMOS and water, the solution of the potassium salt of the anionic cluster [Ru3H(CO)11] was refluxed up to incipient gelation, before transferring it in the vessel for the evaporation of the solvent. Finally, another solution of the potassium salt of the anionic Ru cluster was neutralized with concentrated HNO3 (0.176 mmol) before addition of TMOS and water for the gelation. (b) Ru3(CO)12 (0.05 g) was dissolved in dry THF (40 ml) at room temperature, then TMOS (2.88 ml) and, after 1 h, distilled water (2.43 ml) were added dropwise, under continuous stirring, under a nitrogen stream. In the first preparation, N H ~ (0.19 retool), as nucleophilic catalyst, was added to the homogeneous sol, which after stirring (2 h) under nitrogen, was transferred in a vessel suitable for the evaporation of the solvent and the gelation. A homogeneous gel was obtained in 4 h, which was successively treated as reported in (a). In another type(b) preparation no catalyst was added and the initially yellow solution, transferred in a vessel for the evaporation of the solvent, progressively turned in 8 days to a whitish gel, which was then treated as reported in (a). This same preparation was repeated starting from a higher amount of Ru3(COh2 (0.125 g) dissolved in dry THF (100 ml). Finally, a little amount of the gel, obtained without catalyst, was impregnated with a solution of KOH (0.09 mmol) in methanol and evaporated to dryness at room temperature in order to prepare a catalyst modified with about 1% of K+. 1.6 Activation

Catalysts prepared from Ru3(CO)12 or [Ru3H(CO)11]- were all activated as follows: a weighed amount of supported cluster was charged into a flow microreactor and gradually heated (2 K min -1) up to 573 K in a helium stream (50 ml min-1). The effluent gas was continuously analysed by a GC apparatus in line. Upon heating in an inert gas stream, silica-supported Ru3(CO)12 (or [Ru3H(CO)11]) was decarbonylated in at least two steps [14-15], first giving the grafted ruthenium cluster HRu3(CO)10(O-Si-), which progressively evolved to Ru(II)(CO)n (n = 2, 3) species above 393 K [18, 23-24, 35-36]. The reduction to Ru metal was then performed by feeding hydrogen (75 ml min -1) at 573 K for 2 h. A part of CO developed upon heating under helium stream probably

223 disproportionate to CO2 and C by the Boudouard reaction and the carbon finely deposited on the catalyst surface enhances a high dispersion of Ru particles after reduction [12].

1.7. Characterization of catalysts The Ru/SiO2 catalysts prepared in this work were characterized by FT-IR spectroscopy, dispersion measurements, surface area determinations, TEM analysis, and catalytic activity tests. FT-IR spectra of the supported Ru catalysts before and after activation in the range 4000-400 cm -1 were recorded by using a Nicolet 5 PC spectrophotometer. The dispersion measurements of the metal Ru particles on the surface of the activated catalysts were performed by the gas chromatographic pulse method described elsewhere [12], based on the chemisorption of CO at 423 K under a helium stream (60 ml rain-l). Surface area of the activated Ru catalysts were measured by a Micromeritics 2200 analyzer, based on nitrogen adsorption at the temperature of liquid nitrogen, after degassing the samples at 573 K for I h under a nitrogen stream. TEM analyses were performed at 200 keV with the Jeol JEM 2000EX instrument of the MASPEC Institute, CNR, Parma. Catalytic activity in the FTS reaction, in the temperature range 473-573 K, under atmospheric pressure, was tested by a flow microreactor, containing about 0.5 g of activated catalyst, by feeding a CO-H2 1:1 gas mixture (10 ml min-1). The effluent products were periodically analysed by a sampling device connected to a Dani 3400 GC apparatus. 2. RESULTS AND DISCUSSION

2.1 Impregnated catalysts FT-IR spectra of the Ru3(CO)12/SiO2 samples prepared via impregnation are generally identical to solid Ru3(CO)12 in the carbonyl bonds stretching range 22001900 cm -1, exhibiting i.r. bands at 2061vs, 2054m, 2040s, 2026m, 2017s, 2000vs and 1978s cm-1; then, ruthenium cluster is initially only physisorbed on silica. The anionic clusters prepared from Ru3(CO)12 either with NaBH4 in THF or with KOH in methanol give the same FT-IR spectra, with two large carbonyl bands at 2039-2041 and 1972-1973 cm -1, which both move to higher frequencies (of about 20 cm -1) after impregnation, giving some evidence of a chemical interaction with silica. Table I shows the results of the measurements of dispersion, surface area and catalytic activity for the activated Ru/SiO2 catalysts prepared by impregnation. The dispersion degree (D%) is calculated by the chemisorbed CO (mol/mol Ru), based on the hypothesis of the formation of dicarbonylated Ru species by chemisorption at 423 K. Surface areas (SA) are expressed in m 2 g-1. The catalytic activity (CA) is expressed as the specific rate of production of hydrocarbons (rnl h1/g of catalyst) at 573 K; the only hydrocarbons revealed by the GC column used are methane, ethane and, rarely, propane.

224 Table 1 Characterization data (see text) for the impregnated catalysts Catalyst (2% Ru)

Solvent

D%

Ru/SiO2 . . . . . . . . . . . . . . Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 RuNa(0.68) / SiO2 RuK(0.82) / SiO2 Ru/K(0.60)SiO2

Pentan'e' Hexane Cyclohexane Dichloromethane Trichloromethane Methanol Tetrahydrofuran Tetrahydrofuran Methanol Hexane

63 61.5 64 55 52.5 45 35 65 81 56.5

SA '

'490 . . . . . 500 510 450 470 455 465 480 440 515

CA 597 614 491 312 153 49 148 26 13 32

The content of Ru metal is always 2.0% by weight on the silica support. The contents (wt% on silica) of Na + or K + are specifically indicated in the table, along with the solvent used.

The above results show the effect of the solvent on the impregnation: higher dispersions and consequent higher catalytic activities are obtained in the order: non polar solvents (hydrocarbons) > chlorinated solvents > polar solvents, probably according to the wetting degree of the silica surface in the presence of different types of solvent. The use of an anionic cluster, such as [Ru3H(CO)11]-, strongly enhances the metal dispersion, but the presence of alkali cations negatively influences the catalytic activity. However, in this case, a product distribution with less methane and more ethane is observed.

2.2 Anchored catalysts The FT-IR spectra of the products obtained by the reaction of Ru3(CO)12 with a thiol- or amino- silane ligand show carbonyl bands in good agreement with the literature data [22,28]: 2106m, 2065vs, 2056s, 2026vs, 2012m cm -1 and 2061vs, 2030vs, 20!0m cm -1, respectively. These bands appear b r o a d e n e d after impregnation on silica. Both the anchored Ru clusters prepared by the reaction of [Ru3H(CO)ll]- with functionalized silica show two bands at about 2040-2045 and 1970-1975 cm -1, very similar to the unsupported anionic cluster, then inducing to suppose a simple physisorption. Table 2 shows the results of the measurements of dispersion, surface area and catalytic activity on the activated Ru/SiO2 catalysts prepared by anchoring on functionalized silica. The content of metal Ru is always 2.0% by weight on the functionalized silica. The contents (wt% on functionalized silica) of Na + or K+ are specifically indicated in the table. The ligand type and the anchoring procedure used are also specified.

225 Table 2 Characterization data for the anchored catalysts catalyst (2% Ru). . . .

Ligand/Method

D% . . . .

SA

..... c A

Ru/Si()2 . . . . . . . Ru/SiO2 Ru/SiO2 Ru/SiO2 RuNa(0.84) / SiO2 RuK(0.82) / SiO2 RuK(0.82)/SiO2

Ti~iol + Si()2 . . . . . . . . . . Thiolated SiO2 Amine + SiO2 Amine + TMOS + SiO2 Aminated SiO2 Aminated SiO2 Amine + SiO2

i015 ....... 43 30.5 52 84 91 68.5

400 . . . . . 417 300 5 550 282 200 267 184 71 200 54 485 65

The results summarized in Table2 suggest that the use of the anionic cluster as precursor strongly enhances the metal dispersion, but the presence of alkali cations and, more generally, of basic substituents negatively influences the catalytic activity and moves the FTS reaction to higher paraffins. Sulfur groups seem to explicate a poisoning effect on the catalytic activity.

2.3 Sol-gel prepared catalysts The FT-IR spectra of Ru3(CO)12 dissolved in THF shows three carbonyl bands at 2060vs, 2029vs and 2015m cm -1, which are not influenced by the addition of TMOS and water. After gelation, with or without catalyst, two very strong i.r. carbonyl bands at 2065 and 2000 cm -1 appear, which are probably due to the formation of dicarbonyl- Ru(II) species Ru-O-bonded to silica. This suggests that strong interactions between cluster and the incoming support take place during the hydrolysis and condensation steps of the gelation process. Table 3 shows the results of the measurements of dispersion, surface area and catalytic activity on the activated Ru/SiO2 catalysts prepared by sol-gel techniques. The content of metal Ru is generally 2.0% by weight on the silica. The contents (wt% on silica) of Na + or K + are specifically indicated in the table. The Ru and SiO2 precursors and the type of catalyst used in the gel formation are also specified. As elsewhere stated [12], the use of Ru3(CO)12, rather than RuC13, as Ru precursor, affords very higher metal Ru dispersions, corresponding in some cases to the chemisorption of two mole of CO per mole of Ru (100% in the adopted scale). The highest catalytic activity in the FTS reaction is obtained with the solgel Ru catalyst prepared without addition of catalyst, probably because the presence of F- ions negatively influences the catalytic activity, as Na + or K+ actually do. The apparent lower activity of the 5.0% Ru catalyst is due to the complete consumption of H2 before 573 K and the consequent partial deactivation by disproportionation of CO and formation of carbonaceous deposits; in fact, the specific activity values reported in Table 4, related to the whole temperature range from 473 to 573 K, illustrate that this catalyst is the most active up to 523 K.

226 Table 3 Characterization data for the sol-gel catalysts Catalyst (2% Ru)

Precursors/catalyst

D%

SA

CA

Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru/SiO2 Ru(5.0)/SiO2 Ru/SiO2 Ru/K(1.0)SiO2 RuNa(0.68) / SiO2 RuK(0.82)/SiO2 RuK(0.82) / SiO2 RuK(0.82) / SiO2

RuC13, TEOS/Nil RuC13, TEOS/HC1 RuC13, TEOS/NH4OH R u3(CO)12, TMOS/Nil R u3(CO)12, TMOS / Nil R u3(COh2, TMOS/ NH4F R u3(CO)12, TMOS / Nil [Ru3H(CO)11]-, TMOS/Nil [Ru3H(CO)11]-, TMOS/Nil [Ru3H(CO)11]-, TMOS/Refl. [Ru3H(CO)11]-, TMOS/HNO3

30.5 7.5 30 100 25 100 72 68 100 80 15

600 495 660 339 320 476 300 660 750 320 0

n.d. n.d. 152 879 374 424 24 65 29 n.d. 81

Table 4 Catalytic activity of Ru/SiO2 catalysts (FTS reaction) at different temperatures Type of preparation Ru/SiO2 Ru/SiO2 Ru / SiO2 Ru / SiO2 Ru / SiO2 Ru(5.0)/SiO2

Impregnated, hexane sol. Anchored, amine + SiO2 RuC13, TEOS/NH4OH R u3(CO)12, TMOS/NH4F Ru3(CO)12, TMOS/No cat. Ru3(CO)12, TMOS/No cat.

HC Production rate (ml 473 498 523 548 15 29 90 304 3 17 29 96 3 7 13 38 6 19 78 204 22 50 170 462 35 86 202 287

h -1 g-l) 573K 614 282 152 424 879 374

Table 4 also allows to make a comparison between the best catalysts prepared by the different methods tested in this work, from which the superiority of the sol-gel method clearly results. The most active catalyst, i.e. Ru(2.0%)/SiO2 prepared via sol-gel from Ru3(CO)12 and TMOS, without catalyst added in water to accelerate the gelation process, has been analyzed by transmission electron microscopy (TEM). The photographs of the sample, such as showed in Figure 1, clearly evidenced a lot of very homogeneously dispersed, spheroidal metal particles, whose average diameter is about 1 nm. Instead, the average values given in literature [33] for the metal particles of a Ru(1.0%)/SiO2 catalyst prepared via sol-gel from RuC13 are about 3-5 nm.

227

Figure 1. TEM image of a Ru(2.0%)/SiO2 sample prepared by the sol-gel method from Ru3(CO12, dissolved in THF, TMOS and H20, with no catalyst added, after activation in helium stream up to 573 K and reduction with H2. 3. CONCLUSIONS The following conclusions can be drawn out from this work. (a) Very high dispersed metal Ru particles on silica can be obtained by using the sol-gel method starting from a cluster compound, such as Ru3(CO)12, or an anionic cluster, such as [Ru3H(CO)11], rather than from a Ru salt. (b) Ru/SiO2 sol-gel prepared from Ru3(CO)i2, free of alkali ions, is a very active catalyst for the Fischer-Tropsch synthesis reaction at relatively low temperatures (473-573 K) and at atmospheric pressure. ACKNOWLEDGMENTS The authors are indebted with Dr. G. Salviati (MASPEC Institute, C.N.R., Parma) for TEM analyses, and with Mr. Pier Antonio Bonaldi for technical assistance. Financial support from M.U.R.S.T. (Rome) is gratefully acknowledged. REFERENCES

1. 2. 3. 4.

M.A. Vannice, J. Catal., 37 (1975) 449. P. Kluson and L. Cerveny, Appl. Catal. A, 128 (1995) 13. D.L. King, J. Catal., 51 (1978) 386. C.S. Kellner and A.T. Bell, J. Catal., 75 (1982) 251.

228 5. T. Okuhara, T. Kimura, K. Kobayashi, M. Misono and Y. Yoneda, Bull.Chem. Soc. Jpn., 57 (1984) 938. 6. J.A. Mieth and J.A. Schwarz, J. Catal., 118 (1989) 203. 7. A.A. Adesina, Appl. Catal. A, 138 (1996) 345. 8. F. Stoop, A.M.G. Verbiest and K. Van der Wiele, Appl. Catal., 25 (1986) 51. 9. T. Okuhara, K. Kobayashi, T. Kimura, M. Misono and Y. Yoneda, J. Chem. Soc., Chem. Commun., (1981) 1114. 10. J.M. Basset and A. Choplin, J. Mol. Catal., 21 (1983) 95. 11. R. Pierantozzi, E.G. Valagene, A.F. Nordquist and P.N. Dyer, J. Mol. Catal., 21 (1983) 189. 12. P. Moggi, G. Albanesi, G. Predieri and G. Spoto, Appl. Catal. A, 123 (1995) 145. 13. V. Ragaini, R. Carli, C.L. Bianchi, D. Lorenzetti, G. Predieri and P. Moggi, Appl. Catal. A, 139 (1996) 31. 14. J. Robertson and G. Webb, Proc. R. Soc. Lond. A, 341 (1974) 383. 15. R.A. Sanchez-Delgado, I. Duran, J. Monfort and E. Rodriguez, J. Mol. Catal., 11 (1981) 193. 16. D.J. Hunt, R.B. Moyes, P.B. Wells, S.D. Jackson and R. Whyman, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 189. 17. V.L. Kuznetsov, A.T. Bell and Y.I. Yermakov, J. Catal. 65 (1980) 374. 18. D.K. Chakrabarty and A.A. Desai, Inorg. Chim. Acta, 133 (1987) 301. 19. S. Uchiyama and B.C. Gates, J. Catal., 110 (1988) 388. 20. E. Rodriguez, M. Leconte, J.-M. Basset and K. Tanaka, J. Catal., 119 (1989) 230. 21. A.F. Simpson and R. Whyman, J. Organomet. Chem., 213 (1981) 157. 22. J. Evans and B.P. Gracey, J. Chem. Soc., Dalton Trans., (1982) 1123. 23. J. Evans and G.S. McNulty, J. Chem. Soc., Dalton Trans., (1984) 1123. 24. N. Binsted, J. Evans, G. Neville Greaves and R.J. Price, Organometallics, 8 (1989) 613. 25. J.G. Goodwin Jr. and C. Naccache, Appl. Catal., 4 (1982) 145. 26. B.F.G. Johnson, J. Lewis, P.G. Raithby and G. Stiss, J. Chem. Soc., Dalton Trans., (1979) 1356. 27. Y. Doi, H. Miyake, A. Yokota and K. Soga, Inorg. Chim. Acta, 105 (1985) 69. 28. U. Kiiski, T.A. Pakkanen and O. Krause, J. Mol. Catal., 50 (1989) 143. 29. T. Lopez, P. Bosch and R. Gomez, React. Kinet. Catal. Lett., 41 (1990) 217. 30. T. Lopez, R. Gomez, O. Novaro, A. Ramirez-Solis, E. Sanchez-Mora, S. CastiUo, E. Poulain and J.M. Martinez-Magadan, J. Catal., 141 (1993) 114. 31. T. Lopez, A. Lopez-Gaona and R. Gomez, J. Non-Cryst. Solids, 110 (1989) 170. 32. T. Lopez, P. Bosch, M. Asomoza and R. Gomez, J. Catal., 133 (1992) 247. 33. T. Lopez, L. Herrera, R. Gomez, W. Zou, K. Robinson, R.D. Gonzalez, J. Catal., 136 (1992) 621o 34. T. Lopez, L. Herrera, J. Mendez-Vivar, P. Bosch, R. Gomez and R.D. Gonzalez, J. Non-Cryst. Solids, 147&148 (1992) 773. 35. A. Theolier, A. Choplin, L. D'Ornelas, J.M. Basset, G.M. Zanderighi, R. Ugo, R. Psaro and C. Sourisseau, Polyhedron, 2 (1983) 119. 36. G.M. Zanderighi, C. Dossi, R. Ugo, R. Psaro, A. Theolier, A. Choplin, L. D'Ornelas and J.M. Basset, J. Organomet. Chem., 296 (1985) 127.

91998ElsevierScienceB.V.All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

229

T h e i n t e r r e l a t i o n of t h e p r e p a r a t i o n m e t h o d a n d a c t i v i t y of t h e Co-Ru/SiO 2 c a t a l y s t s M. Niemel~i, M. Reinikainen and J. Kiviaho VTT Chemical Technology, P.O. Box 1401 02044 VTT, Finland 1. I N T R O D U C T I O N

Cobalt and ruthenium catalysts have been studied extensively [1-4] due to their high activity in Fischer-Tropsch (FT) synthesis. The characteristics and the performance of the catalysts has depended greatly on the precursor used [1,2,5], on the method of preparation [3,4,6-8] as well as on the pretreatment procedures [9,10]. Over the recent years a lot of effort has been directed particularly on the utilisation of carbonyl compounds as precursors for catalysts with well dispersed metallic sites of unique performance. Since Fischer-Tropsch synthesis typically yields a wide array of products, these catalysts with tailored active sites have been seen as potential candidates for improving the selectivity [11]. Accordingly, we have also investigated cobalt, ruthenium and rhodium carbonyls as catalyst precursors with the emphasis of determining the effect of catalyst preparation method on its performance in synthesis gas reaction. 2. CATALYST P R E P A R A T I O N We have prepared monometallic and bimetallic cobalt and/or ruthenium catalysts on silica support by various techniques [12-15]. Firstly, we have prepared catalysts on partially dehydroxylated silica support (pretreated at 600~ vacuum) from tetranuclear carbonyl precursors under air-free conditions by impregnation [12] and reflux method [13]. Now, for comparative purposes, we are also reporting data for catalysts prepared from commercially available carbonyls on silica support by air-free impregnation [14]. In addition, we prepared catalysts by ionic adsorption from salt precursors [15], since this method has also been suggested to yield catalysts with well dispersed active sites [16]. The recipes and pretreatment procedures of the bimetallic catalysts are summarised in Table 1. It should be noted that the same methods (impregnation, reflux, ionic adsorption) were used also for the preparation of monometallic cobalt and ruthenium catalysts. None of the catalysts were calcined to avoid oxidation and/or agglomeration of the active sites [12-15].

230 Table 1. S u m m a r y of the catalTst preParation methods. Precursor Method Solvent Co wt.% Ru Wt-% Reduction fresh used ~ fresh used 1 in H~. Co2Ru2H2(CO)12 impr. DCM 2, rt 2 2 . 6 3 n.a. 4 4.53 n.a. 4 300~ 2h Co2Ru2(CO)I~ Co4(CO)~2+ Ru4(CO)~ Co2(CO)s+ Ru~(CO)~2 Co2Ru2H2(CO)~2

reflux

Co2Ru2(CO)13

reflux

Co4(CO)~2+ Rut(CO)~2 Co2Ru2H2(CO)12

reflux

Co2Ru2(CO)I3

impr. impr.

DCM, rt. DCM, rt.

2.63 2.63

2.0 1.8

4.5 3

2.0 0.7

300~ 300~

2h 2h

impr.

n-hexane, rt n-hexane, 5h bp. n-hexane, 5h bp. n-hexane, 5h bp. n-hexane, 5h rt. n-hexane, 5h rt. 8% NH 3

5

4.1

2.7

2.2

400~

2h

1.6

1.8

1.8

2.6

3O0~

2h

0.6

0.6

0.6

0.6

300~

2h

0.4

0.3

2.6

2.7

300~

2h

0.8

n.a. 4

1.2

n.a. 4

n.a. 4

0.2

n.a. 4

0.2

n.a. 4

n.a. 4

3

1.7

ads. ads.

Co(NH3)~C13+ ionic 3 2.0 RuCl~ ads. 1used = after reduction or after reaction 2 DCM = dichloromethane, rt = room t e m p e r a t u r e 3 loading 4 n.a. = not available

4.53

450~

3h

3. C A T A L Y S T C H A R A C T E R I Z A T I O N The catalysts have been characterised by A A S , XRF, TPR, TPD, FT-IR, a n d XPS [15, 17-19]. The m e t a l analyses revealed t h a t in case of t e t r a n u c l e a r carbonyls cobalt was not attached on the support by the reflux m e t h o d if cobalt was used alone, w h e r e a s some cobalt was attached on the support in connection w i t h s i m u l t a n e o u s use of cobalt and r u t h e n i u m carbonyl or in case of b i m e t a l l i c carbonyls. Thus, the presence of r u t h e n i u m either as a component of a b i m e t a l l i c cluster or in a physical mixture of the two metals was e s s e n t i a l for the adsorption of cobalt on silica. In other words, the interaction of t h e r u t h e n i u m species w i t h the support was stronger t h a n t h a t of the cobalt species, a n d it w a s r u t h e n i u m t h a t was a t t a c h e d on the support and cobalt was a t t a c h e d on top. In the presence of r u t h e n i u m some cobalt was adsorbed on the c a t a l y s t also a t room t e m p e r a t u r e , a l t h o u g h cobalt alone has been found not to be adsorbed on silica in an significant a m o u n t [11,20]. This observation allows us to s u g g e s t t h a t in the i m p r e g n a t i o n method, the interaction proceeds in the same way.

231 In regard to the different methods (impregnation vs. reflux), we observed that the metal contents of the fresh catalysts prepared by reflux were lower than those loaded on catalysts prepared by impregnation [13]. Yet, the amount of tightly bound metal was higher in conjunction with reflux at elevated temperature t h a n with adsorption at room temperature, i.e. with increasing temperature the a m o u n t of metal attached on the support increased. Thus, we m a y conclude t h a t at room temperature, during impregnation, the tetranuclear carbonyls are not chemisorbed on the silica surface in a high amount - a conclusion supported by literature [11]. Consequently, these physisorbed precursor species are quite susceptible to mobilization during drying and reduction as also suggested by Neimark et al. [21]. Accordingly, H u n t et al. [22] report that the r u t h e n i u m species on the support were not evenly distributed after impregnation, but were spread during thermal t r e a t m e n t to result in an even distribution. The studies on reflux method [13] revealed yet another very interesting result: the two tetranuclear carbonyl compounds, Co2Ru2H2(CO)12 and Co2Ru2(CO)I~, behaved very differently, i.e., significantly more of Co2Ru2H2(CO)12 was adsorbed on silica both under reflux and at room temperature, see Table 1. Since the difference is evident at both temperatures, it can not be entirely related to the different t h e r m a l stability of the clusters. Therefore, we presume that the Co2Ru~H2(CO)~2 cluster was more susceptible to structural change upon contact with partially dehydroxylated silica t h a n was Co2Ru2(CO)~3, because the hydrides in the hydrido cluster are known to be mobile [22, 23], whereas the structure of Co2Ru2(CO)I~ might be more stable. Namely, the Co2Ru2(CO)~a cluster is prepared from RuCo2(CO) n by decomposing it in hexane at 65~ [24], i.e., it is stable even at elevated temperatures. Consequently, were assume that the restructured Co2Ru~H2(CO)12 cluster was at least in part chemisorbed on the surface, whereas the adsorption of Co2Ru2(CO)~a was dominated by physisorption. Indeed, the essence of the right choice of a precursor material to obtain sufficient interaction between the support and the catalyst precursor complex has been described in detail in literature [16]. Therefore, the precursor chosen for preparation by ionic adsorption was known to interact strongly with the support, and consequently the metal loading of the catalyst was according to XRF as aimed, 3 wt%. It should be noted that also during ionic adsorption the ruthenium species appeared to be more strongly bonded on the support than cobalt, and thus the surface of a bimetallic catalyst again appeared cobalt rich. The metal contents of the fresh catalysts are, however, only a first implication of the success of the catalyst preparation, since the catalyst surface is significantly altered during the pretreatment [18,19]. In case of the carbonyl precursors calcination is omitted in order to keep the metal in the precursor in zero valent, well dispersed, state. During reduction, however, we have commonly observed formation of metallic mirrors inside the quartz reactor tubes, i.e. some of the metal has sublimated from the support. Accordingly, the analyses indicate

232 t h a t the metal content of the reduced/used catalyst is much lower than the loading implies. This is a significant drawback for the utilisation of carbonyl compounds as catalyst precursors - the precursors are tedious to prepare and thus significant losses of the valuable compounds during preparation cannot be accepted. 4. P E R F O R M A N C E I N F T S Y N T H E S I S

In terms of activity the results for bimetallic catalysts are very difficult to interpret since the metal contents as well as the ratios of the two metals are so different. Therefore, the interrelation of preparation and performance is illustrated by catalysts containing only cobalt or ruthenium. The results shown in Table 2 indicate t h a t the activities of the catalysts varied significantly. The differences in activity were due to the catalyst preparation method as well as due to the differences in the testing conditions. Nevertheless, the results indicate t h a t the catalysts of carbonyl origin were much more active t h a n the ones prepared by ionic adsorption from salt precursors - a finding in agreement with the significantly higher chemisorption capacity, respectively. Table 2. Activities of the catalyst during FT synthesis per 1 g of catalyst. Precursor/Method wt% T P GHSV, (h 1) in ~ MPa react. Co,(CO)Jimp. 3.5 233 2.1 12700 Co2(CO)Jim p. 3.5 220 2.1 2000 Co(NH3)6ClJi.ads. 2.5 282 2.0 2500 Ru~(CO)lfimp. 2.7 250 2.1 2000 Ru4H4(CO)~2/imp. 1.4 233 2.1 14500 Ru4(CO)Jref. 3.0 233 2.1 14000 RuC1/i.ads. 1.1 230 2.0 2500

X, % 3.0 10.3 1.4 16 3.0 3.0 8.0

For catalysts of carbonyl origin, the method of preparation and the precursor used also played a role in the catalyst activity: the cobalt catalyst prepared from the tetranuclear carbonyl appeared significantly more active t h a n that prepared from C%(CO) 8. However, another comparative study [5] indicated that the difference between the two cobalt precursors was much less profound. Thus, the catalyst preparation in terms of performance was not well reproducible and the superiority of either of the precursors in connection with impregnation could not be unambiguously determined. Very clearly, however, for cobalt precursors, the reflux method was inferior to impregnation, because no cobalt was attached on the support during reflux. In case of ruthenium carbonyls, the results indicate t h a t the method of preparation did not effect the catalyst performance, since both of the carbonyl based r u t h e n i u m catalysts were highly active, and clearly more active t h a n the catalyst prepared by ionic adsorption. It is also important to mention t h a t the r u t h e n i u m precursors are not as air sensitive as are cobalt

233 carbonyls, and thus the catalyst preparation from ruthenium carbonyls would be facile also in larger scale. In Fischer-Tropsch synthesis the selectivity of the catalysts is, however, even more important than the activity. In connection with Co-Ru catalysts, some previous studies had suggested that higher C5+ selectivities could be achieved by adding ruthenium into cobalt catalysts [3,4] whilst others claimed that the catalysts derived from bimetallic clusters exhibited higher selectivities for the higher oxygenates than did catalysts prepared by mixing the monometallic clusters or metal chloride precursors [25-27]. Yet, our own results obtained for the impregnated catalysts supported neither of these findings: the bimetallic catalysts prepared from the tetranuclear clusters performed no better in the formation of oxygenates than did the monometallic ones or those prepared by mixing the monometallic clusters, see Figure 1. In the same way, the data for the catalysts prepared by the reflux method indicated no enhancement in the formation of oxygenates, see Figure 2.

Figure 1. The results in FT synthesis: impregnated catalysts at 2.1 MPa, 233~ and CO conversion of 3 % using CO:H2:Ar 3:6:1 [10].

234 In yet a further regard to cluster based catalysts, we discovered that the catalysts prepared from the commercially available clusters by impregnation performed well in comparison to the ones prepared from tetranuclear clusters, see Figure 2. For example, the oxygenate yield (particularly methyl acetate) was significantly higher for the catalysts prepared from the commercial clusters, and they maintained their original metal content better than did the catalysts prepared from tetranuclear clusters, see also Table 1. Very interestingly, in regard to the selectivities, all the bimetallic catalysts were clearly more cobalt-like than ruthenium-like, see Figures 1 and 2. Thus, the reactivity results provide further evidence for our suggestion of ruthenium being bonded on silica and cobalt being located on top of ruthenium. In other words, the working surfaces of all bimetallic catalysts did appear cobalt rich as we expected.

Figure 2. The results in FT synthesis: catalysts prepared by reflux method and the reaction carried out at 2.1 MPa, 233~ and CO conversion of 3 % [13].

235 Concerning the bimetallic synergy, however, a very intriguing finding was made in case of the catalysts prepared by ionic adsorption. Namely, the Co-Ru catalyst was much more selective in the formation of oxygenates (particularly methanol) than either of the monometallic or any of the carbonyl based catalysts, see Table 3. This extraordinary selectivity is a novel finding, and it indicated very clearly that the two metals interacted during catalyst preparation to form active sites with unique characteristics. We presume that a positively charged mixed ensemble was formed to account for the observed synergistic enhancement in the formation of methanol.

Table 3. The results obtained for CO hydrogenation at 250~ CO:H2:Ar molar ratio of 3.6:1, pressure of 2.0 MPa. and 1 g of catalyst with GHSV 2000 h 1. Catalyst Co2 Ru 2 X CH 4 C~.s C8. MeOH EtO PrO CO 2 SiO~ wt wt % (%) (%) (%) (%) H H (%) .

Co(NH8)6C131 2.5 Co(NH~)6C13 + RuC133 Co(NH3)6CI~ and RuC134 RuC13

.

.

.

.

2.1

71.4

22.3

2.3

3.8

0.1

-

9.6

36.1

38.2

5.5

15.7

1

3.4

1.35 0.65 4.5

50.2

41.3

4.2

4.0

0.22

-

40.9

22.4

0.5

0.1

-

2.0

1.7

1.1

18.7 36

-

1The reaction carried out at 300~ ~Metal contents of the used catalysts, analysed by AAS. 3Simultaneous ion exchange 4Physical mixture of the catalysts prepared by ion exchange 5Estimate based on the analysis of the respective monometallic catalysts

In all, the results indicate that the catalyst performance may be significantly influenced not only by the choice of the right combination of metals but also by the preparation method. Consequently, in our case, the CoRu/SiO 2 catalyst prepared by ionic adsorption exhibited unique selectivity in FT synthesis in regard to similar catalyst compositions prepared by impregnation or reflux method.

REFERENCES

1. 2. 3. 4. 5.

L. Guczi (ed), Stud. Surf. Sci. Catal. 64 (1991) Kodansha, Progress in C1 Chemistry in Japan, Elsevier, 1989 E. Iglesia, S.C. Reyes, R.J. Madon, Adv. Catal. 39 (1993) 221 E. Iglesia, S.L. Soled, R.A. Fiato and G.H. Via, J. Catal. 143 (1993) 345 M.K. Niemel~i, A.O.I. Krause, T. Vaara, J. Kiviaho and M.K.O. Reinikainen, Appl.Catal. A General, 147 (1996) 325

236 6. 7. 8. 9.

R.C. Reuel and C.H. Bartholomew, J. Catal. 85 (1984) 63 R.C. Reuel and C.H. Bartholomew, J. Catal. 85 (1984) 78 B.A. Sexton, A.E. Hughes and T.W. Turney, J. Catal. 97 (1986) 390 M.K. Niemel~i, L. Backman, A.O.I. Krause and T. Vaara, Appl. Catal. A General, 156 (1997) 319 10. J. Kiviaho, M.K. Niemelfi, M. Reinikainen, T. Vaara, T.A. Pakkanen, J. Mol. Catal. A:Chemical, 121 (1997) 1 11. A. Zecchina 12. J. Kiviaho, M. Reinikainen, M.K. Niemelfi, K. Kataja and S. Jfifiskelfiinen, J. Mol. Catal. A:Chemical 106 (1996) 187 13. M. Reinikainen, J. Kiviaho, M. KrSger, M. Niemel~i and S. Jfi~iskel~iinen, J. Mol. Catal. A:Chemical, 118 (1997) 137 14. M. Reinikainen and Iwasaki, Unpublished data 15. M. Reinikainen, M.K. Niemelg and N. Kakuta, manuscript 16. R.D. Gonzalez and H. Miura, Catal. Rev. Sci. Eng. 36 (1) 1994 145 17. J. Kiviaho, M.K. Niemelfi, M. Reinikainen and T.A. Pakkanen, Appl. Catal. A:General, 149 (1997) 353 18. J. Kiviaho, PhD. Thesis, VTT Publications 290, Espoo, 1996 19. M. Niemel~i, Dr.Tech. Thesis, VTT Publications 310, Espoo, 1997 20. A. Ceriotti, S. Martinengo and L. Zanderighi, J. Chem. Soc. F a r a d a d a y Trans. 1 84 (1984) 1605 21. A.V. Neimark, L.I. Kheifez and V.B. Fenelov, Ind. Eng. Chem. Prod. Res. Dev. 20, (1981) 439 22. D.J. Hunt, R.B. Moyes, P.B. Wells, S.D. Jackson and R. Whyman, J. Chem. Soc. Faraday Trans. I, 82 (1986) 189 23. J.N. Nicholls, Polyhedron 3 No 12 (1984) 1307 24. E. Roland and H. Wahrenkamp, Chem. Ber. 118 (1985) 1133 25. M. Ichikawa, F-s. Xiao, C.G. Macpanty, A. Fukuoka, W. Henderson and D.F. Schriver, Stud. Surf. Sci. Catal. 61 (1991) 297 26. F-s. Xiao, A. Fukuoka and M. Ichikawa, J. Catal. 138 81992) 206 27. F-s. Xiao, J. Nat. Gas Chem. 3 (1994) 219

91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsV I I B. Delmonet al., editors.

237

Preparation o f the Metal C o m p l e x Catalysts Immobilized on Chitosan for Carbonyl C o m p o u n d s Transfer Hydrogenation V.Isaeva, V.Sharf, N.Nifant'ev, V.Chernetskii, Zh.Dykh N.D. Zelinsky Institute for Organic Chemistry Russian Academy of Sciences Leninsky Pr.47, Moscow, 117913, Russia

Abstract

Novel catalysts of reduction reactions were prepared by immobilization of binuclear Rh(II) and Ru(II, III) teraaacetate complexes with metal-metal bond on original chitosan and succinamide chitosan derivative. Obtained metal complex systems catalyzed transfer hydrogenation of carbonyl group of cyclohexanone and acetophenone in the liquid phase under mild conditions (82.4~ Ar). 2-Propanol was a hydrogen donor and reaction promoted by KOH in 2-propanol solution. The preparation procedure (solvent, time of the complex deposition, size of chitosan corpuscles) strongly influenced the activity of the prepared catalysts. The metal complex structures formed on the carrier surface were examined by IRand electronic spectroscopy. Introduction.

Metal complexes immobilized on the solid carriers are very attractive for the researchers due to combination of the activity and selectivity of the homogeneous catalysts and the simplicity of the recycling and recovery of heterogeneous ones. Currently, the significantly efforts are focused on the development and design of the fitting materials for the carriers. Presently, various synthetic polymer carriers are successfully applied for the complex immobilization. Simultaneously, the natural carriers have stimulated much interest due to their availability, biodegradability and higher thermal stability in comparison with the most synthetic resins. Generally, the natural carrier application for the metal complexes immobilizaton is restricted by cellulose [ 1]. We suggest that very promising results will be achieved using chitosan as carrier [2]. In contrast with cellulose, monomer of chitosan, deacetylated chitin derivative, contains aminogroup and therefore can be used without preliminary functionalization. It has been shown previously [3,4] that the nature of the ligand environment of immobilized binuclear Rh(II) and Ru(II, III) complexes determines the structure and properties of the catalytic systems being formed. After immobilization on qt-aminopropyl groups containing silica and synthetic polymers modified by heterocyclic amine groups complexes with tetraacetate ligands retaine the binuclear structure with metal-metal bond. Immobilized binuclear Rh(II) and Ru(II,III) complexes differ from mononuclear ones by higher activities in the several reduction reactions [3-5]. To continue the investigations along this line we studied

238 the effect of the structure of Rh and Ru complexes immobilized on original chitosan (Figure 1) and succinamide chitosan derivative (Figure 2) on the activity of the resulting catalysts.

OH

OH

0

0

/0

0

~N

i O~ C--L.coo_Na+

Figure 1. Original chitosan

Figure 2. Succinamide chitosan derivative

Thus the aims of our work were: 1) to investigate the immobilization of Ru and Rh complexes on chitosan and succinamide chitosan derivative; 2) to examine the obtained catalytic systems in the transfer hydrogenation of cyclohexanone and acetophenone; 3) to elucidate the effect of carrier and catalyst preparation conditions on the metal complex structures formed on the carrier surface and by this on the catalytic activity.

Experimental. Purified chitosan and succinamide chitosan derivative were applied for the complex immobilization. [Rh2(O2CCHa)4] and [Ru2(O2CCH3)4] were prepared according to the reported procedures [6,7]. Rh and Ru tetraacetates were deposited from aqueous and 2-propanol solution under Ar. The quantity of Rh/Ru deposited were calculated from the difference between the initial concentration of metal in the solution and the concentration after filtration and washing of the catalyst, as determined by atomic absorption spectroscopy. The experiments were carried out in a reactor equipped with a magnetic stirrer, a water jacket, a reflux condenser, and facility for sampling [3]. 2-propanol was used as hydrogen donor. A catalyst (0.05-0.1 g, 4.96 xl 0.6 moles of Rh/Ru) were placed in reactor, the system was filled with Ar, and 10 ml of 2-propanol was introduced along with a promoter (solution of KOH in 2-propanol). The reaction mixture was stirred and heated to 82.4~ after which a solution of substrate (4.84-9.68)xl 0.4 mole in 5 ml of 2-propanol was added. The composition of the catalysate was determined by GLC using a Biochrom-21 chromatograph with a flame ionization detector at 100-170~ (with N~ as the carrier gas, and a 3mx3mm stainless-steel column filled with Triton on X-545 Celite). The activities of the catalysts were characterized by the initial specific rates of the cyclohexanol (W0~) and 2-phenylethanol (W02) formation determined graphically (mols mo1-1 of M minl). IR-spectra of original and immobilized

239 complexes were recorded on a Specord-M 80 instrument in the form of pressings with KBr. The electronic spectra were taken in a Specord M-40 instrument.

Results and Discussion. Catalytic behavior of immmobilized binuclear tetraacetate Rh(ll) and Ru(ll,lll) complexes Binuclear [Rh2(O2CCH3)4]and [Ru2(O2CCH3)4] were chosen for immobilization due to excellent ability of metals in these complexes to coordinate with N-containing functional groups without leaching [3,4]. Actually, any metal complex remove from carrier surface was observed after immobilization as well as during catalytic process. Transfer hydrogenations of cyclohexanone and acetophenone were examined as model reactions. Reduction of both cyclohexanone and acetophenone proceeded selective without byproduct formation, with yield 99.6%. The reaction of transfer hydrogenation promoted by KOH in 2-propanol solution. It can be seen from Table 1 that activity of Rh catalysts surpassed one of Ru catalytic systems in examined reaction of transfer hydrogenation. Acetophenone was reduced rather slowly then cyclohexanone. It could be explained by more solid coordination of acetophenone with metal in intermediate complex formed during transfer hydrogenation (like in Meerwein-PonndorfVerley reactions). The nature of N-containing group (amino- or succinamide group) of the carrier did not considerably influence in the catalytic activity in this reaction. In contrast, the preparation conditions (solvent, time of the complex deposition, size of chitosan corpuscles) strongly influenced the activity of the prepared catalysts. Effect of the chitosan corpuscle size. The chitosan corpuscle sizes were regulated by subsequent solvation and repricipitation from diluted CH3COOH and NaOH aqueous solutions. The decrease of the chitosan corpuscle size from 3 to 0.1 mm leaded to the significant rise of the initial rate of carbonyl group transfer hydrogenation for both Rh and Ru catalysts (Table 2). Effect of the metal complex deposition conditions The metal complex preparation conditions influenced essentially in the catalyst activity. The data from Table 1 demonstrate that the nature of the solvent from which metal complex deposition carried out considerably effected the catalytic properties of the prepared catalyst. Catalytic activity of immobilized metal complex system on the basis of Rh and Ru complexes deposited from 2-propanol solution surpassed one of these complexes deposited from aqueous solution. It can be seen from Table 1 that decrease of complex deposition time from 72h to 2h leaded to the rise of the initial rates of transfer hydrog~,ation about one order for both Rh and Ru metal complex catalysts (in the case of the deposition from aqueous solution). These catalytic data could be explained by the difference in the structures formed on the carrier surface under different deposition conditions of metal complex. In the case of the deposition from aqueous solution, the hydrolysis of original binuclear metal complex into mononuclear one could be prevented by deposition time shortening. This suggestion was confirmed by IRand electronic spectroscopic examinations.

240 Table 1. Transfer hydrogenation of cyclohexanone and acetophenone in the presence of immobilized Rh and Ru tetracetates (4.9xl 0 .6 moles of Rh/Ru, 4.8x104 moles of substrate, 4.9x104 moles of KOH, 82.4~ 15 ml of 2-propanol Metal complex Carrier Solvent, from Metal W0~, mols W02,mol 1 which metal complex mol ~ of M o f M min ~ complex deposition deposition min ~ was carried out time, h [Rh2(O2CCH3)4] original H20 2 3.7 3.1 chitosan . . . . . . 72 0.3 0.1 . . . . 2-propanol 2 9.5 8.2 . . . . . . 72 9.3 8.1 " succinamide H20 2 3.4 2.1 chitosan derivative . . . . . . 72 0.3 0.1 . . . . 2-propanol 2 10.1 9.6 . . . . . . 72 10.2 9.5 H20 2 1.6 1.2 [Ru2(O2CCH3)4] original chitosan . . . . . . 72 0.2 0.1 . . . . 2-propanol 2 5.1 4.8 . . . . . . 72 4.8 4.4 " succinamide H20 2 1.0 0.8 chitosan derivative . . . . . . 72 0.2 0.1 . . . . 2-propanol 2 5.4 5.1 . . . . . . 72 5.2 4.7

Structure of Immobilized Rh(ll) complexes When the tetraacetate binuclear complexes of rhodium(II) are deposited on chitosan, four different types of surface structures may be formed, as it suggested for immobilization on 7aminopropyl containing silica [3]. One of them is bound to the carrier through the equatorial coordinate (A), two of them through the axial coordinate (B, C). These structures retain their dimeric form. Also possible is the formation of structures with a mononuclear nature (D). The formation of surface structure depends on the deposition conditions of metal complex and the flexibility of the hydrocarbon fragments of the carrier. In the case of Rh teraacetate deposited from 2-propanol solution and from aqueous solution (deposition time 2 h) on original chitosan, the color of the carrier with immobilized complex became lilac. In the case of the Rh teraacetate deposited under the same conditions on succinamide chitosan derivative, the color of carrier changed during the deposition process

241 from lilac to green. A lilac color is observed for the acetate complexes of Rh(II) in the case in which a nitrogen-containing ligand is coordinated through the axial site. The acetate complex takes on a green color when one or two acetate bridge group are split out [8]. In the case of succinamide chitosan derivative, we can assume that the succinamide groups of the Table 2. Influence of the chitosan corpuscle size in the catalytic activity in transfer hydrogenation of cyclohexanone and acetophenone in the presence of immobilized Rh and Ru tetracetates (4.9x10 6 moles of Rla/Ru, 4.8x10 -5 moles of substrate, 4.9x104 moles of KOH, 82.4~ 15 ml of 2-propanol W02, mols mol l of M Metal complex Chitosan corpuscle W0~, mols mol l of M size, mm min ~ min-1 min ~ rain- 1 [Rh2(O2CCH3)4] 0.1 9.5 8.2 " 0.5 5.8 4.9 " 1 1.2 0.8 " 3 0.6 0.3 [Ru2(O2CCH3)4] 0.1 5.2 4.0 " 0.5 2.2 1.7 " 1 0.7 0.5 " 3 0.2 0.1

macroligand are initially coordinated through two axial sites of the complex, and then through equatorial sites, at the expense of splitting out part of the acetate groups. In the electronic reflection spectra of these samples was present a broad absorption band with a maximum in the 16.000 cm ~ region, indicating preservation of the binuclear structure of the complex Rh(II) with an Rh-Rh bond, and also indicating the presence of acetate bridge groups, the number of which varies from 2 to 4 [9]. In the case of original chitosan, we suggested that the binuclear structure (B) was realized on the surface, and in the case of succinamide chitosan derivative, the binuclear structure (A) was realized. In the case of metal complex deposition from aqueous solution with prolonged deposition time (72 h) the color of carrier changed during the deposition process from lilac to grey and the characteristic absorption band was absent in the elrectronic spectra. Thus, possibly, the formation of the mononuclear structure (D) took place on the carrier surface due to hydrolysis of initial binuclear complex. After immobilization of Rh(II) tetraacetates on original chitosan (deposition from 2propanol and from aqueous solution with deposition time 2h), absorption bands (AB) Vas (COO) 1600 cm ~ and v,(COO) 1425 cm ~ were presented in the region of the carboxyl group vibrations (see Figure 3) in IR-spectra. The same AB were presented in initial complex (1585 and 1425 cm~). The frequence 16000 cm ~ was rather higher then in original complex. It could be explained by Rh coordination changing or electronic density. The intensity of these AB as well as the absence of shifts for the COO vibrations (Figure 3), indicated that complexes retaining four acetate bridges were found on the carrier surface (structure B). In the case of deposition from aqueous solution with prolonged deposition time on original chitosan or on

242 succinamide chitosan derivative the characterisic AB of carboxylic group vibrations were absent. It could be explained by the absence of acetates groups or theirs very low concentration. Acetate groups could be replaced by succinamide groups of the carrier or by water during deposition (structure A or D). This agrees with the data obtained by electronic spectroscopy. Thus, our data demonstrated, that when the shorter deposition time during immobilization of metal complex from aqueous solution, the binuclear structure of original complex was preserved. In the case of the deposition time lengthening, binuclear structure degraded to mononuclear one with the substitution of acetate groups of original complex with aminogroups of chitosan.

Figure 3. IR spectra: 1) Chitosan; 2) [Rh2(O2CCH3)4]/original chitosan, deposition from 2-propanol solution; 3) [Rh~(O2CCH3)4]/succinamide chitosan derivative, deposition from 2-propanol solution; 4) [Rh2(O2CCH3)a]/original chitosan, deposition from aqueous solution, deposition time 72 h; 5) [Rh2(O2CCH3)4]/original chitosan, deposition from aqueous solution, deposition time 2 h

Conclusions

1. Original chitosan and succinamide chitosan derivative were applicated for the preparation of the novel transfer hydrogenation catalysts on the basis of immobilized of binuclear Rh and Ru tetraacetates. 2. IR- and electronic spectroscopic examinations demonstrated that type of the metal complex structure formed on the carrier surface depended on the catalyst preparation procedure (mononuclear or binuclear immobilized complex) as well as on the macroligand nature

243 (coordination type of binuclear metal complex with carrier: through equatorial or axial coordination sites). 3. Metal complex immobilized system with binuclear structure possessed more catalytic activity in examined reaction, then mononuclear one.

References.

1. K. Kaneda, H. Yamamoto, T. Imanaka, S. Teranishi, J.Mol.Catal. 29 (1985) 99. 2. V.N. Chemetskii, N.E. Nifant'ev, Mendeleev Chemistry Journal, 41 (1997) 80. 3. Isaeva V.I., SharfV.Z., Zhilyaev A.N., Bull.Acad.Sci. of USSR, Chem. div. 40 (1991) 257. 4. V.I. Isaeva, V.Z. Sharf, Y.V. Smirnova, Zh.L. Dykh, G.N. Baeva, T.A. Fomina, A.N. Zhilyaev, I.B. Baranovskii, Rus. Chem. Bull. 44 (1995) 64. 5. V.I. Isaeva, V.Z. Sharf, Zh.L. Dykh, L.I. Lafer, V.I. Yakerson, Bull. Acad. Sci. of USSR, Chem. div. 41 (1992) 49. 6. B.C.Hui, G.L. Rempel, J. Chem. Sot., Chem. Commun. (1970) 1195. 7. F.A. Cotton, Inorg. Chem. 27 (1988) 43. 8. A.N. Zhiljaev, A.T. Fal'kengof, M.A. Golubnichaya, et al., Koordinats. Khim. 12 (1986) 1862. 9. V.Z. Sharf, V.I. Isaeva, A.N. Zhilyaev, I.B. Baranovskii Bull. Acad. Sci. of USSR, Chem. div. 38. (1990) 1796.

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1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmonet al., editors.

245

Preparation and Evaluation of Novel Hydrous Metal Oxide (HMO)-Supported Noble Metal Catalysts Timothy J. Gardner a, Linda I. McLaughlina, Lindsey R. Evansa, and Abhaya K. Datyeb aCatalysis and Chemical Technologies Department, Sandia National Laboratories, Albuquerque, NM 87185-0710" ~Department of Chemical Engineering and Center of Microengineered Materials, University of New Mexico, Albuquerque, NM 87131 ABSTRACT Hydrous Metal Oxides (HMOs) are chemically synthesized materials that, because of their high cation exchange capacity, possess a unique ability to allow the preparation of highly dispersed supported-metal catalyst precursors with high metal loadings. This study evaluates high weight loading Rh/HMO catalysts with a wide range of HMO support compositions, including hydrous titanium oxide (HTO), silica-doped hydrous titanium oxide (HTO:Si), hydrous zirconium oxide (HZO), and silica-doped hydrous zirconium oxide (HZO:Si), against conventional oxide-supported Rh catalysts with similar weight loadings and support chemistries. Catalyst activity measurements for a structure-sensitive model reaction (n-butane hydrogenolysis) as a function of catalyst activation conditions show superior activity and stability for the ZrO2, HZO, and HZO:Si supports, although all of the Rh/HMO catalysts have high ethane selectivity indicative of high Rh dispersion. For the TiOz-, HTO-, and HTO:Sisupported Rh catalysts, a significant loss of both catalyst activity and Rh dispersion is observed at more aggressive activation conditions, consistent with TiOx migration associated with SMSI phenomena. Of all the Rh/HMO catalysts, the Rh/HZO:Si catalysts appear to offer the best tradeoff in terms of high Rh dispersion, high activity, and high selectivity. 1. INTRODUCTION Hydrous Metal Oxides (HMOs) are chemically synthesized materials that contain a homogeneous distribution of ion exchangeable alkali cations that provide charge compensation to the metal-oxygen framework. For catalyst applications, the HMO material serves as an ion exchangeable support which facilitates the uniform incorporation of catalyst precursor species. Because of their high cation exchange capacity with respect to conventional oxide supports these materials possess a unique ability to allow the preparation This work was supportedby the United StatesDepartmentof Energyunder ContractDE-AC04-94AL85000. Sandia is a multiprogramlaboratoryoperatedby Sandia Corporation, a LockheedMartin Company,for the United States Department of Energy. *

246 of highly dispersed supported-metal catalyst precursors with high metal loadings via cation exchange. Considerable previous work on HMO materials has emphasized various forms of hydrous titanium oxides (HTO) as catalyst supports for direct coal liquefaction, hydrotreating, hydrogenolysis, and hydrogenation/dehydrogenation applications. ~'8 These studies have yet to show a definitive advantage for HMO-supported metal catalysts at high weight loadings relative to conventional oxide-supported metal catalysts, although HMO-supported noble metal catalysts have not systematically been investigated. Our recent study compared high weight loading (> 10 wt.%) HTO- and conventional oxide-supported Rh catalysts, 9 showing that although high Rh dispersions could be attained under certain low temperature activation conditions in H2, exposure to higher temperature reduction conditions resulted in strongmetal-support-interaction (SMSI) phenomena similar to that reported for highly dispersed titania-supported Rh catalysts, x~ Our current work represents an extension of our previous supported Rh catalyst work to other HMO support compositions, such as silica-doped hydrous titanium oxide (HTO:Si), hydrous zirconium oxide (HZO), and silica-doped hydrous zirconium oxide (HZO:Si). Catalyst activity and selectivity measurements for a structure-sensitive model reaction (nbutane hydrogenolysis) will be correlated with hydrogen chemisorption measurements and microstructural analysis via high resolution transmission electron microscopy (HREM). The sensitivity of these HMO-supported Rh catalysts to different activation conditions and HMO support chemistry will be examined, with the overall goal of preparing stable, highly dispersed supported noble metal catalysts with a range of metal loadings. 2. EXPERIMENTAL PROCEDURE

2.1. HMO Synthesis Chemistry HTO- or HTO:Si-supported catalyst preparation involves a multiple step chemical procedure that begins with the synthesis of a bulk Na form HTO or HTO:Si powder. 24'~2 Previous work has demonstrated that SiO2 additions (Ti:Si mole ratio = 5" 1) to HTO materials act to stabilize support surface area at high temperature (> 773 K)3"6 without altering the ion exchange behavior. 3'~3 A brief review of the synthesis of both HTO and HTO:Si materials with a maximum cation exchange capacity (Na:Ti mole ratio = 0.5) will be given here as an example. In both cases, changes in the Na:Ti stoichiometry (0 < Na:Ti < 0.5) are easily accommodated. The first step of the reaction scheme involves adding the Ti (or mixed Ti-Si) alkoxide to a dilute (-~10 wt.%) solution of sodium hydroxide in methanol, resulting in the production of a soluble intermediate species. Structural studies completed to date indicate that the soluble intermediate is composed of a highly crosslinked Na-Ti (or Na-Ti-Si) polymer with alkoxides bridging the various metal centers. ~4 To prepare bulk Na form HTO or HTO:Si powders, the soluble intermediate is hydrolyzed in a water/acetone solution. The amorphous HTO or HTO:Si precipitate is filtered, washed, and vacuum dried at room temperature to produce the material for subsequent acidification/ion exchange processing. HZO and HZO:Si supports are prepared initially in Na form utilizing similar alkoxide/solgel chemistry to that described above, with zirconium n-propoxide used as a Zr precursor. An alcohol exchange reaction between zirconium n-propoxide and ethanol (2:1 mole ratio of C2Hs:C3H7 is used to facilitate the formation of zirconium ethoxide. 4 The synthesis and

247 hydrolysis of the soluble intermediate to form the bulk Na form HZO or HZO:Si powder is identical to that previously described for the HTO or HTO:Si systems.

2.2. Catalyst Preparation High weight loading Rh/HTO and Rh/HTO:Si catalyst precursors were prepared utilizing a controlled ion exchange procedure. First, a Na form HTO or HTO:Si/H20 slurry was acidified to pH 4 using 10 wt.% HNO3, resulting in the exchange of a significant portion of Na § with IT. Subsequently, a concentrated (-~10 wt.% Rh) Rh(NO3)3 solution was added in small (1-2 ml) aliquots to affect Rh +3 ion exchange with IT or Na +. Sufficiem Rh(NO3)3 was added to ensure a 50% excess beyond the total cation exchange capacity of the Na form HTO or HTO:Si. Because of the extreme acidic nature (4-6 M excess HNO3) of the concentrated Rh(NO3)3 solution (pH < 1), small (1-2 ml) additions of 0.6 M NaOH were required between Rh(NO3)3 aliquots to return the slurry pH to the optimum value of--4. After complete addition of the Rh(NO3)3, a 1 h mixing period at pH 4 (with further NaOH additions as necessary) facilitated complete ion exchange. These solution chemistry conditions (i.e., pH) were chosen to maximize the cation exchange capacity of the HTO/HTO:Si support while avoiding precipitation of Rh(OH)3, which occurs at pH values > 4.5. ~5 After washing (with H20 and acetone) and filtering, the catalyst was dried at room temperature under vacuum (75130 mm Hg) for a minimum of 16 h. Similar procedures were used to prepare ion exchanged Rh/HZO and RB/HZO:Si catalyst precursors, although initial acidification was performed at pH 6 due to the higher isoelectric point of the HZO and HZO:Si materials. High weight loading oxide-supported Rh catalysts precursors were also prepared by wet impregnation using an aqueous Rh(NO3)3 solution and as-received TiO2 (Degussa, P25 grade), ZrO2 (Degussa, VP grade), and SiO2 (Cabot Corporation, Cab-O-Sil, HS-5 Grade) as raw materials. A fresh 0.3 M Rh(NO3)3 aqueous solution (from Rh(NO3)3*2H20) was used for impregnation rather than the Rh(NO3)3 solution described above. Oxide support dissolution was minimized at the pH of this impregnating solution (-~1.8), while extensive Rh hydrolysis was avoided due to the minimal time between solution preparation and use. The preparation procedure consisted of aqueous slurry (salt solution + oxide) preparation, drying at 373 K, grinding/mixing via mortar and pestle, and sieving to < 250 ~tm particle size. Table 1 shows the final high weight loading catalyst compositions produced for this study. The Rh loadings obtained for the Rh/HTO and Rh/HTO:Si catalyst samples roughly correspond to 100% Rh +3 exchange for Na +, while the yields for the Rh/HZO and Rh/HZO:Si samples were less than theoretical. The Rh§ ion exchange procedure removed/replaced virtually all of the Na § (> 99 %) from the original Na form HMO ion exchangers. The compositions of the oxide-supported Rh catalysts prepared by incipient wetness impregnation were targeted to match the composition of the HMO-supported Rh catalysts prepared by ion exchange.

2.3. Catalyst Activation, Testing, and Characterization Catalyst activation was performed in situ in a microflow reactor system prior to catalyst activity measurements. The microflow reactor system was composed of a 0.635 cm OD quartz U-tube packed with quartz wool supporting the catalyst (fine powder form). For all activation procedures and activity measurements, total gas pressure in the reactor was 630 mm Hg. For these activity studies, a single as-prepared catalyst sample (0.020-0.050 g) was activated in flowing UHP H2 (20 seem), and evaluated intermittently as a function of time (132 h) at a given activation temperature, with the activation temperature increased stepwise

248 Table 1. Supported Rh Catalyst Compositions Catalyst Rh/SiO2 Rh/TiO2 Rh/ZrO2 Rh/HTO Rh/HTO:Si Rh/HZO Rh/HZO:Si

.....

Wt. % Rh +

Wt.% Na +

Wt.% Rh (Final)*

9.2 9.8 7.1 12.7 13.8 7.3 7.6

NA NA NA 0.03 0.09 0.04 0.02

11.5 12.4 8.9 15.0 16.0 9.0 9.4

Elemental compositions determined by AAS, with weight loadings expressed on an asprepared basis. * Final catalyst compositions assume the removal of all volatiles and that all Rh exists in the metallic state. from 423 K, to 573 K, to 673 K. In certain cases, oxidative heat treatments at various temperatures utilizing He:O2 gas mixtures (20:2 sccm He:O2) were either performed prior to 1-12reduction or used as part of additional oxidation/reduction cycling experiments. The model reaction used to determine the activity of the catalyst materials was n-butane hydrogenolysis, a known structure-sensitive reaction. 16'17 Under steady state reactor conditions, UHP HE and research purity n-butane flow rates to the reactor were 20.0 and 1.0 sccm, respectively, and were selected to prevent catalyst deactivation. Reaction temperatures investigated ranged from 393 to 503 K, depending on the particular catalyst and its state of activation. After 10 min of reactant flow over the catalyst at the desired reaction temperature, product gas analysis (relative to the pure n-butane reactant feed) was determined by on-line gas chromatography with a flame ionization detector. For all activity measurements, the reactor was assumed to operate in a differential mode. After careful passivation, selected catalyst samples were characterized by x-ray diffraction and/or H R E M . 9 HE chemisorption measurements were performed at room temperature (after evacuation to -~10-5 Pa) using an automated gas chemisorption unit (Coulter Omnisorp 100CX). Catalyst activation treatments were performed in situ on as-prepared supported Rh catalyst samples (0.5-0.7 g) using flowing UHP HE (-30 sccm) with an activation temperature/time sequence identical to that used for n-butane hydrogenolysis activity testing. Rh dispersion values were calculated from the total 1-12chemisorption monolayer capacity. 3. RESULTS AND DISCUSSION The results for the various catalysts will generally be grouped in three different sets: titania-supported catalysts (including TiOz, HTO, and HTO:Si supports), zirconia-supported catalysts (including ZrO2, HZO, and HZO:Si supports), and the silica-supported catalyst. Inclusion of the silica-supported Rh catalyst with the other two oxide supports (TiO2 and

249 ZrO2) helps provide a reference point for comparison with the silica-doped HMO (HTO:Si and HZO:Si) supports and to the existing literature. Figure 1 shows the activation behavior in H2, with respect to normalized (per g~) n-butane hydrogenolysis activity measured at a reaction temperature of 423 K, of several oxide- and HMO-supported Rh catalysts. Single samples of the respective catalysts were used to obtain the complete data set, except in the case of the Rh/TiO2 catalyst, where separate aliquots were tested as a function of time at 423 K, 573 K, and 673 K. Previous work with high weight loading Rh/HTO catalysts has shown little difference in catalyst activity between these two testing protocols with the n-butane hydrogenolysis model reaction. 9 For the oxide-supported Rh catalysts, catalyst activation in H2 was fairly rapid at low temperature (423 K), which was consistent with TGA data that showed TiO2-supported Rh(NO3)3 decomposition in H2 is complete within 1 h at 423 K. Once fully activated, both the TiO2- and ZrO2-supported Rh catalysts were relatively stable against deactivation, both as a function of time at a given activation temperature and to some extent with increasing temperature. However, unlike the stable Rh/ZrO2 catalyst, catalyst activity for the Rh/TiO2 catalyst was found to decrease significantly at 673 K. In contrast to the extensive data obtained for the other oxidesupported Rh catalysts, a single data point was obtained for the Rh/SiO2 catalyst atter a 573 K/H2/lh activation treatment. Although this data point is not shown in Figure 1, the normalized n-butane hydrogenolysis activity observed for the Rh/SiO2 catalyst (-7 lamoles nbutane reacted/gp,h.s) was very similar to that observed for the Rh/TiO2 and Rh/ZrO2 catalysts under similar activation conditions.

100 "~ o.=

n,=g ,----

o~

10

.~ ,->"5 o..~

1

~ ~ -o'~

0.1

m

d::

0.01

--O-- 9.2 wt.% Rh/TiO 2 ---E>-- 12.0 wt.% Rh/HTO 13.8 wt.% Rh/HTO:Si ---0-- 7.1 wt.% Rh/ZrO 2 --EF- 7.3 wt.% Rh/HZO 7.6 wt.% Rh/HZO:Si

~ lh8h

~

I lh

8h 32h

/

lh8h

Activation Conditions

Figure 1. Normalized n-butane hydrogenolysis activity as a function of activation conditions for various high weight loading oxide- and HMO-supported Rh catalysts. The activation behavior of high weight loading Rh/HTO and Rh/HTO:Si catalysts proved to be both interesting and complex, and can best be summarized by separation into distinct

250 regimes. First, similar to the oxide-supported Rh catalysts, both catalysts are easily activated at low temperatures (423 K) and short times (-~1 h) in H2. Catalyst activity is stable as a function of activation time at these temperatures. Second, at higher temperatures (573 K), catalyst deactivation is observed as a function of heat treatment time at a given temperature. Finally, in the case of the Rh/HTO catalyst only, at higher temperatures (673 K) some reversal of catalyst deactivation is observed. In contrast, the activity of the Rh/HTO:Si catalyst continues to decline with increasing activation temperature and time in 1-12. Although not shown in Figure 1, the minimum activity observed for the Rh/HTO:Si catalyst was --2 orders of magnitude lower than the Rh/HTO catalyst at the most aggressive activation conditions. It should be noted that the activity values shown in Figure 1 for the Rh/HTO and Rh/HTO:Si catalysts after activation at 673 K were too low to be accurately measured. Instead, they were calculated from data taken at higher reaction temperatures using the calculated Arrhenius parameters for the specific catalyst at those activation conditions. The activity data for the Rh/HZO and Rh/HZO:Si catalysts were significantly different from the Rh/HTO and RN O:Si catalysts. Both of these catalysts showed no degradation in activity with more aggressive activation conditions. In fact, a increase in catalyst activity was observed upon increasing the activation temperature from 423 to 573 K. The Rh/HZO:Si catalyst was significantly more active than the Rh/HZO catalyst, being the most active catalyst tested in this study, even with respect to the oxide-supported Rh catalysts. It is well known that highly dispersed Rh particles result in preferential central bond scission for both n-butane and n-pentane hydrogenolysis. 18'19 Therefore, analysis of the product distribution under similar activation conditions can provide helpful insight regarding the state of Rh dispersion on the various supports. For conventionally prepared catalysts, it would be expected that the metal dispersion would decrease significantly with increased metal weight loading due to increases in metal particle size. Since changes in particle size can result in significant changes in the number of surface atoms exposed on various low index crystallographic planes, 2~marked changes in product selectivity can result. A recent study has examined the changes in activity and product selectivity for n-butane hydrogenolysis over single crystal and supported Rh catalysts. TM Ethane selectivity was shown to decrease from -~80 mole % ethane (for highly dispersed Rh) to --50 mole % ethane (for single crystal Rh (111) surfaces) with increasing Rh weight loading (and therefore particle size). Table 2 shows the ethane selectivity of the oxide- and HMO-supported Rh catalysts as a function of activation conditions (temperature and time) in H2. Similar to the data shown in Figure 1, the ethane selectivity values correspond to a reaction temperature of 423 K. Under these conditions, the ratio of methane to propane was approximately unity (-~1.0-1.2) in all cases, which is consistent with a single hydrogenolysis event. For low temperature activation (< 573 K), ethane selectivity was found to be relatively independent of activation time and extremely low values for ethane selectivity (--10-15 mole % ethane) were observed for the TiO2- and ZrO2-supported Rh catalysts. Some improvement in the ethane selectivity of these catalysts was observed with more aggressive activation conditions. In contrast, the ethane selectivity of the Rh/SiO2 catalyst was higher (--50 mole %), similar to that predicted from previous work. TM Additional experiments indicated that the abnormally low ethane selectivity data observed for these Rh/TiO2 and Rh/ZrO2 catalysts cannot solely be attributed to activation in 1-12, since similar data were observed after a more traditional activation procedure (i.e., calcination at 773 K followed by H2 reduction). 9 It is therefore likely that the choice of Rh precursor, the

251 resulting wet impregnation solution chemistry conditions, or the reduction temperature used in the catalyst activation procedure had a significant effect on the resulting ethane selectivities. Table 2. Ethane selectivity values (mole %) for n-butane hydrogenolysis at a reaction temperature of 423 K for various supported Rh catalysts over a range of activation co.nditions.

Activation 423 K/1 h 423 K/8 h 573 K/1 h 573 K/8 h 573 K/32 h 673 K/1 h 673 K/8 h

SiO2 NA NA 49.0 NA NA NA NA

Ti02 10.1 11.5 18.2 18.4 16.3 24.5 25.9

Rh Catalyst Support HTO HTO:Si ZrO2 70.5 71.7 14.8 NA NA 16.8 79.5 79.1 20.6 NA 80.4 19.9 76.6 78.7 18.4 54.9 * 18.3 56.0 * 18.1

I-~O 47.1 52.1 51.2 51.4 52.1 50.1 43.6

HZ0:Si 47.9 61.5 66.1 67.3 67.7 69.9 69.5

NA Not applicable. Testing not performed for these specific activation conditions. * Catalyst activity was too low to allow testing at a reaction temperature of 423 K. For all activation conditions, the HMO-supported Rh catalysts proved to be fairly selective to ethane. The minimum and maximum ethane selectivity values observed were --50 mole % (for Rh/HZO) and --80 mole % (for Rh/HTO:Si). In contrast to the oxide-supported Rh catalysts, these high ethane selectivities were obtained under mild activation conditions and found to be relatively stable after exposure to more aggressive conditions. These high ethane selectivities are indicative of well dispersed Rh particles. TM At the highest activation temperature (673 K), the ethane selectivity of the Rh/HTO and Rh/HZO catalysts decreased, but was still superior to that of the Rh/TiO2 and Rh/ZrO2 catalysts. The decrease in ethane selectivity at these conditions may be consistent with some Rh particle growth. H2 chemisorption measurements were performed on the oxide- and HMO-supported Rh catalysts to determine the Rh dispersion and the total number of active sites as a function of catalyst activation in H2. The Rh dispersion data for the different supported Rh catalysts are shown in Figure 2, and illustrate several interesting trends. First, under low temperature (423 K) activation conditions in H2, the Rh dispersions are fairly high (-25-60 %) for all of the catalysts, and remain stable as a function of activation time. The Rh dispersion for the Rh/HMO catalysts is superior to the oxide-supported Rh catalysts at these low temperature activation conditions. Significant differences are observed among the different support chemistries upon exposure to more aggressive activation conditions. For the TiO2-, HTO-, and HTO:Si-supported Rh catalysts, Rh dispersion is observed to significantly decrease with increasing activation temperature and time at a given activation temperature. This behavior is most pronounced for the Rh/HTO and Rh/HTO:Si catalysts, which both exhibit large decreases in Rh dispersion (factors of 20-50x) after 673 K activation relative to 423 K activation in H2. For comparison, the Rh/TiO2 catalyst exhibits a factor of-2.5x decrease in Rh dispersion over the same range of activation conditions.

252 100

r--F---r

Legend

lO

9.2 wt.% Rh/SiO 2

g

- - O - - 9.8 wt.% Rh/TiO 2 12.0 wt.% Rh/HTO - - t - - 13.8 wt.% Rh/HTO:Si - - D - - 7.1 wt. % Rh/ZrO 2 ---EF- 7.3 wt.% Rh/HZO --I7.6 wt.% Rh/HZO:Si

u'J

i::3

1

0.1

lh8h

l h 8h 32h l h 8 h Activation Conditions Figure 2. The effect of activation conditions on dispersion values for various high weight loading oxide- and HMO-supported Rh catalysts. In contrast to the behavior observed for the TiO2-, HTO-, and HTO:Si-supported Rh catalysts, the Rh dispersion of the Rh/ZrO2, Rh/HZO, and Rh/HZO:Si catalysts was very stable over a wide range of activation conditions, comparing well with that of the Rh/SiO2 catalyst. The HZO- and HZO:Si-supported Rh catalysts appear to offer a slight benefit in terms of higher dispersion than the Rh/ZrO2 catalyst. Combining this data with the n-butane hydrogenolysis activity data shown in Figure 1 clearly indicates that the Rh/HZO and Rh/HZO:Si catalysts are superior to the Rh/HTO and Rh/HTO:Si catalysts for applications requiring catalyst stability in reducing environments. Compared to the oxide-supported catalysts, the Rh/HZO:Si material offers enhanced selectivity with no penalty in terms of catalyst activity. Considering that the trends and magnitudes of change between Figures 1 and 2 are very similar over the range of activation conditions, one could infer that turnover frequency values for n-butane hydrogenolysis based on the number of active sites determined by H2 chemisorption would not be significantly different for the various supported Rh catalysts at any selected activation condition, which was indeed the case. Two explanations exist which could possibly explain the significant decrease in both catalyst activity and Rh dispersion at the higher temperature activation conditions for the TiO2-, HTO-, and HTO:Si-supported Rh catalysts: 1) strong metal-support interaction (SMSI) phenomena; and 2) significant changes in Rh particle size. The existence of SMSI phenomena in transition metal oxide-supported metal catalysts after high temperature reduction was first characterized by a near total loss of metal chemisorption capacity for H2 and CO. 1~ As a consequence of the reduced chemisorption in the SMSI state, large decreases in reaction rates, otten several orders of magnitude, have been observed for alkane hydrogenolysis reactions utilizing transition metal oxide-supported metal catalysts. 22'23 For Rh/TiO2 catalysts, it has been shown by numerous investigations that high temperature reduction at 773 K results in significant TiOx migration onto Rh particles, thereby blocking active sites and causing a loss of activity in a number of catalytic reactions. 24"27 SMSI

253 phenomena has primarily been observed with highly dispersed Rh/TiO2 catalysts, since smaller Rh particle size allows more rapid active site blockage by TiOx migration. However, the work of Miessner, et. al., 28 has shown that SMSI effects are observable via H2 chemisorption even for very high weight loading (> 25 wt.% Rh) Rh/TiO2 catalysts. The lack of observed SMSI phenomena in this study for the ZrOz-, HZO-, and HZO:Si-supported Rh catalysts is indicative of a lack of reducibility of the zirconia support, consistent with previous work. 2~ In order to further probe these potential explanations, HREM was performed on both the Rh/TiO2 and Rh/HTO catalyst samples exposed to both low (423 K) and high (673 K) temperature activation in H2. These mierostructural results, which are not shown herein due to space limitations, did not show a significant difference in Rh particle size with increasing activation temperature for either the high weight loading Rh/TiO2 or the Rh/HTO catalysts. 9 If the decrease in H2 chemisorption uptake observed on increasing the activation temperature from 423 to 673 K was due solely to Rh particle growth, large increases in Rh particle size (at least of a factor of-~4x) would have been expected. Since no significant increase in Rh particle size was observed by HREM, the decrease in H2 ehemisorptive uptake can most likely be attributed to SMSI-related phenomena, i.e., TiOx migration over Rh particles. Another supporting factor for the lack of significant Rh particle growth is that relatively little change was observed in ethane selectivity values for the Rh/HTO and Rh/HTO:Si catalysts as activation temperature was increased. An interesting aspect of the HTO and HTO:Si supports is that both of these initially amorphous materials are easily reducible and susceptible to the TiOx migration associated with SMSI phenomena even at low temperatures (573 K) relative to conventional crystalline TiO2 supports. 9'29'3~ Consistent with previous work, 25'27'3~various oxidation procedures, followed by low temperature reduction, were used to destroy the SMSI state, resulting in normal catalytic activity and higher Rh dispersions. In the ease of the Rh/HTO catalysts, we believe that the partial reversal of the SMSI state also results from crystallization of the amorphous HTO support into anatase or rutile, which explains the reversal of the decrease in n-butane hydrogenolysis activity observed as a function of activation time at 673 K (see Figure 1).9 No such reversal occurs for the Rh/HTO:Si sample because it remains amorphous under these activation conditions due to the presence of the SiO2 dopant. 4. SUMMARY This purpose of this study was to evaluate high weight loading HMO- and conventional oxide-supported Rh catalysts. Over a wide range of HMO support compositions, highly dispersed supported Rh catalysts were produced after low temperature activation in H2 as evidenced by high n-butane hydrogenolysis activity, high ethane selectivity, and high H2 chemisorptive uptake. In particular, the ethane selectivity values were significantly higher for the HMO-supported Rh catalysts than the TiO2- and ZrO2- supported Rh catalysts. For the TiO2-, HTO-, and HTO:Si-supported Rh catalysts, a significant loss of both catalyst activity and Rh dispersion was observed at more aggressive activation conditions, consistent with TiOx migration typically associated with SMSI phenomena. Superior activity and stability were observed for the ZrO2-, HZO-, and HZO:Si-supported Rh catalysts over a wide range of activation conditions in H2. Of all the Rh/HMO catalysts, the Rh/ O:Si catalysts appear to offer the best tradeoff in terms of high Rh dispersion, high activity, and high selectivity.

254 5. R E F E R E N C E S

1. H. P. Stephens, R. G. Dosch, and F. V. Stohl, Ind. Eng. Chem. Res. Devel., 24 (1985) 15. 2. H.P. Stephens and R. G. Dosch, Stud. Surf. Sci. Catal., 31 (1987) 271. 3. R. G. Dosch, H. P. Stephens, and F. V. Stohl, Sandia Report, SAND89-2400, Sandia National Laboratories, Albuquerque, NM (1990). 4. R. G. Dosch and L. I. McLaughlin, Sandia Report, SAND92-0388, Sandia National Laboratories, Albuquerque, NM (1992). 5. S. E. Lott, T. J. Gardner, L. I. McLaughlin, and J. B. Oelfke, Fuel, 75 (1996) 1457. 6. T. J. Gardner, C. H. F. Peden, and A. K. Datye, Catal. Lett., 15 (1992) 111. 7. S.L. Anderson, A. K. Datye, E. J. Braunschweig, and C. H. F. Peden, Appl. Catal. A, 82 (1992) 185. 8. A. G. Sault and E. P. Boespflug, Sandia Report, SAND96-1889, Sandia National Laboratories, Albuquerque, NM, 1997. 9. T. J. Gardner, Ph.D. Dissertation, University of New Mexico, 1994. 10. G. L. Hailer and D. E. Resasco, Adv. Catal., 36 (1989) 173. 11. S. A. Stevenson, J. A. Dumesic, R. T. K. Baker, and E. Ruckinstein (eds.), Metal-Support Interactions in Catalysis, Sintering, and Redispersion, Van Nostrand Reinhold, New York (1987). 12. R. G. Dosch, H. P. Stephens, F. V. Stohl, B. C. Bunker, and C. H. F. Peden, Sandia Report, SAND89-2399, Sandia National Laboratories, Albuquerque, (1990). 13. T. J. Gardner and L. I. McLaughlin, Mater. Res. Sor Symp. Proc., 432 (1996) 249. 14. T. J. Boyle, T. L. Alam, and T. J. Gardner, unpublished work. 15. C. F. Baes, Jr. and R. E. Mesmer, The Hydrolysis of Cations, John Wiley and Sons, New York (1976). 16. M. Boudart, Adv. Catal., 20 (1968) 153. 17. J. R. Engstrom, D. W. Goodman, and W. H. Weinberg, J. Am. Chem. Sor 110 (1988) 8305. 18. D. Kalakkad, S. L. Anderson, A. D. Logan, J. Pena, E. J. Braunschweig, C. H. F. Peden, and A. K. Datye, J. Phys. Chem., 97 (1993) 1437. 19. H. C. Yao, Y. F. Y. Yao, and K. Otto, J. Catal., 56 (1979) 21. 20. R. Van Hardeveld and F. Hartog, Surf. Sci., 15 (1969) 189. 21. S. J. Tauster, S. C. Fung, R. T. K. Baker, and J. A. Horsley, Science, 211 (1981) 1121. 22. E. I. Ko and R. L. Garten, J. Catal., 68 (1981) 233. 23. D. E. Resasco and G. L. Hailer, in B. Imelik, et. al. (eds), Metal-Support and Metal Additive Effects in Catalysis, Elsevier, Amsterdam (1982) 105. 24. D. E. Resasco and G. L. Hailer, J. Catal., 82 (1983) 279. 25. P. Meriaudeau, O. H. Ellestad, M. Dufaux, and C. Naccache, J. Catal., 75 (1982) 243. 26. D. E. Resasco and G. L. Hailer, J. Phys. Chem., 88 (1984) 4552. 27. J. B. F. Anderson, R. Burch, and J. A. Cairns, Appl. Catal., 25 (1986) 173. 28. H. Miessner, S. Naito, and K. Tamaru, J. Catal., 94 (1985) 300. 29. A. G. Sault, C. H. F. Peden, and E. P. Boespflug, J. Phys. Chem., 98 (1994) 1654. 30. A. G. Sault, J. Catal., 156 (1995) 154. 31. E. J. Braunschweig, A. D. Logan, A. K. Datye, and D. J. Smith, J. Catal., 118 (1989) 227.

91998 Elsevier Science B.V, All fights reserved, Preparation of Catalysts VII B. Delmonet al., editors.

255

Rhodium carbonyl catalysts, immobilized on polymeric supportes in the hydroformylation of olefins. G. V. Terekhova, N. V. Kolesnichenko, E. D. Alieva, N. I. Truhmanova, A. T. Teleshev, N. A. Markova, E. I. Alekseeva, E. V. Slivinsky, S. M. Loktev and O. Yu. Pesin. A. V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences, 29. Leninsky prospect, 117912 Moscow, B-71, Russia* Abstracts

The hydroformylation of olefins in presence catalytic systems on a basis RhC13*3H20 and a copolymer of styrene with 4-N-pyrrolidinopyridine units in a main chain and acacRh(CO)2 and polymeric organosiloxanes was investigated. These catalytic systems have high stability and activity. The hydroformylation of isobutylene in the presence of such systems proceeds with high speed in conditions, when traditional homogeneous catalysts unactivity. It was showed by ESCA, that complexing of rhodium with pyrrolidinopyridine units of copolymer occurs on atom of nitrogen of pyridine ring. The influence of composition and structure of polymeric supports on creation of active centers of catalysts and their catalytic properties was investigated. The influence of solvents on catalytic properties of polymeric catalysts was studied. Introduction

One of the important oxosynthesis directions is the development of ways heterogenization of catalysts allowing to involve in this process medium- and high- molecular olefins. There is a set of the approaches to designing of immobilized metallocomplexes. In the present paper the most perspective of them are used" immobilization of metal at the expense of functional groups of the support and inclusion metallocomplexes .

.

.

.

.

.

*This work is supported by Russian Basic Research Foundation (RBRF), Grant 97-03-32352a

256 in a polymeric matrix. The chemical modification copolymer of styrene with maleic anhydride by condensation with 4-aminopyridine leads to obtain the polymeric support with a various amounts of 4-N-pyrrolidinopyridine units. Since the nitrogen atom in the pyrrolidinopyridine units of such polymer has a high basicity and capable to derivate complexes of different structures with transition metals, one can expect that this matrix not only will firmly hold the rhodium atoms, but it will also effect on catalytic properties. Other interesting method immobilization is overlapping a stage of synthesis of polymer and immobilization metallocomplex, when the complex of metal initiating of polymerization remains in the composition of polymer. An example of such immobilization are rhodium complexes wit.h polyorganosiloxanes. This method allows to regulate a dispersibility of complexes on the supports and to create a polymeric shell around of isolated ions or agglomerates. In the present paper is studied the hydroformylation of olefins 1. in presence catalytic systems on a basis RhCla and copolymer styrene with 4-N-pyrrolidinopyridine units in a main chain with molecular weight 70000. The general formula of polymer is: 2. in presence catalytic systems on a basis acacRh(CO)2 and polyorganosiloxanes. The influence of composition and structure of polymeric supports on creation of active centers of catalysts and their catalytic property is investigated. The effect of solvents on catalytic properties of polymeric catalysts is studied.

Experimental part Hydroformylation of 1-hexene was carried out in a 0.25-L stainlesssteel autoclave equipped with a stirrer. The reaction was performed in the batch mode at 120~ and 6 MPa. The CO:H2 molar ratio was 1:1. The reaction rate was monitored taking into account the amount of converted gas, which was measured from the decrease in the pressure in a calibrated vessel (gas was fed from this vessel to the autoclave as gas was consumed in the reaction mixture). The reaction rate was determined in the kinetic region of the process. The reactions were conducted until 95% conversion of olefins. The reaction products were analyzed on a Chrom5 chromatograph (capillary column, 50 m; p h a s e - PEG 20 M, helium as carrier gas, T - 110~ RhCI3 93H20 and acacRh(CO)2 were ltsed as precursors of the rhodium catalyst of hydroformylation. Rhodium acetylacetonate [1], 4-ethyl-2,6,7-triioxa-l-phosphabicyclo-[2,2,2]-octane

257 (ETPO) [2] were prepared using known procedures. R h o d i u m - containing polyorganosiloxane (PS) catalysts were prepared from the corresponding compounds: from oligovinylsiloxanes with methyl and phenyl substituents at the Si atom, oligohydrosiloxanes with methyl and phenyl substituents at the Si atom and the rhodium complex acacRh(CO)2. Oligoorganosiloxanes were prepared according to the known procedure [.3]. Results

Polymeric pyrrolidinopyridines (SPP) with 4-N-pyrrolidinopyridine (PP) groups in the backbone were synthesized [4] by the condensation of the alternating copolymer of styrene and maleic anhydride (MM = 70000) with appropriate amounts of 4-aminopyridine followed by the reduction of the carbonyl groups of the pyridilmaleimide rings. Starting copolymer of styrene and maleic anhydride (STM) was synthesized by radical copolymerization of styrene and maleic anhydride [5] with 0.1 wt% benzoyl peroxide as initiator in the absence of solvent at 60~ STM is dissolved in THF and DMF, q=5.29 g/dl (DMF, 25~ Frequences of the main bands of functional groups in the FTIR absorption spectra of synthesized polymers are given in Table 1. Table 1 Frequences of main bands in the FTIR-spectra of polymers Wavenumbers, c m Carboxyanhydride Polymers groups

1

"

_ N( / \

c=o of

styrene 1780,1850 720, 750, 1470,1500 1600,1650 1770,1720 . . . . . . .

STM STMP SPP

925, 1125 1380

1600,1650

-"-

In the F T I R - spectra of STMP have been observed the appearance of the absorption bands of nitrogen of pyridine cycles at 1600 and cm -1, of C-N

258 bonds of imide groups (1380 cm -t) and disappearance of the absorption bands of carboxyanhydride groups (925, 1225 cm-1). In the F T I R spectra of SPP are distingly observed the disappearance of the absorption bands of C=O groups (1720, 1770 cm -1) along with the retainment of absorption bands of pyridine nitrogen and C-N bond of imide groups. The composition and the properties of the resulting polymers were studied These compounds represent themselves the linear alternating copolymers, which are capable of formation the partially crosslinked, swelling in nonpolar media polymeric systems. It is found that SPP has a high basicity (pKa =9.9) and significant thermic stability (the temperature of initial weight loss was above 350~

180 - 160 - l,'-

140

-

.o 1 2 0 - 0 .13

100

.~

80

~

60

CO

+ 2

- -

~ 3

-

--x 4

40 20

.-----x

0

, 0

0,5

I 1

2

3

timelhs

Figure 1. Hydroformylationof isobutylene in the presence of catalytic systems- 1, RhCla-SPP; 2, acacRh(CO)2-SPP; 3, RhC13-PP; 4, acacRh(CO)2+

ETPO (P/Rh=9). The complexing of rhodium compounds with the SPP was carried out in the water or methyl alhogol. It was found that the composition of complexes had not more three units containing nitrogen atoms. The composition of complexes is changed in dependence of a degree of poty-

259 mer functionalization. The ratio of metal to macroligand is differed in connection with reduced or unreduced forms of initial copolymers. The degree of intermolecular interaction of metal with polymer is incre&sed in the case of unreduced form. It was shown by ESCA that complexing of rhodium with SPP and their low-molecular analog (PP) took place at nitrogen of pyridine cycle. The bond energy N1S increases (0.4-0.7 eV in dependence of the structure of polymer ligand). The complexes of RhCla, 3H20 and acacRh(CO)2 with SPP were studied in the hydroformylations of isobutylene and hexene-1. As can be seen from Fig.1 (curves 1 and 2), the hydroformylation of isobutylene in the presence of rhodium complexes with SPP proceed at a high rate independently of the catalyst precursor. 160 140 c-

120

O ..,,,.

~.100

-*--1

0 U)

+2

.a 8 0 ~l - -

:

3

60 40 20

0

0,5

1

1,5

2

3

time/hs

Figure 2. Hydroformylation of hexene-1 in the presence of catalytic systerns" 1, acacRh(CO)2+ETPO (P/Rh=9; 2, RhCla-SPP; 3, RhCla-PP (PP/Rh=2). Catalytic systems do not lose their activity after the reaction products are eliminated and recycled. A different picture is observed in homogeneous hydroformylation in the presence of Rhacac(CO)2, modified by

260 ETPO. In this case the hydroformylation of isobutylene proceeds at a low rate (Fig. 1, curve 4). The hydroformylation of hexene-1 (Fig.2, curve 2) in the presence of RhC13-SPP has an induction period; in this case the synthesis rate is substantially lower than with the use of Rhacac(CO)2+ ETPO (Fig.2, curve 1). The hydroformylation of hexene-1 proceeds at a higher rate (Fig.2, curve 1) than the hydroformylation of isobutylene in the presence of conventional Rh-catalysts (Fig.l, curve 4). The picture is changed by heterogenization of the Rh-complexes: hydroformylation of hexene-1 in the presence of the RhCla-SPP system proceeds more slowly than hydroformylation of isobutylene. This inversion occurs for a number of reasons, in particular, because of the different affinity of the polymer for the substrate, the high basicity of nitrogen in the pyrrolidinopyridine fragment [1], etc. In this connection, it was of interest to compare hydroformylation of olefins in the presence of heterogeneous RhCla-SPP catalysts with hydroformylation in the presence of the rhodium complex of pyrrolidinopyridine, a low-molecular analog of the polymeric catalytic systems under study. The rate of hydroformylation of isobutylene and hexene-1 in the presence of RhCla-PP (Figs.1 and 2, curves 3), all things being equal, was established to be significantly lower than in the presence of RhCla-SPP. In this case hydroformylation of isobutylene also proceeds at a higher rate than hydroformylation of hexene-1. The result obtained allows one to assume that the high basicity of the nitrogen atom in the pyrrolidinopyridine fragment of the copolymer significantly affects the ra.tio of the rates of hydroformylation of hexene-1 and isobutylene. The fa.ct that the rates of hydroformylation of olefins in the presence of the RhC13-PP catalytic system are lower than that with a polymeric catalyst can be associated with the fact that the amount of pyrrolidinopyridine is insufficient to prevent transition of the active rhodium complex into polynuclear complexes [6]. The effect of solvents on the catalytic properties of RhC13-SPP was studied. Replacing p-xylene with pyridine results in a sharp increase in the reaction rate and in a decrease in the induction period. The reaction rate is significantly decreased in the presence of other solvents. The high activity of RhC13-SPP in the presence of pyridine and p-xylene can be explained by the good swelling of the polymer in these solvents, which makes it easier for the substrate access to active centers. The other interestig catalysts are rhodium complexes immobilized on the polymeric organosiloxanes. These catalysts were prepared by the polyaddition reaction of oligovinylsiloxanes and oligohydrosiloxanes. The solid-state 29Si NMR spectrum exhibited two tipes resonanses. The peak

261 at -18.00 ppm was assigned to the-O[(CH3)(-CH2CH2-)SiO]- units. The signal at -66.12 ppm was assigned to the-O[(CH3)(O-)SiO]- units [7], which may be formed due to dehydrogenative coupling followed by oxidation of the Si-Si bond. The ESCA showed that rhodium presented in the polymeric matrix as Rh(I) complexes. Rh-PS catalysts were studied in the hexene-1 hydroformylation. Heterogeneous catalysts have the advantage that they can be used many times without loss of their activity. In this connection~ all catalysts were tested for stability in a series of successive runs with intermediate separation of the reaction products from the catalyst. Preliminary studies of swelling of the polymers in various solvents demonstrated that the polymers exhibit high affinity for aromatic and aliphatic solvents. The weight of the polymer in p-xylene~ hexane, and hexene-1 was almost doubled even in lh, whereas in aldehydes, the identical increa.se in the weight was attained in 24 h. Polymers virtually did not swell in dioxane, water~ and allyl alcohol. 2,5

"

g

"7

E

"7, _..! 0

1,5

"

1

E

0

.,

-=-2

m

---=- 3 • 4

i,,,-

0,5--

0

I

I

I

1

2

3

'"

I

' I

4

5

number of recycles

Figure 3. Effect of the concentration of acacRh(CO)2 in the phenylcontaining polymer on the catalytic properties of the system in hydroformylation of hexene-1.

262 The effect of the concentration of acacRh(CO)2 in the polymer on the synthesis and the catalytic properties of acacRh(CO)2,PS was studied. Phenyl-containing oligosiloxanes were used as components of the catalytic system. The results are given in Fig.3. The reaction rate increased as the rhodium content of the polymer increased. However, durable hydroformylation of hexene-1 can be carried out only in the presence of catalytic systems in which the rhodium content is (0.03-0.35),10 -3 g of Rh/g of the polymer. At higher concentrations of rhodium, potyaddition was accompanied by foaming, resulting in formation of nonhomogeneous samples. In this case, the polymer lost elasticity and became more friable. A decrease in the stability of the catalyst at a low rhodium content (0.03,10 .3 g of Rh/g of PS) is apparently associated with the fact that the polymer was partially converted to the gel state. Apparently, at low concentrations of rhodium, the polymer formed has a lower

,5 --

m

"7 e-

E

1,5-

,,,,.,.

~2

o

E

O

T,...o

1"

0,5o |

| l

i I

I |

i II

1

2

3

4

5

i i

i II

number of recycles

Figure 4. Effect of the functional groups of organosiloxane on the catalytic properties of the system acacRh(CO)2,PS in hydroformytation of hexene-l" 1, phenyl groups; 2, methyl groups. degree of cross-linking and becomes soluble.

At the rhodium content

263 of > 0 . 3 5 . 1 0 -3 g of Rh/g of PS, the polymer lost its stability due to the polymer destruction and formation of rhodium complexes inactive in hydroformylation [6]. The catalytic properties of the systems are substantially affected by the nature of the oligomers. As can be seen from Fig.4, the use of oligomers with methyl substituents instead of phenyi-containing oligosiloxanes resulted in a decrease in the activity and loss of stability. Apparently, this is associated with the fact that when phenyl substituents were replaced by methyl substituents, more rigid and friable polymers were formed with a lower degree of swelling in organic solvents. A change in the physical properties of the polymer is reflected in the catalytic properties of the system. A low degree of swelling results in a decrease in the activity, and friability leads to mechanical destruction of the catalyst in the course of the reaction and deactivation of the rhodium complexes. Polyaddition of oligosiloxanes can be carried out with the weight ratio of oligomers varied over a wide range. However, a deficiency or an excess of oligohydrosiloxanes has a substantial effect on the physical properties of the polymer obtained. The best results were obtained when we used catalytic systems based on polymers with oligovinylsitoxane 9oligohydrosiloxane weight ratios in the range from 1"1 to 1:2. When the vinyl oligomer content was increased, the catalyst, which was noticeably soluble in the reaction mixture at high temperature, formed. In the case of an excess of the hydro-containing oligomer, the polymer became friable, which also impaired the stability of the catalytic system. References

1. Yu. S. Varshavskii and T. G. Cherkasova, Zh. Neorg. Khim., 12 (1967) 1709. 2. E. E. Nifant'ev and I. M. Petrova, Zh. Obshch. Chim., 40 (1970) 2196. 3. E. I. Alekseeva, S. R. Ninush'yan, and A. B. Polees, Khim. promyshl.,

(1955)53. 4. E. D. Alieva, N. I. Truhmanova, S. A. Shuvalova, and N. A. Plate' Docl. Chem., (1990) 314. 5. D. J. Hill, J. O'Donell, and P. W. O'Sullivan, Macromolecules, 18 No.1 (1985) 9. 6. Yu. B. Kagan and E. V. Slivinskii, Neftekhimiya, 25 (1986) 791. 7. N. Satyanarayana and H. Alper, Macromolecules, 28 (1995) 281.

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91998ElsevierScienceB.V.All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

265

Novel Preparation Method for Supported Metal Catalysts Using Microemulsion - Control of Catalyst Surface Area M. Kishida, T. Hanaoka, H. Hayashi, S. Tashiro, and K. Wakabayashi. Chemical Engineering Group, Department of Materials Process Engineering, Graduate School of Engineering, Kyushu University, Higashiku, Fukuoka, 8128581, Japan. In this work, it was found that the surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600m2/g by using the preparation method which we have developed using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. By using these catalysts, it was also found that turnover frequencies for CO2 hydrogenation increased linearly with the catalyst surface area.

1. I N T R O D U C T I O N Although it is important, it is difficult to study the effect of catalyst surface area on the catalytic behavior because it has been impossible to control only catalyst surface area by using a conventional method for catalyst preparation. For example, the metal particle size of the catalyst probably becomes small if the metal salts were impregnated on the support with a large surface area. On the other hand, we have developed a novel method for catalyst preparation using water-in-off (w/o) microemulsion. By using our method, the metal particle size of the catalyst has been controlled regardless of metal content [1,2]. The objectives of this work are to control the catalyst surface area of silica-supported rhodium catalysts regardless of rhodium particle size.

2. E X P E R I M E N T A L Rh/SiO2 catalysts were prepared using c e t y l t r i m e t h y l a m m o n i u m chloride (CTAC) / 1-hexanol / RhC13 aq. w/o microemulsion in the same manner as reported previously [1-3]. The concentration of CTAC in 1-hexanol and the aqueous concentration of RhC13 were 0.5 and 0.19 mol/dm 3, respectively. The water-to-surfactant molar ratio in the s t a r t i n g m i c r o e m u l s i o n was 12. The Rh-N2H4-complex nanoparticles were formed in the microemulsion by adding hydrazine directly. After tetraethylorthosilicate (TEOS) as the silica source and a diluted ammonium solution were added to the microemulsion containing the nanoparticles, silica gel containing the nanoparticles were precipitated by the hydrolysis of TEOS. The pre-

266 cipitates were filtered, thoroughly washed by ethanol, dried at 80 ~ overnight, and calcined under air flow at 500 ~ for 2 h. The catalyst thus obtained was pelleted, crushed, sized to ca. 16-24 mesh, and reduced at 450 ~ for 2 h. This preparation method will be denoted by ME method. The catalyst surface areas were determined using the BET equation from the nitrogen isotherms at 77 K. The rhodium particle was characterized by X-ray diffraction (XRD, Rigaku, RINT2500), transmission electron micrography (TEM, Nihon Denshi, JEM-2000FX). Rhodium particle size was determined by the broadening technique. 3. R E S U L T S AND D I S C U S S I O N 3.1 E f f e c t s o f h y d r o l y s i s c o n d i t i o n s o n c a t a l y s t s u r f a c e a r e a In the ME method, silica as a support was formed by the hydrolysis of TEOS. Thus, in order to control a catalyst surface area, it is necessary to prepare catalysts changing the hydrolysis conditions of TEOS and to investigate the change in surface area of the resulting catalysts. Table 1 shows the relationship between hydrolysis conditions and the catalytic properties. The amount of TEOS charged considerably affected the BET surface area of the catalysts prepared by the ME method. It was found that the BET surface area increased with decreasing the amount of TEOS. The silica yield, which is defined as {weight of silica obtained}/{weight of silica w h e n all the TEOS charged were converted into silica}, was independent of the a m o u n t of TEOS. This result indicated t h a t the a m o u n t of silica obtained was approximately proportional to the

Table 1 Catalyst preparation condition and the catalytic properties.. Catalyst

Composition before hydrolysis Solution [cm 3]

TEOS [g]

NH3 aq. a) [Vol%]

[NH3]aq a) [mol/dm 3]

Silica yield [%]

Rh cont. BET S.A. [wt%] [m2/g]

191 191 191 191

10 20 30 50

30 30 30 30

13.5 13.5 13.5 13.5

94 94 98 93

6.3 3.1 2.1 1.2

420 240 140 31

191 191 191 191 191 191 191

50 50 50 50 50 50 50

30 30 30 39 30 39 39

2.9 4.7 6.8 8.8 10.3 11.7 13.5

55 69 78 80 92 86 93

2.0 1.6 1.4 1.4 1.2 1.3 1.2

700 605 413 327 141 68 31

267 amount of TEOS charged. Here, the amount of rhodium contained in the s t a r t i n g solution was kept constant at 4.56x10 -3 tool. Consequently, the rhodium content of the resulting catalyst increased with decreasing the amount of TEOS. The aqueous concentration of NH3 before adding TEOS also greatly affected the BET surface area of the resulting catalysts. It was noteworthy that the BET surface area extremely increased with decreasing the concentration of NH3 and was 700 m2/g at the NH3 concentration of 2.9 mol/dm 3. Here, the amount of TEOS charged was always 50 g, but the rhodium content was changed because silica yield was changed with the NH3 concentration. As it was generally known t h a t both increasing the amount of TEOS and the concentration of NH3 promote the hydrolysis rate, these results show that the silica prepared at a faster hydrolysis rate has a smaller BET surface area. The surface area of the catalyst was changed in this way, however the rhodium content was also changed at the same time. Thus, we made an a t t e m p t to control only the surface area at a constant rhodium content. From Table 1, the catalyst surface area and the rhodium content were found to be a function of both the amount of TEOS and the NH3 concentration as follows; (Surface area) = -2.2 x (amount of TEOS) + 62 x (NH3 conc.) + 1071 (Rh cont.) = -0.027 x (amount ofTEOS) - 0.024 x (NH3 conc.) + 2.91

(1) (2)

The amount of TEOS and the NH3 concentration were determined by the eqs. (1) and (2) for the purpose of controlling the catalyst surface area with a constant Rh content. Consequently, as shown in Table 2, the catalyst surface area could be controlled in the range between 68 and 605 m2/g. The rhodium particle sizes and the rhodium contents of all the catalysts were approximately 5 nm and 1.6 wt%, respectively. 4. C O N C L U S I O N The catalytic surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600 m2/g by the novel preparation m e t h o d

Table 2 Control of catalyst surface area. Target Rh cont. BET S.A. [wt%] [m2/g] 1.6 1.6 1.6 1.6 1.6

50 150 300 400 600

Condition

Result

TEOS [g]

[NH3]aq [mol/dm 3]

Rh cont. [wt%]

BET S.A. [m2/g]

Rh size [nm]

32.4 33.9 36.1 37.6 40.6

15.4 13.7 11.1 9.6 6.0

1.5 1.6 1.6 1.7 1.6

68 118 299 371 605

4.9 5.0 4.4 4.8 5.0

268 using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. REFERENCES

1. M. Kishida, K. Umakoshi, W.Y. Kim, T. Hanaoka, H. Nagata, K. Wakabayashi, Kagaku-kogaku Ronbunshu, 21,990 (1995). 2. M. Kishida, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Catal Today, 29, 355 (1996). 3. M. Kishida, T. Fujita, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Chem. Soc., Chem. Commun., 1995, 763.

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

269

Study of rhenium deposition onto Pt surface with electrochemical methods S. Szab6 and I. Bakos Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest P.O. Box 17, Hungary

ABSTRACT

The deposition of rhenium species from ReO'4 ion solutions on polycrystalline Pt surface has been studied. The results suggest that both adsorbed and bulk ReO2 as well as bulk Re03 are deposited. It has been demonstrated that rhenium oxides deposited onto the Pt surface may act as a hydrogenating catalyst. CH3-CN and HCHO were hydrogenated on a rhenium oxide electrode. Deposition of rhenium oxides onto the Pt surface from the solution of ReO4 ions by reduction of ReO4 ions with gaseous hydrogen has been carded out.

1. INTRODUCTION At the very beonning, in the study of Re deposition onto Pt surface it was supposed that ReO-4 ions were reduced to zero valent Re atoms adsorbed on the Pt surface [1-4]. A more detailed study of electroreduction of ReO-4 ion, however, revealed that instead of adsorbed Re atoms the species deposited at submonolayer level were probably ReO2 molecules [5]. Similar conclusions were drown and published also in our earlier papers [6, 7]. It follows from the above that our current knowledge of rhenium deposition is still far from complete. Therefore, the study of Re deposition onto Pt surface has been resumed. In this paper, we report our results attained in the study of Re deposition on the Pt surface and the catalytic characteristics of Pt covered with species.

2.EXPERIMENTAL Our experimental technique was described in detail in an earlier publications [8]. The cell solutions in the main compartment can be exchanged by the exclusion of air, consequently, the properties of rhenium covered Pt could be studied in the absence of the precursor compound. Merck P.A. reagents and triply distilled water were used in our experiments. The cell solutions were deoxygenated and agitated by bubbling purified nitrogen through the cell.

270 A platinized Pt electrode was used as the working electrode(apparent surface area was about 2 cm2 ). All measurements were carried out at room temperature. The potential of the working electrode was measured against a hydrogen electrode in the same electrolyte used in the main compartment of the cell. ReO4 ions were added to the electrolyte of the main compartment (of approx. 50 ml vol.) ofthe cell in the form of Re207 oxide.

3. RESULTS AND DISCUSSION

3.1. Rhenium deposition via ionization of hydrogen adsorbed on Pt

First the potential sweep of the platinized pt electrode was determined in 0.5 M H2804 solution (curve 1 in Fig. 1) and then the Pt electrode was resaturated with hydrogen. When the potential reached 0.05 V, 10 mg Re207 was introduced into the main compartment of the cell. As a result, the potential of the electrode rose to 0.425 V in 30 min. After Re deposition the cell was washed free of ReO4 ions with deoxygenated 0.5 M H2SO4 and the potential sweep of the electrode covered with adsorbed rhenium species was determined also in 0.5 M H2SO4 (curve 2 in Fig. 1). Since the integrals of the hydrogen region of curve 1 and the rhenium region of curve 2 are the same, adsorbed hydrogen was replaced by rhenium species without any loss of charge. It also follows from this result that the adsorbed material was reoxidized into ReO-4 ions also quantitatively.

2 1 r'--

0

0./+

O.8

E/V

Fig.1. Potential sweep of the platinized Pt electrode in 0.5 M n2so4 (1). Potential sweep of the same electrode also in 0.5 M H2SO4 after Re deposition via ionization of the preadsorbed hydrogen (2). Sweep rate: 2 x 10 3 V/s.

Potential of oxidation of adsorbed species is practically the same as published earlier [5]. It follows from this result that in this case ReO2 deposition may also be assumed

271 3.2. Electrodeposition of bulk rhenium species

Electrodeposition of rhenium species on the Pt surface has been studied by Schrebler et al. [5] but only at submonolayer level. As a subsequent step, we carried out bulk deposition of rhenium (-oxide) species onto a Pt surface in sulphuric and hydrochloric acid media in the presence of 2 g/1 R e g _ O 7 . As a result of multilayer formation of rhenium species, the color of the electrode turned black. The black bulk rhenium species deposited onto the Pt surface could be dissolved in deoxygenated concentrated HC1 without hydrogen evolution, consequently, the deposit could not be metallic rhenium but merely some mixture of oxides. Except for the bulk deposit deposited in diluted HC1, the reoxidation of bulk and adsorbed rhenium deposits by the potential sweep method resulted in three peaks in the curve. On the basis of experimental data it may be assumed that the peaks are due to the oxidation of bulk and adsorbed ReO2 and bulk KeO3.

3.2.1. Rhenium deposition on Pt in 0.2 M HCI solutions of perrhenic acid

0.2 M HC1 was chosen as a supporting electrolyte to model the processes which might take place during the preparation of the bimetallic Re/Pt catalyst via ionization ofpreadsorbed hydrogen (,,recharge" method [3]) or by reduction of the ReO'4 ion by gaseous hydrogen (,,catalytic" reduction [3]). First the potential sweep of the platinired Pt electrode was determined in 0.2 M HC1 (dotted line in Fig. 2) and then 0.1 g Re207 was added to the solution in the main compartment ofthe cell. Rhenium deposition was carried out with 0.15 mA for 30 rain, starting from 0.6 V. During this period of polarization the potential decreased to 0.31 V. From this potential, the sweep of the rhenium covered electrode is shown in curve 1 in Fig. 2. The time of polarization was then raised to 2 h (curve 2), then to 5 h (curve 3), to 15 h (cm've 4), and finally to 32 h (curve 5 ), Fig 2. From the standard potentials of different rhenium redox systems [9, 10], it can be stated that the deposit oxidized at a potential higher than 0.7 V can only be ReO3 as the standard potential for ReO'4/ReO3 is 0.768 V. According to the results of Schrebler et al. [5], the peaks at about 0.5-0.6 V in curves 1-5 in Fig. 2 result from oxidation ofmultilayer geO2 adsorbed on platinum Since all of the rhenium deposit could be dissolved in deoxygenated concentrated HC1, it may be concluded that the rhenium deposit oxidized at 0.36 V and 0.49 V must also be some oxide. RedO3, ReO:, as well as their hydrates or chloride complexes might also be taken into consideration. The peak at 0.49 V may be attributed to the oxidation of some chloride complex of rhenium because no such peak can be observed if 0.1 M H2SO4 is used as the supporting electrolyte. Since Re203 can readily be oxidized into ReO: , with RedO3 on the surface, three peaks should have preceded the peak of oxidation of adsorbed ReO2 . The

272 oxidation o f Re203 to R e O 2 , ReO2 to R e O 4 and the oxidation of the above mentioned chloride complex. It follows from this that the peak at 0.36 V is due to the oxidation of bulk ReO2 on the surface because there are only two peaks before the peak of oxidation of adsorbed ReO2 at about 0.5-0.6 V.

limA

'6

0'8

10 E/V

Fig.2. Potential sweep ofthe platinized Pt electrode in 0.2 M HC1 (dotted line). Potential sweep of the same electrode covered with rhenium species by polarization with 0.15 mA for 0.5 h (1), 2 h (2), 5 h (3), 15 h (4) and for 32 h (5) in 0.2 M HC1 containing 2 g/1 Re207 . Sweep rate: 2 x 10 -3 W/s.

3.2.2. Rhenium deposition on platinum in 0.5 M H2804 solutions of perrhenic acid

In this case, firstly the platinized Pt electrode was polarized for 65 h with 0.05 mA in 0.5 M H2SO4 containing 2 g/1 Re207 . Afcer the long period of polarization the color of the Pt surface turned black, and its potential decreased to -0.015 V by the end of rhenium deposition. The potential sweep of the Pt surface covered with rhenium species from 0.0 V in R e O 4 ion free 0.5 M H2SO4 is curve 1 in Fig. 3. This curve is characteristic of the Pt surface covered with bulk rhenium species. According to considerations outlined above the peak at 0.375 V is the result of oxidation of bulk ReO2, the central peak at 0.585 V is brought about by multilayer adsorption of ReO2 on the Pt surface. As pointed out earlier, the peak at 0.81 V can only be the oxidation of ReO3 to ReO4 ion.

273 If polarization is carried out under the same conditions but it took place only for 320 min with 0.025 mA, then the potential sweep of rhenium covered electrode is curve 2 in Fig. 3. In this case only one peak can be seen in the curve: the peak of oxidation of multilayer Re| adsorbed on Pt sin-face.

I

1mA

o

0.8

E/V

Fig.3. Potential sweep in rhenium free 0.5 M H2SO4 of the platinized Pt electrode covered with rhenium oxide species by polarization with 0.05 mA for 67 h (1) and 320 min (2) in 0.5 M H2SO4 containing 2 g/1 Re2| Potential sweep of the same and Re free electrode in 0.5 M H2SO4 (3). Sweep rate: 2 x 10-3 V/s.

It can be concluded from the results depicted in Fig. 3, the deposition of adsorbed Re| is completed more or less in 300 min and in case of further polarization, slow deposition of bulk rhenium oxides takes place. According to Schrebler et al. [5], the number of hydrogen adsorption sites (S) covered by one adsorbed Re| molecule is 2 . With the use of this result, the coverage of Re| adsorbed on the Pt surface can be calculated by applying the following equation:

|

QMo z

(1)

Q~

where | is the coverage of Re| on Pt, z is the number of electrons (in this case 3) transferred during oxidation of adsorbed species, Quc is the charge required for the oxidation of adsorbed Re| and Q~ is the charge required for the oxidation of hydrogen adsorbed on Pt free of rhenium

274 For curves 1 and 2 the calculated values of | are 2.8 and 2.3 ad sorbed monolayers of ReO2 on the Pt surface.

3.3 Electrocatalytic activity of platinum covered with rhenium oxides

One of the best control reactions for studying Pt catalysts covered with rhenium oxides is hydrogen evolution reaction. It has served as a model for electrocatalysis studies over many years. In this study, first a Pt electrode covered with bulk rhenium oxide was tested in ReO4 ion free 0.5 M H2SO4 (curve 1 in Fig. 4) . After determination of Tafel plot , the cyclic voltammogram of the same electrode between -0.1-0.3 V potential limits was also measured in pure 0.5 M H2SO4 (Fig. 5), and finally, the potential sweep of the Pt electrode covered with rhenium oxide species was determined (curve 1 in Fig. 3).

IlmA

\

L

1.0

\

0.1

0.01 -8o

'

-~

\

2mA

\

II

O-E/mY t~O

Fig.4. Tafel plot ofthe rhenized (bulk (1) adsorbed (2)) and rhenium flee (3) platinized Pt electrode in 0.5 M

H2SO4.

Fig.5. Cyclic voltammogram of the platinized Pt electrode covered with bulk rhenium species, in 0.5 M H2SO4. Sweep rate: 5 x 10-3 V/s.

Upon completion of the above experiments, a Pt electrode covered with adsorbed ReO2 was prepared and the Tafel plot was measured once again (curve 2 in Fig. 4) in ReO-4 ion free

275 0.5 M H2804 supporting electrolyte. At the end of this series of experiments the Tafel plot of the rhenium t~ee Pt electrode was measured also in ReO4 ion free 0.5 M H2SO4 (curve 3 in Fig. 4). As can be seen, the slopes of the curves in Fig. 4 are exactly the same, consequently, the mechanism of hydrogen evolution must also be identical. From the slopes (-0.031 V/decade) of curves in Fig. 4 the conclusion can be drawn that hydrogen deposition takes place with the Tafel mechanism, i.e., the rate determining step of hydrogen evolution reaction is:

2R

-- n~

(z)

It follows from this result that rhenium oxides deposited on the Pt surface may adsorb (or absorb) hydrogen atoms and the substance oxidized between 0.0-0.3 V (curve 1 in Fig. 3) is adsorbed (or absorbed) hydrogen. The existence of hydrido compounds of rhenium makes plausible the concept of absorption of hydrogen atoms by rhenium oxides deposited onto the Pt surface [11]. Another surprising result of these experiments is that that ReO2 adsorbed on Pt surface does not effect hydrogen overvoltage (curves 2 and 3 in Fig 4 are identical), and the rate of hydrogen evolution on a Pt electrode covered with bulk Re-oxide species is only one order of magnitude lower that on the same electrode without bulk rhenium species on the surface (compare curve 1 with curve 3 in Fig. 4).

3.4. Rhenium oxide as a hydrogenating catalyst

From the results of electrocatalytic studies of platinum covered with adsorbed and bulk rhenium oxide species the conclusion can be drawn that rhenium oxide species may act as a hydrogenating catalyst. In the case of aceton and maleic acid no hydrogenation was observed (on the other hand, this result shows that rhenium oxides offer perfect coverage on Pt as these compounds are readily hydrogenated on the Pt surface). Acetonitrile and formaldehyde, however, could be hydrogenated on a rhenium oxide electrode. In addition to the hydrogenation of organic compounds, hydrogenation of lt,eO4 ion with gaseous hydrogen was also tested. The reaction was carried out under the same conditions as described by curve 1 in Fig. 3, with the exception that -as mentioned abovepolarization was carried out with gaseous hydrogen and not with electric current. The result was similar to that depicted by curve 1. in Fig. 3, which verifies that aRer the formation of the adsorbed ReO2 layer on Pt surface adsorption, dissociation and ionization of hydrogen may still take place. The data of deposition of rhenium from the solution of geO4 ions onto a parent catalyst by reduction of geO-4 ions with gaseous hydrogen (,,catalytic reduction" [3]) are in accordance with the results given above.

276

Acknowledgements. The financial support of the Htmgafian Scientific Research Fund (OqKA), No. T-015828 is gratefully acknowledged.

REFERENCES 1. F. Kadirgan, B. Beden and C. Lamy, J. Electroanal. Chem_, 143 (1983) 135. 2. C.L. Pieck, P. Marecot, C.A. Querini, J.M. Parera, J. Barbier, Appl. Cat., 133 (1995) 281. 3. C.L. Pieck, P. Marecot, J. Barbier, Appl. Catal., 134 (1996) 319. 4. C.L. Pieck, P. Marecot, J. Barbier, Appl. Catal., 141 (1996) 229. 5.1L Schrebler, H. Gomez and 1L Cordova, Electrochim. Acta, 34 (1989) 1405. 6. I. Bakos and G. Horanyi, J. Electroanal. Chem_, 375 (1994) 387. 7. G. Horanyi and I. Bakos, J. Electroanal. Chem_, 378 (1994) 143. 8. S. Szabo and F. Nagy, J. Electroanal. Chenl, 70 (1976) 357. 9. M. Pourbaix, Atlas of Electrochemical Equilibria, CEBELCOR, Brussels, 1974. 10. A.J. Bard, 1L Parsons and J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker, Inc. New York and Basel, 1985. 11. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Fifth Adition, John Wiley and Sons, Page 1097.

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al,, editors,

277

Deposition of gold nanoparticles on silica by CVD of gold acethylacetonate Mitsutaka Okumura, Shyun-ichi Nakamura a, Susumu Tsubota, Toshiko Nakamura, and Masatake Haruta OsakaNational Research Institute, AIST, MITI Midorigaoka 1-8-31, Ikeda 563, Japan a Department of Chemistry, Faculty of Science, Osaka University, Toyanaka, 560, Japan

Although Au/SiO2 catalysts prepared by liquid phase methods are poorly active for the oxidation of CO and of 1-12,the catalysts prepared by chemical vapor deposition of organo gold complex exhibit remarkably enhanced catalytic activities. In order to investigate the surface processes occurring during the adsorption and decomposition of dimethyl gold acetyl acetonate [Me2Au(acac)] on the surface of SiO2, FT-IR, TG-TDA analyses, and the catalytic activity measurements for CO and H2 oxidation were carried out. Ab initio DFT (Density Functional Theory) calculations were also carded out to estimate the adsorption state of the gold precursor. The results of the above experiments indicated that the gold precursor was adsorbed weakly through the interaction of the electron-rich oxygen in acetylacetonate with the OH group on the surface of SiO2 and was decomposed into metallic particles during the following calcination process.

1. INTRODUCTION Hydrogen oxidation takes place at lower temperatures than CO oxidation over unsupported gold powder [1] and appears to be much less influenced by metal-support interaction. Therefore, the catalytic activity for 1-12oxidation can be primarily related to the exposed surface area of gold. On the other hand, the catalytic activity for CO oxidation is markedly dependent on the preparation methods, the mean diameter of Au particles, and the contact structure between the Au particles and the support metal oxides[2]. When supported gold catalysts are prepared by liquid phase methods[3,4], metal oxides suitable as a support are limited to a group of reducible metal oxides having p-type or n-type semiconductivities. In fact, gold supported on insulating, covalent bonded metal oxides like SiO2 was much less active for CO oxidation than Au/TiO2, Au/Fe203, Au/C0304, and Au/NiO. Because of the low point of zero charge (pH 2) of SiO2, it was difficult to deposit Au(OI-I)3 on the surface of SiO2 by the liquid phase methods such as coprecipitation and deposition precipitation. Impregnation method was the only one that could prepare Au/SiO2 catalyst, however the mean diameter of gold particles were over 10nm, and the catalytic activities for the oxidation of CO and of H2 were low [ 1]. In order to make it clear whether SiO2 is really not effective as a support or the inferior activities are simply caused by the weak interaction of the gold particles with the

278 supports and accordingly by poor dispersion, we have attempted to deposit gold on SiO2 by chemical vapor deposition of an organo gold complex. This method has already been proved to be effective to prepare Au/TiO2 catalysts [5]. In the present work, TG-TDA and FT-IR analyses were conducted to examine the adsorption state and the decomposition of Me2Au(acac) on SiO2. Ab initio DFT (Density Functional Theory) calculations were also carried out to estimate the electronic structure of the gold precursor. The difference in the interaction between gold particles and SiO2 prepared by impregnation method and CVD method is discussed based on the catalytic activities for CO oxidation and H2 oxidation.

2. EXPERIMENTAL

2.1. Ab initio DFT calculation The charge density and IR frequency of gold precursor were obtained by ab initio density functional calculations using Gaussian 94 program [6]. For DFT calculations, Becke 88 functional was used as an exchange functional and LYP (Lee, Yang, and Parr) functional was used as a correlation functional. The basis sets used in these calculations were double zeta effective core potential basis set (I.anl2DZ) on Au atom and Dunning/Huzinaga full double zeta basis set (D95) on H, C, and O atoms.

2.2. TG-DTA, FT-IR analysis and TEM observation TG-DTA analysis for Me2Au(acac) alone and Me2Au(acac) adsorbed on SiO2 were carried out by using RIGAKU TG8101D in the temperature range from 300K to 873K in air. The state of adsorption and decomposition of Me2Au(acac) on the SiO2 surface was monitored by means of FT-IR (Nicolet 20SXC). The dispersion of gold particles was observed with a transmission electron microscope (TEM, Hitachi H-9000).

2.3. Catalyst preparation The metal oxide used as a support is SiO2 powder (Fujisilycia Chemical Ltd., type G, specific surface area 310m2/g). As a gold precursor, (CH3)2Au(CH3COCH2COCH3), abbreviated to Me2Au(acac), is used without further purification of a reagent commercially available. The CVD experimental setup made of hard glass was described elsewhere [3]. The lowest pressure that could be reached in the apparatus is about 10-3 torr. Support metal oxides were evacuated at 473K for four hours to remove water physically adsorbed and were then treated with 20 torr oxygen gas at 473K for 30 min. to remove organic residue and to oxidize the surface. The precursor vessel was heated to a fixed temperature of 306K to gradually evaporate a measured amount of Me2Au(acac). The precursor adsorbed on the metal oxide supports which were mounted in the reaction vessel was calcined in air at a fixed temperature in the range of 473K~773K to decompose into metallic gold particles on the support surface.

2.4. Catalytic activity measurements and TOF calculation Catalytic activity measurements were carried out by using a conventional fixed-bed flow reactor. A powder sample (100mg) sieved between 70~120 mesh (212 ~ 125~m) was placed on a ceramic wool plug in a quartz tube with an inner diameter of 6mm. The reactant gas (1 vol% CO or lvol% H2 in air) was passed through the catalytic bed at a flow rate of

279 33ml/min. (SV=20,000hI ml/g-cat.). The activation energies were determined by Arrhenius plots. A powder catalyst sample (20~30mg) mixed with quartz powder, both sieved between 70~120 mesh, was used for the activity measurements. The flow rate of reactant gas (1 vol% CO in air) and catalyst temperature was varied to keep the CO conversion between 10 ~ 20%. As the rate of CO oxidation was almost independent of the concentrations of CO and O213], it was calculated based on a zero-th order reaction. The number of gold atoms exposed to the surface was calculated from the mean diameter of Au particles and the actual Au loading obtained by inductively coupled plasma spectrometry (made by Sumika Chemical Analysis Service Co. Ltd.). Turnover frequency was calculated by dividing the reaction rate with the number of surface gold atoms.

3. RESULTS AND DISCUSSION 3.1. ab initio DFT calculation Me2Au(acac) Fig.la shows the charge density of MegAu(acac). The signs of atomic charges change alternately on the acethylacetonate ligand and the large negative charges are observed on the oxygen atoms, suggesting that the interaction between the gold precursor and the support surface occurs mainly between oxygen atoms of Me2Au(acac) and the OH group of SiO2 surface. Such adsorption models are shown in fig.lb and c. The obtained IR frequency of Me2Au(acac) obtained by ab initio DFT calculation is shown in fig.2. The C-H stretching of -CH3 groups and the C-H group of Me2Au(acac) are observed in the range between 2950 ~3200 cm 1. The peaks in the range of 1000 ~ 1550 cm 1 can be ascribed to the vibration modes related to acetylacetonate ligand.

H3C~

-0.34 0.39/CH 3

I," ",k 0.36 Aul -o.44)CH (0.20) 1/ / \\,~

H3C

~ C ,

/CH3 -- ..C~,~ ,/CH

H3C\Au\/r

o.3~\CH3

/CH3 /2.._..:C2,," Au ~,~ ~ CH

/ '",.

\OH 3

H

?

0--7- C -0.34

H3C \

-- ~ 1

~I~a~

I ~i~ ! ~ ill~

~i ~i ~

n3c

~ ~ ~ C\CH3 H'

?S -/~

~ii i i i i i i i i i{!

(a) (b) (c) Fig.1. Schematic diagram of Me2Au(acac ). (a) charge densities for Me2Au(acac ). The charge density of hydrogen atom is given in a parentheses, (b)and (c) adsorption models of Me2Au(acac) on SiO2 surface

280

3000

2500

2000 Wavenumber/cm-1

1500

1000

Fig.2. Theoretical IR spectrum for Me2Au(acac) obtained by ab initio DFT calculation. 3.2. TG-DTA analyses of Me2Au(aeae) before and after adsorption on SiOz In the DTA curve of Me2Au(acac) shown in fig.3, two peaks are observed. The first endothermic peak at 355K is due to the melting of Me2Au(acac) and the second exothermic peak at temperatures above 400K can be ascribed to the decomposition of the gold precursor. This peak is a little tilted to the higher temperature side because the large exothermicity of the reaction resulted in a sharp increase in the sample temperature. The TG curve shows a gradual decrease from 353K and reaches a steady value above 473K, indicating that the decomposition of Me2Au(acac) is completed up to 473K. While the theoretical weight loss is calculated to be 39 %, the weight loss observed was about 70 %. This is probably because of the vaporization of part of Me2Au(acac) during heating procedure. In the case of Me2Au(acac) adsorbed on SiO2, as shown in fig.4, three peaks are observed. The first sharp exothermic peak which is observed at a temperature a little lower than that of Me2Au(acac) itself can be ascribed to the decomposition of gold precursor. It appears that the decomposition might be enhanced on the SiO~ surface. The second and third exothermic peaks are considered to be due to the combustion of the ligand residues. The decomposed residues might remain on the support surface up tO 573K. This suggests that calcination at a temperature above of 573K is needed to prepare Au catalysts. The gradual decrease in weight in the TG curve above 573K is due to the dehydration of SiO9 itself, as the same tendency is observed in the case of SiO2 alone. 1 ,,,--,i,,,, i,,,, i,,,,i,,,, i , , ~, 250 \ 0.9 .~--', ~ 200 ~0.8 Decomposition 150~ 0.7 ~D .~ 0.6 100:~ 0.5 50 < 0.4 ,' 0.3 " ~ M lting po~'l -50 0.2 ~ 300 400 500 600 700 800 900 Temp./K Fig.3. TG-DTA curves of Me2Au(acac)

~

0

1[,~,,,1 .... i .... ~,,,,, a,r,,,i .... 100 .~

0.99 I-<-~J I J

Combustion of

.~0.981o0.971 1",~1" surfaceredidue "~0.961 ~'~ 0 . 9 5 ~ ~

00; !D ]

]

50~

],~, "~~---~

"~ 0

mpor!t!on, ~ , - 5 0

300 400 500 600 700 800 900 Temp./K Fig.4. TG-DTA curves of Me2Au(acac) adsorbed on SiO2 surface

281

3.3. FT-IR spectra of Me2Au(acac) adsorbed on SiO2 Fig.5 shows the VI'-IR spectra of unsupported Me2Au(acac). Several peaks are observed in the range of 2800~3000cm-1 and of 1000~1600cm -1. The former peaks are due to the C-H stretching of-CH3 groups and of the C-H group of Me2Au(acac) and the latter peaks are due to the vibration modes of acetylacetonate ligand. The experimental results are in a good agreement with the result of ab initio D F r calculation. Fig.6 shows the FT-IR spectra of silica-supported Me2Au(acac), which are not so different from that of the unsupported Me2Au(acac). The spectra showed a band at ca. 1600 ~ 1680 cm -1 which is due to water physically adsorbed on the support surface at room temperature. These VI'-IR spectra resemble the spectra of silica-supported Pt(acac)2 reported by S. Kohler et al. [7]. Me2Au(acac) absorbed on the SiO2 surface might be weakly interacted and maintain its original structure. In fig.7, the Fr-IR spectra of silica-supported Me2Au(acac) obtained after calcination in air at 473K showed no peaks related to Me2Au(acac) compound. This result together with TG-DTA results indicate that the ligand was already decomposed or vaporized during calcination at 473K for 4 hours but partly remained as carbon residues. |

3000

i

i

|

[

i

|

|

i

i

2500

2000 1500 Wavenumber/cm-~ Fig.5. IR spectrum of Me Au(acac)

1000

2500

1000

2

3000

2000 Wavenumber/cm-1

....... 1500

Fig.6. IR spectrum for Me Au(acac) adsorped on SiO surface 2

3000

2

2500

2000 1500 Wavenumber/cm1 Fig.7. IR spectrum for Au/SiO calcined at 473K in air for 4hours 2

1000

282 3A. TEM observation of Au/SiO2 A TEM photograph of Au/SiO2 is shown in fig.8a. A large number of gold particles in Au/SiO2 (CVD) calcined at 673K remain to be smaller than 10rim in diameter. Fig.8b shows the Au particle diameter distribution. The mean diameter of gold particles is calculated to be 6.6nm. The gold particles are smaller by three times than those of Au/SiO2 prepared by impregnation method (IMP).

3.5. Catalytic acthities of AulSiQz catalyst6 for the oxidation of Hz and of CO Fig9 and I0 show the reaction curves for Au/SiQz catalysts prepared by IMP with different Au Ioadings in the oxidation of HI and of CO, respectively. All of them are poorly active for both 1-12 and CQ oxidation. It should be noted that they are more active for H2 oxidation than for CO oxidation and the catalytic nature of which was similar to unsupported gold particles and completely different from the catalytic nature ofAu/SiOz pwpared by CVD.

283 Fig.11 and fig.12 show the conversion vs. temperature curves of CO oxidation and of H2 oxidation are shown, respectively. The catalytic activities of Au/SiO2 prepared by CVD were improved with an increase in calcination temperature. In the case of calcination at 473K, the catalytic activities of both CO oxidation and H2 oxidation are both very poor. Even though the calcination temperature was raised to 573K and 673K, the catalytic activities of H2 oxidation did not appreciably change. This means that the exposed Au surface area did not change between these two different calcination conditions. On the other hand, the catalytic activities for CO oxidation were remarkably enhanced. Judging from our previous works [8], calcination conditions at higher temperatures might strengthen the interaction between Au particles and the support, giving rise to longer inteffacial perimeter. The catalytic properties of typical Au/SiO2 prepared by IMP and CVD are summarized in table l. In the case of calcination at 673K, Au/SiO2 prepared by CVD exhibits high catalytic activity for CO oxidation with T1/2values lower by about 100K than for H~ oxidation. Therefore, hemispherical nanoparticles of Au may be deposited on the SiO2 support in such a contact structure that SiO2 support can contribute to catalytic reaction by CVD. The TOF and activation energy of Au/SiOz (IMP) were not obtained because the catalyst was inactive at 273K. In contrast, Au/SiO2 (CVD) method exhibits catalytic activity for CO oxidation as high as Au/TiO2. The apparent activation energy of this catalyst is 17kJ/mol and TOF at 273K is 0.02. 100 80 60

~9 40 20 I

0150

t

I

250 300 350 400 Catalyst temperature/K Fig.11. Conversion of CO over 6.6wt%Au/SiO2 prepared by CVD and by calcination at different temperatures

100

_

=

]

.

80

o

200

.

.

......

.

.

i,

I

'

'

'

I

,,

'

'

i

,

~

-'

'

~

"

450

, A,

I

,,

c%

'

- I ----O~ ~. 473K 573K

60 II

4O o 20 L) 0

280

300

320

340 360 Catalyst temperature/K

380

400

Fig.12. Conversion of H2 over 6.6wt% Au/SiO2 prepared by CVD and by calcination at different temperatures

420

284 Table 1 Mean diameters of gold particles in Au deposited on SiO2 and their kinetic ..parameters for the oxidation of H2 and of CO Method Au loading Calc.temp. Meandiameter Tit2(H2) Tl/2(CO) TOF(273K) Ea(CO) ......[wt%] [K] of Au [nm] [K] . [K] [sl] . [kJ/mol] CVD 6.6 673 6.6+__3.8 329 227 0.02 17 IMP 14.7 673 20. 589 668 'T()F(CO)[273K]:Tumover frequency for CO oxidation at 273K, Ea(CO):Apparent activation energy for CO oxidation The extremely low catalytic activity of Au/SiO2 (IMP) for CO oxidation can be ascribed to the large Au particles simply loaded on SiO2 without significant interaction. This is because the interaction between HAuC14 crystallites and the support is weak, gold particles which are formed during calcination can migrate on the support surface and grow to larger spherical particles through coagulation.

4. CONCLUSION From the ab initio DFT calculation and FT-IR analysis, it can be assumed that dimethyl gold acetylacetonate is adsorbed on SiO2 surface through the interaction between the electronrich oxygen atoms of acetylacetonate and the surface OH groups. TG-DTA analyses showed that the adsorbed precursor was decomposed at temperatures below 423K and carbon deposit might remain up to 573K. Although the catalytic activities for H2 oxidation and CO oxidation of Au/SiO2 prepared by CVD of Me2Au(acac) and by calcination at 473K were still very poor, they were greatly improved by calcination at higher temperatures, 573K and 673K, suggesting that once the strong interaction between gold particles and S iO2 was obtained, even Au/SiO2 exhibited high catalytic activity for CO oxidation. Accordingly, it may be concluded that metal oxide support has an important role for the genesis of the lowtemperature catalytic activities for CO oxidation only when the gold particles are strongly attached to the supports.

REFERENCES

1.

S. Tsubota, D. A. H. Cunningham, Y. Bando, and M. Haruta, Preparation of Catalysts VI, eds, G. Poncelet et al., Elsevier, Amsterdam, 227(1995). 2. M. Haruta, Catal. Today, 36(1997)153. 3. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. Genet, and B. Delmon, J. Catal., 144(1993)175. 4. M. Haruta, N. Yamada, T. Kobayashi, and S. Iijima, J. Catal., 155(1989)301. 5. M. Okumura, T. Tanaka, A. Ueda, and M. Haruta, Solid State Ionics, 95(1997)143. 6. Gaussian 94 (Revision El), M. J. Frish, et al., Gaussian, Inc., Pittsburgh PA, 1995. 7. S. Kohler, M. Peiche, S. Frobel, and M. Baerns, Preparation of Catalysts VI, eds, G. Poncelet et al., Elsevier, Amsterdam, 1009(1995). 8. S. Tsubota, T. Nakamura, M. Haruta, Catal. Lett., acceptedl

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts V I I B. Delmon et al,, editors.

285

Influence of the p r e c u r s o r s on the formation of a trimetallic defined structure. Application on Ni catalysts used for syngas obtention. H. Provendier, C. Petit, A.C. Roger and A. Kiennemann Laboratoire de la R~activit~ Catalytique, des Surfaces et des Interfaces, U.M.R.C.N.R.S. 7515, E.C.P.M., 1 rue B. Pascal 67008 Strasbourg, France. Preparations by a sol-gel like method have been studied to form trimetallic perovskites La-Ni-Fe, as catalysts for the obtention of syngas by methane reforming. On the nature of the precursors, identified by infrared spectroscopy, depends the formation of the mixte perovskite or of nickel oxide deposited on LaFeO3, as shown by X-ray diffraction. These structures are active in partial oxidation of methane to CO + H2. However, a large difference in behaviour is observed in steam reforming of methane versus the starting materials used for the preparation of the catalyst and evidences the advantage of defined structures, in preventing carbon formation. 1. I N T R O D U C T I O N

Natural gas is the most abundant, clean and easily extractable energy source [1,2]. However, the resources of this gas (mainly composed of methane) are located far away from the consumption sites. It is desirable to convert methane into liquid products for more efficient utilization [3]. The conventional way to liquid fuel production is through the syngas intermediate. Syngas is typically produced by reaction of CH4 with H20 at about 800~ but can also be obtained by the direct catalytic oxidation of methane with oxygen [4] and carbon dioxide [5,6]. It is subsequently found that metals including nickel [7], cobalt, iron and the platinum group [8] can catalyse these reactions. Nickel catalysts belong to the most practical ones for their cost, long-term stability and fast turnover rates. Studies performed on bimetallic definite structures (LaNiO3 [9] and La2Ni204 [10]) evidenced that the insertion of nickel into this type of structures could stabilize the active metal at high temperature and limit its sintering and the coke formation. Even trimetallic defined compounds such as La(Ni,AI)O3 [9], C a T i o . 5 N i o . 5 0 3 [11] and Cao.8Sro.2Tio.8Nio.203 [12] show an improved stability compared to bimetallic structures. The core of this work is to master the formation of the catalytic species (metallic nickel) by the understanding of the nature of the oxide structures formed depending on the choice of the starting materials (nitrate, oxide, metal ...). This point will be presented in the particular case of LaNio.3Feo.703. Correlations between the nature of the precursors, induced by the starting materials, and the obtention of the expected phases will be made. Furthermore the subsequent effect on the structural evolution of the catalyst under test, thus, on its catalytic behaviour, will be discussed.

286

2. EXPERIMENTAL PART 2.1 Preparation Catalysts with the determined Ni, Fe, La molar composition: La:l, Ni:0.3 and Fe:0.7, were prepared starting from various initial compounds (nitrates, acetates, oxides or metals) of La, Ni and Fe, dissolved separately in hot propionic acid under stirring. The three obtained solutions were mixed together and after a 30 minutes stirring, the resulting solution was evaporated until the formation of a resin or a foam. A highly exothermic reaction occured at the end of the evaporation of the solvent in the presence of a large amount of nitrate salts. Free NO2 was formed and explosion phenomenon could appear through a reaction between NO2 and the organic solvent. With the other initial compounds a transluscent resin could be obtained after the soft evaporation of propionic acid. This resin or foam was then calcined at 750~ for 4 hours with a ramp of 3~ 2.2 Identification of the precursors The precursors, formed in solution were precipitated by addition of hexane into the propionic solutions, followed by a cooling down, and were then filtered. The precipitates, after careful washing with hexane and drying, were analysed by FT-IR in a KBr matrix. 2.3 Characterization The catalysts were characterized before test by X-Ray Diffraction (X.R.D.) with a Siemens DS000TT diffractometer using a Cu Ko~ radiation, by specific surface area measurements (B.E.T.), by Scanning Electron Microscopy (S.E.M.), using a JEOL JSM 840 microscop. Temperature Programmed Reduction (T.P.R) experiments were performed on 50 mg of sample placed in a U quartz reactor (6.6 mm ID). A mixture of H2 (15 rnL.min-1) and Ar (50 mL.min-1) was flowing through the reactor, while it was heated up to 900~ at a rate of 15~ After the tests, the catalysts were analyzed by XRD, observed by S.E.M., and the specific surface areas were once more measured. 2.4 Reactivity conditions The operating conditions were the following: 200 mg of catalyst in a fixed bed quartz reactor (6.6 mm ID); inlet temperature: 400-800~ feed flow rate: 0.4 L.h-1 of CH4 with a ratio CH4/O2=2 or CH4/CO2=1 or CH4/H20:l/3 depending on the nature of the tested reaction. A complementary flow of argon was added to obtain a total gas flow of 3 L.h-l. The outlet gas was analysed by two on-line gas chromatographs (one giving the amount of remaining CH4 and of produced CO, CO2; the other quantifying CO and H2 production). Using oxygen and carbon dioxide as oxidants, the catalytic systems became active under the gas mixture and under heating. The temperature program for these two reactions was composed of three cycles. The first cycle was formed by a continuous temperature increase to 800~ with a ramp of 3~ during which chromatographic analyses were carried out every 60~ followed by a stage of one hour at 800~ and by a stepwise decrease every 50~ down to 400~ This first cycle was carried out to activate the catalyst. The second temperature cycle was performed like the first one, and the characteristic catalytic behaviour was obtained in the second temperature increase. The third temperature increase confirmed the results obtained in the second one.

287

The CH4+H20 reaction required a preliminary reduction of the catalyst under hydrogen flow and under heating up to 600~ with a ramp of lO~ After 30 minutes purge under argon, the gas mixture of CH4 and H20 flowed through the reactor. For the test, the temperature increased from 600 to 800~ with steps of 40 minutes every 50~ The total amount of carbon deposited on the catalysts after test was measured by elemental and thermogravimetric analyses.

3. RESULTS AND DISCUSSION The preparation mode of the different catalysts tested with the particular molar composition La:l, Ni:O.3, Fe:O.7 is a sol-gel like method [13,14]. Preliminary results have been obtained on catalysts prepared by the use of a mixture of three nitrate salts of La, Ni and Fe. XRD and transmission electron microscopy coupled to energy dispersive X-ray spectroscopy studies have shown that this preparation gave a highly homogeneous solid solution of LaFe03 and LaNi03 [15]. These systems, named La(NixFel-x)03 with various x data (0 _<x_ 1) have been tested under the 2CH4 + 02 reaction and their activities have been compared [16]. The structure with the ratio Ni/Fe= 0.3/0.7 was retained to be very active for this reaction with a low evolution of the trimetallic relative concentration in the structure. Only a little part of the nickel was reduced and became responsible for the high CO + 2H2 mixture formation (Yield= 90% at 800~ But as presented before, the described preparation can only be used for a limited amount of catalyst. For this reason, other starting materials were tested for the sol-gel like preparation to limit the use of nitrate salts for the preparation of La(Nio.3Feo.7)03. The scheme presented in Figure 1 shows the different steps of the preparation starting from the six possible compounds. La nitrate .... or La oxide

,0O~ .

.

.

Clear solution

IR analysis of the precursors

Clear solution

.

I

.....

/

"

I

Ni nitrate or Ni acetate

add.itiono, ~100 mLofp.ropionic acid~

Clear .solution .

Fe ni'trate or Fe metallic

mixing Of the three solutions

/

solvent evaporation

~ by heating (140~ o btention of the resin or foam

Calcination 4 h formaiion Of the catalys! (5cj) '

I

at 750~ I

Figure 1. Preparation mode of catalysts with the composition La:l, Ni:O.3, Fe:O.7.

288

We chose lanthanum oxide, nickel acetate and metallic iron, because neither metallic nickel, nor nickel oxide, nor iron oxide were soluble in propionic acid. The eight combinations of La (nitrate or oxide), Fe (nitrate or metallic) and Ni (nitrate or acetate) tested are given in Table 1. The prepared samples were first analysed by S.E.M.. The catalysts prepared with at least one nitrate salt, exhibit holes due to NO2 evaporation, as seen on the S.E.M. photograph (Figure 2a), whereas the catalyst prepared without any nitrate salt presents compact bulk (Figure 2b).

Figure 2a. Photograph of catalyst (n~ with three nitrate salts.

Figure 2b. Photograph of catalyst (n~ without nitrate salt.

This difference between both preparations 1 and 8, observed by S.E.M. has been completed by XRD analyses. For the preparations 7 and 8, three cristalline phases were observed by XRD: LaFeO3, and traces of NiO and La203. in the other cases (n~ only one perovskite phase offset to higher 2e values (closer to LaNiO3) in comparison to LaFeO3 was evidenced. This shift in the perovskite peak position can be seen (Figure 3) in the area of the most intensive peak (2e between 32.0 and 33.0), situated between LaFeO3 and LaNiO3 reference peaks, and corresponds to a trimetailic phase La-Ni-Fe. For the solid solution LaNixFel-xO3, the lattice parameter has been calculated and reported versus the relative concentration of Ni, noted x [15] (Figure 4). According to this calibration curve, x is equal to 0.3 for the preparations 1-4, equal to 0.16 for 5 and 6, and equal to 0 for 7 and 8. This evidences the obtention of the solid solution LaNio.3Feo.703 for the catalysts prepared from lanthanum nitrate (n~ and the presence of free nickel oxide in the other cases (n~ Especially NiO supported on LaFeO3 and La203 was obtained by the preparation without nitrate salt (n~

289 a (Ao}

~

r

'

i

'

~

',

'

3.93 3.92 3.91 3.9

I"

~" 3.89

, ~

i 32,0

4

32.2

32.4

32.6

32.8

Figure 3. XRD of catalysts in function of the initial compounds with x=0.3.

2e

3.88 3.87 3.86 3.85 3.84

v

0

0.2

0.4

0.6

0,8

1

x

Figure 4. Evolution of the lattice parameter of the structures LaNixFel-xO3 versus x.

These results have been confirmed by TPR measurements. The TPR curves of the mixte perovskite structures La(NixFel.x)O3 with x __.0.3, showed two major reduction peaks: one corresponding to the partial loss of oxygen from the perovskite structure at 380~ and the other corresponding to the reduction of the perovskite structure above 800~ But the presence of an additional reduction peak at 420~ corresponding to the reduction of NiO, confirmed the presence of free nickel oxide for the two catalysts n~ and 8. The percentage of nickel, which remains free as NiO (%NiO), has been calculated from the volume of hydrogen (VH2) consumed for the NiO reduction peak, according to Equation 1, where Vmol is the molar volume of ideal gases, Wsample the weight of the sample and Molar Wsample its molar weight. %NiO -- nNiO/nNi total - Wsample * 0.3 * VH2 * 100 / (Molar Wsample * Vmol)

(1)

For all systems prepared, the nickel content x in the perovskite structure La(NixFel-x)O3, obtained from the XRD calibration curve (Figure 4) and the percentage of free nickel, calculated via Equation 1, are reported in Table 1. For a better understanding of the preparation process in order to explain the differences observed, the precursors, prepared as explained in the experimental part, were studied by infrared spectroscopy. Nitrate salts of La (Figure 5a), Ni and Fe gave similar spectra after solubilization in propionic acid, showing characteristic bands of nitrates (underlined - - - at 1380 and 830 cm-1) and few propionate species, whereas La203, Ni(C2H302)2 and Fe led to the formation of propionate species (underlined --- at 1570 or 1520 cm-1 for vasCOO and 1410 or 1420 cm-1 for vscoo, depending on the mono or bidentate propionate formed), as shown for La203 in propionic acid in Figure 5b. This shows, that nitrate salts form only few propionates in propionic acic, whereas in the absence of nitrates, propionates are easily obtained. Nitrates seem to create a protection sphere around the metallic cations in propionic acid solution, preventing the formation of propionates.

290

4.0 3,5

!

3,0 2,5 o 2,0 1,5

_1384 1522~469 ~11434,L,, ~,

4,0t

~~, ~~/129~/1078 .1679 ~1lll t ' " " ~ [ 896

3,0

~3~ /li,ii // /t82_5 ~]Y It / /

~2,5 o 20

//642

1,0 ,

}'V

~ /

1467

--'---

l

1296

/1012 // 893

1716

"~

VII / / [ / / ~

1233 1079

1378

/,/815 i;

7/643

0,5 2000

1000

cm-1

Figure 5a. IR analysis of precursors obtained with La nitrate in C2HsCOOH.

2000

10(?0 cm-1

Figure 5b. IR analysis of precursors obtained with La oxide in C2H5COOH.

All the mentionned data concerning the eight preparations are gathered in Table 1. Table 1 Evolution of the properties of La, Ni, Fe oxide catalysts in function of the initial compounds. Ratios La:l, Ni:0.3, Fe:0.7. n~

initial compounds La

Fe

1.... nitrate nitrate 2 .......n!trate nitrate 3 nitrate . metallic 4 nitrate . metallic 5 oxide nitrate 6 oxide nitrate 7 oxide metallic 8 . o x i d e metallic

Propionate formation

XRD

TPR

Ni

La

Fe

Ni

x

nitrate acetate nitrate acetate nitrate acetate nitraie acetate

few few few .few yes yes yes y.e.s

f'ew few yes ye s few few yes .yes ...

few yes few yes few yes few yes

0.3 0.3 0.3 0.3 0.16 0.16 0 0

% of free NiO

420oc

at

''0

0 0 0 46 48 95 ....... 98

As shown in Table 1, the type of precursors present in propionic acid solution determines the formation of the mixte perovskite or the presence of free nickel oxide. The addition of three propionate species of La, Ni and Fe gave the formation of NiO on La203 and LaFeO3, as confirmed by the characteristic presence of FeLaO3 spectrum by XRD [17] and the NiO reduction peak in TPR. The presence of nitrates in the solution was not sufficient to obtain the solid solution La(NixFel-x)O3

291

with x > 0 and the proximity of La and Fe propionates led to the formation of LaFeO3, as noticed for the catalyst n~ For the preparation of a mixte perovskite, it was necessary to start with La and/or Fe nitrate salts in order to limit else La or Fe propionate formation. However, the obtention of LaNio.3Feo.703, corresponding to the total insertion of nickel into the structure, requires the presence of lanthanum nitrate, it seems to be important to curb the formation of FeLaO3 perovskite, if the total insertion of nickel into the structure is wished. Indeed, from the three propionate species, the LaFeO3 perovskite, which is more stable and easier to form than LaNiO3 [18, 19], will be obtained. An important point is to look if the differences observed on the preparations using various initial compounds influence the catalytic activity. For this study, the catalyst prepared with only nitrate salts (n~ resulting in LaNio.3Feo.703, and the one prepared without nitrates (n~ resulting in NiO/LaFeO3, have been tested in the three reactions of methane reforming. In the oxyreforming and in the carbon dioxide reforming of methane, for which the CO yields are given versus the temperature respectively in Figures 6 and 7. No significant difference in activity was observed between both preparations and no carbon formation was seen. However, LaNio.3Feo.703, showed a slightly higher activity at 800~ than NiO/LaFeO3 (CO yields at 800~ respectively of 88.9% and 85.2% in the oxyreforming and of 94.0% and 91.2% in the CO2 reforming of methane). In the 2CH4 + 02 reaction, the activity started, for both systems, at higher temperature during the up temperature ramp than it stopped during the down temperature ramp, because of the in situ reduction of the catalysts (hysteresis curve). However, in the case of NiO/LaFeO3, nickel is less in interaction with the perovskite phase than the nickel inserted in the LaNio.3Feo.703 structure, so it can be reduced at a lower temperature (TPR results). This explains why the activity starts already at 630~ for NiO/LaFeO3 and only at 690~ for LaNio.3Feo.703 under the 2CH4 + 02 test. 100 ..................................................................................

8O 70 0 (J .-. 0

"0 >o l

B

B

.." &"

60 50 40

9

I ~ LaNi0.3Fe0.703~ p[--A.--NiO/LaFeO3 . s

30 20

;

0

~

400

500

600

700

800

Temperature (~

Figure 6. Yield of CO in 2CH4+ 02 versus the temperature for the catalysts LaNio.3Feo.703 (n~ and NiO/LaFeO3 (n~

292

The profiles observed in the 0 0 2 + CH4 test are very similar for both preparations and activity is noticed above 400~ Under the reductive conditions of this test, the active nickel species was already reduced during the first temperature cycle and even part of iron contained in the LaFeO3 perovskite was reduced, to form a stable nickel iron alloy. This Ni-Fe alloy has been observed by the shift of the hyperfine field from 33T for o~-Fe to 29.4T for the alloy in M6ssbauer measurements. The presence of this Ni-Fe alloy seems to limit the sintering of nickel and avoid the carbon deposition. 100 90

A

=~ 0 ,. o .=

,~_

8O 7o 60 50 40

--.e-.- LaNi0.3Fe0.703 - - ~ -- NiO/LaFeO3

30 2O

10 -

400

',

500

!

. . . . . . . . . .

600

i

i

9

700

Temperature

80O

(~

Figure 7. Yield of CO in OH4+ 0 0 2 versus the temperature for the catalysts LaNio.3Feo.703 (n~ and NiO/LaFeO3 (n~ Nevertheless, important differences were evidenced in the CH4 + H20 test (Figure 8). 100 .................................................. 90

8o

i

~

7o

g

6o

~

50

-a

40

~.

30 2O 10

--

0~-

600

....

......

I ............

650

"'A.

I

I

1I

!

.........

700 Temperature

LaNi0.3Fe0.703 i -Jr- - N i O / L a F e 0 3

I

750

800

(~

Figure 8. Yield of CO in CH4+ H20 versus the temperature for the catalysts LaNio.3Feo.703 (n~ and NiO/LaFeO3 (n~

293

For this reaction, a previous reduction of the catalyst under H2 at 600~ is required. The catalytic system NiO/LaFeO3, that presented after reduction metallic nickel particles, was already active in the steam reforming of methane at 600~ and an important coke formation was observed (26% wt of C of the total amount of catalyst). In the contrary, the slow in situ reduction of LaNio.3Feo.703 permitted to increase the activity with increasing temperature and to reach a CO yield of 92% at 800~ and less than 0.3% of C in weight was found on the catalyst after test. This drastic difference can be explained by formation of larger Ni ~ particles (mean diameter higher than 100nm compared to 30nm), favouring carbon deposition, or by lower interaction between nickel and the support for NiO/LaFeO3. Free NiO is reducible at low temperature (420~ and forms large particules by sintering [20]. In Table 2 are reported the CO yields at 800~ (Yco), the amount of C (%C) measured after test (% of the total catalyst weight) and the cristailine phases detected after test by XRD for both systems LaNio.3Feo.703 and NiO/LaFeO3. Table 2 Activity and final states of the LaNio.3Feo.703 and NiO/LaFeO3 catalysts in the three reactions of methane reforming. Catalyst Reaction 2CH4+O2 CH4+CO2 CH4+H20

YCO 88.9 94.0 92.1

LaNio.3Feo.703 %C XRD 0 LaNiyFel.yO3 0.30 Ni-Fe+LaFeO3 0.28 LaNiyFel.YO3

Yco 85.2 91.2 0

NiO/LaFe.O3 %C XRD 0.17 NiO+LaFeO3 0.70 Ni-Fe+LaFeO3 26.6 Ni+LaFeO3

In the case of La(Nio.3Feo.7)O3, only a small part of nickel becomes free and reduced under the test conditons of the reactions between methane and oxygen or steam, giving a new perovskite phase corresponding to the formula La(NiyFel.y)03 with y slightly lower than 0.3. This phase permits to stabilize the nickel andlower its sintering by interacting with the perovskite. This explains why this catalyst is active without coke formation. Moreover, for La(Nio.3Feo.7)O3, the part of nickel reduced under test can be reinserted into the La(NiyFel.y)03 structure by a new calcination at 750~ In the CH4+CO2 reaction, the formation of a Ni-Fe alloy leads to a dispersion of nickel, and thus, limits the sintering and the carbon deposition. Finally, for the preparation of this performing catalyst in methane reforming, with a limited amount of nitrate species, lanthanum nitrate, metallic iron and nickel acetate should be used. 4.

CONCLUSION

We have evidenced the large influence of the preparation on the formation of the LaFeO3, LaNiO3 solid solution, it must be noted that it is necessary to curb the formation of stable LaFeO3, in order to permit the insertion of nickel into the structure. For the obtention of high CO, H2 yields in methane reforming, without carbon deposition, well defined structures are very suitable. These catalysts are stable under test and regenerable by a reversible insertion of well dispersed nickel into the perovskite via a new calcination, but their preparation requires a careful choice of the precursors formed, thus, of the starting materials.

294

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

P.R. Bauquis, Rev. Institut Frans du Petrole, 51 (1996) 615. P. Chaumette, Rev. Institut Frans du P~trole, 51 (1996) 711. J.P. Hindermann, G.J. Hutching and A. Kiennemann, Catai. Rev.-Sci. Eng., 35 (1993), 1. D.A. Hickman and L.D. Schmidt, Science, 259 (1993) 343. Z. Zhang, X.E. Verykios, S.M. Mac Donald, S. Affrossman, J. Phys. Chem., 100 (1996) 744. S.C. Tsang, J.B. Claridge and M.L.H. Green, Catai. Today, 23 (1995) 3. J. De Deken, P.G. Menon, G.F. Froment and G. Haemers, J. Catal., 70 (1981) 225. J.R. Rostrup-Nielsen, Catalysis, Science and Technology, 5 (1984) 1, J.R. Anderson and M. Boudart (eds.), Springer, Berlin. A. Slagtern and U. Olsbye, Appl Catal. A, 110 (1994) 99. M. Stojanovic, C.A. Mims, H. Moudallal and A.J. Jacobson, 1lth International Congress on Catalysis June 30-July 5 (1996) Baltimore USA Po 348. S. Rapagna, Thesis, University of Strasbourg, France (1996). T. Hayakawa, H. Harihara, A.G. Andersen, A.P.E. York, K. Suzuki, H. Yasude and K. Takehira, Angew. Chem. Int. Ed., 35 (1996) 192. J.L. Rehspringer and J.C. Bernier, Mat. Res. Soc. Symp. Proc., 72 (1986) 67. A.C. Roger, C. Petit and A. Kiennemann J. Catai. 167 (1997) 447. H. Provendier, C. Petit, G. Ehret and A. Kiennemann accepted in J. Chimie Physique. C. Petit, A. Kiennemann, P. Chaumette and O. Clause, US Patent No. 5 447 705 (1995). A. Golub, L.S. Sidorik, S.A. Medicko and M. Fedoruk, lnorg. Mater., 14 (1978) 1449. M.J. Martinezlope and J.A. Alonso, European J. Solid State and Inorg. Chem., 32 (1995) 361. A.E. Goeta, G.F. Goya, R.C. Mercader, G. Punte, H. Falcon and R. Carbonio, Hyperfine Interactions, 90 (1994) 371. E. Ruckenstein and S.H. Lee, J. Catal., 86 (1984) 457.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

295

Characterization of Zircolfia-supported Nickel Catalysts Prepared by Multiple Ion-exchange M. Mihaylova, K. Hadjiivanov a, N. Abadjieva a, D. Klissurskia and L. Mintchev b alnstitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia - 1113, Bulgaria blnstitute of Catalysis, Bulgarian Academy of Sciences, Sofia - 1113, Bulgaria

ABSTRACT Using IR spectroscopy of probe molecules, it has been established that the deposition of nickel ions on zirconia by ion-exchange results in blocking of part of the surface Zr 4+ cations. Reduction of the Ni 2+ leads to liberation of the ion-exchange active sites and thus a next ion exchange can be performed with the reduced sample. Samples, prepared by single (Ni-l-Zr), double (Ni-2-Zr) and triple (Ni-3-Zr) ion exchange have been characterized by different techniques. It has been shown that with the increasing number of exchanges the nickel concentration and the average diameter of the nickel particles (2 nm < day < 16 nm) increases. The nickel particles in the Ni-1-Zr sample are encapsulated in zirconia, however the increase in the number of exchanges results in exposure of metal nickel on the surface. It is shown that the multiple ion exchange might be utilized for preparation of zirconia-supported nickel catalysts of uniform dispersed metal particles with a preset diameter. 1. INTRODUCTION The main reactions where nickel catalysts have found industrial application are hydrogenation and hydrogenolysis, reducing cyclization of polyfunctional compounds and reductive amination of carbonyl compounds [ 1-3 ]. It is known that the support greatly affects the catalytic performance of the supported phase. In particular, Ni/ZrO2 catalysts show promising properties in a series of reactions. Thus, while most of the supported nickel catalysts produce methane in the CO + H2 reaction, higher hydrocarbons (including olefins) are obtained over Ni/ZrO2 catalysts [4,5]. In addition, Ni/ZrO2 catalysts show a good activity in the methane steam reforming [6,7], methane CO2 reforming [8] and methane partial oxidation into synthesys gas [9]. Important advantages of Ni/ZrOz catalysts are that they have a great sulfur tolerance [ 10], are stable in a large temperature interval [ 11], and do not form NixZryOz bulk compounds [12]. For instance, a big part of the nickel in Ni/A1203 is irreversibly lost after calcination as a result of formation of NiAI204 [12]. For that reason, nickel usually keeps a high dispersion even after a high temperature treatment when supported on zirconia [ 12].

296 The synthesis of supported metal catalysts is generally aimed at reaching maximum dispersion of the metal with a view to its more efficient participation in the catalytic reaction. With structure sensitive reactions the turn-over frequency (TOF) depends on the average diameter of the particles [ 13]. Very ot~en this dependence passes through a maximum. That is the reason for which the development of techniques for synthesis of supported metal catalysts, characterized by metal particles with a preset diameter, is important. The most conventional technique for catalyst preparation is impregnation. However, this method ot~en results in inhomogeneous distribution of the active phase. Common technique usually resulting in dispersed materials is the ion exchange [ 13,14]. Disadvantage of the latter is that the metal loading is restricted by the capacity of the support utilized. This inconvenience may be surmounted using the so called multiple ion exchange (MIE) [15,16]. According to this technique, the metal ions are deposited by ion exchange and then reduced. During the reduction, part of the active sites for ion-exchange are liberated, which allows performance of the next exchange. MIE possesses some of the advantages of the simple ion exchange, e.g. it results in formation of homogeneously distributed and highly dispersed metal particles and permits preparation of catalysts with a desired metal concentration. In the literature there are no data on whether the MIE can be applied to other supports besides the originally proposed titania. It is not certain whether reduction would lead to regeneration of the ion-exchange sites or eventual reoxidation would result in reblocking of these sites. The aim of this work is to establish the applicability of MIE to the synthesis of Ni/ZrO2 catalysts and to characterize the samples prepared by this technique. 2. EXPERIMENTAL

2.1. Supports and Catalysts To prepare the support, 100 g ZrC14 were dissolved in 1500 ml water and 25% ammonia solution was added drop-wise until pH 10.5 was reached. During this procedure the temperature was kept at about 0~ by consistent cooling with ice. The precipitate was washed thoroughly with water, dried and calcined at 400~ Then the product was successively washed with 1% ammonia solution and water in order to remove residual chlorine ions, and finally calcined for 1 h at 500~ The sample thus obtained was a monoclinic zirconia with a B.E.T. surface area of 57 m 2 g-1. The ion exchange was carried out using a 0.1 M Ni 2+ solution containing 12.5 wt. % ammonia: 10 g of ZrO2 were suspended in 50 ml Ni 2+ solution, and after 10 min. the liquid phase was removed and 100 ml of the initial solution were added again. After 50 min. agitation the precipitate was filtered, washed thoroughly with water (to remove the reversibly adsorbed cations), dried and calcined for 1 h at 340~ The reduction of the supported nickel was carried out at 500~ in a flow of 10 vol. % H2 in Ar at a flow rate of 50 ml min1. The sample was placed in this flow at ambient temperature, while the reduction temperature was attained with a heating rate of 5 K min1. After the reduction the samples were cooled to room temperature in the hydrogen-argon mixture. Doubly (triply) exchanged catalysts were synthesized according to the same procedure, but in these cases reduced catalyst samples obtained by a single (double) ion-exchange were used instead of the pure support. All the other procedures and reduction conditions were as described with the single ion-exchange.

297 For the sake of comparison, a Ni/ZrO2 catalyst prepared by impregnation and two Ni/SiO2 samples synthesized by ion exchange and by impregnation, respectively (a commercial aerosil with a surface area 336 m2 g-1 was utilized as a support), were also studied. The impregnated samples were prepared using a Ni(NO3)2 solution as a precursor and calcined at 450~ in order to attain complete decomposition of the nickel nitrate. The reduction temperature for those samples was 500~ The Ni/SiO2 ion-exchanged sample was prepared with the same Ni 2+ solution as that used for the zirconia-supported samples. In this case, however, the reduction temperature was 700~ The nickel concentration in the samples studied as well as the notation that will be used further on in the text are presented in Table 1.

Table 1 Concentration of Ni in the samples studied Notation

Preparation technique

Ni-1-Zr

Ni 2+ ion-exchange on ZrO2

0.40

Ni-2-Zr

Ni 2+ ion-exchange on reduced Ni-1-Zr

0.80

Ni concentration, wt. %

Ni-3-Zr

Ni 2+ ion-exchange on reduced Ni-2-Zr

1.19

Ni-i-Zr

impregnation of ZrO2 by Ni(NO3)2

0.40

Ni-l-Si

Ni 2+ ion-exchange on SiO2

1.72

Ni-i-Si

impregnation of Si92 by N!(NO3)2

1.70

2.2. Apparatus The TPR experiments were performed with 10 vol. % hydrogen in argon (30 ml min-1) at a heating rate of 10 K min1, the hydrogen consumption being measured by a catharometric detector. All TPR profiles were normalized for the same catalyst mass. The IR spectra were recorded by a Specord-M-80 apparatus. Prior to the experiments, the samples were pressed into self-supporting pellets and heated directly in a vacuumabsorption apparatus. The activation was carried out by successive thermooxidative (1 h, 150 Tort oxygen) and thermovacuum (1 h) treatments at 400~ The FMR measurements were performed with PS 100X CentroSpectr (Minsk) spectrometer using 100 kHz modulation of the magnetic field. The g-values were determined relative to DPPH. The X-ray phase analysis was made by a DRON-3 apparatus with a CuI~ radiation source. The nickel content of the catalysts was determined by flame atomic absorption spectrometry at the resonance wave length using a PYE-UNICAM SP 1950 apparatus. The specific surface area of the samples was measured by low-temperature nitrogen adsorption according to the conventional B.E.T. technique.

298 3. RESULTS AND DISCUSSION

3.1. Thermoprogrammed Reduction TPR is a favourable technique for studying supported nickel catalysts. NiO is characterized by TPR peaks at 400~ [17], whereas when the nickel ions interact with the supports (e.g. forming surface compounds) their reduction temperature is considerably higher [17,18]. A "NiO'-like phase, produced by oxidation of metal nickel, is reduced in the temperature interval of 110-355~ the higher the temperature of preliminary calcination, the higher the reduction temperature [ 17]. The TPR profile of the Ni-i-Zr sample (Fig. 1) consists of two peaks with maxima at 355 and 470~ assigned to reduction of two different kinds of Ni 2+ ions to metallic nickel. The same two peaks, however slightly shifted towards higher temperatures (392 and 508~ respectively) were detected with the Ni-l-Zr sample. This suggests a somewhat stronger interaction between Ni2+ and the support in the case of the ion-exchanged catalyst. It may be assumed that a small part of the hydrogen consumption is due to reduction of Zr 4+ to Zr 3+ facilitated by the presence of Ni ~ [ 19]. However, estimation of the hydrogen consumption has shown that this reduction, if any, occurs to a negligible extent.

cO

.m.

o_

E

o o

0

Cq

-I-

'

0

I

200

'

I

400

'

I

600

'

800

Temperature / 0C Figure 1. TPR profiles ofNi-i-Zr (a), Ni-1-Zr (b), reduced and reoxidized Ni-1-Zr (c) and Ni-2-Zr (d)

The TPR profile of Ni-l-Zr, already reduced and than exposed to air for 1 h at ambient temperature, exhibits only one intense peak at 257~ This peak is attributed to reduction of a "NiO"-like phase, produced by oxidation of Ni ~ [17]: it has been estimated that about half of the metallic nickel is reoxidized after coming in contact with oxygen. Two intense peaks, at 325 and 468~ have been registered in the TPR pattern of Ni-2-Zr. The lowtemperature peak is more intense and is assigned to reduction of reoxidized nickel deposited by the first ionexchange. The higher temperature peak characterizes reduction of cations loaded during the second ionexchange. Thus, it appears that the supported Ni~+-containing phase becomes easier to reduce with increasing number of exchanges.

299 3.2. IR

Spectroscopy

In order to establish the state and localization of nickel in the different samples, the latter have been tested with CO and NO. Carbon monoxide is a probe molecule which allows separate monitoring of Ni >, Ni + and Ni ~ on the surface of supported catalysts. Carbonyls of the type Ni>-CO are unstable and produce bands at 2220-2180 cm1 [20-26]. With Ni + ions carbon monoxide forms stable monocarbonyls (2160-2110 cm1) [22-25], which may be converted into dicarbonyls [vs(CO) at 2145-2131 cm1 and Vas(CO) at 2100-2081 cm~] under equilibrium CO pressures [22,23]. The typical bands for linear carbonyls of Ni~ are at about 2060 cm~, whereas the bridged nickel carbonyls are visible below 2000 cm-~ [20,22,25,26]. Unfortunately, Zr4+-CO species are detected in the same region as Ni2+-CO carbonyls [27]. Thus, to distinguish between Zr 4+ arrd Ni > cations adsorption of other probe molecule, e.g. NO, has to be studied. It is reported that NO is either not adsorbed on ZrO2 [28], or produces Zr4+-NO nitrosyls absorbing around 1900 cm1 [29] and 1760 cm-1 [29,30]. On the contrary, Ni2+-NO (or Ni2+(NO)2) species are detected around 1870-40 cm~ for the cases of supported nickel cations [31-35]. In agreement with data from the literature [27], adsorption of CO (32 Torr equilibrium pressure) on the zirconia support leads to the formation of mainly one type of Zr4+-CO complexes manifesting band at 2190 cm-~. Adsorption of CO (32 Torr equilibrium pressure) on Ni-l-Zr leads to appearance of a band at 2192 cm -1 which decreases with the equilibrium pressure and almost disappears after evacuation (Fig. 2). This band is due to the C-O stretching modes of Zr4+-CO and/or Ni>-CO type surface carbonyls. In addition, a weak and broad feature at 2120 cm1 is also visible. The latter is not observed with the pure support and originates from small amounts of (di)carbonyls of Ni § CO adsorption on reduced Ni-l-Zr also causes the appearance of the band at 2192 cmI, however with a considerably lower intensity. This confirms the supposition that the 2192 cmq band for the oxidized sample, consists of two components, one due to Ni2+-CO species, and the other to Zr4+-CO carbonyls. No metal nickel has been monitored, which suggests that the nickel particles are encapsulated in zirconia. CO adsorption on a sample which has been reoxidized by oxygen at ambient temperature results in a spectrum very similar to that registered with the unreduced sample. This means that, although the metal nickel is not surface situated, part of it can be reoxidized to Ni 2§ ions. In order to verify the existence of Ni> ions on the sample surface, the adsorption of NO has also been studied. Introduction of NO (10 Torr) to ZrO2 results in the appearance of two weak bands with maxima at 1912 and 1763 cm1 (Fig. 3). The first band is unambiguously due to the N-O stretching modes of Zr4*-NO nitrosyls [29-30], whereas the latter probably corresponds to bent species. Adsorption of NO (10 Torr) on the Ni-l-Zr sample leads to the appearance of a strong band at 1848 cm1 which loses some of its intensity during the pressure decrease, but does not disappear after evacuation. A weak band at 1763 cmq, already attributed to NO bound to the support, is also visible. No band at 1912 cmq has been detected, which evidences that the respective Zr 4+ sites have been blocked after the ion exchange. The band at 1848 cm~ is evidently due to nitrosyls formed with the participation of Ni 2§ ions. Although there is no agreement in the literature about the interpretation of this band (to Ni2+-NO [31-33] or Ni>(NO)2 [34,35] species), it proves the existence of nickel ions on the sample surface.

300

8

o c...

o

0 .13

< e

t,.O I",-. i l

2250

2200

2150

2100

2050

Wavenumber / cm-1 Figure 2. IR spectra of CO adsorbed on: ZrO2 (32 Torr CO) (a); Ni-l-Zr: 32 (b), 5 (c) and 0.5 (d) Torr CO and after evacuation (e); reduced Ni-l-Zr (32 Torr CO) (f).

|

|

I

i

i

1950 1900 18;0 1800 17;0 1700

Wavenumber / cm-1 Figure 3. IR spectra of NO adsorbed on: ZrO2 (10 Torr NO) (a); Ni-l-Zr: 10 (b) and 1 (c) Yorr NO and after evacuation (d).

The spectra of CO adsorbed on the reduced Ni-i-Zr and Ni-2-Zr samples (Fig. 4) have similar features and strongly differ from the spectrum of CO adsorbed on Ni-l-Zr. In both cases five intense bands have been registered: at 2188 cm 1 (due to Zr4*-CO species), at 2133 cm 4, 2070 cm 1 (with a shoulder at 2090 cm-1), 2035 cm1 and 1972 cm1, the latter four evidently arising from some kind(s) of nickel carbonyls. After evacuation only one band at 2100 cm 1 remains in the spectrum. This band, according to literature data [25,26], is assigned to linear nickel(0) carbonyls. Subsequent introduction of CO restores only the Zr4+-CO band and does not change the spectrum in the 2150-1900 cm~ region. These results indicate that: (i) the band at 2133 cm 1, although coinciding in position with that for Ni+-CO species, is due to another compound, since the carbonyls of Ni + are stable towards evacuation, and (ii) the band quartet arises from volatile species that are removed by evacuation. We assign the four bands to Ni(CO)4 carbonyls [36]. Thus, the results evidence that the application of a second ion exchange results in a Ni/ZrO2 sample where the nickel particles are not encapsulated in the support.

301

L0., I C

o (-. .Q L_

o ..Q t~

<

i

3.3. X-ray Diffraction The samples were studied by X-ray diffraction. XRD provided no information about the nickel particles, since the strong zirconia bands masked the Ni-lines. This technique, however, allowed calculation of the nickel particles in the Ni/SiO2 reference samples: Ni-l-Si was found to have a diameter of about 2 nm, whereas the average metal particle diameter in Ni-i-Si was 16 nm.

3.4. Transmission Electron Microscopy

i

i

2200

'

2100

i

'

'"

i

2000

1900

W a v e n u m b e r / cm -1 Figure 4. IR spectra of CO (32 Torr) adsorbed on reduced: Ni-i-Zr (a), Ni-l-Zr (b) and Ni-2-Zr (c).

With TEM there was a similar situation: no metal nickel was detected on the Ni/ZrO2 samples. This is most probably due to the poor contrast between the metal and the support as observed e.g. for nickel supported on titania [37].

3.5. Ferromagnetic Resonance One of the techniques giving intbrmation on the dispersion of supported ferromagnetic metals is FMR [38,39]. The exchange interaction in fbrromagnets is much more pronounced than that in paramagnets. As a result, even with ferromagnetic particles having an average diameter of dozens of nanometers, it is possible to obtain a resonance line which is narrow enough to be detected. The increase in metal particle size leads to an increase in the magnetization and the anisotropic effect [38]. As a result, the lines broaden and the g-factor is shit~ed. However, with very small particles (d _< 1 nm) the dipole-dipole interaction is very important and the lines also become very broad and might not be detected [38]. Studies on supported Ni~ particles [39] have shown that when the particle diameter is below ca. 12 nm: the FMR signal intensity increases with the particle size, whereas above 12 nm the signal remains unchanged.

302

.'~

d

.-

,.'" o," sp f'~

c

o

_.= O ,. i..

o'"

Figure 5. FMR spectra of reduced: Ni-l-Zr (a), Ni-2-Zr (b), Ni-3-Zr (c), Ni-i-Zr (d), Ni-l-Si (e) and Ni-i-Si (f).

The FMR spectra (normalized per unit sample mass) of the reduced e x s i t u Ni/ZrO2 samples show a broad and symmetric signal with a g-factor of 2.28 due to dispersed metal nickel (Fig. 5). It is seen that with the increase in the number of exchanges, the lines become broader. For the Ni-1-Zr, Ni-2-Zr and Ni-3-Zr samples line width, z~H, is 990, 1150 and 1420 G, respectively. The Ni-i-Zr catalyst is characterized by g = 2.30 and M-I = 1190 G, i.e. the impregnated sample, having the same nickel concentration as Ni-1-Zr, shows a lower metal dispersion. The Ni-l-Si reference sample (with a metal particle diameter of 2 nm) has demonstrated a rather narrow signal (AH = 320 G), with a g-factor of 2.22. The other reference sample, Ni-i-Si (d = 16 nm) manifested a signal with a g-factor of 2.35 and M-I = 1540 G. The increase in the integral intensity per unit mass of nickel for the different samples is as follows: Ni-1-Si (2 nm)< Ni-l-Zr < Ni-i-Zr = Ni-2-Zr < Ni-3-Zr < Ni-i-Si (16 nm). Thus, on the basis of the FMR measurements, the following conclusions can be made for the exchanged Ni/ZrO2 samples: (i) the nickel particle diameter for all of them is between 2 and 16 nm and (ii) the average metal particle diameter increases with the number of exchanges.

303 4. CONCLUSIONS: 9 Zirconia active sites, blocked by Ni 2. during ion exchange, are liberated after reduction of the deposited cations. The partial reoxidation of Ni~ does not lead to re-blocking of these active sites, which allows performance of a next ion exchange. 9 Each successive ion exchange leads to an increase in nickel concentration (by the value achieved after the first exchange) and results in growth of the metal particles. 9 The metal nickel particles in the reduced Ni-l-Zr sample are encapsulated in zirconia, whereas already the second ion-exchange results in exposure of metal nickel on the sample surface. 9 Multiple ion exchange can be applied to the preparation of zirconia-supported nickel catalysts with the desired metal particle size.

REFERENCES

1. .

3. .

5. 6. 7. 8. .

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

G. Zakumbaeva, N. Zakharina, L. Beketaeva and V. Naidin, Metal Catalysts (D.V. Sokolskyi, ed.), Nauka, Alma-Ata, 1982. B. Dolgov, Catalysis in Organic Chemistry, Gosudrstvenoe Khimitcheskoe Izdatelstvo, Moscow, 1949. C.L. Thomas, Catalytic Processes and. Proven Catalys.ts, Academic Press, New York, 1970. L.A. Bruce and J.F. Mathews, Appl. Catal., 4 (1982) 353. L.A. Bruce, G.J. Hope and J.F. Mathews, Appl. Catal., 8 (1983) 349. A.V. Macunin, S.I. Serdyukov and M.S. Safonov, Nefiekhimiya, 36 (1996) 418. A.S. Ivanova, I.I. Bobrova, E.M. Moroz, V.A. Sobyanin and V.Yu. Gavrilov, Kinet. Katal., 38 (1997) 114. J.A. Lerher, J.H. Bitter, W. Hally, W. Wiessen and K. Seshan, Proc., 11th Int. Congr. Catal., Stud. Surf. Sci. Catal, 101 (1996)463. Y. Zhang, Z. Ma, W. Lin, Cuihua Xuebao, 17 (1996) 375. R.A. Dalla Betta, A.G. Piken and M. Shelef, J. Catal., 40 (1975)173. T. Yamaguchi, Catal. Today, 20 (1994) 199. G.R. Gavalas, C. Phichitkul and G.E. Voecks, J. Catal., 88 (1984) 54. M. Che and C.O. Bennett, Adv. Catal., 38 (1989) 55. M. Arai, S.L. Guo and Y. Nishiyama, Appl. Catal., 77 (1991) 141. K. Hadjiivanov, J. Saint-Just, M. Che, J.-M. Tatibou~t, J. Lamotte and J.-C. Lavalley, J. Chem. Soc. Faraday Trans., 90 (1994) 2277. K Hadjiivanov, E. Vassileva, M. Kantcheva and D. Klissurski, Mater. Chem. Phys., 28 (1991) 367. B. Mile, D. Stifling, M.A. Zammitt, A. Lovell and M. Webb, J. Catal., 114 (1988) 217. P. Burattin, M. Che and C. Louis, J. Phys. Chem., B 101 (1997) 7060. G.L. Haller and D.E. Resasco, Adv. Catal., 36 (1989) 173. A.G. Sault, C.H.F. Peden and E.P. Boespflug, J. Phys. Chem., 98 (1994) 1652.

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21.

91998 Elsevier Science B.V. All rights reserved.

Preparationof CatalystsVII B. Delmon et al., editors.

305

New Heterogeneous Nickel Catalysts For Enantioselective Hydrogenation. S. David Jackson*, E. Allan a, G. Webb a, and N. C. Young al. ICI Katalco, Research, Technology and Engineering Group, PO Box 1, Billingham, Cleveland TS23 1LB, U.K. adept, of Chemistry, The University, Glasgow G12 8QQ, Scotland, U.K.

1.

INTRODUCTION

Although tartaric acid modified nickel catalysts are well known for their use in [3-keto ester hydrogenation, there have been few examples of other nickel catalysts that have been effective in catalysing enantioselective reactions [ 1]. Therefore we set out to develop new modifier/nickel/support systems, that would be effective in hydrogenating systems other than [3-keto esters. In the vast majority of previous research there has been little attempt to define the modifier/metal/support system. However we believed it to be important to characterise the preparation of the catalysts so that we understood the processes occurring during catalyst manufacture, to enable an understanding of the catalysis with respect to its relationship with the structure and chemistry of the catalyst. To this end we have prepared, characterised and tested catalysts developed from substituted binaphthalenes/nickel/silica. Chiral binaphthalene derivatives were chosen as they had already been shown to very successful modifiers in asymmetric homogeneous catalysis [2, 3]. The preparation of the catalysts was divided into four stages, 1) preparation of the nickel salt/silica, 2) reduction of the nickel salt/silica, 3) treatment of the reduced nickel/silica with the binaphthalene derivative, and 4) washing the binaphthalene/nickel/silica material to remove weakly adsorbed organic. The catalyst was characterised at each stage by a variety of techniques so that as much as possible was known about a given state before the next process was initiated. At the end of processing steps 2, and 4, the catalyst was tested for its efficacy in an asymmetric hydrogenation reaction. The reactions chosen were the hydrogenation of methyl tiglate and the hydrogenation of tiglic acid. 2.

EXPERIMENTAL

The metal content of the catalysts was determined by atomic absorption spectrometry using a Perkin Elmer 1100B spectrophotometer, after dissolution of the nickel from the catalyst in a 1Current address, ICI Katalco, Research, Technology and Engineering Group, PO Box 1, Billingham, Cleveland TS23 1LB, U.K.

306 solution of nitric acid. Temperature programmed reduction (TPR) was performed by taking 0.2 g of unmodified catalyst and following the reduction over the range 273 - 973 K. The catalyst was heated at 5 K.min ~ in 6% H2/N2 (40 cm3.min~). The effluent gas was monitored by a thermal conductivity detector. Metal surface area was determined by both CO and 02 pulse chemisorption. A sample was reduced in flowing H2 and then purged with helium and cooled to 293 K. Pulses (typically 15.78 gmoles) were passed across the sample until saturation. The quantity adsorbed was determined by a difference method between the quantity of adsorbing gas in the effluent stream and that in a reference pulse. Transmission electron microscopy measurementswere conducted using a Phillips 1200 electron microscope. Modification of the basic catalysts was performed using R(+) or S(-) -2,2'dihydroxy- 1, l'-binaphthalene, R(+) or S(-) -2,2'-diamino- 1, l'-binaphthalene, and (_+)-dimethoxy-l,l'-binaphthalene. Allmodifiers were greater than 99% pure. Hydrogenation of the tiglic acid and the methyl tiglate was performed in the liquid phase, at 293 K and 1 atm., using tetrahydrofuran (THF) as the solvent. The catalysts were reduced in flowing dihydrogen at 723 K, modified with the appropriate binaphthalene, washed with solvent (THF), then the reactant added. Dihydrogen was bubbled through the stirred solution continuously during the reaction. The reactants and products were analysed by chiral gas chromatography. 3.

RESULTS

3.1

Preparation of Ni(NOB)z/silica

To prepare the impregnated catalysts sufficient solution of Ni(NO3)2 was added to the silica supports (Fuji Silysia Q 10, and Fuji Silysia Q30). The resulting suspension was evaporated to dryness, heated in air at 373 K for 16 h, and finally 393 K for 3.5h. The catalysts were then calcined in air at 723 K for 2h. A third catalyst was prepared by a precipitation/deposition route. To an aqueous slurry of silica (Cab-O-Sil 5M) was added nickel nitrate and urea in a molar ratio of 0.36. The pH of the solution was adjusted to 2.4. The slurry was then heated to 363 K and held for 5 h at this temperature with constant stirring. The suspension was filtered, washed, and dried at 387 K for 22 h. The dried material was calcined in air at 723 K for 5 h. The amount of nickel on each catalyst was determined by atomic absorption spectroscopy. Table 1. Properties of as-prepared catalysts. Catalyst

% w/w Ni loading

Ni/Q 10

9.2

Support surface area Support pore diameter 307 mZgl

9.9 nm

Ni/Q30

8.4

106 m2g~

31.5 nm

Ni/Cab-O-Sil

6.8

194 mZg1

non-porous

307 3.2

Reduction of Ni(NO3)Jsilica

TPR was performed on the three samples and the up-take of dihydrogen recorded. The two impregnated catalysts gave very similar TPR profiles. The Ni/Cab-O-Sil however gave a different profile with a high temperature reduction peak at 879 K. It is noticeable that the temperature where reduction ceased after the first reduction peak (Ttanal) is similar for both the impregnated and the deposition/precipitation systems. The number of exposed metal surface atoms, available after reduction, was determined by CO and 02 chemisorption and is reported in Table 3. The reduced catalysts were also examined by HRTEM and average particle diameters determined. These were found to be in excellent agreement with those calculated from the chemisorption data. The Ni/Q 10 had an average particle diameter of 207 nm from HTREM and 184 nm from chemisorption, the Ni/Q30 had an average particle diameter of 106 nm from HTREM and 94 nm from chemisorption, while the Ni/Cab-O-Sil had an average particle diameter of 7 nm from HTREM and 5 nm from chemisorption. Table 2. TPR of catalysts Catalyst

Tmax (K)

Tt~n~(K)

Ni/Q I 0

641

668sh

723

Ni/Q3 0

618

643sh

723

Ni/Cab-O-Sil

603

879

703

Table 3. Chemisorption of carbon monoxide and oxygen

3.3

Catalyst

% Dispersion CO adsorption

% Dispersion Oz adsorption

No. surface metal atoms xl 0~Sg~

Ni/Q 10

0.45

0.62

5.19

Ni/Q30

0.98

1.23

9.24

Ni/Cab-O-Sil

18.04

19.71

137.39

Modification of Ni/silica with Substituted Binaphthalenes

The adsorption of the three substituted binaphthalenes (diol, diamine, and dimethoxy) was studied on each catalyst and support sample. After reduction the catalyst was cooled to 293 K in a flow of dinitrogen. THF solutions containing a substituted binaphthalene were then added to the catalyst. The dinitrogen atmosphere was maintained and the catalyst was left in contact with the modifier solution for 24 h with constant stirring. After modification the

308 concentration of the remaining solution was determined by HPLC. Polarimetry was used to determine whether the modifier had retained its chirality after being in contact with the catalyst. The supports, in the absence of the nickel, did not adsorb any substituted binaphthalene. Similarly the dimethoxy species did not adsorb on any of the catalyst samples. When the three catalysts were subjected to modification of a solution containing the diol, it was found that there was corrosive chemisorption. The solution changed colour to yellow/green and UV-visible spectroscopic analysis revealed bands at 439 nm, 397 nm, and 373 nm indicative of a Ni (II) ion. Quantitative analysis was inconclusive indicating a concentration of metal < 1ppm. The amount of diol adsorbed is reported in Table 4. The chirality of the diol was retained throughout the modification procedure. When the diamine was used as the modifier there was no coloration of the solution. The amount adsorbed is also reported in Table 4. The chirality of the diamine was retained throughout the modification procedure.

3.4

Washing of Modified Binaphthalene/Ni/silica

After the catalysts had been modified by a substituted binaphthalene the modifying solution was removed and fresh THF added. The suspension was stirred and after 1h the THF was analysed for substituted binaphthalenes. This process was repeated for up to 24 h, however all the weakly held binaphthalene was desorbed within the first 1 h. The extent of the weakly held material is reported in Table 4.

Table 4 Adsorption and desorption of substituted binaphthalenes Catalyst

Modifier

Initial adsorption ~mole.g 1

% removal on washing

Final modifier: Nis ratio

Ni/Q 10

R-diamine

2.04

15

0.2

S-diamine

2.03

16

0.19

R-diol

7.34

7

0.79

R-diol

8.39

6

0.52

S-diol

8.48

3

0.53

Ni/Q30

Ni/Cab- O- Sil

R-diamine

1.81

6

0.11

S-diamine

2.16

32

0.1

S-diol

3.07

0

0.01

S-diamine

4.85

0

0.02

309 3.5

Testing of catalyst samples

The catalysts were tested for their efficacy in hydrogenating methyl tiglate to 2-methyl butyric acid methyl ester, and tiglic acid to 2-methyl butanoic acid. The supports were inactive for both hydrogenation reactions. The unmodified nickel catalysts were inactive for the hydrogenation of the methyl tiglate, as were the diol and diamine modified systems. However the Ni/Cab-O-Sil did hydrogenate tiglic acid to 2-methyl butanoic acid. If at any stage after the reduction the catalyst is exposed to air, all activity is lost.

Table 5 Hydrogenation of tiglic acid Catalyst

Modifier

Amount adsorbed ~mole.g"

% Conversion

% e.e.

Ni/Cab-O-Sil

None

0

45.6

0

R-diamine

1.38

7.3

3

S-diamine

4.85

0.5

3

4. DISCUSSION The preparation of catalysts for use in chiral catalysis has often been marked by a lack of characterisation of the intermediate stages and indeed of the final catalyst. In this study we have tried to characterise the samples at each stage of the preparation. The TPR of the as prepared Ni/Q 10 and Ni/Q30 materials gave very similar profiles. The main peak at approximately 623 K is assigned to the reduction of bulk nickel oxide [4]. While the shoulder indicates a second nickel (oxide) species which is slightly more difficult to reduce. The peak at 603 K with the Ni/Cab-O-Sil is also assigned to nickel oxide (hydroxide). The high temperature peak (879 K) can be assigned to the reduction of a nickel species that is likely to be the nickel hydrosilicate [5]. By setting the reduction temperature at 723 K all the nickel oxide (hydroxide) species were reduced, however some of the nickel, present as nickel hydrosilicate, in the Ni/Cab-O-Sil sample would not be reduced. The difference in the average particle size of the nickel between the catalysts was revealed by chemisorption and HRTEM. This variation in particle size allowed the effect of particle size on adsorption of the substituted binaphthalenes to be investigated. Previous studies had indicated that large crystallites were preferable for chiral catalysis [6]. When the catalysts were put in contact with the solution containing the diol there was leaching of the nickel into solution. Given the yellow coloration and the absence of bands above 1000 nm the UV-visible spectra of the solutions suggest that the species in solution is a Ni (II) ion in a square planar geometry. Note that no coloration was found with the silicas in the absence of nickel and that once the initial quantity was solubilised, no further leaching was observed. It is worth noting that only with the diol modifier was leaching observed. No

310 leaching was detected with the diamine, dimethoxy, or hydroxy-methoxy binaphthalenes, nor with naphthalene itself. These results suggest that the adsorption was through both OH groups in the diol as if only one hydroxy group was involved it would be expected that the hydroxy-methoxy derivative would also have initiated leaching. The cause of this leaching however is less clear. A simple interpretation would be that the diol extracts nickel species from the metal matrix that are not fully reduced and indeed that may be the case. Clearly there is no leaching of the hydrosilicate species. However leaching with the diol binaphthalene also occurred with palladium derived catalysts [7] and although non-reduced nickel species may be not unexpected it is less expected with palladium.

Figure 1. Mode of adsorption of-OH and -NH2 2,2'-substituted binaphthalenes.

311 If we now examine the adsorption of the modifiers we find that dimethoxy did not adsorb on any of the samples. Similarly naphthalene itself was found not to adsorb. However the diol and diamine both adsorbed. From these results it would appear that the mode of adsorption is through the substituent groups and not through the naphthalene ring system (Figure 1). When the substituents are bulky and cannot dissociate we find that there is no adsorption. Molecular modelling of the adsorption on a planar surface [7] suggests that the modifier:Ni, ratio should be 0.2 or less. From Table 4 we can see that the catalyst with the largest particle size (Ni/Q 10) has a modifier:Nis value of 0.2 whereas as the particle gets smaller the packing of the modifier on the surface is less efficient and the modifier:Ni~ ratio decreases. It should also be noticed that with the low dispersion catalysts the adsorption of the diol gives a modifier:Nis value in excess of 0.2. This may be due to the corrosive chemisorption process; by removing atoms from the surface it is possible to generate a surface with a higher site density by producing a defective surface in situ. This would lead to an underestimate of the number of surface Ni atoms and hence an artificially high ratio. The washing of the catalysts to remove weakly held material was an important step in confirming the final state of the catalyst before use in the catalytic reactions. In this way the absence of both modifier and metal in the solution could be ensured. Washing the modified catalysts does lead to the removal of weakly adsorbed material. This desorbed material does retain its chirality through the adsorption/desorption process. The extent removed is small overall and does not show any noticeable trends. Only the Ni/Cab-O-Sil shows no evidence for weakly held species. Given that the silica supports do not adsorb the modifiers it is likely that the weakly held material is associated with the metal. Two possibilities exist, one that in the presence of modifier in solution the adsorbed layer is compressed to accommodate a higher surface concentration. When the modifier is removed from solution the system relaxes desorbing the excess over the ideal monolayer coverage. This behaviour has been seen in gas phase adsorption [8, 9] where it is detectable through isotopic exchange. The second possibility is that the strongly adsorbed species can weakly adsorb in their own right by a n - n stacking interaction as is found when binaphthyl molecules are used in chiral recognition systems [10, 11 ]. When the nickel catalysts were tested for hydrogenation of methyl tiglate no activity was discerned. Not surprisingly then, when the modified systems were tested for the same reaction no activity was found. The hydrogenation of tiglic acid however was more successful. The unmodified catalyst gave a conversion of 45.6 % with no enantiomeric excess. This difference between the two reactants suggests that adsorption takes place through the acid functionality. When modifier is added (Table 5) we see that there is a significant drop in activity. As the amount of modifier is increased the activity decreases, indeed when the modifier is at monolayer coverage the activity is reduced by 99 %. Therefore the modifier is acting in a site blocking mode inhibiting the reaction. However there is residual activity and enantioselectivity has been generated. Therefore not only does the modifier block most of the sites it also directs the adsorption and hydrogen addition in the sites left. It is likely that this process occurs via a n - n interaction between the naphthalene ring and the delocalised z~-system of the adsorbed tiglic acid coupled with a hydrogen bonding interaction between the C-O of the tiglic acid and the hydrogen and the

312 hydrogen on the amine function. Such an interaction forces the adsorbed acid into a specific conformation and lowers the surface energy of the whole complex. Hydrogen addition is then controlled such that only a single enantiomer can be produced.

5. CONCLUSIONS In conclusion we have produced an active enantioselective catalyst which has been characterised at each stage of its preparation. The effect of the preparation method is seen on the particle size of the nickel crystallites, the size of the crystallites is seen to influence the adsorption of the modifier, while the extent of modifier adsorption is seen to effect the activity/selectivity of the final catalyst. In this study we have shown that it is possible, and desirable, to characterise at each stage of the preparation, and how this can be used to understand the current process and hence to design new and improved catalysts.

6. REFERENCES

1. G. Webb and P. B. Wells, Catalysis Today, 12 (1992) 319. 2. A. Miyashita, H. Takaya, T.Souchi, and R. Noyori, Tetrahedron, 40 (1984) 1245. 3. H. Muramatsu, H. Kawano, Y. Ishii, M. Saburi, and Y. Uchida, J. Chem. Soc., Chem. Commun. (1989) 769. 4. S. D. Robertson, D. D. McNicol, J. H. DeBaas, and S. C. Kloet, J. Catal., 37 (1975) 424. 5. J. T. Richardson and R. J. Dubus, J. Catal., 54 (1978) 207. 6. Y. Nitta, F. Sekine, T. Imanaka, and S. Teranishi, Bull. Chem. Soc. Jpn., 54 (1981) 980. 7. N. C. Young, Ph.D. Thesis, University of Glasgow, 1995. 8. S. D. Jackson, React. Kinet. Catal. Lett., 34 (1987) 239. 9. S. D. Jackson, J. Catal., 115 (1989) 247. 10. J. Reeder, P. P. Castro, and C. B. Knobler, J. Org. Chem., 59 (1994) 11. 11. P. Qian, M. Matsuda, and T. Miyashita, J. Am. Chem. Soc., 115 (1993) 5624.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

313

N e w Preparation Method of Asymmetrically Modified Supported Nickel Catalysts for the Enantio-differentiating Hydrogenation of Methyl Acetoacetate Tsutomu Osawa *, Shinya Mita, Akiko Iwai, Osamu Takayasu, Tadao Harada a, and Ikuya Matsuura Faculty of Science, Toyama University, Gofuku, Toyama 930-8555, Japan a Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520-2194, Japan

Supported nickel catalysts were prepared from nickel acetylacetonate, and then modified in a solution containing tartaric acid and NaBr. The effects of the catalystpreparation conditions on the optical yields and those of the Ni surface modification process on optical yields were examined in the enantio-differentiating hydrogenation of methyl acetoacetate. The maximal optical yield of 86% was attained when zeolite was used as the support. 1. INTRODUCTION The enantio-differentiating hydrogenation over asymmetrically modified heterogeneous metal catalysts has been attracting much attention. Modified nickel catalysts [1-6], modified platinum catalysts [7, 8], and modified palladium catalysts [9, 10] have been intensively studied and succeeded in obtaining optically active products. In the enantio-differentiating hydrogenation of I~-ketoesters and alkanones, tartaric acidNaBr-modified nickel catalysts are the best system for attaining a high optical yield [11, 12]. '~,,.OCH3 O

O

H2

~. ~ " ~

TA-NaBr-modified Ni catalyst

pOCH3

OH O

Various nickel sources were tested in the preparation of the modified nickel catalysts. For example, Raney nickel, nickel powder prepared from nickel oxide, and commercial nickel powder were used as the unsupported catalysts. It is known that a number of catalyst preparation variables affect the optical yield. In the case of the Raney nickel catalyst, the Raney alloy has several Ni-AI phases and optical yield depends on the composition of these phases [13]. The temperature and pH of the modification solution affect the content of residual aluminum on the surface of the Raney catalyst [ 14]. The residual aluminum compounds on the surface reduce the optical yield, but aluminum on Raney catalysts can not be completely removed. Nickel catalysts prepared from nickel oxide and commercial nickel powder have the problem that the resulting modified catalysts showed scattered optical yields according to the manufacturers of the nickel oxide and nickel powder [15-17]. Therefore, although tartaric acidNaBr-modified unsupported nickel catalysts generally gave high optical yields in the

314 hydrogenation of 13-ketoesters and alkanones, it would be difficult in attaining higher optical yields than the values reported so far. On the other hand, nickel catalysts on various supports were also used for the preparation of asymmetrically modified nickel catalysts and the preparation variables were studied in order to attain the highest optical yield, but the resulting optical yields were lower than those obtained by the modified unsupported nickel catalysts [6, 18-19]. Additional effects by the supports would be expected when supported catalysts were used. For preparing a catalyst having a higher activity in the hydrogenation reaction and higher enantio-differentiating ability than those of the catalysts reported so far, a breakthrough in the catalyst preparation method is required. In this study, nickel catalysts supported on various supports were prepared from the mixture of nickel acetylacetonateand various supports. The hydrogenation of methyl acetoacetate was carried out over the resulting modified supported nickel catalysts. 2. E X P E R I M E N T A L

The GLC measurement of the products was carried out using a Hitachi 263-30 gas chromatograph. The optical rotations were measured with a JASCO DIP-1000 polarimeter. The X-ray diffraction patterns of the catalysts were measured with a Shimadzu XD-3A diffractometer. Materials All the chemicals except ct-A1203 and SiO2 were used as received, ct-A1203 was obtained by the treatment of "~-Al203 at 1150 ~ for 6 h. SiO2 was obtained by drying silica sols (Snowtex 30, Nissan Chemical Co.) at 110 ~ Preparation of supported nickel catalyst Nickel acetylacetonate(3.3 g) and a support (1 g, in the case of 40 wt%) were mixed well, and formed into a pressed tablet at 200 kg/cm2. This tablet was treated in an Ar stream typically at 270 ~ for 2 h and at 370 ~ for 1 h. After the decomposition of Ni-acetylacetonate, the sample was crushed into a 25-30 mesh powder and then treated in a H 2 stream typically at 500 ~ for 3 h. Modification of the catalyst The supported nickel catalyst was soaked in 100 cm3 of aqueous solution containing (R,R)tartaricacid (1 g) and NaBr (amounts were described in the text) at 0 ~ or at 100 ~ for 1 h. The pH of the modification solution was adjusted in advance with 1 mol dm 3 NaOH solution. After the modification, the catalyst was separated from the modification solution by centrifugation. The catalyst was successively washed once with 45-cm3 water, twice with 45cm3 methanol, and twice with 45-cm~ THF. Hydrogenation of methyl acetoacetate The catalyst thus obtained was employed for the hydrogenation of methyl acetoacetate (10 g) in THF (20 cm3) containing acetic acid (0.2 g) in an autoclave. The initial hydrogen pressure was 10 MPa and the hydrogenation temperature was 100 ~ The hydrogenation was completed within 12 h using a-A1203 and zeolites as the support. The modified catalysts

315 prepared from y-A1203 needed 3 days and that from SiO2 needed 5 days for the completion of hydrogenation. A simple distillation gave the hydrogenation products with chemical purities of more than 98 % (GLC analyses: 90 ~ 5 % Thermon 1000 on Chromosorb W). Measurement of mean crystallite size The mean crystallite size of the nickel particles on a modified catalyst after a reaction was obtained from the half-width of the Ni (111) peak using Scherre' s method. Determination of optical yield The optical purity of methyl 3-hydroxybutyrate was determined by polarimetry. Optical purity (%) =

[(~]2~ hydrogenation product X100 [ct]~~ optically pure methyl 3 -hydroxybutyrate

The specific optical rotation of optically pure

[14].

(R)-methyl 3-hydroxybutyrate is -22.95 ~ (neat)

3. R E S U L T S A N D D I S C U S S I O N

The preparation scheme of the asymmetrically modified supported nickel catalysts prepared from nickel acetylacetonate is illustrated in Figure 1. In the catalyst preparation process, an active nickel catalyst was prepared from nickel acetylacetonate and a support. In this process, the important parameters affecting the optical yield were the temperature of the Ni acetylacetonate decomposition, hydrogen treatment temperature of the precursor, Ni weight percentage of the catalyst, and the type of support. In the modification process, the modification pH, the modification temperature, and the amount of NaBr in the modification solution affected the optical yield.

Source of nickel catalyst (Ni acetylacetonate and support) Catalyst preparation process

Decomposition of Ni acetylacetonate Hydrogen treatment

Supported nickel catalyst Modification process

Immersion in the solution of tartaric acid and NaBr

Modified supported nickel catalyst Figure 1. Preparation scheme of modified supported nickel catalyst

316

3.1.

Conditions for catalyst preparation process

Preliminary experiments showed that the decomposition temperature affected the optical yield and that the decomposition at the temperature just above the melting point of nickel acetylacetonate (270 ~ m.p. of nickel acetylacetonate hydrate: 230260 ~ dec.) tot 2 h and followed by a higher temperature (370 ~ for 1 h gave a high optical yield. Figure 2 shows the relation between the initial Ni weight percentage and optical yield, when otAI203 was used as the support [20]. The optical yield increased with the increase in the Ni loading percentage and reached a plateau. It is recognized that more than 40 % of the initial Ni loading percentage is necessary to attain more than 80 % optical yield. After the decomposition of Ni acetylacetonate, this precursor was treated with hydrogen gas for obtaining the active nickel catalyst. The effect of the Ni/c~-Al203 precursor hydrogen treatment temperature on optical yield is shown in Table 1. The optical yield increased with the increase in hydrogen maximum value was attained at 500-600 ~ Table 1 also shows the effect of

1O0 O 80 o~ "o 60 ._o "~ 40 "~a.

l

0 20

10

,,1,,

t

!

t

I

i

20

30

40

50

60

70

L

80

90

Percentage of Ni loaded / % Figure 2. The relation between the Ni weight percentage of Ni/et-Al203 " catalyst and the optical yield b [20] This was the initial Ni weight percentage in the preparation of the supported nickel catalyst. A small amount of Ni was dissolved into the solution during the modification. b Modification was carried out in the 100 cm 3 aqueous solution containing 1 g tartaric acid and 6 g NaBr at pH 3.2 and 100 ~ for 1 h.

treatment temperature from 300 ~ to 600 ~ and the ~ However, the optical yield decreased beyond 600 hydrogen treatment temperature on the mean nickel

Table 1 Effect of t.he hydrogen treatment temperature on optical yiel d" Support Hydro~;en treatment temp. / ~ Optical yield / %

Mean crystallite size / nm

a-Al203

300

32

8

et-Al203 et-Al203

400 500

56 83

12 53

a-Al203 600 85 51 c~-Al20.~ ..... 700 76 . 68 a Initial Ni percentage loaded: 40 wt%. Modification was carried out in the 100 cm 3 aqueous solution containing I g tartaric acid and 6 g NaBr at pH 3,2 and 100 ~ for 1 h.

317

crystallite size. When cx-A1203 was used as the support, the increase in hydrogen treatment temperature increased the mean crystallite size. The mean crystallite size of about 50 nm gave over 80 % optical yield (hydrogen treatment temperature: 500 and 600 ~ Although the mean crystallite size was large, the hydrogenation was completed within 12 h. It has been known that a large nickel mean crystallite size was generally favorable for attaining a high optical yield [21]. Nitta et. al. demonstrated that modified nickel with a 26 nm crystallite size on SiO 2 gave a 62% optical yield [22]. The enantio-differentiating hydrogenation of ethyl acetoacetate over 60 % Ni/aerosil with a 120-180 nm mean crystallite size has also been reported. The optical yield of this reaction was 9.3 % [23]. The crystallite size of about 50 nm would provide a suitable surface for the effective enantio-differentiation with the aid of tartaric acid. Table 2 shows the results of the enantio-differentiating hydrogenation of methyl acetoacetate over various modified supported nickel catalysts. The results of the hydrogenation over the Table 2 Effects of Ni sources and supports on optical yields Ni source

Support

.

.

.

.

Initial Ni percentage

Optical yield

loaded / wt % Ni acetylacetonate

without support

Ni acetylacetonate Ni acetylacetonate

ct-A1203 ~_A120~

Ni acetylacetonate Ni acetylacetonate

y- A120~ zeolite (Si/AI= 15)

.

f

.

.

/% .

.

.

a, d,e

40 c 40 c

81 a, 83 b

80 c 40 c

78 b 86 ~' ~

Ni acetylacetonate Ni acetylacetonate Ni acetylacetonate

zeolite (Si/Al=46) g zeolite (Si/Al=123) h zeolite (Si/AI=215) i

40 ~ 40 c 40 c

83 a, o 84 a, 84 a,

Ni acetylacetonate

SiO 2

40 ~

73 a,

Ni acetylacetonate j ct-A1203 40 81 a Ni(NO3)2 k ~-A120s 40 71 a' e Raney nickel without support --86 b, o a Modification was carried out in the 100 cm 3 aqueous solution containing 1 g tartaric acid and 1 g NaBr at pH 3.2 and 100 ~ for 1 h. b Modification was carried out in the 100 cm 3 aqueous solution containing 1 g tartaric acid and 6 g NaBr at pH 3.2 and 100 ~ for 1 h. c Initial Ni percentage loaded giving maximal optical yield. d Hydrogenation reaction did not take place. eReference [20] f Siid Chemie, H-type Pentasil powder (EX-122). g Stid Chemie, H-type Pentasil powder (T-4480). h Stid Chemie, H-type Pentasil powder (EX-717). i Stid Chemie, H-type Pentasil powder (EX-504). J Ni acetylacetonate and a support were suspended in THF and then evaporated. (decomposition condition was the same as for the case of making a tablet without solvent) k The catalyst was prepared by a precipitation method [24].

318 modified RNi cmalyst and the modified supported nickel catalyst prepared from Ni(NO3) 2 solution by the precipitation method are also listed for comparison. When a-A1203 or zeolite was uscd as a support, an optical yield of over 80 % was attained. The maximal optical yield 86 % was achieved over the catalyst with a zeolite support. This is the highest optical yield of thereported values using an asymmetrically modified supported Ni catalyst so far and is comparable to the values obtained by the modified Raney nickel catalysts. When nickel acetylacetonate and a support were suspended in a solvent (THF) and evaporated in vacuo instead of making a tablet without solvent, the same optical yields (81%) were obtained. The modified nickel catalyst prepared from Ni(NO3) 2 by the precipitation method, which is the conventional method for preparing supported catalysts, gave only a 7 1 % optical yield. When a nickel catalyst was prepared from nickel acetylacetonate without a support, the resulting modified catalyst had a very low hydrogenation activity. 3.2.

C o n d i t i o n s for the p r o c e s s of Ni s u r f a c e m o d i f i c a t i o n

It has been demonstrated that a modification pH and a modification temperature are the key parameters for attaining high optical yield with the 8O modified Raney nickel catalysts [25, 26] and modified supported nickel Q x~ 60 catalysts reported [6]. .ca >, Figure 3 indicates the relation o 40 between the modification pH and optical yield. When pH was not 0 adjusted with NaOH solution (pH 1.9), 2o a low optical yield was obtained. The pH 3.2 modification gave the maximal 0 I i I I ,I I I I I I I optical yield, while optical yield was 1 2 3 4 5 6 7 8 9 10 11 12 13 decreased beyond pH 3.2. These Modification pH phenomena were similar to the case of Figure 3. The relation between the modification the Raney nickel catalyst modified at pH and the optical yield a 100 ~ [14]. However, the pH Support: ct-Al203. Initial Ni percentage loaded: dependence of optical yield using 40 wt%. Modification was carried out in the 100 supported nickel catalysts reported in cm3 aqueous solution containing 1 g tartaric acid the literature are different from that of and 6 g NaB r at 100 ~ for 1 h. the Raney nickel catalyst [6, 27]. Table 3 shows the effect of a modification temperature on optical yield. Modification at 100~ gave higher optical yield than 100

.m

Q.

Table 3 Effect of the modification temperature on optical yield a Support Modification temperature / ~ Modification time / h

Optical .yield / %

ct-Al203 0 1 63 a-AlzO3 100 1 81 . . . . . . "Initial Ni percentage loaded: 40 %. Modification was carried out in the 100 cm3 aqueous solution containing 1 g tartaric acid and 1 g NaBr at pH 3.2 for 1 h.

319 0 ~ modification. According to the studies of modified RNi catalyst, following two factors are important for attaining high optical yield. 1. Adsorption species on the catalyst surface are sodium tartrate or disodium tartrate [28, 29]. 2. The surface was made using pure nickel or regularly arranged nickel [30]. These two factors would be achieved by a pH 3.2 modification at 100 ~ It would be assumed that in the case of the modified supported nickel catalyst prepared in this study, the same surface condition and adsorbed species as that of the modified RNi would provide the effective enantio-differentiation. Inorganic salts in a modification solution increase the optical yield in the hydrogenation over modified Raney nickel [31] or modified supported nickel catalysts [32, 33]. The consensus of the role of NaBr has not yet been achieved [32, 33, 34]. We proposed that inorganic salts selectively adsorbed and deactivated the non-enantio-differentiating site (site where racemic products are produced) [34]. NaBr is known to be the most effective inorganic salt improving the optical yield. Figure 4 shows the relation between the amount of NaBr in the modification solution and optical yield. The addition of NaBr increased the optical yield which reached a plateau when using more than 1 g of NaBr. The difference between the optical yield without NaBr and optical yield after reaching the plateau is smaller than for the case of the Raney nickel 100 catalyst. From these results, the percentage of the enantio-differentiating site of the modified Ni/et-Al203 catalyst 80 O prepared in this study was higher than that of the modified Raney nickel "co 60 catalyst. ,,.,.

40

4. C O N C L U S I O N S

20

Optical yields attained in this study were much higher than those obtained using the conventional modified supported nickel catalysts prepared in a Ni ion solution. The results obtained by the present work demonstrate that a preparation from nickel acetylacetonate is a promising method for the preparation of an asymmetrically modified nickel catalyst. This method would produce nickel particles, the surfaces of which are suitable for the formation of enantiodifferentiating sites with the aid of tartaric acid, on the surface of the supports.

Q.

0

0

0

I

I

I

I

I

I

1

2

3

4.

5

6

NaBr / g

Figure 4. The relation between the amount of NaBr and the optical yield a a Support: o~-AI203. Initial Ni percentage loaded: 40 wt%. Modification was carried out in the 100 cm 3 aqueous solution containing 1 g tartaric acid and NaBr at pH 3.2 and 100 ~ for lh.

ACKNOWLEDGMENTS This research was supported by a Science Research Grant-in-Aid, No. 08232239, from the Ministry of Education, Science and Culture, Japan.

320 REFERENCES

1. Y. Izumi, Adv. Catal. 32 (1983) 215. 2. A. Tai and T. Harada, Tailored Metal Catalysts, Reidel, Dordrecht (1986) 265 and references therein. 3. W.M.H. Sachtler, Chem. Ind., 22 (1985) 189 and references therein. 4. Y. Nitta, M. Kawabe and T. Imanaka, Appl. Catal., 78 (1987) 141 and references therein. 5. E.I. Klabunovskii, A.A. Vedenyapin, E.I. Karpeiskaya, V.A. Pavlov, Proceedings of 7th International Congress on Catalysis (1980) 390. 6. M.A. Keane and G. Webb, J. Catal., 136 (1992) 1. 7. H.U. Blaser, H.P Jalett, M.Mtiller, M Studer, Catalysis Today, 37 (1977) 441 and references therein. 8. A. Baiker, J. Mol. Catal. A: Chemical, 115 (1997) 473 and references therein. 9. Y. Nitta, K. Kobiro, and Y. Okamoto, Stud. Surf. Sci. Catal., 108 (1997) 191 and references therein. 10. K. Borszeky, T. Mallat, and A. Baiker, Catalysis Letters, 41 (1996) 199. 11. T. Sugimura, T. Osawa, S. Nakagawa, T. Harada, and A. Tai, Stud. Surf. Sci. Catal., 101 (1996) 231. 12. T. Osawa, T. Harada, and A. Tai, Catalysis Today, 37 (1977) 465 and references therein. 13. J. Masson, P. Cividino, and J. Court, J. Mol. Catal. A:Chemical, 111 (1996) 289. 14. T. Harada, M. Yamamoto, S. Onaka, M. Imaida, H. Ozaki, A. Tai, Y. Izumi, Bull. Chem. Soc. Jpn., 54 (1981) 2323. 15. T. Harada, Y. Imachi, A. Tai, Y. Izumi, Metal-Support and Metal-Additive Effects in Catalysis, Lyon (1982) 377. 16. T. Osawa, T. Harada, A. Tai, O. Takayasu, and I. Matsuura, Stud. Surf. Sci. Catal., 108 (1997) 199. 17. H. Brunner, M. Muschiol, and T. Wischert, Tetrahedron: Asymmetry, 1 (1990) 159. 18. Y. Nitta, T. Imanaka, and S. Teranishi, J. Catal., 96 (1985) 429. 19. A. Hock and W. M. H. Sachtler, J. Catal., 58 (1979) 276. 20. T. Osawa, S. Mita, A. Iwai, T. Miyazaki, O. Takayasu, T. Harada, I. Matsuura, Chem. Lett., (1977) 1131. 21. Y. Nitta, F. Sekine, T. Imanaka, and S. Teranishi, Bull. Chem. Soc. Jpn., 54 (1981) 980. 22. Y. Nitta, E. Utsumi, T. Imanaka, and S. Teranishi, J. Catal., 101 (1986) 376. 23. A.A. Vedenyapin, B.G. Chankvetadze, L.K. Tsivinskaya, V.M. Akimov, and E.I. Klabunovskii, React. Kinet. Catal. Lett., 33 (1987) 53. 24. Y. Nitta, F. Sekine, T. Imanaka, and S. Teranishi, J. Catal., 74 (1982) 382. 25. T. Harada, Bull. Chem. Soc. Jpn., 48 (1975)3236. 26. G. Wittmann, G.B. Bart6k, M. Bart6k, and G.V. Smith, J. Mol. Catal., 60, (1990) 1. 27. A.A. Vedenyapin, B.G. Chankvetadze, and E.I. Klabunovskii, React. Kinet. Catal. Lett., 24 (1984) 77. 28. A. Tai, T. Harada, Y. Hiraki, and S. Murakami, Bull. Chem. Soc. Jpn., 56 (1983) 1414. 29. Y. Inoue, K. Okabe, and I. Yasumori, Bull. Chem. Soc. Jpn., 54 (1981) 613. 30. T. Harada, Bull. Chem. Soc..lpn., 53 (1980) 1019. 31. T. Harada and Y. Izumi, Chem. Lett., (1978), 1195. 32. L.J. Bostelaarand W.M.H. Sachtler, J. Mol. Catal., 27 (1984) 387. 33. M.A. Keane and G. Webb, J. Mol. Catal., 73 (1992) 91. 34. T. Harada, A. Tai, M. Yamamoto, H. Ozaki, and Y. Izumi, Proceedings of Seventh International Congress on Catalysis, (1980), 364.

91998ElsevierScienceB.V.All rightsreserved. Preparation of Catalysts VII B. Delmonet al., editors.

321

F e i t k n e c h t c o m p o u n d u s e d a s t h e p r e c u r s o r of t h e c a t a l y s t for t h e c a t a l y t i c g r o w t h of c a r b o n f i b e r s f r o m m e t h a n e Yongdan Li*, Jiuling Chen, Liu Chang, Jiusheng Zhao Department of Catalysis Science and Technology and State Key Lab on C 1 Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China The catalytic growth of carbon fibers from methane over a nickel-alumina composite catalyst has been investigated. The effect of doping with iron is also discussed. The catalysts used in this work were prepared from Feitknecht compound precursor obtained in coprecipitation process. It has been found that, a strong interaction of nickel with alumina is essential for the low temperature activity of the catalysts. The crystallinity of the precipitates and that of the mixed oxides after calcination have rather strong effects on the activity and stability. The reaction temperature has different effects on samples having different alumina content, which indicates that alumina not only changes the structure and texture of the active component, but also promotes its intrinsic activity. Doping with other group VIII metals has a strong effect on the catalytic performance. The diameter of the carbon fibers formed is controlled by the size of the nickel particles after reduction, which is also related to the catalyst composition. Key words: catalytic growth of carbon fiber, Feitknecht compound, hydrotalcite, nickel-alumina catalyst, methane

I. I n t r o d u c t i o n Nickel catalysts prepared by coprecipitation technology have been widely used in hydrogenation and dehydrogenation reactions of hydrocarbons. In these processes, carbon formation on the catalysts has been generally considered harmful that it causes the catalyst to deactivate [1]. However in recent years, such carbon, often with a filamentous or fibrous texture, becomes the purpose of production due to its special physical and chemical property [2]. Nickel and iron have been used as the major components of the catalyst for carbon fibers (CFs) production [3-6]. Nickel-based catalysts can produce CFs in a temperature range near 773 K [7], while iron-based catalysts are often used at a temperature as high as 1273 K [5, 6]. On nickel catalyst, the CFs tend to form bulk sized lumps, which can be crushed further into suitable sized particles. It was reported that the bulk crushing strength (BCS) of the CFs lumps produced at 843 K on a nickel catalyst from * Corresponding author, e-mail:[email protected]

322 m e t h a n e is c o m p a r a b l e to the BCS of commercially available s u p p o r t materials [8]. The surface a r e a of the CFs can reach 700 m2/g after a careful activation in c a r b o n dioxide [2]. It h a s been found that the CFs t h u s formed have good a d s o r p t i o n properties to some organic molecules, s u c h as benzene a n d toluene, a n d inorganic molecules, such as CO and NO [9]. A s t r o n g p r o m o t i o n effect on the CFs growth reaction of a l u m i n a to nickel h a s been observed, w h e n incorporated by coprecipitation [3, 7]. Copper h a s an effect on the activity of nickel too, b u t when there is no a l u m i n a , the alloy of nickel a n d copper h a s no activity to the CFs formation from m e t h a n e [10]. Avdeeva et al. [3] reported t h a t coprecipitated nickel-alumina a n d nickelc o p p e r - a l u m i n a catalysts with a high nickel content h a s a high activity [3]. A m a x i m u m a m o u n t of 250 gC/gNi of CFs was produced by the d e c o m p o s i t i o n of m e t h a n e at 823 K, with a reaction rate near 10 gC/gNi.h [3, 11]. A nickela l u m i n a composite catalyst derived from the activation of coprecipitated Feitknecht c o m p o u n d (FC) h a s been designed and tested in our lab [7, 13]. This catalyst grows CFs from m e t h a n e with a high rate at 773 K [7]. It h a s been p r o p o s e d t h a t a strong interaction of nickel and a l u m i n a inherited from FC s t r u c t u r e e n h a n c e d the activity [7]. FC was found as a kind of layered s t r u c t u r e d clay, which h a s brucite-like layers c o n t a i n i n g octahedrally coordinated bivalent and trivalent cations, as well as interlayer a n i o n s a n d water. In its brucite-like layer, the cations are s u p p o s e d to have a uniform distribution [12]. Due to the difficulty of solid p h a s e diffusion of metal ions, this precursor results in a p h a s e of well mixed oxides containing bivalent a n d trivalent metal ions after careful calcination, a n d therefore, r e s u l t s in a m a x i m u m interaction between its c o m p o n e n t s . D u r i n g reduction, a highly active metal phase can be obtained, which w a s s u p p o s e d to be paracrystalline [14]. Some preliminary results on nickela l u m i n a catalysts derived from FC precursor and on the effect of copper have been p u b l i s h e d elsewhere [7, 13]. This paper gives some more u n d e r s t a n d i n g s on the activity and the stability of this catalytic s y s t e m a n d the effect of i n t r o d u c t i o n of trivalent iron.

2. E x p e r i m e n t a l 2.1. Catalyst samples All the p r e c u r s o r s with FC s t r u c t u r e were o b t a i n e d by coprecipitation of a mixed a q u e o u s solution of nitrates by s o d i u m carbonate. The p r e c u r s o r s of p u r e oxides were p r e p a r e d with a same condition. Then the precipitates were w a s h e d with water, dried in air at 393 K for 5 h o u r s a n d calcined at 723 K for 10 h o u r s . The powder t h u s formed was p r e s s e d at 1.5 k b a r into cylindrical

Table 1 Samples used in this work sample Composition 1 2 3 4 5 6 7

Ni " A1

23:77

Ni " A1

2:1

Ni:A1

3:1

Ni A1 9

9:1

Ni" Fe" A1 7 5 : 1 5 : 1 0 Ni

oO

Fe

oO

323 tablets. Then the tablets were c r u s h e d into a particle size of 2 6 0 - 2 7 0 mesh. Table 1 lists the composition of the catalysts in mole ratio u s e d in this work. 2.2. Characterization A D/MAX-2038 x-ray diffractometer with Fe K(z was u s e d to get the XRD profiles of the catalyst p r e c u r s o r s . A CHEMBET-3000 w a s u s e d to m e a s u r e the specific surface a r e a of the mixed oxides in 40-60 m e s h with nitrogen as the adsorbate. The m o r p h o l o g y of the CFs grown on catalyst was observed with a Philip 4 0 0 s t T r a n s m i s s i o n Electron Microscopy (TEM). TPR of the catalysts w a s carried out in a "U" s h a p e d reactor with an internal diameter of 4 m m at a h e a t i n g rate of 10 K / m i n , a n d with a total flow of 50 m l / m i n (STP) of h y d r o g e n a n d nitrogen (in 1:9). The size of particles u s e d in TPR was between 40 a n d 60 m e s h e s , a n d the a m o u n t of catalyst for each experiment was a b o u t 200 mg. 2.3. Reaction conditions The e x p e r i m e n t s were carried out with an in-situ t h e r m a l balance. A vertical q u a r t z reactor with a d i a m e t e r of 20 m m a n d a length of 160 m m was employed. The catalyst particles were retained on a q u a r t z b a s k e t with a diameter of 12 m m a n d a height of 12 m m h a n g i n g in the middle of the heated zone. The a m o u n t of catalyst in oxidized state u s e d for each experiment w a s a b o u t 3 mg. The reactor system was first q u e n c h e d with nitrogen. It was then h e a t e d at a rate of 5 K / m i n to the reduction t e m p e r a t u r e in a mixed gas flow 45 m l / m i n (STP) of h y d r o g e n a n d 9 A nitrogen (in 1"3). The t e m p e r a t u r e was 9 9 held a b o u t thirty m i n u t e s for reduction. Then h y d r o g e n w a s s t o p p e d a n d the t e m p e r a t u r e in the reactor w a s a d j u s t e d to the preset value in a n a t m o s p h e r e of nitrogen. Afterwards, a mixture of m e t h a n e a n d nitrogen (in 1:2) with a flow rate 45 m l / m i n (STP) w a s switched into the reactor a n d the reaction began. When C the reaction w a s ended, m e t h a n e w a s stopped first a n d nitrogen c o n t i n u e d to flow until the t e m p e r a t u r e of the reactor was lowered to ambiance. 10 30 50 70 90 1

I

I

1

i

f

_

2-Theta

3. Results

Figure 1, selected XRD patterns of the o f t h e precipitated precursors. A: samplel" B: sample precursors 3; C: sample 5. ~: Ni6A12CO3(OH)lo'4H20; Figure 1 illustrates three XRD profiles O: AI(OH)~ (Baverite) 3.

I.

Structure

and

texture

324 of the p r e c i p i t a t e d p r e c u r s o r s before calcination. It s h o w s t h a t s a m p l e s 3 a n d 5, with a M2+/M3+=3, have a typical A FC s t r u c t u r e . While in s a m p l e 1, w h i c h was precipitated u n d e r a exactly s a m e condition, b u t with a m u c h h i g h e r B content of aluminum, hydrated a l u m i n a , bayerite, c a n be identified. C The XRD profiles of the mixed oxides after calcination are s h o w n in figure 2. The profile of s a m p l e 6 is given for comparison, w h i c h is a p u r e NiO phase. The profiles of s a m p l e s 3 a n d 5 show one identical p h a s e , its m a j o r 20 40 60 80 100 p e a k s are n e a r to t h o s e of NiO b u t 2-Theta widened t h a n t h a t of NiO, a n d show no p e a k s visible for A1203 a n d Fe203. In Figure 2, The XRD patterns of the contrast, the profile of s a m p l e 1 s h o w s mixed oxides. A: sample 1; B: sample 3; a m i x t u r e of p h a s e s of q- & y-A1203 a n d C: sample 5; D: sample 6. II,: NiO; 0: 3' NiO. C o m p a r e d with s a m p l e 6, the & q-A1203 crystallinity of NiO in the s a m p l e s with a l u m i n u m is poor. The BET a r e a of the mixed oxides s a m p l e s 1-4 were m e a s u r e d , their values are 256, 173, 158, 112 m 2 / g respectively, showing r o u g h l y inverse proportional to the c o n t e n t of a l u m i n u m in them. 3 . 2 . TPR r e s u l t s o f t h e m i x e d o x i d e s Figure 3 depicts the TPR c u r v e s of samples 3, 5 a n d 7. S a m p l e 3, which was p r e p a r e d from FC p r e c u r s o r a n d h a s a ratio of Ni:AI=3, h a s one peak, s t a r t i n g at a r o u n d 663 K a n d ending at n e a r 1243 K with a m a x i m u m rate in the range 1032-1093 K. S a m p l e 5, w h i c h was p r e p a r e d from s a m e p r e c u r s o r b u t h a s a c o n s i d e r a b l e a m o u n t of iron in a ratio of Ni:Fe:Al=75:15"10, h a s also one peak, b u t in a m u c h lower t e m p e r a t u r e range. While s a m p l e 7, which is a p u r e Fe203, r e d u c e d in two stages. These indicate t h a t the i n c o r p o r a t i o n of o t h e r metal ions through FC precursor attained the maximum interaction between the c o m p o n e n t s .

o r

s

500

700

900

11 O0

1300

T/K

Figure 3, TPR curves of the samples. A: sample 3; B: sample 5; C: sample 7

325

3 . 3 . CF g r o w t h o n t h e s a m p l e s Figure 4 s h o w s the weight gaining process of the first five s a m p l e s at a fixed t e m p e r a t u r e of 773 K and the corresponding rate of CFs growth. The growth p r o c e s s of CFs on the samples 1, 2, 3, a n d 5 can be r o u g h l y divided into two stages, rate increasing and rate decreasing. But on s a m p l e 4, there was a r a t h e r long period of steady rate of CFs growth. At the end of the reaction, the weight of CFs produced on s a m p l e s 1, 2, 3, 4 a n d 5 reached values of 37, 69, 88, 123 and 35 times t h a t of nickel in the samples, respectively. The m a x i m a of the growth rates of CFs on the s a m p l e s are 0.525, 0.365, 0.365, 0.241 and 0.276 mgC/mgNi.min, respectively. The curves of weight gaining and rate of CFs growth on s a m p l e s 1, 3, 4, 5, at 873 K are s h o w n in figure 5. At the end of the reaction, the weights of CFs p r o d u c e d on t h e m reached values of 19.5, 42.8, 9.5 a n d 10.6 times t h a t of nickel in the samples, respectively. Before deactivation, the m a x i m a of the growth rates of CFs on t h e m are 1.56, 1.36, 1.90 and 0.363 m g C / m g N i . m i n , respectively. It m a y need to mention that, on samples 6 a n d 7 no reaction h a p p e n e d or no CFs were p r o d u c e d at 773 K and 873 K.

~

._~ 0.6 E

140

D

120

- 100

E

0.5

b 0.4

C

~E 80

E

v

0.3

t~ L-

~_ 40 ~0 20

A

.= 0.2

E

0.1

0

1

0

200

400 time/min

600

800

0

200

400 time/min

600

800

Figure 4, amount of CFs formed and CFs growth rate as a function of time at 773 K. Left: amount of CFs formed; Right: CFs growth rate. A: sample 1; B: sample 2; C: sample 3" D: sample 4; E: sample 5 3 . 4 . TEM o b s e r v a t i o n of t h e CFs The CFs grown on samples 1, 3 and 4 at 773 K were e x a m i n e d by TEM to determine their morphology. Typical photos are shown in figure 6. It was found t h a t all the CFs are curved and hollowed tubes e a c h with a catalyst particle located at its tip. The diameters of the CFs are governed by the size of the a s s o c i a t e d catalyst particle. The diameters of CFs grown on the three samples are typically in the ranges of 10 to 31 nm, 15 to 70 n m a n d 38 to 100 nm, respectively.

326 45

r--

z 40

E

E 35

1.6

E ~ 1.2 g

30 25 2O ul

2.0

~ 0.8

15

u.. 10 o 5

~0.4 i

0

20

time/min

40

0.0 60

0

20

time/rain

40

60

Figure 5, amount of CFs formed and CFs growth rate as a function of time at 873 K. Left: amount of CFs formed; Right: CFs growth rate A: 9 sample 1; B" sample 3; C" sample 4; D" sample 5

Figure 6, TEM photos of the CFs grown. A: on samples 1; B: on sample 3" C: on sample 4.

4. Discussion 4.1. The structure of the precursors and the catalysts

The XRD profiles of the coprecipitated p r e c u r s o r s p r e s e n t e d h e r e s h o w t h a t i n c o r p o r a t i o n of iron does n o t c h a n g e m u c h t h e crystallinity of the FC s t r u c t u r e , b u t too m u c h a l u m i n u m leads to the f o r m a t i o n of a s e p a r a t e d AI(OH)3 p h a s e . Referring to [7], w h e n the ratio M2§ 3§ is a s high a s 9:1, the s t r u c t u r e of the precipitates c a n still be a t t r i b u t e d to FC p h a s e , b u t the crystallinity of it is very poor. Although the allowed M 2§ a§ ratio of t h e s e FC

327 p h a s e s o b t a i n e d in o u r l a b o r a t o r y [7, 13] is m u c h wider t h a n reported in literature [12], t h e s e r e s u l t s suggest t h a t an a p p r o p r i a t e ratio of M2+/M a§ is n e e d e d to o b t a i n a well crystallized FC p h a s e w h e n the coprecipitation condition is k e p t u n c h a n g e d . The allowed range of this ratio is a d e p e n d a n t factor of the coprecipitation condition. In our recent work [13], copper was i n c o r p o r a t e d into the s t r u c t u r e u n d e r a s a m e coprecipitation condition. It w a s observed t h a t copper, w h e n at a s a m e ratio with the iron in sample 5, leads to m o r e distorted FC s t r u c t u r e t h a n iron. Combine the results p r e s e n t e d in [7, 13] a n d here, it can be realized that, u n d e r our preparation conditions, w h e n M2+/M a§ is too small, s e p a r a t e AI(OH)3 p h a s e exist, which h a s a completely different s t r u c t u r e from FC, b u t at the other end, when M2+/M a§ grows higher u p to 9:1, seriously distorted FC p h a s e forms. This is interesting t h a t a small a m o u n t of trivalent ion can help to hold the FC s t r u c t u r e , a n d t h a t c a t a l y s t s with high nickel content a n d same s t r u c t u r e c a n be p r e p a r e d . The XRD profiles of the mixed oxides show again the different features at the low a n d high end of M2+/M a§ ratio. W h e n M2§ a§ is very small, as in s a m p l e 1, s e p a r a t e A12Oa is formed. While M2+/M a§ is high, as high as 9:1 [7, 13], the mixed oxides formed d u r i n g calcination of the p r e c u r s o r s have a similar s t r u c t u r e as NiO. Even w h e n copper or iron was incorporated into the p r e c u r s o r in a r a t h e r large ratio, the s t r u c t u r e of the mixed oxides r e m a i n the s a m e [7, 13], the p h a s e s often m e n t i o n e d in literature such as spinel, Fe2Oa, CuO a n d A12Oa are none existing. The mixed oxides exist in a uniform NiO-like p h a s e , w h i c h shows more distortions as the decrease of M 2§ a§ ratio, c h a r a c t e r i z e d by the widened XRD peaks. This p h e n o m e n o n is c o n s i s t e n t with the model p r o p o s e d by Puxley et al. [14], which suggests t h a t the mixed oxides o b t a i n e d by calcination of n i c k e l - a l u m i n u m FC are dispersed uniformly a n d t a k e a m e t a - s t a b l e a n d distorted s t r u c t u r e having the features of both NiO a n d NiA1204[14]. A conclusion [7] h a s been drawn t h a t a well crystallized FC r e s u l t s in a seriously distorted NiO structure, a n d t h a t the i n c r e a s e d a m o u n t of the divalent cations decreases the crystallinity of FC but i n c r e a s e s the crystallinity of the mixed oxides. These results tell also t h a t the mixed oxide is a real mixture, which retains irreducible A12Oa in its distorted NiO-like crystal lattice. The BET surface of the s a m p l e s containing nickel and a l u m i n a shows a t r e n d r e a s o n a b l y t h a t less defected NiO-like p h a s e h a s a smaller surface a r e a a n d vice versa, more defected NiO-like p h a s e h a s a larger surface area. The TPR c u r v e s p r e s e n t e d in figure 3 show t h a t n i c k e l - a l u m i n a composite oxides in a specific ratio p r e p a r e d from FC p r e c u r s o r are extremely difficult for reduction. Referring to the d a t a in [13], the t e m p e r a t u r e for finishing the reduction of nickel in s a m p l e 3 is a b o u t 470 K higher t h a n t h a t in pure NiO. Incorporation of iron t h r o u g h FC p r e c u r s o r lowers the complete reduction t e m p e r a t u r e , a n d shows s i m u l t a n e o u s reduction of nickel and iron. These p h e n o m e n a indicate the existence of very strong effect of irreducible A12Oa a n d very s t r o n g i n t e r a c t i o n between the c o m p o n e n t s in the mixed oxides from FC p r e c u r s o r .

328 4 . 2 . F o r m a t i o n o f CFs o n t h e s a m p l e s The d a t a p r e s e n t e d in figure 4 show several points, 1). All these catalysts have r a t h e r high activity at 773 K at the beginning of the reaction, a n d need no i n d u c t i o n time. 2). All the catalysts deactivate after different periods of high activity reaction. 3). Rather large differences in activity a n d a m o u n t of CFs formed on different catalysts exist, which m e a n s the nickel in these c a t a l y s t s is very sensitive to its environment. 4). Samples 1, 2, 3, 5 have m a x i m u m r a t e s in their activity curves, while sample 4 h a s a r a t h e r long time of c o n s t a n t rate reaction. 5). The curves show that the higher the initial reaction rate is the faster the deactivation. For instances, sample 1 h a s the h i g h e s t reaction rate at its m a x i m u m a m o n g these samples a n d produces the lowest a m o u n t of CFs before deactivation, while the reaction rate on sample 4 is the lowest b u t the a m o u n t of CFs formed on it is the highest. 6). The r e s u l t s of s a m p l e s 1-4 p r e s e n t also t h a t the increased a m o u n t of a l u m i n a i n c r e a s e s the activity of nickel b u t also increases the rate of deactivation. 7). The incorporation of iron lowers both the activity and the a m o u n t of CFs formed. This indicates t h a t iron is not a suitable c o m p o n e n t for this catalyst system. It can be observed from the TEM p h o t o s of the CFs formed on the three s a m p l e s at 773 K t h a t the diameter of the CFs increases with the increase of nickel c o n t e n t in the catalyst. As non-catalytic thickening of the fibers or c a r b o n deposition on the surface of the fibers at this reaction t e m p e r a t u r e can be excluded, the diameter of the CFs is governed by the size of the nickel particles on which the fibers grow. So that the nickel particle size after r e d u c t i o n in these s a m p l e s h a s a sequence: size in sample 4> in sample 3 > in s a m p l e 1. This result is c o n s i s t e n t with the results of XRD a n d BET m e a s u r e m e n t s t h a t less a l u m i n a favors better crystallized NiO, better crystallized NiO h a s smaller surface area a n d produces larger nickel particles after reduction. The catalytic p e r f o r m a n c e s of these catalysts at 873 K are illustrated in figure 5. C o m p a r e d to the results of 773 K presented in figure 4, the s e q u e n c e a n d the values of the reaction rates a n d the a m o u n t of CFs formed all c h a n g e d very m u c h , showing the strong effect of reaction t e m p e r a t u r e . It s h o u l d be noticed t h a t the reaction rate of all the samples increased m u c h c o m p a r e d to 773 K reaction, while the a m o u n t of CFs formed of all the s a m p l e s d e c r e a s e d m u c h . For the three nickel-alumina catalysts, it shows again t h a t the least active catalyst, this time sample 3, p r o d u c e s the m a x i m u m a m o u n t of CFs before deactivation due to its long life. The catalyst sample 4, which p r o d u c e d the largest a m o u n t of CFs at 773 K, p r o d u c e s the lowest a m o u n t of CFs at 873 K a m o n g these nickel-alumina catalysts. 5. Conclusions

The c o m p o s i t i o n of the FC s t r u c t u r e can be varied in a r a t h e r large range, which provides a r a t h e r large room for the modification of the catalytic properties. It h a s been found t h a t the allowed M2+/M a§ ratio in the s t r u c t u r e

329 is a d e p e n d e n t factor of the coprecipitation conditions. Under our p r e p a r a t i o n conditions, w h e n M2+/M a§ is as low as 23:77 separate bayerite p h a s e forms, w h i c h t r a n s f o r m s to separate a l u m i n a p h a s e during calcination. In c o n t r a s t , w h e n M2§ a§ is as high as 9:1, defected FC phase forms, which r e s u l t s in a NiO-like p h a s e during calcination. Incorporation of Fe z+ does not c h a n g e the s t r u c t u r e b u t lowers the reduction temperature. For the s a m p l e s c o n t a i n only nickel a n d alumina, their BET surface area are roughly inverse proportional to the content of alumina. The r e s u l t s of CFs growth reaction show that these catalysts prepared from FC p r e c u r s o r are highly active. All the catalysts deactivate after different period of high activity reaction. The results at both 773 K and 873 K show a same t r e n d t h a t the higher the m a x i m u m activity is, the shorter the life is, a n d the less the CFs p r o d u c e s on the catalyst. This indicates that the e n c a p s u l a t i o n of the active sites by inactive carbon due to its fast formation is the r e a s o n for deactivation. The diameters of the CFs formed are controlled by the size of the nickel after reduction, which h a s a strong relationship with the ratio of a l u m i n a in the sample. The irreducible a l u m i n a in the catalysts h a s two major effects. It e n h a n c e s the dispersion of nickel a n d increases the surface area. It changes the energetic state a n d p r o m o t e s the intrinsic activity of nickel, this is evidenced by the fact t h a t different samples with different a m o u n t of a l u m i n a have different t e m p e r a t u r e effect. Iron w h e n introduced into the FC structure, helps to keep the s t r u c t u r e , and shows ensemble effect with nickel, but has not p r o m o t i o n effect to the fibers formation.

Reference ~

2. 3.

,

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

D. L. Trimm, Catal. Rev. Sci. Eng., 16 (1977) 155 N. M. Rodriguez, M. S. Kim and R. T. K. Baker, J. Phys. Chem., 98 (1994) 13108 L. B. Avdeeva, O. V. Goncharova, D. I. Kochubey, V. I. Zaikovskii, L. M. Plyasova, B. N. Novgorodov and Sh. K. Shaikhutdinov, Appl. Catal. A, 141 (1996) 117 M. S. Kim, N. M. Rodriguez and R. T. K. Baker, J. Catal., 131 (1991) 60 G. G. Tibbetts, M. Endo and C. P. Beetz, Jr., Sampe J., september/october (1986) 30 M. Endo, Chemtech, september (1988) 568 Y. D. Li, J. L. Chen, L. Chang, Appl. Catal. A, 163 (1997) 45 G. S. Hoogenraad, Thesis, Universiteit Utrecht, The Netherlands, 1995 V. B. Fenelonov, L. B. Avdeeva, V. I. Zheivot, Kinetika I Kataliz, 34 (1993) 542 M. S. Kim, N. M. Rodriguez and R. T. K. Baker, J. Catal., 134 (1992) 253 Sh. K. Shaikhutdinov, L. B. Avdeeva, O. V. Goncharova, D. I. Kochubey, B. N. Novgorodov and L. M. Plyasova,, Appl. Catal. A, 126 (1995) 125 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 11 (1991) 173 Y. D. Li, J. L. Chen, L. Chang, Yongning Qin, Submitted to J. Catal. D. C. Puxley, I. J. Kitchener, C. Komodromos and N. D. Purkgns, Stud. Sur. Sci. Catal. (Prep. of Catalysts III), 16 (1983) 227

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

331

T h e S y n t h e s i s of Z e o l i t e Z S M - 5 o n R a n e y Ni: A N o v e l C o m p o s i t e C a t a l y s t Precursor B. Zong*, M. Muhler #+, and G. Ertl # Ruhr-Universit~it Bochum, D-44780 Bochum, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Zeolite ZSM-5 was grown onto Raney Ni powder, foil and wire gauze. For the preparation of the composite material, the leaching of Raney Ni, the pH of the synthesis mixture, and its A1 content had to be optimized. The preparation of the composite ZSM-5/Ni net demonstrated the industrial applicability as catalyst packing. The Cu-exchanged ZSM-5-0/Ni powder grown without additional A1 source in the synthesis mixture was used as catalyst for the selective reduction of NO with NHa in the presence of 02. The composite catalyst was found to be highly stable against steaming and high-temperature treatments. 1. Introduction

Zeolites grown on various supports are becoming increasingly interesting for numerous scientific and industrial applications. Recently Calis et al. [ 1] used Cu-exchanged ZSM-5 grown on wire gauze as catalyst packing for a dustproof, low pressure drop DeNO, process. Shan prepared composite ZSM-5 / stainless steel packings for catalytic distillation [2]. Haag et al. [3] synthesized dense layers of ZSM-5 on e.g. polytetrafluoroethlyene used as microporous membrane for separation after removal of the forming surface. For catalytic applications in fixed beds, the hydrothermally synthesized zeolite crystals with a typical diameter of 1 ~m are usually embedded in an ceramic binder to obtain larger particles with a diameter of about 1 mm in order to reduce the otherwise unacceptable pressure drop. An alternative way to overcome the pressure drop problem is to prepare shell catalysts by growing thin layers of ZSM-5 on a preformed SiO2 support [4]. The advantage of growing the catalytically active zeolite directly on the support is the significantly higher mechanical strength of the composite catalyst, and the more efficient utilization of the undiluted zeolite crystals which are fully exposed on the surface of the support. Jansen et al. [5] grew MFI-type zeolite films on different substrates through heterogeneous nucleation. The authors showed that the nuclei were formed at the substrate surface, and therefore attribute the high mechanical strength to the chemical anchoring of the zeolite crystals [5]. For applications with high space velocities which require low pressure drops, ceramic or metallic honeycomb structures are used also to prevent rapid plugging by dust. Calis et al. [ 1] were able to grow ZSM-5 on a stainless steel gauze preshaped in rolls which was subsequently impregnated with an aqueous solution of copper acetate. The composite catalyst was used in a gradientless recycle reactor for the *on leave from Research Institute of Petroleum Processing, Sinopec, RR. China t to whomall correspondenceshould be addressed

332 selective catalytic reduction (SCR) of NO~ with NHa in the presence of 02. Ito et al. [6] used ZSM-5 and mordenite directly grown on a stainless-steel metal gauze support and subsequently exchanged with cerium as catalysts for the same reaction. In the present study, ZSM-5 was grown on Raney Ni by heterogeneous nucleation for the following reasons: first, due to the highly porous Raney Ni substrate, high zeolite loadings should be accessible. Second, the nucleation at the rough Raney Ni surface should lead to strongly bound zeolite crystals. Third, Raney Ni itself can serve as source for A1 under the hydrothermal synthesis conditions. Thus no additional A1 compound has to be added to the aqueous synthesis solution leading to high Si/A1 ratios. It is known from ultrastable zeolite Y (USY) that high Si/A1 ratios are beneficial for enhanced hydrothermal and high-temperature stability. Raney Ni was used either as freshly prepared powder, or as pretreated Ni foil and wire gauze in the present study. A 1000 nm thick A1 film was sputtered onto the foil or the net followed by heating in Ar at 1153 K to synthesize a Ni50-A150 layer which was leached under the same conditions as the bulk Ni50-A150 alloy used for the synthesis of Raney Ni powder. The powder composite was used to investigate the high-temperature and hydrothermal stability by applying it in the Cu-exchanged form as catalyst for the SCR of NO~ with NH3 in the presence of 02. The foil composite served as flat model system for XPS/UPS/ISS investigations to be reported elsewhere [7], whereas the preparation of the composite ZSM-5/Ni net is intended to demonstrate the industrial applicability as catalyst packing which would require additional shaping of the net to e.g. corrugated or folded sheets.

2. Experimental The preparation of Raney Ni powder, foil and wire gauze For the preparation of Raney Ni powder, 1 g of the Ni50-A150alloy (Merck) was leached with 50 ml NaOH (Merck, 25 wt %) for 1.5 h at the temperatures specified in table 1. The Raney Ni powder was washed with deionized water until the washing water was neutral. After additional washing three times with acetone, the Raney Ni powder was heated to 353 K for 2 h in vacuum, and subsequently transfered to a glove box to determine the weight. The results of the weighing, the elemental composition and the BET areas are summarized in table 1. Ni foil (Goodfellows, 18 mm x 14 mm x 0.4 mm, purity 99.99 %) and electrolytically formed Ni net (Goodfellows, 63 mm x 32.5 mm, wire diameter 18 ~m, 70 wires per inch) were pretreated with 0.1 N HC1 followed by washing with deionized water. A 1000 nm thick A1 layer was sputtered onto the Ni foil and onto both sides of the Ni net. Subsequently, both samples were heated in Ar to 1153 K for 1 h to synthesize a layer of Ni50-A150. Then both samples were leached with 50 ml NaOH (25 wt %) for 1.5 h followed by leaching in an autoclave with 15 ml 1 N NaOH at 453 K for 48 h.

The growth of ZSM-5 on Raney Ni and the Cu ion exchange The synthesis mixture consisted of aqueous solutions of sodium silicate (Merck, 27 wt % SiO2, 14 wt % Na20), NaA102 (Wako Pure Chemicals), tetrapropylammonium bromide (TPABr) (Merck) using deionized water. The pH of the solution was adjusted by adding NaOH. The synthesis was carried out in the molar composition range of Na20" 0-1 A12Oa 60-100 9 SiO2 10 9 TPABr 7000 9 H20. 15 ml synthesis solution and the amount of Raney Ni obtained by leaching 1 g of Ni50-A150 or

333 the leached Ni foil or net were transfered into a 25 ml teflon-lined autoclave. The synthesis was performed at 453 K for 48 h at autogenic pressure. The resulting composites were washed with deionized water, dried at 393 K for 12 h and calcined in air at 773 K for 12 h. The Na-form of the zeolite composite was transformed into the H-form by ion exchange with aqueous NH4OH (1 M, Merck), followed by washing with deionized water. The Cu-ZSM5 composites were prepared by ion exchange using three times aqueous solutions of copper acetate (0.02 M, Cu(CHaCOO)2, Merck) at room temperature for 24 h, followed by washing with deionized water, drying in air at 393 K for 12 h, and calcination in air at 773 K for 12 h. The hydrothermal and the high-temperature stability of the composite powder was studied by steaming in a mixture of 3 mol % water in Ar using a flow rate of 100 Nml/min for 10 h at 873 K, and by heating in air for 4 h at 1173 K, respectively.

Characterization and catalytic testing The elemental composition was determined by atomic absorption spectroscopy (AAS). The BET areas were obtained with a Quantasorb Jr. set-up (Quantachrome). Prior to the BET measurements, the samples were outgassed at 353 K for 2 h in vacuum and transfered into the sorption cell in a glove box. Powder X-ray difraction data were collected with a Siemens D 5000 diffractometer using Cu Ka radiation. The scanning electron microscopy (SEM) investigations were carried out with a Hittachi 4000 microscope. A stainless-steel microreactor set-up equipped with six gas lines (Ar, NO in Ar, NH3 in Ar, 02 in Ar, N2 in Ar, N20 in Ar) was used for the catalytic testing. The feed composition was 500 ppm of NO, 500 ppm of NHa, and 8600 ppm of 02 in Ar. Typically, 100 mg of the composite catalyst were placed in a quartz U-tube and exposed to a total flow rate of 120 Nml/min. Prior to each run, the catalyst was dried in flowing Ar at 673 K for 2 h. A calibrated mass spectrometer and a combined non-dispersive IR/UV detector for NO and NO2, respectively, were used as detectors. 3. Results and Discussion

The influence of the Raney Ni leaching conditions on the zeolite synthesis The influence of the pretreatment conditions of the Raney Ni on the growth of the zeolite was studied using a composition of 5 Na20:100 SiO2 : 10 TPABr : 7000 H20. The absence of NaA102 in the synthesis mixture implies that the residual A1 content of the Raney Ni acted as the only A1 source for the zeolite. The properties of the Raney Ni as a function of the leaching conditions are summarized in table 1. Table 1 Weight, composition and BET Leaching conditions with 50 ml NaOH (25 wt %) 323 K, 1.5 h 343 K, 1.5 h 363 K, 1.5 h additionally 453 K, 48 h, 1 N NaOH

area of Raney Ni powder obtained by leaching 1 g Ni-A1 alloy Weight Composition BET area g w t % N i wt%A1 m2/g 0.599 67.9 0.552 71.1 0.530 92.4 7.6 86.7 0.506 98.8 1.2 12.5

334 Table 2 The effect of the Si/A1 ratio of the synthesis mixture on the growth Sample Composition Zeolite detected A1203 :SiO2 :TPABr: H20 ZSM-5-40/Ni 1:40:10:7000 ANA + ZSM-5 ZSM-5-60/Ni 1:60:10:7000 litle ANA + ZSM-5 ZSM-5-100/Ni 1:100:10:7000 ZSM-5 ZSM-5-0/Ni 0:100:10:7000 ZSM-5

of the zeolite wt % zeolite on Raney Ni 26.8 27.8 32.9 19.3

The XRD data of the resulting composites revealed that the leaching at 453 K was necessary to grow pure ZSM-5 on Raney Ni. In all other cases, the zeolite analcime was detected additionally which has a rather high Si/A1 ratio of 2/1 [8]. Under the hydrothermal synthesis conditions, the residual A1 content of the Raney Ni is obviously dissolved by OH- resulting in large amounts of soluble anionic A1 species which participate in the zeolite synthesis. The leaching of the Raney Ni at 453 K was performed under the same conditions as the subsequent zeolite synthesis. Hence the amount of A1 ions available for the synthesis should be strongly reduced, leading to ZSM-5 with a much higher Si/A1 ratio than analcime. For all the investigations following below, only Raney Ni leached at 453 K was used.

The influence of the pH and of the A1 content on the zeolite synthesis In addition to the leaching conditions of the Raney Ni, the pH of the synthesis mixture is an important parameter for the growth of ZSM-5. Generally, an increase in pH is known to accelerate the growth of the crystals and to shorten the induction period by an enhanced reactant concentration [9]. The pH of the synthesis mixture was varied between 10.0 and 11.9 using Raney Ni leached at 453 K. It turned out that the pH had to be kept ___ 11.5 in order to ensure complete crystallization of ZSM-5 on the Raney Ni. At lower pH, ZSM-5 was found to crystallize outside the porous Raney Ni structure. Finally, the effect of the A1 content of the synthesis mixture on the growth of the zeolite was studied. The A1 content was varied between c(A1203) = 0 and c(SiO~)/c(A12Oz) = 40 using Raney Ni leached at 453 K. The quantitative results are summarized in table 2, and the corresponding XRD patterns are shown in fig. 1. It is evident that SiO2/A12Oz ratios lower than 100 lead to the additional growth of analcime. Without additional A1 source in the synthesis mixture, about 19 wt % of ZSM-5 can be grown on Raney Ni. SEM characterization of the composites The SEM image of ZSM-5 grown on Raney Ni powder (fig. 2, top image) reveals that the ZSM-5 crystals are densely packed and intergrown, thus forming a continuous layer. The lower image in fig. 2 originates from ZSM-5 grown on Raney Ni foil. With this model system, it can be seen more clearly that the ZSM-5 crystals exhibit the typical elongated prismatic shape. Furthermore, it is obvious that the growth of the crystals started at the rough surface of the Raney Ni leading to a random orientation of the prismatic crystals. It was hardly possible to remove the zeolite layer from the Raney Ni foil mechanically, indicating strong chemical bonds to the support in agreement with the heterogeneous nucleation model [5]. Fig. 3 displays two SEM images of ZSM-5 grown on Raney Ni wire gauze. The upper image

335 i

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Si/AI=100

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i

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Figure 1. XRD patterns of zeolite Cu-ZSM-5 (trace A), Cu-ZSM-5-0/Ni (trace B), Cu-ZSM-5100/Ni (trace C), Cu-ZSM-5-60/Ni (trace D) and Cu-ZSM-5-40/Ni (trace E). The SiO2/A12Oa ratio was varied between 40 and c(A1203) = 0.

shows that the Ni wire diameter of originally 18 ~tm increased up to 80 #m due to the coating by ZSM-5. The lower image reveals that the coating is dense consisting of intergrown ZSM-5 crystals. The high mechanical stability of the composite net allowed us to roll the composite to fit it into a tubular reactor in order to simulate a monolith catalyst.

Catalytic testing Fig. 4 displays the influence of the reaction temperature on the conversions of NO and NH3 and on the yields of N2 and NO2 using the Cu-ZSM-5-0/Ni powder catalyst. The conversions of NO and NHa started at about 423 K yielding N2 only. The conversion of NH3 reached almost 100 % at 573 K, whereas the conversion of NO went through a maximum of about 50 % at 573 K and decreased with increasing temperature. The yield of N2 showed the same trend as the conversion of NO as a function of temperature. This is due to the oxidation of NH3 by 02 yielding NO as side reaction which occurs at higher temperatures. However, the conversion of NO was found to be increased subsequent to steaming the CuZSM-5-0/Ni catalyst at 873 K for 10 h as shown in fig. 5. The steamed composite catalyst

336

Figure 2. SEM images of ZSM-5 grown on Raney Ni powder (top) and on Raney Ni foil (bottom).

337

Figure 3. SEM images of ZSM-5 grown on Raney Ni net.

338 100 2

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Figure 4. Conversion of NO (trace A) and NHa (trace B) and the yields of N2 (trace C) and NO2 (trace D) using 100 mg of Cu-ZSM-5-0/Ni and a total flow rate of 120 Nml/min containing 500 ppm NO, 500 ppm NHa and 8600 ppm 02.

was even more active than a physical mixture of conventionally prepared Cu-ZSM-5 and Raney Ni. Heating to 1173 K decreased the activity of Cu-ZSM-5-0/Ni leading to a shift of the maximum NO conversion to higher temperatures by about 100 K. It has to be pointed out that the conventionally prepared Cu-ZSM-5 catalyst lost half of its activity upon steaming, and almost all of its activity upon heating to 1173 K. It is commonly accepted that the loss of activity due to wet aging originates from dealumination and correlated changes of the nature of the ionic Cu species present in ZSM-5 [ 10]. Thus, our results suggest that the composite Cu-ZSM-5-0/Ni catalyst is highly resistant against dealumination which may be due to the absence of additional A1 sources in the synthesis mixture except residual A1 on the surface of Raney Ni. 4. Conclusions

In the present study, ZSM-5 was grown on Raney Ni by heterogeneous nucleation following the concept by Jansen et al. [5] for the following reasons: 9Due to the highly porous Raney Ni substrate, high zeolite loadings were accessible. 9The nucleation at the rough Raney Ni surface lead to strongly bound zeolite crystals which were mechanically stable.

339 ,,

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400 500 Reaction Temperature / K

600

Figure 5. NO conversion data obtained with Cu-ZSM-5 (trace A), Cu-ZSM-5-0/Ni (trace B), Cu-ZSM-5-0/Ni steamed at 873 K (trace C) and Cu-ZSM-5-0/Ni thermally treated at 1173 K (trace D).

9Raney Ni itself can serve as source for A1 under the hydrothermal synthesis conditions. Thus no additional A1 compound had to be added to the aqueous synthesis mixture. The Cu-exchanged composite powder was used as catalyst for the SCR of NO. with NH3 in the presence of 02. Contrary to unsupported Cu-ZSM-5, it was found to be hydrothermally and high-temperature stable.

Acknowledgement The authors benefited from discussions with J.C. Jansen, H.G. Karge, and K. K6hler, and are grateful to J. Find, D. Herein, H. Schneider, B. Sulikowski and G. Weinberg for technical support. REFERENCES 1. H.E Calis, A.W. Gerritsen, C.M. van den Bleek, C.H. Legein, J.C. Jansen and H. van Bekkum, Canadian J. Chem. Eng. 73 (1995) 120. 2. Z. Shan, The Preparation of Zeolite /Steel Composite Packings for Catalytic Distillation (PhD thesis, 1995). 3. W.O. Haag and J.C. Tsikoyiannis, U.S. Patent 5,100,596 (1992). 4. A. Danner and K.K. Unger, Chem.-Ing.-Tech. 62 (1990) 487.

340 5. 6. 7. 8. 9. 10.

J.C. Jansen, W. Nugroho and H. van Bekkum, Proc. 9th Int. Zeolite Conf., Montreal, (1992) p. 247. E. Ito, R.J. Hultermans, H.E Calis, J.C. Jansen, H. van Bekkum and C.M. van den Bleek, Catal. Today 27 (1996) 123. B. Zong et al., in preparation. G. Ferraris, D.W. Jones and J. Yerkess, Z Kristallogr. 135 (1972) 240. E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Stud. Surf. Sci. Catal. 84 (1994) 3. G. Centi and S. Perathoner, Appl. CataL A 132 (1995) 179.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

341

Effect of texture of cobalt oxide catalyst on its properties in ammonia oxidation* Jan Petryk, Ewa Kotakowska, Krzysztof Krawczyk and Zbigniew Kowalczyk Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, PL 00-664 Warszawa, Poland

ABSTRACT The selectivity of cobalt oxide catalysts with different porosity characteristics was studied in ammonia oxidation process. It has been found that the selectivity of NH3 oxidation to NO depends significantly on the total porosity and pore size of the catalyst. An advantageous effect of large pores of several micrometers was observed. Low porosity of catalysts favoured the formation of N20. The presence of fine pores stimulates the formation of N2 molecules. 1. I N T R O D U C T I O N One of the reasons of great interest in cobalt oxide catalysts for ammonia oxidation is the possibility of obtaining the selective processes featured by small yields of unwanted N20. In general opinion nitrous oxide is a greenhouse gas, which is also responsible for destruction of ozone in upper atmosphere layers. The process of ammonia oxidation over oxide catalysts proceeds in the region of external diffusion [1, 2, 3]. One should expect, therefore, that the inner structure of the porous catalyst grains should influence markedly neither the rate nor the direction of the catalytic reactions. It has been found, however, that the inner surface area, total porosity, and pore size distribution have a strong bearing on the course and the final effect of ammonia oxidation. The properties of the catalysts are formed in the process of their preparation, the temperature of preliminary treatment being one of the most important preparation parameters. The temperature may influence the development of texture parameters, such as the inner surface area, porosity and pore size distribution and it may determine the initial phase composition of the catalyst, as well as the kind and the extent of crystal defects in the oxide structure. Obviously, such effects are not limited to catalysts used for ammonia oxidation [4].

*This work was granted by the State Committeefor Scientific Researchin Poland, Project No 3 T09B 034 11

342 It has been stated formerly [5], that the sintering of cobalt oxide at different temperatures gives catalysts differing in both the activity and the selectivity in ammonia oxidation. In the present work an attempt was made to explain, whether the selectivity of the catalyst is determined by its grain texture or it results from other properties settled during the heat treatment. Two groups of catalysts sintered at the two temperatures have been studied. Each group comprised catalysts differing in total porosity and pore size.

2. E X P E R I M E N T A L All the catalysts were obtained from the same raw material, which was high purity cobalt. The metal was dissolved in nitric acid and pink cobalt(II) hydroxide was precipitated by means of aqueous ammonia. The hydroxide was decomposed on heating to give Co304 [6]. The obtained oxide was pressed into tablets and sintered for 48 h in air at 850 ~ (the stable phase is Co304). The content of oxygen in the product, as determined by reduction with hydrogen, corresponded to the general formula of Co304,03. The tablets were grounded and sieved to separate the fraction of 2 - 3.15 mm. Samples differing in porosity were obtained by additional impregnation of the base Co304 catalyst with aqueous solution of cobalt(II) nitrate, which was then decomposed on thermal treatment. Several-fold repetition of the procedure gave catalysts of still lower and lower porosity but retaining almost identical pore diameters. One part of each of the catalysts obtained was used for the study of selectivity in ammonia oxidation, and the other part was sintered again in air for 24 h at 1200~ Under these conditions the samples were transformed into CoO [5]. During the subsequent cooling in air the catalysts were partly reoxidized and the materials of mean composition Co303.98were obtained. The texture of the catalysts was characterised by the mercury porosimetry method. The surface area was calculated from the measurements of pore volume distribution vs. diameter, assuming the cylindrical shape of the pores. The oxidation of ammonia was carried out in a flow reactor with a stationary catalyst bed. The tubular reactor of diameter 19 mm was made of quartz glass. The catalyst layer thickness was 22 mm. The measurements were performed at 770-780~ the reaction mixture of air with ammonia contained 10 % by vol. NH3, and the flow rate was 400 dm3/h (STP). The contents of NO, N20 and NH3 were determined in the outlet gas. NO was determined by combined volumetric and gravimetric method, NH3 was determined spectrophotometrically with Nessler reagent, and N20 was determined by chromatographic method. The conversion of ammonia to molecular nitrogen was calculated from the mass balance. 3. R E S U L T S

AND DISCUSSION

Table 1 presents the textural parameters of all the catalysts studied. The detailed porosimetrics characteristic of the samples sintered at 850~ are shown in Fig.I, as the

343 Table 1. Porosity and dominating pore size in the catalysts ....Catalyst No. Finalsintering temperature, ~ Porosity, %

1 2 850 850 ..... 32.4 25.4

3 850

Dominating pore size, [xm .

1.3 1.2 1.0 . . . . .

4 850

5 850

6

7

8

9

10

11

12

850 1200 1200 1200 1200 1200 1200

17.8 40.3 33.3 24.3 23.5 21.2 10.2 22.2 15.2 0.3 0.35 0.35 . .

3.5

3.0

3.0

1.5

1.5

5.7 1.3

relationships between the total pore volume (Fig. 1A), the incremental pore volume (Fig. 1B), the pore surface area (Fig. 1C), and the incremental pore surface area (Fig. 1D) on one side and pore diameter on the other. Attention should be paid for the very small specific surface areas of the samples. However, the pore diameter distribution is rather narrow, with a distinct dominating value. Hence, the catalytic properties of the material may be referred to its definite pore size. Figure 2 shows analogous porosimetric characteristics of the catalysts resintered at 1200~ after previous sintering at 850~ Catalysts nos. 7, 8 and 9 were obtained by resintering of those ofnos. 1, 2, and 3, respectively (see Table 1)~ whereas catalysts nos. 10-12 were obtained from catalysts nos. 4 - 6. Resintering at 1200 C reduced considerably the total porosity of catalysts and increased the pore size. The pore diameter increased almost tree-fold but, like in the catalysts sintered at 850~ the distribution of pore diameter was kept in a very narrow range. The reduction of porosity associated with increasing pore size resulted in further decrease of specific surface area of the catalysts. Microscopic observations show [5] that the image of inner surface of catalysts sintered at lower temperatures is much more developed than that of catalysts heated of higher temperatures. It may be assumed, therefore, that the decrease of specific surface of the catalysts is even greater than that measured by Hg-porosimetry. Despite of the low specific surface areas the catalysts studied were featured by the high activity, at least in the initial period of operation. At space velocity (SV) exceeding 60 000 h -1 only 0 - 0.1% ammonia remained unoxidized. No correlation was found, however, between concentration of N H3 in the outlet gas (remained ammonia) and the texture of the catalysts tested. The overall porosity and the pore size of the catalysts influenced considerably the selectivity of ammonia oxidation to the individual final products. Figure 3 shows the effect of porosity of the catalysts on the selectivity of oxidizing to NO. In the case of catalysts sintered at 850~ (samples 1 - 6) higher yields of NO were obtained on samples with larger pore diameters (see Fig. 3, samples 1 - 3). The selectivity increased with increasing total porosity of the catalysts. In the case of catalysts with smaller pore diameters (about 0,35 ~tm) less ammonia was oxidized to NO, and the selectivity decreased with increasing porosity. Similar relationships were observed for catalysts sintered at 1200~ More ammonia was oxidized to NO on the samples with pore diameter about 3 ~tm (samples 7 -

344

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Fig. 1. Catalysts 1,2 and 3. Pore volume, incremental pore volume, pore area and i n c r e m e n t a l pore area vs. pore diameter; sintering temperature 850~

345

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Fig. 2. Catalysts 7,8 and 9. Pore volume, incremental pore volume, pore area and incremental pore area vs. pore diameter; sintering temperature 1200~

346 98-

1

L

- - O - - s a m p l e s 1,2,3 O" 97

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POROSITY, %

Fig. 3. Selectivity toNO vs. porosity of the catalysts.

_

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POROSITY, %

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~

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POROSITY, % Fig. 5. Selectivity to Nz vs. porosity of the catalysts.

9) than on those with smaller pores (samples 10 - 12). In both cases the increase in porosity favoured the oxidation to NO. The selectivity was even higher for the catalysts sintered at higher temperatures than those sintered at lower temperature. Hence, an additional sintering at 1200~ increased the selectivity of the catalyst in reaction of ammonia oxidation to NO, probably due to about three-fold increase of the pore size. Catalysts with similar porosity characteristics but differing with temperature of final sintering (nos. 2,3 and 10,11) give different results. The selectivity of oxidation to NO is lower for catalysts sintered at higher temperatures. Fig.4 presents the results of selectivity iof oxidation to NzO as obtained for the catalysts studied. The effect of the total porosity on the selectivity to N20 is observed in every series of catalysts differing in pore size and temperature of final heat treatment. However, the sintering temperature influences considerably the share of N20 in the reaction products. The catalysts denoted as 2 and 3 (sintered at 850~ have both the total porosity and the pore size similar to those of catalysts nos. 10 and 11 (sintered at 1200~ but in the latter case almost twice as much ammonia is oxidized to nitrous oxide, as on the samples 2 and 3. A similar selectivity to N20 formation is observed in catalysts of the series 4 - 6 and 7 - 9. However, in this case the pore diameter of catalysts sintered at higher temperature exceeds almost 10-fold the diameter of catalysts sintered at lower temperature. Among catalysts sintered at identical temperature lower yields of nitrous oxide are obtained on catalysts with larger pores.

348 The selectivity in oxidation of ammonia to elementary nitrogen was calculated from the mass balance. In this case (Fig. 5) the selectivity increases with increasing porosity, and this effect if the most pronounced in catalysts with the smallest pore diameters. The presence of small pores favours the formation of free nitrogen, and the effect of sintering temperature on the selectivity of this reaction is much smaller. 4. C O N C L U S I O N S The selectivity of ammonia oxidation over cobalt oxide catalysts under defmite conditions of the process depends on total porosity of the catalyst, its pore size, and temperature of final heat treatment. The effect of sintering temperature is not limited to its influence on textural parameters of catalyst grains, such as total porosity and pore size. The conditions of heat treatment may influence also the structure of solid surface, as well as the some structural parameters, such as the kind and the number of the oxide crystal lattice defects. The results of the experiments performed have shown that the selectivity of ammonia oxidation to NO is influenced unfavourably by the small porosity of catalyst grains and the presence of pores of small diameter. Low porosity of the catalysts favours the oxidation of ammonia to N20. High porosity, particularly if due to free pores, increases the yield of free nitrogen in the reaction products. The oxidation of ammonia proceeds in the region of external diffusion. The rate of ammonia oxidation referred to unit external surface will decrease with decreasing porosity. One may assume that it will result in increase of ammonia concentration at the catalyst surface, thus leading to increase of the rate of N20 formation. The presence of fine pores may favour secondary reactions, such as dissociation of NO formed and reaction of NH3 with NO. Either of these reactions leads to the formation of elementary nitrogen. An effective catalyst for selective ammonia oxidation to NO should exhibit large pores and high total porosity. It should not be treated at excessively high temperatures. REFERENCES 1. M.I. Tiemkin, N.M. Morozov, et al., Probl. Fiz. Chim., 1959, No 2, 14. 2. G. Bliznakov, D. Klissurski, Z. anorg, allg. Chem., 1963, 323, 89. 3. J. Petryk, J. Sadren, Przem. Chem., 1998, 77, 11. 4. C. Oliva, L. Forni, L. Formaro, Appl. Spectroscopy, 1996, 50, 1395. 5. K. Krawczyk, J. Petryk, K. Schmidt-Szatowski, Preparation of Catalysts VI, Scientific Bases for the Preparation of Heterogeneous Catalysts, Elsevier, Amsterdam 1995, 683699. 6. J. Petryk, K. Krawczyk, Manufacturing of the Precursor of Cobalt Catalyst, PL Patent No 149619 (1990).

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

349

Supercritical AntiSolvent precipitation: a novel technique to produce catalyst precursors. Preparation and characterization of samarium oxide nanoparticles. E. Reverchon a, G. Della Porta a, D. Sannino a, L. Lisib and P. Ciambellia aDipartimento di Ingegneria Chimica e Alimentare, Universit/t di Salerno, 84084 Fisciano (SA), Italy bIstituto di Ricerche sulla Combustione, CNR, Napoli, Italy

A novel technique using supercritical CO2 was applied to the preparation of catalyst nanoparticles. Thermal decomposition of samarium acetate prepared by Supercritical AntiSolvent precipitation produced no porous samarium oxide particles of about 100 nm. Catalytic test of ethane oxidative dehydrogenation showed higher selectivity to ethylene in comparison with samarium oxide samples obtained by decomposition of commercial acetate. 1. INTRODUCTION The Supercritical AntiSolvent (SAS) precipitation has been applied to the production of micronic and submicronic particles of high explosives [1], polymers [2, 3], biopolymers [4], pharmaceutical compounds [5-7] and superconductor precursors [8]. It is an alternative to the liquid antisolvent precipitation when the solute is insoluble in the supercritical fluid whereas the liquid is completely miscible in it. Among candidate antisolvents CO2 is especially adequate, being cheap, non toxic, non flammable. Moreover its critical parameters are readily obtained (To = 31.1 ~ pc = 73.8 bar). The key property of supercritical fluids in the antisolvent process is the diffusivity. Indeed, diffusivities of supercritical fluids are about two orders of magnitude larger than those of liquids. Therefore, mass transfer from supercritical to liquid phase is so fast that allows the production of very small particles of the solute contained in the liquid phase and the control of the particle size distribution. Complete elimination of the antisolvent is also obtained when the precipitation chamber is depressurized and the antisolvent returns to the gaseous state. In this work the SAS process has been applied to the production of catalyst particles. We attempted the production of nanoparticles of samarium acetate to be used as a precursor of samarium oxide catalyst. 2. EXPERIMENTAL 2.1. Materials and characterization

Samarium acetate (AcSm) and dimethylsulphoxide (DMSO), both 99.9% purity, were supplied by Aldrich. Particles diameter of AcSm ranged from about 20 to 200 ~tm. Gaseous CO2 99.9% was given by SON. Samarium acetate by SAS process, AcSm(SAS), was obtained

350 as described under Apparatus and methods. Calcination of AcSm(SAS) at 700~ in air produced samarium oxide particles OxSm(SAS). Physico-chemical characterization of AcSm(SAS) and OxSm(SAS) was performed by different techniques. X-ray diffraction (XRD) spectra were obtained with a PW 1710 Philips diffractometer, thermogravimetric analysis (TG) was carried out with a Netzsch TA209 thermoanalyzer, FT-IR spectra with a Bruker IF 66 spectrophotometer. Surface area and porosity characteristics were obtained by N2 adsorption at 77K with a Thermoquest Sorptomatic 1990. Scanning Electronic Microscope (SEM) pictures were observed with a LEO 420 microscope. SEM samples were covered with a gold film 250A thick using a sputter coater (Agar mod. 108A). Particles Size (PS) and Particle Size Distributions (PSD) were measured using Sigma Scan Pro software (Jandel Scientific). More than 500 particles were considered in each PSD measurement. Thermal decomposition of AcSm(SAS) was also investigated by temperature programmed desorption of carbon oxides (CO and CO2 TPD). Samples of AcSm and OxSm obtained upon calcination of AcSm were also characterized for comparison. 2.2. Apparatus and methods

The SAS apparatus is schematized in Figure 1. It consists of three I-IPLC pumps equipped with pulse dampeners to deliver the liquid solvent, the liquid solution and the supercritical CO2, respectively. A cylindrical vessel of about 500 cm3 I.V. (L = 25 cm, D = 5 cm) was used as precipitation chamber. The liquid solvent or the liquid mixture were fed to the precipitation chamber through a 60 ~tm diameter, 800 ~tm length stainless steel nozzle.

P2

C$ I

t

VM

P3

Pl

Figure 1. Schematic representation of the SAS apparatus. P1, P2 ,P3 are high pressure pumps used for supercritical antisolvent, liquid solvent and liquid solution, respectively; CS is precipitator vessel; VM micrometering valve, SL separator, BP backpressure valve.

351 Supercritical CO2 was fed through another inlet point located on the top of the chamber. Before entering the precipitator, CO2 was heated by an electric cable connected to a temperature controller. The precipitation chamber was electrically heated using thin band heaters connected to another temperature controller. Pressure in the chamber was measured by a test gauge manometer and regulated by a micrometering valve located at the exit (bottom) of the chamber. This valve was heated by a cable heater connected to a temperature controller, to avoid valve clogging and to maintain a constant outlet flow. A stainless steel flit was put at the bottom of the chamber. It collected the solid product but allowed the CO2-organic solvent solution to pass through. A second collection chamber located downstream the micrometering valve was used to recover the liquid solvent. At the exit of the second vessel a rotameter and a wet test meter were used to measure the CO2 flow rate and the total quantity of antisolvent used, respectively. A SAS experiment began by feeding CO2 to the precipitation chamber until the desired pressure was reached. After steady flow of antisolvent was established, the liquid solution of AcSm in DMSO was delivered through the nozzle at 1 ml/min flow rate. During this part of the experiment, when adequate precipitation conditions were chosen, nanometric particles were precipitated on the flit located at the bottom and to a minor extent on the walls of the chamber. The experiment ended when the feeding of liquid solution to the chamber was interrupted. However, supercritical CO2 continued to flow for further 90 min to wash the chamber for the residual content of DMSO solubilized into the supercritical antisolvent. When the washing process was completed, CO2 flow rate was stopped and the chamber was depressurized down to atmospheric pressure. We explored the following ranges of operating conditions for SAS: temperature between 35 and 70~ pressure between 60 and 160 bar, concentration of liquid solution between 5 and 65 mg AcSm/ml DMSO. Liquid flow rate ranged between 0.75 and 1.5 ml/min and feed ratio solution/antisolvent was between 1/20 and 1/30.

2.3. Catalytic activity tests The experimental apparatus for the catalytic activity tests, carried out at atmospheric pressure and 0.0078 g s cm3 contact time, is described in detail elsewhere [9]. The reactor outlet gas was analyzed with a Hartmann & Braun URAS 10 E electrochemical/IR continuos photometer for 02, CO and CO2 and with a Hewlett Packard 5890A gaschromatograph equipped with a flame ionization detector for C2H6 and C2H4. The reaction temperature ranged from 550 to 650~ The feed composition was 4% C2H6 and 2% 02 in helium Carbon balance was always closed to within + 2%. In the absence of catalyst no ethane conversion was observed up to 700~ In the catalytic tests the oxygen conversion was kept always <100% and the temperature increase, due to the exothermal reactions, was negligible. All the catalysts produced C2H4, CO and CO2. 3. RESULTS AND DISCUSSION

3.1. Materials preparation and characterization The effectiveness of SAS technique is connected to the solubility of the carrier solvent in the supercritical antisolvent. Indeed, if the liquid solvent is not completely miscible in CO2 two fluid phases can form in the precipitation chamber. A measure of the liquid solvent solubility is given by the volumetric expansion of the liquid phase in the presence of the antisolvent at a given value of temperature and pressure.

352 Volumetric expansion of the couple DMSO/CO2 has been measured at 22 and 35~ [6]. Volume expansion of DMSO with supercritical CO2 has also been calculated at 25 and 30~ [10]. The range of temperatures in which expansion curves are available has then extended [11] by interpolation and extrapolation of the experimental data using a modified PengRobinson equation of state. The yield of the SAS process, i.e. the ratio between the amount of solute collected in the precipitation chamber and the amount of fed solute resulted always larger than 98%. Taking into account that it is not simple to completely recover the material that covers the walls of the precipitation chamber, this means that a negligible quantity of solute passed through the metallic frit at the bottom of the precipitator. At the lowest levels of volume expansion solute was found only on the frit at the bottom of the precipitation chamber. These results can be explained remembering that at small volume expansions two fluid phases can form in the precipitation chamber due to the low solubility of the liquid solvent in supercritical CO2. Therefore, particles can have been formed by supersaturation of the DMSO rich phase at the bottom of the chamber. When the SAS process was operated at higher volume expansion very large particles (balloons) were collected on the bottom of the precipitator but also (though for a minor extent) on the walls of the chamber. In Figure 2 a typical SEM image of AcSm balloons is reported. In some cases we also observed broken balloons. This fact demonstrates that the observed aggregates are empty shells, i.e. the solid is segregated on a spherical surface that can have a diameter of several microns. The formation of these empty shells depends on the rapid expansion of the liquid droplets (that were produced by the injection nozzle), due to the fast diffusion and subsequent solubilization of supercritical CO2 in DMSO. The precipitation of the solid material starts on the droplet surface where there is the lowest liquid concentration. After the onset of precipitation, solute contained inside the droplet migrates towards the surface thus producing the empty shell.

Figure 2. SEM image of AcSm(SAS) particles produced at 95 bar, 40~ DMSO (5,000 x magnification).

mg of solute/ml of

353 At the highest volume expansions very small spherical nanoparticles were produced by explosion of balloons. In this case all the internal surface of the precipitator was covered by particles though they were prevalently deposited at the bottom of the chamber. For example, in Figure 3 we reported the SEM image obtained for AcSm at 150 bar, 40 ~ 10 mg solute/ml DMSO. These particles have a mean diameter of 144 nm. The corresponding PSD is reported in Figure 4. A standard deviation of 59 nm was measured. Using other materials we also observed particles with a mean diameter lower than 50 nm [ 12].

Figure 4. Example of particle size distribution for AcSm(SAS) obtained at 150 bar, 40~ mg of solute/ml DMSO. Particles mean diameter is 140 nm; standard deviation is 60 nm

10

354 This kind of particles has an interesting morphology from the point of view of industrial application: they are very small and their mean particle diameter can be varied from about 100 to 300 nm modifying some process parameters. We performed experiments at 150 bar and 40~ and varying the AcSm concentration from 5 to 65 mg/ml DMSO. The mean particle size of AcSm(SAS) particles increases from about 100 to about 300 nm in the range of concentrations explored, while a larger increase was observed in the particle size distribution. In Figure 5, a SEM image is reported for AcSm(SAS) at 40 mg solute/ml DMSO that, when compared to the sample of Figure 3 (obtained at the same temperature and pressure but at a lower concentration) well demonstrates the enlargement of the particle size distribution connected to an increase of concentration. Thus, the increase of solute concentration in the liquid carrier largely influences the polydispersity of particles.

Figure 5. SEM images of AcSm(SAS) particles produced at 150 bar, 40~ ml of DMSO (30,000 x magnification).

40 mg of solute /

The XRD analysis of AcSm shows the pattern of hydrate crystalline acetate, Sm(CH3COO)3, whereas AcSm(SAS) is amorphous. The FT-IR spectra reported in Figure 6 show the bands of acetate salt for AcSm sample, with main peaks at 1543 and 1426 cm"1, due to asymmetrical and symmetrical stretchings of carboxyl groups. Moreover the SAS sample contains a great amount of samarium basic carbonate, as indicated by the apparison of bands at 1501 and 845 cm"1 . Upon calcination in air at 700~ these differences disappear, both calcined samples showing typical patterns of crystalline oxide, Sm203, as confirmed by XRD and FT-IR analysis. However, the characteristics of surface area and porosity are different. For example, OxSm(SAS) has a specific surface area of about 8 m2/g, microporosity being absent, whereas OxSm exhibits micro-mesoporosity resulting in 80 m2/g surface area. Since the average size of OxSm(SAS) is about 100 nm (Figure 6), the geometric surface area of these particles is 7 m2/g, in very good agreement with the measured area. It must be also pointed out that the surface area of the precursor acetate is 14 m2/g for AcSm(SAS) and 12 m2/g for AcSm. This

355 result indicates that the thermal treatment of AcSm(SAS) does not induce any porosity in the particles, at variance of that found in the case of AcSm

OxSm (SAS) ID

J ID

' 4000

'

I

3000

'

I

2000

'

w a v e n u m b e r s , cm-1 Figure 6. FT-IR spectra of AcSm, AcSm(SAS) and OxSm(SAS).

Figure 7. SEM image of OxSm(SAS) (40,000 x magnification).

I

1000

'

356 The investigation of the decomposition process by TG and TPD indicates that the formation of samarium oxide passes through the intermediate formation of samarium oxycarbonate, but the latter decomposes differently from the basic carbonate originally present in AcSm(SAS).

3.2 Catalytic activity in ethane oxidehydrogenation The conversion of ethane on OxSm(SAS) increases with the temperature reaching 22 % at 680~ (Figure 8). Also the selectivity to ethylene increases with increasing the reaction temperature (58% at 680~ due to progressive reduction of conversion to CO2 and, at the higher temperature values, to CO, in agreement with [13]. The reaction rate of ethane evaluated up to 590~ i.e. for conversions lower than 10%, ranged from 13x10 "6 (550~ to 19x10 "6mol g-1 s-1 (590oc). The comparison with the performances obtained with OxSm shows that the consumption rate of ethane on the latter sample was three times higher than on OxSm(SAS) when referred to the unit mass of catalyst, but three times lower with reference to the unit surface area. 80 ,,,-,,,,

>, 6 0 -

>

, m

(D

m

co 4 0 c" 0 t_

(D > tO

20-

o

0

i

500

I

I

550 600 650 Temperature (~

Figure 8. Effect of temperature on C2H6 (o) and 02 and C02 (A) selectivity on OxSm(SAS) catalyst.

700

(I) conversion and on C2H4 (O), CO (!-'!)

A comparison of catalyst performances reported in the specific literature on ethane oxydehydrogenation on samarium oxide can not be easily performed, due to significant differences of experimental conditions investigated by other authors [13,14]. Using greater ethane concentrations in the feed reaction rates of 9x10 7 mol m2 s-1 at 510~ [13] and of 1.1xl0 "6 mol m"2 S~ a t 500~ [14] have been reported. Taking into account the different conditions it appears that the activitiy for ethane conversion is not so different from that exhibited by OxSm(SAS). A further comparison in terms of ethylene yield is shown in Figure 9. It is observed that the ethylene yield obtained with OxSm(SAS) is higher than with OxSm at a given value of ethane conversion. Moreover, also the ratio of CO and CO2 produced is higher in the case of AcSm(SAS). The more favoured formation of CO2 on AcSm could be related to diffusion

357 limitations, likely expected with OxSm sample as result of the presence of micro- and mesoporosity exhibited by that sample with respect to no porous OxSm(SAS). 20 . . . . .

1.0

16-

A

A

-0.8

.~A

ll

"~ 1 2 -

~

-

0.6 .>

- 0 . 4 co

8-

O4

0

o

-ro4

0

o

rr

i-

4-

-0.2 o0

0 4

I

I

I

I

8 12 16 20 C2H 6 Conversion (%)

0.0 24

Figure 9. Ethylene yield (re,El) and C O / C O 2 selectivity ratio (e,O) versus ethane conversion on OxSm(SAS) (solid symbols) and on OxSm (open symbols) catalysts.

4. CONCLUSIONS Samarium acetate nanoparticles were successfully produced by SAS process obtaining mean diameters down to 100 nm. Very narrow particle size distributions were also obtained especially when low solute concentrations were used. Acetate decomposition resulted in the formation of no porous samarium oxide particles that showed high selectivity to ethylene in the oxidative dehydrogenation of ethane. REFERENCES l. 2. 3. 4. 5. 6.

P.M. Gallagher, M. P. Coffey, V. J. Krukonis and W. W. Hillstrom, J. Supercrit. Fluids 5 (1992) 130. J. Bleich, B. W. Muller, and W. Wal3mus, Intern.. J. Pharm. 97 (1993) 111. D.J. Dixon, G. Luna-Bercenas and K. P. Johnston, Polymer 35 (1994) 3998. L. Benedetti, A. Bertucco and P. Pallado, Biotech. Bioeng. 53 (1997) 232. S. Jaarmo, M. Rantakyla and O. Aaltonen, Proceed. 4th Intern.. Symp. on Supercritical Fluids, K. Arai Ed., Sendai (Jap), 1997, 263. S.D. Yeo, G. B. Lim, P.G. Debenedetti and H. Bernstein, Biotech. Bioeng. 41 (1993) 341.

358 7. 8.

P. York and M. Hanna, Intern. Patent C07C215/60 (1994). E. Reverchon, G. Della Porta, C. Cdano, S. Pace and A. Di Trolio, J. Mater. Res. (1998) in press. 9. P. Ciambelli, L. Lisi, G. Ruoppolo, G. Russo and J.C. Volta, Proc. 3rd World Congress on Oxidation Catalysis, Studies Surf. Sci. Catal. 110 (1997) 285. 10. A. Kordikowski, A.P. Schenk, R. M. Van Nielen and C. J. Peters, J. Supercrit. Fluids 8

(1995) 205. 11. E. Reverchon, G. Della Porta, S. Pace and A. Di Trolio, Ind. Eng. Chem. Res. (1998) in press. 12. E. Reverchon, G. Della Porta, D. Sannino and P. Ciambelli, Powder Technol., submitted. 13. E. M. Kennedy and Cant N. W., Appl. Catal. 75 (1991) 321. 14. G. A. Martin.,S. Bemal, V. Perrichon and C. Mirodatos, Catal. Today 13 (1992) 487.

91998ElsevierScienceB.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

359

Soft chemical synthesis o f mixed metal molybdate oxidation catalysts and their structural relationship to hydrotalcite S. Soleda, D. Levinb'', S. MiseC, J. Yingb aCorporate Research Laboratories, Exxon Research and Engineering Co., Rt. 22 East, Annandale, NJ 08801 bDept, of Chem. Engineering, Massachusetts Institute of Technology, Cambridge, Mass 02139 Low temperature soft chemical syntheses, which allow preparation of metastable materials, have been commonly used in preparing zeolites, but have not been studied extensively for other mixed oxide catalyst systems. We have found a low temperature solution synthesis route to convert some hydrotalcite-related phases into ammonium mixed-metal molybdates that crystallize with a hydrotalcite-related superstructure and which are useful precursors for high surface area mixed metal oxides. 1. INTRODUCTION Hydrotalcite related phases (also referred to as layered double hydroxides (LDH's), or anionic clays) resemble more familiar smectite clays but contain exchangeable anions rather than cations. They are derived from simple layered hydroxides such as brucite, Mg(OH)2, with partial substitution of trivalent cations (like A1+3) for the divalent host cation. Interlayer (gallery) anions provide charge compensation, so the LDH's can be represented by the general formula H

HI

x+

n-

[M l.xM x(OH)2] [A ~s..zH20]. The mineral hydrotalcite itself, Mg6A12(OH)16CO3.4H20, is probably the best known material in this class; its synthesis, structure, and properties have been extensively studied 1,2'3'4. Interlayer carbonate anions (and water) bridge the positively charged Mg-A1 hydroxyl sheets. The structure of LDH's, shown schematically in Figure 1, resembles that of brucite, Mg(OH)2, where Mg2+-octahedra (6-fold coordinated to OH-) share edges to form infinite sheets. These sheets are stacked on top of one other, and are held together by van der Waals forces. Isomorphous substitution of Mg§ by AI+3 creates a positive charge in the hydroxyl sheet. This net positive charge is compensated for by incorporation of hydrated anions in the interlayer region; in the parent Mg-A1 hydrotalcite structure, the oxygens of water and carbonate are disordered, partially occupying one of six available sites 5. A large variety of "current address: Mobil TechnologyCompany,StrategicResearchCenter, Paulsboro, NJ 08066.

360 Fig. 1. LDH Structure, containing octahedral sheets with anions (and H20 ) occupying interlayer gallery positions.

divalent and trivalent cations are found in LDH's including Mg 2+, ZR§ Ni § Fe§ Co+z, and AI§ Cr+3, Co+a, and Fe+3. Recently, Velu et al. have reported the successful incorporation of tetravalent Zr+4 ions into the cation sites 6. Typically, the carbonate anion is incorporated into the structure during synthesis, and is tenaciously held. Other simple anions such as sulfate, nitrate, hydroxyl, and halide can be found in the structure, and more complex isopoly(i.e. Mo70246") 7 or heteropolyanions (SiMo120404-) 8 have, with considerable synthetic skill, been introduced into the gallery. Considerable attention has been focused on pillaring of LDH's with Keggin type [XM12040] m" polyoxometalate (POM) anions. Two approaches have been investigated: i) direct ion-exchange 8'9,1~ of polyoxometalate (POM) anions in [Zn2AI(OH)6]NO3.zH20]; and ii) direct synthesis of pillared LDH derivatives in a single-step 12. Results of pillaring reactions by ion-exchange showed that nitrate LDH's could be successfully pillared providing that the pillaring Keggin anion has a relatively high charge density (i.e. m>6). Recently Wang et al. showed that (-3) Keggin ions could be pillared by direct synthesis if the LDH was first swelled with organic reagents 13. Drezdon et al. at Amoco reported a novel approach to the synthesis of polyoxometalate-pillared LDH's that involves the initial synthesis of an organic anion-pillared precursor which is subsequently exchanged with the appropriate isopolymetalate under mildly acidic conditions~4,~5. In this strategy, an organic anion slightly larger than the polyoxometalate is used to prepare the initial precursor. Acidification of the organic pillared LDH protonates the organic anion, weakening the electrostatic interaction between the brucite-like layers and the organic species, and allows the migration of the organic acid out of the interlayer. Partial exchange of the organically-pillared LDH by the polyoxometalate, followed by calcination to remove the remaining organic material enables the synthesis of pillared LDH's with large gallery heights ~6. Alternate synthetic strategies for the synthesis of pillared LDH's uses the ability of a mildly calcined LDH to reconstitute its original layered structure in the presence of an aqueous anionic solution. By calcining hydrotalcite at 450~ V100286" and Mo70246" anions were intercalated into the LDH structure upon reconstitution ~7. This procedure appears to have a variable success rate, as all the reported x-ray patterns are marginally crystalline, and in many cases the authors concede that it is preferable to first spread the layers with an initial intercalation of organic anions TM. In contrast to all the materials just described, we will

361 show that the phases reported here have isolated MoO4 = anions, rather than polymeric oxyanions, and these MoO4= anions are actually bonded to the divalent hydroxyl layer. LDH's and their derivatives have found applications as heterogeneous catalysts, anionic exchangers and adsorbers 3,19. They have aroused interest as solid bases2~ and as supports for metals 21.

2. EXPERIMENTAL 2.1 Preparation The chemie douce syntheses of the Zn-molybdate phase involved a multistep procedure. First, the Zn-A1 hydrotalcite phase was prepared, then calcined to form a metastable mixed oxide precursor related structurally to ZnO, and finally this solid was reacted at 55~ with a solution of ammonium molybdate. This pathway is represented as" Precipitation Zn,A1 salts

==->

Zn/A1-LDH

Calcination ==>

Chimie douce

Zn/A1Mixed Oxide ==>

Zn-Molybdate

The initial LDH, with a composition of Zn4AI2(OH)12CO3"zH20 was coprecipitated at a constant pH of 9 by simultaneously adding 1 molar solutions of Zn(NOa)E.6H20 and Al(NOa)a.9H20 to a solution of KOH and K2CO3 at 55 +2~ Following precipitation, the material was aged in the mother liquor overnight at 70~ and then filtered, washed, and dried at 110~ This LDH phase was calcined in air at 500~ for 3.5 hours to form a metastable Zn(AI)-O. The chemic douce synthesis was accomplished by placing the metastable Zn(Al)-O oxide in a 0.05 molar ammonium heptamolybdate solution held at 55~ Progress of the reaction was followed by monitoring the pH. A typical pH profile over the course of the reaction is shown in Figure 2. The pH variation represents a complex series of events including alumina extraction and hydrolysis, and reaction of the dealuminated zinc oxide with molybdate anions. Attainment of a constant pH marks the completion of the chimie douce reaction. Following this, the fine white powder was filtered, washed with deionized water, and dried overnight at 110 ~ As described below, the analogous Ni-Mo phase, which had been prepared and reported by Teichner and Astier22 in 1988, was synthesized by the boiling precipitation technique reported by them- a procedure that does not involve hydrotalcite precursors.

2.2 Characterization Elemental analysis of the LDH and the Zn-Mo chemic douce phase was determined by X-ray fluorescence (Oneida Research Services). Both Fourier-transform infrared (FTIR) and Raman spectra were obtained on BioRad spectrometers. Solid state A127MASNMR spectra of the Mg-A1 and Zn-Al LDH phases were acquired at room temperature on a Chemagnetics CMX-5000 operating at 130.27 MHz. Powder X-ray diffraction spectra collected on a Siemens D5000 diffractometer operated at 45 kV and 40 mA with CuK~ radiation were recorded. A step-scanned diffraction pattern was collected from 10 to 90 ~ 20 (0.02 ~ step, 2.5 see/step, sum of 3 scans) for use in Rietveld refinement of the Zn-Mo chemic douce structure.

362

In addition, a diffraction pattern of the Ni-Mo analog was collected at the Brookhaven National Synchrotron Light Source. Both data sets were used to obtain least square fits of unit cell parameters and the isomorphous crystal structures were solved and refined by Rietveld methods.

58

..........

i

5.6

i....... t

i mt i

pH 5.4

I

I

i . . . . . . . . . .

!_ 4.8

' i

i

0

....

-t~~

I

L .............. 1

!

i

i

i

i

i

i ....

t ....

i ....

i ....

i ....

i ........

25

'

50

75

100

time (min)

125

150

L"

175

200

Figure 2. pH variation with time during chemie douce synthesis of Zn-Mo phase.

3. RESULTS AND DISCUSSION The Zn4A12-LDH analyzed as 41.2% wt. Zn and 8.02% wt. A1, close to the initial Zn/A1 molar ratio of 2/1. Surprisingly, following chimie douce synthesis with an ammonium heptamolybdate solution, the Zn-Mo chemie douce product analyzed at 33.8% wt. Mo, 0.63% wt. A1, and 24.6% wt. Zn. The Zn/Mo molar ratio was close to unity (1.1) and the low aluminum content suggested that A1 was not retained in the chemie douce product. The x-ray diffraction patterns showing the relationship among the starting LDH, the calcined Zn-A1-O phase, and the final Zn-Mo chemie douce product are shown in Figure 3. Figure 4 compares the Zn-Mo phase with the spectrum obtained from a synthesized version of a partially characterized (NHx)-Ni-Mo-O phase reported by Teichner and Astier in 198822 (PDF Card 40674). These spectra show the close similarity of the Zn-Mo phase prepared by chemie douce techniques and the (NHx)-Ni-Mo-O phase prepared by the previously reported boiling precipitation procedure without using LDH-related precursors. However, none of our attempts to prepare the Ni-Mo analog of the chimie douce Zn-Mo phase starting from a Ni-A1 LDH precursor were successful, which provided yet another clue about this chemistry. The A127 MASNMR of the hydrotalcite Zn-AI and Mg-A1 precursors shed some light on this finding. As seen in Figure 5, the Zn-A1 phase contains A1 primarily in tetrahedral sites, whereas Mg-A1 LDH contains A1 primarily in octahedral sites. Apparently the relative instability of these tetrahedral AI atoms in the ZnO phase is a prime driving force for producing the chimie douce reaction. During the chemic douce reaction, the AI atoms are expelled as the solid reacts with the ammonium heptamolybdate solution. The more stable

363 octahedral AI present in the rock salt structure of MgO creates a refractory mixed oxide precursor that remains tmreactive at the chemie douce conditions.

11 I I

"chimie douce" phase Zn-Mo chemie douce prepared phas

+ (NH4)6Mo7024 (50~ suspension) ~___J

_

__

.J

(D

|_--.

"ZI2AllO3.5" 500~

10

20

I I I I

z~i2(o~2(co3) l/z I ~ L _ L 1~_i 40 50 60 70

30

10

two theta (degrees)

20

Figure 3. X-ray diffraction spectra of Zn-Mo chemie douce sequence

30

40

two theta (degrees)

50

60

Figure 4. Comparison of Zn-Mo and Ni-Mo phases

A

f~

]/

calcined 12CO3"4HzOZn4AIz (riOH)

]\

calcined Ai primarily octahedral

.

.

. 200

.

.

.

'"

.

.

150

.

.

,

''

100

,

,

,

,

5t0

i

,

,

,

| -o

i

,

,

~

I -50

'

'

'

i

f"', -100

b

,

,

I -150

ppm '

'

Figure 5. A127MASNMR comparing Zn-Al and Mg-A1 HTC precursors The DRIFT spectrum shown in Figure 6 for the closely related Ni-Mo phase prepared by solution boiling confirmed the presence ofNH4 + ions in the structure, as evidenced by the stretches at 3300-3050 cm-1 at 1408 cm"1. The Raman spectrum of this phase (Fig. 7) showed the presence of the tetrahedral molybdate ion identified by the two strong bands at 904 and 321 cm l. The x-ray diffraction pattern of the chimie douce-prepared phase is significantly different from ZnO and bears no resemblance to that expected from an LDH pillared with the heptamolybdate anion as reported by Drezden 7. Because of the similarity of the Zn-Mo chemie douce product and the NHx-Ni-Mo-O phase reported by Astier et al., we initially

364 solved the crystal structure of the NHx-Ni-Mo-O phase. The monoclinic cell reported by Astier 22 was actually found to reduce to a hexagonal cell in the same R3(bar)m space group (No. 166) as occurs with the rhombehedral form of hydrotalcite. The patterns were indexed and lattice constants were refined by least squares. The unit cell constants for the Ni-Mo and Zn-Mo phases are and ao=6.015 A and Co=21.881A and ao=6.107 A and Co=21.641 A respectively. This compares to the Zn LDH with cell constants ao= 3.06A and Co=22.65 A. Relative to the LDH phases, there is an approximate doubling of the ao axis, suggesting a crystallographic ordering and superstructure formation. The substantial (-~1 A) contraction along the Co axis, with the substitution of the large molybdate anions for the smaller carbonate ions was initially puzzling, but it clearly indicated that this structure did not coordinate the anions in the same manner as in the LDH phases. The explanation for this contraction became apparent as the structure solution proceeded.

j i J

4OOO

1500 Wavenumbers (crn "1)

Fig. 6. IR spectra of (NHx)-Ni-Mo-O HTCrelated phase.

1300

1100

900

700

500

Wavenumbers (cm -l)

300

100

Fig. 7. Raman spectra of (NHx)-Ni-Mo-O HTC-related phase.

The ab initio structure determination of the ammonium nickel molybdate analog was accomplished first using a combination of three-dimensional Patterson methods, difference Fourier syntheses, and Rietveld refinement of synchrotron X-ray powder diffraction data. Following that solution and refinement, a similar refinement from CuK~ x-ray data on the ZnMo chimie douce phase was done. Knowing that the Zn-Mo chemic douce phase was related topotactically to the parent LDH Zn-AI phase simplified the structure solution. The Zn-Mo and Ni-Mo phases were isomorphous. Crystal Structure of Zn-Mo Hydrotalcite Related Chimie Douce Phase Figure 8 illustrates a three-dimensional polyhedron representation of the crystal structure of the Zn-Mo chemie douce phase. The framework of the ammonium zinc molybdate consists of sheets of distorted zinc octahedra to which tetrahedral molybdate groups are bonded. Ammonium ions are interspersed in the gaps between the Zn-Mo-O-OH sheets. The Zn-MoO-OH layers are stacked in the Co direction and are held together by hydrogen bonding. The zinc atoms defining these layers are located at site 9(e) of space group R3barm, and can vary in occupancy from 89 to 1. This fact rationalizes the experimental finding of Astier et a122that during preparation studies, similar phases formed even as the Ni/Mo ratio in the preparation

365 solution changed. If the occupancy of this Zn site is one, the arrangement of zinc atoms can be considered as a pattern of two alternating strings, one being Zn-Zn-Zn, the other being Zn Zn, where represents an ordered cation vacancy. This ordered cation vacancy is present independent of the zinc occupancy. As the zinc occupancy deviates from unity, additional disordered vacancies appear in the sheet. The ordered vacant octahedral sites are capped, both above and below, by tetrahedral molybdate groups as seen in Figure 9. The ordering of the Mo tetrahedra and the resulting doubling of the Co axis are shown, but no disordered vacant octahedral sites resulting from a reduction in the zinc occupancy from unity are shown. It should be noted that these random vacancies are not capped by tetrahedral molybdate groups. For the compounds studied, the Zn/Mo and Ni/Mo ratios are close to one, so that three of every nine zinc sites are vacant. The coordination of oxygens about the zinc atoms is distorted octahedral. The zinc atoms are linked to each other through double hydroxyl bridges. Each zinc octahedron shares edges with a maximum of four adjacent octahedra, thereby creating octahedral sheets perpendicular to the Co axis. That the molybdate anions are actually bonded to the zinc oxide-hydroxide sheets accounts for the curious shortening of the Coaxis relative to the Zn-A1 LDH. Each molybdate tetrahedron shares comers with six different zinc octahedra, generating the hexagonal arrangement shown in Figure 9. Between each Zn-Mo-OOH layer, in the space defined by oxygens of six tetrahedral molybdates, lie ammonium ions. A short NO distance of 2.98 A suggests that the ammonium ions are anchored via a hydrogen bonding mechanism. We attempted to site the mysterious AI atoms, which appeared to be expelled during synthesis of the ZnMo chemie douce structure. When A1 atoms were placed in the Zn sites and the fractional occupancy set on the basis of the elemental analysis, the thermal parameter for AI increased immediately to 0.400, the maximum that GSAS (the refinement program) will allow after a few cycles of refinement. Alternatively, when the thermal parameter of the A1 was constrained to equal that of the Zn, and the fractional occupancy was allowed to vary, the occupancy refined to zero. This lead to the conclusion Fig. 8. Crystal Structure of (NH4)HZn2(OH)2(MoO4). Zn Edge-Shared Octahedral with Mo-Containing Tetrahedra Capping Vacancies and Separated by NH4 + Ions. that the A1 atoms are not actually in the structure itself, but are probably present as a small amorphous phase, also consistent with the 12 terms required to satisfactorily fit the background during the structure refinement. Hydrogen positions were assigned on the basis of difference Fourier maps, as discussed elsewhere 23. This assignment of protons led to the chemical formula for the Zn-Mo

366 chemie douce phase (with Zn/Mo N1) as (NH4)HZn2(OH)2(MoO4) 2. The variable proton content and hydroxyl group population on the Zn octahedral provides a mechanism to

Fig. 9. Arrangement of Zn-containing octahedra in sheets perpendicular to (001) showing ordered vacancies and capping by molybdena tetrahedra that results in doubling ao. maintain charge neutrality when changes in the Zn/Mo ratio (0.75/1=>1.5/1) occur. details of this flexibility in stoichiometry are outlined elsewhere 23.

The

ACKNOWLEDGMENTS

The authors wish to thank NSF, the Exxon summer intern program, as well as M. Leonowicz, J. Millar, B. Zhang, T. Sun, B.J. Wuensch, D. Benn and J. Holmes for help with this work. REFERENCES

' W. T. Reichle, Chemtech, 16(1) (1986) 58. 2 D. E. Laycock and R. A. Newman, Stud. Surf. Sci. Catal., 73, K.J. Smith and E.C. Stanford (Eds), (1992) 269. 3 F. Cavani, F. Trifiro, and A. Vaccari, Catal. Today 11(2), (1991) 173. 4 F. Trifiro and A. Vaccari, Compr. Supramol. Chem., 7, Editor(s): G. Alberti and T. Bein. Publisher: Elsevier, Oxford, UK (1996) 251. 5 H.F.W. Taylor, Mineral. Mag., 39 (1973) 377.. 6 S. Velu, V. Ramaswamy, and S. Sivasanker, Chem. Commun. 21 (1997) 2107. 7 M. A. Drezdon, Inorg. Chem. 27, (1988) 4628. 8 T. Kwon, G. A. Tsigdinos, and T. J. Pinnavaia, J Amer. Chem. Sot., 110 (1988) 3653.

367 9 T. Kwon and T.J. Pinnavaia, Chem. Mater. 1 (1989) 381. 10M. Doeuff, T. Kwon and T.J. Pinnavaia, Synthetic Metals, 34 (1989) 609. 11T. Kwon and T.J. Pirmavaia, J. Molec. Catal. 74 (1992) 23. n j. Wang, Y. Tian, R.-C. W., J.L. Col6n and A. Clearfield, Mat. Res. Soc. Symp. Proc., 233, (1991)63. 13j. Wang, Y. Tian, R.-C. W. and A. Clearfield, Chem. Mater., 4, (1992) 1276. 14M. A. Drezdzon, US Patent 4,774,212 (1988). 15M. A. Drezdzon, Inorg. Chem. 27 (1988) 4628. 16M. A. Drezdzon, ACS Symp. Series 437 (1990) 140. 17K. Chibwe and W. Jones, Chem. Mater., 141 (1989) 489. 18K. Chibwe and W. Jones, J. Chem. Sot., Chem. Commun., (1989) 926. 19W. T. Reichle, Solid State Ionics, 22 (1986) 145. 20j. Shen, J.M. Kobe, Y. Chen, and J. A. Dumesic, Langmuir 10(10) (1994) 3902-8. 21I. Ivanova, A. Pasau-Claerbout, M. Seirvert, N. Blom, and E.G. Derouane. J. Catal. 158(2) (1996) 521-36. 22M. P. Astier, G. Dji, and S. J. Teichner, Ann. Chim. Ft., 12 (1987) 337. 23D. Levin, S. L. Soled and J. Y. Ying, Inorg. Chem. 35(14) (1996) 4191.

This Page Intentionally Left Blank

91998 Elsevier Science B.V' All rights reserved.

Preparation of Catalysts VII B. Delmon et al., editors.

369

Preparation of supported SO42-ZRO2 for isomerization of n-butane C.R. Vera a'b, C.L. Pieck", K. Shimizu b and J.M. Parera a aInstituto de Investigaciones en Cat/tlisis y Petroquimica -INCAPE-, Universidad Nacional del Litoral - CONICET, Santiago del Estero 2654, 3000 Santa Fe, Argentina. bNational Institute for Resources and Environment -NIRE-, AIST-MITI, 16-30nogawa, Tsukuba, 305 Japan.

Supported SO42-ZRO2 was prepared by the technique of controlled hydrolysis of alkoxydes and its activity for isomerization of n-butane was assessed. When deposited over silica or alumina, the growth of the monoclinic phase was suppressed and the zirconia crystals were fluoritic (cubic). Stabilization of this crystal habitat was essential for the onset of catalytic activity in the sulfated supported materials. The metastable defective fluoritic lattice was thought to produce the redox sites needed for the onset of a bimolecular isomerization mechanism. The amount of crystalline defects was increased by further reducing the support before sulfation, producing catalysts with enhanced activity, similar to that of bulk sulfated zirconia.

1. INTRODUCTION The dispersion over commercial supports of transition metal oxides (ZrO2, TiO2, Fe203) which develop high acidity upon promotion with sulfate, have attracted the attention of some researchers in the past [ 1,2] because the number of active sites of these oxides can be limited by the available specific surface area. Other advantages may result from the lower cost or the enhanced mechanical resistance of the host support. The preparation of supported SO42--ZRO2 (SZ) and the study of its activity in isomerization of n-butane are undertaken in this paper. With respect to the sulfate promotion two possibilities were considered: (I) pre-sulfation of the untreated, uncalcined supported hydroxide; (II)pos-sulfation of the zirconia calcined till crystallization. Calcination is important because in the case of SZ catalysts the active materials are crystalline and mainly tetragonal [3]. From the standpoint of dispersion, calcination is also related to the sintering of the dispersed phase, which produces a loss of active surface area. For technique (II) the influence of pretreatment in hydrogen was also studied. Vera et al [4,5] have proposed a model of active sites based on the presence of bulk and surface defects (oxygen vacancies and Zr 3+ cations) and adjacent sulfate groups. The concentration of such defective sites could be enhanced by eliminating lattice oxygen by reduction at high temperatures.

370

2. EXPERIMENTAL

2.1 Catalyst preparation Supported zirconia was prepared in a similar fashion to the technique reported by MarquezAlvarez et al [6]. Samples of zirconia supported on silica and alumina were prepared by the method of controlled hydrolysis of alkoxyde, where the OH groups of the support were used for the removal of alkoxy groups from a Zr alkoxyde. An alumina carrier from Ketjien (CK300, 180 m2/g) and a silica carrier from Alfa Morton Thiokol (wide pore, 300 m2/g) were used in their commercial form with no further treatment, except for the grinding and sieving to 3580 mesh. The zirconium precursors of supported zirconia were zirconium alkoxydes: Zr nbutoxide (Strem, Zr(n-BuO)4, stabilized in n-butanol, 76-80%); Zr i-propoxide (Strem, adduct with i-propanol, FW-387.67 g/mol); Zr n-propoxide (Fluka, -70% in n-propanol, FW=327.58 g/mol, density=l.058 g/ml). The variety of precursors used was due to the preliminary screening of preparation conditions. The solvents used was n-hexane (Dorwil, commercial grade), which showed similar solubilization properties than i-propanol and higher than nbutanol, and no retention of water, which was to be avoided during impregnation. It was useful to express all surface coverages and Zr contents in terms of the monolayer content. The bidimensional density of a zirconia layer of one atom thickness (8) was estimated as the average of the cationic density in the <111> and <001> planes oftetragonal zirconia [7], and was equal to 8 Zr/nm 2. This amounts to 13.29 gmo! Zr/m 2. This value does not coincide with that used by M/trquez-Alvarez et al [6] which is much lower (calculated from their paper as 6.09 gmol/m2). Asakura et al [8] used the value of the bidimensional size of the unit cell of ZrO2 (0.13 nm2), yielding 15=7.7 Zr/nm 2, which is similar to our estimation. As a result of the first screening of preparaton conditions, a final method was adopted which consisted of two succesive impregnation steps of zirconium n-propoxide, using an amount equal to the monolayer content, and using n-hexane as a solvent. After each deposition, the sample was hydrated by immersion in water for 8 h and then was dried in a stove at 110 ~ overnight. The samples were denominated Z/Si (21% Zr) and Z/A1 (24% Zr). For the sake of comparison, bulk SO42-ZRO2catalysts were prepared. Zr(OH)4 from ZrOC12.8H20 (Strem, 99.9998%) was obtained with a technique described elsewhere [9] and denoted as ZOH. Zr(OH)4 from n-propoxide (ZOH-Pr) was obtained by dissolving the alkoxyde in n-hexane in a 1:20 volume ratio, and adding slowly distilled water under constant stirring until all the alkoxyde was precipitated. Finally it was filtered and dried overnight at 110 ~ Crystalline zirconia was obtained by calcination of ZOH and ZOH-Pr at 600 ~ in still air for 3 h (samples ZX and Z-Pr ). Also a standard calcined material was used (ZrO~, Strem, commercial grade), and denoted as Z-W. Two different sulfate promotion techniques were used: traditional wet impregnation at ambient temperature (WI) with no previous pretreatment, and sulfation by wet impregnation preceded by a hydrogenation treatment at high temperature (HS). In the WI method, the samples were dipped in H2SO4 1 N (20 ml/g) for 1 h in the case of the supported samples (to avoid acid leaching of Zr) and for 2 h in the case of bulk zirconia. Impregnation with previous hydrogenation was chosen for pos-sulfation of calcined supported zirconia because it has been recently found [4,5] that crystalline zirconia promoted in this way is active in isomerization of n-butane.

371 Supported sulfated zirconia catalysts were obtained by combining different synthesis steps: CA=calcination in air at 620 ~ for 3 h (for the production of crystalline ZrO2 or the remotion of excess adsorbed sulfate); CH=hydrogenation at 500 ~ for 3 h (for the production of oxygen vacancies and surface Zr3+); WI=impregnation with H2SO4 1 N, immersion for 2 h, followed by filtering and drying at 110 ~ Three different promotion routes were used. These routes and the corresponding catalysts are: SZ/AI(1)-SZ/Si(1)=WI,CA; SZ/AJ (2)SZ/Si(2)=CA,WI, CA; SZ/Si(s)-SZ/AI(3)=CA,CH, WI, CA. Finally, bulk sulfated zirconium hydroxide (SZOH) was prepared by wet impregnation of the ZOH support and bulk sulfated crystalline zirconia (SZX) by impregnating the ZX support.

2.2 Catalyst characterization TPD (in pure Ar) and TPR (5% H2 in Ar mixture) tests were performed in a Ohkura TP2002-S equipment, using a heating rate of 10 ~ DRX spectra were collected in a Shimadzu DX-1, using radiation CuKc~ filtered with Ni. Surface area and pore volume were obtained in a Quantachrome NOVA-1000 by nitrogen adsorption at 77 K. The catalytic activity was assessed with the reaction of isomerization of n-butane performed in a pulse reactor [7] (0.2 g of catalyst, 8 ml/min N2 -cartier-, 0.5 ml n-Ca/pulse). The surface charge developed in solution was measured by "fast" potentiometric titration (FPT), using solutions of (0.1 N KOH/0.1 N KNO3) and (0.1 N NO3H/0.1 KNO3). TG-DTA tests were performed in a Rigaku Thermoflex equipment using a 10 ~ heating rate and nitrogen as carrier gas. Ammonia TPD tests were performed in a flow apparatus with a TCD.

3. RESULTS A first screening of preparation conditions was performed by impregnating silica and alumina with different alkoxydes. Their hydrolizability was related to the reported order of stability of alkoxydes [10] ( Zr(OMe)4-~ Zr(OEt)4 >> Zr(OPr)4 >> Zr(OBu)4 ). Zr n-butoxide was highly reactive which was good for the anchorage onto the supports, but it hydrolyzed rapidly even with air humidity and its handling was troublesome. Zr i-propoxide was more stable but the adduct with i-propanol was difficult to hydrolyze and put into solution. Zr npropoxide displayed good reactivity and was fairly stable when stored and handled in a dessicator or a glove box; it was therefore preferred for the synthesis of the catalysts and its use is implicit in the rest of the Table 1 article. In a second step of the Amount of Zr leached during sulfation. Influence of screening (Tables 1 and 2), both hydrothermal pretreatment. the amount of precursor loaded in Sample Zr impregnated Zr leached (mgZr/g sample) a one-step impregnation and the (monolayers) Untreated Stabilized treatment before the step of Z/A1 1 8.90 3.81 sulfation were varied. When the 2 49.5 16.88 impregnated support was only 3 41.4 16.50 filtered and dried, a big amount of Z/Si 0.30 24.0 12.4 Zr was leached from the support. 0.61 31.0 10.5 Conversely, the amount of leached 1.21 42.0 12.4 zirconium was reduced to about 1.82 56.0 16.2 one-third when an intermediate

372 Table 2 Amount of ZrO2 (monolayers) deposited over alumina. Impreg- After After sulfation nated filtering Untreated Stabilized 1.0 0.84 0.45 0.80 2.0 1.24 0.89 1.06 3.0 1.52 0.90 1.30

step of hydrothermal stabilization (immersion in water and evaporation at 110 ~ was performed. Table 2 also shows that the content of Zr over the support is limited by the capacity of alkoxyde hydrolysis of the reactive groups on the surface of the support. Contents higher than a monolayer were not favored since the zirconia layer formed during impregnation is supposed to have only alkoxy groups exposed to the solution, and hence scarce or no reactivity towards other alkoxyde molecules. For this reason, impregnations with a nominal amount of one (1) monolayer were adopted. Succesive impregnations with intermediate hydration were used for obtaining higher loadings. In the case of silica, the amount of Zr grafted during impregnation was smaller than that of alumina, a fact possibly related to the smaller OH density of this support. In any case, two successive impregnation were necessary for obtaining a content of approximately one monolayer on the support. According to DRX data (Figure 1), the supported zirconia samples crystallized in a fluoritic phase with different shifts in the temperature of crystallization if compared to bulk ZrO2 (375 ~ Z/A1 (550-650 ~ Z/Si (400-500 ~ Crystallization was assessed by appearance of the main peak of the fluoritic phases at 20=30-30.5 ~ The higher shift in the case of alumina was related to a greater interaction between zirconia and alumina. For supported samples, a clear assignment of the crystalline phase was difficult, mainly ..............J~./L.,_ (h) because of the interference of the 8 support. However, the cubic phase was e3 a priori assigned (main peaks at c~ (g) 20=30.3 ~ 50.5-51~ 60.5 ~ based on previous reported results [6] and on the (f) e8 analysis of intensity ratios for the main (e) peaks. The sulfation treatment did not modify the crystal structure of any of the _.= samples, or their sintering behaviour. In (c) the case of the catalyst SZOH calcined at 2 ~ . (b) 550 ~ the crystalline phase was also fluoritic. The tetragonal habitat has been assigned on the basis of XRD and Raman 20 30 40 50 60 70 80 spectrum analysis [ 11 ]. ZOH-Pr calcined at 600 ~ SZX prepared by sulfation of 2 Theta[deg] the hydroxide calcined at 600 ~ and commercial Z-W, all had a similar Figure 1 XRDresults. (a) Silicagel; (b) Alumina; spectrum (g), which was mainly (c) Z/A1 calcined at 500 ~ (d) Z/A1 calcined at monoclinic. 700 ~ (e) Z/Si calcined at 500 ~ (f) Z/Si In accordance with the XRD data calcined at 700 ~ (g) ZOH-Pr calcined at 600 ~ showing the shifts in the crystallization (h) SZOH calcined at 600 ~ 0

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

"

.

.

.

.

.

.

.

.

.

.

-

&6

~

(d)

(a)

373 peaks, the TPR plots (Figure 2) of supported Z/Si and supported 4000 Z/A1 indicated that the process of formation of zirconia particles 3000 occurs in different temperature .,~" / Z - W 2000 Z / S i l / ."! _~ intervals depending on the base support. "6 1000 The first broad negative peak ,...-..-~=~-.-.-.-~..- ,-.---'-~:':. . . . . . . ":_. . . . . . . . . . . . . . . 0 .... --__L-.;,-'.. ...,." ',,.. was addressed to the desorption - 1000 of water and release of adsorbed 100 200 300 400 500 600 700 800 alkoxyde. The second peak may Temperature be related to the reduction of Zr and elimination of lattice oxygen Figure 2: TPR plots of bulk and supported zirconia. Z/A1 and Z/Si: supported hydroxide; ZOH-Pr: bulk hydroxide; during the sintering of the Z-W: bulk oxide, monolayer to form the first crystals. The TPR plot of a bulk crystalline material (Z-W) displayed a peak at 550 ~ assigned to the surface reduction of zirconia (elimination of lattice oxygen and Zr 4+ to Zr 3+ reduction) and another broad peak at 580-680 ~ addressed to the reduction of bulk zirconia, displaced with respect to the other because of diffusion constraints. In the case of the ZOH-Pr sample, a peak at 360 ~ coincided with a peak in the DTA plot (not shown), and was related to the removal of oxygen in the "glow exotherm" [12,13]. The peaks at 500 ~ and 550-700 ~ have the same meaning as in the case of Z-W. One noteworthy feature in the supported materials was the existence of only one positive peak. This result can be related to e reduction occurring mainly during the crystallization of small crystallites, which can not undergo a further loss of oxygen at higher temperatures. With respect to the different interaction between dispersed zirconia and the supports, this can be explained by the different surface cationic density of the interacting 5000

ZOH-Pr

oxides: ZrO2=8 Zr/nm2; A1203>10 A1/nm2; SIO2=5 Si]nm2. FPT results (Figure 3) revealed that when dispersed over silica, ZrO2 lost its amphoteric properties (shift of the isoelectric point -ZPC-, from 7.0 to 3.0 approximately). Similar results have been found in the case of mixed oxides of silica and zirconia, indicating that silicon atoms 0.3 bonded to Zr change its acidbase behaviour. Figure 3 also ~E 0.2 shows that calcination at high temperatures produces extensive E o 0.1 sintering and a great loss of 0 surface in the supported material, o o.0 d~ since the surface charge of Z/Si '-- -0.1 calcined at 600 ~ is practically o equal to that of pure SiOz. The -0.2 2 4 6 8 10 12 shift in the ZPC of the dispersed hydroxide, supposed to have full pH monolayer coverage, highlights Figure 3' Potentiometric titration curves of SiO2 gel (D), the fact that for non-crystalline Zr(OH)4 gel dried at 100 ~ (ZOH, A), Z/Si dried at 100 supported zirconia ZPC ~ (O) and Z/Si calcined at 600 ~ (m). measurements cannot be used for rl

r

r-

374 quantification of the surface coverage (0) ,. since in this case ZPC will not obey the linear equation: zpcZire~176

x Z P C supp~ + 0 X ZPC zire~

(1)

For crystalline particles, however, the method has been useful for measuring dispersion in other catalysts based on supported transition metal oxides [ 14,15]. Table 3 shows the results of catalytic activity of the samples. It can be seen that in the case of supported catalysts, method (3) yields the most active catalysts, with a conversion similar to bulk SZ and in the case of Z/Si with a higher selectivity (all catalysts of Fig. 4 and Table 3 have the same mass of Zr). The leaching of the dispersed Zr hydroxide during impregnation, and the loss of sulfate due to the the sintering of ZrOz Table 3 during the activation of Z/Si (1) and Z / h l (1) , are Initial activity of bulk and supported thought to be responsible for a supposed loss of sulfated zirconia catalysts (first pulse, active sites and hence for the low activity in these activation at 620 ~ in air). samples. One noteworthy result is that SZX, Sample Conversion Selectivity to which was prepared by sulfation (WI method) of a [%] i-butane [%] crystalline (mainly monoclinic) sample, is practically inactive, while the supported posSZX 0.71 85.32 sulfated samples which were also crystalline, but SZOH 29.7 69.9 SZ/Si (1) 0.95 90.3 fluoritic, were active. The activity found when SZ/Si (2) 9.81 83.0 supported crystalline ZrO2 was sulfated was thus associated to the stabilization of a fluoritic phase SZ/Si @ 27.9 86.2 [4]. SZ/A1(a~ 0.66 100.0 The plots in Figure 4 show that both Z/Si (3) and SZ/A1(2) 4.86 90.5 Z/AI(3) have a higher thermal resistance than bulk SZ/A1(3) 25.4 69.7 SZ. Presintering and the interaction with the support are thought to dimminish the surface ionic mobility and therefore to enable the sulfatezirconia bond to be stable at higher temperatures. Z/A1(3) displayed a continuous growth of the catalytic activity up to T > 700 ~ although it deactivated much faster than the other catalysts, activity being nil after a few pulses. Activity of Z/Si and bulk SZOH was much more stable. In the case of SZOH the drop in activity with increasing temperature is thought to be due to the total removal of sulfate in a process of collaborative collapse in which the removal of the first sulfate groups leads to sintering of the metastable network beneath and then

100 o~ ~, "N ,_..._,

80

e........

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

, , . .

"~ 09, = o ",~

60 40 20

~'o O

0 550 600 650 700 750 500 Activation temperature [~ Figure 4" Initial conversion (0, A, II) and selectivity to i-butane (O, D) as a function of the temperature of activation in air. SO42-Zr(OH)4 (A), Z/Si (3) (11, [-]), Z/AI(3)(O, O).

375 to unstabilization of the whole sulfate layer. The ammonia TPD spectrum of SZOH calcined at 600 ~ showed two main peaks at 330 and 450 ~ related to sites of medium and high acid strength. In the case of SZ/A1(3) only one main peak at 350 ~ could be found, with two small and broad shoulders at 450 and 550 ~ This indicates that mainly sites with medium acid strength are present on supported sulfated zirconia, a fact that may explain the low cracking activity of the samples and their high selectivity to i-butane when compared to SZOH (see Table 3).

4. DISCUSSION

The stabilization of the cubic phase of zirconia either calcining in air or hydrogen, might be related to the difference in surface energy between the monoclinic phase and the fluoritic phases [16]. A certain amount of energy is necessary to grow a particle and counteract the interaction with the support. The higher surface energy of monoclinic zirconia might pose an obstacle for normal crystal growth when the hydroxide is dispersed over a support. Formation of vacancies can stabilize even further the metastable fluoritic phases by lowering the average coordination number of Zr [17]. When calcining in air or an inert gas, vacancies are probably produced according to equation (2), and according to (3) when calcining in hydrogen: O 2" ....... >

89 02 (evolved) + [3 (oxygen vacancy) + 2 e

H2 + 02- - . . . . . . > H 2 0

(evolved) + E] (oxygen vacancy) + 2 e

(2) (3)

Vacancies might react readily with sulfuric acid, since SO3 has been found to react stoichiometrically with them [18]. In sulfated zirconia crystalline defects (vacancies and Z r 3+ cations) have been suggested to produce a redox couple and a network for charge transfer and charge stabilization [5]. The redox sites are responsible for the formation of olefins and they enable the onset of a bimolecular mechanism of isomerization of n-C4 of low activation energy. From a practical point of view, one restriction in the use of.supported SZ seems to be the control of the dispersion of the particles over the support. This is related to temperature dependent processes which influence the final activity and are antagonic in nature: promoting (crystallization, loss of lattice oxygen, formation of fluoritic phases) and inhibiting (sintering with loss of ZrO2 surface area or with loss of grafted sulfate).

5. CONCLUSIONS Z r 0 2 supported over silica or alumina crystallizes in the cubic phase, with a shift in the crystallization point which seemingly depends on the surface cationic density of the support. The stability of the crystals on the support gives presintered supported zirconia catalysts a higher thermal resistance than bulk sulfated zirconia. Sulfated prereduced zirconia is highly active, a fact possibly linked to an increase in the population of surface and bulk lattice defects.

376 Even in the case of crystalline non-reduced supported catalysts sulfation promoted the onset of catalytic activity in isomerization of n-butane, indicating that crystallinity is not an obstacle for the production of active sites. Activity is seemingly only related to the crystal habitat of the material. Fluoritic phases are active while monoclinic phase is inactive. Supported SO42--ZRO2 materials are promising catalysts for the conversion of short paraffins, but the control of the dispersion of the supported particles is foreseen as a major problem to cope with.

REFERENCES

.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

T. Yamaguchi, T. Jin, T. Ishida and K. Tanabe, Mater. Chem. Phys., 17 (1987) 3. T. Ishida, Chem. Lett. Chemical Society of Japan, (1988) 1869. R.A. Comelli, C.R. Vera and J.M. Parera, J. Catal., 151 (1995)96. C.R. Vera, C.L. Pieck and J.M. Parera, 2nd. JICA-CENACA Symposium on Catalysis, Santa Fe, Argentina, (1997) 47. Vera, C.R., Yori, J.C. and J.M. Parera, Appl. Catal. A: General, in press. C. M/trquez-Alvarez, J.L.G. Fierro, A. Guerrero-Ruiz and J. Rodriguez-Ramos, J. Coll. Interf. Sci., 159 (1993) 454. C.R. Vera. Doctoral Thesis. Universidad Nacional del Litoral, Santa Fe, Argentina (1995). K. Asakura and Y. Iwasawa, J. PhyS. Chem., 96 (1992) 7386. J.M. Parera, Catal. Today, 15 (1992) 481. Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, Third edition, Vol. 2, p. 1. R. Srinivasan, T.R. Watkins, C.R. Hubbard and B.H. Davis, Chem. Materials, 7 (1995), 725. M. Osendi, J.S. Moya, C.J. Serna and J. Soria, J. Am. Ceram. Soc., 68 (1985) 135. B. Djuricic, S. Pickering, D. Mc Garry, P. Glaude, P. Tambuyser and K. Schuster, Ceramics International, 21 (1995) 195. C. Knapp, F.J. Gil-LLambias, M. Gulppi-Cabra, P. Avila and J. Blanco, J. Mater. Chem., 7 (1997) 1641. J.C. Yori, C.R. Vera and J.M. Parera, Appl. Catal. A: General R.C. Garvie, J. Phys. Chem. 69 (1965) 1238. P. Li, I-W. Chen and J.E. Penner-Hahn, J. Am. Ceram. Soc. 77, (1994) 118. R. Silver, C.J. Hou and J.G. Ekerdt, J. Catal. 118, (1989) 400.

r 1998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

377

SbVO4: the chemistry of preparation F. Cavani, S. Ligi, S. Masetti and F. Trifirb Dipartimento di Chimica Industriale e dei Materiali, Universit~t di Bologna, V.le Risorgimento 4, 40136 Bologna, Italy

Different starting materials and procedures of preparation (solid state reaction, synthesis with H202 and coprecipitation) suited to obtain the rutile-type SbVO4 are analyzed in this paper. 1. INTRODUCTION Acrylonitrile is at present being produced by propene ammoxidation, nevertheless in recent years some companies have decided to invest in research on the propane ammoxidation. One of-the most interesting catalytic systems for the direct functionalization of the paraffin is Sb/WO [1-3], characterized by the futile-type SbVO4. From the 1988 until now, Standard Oil (now BP America) has published a series of key patents on Sb/V/O based catalysts, reporting high yields and productivities in acrylonitrile from propane. The catalysts were prepared by redox reaction of Sb203 and V205 in an aqueous medium, followed by the addition of promoters and supports [4, 5]. The use of V205 or NH4VO3 as a source of vanadium and the nature of the heat treatment were found to have a considerable effect on the formation of acrylonitrile. Good catalytic results were obtained only for Sb/V ratios higher than 1.0. An alternative patented method [6] consists of reacting Sb203 wiih a monoperoxovanadium cation VO(O2) +, obtained by reaction of V2Os with H202. The claimed advantage of this method is the possibility to obtain catalysts with good hardness and greater resistance. Several other patents have been published by this company on the modification of the catalysts by various doping elements, on the use of a dual-bed catalyst in order to improve the conversion of the intermediate propylene to acrylonitrile and on various technological aspects of the process, but new specific aspects of the preparation method were not reported. Another method for the preparation of SbVO4 is by coprecipitation: this procedure, utilized for the preparation of Sn/V/Sb/O systems active in the direct ammoxidation of propane to acrylonitrile [7, 8], can be used also for the Sb/V/O catalysts. An aqueous or ethanolic solution of SbC15 and VO(acac)2 is coprecipitated in an aqueous buffer, keeping constant the pH at 7.0 by adding ammonia. The advantage of this method is to obtain a high intimate mixture of the catalyst elements. In this paper, the chemistry of SbVO4 formation is analyzed. In particular different methods of preparation reported in the patents and in the open literature are utilized: solid state reaction between oxides, preparation with hydrogen peroxide and coprecipitation methods.

378

2. EXPERIMENTAL 2.1. Preparation of catalysts Even if catalysts with an excess of antimony show higher acrylonitrile selectivity, all the samples have been prepared with an equimolar Sb/V ratio in order to better evidence the formation of the desired compound. Even if calcination temperature of about 700~ are necessary to obtain the SbVO 4 with the solid state reaction method, in order to better evidence the different reactivity of each couple of compounds, all the samples have been calcined first at 400~ for 24 hours. Table 1. Samples analyzed wit h corresponding preparation method and compounds utilize d Sample Preparation Compounds Sample Preparation Compounds method utilized method utilized 1 S.S.R. V205 Sb203 9 H202 N H 4 V O 3 Sb203 2

S.S.R.

V205

ot-Sb204

l0

H202"

3

S.S.R.

V205

Sb6Ol3

11

Coprec.

VCI3

SbCl3

4

S.S.R.

V203

Sb203

12

Coprec.

VO(acac)2

SbCl3

5

S.S.R.

V6Ot3

Sb203

13

Coprec.

NH4VO3

SbCI3

6

H202

V205

Sb203

14

Coprec.

VCl 3

SbCls

VO(acac)2

7

H202

V20.s

cz-Sb-,O4

15

Coprec.

8

H202

V205

Sb6Ol3

16

Coprec.

N H 4 V O 3 Sb203

NH4VO3

SbCI5 SbCl5

* With acidified aqueous solution.

1) Solid state reaction (S.S.R): samples are prepared by mechanical mixing and grinding of a couple of different vanadium and antimony oxides (Table 1, samples 1-5). Part of the samples 1 and 2 (respectively V205/Sb203 and V205/~-Sb204) are calcined at 500~ and 600~ for 24 hours too. 2) Preparation with hydrogen peroxide (H202): a 30% aqueous solution of HzOz (37ml of HzO2 for 10g of V205) is added to an aqueous solution containing V205. When the brick-red slurry turns to a dark-red solution (with formation of the monoperoxovanadium cation VO(O2)+), the antimony oxide is added (Table 1, samples 6-8). The solution is maintained under reflux and stirred for 4 hours at about 100~ Hence, water is evaporated on a hot plate and dried overnight at 120~ Sample 9 is prepared in the same way, utilizing NH4VO3 instead of V205. In the case of sample I0, the aqueous solution of NH4V.O3 is acidified with several drops of concentrated HCI, before adding HzO2. 3) Coprecipitation (Coprec.): each couple of vanadium and antimony starting materials (Table 1, samples 11-16) is dissolved in an acidic aqueous solution (about HC1 3M). This solution is added dropwise to an aqueous solution of CH3COONH4, keeping constant the pH at about 7.0 by adding concentrated ammonia solution. The resulting precipitate is filtered, washed with water and dried overnight at 120~ All the samples were calcined at 400~ in air for 24 hours.

379 2.2. Characterization

X-ray diffraction patterns (powder technique) were obtained using Ni-filtered CuKo~ radiation (9~=1.542A) with a Philips computer controlled instrument (PW 1.050/81). Fourier transformed infrared (FT-IR) spectra in transmission were recorded using a Perkin-Elmer 7200 Fourier transfon'n spectrometer and KBr disk technique. Surface areas were determined using the B.E.T. method with nitrogen absorption at-I 96~ on a Carlo Erba instrument. 3. RESULTS AND DISCUSSION Reported in Figure 1 are respectively the XRD and the FTIR spectra of the samples obtained by solid state reaction (1-5), after calcination at 400~ There are no evidence of SbVO 4 in every sample from both the characterization techniques. This calcination temperature is too low in order to obtain a reaction between each couple of oxides. The unique difference is the change of valence state of the oxides in the samples 1, 4 and 5, due to oxidation by air; the samples 2 and 3 show the presence of the two couples of starting oxides. The samples 1, 2 and 5 are similar: in the FTIR spectra are visible the absorption bands due to c~-Sb204 (760, 745,650, 605 and 528 cm ~) and V205 (1020 and 824 cm~); the XRD show the presence of the same phases. Sample 3 is characterized by the presence of V-,O5 and Sb6013, the latter visible only by XRD. Sample 4 is characterized by ~-Sb204 (perfectly revealed by

o

o

4,

A

II o o

lI,t

g 1

1~

[]

1100 1000 900

ZO

4,0

60

800 700 CM-1

Figure 1. XRD and FTIR spectra of samples 1,2, 3, 4 and 5 (solid state reaction). (O) ~-Sb204" (,,1/) V205; (r-l) Sb6013.

600

500

400

380 XRD and FTIR) and by a vanadium oxide different from V205, probably V204. In fact, in the FTIR spectra is visible a band at 1010 cm -l, typical of more reduced species with respect to V205; moreover the bands at 825 cm -~ (typical of V2Os) and at 890 cm -~ (typical of V6013 ) are absent, while it is visible an intense band at 760 cm -l, typical of V-,O4. The samples 1 and 2 (respectively V2Os/Sb203 and V2Os/~-Sb204) are also calcined at 500~ and 600~ in order to study the different interactions between these two couples of oxides. Reported in Figure 2 are respectively the X R D and the FTIR spectra of sample 1 at increasing calcination temperature. Traces of SbVO4 at 500~ and the complete formation of this compound at 600~ are visible for the sample 1; on the contrary, traces of SbVO4 are visible only at 600~ with the sample 2. In fact, the sample 1 calcined at 500~ is characterized by the disappearance of the reflections of V'205 and by the appearance of SbVO4; by calcining at 600~ it is visible intense reflections of SbVO 4 and few traces of czSb204 . This futile-type SbVO 4 phase is similar to that cationic-deficent Sb0.92V0.9204 characterized by Birchall and Sleight [9], Berry [10, 11] and Andersson [12, 13]. As said before, in the FTIR spectra of sample 1 calcined at 400~ it is possible to see the bands due to V205 and o~-Sb204. This sample calcined at 600~ (Figure 2) is characterized mainly by the bands due to SbVO4 (740, 660 and 550 cml) 9moreover by a band and a shoulder (respectively 1020 and 825 cm l ) due to stretching of vanadium oxygen bonds, which could be attributed to the presence of V205 (that is not detected by XRD) or to the surface vanadyl and

A

4:>

,,0,

''00

2 500

.lit j

2-500

~____.__2-400 . . . . . . . . . . . . . .

'-'

"z"o

.... '

"

' ........'

'

-

'

.

.

.

.

.

.

.

1100 1000 900

.

'4"o

. . . .

'

800 700 CM-1

Figure 2. XRD and FTIR spectra of sample 2 at increasing calcination temperature. (O) C~-Sb204; (',1/) V205; (#) SbVO4.

600

500

400

381 internal vanadium-oxygen group of SbVO4. Finally there is a shoulder at 890 cm l which could reasonably indicate the presence of a distorted SbS+-oxide [3, 12]; the presence of Sb 5§ in SbVO4 is confirmed by MOssbauer spectroscopy [9]. On the contrary, only traces of SbVO4 are visible in the sample 2 after calcination at 600~ the XRD and the FTIR spectra of this sample calcined at 600~ are similar to the diffractogram and spectra of sample 1 calcined at 500~ Higher calcination .temperature is necessary to obtain the complete formation of SbVO4, utilizing ~-Sb204 instead of Sb203. Even if only indirect data (especially for vanadium) exist about the real valence state of the elements in the SbVO4, the "formal" valence state of the non-stoichiometric cationic-deficent SbVO4 phase is three and four for vanadium and five for antimony (S ~-5§ ,a ~. t., 0.92 ,tr4+ v 0.64 xr3+ v 0.28x.J4), however a certain amount of Sb 3+ on the surface of the rutile-type matrix (probably due to low crystalline Sb204 spreaded on its surface) was observed by XPS characterization [11]. The solid state reaction, that brings the formation of SbVO4, takes place through the two-electrons redox reaction between V 5+ and Sb 3+ to form V 3+ and SbS+; nevertheless the redox reaction between V ~+ and V 3+ to form two V4+ ions competes with the reduction by Sb3+. In this way, by calcining V205 and Sb203, at the right temperature, it is possible to obtain the desired 5+ 4+ 3+ 5+ 3+ product, which is a mixture of Sb , V and V , and traces of V205 ( V ) and Sb204 (Sb and Sb5+). By utilizing 0~-Sb204 instead of Sb203, we begin with a half amount of Sb 3+ and the redox reaction takes place with more difficulties. In order to obtain the formation of SbVO4, it is necessary to calcine the mixture at higher temperatures, where the ions mobility is higher. The preparation with H202 is an interaction between the monoper0xovanadium cation (VO(O2) +) and the antimony oxide at 100~ in water. In fact, hydrogen peroxide has the function to dissolve V205 in water; the formation of the soluble peroxo-ion is easily visible due to the change from the red-brick slurry (vanadium pentoxide) to the dark-red.solution (peroxo-ion). In order to analyze the reactivity of this monoperoxovanadium cation, sample 6, 7, and 8 (respectively with Sb203, ~-Sb204 and Sb6OI3) were prepared. Reported in Figure 3 are respectively the XRD and the FTIR spectra of these three samples. In the XRD it is possible to see the formation of a low crystalline SbVO4, just after calcination at 400~ only in the case of sample 6 (V2Os/Sb203); while samples 7 and 8 are characterized by the starting oxides (V205 and respectively ~-Sb204 and Sb6013). Also the FTIR spectra of samples 7 and 8 are similar to the spectra of samples 2 and 3, prepared with the same couples of oxides, but without H2Oz. In the FTIR spectra of sample 6 are visible the bands due to SbVO4 (740, 660 and 550 cm "~) and the shoulder due to SbS+-oxide (885 cm-Z); moreover the bands at 825 cm -z is disappeared and the band due to the vanadyl group is shifted to 990 cm l with respect to 1020 cm z (due to pure V~_O5). This shifting could be attributed to the surface V=O groups of the SbVO,~ partially distorted or to a partially reduced vanadium oxide. In this kind of preparation the valence state of the reagents is important too, in fact the desired product is obtained only utilizing Sb203. The hydrogen peroxide has mainly the function to dissolve V205, creating a monoperoxovanadium cation that is highly reactive (it is possible to obtain SbVO4 at low temperature of calcination). After the addition of Sb203 to the dark-red solution and heating at the reflux temperature, the solution becomes dark-green in about 1-2 hours, clearly indicating the reduction of V5+ by reaction with Sb 3+. Notwithstanding the initial addition of HzO2, the results are similar to those obtained by direct reaction of V5+ and Sb3+ (calcined at higher temperatures): the effect on the change of the valence state of antimony and vanadium is secondary.

382 In order to better understand the interaction between vanadium and hydrogen peroxide, the sample 9 was prepared, utilizing NH4VO3 (the valence state of vanadium is still 5) instead of V205. After the addition of H202, the red-brick slurry turns to a yellow-orange solution, due to the formation of another kind of peroxo-ion: the diperoxovanadium anion VO2(O2)23-. This peroxo-anion is inactive, in fact after the addition of Sb203 to the yellow-orange solution and the heating at the reflux temperature, the precipitate is not black (due to the reduction of vanadium and the formation of a precursor of SbVO4), but white. This powder was analyzed by XRD and FTIR techniques (Figure 3) and it results to be Sb203. In order to verify the existence of this diperoxovanadium anion, an excess of H202 and NH4OH was added to the yellow-orange solution, keeping the solution temperature at 0~ and obtaining a blue-violet precipitate, which is the salt (NH4)3V(O2)4, as described by Greenwood [14]. This diperoxovanadium anion predominates in alkaline solution, while the monoperoxovanadium cation in acid solution. As verification, sample 10 was prepared by acidifying with HC1 the aqueous solution of NH4VO3 before adding H202. In this way a dark-red solution was obtained and, after adding Sb203 and heating at the reflux temperature, the SbVO4, as characterized by XRD and FTIR techniques (Figure 3). The last method of preparation analyzed in order to obtain SbVO4 was the coprecipitation from a solution containing soluble compounds of antimony and vanadium in a pH-controlled aqueous solution. The first sample prepared was 15, utilizing SbC15 and VO(acac)2, as

A

I

9"

m

[]

in

i,

[]

[]

[]

? ,

'J

0 ,

toI oo.o

0 0 .

o o

! 1100 1000

2O

40

2U

900

6O

800 700 CM-1

600

Figure 3. XRD and FTIR spectra of samples 6, 7, 8, 9 and 10 (preparation with H202).

(O) o~-Sb204; (@') V205" (F']) Sb6013; (11)Sb203" ('~') SbVO4.

500

400

383 described in the literature [7, 8]. This catalyst was characterized by XRD and FTIR techniques (Figure 4), after calcination at 400~ and, as for the sample 6 prepared with H202, it results to be SbVO4. In fact, in the diffractogram only the reflections of the futile compound are visible, even if it is more crystalline with respect to sample 6, and in the FTIR spectra the bands due to SbVO4 (740, 660 and 550 cm-I), the shoulder due to SbS+-oxide (885 cm t ) and the band at 1015 cm j due to V=O bond. This last band is shifted to higher wave numbers with respect to sample 6, probably due to a higher amount of V 5+. In order to understand if the valence state of the starting compounds of antimony and vanadium is an important key in order to obtain SbVO4 with the coprecipitation method too, samples 11-16 were prepared, as described in Table I. Nevertheless, all these samples are quite similar: characterized by the presence of the desired rutile-type compound. The XRD and FTIR spectra are the same. Sample 15 was also characterized after the drying step, in order to understand the kind of interaction occured between antimony and vanadium. The dried coprecipitate is totally amorphous, as characterized by XRD in Figure 4, but contains both antimony and vanadium as described by FTIR in Figure 4 (wide bands at 6 I0 and 740 cm -z due to antimony and 990 -1 cm due to vanadium). Nevertheless SbVO4 is not yet created. In fact, this kind of preparation leads to a co-precipitate of hydroxides, which, after the drying step, is not already SbVO4, but a not well defined oxo-hydrate compound with antimony and vanadium at intimate contact.

<3-

A

43,0"

i,,c

1t J 15D

'

. . . .

. . . .

"

. . . .

..........' ..........." '

..... ' 2 ' 0 '

" %

1100 1000

900

800 700 CM-1

600

500

400

Figure 4. XRD and FTIR spectra of samples 6 and 15 after the drying (D) and calcination (C) steps. (8) SbVO4.

384

This compound evolves easily to SbVO4, even by calcination at 400~ On the contrary, sample 6, prepared with H2Q, V205 and Sb203, is characterized by SbVO4, even after the drying step, as evidenced by XRD and FTIR (Figure 4). The monoperoxovanadium cation, formed byodissolution of V205 with H202, is a very reactive specie that interacts with Sb203 during the reflux step, bringing to the direct formation of SbVO4, as visible during the preparation, thanks to the colour change. The SbVO4 obtained by preparation with H202 is different from the one obtained by coprecipitation. The latter is more crystalline and has a lower surface area (15 m~-/g) with respect to the former (118 m2/g). The reaction between the peroxovanadium cation and the antimony oxide could be described as a corrosion of the oxide bulk by the reactive peroxoions. In this way, a large number of nucleation centers are created, and a poorly crystalline SbVO4, even if just after the drying step, is formed. On the contrary, in the preparation by coprecipitation, after the drying step, SbVO4 is not formed yet, but a not well defined oxohydrate compound with antimony and vanadium at intimate contact. During the calcination step, the crystallites increase in dimension, and the SbVO4 obtained is more crystalline with respect to the sample prepared with H202, due to a lower number of nucleation centers. All the three methods of preparation are suitable in order to obtain the rutile-type S b V Q , nevertheless they give compounds with different peculiarity. The solid state reaction needs a higher calcination temperature. The methods with H202 gives the desired sample, even at very low calcination temperature, characterized by high surface area. The coprecipitation method instead needs intermediate calcination temperature and gives the desired sample with low surface area.

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

9. 10. 11. 12. 13. 14.

G. Centi, R.K. Grasselli and F. Trifirb, Catal. Today, 13 (1992) 661. G. Centi, R.K. Grasselli and F. Trifir6, Chim. Ind. (Milan), 72 (1990) 617. G. Centi and S. Perathoner, in Preparation of Catalysts VI, G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P. Grange (Eds.), Stud. Surf. Sci. Catal. 91 (1995) 59. A.T. Gutmann, R.K. Grasselli and J.F. Bradzil, US Patent 4,746,641 and 4,788,317 (1988), assigned to The Standard Oil Co. L.C. Glaeser, J.F. Bradzil, D.D. Suresh, D.A. Orndoff and R.K. Grasselli, US Patent 4,767,739 and 4,788,173 (1988), assigned to The Standard Oil Co. M.A. Toft, J.F. Bradzil and L.C. Glaeser, US Patent 4,784,979 (1988), assigned to The Standard Oil Co. S. Albonetti, G. Blanchard, P. Burattin, F. Cavani and F. Trifir6, France Patent 94-07982 1994, assigned to Rhone Poulenc Chimie. S. Albonetti, G. Blanchard, P. Burattin, S. Masetti and F. Trifir6, in 3rd World Congress on Oxidation Catalysis, R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Eds.), Stud. Surf. Sci. Catal. 110 (1997) 403. T. Birchell and A.W. Sleight, Inorg. Chem., 15 (1976) 868. F.J. Berry, M.E. Brett and W.R. Patterson, J. Chem. Soc. Dalton Trans., (1983) 9 and 13. F.J. Berry, M.E. Brett, R.A. Marbrow and W.R. Patterson, J. Chem. Soc. Dalton Trans., (1984) 985. S. Hansen, K.Stahl, R. Nilsson and A. Andersson, J. Solid State Chem., 102 (1993) 340. R. Niisson, T. Lindblad and A. Andersson, J. Catal., 148 (1994) 501. N.N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon, 1989, p. 1157.

91998ElsevierScienceB.V.All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

385

M e c h a n o c h e m i s t r y in p r e p a r a t i o n o f VPO c a t a l y s t s for p a r a f f i n s oxidation V A . ~ g a l o v 1 % J.Haber Ib, J.Stoch Ib, A.I.Kharlamov 2, I.V.Bacherikova Is and L.V.Bogutskaya la iUkrainian-Polish Laboratory of Catalysis: aInstitute of Physical Chemistry, National Academy of Sciences of Ukraine, Prospekt Nauky 31, Kyiv-22, 252022, Ukraine bInstitute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 1, Krakow PL-30239, Poland 2Institute for Materials Science Problems, National Academy of Sciences of Ukraine, vul. Krzhizhanovskogo 3, Kyiv, 252680, Ukraine It is shown t h a t mechauochemical t r e a t m e n t can be a progressive method to activate both VPO precursor and starting compounds for VPO or MeVPO catalysts synthesis to get a noticeable i m p r o v e m e n t in VPO catalytic properties.

1, INTRODUCTION

It is well known, t h a t there is a strong dependence b e t w e e n the activity and selectivity of any catalyst and the method, following which it can be obtained. Thus, for example, VPO catalyst of n - b u t a n e oxidation to maleic anhydride was found to be of different performance w h e n it was prepared by different techniques. The most efficient m e t h o d to produce VPO catalyst has proved to be t h a t realized in organic solution which b e c a m e the industrial one. However, at present the yield of maleic anhydride over the best VPO catalyst in industry is not so high (52-55 mol.~ a n d there is m u c h to be desired. That is why the study to improve the catalytic performance is still being advanced. Some recent publications are devoted to the development of the novel alternative methods to prepare V t ~ catalysts, s u c h as their flame synthesis or aerosol processing [1,2 ]. On the other hand, the catalyst efficiency can be improved by m e a n s of the metal additives introduction into its composition a n d by changing of the

386 surface morphology [3-6]. Typically the additive requires to be introduced by the liquid-phase reactions s u c h as coprecipitation, impregnation or grating. One of the methods being alternative to t h e traditional technique for the catalysts synthesis and their modification can be m e c h a n o c h e m i s t r y (MCA). There are few publications and p a t e n t s concerning ufili~iug of the MCA for the VPO catalysts properties improvement [7-12]. Thus, the t r e a t m e n t at not high energy load {up to 400 rpm) led to decrease of the catalyst particles size, t h a t is increase of the sample specific surface a r e a a n d corresponding growth in the catalytic activity. However, the a u t h o r s of t h e Europ. Pat. [8], gave a preference to dry milling of VOHPO4 0.5H20 precursor and, at t h e same time, the wet milllng process in t h e presence of dispersant w a s indicated as the best way to a t t a i n a good r e s u l t in [7]. Our results [13,14] have showed t h a t t h e m o s t perspective is ufili~iug of high-speed planetary mill to get VPO precursor a n d V~O5 properties modified. The present paper deals with a n effect of MCA conditions on the process of VPO precursor synthesis a n d t h e properties of t h e obtained from it catalyst. 2. ~ E R I M Z N T A L

V2Os (pure for analysis), VO2 (pure), H3PO4 (pure), Bi203 (pure), ZrO~. (pure) were u s e d in the work as initial compounds for synthesis. Preparation of VPO precursors was performed following well-known technique [15] in butanol m e d i u m in the presence of reducing agent or w i t h o u t it. The final solid, separated from mother's solution by filtration,was heated step by step up to 270 0C in v a c u u m (totaltime was 60 11). Mechanochemical activation was carried out by two ways: i) in a planetary mill at 3,000 rpm (HE}, ii)in a vibratory mill (LE~. In so doing, a dispersant nature (water, ethanol} and the treatment time were varied. Phase composition of the samples was analyzed using D R O N - 3 M X-ray diffractometer with C u K~ irradiation. The specific surface area {SSA} of the solids was measured by B E T method on Oasochrom-1 device. Thermal analysis was carried out with thermoanalytical instrument Derivatograph Q-1500 D in helium atmosphere at a heating rate of 10 K / m in. Crystal optical study was performed on micriscope MIN-8 in transmitted light. The surface composition was examined using V G ESCA-3 X-ray photoelectron spectrometer {AI K~1,2 ). The obtained spectra were calibrated against C Is line (284.8 eV) as a standard in the binding energy determination. Catalytic properties of the synthesized samples were examined in t h e reactions of the n - b u t a n e oxidation (1.65 vol.% CcHlo in air). Catalytic tests were performed in the flow system w i t h the C ~ control of the reaction products. The precursor activation w a s done in the reactor directly before catalytic testing. In so doing, the reaction mixture w a s u s e d at step by step rise of the t e m p e r a t u r e up to 440 oC for 72 h.

387 3 . R F . ~ U L T 8 AND D I S C U S S I O N 3 , 1 , S y n t h e s i s o f VPO p r e c u r s o r u s i n g m e c h a n o c h e m i c a l l y and V02,

treated V20s

Previously it h a s b e e n s h o w e n by u s [14] t h a t MCA of V2Os in e t h a n o l m e d i u m allows to modify significantly its morphology a n d v a n a d i u m valency at t h e surface. We a t t e m p t e d to p r e p a r e VPO p r e c u r s o r starting from s u c h t r e a t e d oxide. In t h e p r e s e n t work it is considered in details an influence of b o t h v a n a d i u m oxide r e d u c t i o n degree a n d its morphology on VPO p r e c u r s o r synthesis r u n a n d t h e properties of t h e obtained catalyst. Table 1 lists t h e m a i n characteristics of v a n a d i u m (VJ oxide u n d e r g o n e to MCA in different conditions. Not paying m u c h a t t e n t i o n to r e a s o n s c a u s e d the o b s e r v e d c h a n g e s , it s h o u l d be pointed o u t t h a t d u r i n g the t r e a t m e n t in e t h a n o l it is initiated plastic anisotropic deformation of t h e crystals along (010) plane. Dry m{ll{ng leads to crystals chaotic d e s t r u c t i o n with their size decrease by m o r e t h a n order. The short-time t r e a t m e n t in the presence of w a t e r a t a h i g h energy load (HE) also causes chaotic destruction of t h e c r y s t ~ s . Power supply r a t e is higher t h a n its spreading and, as a result, t h e freshly formed defects h a v i n g s u r p l u s energy catalyzes w a t e r decomposition a c c o m p a n i e d w i t h t h e oxide p h a s e reduction. At a low energy (LE) t r e a t m e n t in w a t e r anisotropic deformation of t h e oxide grains t a k e s place. Table 1 Properties of t h e v a n a d i u m oxides after their m echanochemical activation ,

Sample

,

,,

i

,,,

,

Treatment 88/% conditions M e d i u m Timel m 2] g re.in .

V~Os-I V2Os-A 1 V2Os-A2 V2Os-EI V2Os-E2 V2Os-W1 V2Os-W2 V2Os-W3 V205-W4 VO2-II

Air 12 Air 30 E t h a n o l I0 E t h a n o l 30 Water 10 Water 30 Water 60" Water 120"

VO2-AA Air

30

.

.

.

.

.

.

.

XRD XPS

.

.

V 4+ content, % . . . . I!olo] O Is V 2p(1) V 2p(2) Surface Bulk Iillq]

.

3.8 13.8 19.2 5.6 8.8 5.3 1.6

.

.

.

.

.

.

.

1.33 531.0 517.8 0.85 531.1 516.3 517.6 0.80 531.1 516.2 517.7

8.6

6.9 2.7 14.2

.

2.15 4.33 0.78

531.2 530.6 530.9

**

530.5 516.2 517.6

3.30 4.33 ** **

531.2 531.1 529.6 529.5

.

.

.

.

.

.

.

.

.

.

515.5 515.1 515.7 516.5 516.4 516.0 516.0 .

.

.

.

517.8 517.5 517.8 517.8 517.6 517.5 ,

.

.

0 8 7

0 I0 I0

6 27 12

5 20 15

32

74

8 9 100 90

II 12 100 98

,

* LE t r e a t m e n t ; for other samples HE MCA ** There were only VO2 p h a s e reflections observed Finally, t h e s a m p l e s h a v i n g a b o u t t h e same V 4+ ions concentration on t h e surface a n d in t h e b u l k w e r e formed (/%1, A2, El, W3, W4 - Table 1). The

388 relative content of (010) plane in these samples is different a n d changes from 0.8 to 4.33. On the other hand, the samples with the s a m e morphological factor (E2 and W4) and rather different V4§ ions content were also obtained. Using the traditional technique for VPO precursor (VOHPO4 0.5 H~O phase) synthesis, in 6-8 hours after the reagents mixing a n d keeping the mixure u n d e r stiring and reflux at 105-107 0C it is observed the colour change to t h a t attributed to VPO compound. For the synthesis completion the conditions should be maintained for another 8-10 h o u r s (total time is about 16 11). Properties of the samples prepared in such w a y using u n t r e a t e d and mechanochemically activated V~Os are presented in Table 2. Table 2 Properties of VPO precursors synthesized by traditional m e t h o d Sample

Initial vanadium oxide ,

VPO-I Vt~-A1 VPO-A2 VPO-E1 VPO-E2 VPO-W1 VPO-W2 VPO-W3 VPO-W4 VPO-II VPO-AA

V205-I V205-A1 V205-A2 V205-E1 V205-E2 V205-W1 V2OS-W2 V205-W3 V205-W4 YO2-II VO2-AA

SSA m2/g

XRD XIXJ Io.57o . . . Io.~ O Is V 2p

,

20.2 19.5 20.0 28.5 4.6 14.3 9.6 27.0 25.3 9.5 10.2

,

0.75 I. 11 1.17 1.72 0.73 1.02 0.71 1.52 1.60 0.42 0.54

,,,

,,

532.3 532.2 532.2 532.1 532.4 532.3 532.4 532.2 532. I 532.4 532.3

.

DTA . . Tenao,~ P 2p (P/V)s

,

,,

517.5 517.4 517.4 517.4 517.5 517.4 517.5 517.3 517.3 517.5 517.4

133.5 133.6 133.9 133.7 133.7 133.5 133.5 133.8 133.8 133.6 133.6

2.08 2.00 2.07 2.05 3.30 1.92 2.82 2.10 2.17 2.90 2.75

,,

,,

448 435 433 425 458 440 450 430 425 465 452

It has been observed t h a t in the case of V2Os-E2, -W2 and VO2-II, -AA v a n a d i u m oxides use, on the contrary to others, the colour change showing VPO precursor formation happened not after 5-8 hours of the reaction b u t during the first hour. Thus, taking into account the Table 1 data, one can conclude t h a t the presence of sig~fl~cant a m o u n t of the reduced v a n a d i u m ions in a starting material for the synthesis favours quick VPO precursor formation. After XRD data the obtained VPO samples have a relatively low concentration of (001) crystallographic plane (d = 0.570 nm). Besides, the solids have low specific surface area. As for the VPO s~mples synthesized from mechanochemically t r e a t e d V2Os having low concentration of V 4§ ions, it seems to be a definite correlation between (010) plane content in V~O5 and (001) plane in the p r e p a r e d precursor (Figure 1). The samples have high specific surface area (Table 2). These results are in an agreement with those published in [ 16]. We have also synthesized VPO precursor by the s t a n d a r d method u s i n g mechanical mixtures (50]50) of V2Os El+A1 and W2+A1 as initial reagents.

389

I(oo~) I(2:~)

I Tendo, ~ I * 460

9j .

1.5 440 t

*

420

1.0 l 1

! 2

I 3

1

.... l..... 4

I

0.5

Ratio I{o:tol]I0.:tol -')' Figure 1. Relation between the relative content of (010) surface plane in the initial v a n a d i u m oxide and (001) plane in VPO precursor

t

!_

1.0 1.5 Ratio lloo:]/I{22o1 -9'

Figure 2. An influence of the relative content of (001) surface plane in VPO precursor on the t e m p e r a t u r e of its phase transformation into (VO)2P207

Some properties of the p r e p a r e d solids are listed in Table 3. It is seen from the data in, mixing of the v a n a d i u m oxide having high content of (010) plane with another one of r a t h e r lower its content leads to slightly lower content of (001) plane in the obtained VPO precursor (samples VPO-MM1 and VPO-E1). At the same time, dilution of V2Os having high concentration of V4+ with less reduced one provokes a n increase of (001) in VOHPO4 0.5H~O (samples VPOMM2 and VPO-W2). Thus, one can conclude t h a t the experimental results show an important role playing by s t r u c t u r e and reduction degree of the starting v a n a d i u m oxide for morphology of the formed VPO precursor. Table 3 Properties of the VPO precursors synthesized from v a n a d i u m oxides mechanical m ~ q l r es i

Sample

VPO-MMI VPO-MM2

Taken v a n a d i u m oxides

El +A1 W2 + A1

,

,

i

SSA m2] g

XRD Io.s~o Io.293

XI~ O Is

V 2p

P2p

27.2 1 I. 1

1.68 0.87

532.2 532.1

517.4 517.3

133.8 2.12 133.6 2.45

(P/V).

It should be noted, t h a t in [17] VPO precursor with preferable development of (220) plane (d = 0.293 nm) h a s been also prepared starting from the r e d u c e d v a n a d i u m oxide (50 % V4§ Therefore, it m u s t be general rule t h a t high rate of VPO precursor Ibrm~ng in the presence of significant a m o u n t of V4§ in the v a n a d i u m oxide t a k e n for the synthesis and some reducing agent in butanol m e d i u m does not favour obtoi,ing of VOHPO4 0.5

390

H20 with high exposure of (001) plane. On the other h a n d , low concentration of V 4§ ions in the oxide gives a positive effect in growth of (001) vanadyl plane in the p r e c u r s o r (see samples VPO-E ], VPO-A 1, VPO-A2). In [ 17] m u c h a t t e n t i o n w a s paid to the precursor crystallites of large size yielded from t h e r e d u c e d v a n a d i u m oxide in the presence of reducing agent. Obviously, a t t h e high r a t e of VPO forming it a c c u m u l a t e s on the oxide surface in t h e form of n u m e r o u s similar particles ( s t r u c t u r e of white-yolk type) which c a n be c o n d e n s e d possibly involving phosphoric acid molecules. Agglomeration of t h e particles does n o t allow to r e a c h uniform composition in the large grains b u l k and, finally, pellets consist of v a n a d i u m oxide inside and have s u r p l u s a m o u n t of p h o s p h o r u s on the surface (Table 2}. The crystal optical study of t h e s a m p l e s confirmed the presence of white-yolk s t r u c t u r e with internal p a r t h a v i n g refraction coefficients being equel to np = 2A6-2.9d and nQ - 2.55-2.50 a n d corresponding to those of v a n a d i u m oxide. To decrease t h e VPO p r e c u r s o r forming r a t e in [17] it w a s proposed to carry out t h e synthesis w i t h o u t any reducing agent (benzyl alcohol) at a dropwise addition of H3PO4. We have also applied the modified technique for VPO synthesis w i t h o u t the reducing agent in the reaction mixture (total duration of t h e synthesis w a s t h e s a m e 16 h). The properties of the obtained samples are listed in Table 4. It is seen, t h a t s u c h modification of the p r e p a r a t i o n m e t h o d positively influences on VPO morphology and the relative intensity of (001) p l a n e reflection rises (see Tables 2 and 4). The samples specific surface a r e a is also grown. Table 4 Properties of t h e VPO p r e c u r s o r s synthesized by modified m e t h o d (without reducing agent) ,,

,

,,

Sample

,,,,,

Initial

vanadium

,

,,,

VPO-AIM VPO-EIM VPO-E2M VPO-WIM VPO-W2M VPO-IIM VPO-AAM .

.

,

V2OS-AI V205-EI V205-E2 V2OS-WI V2OS-W2 VO2-11 VO2-AA .

.

.

.

.

.

,

XRD

m2/g

oxide

.

,,

SSA

21.4 30.2 14.1 24.6 17.5 12.8 18.7 .

,

,,,,

,,

,,

,

XPS

_Io.s'zo

.

Io.~.r3

.

.

.

.

O Is

.

.

V 2p

P 2p

, .

* 1.92 1.32 1.47 0.96 0.82 1.02

,.

532.1 532.1 532.3 532.3 532.2 532.1 532.1 ,

.

.

,,

DTA

.

,

517.6 517.4 517.5 517.4 517.3 517.4 517.3 .

.

.

(P/V)~ .,

133.7 133.6 133.7 133.6 133.6 133.7 133.8 .

Te.do,~

2.10 2.04 2.05 1.92 2.24 2.32 2.18

420 438 430 443 445 445

.

*X-ray a m o r p h o u s sample As for X-ray a m o r p h o u s sample VPO-A1M, it h a s b e e n established t h a t continuation of t h e synthesis for a n o t h e r 3 h leads to a n a p p e a r a n c e of s h a r p reflections a t t r i b u t e d to VOHPO4 0.5 H20 phase. The intensity of all the reflections i n c r e a s e s w i t h the reaction time a n d the m o s t intense one is 0.293 n m at a n y point of t h e synthesis. Comparison of t h e d a t a for VPO-A1M

391 and VPO-E 1M samples prepared from V2Os of the s a m e reduction degree one more t~me shows a decisive role of (010) plane of the oxide for the VPO precursor morphology [content of (0O 1) plane]. In its t u r n , as it can be seen from Figure 2, relative content of (00 I) plane in VPO precursor greatly influenses on the t e m p e r a t u r e of its p h a s e transformation into (VO)2P~07 phase. Along the increase of the plane exposure the t e m p e r a t u r e of such phase transformation becomes lower. The properties of the prepared VPO samples in catalytic oxidation of nb u t a n e are shown in Figure 3. It is seen t h a t the preliminary MCA of v a n a d i u m oxide as an initial reagent for VPO precursor preparation mostly improves the catalytic performance.

X ~

%

==

I-

70 60

m

B

=

5O w

_ _

_

E IE2 WIW2 W3 W4 II AA A-1M E IM E:iM WIM W2M IIM-AAM

I A1A~

S, %

b

.

.

.

.

I AIA2 EIE2 W I W 2 w a w 4

.

.

.

.

.

.

II A A A I M E l M E 2 M W I M W 2 M

.

IIMAAM

Figure 3. Activity (a) and selectivity (b) of VPO samples synthesized from MCA V~Os (abbreviations are the same as in Table 2 a n d 4).

A decreased paraffin conversion is observed j u s t due to the sample surface a r e a less t h a n t h a t for the sample prepared from u n t r e a t e d v a n a d i u m oxide (VPO-I). It is also illustrated in Figure 4, although clearly defined correlation between these p a r a m e t e r s cannot be found. The selectivity of maleic anhydride formation for the majority of the synthesized

392 samples is higher t h a n t h a t of traditionally p r e p a r e d VPO-I catalyst. It is noteworthy, t h a t there is clear correlation b e t w e e n the achieved selectivity a n d the relative content of (001) in VPO precursor (Figure 5).

X, %

l

80

S

7~ / 70

60 I_.

,/ 60

- ~/~i

i , 10 20 SSA, m21 g

,,!,

"S

~//*

50=

30 -)

Figure 4. Dependence between file n - b u t a n e conversion a n d the surface specific a r e a of VPO catalysts

0.5

! ! 1.0 1.5 Ratio lioo:i/1122oi-)

Figure 5. D e p e n d e n c e b e t w e e n the selectivity to maleic anhydride a n d t h e relative content (001) p l a n e in p r e c u r s o r s

3,2. M e c h a n o c h e m i c a l activation of the promoter and s y n t h e s i s of

M e V P O precursor by m e c h a n o c h e m i s t r y

The p r e s e n t investigation deeds with the results obtained a t the u s e of Bi203 and ZrO2 as promoting additives in atomic ratio B~Zr)JV = 0.2. The oxides were u s e d both u n t r e a t e d a n d after preffminary dry milllng in t h e p l a n e t a r y mill. It is seen from the Table 5, m e c h a n o c h e m i c a l synthesis of t h e promoted MeVPO catalysts starting from the m~xture of VPO p r e c u r s o r a n d the a d d i t ~ e oxide powders allows to grow a n efficiency of t h e VPO sample in n - b u t a n e oxidation. Due to a d d i t ~ e hltroduction it is h a p p e n e d a n i n c r e a s e of the r e l a t ~ e content of (001) plane in VPO precursor. It should be also noted, t h a t at the short time t r e a t m e n t the additive c o n c e n t r a t i o n on t h e surface is lower t h a n t h a t in the bulk. Continuation of t h e t r e a t m e n t leads to ~-lcrease the surface ratio MeJV which can be explaiued by different plastic properties of the precursor p h a s e a n d b i s m u t h or zirconium oxide. It could be hypothesized t h a t more plastic VOHPO4 0.5 H20 p h a s e being experienced anisotropic deformation spreads over the surface of h a r d e r oxide ~ v i n g MeVPO surface compound. Then this newly formed p h a s e slides down from the surface to clean it for another spreading process. Preliminary MCA of the oxide before its mixing w i t h VPO p r e c u r s o r favours an additional improvement of catalytic performance. In this case t h e surface ratio MeJV is found to be higher t h a n t h a t for t h e p r e p a r a t i o n n o t involving the p r e t r e a t e d oxide. This fact is in a n a g r e e m e n t w i t h the s t a t e d above hypothesis and is explained by decrease of t h e oxide particles dimensions (that is increase of their number).

393 Table 5 Properties of mechanochemically promoted MeVPO precursors ,,,,,

,,,

Sample*

,, ,,,,,,,

VPO VPO/Bi2Cb VPO/Bi2Cb VPO/Bi~CYjM VPO/ZrO2 VPO/ZrO~M .

.

.

.

.

,

,,

L

,

,

,

,

,

,

Treatment SSA XRD XPS n-C4Hm oxidation tim e, min m ~-/g Io.sTo/Io.~,~ ................. P/V Me/V GHSV, h -z X,% S,%

.

I0 30 30 30 30

.

6.0 9.2 12.1 12.9 13.4 14.3 ,,

~

.

.

.

0.74 I. 16 1.32 1.38 1.47 1.51

1.43 1.98 1.76 1.82 1.88 1.75

.

,,

.

.

.

.

.

.

.

.

.

.

0.08 0.15 0.21 0.13 0.18 .

.

2400 3000 3000 3000 2400 3000

.

48 57 63 68 62 61

62 66 70 73 70 74

,

*M m e a n s preliminary MCA of the oxide before mechanochemical t r e a t m e n t of the mixture VPO + MexOz

3.3. Mechancchemical

activation

of VPO precursor

Table 6 contains experimental results on comparable t r e a t m e n t of the precursor at a high (HE) and low (LE) energy load. Low energy activation leads to increase of the specific surface a r e a and corresponding decrease of the particles size without any remarkable change of morphological factor. In accordance with this, it is observed the increased activity of the samples in n - b u t a n e oxidation while the selectivity remains almost unchanged. The MCA in the planetary ml]1 (HE) besides the sample surface increase provokes remarkable change of (001) plane exposure in VPO precursor. As a consiquence, in parallel with its catalytic activity increase there is also a rise in the selectivity towards maleic anhydride. Some combined t r e a t m e n t allows to get all the more efficient catalyst for n - b u t a n e selective oxidation. Table 6 Properties of the mechanically activated VPO precursor Sample

Treatment time, h

VPO VPO/LE VPOI LE VPO/HE VPO/HE VPO/HE+LE .

.

.

.

.

.

.

.

.

,

.

.

SSA m2/g

6.0 12.8 22.6 8.8 14.2 18.9

-

2 4 0.16 0.33 0.88+2 .

.

.

.

.

.

.

XRD Io.s7o/Io.~y3

XPS PIV

0.74 0.78 0.55 0.95 1.28 1.25

1.43 1.48 1.52 1.62 1.80 1.82 ,,

,

n - B u t a n e oxidation X, %

J,

S, %

48 58 57 52 57 65

62 61 60 66 73 72 ,,

394 Thus, the presented results show that the mechanochemical activation in all used its variants : the precursor modification, additives introduction into it and the pretreatment of the starting materisls for the catalyst synthesis, is a promissing method to improve the VPO catalytic properties. RF~E, RZI~CF~

1. P.M.Michalakos, H.E.Bellis, P.Brusky, H.H.Kung, H.Q.Li, W.R.Moser, W.Partenhelmer and L.O.Satek, Ind. Eng. Chem. Res., 34 (1995) 1994. 2. P.F.Miquel, E.Bordes and J.L.Katz, O. Solid State Chem., 124 (1996) 95. 3. G.Centi (Editor), Vanadyl Pyrophosphate Catalysts, Catal. Today, 16 (1993) I- 147. 4. G.J.Hutchings, Appl. Catal., 72 ( 1991) I. 5. F.Cavani and F.Trifiro, Preparation of Catalysts VI, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 1. 6. E.Kesteman, M.Merzouki, B.Taouk, E.Bordes and R.Contractor, Preparation of Catalysts VI, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 707. 7. R.Higgins and G.J.Hutchings, Production of maleic acid and anhydride, US Patent No.4 317 777 (1982). 8. C.B.Hanson and C.R.Harrison, Production of catalysts, Euxop. Patent No. 0 098 065 (1984). 9. G.D.Suciu, G.Stefani and C.Pumagali, Improvements in or relating to catalysts containing mixed oxides of vanadium, phosphorus, and atuminium and/or boron, UK Patent No. 2 145 009A (1984). 10. H.S.Horowitz, C.M.Blackstone, A.W.Sleight and G.Tenfer, Appl. Catal. 38 (1988) 193. I I. K.Sl~ima, M.Ito, M.Murayama and M.Hatano, Science and Technology in Catalysis, 1994, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 92 (1995) 355. 12. G.J.Hutchings and R.Higgins, Appl. Catal. A, 154 (1997) 103. 13. V J I . Z ~ g a l o v , J.Haber, J.Stoch, L.V.Bogutskaya and I.V.Bacherikova, 11th Int. Congr. Catal., Stud. Surf. Sci. Cata]., Elsevier, Amsterdam, I01 (1996) 1039. 14. V . A . ~ g a l o v , J.Haber, J.Stoch, A.I.Kharlamov, I.V.Bacherikova and L.V.Bogutskaya, 3 rd World Congr. Oxid. Catal., Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 1 I0 ( 1997] 337. 15. V.A.Zazhigalov, J.Haber, J.Stoch, A.I.Pyatnitskaya, GJI.Komashko and V.M.Belousov, Appl. Catal. A, 96 (1993) 135. 16. N.Guilhaume, M.Roullet, G.Pajonk, B.Grzybowska and J.C.Volta, New Develop. Sel. Oxid., Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 72 (1992) 255. 17. T.Miyake and T.Doi, Appl. Catal. A, 131 (1995) 43.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

395

Effect of the preparation procedure and parameters on the physico-chemical properties of Higher Alcohol Synthesis ZnCrO catalysts P.L.

Villa I

and L. Lietti

Politecnico di Milano, Dipartimento di Chimica Industriale ed Ing. Chimica "G. Natta", P.zza L. da Vinci 32, 20133 Milano (Italy)

The effects of the preparation procedure and parameters on the physico-chemical properties of ZnCrO catalysts have been addressed. For this purpose, samples with a 1/1 theoretical Zn/Cr atomic ratio have been prepared by means of the co-precipitation techniques and the influence of several parameters controlling the coprecipitation, aging, washing and calcination steps on the characteristics of the final samples have been investigated. It has been found that catalysts with Zn/Cr atomic ratio close to the theoretical 1/1 ratio constituted by a ZnCrO micro-crystalline spinel phase and high surface area could be synthesized by co-precipitation at pH near 6. These catalysts are active in the synthesis of higher alcohols from CO + H2.

1. INTRODUCTION The possible use of higher alcohol mixtures as high octane blending stocks for gasoline and as possible precursors of octane enhancers ethers (e.g. Methyl Tert-Butyl Ether, MTBE) has motivated in recent years the reinvestigation of the direct synthesis of higher alcohols (HAS) from CO and H 2 [1]. In this respect, modified high temperature ZnCrO methanol synthesis catalysts appear promising among the presently available HAS catalysts because of their peculiar high productivity to branched alcohols, and specifically isobutanol (2-methyl-1propanol), that is a potential precursor of MTBE [2]. These catalysts are constituted by a ZnCr204 spinel phase dispersing the excess of ZnO, that has been suggested to be responsible for the catalytic activity [3,4]. The characteristics of the final catalyst strongly affect the catalytic performance, and specifically the selectivity of the reaction and the rate of deactivation with time on stream. In particular, during the HAS it is of primary importance to maximize the overall oxygenates C2+ yield while minimizing the selectivity of methane, hydrocarbons and dimethyl ether the formation of which decreases the process efficiency and may lead to uncontrolled temperature increases in the reactor. However, some irreproducibility of the performances of the catalysts

i Present address: Dipartimento di Chimica, Ingegneria Chimica e materiali; Universit/l di L'Aquila, 67040 Monteluco di Roio, L'Aquila

396 was observed, in particular for what concems the selectivity to methane [5]. This suggests a detailed analysis of the preparation procedure and parameters and of their impact on the catalyst performances. As a matter of facts, it has been reported that the presence of free oxides of Cr catalyze the formation of methane and higher hydrocarbons [6], and that high values of the surface area can guarantee high overall productivity to oxygenates [7]. ZnCr oxide catalysts are prepared in radically different ways. Commercial catalysts were prepared in the past by kneading a paste made of finely ground ZnO powder, CrO3 and water, then drying and calcining: the catalyst powder grains finally consist of a ZnO core and an outside shell of ZnCr204 spinel [8]. Catalysts with a more uniform local composition are usually obtained by co-precipitation of Zn-Cr hydroxicarbonates such as hydrotalcites with Na, K or NH4 carbonates at about 80~ [9, 10]. Hydrotalcite are anionic clays which may allocate bivalent and trivalent metallic cations between the brucite layers thus achieving an excellent interspersion of the cations which is preserved after the calcination step necessary to obtain the final oxides. Considering that the preparation procedure should also be suitable to be scaled-up to a commercial manufacture of the catalyst, we planned to study the effects of the preparation parameters by decomposing it in typical unit operation [ 11]. The most critical steps were found to be the coprecipitation, aging, washing and calcination, which are here reported. Catalysts with a Zn/Cr atomic ratio of 1 were reported to display a low rate of deactivation due to a slower segregation and crystallization of the ZnO phase from the Zn-Cr defective spinel phase [3]. Therefore the catalysts here reported were prepared by co-precipitation starting from solution with Zn/Cr = 1.

2. EXPERIMENTAL

2.1. Catalyst preparation ZnCr oxide catalysts were obtained by means of the co-precipitation techniques starting from a Zn and Cr nitrate solution the amount of which was suitable to obtain theoretically 50 g of oxides. In a typical preparation procedure, a solution of Zn and Cr nitrate (Zn + Cr = 3 M) was simultaneously dropped along with a (NH4)2CO3 solution (2.5 M) in a beaker containing 0.5 1 of a solution of (NH4)2CO3 (1.5 M) whose pH was kept constant during co-precipitation using a Metrohm 632 pH-meter and an Ingold 405-EQ-$7/120 electrode. (NH4)2CO3 was used as precipitating agent because Na and/or K carbonates may leave uncontrolled amounts of Na and/or K cations which may affect in an irreproducible way the performance of the catalyst. Indeed it is known that alkali are necessary to increase the selectivity towards higher alcohols: therefore, to obtain a better reproducibility, we preferred to add dosed amounts of K by impregnation of the calcined oxides with a CH3COOK solution. The temperature was set to 78~ and the coprecipitation was carried out under vigorous stirring by using a Heidolph Diax 600 stirrer. The coprecipitation lasted within about 45', then the slurry was aged for 6 h at 78~ under stirring at constant pH, by addition of (NH4)2CO3 (2.5 M). The precipitate was then filtered under pressure, washed 4 times by re-slurring for 45' the cake in 800 ml of distilled water at 80~ dried in a oven at 110 ~ for 24 h and calcined at 400 ~ for 4 h in air.

397

2.2. Catalyst characterization The catalyst precursors and the final calcined samples were characterized by means of IR spectroscopy, surface area measurements, X R and chemical analysis. Surface Area were measured by N2 adsorption at 77 K with the BET method using a Carlo Erba Sorptomatic 1900 Series instrument. Powder X -ray diffraction analyses were performed with a Philips vertical goniometer PW 1050/70 and Ni-filtered CuKct radiation. The FTIR spectra of all samples (KBr pressed disk method) were recorded by a Perkin-Elmer 147 FTIR spectrometer. 2.3. Catalytic activity tests Catalytic activity runs in the synthesis of higher alcohols from CO + H2 were performed in a copper lined fixed bed tubular reactor (internal diameter 6 ram) operating at 8 MPa, with a 1/1 CO/H2 mixture and in the temperature range 633-723 K. 1.5 g of catalyst (120-200 mesh) were used in each run, diluted three times its volume with glass beads. Analysis of the reaction products was performed by condensation of the reactor effluent in a dry-ice cooled trap, followed by capillary GC analysis of the collected liquid samples. Known amounts of n-propyl acetate have been added to the liquid fraction as internal standard. Non-condensable species were analyzed by on-line GC. Identification of the various reaction products was performed by comparison of their retention times with those of known standards. The carbon balances always closed within + 10 %.

3. RESULTS AND DISCUSSION

3.1. Influence of the precipitation pH on the catalysts structural and morphological properties The influence of the value of the pH of the coprecipitation step on the characteristics of the final catalysts was investigated in the range 4 < pH < 9. To operate at pH close to 9, a value which is higher than that of the (NH4)2CO3 solution itself, gaseous ammonia was also bubbled inside the (NH4)2CO3 solution. Preliminary experiments have been performed with the aim of ascertaining the pH precipitation range of Cr and Zn. Accordingly, a solution of K2CO3 was dropped in a solution of Zn and Cr nitrate (1 M) at 78~ while monitoring the pH (Figure 1). It appears that Cr precipitates already at pH lower than 3.5, while the precipitation of Zn is completed at about pH = 5.5. The most relevant catalyst characteristics obtained by varying the precipitation pH in the range 4-9 and the after calcination at 400 ~ are reported in Figures 2 and 3 and Table 1. The XRD analysis performed over the samples obtained after coprecipitation, aging and drying (here not reported) showed features typical of hydrotalcite-like anionic clays. On the other hand, the samples prepared at pH < 8.9 and calcined at 400 ~ in air (Figure 2) are monophasic, showing only the presence of a ZnCrO micro-crystalline spinel phase with crystal size in the range 40-60 A (Scherrer method). The spinel phase shows a certain degree of inversion, as calculated by profile fitting analysis, particularly in the samples obtained at pH 6 and 7. The segregation of ZnO is completely absent at 400 ~ after calcination at 500 ~ ZnO

398 14-

is present only in trace amounts. In the case of the 12 sample prepared at the highest investigated pH 10 (8.9), the presence of well 8 crystallized (~-Cr203 has been observed, along with the micro-crystalline ZnCrO spinel. These data 4imply that coprecipitation should be performed at 2 reasonably low pH values 4:.0 in order to avoid the o.o ols 1.o 1.5 2.0 a.o 3'.5 presence of free a-Cr203 which may lead to equivalent C 0 3 = / e q u i v a l e n t Z n §247 + equiv. Cr ~ methane formation during the synthesis [6] and to Figure 1 - Neutralization of a Zn ++Cr3+ solution (1 M) with increase the amount of Zn K2C03 at 78 ~ pH vs. K2CO3 equivalents. in octahedral coordination, which was claimed to be A the active species in A,~I A the alcohol formation A i~ A A 1 tA [3]. p H 8.90 Figure 3 and Table 1 report the pH 8 variation of the Zn/Cr atomic ratio, the precipitation yield pH 7.72 (weight of calcined catalyst / theoretical weight) and the pH 7 ,,,w- ~ , , l w w r r ~ -.- ..,. surface areas estimated on the pH 6 samples calcined at 400 ~ obtained by precipitation at pH 5 different pH values. It appears that the pH 4 precipitation pH has a significant influence on these 1 ' I I I 1 I '1 i " I I i i i characteristics: in 10 20 30 40 50 60 70 [020] particular a maximum in the Zn/Cr atomic Figure 2 - Effect of the precipitation pH on the XRD spectra of the ratio and in the samples calcined at 400 ~ ( * = spinel phase; A = ct-Cr203) =

"

I

9

I

9

.....

I

"

9

I

9

399 ,.

.,

,

....

,

.

,

.

,

100

precipitation yield is observed if the co1.0 precipitation is carried out 90 near an optimum pH value of 6. Indeed in this 0.8 case high precipitation 0 80 ", yields can be attained (> O 90 % of the theoretical 0 0.6 I11 value) and the prepared 70 ,-,E samples after calcination 0 O o in air at 400 ~ show 0.4 satisfactory high surface 0 60 c~ t= area values. On the other N hand, if the precipitation step is carried out at 0.2 50 higher pH values (pH >_ 8), the yield of the precipitation is not 40 0.0 quantitative (lower than 50 % of the theoretical value, primarily due to Zn loss) and the presence of Figure 3 - Effect of the precipitation pH on the precipitation crystalline c~-Cr203 is yield and Zn/Cr atomic ratio (samples calcined at 400 ~ observed (Figure 2), as already mentioned. Furthermore, high values of pH of co-precipitation requires relevant amounts of (Nt-h)2C03 solution. On the other hand, Figure 3 clearly shows that by performing the precipitation step at low pH values (e.g. pH = 4 or 5), lower precipitation yields and Zrger atomic ratios have been measured if compared with the samples obtained by precipitation Table 1 - Surface areas of the various samples at pH = 6, in line with the results precipitated at different pH and calcined at 400 ~ reported in Figure 1 showing that Zn precipitation occurs above pH values of 5. Accordingly, it is Precipitation pH Surface area (m 2/g) concluded that the precipitation 4 80 5 65 step should be performed at or 6 82 near pH = 6 in order to obtain 7 120 micro-crystalline ZnCrO catalysts 7.8 105 with high yields, high surface 8 145 areas and Zn/Cr atomic ratios 8.9 100 close to that of the starting solution.

//

. =.,.=

""c

,,m,

L_

n , , ,

~

,

L_

/

"|

,

"

i

.

pH

3.2. Effects of the ageing of the precipitate inside the mother liquors Once the eopreeipitation step was completed, the slurry was aged at 78 ~ at constant pH

400 and samples of the slurry were withdrawn at different times and filtered to analyze to content of Zn and Cr in the solution. It appeared that the Zn and Cr concentration in the mother liquors decreased with time, and reaches an asymptotic value after 6 hours of ageing. Accordingly a minimum aging time of 6 hours should be employed in order to give rise to a more complete precipitation. Notably, the sample prepared at pH = 8.9, after the normal ageing procedure, was divided into 2 portions, A and B: portion A was processed in the standard way, while portion B was further aged overnight in contact with the mother liquor at room temperature with no addition of gaseous ammonia. While the oxide obtained from portion A after calcination at 400 ~ shows the presence of ot-Cr203, as already reported in Figure 2, the corresponding sample obtained from portion B shows only the presence of the spinel phase with no (x-Cr203. Therefore i) the formation of ct-Cr203 should be ascribed to the high pH values of the coprecipitation step; ii) the repeated washing with hot distilled water are not sufficient to eliminate the presence of (x-Cr203 in the final catalysts; and iii) a prolonged ageing at room temperature leads to the disappearance of ot-Cr203, probably due to the lowering of the pH overnight. -~ 2.0 " o E co

=

A 1.5

__.,__ NH4+

L--

NO

e" 0

= o

1.0

z

0.5

r

Z

0.0 ~

--o

, ~ ~ , ~ - ~ ~ 7

,, ~,

3

B

v

t~

2

o

e--

c ID c o

?

Z

0

i

After aging

1

1~ washing

9

i

2~ washing

i

3~ washing

,,t.

4~ washing

Figure 4 - Concemrations of NO3" and NH4+ ions in the liquid phase (A) and in the solid 03) after aging and after the various washings.

3.3. Effects of the washing procedure

The effect of the washing procedure has also been investigated. This step is relevant in order to eliminate NO3" and NH4+ ions that may lead to local over-heating of the catalyst due to their exothermic decomposition upon calcination in air. The concentration of NO3" and NH4 + ions was analyzed both in the solution and in the solid alter the aging step and after each washing step up to the fourth. Zn and Cr content was also determined in the solutions. Figure 4 shows the concentrations of the NO3" and NH4§ ions in the solid and in the liquid phase after the various steps. From Figure 4 it appears that the concentrations of both NO3"

401 and NH4+ ions in the solution is negligible after the third washing step, while the N content in the precipitate (and specifically the NH4+ ion content) remains significant even after the last washing step. However, no further washing steps were performed due to the low content of Nt-h + ions in the solution. The analysis of Zn and Cr shows negligible losses of these cations during the washing procedure and therefore the losses of Zn and Cr in the final catalysts should be almost exclusively ascribed to their presence ion the mother liquors due to an incomplete coprecipitation.

3.4. Catalytic performances of the ZnCrO catalyst in the HAS The ZnCrO sample obtained by precipitation at pH = 6 was tested in the HAS synthesis at 400~ 8.5 MPa and with CO/H~ = 1/1, after being promoted with 3% w/w of I<20 by dry impregnation starting from a CH3COOK solution. After impregnation, no modifications in the catalyst chemico-physical characteristics have been pointed out on the sample re-calcined at 400 ~ The results of the catalytic activity data of this catalyst are reported in Figure 5 in terms of 100 productivity of the various reaction 90 products. As shown 8O I Z r # K ) + K, pH = 6 in the Figure, the 57O [] ~ c i a l ZnCrO + K catalyst sample is active in the .>_ 50 synthesis of o =40 methanol and higher o "-30 IX. alcohols from 2O CO/H2, with 10 methanol and isobutanol as the I -i" -I-I-r -r ,~ 0 0 most abundant 0 0 0 0 0 0 "I" -I"I" oxygenate species. x "' ~, m m ~ ~ + ""9 E E E 0 After 50 h of time on stream the Figure 5 - Results of catalytic activity runs in the synthesis of alcohols catalyst was from syngas over K-doped ZnCrO catalysts. Sample A: ZnCrO discharged from the catalyst prepared in the present study at pH=6; Sample B: commercial reactor and further ZnCrO catalyst with Zn/Cr=3/1. analyzed. XRD Experimental conditions: T = 400~ P = 8.5 MPa, CO/H2= 1/1. analysis showed a MeOH: methanol; EtOH: ethanol; 1-PrOH: 1-propanol; 2mePrOH: substantially 2-methyl-1-propanol; 2meBuOH: 2-methyl-1-butanol; 2mepeOH: unmodified ZnCr 2-methyl-l-pentanol; CH4: methane; C2 HC: C2 hydrocarbons; C2+ HC: spinel phase, with C2+hydrocarbons. no presence of ZnO. Only a slight modification in the value of the catalyst surface area has been observed, decreasing from 82 to 73 m2/g. This confirms that catalysts with Zn/Cr ratio = 1 are stable under HAS conditions. Figure 5 also compares the catalytic activity data of the sample prepared in this study with

402 those obtained under the same experimental conditions with a commercial ZnCrO oxide catalyst (Harshaw) having a Zn/Cr atomic ratio = 3/1 and promoted with the same amounts of alkali. From figure 5 it appears that the catalyst prepared in tghe present study shows better catalytic performances with respect to the commercial reference sample, and specifically a greater production of isobutanol and 2-methyl-branched species. Accordingly it is concluded that catalysts with Zn/Cr atomic ratio around 1 and prepared according the procedure here described leads to catalysts which are promising in the HAS.

Acknowledgments The authors wish to tank SNAMPROGETTI for financial support and Dott. G. Del Piero (ENIRICERCHE) for profile fitting analysis.

REFERENCES 1. 2. 3. 4.

P. Zhou, "Summary ofthe HAS Workshop", U.S. Dept. of Energy, Pittsburg, Febr. 1994. Forzatti, E. Tronconi, I. Pasquon, Catal. Rev.-Sci. Eng., 33, 109 (1991). Del Piero, F. Trifir6 and A. Vaccari, J. Chem. Soc., Chem. Comm., 10 (1984)656. Di Conca, A. Riva, F. Trifir6, A. Vaccari, G. Del Piero, V. Fattore, F. Pincolini, Proc. 8th Int. Congress on Catalysis, Berlin, Vol. 2, Veflag Chemic, Weinheim (1984) 173. 5. Unpublished results from our laboratories. 6. Tronconi, C. Cristiani, N. Ferlazzo, P. Forzatti, P.L. Villa, I. Pasquon, Appl. Catal., 32 (1987) 285. 7. Forzatti, C. Cristiani, N. Ferlazzo, L. Lietti, E. Tronconi, P.L. Villa, I. Pasquon, J. Catal., 111 (1988) 120. 8. Stiles, in "Catalyst manifacture", Chemical Industries (1983) 126. 9. Bertoldi, B. Fubini, E. Giamello, G. Busca, F. Trifir6, A. Vaccari, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 1405. 10.Riva, F. Trifir6, A. Vaccari, L. Mintchev, G. Busca, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 1423.

91998ElsevierScienceB.V. All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

Copper-zinc c a t a l y s t s . U s e of n e w comparison with co-precipitation method

403

bimetallic

precursors

and

R. Brahmi a, C.Kappenstein b, J. Cernak c and D. Duprez b aLaboratoire de Physico-Chimie des Mat6riaux, Facult~ des Sciences, Universit~ Chouaib Doukkali, E1 J a d i d a , Maroc, Fax 212 3 34 21 87 bLaboratoire de Catalyse en Chimie Organique, UMR 6503, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France, Fax 33 (0)5 49 45 40 20, e-mail [email protected] CDepartment of Inorganic Chemistry, P.J. Safarik University, Moyzesova 11, 04154 Kosice, Slovakia, Fax 421 95 62 221 24, e-mail [email protected]

New Cu-Zn bimetallic compounds containing Cu(I) and Zn(II) associated with cyanide, ammonia or ethylenediamine ligands, have been used as precursors for the preparation of methanolization catalysts. Air calcination at 450~ leads to the formation of CuO and ZnO and H 2 treatment at 300~ to quantitative reduction of copper. The copper surface area is similar to a coprecipitated sample, but the catalytic activity for the formation of methanol from CO + CO 2 + H 2 mixtures is lower. The sample originated from ammonia containing precursor, displays no metallic area and no activity and shows Zn(CN) 2 and ZnCN 2 after the catalytic test, disclosing the presence and the poisonous effect of remaining cyano group ; a calcination at higher temperature is necessary for catalytic activity. 1. I N T R O D U C T I O N

Methanol synthesis through hydrogenation of CO2 or CO over Cu/ZnO based catalysts is an important industrial process. Fundamental aspects of this process and associated catalysts have been subjected to considerable studies in both academic and industrial research laboratories and the results are summarized in reviews [1,2,3]. However, there are still some controversies concerning m a n y important questions such as the role of ZnO and the nature of the active sites. The active sites are frequently proposed to be Cu ~ [4,5,6]. Alternatively, other researchers claim t h a t Cu § species associated to the zinc oxide phase are the active centers for methanol synthesis [1,7,8]. Recently, some workers suggested that Cu § and Cu ~ species are both essential for the methanol formation [9]. The conventional preparation methods of Cu/ZnO is mainly based on the coprecipitation of Cu(II) and Zn(II) nitrates with sodium carbonate [10]. In recent years, a number of alternative catalysts have been reported [11,12,13] with the requirement to product the finest interdispersion between the metallic elements.

404 As this elemental interdispersion at the atomic level is already present in bimetallic complexes, these compounds are good candidates for catalyst precursors. To have a renewed view concerning the influence of zinc and the role of Cu § species in such catalysts, we have prepared and used as precursors new Cu-Zn bimetallic compounds containing Cu(I) and Zn(II). These compounds have been also ascertained by single crystal X-ray structural studies [14]. Such bimetallic complexes with known crystal structure are no very numerous and contain mainly Cu(II) [15] ; only one other compound [(CH3)4N][ZnCu(CN)4] exhibits copper in the oxidation state (I) [16].

2. E X P E R I M E N T A L 2.1. P r e p a r a t i o n of precursors and activation procedure The studied compounds were prepared as described in [17] and are listed in table 1. A coprecipitated sample, included for comparison purposes, was obtained by adding 10 mL Na2CO3 aqueous solution (1 mol/L) dropwise during 2 hours to a mixture of Cu(NO3)2 (5 mL, 1 tool/L) + Zn(NO3)2 (5 mL, 1 mol/L) at 85~ under continuous stirring. The precipitate was then filtered out, washed with cold distilled water and dried at 80~ overnight. Final Cu/Zn molar ratio was 50/50. The dried powders were calcined in air at 450~ or 600~ (4~ plateau 3 h) and reduced in He flow (5% He in N2, 4~ plateau at 300~ 3 h). Prior to reaction, the catalysts were reduced under the same conditions. 2.2. Characterization * XRD: phase analysis was carried out on a Siemens D-500 diffractometer, using Cu radiation back-monochromatized by graphite monochromator (~.Kal = 1.54060/k). The diffractograms were recorded in step mode (0.02 or 0.04 ~ 1 s). The present phases were identified by use of the PDF library [18] associated with Search program ; data handling was carried out using the software package Eva and Profile (Socabim, France). The size of the crystallites was determined from the integral width [~ (area/height) of the different peaks using the Scherrer relation [19] : D = Lt~*.cos0, where 0 is the Bragg angle, D the crystallite mean size and ~* the corrected integral width (LaB~ standard, pseudo-Voigt function). * B . E . T a n d C u a r e a : the surface area of the catalysts was determined by the BET method (N2 physisorption) Flowsorb-II Micromeritics. The copper specific surface area was determined by N20 chemisorption in a pulsed chromatographic apparatus according to the method described in ref. 20. Pulses of N20 (0.25 cm 3) were injected every other minutes on the reduced catalysts (300~ 3 h, H2, 1 bar). The surface area was calculated from the following stoichiometry (Cu/N20=2/1) using as mean surface density of copper 1.46x1019 at/m 2. * T P R "pulsed chromatographic a p p a r a t u s - H2 pulses (0.22 cm 3 , every other minutes, T ramp 4 ~ range 20-500~

405

2.3. Catalyst testing The catalytic activity for methanol synthesis was measured in a flow system under atmospheric pressure at 220~ (catalyst 300 mg ; space velocity = 1900 h -1" reactant mixture 25 % CO + 8 % CO2 + 67% H2). The feed exit gases were analyzed by gas chromatography (Porapak T column, FID). 3. RESULTS AND DISCUSSION

3.1. P r e c u r s o r s The notation of the bimetallic precursors (Table 1) is based on the following codes: Z = zinc, C = copper, A = ammonia, E = ethylenediamine, CC = Cu(I) + Cu(II). The samples were calcined at 450~ except ZCA-600, heated at 600~ Table1 List and notatio n of the precursors. precursor formula Zn(NH3)2Cu(CN)3 Zn(NI-I3)2Cu(CN)3 [Zn(en )] [Cu(CN)3] [Znl_xCux(en)] [Cu(CN)3] (x-0.03) [Zn(en)3] 6[Cus(CN)17].nH20, n=8.4 co-precipitated.

notations ZCA ZCA-600 ZCE1 ZCCE1 ZCE3 CP

(Zn/Cu)at. 50:50 50:50 50:50 48:52 55:45 50:50

ref. [14] [17] [17] [14] [21]

The bimetallic precursors display polymeric network (ZCA, ZCE1, ZCCE1) or discrete ions (ZCE3), without direct contact between Cu and Zn atoms. For example, figure I shows the polymeric nature of ZCA with bridging cyanide ligands and tetrahedral geometry around copper(I) and zinc(II).

N4 Zn

Figure 1. Drawing of the polymeric network of ZCA [14].

2

u

N3

C1

406

3.2. Characterization of copper and zinc oxides after calcinatiow All the samples display the formation of major CuO and ZnO phases, t h u s exhibiting the quantitative oxidation of copper(I) to copper(II) for the bimetallic precursors (figure 2 and table 2). Ethylenediamine ligands lead to better B E T surface area by comparison with ammonia ligands. Table 2 Properties of CuO and ZnO phases after calcination of precursors at 450~ (except ZCA-600) and TPR results. Crystallite size a intensity ratio b B E T area TPR max. H/Cu Precursor CuO/ZnO (/k) I(CuO)/I(ZnO) (m2.g -1) (~ ZCA 200/100 0.69 6.6 345 2.00 ZCA-600 250/200 0.81 3.2 330,390 2.06 ZCE 1 140/100 0.90 10.1 340 2.50 ZCCE1 130/90 0.82 10.9 340 2.16 ZCE3 150/90 0.98 8.9 340 1.92 CP 130/190 1.00 14.0 (4.6) 330 2.00 a) estimated relative standard deviation 10% ; b) calculated from the neighbor peaks at 47.5 ~ (ZnO) and 48.8 ~ (CuO).

6000

'

'

~

'

t

. . . .

1

. . . .

I

. . . .

I

. . . .

_ Intensity/cps o* o

I

'

'

*oC: 0

'

'

I

t

: 4000

2000 -

020 . . . .

~

0

I ....

30

i

I ....

40

I ....

50

I ....

60

I ....

70

80

2 t h e t a (deg.) Figure 2. XRD p a t t e r n s of ZCA and CP samples after calcination at 450~ 600~ The shifted diffractograms are on the same scale.

or

For ZCA sample (figure 2) there remains small amounts of Cu(I) as Cu20, in agreement with an intensity ratio I(CuO)/I(ZnO) obviously lower t h a n the corresponding values for the other samples (eventually corrected from the initial

407 ratio). We can conclude t h a t for sample ZCA (and to a lesser extent for the other bimetallic precursors), a significant part of copper is not present as well crystallized CuO. The crystallite sizes are between 90 /k (ZnO) and 2 5 0 / k (CuO) ; for all the bimetallic precursors, the CuO particles are larger t h a n ZnO ones, the reverse being observed for the CP sample. The BET areas follow the crystallite size except for the CP sample ; they are much lower than the values calculated on the basis of the crystallite size (by a factor 5 to 10). The TPR profiles are shown on figure 3. The samples calcined at 450~ display a broad reduction peak centered around 340~ while ZCA-600 sample exhibits a shoulder in the s a m e range and a maximum shifted to 390~ This shift can be related to the sintering effect of the higher calcination t e m p e r a t u r e (i.e. 600~ on CuO crystallites and a possible explanation could be the presence of a dual distribution of CuO particle size. The hydrogen consumption agrees with the quantitative reduction of CuO to metallic copper; an hydrogen excess can be observed for sample ZCE1 which could be attributed to residual carboxy species.

600

'

'

'

'1

'

'

'

'

t

'

'

'

'

I

'

H consumed (a.u.) 2

a

w

'

'

'

I

'

'

'

-'" .'" . 9

i;7 '/

200

_

~

- " ..."

0 100

200

y

t

'

'

'

'

I

'

~

ZCA -~-- ZCA600 - - A- - Z C C E 1 .... ~"" C P

t

. :' ~_.c~, ~-~

400

'

\,

I

"i. i\

300

400

:

.-........ X " ' o ...... +'-'4

500

Figure 3. TPR profiles of CP and ZCCE1 samples calcined at 450~ sample calcined at 450 and 600~

>

T (~ and for ZCA

3.3. Characterization of reduced samples The samples were reduced under H2 (5% in N2) at 300~ during 3 h. Only ZnO and metallic copper phases can be detected by XRD (figure 4) showing the fiall reduction of CuO. For the bimetallic precursors, the sintering effect of the t h e r m a l t r e a t m e n t is revealed by a higher crystallite size of copper phase by comparison with the size of the CuO particles (table 3), whereas the ZnO crystallites display a good stability with only a slight size increase (compare tables 2 and 3).

408

Table 3 Characterization of reduced samples. precursors copper area crystallite size a specific activity (N20) m2.g1 Cu/ZnO ( / k ) [tmol.h-l.g1 ZCA # 0 250/140 # 0 ZCA-600 1.4 270/180 1.7 ZCE1 3.2 230/120 2.0 ZCCE1 3.0 1.8 ZCE3 3.7 230/100 1.0 CP 3.9 120/220 6.1 a) estimated relative standard deviation : 15% for Cu and 10% for

intrinsic activity [tmol.hl.m2Cu # 0 1.2 0.6 0.6 0.3 1.6 ZnO.

On the other hand, the CP sample shows a decrease of Cu crystallite size by comparison with CuO; for the reduction of one CuO particle into one Cu particle, the experimental size ratio 0.91 compares very well with the calculated ratio 0.83. Despite copper crystallite size of the same order, the bimetallic precursors display a dramatic difference in copper surface area with a value n e a r zero for ZCA sample ;for this precursor the calcination at 600~ (ZCA-600) leads to a limited rejuvenation of the copper surface. For the other samples, the copper surface a r e a is of the same order, although the CP sample shows a m u c h lower copper size (Table 3).

6000

-lntensity/cps

. ZnO o Cu

o ~

0

~

_

4000

0

-

2000

0

~

20

~

I

,...I

30

i

l

p

I

I

40

I

50

60

70 80 2 theta (deg.)

Figure 4. XRD results of ZCA and CP samples after calcination and reduction.

409 3.4. A c t i v i t y o f c a t a l y s t s

The catalytic activity defined as the rate of CH3OH formation is given on table 3. The specific and intrinsic activities are higher for the CP sample in relation with the lowest copper crystallite size (table 3). The very poor activity of ZCA can be related to the zero copper area of this sample, whereas the calcination at 600~ of the same precursor leads to a specific activity of the same order than the other samples. No obvious relation can be evidenced between copper area, as measured by N20 reaction and intrinsic activity. To understand the peculiar behavior of ZCA samples, we have determined the diffractograms of these samples after catalytic test and the results are presented on figure 5. Beside Cu and ZnO, new crystalline phases appear clearly and have been identified to be zinc cyanide Zn(CN) 2 and zinc cyanamide ZnCN2, whereas few peaks remain unidentified. The presence of such compounds discloses the fact that after the calcination, an important part of the cyanide species remains on the catalyst surface, probably associated with copper. Such compounds are strong poisons of the reaction, thus the need of a higher calcination temperature is necessary for such type of catalysts as it is evidenced for the ZCA sample calcined at 600~ 8000 Intensity/cps

o

, ZnO o Cu

-

,

-

9ZnCN 2

6000

/ i 4000

:ZCA-6O0

o

/l Jill

~

*0

2000

,

oZn(CN)2 L

A

0

~

0

,~,

o

_-

.

0 10

20

30

40

50

60

70 80 2 theta (deg.)

Figure 5. XRD diffractograms of ZCA samples before and after catalytic test. 4. CONCLUSION Our objective to prepare catalysts for methanol synthesis from new bimetallic precursors was only partially fulfilled as all the samples have shown a lower activity than a classical coprecipitated catalyst. The deleterious effect of cyanide

410 associated with ammonia ligands was demonstrated by sample ZCA which showed no copper surface area and no catalytic activity as well as the formation of zinc cyanide and cyanamide during or after the catalytic test, from previously not well crystallized residual species. The poisonous effect of these species can be overcome by calcination at higher temperature.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

K. Klier, Adv. Catal. 31 (1982) 243. G. Giotti and F. Boccuzzi, Catal. Rev. Sci. Eng. 29 (1987) 151. J.C.J. Bart and R.P.A Sneeden, Catal. Today 2 (1987) 1. B.S. Clausen and H. Topsoe, Catal. Today 9 (1991) 189. B.S. Clausen, G. Steffensen, B. Fabius, J. Villadsen, R. Feidenhas'l and H. Topsoe, J. Catal. 132 (1991) 524. D. Duprez, Z. Fehrat-Hamida and M.M. Bettahar, J. Catal. 124 (1990) 1. K. Klier, Appl. Surf. Sci. 19 (1984) 267. V. Ponec, Catal. Lett. 11 (1991) 249. T. Fujitani, M. Saito, Y. Kanai, T. Kakumoto, T. Watanabe, J. Nakamura and T. Uchijima, Catal. Lett. 25 (1994) 271. D. Waller, D. Stifling, F.S. Stone and M.S. Spencer, Faraday Disc. Chem. Soc. 87 (1989) 107. E.G. Baglin, G.B. Atkinson and L.J. Nicks, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 87. G. Owen, C.M. Hawkes, D. Lloyd, J.R. Jennings, R.M. Lambert and R.M. Nix, Appl. Catal., 33 (1987) 405. D. Andriamasinoro, R. Kieffer, A. Kiennemann and P. Poix, Appl. Catal. A, 106 (1993) 201. J. Cernak, C. Kappenstein, J. Chomic and M. Dunaj-Jurco, Z. Krist., 209 (1994) 430. F.H. Allen et al., Cambridge Structural Database System (CSDS), version 5.04, 1992, Univ. Cambridge, England. B.F. Hoskins, R. Robson, J. Am. Chem. Soc. 112 (1990) 1546. J. Cernak, J. Chomic, C. Kappenstein, R. Brahmi and D. Duprez, Thermochim. Acta 276(1996) 209. X-ray Powder Data File : 4-836 for Cu ; 36-1451 for zincite ZnO ; 41-254 for tenorite CuO. R.J. Matyi, L.H. Schwartz and J.B. Butt, Catal. Rev.-Sci. Eng., 29 (1987) 41. B. Dvorak, J. Pasek, J. Catal., 667 (1970) 108. R. Brahmi, Thesis, University of Poitiers, n ~ 631, 1994.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

411

Effect of coprecipitation conditions on the surface area, phase composition, and reducibility of CeO2-ZrO2-Y203 materials for automotive three-way catalysts O.A. Kirichenko a, G.W. Graham b, W. Chun b and R.W. McCabe b aMaterial Sciences Department, Institute of Solid State Chemistry SB RAS, Kutateladze, 18, Novosibirsk, 630090, Russia b Chemical and Physical Sciences Laboratory, Ford Motor Company, P.O. Box 2053, Dearborn, MI 48121, USA

Abstract Yttria-modified ceria-zirconia powders, prepared by various coprecipitation methods, were characterized by BET surface area measurements, X-ray diffraction (XRD) and hydrogen temperature-programmed reduction (TPR). Variations in preparative method strongly affect the BET areas, TPR features, and phase homogeneity of the materials. A new synthesis method based on reduction-coprecipitation produced materials with greater surface areas and ease of reduction after 1000~ aging than obtained for conventionally prepared counterparts.

1. INTRODUCTION Ceria-zirconia mixed oxides are becoming widely used as oxygen storage components in automotive three-way catalysts. Large oxygen storage capacities (OSC) and specific surface areas (S, m2/g) up to 1000~ as well as low reduction temperatures, have all been obtained for the compositions Cel.xZrxO 2 with x in the range of 0.2-0.5 depending on the method of catalyst preparation [1-6]. The general goal in synthesizing such materials has been to produce samples which, even after high-temperature aging, retain a phase structure which is single-phase (by XRD) and characteristic of the cubic fluorite structure of ceria (or closely related tetragonal phases). The ceria-zirconia solid solutions with cubic structure are unstable in air at temperatures of 1050 ~ and higher, decomposing to Ce3ZrOs and t-ZrO2 [7]. Introduction of yttria (20 mol.%) stabilizes the cubic structure solid solution up to 1400 ~ [8]. It is known that even smaller concentrations of yttria stabilize cubic zirconia. Thus, the present study focuses on the preparation of ceria-zirconia modified with 4-16 mol% yttria.

Table 1. Coprecipitation conditions and effect treatment on surface area (S) and crystal size (d) of,*.=.,.~~~.-,* ceria-zirconia materials ,.,,- of heat,",,-~" ,,.,.,,... /~.r*I,~".-"_YY.~,.."j.l..-F.l..l-.., ~ ~ " l".""l.,"* ..,. ..,,. *,,,,--".*..,., .... S, d, Ref Composition, Precursors Solution Supporting pHof Washing/ Drying T, (mol%) reagent precipi- dehydrating nm oc m2/g tation -- media __ -__ dinitrodiamine450 120-150 5 C e 0 2 (20-60) Ce, Zr, Y Zr02 (30-60) butoxides Pt; Pd, Rh 1000 25-35 YO1 5 (10-20) nitrate (w)

.l.-.,...*l,",l^"-. .X.^I"..II-~.."~..,,".".~..""..~. ...^"-~--. , l " ~

-

'..".,~,l~l.*,~"l,~..

,In,."/."

".I(.I-

I,LII"-..I"*,^-*rr".Y.^I.i

,lYl..l.

I

I

20 wt% (w)

CeO2 (5-75) Zr02 (25-95) CeO2 (2-12) ZrOz (88-95) Yoi.5 (4-6)

ZrO(N03)2, 0 5M (w) pH=2 Ce(N03)3 Ce, Zr, Y chlorides

zroc12,

Ce02 (10) Zr02 (84-90) yoi.5(1-6)

Ce(N03X, y(No3)3

Ce02 (5-75) ZrOz (25-95)

ZrOCl2, Ce(N03k

(w)

rnOH

10

w, ethanol

supercritical

450 800 1000

124 52 39

6 11 14

6

(NH2)Z.H20, 80%

2-8

W

120°C

400 900

-

7

NH4OH, 25%

(w)

N&OH

0 25M (w)

NH40H concentrated

58g of oxide NH40H concentrated per 1 (w) 1M (w)

oxalic acid 1M

11

w-m,

1000

110 80 40

1100 550

95-125

810

9

12

10

8

11

1

ethanol

9.1-9.2

w, acetone, alcohol, ethyl ether

IR radiation

500

82-90

W,

110°C

500

-

1300

alcohol

9.3

W,

methanol

42

supercritical

300

120

-

13

40°C

460 600

35 25

-

16

2

h)

413 The variation of the specific surface area with composition in the ceria-zirconia system depends strongly on the preparation procedure. The ordinary ceramic technique gives S less than 1 m2/g. Using pyrolysis of the precursor solutions, values of S between 18 and 20 m2/g have been obtained [8]. High-energy mechanical alloying has produced solid solutions with S in the range 6-30 m2/g [6]. Even larger S values have been obtained by coprecipitation techniques, with values in the range of 80-150 m2/g depending on the precursors and preparative conditions [5,7,9-13]. Some of the published data on coprecipitation procedures used to prepare catalytic and ceramic materials are summarized in Table 1. Generally, heating the high-surface-area materials at temperatures above 700~ greatly decreases the surface area- the extent of loss depending strongly on the precursor and the preparation procedure (Table 1). In addition to surface area stability, the phase composition of the mixed oxide materials is believed to play a large role in performance characteristics such as reducibility and its related property - oxygen storage capacity (OSC). In some cases [4], good performance characteristics have been attributed to the formation of single-phase cubic solid solutions. High-temperature calcination causes the single phase materials to segregate into ceria-rich and zirconia-rich phases. Questions remain, however, concerning the importance of preparing true solid solutions. Recent studies have shown that 1) significant performance benefits can be obtained even in cases where ceria is simply supported on zirconia [5] and 2) materials which appear to be single phase by conventional X-ray diffraction show evidence for phase segregation on the nanometer scale when analyzed by higher energy diffraction methods [ 14]. In the present work the effects of various coprecipitation procedures on surface area stability, phase composition, and reducibility were studied.

2. EXPERIMENTAL The yttria-modified ceria-zirconia powders were prepared by various coprecipitation procedures, with selection of precursors, precipitating agents, solvents and pH values based on the analysis of data published previously [5,6,9-13,15,16]. Three broad techniques were used: Conventional Method (I) - Zirconium oxynitrate ZrO(NO3)2oxH20 (99%), cerium nitrate Ce(NO3)3o6H20 (99.5%, Johnson Mat'they) and yttrium nitrate Y(NO3)3e6H20 (99.9%, Aldrich Chemical Company) were used as precursors. Mixtures of 0.25M solutions of 1) zirconium oxynitrate in concentrated nitric acid and 2) cerium and yttrium nitrates in water were hydrolyzed in aqueous acetone solution at pH=9.1-9.3 by slowly adding concentrated aqueous ammonia solution. The resulting precipitate was allowed to settle for several hours, then filtered, washed with water-acetone-ammonia solution (pH=9.2) and dried at room temperature. Although the method is referred to as "conventional", the use of acetone as a solvent represents a departure from traditonal syntheses which are usually water based. Reduetion-coprecipitation (II) - Two types of starting 0.25M solutions of zirconium salts were used. One was prepared by adding oxalic acid and ammonium oxalate into aqueous zirconium oxynitrate suspension while heating and stirring. The other was an aqueous

414 solution of zirconium(IV) citrate ammonium complex (CAC) (26.3% Zr, Aldrich Chemical Company). The cerium/yttrium starting solution was also 0.25M and contained cerium (IV) ammonium nitrate (NHa)2Ce(NO3) 6 (99.5%, Johnson Matthey) and yttrium nitrate in either pure water or 2-propanol/water solution. The solutions of cerium, yttrium and zirconium salts were prepared separately and then quickly combined while stirring, resulting in essentially simultaneous reduction and coprecipitation. Separated precipitation ( m ) - In a variation of the reduction-coprecipitation method, separate precipitation of Ce/Y and Zr oxides (within the same vessel) was effected by admixing the respective precursor solutions into an acetone-water suspension of ammonium bicarbonate. Throughout the paper, samples are identified by their basic method of preparation (i.e. I, II, or HI) together with a second numeral indicating variations within a series (such as pH differences). To establish a calcination regime for stabilizing the as-prepared materials, thermogravimetric analysis was carried out on the air-dried coprecipitates. TGA revealed that a temperature of 300~ was sufficient to decompose precursor species for type-I materials while 500~ was required for type-II and HI materials. Thus, samples calcined at 300 or 500~ respectively, are referred to as "initial" samples. Other samples were calcined at 600, 900 or 1000~ for 6 hrs to evaluate their thermal stability. BET surface area analyses were carried out with a Micromeritics ASAP 2400 instrument, with sample outgassing at 350~ for 1 h prior to obtaining the adsorption isotherm. X-ray phase analysis was performed with a Scintag X1 diffractometer, using Cu-Ko~ radiation. Temperature-programmed reduction (TPR) was carried out on an Altamira system after pretreating the samples at 500~ in 10% O2/He for 1 h. The TPR traces were obtained by heating approximately 100 mg of sample from room temperature to 900~ at a rate of 10~ in a stream of 10% H2/Ar(25 cm3/min).

3. RESULTS AND DISCUSSION 3.1 Exploratory studies A series of exploratory experiments were first carried out to assess the general characteristics of the three techniques prior to selecting one of the techniques for further development. Results of the exploratory experiments are presented in Tables 2-4 and Figs. 1 and 2. Formation of crystalline phases was observed over the temperature range of 300500~ with the ammonia-precipitated samples (I) undergoing phase nucleation at the lowest temperatures. The majority of the initial and 600~ samples gave XRD patterns with broad peaks, consistent with the fluorite structure of ceria. Table 2 summarizes results for a (56.1)/(37.4)/(6.5) CZY composition prepared by the methods I-HI, the two reductioncoprecipitated materials (II-1 and 11-2) differing only in the way the starting solutions were combined.

415 Table 2. Effect of the preparation method on CEO2(56.1) - ZRO2(37.4) - YO3/2(6.5) . .Sample . . . . . . . . . . . ..... . . . . . . a ......... ~ .......... S~-. . . . . ------I-I-~uptak'-~--e--~ (nm) (nm) (m2/g) (txmol/g) 600~ 1000~ 600~ 1000~ 600~ 900~ 1000~ 600~ 1000~ I-1

0.535

0.533

9.0

14.0

68

15

1.3

1042

827

I1-1

0.538

0.540 0.527

5.3

-

54

16

10

1270

717

11-2

0.538

0.540 0.528

6.4

-

39

11

8

1187

918

III-1

0.540

0.5396

8.2

30

27

14

11

1533

1704

5.42 r

"

.

9

a,A

160

,t

5.38

: _.o- A

d,A 120

,din,.

5.34

~np~l,~ i ,

5.26 200

,

400

,

,

,

600

ia, I-] 80

5.30

Oa, I T - 2 ,kd, I

9

,

40 800

i000

1200

T,C Fig. 1. Lattice parameter (a) and crystal size (d) vs calcination temperature Focusing on the lattice parameter (a) and XRD line-broadening particle diameter (d) presented in Table 2 and in Figure 1, the following observations can be made: 1) the 111-1 sample gives a lattice parameter only slightly less than that of pure ceria (0.5409 nm), indicating little formation of solid solution between ceria and zirconia (both initially and after calcination at 1000~ 2) the I-1 sample appears to be largely single-phase, both as initially prepared and after calcining at temperatures up to 1050~ (Fig. 1); 3) the type-II materials appear to be single-phase initially, but after calcining at temperatures ca. 800~ or higher, reveal peak-splitting in the XRD patterns, characteristic of segregation into zirconia-enriched and almost pure ceria phases (this can be seen in XRD trace 1 in Fig. 3a which was obtained from the 11-1 sample after 1000~ aging). TPR data are shown for the same set of materials in Fig. 2 after calcining at 600 and 1000 ~ The key observation from Fig. 2a (600~ aged) is that both the type I and II samples

416 show a main peak between 500-600~ and a smaller peak near 800~ These traces are characteristic of ceria-zirconia solid solutions [ 17], where the main peak between 500-600~ is attributed to reduction of the mixed oxide material and the peak at 800~ is attributed to bulk reduction of ceria not in solid solution. The III-1 material, in contrast, shows a broad TPR trace, characteristic of pure ceria, but with additional low temperature features. In all materials, the main effect of calcining at 1000~ (Fig. 2b) is to shift most of the reduction to temperatures above 600~ representing either 1) kinetic limitations for reducing the mixed oxide material or 2) phase segregation (resulting in high temperature ceria reduction). Among the materials prepared in the exploratory stage, all retained surface areas near 10 m2/g up to 1000~ except for the type I material which showed a precipitous decrease from 15 m2/g at 900~ to 1.3 m2/g at 1000~ (Table 2). Additional samples were prepared by method I under slightly varying conditions, as summarized in Table 3, in an attempt to improve the thermal stability. However, surface areas greater than 3.5 m2/g after 1000~ aging were not obtained.

TCD signal

a

11I-1

/

~

2

II-

I

~

100

I

300

'

!

500

~

I

"

I

I

700 900 T,C

100

'

I

I

'

I

'

1

I

500 700 900 T,C

300

Figure 2. TPR traces of materials calcined at 600 (a) and 1000~ (b).

Table 3. Effect of coprecipitation conditions on surface area, m2/g ........

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

p / i

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

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

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

- .........

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

I - 1 I- 2 I - 3 I - 4

9.1 - 9.2

-

108

9.1-9.3 9.1 - 9.3 9.4 - 9.5

Drying with acetone Aging for 20 hrs

81

68 62 46 80

1.3

3.0 1.1 3.5

417 The fractional surface area and volume associated with the micropores in these samples are too small to attribute the drop in S after high temperature treatment to annealing of micropores alone. This can be seen from the small micropore volumes Wmic and associated micropore surface areas Stoic listed in Table 4. Moreover, the volume of pores between 1.7 and 300 nm (V,/a) is smaller than the water capacity of the samples V, and the average size of the pores (as measured by either Da or Da) is large. The difference between the particle size calculated from the surface area data (dBET) and X-ray crystal size (dx-ray) is greater than that which can arise from interparticle necking during sintering. Thus, the materials formed by all three techniques have a high degree of macroporosity produced by aggregates of primary particles less than 10 nm in diameter.

Table 4. Structural characteristics of materials. Sample T SnET Smic Va/d* Vmie ~ m2/g m2/g cc/g cc/g I-4 300 81 16 0.058 0.007 I-1 300 108 21 0.038 0.008 900 15 1.2 0.037 0.0003 1000 1.3 0.4 0.054 0.0002 II-1 500 54 4.2 0.16 0.001 1000 10 1.5 0.10 0.0006 III-1 110 19 2.9 0.09 0.001 500 29 4.2 0.07 0.0017 600 27 4.0 0.09 0.0017 1000 11 1.5 0.056 0.0006 *Average cumulative adsorption/desorption pore volume **Water capacity

V** cc/g 0.13 0.12 0.09 0.47 0.45 2.1 1.8 1.2 0.7 of pores

Da[Dd

dBZT nm nm 5 / 4.3 12 29 / 26 9 87 / 36 62 21/4.8 720 10 / 8.0 17 40 / 32 93 20 / 20 12 / 11 32 14 / 13 35 21 / 21 85 between 1.7 and 300

dx-ray nm 6.9 11 14 5.3 8.2 15 30 nm

3.2 Development studies Despite the phase segregation observed in the reduction-coprecipitation materials after aging above 800~ the combination of good surface area retention and maintenance of relatively good low-temperature reducibility after high temperature aging suggested that further development of the reduction-coprecipitation method was warranted. The approach used was to first vary the CZY composition (Table 5) and then select one of the compositions for further study varying both the Zr-precursor and the solvents (Table 6). Considering first the effects of composition, it can be seen that samples 10, 4, and 11, having the highest ceria contents (62-65%) showed much less tendency for phase segregation after 1000 ~ aging than their counterparts with lower ceria concentrations, regardless of yttria content.

418 Table 5. Effect of CZY ratio on the features of C e O 2 ( x ) - Z r O 2 ( Y ) - Y O 3 / 2 ( z ) .......... S ................ Sample (x) (y) (z) a nm m2/g 1000~

9

(46) (46) (8)

1

(56.1) (37.4) (6.5)

10

(62) (22) (16)

4

(63) (29) (8)

11

(65)(31)(4)

,,

600~

0.540 O.525 0.5398 0.527 0.540 O.535 0.538 0.533 0.535

1000~

(method II) H 2 uptake ..... ~tmol/g 600~

1000~

31

13

1080

487

54

10

1270

717

61

12

-

922

70

13

-

668

14

1029

1499

47

,

,

,,,

,

,,

11 "m

4

C M m

r~ i,=

I

27

30

33 20

36

200

400

600 T, oC

800

Fig. 3. XRD pattems (a) and TPR traces (b) of samples of Table 5 after 1000~ aging. Figure 3a, for example shows progressively less peak splitting in the XRD patterns in going from the bottom trace (9) to the top trace (11). All of the compositions displayed surface area stability similar to that observed earlier in the exploratory studies (10-14 m2/g after 1000~ aging). In keeping with the XRD trend toward more single-phase character with increasing ceria content, the high-ceria materials also gave TPR spectra more characteristic of ceria-zirconia mixed oxide solutions with their largest peak between 500 and 600~ (Fig. 3b).

419 Table 6. Effect of Zr-precursor and solvent on CeO2(63)-ZrO2(29)-YO3/2(8) (method II) Sample ........ Zr solution ........ Solvent for .... S (m~/g).... a* H2 uptake* Precursor Solvent Ce-Y soln. 600 ~ 1000~ (nm) (~tmol/g) 3 ZrO(NO3)2 HNO3 H20 40 2.7 0.538 938 0.531 I-/20 70 13 4 ZrO(NO3)2 H20, H2Ox, 0.538 668 (NH4)2Ox 0.533 5 ZrO(NO3) 2 1-/20,H2Ox, H20: 64 17 0.535 814 (NH4)2Ox 2-propanol 1:1.6 6 ZrO(NO3)2 H20, H2Ox, 1-120: 45 15 0.538 1070 (NHa)2Ox 2-propanol 0.534 1:4 8 Zr-CAC H20 H20: 27 12 0.537 1040 2-propanol 0.533 1:4 *Samples calcined at 1000~ . . . . . . . . . . . . . . . . Amongst the group of the three high-ceria samples, 10, 4, and 11, the composition of sample 4 (containing 8% yttria) was selected for further processing variations with respect to zirconia precursor and choice of solvents. The matrix of materials prepared is shown in Table 6, along with some of the characterization data. The use of nitric acid as a dissolving agent for the zirconium oxynitrate had a strong detrimental affect on surface area stability, phase homogeneity and TPR characteristics (sample 3 in Table 6 and Figs. 4a&b, respectively).

$

~

6

[5 4

27

30

33 20

36

200

400

600

800

T, oC

Fig. 4. XRD patterns (a) and TPR traces (b) of samples of Table 6 after 1000~ aging.

420 Otherwise, the remaining samples were similar in their XRD, surface area, and TPR characteristics. Sample 5 can be seen in Fig. 4a to have the narrowest XRD peaks of all the 8% yttria samples and is similar to sample 11 (containing 4% yttria).

4. CONCLUSIONS A new method of reduction-coprecipitation has been developed to produce ceria-zirconiayttria materials of phase purity comparable to similar materials prepared by the conventional method of coprecipitation with ammonia. The new method produces materials which retain BET surface areas ca 15 m2/g after 1000~ calcination, compared to 1-3 m2/g for the conventionally prepared counterparts. Moreover, the main reduction peak of the reductioncoprecipitation materials is more than 100~ below that of the conventionally-prepared material, suggesting that CZY materials prepared by the reduction-coprecipitation technique may be of value in certain catalytic applications such as automotive exhaust catalysis.

5. REFERENCES

.

.

.

9. 10. 11. 12. 13. 14. 15. 16. 17.

P.Fornasiero, R. DiMonte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli, and M. Graziani, J. Catal. 151 (1995) 168. C.J.Norman, Soc. of Automotive Engineers, Paper No. 970460 (1997). J.-P. Cuif, G. Blanchard, O. Touret, A. Seigneurin, M. Marczi, and E. Quemere, Soc. of Automotive Engineers, Paper No. 970463 (1997). J.-P. Cuif, G. Blanchard, O. Touret, M. Marczi, and E. Quemere, Soc. of Automotive Engineers, Paper No. 961906 (1996). T. Yamada, T. Kobayashi, K. Kayano, and M. Funabiki, Soc. of Automotive Engineers, Paper No. 970466 (1997). F. Zamar, A. Trovarelli, C. Leitenburg, and G. Dolcetti, Stud Surf. Sci.& Catal. J.W. Hightower et al., eds., Elsevier Sci. B.V. 101 (1996) 1283. A.Kawabata, S. Hirano, M. Yoshinaka, K. Hirota, and O. Yamaguchi, J. Mater. Sci. 31 (1996) 4945. L. Podda and F. Ricciardiello, Adv. Sci. Technol. 3B (1995) 1465. M.M.R. Boutz, A.J.A. Winnubst, and A.J. Burggraaf, J. Europ. Ceram. Soc. 13 (1994) 89. J.G. Duh and J.U. Wan, J. Mat. Sci. 27 (1992) 6197. J.G. Dub and M.Y. Lee, J. Mat. Sci. 24 (1989) 4467. Y. Sun and P.A. Sermon, J. Mater. Chem. 6 (1996) 1025. G.H. Einarsdottir, E.L. Sveinsdottir, F. Thorsteinsson, and G. Gunnarsson, Br. Ceram. Proc. 47 (1991) 55. T. Egami, W. Dmowski, and R. Brezny, Soc. of Automotive Engineers, Paper No. 970461 (1997). R.D. Maggio et al. Adv. Sci. Technol, 7 (1995) 37. T. Settu and R. Gobinathan, J. Eur. Ceram. Soc. 16 (1996) 1309. T. Murota, T. Hasegawa, S. Aozasa, H. Matsui, and M. Motoyama, J. Alloys & Compounds 193 (1993) 298.

91998 ElsevierScience B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

421

Preparation, E x t r u s i o n and Characterization of P e r o v s k i t e Catalysts H.-G. Lintz and S. Ztihlke Institut fur Chemische Verfahrenstechnik der Universit~it Karlsruhe (TH) Kaiserstr. 12, D-76128 Karlsruhe (Germany), Tel.: +49-721-6083958, Fax.: +49-721-6086118 e-mail: simon.zuehlke @ciw.uni-karlsruhe.de 1. INTRODUCTION Perovskite-type lanthanides with the general formula LaBO3, with B = Mn, Co, Fe, Ni, Cr occupy a prominent place under the known ternary systems, due to a series of interesting and useful properties and applications. For example LaMnO3 is used as a catalyst for the oxidation of halogenated and non halogenated Volatile Organic Compounds (VOC' s) in industrial waste gases [ 1-3]. Partially substituted perovskites e.g. with the formula Lal_xSrxMnO3 are also used as electrodes in Solid Oxide Fuel Cells (SOFC's) [4]. These oxides have a higher thermal and chemical stability as well as a lower price than noble metal catalysts used equally for such applications. The catalytic properties of perovskites were systematically studied first by R.J.H. Voorhoeve in 1977. During the total oxidation of VOC's he detected a co-operation of the lattice oxygen and the oxygen adsorbed on the catalyst surface. Therefore the typical perovskite crystal structure as well as a large surface area are required for their catalytic use [5,6]. The preparation of oxides with perovskite structure and large surface area is a generic problem. On the one hand thermal energy is necessary to convert the precursors (oxides, nitrates, etc.) in a reasonable time into the crystal structure wanted. On the other hand high temperatures lead to a sintering of the primary particles and to a loss in surface area. At present the largest surface areas of about 60 m2/g can be generated by precipitation of the appropriate hydroxides and a subsequent decomposition at 600~ [ 1]. It is a decisive disadvantage of this method, that a lot of work and time is needed for the generation of relatively small amounts of catalytically active material. In the work presented here a method called drip-pyrolysis is used to obtain perovskite-like oxides (here: LaMnO3) continuously with the wanted specifications for the total oxidation of VOC's in technical interesting amounts. The as-prepared finely powdered perovskites have to be adequately shaped to be used in continuously operated flow reactors. For this objective there are in principle two configurations, pellets or granules and honeycomb structures, depending on the permissible pressuredrop and the dust loading of the exhausts. For both varieties there are again two possibilities, coating of a support by the active component or preparation of bulk catalysts. The aim of the present study is the systematic investigation of the mutual interference of catalyst preparation, shaping and activity. Therefore the prepared perovskite powders were mixed with 7-A1203, pseudoboemite, alginate and a binder or peptizer to obtain a plastic paste. Hollow cylinders (5x2xl0mm) with great porosity were extruded with subsequent thermal treatment. The generated samples were characterized by XRD, BET, REM and Hg-porosimetry. Their catalytic activity was qualified using the oxidation of acetone and their mechanical resistance was characterized by determination of their compressive strength.

422

2. EXPERIMENTAL 2.1. Preparation of LaMnOa by drip.pyrolysis The perovskite oxide was produced in this work via the so called drip- pyrolysis. Herein a stoichiometric aqueous solution of the metal nitrates (2 mold) containing glucose in a molar ratio of 1:1 with respect to the total metal cation content was continuously dripped into the hot zone of a stainless steel-tube rotary furnace shown in Figure 1. The glucose decomposed abruptly on the hot (T = 700~ surface with an increase in volume. Thereby the drops were divided in smaller droplets and/or were foaming and simultaneously the perovskite structure was built up. In the work presented the residence time in the hot zone of the furnace was 10min and the production rate for the sample Figure 1. Scheme of the tubular rotary furnace used for drip-pyrolysis, was about 1 g/rain and therefore much higher compared to other methods. The sample leaving the tubular rotary furnace could be described as fine, free-falling powder showing visible agglomerates with diameters of about 0.1 to 2 mm. Some 300 g of the perovskite oxide were produced for all extrudates in this work.

2.2. Shaping of perovskite catalysts by extrusion Referring to the work of Roth catalytically active hollow cylinders (5x2x10 mm) were prepared by extrusion of plastic pastes [7]. These pastes are based on about 300 g of an alumina-mix (~,-A1203:A10(OH), 2:1) and an equivalent amount of a plasticizer (alginate). A10(OH) (pseudoboemite) forms fibres during the thermal treatment and thus a porous texture with great mechanical strength is obtained. ~,-A1203 is needed as lean material to avoid a severe shrinkage which would lead to fissures in the extrudates. LaMnO3 was added in different proportions up to 10 wt.-% with respect to the total solid weight by substitution of the corresponding amount of 7-A1203. Thus about 600 g of a plastic paste were taken for the extrusion of one catalyst. In order to improve the mechanical strength of the extrudates a peptizer (acetic acid) or a binder (colloidal Si(OH)4-sol, 14 wt.-% solid content) was added to some of the pastes. The final composition of the plastic pastes was varied a little in order to get a successful shaping. The detailed compositions are specified in Table 1. For the preparation of the alginate the ammonium salt of alginic-acid was mixed 24 hours before use with distilled water in a mass ratio of 13:100. To add the binder the ammonium salt was prepared with the colloidal silica acid in a mass ratio of 10:100 (*). In this case some 100 g of the peptizer were needed additionally to obtain pastes suitable for extrusion. After drying at room temperature for 24 hours, the pellets were calcined at 900~ for 3 hours under air.

423 Table 1. Final composition of the pastes and designation of the extruded catalysts Catalyst name E0 E2 E5 El0 E0s01 E2sol E5sol E10sol E0aac E2aac E5aac E10aac

7-A1203 200 196 190 170 200 194 185 170 200 194 185 170

A10(OH) 100 98 95 100 100 100 100 100 100 100 100 100

Paste composition / g LaMnO3 alginate 0 300 6 300 15 320 30 320 0 410" 6 410* 15 410* 30 410" 0 320 6 320 15 320 30 340

CH3COOH

10 10 10 10

2.3. Characterization The solid materials used for extrusion were investigated by XRD, BET, REM and particle size analysis. The extruded catalysts were additionally characterized by Hg-porosimetry. As characteristic feature for their mechanical resistance the compressive strength of the hollow cylinders was determined, measuring the force needed for their destruction. The catalytic properties of the extrudates were determined by kinetic measurements in an isothermal tubular reactor using the oxidation of acetone. The measurements were carried out by monitoring the gas phase composition along the length of a fixed bed of catalyst through distributed local sampling in a tubular reactor (Figure 2) which was divided into 7 segments with sampling ports and individual temperature control. Nitrogen was loaded with acetone in a saturator (S) at constant temperature and mixed with oxygen and additional N2 to obtain a flow rate of synthetic air with 0.2 Vol.-% acetone of about 100 ml/s , waste gas (NTP) at the reactor inlet. The acetone concentration could be checked through a carbon balance by use of a 2 nd r e a c t o r (TO) containing a sufficient quantity of catalyst to oxidize all carbon containing compounds to CO2 and H20. The analytical section consisted analysis of a GC with capillary column and gas chromatograph 02 IR-spectmmeter FID-detector and two non-dispersive IR-spectrometers to determine the CO and CO2 concentrations. Four measurements at each catalyst were made N2 in the temperature range between T 260~ and 350~ The reactor operation and sampling were automatically Figure 2. Experimental set-up: controlled using a computer. fixed bed reactor with side stream exits.

ii

424

The measurements were carried out in the open system at steady state. Each quantity can be determined at the location of each of the seven analytical ports z distributed along the catalyst bed. The conversion of acetone is defined as the difference between the molar flow rate of acetone at the inlet (z = 1) and the flow rate at port z divided by the inlet flow rate: 9

Xz

9

-- Ilacetone,in-

I'lacetone, z

(1)

I1 acetone, in

A normalized dimensionless concentration Yi denotes the distribution of the total carbon in the different species i and is related to the inlet carbon flow. yi is identical to the yield Yi of the species i: 1~i

s9

z

Yi,z =

(2) gacetone " flacetone,in

In this equation is Ei the number of carbon atoms in the molecule i (e.g. Eacetone = 3). The modified residence time is related to the mass mcat,z of the catalytically active compound

t v'"

_ m cat,z ... ~ , 1 " [tv] = kg s9/ m 3. V - GHSV

(3)

3. RESULTS AND DISCUSSION XRD-measurements of the powder obtained by drip-pyrolysis indicated only the perovskite structure LaMnO3.15. The XRD-patterns of all extrudates were characteristic of the original solid materials used for extrusion. REM-micrographs of the perovskite oxide with a magnification of 1:5000 showed a very porous, spongy texture whereas micrographs with an intermediate magnification of 1:200 showed a primary particle size distribution between 1 gm and 100 gm. Thus the relatively large surface area of the sample prepared by drip-pyrolysis can be referred to the high porosity of the primary grain. The specific surface area measured with the BET-method (N2-adsorption at 77 K) of the perovskite oxide amounted to 17 m2/g, of 7-A1203 to 215 m2/g and the surface area of pseudoboemite (A10(OH)) was 259 m2/g. The average particle sizes of the solid matters used for extrusion determined with laser diffraction were very similar: LaMnO3:30 gm, ~,-A1203:39 gm and A10(OH): 45 gm.

3.1. Compressive strength of the extrudates A sufficient mechanical strength of the extrudates is a prerequisite for their use in a fixed bed reactor.

425 Fig. 3 shows the effect of the LaMnO3 content on the compressive strength (force per length) of the extruded hollow cylinders. In addition an empirically determined range between 0.17 N/mm and 0.2 N/mm is marked. Hollow cylinders having a resistance underneath this range were destroyed in great number while filling up the tubular reactor used for kinetic measurements. No damages were noticed at values higher than 0.2 N/mm.

0.6

,

,

i 0.5 ................... i ............. ~ ~/~~O._ A-~~ ~ 0.4 i Z

~

~ i!

!

[ --m-- without any add. " i ....... I --O--add. of Si(OH)4-sol [ - - A - -::a "-.~,., dd. .,,,,, ,,,,, of acetic acid

ii................ " ~ i............... i . . . . . . . . . . . . 2 ~ ' ; :

0.30.2 0.1

-!

0.0 0

2

4

6

8

10

content LaMnO 3 / %

Figure 3. Compressive strength (FD) of extruded hollow cylinders with different compositions versus LaMnO3 content. Without the addition of a binder or peptizer the strength of the extrudates declines clearly with increasing content of perovskite. This indicates that already a small amount of the porous perovskite oxide prevents the formation of a texture with the strength of the pure aluminamix. However, the compressive strength raised significantly after the addition of 3 wt.-% of acetic acid as peptizer up to a LaMnO3 content of 5 %. This effect is based on the dissolving of a small amount of the pseudoboemite and/or y-A1203with a subsequent formation of solidsolid links during thermal treatment [8]. The decreasing strength with a higher LaMnO3 content than 5 % is caused by difficulties during extrusion resulting in a crack formation in the shaped pieces. The addition of some 33 g colloidal SiO2 as a Si(OH)4-sol results also in a raise of the compressive strength, again caused by the formation of solid-solid links, here by SiO2particles [8]. With this binder, a systematic dependence of the catalyst resistance on the perovskite content can not be observed. Measurements obtained with Hg-porosimetry showed no differences in the morphology of the extruded catalysts. The porosities were nearly constant between 68 % and 71% independently of the catalyst composition. Work in progress aims at an increase of the proportion of the catalytically active perovskite without loosing the mechanical strength obtained so far.

426 3.2. Catalytic properties of the extrudates

Typical results of the kinetic investigation are shown in Figure 4. The dimensionless concentrations are plotted against the residence time for the oxidation of acetone on perovskite extrudates (here: E5aac). All oxygenated intermediates except CO, i.e. acetaldehyde, acrolein, acetic acid, propene and formaldehyde, are lumped together to a pseudo species 'partially oxidized products (PO)'. As CO was the main partially oxidized product, it has been treated separately. 1 ..... ~.--._~S7___--_---i---.-----------_------_C::Z: I

acetone-~:2-

0.5

o

C O ~

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

.................... CO

0.10.05 [--f

~...~:r-PO

~

~ ........

6

8

5aac m~

I 1

0.01 -~

0.005 2

4

10

residence time tv/(kg,s,m 3)

Figure 4. Dimensionless normalized concentrations yi versus residence time tv. The results illustrate the continuous decrease of the acetone concentration and it is clearly shown that the main part of the carbon content of the solvent is found in the deep oxidation product CO2. However, it is equally indicated that intermediate products are formed during the reaction. Their concentration passes through a typical maximum before declining. The typical pattern of Figure 4 has always been obtained, irrespectively of the perovskite content and additives used, though there are some quantitative differences. The extrudates consisting of pure alumina-mix without perovskite showed only a low activity in the oxidation of acetone. The combustion of acetone on all perovskite catalysts can be described by a simplified parallel-consecutive reaction network in the temperature range studied (260 ~ - 350~ This network includes acetone, the observed partially oxidized intermediates CO, PO and the total oxidation product CO2 (Figure 5). They are linked by three parallel and two consecutive reactions. Five mass related reaction rates interfering in four coupled differential equations are therefore considered: dci

d

ffmcatdyst/ 9

= dci = s dtv ij

Vi,ij"rij

(4)

427

r12 = k~2" C1

C O (3) J (CH3)2CO

r~3 = k~3. c~

"~-~34 , C O 2 (4)

(1)

po(2

~

1724

r~4 = k,4, c , / ( 1

+b- c,)

r=4 = k24. c 2 / ( l + b , I"34 =

c~)

k34" c3/(l+b, c,)

Figure 5. Reaction network for the oxidation of acetone on perovskite catalysts with mass related rate equations for the coupled parallel and consecutive reaction steps. The description holds within the whole temperature range studied. We thus need five mass related kinetic coefficients kij as well as the parameter b fitted to the experimentally determined concentration curves. The lines drawn in Figure 4 are calculated after curve fitting. The agreement between the experimental data and model calculations is quite satisfactory. Thus we may safely use the kinetic coefficients to compare the experimental results obtained for the different catalysts and to analyze the influence of each component. For each catalyst Arrhenius-type plots were made to determine the temperature dependence of the single rate constants. To achieve this measurements at 4 different temperatures were carried out in a range between 260 ~ and 350~ The amount of perovskite extrudates was chosen in that way to have at each temperature an acetone conversion between 70 % and 99 % at the end of the reactor. If necessary the extrudates were mixed homogeneously with catalytically inert steatite balls (d = 3 mm) to have a fixed bed in the tubular reactor. The cleaning of solvent containing air is aimed at the deep oxidation of VOC's to CO2 and H20. However, in many cases the formation of toxic intermediates has been observed [9]. Therefore one focus of the present study is to prepare catalysts with both high activity and selectivity for solvent combustion, resulting in high values of k14 and low values of k12 and k13 of the observed reaction network (see Figure 5). Figure 6 shows plots of the natural logarithm of the mass related rate constants for the reaction steps of acetone to the deep oxidation product CO2 or to the partially oxidized intermediates CO and PO respectively versus the reciprocal reaction temperature. Because of the broken order of the reaction rate 14 of acetone to CO2 rate constants of pseudo first order (kl4*) were used for comparison [10]. On the left side the results obtained at extrudates prepared with a certain amount of Si(OH)4-sol as binder, on the right side with acetic acid as peptizer are shown. Catalysts with a LaMnO3 content of 2 wt.-%, 5 wt.-% and 10 wt.-% (Exsol and Exaac) are compared. On the left side one can observe a decrease of the activity in the deep oxidation of acetone as well as in the reaction to the partially oxidized intermediates with an increase of the perovskite content of the catalysts. As the kinetic coefficients were related to the mass of the catalytically active material they should not differ. In a previous work we have clearly repor-

428 ted an influence of this binder on the preparation of egg-shell catalysts by shielding a certain amount of LaMnO3 [ 11 ]. An explanation of the results here may take into account additionally the BET-measurements: The specific surface area of the extrudates prepared with Si(OH)4-sol decreases with an increase of the LaMnO3 content (see Table 2). This can be attributed to the substantially smaller surface area of the perovskite powder compared to the alumina species. As in this work for all catalysts the same amount of binder was added (Table 1), a greater part of the perovskite content could have been covered by the binder during the preparation.

temperature / ~ 350 I

320 ,

I

,

.

I

~

290

I

I

~

I

, .....

]

340

.

..,

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~

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,

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,~

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em

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

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9

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9

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.

,4

.

.

.

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.

.

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.

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9

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,~

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2 0.0016

0.0018 0.0016

0.0017

0.0017

0.0018

0.0019

reciprocal temperature / K "~ Figure 6. Mass related rate constants for the reaction steps of acetone to CO + PO (kl2 + kl3) or CO2 (k14) versus reciprocal temperature; left side: extrudates prepared with Si(OH)4-sol as binder (Exsol), right side: extrudates prepared with acetic acid as peptizer (Exaac).

Table 2. Specific surface areas of the extrudates depending on their LaMnO3 content

without add. add. of binder add. of peptizer

0 wt.-% LaMn03 119 m2/g 146 m2/g

2 wt.-% LaMn03 135 m2/g 147 m2/g

5 wt.-% LaMn03 131 m2/g 134 m2/g

10 wt.-% LaMn03 119 mz/g 101 m2/g '

137 mZ/g

136 m2/g

134 m2/g

121 m2/g

429 The rate constants of the catalysts prepared with a certain amount of peptizer but different proportions of LaMnO3 (Exaac) are shown on the right side of Figure 6. One cannot observe differences in the activity of the reaction of acetone to the partially oxidized intermediates CO and PO. Nevertheless the values of the rate constants for the deep oxidation of acetone towards CO2 differ a little. Here the catalyst with 10 wt.-% LaMnO3 (E 10aac) has a higher activity in the whole temperature range. Visibly in this case no shield effect is possible, as the action of the peptizer leads to solid-solid links of the material itself. Figure 7 shows plots of the natural logarithm of the mass related rate constants of the reaction steps in the same way as in Figure 6. Extrudates with a LaMnO3 content of 2 wt.-% but different extrusion additives (E2xy) are compatemperature / ~ red. The pattern shown 350 340 330 320 310 300 290 in Figure 7 was always l , I . ! . I I ,. I , I , obtained independently "7r.r of the perovskite con-1 tent. The extrudate pre--2 pared with a certain amount of Si(OH)4-sol -3 as binder (E2sol) shows + ................. i. . . . . . . the lowest activity in the c-I oxidations of acetone to"l acetone --> CO + PO I..... : ....................... ::":"~..... ....... - 4 , wards the different species in the whole tempe[] E2 9 E2aac z E2sol -5 rature range. This indi1 cates again a partially deactivation of the pe'Tr/J : . . rovskite by the binder. 9 ".......... -A. i i i ";'~/) ................................................. Viewing only the reaction of acetone to rn the partially oxidized intermediates CO and PO the catalyst E2 prepared without an additive is _> -4 --q9 -Iacetone C0~l '~more active than E2aac ', , although having the . . / . . . . / . . -5 0.00160 0.00165 0.00170 0.00175 0.00180 same specific surface area. In the deep oxidareciprocal temperature /K-' tion of acetone no diffeFigure 7. Mass related rate constants for the reaction steps of rences can be observed. acetone versus reciprocal reaction temperature; extrudates with Related to the LaMnO3 2 wt.-% LaMnO3 and different additives (E2xy). content E2aac prepared with acetic acid is the most suitable catalyst for the oxidation of acetone, showing in addition a sufficient mechanical strength almost three times higher than E2.

g

i

-2

430 4. CONCLUSIONS Perovskite-type LaMnO3 used for the oxidation of VOC's in industrial waste gases can be prepared continuously and in technical interesting amounts with a relatively large specific surface area of almost 20 m2/g by a method called drip-pyrolysis. The as-prepared finely powdered perovskite was shaped by extrusion in order to be used in a continuously operated flow reactor. To obtain extrudates with a sufficient mechanical strength additives are needed, knowing that every additional material mostly has a negative influence on the catalytic properties of the shaped pieces compared with the pure catalytically active material. Using pseudoboemite and ~,-A1203 as lean material and alginate as plasticizer extrudates with a perovskite content up to 10 wt.-% and high porosity of about 70 % were obtained in this work. The mechanical strength declines clearly with increasing LaMnO3 content. However, adding Si(OH)4-sol as binder or acetic acid as peptizer before extrusion the compressive strength of the shaped pieces can be raised significantly. The experimental results have shown that the oxidation of acetone at all extrudates can be described by a simplified reaction network of three parallel and two consecutive reactions with simple rate equations for each reaction step. The kinetic parameters can be used safely to compare the catalytic properties of the extrudates and for the investigation of the interference of catalyst preparation, shaping and activity. The catalytic combustion of solvent containing air is aimed at the deep oxidation of VOC's towards CO2 and H20. The results indicate that the Si(OH)4-sol seems to cover a certain amount of the catalytically active material's surface resulting in a lower activity, whereas the addition of acetic acid results in a higher selectivity towards the formation of the deep oxidation product CO2. The use of acetic acid as peptizer in the preparation of LaMnO3 extrudates seems therefore most interesting with respect to the application in catalytic combustion.

REFERENCES

1. 2. 3. 4. 5.

R. Schneider, D. Kiel31ing, P. Kraak, M. Haftendorn, G. Wendt, Chem.-Tech. 4 (1995) 199 H. Arai et al., Appl. Catal. 26 (1986) 265-276 T. Seiyama, Catal. Rev.-Sci. Eng., 34(4) (1992) 281-301 N. Christiansen, P. Gordes, 2nd Int. Symposium on Solid Oxide Fuel Cells (1991) R.J.H. Voorhoeve, D.W. Johnson, Jr., J.P. Remeika and P.K. Gallagher, Science, 4(195) (1977) 4281 6. R.J.H. Voorhoeve, Advanced Materials in Catalysis, J.J. Burton and R.C. Carter (eds.) Academic Press Inc., New York (1977) 129-180 7. A. Roth, Dissertation, Universit~it Karlsruhe (TH) 1991 8. Hollemann-Wiberg (ed.), Lehrbuch der Anorganischen Chemie, Verlag Walter de Gruyter, Berlin, New York, 91.-100. Auflage (1985) 877; 760 9. H.-G. Lintz and K. Wittstock, Catal. Today 29 (1996) 457-461 10. K. Wittstock, Dissertation, Universit~it Karlsruhe (TH) 1995 11. H.-G. Lintz and S. Ztihlke, R6cents Progr~s en G6nie des Proc6d6s 11/54 (1997) 13-18

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

431

N e w methods to prepare perovskite-type La0.sSr0.2CoO3 catalyst at low temperature Zongping Shao, Guoxing Xiong*, Shishan Sheng, Hengrong Chen, Lin Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, Dalian 116023, China

A pure phase perovskite-type composite oxide La0.sSr0.zCoO3(LSCO) was prepared by three methods: sol-gel method using EDTA as ligand, environmentaly safe metal-EDTAcellulose complexing method and metal-citric-cellulose complexing method. The formation processes of LSCO by those methods were investigated by TG-DTA, XRD and IR spectroscopy. The results indicated that when EDTA complexing method was used, at optimized condition, pure phase perovskite-type LSCO was acquired after the precursor was fired at 800~ for 2hr. In the process of forming perovskite LSCO, SrCO3 was formed and calcination temperatures higher than 800~ was needed to get rid of it, similar to the citric complexing method reported. When cellulose was introduced, the minimum calcination temperature to acquire pure phase LSCO was significantly lowered to around 6000(2 for metalEDTA-cellulose method and 540~ for metal-citric-cellulose method. Low calcination temperature led to relatively high BET area for 21m2/g and 22m2/g respectively. But the surface area were significantly reduced to 12.7m2/g and 12.1m2/g respectively when they were further fired at 620~ for 2hr.

1. INTRODUCTION Mixed ionic and electronic conducting perovskite Lal.xSrxCoO3 are important kinds of composite oxides. They can be used as catalysts for the complete oxidation of hydrocarbons, carbon monoxide and decomposition of environment pollutant NOx [1-3]. They also attracted wide attention for their potential applications as oxygen semipermeable membranes and electrodes in solid oxide fuels cells (SOFC) and oxygen sensors[4-6]. Partial substitution of La3+ in LaCoO3 by Sr2+ increase Co4§ and oxygen vacancy which lead to high oxygen permeation of those materials at high temperature without any outside electrode. The oxygen vacancy also has an important role in the high catalytic activity of the materials in the above mentioned reactions.

Author 9 to whom all correspondence should be addressed

432 There are several methods developed that can be used to synthesize perovskite-type composite oxides. Solid state reaction and coprecipitation techniques are often used, but those methods need high temperatures (usually >1000~ for solid state reaction), which lead to low surface areas. A further drawback is the lack of homogeneity for an incomplete reaction of the precursor. It was reported that the catalyst powder Lao.sSr0.2MnO3 synthesized by those methods which can resuk in higher surface areas and homogeneity had higher catalytic activity for CO oxidation[7]. Surface area depends mainly on the minimum temperature necessary for complete reaction. The greatest losses in surface area by sintering were obtained in the temperature range 700~176 In order to synthesize ultrafine and uniform catalyst powder with high specific surface area, it is necessary to lower the minimum firing temperature. New methods have been developed, such as heteronucleur complex thermal decomposition techniques and sol-gel type technique [8-10]. Recently, pyrolysis of salt-cellulose composites has gained acceptance for the preparation of mixed oxide powder such as YSZ [11]. This method has the advantage to control the morphology and particle size of the resultant powders. In this paper we report the preparation of Lao.sSr0.2CoO3 by three methods: EDTA complexing sol-gel type technique, metal-citric-cellulose complexing method and metalEDTA-cellulose complexing method. IR, TG-DTA and XRD were used to characterize the products calcined at different temperatures, the formation processes of LSCO were investigated, the BET area of the result powders were also determined.

2. EXPERIMENTAL

2.1 Preparation of LSCO powder by EDTA complexing sol-gel method Metal nitrates of La(NO3)3.6H20, Sr(NO3)2 and Co(NO3)2.6H20 were dissolved in double-distilled water. Their precise concentrations were determined by EDTA titration. Stoichiometric moles of them were mixed, then added in complexing agent (NH3.H20 solution of EDTA acid) and dispersant (when needed) to make a transparent solution, NH3.H20 was used to adjust the pH value of solution to between 5-6. The solution was kept at 80~ to evaporate the water and stirred by a magnetic stirrer. The purple viscous gel was formed through sol process by pretreating the gel at 150~ in a hot-plate for several hours, and finally calcined in air at different temperatures for 2hr. 2.2 Preparation of LSCO by metai-EDTA-Ceilulose and metal-citric-cellulose methods. Amorphous cellulose was treated wkh strong HNO3 acid for about lhr then washed by distilled water to get rid of the possible metal salts existing in the raw cellulose material and also to make it more adsorptable for the aimed salt ions. Stoichiometric mole of La(NO3)3, Sr(NO3)2 and Co(NO3)2 solution were mixed and stirred for 6hr, then added to the dried activated cellulose, vibrated fiercely for 6hr at 50~ in a vibrator to prompt and make the saks adsorbed in the cellulose surface more strongly and homogeneously, then added in citric acid or EDTA ammonia solution ( complexing agent: total metal ions = 1.2:1, mole ratio ), vibrated for another 8hr, dried at 80~ finally calcined at various temperatures for 2hr in muffle oven under air atmosphere.

433

2.3 Characterization of precursor and resulted powders XRD characterization was carried out using a Riguku D/Max-RB X-ray diffractometer with Cu Kcx radiation at 40KV• and scanning speed of 5~ from 10~ to 75 ~ for crystalline phase detection. IR spectra of products calcined at different temperatures were performed on a Nicolet Impact 410 FT-IR spectrometer. The thermal gravimetry and thermal differential analytical analysis (TG-DTA) were performed with a Perkin-Elmer TGS-2 and DTA 1700 instrument. The TG profiles were recorded and treated by a Perkin-Elmer 3600 working station at a programmed temperature velocity of 10K/min in air with the flow rate of 30ml/min. The specific surface areas were obtained from the nitrogen adsorption and desorption isotherms at 77K, using a BET apparatus ( Couker 100CX, USA ).

3. RESULTS AND DISCUSSION

3.1 Formation of perovskite phase 3.1.1 Formation mechanism of LSCO prepared by EDTA complexing method Fig. 1 shows the TG-DTA profiles of the precursor prepared by EDTA complexing method. Weight loss took place mainly between 240-630~ and it seemed to be completed around 650~ There was an endothermic peak around 180~ in the DTA curve which was assigned to the elimination of hydration water. The broad exothermic peak with several shoulder peaks between 300-460~ was attributed to the burn-out of nitrate, ionized EDTA and complexed EDTA. Fig. 2 shows the TG profiles of samples prefired at different temperatures for 2hr. There were still some weight losses of the samples fired below 800~ The main weight losses took place around 600-730~ The weight losses end around 760800~ No more weight loss was observed of the sample fired at 800~ for 2hr. The IR results of sample fired at different temperatures for 2hr are shown in Fig. 3. The peak around 1450crnI was

100

80

~ 60 ~

4o 20

TG

0 0

200

400 600 Temperature (oC)

800

Figure 1 TG and DTA curves of precursor prepared by EDTA complexing method

434

5% q)

o r o

"T,

9

0

i

I

,I

'

200

.

,

I.

.

.

,

,,I

400 600 Temperature (oC)

,,

,

,

I

i

SO0

Figure 2. TG curves of samples calcination at different temperatures for prepared by EDTA complexing method a: 280~ b: 400~ c: 600~ d: 800~

50

40

a

% T

30

b

20

10

2000

e

"1500

1000

500

Wavenumbers (cm ~)

Figure 3.IR profiles of samples fired at various temperatures for 2hr prepared by EDTA complexing method, a: 280~ b: 400~ c: 600~ d: 800~ e" 900~

435 assigned to the C-O stretching vibrations of ionized carboxylate, the peaks around 1383crn1 and 853crn"1 to ionic nitrate, peaks around 590cm"~ and 420crn~ were assigned to the Co-O stretching vibration of perovskite LSCO in which 02- ions displace along the Co-O-Co line and perpendicularly to the Co-O-Co line respectively. From the IR, it shows that organic compounds were removed almost completely without any trace of C-H vibration peaks observed for the sample fired at 280~ for 2hr, but the ionized carboxylate appeared and ionic nitrate was observed. With the rise of firing temperature, the intensity of CO32" and NO3 peaks became weak. When the precursor was fired at 800~ for 2hr, NO3- and CO32 peaks all disappeared, the only peaks observed were around 420crn~ and 590crn-~. It indicated that purephase LSCO might have formed then. The results of XRD analysis are shown in Fig. 4. When the precursor was fired at 280~ for 2hr, the perovskite structure was already formed with some second phases. SrCO3 phase was still observed for the samples fired at 400~ or 600~ for 2hr. Pure-phase perovskite was acquired for the sample fired at 800~ for 2hr, which agreed with the results of TG and IR well. So we can see that the EDTA complexing method was similar to citric complexing method[12], where in the processes of forming perovskite LSCO, SrCO3 formed and firing temperature higher than 800~ was needed to get rid of it. The weight losses around 600-760~ of the TG in Fig. 2 were assigned to the decomposition of SrCO3.

09

e

f=

q) >

9,....i

jb~ ,.~,~,~,.~.,,~.~.~ i

10

,I

20

a ''

,

~

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-

.

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,

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~

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,

40

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50

.

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,

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I

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2-theta

Figure 4. XRD profiles of sample fired at various temperatures for 2hr, a: precursor, b: 280~ c: 400~ d: 600~ e: 800~ ( by EDTA complexing method )

436

3.1.2

Formation of LSCO prepared by metal-EDTA-cellulose complexing method

One ticklish problem of EDTA or citric complexing method is that the gel spills out from the container during pyrolysis because the enormous gas produced from decomposition of nitrate cannot be evacuated in time and make the volume of gel expanded tremendous. When cellulose was introduced, this problem was eliminated for the texture of cellulose provided effective evolution passage for the produced gas. The minimum temperature needed for the completion of thermal oxidation was also lowered when cellulose was introduced. The thermal oxidation of precursor went to completion around 450~ (Fig. 5) compared with 630~ for EDTA complexing method and 570~ for activated cellulose. It was obvious that there were some kind of interaction between cellulose and EDTA which led to lower minimum calcination temperature needed for completion oxidation of precursor. The XRD results (Fig. 6) indicates that when the precursor was fired at 180~ or 350~ for 2hr the structure of the products were still amphormous. The perovskite structure was formed with some second phases of SrCO3 and Co304 when the precursor was fired at 450~ for 2hr, the pure phase LSCO was formed between 540~ and 620~ and no any other phase appeared in the XRD profiles except of perovskite when the precursor was fired at 620~ for 2hr; this temperature was significantly lower than other methods. In order to investigate the influence of sequenceof materials added in the preparation to the properties of ultimate powder acquired. We tried three sequences.These were a: salt solution added to the cellulose first then added in EDTA solution, b: EDTA was premixed with salt solution first then added to the cellulose and c: EDTA solution was added to cellulose first then added in salt nitrate solution, XRD results showed that the first methodwas most effective to acquire pure phase LSCO. when all the precursors fired at 540~ for 2hr, pure phase was acquired for the sequence a, but for the other two methods, trace of SrCO3 was still observed in the XRD profile (not shown). So the sequence a is recommended.

100 C

80

o

60 40 20 .

0

,

.

,_

I

200

....

,

.

,

!

.

,

,

400

,.

600

,

,_.

I

800

Temperature (oC)

Figure 5. TG curves of a: precursor prepared by metal-EDTA-cellulose method, b: precursor prepared by metal-citric-cellulose method and c: activated cellulose

437

.!.=1

e

op,,~

b

,

10

I

20

,

I

30

,

,

I

,

40

I

50

,

I

60

,

,

....... I

,,

70

2-theta Figure 6. XRD profiles of samples fired at various temperatures for 2hr, a: 280~ c: 450~ d: 540~ e: 620~ (prepared by EDTA-cellulose method)

b: 360~

3.1.3 Formation of LSCO prepared by metal-citric-cellulose complexing method It was interesting to observe that when the precursor prepared from metal-citriccellulose method was being dried in a stirred air oven at 80~ self-ignition occurred. A black product with the morphology of the precursor was acquired which was very flocculent and really easy to pulverized. XRD result indicates that the almost pure phase perovskite was already formed (Fig. 7). It is the lowest temperature for the oxidation of precursor to initiate reported. We attributed it to that the mutual action between cellulose and citric lowered the initial oxidation temperature of the compound, so exothermic redox decomposition of metal nitrate easily ignited the cellulose-citric-metal complexing compound. The self-combustion process was auto-catalytic and self-propagating in nature which resulted in instant high temperatures (higher than 1000~ that helped to form perovskite structure. TG curve (Fig. 5) shows that the oxidation of the precursor went to completion as low as 370~ much lower than other methods. XRD results shows that pure phase perovskite LSCO was acquired when the precursor fired at 540~ for 2hr (Fig. 7). It was reported that it was very difficuk to acquire pure phase LSCO by citric complexing method, even when fired at 850~ for several hours. The resulting powder was composited of many phases[12]. The low minimum calcination temperature needed to acquire pure-phase LSCO by metal-citric-cellulose complexing method was possibly due to the easy evolution of produced gas preventing the formation of SrCO3 which was the main second phase of the sample prepared by citric complexing method.

438

.b r,,O

b i

10

I

20

,

i

30

,,

I

J

..... I

4O

50

,

I

60

~

I

7O

2-theta Figure 7. XRD profiles for the sample fired at different temperatures for 2hr, a: 80~ b: 180~ c: 360~ d: 450~ e: 540~ ( by metal-citric-cellulose method) 3.2 Specific area determination Fig. 8 shows the specific surface area of LSCO powders prepared by the three methods and calcined at different temperatures. For the EDTA complexing method the BET areas were around 10m2/g when the firing temperatures were lower than 800~ A surface area of 5.7m2/g was obtained of the powder fired at 900~ for 2hr. For the EDTA-ceUulose-metal complexing method when the precursor fired at 350~ for 2hr, a surface area of 25.7m2/g was investigated, with a little loss to 21.2m2/g when the precursor was fired at 540~ for 2hr. It was significantly lowered to 12.7m2/g when the sample which was prefired at 540~ was further fired at 620~ for 2hr. By metal-citric-cellulose method, the samples which were fired at the temperatures lower than 540~ had a surface area of about 22m2/g with modest BET area gain with the rise of firing temperature. We attributed it to that the remnant trace of carbon possibly blocked the micropores of the result powder. With the rise of calcination temperature the carbon was gradually burn out and the pores were exposed. It was also interesting to observe that when the sample prefired at 540~ for 2hr was further fired at 620~ for 2hr, the specific area was lowered to 12.1m2/g. It appeared that the calcination of resuk powder LSCO took place near 600~ by metal-EDTA (citric)-cellulose complexing method. In order to acquire pure LSCO powder with high surface area , a long-time calcination below 600~ is more recommendede than calcination at a temperature higher than 600~ for a relative short time.

439

30 25

~

20

~5

0

0

.

I

200

..

I

,

I

.

400 600 Firing temperature(oC)

9

I

800

,

......

1,000

Figure 8. Specific surface area of LSCO powders prepared at different temperatures from o: metal-citric-cellulose method. 9 EDTA 9 complexing method and O: metal-EDTA-cellulose complexing method. ACKNOWLEDGMENTS Acknowledgement is made to the Donors of the Natural Science Foundation of China, Chinese Academy of Science and State Science and Technology Commission. REFERENCES .

2. 3 4 .

6. 7. 8. 9. 10. 11. 12.

T. Nakamura, M. Misono, T. Uchijima etc., Nippon Kagaku Kaishi, (1980) 1679 J. G. McCarty and H. Wise, Catalysis Today, 8 (1990) 231. Y. Teraoka, H. Fukuda, S. Kagawa, and N. Yamazoe, Chem. Lett., (1990) 1. C. H. Chen, H. J. M.Bouwmeester, R. H. E van Doom etc., Solid State Ionics, 98 (1997) 7. Y. Teraoka, H. Zhang, S. Furakawa and N. Yamazoe, Chern.Lett., (1985) 1743. N. Itoh, T. Kato, K. Uchida etc., J. Membr. Sci., 92 (1994) 239. T. Kudo, Catalysis Today, 8(1990)213. K. Tabata and M. Misono, Catalysis Today, 8 (1990) 249. Y. Muto and F. Mizukami, Preparation of Catalysis VI, P627-636. S. Nakayama, M. Sakammoto and Y. Sodaoka, Chem. Lett., (1992)2145. L.V. Solov'eva, I. A. Bashmakov, V. P. Novikovetc and F. N. Kaputskii, Inorganic Material 1995(31), 1416. H. Zhang, Y. Teraoka, M. Nagao, Chem. Lett., (1987) 665.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. DeMon et al., editors.

441

An important principle for catalyst preparation--spontaneous monolayer dispersion o f solid c o m p o u n d s onto surfaces o f supports Youchang Xie, Yuexiang Zhu, Biying Zhao and Youqi Tang Laboratory for Structure of Matter, Institute of Physical Chemistry, Peking University, Beijing 100871, E-mail: [email protected]

Spontaneous monolayer dispersion of oxides and salts onto supports is quite a widespread phenomenon and an important principle for the preparation and design of heterogeneous catalysts. This paper summarizes the main results developed in our laboratory. It contains the following points: (1) Preparation of highly active catalysts by dispersing active components on supports as a monolayer. (2) Thermal dispersion of active components onto supports as monolayer can be a simple method to make some catalysts. (3) Monolayer-dispersed oxides or salts play a role as surface modifiers. (4) Monolayer-dispersed oxides or salts on supports are better precursors of supported metal catalysts. (5) Regeneration of supported metal catalysts based on spontaneous monolayer dispersion of oxide and salt of the metals. (6) Hindering sinter of supports with monolayer-dispersed oxide or salt to make catalysts with very high surface area. (7) Modification of zeolites by spontaneous dispersion of oxides or salts to zeolites.

1. INTRODUCTION We have found that many solid compounds such as oxides or salts can disperse spontaneously onto surfaces of supports to form a monolayer or submonolayer[1-5]. This is quite a wide spread phenomenon. For a compound with not too high melting point, its monolayer dispersion on a support can be done by heating the compound mixed with the support at a suitable temperature well below its melting point. For compounds with high melting points, although their monolayer dispersion on supports can not be done by just heating the mixtures, it can be done by impregnation and thermo-decomposition or other methods. This monolayer dispersion phenomenon has been confirmed by XRD, EXAFS, XPS, ISS, SIMS, TEM, HEED, UV, IR, LRS, Mt~ssbauer Spectra, STM, DTA, TPR, adsorption and catalysis, etc [1,4, 8, 9]. The reason for the spontaneous monolayer dispersion is that the dispersion would make Gibbs free energy of the whole system decrease. Monolayer dispersion of a compound onto a support always makes entropy of the whole system increase considerably; while the strength of surface bonds formed between the atoms, ions or molecules of the monolayer-dispersed compound and the surface of the support are usually comparable to that of the bonds in the undispersed compound, so the change of energy and enthalpy of the whole system would be not much in the monolayer dispersion process. Since Gibbs free energy includes both enthalpy and

442 entropy, the change of free energy for a monolayer dispersion process would be AG = AH - TAS < 0, the monolayer dispersion is a thermodynamically spontaneous process. Spontaneous monolayer dispersion as a widespread phenomenon and a principle is very useful for the preparation and design of heterogeneous catalysts. The following sections will illustrate this point. 2. A P P L I C A T I O N S 2.1. Preparation of highly active catalysts by dispersing active components on supports as a monolayer Dispersing an active component into monolayer-dispersed state is an important measure to enhance the activity of a catalyst. A striking example is the highly active catalysts for polymerization of ethylene and propylene. The main active component of the early ZieglerNatta catalyst, invented in 1950s, was TIC13 crystalline. A new generation catalyst TiC13/MgCI2, for ethylene and propylene polymerization with much higher activity was developed in 1970s [10]. We have shown that the TIC13 supported on MgCI2 is in a monolayer state [ 11 ]. Many supported catalysts are metal oxides or salts as active components on supports in monolayer-dispersed state. The optimal contents of the active components in these catalysts are often near their monolayer dispersion capacities. For example, nickel sulfate can disperse on the surface of alumina as a monolayer and NiSO4/7-A1203 is a good catalyst for oligomerization of olefins [ 12]. Figure 1 shows the catalytic activity of oligermerization of olefins in a liquefied petroleum gas as a function of NiSO4 loading in the NiSO4/~'-AI203 catalysts. There is a maximum activity at a loading of 0.27g NiSO4/g 7-A1203. An XRD quantitative phase analysis for the NiSO4/7-A1203 system also shows a monolayer dispersion capacity at the same loading, 0.27 g NiSO4/g 7-A1203, below which no crystalline phase of NiSO4 can be observed while above which crystalline NiSO4 increases linearly with NiSO4 loading[12]. Cuprous 15

0.15 ~ , 9

55 so

[I.10g

/

.~ 10 r~

~D

'~, 45

Z .9 -0.05 -=

40

/

g

ra~

i" rj

35 ,

0

i

0.10

9

i

Ill

0.20

!

93

0.30

0.40

0.:~0

NiSO 4 content (g/g ,/-A1203)

Figure 1. Liquid yield of oligermerization of olefins in a LPG and XRD phase analysis result for NiSO4/7-A1203 catalysts.

I

0

I,

I

r

0.1; 0.20 0.30 CuCI content (g/g 3,-A1~O3)

Figure 2. Activity of NO decomposition for CuC1/,I-AI2O3catalyst as a function of CuC1 loading.

443 chloride can disperse on 7-A1203 to form a monolayer dispersion catalyst which has NO decomposition activity [ 13]. Figure 2 shows that the CuC1/7-A1203 catalysts has maximum NO decomposition activity at a loading of 0.21 g CuC1 / g 7-A1203 which is close to the monolayer dispersion capacity, 0.23g CuC1/g 7-A1203 obtained by XRD phase analysis. Similar results have also been observed for other supported catalysts with monolayerdispersed active components, such as CuC12/q,-A1203 catalyst for oxychlorination of ethylene [14], HgC12/active carbon catalyst for synthesis of vinyl chloride from acetylene and HC1 [ 1], ZnAc2/active carbon catalyst for synthesis of vinyl acetate from acetylene and acetic acid [15] and A12(SO4)3 catalyst for oligermerization ofbutene [16], etc. It is well known that MoO3-Co304/y-A1203, MoO3-NiO/y-A1203 and WO3-NiO/7-A1203 are precursors of commercial catalysts for HDS and HDN. The supported metal oxides, MOO3, WO3, Co304, and NiO in the precursors are also in monolayer dispersion state and their contents are near or lower than their monolayer dispersion capacities. 2.2. Thermal dispersion of active components onto supports as monolayer can be a simple method to make some catalysts

It has been found that a HDS catalyst obtained by heating an extrudate of mechanical mixture of MOO3, Co(NO3)-6H20 and y-A1203 at 450~ for several hours has the same activity as a catalyst with similar composition made by impregnation and therrno-decomposition method. The reason is that both the heating mixture and impregnation method can obtain similarly monolayer-dispersed MoO3 and CoO on the surface of y-A1203. Similar results have been observed in the preparation of other supported catalysts with monolayer-dispersed active components. An ethylene oxychlorination catalyst, CuC12/,/-A1203 [I] made by heating a mixture of CuC12 and 7-A1203 at 350~ for several hours has nearly the same activity as the one with the same composition made by impregnation method. A ZnAc2/active carbon catalyst for synthesis of vinyl acetate from acetylene and acetic acid can be obtained by heating a mixture of ZnAc2 and active carbon at 150~ for several hours [15]. It has almost the same activity as the one made by impregnation method. In short, a highly active monolayer-dispersed catalyst can be b" prepared by calcining a mechanical mixture of the active component and the support at an appropriate temperature which is well below the a melting point of the active component, if the a I I 20 25 30 20~ active component is a salt or oxide with not too high melting point. It is interesting to note that Figure 3. XRD patterns of HgC12 on even at ambient temperature an active component active carbon with specific surface can disperse onto the surface of a support to form 1000m2/g a monolayer, if it is a low-melting-point salt or (a) Active carbon, (b) Fresh mixture of oxide. For example, HgC12 has melting point at 0.13g HgC12/g active carbon, (b') Sample 298~ When HgC12 is mixed with active carbon b after holding at room temperature for 1 and held at ambient temperature for several hours, hour, (b") Sample b after holding at room it can disperse onto the surface of the active temperature for 4 hours

444 carbon and crystalline phase of HgC12 can disappear as shown by XRD pattems in figure 3. Then a highly active catalyst, HgClz/active carbon, for synthesis of vinyl chloride from acetylene can be obtained. It has activity similar to the catalyst obtained by impregnation method. Hydrated nitrates and chlorides, which are often used for preparation of catalysts, are lowmelting-point salts. They can also disperse spontaneously at ambient or mild temperatures onto the surfaces of supports as monolayer [1-3]. Instead of wet method (impregnation), a dry method can be used to disperse low-melting-point salts or oxides onto the supports for the preparation of catalysts. The dry method might have advantage over conventional wet methods in some cases. 2.3. M o n o l a y e r - d i s p e r s e d oxides or salts play a role as s u r f a c e m o d i f i e r s

Oxides or salts are often used as promoter or additive in supported catalysts. Their function in various catalysts can be very widely and is too complicated to have been adequately elucidated so far. However, we have found that they often disperse on the surfaces of supports as monolayer or submonolayer and play a role of surface modifier. For example, adding a small amount of rare earth oxides such as La203 to the Ni/A1203 catalysts for methanation of CO and CO2 or methane reforming with steam or CO2 can significantly improve their activity and thermal stability. We have proved that the La203 on A1203 surface is in a monolayer dispersion state, and the surface property of the support is modified by the monolayer-dispersed La203 [17]. Figure 4 shows the effect of La203 on XRD peak 111 of Ni metal in the catalysts. Obviously, the peak become much broader, i.e. the size of the nickel crystallites decreases markedly as the addition of La203 to the catalysts [17]. The catalysts was prepared by impregnating 7-A1203 with Ni(NO3)2 and La(NO3)3 solutions followed by calcination at 450~ for one hour and reduction with a hydrogen. After the calcination, Ni(NO3)2 and La(NO3)3 changed to monolayer-dispersed NiO b and La203 on the surface of the support. After the reduction, NiO changed to nickel metal crystallites while La203 remained unchanged. I I I In the presence of monolayer-dispersed La203 55 60 65 20~ on the surface of ~/-A1203, NiO would be reduced to nickel crystallites of smaller size Figure 41 Effect of adding La203 on XRD and higher thermal stability. We can call this as peak 111 of supported Ni catalysts monolayer-dispersed modifier effect. Similar (a) 0.10g Ni/g y-A1203, (b) 0.10g Ni/0.05g effect have also been observed for the LazO3/gNiO/gq,-A1203, (c) 0.10g Ni/0.30g additives La203 on Pt/7-A1203 and MgO on LazO3/g 7-A1203 Ni/~/-A1203 [ 18, 19].

445 2.4. Monolayer-dispersed oxides or salts on supports are better precursors of supported metal catalysts

Contrary to oxides and salts, metals can not disperse onto surface of common supports as monolayer owing to the weak bonding between metals and the supports. However supported metal catalysts are often made by reducing their precursors which are oxides or salts on supports. If a metal oxide or salt on a support is in monolayer or submonolayer state (i.e. the loading of the oxide or salt on the support is equal to or lower than its monolayer dispersion capacity), after reduction, the catalyst will have metal crystallites smaller than that from reduction of the oxide or salt in crystalline state. For example, figure 5 shows the XRD peak 111 of Nickel of a supported nickel catalyst for methanation obtained by reducing a precursor NiO/y-A1203 with NiO loading below its monolayer dispersion capacity(Figure 5a) is much broader (i.e. nickel crystallites much smaller) than that above the monolayer dispersion capacity(Figure 5b). The result is also consistent with electron microscopy observation. Similar results have also been observed for the CuO/7-A1203 system. 0

__/k

3 o 4 r~ O

5 6

I

43

I

44

, ,,I

45

,I,

46

I

47

'20 o

Figure 5. XRD peak 111 of Ni in catalysts obtained by reducing different precursors (a) 0.15g NiO/g y-nl203 , (b) 0.60g NiO/g y-n1203. The monolayer dispersion capacity is 0.25g NiO/ g y-AI203

,

,

I

I

I

300

400

500

I

600 t(~

Figure 6. The TG and DTG curves recorded in a temperature-programmed reduction for a sample of 0.70g NiO/g y-A1203 (a) TG curve, (b) DTG curve

The other reduction properties such as reducing rate and temperature for starting reduction of monolayer-dispersed oxides or salts are also different from those of crystalline one. For example, it has been fotmd that the NiO or CuO supported on 7-A1203 in monolayer state is much more difficult to be reduced than that in crystalline state. Figure 6 shows the thermogravimetry (TG) and differential thermogravimetry (DTG) curves recorded in temperature-programmed reduction of a sample containing 0.70g NiO/g 7-A1203. This sample contains both monolayer-dispersed phase and crystalline phase of NiO. They behave differently during the process of reduction. The weight loss in the range 300-390~ on the TG curve and the distinct peak at 337~ on the DTG curve are due to the reduction of crystalline NiO. The second weight loss shown on the TG curve in the high temperature range 420-550~ and the broad peak in this range on the DTG curve can be ascribed to the reduction of the monolayer-dispersed portion of NiO.

446 2.5. Regeneration of supported metal catalysts based on spontaneous monolayer dispersion of oxide and salt of the metal Metal particles in supported metal catalysts may sinter during use at elevated temperature. It would cause deactivation of the catalysts. The deactivated catalysts can be regenerated by reoxidizing the metal into oxide or salt, which will spontaneously disperse onto the support as monolayer, then reducing the monolayer dispersed oxide to get highly dispersed metal particles on the surfaces of the supports. It has been reported by Yao et al. [20] that for the catalyst Pt/AI203, redispersion of Pt metal particles on the support can be effected by heating the catalyst in an air atmosphere at 500~ and then by reducing in H2. Most likely the Pt metal particles is oxidized to PtO2 and the latter is spontaneously dispersed onto the surface of the support as monolayer. Then the reduction of the monolayer-dispersed PtO2 leads to the formation of highly dispersed particles of Pt metal. Similar redispersion phenomenon was also reported for a catalyst k/qt-A1203 [21,22]. Redispersion of metal particles by an oxidationreduction cycle would be an effective way for regeneration of deactivated catalysts if the deactivation is owing to the sinter of supported metal particles. In addition to oxygen, chlorine can also be used as oxidation agent. It can oxidize the metals to monolayer-dispersed metal chlorides, which can also be reduced to get highly dispersed metal particles.

2.6. Hindering sinter of supports with monolayer-dispersed oxide or salt to make catalysts with very high surface area Oxides are often used as catalyst supports or catalysts. In general, oxide supports with higher specific surface are better than that with lower specific surfaces. For the preparation of a supported catalyst, usually an oxide support is made by thermo-decomposition of its hydroxide at first, then the active component is put on the surface of the oxide by impregnation. However, it has been found [23,24] that direct thermo-decomposition of a hydroxide mixed with an active component, oxide or salt, often can obtain a supported catalyst with surface area much higher than the oxide support obtained by just heating its hydroxide at the same temperature without the presence of the oxide or salt. Because the thermo-decomposition of the hydroxide and monolayer dispersion of the oxide or salt take place simultaneously and the monolayerdispersed oxide or salt on the fresh surface of the forming oxide can hinder the sinter of the support. For example, Table 1 shows that MoO3/ZrO2 catalysts, obtained by impregnating Zr(OH)4 with 0NII-'I4)6Mo7024solution then drying and heating them at 550~ or 750~ respectively, have surface area much higher than the ZrO2 support obtained by just heating Zr(OH)4 at the same temperature without the presence of ('NH4)6Mo7024. At a certain calcination temperature, there is a suitable loading of an active component on a catalyst which has a maximum specific surface area. The catalyst with 0.26g MoO3/g ZrO2 calcinated at 550~ has a highest surface area, 224 m2/g, which is four times more than the surface area (52 m2/g) of the ZrO2 obtained by heating the pure Zr(OH)4 at the same temperature. The catalyst with 0.16g MoO3/g ZrO2 calcinated at 750~ has a maximum surface area, 101m2/g, which is even ten times more than that of the corresponding ZrO2. The XRD, XPS and LRS studies has also showed that the best loading of an active component on a support to get a highest surface area for the catalyst is close to the monolayer dispersion capacity of the system [23,24]. Similar effects has also been observed for the systems such as WO3/ZrO2, CuO/ZrO2, MoO3/TiO2, WoOa/TiO2, NiO/ZrO2, Fe203/ZrO2, Fe2(SO4)a/ZrO2, NiSOa/TiO2, Ni/TiO2 and V2Os/TiO2 etc as shown in Table 1.

447

Table 1 Specific surface areas of supports and supported catalysts obtained by heating hydroxides with addtives at various conditions Content of Calcination temp. Surface area Calcination temp. active component (~ ) (m2/g) (~ ) (g/g support) 0 550 52 750 MoO3/ZrO2 0.03 87 0.09 140 0.16 186 0.26 224 0.42 159 0.51 104 0 500 66 800 WO3/ZrO2 0.05 76 0.15 175 0.30 225 0.40 227 0 500 58 650 CuO/ZrO2 0.04 76 0.06 139 0.09 147 0.11 137 0.14 115 0.14 500 80 NiO/ZrO2 0.10 500 87 Fe203/ZrO2 500 143 Fe2(SO4)3/ZrO2 0.19 0 500 80 600 MoO3/TiO2 0.10 94 0.20 127 0.25 140 0.30 135 0.32 127 0 600 62 800 WO3/TiO2 0.10 70 0.15 72 0.20 87 0.25 96 0.30 84 0.11 500 112 NiSO4/TiO2 0.09 125 NiO/TiO2 0.05 136 V2Os/TiO2

Catalysts

Surface area (m2/g) 10 52 96 101 97 78 47 10 44 73 57 50 43 45 46 58 48

62 74 104 72 70 65 5 27 17 14

448 2.7. Modification of zeolites by spontaneous dispersion of oxides or salts to the surfaces of zeolites Many oxides or salts can also disperse spontaneously to the internal and external surface of zeolites [ 1,4,]. This point has been verified by the dispersion of many oxides and salts such as Sb203, B203, MgO, La203, ZnO, CuO, LaOC1, LiC1, NaC1, KC1, CuC1, CuC12, etc., to the zeolites such as NaY, NaX, 4A, 5A, ZSM-5, mordenite, etc., by heating the mixtures of the oxides or salts and the zeolites at suitable temperatures or by impregnation method [ 1,4,25-27]. Catalytic properties of zeolites can be modified by the dispersion of oxides or salts to the surfaces, channels and pores of zeolites. For example, it has been reported [26-29] that HZSM-5 modified with certain amount of MgO, B203 and Sb203 respectively can significantly improve its catalytic selectivity for making paraxylene from toluene and methanol. Figure 7 shows the paraxylene selectivity for MgO/HZSM-5 catalyst as a function of MgO loading on the catalyst. The 100 para-selectivity reaches more than 90 %, when the | 9 8 loading of MgO in the catalysts close to 0.12g 80 MgO/g HZSM-5. The dispersion capacity of MgO on HZSM-5 obtained by XRD and XPS "7, 60 analysis is also 0.12g MgO/g HZSM-5. Similar r ~D results are also observed for the B203/HZSM-5 ~ 4o and Sb203/HZSM-5 catalysts. The dispersion ~ 20' capacities of BEO3 and Sb203 on HZSM-5 are 0.11g B203/g HZSM-5 and 0.25g SbaO3/g HZSM-5 respectively. When the loadings of the o0 o.lo o.A 0. o 0.4'0 oxides on the catalysts are close to these two MgO content (g/gHZSM-5) values, the catalysts also have para-selectivities more than 90 %. The significant improvement of Figure 7. Para-selectivity in toluene paraxylene selectivity for the HZSM-5 catalysts methylation with methanol as a function modified by the oxides can be ascribed to that the of MgO loading on HZSM-5 catalyst. dispersed oxides make the channels of the zeolite The catalysts are prepared by become narrow, leading to shape-selective impregnating HZSM-5 with Mg(NO3)2 catalysis for producing paraxylene. and calcinating at 500~ for 3 hours.

3.

CONCLUDING REMARKS

Spontaneous monolayer dispersion is a basic principle for the preparation and design of heterogeneous catalysts. Its applications to catalysis are not limited to the above points. It can be also used in the related field of material science. Some fundamental respect such as the surface structure of monolayer dispersion state and kinetics of the dispersion are need to further study. The work on these subjects are continuing in our laboratory. ACKNOWLEDGMENT The authors are grateful to the colleagues and graduate students of surface structure group in Physical Chemistry Institute at Peking University for their long term contributions in this work. The project is supported by the National Science Foundation of China.

449 REFERENCES

1. Y. Xie and Y. Tang, Adv. in Catal., Vol. 37 (1990) 1. 2. Y. Xie, L. Gui, Y. Liu, B. Zhao, N. Yang, Y. Zhang, Q. Guo, L. Duan, H. Huang, X. Cai, and Y. Tang., Proc. 8th. Int. Congr. Catal., Berlin, 1984, G. Ertl Ed., Vol.5, 147. 3. Y. Xie, N. Yang, Y. Liu and Y. Tang, Scientia Sinica (series B), Vol. 26 (1983), 337. 4. Y. Xie, X. Wang and Y. Tang, Study in Surface Science and Catalysis, 112, Spillover and Migration of Surface Species on Catalysts, Proceedings of 4th International conference on Spillover, Dailian, China, 1997. Can Li and Qin Xin ed., 49. 5. Y. Tang, Y. Xie and L. Gui, Advances in Natural Science (China), 4 (1994) 642. 6. Y. Liu, Y. Xie, J. Ming, J. Liu and Y. Tang, J. Catal. (China), 3 (1982) 262. 7. Y. Xie, L. Gui, Y. Liu, Y. Zhang, B. Zhao, N. Yang, Q. Guo, L. Duan, H. Huang, X. Cai and Y. Tang, Adsorption and Catalysis on Oxide Surface, M. Che and G. C. Bond Eds., 1985, Elsevier, Amsterdam, 139. 8. Y. Xie, X. Xu, B. Zhao and Y. Tang, Catalysis Letter, 13 (1992) 239. 9. X. Cai, K. Lu, Y. Xie, X. Xu, J. Dong and Y. Tang, Proc. 7th Int. X-ray Absorption Fine Structure, Kobe, Jpn. J. Phys. Vol. 32 (1993) Supp. 32-2, 505. 10. J. J. Stevens, Hydrocarbon Processing, 49 (1970) No. 11, 179. 11. Y. Xie, L. Gui, W. Liu, N. Bu and Y. Tang, Scientia Sinica (Series B), 22 (1979) 1045. 12. D. Wang, J. Wang, X. Tao Y. Zhang and Y. Xie, Acta Petrolei Sinca (Petroleum Processing Section), 9 (1993) 48. 13. Y. Zhu, Y. Xie and Y. Tang, 7th Japan-China-USA Symposium on Catalysis, Tokyo, Japan,1995, 107. 14. X. Cai, Y. Xie, L. Gui and Y. Tang, Petrochem. Technol. (China), 21 (1992) 222. 15. S. Chen, G. Li, Y. Wang and B. Yu, J. Catal. (China), 7 (1986) 155. 16. D. Wang, W. Zhang, X. Tao, T. Yang and T. Cai, J. Mol. Catal. (China), 8 (444) 443. 17. Y. Xie, M. Qian and Y. Tang, Sciemia Sinica, (Series. B ) 27 (1984) 549. 18. J. Yang, and W. E. Swartz, Spectros. Lett. 17 (1984) 331. 19. Y. Zhang, Y. Xie, N. Xiao, W. Han and Y. Tang, Petrochem.Technol. (China) 14 (1985) 141. 20. H. C. Yao, M. Sieg and H. K. Plummer, J. Catal. 59 (1979) 365. 21. R. M. J. Fiedorow, B. S. Chahar and S.E. Wanke, J. Catal. 51 (1978) 193. 22. G. B. Mcvicker, R. L. Garten and R.T.K. Baker, J. Catal. 54 (1978) 129. 23. B. Zhao, X. Xu, H. Ma, J. Gao, R. Wang D. Sun and Y. Tang, Acta Physico Chimica Sinica, 9(1993) 8. 24. B. Zhao, X. Xu, H. Ma, D. Sun and J. Gao, Catal. Lett., 45 (1997) 237 25. C. B. Wang, Y. C. Xie, Z. W. Liu and T.Q. Tang, Proc.12th Int. Zeolite Conference, Baltimore,1998. to be published. 26. X. Long, B. Zhao and Y. Xie, Acta Physico-Chimica Sinica, 13 (1997) 302. 27. X. Long, B. Zhao, H. Huang and Y. Xie, Acta Petrolei Sinica (Petroleum Processing Section), Oct. 1997, 66. 28. W. W. Kaeding, C. C. Chu, L. B. Young et al, J. Catal. 67 (1987) 159. 29. C. C. Chu, US Patent No. 4250345 (1981).

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

C a t a l y s t g r a n u l e p r o d u c t i o n in a s p o u t e d creative catalyst design

451

bed:

Opportunities

for

A.J.Kamphuis, J.R.Walls Department of Chemical Engineering, University of Bradford, BD7 1DP, United Kingdom

Bradford,

ABSTRACT

The formation of agglomerates (pellets, granules) is an important operation in the production of catalysts. In this paper the results obtained when using a spouted bed granulator with a powder feed to granulate catalysts are reported. The granulation method was found to be suitable for the production of catalysts and similar materials, producing catalysts with mechanical and physical properties comparable to those of industrial catalysts. Because of the way catalyst agglomerates are build up in this granulator, it is expected that this method is especially suitable for the production of layered catalysts with different catalytic functions or physical or mechanical properties in each layer.

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

One of the final steps in catalyst production, the agglomeration of catalytic agent, support, inerts and binders into an industrially usable catalyst, is not often considered in academic studies. The main reason for this is the proprietary nature of much of the know-how. Consequently, little information on the production and properties of catalyst granules (also known as pellets) is available in the open literature. Many non-industrial catalytic studies therefore stop at the stage of powder production. Larger scale reactor design studies in realistic conditions are often only done with off-the-shelf catalysts.

2. S P O U T E D B E D G R A N U L A T O R

In order to be able to produce catalyst granules to a wide range of specifications we have developed a new method of catalyst agglomeration. This allows the granulation of relatively small quantities of powders (typically 1 kg) to form granules with properties comparable to industrial catalyst, so that their

452 catalytic, physical and mechanical properties can be tested. The agglomeration takes place in a spouted/fluidized bed granulator, sketched in figure 1. Spouted bed granulators are not uncommon in the particle technology literature (Pietsch, 1991) but nearly all require a liquid feed (melt, solution, suspension), ruling out the application of fluidized or spouted bed granulators for the agglomeration of many catalysts. The design of the present gramflator is such that fresh material can be fed as a powder. This makes the spouted/fluidized bed granulation method interesting from a particle technology point of view as well, because there are very few applications of the spouted bed as a granulation device using a direct powder feed reported. A spouted bed can be seen as a packed bed of particles with a strong gas jet in the centre, transporting particles upwards. As soon as they leave the bed the particles form a fountain and are deposited to the top of the bed. Under gravity they slowly move down and towards the centre, until they are entrained once again in the gas jet. This method allows good contact of gases and solid particles and also provides mixing of the solids. A spouted bed is usually employed to handle particles larger than can be dealt with in a fluidized bed. An advantage of using a spouted bed granulator for the formation of catalyst granules is that due to the high shear forces in a spouted bed particle growth only takes place by

Powder

l

Spouted bed section

Powder entrainment section

/

1

Auxiliary Air Spouting Air k

water

Figure 1" Schematic drawing of the spouted bed granulator

453 layering. This means that approximately spherical products are formed with a uniform structure and good strength. The granulation itself is started with small seeds. These can be undersize material from a previous granulation, formed in another way (for instance in a mixer), or an inert core, for instance small alumina beads. These seeds are spouted with air and slowly wetted with water. When they have the right moisture content fresh catalyst or precursor powder is fed into the lower section of the granulator, the powder entrainment section. In this section the powder is entrained in the auxiliary air flow, and transported upward into the spouted bed section, where the actual agglomeration takes place. The design of the base of the spouted bed section is important here, because it has to be permeable to the airborne powder, but it also has to keep the granules in the spouted bed section. The dry powder that is transported by the auxiliary air into the spouted bed of moist granules adheres to the surface of the granules. Due to the circulating action of the spouted bed the powder is compacted on the particles, which grow in size by the continuous addition of powder and get an approximately spherical shape. A low water flowrate is maintained to keep the particles at the right moisture content. The granulation is stopped when the particles have reached the desired size, or when the spouted bed has reached the maximum spoutable height. A typical batch time is approximately 30 minutes. A mathematical model of the granulation has been developed (Kamphuis 1998, Kamphuis and Walls 1998), allowing prediction of the size distribution of the product granules. The following materials have so far been granulated successfully in this granulator: AI(OH)3 to form A1208 catalyst supports Zr(OH)4 to form ZrO~ catalyst supports ZnO in various formulations (as ZnO or by calcination of basic zinc carbonate) NiO on AI(OH)3 to form Ni/A1203 catalysts Lio.gNio.~Coo.502-x (catalyst for oxidative coupling of methane) on Al(OH)3 In all cases a high-alumina cement (Lafarge Ciment Fondu or Secar 71) was used as a binder. This is a common binder in catalysts, used in, among others, steam reforming catalysts, methanation catalysts, methanol synthesis catalysts and ZnO desulfurisation sorbents (Twigg 1989). When the granules come out of the bed they have a paste-like consistency. They need to be kept under water or in a moist atmosphere for at least 12 hours to give the cement time to react, and thus develop its full strength. After this the granules can be dried and are ready for subsequent processing steps such as impregnation, calcination and reduction.

3. P R O P E R T I E S OF THE P R O D U C T GRANULES

Before and during use an industrial catalyst is subjected to a variety of mechanical stresses. For this reason the mechanical properties of a catalyst are

454 Table 1 Properties of the various catalyst and similar gramfles produced in the spouted/fluidized bed granulator (all granules contain 20 % w/w cement, except ZrO~ which contains 35% w/w cement) Material Crushing modulus Porosity Surface area _ (MPa) (%) m2/g ZrO2t 3.5 nd nd ZrO2$ 1.0 57 158 A1203 6.4 49 136 30% NiO/A1203 7.1 53 176 35% Lio.gNio.sCoo.502-~/A1203 9.9 50 130 ZnOt 5.0 61 69 ZnO$ 1.7 61 62 Cement 15.0 19 21 tcured for min 24 hrs in moist environment $dried immediately after production _

of importance. They determine the limitations to the use of the catalyst. Denny (1989) separated the stresses that a catalyst in a packed bed is subjected to into three groups: stresses during processing and transportation, stresses during loading of reactors and stresses during use (static stresses). The first two types of stresses are due to particle movement and to particle impact. They are also important for catalysts used in fluidized bed and other types of reactor with moving particles. The quality of the catalyst with respect to resistance against these stresses can be measured independently, but often only the static mechanical properties are considered. The reasons for this are that various authors (Le Page and Miquel, 1976; Denny, 1989 and Benbow and Bridgwater, 1987) find strong correlations between the measured values for resistance against each of the stresses and that static crushing strengths are easy to measure. No single value for the static crushing strength required of industrial catalysts can be given because it depends on the application and the reactor design. An overview from various literature sources can be found elsewhere (Kamphuis 1998), and from this it can be concluded that catalyst particles with a crushing modulus (force per cross-sectional area) of the order of 1-3 MPa are generally strong enough for industrial use. The granules produced in the spouted bed granulator have crushing moduli as given in table 1. These granules were all calcined at 350 ~ to ensure thermal transformation of the precursors and the reaction products of the cement hydration, AI(OH)3 and Ca3(AI(OH)6)2. It can be seen that all of the produced granules have mechanical strengths high enough for most industrial applications. Different after-treatments or calcination temperatures can alter these values slightly. An important factor in determining the mechanical strength are the conditions in which the granules are kept immediately after production. The hydration reaction of cement is relatively slow, so a marked difference in the properties of fast dried and properly cured granules can be seen,

455 which can mean the difference between rejection and acceptance. This is illustrated by the values of the crushing moduli for ZrO2 and ZnO in table 1. The porosities and surface areas of the granules are also given in table 1. They were determined by N2 adsorption. It can be seen that the porosities of the granules are relatively high, making them suitable catalysts. The surface areas are sufficiently large as well. It is important to note that the surface areas are comparable to those of the ungranulated powders. This shows that little or no blockage of pores by coating of the binder on the primary particles takes place. The limitations of the spouted bed granulator lie in the materials that are used as a granulating and binding aids, in this case water and cement. The catalyst precursors that are used have to be compatible with these materials. An example in which this is not the case is for instance the superconductor YBCO, which is under investigation as an oxidation catalyst (Sun and Lee, 1995; Ovenston et al. 1994). This material can not readily be granulated in the spouted bed granulator, because it decomposes in contact with water.

4. O P P O R T U N I T I E S F O R CREATIVE CATALYST D E S I G N Catalysts with a non uniform radial distribution of active phase, so called egg shell, egg yolk and egg white catalysts, are receiving increasing attention. The reasons for this are that they can improve selectivities by using differences in diffusion, have an improved lifetime or represent a saving on raw material. With the spouted/fluidized bed granulator the production of these layered catalyst granules becomes easier than before. The classical method of producing layered catalyst particles involves sequential steps of impregnation with catalytic agent and site-blocking species. There are several disadvantages to this method. 9It requires prior experimentation to determine the adsorption equilibria of the precursor and additional species (such as site-blocking agents) involved and correlate the final distribution of active phase with the preparation method. It is not always possible to deposit the catalyst precisely at the desired location, or even to measure exactly the radial position of the active phase (Pernicone et al., 1997). ~ For the impregnation to work the precursor has to be soluble in a suitable impregnating liquid. ~ There is a maximum concentration of active phase that can be achieved this way due to the impregnation technique. The production of catalyst with multiple steps in the active phase distribution or with several different active materials is clearly even more complex. It has recently been established that in some circumstances a double step function in a catalyst pellet is optimal (Baratti et al., 1997). In the spouted bed granulator a particle is built up from the seeds outward. Therefore it is straightforward to build up a particle layer by layer, to exactly the specifications required, with each different layer at the desired radial position. The advantages of this are:

4~0 9The method is simpler and more straightforward, and doesn't require prior determination of adsorption equilibria. 9Higher concentrations of the active phase can be obtained, by first producing the required active phase or precursor in a powder form by any method, and incorporating it into the catalyst pellet later. 9The radial positions of the different layers are directly controlled during production and are sharply defined. 9Especially when several different layers are desired the current method enables the production of the particles which might otherwise be impossible. 9With this method it is not only possible to obtain a non-uniform distribution of the active material, but also to determine the radial distribution of surface areas and pore sizes. Due to these advantages it is for instance relatively easy to incorporate a poisoncatching layer into a catalyst particle. Altering the mechanical properties of the catalyst pellets, either by adding an abrasion resistant layer, or by encapsulating a mechanically weak catalyst in a strong shell is also a possibility. An example of an egg shell catalyst made in this granulator is the Li-Ni-Co oxide catalyst from table 1. The reason for making this catalyst as an egg shell catalyst was the limited availability (relatively high price) of the active material. A picture of these granules is shown in figure 2. The core of this catalyst is A1203. In the crushing strength measurements of this catalyst it was observed that breakage only occasionally occurs at the interface between the two layers. This indicates that the strength of the binding between the layers is of the same order of magnitude as the binding strength within the layers.

Figure 2:Lio.gNio.~Coo.502-JA1203 egg shell catalyst

457 CONCLUSIONS A small spouted/fluidized bed granulator was developed and used successfully for the granulation of catalysts and similar materials. The physical and mechanical properties of the catalysts and supports produced in this granulator are good. Due to the way the granules are grown in the spouted bed granulator, it is particularly suitable for the production of layered catalysts, in which different layers can have different functions and properties. This opens up new possibilities for advanced catalyst design.

ACKNOWLEDGEMENT

The financial support for this project from the EPSRC is gratefully acknowledged.

REFERENCES

Baratti, R., Feckova, V. Morbidelli, M. and Varma, A., Ind. Eng. Chem. Res. 36, 3416-3420 (1997) Benbow, J.J. and Bridgwater, J., Chem. Eng. Sci. 42(4), 753-766 (1987) Denny, P.J. 5th Int. Syrup. Agglom., Brighton (UK), 403-411 (1989) Kamphuis, A.J., Spouted bed granulation of catalysts, Thesis University of Bradford, UK (1998) Kamphuis, A.J. and Walls, J.R., The granulation of insoluble powders in a spouted bed, accepted at: World Congress on particle Technology 3, Brighton, UK, July 1998 Le Page, J.F., and Miquel, J., in: Preparation of Catalysts I, Delmon, B. et al. (eds.), Elsevier, Amsterdam, 39-50 (1976) Ovenston, A., Walls, J.R., Allen, A. and Armstrong, W., J. Mater. Sci. 29(5), 1358-1367 (1994) Pernicone N., Cerboni, M. and Prelazzi, G., 3rd Eur. Congress on Catalysis (EuropaCat-3), Krakow 1997, Book of Abstracts Vol. 2, 736 (1997) Pietsch, W., Size enlargement by agglomeration, Wiley (1991) Sun, Y.K. and Lee, W.Y., Kor. J. Chem. Eng. 12(1), 36-38 (1995) Twigg, M.V. Catalyst Handbook, Wolfe Publishing, London, (1989)

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91998 Elsevier ScienceB.V. All rights reservea. Preparation of Catalysts VII B. Dehnon et al., editors.

459

In situ R a m a n evidence for a b a r i u m solid state p h a s e that is active in nitric

oxide decomposition: influence o f preparation p a r a m e t e r s Gerhard Mestl I, Shuibo Xie, Michael, P. Rosynek, and Jack. H. Lunsford* Department of Chemistry Texas A&M University College Station, Texas 77843, USA

Barium on MgO and CaO and, to a lesser extent, pure barium are effective catalyst for NO decomposition. A very distinct catalytic activity is observed on MgO and CaO, depending on the barium loading. Identical Ba loadings on amphoteric A1203 or basic La203 with comparable specific surface areas give inactive materials. Barium loadings on MgO and CaO, exceeding 12 mol%, and pure BaO, result in an unusual steady-state behavior. The N2 formation rate goes through a sharp maximum. This behavior is not observed for comparable Ba loadings on A1203 or La203 or barium loadings below 12 mol% on MgO and CaO. The exact temperature, at which the maximum conversion occurs, depends on the NO concentration: it increases with increasing NO pressure from 630~ in 1% NO/He to 700~ in 4% NO/He over 14 mol% BaJMgO. The rate of reaction also undergoes an unusual transient effect with respect to changes in the NO partial pressure. In situ Raman spectroscopic characterization of 14 mol% Ba/MgO, BaJCaO and pure BaO shows the presence of a coordinatively bound nitro species (Ba-NO2) on the catalyst. Its Raman intensity varies on the same time scale as the rate of N2 formation. This strong temporal correlation of the Raman intensity variation and the N2 formation rate suggests that the Banitro species plays an active role as an intermediate in the catalytic reaction. The Raman intensity variations of ionic Ba(NO3)2 and nitrito species (Ba-ONO) do not correlate with the changes in NO conversion. These species, therefore, are considered to play no active role in the catalytic reaction. At the critical temperature, at which the sharp decrease in catalytic activity is observed, the Raman bands of all three species exhibit a sharp decrease in intensity and the spectrum of BaO is detected. Therefore, the sharp reduction in catalytic activity is related to a type of phase transition, which only can occur within Ba particles of considerable size. This active Ba-nitro species and the related phase transitions could not be detected in Ba/ A1203 and Ba/LazO 3 materials, despite comparable specific surface areas. The importance of support specific surface area and support basicity/acidity, thus, can be excluded for catalytic

IAbt. Anorg. Chemie, Fritz-Haber-Institutder Max-Planck-Gesellschaft, Faradayweg4-6, 14195 Berlin, Germany. G. Mestl gratefully thanks the Alexander-von-Humboldtfoundation for the Feodor-Lynen Fellowship which made possiblehis research stay at Texas A&M University. *to whom correspondenceshouldbe addressed

460 activity. The support structure, therefore, is suggested to play an important role in the development of active NO decomposition catalysts. 1. INTRODUCTION In the past 20 years increasing effort has been made to prevent the emission of nitric oxide into the atmosphere, and several catalytic and noncatalytic processes have been developed for this purpose. Cu-ZSM-5 zeolite was found to be the most active catalyst, however, other materials including strongly basic oxides are also active [1,2]. Several spectroscopic studies have suggested that surface nitrosyl species may be reactive intermediates in N-N bond formation [3]. 2. EXPERIMENTAL The barium catalysts were prepared by impregnating the respective supports (0~-A1203, Girdler, specific surface area 4 m2/g; La203, Aldrich, specific surface area.., m2/g; CaO, Aldrich, specific surface area.., m2/g) with the appropriate amount of Ba(NO3)2 (Baker). After drying in a rotary evaporator, the material was pressed under 400 kg/cm 2, crushed and sieved to 20 - 40 mesh size. BaO was used as purchased from Alfa (88 - 94% purity) without any further purification. Prior to the Raman characterization, this material was calcined ex situ at 800~ in 4% OJHe for 2 h, and cooled to room temperature in a flow of He, in order to remove carbonates. Subsequently, the BaO was rapidly transferred into the in situ Raman cell. All gases used (10% OJHe, and He) came from Matheson and had ultra high purity. The gas flow through the catalyst bed in the in situ Raman cell was set at 40 ml/min, thus identical to the flow applied in the catalytic tests. The experimental conditions were chosen to be close to those under which the high activity for NO decomposition was observed. The in situ Raman spectra were recorded with the Holoprobe | spectrometer (Kaiser Optical). The resolution of this instrument is 5 cm -1 and the wavelength reproducibility is better than 0.5 cm -1. The exciting light source was a NdYAG laser which was frequency doubled to 532 nm. All Raman spectra were recorded with a laser power of 2.5 mW measured at the sample position leading to laser heating of 5~ The in situ Raman cell, which was designed for this study, and the optical setup are described in detail elsewhere [4]. 3. RESULTS AND DISCUSSION 3.1. Barium on MgO

We have observed that BaO supported on MgO is an effective catalyst for NO decomposition [5]. A very distinct catalytic activity was observed depending on the barium loading on MgO,. When the barium loading on MgO was above 12 mol%, the catalysts exhibited an unusual steady state behavior in that the N2 formation rate for the 14 mol% Ba/MgO went through a sharp maximum at 630~ with 1% NO/He (Fig. 1). The exact temperature, at which the maximum conversion occurred, depended on the concentration of NO. This temperature increased with increasing NO partial pressures. Over 14 mol% Ba/MgO, maximum conversion was observed at 700~ with 4% NO/He, and the rate of N2 formation was 2.5 gmol g-1 s-1. It was possible to obtain 96% NO conversion to

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stoichiometric amounts of N: and O: at 650~ with 2 g of catalyst and a flow rate of 40 ml/min. The rate of reaction also underwent an unusual transient effect with respect to changes in NO partial pressure. At 600~ when the NO concentration was decreased from 4% to 1% in He, the rate rapidly decreased ca. 4 fold, and then increased to the value shown in Fig. 2A over a period of 30 rain. When the NO concentration was increased from 1 to 4% in He, the rate showed the opposite behavior; it rapidly increased 4 fold to later stabilize at a considerably lower level. This process also required

about 30 min at 600~ At temperatures above the discon-tinuity and over low-loaded Ba/MgO catalysts (8 mol% Ba), the decom-position of NO occurred via another reaction mechanism as revealed by catalytic tests [5]. The activation energy determined for this reaction was 13 kcal/mole, while an activation energy of 43 kcal/mole was determined over highly loaded materials. In addition, N20 was identified as an intermediate in the decomposition of NO only over low loaded catalysts. In situ Raman spectroscopic characterization of a 14 mol% Ba/MgO sample showed that Raman bands at 1323, 804, 400, 245 and 145 cm 1, attributed to a coordinatively bound nitro species (Ba-NO2) on the catalyst, grew in intensity on the same time scale as the rate of N2 formation, following the decrease in NO concentration (Fig. 2B). This strong temporal correlation of the intensity variation of the Ba-nitro complex bands and the N2 formation rate suggests that this species plays an active role as an intermediate in the catalytic reaction. The intensity variations of additional bands at 1048, and 714 cm ~ and at 1335 and 805 cm 1, attributed to ionic Ba(NO3) 2 and nitrito species (Ba-ONO), respectively, did not show a temporal correlation with the catalytic NO conversion. These species are, therefore, considered not to play an active role in the catalytic reaction. The Raman bands of this Ba-nitro species were only observed when high barium loadings were used. In situ Raman spectra of all the other catalyst materials, which did not exhibit this pronounced activity, only showed the presence of nitrates, nitrites or Ba-nitrito species. This observation again confirms the active role of Ba-nitro species in the pronounced catalytic activity. At the critical temperature, at which the sharp decrease in catalytic activity was observed (Fig. 1), the Raman bands of all three species exhibited a sharp decrease in intensity and the spectnma of BaO was detected (Fig. 3). The sharp reduction in catalytic activity is, therefore, related to a type of phase transition. The strong temporal correlation of the Raman intensity variations of the Banitro species with the catalytic activity again indicates that the Ba-nitro species is an intermediate in the NO decomposition.

462 Gas phase or weakly adsorbed NO is suggested to react with this interme0.4 t 4%N0 I 0,5%N0 diate to form N2 and 02. L % Since this mechanism in02 t O,,, 0 n ~ O O O O O volves the reaction between E I an NO molecule and a surface Ba-nitro species, a 0,05 ' 'i ._o n m,..l-l-I-I It decrease in NO pressure _,~i-I E V imai $ results in an immediate r i decrease in N2 formation i rate. Higher loadings on MgO are required to form a special barium nitro/nitrito/ , 0,00 .... ,. . . . , , nitrate phase which is able 0 50 1 O0 150 200 to stabilize the Ba-nitro T i m e [min] complexes. Peroxide deB fects in or on defect-rich BaO [6,7] were suggested to be responsible for the activation of NO v i a the formation of the intermediate Ba-nitro complex. The theoretical monolay-er coverage for BaO on MgO is exceeded at a loading of 2 mol% barium. Thus, in terms of monolay-er formation, the difference in the catalytic 1000 1200 1400 1600 behavior of highly loaded Ftaman Shift (cm-1) Ba-catalysts, as compared to low loaded materials, Fig. 2 A: Change in N2 formation rate upon change in NO cannot be understood. partial pressure. Exploiting the special B: Spectral variation induced by reduction in NO design of our Raman probe partial pressure from 4 to 0.5%. holder [4], we were able to show that catalysts containing 4 mol% Ba possessed an inhomogeneous surface composition [1 ]. Out of ten randomly chosen spots on the sample, only one spot led to the detection of the characteristic Raman spectrum of the catalytically active phase, while all other investigated spots on the specimen only showed the weak Raman scattering of a monolayer-type Basurface species (spectra not shown). I n s i t u Raman spectra recorded of such larger particles in the 4 mol% samples revealed that identical phase transitions occurred in this crystallites as in those observed on highly loaded catalysts (spectra not shown). A comparable behavior could not be detected for the monolayer-type areas on 4 mol% Ba/MgO. This observations suggest A

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463 the following: a) excess Ba(NO3) 2 on MgO does not form homogeneous overlayers, but crystallizes into larger particles, b) only large crystallites stabilize the catalytically active phase, and c) the ratio of large crystallites to monolayer-type surface species determines the ,..,~ -...,.. 150 macroscopic catalytic behavior. _1 I I ..I . i . ! . I ! _ .I.. "1I 0 ,,.,~,~t'" If this remarkable 100 200 300 400 500 600 700 800 900 1000 ca-talytic activity for Raman Shift (cm-1) de-composition of NO over highly loaded Ba/ Fig. 3 Decomposition of catalytically active phase into BaO at MgO catalysts is rela630~ in 1%NO/He. ted to the crystallite size of the barium species, than pure BaO or barium supported on other supports should exhibit a comparable behavior. \

3.2. P u r e B a r i u m

-

Oxide.

If the stabilization of the active phase requires the presence of large crystallites, then one should be able to observe phase transitions in pure barium oxide similar to those detected in supported 14 tool% Ba/MgO catalysts. Indeed, catalytic tests of pure BaO for the direct decomposition of NO showed a similar activity behavior, i.e., a sharp fall-off in the conversion when a particular temperature was exceeded (data not shown). In situ Raman spectroscopy was applied to characterize the structural evolution of BaO as a function of the gas phase (spectra not shown). BaO was calcined in 10% 02 at 600~ for 9 h. Thereafter, the gas mixture was switched to 1.5% NO/He. From the phase diagram [4], we l~ow that this NO partial pressure is sufficient to lead to the formation of the active species. After 10 min, the band at 830 cm 4 due to barium peroxide was completely lost. New bands developed at 143, 244, 400w, 1047, and 1322 cm 4, and the band at 807 cm -~ increased in intensity. The new band at 1047 is assigned to ionic nitrates. This is in line with the observations made for Ba/MgO. Thus, the catalytic active phase was formed in this experiment during 40 min time on stream. This experiment confirmed data obtained on supported Ba/MgO that Ba-nitro species are formed from the reaction of peroxide ions with NO [6]. In summary, it is shown that pure BaO is also active in the direct decomposition of NO, and, in accordance, the catalytically active Ba-nitro species is formed. We conclude, in combination with the results obtained for supported materials, that large crystallites of BaO are required to accommodate the active phase. The small active surface area of unsupported BaO explains the observed smaller maximum conversion of NO as compared to Ba/MgO

464

catalysts. This points to the role of the support material: the support stabilizes a high degree of dispersion of the active crystalline material. If MgO exerts only this role, then other usual supports, like A1203, loaded with high amounts of Ba should also give active catalysts for the direct decomposition of NO. 3.3. Barium on ct-AlzO3 To prove this argument, we prepared 14 mol% Ba on a-A1203 of a comparable specific surface area. Surprisingly, this material was absolutely inactive for the direct decomposition of NO (data not shown). In situ Raman spectroscopy was applied to characterize the evolution of the surface Ba(NO3)2 precursor phase during pretreatment and in an atmosphere of 4% NO. For the ct-A1203 supported barium phase, only the formation of an amorphous phase containing ionic nitrates and nitrites was observed at 500~ in He. During this process of precursor decomposition, the nitrate bands at 725 and 1047 cm -1 continuously lost their intensity. Weak bands at 810 and 1336 cm ~ due to ionic nitrites remained at a low level. No further Raman spectral changes could be observed up to 600~ in He (spectra not shown). A switch of the gas phase at 600~ from He to 4% NO and back did not result in any changes in the Raman spectra, neither in the nitrate, nor in the nitrite related bands (spectra not shown). We, therefore, conclude that the surface species generated at the elevated temperatures had reacted with alumina to catalytically inactive mixed phases. This experiment showed that the specific surface area cannot be a major factor controlling the formation of the catalytically active species. Compared to MgO, alumina is more acidic. The support acidity/basicity, thus, could be the factor which changes the interaction of the barium phase with the support. If the support basicity is the governing factor, at comparable specific surface area, a basic, low surface area support, e.g., La203 should result in the generation of active catalysts. 3.4. Barium on La203 In order to prove this point, a catalyst was prepared containing 14 mol% Ba on La203. This catalyst showed no activity in the direct decomposition of NO that was comparable to the one observed for MgO supported catalysts (data not shown). The evolution of the bm'ium phase on La203 was also investigated by in situ Raman spectroscopy. A difference in the behavior of Ba/La203, as compared to pure La203, was observed, when both materials were heated in He to 500~ (spectra not shown). Lanthanum hydroxide overlayers on pure La203 decomposed between 300 and 400~ This process occurred at the higher temperature of 500~ in the BaJLa203 material. At 600~ in He, a phase transition occurred in pure La20 ~ (spectra not shown). This process was characterized by the strong intensity reduction of the band at 303 cm 1, the loss of the weak shoulder at about 500 cm ~, and a strong increase of the bands at 100, 179, and 393 cm ~. In addition, the last observable traces of carbonates at 1061 cm 1 vanished at the same time. The observed changes in the lattice mode regime are attributed to a transition from lanthanum oxy-carbonate to La203. Upon further heating up to 725~ no further changes in the in situ Raman spectra of La203 could be detected (spectra not shown), irrespective of the gas atmosphere applied A different behavior was observed for the La203 catalyst containing 14 mo1% Ba. At 600~ in He, a growth of the band at 393 cm z was observed within 60 min, but the band at

465 303 cm -1 did not lose its intensity (spectra not shown). Upon heating to 725~ in He, the same structural change as in pure La203 was observed to occur within 10 min at the lower temperature of 600~ (spectra not shown). However, opposite to the pure support, continued heat treatment at 725~ in He for 14 h resulted in further structural changes of the material (spectra not shown). When the temperature subsequently was lowered to 600~ and the gas phase was switched to 4% NO, the structure of the Ba~a203 catalyst transformed back within 30 min as confirmed by Raman bands (spectra not shown) at 99, 127sh, 175, 393, 690, 807, 1048, and 1334 cm ~. The bands at 807 and 1334 cm 1 are characteristic for the presence of ionic nitrites. Absolutely no catalytic activity for direct NO decomposition was observed for this material. This is direct proof that the presence of ionic nitrites does not lead to catalytic activity. We have to conclude from the changes that occurred in the Raman spectra of 14 mol% Ba/La203, as compared to those observed for pure La203,that barium forms a compound with La203, which was able to incorporate ionic nitrites. The presence of this ionic species, however, did not result in catalytic activity for the direct decomposition of NO. If the specific surface area, and the support basicity are not the main factors which control the catalytic activity of barium catalysts, the crystalline structure of the support oxide may possibly exert this role. 3.5. B a r i u m on C a O

Magnesium oxide, like barium oxide, crystallizes in the rock salt structure. Its lattice dimensions, however, are much smaller as compared to BaO. This much smaller lattice constant is thought to inhibit the formation of a mixed oxide of the general stoichiometry Ba~Mg~xO. The MgO surfaces are hydroxylated when in contact with H20, and deep overlayers of Mg(OH)2 are formed. During impregnation with Ba(NO3)2 solutions the intermixing of the barium and the magnesium phases may occur in this hydroxide overlayers. However, Raman spectra of this class of catalysts only showed the presence of crystalline Ba(NO3) 2 and Mg(OH)2. No evidence of an intermixed phase could be deduced from the Raman spectra [4]. The Mg(OH)2 decomposition at 300~ did not affect the Raman spectrum of the crystalline Ba(NO3)2 phase [4]. This observation again pointed to the fact that no intermixing occurred between the Mg(OH)2 overlayer and crystalline Ba(NO3) 2. CaO also crystallizes in the rock salt structure and is strongly basic. It may be suggested that a CaO support should result in an active catalysts for the direct decomposition of NO. Moreover, the much larger lattice constant of CaO as compared to MgO may help in determining whether or not a compound between barium and the support may be formed. If undetected mixed oxides are formed between barium and magnesium oxide, then such a compound formation should occur much easier in the case of calcium oxide which has a lattice constant closer to that of BaO. If no additional Raman bands are detected for barium supported on CaO as corn-pared to pure BaO or Ba/MgO, then compound formation can be ruled out. In fact, a catalyst prepared from 14 mol% Ba(NO3)2 on CaO exhibited the same behavior in the catalytic direct decomposition of NO as the MgO supported catalysts or unsupported BaO, i.e., the catalytic activity strongly decreased above a certain limiting temperature (data not shown). In situ Raman spectroscopy identified the surface species which were present in the working Ba/CaO catalyst. During a time period of 15 h in 4% NO/He at 550 ~ bands grew in at 723, 812, 1052, and 1337 cm ~, assigned to ionic nitrates and BaCO3, and nitrites,

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respectively. The complete absence of any bands or shoulders in the lattice mode regime indicates that this material is an amorphous phase [4]. The phase diagram which was determined for Ba/MgO suggests that a decrease in the NO concentration from 4 to 0.75% NO/He at this temperature should lead to the formation of the active Ba-nitro species. The series of time resolved (10 min) in situ Raman spectra, which were recorded during this experiment, are displayed in Fig. 4A. Ten minutes after the NO partial pressure was reduced to 0.75% NO, the characteristic bands of the Ba-nitro complex began to appear at 244 and 400w cm ~ in the low frequency regime. After about 10 min on stream, these bands reached maximum intensity. A further reduction in the NO partial pressure to 0.2% NO should lead, according to the phase diagram [4], to decomposition into BaO. Fig. 4B depicts the series of in situ Raman spectra recorded after this reduction in the NO concentration. This decomposi-

tion occurred in line with the prediction from the phase diagram,. A switch back to 0.75% NO led to the reformation of the Ba-nitro complex (spectra not shown). No Raman spectral evidence could be obtained that there was compound formation between Ba and CaO, as in the case of MgO supported catalysts. We, therefore, exclude compound formation as being responsible for the development of catalytic activity. Fig. 4 In situ Raman characterization of the formation (A) and decomposition (B) of the catalytically active Ba-nitro species on CaO.

4. SUMMARY Barium on MgO and CaO and, to a lesser extent, pure barium are effective catalyst for the direct decomposition of NO. The catalytic activity is observed to strongly depend on the barium loading on MgO and CaO. Identical Ba loadings on amphoteric A1203 or basic La203 with comparable specific surface areas, however, gave inactive materials. Barium loadings on MgO and CaO, exceeding 12 mol%, and pure BaO, resulted in an unusual steady-state behavior. The N2 formation rate showed a sharp maximum. This behavior was not observed

467

for barium loadings below 12 mol% on MgO and CaO. Thus, this distinct behavior depended on the Ba loading. The exact temperature, at which the maximum conversion occurred, was also a function of the NO partial pressure: it increased with increasing NO concentration from 630~ in 1% NO/He to 700~ in 4% NO/He over 14 mol% Ba/MgO. The rate of reaction also underwent an unusual transient effect with respect to changes in the NO partial pressure. In situ Raman spectroscopic characterization of 14 mol% Ba/MgO, Ba/CaO and pure BaO revealed the presence of a coordinatively bound nitro species (Ba-NO2) on the catalyst. Its Raman intensity varied on the same time scale as the rate of N2 formation upon changing the NO partial pressure. This strong temporal correlation of the Raman intensity variation and the N2 formation rate suggests that the Ba-nitro species plays an active role as an intermediate in the catalytic reaction. The Raman intensity variations of ionic Ba(NO3)2 and nitrito species (Ba-ONO), however, did not correlate with the changes in NO conversion. These species, therefore, are considered to play no active role in the catalytic reaction. At the critical temperature, at which the sharp decrease in catalytic steady state activity was observed, the Raman bands of all three species exhibited a sharp decrease in intensity and the spectrum of BaO was detected. Therefore, the sharp reduction in catalytic activity is related to a type of phase transition, which only can occur within Ba particles of considerable size. This larger Ba particles are also required to accommodate the catalytically active Banitro species. This active Ba-nitro species and the related phase transitions could not be detected in Ba/ A1203 and Ba/La203 materials, despite comparable loadings on comparable specific surface areas. The importance of the support specific surface area and support basicity/acidity, thus, can be excluded for catalytic activity. The support structure, on the other side, is suggested to play an important role in the development of active NO decomposition catalysts. References

1. M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, and S. Kagawa, J. Phys. Chem., 95 (1881) 3727. 2. T. Yamashita and A. Vannice, J. Catal., 163 (1996) 158. 3. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. 4. G. Mestl, S. Xie, M. P. Rosynek and J. H. Lunsford, J. Phys. Chem., 101 (1997) 9321. 5. S. Xie, G. Mestl, M. P. Rosynek and J. H. Lunsford, J. Am. Chem. Soc., 119 (1997) 10186 6. G. Mestl, S. Xie, M. P. Rosynek and J. H. Lunsford, J. Phys. Chem., 101 1997)9329. 7. G. Mestl, S. Xie, M. P. Rosynekand J. H. Lunsford,acceptedfor J. Phys. Chem.

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91998 Elsevier Science B.V. All rights reserved.

Preparationof CatalystsVII B. Delmon et al., editors.

469

Further studies on ligand-promoted oxide dissolution during supported catalysts preparation" evidence for the formation of aluminoheteropolytungstates in the case of the W O x / 7 - A l 2 0 3 system. X. Carrier, J.F. Lambert* and M. Che § Laboratoire de R6activit6 de Surface, (UMR 7609), Universit6 P. & M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France. This study deals with the importance of alumina dissolution on the speciation of tungstic species encountered in WOx/A1203 catalysts preparation by the equilibrium adsorption method. The dissolution of the support is enhanced by the formation of soluble aluminoheteropolytungstates in solution and this can be rationalised in the words of geochemists as a "ligand-promoted dissolution" effect. We have first synthesised different types of aluminoheteropolytungstates and obtained 27A1 NMR fingerprints of these compounds. All of these compounds adopt a Keggin-type structure. The aluminium ions exist in two different environments: either tetrahedral (8 - 71 to 73 ppm) or octahedral (8 - 5 to 12 ppm). Some of these structures are lacunary by removal of some tungsten octahedra. By comparison with the 27A1 NMR fingerprints of these reference compounds, we show here that during the preparation of WOx/A1203 catalysts, alumino-heteropolytungstates are formed in solution by reaction between dissolved aluminium ions and the tungstic precursors (mono, para, or metatungstate) even at near neutral pH where alumina solubility is supposed to be negligible. The situation appears to be much more complex than with molybdates since several types of compounds are formed and at least three factors influence the chemistry of the system: the pH, the nature of the tungstic precursors and the oxide/water ratio. This system, as well as the previously studied MoOx/A1203, are clear examples showing that the support is not chemically inert. It can even play the role of a stoechiometric reagent. 1. I N T R O D U C T I O N Phenomena occurring at the oxide-water interface are particularly relevant in many fields of science (geochemistry, catalysis, materials science...) and much progress can be expected by bringing these areas together. Especially geochemists are very close to the preoccupations of chemists interested in heterogeneous catalysts preparation as nicely demonstrated in a thorough review on the chemistry of the solid-water interface by Stumm [ 1]. One of the major lessons which can be drawn from the field of geochemistry is that one has to be very careful when considering the oxide supports used in heterogeneous catalysis (SiO2, A1203) as chemically inert. Indeed, geochemists have shown that organic ligands are able to promote the kinetics of dissolution of oxides by weakening the metal-oxygen bonds when binding on the surface of oxides [1]. This is for example demonstrated in the goethite (FeOOH)/EDTA (EthyleneDiamineTetraAcetate) system where EDTA promote the dissolution of goethite by forming binuclear and mononuclear complexes on the surface of the oxide [2] and thus increasing the dissolution rate. But organic ligands can also promote the thermodynamics of the * [email protected] + Institut Universitaire de France

470 dissolution of oxides by complexing the metal ions released in solution and thus shifting the solubility reaction to the right (oxide <-> dissolved metal ions). This is, for example, the case of the alumina/NTA system (NTA - NitriloTriAcetate) where the solubility of 3'-A1203 is increased by more than one order of magnitude [3]. This can be explained by the formation of strong complexes between A13+ and NTA 3- in solution. One has to notice that this effect is especially pronounced in the neutral pH region, i.e., where alumina solubility is supposed to be negligible [4,5]. In line with this, we have recently shown that these basic ideas can be successfully applied to the preparation of MoOx/A1203 catalysts by the Equilibrium Adsorption method [6]. During the preparation of these catalysts, A13+ ions are released from alumina in much larger amounts than predicted by the solubility product, at pH close to neutral; these A13+ ions react with the heptamolybdates initially present in solution to form an Anderson-type heteropolyanion [7]: Al(OH)6Mo60183-. This reaction occurs on the time scale of catalysts preparation as the heteropolyanion is identified in solution after only one hour of equilibrium at pH 4. Furthermore, using 27A1CP-MAS NMR, we showed that this heteropolyanion is deposited on the support rather than the isopolyanion Mo70246- as frequently repol"ted in the literature. This process is illustrated in Figure 1.

Figure 1. Schematic representation of ligand-promoted alumina dissolution by molybdates. This is a clear evidence that the oxide support is not chemically inert but can act as a stoechiometric reagent and thus provide very strong metal-support interactions from the first steps of catalysts preparation. Our previous study [6] showed that it is of outmost importance to characterise not only the solid catalyst obtained but also the liquid phase used in the preparations. This is also pointed out by Ludwig et al. [8] when they write "...the detailed spectroscopy of dissolved metal-ligand complexes can be used to infer structural characteristics of surface and activated complexes". In this work, we want to show that the idea of ligand-promoted support dissolution can be extended to related systems such as WOx/3t-A1203 catalysts and we will thus focus on the characterisation of the liquid phases used in these preparations. Alumino-heteropolytungstates have been reported in the literature and we can therefore foresee that tungstates as well as molybdates can act as ligands for aluminium ions. We can, thus, expect the same type of behaviour for tungstates and molybdates. In contrast with the A1-Mo system, the AI-W system is not known to lead to Anderson-type alumino-heteropolymetalates : only Keggin-type species have been reported in the literature [7,9]. To the best of our knowledge, the only "pure" alumino-heteropolytungstate reported [7,9-11] is AlW 120405-. This compound adopts a Keggin structure made of a central A104 tetrahedron surrounded by twelve WO6 octahedra arranged in four groups of three edgesharing octahedra, W3013. These groups are linked by shared corners and to the central A104 tetrahedron. But there are also "mixed" heteropolytungstates which contain aluminium in an octahedral environment (substituting one tungsten atom) and another atom (most frequently phosphorus or

471 silicon) in the central tetrahedron. These compounds are derived from the lacunary phosphotungstate PWllO397- [12,13] and have the formula PAl(H20)W110394- [14]. Such compounds have also been reported without any central atom in the tetrahedron provided that fluorine atoms substitute the central oxygens, yielding the formulas HW 11A1F3036(H20) 5and H2Wl 1A1F2037(H20) 5- [ 15,16]. We will first report on the synthesis of different types of reference aluminoheteropolytungstates (with the aluminium either in tetrahedral or octahedral environment) and the 27A1 NMR fingerprints of these compounds. We will then show that some of these compounds can be identified in the liquid phase during the preparation of WOx/7-A1203 catalysts. We will also briefly discuss the possible occurrence of these compounds on the surface of the dried catalysts.

2. EXPERIMENTAL 2.1 Reference Compounds Synthesis The reference heteropolyanion, A1W 120405-, was synthesised according to the procedure of Mair and Waugh [ 17]. Sodium tungstate (28.06 g) was dissolved in 200 mL of water followed by the addition of 97.2 mL of 1 M nitric acid. The solution was then heated (60-70~ and 250 mL of aluminium nitrate (5.3 10-2 M, Merck) were slowly added dropwise during 20 hours. The solution was then concentrated to 70 mL, cooled to 4~ for 24 hours and filtered. The 27AIliquid-state NMR spectrum of the solution was recorded. In order to isolate the free acid of the heteropolytungstate, the filtrate was then mixed with 200 mL of diethylether in an ice bath and 200 mL of sulfuric acid (6 M, Prolabo) were slowly added to the solution. The mixture was transferred to a separating funnel and the lowest layer corresponding to the etherate of the heteropolyacid was separated and washed again with 100 mL of diethylether. This was repeated twice in order to remove any aqueous phase. The etherate (8 mL) was transferred to a crystallising dish with 8 mL of water and left to evaporate. 10 g of a yellow powder were obtained. 4 g of this powder were mixed with 100 mL of water and the suspension was stirred for 48 hours before being filtered in order to remove any insoluble residue. The 27A1 liquid-state NMR spectrum of the filtrate was recorded. It will be further referenced as AlTdW12. The subscript Td refers to an aluminium ion located in the central tetrahedron of the Keggin structure. The solid reference compound was then obtained by adding caesium nitrate in order to precipitate the caesium salt of the Keggin tungstoaluminate ion. It will be further referenced as Cs5AlTdW12. The 27A1 solid-state NMR spectrum of this compound was recorded. Cs5A1TdW12 was then dissolved in water. The pH of the solution was increased by adding K2CO3 (0.1 M) and the solution was heated until no solid phase remained. The final pH of the solution was 8.1. The 27A1 liquid-state NMR spectrum was recorded. The synthesis of A1W120405- was repeated at room temperature, i.e. the aluminium salt was added without heating the solution. The 27A1 liquid-state NMR spectrum of the filtrate was recorded after concentration of the solution. The reference heteropolyanion, PAI(H20)W 110394", was synthesised according to the procedure of Zonnevijlle et al. [14]. First of all, the potassium salt of the phosphotungstate heteropolyanion (PW 120403-) was prepared in a classical way [9]. The lacunary polyanion was then prepared by dissolving K3PW 12040 in hot water followed by alcalinisation of the solution to pH 6 with a 0.1 M solution of KHCO3 [13]. The potassium salt with formula K7PW 11039 was obtained by adding KC1 in excess. PAI(H20)W 110394- was then prepared

472 by adding 1.10 -4 mol of K7PW 11039 to 1.10 -4 mol of aluminium nitrate in 10 mL of hot water (80-90~ The solution was stirred for 10 minutes and then cooled down in an ice/water bath before adding 10 mL of methanol. The white precipitate which immediately appeared was filtered and left to dry under air. This salt was dissolved in water and its 27A1 liquid-sate NMR spectrum was recorded. It will be further referenced as PAlohWll. The subscript Oh refers to an aluminium ion which substitutes one tungsten in an octahedral environment of the Keggin structure.

2.2 Catalysts Preparation The support was a 7-alumina from Rh6ne-Poulenc, with a surface area of 189 m2.g - 1 and a pore volume of 0.64 cm3.g - 1. Prior to use, the alumina pellets were ground in a mortar and the 150 < ~ < 400 gm fraction was collected by sieving. The catalysts were prepared by the equilibrium adsorption method.Three different tungstate salts (Fluka) were used as precursors: sodium monotungstate (Na2WO4), ammonium paratungstate ((NH4)10H2W 12042) and ammonium metatungstate ((NH4)6H2W 12040). Solutions of the monotungstate precursor were prepared by dissolving sodium monotungstate with a concentration of 0.15 M (pH = 9.5). The solutions of the less soluble paratungstate precursor were prepared by dissolving ammonium paratungstate with a concentration of 0.1 M in hot water, then cooled to room temperature and left to stand for 24 hours (pH = 6.2) before contact with the alumina suspension. Solutions of the metatungstate precursor were prepared by dissolving ammonium metatungstate with a concentration of 0.15 M (pH = 4.3). Ground alumina was then suspended in each solutions with an oxide/water ratio of 1 g/100 mL. The resulting tungstate/alumina suspensions were then stirred at room temperature for 168 hours. The pH was kept constant to either 4.5 or 6.8 throughout equilibration by addition of either concentrated HNO3 or NH3 respectively. One experiment was performed with ammonium paratungstate by changing the oxide/water ratio to 2 g/100 mL. After filtration, the solid phases were washed three times with distilled water and left to dry in air at room temperature. The filtrates were kept for 27A1 NMR measurements.

2.3 Characterisation Liquid state NMR spectra were obtained at 9.4 Tesla over a Bruker MSL 400 spectrometer. Simple one-pulse sequences with phase cycling were used. For 27A1, the Larmor frequency was 104.26 MHz; the pulse length was 1 gs and the recycle delay was 500 ms. A 0.1M solution of aluminium nitrate was used as the 0 ppm reference. The solid state 27A1 NMR experiments were carried out at 7.05 Tesla over a Bruker MSL 300 spectrometer fitted with a high-speed MAS probe. The Larmor frequency was 78.205 MHz. We used single pulse MAS experiments; the pulse length was 1 gs, the recycle delay was 1 s, and the MAS spinning rate w a s 0)rot = 15 k H z .

3. R E S U L T S AND D I S C U S S I O N

3.1 Reference Compounds AITdW 12 synthesis The spectra recorded during A1TdW 12 synthesis are shown on Figure 2-a and 2-b. They are similar to those obtained by Akitt and Farthing (at 23.45 MHz and 2.11 T) [18]. These authors attributed the narrow peaks at 71.8 and 71.4 ppm to the A1TdW12 polyanion in two slightly different arrangements. One possible explanation for these different arrangements could be the existence of a protonated species to yield H3A1W 120402- instead of A1W 120405- [ 18]. After diethylether extraction, there is no more free aluminium in the solution (Al 3+ at 0 ppm) and only the 71.4 ppm resonance remains. It is also the only resonance present in the solid

473 state (Figure 2-c). Note that differences in chemical shift between liquid-state and solid-state spectra are not surprising. This was already observed in the case of Al(OH)6Mo60183- in liquid-state (15.5 ppm) and K3Al(OH)6Mo6018 in solid-state (13.2 ppm) [6]. A

B (AI in octahedral coordination)

(AI in tetrahedral coordination)

6.6

d

e

....

cd

(Intensity / 10 with respect to Fig. I-a, part A) 17

... g . .

78

,I,

76

,

,, i

,

74

72

8 (ppm)

70

68

20

15

b

~,1

JL

i

i

I

10

5

0

a

-5

-10

8 (ppm)

Figure 2.27A1NMR of a) A1TdW 12 synthesis before diethylether extraction (liquid-state), b) A1wdWl2 synthesis after diethylether extraction (liquid-state) c) Cs5A1TdW12 (solid-state at 7,05 T), d) alcalinisation of A1TdWl2 e) A1TdW12 synthesis at room temperature and f) PA1ohW 11 (liquid-state). The broader resonance at 73.3 ppm (Figure 2-a) is tentatively assigned by Akitt and Farthing [18] to an aluminium ion in a tetrahedral environment in a Dawson-type aluminoheteropolytungstate. This would explain the broader resonance as compared to A1TdW12 since the tetrahedral environment is less regular in Dawson-type compounds. This hypothesis seems doubtful since, to our knowledge, no Dawson-type Alumino-heteropolytungstate has ever been reported in the literature. A more likely hypothesis would be the existence of a lacunary species, A1TdW11, which is reported in the literature (table 5.1 in [7]) and which could also explain the broad resonance observed. Indeed, the departure of one tungsten would leave a distorted tetrahedron in the lacunary species. This hypothesis is confirmed by the alcalinisation of the A1TdW12 solution (Figure 2-d) and by the A1TdW 12 synthesis performed at room temperature (Figure 2-e). Indeed, the alcalinisation of A1TdW12 leads to the appearance of a peak at 73.7 ppm in addition to the one at 71.8 ppm. In the experiment performed at room temperature, only the 73.2 ppm resonance is observed (in good agreement with the result of Akitt and Farthing [18]) and the pH of this solution is basic (pH = 8.4). Now, it is well known that the basification of Keggin units leads to the formation of lacunary ions. For example PW12 leads to PW11 around pH 6 [9,13]. Thus we tentatively assign the resonance around 73 ppm (73.2-73.7 ppm) to the lacunary species A1TdW11. The resonance at 5.6 ppm is not attributed by Akitt and Farthing [18] but is discussed below. P A I o h W 11 synthesis This synthesis yields no peak in the region associated to A1 in tetrahedral coordination (Figure 2-f, part A). Only peaks in the octahedral region (Figure 24, part B) can be identified at

474 12.3 and 6.6 ppm. The first one can be tentatively attributed by reference to the work on fluoropolytungstates [15,16]. Indeed HWl 1A1F3036(H20) 5- exhibits one peak at 10.6 ppm which is associated to the aluminium ion in an octahedral environment. Thus, the peak observed at 12.3 ppm would be associated with the A1 in PA1ohWl 1. The peak observed at 6.6 ppm has to be compared with the one at 5.6 ppm in Figure 2-d. It is reasonable to attribute both peaks to the same species. Since the chemical shift is in the range of octahedral aluminium, we could tentatively assign it to a lacunary Keggin ion with an aluminium substituting one tungsten ion with possible formula A1ohW 10. The presence of the peak at 5.6 ppm in addition to the one at 73.2-73.7 ppm (Fig. 2-d and 2-e) could also be explained in the same manner, that is to say, a lacunary Keggin unit with an aluminium acting both as the primary (tetrahedral) and secondary (octahedral) heteroatom [7]. The proposed formula of this third type of "mixed" Keggin species would be A1ohA1TdW10. Work is being performed in order to confirm this hypothesis. In summary, we propose to distinguish two types of Keggin-type aluminoheteropolytungstates. In the first type, the aluminium ions are in tetrahedral coordination in the centre of the Keggin unit. The polyanion can be either complete A1TdW12 (71.4-71.8 ppm) or lacunary A1TdW 11 (73.2-73.7 ppm). In the second type, the aluminium ions are in octahedral coordination and substitute one tungsten atom. In the same manner, the polyanion can be either complete A1ohWl 1 (10.6-12.3 ppm) or lacunary A1ohWl0 (or A1ohA1TdW10) (5.6-6.6 ppm).

3.2 Catalysts Preparation

A (pH 6.8)

B (pH4.5)

73.4

73.~fl.4 G"

5.9

&

~0.4

,._.,

-~=

~D 12

b' b a

80

60

40

5 (ppm)

20

0

80

60

40

20

8 (ppm)

0

Figure 3. 27A1liquid-state NMR spectra of filtrates obtained after equilibrium adsorption, A) pH 6.8 with a) metatungstate b) paratungstate b') paratungstate (oxide/water ratio of 2 g/100 mL) and c) monotungstate. B) pH 4.5 with a) metatungstate b) paratungstate and c) monotungstate. The 27A1 liquid-state NMR spectra of filtrates obtained after equilibrium adsorption are shown in Figure 3. By comparison with the reference compounds (Figure 2), it is obvious that there is formation of alumino heteropolytungstates during WOx/A1203 catalysts preparation. But the situation appears to be much more complex than in the case of molybdates. The preparations performed at near neutral pH with metatungstate and paratungstate as tungstic precursors yield no 27A1 NMR signal (Figure 3-a and 3-b, part A), indicating that

475 alumina dissolution was insignificant in these preparations. The experiment was repeated with paratungstate by changing the oxide-water ratio to 2 g/100 mL and we can now identify (Figure 3-b', part A) two peaks at 73.4 ppm and 5.9 ppm in these filtrates. Along previous assignments, these peaks can be attributed (Figure 2) to Keggin-type aluminoheteropolytungstates. They were described as lacunary species with aluminium either in the central tetrahedron or substituting tungsten ions in the polyanion framework. The intensity of both peaks is even higher for the synthesis performed with monotungstate (Figure 3-c, Part A). The occurrence of lacunary species is not surprising because they are favoured over regular species at this pH. For the preparation performed at pH 4.5 with metatungstate (Figure 3-a, Part B), there are no peaks which can be attributed to alumino-heteropolytungstates but only one peak at 0 ppm which corresponds to free aluminium (Al(H20)63+). With paratungstate (Figure 3-b, Part B), we can identify one peak at 10.4 ppm which can be attributed to Keggin-type aluminoheteropolytungstates with one aluminium substituting one tungsten. The proposed formula would be A1ohW 11. For the monotungstate, the peak corresponding to the former species is also identified in solution but in addition two peaks appear at 73.3 and 71.4 ppm which correspond to A1TdWl1 and A1TdW12 respectively. We evidence here the formation of several alumino-heteropolytungstates in solution during the preparation of WOx/A1203 catalysts by reaction between tungstates and aluminium ions in solution. This reaction takes place even at near neutral pH (6.8) where the solubility of alumina should be negligible. But the situation appears to be much more complex than in the case of molybdates. Indeed, at least three factors influence the chemistry involved in these systems. Of course the pH appears to be a key factor (see the difference for the synthesis with monotungstate at pH 6.8 and 4.5, Figure 3-c, Part A and Part B), but the tungsten precursors seems also to be important and this is illustrated by the difference between the preparation with metatungstate and monotungstate at the same pH (see Figure 3-a and 3-c, Part A). Although the equilibrium form of tungstates in solution should depend only on pH and concentration, different forms are obtained when starting with different precursors, possibly due to the wellknown kinetic inertness of tungstates [7]. Finally, the oxide/water ratio seems also crucial. Indeed, there is a dramatic change between the preparations performed with 1 g of alumina for 100 mL (Figure 3-b, part A) or 1 g for 200 mL (Figure 3-b', part A). This may constitute evidence for the fact that these reactions are under a kinetic regime: the amount of solid oxide should not modify the thermodynamics and thus the aluminium concentration in solution. There is at present no direct evidence that the heteropolytungstates formed in solution are adsorbed as such on the support surface. The next step would be to show that these structures are retained on the surface of the catalysts. This has been actually demonstrated for molybdates [6]. This is also the case for the (FeOOH)/EDTA system [2]. EDTA can form binuclear or mononuclear complex with the surface atoms (Fe 3+) and thus, increase the dissolution rate of the oxide. After dissolution of this Fe-EDTA surface complex the authors stated that there was readsorption of some of these complexes on the surface of the oxide. The readsorption of the dissolved species seems, thus, very likely and this clearly demonstrates that it is of outmost importance to characterise the liquid phase used during the preparation of catalysts. We show on Figure 4 a schematic representation of the chemistry involved in the preparation of WOx/A1203 catalysts with monotungstate as precursor and at pH 6.8. 4. C O N C L U S I O N The preparation of WOx/A1203 catalysts by the equilibrium adsorption method leads to the formation of alumino-heteropolytungstates in solution by dissolution of the support and subsequent reaction of the A13+ ions released with tungstic species. These heteropolyanions identified by comparison with reference compounds with the help of 27AI liquid state NMR

476 adopt a Keggin-type structure with the aluminium either in tetrahedral or octahedral environments. This process which seems to depend on several factors including pH, nature of the tungstic precursor and oxide/water ratio, is effective even at near neutral pH where alumina solubility is supposed to be negligible. In line with our previous study on molybdates [6], this can be rationalised as a ligandpromoted dissolution effect; the ability of tungstates to complex aluminium greatly enhances the solubility of alumina. Work is underway to show that these species are not only formed in solution but also are retained on the surface of the support.

Figure 4. A likely picture of tungstate/alumina interactions in the preparation of WOx/A1203 catalysts.

REFERENCES ~

.

3. 4. ~

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

W. Stumm, Chemistry of The Solid-Water Interface: Process at the Mineral-Water Interface and Particle-Water Interface in Natural Systems, J. Wiley & Sons, New York, 1992, pp 165-169. B. Nowack and L. Sigg, Geochim. Cosmochim. Acta, 61 (1997) 951-963. H. A. Helliot and C. P. Huang, J. Coll. Interf. Sci., 70 (1979) 29-45. E. Baumgarten, F. O. Geldsetzer and U. Kirchausen-D~sing, J. Coll. Interf. Sci., 173 (1995) 104-111. J. P. Brunelle, Pure and Appl. Chem., 50 (1978) 1211-1229. X. Cartier, J. F. Lambert and M. Che, J. Am. Chem. Soc., 119 (1997) 10137-10146. M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, 1983. C. Ludwig, W. H. Casey and P. A. Rock, Nature, 375 (1995) 44-47. P. Souchay, Ions Min6raux Condens6s, Masson et Cie, Paris, 1969, pp 83-119. D. H. Brown, J. Chem. Soc., 8 (1962) 3281-3282. L. G. Maksimova, T. A. Denisova and N. A. Zhuravlev, Russ. J. Inorg. Chem., 42 (1997) 350-354. G. M. Maksimov, R. I. Maksimovskaya and I. V. Kozhevnikov, Russ. J. Inorg. Chem., 37 (1992) 2279-2286. C. Tourn6 and G. Tourn6, Bull. Soc. Chim. Fr., 4 (1969) 1124-1136. F. Zonnevijlle, C. M. Tourn6 and G. F. Tourn6, Inorg. Chem., 21 (1980) 2742-2750. F. Chauveau, P. Doppelt and J. Lefebvre, Polyhedron, 3 (1982) 263-267. F. Chauveau, Bull. Soc. Chim. Fr., 2 (1986) 199-217. J. A. Mair and J. L. T. Waugh, J. Chem. Soc., (1950) 2372-2376. J. W. Akitt and A. Farthing, J. Chem. Soc., Dalton Trans., (1981) 1615-1616.

91998ElsevierScienceB.V. All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

477

Preparation of heteropolyacid-imbedded polymer film catalysts by membrane technology and their modified catalysis Gyo Ik Park", Seong Soo Lim a, Jun Seon Choi", In Kyu Song b and Wha Young Leea* aDepartmem of Chemical Engineering, Seoul National University, Seoul 151-742, Korea bDepartmem of Industrial Chemistry, Kangnung National University, Kangwondo, Korea

Heteropolyacid-imbedded polymer film catalysts were prepared using homogeneous heteropolyacid-polymer solutions by membrane preparation technology with the variation of solvents and polymer materials. They were characterized by SEM, XRD and DSC. It was found that heteropolyacid was finely and uniformly dispersed throughout the polymer matrix. They were applied to the vapor-phase ethanol conversion and MTBE (methyl tert-butyl ether) decomposition. The film catalysts showed enhanced conversions and product yields in the model reactions. Porosity of the film catalysts was controUed by a phase inversion method. The blending patterns and crystal structures of HPAs were also confirmed to ensure the modified catalysis ofHPA-imbedded polymer film catalysts.

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

The acidic and redox properties of heteropolyacids (HPAs) have been conventionally modified by replacing the protons with metal cations and/or by changing the heteroatom or the framework transition-metal atoms [1-3]. Novel catalysis of HPAs has been also modified by combining HPAs with polymer materials. H3PMo12040-doped polyacetylene film catalyst showed a higher oxidation activity and a higher acid-catalytic activity for the ethanol conversion than bulk H3PM012040 [4]. Another work [5] was done on the H3PW12040dispersed polyaniline catalyst which was prepared by polymerizing aniline together with HPA. HPA-polyaniline catalyst showed a higher oxidation activity but a lower acid-catalytic activity for the isopropanol conversion than the mother acid. HPAs are inorganic acids as well as oxidizing agents [6]. They are highly soluble in polar solvents such as water, alcohols and amines, but some HPAs are insoluble in non-polar chemicals such as benzene and olefms [7]. Taking advantage of these properties, HPAs have been blended with polymer materials to form membrane-like film catalysts for the modified catalysis of HPAs [8-11]. It is expected that acidic and redox properties of HPAs can be easily modified by this method to meet the need for the specific reactions. Membrane-like film catalysts were prepared by membrane preparation technology using homogeneous HPApolymer solutions in this work. Preparation methods and characterization results of HPAimbedded polymer film catalysts depending on polymer materials and solvents were

478 investigated. They were used as fixed-bed catalysts for the vapor-phase ethanol conversion and MTBE decomposition in a continuous flow reactor.

2. EXPERIMENTAL 2.1. Preparation of HPA-imbedded polymer film catalyst H3PMo12040 (PMo, Aldrich) was used as a catalyst after purification. Polysulfone (PSF, Udel-1700 from Union Carbide), polyethersulfone (PES, Victrex 5200P from ICI), and polyphenylene oxide (PPO, poly-2,6-dimethyl-l,4-phenylene oxide from Aldrich) were used as blending polymers. Dimethylformamide (DMF), methanol (M)-chloroform (C) mixture or methanol (M)-benzene (B) mixture was used as a solvent. A homogeneous PMo(4.76 wt%)PSF(23.81 wt%)-DMF(71.43 wt%) solution, PMo(1.22 wt%)-PSF (or PES or PPO) (6.90 wt%)-M(4.41 wt%)-C(87.47 wt%) solution or PMo(2.33 wt%)-PSF(23.29 wt%)-M(6.15 wt%)-B(68.23 wt%) solution was casted on a glass plate with a constant thickness in ambient condition (56% relative humidity) to form a corresponding membrane-like film catalyst, and was subsequently dried for 4-5 hrs under the same condition. The thickness of the film catalysts was 0.017 mm. All the film catalysts were thermally treated at 170~ before reaction and characterization. Porosity of PMo-PSF-DMF film catalysts was controlled by modulating water vapor concentration (relative humidity). 2.2. Reaction and characterization Vapor-phase ethanol conversion and MTBE decomposition were carried out in a continuous flow reactor. The film catalyst was cut into small pieces (2 mm x 2 ram) and used as a fixed-bed catalyst. All the film catalysts were treated at 170~ for 1 hr by passing air (5 cc/min) before the reaction. Ethanol (or MTBE) was preheated for vaporization and fed to the reactor together with a carrier gas (air for ethanol conversion and helium for MTBE decomposition). The products were analyzed under a steady state condition with an on-line GC (HP 5890 II). The film catalysts were characterized by TPD, SEM (Jeol JMS-35), EDX (Philips PV-9900), DSC (TA Instruments TA200), XRD (Jeol JDX-5P) and ESCA (PerkinElmer PHI 581). All the film catalysts were thermally stable during the reaction because the reaction was carried out at temperatures below the glass transition temperatures of polymers.

3. RESULTS AND DISCUSSION 3.1. Characteristics and activities of PMo-PSF-DMF for ethanol conversion Thermal analysis revealed that the glass transition temperatures (Tg) of PSF-DMF (PMofree PSF film) and PMo-PSF-DMF were 187~ and 174~ respectively [9]. The decreased Tg of PSF after blending with PMo means that PMo in PMo-PSF-DMF acted as an impurity for PSF and that the blending of PMo with PSF was physical. The physical blending was also confirmed by observing the typical Keggin structure of PMo in PMo-PSF-DMF film by FT-IR. Oxidation state of molybdenum in bulk PMo and in PMo-PSF-DMF film was measured by ESCA. Only one type of molybdenum (VI) in both bulk PMo and PMo-PSF-DMF was confirmed. The binding energies of Mo 3d3/2 and Mo 3d5/2 in both catalysts were found to be 235.3 eV and 232.1 eV, respectively [8].

479 Fig. 1 shows the cross-sectional SEM images of PMo-PSF-DMF film catalysts. PMo-PSFDMF-1 and PMo-PSF-DMF-2 were prepared at 56% and 85% relative humidity, respectively. Two film catalysts showd well-developed macropores with no skin-layers. The porosity of PMo-PSF-DMF-1 and PMo-PSF-DMF-2 were 14% and 56%, respectively. No visible evidence representing PMo was found in SEM images of PMo-PSF-DMF films and there was no distinctive difference in SEM images between PSF-DMF and PMo-PSF-DMF. This indicates that PMo in PMo-PSF-DMF was not recrystallized into large particles but was highly dispersed as fine particles throughout PSF matrix. The uniform distribution of PMo in the PMo-PSF-DMF film was also confirmed by EDX analysis.

Figure 1. Cross-sectional SEM images of(a) PMo-PSF-DMF-1 and (b) PMo-PSF-DMF-2. The different pore characteristics of PMo-PSF-DMF film catalysts shown in Fig. 1 can be understood in terms of a phase inversion process [12]. The phase inversion is a process whereby a polymer is transformed in a controlled manner from a liquid to a solid state. Changes in temperature or composition lead to .the thermodynamical instability of the homogeneous polymer solution, and subsequently lead to the liquid-liquid demixing ; a polymer-rich phase and a polymer-lean phase. In this study, PMo-PSF-DMF film was exposed to the controlled water vapor concentration. DMF (solvent for PSF) and water (non-solvent for PSF) molecules exchange by diffusion. After a given period of time, the exchange of DMF and water vapor has proceeded so far that the solution becomes thermodynamically unstable and the polymer solution demixes into two liquid phases. Droplets of PSF-lean phase which will be pores in the long run grow further by gaining water from the vapor phase and by gaining DMF from the surrounding PSF-rich phase. The PSF-rich phase loses DMF to the droplets of PSF-lean phase and to the water vapor phase until the PSF-rich phase is precipitated into the cominuous PSF network. It is believed that PMo in polymer-rich phase migrates to the droplets of PSF-lean phase together with the migration of DMF because PMoDMF solution ~om PSF-rich phase is completely miscible with water. PMo concentration in the droplets increases during this entire process. It is inferred that a majority of PMo in PMoPSF-DMF catalyst exists on the surface of pore walls as an encapsulated and physisorbed state. The porous PMo-PSF-DMF film catalyst may be considered as a highly dispersed PMo catalyst supported on PSF matrix from this point of view. Pore characteristics of PMo-PSFDMF film catalysts were controllable by modulating water vapor concentration, as observed in Fig. 1.

480 Fig. 2 shows the XRD patterns of bulk PMo and PMo-PSF-DMF. Film catalyst and bulk PMo were treated at 170~ and 300~ respectively, under the air stream for 1 hr before XRD measurements. PMo-PSF-DMF had a different secondary crystal structure from bulk catalyst. The XRD peaks of PMo-PSF-DMF were attributed to only PMo and DMF because PSF was an amorphous polymer. This means that PMo in PMo-PSF-DMF existed as a modified secondary crystal structure. A DMF-TPD experiment on bulk PMo revealed DMF (organic base) was strongly adsorbed on the acid sites of PMo during the blending, and remained in the film catalyst during the reaction at 170~

l

(a)

20

Figure 2. XRD patterns of (a) bulk PMo and (b) PMo-PSF-DMF. Typical catalytic activities for ethanol conversion over bulk PMo and PMo-PSF-DMF film catalysts are summarized in Table 1. Ethylene and diethylether are formed by acid-catalyzed reaction whereas acetaldehyde is formed by oxidation reaction [10]. PMo-PSF-DMF film catalysts showed higher ethanol conversions than bulk PMo. PMo-PSF-DMF film catalysts showed remarkably enhanced yields for acetaldehyde, but they showed drastically decreased yields for ethylene and diethylether compared to bulk PMo. The enhanced ethanol conversions over PMo-PSF-DMF film catalysts were attributed to the enhanced oxidation activity over highly dispersed PMo catalyst throughout PSF matrix. PMo-PSF-DMF-2 showed a higher conversion than PMo-PSF-DMF-1. This may be attributed to the reduced mass transfer resistance of reaction species through the well-developed pores, as observed in Fig. 1. The suppressed activity for the acid-catalyzed reaction over PMo-PSF-DMF was due to strongly adsorbed DMF on the acid sites of PMo. The effect of DMF on the acid catalytic activity of PMo could be confirmed from the catalytic activity of PMo-DMF in Table 1. It is concluded that PMo can be modified by blending it with PSF using DMF to show selective oxidation activities at low reaction temperatures by suppressing acid-catalyzed reaction and by dispersing PMo finely throughout porous PSF matrix.

481 Table 1 Catalytic activity of PMo-PSF-DMF for the ethanol conversion at 170~ Catalyst EtOH conversion Amounts of EtOH converted to product (%) (xl 04 moles/g-PMo-hr) CH3CHO

C2H4

C2HsOC2H5

Bulk PMo a) 2.7 0.69 0.42 3.0 PMo-DNff"b) 1.5 1.50 0.19 0.50 PMo-PSF-DMF- 1~ 6.2 7.44 0.49 1.39 PMo-PSF-DMF-2 a) 9.6 11.50 0.63 2.23 W/F=66.73 g-PMo-hr/EtOH-mole ; air=5cc/min ; film thickness=0.017 mm; a)bulk PMo was treated at 300~ ; b) PMo was recrystallized from dimethylformamide and then treated at 170~ ; ~ it was prepared at 56% relative humidity ; a) it was prepared at 85% relative humidity

3.2. Characteristics and activities of PMo-PSF(PES, PPO)-MC for ethanol conversion HPAs and polymers can be easily blended if both materials are soluble in a common solvent as in the case of PMo-PSF-DMF. Although HPAs and polymers are not soluble in the same solvent, if a solvent dissolving HPAs and another solvent dissolving polymers are miscible, HPAs and polymers can be blended using solvent mixture. Methanol (M) was used as a solvent for PMo whereas chloroform (C) was used as a solvent for PSF (or PES or PPO) for the blending of these two materials because two solvents were miscible [ 11 ]. Thermal analysis revealed that the glass transition temperature of PSF (or PES) was decreased after blending with PMo while that of PPO was increased after blending with PMo. This means that there was neither interaction nor chemical bonding between PMo and PSF (or PES) and that PMo was physically blended with PSF (or PES) as in the case of PMo-PSFDMF. The above result also means that there was a certain interaction between PMo and PPO, and that PMo was physicochernically blended with PPO. Although chemical state of PMoPPO-MC is not clear, it is believed that PMo-PPO-MC may show a different catalytic activity from PMo-PSF-MC and PMo-PES-MC. Fig. 3 shows the SEM images of PMo-PSF-MC and PMo-PPO-MC. There was no distinctive difference in surface morphology between PSF-MC (PMo-free PSF film) and PMoPSF-MC, and no visible evidence representing PMo was found in PMo-PSF-MC. It was also observed that PES-MC (PMo-free PES film) and PMo-PES-MC showed the same surface morphology with PSF-MC and PMo-PSF-MC in SEM images. Above results mean that PMo was not recrystallized into large particles but was physically dispersed as fine particles throughout PMo-PSF-MC and PMo-PES-MC. PMo-PSF-MC and PMo-PES-MC film catalysts had no pore-like feature and their surfaces were dense and clean. Although PPO-MC (PMo-free PPO film) was a dense film having no pores like PMo-PSF (or PES)-MC, there was a distinctive difference in surface morphology between PPO-MC and PMo-PPO-MC. PMo in PMo-PPO-MC existed as large particles having diameters of 1 gm or less. This might be due to the different blending pattern of PMo-PPO-MC from PMo-PSF(or PES)-MC as evidenced by thermal analysis. All the film catalysts prepared in ambient condition had no macropores. It is clear that non-porosity of PMo-PSF (or PES or PPO)-MC was due to the immiscible nature of chloroform (a major component of the mixed solvent) with water vapor. Water vapor was not efficient for the pore formation through these film catalysts.

482

Figure 3. SEM images of (a) PMo-PSF-MC and (b) PMo-PPO-MC. Fig. 4 shows the XRD patterns of PMo-PSF-MC, PMo-PES-MC and PMo-PPO-MC. These film catalysts were treated at 170~ for 1 hr before XRD measurements. PMo in PMoPPO-MC existed as a modified crystal structure. On the other hand, however, PMo-PSF-MC and PMo-PES-MC showed no characteristic XRD peaks. This means that PMo in PMo-PSFMC and PMo-PES-MC film catalyst did not exist as a crystal structure but as an amorphous state. It is inferred that there may be some unidentified interactions among the blending components and that PMo may be dispersed as very fine particles throughout PSF and PES matrix. It is evident that PMo in PMo-PSF-MC and PMo-PES-MC had structural flexibility which was provided by forming a pseudo-liquid phase.

I

0

'

I

10

'

I

20

'

I

30

...... '

I

40

'

I

50

'

i

60

20

Figure 4. XRD patterns of (a) PMo-PSF-MC, (b)PMo-PES-MC and (c) PMo-PPO-MC. Ethanol conversions and product selectivities over the film catalysts are listed in Table 2. All the film catalysts showed higher ethanol conversions than bulk PMo. The enhanced conversions over the film catalysts were believed to be resulted from the enhanced PMo dispersion. The conversions were in the following order ; PMo-PSF-MC > PMo-PES-MC >

483 PMo-PPO-MC > PMo. PMo-PPO-MC showed the smallest conversion among three film catalysts. This may be partly resuked from partial agglomeration of PMo throughout PPO matrix. The fact that bulk PMo and PMo-MC showed similar ethanol conversion and product selectivity means that the mixed solvent had no influence on the catalytic activity of PMo unlike DMF of PMo-PSF-DMF. PMo-PSF-MC and PMo-PES-MC showed enhanced oxidation and acidic catalytic activities compared to the bulk PMo. The selectivity to oxidation over PMo-PPO-MC was three times or more compared with the other two film catalysts and that to dehydration was 50% or less. Lower surface area of PMo-PPO-MC may be responsible for the lower activity. It is believed that the interaction between PMo and PPO contributed to the inhibition of acidic activity of PMo-PPO-MC. Permeabilities of reaction components through the film catalyst also affected the product selectivity. Perm-selectivity of O2/ethanol measured by membrane technique at 80~ and acetaldehyde selectivity in Table 2 showed the same trend in the following fashion ; PMo-PPO-MC > PMo-PSF-MC > PMo-PES-MC. The structural flexibility of PMo may be responsible for the enhanced catalysis of non-porous PMoPSF-MC and PMo-PES-MC. It is believed that the pseudo-liquid phase of PMo in PMo-PSFMC and PMo-PES-MC which had more structural flexibility than the crystal structure may offer selective penetration sites for reaction species with low mass transfer resistance. Table 2 Catalytic activity of PMo-polymer-MC for the ethanol conversion at 170~ Catalyst EtOH conversion Amount of EtOH converted to product (%) (xl 04 moles/g-PMo-hr) (carbon selectivity) CH3CHO C2H4 C2HsOC2H5 Bulk PMo a) 6.9 0.52(12.8) 0.34(8.4) 3.22(78.8) PMo-MC b) 7.4 0.46(10.5) 0.38(8.6) 3.54(80.9) PMo-PSF-MC ~ 39.5 4.67(20.0) 3.76(16.1) 14.93(63.9) PMo-PES-MC r 33.7 1.79(9.0) 6.46(32.4) 11.68(58.6) PMo-PPO-MCo) 13.4 4.71(59.4) 0.78(9.8) 2.44(30.8) W/F=169.1 g-PMo-hr/EtOH-mole ; air=5cc/min ; film thickness=0.017 mm; a)bulk PMo was treated at 300~ ; b) PMo was recrystallized from methanol-chloroform mixture and then treated at 170~ ; r film catalyst was treated at 170~

3.3. Characteristics and activities of PMo-PSF-MB for MTBE decomposition PMo-PSF-MB film catalyst was also successfully obtained using methanol (M)-benzene (B) mixture as in the case of PMo-PSF-MC. Benzene was used as a solvent for PSF while methanol was used for PMo. The glass transition temperatures of PSF-MB (PMo-ffee PSF film) and PMo-PSF-MB were found to be 193~ and 179~ respectively. This means that the blending of PMo-PSF-MB was physical. SEM analysis revealed that there was no difference in surface morphology between PSF-MB and PMo-PSF-MB, and their surface morphologies just looked like the image of Fig. 3(a). PMo was also finely dispersed throughout PMo-PSF-MB. XRD measurement represented that PMo in PMo-PSF-MB also existed as an amorphous phase. Characteristics of PMo-PSF-MB were very much similar to those of PMo-PSF-MC.

484 Table 3 Catalytic activity of PMo-PSF-MB and bulk PMo for the MTBE decomposition Temp.(~ MTBE conversion (%) Bulk PMo PMo-PSF-MB al 130 15.0 37.7 140 20.0 43.3 150 21.7 65.0 160 28.3 71.1 W/F =17.15 g-PMo-hr/MTBE-mole ; helium=5 cc/min ; a)thickness=0.017 mm

Table 3 shows the catalytic activity of PMo-PSF-MB film catalyst for the vapor-phase MTBE decomposition. The PMo-PSF-MB showed enhanced MTBE conversions compared to bulk PMo. The catalytic activities of PMo-PSF-MB were enhanced due to the fine dispersion of PMo throughout PSF support and the structural flexibility of PMo in the film catalyst which was provided by forming a pseudo-liquid phase.

4. CONCLUSIONS HPA-imbedded polymer film catalysts were prepared by membrane preparation technique. They were applied to the vapor-phase ethanol conversion and MTBE (methyl tert-butyl ether) decomposition. Porosity of the film catalysts was successfully controlled by the phase inversion method. It was found that PMo was finely and uniformly dispersed throughout polymer matrix. Conversion and product yield over film catalysts were affected by the nature of solvent and polymer used. The film catalysts could be applied to the low temperature oxidation reactions with high yield and selectivity for oxidation products by enhancing catalyst dispersion and by suppressing acid-catalyzed reaction. It was also revealed that acid catalysis of HPAs could be enhanced by the mutual action of solvent and polymer used. Choosing suitable solvent and polymer was the key step for the modification of novel acid catalysis of HPAs. It is expected that HPA-blended polymer film catalysts can be applied to the liquidphase heterogeneous reactions to meet the need for environmentally benign process.

REFERENCES

1. M. Ai, Appl. Catal., 71 (1981) 88. 2. H. C. Kim, S. H. Moon and W. Y. Lee, Chem. Lett., (1991) 447. 3. M. Misono, Catal. Rev.-Sci. Eng., 29 (1987) 269. 4. J. Pozniczek, I. Kulszewicz-Bajer, M. Zagorska, K. Kruczala, K. Dyrek, A. Bielanski and A. Pron, J. Catal., 132 (1991) 311. 5. M. Hasik, W. Turex, E. Stochmal, M. Lapkowski and A. Pron, J. Catal., 147 (1994) 544. 6. M. Misono, Mater. Chem. Phys., 17 (1987) 103. 7. I. V. Kozhevnikov, Catal. Rev. -Sci. Eng., 37 (1995) 311. 8. I. K. Song, S. K. Shin and W. Y. Lee, J. Catal., 144 (1993) 348. 9. I. K. Song, J. K. Lee, G. I. Park and W. Y. Lee, Stud. Surf. Sci. Catal., 110 (1997) 1083. 10. J. K. Lee, I. K. Song, J. J. Kim and W. Y. Lee, J. Mol. Catal., A104 (1996) 311. 11. J. K. Lee, I. K. Song and W. Y. Lee, J. Mol. Catal., A120 (1997) 207. 12. C. Huang, M. O. Delacruz and B. W. Swift, Macromol., 28 (1995) 7996.

91998ElsevierScienceB.V.All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

485

R e a c t i v e s p u t t e r i n g as a tool for p r e p a r i n g p h o t o c a t a l y s t s D. Dumitriua, A. R. Bally b, C. Ballif~, V. I. Parvulescu c, P. E. Schmid b, R. Sanjin~s b and F. I~vy b aInstitute for Nonferrous and Rare Metals, Bd. Biruintei 102, 73957, Bucharest, Romania bEcole Polytechnique F~d~rale de Lausanne, Institut de Physique Appliqu~e, D~partement de Physique, PHB - Ecublens CH - 1015, Lausanne, Suisse cUniversity of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Republicii 13, 70346, Bucharest, Romania

TiO2 thin films with different thickness (0.4 ~m, 1 ~m, 1.7 ~m) deposited over various substrates (glass, quartz, alumina) were prepared by DC reactive sputtering. The structural characteristics of the films and their morphology, obtained by X-ray diffraction and by atomic force microscopy, are related to the deposition parameters. The catalytic properties of TiO2 thin films in the oxidation of toluene, and in particular their deactivation behavior, show a sensitivity to the morphology of the thin film.

1. INTRODUCTION TiO2 has been extensively employed in photocatalytic degradation of pollutant chemicals because of its high activity and chemical stability, and its practical application has been seriously considered [1-7]. Studies made up to date showed that crystallographic structure and surface properties, which are strongly related to the preparation method, are very important factors [5, 8-11]. Generally, it is shown that rutile is a less efficient photocatalyst than anatase [8-10], this feature being mainly attributed to a higher electron-hole recombination rate. When rutile is obtained at a low calcination temperature, a high concentration of OH groups is present on the surface and thereby the catalytic activity is improved [11]. The primary role of the OH groups seems to consist in stabilizing the holes in the valence band and electrons in the conduction band and preventing their recombination. Many attempts have been made to immobilize the catalyst on rigid supports, because it is a technological requirement to avoid the catalyst filtration step [7, 12-14]. However, due to the complexity of the photocatalytic process, more work

P

Table 1 Parameters of Ti02 thin films deposition in mixed reactive gas (Ozor HzO) and Ar atmosphere; sputtering voltage: 1250 V; target: metal titanium, purity 99.5 %. Structural characteristics of supported films ( wt.% of each crystalline phase and averaged sizes of crystallites, nm)

A

B

C

D

E

51-115 (no heater) 0.001

33-115 (no heater) 0.001

250

250

0.001

50-147 (no heater) 0.001

0.001

0.0007

0.0007

0.0007

0.0007

0.0007

Deposition Temperature

(OC)

Total pressure (mbar) Ar pressure (mbar) Reactive gas pressure (mbar) Deposition rate

(02)

0.0003

(02)

0.0003

(02) 0.0003

0.0003

(02)

(HzO) 0.0003

0.78

0.76

0.37

0.73

0.32

0.4

1

0.4

1.7

0.4

amorphous

amorphous

TiOdquartz

amorphous

amorphous

TiOdAlz03

amorphous

amorphous

anatase 100% 25 nm anatase rutile 51% 49% 44nm 10.2nm anatase rutile 92% 8% 30nm 8.5nm

(W

Film thickness (clm) Structural characteristics TiOdglass

anatase 33% 19 nm anatase 36% 31.4 nm anatase 61% 24 nm

rutile 67% 7 nm rutile 64% 7 nm rutile 39% 6 nm

anatase rutile 82% 18% 15 nm 30 nm anatase rutile 47% 53% 25.3 nm 11.4 nm anatase rutile 5% 95% 9.8 nm 37 nm

00

o!

487 is necessary to elucidate how the characteristics of the material influence the catalytic process. Chemical deposition techniques, well known as inexpensive procedures, do not offer a real possibility to prepare a catalyst film in a controlled manner [15]. The aim of this study was to prepare TiO2 thin films by DC reactive sputtering over glass, quartz and alumina (corundum) substrates. The sputtering technique provides good perspectives for the elaboration of a physical model describing the behavior of thin films in photocatalytic processes, because film thickness, crystallinity, morphology and dopant concentration can all be well controlled by sputtering, and various materials can be used as substrates [16-18].

2. E X P E R I M E N T A L Thin films depositions were carried out using a Sputtron II, Balzers equipment. The deposition parameters are presented in Table 1. For each deposition, various substrate materials were introduced in the deposition chamber such as: glass (microscope slides, RE-WA Lehmann-Schmidt), quartz (Heraeus Quartzglass) and alumina (polished alumina, corundum structure, Raymond & Co SA). Thin films thickness measurements were realized with an Alphastep 500, Surface Profiler, Tencor Instrument. XRD patterns were obtained with a Rigaku diffractometer in the theta-2 theta mode using CuKa radiation. The anatase/rutile ratios were deduced from the relative areas under the anatase (101) and rutile (110) diffraction lines, using a weighting factor of 1.265 for the rutile line. The full widths at half maximum of these XRD lines provided the crystallite sizes for each phase, based on the Scherrer equation [19]. Atomic force microscopy (AFM) observations were performed using a Topometrix Explorer AFM in the non-contact mode. The catalytic experiments consisted in the oxidation of toluene in the gas phase, in a recycling batch reactor, under near-UV radiation. A 100-W black light (UVP) lamp was used to provide near-UV illumination. All experiments were performed at 22-24oc. An air stream containing 100 ppm toluene and 20 % relative humidity was passed over the catalyst film (with an average geometrical section about 1 cm 2) with a recirculation time of 20 s. After the air recirculation had stabilized, water was injected and allowed to evaporate and circulate until the adsorption equilibrium was reached. At that time, toluene was injected in liquid form and allowed to evaporate, circulate and reach the gas-solid equilibration. Then the light was turned on, and the gas-phase was sampled periodically (every 15 rain.). All air samples were analyzed by gas chromatography (Perkin-Elmer Sigma Series 1) operating with a flame ionization detector (FID) and an Alltech C-5000 column, with AT-1000 Chromosorb W-AW 80/100 packaging. Carbon dioxide was measured with an infrared gas analyzer (Horiba Model PIR-2000).

488 3. R E S U L T S AND DISCUSSION The textural and structural characteristics of TiO2 films have been correlated with the following deposition parameters: substrate temperature, deposition time, nature of the reactive gas and nature of the substrates. 3.1. I n f l u e n c e of substrate t e m p e r a t u r e A simple way to obtain TiO2 deposited samples with the expected structural characteristics is to control the substrate temperature during deposition. To a large degree the deposition temperature defines which of the crystalline or amorphous forms of titania will be present in the thin film. For example, depositions A and B were carried out in a low temperature range (below 115oc). No heater was used in this case, the temperature increase being due only to exothermic processes during deposition. As a result, no crystalline structure was evidenced by XRD for these samples. Only amorphous titania was deposited over all the substrates used, because in this temperature range, the mobility of surface atoms is low enough to prevent crystalline growth during deposition. In case of deposition C, the temperature of the substrates was controlled and maintained very close to 250~ This resulted in a crystallization predominantly in the anatase of TiO2, as shown in the X-rays diffractograms (Figure 1). 800

250 200

600

150

400

100

200

"1302/quartz (C)

TiO2/glass (C) .-,--~,,

30

2O

2000

40 2 Theta

50

60

20

30

40 2 Theta

TiO2/AI203 (C)

1500 1000 500

.... ~_~ i

20

30

40 2 Theta

50

60

Figure 1. XRD patterns of TiO2 films resulted from deposition C.

50

60

489 The composition of the films in wt % of each detected crystalline phase and the averaged size of the crystallites, calculated from the XRD data, are presented in Table 1. On glass substrates, only the anatase phase was detected, while on alumina or quartz, crystallization of rutile is well evidenced. In these cases, the anatase crystallites are larger than rutile crystallites. This is an expected phenomenon, considering that rutile is the crystalline structure generally obtained by calcination of TiO~ at higher temperature.

3.2. Influence o f d e p o s i t i o n time Using the same TiO2 deposition rate (0.76-0.78 ~/s), the increase of the film thickness was obtained by prolonging the deposition time. Samples A, with an approximate 0.4 ~m thickness, and samples B, with an approximate 1 ~m thickness, were obtained in 85 rain., and respectively 219 rain. No structural differences were detected by XRD between these samples, all of them showing an amorphous structure. With the intention of preparing thicker films, in case of samples D, a longer deposition time was used (388 rain). This had a remarkable effect on crystalline structure of the deposited TiO2. So, even at low temperature (50-147oc), but after a prolonged time, in case of samples D, crystallization of TiO2 was observed, with a predominance of the rutile phase. XRD data recorded for these samples are showed in Figure 2, and calculated characteristics are presented in Table 1. 800 ,

250

-

200

600 400

200

t

150 100 _j~

20

-

TiO2/glass (D) "'

i

J

i

30

40 2 Theta

50

"1302/quartz (D)

50 0

-

60

~

20

30

J

40 2 Theta

2000 TiO2/AI203 (D)

1500 1000 500

i

20

30

40

2 Theta

50

60

Figure 2. XRD patterns of TiO2 films resulted from deposition D.

a

50

60

490 The sizes of rutile crystallites are small (6-7 nm), even if this phase is predominant (wt%) - for D TiO2/glass, D TiO2/quartz. A high rutile nucleation site density during film growth is suggested by these results. Energetic constraints, resulted from local caloric accumulation, could be the reasons of this feature. 3.3 I n f l u e n c e o f t h e r e a c t i v e g a s Changing the composition of the reactive gas resulted in both structural and morphological modifications. Samples C were deposited with 02, while samples E were deposited with H20 at the same partial pressure (0.0003 mbar). The other deposition parameters were identical in both cases. XRD data recorded for samples E are presented in Figure 3, and the corresponding crystallites sizes and phase composition are included in Table 1. A tendency to rutilization is visible especially for sample E TiO2/glass, for which large rutile crystallites were detected in comparison with sample C TiO2/glass. 800

250

600

200 150

400

100

200

.....L

50

........ _ i l l .....TiO2/gl.ssa (E) 30

20

4O

2 Theta

5O

60

5O

60

20

30

40 2 Theta

50

60'

2O0O TiO2/AI203 (E)

1500 1000 5OO

J

2O

30

40 2 Theta

Figure 3. XRD patterns of TiO2 films resulted from deposition E. AFM characterizations of samples C and E of TiO2/glass show very different morphologies. For TiO2/glass sample C, a continuous, rough film, was observed (Figure 4). For TiO2/glass sample E, the films reveals a network of pores between particles with an approximately monodisperse size distribution (Figure 5).

491 The morphology of the films is related with both the energy and the nature of the depositing species. For sample E, randomly directed neutrals (atoms or radicals), and maybe in a higher energy state particles bombarding the substrate could be responsible for the small grain sizes and void formation. Heating effects are responsible for the observed rutile crystallographic modification.

Figure 4. AFM characterization of sample C TiO2/glass.

Figure 5. AFM characterization of sample E WiO2/glass.

3.4. I n f l u e n c e o f the substrate Examining the XRD results, the effects induced by substrate nature on the structural characteristics of the deposited films are as follows: alumina (corundum) substrates used in our experiments induce a preferential anatase crystallization, while quartz seems to keep the anatase/rutile (wt %) ratio close to 1:1. The deposition of TiO2 films on glass appears to be more difficult to control than the deposition on quartz or alumina. The adhesion of the films to the substrate, their crystallographic and morphological features could be the subject of further investigations as a function of the deposition rate, of the smoothness, the surface composition and the structure of the substrate.

3.5. C a t a l y t i c p r o p e r t i e s vs. m o r p h o l o g y of TiO~ t h i n films The photocatalytic properties of the TiO2 films were investigated by testing the oxidation of toluene in a wet air stream, under UV irradiation. The toluene concentration decrease in the air stream vs. irradiation time, in successive experiments for TiO2/glass samples C and E, are presented in Figures 6 and 7. In spite of the fact that it contained not only the anatase, but also the rutile phase, TiO2/glass sample E was more active in the first run of experiments than sample C. A higher surface area of sample E could be a reason of this behavior.

492 CO2 was found to be the main reaction product, trace benzaldehyde, as partial oxidation product, being also present in the gas phase. Catalytic experiments carried out in repeated runs without catalyst regeneration before each reuse showed deactivation tendencies which were different for the TiO2/glass C and E films, mainly according to their morphology. For sample E, a more pronounced deactivation tendency appeared, while sample C was less sensitive to this phenomenon. The textural properties of TiO2/glass sample E seem to promote the formation of carbonaceous deposits (known as the source of photocatalysts in gas-phase deactivation [20, 21]) during photocatalytic reactions. Benzaldehyde, which is more abundant in the reaction effluent in experiments carried out over TiO2/glass sample C than over sample E, could be one of the precursors of such deposits. These results are in agreement with the observations of Ollis et al. [22]. Steric constraints, imposed by the porous structure of the films, could favour the readsorbtion of desorbed benzaldehyde, and its overoxidation. E 100 ,.-

90

~

80

0

I

E

~ooOn.,,.

t.J

80

r0c-9 70 O o 60 e--

~

0 ~

oo O

r~

50 9

40

0

100 ~~-~~~oAAA

l

{

100

200

t

70

Figure 6. Toluene concentration in gas stream vs. irradiation time, for sample C TiO2/glass; 0 first run; osecond run;athird run.

000

~

r'A~AA A

oo

nO 0000

50

40

Time (min)

-OO"OOn ~ o O~

-

60

~0 t 300

~A,~,~AAA

0

i i 100 200 Time (min)

300

Figure 7. Toluene concentration in gas stream vs. irradiation time, for sample E TiO2/glass; 0 first run; o second run;~third run.

4. CONCLUSIONS DC reactive sputtering is an effective technique to prepare thin film, dispersed, nanostructured, TiO2 photocatalysts. The key parameters, controlling both the crystallinity and morphology of the films, are the substrate temperature, the nature of reactive gas and nature of the substrate. The photocatalytic properties, and in particular the deactivation behavior, are sensitive to the film morphology and can be optimized by a proper choice of deposition parameters.

493 REFERENCES

1. G. Ruppert and R. Bauer, Chemosphere No. 8 (1994) 1447. 2. L. Minsker, C. Pulgarin, P. Peringer and J. Kiwi, New J. Chem., 18 (1994) 793. 3. U. Stafford, K.A. Gray and P.V. Kamat, Heterogeneous Chemistry Reviews, 3 (1996) 77. 4. H. Idriss, A. Miller and E. G. Seebauer, Catal. Today,~33 (1997) 215. 5. A.P. Rivera, K. Tanaka and T. Hisanaga, Appl. Catal. B: Env., 3 (1993) 37. 6. C. Bouquet-Somrani, A. Finiels, P. Graffin and J-L. Olive, Appl. Catal. B" Env., 8 (1996) 101. 7. S.A. Larson, J. A. Widegren and J. L. Falconer, J. Catal., 157 (1995) 611. 8. S.-I. Nishimoto, B. Ohtani, H. Kajiwara and T. Kagiya, J. Chem. Soc., Faraday Trans., 181 (1985) 61. 9. D.F. Ollis and H. A1-Ekabi (eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. 10. M.A. Fox and M.T. Dulay, Chem. Rev., 93 (1993) 541. 11. S-J. Tsai and S. Cheng, Catal. Today, 33 (1997) 227. 12. I. Sopyan, S. Murasawa, K. Hashimoto and A. Fujishima, Chem. Lett. (1994) 723. 13. A. Fernandez, G. Lassaletta, V.M. Jim~nez, A. Justo, A.R. Gonz~lez-Elipe, J.-M. Herrmann, H. Tahiti and Y. Ait-Ichou, Appl. Catal. B: Env., 7 (1995) 49. 14. M. L. Sauer and D. F. Ollis, J. Catal., 158 (1996) 570. 15. P.V. Kamat and D. Meisel (eds.), Semiconductor Nanoclusters - Physical, Chemical and Catalytic Aspects, Stud. Surf. Sci. Catal., Elsevier, 1997. 16. R. Messier, J. Vac. Sci. Technol. A. No.3 (1986) 490. 17. H. Tang, K. Prasad, R. Sanjin~s, P.E. Schmid and F. L~vy, J. Appl. Phys., 75 (1994) 2042. 18. A. R. BaUy, P.E. Schmid, F. L~vy, J. Benoit, C. Barthou and P. BenaUoul, Jpn. J. Appl. Phys., 36 (1997) 5696. 19. Wiley & Sons (eds.), X-Ray Diffraction Procedures, New-York, 1974. 20. J. Peral and D. F. Ollis, J. Mol. Catal. A: Chem., 115 (1997) 347. 21. S.A. Larson and J. F. Falconer, Catal. Lett., 44 (1997) 57. 22. Y. Luo and D. F. Ollis, J. Catal., 163 (1996) 1.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

495

P r e p a r a t i o n of zirconium oxide particles for catalyst supports by the m i c r o e m u l s i o n technique. Characterization by X - R a y diffraction, BET, S E M - E D X , F T - I R and catalytic tests. Magali Boutonnet Kizling and Francesco Regali Department of Chemical Engineering and Technology, Chemical Technology, The Royal Institute of Technology Teknikringen 42, S-10044 Stockholm, Sweden SUMMARY

In this ~work the preparation of zirconium oxides designed for catalyst supports was studied. The platinum promoted sulfated zirconia catalyst is prepared using microemulsion systems (anionic and nonionic) and precipitation-impregnation procedures. Catalyst particles were characterized in detail using X-ray, BET, SEM, EDX and FT-IR. Zirconia particles prepared in microemulsion have a pure tetragonal structure and are smaller than precipitated particles. Infrared spectra show that the sulfate species are more homogeneously distributed over the zirconia surfaces when the sulfating process is carried out in the microemulsion, than when the sulfating is performed by impregnation. Catalytic tests for isomerization reactions of hexanes show that the supports prepared by the microemulsion method have higher selectivity towards isomers but lower activity than the precipitation-impregnation method supports. Platinum promoted catalysts prepared in microemulsions show higher selectivity and slightly lower activity than those prepared by impregnation in a water solution. 1. INTRODUCTION The subject of this work is the preparation and assessment of zirconium oxide supported metal catalysts using the microemulsion technique. This novel technique, which exploits the compartmentalizing ability of microemulsions, was used to prepare particles of Pt, Pd and Rh in the nanometer size range with a very narrow size distributions [ 1]. While well established for producing metal catalyst particles, microemulsions have not yet been yet used to prepare the metal oxide particles for supported catalysts. The possibility of microemulsions to control the particle size at the nanometer level can be utilized in catalyst support synthesis. Of particular interest is the preparation of metal promoted sulfated zirconia supports. The sulfated zirconia was promoted with platinum metal particles, using a microemulsion as both the synthesis and deposition medium. The metal promoted sulfated zirconia has attracted attention of many researchers as it has shown good catalytic activity for the isomerization of hydrocarbons. So far there are no publications on this subject. Two different microemulsion systems and the traditional preparation are compared; the oxide particles are studied with respect to their physical

496 properties, such as crystalline structure, chemical composition and their dependence on thermal treatment. The usual sulfating procedure for zirconium oxide is impregnation with sulfuric acid, followed by evaporation of the solvent and calcination [2]. Catalytic activity tests, i.e. isomerization of hexanes, are carried out on the platinumpromoted sulfated zirconium oxide. Many of the catalysts currently used in the isomerization of paraffins require high temperatures to achieve a reasonable reaction rate. However, the platinum promoted sulfated zirconia has been able to convert linear alkanes to branched ones at much lower temperatures (150~ or even less) [3]. The superacidic characteristics of sulfated zirconia, enhanced by the presence of metal sites, lead to superior selectivities towards isomerization. 2. E X P E R I M E N T A L

2.1. Catalyst Preparation Two different zirconia sulfating procedures were compared i) the incorporation of sulfate ions in the water domains of microemulsions as in Ref.[4]; and ii) the impregnation with sulfuric acid of the zirconia particles prepared in microemulsion. Platinum particles were prepared in a microemulsion system previously described in [ 1]. For comparative purposes, the preparation of the platinum promoted sulfated zirconia catalysts were also performed by a traditional precipitation-impregnation method. 2.1.1 Preparation of Platinum Promoted Sulfated Zirconia 2. 1.1.1 Formation of SO42"-ZRO2in Microemulsion in Presence of Sulfate (System

z) A solution consisting of nonionic surfactant (Nonylphenyl ethoxilate), cyclohexane and an aqueous solution of sulfuric acid was mixed with a solution containing zirconium npropoxide rather than sulfuric acid (Table 1). A microemulsion is formed after mixing. This microemulsion was stirred for approx. 72 hours to achieve hydrolysis of the zirconium alkoxide. The hydrous sulfated zirconia was separated after the addition of tetrahydrofuran, repeated centrifugation and washing with ethanol. The product was dried overnight at 100~ and calcined at 600~ for 6 hours, it was then used as a support in catalyst K1 (See section 3). Table 1. Composition and parameters for Z, A and P systems. iS~vstem Composition Weight % Molar Ratios Z Nonionic surfact. (Berol 02) 18.4 water/surfactant Solvent (cyclohexane) 73.6 water/alkoxide Water 6.1 H2SO4/surfactant H2SO4 0.66 Zirconium prop0xide ....... 1.25 Anionic surfactant (AOT) 8.6 water/suffactant A Solvent (isooctane) 48.7 water/alkoxide Water 4.2 Zirconium propoxide 38.5 Surfactant (PEGDE) 19 water)surfactant Solvent (n-octane) 75.9 Water 5 Platinum 0.1

-10 -90 0.17

-2 -10

-7

497

2.1.1.2 Formation of ZrO2 in Microemulsion and Impregnation with H2SO4 (System A). The microemulsion used in this preparation consists of an ionic surfactant sodium bis (2-ethyl-hexyl) sulfosuccinate surfactant, isooctane and water (Table 1). Zirconium npropoxide (diluted with propanol) was added directly to the microemulsion during ultrasonic treatment. The resulting gel-like solution was stirred for approx. 72 hours to achieve hydrolysis. Hydrous zirconia was recovered by repeated centrifugation and washing with ethanol, it was then dried at 100~ overnight. Sulfating of this powder was performed by treatment with 2 ml of 0.5 M H2SO4per gram of powder for 5 hours. After drying overnight at 100~ the powder was calcined at 600 ~ for 6 hours and used as a support for catalyst K2. 2.1.1.3 so4Z'-ZrO2 by Precipitation-lmpregnation Method Zirconium hydroxide was synthesized by hydrolysis of ZrOC12. 8H20 with 4M NaOH. It was recoverd by repeated centrifugation and washing with H20 (Table 1). The sulfating and calcination processes were similar to that described in the previous section. This material was used as a support in K3 and K4 catalysts. Platinum nanoparticles were synthesized in a microemulsion medium (System P) as described in [4] (Catalyst K3). The deposition of platinum on the zirconia support was also carried out using the incipient wetness method. In this case, the reduction was performed directly in the reactor before the catalytic test (Catalyst K 4). All the catalysts were prepared with a metal loading of 2 wt%. 2. 2 Particle Characterization Diffraction patterns were obtained on a Philips D5000 Diffractometer. Specific surface area measurements were carried out by the BET nitrogen adsorption method using a ASAP 2000 from Micrometrics. The SEM-EDX technique has been used for particle size estimation and elemental composition. The instrument was a Zeiss Digital Scanning Microscope DSM 940, equipped with the energy dispersive spectrometer for X-ray analysis Link QX 2000. FTIR spectroscopy for sulfate analysis in the sulfated zirconia was carried out on a Perkin-Elmer FT-IR Spectrometer 1760X in the transmission mode. 2. 3. The Catalytic Test Reaction The catalytic activity and selectivity of the platinum promoted sulfated zirconia catalysts were tested for the isomerization of hexanes. The Table 2 summarizes the preparation procedures for the catalysts tested. Table 2 _Catalyst* K1 K2 K3 _K4 * Calcination

Support (ZrO2) ME system Z ME system A Precipitation Precipitation of the support: 600~

Sulfating ME system Z Impregnation Impregnation Impregnation h; H2 pretreatment:

P.t deposition ME system P ME system P ME system P Impregnation 200~ lh.

10 % Pt/AI203 particles prepared by the impregnation technique and reduced in Ha at 200~ for 24 hours were used as a reference. Isomerization of 2-methylpentane and hydrogenolysis of methylcyclopentane were studied in a gas flow reactor at atmospheric

498 pressure. The catalysts were activated in situ prior to the reaction in hydrogen for 1 hour at 200~ Isomerization of 2-methylpentane was carried out at 300 ~ The products were analysed by gas chromatography using a HP 19091F-112 column. 3. RESULTS AND DISCUSSION

3.1. Mode of preparation The preparation of zirconia particles by precipitation is straight forward as the solution is water based and the process is fast. On the other hand the microemulsion based method has the advantage of producing monodispersed particles of high purity. Furthermore, the hydrolysis of the zirconia precursor in the presence of sulfate ions can only be performed in microemulsions, as sulfate ions would be washed away in the precipitation procedure. In addition, replacing a two step preparation by a one step procedure is a clear advantage of the microemulsion mediated synthesis. The zirconia prepared by precipitation contained both chloride and sodium ions, hence care was taken to wash the samples thoroughly. Sulfating by the incipient wetness impregnation method was preferred to soaking with excess sulfuric acid as better reproducibility was obtained. Comparing the zirconia obtained from the system Z (Berol 02/cyclohexane/sulfuric acid) with the system A (AOT/isooctane/water) the following comments can be made: For system Z, separation of the product was difficult. The separation was achieved by dilution with tetrahydrofuran and centrifugation. Washing the hydroxide gel with ethanol failed to remove all the surfactant. Thermal treatment was necessary to remove the remaining surfactant. The drawback of the microemulsion system Z is its low capacity: one liter of solution was needed to synthesize a few grams of sulfated zirconia. Concerning system A, the microemulsion was easier to destabilize. Repeated centrifugation was enough to recover most of the product. The ionic surfactant was completely removed by three or four washings in ethanol. System A had a higher product-to-microemulsion volume ratio since the zirconia precursor cons-tituted 38 % of the total weight, compared with the 1.25% in the system Z. Platinum deposited on the sulfated zirconia support synthesized by the microemulsion method was reduced at room temperature, while platinum impregnated in aqueous solution required temperatures of 200~ for reduction. The microemulsion method permits control of the size of metal particles, whereas the size of the metal particles obtained by impregnation is controlled by the pore size of the support and the concentration of salt in the solution. Catalysts K3 and K4 differ only in the platinum deposition method, thus comparison of these catalysts allows the influence of the preparation techniques on the properties of the catalysts to be examined. 3.2. Characterization The studied zirconia samples are listed in Table 3. Table 3 Sample SZMEA SZMEZ SZp. i. ..

Preparation method . . . . . Microemulsion system A + Impregnation Microemulsion system Z Precipitation-Impregnation

Support for catalyst K2 K1 K3, K4

499 Zirconium oxide can have two crystalline structures, tetragonal and monoclinic. The tetragonal phase is preferred for catalytic purposes due to its larger surface area. X-ray diffraction analyses showed that tetragonal zirconia is the prevalent crystal phase in all the prepared catalyst supports (Fig.l). The tetragonal phase is characterized by a major peak at 20 = 30 ~ Neverthless, sulfated zirconia obtained from precipitation-impregnation method (SZp.i.) contains a small amount of the monoclinic phase (Peaks at 20 -28,2 ~ and 31,3~ Microemulsion system A (SZMEA) yielded the highly crystallized zirconia, yet system Z (SZMEZ) showed exclusively tetragonal zirconia pattern, indicating that when the synthesis of zirconia takes place at the same time as the sulfating process the formation of the desired tetragonal phase is favored (Fig. 1). Samples produced from the microemulsion system A were calcined at various temperatures and analyzed by tetragonal phase XRD. The synthetized material remains amorphous up to 400~ while at ./ 500~ it showed a crystalline structure onocl~nic phase consisting mainly of the tetragonal a) zirconia phase. A sample of zirconium oxide was .... ---b) prepared from the micro-emulsion system Z. The monoclinic phase (peak at 20 -- 28,2 ~) was found in this 20 40 60 2 theta sample, while the sulfated zirconia (SZMEZ) showed solely the tetragonal Fig.1. XRD spectra. phase pattern (Fig.2). Thus, as above, a) Microemulsion,system A; the presence of sulfate ions favors the b) Precipitation, impregnation; formation of the tetragonal phase. c) Microemulsion, system Z.

Figure 2. XRD spectra. a)Microemulsion, system Z; b) Microemulsion, system Z without sulfate.

20

40

2 theta

60

In Table 4 we present the surface areas of the different zirconium oxides. All the samples were calcined in air at 600~ for 6 hours.

500 Table 4 Sample SZMEA SZMEZ SZp.i.

Preparation method Microemulsion system A + Impregnation Microemulsion system Z Precipitation-Impregnation

Support for catalyst K2 K1 K3, K4

BET Surface Area [mZ/g] 41 29 13

The low surface area of the sample SZp.i. confirms the presence of the monoclinic phase. For the SZMEA sample, the specific surface area was measured after calcination at different temperatures. Measurements were also done on zirconium oxide, before impregnation with sulfuric acid, after drying at 100 ~ and then after calcination at 600~ Results are presented in the Figure 3. The sample surface area decreases with calcination temperature due to the transition of the amorphous material into crystalline material. However, despite a substantial decrease in surface area between 250~ and 400~ no crystalline structure of the material was detected by X-ray diffraction. The non-sulfated sample showed a much high surface area after drying at 100~ (318 m2/g) than the sulfated sample BET Surface ~ [rn2/g] (198 m2/g). The additional drying at 350 '31 "-~ ~o~ sulfate 100~ after the sulfate impregnation 300 3~ could explain this difference. As expected the surface area of the calcined (600~ non-sulfated zirconia SZMEA is much lower (nearly half) than that of 97 the sulfated tetragonal zirconia 41 (SZMEA). 100

300 400 500 200 Calcination Temperature [~

22 600 Fig.3. Influence of temperature and sulfate ions on BET surface area

EDX spectra of the samples SZp.i. and SZMEZ are presented in Figures 4a and 5a. Elemental analysis showed a high content of zirconium and oxygen, as expected. The sulfate peak appears as a small shoulder on the right side of the major zirconium peak. It is, however, not visible for all the studied sulfated zirconia samples. A small peak in the spectrum for SZp.i. (Fig.4a) produced by the precipitation-impregnation method, attributed to sodium ions, reveals that these ions were still present despite the extensive washing. SEM micrographs of the impregnated sample (SZp.i.) and the sulfated zirconia from microemulsion (SZMEZ) after calcination at 600~ are shown in Figures 4b and 5b. These pictures reveal there is an essential difference in the size of the particles in these two samples. Unfortunately, neither the size nor size distribution could be evaluated. However, particles from microemulsion (SZMEZ) are uniform in size unlike the particles in the precipitated sample.

501

Figure 5: a) EDX spectra and b) SEM micrograph of the the SZMEZ sample (lcm= 10[.tm) The sulfated zirconia samples prepared from the microemulsion system Z (Fig.6,a) and by the precipitation-impregnation method (Fig.6,b) were analyzed with infrared spectroscopy. The spectra were similar to spectra of sulfated zirconia published in references [4, 5, 6, 7]. The region of interest for sulfate species is between 1450 and 850

502 cm 1. The broad band at ---1600 cm -1 in Figures 6,a and 6,b is assigned to the adsorbed water molecules. All the specimens show an intense band at 1400-1350 cm 1, corresponding to the S=O stretching mode. It appears as a single peak for the microemulsion samples, while for the impregnated zirconia samples consists of several components. The band is more intense for the calcined samples, probably due to the absence of water. In the S-O stretching region (1150-850 cm -1) a multiplicity of bands is apparent in all the spectra due to the presence of different types of the sulfate surface species. Yet the microemulsion samples show somewhat better defined peaks, indicating higher homogeneity of the sulfate surface species on the extremely pure tetragonal zirconia.

Absorbance //~alcined

I

3800

a) at 600 ~

\

3200

Absorbance 131~61

.

2600

2000

~ . . ~ i n e d

1081.

1400

b) at 600 ~ 1140

/-'N.

/

800 3800

W a v e n u m b e r (cm-1)

1140~1

-~'x

dried

~05~ 3

3200

2600

2000

1400

9

800

W a v e n u m b e r (cm-1)

Figure 6. FTIR spectra of sulfated zirconia a) prepared by microemulsion, system Z. b) samples prepared by precipitation-impregnation. Comparison of the obtained spectra with literature suggests that the predominant surface species on tetragonal sulfated zirconia, which is responsible for superacidity hence the catalytic activity, is:

-O~/~O SN 0 O 3.3. Catalytic Isomerization of Hexanes Before testing the supported metal catalysts in the isomerization reactions of hexanes, the 2-methylpentane reactant was run on blank samples, i.e. catalyst supports without metal loading. Unpromoted sulfated zirconia did not show catalytic activity at 300 ~ Isomerization reactions of 2-methylpentane at 300 ~ on the tested catalysts yielded isomers (methylcyclopentane, n-hexane and 3-methylpentane) and cracking products. The results are shown in Table 5.

503 Table 5 Catalyst K1 K2 K3 K4 Ref.

Support (Zr02) ,. ME system Z ME system A Precipitation Precipitation 10% PtA1203

Cracking products

Catalyst

K1 K2 K3 K4 Ref.

Preparation Sulfating ME system Z Impregnation Impregnation Impregnation .

C1+C5 6,53 29,17 13,92

C2+C4 12,18 16,67 10,02

2 C3 7,01 0,00 4,05

Pt deposition ME system P ME system P ME system P Impregnation Impregnation

Reaction rate [mol/g/s] 107 7,26 4,52 19,20 18,28 . 1,43

Isomerization products 3MP 4,50 20,09 6,98 16,67 39,51

n-HEX 76,88 24,82 57,49 8,33 25,31

MCP 18,62 55,10 13,32 29,17 3,31

Selectivity % 100 100 77,8 54,2 67,9 3MP/ n-HEX ratio 0,06 0,81 0,12 2,00 1,57

Reaction rates for the tested catalysts were generally high. Despite the lower platinum content, the sulfated zirconia supported catalysts showed, on average, activities larger by one order of magnitude than the reference catalyst. This is in agreement with literature, stressing the importance of surface sulfate which enhance the catalytic active sites of Platina. The microemulsion prepared catalysts seemed to have lower activity than the precipitated catalysts. On the other hand, selectivity towards isomerization is higher for these catalysts, reaching 100 %. The catalysts based on precipitated support (K3 and K4), have similar selectivity towards isomerization as the reference (Table 5) but have tenfold higher activity. The method of platina deposition seems to influence primarily the selectivity towards specific isomers. The amount of n-hexane formed on K3 (microemulsion) is 57.5% of the total conversion, while on K4 (impregnation) it is 8.3% (Table 5). Impregnation of the support by sulfate ions in catalyst K2 seems to decrease the selectivity towards hexane, yielding 24.8%. The main difference between the catalysts is reflected by the ratio of isomers, n-hexane is predominant on (K1), the catalyst prepared by the microemulsion method. All the tested catalysts produced relatively large amounts of methylcyclopentane, indicating that the isomerization reactions proceed by a cyclic mechanism. The 3MP/n-HEX ratio has the largest value (2) for the K4 catalyst, implying a preferential breaking of the cyclopentane ring bonds between secondary carbons thus a selective mechanism. This is often encountered on large metal particles, as it is the case of the K4 catalyst, prepared by the impregnation method. All the catalysts prepared with platinum deposition by the microemulsion method showed a low 3MP/n-HEX ratio, confirming the expectation that metal particles produced by the microemulsion method are smaller in size. 4. CONCLUSION The microemulsion technique was successfully applied to the preparation of sulfated zirconium oxide catalyst supports. The nonionic microemulsion shows high stability. But, a drawback of this system is its low production capacity. Advantages of the ionic system were its higher capacity of solubilization and the ease in destabilizing the microemulsion to recover the oxide particles. The major drawback was the need of an

504 separate sulfating procedure. X-ray diffraction patterns showed that the particles of zirconium oxide exist in only the tetragonal phase, while precipitation-impregnation synthesized zirconia contains a small amount of the monoclinic phase. Zirconia particles prepared by the microemulsion method are small compared with precipitated particles. Sulfated zirconia prepared from a nonionic microemulsion consisted solely of tetragonal zirconia. Infrared spectra indicate that sulfate ions were more homogeneously distributed over the zirconia surfaces. Zirconia particles prepared from the ionic microemulsion system, subsequently sulfated by incipient wetness impregnation, have a high degree of crystallinity and showed the highest specific surface area. Nevertheless, the catalytic activity was somewhat lower than that of the catalyst prepared from nonionic system, despite using the same promotion method. Despite the lower platinum content, the sulfated zirconia supported catalysts showed much higher activity than the alumina supported platinum catalyst used as reference. The platinum promoted sulfated zirconia prepared by the various methods showed different catalytic behaviors, indicating the influence of the preparation method on catalytic behavior. From the results of the activity tests, it can be concluded that the preparation of the zirconia supports by the microemulsion technique enhances the isomerization selectivity at the cost of decreased activity. Even f-alumina prepared in the ionic microemulsion system has shown interesting properties. This was an additional proof of the potentiality of microemulsions as a preparation substrate. These results will be the subject of a future publication. REFERENCES

1. M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Coil.Surf., 5 (1982) 209-225. 2. D. Farcasiu, J. Qi Li, Appl. Catal. A: General 128 (1995) 97-105. 3. B.H. Davis, R.A. Keogh, R. Srinivasan, Catal. Today 20 (1994) 219-256. 4. Takeshi Kawai, Atushi Fujino, Kijiro Kon-No, Coil. Surf. A. 109 (1996) 245-253. 5. E. Escalona Platero, M. Pefiarroya Mentruit, C. Otero Are~tn, A. Zecchina, J. Catal., 162 (1996), 268-276. 6. C. Morterra, G. Cerrato, F. Pinna, M. Signoretto, J. Catal., 157 (1995), 109-123. 7. G. Larsen, E. Lotero, M. Nabity, L.M. Petkovic, D.S. Shobe, J. Catal., 164 (1996), 246-248.

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

505

Influence of the i m p r e g n a t i o n order of m o l y b d e n u m and cobalt in carbon supported catalysts for h y d r o d e o x y g e n a t i o n reactions M. Ferrari, C. Lahousse, A. Centeno a, R. Maggi, P. Grange, B. Delmon Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, P1. Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium apresent address, Escuela de Ingenieria Quimica, Universidad Industrial de Santander A.A. 678, Bucaramanga, Colombia This work concerns the role of the impregnation order of molybdenum and cobalt in carbon supported catalysts. Four activated carbons have been used as supports and the effect of the impregnation order (either cobalt or molybdenum at first) has been considered. The samples were characterized by N2 physisorption, XPS and SEM. The sulfided catalysts have been tested in hydrodeoxygenation, using a model mixture containing: 4-methylacetophenone, ethyldecanoate and guaiacol. Both series of samples exhibit a low dispersion of the active phase, but CoMo is more uniformly distributed than MoCo. This result was interpreted to be consequence of the position occupied by the metal impregnated at first and of the strong interactions existing between cobalt and molybdenum. In the samples containing only one metal, cobalt is mainly impregnated at the exterior of the grains, while molybdenum is impregnated at the interior of the grains, causing micropore blocking. When the second metal is added, cobalt seems to bring about the remobilization of molybdenum outside the grains in CoMo catalysts and the formation of molybdenum oxide on the external grain surface in MoCo. Concerning catalytic activity, CoMo catalysts show higher hydrogenation properties for the conversion of ketonic groups and higher decarboxylation selectivity in the conversion of the ester. 1. INTRODUCTION So called bio-oils, namely liquids obtained from the pyrolysis of organic materials (biomass), are contemplated as alternative to fossil fuels (1). However, they are far from reaching the quality standards of the latter. Their high oxygen content (O = 26-47 %wt) is the cause of high viscosity, bad thermal stability, poor heating value and substantial corrosivity. Bio-oils properties can be upgraded by partial or total oxygen elimination by catalytic hydrotreating (2), more precisely by hydrodeoxygenation (HDO), performed at moderately high temperature and under hydrogen pressure. The reactions occurring are the elimination of oxygen as water and the hydrogenation-hydrocracking of various chemical functions contained in the molecules constituting the pyrolysis products. Some authors (3-5) have demonstrated the feasibility of bio-oils deoxygenation by catalytic hydrotreatment. Laurent et al (6, 7) have elucidated the reaction pathway and studied the influence of operational parameters using model compounds which contain the oxygenated functions responsible for bio-oil instability (carbonyl, carboxylic esters and phenolic ethers). All these studies have been carded out using industrial hydrodesulfurization catalysts, such as sulfided NiMo/A1203 or CoMo/A1203. Although they presented a good activity, they were quickly deactivated by coke formation, due to the acidity of the alumina support (8, 9). This tendency to coke formation was found to be due particularly to phenol type molecules containing two or more oxygenated substitutes, like guaiacol and catechol. The utilization of

506

neutral supports, such as activated carbons, has been shown to limit or to avoid the formation of condensation products acting as coke precursors (8, 9) and to facilitate the elimination of oxygenated groups from guaiacol and catechol. Active carbons as catalyst supports for hydrotreating processes have received much attention in the last years (10, 11). Some advantages of carbon over alumina based catalysts have been highlighted, like a lower coking propensity (12-14) combined with a higher catalytic activity (14-18). The carbon characteristics are quite different from those of alumina, especially as regards: i) textural properties (surface area, total pore volume, micropore volume, average pore size), ii) nature and quantity of surface groups, iii) thermal stability (10, 15, 19, 20). These differences can strongly influence metal-support interactions, the metal dispersion, the accessibility to the surface by impregnating solutions and to the active phase by reactants during catalysis. Carbon and alumina supported catalysts certainly require different preparation methods to optimize activity. In this paper we present the results of textural and structural characterizations of two series of catalysts containing cobalt and molybdenum. Four commercial activated carbons have been chosen as support and two impregnation orders have been used for the preparation: cobalt first, then molybdenum, or molybdenum first, then cobalt. Their catalytic activity in hydrodeoxygenation (I-DO) reactions has been tested. The objective of the work was to acquire new knowledge on the parameters which control the active phase distribution on carbon supported catalysts. The optimization of the preparation procedure and the increase of the catalytic activity in HDO reactions are the final goals of our research. 2. E X P E R I M E N T A L

2.1 Catalyst preparation Four commercial activated carbons (Merck, Norit RO-08, Chemviron F-300, BKK-100) were used as catalyst support. The carbon particles (diameter size between 0.315 and 0.500 mm) were impregnated with aqueous solutions of ammonium heptamolybdate ( ( N H 4 ) M o 7 0 2 4 " 4 H 2 0 ) and cobalt nitrate (Co(NO3)2"6H20,) both from Merck. The concentration was 0.70 mol 1-1 for molybdenum and 0.26 mol 1"1 for cobalt; the pH of the two solutions was 6 and 5 respectively. For the preparation a 'repeated' incipient wetness impregnation method was used; a volume of solution equal to three times the pore volume of the support was added in several steps (between 6 and 8) to the carbon. Between each impregnation step the catalyst was dried at 110~ in air for 15-20 min. After the last one it was dried at 130~ (heating rate = 3~ min- 1) under a flow of dry air (100 ml min- 1) for 16 h. The final catalysts contained 15 % wt of MoO3 and 3% wt of CoO. Depending on the metal impregnation order, the catalysts have been denoted as CoMo, if molybdenum was impregnated at first, or as MoCo if cobalt was impregnated at first. 2.2 Textural characterization Nitrogen adsorption experiments were performed at 77 K (-196~ using a Micromeritics ASAP 2000 apparatus. The sample (100 mg) was previously outgassed overnight at 200 ~ at a pressure of 0.1 Pa. At low relative pressure (10 -~ < P/P0 < 4-10-z), fixed dose amounts of nitrogen (0.5 ml g-l) were introduced and the residual pressure was recorded. At higher pressure (4.10 "z < P/P0 < 1), nitrogen was added and the volumes required to achieve a fixed set of P/P0 were measured. Specific total surface area were calculated using the Langmuir equation (0.01 < P/P0 -< 0.05). The total pore volume was estimated by the amount of nitrogen adsorbed at P/P0 = 0.985. The microporous volume ~-Pore) was calculated from the Horvath-Kawazoe model (interaction parameter = 2.5 x 10'*~ erg cm '~, maximum micropore size = 2 nm, P/P0 = 0.27) and from the Dubinin-Astakov method (2 x 10-o _

507

2.3 X-ray photoelectron spectroscopy (XPS) The XPS analysis were performed on a SSI X-probe (SSX 100/206) spectrometer from FISONS equipped with a monochromatised micro-focused A1 K s X-ray source (1486.6 eV). The analysis pass energy was set at 150 eV and the analysed area was 0.5 mm z. In these conditions the energy resolution determined by the Au 4f7/2 full width at half maximum (FWHM) of gold was 1.4 eV. The general spectrum and the spectra of the peaks of Cls, Ols, Mo3d and Co2p3/2 were registered. Binding energies were calculated by reference to the binding energy of the C-(C,H) component of Cls considered to be at 284.16 eV. Intensity ratios were converted into atomic concentration ratios (Mo/C x 100 and Co/C x 100) by using the sensitivity factors given by the manufacturer. The analysis were done on the catalysts impregnated with molybdenum or cobalt alone and CoMo or MoCo. In each case the not ground samples (not gr.) (particle diameter between 0.315 and 0.500 mm) and the finely ground samples (gr.) were analysed. The atomic ratios obtained for the not ground samples are representative of the active phase distribution on the external grain surface and the difference between the atomic ratios for the not ground and ground samples indicates whether a preferential deposition of the active phase at the exterior of the grains occurs.

2.4 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was performed with a Philips ESEM-FEG-XL30 electron microscope. An electron voltage of 25 kV was used. Studies were done at several magnifications (20000 x, 10000 x, 5000 x and 1000 x).

2.5 Catalytic tests Before the reaction test, the catalysts were activated by reduction-sulfidation in a glass tubular reactor at atmospheric pressure. 1.5 g of catalysts were dried at 130 ~ (heating rate = 3 ~ min -1) under a flow of argon (100 ml min-l) during 1 hour. The gas was then switched to a mixture of 15% vol of H2S in H2 (100 ml min-l). The temperature was increased up to 400 C (heatm " g rate = 3 C r m"n -i ) and mmtaa a" "ned at this value for 3 hours. The reactor was rapidly cooled down (30 min) to room temperature under the same gas mixture, then the flow was switched back to argon. The catalyst was protected from air contact until it was poured into the reactant solution. The reaction tests were performed in a 570 ml stainless steel batch reactor, at 280 ~ and under a 7 MPa pressure of hydrogen, under vigorous agitation. Ten samples were withdrawn at 0, 10, 20, 30, 40, 50, 70, 90, 120 and 150 minutes after the moment when the reaction conditions (temperature and pressure) were reached and the agitation started. The catalytic tests were carried out using a model compound solution selected on the basis of an in depth chemical characterization of bio-oils (23). The composition, as reported in previous works (6, 9), was as follows: 4-methylacetophenone, MA (0.218 mol 1-1), ethyldecanoate, ED (0.147 mol 1-t), guaiacol, GUA (0.218 mol 1"1). Pentadecane was added as internal standard for the chromatographic analysis and p-xylene was used as solvent. Carbon disulfide (0.018 mol 1-1), which rapidly decomposes in the presence of hydrogen, was added as a precursor of H2S. The volume of the reactant solution was 170 ml. The reactants were from Aldrich or Acros and they had a purity of 98% at least. Liquid samples were analysed by a gas-chromatograph equipped with a FID detector and a capillary column (stationary phase CP-Sil-8CB, length = 25 m). The hydrodeoxygenation pathways of the model compounds have been established in previous works (6). They are recalled in figure 1. The only product obtained from 4-methylacetophenone was p-ethylmethylbenzene. The intermediates r and 4-methylstyrene were never detected under our reaction conditions.

508 Ethyldecanoate can react following two pathways, the first is the hydrogenation of the carboxylic group to a CH3 group, which produces decanol and ethanol, followed by a dehydration of decanol to give olefins (decene) which are subsequently hydrogenated to decane; the second is the direct decarboxylation to produce nonane. The carboxylic acid can also be formed via a de-esterification reaction; it is an intermediate product that can follow the hydrogenation or decarboxylation pathway. Guaiacol can be demethylated to give catechol which can be transformed in phenol with a cleavage of the CAromatic-O bound; benzene, cyclohexene and cyclohexane are obtained from phenol. Phenol can also be directly formed from guaiacol by a demethoxylation reaction (cleavage of CAromatic-O bound). O il

~ ]

~ C" CH3

-

~ v

~

H3C

CH2CH3

H3C

4-Methylacetophenone

p-Ethylmethylmenzene

Decanol C10H21 OH "

Dehydration ~

Decene C8H17 "H C-----CH2

Hydrogenation

Hydrogenation O II C9H19 - C. OC2H5

o

/

Hydrogenation

Deesterification C9H19 " C - - OH

Ethyldecanoate

~arboxylation C9H2o

Decarboxylation OH

CloH22

Decane

OH

Nonane OH

~

( Benzene Cyclohexene Cyclohexane

,,

Guaiacol

Catechol

Phenol

Figure 1. Hydrodeoxygenation pathways of 4-methylacetophenone (MA), ethyldecanoate (ED), guaiacol (GUA). For the evaluation of the catalytic activity in the reaction of the different molecules, pseudo first order kinetics are assumed. The rate constants were calculated according to the kinetic equation (1 and 2) used by Gevert et al. (25):

-In C--L= kWf(t / V) CO where f(tN) is:

(1)

509

n

f(t / V) = E ti-ti-1 (2) i=1 ti Ci and CO are the reactant concentration in the sample at the time i and at the initial time 0, respectively; k is the pseudo first order rate constant (ml min- 1 g- 1); W is the catalyst weight (g); n is the number of the sample taken; t is the time (rain); V is the solution volume (ml). The function (tN) takes into account the fact that the reactant volume decreases after each sampling and consequently the ratio between the catalyst and reactant weight decreases. In the conversion of the ethyldecanoate the selectivity for the deoxygenation reaction of the ester group is expressed as a decarboxylation percentage. It is calculated as the ratio between the concentration of nonane and the sum of the concentration of all the hydrogenated and decarboxylated products (decane + decene + decanol + nonane).

x 100 ..... C non . . . . . . . Sdeearb --" Cnon + C deca q- Cdece -i- Cdecano1 3 RESULTS

3.1 Characterization of fresh catalysts 3.1.1 Textural characteristics The results of the textural characterization of the four supports and of the MoCo and CoMo catalysts are reported in table 1. For two selected supports, namely Merck and BKK, the samples impregnated with only one metal were also analyzed. The difference between the total and the microporous volume (A Volume) will generally be indicated as "non-micropous" pore volume and assumed to be representative of the mesoporosity. The four supports have similar properties, however some differences emerge: Merck and Norit are characterized by the highest surface area and microporous volume, BKK has the most developed non-microporous volume. The impregnation of cobalt and molybdenum does not significantly change the micropore diameter, but it affects all the other characteristics, both the total and micropore volumes, as well as the surface area. The decrease of the total pore volume is higher than the one theoretically occupied by the active phase, especially in the case of the catalysts supported on carbon Merck, Norit and Chemviron. On the basis of structural data, the volume of molybdenum oxide can be estimated between 0.06 and 0.08 cm 3 g" 1 (26). For the catalysts supported on Norit and Chemviron, this decrease of the micro and total pore volumes is slightly more important for the CoMo than for the MoCo series. In the case of Merck and BKK similar values are observed for CoMo and MoCo. The change of the microporous volume accounts for most of the total pore volume decrease for the samples impregnated on carbon Merck, Norit and Chemviron. For the samples impregnated on carbon BKK, both CoMo and MoCo, the decrease of the total pore volume is due to an equal decrease of the micro and non-microporous volume. As regards the monometallic samples, the impregnation of molybdenum causes a more important decrease of the surface area and pore volume than cobalt; this is in accordance with the different metal loading in the two cases: 15% wt of MoO3 and 3% wt of CoO. The decrease of the micro and total pore volume is more important for Merck than for BKK.

510 Table 1. Textural characteristics of the activated carbons (Merck, Norit, Chemviron, BKK) and of the impregnated catalysts. Support

Catalyst

Surface area (m2g-1)

Merck

. ,

1220 1042 772 790 754 1158 758 705 924 475 385 1069 967 789 680 677

Co/Merck Mo/Merck MoCo/Merck CoMo/Merck

,,,

Norit MoCo/Norit CoMo/Norit Chemviron MoCo/Chem CoMo/Chem " ' BKK" Co/BKK Mo/BKK MoCo/BKK CoMo/BKK

Tot Pore Volume (cm3g-1)

[.t Pore Volume (D.A.) (cm3g.])

A Volume

0.45 0.38 0.29 0.30 0.29 0.43 0.29 0.26 0.35 0.19 0.16 0.39 0.35 0.29 0.25 0.25

0.18 0.17 0.12 0.12 0.12 0.18 0.12 0.12 0.16 0.11 0.10 0.27 0.26 0.24 0.14 0.16

0.64 0.55 0.41 0.42 0.41 0.61 0.41 0.38 0.51 .... 0.30 0.26 0.66 0.61 0.53 0.39 0.41

i

T~ (crn3g-1)

[.t Pore Diameter (/~) 4.9 5.0 5.0 5.0 5.0 4.3 ..... 4.3 4.3 4,7 5.0 5.0 4.4 4.5 4.4 4.5 4.5

3.1.2 X-ray photoelectron spectroscopy (XPS) The results of the XPS analyses of the samples containing molybdenum or cobalt alone and of MoCo and CoMo catalysts are reported in table 2. The results are discussed in terms of Mo/C and Co/C atomic ratios for not ground and ground samples. Table 2. Mo/C and Co/C atomic ratio.

Merck Norit Chemv BKK

.

Mo/Cx 100 atomic ratio Mo MoCo CoMo " notgr. 81". notgr, gr. notgr, gr, 1.0 1.0 3.1 1.9 1.5 1.4 2.7 1.7 5.2 1.9 3.3 1.8 2.8 1.7 3.4 2.3 3.5 1.8 2.6 2.1 4.5 2.1 3.2 2.0 .

.

.

.

.

Co/Cx 100 atomic ratio Co MoCo CoMo " notgr, gr. notgr, gr. notgr, gr. 3.1 1.7 0.9 0.7 0.5 0.5 6.3 2.4 1.9 0.5 1.4 0.7 1.3 0.8 1.8 0.7 2.2 1.5 1.2 0.6 2.0 0.6 _

For both molybdenum and cobalt, in mono and bi-metallic samples, the metal/carbon atomic ratios are higher for not ground than for ground samples. The difference between Mo/C ratios in not ground and ground samples is higher for MoCo than for CoMo catalysts. In the samples containing only cobalt, very large differences exist between the atomic ratios for not ground and ground. When molybdenum is impregnated after cobalt (MoCo) the Co/C ratio decreases. When the Mo/C ratio is considered, higher differences between not ground and ground are observed as compared with the samples containing molybdenum alone. In the samples containing molybdenum alone, the differences between not ground and ground are weaker than in the case of cobalt alone, this indicates that molybdenum is more easily impregnated at the interior of carbon grains than cobalt.

511 The impregnation of cobalt over molybdenum (CoMo) results in an increase of Mo/C atomic ratio for not ground samples. The Co/C ratio is lower than the one observed in samples containing cobalt alone.

3.1.3 Electron Microscopy studies Figure 2 shows the SEMs micro graphs (20000 x) of active carbon BKK (A), Mo/BKK (B), CoMo/BKK (C) and MoCo/BKK (D). The study has also been done on the catalysts supported on the other carbons. The pictures obtained are very similar to the ones reported hereafter. Comparing the samples containing molybdenum alone (B), with the pure support (A), no formation of crystallites is visible; however a difference in the contrast can be observed. When cobalt is added to molybdenum (CoMo) (C) square parallelepipedic crystals are formed; they are spread all over the sample, but it is still possible to see the carbon surface. When the catalyst is prepared with the other impregnation order (MoCo) thick agglomerates of prismatic hexagonal crystals, typical of MOO3, are formed (D). They cover a large portion of the external surface of the carbon grains.

Figure 2. SEM micrographs for carbon BKK (A), Mo/BKK 03), CoMo/BKK (C) and MoCo/BKK (D) samples.

3.2 Catalytic tests We recall that the reaction of the three model molecules was tested simultaneously. However, for the sake of clarity the results of the catalytic tests will be presented separately for each model molecule.

512

513

4 Discussion As shown in table I the decrease of the specific surface area and of the total pore volume is due to a decrease of both the micro and non-microporous volume; however the volume of the impregnated active phase, calculated on the basis of structural data (26), accounts for only a part of this diminution. The micropore diameters remain constant in all cases after impregnation, thus micropore blocking seems to take place. On the whole, no considerable difference exists between the catalysts impregnated on the four carbons. Only the textural changes of the samples impregnated on carbon BKK slightly differ from the ones occurring on the other supports. The decrease of the non-microporous volume accounts for half of the total pore volume reduction in the case of BKK, while the decrease of the total pore volume is almost exclusively due to micropore blocking in the other cases. These results indicate that, on carbon B KK, the active phase is deposited in both the micro and non-micropores. The catalysts of the MoCo series prepared with carbon Norit and Chemviron present textural values higher than those found for the CoMo series. These results could suggest that when molybdenum is impregnated at first the active phase has a higher tendency to be located in the micropores. The situation is different for the BKK carbon; this may be due to the fact that on this support the impregnation in the non-microporous volume is much more important for both CoMo and MoCo. The XPS results show that Mo/C atomic ratios are always lower for samples which contain molybdenum alone than for CoMo and MoCo. The molybdenum concentration at the exterior of the grains (represented by Mo/C ratios for not ground samples) is higher for MoCo than for CoMo. These results are in agreement with those of Duchet et al. (16). In CoMo catalysts the active phase is better distributed than in MoCo. When molybdenum is impregnated alone it seems to penetrate more deeply inside the grains. When cobalt is impregnated after molybdenum (CoMo series) an increase of the Mo/C ratios, compared to the samples containing molybdenum alone, is observed. At the same time lower Co/C ratios compared to samples containing cobalt alone are obtained. A possible explanation could be that cobalt interactions with molybdenum are stronger than those existing between molybdenum and carbon, so inducing the partial migration of the latter towards the exterior of the grains. The large amount of water used during the multistep incipient wetness impregnation could also facilitate the remobilization of molybdenum. When cobalt is impregnated alone it seems to be preferentially located on the exterior of the grains. After molybdenum impregnation, cobalt is covered by the second metal, as its signal decreases very much. This indicates that molybdenum is deposited at the exterior of the grains, probably in the form of agglomerates. The microscopic analyses show the formation of crystals at the exterior of the grains in both bimetallic samples. The shape, the amount and consequently the nature of these crystals is completcly different for CoMo and MoCo. The ones observed in MoCo samples can be attributed to MOO3. What emerges from the characterization analyses can be schematically represented (figure 7) and summarised as follows. In monometallic samples cobalt is preferentially located at the exterior of the grains, while molybdenum is at the interior, mainly blocking the micropores. When the second metal is impregnated, the interactions between cobalt and molybdenum seem to be stronger than the ones between molybdenum and carbon. As a consequence, cobalt brings about the remobilization of molybdenum outside the grains in CoMo and the impregnation of molybdenum oxide at the exterior of the grains for MoCo catalysts. The different active phase distribution in CoMo and MoCo catalysts considerably affects their hydrodeoxygenation activity. The catalysts in which MoO3 crystals are formed at the exterior of the grains, namely MoCo, show a lower hydrogenation activity than CoMo in the case of ketonic group and a higher hydrogenation selectivity for the conversion of ED. This behaviour is very similar to the one indicated by Centeno (26) in the case of monometallic catalysts, containing molybdenum alone, or in the case of CoMo catalysts with low Co/(Co+Mo) ratio. Actually molybdenum supported catalysts have a lower hydrogenation

514 activity for MA than the ones also containing cobalt. This is similar to the resuks obtained with our MoCo catalysts. Moreover these catalysts are not at all selective for the decarboxylation of ED. For CoMo catalysts, it was observed (27) that cobalt had a promoting role for molybdenum; MA conversion shows a maximum for a catalyst having a Co/(Co+Mo) ratio equal to 0.3; also the decarboxylation selectivity has been shown to depend from the Co/(Co+Mo) ratio, it increased with cobalt content. The results obtained with our MoCo catalysts (Co/(Co+Mo) =0.3) are very similar to those obtained by Centeno with samples having Co/(Co+Mo) ratios equal to 0.0-0.2. Actually it seems that molybdenum could not interact with cobalt.

Figure 7. Schematic representation of CoMo/C and MoCo/C catalysts. 5 CONCLUSIONS The aim of our work was to evaluate the influence of the impregnation order (cobalt at first: samples MoCo; molybdenum at first: samples CoMo) in the preparation of carbon supported catalysts. This impregnation order is very important. It controls the location and the distribution of the active phase (CoMo is better distributed on the carbon grains) and thus the catalytic activity. The differences observed when changing the impregnation order are a consequence of the position occupied by the metal impregnated at first and of the strong interactions existing between cobalt and molybdenum. In the samples containing only one metal, cobalt is mainly impregnated at the exterior of the grains, while molybdenum is impregnated at the interior of the grains, causing micropore blocking. When the second metal is added, cobalt seems to bring about the remobilization of molybdenum outside the grains in CoMo catalysts and the formation of molybdenum oxide on the external grain surface in MoCo. CoMo catalysts show a higher hydrogenation activity than MoCo in the case of MA, but a lower hydrogenation selectivity in the case of ED. The textural properties, more precisely the micro- and meso-porosity, also play. a fundamental role on the impregnation of the active phase. A less developed micro porosxty seems to allow a better distribution and accessibility of the active phase itself with consequently a probable benefit for the catalytic activity, especially when steric hindered molecules, like guaiacol, are involved.

515 ACKN OWLEDG E MENT S The financial support of the European Union (Contract No. JOR3-CT95-0025) is gratefully acknowledged. Special thanks are also due to the staff of CERTECH-CrAS Division (Seneffe, Belgium) for their assistance in the realization of the SEM experiments. 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.

G. Grassi, T. Bridgwater, in "Biomass for energy and environment, agriculture and industry in Europe", Esagono Ed., (1992). E. Laurent, B. Delmon, in "7th European Conference on Biomass for energy industry and environment, 1992", G. Grassi et al. Eds. (1992). D. Elliott, E. Baker, in "Thermochemical biomass conversion", A.V. Bridgwater, J.L. Kuester, Eds. (1988) 883. E. Churin, P. Grange, B. Delmon, in "Research in Thermochemical Biomass Conversion" A.V. Bridgwater and J. L. Kuester (Eds), Elsevier London and New York (1988) 878. J. Gagnon, S. Kaliaguine, Ind. Eng. Chem. Res., 27 (1988) 1783. E. Laurent, B. Delmon, Appl. Catal. A, 109 (1997) 77. E. Laurent, B. Delmon, Appl. Catal. A, 109 (1997) 97. E. Laurent, A. Centeno, B. Delmon, in "Proceedings of the 6th International Symposium of Catalysts Deactivation" 1994, B. Delmon and G. Froment (Eds.), Elsevier Amsterdam (1994) 573. A. Centeno, E. Laurent, B. Delmon, J. Catal., 154 (1995) 288. H. Jungten, Fuel, 65 (1986) 1436. G.M.K. Abotsi, A.W. Scaroni, Fuel Process. Tech., 22 (1989) 107. V.H.J. De Beer, F.J. Derbyshire, C.K. Groot, R. Prins, A.W. Scaroni, J.M. Solar, Fuel, 63 (1984) 1095. J.P.R. Visser, C.K. Groot, E.M. van Oers, V.H.J. De Beer, R. Prins, Bull. Soc. Chim. Belg., 93 (1984) 813. B. Scheffer, P. Arnoldy, J.A. Moulin, J. Catal., 112 (1988) 516. J.P.R. Visser, B. Scheffer, V.H.J. De Beer, J.A. Moulin, R. Prins, J. Catal., 105 (1987) 105. J.C. Duchet, E.M. van Oers, V.H.J. De Beer, R. Prins, J. Catal., 80 (1983) 386. J.Laine, F. Severino, M. Labady, J. Catal., 147 (1994) 355.Fuel, 71 (1982) 87. C.K. Groot, V.H.J. de Beer, R. Prins, Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 522. J.M. Solar, C.A. Leon y Leon, K. Osseo-Asare, R. Radovic, Carbon, 28 (1990) 369. A. Martin-Gullon, C. Prado-Burguete, F. Rodriguez-Reinoso, Crabon, 7 (1993) 1099. A. Gil, P. Grange, Langmuir, 13 (1997) 4483. A. Gil, P. Grange, Colloids and Surfaces, 113 (1996) 39. G. Horvath, K. Kawazoe, J. Chem. Eng. Jpn., 16 (1983) 470. R. Maggi, B. Delmon, Fuel, 73 (1994) 671. B.S. Gevert, J.E. Otterstedt, F.E. Massoth, Appl. Catal., 31 (1987) 119. J. Sonnemans, P. Mars, J. Catal., 31 (1973) 209. A. Centeno, in "Hydrodesoxygenation catalytique de composes modeles: contribution a l'etude de la stabilisation des huiles de pyrolyse de la biomasse", PhD Thesis, UCL, Belgium (1997).

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91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

517

Influence of phosphorus on the preparation of CoMo/A1203 hydrotreating catalysts R. de Back a , F. Croonenberghs and P. Grange Unit6 de Catalyse et de Chimie des Mat6riaux Divis6s, U.C.L. Louvain la Neuve, Place Croix du Sud 2/17, 1348 Louvain la Neuve, Belgium

This paper deals with the preparation, testing and characterisation of two series of phosphorus containing, A1203 supported cobalt-molybdenum hydrotreating catalysts. Co and Mo loadings were fixed at 3.8 % wt and 11.6 % wt respectively, whereas P content was varied between 0 and 3 % wt. Model reactions studied were the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of naphtalene (NAPHT). When P was impregnated prior to Co and Mo (the latter two in one step, double impregnation, D.I.) HDS and HYD activities were lower than for the P-free catalyst and decreased with increasing amounts of P. Catalysts prepared by impregnating P, Co and Mo in one single step (coimpregnation) are more active than the P-free catalyst and HDS and HYD activity increased with P loading. Characterisation by XRD, BET, XPS and SEM/EDX revealed that the P-free and D.I. catalysts exhibited preferential deposition of Mo and Co at the exterior support surface, whereas for the C.I. catalysts a more homogeneous dispersion is achieved, explaining the differences in catalytic behaviour. 1. INTRODUCTION Reduction of sulphur, nitrogen, aromatics and metal contents of oil-fractions is commonly achieved industrially in the so-called hydrotreating processes, using high H2 pressures, medium to high reaction temperatures and sulfided catalysts. More severe environmental legislation, increased fuel demand coupled to a shit~ towards transportation fuel products continuously stimulates the oil industry to look for more active hydrotreating catalysts.

a

RdB gratefully acknowledges financial support by BP Oil (Sunbury-on-Thames, England)

518 The active phase of these supported catalysts is thought to be located on the edges and corners of small MoS2 or WS2 slabs and the addition of small amounts Co or Ni (as sulphides) to the active phases promotes their activity markedly (synergy). Although no real consensus is reached at the moment about the origin of the synergy between active phase and promoter, it is well established that an intimate contact of the two well dispersed sulphide phases is necessary to obtain highly active catalysts. Apart from the three basic ingredients (support, active phase and promoter), other elements, called additives, are found as well in the formulation of commercial hydrotreating catalysts. Phosphorus is frequently used as additive, but due to the various levels at which this element can interfere in the functioning of hydrotreating catalysts, no tmivocal explanation has been given. Besides improved mechanical and thermal resistance of the support [1,2], the beneficial action of phosphorus has often been related to a more easy catalyst preparation due to improved solubility of metal salts and increased stability of the impregnation solution [3]. Furthermore, a change of support properties leading either to different stacking of the active phase [4], to a decrease of the amount of promoter ions diffusing into the support lattice [5,6] or to a more homogeneous distribution of the active phase over the support pellets [7] have been reported to occur on P modified catalysts. Decreased coke formation and the production of a more hydrogen rich coke [8] or a direct participation of phosphorus in the constitution of the active phase [9] have also been put forward to explain the beneficial effects of phosphorus on hydrotreating catalysts. The importance of preparation method and the amount of phosphorus will be shown by the following non-exhaustive list of examples. On P-modified unpromoted Mo/Ai203 catalysts, Fierro et.al. [10] observed a larger increase for thiophene HDS activity than for alkene HYD for co-impregnated samples, whereas the opposite occurred when P was impregnated first (sequential impregnation). Cruz Reyes et.al. [4] observed an increase in activity during the simultaneous gas-oil HDS and pyridine HDN for tmpromoted P-modified W/A1203 catalysts prepared by co-impregnation. However, HDS activity was increased more than HDN activity upon increasing the P-content. Chadwick et.al. and Atanasova and colleagues studied the HDS of thiophene on NiMoP/A1203 catalysts (4 % wt NiO, 15 % MOO3, various amounts of P) prepared by co-impregnation [11,12]. In both studies an increase of activity was observed upon increasing the P content up to 1 % wt P and a decrease at higher levels, however, all P containing catalysts were more active then the P-free sample. Tischer et.aL have studied the effect of P on the activity of NiMoP/AlzO3 catalysts in the upgrading of coal-liquefaction oil [13]. The catalysts were prepared by co-impregnation of P and Mo followed by calcination and subsequent Ni impregnation. Activity for the conversion of the heavy oil into fighter products increased up to approximately 3.6 % wt P, whereas HDS and HDN activity were constant. Above 3.6 % wt P, upgrading activity decreased to values well below those for the P-free catalyst, indicating phosphorus poisoning at higher P levels. Using NiMoP/AI203 catalysts prepared by sequential impregnation procedures (in the order P, Mo and Ni), Eijsbouts et.al, observed an increase in quinoline HDN, whereas thiophene HDS remained practically unaffected [ 14]. Poisoning effects for thiophene HDS have been reported for carbon supported CoMo catalysts prepared by sequential impregnation (in the order P, Mo, Co) [ 15,16], whereas in the latter study sequentially impregnated AlzO3 supported CoMo catalysts used in the same reaction were not affected by the presence of phosphorus. Based on the formation of phosphomolybdate anions in solutions containing HaPO4 and ammonium heptamolybdate, the use of 12-molybdophosphoric acid or Co(Ni)-containing

519 phosphomolybdates as starting material for the preparation ofhydrotreating catalysts has been investigated as well in the literature [ 17-19], leading to encouraging results. From the above mentioned literature data it can be concluded that the effect of P is far from being well understood. In order to shed more light on this complex topic, two series of catalysts have been prepared with fixed Co and Mo content but with different P loading, either by sequential impregnation (two steps) or by co-impregnation (one step). They have been tested and characterised by XRD, XPS, BET and ESEM/EDX. 2. EXPERIMENTAL

2.1. Catalyst preparation The catalyst support was a commercial trilobed A1203 (specific surface area 273 m2/g, pore volume 0.63 cm3/g, average pore diameter 58 A), in the form of 10xl mm extrudates which have been crushed and sieved to obtain the fraction between 0.3 and 0.5 mm, designated in the text as 0.4 mm particles or grains. Prior to impregnation, the grains have been dried at 125~ overnight and recalcined at 500~ (ramp 5 ~ under 100 ml/min (STP) air flow (Air Liquide) for 4 h. Two series with fixed Co and Mo loading (3.8 % wt Co and 11.6 % wt Mo) and with va13ring phosphorus contents (1, 2 and 3 % wt P) have been prepared by incipient wetness impregnation as well as one phosphorus-free catalyst with the same Co and Mo content. Mo loading corresponds to approximately 75 % of theoretical monolayer coverage of A1203 by MoO3 [20]. P loading corresponds to maximally 0.5 theoretical monolayer coverage, assuming P adsorption as P205 and a P205 monolayer value of 4.7 P atoms/nm: [21]. For all preparations, the volume of solution used was in slight excess of A1203 pore volume. After impregnation, the grains were left for one hour prior to drying and calcination. In the first series (D.I. catalysts), phosphorus was impregnated on the freshly recalcined support by an aqueous solution with the appropriate amount of ammonium dihydrogenophosphate ((NH4)H2PO4, Fluka, 99+ %) followed by drying in a rotary-evaporator at 65~ under reduced pressure, drying-overnight in a glass reactor under flowing air (100 ml/min STP) at 125~ over~'ght and finally calcination at 500~ under the same air flow (5 ~ for 4 h. After calcination, the P-modified support has been impregnated in a single step with a solution containing the appropriate amounts of cobalt nitrate (Co(NO3)2.6HgO, Vel, 97+ %) and ammonium heptamolybdate ((NH4)6MoTO24"4H20,AHM, Merck, 99+ %) followed by drying and calcination as described above. Due to the two impregnation steps used, this series will be called double impregnation (D.I.). The second series of P-containing catalysts has been prepared by a one-step procedure, using aqueous solutions of appropriate amounts of ortho-phosphoric acid (H3PO4,Fluka, 85 + %), Co(NO3)2-6H20 and AHM followed by drying and calcination as described above. This series will be nominated co-impregnation (C.I.). The P-free catalyst was prepared on fleshly recalcined A1203 by one-step impregnation of Co(NO3)2.6H20 and AHM followed by drying and calcination as above. It must be noted that the impregnation solution containing AHM and Co(NO3): is very concentrated; the actual AHM concentration of 0.303 g AHM/ml 1-120is approximately half of the maximum AHM solubility at 25~ (0.655 g AHM/ml 1-120) [22]. Furthermore, concentrated AHM solutions are known to be unstable and variations of temperature and pH

520 may yield precipitates [22] whereas the presence of Co 2+ ions in the solution could also influence stability of the solution. It was indeed observed that acidifying the Co and Mo containing solution with both concentrated and diluted HNO3 immediately led to a precipitate and the same was observed (but to a lesser extend) with (NH4)H2PO4. However, acidifying with H3PO4 led to stable solutions, which is in agreement with the literature [3]. 2.2. Catalyst characterisation All catalysts have been characterised by XRD, BET and XPS. XRD and XPS analysis have been performed on oxide samples and on catalysts after test. Both 0.4 mm grains and powders obtained by grinding the grains have been analysed by XPS in order to examine eventual differences in composition between the exterior surface of the grains and their interior surface. Only oxide samples have been analysed by BET. One specially prepared sample (D.I. 1 %P on extrudates instead of on 0.4 mm grains) has been analysed by environmental scanning electron microscopy coupled to energy dispersive X-Ray microanalysis (SEMfEDX).

2.2.1. XRD The samples have been fmely ground to powder and placed on a sample holder. The machine was a Siemens Kristalloflex D5000 diffractometer using monoctu'omatic CuKc~ radiation. 20 angles between 10 and 80 ~ have been analysed, with a step of 0.05 ~ and a scantime of 22 seconds per step, resulting in a sample analysis time of 8 89 h. 2.2.2. SEM/EI)X The scanning electron microscope was a Philips IPS-XL 30 ESEM-FEG machine, operating at an accelerating potential of 5 and 10 kV and a working distance of 10 mm The detector was of the GSE type. The microscope was used to study special structures on the surface of the catalyst and their compositions were examined by EDX. Furthermore, 100 points on the section of the pellet were analysed by EDX to obtain information on the radial composition of the catalyst particles. 2.2.3. BET The textural properties of the solids have been obtained at -196~ (77 K) on a Micromeritics ASAP 2000 machine, following sample outgassing at 150~ and 10-3 mm Hg pressure.

2.2.4. XPS Samples have been analysed with a Surface Science Instruments SSX-100 model 206 using monochromatised AlKot radiation (1486.87 eV). A residual pressure inferior to 10-8 Torr existed in the analysis chamber. A floodgtm at 10 eV compensated for sample charging. Samples have been pressed into stainless steel sample holders and introduced in the preparation chamber of the XPS machine for outgassing (1.5x10 "6 Torr overnight). Sulphided samples were protected from air by iso-octane during storage, sample preparation and introduction. Besides the general spectrum, the following analytical photopeaks have been monitored C9 is Ols- Al2p - MO3d - (S2p) - Co2p3/2 - P2p. The binding energies (BE) of the peaks have been determined by fixing the C ls peak at 284.8 eV. Surface concentrations have been determined using a standard peak deconvolution method. Atomic ratios of Mo, Co, P and S to AI (Mo/A1,

521 Co/AI, P/A1 and S/A1), representing a measure of their dispersion over the support, were calculated. 2.3. Catalytic activity The catalysts have been tested in the simultaneous HDS of DBT (Acros 99 %) and HYD of NAPHT (Aldrich 99 %) in a fixed bed trickle flow reactor operating at 280~ and 30 bar total pressure. The reactants (DBT, 0.572 % wt and NAPHT, 3.734 % wt) were dissolved in hexadecane (C16, Acros 99 %). An H2S partial pressure was generated by the rapid and complete decomposition of dimethyl disulphide (DMDS, CH3-S-S-CH3, Aldrich 99 %) during the reaction. DMDS/DBT molar ratio in the feed equalled 10, meaning that the generated H2S pa1~tial pressure can be considered independent of DBT conversion level. DBT is converted by a triangular reaction scheme into biphenyl (BiPh, direct hydrogenolysis pathway) or into cyclohexylbenzene and bicyclohexane (CHB and BiCH, hydrogenation of DBT ring prior to C-S bond breaking) [23], however it has been observed that BiPh hydrogenation into CHB or BiCH took place to a minor extend in our conditions (< 5-7 %). NAPHT is converted via two consecutive reactions into tetrahydronaphtalene (THN) and cis and trans-decahydronaphtalene (DHN) respectively [24]. Prior to activation and reaction, the oxidic catalysts were recalcined at 500~ for 4 h in a glass reactor under 100 ml/min air flow. 4 g of recalcined catalyst was loaded in the high pressure reactor and dried at 125~ overnight under Ar flow (Air Liquide, N 46). The sulphidation mixture, 5 % H2S in 1-I2 (Air Liquide, H2S N 27, 1-12 N 40) was introduced, pressure increased to 30 bar at 2 bar/min and H2 flow set at 4.5 Nl/h (STP) and after a stabilisation period of 1 89 h, the temperature was increased to 400~ at a rate of l~ These conditions were held overnight followed by decreasing the temperature to 280~ switching to pure H2 and introduction of the liquid feed. The flow rates were respectively 8 Nl/h and 32 ml/h for 1-I2and liquid. Liquid samples have been collected at regular time intervals (+1-2 h) and have been analysed by gas chromatography on a Chrompack CP9001 machine, equipped with a 25 mx0.25 mm capillary column (Chrompack, CP-SIL 5CB) and a flameionisation detector which has been calibrated by injection of solutions of known concentration of reactants and products. Conversion and selectivity data reported here are obtained in the steady-state regime (after 15-20 h on stream) on at least three samples. 3. RESULTS 3.1. Visual observations After calcination, the P-fi'ee sample and the catalysts prepared by D.I. showed a very heterogeneous aspect. Some grains were covered by a layer of white material, whereas some grains were not or less. The interior of all grains was bluish, an indication of the presence of Co aluminate species. Visual inspection of the calcined C.I. catalysts showed that they were much less ( 1 % P) or not at all heterogeneous. The colour was bluish at the outside as well as in the interior of the grains. 3.2. XRD

The XRD patterns of the supports and of the catalysts before test are shown in Figures la and lb; they indicate clear differences between the two series. The diffraction patterns of the D.I. series (Figure la) contained beside the broad bands of the support some well defined

522 diffraction peaks, which have been attributed to bulk phases of MoO3 and A12(MoO4)3by comparison with diffraction patterns in the ASTM files. It seems that MoO3 was present in more important amounts than A12(MoO4)3.For the C.I. series (Figure lb) on the contrary, only the 1 % P catalyst showed traces of crystallised A12(MoO4)3and an even smaller amount of clystalline MOO3,but for the other two C.I. solids these contributions were absent and only the typical diffraction pattern of the bare support was observed. Other peaks belonging to Co or P containing phases have not been detected for both series. All catalysts were X-ray amorphous after test (not shown), indicating that the sulphidation procedure might have transformed the original large sized MoO3 crystals into smaller MoS2 crystallites, not detectable by XRD. However, air exposure during sample preparation and analysis may also have contributed to the absence of crystalline material after test. X R D s p e c t r a D.I. catalysts

X R D s p e c t r a C.I. c a t a l y s t s

'+~l'''{'lJ,,,l +,,,I~,,,I,,,, I ~+," ~. C.I. 3 % P,,

-i

CI"92%P

!

l 10

20

30

40 50 60 2 theta (deg)

70

80

10

20

30

40 50 60 2 theta(deg)

70

80

Figures la and lb XRD 9 spectra of oxidic catalysts prepared by D.I. (la) and C.I. (lb) 3.3. SEM-EDX

The SEM analysis on an oxidic catalyst prepared by D.I. containing 1 % P on extrudates clearly showed that the exterior pellet surface was covered with large and thin hexagonal shaped crystals (Figure 2). The bare support (not shown) did not show these features. Crystal particles with basal plane dimensions up to 10 gm have been observed. By EDX analysis, it was shown that these crystals were very rich in Mo and O; based on the previous XRD data on oxidic catalysts, it can be concluded that we dear with more or less basal plane oriented MoO3 crystals. Some of these crystals were also found in pores in the interior of the pellet, but the majority was found at the exterior surface. The radial concentration profiles of P, Co and Mo (shown in Figure 3) indicated that Mo was located in high amounts on the exterior surface and possessed a much lower but more or less homogeneous distribution throughout the interior surface of the support. Co seemed to

523 be rather uniformly distributed, but some local fluctuations occurred. P was located more at the outside of the pellet and its concentration gradually decreased when approaching the centre of the pellet. However, this result must be considered carefully as 1 % was close to the detection limit of the machine.

Figure 3 : Concentration profiles of Mo, Co and P over extmdate section

524 3.4. B E T

The results of the texture analysis are shown in Table 1. S.A. has been determined with the BET method, whereas pore volume and pore diameter data have been calculated with the BJH method. Table 1 : BET analysis on oxidic catalysts S.A. (m2/g) Wp(cm3/g) Catalyst A1203 273 0.630 240 0.582 1 % P/AI203 3 % P/AI203 228 0.541 CoMo 0 % P 231 0.491 D.I. 1 % P 246 0.514 D.I. 2 % P 234 0.522 D.I. 3 % P 213 0.505 C.I. 1 % P 236 0.514 C.I. 2 % P 228 0.510 C.I. 3 % P 216 0.471

Av. pore diameter (A) 58.0 62.5 62.7 61.4 59.2 61.1 62.8 61.5 63.6 56.7

The following comments can be made : 1. Impregnation of the bare support with increasing amounts of P led to a si,L,nificant decrease in surface area (S.A.) and pore volume (Vp).A small increase in pore size is observed, due to blocking of micropores. The decrease of S.A. with P content indicates an increasing penetration into and coverage of the Al203 support by P species. It must be recalled that at the maximum P loading, only half theoretical monolayer coverage is possible. 2. Impregnation of Co and Mo on the P-flee support resulted in a considerable loss of surface area and pore volume, implying that at least some of the Co and Mo had been deposited in the interior of the support. 3. Impregnation of Co and Mo on P-modified supports led to a supplementary loss of S.A., but far less than for the P-free support at constant pore volume, leading to a small increase in pore size. During the second step of the D.I. procedure, Co and Mo penetration and adsorption still occurred, but apparently to a lesser extend than on the P-flee support. 4. Also for the C.I. catalysts, S.A. decreased when P content increases. The pore volume of the 2 and 3 % P were lower than for the D.I. counterparts. The significantly decreased pore volume at 3 % P indicated that at this P loading, Mo and Co penetrated to a higher extend into the pore system of the Al203 support compared to the D.I. system. 3.5. XPS The A12pspectra consisted of a single peak, whereas the asymmetric O1s spectra were deconvoluted with the aid of two peaks. The Mo3d spectrum of these oxidic catalysts consisted of a single doublet with B.E. corresponding to oxidic Mo 6+ species. The Co203/2 spectrum showed the main peak at B.E. between 780.8 and 781.6 eV corresponding to Co-oxide or Coaluminate species and the shake-up peak at somewhat higher B.E. For some 0.4 mm samples (sulphided and oxide), an increased as3nnmetry was observed for the O1s spectrum coupled to the apparition of a second Al2p peak at 2-3 eV higher B.E.. This may be interpreted as a coverage of the AlzO3 support by a thick layer of another material or to differential charging effects; however, no peak doubling has been observed for the other elements so the former

525 explanation seems to be favoured. A12p peak doubling or increased Ols peak deformation has not been observed for the powdered samples. The MO3d spectrum of the sulphided catalysts consisted again of a single doublet, however, partial overlap with the S2s peak at 226 eV occurred. A shift of the MO3ds/2 B.E. to values between 228.8 and 229.5 eV indicated the presence of MoS2 like species. No residual Mo 6§ nor Mo 5+ species needed to be considered for peak deconvolution purposes. The B.E. of the main Co2p3/2 peak is representative of sulphided Co species, but some non sulphided Co species were still present (peak at 781 eV). A small shift is also observed for the P2p and A12p B.E. in the sulphided samples. The results of the quantitative XPS analysis on the oxidic are shown in Table 2, whereas Table 3 groups the results obtained on catalysts after test. Both the atomic percentages of AL Mo, Co, P and S, as well as the atomic ratios of Mo/A1, Co~A1, P/AI and S/A1 are reported in both tables. Table 2 Quantitative 9 XPS data on oxide catalysts n.d. scanned 9 but not detected n.a. "not analysed C (%) O (%) A1 (%) Mo (%) Co (%) P (%) C/A 1 O/A 1 Mo/A I Co/A 1 P/A I Oxide OUT IN OUT IN OUT IN OUT IN OUT IN OUT IN A1203 18.4416.6349.5651.3532.0032.03 n.a. n.a. n.a. n.a. n.a. n.a. 0.5760.5191.5491.603 0.93 0.74 P/AI203 14.0113.5555.5456.3029.5229.42 n.a. n.a. n.a. n.a. 0.0315 0.0252 1 % P 0.475 0.4611.8811.914 2.87 2.68 P/A1203 13.2713.4553.8654.2130.0029.67 n.a. n.a. n.a. n.a. 0.0957 0.0903 3 % P O.442 O.4531.7951.827 CoMo 19.9216.5352.2251.25 15.2825.20 11.45 5.89 1.12 1.12 n.d. n.d. 0 % P 1.3040.6563.4182.034 0.749 0.234 0.0733 0.0444 D.I. 16.1416.3551.5251.2121.2427.24 9.2 3.78 1.14 0.78 0.93 0.77 1%P 0.7600.6002.4261.880 0.433 0.139 0.0537 0.0286 0.0439 0.0283 D.I. 17.4715.7453.8052.5513.8625.33 13.24 4.33 0.88 0.77 0.76 1.28 2 % P 1.26oo.6213.8822.075 0.955 0.171 0.0635 0.0304 0.0548 0.0505 D.I. 15.0015.0954.3252.6418.9224.94 8.37 4.38 2.00 0.92 1.39 2.04 3~ 0.7930.6052.8712.111 0.442 0.176 0.106 0.0369 0.0735 0.0818 C.I. 17.9018.8451.8752.0924.6525.20 4.10 2.35 0.76 0.69 0.70 0.84 1 % P 0.7260.7482.1042.067 0.166 0.09330.0308 0.0274 0.0284 0.0333 C.I. 9.76 10.2355.2954.7529.3929.87 2.82 2.65 0.77 0.86 1.98 1.65 2 %P 0.3320.3421.8811.833 0.09600.08870.0262 0.0288 0.0674 0.0552 C.I. 13.2914.9253.0052.8327.7427.13 2.67 2.38 0.58 0.71 2.72 1.99 3 %P 0.4840.5501.9111.947 0.09630.08770.0209 0.0262 0.0981 0.0734 Some comments need to be made for the oxidic catalysts : 1. The XPS "visibility" of the A1203 support was not greatly modified by the addition of P. The P-modified A1203 supports displayed a rather homogenous distribution of P throughout the whole particle and the visibility of the A1203 support was almost not modified.

526

2. After impregnation of Co and Mo in the second impregnation step for D.I. catalysts, much less A1 was detected at the outer surface of the grains compared to the P-modified support, pointing to a pronounced coverage of the external AlzO3 support surface by Co and Mo containing phases. Furthermore, the composition of the external surface fluctuated in a noncoherent sense (in fimction of P-content); these two observations are in agreement with the visual inspection, which indicated that the surface of some grains were covered by a layer of white material whereas some where not, or less. 3. An elevated Mo/A1 ratio seemed to be linked to a high O/A1 ratio, which is a supplementary indication for a layer of a Mo-oxide phase situated above the support. 4. The interior surface of the D.I. grains showed a more homogeneous composition and a much higher visibility of the support. 5. For 1% and 2 % P D.I. catalysts, P seemed to be located preferentially on the outer surface of the catalyst after the second impregnation step, which is in agreement with the EDX results. For the 3 % P D.I. sample however, P seemed to be located preferentially on the inner surface of the particles. The lesser visibility of P on the external surface could not be explained solely by the coverage of the surface by large amounts of Mo and Co, as the P/A1 ratios are different as well for the P-modified support and the D.I. catalysts. 6. For the C.I. samples the behaviour is quite different, in agreement with theh" more homogeneous aspect. Practically no differences in support visibility were observed between the inner and the outer surface of the catalysts. 7. The Mo and Co enrichment at the outer surface decreased significantly when introducing P ( 1 % P) and disappeared when increasing P content. 8. With increasing P loading, P seemed to be located more at the outer surface than at the inner surface in contrast to the tendency observed on D.I. samples. Table 3 : Quantitative XPS data on catalysts after test n.a. :not analysed 0 (%) A1 (%) Mo (%) Co (%) P (%) S (%) O/A 1 Mo/A 1 Cola 1 P/A l S/A 1 After test OUT IN OUT IN OUT IN OUT IN OUT IN OUT IN CoMo 31.34 43.36 14.19 20.40 7.02 4.05 1.58 1.14 n.a. n.a. 15.32 9.09 0%P 2.209 2.125 0.495 0.199 0.111 0.0559 1.08 0.446 D.I. 1 % P 30.28 43.29 16.51 16.35 10.13 4.30 1.25 1.10 n.a. 0.49 20.53 10.05 1.834 2.648 0.614 0.263 0.0757 0.0673 0.0300 1.24 0.615 D.I. 2 % P 23.68 50.33 8.50 20.14 12.27 2.53 1.66 0.87 0.26 1.00 25.00 5.06 2.786 2.500 1.44 0.126 0.195 0.0432 0.0306 0.04972.941 0.251 D.I. 3 % P 33.71 45.72 18.55 22.41 5.85 1.98 3.05 1.12 0.65 1.87 15.07 4.78 1.817 2.040 0.315 0.0884 0.164 0.05000.03500.08340.812 0.213 C.I. 1 % P 27.64 46.11 11.89 22.27 12.37 2.02 1.48 0.77 0.17 0.63 21.15 4.74 2.325 2.070 1.04 0.0907 0.124 0.03460.0143 0.0283 1.78 0.213 C.I. 2 % P n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. C.I. 3 % P 42.26 49.86 18.58 22.80 4.23 1.68 1.48 0.76 0.67 1.80 9.93 3.95 2.274 2.187 0.228 0.0737 0.0797 0.0333 0.0361 0.07890.534 0.173

527 For the samples after test some comments need to be made as well : 1. The A1 concentrations found after test were generally lower than for the corresponding oxide sample, the difference being much larger for the exterior surface. 2. Carbon content (not shown) was 5-8 % higher after test than before test (oxide) due to coke deposition during test and to the use ofiso-octane to protect the sulphided samples. 3. Only Mo species in a +4 state in a sulphur environment, corresponding to MoSz, were observed after test, but besides the sulphided Co species, some non-sulphided Co species were still found after sulphidation and reaction. 4. The actual S san-face concentration was very close to the stoichiometric amount needed for the formation of MoSz and Co9S8, implying that the catalysts, or at least that part analysed by XPS, were well sulphided. 5. All catalysts showed a very important depletion of the P on the exterior surface of the grains after test (p concentrations and P/A1 ratios), which was more important for the catalysts with higher P content. 6. Si~ificant lower P concentrations were also observed after test for the powdered samples, compared to the corresponding oxide samples, but the P/A1 ratio were constant, indicating that P dispersion on the interior surface was not changed after sulphidation and test. 7. All catalysts showed pronounced A1, Mo and Co concentration differences between the inner and outer surface after test. 8. Mo/A1 and Co/A1 ratios for the powdered samples were generally lower after sulphidation and test, compared to the oxide samples, which is in agreement with the slight uncovering of the support commonly observed during sulphidation ofhydrotreating catalysts. 9. For the C.I. catalysts, Mo/AI ratios on the exterior surface were higher after test than before test, which can partly be explained by the decreased visibility of the support observed after test. 10.For all catalysts, Co/A1 ratios on the exterior surface were generally much higher after test than for the corresponding oxide catalysts.

3.6. Catalytic activity Based on the resuks of the catalyst characterisation, differences in catalytic activity can be expected. HDS and HYD data reported in Table 4 indeed show that the two series exhibited completely opposite behaviour : for D.I. catalysts activity decreased with P content, whereas for C.I. catalysts activity increased. Table 4 HDS 9 and HYD activities and selectivities for D.I. and C.I. catalysts XDBT SBiPh SCHB+ SBiCH XNAPHT STHN SDHN

Catalyst

CoMo 0 % P D.I. 1 % P D.I. 2 % P D.I. 3 % P C.I. 1% P C.I. 2 % P C.I. 3 % P

(%) .... 45.50 33.90 26.93 25.10 50.42 51.93 54.75

(%)

(%)

(%)

(%)

(%)

71.66 68.79 64.08 63.23 71.12 70.03 70.05

27.12 29.02 32.68 35.08 28.10 28.83 29.19

47.72 43.92 39.80 38.93 56.99 60.92 63.14

98.73 98.72 98.77 98.67 98.11 97.64 96.44

1.27 1.28 1.23 1.33 1.89 2.37 3.56

528 The following comments can be made : 1. For the P-free catalyst, HDS and HYD conversions are practically identical. 2. For the D.I. catalysts, HDS and HYD activity fell drastically upon introduction of P and decreased steadily when the concentration of P increased. HDS activity is much more affected than HYD and a small but si,maificant decrease of the BiPh selectivity indicated that, for DBT, the hydrogenation-prior-to-hydrogenolysis pathway was slightly favoured when increasing the P content. A decrease in conversion can not explain this observation, because the selectivity ratio SBiPh/(Sc~ + SBiCH)should be independent on conversion (parallel reactions). No change in NAPHT selectivity towards the completely hydrogenated DHN was observed. 3. For the C.I. series, a sharp increase in HDS and HYD activity is found when P is introduced and activity grew monotonously when the P-content increased. Again, HYD activity is more affected than HDS. In this case, no change in BiPh selectivity is observed, however, DHN selectivity increased sharply when increasing the P concentration. This latter increase can not be explained entirely by an increase of NAPHT conversion (consecutive reactions), as experiments with commercial catalysts in the same conditions yielded similar or higher NAPHT conversions with DHN selectivities around 1-1.5 %. These resuks indicate that the impregnation method (D.I. or C.I.) played a crucial role in detennining the activity of P-modified hydrotreating catalysts. According to the impregnation procedure, an increase or a decrease of activity is observed when introducing P in the catalysts formulation by co-impregnation or by sequential impregnation respectively. These trends were further accentuated by increasing the amount of P. Based on the activity and selectivity data, it can be said that not only the amount but also the nature of the sites changed (slightly) upon the amount and mode of introduction of P. 4. DISCUSSION Based on the characteristics of pore-filling impregnation on freshly activated A1203 support and on the aforementioned results on catalysts characterisation and testing, a consistent picture of the preparation of (P-modified) catalysts arises; the P-free CoMo/AI203 catalyst will be treated first, followed by the D.I. ones and finally the C.I. samples. Water absorption during pore-filling impregnation being an exothermic process, the first drops of the concentrated AHMJCo(NO3)2 solution added to the freshly recalcined support led to a temperature increase and pal~ial evaporation of the solvent, decreasing the stability of the concentrated impregnation solution, leading to a precipitate on the exterior surface of the grains. Moreover, van Veen et.al, have drawn the attention to the fact that higlfly concentrated AHM solutions used for the preparation of Mo/AI203 or Mo/TiO2 catalysts may lead to precipitation even for catalysts prepared with an excess of solvent [25]. Precipitation formation could furthermore be accentuated due to the strong interaction between molybdate species and the alumina support. The precipitate transformed into bulk MoO3 and A12(MoO4)3 upon calcination as found by XRD on the P-free catalyst. XPS on the same catalyst indicated a ve12r pronotmced composition difference between the exterior and the interior surface indicating that the P-free catalyst did not display an homogeneous active phase distribution ffuoughout the catalyst particle and that Co and Mo were not deposited as a monolayer on the alumina suppox~. For the D.I. catalysts, the presence of P on the support led to less Mo adsorption sites, as they can adsorb on the same kind of sites, increasing thus the amount of Mo deposited at the

529 exterior surface compared to the P-free catalysts. The increase of the P content still aggravated this process, resulting in poorly dispersed Mo and Co (XPS, EDX) and in the presence of bulk MoO3 and A12(MoO4)3phases (XRD, SEM). As the total amount of Co and Mo added to all catalysts is the same, a deposition of more active phase on the exterior of the grains must lead to less active phase in the interior and hence to less active catalysts, which has effectively been observed. These observations are totally in agreement with L6pez Cordero et.al, who also found the presence of A12(MoO4)3and MoO3 for P-Mo/AI203 catalysts prepared by sequential impregnation whereas their P-free catalyst (with only 9.5 % wt Mo) was X-Ray amorphous [26]. For the C.I. catalysts, the addition of H3PO4 to AHM and Co 2+ containing solutions may have led to the formation of H2P2MosOz34 anions [27] and Co-Mo-P type compounds [19,28-30]. Due to the higher metal solubility and stability of these H3PO4 containing solutions compared to P-free CoZ+/AHM solutions, less Mo and Co species were deposited on the exterior surface during the addition of the first drops of impregnation solution to the freshly activated support. Furthermore, compared to PO43-and Mo70246-anions, H2P2MosO234"anions are known to interact less strongly with the AlzO3 support so the (cobalt containing) phosphomolybdate anions can migrate more freely throughout the support particle before being adsorbed, resulting in a more homogeneous distribution of Mo throughout the catalyst particle, as has been shown by Jian and Prins for PMo/A1203 catalysts prepared by coimpregnation [31]. Complete transformation of phosphate and molybdate species into H2P2MosO234 anions demands a P/Mo molar ratio of 0.4, which is, in the present case, only satisfied for the C.I. catalysts containing 2 and 3 % wt P. For the 1 % C.I. catalyst (P/Mo < 0.4), all P atoms can considered to be bound in H2P2MosO234 anions and thus approximately 3 5 % of the original amount of Mo still existed in the form of Mo70246"anions in the solution. For the reasons stated above for the P-free catalysts, these latter anions may have been responsible for the presence of crystalline A12(MoO4)3 phases and the composition differences between the outer and inner surface. However, these composition differences were less important than for the P-free or D.I. samples, explaining that the palatial transformation of the AHM into phosphomolybdates (C.I. 1 % P) ah'eady resuked in a better distribution of the Co and Mo species throughout the support particles and therefore in increased activity. The P/Mo ratios in the impregnation solutions for C.I. catalysts with 2 and 3 % wt P were above 0.4 indicating that all Mo would have been bound as (Co containing) H2P2MosOz34- anions, the excess P being present as phosphate species in solution. These excess phosphate species ascertain a low impregnation pH which increases the anion adsorption capacity of the support. They would also inhibit the decomposition of H2P2MosO234 anions into phosphate and molybdate species [31], leading to a more homogeneous distribution of the Co and Mo species throughout the catalyst particles and to the absence of crystalline material deposited on the outer surface. The discussion for the catalysts after test is less straightforward. It has been obsel-ved by XRD that all catalysts were X-ray amorphous after test, indicating a decrease in crystaUite size during sulphidation and testing. It is found by XPS that the Mo species were in the sulphided state after sulphidation and test, and that no other valence states needed to be considered. Non sulphided Co-species were found however after test, but no link existed between its amount and activity. In agreement with the literature, it is found that the dispersion and active phase distribution over the carrier particles in the oxide state is important in the determination of the resuking catalytic activity in the sulphided state. The distribution of the

530 active phase (e.g. the difference in composition between the exterior and interior surface) in the oxide state seemed to be linked with activity: the more homogeneous the distribution, the more active the catalyst. The more homogeneous distribution for the C.I. catalysts seemed to be lost after sulphidation and test as for all sulphided catalysts, pronounced concentration differences of A1, Mo, Co and P have been observed by XPS between the interior and the exterior support surface. This can only partly be explained by a decreased visibility of the support due to carbon depositions during test and sample storage and the relatively high amounts of S. However, this better distribution for C.I. catalysts might have been maintained after sulphidation and reaction; but the simple comparison between XPS results on grains and powder obtained by crushing the grains seemed then not to be the appropriate method to evidence eventual concentration differences between exterior and interior support surface. EDX analysis on the section of the grains after test would yield valuable information, but these experiments have not been performed. 5. CONCLUSIONS It has been shown that the catalytic properties of P-containing CoMo/AI203 catalysts were greatly modified by the impregnation method (sequential or co-impregnation) and by the amount of P added. Introduction of P prior to Mo and Co (D.I.) led to badly dispersed active phases, because P occupied a part of the anchoring sites of the molybdate anions on the support. Hence, a decrease in activity was observed, which was still aggravated by an increase of the P content. When P, Mo and Co were impregnated in one step (C.I.), homogeneously dispersed catalysts were obtained, leading to increased activity. The increased dispersion was rationalised through the formation of (Co containing) phosphomolybdate anions, capable of penetrating more deeply into the pore system of A1203 and the lesser competition between P and Mo for adsorption on the support. REFERENCES

1. 2.

S. Stanislaus, A. Asi-Habi and C. Dolona, Appl. Catal. 39, 239 (1988) R. Prada-Silvy, Y. Romero, J. Guaregua and R. Galiasso in "Preparation of Catalysts V", (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Eds.), Elsevier Science Publishers B.V., Amsterdam, 1991, p. 37 3. J.D. Colgan and N. Chomitz, US Patent 3.287.280, (1966) 4. J. Cruz Reyes, M. Avalos-Borja, R. L6pez-Cordero and A. L6pez-Agudo, Appl. Catal. A: General 120, 147 (1994) 5. A. Morales, M.M. Ramirez de Agudelo, Appl. Catal. 12, 381 (1984) 6. P. Atanasova and T. Halachev, Appl. Catal. 48, 295 (1989) 7. M.A. Goula, Ch. Kordulis and L. Lycourghiotis, J. Catal. 133, 486 (1992) 8. C.W. Fitz and H.F. Rase, Ind. Eng. Chem. Prod. Res. Dev. 22, 40 (1983) 9. O. Poulet, R. Hubaut, S. Kasztelan and J. Grimblot, Bull. Soc. Chim. Belg. vol. 100, 857 (1991) 10. J.L.G. Fierro, A. L6pez Agudo, N. Esquivel and R. L6pez Cordero, Appl. Catal. 48, 353 (1989)

531 11. D. Chatwick, D.W. Aitchinson, R. BadiUa-Ohlbaum and L. Josefsson in "Preparation of Catalysts HI", (G. Poncelet, P. Grange and B. Delmon, Eds.), Elsevier Science Publishers B.V., Amsterdam, 1983, p. 323 12. P. Atanasova, T. Halachev, J. Uchytil and M. Kraus, Appl. Catal. 38, 235 (1988) 13. R.E. Tischer, N.K. Narain, G.J. Stiegel and D.L. Cillo, Ind. Eng. Chem. Res. 26, 422 (1987) 14. S. Eijsbouts, L. van Gruijthuijsen, J. Volmer, V.H.J de Beer and R. Prins, in "Advances in Hydrotreating Catalysts", (M.L. Occelli and R.G. Anthony, Eds.), Elsevier Science Publishers B.V., Amsterdam, 1989, p. 79 15. S.M.A.M. Bouwens, J.P.R. Vissers, V.H.J de Beer and R. Prins, J. Catal. 112, 401 (1988) 16. S.M.A.M. Bouwens, A.M. van der Kraan, V.H.J de Beer and R. Prins, J. Catal 128, 559 (1991) 17. Y. Okamoto, T. Gomi, Y. Mori, T. Imanaka and S. Teranishi, React. Kinet. Catal. Lett., 22, 417 (1983) 18. S. Damyanova and J.L.G Fielxo, Appl. Catal A: General 144, 59 (1996) 19. A. Griboval, P. Blanchard, E. Payen, M. Foumier and J.L. Dubois in "Hydrotreatment and hydrocracking of oil fractions", (G.F. Froment, B. Delmon and P. Grange, Eds.) Elsevier Science B.V., Amsterdam, 1997, p. 181 20. J. Sonnemans and P. Mars, J. Catal. 31, 209 (1973) 21. H. Kraus and R. Pfins, J. Catal 164, 260 (1996) 22. G.A. Tsigdinos, H.Y. Chen and B.J. Streusand, Ind. Eng. Chem. Prod. Res. Dev., 20, 619 (1981) 23. M. Houalla, N.K, Nag, A.V. Sapre, D.H. Broderick and B.C. Gates, A.I.Ch.E.J. 24-6, 1015(1978) 24. C. Moreau and P. Geneste in "Theoretical Aspects of Heterogeneous Catalysis", (J.B. Moffat, Ed), Van Nostrand Reinhold Catalysis Series, 1990, p. 256 25. J.A.R. van Veen, H. de Wit, C.A. Emeis and P.A.J.M. Hendriks, J. Catal. 107, 579 (1987) 26. R. L6pez Cordero, S. L6pez Guerra, J.L.G. Fierro and A. L6pez Agudo, Appl. Catal. 48, 341 (1989) 27. H. Kraus and R. Prins, J. Catal. 164, 251 (1996) 28. H. Kraus and R. Prins, J. Catal. 170, 20 (1997) 29. P. Atanasova and T. Halachev, Appl. Catal. 48, 295 (1989) 30. P.J. Mangnus, J.A.R. van Veen, S. Eijsbouts, V.H.J. de Beer and J.A. Moulijn, Appl. Catal. 61, 99 (1990) 31. M. Jian and R. Prins, Bull. Soc. Chim. Belg. 104, n ~ 4-5, 231 (1995)

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

533

P o l y m e r s u p p o r t e d m e t a l complex c a t a l y s t s w i t h m e m o r y A.A.Efendiev a and T.N.Shakhtakhtinsky b aAzerbaijan Academy of Sciences Institute of Polymer Materials, Samed Vurgun str.124, Sumgait, 373204, Azerbaijan Republic bAzerbaijan Academy of Sciences M.F.Nagiyev Institute of Theoretical Problems of Chemical Technology, Gusein Javid Prospect 29, Baku, 370143, Azerbaijan Republic

INTRODUCTION Interest in solid supported metal complexes has largely grown in the last two decades [1]. These catalytic systems can combine the advantages of heterogeneous catalysts, such as simplicity of separation from the reaction media and high stability with the advantages of homogeneous catalysts, such as high activity and selectivity and possibility of handling more exact information about the structure of active centres. The use of polymer complexons as solid supports opens up new possibilities to vary ligand surrounding and to control catalytic properties of complexes [2,3]. This is due to macromolecular nature of the polymer complexons, i.e. the flexibility of macromolecules, their ability of acquiring different conformations and possibilities of varying locations of functional complexing groups both in macromolecular chains and in a three-dimensional polymeric matrix. This paper summarizes our results on synthesis, investigation and use of polymer supported metal complex catalysts with memory to the catalyzed substrate. RESULTS AND DISCUSSION Earlier we have developed a principle of preparation of complexing polymers based on use of memory of polymer composition [4-8]. The principle consists in conformational prearrangement of macromolecules of noncrosslinked complexing polymers to a favorable for complexing metal position followed by fixation of conformations optimum for the complex formation by means of intermolecular crosslinking. Crosslinked macromolecules might keep in mind conformations favorable for complexing substrate and this leads to an essential improvement of complex-forming ability of polymers. It has been also shown that prearranged complexes are efficient catalysts for various chemical reactions in comparison with nonprearranged complexes.

534 Using this principle, a number of crosslinked complexing polymers and metal polymer complex catalysts containing phosphorylic, carboxylic, pyridine, amine and imine functional groups were prepared and they have been prearranged for complex formation with copper, cobalt and nickel and also with ethylbenzene, isopropylbenzene, n-decane, allyl alcohol and propyl alcohol. The prearranged complexes were used as catalysts in oxidation and hydrogenation reactions of hydrocarbons. As an example, reaction of liquid-phase oxidation of ethylbenzene (EB) has been chosen for investigation. Copolymer of diethyl ester of vinylphosphonic acid (DEVPA) and acrylic acid (AA) has been synthesized, prearranged for complex formation with cobalt and used as catalysts. The reaction was carried out in a glass thermostatted reactor without solvent. Gaseous oxygen was used as an oxidizer at atmospheric pressure. The catalytic activity of the catalyst was evaluated according to the oxygen absorption rate and the yield of the reaction products - ethylbenzene hydroperoxide (EHP), methylphenylcarbinol (MPC) and acetophenone (AP). It has been shown that the reaction in the presence of nonprearranged complexes does not practically take place. It should be noted that prearranged complexes can accomodate up to 4,2 meq of cobalt/g whereas in nonprearranged complexes cobalt content does not exceed 1,2 meq/g [9].

r~

~

2

4

O

<:

4

2 TL~te, tt

Fig. 1. Kinetic curves for oxidation of ethylbenzene in the presence of cobalt complexes with copolymer of DEVPA and AA. Concentration of the catalyst, g/l: 1 - 0.25; 2 - 0.5; 3 - 0.75; 4 - 1.0 (cobalt benzoate). To determine the effect of the amount of catalyst on the reaction proceeding, catalyst concentrations have been ranged from 0.25 to 0.75 g/1 which corresponds

535 to cobalt content from 1.05 to 3.15 meq/1. The temperature of the reaction was 90~ - at this temperature the amount of oxygen consumed had been found to be maximum. The results are shown in Figure 1. It can be seen from the Figure that the highest rate of the reaction is achieved at catalyst concentration between 0.5 and 0.75 g/l which corresponds to cobalt content in the range of 2.1 - 3.15 meq/g. The rate of oxidation is comparable with that in the presence of homogeneous cobalt-containing catalyst - cobalt benzoate which has been taken in an amount of lgfl to provide maximum reaction rate. This amount corresponds to cobalt content of 8.2 meqfl. The results also show an essential advantage of prearranged cobalt polymer catalyst over cobalt benzoate. First, about three times less amount of cobalt is needed to achieve approximately the same activity of the catalyst. Second, cobalt polymer complexes can be separated and used repeatedly, practically without loss of catalytic activity. As the next example, complexes of copper with the polymer prepared from polyethylene polyamine (PEPA) and epichlorohydrin-ammonia oligomer (ECHA) have been studied. Molar ratio of PEPA to ECHA was 1:1. They have also been prearranged for the complex formation with copper and both nonprearranged and prearranged complexes have been used in the same reaction. Preparation and prearrangement of PEPA and ECHA based polymers are described in our paper [ 10]. The catalyst was taken in an amount of 1 gfl. The content of copper in nonprearranged and prearranged complexes was the same - 1.7 meq/g. Results are presented in Table. Table. Oxidation of ethylbenzene in the presence of nonprearranged and prearranged PEPA based copper complexes

Catalyst

Conversion, % mol

Yield, % mol . . . . . . EHP

MPC

AP

Nonprearran_ ged copper complexes

10.5

3.4

2.1

5.0

Prearranged copper complexes

18.3

6.4

2.9

9.0

It can be seen that prearranged complexes demonstrate higher catalytic activity in comparison with nonprearranged ones, even with similar compositions of both catalysts. Obviously the improvement of catalytic properties of the prearranged complexes is related not only to higher content of metal but also to essential structural changes of the complexes. It must be noted, however, that

536 interconnection between the structure of the complexes, especially the prearranged ones, and their catalytic properties has not been studied. We investigated nonprearranged and prearranged copper complexes with polymers based on PEPA and their low molecular weight analogues using ESR. Investigation has shown a significant changes in the structure of complexes of prearranged polymers compared with nonprearranged ones. Prearrangement provides formation of complexes with uniform structure similar to their low molecular weight analogues in solution. It is well known that formation of uniform active centres of similar structure is an important condition for preparation of active and selective catalysts [ 11]. An attempt has been made to a further increase of activity of metal polymer complexes by means of formation and subsequent fixation in memory of the catalyst of the structure of active centres favorable for hydrocarbon substrate. Above mentioned complexes of cobalt with the copolymer of DEVPA and AA have been chosen first for such treatment and liquid-phase oxidation of ethylbenzene was taken as a model reaction. The approach proposed is based on the principle suggested for the prearrangement of polymers to a complexing metal. The method is based on interaction of noncrosslinked metal polymer complexes with the hydrocarbon substrate or an intermediate product of its transformation in fairly dilute solution, i.e. under conditions when macromolecules are still sufficiently mobile, followed by fixation of the structure by crosslinking and removal of the template substrate from the crosslinked catalyst [6 - 8]. Solution of complexes in ethanol was mixed with excess amount of ethylbenzene or isopropylbenzene, quickly frozen and, after the removal of the solvents by freeze drying, complexes were crosslinked using NI, N ~- methylenediacrylamide [8,9]. Finely ground complexes were used as catalysts. The reaction was carried out in the same conditions as described above. For comparison, complexes with identical composition crosslinked without prearrangement for hydrocarbon were also used. Kinetic curves for the ethylbenzene oxidation in the presence of nonprearranged and prearranged for ethylbenzene and isopropylbenzene complexes of cobalt with the crosslinked copolymer of DEVPA and AA are given in Figure 2. It can be seen from the Figure that the rate of reaction in the presence of prearranged complexes is much greater than that in the presence of nonprearranged complexes. Prearrangement for isopropylbenzene also leads to an increase of the reaction rate but less marked than in the case of prearrangement for ethylbenzene. The next step was preparation of prearranged for n-decane PEPA and ECHA based copper complexes. The prearrangement procedure was similar to that used in the case of prearrangement of the complexes to ethylbenzene. Catalytic properties of nonprearranged and prearranged for n-decane complexes were investigated in reaction of liquid-phase oxidation of n-decane. Both catalysts contained 1.7 meq of Cu/g. The amount of the catalysts taken in

537 both cases was 0,8 g/1. The reaction was carried out in the same reactor using gaseous oxygen at atmospheric pressure.

I

..4

inK.

0 r

<:

4O

4

2

3 4 Time, It

Figure 2. Kinetic curves for oxidation of ethylbenzene in the presence of nonprearranged (1) and prearranged for ethylbenzene (2) and isopropylbenzene (3) complexes of cobalt with copolymer of DEVPA and AA. Kinetic curves for the oxidation of n-decane in the presence of nonprearranged and prearranged for n-decane complexes are shown in Figure 3. It can be seen from the Figure that in this case prearrangement does not lead to increase in the reaction rate. On the contrary, complexes prearranged for ndecane are less active in oxidation reaction than nonprearranged ones. It is known that catalytic oxidation of hydrocarbons might proceed by two mechanisms. In the first case, hydrocarbon is adsorbed on the active centres of catalysts and activated. The activated hydrocarbon reacts with oxygen and is oxidized. The oxidation reactions previously mentioned proceed according to this mechanism. Prearrangement of complexes for hydrocarbons results in easier adsorption of the substrate on the active centres which in turn improves catalytic activity of the catalyst. In case of the second mechanism oxygen molecules are adsorbed on the active centres of catalysts and activated. The activated oxygen then oxidizes the hydrocarbon.

538

O

4o-

~'J 6 -

4

2 ~ 4 56 TLme, it

Figure 3. Kinetic curves for oxidation of n-decane in the presence nonprearranged (1) and prearranged for n-decane (2) copper complexes.

of

Presumably, the oxidation of n-decane proceeds according to the second mechanism. Thus, the prearrangement for hydrocarbon creates hindrances to adsorption of oxygen instead of facilitating it. That is why in this case decrease in catalytic activity of complexes is observed. We also carried out hydrogenation of allyl alcohol using poly-4-vinylpyridine nickel complexes prearranged for allyl alcohol and for the product of the reaction - propyl alcohol. Poly-4-vinylpyridine ~VP) was prepared by polymerization of conventional 4vinyl-pyridine (((Merck~)). An ethanol solution of PVP (1 mol) was mixed with an ethanol solution of NiC12 (0,5 mol) to give PVP nickel complexes. The complexes were crosslinked using NI, N I - methylenediacrylamide as a crosslinked agent. The procedure of polymerization and preparation of nonprearranged and prearranged for allyl alcohol and propyl alcohol complexes was described earlier

[8l. Hydrogenation of allyl alcohol was carried out in a gasometric unit at 25~ using gaseous hydrogen which was bubbled through the reaction medium at atmospheric pressure. Crosslinked PVP nickel catalyst was taken in the amount of 1,4 g/l. Content of nickel in all catalysts was 4 meq/g. Kinetic curves for hydrogenation of allyl alcohol to propyl alcohol in the presence of both nonprearranged and prearranged for allyl and propyl alcohol PVP nickel complexes with different degrees of crosslinking are given in Figure 4.

539

42

~

8

4

2

5

~. 5 TLme, h

Figure 4. Kinetic curves for hydrogenation of allyl alcohol into propyl alcohol in the presence of nonprearranged (1) and prearranged for propyl alcohol (2) and allyl alcohol (3) complexes. Crosslinking degree- 15%. It can be seen from the Figure that the rate of hydrogenation in the presence of complexes prearranged for allyl alcohol, is much higher in comparison with nonprearranged complexes. On the contrary, the rate of the reaction in the presence of complexes prearranged for propyl alcohol is lower than that in the presence of nonprearranged complexes due to blocking of active centres from complexation with allyl alcohol. During formation of complexes between substrate and catalyst, bonds both in the substrate and in the catalyst become more rigid. The substrates which are bound more loosely are less rigid and therefore less reactive compared to those with stronger bonds. Naturally, the strength of the bonds should have an optimum value. Specificity might influence activation entropy of transition complex because the conformation of the substrate-catalyst complex upon sorption is close to conformation of catalyst within the activated complex. This possibility is stipulatedby rigid valence angles and bonds and also restriction of rotation. In complexes with ~goocb~ substrates, i.e. those strongly bound and easily reacted the conformation is fixed very close to that in transition state. The situation is different in the case of ~bacb~ substrates. Rotation around the critical bond is more free and favorable conformations occur less often. As a result of prearrangement of metal polymer complexes, substrate stereocorrespondence similar to that in enzymic reaction takes place.

540 CONCLUSION The results obtained show that the special prearrangement of active centres of polymer supported metal complexes for the hydrocarbon substrate leads to an essential increase in activity and selectivity of the catalysts prepared on their base. The prearrangement can also be used as an additional instrument for investigation of mechanism of catalytic processes with the use of polymer supported metal complex catalysts. ACKNOWLEDGEMENT The results discussed have been obtained in collaboration with Dr.J.J.Orujev, Dr.E.B.Amanov and Dr.Yu.M.Sultanov of Azerbaijan Academy of Sciences M.F.Nagiev Institute of Theoretical Problems of Chemical Technology. The authors thanks Professor V.A.Kabanov of Moscow State University for valuable advices and fruitful discussions. REFERENCES

1. Yu.I.Yermakov, V.A.Zakharov, B.N.Kuznetsov, Oxide Supported Complexes in Catalysis, Nauka, Moscow, 1980. 2. ((Polymer - Supported Reactions in Organic Synthesis)), P.Hodge and D.C.Sherrington, eds., John Willey&Sons, Chichester, New York, Brisbane, Toronto, 1980. 3. F.R.Hartley, ((Supported Metal Complexes)), D.Riedel Publishing Company, Dordrecht, Boston, Lancaster, Tokyo, 1985. 4. V.A.Kabanov, A.A.Efendiev, J.J.Orujev, Auth. Cert.USSR 502907 (1974), Bull. Inv.6, (1976) 58. 5. A.A.Efendiev, V.A.Kabanov, Pure & Appl.Chem. 54, (1979) 2077. 6. A.A.Efendiev, J.J.Orujev, T.N.Shakhtakhtinsky, V.A.Kabanov, Kinetika i Kataliz, 27, (1986) 520. 7. A.A.Efendiev, Macromol.Symp. 80, (1994) 289. 8. A.A.Efendiev, J.J.Orujev, E.B.Amanov, Yu.M.Sultanov, in: ((Metal Containing Polymeric Materials)), Plenum Publishing Corporation, New York, N.Y. (1996) 255. 9. A.A.Efendiev, T.N.Shakhtakhtinsky, L.F.Mustafaeva, H.L.Shick, Ind.Eng. Chem. Prod.Res.Dev.19, (1980) 75. 10. A.A.Efendiev, Yu.M.Sultanov, B.G.Abbasova, S.I.Bagbanly, V.A.Kabanov, Vysokomol.Soyed. B 27, (1985) 264. 11. V.A.Kabanov, L.S.Molochnikov, S.A.Ilichev, O.N.Babkin, Yu.M.Sultanov, J.J.Orujev, A.A.Efendiev, Vysokomol.Soyed. B 28, 11, (1986) 2459.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

541

Development of a catalytic reactor with annular configuration A. Beretta, P. Baiardi, D. Prina and P. Forzatti Dipartimento di Chimica Industriale e Ingegneria Chimica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, ITALY

A structured reactor configuration with annular geometry is obtained by depositing the catalyst onto a ceramic tubular support, which is then placed internally to a quartz tube. The present catalyst deposition is a much easier procedure than washcoating or paste extrusion of a monolith. The single steps of preparation of the catalyst layer are herein described. Also, characterization of the deposited layer is presented; it indicates that the same morphological and structural properties of the initial catalyst powders are retained atter deposition. I. INTRODUCTION In a previous paper [1] the authors have presented a novel reactor configuration with annular geometry, wherein the catalyst is used in the form of a thin and short layer deposited onto a ceramic tubular support. As schematically represented in Figure 1, the ceramic tube is then inserted and properly assembled into a quartz tube.

Quartz tube Gas Flow

t, I~!tl

-

Muuite tube

Figure 1. Representation of the reactor section

The gas stream flows in laminar regime across the resulting annular duct and contacts longitudinally the catalyst layer. Opposite to the case of standard fixed-bed reactors wherein pressure gradients arise along the catalyst packed bed, in the present annular reactor pressure drop is always negligible, even in the case of very high flow rates. Also, very small amounts of catalyst can be deposited (down to few milligrams). Consequently, gas hourly space velocities as high as 106 L(NTP)/kgcat/h can be realized, which is far beyond the operating conditions of

542 standard packed bed reactors. This feature can be exploited for extending to higher temperatures the field of investigation for kinetic studies in catalysis. In fact, by using very high space velocity, that is very short contact time, conversion of the reactant can be moderated and kept below the total conversion even at high temperature. For instance, in our laboratory CO catalytic combustion over a hexa-aluminate material has been investigated at temperatures between 200 and 450~ (value at which CO conversion was complete) by using a standard fixed bed reactor operating at GHSV = 50.000 h"1 [2]; the same reaction could be studied then in the T-range 400-900~ by using the present annular reactor at GHSV = 300.000 1.200.000 h'l[1]. Kinetic information could be thus directly obtained (and not extrapolated) within the range of temperature which is relevant for gas turbine catalytic combustion [3]. In this work, attention is focused on the deposition of the catalyst layer. The methodology herein developed was inspired by the literature of monolith washcoating It is based on the acid pretreatment of the ceramic support, the preparation of very fine catalyst powders and their transformation into a slurry-phase. While monolith supports are usually dipped into the slurry with a subsequent step of washing or vacuum treatment which removes the excess material inside the channels, in the case of the mullite tubular support the catalytic layer is obtained by simply dropping the slurry. In the following the single steps of the preparation of the catalyst layer are presented: mullite pre-treatment, preparation of the slurry, deposition and thermal treatment of the catalyst layer. A combined characterization of the initial powders and of the final deposited layer is also presented. Finally the performance of the annular reactor is exemplified.

2. PREPARATION OF THE SUPPORTED CATALYST LAYER

2.1. Support pre-treatment The ceramic tubes used in this study are commercially produced by extrusion of a paste with composition 80% mullite and 20% glass fibers by weight. Chemical analysis shows that the mullite phase is mainly formed by A1203 (55.4% w/w) and SiO2 (42 % w/w); other oxides are present in small amounts. The tubes are gas tight, highly resistant to compression and are supposed to have a maximum working temperature of 1650~ In our experience, however, the muUite tubes tend to break under thermal fatigue, that is when they are subject to fast temperature cycles; consequently, during the activity tests very slow rates of increase or decrease of temperature were adopted (~ l~ This allowed to operate the reactor at temperature higher than 700~ for extended periods without damaging the catalyst supports. A morphological characterization indicated that the tubes are non-porous with practically zero BET surface area. Prior to deposition, the ceramic tubes were pre-treated in acid bath (a 2% HF aqueous solution) at ambient temperature for a period of 2-3 hours. As originally patented by Hegedus [4] for the washcoating of cordierite honeycomb monoliths used in automotive applications, the etching treatment resulted in enhanced surface roughness of the mullite surface. This is shown in Figure 2 where the SEM images of the ceramic tube surface before and after the acid

543 treatment are compared. Part of the superficial material appeared removed by the acid attack and a more irregular surface arose.

Figure 2. SEM images of the mullite surface before (a) and after (b) the etching treatment

The mullite pre-treatment improved significantly the quality of the final catalyst layer; it resulted in fact in a better adhesion of the slurry to the ceramic support and a more homogeneous catalyst deposition.

2.2. Slurry preparation The catalyst used in the present study was a Ba-Mn substituted hexa-aluminate,

BaMnlA111019 prepared via coprecipitation from a solution of barium, aluminum and manganese nitrates [5]. The catalyst was calcined at 1300~ for 10 hr. X-ray diffraction of the catalyst powders revealed the presence of the desired 13-A1203 phase wherein crystals are highly unisotropic with mean crystal size equal to 440 _~. Concerning the material structure, which is discussed in more detail elsewhere [5], it is noted that Mn is mostly present as Mn 2+ ions, located in tetrahedral A1 sites; Mn 3+ ions are also present and located in octahedral A1 sites. The calcined material herein used had a BET surface area equal to 18 m2/g and the pore size distribution (measured by Hg-intmsion) reported in Figure 3; this shows the presence of mesopores with average radius = 620 ~ and a porous contribution in the range 10,000 -100,000 A, presumably associated to intra-particle voids. The catalyst powders were ground several times and then sieved with a 325 mesh sieve (corresponding to the maximum particle diameter of 44 ~tm). The particle size distribution was measured with a Laser-ray HR850 CILAS Granulometer and the analysis is reported in Figure 4.

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The distribution included a significant fraction of very fine particles. It resulted in fact that 20% by weight of the particle population had dimension dp < 3 gm; 50 % of the population had dp _< 16 gm; 90% of the population had dp < 30 gm. The presence of small particles was decisive for the formation of a dense and compact catalyst layer, well adherent to the mullite surface, once dried and calcined. The slurry was prepared by simply impregnating and then wetting the sieved catalyst powders with increasing amounts of deionized water. A volume of 2 ml water was added to 1 g catalyst to obtain a slurry fluid enough to be dropped and easily distributed onto the mullite

545 support but still dense and viscous enough to obtain a catalyst layer of desirable thickness (50100 lain).

2.3 Deposition The catalyst deposition was obtained by simply dropping the slurry onto a portion of the mullite tube while this was kept in rotation. Guards made of Teflon tape were used to define the portion of mullite corresponding to the desired catalyst layer length. They were removed after the catalyst had been dried in air for 48 hours; no shrinkage of the catalyst layer was observed after drying. Finally, the catalyst-coated mullite tube was calcined in air at 700~ for 5 hours. The thermal treatment increased the compactness of the catalyst layer; a~er calcination, the catalyst resulted in fact less dusty and more adherent to the support. 3. CHARACTERIZATION OF THE CATALYST LAYER The amount of catalyst deposited onto the mullite tube was measured by difference of the support weighs after and prior deposition. Once performed the activity tests and discarded the coated tube from the reactor, the effective catalyst weigh could directly verified by scratching the catalyst layer from the support. Catalysts loads varying from few to 100-200 milligrams were deposited depending on the length of the catalyst coatings (which could be varied from few mm to several cm). Concerning the thickness of the catalyst layer, this was measured based on magnified images of the optical microscope. An example is provided in Figure 5. The diameter of the coated portion of mullite (unknown size Y) was compared with diameter of the support (known size

Figure 5. Estimate of the coating thickness by means of optical microscope images.

546 The morphology of the calcined catalyst layer was examined by electron microscopy. Two SEM images of the catalyst coating are reported in Figures 6 and 7. Figure 6 shows the grain distribution of the coating. Most of the agglomerates have diameter of 15-20 Ixm; the biggest particles are nearly 50 lxm large, while the smallest ones have size of few microns. This is remarkably similar to the particle size distribution of the initial catalyst powders used for the slurry preparation. Wetting of the powders, deposition and subsequent thermal treatments have not changed the dimension of the particle agglomerates; in particular, the fraction of very fine particles was retained, which allowed the packing of the agglomerates into a compact layer. Besides, Figure 6 shows the presence of intra-particle voids with size ranging between 1 and 10 lxm; this is also congruent with the pore size distribution of the initial catalyst powders prior to deposition shown in Figure 3, which included a significant void fraction of macro-porosity.

Figure 6. SEM image of the catalyst layer: material grain distribution Figure 7 further magnifies the coating image and shows the sponge-like structure of the particle agglomerates. It could be appreciated that the layer grains are indeed formed by crystallites smaller than 0.1 ~tm, this is in line with the mean crystal size estimated from XRD of the calcined catalyst powders, as previously mentioned, as well as with the presence of meso-pores as indicated by Hg-porosimetry. Thus, it can be concluded that the present procedure for catalyst deposition does not alter the intrinsic catalyst properties, including the morphological characteristics as well as the particle size distribution. Accordingly, the characterization of the initial powders is fully representative of the final structured catalyst used in reaction. This is a considerable simplification with respect to other techniques reported in the literature for the preparation of structured catalysts, both in the ease of application of a catalyst

547 coating to a dense monolith support and in the case of bulk extrusion of a catalytic monolith the procedures involved are laborious and the characteristics of the initial catalyst powders can be significantly altered in the final catalytic element.

Figure 7. SEM image of the catalyst layer: material mophology.

4. PERFORMANCE OF THE ANNULAR REACTOR

This section provides an example of the performance of the annular reactor. As mentioned above the main advantage given by the present configuration is represented by the possibility of realizing very high space velocities. Figure 8 reports the results of CO combustion experiments which were performed over the BaMnAll~O19 catalyst (a 5 cm long and 110 lxm thick layer) at GHSV values ranging between 300,000 and 1,200,000 L(NTP)/kg~at/h. These results are compared with those obtained by testing the same catalyst inside a standard lab-scale packed-bed reactor, wherein GHSV values in the order of 50,000 L(NTP)/kgcat/h are typically used as a compromise between minimum amount of catalyst to avoid by-pass and maximum flow rate to contain pressure drop. The data show that while in the fixed bed micro-reactor CO conversion was complete at 450~ in the case of the annular reactor, where much shorter contact times were realized, the range of intermediate CO conversions (that is the range which is relevant for kinetic measurements) was shifted 300-400~ towards higher temperatures. When using a GHSV = 1.2 * 106 L(NTP)/kg~/h, conversion amounted to 65% at a temperature value as high as 900~

548

% CO Conversion

/-

100

80

60 /

40 /

20

--~1~-- Packed-bed reactor, GHSV = 54,000 NL/kg/h 9 Annular reactor, GHSV = 314,300 NL/kg/h 9 Annular reactor, GHSV = 628,600 NL/kg/h 9 Annular reactor, GHSV = 1,257,100 NUkg/h

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Utilization of the present annular reactor opens the field of high temperatures to mechanistic and kinetic studies. Especially in the case of reactions which are industrially realized at high temperature and high space velocity, like the catalytic combustion for gas turbines, this is a very desirable feature; scale up of the lab-scale results and process design can in fact greatly benefit from the availability of kinetic information which pertain to the operating field of industrial application, rather than extrapolation of kinetic data obtained at much lower temperature. REFERENCES 1. A. Beretta, P. Baiardi, D. Pfina and P. Forzatti, Chem. Engng. Sci., submitted, 1997. 2. G. Groppi, L. Lietti, E. Tronconi, and P. Forzatti, Catalysis Today, in press 1998. 3. M. F. M. Zwinkels, S. G. Jaras, and P. G. Menon, Catal. Rev.- Sci. Eng. 35 (1993) 319. 4. L. Hegedus, U.S. Patent 4,199,477 (1980). 5. G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti and P.L. Villa, Appl. Catal. A:General, 104 (1993) 101.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

549

Effects o f Silica and Titania Supports on the Catalytic Performance o f V a n a d i u m P h o s p h o r u s - O x i d e Catalysts # M. Ruitenbeek ~ , A.J. van Dillen, D.C. Koningsberger, and J. W. Geus Utrecht University, Debye Institute, Department of Inorganic Chemistry, PO Box 80.083, 3508 TB, Utrecht, The Netherlands Silica - and titania-supported V-P-O catalysts (-8 wt.% V) were prepared by means of homo-geneous deposition precipitation and compared with an 'organic' bulk V-P-O catalyst. All samples were tested in the selective oxidation of n-butane to maleic anhydride (MA), and characterised with X-ray absorption spectroscopy (EXAFS). The catalytic performance of VPO/TiO2 on the one hand and VPO/SiO2 and VPO/bulk on the other hand is remarkably different. EXAFS data analysis revealed that these differences can be explained by structural differences between the V-P-O phases in the different catalysts. However, VPO/SiO2 and VPO/bulk, which display comparable yields to MA, show a different EXAFS spectrum. This indicates that the active phase in this reaction is not vanadylpyrophosphate.

1. INTRODUCTION Catalysts based on Vanadium-Phosphorus-Oxide (V-P-O) are industrially used for the production of maleic anhydride (MA) from n-butane. The V-P-O system has been under investigation for the last three decades, and although it is widely accepted that vanadylpyrophosphate, (VO)zP207, is the main component in the active catalyst, still little is known about the exact nature of the catalytic active site (1-3). At Utrecht University much effort is devoted to the development of supported V-P-O catalysts (4-6). It can be expected that the supported V-P-O catalysts have superior characteristics over bulk V-P-O, such as a cheap and reproducible preparation procedure, a large active surface area, a short activation period and a high mechanical strength. Therefore, application of supported V-P-O catalysts in a fluidised-bed process could be very promising. Our newly developed catalysts show interesting catalytic properties. For instance titaniasupported V-P-O is already active at low temperatures (250~ although the selectivity is low

t

Netherlands Institute for Research in Catalysis(NIOK)publication # UU 98-1-04 and UU 98-2-01 Corresponding author, e-mail: [email protected]

550 (4,6). Silica-supported V-P-O catalysts, on the other hand, show reasonable yields in maleic anhydride (5), and these systems are being further optimised in our laboratory. In literature, only a few other examples of deposition of V-P-O on silica (7,8), titania (9), allumina (9-12) and A1PO4 (13) have been described. In general, with supported V-P-O catalysts, the lack of long-range order of the supported V-P-O particles makes a proper characterisation of the supported V-P-O phase with common techniques like X-ray diffraction or FTIR spectroscopy difficult or even impossible. Nevertheless, most supported V-P-O catalysts are reported to consist of a phase that strongly resembles VS§ mostly ),VOP04 (14,15). Generally, the catalytic activity of the V-P-O catalysts has been related to the local structure of surface vanadyl groups in vanadylpyrophosphate. However, with most techniques, no explicit information about this active surface is obtained. The structure of crystalline vanadylpyrophosphate consists of pairs of edge-sharing pseudo octahedral co-ordinated vanadium ions at a distance of 3.23 A, isolated from other pairs by pyrophosphate groups (16). This structural unit is often used to model the active sites in V-P-O catalysts (2,17,18). Moreover, in the literature, various other models have been proposed, i.e. interfaces between different VOPO4 phases and (VO)2P207 (14,15), V s§ sites on the surface of (VO)zPzO7 (2), V 5§ species in interaction with VO(PO3)2 (19), and amorphous V4§ and/or V 5§ phases supported on crystalline (VO)zP207 (20,21). In this paper we have studied the structure-activity relationship of our supported V-P-O catalysts. To this end we have applied catalytic measurements and X-ray absorption spectroscopy (EXAFS). For this technique crystaUinity of the samples is not a prerequisite.

2. EXPERIMENTAL

2.1. Catalyst preparation The catalysts used for this study were bulk V-P-O, titania-supported V-P-O (8.2 wt.% V), and silica-supported V-P-O (7.5 wt.% V). All catalysts were prepared with a P/V ratio of 1.1. Bulk V-P-O was prepared in i-butanol according to a well known procedure (22). This catalysts will be denoted VPO/bulk. Supported V-P-O catalysts have been prepared on TiO2 (Degussa P25) and SiO2 (Engelhard C500-20) according a procedure described by Overbeek et al. (4,5). This method is based on homogeneous precipitation (HDP) of V(III) species in the presence of NI~H2PO4. To this end V v was electrochemical reduced first to V m ions in diluted hydrochloric acid solution. In a typical experiment 10 grams of the support material was suspended in 1500 ml demineralised water and stirred vigorously. Secondly, the pH of the solution was decreased by addition of concentrated hydrochloric acid (pH~2). Next, an appropriate amount of V(III) electrolyte was added, followed by NI-LH2PO4 (P/V ratio 1.1). The pH was homogeneously increased by addition of a 5 wt.% ammonia solution at a rate of-0.1 mmol OH" per minute. Addition was stopped at pH~7. The suspension was filtered and dried (air, 120~ 16 hours). For the preparation of silica-supported catalysts this method was slightly adapted, because of the poor interaction of the V-P-O with silica (5,23). First an amount of vanadium oxide was

551 precipitated, which was subsequently impregnated with diluted phosphoric acid to obtain silicasupported V-P-O. The supported catalysts will be referred to as VPO/'I'iO2 and VPO/SiO2 respectively. Finally, catalyst precursors were calcined in N2 at 450~ for 16 h prior to testing and characterisation.

2.2. Catalyst performance testing After calcination, both bulk and supported V-P-O catalysts were tested in the selective oxidation of n-butane using a 1.5% n-butane, 20.5% 02, 78.5% Ar flow (1.5 ml catalyst, 50 rnl/min, GHSV=2000 h-~) at atmospheric pressure. Formed gaseous products, as well as unconverted reactants were analysed using an on-line Balzers QMA-420 mass spectrometer operating at 150 ~ to avoid condensation of products. Carbon mass balances were in the range of 0.98 to 1.02. A detailed description of the experimental conditions is given in earlier publications (4,5).

2.3. Catalyst characterisation All catalysts have been characterised with X-ray absorption spectroscopy (XANES/EXAFS). EXAFS data were collected at Station 8.1 of the SRS facility in Daresbury (UK). The energy of the electron beam amounted to 2 GeV (average current ~ 150 mA). The Si [111] double crystal was detuned to 70 % intensity to minimise the presence of higher harmonics. The measurements were all carried out in transmission mode using optimised ionisation chambers as detectors. To minimise noise the counting time per data point was taken 1000 ms, and at least three scans were recorded and averaged. All samples were recorded exsitu in He at -196~ after calcination of the fresh precursor in nitrogen at 450~ for 16 h. The energy calibration was performed by means of a V-foil (5l.tm). The absolute value of the vanadium-edge is 5465 eV. The catalyst samples were pressed into self-supporting wafers and mounted in an in-situ EXAFS cell (24). The thickness of the wafer was chosen in such a way as to give an absorbance (l.tx) of 2.5 for optimal signal-to-noise ratio. To prevent self-absorption by the catalysts the amount of sample was chosen to obtain a step in absorbance of 1.0 in the edge region (Al.tx=l). If necessary, samples were diluted with boron nitride (BN). Standard procedures were used to extract the EXAFS data from the measured absorption spectrum The background was subtracted using cubic spline routines (25). Normalisation was done by dividing the data by the height of the absorption at 50 eV.

3. RESULTS AND DISCUSSION

3.1. Catalyst Performance Testing In Figure 1 the conversion as a function of temperature of VPO/bulk, VPO/TiOz and VPO/SiOz are represented. It is obvious that VPO/TiOz is the most active catalyst, showing conversion at 250~ already. In general, our titania-supported catalysts are even more active at lower loadings (4). Despite the lower amount of V-P-O present in the silica-supported catalyst,

552 this sample is more active than the bulk catalyst. This difference can be explained by the higher specific surface area of the active component in the supported catalysts. Both VPO/bulk and VPO/SiO/show about the same selectivity to maleic anhydride (Figure 2). However, for VPO/SiOz the selectivity decreases much more when the conversion is raised. The selectivity of VPO/TiO2 at low conversions is substantially below that of the other catalysts and only a yield of 6% is obtained at a conversion of 30%. The differences in activity and selectivity between VPO/TiOz on the one hand and VPO/SiO2 and VPO/bulk on the other hand have been explained by the strong interaction between V-P-O and titania (4,6). Therefore we will focus on the structural characterisation of the samples in the remaining part of this paper.

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3.2. EXAFS In Figure 3 the k 2 Fourier Transform of VPO/bulk and VPO/SiOz is shown (0.5A
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Recently, Nguyen et al. have revealed the structure of crystalline vanadylpyrophosphate (16). Our EXAFS data of VPO/bulk could be fit up to a distance of 3.5/~ with the single crystal Xray data ofNguyen et al. (16). Above this threshold too many contributions must be included in

554 the calculations. This results in a low reliability of the final fit. The results of the EXAFS data analysis of VPOfoulk have been published elsewhere (27). Although an appreciable amount (+30%) of our VPO/bulk consists of an amorphous V-P-O phase (28), the EXAFS spectrum is dominated by the structure of the ideal vanadylpyrophosphate structure. This is the major constraint for the application of techniques like XRD or EXAFS to study active bulk V-P-O catalysts. Figure 4 represents the k2 Fourier transforms of the EXAFS data of both supported catalysts. Again these spectra are totally different for both the first co-ordination shell and for the higher shells. It is important to note that the spectrum of VPO/TiO2 also deviates from that of VPO/bulk. As stated above EXAFS is a bulk technique. Since the EXAFS spectrum of VPO/SiO2 is not suffering from interfering dominant bulk contributions, we think that the EXAFS spectrum of VPO/SiO2 represents the structure of the surface phase that is present in VPO/bulk.

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4. CONCLUSIONS Supported V-P-O catalysts have been tested in the n-butane oxidation reaction to maleic anhydride, and characterised with X-ray absorption spectroscopy. Our results indicate that the differences in catalytic performance between VPO/TiO2 and VPO/SiO2 can not only be explained by the difference interaction between V-P-O and the respective supports. The combination of characterisation methods has revealed that the structure of the supported V-P-

555 0 phase does not match crystalline vanadylpyrophosphate. Furthermore there are large differences between the structures of the supported compounds. Since VPO/SiO2 is exhibiting a yield close to the yield of our bulk catalyst we think that the oxidation of n-butane to maleic anhydride takes place over an (amorphous) surface V-P-O phase. Therefore it will be interesting to study the structure of the VPO/SiO2 catalyst in more detail, and with different techniques. A detailed analysis of the EXAFS data of the silicasupported V-P-O catalyst (26) can give more insight in the nature of the real active site of the maleic anhydride catalyst.

REFERENCES

B.K. Hodnett, Catal. Rev. -Sci. Eng. 27(3) (1985), 373 G. Centi, F. Trifirb, J.R. Ebner, and V.M. Franchetti, Chem. Rev. 88 (1988), 55 G. Centi, Catal. Today 16, (1993), 5 R.A. Overbeek, P.A. Warringa, M.J.D. Crombag, L.M. Visser, A.J. van Dillen, J.W. Geus, Applied Catalysis A: General 135 (1996), 209 5. R.A. Overbeek, A.R.C.J. Pekelharing, A.J. van DiUen, J.W. Geus, Applied Catalysis A: General 135 (1996), 231 6. M. Ruitenbeek, R.A. Overbeek, A.J. van DiUen, D.C Koningsberger, J.W. Geus, Recl. Trav. Chim. Pays-Bas 115 (1996), 519 7. V.A. Zazhigalov, Yu. P. Zaitsev, V.M. Belousov, B. parlitz, W. Hanke, G. Ohlman, React. Kinet. Catal. Lett. 32 (1986), 209 8. K.E. Birkeland, S.M. Babitz, G. K. Bethke, H.H. Kung, G.W. Coulston, S.R. Bare, J. Phys. Chem. B 101 (1997), 6895 9. M. Martinez-Lara, L. Moreno-Real, R. Pozas-Tormo, A. Jiminez-Lopez, S. Bruque, P. Ruiz, G. Poncelet, Can. J. Chem 70 (1992), 5 10. M. Nakamura, K. Kawai, Y. Fujiwara, J. Catal. 34 (1974), 345 11. N.T. Do, M. Baerns, Appl. Catal. 45 (1988), 1 12. A. Ramstetter, M. Beams, J. Catal 109 (1988), 303 13. P.S. Kuo, B.L. Yang, J. Catal. 117 (1989), 301 14. E. Bordes, Catal. Today 1 (1987), 499 15. G.J. Hutchings, A Desmartin-Chomel, R. Olier, J.C. Volta, Nature 368 (1994), 41 16. P.T. Nguyen, R.D. Hoffman, and A.W. Sleight, Mater. Res. Bull. 30 (1995), 1055 17. P.A. Agaskar, L. DeCaul, R.K. Graselli, Catal. Lett. 23 (1994), 339 18. B. Schiott, K.A. Jorgensen, Catal. Today 16 (1993), 79 19. M.T. Sananes, A. Tuel, J.C. Volta, J. Catal. 145 (1994), 251 20. N. Harrouch Batis, H. Batis, A. Ghorbel, J.C. Vedrine, J.C. Volta, J. Catal. 128 (1991), 248 21. G. Bergeret, M. David, J.P. Broyer, J.C. Volta, G. Hecquet, Catal. Today 1 (1987), 37 22. K. Katsumoto, D.M. Marquis, US Patent 4,132,670 (1970) 23. R.A. Overbeek, E.J. Bosrna, D.W.H. de Blauw, A.J. van DiUen, H.G. Bruil, J.W. Geus, Applied Catalysis 163 (1997), 129 1. 2. 3. 4.

556 24. M. Vaarkamp, B.L. Mojet, J.T. Miller, D.C. Koningsberger, J. Phys. Chem. 99 (1995), 16067 25. J.B.A.D. van Zon, D.C. Koningsberger, H.F.J. van Blik, D.E.J. Sayers, J. Chem. Phys. 82 (1985), 5742 26. M. Ruitenbeek, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, to be published 27. M. Ruitenbeek, R.A. Overbeek, D.C. Koningsberger, J.W. Geus, in Catalytic Activation and Functionalisation of Light Alkanes, E.G. Derouane et al. (eds.), Kluwer Academic Publishers, Dordrecht (1998), 423 28. M. Ruitenbeek, A. Barbon, E.E. van Faassen, J.W. Geus, submitted to Catalysis Letters

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

557

Design and development o f novel copper chromite catalysts (tmsupported/supported) with enhanced activity D. Mohan*, R. Prasad** and K.S. Karki* *Department of Civil Engineering, Institute of Technology, Banaras Hindu University, Varanasi - 221 005, Uttar Pradesh, INDIA. **Department of Chemical Engineering & Technology, Institute &Technology, Banaras Hindu University, Varanasi - 221 005, Uttar Pradesh, INDIA. ABSTRACT More than one hundred and fifty chemicals have been identified in the gaseous emissions of automobiles. Noble metal based three way catalytic converters are being employed to treat the exhaust gases from gasoline powered engines. Base metal catalysts using copper chromite have shown potential to provide an attractive alternative to these noble metal catalysts. The study presented in the paper suggests a new route which bypasses precipitation as well reduction steps being followed in the traditional method of copper chromite catalysts preparation. The paper also discusses about the improvement achieved in the efficiency of the catalysts prepared following the new path. A bench scale tubular reactor was used in the study with the temperature varying from 323 - 573 K, CO/air ratio as 1/40 and W/F equal to 27.1 g.h.mol1. The compositions of the catalysts under comparison were kept identical having 20% total metal with 10% BaO over 7-alumina support. 1. INTRODUCTION One of the major sources of air pollution in large urban areas are automobiles [1]. The automotive exhaust contains carbon monoxide (CO), unbumt hydrocarbons (UBHC), nitrogen oxides (NOx) and particulate matters (consisting of carbon and lead) etc.. CO can cause health hazards without giving any physical indication because it is a tasteless, colourless and odourless gas. It is chemically inert under normal conditions with estimated atmospheric mean life of about 2.5 months [2]. The total emission of CO on a mass basis accounted for slightly over half (53%) of all the anthropogenic air pollutants. At higher concentrations, it can seriously affect human aerobic metabolism because of its affinity to haemoglobin being 210 times greater than that of the oxygen and so, if its concentration becomes higher than 0.0998% by volume in atmosphere, it can lead to the failure of the process of combination of oxygen with the haemoglobin resulting ultimately in the respiratory failure and consequent death. It forms a very stable compound called carboxyhaemoglobin (COHb) in blood whose 1-2% concentration can cause headache, fatigue, drowsiness and may have effect on human

558 behavioural performance. 2-5% concentration may cause impairment of time interval discrimination, visual acuity, brightness discrimination and certain other psychomotor functions which may lead to the accidents on the roads. 5-10% concentration can lead to changes in cardiac and pulmonary functions and 10-80% can cause coma, respiratory failure and death [3]. It can also lead to an increased frequency of acute bronchitis among infants and school-going children. It not only affects human beings, but also vegetation by interfering with respiration and nitrogen fixation processes of the plants. The oxidation of CO with the objective of reducing urban air pollution becomes a very important consideration when one plans the strategy for automobile emission control. The catalytic system CuO.CuCr204, known as copper chromite catalyst is observed to be an attractive alternative to noble metals currently used in the vehicular exhaust catalytic converters in view of their relative abundance, lower cost and highly comparable activity to those of precious metal based catalysts [4,5]. As fuel and engine modifications and maintenance alone can not meet the stringent regulatory limits of pollutant concentrations in the exhaust, additional measures like exhaust treatment devices are to be employed. Incorporation of catalytic converter in the exhaust system has been proposed long back and is still, perhaps, the most promising one [6]. The catalysts convert pollutants to harmless or less hazardous products such as carbon dioxide, water and nitrogen. CO is converted in CO2, abundantly found in the atmosphere and useful for the survival of the major part of the plant kingdom. CO + (1/2) 0 ........... > C02,

A H = -67.64 Kcal/mol

(1)

Commercial catalytic converters being used in megacities contain the noble metal catalysts like Pt, Pd alongwith Rh and Ru which are very expensive and presently, being imported causing heavy burden on the exchequer of India. This catalyst has also been found to be active for the oxidation of CO alongwith the reduction of NOx [7], another perilous auto-exhaust pollutant. Several studies on the oxidation of CO have been reported over unsupported as well as supported copper chromite catalysts [8,9]. The concentration of the most efficient catalyst, with regard to both activity and stability, has been reported to be between 5 to 20% by weight on alumina, with a metal composition close to the stoichiometry of Cu.CuCr204 rather than CuCr204 [9]. Studies have also been reported employing different catalyst pretreatments in reducing [ 10] and oxidizing [ 11 ] environments. These pretreatments produce catalysts having different physico-chemical characteristics which lead to the variation in their performance in the reaction as CO oxidation is a catalyst-structure sensitive reaction [ 12]. The widely used traditional method of preparation of copper chromite catalysts is the calcination (eqn. 2) of a precursor BCAC [13], followed by the reduction for improved activity [14,15] for CO oxidation. calcination 2CuOHN4CrO4 ............ >

CuO.CuCr204 + N2 + 5H20 (highly exothermic)

(2)

Reduction of copper chromite catalyst (eqn.3) improves catalytic activity [16] for CO oxidation.

559 Reduction CHO.CuCr204

-]- C O . . . . . . . . . . . .

>

Cu.CLICr204+ CO2 (highly exothermic)

(3)

The surfaces of the reduced catalysts has been shown to consist of copper crystallites essentially, which is identified as the main active species for this oxidation reaction [ 17]. In the conventional methods, the calcination of the precursors and the reduction of the catalysts are highly exothermic processes which always have adverse effect on the morphology of the catalysts desired for the reactions [18]. In a recent study, a new complex precursor [19], copper amino oxalate chromate (CAOC), for the preparation of novel copper chromite catalysts has been reported. The essence of the novelty of the process developed is that the catalyst is directly produced in active state (bypassing highly exothermic reduction step) through an almost isothermal calcination of CAOC in nitrogen atmosphere enabling the catalysts to possess a morphology more favourable for the oxidation reaction. In the present study, oxidation of CO was studied comparatively over the T-alumina supported novel catalysts and the catalysts (unreduced/reduced) prepared following the conventional route with the main aim to design and develop new copper chromite catalysts with improved surface texture to achieve higher efficiency for the CO oxidation. 2. EXPERIMENTAL 2.1. Catalyst Preparation The complex precursor, CAOC was synthesized by mixing the requisite amounts of ammoniacal solutions of copper amino oxalate (CAO) and ammonium dichromate. This precursor solution was dispersed over T-AI203 by incipient wetness method followed by drying. A total of twenty-one unsupported and y-alumina supported catalysts (as shown in Table 1 and Table 2) were prepared using CAOC solution, followed by drying, calcination in air/nitrogen and reducing the conventional catalyst with CO at 493 K. 2.2. Characterization The precursor CAOC was examined by elemental analysis and by simultaneous TGA/DTA under different environments (air/CO/nitrogen). The catalysts were characterized by X-ray diffraction, particle size analysis and low temperature adsorption techniques also. 2.3. Oxidation of Carbon Monoxide The oxidation of CO was studied in a bench scale down flow, continuous, fixed bed,tubular reactor made of coming glass. The reaction conditions maintained were: 100 mg catalyst, 100 - 150 mesh size diluted to 2.0 g with y-alumina of the same size, temperature variation from 323 to 573 K, CO/air ratio as 1/40, gas flow rate equal to 60 ml/min, and pressure of 1 atmosphere. Mixture of CO and air in required proportion was fed to the reactor after drying it (the mixture) over P205. The gas flow rate was monitored with the help of a bubble flow meter. Reactants and products were analyzed to determine the concentration of CO2, 02 and CO with the help of an Orsat Apparatus. The conversion was calculated on the basis of values of CO concentration in the reactor input and output samples.

560 Table 1 Nomenclature and composition of unsupported catalysts. Catalyst Precursor used Composition (atomic ratio)

Xla/Xln ?(2

X3a/X3n Yln Y2a/Yzn

Y3ar Y4n Ys, Y2np

BCAC Commercial copper chromite Cupricchromate Copper amino oxalate + Ammonium chromate Copper amino oxalate + Ammonium chromate Reduced species of Y2a Copper amino oxalate + Ammonium chromate Copper amino oxalate + Ammonium chromate Copper amino oxalate + Ammonium chromate

Cu/Cr Cu/Cr Cu/Cr Cu/Cr

= = = =

1 : 1.0 1 : 1.0 1 : 1.0 1:0.5

Calcination/reduction temperature (K) and the environment. 623, Air/nitrogen 623, Air/nitrogen 623, Nitrogen

Cu/Cr = 1 : 1.0

623, Air/nitrogen

Cu/Cr = 1 : 1.0 Cu/Cr = 1 : 1.5

493, CO 623, Nitrogen

Cu/Cr = 1 : 2 . 0

623, Nitrogen

Cu/Cr = 1 : 1.0

623, Nitrogen

* Suffixes a, n, r and p denote air, nitrogen, reduction and promoter respectively.

**Yla refers to the Catalyst produced by thermal decomposition in air. Y1, refers to the Catalyst produced by thermal decomposition in nitrogen environment. Y3arrefers to the Catalyst Y2a reduced with CO. Y2np refers to the Catalyst Y2n promoted with BaO.

Table 2 Nomenclature and composition of alumina supported catalysts (Cu/Cr = 1" 1.0). Catalyst Precursor used % composition (by weight) Calcination temperature (K) in nitrogen environment Total Metal BaO Zln CAOC + 7 alumina 10 623 Z2n CAOC + 7 alumina 15 623 Z3n CAOC + 7 alumina 20 623 Z4, CAOC + 7 alumina 25 623 Zs~ CAOC + 7 alumina 30 623 Z3~pl CAOC + 7 alumina + BaO 20 4 623 Z3,~2 CAOC + 7 alumina + BaO 20 8 623 Z3~p3 CAOC + 7 alumina + BaO 20 10 623 Z3np4 CAOC + 7 alumina + BaO 20 12 623 Z3~p5 CAOC + 7 alumina + BaO 20 16 623

561 3. RESULTS AND DISCUSSION 3.1. Elemental Analysis of Precursor The quantities of carbon, hydrogen and nitrogen in CAOC were determined by C.H.N. analyzer and those of copper and chromium by atomic absorption spectrophotometer (A.A.S.). Quantities of oxygen were determined by the difference. The results of the analysis are mentioned in Table 3. The ratio of nitrogen and hydrogen experimentally found to be 4.61 shows that they were only present as NH3 (theoretical value = 4.67). The other results of weight losses obtained by elemental analysis and by thermogravimetry suggested a chemical composition of the precursor CAOC, closely approximating the formula, Cu(NHa)2C204CrO4.

Table 3 Elemental Analysis of Copper Amino Oxalate Chromate Element Per cent Composition Theoretical Experimental Carbon 7.96 7.87 Hydrogen 1.99 2.00 Nitrogen 9.28 9.22 Copper 21.08 21.13 Chromium 17.25 17.18 Oxygen 42.44 42.60 (by difference) N/H ratio " experimental =4.61 theoretical in NH3 =4.67 Decomposition losses : By elemental analysis = {C + H + N + (3/4) O} By thermal analysis in nitrogen .... By the~al analysis in air

= 51.04% = 51.02% = 48.38%

3.2. Thermal Analysis of CAOC The TG curve for thermal decomposition of CAOC in nitrogen environment as shown in Fig. 1(c) gives a continuous loss in the weight of the sample reaching a final value of 51.02%, suggesting the final product to be Cu.CuCr204 (theoretical loss 51.06%) as mentioned in eqn. 4.

calcination(in nitrogen) 6Cu(NH3)2C204CFO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . >

3Cu.CuCr204 + 4NH3 + 12CO2 + 4N2 + 12H20 (almost isothermal)

(4)

An intermediate increase in sample weight was observed during the thermal decomposition of CAOC in air as given in Fig. 1(a) and the total weight loss of only 48.38% was recorded. Since the increase in weight was observed in the presence of oxygen, this would be ascribable

562 to the oxidation of the resulting solid product, suggesting the final product to be CuO.CuCr204 (theoretical loss 48.41%) as given in eqn.5. calcination ( in air) .> 2CuO.CuCr204 + 8CO2 + 4Nz + 12H20 (almost isothermal)

4Cu(NH3)2C204CrO4 + 302

(5)

The DTA curves show that the thermal decomposition of CAOC was almost isothermal in nitrogen whereas calcination in the air was highly exothermic process as depicted in Fig 1(a). Further, the reduction of the conventional catalysts in CO was also an exothermic process as idicated by eqn. 3 and shown in Fig. l(b).

,;;'\

Ca)

DTA

15 30

TGA

/,S

"-" 0._1

i

6O 0

I

I

I

I

I

I

I

,,', ...-'

6

" "-o.i~nd " I I

(b)

"~.i~_---- . . . . . . . . .

~

-

I.

I

.

I

D TA

I

I

TGA

,

(r

w 15

DTA

30

Z~S

TGA 73 TEMPERATURE

Fig. 1. T h e r m o g r a m s ,

heating

(a) CAOC in a i r , (b)

Ylmr

I

I 7?3

,

873

(K) rate

10 9 K/rain.

in C O , ( c ) C A O C in N 2

563 Thus, calcination of CAOC in nitrogen resulted in a novel catalyst directly as indicated by eqn. 4. The X-ray diffraction studies revealed that the phases present in the novel catalysts were identical to the conventional reduced catalyst as obtained in eqn.3. X-ray diffraction studies of the reduced catalysts and novel catalysts revealed the presence of very fine crystallites of cuprous chromite as well as of copper metal by weak and broad diffraction bands. A substantial fraction of the finely divided nascent crystallites of copper, produced in the process of reduction of the catalysts has a large surface energy and a high specific area and reacts almost immediately with cupric chromite, producing cuprous chromite.

(6)

Cu.CuCr204 . . . . . . . > Cu2Cr204

The remaining copper, not incorporated in Cu2Cr204, is left well dispersed over residual chromite and it accounts for the observations in the X-ray diffraction. Thus, the surface composition of the novel catalysts, directly produced without reduction within few minutes of calcination of CAOC in nitrogen (eqn.4) is identical with that resulting after hours of reduction of the conventional catalysts (eqn.5).

3.3. Catalyst Morphology Adsorption-desorption isotherms of CC14at 273K for the catalysts not calcined in an inert atmosphere (Nitrogen) showed hysteresis loops [19] while the desorption curve for the catalysts calcined in the inert environment coincides with the adsorption branch and therefore, shows an absence of hysteresis. This difference may be due to the complicated pore structure (constricted pore mouth) present in catalysts mentioned earlier as a result of pore coalescence and recrystallization caused by high heat of decomposition of CAOC in air and reduction of the catalyst particles. Uncontrolled decomposition of the precursor and the reduction of the catalysts often produce local temperatures exceeding 873 K [20], higher than the recrystallization temperature of about 698 K for copper [21]. The exothermic processes simultaneously caused sintering of the particles of catalyst reduced in air/CO to bigger aggregates. Bidispersity of the particles of the catalysts unreduced and reduced in air or CO (Table 4) indicates the recrystallization of the original particles. But, almost isothermal decomposition of CAOC in an inert atmosphere resulted in open textured pores due to bursting of the original particles where a large amount of compound (> 50%) is released and particles with highly heterogeneous surfaces are produced. The highest specific surface area of the catalysts reduced in nitrogen atmosphere (Table 4), open textured pores and monodispersity of the particles indicate that the finely divided particles remain unsintered. Thus, calcination of CAOC in nitrogen produces an ideal copper chromite catalysts. Table 4 Morphological Parameters of the Catalysts Catalyst Empirical formula Unreduced CuO.CuCr204 Reduced in carbon monoxide Cu. CuCr204 CAOC decomp.osed !n nitrogen Cu.CuCr204

Surface area, (m2/g) 22.6 28.7 39.2

Probable particle size (~tm) 12, 45 12, 35 10

564 3.4 Activity and deactivation of the catalysts in CO oxidation

Different indices for the comparison of the activity of the catalysts have been reported in the literature. Activation energy, temperature required for the reaction to attain an arbitrary conversion (or rate) and conversion (or rate) at certain fixed temperature are widely used quantitative measures of the catalytic activity. The term, activity of the catalysts, means their ability to convert CO to CO2 by oxidation. The percent conversion values of CO on different catalysts at temperature ranging from 340 to 580 K are recorded after an establishment period of 2 hours and are shown in Fig.2. It can be seen that the activities of all the catalysts increased at a slow rate initially with increase in the temperature but at a certain temperature, known as light-off temperature, conversion increases rapidly to give conversion above 80%. 100

8o

8 6o

z

8 2o 0 340

380

1,20

460

500

540

580

TEMPERATURE (K)

Fig. 2.

Effect of catcination atmospheres on CAOC for the preparation of unsupported copper-chromite cata[ysts,feed composition: 2.5% CO by votume, W/Fco ' 27.1 gm cot-hr/gm,mote. (o)N 2 , (e)Air

Light-off temperature varies with the catalysts used. It was lowest (about 420 K) for the catalysts reduced in nitrogen atmosphere and highest for the unreduced catalysts (about 450 K). The value of light-off temperature for the catalysts reduced in air/CO was 440 K. Above light-off temperature, differential operation of the reactor was invalidated. This sudden transition to much higher reaction rates resembles with the ignition behaviour observed in case of packed bed reactors and the detection of this phenomenon in very small bed employed in this work demonstrates the enhanced activity of the catalysts and highly exothermic nature of the reaction. All the catalysts showed similar ignition behaviour. Three distinct zones can

565 be visualized in Fig.3. In the first region - A below light-off temperature, rate is controlled by the kinetics of the chemical reaction. Catalyst performance in this region is dictated by the specific activity of the catalyst and by the dispersion of metal. As the rate increases, heat builds up due to exothermic oxidation to the point where the catalyst lights off (region - B). Eventually, the transfer of heat becomes limited and the system reaches steady state condition (region - C) at very high conversion. In this region, mass transfer of gases to the catalyst is the rate determining step. It is also observed that the conversion of CO by cat-Y was higher than that by catLight off region ~ " X and it was the highest by 80 -C- S teody state open'crli~ cat-Z at all temperatures =,~ (moss transfer c~miled 0 region) considering the catalysts represented collectively by u 6 0 Lt. the symbols X, Y and Z (in o the Table 1 and 2) as z groups. The catalysts Z3np3 _o ~ 0 03 (Table 2) is found to be the OC LO most active which has the Z 20total metal concentration 0 ,/ ~ .Light off. (by weight) as 20 percent . ~ te~npemiure AA _ o " with the atomic ratio of I.~...-4D"P. m I ,I Cu/Cr equal to 1 : 1.0 340 38"6 420 460 500 $40 580 alongwith 10 percent BaO T E M P E R A T U R E (K) as promoter. The durability of the catalysts was tested F i g . 3 . Curve showing t i g h t - o f f r - e g i o n . by carrying out the reaction for 50 hours at 500 K.In the beginning, exothermic effect of the reaction increased the bed temperature above the preadjusted value of 500 K.Aiter the temperature was stabilized, a progressive deactivation of the catalysts was observed that the activities of all the catalysts dropped down by about 6 8% during 50 hours of operation. Cat- Z3,p3 (Table 2) maintained about 95% conversion efficiency even after 50 hours of continuous use. Since, the possibility of poisoning of the catalysts by sulfur, lead, halogens and heavy metals was absent, the drop in the catalytic activity might be probably due to textural and surface modifications. Prolonged exposure to elevated temperatures resulted in the sintering of active material, increase in the particle size and decrease in active sites. -

4. CONCLUSION It can be concluded that the traditionally reduced (in air/CO) catalysts are more active in CO oxidation than unreduced catalysts. The novel catalysts reduced in nitrogen is found to be the most promising one (Z3np3). The poor performance of the Cat-X and Cat-Y groups may be due to the fact that the highly exothermic processes of precursor decomposition and the reduction thereafter, resuked to some extent, in the sintering of the active component (loss of active area), produced narrow neck structure of pores (causing high diffusion resistance) and

566 smoothened down the surface roughness, edges, comers and tended to bring down the disordered structure towards equilibrium (decrease in the density of active sites). But, Cat-Z was free from all these ill-effects, in which fine crystallite of copper were directly produced through almost isothermal decomposition of CAOC in an inert (nitrogen) atmosphere. REFERENCES

1. M. Shelef, K. Otto and N.C. Otto, Adv. Catal., 27, 311 (1978). 2. Medical and Biological Effects of Environmental Pollutants - Carbon Monoxode, National Academy of Sciences, Washington, D.C. (1977). 3. P.C. Wolf, Carbon Monoxide Measurement and Monitoring in Urban Air Environment, Sci. Tech., 5(3), 231(1971). 4. M. Shelef, K. Otto and H. Gandhi, The Oxidation of CO by 02 and by NO on Supported Chromium Oxide and Other Metal Oxide Catalysts, J. Catal., 12, 361-375(1968). 5. J.T. Kummer, Oxidation of CO and C2H4 by Base Metal Catalysts Prepared on Honeycomb Supports, Adv. Chem. Ser. No. 143, 178-192(1975). 6. K.C. Taylor, Studies in Surface Science and Catalysis, (A. Crucq and A. Frennet, Eds.), Elsevier (Amsterdam), 30, 97(1987). 7. R. Hierl, H. Knozinger and H.P. Urback, J. Catal., 69, 475(1981). 8. R.J. Farrauto, K.E. Hoekstra, R.D. Shoup, U.S. Patent No. 3, 870, 656(1975). 9. Y.F.Y. Yao and J.T. Kummer, A Study of High Temperature Treated Supported Metal Oxide Catalysts, J. Catal., 46, 388-401 (1977). 10. J.C.W. Frazer and C.G. Albert., J. Phys. Chem., 40, 101(1936). 11. W.L. Morgan and R.J. Farrauto, J. Catal., 31, 140(1973). 12. T.J. Huang, T.C. Yu and S.H. Chang, Effect of Calcination Atmosphere on CuO/gammaAl203 catalyst for CO Oxidation, Appl. Catal., 52, 157-163(1989). 13. F. Hanic, I. Horvath and G. Plesch, Thermochem. Acta., 145, 19(1989). 14. F. Severino and J. Laine, Effect of Composition and Pretreatment on the acidity of Copper-Chromium Based Catalysts for the Oxidation of CO, Ind. Eng. Chem. Prod. Res. Dev., 22, 396-401 (1983). 15. A.L. Agudo, J.M. Palacios and J.L.G. Fierro, Activity and Structural Changes in Alumina Supported CuO and CuCr204 Catalysts during CO Oxidation in the Presence of Water, Appl. Catal., 91, 43-55 (1992). 16. J. Laine, J. Brito, F. Severino, G. Castro, P. Tacconi, S. Yunes and J. Cruz, Surface Copper Enrichment by Reduction of Copper Chromite Catalyst Employed for CO Oxidation, Catal. Lett., 5, 45(1990). 17. S.P. Tonner, M.S. Wainwright, D.L. Trimm and N.W. Cant, Characterization of Copper Chromite Catalysts for Methanol Dehydrogenation, Appl. Catal., 11, 93(1984). 18. R. Prasad, A New Copper Catalyst for Dehydrogenation of Alcohol, Indian Chemical Engr., 31(3), 57(1989). 19. R. Prasad, Adsorption of CC14 : A Convenient Method for Characterization of Adsorbent and Catalysts, Indian J. Tech., 30, 369-374 (1992). 20. J.R. Monnier, M.J. Hanrahan and G. Apai, A Study of the Catalytically Active Copper Species in the Synthesis of Methanol Over Cu-Cr Oxide, J. Catal., 92, 119(1985). 21. Catalyst Handbook (ICI), Wolfe Scientific Books, London, WC 2, 111 (1970).

91998 Elsevier Science B.V. All rightsreserved.

Preparationof CatalystsVII B, Delmon et al,, editors,

567

Zeolite modification by in-situ formed reactive gas-phase species. Preparation and properties of Mo-containing zeolites A.V. Kucherov and A.A. Slinkin

Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow B-334, Leninsky Prosp. 47, Russia. E-mail: [email protected]

New way of preparation of Mo-containing zeolites is discussed being based on the use of the active gas-phase species formed in-situ upon thermal treatment of the mixture [H-zeolite + MOO3] with an air flow containing CC14. Chemical transport reaction, with formation of reactive and mobile oxychloride fragments in the zeolitic bed, provides effective dissipation of the oxide phase and migration of active species into zeolitic channels at temperatures as low as 150 - 200~ ESR is used as an informative and sensitive method to monitor transformations in the systems containing Mo 5§ ions. Peculiarities of introduction of Mo into H-forms of several zeolites (ZSM-5, beta, ferrierite, USY) differing in the channel size/structure are discussed.

1. INTRODUCTION Atomic-scale engineering of catalytic functions of isolated redox sites located in confined environments of zeolitic voids ('biomimetic' systems) is of great interest. However, a conventional ion exchange from solutions is of limited usefulness for introduction of multicharged ions into small cavities of different zeolites. The majority of in-going transition metal cations is prevented by its hydration shell from penetrating into narrow channels of the zeolite structure. In some cases, a solid-state reaction between zeolites and different compounds can be used for introduction of one or several transition metal ions into cationic positions of zeolites [1, 2] but this method usually requires a high-temperature calcination (_>500~ and can be used only for the most thermostable high-silica zeolites (H-ZSM-5, mordenite) [3-19]. However, in studying the influence of different factors on the processes of the solid-state exchange it was noted that the temperature required for providing a noticeable rate of reaction depends on the nature of both the cations and anions involved [610]. For example, use of chlorides permits to reduce drastically the temperature of the solid-state exchange [1-6, 12, 13]. In some cases, the nature of the gas phase influences noticeably the process of ion migration upon calcination of zeolite/oxide mixtures [2, 8, 16, 18]. Hence, the question arises of whether it is possible to extend the range of the method by use of in-situ chemical transport reactions provided by gas phase reactants.

568 The aim of the present work is to study how the presence of CC14molecules in the gas phase enhances the possibility of "solid-state" introduction of Mo 5+ ions in different zeolites due to the change in mobility of cationic fragments by in-situ formation of active gas-phase species.

2. EXPERIMENTAL 2.1. Preparation of the samples Molybdenum-containing samples, with 2.0 wt% of MoO3, were prepared by mechanical mixing of precalcined MoO3 with H-forms of different zeolites (ZSM-5, Si/Al = 15; beta, Si/Al = 8; ferrierite, Si/A1 = 4; USY, Si/A1 = 5 ; crystallinity of all the zeolites > 90%) in a mortar by the pestle. Thoroughly mixed powders were pressed without binder and crushed into 5-10 mm pieces. A reference sample, 2 %MoOJH-ZSM-5, was prepared by incipient wetness impregnation (0.8 cm 3 per 1 g of zeolite) of H-ZSM-5 by water solution of ammonium molybdate, (NH4)6MOTO244H20 (analytically pure grade), with subsequent drying at ~120~ Two samples, with 1% and 20 wt% of MoO3 supported on amorphous A1203 (S ~_ 120 m2/g), were prepared by the same method for reference. Prepared granulated samples were placed in a quartz reactor tube and calcined at 500~ in an air flow for 2 h. After reactor cooling to 20~ the air flow was switched to the bubbler filled with CC14, the reactor temperature was raised during 1 h to a given value (between 150 and 300~ the sample was calcined in the stream of air saturated with CC14vapor (Pcc~4 = 80 - 100 Torr at 20 - 23~ for 2 h, cooled to room temperature, and purged with pure dry air for 2-3 h. 2.2. ESR measurements The ESR spectra were obtained at 200 ~ 20 ~ and -196~ in the X-band 0~ = 3.2 cm) on a reflecting type spectrometer equipped with a heater permitting the sample heating up to 400~ in the cavity. The ESR signal from DPPH (g = 2.0036) was used as an internal standard. The Origin 3.5 program for Windows was used for the treatment (baseline correction, double integration, and subtraction) of the recorded spectra (1024 or 2048 points). The ESR signals of Mo 5+ were registered in the field region from 2500 to 3500 G (one scan with a sweep time o f - 3 . 5 min). Resonances for various levels of microwave power were recorded to verify the lack of sample saturation. The samples were crushed into 0.1 - 0.2 mm pieces, and 25 - 50 mg of a sample were placed in a glass ampoule with an inner diameter of 3 mm for ESR measurements. Two modes of ESR study were used: Comparison of different samples pretreated in identical conditions. The samples pretreated in an [air + CC14]flow were placed in a glass ampoule, evacuated to 10.2 Tort, and sealed off. To provide the maximum accuracy of the ESR measurements, carried out at 20 ~ or -196~ the packing height of the ampoule was 25 mm in all cases, with the center of the sample positioned in the center of the ESR cavity. ln-situ ESR monitoring of the system transformation. The oxide samples precalcined at 450~ on air were placed in the glass ampoule positioned in the ESR cavity heater and

569 sealed to a static adsorption system permitting evacuation of the sample to 10-6 Torr and admission of different gases. This system permits in-situ ESR monitoring of the transformation of the starting sample upon heating at temperatures up to 400~ in an atmosphere of different gases (02, H2, CC14, C2H4). In-situ ESR-testing of the samples at 500~ in flowing gas mixtures was done by A.Kucherov on a Bruker ESP300 spectrometer equipped by a high temperature cavity ER 4111 HT-VT and co-axial quartz flow cell (Ford Scientific Research Labs., Dearborn, USA).

3. RESULTS AND DISCUSSION In a blank experiment interaction of pure H-ZSM-5 with an [air + CC14] flow was studied at 300~ A rather weak ESR spectrum appears as a result of the treatment. The signal formed, with gxx = 2.017; gyy = 2.010; gzz = 2.004, depends markedly on the 02 pressure and can be unambiguously assigned to the signal from 02- radical species stabilized on the defect sites of the zeolitic lattice. Thus, the rupture of some stressed framework bonds takes place upon the sample treatment in [air + CC14] mixture. However, the number of the defect sites formed (being estimated by the intensity of this ESR signal) comprises less than 1% of the number of the lattice A13+ ions in the sample. Thus, only minor dealumination of the zeolite occurs as a result of the treatment used in the further investigation. 3.1. Molybdenum introduction into zeolites

(a) Calcination of oxide samples. As was shown earlier, the solid-state interaction at T _< 600~ between mordenite or ZSM-5 and MOO3, in air or in vacuum, did not result in an ESR signal from Mo(V) ions [2, 18]. The same is true of the other zeolites studied in the present work. Reduction of both starting and air-calcined mixtures by H2 at 400~ produces identical weak Mo(V) ESR signals due to reduction of the surface of the MoO3 phase (2 wt%) added to zeolite. Therefore, no disintegration of bulk MoO3 admixture takes place upon oxidative calcination. It is known that MoO3 in vapor phase forms polymer fragments and does not form cationic species. Therefore, it could be assumed that Mo(VI) ions can not enter the zeolitic channels despite the rather high volatility of MOO3. On the other hand, heating at 150~ in vacuum of the mixture [H-ZSM-5 + MoC15] generated a strong ESR signal from isolated Mo5+ ionic species [2, 18]. Sublimation of the excess MoC15 even at 350~ was not accompanied by a substantial loss in the intensity of the ESR signal. Admission of air caused reversible broadening of the ESR signal due to dipole-dipole interaction of isolated paramagnetic MoS+-sites with paramagnetic 02 molecules. Thus, a solid-state interaction of MoC15 in vacuum with H-ZSM-5 resulted in effective dispersion/stabilization of isolated Mo(V) ions in the zeolite. However, Mo(V) bonding in cation positions of H--ZSM-5 was not stable upon oxidative treatment: the calcination of Mo(V)/H-ZSM-5 in air at 300~ resulted in the irreversible disappearance of the Mo(V) ESR signal [18].

570

(b) CCl4-assisted interaction between molybdena and zeolites. First it is necessary to note that the effectiveness of the CCl4-assisted process depends noticeably on conditions of treatment : volatile Mo-compounds formed upon too severe heating Of the sample could provide sublimation of Mo out of the hot reactor zone, with condensation of the ring of Mo compounds on the cold wall of the reactor. This result demonstrates unambiguously that the ion exchange is effected not only by surface diffusion inside the sample bed but is mediated through the gas phase (at least at higher temperatures). At a given temperature, this removal of Mo from the sample is much more pronounced in an air stream than with a static treatment as used in our in:situ ESR measurements. The rate of Mo removal depends on the air flow rate. This means that for MoOJzeolite mixtures the upper limit of the calcination temperature is 200~ at an air flow of 50 - 100 cm3/min. Starting MoOa/zeolite mixtures show no ESR signals. Treatment of different samples at 200~ in the [air + CC14] flow (50 cm3/min, 2 h) results in the appearance of ESR spectra typical of isolated Mo s ions (Fig. 1). ;o = 1 . 9 5

DPPH

1

/ i . . . .

i

b.

do

X/0

_.:_

Fig. 1. ESR spectra, obtained at 20~ of 2 % MoO3/zeolite mixtures treated at 200~ for 2 h in the [air+CCl4] flow of 50 cc/min : (a) - H-beta; (b) - H-ZSM-5; (c) - ultrastable H-Y; (d) - H-ferrierite.

571

In spite of identical treatment conditions, the result obtained depends drastically on the type of zeolite: for both H-ZSM-5 and H-beta strong ESR signals from Mo 5+ ions are obtained (Fig. la,b), a moderate ESR spectrum is seen for USY (Fig. lc), and only a weak Mo 5+ spectrum is observed for H-ferrierite (Fig. l d). Samples differ also in color change caused by the treatment. Although all starting mixtures are white, Mo/ZSM-5 and Mo/beta take on a quite intense blue color after treatment (suggesting formation of disperse phases of socalled 'molybdenum blues'), whereas the two other ones retain its white color. Thus, the process of MoO3 dissipation takes indeed place in the chosen conditions but stabilization of mobile Mo-species depends strongly on the type of the zeolite. A negligible stabilization of isolated Mo 5+ ions by H-ferrierite (a two-orders below those typical of H-ZSM-5 or H-beta) can be related to an essential difference in the kinetic diameter of this zeolite compared to the others (0.8 nm for Y, 0.6 nm for ZSM-5 and beta, and 0.4 nm only for ferrierite). One can assume that the difference in the pore sizes becomes critical just for migration of oxychloride Mo-species into a narrow pore structure of ferrierite, and a weak ESR signal (Fig. ld) is associated with Mo 5+ ions stabilized only on the outer zeolitic surface of ferrierite. A noticeable difference in the ability of USY and H-ZSM-5 (or H-beta) to stabilize Mo 5§ upon identical treatment cannot be explained by the geometrical factor: as mentioned above, the effective size of entrances in H-Y exceeds noticeably the diameter of channels in HZSM-5 or H-beta. Therefore, a rather low concentration of isolated Mo 5§ ions stabilized by USY is not related with steric hindrance for migrating species. It is impossible to correlate it with the total number of protonic (acid) sites in the zeolites. Rather, a relationship between the Mo 5+ concentration and the fraction of the strongest acid sites, capable of firmly entrapping the cationic species, could be assumed. It is important to note, however, that the narrow ESR line, with g = 1.95 and AH -_- 50 G, is typical of well isolated, noninteracting Mo(V) ions only. Interaction between closely arranged paramagnetic Mo(V) ions results in broadening and loss in intensity of the MoS§ signal. Therefore, the comparative analysis of the results obtained (Fig. 1) point to a noticeable aggregation of Mo 5+ in CCl4-treated MoO3/H-Y. In our opinion, formation of a noticeable portion of clustered Mo 5+ species is most likely to occur inside large pores of the zeolite Y. The crucial role of the nature/structure of the support in the effectiveness of the CC1 aassisted Mo dispersion is confirmed by the comparative study of MoO3/alumina samples. Only a weak effect is observed for the sample with 20% of M003, and no stabilization of Mo 5+ species is found in the 1%MoO3/A1203 treated with [air + CC14] at 200~ The lack of the effect cannot be explained by reduced MoO3 content : the surface concentration of Mo in the 1%MoO3/A1203 sample exceeds noticeably those in 2 %MoO3/zeolite samples due to the difference in specific area of amorphous alumina and zeolites (120 and -500 m~-/g, respectively). It is likely that the difference observed is representative for sharp differences in the nature of the support surfaces : in contrast to the zeolites used, A1203 surface contains negligible amounts of protonic (Broensted) acid sites. Let us now discuss in more detail the two zeolitic systems providing the most effective CCl4-assisted stabilization of isolated MoS+-species : H-ZSM-5 and H-beta. These two

572 samples demonstrate intense (-10 -'~ spin/g) and narrow ESR lines from well separated Mo s+ ions (Fig. la,b). It seems that just these two zeolites, with intermediate channel size, both allow a quite easy migration of reactive Mo-species to cationic positions inside the channels and provide preferential mono-atomic fixation of well separated MoS+-species in restricted inner voids. It is difficult to imagine for high-silica zeolites with large distances between neighboring framework AI atoms that sites occur capable of coordinating polyvalent cation without additional ligands. Therefore, we suggest that isolated complex species [(MOO2)+, (MoOCI2) + or (MoCI4)+], and not isolated Mo s+ ions are coordinated in cation positions. The signal obtained immediately after the MoOJH-ZSM-5 treatment in [CC14 + air] flow at 200~ demonstrates the presence of an additional quite weak and narrow triplet This triplet is identical with the spectrum from CCl4-treated pure H-ZSM-5 and shows the same dependence of the signal shape from the oxygen pressure. Therefore, this signal, with gx~ = 2.018; gyy -- 2.009; g~z = 2.003, demonstrates stabilization of 05. radical species on a minor part of defect framework sites formed upon the sample treatment. It seems that no essential interrelation between these sites and Mo 5+ stabilization exists. 3.2. Properties of Mo-containing zeolites The Mo(V) state stabilized by the ZSM-5 or beta matrixes is quite stable at room temperature: after the 5-day's stay of the sample on air the transformation of the MoS+-ESR signal occurs without substantial loss in the integral intensity. Noticeable narrowing of the

o = 1.95

Fig. 2. ESR spectra, taken at -196~ of 2 %MoO3/beta : (a) -treated at 200~ in the [air + CC14] flow, (b) - impregnated with water at 20~

1. IOOG H

573 line could be explained by a gradual saturation of the zeolite with water absorbed from the wet air, so that H20 molecules filling the zeolitic channels contribute to an additional separation of Mo 5§ species from each other.As illustrated by Figure 2, even the sample impregnation with water, in air presence, does not cause an immediate disappearance of the MoS*-ESR signal. Bonding with strong H~.O ligands results in transformation of the Mo 5§ coordination only, as shown by drastic change in the ESR signal shape (Fig. 2a ~ 2b). However, the Mo(V) oxidation state is not resistant to oxidative calcination, and the sample treatment in air at 200-300"C results in the total disappearance of the MoS+-ESR signal pointing to a quantitative oxidation of Mo(V). Reduction of the oxidized samples by H2 (-10 Torr) at 400~ leads to the appearance of a new Mo 5§ ESR spectra, shown in Fig. 3. Reduction of the pre-oxidized sample Mo/beta by H2 leads to the appearance of the Mo 5§ ESR spectrum shown in Fig. 3a. Some trace shf-splitting can be distinguished on this less intense and broadened ESR-line from Mo 5§ cations stabilized in the structure of the zeolite beta.

al

Fig. 3. ESR spectra, taken at 20~ of MoO3/beta (a) and "-" MoOJH-ZSM-5 (b) treated at 200~ in the [air + CC14] flow, oxidized at 400~ in air for 1 h, and reduced by H2 (-10Torr) at 400~ for 2 h.

bo

I

,

3200

I

,,, I

3400

t

I

3500

,,

,I

[G]

,,

I

3800

The ESR-line from isolated Mo 5+ ions in the Mo/ZSM-5 reduced by H2 demonstrates a very informative peculiarity : an additional superhyperfine splitting (shfs), with AH --- 7-8 G, can be distinguished (Fig. 3b). The formation of the extra structure may be due only to a shfinteraction of the unpaired electron of Mo 5+ with an outershell 27AI (nuclear spin 5/2). Earlier shfs of this type was detected for Cr 5+ and V4+ species located in cationic positions

574 of H-ZSM-5 [2, 9, 10]. Therefore, in spite of the non-perfect resolution of lines, the presence of just the shfs of this type gives an evidence of cationic location of the Mo 5+ species, with the lattice AI 3+ of the zeolite positioned in the second coordinative sphere. To account for the lack of resolution in the starting MoS+-ESR signal, it is necessary to take into consideration that the sample after treatment at 200~ in an [air + CC14] flow contains residual H20 and CC14 molecules coordinated with Mo 5+ cations. We assume that only a severe oxidative-reductive treatment at 400~ removes these additional ligands and provides conditions for electronic interaction of the cation with the lattice A13§ Reduction of the CC14-treated Mo/H-ZSM-5 by C2H 4 (-10 Torr) at 400~ results also in formation of the less intense and slightly asymmetric ESR-line from isolated Mo 5+ ions resembling the spectrum of the sample reduced by H2 (Fig. 3b) but shf-splitting is absent in this case. The lack of the shfs could be explained by the fact that residual CCI 4 is not removed before the reduction, and further treatment with hydrocarbon molecules is accompanied by formation of the "coke" residue preventing electronic interaction of the Mo 5+ cations with the lattice A13§ ions. Deposition of the coke of the sample is confirmed by appearance of the narrow ESR line with g -_- 2.002.

3.3 ln-situ ESR monitoring of Mo s+/ZSM-5 and Mo 5+/beta in flowing gas mixtures The MoS+-ESR signal is not altered at 500~ in He flow save for an intensity decrease due to the Curie-Weiss law. When treated with a flow of [1% vol H2 + He] the ESR signal associated with the isolated Mo 5+ ions did not change the shape in the temperature range from 20 to 500~ Switch to [0.4 %NO + He] flow at 20~ is accompanied by fast appearance of a new signal from paramagnetic species, with gl __ 2.041 and g2 = 2.011. The intensity of this signal reaches a maximum in the first 2-4 min in stream, and a monotonic drop of this signal is observed during the next 1 h. At the same time, the signal from isolated Mo s+ ions, with gl = 1.956 and gll = 1.895, disappears gradually. Back switch to pure He does not restore the starting MoS§ signal. Therefore, irreversible change of the paramagnetic Mo(V) sites occurs upon interaction of Mo/ZSM-5 with NO at 20~ One can assume that intermediate formation of paramagnetic complex specie (NO2-?) takes place, with subsequent quenching of the two unpaired spins and oxidation of Mo(V) to Mo(IV). It seems that oxidation of the Mo(V) state at room temperature by NO occurs noticeably easier than oxidation by the oxygen of air. At temperatures > 200~ the complete oxidation of Mo(V) by the [0.4 %NO + He] flow occurs very fast, and no formation of intermediates can be registered. At 500~ in a flow of [He + H2 + NO] at different stoichiometries, going from reducing to oxidizing conditions, the signal from Mo 5§ ions does not decrease when going from a stoichiometric ratio, H J 2 N O , of -7 to 1.2 (Fig. 4). Going to stronger oxidizing conditions results in noticeable drop of this signal, as shown by Fig. 4, but the signal decrease is completely reversible upon back switch to the reducing gas mixture. Therefore, at high temperature in the reaction mixture of NO and H2 the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(IV) ~ Mo(V) seems to be strongly shifted to Mo 5+.

575

112

100

El--O-

80

o-e

6O

=o 40

1~ -

:S

20FI[[_I 0 0

Fig. 4. Change in the integral intensity of the Mo 5+ESR signal, at 500~ in flowing mixture of

. [ i i , 1

,

~ . , 2 3

.

A , ! 4 5

9, 6

-

different[He + H2 + NO] at

9

stoichiometries 9 (EI)-Mo/ZSM-5" ( O ) - Mo/beta.

7

H21NO ratio

4. CONCLUDING REMARKS Modification of zeolites with use of active gas-phase species formed in-situ upon thermal treatment of the mixture IH-zeolite +MOO31 with an lair + CCI 4] flOW can be treated as a promising way of introduction of Mo ~+ -species into zeolitic channels. The presence of CCI 4 molecules enhances the possibility of "solid-state" introduction of polycharged ions into zeolites due to a sharp increase in the mobility of cationic fragments by in-situ formation of active species. Chemical transport reactions with formation of reactive and mobile oxychloride fragments in the zeolitic bed provides effective dissipation of the MoO3 phase and migration of active species into zeolitic channels in mild conditions.

5. ACKNOWLEDGMENT This work was supported by the Grant of Russian Foundation for Basic Research.

576 REFERENCES.

1. 2. 3. 4.

Karge, H.G. and Beyer, H.K., Stud. Surf. Sci. Catal., 69 (1991) 43. Slinkin, A.A. and Kucherov, A.V., Catal. Today, 36 (1997) 485. Rabo, J.A. and Kasai, P.H., Progr. Solid State Chem., 9 (1975) 1. Clearfield, A., Saldarriaga, C.H., and Buckley, R.C., Proc. 3-rd Int. Conf. Molec. Sieves, Sept. 3 - 7 , 1973, Zurich, Switzerland, Recent Progress Reports, J.B.Uytterhoeven, (Ed.) Univ. Leuven Press, 1973, Paper 130, p. 241. 5. Karge, H.G., Zhang, Y., and Beyer, H.K., Catal. Lett., 12 (1992) 147. 6. Kucherov, A.V. and Slinkin, A.A., Zeolites, 6 (1986) 175. 7. Kucherov, A.V. and Slinkin, A.A., Zeolites, 7 (1987) 38. 8. Slinkin, A.A., Kucherov, A.V., Gorjashenko, S.S., Aleshin, E.G., and Slovetskaja, K.I., Zeolites, 10 (1990) 111. 9. Kucherov, A.V., Hubbard, C.P., and Shelef, M., Catal. Lett., 33 (1995) 91. 10.Kucherov, A.V. and Slinkin, A.A., Zeolites, 7 (1987) 583. 11.Marchal, C., Thoret, J., Gruia, M., et al., Fluid Catalytic Cracking, vol. II. Concepts in Catalysis Design, M.L.Occelly Ed., ACS Symp. Ser., 1990, p. 452. 12.Wichterlova, B., Beran, S., Bednarova, S., Nedomova, K., Dudikova, L., and Jiru, P., Stud. Surf. Sci. Catal., 37 (1987) 199. 13.Beran, S., Wichterlova B., and Karge, H.G., J. Chem. Soc., Faraday Trans. 1, 86 (1990) 3033. 14.Kucherov, A.V. and Slinkin, A.A., Zeolites, 8 (1988) 110. 15.Yang, Y., Guo, X., Deng, M., Wang, L., and Fu, Z., Stud. Surf. Sci. Catal., 46 (1989) 849. 16.Price, G.L. and Kanazirev, V., J. Catal., 126 (1990) 267. 17.Zhu, Jian-hua, Cuihua Xuebao (Chines J. Catal.), 14 (1993) 294. 18.Kucherov, A.V. and Slinkin, A.A., Zeolites, 7 (1987) 43. 19.Kucherov, A.V., Slovetskaya, K.I., Goriaschenko, S.S., Aleshin, E.G., and Slinkin, A.A., Microporous Materials, 7 (1996) 27.

91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

577

Influence of the Preparation M e t h o d o l o g y on the Reactivity and Characteristics of F e - M o - o x i d e N a n o c r y s t a l s Stabilized inside Pentasyl-type Zeolites G. Centi,a'b F. Fazzini a, J.L.G. Fierro, c M. L6pez Granados ~, R. Sanz d and D. Serrano d Dept. Ind. Chemistry and Materials, V.le Risorgimento 4, 40136 Bologna, Italy. Fax. +39-51-644.3680, e-mail: centi @scirocco.unime.it b Dept. of Industrial Chemistry, Salita Sperone 31, 98166 Messina, Italy. c Inst. de Cat~.l. y Petroleoquim., CSIC, Campus UAM, Cantoblanco 28049, Madrid, Spain. d Dept. Ingenierfa Qufmica, Fac. Ciencias Quimicas, Univ. Complutense, Madrid, Spain. a

The CVD (Chemical Vapor Deposition) preparation of small Fe-Mo oxide clusters inside pentasyl-type zeolites (ZSM-5, boralite, silicalite and Fe-silicalite), the effect of preparation variables, and the differences with respect to preparation by impregnation or grafting procedures were studied with reference to the reactivity of the samples in p-xylene selective oxidation. The results indicate that small Fe-Mo-oxide particles inside the zeolite channels have significantly better properties of selectivity in mono- and dialdehyde formation than bulk Fe-Mo-oxide, but the catalytic behavior depends considerably on both the preparation methodology and the nature of the zeolite.

1. INTRODUCTION Oxide nanocrystals often possess quite different and particular reactivity properties in comparison to the corresponding bulk oxide, as indicated by differences in the crystalline habits, morphology, the bulk oxygen transport properties, electronic properties, etc. Various methodologies have been developed to synthesize these oxide nanocrystals (1), but usually it is very difficult to control the characteristics of the nanocrystals obtained and especially to avoid recrystallization during catalytic runs. A interesting possibility is to stabilize these oxide crystals inside a solid ordered matrix such as zeolites. Some attempts in this direction have been made recently, but it remains a quite unexplored field of research. When oxide crystals are stabilized inside a solid ordered matrix such as a zeolite it may be expected that the growth and crystalline habit of the guest nanoxide is influenced by the host zeolite, but also that additional factors contribute to change the reactivity of the oxide, such as the presence of a strong electrostatic field inside the zeolite channels, the possibility of synergetic cooperation between isolated transition metal ions and oxide nanoparticles and the possible presence of shape selectivity effects. The study of the characteristics and reactivity of guest oxides inside ordered crystalline host matrices (zeolite, in particular) is thus an interesting new opportunity, but background knowledge on the method of preparation of these solids must be first developed.

578 In the synthesis of catalytic active oxides within ordered matrices especially critical are the problems of preparation, such as: (i) avoiding external deposition on zeolite crystals, (ii) avoiding deposition of the oxide in a way as to inhibit diffusion of reactants or reaction products, (iii) avoiding the unselective catalytic role of the active sites already present in the zeolite matrix (especially acid sites), (iv) controlling the effective local structure and composition of nanoxide particles within the zeolite, (v) deposition of a sufficient amount of active components inside the zeolite pores to have reasonably high reaction rates, etc. Recently it has been reported in the literature that Fe-Mo/DBH (where DBH indicates a deboronated borosilicate in the H form) zeolites prepared by chemical vapor deposition (CVD) show interesting reactivity and selectivity characteristics in the selective oxidation of p-xylene to the corresponding aromatic aldehydes (2-5). Although preparation of these catalysts is very complex, little attention was given to control of the preparation and dependence of the catalytic behavior on preparation variables, nature of the zeolite, etc. The aim of the study reported in this communication was to analyze the details of the preparation of Fe-Mo/zeolite (pentasyl type) in order to evidence which factors in the preparation determine the catalytic behavior and reproducibility of the reactivity data, and analyze the relationship between the nature of the Fe-Mo-oxide species inside the zeolite channels and catalytic behavior. Special attention is given in this work to the influence of the nature of the zeolite host matrix and the method of introduction of Fe and Mo inside the zeolite on the catalytic behavior, because it is suggested that these aspects are of more general relevance for the design of catalytic active oxides in host structured matrices. 2. EXPERIMENTAL

2.1 Synthesis and characteristics of the host zeolites Z5 and BH are commercial samples (SM-27 and HAMS-1B-3 code names, respectively), from ALSI-Penta Zeolithe GmbH and PQ Company, respectively. The BH zeolite is the same indicated by Yoo et al. (2-5) as the preferable starting zeolite to prepare Fe-Mo/zeolite samples. Z5 is a crystalline ZSM-5 sample with SIO2/A1203 = 27 (primary crystal length ~m 1-3) and BH is a borosilicate sample (MFI crystal structure) containing 1.7% wt. B. BOR is a crystalline Boralite sample (SIO2/B203 = 22, BOR-C structure) synthesized by hydrothermal crystallization at 165~ (12 days) following the procedure given by Perego et al. (6). DSI and SIE are both Silicalite-1 samples. The first is prepared with a method leading to a high content of defects (silanol nests) and the second (SIE) has very small crystals (<100 nm) and a high fraction of external surface and silanol groups (about 64 m2/g external surface with respect to 458 m2/g total surface). DSI: a SIO2-V205 xerogel with Si/V=50 [obtained by a two step process (acid hydrolysis-basic condensation), using TEOS and V-isopropoxide as Si and V sources] has been impregnated by wetness method with 1M TPAOH solution and subsequently crystallized in autoclave at 170~ for 4 days. The sample shows a Si/V ratio of 878 and high numbers of defects, due to the presence of vanadium easily removed from the framework (7). SIE: the nanocrystalline silicalite was prepared mixing TEOS, 1 M TPAOH and water at room temperature, the mixture being stirred for 16 h. Thereafter, the solution was heated at 80~ to reach the molar composition 1 SiO2:0.36 TPAOH: 13.3 H20. The liquid gel was charged into a Teflon-lined autoclave and the synthesis carried out under autogeneous pressure and static conditions at 170~ for 3 days. After the synthesis, both

579

samples were dried at 110~ and calcined in air at 550~ FeSI (a,b,c) are Fe/silicalite zeolites with increasing iron contents (SiO2/Fe203=300, 200 and 100, respectively), synthesized (8) using the following conditions: TPAOH/SiO2=0.25 and H20/SIO2=40. FeSIa: TEOS (0.05 mol) in EtOH (0.05 mol) was added very slowly to a solution containing Fe(NO3)3.9H20 (0.00033 tool) in H20 (1.44 mol), under vigorous stirring. After 1 h TPAOH (0.0125 mol, 1 M aq. solution) was added to the previous solution very slowly, obtaining a gel. During the addition of TPAOH, a white gel was formed which finally dissolved again to give a clear liquid. Then the liquid was transferred to a Teflon-lined rotating autoclave (170~ 3 days). The white crystalline product obtained was filtered, washed with deionized water (different times to obtain pH=8), dried at 120 ~ for 2 h and calcined at 500 ~ for 12 h. FeSIb and FeSIe: prepared as for FeSIa, but using 0.0005 mol or 0.001 mol Fe(NO3)3.9H20, respectively.

2.2 Addition of Fe and Mo by Chemical Vapor Deposition (CVD) The initial CVD method adopted to introduce Fe and Mo in the zeolite channels was the same as that reported by Yoo (2-5), later modified as discussed in the text: 1. Ammonium acetate (0.44 g) is dissolved in deionized water (25 ml), then the zeolite (1 g) is added and the resulting white slurry is stirred at r.t. for one-half hour. The slurry is then filtered, washed with H20 (3 times), air dried at 120~ and calcined at 660~ overnight. 2. The zeolite (1 g) is loaded into a flow quartz reactor and dried at 400~ for 4 h under a nitrogen flow (20 ml/min). 3. The reactor is cooled down to r.t. and FeCI3 (0.2 g) is put on the bottom of the reactor. The zeolite is located above the iron-chloride (at about 1-2 cm and separated with a quartz septum). The reactor temperature is increased to 460~ in lh under flow of N2 (20 ml/min), held at temperature for lh, and then the flow of nitrogen is inverted for 1 h in order to make uniform the Fe composition along the axial reactor profile. 4. The reactor is cooled down to r.t. and the catalyst, after extraction from the reactor, is washed with H20 three times, filtered and dried at 120~ overnight. This procedure is necessary to eliminate chlorine ions and form iron-hydroxide-type species. 5. The solid is put again in the reactor which is heated at 400~ for 4h in a flow of N2 (20 ml/min) to remove adsorbed water. 6. The reactor is cooled down to r.t. and then MOO2C12 (0.2 g) is put on the bottom of the reactor. The zeolite already containing iron is located above the solid MOO2C12 and separated by a quartz septum. 7. The reactor temperature is increased to 200~ (in about 15 min) and then maintained at this temperature for 4h. After this treatment, the flow of nitrogen is reversed, and the reactor temperature increased from 200 ~ to 300~ in half an hour and kept at 300~ for 12 h under flow of N2 (20 ml/min). 8. The solid is calcined in air at 300~ (8h) and then at 650~ (8h). 9. The solid is washed with deionized water three times, filtered and dried again at 120~

2.3 Synthesis of reference bulk iron-molybdate A reference bulk Fez(MoO4)3 sample was prepared using the method reported by Carbucicchio and Trifir6 (9). Hereinafter this sample is called FeMo/BULK.

2.4 Catalytic Tests and Catalyst Characterization Catalytic tests were made using a quartz continuous flow reactor (5 mm inner diameter)

580

ZEOLITE PRETREATMENT Step 1

CVD with FeCI3 Step 2

treatm.ammoniumacetatei washing i drying,calcin.(660~ /AI

i~. (dehydration(4OO~ N1 reactionwith FeCI3 N2flow)

Q

CI\ /C1

HYDROLISIS Step 3 ~- hydrolysis / "~drying J OH,, /OH

)H ////

FINALTREATM. Step 5 Scheme 1

~_

Icalcination washing r dehydrationJ

CVD with MoO2C!2 Step 4

O\ /0 o

~-dehydration(400~ N2)~.-/-i/ reactionwith MoO2CI2 ""

J"

loaded with 0.5 g of catalyst in the form of small pellets (0.1-0.3 mm range). The feed composition was 0.5% p-xylene, 5% Oz and 94.5% helium. The space-velocity was set to 20.000 h -1 to avoid mass transfer limitations on the reaction rate. Tests were made in the 300500~ range. Above the upper temperature limit deactivation of the catalysts was observed due essentially to the formation of carbonaceous-type deposits, whereas below 450-500~ the catalysts were found to show a stable catalytic behavior at least on a time scale of about 20 hours of time on stream. The axial temperature profile was determined by a thermocouple inserted in the catalytic bed. Preliminary experimental tests were made to ensure the absence of mass and heat diffusion limitations on the reaction rates. In the absence of catalyst it was also verified that p-xylene does not react at a temperature of 500~ or lower. Before the catalytic tests the catalysts were first dried in an inert flow at 300~ for 2h, then conditioned at 450~ for 6 h in the presence of the reaction feed. The reactor temperature was then decreased to 300~ under the reaction feed and the catalytic activity monitored at increasing temperatures from 300 ~ to 500~ waiting about 20-30 rain for each analyzed temperature before recording catalytic behavior in order to stabilize activity. After reaching the higher temperature (500~ the catalytic behavior was monitored at decreasing temperatures in order to check possible variations in the performances. The analysis of the feed composition and of the reaction products was made using two gas chromatographs (GC), the first on-line for the analysis of incondensable gases (N2, O2, CO, COs). All organic products and water were absorbed in two traps containing acetone as the solvent and maintained at about-5~ The products in the absorption traps were analyzed using a GC equipped with a mass quadrupole detector after adding tridecane as an internal standard. The first GC uses a TCD detector and a 3m long Carbosphere packed column, and the second a HP-5 5% PhMe Silicone 30m long capillary column. UV-Visible-NIR diffuse reflectance spectra (DRS) were measured in air using a Perkin Elmer Lamda 19 instrument after transformation of reflectivity (R) according to the Kubelka-

581 Munk function: F(R) = (1-R)2/2R=. BaSO4 was used as a reference. Photoelectron spectra were acquired with a VG ESCALAB 200R spectrometer (MgKa 120 W X-ray source). The spectra were collected for 20 to 90 min, depending on the peak intensities, at a pass energy of 20 eV (1 eV = 1.602.10 -19 J) which is typical of high resolution conditions. The intensities were estimated by calculating the integral of each peak after smoothing and subtraction of the "S-shaped" background and fitting the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportion. All binding energies (BE) were referenced to the adventitious C ls line at 284.9 eV. This reference gave BE values within an accuracy of + 0.2 eV. Raman spectra were recorded with Renishaw Raman Spectrometer system 1000 equipped with a laser of Ar as exciting source (514.5 nm). Samples were calcined in situ under air at 773 K and spectra were also recorded at 773 K. The zeolite crystallinity and structure were determined by X-ray diffraction (Philips PW 1710 diffractometer (CuK~ radiation) and FT-IR (Perkin Elmer 1750 instrument) analyses. 3. RESULTS AND DISCUSSION

3.1 Role of the various preparation steps in determining the catalytic behavior The method of preparation of Fe-Mo/zeolite catalysts by CDV reported by Yoo et al. (2-5) and summarized in Scheme 1 involves several reaction steps. In Scheme 1, together with the indication of the sequence of preparation steps (details are given in the experimental part), the species formed in the various steps according to Yoo et al. (2) are also indicated. When this procedure of preparation is followed closely, however, catalysts with poor catalytic performances are obtained. This is shown in Table 1 which reports the activity and selectivity obtained for samples prepared using the procedure indicated in Scheme 1 and the experimental part for three zeolites: ZSM-5 (Z5), pure Boralite (BOR) and the same Borosilicalite (BH) cited by Yoo et al. (2) as the pentasyl-type zeolite giving the better catalytic performances. The reaction conditions in terms of both feed composition and spacevelocity are also analogous to those used by Yoo et al. (2-5). Table 1 Catalytic tests in p-xylene conversion to p-tolualdehyde (PTA) and terephthaldehyde (TFA). Reaction conditions as indicated in the experimental part.

Catalyst

Reaction Temp., ~ ..... 350 FeMo/Z5-a 400 450 300 FeMo/BOR-a 350 4OO 300 FeMo/BH-a 350 400 ,

,,

Fe, % wt. 1.9

2.2

Mo, % Mo/Fe Cony. pwt. ratio xylene, % 4 3.3 1.1 48 ' 59 2 ,

1.7

0.5

. . . . .

.

.

.

.

4.2

4.4

0.6

5 23 6 36 66

Selectivity to PTA, % 8.5 6.5 " 2.4 ' 4] 35 ...... 10 54 34 0 ,

,.

....

. . . .

,

Selectivity to TFA, % 0 0 1 0 0 0 7 18 1 ,

_

.

The catalyst prepared starting from the Z5 host zeolite shows very low selectivity to both aldehydes. The sample prepared starting from the BOR zeolite shows better selectivity behavior, but no terephthaldehyde formation and low activity. Using the BH host zeolite, a

582 10 F(R) 8

0 50000 40"

'

40000

'

30000

'

20000

'

cm-1

10000

F(R)

20

b 10"

31000 ,

0 50000

.

40000

i

,

30000

,

,

20000

,

cm-1

,

10000

Fig. 1 UV-Visible-NIR diffuse reflectance spectra of Fe/BH samples (top) and Fe/Z5 samples (bottom) after grafting (CVD) with FeC13 and subsequent hydrolisis treatment. Top spectra: FeC13 CDV at 460~ (a) or at 300~ (b,c: the latter on a BH sample treated 5 times with ammonium acetate before iron addition step, whereas the former only 1 time). Bottom spectra" FeC13 CDV at 300~ but on Z5 sample calcined at 600~ (a) or 800~ (12h) (b) before the iron addition step. higher selectivity and activity is observed. In this case, terephthaldehyde also forms. These catalytic results thus show the considerable dependence of the catalytic behavior in p-xylene conversion on the nature of the host zeolite. However, in the case of the better sample (FeMo/BH-a) the catalytic performances are much worse with respect to those reported by Yoo et al. (2-5) for exactly the same preparation and host zeolite. A closer analysis of the influence of preparation steps on the nature and reactivity of final catalysts is thus necessary. Addition of Fe by CVD_. This step of the preparation involves the "formal" grafting of FeC13

583 to Br6nsted acid sites of the zeolite, followed by room temperature (r.t.) hydrolysis of the grafted species to the equivalent hydroxide species (Scheme 1). The grafting reaction is carried out at 460~ according to Yoo et al. (2), but at this temperature, small amounts of 02 and H20 in the carrier nitrogen flow (due to small leaks their presence in the N2 flow is often observed) may lead to conversion of the anchored iron-chloride to form iron-oxide species that are not bound to Br6nsted sites. This is demonstrated in Figure 1 (top spectra) showing the diffuse reflectance UV-Visible-NIR spectra of FeMo/BH samples prepared by FeC13 CVD at 300~ and 460~ (spectra a and c, respectively). The UV-Visible diffuse reflectance spectrum of Fe-silicalite with iron in the framework position (after calcination at 500~ (10) is characterized by two intense charge-transfer (CT) bands at 46500 and 40500 cm -1 (tl --~ t2 and t~ --~ e, respectively) of the [FeO4] tetrahedral group, and weak d-d transitions in the 20000-30000 cm -l region, largely masked by the tail of CT bands. At higher calcination temperatures (700~ the 46500 cm -1 band decreases and growth of a strong asymmetric peak with a maximum at 36000 cm 1 occurs indicating the partial Fe 3+ migration from the framework toward extraframework positions forming clustered oxide Fe 3+ species (probably hydroxide type), although different from Fe203 particles (massive oxide microaggregates) which give a broad band, asymmetric towards high frequencies, centered at about 20000 cm -l . When FeC13 grafting is made at 460~ (spectrum c in top Figure 1) two peaks at 46000 and 37500 cm -1 are observed which may be assigned to Fe ~ O (or C1) CT bands similar to those for framework [FeO4] tetrahedral groups (10). The presence of a C1 ligand instead of a O (framework) ligand it is not expected to change markedly the position of these CT bands (11). In addition, a very broad band in the 30000-18000 cm I region is observed which may be assigned to the formation of small clusters of iron-oxide. When the temperature of FeC13 grafting is lowered to 300~ (spectrum a in top Figure 1), a considerable increase in the intensity of CT bands is observed with also a small shift of the band at 37500 cm -1 to 39000 cm -l. The broad band in the 30000-18000 cm -l region also disappears, but a clearly evident band centered at 29000 cm -1 appears. This band appears when the sample after the FeC13 grafting is then put at r.t. in water for the hydrolysis step. Bordiga et al. (10) observed that 7FeOOH or Fe(OH)3 diluted in SiO2 is characterized by an asymmetric band centred in the 30000-25000 cm -1 region and thus the band at 29000 cm -l in spectrum a (top Figure 1) may be assigned to an iron-hydroxide which forms by hydrolysis of-OFeC12 anchored species. The question is whether the species generated by hydrolysis is the equivalent hydroxidespecies [-OFe(OH)2] of the starting grafted species or instead the hydrolysis generates an hydroxide species not directly anchored to the zeolite cage, but simply supported on the zeolite channel walls. To answer this question is useful to analyze the spectrum of an analogous Fe/BH sample (FeC13 grafting at 300~ the host BH zeolite of which has been treated several times with ammonium acetate. This treatment followed by direct calcination at high temperature (660~ first step in Scheme 1) causes the removal of part of the framework boron, as indicated by chemical analysis (2), reasonably creating zeolite framework defects (hydroxyl nests). During consecutive CVD with FeC13, iron may insert into these hydroxyl nests, formally replacing the extracted boron ions. It is expected that this multiple interaction of iron ions with lattice zeolite oxygens stabilizes it with respect to formation of iron hydroxide-type species simply supported on zeolite channel walls. In fact, the spectrum of the F e ~ H zeolite after extensive ammonium acetate treatment (spectrum b in

584 top Figure 1) clearly indicates a decrease in the intensity of the 29000 cm -1 band. To obtain further support for this concept of the role of reaction of Fe with hydroxyl nests (zeolite defects) versus grafting with Br6nsted sites regarding the stability of formed species to hydrolysis conditions, it is useful to analyze the spectra of Fe/Z5 samples (bottom Figure 1). It is known that the strength of Br6nsted acid sites in ZSM-5 samples is higher than in Boralite samples (12) and thus more stable iron grafted complexes may be expected using Z5 host zeolite than using BH. On the contrary, the Fe/Z5 sample, which is pale yellow after FeC13 CVD similarly to Fe/BH, becomes brown after the hydrolysis treatment. The UVVisible-NIR spectrum (spectrum a, bottom of Figure 1) clearly indicates the presence of a very intense band centered in the 32000-28000 cm ~ region (attributed to small particles of iron-hydroxide supported on the zeolite channel walls) as well as a shoulder at 19000 cm -1, suggesting the partial formation of iron-oxide particles. A1 ions in ZSM-5 are more stable than B ions in Boralite and thus significant dealumination by the zeolite pretreatment as in step 1 (Scheme 1) is not expected. However, calcination in more severe reaction conditions (800~ causes dealumination (13) and creation of zeolite defects. When a Z5 sample calcined at 800~ before FeC13 CVD is used instead of the untreated Z5 host zeolite, the intensity of the band attributed to iron-hydroxide (at around 30000 cm -~) decreases considerably (spectrum b, bottom of Figure 1) and the overall spectrum becomes more similar to those of the Fe/BH series. These indications provide a basis for the analysis of the results of the catalytic behavior reported in Table 1 and the considerable effect of the host zeolite on them. In the FeMo/Z5 sample, FeC13 reacts with Br6nsted acid sites, but the complex is not stable and decomposes during the consecutive r.t. hydrolysis step to form supported iron-hydroxide(oxide) and Br6nsted sites. This is further demonstrated by the fact that in catalytic tests using FeMo/Z5, typical by-products of acid catalyzed reactions (trimethylbiphenylmethane, toluene and 1,2,4trimethylbenzene) were observed and in an amount markedly higher than for BH based samples. The supported iron-hydroxide(oxide) species do not react (or only in part) with molybdenum during the successive MoO2Cl: CVD and further steps (Scheme 1) and thus a selective iron-molybdenum complex does not form. The iron oxide particles inside the zeolite channels are reasonably responsible for the total oxidation activity. In the FeMo/BH sample, the initial zeolite pretreatment (step 1 in Scheme 1) creates zeolite defects with which iron reacts forming iron-hydroxide species still anchored to the zeolite structure and which react effectively with MoOzCI2 during the consecutive step to form iron-molybdenum-oxide selective species. Those iron complexes formed by the grafting reaction with Br6nsted sites, instead, have a behavior analogous to that described above for Z5-based samples. Using a crystalline synthetic Boralite (BOR) instead of the commercial sample (BH) B ions are less easily removed from lattice positions during ammonium acetate extraction and successive calcination at 660~ as confirmed by a very small change in B content after the treatment. This sample thus behaves similarly to Z5, but probably due to lower acid strength of the Br~3nsted sites, anchors a smaller amount of molybdenum during the MOO2C12 CVD step (Table 1) and is thus less active. Similarly to FeMo/Z5, FeMo/BOR also does not form terephthaldehyde, differently from FeMo/BH. Role of Fe in framework positions.: The results discussed in the previous section point out the critical role of the nature of the host zeolite (including modifications induced on it by pretreatment) in determining the type of iron species formed by FeC13 CVD and their stability

585

towards the consecutive step of hydrolysis. In particular, it was indicated above that in order to have stable iron species during the hydrolysis step and avoid the formation of supported iron-hydroxide(oxide) species, both critical factors regarding the catalytic behavior after the step of addition of molybdenum, it is necessary that the iron react with zeolite defects (hydroxyl nests) to formally form a Fe-silicalite zeolite having iron in framework positions. It is thus interesting to analyze the behavior of Fe-silicalite (FeSI) as the host zeolite, but starting from step 4 (MOO2C12 CDV) in the general preparation method (Scheme 1). Reported in Table 2 is the catalytic behavior of Mo/FeSI samples at increasing iron content. There is a significant improvement in the catalytic behavior, both in terms of global selectivity and specifically selectivity to terephthaldehyde of all these samples with respect to the data shown in Table 1. This confirms the previous suggestion of the necessity of having iron ions in a framework-like position to form iron-molybdenum-oxide species selective especially to the dialdehyde. However, increasing the Fe content in the Fe-silicalite, decreases the amount of Mo deposited during the consecutive step with a consequent drastic decrease in the Mo/Fe molar ratio (Table 2). According to Yoo et al. (2-5), catalysts selective towards the dialdehyde must have a Mo/Fe molar ratio close to 2-2.5. The change in the catalytic performances in FeSI is thus probably related to the change in the Mo/Fe ratio. More difficult to explain is why the increase in iron content in FeSI decreases the amount of molybdenum deposited by CVD. The possible grafting reaction of MOO2C12 with framework iron requires in the latter the generation of hydroxyl groups. Bordiga et al. (10) noted in Fe-silicalite that the vacuum removal of adsorbed water gives bridging hydroxyl groups between Si and Fe ions. This is accompanied by a distortion in the [FeO4]- unit which looses its tetrahedral structure. The acidity of the Br6nsted acid site depends on this distortion and the effective charge on the oxygen atoms. Thus it is expected that as the amount of iron in the framework increases, the acid strength of these bridging hydroxyl groups decreases, similarly to that observed for ZSM-5 zeolites as the Si/A1 ratio decreases. It is thus reasonable to attribute the change in the amount of Mo deposited by CVD as a function of the iron content in FeSI samples to a change in the acid strength of the Br6nsted acid sites and possibly in the number of Br6nsted acid sites which is reflected in a decreased amount of Mo able to be grafted during the MOO2C12 CVD step. Table 2 Catalytic tests in p-xylene conversion to p-tolualdehyde (PTA) and terephthaldehyde (TFA) of Mo/FeSI samples. Reaction conditions' see experimental part. Catalyst Mo/FeSI-a

Mo/FeSI-b

Mo/FeSI-c

Reaction Temp., ~ 300 350 400 300 350 400 300 350 400 , ,

Fe, % wt. 0.6

1.8

3.0

Mo, % Mo /Fe Conv. pwt. ratio xylene, % 4 27 5.6 5.4 63 12 44 4.2 1.4 59 11 42 2.1 0.4 81

Selectivity to PTA, % 50 27 15 51 32 7 34 18 5

Selectivity to TFA, % 10 11 6 19 17 3 2 5 1

586 Although formally equivalent, true framework iron may have different characteristics from those of iron ions anchored at zeolite defects (hydroxyl nests), in terms of (i) reactivity with Mo during the CVD step, (ii) availability to form iron-molybdate like species during consecutive heating treatments and (iii) influence on molybdenum reactivity during catalytic tests. It is thus interesting to analyze the behavior of a sample prepared by the CVD method starting from a highly defective silicalite (DSI) host zeolite. Reported in Table 3 is the catalytic behavior of the FeMo/DSI catalyst evidencing the significant improvement in catalytic behavior, both in terms of global selectivity to the two side-chain aldehydes (global selectivity over 70% up to about 40% of p-xylene conversion) and especially of formation of the dialdehyde, notwithstanding the low Mo/Fe ratio in the catalyst (Table 3). This suggests that a large part of iron, reasonably incorporated in defects of the zeolite, is inactive towards reaction with molybdenum, but at the same time does not play a main catalytic role in either the selective or unselective oxidation of p-xylene, differently from iron-oxide microparticles supported on the walls of zeolite channels responsible for unselective oxidation to carbon oxides as discussed above. Table 3 Catalytic tests in p-xylene conversion to p-tolualdehyde (PTA) and terephthaldehyde (TFA) of FeMo/silicalite samples. Reaction conditions: see experimental part.

Catalyst FeMo/DSI

FeMo/SIE

Reaction Temp., ~ 300 350 400 400 450 500

Fe, % wt.

Mo, % Mo /Fe Conv. p- Selectivity wt. ratio ~lene, % to PTA, % 7 52 37 32 2.5 0.4 44 9 16 24 1.4 0.5 54 I 12 69 4 ......

4.3

,,

1.8

....

Selectivity to TFA, % 41 43 16 0 25 17 .

.

.

.

.

.

Role of external surface: The role of the external surface of the zeolite crystals was analyzed using as host zeolite a Silicalite-1 sample (SIE) having a very high external surface area, due to the presence of microcrystals. Due to the presence of small crystals, this sample also contains a large amount of defects similarly to DSI and thus may be compared with it. The catalyst behavior of FeMo/SIE is also reported in Table 3 together with that of FeMo/DSI. With respect to the latter sample, FeMo/SIE shows a much lower reactivity (it is necessary to increase the reaction temperature by about 100~ to have comparable conversions). It also forms terephthaldehyde, but the global selectivity is markedly lower, notwithstanding the analogous Mo/Fe molar ratio. The X-ray photoelectron spectroscopy (XPS) characterization of these two samples (Table 4) gives useful indications. Table 4 XPS characterization of FeMo/silicalite samples. Binding energy (eV) of core electrons and surface atomic Mo/Si and Fe/Si ratios.

Catalyst

Si2p

Mo3ds/2

Fe2p~/2

Mo/Si

Fe/Si

FeMo/DSI FeMo/S1E

103.4 103.4

232.0 232.1

710.3 710.4

0.014 0.173

0.0101 0.011

In the FeMo/SIE sample there is considerable enrichment in molybdenum at the external

587 surface of the zeolite crystals and partially also of iron (the Fe/Si surface iron in FeMo/SIE is slightly higher than in FeMo/DSI, even though the total amount of iron - see Table 3 - is only 40% of the other). The SIE host zeolite is microcrystalline and is characterized by an enhanced fraction of external surface area (about 15% with respect some few % or less for normal crystalline zeolites) and thus of external silanol groups (isolated and nested) which can react with iron and molybdenum during CVD. XPS data on this sample (Table 4) confirm this indication of a surface enrichment of both iron and molybdenum, but indicate that the effect is especially remarkable for molybdenum. This suggests that probably due to diffusional limitations, the molybdenum oxychloride reaction with silanol groups on the outer surface of the crystals is faster than with those inside the pore structure. To verify this indication, a sample was prepared using a grafting procedure in liquid anhydrous tetrahydrofuran (THF), and the same FeC13 and MOO2C12 reactants used for vapor phase CVD. While the amount of iron grafted to zeolite is comparable to that obtained in vapor phase CVD, the amount of molybdenum is nearly negligible confirming the probable steric limitations of the reaction of molybdenum oxychloride with internal Bri3nsted sites. The comparison of the catalytic results of FeMo/DSI and FeMo/SIE samples (Table 3) indicates that the latter is markedly less active and selective, thus indicating that when iron and molydenum deposit on the external surface of the zeolite crystals, less active and selective species form.

3.2 Effect of the inclusion of Fe-Mo-oxide inside the host zeolite on its reactivity As shown below, Fe2(MoO4)3 (plus minor amounts of MOO3) is the only species detected 100 -

FeMo/BULK

I'Conv.

FeMo/DSI

1 /"

-- 80

80-

- 60

o~

< 0

60-

~,

9~ -

~

CO

0 L3

40

40-

-o ~<-79

20

--

0300

350

400

300

350

~ 4110

20

0

Reaction temperature, ~ Fig. 2 Catalytic behavior in p-xylene oxidation of FeMo/BULK and FeMo/DSI. S-TPA: selectivity to p-tolualdehyde, S-PTA: selectivity to p-terephthaldehyde. Reaction conditions as reported in the experimental part.

588 by Raman spectroscopy in FeMo/zeolite samples and therefore the activity of a bulk Fe2(MoO4)3 (FeMo/BULK) was compared with that of the FeMo/DSI sample. The results of the catalytic tests are summarized in Figure 2. FeMo/DSI is less active at the higher temperatures in comparison to F e M o / B U L K probably due to limitations on diffusion of the reactants inside the channels. Instead below about 350~ FeMo/DSI is slightly more active than the bulk oxide, even though the amount of active component (FeMo-oxides) is about 15 times lower. Furthermore, the selectivity behavior changes considerably: Not only is the global selectivity higher, but especially the dialdehyde (terephthaldehyde) forms with good selectivity, while the dialdehyde does not form over bulk Fe-Mo-oxide. This shows that the reactivity properties of a mixed oxide can be improved considerably by a controlled inclusion inside a zeolite pore structure, as well as suggests that the nature of the Fe-Mo-oxide species inside the zeolite pore structure should present particular structural differences with respect to bulk iron-molybdate oxide. In the previous section it was shown how both the activity and selectivity behavior depends considerably on the preparation and thus the differences in the catalytic behavior cannot be attributed to the influence of the zeolite host structure (shape selectivity effects, presence of an electrostatic field, diffusion effects on the reaction rates, etc.) on the catalytic performances, but rather to the possibility of stabilizing inside the host zeolite different mixed oxide species having different reactivity and selectivity from those of bulk oxides.

3.3 Nature of the Fe-Mo-oxide species inside the host zeolite Characterization of the nature of the Fe-Mo-oxide species formed inside the host zeolite is made difficult by the small amount of these species present and the interference in the spectroscopic analyses by the host zeolite. However, some useful indications are given by Raman characterization of the samples. Reported in Figure 3 are the Raman spectra in the 200-1200 cm -~ region of two samples with low selectivity (FeMo/Z5-al and FeMo/BOR-al; in these samples the temperature of FeC13 CVD was 300~ as for other samples in Figure 3; with respect to the data reported in Table 1 only the catalytic behavior is not markedly different), and three samples all selective to terephthaldehyde, although with different behaviors (see Tables 2 and 3). For comparison, the Raman spectrum of MoO3 is also reported, together with the main bands for Fe2(MoO4)3 (9). It can be noted that in all samples the presence of Fe2(MoO4)3 and MoO3 (in minor quantities) only can be detected. No other phases such as FeMoO4 and iron oxides (o~-Fe203, 7-Fe203, Fe304, etc.) are observed. The data in Figure 3 suggest that independently of the preparation, Fe3+-molybdate forms in all samples. In the bottom two samples (on Z5 and BOR host zeolites), where it was suggested that iron is mainly present as iron-hydroxide(oxide) species after the hydrolysis and thermal treatment, molybdenum oxide probably reacts by solid state reaction to form the iron-molybdate. Apparently, therefore the same species is present in all zeolites, independently of the preparation and nature of the host zeolite, although catalytic data (Tables 1-3) clearly indicate the high sensitivity of the catalytic behavior to these two parameters. The Raman spectrum of Fe-Mo-oxide species inside the host zeolite is also not remarkably different from that of bulk Fe2(MoO4)3 catalysts (9), although also in this case the

389 e~ co oo ~ . ~ o ~

~-4

,.,-, o

0 0

:~ ,'- i :~

CO

O

t

;

u..

,

I

l I

i

I..!_

iJL__ Mo__Ets o

i

i-b

,,.

/'~

FeMo/SIE

/~

FeMs

E

i

/~j,

,!

i

!

,, 200

, !

I

400

,

I

600

,

,

,

I[

! i

,

800

Wavenumbers,

II

1000

....xa ,

........ J

1200

cm "1

Fig. 3 Raman spectra of MOO3, Mo/FeSI-b, FeMo/DSI, FeMo/SIE, FeMo/BOR-al and FeMo/Z5-al samples. The principal bands for bulk Fe2(MoO4)3 are also reported. results shown in Figure 2 point out the differences in the catalytic behavior. A closer inspection of the Raman spectra (Figure 3), however, shows that in the more selective catalysts a new broad band appears at about 380 cm -I with possibly a second broad and weak band centered at about 450 cm -~. It may be also noted that the peaks of ironmolybdate species are broaden than in the other two, less selective, samples. It is reasonable that due to space-constraints inside the pores, the iron-molybdate does not grow as in bulk oxide, but rather is modified. The 300-500 cm l region corresponds to the bending vibrations of Mo-O-Fe bonds in iron-molybdate and thus it is reasonable to expect that due to both the partial incorporation of iron ions into the zeolite framework (by inclusion in defects) and steric constraints in the growth of the iron-molybdate, deformations in the Mo-O-Fe bonds create, although the general arrangement of polyhedra should remain

590

Fig. 4 Model of the possible structure of iron-molybdate nanocrystal species inside host pentasyl zeolite. substantially unchanged [with respect to bulk Fe2(MoO4)3] as indicated by the Raman spectra. A tentative model for the possible structure of the iron-molybdate species is shown in Figure 4. The local arrangement of iron and molydenum polyhedra is analogous to that of bulk Fez(MoO4)3 (14), but part of the iron tetrahedra formally belong to the zeolite framework, being inserted at zeolite defects. This induces small changes in the spatial arrangement of molybdenum polyhedra (as reflected by the Raman spectra), providing possibly a better geometrical arrangement for the coordination of the p-xylene molecule, with the simultaneous insertion of oxygen into the two methylic groups of p-xylene, as suggested by Yoo et al. (2-5). It may be noted, on the other hand, that the redox characteristics of iron ions in zeolite framework positions should be different from those in bulk iron-molybdate, because Si-O-Fe bonds are present. Reasonably this factor also contributes in determining a change in the reactivity/selectivity characteristics of the Fe-molybdate nanocrystals inside the host zeolite with respect to corresponding bulk species. Finally it may be remarked that the tentative model outlined in Figure 4 suggests a critical role of the spatial arrangement of the zeolite defects (and thus of iron framework species after FeC13 CVD) in determining the effective local structure of iron-molybdate nanocrystals and that the Mo/Fe ratio may not necessarily be close to 1.5 [Mo/Fe molar ratio in Fe2(MoO4)3] to have a local structure similar to that of the iron-molybdate. Both indications are in agreement with the results obtained in this work, but a more detailed study on the nature of Fe-

591 molybdate species inside host zeolites is necessary to confirm these indications. Finally, it also should be pointed out that these results indicate that both the chemistry of preparation and the nature of the active species are more complex than that suggested by Yoo et al. (2-5) and briefly outlined in Scheme 1. On the other hand, the present data also indicate that although complex, control of the preparation of mixed oxides inside the host zeolites can be an interesting new opportunity to synthesize mixed oxides having different catalytic properties from those of the parent bulk oxides.

ACKNOWLEDGEMENTS This work was carried out within the framework of an EuRam-Brite III project (BRPRCT95-0062) financially supported by the E.C., support which is gratefully acknowledged. ALSI-Penta Zeolithe GmbH and Amoco are also gratefully acknowledged for providing samples of SM-27 and HAMS-1B-3 (from PQ Company) zeolites, respectively. Dr. A. Tuel (IRC-CNRS, Lyon, France) is also acknowledged for this helpful suggestions and discussion regarding the synthesis of Fe-silicalite samples. REFERENCES 1. W.R. Moser (Editor), Advanced Catalysts and Nanostructured Materials - Modern Synthetic Methods, Academic Press, San Diego CA 1996. 2. J.S. Yoo, J.A. Donohue, M.S. Kleefish, P.S. Lin, S.D. Elfine, Appl. Catal. A, 105 (1993) 83. 3. J.S. Yoo, C. Choi-Feng, J.A. Donohue, Appl. Catal. A, 118 (1994) 87. 4. J.S. Yoo, Appl. Catal. A, 143 (1996) 29. 5. G.W. Zajac, C. Choi-Feng, J. Faber, J.S. Yoo, R. Patel, H. Hochst, J. Catal., 151 (1995) 338. 6. G. Perego, M. Cesari, G. Allegra, J. Appl. Cryst., 17 (1984) 40.3 7. G. Centi, S. Perathoner, F. Trifirb, A. Aboukais, C.F. A?'ssi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. 8. P. Ratnasamy, R. Kumar, Catal. Today, 9 (1991) 403. 9. M. Carbucicchio, F. Trifirt), J. Catal., 45 (1976) 77. 10. S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal., 158 (1996) 486. 11. C.K. Jq~rgensen, Struct. Bonding (Berlin)., 1 (1966) 3. 12. J. Datka, Z. Pivowarska, J. Chem. Soc. Faraday Trans. I, 85 (1989) 47. 13. W. Lutz, B. Zibrowius, E. L6ffler, In Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp, G. Karge, H. Pfeifer and W. H/31derich Eds., Studies in Surface Science and Catal. Vol. 84, Elsevier Science Pub.: Amsterdam 1994, p. 1005. 14. G. Fagherazzi, N. Pernicone, J. Catal., 16 (1970) 321.

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91998Elsevier ScienceB.V. All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

593

S C I E N T I F I C BASES FOR PREPARATION OF OXIDE S U P P O R T S AND CATALYSTS VIA SOL-GEL METHODS

Oleg P. Krivoruchko B o r e s k o v Institute o f Catalysis, Pr. A k a d . L a v r e n t i e v a 5, N o v o s i b i r s k 630090, R u s s i a 1. I N T R O D U C T I O N Poorly soluble hydroxides are widely used to prepare catalysts, supports, adsorbents and other materials. An understanding of the mechanism of hydroxide formation at the level of sol-gel processes and gel evolution on its ageing in mother solutions permits one to control their properties such as phase composition, dispersion, and porous structure that are important for catalysis. Before the publication of our works, the mechanism of hydroxide formation was usually treated from two standpoints. Regarding the first one, hydrolysis of aqua cations in solutions is a step-wise process and "monomer molecules" of hydroxides form at the initial stage. When the rate of such "molecules" formation is high, they produce hydroxide precipitates in the amorphous state/phase. The second view holds that hydroxides form via a permanent growth of inorganic polymer chains to the point when their size and molecular weight range up to that of colloid particles. The latter approach suggests a wide range of polymer complexes in the solution at every instant. It is well known that hydrolysis is always complicated by formation of polynuclear hydroxo complexes (PHC) on the interaction between aqua cations (III, IV) and bases. For a long time, complex hydrolytic reactions occurring in solutions and the mechanism of hydroxide formation were studied in parallel as two independent problems. We have combined both studies from a conceptual point of view, since the results of these studies exhibit the regulations of separate stages of a single process. We have established [1] a new, not known earlier mechanism of formation of poorly soluble hydroxides and formulated the theoretical concepts, providing an explanation for relations and dependences between the stages of genesis of hydroxides and oxides on catalyst synthesis. Thus, we state that PHC of precipitated metals act as intermediate materials on hydroxide formation. As was shown, a limited number of complexes forms on polycondensation. In the course of mutual conversions, "key" polynuclear hydroxo complexes (KPHC) are formed and accumulated in solutions to produce sol particles. Consequently, the "key" complexes are not engaged in the formation of complexes with a different nuclearity and structure at the following steps of the hydroxide formation. In this report, we present the data related to formation of individual and binary hydroxides of AI(III), Fe(III), and Cu(II) using the sol-gel methods.

594 2. F O R M A T I O N PROCESSES

OF INDIVIDUAL

HYDROXIDES

VIA SOL-GEL

2.1. Hydroxides of AI(III) Aluminium hydroxides are extensively used to manufacture supports and catalysts. For this reason, they are the subject for numerous studies, primarily, of empirical nature. Nevertheless, the mechanism of formation and the structure of amorphous AI(III) hydroxides are not sufficiently elucidated. For any method and precipitation conditions, the interaction between a precipitated metal salt and an alkaline, the system passes through a number a states which are characterized by various a~ = [OH-]/[AP+]. A particular distribution of AI(III) complexes with a different nuclearity corresponds to every aa. 27A1 N M R permits one to follow separately the complexes containing 1, 2, and 13 atoms of aluminium (All, Alz, and Al~3, respectively). The structure of complexes AI2 and AI~3 have been determined elsewhere [2,3]. ZVAI N M R lines were attributed in [4]. On polycondensation of concentrated solutions of AI(III) salts, Alp forms which is not observed in the 27A1 N M R spectra. We suggest the following experimentally substantiated structural and crystallochemical model of this complex in solution: AlyO2(OH)14(H20)103+ ( denoted as AIT). The complex contains 1 atom of aluminium in a tetrahedral coordination and 6 atoms in an octahedral coordination with respect to oxygen [5]. For diluted salt solutions ([AI(III) < 0.3 mol/l), as aa increases, a fraction of (x (All + A12) decreases and a fraction of the key complex ix(All3) increases. The limiting value is 2.3. When a~ is higher than the limiting value, sol particles (SP) of hydroxide begin to form from the key PHC of AI(III). This is evidenced by an increase in the relation between longitudinal and cross times of nuclear relaxation of water deuterium (T1/T2), a broadening of the 3sC1NMR line and by a decrease in the fraction of complex Al~3 in solutions (Fig. 1). The products of polycondensation of aqua ions of AI(III) are different in concentrated solutions. Here, AI7 is the key complex. However, polycondensation processes proceed in a similar way [6]. Af,

I

v,

~~-"~'-~-~ ~ "-vTi - - - v of.,

1

"-5

"-"

i

1 ' I +------%

,

0

.

2.,..,.f-

.

i

!

"

,

1

.

Hz I

.Ip

9

Tq, T 2 ,ITIS 50O

1 Ill I/

.

.

.

,

2

ee

Figure 1. Polycondensation in solutions at [A1C13] = 0.2 mol/l and parameters of N M R spectra as a function of a~ - [OH-]/[Al(III)]: 1. tx (Air + A12); 2. ct(Alt3); 3. Af(35C1); 4,5. 2D(T~, T2). pH = 3.0-3.7 (I), 3.7-4.0 (II) and 4.0-4.2 (III).

595 When a base is added to an aluminium salt solution, polycondensation may follow the routes (1) Al(H20)63+ ~ Alz(OH)2 (H20)84+ +--~A1,304(OH)24(H20)i27+ --+ sol particles (SP~) and (2) AI(H20)63+ ~ A12(OH)2(H20)84+ 4+ A1702(OH)14(H20)lo 3+ --+ sol particles (SP2). Schemes (1) and (2) exhibit the limiting cases for diluted and concentrated solutions of AI(III) salts. In the general case, both KPHC of AI(III) exist on polycondensation in solutions. The key complexes of AI(III) form true solutions. At the stage preceding the formation of sol particles, the key complexes of AI(III) strongly bonded with anions of the mother salt pass together into a volume of SP. These anions are structural elements of SP and can not be removed by sediment washing. The anions and SP surface form a weakly bonded layer of counter ions, which is in the state of quick exchange with solution anions (a broadening of line Af(35C1) in Fig. 1) and can be easily removed on precipitate washing. It was found that the chemical nature of the initial salt anions affects essentially both the direction of polycondensation processes of aqua cations and the whole ensuing evolution of hydroxides on their ageing. Thus, sulfate ions hinder formation of Al13 complexes, and can completely suppress it when the concentration is sufficiently high. Anions that can enter the first coordination sphere of the aqua cation considered affect most essentially the direction of polycondensation processes and the structure of PHC formed. The effect of anions on the polycondensation of aqua ions of AI(III) and the properties of gels decreases in a series: SO42- >> CI- = NO3- > C104-. Using the RDA technique, calculations of simulated models [5] and SAXS [7], we have shown that AI~3 and AI7 pass from the solution into gels of AI(III) amorphous hydroxides with no detectable changes in their structure and size (18 and 15 .~, respectively) at pH = 4-10 and T < 50 ~ Hence it follows that (1) a minimum, theoretically attainable size of sol particles is determined by sizes of key complexes of precipitated metals, and (2) gels of AI(III) hydroxides "inherit" the structure of key complexes of AI(III). Note that an interaction between several key complexes yielding particles of AI(III) hydroxide sols 30-50 A in size is more typical. In later works, the authors used [8-10] our methodology to study experimentally polycondensation of aqua ions of AI(III) and reproduced a number of our results. At the close of gel formation at pH - 7-10, T > 60 0C, a series of complex physicochemical processes begins spontaneously. Crystallization of AI(III) hydroxides is the most important one. The process is accompanied by significant changes in the chemical and phase compositions, particle dispersion, specific surface, and porous structure of hydroxides. We have established the mechanism of crystallization of poorly soluble hydroxides. Crystallization proceeds in a volume of every primary particle via the rearrangement of its polymer structure and growth of secondary crystals and by oriented accretion of the primary, partially crystallized particles over the similar faces of the other particles [1], rather than via dissolution of highly disperse primary particles. The phase composition of crystallization products is determined by the nature of key complexes yielding an amorphous hydroxide gel. Thus, the gels obtained from Al~3 or A17 transform into bayerite and pseudoboehmite, respectively, during ageing, the other conditions being the same [11].

596

2.2. Hydroxides of Fe(llI) The main stages of formation of gels of amorphous hydroxides of Fe(III) are (3) Fe(H20)63§ ~ Fe2(OH)2(H20)84§ ++ Fep --> sol particles (primary particles, PP) --+ disordered aggregates of PP (gel). The key complexes of Fep composed of double chains of octahedrons [FeO6] are joint at the edge (Fig. 2). According to the number of oxo and hydroxo groups, and coordinately bonded water per 1 atom of iron, the composition of Fep complexes is FeOOH H20. 9 Antiferromagnetism of PHC of Fep indicates a high ordering in the state of Fe(III) cations in complexes. When polynuclear hydroxo complexes Fep transfer to the volume of sol particles, their structure and composition do not practically change. Gels of amorphous hydroxides of Fe(III) are composed of disordered aggregates of PP with a narrow size distribution (35-45/~). The size of PP remain almost constant (40 A) under a wide range of precipitation conditions: pH = 4-12, T = 25-100~ and [Fe(III)] = 0.01-2.0 g-ion/1. The mechanism of formation of amorphous hydroxides of Fe(III) from PHC of Fep permits one to predict that the volume of primary particles, visually observed as a unit, contains some substructure. Analyzing the curves of radial atom distribution, and using the bright image and dark image of TEM techniques, we have discovered local, structurally ordered regions 10/k in size. The atom distribution in such regions is characteristic of cz-FeOOH (goethite) [12]. These ordered regions are not oriented relative to each other in the volume of primary particles, which predetermines the possibility of micropore formation on dehydration of Fe(III) hydroxides.

A

OH2

I~o-

A

i I

~o

H

I I

OH2

~''" ' ~ o

H

OH2

o

1

H

_o....

-~I" i ~ o / .

H O

i H~O--F 9 -- i

1 I

l i I

H

OH2

0 -- ~ ~c~O

p

.

H

OH2

H

OH2

6 H

Figure 2. A structure fragment of key complex Fep. A: the projection of complex Fep along its axis. As in the case of Al(III) hydroxides, the anions of the initial salt participate in polycondensation of aqua ions of Fe(III) and SP formation yielding strong and weak bonds. The anions strongly bonded with the key Fep complexes pass to a SP volume, and weakly bonded anions form a layer of counter ions on the SP surface. On coagulation of sol particles, the latter carry away such anions to the gel volume. The nearest order of atom distribution in the structure of key Fep complexes is similar to that of goethite crystals. So, we have predicted and experimentally proved that on ageing of such gels, precisely hetite forms but not some other structural

597 modification of Fe(III) hydroxides [12,13]. On the basis of the theoretical concept on the mechanism of crystallization of poorly soluble hydroxides, we have developed an approximate kinetic model: do~/dt = K(1 - c~)2 (c~ is a fraction of the crystal goethite phase). This model adequately describes all experimental data on gel ageing in alkaline media at 25-90 ~ Parameters Ko = 7.085 10 ~ and E = 76.8 + 2.6 kJ/rnol were determined elsewhere [13]. After drying at 110 ~ gels of Fe(III) hydroxides (xerogels) composed only of disordered PP aggregates have a surface of 400-500 mZ/g and micropores 10-20 A in size. In the course of ageing, rough secondary crystals (SC) are formed and accumulated in gels, which leads to appearance of meso- and macropores in xerogels and a decrease in their surface area down to 20-50 m2/g. The surface area of such samples is proportional to the content of highly disperse PP of amorphous Fe(III) hydroxide. Thus, a knowledge of the mechanism and kinetics of crystallization of amorphous Fe(III) hydroxides permits one to manufacture (using one technological scheme) a wide range of hydroxides and even oxides with the following parameters: S = 20-500 m2/g, the overall pore volume is 0.1-1.0 cm3/g, size of pores ranging from 10 to thousands A. 3. F O R M A T I O N PROCESSES

OF BINARY HYDROXIDES

USING SOL-GEL

3.1. Hydroxides of AIOII) - Fe(IH) The results above indicate that the mechanism of formation of amorphous poorly soluble hydroxides of AI(III) and Fe(III) has much in common, although the composition and structure of key complexes have a number of distinctions, resulting from a difference in the chemical properties of the precipitated aqua cations. Both facts are important for understanding the mechanism of formation of hydroxides from mixed solutions of initial salts of AI(III) and Fe(III). Because the PHC formed on separate polycondensations of aqua ions of AI(III) and Fe(III) differ essentially in their composition and structure, the primary particles comprising both cations should not form in mixed solutions. The data on the NMR studies, NM relaxation and magnetic susceptibility (7;) show (Table I) that polycondensation of aqua ions of Fe(III) begins first in mixed solutions of Al(III) and Fe(III) salts. It proceeds up to formation of sol particles of Fe(III) hydroxide. Then polycondensation of aqua ions of AI(III) begins. This is proved by the facts such as decrease in the relative magnetic susceptibility (1), increase in a line width Af (3sC1) and the T~/T2 ratio for ~H (2), decrease in a relative intensity (I) of a~ 35C1 N M R line (3), and absence of polycondensation processes of AI(III) aqua ions (4), with a ranging from 0 to 0.9. As in the case of individual solutions, the polycondensation of every component of mixed solutions proceeds via the same stages and intermediate complexes. Sol particles of Fe(III) hydroxide contain no detectable amount of AI(III) ions and vice verse. When a molar ratio is AI/Fe > 1, hydroxo complexes of AI(III) stabilize sol particles of Fe(III) hydroxide . The latter begin to coagulate when the larger part of AI(III) particles converts to sol particles of AI(III) hydroxide. At AI/Fe <_ 1, a sol of Fe(III) hydroxide is not stabilized and begins to precipitate even before the polycondensation of aqua ions of AI(III). Thus, the interaction of precipitated components in the AI(III)-Fe(III) system is at the level of primary particles of the individual hydroxides of AI(III) and Fe(III) within the

598 framework of mixed aggregates of the primary particles. For A1/Fe > 1, the composition of mixed aggregates (forming gels) corresponds to a ratio of precipitated metals in the initial solutions. When AI/Fe <_ 1, two types of aggregates are precipitated. The first is enriched with PP of Fe(III) hydroxide, the other - with PP of AI(III) hydroxide [14]. On ageing, gels of AI(III)-Fe(III) hydroxides undergo the processes as follows: (1) dehydration, (2) partial crystallization in the volume of PP, (3) decomposition of the mixed aggregates of primary particles, yielding segregation and interaction of particles with a similar chemical nature and structure, (4) formation and growth of secondary crystals via the mechanism of oriented accretion. Table 1. Properties of chloride solutions with A1 : Fe = 3:1 ratio regarding aa = [OH-]/[AI(III)] + [Fe(III)]. [AI(III)] + [Fe(III)] = 0.5 g-ion/1 a?

pH

~

NMR27AI oc(Allq-A12) oc(All3)

NMR3SCI

IH- R,ms

I

Af, Hz

T~

T2

0

1.60

1.00

0.95

0

1.00

240

1.30

0.5

0.3

1.70

0.70

1.00

0

0.80

560

2.55

0.8

0.6

2.20

0.35

1.00

0

0.50

785

29.5

5.5

0.9

3.30

0.30

0.80

0

0.50

940

260

18.5

1.2

3.40

0.30

0.75

0

0.55

790

295

13.5

1.5

3.55

0.30

0.65

0.10

0.50

940

330

15.5

1.8

3.65

0.30

0.35

0.15

0.45

1165

285

14.5

2.0

3.70

0.30

0.25

0.30

0.50

1285

270

11.0

2.2

3.85

0.30

0.15

0.35

0.50

1450

240

10.5

2.4

gel

0.30

0.05

0.25

0.40

1300

160

5.5

3.2. Hydroxides of Al0II)-Cu(II) Poorly soluble binary hydroxides of two- and three-valence metals (M(II)M(III)) are very interesting as precursors for synthesis of complex oxide supports and catalysts which have, in particular, a spinel-like structure. To elucidate the mechanism of such systems formation, one should study the polycondensation of aqua cations in mixed solutions of their salts. It is well known that mononuclear hydro• complexes and dimers are dominating on hydrolysis of individual salts of M(II). For polycondensation of M(III), formation of multinuclear complexes is typical. This difference should be also observed for polycondensation of mixed solutions of salts of two- and three valence metals, and consequently, at the stage of formation of gels of coprecipitated hydroxides. Consider sol-gel processes proceeding on polycondensation of mixed solutions of salts AI(III)-Cu(II) (Table 2). At the initial stage of polycondensation, aqua cations form labile aluminium-copper complexes with an existence of about ms. This is

599 indicated by a five-fold increase in the line width Af(All) at m = 0-0.5, which is associated with a chemical exchange of 27A1nuclei between the states: (4) AI(OH) 2+ + Cu(OH)l++-> AI(OH)2Cu 3+ Further the nuclei rearrange themselves into some other complexes. At the stages preceding formation of gels of binary hydroxides, the key complexes A113 and AI7 , and heteropolynuclear hydroxocomplexes (HPHC) Alp-Cu(II) are formed. It is interesting that at pH ranging from 3.0 to 5.0, the gels of such hydroxides are formed by aggregation of the above key complexes, bypassing the stage of sol particle formation. This is evidenced by constant Af(14N) and T1/T2 at ~ = 0 to 2.7 (Table 2). On ageing of gels at pH = 6.0-11.0 and T = 25-90 ~ All3, AI7, and Alp-Cu(II) complexes produce bayerite, pseudoboehmite, and hydroxoaluminate of Cu(II), respectively, with a structure of hydrotalcite. Table 2. N M R and ESR spectra of mixed salt solutions at [A10NIO3)3] = 0.3 and [Cu(NO3)2]= 0.025 mol/l regarding ee = [OH-]/[AI(III)]. a~

pH

NMR 14N, Af, Hz

NMR27AI

EPR Cu(II), I

oc(Al,+Alz) oc(Alt3) Af(All),Hz

2D- R, ms

Tt

T2

0

2.80

1.00

0

56

19

1.00

308

315

0.5

3.55

0.85

0

276

17

1.20

-

325

1.0

3.65

0.75

0.05

318

-

0.50

324

290

1.5

3.75

0.60

0.20

366

-

0.65

340

340

2.0

3.90

0.30

0.45

418

17

0.70

-

325

2.2

3.90

0.30

0.40

452

-

0.70

350

340

2.5

4.05

0.05

0.70

500

-

0.55

360

355

2.7

4.10

0.01

0.80

753

21

0.55

376

345

4. C O N C L U S I O N In this report, we have considered the sol-gel processes occurring on the formation of individual and binary, poorly soluble hydroxides, materials-precursors of supports and catalysts, which contain AI(III), Fe(III) and Cu(II). Sol particles and hydroxide gels form from the key complexes of the precipitated metals. We have established a very important phenomenon of "inheritance" of the structure of key polynuclear hydroxocomplexes by hydroxides, which determines a genetic relation of the nearest order between gels of amorphous hydroxides and the structure of the corresponding PHC. The structure, chemical composition, and arrangement of the functional atom groups of the key PHC carry an information about

600 the evolution of amorphous hydroxides on ageing up to the point when they transform to the crystal state. The number of crystal phases formed on ageing corresponds to the number of structurally and chemically detectable versions of amorphous hydroxides in precipitates. When one knows the structure of the key PHC and their concentration in the parental solution, it is possible to predict quantitatively a phase composition of the ageing products.

References 1. 2. 3. 4.

R.A.Buyanov and O.P.Krivoruchko, React. Kinet. Catal. Lett.,35(1987)293. G.Johanson, Acta Chem. Scand., 16(1962)403. G.Johanson, Arkiv. Kemi, 30(1963)321. J.W. Akitt, N.N. Greenwood, B.L. Khandelwal and G.D. Lester, J. Chem. Soc. Dalton Trans.,No. 5(1972)604. 5. T.A. Kriger, O.P. Krivoruchko, L.M. Plyasova and R.A. Buyanov, Izv. SO AN SSSR, Set. khim. nauk, No. 7(1979) 126 (in Russian). 6. M.A. Fedotov, O.P. Krivoruchko and R.A. Buyanov, Zh. Neorg. Khim. 23(1978)2326 (in Russian). 7. O.P. Krivoruchko, V.N. Kolomiichuk and R.A. Buyanov, Zh. Neorg. Khim., 30(1985)306 (in Russian). 8. J.Y. Bottero, J.M. Gases, F. Fiessinger and J.E. Pokier, J. Phys. Chem., 84(1980)2933. 9. D. Muller, W.Gessner, S.Schonherr and H. Gorz, Z. anorg, und allg. Chem.,483(1981)153. 10. R. Bertman, W. Gessner, D. Muller, H. Gorz and S. Schonherr, Z.anorg. und allg. Chem., 525(1985)14. 11. O.P. Krivoruchko, R.A. Buyanov, M.A. Fedotov and L.M. Plyasova, Zh. Neorg. Khim.,23 (1978) 1798. 12. T.A. Kriger, O.P. Krivoruchko and R.A. Buyanov, React. Kinet. Catal. Lett., 24(1984)401. 13. O.P. Krivoruchko, V.V. Malahov, A. Ermakova, R.A. Buyanov and L.F. Lokotko, Kinetika i Kataliz, 28(1987)442 (in Russian). 14. M.A. Fedotov, O.P. Krivoruchko, A.V. Golovin and R.A. Buyanov, Izv. AN SSSR. Ser. khim., No.2(1977)473 (in Russian).

1998 Elsevier Science B.V. Preparation of CatalystsVII B. Delmon et al., editors,

601

TECHNIQUES FOR PREPARATION OF MANGANESE-SUBSTITUTED LANTHANUM HEXAALUMINATES A. G. Ersson, E. M. Johansson, and S. G. J~irfts KTH - Royal Institute of Technology, Chemical Technology, S-100 44 Stockholm, Sweden.

ABSTRACT Manganese substituted lanthanum hexaaluminates (LaMnAll~O~9) have been prepared by two different co-precipitation techniques, i.e. precipitation as hydroxides and carbonates, and by a sol-gel technique. Pore size distribution after calcination up to 600 ~ could be controlled with the use of a modifier, acetyl-acetone (Acac), that was added to the alkoxide solution during the sol-gel synthesis. Two different drying techniques were used in the test, oven drying at low pressure yielding xerogels and supercritical drying yielding aerogels. The crystal-phase, surface area and pore size were compared after consecutive calcinations at 600 ~ 1000 ~ and 1200 ~ The samples prepared with the carbonate and sol-gel methods showed pure hexaaluminatephases at 1200 ~ The highest thermal stability, 28 m2/g at 1200 ~ was found for the aerogel samples. It was shown that a high initial surface area did not correspond to a high surface area after calcination to high temperatures. To retain a high surface area at high temperatures, an initial broad pore size distribution was needed. Small pores, below 50 A, seemed to collapse more readily than larger pores and should be avoided if high thermal stability is desired. The sol-gel technique combined with supercritical drying gave the most promising material, although the carbonate method also showed promising results. 1. INTRODUCTION High-temperature catalytic combustion have been shown to be a promising technique to reduce emissions of nitrogen oxides, carbon monoxide and hydrocarbons from combustion processes [1]. One of the main hurdles to overcome is to develop high temperature stable catalysts that will withstand temperatures above 1000 ~ One of the most promising groups of materials are hexaaluminates. The hexaaluminate structure consists of blocks of close packed aluminium oxide with spinel structure separated by a large cation [2]. This will suppress crystal growth along the c-axis, giving rise to thin plate-like crystals and thereby maintaining a material with high surface areas even at high temperatures, e.g. above 10 mVg at 1200 ~ The large cation can be an alkali, alkaline earthmetal or rare earth-metal. The aluminium ions could be substituted by transition metal ions with similar radii and charge. It has been shown that the choice of both ions can effect the catalytic activity and the thermal stability of the resulting substituted hexaaluminates. Manganese substituted lanthanum hexaaluminate, LaMnAI~IO~9, have shown to have both high activity and thermal stability [3-4].

602

There are several methods for preparation of substituted hexaaluminates, the most common ones are different co-precipitation methods and sol-gel methods. In the co-precipitation methods the components are precipitated as insoluble metal salts, e.g. hydroxides and carbonates, often at a high pH. The amorphous precipitate is then calcined to allow the crystallisation of the final compound. The sol-gel methods represents a very versatile preparation route for substituted hexaaluminates [5] with which most compositions can be prepared from alkoxide precursors. With the use of modifiers like acetic acid or acetyl-acetone (Acac), the rate of hydrolysis can be controlled [6], and hence it is possible to control the pore structure and morphology of the precursor compound, which are of importance for the thermal stability of the final hexaaluminate material. The effects of Acac on cerium isopropoxide have been described by Sanchez er al. [7]. Low additions of Acac, i.e. Acac/Ce ratio of 0.15, would give a combination between a polymeric network and a precipitate, a so called closo structure. At Acac/Ce=0.5 a branched polymeric network is formed, and at Acac/Ce=l molecular clusters are formed. The objective with the present work was to compare different techniques for preparation of manganese substituted lanthanum hexaaluminates to be used as high temperature combustion catalysts. Carbonate and hydroxide precipitation methods were compared with a sol-gel method. Moreover, the effect of additions of Acac was studied and supercritical drying was compared to oven drying for the sol-gel samples.

2.

EXPERIMENTAL

2.1. 2.1.1.

Catalyst preparation THE CARBONATE-METHOD

In this method the intermediate precursor mixture contained metal carbonates, hydroxycarbonates and aluminium hydroxide. The method used was similar to the one described by Groppi et al [8,9]. Ammonium carbonate was dissolved in deionised water and pH was adjusted to 9.0 by additions of ammonia. Aluminium, lanthanum and manganese nitrates were dissolved in deionised water and were added slowly to the carbonate solution under vigorous stirring. The pH was kept constant by additions of ammonia. The precursor mixture formed a whitish precipitate. The precipitate was separated from the excess liquid by centrifugation and washed twice with acetone to remove any impurities. The precipitate was dried at reduced pressure for about 8 h. The dried precipitate was then crushed to a powder and heated to 300 ~ for 1 h, to remove eventual nitrates and ammonium compounds in the sample, and then to 600 ~ for 4h. 2.1.2. THE CO-PRECIPITATIONAS HYDROXIDES The technique used was similar to the one used by Marti et aI [10]. The intermediate precursor compound was an amorphous mixture of insoluble metal hydroxides. A mixture of isopropanol and ammonia with a pH of 8.5 was prepared. Stochiometric amounts of metal nitrates were dissolved in 0.1 M EDTA in isopropanol. The metal nitrate solution were then added slowly, to the vigorously stirred isopropanol/ammonia solution. The pH was kept constant by small additions of ammonia. The precipitate was then treated in the same way as in the carbonate method.

603 2.1.3. THE SOL-GEL METHOD: The precursor compounds for the sol-gel syntheses were aluminium isopropoxide, lanthanum nitrate and manganese nitrate. Acac was added to some of the batches in amounts corresponding to an Acac/Al-ratio from 0.2 to 1. The metal nitrates were dissolved in a water/isopropanol solution. The aluminium isopropoxide powder was dissolved in warm, 70 ~ isopropanol together with eventual Acac. The metal nitrate/water solution was then added to the aluminium isopropoxide solution. The formed gel/sol was allowed to age for 12 h and was then centrifuged and washed twice with isopropanol. In one case, xerogel-0a, metal acetates were used instead of metal nitrates. For the xerogel-1 p the pH was adjusted to 8.5 after adding the nitrate/water solution. The different samples are shown in Table 1. The xerogel samples were dried in a vacuum oven at 70 ~ for 8 h at 0.2 atrn pressure. The aerogel samples were dried in isopropanol under nitrogen atmosphere at 70 bar and 255 ~ Both xerogel and aerogel samples were then heated to 300 ~ for 1 h and 600 ~ for 4 h. Table 1 The different preparation and drying techniques for the samples. Sample

Preparation technique

Carbonate Carbonate Hydroxide Hydroxide Xerogel-0a Sol-gel x Xerogel-0 Sol-gel Xerogel-0.2 Sol-gel Xerogel-0.5 Sol-gel Xerogel- 1 Sol-gel Xerogel- 1p Sol-gel 2 Aerogel-0 Sol-gel Aerogel-0.5 Sol-gel Aerogel-0.7 Sol-gel . . . . . . . 1 acetate precursors 2 pH adjusted to 8.5 after synthesis 2.1.4.

Acac/Al-ratio

Drying process

0 0 0.2 0.5 1.0 1.0 0 0.5 0.7

Vacuum oven Vacuum oven Vacuum oven Vacuum oven Vacuum oven Vacuum oven Vacuum oven Vacuum oven Supercritical Supercritical Supercritical

'"'

CALCINATION OF THE POWDERS.

The powders were consecutively calcined in a tubular furnace at 1000 ~ for 4 h, 1200 ~ for 4 h and some to 1400 ~ for 4 h in air. Samples were taken at each temperature for analysis.

2.2.

Characterisation.

Surface area and pore size distribution were measured on a Micromeritics ASAP 2000/2010 using N2 adsorption/desorption at liquid N2 temperature. The crystal-phases were identified by powder X-ray diffraction (XRD) using a Siemens Diffraktometer 5000 scanning 20 from 10~ ~ using monochromatised Cu-K~ radiation. The crystal-phases were identified using JCPDS-files.

604

3. 3.1.

RESULTS AND DISCUSSION Crystal structure

The XRD-patterns showed that almost all the prepared samples remained amorphous at 1000 ~ Only the xerogel samples with an acac:A1 ratio = 1 showed a crystalline structure, corresponding to the LaAlllO~8-phase, at 1000 ~ At 1200 ~ all of the samples had crystalline phases. The crystal phases for the different samples are shown in Table 2. The hydroxide and the xerogel-0a samples were the only ones which did not show pure hexaaluminate phases at 1200 ~ This was probably due to poor precipitation or an uneven distribution of lanthanum ions in the precursor mixture. The xerogel-1 was almost entirely consisting of LaAl~lOl8, probably due to poor incorporation of manganese ions in the small clusters formed. This could be circumvented by increasing the pH, which allowed precipitation of manganese hydroxide, as can be seen for xerogel-1 p. Table 2 BET surface area after drying and calcination at 600/1200 ~ and crystal phases at 1200 ~ HLM=LaMnAllxOI9.a, HLA-LaAll IOf 8, PLA=LaA103, n . d . - not determined. Method

BET surface area [m2/g] 600 ~ 1000 ~ 1200 ~

Carbonate 229 70 Hydroxide 109 35 Xerogel-0 80 33 Xerogel-0a 128 42 Xerogel-0.2 269 1 Xerogel-0.5 283 11 Xerogel-1 16 3 Xerogel- 1p 168 n.d. Aerogel-0 408 64 Aerogel-0.5 421 91 ....... Aer0ge!-0.7 ............................ 3.9.1............................82

18 8 17 16 n.d. <1 n.d. n.d. 29 28 ........................... 28

Crystal phase (XRD) at 1200 ~ HLM PLA, HLA HLM HLA, PLA HLM HLM HLA HLM HLM HLM ............... HLM ...........

3.2. Surface area and pore size The surface areas after calcination at 600 ~ and 1200 ~ for the different samples are shown in Table 2. As can be seen, the highest surface areas were achieved for the aerogel samples. 3.2.1. THE EFFECT OF ACAC. When no Acac was present in the solution a whitish precipitate formed. If Acac was present corresponding to an Acac/Al-ratio
605

ero e,-0. ~14 ~-" 12 l0 T , ~ " \ 6 Jm

~Xerogel-0

i ~',, / X e r ~ ' ~. ", "

4 .

~

.

r

. .

. . . b

. . . . . . . .

A . . . . . . .

0 0

100

200 300 Pore diameter [,~]

400

500

Figure 1. The surface area versus pore diameter for different additions of Acac for the xerogel samples after calcination at 600 ~

100

-

90-

80 70 60 '- 5 0 40= 3020 10

,.

Aerogel-0.7 Aerogel-0.5 Aerogel-0

i .,..;,

! N

.-

i

0

200

...,h ... :.... ,." "-,-,,

i

i

400 60g Pore diameter [A]

i

i

800

1000

Figure 2. The surface area versus pore diameter for different additions of Acac for the aerogel samples after calcination at 600 ~ The pore size distribution was also highly affected by the Acac additions, as shown in Figure 1 and 2. For the xerogel samples, without any additions of Acac, the pore size distributions are broad and very similar to the carbonate precipitate. When Acac was added, in minor amounts (xerogel-0.2), it resulted in a narrower pore size distribution with a high degree of small pores. When more Acac was added (xerogel-0.5), the pore size distribution became narrower, with almost the entire surface area in pores below 40 *. This is due to the formation of a precursor gel with a branched polymeric network. If the additions of Acac was large (xerogel-1) the pore size distribution was narrow, due to formation of very small particles or clusters. For the aerogel samples the situation was somewhat different, as can be seen in Figure 2. Here the aerogel-0.5 yielded a material with a larger pore size than both the aerogel-0 and

606 aerogel-0.7 samples. The pore size for the aerogels is larger than for the xerogels. This difference between the aero- and xerogels is due to the special features of the supercritical drying process. It preserves the original pore structure, keeping the expanded structure of the gel, while oven drying yields a denser structure. The larger pore size of the aerogel seems to be, as will be described below, better suited to preserve a high surface area at high temperatures compared to the relative dense structure of the xerogels. 3.3.

S u r f a c e area versus pore size distribution.

The initial surface area at 600 ~ was much higher for the aerogel samples (around 400 m2/g) than for the other samples. It was also fairly high for the carbonate, the xerogel-0.2 and xerogel-0.5 (above 200 m2/g). The surface area, for all samples, showed a significant decrease when calcined at 1200 ~ cf. Figure 3. The sintering was especially severe for the xerogel-0.2 and xerogel-0.5, which almost totally lost their surface area. This while the xerogel-0 and xerogel-0a samples, with an initial surface area of about 100 m2/g, retained a surface area of 17 m2/g. This is almost as high as the surface area retained by the carbonate sample, which initial surface area were twice as high. This implies that not only a high surface area in the starting material is needed to prepare a high surface area material at high temperatures. 1000 100 10 1 0 600

800 1000 1200 Calcination temperature [~

1400

Figure 3 The BET surface areas after consecutive calcinations at 600 ~ 1000 ~ 1200 ~ and 1400 ~ for hydroxide [], carbonate A, xerogel-0 !~, xerogel-0.5 x and aerogel-0.5 0. The large difference in thermal stability seems to depend upon the pore size of the different samples. The pore size distribution for the xerogel-0 and the xerogel-0.5 is shown in Figure 4. As can be seen the xerogel sample shows a very narrow pore size distribution with almost all pores with a diameter below 50 A, while the carbonate samples has a much larger part of its surface area in pores with a diameter above 50 ~,. After being heated to 1000 ~ the surface area found in pores below 50 A decreases dramatically while the pores above 50 A survives to a much greater extent and thereby preserves the surface area. This explains the thermal stability of the xerogel-0 samples where only around 45% of the pores have an diameter below 50 A in the initial material, while the xerogel-0.2 and xerogel-0.5 have above 85%.

607

4 0

--

l140 mVg

35--

Xerogel-0.5 (600~

"~ 30-~25 20-

r

/

Xerogel-0 (1000~

15-

Xerogel-0.5 ( 1ooo~

= 105 '/

0 0

100

i,,_- 7 ""'~ - - "~ -

200 300 Pore diameter [~]

400

-i

500

Figure 4. The surface area versus pore diameter for xerogel-0 and xerogel-0.5 samples after sintering at 600 ~ and 1000 ~ 4.

CONCLUSIONS Manganese substituted lanthanum hexaaluminate (LaMnAl~1019) have been prepared by two different co-precipitation techniques, i.e. precipitation as hydroxides and carbonates, and by a sol-gel technique using aluminium isopropoxide as a precursor. The crystal-phase, surface area and pore size distribution were compared after consecutive calcinations at 600, 1000 and 1200 ~ for 4 hours in air. The crystal-phase analysis showed that the carbonate sample and all sol-gel samples were more or less amorphous up to 1000 ~ and formed pure lanthanum manganese hexaaluminate-phases at 1200 ~ except for xerogel-1 and xerogel-0a. The hydroxide sample and xerogel with acetate precursors showed impurities of lanthanum aluminate perovskite and lanthanum hexaaluminate, while the xerogel-1 showed an almost pure phase of lanthanum hexaaluminate. A comparison between the three different methods showed that the hydroxide precipitation was unfavourable, since the crystal phase was not pure and the final surface area at 1200 ~ was poor (8 mZ/g). The sol-gel method followed by supercritical drying gave the most promising materials (29 m2/g), but the results were less favourable with only vacuum oven drying (17 mZ/g). The carbonate method gave materials with comparable surface area (18 mZ/g) after oven drying, and is also relatively easy to use. The pore size distribution and surface area could be controlled by the choice of synthesis method, which influenced both the initial (600 ~ and final (1200 ~ specific surface area strongly. On the one hand, materials with an initial high fraction of small pores below 50 A and a high fraction of the initial specific surface area in these pores, i.e. xerogel-0.5 and xerogel-0.2, lost a large part of the small pores upon calcination at temperatures at and above 1000 ~ On the other hand, hexaaluminate powders with an initial pore size distribution with a large fraction of pores between 80 and 600 A, such as the carbonate and the aerogels, could preserve these pores and consequently retain higher specific surface areas. The small pores seemed to collapse more readily than the larger pores after high temperature calcination. Hence, material with sharp small pore size distributions should be avoided and materials with

608 broad pore size distributions in the 80 to 600 A range should be favoured, if high surface areas at and above 1200 ~ is desired. For the series of sol-gel samples, acetyl-acetone (Acac) was used to alter the reactivity, i.e. the rate of hydrolysis, of aluminium isopropoxide, and thereby change the morphology of the powders formed. The study showed that additions of Acac could be used to control the pore size distribution of the low temperature materials (600 ~ In addition to the Acac studies, a comparison between vacuum oven drying yielding xerogels and supercritical drying yielding aerogels was made. It showed that supercritical drying gave materials with higher surface areas, at both low and high temperatures, and a broader pore size distribution of the initial powders. It should be emphasised, that the methods described here to control and prepare aluminatype materials with a very narrow and small pore size distributions could be advantageous for other catalytic processes, e.g. when selectivity and not high-temperature stability is important characteristics. In conclusion, sol-gel synthesis with supercritical drying gives the most promising surface areas. In addition, the carbonate method showed promising results. The work described here has showed how the pore size distribution at low temperatures can be controlled during the first preparation step and how this can be used to improve the high temperature stability of these materials. 5.

ACKNOWLEDGEMENT Funding from the European Union, contract JOR3-CT96-0071, and Foundation for Strategic Research, CECOST project PROJ9701-SOT, are gratefully acknowledged.

REFERENCES [ 1]

M . F . M . Zwinkels, S. G. J~ir~s, P. G. Menon, T. A. Griffin, Catal. Rev.-Sci. & Eng., 35, (1993) 319. [2] H. Inoue, K. Sekizawa, K. Eguchi, H. Arai, J. Solid State Chem. 121, (1996), 190. [3] E.M. Johansson, S. G Jg.r~s, Preprints ACS Symp., San Francisco, 42(1) (1997) 146. [4] K. Eguchi, H. Takahara, H. Inoue, K. Sekizawa, H. Arai, Preprints ACS Symp., San Francisco, 42(1) (1997) 180. [5] M. Machida, K. Eguchi, H. Arai, J. Catal., 103(2) (1987) 385. [6] C.J. Brinker, G. W. Scherer, Sol-gel science: the physics and chemistry of sol-gel processing, Academic Press, San Diego, 1990. [7] C. Sanchez, F. Ribot, S. Doeuff, Inorganic and Organometallic Polymers with Special Properties, Laine R. M. (ed.), Kluwer, Dordrecht, 1992, 267pp. [8] G. Groppi, C. Cristiani, P. Forzatti, J. Mat. Sci., 29 (1994) 3441. [9] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, Appl. Catal. A. 104 (1993) 101. [10] P.E. Marti, M. Maciejewski, A. Baiker, "Preparation of Catalysts IV", Poncelet, G. et. al. (Eds.), Elsevier, Amsterdam, 1995, 617pp.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

609

F o r m a t i o n of the porous structure of t i t a n i u m dioxide prepared by a sol-gel method. V. Yu. Gavrilov, G. A. Zenkovets and G. N. Kryukova Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk, 630090 Russia. Effect of the precipitate conditions and ageing of titanium dioxide hydrogel on the formation of xerogel porous structure was investigated. Results obtained demonstrated that hydrogel precipitation with ammonia solution in a sol-gel method of hydrogel TiO2 synthesis, independently from pH, provides limiting dispersion of the primary oxide particles (5 nm). Porosity and the character of the porous volume distribution are determined by ions adsorption at the stage of hydrogel synthesis. Xerogels, received close to IEP, have the big porosity (0.78-0.79) and heterogeneous porous structure. A deviation of pH from IEP results in the formation of similarly fineporous xerogel. Hydrogel precipitated by NaOH solution leads to the formation of xerogels with a greater porosity (0.84-0.85) and smaller surface value. During ageing of the TiO2 hydrogel, firstly, the appearance of anatase results to disordering the skeleton of amorphous titania, decreasing the porosity and raising the xerogel strength. Long ageing is accompanied by aggregation of crystalline particles, resulting already to decrease of mechanical strength. 1. INTRODUCTION Titanium dioxide is widely used in catalysis, adsorption, an ion exchange, electronic engineering and chemistry of sensors [1, 2]. Such variety of application areas is caused by a possibility to vary chemical and phase compositions and mechanical properties of the xerogel. In turn, this make possible due to a wide range of the initial substances, changeability of the compounds formed, great number of the factors influenced on the process of hydrogel formation and routes of the synthesis. The greatest opportunities are given by a sol-gel technology that enables to perform a definite stages of formation of the primary particles of oxide phase upon given conditions, to establish control over processes of their aggregation and crystallization. The subject of this paper is to study a mechanisms of the formation of porous space of the titania xerogel prepared by a sol-gel technology. 2. EXPERIMENTAL

Sol-gel technology implies time or spatial separation between stages of the formation of the oxide phase precursors and appearance of the primary particles of the phase in sol and/or aggregated hydrogel forms. In the case of titanium dioxide prepared by precipitation from TIC14 solutions this condition is obeyed due to high velocity of the heterogeneous

610

nucleation in comparison with the different oxides of metals of the IV group [3]. This leads to the critical displacement of the equilibrium in the zone of hydrolysis in the direction of the formation of particles of a definite phase. Titania hydrogels were prepared via precipitation of TIC14 solution with a 25 % water solution of NH4OH or 30 % solution of NaOH at constant pH varying from 2 to 9.5 at 70 ~ C [4]. In a number of cases solutions of LiOH and KOH were used. Deposits received were washed out up to the absence of CI, NH4+ or Na + ions in wash waters, then dried at 150o C for 12-14 hours. Concentration of ions contained in xerogels was determined by chemical methods. Hydrogel precipitated by NH4OH solution at pH 7 was exposed to ageing in water at 80~ for a definite time. Porous structure of xerogels was studied by method of low temperature adsorption of nitrogen with Digisorb-2600 Micromeritics. To evaluate volumes of macro- and supermacro pores in samples in question mercury porosimetry was applied using PoreSizer-9300 Micromeritics. True density 9 (g/cm3) was determined with He on the Autopycnometer-1320 Micromeritics. Total pore volume Vz (cm3/g) and porosity a (cm3/cm 3) were calculated on the base of data of apparent and true densities. Mechanical strength of granules on the end-face crushing P~ (kg/cm 2) was measured with MP-2C instrument. The X-ray analysis was performed with DRON-1.5 diffractometer on CuK~ radiation. Content of anatase in xerogel was determined by the (200) reflection. Transmission electron microscopic study was carried out on JEM-2010 electron microscope (resolution power 0.14 nm, accelerating potential 200 kV). 3. R E S U L T S AND DISCUSSION The formation of titania porous structure begins from the moment of the formation of primary particles (globules), the size and morphology of those define the initial value of a xerogel specific surface area, and character of their packing affects the volume and pore sizes distribution. This statement was advanced for corpuscle systems [5], therefore we shall apply mechanisms of the appearance and change of particles of the definite phase and their subsequent aggregation to mechanisms of the formation of titania texture. Table 1 Data in the porous structure of the samples precipitated with NHgOH. pH 2 3.5 5 6 7 8 9.5

[NH4+] [CI-] i:) S, ~:, ~, y, wet.% g/cm! m2/g cm3/cm 3 ........... 0.04 9.05 3.15 393 0.44 24 6.5 0.07 6.76 3.17 398 0.56 15 4.7 1.33 0,21 3,03 414 0.79 3 1.6 2.03 0.20 2.98 415 0.76 4 2.6 2.02 0.20 2.98 396 0.74 6 2.7 2.67 0.03 2.96 392 0.59 15 3.8 2.65 0.02 2.90 417 0.47 - 3.5

Table 1 gives data on the porous structure of xerogels prepared by precipitation with NHeOH solution. As one can see from this Table, value of the specific surface area (S) does

611 not practically depend on pH of precipitation and makes about 400 m2/g; the size of primary particles is close to one extremely possible for oxides (5 nm) [6]. At the same time, porosity changes at a great extent and reaches the maximum (0.78-0.79) at pH 5-5.5 that corresponds to the location of an isoelectric point (IEP) of TiO2 dispersion [7,8]. Thus, porous structure of the xerogel possesses heterogeneous character. When pH of precipitation deviates from IEP and sorption of ions on a surface of titania particles increases correspondingly, growth of the compression degree of the interparticle volume during process of the transformation of hydrogel into xerogel takes place. Value of the compression degree may be written as: = Ex/Eh (lX/lh) 3

(1)

where ~ and 1 are porosity and linear size of granules, x and h indexes concern to the xerogel and hydrogel, respectively. Most essential compression occurs when NH4OH is used as a precipitate, whereas the application of NaOH, KOH or LiOH is accompanied by decrease of the surface area and, as a rule, raising the porosity, in comparison with the first case [9]. Appearance of the superficial charge of colloidal particles as a result of the sorption of ions on the particle surface as well as following increase of the charge due to the deviation of pH precipitation from IEP prevents the aggregation of particles under influence of van-der-vaals forces and promotes the hydrogel condensation under influence of capillary forces at drying. As a result, monomodally mesoporous xerogel is formed. Experimental dependence of the compression degree of porous space 13on the calculated value of the superficial charge density of dispersion particles Y. This may be written as: 7 --- ([A1 - [K])/S

(2)

where [A] and [K] - equivalent concentration anions (CI) and kations (NH4§ on a surface, is shown in Fig. 1. Good correlation between these parameters allows to argue on their internal relationship. Table 1 gives textUral (13) and calculated (7) parameters of TiO2 prepared by precipitation with NH4OH at changing pH. 25--

Figure 1. Relationship between compression degree of the interparticle volume (13) and the superficial charge density of dispersion particles (Y) for TiO2 samples precipitated with NH4OH.

13

20 m

15.,..-

2"

10--

5

0

"

I ........

2

I ....

I" 4 3,

I

I 6

I

Use of NaOH, KOH or LiOH as precipitates results in the formation of bimodal mesoporous structure as evident from Fig. 2. In our opinion it may be associated with nonuniformity of the charge distribution over the surface and smaller value of charge density 7, in comparison with a case of NH4OH

612

0.12

application, that promotes aggregation of particles.

--

-2

0.08

--

- -@-

- 3

- - 0 -

- 4

Figure 2. Curves of incremental pore volume distribution for TiOz samples precipitated at pH 3 with : 1- NH4OH; 2NaOH; 3- KOH; 4-LiOH.

It should be noted, that the general case of non-uniform distribution of superficial charge in dispersions and influence such heterogeneity on properties of double electrical layers of colloidal particles is 0.00 considered recently by authors [10]. It 10 100 was shown that really existing D, nm heterogeneity of charge distribution produces an effect on the electrosuperficial phenomena determining behavior of colloidal particles. The authors [10] have theoretically proved, that in accordance with increase of a heterogeneity degree of distribution ~5, define by them by relative meaning of a root-meansquare deviation of a charge value Og from its homogeneous part o0 do additional a multiple moments appear. In this case 8 value may be written as: 0.04

--

- {(~g)2/(~s0)2}0,5

(3)

Occurrence of new moment of force in originally random and, apparently, similar distribution of an ensemble of particles in intermicelliar medium, results in their mutual orientation connected to compensation of induced multiple moments. And, the collective effects of behavior of particles are proportional to a square of a heterogeneity degree of a charge distribution 82. It should be emphasized, already in intermicelliar medium there are the additional factors of aggregation of colloidal particles, which can hereinafter be kept in a xerogel. Moreover, process of transfer of the low molecular forms (LMF), precursors of the TiO2, may also influence. The equilibrium concentration of LMF in the solutions of these hydrogels is higher than that of prepared with NH4OH that is proved by data of atomabsorption spectroscopy. The migration of LMF on the surface of oxide phase provides hardening the aggregates of primary particles and their preservation at the condensation during the process of hydrogel drying. Simultaneously, the reduction of the specific surface value due to the expense of LMF precipitation in the regions possessing increased chemical potential, i.e. zones of contacts between primary particles, takes place. X-ray diffraction data and measurements of true density indicate that titanium dioxide prepared in alkaline or neutral ranges of pH is amorphous whereas TiOa precipitated at sour range contains insignificant quantity of anatase. The ageing of hydrogel leads to the formation of an anatase phase with a strong transformation of the xerogel texture.

613 Table 2 Effect.0f ageing..tim9 on the texture-mechanical propert!es of TiO2 samples PT/(l-e), N ~, S, 9, Vz, ~, ~, d, (~ PT, . h ....m2/g._g/cm3., cm3/g cm3/cm3 nm kg/cm ~ kg/cmZ 0.74 0 0 20 76 1 0 396 2.98 0.94 0.69 0.023 4.5 0.005 40 128 2 1 395 2.99 0.74 0.64 0.171 4.0 0.050 196 542 3 5 396 3.10 0.57 0.45 0.358 5.2 0.165 285 520 4 10 350 3.24 0.27 0.51 0.554 6.0 0.238 230 474 5 12 320 3.40 0.31 0.52 0.581 6.4 0.248 232 484 6 16 308 3.42 0.32 0.56 0.640 6.6 0.255 154 353 7 24 296 3.48 0.37 0.58 0.662 7.0 0.253 111 265 8 34 281 3.50 0.40 Textural parameters of xerogels prepared from hydrogel with different ageing time "c (h) are given in Table 2. The weight share of anatase formed in a course of ageing (~) is calculated from the additivity of specific volumes of the crystalline and amorphous parts of the skeleton using an obvious equation: gr = (Pi-P~) Per/(Per- Pa ) Pi,

(4)

where Pi is a density of the sample in question, p.~ is a density of an amorphous phase equal 2.98 g/cm3; Pcr is anatase density equal to 3.94 g/cm3. From Table 2 it is evident that the weight share of anatase increases gradually and reaches 66 % at ageing for 34 h.. The final formation of the porous structure is observed at the stage of hydrogel drying; reduction of gel aggregation results in a stronger condensation of particles under influence of capillary forces. Therefore, all physicochemical factors affecting the drop of the degree of particle aggregation, result also in the reduction of xerogel porosity. Data given in Table 2 indicate that at ageing below 10 h gel porosity decreases and then begins to grow again. Let us analyze a reasons for the observed tendency. Formation of the anatase particles in an amorphous titania skeleton should result in destruction of initial units, because of essentially various interactions between amorphous particles (i) and between amorphous and crystalline particles (ii). In other words, the particles of anatase break off "chains" of particles consisted of amorphous TiO2, thus shortening them. As a result, more densely packed TiO2 is formed. Increase of the weight share of anatase during first 10 h of ageing is accompanied by the decrease of porosity from 0,74 to 0,45, i.e. porosity becomes close to the dense casual packing of the individual particles. In this case anatase formed may be considered as a factor responsible for the destruction of the skeleton of an amorphous phase, whereas the coalescence between anatase particles is not appreciably displayed. The opportunity of formation of aggregates is determined by a probability of the appearance of contacts between particles of the given phase, which in turn depends on the volumetric share of the latter. According to data given in [11,12], none zero probability of formation of extended cluster (including one consisting of 2-3 particles) is possible at 15-16 vol. % of the given phase, when particles are distributed uniformly over the sample volume. Hence, when the volumetric share of anatase will exceed this value, it seems reasonable to expect the aggregation between particles of crystalline phase simultaneously with increasing

614

the xerogel porosity. Table 2 dives results of the calculation of a volumetric share of anatase in samples obtained at different ageing times. For calculations following equation was used: q~- ~ Pi (l-e)/p~

(5)

Value of q~, close to one marked above, corresponds to the minimum value of the porosity (0.45). Thus, the growth of porosity at rather large ageing times is associated with aggregation of anatase particles. When ageing time lies between 10 and 34 h there are two kinds of aggregates consisting of amorphous and crystal particles in the TiO2 skeleton. Further increase of ageing time will diminish value of the share of an amorphous phase below critical meaning (15 -16 vol. %), and one could expect that aggregation of crystalline anatase particles will affect strongly the sample porosity.

0 , 0 1 3 ~ - - - - Figure 3. Curves of pore volume distribution for the fresh TiO2 sample (1) and aged samples for a definite time: 2- 3 h; 3- 10 h; 4- 34 h.

0,011

o,oooF;. / t

Curves of pore sizes distribution for flesh and ageing samples are shown in Fig.3. During o,ooa V u-~g - , ~ ~ , , . , , 1 ageing, firstly, largest pores disappear; at increasing of ageing 2 5 9 14 20 30 40 50 time up to 10 h the porous structure D, nm comes nearer to similarly porous with an average pore diameter of 4 nm. This may be considered as a good justification of the conclusion about disaggregation of the titanium dioxide amorphous skeleton at ageing. Undoubtedly, fall of the aggregation degree should results, first of all, in the disappearance of the largest pores. The ageing of gel more than 10 h leads to the growth of the total pores volume (Table 2) and some increase of their average size due to the aggregation of anatase particles. It should be noted that the anatase aggregation (at ~ < 0,3) does not result in occurrence of the large pores, i.e. the samples have similar porous structure. Data of the electron microscopy fit well with previous conclusions. Typical morphology of the particles of initial sample and those after ageing are shown in Fig. 4. As evident from these micrographs, formless aggregates of the rounded particles are typical for the fresh sample. At the first stage of ageing the separate anatase crystals are observed, whereas at long ageing time formation of the aggregates between anatase particles is registered. It is well-known that the mechanical strength of porous bodies is determined by their structure. Obviously the strength of the samples possessing nonuniform porous structure is mainly determined by the volume of the largest pores. Indeed, the large pores are the basic defects, resulting eventually in a total destruction of the material. As a rule, the mechanical strength correlates with a volume of the large pores; it is reduced with growth of their volume. For similarly porous materials the strength is defined by two independent parameters, i.e. a

"

o,oo

615 total porosity (or, in other words, number of contacts between particles in the unit of section) and strength of the individual contact. A few models which differ from each other mainly by details in the calculations of the number of contacts and their strength were proposed for the description of strength of similarly porous materials (see, for example [13]).

As already was mentioned above, strength of 1 and 2 samples cannot correlate with the total porosity; it depends on the volume of largest pores. For all other samples the specific strength is determined by the inherent strength of a material of the skeleton. Hence, the strengths of the skeletons of 3-6 samples are approximately identical, though a ratios between amorphous and crystalline phases in these samples are variable. Since the only moment, when the anatase weight share exceeds a share of the amorphous TiO2, strength of the skeleton begins to decrease. It seems likely that this is associated with a fact that the amorphous phase

616 and anatase skeleton bring a basic contribution to the strength of 3-6 and 7-8 samples, respectively. Thus change of conditions of hydrogels precipitate and ageing allows purposefully to vary texture-mechanical properties of titania xerogels.

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

M.D.Amiridis, B.K.Na and E.I.Ko, ASC Symposium Series 587 (1995) 32. Y.Yanagisawa and T.Samimoto, Appl. Phys. Lett. 64 (1994) 3343. V.Yu.Gavrilov, Kinet. Catal. 38 (1997) 762. V.Yu.Gavrilov and G.A.Zenkovets, Kinet. Catal. 31 (1990) 168. A.P. Karnaukhov, Kinet. Catal. 12 (1971) 1025. V.A. Dzisko, A.P. Karnaukhov and D.V.Tarasova, Physicochemical bases of synthesis of oxide catalysts, Nauka, Novosibirsk, 1978. (In Russian). 7. G.A. Parks, Chem.Rev. 65 (1965) 177. 8. E.A. Barringer, and H.K. Bowen, Langmuir 1 (1985) 420. 9. G.A.Zenkovets, V.Yu.Gavrilov and G.G. Zakharova, Pat. RU. No 1646992 (1991). 10.I.A. Razilov, L.K. Koopal and S.S. Dukhin, Colloid J. 59 (1997) 380. 11.H. Scher and R. Zallen, J.Chem.Phys. 53 (1970) 3759. 12.B.I. Schklovskii and A.O. Efros, Uspekhi chim.nauk 117 (1975) 401. (In Russian). 13.H.Rumpf, Chem.Ind.Technik. 30 (1958) 144.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

617

Structure Control of Si02 Sol-Gels via Addition of PEG Jihong Sun

Wenhao Fan

Dong Wu

Yuhan Sun*

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, RO.Box 165, Taiyuan, 030001, PRC

Abstract: Base-catalyzed silica sols and then xergels have been prepared from TetraEthyl OrthoSilicate (TEOS) in ethanol by using PEG200, PEG400, PEG600 as organic modifier. The addition of PEG to the SiO2 sols led to an increase in the particle size and then the formation of the networks with ring-like structure in short-order due to the linkage of PEG with sol particles. As a result, SiO2 gels modified by PEG were shown to be abundant of large pores. Keywords:

Sol-gel

SiO2

PEG

Porous xerogel

1, Introduction The sol-gel method, which comprises controlled hydrolysis and subsequent condensation of metal alkoxides, offers a new route to the production of highly-dispersed porous oxides, especially that of structure-controlled particles (such as high purity and homogeneity down to the molecular level, high BET surface area, adjustable pore size distribution) for catalysts I~'21 The importance of SiO2 in catalysis led to a deep understanding of the hydrolysis-condensation of TEOS TM. Besides an extensive research on TEOS sol-gel process I471, the addition of drying control chemical additives (such as formamide, N,N-dimethylformamide, N,N-dimethylaceta, ethylene glycol and organic acids) was used to control the rate of hydrolysis and condensation and then the structure of SiO2 sol-gelstS~2k Recently, organic templates and supramolecules have been made to control the structure of xerogel at a molecular or nano-technological level I~3~sl, the focus of which was onto sol-gel processing of alkoxides for structure-controlled catalysts. However, less attention was paid to the interaction between sol particles and then the control of gel structure 091. In the present work, sol-gel processing of TEOS with PEG as organic modifier was investigated in order to control the sol interaction for structure-oriented SiO_~.

*To whom all correspondence should be addressed

618

2. Experimental

2.1. Sol-gel process The SiO2-PEG sols were made with the composition of TEOS: H20: C2HsOH: NH4OH = 1:2: 40: 0.0005 (molar ratio); the amount of PEG 200, 400 and 600 were added up to 10 mol% of TEOS (see Figure 1). TEOS (analytical grade, distillated) and ethanol (analytical grade, distillated) were mixed at 330K for 30 min in a water bath to form solution A; a certain amount of NH4OH and PEG was mixed with the same volume of ethanol to make solution B, which was then added into solution A to produce a homogenous and transparent solution. After 30 min stirring, the solution was kept at 330K in the oven for 7 days; and the gels were formed under open condition (with a few pinholes on the container cap). After gelation, the gels were dried either in air at 393K or in vacuum at 473K for 3 hrs.

!

A TEOS+EtOH i[

B E't()H +

PEG + H2O + NH4OH

Figure 1. Schematic diagrams of the preparation

2.2. Characterization

The size distribution of the primary sol particles and their networks (i.e. the secondary particle) was in-situ determined by using Counter N4 plus; The particle size was observed by transmission electron microscopy (using Hitachi H-600). The pH of the SiO2 sols was measured with S-25 pH meter. The surface area and the porosity of the xerogels dried were obtained from N2 adsorption at 77K (using Micromeritics ASAP2000).

619 3. Results

3.1. TEM of S i O 2 sois

Figure 2. TEM of SiO2 sols without PEG(a) or with 5%PEG200(b), 400(c), 600(d)

The transmission microscope micrograph indicated the significant differences in the structure of the sols in which PEG200, PEG400 and PEG60C were added as organic modifier (see Figure 2). With the presence of PEG, the size of the primary particles was in the range of 10-20nm and the secondary particles (composed of the primary particles) showed ring-like network. In the particular, these ring-like structure of the SiO2-PEG sols looked more homogenous and short-ordered as the molecular weight of PEG increased. However, such characteristics were not observed without PEG in SiO2 sols, which showed an irregular distribution of the sol particles. Thus, it was possible to control the particle size and the pore structure of SiO2 xerogels via the addition of PEG

620 3.2. Formation and growth of sol particles

Figure 3. Relationship between particle size distribution and ageing time in SiO2 sols

Figure 3 clearly shows that the particles grew with the ageing time. The growth of the particles could be considered in two stages: the growth of the primary particles and the formation of the particle networks. Initially, the primary particle gradually increased in size, and the presence of PEG only speeded up the growth of sol particles. This implied that PEG improved the linkage between the particles via its interaction with the surface group of sol particles. However, the formation of the secondary particles or the networks resulted form the complicated interaction. Three types of particle networks appeared in different size with the presence of PEG, and finally the biggest particle networks were predominant in the system. On the other hand, two types of particle networks were present almost at the same percentage without PEG. This indicated that PEG indeed improved the formation of homogeneous particle networks via its linkage with sol particles.

621

3.3. pH of SiO 2 sol solution

9.4 5%PEG200

9.2 =

without PEG

9

O

g8.8 ,.--t 0

~8.6 8.4 8.2 0

i

I

50

100

I

I

f

150

200

250

300

ageing time (hr)

Figure 4. pH change with ageing time at 60~

At the initial stage, the pH sharply decreased and then remained the same until the decrease of the pH took place (see Figure 4). With less PEG, the pH decreased very fast and was lower than that without PEG. However, the pH showed different change with more PEG, which slowed down the decrease of the pH. Thus, the role of PEG was dependent on its amount in the sol solution. 3.4. Texture of SiO2-PEG xerogels

Table 1 The effect of PEG on texture of SiO2 gels

samples

SiO2 SiO2-PEG200 SiOz-PEG200 SiO~-PEG200* SiO2-PEG200 SiO2-PEG400 SiO2-PEG600

PEG Amount of PEG (molecular weight) (mol%)

200 200 200 200 400 600

* heated in vacuum at 473K

1% 5% 5% 10% 5% 5%

Surface area (cm2/g)

Pore volume (cc/g)

579.6 239.3 209.1 459.7 150.3 97.6 16.6

0.49 0.47 0.48 0.69 0.35 0.23 0.04

622 3.5

3.5 --,-'o.

3.0 ~-c,..) 2.5 o

.2.0 o $..l

!

1.5

with5% PEG200 with5% PEG400 with5% PEG600 wiN0ut PEG

,

o

;

/.

E• 1.5

// ,/,

2.0

-'

with I% PEG200 1

-.

with5%PEG200* !

---.

with 5% PEG200 l

.

2.5

'

t i

--,- wi~ 10% PEG200 " !

,-O-,~ 1.0

g.,

1,0

o a

0.5 0.0

,_, 3.0

/o \ll-li

,

:,

,

\'

. _.:,:,..-'--,~ . . . . . . . . . . ,

10

v

,.

,

[

100 Pore diameter

--~.~ !

1000 (A)

0.5 0.1 10

100 Pore diameter

1000 (A)

Figure 5. The effect of PEG on the pore distribution of gels

PEG had a strong effect on the texture of SiO2 xerogels (see Figure 5 and Table 1). The xerogel showed a concentrated pore distribution around 4nm without PEG, but the pore distribution shifted towards 6-7nm with the presence of PEG, which was dependent on the molecular weight. Therefore, the addition of PEG resulted in SiO2 xerogels with large pores probably due to the formation of ring-like network (see Figure 1) For the same kind of PEG, the pore distribution showed a relatively slight difference with the amount of PEG up to 10%. Table 1 indicated that the total surface area decreased gradually as the amount of PEG200 increased from 1% to 10% and the molecular weight of PEG from 200 to 600, while the pore volume was found to show a maximum at 5%PEG200. However, the post-treatment led to high surface area and large pore volume. This demonstrated that part of PEG remained in the pore of the xerogels.

3. D i s c u s s i o n The structure of SiO2 sol-gels depended on the pH at which TEOS was processed, and the base-catalyzed sol-gel processing of TEOS usually produced the porous xerogels via the linkage of sol particles t~l. In any cases, the lower pH favored a rapid hydrolysis followed by a slow condensation, and the higher pH suppressed the hydrolysis but improved the condensation. Initially, the hydrolysis of TEOS and then the condensation was predominant, the pH of sol solution therefore decreased sharply down to a certain level due to the release of ethanol with a slow growth of the primary particles (see Figure 2 to 4). On the further hydrolysis-condensation, the primary particles started cross-linking to produce the secondary

623 hydrolysis-condensation, the primary particles started cross-linking to produce the secondary particle networks, which led to the formation and the further growth of the networks and then the gradual decrease of the pH, producing the porous structure (see Figure 2 to 5). PEG strongly modified the sol-gel process of TEOS. The formation of the ring-like structure demonstrated that PEG had a strong interaction with sol particles. The interaction oriented the growth of the networks and then controlled the final texture. This was probably caused by the formation of bridging PEO through the occurrence of Si-O-C chemical bonds t2~ However, VongI2~jsuggested that the interaction between the hydrated PEG and the Si-O-Si network took place by hydrogen bonding. In our opinion, both chemical and physical interactions should be involved in the system. Otherwise, the pH of the sol solution hardly changed with the amount of PEG (see Figure 4). The pH of SiO2-PEG sol solution was observed lower than that of SiO2 sol solution when small amount of PEG was added, which indicated that the chemical interaction took place to release ethanol in this case. With more PEG in the solution, PEG adhered onto the sol particles via hydrogen bonding to prevent the fast hydrolysis, and then the pH of such solution was higher than that of SiO2 sol solution. Consequently, the interaction between sol particles was modified by PEG, which strongly influenced the formation of highly cross-linked network. The interaction between PEG and SiO~ sols could be considered as follows: _

+PEG 9. . .

O O 0 0 ~ OOO0

~

Sol particles PEG in the Sol

SiO2-PEGparticles

SiOz-PEG network

Figure 6. The interaction between PEG and SiO2 particles

Clearly, PEG initially interacted chemically with the sol particles to produce SiO2-PEG particles. Those particles were then oriented into ring-like network in short-order, which led to the concentrated pore distribution. However, part of PEG interacted physically with sol particles would remain in the pore channel, which could be removed by heating so-produced xerogels in vacuum at relatively lower temperature (see Figure 5 and Table 1).

624

5. Conclusion

The texture of SiO2 xerogles could be controlled by adding PEG. The introduction of PEG to the SiO2 sols led to an increase in the particle size and then the formation of the secondary particles with ring-like structure in short-order. This was due to the linkage of PEG with sol particles via the interaction between PEG and sols. As a result, SiO2 gels modified by PEG were shown to be abundant of large pores. However, the interaction between PEG and sols was complicated very much, and the investigation of its nature is still under the way.

Reference 1. C.J. Brinker and G. W. Scherer, The Physics and Chemistry of Sol-Gel Science, New York, 1990. 2. L.L. Hench and J. K. West, Chem. Rev., 90 (1990) 33. 3. R.K. lier, The Chemistry of Silica, New York, t979. 4. H. Schmidt, H. Scholze and A. Kaiser, J. Non-Cryst. Solids, 63 (1984) 1. 5. C . J . Brinker, and G. W. Scherer, J. Non-Cryst. Solids, 70 (1985) 301. 6. E.J.A. Pope and J. D. Mackenzie, J. Non-Cryst. Solids, 87 (1986) 185. 7. Y.Y. Wang, Y. B. Gao, Y. H. Sun and S. Y. Chen, Catal. Taday, 30 (1996) 171. 8. L.L. Hench, Science of Ceramic Chemical Processing (edited by D. R. Ulrich and L. L. Hench), New York, (1984) 52. 9. S. Wallace and L. L. Hench, Mater. Rec. Soc. Syrup. Proc., 32 (1984) 47. 10. N. Uchida, N. Ishiyama, Z. Kato and K. Uematsu, J. Mater. Sci., 29 (1994) 5188. 11. T. Katagiri and T. Maekawa, J. Non-Cryst. Solids, 134 (1991) 183. 12. R K. Sharma and A. Ramanan, J. Mater. Sci., 31 (1996) 773. 13. R T .Tanev, T. J. Pinnavaia, Science, 271 (1996) 1267. 14. H. Yong. A. Kuperman, N. Coombs, et al, Nature, 379 (1996) 703. 15. M. Ogawa, J. C. S. Chem. Commun., (1996) 1149. 16. J. M. Smith and D. E. Katsoulis, J. Mater. Chem., 5 (1995) 1899. 17. J. S. Reddy and A. Sayari, Catalysis. Letters, 38 (1996) 219. 18. G. Larsen, E. Lotero, M. Nabity, et al, J. Catalysis, 164 (1996) 246. 19. N. K. Raman, M. J. Anderson, C. J. Brinker, Chem. Mater., 8 (1996) 1682. 20. D. Ravaine, A. Seminel, Y. Charbouillot, et al, J. Non-Cryst. Solids, 82.(1986) 210. 21. M. S. W. Vong, N. Bazin, R A. Sermon, J. Sol-Gel. Sci & Tech., 8 (1997) 499.

Acknowledgment i-he financial support from the National Natural Science Foundation of China (NFSC) is greatly acknowledged.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

Sulfated zirconia precipitation

spheres

and

625

microspheres

b y gel s u p p o r t e d

F. Pinna a, M. Scarpa a, M. Signorettoa,G. Strukula, M. Marella b, M. Tomasellib, L. Meregallib and R. Scattolinb aDepartment of Chemistry, University of Venice Calle Larga S. Marta 2137, 30123 Venice, Italy bEnirisorse - Centro Ricerche Venezia, Via delle Industrie 39, 30175 P.to Marghera, Venice, Italy

ABSTRACT Sulfated zirconia spheres and microspheres with variable, though strictly controlled, diameters up to several millimeters have been prepared by a new sol gel procedure. The spheres have been also promoted with 1% Pt and have been characterized after calcination at 550 ~ and 600~ The Pt modified catalysts are very active and stable in the isomerization of n-butane

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

Because of the increasing need for environment friendly processes, sulfated zirconia (SZ)-based systems have attracted much attention as potential process catalysts since the discovery of their strong acidity[l]. The synthetic methods for this family of catalysts are numerous and several studies on the characterisation of its physicochemical properties and catalytic performance in different reactions are available in the literature [2,3]. The preparation procedures can be classified into two groups, based either on two-step or on one-step methods. The first one involves the impregnation with aqueous H2SO4 or (NH4)2SO4 of previously prepared Zr(OH)4 (either by alkaline precipitation or by sol-gel method), followed by drying and possible subsequent impregnation with transition metal promoters and final calcination. The one-step method [4] makes use of a sol-gel technique, in which an alcogel is obtained by mixing an isopropanol solution containing a zirconium alkoxide, H2SO4, and possible promoters with another solution containing water and isopropanol. The alcogel is aged and supercritically dried. To our knowledge, all the so far reported catalysts are in the form of powder. For commercial applications, the pelletization of these powders with retention of a large surface area and a proper shape is a very important point: this appears to be a hard task to achieve, for which very little information is available in the

626 literature. A versatile sol-gel procedure (GSP) for the preparation of zirconia based catalyst supports, e.g. spheres, microspheres and washcoats, was previously developed by some of the authors [5-7]. In this work we report a modification of the previous procedure which allows to obtain spheres and microspheres of sulfated zirconia with variable, though strictly controlled, diameters up to several millimeters. 2. EXPERIMENTAL 2.1 P r e p a r a t i o n By GSP method a zirconium sol was prepared as disclosed elsewhere [7]. The procedure is based on the precipitation of the hydroxide in a basic solution, but in the presence of an organic additive that immediately gelifies, thus acting as a template for the precipitation. The starting sol was stable at room temperature for a long time. For the preparation of mm-size spheres the sol was dripped by a multicapillary a p p a r a t u s into the gelation bath. The capillary diameter can be varied between 0.8 mm and 5 mm. After ageing (usually less t h a n 30 min) the spheres were filtered and washed with H20 down to pH = 9. With a similar procedure it is possible to obtain gel microspheres (20-60 ~n) by atomizing the sol with a rotating (8,000 rpm) cup atomizer. The rinsed gel spheres were divided into two batches, A and B, which underwent two different sulfation routes. Batch A was dried at ll0~ for 24 h to give mechanically resistant spheres without defects, t h a t were impregnated with either a 0.5 M H2SO4 or 1 M ( N H 4 ) 2 S O 4 solutions. Then the same impregnation procedure, but with (NH4)2SO4 solution only, was performed on the same batch, precalcined at 280~ and 350~ (A-SZ sample) respectively. They were dried at l l 0 ~ and then calcined at 550~ Promotion of the A-SZ sample with Pt was obtained by incipient wetness impregnation by using a proper amount of aqueous solution of H2PtC16 so as to obtain 1.0 % wt of Pt. The sample was then calcined at 600~ (APtSZ sample). Batch B was impregnated with an aqueous 0.35M (NH4)2SO4 solution [8]. The impregnated gel spheres were dried at ll0~ for 24 h. The xerogel was precalcined at 350~ to give sample (B-SZ). The further promotion with Pt and calcining (BPtSZ sample) was carried out as previous said for batch A. In one case the Pt promotion was perfomed on the xerogel (batch B) precalcined at 550~ (B1-PtSZ) 2.2. Characterization Quantitative determination of sulfates was performed by ion chromatography Dionex LC20 following a procedure reported elsewhere [9]. Simultaneous differential thermal and gravimetric analysis (DTAfrG) were carried out on dried samples with a Netzsch STA 409 equipment. The m e a s u r e m e n t s were performed in air (18 l/h), from room temperature to 1000~ at a heating rate of 10~ The morphology was studied by Scanning Electron Microscopy (SEM) with a Philips XL 30 instrmnent. X-ray diffraction (XRD) patterns were obtained by a Siemens D5000 powder diffractometer equipped with a graphite crystal monochromator using a Copper

627 K s X-ray radiation source. Experiments were run in step-scan mode with a step interval of 0.02 ~ 20 and a count rate of 1 second per step over the range 5 ~ to 90 ~ The fraction of tetragonal to monoclinic form was determined by Rietveld method [10,11] Nitrogen adsorption and desorption were measured with a Micromeritics ASAP 2010 apparatus. The samples were pretreated at 300~ for 3 h under vacuum (1.33 Pa).

2.3 Catalytic measurements Catalytic activity tests in the isomerization of n-butane were performed in a glass flow microreactor (200 mm long, 8 mm i.d.) externally heated by an oven. The temperature was controlled with a thermocouple immersed in the catalytic bed. The reaction was carried out at atmospheric pressure at either 150~ or 250~ The feed was dependent on 1527 .'action temperature although the butane space velocity was kept constant ~ ~00 h -1. At 150~ the feed (total flow 25 ml/min) was made of a mixture of n-butane and helium (C4Hlo/He=I/4); at 250~ it was made with n-butane and hydrogen (C4Hlo/H2=l/4). Prior to the reaction, catalysts (0.5 g) were activated in dry air (25 ml/min) at 450~ for 2 h. Isobutane was analyzed on line by gas-chromatography. Reaction rates were defined as mol n-butane converted per gram catalyst per unit time. 3. R E S U L T S AND DISCUSSION Initial attempts have been made to apply the GSP technique to the synthesis of sulfated zirconias in a single stage. To this purpose ZrOS04 was added to the zirconium sol prior to dripping into the gelation bath. No gel formation was observed and no further attempts to improve the method were performed. Similarly, impregnation of dried spheres of batch A with either a 0.5 M H2SO4 or 1 M (NH4)2SO4 solutions resulted in their quick mechanical collapse already in the presence of the sulfate solution. Precalcination of the dry spheres at 280~ and 350~ (A-SZ sample) proved useful to achieve the sulfate impregnation without incurring in mechanical collapse. However, during the final calcination in dry air at 550~ the spheres broke in approximately 70% and 40% amount for the 280 ~ and 350~ precalcined samples respectively. Addition of Pt on the A-SZ sample caused a moderate breaking of the spheres already during impregnation while >80% were found broken after calcining at 600oc. At variance with the previous samples, the preparation procedure applied to batch B did not show any problems as far as the mechanical properties of the spheres were concerned. In Table 1 is reported a summary of the analytical and morphological p a r a m e t e r s of the two SZ samples and the corresponding, Pt promoted, Pt-SZ samples, calcined at 550 ~ and 600~ respectively. All samples show similar high BET surface area in comparison with the value previously obtained (82 m2/g) for plain unsulfated zirconia spheres prepared with the same procedure [5]. This phenomenon has been already reported for powder samples in the literature [12] as due to the presence of sulfate groups. The sulfur content is similar in all

628 samples and ranges between 2.9 and 3.8 SO4 groups/nm 2 i.e. a little lower than 4, the calculated value for the full coverage at the monolayer [13]. Table 1 Analytical and morphological parameters of the samples investigated Sample SO4 SO4 SBET Vp rp max % wt groups/nm2 (m2/g) (ml/g) (run) A-SZ . . . . . . . . 8:1 3:2 163 0.20 3'.9 B-SZ 9.3 3.8 156 0.12 3.4 A-PtSZ 7.6 2.9 166 0.16 3.3 B-PtSZ 8.5 3.6 148 0.13 3.8

Vm

(ml/g) 0.14 0.09 0.13 0.09

DTA carried out in air show similar features for all samples. As an example, in Figure 1 is reported the DTA plot of the B-SZ sample (curve a) in comparison with the plot of a plain unsulfated zirconia sample (curve b). In both plots a first broad endothermic peak appears, centered around 190~176 due to water evolution. Residual NOx and organics used as thickening agents evolve up to around 500~ Finally an exothermic band, due to the transition between the amorphous and crystalline phase (tetragonal), appears at higher temperature. This band is very sharp and centered at 439~ in the case of pure ZrO2 (curve b), whereas it shifts to higher temperatures for the B-SZ sample (curve a). In this case the broader appearance can be explained by the overlapping with the decomposition band of the surface sulfate groups, and is supported by the observation of a simultaneous weight loss of the sample.

a

n Li..I

u/

m

b

Ii

I/3

....

I_

200

..,

I

400

..

~

I

i

600

I

800

,

1000

TEMPERATURE (~ Figure 1. DTA plot of B-SZ sample (curve a) and unstflfated ZrO2 (curve b)

629 The crystal phase composition of the B-SZ sample calcined at 550~ was determined by XRD. Rietveld analysis gave 100% tetragonal phase . This behaviour is opposite to what is found for plain zirconia, but is in line with the well known effect of sulfate on the crystal phase composition of zirconia. SEM micrographs of microspheres and spheres, after calcination at 550~ show near perfect spheres with a narrow size distribution for the former (Figure 2a: 10-25 lma) and a constant diameter for the latter (Figure 2b: ~ 2.5 ram).

Figure 2. SEM micrographs of B-SZ sample microspheres (a) and spheres (b) The nitrogen adsorption-desorption isotherms of the SZ and PtSZ samples calcined at 550~ and 600~ respectively are reported in Figure 3. All isotherms are of type IV and H2 hysteresis according to IUPAC convention [14]. A summary of the morphological parameters of the spheres has been reported in Table 1. The total pore volume (Vp) computed from the adsorption at P/Po=0.99 ranges from 0.12 to 0.20 with lower values for the B-type samples. 150

150 b

a

13. !-oo100

A-SZ

A-PtSZ

O3

~100

, .....

i

B-SZ

to

so

J

B-PtSZ

~5ol

E3 <

,,

121 < S:0

" 012 " 0:4 0:6 ' 0:8 ' 1.0 RELATIVE PRESSURE(P/Po)

g.o

0'.2

' 024 " 016

' 0'.8

' 1.0

RELATIVE PRESSURE (P/Po)

Figure 3. Nitrogen adsorption-desorption isotherms of samples SZ (a) and PtSZ (b) respectively.

630 A comparison between the total pore volume (Vp) and the micropore volume Vm, calculated by the t-plot method, shows that the samples have strong contribution of micropores. The pore size distribution was obtained from the desorption branch of the isotherms following the BJH method. (Figure 4). The mesopore size distribution is essentially unimodal with the pore radius centered at the rpmax values reported in Table 1.

1.5

E3

"~ 1 0 E

"

o 0.5

"0

"0

o.o

!

9

O ~ a ~ - - m

~

Pore D i a m e t e r (A)

:~-":

lOO

Figure 4. Pore size distribution (dV/dlogR) vs. the average pore radius of samples A-SZ (squares) and B-SZ (circles). The activity of the catalysts has been tested in the isomerization of n-butane. SZ samples have been checked at 150~ and in the absence of H2 (Figure 5a). The type of profile shown is typical: after an initial fast decay the activity declines slowly as a consequence of a deactivation process suggested by the observation of minor amounts of some cracking products (propane <3% and methane <1%). The initial activity can be completely restored after calcining the samples in dry air at 450~ The two catalysts seem to behave in a similar manner, but sample B-SZ is more active. In Figure 5b the corresponding PtSZ samples checked at 250~ and in the presence of H2 in the feed are reported. The catalysts are more active than the corresponding unpromoted samples, in particular B-PtSZ which appears also remarkably stable for reaction times up to 20 h. In both cases the impregnation of sulfate on the wet ZrO2 samples with the new procedure (Batch B), yields catalysts with superior activity, especially when Pt is present. This higher activity is retained even when Pt is added after calcination at 550~ (Figure 5b; B1-PtSZ sample), instead of the usual 350~ a procedure that generally leads to a severe loss of catalytic activity, as is observed in Figure 5b also in this case. The reasons for this activation effect on butane isomerization following the new synthetic procedure (Batch B) are not presently clear, since the composition and morphological parameters reported in Table 1 appears very similar and do not

631 seem to justify the large differences observed for the Pt promoted samples. //

4

r v

0~-0-0-0.0

3 ~

~2

c~2

. ,~m..mt.~.A~A~A.A_.A

"U'u'n-n-n-u_nnn

-:-: : : :-:-:-: : :," 0

o

' ~'o

' 16o ' 1,~o ' 26o

Time (min)

o ' sb

I do' I ~ o 2 6 0 Time (rain)

//'

Io'oo

Figure 5. n-butane isomerization rate vs. time on stream plots measured at 150~ (Fig. 5a) over B-SZ (circles), A-SZ (squares) and at 250~ with H2 (Fig. 5b) over BPtSZ (circles), A-PtSZ (squares) and B1-PtSZ (triangles) 4. CONCLUSIONS The procedure for obtaining sulfated zirconia spheres and microspheres with a large range of diameters and active in the isomerization of hydrocarbons, was developed by a modified gel supported pre_cipitation technique. The technique here described can be extended to the preparation Of different inorganic oxides starting from suitable water soluble precursors, so that the preparation of sulfated zirconias with different type of modifiers can be predicted. REFERENCES

1. 2. 3. 4. 5.

M. Hino and K. Arata, J. Chem. Soc. Chem. Commun. 851 (1980). A. Corma, Chem. Rev., 95 (1995) 559. X. Song and A. Sayari, Catal. Rev.- Sci. Eng., 38 (1996) 329. D.A. Ward and E. I. Ko, J. Catal. 145 (1994) 18. M. Marella, M. Tomaselli, L. Meregalli, M. Battagliarin, P. Gerontopoulos, F. Pinna, M. Signoretto and G. Strukul; in "Preparation of Catalysts VI", Elsevier, Amsterdam 1995, p.327. 6. G. Centi, M. Marella, L. Meregalli, S. Perathoner, M. Tomaselli, and T. La Torretta, in "Advanced Catalysts and Nanostructured Materials", Ed. W. R. Moser, Academic Press, 1996, p. 63. 7. M. Marella, L. Meregalli and M. Tomaselli, EP No. 718239 (1996).

632 8. M. Marella, M. Tomaselli L. Meregalli and F. Pinna, Italian Patent application submitted. 9. C. Sarzanini, G. Sacchero, F. Pinna, M. Signoretto, G. Cerrato and C. Morterra, J. Mater. Chem. 4 (1995) 353. 10. H.M. Rietveld, J. Appl. Cryst., 2 (1969) 65. 11. C.J. Howard and R.J. Hill, J. Mater. Sci., 26 (1991) 127. 12. A. Clearfield, G. P. D. Serrette, A.H. Khazi-Syed, Catal. Today 20 (1994) 295. 13. M. Waqif, J. Bachelier, O. Saur and J. C. Lavalley, J. Mol. Catal., 72 (1992) 127. 14. K.S.W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl. Chem. 57 (1985) 603.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

633

T i t a n i a - a l u m i n a p r e p a r e d b y sol-gel m e t h o d : I n f l u e n c e o f p H and drying on textural a n d structural p r o p e r t i e s S.S.X. Chiaro a, J.L. Zotin a and A.C. Faro Jr. b a Petrobras S.A. - Centro de Pesquisas e Desenvolvimento - Divis~o de Catalisadores Cidade Universitfiria- Qd 7 - Ilha do Fundgo - 21949-900 - Rio de Janeiro - Brazil b Universidade Federal do Rio de Janeiro - Instituto de Quimica- Dept. de Fisico-Quimica Cidade Universitfiria- CT - B1. A - Ilha do Fund~o - 21949-900 - Rio de Janeiro - Brazil ABSTRACT The influences of synthesis conditions (pH) and drying (oven or freeze-drying) on textural and structural properties of titania-aluminas prepared by hydrolysis of alkoxides were studied. Titania-aluminas with a 1:1 molar ratio were prepared by hydrolysis of alcoholic solutions of titanium and aluminium isopropoxides. The syntheses were conducted in acidic (pH 1.4) or basic hydrolysis conditions (pH 9). Solvent and water were removed by freeze-drying or oven-drying at 393 K, followed by calcination at 773 K or 1073 K. The solids were characterized by nitrogen adsorption, X-ray diffraction, 27A1NMR, DTA and XPS. The results suggested that the textural properties of titania-aluminas prepared by the sol-gel method were mainly determined by the pH of hydrolysis. However, structural properties were very sensitive to the drying procedure. A less stable structure, with a more random distribution of titanium and aluminum, was formed by freeze-drying than the one obtained by oven-drying, which led to separate alumina and titania domains. 1. INTRODUCTION New supports for hydrotreating catalysts have been recently studied and titania-alumina mixed oxides are among the ones found to be promising for the production of more active catalysts [1,2]. However, knowledge of the preparation parameters which control the physical and chemical properties of these supports is still limited, when compared to the one available for more conventional carriers, like alumina or silica-alumina. Growing interest is being devoted to use of sol-gel methods for the preparation of mixed oxides, due to the possibility of obtaining higher surface areas and purity, better control of pore volume and pore size distribution and a more homogeneous distribution of the elements in the solid. In recent review, Narendar and Messing [3] discuss the mechanism of phase formation in solids obtained by gel-based synthesis. According to these authors, the control of the hydrolysis and condensation reactions during synthesis is critical for directing the synthesis of the mixed oxides with respect to heterocondensation or homocondensation. When large pore volumes or surface areas are desired, it is necessary to preserve the textural properties of the wet gel by adopting appropriate drying methods which avoid collapsing of the pore structure due to capillary effects. The supercritical drying process as well as freeze-drying method are suitable for this [4].

634 Several papers report the use of the sol-gel method for the preparation of titania-aluminas, but results on the influence of synthesis conditions on the properties of the final products are scarce [5,6,7,8]. For example, Pacheco-Malagon et aI. [7] characterized titania-aluminas by NMR, UV-VIS DRS and DTA and proposed the existence of separate alumina and titania domains in their materials, originating from a homocondensation process. Toba et al. [8] report the obtention of homogeneous solids using moderators of alkoxide hydrolysis during the sol-gel synthesis. The purpose of our study was to analyze the influence of synthesis conditions (pH) and drying (oven or freeze-drying) on textural and structural properties of titania-aluminas prepared by hydrolysis of alkoxides.

2. EXPERIMENTAL 2.1. Preparation of the solids Titanium isopropoxide [546-68-9] and aluminum isopropoxide [555-31-7] were supplied by Aldrich. All titania-aluminas were prepared with a 1:1 molar ratio, relative to the oxides. The alkoxides were manipulated in a glove box, in order to avoid contact with air moisture, until the addition to the solvent (isopropanol). Acetylacetone (acac) was added to the titanium isopropoxide in order to decrease its hydrolysis rate [9] and this mixture was dissolved with 1/5 of the total isopropanol volume. Aluminum isopropoxide was dissolved in xilene at 333 K, and diluted with another 1/5 of the total isopropanol volume. Both solutions were then simultaneously added to the reaction flask. When the preparations were carried-out in acidic medium (A series), the alkoxide solutions were added to the reactor containing a mixture of water acidified with nitric acid to pH 1.4 and isopropanol, at 353 K with vigorous stirring, and kept under reflux for 15 min. For preparations in basic medium (B series), the precipitation was carried-out at pH 9, adjusted by addition of ammonium carbonate to the isopropanol/water mixture, at room temperature, in order to avoid the decomposition of the ammonium carbonate; stirring was kept for 15 min after complete addition of the alkoxides. All of the solids obtained in acidic conditions were stable sols, whereas those prepared in basic conditions precipitated immediately. The hydrogels obtained in both cases were aged at ambient temperature for 72 h. Pure alumina and titania solids, used as reference materials, were synthesized as described above for the A series, except, of course, that only one of the alkoxide solutions was used in each case. Table 1 summarizes the molar ratios of reactants and solvents used in the preparations. Table 1 - Molar ratios of reactants and solvents TiO2/A1203 TiO 2 30 30 H20/alkoxide 50 50 isopropanol/alkoxide (a) 0.002 0.002 HNO3/H20 0.003 (NH4)2CO3 4.5 xilene/Al-alkoxide 1 acac/Ti-alkoxide a- total amount of alcohol used in the synthesis

A1203 30 50 0.002 4.5

635

2.2. Drying and calcination The solids obtained by both preparation procedures were dried by two different methods: freeze-drying (F series) and oven-drying (O series). Freeze-drying was carried-out in a commercial equipment from Edwards, where the water was removed by sublimation at 0.13 Pa. In the oven-drying procedure, the samples were heated to 353 K in a vacuum oven at 2.2 kPa for 6 h and then for 6 h more at 393 K. The freeze-dried samples were further dried under the same conditions used in oven-drying. The calcinations were carried-out in three stages. First, the solids were heated from room temperature to 393 K and kept at this temperature for 1 h, then at 573 K for 2 h and finally at 773 K for 3 h. Some samples were also calcined at 1073 K for 3 h, in order to assess the thermal stability of the solids 2.3. Characterization The adsorption-desorption of nitrogen was used in surface area and pore size distribution measurements, using a Micromeritics ASAP 2400 equipment. X-Ray powder diffraction patterns were measured with a JEOL JDX-8030 diffractometer with Cu K(x radiation. A VARIAN VRX-300 NMR spectrometer was used to obtain 27A1MAS/NMR spectra at 78,4 MHz, with 0,7 ps (n/12) pulses, 7 kHz spinning rate and spectral window of 100 ldtz. The reference material was aluminum chloride hexahydrate. Impregnation with acetylacetone (acac) was used to check for the presence of disordered hexa-coordinated aluminum species, invisible to NMR. Differential Thermal Analysis (DTA) was performed in a Rigaku Model 8150 equipment. The sample was heated up to 1273 K (heating rate of 10 K.min 1) in a 40 cm3.min1 flow of dry nitrogen. X-Ray Photoelectron Spectroscopy (XPS) was performed in a VG Scientific ESCALAB MK II spectrometer, with A1 Ka radiation. Binding energies were referenced to the binding energy of Cls considered to be 285 eV. Atomic concentration ratios were calculated by correcting the peak intensity ratios with the theoretical sensitivity factors based on Scofield cross sections. 3. RESULTS 3.1. Textural properties The textural properties of the solids calcined at 773 K are shown in Table 2. Except for pure titania, all of the solids prepared had high surface areas ( > 220 m2.g-~) and there was no clear influence of the pH of the preparations or the drying method on the surface areas of the mixed oxides. However, pore volumes and mean pore diameters were strongly dependent on the pH of precipitation: the mixed oxides of the A series had lower pore volumes and pore diameters than those of the B series, independently of the drying method used.

3.2. X-Ray Diffraction The X-ray diffraction patterns of the mixed oxides calcined at 773 K and 1073 K are shown in Figure 1 and 2, and the corresponding crystalline phases are summarized in Table 3. The mixed oxides calcined at 773 K were essentially amorphous and traces of 7-alumina or anatase were observed for the oven-dried samples, TA-A-O, TA-B-O1 and TA-B-O2. The pure titania was obtained as a mixture of rutile and anatase and the pure alumina displayed a low crystallinity 7-phase.

636 After calcination at 1073 K, the alumina was still a 3,-phase and the titania was completely converted to rutile. Titania-aluminas presented, in general, a mixture of rutile and anatase. However, the freeze-dried samples displayed characteristic peaks of an c~-alumina phase and a higher proportion of rutile, as compared to the oven-dried samples. The presence of c~-alumina in the oven-dried titania-aluminas and in the pure alumina was observed only after calcination at 1273 K. Table 2 - Textural characterization of the solids calcined at 773 K. Sample Surface Area Pore Volume Average Pore (m2.g-1) (cm3.g-1) Diameter ( A ) TI-A-O 22 0.063 73 TA-A-F1 TA-A-F2 TA-A-O

296 324 334

0.41 0.37 0.47

41 36 41

TA-B-F TA-B-O1 TA-B-O2

376 303 222

0.81 0.58 0.54

81 59 74

AL-A-F

223

0.17

35

t

+

+

+\![ 9

O

Q

i II

-+

TA-A-O

+ TA-A-F 1

1073 K

n

773 K 'III II"I"ll II'I I I Ill Ill I IIIII I IIII[I I II['III111 III I IIIII IIIIIIIIIIIIII

22

42

62

I'I'II

20( ~)

Figura 1 - X-ray diffraction of titania-aluminas prepared in acidic condition and calcined at 773 K and 1073 K - O-anatase; 0-rutile; m-a-alumina

637

it

T

I

.

TA-B-F

1073 K ~

f

Ib

.,.,.

O

~ ,~,,,~.

TA-B-O 1

TA-B-F 773 K

,

.~

~

Illllilliilllllllllllllll

~-,~--~,..~._~.~_~... IIIIIIilllllllllllllllllllilll

22

42

-

-

IIII1[11111

62

_J

I Ii

20( ~)

Figure 2 - X-ray diffraction of titania-aluminas prepared in basic condition and calcined at 773 K and 1073 K - e-anatase; #-ruffle; I - a - a l u m i n a

Table 3 - Crystalline phases as determined by X-ray diffraction Sample

Crystalline Phases (a)

.....

1073 K (b)

TI-A-O

773 K (b) Rutile+Anatase

TA-A-F1 TA-A-F2 TA-A-O

amorphous amorphous amorphous + 7-alumina(tr)

ruffle + anatase + cx-alumina na rutile + anatase (tr)

TA-B-F TA-B-O1 TA-B-O2

amorphous amorphous + anatase (tr) amorphous + anatase (tr)

ruffle + anatase + c~-alumina anatase + rutile (tr) na

.

.

AL-A-F (a) - tr: traces

.

"'

.

7-alumina (b) - Calcination temperature

Ruffle

,/-alumina na - not analysed

3.3.27A1 MAS-NMR The NMR spectra of the solids showed, for all the samples, the characteristic peaks of octahedral aluminum [A1]o, with chemical shift in the range 1.5 to 10 ppm, and tetrahedral aluminum [A1]t in the range 60 to 65 ppm. In several samples, a third peak with chemical shift between 32 and 35 ppm was observed, which is attributed in the literature to

638 pentacoordinated aluminum species [A1]p [7,10]. Figure 3 shows some representative spectra of alumina and titania-alumina samples.

i

i

t

I

100

'

,'

t

'

50

'l

l"

i

i

i

j

0

J

J

i

..... i

-50

'J

i

i

J

i

I

-100

(ppm)

Figure 3 - Typical 27A1NMR spectras of alumina and titania-aluminas

The distribution of the different aluminum species, obtained by decomposition of the NMR spectra, are shown in Table 4 for the solids calcined at 773 and 1073 K. The oven-dried mixed oxides presented a lower amount of pentacoordinated species and proportions of tetrahedral and octahedral species similar to those observed for the pure alumina. In the freeze-dried samples, the amount of pentacoordinated aluminum increased together with a decrease of the octahedral species. This behavior was observed independently of the pH used in the preparations. So, these results indicate that the drying method influences the nature of the aluminum species present in the calcined mixed oxides. Complexation by acetylacetone (acac) is commonly used to convert disorganized aluminum species, invisible to NMR analysis, into octahedral species detectable by this technique [11 ]. The amount of the different aluminum species present in the solids calcined at 773 K after complexation with acac are also shown in Table 4 (values in parentheses). The freeze-dried samples showed a marked increase in the proportions of octahedral species due to acac impregnation, as compared to the oven-dried samples, indicating that freeze-drying led to more disorganized aluminum species in the final mixed oxides. Some samples calcined at 1073 K were also analyzed by NMR. In the freeze-dried titaniaalumina prepared in acidic conditions, the pentacoordinated aluminum disappeared and most of the aluminum species were octahedral, in agreement with the presence of the (z-alumina phase detected by X-ray diffraction. With the oven-dried mixed oxide prepared in basic conditions, as well as with the pure alumina, no significant changes were observed in the proportion of the aluminum species.

639 Table 4 - Aluminum species determined by 27A1NMR analysis

27A1NMR (a~ .

.

.

.

.

.

.

.

.

.

773 K (b) Sample TA-A-F 1 TA-A-F2 TA-A-O

[A1]t 27.6 (18.6) 26.3 (15.7) 27.9 (na)

[A1]p 14.5 (9.6) 17.7 (12.5) * (na)

[A1]o 57.9 (71.8) 56.0 (71.8) 72.1 (na)

[A1]t 3.9 na na

TA-B-F TA-B-Oa TA-B-O2

21.4 (11.6) 25.7 (21.3) 24.0 (na)

17.3 (10.2) * (-) * (na)

61.3 (78.2) 74.0 (79.0) 76.0 (na)

na 19.0 na

AL-A-F

.

.

1073 K (b~ [A1]p na na na na

[A1]o 96.1 na na na 81.0 na

22.9 76.5 76.5 (75.5) 23.5 (24.5) - (-) (a) - Percent distribution of aluminum species- ..... [A1]t: tetrahedral; [Al]o: octahedral; [A1]p: pentacoordinated aluminum species Values in parentheses: after complexation with acetylacetone (acac) (b) - Calcination temperature (*) - present but under limits of integration (-) - not present na - not analysed ........

3.4. DTA The thermograms of some solids previously calcined at 773 K are shown in figure 4. All of the samples, except for titania, showed an endothermic peak at ca. 373 K, attributable to physisorbed water. Pure titania had a discrete exothermic transition at ca. 1030 K and the transitions of pure alumina were essentially athermal, as previously reported in the literature [12]. The oven-dried mixed oxide prepared under basic conditions displayed small and very broad exothermic peaks in the range 1040 to 1300 K. However, with the titaniaalumina prepared under acidic conditions, marked exothermic peaks at 1037, 1212 and 1246 K were observed. These transitions can be related to the appearance of rutile and (zalumina in this sample, as indicated by the preceding characterization techniques.

3.5. XPS The XPS spectra in the Ols photoemission region for samples AL-A-F, TI-A-O, TA-A-F2 and TA-B-O2 are shown in figure 5. The binding energy for O1~ in pure alumina (531.6 eV) was 1.4 eV higher than the value obtained for pure titania (530.2 eV). The mixed oxides presented different behaviors, according the preparation conditions. The oven-dried titaniaalumina prepared in basic conditions had the O1s binding energy (531,5 eV) similar to that observed for pure alumina, whereas in the freeze-dried material prepared in acidic conditions the O~ binding energy (530.8 eV) was between the ones obtained with the pure oxides. There were no significant differences among the binding energies of Alzp and Tizp for these samples. The Ti/A1 ratio measured for TA-A-F2 and TA-B-O2 were, respectively, 0.33 and 0.23, indicating a higher amount of titanium at the surface for the sample TA-A-F2. However, in both cases, the Ti/A1 ratio at the surface was smaller than the bulk Ti/A1 ratio of 0.5.

640

TI-A-O

--... TA-A-F~2

""~'-~x, -A-F

TA-B-O2

\._j AL-A-F

y / I

l

I

573 873 1173 Temperature (K)

i

1473

Figure 4 - DTA of some titania-alumina samples

I

536

~ . . TI-A-O I

!

532

I

528

B.E. (eV)

Figure 5 - XPS spectra of O1s level of titaniaaluminas samples

4. DISCUSSION

Our results show that the texture and structure of titania-aluminas prepared by hydrolysis of alkoxides are strongly influenced by the choice of the experimental conditions, such as the pH of hydrolysis and the drying method. The results demonstrate that the solids prepared under basic conditions had larger pore volumes and average pore diameters than the ones obtained under acidic conditions. The surface areas, however, did not vary in any systematic way, neither with the hydrolysis pH nor with the drying method. This may be taken as an indication that these parameters do not have a systematic influence on the sizes of the primary particles, which basically determine the surface areas of the materials, buy that they do influence the way in which these particles aggregate, which determines the pore structures of the materials. The fact that a stable sol is obtained in the preparation under acidic conditions may be explained if the primary particles are highly dispersed in the reaction medium. They would then aggregate only during the drying step, producing a more densely packed structure. On the other hand, in the preparation under basic conditions, condensation of the primary particles might occur already in the reaction medium, producing a more open pore structure after drying and calcination. It has been reported that, in the sol-gel preparation of silica, hydrolysis reactions are favored under acidic conditions, leading to the formation of dispersed primary particles, whereas under basic conditions, condensation reactions are favored [ 13].

641 The sol-gel method used in the present work led to pure alumina and titania samples with structural and textural properties typical of these materials prepared by other methods. In other words, the fact that a sol-gel method was employed apparently did not confer any special properties to these materials. For example, the titania was obtained as a mixture of anatase and rutile after calcination at 773 K, and was almost completely converted to rutile by calcination at 1073 K; the alumina had the gamma structure and was transformed to a mixture of alpha and delta phases only after calcination at 1273 K. Therefore the fact that the properties of the mixed-oxides are not a linear combination of the properties of the pure ones is not a direct consequence of the preparation method itself, but of the existence of interaction between alumina and titania in these materials. For the oven-dried mixed oxides, only traces of anatase or 3,-alumina superimposed on an amorphous pattern were observed for the solids calcined at 773 K. After calcination at a higher temperature (1073 K), rutile and anatase phases were clearly present. The freeze-dried titania-aluminas had a clearly different behavior and the structural changes were more pronounced. Although they were amorphous after calcination at 773 K, (z-alumina mixed with rutile and anatase were present after calcination at 1073 K. It is important to remember that oralumina, the most stable phase of alumina, did not appear at this temperature with the other solids, therefore indicating a lower structural stability of the freeze-dried titania-aluminas, when compared to the oven-dried ones and the pure alumina. The 27A1NMR spectra provide further insight into the structure of the mixed oxides. With the oven-dried samples, despite the presence of pentacoordinated aluminum in small amounts, the distribution of aluminium ions between tetrahedral and octahedral sites was essentially the same as that of the pure alumina sample, typical of its distorted spinel structure [14,15]. This result suggests the presence of separated domains of alumina and titania in these samples. This hypothesis is supported by the X-ray diffraction results, as separate alumina and titania phases were identified in the oven-dried mixed oxides and the phase transitions with increasing calcination temperature follow the same trends as with the pure oxides. The NMR spectra of the freeze-dried samples are clearly different from that of the pure alumina: they display an important proportion Of pentacoordinated aluminum, as well as the presence of"invisible" aluminum, generally attributed to highly disorganized hexacoordinated aluminum species [11 ]. It is thus likely that these solids have a more random distribution of aluminum and titanium ions in their structure, as compared to the oven-dried materials. The results suggest that primary particles with similar structures may be formed during the sol-gel synthesis, independently of the pH of reaction, the main difference being in the way by which these particles associate to form solids with different textural properties. During ovendrying, a substantial part of the water must remain in the pore structure of the solid up to temperatures above 373 K, due to capillary condensation. This may cause hydrothermal effects, such as rearrangements of the structure, leading to separate alumina and titania domains. On the other hand, during freeze-drying, most of the water is removed at low temperature by sublimation, and the original structure of the particles is likely to be preserved, with a more random distribution of titanium and aluminum in the structure. The DTA and XPS results, although so far unfortunately limited to a few samples, seem to support these ideas. During DTA, the phase transitions with the pure oxides and with the oven-dried mixed-oxide TA-B-O2 were nearly athermal, whereas with the freeze-dried material TA-A-F2 they were strongly exothermal. Indeed, the more random distribution of the

642 metals proposed for the freeze-dried materials would require a more extensive rearrangement of the structure during the formation of the titania and alumina phases and therefore to higher exothermicity as detected by the DTA experiments. The XPS results clearly show that the oxide anions in the structure of the freeze-dried sample TA-A-F2 are in an intermediate chemical environment between those of the pure titania and the pure alumina, which supports the idea of a more random distribution of aluminum and titanium in the structure. On the other hand, with the oven-dried sample TA-B-O2, a strong surface enrichment in alumina and an O~s chemical shift close to that of the pure alumina were observed. This is consistent with segregation of alumina and titania domains in this material, with surface enrichment in alumina, which might be caused by the hydrothermal effect during oven-drying. 5.CONCLUSIONS The textural and structural properties of titania-aluminas prepared by the sol-gel method are strongly influenced by the pH of synthesis and drying method. The textural properties are mainly determined by the pH of the reaction, the synthesis in basic medium leading to higher pore volumes and pore diameters than the one in the acidic medium. The drying-method plays a major role in determining the structure and the nature of the aluminum species present in the mixed oxides. Freeze-drying seems to lead to a more random distribution of titanium and aluminum in the structure of the solids, to a higher proportion of pentacoordinated and N M R invisible aluminum and to lower thermal stability, as compared to oven-drying. With oven-drying, a solid containing separate titania and alumina domains are formed, probably due to an hydrothermal effect. The influence of these properties on surface acidity and catalytic activity are currently under study. REFERENCES

1 - W. Zhaobin et al., - Appl. Catal., 75 (1991) 179-191. 2 - J. Ramirez et al. - Appl. Catal., 94 (1993) 45-59. 3 - Y. Narendar and G.L. Messing - Catal. Today, 35 (1997) 247-268. 4 - G.M. Pajonk- Catal. Today, 35 (1997) 319-337. 5 - O. Rodriguez et al. - Catal. Today, 14 (1992) 243-252. 6 - J. Ramirez et al. - Appl. Catal., 94 (1993) 45-49. 7 - G. Pacheco-Malagon et al., J. Mater. Res., 10 (1995) 1264-1269. 8 - M. Toba et al., J. Mater. Chem., 4 (1994) 585-589. 9 - J. Livage et al. - Prog. Solid State Chem., 18 (1988) 259-341. 10 - J.P. Gilson et al., J. Chem. Soc., Chem. Comm., (1987) 91-92. 11 - G. Engelhardt & D. Michel - High Resolution Solid State N M R of Silicates and Zeolites. John Wiley & Sons, New York, 1987. 12 - K. Terabe et al., Ceramic Trans., 22 (1991) 425-430 13 - R.D. Gonzalez et al., Catal. Today, 35 (1997) 293-317. 14 - N.C.M. Alma et al., Appl. Catal., 6 (1983) 341-346. 15 - J.J. Fripiat et al., J. Catal., 133 (1992) 263-278.

1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmonet al., editors.

Zirconium-Titanium Phosphate Acid Catalysts Synthesized by Sol Gel Techniques Nancy B. Jackson, Steven G. Thoma, Steven Kohler, and Tina M. Nenoff Sandia National Laboratories PO Box 5800, MS 0710, Albuquerque, NM 87185, United States ABSTRACT Recently a large effort has been put into identifying solid acid materials, particularly sulfated zirconia and other sulfated metal oxides, that can be used to replace environmentally hazardous liquid acids in industrial processes. We are studying a group of mixed metal phosphates, some of which have also been sulfated, for their catalytic and morphological characteristics. Zirconium and titanium are the metals used in this study and the catalysts are synthesized from alkoxide starting materials with H3PO4, H20 , and sometimes H2SO4 as gelling agents. The measurement of acidity was achieved by using the isomerization of an olefin as a model reaction. The phosphate stabilized the mixed metal sulfates, preventing them from calcining to oxides boosting their initial catalytic activity. The addition of sulfate prevented the formation of the catalytically inactive mixed metal pyrophosphates when calcined at high temperatures (>773 K). 1. INTRODUCTION The desire to replace mineral acids in industrial processes has lead to a significant amount of work on solid acid catalysts. Recent work has focused on sulfated zirconia and other sulfated metal oxide catalysts. These sulfated metal oxides have proved disappointing (particularly those without a platinum promoter) because of their rapid deactivation and because of the suspicion that the sulfate acts as an oxidizing material rather than a superacid, making the sulfate a stochiometric reagent rather than a true catalyst. ~ Metal phosphates have also been investigated as potential as acid catalysts. For zirconium phosphate, the most extensively investigated crystalline form is the a-layered salt, zirconium bis(monohydrogen orthophosphate), ct-Zr(HPO4)2. H20. z'3 Surface P-OH groups in metal phosphates have been identified as the source of Lewis and Bronsted acid sites. 4'5 Based on the study of zirconium phosphate, others have suggested that it is the electron withdrawing capability of the bulk phosphate groups that enhance the acid strength of surface P-OH groups, the electrons being withdrawn into the bulk via P-O-P bonds. 3 The work that we report on here examines metal phosphate materials synthesized using sol gel techniques that give different metal phosphates than have been previously studied for their catalytic activity.

643

644 The activity of sulfated zirconia (SZ) is particularly sensitive to the synthesis process. As a consequence, the synthesis of sulfated zirconia has been well studied. 61~ When SZ is synthesized from the precipitation of inorganic salts (ZrC14 or ZrO(NO3)2) , the sulfate is added to the amorphous Zr(OH)4 prior to calcination in the form of H2SO4. The zirconiasulfate mix is calcined and a tetragonal ZrO2 is formed which gives a catalytically active SZ. If the Zr(OH)4 is calcined without sulfating first, the monoclinic form of ZrO2 forms and later sulfating with H2SO 4 does not produce an effective catalyst. When SZ is made using sol gel methods, starting with zirconium isopropoxide or some other zirconium alkoxide, the point of addition of U2SO 4 is less significant. It can be added with the gelling water, or the zirconia can be sulfated after drying and before calcination. Although some authors report there is a difference in textural characteristics depending upon when the sulfate is added, the conclusion of its effects on reactivity are varied. 9.~0 When water and sulfuric acid are added to a zirconium alkoxide/alcohol solution, zirconium sulfate may initially form, however, calcination causes the oxide to be the dominant phase. Acidity was measured using the isomerization of 2-methyl-2-pentene ~2-~4as a test reaction. This acid catalyzed reaction can lead to 17 different hexene isomers; however, only three major products emerge. The total activity of the catalyst is thought to be indicative of the ability of the catalyst to protonate the olefinic reactant. 2. E X P E R I M E N T A L The mixed metal phosphates were made using sol gel techniques by mixing two miscible alkoxides together: in this case primarily titanium isopropoxide (TIPT) and zirconia npropoxide in isopropanol. Next, a 5% H3PO4 solution in H20 was added as a gelling agent. The phosphate is then dried and calcined. The sulfated metal phosphates were made using two different methods. The first method added sulfate following calcination: 1) Two miscible alkoxides are mixed together in isopropanol. A 5% HBPO4 solution in H20 is added as a gelling agent. The phosphate is dried and calcined. It is re-dissolved in concentrated H2SO 4, dried and re-calcined. The second method added sulfate during the gelling process: 2) Two miscible alkoxides are mixed together in isopropanol. Along -with the 5% HBPO4 solution in H20 that is added as a gelling agent, H2SO4 is also added. The precipitated material, which is extremely hydroscopic at this point, is dried (523 K for 3-4 days) and calcined. The sulfated zirconia was supplied by Magnesium Electron, Inc. (MEI). Crystal phase was identified using powder x-ray diffraction at room temperature on a Siemens Model D500 diffractometer, with O-20 sample geometry and Cu K-a radiation, between 2 0 =5 and 60 ~ BET measurements were performed on a Quantachrome Autosorb automated gas sorption system. Chemical analysis was performed via DCP

645 using an ARL SS-7 DCP, with the exception of sulfur analysis which was performed by Galbraith Laboratories. A flow-through microreactor system was used for the isomerization reaction. Approximately 50 mg of sample was positioned vertically on a glass frit and topped with glass wool as a fixed bed in a 6 mm O.D., 40 cm long Pyrex tube. The samples were pretreated at elevated temperatures in either ultra-high purity helium (99.999%) or flowing hydrocarbon-free air flowing at 40-60 cc/min. All pretreatments and reactions were at atmospheric pressure. A 10 cc/min He flow was used to carry the 2-methyl-2-pentene from the 273 K saturator to the reactor bed which was held constant at 423 K. The products flowed through heated lines to a HP 5890 series II gas chromatograph equipped with an FID and an automatic sampling valve. 3. RESULTS To study the effect of calcination temperature and sulfating on catalyst activity, an active zirconium-titanium phosphate mixture was chosen and systematically studied as shown in Figure 1. The morphology, surface area, and catalytic activity for the catalysts in Figure 1 are reported in Table 1. The catalytic activity for materials whose final calcination temperature is 773 K is the lowest. As pyrophosphates, these inactive catalyst also have a morphology different from the other catalysts. The condensation of the phosphates to pyrophosphates significantly decreases the activity of the catalyst, which indicates the importance of the P-OH bonds for acidic catalytic activity. Although P-O bonds are still present in pyrophosphates, the number of P-OH bonds have significantly diminished. Sample A: Zr/Ti~O 4 Ti/Zr = 1.5 P/Metal = 0.8 Dry at 368 K

Sample B Calcine 573 K I Sulfated with H2SO4

Sample C Calcine 673 K

Sample D Calcine 773 K

I Sulfated with H2SO4

I Sulfated with H2804

+

4,

Sample SB Calcine 673 K

Sample SC Calcine 773 K

Sample SD Calcine 773 K

Figure 1. The synthesis scheme for zirconium/titanium phosphate and sulfated zirconium/titanium sulfate.

646 The number of P-O-P bonds and P-O-M bonds increase as the material condenses to the pyrophosphate. Although the sulfated catalysts are much more active than the "nonsulfated" phosphates, the deactivation rate is much slower for the phosphate catalysts. Figure 2 compares the activity versus time of C with SB. In addition, the isomerization activity of sulfated zirconia was measured and is recorded on the same plot for reference. The sulfated form of the phosphate initially gives a very active catalyst for this acid catalyzed reaction on a surface area basis. Since the sulfate clearly contributes a hefty portion of the acidity to the catalyst, we attempted to prepare a Ti-only catalyst like A, except sulfuric acid was used, along with 1-120, as the gelling agent in place of phosphoric acid. This synthesis is similar to what is reported in References 8,9, and 10 for sulfated zirconia using sol gel techniques. In calcining up to 773 K, the crystal structure was amorphous. Above 773 K the sulfated titanium material crystallized into the anatase form and at 973 K it started forming the rutile phase. When zirconia is prepared this way, calcination also produces the oxide phase. Like the sulfated titanium, there is enough sulfate left behind on the zirconium material to dramatically effect the catalyst activity. If the Ti-only catalyst is synthesized with using both sulfuric and phosphoric acid with the water as a gelling agent (synthesis 2 described above except with only one metal alkoxide, titanium tetra-isopropoxide), then the catalyst remains amorphous even after calcination at 973 K. The synthesis and morphologies are summarized in Table 2. Table 1 Initial isomerization rate of 2-methyl-2-pentene, morphology and surface area for the zirconium/titanium phosphate catalysts and their sulfated analogs. Sample Calcination Initial Rate XRD Results BET Surface K Mol/min/m 2 x 10.7 Area m2/g A 368 2.1 amorphous 510 B

573

5.5

amorphous

434

C

673

3.2

TixZr2_x(PO4)3

400

D

773

0.3

TixZrl.xP207

367

SB

573

23.9

TixZr2.x(PO4)3(SO)4 51 (NASICON type)/ amorphous

SC

673

39.0

TixZr2.x(PO4)3(SO)4 40 (NASICON type)/ amorphous

SD

773

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

1.5

TixZr2.x(PO4)3(SO)4 47

,., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

,

,

,,

647

1.2E-06

II, "~-----~Zr/Ti/PO4/SO4 (51 m2/g)

1.0E-06

A

i

8.0E-07 "

r~

-'~

It

' Z r O 2 / S O 4 ( 1 0 0 rn2/g)

-~ 6.0E-07 !

~k

4.0E-07

"'"lg. ""

2.0E-07

"Ill- -If.

Z-r/

0.0E+00

. . . . m. . . . . [] l

20

"

l

40

I

;

60 80 Time (minutes)

!

I

100

120

Figure 2. Rate of isomerization of 2-methyl-2-pentene for Zr/Ti phosphate and sulfated Zr/Ti phosphate. These are compared to sulfated zirconia.

Table 2 The effect of calcination on the morphology of a sulfated and sulfated plus phosphated titanium catalyst. Catalyst Titanium, sulfate catalyst Titanium, sulfate, phosphate Ti:S = 2:3 (Ti2(SO4)3) Wi:S = 2:3 (Ti2(SO4)3) Reagents TIPT/H2SO 4 TIPT/H2SO4/H3PO 4 Calcination K Crystal phase Calcination K Crystal phase 773 Amorphous, 773 Amorphous, Ti2(SO4)3 Ti2(SO4)3 883 Anatase 883 Amorphous 993 Anatase/rutile 993 Amorphous 1073 Rutile The sulfur content of the sulfated-phosphorous catalysts is significantly greater than the sulfated metal oxides. The sulfated metal phosphates made in our laboratory by the method described above consistently had a sulfur content of 10-15 weight percent sulfur 15 compared to the 1-3 wt% sulfur found on sulfated zirconia. The amorphous nature of the catalyst and the large weight percent of sulfur present indicates the sulfur has been incorporated into the bulk catalyst. 4. D I S C U S S I O N

The mixed metal phosphate catalysts made in this study were a very different species than the catalytic phosphates previously studied. This is most likely from the significantly different synthesis route which we took that was based on sol gel techniques. Exposure to high calcination temperatures (> 773 K) decreased the catalytic activity of the metal

648 phosphates and cauged the transformation of metal phosphate into metal pyrophosphate. This transformation condenses the phosphate, which has many P-OH bonds available for catalyzing a reaction, to the M2 P207 form which does not have as many hydroxyl groups as the phosphate. In addition, the M2P207 is a very stable structure that does not lend itself well to catalytic activity. Although the amorphous phosphates (B and C) are low in relative activity based on surface area, these are very high surface area materials and on a per gram basis are quite active. Their slow deactivation compared to the sulfated zirconia and other sulfated catalysts is a beneficial characteristic. Unfortunately, the transformation to pyrophosphate at higher temperatures is an unavoidable characteristic for these phosphates. However, as can be seen in Tables 1 and 2, the addition of sulfate, prevents the formation of the pyrophosphates at higher temperatures. In fact, XRD analysis of the sulfated mixed metal phosphates showed a crystallinity that was from the NASICON system which have structures of the type MlxM2(z.x)(PO4)2(SO4). The x-ray diffraction peaks for the SB and SC are very broad and set in the low humps generally associated with amorphous materials, suggesting that the material is not fully crystalline. 15 5. CONCLUSIONS Sulfating a metal such as titanium or zirconium may produce a sulfate initially, but upon calcining the oxide will form. Addition of phosphoric acid disrupts the formation of the oxide and stabilizes the sulfate in the structure. It allows crystallization of sulfate as opposed to the oxide. Ti-O-P bonds appear to be preferred over Ti-O-Ti bonds and Ti-OS bonds. This same chemistry appears to take place with zirconium as well as titanium. Preventing the formation of the oxide allows more sulfate to remain in the bulk of the catalyst, effecting its catalytic properties at higher temperatures. Also, the addition of sulfate to the titanium-zirconium phosphate appears to prevent the formation of pyrophosphate at higher temperatures and actually causes a more crystalline material to form at lower temperatures than what would form without the sulfate. (See Table 1.) This results in the catalyst being active at higher temperature. This correlates with other observations made that associate the acidity of metal phosphates with the surface P-OH groups. 4'5 Additionally, since the electron withdrawing capability of the bulk phosphate groups enhances the acid strength of surface P-OH groups 3, it is not surprising to find the phosphate catalysts more acidic than the condensed pyrophosphates. ACKNOWLEDGMENTS This work was supported by the United States Department of Energy under Contract DEAC04-94AL850000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

' D. Farcasiu,, A. Ghenciu, and Jing Qi Li, J. Catal., 158, (1996) 116.

649 2 j. M. Troup anad A. Clearfield, Inorg. Chem., 16 (1977) 3311. 3 K. Segawa, S. Nakata, and S. Asaoka, Mat. Chem. and Phys., 17 (1987) 181. 4 T. Hattori, A. Ishiguruo, and Y. Murakami, J. Inorg. Nucl. Chem., 40 (1978) 1107. 5 A. Clearfield and D. S. Thakur, J. Catal., 65 (1980) 185. 6 C. Morterra, G. Cerrato, F. Pinna, M. Signoretto, and G. Struckul, J. Catal., 149 (1994) 181. 7 R. A. Comelli, C. R. Vera, and J. M. Parera, J. Catal., 151 (1995) 96. 8 D. A. Ward and E. I. Ko, J. Catal., 150 (1994) 18. 9 D. A. Ward and E. I. Ko, J. Catal., 157 (1995) 321. 10D. Tichit, B. Coq, H. Armendariz, and F. Figueras, Cat. Lett., 38 (1996) 109. ll A. Corma, A. Martinez, C. Martinez, Appl. Catal. A: General 144 (1996) 249. ~2G. M. Kramer, G. B. McVicker, and J. J. Ziemiak, J. Catal, 92 (1985) 355. 13G. M. Kramer and G. B. McVicker, Acc. Chem. Res., 19 (1986) 78. 14j. F. Brody, J. W. Johnson, G. B. McVicker, and J. J. Ziemiak, Solid State Ionics, 32/33 (1989) 350. ~5S. G. Thoma, N. B. Jackson, T. M. Nenoff, R. E. Maxwell, Proc. Mat. Res. Sot., (1997) in press.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

651

Methane Combustion over NiO/BaO/ZrO 2 Catalysts Yuan-Yang Wang" Yin-Ben Gao '~ Yu-Han Sun~* and Song-Ying Chen b " State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, P. R. China

b

Institute of Catalysis, Hangzhou University, Hangzhou 310028, P. R. China

1. INTRODUCTION As the alternative fossil fuel, natural gas becomes much important and even predominant in the energy source structure of next century[ 1]. However, the combustion of natural gas leads to the production of pollutants such as nitrogen oxides and carbon monoxide at high temperature. Catalytic combustion of methane is therefore attracting much attention due to its high efficiency and less or even no pollution[2]. Two types of catalysts were developed for methane combustion. One was supported noble metal, the other was peroviskite-type oxides. The later showed lower activity than the former although it was relatively cheaper[2, 3]. In the previous work, a new type of non-noble metal oxide --- NiO/ZrO~ catalyst was found to be highly active towards methane combustionwith full conversion at about 973K[4]. But the catalyst appeared not stable enough at high temperature. On the other hand, La, Ce and Ba were reported to be the good stabilizer for ZrO 2, and Ba was tbund to be the best additive for NiO/ZrO, ill methane combustion [4]. NiO/BaO/ZrO 2 was therefore investigated via orthogonally-designed preparation in the present work. 2. EXPERIMENTAL

Catalysts were prepared by sol-gel method involving supercritical drying with orthogonal design[5, 6](see Table 1). The nickel was either impregnated (IM) onto BaO/ZrO,. which was prepared by supercfitically drying hydrogels (at 533K and 7.5MPa) produced via the titration of ZrOC1/Ba(NO3) 2 by ammonia (to pH=10.0) or incorporated with ZrOC1/Ba(NO3) 2 before * To whom all correspondence should be addressed.

652 (BG) and after gelation (AG) and then supercritical drying. All samples were calcinated at 773K for 3 hours. Table 1 The characteristic data of methane combustion over each catalyst No.

precursor

1 2 3 4 5 6 7 8 9

NiClo Ni(NO3)2 Ni(AC), NiClo Ni(NO3) 2 Ni(AC)~ NiCI~ Ni(NO3) 2 Ni(AC)o

NiO method BaO (mol%) (tool%) 5 5 5 10 10 10 15 15 15

IM AG BG BG IM AG AG BG IM

6 18 12 18 12 6 12 6 18

TCM(K)

TD(K)

CO(%)

801.86 793.86 795.70 780.85 778.49 787.21 789.16 785.92 769.80

75.78 4.32 7.25 7.53 8.30 4.76 14.85 13.85 13.34

1.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

The catalysts of 200 mg were tested at the ratio of 25 of air to methane and the flow rate of 60 ml/min with the rise of the temperature up to 1123K and then down to room temperature at 2K/rain.The activity was expressed by the Temperature at which the Conversion of Methane reached 50% (TCM), and the stability by the Temperature Difference (TD) of the TCM when heated and cooled. XRD Characterization was executed with a D/max-qtA X-ray diffractometer using MoKc~ radiation(40Kv,50mA). TPR of 50mg catalyst was carded out from 60~ to 880~ at a heating rate of 15K/min with 5% H JAr mixture at a flow rate of 60ml/min.

3. RESULTS 3.1. Catalytic Performance The catalysts were found to be very active, and no CO was detected besides catalyst 1. Figure 1 shows the typical performance of NiO/BaO/ZrO 2 catalysts (catalysts 1, 6 and 9) in methane combustion on heating (solid) and cooling (dash). The data listed Table 1 indicated that most of the catalysts appeared to be very stable after runnig at 1123K. 3.2. Orthoganol Analysis of Preparation Parameters The effect of preparation parameters on reaction performance of catalysts were orthogonally analyzed as shown in Figure 2, the activity was influenced by different preparation pm'ameters as follows: Ni(AC)~ > Ni(NO3) 2 > NiC12

653 100

100 ~

v

~ . , ~

.

Catatyst 6

80

0 ~9

~

6o

.=_

-

~

CO 2

40

=

20

6

0 623

723

823

923

1023

1123

62:

723

823

Temperature (K)

923

1023

1123

Temperature (K) ~

Catalyst 9

~, 8o ,,

co,

60

40

~o ~_o L) 0 623

723

823

923

1023

1123

TemperatLu'e (K)

Fig.1

The catalytic combustion of methane over typical NiO/BaO/ZrO 2 catalysts 15% N i O > 10% NiO > > 5% N i O IM > B G > A G 18% B a O > 12% B a O > 6% B a O

and the stability 9 Ni(AC) 2 > Ni(NO3)~ >> NiC1,. 10% NiO > 15% N i O > 5% NiO

AG > BG >IM 18% B a O > 12% B a O > 6% BaO .';

803

-~'~z z z

~'~~'=

2 >~'

~' ~n

,~ 323

-~ g

o

z z z '-'

-

,,,~-

~:>~

NiO

Melhod

~,'r5

O~

3o6

793

@ o tr+

8

~. "/83

290 g

E r. 773 Precursor

NiO

Melhod

BaO

~, 273 b,

Precursor

Fig.2 The orthogonal analysis of prepartion parameters

BaO

-

554 Among the parameters, besides the introduction method of nickel, the other parameters almost showed the same influence on the pert'onnance of catalysts. It was interesting that the activity and the stability changed with the introduction method in an opposite way.

3.3. XRD XRD patterns of catalysts 1, 6 mad 9 before and after methane combustion (see Figure 3a and 3b) illustrated that no obvious NiO peak was abserved on three typical catalysts before reaction, but it appeared after the reaction, being indicative of the growth of nickel oxide pa~xicles at high temperature. On the other hand, the phase formation and transformation of tetragonal to monoclinic (t --> m) took place [7]. The ZrO 2 in catalyst 9 was in an amorphous state and tetragonal in catalysts 1 and 6 before the reaction. After the reaction, the teu'agonal ZrO 2 mainly appeared in catalysts 1 and 9 whereas the monoclinic ZrO 2 was predominant in catalyst 6.

j9

b

V --- t-ZrO2

X1 V

O --- NiO

~6

~ I

20

I

V

I

1

i 30

~9

[

v

I

I

V --- t-ZrO2

X

X --- rn-ZrO 2

X --- m-Zr% O --- NiO

V

V

I

I

I 40

.I

I

20

I

I

I 50

I

I

i

I

I

60

,

I

i

l

i

!

I

[

!

i

i

,

30

70

i

i

i

,

|

40

I

I

i

i

i

50

i

i

|

i

60

i

70

20

Fig. 3 The typical XRDpatterns of NiO/BaO/ZrO 2 catalysts before(a) and after(b) rection 3.4.TPR TPR patterns of catalyst 1, 6 and 9 were shown

OC-Ni[

NiO

in Figure 4. Three hydrogen-consumed peaks appem'ed for all catalysts, although the peak at 823K was

~_

weakly observed for catalyst 1. According to the

~"

literature [8] and DRS characterization, the peak at

=

-,~

683K was attributed to the reduction of NiO, that at 823K was caused by tetrahedron-coordinated nickel oxide(TE-Ni), and that after 1153K was due to the

Catalyst 6

__J ~'"'"~.~.~,"~

Catalyst 1

=

i

353

t

i

i

553

f

|

|

i

!

T

753

i

I

953

!

i

i

1153

Temperature (K)

reduction of octadetron-coordinated nickel oxide

Fig. 4 The typical TPR patterns of

(OC-Ni).

NiO/BaO/ZrO 2 catalysts

655 4. DISCUSSION 4.1. The Active Sites for Methane Combustion The predominance of NiO present in catalyst s 6 and 9 (see Figures 3 and 4) indicated that NiO was one of the active sites in methane combustion. However, catalyst 1 with less NiO also showed to be active in the reaction, implying that another kind of active sites should exist. TPR patterns illustrated that OC-Ni peak might be responsible for the activity of catalyst 1 (see Figure 4). Forthremore, catalysts 6 and 9 with more NiO led to the full conversion of methane at about 923K without the production of CO, while catalyst 1 fully converted at 1073K and produced CO (see Figure 1 and Table 1), indicating that NiO was more active than OC-Ni in methane combustion. On the othre hand, TE-Ni greatly influenced the stability of catalysts. Catalyst 6 with more TE-Ni sites showed a good stability whereas catalyst 1 with less one deactived seriously (see Figures 1 and 4). Although NiO was available to the activity of catalysts, it might be unstable. As a result, catalyst 9 ahowed a lower stability than catalyst 6 (see Figure 1 and Figure 4). In the meantime, the phase state also influenced the stability of catalyst. The monoclinic ZrO~ ( catalyst 6) showed more stable than the tetragonal ZrO 2 (catalyst 9). 4.2. Tile Effect of Preparation Parameters on Surface Active Sites As mentioned above, each types of surface sites had their own contribution to the performance of catalysts in methane combustion. Obviously, the effect of preparation parameters could be understood in terms of the change of the surface sites. There was a shreshold value for nickel oxide in the catalysts, which might be closely related with the monolayer-dispersion of NiO ola surpport surface[9]. The shreshold of NiO on ZrO 2 support might be between the content of 5% and 10%. Obviously, this type of nickel oxide was active but unstable. Free NiO could be present on the surface only when the content was over the shreshold. So catalysts 9 and 6 exihibited more NiO sites whereas catalyst 1 exhibited less one (see Figure 4). The different performance of precursors resulted fi'om their anions. AC- and NO 3- could be almost removed because all catalysts were calcined at 773K for 3 hours, while most of CI might remain in catalysts. Obviously, CI showed a negative effect on catalytic performance of catalysts possibly either by its reaction with methane or by its interaction with the catalyst surface. Thus, catalysts 1, 4 and 7 with NiC12 as precursor showed lower activity and stability. BaO appeared to be a good stabilizer to NiO/ZrO 2, its effect was closely related with the content. NiO/ZrO~ prepared by sol-gel method involving supercritical drying, as well as ZrO2 by same method, generally appeared in the tetragonal phase or the amorphous state, and the phase transformation of t --> m occurred at the temperature over 673K. When BaO was introduced into NiO/ZrO, catalyst, the t--> m transformation was retarded, which depended on the amount of BaO in the catalysts. With BaO content as low as 6% (catalysts I and 6), BaO could not prevent the phase transformation and most of ZrO 2 was monoclinic. Only when BaO content was such high

656 as 18% (catalyst 9), the tetragonal ZrO~_ could remain predominant in catalyst(see Figure 3). However, the monoclinic ZrO, appeared better than the tetragonal one in the reaction, which should be closely related to the interaction between NiO and ZrO 2. The introduction method influenced the activity and stability of catalysts in an opposite way. IM increased the activity but lowered the stability, whilethe effect of BG and AG were totally different. It was reasonable if the dispersion or explosure of NiO on catalyst surface and the interaction between NiO and BaO/ZrO, were considered. Obviously, IM led to high dispersion of NiO on the surface while BG and AG resulted in the presence of NiO in the body of catalysts. On the other hand, the stronger interaction bwtween NiO and BaO/ZrO, via AG or BG existed because they were mixed at a molecular level. Therefore, more TE-Ni and OC-Ni formed ( see catalysts 6 and 9 in Figure 4). 5. CONCLUSION Barium oxide was shown to be an effective promoter to NiO/ZrO 2in methane combustion. The complete conversion of methane into CO~ could be realized over NiO/BaO/ZrO 2 catalysts at relatively lower temperature by optimizing the parameters. In such case, the temperature correponding to the full conversion of methane was about 923K. Three types of nickel oxides were present on the catalysts. NiO and octadetron-coordinated nickel oxide were responsible for the activity of catalysts (the former showed to be more active than later), while tetrahedron-coordinated nickel oxide and well partially-stabilized ZrO 2 for the stability of catalysts. ACKNOWLEDGEMENT The financial support from the National Natural Science Foundation of China is greatly acknowledged. REFERENCES 1. Soffanko, Stud. Surf. Sci. Catal., 81(1994)93. 2. Joo H. Lee and David L. Trimm, Fuel Processing Technology, 42(1995) 339. 3. M. F. M. Zwinkels, S. G. Jaras and P. G. Menon, Catal. Rev.-- Sci. Eng., 35(1993)319. 4. Y. Y. Wang, Y. B. Gao, Y. H. Sun and S. Y. Chen, Proceeding of 14th Pitsburg Conference,

S-p715, Taiyuan, 1997. 5. Y. Y. Wang, Y. B. Gao, Y. H. Sun and S. Y. Chen, Catal. Today, 30(1996)171 6. Mathmatical Institute of Academia Sinica, Experimental Method with Orthogonal Design, Press of People's Education, Beijing, (1975)69. 7. P. D. L. Mercera, J. G. V. Ommen and E. B. M. Doesburg, Appl. Catal., 78(1991) 79. 8. S. Contarini, J. Michalik and M. Narayana, et. al., J. Phys. Chem., 90(1986)4586. 9. Y. C. Xie and Y. Q. Tang, Adv. in Catal., 37(1990)1.

1998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

657

CONTROL PREPARATION OF ALUMINIUM CHROMIUM MIXED OXIDES BY SOL-GEL PROCESS L. Baraket and A. Ghorbel Laboratoire de Chimie des Mat6riaux et Catalyse. D6partement de Chimie, Facult6 des sciences de Tunis, Campus Universitaire 1060 Tunis.

Abstract Gels of mixed chromium and aluminium oxides were prepared by hydrolysis and polycondensation of organic precusors of metal. In this work, we try to understand the chemistry involved in the preparation with the aim to better control the formation of final material. Evolution of UV-vis, ESR, IR and 27A1 NMR spectra as a fonction of stirring time of organic metals mixture, shows a modification of Cr(lII) and AI(III) ions environment, wich reflect the formation of mixed complex Cr-A1 in solution. The synthesis parameters examined in this work are the chromium loading Cr/(Cr+AI)(%), stirring time (Ta) and the hydrolysis ratio k=[CH3COOH] / [Al(O-s-Bu)3]. We found that the aging time (Ta) of organic precursors governs the distribution of chromium between the surface and the bulk, and affect the symmetry of surface chromate species. The amount of acetic acid added affects considerably the solid texture and we note an increase of pore size with the K ratio, whereas chromium amount affects the dispersion and chromium oxidation state at the surface. Introduction Sol-gel process provides a new approach to the preparation of new materials. This process allows a better control of the whole reactions involved during the synthesis of solids. Homogenous multi-component systems can be easily obtained, particulary homogenous mixed oxides can be prepared by mixing the molecular precursors solutions (I). The chemistry of the sol-gel process is based on hydrolysis and polycondensation reactions. Metal alcoxides [M(OR)3] are versatile molecular used to obtain oxides, on account of their ability to form homogeneous solution in large variety of solvents and in the presence of other alcoxides or metallic derivatives and also for their reactivity toward nucleophilic reagents such as water (2). The essential aim for the most investigations reported was the final product and its applications without interested to the synthesis conditions and reactionnel mechanism involved to obtain gels, although the properties of a gel and its behaviour on heat treatment could be very sensitive to the structure already created during the sol stage. Therefore, the formation of colloidal aggregates determines the main properties of resulting powder. By varing the chemical conditions under which materials is polymerized, the structure and the morphology of samples formed are drastically affected. However, many earlier investigations try to developp a good understanding of gelification phenomenon.The purpose of this work is to better understand the chemistry involved during the preparation of mixed oxides by sol-gel process, in order to have a good control of the properties of the final material. Experimental section 1. Catalysts preparation two steps are involved to obtain the gel, a one compexation followed by a second polymerisation reaction. The synthesis was carried out with a mixture containing aluminium sbutoxide [Al(O-s-Bu)3] (strem, purity 98%) and chromium acetylacetone (strem, purity 97,5%)

658 in butanol-2 as solvant (Merck, purity 99%). The alcoxide concentration relative to the alcohol is constant and equal to lmol/dm 3. This mixtures was then stirred under reflux condition during a definite time (Ta) namely a~ng time. Gels are obtained after hydrolysis and condensation in presence of water. In our case, water will be formed in situ after supply acetic acid (prolabo, purity 99,7%) to the mixture. Acetic acid played important roles in both the hydrolysis and condensation. It reacts rapidlly with AI(O-s-Bu)3 to give AI(O-s-Bu)3_x(OCOCH3)x, which is much less reactive than Al(O-s-Bu)3 in hydrolysis. The amount of acetic added is represented by k=[CH3COOH] / [Al(O-s-Bu)3]. The preparation parameters studied in this work are the chromium loading Cr/(Cr+AI)(%), aging time (Ta) and hydrolysis ratio K. Gels already prepared, dried overnight at 340 K in air and then heated under air flow at 723 K, lead to chromium and aluminium mixed oxides catalysts.

2. Experimental techniques In order to study the precursors behaviour in solution under reflux condition, we followed the complexation step of organics sels by: IR, UV-visible, ESR and 27A1NMR spectroscopies. The spectra were recorded as a fonction of a~ng time. Infrared spectra were mesured with Perkin-Elmer FTIR paragon 1000 PC, the solution was placed in a thin film between two KBr windows. Electronic spectra were mesured at room temperature with Perkin Elmer lamda 2. ESR study was realized by a spectrometer Bruker ER 200tt at 77 K. All spectra were measured at a microwave frequency 9,47 GHz. The 27A1 NMR spectra were recorded with a Bruker AC 300 MHz, AICI3 was used as an external reference. Thermogravimetric (TG) and differential thermal (DTA) analyses were performed on dried (uncalcined) samples by heating up to 1073 K in air stream, with a heating rate of 5~ The equipment used is a SETARAM. XRD was used to study the structure of the samples after calcination at 723 K. The diffractograms were recorded on a philips equipment using CuKet radiation. BET Surface areas and pore volumes of the samples treated at 723 K were detemlined by adsorption of N2 at 77 K, the apparatus used is a Micromeritics ASAP 2000. Catalytic activity was performed at low conversion (<5%) in paraxylen partial oxydation on 70 mg of sample, in dynamic regime between 523 K and 643 K, under atmospheric pressure. The catalysts were prior activated under flow oxygen for 14 hours before use in the catalytic reaction. Subsequently the catalyst was contacted with the reactant mixture flowing at a rate of 7,2 1/h.

Results and Discussion 3. Complexation reaction study 3.1. UV Visible The first preparation stage was examined in transmission to follow the evolution of Cr(III) coordination sphere. This study was carried out for solution with 5% chromium loading. figure 1 present the UV-vis spectra recorded at different stirring times. UV-vis spectrum of Cr(acac)3 in butanol-2 (figure l-a) present intense band at 380 nm assigned to ~r --~:r* or n----zr* transitions in the acetylacetone ring and a broad band at 559 nm due to d-d transition (4A2g----~4T2g(T)) of Cr(IIl) in an octahedral environment, as a matter of fact, the two other Cr(III) transitions bands are masked by the charge transfer corresponding to the organic ligand (3, 4). The spectra of mixture undergo modification as fontion of aging time as revealed by the shift of the broad transition band at 559 nm toward lower frequency to steady at 615 nm. This reflect a modification in Cr(III) coordination sphere with preserving an octahedral symmetry, due to the decrease of the ligand field strength of chromium complex (5, 6). In the other hand, the intensity of the band at 350 nm decrease with stirring time (Ta).

659 Therefore, the changes induced by increase of aging time of the mixture, can be interpreted on the basis of partial exchange of organic ligands chromium ions to be anchored on the aluminium. We note also that the spectrum aluminium acetylacetonate shows no bands in this region. 2.0

. . . . .

1.5

0.5

0.0

~

t_

3041

2(X}

400

500

:Yr'~,---_

6(kq

700

I.ONGI IEUR D ' O N D E

Figure 1. UV-vis tranmission spectra of mixture containing 5% chromium as a fonction of aging time. (a): Cr(acac)3 in butanol-2. (b): after 1 day (c): after 20 days.

3.2. Infrared study The infrared spectra of the mixture containing 5% chromium for different aging times are shown in figure 2. IR spectrum assigned to Cr(acac)3 (figure 2-a ) shows two strong absorption bands at 1577 and 1523 cm-I assigned to the C=O and C=C stretching modes respectively, relative to ligand acetylacetone (7). For the molecular precursors chromium and aluminium mixture, the spectra show two news bands at 1600 and 1534 cm-i, which increase in intensity while those corresponding to the chromium acetylacetonate decrease when the aging time is extended. As infrared spectrum of aluminium acetylacetonate present two caracteristics bands at 1600 and 1535 cm-t (7) we can therfore attribut the two news bands to ligand acetylacetonate anchored to aluminium.

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

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----\,

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.

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,.

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,,,

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.

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.

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.-,:

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.... ,,1i

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\ \" i/i " ." "'..:'..,\ /, ......... ..' ..................... ".. ".,'.\ i.'..,.j : ....,... .',\ 9 i,; " .... .." ',, ..+' ,"

..... "'-. " ',X \\

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i , ,/- , . ' \ ?",..._'1,, ,,." .

: : ',,.7,,

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....

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.......

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.

...........

. . .

'~"-

', \

". "

',",

/

,"

.' ..... 9 ."

,,:;; ......( I '.'''o' 1533 cm i

Figure 2. Infrared spectra of mixture containing 5% chromium as a fonction of aging time. (a): Cr(acac)3 in butanol. (b): 3 hours. (c): 1 day. (d): 10 days (e): 18 days. (f): 30 days.

660

4.2. Electron Spin Resonance Cr(lll) is a good paramagnetic probe when is used in a diamagnetic matrix. Indeed, Cr(llI) is a d3 transition metal ion with three unpaired electrons (S=3/2) and exhibits energetic levels very sensitive to small variation of the local crystal field (8). The ESR spectrum recorded for the precursor Cr(acac)3 in butanol-2 (figure 3-a) exhibits two main signals around g= 1,95 and g= 3,93. It is closely the same spectrum already rapported by S. Doeuff and al (9). On the basis of theoretical analysis, these authors attribute this spectrum to chromium ions axial symmetry with a slightly orthorhombic ligand field. A drastic modification is however observed for the spectra when the solution containing 10% chromium is refluxed, indicating ligand movements in the first coordination sphere of the chromium ion (10). Indeed, we note the apparition of two signals at g= 5,36 and g= 4,48, whereas the intensities of the signals assigned to Cr(acac)3 at g values of 3,9 and 1,95 are reduced. This change can be interpreted by a chemical interaction between both molecular precursors Cr(acac)3 and AI(OR)3. f=3.91

*..

Cr(acac}3 in bmanol-~

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Figure 3. ESR spectrum for mixture at loading chromium 10% as a fonction of aging time.

4.2. 2 7AI Nuclear Magnetic Resonance By means of 27A1 chemical shifts, we can distinguich the environmental symmetry of aluminium atoms (11,12). Hence, we have try to follow the evolution of aluminium coordination sphere by 27A1 NMR investigation. The spectrum of aluminium s-butoxide in butanol-2 shows only a broad resonance at 58 ppm attributed to tetrahedral coordinated aluminium (figure 4 ) (13). The spectrum recorded after 24 hours stirring of 5% chromium mixture, present a new narrow resonance at 4,07 ppm, the value of this chemical shift indicate the presence of aluminium in octahedral symmetry. The same narrow peak appears when we sup.ply acetylacetone (Hacac) to the solution of aluminium s-butoxide. In fact, acetylacetone is a bidentate ligand which can react rapidly with aluminium alcoxide (7) according to the following reaction: AI(OR)3 + xHacac ~ Al(OR)3(acac)x The symmetry of aluminium undergo an increase from tetrahedral to octahedral coordination. The increase of chromium loading to 10% deplaces the chemical shift of the

661 narrow peak toward high values. The chemical shift of the six-coordinated aluminium from 4,07 to 5,22 ppm observed with increasing loading chromium, indicates that chromium atoms could be located in the second coordinated sphere. In a previous study (14), it was shown the contribution of the outer-coordination sphere on the chemical shift when the ratio Si/A1 change.

f

9

~,

,~

,~,

,,~L,

Figure 4. 27A1 NMR spectrum: (a) Al(O-s-Bu)3 in butabol-2. (b): Al(O-s-Bu)3+Hacac in butanol-2. (c): 5% content chromium solution. (d): 10% content chromium solution.

4. Caracterisation catalysts Obtained xerogels were designed as X-Y-Z: X: weight percent Cr/(Cr+AI)(%) Y: stirring time Ta (day) Z: K ratio (K=[CH3COOH]/[AI(OR)3])

4.1. X-Ray analysis XR powder diffractograms for all the catalysts treated at 723 K have the same aspect which indicates an amorphous structure.

40 '

30 20 ' ' 2O Figure 5. Shape of XR-diffractogram relative to catalysts.

1'o

5

2.2.2. T h e r m a l analysis All the DT and TG curves of the uncalcined samples, exhibit the same shape ( figure 6 ). The DT curve show two weak endothermics peaks around 373 and 533 K. These endothermic effects involve departure of physically adsorbed water molecules (370-383 K), and the elimination of -OH groups and water molecules strongly adsorbed at the network (15) at higher temperature (533 K). The intense exothermic peak around 673 K corresponds to the elimination of organic ligands, which undergo combustion reaction with oxygen. Morover, the curves

662 don't present a thermic accident above 673 K until 1073 K. This result suggest the formation of stable oxides which occures close to 693 K.

~

3

3

Figure 6. Shape of DT and TG curves for chromia-alumina catalysts. 4.3. Surface areas m e s u r m e n t s

Table 1. BET s ufface area and porous volume of zhromia-alumina catalysts. Sam lp.~ BET Surface area m2_./.g Porous volume crn~_g 5-1-1

382

0,211

5-2-1

378

0,225

5-3-1

341

0,503

10-2-1

374

0,265

10-2-5

373

0,199

10-2-20

381

0,219

18-2-1

384

0,225

25-2-1

445

0,265

30-2-1

462

0,228

35-2-1

256

0,166

,

The textural properties of all the samples are presented in the table 1. We note that BET surface area decreases rapidly for sample containing 35% chromium, this is due to the agglomeration of chromium oxide under the effect of temperature at 723 K. The amount of acetic acid does'nt affect the BET area, whereas the porous volume increases with the K ratio. This result can be interpreted on the basis of the enhance exchange between acetate ions and -OR when K increases as observed by IR spectroscopy (16). At high temperature the departure of this organic ligands induces a large porosity for important K ratio used. 4. 4. Electronic Spin Resonance

Supported oxides present different dispersions with the presence of variable oxidation state chromium ions. ESR is a good technique to bring informations about the dispersion state and oxidation degrees of the paramagnetic species. Figure 7 present ESR spectra for chromia alumina samples with different chromium loading. The spectra show at least three kinds of chromium ions. A narrow signal at g= 1,98 with a AH width of about 50 G, this signal is identified as the y-phase of supported chromium (17), a broad symmetrical band centred arround g=1,98, having a width AH=1287 G and a strong peak at g=4,89, this two signals has been designated as 13-phase and b-phase chromium

663

respectively (17). It was demonstrated that the chromium in both the f~ and b-phases was in 3+ valence state. The 6 phase chromium resonance is attributed to isolated Cr3+ ions in a fairly strong axial field with distorsions of lower symmetry, situated either in the bulk of the support or on its surface, while the resonance 13is due to clumped Cr3+ ions. The ~,-phase is identified with Cr(V) ions at the surface, or often attributed to an oxidised phase containing coulped Cr3+ and Crr+ species at the surface of supported chromium and of amorphous chromia. Schematically the model consists of-Cr6+-O2--Cr3+-O2--Cr6+ species linked by exchange forces (18). The spectra show a dominated broad I3-signal at high content chromia and a y-phase at low chromia loading. These results display that the chromium is well dispersed for the low loading and is aggregated for high content. phase't..,./~ /

phase "/

(a)

I I ~ ~ .......

./

gain=8 104

(b)

gain= 8 102

(a)

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(c) gain= 4 104

! I

I Y;X)

IIXX) 15t'D 2t'X~ ~

~

V

3~X) 4(X~)

3700

..~~,)0

FigureT. Evolution of ESR spectra as fonction of chromium loading (a): 18-2-1. (b): 10-2-1. (c):5-2-1. Figure 8. shows ESR spectra for 10% containing chromium samples for different aging time of the solution. The experimental results show that the spin Cr(V) number at the surface decreases with the increases of aging time due probably to the incorporation of chromium within the network. This incorporation is related to the formation of a stable mixed complex chromium-aluminium in the solution. ph....

"x /'t tt

f i

llliu'~t~ b

(hi

gains4.104 500

lfX)0

15130

2O00

.....

./

L

.... i /

....

'\/t

2500 .~

Figure 8. Evolution of ESR spectra as fonction of aging time. (a): 20 days. (b): 5 days. (c): 1 day

.~.~,,

I/-----

V

~h-,

gain: 3,L.IflI .doo

664 The amount of acetic acid affects the number of Cr(V) at the surface, which increases with the K ratio. 4. 5.

UV-vis reflectance diffuse

Figure 9 shows the diffuse reflectance spectra of mixed oxides containing 5% chromium for three aging time of the solution. The spectra present a band at 620 nm attributed to Cr(III) d-d transition in octahedral symmetry. Strong bands situated at 285-300 nm and 375 nm attributed to tetrahedral chromate transition (19) with a shoulder at 420 nm also assigned to Cr(VI) transition. Since such transition is symmetry forbidden in Td symmetry (chromate anion CRO42-) (19,20), the presence of this band indicates the distorsion of Td symmety of Cr(VI) caused by anchoring to the support surface or formation of a dichromate species. Nevertheless, Raman spectra of samples don't exhibit band caracteristic of polymerised chromate species, only bands attributed to chromate species with a low symmetry are observed. When stirring time of the mixture is extended, the intensity of the band at 420 nm increases which indicates a pronouced anchoring of chromate species at the surface. When the chromium loading increases toward 30% th peak at 425 nm became non-negligible as shown in figure 10. The intensification of this band is caused by the extent of chromate species polymerisation. Hence the increase of chromium concentration improves the crystallization of Cr203 as it was yet shown by ESR study where 13-signal dominates the spectrum.

/

',

A

,:' ,:': i :f

!/

i

i i

i,

~ ",...."'"",, i i

t

Figure 9.

r- ...-'...'..i \~ " :/ "i ._.',.t. !-._ i/ ",,\" i V/ "/ "r '..

,

"1

'',,

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\,X\

:

"i .;

"~ :.

",,~

.... ~.

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L ............. ..

Figure 10.

Figure 11.

Figure 9. Evolution of UV-vis spectra as fonction of aging time. (a): 10-2-1 (b): 10-2-5. (c): 10-2-20. Figure 10. Evolution of UV- vis spectra as fonction of chromium content. (a): 35-2-1. (b): 30-2-1. (c): 25-2-1. (d): 18-2-1. (e): 10-2-1

(f):5-2-1.

Figure 1 I. Evolution of UV-vis spectra as fonction of K ratio. (a): 5-1-1. (b): 5-2-1. (c): 5-3-1. Figure t 1 presents the evolution of UV-vis spectra as a fonction of K ratio. For samples with ratio equal to 1 and 3 the spectra present two independent bands at 275 and 370 nm similar

665 to those observed wil~hCRO42-solution (19). Thus for these samples the chromate species are in Td symmetry which decreases for K=2 sample where the chromates are likely more anchored at the surface.

Catalytic activity The xerogels chromia alumina are tested in paraxylen partial oxydation reaction. The reaction products detected are carbon dioxide and paratolualdehyd. The results given in this paper correspond to a reaction temperature of 613 K. The effect of variation of aging time on the PTA activity and selectivity is summarized in figure 12. It is seen that an extent of stirring time is unfair to aldehyd formation. This result is correlative with the ESR study which display that a prolongation of stirring reduces the number of Cr(V) ions at the Surface caused by the formation of stable chromium-aluminium complex. Indeed, many investigations pointed out that active sites in partial oxydation are surface CrS+ ions (22, 23). .. ................ i!li

60 u PTA activity 9PTA selectivity

50 ~. 3.

40

t t

2.

30

1

.

.

0

.. , .r , 10 20 s t i m n g time (days)

20 30

Figure 12. The effect of stirring time on the F'TA activity and selectivity The catalytic activity and selectivity of oxides having various Cr contents are illustrated in figure 13. We found that the PTA activity increases with the content chromium, whereas the selectivity remains almost constant with catalysts having low content chromium (5-18%), and falls for higher loading samples. Therfore, a high chromium content unfairs the chromium dispersion at the surface and reduces the number of actif Cr(V) sites in partial oxydation. For those concentration Chromium will be found mainly in Cr203 amorphous phase, wich Cr3+ ions actifs for total oxidation .

. .

.

o .....

i4 !2

.

. . . . " PTA activity .p.o PTA selectivity ~ ~,

60 50 .-.

4o :~. 3o "~ x

aal

'

"to"

.~

'

20

"

Cr/Cr+AI (%)

Figure 13. The effect of loading chromium on the PTA activity and selectivity

666 The results pointing out the effect of the acetic acid amount added for gelification are presented in figure (14). The catalytic properties are improved with the increase of K ratio. This result is correlative with the ESR study, which shows exaltation of paramagnetic Cr(V) species with the amount of acetic acid supply (16). 3

.

.

.

.

100

.

x PTA activity e PTA selectivity

r

,,,.

90--,

80 ;~ lJ

/

9

oo

.

,

.

I/ '// a

i

70 60 ,

;

,

so

Ratio K

Figure 14. The effect of K ratio on the PTA activity and selectivity

Conclusion Infrared investigation show the presence of acac ligand linked to aluminium atoms with the same ligand attached to chromium, thus the exchange of this ligand is not total. Under this condition the coordination of chromium atoms is always the same but the ligand field strength decreases. An insight into the molecular interaction between the chromium and aluminium precursors is given by ESR study wich display a modification of ligand field. In the same way Al atoms undergo an increase in its coordination sphere due to the exchange with acac ligand. All this results converge to the formation of mixed stable complexe in solution between the two precursors of chromium and aluminium. Gels obtained dried at 340 K, then treated at 723 K lead to amorphous chromia-alumina catalysts. The caracterization of these samples as fonction of the synthesis parameters allow to conclude that the synthesis conditions have a considerable effects upon texture, structure and catalytic performances. The aging time (Ta) of organic precursors under reflux governs the distribution of chromium between the surface and the bulk. Thus, when Ta increases the chromium is more incorporated within the lattice due to the formation of mixed complexe, this will induce a falls of the activity and selectivity, whereas the textural properties are not changed. The rise of chromium content affects the dispersion and the oxidation state of chromium at the support. Catalytic activity enhances when the chromium loading increases. On the other hand, the PTA selectivity decreases for high content chromium samples due to chromium agglomeration at the surface. The amount of acetic acid supply affects considerably the solid texture, and we note an increase of pore size with the K ratio, while the surface area remains almost constant. In the same way, the catalityic properties are improved due to the exaltation of actif sites Cr(V) at the surface. REFERENCE 1. S. R. Gurkivich and J. B. Blum, Ferroelectrics., 62 (1985) 189. 2. J. Livage, M. Henry and C. Sanchrz, Prog. State. Chem., 18 (1988) 259. 3. T. S Piper and R. L. Carlin, J. Chem. Phys., 36 (1962) 3331. 4. D. W. Brown, J. Inorg. Nuclear. Chem., 21 (1961) 221; 22 (196) 183. 5. D. A. Brown, D. Cunningham and W. K. Glass, J. Chem.Soc., A (1968)1563. 6. K. Rajeshwar, J. App. Electrochem., 15 (1985) 20.

667 7. M. Duval, R. Freymannand J. Lecomte, Societe chimique de France., 17 (1952) 106. 8. Mc. Garvey, B. R. J. Chem Phys., 40 (1964) 809. 9. S. Doeuff, M. Henry, C. Sanchez and J. Livage, J. Non-Cryst. Solids., 89 (1987) 84. 10. R. Karthein, H. Motschi, A. Schweigr, S. Ibric, B. Sulzberga and W. Stumm, lnorg. chem., 30 (1991) 1606. 11. D. Muella, W. Gessner, H. J. Behrens and G. Schelr, chem. phys. Letteers., 79 (1981) 59 12. D. Freude and H. J. Behrens, tryst. Res. Techn., 16 (1981) K36. 13. J. Akitt, W. Duncan, J. Magn. Reson., 15 (1974) 162. 14. D. Mueller, D. Hoebbel and W. Gessnr. chem.phys. Letters., 84 (1981) 1. 15. Ch. Ben Amara, N.Gharbi and H. Zarrouk, Journal de societe chimique de tunisie III (1988) 8. 16. S. Rezgui, A. Ghorbel, J. chim. Phys., 92 (1995) 1576. 17. K. Ktihler, C. W. Schl~ipfer, A. Von. Zelewsky, J. Nickl, J. Engweiler and A. Baiker, J. catalysis., 143 (1993) 201. 18. R. B. Fahim, M. I. Zaki and R. M. Gaber, Appl. catalysis., 4 (1982) 189. 19. A Iannibello, S. Marengo and P. Tittarelli, J. chem. Soc Faraday. Transl., 80 (1984) 2209. 20. F. Cavani, M. Koutyrev, F. Trifiro, A. Bartilini, D. Ghisletti, R. lezzi, A. Santucci and G. Del Piero., J. Catalysis 158 (1996) 236. 21. M. Zaki and N. E. Fouad., Appl. Catalysis 21 (1986) 359. 22. I. Mathur, N. N. Bakhshi and J. F. Mahews. Canadien., J. chem. Enginee. 58 (1980) 601. 23. S. Zinc, A. Sayari and A. Ghorbel, Canadien J. Chem. Enginee., 65 (1987) 127

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1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmonet al., editors,

669

V205-SIO2 Catalyst P r e p a r e d by the sol-gel P r o c e s s in the O x i d a t i v e D e h y d r o g e n a t i o n o f n-butane E.L. S h a m ab, V. Murgia ab, J.C. Gottifredi ab and E.M. Farffin-Torres a a INIQUI-CONICET, Consejo de Investigaci6n

b Facultad de Ingenieria Universidad Nacional de Saita, Buenos Aires 177, 4400 - Saita, Argentina ABSTRACT Vanadium silicate gels (V205-8iO2) were prepared by the sol-gel method by hydrolysis of vanadium acetilacetonate and silicon alkoxide. The changes in the vanadium species and its interaction with SiO2 network upon heat treatment at various temperatures was studied by means of XRD, XPS and IR. The catalytic behavior for the oxidative dehydrogenation of nbutane was studied. It was found that hydration degree of gel derived catalyst strongly influenced its catalytic activity. 1. INTRODUCTION Sol-gel methods have been used by different authors working on catalysts preparation (1). The advantages of the applications of these methods to the design of catalytic materials have been already described in the literature (1,2). Catalysts based on supported vanadium oxide are frequently used in selective oxidation reactions due to their considerable activity for the oxidation of aromatics, alkanes, and alcohols (3-5). Differences in their catalytic behaviour are generally explaines on the basis of the nature and distribution of vanadium species, which are influenced by the vanadium loading, the preparation procedure and the acid-base character of the support (6-9). The interaction of vanadia with silica and the structure of the VOx units on the support have been studied by a number of investigators (5,7,9-17). It has been observed that in general highly dispersed monomeric surface vanadia species were found on silica-supported catalyst with very low loadings of vanadia (7,12-13). Crystalline V205 has been detected on catalysts well bellow the so-called "monolayer" coverage as the vanadium loading was increased. The formation of crystalline V205 in the V205/SIO2 system reflects the weaker interaction of vanadium oxide with the SiO2 supports relative to others like A1203 and TiO2 (5,14,18).

670

To obtain a catalyst with V-SiO2 interactions stronger than those developed in impregnations solids we have studied the preparation of these materials by means of the solgel process. Neuman et al. have previously prepared amorphous metallosilicalite xerogels by the sol-gel method using metal alkoxides precursors (4). This work have shown that vanadium silicate xerogel are active catalysts for the activation of aqueous hydrogen peroxide for a variety of reactions including epoxidation of alquenes, oxidation of secondary alcohols to ketones and the hydroxylation of phenol. In the present work we report the obtention of VOx-SiO2 catalyst from vanadium acetyl acetonate and tetraethoxysilane. These catalysts were characterized and evaluated in the oxidative dehydrogenation of n-butane.

2. E X P E ~ M E N T A L

2.1. Catalyst Preparation Tetraethoxysilane (TEOS) and vanadium acetyl acetonate (VAA) were used as precursor to prepare gels containing 1.4 weight % of vanadium. The catalysts preparation is based on a two-step procedure. In the first stage VAA was dissolved in methanol, given rise to a clear green solution, and mixed under stirring with TEOS. In the second stage this mixture was refluxed at least two hours. Then hydrolysis and condensation were completed to form the gel by the addition of one water equivalent and ammonium hydroxide (25%). The gel is then washed with water and dried at 60 ~ to obtain a fine yellow green powder. The dried gel was calcined in air increasing the temperature with a speed of 10~ min -i and then kept at a fmal temperature of 500, 600 or 700~ during 16 hours. Samples prepared in this way were named SiVA-x ~ were x ~ is the calcination temperature or SiVA-TS. The gel was also activated by a sudden heating at 700 ~ during a few minutes and then calcined at 500 ~ and named SiVA-TS (TS = thermic shock). A reference catalyst with the same vanadium content was also prepared by impregnating SiO2 (Aldrich 300 m 2 g-l) with ammonium methavanadate solutions. The solid named 1.4 V/SiO2 was dried at 60 ~ and calcined for 16 hours at 500~

2.2. Catalyst Characterisation X-ray diffi'action (XRD) patterns were collected in a Rigaku diffractometer DMax-IIC using Cu kc~ radiation (3~ = 0.1549 nm). Diffuse reflectance spectra (DR) in the UV-Visible region were obtained with a GBCUV/Vis 918 equipped with an integer sphere and using BaSO4 as blank reference. Infrared spectra were recorded between 4000 and 400 cm-1 with a Bruker IFS 88 on samples dispersed in KBr and pressed into thin wafers. The XPS spectra were taken in a Physical Electronics 5700 ESCA spectrometer. The exciting radiation was Mg Kc~ (1253.6 eV). The binding energies (BE) were calculated with respect to C 1s peak set at 284.5 eV.

671 2.3. Catalytic Test N-butane oxidehydrogenation (ODH) studies were performed in an isothermal stainless steel microreactor (1I) 0.9 cm) fed with a mixture 15/20/65 of oxygen, n-butane and nitrogen respectively at a constant rate of 153 ml min l. In all cases a constant amount of catalyst (0.4 g ) diluted in 0.8 g of quartz was charged in the reactor. Particle sizes were 500-800 g m . Two layers (0.5 cm height) of quartz particles one above and one below the bed completed the reactor. Temperature was recorded from a moving centreline thermocouple (type K). The resulting catalytic volume and W/F were 1.2 cm 3 and 6.26 g cat h/tool (n-butane) respectively. The reaction temperature range was varied (400-600 ~ to achieve different conversion levels. Blank tests on the same reactor showed no activity in all the range of temperatures studied. Reactants and reaction products were analysed by on-line chromatography using a Varian 3700 and a Shimadzu GC-3BT chromatographs for heavy and light products respectively. A 25% dimethyl sulfolane in Chomosorb P 60/80 (15m) and 32% silicone oil DC 200/50 in Chomosorb W 60/80 (5m) nylon column was used in the first case and a molecular sieve 5A 60/80 stainless-steel column (2 m) in the second case.

3. RESULTS

3.1. Catalyst Characterisation The 1.4 V/SiO2 catalyst and SiVA calcined materials show XRD patterns of amorphous solids with a very broad diffraction line centre near 4.06 A characteristic of amorphous silica. In the case of the gel dried at 60 ~ (SiVA-60~ four low intensity diffraction lines at 5.876 A; 4.131 A; 3.761A. and 3.160 A were also observed. These lines corresponds to a vanadium hydrated gel developed on the surface of SlOE (19-20). The DR spectra in the UV-Vis region of SiVA solids are shown in Fig.1. SiVA-60~ is a green yellow powder, (Fig. 1.A) and shows an broad absorption band at 229.3 nm related to SiO2 gels, and two shoulders centred at 288.5 and 347.8 nm corresponding to V 5+ in tetrahedral environment (3,4). After calcination at 500 ~ a broad absorption band with two maxima at 265.0 and 389.3 nm are observed corresponding to V 5+ ions with tetrahedral and octahedral co-ordination respectively. When the calcination temperature is increased at 600~ the presence of only an absorption band at 280.0 nm shows that the V 5+ ions in tetrahedral environment are the principal species present in this solids.( Fig. 1C and 1D). The DR spectrum of SiVA-TS sample (Fig. 1E) is very similar to that of SiVA-500~ showing that the thermal treatment employed in this case does not produce a noticeable change in the developed vanadium species. UV-Vis DR spectra of SiO2 and 1.4V/SIO2 catalyst before and after calcination together with SIVA-500~ are presented in Fig.2. 1.4V/SiO2 before calcination (Fig. 2B) present an absorption band typical of SiO2 (213 nm) and the presence of both tetrahedral (268 nm) and octahedral (broad band in the 350-500 nm region) V s+ species. After calcination (Fig. 2C) SiO2 band disappear and tetrahedral and octahedral V 5+ species are only observed. This spectrum is very close of SiVA-500~

672

till (J

z

-D

El

D~ 0

9C -B

w

~D

2OO

4OO

WAVELENGTH

600

{n m )

A

BOO

Fig.1 DR UV Vis spectra of (A) SIVA60~ (B) SiVA-500~ (C) SiVA-600~ (D) SiVA-700~ and (E) SiVA-TS

----- A

260

I

4o0

....

I

I

'

WAVELENGTH {nrn)

obo

Fig.2 DR UV Vis spectra of (A) SiO2; (B) 1.4V/SiO2 60~ (C) 1.4V/SiO2 500~ and (D) SiVA-500~

Infrared spectra of SiVA materials in the 2000 to 400 cm -1 region are depicted in Fig.3. On the SiVA-60~ spectrum (Fig.3A) we can see the following absorption bands: 1636 cm l belong to physisorbed water; 1400 cm -1 and 669 cm 1 belong to residual ammonia, 1080 cm l and the shoulder at 1217 cm-1 assigned to v -Si-O-(Si) vibration; 958 cm l assigned to v-SiO-(H) and v-Si-O-V, 800 cm -1 belong to ring structures and valence vibration of silicon; and 519, 509 and 459 cm-1 assigned to O-Si-O bending deformation (22,23). The IR spectra of calcinated SiVA show that the intensity of the band at 1639 cm ~ decrease for sample heated at 500~ (Fig.3B) and disappear for higher calcination temperatures. The bands corresponding to v-Si-O-(H) vibrations becomes narrower and shift to 1082, 1103, and 1107 cm -1 for SiVA-500~ SiVA-600~ and SiVA-700~ respectively. The band corresponding to v-Si-O-(H) is very sensible to the heat treatment and shift to 970 cm l for SiVA-500~ and to 933 cm -] for samples calcined at higher temperatures. The intensity of bands belonging to residual ammonia (1400 and 669 cm -1) decrease with the calcination temperature being completely absent for SiVA-700~ (Fig.3D). In any case the Si-O-Si bending deformation bands shift from the corresponding values observed for SIVA60~ The IR spectrum of SiVA-TS sample (Fig.3E) is very similar to that of SiVA-500~ but bands belonging to residual ammonia are completely absent in this case. Infrared spectra of SiO2 and calcined V1.4/SiO2 are presented in Fig.4. Commercial SiO2 support present the same bands observed for SiVA-600~ (Fig.4A). When vanadium is

673 introduce on the support the v-Si-O-(Si) and v-Si-O-(H) band shift slightly to lower wavenumbers due to the weak interaction of vanadia with the silica support.

E la,l m

m

2006 ..... ~6bo ' ~2~ ' 8 o o WAVENUMBER (r

'

Fig. 3. IR spectra of (A) SiVA-60~ (B) SiVA-500~ (C) SiVA-600~ (D) SIVA700~ and (E) SiVA-TS

4oo 2oo0

~

~2oo

WAVENUMBER

800

(era - I )

400

Fig. 4. IR spectra of (A) SiO2; (B) 1.4V/SiO2 500~ and (C) SiVA-500~

Bands related to vanadia species like g o 4 or V=O, could not be distinguish in the IR spectra due to the overlapping of this bands with the corresponding to the SiO2 support. The binding energies determined from XPS spectra together with the V/Si atomic ratios are presented in Table 1. Table 1. Binding energies and V/Si atomic ratios determines for SiVA and 1.4V/SIO2 catalysts BE (eV) Catalyst

V2p3/2

Si2p

SiVA-60~ SiVA-500~ SiVA-600~ SiVA-700~ SiVA/TS 1.4V/SiO2/60~ 1.4V/SiO2/500~

515.50 516.30 516.20 516.70 516.40 516.40 516.20

102.60 103.12 103.10 103.00 103.25 102.50 102.80

Surface Atomic Ratio V/Si 0.38 0.048 0.054 0.066 0.050 0.057 0.050

674

The most relevant features of the BE values of the V2p3/2 levels for the SiVA catalysts are as follows: (i) For SiVA-60~ the low BE values observed indicate that VO + species can be present in dried gels before calcination (24). ii) When SiVA gel is calcined BE corresponding to VO4 species were observed for all the calcination temperatures, iii) For impregnating catalyst BE of VO4 units were also determined. Regarding to BE of the Si 2p levels it can be observed that in the case of SiVA-60~ gel silicon may be present forming ring or chain silicates (24). When SiVA gel is calcined BE typical of SiO2 gel were detected. The atomics ratios V/Si were determined from the integral of the signal corresponding to V2p3n and Si 2p levels. These atomic ratios can be taken as a measure of the relative dispersion of vanadium in this support (25). From values reported in Table 1 we can observed that SiVA-60~ gel present a high vanadium surface concentration showing that vanadium is essentially distributed on the surface of SiO2 particles or the silicates formed upon hydrolysis. After calcination at 500~ the V/Si ratio strongly decrease indicating that a new solid organisation is achieved and V is now also dispersed in the pores of a S i Q network. When calcination temperature is increase to 600 and 700 ~ an increase of surface vanadium dipersion is produced as consequence of the loss of specific surface area due to dehydration of SiO2 network and also to vanadiumsegregation from the micropores to the surface of the support 1.4V/SiQ catalyst calcined at 500 ~ and SiVA-500~ present near the same vanadium dispersion, so we can expect to observe similar catalytic behaviours for these two materials. 3.2. Oxidative Dehydrogenation of n- butane The predominant products in the oxidation of n-butane were dehydrogenation products (1butene, cis- and trans-2-butene and 1,3-butadiene) and combustion products (carbon oxides). Small amounts of lower hydrocarbons were also detected. Figure 5 shows the dependence of conversion on reaction temperature. Over the range of studied temperatures the conversion was found to be higher on the SiVA-500~ and SiVA-TS catalyst than the SiVA-600~ SiVA-700~ and 1.4V/SIO2 samples.

30 25. 20o

"~ 15 L_

> C 10 o

35O

i

~

- -

zOO

i

|

5O 500 55O Temperature (*C)

600

Fig.5. Variation of n-butane conversion % with the temperature of" (11) SiVA-500~ SiVA-600~ (A) SiVA-700~ (v) SiVA-TS and (O) 1.4V/SIO2

(O)

675 Table 2 shows the DHG selectivity values measured at the reaction conditions investigated for these catalysts. As indicated in this table, the 1.4V/SiO2 sample was the more selective catalyst, followed by SiVA-500~ solid. However, the conversion levels were lower for the impregnated catalyst than those observed for SiVA-500~ sample. SiVA-TS catalyst developed near the same activity of SiVA-500~ solid but its DHG selectivity is lower. SiVA-600~ and SiVA-700~ were the less selective catalyst of the studied series. Table 2. Reaction Data for SiVA and 1.4V/SiO2 Catalyst Catalyst

Temp (~

Conversion %

Selectivity % Cracking 1.14 4.72 4.71

DHG 44.16 54.54 53.74

SiVA-500~

494 567 575

17.28 26.29 26.01

Combustion 54.7 40.74 41.55

SiVA-600~

492 566 574

5.39 16.43 17.93

92.74 83.55 77.7

1.77 1.,89

7.26 14.68 20.41

SiVA-700~

494 567 575

2.77 11.63 12.71

87.75 82.09 73.15

2.59 2.49

12.25 15.53 24.37

SiVA-TS

494 567 574

15.49 27.07 28.11

66.06 47.63 46.05

1.29 3.7 3.83

32.65 48.67 50.12

1.4 V-SiO2

490 567 574

2.87 15.53 17.16

23.34 32.67 32.84

2.83 3.29

76.66 64.5 63.87

4. D I S C U S S I O N

From DR, FTIR and XPS results we can conclude that the vanadium acetyl acetonate is completely hydrolysed under the reaction condition employed in the synthesis of vanadium silicate gels. A vanadium oxihydroxide layer interacting with a silicate oligomer derived from TEOS hydrolysis is obtained after drying the gels at 60 ~ After heat treatment at 500~ a rearrangement of gel network takes place and a dispersion of VO4 unit in a SiO2 gel network is obtained. From IR results it can be observed that the absorption band belong v-Si-O-(H) which normal absorption takes place around 980 cm -1 (for highly ordered dehydrated silica) is strongly shift to 958 cm ~ for SiVA-60~ This shifting can be related to the substitution of hydrogen for vanadium in the silanol groups, which can produce a change in the position of

676

the band more pronounced than in the case of deuteration of silanol groups (26). However the observed absorbance position shift could also be due to the formation of metastable intermediate products of polycondensation of silica gel, for example trimmer or tetramer (23). This is the most probable situation because when vanadium silicate gel is calcined at 500~ (SiVA-500~ a shift to 970 cm -1 was observed for v-Si-O-(H) band related with a polycondensation of silica gel network and the formation of VO4 units on the surface of support (23). While, if the absorption band at 958 cm -1 should be related with Si-O-V groups this mild heat treatment must not produce any change on its position and intensity. Furthermore XPS and DRX results agree with these conclusions, because for SiVA-60~ material the BE energies of V2p3/2 level correspond to the presence of VO + species (probably a vanadium oxihydroxide from XRD), and the BE for Si2p belong to tetramers of SiO2. The SiVA-60~ powder is then a silicate highly hydrated cover by a layer of a vanadium oxihydroxide gel, as denoted by the position of the broad absorption band at 1080 and 1216 cm -1 belong to v-Si-O-Si in tetrahedron silica well organised network and the high V/Si surface atomic ratio determined from XPS studies. The calcination of SiVA materials at 500 ~ produce only a slight shift of the v-Si-O-Si band to 1082 cm -1 without changes in the band intensity. However, the v-Si-O-H band shifts to 970 c m 1 showing that at this temperature a solid state reaction between the vanadium oxihydroxide layer and the silica oligomers takes place. In consequence the v-Si-O-H band loss intensity and shifts to wavenumbers values typical of VO4 bridged by water molecules on silica (3). This is also confirm by the BE of V2p3/2 level and the disappearance of the vanadium oxihydroxide diffraction lines in the XRD patterns. This model is consistent with the conclusions of Yoshida et al. (15) deduced from XANES/EXAFS spectroscopy and also with that of Narayama et al. (27) by spin-echo modulation. However, octahedral species are also present, because a broad band in the region of 400-550 nm was observed in the UV-Vis spectrum of SiVA-500~ (Fig.lB), which is assigned to a change transfer band in VO6 clusters (28). From XPS, DR and IR studies it can be seen that the catalyst obtained by calcination of SiVA gel at 500~ presents the same surface vanadium dispersion and is structural similar to the reference catalyst 1.4V/SIO2/500~ The principal differences between both catalyst are the structure arrangement and hydration degree between the SiO2 network obtained by the solgel synthesis and the commercial silica. When the calcination temperature increase the v-Si-O-Si band becomes more intense and narrow and shift to higher wavenumbers values (1102-1103 cm-1). The observed increase in the definition and intensity of this band is produce when water is release from SiO2 network producing that S i-O-H groups change to S i-O-Si groups. These results show that the silica network developed by sol-gel process is really well organised, because only in this case this behaviour is observed. For randomly crosslinked SiO2 gels negative shift of the band and a strong intensity decrease must be observed. Another fact that confirm the former observation is that the 800 cm l band related with ring silicate structures is unaltered upon heat treatment. For SiVA-600~ and SiVA-700~ catalyst it is also observed that the band at 970 cm -l belong to v-Si-O-V absorption shift to 933 cm -1. As in these solids the dehydration degree is strong we can assign this band to V=O species adsorbed on basic sites. This band is more

677 intense and narrower than that observed for SiVA-500~ because there no more or very few contribution of silanols groups to the absorption as was confirmed from v-Si-O-Si band shape. SIVA-TS catalyst is very similar in structure and vanadium dispersion to SiVA-500~ oxide. The only difference between these materials is that the hydration degree seems to be lower in the SiVA-TS solid as shown in IR and DR results. In DR spectra, it can be seen that a more important contribution of tetrahedral vanadium species seems to be present. This probably can arise because the short heat treatment at 700~ produce an important dehydration allowing that a more important fraction of octahedral species transforms to tetrahedral co-ordination. Reaction Studies

As was shown from characterisation results SiVA-500~ SiVA-TS and 1.4V/SiO2/500~ present almost the same surface vanadium species and dispersion. It can therefore be expect to observe the same catalytic behaviour for this three solids. As predicted selectivity towards dehydrogenation products is very close for the three catalysts. However, from Fig.5 it can be seen that 1.4V/SiO2/500~ is markedly less active than both SiVA catalyst. This difference in catalytic activity can be related with the hydration degree of SiO2 supports. Results reported by Le Bars et al. (29) show that acidic hydroxyl groups present on V/SiO2 catalyst could be related with hydrated forms. These authors also report that an increase in the acidity of silica-supported vanadium oxides enhance the rates of oxidative dehydrogenation of ethane but does not affect the selectivity to ethene. Then, as in SiVA-500~ and SiVA-TS catalyst the hydration degree is higher than that observed for 1.4V/SiO2 oxide, the SiVA oxides must be more acidic and consequently more active as was determined. For SiVA oxides calcined at 600 and 700~ the hydration degree is lowered and SIVA700~ oxide behave as impregnated catalyst, confirming that activity is controlled by the acidic hydroxyl groups content. In the case of SiVA-600~ and SiVA-700~ catalyst changes in selectivity were also observed. However, the vanadium dispersion is higher for these solids. Then, changes in selectivity must be correlated with the increase in VO4 groups after dehydration. Then it becomes clearly that both octahedral and tetrahedral species must be present on vanadium-silica catalyst to achieved good selectivities. CONCLUSIONS Our research results show that vanadium-silica catalyst prepared by sol-gel method are active catalyst for the oxidative dehydrogenation of n-butane. This catalyst was more active that an impregnated catalyst with the same vanadium loading. Differences in activity were attributed to the development of a hydrated vanadiasilica catalyst when sol-gel process is used. Derived sol-gel catalyst with good activity and selectivity are obtained when the vanadiumsilica gels were calcined at 500~ Selectivity toward dehydrogenation products was proven to depend on the ratio of tetrahedral to octahedral vanadium species. Both type of species must be present to obtain good selectivities to olefin production.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

679

Synthesis, structure and catalytic activity of CuO/TiO2 mixed oxides obtained by alkoxo-methods in CO oxidation M.V. Tsodikov a.., Ye.A. Trusova a, Ye.V.Slivinski, G.G.Hemandez a, D.I. Kochubeyb, V.G.Lipovich " and J.A. Navio ~'* "A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Prospekt, 29, Moscow l17912(Russia). ~mstitute of Catalysis, Siberian Branch of Russian Academy of Sciences, Acad. Lavrent6v Prospekt, 4, Novosibirsk 630090 (Russia). CInstituto de Ciencia de Materiales de Sevilla, Centro de Investigaciones Cientfficas "Isla de la Cartuja", Avda. Am6rico Vespucio, s/n. 41092-Sevilla (Spain). * Corresponding authors

Structural, electronic and catalytic properties of Cu-O/TiO2 mixed oxide catalysts tested for CO oxidation, wich were obtained by a modified alkoxysynthesis method from Ti(O"Bu)4 and Cu(C17H35COO)2, have been studied by X-ray diffraction, EXAFS, XPS, magnetic susceptibility techniques and kinetic measurements. Partial chelation of Ti(O"Bu)4 by acetylacetone was shown to stabilise sols and to ensure anatase-based single phase Cu~Tix. 0.5~02 (x<_0.06). For x20.06, the formation of cupric oxide and titania gives evidence of phase separation. At temperatures T<150~ an overall CO conversion over the Cu~Til-0.sxO2 depends on the parameter x and on mean crystal size : by increasing x and decreasing , there is a marked increase in activity. It is suggested that terminal (non-bridged) oxygen of the bond Cu=Os, surface anion vacancy Vs, related to the reduced Cu+1 state and surface labile species Os participate in CO oxidation. At temperatures T >150-200~ the participation of lattice oxygen in the reaction mechanism is postulated. 1. INTRODUCTION The development of highly efficient mixed oxide exhaust catalysts, free of noble metals, is one of the principal problems in the field of ecological catalysis [ 1]. For the gas phase reaction of CO oxidation, a fundamental question is to elucidate the role of structure and surface oxygen in the reaction mechanism. Over a number of perovskites ABO3, obtained by high temperature synthesis, the change of overall activity in CO oxidation is affected by the binding energy of oxygen with the B ions: the decrease in B-O binding energy corresponds to an increase in catalytic activity [ 1,2]. Thus, the participation of surface oxygen species in a redox mechanism is highly probable. Oxidation/reduction steps proposed for the reaction mechanism suggest an appearance of the point defects in the oxygen sublattice.

680 Alumina and titania based Cu-containing catalysts, prepared by precipitation and manifested as attractive catalysts for CO oxidation, have been extensively studied by structural and kinetic methods [3,4]. In catalyst production technology and research into their properties chemical purity of the oxides synthesised is of great importance together with the development of the surface and the formation of the desired structure. In this connection recently developed alkoxotechnology methods of complex oxide synthesis merit especial attention. These methods permit the production of uniform complex and highly dispersed metal oxide systems [5,6]. One might expect in this connection that the alkoxo-technique would provide the most favourable conditions for the set-up of the complex metal oxide structure and its catalytic activity interconnection investigation [7]. In the present work the results are reported concerning synthesis and structure investigation of copper titanate oxides prepared by means of alkoxosyntheses and an investigation of the relationship between structure and catalytic activity in low temperature CO oxidation will also be reported.

2. EXPERIMENTAL 2.1. Catalyst preparation Two groups of Cu-O/TiO2 catalysts were synthesised by modified sol-gel methods, using titanium n-butyrate Ti(OnBu)4 and cupric stearate Cu(C~vH35COO)2. The first group of samples was obtained from gels containing different amounts of Cu in the following way: (1) firstly a partially chelated by acetylacetone Ti(OnBu)4 in benzene solution was obtained. For this purpose 10% benzene solution of Ti(OnBu)4 and acetylacetone, taken in the molar ratio 1:1, were boiled with the reverse condenser for 30 min. (2) Monoethanolamine solutions of Cu(C~TH35COO)2 containing different amounts of Cu, were prepared next. (3) The two above solutions were mixed, boiled for 30 min. and finally cooled to room temperature, under intensive stirring. The solution obtained was denoted as parent solution (PS). (4) Mother liquors were obtained by mixing PS and 80% ethanol-water solution followed by intensive stining and boiling for 30-40 min. The molar ratio of Ti(OnBu)4 and water during this stage was kept at 1:4. (5) The solid gels formation period lasted up to more than one month. In this connection, the freshly prepared gel was made up after the organic solvent was evaporated. After removing of organic solvents, drying in air, successive drying in v a c u o (2 mm Hg) at 80~ careful mixing and thermal treatment at T ~ 5 0 0 - 5 5 0 ~ in air for 6h., a set of mixed oxide catalysts containing 0.5; 1.2; 2.3; 5.0; 8.2; 11.0 and 14.2 wt.%Cu was obtained, named hereafter samples 1, 2, 3, 4, 5, 6 and 7, respectively. In the second set of Cu-O/TiO2 catalysts, each sample always contained-5 wt.%Cu. The catalysts 8, 9 and 10 were obtained after the mother liquors were exposed (ageing) at room temperature for 24, 72 and 144 h., respectively, followed by catalyst extraction as described above. Catalyst 11 also contained-~5 wt.%Cu and was prepared from non-chelated by acetylacetone Ti(O'lBu)4 . Here, the solid product precipitation was observed when adding the alcohol-water mixture to organic solution. Once a precipitation was completed, the organic solution was evaporated and the solid gel was subjected to a treatment procedure as described for samples 1-9. The copper-free titania, sample was also prepared by the sol-gel method using Ti(0 n Bu)4 and will be considered as a blank sample (sample 0).

681 2.2. Catalytic experiments Gas phase CO oxidation was carried out at temperatures 50-200~ at atmospheric pressure in a V-type quartz flow reactor supplied by a stainless steel sheathed ChromelCoppel thermocouple. The reactor was placed in a vertical resistively heated oven, the temperature of which was regulated automatically by a controller. It was verified in a special blank run that neither thermocouple nor reactor walls showed any catalytic activity. The standard loading in each run was 0.5 cm 3 catalyst of fraction 0.1-0.25 mm. Initial gases were mixtures of 0.36-0.40 vol.%CO in air. The gas flow was controlled by mass flow controllers. A flow rate of an hourly space velocity of 7000-50000 was used. Catalytic runs were performed in a regime of step-wise temperature rise and drop whereby the duration of each run at a given (fixed) temperature was not less than 30 min. The oven temperature was ramped up and down at a speed of 5~ Before the run each catalyst was pretreated in a helium flow at 300~ for 3 h. The gas mixture from the reactor was checked automatically by IR in situ cell (Riken-Fine IR-5500) which allowed an immediate evaluation of CO to CO2 in the gas effluent. 2.3. Catalyst characterisation All synthesised catalysts were dispersed powders with grain size 3-60 p. Cu content was evaluated using a Perkin Elmer spectrometer AS-400. X-ray diffraction was used to determine the crystalline phases present and to estimate the mean size of microcrystals. A DRON-3M diffractometer with filtered Cu K~ radiation was used. Room temperature EXAFS spectra for near K-edge absorption bands of Cu and Ti in Cu-O/TiO~_ were obtained. Catalysts were recorded on the VEPP-3 electron storage rings of the Institute of Nuclear Physics (Novosibirsk) and analysed as in references [8]. Magnetic properties of copper-containing oxides were studied by the Faraday method on the installation described previously [9]. The temperature dependence of the static magnetic susceptibility z(T) was determined in the 77 to 300K temperature range. A study of the surface compositions and valence state of Cu was performed by X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were obtained by a PHI-5500 PerkinElmer device with a hemispherical electronic energy analyser with monochromatic A1 K~ X-ray beam (1486.6 eV). An estimate of valence state of Cu was performed via combined analysis of binding energy and curve shape [10,11]. Spectra of O(ls) were divided on the peak of oxygen relating to the lattice oxygen OL and oxygen of the water included in the structure with a known mean of binding energy of 529 eV and on the peak of the surface oxygen Os with a binding energy of 531,5eV. [12]. The calculation of atomic concentrations based on XPS-spectra was carried out according to standard programs. The study of gels by IR-spectroscopy was carried out on a Spekord-8000 device in the 800-3700 cm -1 range. The values of specific surface areas were calculated on basic adsorption isotherms (desorptions curves) obtained on a vacuum installation equipped with the Mack-Benn balance at 300K by BET method. Purified N2 was used as adsorbate.

3. RESULTS 3.1. Catalytic and structure data Table 1 summarises the data on copper content, phase composition, average crystallite size, specific surface area and the overall CO conversion obtained at an hourly space

682

Table

1

Copper content, phase composition,

size o f c r y s t a l l i t e , specific s u r f a c e a r e a (Sap), a n d C O

c o n v e r s i o n ( % ) o v e r C u - O / T i O 2 catalysts at d i f f e r e n t t e m p e r a t u r e s

a n d at a h o u r l y s p a c e

velocity 50000.

Cat

Cu

Phase composition,



S,,

cont.

empirical formula

(nm)

m2g

CO conversion (+1%) at temperature"

wt. %

100~

0

' 0.0

'Anatase

I

' 0.5

' Anatase

' .

122

,

50

120~

[ 120 !<0,5 i

120

,

3

140~

<0,5 ,

<0,5

150~

170~

<0,5

<0,5

200~ <0,5

,,,

7

'

14

19

24

27

22

26

32

49

72

' 86

100

46

66

90

100

60

74

84

96

50

73

93

100

Cuo.006Tio.99702 1.2

Anatase

42

120

8

Cuo.olTio.9902

2.5

Anatase Ctlo.ozTio.99Oz

4

' 5.0

i Anatase

i 8.2

' Anatase

!

!

30

118 i 16

39

i 61

17

118

49

89

I 100

14

110

56

92

' 100

7

'21

i I

I

Ctlo,o6Tio.9702

Cuo.ogTio.9502 CuO (tr) 6

' 11.0

'Anatase-80~

'

' 80

'

'

Rutile-9%, CuO-I 1% 14.2

i

Rutileo36%~"

i 70

9

28

12

22

Anatase-50%, CuO- 14% 5.3

Anatase

i

19

110

' Anatase,CuO (tr)

'

25

~ 100 '

,

i

'

CLlo.o6Tio.9702

9

5.1

i0

5.4

11

' 5.2

Anatase-23 ~

2

8

'-' 15

20

2

8

''17

25

62

96

7

18

92

,

-

90

'40

92

, ,

Rutile-73%,CuO-4% i

i

Ruffle, CuO Anatase

30

1

: 3

(tr)

v e l o c i t y o f 5 0 0 0 0 h 1. A s c a n b e seen, u n l o a d e d C O o x i d a t i o n at a n y t e m p e r a t u r e .

TiO2 ( s a m p l e 0) is p r a c t i c a l l y i n a c t i v e in

On loading 0.5-8 wt.%Cu

(samples

1-5) a s i g n i f i c a n t

i n c r e a s e in C O c o n v e r s i o n w a s o b s e r v e d . F u r t h e r l o a d i n g u p to 14,2 w t . % C u ( s a m p l e 7) r e s u l t s in a c o n s i d e r a b l e a c t i v i t y d e c r e a s e e s p e c i a l l y n o t i c e a b l e f o r t h e l o w t e m p e r a t u r e s

683 100-140~ For catalysts 8-10, obtained from the ageing gels by (4) during the preparation, a noticeable decrease in activity is observed, if it is compared to the activity shown by catalyst 4, obtained from the fresh gel. The most remarkable effect is observed for sample 11 at low temperatures 100-150~ When non-chelating by acetylacetone (stage1 during the preparation), the obtained catalyst 11 also shows a decreased index of CO conversion compared to sample 4. Thus, the results obtained clearly show that the activity of Cu-O/TiO 2 mixed oxide catalysts in CO oxidation in the low temperature region is being strongly influenced by the extent of copper loading and by the conditions under which the catalysts are prepared. Unloaded polycrystal TiO2 (sample 0) is a tetragonal modification of titania (anatase) with a mean crystal size of -122 nm and lattice parameters close to those given in reference (a =0,377A; c =0,945A) [8]. For samples 1-4 containing 0.5-5.0 wt.%Cu, X-ray diffraction patterns also exhibited the anatase structure, whereby the characteristic diffraction peaks of the doping element are not observed. There is practically no change in the TiO2 lattice parameters upon loading, while the mean crystal sizes are considerably decreased. For sample 5 (8wt.%Cu), the presence of traces of CuO together with the host TiO2 (anatase) give evidence of phase separation. For further copper loading, in sample 7 (14,2 wt.%Cu), the formation of roughly dispersed CuO and TiO2 (rutile) structures are observed. For catalysts 8,9, containing ~5 wt.%Cu - obtained by exposure of the mother liquor for 24 and 72 h. during the preparation-, an increase in the mean crystal size of copper-containing TiO2 (anatase) compared with sample 4 from 17 to 25 nm is quite apparent. Ageing for 144 h. (sample 9) causes a considerable phase separation with the formation of anatase (~86 nm), rutile and CuO structures. For doped TiO2 samples (2 and 5 wt.%Cu), the temperature dependencies of the static magnetic susceptibility, Z-~(T), showed linear behaviour obeying Curie's law. The means of effective moment, ~fr are equal to ~1.95-2,15 gB. This value seems to correspond to isolated, non-interacting Cu 2+ ions (d 9) in an octahedral surrounding of oxygen anions [ 13]. EXAFS data showed that all dispersed catalysts 1-5 are in the form of Cu-doped anatase where the interatomic distances for anatase were identical with those for the crystalline oxide [12]. Figure 1 (a,b) shows the typical copper edge EXAFS spectrum of catalyst 4 (5 wt.%Cu) and the corresponding modules of the Fourier transform [FT(R)[. According to the EXAFS data copper ions have regular surroundings by 6 oxygen atoms at a mean distance of~1.95 A. Beside this distance there is only one peak at ~3.0 A, which is not typical for CuO compound with the Cu-Cu distance equal to 2.88 A [14]. Moreover, the contribution at ~3.0 A cannot be due to the presence of a small domain of Cu20 phase because the Cu-O bond in this compound has the coordination number 2 [14]. These data are in agreement with data obtained by magnetic measurements showing the presence of paramagnetic ions Cu 2+. Table 2 presents two possible models describing the distance at ~3.0 A in the EXAFS spectrum. The first model describes the Cu-Ti distance of ~3.04 A as an interatomic distance in the first coordination shell of copper, involving copper-doped anatase. The second model deals with the Cu-Cu distance of ~2.99 A of the copper dimmers situated within the CuO structure. For both models, the coordination number is estimated as equal to -1.

684

CuO/TiO.2 o,.78

s wt

1 20

i

I

b

~176176

'

t

I 0.0a

'-i

I

Cu

0.80 - -

1 -I

0.00

0.40

'-i I

i

I

-0.04

' ! 0.00

1 s.oo

1o.oo

12.oo

o.oo

2.00

k.A-1

4.00

6.00

8.00

10.00

R-B,A

Figure 1. (a) EXAFS structure, %(k), vs. X-ray photon energy (k, A -1) and (b) modules of the Fourier transform for anatase-based catalyst 4 (5 wt.%Cu).

Table 2 EXAFS data of coordination number, Debye-Waller factors (AcF), R-factors and interatomic distances involving immediate copper environment according to the two proposed models for the Cu-O/TiO2 catalyst 4 (5 wt. %Cu) . Model

Interatomic distances, A

....

Effective coordinatio n number

A(y 2

(xlO 3)

Bond type

R-factor

26.9

1.95

6.2

2.6

Cu-O

3.04

0.74

2.4

Cu-Ti

1.95

6.2

2.6

Cu-O

2.99

0.72

2.3

Cu-Cu

26.4

685 The analysis shows that preference should be given to the first model because: (i) this model is consistent with magnetic data indicating the presence of isolated and noninteracting paramagnetic cupric ions, and (ii) the second model does not correspond with the data on CuO in which, for distorted CuO6 polyhedrons, four Cu-O bonds with mean distances of-1.95 A and two Cu-O bonds with distances exceeding 2.2 A are typical [ 14]. The possibility of the estimated coordination number equal to -1 by EXAFS was also demonstrated for Cu-Ti bonding in the mixed oxide Cu3TiO 4 [ 15]. Table 3 summarises the XPS data for the Cu-O/TiO2 catalysts 4,9 and 11 with -5 wt.%Cu obtained in different conditions and showing different catalytic ability. For all these samples the XPS scans showed that copper is mainly present as Cu § (-90% rel.) ( the binding energies of 935; 942,5- 943,0 and 956 eV) and a small quality Cu l§ (-10% rel.) (the binding energies of 932,5 and 952,5 eV) [11,12]. The divalent-Cu has practically been converted into Cu +l during 30 min of XPS-procedure.

Table.3 XPS results of surface composition (% atomic) for the Cu-O/TiO2 catalysts 4, 8 and 11. 4

8

1i

20.6

21.3

22.'8

2.8

6.1

5.9

O Lat. + H 2 0

63.1

60.5

61.1

Os

13.5

12.1

10.3

Os / Cu

4.9

2.0

1.7

Composition ........

Ti

It is noticeable that the surface copper concentration for sample 8 is significantly higher than for sample 4 refined from the fresh gel. Probably the process of surface enrichment by copper can be strengthened during mother liquor ageing and at least it can lead to microheterogenisation of the systems; as a result CuO formation can be seen in the phase composition of samples 9 and 10 (Table. 1). It is worth noting the important result that the difference in the surface composition of the catalysts studied is the various values of the surface oxygen (ls) to cuprous ions ratio. 3.2. Activity-structure relationships

The data obtained show that at low temperatures the overall CO conversion depends on the structure properties of the mixed oxide catalysts. Thus, an increase of copper loading causes the decrease in the mean crystal size from -122 nm for unloaded TiO2 to -14 nm for the Cu-doped anatase with 8.2 wt.%Cu (Table 1). Moreover the relationship between microcrystal size and catalytic activity can be observed only for Cu-containing oxides because the copper free TiO2 was not active in this temperature range. For catalysts, contained ~5% wt. of Cu, the dependence of CO catalytic ability (C.A.) on , reported in Table 1, clearly shows a pronounced tendency for C.A. to fall with mean crystal size. The rise in is mostly caused by gel ageing.

686 Size effects in CO oxidation have been previously studied in [16] for a series of perovskites. It was shown that a further increase of by 2 nm causes the significant decrease in activity. Thus, the results obtained show that at low temperatures 100+140-150~ the active sites of the Cu-O/TiO2 catalyst are mostly formed on the base of single phase Cu-doped anatase whereby the catalytic activity increases with copper loading. On the contrary, phase separation does not favour catalytic ability.

4. DISCUSSION It is well known that an ageing of the gels obtained from the parent solution (PS) is accompanied by the condensation and polymerization of the hydrated colloid system affecting the structure of the target material [17,18]. The gels in the present work, unlike gels obtained by conventional sol-gel methods from inorganic compounds in water solutions, preserve the amorphous structure. The ageing of organometallic gel-precursors causes an increase in the mean size of TiO2 microcrystals which are accompanied by the phase separation and formation of the thermodynamically stable structures of rutile and cupric oxide. The metastable anatase may be only stabilised from the fresh gels suffering no ageing (see Table 1). During the first stage of the synthesis, the partial chelation of Ti(O"Bu)4 by acetylacetone favours the sol stabilisation and formation of anatase-based single phase CuxTil_0.sx02 (x<0.06). Partial chelation of titanium alkoxide by acetylacetone is known to result in an exchange of alkoxyl groups for acetylacetonate ligands, which prevents the following substitution of alkoxyl groups by hydroxyl groups [17,18]. Thus, the rate of hydrolysis is significantly decreased after addition of alcohol-water solution. In fact, the IR-spectra of Cu-containing gels, obtained by partial chelation, showed strong band at v=1520-1530 c m -1 typical of CO-bond in the chelated groups [19]. Moreover, the addition of water-alcohole solution to the partially chelated system does not result in precipitation. In this case the time of gel formation lasts more than month. According to X-Ray data the decrease in the mean crystal size with an increase in copper loading was not accompanied by any change in the anatase crystal parameters. The results presented are similar to results which have been in the previously work [8]. In this study it has been shown by X-Ray and EXAFS methods that the atomic distances and microcrystallite parameters of the anatase practically not change with Fe loading. It can be noted that the ionic radius of Cu 2§ (0,72 A) is quite close to Ti4+(0,68A). Therefore, one can suggest that the titanium ions are,in part, isomorphic substitutes for Cu 2§ ions on the interface planes, thus preventing their growth. In fact, due to the EXAFS data, cupric ions most probably raise an anatase surface but do not impenetrate in the bulk. Phase separation takes place when the copper concentration achieves the means more than 5%wt.(x> 0,06 ) probably due to come nearer and CuO domains formation as well. It has been pointed out earlier in [20] that for a series of transition metal oxides (M=V, Mn, Fe, Cr, etc.) supported on inorganic carriers, non-bridged terminal oxygen of the M=Os bonds may be considered as active centres in oxidative catalysis. An easy shift in electron density from 02. to M n+ during electron-phonon excitation of M=Os may result in anionradical intermediates which are effective in heterogeneous free radical generation at low temperature (T-90 ~ hydrocarbon oxidation [7,20,21]. At the same time, thermal excitation of the M=Os bonds at temperatures (T>200 ~ because of high covalence of the bond, may ensure great mobility of the surface oxygen resulting in its extraction from the

687 lattice and formation of an easy bound surface species Os. The latter seems to be of great importance during active site formation under CO oxidation. It was shown, for a series of perovskites ABO~, that a noticeable correlation exists between catalytic ability in CO oxidation and the strength of the B-O bond [1,2]. Over perovskites at low temperatures, a small decrease in the bond energy causes the pronounced increase in overall rate of carbon monoxide oxidation. We suggest that the surface oxygen, observed by XPS on the surface of single-phase anatase-based catalysts, acquires properties of an easy bound surface species Os. For the catalysts CuxTil_0.sxO2, the relative amount of Os exceeds that for the phase separated system by a factor of 3.5 (see Table 3). These species may arise either under ultrahigh vacuum treatment of the catalyst surface in a chamber of the XPS device or by annealing the catalyst in an atmosphere of inert gas, or by interaction of the catalyst with CO. At the same time, the ionic radius of Cu I+ (0,93 A) is considerably more than one for Ti 4+. In this connection the Cul+-ions can only grow in [111 ] plane of single-phase oxide. In this case the non-bridged oxygen ion is mostly weakened and accessible for interaction with CO chemisorbed. A model fo the copper-content centre transformation in redox process is illustrated in Fig.2.

i f

I

21 I

I

6

f

I

1

I .i

7

"

I

I

'-T6 CO Cu§ ]

#

J"

J"

I ~"

X

"*

XI

Figure 2. The model of the local copper-content polyhedron transformation in redox process over a Cu-titania catalyst.

688 Schematically this process can be written as a sequence of the following proposed steps: Cu2+__.Os

excitation_>

Cu2+-O s + CO Cu 2+- [Os - C -O]

---> --->

Cu2+-Os,

( 1)

Cu 2+- [Os - C -O] Cul+-Vs + CO2

(2) (3)

where C u 2+ = 0 S is the surface mixed with the terminal oxygen in its ground state and Cu2+Os is a thermally excited state acquiring anion-radical property with an easy bound surface oxygen; Vs is the surface anion vacancy stabilising the reduced form of copper which is, in fact, the new active site. Interaction of Vs with oxygen from the gas phase can cure the surface defect and generate an easy bound surface oxygen (in its excited anionradical form) and labile surface species Os, which oxidises CO, etc. Cu1+-Vs + Os +

02 CO

.... > .... >

Cu2+-Os + Os CO2

(4) (5)

At temperatures (T>150-170 ~ the overall CO conversion is extremely high, independent of the doping content, of mean crystal size and phase composition of the anatase-based catalyst (Tables 1,2). The pronounced increase in CO conversion may be caused by the participation of the volume lattice oxygen (OL) in the reaction mechanism. ACKNOWLEDGEMENTS The authors thank Dr.O.Ellert for making magnetic measurements and interpretation of the results. Also, authors thank the Russian Science Foundation (Project 97-03-32028a) and NATO High Technology Foundation (Project Ref. HTECH. LG 960967) for financial support of this work. J.A.Navio acknowledge the subsidy received from the "Direcci6n General de Ensefianza Superior/Ministerio de Educaci6n y Cultura, Spain" Project PB961346, for supporting part of this work. REFERENCES

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

B. Viswanathan, Catal. Rev.-Sci. Eng., 34 (1994) 337. T. Arakava, A. Vosida and J. Shiokava, Mat. Res. Bull., 15 (1980) 347. S. Nansen, J. Otamiri, J.O. Bovin and A. Anderson, Nature, 334 (1988) 14. A.A. Davydov and A.A. Bodneva, React. Kinet. Catal. Lett., 25 (1984) 121. D.C.Bradley, Chem.Rev., 89 (1989),1317. M.V.Tsodikov, O.V. Bukhtenko,.O.G. Ellert, D.I. Kochubey, S.M. Loktev and S.I. Kucheyko, Structural Ceramic Processings, Microstructure and Properties, eds. J.J. Bendzen, J.B. Bilde-Sorensen, N. Christiansen, A. Horsewel and B. Ralph, Proceedings of.ll-th Intern. Symp. on Metallurgy and Material Science, Roskild, Denmark, 1990, p.505. 7. M.V.Tsodikov, V.Ya.Kugel, Yu.V.Maksimov, O.G.Ellert, V.M.Shcherbacov, and O.V.Bukhtenko, J.Catal., 148 (1994) 113. 8. M.V.Tsodikov, O.V. Bukhtenko, O.G. Ellert, V.M. Shcherbakov and D.I. Kochubey, J. Mater. Sci., 30 (1995) 1087. 9. V.T. Kalinnikov and Yu.V. Rakitin, Vvedenie v Magnetokhomiyu, Nauka, Moscow, 1980, p.53.

689 10. F.M. Capece, V. Dicastro, F. Furlani, G.Mattogno, C.Fragale, M.Gargano, M.Rossi, J. Electron Spectrosc. Relate Phenom., 27 (1982) 119. 11. N.S. Mclntyre, S. Sunder, D.W. Shoeswith, F.W. Stanchell, J. Vac. Sci. Technol., 18 (1981) 714. 12. J.H.Burness, J.G.Dillard, L.T.Taylor, J.Am.Chem.Soc.,97 (1975), 21, 6080. 13. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Academic Press, New-York-London-Sydney, 1965, p.420. 14. R.W.G. Wycoff, Crystal Structures, W. Sons (Editor), New-York., 1963, 358p. 15. K. Hayashi, N. Mizutani and M. Kato, J. Chem. Soc. Jap.(Chem. and Ind. Chem.), 6 (1974) 6. 16. M.Funabiki, T.Yamada and K.Kayano, Catal.Today, 10 (1991) 237. 17. G. Yi and M. Sayer, Ceram. Bull., 70 (1991) 1173. 18. D.C. Bradly, R.C. Mehrotra and D.D. Gaur, Metal Alkoxides, Academic Press, New-York (1978) p.213. 19. K.Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds. A Wiley-Interscience Publication, 4th. Edition, 1986, p.484. 20. V.B.Kazanskiy, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds), Proceedings of the 6-International Congress on Catalysis, London, 1976, 1, p.50. 21. Yu.V.Maksimov, I.P.Suzdalev, M.V.Tsodikov, V.Ya.Kugel, O.V.Bukhtenko, Ye.V. Slivinski and J.A. Navio, J.Mol.Catal.A: Chem., 105 (1996) 167.

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91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

691

C o - N b 2 O J S i O 2 sol-gel catalysts: preparation implications on the texture and acidity o f the support and d i m e n s i o n o f the metal particle V. Parvulescu a, R. Craciun b, F. Tiu c, S. Coman c, P. Grange d and V. I. Parvulescu c a Institute of Physical Chemistry, Spl. Independentei 202, Bucharest 77208, Romania b _ University of Pennsylvania, Department of Chemical Engineering, 220S, Philadelphia, PA 19104, USA c _ University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Republicii 13, Bucharest 70346, Romania - Universite Catholique de Louvain, Unite de Catalyse et Chimie des Materiaux Divises, Place Croix du Sud 2/17, 1348 Louvain-la Neuve, Belgique Colloidal and polymeric sol-gel, an intermediate technique between the colloidal and the polymeric sol-gel and the clasical incipient wetness impregnation method were used to prepare a set of Co-Nb2Os/SiQ catalysts with various characteristics. Modifications in the preparation procedure and its parameters have determined significant changes of the texture and the acidity of the support as well as of the Co metal particle size. Textural and structural characterization and catalytic data from butane hydrogenolysis were used to reveal the main differences among the catalysts obtained by the various techniques applied for preparation. 1. INTRODUCTION The use of niobium oxide as catalysts support or as promoter received an increased interest because of the significant effects exerted by metal-oxide interactions on both chemisorption and catalytic properties [ 1-3 ]. The preparation of the supported niobium oxide catalysts followed various routes such as simple impregnation of niobium oxide with the metal salts [4] or very elaborate procedures like double decomposition of an aqueous metal solution with a niobium alcoxide or potassium orthoniobate [1] and glycothermal treatment, when niobium alcoxide is mixed with a metal alcoxide, acetate or acetylacetonate in a 1,4-butanediol and decomposed in autoclave, at 300 ~ under nitrogen [5]. Other complex techniques which were used for niobia-based catalysts preparation are the colloidal [6,7] and polymeric [8-11] sol-gel methods. Thermal methods were also utilized for Nb-based catalysts preparations. Following this procedures, bicomponent catalysts were prepared by decomposition of mixed citrates of niobium and the active metal [12] or by calcination the bulk mixed oxides at a given temperature [3]. The typical procedures for the synthesis of the niobia-promoted metal/silica catalysts were based on a stepwise impregnation method of the precalcined silica with niobium and the other active metal components [3] and on the double decomposition of the aqueous salts of the metal components and of the niobium compound in the presence of SiO 2 [13]. Simple grafting deposition of niobium on the silica, when the interaction between a niobium complex ( N b ( r l 3 - C3H5)4 or [Nb(rl5 - C5H5) H- g- (q5,111 C5H4)] 2 and SiO 2 occurs through surface hydroxyl groups [ 14,15] has been also reported. _

692 In this paper, we report the influence of the preparation conditions upon the texture and the acidity of Co-Nb2Os/SiO2 catalysts obtained via the sol-gel techniques. A particular interest was given to the relation between the parameters of the sol-gel variant and the characteristics of the resulted catalysts (surface area, pore size distribution, reduction degree and particle size of the metal). 2. EXPERIMENTAL SiO 2 supported Co-niobium oxide catalysts were prepared usiing different sol-gel procedures: i) colloidal (designated as #A, and #B), ii) polymeric (designated as #D) and iii) a novel intermediate procedure between the colloidal and polymeric sol-gel method (designated as #C). For comparison, samples obtained via classical incipient wetness impregnation were prepared as well (designated as #E). NbC15 and niobium ethoxide were used as niobium source, SiC14 and tetraethylorthosilicate (TEOS) as silicon source and Co(NO3) 2 as cobalt source. Procedures #A, #B and #C used NbC15 and SiC14 precursors. In the first two procedures, the precursors were prepared as colloidal suspensions in anhydrous ethanol. Niobium hydroxide and silica were independently precipitated with NaaCO 3 solutions (#A) or H20 (#B). In this way, precipitates exhibiting various colloid sizes were obtained and peptized in the acid conditions (HC1). The aqueous Co(NO3) 2 solution was added to the resulted colloidal suspensions. Procedure #C used NbC15 and SiC14 dissolved in a mixture of anhydrous ethanol with CC14. Partial hydrolysis of the salts and the gelation process were carried out at an acid pH (HC1) in the presence of the H20 traces resulted in the alcoholic phase. The distribution of cobalt and the porous structure were controlled by adding together the alcoholic solution, Co(NO3)2, and tempating agents (PEG's 1500, 2000, 6000). The polymeric sol-gel procedure (#D) used TEOS as silicon precursor and niobium ethoxide as niobium precursor. The alcoxides were separately hydrolyzed at an acid pH (HC1), in the presence of different amount of water, and then mixed after different prehydrolysis time. Co(NO3) 2 was added as an alcoholic solution in the gelation phase of the sols mixture. In all procedures, the elimination of the solvent was carried out in the 80-110 ~ range followed by calcination at 400-600 ~ range, in the reductive conditions (under 30 mlmin -1 H2). Following these procedures, samples containing 10 wt.% Co and 10 wt.% niobium oxide (expressed as Nb2Os) were obtained. For each procedure, the modification of the texture, acidity and metal dispersion were monitored by modifying the parameters of peptization, polymeryzation and sintering of the materials as well as by using the templating agents with different masses. The characterization of the resulted catalysts was performed using various techniques. N2 adsorption and desorption curves at 77 K were obtained with a Micromeritics ASAP 2000 apparatus. NH3 and H2-chemisorption were carried out using a Micromeritics ASAP 2010C. H2-chemisorption data considered the reduced Co species to be those which were determined in conditions of temperature-programmed oxidation (TPO) experiments. These experiments were performed using a Micromeritics PulseChemisorb 2705 apparatus at a 50 ml rain l 02 (5%) - He flow. Reduced cobalt was determined assuming that at 500 oc all Co was converted to Co304. This assumption was confirmed also by XRD and XPS. The actual fraction of Co was used to determine the metal dispersions. Temperature-programmed reduction (TPR) was carried in the same setup as that used for TPO experiments. XPS spectra were recorded using a SSI X probe FISONS spectrometer (SSX-100/206) with mono-

693 chromatic AI-Kct radiation. It were followed the bands assigned to Co3p , NbEp, SiEp, and Ols. XRD measurements were made with a SIEMENS D-5000 diffractometer. The diffractograms were recorded in the 0-800 20 range, using CuKa radiation (1 = 1.5418 A~ Solid state 1H MAS-NMR spectra were obtained at room temperature using a Varian VXR-400S spectrometer with an Oxford cryomagnet, operated at a Sun workstation network with a VNMR operating system version 4.1. The Fourier transform of free induction decay (FID) was observed using a quadrature detection at a spining rate of 6000 rot min -1 for more than 2 hours acquisition time. About 0.5 g catalyst powder, outgassed by heating at 125 oC, was introduced into the spectrometer sample holder and tuned at the minimum signal possible for proton analysis. The chemical shift of the peaks observed from the spectra were corrected using the same reference for each sample (- 133 ppm), so that the observed shift to be in the 0-10 ppm region, specific to protons. The various catalyst samples were tested in butane hydrogenolysis. Standard experiments were carried out in a quartz microreactor, at atmospheric pressure and HE:butane = 10:1 ratio. The exit of the reactor was connected on-line to a Carlo-Erba gas chromatograph equipped with a FID detector and a 6 m packed column containing Chromosorb W coated with silicon oil. The reaction was monitored using the reactant concentration and conversion was expressed as a percentage of the total carbon fed to the reactor. Each experiment used 0.3 g catalyst. Reaction rates were expressed as a mmole alkane reacted, (gCol)h "l. The selectivity of product j was defined using Bond's formula [ 16]" Sj = cj/A, where cj is the molar fraction of products containing j carbon atoms (j<4), and A the number of moles converted. 3. RESULTS AND DISCUSSION 3.1. Modification of the catalysts texture Figure 1 shows the variation of the surface area as a function of catalysts preparation procedure. From this figure can be observed that, using the colloidal (#B) and polymeric (#D) sol-gel procedures, high surface area catalysts have been obtained. Using the other procedures, the surface area decreased in the order: #C > #A > #E.

Figure 1. Variation of the surface area as a function of the preparation procedure

Figure 2. Pore volume distribution versus pore diameter

694 The average pore diameter (Figure 2), as determined from the desorption branch of the N 2 isotherms, indicated various texture as a function of preparation method. Thus, colloidal (#B) and polymeric (#D) sol-gel procedures led to a mesoporous-type solids while the other two procedures (#A and #E) led to mixed pores size (large and mesopores) solids. The distribution of the pore volume as a function of the pore diameter shows that the solids exhibit a narrow pore distribution for the #B, #C, and #D samples. Samples obtained using #A and #E procedures exhibit a broad pore size distribution. The type of the isotherms also accounts for this behavior. Thus, the shape of the isotherms associated to procedures #B, #C, and #D corresponds to a texture characteristic to mesoporous solid materials (type IV), whereas the shape of the isotherms associated to #A and #E procedures to a texture characteristic to a material having slit-shaped and large pores (type H4) [ 17]. The observed differences between the #A and #B samples texture can be attributed to the specific precipitation procedure used during catalyst preparation. The peptization, similar for both type of samples #A and #B, was carried out in the presence of variable amounts of distilled water and HC1 (pH 1-4 for #A and 1-2 for #B). High pH and amount of water were found to increase the surface area and diminish the average pore diameter to a narrow pore size distribution. In the case of the catalysts obtained following the polymeric sol-gel procedure (#D), the mixing of the niobia and silica sols was made after various gelation times" 30 min, 1 h or 2 h. It was found a direct dependence between the surface area of the resulted materials and the gelation time. The increase of the surface area was accomplished by a decrease of the pore diameter. This behavior could be explained by less homogeneity in the final polymer as a consequence of the increasing part corresponding to separate niobia and silica unities. The increase in the sol-gel temperature, from 20 to 80 ~ have significantly reduced these differences. Evidences in this sense were obtained from XRD and XPS measurements.

Figure 3. Influence of the surfactant molecular mass upon the pore diameter (#C)

Figure 4. Modification of the acidity as a function of preparation procedure

Another parameter which had a strong influence upon the texture of the cobalt-niobiaSiO 2 catalysts was the nature of the templating agent. The increase of the molecular mass of

695 polyethylenglycohol from 1500 to 6000 amu, for the same calcination temperature, led to an increase of the average pore diameter (Figure 3). 3.2. Modification of the acidity Figure 4 shows the variation of the catalysts acidity for samples obtained following the procedures #A - #E determined from NH 3 chemisorption at room temperature and at 200 oC, respectively. The generation of the acidity is the consequence of the mixing of niobium and silicon unities. However,the acidity exhibited by niobium oxide itself mainly when calcinated at low temperature can not be neglected [ 18]. In the present case the catalysts were reduced in hydrogen atmosphere in the range 400 - 600 oC. The samples obtained via procedure #D exhibit the highest number of acid sites while the acidity of the the samples obtained via procedure #E, was the lowest. Normalized acidity on number of sites per gram of sample led to differences of about one order of magnitude be~;een #D and #E samples. The variation of the total acidity parallels the homogeneity of the samples prepared via the above procedures. XRD patterns indicated a decrease in the sample crystallinity of the samples in the order #D > #C > #B > #A > #E. The individual lines almost dissapeared in the case of samples prepared by wet impregnation (#E). Bronsted acid sites correspond to the protons situated in the close proximity of niobium. In the same time, these positions represent a docking site for cobalt. More homogeneous samples will have less sites blocked with Co and, as a consequence, exhibit more free protons and niobia sites. The low acidity observed for samples prepared via #E can be related to a large interaction of Co with niobia sites. But a high amount of acidic sites does not imply a high population of strength acidic sites. The population of the strong acid sites was approximately the same for the samples obtained via #B, #C, and #D. These acid sites are mainly Lewis type and seems to indicate a similar distribution of niobia-silica interaction in these samples.

D

C q"~

~ .... I .... I'"'1'"'1

12

10

--_

.... I"',l,'~'l

8

6

.... t .... 1.... I .... I""i'"'L'"'i'"'l""l",'l

4

2

0

-2

.... in"

-4

-6

_

i' '.'l"";"l-

',1 , ' r

-8 ppm

Figure 5.1H-NMR spectra of the samples obtained using procedures #B - #E

The MAS 1H-NMR spectra obtained for the samples prepared via different procedures are presented in Figure 5. As observed, the NMR spectra of each catalyst shows signals specific to protons of various configurations. However, analysis of chemical shift and the

696

corresponding structural information about the origin of these signals are hard to be explained. Under these limitations, the shape of the spectra presented in Figure 5 indicates for all the catalyst samples the co-existence of different proton species exhibiting different chemical shifts compared with the spectra of the samples obtained via #E, which exhibit diffuse spectrum and poor resolution of the signals. Modification in the calcination temperature affected the total amount of acid sites. High temperatures determined the presence of low number of acid sies. This behavior can be explained by the fact that at high temperatures the contribution of the Bronsted acid sites decreased. Thus, one can conclude that the total amount of acid sites corresponds to the sum of Bronsted and Lewis sites whereas the strong acid sites correspond only to Lewis ones. 3.3. Modification of the metal dispersion Table 1 gives the H2-chemisorption data for the representative catalysts obtained following procedures #A - #E. The degree of reduction is typical for such high Co loading present on oxidic supports. However, modifications in the preparation method have led to metal species in various positions (hidden or not), and therefore to more or less reductible species. Based on this observation, one can observed that the homogeneous samples contain highly dispersed cobalt. Thus, there is a 5 time higher Co dispersion in the catalyst sample obtained via polymeric sol-gel (#D) compared to that prepared via traditional wet impregnation (#E) method. Small Co particles as about 90 A (#D) can not be generally obtained at such high metal loading. Table 1 H 2 -chemisorption data for catalysts obtained using different preparation procedures Property Sample #A #B #C #D #E 400~ 600~ 400oC 600~ 400oC 600~ 400oC 600~ 400~ 600~ difference 0.09 0.12 0.09 0.13 0.10 0.15 0.10 0.16 0.06 0.09 volume average, cm 3

reduction degree, % % reduced metal in the sample %metal dispersion d,A

5.8

7.3

3.2

4.8

2.7

4.2

2.5

4.0

8.6

11.2

0.58

0.73

0.32

0.48

0.27

0.42

0.25

0.40

0.86

1.12

4.08

4.32

7.40

7.13

9.74

9.40

10.52

10.53 1.86

2.11

238.0

224.8

1 3 1 . 2 136.2

99.7

1 0 3 . 3 92.3

92.2

460.2

522.0

3.4. Hydrogenolysis of butane Hydrogenolysis of butane is a structure sensitive catalytic reaction. Previous work have showed that an increased in the support acidity has a positive influence upon the stability of the catalyst [6,7]. The catalytic activity was low regardless of the preparation route, however, clear differences have been observed (Figure 5). It appears that the catalysts with high Co

697 dispersion exhibit high reaction rates. The selectivity toward methane exhibits a maximum for the #D samples which have the highest dispersion and homogeneity.

16

12

,

\

--

_]_lOO L

-\

i 8~

, I / '

60

"~

8

4

,M

.40

[

i

I"4

20 O

0

1. . . . A

D B C Preparation procedure

,

E

0

Figure 6. Variation of the reaction rate (ll~) and of the selectivity to C 1 ( t ) and C2 ( ~ )

Catalysts stability follows the trend observed for the variation in the acidity. Therefore, it can be considered that this is in good agreement with our previous assumption [7] which considered that strong acid sites sorrounding small metal particles facilitate the spill over transfer of the cracked part. As a consequence of this phenomena, the catalyst surface can be maintained clean. 4. CONCLUSIONS Modifications in the preparation procedure led to variations in the support texture and acidity as well as in the metal particle size. In this work, it was shown how the above mentioned properties of a Co-Nb20/SiO2 catalyst can be monitored by using different sol-gel procedures (colloidal, polymeric and the intermediate between the colloidal and polymeric sol-gel methods) in comparison with the classical incipient wetness impregnation technique. REFERENCES 1. K.Kunimori, H.Shindo, H.Oyanagi and T.Uchijima, Catal.Today, 16 (1993) 387. 2. D.A.G.Arande, A.L.D.Ramos, F.B.Passos and M.Schrnal, Catal.Today, 28 (1996) 105. 3. T.Beutel, V.Siborov, B.Tesche and H.Kn6zinger, J.Catal., 167 (1997) 379. 4. T.Ushikubo, Y.Hara and K.Wada, Catal.Today, 16 (1993) 525. 5. H.Kominami, M.Inoue and T.Inui, Catal.Today, 16 (1993) 309. 6. V.Pgrvulescu, M.Ruwet, P.Grange and V.I.PS.rvulescu, Solid State Ionics, in press. 7. V.P~rvulescu, M.Ruwet, P.Grange and V.I.P~rvulescu, J.Mol.Catal., in press. 8. P.Griesmar, G.Papin, C.Sanchez and J.Livage, Chem.Mater., 3 (1991) 335.

698 9. S.M.Maurer and E.I.Ko, J.Catal., 135 (1992) 125. 10. G.R.Lee and J.A.Crayston, Adv.Mater., 5 (1993) 434. 11. E.C.DeCanio, V.P.Nero and E.I.Ko, J.Catal., 146 (1994) 317. 12. Y.G.Yin, T.Wakasugi, H.Shindo, T.Uchijima and K.Kunimori, Bull.Chem.Soc.Jpn., 6 (1992) 3218. 13. K.Kunimori, H.Shindo, H.Oyanagi and T.Uchijima, Scokubai, 29 (1987) 5. 14. J.-M. Jehng and I.E.Wachs, Catal.Today, 16 (1993) 417. 15. N.Ichikuni and Y.Iwasawa, Catal.Today, 16 (1993)427. 16. G.C.Bond and J.C.Slaa, J.Mol.Catal. A: Chemical, 106 (1996) 135. 17. K.S.W.Sing and S.J.Gregg, Pure Appl.Chem., 57 (1985) 603. 18. K.Tanabe, CHEMTECH, 21 (1991) 628.

91998ElsevierScienceB.V.All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

699

S i l i c a - s u p p o r t e d b i s m u t h m o l y b d a t e catalysts o b t a i n e d b y the sol-gel process via silicon alkoxides D. Cauzzi a, M. Deltratti a, M. Devillersb, G. Predieria, O. Tirions b and A. Tiripicchio a a Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universit/~ di Parma, Viale delle Scienze, 1-43100 Parma, Italy b Laboratoire de Chimie Inorganique et Analytique, Universit6 Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium The sol-gel route is applied to the preparation, in a one-step process, of bismuth molybdates highly dispersed into silica xerogels produced by hydrolysis/condensation reactions involving silicon alkoxides. The samples correspond to different Bi/Mo and (Bi+Mo)/Si stoichiometric ratios, according to the different bismuth molybdate phases (c~-Bi2Mo3012, ~-Bi2Mo209, ~/-Bi2MoO6) and for various metal (Mo+Bi) loadings (2, 5, 10, 25 or 50 mol.% with respect to silica). The samples were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray microanalysis coupled with scanning electron microscopy (EDX-SEM). The xerogels containing low molybdate loadings (2, 5, 10 tool %) exhibited high SBET values (300 to 700 m2/g), but the expected phase was never detected as pure in these materials. The XRD spectra of the xerogels characterized by high molybdate loadings (25, 50 tool %) clearly showed the formation of complex mixtures, the m-phase being predominant at 600~ when the starting Bi/Mo ratio was 0.66, while phase mixtures were observed in all the other cases. Whereas the Bi/Mo ratios determined by XPS were systematically much lower than the expected ones, more satisfactory values were observed using EDX-SEM. This apparent discrepancy reflects the difference in surface depths explored by the two techniques and suggests Mo enrichment of the outer surface layers. This behaviour is thought to be related to the selective migration of ions throughout the incoming silica network, resulting in heterogeneous distributions of Bi-based and molybdate ions, that could actually lead to different phases and anomalous surface Bi/Mo ratios. 1. INTRODUCTION Metal molybdates are among the most active catalysts for olefin oxidation and ammoxidation, possessing key properties including optimum metal-oxygen

700 bond strengths and suitable solid state redox couples [1-3]. Generally, metal oxide catalysts have been used either as pure compounds or supported on a suitable solid matrix, like molybdena [4], vanadia [5] or mixed oxides [6] on silica or alumina. In some instances, however, the supported systems are preferred because of the decisive improvement of the catalytic properties [1], owing possibly to the increase of surface area, the easier removal of the reaction heat from the active sites and the stabilization of a wanted oxide phase, as a result of strong metal oxide-oxide support interactions. In particular, silica-supported bismuth molybdates are among the most important catalysts for the selective oxidation of olefins. They are classically prepared by a precipitation-deposition procedure from highly acidic aqueous solutions of bismuth(III) and molybdenum(VI) inorganic salts, but this method does not allow the adequate control of dispersion and nature of the formed phases. These drawbacks can be overcome by using coordination compounds, e.g. bismuth(III) and molybdenum(II) acetates or benzoates, as precursors, and incorporating them on the support according to a deposition procedure from an anhydrous organic medium [7]. Another convenient preparation method of silica-supported catalysts is the sol-gel route [8] via silicon alkoxides; it allows a good control on the composition, homogeneity and textural properties of the final products. The nanoscale chemistry involved in sol-gel methods appears actually as the most straightforward way to prepare tailored nanocomposites, including organicinorganic hybrid materials. The applications of the sol-gel process in catalysis, which were recently reviewed [9], concerned the preparation of both tethered metal complexes [10] and supported small metal particles [11]. This paper deals more specifically with the application of the sol-gel route to the production of silica-supported bismuth molybdates, with their bulk and surface characterization using X-ray diffractometry, X-ray photoelectron spectroscopy and energydispersive microanalysis. 2. EXPERIMENTAL 2.1. Materials and equipments

Solvents, organic and inorganic reagents were pure commercial products and were used as received. B.E.T. specific surface areas were determined from nitrogen adsorption at 77 K using a Micromeritics ASAP 2000 instrument. FT-IR spectra were recorded on a Nicolet 5PC spectrometer. XRD spectra were obtained with a Philips PW 1050 diffractometer using the Cu-Kc~ radiation. Energy dispersive X-ray microanalyses were performed with the SEM equipment JEOL 6400 EDS Tracor at the Istituto di Mineralogia, Universit/~ di Parma. X-ray photoelectron spectra were registered on a SSI-X-probe (SSX-100/206) spectrometer from FISONS, using a monochromatized A1-K~ radiation (E = 1486.6 eV) and a ceramic sample holder. Charge compensation was achieved by the use of a flood gun fixed at 6 eV. The binding energy scale was calibrated with respect to the Si 2p photopeak fixed at 103.5 eV.

701 2.2. P r e p a r a t i o n o f s i l i c a - s u p p o r t e d b i s m u t h m o l y b d a t e s

The adopted procedure is the result of several attempts carried out under various experimental conditions, using different starting materials, such as Si(OEt)4 (TEOS)or Si(OMe)4 (TMOS). Different Bi/Mo and (Bi+Mo)/Si stoichiometric ratios were selected according to the three main different bismuth molybdate phases (~-Bi2Mo3012, ~Bi2Mo209 and ~-Bi2MoO6), and to different mixed-oxide loadings (between 2 and 50 mol % active metals with respect to silica). Nevertheless, the synthetic procedure, described hereafter for 2% c~-phase (2-~), is the same for all n-X cases (n = 2 or 5, X = ~ or ~ or ~; n = 10, X = ~; n = 25 or 50, X = ~ or ~,. A solution (6 ml) of (NH4)6Mo7024"4H20 (95 mg) was added dropwise, under thorough stirring, to a solution of Bi(NO3)3.5H20 (174 mg) and nitric acid (4 ml) in water (10 ml). Methanol (4 ml) was added and subsequently nitric acid (2 ml) to maintain the solution clear. After addition of Si(OMe)4 (3.3 g ) t h e colourless solution turned yellow and gelation occurred in ca. 2 h, giving a pale yellow transparent gel. The gelled solid was allowed to rest for 24 h and then aged under vacuum at 303 K for 16 h. Finally, the gel was heated in an oven at 773 or 873 K (20 h), giving yellowish powders in both cases. FT-IR (KBr): 1105vs, 806m, 468m cm -1.

3. RESULTS AND DISCUSSION 3.1. S y n t h e t i c a s p e c t s

The primary objective of this research was to prepare in a one-step process bismuth molybdates highly dispersed into silica xerogels produced by hydrolysis and condensation reactions involving silicon alkoxides. For this purpose, the hydro-alcoholic reaction medium has to be suitable to allow the growth of the colloidal silica particles and to prevent the precipitation of molybdate crystaUites before the sol-gel process has come to completeness. This goal has been reached by dissolving the bismuth salt in a nitric acid solution and then adding the molybdate solution dropwise; this addition should be sufficiently slow to allow the complete dissolution of a white precipitate that begins to form, but disappears within few seconds. By this way it is possible to obtain a clear solution, which tolerates the addition of the silicon alkoxide (TMOS); if segregation of some solid particles occurs, it is suficient to add a small amount of nitric acid to restore the clearness. The final result is an homogeneous, transparent, yellow xerogel, which is subjected to rapid aging (16 h in vacuo); this is preferable, in order to avoid separation of inorganic salts carried by the inner solvent. The aged xerogel is subsequently heated at 773 or 873 K, causing the formation of bismuth molybdate crystallites well evidenced by X-ray diffraction investigations. Thermal treatment has beencarried out in two ways, by heating gradually (160 K/h) the samples starting from room temperature, or by introducing them in the pre-heated hot furnace.

702

3.2. Characterization of the samples at low loading (2, 5,10 mol % active phase)

XRD and BET results (Table 1) As stated in the experimental part, various xerogels were p r e p a r e d , which are characterized by different Bi/Mo atomic ratios (2:3, alpha-phase; 1:1 beta; 2:1 g a m m a ) and mixed-oxide percent contents (2, 5, 10, 25, 50%). The gels containing low molybdate loading are characterized by high surface area values, particularly w h e n treated at the lower temperature (773 K, see Table 1). These values are significantly smaller than those found for a pure silica s a m p l e (852 m 2 / g ) p r e p a r e d from TMOS under the same conditions. O w i n g to the low content of bismuth molybdate, the XRD spectra of these materials are not particularly useful, showing only one low-intensity p e a k not attributable to a particular phase. Table 1 XPS and BET results. (a) (b) Bi+Mo Tcalc SiO2 (K) ,

0.50

0.25

,

,,,,

,

(a) Bi = 0.66 Mo

(a)

Bi = 2 . 0 Mo XRD Detected phases (d)

SBET (m2/g)

773

7 = [3 = (Bi203) >> (o0

249

873

XRD SBET Detected phases (d) (m2/g)

7 --- 13= (Bi203) >> (o0

o~= ~ >> (Bi203) (x

95 26

773

y = 13> (z > (Bi203)

o~ > 13

168

873

Bi203 > y = [3

o~ >> 13

44

0.10

773 (c)

13> ~,7, (Bi203)

464

-

-

0.05

773 (c)

([3 = 7), (Bi203)

578

amorphous

580

13> y = (o0 amorphous

660 521

Bi203 = 13> (obT) Bi203 > 13>> (o0

594 329

773 873

0.02 773 (c) amorphous 691 (a) Engaged molar ratios. (b) Calcination temperature. (c) With linear temperature increase (160 K/min). (d) Y= Bi2MoO6, 13= Bi2Mo209, o~= Bi2Mo3012 ; phases between parentheses are low abundant or hypothetical.

-

Discussion of XPS data (Table 2) In all the investigated samples (low and high loading), the O ls, Mo 3d and Bi 4f b i n d i n g energy values correspond to those observed typically for silicas u p p o r t e d bismuth molybdates. In particular, the carbon signal appears at 284.6 + 0.3 eV and its low intensity (C/Si ratio close to 0.1) is similar to that m e a s u r e d in silica-supported Bi-Mo-O catalysts obtained by precipitation-deposition from carbon-free inorganic precursors in aqueous solution [7a], s h o w i n g that there is

703 no additional carbonaceous contamination resulting f r o m the p r e p a r a t i o n procedure. The O(I)/Si ratio corresponding to the silica matrix is close to 2.0. Table 2 XPS data. (a) (a) (b) Bi Bi+Mo Tcalc Mo Si02 (K) Bi Mo 0.50

2.0

0.66

0.25

2.0

Atomic intensity ratios (c)

O(I) Si

C(I) Si

Bi Siexp

Mo Siexp

Bi Sith

Mo Sith

773

1.10 2.25 1.18(e)

0.11

0.0477 0.0434 0.3333 0.1667

873

1.06 2.04 1.76(e)

0.11

0.0572 0.0541 0.3333 0.1667

773

0.31 2.02 0.77(e)

0.12

0.0364 0.1170 0.2000 0.3000

873

0.50 2.06 0.70(e)

0.14

0.0448 0.0889 0.2000 0.3000

773

0.95 2.22 1.40 (e)

0.16

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0.02 2.0 773 (d) 1.63 2.08 0.09 0.0146 0.0090 0.0133 0.0067 (a) Engaged molar ratios (b) Calcinationtemperature (c) Analytical photopeaks are Cls, Ols, Mo3d et Bi4f ; C(I): hydrocarbons "CHx"; O(I): oxygen of silica; exp=experimental, th=theoretical (d) With linear temperature increase (160K/h) (e) EDX-SEM data

704 In general, the Bi/Mo atomic intensity ratios are significantly different from the theoretical values expected for the bulk, confirming thereby the XRD observation that mixtures of several phases are present. However, at low total loading (5 mol. %), the theoretical and experimental Bi/Mo ratios are in agreement, suggesting that the purity of these samples is higher at the surface level. It should nevertheless be remembered that this agreement does not imply the presence of a unique phase, but could merely result from an average effect of several coexisting phases characterized by different ratios. As far as the Bi/Si and Mo/Si ratios are concerned, the experimental values listed in table 2 appear significantly smaller than the "theoretical" values, i.e. those calculated from the molar quantities of Bi and Mo engaged in the syntheses. These theoretical values actually refer to a uniform and homogeneous distribution of silica and bismuth molybdate particles, which would be equally accessible to the XPS analysis. Experimental values lower than the latter indicate that Bi- or Mo-containing aggregates are present. Experimental ratios exceeding the theoretical values suggest that the metal-containing particles are finely and homogeneously divided with respect to the silica particles. It should also be remembered that these ratios are sensitive to the particle size, the smaller the particles containing Bi and Mo, the higher the Bi/Si and Mo/Si ratio. Indication that the bismuth molybdate particles are satisfactorily divided appears only in the sample with the lowest metal loading (2 mol.%). It seems also that this behaviour is favoured by a linear temperature increase during the activation step. These observations have to be analyzed with some greater care, because the silica support generated by the sol-gel technique according to the procedure described above displays a very high specific surface area (between 700 and 850 m 2 / g according to the calcination temperature). The obtention of experimental Bi/Si and Mo/Si ratios similar to the theoretical values calculated from the overall composition of these samples assumes therefore that the particle size achieved for the bismuth molybdates be comparable to, or smaller than that of the support, a condition which is hard to be satisfied. 3.3. Characterization of the samples at high loading (25, 50 mol % active phase) XRD and BET results (Table 1) As expected, the surface area dramatically decreases for the high-loaded samples, where the massive presence of crystalline bismuth molybdate appears to hide the silica pores. Actually, these gels were prepared in order to stress the ability of the porous material to support high amounts of active species (without losing its peculiar characteristics), considering also that the 5%-loading is too low to cover properly the high-area silica surface. The XRD spectra of these xerogels clearly show the formation of complex mixtures, the alpha phase being predominant when the Bi/Mo starting ratios is 2:3. In the case of the 2:1 ratio, the gamma phase is present but mixed with the beta modification, Bi203 and sometimes with the alpha phase.

705 Discussion of XPS and EDX-SEM data (Table 2) The XPS spectra show unsatisfactory Bi/Mo atomic ratios, that are systematically much lower than the expected ones. On the other hand the same samples, examined by EDX with a SEM equipment, exhibit more satisfactory Bi/Mo ratios that are systematically larger than those measured in XPS (Table 2); in the case of the expected alpha-phase, these ratios are in good agreement with the theoretical value. This apparent discrepancy between the two sets of data could be due to the different surface depths explored by the two techniques (ca. 4 nm for XPS and ca. 1000 nm for EDX) indicating that the molybdenum concentration is higher in the outermost layers, associated with a bismuth enrichment in the bulk. This heterogeneity can be understood when considering the microporous nature of the present support. During the gelation and aging stages, the rapid evaporation of the inner solvent at the solid-gas interface generates high salt concentrations gradients which could effectively cause the selective migration of ions throughout the incoming silica network. If one assumes that the major part of the pores has a diameter smaller than 2 nm, access to them would actually be very restricted for many of the major species which are present as precursors in the starting solution. This would obviously be the case for polynuclear entities like [Mo7024] 4-, [Mo8026] 4- and others, which are the more abundant Mocontaining moieties present at highly acidic pH. On the other hand, smaller species like essentially those containing bismuth could reach more easily the inner region of the material, where the salt concentration is lesser. Because of these sterical differences, the achievement of heterogeneous distributions of bismuth and molybdate ions, enhanced by their high concentrations, could be responsible for the generation of several molybdate and binary phases during the activation step, and to anomalous local Bi/Mo ratios.

4. CONCLUSIONS This preliminary investigation has shown that it is possible to produce silica-dispersed bismuth molybdates by the sol-gel process. Low-loaded materials exhibit high specific surface areas and in some cases, there is evidence of an homogeneous dispersion with finely divided mixed oxide particles. By contrast, the high-loaded catalysts show low surface areas and a clear difference between the surface Bi/Mo atomic ratios and the corresponding bulk values.

ACKNOWLEDGEMENTS The authors greatly acknowledge financial support from the M.U.R.S.T. (Rome), the Belgian National Fund for Scientific Research (F.N.R.S.. Brussels), and the F.R.I.A., Brussels, for the fellowship allotted to O.T.

706 REFERENCES

1. H. H. Kung, Transition Metal Oxides: Surface Chemistry and Catalysis, Elsevier, Amsterdam, 1989. 2. R. K. Grasselli, J. Chem. Ed., 63 (1986) 216. 3. (a) J. D. Burrington, C. T. Hartisch and R. K. Grasselli, J. Catal., 81 (1983) 489; (b) J. D. Burrington, C. T. Hartisch and R. K. Grasselli, J. Catal., 87 (1984) 363; (c) K. Aykan, D. Halvorson, A. W. Sleight and D. B. Rogers, J. Catal., 35 (1975) 401; (d) B. Grzybowska, A. Mazurkiewicz and J. Sloczinsky, Appl. Catal., 13 (1985) 223; (e) U. Ozkan and G. L. Schrader, J. Catal., 95 (1985) 120; (f) S. R. G. Carrazan, C. Martin, V. Rives and R. Vidal, Appl. Catal. A, 135 (1996) 95. 4. M. A. Bafiares, H. Hu and I. E. Wachs, J. Catal., 150 (1994) 407. 5. (a) L. Owens and H. H. Kung, J. Catal., 144 (1993) 202; (b) P. M. Michalakos, K. Birkeland and H. H. Kung, J. Catal., 158 (1996) 349. 6. R. Grabowski, J. Sloczynski, K. Direk and M. Labanowska, Appl. Catal., 32 (1987) 103. 7. (a) O. Tirions, M. Devillers, P. Ruiz and B. Delmon, Stud. Surf. Sci. Catal. 91 (1995) 999; (b) M. Devillers, O. Tirions, L. Cadus, P. Ruiz and B. Delmon, J. Solid State Chem. 126 (1996) 152; (c) L. T. Weng, P. Bertrand, O. Tirions and M. Devillers, Appl. Surf. Sci. 99 (1996) 185. 8. L. L. Hench and J. K. West, Chem. Rev., 90 (1990) 33. 9. (a) M. A. Cauqui and J. M. Rodriguez-Izquierdo, J. Non-Cryst. Solids, 147 (1992) 148; (b) U. Schubert, New J. Chem., 18 (1994) 1049. 10. (a) H. S. Hilal, A. Rabah, I. S. Khatib and A. F. Schreiner, J. Mol. Catal., 61 (1990) 1; (b) E. Lindner, R. Schreiber, T. Schneller, P. Wegner, H. A. Mayer, W. G6pel and Ch. Ziegler, Inorg. Chem., 35 (1996) 514; (c) D. Cauzzi, M. Lanfranchi, G. Marzolini, G. Predieri, A. Tiripicchio, M. Costa and R. Zanoni, J. Organomet. Chem., 488 (1995) 115. 11. (a) W. M6rke, R. Lamber, U. Schubert and B. Breitscheidel, Chem. Mater., 6 (1994) 1659; (b) T. Lopez, L. Herrera, J. Mendez-Vivar, P. Bosch, R. Gomez and R. D. Gonzales, J. Non-Cryst. Solids, 147/148 (1992) 773; (c) D. Cauzzi, G. Marzolini, G. Predieri, A. Tiripicchio, M. Costa, G. Salviati, A. Armigliato, L. Basini and R. Zanoni, J. Mater. Chem., 5 (1995) 1375.

91998 Elsevier ScienceB.V. All rights reserved. Preparation of Catalysts VII i B. Delmonet al., editors.

707

Pd-Ag/SiO) sol-gel catalysts designed for selective conversion of chlorinated alkanes into alkenes B. Heinrichs a'* , P. Delhez a, J.-P. Schoebrechts b, and J.-P. Pirard a aLaboratoire de G6nie Chimique, B6a, Universit6 de Liege, B-4000 Liege, Belgium bLaboratoire Central, Solvay, S.A., Rue de Ransbeek, 310, B-1120 Brussels, Belgium

Aerogel-like Pd-Ag/SiO2 catalysts were prepared in a one-step sol-gel process by using Pd and Ag complexes containing an alkoxide moiety with ensuing ordinary drying under vacuum. Although they are trapped inside microporous silica particles which makes them sinter-proof, the resulting bimetallic particles are completely accessible. The formation of Pd-Ag alloy crystallites allows to obtain a very high selectivity in ethylene during hydrodechlorination of 1,2-dichloroethane to the detriment of ethane which is the main product when pure Pd is used.

1. INTRODUCTION The common industrially important preparation methods of dispersed metal catalysts are multistep processes consisting of [ 1, 2]: (a) preparation of the support; (b) distribution of the active component precursor over the support surface (by impregnation, ion exchange, anchoring, ...); (c) drying, calcination and reduction of the precursor compound into the active metallic phase. The application of the sol-gel process to catalysis allows to prepare a solid from a homogeneous solution which includes not only the metal precursor, but also the support precursor [3]. Steps (a) and (b) corresponding to classical methods are then gathered in a single step. Several authors synthesized metallic supported catalysts by the sol-gel route and showed its ability to highly disperse catalytic metals on gels whose texture is finely controlled. Most of the time, 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.) [36]. Schubert and coworkers used a particularly interesting method to homogeneously disperse nanometer-sized metal particles in a silica gel [7-10]. These authors used alkoxides of the type (RO)3Si-X-A in which a functional organic group A, able of forming a chelate with a cation of a metal like palladium, nickel, silver, copper, etc., is connected to the hydrolysable alkoxide moiety (RO)3Si- via an inert and hydrolytically stable spacer X. The cocondensation of such *E-mail: [email protected]

708 molecules with a network-forming reagent such as Si(OC2H5)4 (TEOS) results in materials in which the catalytic metal is anchored to the SiO2 matrix. Schubert et al. applied this method to the preparation of monometallic and bimetallic catalysts supported on silica. In a previous work, we used this method for the preparation of transition metal aerogelsupported catalysts [11]. In the case of a metallic cation with a coordination number of 4 complexed by [3-(2-aminoethyl)aminopropyl]trimethoxysilane H2NCH2CH2NH(CH2)3Si(OCH3)3 (EDAS), the introduction of the active component precursor in the silica framework proceeds as shown in Figure 1. CH2-CH2 / ',, H5020,, /,0C2H5 H2N",. n4L"NH-(CH2}3-Si(OCH3]3 NH3/H20 \Si + ,M, ~ ~ / / "xX ,., CHaOH H5C20 0C2H5 (H3CO}3Si-(CH213-HN. 'NH2 - C2HsOH CH2-CH2 ,,

/ "" Si-----

CH2-CH2 / ',

"', rig" \ ,.M,,. /oJS~-(CH2}~-HI~" iNH2 0

0

\

-s~ / 0 /

/

0 /

CH2-CH2

-----Si-I

Figure 1. Introduction of Mn+(EDAS)2 in a SiO2 framework This picture points out the risk of confining the metal inside the silica matrix and making it inaccessible or difficult to reach for a fluid phase. The case of Pd/SiO2 aerogel catalysts was examined in detail [ 12]. Figure 2 represents schematically TEM pictures of those samples after supercritical drying. It appeared that the cogelled catalysts (the term "cogelled" refers to the cocondensation of pd2+(EDAS)z with TEOS) are composed of palladium crystallites with a diameter of about 2 nm located inside quasi-monodisperse silica particles whose diameter is between 10 and 20 nm depending on the considered sample. This metal localization inside SiO2 particles is probably a consequence of the ligand used: the hydrolysable functions in EDAS allow the formation of Si-O-Si bonds all around the complex (Figure 1) and the observation of TEM pictures suggests that a group of such complexes can Figure 2. Schematic representation act as a nucleation agent which leads after all to silica of TEM micrographs of Pd/SiO2 particles with a palladium heart. In order to assess the aerogel catalysts validity of this nucleation by palladium complex hypothesis, the relation between silica particle volume (Vp) and TEOS, EDAS and Pd 2+ concentrations ([TEOS], [EDAS], [pd2+]) was examined. It was shown that Vp is directly proportional to the ratio ([TEOS]+[EDAS])/[Pd 2+] which is in agreement with a nucleation phenomenon [12]. It was then reasonably assumed that gel formation occurs via SiO2 particle nucleation by a set of Pd 2+ complexes, particles growth thanks to hydrolysis and condensation of methoxy groups of EDAS and TEOS, and finally particles aggregation.

709 A very important concern about cogeUed catalysts is the accessibility of the active centers. Because palladium is located inside silica particles, there is a risk that it may not be accessible. By performing drying under supercritical conditions, the goal was to maintain the porosity and thus the accessibility as high as possible. In order to check the accessibility of Pd crystallites, their mean size were measured by TEM as well as by carbon monoxide chemisorption. The two techniques gave the same size which was the proof that all Pd particles are accessible for CO and then for any reactants (provided that the molecules are not too large) in a catalytic system. A detailed analysis of the texture showed that the samples exhibit a continuous macroand mesopore size distribution located in voids between silica particles and a narrow monodisperse micropore distribution centered on a pore size of 0.8 nm located inside SiO2 particles. Pd crystallites are then completely accessible via this micropore network. This particular structure is of great importance to the behavior of cogelled aerogel catalysts in relation to sintering. Because they are larger than the micropores of the silica particles in which they are located, the palladium particles are trapped and are then unable to sinter by migration and coalescence [13]. In consequence, those catalysts are sinter-proof during supercritical drying. It was indeed shown with impregnated catalysts, in which Pd crystallites are dispersed on the outer surface of SiO2 particles, that supercritical drying leads to an extensive sintering when the metal particles are not trapped. Knowing that sintering of metals loaded on a support is one of the main causes for deactivation of industrial catalysts [ 14], cogelled catalysts could be potential candidates for high temperature applications. The cogelation method described above was adapted to the preparation of bimetallic catalysts composed of metals from groups VIII and IB supported on xerogels in order to use them for the selective hydrodechlorination of chlorinated alkanes into alkenes [ 15]. Ito et al. [16] indeed recently demonstrated the ability of bimetallic catalysts from groups VIII and IB to convert chlorinated alkanes into less-chlorinated or unchlorinated alkenes. This new process is economically very attractive since it allows the recycling of alkenes which are raw materials for numerous industrial reactions. The purpose of this paper is to explain the mechanisms of formation of those bimetallic xerogel catalysts and to show that many conclusions relative to the above described metal aerogel-supported catalysts still apply. The interest of those materials for selective hydrodechlorination is also emphasized.

2. EXPERIMENTAL

2.1. Catalysts preparation Six samples containing various amounts of Pd and Ag were prepared. The synthesis parameters are presented in Table 1. For each sample, X or A denote the way used to dry the gel, xerogel dried under vacuum or aerogel dried under supercritical conditions, respectively, followed by the nominal overall atomic percentage iof Ag in the sample. The general synthesis procedure was as follows. For mixture A, to a suspension of Pd acetylacetonate powder (Pd(acac)2) in a quart of the total volume of ethanol, EDAS is added; for mixture B, to a suspension of Ag acetate powder (AgOAc) in another quart of ethanol, 3(aminopropyl)triethoxysilane, HEN(CH2)aSi(OC2Hs)3 (AS) is added. For the monometallic samples X0 and X100, only one mixture was prepared in half the total volume of ethanol.

710 Mixtures A and B were stirred at ambient temperature during 1 h (formation of Pd and Ag complexes) after which they were mixed together. Tetraethoxysilane (TEOS) was then added. Finally, a solution containing aqueous 0.18N NH3 in the remaining ethanol, was slowly added under vigorous stirring. The vessel was then closed tightly and heated to 70~ for 72 h (gelation and aging). In all gels, the molar ratios EDAS / Pd and AS / Ag = 2 and in the pure silver sample X100, the addition of EDAS was necessary to obtain gelation of the solution. After aging, gels X0 to X100 were dried under vacuum at 150~ for 72 h. The resulting samples are xerogels. Gel A67, which is identical to gel X67, was dried under supercritical conditions at 327~ and 13 MPa and is an aerogel. All gels were subsequently calcined under air at 400~ during 12 h and reduced under 1-I2at 350~ during 3h. Table 1 Synthesis parameters Nominal Ag Ag/(Pd+Ag) Gel time Catalyst Pd(acac)2 Ag(OAc) EDAS AS Pd (wt%) (at%) (min) (mmol) (mmol) (mmol) (mmol) (wt%) 0 1.5 0 0 26 5 36 X0 2.69 0 2.77 1.5 0.75 33 15 541 X33 2.70 1.35 5.40 1.5 1.5 50 15 5 48 X50 2.75 2.70 11.06 1.5 3.0 67 13 5.55 X67 2.76 5.48 5.32 0 1.5 100 30 5.32 X100 0 2.65 11.06 1.5 3.0 67 12 5.55 A67 2.77 5.47 TEOS: 300 mmol; 1-120:1500 mmol; NH3:4.9 mmol; C2HsOH: 3100 mmol

2.2. Catalysts characterization and catalytic experiments The real metals contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The composition and size of the metallic particles were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDX). Texture was examined by N2 adsorption at 77 K, Hg porosimetry, and TEM. The hydrodechlorination of 1,2-dichloroethane was conducted in a stainless steel tubular reactor at 300~ and 0.3 Mpa. For each experiment, 0.11 g of catalysts pellets were used. The flow of the reactant mixture consisted of CH2C1-CH2C1 (1 N-l/h), H2 (2 N1/h), and He (37 N1/h). More details about catalysts preparation~ characterization and test can be found in [ 17].

3. RESULTS In all xerogels, metals contents measured by ICP-AES are higher than the nominal values and the gap between nominal and experimental contents decreases when the silver loading increases (Tables 1, 2). However, the weight ratio Ag/(Pd+Ag), which is equal to the atomic ratio since Pd and Ag have almost the same atomic weight, remains identical to the nominal ratio. Metals loadings higher than nominal values could therefore be due to a loss of unreacted

711 TEOS during drying. In aerogel A67, nominal and experimental metals contents are equal which could be due to the ability of supercritical drying to lead reactions to completion. Table 2 Characterization of metallic phases . . . . ICP-AES (wt%) Ag/(Pd+Ag) (at%) Particle size (nm) XRD XRD TEM Catalyst Pd Ag ICP-AES Phase 1 Phase 2 Phase 1 Phase 2 Small Large X0 3.3 0 0 0 7.7 2.3 28 X33 2.2 1.1 33 31 100 1.9 >30 2.4 9 X50 2.3 2.2 49 43 100 2.1 >30 3.0 7 X67 1.9 3.7 66 46 100 2.0 18 3.0 10 X100 0 1.7 100 100 10.3 1 to 20 A67 1.5 3.0 67 8 100 2.7 6 2.4 10 XRD spectra of bimetallic xerogels X33, X50 and X67 exhibit a broad peak between the Bragg lines of pure Pd and Ag which demonstrates the presence of a solid solution (SS) Pd-Ag (phase 1). The three samples, but particularly catalyst X67, show also peaks characteristic of unalloyed Ag (phase 2). The composition of the solid solution was calculated from the unit cell parameter corresponding to the SS peak [ 18]. Results in Table 2 indicate that the composition of the bimetallic particles is almost the same as the overall ICP-AES composition (= nominal composition) in xerogel X33, but it deviates increasingly from the overall composition for growing Ag loading. The main reason for this composition gap is probably the presence of a pure silver phase but the shift toward a lower silver percentage may also result from surface enrichment with Ag [ 19]. This enrichment results from the lower surface energy of Ag with respect to Pd [20]. The very wide SS peak are due to the small size of the particles and/or to a distribution of composition [ 19]. With the assumption that all the bimetallic particles have the same Pd-Ag composition, their size can be calculated from the peak broadening thanks to the Scherrer equation [18]. Bimetallic particles in xerogels X33, X50, and X67 are found to be finely dispersed, whereas pure silver particles in these samples are much larger. XRD analysis of aerogel A67 shows an almost total segregation of Pd and Ag with small Pd-containing particles and large pure Ag particles. In figure 3 are schematically represented TEM pictures of bimetallic catalysts. TEM analysis show that all Pd containing samples exhibit metal particles distributed in two families of different size (Table 2). Bimetallic xerogels X33, X50, X67 and aerogel A67 exhibit small and large particles with diameters of 2 to 3 nm and 7 to 10 nm, respectively. It appears that there are more large particles in A67 than in X67 and in X67 than in X33 and X50. In the pure Pd sample X0, small particles are about the same size (2.3 nm) but large particles are much larger, with a mean diameter of 28 nm. The pure Ag catalyst X100 exhibits a broad distribution of metal particle sizes from 1 to 20 nm. Except in X100, it appears that the catalysts are composed of silica particles arranged in strings or aggregates, and although TEM gives only a 2D view, it seems that small metal particles are located inside silica particles, whereas big metal particles are located at their external surface.

712 It is interesting to compare the crystallite size determined by TEM and XRD in Table 2. For the monometallic samples X0 and X100, XRD gives an intermediate value consistent with the limits given by TEM. In the bimetallic xerogels as well as in aerogel A67, the size given by XRD for the solid solution (phase 1 in Table 2) is always close to the size of the small particles detected by TEM. Both methods detect a second family of bigger particles, and the big particles detected by XRD correspond to pure silver. These results lead to the conclusion that the small metal crystallites located inside the silica particles would be Pd-Ag alloy crystallites, whereas the big crystallites located outside the silica would consist of pure Ag. This conclusion was confirmed by an additional STEM-EDX analysis of xerogel X67 and aerogel A67. Due to their sufficient size, the focusing of the electron beam on individual big particles was possible and showed that they are composed of pure silver in both samples. On the contrary, it was not possible to focus the beam on individual small particles but X-rays emitted by groups of small particles in sample X67 were characteristic of Pd and Ag. In sample A67, only X-rays corresponding to Pd were observed which is in agreement with the nearly pure small Pd crystallites observed by XRD.

Figure 3. Schematic representation of TEM micrographs of Pd-Ag/SiO2 sol-gel catalysts

I0~ micropores ] - - p o ~ ~ . . . _ _ / f XO "~

0.I-

-~ 0.01 r..)

0.001

I 0.1

1

10

I

I

I

100

1000

10000

Pore size (nm) Figure 4. Pore size distributions

The texture examination by N2 adsorption as well as Hg porosimetry shows that the catalysts prepared in this work exhibit very broad pore size distributions. The cumulated distributions over the complete pore size range shown in Figure 4 were obtained by applying a combination of various methods to their respective validity domains and by additioning the porous volumes corresponding to those domains. The distributions of micropores were calculated by Brunauer's

713 method, which is based on the multilayer adsorption assumption, applied to the t-plots derived from nitrogen adsorption isotherms [21]. The distributions of mesopores smaller than 7.5 nm were calculated by the Broekhoff-de Boer method based on N2 capillary condensation [21 ]. Mesopores larger than 7.5 nm and macropores were examined by Hg porosimetry using the collapse model at low Hg pressure and the intrusion model at high pressure [22, 23]. Palcontaining xerogels (X0, X33, X50 and X67) exhibit similar distributions an example of which is given in Figure 4 with sample X0. In the micropore domain, those four catalysts exhibit a very narrow pore size distribution centered on a mean value of about 0.8 nm which corresponds to the steep volume increase followed by a plateau. Contrary to Pd-containing xerogels, the pure Ag sample X100 exhibits a broad micropore size distribution from 0.7 to 2 nm, and aerogel A67 does not contain any micropores. In the range of meso- and macropores, one observes that all samples exhibit a broad distribution and that for xerogels this distribution shifts toward the large pores from X0 to X100. It is particularly interesting to note that the pore volumes VHg of those xerogels, contained between 2 and 6 cm3/g, are in the same order of magnitude as the pore volume of aerogel A67 (9.2 cm3/g) or other aerogels described in literature [24]. For this reason, those materials are called "low-density xerogels". The texture was also examined by TEM. Micrographs of samples X0, X33, X50, and X67 show that when Ag loading is increased, the xerogel structure evolves from an arrangement of silica particles in strings to an arrangement in aggregates which become more compact. This is schematically represented in Figure 3. Aerogel A67 exhibits dense aggregates comparable in size to X67, and xerogel X100 contains much bigger aggregates where silica particles do not appear clearly as it is the case in Pd containing samples. Pure silver xerogel X100 and aerogel A67 were completely inactive for the hydrodechlorination of 1,2c2 dichloroethane. On the ~ 75 ".~ contrary, xerogels X0, X33, X50, and X67 convert ~ 5o CH2C1-CH2C1 with an increasing selectivity in g ethylene. Figure 5 shows .~ 25 conversions and selectivities e =o measured at 300~ and at 0 stationary state obtained after 0 20 40 60 80 16 h. The pure palladium Ag/(Pd+Ag) (overallat%) sample X0 mainly produces ethane with a selectivity of Figure 5. Hydrodechlorination conversion and selectivities about 85 %. Ethyl chloride (o: 1,2-dichloroethane conversion; o: ethylene selectivity; CH3-CH2C1, which is not observed with the other o: ethane selectivity; A: ethyl chloride selectivity) catalysts, is the secondary product. The introduction of silver into the catalyst leads to a drastic change in selectivity towards C2H4, and when the Ag loading is high enough, this selectivity reaches 100 %.

714 4. DISCUSSION As mentioned above, the examination of Pd containing xerogels shows a structure with increasingly compact aggregates of SiO2 clusters or particles as the Ag content is increased (Figure 3). In agreement with the work of Brinker and Scherer [25], we assume that gel formation occurs in two successive steps: 1- formation of silica particles by hydrolysis and condensation of TEOS and 2- gelation by aggregation of those clusters. Let us consider the second step first. As explained in [25], if the colliding clusters always stick together (sticking probability = 1), the rate of aggregation is determined by transport kinetics, and the process is known as diffusion-limited cluster-cluster aggregation (DLCCA). In this case, attachment tends to occur at the cluster periphery which leads to an open structure. In many cases, the sticking probability is much lower than unity; thus many collisions will occur before two clusters link together. This process, called reaction-limited cluster-cluster aggregation (RLCCA), allows more opportunity for the clusters to interpenetrate and leads to more compact aggregates. The RLCCA mechanism is expected to dominate when a repulsive electrostatic barrier, which decreases the sticking probability, is present between the colliding particles. In his study of the stability of aqueous silica sols, Iler indicates that pH 2 is approximately the point of zero charge [26]. Above this value, the surface of the silica particles becomes negatively charged by deprotonation, and their mutual repulsion increases with pH. This repulsion effect is also observed for silica gel synthesized from TEOS in mixed alcoholwater systems [25]. In precursor solutions of Pd-containing gels, the AgOAc and AS concentrations and, therefore, the basicity are increased from X0 to X67 (Table 1). Silica particles in those solutions are then characterized by a growing repulsive negative electrostatic barrier, and their aggregation is limited by an increasingly slow reaction. As explained above, this progressively slower RLCCA mechanism would have to lead to more compact aggregates which is in agreement with TEM observations schematized in Figure 3. If we consider the aggregation step only, gel time is supposed to increase with the concentrations of AgOAc and AS. On the contrary, gel times reported in Table 1 indicate that gel formation is faster with increasing Ag content. This initially paradoxical result can be explained by considering the first step of gel formation, that is the formation of silica particles by hydrolysis and condensation of TEOS. These two reactions are accelerated by an increasing basicity [25], and the gel time decrease leads to the conclusion that the formation of silica particles is probably the rate determining step in the overall process of gel formation. It was shown previously that the complex pd2§ seems to act as a nucleation agent in the formation of silica particles (see introductory part and [12]). This nucleation effect is observed again in the pure Pd sample X0: TEM shows small Pd crystallites located inside SiO2 particles. Nevertheless, in this xerogel, big Pd crystallites are also observed at the surface of SiO~ particles. As explained before, this might result from migration and coalescence during thermal treatments of smaller Pd particles which are not trapped inside silica particles [ 12, 13]. On the contrary, in the pure Ag sample, a nucleation effect by Ag is not observed. Whereas Pd-containing samples exhibit strings or aggregates of SiO2 particles enclosing metal crystallites, this structure does not appear in sample X100. Ag particles are distributed over a broad range of sizes. Small crystallites seem to be located inside a porous SiO2 matrix but not inside individual silica particles having a monodisperse micropore size distribution (Figure 4). Unlike the Pd complex, the Ag complex is, therefore, supposed not to act as a nucleation

715 agent. In the wet gel, this molecule would be spread rather randomly through the SiO2 network. The bimetallic xerogels X33, X50 and X67 exhibit a structure similar to the pure Pd xerogel X0. TEM shows small Pd-Ag alloy crystallites located inside SiO2 particles with a monodisperse micropore size distribution (Figure 3 and 4). Several larger pure Ag particles are also observed at the surface of the SiO2 particles. A careful examination of aerogel A67 is helpful in order to understand the mechanism of Pd-Ag alloy formation. It has been shown that this sample contains small, nearly pure Pd crystallites (8 at% of Ag) located inside SiO2 particles, as well as numerous large pure Ag crystaUites (Table 2). Its texture is very different from that ofxerogel samples, since no micropores are present (Figure 4). Finally, A67 does not exhibit any catalytic activity which leads to the conclusion that Pd is not accessible for the gas phase. Aerogel A67 is then probably composed of small, nearly pure palladium crystallites confined inside non porous silica particles and big pure silver crystallites (inactive for hydrodechlorination) spread over their surface. All these results are in favor of the following mechanism for the Pd-Ag alloy formation in bimetallic xerogels: in wet gels, Pd complex groups would be located inside porous SiO2 particles because of the nucleation effect, while the Ag complex would be spread randomly through the silica network. During subsequent thermal treatments, silver would migrate through micropores inside SiO2 particles and combine with trapped palladium to form small alloy particles. In the course of their migration, silver atoms and/or particles could meet and form bigger particles. Some of them, observed experimentally, would become too large to diffuse through the micropores and to combine with palladium. In aerogel A67, the same mechanism would probably occur, but, for an unknown reason, micropores inside SiO2 particles would close before the majority of silver reaches palladium. This leads to the observed nearly complete segregation of the two metals. The catalytic resuks shows that the formation of alloy particles is a determining factor for the obtaining of a high selectivity in C2H4during hydrodechlorination and the role of this alloy in the reaction mechanism is at present under investigation.

5. CONCLUSIONS When dried under vacuum, the Pd-Ag/SiO2 xerogels exhibit a texture similar to that of Pd/SiO2 aerogels dried under supercritical conditions with pore volumes in the range of 2-6 cm3/g and pores ranging from micro- to macropores of several hundred nanometers. As in aerogels and due to the nucleation by Pd2+(EDAS)2 effect, completely accessible metal crystallites are trapped inside microporous silica particles which makes them sinter-proof. Formation of Pd-Ag alloy particles by migration of Ag toward Pd allows to obtain a very high selectivity in ethylene during hydrodechlorination of 1,2-dichloroethane. The possibility to obtain an aerogel structure by avoiding the difficult step of supercritical drying is of great importance for the development of such materials in the future.

REFERENCES 1. K. Foger, in "Catalysis: Science and Technology", J.R. Anderson and M. Boudart (eds.), Vol. 6, p. 227, Springer-Verlag, Berlin, 1984.

716 2. M. Che, O. Clause and Ch. MarciUy, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Knrzinger and J. Weitkamp (eds.), Vol. 1, p. 191, Wiley-VCH, Weinheim, 1997. 3. K. Balakrishnan and R.D. Gonzalez, J. Catal., 144 (1993) 3 95. 4. T. Lopez, M. Asomoza, P. Bosch, E. Garcia-Figueroa and R. Gomez, J. Catal., 138 (1992) 463. 5. J.N. Armor, E.J. Carlson and P.M. Zambri, Appl. Catal., 19 (1985) 339. 6. T.Lopez, P. Bosch, M. Asomoza and R. Gomez, J. Catal., 133 (1992) 247. 7. B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater., 3 (1991) 559. 8. U. Schubert, New J. Chem., 18 (1994) 1049. 9. W. Mrrke, R. Lamber, U. Schubert and B. Breitscheidel, Chem. Mater., 6 (1994) 1659. 10. A. Kaiser, C. Grrsmann and U. Schubert, J. Sol-Gel Sci. Technol., 8 (1997) 795. 11. B. Heinrichs, J.-P. Pirard and R. Pirard, Transition Metal Aerogel-Supported Catalyst, US Patent No. 5 538 931 (1996). 12. B. Heinrichs, F. Noville and J.-P. Pirard, J. Catal., 170 (1997) 366. 13. S.A. Stevenson, J.A. Dumesic, R.T.K. Baker and E. Ruckenstein, Metal-Support Interactions in Catalysis, Sintering, and Redispersion, Van Nostrand Reinhold, New-York, 1987. 14. G.F. Froment and K.B. Bischoff, Chemical Reactor Analysis and Design, John Wiley & Sons, New-York, 1990. 15. P. Delhez, B. Heinrichs, J.-P. Pirard and J.-P. Schoebrechts, Procrd6 de prrparation d'un catalyseur et son utilisation pour la conversion d'alcanes chlorrs en alcenes moins chlorrs, Demande de brevet europeen EP 0 745 426 A1 (1996). 16. L.N. Ito, A.D. Harley, M.T. Holbrook, D.D. Smith, C.B. Murchison and M.D. Cisneros, Processes for Converting Chlorinated Alkane Byproducts or Waste Products to Useful, Less Chlorinated Alkenes, International Patent Application WO 94/07827 (1994). 17. B. Heinrichs, P. Delhez, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 172 (1997) 322. 18. J.H. Sinfelt, Bimetallic Catalysts- Discoveries, Concepts, and Applications, John Wiley & Sons, New-York, 1983. 19. A. El Hamdaoui, G. Bergeret, J. Massardier, M. Primer and A. Renouprez, J. Catal., 148 (1994) 47. 20. E.G. Allison and G.C. Bond, Catal. Rev. 7 (1972) 233. 21. A.J. Lecloux, in "Catalysis: Science and Technology", J.R. Anderson and M. Boudart (eds.), Vol. 2, p. 171, Springer-Veflag, Berlin, 1981. 22. R. Pirard, S. Blacher, F. Brouers and J.-P. Pirard, J. Mater. Res., 10 (1995) 2114. 23. R. Pirard, B. Heinrichs and J.-P. Pirard, in "Characterisation of Porous Solids IV", B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (eds.), p. 460, The Royal Society of Chemistry, Cambridge, 1997. 24. G.M. Pajonk, Appl. Catal., 72 (1991) 217. 25. C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. 26. R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979.

91998 Elsevier ScienceB.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

717

Pore-wall modified metal/ceramic catalytic membranes prepared by the sol-gel method Hongbin Zhao a*, Guoxing Xiong b+ and G.V. Baron a aDepartment of Chemical Engineering, Vrije Universiteit Brussel Pleinlaan 2 B-1050 Brussel, Belgium bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China ABSTRACT The sol-gel method has been explored to prepare a catalytic membrane. Photocorrelation spectroscopy, nitrogen adsorption, scanning electron microscopy (SEM), scanning electron microscopy wave dispersive X-ray analysis (SEM-WDX), and gas permeation measurement were used to characterize the preparation of the catalytic membrane. In the sol-gel process, the noble metal ion-modified boehmite sols were produced by adsorption of the noble metal complexes at the liquid/solid interface of the boehmite sol particles. The thickness of the catalytic membrane was increased by a multiple sol-gel process. It was confirmed that the distribution of the catalytst precursors exhibited a uniformity in the direction of the thickness of as well as along the surface of the catalytic membrane. The gas permeation measurement showed that the catalytic membrane prepared had a good thermostability. 1. INTRODUCTION Asymmetric pure alumina membranes with pore sizes up to micrometers were already commercially available in the early 1980's, and were mainly applied for ultrafiltration and microfiltration. Since then, an impetus has been given to investigations of application of these ceramic membranes in catalytic reactions [1]. For the catalytic applications, the ceramic membranes usually need to be modified catalytically. Many techniques (impregnation, reservoir method, precipitation-deposition, ion-exchange, grafting, and metallic cluster deposition) have been exploited to incorporate the catalysts in the ceramic membranes [2]. With the above methods, the catalysts can be deposited inside or/and outside the ceramic membranes. However, the techniques can not always be made to yield the desired activephase loading and distribution. It is worthwhile to consider alternative methods. This contribution focuses on in-situ incorporation of the catalysts in the ceramic membrane during the sol-gel process, resulting in a pore-wall modified metal/ceramic catalytic *On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China. +The corresponding author.

718 membrane. The catalyst loading, location and dispersion in the catalytic membrane were investigated. The permeation of hydrogen and nitrogen through the catalytic membrane was determined. 2. EXPERIMENTAL 2.1. Determination of the amount of adsorption as a function of pH ~,-A1203 powder was prepared by calcining commercial PURAL SB powder at 600~ Fifteen samples of 7-A1203 powder of 0.3000 g were weighed, and put into conical flasks with standard solutions of 100 ml containing 10 -4 molfl of the metal ions. The pH difference between the two neighboring samples was adjusted to half a pH unit over a pH range of 1-10 with diluted HNO3 and NaOH. The samples were shaken at room temperature in order to attain a chemical equilibrium of the adsorption of the metal ions on the 7-A1203 particles. Then, the pH values of the samples were measured, respectively. The solutions were separated for the determination of the concentration of the metal ions by spectrophotometric method. The amount of the metal ions adsorbed was calculated by subtraction of the concentration determined from the total concentration of the metal ions added. The adsorption percentage is defined as a ratio of the adsorbed metal ions to the added metal ions. 2.2. Preparation of catalytic membrane The preparation of the catalytic membrane is schematically indicated in Figure 1. A boehmite sol was produced by peptization of the boehmite powders with the diluted nitric acid at 80~ Then the solution containing the metal complexes was added dropwise to the sol under stirring, resulting in the metal ion-modified sol by adsorption. The casting sol consisted of the modified sol (0.5 tool/l), polyvinylacohol 72000 (PVA) and polyethylenglycol 400 (PEG) (2-5wt%). The PVA and the PEG were used to enhance the strength of the gel-coating. Using spin-coating, the casting sol was deposited onto the flat-shaped o~-A1203 substrate of 1.6-gm average pore size and 48% porosity. The coating was dried at 5~ and 65% relative humidity for two days, followed by calcination at 600~ at heating and cooling rates smaller than 1~

boehmite suspension I[ peptizat

i~

calcination

......

boehmite sol II ads~176---II modified sol

I

organic additives ~....

I activation [ I_.. drying " ' catalytic membrane [ ~ igelmembranel-., i castings~ I Figure 1. Schematic of preparation of the metal/ceramic catalytic membranes 2.3. Catalytic membrane characterization The particle size of the sols was measured by photocorrelation spectroscopy (Malvern Zetasizer 3). The as-prepared sols were diluted with distilled water to obtain the samples with identical spheres.

719 The nitrogen adsorption-desorption isotherms of the catalytic membrane materials at 77 K were obtained with Micromeritics ASAP 2000. The adsorption isotherms were used to calculate the specific surface area by the BET theory. The pore size and pore size distribution were produced from the desorption isotherms using BJH method. The morphology of the catalytic membrane was examined by scanning electron microscopy (SEM Stereoscan 120). The metal distribution in the catalytic membrane was determined by scanning electron microscopy-wave dispersive X-ray analyzer (SEM-WDX) (WDX Mapping $200). The conductive treatment of the catalytic membranes was done before performing the SEM and SEM-WDX analyses. The pure gas permeation through the catalytic membrane was determined using the variable-volume method on the home-made set-up. A flat-shaped permeation cell was employed. A 30-mm diameter catalytic membrane was connected with the stainless-steel body using graphite gaskets. The side edge of the catalytic membrane was sealed by a commercial ceramic glaze (UHLIG Kera-Dekor), which can resist to high temperatures up to 800~ 3. RESULTS AND DISCUSSION 3.1. Metal complexes for modification of boehmite sol and metal ion-modified boehmite sol for the sol-gel process The boehmite sol maintains its dynamic stability in the pH range of 3.5-4. Adsorption of metal ions on the boehmite sol particles can only be performed in the above pH range, but the adsorption can still be optimized by proper choice of ligand, resulting in a stable metal ionmodified boehmite sols. Therefore, the adsorption of Pd(H), Pt(lI), Pt(IV) and Rh(ffD on the yA1203 particles as a function of pH was determined, and the effect of the ligands including CI-, NH3 and EDTA on the adsorption was addressed. Based on the above investigation, the metal complexes suitable for modification of the boehmite sol can be found. A: Pt(IV)-C1 B: Pt(II)-C1 C: Pt(II)-NH3 D: Pt(II)-EDTA 100

.o

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~

00

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Figure 3. The adsorption of Pt(ll) and Pt(IV) complexes as a function of pH.

720 The adsorption of three kinds of the Pd(lI) complexes as a function of pH is shown in Figure 2. From Figure 2, the adsorption percentages of Pd(NH3)42+ and PdEDTA 2- reached more than 80% over the pH range of 3.5-4. Figure 3 plots the adsorption percentages of the Pt(ll) and Pt(IV) complexes versus the pH values of their solutions. As indicated in Figure 3, the adsorption of PtEDTA 2-, PtC142-and PtC162- occurred significantly in the above pH range. The adsorption percentages of RhCI63- versus pH curve is given in Figure 4. From Figure 4, the adsorption percentages of RhC163- became as much as 85% in the pH range of 3.5-4. According to the requirement of the modification of the boehmite sol by adsorption at the liquid/solid interface, Pd(NH3)42+, PdEDTA 2-, PtEDTA 2, PtC142-, PtC162-, and RhCI63- would become candidates for the modification of the boehmite sol. Furthermore, it was shown that the ligands of CI-, NH3 and EDTA had a significant effect on the adsorption of the metal ions (Pd(ll), Pt(ll), Pt(VI) and Rh([ll)). According to the surface complex theory [3], the effects would be caused by change of the apparent charge or/and valence electronic structure of the central metal ions with the ligands. Therefore, ligand change can be one of the effective means by which the adsorption of the metal ions can be tailored to a specific application besides pH. 100 .

.

.

.

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0"00

50 100 150 200 250 300 Sol particle size (nm) Figure 5. The particle size distribution of the Pd(II)-modified boehmite sol.

As an example, a porous Pd/7-A1203 membrane was prepared and characterized. In this work, Pd(NH3)42§ was used to produce the Pd(ll)-modified boehmite sol. The effect of the adsorption of Pd(NH3)42§ on the particle size and particle size distribution of the modified boehmite sol was evaluated, and the result is listed in Table 1. As the concentration of Table 1 Pd(NH3)42+ increased, the average particle The average particle size of the Pd(II)size of the modified boehmite sol modified boehmite sols increased. This would be attributed to the Pd content Sol particle size adsorption of Pd(NH3)42+ on the boehmite (Pd/~'-A1203 wt%) (nm) sol particles. It should be mentioned that 1 43.7 the 4 wt% Pd(ll)-modified boehmite sol 2 66.2 exhibited smaller particle size than 3 wt% 3 73.0 one, and this remains unplain. 4 59.5

721

The particle size distribution of the 2 wt% Pd(l/)-modified boehmite sol is shown in Figure 5. It illustrates that the modified sol exhibited a narrow particle size distribution. From the particle packing model [4], a porous ceramic membrane with narrow pore size distribution can be produced by free close-packing of sol particles with narrow particle size distribution. Accordingly, the synthesis of the Pd(ll)-modified boehmite sol with narrow particle size distribution is one of the key steps to the catalytic membrane with narrow pore size distribution.

3.2. Structure of catalytic membrane The pore structure of the Pd/T-A1203 membrane materials with different loading of palladium was determined, and the result is listed in Table 2. The pore size of the Pd/T-A1203 membrane materials increased with increasing the loading of palladium, but the BET specific surface area did not exhibit significant change. The pore size distribution of the Table 2 Pd/T-A1203 membrane material (2 wt% The average pore size of the Pd/7-A1203 membrane materials Pd/T-A1203) is given in Figure 6. As expected by the particle packing model, Pd content Pore size Surface area the Pd/7-A1203 membrane material with (Pd/7-A1203 wt%) (nm) (m2/g) narrow pore size distribution was made 0.5 5.48 267 from its precursor Pd(ID-modified 1.0 5.56 271 boehmite sol with narrow particle size 1.5 6.45 240 distribution. Furthermore, the adsorption 2.0 6.47 253 of Pd(NH3)41+ on the boehmite sol particles had effect on packing of the Pd(l/)-modified boehmite sol particles, leading to an increase in the pore size of the Pd/7-AI203 membrane materials with increasing loading of palladium.

|,

,,

i

i, i

,,

,,

,

,

Figure 6. The pore size distribution of the Pd/T-A1203 membrane material.

Figure 7. The SEM micrograph of the cross section of the Pd/7-A1203 membrane.

The Pd/7-A1203 membrane was prepared by a multiple sol-gel process. The SEM micrograph of the cross section of the Pd/7-AI203 membrane produced by six-time sol-gel process is shown in Figure 7. From Figure 7, the thickness of the Pd/7-A1203 layer grew to 19

722 lxm. The interfaces among the six successive layers were not identified as also reported by Zaspalis et al [5] for their TiO2 membrane. Therefore, the multiple sol-gel process can be used to tailor the thickness of a catalytic membrane to a specific catalytic reaction. Furthermore, in the multiple sol-gel process each layer may have different active components, resulting in a multilayer catalytic membrane, or a catalytic membrane with nonuniform catalyst distribution [6]. Figure 8 shows the SEM-WDX result of the Pd/y-AI203 membrane prepared by six-time sol-gel process (4 wt% Pd/y-A1203). The right side is the SEM micrograph of the catalytic membrane sampled, and left side is the WDX result of the corresponding catalytic membrane on the micrograph. From the distribution of the white points, there was a sharp interface between the Pd/y-A1203 layer and the support, and palladium was located in the top layer. Furthermore, palladium exhibited a uniform loading in the direction of the thickness of as well as along the surface of the Pd/T-A1203 layer. The result confirmed that the penetration of the metal ion to the support during deposition of the sol was prevented, and the uniform loading of the active components was realized with the proposed procedure. This is because the metal precursors were anchored on the sol particles by adsorption at the liquid/solid interface.

Figure 8. The SEM-WDX micrograph of the cross section of the Pd/7-A1203membrane.

3.3. The gas permeation of catalytic membrane The effects of transmembrane pressure and temperature on the permeation of hydrogen and nitrogen through the Pd/7-A1203 membrane obtained by six-time dipping were investigated. The result is given in Figure 9. From Figure 9, the permeation rates of nitrogen remained unchanged over the pressure range at room temperature and 503~ respectively. According to the gas transport theory in porous medium [7], the permeation of nitrogen occurred in the

723 Knudsen regime at the two temperatures, as the dimensions of the gas passage of the catalytic membrane were smaller than the mean free path of nitrogen. The catalytic membrane remained thermostable without formation of any microcracks at the elevated temperature. 6.0 ~. 5.0 ~: :, -o-: ~:

4.0

ee~

3.0 ~

= 0

18Celsius, hydrogen : 503Celsius, hydorgen 18Celsius, nitrogen 503Celsius, nitrogen

2.0 1.0 0.0

.,.

I

l,

!

]

I

t

I

I

l

0.0 0.2 0.4 0.6 0.8 1.0 Transmembrane pressure (bar) Figure 9. The gas permeation through the Pd/qt-A1203 membrane. As shown in Figure 9, the observed ratios of hydrogen flux to nitrogen flux were comparable to their Knudsen ratio (3.74) over the pressure range at the two temperatures, respectively. It has been suggested that the presence of surface diffusion can be detected as extra contribution to gas separation over the Knudsen separation [7], but this did not occur with this membrane. 4. CONCLUSIONS The sol-gel method has been shown to be an attractive way to produce catalytic membranes. As an example of the method, the Pd/7-A1203 membrane was prepared. The Pd/7A1203 membrane material exhibited a narrow pore size distribution with an average pore diameter of 6 nm up to 4 wt% the metal loading. Palladium was uniformly dispersed in the top layer of the Pd/7-A1203 membrane. Furthermore, a multiple sol-gel process can be used to obtain a thick Pd/7-A1203 membrane. The catalytic membrane remained intact at elevated temperature. ACKNOWLEDGMENT Support for this research work by the National Sciences Foundation of China, German BMBF, FhG-IGB and IUAP4-11 (Interuniversity Poles of Attraction, Belgium) is gratefully acknowledged. The authors wish to thank Miss M. Riedle for taking SEM micrographs, Mr. S. Thdyka for helping with SEM-WDX analysis and Mr. Sheng for constructing the gas permeation set-up.

724 REFERENCES

1 J.N. Armor, CHEMTECH, (1992) 557. 2 G. Saracco and V. Spechia, Catal. Rev.-Sci. Eng., 36 (1994) 305. 3 W. Stumm (ed.), Chemistry of the Solid-Water Interface, John Wiley & Sons, Inc., New York, 1992. 4 C.J. Brinker, R. Sehgal, S.L. Hietala, R. Deshpande, D.M. Smith, D. Loy and C.S. Ashley, J. Membrane Science, 94 (1994) 85. 5 V.T. Zaspalis, Ph.D. thesis, Catalytically Active Ceramic Membranes: Synthesis, Properties and Reactor Applications, University of Twente, the Netherland, 1990. 6 K.L. Yeung, R. Aravind, R.J.X. Zawada, J. Szegner, G. Cao and A. Varma, Chemical Engineering Science, 49 (1994) 4823. 7 R.J.R. Uhlhorn, K. Keizer, and A.J. Burggraaf, J. Membrane Science, 46 (1989) 225.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

725

Preparation of catalysts supported on Y-PSZ fibers via sol-gel technology M. MARELLA, M. TOMASELLI, J.F. ALLAIN and F. GEROLIN Enirisorse-Centro Ricerche Venezia, via delle industrie 39 - 30175 P.to Marghera (VE), Italy. Tel: +39 (41) 5317322, Fax: +39 (41) 5316756 e-mail: marell@portve, interbusiness, it ABSTRACT A new sol-gel route for synthesizing Y-PSZ fibers is shown. Their functionalization for catalytic applications by coating an additional high-surface-area layer is also shown. Pt containing fibers are very active towards CO oxidation and a possible environmental field application is envisaged. 1. INTRODUCTION A sol-gel innovative process for synthesizing microspheres, beads and washcoats was developed by the authors [1]. However, for some special applications, the possibility of supporting catalytically active species on fibers represents an interesting technological solution. The different sol-gel procedures for the synthesis of ZrO2 fibers found in the literature [2-5] do not take into consideration the chemically induced gelation after extrusion of the sol into a gelation bath [6]. In this paper we show some outstanding features of the developed sol-gel technology in function of envisaged environmental catalytic applications. 2. EXPERIMENTAL DETAILS 2.1. Preparation of Y-PSZ fibers The sol was prepared according to the procedure described elsewhere [6]: 9ZrOz sol (94.5 wt%) prepared from a nitrate precursor of zirconium ([NO3]/[Zr 4.] = 0.72, molar ratio). 9Y20~ sol (5.5 wt%) prepared from a nitrate precursor of yttrium. 9Thickening agent. 9 Final concentration of the sol: 50 g/I as oxides.

EU Research Training Grant ERBBRMACT965008

726 200 ml of this final sol were intensively stirred at room temperature for one hour and then centrifuged at 10,000 rpm for half an hour in order to homogenize and deaerate the mixture. Figure 1 shows the experimental apparatus used for the drawing of the Y-PSZ fibers. 100 ml of sol are introduced in the stainless steel cylinder (a). The sol is then forced through the single hole precious metal spinnerette (b) by a piston (c) with a regular and monitored rate of extrusion. The extrusion load is monitored by a straingauge load cell. After gelation in the bath of diluted aqueous solution of ammonia (d), the fibers are taken by a first take-up unit (e) and washed in a water bath (f). They are then taken by a second take-up unit (g), partially dried in a countercurrent hot air stream (h) and wound onto an alumina roller (i). Finally, they are fired in inert atmosphere at 1473K for 30 min. The gelation occurs immediately in the NH4OH bath since the residence time of the filament into this bath does not exceed 2 or 3 s. However, this gelation doesn't occur inside the cylinder (before the spinnerette) as revealed by the low value of the extrusion load (30 kg).

r-q

Extrusionr---]

_•1

First take up unit (e)

load speect~

Secondtake up unit (g)

Alurrinaroller 0)

L_Jo

~o

controlle

,,~z

r--I Gelation b ' ~ )

Washingbath (f)

~

Ho air s eam ( )

iston (c)

~ I, Stainlesssteel ~, . , ~ Cylinder (a)

O OOUS More detailed representation of the cylinder containing the sol.

Figure 1. Schematic representation of the drawing apparatus used for the synthesis of Y-PSZ fibers.

727

2.2. Preparation of fiber coating According to previous expertise [6], the sol was prepared in the following experimental conditions : 9ZrO2 sol (90 wt%) prepared from a nitrate precursor of zirconium ([NO3-]/[Zr~*] = 0.72, molar ratio). 9A!203 sol (10 wt%) prepared from a nitrate precursor of aluminum. 9Thickening agent. The sol was then intensively stirred for one hour at room temperature. After coating by a dip-coating procedure, the fibers were caicinated at 823K for 3 hours in air. Figure 2 shows the lab-scale apparatus utilized for the dip-coating of the Y-PSZ fibers. (The rate of extraction of the Y-PSZ fibers depends on the speed of this motor and on the diameter of this roller) ,/.~

~ ~ ~ M

otor (a)

Box protecting fibers from the dust.

Sols for the dip coating

Figure 2. Schematic representation of the lab-scale apparatus utilized for the dip-coating of the Y-PSZ fibers. This high-surface-area coating can support a great variety of catalitically active metals and oxides, e.g. Pt, Pd, CuO, NiO, MOO3, etc. Pt/ZrO2-AI2OJY-PSZ fibers (1.5 wt% Pt) were prepared by dip-coating, adding the desired amount of Platinum (as H2PtCI6) to the ZrO2/Ai203 (10 wt%) sol followed by calcination at 823K for 3 hours in air.

728

2.3. Characterization Simultaneous differential thermal and gravimetric analyis (DTA/TG) were carried out on dried fibers in a Netzsch STA 409 instrument. The measurements were performed either in air or in argon (18 l/h), from room temperature to 1473K at a heating rate of 2K/rain. The morphology was studied either by Scanning Electron Microscopy (SEM) with a Philips XL 30 equipment or by Atomic Force Microscopy (AFM) with an AFM EXPLORER Topometrix apparatus. X-ray diffraction (XRD) patterns were obtained by a Siemens D 5000 powder diffractometer equipped with a graphite crystal monochromator using a Copper K~ X-ray radiation source. Experiments were run in step-scan mode with a step interval of 0.02 ~ 20 and a count rate of 1 second per step over the range 5 ~ to 90 ~ 20. Nitrogen adsorption and desorption were measured at 77K with a Micromeritics ASAP 2010 apparatus. The samples were pretreated at 573K under vacuum (1.33 Pa). Tensile strength measurements were carried out on single Y-PSZ fibers by an in-lab-developed apparatus in the following experimental conditions : Load range : 0-20 N ; Load rate : 0.3 N/s ; Gage length : 30-55 mm ; Cross-head speed, typical : 0.04 ram/rain. 2.4. Catalytic measurements Catalytic measurements were carried out by an in-lab developed stainless steel reactor as shown in figure 3. The experiments were performed using 1.5 g of 1.5Pt/ZrO2-AI20~(-PSZ fibers and a flow rate of 100 ml/min (GHSV = 6,000 hl). The feed stream contained 5 vol% CO and 50 vol% 02 in a He background. Before testing, the catalyst was reduced in flowing Hz for 2h at 523K. The reaction mixture was analyzed using an on-line gas chromatograph.

Figure 3. Schematic representation of the reactor utilized for the catalytic tests.

729

3. RESULTS AND DISCUSSION

DTNTG plots, figure 4, show the following properties of xerogel fibers during firing 9i) a relevant weight loss, 34 % at the final sintering temperature and ii) a sharp exothermic crystallization peak at 768K. O

A

_.~

,,

-10

,

o

==

,,,

o -20

-40 273

473

673

873

1073 1273 1473 1673

Temperature (K) Figure 4. DTA/TG plots on xerogel fibers in flowing argon. The optimization of the experimental conditions for the drawing of the Y-PSZ fibers (diameter of the spinnerette, rate of extrusion, relative speed of take-up units, thermal cycle, atmosphere, etc) allowed to obtain very regular fibers with a diameter equal to 20 IJm (figure 5).

Figure 5. SEM micrographs of the Y-PSZ fibers.

730

The external surface of the Y-PSZ fibers is very smooth and, when they are fractured, a near-full density microstructure is revealed. The average diameter of the grains is below 0.5 pm, due to the relatively low temperature of sintering. More precise investigations made by Atomic Force Microscopy show that their diameter is in the range 300-400 nm. The X-ray pattern of Y-PSZ fibers shows that zirconia is in the tetragonal form. The pattern also shows a shift (2e = 0.5 ~ of the peaks which can be explained by the fact that instead of pure zirconia a sol mixture with 5.5 wt% yttria is utilized. Yttrium helps to stabilize the tetragonal lattice of zirconia though creating some lattice distortions. The average value of the tensile strength (0.5 GPa) is comparable with that of calcia stabilized zirconia fibers (11CaO-89ZrO2) obtained with a different procedure [7]. As expected from morphological considerations, the specific surface area of our Y-PSZ fibers is very low (below 1 m2/g). This low value is comparable, however, to those found in the litterature [8] for a temperature of sintering equal to 1473K. So, the use of Y-PSZ fibers for catalytic applications seems to be problematic. In order to check the possibility of supporting catalytically active species (i.e. Platinum), we tried to impregnate our fibers by hexachloroplatinic acid (H2PtCI6) solutions (figure 6).

Figure 6. SEM micrographs of Pt impregnated Y-PSZ fibers. Even if there is sometimes a fine dispersion of platinum in thin islands, it often agglomerates into big particles even of the order of 1pm of thickness. in order to increase the specific surface area of Y-PSZ fibers, we examined the possibility of coating them with a high-surface-area additional layer. We decided to utilize a dip-coating procedure, extracting the Y-PSZ fibers from a more or less diluted sol of final oxide composition ZrO2/AI203 (10wt%). By modification of one parameter (i.e. the amount of thickening agent), we can obtain, after calcination at 823K for 3 hours in air, a well dispersed additional layer showing a thickness varying from 4 pm to 15 IJm (figure 7).

731

Figure 7. SEM micrographs of Y-PSZ fibers impregnated by a sol of final oxide composition ZrO2-AI203 (10wt%). a) : 1 wt% thickening agent, thickness of the coating layer = 4 pm. b) : 2 wt% thickening agent, thickness of the coating layer = 15 pm. The amount of this additional layer, determined by measurement of the amount of AI203 deposited on the Y-PSZ fibers (by Atomic Absorption Spectroscopy), is around 35 wt% (with respect to the Y-PSZ fiber) for a thickness equal at 4 pm. By this way, the specific surface area of the Y-PSZ fibers has been increased from below 1 m2/g to around 10 m2/g.. Even if this specific surface area remains lower than the one of Pt impregnated.sol-gel zirconia spheres (ca. 80 m2/g for a temperature of calcination equal at 823K [9]), it is comparable to those of the catalysts used by P.-O. Larsson et al. [10]. 1.5 g of 1.5Pt/ZrO2-AI20~rY-PSZ fibers with a coating of 15 pm were introduced in the reactor already described in the Experimental. That means that we really had 500 mg of catalyst (1.5 wt% Pt/ZrO2-Ai203) and 1.0 g of support (Y-PSZ fibers). The CO oxidation activity of the catalyst was determined by temperature runup experiments with fixed feed composition. Figure 8 shows the evolution of the conversion of the CO as a function of the temperature. The onset of the reaction was followed by a gradual increase of CO conversion covering a temperature range of about 100K, indicative of absence of light-off of the catalyst. Under the experimental conditions utilized our catalyst seems to be rather active since the full conversion to carbon dioxide is obtained at around 420K. A direct comparison of this activity with those shown in the litterature [10-12] is difficult since the choice of the experimental parameters is relatively different, however the activity of our Pt/ZrO2-AI20~rY-PSZ fibers remains comparable to these results.

732

100

Ii

A

C O

i i m

> 0

0

80

--

60-40-20-0 250

,Al

300

,,i

350

i

,

400

450

Reaction temperature (K) Figure 8 Temperature 9 dependence of CO oxidation activity over 1.5Pt/ZrO2-Ai203/Y-PSZ fibers. 4. CONCLUSION We have shown the synthesis and characterization of new sol-gel Pt/ZrO2-Ai2OJY-PSZ fibers. The preparation flow-sheet is economically interesting due to the relatively low cost of the raw materials when compared to that of other solgel manufacturing processes of fibers. The high-surface-area zirconia base coating is a versatile support where, beyond platinum, other noble metals (i.e. Rh, Pd) and also transition metal oxides (i.e. CuO, NiO, MOO3, etc) can be efficiently dispersed. The possible applications of this new kind of catalysts concern the environmental field, e.g. the catalytic combustion of volatile compounds in fume hoods, as demonstrated by their activity for CQ oxidation. ACKNOWLEDGMENTS The authors wish to thank Professor F. PINNA (Department of Chemistry, University of Venice) for helpful discussion and fruitful collaboration.

733

REFERENCES

1. M. Marella, L. Meregalli and M. Tomaselli, E.P. Patent No. 718239. 2. K. Yokoi and T. Mizuno, Japanese Patent Application No. 01230443 A2. 3. E. Leroy, C. Robin-Brosse and J. P. Torre, Comm. Eur. Communities Rep. EUR 9210, Ceram. Adv. Energy Technol. (1984) 501. 4. K. H. Hwang, T. K. Yoon, K. J. Koh and E. H. Kim, Yoop Hakhoechi, 29 (1992) 544. 5. M.K. Naskar and D. Ganguli, J. Mater. Sci, 31 (1996) 6263. 6. M. Marella, L. Meregalli and M. Tomaselli, Italian Patent Application No. MI 97 A 001464. 7. K. Kamiya, K. Takahashi, T. Maeda, H. Nasu and T. Yoko, Journal of the European Ceramics Society, 7 (1991) 295-305. 8. D. B. Marshall, F. F. Lange and P. D. Morgan, Journal of American Ceramic Society, 70 [8] (1987) C-187. 9. M. Marella, M. Tomaselli, L. Meregalli, M. Battagliarin, P. Gerontopoulos, F. Pinna and G. Strukui, "Preparation of catalysts VI, Scientific Bases for the Preparation of Heterogeneous Catalysts", G. PONCELET et al. (Editors), (1995), 327-335. 10. P.-O. Larsson, H. Berggren, A. Andersson and O. Augustsson, Catalysis Today, 35 (1997) 137-144. 11. Y. Yuan, K. Asakura, H. Wan, K. Tsai and Y. iwasawa, Catalysis Letters, 42 (1996) 15-20. 12. N.W. Cant and N.J. Ossipoff, Catalysis Today, 36 (1997) 125-133.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

735

Preparation of catalyst matrixes of controlled porous texture from silica and alumina sols. J.P. Reymond*, G. Dessalces** and F. Kolenda. UMR CNRS-IFP 36, IFP-CEDI, BP 3,69390 Vernaison, France

In this work, sol-gel transition is used in order to prepare silica and silica-alumina with tailormade texture for catalyst matrix (or support) purposes. Silica and silica-alumina are obtained from commercial sols of silica (Ludox) and alumina (Chlorhydrol) destabilized with electrolyte addition or drying. The influence of preparation operating parameters on gelation time and texture of final products is studied. In particular, the main role of the diameter of elementary silica particles on xerogel texture is evidenced. Key words : catalyst, sol-gel transition, silica, alumina, preparation, sol, texture. 1. INTRODUCTION The final quality of an heterogeneous catalyst depends on all steps of its preparation process : synthesis step (cristallisation, precipitation ...), shaping step (drying, grinding, pelleting, extrusion...), purification step (washing, filtration...),.... Mastering the relationship product properties versus process parameters requires an in-depth study of the phenomena at a macroscopic scale as well as microscopic scale. In a previous work (1) we have described the preparation of catalyst matrix based on silica and silica-alumina with controlled porous texture (i.e. size, volume and surface of pores), obtained from acid neutralization of aqueous solutions of sodium silicate (water glass). During this partial neutralization of sodium silicate, formation of a silica hydrosol occurs, followed by the formation of a silica hydrogel (gelation step). The silica hydrogel texture and the texture of the corresponding silica xerogel (in spite of the material shrinkage observed during the drying) depend on the operating parameters of the gelation procedure and particularly on the size of silica colloidal particles. Addition of aluminium sulfate and ammonia solutions to the silica hydrogel leads to the precipitation of alumina under several forms : A104 tetraedrons isomorphically incorporated in the chains of SiO 4 tetraedrons; alumina deposits anchored on silica and free amorphous alumina agregates located in the cavities of the silica network. The silica hydrogel network is the framework of the silicaalumina hydrogel. Consequently, the texture of silica-alumina xerogels depends on the characteristics of the initial silica sol, on the operating parameters of the silica sol-gel transition and of the alumina precipitation. Applied to this particular mode of preparation of silica and silica-alumina, experimental design theory led to a predictive mathematical model * To whom all correspondance should be adressed to the present adress : LGPC, CPE, 43 Bd 11 Novembre, 69616 Villeurbanne cedex, France ** Present adress LMOPS, BP 24, 69390 Vernaison, France

736 of the silica-alumina texture, validated in a given experimental range (2). Thus, for a given set of process operating parameters, the texture of silica and silica-alumina can be predicted. Unfortunately, the followed mode of pre-paration presents several drawbacks. It leads to the formation of side-products (ammonium sulfate and sodium sulfate) which have detrimental effects on the texture of solids and are difficult to remove (1). Furthermore, the colloidal particles of silica sols formed during the acid neutralization of sodium silicate solutions have wide diameter distributions and are not stable. Consequently, the resulting silica and silicaalumina hydrogels and xerogels have a wide and complex pore size distribution. To better control the xerogel texture, the use of silica sols with narrow particle size distributions, centered on various diameter values, appears of great interest. Nevertheless, stable silica sols are difficult to prepare (3), but some are commercially available. Although commercial silica sols and alumina sols are raw materials more expensive than sodium silicate or aluminium sulfate, their use presents some advantages. The number of preparation steps is minimized (for example gel filtration can be eliminated) and the materiel cost investment is lower. The high purity of commercial sols and limitation of by-product formation reduce the difficult problems of elimination of impurities (1) and lead to purer end products. 2 EXPERIMENTAL 2.1. Raw materials

Among available commercial silica sols, the well known Ludox| AS40, HS40 and SM30 grades from Du Pont de Nemours (4) have been selected. Table I depicts some properties of these sols. Table 1 Properties of Ludox | silica sols. Sol ~Specific ....................bsears surface area surface m2/g m2/g

Mean particle diameter c nm

Silica content

Stabilizing counter ion

pH (25~

wt%

AS 40

140

128

22

40

NH+4

9.1

HS 40

240

245

12

40

Na +

9.7

SM 30

360

286

7

30

Na +

10

a : values from Du Pont. b )our vaiues~ nfrom S earsmeih~~easuremenis~ c ~iascalcui~e~d from Sears measurements with silica density of 2.2 g/cm 3. As alumina sol, we have used Chlorhydrol| from Reheis Chemical Company (5). It is assumed to be a colloidal solution of basic polymeric aluminium chloride having the empirical formula [A12[OH]sC1]x, with x=2 to 4 (6). The commercial aqueous solution contains 23 to 24% of alumina, 7.9 to 8.4% of chloride and its pH value is approximately 3.5 4. The diameter of colloidal particles is not given by the manufacturer and we have not been able to measure it. When heated chlorhydrol releases water and hydrochloric acid. As the temperature increases there is formation of aluminium hydroxyde, then ,/-alumina and, at high temperature, m-alumina. This compound is used, for example, as a binder in the preparation of

737 fluid cracking catalysts (7), leading to very high attrition-resistant particles (formation of aA1203).

2.2. Preparation of hydrogels and/or xerogels Ludox silica sols are stabilized by addition of cations (small amounts of ammonia or sodium hydroxyde solution, see table 1) to prevent gelation. The surface of silica particles becomes negatively charged and the particles repel each other. Agglomeration of silica sol particles (gelation) can be obtained by decreasing the sol pH value, increasing the ionic force of the solution (addition of a salt solution) or evaporation of water solution (drying). Addition of an acid to a silica sol does not give satisfactory results (very long gelation times) and has not been used in this work. Sample preparations took place at two scales. Small quantities of suspension (100 to 150 cm3), heated and stirred with a magnetic stirrer, were prepared in glass beakers. Operating parameters of sol destabilization have been studied from these small scale experiments. Spray-drying has been chosen as the drying technique. It allows simultaneous drying and shaping of the material, leading to a representative solid. However, operating parameters of a spray dryer, even a small apparatus, require a large amount of material to dry. Thus, sample preparations took place in a five liter glass reactor, equipped with sensors (temperature and pH). A torquemeter mounted on the stirring shaft allows to record viscosity variations of the reactor content. All xerogels mentioned in the text below have been obtained in drying hydrogels or hydrosols in a Niro Minor spray-dryer equipped with a centrifugal atomizer. This dryer leads to microspheres of xerogels whose mean diameter is approximately 20-30 micrometers. In the case of this small spray-dryer, operating parameters have weak effects on the texture characteristics of the particles. Consequently, in all cases the drying conditions were : inlet temperature 300 ~ outlet temperature 110 ~ feed flow 1.5 kg/h. 2.3. Techniques of characterization Sample texture has been evaluated from nitrogen adsorption-desorption isotherms at 77K (Accusorb 2100 E, Micromeritics). This method gives the specific surface area (BET), pore volume and pore diameter distribution (BJH). Xerogel samples, coming from the spray-dryer, are out-gassed under vacuum at 300 ~ during 15 hours before texture measurements. Complementary measurements have been done using mercury porosimetry (Autopore II, Micromeritics), helium pycnometry (Autopycnometre 1320, Micromeritics) and Sears method

(8). The latter is a titration method applying only to silica, colloidal silica and powdered silica. Surface hydroxyl ions of silica are counted from measuring the sodium hydroxyde amount poured in a silica suspension (1.5 g of silica in a 20 wt% aqueous solution of sodium chloride) to increase the pH value from 4 to 9. The specific surface area, S (mZ/g), is given by the equation S = 26.5(Vt-Vb) , where Vt (cm 3) is the sodium hydroxyde volume poured in presence of silica, and Vb is the sodium hydroxyde volume poured in absence of silica (blank).

738 3. RESULTS AND DISCUSSION

3.1. Preparation of silica xerogels from drying of silica sols The most convenient method to prepare a silica xerogel from a silica sol is to evaporate the water surrounding the colloidal particles. Applied to the Ludox sols spray drying leads to results depicted in table 2. Despite some differences, BET and Sears methods lead to the same specific surface area values As demonstrated by nitrogen adsorption-desorption isotherms (type IV) silica xerogels are mesoporous materials and their hysteresis loops (type E according to De Boer) are typical of sphere assemblies (9). As shown in table 2 the specific surface area of a sample depends on the diameter of initial sol particles :the greater the particle diameter, the smaller the area value. On the otherhand, in all cases pore volume (< 0.2 cm3/g) and mean pore diameter are small. As determined by helium pycnometry the density of those xerogels is between 2.201 (Ludox AS 40) and 2.228 (Ludox SM30). Mercury porosimetry confirms the presence of mesopores (- 0.18 cm3/g) and underlines the absence of intra-granular macropores. Texture of silica supports can be mastered in a simple way, from direct spray-drying of silica sols having elementary particles with defined diameters. Table 2 Texture characteristics of dried silica sols (Ludox| ....giiic-a"so-i-" .....S-pecifi---c-surfac~..... P ~ r ~ ...................i~ore-.............................~ e a ..............Pa~icie ....... Isotherrne area (BET) volume diameter (Sears) diameter a type m2/g cm3/g nm m2/g nm AS 40

96

0.198

6.1

105

26

IV (E)

HS 40

167

0.154

3.6

154

17.7

IV (E)

SM 30

205

0.140

3.4

230

11.3

IV

calculated from Sears measurements with slhca density of 2.2 cm/g. If the main goal is to prepare silica-alumina supports with acid properties the best way seems to prepare silica hydrogels in which alumina would be incorporated. To achieve this aim, preparation of silica hydrogels from sols have been studied.

3.2. Preparation of silica hydrogels from destabilization of silica sols by means of electrolyte addition. A second way of destabilizing a sol is the addition of an electrolyte which ions are able to reduce repulsive forces which maintain colloidal particles away one from another. Gelation rate of the sol depends on numerous parameters. The efficiency of the added electrolyte can be evaluated from the measurement of the gelation time of the sol. Silica sol destabilization has been studied with regard to the gelation time which must remain compatible with preparation process constraints. A too short gelation time (< 2 minutes) does not allow a real mastering of the gelation process and to avoid local precipitations in the reactor. A long gelation time (>300 minutes) is not economically interesting.

739

3.2.1. Influence of the gelation time In the case of the five liter stirred reactor the gelation time is determined by means of the torquemeter mounted on the stirring shaft : a fast viscosity increasing points out the gelation. In the case of experiments made in a beaker with small reactant quantities, the stop of the magnetic bar gives the gelation time. Simultaneous experiments realized with the same reactant mixtures (sol + electrolyte), in a beaker and in the five liter reactor, lead to a quite good agreement between the two gelation time measurement methods. As seen above, the knowledge of the gelation time is of great interest to define a preparation process from silica sols. So, the influence of several parameters on silica sol gelation time has been studied : electrolyte type: anions (CI, NO3, 8042-) and cations (Na +, K 2+, NH4 +, A13+) - sol pH value (4 to 12) and temperature (20 ~ ~ - colloidal particle diameter (7 to 22 nm) - silica concentration in the sol (5 to 20 %) - salt solution molarity (0.05M to 0.5M) - value of [SiO2]/[salt] ratio. Experiments have been realized with silica sols and various solutions of electrolytes (mono-, di- or trivalent ions) having a common cation and different anions or a common anion and different cations. The amount of salt added to the silica sol is calculated to obtain a salt concentration of 0.1M. The pH value of the mixture is the resulting from the mixing of the two solutions. From these experiments a classification of anions and of cations with regard to the gelation time is established according to the efficiency of the electrolyte (gelation time decreasing). The tested cations are classified as Na 9 + > Mg 2+ > A13+ >K + > Ca 2+ (gelation time for Na + is smaller than gelation time for Mg 2+, ...). Gelation time increases with the atomic weight of the cation. In the case of ammonium salts the gelation time is small. Due to its complexing action, ammonium cation makes easier the gelation of silica sols. For a given cation classification of anions is SO42 9 > N O 3 > CI Gelation time decreases when the anion size increases. -

"ff

4O

"~ 30 20

0 0

2 4 SiO2/K2SO4

6

Figure 1 "Influence of the ratio silica/electrolyte on the gelation time (10 %silica; pH=7)

Y40 From others sets of experiments it appears that gelation time decreases when : - electrolyte concentration increases - diameter of elementary silica particles of the sol decreases - silica content increases. In the case of the reacting system ludox SM 30 + K2NO4 figure 1 points out the influence of K2SO4 concentration for constant silica content (10 %). The gelation time of the system increases when the electrolyte concentration decreases. With the same reacting system, figure 2 depicts the influence of the silica content for a constant electrolyte concentration (0.1M potassium sulfate) : the gelation time of the system decreases when the silica content increases for a constant electrolyte concentration. In order to vary the ratio silica/salt in a large range and to have short gelation times the silica content in experiments leading to results depicted in figure 2 is lower than in experiments used for figure 1. This explain the difference observed on figures 1 and 2 between the two gelation time scales.

400 A

=9

E

E

300 200

O

,,,,,,=

L~

100

0

5 10 SIO21K2SO4

15

Figure 2 Influence 9 of the ratio silica/electrolyte on the gelation time (0.1M K2SO4; pH=7; Ludox SM30).

In another set of experiments, pH of the reacting system (sol + electrolyte) is adjusted to given values by means of addition of acidic or basic solutions. Figure 3 represents the variations of sol gelation time in function of pH value and of diameter of silica particles for Ludox AS40 (d=22 nm), HS 40 (d=12 nm) and SM30 (d=7 nm) in presence of 0.3M potassium sulfate.

741 2000

--*--AS 40 - o - H S 40

A

1500

=.m

E (D

E

1000

==., dk,a

O =.=

500

t9

I

I

5

7

pH

9

11

13

Figure 3 Influence 9 of pH and diameter of silica particle on gelation time Figure 4 depicts, in the case of Ludox AS40, the influence of pH value and of silica content on gelation time in presence of 0.3M K2SO4 9000 _ ~ A

5 % silice

C

E -~

10 % s i l i c e

6000_

sili

E

==.=

O

=.=,

4.a

(D

3000 _

(3

--

,,,

3

5

g=

,m

7

9

11

13

pH

Figure 4 Influence 9 of pH and silica content on gelation time. Gelation time falls to a minimum value for pH values near 8. For acid pH values (<4) gelation time is extremely long (several weeks), so this pH value range is of poor interest. For basic pH value (>12) siloxane bonds are not stable and the silica solubility increases. The sol particle diameter decreasing can probably explain the decreasing of gelation time observed for pH=l 3 (the effect is more important for SM30 particles which are the smallest ones). All these gelation time measurements have led to specify the best conditions of destabilization of silica sols in order to master the building of the gel network.

742 3.2.2. Texture eharacterisation of silica xerogels Silica xerogels have been prepared from addition of a given amount of an electrolyte solution to silica sols. Resulting hydrogels are spray-dried (drying conditions are indicated above) and the texture properties of the xerogels are evaluated. Table 3 presents results obtained for each silica sol in various destabilization conditions. In each case, silica content is 10 wt%. As indicated by the shape of nitrogen adsorption-desorption isotherms (type IV (9)), silica xerogels are mesoporous solids resulting from sphere aggregation (nevertheless, presence of ink-bottle pores cannot be excluded). As a general rule xerogel texture depends mainly on the diameter of elementary silica particles of the destabilized sol. This result is a satisfactory confirmation of our previous work (1). The choice and the amount of the destabilizing agent is also predominant as pointed out by comparison of effects of KNO3 and (NH4)2SO4 salts (table 3). In each experiment, the amount of silica is maintained to 10 wt%. The change of ion nature and amount affects the double electrical layer around each silica sol particle and the final aggregation of these charged particles. From the analysis of the xerogel texture properties, it seems that we can obtain more porous and open solid architecture when the amount of ions is decreased or when interaction between particles in the sol media are weaker because of a more diffuse double electrical layer. The effect of the chemical nature of ions is of a second order. Table 3 Properties of silica xerogels (pretreatment 300 9 ~ 15 h, under vacuum) . . . . . . . Initial salt Gelation SiOJSalt Specific Pore Pore sol time wt/wt area (BET) volume diameter min ma/g cm3/g nm

Sears area ma/g

1

AS40

KNO3 (0.3 M)

32

3.30

60

0.09

20.3

94

2

HS40

KNO 3 (0.3 M)

65

2.89

80

0.28

9.8

139

3

SM30

KNO 3 (0.3 M)

78

3.30

95

0.26

7.2

173

4

SM30

KNO3 (o.1 M)

19

5.74

199

0.39

6.4

171

5

SM30

K2SO4 (0.1M)

80

9.9

197

0.45

5.7

136

6

AS40

54

2.53

79

0.16

19.5

633

7

HS40

23

2.53.

133

0.30

5.5

622

8

SM30

10

2.53

137

0.37

9.6

687

9

SM30

(NH4)2SO4 (0.3 M) (NH4)2804 (0.3 M) (NH4)2804 (0.3 M) (NH4)2SO 4 (0.1 M)

31

7.57

254

0.43

6.8.

307

This preparation mode of silica xerogels presents some defects evidenced by comparison between BET and Sears measurements. To obtain short gelation times, quite large electrolyte concentration have been used, which induces presence of impurity (the destabilization salt) in the hydrogels. In order to simplify the preparation process, hydrogels were not washed before drying, impurities remain in the solid and can alter texture measurements. For xerogels prepared with addition of potasssium nitrate, BET specific surface area values are lower than

743 values given in table 2 for dried sols. It can be assumed that during water evaporation there is formation of solid deposits of KNO 3 on the solid which leads to total or partial closing of pores, limiting the nitrogen accessibility to the surface and minimizing surface area and pore volume. The decrease of added KNO3 amount (compare line 3 and 4) leads to an increase of the measured BET surface area. Presence of potassium nitrate deposited on the solid does not affect the Sears titration with sodium hydroxyde. In the case of destabilization of a sol with addition of ammonium sulfate, deposition of this salt also occurs during drying and BET surface area measurements are lower than values in table 2. On the other hand, Sears measurements are strongly altered. The presence of sulfate ions adsorbed on the solid is likely while ammonium ions are decomposed during drying or during sample pretreatment before BET measurement. Thus, in Sears titration, part of added sodium hydroxyde reacts with sulfate ions, altering the titration and increasing the measured surface areas. Ions specifically adsorbed on the surface can greatly modify Sears titration as in the case of sulfate ions. When using texture measurements it must be taken into account the possible interactions between the molecule probe and the species present on the surface of the solid.

3.3. Preparation and characterization of silica-alumina xerogels Three procedures have been used to prepare silica-alumina. In all cases, the preparation takes place in the five liter glass reactor with fixed parameters : temperature is 40~ silica concentration in solutions is 4 wt%; alumina/(silica+alumina) ratio is 33 wt%; pH value of the reactant mixture is 6. 1 Silica sol + alumina salt (sol-salt route) : a diluted aluminium sulfate solution is added to a silica sol, leading to a gel which is spray-dried. 2. Silica sol + alumina sol (sol-sol route) : diluted alumina sol (chlorhydrol) is added to diluted silica sol, and the resulting "sol" is dried. 3. Silica gel + alumina sol (gel-sol route) : a silica hydrogel is prepared from destabilisation of a silica sol by a diluted electrolyte ((NH4)2SO 4 , 0.1 M). Then, the diluted alumina sol is added to this silica gel and the resulting mixture is dried. The texture characteristics of some silica-alumina xerogels are depicted in table 4. Table 4 Properties of silica-alumina xerogels. Silica sol Preparation Reactants mode

AS40 AS40 HS40 SM30 SM30

Specific surface area (BET) m2/g

......Pore volume cm3/g

-i~ore............. diameter nm

sol+salt sol+sol sol+sol sol+sol gel+sol

(a) A12(SO4)3.18H20 49 0.177 10 (a) Chlorhydrol 93 0.3 12.8 (a) Chlorhydrol 142 0.33 9 (a) Chlorhydrol 320 0.51 5 25 0.11 12 (b) (NH4)2SO 4 Chlorhydrol (a) pH adjustment with ammonium hydroxyde. (bi pHadjustment with s u l f u r - - - - - With regard to the silica-alumina texture the only interesting preparation scheme is the sol-sol route. Sol-salt route and gel-sol route lead to xerogels having small surface areas (49

744 and 25 m2/g respectively), small pore volume (0.18 and 0.11 cm3/g) but their mean pore diameters are quite large (10 and 12 nm). When alumina sol is added to a silica gel (gel-sol route) the alumina sol diffuses into the pores of the gel filled with aqueous ionic solution where it can precipitate. This phenomenon can be enhanced by drying and results in the partial filling or plugging of pores. Consequently, the surface area and the pore volume of the material decrease, but the largest pores remain open (or partly open). Similar events occur in the case of the sol-salt route. A similar trend is observed as for destabilization of silica sols. Textural characteristics of silica-aluminas prepared from the sol-sol route depend on the diameter of their initial silica sol : smallest surface, largest pores and smallest pore volume of xerogels correspond to the larger sol particles (Ludox AS 40). Hysteresis loops on nitrogen adsorption-desorption isotherms are of type E in the classification of De BOER. This corresponds to sphere assembly. The following assumptions can explain the results observed in the sol-sol way : when pH value of silica sol is 6, silica particles are negatively charged and can adsorb cations like those mentionned in the litterature (10, 11) for alumina sol in aqueous solutions ([A11304(OH)24(H20) 12]7+ or [Als(OH)zo(H20 ) lo]4+), or larger clusters. Alumina precursors play, here, the role of a binder and initiate a more open structure than the ones observed without alumina. Drying will strengthen this solid network resulting from the agglomeration of silica spheres partially coated with alumina precursors. CONCLUSION Mastering of silica xerogel texture can be obtained from silica sol destabilization from direct spray-drying of the sol or electrolyte addition. However, formation of by-products due to the presence of destabilization ions can induce some problems in the evaluation of texture characteristics of xerogels. The agglomeration of spherical silica particles, negatively charged in the pH range 4-10, can be affected by two dominant factors : the size of the initial sol particle and the ionic strength of the surrounding liquid. The decrease of the ionic strength results in a looser and more open solid structure. REFERENCES : 1. I. Biay, G. Dessalces, C. Hypolite, F. Kolenda and J.P. Reymond, Studies in Surface Science and Catalysis, 63 (1991), 1. 2. G. Dessalces, I. Biay, F. Kolenda, J.F. Quinson and J.P. Reymond, J. Non-Crystal. Solids, 147-148 (1992), 141. 3. R.K. Iler in "'The Chemistry of Silica" (John Wiley and Son, New-York 1979). 4. Ludox | Colloidal Silica, E.I. Du Pont de Nemours Technical Publication. 5. Chlorhydrol | 50% w/w Solution, Reheis Chemical Company Technical Data 6. Ullman's Encyclopedia of Industrial Chemistry,vol. A1 (1985), 538. 7. U.S. Patent,4 458 023 (W.R. Grace & Co), Jul. 3, 1984. 8. G.W. Sears, J. Anal. Chem., 28, (1956), 1981. 9. S.J. Gregg and K.S.W. Sing, in "Adsorption, Surface Area and Porosity" (Academic Press, London, 1982). 10. Compos6s Inorganiques de l'Aluminium, Hoechst Technical Data. 11. T.J. Pinnavala, M-S Tzou, S.D. Landau and R.H. Raythatha, J. Mol. Cat., 27 (1984), 195.

91998 ElsevierScience B.V. All rights reserved,

Preparationof CatalystsVII B. Delmonet al., editors.

Properties Needles

745

of A l u m i n a Catalysts P r e p a r e d from B o e h m i t e

Hironobu OHKITA, Shun-ichi KURAMOTO, Takanori MIZUSHIMA, and Noriyoshi KAKUTA* D e p a r t m e n t of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi 441-8580, J a p a n A preparation of CuO/A12Oa catalyst has been attempted using boehmite needles as a precursor. The boehmite needles (ca. 100 nm x 10 nm), which were derived from the peptization and hydrolysis of aluminum alkoxide (AI(OCaHT)3), were employed for this purpose. XRD results showed that copper compounds were not measured up to the calcination temperature of 1203 K although (z-A1203 was realized, but CuA1204 and (z-A120 ~ were clearly detected at 1223K. Copper contents did not change during those treatments. TPR spectra showed one reduction peak and the peaks shifted from ca. 550 K to ca. 500 K with an increase in calcination temperatures up to 1203 K. The catalyst calcined at 1073 K was attractive for the direct decomposition of N20 above the reaction temperature of 800 K, and N20 less t h a n 1000 ppm was completely decomposed even in the presence of 5 % 02. The activity was not improved under the coexistence of 02 and CaHG. No obvious differences in XRD spectra before and after the reactions were observed. Therefore, oxidized copper species, which were amorphous in XRD, are presumed to play an essential role to release the oxygen species from N20 for the catalytic decomposition. 1. I N T R O D U C T I O N It is well-known that a new physico-chemical property appear when a particle diminished in size. This is so-called particle size effect and originates from the unusual surface structures on the small particle. The catalytic activity thus is expected to change with the particle size, too. Indeed, studies on the particle size effect, mainly focused to active species on supported catalysts, have been carried out widely to discover the new catalysis[I].

746

However, few investigations applied the particle size effect to the support have been performed. Here the particle size effect of support means the generation of u n u s u a l materials on the surface of support, where the highly reactive surface created by a diminution in size plays an important role for the solid-solid reaction. One of the attractive points is t h a t the small amounts of additive are enough for the u n u s u a l materials because those are formed only on the surface not in the whole supports. Therefore, it might be interested to investigate the activity as the catalyst. The effects, unfortunately, have not been detected yet. The reason seems t h a t the properties originated from the new surface materials are lost completely for the particle growth by the co-aggregation and sintering of small particles during the activation procedure, even if the catalyst precursors are successfully prepared. We thus considered t h a t the size, especially shape, of the particle is the important key to suppress the particle growth and to evaluate the particle size effect of support. In order to prove the hypothesis, we have chosen boehmite needles prepared by a hydrolysis of aluminum alkoxide as the support precursor. The boehmite needles prepared are about 100 nm in length and 10 nm in diameter and dispersed as a sol solution. We had attempted to prepare thermally resisted alumina by addition of Ba ions[2] because BaO.6A1203 crystallite is known as the thermally stabilized structure as high as 1473 K.[3] The thermally resisted alumina was achieved successfully at lower Ba concentration t h a n the theoretical concentration for BaO.6A1203 structure. This was attributed that the alumina surface was covered with BaO.6A1203 crystallites since the Ba content observed was

consistent

with

the

Ba concentration

estimated

from the

assumption t h a t the surface of boehmite needle was fully covered with Ba(OH)2. It was proved by thin film XRD t h a t BaO.6A1203 phase existed only the surface. This means t h a t the surface reaction occurs smoothly on the boehmite. We have also successfully prepared the nonstoichiometric cerium oxide(CeO2.x) supported on alumina.[4] The formed CeO2.x species were stable even at 773K under methane oxidation and acted as the active sites for the oxygen activation. The novel property for oxygen activation was enhanced by the noble metal such as Pd[5]. We thus believe t h a t those are correlated to the particle size effect and this is useful for the preparation of new alumina catalysts. In this study, the properties of copper species on CuO/A1203 catalysts were investigated using XRD and TPR methods. The catalytic activities were also evaluated through a NO reduction and a catalytic decomposition of N20.

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

2.1 preparation of boehmite needles This method was developed by Yoldas[6] and extended to the production of transparent and porous alumina substrates[7]. Buining et al. also attempted to prepare colloidal boehmite needles with a high aspect ratio using aluminum alkoxide precursors.J8] In our preparation, a reagent for the peptization was used an aqueous HNO 3 solution instead of an aqueous HC1 solution to avoid the contamination of C1 species. Aluminum alkoxide(AIP, AI(OC3HT)s) was introduced into hot water(353-363K) under vigorous stirring. After the hydrolysis for 4 h, the critical amount of HNO8 was added to the slurry for the peptization, and then the slurry became a clear sol. Note that the molar ratio of HNOJAIP was 0.11. The sols were observed by transmission electron microscope(TEM, Hitachi-800) operating at 200 kV with a magnification of 5x104. Figure 1 shows that the shape of boehmite sols is a needle and the average size of the needle was estimated to be about 100 nm in length and 10 nm in diameter.

Fig.1

2.2

TEM photograph ofboehmite needles

Preparation of catalysts

Alumina powder was obtained from the elimination of the solvents in the clear sol under reduced pressure, drying, and calcining at 773K for 4 h. The alumina powder was immersed in an aqueous solution of Cu(NOs)2. CuO/A1208 catalyst (Cat I) was finally obtained drying at 383 K and calcining at 573K for 4 h. The other catalyst(Cat B) was also prepared by the addition of an aqueous Cu(NOs)2

748 solution to the clear sol. The powder was dried and calcined in the same way. Copper content was evaluated using a X-ray fluorescence method. CuO/A1208 catalysts containing 10 wt% Cu were employed in all experiments because the surface might be fully covered with copper compounds.

2.3 Characterization of CuO/A1203catalyst Structural analyses of CuO/A120~ catalysts after the heat t r e a t m e n t

at

temperature from 773 to 1273 K for 4 h were carried out by X-ray diffraction (XRD, Rigaku RINT2000), operated at 30 kV and 20 mA using a Ni filter for CuKa irradiation. Changes in copper loadings by the heat t r e a t m e n t s were also monitored by a X-ray fluorescence analyzer. In order to characterize the reduction property of copper species, t e m p e r a t u r e programmed reduction(TPR) measurements were carried out, elevating the temperature up to 873 K with a temperature raising rate of 10 Kmin 1. During the measurements 10 vol% H2/Ar was introduced into the reactor, where 0.25g of the catalyst powder was placed, with a flow rate of 100 ml min 1. The a m o u n t of hydrogen consumed, where water formed was eliminated by molecular sieve 5A, was recorded using a gas chromatography with a TCD detector.

2.3 Catalytic activity for N20 decomposition Activity measurements were carried out using a conventional flow reactor. A gas chromatography with a TCD detector was used to evaluate N20 and hydrocarbon conversions. Several gas streams consisting of N20, 02, C~H~ and He, as a balance, gas were employed. The reaction temperature was varied from 373K to 973K. Before the catalysts were subjected to the N20 decomposition, NO reduction activities were examined using the flow reactor connected directly to a NOx meter with a chemiluminescence detector. The reaction was carried out at the temperature range from 473 to 873 K. The space velocity was ca. 50000 h ~ in all experiments.

3. R E S U L T S AND D I S C U S S I O N 3.1 Structure of CuO/A1203 catalysts In table 1 are given BET surface area and copper content of both 10wt% CuO/A120a catalysts calcined up to 1273K. No obvious difference in the copper content was observed except that the Cat I showed slightly higher content t h a n the Cat B, suggesting that the copper species exist on the alumina support even

749 after the calcination at 1273 K. The surface area in both catalysts lowered with the calcination t e m p e r a t u r e s and the decrease might be due to the crystallization of copper compounds and/or phase transformation of alumina. The preparation procedures of Cat I and Cat B do not affect to the behavior of copper species and alumina supports with respect to the surface area and copper loading. Table 1

BET surface area and Cu content of 10 wt%CuO/A1203 catalysts

.....Calcination temp.

BET surface area (m2g1)

Cu content (wt%)

(K)

Cat I

Cat B

Cat I

Cat B

773 873 973 1073 1173 1193 1203 1223 1273

177 174 150 130 78 72 6O 21 7

160 135 139 100 54 47 37 10 8

10.4 10.2 9.8 10.5 10.6 10.8 10.5 10.2 10.2

9.4 9.3 9.2 9.1 9.5 9.7 9.7 9.2 9.3

.

.

.

.

.

.

.

Figure 2 shows XRD spectra corresponding to the measurements in Table 1. The formation of CuO crystallite was not observed even after the calcination at 1193K but the peaks assigned to copper compounds(CuA1204) were detected at the temperature of 1203K. In XRD profiles above 1173 K, the peaks however became clear and sharp. Ozawa et al. have reported the formation of "Cumodified A1203" with d values of 0.195-0.2, 0.241 and 0.268 nm when the catalyst was calcined at the t e m p e r a t u r e s of 1103-1203K.[9] I n this experiment, the novel peaks were not measured in both catalysts at the temperature ranges of 11731203K. In order to obtain the information about the surface copper compounds, the catalysts t h u s were subjected to further characterization by thin film X-ray diffraction. The results were almost the same profiles in Fig. 2. However, the tiny peaks corresponding to CuA102, Cu20, and CuO species were measured at 1073K although the peaks of those compounds were not clearly distinguishable. The XRD data indicate that most of the copper species deposited on alumina are the amorphous phase and the tiny part of them seems to be crystallites such as Cu20, CuO etc around 1073 K, and then the amorphous copper compounds react with alumina to form Cunl204 or the phase transformation of 7-A1208 is induced above 1173 K.

The solid-solid reaction and phase transformation are supported by the

drastic changes in surface areas of the catalysts calcined at 1073 K to 1173 K in

750 '

Cat

I

I

'

I

'

I

I

'

'

~

Cat

I

'

I

'

I

'

B

e t, 9

e

e

9

1273

~,

e=== m

d L~ .,==~

1223K

~D

~i~r v "l~mi,~a~" r162162 --" ~ - " : : : - ' - ~ - ~ - ~

973K 873K 773K 573K

0

20

40 2 0 [deg.]

60

80

0

20

40 2 0 [deg.]

60

80

[-l:Boehmite; ~ : 0l-Al203; A,: "/" .Al203; /~:CBAI204; (~. (~-Al203.

,~: "f-A1203; ~ : a-Al203; (~: t~ -Al203; /~'CuAl204.

Fig. 2 XRD profiles of CuO/A1203 catalysts calcined at 773 to 1273 K

'

Cat

I

CaI.1273K "7.

C--at B

I

_

~A,~

4K

~--507~4K

CaI.1223K

o

'

'

'

'

.~.-.719K

CaI.1223K ~504K

<--704K

Cal.1203K J~__~506K

CaI.1203K ~ , ~ 507K

Cal.1193K _~__'~---508K

Cal.l193K ~

Cal.1173K ~__~"-515K

e0

'

CaI.1273K

511K

.

518K

Cal.1173K ~ "

eo Cal.1073K_~ ~ ' " 529K

CaI.1073K L ~ ' 5 1 8 K Cal.973K A~"'518K

Cal.973K______~~ ' " 535K

Cal.873K

Cal.873K

_ . j ~ ' " 535K

Cal.773K

~

~"

519K

Cal.773K ~ ,

200

I

400

,

525K I

,

600

Temperature [K]

I

800

,

1000

200

550K

I

I

400

600

,

I

,

800

Temperature [K]

Fig. 3 TPR profiles of CuO/A1203 catalysts calcined at 773 to 1273 K

1000

751 Table 1. It thus can be presumed that the copper species deposited are highly dispersed and interacted strongly with alumina support up to 1073K. Figure 3 displays the TPR profiles of both CuO/AI~O~ catalysts corresponding to the XRD measurements. One reduction peak, probably the reduction from Cu 2§to Cu ~ was observed and the peaks appeared around 500K or 700K in the both catalysts. Copper species became gradually unstable and active for the reduction since the peak temperature decreased with the increase in the calcination temperatures. However, the property of copper species was changed by the formation of CttA1204 and the reduction peak was found to be at the temperature of ca. 700K. The relatively high reduction temperature of Cat B calcined at 773K suggested that the copper species are more interacted with the alumina support at the initial stage. XRD and TPR observations suggest that the copper species exist as the amorphous phase up to 1073 K and the amorphous species change to the reducible state with the increase in calcination temperatures.

3.2

Catalytic activity for N20 decomposition

In order to evaluate the activity of copper species over CuO/A120~ catalysts, the selective catalytic reduction(SCR) of NO was at first performed using the gas stream containing NO(1000 ppm), 02(5%), C3H~(2000 ppm) and N2 as balance gas. The activity profiles were almost the same in both catalysts. The maximum conversion of NOx(ca. 40%) was obtained at the temperature of ca. 673 K and on the catalysts calcined at 1073K. No N20 was detected. The SCR activity was consistent with the reduction profile of copper species and the catalyst calcined at 773K provided the lowest SCR activity. On the other hand, when the calcination temperature exceeded 1073K, the optimum temperature was higher than 673K and the SCR activity further lowered, indicating that the calcination treatments above 1073 K serve negatively to the SCR activity of copper species and CuA1204 is not active species for NO reduction. Based on the results, CuO/A120~ catalyst(Cat I) calcined at 1023K was used for catalytic decomposition of N20.

Figure 4 shows the catalytic activities when the

concentration of N20 varied from 100 to 1000 ppm. N20 decomposition started around 773K and was attainable completely above 873K. The concentration of N20 did not so much contribute to the catalytic activity. Figure 5 also shows the catalytic activities in the presence of 5% 02. The activities seem to be lowered slightly by 02 but major differences in the activities were not realized from Figs.

752 i

100

I

'

'

"

I

A

'

A

80

9

o

. ,...~

O A

60 o

40

O~'4 Z

O

2O

200

400

(3

600 800 Temperature [K]

1000

/%" 100 ppm N20 + He; O: 500ppm N20 + He; O" 1000ppm N20 + He. Fig. 4 N20 decomposition activity without 02 on CuO/A1203 catalyst

100

I

I

'

I

'

I

80 9 ~

60

O

0 ~ 40 0 2:20

9

~ 200

400

2

9 i

I

600 800 Temperature [K]

I

,

1000

/%" 100ppm N20 + 5% 0 2 + He; Q" 500ppm N20 + 5% 02 + He; 0" 1000ppm N20 + 5% O2 + He. Fig. 5 N20 decomposition activity with 02 on CuO/A1203 catalyst

753

4 and 5, indicating t h a t the effect of 02 is negligible for N20 decomposition. Note t h a t bare alumina gave poor activities in the same conditions. This means that the active sites for N20 decomposition are the oxidized copper species. In addition, no hysterisis of the activity profiles was observed when the reaction temperature continuously increased and decreased from 373 to 973 K. This suggests that the reaction sites formed around 900K are lost their activities with the decrease in t e m p e r a t u r e s and the selection of the reaction temperature is essential for the promotion of catalytic decomposition of N20. We speculate that the rate determining step might be the desorption of oxygen species from N20 and the t e m p e r a t u r e as high as 900K might be necessary as the desorption energy of 02. Indeed, the continuous evolutions of N2 and 02 were detected at 973 K by mass spectrometer. The deactivation of catalyst was not observed at 973 K for 24 h under the gas s t r e a m consisting 1000 ppm N20 in He, and the activity was kept 100% conversion of N20. This means that the oxidized copper species promote the catalytic decomposition of N20 at 973 K; the oxygen species deposited are released smoothly from the active sites. Next the effect of hydrocarbon was examined by the addition of 1000 ppm C3H6 as a reducing agent. The result is displayed in Fig. 6. The activity was improved

100

,

~

~

~- '

"'

i

~ l

m

80 0 .,.-i ~

60

0

~9 40

A

2:20

200 O: Q: A: A: Fig. 6

0

~ i

,IlL i

400

A 0

~i~ ,

,,~i

i

I

600 800 Temperature [K]

i

1000

1000ppm N2~ + He; 1000ppm N 2 0 + 1000ppm C3H 6 + He; 1000ppm N20 + 1000ppm C3H 6 + 5% 02 + He; 1000ppm N20 + 5% O2 + He.

N20 decomposition activity with C3H6 on CuO/A1208 catalyst

754 by the addition of 1000 ppm C3H6 to 1000 ppm N20 and He, but no remarkable change in the activity was detected when 5% 02 was present. This indicates that C~H6 is the attractive reductant for N20 in the absent of 02 but C3HG reacts selectively with 02 in the place of N20. XRD spectra after the reactions showed the same spectrum before the reaction, leading that the N20, 02 and C3HGdo not participate for the crystallization of the copper compounds on alumina at 973 K. Further experiments are necessary to understand the roles of the copper compounds as the active species in connection with the reduction property as shown in Fig. 3. Especially, oxidation states of copper species and the evolution mechanism of 02 from N20 are important. Consequently, the copper species on the CuO/A1203 catalysts prepared from boehmite needles are dispersed as amorphous species up to 1023 K and the reduction of amorphous species are facilitated with the increase in calcination temperatures. Since the CuO/A1208 catalyst has a potential for the catalytic N20 decomposition at the temperature as high as 973K without any deactivations, the CuO/A1203 catalyst might be effective for the N20 removal from the exhaust gases at high temperatures. ACKNOWLEDGMENT A part of this work was financially supported by a Grant-in-Aid for Scientific Research on Priority Areas 'Extreme Environment Catalysts' from The Ministry of Education, Science and Culture.

REFERENCES 1. G. Wedler, in "Handbook of Herogeneous Catalysts"(G. Ertl, H. Kn6zinger, and J. Weitkamp, Eds.), Vol. 2. p792. Wiley-VCH, 1997. 2. M.Ushijima, D.Kobayashi, T.Ishikawa, N.Kakuta, and A.Ueno, Nippon Kagakukaishi, (1991) 1332. 3. H.Arai, and M.Machida, Catal.Today, 10(1991)81. 4. M.Haneda, T.Mizusima, and N.kakuta, J.C.S., Faraday Trans., 91(1995)4459. 5. M.Haneda, T.Mizusima, and N.kakuta, J.C.S., Faraday Trans., submitted. 6. B.E.Yoldas, Amer.Ceram.Soc.Bull., 54(1975)289. 7. B.E.Yoldas, Amer.Ceram.Soc.BuU., 54(1975)286. 8. P.A.Buining, C.Pathmamanoharan, J.B-H.Jansen, and H.N.W.Lekkerkerker, J.Am.Ceram.Soc., 76(1991) 1303. 9. M.Ozawa, H.Toda, O.Kato, and S.Suzuki, Appl.Catl. B Env., 8(1996)123.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

755

The structure of silica particles prepared by acid treatment of olivine: a nitrogenp h y s i s o r p t i o n and 29Si-MAS N M R study. D.J. Lieftink*, B.G. Dekker and J.W. Geus Department of Inorganic Chemistry, Debye Institute, Utrecht University, P.O.Box 80083, 3508 TB Utrecht, the Netherlands

The structure of silica particles, produced by the acid decomposition of olivine grains is studied. Nitrogen physisorption measurements reveal the presence of both meso- and micropores. 29Si MAS-NMR analysis shows a high silanol content of the silica samples. The combination of the results of both techniques indicates that silica particles with a size between 8 and 20 nm consist of smaller particles. The smaller particles can accommodate the high silanol content and give an explanation for the high microporosity of the olivine-silica. 1. INTRODUCTION Silica is an important material in catalysis, as it is used as a support itself and it is a silicon source for the synthesis of zeolites and clays. State of the art of the preparation of silica is by neutralising basic waterglass solutions (sodium silicate) or the pyrolysis of silicon tetrachloride at high temperatures. With both methods it is possible to tailor the specific surface area, the pore volume and the pore size distribution suitable for many different applications. The reaction between the naturally occurring magnesium silicate olivine and sulphuric acid also provides silica. Olivine was chosen because the structure of the silicate consists of loose silica tetrahedra. Most other silicates consist of chains, plates or three dimensional silicate structures, which are less easy to break down. Olivine is abundant available on the earth's surface. Originally, the reaction was proposed to neutralise industrial waste acids [1], providing a magnesium sulphate solution which contains colloidal silica. (Mg,Fe)2SiO4 + 2H2SO4 --> 2(Mg,Fe) 2+ + 25042 + Si(OH)4

(1)

When the silica monomers have entered the solution they start to polymerise to colloidal particles. This contribution deals with the preparation of silica produced by the olivine dissolution. An extensive description of the dissolution of olivine and the production of silica thereof is given by Lieftink [2] and Jonckbloedt [3]. Main difference with the waterglass process is the high acidity at which the silica is produced during the dissolution of olivine. The pH during the preparation of silicas from waterglass normally is between 2 and 8 [4], whereas the pH in e-mail: [email protected]

756 the olivine dissolution reaction is between-0.5 and 2. From literature it is known that silica produced from waterglass at a pH level below 2 is microporous and contains particles of about 2 nm [5-7]. Aim of this study is to describe the texture of the produced silica particles. The silica samples prepared by the olivine dissolution will be studied by nitrogen physisorption experiments and 29Si MAS NMR. Both techniques provide information about the preparation of the texture of the silica samples.

2. EXPERIMENTAL METHODS The silica samples have been prepared by dissolving stoichiometric amounts of olivine in sulphuric acid in a double walled, glass reaction vessel. Olivine sand has been obtained form Hoogovens in IJmuiden, The Netherlands. The olivine sand has been sieved to a grain size fraction of 425 < O < 1000 ~tm. The concentration of the sulphuric acid was three molar. A stoichiometric amount of olivine was added to 1.5 1 of sulphuric acid. During the reaction the pH was measured constantly. From the pH the concentration of produced monomers of silica can be calculated. The reaction equation as represented in eq.1. The reaction temperature was maintained stable at 70~ The reaction mixture must be vigorously stirred to avoid settling of the heavy olivine grains at the bottom of the reaction vessel. The reaction was stopped at a pH of 1.5 to prevent precipitation of iron hydroxides on the silica. During the experiment, samples of the colloidal solution were taken. The samples were filtered and washed to remove the magnesium sulphate. The silica samples were dried in an oven at 120~

2.1 Analytical techniques The silica samples were analysed using nitrogen physisorption at -196~ using a Micromeritics ASAP 2010 apparatus. Before analysis the samples were outgassed at 120~ for one night, until the pressure was stabilised. The specific surface area was determined according to the BET theory. Also, a t-plot [8] was constructed from which the specific t surface area and the micropore volume could be calculated. The micropore volume of the samples was calculated according to the method of Dubinin too. Silica samples were analysed using 29Si MAS NMR to determine the amount of hydroxyl groups present in the silica samples. The NMR measurements were performed on the Bruker AM-500 spectrometer (11.7 T) of the SON HF-NMR facility of the University of Nijmegen, The Netherlands. Magic angle spinning was performed at 5000 Hz. The relaxation delays used are 300 s. For each spectrum, 75 scans were recorded to obtain a good signal-to-noise ratio. The chemical shifts are reported in ppm relative to TMS (tetramethylsilane). 3. RESULTS The course of the pH durign the reaction against time is shown in figure 1. The black squares represent the samples studied. The reaction rate decreases from the beginning following an exponential course. In the beginning of the dissolution reaction, the temperature of the reaction mixture rises, because of the dissolution of olivine is exothermic. After about one half hour the solution becomes very viscous. This viscous phase lasts about 1 hour, then the viscosity is normal again. After the viscous phase, colloidal particles can be observed.

757 1,6

~- 1.41,2.o

'~ 0,8 a 0,6 (~ 0,4 0,2 I

0

5

I

I

10 (hours)15

I

20

25

Figure 1. Silica production curve of the dissolution experiment at 70~ using olivine with a grain size between 425 and 10001.tm. The squares represent the samples taken. 3.1. N i t r o g e n - p h y s i s o r p t i o n

Figure 2 represents a typical nitrogen-physisorption isotherm for silica obtained from the dissolution of olivine. From the shape of the isotherm at low relative pressures the presence of micropores can be expected. The capillary condensation at higher relative pressure indicates mesoporosity. The isotherm is of type IV according to the IUPAC classification [9] and the hysteresis loop resembles of type H1 indicating a packing of spheres. 900 8OO 700 5oo

g4oo ~2 3OO 200100

01 0

0,2

0,4

P/Po

0,6

0,8

1

Figure 2. Nitrogen-physisorption isotherm of the silica sample 3 For all samples a t-plot [8] has been constructed and the specific t surface area has been calculated. The micropore volume of all silica samples has been calculated from the t-plot, PV(t-plot), and according to the method of Dubinin, PVDubi,in, respectively. The results of the nitrogen-physisorption of all seven samples are listed in table 1. The specific BET surface area (SBET) and the specific t surface area (St) decrease as the dissolution reaction proceeds.

758 Table 1 Textural data derived from nitrogen-physisorption isotherms for silica samples, prepared from the dissolution of olivine sample SBET(m2/g) St (m2/g) PV(t-plot) (ml/g) PV(Dubinin)(ml/g) 1 522 461 0.03 0.16 2 513 420 0.04 0.16 3 464 334 0.06 0.15 4 442 314 0.06 0.15 5 397 263 0.07 0.13 6 378 225 0.07 0.13 7 352 178 0.07 0.12 ,

From the textural data it is obvious that the specific surface area of the silica decreases during the dissolution reaction. This can be explained by the growth of the silica particles. The micropore volume calculated using the method of Dubinin is larger than when the micropore volume is derived from the t-plot.

3.2 29Si MAS-NMR A typical 29Si MAS-NMR spectrum of olivine silica is shown in figure 3a. For comparison a spectrum of a silica prepared from waterglass (KS 404 provided by AKZO-PQ) is represented in figure 3b. Deconvolution of the spectra provides three peaks. These peaks represent three different environments of the silicon atoms in the silica. The peak at -90 ppm represents the Q2 site, i.e. a silicon atom with two neighbouring siloxane (Si-O-Si) bonds and two silanol groups (Si-O-H). The peaks at-100 and-110 ppm are related to silicon atoms with three (Q3) and four neighbouring siloxane bonds (Q4), respectively [10]. The intensity of the deconvoluted peaks is representative for the relative amounts of the Q sites in the silica sample.

p

-,

,

~

i

~

~

.

~

-90 -100 -110

8 (ppm)

.

~

,

.

.

.

.

,

.

~

~

.

,

.

-90 -100 -110 8 (ppm)

Figure 3a) 29Si MAS-NMR spectrum of olivine silica sample 3. and b) of KS 404. Both the measured spectra and the deconvoluted peaks are shown.

759 When the spectra in the figure 3a and 3b are compared it is obvious that the silica prepared by the dissolution of olivine contains relatively more Q3 silicon atoms than the silica prepared from waterglass. It appears that the silanol fraction, i.e. the amount of hydroxyl bearing silicon atoms which is (Q2+Q3/Q2+Q3+Q4), remains at a constant value during the dissolution reaction. The silica samples prepared by olivine dissolution at 70~ using the grain size fraction between 425 and 1000 l.tm have a silanol fraction of 0.37. The waterglass silica has a silanol fraction of 0.27.

4. DISCUSSION The results of the nitrogen-physisorption experiments indicate that the silica particles that are produced during the dissolution are growing. From the t-surface, which is the external surface area, the particle size can be calculated. The relation between the external surface area (St) and the particle diameter (dp), using a density of the silica of 2.2 g/ml is: St (mZ/g)= 2720/dp (nm)

(2)

The particle size increases from 6 to 15 nm during the dissolution reaction. The higher values of the micropore volume obtained with the Dubinin method are confirmed by calculations of the micropore volume according to the method of Saito and Foley [11]. The same silica samples have been analysed using CO2 as adsorptive [2]. CO2 is often applied in the case of microporous materials. The micropore volumes using the CO2 adsorption were equal to the results using nitrogen, both calculated according to the method of Dubinin. The difference in the micropore volume using the t-plot and the method of Dubinin may be attributed to the fact that the standard t-curve used in the t-plot procedure is based on spherical, non-porous particles. The specific micropore surface area can be obtained by subtracting St from SBEr. Assuming a monolayer coverage of nitrogen, the specific micropore surface area can also be calculated from the micropore volume. The specific micropore surface area is between 60 and 170 m2/g. When the micropore volume is calculated according to the method of Dubinin the specific surface area is about 300 m2/g. From the 29Si MAS-NMR data the high silanol content is remarkable. We are sure that the T1 of the NMR measurements was long enough to measure reliable spectra. Spectra recorded using 100 s as relaxation delay showed the same intensity ratios. Zhuravlev determined the silanol density of a silica surface at 4 . 9 0 H / n m 2 [12]. This silanol density is applicable for precipitated silicas and silica gels. Pyrogenic silicas show lower silanol densities [13]. The silanol fraction in the silicas samples derived from olivine dissolution is to high to be accommodated by the specific surface area as found by the nitrogen-physisorption. Using the specific BET surface area, the silanol density on the surface of silica from olivine should be between 9 and 18 OH/nm 2. L6onardelli et aL used the value of 4.9 OH/nm z to show the relation between the specific surface area and the amount of silanol groups in the silica sample [14]. They derived the following equation: = (f~(1+fs)/S). (NA/(60+9(fs(1 +fg))))

(3)

760 where t~ is the silanol density, f~ is the amount of Q2 silicon atoms relative to the total amount of hydroxyl-bearing silicon atoms (Q2/Q2+Q3) and fs is the amount of hydroxyl-bearing silicon atoms, Qa+Q3. NA is Avogadro's Number; S is the specific surface area. Using this relationship we can calculate the specific surface area, which is necessary to accommodate all the silanol groups measured by NMR. A silanol fraction of 0.37 as derived from the 29Si MAS NMR analysis requires a specific surface area of about 790 m2/g. The specific surface area as calculated from the silanol content requires, using relation (2), a particle size of about 3.5 nm for all silica samples derived from olivine dissolution at 70~ with the used grain size fraction. The specific surface area as derived from the silanol content is also similar to the specific t-surface area together with the specific micropore area as calculated using the method of Dubinin from the nitrogen-physisorption data. The combination of the NMR and the physisorption data reveals that the silica particles which form a range of the external surface area of 460 -178 m2/g consist of smaller particles of about 3.5 nm. This particle size is conform the data from literature [5-7], where particles of 2-3 nm are reported for silica prepared at a pH of 0. The silica particles grow by the addition of small particles as is described by Bogush and Zukoski [15]. They found nucleation of silica particles occurs constantly during the reaction. From literature it is known that small colloidal particles tend to aggregate to larger particles more easy than to particles of equal size [ 16]. Therefore the particles grow by aggregation and not by the monomer addition model as proposed by Matsoukas and Gulari [ 17]. When silica is prepared from waterglass, under the basic conditions, the small particles are subjected to fast ageing and the micropores between the 3 nm particles are removed. Under the strong acid condition of the olivine dissolution the micropores are removed very slowly. The slow ageing of the silica, probably due to the high acidity or the high salt concentration of the solution, causes the high microporosity. 5. CONCLUSIONS The dissolution of olivine in sulphuric acid provides a micro- and mesoporous silica. The silica particles, which form the mesoporous structure, grow when the dissolution reaction proceeds at 70~ and using an olivine grain size fraction of 425 to 1000 ~tm. When the micropore volume is calculated according to the method of Dubinin the value is much higher than when it is calculated after the t-method. The standard t-curve may be not valuable for the microporous silica. a9si MAS-NMR measurements show a high silanol content of the silica. The high silanol content is explained by small particles of about 3.5 nm. The growth of the particles is achieved by aggregation of these small particles. Because of the high acidity and/or the high salt concentration of the solution the micropores between the 3.5 nm particles have not been removed by ageing.

AKNOWLEDGEMENTS The authors wish to thank Mrs. G. Nachtegaal and Dr. A.P.M. Kentgens of the SON-HFNMR facility at the University of Nijmegen, The Netherlands for the 29Si MAS-NMR measurements. Mr. P. Elberse and Mr J. Raaymakers are thanked for all the nitrogen physisorption measurements.

761 REFERENCES

1. R.D. Schuiling, J. Van Herk and H. Pietersen, Geol. en Mijnb. 65 (1986) 243. 2. D.J. Lieftink, The preparation and characterization of silica from acid treatment of olivine. Ph.D. thesis, Utrecht University (1997) 3. R.C.L. Jonckbloedt, The dissolution of olivine in acid. Ph.D. thesis, Utrecht University (1997). 4. R.K. Iler, The chemistry of silica. John Wiley and sons, New York, 1979. 5. C.Okkerse and J.H. De Boer, J.Chim.Phys.Rt.Phys-Chim.Biol. 57 (1960) 534. 6. K.S. Rao and B.C. Nayar, Ind.J.Chem. 16 (1978) 120 7. M.K.Titulaer, M.J. den Exter, H. Talsma, J.B.H. Jansen, and J.W. Geus. J.Non-cryst. So1.170 (1994) 113. 8. J.H. De Boer, B.G. Linsen, Th. van der Plas and G.J. Zondervan. J.Catal. 4 (1965) 649. 9. K.S.W. Sing, Pure and appl. Chem. 54 (1982) 2201. 10. D.W. Sindorf and G.E. Maciel, J.Am.Chem.Soc. 102 (1980) 7606. 11. A. Saito and H. Foley, AIChE.J. 37 (1991) 429. 12. L.T. Zhurvlev, Langmuir, 3 (1987) 316 13. B.A. Morrow and A.J. McFarlan, Langmuir, 7 (1991) 1695 14. S. L6onardelli, L. Facchini, C. Fretigny, P. Tougne and A.P. Legrand, J.Am.Chem.Soc. 114 (1992) 6412. 15. G.H. Bogush and C.F. Zukoski IV, J.Coll.Interf.Sci. 142 (1991) 19 16. P.J. Feeney, D.H. Napper, R.G. Gilbert, Macromolecules, 17 (1984) 2520 17. T. Matsoukas and E. Gulari, J.CoU.interf.Sci. 132 (1988) 13.

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91998ElsevierScienceB.V. All rightsreservect. Preparationof CatalystsVII B. Delmonet al., editors.

763

P r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of niobia a n d silica-niobia systems S. Morselli a, P. Moggi a, D. Cauzzi b and G. Predieri b aDepartment of Organic and Industrial Chemistry, University of Parma, Viale delle Scienze, 43100 Parma, Italy bDepartment of General and Inorganic Chemistry, Analytical Chemistry and Physical Chemistry, University of Parma, Viale delle Scienze, 43100 Parma, Italy

Niobia (Nb205) and niobia-based catalysts have recently attracted increasing attention by several research groups, as testified by numerous communications, papers and two special issues of Catalysis Today devoted to it [1-2]. Niobium compounds and mixed oxides containing niobia are not only useful as promoters and supports for catalysts, but also as solid acid catalysts or selective oxidation catalysts themselves [3]. Silica-supported Nb205 has been proved to be a very active and selective catalyst for various aldol condensation reactions in the gas phase, i.e. methyl methacrylate by condensation of formaldehyde with methyl propionate [4-5] and 2-ethylhexenal by the self-condensation of n-butyraldehyde [6]. Among the different methods of preparation of niobia and niobia-silica systems, particular attention has been devoted in this work to the sol-gel procedures, in order to prepare highly homogeneous and thermally stable niobium materials having high surface area and acidity, possibly useful as supports or solid acid catalysts. 1. PREPARATION OF NIOBIA

Nb205 can be prepared by calcining niobic acid, HsNb6019 xH20, 9 which can be obtained by the following literature methods. (a) Precipitation by hydrolysis of water-soluble niobium complexes; they are usually prepared by fusing Nb205 with an alkali-metal pyrosulphate or hydrogen sulphate, to form a soluble sulphato-complex, then diluted with sulfuric acid or with a solution of another complexing agent, such as oxalic acid. Niobic acid is then precipitated by boiling the solution in sulfuric acid or by addition of NH4OH [7] or KOH [8] to the oxalic acid solution. (b) Precipitation by hydrolysis of NbC15 in aqueous solution by NH4OH at pH 7 [9101. (c) Precipitation by acidification with HC1 of niobate solutions, which have been

764 prepared by fusing Nb205 with an excess of potassium hydroxide or carbonate [7]. (d) Gel formation by thermal destabilization of the aqueous colloidal solution derived from the dissolution of the gelatinous solid first precipitated by mixing NbC15 into water, after evolution of HC1 gas [11]. (e) Gel formation by evaporating the solvent from the solution prepared by dissolving Nb(OEt)5 into an ethanol solution containing enough water for the complete hydrolysis of all the alkoxy groups [11], or by slowly adding increasing amounts of water until gelation [12]. (f) Gel formation by hydrolysis of chloroalkoxides, easily obtained by adding NbC15 to an alcohol (methanol, ethanol or 2-propanol) [11]. (g) Preparation via sol-gel from Nb(OEt)5 dissolved in sec-butyl alcohol and then quickly added to a well mixed solution of the same alcohol, water and nitric acid [13-14]. (h) Preparation via sol-gel from Nb(OPentn)5, synthesized via alcohol interchange between Nb(OEt)5 and n-pentanol in dry cyclohexane solution by removing ethanol via azeotropic distillation; Nb(OPentn)5 was before further stabilized by adding glacial acetic acid in 1/1 molar ratio, then gelation was performed by adding water diluted in n-pentanol [15]. Some of these ways were also adopted in this w o r k in order to compare them with new sol-gel preparations, which will be described in the following paragraphs. 1.1 Preparation via fused Nb205 A mixture of Nb205 (Fluka AG) (1.06 g) and K2CO3 (4.21 g), previously dried for 2 h at 403 K, was fused in a furnace by gradually heating: I h at 773 K, I h at 923 K, 1 h at 1073 K, then maintaining for 14 h at 1123 K. After cooling to room temperature in the furnace, the fuse was leached from the crucible with boiling water; hydrous niobium oxide was then precipitated by adding HC1 1 N to the filtered solution up to pH 5. The milky white solid was thoroughly washed with distilled water, to eliminate residual chloride ions, then dissolved in a 0.25 M oxalic acid solution. Niobic acid was finally precipitated from the hot solution of the oxalate complex by adding NH4OH 14 N up to pH 8. In a different preparation, finely ground Nb205 (1.02 g) and KHSO4 (8.94 g), dried for 2 h at 403 K, were fused in a furnace by gradually heating: 1 h at 873 K, 1 h at 1023 K, and I h at 1123 K. After cooling to room temperature in the furnace, the fuse was leached from the crucible with boiling water; the filtered solid was then washed with HC1 3 N, and distilled water to completely eliminate residual chloride ions, then dissolved in an oxalic acid solution 2.8 M. Niobic acid was finally precipitated from the hot solution of the oxalate complex by adding NH4OH 14 N up to pH 8. Both niobic acid precipitates were washed with HNO3 0.5 M, then with distilled water to completely eliminate residual nitrate ions; the solids were finally dried in vacuo at 323 K for I h, then 4 h at 423 K at atmospheric pressure.

1.2 Preparation via sol-gel Nb20 5 gels were made starting from Nb(OEt)5 (Aldrich Chemie) or NbC15 (Acros) by the following synthetic routes.

765 (a) To a stirred mixture of Nb(OEt)5 (0.51 g) and ethanol (1.6 ml), a solution of 70% HNO3 (0.06 ml) in water (0.29 ml) and ethanol (1.6 ml) was added dropwise; the solution turned bluish in few seconds, then an opaque gel was obtained by curing in the air for 4 days. Similar preparations were performed by using 2butanol or 3-methyl-2-butanol as diluting alcohol, and HC1 or H2C204 as acidic catalyst. On the contrary, addition of CH3COOH, NH4OH, N H ~ , or of HNO3N H ~ or H2C204-NH~ mixtures always led to a precipitate. (b) Nb(OPri)5 was prepared by the alcoholysis reaction of NbC15 (6.50 g) with 2propanol (40 ml) in the presence of triethylamine (16.9 ml), by refluxing for 5 h to remove most of HC1. Nb205 gels were obtained from the alkoxide by the same way described above with Nb(OEt)5, using ethanol as diluting agent, and HNO3, HC1, H2C204, or the H2C204-NH~ mixture as catalyst. (c) NbC15 (1.04 g) was dissolved in water (10 ml); gelatinous niobic acid suddenly precipitated by hydrolysis, then a stable colloidal solution was obtained after decanting the mother liquid by washing and adding more water (15 ml). Solgel transition simply occurred after standing for 24 h, by heating at 333 K for I h. (d) Nb205 gels were also obtained from NbC15 as precursor, by the intermediate formation of an alkoxychloride. In this case, NbC15 (4.7 g) was dissolved under vigorous stirring into dry ethanol (30 ml), keeping at low temperature with an ice-bath; then the solution was refluxed for 2 h until it turned clear pale yellow. A stable sol was then obtained by adding a water (0.4 ml)-ethanol (4 ml) solution to the alkoxychloride solution (2 ml); finally the gel was made after evaporating the solvent in the air. The same procedure was originally adopted by using 2propanol, 2-butanol and 3-methyl-2-butanol as alkoxychloride precursors. In the case of the isopropoxychloride, a gel preparation was also made by adding NH4OH as catalyst, while with the 2-butanol-or 3-methyl-2-butanol-derived alkoxychlorides, the Nb205 gels were also quickly obtained by adding NH4F.

2. PREPARATION OF NIOBIA-SILICA SYSTEMS

Different synthetic routes to Nb205-SiO2 systems have been reported in the literature as evidenced by the following list of methods. (a) Impregnation of silica to incipient wetness with a niobium oxalate/oxalic acid aqueous solution prepared from commercial niobium oxalate [16-17] or by dissolving hydrous niobic acid in oxalic acid [6, 18]. (b) Impregnation of silica with a solution of Nb(OEt)5 in hexane to incipient wetness [19-20], or under reflux condition in inert atmosphere [21]; (c) Coprecipitation by adding HC1 (up to pH 5) to an aqueous mixture obtained from colloidal silica and the hexaniobate salt K8Nb6019.16H20 (prepared by the alkali fusion method) dissolved in boiling water [5]. (d) Coprecipitation by adding NH4OH (up to pH 7) to a methanol solution of SIC14 and NbC15 [20]. (e) Functionalization of silica by the reaction of the surface OH groups of SiO2 with a niobium organometallic precursor, such as Nb(II3-C3H5)4 or Nb(OEt)5 [22]. (f) Sol-gel synthesis from Nb(OEt)5 and Si(OEt)4 in methanol, by adding doubly deionized water and nitric acid as catalyst for the hydrolysis and condensation

766 steps [231. In this work an original sol-gel method of preparation was developed, starting from different niobium precursors and catalytic systems and new methods of impregnation and precipitation/gelation were also introduced.

2.1 Preparation via sol-gel Mixed Nb2Os-SiO2 gels were prepared starting from Nb(OEt)5 or NbCls, as niobium precursor, and Si(OEt)4 or Si(OMe)4 (Aldrich Chemie), as silica precursor, by different procedures: (a) To a mixture of Nb(OEt)5 (18 ml) and Si(OEt)4 (3.34 ml) in ethanol (4 ml) a solution of 70% HNO3 (0.03 ml) in water (1.2 ml) and ethanol (4 ml) was added dropwise, under stirring; a stable sol was obtained, which slowly gelified in about 30 days. The same procedure was repeated to prepare gels with Nb205/SiO2 molar ratios of 1:1, 1:2, 1:5, 1:10, 1:20, 1:30 and 1:50, even substituting Nb(OEt)5 with Nb(OPri)5 (prepared apart from NbC15,) or Si(OEt)4 with Si(OMe)4. Very faster gelation processes were performed by mixing oxalic acid in ethanol to the ethanol solution of the precursors, before adding the water-ethanol mixture or by using a NH~-water-ethanol mixture, as modified catalytic systems. 0a) Si(OEt)4 (1.41 ml), diluted in ethanol (10 ml), was mixed to the niobium ethoxychloride solution (0.5 ml), prepared as described in 1.1(d), then a water (0.8 ml)-ethanol (10 ml) solution was added dropwise, forming a clear stable sol, which transformed in a transparent gel in few days. The same procedure was repeated to prepare gels with Nb2Os/SiO2 molar ratios of 1:1, 1:2, 1:5, 1:10, 1:20 and 1:50, even substituting the ethoxychloride with the isopropoxychloride solution, also apart prepared from NbC15, or Si(OEt)4 with Si(OMe)4.

2.2 Preparation via precipitation/gelation Si(OEt)4 (1.86 ml), diluted in ethanol (9 ml), was added to a mixture of water (1.2 ml), aqueous N H ~ (0.16 mmol) and ethanol (9 ml). After few minutes, w h e n the solution was turning translucent due to the incipient gelation, Nb(OEt)5 (0.42 ml) under stirring was quickly added. Niobic acid immediately precipitated, being dispersed in the viscous mass of the gelling silica, finally cured for 4 days before evaporating the solvent in vacuo. T h e same procedure was repeated to prepare mixed oxide systems with Nb2Os/SiO2 molar ratios of 1:5, 1:10, 1:20 and 1:50.

2.3 Preparation via impregnation Commercial silica (Aldrich Chemie) (0.5025 g), which had been dried in an oven at 773 K for 10 h under a nitrogen stream, was suspended in ethanol (15 ml); to the heated suspension oxalic acid in ethanol (1.2 mmol) and the niobium ethoxychloride solution (0.66 ml, 0.4 mmol) were added under continuous stirring. After 30 minutes 25% NH4OH (2 ml) was added to precipitate niobic acid on silica; the solid was decanted, filtered, washed with water to eliminate chloride ions, then with acetone and finally dried at 423 K for 4 h. The same procedure was repeated to prepare niobia/silica systems with Nb205/SiO2 molar ratios of 1:1, 1:5, 1:20 and 1:50.

767 3. METHODS OF CHARACTERIZATION Nb205 and Nb2Os/SiO2 systems prepared in this work were characterized by surface area determinations, surface acidity titrations, XRD, FT-IR, DRIFTS spectra and TEM investigations. Surface area was determined by a Micromeritics 2200 analyzer, based on nitrogen adsorption at the temperature of liquid nitrogen, after thermally treating the samples at 423, 623, 773 or 873 K for 4 h. Surface acidity was measured by a titration method based on: a) adsorption of ammonia till to saturation from a streaming gas mixture of about 5%NH3-95%He at room temperature on a sample thermally pretreated at 473 K for 2 h in helium; b) thermal desorption under a helium stream at increasing temperatures in two steps: from room temperature till to 523 K (weak acid sites), and from 523 K till to 873 K (strong acid sites) with bubbling of the desorbed ammonia in HC1 0.01 N; c) titration of HC1 in excess with NaOH 0.01 N and phenolphtalein as indicator. XRD spectra of thermally pretreated samples from 773 to 1473 K were recorded on a Philips PW 3710 diffractometer equipped with a Cu Kc~ source and a scintillation detector. FT-IR spectra in the range 4000-400 cm -1 were recorded on a Nicolet 5 PC spectrophotometer. The same instrument, equipped with a DRIFTS cell was used to collect the diffuse reflectance spectra of some samples after chemisorption of pyridine or carbon dioxide to investigate on the presence of BrOnsted and Lewis acid sites, and of basic sites, respectively. TEM analyses were performed with a Jeol JEM 2000EX instrument at 200 keV. 4. RESULTS AND DISCUSSION 4.1 Niobia

Table 1 shows the surface area values observed for the niobia samples prepared by the fusion/precipitation methods and thermally treated at different temperatures. XRD spectra show that a monoclinic Nb205 crystalline phase begins to appear at 1073 K for the samples prepared by alkali fusion, and at 873 K for those prepared by acid fusion, according to the pattern of surface area v s temperature. Surface acidity, measured on the samples thermally pretreated at 523 K, is higher for the sample prepared by alkali fusion (0.76 mmol NH3 g-l) than for the sample prepared by acid fusion (0.52 mmol NH3 g-l); then both strongly decrease over 523 K, according to the disappearance of the -O-H bending band at 1650 cm -1 in the FT-IR spectra. Table 2 shows the surface area values observed for niobia samples prepared via sol-gel methods and thermally treated at 423 or 523 K. Nb205 gels prepared from Nb(OEt)5 generally exhibit very low surface area values even after a simple drying at 423 K, while those prepared from NbC15 generally have surface area values comparable to (or higher than) those observed for Nb205 prepared via fusion/precipitation. However, after heating at 773 K, surface area values of Nb205 gels are generally lower (about 30 m 2 g-l).

768 Table 1 Surface area values (m2 g-l) for niobia samples prepared via fusion/precipitation Temperature (K)

423

523

773

873

Nb205 (alkali fused) Nb205 (acid fused)

229 171

205 150

148 80

78 18

Table 2 Surface area values (m2 g-l) for niobia samples prepared via sol-gel Temperature (K)

423

523

Nb2Os, from Nb(OPri)5 (+ HNO3) Nb2Os, from Nb(OPri)5 (+ HC1) Nb2Os, from NbC15 (+ H20) Nb2Os, from ethoxychloride Nb2Os, from isopropoxychloride Nb205, from isopropoxychloride (+ NH4OH) Nb205, from sec.butoxychloride Nb2Os, from isoamyloxychloride Nb205, from isoamyloxychloride (+ N H ~ )

231 301 274 140 195 246 219 272 330

152 268 204 99 176 196 192 195 288

The surface area values of the gels prepared via alkoxychloride clearly show the influence of the size of the alkoxy group and the effect of a catalyst added to promote a faster gelification. Nb205 gels thermally treated at 523 K generally have a lower surface acidity than samples prepared via fusion/precipitation. XRD spectra of Nb205 gels yet show the appearance of a crystalline phase at 773-873 K, according to the observed reduction of surface area.

4.2 Niobia-silica systems Table 3 shows the surface area values for niobia-silica samples with different Nb205/SiO2 molar ratio prepared via sol-gel, precipitation/gelation and impregnation methods, all thermally pretreated at 423 K. The first column also indicates precursors and catalysts. Regarding the sol-gel method, the best procedure is probably that starting from Nb(OEt)5, as Nb205 precursor, in combination with Si(OMe)4, as SiO2 precursor, and H2C204 (+ N H ~ ) as catalyst. The mixed catalyst H2C204 + NH4F affords higher surface areas for Nb205/SiO2 molar ratio < 1-10, which are only lower than those exhibited by the systems prepared by precipitation/gelation. The systems obtained with H2C204 as sole gelation catalyst have high surface areas in all the range of compositions.

769 Table 3 Surface area values (rn2 g-l) of niobia-silica systems thermally pretreated at 423 K Nb2Os/SiO2 molar ratio

1:50

1:20

1:10

1:5

Sol-~el preparations from: Nb(OEt)5 + Si(OEt)4 + HNO3 Nb(OEt)5 + Si(OMe)4 + HNO3 Nb(OPri)5 + Si(OEt)4 + HNO3 Nb(OEt)5 + Si(OMe)4 + H2C204 Nb(OPri)5 + Si(OEt)4 + H2C204 Nb(OEt)5 + Si(OMe)4 + H2C204 + N H ~ Ethoxychloride + Si(OEt)4 Ethoxychloride + Si(OMe)4 Isopropoxychloride + Si(OEt)4

280 363 390 410 591 628 260 352 402

139 28 388 388 311 618 256 349 413

0 0 ............... ............... 355 406 ............... 172 13 389 11 261 46 ..... 216

Pre .cipitation/gelation from: Si(OEt)4 + N H ~ + Nb(OEt)5

947

710

612

508

Impregnation from: SiO2 + Ethoxychloride + H2C204 + NH4OH 498

351

272

250

1:1

0

203 44 0 0 155

Regarding the systems obtained from alkoxychlorides, as Nb205 precursors, higher surface areas are even observed by increasing the size of the alkoxy group. It is interesting to note, in the case of sol-get prepared systems, that quite similar surface areas are generally obtained with Nb2Os/SiO2 molar ratio of 1:50 and 1:20, while the samples obtained by the precipitation/gelation or the impregnation method show continuously decreasing surface area values at increasing Nb2Os/SiO2 molar ratio. This behaviour suggests the presence of strong interactions between the Nb205 and SiO2 growing networks during the sol-gel transformation processes, probably involving the formation of Nb-O-Si bonds. This is supported by the presence of a band at 920-930 cm -1 in the FT-IR spectra of Nb2Os-SiO2 systems prepared via sol-gel, which disappears after heating at higher temperatures, when crystalline Nb20 5 begins to segregate from the amorphous mass of SiO2. The sol-gel way of preparation seems also to stabilize Nb2Os-SiO2 systems in an amorphous texture up to higher temperatures than the other methods; in fact, the XRD spectra of sol-gel prepared systems with Nb2Os/SiO2 molar ratio of 1:20 show the appearance of crystalline Nb205 phases only over 1273 K. On the other hand, XRD peaks due to crystalline phases are already evident at 1173 K for the systems of the same composition prepared by precipitation/gelation and at 873 K for the impregnated ones. TEM images reported in Figure 1 show, on the left, a Nb205 nanocrystal (15 nm diameter, indicated by an arrow), encapsulated in the amorphous texture of the gel, and, on the right, a crowd of Nb205 prismatic crystallites (0.7 ~tm maximum length) grown in the Nb205-SiO2 1:2 gel, by heating at 1473 K for 10 h.

770

Figure 1. TEM images of: (on the left) a Nb2Os'-SiO2 1:10 gel, thermally treated at 523 K for 4 h; (on the right) a Nb2Os-SiO2 1:2 gel, after heating at 1473 K for 10 h. Sol-gel prepared niobia-silica systems, even thermally treated, keep noteworthy acid and basic characteristics, which indicate them as promising catalysts for many acid- or base-catalyzed reactions. Table 4 illustrates the surface acidity values of some niobia-silica systems, prepared by different ways and thermally treated at 523 K, measured by the ammonia titration method. The Nb2Os-SiO2 1:20 gels, both prepared from Nb(OEt)5 or the ethoxychloride, by far show the highest acidity, confirming the special behaviour of this composition. DRIFTS spectra of Nb2Os-SiO21:20 gels, collected after pyridine chemisorption both show the permanence of Lewis and BrOnsted acid sites at 873 K, while spectra collected after carbon dioxide chemisorption on the same gels show the presence of basic sites too, which persist up to 523 K.

771 Table 4 Surface acidity values (mmol NH3 g-l) of niobia-silica systems thermally pretreated at 523 K .

.

.

.

.

.

.

Nb205/SiO'2 molar ratio

Surface A~'idity (mmol NH3 g-l)

Sol-gel p r e p a r a t j o ~ from: Nb(OEt)5 + Si(OMe) 4 + H2C204 + NH4F Nb(OEt)5 + Si(OMe)4 + H2C204 + NH4F Nb(OEt)5 + Si(OMe)4 + H2C204 + N H ~ Ethoxychloride + Si(OEt)4

1:50 1:30 1:20 1:20

0.21 0.42 1.39 1.22

Precipitation / gelation fr0~m~ Si(OEt)4 + N H ~ + Nb(OEt)5 Si(OEt)4 + N H ~ + Nb(OEt)5

1:10 1:5

0.40 0.08

Impregnation. from." SiO2 + Ethoxychloride + H2C204 + NH4OH

1:5

0.07

5. CONCLUSIONS The following conclusions can be drawn out from this work: (a) The sol-gel way is a very useful and versatile method of preparation both for pure niobia and mixed niobia-based oxide systems, such as Nb2Os/SiO2. (b) Niobia-silica systems in a wide range of composition can be prepared via solgel from different precursors and catalysts, affording materials with high surface area and variable acidity, very promising for their application in catalysis. ACKNOWLEDGMENTS

The authors are indebted with Dr. G. Salviati (C.N.R., MASPEC Institute, Parma) for skillful assistence in TEM analyses, Mrs. Barbara Furlotti for laboratory work and Mr. Pier Antonio Bonaldi for precious technical assistance. Financial support from M.U.R.S.T. (Rome) is gratefully acknowledged REFERENCES

1. K. Tanabe (Editor), Catal. Today, 16(3-4) (1993). 2. K. Tanabe (Editor), Catal. Today, 28(1-2) (1996). 3. K. Tanabe and S. Okazaki, Appl. Catal. A, 133 (1995) 191. 4. P. Moggi and G. Albanesi, React. Kinet. Catal. Lett., 22 (1983) 247. 5. G. Albanesi and P. Moggi, Appl. Catal., 6 (1983) 293.

772 6. P. Moggi and G. Albanesi, Appl. Catal., 68 (1991) 285. 7. F. Fairbrother, The chemistry of niobium and tantalum, Elsevier, Amsterdam, 1967. 8. T. Ushikubo, T. Iizuka, H. Hattori and K. Tanabe, Catal. Today, 16 (1993) 291. 9. A. Ouqour, G. Coudurier and J.C. V6drine, J. Chem. Soc., Faraday Trans., 89 (1993) 3151. 10. J.C. V6drine, G. Coudurier, A. Ouqour, P.G. Pries de Oliveira and J.C. Volta, Catal. Today 28 (1996) 3. 11. C. Alquier, M.T. Vandenborre and M. Henry, J. Non-Cryst. Solids, 79 (1986) 383. 12. M.T. Vandenborre, B. Poumellec, C. Alquier and J. Livage, J. Non-Cryst. Solids, 108 (1989) 333. 13. E.I. Ko and S.M. Maurer, J. Chem. Soc., Chem. Commun., (1990) 1062. 14. S.M. Maurer and E.I. Ko, J. Catal., 135 (1992) 125. 15. P. Griesmar, G. Papin, C. Sanchez and J. Livage, Chem. Mater., 3 (1991) 335. 16. J. Datka, A.M. Turek, J.M. Jehng and I.E. Wachs, J. Catal., 135 (1992) 186. 17. T. Tanaka, S. Takenaka, T. Funabiki and S. Yoshida, Chem. Lett., (1994) 809. 18. T. Tanaka, H. Nojima, H. Yoshida, H. Nakagawa, T. Funabiki and S. Yoshida, Catal. Today, 16 (1993) 297. 19. E.I. Ko, B. Rafrali, N.T. Nuhfer and N.J. Wagner, J. Catal., 95 (1985) 260. 20. P.A. Burke and E.I. Ko, J. Catal., 129 (1991) 38. 21. K. Asakura and Y. Iwasawa, Chem. Lett., (1986) 859. 22. N. Ichikuni, M. Shirai and Y. Iwasawa, Catal. Today, 28 (1996) 49. 23. S.M. Maurer and E.I. Ko, Catal. Lett., 12 (1992) 231.

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII ' B. Delmonet al., editors.

773

T h e U l t r a s o n i c S y n t h e s i s o f N a n o s t r u c t u r e d M e t a l O x i d e Catalysts S. C. Emerson, C. E Coote, H. Boote III, J. C. Tufts, R. LaRocque, and W. R. Moser* Department of Chemical Engineering, Worcester Polytechnic Institute 100 Institute Road, Worcester, MA 01609-2280, USA This paper describes the synthesis of a wide range of metal oxide and supported metal catalysts to determine whether ultrasonic treatment during precipitation affords any advantages for the preparation of nanostructured materials compared to identical syntheses by classical precipitation. The investigation concentrated on a determination of ultrasound effects during synthesis on reducing and controlling grain sizes, improving phase purities of complex metal oxides, improving the degree of alloy formation for supported bimetallic catalysts, and the capability to systematically vary and control grain sizes of the metallic particles. Based on current experimental results, it seems that the general effect of ultrasound on these syntheses was relatively minor, and in only a few cases consequential. In some cases, such as ceria and titania, the effect was more dramatic; however, the degree of variability of grain sizes and phase purity were less than expectations. 1. INTRODUCTION Several research groups over the last few years have reported both beneficial effects in catalyst performance and novel effects in materials synthesis by the application of high powered ultrasound [ 1-4]. Other recent studies demonstrated that the rates of certain catalytic reactions were accelerated by factors of 10-100 when the grain sizes of the catalysts were systematically reduced into the 2-10 nm range [5-8]. The focus of this report is to examine a wide range of metal oxide and supported metal catalysts to determine whether ultrasonic treatment afforded any advantages for the preparation of nanostructured materials as compared to their identical synthesis by classical precipitation technology. The investigation concentrates on a determination of ultrasound effects during synthesis on reducing and controlling grain sizes, improving phase purities of complex metal oxides, improving the degree of alloy formation for supported bimetallic catalysts, and the capability to systematically vary and control grain sizes of the metallic particles. The strategy used during synthesis was to precipitate all components of the catalyst's composition, including support materials, within the zone of highest intensity of the ultrasound horn. Our expectations based on the well known dynamics of acoustic cavitation generated by ultrasound energy was that both high shear and high local temperatures would result [9-12]. The effect of high shear on the incipient formed gel would be to efficiently mix all metal components so that the final catalyst would demonstrate a high phase purity, and the shear forces would also result in the formation of nanostructured materials. The effect of the *Authorto whomcorrespondence should be addressed

774 high local temperatures would result in an in situ calcination of the metal oxide grains preventing their further grain growth. A variety of material property modifications have been attributed to the use of ultrasound during synthesis. Suslick and co-workers examined a wide range of alloy and nanostructured metal particle syntheses by irradiating solutions of the corresponding metal carbonyls with ultrasound [13]. Nanostructured iron-cobalt alloys prepared in this way resulted in greatly improved activity and selectivity in cyclohexane dehydrogenation to benzene [13]. Materials produced in the presence of ultrasound have exhibited higher surface areas compared to materials synthesized in a conventional manner [ 1,14]. Crystallization and gelation times for zeolites and gels have been dramatically increased by ultrasound treatment [14, 15]. In addition, exposure to ultrasound can modify catalyst surfaces, removing undesirable impurities and increasing the dispersion of precious metals on a support [3, 16]. Recent findings by Dhas et al. showed that ultrasound treatment of aqueous ammonium dichromate solutions resulted in an in situ reduction forming Cr203 as amorphous fine grains [ 17]. These modifications to conventional catalyst preparations normally result in increased catalytic activity. Bosch et al. noted that silica supported platinum catalysts which were treated with ultrasound during synthesis and reduced at 673 K exhibited three times the activity for the hydrogenation of phenylacetylene than classically prepared materials [2]. Bernal et al. prepared a series of TiO2-SiO2 gels as supports for rhodium catalyzed n-butane hydrogenolysis [14]. The activities of the catalysts, where the support was synthesized with ultrasound, were higher than those of the conventional supported materials, most likely due to increased dispersion of the rhodium. Bianchi noted similar results for the reduction of acetophenone with palladium supported catalysts [3]. The use of ultrasound during metal oxide precipitation to produce advanced catalysts and materials has been investigated before in several laboratories [ 1,3,14,18]. The ability of acoustic cavitation to synthesize materials of varying small grain sizes with high phase purity was of particular interest for this study; however, it was important to demonstrate that catalysts prepared by ultrasound possessed properties that could not be achieved by classical methods of synthesis. For this reason, the sonication syntheses reported here were directly compared to classical syntheses under identical synthesis conditions within the same equipment but with the ultrasound power turned off. Based on the effects of acoustic cavitation, it is expected that the precipitation of catalysts within an ultrasound interaction zone should result in high phase purity, nanometer-size particles which can be adjusted over a wide range of grain sizes from 1-50 nm. From frontier electron theory concepts [ 19], it is our expectation that every catalytic reaction would have a unique grain size within the 1-50 nm region where maximum rates are observed. Several catalytic systems synthesized in the 2-8 nm range have demonstrated sharply enhanced rates within this range as compared to conventionally prepared materials [5, 7]. Enhanced rates within this range could also be due to a much higher edge to stable basal plane concentration of any crystallite as the grain size is reduced, leading to more reactive sites of low coordination [20].

775

2. EXPERIMENTAL 2.1. General synthesis procedure Two different experimental setups were used in the synthesis of metal oxides: a flow reactor and a semi-batch reactor. The flow reactor setup is shown in Figure 1. For this setup, a process stream containing a metal salt or alkoxide solution was metered into the inlet of the reactor section using a peristaltic pump. This stream was mixed with a second process stream containing a precipitating agent, typically ammonium or sodium hydroxide. The result was the formation of a precipitated gel of the metal oxide components immediately before the inlet to the synthesis reactor.

Ultrasonic probe

Precipitating agent

Product

l, i

\Inter. tio. zo.o

_/

-j

I--

Precipitating I agent recycle[ <

Metal salt / solution

V

/

~Peristaltic pumps

Figure 1. Schematic of ultrasonic continuous flow setup.

The flow reactor consists of a 20 kHz, 600-Watt Sonics & Materials Vibracell high intensity ultrasonic processor and a solid Teflon shell. The titanium alloy ultrasonic probe is screwed into the shell, which has an inlet port opening directly in front of the probe tip and an outlet port positioned above the level of the probe tip. The shell was designed such that the internal volume through which the precipitated gel must pass encounters the full intensity produced by the ultrasonic probe. At the start of a typical experiment, the precipitating agent metering pump was turned on to fill the interaction zone of the reactor with precipitating agent. Once flow was established within the cell, the ultrasonic probe was activated. Unless otherwise noted, all runs reported in this paper were made with the probe amplitude set at 100%. After the probe was turned on, the salt solution feed pump was activated to begin the generation of a precipitated metal oxide gel. Immediately upon formation, the gel encountered the ultrasonic field fl'om the probe. The field induced acoustic, transient cavitation which could be audibly detected by the "frying bacon" sound which was produced [ 10]. After encountering the acoustic field, the gel solution flowed

776

past the ultrasonic probe tip and exited the reactor where it was either collected directly, or, in the case of a recycle run, redirected back into the inlet of the reactor. During the recycle runs, the temperature of the recirculating material could be regulated by the use of an ice bath. A schematic of the semi-batch reactor setup is shown in Figure 2. For this configuration, the precipitating agent was placed into a stainless steel reactor with a magnetic stir bar. The ultrasonic probe was then lowered into the precipitating agent to cause acoustic cavitation. The salt solution was then pumped into the reactor such that the flow was directed upwards toward the probe tip. The entire reactor was typically immersed in an ice bath to maintain a low temperature.

Ultrasonic ~robe

Metal salt solution A ' ~

/

Metal salt / solutionB

\

-Z-

_= m

4 Precipitating agent

\

Magnetic stirrer

Peristaltic pumps

Figure 2. Schematic of semi-batch reactor setup.

At the end of a synthesis, the product slurry was collected and then centrifuged three times, at 10 000 rpm, in a Beckman model J2-21 centrifuge equipped with a JA-10 rotor. After the first centrifugation, the supernatant solution was decanted from the concentrated slurry. For additional centrifugations, the supernatant was decanted and replaced with fresh, deionized water to wash the product. The washed solid was then dried overnight at 105 ~ prior to x-ray diffraction (XRD) analysis. The dried material was then calcined to a higher temperature (e.g. 600 ~ to verify the identity of the product and to make a particle size estimate. All XRD patterns were collected on a Rigaku Geigerflex x-ray diffractometer using a Cu K~ (L = 1.5418 A) radiation source with a nickel filter. Particle size measurements obtained from the XRD patterns were calculated using the Scherrer equation [21 ]. Experiments were conducted using the flow reactor where the addition of the salt solution to the precipitating agent was done over 30 minutes while the precipitating agent and resultant gel were continuously recycled back into the ultrasound reactor. Other experiments precipitated the metal salt solution in one pass, i.e. without recycle, in the ultrasound reactor where the salt solution was added to the precipitating agent over a period of 30 minutes. Using this mode of operation, the effect of ultrasound treatment on the immediately formed gel was examined by

777 passing the entire gel slurry back through the ultrasound reactor without recycle. This procedure was done several times, and the effect on the number of passes on grain study was studied by XRD analysis on the dried and calcined samples taken after each pass. Since some studies showed a grain size reduction as the number of passes were increased, most of the studies described here were carried out in the semi-batch reactor shown in Figure 2, where the total irradiation time was longer than that employed in the continuous flow reactors. 3. RESULTS

3.1. Synthesis of ceria The synthesis of ceria was studied for both its usefulness as a catalytic support (e.g. three-way automotive catalysts) [22, 23] and the fact that it forms grain sizes which are easily discernable by XRD upon drying. The majority of the materials synthesized in this investigation exhibited very broad x-ray diffraction patterns after drying at 105 ~ This makes the determination of any grain size reduction via ultrasound unreliable. As a result, the materials were calcined to progressively higher temperatures which resulted in coarsening and grain growth to a size which permitted grain size determination by XRD. Typical XRD patterns for ceria synthesized in the presence of ultrasound and subsequently calcined to higher temperatures are shown in Figure 3. The initial grain size after drying was approximately 7 nm and increased to 13 nm upon calcination to 600 ~ This result demonstrates that control over grain size can also be maintained in materials produced via acoustic cavitation by choosing the appropriate calcination temperature.

3000 11

o -~

2500

9 Ultr:t~ound Cla.~sical Microthndi:,er. 21 ()lgl psig

10 ~9

I

2000

9

1500

i"~

"-9 r.n

A

lOOO ' - . - - , . ~

10

20

7

/t,

200 "C

A

<Jk_ ......... ~ ' ~

30

8

.E

40

^~+t

50

.....

6

.........

60

70

ao

90

20 (Degrees)

Figure 3. X-ray diffraction patterns for ceria synthesized with 20 kHz ultrasound and calcined to different temperatures.

.10

0.15

0.20 Cerium

0.25

0.30

Concentration

0.35

0.40

0.45

0,50

(tool/L)

Figure 4. Grain size differences at 200 ~ between 20 kHz ultrasound, Microfluidizer, and classical flow syntheses.

In addition to temperature control, the concentration of tile metal salt in solution was also used to study the variability of grains. Figure 4 shows the crystallite sizes of ceria as a function of the cerium concentration after the materials were calcined to 200 ~ Three different experimental

778 setups were used to produce this data: the ultrasonic flow reactor, a classical synthesis technique, and hydrodynamic cavitation using a Microfluidizer. The classical synthesis was performed under the same conditions and reactor geometry as the ultrasound experiments, except that the probe was turned off. The single Microfluidizer experiment was conducted using hydrodynamic cavitation at pressures alternating between atmospheric and 21 000 psig. The use of hydrodynamic cavitation to produce nanostructured materials has been discussed previously [24]. From Figure 4 it can be seen that as the concentration of cerium was decreased, a greater difference was observed in the crystallite size between the classical and ultrasonic syntheses (approximately 3 nm at 0.15 mol/L). It would also appear that the ultrasonic flow reactor setup produces materials with grain sizes comparable to hydrodynamic cavitation. To increase the exposure time of the precipitated ceria to the ultrasound, and to simplify the system geometry, several ceria experiments were performed in the semi- batch reactor. Since the effects of metal concentration and calcination temperature were known, a response surface study was initiated to determine the effects of the salt solution feed rate coupled with the ultrasound amplitude on the ceria grain size. The salt feed rate was varied between 2 and 48 mL/min while the amplitude was adjusted from 0% (classical synthesis) to 100%. The results of this study are plotted in Figure 5.

/

'~

:

!

1 3500 100% amplitude, 7 mn

2000 .~

"

o

~

'

.

:

~ 60

"

/3o ,s

........

............. 7:............. /lo 80

100

1500 1000 5OO

~-

A~nplitude (%)

10

20

30

40

50

60

70

80

90

20 (Degrees)

Figure 5. Ceria grain size (at 105 ~ response surface study as a function of amplitude (20 kHz ultrasound) and flow rate (mL/min).

Figure 6. X-ray diffraction patterns for ceria synthesized with and without 40 kHz ultrasound, dried at 105 ~

At low salt solution flow rates, the effect of amplitude was rather moderate, but consistent with the flow reactor results (approximately 5 nm material at 100% amplitude and 8 nm at 0% amplitude). At intermediate flow rates, it appeared that the effect of ultrasound was nil. However, at the highest flow rate (48 mL/min), the largest difference between ultrasound and

779 classical experiments was observed (6 nm at 100% versus 12 nm at 0%). The reasons for this behavior are unknown, although the change in the effect as a function of flow rate suggested that nucleation and growth rates, as well as mass transfer issues were a factor in this phenomenon. To examine the effect of higher frequency ultrasound, parallel experiments were conducted using 20 kHz and 40 kHz probes. The results of two experiments conducted at a flow rate of 4 mL/min with and without 40 kHz ultrasound are shown in Figure 6. The grain sizes were 12 nm for the classical material and 7 nm for the sonicated sample. These results are more consistent with the 20 kHz results for the 48 mL/min flow rate, suggesting that the change from 20 kHz to 40 kHz also alters the nucleation and growth of ceria. Our conclusions for the ceria studies are: (1) that ultrasound processing coupled with other synthesis parameter variations permits one to synthesize sets of metal oxides having variable grain sizes in the 2 nm to 20 nm range; (2) the degree of shear induced by cavitation in this type of experiment is not large enough to cause a radical difference in grain size compared to classical syntheses; and (3) the major changes occurring are on the surface rather than in the bulk, since nearly all of the metal oxide systems studied resulted in slight color changes between sonicated and classically prepared compounds. For example, the ceria synthesized classically were yellow powders while those prepared with ultrasound were green-colored.

3.2. Synthesis of titania Titania was synthesized from solutions of Ti2(SO4)3 by precipitating with ammonium hydroxide. The differences in grain size between sonicated and classical materials were small, on the order of 1-2 nm. To determine whether there were any synthesis parameter areas of consequential effects due to acoustic cavitation, a response surface study was conducted with feed flow rate and amplitude as the dependent variables. The grain size results for this study are shown in Figure 7. At all extreme conditions (e.g. high flow rate and no ultrasound or low flow rate and 100% amplitude), the grain sizes of materials calcined to 300 ~ were approximately 10 nm. However, as the conditions were adjusted toward the "middle" of the response surface (50% amplitude, 25 mL/min), the grain size was reduced below the level at which the peaks could reliably be measured (approximately 2 nm). Selected area diffraction analysis (SAD) showed that the 50% amplitude, 25 mL/min samples were amorphous with a negligible amount of crystalline titania. Examples of two XRD patterns from this response surface are shown in Figure 8. The middle portion of the response surface was reproduced by several experiments and represents one of the most striking effects of ultrasound on metal oxide grain size. The use of a 40 kHz probe at a flow rate of 4 mL/min showed a very slight difference (approximately 1-2 nm) between acoustic cavitation and classical synthesis. However, this slight difference is negligible compared to the dramatic interaction between flow rate and amplitude demonstrated in the 20 kHz study.

3.3. Synthesis of titanium supported Au-Pt alloys Although this study concentrates on determining whether the application of acoustic cavitation in metal oxide synthesis has the capabilities to synthesize nanostructured materials having systematically controllable grain sizes, the efficacy of mixing of the synthesis components is also an important question. To evaluate the capabilities of ultrasound to mix the components of a synthesis mixture by acoustic cavitation, a variety of gold-platinum alloys supported on titania were synthesized. The objective of the synthesis study was to determine whether the

780

12

]

10

8

"

:

:

-

-

":

'

.

~>, ._

~.

500 Cla.ssical ( 0 % A m p l i t u d e ) . 4 8 m L / m i n . 9_+1 m n

40o loo

..... -........... I./....-

'...../ 8 o

.... -. . . . . . . :. . . . . / o o

o- ~ 4 0 ~ " " - - - - . . , . ~ .

-" " '

a~ 2 0 " ' ~ - ~ "

"-

. .

:

.-~

300

e,~ ~

200

' , //~4 400

/20 1o,---.-.......~o

~4>~

FlowRate011L/11]i0)

100

10

20

30

40

50

60

70

80

90

20 (Degrees)

Figure 7. Titania grain size (at 300 ~ response surface study as a function of amplitude (20 kHz ultrasound) and flow rate (mL/min).

Figure 8. X-ray diffraction patterns (at 300 ~ for titania response surface study.

precipitation of Au and Pt salts in an ultrasonic field while titania was also being precipitated would result in a higher degree of alloy formation as contrasted to a classical synthesis. It was expected that shear induced during the acoustic cavitation experiment would be significantly greater than that obtained during a classical preparation and that this higher degree of mixing of the precipitating components would lead to a higher degree of alloy formation. Furthermore, it was expected that both the alloy and the support would be formed as nanostructured materials even when calcined to high temperatures. To test these concepts, catalysts were prepared having different compositions of Au and Pt supported on titania having 2 wt% Au with Pt/Au atom ratios of 0.25, 0.50, 0.75, and 1.0. In addition, a catalyst was synthesized having the same amount of Pt as the 1/1 Au/Pt material but with no Au. All of the syntheses were carried out in the semi-batch ultrasound reactor using a flowing stream of Ti2(SO4)3 in water which mixed with a second stream of H2PtC16-6H20 and HAuC14.3H20 just before the two streams were mixed with a sodium hydroxide solution which was contained in the reactor vessel. To precipitate the Au and Pt salts, two moles of hexamethonium bromide monohydrate per mole of Pt and Au was contained in the sodium hydroxide solution. The synthesis used the 20 kHz ultrasonic probe and a 30 minute precipitation time under ambient conditions. All of the materials synthesized were dark blue in color after filtration and drying at 120 ~ XRD analysis indicated titania primary particle grain sizes of 3-8 nm. Transmission electron microscopy (TEM) analysis also indicated the titania grains were in this range. Selected area electron diffraction analysis (SAD) showed these materials to be anatase, and that the Au-Pt grains were in the range of 6-10 nm. Energy dispersive x-ray analysis on the particles in the

781 1/1 Au/Pt samples showed that all particles analyzed contained both Au and Pt. Detailed XRD analysis of the entire species of alloys, where the extent of alloy formation was evaluated from their shift from the pure gold XRD reflection, demonstrated that the degree of alloy formation was consistent with the fraction of each metal in the applied synthesis solution. For example, modeling of the alloy reflection at 45.430 ~ measured on the 2% Au sample with a 1/1 Au/Pt ratio resulted in d-spacing shifts in the XRD analysis corresponding to a 45% Au- 55% Pt alloy. This analysis also indicated that the best fit showed the presence of little free gold or platinum. When the identical classical synthesis was carried out with the exception that the ultrasound power was turned off, the XRD analysis as indicated above showed only a 62% Au-38% Pt alloy formation which constituted about 50% of the total metal along with reflections for free gold and platinum. We take this data to indicate that the ultrasound experiment resulted in a far greater degree of mixing of components than the classical experiment as a result of acoustic cavitation. The XRD analysis indicated that the grain size of the alloy was in the range of 3-5 nm.

100

.

9 2% o 2% 9 2% v 2% m 2% [] 2%

80

60

.

.

.

.

v

Au, 1/1 Au/Pt, 100% amplitude Au, 1/1 Au/Pt, Classical Pt, 100% amplitude Pt, Classical Au, 1/0.5 Au/Pt, 100% amplitude Au, 100% amplitude

,

,

v

. o,.,,,~ :>

40

0 v

v

v

20

O

o

~ 300

350

~ 400

t

450

-~ 500

"

[]

[] 550

600

Temperature (~

Figure 9. Microreactor results for titania Au/Pt catalysts synthesized with and without 20 kHz ultrasound.

All of the catalysts were examined in a micro-reactor for their activities for the total oxidation of methane in a 3.8% methane in air stream between 350 and 600 ~ The catalytic results are demonstrated in Figure 9. The figure shows that all of the gold containing catalysts were substantially less active compared to the pure platinum catalysts. Furthermore, the 2% Au I/1 Au/Pt catalyst prepared by ultrasound and classical techniques demonstrated exactly the same reactivity profile, as did the pure Pt on titania prepared by ultrasound and classical techniques. Our conclusion from the synthesis and catalytic reactivity data is that acoustic cavitation does not lead to catalytic surfaces having important differences for methane total oxidation despite

'/82 the fact that the alloy composition of the two catalytic surfaces were significantly different. 3.4. Synthesis of cobalt oxide An interesting side effect of exposure to ultrasound for all oxide systems is a change in the shade or color of the samples. For example, in the ceria system the typical classical synthesis produced a yellow solid. When the same synthesis was performed in the presence of ultrasound, the final solid was a shade of green. Although these color changes could be due to changes in grain size, they might also be due to differences in the surface defect concentrations, as ultrasound is known to provide surface roughening [3, 15]. One system not only exhibited a color change, but a phase transformation as a result of exposure to ultrasound. Figure 10 shows the XRD patterns for the synthesis of cobalt oxide with and without 20 kHz ultrasound using Co(NO3)2.6H20 as the metal salt. When the material was synthesized in the presence of ultrasound and dried, the final product was dark brown and exhibited an XRD pattern which was indexed as cobalt oxide, Co304. However, the dried, classically synthesized sample was light brown and was identified by XRD to be/3-cobalt hydroxide. Further calcination of the classical sample to higher temperatures transformed the sample to cobalt

oxide, C0304. This interesting formation of the oxide, rather than the hydroxide may be evidence of some in situ heating which has been attributed to acoustic cavitation. The bulk temperature during the ultrasonic synthesis was approximately 56 ~ due to the energy input by the probe, while the classical synthesis bulk temperature was 20 ~ This difference in temperature was negligible compared to the heating at 105 ~ which both samples endured prior to XRD analysis.

1800 1600 1400

~

.~ 1200

'

,.,,x. ~

,,..,

250 200

1ooo

1kHzU

150

800 6O0

100

111I)%Amplitude C~baltOxide

4OO

~

t Classical.41111~

20O

10

20

30

40

50

60

70

80

90

20 (Degrees)

Figure 10. X-ray diffraction patterns for cobalt compounds (dried at 105 ~ synthesized with and without 20 kHz ultrasound in the semi-batch reactor.

10

20

30

40

50

60

70

20 (Degrees)

Figure 11. X-ray diffraction patterns for bismuth molybdate synthesized via hydrodynamic cavitation (Microfluidizer), acoustic cavitation (40 kHz ultrasound), and classical means.

783

3.5. Synthesis of fl-bismuth molybdate The synthesis of fl-Bi2Mo209 using hydrodynamic cavitation (i.e. Microfluidizer) was previously shown to give higher phase pure materials as compared to conventional synthesis which yields mixed phases [24]. Thus, the synthesis of bismuth molybdate is an indicator as to the quality of mixing during syntheses which use two or more metal ions. Figure 11 shows XRD patterns for classical, 40 kHz, and Microfluidized syntheses. These materials were synthesized by preparing two separate solutions of bismuth nitrate pentahydrate (Bi(NO3)3.5H20) and ammonium molybdate tetrahydrate ((NH4)6Mo7024.4H20) and co-precipitating them with isopropyl alcohol. The figure shows that the classically prepared material has the largest degree of phase impurities, and the ultrasound treated material showed a modest improvement. The hydrodynamic cavitation prepared sample resulted in a material of exceptionally high phase purity as compared to the calculated XRD pattern [24]. 4. CONCLUSIONS Based on our current experimental results, it seems that the general effect of ultrasound on these syntheses is relatively minor, and in a few cases consequential. In some cases, such as ceria and titania, the effect is more dramatic; however, the degree of variability of grain sizes and phase purity were less than expectations. This conclusion is in contradiction with much of the literature which reports reduced crystallization times and reduced particle size distributions for syntheses involving ultrasound [1, 14, 15]. This study examined a much wider variety of metal oxide systems than reported here along with parallel classical syntheses, and in all cases this comparison led to modest changes as reported for the above described systems. However, it would appear that for a majority of the systems in which ultrasound has been reported to have a beneficial effect, the materials affected either would not have been produced without the intervention of ultrasound, or they are very slow (e.g. zeolite synthesis and the formation of sol gels). In such systems, it is expected that acoustic cavitation would increase the rate of precipitation/crystallization and even reduce particle size distributions due to an increase in mass transfer and high energy collisions. The increase in mass transfer rates could be seen as due to the increase in the interfacial area as well as the micromixing and acoustic streaming produced by ultrasound [9]. For the systems studied in this paper, the rate of precipitation is extremely fast. Based on our work with hydrodynamic cavitation, it was desired to produce a soft metal oxide gel immediately and then subject it to the high shear cavitation created by a Microfluidizer. This methodology was naturally extended to the work with acoustic cavitation, which may be exactly why the acoustic cavitation results were so poor. In a Microfluidizer, the newly formed soft gel is forced through a very small volume, high shear environment. Thus, the particles are forced to collide with one another. In both of the ultrasonic reactor designs presented in this paper, it may have been possible for the bulk of the newly precipitated metal oxide precursors to avoid the effects of the ultrasound for two reasons. First, the reactor volumes are fairly large. It may be possible in the flow reactor, with a volume on the order of 1-2 mL, for particles to take a flow path such that they do not "feel" the maximum amplitude of the probe. Certainly, in the semi-batch reactor, the wall of the reactor is a few centimeters from the probe, and thus it is possible the acoustic field is not strong very far from the probe. Second, the ultrasonic probe produces an acoustic force which may propel

784 particles away from it. If one pours a small amount of water onto an ultrasonic probe and then activates it, the water is propelled rapidly from the surface of the probe. Similarly, it is possible to make light objects move by bringing the tip of an ultrasonic probe close to them. Thus, it is not inconceivable to imagine newly formed precipitates in solution being pushed away from an ultrasonic probe and therefore avoiding the effects of acoustic cavitation. Based on these arguments, it would seem that the rate of precipitation in the current synthesis schemes is much greater than the rate of mixing produced by acoustic cavitation. To test this hypothesis, future work will involve the design of a new flow reactor in which the volume is very small and the residence times much higher. These studies may be able to verify whether acoustic cavitation can be a general purpose method for the synthesis of nanostructured materials of controllable grain sizes in high phase purity. 5. ACKNOWLEDGEMENTS

We wish to thank Sonics & Materials, Inc. and Branson Ultrasonics for the loan of equipment used in the synthesis of materials reported in this paper. The financial support of Bayer AG Leverkusen for part of this work is greatly appreciated. We also want to thank Bridget Smyser of the WPI Materials Science Department for help with TEM, as well as Ivo Krausz, Greg Krueger, and Andrew Schnellinger for their assistance. REFERENCES

1. Kunz, U.; Binder, C.; Hoffman, U. Stud. Surf. Sci. Catal.; Poncelet, G. et al., Eds.; Elsevier: Amsterdam, 1995; Vol. 91,869. 2. Bosch, E; L6pez, T.; Asomoza, M.; G6mez, R.; Cauqui, M. A.; Rodrfguez-Izquierdo, J. M. Langmuir 1995, 11, 4328. 3. Bianchi, C. L.; Carli, R.; Fontaneto, C.; Ragaini, V. Stud. Surf Sci. Catal.; Poncelet, G. et al., Eds.; Elsevier: Amsterdam, 1995; Vol. 91, 1095. 4. Suslick, K. S.; Choe, S.; Clchowlas, A. A.; Grinstaff, M. W. Nature 1991 353, 414. 5. Busser, W. G.; van Ommen, J. G.; Lercher, J. A. Advanced Catalysts and Nanostructured Materials; Moser, W. R., Ed.; Academic: San Diego, 1996; 213. 6. Wilcoxon, J. P.; Martino, A. J.; Baughman, R. L.; Klavetter, E.; Sylwester, A. E Mater. Res. Soc., Symp. Proc., 1993; Vol. 286, 131. 7. Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M.; Delmon, B. J. Catal. 1993, 144, 175. 8. Sze, C.; Gulari, E.; Demczyk, B. G. Mater. Res. Soc., Syrup. Proc., 1993; Vol. 286, 143. 9. Young, E R. Cavitation; McGraw-Hill: London, 1989. 10. Atchley, A. A.; Crum, L. A. Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: New York, 1988; 1. 11. Rooney, J. A. Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: New York, 1988; 65. 12. Suslick, K. S. Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: New York, 1988; 123. 13. Suslick, K. S; Hyeon, T.; Fang, M.; Clchowlas, A. A. Advanced Catalysts and Nanostructured Materials; M0ser, W. R., Ed.; Academic: San Diego, 1996; 197.

785 14. Bernal, S.; Calvino, J. J.; Cauqui, M. A.; Rodrfguez-Izquierdo, J. M.; Vidal, H. Stud. Surf Sci. Catal.; Poncelet, G. et al., Eds.; Elsevier: Amsterdam, 1995; Vol. 9l, 461. 15. Lindley, J. Ultrasonics 1992, 30, 163. 16. Tai, A.; Kikukawa, T.; Sugimura, T.; Inoue, Y.; Abe, S.; Osawa, T.; Harada, T.New Frontiers in Catalysis: Proceedings of the lOth International Congress on Catalysis; Guczi et al., Eds.; Elsevier, 1993; 2443. 17. Dhas, N. A.; Koltypin, Y.; Gedanken, A. Chem. Mater 1997, 9, 3159. 18. David, L. D. U.S. Patent 4 588 575, 1986. 19. van Santen, R. A.; Niemandverdriet, J. W. Chemical Kinetics and Catalysis; Plenum: New York, 1995; 224. 20. van Hardeveld, R.; Hartog, E Adv. Catal. 1972, 22, 75. 21. Kaeble, E Handbook of X-Rays; McGraw-Hill: New York, 1967. 22. Yao, H. C.; Yu Yao, Y. E J. Catal. 1984, 86, 254. 23. Fajardie, E; Temp6re, J.; Dj6ga-Mariadassou, G.; Blanchard, G. J. Catal. 1996, 163, 77. 24. Moser, W. R.; Marshik, B. J.; Kingsley, J.; Lemberger, M.; Willette, R.; Chan, A.; Sunstrom IV, J. E.; Boye, A. J. Mater Res. 1995, 10, 2322.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

787

On the relations between the rheology of TiO2-based ceramic pastes and the morphological and mechanical properties of the extruded catalysts. by Pio Forzatti ", Carlo Orsenigo", Daniele Ballardini a, Francesco Berti b "Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, P.za Leonardo da Vinci 32, 20133 Milano - Italy b ENEL S.p.A.-SRI-PDM, Via Volta, 1, Cologno Monzese- Italy

A rational and quantitative study of extrusion of TiO2-based ceramic pastes is attempted. First the analysis of rheological measurements, that are used to investigate and foresee the form ability behaviour of the pastes, is illustrated. Then the mixing stage and two of the most relevant preparation parameters, namely liquid content and inorganic additives, are discussed to clarify the correlations between operative parameters, corresponding rheological characteristics of the paste, and morphological, structural and mechanical properties of the final material.

1. INTRODUCTION The extrusion process for forming ceramics is widely employed, particularly in the structural clay products industry and to a lesser extent in the whiteware and refractories sector [l]. Besides extruded monolithic ceramic catalysts are now widely used in environmental applications, and specifically in automotive and stationary NOx emission control [2,3]. They are also considered in other applications including catalytic combustion, chemical processing industries and others [4]. Monolithic or honeycomb materials offer a number of advantages over more traditional pellet-shaped catalysts, namely low pressure drop, high external surface area per unit volume, excellem attrition resistance and low tendency to plugging by dust. Extruded monoliths for the control of NOx emissions from stationary sources consist of homogeneous mixtures of anatase TiO2 as high surface area support, of WO3 (or MOO3)and V205 as active components, and of silico-aluminates and glass fibres as mechanical promoters. The high surface area material represents the major component of the monolith instead being applied as a washcoat, as in the case of automotive emission control honeycombs. These monolithic catalysts are fabricated according to the following steps: i) dry mixing of the solid raw materials; ii) wet mixing and plasticizing with water and organic additives; iii) extrusion of the monolith honeycomb; iv) drying uniformly; v) calcining at about 550-600~ In some cases vanadia is not added in the dry mixing stage but is impregnated on the extruded monolith after calcination. Each of the production steps in the extrusion process influences the quality of the final material. Therefore it is important to know the correlations between process parameters and relevant properties of the catalysts or of the catalyst substrates. This is particularly true for extruded monolithic ceramic catalysts considering that the precision demanded for these

788 materials may be in the range of microns and is much higher than that of traditional items that are also formed by extrusion of ceramic pastes such as bricks and pipes. So far the extrusion of ceramic pastes has been poorly understood, and it has been presented and discussed in the literature mainly on a phenomenological basis. Several aspects of the extrusion process have been addressed, including the relation between rheology and extrudability for alumina-based pastes, the dependence of TiO2-based catalysts on the composition of the corresponding paste, the mathematical modelling of the extrusion of ceramic pastes [5-7]. Besides a wealth of experience concerning the extrusion of ceramic pastes is lying with equipment manufacturers and processing industries. Still a systematic and complete approach to the extrusion of ceramic pastes in the form of honeycomb monoliths is lacking. In the following we shall address first the rheological characterisation of TiO2-based ceramic pastes employed in the preparation of DeNOx-SCR catalysts; than we shall discuss the mixing stage and the effect of two significant preparation parameters, namely liquid content and nature and quantity of inorganic additives, to clarify the correlations between the rheological characteristics of the paste and the morphological, structural and mechanical properties of titania-based honeycomb monoliths.

2. ANALYSIS OF RttEOLOGICAL MEASUREMENTS The extrusion process is generally accomplished in a screw extruder consisting of a feed port, a screw rotating in a barrel and an extrusion die. The ceramic paste is fed through the port, is pumped through the barrel (while the pressure build up necessary for extrusion is generated) and is finally forced through the forming die. Considering that extrusion can be envisaged as an ordinary flow process, rheological methods could be useful in principle to investigate the formability behaviour of ceramic pastes [8]. A capillary rheometer was employed for this purpose in view of its correspondence to the extrusion machines, and the slit capillary devise was preferred to the road capillary one since its geometry makes it possible to analyse a range of shear rate values (from 0 to 100 s-t) that compares well with those estimated inside the honeycomb die land during the extrusion process. In a standard rheological measurement the paste is forced to flow at a definite flow rate Q through the capillary. The resulting pressure drop between two pressure sensors placed at a given distance along the slit capillary axis are used to calculate the apparent shear stress at the wall (~w-- dPxH/2L where L and H are the length and the height of the slit capillary). To ensure negligible entrance effects and careful pressure drop measurements the pressure sensors have to be placed far from the entrance of the capillal3Tand the distance between the pressure sensors must be sufficiently large. When a Newtonian fluid of viscosity rl deforms, the shear is uniform throughout its volume and the viscosity is defined as the ratio of shear stress to the shear rate dT/dt= ~,'. That is:

n= ~/~,

(1)

Ceramic pastes resemble the characteristic of non-Newtonian fluids, for which rl is not independent of x and 3/. To correlate experimental shear stress and true shear rate values for a

789 non Newtonian fluid a simple power law function can be used in line with the literature approach for plastic extrusion [9]. Accordingly: ri= rio 3"t-1

(2)

and thus ~:= K 7't

(3)

The true shear rate 7't of a non-Newtonian fluid is obtained from experimental data by correcting the apparent shear rate 7'N valid for a Newtonian fluid, which depends on the flow rate Q, by the Rabinowitsch equation [10]. That is to say: ?'U (at H/2, corresponding to xw) = 6Q/(WH 2)

(4)

Y' [ dlogy' I 7' t = - - ~ / [_ 2 + ~dlog~:w

(5)

3"

=-~(2+$1

Since ceramic pastes are pseudoplastic fluids, S values are well above unit and thus 7't > 7'NThe rheological parameters in the power law equation (K and n) can be estimated by fitting the characteristic equation of a slit die to the experimental data of flow rate and pressure build-up. The characteristic equation of a slit die for the power law model (eq. (6)) is given in the following [1 ]: nWH 2 HdP) 1/,~ e = 2(1+2n)(2KL

(6)

The parameters n and K are directly related to relevant physical characteristics of the flow behaviour [5,7]: - n value is strictly connected to the form of velocity profile in the die land: lower n values are associated to a more flat velocity profile inside the capillary and thus to more favourable conditions for extrusion with lower or no tendency to generate lamination defects; indeed lamination defects are generally associated with the inadequate joining together of separated mixture fluxes of the paste in the bulk of the material: - in the case of very low n values, K parameter can be regarded as an index of shear stress (or viscosity) values, being K ~ x as n-+0. The simple flow analysis described above deserves the following theoretical and practical comments. First of all, a critical assunaption of the analysis is that of fluid adhesion to the wall. If the paste slips at the wall, a complex velocity distribution emerges, which originates from the superimposition of the velocity profile v* due to intemal deformation of the fluid (that depends on the distance from capillary axis), and a constant velocity Vw, which represents the slip velocity of the fluid in proximity of the wall. It is intuitive that the slip velocity has no influence on the internal deformation, so that the real shear rate has to be calculated from v*

790 alone and not from eq. (5). Experimental methods have been developed to separate the effect of slip at the wall based on rheological measurements performed with different capillaries [11]. Besides many factors have been shown to affect the measure under certain conditions. For example an excessive quantity of solid materials or an inadequate preparation procedure may determine in-homogeneous liquid-solid distribution, so that a direct correlation between pressure drop and volumetric flow rate is not observed any longer: the shear stress function degenerates into a shear stress spectrum. On the other hand if the viscosity of the paste is too low, for instance due to an excessive liquid content or to an inadequate choice of the nature and quantity of the lubricating agents, back-flow may occur and equal that imposed by the rotational speed of the screw and by wall adhesion; as a consequence no net output flow is observed under these conditions. Fortunately the most critical situations for rheological measurements usually coincide with the cases of unsuccessful extrusion. The extrusion process can also be complicated by de-watering process. However if, as in our case, the extrusion is carried out at room temperature and in the presence of tensionactive agents in the liquid phase very low liquid losses are measured in the paste. In our case the liquid loss measured from the mixing stage to the beginning of drying step was less than 2% of the water content. On the base of the above considerations, it can be concluded that the flow processes occurring during extrusion are complex. However capillary rheometry, if critically applied, still represents a powerful and advantageous tool in understanding and developing the extrusion process of ceramic pastes.

3. EXPERIMENTAL METHODS

Typically the honeycomb monoliths have been prepared using tungsta-titania A-DW-1 Beyertitan (TiO2 ~ 90% w/w of the solid powders), minor amounts of bentonite (~6% w/w, Aldrich from South Dakota) and glass fibres (~.~4%w/w, type E, VS 1200 Vetrotex-Italia) as strengthening agents. The powder components were mixed together in the dry state in a Brabender S 300C mixer unit with sigma-shaped blades. Water with small amounts of organic additives (2% w/w of poly-ethylene-glycol, Carbowax 4000 from Merck and 1% w/w of methyl-hydroxy-ethyl-cellulose, Tylose P 6000 Z2 from Hoechst) was then added to the solids to impart plasticity and to provide effective lubrication during extrusion. % wt of the solid and liquid components are given on a wet basis. In some preparations a high shear mixing stage has been applied: the ceramic paste was forced for three times to flow within the screw and subsequently through a die plate with rectangular cross section (2 mm in height, 20 mm in width and 20 mm in length) mounted after the end of the screw. During all the mixing stages the torque moment of the naixer and of the screw shafts was recorded by means of Brabender data processing-plastic corder PL 2000. The rheological measurements were then conducted by means of a slit capillary (20 mm in width, 2 mm in height, 150 mm in length) connected with a Brabender laboratory screw extruder; the corresponding pressure drop is measured by two pressure sensors (Dynisco 420 PTA) placed at 75 mm and 125 mm from the entrance. The ceramic paste was extruded through a die of size and shape appropriate to that of the honeycomb product: monoliths 20-30 cm long and with square cross section of 2.5 cm, a pitch of 7.4 mm and a wall thickness of 1.4 mm (3• channels) were obtained. The extruded

791 monoliths were first dried at room temperature trader moisture controlled atmosphere for several hours then at 110~ for 2 hours; they were finally calcined .at 550~ for 2 hours. The dimensions of TiO2 anatase crystallites have been determined by using the Sherrer equation and X-ray diffraction data collected with a Philips vertical goniometer PW 1050-70 and a Ni filtered Cu-Kct radiation. Surface area and pore size distribution (pore radius < 300 A) have been determined by N2 adsorption-desorption using a Carlo Erba Sorptomatic 1900 Series instrument. Meso- and macro- pore distribution (pore radius > 40/~) has been obtained by the mercury penetration method with a Carlo Erba Porosimeter 2000 Series instrument. The axial crushing strength was measured on honeycomb specimen (25x25x25 ram) by using Instrom F.S. 5 kN instruments. Typically the measurements were replicated over 4 specimen of the same material to secure statistically significant values. The particle size distribution of the powders was determined by a Cilas HR 850 laser granulometer.

4. EXPERIMENTAL STUDIES: RESULTS AND DISCUSSION 4.1. Mixing The first stage employed in the preparation of TiO2-based honeycomb monoliths consists of dry mixing of raw materials, followed by liquid addition (wet lrfixing) and in some cases by high shear mixing.

Table 1 Influence of wet and high shear mixing stages on the morphological characteristics of the final products (paste with 28,% w/w water content). Treatment Starting Powder Wet Mixing High Shear ...... !8 hrs) Mixing, BET .................. surface area (mZ/t~,) ........ 8,3 .... 78 ....... 74 Hg penetration: Mean Radius for 40A<rp<300A (A) 82 82 80 Pore Vol. for 40]k<rp<300i (cc/g) 0.21 0.25 0.21 Mean Radius for rp>300_~ (_~) 1100 400 350 Pore Vol. for rp>300~ (cc/g) , 0:22 , 0:09 0.O2 ,

,, ,,,

,

,

,

Mixing of the powder components in the dry state involves a pure mechanical action and allow to reach an uniform distribution of the solid components. Upon addition of the liquid to the inorganic powders the torque value of the mixer showed a sudden sharp maximum due to the high stresses that develop because of granulation at the very early stage. Then the liquid distributes uniformly and the granules are broken down to smaller particles in view of the high shear stresses that result in the paste; accordingly the torque value gradually decreases To prevent damage of the machine the liquid was initially added only to a part of the solid powder; the addition of the remaining powder was accomplished in few subsequent steps, when the torque value has decreased significantly.

792 Table 1 shows the morphological characteristics of the extruded honeycombs when only wet mixing was applied for 8 hrs and when also the high shear mixing stage was accomplished after wet mixing (for 26 hrs). By comparison with the pore size distribution of the starting powder, that is also given in the Table, it appears that the morphology is modified in consequence of wet mixing and to a greater extent of high shear mixing. The volume and the mean radius of the largest pores decrease whereas the diameter and the volume of smallest pores shows minor variations. The radius of the largest pores (around 800-1000A) is believed to originate from the dimensions of the aggregates, as determined by granulometric measurements (~10000]~); likewise the radius of the smallest pores (around 100A) results from the characteristic crystal dimensions of tungsta-titania, as derived from XRD analysis by using the Sherrer equation (~200A). Accordingly this processing stage does not affect appreciably the surface area, which mainly depend on the radius and volume of the smallest pores (mean pore radius = 70 A for 15~<rp<300~; pore vol. = 0.29 cc/g). It can be concluded that the essential feature of the high shear mixing stage is to generate high shear stresses through the paste, so that the agglomerates are broken down to single particles. It is worth mentioning that the application of high shear stresses during the mixing stage favours effective extrusion and avoids flow perturbation due to particles deagglomeration during extrusion. 4.2. Liquid content in the paste The liquid content was found to affect markedly the rheology of the paste and the morphology and the mechanical characteristics of the honeycomb monoliths. The pore size distribution measured by Hg porosimetry, the BET surface area, the values of the rheological parameters K and n in power law function, and the crushing strength for monoliths obtained from pastes with different water contents are presented in Table 2. Table 2 Effect of water content and high shear mixing stage on the rheology of ceramic pastes and on the morpho!0 ~ andmechanical characteristics of extruded monoliths. 40 water Content (% wt) . . . . . . . . 28 30 30 35 Treatment HSM HSM WM " WM WM BET: Specific Surface Area (mZ/g) 74 , 72 ,, 73 , 76 ,78 .... Hg penetration: Mean Radius for 40_~<rp<300A (~) 80 82 80 80 78 Pore Vol. for 40A<rp<300A (cc/g) 0.21 0.19 0.21 0.21 0.23 Mean Radius for rp>300A (~) 350 400 680 1100 1500 Pore Vol. for rp>300A (co/g) , 0.02 0.06 . . . . ,0.09 , 0.22 .... 0.30 Rheological Parameters: 2.7 103 K (Pa) 4.0 104 3.0 104 3.5 104 1.1 10 4 n 0.143 0.234 0.056 0.213 0.356 Crushing Stren~h.,(K~'cm 2) , ,45,, ' 46 '23 ' ,, 7 .... 3 HSM: application of high shear mixing stage. WM: application of wet mixing stage but not of high shear mixing stage. ,.

,,

.

,

i,

,

,

.

,.

i

,

.

,

.,,.

,.

793 It appears that the volume and the mean radius of the smallest pores are almost unaffected by the reduction in the water content from 40 % to 28% by weight whereas both the volume and the mean radius of the largest pores are drastically reduced. Since, as already discussed, the large and the small pores are associated to the dimensions of particles aggregates and of TiOz anatase crystallites respectively, it is concluded that de-agglomeration of the particles is greatly favoured by a decrease in the water content of the paste. On the other side this affects only marginally the BET surface area that decreases only slightly from 75 to 70 m2/g.; indeed the surface area is primarily associated with the radius and volume of the small pores. On the other side the rheological parameter K increases by one order of magnitude by decreasing the water content from 40% down to 28% w/w. In view of the low values of n and further considering that K--+x as n--+0, it appears that low water contents result in higher shear stresses which, as shown by morphological data, cause extensive de-agglomeration of the particles. As a matter of fact we expect that in order to make a powder extrudeable the liquid has to be added in a quantity sufficient to fill the intra-particle pores and the inter-particle voids when the particles are in contact, so providing a lubricating layer around them. The liquid quantity required to fill the small pores of tungsta-titania, which accounts for about 90% of the solid powders, always corresponds to 0.29 cc/g based on BET measurements (for 15i<rp<300i) and that required to fill the large pores (rp>300i based on Hg penetration) corresponds to 0.02 cc/g and 0.30 cc/g for water content of 28% and 40% w/w respectively. The residual liquid contents can be calculated accordingly. Hg penetration data of the final products prove that the tungsta-titania particles are completely de-agglomerated to single crystallites with a diameter of ~200~_, as indicated by XRD analyses, in the case of low water content whereas they have a diameter of .~10000.~ as determined by granulometric measurements, in the case of high water content since deagglomeration of the particles has occurred to a limited extent. The residual liquid contents mentioned above result in a lubricating layer around the crystallites of the order of 30 i in the former case and around the particle aggregates of the order of 1100 i in the latter case. This picture is consistent with the observed changes in paste rheology and further implies an expected shrinkage of about 20%, which compares reasonably with that measured experimentally of about 10%. Along these lines by comparing the morphological and rheological data in Table 2 for the pastes with 30 % water content when the high shear mixing stage is applied or not it appears the application of this stage results in higher shear stresses (higher values of the rheological parameter K) and causes a significant reduction in the mean radius and the volume of the largest pores. Finally a strict correspondence exists between the radius and the volume of the macropores and the crushing strength of the honeycomb monoliths. As expected superior crushing strength properties were measured as the radius and volume of the largest pores decrease, either because of lower water content or of application of the high shear mixing stage. 4.3. Role and effect of the inorganic additives Clays (bentonite) and glass fibres are used as inorganic additives. SEM photographs demonstrated that the fibres are oriented in the direction of extrusion and crushing strength measurements proved that they are required to provide reasonable mechanical strength to the final materials. Figure 1 shows that the rheological properties of the paste depend on the content of bentonite for the case of low water content (28% w/w) but not for that of high water content

(35%w/w).

794 5,0

i

0

4,8

Q

AA

Q. v

r--==

o0

4,6

rat} !,.,

(1) cO9 =....a 0"1 0 _.1

4,4 4,2 4,0

0,4

J ,

,

,

0,8

,

,

,

,

1,2 1,6 Log[Shear rate] (l/s)

,

",'

2,0

,

2,4

Figure 1. Rheological curves of pastes with different bentonite content (squares: 0% w/w; circles: 4% w/w; triangles: 8% w/w) at two water levels (solid symbols: 28% w/w; open symbols: 35% w/w ). In the former case higher shear stresses are measured at a given shear rate and steeper rheoloNcal curves are observed in a log-log plot diagram. This corresponds to higher values of the rheological parameter n in the power law function (n = 0.32, 0.16 and 0.064), which implies a more pronounced velocity profile in the slit capillary. The morphological and mechanical characteristics of the honeycomb monoliths obtained from these pastes are presented in Table 3. Table 3 Effect of bentonite content on the morphological and mechanical characteristics of extruded monoliths. ,,Water content,(% wt), 28 .... 28 , 28 ....35 , , 35 . . . . . . . 35 Bentonite content (% wt), 0 4 . . . . . . . . 8,, 0 4 8 Treatment HSM HSM HSM WM " WM WM Hg penetration: Mean Radius for rp>300A (.k_) 350 550 1100 1100 1000 Po,re Vol. fo r rv>300A (cc/g) 0 ....... 0.03 0.08 0:21 0.23 0.20 C rushint~ Stren~h,,,(k~cm 2) ....61, 45 ,, 43 ', 10 6' ~', 12 ',' '

'

""'

,,

,

,

,

,, ,, , , ,

,

,

,,

,,,

,

,,

i

,

,

HSM: application of high shear mixing stage.WM: application of wet mixing stage but not of high shear mixing stage. In the case of low water content the volume of the largest pores increases slightly on increasing the clay content, and the crushing strength diminishes accordingly. This apparent contradiction can be rationalised by considering that the effect of higher shear stress is more

795 than offset by that of the more pronounced velocity profile in the pastes. Indeed the later effect may produce inadequate joining of the adjacent layers of the paste during extrusion in the die land; this in turn may cause internal fracture during thermal treatment thus accounting for the higher macroporosity and, because of this, for the lower mechanical properties. In the case of high water content the water excess is so high that the bentonite content does not affect significantly the rheology of the paste, and as a consequence the morphology and the mechanical properties of the honeycomb monoliths as well. However if the pH of the tungsta-titania paste (pH = 3) is modified by addition of NaOH the measured shear stress values present a maximum at pH = 6, as shown in Figure 2 for 7't50 s1.

900-

- 45000

40000

800

6O :::3"

or} IX.

v

t~ 0 0 t~

700

35000 60

600

30000 "13

i

500 2

"

'

'

I

4

'

.....

I

6

~

I

8

'

'~

/

10

25000 12

pH

Figure 2. Effect of pH on rheology of pastes (]tvt--- 50 s'l). In all cases almost constant n values were observed (n ~ 0.16), suggesting that the velocity profile in the slit capillary is not markedly influenced by pH. On the other hand the maximtun in Figure 2 can be explained in relation to the structure of bentonite, which is a smectite clay with crystallographic phase corresponding to montmorillonite. It consists of two tetrahedral layers separated by an octahedral layer; these units are separated by interlayers with hydrated exchangeable cations, such as Ca2+, Mg 2+, Na+,. that are present to counterbalance the weak negative charge in the structure due to the substitution of A13+ cations by Fe2+ or Mg 2+ in the octahedral sheets. Since the natural pH of bentonite is around 9, as the pH is decreased into the neutral range (i.e. pH = 6-7) a large fraction of the platelet faces are neutralised and the particles experience lower repulsive forces, which eventually result in higher viscosity of the paste. If the pH decreases further into the acidic range (i.e. pH = 3-4) the platelets become positively charged, and the viscosity decreases accordingly. The above data suggest that the chemical effect of the bentonite content must be properly taken into account in the preparation of DeNOx-SCR catalysts, considering that pH is expected to affect the impregnation of tungsta-titania powders with the solution containing the vanadium salts.

796 5. CONCLUSIONS The present study has proved that, in spite of the analytical and practical complexities associated with flow processes of ceramic pastes, capillary rheometry when critically applied represents a useful tool to understand the preparation of structured catalysts by extrusion and to foresee the extrudability of ceramic pastes. This is because extrusion can be regarded basically as a flow process of a non-Newtonian fluid. With this in mind, a strict relation has been demonstrated for the specific case of TiOabased honeycomb monoliths between the theological characteristics of the ceramic paste and the morphological and mechanical properties of the extruded material. Such an approach can be can be extended to other ceramic pastes.

Acknowledgements This work was performed under contract with ENEL-SRI-PDM Cologno Monzese. The support of MPI-Roma is also acknowledged.

REFERENCES 1. J. Benbow, J. Bridgwater, "Paste Flow and Extrusioff', Claredon Press, Oxford, 1993. 2. R.M. Heck, R.J. Farrauto, "Catalytic Air Pollution Control", Van Nostrand Reinhold Publisher, New York, 1995. 3. P. Forzatti, L. Lietti, Heterogeneus Chemistry Reviews, 3 (1996) 33. 4. S. Irandoust, B. Andersson, Catal. Rev. Sci. Eng., 30 (1988) 341. 5. M. Miller, R.A. Haber, Ceram. Eng. Sci. Proc., 12 (1991) 33. 6. I.M. Lanchman, J.L. Williams, Catal. Today, 14 (1992) 317. 7. D. Ballardini, L. Sighicelli, C. Orsenigo, L. Visconti, E. Tronconi, P. Forzatti, A. Bahamonde, E. Atanes, J.P. Gomez Martin, F. Bregani, Studies in Surface Science and Catalysis, J.W. Hightower et al. (Editors), 101 (1996) !359. 8. W. Gleissle, J. Graczyk, H. Buggisch, KONA Powder and Particles, 11 (1993) 125. 9. N.G. McCrum, C.P.Buckley, C.B. Bucknall, "Principles of Polymer Engineering", Oxford University Press, 1988. 10.J. Ferguson, Z. Kemplowski, "Applied Fluid Rheology", Elsevier Applied Science, London and New York, 1991. 11. W. Gleissle, E. Windhab, Experiments in Fluids, 3 (1985) 177.

1998 Elsevier Science B.V. Preparation of CatalystsVII B. Delmonet al., editors.

797

The study of formation of supports and catalysts based upon A1203/A1 cermets S.F.TikhoC, V.A.SadykoC, Yu.A.Potapova a, A.N.SalanoC, G.N.Kustovaa, G.S.Litvak~, V.I.Zaikovskii', S.V.Tsybulya', S.N.Pavlova~, A.S.Ivanovaa, A.Ya.Rozovskiib, G.I.Linb, V.V.Luninc, V.N.Ananyind, V.V.Belyaevd. a Boreskov Institute of Catalysis SB RAS, Pr.Lavrentyeva 5, Novosibirsk, 630090, Russia b Topchiev Institute of Petrochemical Synthesis RAS, Leninskii Pr. 29, Moscow, 117912, Russia ~Lomonosov Moscow State University, Vorobyevy gory, Moscow, 119899, GSP, Russia d Institute ofRadiomaterials, Minsk, Belorussia. The regularities of formation of porous metaUoceramic supports and catalysts of a A/AI2OjAI type via hydrothermal oxidation of powdered aluminum in mixture with various dispersed additives (A) have been investigated. The interrelation between the parameters of composites synthesis (temperature and time of processing, type of the aluminum powder and nature of additives) and their properties including phase composition, texture, mechanical and catalytic properties (CO and butane oxidation, methane steam reforming, Fischer-Tropsch synthesis) was analyzed. 1. INTRODUCTION For advanced catalytic materials, the problem of their shaping as complex geometric forms (tubes, monoliths, foams etc) is one of the most demanding. Such forms ensure a number of advantages for catalyst bed design due to controlled pressure drop, tunable regimes of gas flow and heat exchange. As a result, catalytic activities and selectivities can be improved. The traditional approach in preparation of such catalysts consists in coating non-porous metallic or ceramic carriers with secondary supports having developed pore structure and surface area. For this purpose, usually washcoating by alumina and/or silica oxides from their suspensions in water or from organic precursors dissolved in various solvents is used. In such a way, thin (<100 ~tm [ 1]) layers of the secondary supports are obtained. For a number of important applications including precious metals based monolith catalysts for car exhaust converters, catalytic combustion etc, this thickness is quite sufficient due to internal diffusion limitations at operating temperatures. However, for slower processes of industrial importance (methane reforming, Fisher-Tropsch synthesis, partial oxidation etc), thicker films of the secondary supports would be beneficial due to more flexible tuning of the active component loading via impregnation, that is expected to increase the catalysts life-time in some cases. Earlier [2], we have developed a method of metalloceramic supports and catalysts synthesis via hydrothermal oxidation of aluminum -containing mixtures of powders in a confined space. It allows to obtain catalysts with complex shapes as well as to coat the non-porous metallic carriers (internal/external tubes surfaces) with thick (up to several cm) layers of the porous secondary supports. Due to the increase of the solids volume in the course of their oxidation/hydroxilation, their self-pressing within specially designed forms yields monoliths with a high crushing strength. After calcination of the hydrothermally treated samples, a porous

798 cermet is formed with a rather uniform distribution of the remaining metal particles within the oxide matrix (Fig. 1). As a result, a good thermal conductivity is ensured exceeding that of the ceramic supports [3], especially for configurations when cermet is pressed onto the metallic wall. For these systems, the active component can be incorporated either during mixing of the initial powders before hydrothermal treatment, or via impregnation of the cermet support. In the paper presented, for typical AI203/AI -based systems, the impact of synthesis parameters (temperature and duration of hydrothermal treatment, blend composition) on the physicochemical and catalytic properties of cermets thus obtained are considered.

Fig. 1. Scheme of the cermets synthesis via hydrothermal treatment with the subsequent calcination. 2. EXPERIMENTAL As starting compounds for cermet synthesis, two types of AI powders (PA-4 and PA-HP) were used with the mean particles sizes around 21-22 ~tm. The second type of powder has a higher purity and a higher share of small particles with sizes less than 5 ~tm. A number of components including zirconias partially stabilized by Ca, Sr, Ba (PSZ); basic cobalt carbonate (BCC, pure grade); industrial fused potassium promoted iron catalyst of Fischer- Tropsch synthesis; hydride of the Fe-Zr intermetallide, as powdered (fraction, <0.25 mm) solids were mixed with AI powder before hydrothermal treatment. Hydrothermal oxidation was carried out in the <>type autoclave (working volume, 2 / ) in the isothermal regime for required periods of time. To stop the process, the pressure was released. The catalysts of methane steam reforming were prepared via impregnation of PSZ-containing cermets with the ethanol solution ofRu(OH)Cl2 (Ru loading 0.5 wt. % ). Catalytic activities in the reactions of CO and butane (mixed with air) oxidation were determined in the batch-flow systems using catalysts fraction 1-2 mm and were characterized either by rate at 400~ (0.2 vol. % steady state butane concentration) or by a temperature at which 50% of CO conversion was achieved (GHSV 10,000/h). Catalytic activity in the reaction of steam reforming was characterized by a degree of CH4 conversion measured at 750~ and/or by an efficient first order rate constant using the same type of catalytic installation with the reaction mixture CH4:H20 = 1:2 fed with velocity 2 ml/s at atmospheric pressure into the reactor equipped with 0.5 g of catalysts diluted by quartz in the 1"10 ratio. Before measurements, the catalysts were activated in a stream of hydrogen with a temperature ramp from the ambient temperature to 750~ and keeping at this temperature for 1 h. Catalytic performance in the reaction of Fischer- Tropsch synthesis was estimated using the batch-flow high-pressure installation of the type described in [5] for the reaction mixture composition (vol. %): CO2 - 4-5; CO - 35-38; 1-12- 55-60; balance N2 at 8-10 MPa. Before measurements, the catalysts (fraction, 3-5 mm) were reduced in the flow of hydrogen at the same

799 pressure. The rate of production (yeld) was estimated without taken into account the carbon dioxide and methane formation. Some typical reaction parameters are included into Table 3. The particle size distribution was investigated by Coulter method using TA-2 machine and DAS computer program [6]. The thermal analysis of samples pre-dried at 120~ was performed in a stream of air using Q-1500D derivatograph. The amount of the oxide formed was estimated by gravimetry. The phase composition was analyzed with an URD-63 diffraetometer using CuK~ radiation and with IR-75 spectrometer by analysis of the lattice modes. The pore size distribution was estimated by mercury porosimetry for the samples fraction 3-5 mm using AutoPore 9200 Micromeritics. The specific surface area (SSA) was determined by the expressBET method using Ar thermodesorption data. The morphology and textural features of samples were studied by SEM using BS-350 "Tesla" machine. TEM data were obtained using JEM-400 machine. The real density of cermet samples was estimated using helium AutoPienometer 1320. 3. RESULTS AND DISCUSSION 3.1.Formation of ceramometai supports and catalysts Ai203/AI composites. During hydrothermal treatment, oxidation and subsequent hydration of the aluminum particles surface layer occurs accompanied by self-pressing of the powder blend into strong monolith within confined volume of the press-form [2]. Fig. 2 illustrates typical thermal analysis data for pure aluminum powder subjected to HTT at 200~ for various periods of time. Both DTA and DTG curves display effects inherent for metallic aluminum (590~ - partial oxidation, 660~ - melting), along with those of aluminum oxohydroxide with the boehmite structure decomposition at 510-525~ preceded by the effects of water desorption at 120~ [7]. Aiter prolonged (at least for 6 h) hydrothermal treatment, exoeffects at -~ 330~ emerge due to a structural rearrangement of oxohydroxides [2]. The same features were observed for samples treated at 150 n 250~ According to XPD data, hydrothermal oxidation of aluminum was accompanied by accumulation of the boehmite phase. It agrees with the results of infra-red spectroscopy (Fig. 3): for samples dried after synthesis at 120~ absorption bands assigned to stretching (3080 and 3270 cmq) and bending (1065 and 1150 cm"1) modes of the boehmite hydroxyls were detected [8]. Moreover, a group of bands at 730, 610, 490 crn"~ typical for stretching Me-O vibrations in boehmite having water excess [8] were also revealed. Atter calcination at temperatures >520~ these bands disappear, and in the hydroxyls regions, only bands due to stretching and bending modes of adsorbed water (3500 and 1600 cm"~) remain. In the region of the Me-O lattice modes, only broad bands typical for 7-alumina [9] are visible for calcined samples. The process of aluminum hydrothermal oxidation was found to be rather weakly time- and temperature dependent but the initial part of the reaction curve (Table 1). This implies hydrothermal oxidation to be mainly diffusion-controlled. Type of the aluminum powder and admixtures content have a great effect on the dynamics of aluminum oxidation and structural features of the oxohydroxide thus formed. Thus, for PA-HP, all synthesis conditions being the same, the amount of oxide is nearly twice as high compared with cermets formed from PA-4 aluminum. For the former system, DTA curves (Fig. 4) show main endoeffect at 480~ typical for aluminum oxohydroxo -compounds with structure intermediate between that of boehmite and pseudoboehmite, while aluminum oxidation process starts at temperatures for 30-70~ lower than for the latter system (cf. Fig. 2 and 4). IR spectra of the lattice modes of cermet prepared from PA-HP show red shift of the modes assigned to Me-O stretching in

800

/

T.HTr=0h

/

750",,/ ngo~ /

1

!

sJ--1

. . . . . . . . : " " -- ' L - - - - / 1 i ! J

T'HTT=0'5 h

,

SlO ~ .,,,I ,-. . . . . .

_.,

'

'I~'HT T ' ' 6 . 5 h

L

--"" 120"

12

/'

/"i i i,--'/ I 525*

,,/

660'

(640 ~

330" _

/

630 =

//

"

3 1600

;i

II .

--DTG --DTA

4 0 0 0 - 3 5 0' 0

3~

2 5'o0

.....

2000 . . . . . 1500

1do0 500 ' V, cm -1

Fig.2. Differential thermal analysis data for Fig.3. m-spectra of the PA-4 aluminum the PA-4 aluminum powder atter HTT at powder after HTT at 200~ for 0.5 h fol200~ for different periods of time and then lowed by calcination pre-dried at 120~ Table 1. The influence of the parameters of HTT synthesis on some physico-chemical properties of A1203/AIcermet Tm-r (~ 150

200

250

Xrrrr(h) 0.5 2.0 3.5 5.0 6.5 0.5 2.0 3.5 5.0 6.5 0.5 2.0 3.5 5.0 6.5

A1203(wt.%) 12 18 20 21

,

11 19 23 26 23 25 27 31 34 37

SSA(m2/g) 30 32 33 38 37 24 31 32 42 41 36 48 46 58 61

V (ml/g) 0.14 0.11 0.10 0.12 0.11 011 0.11 0.11 008 0.15 011 013 011 0.18 0.11 ,

oxohydroxide (up to that could be due to higher water content in boehmite [8] approaching its structure to that of pseudoboehmite. Hence, variation of the synthesis parameters and type of

801 powdered aluminum allows to control both degree of aluminum oxidation and structural properties of the emerging oxohydroxide thus affecting a texture of the porous cermet (vide infra). Co304/Ai203IA! composites. For these systems, after hydrothermal treatment, the thermal analysis curves (Fig. 4) show endoeffects at 235 and 280~ accompanied by corresponding weight losses typical for the basic cobalt carbonate decomposition [10]. At higher temperatures (480-570~ some variation of the TA curves is also visible implying some impact of BCC on the processes of aluminum hydroxide formation and decomposition. Hence, emerging boehmite seems to interact with BCC. Thus, in cobalt-containing cermets, aider HTT, along with the phases of A1, boehmite and traces of cobalt oxide, XPD data revealed a new hexagonal-type phase with the lattice parameters a = 0.3085(1); c = 0.2271(2) nm different from the initial BCC and tentatively assigned to the mixed cobalt-aluminum hydroxocarbonate with a structure similar to that of Z n 6 A I 2 ( O H ) 1 6 C O 3 4H20 [11]. In agreement with these data, IR spectra of the lattice modes (Fig. 5) for BCC -containing cermets dried after HTT at 120~ revealed only very weak bands of carbonates in the 1300- 1600 cm"1 region, while for pure BCC subjected to HTT these bands were well developed. After calcination of Co-containing cermets at 520~ basic carbonates disappear and only bands due to alumina and cobalt oxide spinels (for the latter, bands at 660 and 570 c m "1 typical to octahedrally coordinated cobalt cations are observed) remain. Hence, even for cermets with encapsulated active components, during HTT, their complex interaction with the aluminum oxohydroxide takes place. I

\ 't ~

v; ..................

//'Y"

./

/

_)J---~. =

~ q / " . - , 1"-

il

......

9

-.". ,~ ""~

~-

~,= ....

1" ' ' / "

.........

i~ - ' - 570

/" Vq ..... ..-

-\v . . . . . . \. ~ J i. . . . . . . . . . . . / """,,('7

.....

il

610

___ ~ O'rG

4

4000

Fig.4. Thermal analysis data for PA-HP based cermets with various content of BCC(in wt. % Co304): 1 - 0; 2 - 8.8; 3 - 20.6.

3200

2400

1600

800

V,CM "1

Fig.5. m-spectra o f Co-containing sampies: 1 - pure BCC; 2 -BCC in cermet

after HTT; 3 -Co304 ; 4 - sample 2 after calcination at 520~

M-Zr-Oy/AI203/Ai composites. The initial PSZ systems are heterogeneous, composed of cubic and/or monoclinic zirconium dioxide solid solutions depending upon the type of the guest cation (Ca, Sr, Ba) and its concentration [12, 13]. Thus, for Ca-PSZ, cubic phase predominates, while for Sr-PSZ its content declines, and Ba-PSZ is mainly monoclinic. As judged by the relative intensity of diffraction peaks, the relative amount of the monoclinic phase was not changed after HTT. Hence, for these composites, in the course of HTT, only encapsulation of the PSZ proceeds without any pronounced chemical interaction.

3.2.Textural and mechanical properties of composite supports and catalysts Pore volume and macroporosity. When aluminum powder completely fills the press-form used for HTT (no mechanical pre-pressing is applied), estimated value of the free volume be-

802 tween the aluminum particles is around 0.4 ml/g of AI. After HTT, this value falls to -~0.18 ml/g independent upon the type of AI powder and synthesis conditions. It implies filling of the free spaces between the aluminum particles by aluminum oxohydroxide generated during HTT. For cermets, the predominant part of pore volume (up to 60-80 %) is due to macropores with typical sizes > 1 lam. This type of pores is mainly affected by the initial blend composition and its packing features [ 17]. Thus, when a blend was pre-pressed to make a denser packing of the AI particles, the AI/AI2Os cermet pore volume declined up to--0.12 ml/g. When the initial blend contains particles of structural promoters such as PSZ not changed during HTT and subsequent air calcination (vide supra), the decrease of the degree of free space filling by aluminum oxohydroxide is expected. Indeed, in this case, cermet pore volume was found to increase up to 0.25- 0.35 ml/g, and similar trend had been observed for such high-temperature additives as perovskites (lanthanum nickelate) [ 17]. Another trend is presented by cermets containing BCC, whose particles are preserved during HTT but decompose into more dense cobalt oxide during air calcination.. For such systems, the cermet pore volume increases up to-~0.5 ml/g (Fig. 6a). It was accompanied by a change of the pore size distribution, in particularly, by a gain in the mesopores (sizes, 40-100 nm) volume, probably, due to pores within the BCC pseudomorphous particles of mixed Co-AI oxide. SEM data (Fig.7) are indeed in favor of pronounced variation of the composites texture for samples with a high cobalt content.

ml P,MPa v,-~0 75. ~

'

aE)-P A-V

\

0,50-15,0 ~ _ . ~

~

Ssp. m= -O'Tx=50% (~

A

,oo

__77 \

0

'

50 '

. H talc W. 10"2(- g.4s 8)

9 ,,]3o

~ s,~ d "

L

b

'

'

'

0

,o

/ v

"

1'o"'

2'o

~

00304, wt%

Fig.6. Textural, mechanical (a) and catalytic (b) properties of Co304/Al203/Al cermets vs Co304 content: O - crushing strength; A - pore volume; C] - specific surface area; 9 oxidation of CO; A - oxidation of butane. Specific surface area (SSA) and microporosity. By using approaches developed in [14], theoretical estimations of the pore volume in AI203/AI composites based upon their apparent and true densities combined with the phase composition could be carded out. For the systems studied, such estimations show the pore volume to be in the range of 0.3-0.5 ml/g substantially exceeding the values obtained by the mercury porometry method. It implies a developed microporosity to be formed in the course of boehmite thermal decomposition. Indeed, a specific surface area increases from 0.5-1.0 m2/g for the initial aluminum powder to 61 m2/g for composites obtained from PA-4 (Table I) and 140 m2/g for those obtained from PA-HP (Fig. 6a). In these composites, specific surface area of the alumina constituent is estimated to be in the range of 150-250 m2/g A1203. TEM data confirmed the existence of micropores. Alumina aggregates were demonstrated to consist of platelets with the most developed (00 I) faces sepa-

803 rated by--- 0.5-1.0 nm pores. The platelets are composed of nearly coherently stacked microblocks with typical sizes up to 5 nm separated by slit-shaped-0 1-2 nm pores. Such texture is quite different from that typical for y-alumina prepared via a precipitation route when the most developed oxide particles faces are usually of the (110) type [ 16]. For PSZ-containing composites, after HTT and air calcination at 800~ specific surface increases from --20 m2/g (alumina/aluminum cermet) to --90 m2/g (Table 2). Detailed analysis shows that for these composites specific surface is higher than the values expected from the simple additive scheme for pure PSZ and alumina/aluminum cermet. It means PSZ hinders aluminum sintering during air calcination [17]. In a similar way, the complex effects of BCC on the texture and phase composition of cermets during HTT are reflected in a non-linear dependence of the cermets specific surface on the BBC content in the blend (Fig.6a).

Fig.7. SEM micrographs of Co-containing catalysts with different content of Co304 (wt.0//O): a 0, b - 5.0, c - 14.7, d- 23.2

804 Table 2. The effect of alkaline-earth metal (M) on the properties of Ru/M-Zr-Oy/A1203/AI steam reforming catalysts ~o_ Promoters M-Zr-Oy Catalysts M

SSA (m2/g)

SSA (m2/g)

Catalytic properties

1

Ca

29

37

XCH4(%) 89.3

Sco (%) 7010

K 10.3 (mlCH4/(gRus)) 3.3

2

Sr

120

90

85.1

69.8

2.3

3

Ba

15

35

81.4

69.8

1.8

Texture and crushing strength. According to SEM data for calcined samples, at the surface of aluminum particles, two morphological types of alumina oxide (oxohydroxide before calcination) texture are observed. At the initial stage of aluminum oxidation, a <<sponge>>-type oxohydroxide layer is formed composed of small particles. At prolonged HTT, especially at higher temperatures, well-crystallized platelets of boehmite are formed (see insets in Fig. 8). These textural types are observed for all types of the aluminum powder used here (cf. Fig.7) for PA-HP). Larger crystals of boehn~te seem to be formed via recrystallization of the smaller particles during HTT. Such types of texture differ by their density (-~2.4 H-~3.6 g/ml, respectively) from each other as well as from y-A1203 obtained via precipitation route (-~3.3 g/m/). Morphology of the alumina particles was found to affect appreciably the mechanical strength of composites. Thus, for samples where one type of alumina morphology dominates, mechanical strength monotonously increases with the A1203 content (Fig. 8). For composites where both types of morphology are present, mechanical strength is much higher decreasing again when only platelets of alumina are mainly present at prolonged HTT (Fig. 8). When component with a low mechanical strength is introduced into the blend, as is the case for BCC, cermets crushing strength monotonously decreases with its content (Fig. 7a). In this case, the shrinkage of the BCC particles during their transformation into oxide at calcination is also responsible for the weak bonding in cermets.

12,0

Crushing strength, MPa B

I' ra

|

E]

6,0

2,0

Fig.8. Crushing strength of h l 2 0 3 / h l pellets after HTT followed by calcination vs HTT time: O - Thtt =150~ V1- Thtt = 200~ A - Thtt = 250~ Insets: models of alumina morphology from the SEM data [2].

805 Hence, variation of the synthesis parameters along with the nature and dispersion of the additives blended with aluminum, allows to broadly regulate the pore volume and pore size distribution in cermets. Macro - and meso-porosity was found to mainly depend upon the blend composition. Microporous structure can be finely tuned by the nature of aluminum powder and HTT parameters such as temperature and duration of treatment. Blend additives could also affect the microporous structure either directly (due to inherent microporosity) or indirectly (via influencing the processes of aluminum oxidation and oxohydroxide growth).

3.3. Catalytic properties As follows from the data presented in Fig. 6b, for cobalt-containing cermets, their catalytic activity in the reactions of CO and butane oxidation monotonously increases with the cobalt oxide loading up to 20%. At higher cobalt oxide content, catalytic activity is nearly constant, probably, due to a less optimum pore structure. If both crushing strength and catalytic activity must be optimized, the best composition is around 15 wt. % of Co304 (Fig.6). The same composition was chosen for catalyst shaped by extrusion of Co304 with the alumina-based binder [18]. At close values of activity, Co-cermet has nearly twice as high crushing strength. In the reaction of methane steam reforming, both catalysts performance and specific activity related to gu loading were found to decrease in the next order: Ca > Sr > Ba independent upon the catalysts specific surface (Table. 2). Earlier [4], in this row of PSZ, the number of the Lewis acid centers -coordinatively unsaturated Zr cations was shown to increase. It was explained by the surface layer reconstruction into a perovskite -type structure accompanied by disappearance of the oxygen vacancies, which is the most favorable for smaller alkaline-earth cations. Hence, specific activity of these catalysts seems to decline with the surface acidity facilitating carbon deposition. A rather weak dependence of activity on the promoter type could be assigned to the leveling effect of the alumina matrix. For the reaction of Fischer- Tropsch synthesis, only unsupported iron catalysts are known to be active and selective, while any attempts to make good supported catalysts were unsuccessful. In this respect, rather unexpected results were obtained in this work showing the increase of the Fe catalysts performance from-0.03 to - 0.24 g/(g h) when active components were introduced into cermet (Table 3). Though a detailed analysis of this phenomenon is still in progress, nevertheless, it clearly demonstrates perspectives of the cermet-based systems for Table 3. Catalytic properties ofFe-containin g cermets !n the Fischer-Tropsh synthesis. Active Reaction conditions Catalytic properties component P,MPa T, V,1/(geat.h) x, h Xco yeld, products ,0~

~;p/(geat.h)

Fe-K

8'i0

260

0C

6.0

100

23

0.24

paraffinsC2-C17,

Fe-Zr

10.0

260

4.3 ,,,,

60

14

0.15

alkohols C6-C20 n-paraffins C6-C20

this reaction, especially for selective synthesis of long-chain hydrocarbons. Such systems as hydrides of Fe-Zr intermetallides were already known for their activity in the F-T synthesis. However, it is necessary to stress that only approach used here allowed to shape them into mechanically strong catalysts with the increase of the catalytic performance from 0.1 to 0.15 g/(g h). Moreover, they were supported as strongly adhering thick layers onto the stainless steel tube surface that allows to efficiently solve the problem of the heat release and trans-

806 fer vital for this reaction. The same approach can be used for all other high heat flux catalytic processes where a high thermal conductivity of the catalyst bed is required. 4.CONCLUSIONS

Basic features of the phase composition and textural properties of the AI2Oa/A1 -based cermets formation via hydrothermal treatment route were studied. These systems were shown to be promising for a wide- scale application as supports and catalysts with tunable pore structure, mechanical and catalytic properties. ACKNOWLEDGMENT

The authors are thankful to V. B. Fenelonov for fruitful discussion. REFERENCES

1. C.J.Peraira, J.E.Kubsh, L.Hegedus, Monolith washcoat having optimum pore structure and optimum method of designing the washcoat, US Patent No. 4 771 029 (1988). 2. S.F.Tikhov, A.N.Salanov, Yu.A.Palesskaya, V.A.Sadykov, G.N.Kustova, G.S.Litvak, N.A.Rudina, V.A.Zaikovskii, S.V.Tsybulya, React.Kinet.Catal.Lett., (1998), [accepted]. 3. V.N.Ananyin, V.V.Belyaev, V.N.Parmon, V.A.Sadykov, S.F.Tikhov, T.G.Starostina in: Proc.25 Mendeleev Cong. General and Applied Chem., v.1, p.37, ~Navuka i Technika,, Belorussia, Minsk, 1993 [in Russian]. 4. V.A.Sadykov, A.S.Ivanova, V.P.Ivanov, G.M.Alikina, A.V.Kharlanov, E.V.Lunina, V. V.Lunin, V.A.Matyshak, N.A.Zubareva, A.Ya.Rozovskii in: Mater.Res.Soc.Symp.Ser. (Advanced Catalytic Materials), MRS, USA, PA, Pittsburgh,No 457 (1997) 199. 5. A.Ya.Rozovskii, G.I.Lin, Scientific Bases of the Process of Methanol Synthesis, ~Chemistry>), Russia, Moskow, 1990, p. 191 [in Russian]. 6. T Allen, Particle Size Measurement, Capt.Hill, London, 1981,590. 7. B.P.Zolotovskii, G.N.Kryukova, P.A.Buyanov, G.S.Litvak, B.E.Loyko, L.M.Plyasova, V.A.Balashov, Izvestiya SO AN SSSR, ser.chim.nauk, 6(1989)111 [in Russian]. 8. H.W.Van der Marel, H.Bektelspaher, Atlas of Infrared Spectroscopy of Clay Minerals and their Admixtures, Elsevier, USA, New York, 1976, 396. 9. A.S.Ivanova, E.V.Skripchenko, E.M.Moroz, G.S.Litvak, G.N.Kustova, O.P.Krivoruchko, Izvestiya SO AN SSSR, ser.chim.nauk, 6(1989)116 [in Russian]. 10. V.P.Chalii, Metal hydroxides, Naukova Dumka, Kiev, 1972 [in Russian]. 11. JCPDS Data File No38-486. 12. JCPDS Data File No27-997. 13. JCPDS Data File No97-1484. 14. V.B.Fenelonov, Kinet.Catal., 35 (1994) 795, [in Russian]. 15. O.P.Krivoruchko, B.P.Zolotovskii, L.M.Plyasova, R.A.Buyanov, React.Kinet.Catal.Lett., 22(1983)375. 16. S.J.Wilson, J.Solid State Chem., 30 (1979) 247. 17. S.F.Tikhov, V.A.Sadykov, A.N.Salanov, Yu.A.Potapova, G.S.Litvak, S.V.Tsybulya, S.N.Pavlova in: Mater.Res.Soc.Symp.Ser. (Advanced Catalytic Materials), MRS, USA, PA, Pittsburgh, No.497(1998) [in press]. 18. S.F.Tikhov, V.A.Sadykov, G.S.Litvak, G.N.Kustova, Yu.A.Palesskaya, L.A.Isupova, G.N.Kryukova, V.N.Parmon, V.N.Ananyin, V.V.Belyaev, Scientific Bases for the Preparation and Engineering of Catalysts. (Proc. 3d Conf. Russia and SIC., Yarosllavl, 1996), Russia, Novosibirsk, 1996, p. 182 [in Russian].

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

807

Tuning o f textural and structural characteristics of A12Oa-TiO2 mixed oxide supports T. Klimovaa, H. Gonzfilez a, R. Hemfindez b and J. Ramirez a aUNICAT, Departamento de Ingenieria Quimica, Facultad de Quimica, Universidad Nacional Aut6noma de M6xico, Cd. Universitaria, C.P.04510, M6xico D.F., M6xico. blnstituto de Fisica, Universidad Nacional Aut6noma de M6xico, Cd. Universitaria, C.P.04510, M6xico D.F., M6xico.

Modification of the textural properties of catalytic supports by chemical methods can lead also to changes in structural and surface chemical properties of these solids. In the present work, we analyze the relationship between the textural changes achieved in A1203-TiO2 supports, by the addition of ammonium carbonate during the preparation procedure, and the changes in chemical structure and surface properties induced by the textural modification. These changes are characterized by surface area, DRX, TGA/DTA, FTIR, FT-Raman, HREM, solid state 27A1 MAS-NMR and surface acidity. The isomerization of 1-octene was also used to asess the importance of the changes in the surface acid properties of the samples.

1. INTRODUCTION Tuning the textural and structural characteristics of catalytic supports is of great importance to the performance of industrial catalysts. For example, for hydroprocessing catalysts, surface area and pore dimensions need to be designed according to the petroleum fraction to be processed. For these catalysts besides the textural properties, the acidity of the support is also important and several ways of modifying the support acidity have been used in the past [1, 2]. Among the new approaches to provide more versatile supports for new hydroprocessing catalysts with enhanced functionalities (HDS, HYD, HDN, HDM), the use of mixed oxides such as AlzO3-ZrO2 [3, 4] and AlzO3-TiO2 [5], among others, has been proposed. However, the tuning of textural and structural properties of mixed oxide supports has only been addressed in few cases. Since it has been found that the use of AlzO3-TiO2 mixed oxide supports enhance the hydrodesulfurization activity of Mo or W based catalysts [6, 7], this work presents a study on the relationship between the structural changes induced in AlzO3-TiO2 supports, and the modifications in the preparation procedure designed to obtain changes in surface area and pore size distribution.

808 2. E X P E R I M E N T A L The AI203-TiO2 mixed oxides with a 1"1 molar ratio were prepared by sol-gel method, with and without the addition of AC in the synthesis procedure, using Ti and A1 isopropoxides as precursors and n-propyl alcohol as a solvent [8]. In the experiments, a solution of a known amount of ammonium carbonate (AC) in water was used to produce the formation of the metallic hydroxides. The resulting precipitate was aged, with slow stirring, for 24 h, filtered under vacuum and washed with water. The solid was then dried at 373 K for 24 h and calcined 24 h at 773 K. Hereafter the mixed oxides will be referred as A1-Ti(C,W), where C is the amount of ammonium carbonate used in the synthesis (grams of AC per g of the fmal calcined oxide), and W- amount of water used for hydrolysis (ml of water per gram of the calcined oxide). For purposes of comparison a pure A1203 sample was prepared by similar procedure (AI(C,W) samples). The resulting textural changes (surface area, pore volume and pore size) were followed by nitrogen physisorption (Micromeritics ASAP 2000 apparatus), and the supports structural changes were determined by different characterization techniques. FT-IR measurements were performed with a Nicolet 510 spectrometer using KBr pressed disks. The IR spectra were recorded at room temperature, with 300 scans and 4 cm -1 resolution. The laser Raman spectra were obtained at a Nicolet 950 FT spectrometer using a Nd-YAG laser as the excitation source and an InGaAs detector. Thermogravimetric and differential thermal analysis were carried out in a Dupont 2000 system, under a nitrogen flow of 100 ml-min 1 and a heating rate of 10 K-min 1. X-ray diffraction patterns were recorded in the 30<20<80 ~ range in a Philips PW 1050/25 diffractometer, using Fe-filtered CuKcz radiation (~ = 1.5418 A) and a goniometer speed of 2 ~ (20)-min "1. High resolution transmission electron microscopy studies were performed using a Jeol 100CX electron microscope. For this characterization the solid samples were dispersed in an n-propanol-water solution (6:1 v/v) and a drop of supernatant liquid was deposited on Cu grids coated with a holey carbon film. A potentiometric method [9] was used to determine, titrating with n-butylamine, the total number of surface acid centers and the maximum acid strength of the surface sites of the prepared solids. Solid state magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra were obtained with a ASX300 Bruker NMR spectrometer. Al(H20)63§ was used as an external 27A1 reference. The 27A1 MAS-NMR spectra were recorded at room temperature at a source frequency of 78.210 MHz and a spinning rate of 12 KHz with radio frequency pulses of 1.0 ~s. The chemical analysis of the alumina and alumina-titania precipitates, dried at 373 K, was made by Desert Analytics (USA). Also, evaluation of the catalytic activity in the 1-octene isomerization reaction was used as a test reaction to assess the changes in the supports acidity induced by the changes in their preparation procedures. The isomerization of 1-octene activity tests were carried out in an stainless steel tubular reactor (1 cm ID, 30 cm length) operating at 523 K and 28 Kg/cm2, using a H2/hydrocarbon molar ratio of 2 and 0.5 g of catalyst. The composition of the feed was 20 vol % 1-octene and 80 vol % n-heptane as a solvent. The reaction products were analyzed by gas chromatography using a capillary HP-PONA column operating with a temperature program consisting of 15 min at 308 K and then a temperature step (1.5 degrees per minute) up to 343 K. To corroborate product identification, the product mixture was also analyzed in a Hewlett Packard GC-IR equipment.

809 3. RESULTS AND DISCUSSION The textu~'al measurements indicate that the addition of ammonium carbonate during the sol-gel preparation of A1203-TiO2 mixed oxides leads to greater surface areas and larger cumulative pore volumes and pore diameters than when no AC additive is used in their preparation [8]. In this respect, the results show that an increase in surface area from 233 to 343 m2/g is possible when AC is used in the preparation (Table 1). Also, an increase in pore volume of 3.6 times and changes in mean pore diameter from 39 to 76 A are produced by the addition of AC. Table 1 Alumina-titania mixed oxides prepared with and without ammonium carbonate as additive Sample Surface area Totai pore volume Mean pore diameter* (mE/g) (cm3/g) (A) A1-Ti(0, 125) AI-Ti(2.4, 125) A1-Ti(6.3, 125)

233 248 343

0.243 0.373 0.874

39 41 76

* Mean pore diameter calculated as 4V/S.

7O0 ~500 t.,.)

~3oo

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~-

....

---':-'t-". " : . - . ~ :

r.~

loo 373

I

I

f

473

573

673

....

773

Calcination temperature, K

Figure 1. Surface areas of samples calcined at different temperatures: (~,) A1(0,125), (m) A1(6.3,125), (.)A1-Ti(0, 125) and (e)A1-Ti(6.3, 125).

Analysis of the changes in textural properties of the A1203TiO2 mixed oxide with the calcination temperature (Figure 1), indicate that the major change in surface area occurs at about 473 K. It is at this temperature that the TGA analysis detects a weight loss in pure alumina samples prepared with the addition of AC (Figure 2). In line with this, the DTA analysis confirms the existence of a decomposition peak at the same temperature. As will be seen below this compound was identified as NH4AI(OH)2CO3 H20.

The DRX of the dried precipitates of pure alumina support prepared with the addition of AC (Figure 3) reveal the presence of a compound of formula NHzAI(OH)2COyH20 (JCPDS29-106). In contrast, the diffractogram of the mixed oxide precipitate reveals a completely amorphous solid. However, the 1:1 NH4+:CO32 ratio deduced from the chemical analysis results of the A1203-TiO2 mixed oxide, indicates that in spite of their amorphous character, an interaction exists between the NH4§ and CO32 ions with the AI-Ti hydroxides.

810

481

too

30 90_ 0.5 -~. 80

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-

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E

-

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1075

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I

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.4 6O I

27:5

67:5

107:5

Temperature (K)

275

I

I

675

I

!

107:5

Temperature ( K )

Figure 2. TGA analysis of alumina and alumina-titania precipitates: (a) AI(0, 125), (b) A1(6.3, 125), (c) A1-Ti(0, 125) and (d) A1-Ti(6.3, 125).

r0'3(1) r

"4"-'--

H-.I

b !

60

!

40

I

20

Degrees(20)

Figure 3. XRD patterns of uncalcined (a) A1 (6.3, 125) and (b) A1-Ti (6.3, 125) samples. The solids were previously dried at 373 K for 24 h.

811 T r a a

n S

b

m i t

C

t a

d

n e

c e

(a.u.)

4000

3000"

2000

i000

Wavenumbers (cm"1)

The interaction between the N H 4 + and CO3 2" ions with the solid was also confirmed by llt

spectroscopy. The FT-IR spectra, in line with the TGA results, reveal the presence of the compotmd with AC up to 473 K. Treatment of the samples at higher temperature cause the disappearance of the infrared bands corresponding to the vibrations of NH4+ and CO32"groups (Figure 4).

Figure 4. IR spectra of A1-Ti-0.5 (6.3, 125) sample calcined for 1 h at (a) 373, (b) 473, (c) 573, (d) 673 and (e) 773 K.

Figure 5. HREM micrographs of dried precipitates obtained with and without ammonium carbonate: (a) A1-Ti(0, 125) and (b) A1-Ti(6.3, 125).

812 In line with this, the precipitates, with and without AC added, observed by TEM, 24 hrs. after the sample was taken, show changes in their morphology (see Figure 5). After calcination, these different precipitates lead to different porous structures.

V

o) x7

o AVl

~>~ LD c(D r'-

v.

V ~ v v o

V

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ov

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I

1273 K

V V xO

X

v

o

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i~

~ >

1073 K

x

875 K

80

60

40

Degrees ( 2 0 )

Figure 6. XRD patterns of (a) A1-Ti(0, 125) and (b) A1-Ti(2.4, 125) samples calcined at 873, 1073 and 1273 K for 3 h. (x - Anatase, v - Rutile, o - (z-A1203). Furthermore, the addition of AC not only modifies the textural properties of the solids but also their structural and chemical properties, as will be seen below. The diffractograms of A1203-TiO2 mixed oxides prepared with and without the addition of AC, calcined at different temperatures, indicate that the AC-modified sample is slightly less crystalline than its AC free counterpart calcined at the same temperature (see Figure 6). For the AI203-TiO2, calcined at 1273 K and 1073 K, the XRD patterns show for all samples, the existence of (x-alumina, anatase and rutile. The samples calcined at 873 K show, on the contrary, an amorphous character.

813 For the same samples, the results from Raman spectroscopy (Figure 7) show in the case of the samples prepared with AC, calcined at 1073 K, the presence of only bands corresponding to anatase (144, 399, 518, and 641 cm1) [10]. The rutile formation in this sample can only be clearly observed at 1273 K (bands at 144, 240, 449 and 611 cml). In contrast, in the sample prepared without AC addition only the bands corresponding to rutile for both temperatures of calcination (1073 and 1273 K) are observed. These results indicate that the anatase-rutile phase transformation is delayed by the use of AC in the preparation of the A1203-TiO2 mixed oxides. Differences in the aluminum structure in the mixed oxide with the addition of AC were detected by 27A1-NMR. The spectra of AC-modified solids showed the presence of pentahedrally coordinated aluminum (Figure 8), in addition to the tetrahedrally and octahedrally coordinated ones found in the AC-free preparations. This pentahedral aluminum structures have been associated in the past with increased Lewis acidity and the delay in the yto cz- alumina transformation [3, 11].

a) 8"

611

449 /-2.4

~4.

~o.

49.I,,,~Af- 40

1073 K 873 K

Raman Shift (cm-')

K

200 I 0

-I00 ~p

--

Raman Shift (cm-~) Figure 7. Raman spectra of alumina-titania samples calcined at different temperatures (3 h): (a) A1-Ti(0, 125) and (b) A1-Ti(6.3, 125)

Figure 8.27A1 MAS-NM~ spectra of alumina titania mixed oxides calcined at 773 K for 24 h: (a) A1Ti(0, 125), (b) A1-Ti(2.4, 125) and (c) A1-Ti(6.3, 125).

814 From the above results it is clear that changing the amount of AC added in the preparation of the catalysts it is possible to alter both, their textural and structural properties. Therefore, it seemed interesting to synthesize A1-Ti catalysts with different typical porous structures (narrow monomodal, wide monomodal and bimodal), changing the amount of AC in the preparation. The changes in acidity, induced in the solids by the changes in preparation procedure, will be assessed by potentiometric titration and catalytic activity in the 1-octene isomerization reaction. The results from the textural and acidity characterization of the prepared samples are shown in Table 2, where it can be seen that, as expected, changes in the textural and surface acid properties of the solids are brought about by the use of AC in the preparation procedure. In particular, the surface density of acid sites (PAS) and the maximum acid strength increase with the amount of AC used in the preparation. Also, the most important change in the density of acid sites is observed with the first addition of AC. Table 2 A1203-TiO2 mixed oxides used in the catalytic activity test and their acidity Sample Surface area Pore size Maximum acid (m2/g) distribution strength (mV) A1-Ti(0, 32)

288

A1-Ti(2.5, 32)

337

A1-Ti(4, 32)

373

Narrow monomodal Wide monomodal Bimodal

PAS 103 (meq/m2)

178

2.6

192

6.4

235

7.4

The results from the activity tests on the isomerization of 1-octene are shown in Table 3. Here it can be seen that, at the conditions of the reaction (523 K and 28 Kg/cm2), only products from the double bond shi~ were detected in the three catalysts. However, the level of total conversion increases from the less to the more acid catalyst. Also, the product distribution shows that only cis- and trans- 2, 3 and 4-octenes were produced. However, it is observed that as the acidity of the catalyst increases, the conversion to isomers where the double bond migrates farther is increased. This is a consequence of the increase in number and strength of acid sites which lead, on one side, to a higher probability of adsorption and, on the other, to a longer lifetime of the carbenium ions. Table 3 Total conversion and product distribution in the 1-octene isomerization reaction Total Cis- and trans- Cis- and trans- Cis- and transCatalyst conversion 2-octenes 3-octenes 4-oetenes A1-Ti(0, 32) A1-Ti(2.5, 32) A1-Ti(4, 32)

(wt. %)

(wt. %)

(wt. %)

(%)

53.5 45.6 44.5

30.2 33.6 33.1

16.3 20.8 22.4

78.0 85.1 89.5

815 All the above results indicate that the addition of different amounts of AC during the preparation procedure of the catalysts studied here leads to changes in their textural (surface area and pore size distribution) and structural properties. Among the later, of importance are the structural promotion in the anatase to rutile phase transformation, and the change in the coordination of the aluminium ions in the solid, leading to an enhanced population of pentahedrally coordinated aluminium ions which do not exist in the free-AC solid. These changes induce others in the surface chemical properties i.e. surface acidity (density and strength of acid sites), which in turn can affect the behaviour of different reactions on the surface of the catalyst.

4. CONCLUSIONS In conclusion, the use of ammonium carbonate in the sol-gel preparation of A1203-TiO2 catalysts allows to tune their textural properties and give more thermal stability to the anatase phase but, at the same time, produce structural changes which should be taken into account when using these solids as catalysts since they lead to the presence of pentahedrally coordinated aluminum, inducing enhanced acidity and structural thermal stability.

ACKNOWLEDGMENTS We acknowledge financial support from CONACyT (Mexico) and the IMP FIES Program.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11.

J. Ramirez, R. Cuevas, A. L6pez Agudo, S. Mendioroz and J.L.G. Fierro, Appl. Catal., 57 (1990) 223. J. Rarnirez, P. Castillo, L. Cedefio, R. Cuevas, M. Castillo, J.M. Palacios and A. L6pez Agudo, Appl. Catal., 132 (1995) 317. J.G. Weissman, E.C. de Canio and J.C. Edwards, Catal. Lett. 24 (1994) 113. E. Payen, L. Gengembre, F. Mauge, J.C. Duchet and J.C. Lavalley, Catal. Today, 10 (1991)521 W.Zhaobin, X. Qin, G. Xiexian, P. Grange and B. Delmon, Appl. Catal., 75 (1991) 179. J. Ramirez, L. Ruiz-Ramirez, L. Cedefio, V. Harle, M. Vrinat and M. Breysse, Appl. Catal., 93 (1993) 163. J. Ramirez and A. Guti6rrez-Alejandre, J. Catal., 170 (1997) 108. T. Klimova, Y. Huerta, M.L. Rojas Cervantes, R.M. Martin Aranda and J. Ramirez, Stud. Surf. Sci. Catal., V. 91, Preparation of Catalysts VI, North-Holland, Amsterdam, (1995)411. R. Cid and G. Pechi, Appl. Catal., 14 (1985) 15. U. Balachandran and N.G. Eror, J. Solid State Chem., 42 (1982) 276. Y Mizushima, M. Hori and M. Sasaki, J. Mater. Res., 8 (1993) 2109.

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91998 Elsevier Science B.V. All rights reserved.

Preparationof Catalysts VII B. Delmon et al., editors.

817

Atomically controlled preparation of silica on alumina M. Lindblad" and A. Root b ~Neste Oy, Corporate Technology, Surface Chemistry, P.O. Box 20, FIN-02151 Espoo, Finland b Neste Oy, Corporate Technology, Analytical Research, P.O. Box 310, FIN-06101 Porvoo, Finland

ABSTRACT A highly controlled growth of silica on alumina support was obtained by applying sequential saturating reactions of vaporized hexamethyldisilazane (HMDS) and air. Different degrees of modification, i.e. amounts of silica, were obtained by repeating the reaction sequence up to eight times. Evaluation of the surface species after each reaction step allowed a-detailed,. growth mechanism to be proposed. Surface structures were characterized by el&nent determination, FTIR and solid state 1H and 298i ~ spectroscopy. Potential reaction sites for HMDS were the original A1-OH groups and pairs of coordinatively unsaturated A1 and O ions (c.u.s. sites) on alumina and the new Si-OH groups formed after each reaction cycle. The high temperature (500-550~ air treatments limited the amount of OH groups on the surface through dehydroxylation and the formation of AI-O-Si and Si-O-Si bridges. Although the reactivity of the Si-OH groups was already high in the second reaction cycle, the reaction sites of the original alumina were still available up to the fifth reaction cycle. It is proposed that silica grows both laterally and vertically forming SiOx-islands, the surface of which consists of siloxane bridges and a few Si-OH groups.

1. INTRODUCTION Amorphous silica-aluminas and silicated aluminas are well-known solid acid catalysts for dehydration, isomerization and cracking reactions. The acidic properties of the catalysts are highly dependent on the 8iO2/A1203 ratio, but also on the method of preparation. Surface modification is regarded as a potential method of tailoring the surface acidity. Chemical vapor deposition (CVD) has been introduced as a means of obtaining uniformly deposited silica layers on supports. Sato et al. [1,2] and Niwa et al. [3-5], for example, have modified alumina with silica using silicon alkoxides such as Si(OMe)nMe4-n (n = 1-4) and Si(OEt)4, where Me is methyl and Et ethyl groups. The silicon alkoxides are decomposed, either thermally or in air, from gas phase onto the support. In thermal decomposition the alkoxides are proposed to form Si-O-A1 bonds with alumina and Si-O-Si bonds through

818 oligomerization reactions [5]. The extent of oligomerization increases with increased reaction temperature and number of alkoxide ligands (n) in the compound. The growth of silica coatings by decomposition of alkoxides is typically carried out at elevated temperatures where a linear dependence between the amount of silica deposited and the amount of alkoxide vaporized is obtained [ 1,3]. Within the scope of CVD, the amount of silica deposited has also been regulated by other means than the dose of alkoxide precursor. Imizu and Tada have grown silica on alumina by repeating a reaction cycle where Si(OMe)3Me is adsorbed onto the support at room temperature, the adsorbed species are thermally decomposed at 500~ and the surface complexes thus formed are oxidized in oxygen at 500~ [6]. Xu et al. have modified the surface of magnesium oxide with silica by repeated adsorption-oxidation cycles using Si(OMe)4 as a silicon precursor [7]. We employ a different approach, making use of saturating gas-soBd reactions in a technique known as atomic layer epitaxy (ALE) [8-11]. In ALE, only chemisorption of the gaseous precursor onto the support is allowed, never decomposition or condensation. Due to surface saturation, the density of the precursor is determined by the adsorption capacity of the surface, not by the dose of the precursor. In the work described, hexamethyldisilazane (Me3SiNH-SiMe3, HMDS) was selected as the silicon precursor because of its high stability. A highly controlled growth of silica was obtained by applying successive reactions of vaporized HMDS and air. Evaluation of the surface species after each reaction step allowed a very detailed growth mechanism to be proposed. The growth of silica using cycles of HMDS and air is introduced for the first time.

2. EXPERIMENTAL

2.1. Reagents The support material was 7-A1203 (Akzo Alumina 000-1.5E) with a surface area of 190 mZ/g, a pore volume of 0.5 cm3/g and an average pore diameter of 110 A. Hexamethyldisilazane ((CH3)3Si-NH-Si(CH3)3, HMDS) purchased from Fluka (purity > 98%) was used as reactant without further purification. Air treatments were carried out with synthetic air. Nitrogen (99.999%) was used as carrier and purge gas in the ALE reactor. 2.2. Equipment The catalysts were processed in fixed-bed flow-type reactors (Microchemistry Ltd.) with heated zones for the reaction vessel and for vaporizing HMDS [ 10]. The processing was done in nitrogen flow at reduced (5-10 kPa) or at ambient pressure (100 kPa). The liquid reactant was introduced into the heated zones of the reactors with liquid pumps. The vaporized reactant was then transported, in nitrogen, downwards through the solid support bed (10-60 g). The heating and gas valving were computer controlled. 2.3. Procedure Silica was grown on alumina by sequential reactions of vaporized HMDS and air. The growth mechanism was studied by preparing a series of SiOx/A1203 samples in an ALE reactor working at reduced pressure. Samples were handled inertly and transferred to the FTIR and

819 NMR instruments under nitrogen atmosphere. The alumina support (10 g) was preheated in a muffle furnace at 600~ for 16 h and, in addition, in the ALE reactor at 450~ for 3 h. HMDS vaporized at 50~ was allowed to chemisorb at 200~ onto the support surface. An excess of HMDS was vaporized to ensure surface saturation. The air treatment was carried out at 500~ leaving silicon in oxide form. For the removal of unreacted reactants, each reaction step was followed by a nitrogen purge at the reaction temperature. Increasing silica concentrations were obtained by repeating the reaction sequence up to six times. The influence of alumina preheat temperature on the interaction mechanism of HMDS with alumina was studied by preheating the support at 200, 300 and 500~ before the reaction of HMDS at 200~ A series of SiO• samples (10-20 g) with different silicon concentrations was also prepared in an ALE reactor working at ambient pressure. Processing steps were the same as those above but temperatures differed somewhat: the preheating in the ALE reactor was carried out for 6-24 h at 200~ HMDS was vaporized at 200~ and the air treatment temperature was 550~ The reaction sequence was repeated two, five and eight times. Five reaction sequences were also carried out on 60 g of alumina. 2.4. Characterization

The carbon concentration after HMDS reaction was determined with a LECO CR12 carbon analyser and the silicon concentration by X-ray fluorescence (XRF) analysis. The NMR measurements were performed on a Chemagnetics CMX Infinity spectrometer operating at 270 MHz. The ~H MAS experiments were carried out as described previously [12]. The 29Si experiments were carried out using 10 mm zirconia rotors spinning at around 4-5 kHz. The r.f. fields employed in the CPMAS experiments were around 36 kHz with a contact time of 5 ms and a recycle delay of 5 seconds. Between 10000 and 20000 transients were acquired. In the 29Si MAS experiments the recycle delay was also 5 seconds and the number of transients was between 30000 and 40000. FTIR spectra were recorded with a Nicolet 730 spectrometer in transmission mode. KBr tablets were prepared inertly in a glovebox. Spectra were obtained at 2 cm -~ resolution with a signal averaging about 16000 scans.

3. R E S U L T S AND DISCUSSION 3.1. Growth rate of silica

Figure 1 shows the growth rate of silica on alumina as a function of repeated reaction sequences of HMDS and air. The silicon concentration grew more or less linearly, at a rate of about 1 wt-% Si per reaction cycle. The same growth was obtained at reduced and at ambient pressure, on up to 60 g of alumina. The slightly curvilinear growth in SiO~/A1203 samples prepared at reduced pressure can be explained by the lower air treatment temperature that was used (500 vs. 550~ The ligand removal was not complete and with increasing number of reaction cycles an accumulation of ligand residues, Si-CH3, was observed in 1H MAS NMR spectra (see Figure 6). The ligand residues may interfere with the reaction of HMDS with the surface, either through sterical hindrance or by occupying part of the bonding sites.

820

o~

10

to

~ .0...i t_

6

r co o

co

4 2

j/

Y

,

i

r

i

1

3

5

7

,..

9

n x (HMDS + air)

Figure 1. Increase in silicon concentration (wt-%) of SiOx/A1203 samples with number of reaction cycles of HMDS and air. Samples were processed at a pressure of 5-10 kPa (+ / 10 g) and 100 kPa ( 0 / 10-20 g and O / 60 g).

3.2. Growth mechanism of silica The growth mechanism of silica on alumina was evaluated by identifying the reaction sites available and surface species formed in each HMDS and air reaction. With its highly controlled sequentially applied gas-solid reactions, the ALE technique lends itself to this kind of stepwise study. Surface structures were characterized by element determinations (Si, C), FTIR spectroscopy and 1H and 29Si MAS NMR spectroscopy. FTIK and 1H NMR were mainly used to study OH groups on the surface. Quantitative values for OH densities were estimated by ~H NMR, whereas the different OH groups were more easily distinguished by FTIR. 29Si (CP)MAS NMR proved very efficient in identifying the various silicon structures, both after HMDS reactions and after air treatment. The support surface changes with each reaction cycle, but, as the first cycle brings about the greatest changes, it will be discussed separately. 3.2.1. First reaction cycle Two types of reactive sites, i.e. hydroxyl groups (A1-OH) and pairs of coordinatively unsaturated A1 and O ions (c.u.s. sites), are available on pure alumina. The ligand exchange reaction of HMDS with OH groups is well known for various supports, whereas the dissociation of HMDS on c.u.s, sites is considerally less well known. Wharry et al. [13] have shown that the reaction on c.u.s, sites occurs on alumina preheated at 600~ The surface species formed in these two reactions are depicted in Scheme 1. Two AI-O-Si(CH3)3 groups are formed in exchange reaction with two OH groups, and simultaneously the NH group is released as ammonia. A1-O-Si(CH3)3 and AI-NH-Si(CH3)3 groups are formed in the dissociative reaction on c.u.s, sites. We observed both interaction mechanisms in the reaction of HMDS with pure alumina.

821

(CH3)3Si-NH-Si(CH3)3 dissociation 9 on pair of coordinatively unsaturated AI and O ions

exchange 9 reaction with OH groups

(6H3)3 (0H3)3

(CH3)3 (CH3)3 H O

I

H O

I

~AI---O---Ai--

Si

...... .~

I O

I

Si

I O

I

Si

+ NH3

\

~AI--O---AI---

I NH

O AI

/

\

AI

I

/

AI

Si

] O

I

AI

..

Scheme 1.

In FTIR spectra measured after the reaction with HMDS the two vibration bands observed in the C-H region around 3000 cm1 (Figure 2b) originated from methyl groups. This together with the C/Si atomic ratio of about 3 determined for these samples suggested that the Si(CH3)3 structure of HMDS stayed intact in the surface species, as proposed in Scheme 1.

TMS

~H

~H"x AZH

~8~176 ~ ~ 1 7 6 ~ ~ 1 7 6d ~ ~

~

~~176

Figure 2. FTIR spectra of alumina (a) preheated at 600~ and after reaction steps of (b) HMDS, (c) HMDS+air and (d) HMDS+ air+HMDS.

TMS AI

. . . . ~0. . . . ~ ; ; ~ ;

Figure 3.29Si CPMAS NMR spectra of HMDS/AIzO3 samples with alumina preheated at (a) 200, (b) 300 and (c) 500~

822 The reaction of HMDS with AI-OH groups was confirmed by FTIR spectroscopy (Figure 2). Unfortunately, part of the A1-OH groups in the HMDS/A1203 samples were perturbed by H-bonding interactions with methyl groups, seen as a broadening and shift of the OH bands towards lower wavenumbers (3650-3500 cm'l). This obscured the decrease in intensity of the OH bands due to exchange reactions. When subsequent air treatment removed the methyl groups, the partial consumption at least of the basic OH groups (3793 and 3774 cm-~) in the reaction with HM S became evident. A1-NH-Si(CH3)3 groups, formed only in the dissociative reaction of HMDS on c.u.s, sites, were detected by 29Si CPMAS NMR spectroscopy (Figure 3). In fact there appeared to be two different sites associated with this reaction since there were two distinct peaks at 17 and 23 ppm which increased in intensity as the preheat temperature increased i.e. as the amount of c.u.s sites increased. There was also evidence for different types of A1-O-Si(CH3)3 sites since the peak maximum for this species moved from 4 to -2 ppm as the preheat temperature increased. Since the amount of Si determined from elemental analysis was about the same for all three samples there would appear to be less A1-O-Si(CH3)3 present at higher preheat temperatures. This loss came from the low field 4 ppm peak and was probably due to HMDS reacted with associated A1-OH groups since these are more abundant at lower preheat temperatures. The peak at -2 ppm could then be due to either A1-O-Si(CH3)3 groups from the reaction of HM S with isolated groups or A1-O-Si(CH3)3 groups from the dissociation reaction with c.u.s sites. The saturation density of silicon in the HMDS/AI203 samples after one reaction cycle was about 1.5 Si atoms/nm2. The amount bonded was constant in the preheat temperature range of 200-600~ even though the reaction mechanism gradually changed with increasing temperature from exchange reactions with A1-OH groups to dissociation on c.u.s, sites. We propose, that the saturation density is mainly determined by the steric hindrance of Si(CH3)3 groups, two such structures being formed from one HMDS molecule in each reaction.

H3 CH3 C CH3 \1/

H O

Si I

H HO O OH \i/ Si I H

I

I

I

0

~

AI Ii

i

[ ....

\

0

0

0

AI

AI

Ai

I

'. . . . . .

! (-75 ppm)

/

Si

H O / \

H

H

/

0

0

0

0

A!

AI

AI

Ai

I

I

I

I

H O ! Si / \

H

H O \ $i / / \

O /

\

0

0

0

0

0

o

o

AI

A!

Ai

AI

AI

AI

AI

I

I

I

I

I

I

I

O I Ai

,. . . . . . . . . . . . . . .

II (-80 ppm)

ill (-85 pprn)

IV (-90 ppm)

V (-95 ppm)

Scheme 2.

In the air treatment of HMDS/A1203 samples the (A1-O-)Si(-CH3)3 configurations of silicon on the surface may oxidize to configurations of the type (AI-O-),Si(-OH)3.,. If two adjacent SiOH groups are able to dimerize, new Si-O-Si bridges will be formed as well. Highly simplified models of monomeric (I, II, III) and dimeric (IV, V) configurations of silicon are presented in

823 Scheme 2, together with the 29Si chemical shifts as proposed by Sato et al. [2]. These models are designed to show the formation of new Si-OH groups and oxygen bridges (A1-O-Si, Si-OSi). The surface of alumina is more complex than depicted, however [14]. Oxygen ions on the surface are not only bonded to one, but, also to two or three aluminium ions. An interesting case not shown here is the ability of the deposited silica to fill the anionic vacancies of alumina, a situation that is often assumed when silica modifications stabilize the structure of A1203 [ 15]. After air treatment of the HMDS/A1203 samples the formation of A1-Si-OH groups was seen in the FTIR spectra (Figure 2c). The wavenumber (3743 cm 1) of these groups was very close to that of Si-OH groups on pure silica (3745 cml). The 29Si CPMAS NMR spectra measured after the first HMDS and air cycle are shown in Figure 5. The signal maximum was between -80 and -85 ppm and this suggested configurations II and III (see Scheme 2) as the most probable ones. Configuration I was not observed. The broadening of the signal towards higher field was an indication of the formation of Si-O-Si bridges on the surface. Since the formation of trimers would be expected to be connected with a 29Si shift at 100 ppm [2], bridging was probably restricted to dimerization.

3.2.2. Subsequent reaction cycles In subsequent reaction cycles, not only the AI-OH group and c.u.s, sites are available for reaction with HMDS, but also Si-OH groups formed in previous reaction cycles. The growth of silica may thus proceed both laterally and vertically.

A 'V Tes

6. cycle

A,I

IIII

J

~It

?

----/"~ ] \ " ~

6. cycle

Z

c c,e 3. cycle

2. cycle

1. cycle

'

PPM''~o ....

~5. . . .

~ ....

;U

I

-60

.

.

.

.

I

-80

.

.

monomers

Figure 4.29Si CPMAS NMR spectra of reaction cycles of HMDS and air. Spectra were recorded after the HMDS reactions.

.

.

PPM

I

-100

.

.

.

.

I

-120

bulk Si(OSi)4

Figure 5. 298i M_AS NM~ spectra of reaction cycles of HMDS and air. Spectra were recorded after the air treatments.

824 The reaction of HMDS with Si-OH groups can easily be followed by 298i NMR since the SiO-Si(CH3)3 signal at 14 ppm can clearly be distinguished from the A1-O-Si(CH3)3 signal at -2 to 4 ppm and the AI-NH-Si(CH3)3 signal at 17 to 23 ppm (Figure 4). After the second reaction of HMDS, i.e. after the reaction ofHMDS+air+HMDS, a strong signal belonging to the Si-OSi(CH3)3 group appeared. Nevertheless, the reaction of HMDS with alumina was still significant. In subsequent reaction cycles, with increasing amount of silica on the surface, the relative importance of Si-OH groups as reaction sites increased and after the fifth reaction cycle HMDS was no longer reacting with sites on the original alumina. The reaction of HMDS with Si-OH groups was also observed in FTIR spectra. After the second reaction of HMDS, H-bonding was not as extensive as after the first one, and the observed decrease in the intensity of the Si-OH vibration band (Figure 2c-d) can be related to the exchange reaction. After air treatment, in contrast to the case after HMDS reaction, the vertical and lateral growths were no longer easily distinguished by 29Si NMR, because the chemical shifts of the various bridged silicon configurations formed in the two cases lie in the region -90 to -100 ppm [2]. The vertical growth can be distinguished only when three-dimensional Si(OSi)4 configurations, with a chemical shift of about -110 ppm, are formed on the surface. The formation of Si-O-Si bridges increased with Si-concentration, as shown by the shift in the maximum of the 29Si signal towards higher field with increasing number of reaction cycles (Figure 5). After six reaction cycles the contribution from Si(OSi)4 configurations continued to be very small, which means that the vertical growth exceeding three atomic layers of silicon was still negligible.

SiOl//HSiMe

AiOH//I1 ~ycles ycle

_

P

P M

'

I

5

'

"

,

I

0

"

,

,

-

Figure 6. ~H MAS NMR spectra of nx(HMDS+air)/A1203 samples, where n = 1 and 6.

825 Figure 6 shows the ~H MAS NMR spectra of SiOx/A1203 samples after one and six reaction cycles of HMDS and air. The peak centred around 2 ppm was assumed to be due to Si-OH groups [12] while the broader peak was probably due to A1-OH groups. There was some ambiguity in these assignments since some A1-OH groups do have resonances at 2.5 and 0 ppm [ 16] so the numbers of Si-OH groups derived from these spectra were the maximum that could be present. The peak at 0 ppm was almost certainly due to Si(CH3)3 groups and this peak increased in intensity with the number of reaction cycles. Table 1 compares the increase in Si-concentration with the number of OH groups determined by 1H MAS NMR as a function of reaction cycles. The total number of OH groups decreased during the first reaction cycle due to the consumption of A1-OH groups in exchange reactions and dehydroxylation forming A1-O-Si bridges. After the first reaction cycle new SiOH groups were formed and the Si-OH:Si ratio was about 1:1. This confirmed the conclusion presented above that the main silicon species at this stage were Si-OH, Si(OH)2 and to some extent Si-O-Si bridges. The total number of OH groups decreased further during the second reaction cycle, but remained more or less constant thereafter. Since the silicon concentration simultaneously increased, the OH:Si ratio declined sharply. The OH density was mainly restricted by the relatively high air treatment temperatures (500-550~ which led to a high degree of dehydroxylation (formation of bridged structures).

Table 1 Number of Si atoms (Si/nm2) and OH groups (OH/nm 2) on SiOx/A1203 samples as a function Of reaction cycles of MDS and a!r. The OH density was determined by 1H MAS NMR. Reaction cycles pure A1203 lxSiO• 2xSiOx 3xSiO• 4xSiOx 6xSiO• pure SiO2

Si/nm2 1.2 2.7 3.9 5.1 6.5

Total OH/nm 2

A1-OH/nm 2

Si-OH/nm 2

-3.0 1.7 0.9

~3.0 0.7 0.2

1.0 0.7

1.1 -1.8

0.3

0.8 ---1.8

4. S U M M A R Y

The stepwise growth of silica on alumina by saturating gas-solid reactions allowed a detailed evaluation of the surface species present at different silica concentrations. A gradual change of the surface from alumina toward silica was observed, with distinct aluminium- and siliconcontaining species in between. The reaction sites available for HMDS in the first reaction cycle were A1-OH groups and c.u.s sites of alumina. New potential reaction sites (Si-OH) were formed after each reaction cycle. Thus, the relative numbers of different reaction sites changed during the growth. The following growth mechanism was thus proposed for silica. In the first reaction cycle HMDS

826 reacted with both AI-OH groups and c.u.s, sites of pure alumina as indicated by the formation of AI-O-Si(CH3)3 and A1-NH-Si(CH3)3 surface species. The latter were formed only in the dissociative reaction of HMDS on c.u.s. Sites. After air treatment, Si-OH groups were present as well. During the succeeding reaction cycles mainly Si-O-Si(CH3)3 surface species were formed in reaction with HMDS, but reaction sites of the original alumina surface continued to be available up to the fifth reaction cycle. These results indicate that silica grew both laterally and vertically. Nevertheless, after six reaction cycles the contribution from bulk silicon configurations, Si(OSi)4, was negligible. After the first reaction cycle, A1-OH and A1-O-Si-OH groups were observed on the surface. The latter occurred mainly in monomeric and dimeric silicon configurations. Reaction with the Si-OH groups led to silica-like Si-O-Si-OH groups. The total amount of OH groups stayed almost constant during the growth, at 1 0 H / n m 2. The relatively small number of OH groups was due to the high temperature used in the air treatment, leading to a high degree of dehydroxylation and the formation of A1-O-Si and Si-O-Si bridges. After six reaction cycles this silica modification most probably was in the form of laterally and vertically grown islands, the surface of which consisted of siloxane bridges and a small number of Si-OH groups.

REFERENCES

1. 2. 3. 4. 5.

S. Sato, M. Toita, T. Sodesawa and F. Nozaki, Appl. Catal. 62 (1990) 73. S. Sato, T. Sodesawa, F. Nozaki and H. Shoji, J. Molec. Catal., 66 (1991) 343. M. Niwa, N. Katada and Y. Murakami, J. Phys. Chem., 94 (1990) 6441. N. Katada, T. Toyama and M. Niwa, J. Phys. Chem., 98 (1994) 7647. N. Katada, T. Toyama, M. Niwa, T. Tsubouchi and Y. Murakami, Res. Chem. Intermed., 9 (1995) 137. 6. Y. Imizu and A. Tada, Chem. Lett., (1989) 1793. 7. B.-O. Xu, T. Yamaguchi and K. Tanabe, Chem. Lett., (1989) 149. 8. T. Suntola, Mater. Sci. Rep., 4 (1989) 261. 9. E.-L. Lakomaa, Appl. Surf. Sci., 75 (1994) 185. 10. S. Haukka, A. Kyt6kivi, E.-L. Lakomaa, U. Lehtovirta, M. Lindblad, V. Lujala and T. Suntola, Stud. Surf. Sci. Catal., 91 (1995) 957. 11. S. Haukka and T. Suntola, Interf. Sci., 5 (1997) 125. 12. S. Haukka, E.-L. Lakomaa and A. Root, J. Phys. Chem., 97 (1993) 5085. 13. S.M. Wharry, S. J. Martin and M. P. McDaniel, J. Catal., 115 (1989) 463. 14. H. Kn6zinger and P. Ratnasamy, Catal. Rev. -Sci. Eng., 17 (1978) 31. 15. B. Beguin, E. Grabowski and M. Primet, J. Catal., 127 (1991) 595. 16. A. Kyt6kivi, M. Lindblad and A. Root, J. Chem. Soc. Faraday Trans., 91 (1995) 941.

ACKNOWLEDGEMENTS Mirja Rissanen is thanked for her contribution to the processing experiments and Alpo Toivo for the FT1R measurements.

91998 Elsevier Science B.V. All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

827

Preparation of titania supported on silica via the vapor phase grafting method: application of statistical and physico-chemical methods R. Castillo, B. Koch a, P. Ruiz and B. Delmon Unite de Catalyse et Chimie des Mat6riaux DivisEs, Universit6 Catholique de Louvain Place Croix du Sud 2/17, 1348 Louvain-la-Neuve. Belgium a Solvay S.A, Laboratoire Central, Rue de Ransbeek 310, 1120 Bruxelles. Belgium.

The parameters influencing the dispersion and the loading of titanium on silica support (support calcination temperature, grafting temperature and TiCh vapor pressure) were studied using two statistical methods. In the first method, a strategy for efficient design of experiments was used. In the second one, the significance of the parameters (or their interactions) was estimated by using a statistical model. A physico chemical characterization (chemical composition, X-ray diffraction, BET surface area, XPS and the OH- group concentration of the support) has been used to correlate the statistical results. From the analysis of the modeled responses (titanium content and dispersion) it can be concluded that an adequate control of the experimental conditions leads to titanium-silica support catalysts with a controlled amount of titanium but with different titanium dispersion. An attempt to correlate the degree of disperson with the type of reaction occurring on the surface of silica during the grafting is proposed

1. INTRODUCTION The morphology as well as the surface properties of the TiO2-silica coated supports depends on the preparation procedure. Several methods of preparation have been proposed in the literature [1-4]. In a previous work [3], a systematic study of the deposition of titanium over a silica support was realized using three different methods of preparation: precipitation with TiCh and ammonium hydroxide in aqueous solution, impregnation with titanium IV isopropoxide in isopropyl alcohol and grafting with a solution of TiCh in n-hexane. Samples were characterized by BET, pore volume, XPS, Zeta Potential, DRS, XRD, SEM and EPMA. It was shown that the method of deposition influenced deeply the morphology and dispersion of the coated oxide. With the precipitation method, TiO2 was deposited at the external surface of the silica. Impregnation provided the best external superficial dispersion of tkania allowing the formation of very small crystaUites. The grafting method gave the best internal TiOz dispersion but with formation of large crystallites at the external surface of silica. A strong interaction was found between titanium and silicon leading to the formation of Ti-O-Si bonds. The vapor phase grafting technique, where TiCh vapor reacts over a silica bed, allows an easier diffusion of the Ti inside the pores of the support particles, producing a catalyst with a high degree of dispersion. The advantage of the vapor phase grafting process over the other methods, is to avoid the use of liquid solvents during its preparation, then damage in the morphology or porosity of the support caused by the solvent. On the other hand, the vapor

828 phase process also avoids the step of recuperation of the solvent in the whole preparation process. The parameters influencing the loading and the dispersion of titanium on silica supports during the vapor grafting process are principally: 1) The calcination temperature of silica, which controls the degree of deshydroxylation of the support. The titanium content of the support is a function of the concentration of the surface hydroxyls groups. 2) The temperature at which the grafting reaction between the TiCh and the hydroxyls groups take place, and 3) the TiCh concentration (vapor pressure) in the grafting reactor. The purpose of this study is to show an approach used for studying the relative effects and their interactions of the different parameters involved during the grafting process. In the optimising step of the work, we used the design of the experiments (DOE) and the statistical analysis of the results. The latter led to mathematical models for interpreting the influence of the preparation conditions over the amount and dispersion of titanium on the silica surface. Complementary with the statistical methods, several physico-chemical methods were used: SBET, chemical composition, concentration of OH group, XRD and XPS. 2. EXPERIMENTAL

2.1. Design of the experiments The objective is to build a model that correlates the response Y (amount and/or dispersion of titania on the surface of silica) as a function of a number of specified variables X which influence the preparation conditions. A linear second order model was chosen which takes into account the first order influences, the interactions and the quadratic terms for the different variables. The model can be represented by the following equation: Y=Cte + F(X,k)+e where : Y is the modeled response X are the variables which influence the preparation Cte is the constant to be estimated F is the mathematical function k are the coefficients of the equation e is the random error variable In order to have a direct comparison scale for evaluating the relative importance of the parameters, all the variables were normalized in the interval (-1, +1). The validity hypothesis test (Student tests) was used to evaluate the statistical value of the different parameters on the Y response. The statistical analysis is valid only if there is no correlation between the variables. This is achieved with a mathematical program, which define the best experimental conditions before starting the experimental works. All these design of experiments, statistical analysis and modeling programs are internal Solvay programs. More details about the methodology adopted is given in [6].

2.2. Preparation of the support for the grafting A silica support from Grace (532) with a specific surface area of 320 m2/g and pore volume of 1.7 cm3/g and particles with average size of 100 mm was used for the preparation of samples. Before grafting, the support was calcinated in an oven, under air, during 12 h at three different temperatures: 300~ 500~ 700~

829

2.3. The grafting procedure A sample of the calcinated silica (20 g) was placed in a glass tube (interior diameter 35 mm) under a flow of dry helium at minimum fluidization velocity (6.9 crn/s). During 2 hours the sample were maintained at the temperature of reaction (or grafting) temperatures of 100, 200 and 300 ~ Then a flow of dry helium saturated with TiCh, at different saturation temperatures of 12.5, 25, 33 ~ was passed during 3 hours. Helium was then passed for 2h to remove the unreacted TiCh. The sample was cooled to room temperature under helium flow. Samples were hydrolyzed with wet helium (saturated with water at 25~ and finally calcined at 500~ in air during 12 h.

2.4. Sample characterization The hydrolyzed and calcined samples obtained in step 2.3 were characterized by the following physico-chemical methods: Specific surface area (BET), pore volume and average pore diameter, chemical composition, ESCA, X-ray diffraction and OH group concentration.

2.4.1. Specific surface area (BET) These measurements were carded out in a Micromeritics Asap 2000 equipment at 78~ using nitrogen. The specific surface area developed by TiO2 in each sample was calculated taking into account the specific surface area of the support and the amount of TiO2 deposited.

2.4.2. Chemical composition The titanium content of the different coated silicas was determined by X-Ray Fluorescence using a Phillips PW1400 apparatus. The chloride content was determined by the volumetric technique.

2.4.3. X-Ray photoelectron spectroscopy analysis (XPS) The X-ray photoelectron spectra were obtained with a Surface Science Instruments SSX100 model 206 (Fisons spectrometer) with a monochromatized A1Ka source, operating at 10 kV and 12 mA. The Cls, Ols, Si2p, and Ti2p lines were investigated and their binding energies were referenced to the Cls line at 284.8 eV. The (Ti/Si)xPS atomic ratios were obtained from the normalized intensities of the Ti2p and Si2p photoelectrons divided by their sensitivity factors, proposed by Wagner [9]. The apparent dispersion "Da" was calculated as the ratio between the atomic (Ti/Si)xPS ratio (calculated fi'om the XPS ratio intensities Ti2p3/Si2p) and the atomic (Ti/Si)Bulk ratio of the bulk solid

2.4.4. X- Ray Diffraction X-Ray diffraction patterns were taken on a Philips PW 1050 diffractometer connected to a Philips computer. The spectra were taken using the CuKtx line. 2.4.5. The hydroxyl group concentration of silica was calculated by using the gravimetric determination described by Van Roosmalen [5].

830

RESULTS AND DISCUSSION 3.1. Design of the experiments Table 1 shows the experimental conditions designed by the DOE method and used for sample preparation. Table 1 Design of experiments determined by the DOE method Support Calcination Grafting Sample Temperature (~ Temperature (~ EG1 EG2 EG3 EG4 EG5 EG6 EG7 EG8 EG9 EG 10 EG 11 EG12 EG 13

500 300 300 700 700 300 700 700 700 500 300 500 300

200 100 300 300 100 300 300 100 300 100 200 200 100

Saturation Temperature of TiC14 (~ 25.0 12.5 12.5 33.0 12.5 33.0 12.5 33.0 25.0 25.0 25.0 33.0 33.0

3.2. Physico-Chemical Characterization Table 2 shows the results of sample characterization. The BET surface area increased after the grafting of titanium. The specific surface areas developed by TiO2 are between 69.0 and 3466 m2 per gram of TiO2. The last two columns of the table shows the results of XPS analysis and the calculated dispersion "Da" of Ti. The best dispersion of TiO2 was found for the samples EG6, EG11 and EG13 (>80%) while a very poor dispersion was found for the samples EG4 and EG9 ( ~ 25%). The binding energies of the Ti 2p3 photoelectron peaks correspond to that of Ti+4 (458-459 eV). Some of the samples show values higher than 459 eV. X-ray dif~action did not detect the titanium oxides (anatase or rutile). The specific surface area (BET), and amount of hydroxyl groups as a function of calcination temperature are presented in Table 3. The silica calcined at low temperature (300~ has the largest amount of hydroxyl groups (2.10H/nm2). From Tables 1 to 3, it is observed that the amount of titanium increases if the amount of the hydroxyl groups diminishes (High calcination temperature of the support). The TiO2 content became constant above certain value of saturation temperature of TiCh. With supports treated at the same temperature, the TiO2 content increases if the grafting temperature increases. The overall structure of the catalysts changes considerably as a function of the preparation parameters. The comparison of samples EG10 (TiO2 BET surface area: 3466 m2/g; apparent dispersion Da: 72%) and EG13 (TiO2 BET surface area: 638 m2/g; apparent dispersion Da: 100%) constitutes a striking illustration of this fact. The apparent contradiction in the above figures comes from the fact that in EG 10, the highly dispersed TiO2 is mainly located inside the

831

pores, whereas relatively large TiOz particles accumulated outside the pores prevent photoelectrons from SiOz from reaching the detector in EG13. Table 2. TiOz content (% wt.), specific surface area BET, pore volume, average pore diameter, TiO2 surface area of grafted samples and X-ray photoelectron spectroscopy analysis ( XPS ). ..... TiOz BET Pore Aver. pore TiO2 % Da B.E Sample % wt. surface area volume diameter surface area (Dispere.V (mE/g) (cm3/g) (A) (m2/g) sion) Ti 2p3 EG 1 3.0 316.8 1.65 208 666 52 458.7 EG2 1.1 321.6 1.70 210 1005 74 459.2 EG3 1.8 356.8 1.87 209 2567 70 459.4 EG4 2.8 301.3 1.58 209 69 25 459.3 EG5 2.0 338.5 1.78 209 1833 57 459.0 EG6 3.0 345.2 1.80 208 1354 81 459.6 EG7 1.7 306.7 1.61 201 232 36 459.1 EG8 3.1 326.3 1.71 203 898 48 458.9 EG9 2.7 324.0 1.69 208 900 19 458.9 EG10 2.0 369.2 1.93 209 3466 72 458.8 EG 11 2.3 316.2 1.66 209 410 91 459.3 EG12 2.2 337.1 1.77 207 1720 44 458.9 EG13 1.7 319.5 1.68 210 638 100 459.1 Table 3. BET surface area, pore volume and hydroxyl concentration as a function of calcination temperature of the silica used. Calcination BET Pore Voi. N ~ OH/rim2 N ~ mol OH/g Temp.(~ ( m 2 / g ) (cm3/g) 2.1 1.08E-03 300~ 314 1.72 1.9 9.68E-4 500~ 306 1.69 1.3 6.4E-4 700~ 308 1.65

4.

STATISTICAL ANALYSIS AND MODELLING

4.1. Modelling of the titanium content For modelling the variations of the titanium content, three design parameters were used: the silica calcination temperature, the grafting reaction temperature and the TiCh solution temperature (determining the vapor pressure). As indicated above, a second order linear model was used in order to reveal the quadratic and the interaction terms. During the modelling, all the statistically non-significant terms were neglected. The variance accounted for this model is approximately 90%. Figure 1 shows the modelled evolution of the grafted titanium content as a function of the calcination and the grafting temperature, for a TIC14 solution temperature of 25 ~ The grafting and calcination temperatures have similar effects. An increase of either of them increases the amount of titanium reacting with the silica surface. Similar results are observed at other temperatures. The highest titanium contents (1,7 to 1,8 % Ti corresponding to approximately 3 % TiO2) are obtained for calcination temperatures temperatures higher than

832

600 ~ and/or grafting temperatures over 275 ~ solution temperature above 25 ~

"~ 300 *.._

Similar results are observed for a TiCh

32.5

1.7 "C. "6 2 Z 5

250

1.7

1.6

1

1.4 1.3

9

150

100 3OO

17.5

12.5 3OO

400 500 600 7O0 Support calcination temperature (oC)

Figure 1. Titanium content (%) as a function of the calcination temperature of the support and the grafting reaction temperature. TiCI4 solution temperature of 25 ~ "G

32.5

400 500 500 7OO Support calcination temperature (~

Figure 2. Titanium content (%) as a function of the calcination temperature of the support and the TiCI4 solution temperature. Grafting reaction temperature is 100 ~ "~

1.8

27.5

15

22.5

2oo

32.5

27.5

1.7 1.5

1.7

22.5

1.6 1.5

17.5

12.5

22.5

------

I ....

300

T3

400 500 600 700 Support calcination temperature (~

Figure 3. Titanium content (%) as a function of the calcination temperature of the support and the TiCI4 solution temperature. Grafting reaction temperature is 300 ~

1.3

.~

1.1 1Z5

12.5

100

150

'

200 250 300 Grafting temperature (~

Figure 4. Titanium content (%) as a function of the grafting reaction temperature and the TiCh solution temperature. Calcination temperature of the support is 300 ~

833 Figure 2 shows the chemisorbed titanium content as a function of the TiCh solution temperature and the calcination temperature for a constant grafting temperature. At a low grafting temperature (100 ~ the most important parameter affecting the titanium content is the calcination temperature. But the TiCI4 solution temperature becomes increasingly important as the grafting temperature increases. This is the only influent parameter at high grafting reaction temperature (300 ~ (Fig. 3). Whatever the calcination temperature, the titanium content increases rapidly when the TiCh solution temperature increases from 12.5 to 20 ~ Above this temperature, a saturation of the support surface occurs. Figure 4 shows the evolution of the titanium content as a function of the TiCh solution temperature and the grafting temperature for a support calcinated at 300 ~ Similar effects have been observed at other temperatures of calcination.

4.2. Modelling the titanium apparent dispersion Figure 5 presents the evolution of the titanium apparent dispersion as a function of the calcination and grafting temperatures for a TiCI4 solution temperature of 25 ~ Similar results are observed at 12.5 ~ and 33 ~ The grafting temperature does not influence the apparent dispersion of titanium when the calcination temperature is lower than 500 ~ whichever the TiCh solution temperature is. The same conclusion as previously emerges: the highest titanium apparent dispersion is obtained for the lowest calcination temperature and, at constant grafting temperature, the titanium apparent dispersion decreases as the calcination temperature increases. Between 12.5 ~ and 33 ~ the maximum titanium apparent dispersion depends on the grafting temperature.

~ 3o0

~ 32.5

'0

5~~

s (11

8 150

100 300

400 500 600 700 Support calcination temperature (~

Figure 5. Titanium apparent dispersion Da as a function of the calcination temperature of the support and the grafting reaction temperature. TiCI4 solution temperature is 25~

~

22.5

~

17.5

12.5 300

1.00 500 600 700 Support calcination temperature [~

Figure 6. Titanium apparent dispersion Da as a function of the calcination temperature of the support and the TiCh solution temperature. Grafting reaction temperature is 100oC.

834 Figure 6 shows the evolution of the dispersion as a function of the calcination temperature and TiCh solution temperatures, for a grafting temperature of 100 ~ The calcination temperature has a larger effect than the TiCh solution temperature regarding the titanium dispersion. The maximum of titanium dispersion is obtained for low calcination and grafting temperatures and high TiCh solution temperature.

5.

MECHANISTIC INTERPRETATION

5.1. Type of reaction The reaction between TiCh and silica occurs principally through the surface silanol (SiOH) groups: n(-SiOH) + TiCl 4

;

n(=SiO)nTiCl4_ n + n HCI

The n value can be between 1 and 4. This value depends on concentration of OH groups, reaction temperature and concentration of TiCh. The reactions that may occur during the process are indicated in Table 4. We mention in the same table the CI/Ti molar ratio which should be found in analysis. 5.2. Correlation between the TiO2 specific surface area and the type of reaction On this basis, it is easy to explain the correlations found. The TiO2 specific surface area depends on the type of reaction between TiCh and the silanol groups. We do not consider here the possibility that more than one monolayer of titanium could be deposited on the silica support or that the hydroxylated surface of TiO2 could also react with TiCh producing Ti "clusters".

Table 4. Types of reaction during the grafting process. REACTION I. I(-SiOH) + TIC14 --->(-SiO)-Ti-C13 + HC1 Then (C1/Ti)molar=3 II. 2(-SiOH) + TIC14 --->(-SiO)2-Ti-C12 + 2HC1 Then (C1/Ti)molar=2 III. 3(-SiOH) + TiCI4 --->(-SiO)3-Ti-C1 + 3HC1 Then (C1/Ti)molar= 1 IV. 4(-SiOH) + TIC14 --->(-SiO)4-Ti + 4HC1 Then (C1/Ti)molar=0

TYPE (OH:Ti) 1:1 2:1 3:1 4:1

If the stoichemistry (OH-TiCh) of the reaction is 1-1 or 2-1 clearly there will be more titanium fixed on the support than for reactions 3-1 or 4-1. At low content of OH groups the reactions 1-1 or 2-1 (OH:Ti) might be favoured. This explains why the samples calcined at 700 ~ have the higher TiO2 contents. At high content the reactions 2-1, 3-1, or 4-1 might be favoured, leading to lower TiO2 contents. On the other hand, it is logical the titanium contents increase if the temperature of reaction and saturation temperature of TiCh increase. This is just because the rates of all reactions increase.

835 The type of grafting reaction could influence the TiO2 specific surface area after the hydrolysis and calcination of the TiOC1X species. If this reaction is carried out by a mechanism such as 1-1 or 2-1, the TiOC13-2 formed species are still reactive and can allow reaction with each other. This would lead to the formation of TiO2 agglomerates and loss of dispersion. On the other hand for reactions 3-1 or 4-1 the probability of the interaction of TiO(OH)1-0 species during the calcination is reduced due to the strong attachment with the support. These assumptions may explain that the samples using supports with high OH populations (lower calcination temperature) show the apparent higher TiO2 surface area after calcination.

6.

CONCLUSIONS

The calcination temperature of the silica, the grafting temperature and the TiCh saturation temperature and their influence on titanium content and dispersion have been studied using the approaches of the design of the experiments and statistical analysis of the resuks. The highest titanium contents are obtained at high TiCI4 saturation temperature, whatever the calcination and grafting temperatures. The highest dispersion of the titanium on the silica is reached for the lowest calcination and grafting temperature but at TiCh saturation temperature between 20 and 30~ The model shows that it is possible to considerably modify the dispersion state of the titanium on the silica surface, keeping the titanium content constant. The control of the experimental conditions permits to obtain a catalyst with controlled titanium content and dispersion. The models can be easily interpreted considering the reactions between TIC14 in the vapor phase and the hydroxyls of the silica surface.

ACKNOWLEDGMENTS

The authors thank the Fonds National de la Recherche Scientifique (Belgium) for financing the laboratory equipment used in this research and the financial support from Solvay S.A. REFERENCES 1. P. Wauthoz, M. Ruwet, T. Machej, and P. Grange, Appl. Catal.,69 (1991) 149. 2. K. Foger, and J.R. Anderson, Appl.Catal., 23 (1986) 139. 3. R. Castillo, B.Koch, P.Ruiz and B.Delmon, J. Mater. Chem., 4(6) (1994) 903. 4. R. Castillo, B.Koch, P.Ruiz and B.Delmon, J. Catal. 161 (1996) 524. 5. A.J. Van Roosmalen, M.C.G. Hartmann and J.C. Mol, J. Catal. 66 (1980) 112. 6. D.M. Himmelblau, Process Analysis by Statistical Methods, John Wiley & Sons, Inc, New York, 1970.

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91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

837

The influence o f preparation conditions on the surface area and phase formation of zirconia A. Calafat Departamento de Bioquimica, Universidad Nacional Experimental del T/tchira, Av. Universidad, Paramillo, San Crist6bal, Venezuela

In this study, zirconia was obtained by precipitation from aqueous solutions of zirconium nitrate with ammonium hydroxide. Small modifications in the preparation greatly affected the surface area and phase formation of zirconia. Time of digestion is the key parameter to obtain zirconia with surface area in excess of 200 m2/g after calcination at 600~ A zirconia that mantained a surface area of 198 m2/g after calcination at 900~ has been obtained with 72 h of digestion at 80~ In addition, samples which had been digested for long times at 80~ are tetragonal and maintain this phase up to 900~ Finally, when the hydrous zirconia is digested at 80~ and for 72 h, the concentration of the zirconium nitrate solution has no effect neither on the surface area nor on the phase formation of the zirconia, maybe because the influence of the temperature and time of digestion is more critical, masking any possible effect of the concentration of the zirconium solution.

1. INTRODUCTION Zirconium oxide and compounds containing zirconium are increasingly being recognized as useful catalytic materials. In particular, zirconia (ZrO2) is an important support material for catalysis, due to its acid-base properties [ 1], and also because it is chemically more stable than the classic materials such 3r-alumina and silica [2]. Zirconia can also function as a catalyst by itself. For instance, it catalyses the hydrogenation of CO, olefins and dienes, as well as the conversion of synthesis gas to isobutane and isobutene [3-7]. Sulfated zirconia is a solid superacid which catalyzes alkylation, acylation and isomerization reactions [8-10] The use of zirconia as a catalyst or as a support is effective only when the compound has a high surface area that remains stable under process conditions. Zirconia prepared by several chemical methods (e.g. co-precipitation method, sol-gel method) is usually in the metastable tetragonal phase [11, 12]. At a temperature of about 600~ zirconia starts to transform from the tetragonal phase to the stable monoclinic phase, a process that is accompanied by a dramatic change in surface area of zirconia [12]. Stabilization of the tetragonal phase can be obtained with sulfated zirconia [13, 14] or when zirconia is doped with lanthana, ceria, yttria or magnesia [2, 15, 16]. It has been found that doping reduces the sintering and grain-growth rates. Also, the tetragonal phase can be stabilized by manipulating the conditions at which the hydrous zirconia precursor is obtained [17, 18]. In this regard, digestion of the precursor seems to be the key step in the preparation of zirconia with high surface area. The present work reports on the different parameters during precipitation

838 of hydrous zirconia from zirconium nitrate with ammonium hydroxide, which influence the surface area and the phase formation of zirconia, i.e., time and temperature of digestion and concentration of the zirconium nitrate solution. 2. EXPERIMENTAL 2.1. Preparation of zirconia Hydrous zirconia was obtained by hydrolysis of zirconium (IV) nitrate with a 1 M ammonium hydroxide solution. Preparations were carried out by adding the aqueous ammonia dropwise, with continuous stirring, to the zirconium nitrate solution (0.20 M, pH < 1) which was kept under reflux at a given temperature. The pH was continuously monitored with a pH meter and the ammonia solution was added until the pH of the resulting solution leveled off at 9.0. During the precipitation, the viscosity of the gel formed reached a maximum at 3.0 before exhibiting a markedly reduced viscosity with further addition of ammonia. To study the effect of precipitation and digestion temperature, the hydrous zirconia was precipitated and then digested, in its mother liquor for 48 h, at room temperature (25~ 50~ and 80~ To study the influence of digestion time, the hydrous zirconia was kept at a constant temperature of 80~ and the time of digestion was varied from 0, 24, 48 and 72 h. Additional ammonia was added throughout to maintain the pH not lower than 8.7. To study the effect of the concentration of the zirconium nitrate solution, the hydrous zirconia was precipitated and then digested for 72 h at 80~ starting from 0.10, 0.20 and 0.40 M zirconium nitrate solutions. For all the experiments, the samples were continuously stirred during the digestion process. The digested samples were vacuum filtered and washed thoroughly with distilled water until the pH of the wash solutions were nearly 7.0. The samples were dried first at room temperature for 48 h and then overnight at 100~ before calcination in a muffle furnace. A heat ramp of 5~ was used to reach the calcination temperature (600-900~ and the temperature was held for 12 h. 2.2. Characterization BET surface areas of the calcined samples were measured in a homemade apparatus, employing N2 as the adsorbate. Adsorption and desorption of the adsorbate, from a N2/He2 mixture at different N2 partial pressures, was followed using a thermal conductivity detector. Before each measurement, the sample was degassed at 150~ for 24 h. For comparison purposes, the surface areas of a few samples were measured using an ASAP 2000 instrument from Micromeritics. X-ray diffraction (XRD) was measured in a Philips PW 1730 instrument, employing Ni filtered Cu Ka radiation. The amount of tetragonal and monoclinic phase present in the zirconia was estimated by comparing the areas under the characteristic peaks of the tetragonal phase (20 = 30.4~ and the monoclinic phase (20 = 28.5 ~ and 31.6~ The height, h, and half-width, w, of these peaks were obtained after Gaussian curves were fitted to the XRD patterns, and then the percent composition of each phase was calculated from the Gaussian areas A = h x w:

% Tetragonal =

Tetragonal Tetragonal+ ~ Monoelinie

x 100

(1)

839

Monoclinic

% Monoclinic

x 1O0

(2)

Tetragonal + ~ Monoclinic

3. RESULTS 3.1. Effect of digestion temperature on surface area and phase formation of zirconia

Figure 1 shows the influence of digestion temperature on surface area and phase formation of zirconia calcined at 600, 700, 800 and 900~ It could be observed that the temperature of digestion had not a marked influence on the surface area of zirconia. For samples calcined at 600~ surface areas vary from 164 to 190 m2/g as the temperature was increased from 25 to 80~ A modest increase in surface area, within this temperature range, has also been reported by Chuah et al. [ 18]. Upon calcining the samples at progressively higher temperatures, the surface areas decreased. Regardless the digestion temperature, a sharp drop in surface area was observed when calcined from 500 to 600~ and subsequently suffered a smaller loss for 700~ upwards. Also, at any digestion temperature, surface area of the zirconia calcined at 700, 800 and 900~ were almost 75, 65 and 62% of the surface area of the sample calcined at 600~ respectively. Thus, the sintering behavior seems to be independent of the digestion temperature. Figure 2 shows that the relative amount of zirconia in the tetragonal phase greatly depends on the digestion temperature. Zirconia exists mainly in the monoclinic structure for samples digested at room temperature. The proportion of the tetragonal phase increased with digestion temperature. The effect was particularly marked when the temperature was raised from 50 to 80~ This phase, however, is not stable and the samples showed increasing amounts of the 230

1130 0600oc [

#-f t'q

t~ er

=oeo

t~ 0~

= r~

17~176176 l t 800~ 1 1900~ 1

80

150

43

' # 600oc |700~ t 800~

I

50

__

0

I

I

I

I

2)

40

60

80

Digestion Temperature (~

22 0

103

IIm

0

20

i

m

40

60

80

lO0

Digestion Temperature (~

Figure 1. Effect of digestion temperature on Figure 2. Effect of digestion temperature on surface area ofzirconia calcined at different phase formation of zirconia calcined at temeratures, different temperatures.

840 monoclinic phase as the calcination temperature was raised. 3.2. Effect of digestion time on surface area and phase formation of zirconia Figure 3 shows that digestion time has a tremendous effect on surface area of zirconia. For samples calcined at 600~ a digestion time of 72 h led to a zirconia with an exceptionally surface area of 240 m2/g. Digestion time also enhances the stability of the solid. When calcined at 900~ the surface area of the zirconia digested for 72 h only decreased 17% (198 m2/g). As the digestion time decreased the loss in surface area upon calcination increased and a sharp drop of 25-40% was observed when calcined from 500 to 600~ The decrease in surface area slowed down above 600~ Figure 4 shows the effect of digestion time on phase formation of zirconia. Samples digested for 24 h or higher showed a marked increase in the tetragonal phase when calcined at 600~ The sample digested for 72 h was 100% tetragonal and maintained this structure even after calcination to 900~ In contrast, if the precursors had been digested for times shorter than 72 h, the tetragonal phase was not stable and increasing amounts of the monoclinic phase were observed as the calcination temperature was increased. 3.3. Effect of concentration of the zirconium nitrate solution on surface area and phase formation of zirconia Figure 5 shows the influence of concentration of the zirconium nitrate solution on surface area and phase formation of zirconia. When the hydrous zirconia is digested at 80~ and for 72 h, the concentration of the zirconium nitrate solution had practically no effect on the surface area of the samples. Surface areas of the zirconia only vary from 243 to 226 m2/g as the concentration increases. The sintering behavior was also unaffected by changes in the

t600~ |700~ t 8oo~ 0900oC

100 250 r

8O 230

~D

150

,6oooci

100

4o

1700~ I t 8oo~ I

50

0900~

. . . . . I

0

3)

I

4O

t

I

60

80

Digestion Time (h)

t

01

100

0

2)

40

60

80

DigestionT/me (h)

Figure 3. Effect of digestion time on Figure 4. Effect of digestion time on phase surface area of zirconia calcined at different formation of zirconia calcined at different temeratures, temeratures.

841 concentration of the zirconium salt. All these samples were 100% tetragonal and stable for calcination temperatures from 600 to 900~

250

4. DISCUSSION 150 In the present study, zirconia with high surface area has been prepared l(I) without addition of dopants. Small modifications in the preparation greatly 50 influence the surface area and the structure obtainable from zirconia. The temperature of digestion was found to have a moderate influence on the 0,2 0,3 Q4 q5 QO 0,1 surface area of zirconia. The increase of [Zr] (M) surface area when the temperature was raised from 25 to 80~ was only of Figure 5. Effect of concentration of the 14% for samples calcined at 600~ As zirconium nitrate solution on surface area it was mentioned above, a modest and ohase formation of zirconia increase in surface area, within this temperature range, has been reported before by Chuah et al. [ 18]. These authors propose that the increase in the surface area is related to the decomposition, upon calcination, of a more dehydrated zirconia precursor, which is formed by elimination of water at digestion temperatures higher than 80~ In agreement with that, it has been found that dimerization of Zr(OH)4 by elimination of water to form oxo bridges takes place above 80~ Thus, an insufficient dehydration of the zirconia precursor due to digestion at T _< 80~ could be the reason for the moderate changes in surface area observed in this study. It is important to notice, however, that the samples prepared here have surface areas unusually high if comparing with those reported before for zirconia [2, 18, 19]. For example, zirconia obtained by digestion at room temperature and calcined at 600~ had a surface area of 164 m2/g, seven times higher than that reported by Chuah et al. using similar preparation conditions [18]. Minor changes in the experimental procedure could be the reason for the differences observed, indicating how sensitive are the properties of the resulting oxide to small modification in the preparation. For example, different for these works, present samples were continuous stirred during digestion. Stirring of the hydrous zirconia precursor could assist in the elimination of nitrate groups from the hydrous zirconia precursor and also prevent the nucleation of zirconia particles, and their extensive growth and agglomeration, leading to samples with high surface areas. Dehydration of the hydrous zirconia precursor could also affect the relative amount of tetragonal zirconia in the calcined samples. An increase in the digestion temperature greatly enhanced the formation of the tetragonal phase, suggesting that a dehydrated precursor leads to the tetragonal structure while a monoclinic phase is obtained when dehydration is not favored, which is expected to occur at low digestion temperatures [18]. However, for the sample digested at 80~ the proportion of the tetragonal phase was not better than 40% when calcined at 600~ indicating that only a partial dehydration of the zirconia precursor is achieved at 80~ In addition, this phase was not stable and the samples showed increasing _

,

I

9

I

. . . . . .

842 amounts of the monoclinic phase as the calcination temperature was raised. The conversion from tetragonal to monoclinic could be related to the number of defect sites present in the zirconia, which is influenced by preparation conditions [18]. Thus, the poor stability of the present samples indicates that a digestion temperature higher than 80~ should be needed to obtain a purer solid, where the tetragonal phase could be stabilized. Contrary to what had been observed varying the digestion temperature, the length of the digestion of the zirconia precursor had a tremendous effect on both the surface area and the phase formation of the final oxide. A digestion time of 72 h led to a zirconia with exceptionally surface areas, which were practically constant even when calcined up to 900~ In addition, these samples were 100% tetragonal and maintained this structure in the calcination temperature range used. As it was mentioned above, a more dehydrated zirconia precursor leads to high surface zirconia atter calcination. Thus, a longer digestion time should result in a bigger loss of water from the precursor, compensating the moderate effect of digestion temperature observed here. In this regard, time of digestion seems to be the key parameter to obtain zirconia with high surface areas. Longer digestion times also enhance the stability of the tetragonal phase obtained from the dehydrated precursor. This stabilization may be due to the effect of digestion on the purity of the sample. Longer digestion times could facilitate the elimination of contaminants that are incorporated within the interior of the crystals during precipitation. This leads to a crystallite growth process with little defect sites, which restrains the conversion from tetragonal to monoclinic during calcination. Decreasing the digestion time increases the number of defect sites and the transformation to the monoclinic phase is favored. Finally, when the hydrous zirconia is digested at 80~ and for 72 h, the concentration of the zirconium nitrate solution has no effect neither on the surface area nor on the phase formation of the zirconia, maybe because the influence of the temperature and time of digestion is more critical, masking any possible effect of the concentration of the zirconium solution. 5. CONCLUSIONS Zirconia with high surface area has been prepared without addition of dopants. Small modifications in the preparation greatly influence the surface area and the structure obtainable from zirconia. The temperature of digestion was found to have a moderate influence on the surface area of zirconia. An increase in the digestion temperature greatly enhanced the formation of the tetragonal phase. However, this phase was not stable and the samples showed increasing amounts of the monoclinic phase as the calcination temperature was raised. Time of digestion is the key parameter to obtain high surface zirconia. A zirconia that mantained a surface area of 198 m2/g after calcination at 900~ has been obtained with 72 h of digestion at 80~ In addition, samples which had been digested for long times at 80~ are tetragonal and maintain this phase up to 900~

Acknowledgments The author wishes to thank Ing. Martin Paz, head of the Decanato de Investigaciones from UNET, and Ing. Guillermo Mufioz for provision of laboratory facilities and valuable

843 assistance. Thanks are also due to Dr. Joaquin L. Brito, Petra Hemb.ndez, Jos6 C~iceres and Mary Labady for their help in characterization studies. REFERENCES l. 2. 3. 4. 5. 6. 7.

K. Tanabe, Mater. Chem. Phys., 13 (1985) 347. R. Gopalan, C.-H. Chang and Y.S. Lin, J. Mater. Sci., 30 (1995) 3075. Y. Nakano, T. Yamaguchi and K. Tanabe, J. Catal., 80 (1983) 307. M. Ye and J.G. Eckerdt, J. Catal., 87 (1984) 381. W.B. Johnson and J.G. Eckerdt, J. Catal., 126 (1990), 146. K. Domen, J. Kondo, K. Maruya and T. Onishi, Catal. Lett., 12 (1992) 127. Z. Feng, W.S. Postula, C. Erkey, C.V. Philip, A. Akgerman and R.G. Anthony, J. Catal., 148 (1994) 84. 8. T. Yamaguchi, AppI. Catal., 61 (1990) 1. 9. F.R. Chen, G. Coudurier, J.-F. Joly and J.C. Vedrine, J. Catal., 143 (1993) 616. 10. D.A. Ward and E.I. Ko, J. Catal., 157 (1995) 321. 11. P.D.L. Mercera, V.J.G. Ommen, E.B.M. Doesburg, A. Burggraaf and J.R.H. Ross, Appl. Catal., 57 (1990) 127. 12. C.-H Chang, R. Gopalan and Y.S. Lin, J. Membrane Sci., 91 (1994) 27. 13. R. Srinivasan, D. Taulbee and B.H. Davis, Catal. Lett., 9 (1991) 1. 14. C.J. Norman, P.A. Goulding and I. McAlpine, Catal. Today, 20 (1994) 313. 15. J.L. Shi, Z.X. Lin and T.S. Yen, J. Europ. Ceram. Sot., 8 (1991) 117. 16. P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burgraaff and J.R.H. Ross, Appl. Catal., 71 (1991)363. 17. K.S. Chan, G.K. Chuah and S. Jaenicke, J. Mater. Sci. Lett., 13 (1994) 1579. 18. G.K. Chuah, S. Jaenieke, S.A. Cheong and K.S. Chan, Appl. Catal. A, 145 (1996) 267. 19. P. Afanasiev, C. Geantet, M. Lacroix and M. Breysse, J. Catal., 162 (1996) 143.

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1998 Elsevier Science B.V. Preparation of Catalysts VII B. Delmon et al., editors.

845

Preparation of monolithic catalysts by dip coating Xiaoding Xu and J.A. Moulijn Industrial Catalysis Section, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands 1. INTRODUCTION Monolithic catalysts have several advantages over catalysts of conventional shapes, e.g. pellets, spheres, powders. The major advantages of monolithic catalysts are the low pressuredrop, easy scale-up and possible selectivity enhancement [ 1,2]. Many methods have been used to manufacture a monolithic catalyst and to incorporate the active species [3-9]. Under many circumstances slurry coating is the most convenient method to incorporate a catalytically active species with known structure and properties into a monolithic catalyst. This method has been used to prepare auto exhaust gas catalysts [6,7] and zeolite on alumina washcoated metallic monolithic catalyst for high temperature combustion [8,9]. A slurry suitable to be used in slurry coating must be stable enough, its viscosity must be in the right range, and the amount of catalyst coated each time should be sufficiently high. Aqueous slurry is the most used in slurry coating. It is recognized that commercial sols are available, which show good properties with regard to stability and viscosity. However, to avoid possible detrimental effect of impurities or additives (e.g. Na or phosphate), only homemade Al-sols were used. We have studied aqueous slurry coating of a Co-La-Al mixed oxide catalyst for N20 decomposition [ 10-12] and a reduced and passivated industrial Ni/AI203 catalyst. Both of them need a special method of preparation and the corresponding monolithic catalysts are impossible to prepare by a single step synthesis. Moreover, the latter catalyst is hydrophobic, making it more difficult to coat. Therefore, attempts were made to prepare the monolithic catalysts by slurry coating and to study the properties of the slurries. 2. EXPERIMENTAL The slurries were made by wet-milling [ 13-16] of a mixture containing a liquid (Al-sol or water) with the catalyst to be coated and other solid phase(s) and optionally additive(s). Two aluminum sols (Al-sols) were used, viz. an Al-sol prepared from pseudo-boehmite, nitric acid and urea (Sol 1) and an Al-sol prepared from hydrolysis of AIC13 (Aldrich Chemie) and aluminum granules (Aldrich Chemie, > 99 %) (So12) [5]. Unwashcoated cordierite monoliths were obtained from Coming (62 cells/cm2). Alumina (ALCOA) and/or pseudo-boehmite (SB 1 from CONDEA) were used as the solid phases. For the dip-coating procedure a reduced and passivated commercial Ni/A1203 hydrogenation catalyst and a mixed oxide (Co-La-A1) catalyst were used. The latter had a Co:La:A1 molar ratio of 3:1:1 and is a decomposition product of hydrotalcite-like compound (referred to as HTLc), prepared by coprecipitation from a common solution of the nitrates by Na2CO3 [ 11,12]. The viscosity was measured using a Contraves concentric cylindrical cone-viscometer at 293 K [5]. The shear rate increased from 0 to 780 rpm in 2 min and the viscosity at the highest shear rate is reported in the present paper.

846

A Fritsch pulverizer (ball miller) was used to grind the powders for various periods (1-3 h). The Zeta Potential (ZP) of diluted slurries was measured by using Model 501 Lazer Zee Meter TM from Pen Kern, Inc. The ZP has been corrected against temperature and is presented as ZP at 293 K. The performance of the Co-La-A1 mixed oxide (ex-HTLc) catalysts in N 2 0 decomposition was tested as described before [ 17] in a fixed-bed reactor at respectively, 473,523, 573 and 723 K, using ca. 50 mg of the catalysts. When a powder sample was used, it was diluted with SiC powder. The total flow rate was 67.53 gmol/s nitrogen containing 386 ppm of N 2 0 (W]FN2o "" 1.918 106 g.s/mol). The free Ni surface areas of the calcined and reduced samples were measured by hydrogen chemisorption using a flow method. After reduction in hydrogen at 648 K for 1 h, it was kept in nitrogen flow at 623 K for 1 h, followed by cooling to 323 K in a nitrogen flow. Subsequently, hydrogen chemisorption was measured at 323 K. The cumulative peak area due to hydrogen adsorption detected by a TCD (Thermal Conductivity Detector) was used in the calculations. 3. RESULTS Preparation of Al-sols Various amounts of pseudo-boehmite, SB 1 (15, 20 or 25 g) were used with a fixed amount of urea (10 g), and nitric acid (0.306 M, 125 ml) to prepare an Al-sol (Sol 1). A less amount of SB 1 leads to a longer gelation time. Sols prepared using this method appeared to be insufficiently stable: they gelated in several minutes to 1-2 h, which made them unsuitable to be used as a component in making a slurry by wet-milling. A1 sol prepared by hydrolysis of A1C13 and aluminum granules (So12) is stable for a period of days to months, depending on the amount of AI concentration and whether an additive is added. Fig. 1 shows an example of the viscosity of such a sol ([A1] = ca. 4.3 M) as a function of time. The sol is stable during at least several hundred hours and the viscosity can be reduced to the desired values by water dilution. The figure shows that addition of water strongly reduces the viscosity. Nickel catalysts A mixture of 1 g of the Ni catalyst powder in 125 ml of 0.306 M HNO3 acid was agitated by a disperser at 13500 rpm. The particles quickly formed a sediment. It appears that the acid could not peptize the Ni/AI:O3 particles in order to form a stable slurry. Subsequently, dip-coating was attempted using a slurry obtained by wet-milling of a mixture containing Al-sol, alumina powder, catalyst powder, and water. Table 1 shows the characteristics of some slurries used in dip coating. Note that slurries 1-4 were based on alumina powder and/or Al-sol, whereas in slurries 5-8, Ni catalyst, pseudo-boehmite (SB 1) and dilute nitric acid were applied. Table 2 shows the weight increase by dip coating using some of these slurries, and the Ni contents in the monolithic Ni catalysts. Depending on the composition, suitable slurries can be obtained which lead to reasonable weight increases (up to 27 wC % of monolith) after one round of dip coating.

847

Table 1 Data of some slurries use d for dip coating. Slurry q~ I"1 pH No. % mPa.s A1203

So12

1 2 3 4 5 6 7 8

10 5 10 -

51.4 44.7 35.8 35.8 12.8 17.1 7.13 9.95

49.81 33.96 7.92 9.06 5.66 28.30 33.96 -

1.94 1.92 1.88 1.94

40 30 20 20 -

Composition/ml H20 Ni/A1203 40 30 35 30 40 40 40 40

2.32 2.32 2.32 2.32 2.32 4.26 2.32 2.32

Tgrina SB 1

h

3.56 3.58 0.75 2.1

3 3 2.5 1 2.5 3 1 1

~p: solid fraction in slurry. So12: Al-sol by A1C1JA1 hydrolysis. Ni/A1203" reduced and passivated catalyst powder (ca. 50 #m). SB 1: pseudo-boehmite. Table 2 Weight increase and corresponding Ni contents in the monolithic Ni catalysts. Slurry No.

AW/W0 %

Ni wt.%

2 3 4 5 6

27.89 21.35 11.85 8.00 9.19

0.42 0.48 0.24 0.96 1.42

It can be seen that the Ni wt.% in the monolithic catalysts is up to 1.4 wt.% of the monolith support, which is obviously low, compared to Ni loadings by other methods [4]. The weight increase is plotted against the viscosity in Fig. 2. It is clear that there is no linear correlation between the initial viscosity of the slurry and the weight increase.

848

100 :3

800

1,

30

q,

600

t

-b

c.

20,

o 400

,,.'~ ,; ,/

>

2/

200

-t-

10 .... -I..t_

_bp t/I e<j ,

/

Water add ed .+

t/-

I-" .q.k-.r

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

I

0

1000

500

I

t

20

40

0 1500

0

t/h

I

60

I

80

100

Vis cos ity/rnPa.s

Figure 1. Viscosity of an Al-sol by hydrolysis of A1C13 and A1 (So12), as a function of time.

Figure 2. Weight increase as a function of slurry viscosity in dip coating.

In order to get insight in the systems, Zeta potential of some samples was measured (Table 3). It appears that the stable slurries (towards agglomeration) showed high ZP values, though sometimes, deposition was observed after a long time of standing (hours). Table 3 Zeta potential values (Z.P.) of some samples. Slurry Z.P. No. mV So12

+63.4

7 8

+70.1 +76.5

Sample Description

Al-sol from hydrolysis of A1 and A1C13, 1629 h after preparation Sample 7 in Table 1 Sample 8 in Table 1

It should be clear that the viscosity should not be too high, otherwise, the slurry cannot be used in dip coating. Fig. 3 shows typical viscosity profiles as a function of time for three slurries. It appears that depending on details of preparation slurries are obtained which viscosity is stable, increasing or decreasing with time.

849 70

Figure 3. Viscosity as a function of time for three slurries. The numbers refer to the slurries mentioned in Table 1.

60 ~ 5O

~ 4o

2

"~ 30 0

.. ~ 2O

10 J

0

i

20

40 ffmin

80

60

Co-La-AI catalysts For the mixed oxide catalyst (Co:La:A1 molar ratio = 3:1:1) prepared from HTI_~, direct peptizing was used. Mixtures of diluted aqueous solution of HNO3 (pH = ca. 1.8), pseudoboehmite (0.5 ml) and various amounts of the mixed oxide were ground for 1-2 h. The slurry obtained was used in dip coating. The quality of the coated layer was good. It appears that for this mixed oxide a grinding period of 1 h was sufficient to obtain particle sizes of ca. 10 ~tm and further grinding did not further reduce the particle size substantially. Hence, in further experiments one hour of grinding was used for this oxide. The weight after dip-coating increases linearly with the amount of mixed oxide (respectively 3, 5 and 15 g) used in the slurry as shown in Fig. 4.

Figure 4. Relative weight increase as a function of the weight of Co-La-A1 mixed oxide used in dip coating, dW/W0/% is the percentage of weight increase with respect to the unwashcoated monolith after one round of dip coating and calcination up to 473 K at a heating rate of 1 K/rain.

20

/ 15

.

.

r/

/

/ / / ....

0

I

I,

5

10

15

Wtmix~ oxide/g The activity in N20 decomposition of three Co-La-A1 samples is given in Fig. 5. It is expressed in activity per gram of mixed oxide.

850

300 250-

/ /

200-

/ a /

150100,/

<

5O

............. ~ . . . . . . . . . . . . . "

0 500

:3

550

_

"

-

/ b , f , - ' " " ,

?~:i. "

C

....................... t-

..4. . . . . . . . . . . . . . . . . . . .

600

650

700

750

Figure 5. Activity as a function of temperature for three Co-La-A1 catalysts. a. Monolithic catalyst block, b. Crushed monolithic catalyst and c. Original mixed oxide powder used for dip coating.

qTK

Hydrogen chemisorption was measured for dried powders of three slurries and the original commercial Ni catalyst. The results are reported in Table 4. Table 4 Chemisorption results of Ni/A1203 samples. Slurry Ni SNi/gcat No. wt.% m2/g Ni/A1203 "~ 5b 6b

16 0.96 1.42

9.7 2.85 4.60

m2/g

SNi/gNi

SNi/lcat m2/lcat

D %

60.7 296 320

9700 466 154.6

9.1 44.6 48.2

a. The original Ni/A1203 powder, b. Calculated from the weight increase and elemetal analysis results, assuming that the Ni content in the monolith is the same as in the dry slurries. 4. DISCUSSION Many excellent papers deal with the stability of suspensions (a slurry is a suspension of solid particles in a liquid) and colloidal systems [ 18-23]. A sol to be used in making a slurry for dip coating should be stable enough. At least, it should not gelate during or immediately after grinding. Due to the short gelation time, Sol 1 from pseudo-boehmite, urea and nitric acid is not suitable for the purpose, whereas So12 from A1C13/A1hydrolysis is sufficiently stable as shown by Fig. 1. Moreover, its viscosity is satisfactory, and, if desired, can be reduced by dilution with water. Fig. 2 shows that some slurries with similar viscosity values lead to quite different weight increases, i.e. there is no satisfactory correlation between the weight increase and the viscosity of the slurry. This result is understandable since viscosity is an intrinsic property of the slurry and the weight increase depends on the interaction between the slurry and the monolith walls, besides the properties of the slurry. There are two types of stability, viz. stability against sedimentation and against flocculation.

Stability against sedimentation The Stokes terminal sedimentation velocity vsed [m/s] can be estimated from [21 ]"

851

v,.,a =

g(p.,. - p t)dp 2

18r/t

(1)

Here, g is the constant of gravity, 9.81 m/s 2, dp, the particle diameter [m], rll the viscosity of the liquid [Pa.s], Ps the solid's density [kg/m 3] and Pl the liquid's density [kg/m3]. When the particles are close to each other, as in the case of slurry with a high solid content, hindered settling occurs. This is often formulated in terms of a correction factor h(~):

%,i =

g(P.,. - Pl )df,2h(q ~)

18r//

(2)

h(q~) is a function of the volume fraction of the particles, d0, in the fluid. For q~from 0 to 0.4, the factor h(~) is given by Happel and Brenner [20]: h(~b) = 3 + 4.5(05'3 _ q~,3)_ 302 3+2~b5/3

(3)

From Fig. 3, the viscosity of slurry 2 increases with time, presumably due to an on-going polycondensation process, which is common in sol-gel systems. The viscosity of slurry 1 decreases fast at the beginning. Later on it becomes stabililized. This can be explained as follows. In a slurry particles of various sizes exist. As smaller particles sediment more slowly than the larger ones [eqs. 1 and 2], the sedimentation velocity becomes smaller as the time on standing becomes longer, i.e. the suspension becomes more stable with the time [ 18], provided that polycondensation process does not proceed at a substantial rate which may lead to the growth of particle size and increase of viscosity. Slurry 3 shows a constant viscosity, due to a high stability. According to Einstein equation, for a dilute system of non-interacting spheres the viscosity of the system, Tlsystemcan be described as follows [ 19]:

1 +0.5~ 7/.,.,..,.,.... = (1 _,~ ) 2 r/l

(4)

Here, Tlsystemis the viscosity of the slurry [Pa.s], rll is the viscosity of the liquid [Pa.s], and 0 is the solid volume fraction (dimensionless). From equations 2-4, it is clear that there are several methods to reduce the sedimentation velocity, Vsed,viz. by reduction of dp (by milling), increase of viscosity, rll, or solid fraction, ~ (by adding suitable additives or use a higher solid fraction). The reduction of particle size by milling is the most effective. In practice, wet milling is often preferred to dry milling [6-9] since it has various advantages, e.g. high energy efficiency, and easy addition of additives [ 14-16].

Stability against flocculation A slurry is thermodynamically unstable, due to its large surface area [21,23]. When the solid powder is dispersed in a solvent, e.g. water, kinetic stability can originate from the electric

852 double layer. According to the double layer theory [ 19, 21-23], the surface of charged particles is surrounded by ions with opposite charges, which increases stability and actually increases the particle diameter. For example, the surface of solid particles is normally charged in water, e.g. metal oxide sols tend to be positively charged [21,22]. The inner shell of charge and the outer atmosphere is called the electronic double layer. The thickness of the double layer depends on the ionic strength, a high ion concentration and high ion strength lead to a thin double layer, leading to instability of the solid particles against coagulation or flocculation. In aqueous solution, the concentrations of charged ions as H + and OH- as well as the charge on the particles are closely related to the pH and the concentration of other ions [21 ]. Here, an important parameter is the so-called electrokinetic potential or Zeta potential. Large values of Zeta potential mean stability of the slurry or colloidal system since electric repulsion between pal~ticles of the same charge stabilizes the system against coagulation. Stability can also be achieved by adsorbing a polymeric long-chain compound with hydrophilic and hydrophobic groups, e.g. a surfactant, leading to steric stabilization [ 19,21,23]. As seen in Table 3, for the three samples measured their Zeta potential values are all high (> +60 mV) and they are also stable. The reduced and passivated Ni catalyst is hydrophobic. As a result, the mixture of the catalyst powder and a dilute nitric acid was not miscible even after dispersion at 13500 rpm. It shows that dispersion of this system at a high shear rate cannot lead to a slurry suitable for dip coating of monolith. In contrast, wet grinding (milling) with alumina powder and/or a suitable sol increases the coatability of the mixture even when the solid to be dispersed is hydrophobic. That the addition of alumina and pseudo-boehmite to the Ni/A1203 catalyst increased its stability can be envisaged as a creation of a high solid fraction and probably the production of AI-O-AI polymeric structure on the surface of the otherwise, hydrophobic Ni catalyst. This leads to steric stabilization. Nevertheless, the condensation reaction of this polymer should not proceed too fast. When the viscosity is too high it is no longer suitable for use in dip coating. The mixed oxide from HTLc is hydrophilic and its slurry is easier to obtain than with the Ni catalyst. Simple peptizing by an acid with the addition of a little pseudo-boehmite (precursor of an Al-sol) leads to a stable slurry, suitable to be used in slurry coating. An advantage of this method is that little extra material is added to the mixed oxide, leading to a minimum change of the structure and a high loading of the catalytically active phase. The observation that the weight change increases almost linearly with the amount of the mixed oxide used (Fig. 4), indicates that a similar volume of the slurry is coated per unit of the monolith surface, since the volume fraction of the mixed oxide in the slurry is proportional to its weight used in preparing the slurry. Fig. 5 shows the performance of three Co-La-A1 catalysts, viz. the original mixed oxide powder used for dip coating, the crushed monolithic catalyst and the monolith block, at various temperatures. Note that the activity is expressed in mole N20 converted per gram of mixed oxide per second. It is obvious that the crushed monolithic catalyst and the monolith block are much more active than the original powdered catalyst, especially at high temperature. An explanation is that the dip coated monolithic catalyst behaves like a catalyst with an eggshell distribution of the active phase. The block of monolithic catalyst showed an activity about five times that of the powdered sample at 723 K, showing the advantage of using monolithic catalysts. For the dip coated Ni/AlzO 3catalyst no performance tests were carried out. Some samples (cf. Table 4) were used to measure the free nickel surface area, which is a crucial parameter with respect to the catalytic activity. It is remarkable that for dried powder of slurries 5 and 6, the

853 dispersion values are much higher than that of the original catalyst after the same pretreatment (44 and 48 % versus 9.1%). A plausible explanation is that the solid from a slurry is more stable towards high temperature reduction. It might also be possible that the structure and dispersion of the nickel in these oxides are modified by the dip coating process. The Ni contents mentioned in Table 4 might actually be lower limit because they have been calculated from the weight increase in dip coating and Ni content in the slurry. It might well be that some Ni enrichment in the monolithic catalyst may take place. When comparing dip coating and impregnation for Ni monolithic catalyst, it is clear that dip coating may lead to higher dispersion but lower loading. As to the composition of slurries, the peptizing method (e.g. slurry 5-8 in Table 1) is preferred to the method involving alumina and Al-sol (slurry 1-4), as in the former case a higher Ni content in the monolithic catalyst is resulted. 5. CONCLUSION Two solid powders, an ex-hydrotalcite-like mixed oxide and a Ni/A1203 catalyst, have been successfully coated onto an unwashcoated monolith using the slurry coating method. Preliminary performance results show that the dip-coated mixed oxide catalyst is much more active than the original catalyst powder in N20 decomposition per weight of the mixed oxide. Stability against sedimentation can be reached by using a high solid volume fraction, a small particle size and a liquid with a high viscosity. Zeta potential gives an indication of the stability of a slurry system towards flocculation. High Zeta potential means a high stability of the colloidal system against flocculation, via electrostatic repulsion between charged particles and/or steric stabilization. An aluminum sol from hydrolysis of A1C13/A1 is stable and suitable to be used in making a slurry for dip coating, due to its suitable viscosity and high stability against gelation. Further research is needed to optimize the properties of the slurry and the eventual catalytic performance of the catalyst by slurry coating. The relatively low nickel loading in the monolithic catalysts remains to be a point of concern.

Acknowledgement We thank Mr. T. Xie for his work on the mixed oxide and Mr. J. Teunisse and Mr. J.C. Groen for the chemisorption measurements.

References 1. S. Irandounst and B. Anderson, Catal. Rev., Sci. Eng., 30 (1988) 341. 2. A. Cybulski and J.A. Moulijn, Catal. Rev., Sci. Eng., 36 (1994) 179. 3. X. Xu and J.A. Moulijn, Transformation of a Structured Carrier into a Structured Catalyst, in A. Cybulski and J.A. Moulijn (Eds) Structured Catalysts and Reactors, Marcel Dekker, New York, 1997, p. 599. 4. X. Xu, H. Vonk, A.C.J.M. van de Riet, A. Cybulski, A. Stankiewicz and J.A. Moulijn, Catal. Today, 30 (1996) 91. 5. X. Xu, H. Vonk, A. Cybulski and J.A. Moulijn, in G. Poncelet et al (Eds), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 1069.

854 6. Z. Hu, C.Z. Wan, Y.K. Lui, J. Dettling and J.J. Steger, Catal. Today, 30 (1996) 83. 7. R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control, Van Nostrand Reinhold, New York, 1995. 8. M.F.M. Zwinkels, S.G. J~ir~s and P.G. Menon, G. Poncelet et al (Eds), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 85. 9. M.F.M. Zwinkels, S.G. J~irfts, P.G. Menon and K.I. Asen, J. Materials Sci., 31 (1996) 6345. 10. F. Kapteijn, J. Rodriguez-Mirasol and J.A. Moulijn, Appl. Catal. B, 9 (1996) 25. 11. C.S. Swamy, S. Kannan, Y. Li, J.N. Armor, T.A. Braymer, US Patent 5407652 (1995). 12. J.N. Armor, T.A. Braymer, T.S. Farris, Y. Li, F.P. Petrocelli, E.L. Weist, S. Kannan, and C.S. Swamy, Appl. Catal. B: Env. 7 (1996) 397. 13. C. Tangsathitkulchai, Powder Technology, 59 (1989) 285. 14. G.C. Lowrison, Crushing and Grinding, the Size Reduction of Solid Materials, London, Butherworths, 1974. 15. R. Verma, R.K. Rajamani, Powder Technology, 1995, 84 (1995) 127. 16. V. Blachou, D. Goula and C. Philippopoulos, Ind. Eng. Chem. Res., 31 (1992) 364. 17. F. Kapteijn, G. Marban, J. Rodriguez-Mirasol and J.A. Moulijn, J. Catal., 167 (1997) 256. 18. R.D. Nelson, Handbook of Powder Technology, vol. 7, Dispersing Powders in Liquid, Elsevier, Amsterdam, 1988. 19. T.C. Patton, Paint Flow and Pigment Dispersions, 2nd ed. John Wiley, New York, 1979, pp. 16, 121,152,183,206. 20. T.A. Ring, Fundamentals of Ceramic Powder Processing and Synthesis, Academic Press, San Diego, 1996, USA. 21. P.C. Hiemenz, Principles of Colloid and Surface Chemistry, 2nd ed. Marcel Dekker, Inc. New York, 1986. 22. J. Lyklema, in C.E. Capes (Ed) Proc. 4th Int. Symp. on Agglomeration, Toronto, 1985, Iron and Steel Soc. USA, Chelsea, MI, USA, pp. 23-36. 23. T.G. Vernardakis, Pigment Dispersion, in D. Sates (Ed), Coating Technology Handbook, Marcel Dekker, Inc. New York, 1991.

91998 Elsevier Science B.V. All rights reserved.

Preparationof CatalystsVII B. Delmon et al., editors.

855

Preparation and characterization of SiC microfibers and Cr3C z with m e d i u m specific surface area for catalytic applications Marc J. Ledoux #, Nicolas Keller, Cuong Pham-Huu, Claude Estourn~s*, Baudouin Heinrich, Helen Lamprell and Elina M. Harlin** Laboratoire de Chimie des Mat6riaux Catalytiques, ECPM, GMI-CNRS, Universit6 Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg, France. # To whom all correspondance should be addressed * Groupe des Mat6riaux Inorganiques, ECPM-CNRS, Universit6 Louis Pasteur, 23, rue du Loess, 67037 Cronenbourg, France ** Helsinki University of Technology, Department of Chemical Technology, Kemistintie 1, PO Box 6100, HUT-2015, Finland

Silicon carbide fibers and chromium carbide with medium surface area were prepared according to a gas-solid reaction between oxidic vapors and solid carbon with the very peculiar property of preserving the original macroscopic shape of the carbon; for this reason this method has been called "the shape memory synthesis". In consequence, no additional shaping was required for catalytical uses. XRD, surface area and pore size measurements, scanning electron microscopy and transmission electron microscopy were used to characterize the materials. Molybdenum oxycarbide supported on the silicon carbide microfibers displayed an equivalent n- heptane isomerization activity and selectivity compared to those obtained on silicon carbide conventionally shaped in the grain form. Chromium carbide or more probably a chromium oxycarbide obtained by modification of the starting material was found to be a good n-butane dehydrogenation catalyst with high selectivity towards C4 olefin molecules.

I. I N T R O D U C T I O N During the last three decades, high surface area carbides have been investigated for their potential catalytic applications, as catalyst supports or active phases, due to their unusual physical and chemical properties allowing the development of catalysts with high performance and stability compared to the traditional ones [1]. Several preparation methods have been developed in order to obtain carbides with high enough surface areas to be efficiently used in the heterogeneous field [ 1-8]. However, a large part of the preparation methods is based on the synthesis of powder materials which are limited in their application because shaping of carbide

856 powders is often very difficult and, in addition, needs a long and delicate monitoring of the different parameters during the carburization period. For the transition metal carbide syntheses, whatever the method used, the final carbide surface is generally polluted by polymeric carbon which necessitates an activation period of decontamination before catalytic use. It is of interest to find new preparation methods which could overcome the different drawbacks cited. Ledoux et al. [5] have developed a "shape memory" concept of synthesis based on a gas-solid reaction which allows the obtention of several types of high surface area carbides with different sizes and shapes depending on their final uses, i.e. catalyst support or active phase. The new concept proceeds from the opposite direction than the conventional synthetic route developed by Boudart and coll. [2,4] for the transition metal carbides i.e. temperature programmed reaction of a metallic oxide treated with a carbon provider molecule (CO, CH 4.... ). In other respects, the conventional synthesis of SiC is a solid-solid reaction between SiO 2 and C, the Acheson Process. The "shape memory concept" is based on the reaction, at relatively low temperature, between a solid pre-shaped carbon and a vapor or a liquid oxide, SiO for SiC or MO x for MCy (MoO 3 vapor for M%C, Cr203 vapor for Cr3C2 or V205 for VsC7) [6,9]. In addition, depending on the reaction conditions employed, the final carbides can be obtained in an unsupported or a supported form, according to the requirements of their end-uses. The aim of the present article is to report the synthesis, the characterization and the catalytic uses of two families of carbides, support and active phase, prepared according to the gas-solid reaction described above. The shape memory concept is applied to the synthesis of microfibers of SiC starting from microfibers of carbon. In fact because of the peculiar mechanism of reaction, the final product is made of hollow fibers which can be easily transformed into microtubes by breaking the fibers and opening the closed extremities. These tubes can easily support an active phase like the conventional SiC carrier in grain form which is demonstrated by the impregnation of MoO 3, its subsequent transformation into Mo oxycarbide and the test reaction of n-heptane isomerization. The method leading to a transition metal carbide with a carbon free surface is applied to the synthesis of high surface area chromium carbide, Cr3C2. This material is tested for n-butane dehydrogenation and compared to more conventional catalysts. The materials are characterized by several techniques such as powder X-ray diffraction (XRD), surface area and pore size measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetry analysis (TGA).

II. E X P E R I M E N T A L SECTION

II.1 Preparation method The principle and the apparatus devices have already been reported in detail elsewhere

[5-6]. Briefly, the carbides were synthesized by a reaction between a solid carbon sample and the chosen oxide vapor, SiO for SiC and CrO 3 for Cr3C 2. The reaction was carried out under dynamic vacuum at temperature between 1200 and 1300~ The reaction duration varied between 7 and 15 h. The gaseous products (CO and/or CO 2) formed during the course of the

857 synthesis were pumped off from the reaction zone in order to displace the equilibrium towards the carbide formation. Sometimes after synthesis a small part of unreacted carbon remained in the core of the material and a subsequent calcination was performed in order to burn off this remaining carbon (obviously, only in the case of SIC).

II.2 Materials The silicon powder (purity > 99.5%) was purchased from Janssen with a granulometry between 10 and 20 mm. The silica powder (purity > 99.9%) was supplied by Merck with a granulometry between 10-30 mm. Two types of carbon source were used : low surface area carbon microfibers (Carbone Lorraine, purity > 99.0%, 0.1 m2.g ~) for the SiC synthesis and high surface area activated charcoal (Fluka, purity > 99.0%, 900 m2.g1) for the Cr3C 2 synthesis.

II.3 Characterization Powder X-ray diffraction (XRD) was carried out on a Siemens model D-5000 diffractometer with CuKa radiation. Surface area and porosity measurements were carried out on a Coulter SA-3100 sorptometer with N: as adsorbant at liquid N 2 temperature using the BET method. Before each measurement the sample was outgassed at 200~ for lh in order to desorb impurities from its surface. Scanning electron microscopy (SEM) was performed on a Jeol model JSM-850 working at 20 kV and 10 mA. The sample was covered by a thin layer of gold in order to avoid problems due to the charge effect during the analysis. Transmission electron microscopy (TEM) was carried out on a Topc0n model E002B UHR operating at 200 kV equiped with a berylium window, with a point-to-point resolution of 0.17 nm. To prevent artifacts due to contamination, no solvents were used at any stage and samples were prepared by grinding the material between glass plates and bringing the powder into contact with a holey carbon-coated grid.

II.4 Catalytic applications II.4.1 Selective isomerization of n-heptane The n-heptane isomerization tests were carried out in a microreactor unit working under medium total pressure (7 to 70 bar). The reactor consisted of a copper-lined stainless steel tube with an inside diameter of 4 mm and of a length of 300 ram. The liquid feed was dosed with a high-pressure liquid chromatography (HPLC) pump (Varian Model 9001) equipped with a non return valve and injected into the H 2 stream. The H 2 flow was regulated by a Brooks 5850 TR flowmeter linked to a Brooks 5876 control unit. The reactant mixture was subsequently vaporized at the top of the reactor. The catalyst sample was placed between quartz wool wads in the center of the reactor. The reactor was heated in a vertical furnace controlled by two thermocouples and the temperature was regulated by a Coreci controller. The reactor pressure was regulated by an IMF back-pressure regulator allowing the reactor to be operated between 7 and 70 bar. The gaseous products were analyzed off-line by gas chromatography (GC)

858 equipped with a flame ionization detector (FID) using a HP-PONA capillary column coated with methyl siloxane (50 m x 0.2 mm inner diameter, film thickness 0.5 mm), allowing the separation of hydrocarbons from C 1 to C10.

II.4.2 Dehydrogenation of n-butane The n-Butane dehydrogenation reaction was carried out at 550~ at atmospheric pressure. The HE:n-C4H10 molar ratio was fixed to 9 at a Weight Hourly Space Velocity (WHSV) of 2h ~. The catalyst, in grain form (0.3-1 mm), was placed on a quartz fritted disk and the reactants were passed upwards through the catalyst bed. The H E and n-butane flow were regulated by Brooks 5850 TR flowmeters linked to a Brooks 5876 control unit. Steam (24 Torr) was provided by a saturator kept at room temperature. The reaction products were analyzed off-line using a PONA capillary column working at -15 ~ allowing the separation of the olefins and saturated C 2 to C 4 molecules. Regeneration was performed under O2-He (20 vol.%) at the reaction temperature for about 30 minutes. The catalyst was flushed with helium before and after each regeneration.

III. RESULTS AND DISCUSSION IlI.l Preparation, characterization and catalytic use of SiC III.l.1 SiC synthesis Depending on the reaction conditions used, i.e. duration, temperature, Si/SiO 2 molar ratio and C/Si+SiO 2 mass ratio, the C ~ SiC conversion varied significantly from 17 to 78% (the conversion was calculated on the basis of converted initial carbon counting that 2 moles of carbon gave 1 mole of SIC). Five examples are given in Table 1. Even with a long reaction duration, ca. 15 h, the conversion was not total and unreacted carbon remained in the core of the material. Such a phenomenon was explained by several factors: (i) the carburization proceeded via a Shrinking Core Model [10,11] where the first layers of SiC acted as a barrier for subsequent S i t vapor diffusion and thus, decreased the apparent carburization rate, (ii) the carbon fibers had a very low surface area (< 1 m2.g l) exhibiting few nucleation sites for the initial SiC formation. A similar observation has been reported by Moene [8] who observed that the number of nucleation sites on activated charcoal was higher compared to that observed on graphite material because of a more grained structure. The highest surface area was obtained at the lowest reaction temperature (low conversion). Such a phenomenon was explained by the surface diffusion process inducing a collapse of the pores as a function of the reaction temperature and duration [ 12]. However, the results obtained for all the experiments have shown that pure SiC with surface area of at least 30 m 2 g-~ can easily be prepared by this method. Such a surface area was expected to be high enough for catalytic applications. The carbide was calcined in air after synthesis at 600~ for 2h to remove the unreacted carbon located in the core of the material and to transform the composite fibers (SiC + unreacted C) into microtubes of SiC.

859 Table 1 Surface area distribution of the SiC microfibers as a function of the synthesis parameters. The surface area ofthe samples after Calcination inair at 600 ~ for 2 his also reported. Duration, temperature, molar ratio Si/SiO 2 and mass ratio (Si/SiO=)/C

run run run run run

1= 2= 3= 4= 5=

15h / 1300~ 15h / 1300~ 15h/1300~ 15h/1200~ 15h / 1300~

/ / / / /

1/ 2/ 1/ 1/ 1/

Conversion

Surface area after synthesis (m 2. g-l)

Surface area after calcination (m 2. g-l)

Pore volume after calcination (cm 3. g-t)

60 66 85 18 46

18 13 18 25 21

45 24 30 66 52

0.111 0.102 0.123 0.139 0.190

8 8 12 8 8*

* the carbon fibers were pre-ground

IlI.1.2. Structural and physical properties The XRD pattern of the material after synthesis is presented in Fig. l a and shows that the only crystalline phase present on the material was/3-SIC crystallized in a cubic structure.

1500

i,

~

i

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0.025

i

i

i iiill

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0.02 -

o

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~ 9 1000-

"

<

t..

I

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S B~_r~18 m2.g "+

Carbon fibers SBET=Im2.g"1

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0.01 -

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i ltll[

r

500-

0

i

-'= aftercalcination SBzr--30 m2.g 1 --O-- aftersynthesis

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= %

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40 60 80 100 Two-Theta Angle (deg,)

120

.

0

i'

1

"'l

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_

~,, j ~ + t ~

100

~

+

+ i Illl

1000

Pore radius (nm) Figure 1" XRD patterns of the (a) SiC fibers after synthesis showing the presence of carbon and (b) SiC fibers after calcination in air at 600~ for 2h.

Figure 2" Specific surface area and pore size distribution of the (a) starting carbon fibers, (b) SiC fibers after synthesis and (c) SiC fibers after calcination in air at 600~ for 2h.

860

Figure 3: SEM micrographs of the (a) starting carbon fibers, (b, c) SiC fibers before calcination and (d) SiC fibers after calcination in air at 600~ for 2h.

861

A careful examination of the diffraction lines, however, revealed the presence of a small amount of a-SiC (hexagonal structure) in the final material. A detailed description of the SiC microstructure obtained by this method has already been published [ 13]. For the sample after synthesis a small hump on the XRD pattern was observed at low diffraction angles which was attributed to the remaining carbon. After air calcination at 600~ for 2h only the SiC phases were detected (Fig. l b), any other phases could only be either in an amorphous form or too dispersed to be detected by the XRD technique. The surface area of the carbon material was significantly increased after carbidation, from < 1 m 2. g~ to 18 m 2. g-~ in Fig. 2 corresponding to run 3 of Table 1, due to the development of mesopores ( 2 to 10 nm) (type IV adsorption isotherm). This behaviour was explained by the fact that during the transformation the morphology of the solid non-porous carbon was transformed by the SiC nucleation in a porous structure, probably created by channels necessary for the extraction of CO formed during the reaction. In fact qualitative observations have shown that the partial pressure of CO in the reaction chamber was playing a significant role on the porosity of the final product. This is well illustrated by the SEM micrographs (Figs. 3) which showed that the smooth surface of the carbon fibers was replaced by holes and cracks on the SiC surface before calcination, while the macroscopic morphology of the material was retained. The surface area of SiC of the non-calcined product was underestimated due to the presence of low surface area residual carbon inside the material as evidenced by XRD and thermogravimetry analysis (not shown in this article). The number of pores and the surface area of SiC after calcination at 600~ were significantly increased (see Table 1 and Fig.2c. The t-plot curve showed a straight line crossing the origin which meant that no micropores were present in the material. The pore size distribution was unchanged when compared to that obtained before calcination, meaning that this calcination only burnt the unreacted carbon without extensive modification of the SiC surface. This implied that the formation of a silica layer on the surface did not influence the increase in surface area. In order to check this, the sample after calcination was treated at 80~ for l h in soda medium to remove the silica layer present on its surface. The surface area obtained after this treatment remained unchanged. Fig. 3c clearly indicates that the carbon contained in the core of the fibers was burnt off, transforming the microfibers into microtubes. n-Heptane isomerization on the MOOxCysupported on SiC Both SiC hollow fibers (or microtubes) and SiC grains (0.4 - 1 ram) were impregnated with an aqueous solution of heptamolybdate, dried and calcined at 500~ in order to obtain about 30% w. of MoO 3 supported on SiC. Using the same techniques as previouly published [14] these two catalysts were tested at 7 bar under a mixture of hydrogen and n-heptane in a ratio 29 to 1. Both catalysts needed about five hours to reach their maximum activity (specific rate -- 170. 10 .7 m o l e / g ' s ) and a stable isomerization selectivity around 90%. Figs. 4a and 4b clearly indicate that there was no difference between the two catalysts. The SiC grains were obtained from activated charcoal while the fibers came from carbon. In both cases the final surfaces were identical and able firstly to disperse MoO 3 and secondly not to interfer chemically with MoO 3 , allowing the formation of the MoOxCy oxycarbide found active and selective for the isomerization of alkanes [14]. This was not the case with conventional supports such as III.1.3.

862 alumina for instance which blocks the formation of the active phase rendering impossible the quasi topotactic transformation of MoO 3 into MOOxCybecause some oxygen atoms are shared between aluminium and molydenum atoms [ 15].

200

I

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I

f l=:

o

100

I

~

90-

150

"~ 80L

r

n,,'

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SiCgrains (0.4-1mm) ,,

I

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2O

70-

25

50

0

- . - o - - SiCmicrofibers ., = SiC grains (0.4-1mm) j 5

110 , 15 Time on stream/h

, 20

25

Figure 4a: Specific rate of reaction of n- Figure 4b: n-Heptane isomerization selectivity heptane on MOOxCy supported on microfibers on MOOxCy supported on microfibers and and grains of SiC. Reaction conditions: total grains of SiC. Reaction conditions: total pressure=7 bar, temperature=350~ H2:n-pressure=7 bar, temperature=350~ H2:nC7H16=29.

C7H16=29.

Figure 5: High-resolution transmission electron micrograph of the SiC microstmcture showing the presence of an amorphous layer on the surface. EDX carded out using a small beam revealed the presence of Si, O and C.

863 The ability of SiC to disperse oxide or metal on its surface was attributed to the presence of a thin amorphous layer of silicon oxycarbide and/or silicon oxide which displays unexpected properties. This amorphous layer is presented in Figure 5.

III.2 Preparation, characterization and catalytic use of CraC ~ III.2,1 CraC 2 synthesis Various parameters and results are summarized in Table 2. Similar observations can be made for these results to those obtained for the SiC synthesis according to the Core Shrinking Model of the gas-solid reaction. The final product constituted a composite material with the same macroscopic shape as the starting activated charcoal, and containing chromium carbide supported on a carbon core. No other chromium phases, i.e. metal or sub-oxides, were ever observed in the XRD patterns.

Table 2 Surface area of Cr3C 2 as a function of the synthesis parameters. The surface area of the initial activatedcharcoal measured by N2 was between 800 and 900 m 2. g-1. .... Duration, temperature and molar ratio C / C r O 3

Run 1 = 7h / 1300~ / 6 Run 2 = 7h / 1210~ / 6 Run 3 = 7h / 1250~ / 5

BET Mesoporous surface area surface area (m 2. g-l) (m 2. g-l)

63 77 91

48 57 68

Microporous surface area (m 2. g-l)

Pore volume (cm 3. g-l)

15 20 23

0.133 0.159 0.180

III.2.2 Structure and physical properties The crystalline phases observed by XRD are presented in Fig. 6. Only Cr3C 2 with a single diffraction peak of graphite could be found. The diffraction lines corresponding to chromium carbide were sharp, meaning that the carbide particles were large and probably did not contribute to a large extent to the overall surface area of the catalyst. An amorphous surface phase, non detectable by XRD, was probably at the origin of the non negligeable mesoporous area, while most of the microporous area was probably due to the remaining unreacted activated charcoal.A similar situation was already observed with the Mo,W or V carbides syntheses [6,9]. This should be confirmed by future high resolution TEM analyses as has been shown on Mo, W or V carbides. Fig. 7 shows that even after reaction at a relatively high temperature most of the pores had a size signicantly lower than 10 nm. However the size distribution was very different from those of the starting charcoal which mainly contained micropores. As shown with the synthesis of SiC from very low surface area carbon fibers, the formation of micro and

864 mesopores during the synthesis process was likely to be quite independent from the microstructure of the starting material, even if the macro shape was kept, and that most of the final porosity was obtained either by channels formed by the diffusion of CO or by the stacking up of regular carbide microcristals.

1800

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.

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.

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.

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Two-Theta Angle (deg.)

~

55

_

60

Figure 6: XRD patterns of Cr3C 2 obtained at 1300~ for 7h. A diffraction line corresponding to graphite phase was also observed in the pattern.

x~9

0.004-

O

0.002-

m

0

. . . .

1

,,,,

~

I0

,

I00

.......

1000

Poredistribution (nm)

Figure 7: Specific surface area and pore size distribution of the (a) starting activated charcoal, (b) Cr3C2 after synthesis.

SEM micrographs of the chromium carbide material are presented in Figs. 8 showing that the smooth surface of the activated charcoal was completely transformed into a rough surface made of several aggregates of carbide.

III.2.3 n-Butane dehydrogenation The n-butane dehydrogenation activity and the C 4 olefin selectivity obtained on the freshly prepared Cr3C 2 catalyst are presented in Fig. 9. The total conversion reached 10-12% with a C 4 olefin selectivity of 80%. As a function of time on stream a small deactivation was observed which could be due to carbonaceous deposition on the catalyst surface. However the deactivation observed in this study was less drastic compared to those observed on unsupported chromium oxide catalysts used in commercial units, where regeneration had to be carried out every 10 minutes [16]. It should be underlined that the industrial process runs without hydrogen in the feed which strongly favors the formation of butadiene even from isobutane, and in consequence carbon deposition on the surface. In addition the initial conversion on chromium oxide is close to the thermodynamic limit, around 50 % at 550~ On the Cr3C 2 catalyst the initial butane conversion of 5 to 6%, slightly increased after short oxidative regenerations at 550~ to reach a stable conversion of 10%. The selectivity was very high and stable at >90%.

865

Figure 8: SEM micrographs of the starting activated charcoal (a, c) and Cr3C2 material (b, d) at different magnifications.

866

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0

OOF-

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-8O

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100

150

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200

Time on stream (min) Figure 9: n-Butane dehydrogenation on the Cr3C 2 catalyst at 550~ and under atmospheric pressure. Reaction conditions: HJn-CgHt0 molar ratio = 9, Weight Hourly Space Velocity (WHSV) = 2, catalyst weight = 0.5g. The regeneration was performed at the same temperature in the presence of a mixture of O2 (20 vol. %) and helium.

In Fig. 10 the XRD pattern of the catalyst after five dehydrogenation tests and oxidative regenerations is reported. Some diffuse diffraction lines were observed and could be attributed either to Cr203 or to chromium oxycarbide, not itemized in the literature.The catalyst surface area, initially at 63 m2.g-1, measured after the five cycles had been decreased to 50 m2.g -l. This loss of area could be attributed either to the disappearance of the micropores contained in the remaining un-reacted activated charcoal or to the sintering of the carbide phase because of the quite agressive reaction conditions, temperature and oxidation steps.

IV. C O N C L U S I O N The shape memory concept of synthesis was successfully applied to the preparation of SiC microfibers with average surface areas of 60 m2.g ~, from low surface area carbon microfibers, < lm2.g l. The material, after calcination at 600 ~ in air, was in the form of hollow SiC microfibers, or microtubes, with a pore size distribution ranging from 10 and 20 nm. This material was able to support and disperse large quantities of MoO 3 subsequently transformed into an active and selective Mo oxycarbide for the isomerization of n-alkanes. A

867 comparison with a more conventional SiC in terms of shape showed that there was no difference between the two catalysts for the isomerization reaction. The new shape - i.e. microtubes, will be tested for very demanding reactions where the mechanical strength (liquid batch) or the shape selectivity is required. In addition the concept is applied to the synthesis of SiC nanostructures which could provide a new family of shape selective catalytical materials. The synthesis of high surface area chromium carbide (supported on un-reacted initial carbon) enlarged the family of these new materials, following the synthesis of Mo, W and V carbides. The concept of reacting a vapor of oxide with a solid sample of carbon was in consequence well established. In addition, the first syntheses were only made on activated charcoal, it is now shown that low surface area carbon can also be used as long as the reaction conditions are modified accordingly. As for most of the catalytic applications found for the other carbides, it seems that for the use of Cr in the dehydrogenating reaction, it is again a carbide containing oxygen (oxycarbide) which is the interesting active and selective catalytic phase. The oxidative treatment allows the increase of the dehydrogenation selectivity from around 70 to 90 % accompanied by a small increase in the conversion. Like most of the other oxycarbides the chromium one has a relatively high resistance towards deactivation and a strong mechanical resistance. A more detailed study will be carried out, using temperatureprogrammed oxidation and XPS techniques, in order to obtain more information about the nature of the catalyst surface and its modifications as a function of the treatment applied.

7001 600 e-

t

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)

r162

'

)

l

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,

t

,,,

/

9

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200 1000

20

I

25

I

30

I

35

I

40

I

45

I

I

50

55

60

Two-Theta Angle (deg). Figure 10: XRD patterns of the catalyst after five cycles of butane dehydrogenation tests at 550~ at atmospheric pressure and oxidative regenerations in air at 550~ for 15 minutes.

868 REFERENCES 1. (a) "The Chemistry of Transition Metal Carbides and Nitrides", Oyama S. T., Ed., Blackie Prof., Chapman and Hall, 1996 and references therein; (b) S. T. Oyama, Catal. Today, 15 (1992) 179. 2. L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 348. 3. D.A. White, S.M. Oleff and J.R. Fox, Adv. Ceram. Mater., 2 (1987) 53. 4. (a) J. S. Lee, S. T. Oyama and M. Boudart, J. Catal., 106 (1987) 125; (b) J. S. Lee, L. Volpe, F. H. Ribeiro and M. Boudart, J. Catal., 112 (1988) 105. 5. (a) M. J. Ledoux, S. Hantzer, C. Pham-Huu, J. Guille and M. P. Desaneaux, J. Catal., 114 (1988) 176; (b) M. J. Ledoux, J. Guille, S. Hantzer and D. Dubots, US Patent No. 4 914 070, 1990. 6. (a) M. J. Ledoux, J. Guille, C. Pham-Huu and S. Marin, Eur. Patent No. 0 396 475 A1 (1989); (b) M. J. Ledoux and C. Pham-Huu, Catal. Today, 15 (1992) 263. 7. P.W. Lednor, Catal. Today, 15 (1992) 243. 8. R. Moene, L. F. Kramer, J. Schoonman, M. Makkee and J. A. Moulijn, Appl. Catal. A, 162 (1997) 181. 9. F. Meunier, P. Delporte, B. Heinrich, C. Bouchy, C. Crouzet, C. Pham-Huu, P. Panissod, J. J. Lerou, P.L. Mills and M. J. Ledoux, J. Catal., 136 (1997) 33. 10. N.W. Hurst, S.J. Gentry, A. Jones and B.D. McNicol, Catal. Rev. Sci. Eng., 24 (1982) 233. 11. J.L. Falconer and K.A Schwartz, Catal. Rev. Sci. Eng., 25 (1983) 141. 12. N. Keller, C. Pham-Huu, S. Roy, M.J. Ledoux, C. Estourn~s and J. Guille, J. of Mater. Sci. (to be published). 13. M. Benaissa, J. Werckmann, E. Peschiera, J. Guille and M. J. Ledoux, J. Mater. Res., 29 (1994) 4700. 14. E.A. Blekkan, C. Pham-Huu, M. J. Ledoux and J. Guille, Ind. Eng. Chem. Res., 33 (1994) 1657. 15. C. Pham-Huu, E. Peschiera, P. Del GaUo and M. J. Ledoux, Appl. Catal., 132 (1995) 77. 16. P.R. Sarathy and G.S. Suffridge, Hydrocarbon Processing, (1993) 43.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

Synthesis of m i x e d g a l l o a l u m i n o p h o s p h a t e oxynitrides nitridation on the ~ A 1 G a P O N rr acido-basic properties

869

9the

influence

of

S. Delsarte a, V. Peltierb , Y. Laurent b, P.Grange a a) Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium. b) Laboratoire de Chimie des Mat6riaux, URA CNRS 1496, ~ Verres et C6ramiques ~, Universit6 de Rennes I, Campus Beaulieu, 35042 Rennes C6dex, France.

In this paper, we present the study of the synthesis parameters and characterisation of the acido-basic properties of mixed galloaluminophosphate oxynitrides, the ~ A1GaPON ~. They were obtained by temperature programmed activation under ammonia flow of amorphous reactive phosphate precursors. Substitution of oxygen by nitrogen modifies the surface acido-basic properties of the ~ A1GaPON ~ and allows their use as basic catalysts in the Knoevenagel-type condensation. We present a systematic study of the link between the nitridation time, the bulk nitrogen content and the acido-basic properties of the ~ A1GaPON ~ evaluated by ammonia chemisorption and by IR spectroscopy of the pyridine adsorption on the ~ A1GaPON ~ surface.

1. INTRODUCTION Research in catalysis has long been guided by the need to develop new catalysts for the petroleum industry. In the last decades, however, large efforts have been dedicated to the discovery of new catalysts for the production of intermediates and fine chemistry. The adjustment of the surface acido-basic properties of those catalysts has often been decisive to their development. Indeed, small changes in the nature, strength and concentration of acid and basic sites induce important modifications of the surface reactivity. This is clearly evidenced in reactions like 1-butanol dehydration [1-2] or ethylenimine synthesis [3] that can proceed through several paths and for which the product composition during the reaction is function of the acid-base properties of the catalyst. The will to control the surface acido-basic properties has been the driving force of a new research area in our laboratory : the study of the influence of the anionic substitution of oxygen by nitrogen on the acido-basic properties of aluminophosphates [4-6] and zirconophosphates [7-9]. It was shown on both systems that the introduction of nitrogen in the anionic network allowed to decrease the acidity of the surface's, to increase its basicity and to prepare solids both weakly acidic and weakly basic. As it appears that, to abstract proton, C-H bond activation requires not only a basic site but an acid site capable of stabilising the anionic

870 product of the C-H bond activation step [10], it was decided to use those two nitrided phosphates as support for platinum in the dehydrogenation of isobutane to isobutene [ 11]. The ~ Pt-A1PON ~ proved active and selective for this reaction. In order to improve their catalytic properties, part of the aluminium was substituted with gallium, supposed to recombine hydrogen in the molecular form during dehydrogenation reactions [12-14]. New mixed gaUoaluminophosphates were thus synthesised and successfully tested in the dehydrogenation of isobutane [11, 15]. As it seems that their acido-basic properties are essential for the reaction, a thorough study of the link between the time of nitridation, which determines the degree of the anionic substitution of oxygen by nitrogen, and the ~ A1GaPON ~ surface properties was carried out and is presented in this paper.

2. EXPERIMENTAL

2.1. Materials

Preparation of the AI~ 5Ga~5P04galloaluminophosphate precursor To prepare a high surface area, amorphous A10.sGa0.5PO4 , a process developed by Kearby [ 16] for aluminophosphates and adapted for ~ (A1, Ga) PO ~ compositions was used [17]. The starting products were GaC13, A1C13 and H3PO4 2M. The solution containing gallium, aluminium and phosphorus with Ga/A1 and (Ga+A1)/P atomic ratios equal to 1 was cooled to 0~ with a dry ice-alcohol bath, then propylene oxide was slowly added under vigorous stirring. In fact, a large excess of propylene oxide was added with respect to the theoretical amount of 3 moles of propylene oxide per mole of the desired product, until the progressively increasing pH reached a value of 2.5. The transformation into a gel occurred after standing several hours at room temperature. The gel was then repetitively washed with isopropanol without any loss of gallium, aluminium or phophorus and progressively dried up to 110~ Finally, the resulting powder was sieved (< 100 l.tm) and calcined 3 hours at 923 K.

Nitridation Nitridation involves a solid-gas reaction at 1023 K between flowing pure ammonia (301/h) and the precursor placed in the isothermal region of a tubular furnace [18]. This treatment led to ~ A1GaPON )~oxynitrides according to the following reaction" Alo.sGao.sPO4 + x NH3

Alo.sGao.sPO(4-3xa)Nx + 3x/2 Hz0"["

Various nitrogen contents, presented in Table 1, were obtained by modifying the time of nitridation from 3 to 89 hours. At the end of the activation process, the samples are cooled to room temperature under pure and dry nitrogen flow. The surface area of the samples were measured by the adsorption of nitrogen at liquid nitrogen temperature by single point BET method (p/po = 0.3) after outgassing 20 min at 523 K using a Micromeritics Flowsorb II 2300 equipment.

2.2. Nitrogen analysis The total amount of nitrogen (bulk nitrogen of nitride-type and hydrogenated NHx surface species) was determined by a method developed by Grekov [19] : the ammonia produced by reaction at 673 K of the nitride ions with melted potassium hydroxide is dissolved in water and titrated with sulphuric acid.

871

2.3. Ammonia chemisorption The ammonia chemisorption experiments were conducted at different temperatures (room temperature, 373, 473 K) using a static volumetric apparatus Micromeritics ASAP 2000 adsorption analyser. The samples (0.2 g) were first degassed at 673 K for 2h at 10-3 Tom A first isotherm allows to evaluate the volumes of ammonia physisorbed and chemisorbed at different pressures. After evacuation of the physisorbed fraction for 0.5 h at the temperatures of analysis, a second isotherm is used to determine the fraction reversibly adsorbed. The volume of ammonia chemisorbed at a given temperature is estimated by subtraction of the two isotherms. 2.4. FTIR measurements Pyridine adsorption spectra were recorded using a Brticker IFS 88 spectrometer. The powders were pressed into self-supporting disks (10mg, 13 mm diameter), placed in an IR cell, and treated under vacuum at 673 K for 1 h before analysis. After cooling to room temperature, a spectra of the powder was recorded. Then, the sample was exposed to pyridine vapor for 30 min. Spectra were recorded after evacuation (7.10 -5 Torr) for lh at room temperature, 423, 523 and 623 K. 2.5. Catalytic evaluation The reaction was carried out in a glass reactor, magnetically stirred, and heated at 323 K in a silicone bath. Four mmol of the two reactants (malononitrile and benzaldehyde) were introduced with toluene as solvent (30ml), then 0.05g of catalyst was added. The course of the reaction was followed by GC (Intersmat Delsi DI 200 equiped with an FID detector and using a capillary column CPSil 180, 50m) by withdrawing small liquid samples with a •tering syringe and analysing them at several times ranging from 5 min to 24 h.

3. RESULTS AND DISCUSSION

3.1. Composition and morphology The synthesis method developed by Kearby [16] and adapted for <<(A1,Ga)PO >> compositions allows to prepare amorphous powders. The mixed galloaluminophosphates show intermediate values of surface area between aluminophosphates which can reach 400 mZ/g and gallium phosphates (180 m2/g max). They seem to be truly mixed phases: while amorphous A1PO4 crystallises in the tridymite form at 1393 K and amorphous GaPO4 crystaUises in the cristobalite form at 1048 K, the mixed galloaluminophosphate composition A10.sGao.sPO4 gives rise to only one crystallisation peak, at 1268 K, which corresponds to the formation of a cristobalite-type solid solution [ 17]. Nitridation of this precursor under ammonia flow leads to amorphous oxynitrides with high surface area whose compositions, presented in Table I, depend on the time of nitridation at 1023 K. Those compositions were established from the total nitrogen contents of the samples : indeed, it has been shown that the (AI+Ga)/P ratio was maintained during nitridation at this temperature, and that the weight loss recorded during nitridation corresponded to the substitution of three oxygen atoms by two nitrogen atoms.

872 Table 1 Characteristics of the oxide and oxynitride powders studied sample

composition

nitridation

total nitrogen specific surface content (wt%) (m2/g)

time (h) temp. (K) AG9

Alo.sGao.sPO4

AG9N2

-

-

-

240

Alo.sGao.sPOs.22No.52 3

1023

5.3

150

AG9N4

Alo.sGao.sPO2.4sN1.04 8

1023

11.0

160

AG9N5

Alo.sGao.sPOl.slN1.46 16

1023

15.9

145

AG9N6

Alo.sGao.sPO0.94N2.0489

1023

23.3

170

3.2. FTIR analysis The comparison of the IR spectra (Figure 1) obtained for the Alo.sGao.sPO4 oxide and for the four nitrided phosphates synthesised from this precursor permits to determine the chemical surface modifications induced by the nitridation. Those spectra were recorded before exposure to pyridine on samples heated for one hour at 673K under vacuum. The presence of a broad band centred around 3550 cml is due to hydrogen-bridged hydroxyl groups (V0H) and indicates the presence of water on the surface, which also explains the molecular water bending band at 1620 crn"1 (SHOrt).

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Figure 1 (A) 9 IR spectra of "A1GaPON" oxynitrides in the 3000-4000 crnl region; (B) IR spectra of "A1GaPON" oxynitrides in the 1450-1700 cmq region 9[AG9 (0 wt% N) (a), AG9N2 (5.3 wt% N) (b), AG9N4 (11.0 wt% N) (c), AG9N5 (15.9 wt% N) (d), AG9N6 (23.3wt% N) (e) ]

873 An intense peak, at 3675 cm "~, assigned to a vibrational mode of OH groups linked to phosphorus atoms (VoH) [20] is visible on the oxide precursor. Its intensity decreases when nitrogen is introduced in the anionic network and it has totally disappeared for the most nitrided sample. On the contrary, the intensity of the two peaks appearing on nitridation around 1550 crn1 and 3320 cm -1 increases with the nitrogen content of the sample. These peaks are assigned respectively to the bending vibrational frequency of a terminal -NHz linked to phosphorus (5 NH2)and to a stretching mode of NHx species [21]. Those observations lead to the conclusion that the P-OH hydroxyl species present at the surface of the oxide are replaced by P-NHx species during the nitridation process, a conclusion similar to the one already mentioned for two other nitrided phosphates, the ~ A1PON ~ [4] and the ~ ZrPON ~ [8].

3.3. Acid properties We show the decrease of the ~ A1GaPON >> acidity with their nitrogen content by adsorption of ammonia and pyridine, regarded as basic probe molecules because they possess a lone pair of electrons at the nitrogen available for donation to other species during bonding, and because they can accept a proton from other species.

3.3.1. Ammonia chemisorption The measurement of the ammonia chemisorption isotherms at room temperature, 373 and 473 K gives information about the surface density of weak, medium and strong acid sites. The number of acid sites corresponding to each strength can be found by suitable subtraction when it is considered that the measured volumes at room temperature, 373 and 473 K correspond respectively to ammonia adsorbed on all acid sites, on medium and strong sites and on strong sites only.

Figure 2 : Total, weak, medium and strong acid sites repartition measured by ammonia chemisorption versus nitrogen content

874 A strong diminution of the number of weak and medium acid sites is observed when the nitrogen content increases from 0 to 11 wt% (Figure 2). Their numbers remain approximately constant for highest nitrogen contents.

3.3.2. Pyridine adsorption The determination of the IR spectra of chemisorbed pyridine in the 1400 to 1700 crn1 region has been used to distinguish between protonic (Bronsted) and aprotonic acidity of the Lewis type. Several workers have discussed the assignments of the infrared spectra of adsorbed pyridine [22-23]. They have shown that pyridine bonds to the surface in three different modes and that the two ring vibration v l9b and v8a can help to distinguish them. These two ring vibration modes are observed at 1447-1464 and 1600-1634 crnl respectively for pyridine coordinatively bonded to Lewis acid sites, 1535-1550 and about 1640 cm 4 for the pyridinium ion formed by transfer of proton from surface hydroxyl to the adsorbate, with the surface acting as a Bronsted acid. The third mode of pyridine adsorption, by hydrogen bonding is the weakest mode of interaction. As hydrogen-bonded molecules can be pumped off readily, they were not observed under our operating conditions. The infrared spectra of pyridine adsorbed on the phosphate precursor AG9, after evacuation at room temperature, 423 and 523 K are shown, after subtraction of the spectrum taken before pyridine adsorption, in figure 3A. At 623 K, all bands typical of the pyridine adsorption on the surface have disappeared. At lower desorption temperatures, the adsorption peaks near 1620, 1575, 1490 and 1450 crn1 demonstrate the presence of Lewis acid sites on the precursor surface. The bands at 1645 and 1545 crnl are very weak and disappear upon heating above 423 K, revealing a very soft Bronsted acidity on the precursor.

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Figure 3 :(A) IR spectra of pyridine adsorbed on AG9 (0 wt% N) after evacuation at room temperature (a), 423K (b) and 523K (c) ; (B) IR spectra of pyridine adsorbed on AG9N4 (11.0 wt% N) after evacuation at room temperature (a), 423K (b), and 523K (c)

875 As shown in Figure 3B, oxynitrides present a Lewis-type acidity. The bands typical of pyridine adsorbed on Bronsted acid sites have almost completely disappeared. The wavenumber of the band at 1600-1634 crn"l is sensitive to the strength of the Lewis acidity" it shifts with increasing temperature of desorption but not with the nitrogen content of the samples (Table 2). Table 2 Position of the vibrational band typical of the strength of Lewis acid sites, for various bulk nitrogen contents after evacuation at room temperature, 423 and 523 K Position of the band typical of the strength of Lewis acid sites (cm"1) sample

desorption temp.: RT

desorption temp." 423K desorption temp.:523K

AG9

1616

1619

1622

AG9N2

1615

1621

1623

AG9N4

1616

1622

1624

AG9N5

1616

1622

1623

AG9N6

1616

1621

1623

The intensity of the band at 1447-1464 crn~ depends on the number of Lewis acid sites 9it decreases when oxygen is substituted by nitrogen in the anionic network and stabilises for nitrogen contents higher than 15wt% (Figure 4). 160

0.8

140

0.7

120

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A

100

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80

1.,,

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0.6 0.5 0.4 0.3 0.2 0.1

20 0

,t

0

5

10

15

20

25

Nitrogen c o n t e n t (wt%)

Figure 4" Evolution of the band characteristic of the total number of Lewis acid sites with the nitrogen content, after evacuation at 423K

0

....

5

I

10

,,

1

~

15

2O

.......... 25

Nitrogen content (wt%)

Figure 5 9 Variation of the conversion after 5h in the Knoevenagel condensation with the nitrogen content

876 The IR observations made before and after exposure to pyridine complete the results obtained by ammonia chemisorption for 9 the lowest nitrogen contents, replacement of P-OH by P-NHx species decreases the Bronsted acidity of the surface. The acidity remaining is of the Lewis-type. Its strength is not affected by the nitrogen content of the surface while its number decreases until it reaches a stable value for nitrogen contents higher than 15 wt%. 3.4. Basic properties A catalytic test was used to demonstrate the increase of the ~ A1GaPON >> basic character with their nitrogen content, namely, the Knoevenagel condensation between a malonate-type ion and an aldehyde, in our study, benzaldehyde and malononitrile, widely described in the literature [24] and used by Lednor to demonstrate the basic properties of another oxynitride system, the <<SiA1ON >>[25]. Although the oxide precursor is very weakly active for this reaction, all oxynitrides selectively catalyse the condensation between benzaldehyde and malononitrile. The only detected product is benzylidenemalononitrile (Eq. 1), no Michael-type addition is observed, which places the <~A1GaPON >>basicity in a pKa range similar to the one observed for the <~AIPON >>" 10.7 < pKa = H- < 11.2 [6]. ~

CN

H

(1)

CN

Figure 5 presents the conversions obtained after 5h of reaction for the different nitrogen contents. The good correlation obtained indicates the major role of nitrogen species on the reaction but does not exclude other basic sites like basic oxygen atoms or hydroxyl groups. The AG9N2 sample presents an activity superior to the one expected from the nitrogen content determination only. A preliminary DRIFT study on this sample indicates that before thermal treatment, this catalyst presents a high concentration of surface chemisorbed ammonia, which rapidly desorbs on heating under helium. Those species could explain the particular reactivity of this sample because catalysts are tested in the liquid phase at 323 K without any thermal pre-treatment.

4. CONCLUSIONS Amorphous and high surface area galloaluminophosphate oxynitrides, the <>have been synthesised by nitridation of reactive galloaluminophosphate oxide precursors. The precursor is prepared by soft chemistry route, using a method developed for the <>and adapted for the <<(A1, Ga) PO >>.This method allows to prepare truly mixed solids. Activation of those precursors under ammonia flow leads to the substitution of oxygen by nitrogen in the anionic network. Control of the time of nitridation allows the adjustment of the O/N ratio and hence of the acido-basic properties of the oxynitrides. A series of four oxynitrides of general formula A10.sGa04PO(4_3x~)Nx with variable nitrogen contents (5.3, 11.0, 15.9 and 23.3 wt%) were obtained by varying the time of nitridation at 1073 K. Their acidity was evaluated by ammonia chemisorption and by IR spectroscopy of the pyridine adsorption and compared to the acidity of the oxide precursor used for their synthesis. They were tested in the Knoevenagel reaction described as requiring basic sites and used to determine the oxynitride basic properties.

877 From this study, we conclude that : the 9 ~>acidity decreases with the nitrogen content of the sample : the highest the nitrogen content, the lowest the pyridine and ammonia quantities chemisorbed on the surface. 9the nitrogen content has a major influence on the ~ A1GaPON >>basic properties which explains that the catalytic activity in the Knoevenagel condensation of an oxynitride is larger than for the corresponding oxide precursor. ACKNOWLEDGMENTS

Authors are grateful to the FNRS for the fellowship awarded to S. Delsarte. They also acknowledge the f'mancial support of the <~R6gion Wallonne >>,Belgium REFERENCES

1. P. Berteau, M. Ruwet, B. Delmon, Bull. Soc. Chim. Belg. 94 (1985) 859. 2. P. Berteau, S. Ceckiewicz, B. Delmon, Appl. Catal. 31 (1987) 361. 3. K. Tanabe, Acidity and Basicity of Solids, J. Fraissard and L. Petrakis (eds.), Netherlands, (1994) 353. 4. P. Grange, Ph. Bastians, R. Conanec, R. Marchand, Y. Laurent, L. Gandia, M. Montes, J. Fernandes, J.A. Odriozola, Preparation of catalysts VI, G. Poncelet et al. (eds.), Elsevier, Amsterdam (1995) 381. 5. J.J. Benitez, J.A. Odriozola, R. Marchand, Y. Laurent, P. Grange, J. Chem. Soc. Faraday Trans., 91 (24) (1995) 4477. 6. A. Massinon, J.A. Odriozola, Ph. Bastians, R. Conanec, R. Marchand, Y. Laurent, P. Grange, Appl. Catal. A. 137 (1996) 9. 7. N. Fripiat, P. Grange, J. Chem. Soc., Chem. Commun. (1996) 1409. 8. N. Fripiat, R. Conanec, A. Auroux, Y. Laurent, P. Grange, J. Catal. 167 (1997) 543. 9. N. Fripiat, R. Conanec, R. Marchand, Y. Laurent, P. Grange, J. Eur. Ceram. Soc., 17 (1997) 2011. 10. E. Iglesia, D.G. Bartion, J.A. Biscardi, M.J.L. Gines, S.L. Soled, Catal. Today, 38 (1997) 339. 11. S. Delsarte, E. Gu6guen, A. Massinon, N. Fripiat, Y. Laurent, P. Grange, Proceedings of the DGMK-Conference, Tagungsbericht 9705, W. Keim, B. Lticke, J. Weitkamp (eds.), Aachen (1997), 235. 12. J.A. Biscardi, E. Iglesia, Catal. Today 31 (1996) 207. 13. I. Nakamura, K. Fujimoto, Catal. Today 31 (1996) 335. 14. E.S. Shpiro, D.P. Shevchenko, R.V. Dmitriev, O.P. Tkachenko, Kh. M. Minachev, Appl. Catal. A., 107 (1994) 165. 15. E. Gu6guen, S. Delsarte, V. Peltier, R. Conanec, R. Marchand, Y. Laurent, P. Grange, J. Eur. Ceram. Soc., 17 (1997) 2007. 16. K. Kearby, Proc. 2nd Int. Cong. Catal., (1961) 2567. 17. V. Peltier, R. Conanec, R. Marchand, Y. Laurent, S. Delsarte, E. Gu6guen, P. Grange, Mat. Sci. Eng. B47 (1997) 177. 18. R. Conanec, R. Marchand, Y. Laurent, High Temp. Chem. Processes 1 (1992) 157. 19. J. Guyader, F. Gr6kov, R. Marchand, J. Lang, Rev. Chim. Min., (1978) 431. 20. B. Rebenstorf, T. Lindblad, L.T. Andersson, J. Catal., 128 (1991) 293. 21. J.B. Moffat, Catal. Rev. Sci. Eng. 18(2) (1978) 199.

878 22. E.P. Parry, J. Catal. 2 (1963) 371. 23. M.A. Kung, H.H. Kung, Catal. Rev.-Sci.Eng., 27(3) (1985) 425. 24. J. March, Advanced organic chemistry, reactions, mechanisms, and structure, fourth edition, Wiley-Interscience Publication, Etats-Unis, (1992) 945. 25. P.W. Lednor, R. Ruiter, J. Chem. Soc. Chem. Commun. (1991).

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

879

Preparation o f aluminovanadate oxynitride catalysts: characterisation o f a dinitrogen intermediate phase by DRIFTS, XPS, TGA. H.M. Wiame, M.A. Centeno ~, L. Legendreb and P. Grange Universit6 catholique de Louvain, Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Place Croix du Sud 2 bte 17,13-1348 Louvain-la-Neuve, Belgium. b Laboratoire de Chimie des Mat6riaux, LIRA CNRS 1496, Verres et C6ramiques, Universit6 de Rennes I, Campus 13eaulieu 35042 Rennes cedex, France.

The nitridation of aluminovanadate oxide precursors in order to obtain new acid-base catalysts is studied. The results show that an intermediate phase appears during thermal nitridation under ammonia flow. This phase, involved in the nitridation mechanism, induces a) DRIFTS bands due to stretching between two nitrogen atoms (vN-N) at 2100 cm -1, b) an XPS N ls binding energy peak at 403 eV, c) an overweight during thermal oxidation followed by TGA. Most probably these fingerprints can be attributed to metal dinitrogen interaction (M-NN), but the presence of azide intermediates can not be ruled out. 1. INTRODUCTION The modification of the reactivity of solid catalysts through nitridation reactions is being more and more used these last years. Lednor 1 et al. have shown on silicon oxynitride that the activity in the Knoevenagel condensation reaction (known to be catalysed by bases) is related to the nitrogen content. More recently Massinon2 et al. on aluminophosphate oxynitride and Fripiat3 on zirconium phosphate oxynitrides, have presented the same kind of trends, showing that the substitution of oxygen by nitrogen modifies the acid-base properties of the catalyst, nitrogen introducing basic surface properties. Another important modification, due to the substitution of oxygen by nitrogen during the nitridation, concerns the early transition metals. Once nitrided, these compounds present pseudo metallic behaviour and therefore are active catalysts in reactions such as dehydrogenation and hydrogenolysis4'5"6, reactions generally catalysed by noble metals. In this paper the thermal nitridation under ammonia flow of an aluminovanadate oxide precursor is studied. It was shown previously7 that the aluminovanadate oxynitride (VA1ON) catalyst synthesised in this way is active in reactions such as the Knoevenagel condensation but also in the dehydrogenation of 1-butanol, indicating that the obtained oxynitrides have basic surface properties. a On leave from Departamento de QuimicaInorganica e Instituto de Ciencia de Materiales de Sevilla. Universidad de Sevilla-CSIC.P.O.Box874. 41071 SeviUa,Spain

880 We present here the characterisation by DRIFTS and XPS of an intermediate phase that appears during the nitridation process. Additional TG-Analysis data of the oxidation of the oxynitrides are also presented. For this purpose a series of 7 VA1ON (A1/V = 2) nitrided at increasing temperatures were synthesised. The results show that this intermediate phase, which appears at low temperatures, can be attributed to a metal dinitrogen interaction (MNN) and it is probably an intermediate species in the nitridation mechanism. 2 EXPERIMENTAL 2.1 Materials: Synthesis of aluminovanadate oxynitride The aluminovanadate oxide precursor powder, with an A1/V=2 (atomic ratio), is obtained by coprecipitation of a solution of aluminium nitrate and ammonium metavanadate. A detailed procedure of the synthesis has been previously described 8'9. Thermal nitridation of 1.5 g of powder is performed in a tubular furnace under a 500 cc/min pure ammonia flow. The heating rate was programmed at 1K/min and the maximum temperature was maintained for three hours. The samples were cooled down to room temperature under N2. The composition of the catalysts of the series as well as a global formula based on chemical analysis, is presented in table 1. No special care was taken during storage and several weeks were spent between the synthesis and the characterisation.

Table 1 Maximum nitridation temperature and chemical composition of the aluminovanadate oxynitride series. Fonnuiation Nitridation OxYgen ' Nit'rogen . . . . temperature(K) content (wt%) content (wt%) AlzV044 IN0.25 623 39.5 1.9 673 38.9 2.2 AlEVO4.3sNo.28 A12VO4.32No.28 723 3 8.8 2.2 773 36.5 3.0 A12VO3.96No.37 823 3 8.2 3.9 AI2VO4.3sN0.sa 823 3 8.0 3.6 AlzVO4.28N0.47 873 36.8 4.6 AlzVO4.13N0.59 923 34.3 4.0 AIzVO3.64N0.49 973 35.4 5.0 A12vo3.90N0.64 The trend is ihat by increasing the nitridation temperature the nitrogen content is'higher. Difference from this tendency can be explained by bad control of the temperature during cooling down. 2.2 Methods 2.2.1 X-ray Photoelectron Spectroscopy The XPS analysis were performed with an SSI X-Probe (SSX-100/206) photoelectron spectrometer by Fisons, working with A1K~ radiation (1486.6 eV) at a pressure ranging from 1 to 7 x 10-9 mbar. The charge neutraliser was adjusted at 10 eV. Before analysis, the samples were heated three times for 15 minutes under vacuum in the pre-treatment chamber and then degassed overnight before being introduced in the analysis chamber. The binding energies were calculated with respect to the C-(C,H) component of the Cls adventitious carbon fixed

881 at 284.8 eV. The spectra were decomposed with the least squares fitting routine provided by the manufacturer with a Gaussian/Lorentzian ratio of 85/15 and after subtraction of a calculated baseline. 2.2.2. Diffuse Reflectance Infrared Fourier Transformed Spectroscopy DRIFTS spectra were obtained from a Bruker IFS 88 spectrometer with a DTGS detector using a temperature and environment controlled chamber (Spectra-Tech 0030-103). Spectra of the pure solids [200 scans, resolution 4cm 1] were recorded at room temperature in the 4000-400 cm ~ range. 2.2.3 Thermo-Gravimetric Analysis The weight change of A12VOxNy powders during oxidation was monitored using a SETARAM TG-DTA 92 balance. 20 mg of sample was placed in an alumina crucible and was heated under a pure O2 flow of 2 dm 3/h at a constant rate of 1K/min from room temperature to 1123 K. 3 RESULTS AND DISCUSSION Figure 1 shows the DRIFTS spectra in the 18502350 ClY1-1 range of the I f I ~ I I ' I ' samples prepared at the 2262 2105 1990 nitridation temperature 2247 indicated. In the literature, the bands present in this region are generally attributed to the stretching vibration between two nitrogen atoms v(N=N), and correspond either to dinitrogen (N2) or to azide species (NAN3 gives a band at 2128 cm -a and KN3 at 2041 cml) TM.For example when N2 is adsorbed on metals Z~ or on oxides t4'I5 it gives IR absorption bands in the range 2000-2250 cm -1. The strength Z30? 2200 2100 2000 1900 of the adsorption between the surface and the dinitrogen species is related to the frequency at which the Figure 1" DRIFTS spectra for the VAION series nitrided at stretching occurs. The higher increasing temperatures the frequency the stronger the bond N-N, the looser the interaction with the surface, M-NN. The position of the bands can also correspond to v(N-N), when N2 is coadsorbed with hydrogen. In this case, bands appear at higher wavenumbers (between 2350-2250 cm 1) and correspond to v(N-N) perturbed by i

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882 the presence of coadsorbed hydrogen. Studying the imeraction of NH3 with 02 at the surface ofMgO, Martra 16 et al. assigned the bands between 2000 and 2100 cm 1 to azide species and the bands at 2197 and 2173 cm 1 to dinitrogen. This is in agreement with the work of Garrone 17 et al. who studied the reaction between NH3 and N20 on MgO and showed the appearance of azide intermediates with bands at 2087 and 2120 cm -~. Those examples are of interests for two reasons. First because they have been realised at high temperature (from room temperature to 623 K), while most of the studies of N2 adsorption are achieved at the temperature of liquid nitrogen. Second, because the nitridation reaction of the VA1ON, that is realised under ammonia flow, corresponds to a substitution of oxygen by nitrogen, meaning that in the first steps of the reaction an interaction between adsorbed ammonia and oxygen is expected, this situation corresponding well to the interactions presented in the work of Martra. Another particularly interesting study was made by Oh-Kita TM et al. who, in order to synthesised ammonia by reaction between a strongly adsorbed N2 and gaseous H2, observed that on K-Ru-alumina catalysts, the bands due to adsorbed N2 appears between 2020-2100 cm -~ the higher the wavenumber the looser the interaction between the catalyst and the dinitrogen, M-NN. The interest is that in this case the reaction is carried out at high temperature (623 K) and then once again fits better with the temperatures encountered in our experiments. We can conclude from this short review of the literature that the bands centred at 2105 cm x for the VA1ON series (figure 1) can be attributed to the stretching vibration between two nitrogen atoms. Unfortunately the results doesn't allow to conclude whether this species contains two (dinitrogen) or three (azide) nitrogen atoms. The other peak centred at 2262 with a shoulder at 2247 cm -1 can be attributed to stretching v(N-N) of a dinitrogen species adsorbed on a metal site perturbed by the presence of hydrogen. This is in agreement with the work of Liu19 et al. who observed those bands in the synthesis of A1N by Vapor Deposition process on alumina and attributed the bands at 2262 and the shoulder at 2247 cm -1 to HAINN with different hydrogen bonding strengths. Those higher frequency bands have also been observed by Kinoshita 2~ et al. when N2 is adsorbed on Co/A1203 with coadsorbed hydrogen. Unfortunately the literature doesn't make any precise descriptions of this hydrogen interaction with the metal and the dinitrogen. These two assignments can explain the evolution observed in figure 1, where the band at 2262 cm -1 was already present at low temperature. The band at 2105 cm 1 increased up to 723 K then decreased at higher nitridation temperature. This suggests that during the nitridation process a site containing one hydrogen and one dinitrogen was first formed and then at higher nitridation temperatures only the dinitrogen species remained. Furthermore, the frequency of the second species (band at 2105 cm ~) corresponds to a stronger interaction between the metal and the nitrogen atom which could be interpreted as a first step in the nitridation mechanism. XPS has been used to obtain a more precise picture of these species. Figure 2 shows the N ls binding energy region for the VAION series. It can be seen that two species are mainly present at low temperature (623K). Increasing the temperature leads to the following modifications. First, a well defined peak at 403 eV grows from 673K, passes through a maximum between 773 and 823K, then decreases. This fingerprint at 403 eV for the N ls binding energy corresponds to a mildly oxidised nitrogen if one considers an oxidation scale for N ranging from 396 eV for N 3 to 407 eV for N in nitrate NO3 (where the oxidation state

883 is Ns+). In the literature, this peak is generally observed when nitrides are oxidised. Milosev 21 et al. observed an XPS N ls peak at 403 eV when TiN is oxidised thermally or electrochemically. The peak is attributed to N2 dissolved in the structure. Following the oxidation of aluminium nitride Wang 22 et al. observed the increase of a peak at 403 eV at the expense of the nitride one at 397 eV when the oxidation was proceeding. The same N1 s peak has been observed by Legendre 23 et al. when oxidising TiON, A1ON, CrON, etc and was assigned, in this latter case, to M-N=N-M. On the VA1ON, contrary to the previous examples, the peak at 403 eV appears during the nitridation reaction, at low temperature of reaction and then is completely removed by higher nitridation temperature - at 1073 K this species has completely disappeared (not shown here)-. It is likely that this peak corresponds to the same dinitrogen species that the one encountered during the oxidation of the nitrides, and could be explained by the oxidation of ammonia into N2 catalysed by vanadium atom in the first step of the nitridation. This hypothesis is supported by fact that vanadium is largely used as catalyst in the reaction of oxidation of ammonia into N224' 25. Nevertheless, XPS doesn't allow us to exclude azide species that would give peaks at 403.7 eV for Na(N_NN) and 399.3 eV for Na(~NLN) 26. The quantitative evolution of the peak at 403 eV can be related to the changes observed by DRIFTS in the 1850-2350 cm q region. Figure 3 plots the area of the 2105 cm 1 band

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~q~,][',w,/~v,~?%~l/?~, 6312:K

~,'~.:~.%r'#~.~,/>,...,:-,~.. ~ ' 404 400 396 408 Binding Energy (eV)

392

Figure 2: XPS N1 s binding energy for the VA1ON series nitrided at increasing temperatures. The areas under the peaks are not normalised.

884 as a function of the XPS N ls 403 eV content (% at.). A good correlation is observed 10 between these two series of data showing that ,~ 8 both correspond to a metal dinitrogen interaction (or metal azide). Conceming the 6 nature of the interaction, we can exclude physisorbed species. First, because the ~ 4 temperature (393K) and the vacuum (7x10 -v .~ mbar) in the pre-treatment chamber of the XPS < 2 would have desorbed any physisorbed species. Second, because heating these VAION o 0 0.01 0.02 o.o3 o.o4 catalysts in the DRIFTS cell up to 773 K under ~s/A~ (~/~t) He flow did not diminish the area of the 2105 cm -I band. Molecular N2 gas trapped in the structure can also be excluded by a close look at Figure 3 Area of the DRIFTS band at 2105 the XPS results. Indeed, a shift from 403 eV to cm 1 as a function of the Nls XPS 403.7 eV occurs when the nitridation peak at 403 eV. temperatures and the nitrogen content increased. This reveals a chemical interaction between this nitrogen and its environment (this behaviour is not expected for a gaseous dinitrogen trapped since no chemical bonds would exist between the surface and the gas), but also a close relation with the nitridation mechanism, suggesting that these species could be nitridation intermediates. This is confirmed by the quantitative evolution of the 403 eV peak which decreased and then completely disappears at higher temperature (not shown here) while the peak attributed to nitrides at 397 eV (VN) increased. Furthermore, this hypothesis could also explain the low temperature (723K) of appearance of the nitrides (N 3-) in these compounds, suggesting another mechanism of nitridation at low temperature than the one based on the decomposition of NH3 into NH2, NH, and N 3-, that occurred significantly above 823K. The presence of molecular nitrogen in interaction with the solids has already been described in the literature. It has been observed during the oxidation process of nitrides for compounds as different as TiON 23, CrON 23, LaTiON 23'27, ZrN 28 , A1ON 23 '29 "30 , or silicon oxynitrides 31. When the oxidation is followed by TGA, the profile of the weight variation as a function of the temperature presents an overweight from what is expected for complete oxidation. Goeuriot 27 et al., Goursat z8 et al., and Veyret 29 et al. have reported this behaviour and suggested the retention of dinitrogen stabilising the partially oxidised structure. Legendre 23et al., in the oxidation of TiON and other oxynitrides, also proposed the retention of dinitrogen species (M-N=N-M). In order to verify if the nature of the phase proposed by these authors is similar to that present in the VA1ON, we have performed TG analysis of the thermal oxidation under pure 02 on the VAION series. We can consider that a general scheme for nitride oxidation can be written as follows: 2 MN + 3/2 02--> M203 + N2(g)

(1)

Two comments can be drawn from equation (1). First the oxide obtained has a molecular weight larger than the original nitride, so a weight increase is expected when following the

885 weight change during the oxidation. Second the thermal 2.0 oxidation is accompanied by the production of N2 in the gas phase. 1.5 Figure 4 plots the weight changes encountered by two 00~oo Owr~ight VA1ON's during thermal ~1.0 o [] ~ ~ ~ I oxidation under 02. As expected, ~.~ O I a weight increase is observed. O II Nevertheless, between 673 ,~0.5 O I % (400~ and 853K (580~ an OI excess of weight from what is 0.0 i ~ ~ ~ AI2VO4.28N0.47 expected for complete oxidation is observed, suggesting the 0 200 400 600 800 retention of intermediate species Temperature (~ before complete obtention of the oxide. This result is very similar Figure 4: TG analysis for thennal oxidation of two to what is reported by the VA1ON samples previously mentioned authors. So we can conclude frorn the TGA curves that during oxidation of the VA1ON, the species that caused the overweight can be assigned to dinitrogen species. Furthermore, the profiles are different for different initial nitrogen contents in the original oxynitride. In figure 4 the open dots correspond to a VA1ON nitrided at 973K while the full ones to a VA1ON nitrided at 823K. The difference between those two being the total nitrogen content, the higher the temperature, the higher the nitrogen content. For the two curves presented here one can see that the overweight is higher for the oxynitride with the lower nitrogen content. In figure 5, the TGA excess weight is plotted as a function of the XPS Nls peak at 403 eV. A good correlation is obtained between these two sets of data, showing that the M-NN characterised by the peak at 403 eV (and then the 2105 cm -I band in DRIFTS since the two 0.o2 ? . . . . . . . . . . . . . . latter are correlated, see figure 3) is probably .~~o 0.015 responsible for the excess weight appearing during oxidation. Another possibility would be 8 O.Ol that the dinitrogen reacts during the oxidation process and gives rise to a new species that ..e 0.005 would be responsible for the overweight. A complete study of the in situ oxidation of 0 VAION is outside the scope of this paper and will be published elsewhere. But one can 0 0.01 0.02 0.03 0.04 already say that during oxidation the DRJFTS Nls/A12p (%at) band at 2105 cm -~ is replaced by another one at 2350 cm -~ and the XPS peak at 403 eV Figure 5: Overweight measure from TGA vanishes, suggesting that the intermediate under oxidation versus XPS N ls phase is most probably transformed into peak centred at 403 eV another species before leaving the solid, this I

'

vo3

I

'

I

42~176

o.Tx.

"

.... I

". . . .

t

'

I

'

886

second species being responsible of the overweight (results not shown). Nevertheless, the point is that the species responsible for the overweight is already present in the solid, before oxidation of the VA1ON. This behaviour is different from the one encountered in the oxidation of other nitrides where the intermediate phase appears during the oxidation. No Nls XPS peaks with a binding energy at 403 eV are present on the original nitrides. This particular behaviour of the VA1ON can probably be explained by the oxidative role played by vanadium that would oxidised ammonia into dinitrogen in the first step of the nitridation reaction. This species being an intermediate before further nitridation. 4. CONCLUSIONS In order to prepare new acid-base catalysts, the thermal nitridation, under NH3 flow, of an aluminovanadate oxide precursor was carried out. This process leads to the formation of an intermediate phase characterised by an XPS Nls peak at a binding energy of 403 eV, a DRIFTS vibration band at 2100 cm -1, and also a particular TGA profile when the oxynitride is oxidised, showing the retention of an intermediate during the oxidation. Correlations between the three sets of data have been possible and reinforce the assumption that these observations are three different ways to characterise the same intermediate. If a metal dinitrogen species is the most probable intermediate, the formation of an azide can not totally be ruled out. If in general this phase appears during oxidation of the nitrides, in this particular case the phase is growing at the beginning of the nitridation process suggesting another route of nitridation than the classical one. ACKNOWLEDGEMENT M.A. Centeno is grateful to the European Union for a Training and Mobility of Researchers (TMR) postdoctoral .grant. The authors also thank the R6gion Wallonne for financial support. REFERENCES

1P.W.Lednor and R. de Ruiter, J. Chem. Soc. Chem. Comm., (1991) 1625. 2 A.Massinon J.A. Odriozola, Ph Bastians, R. Conanec, R.Marchand, Y.Laurent, P.Grange, Appl. Catal., A, 137 (1996) 9. 3 N. Fripiat, R. Conanec, A. Auroux, Y. Laurent, P. Grange, J. Catal., 167 (1997) 543. 4 J.H.Lee, C.E. Hamrin Jr and B.H. Davis, Catalysis Today, 15 (1992) 223. 5 G.Djega-Mariadssou, Proceedings, Vilamoura, Algarve Portugal, May 25 (1997) L 151. 6 S.T.Oyama, <>S.T. Oyama (ed.) Blackie Academic & professional. Publishers (1996). 7 H. Wiame, L. Bois, P. L'Haridon, Y. Laurent and P. Grange, J.Eur. Ceram. Soc., 17 (1997) 2017. 8 H.Wiame, L.Bois, P. L'Haridon, Y. Laurent, P. Grange, Solid State Ionic, 101-103 (1997) 755. 9L.Bois, P. L'Haridon, H.Wiame, P. Grange, Mater. Res. Bull., 33 (1998) 9. 10M. Moskovits and G.A. Ozin, J. Chem. Phys., 58(3) (1973) 1251.

887 11J.K. Burdett, M.A.Graham and J.J. Turner, J.Chem.Soc. Dalton, (1972) 1620. ~2H.P. Wang and J.T. Yates, J.Phys.Chem., 88 (1984) 852. 13E.A. Wovchoko and J.T. Yates, J.Am. Chem. Soc., 118 (1996) 10250. ~4F. Wakabayashi, J. Kondo, A. Wada, K. Domen and C. Hirose, J. Plays. Chem., 97(41) (1993) 10761. ~5J.N. Kondo, S.Shibata, Y. Ebina and K. Domen, J.Phys. Chem., 99 (1995) 16043. 16G. Martra, E. Borello, E. Giamello and S. Coluccia, Stud. Surf. Sci. Catal., 90 (1994) 169. 17E. Garrone, S. Coluccia and E. Giamello, Spectrochimica Acta, 43A (1987) 1567. ~8M. Oh-Kita, K.Aika, K. Urabe and A. Ozaki, J. Catal., 44 (1976) 460. 19H. Liu, D.C. Bertolet, J.W. Rogers, Surf. Sci., 320 (1994) 145. 20 N. Kinoshita, K. Domen, K. Aika and T.Onishi, Appl. Surf. Sci., 18 (1984) 342. z~ I. Milosev, H.H. Strehblow, B. Navinsek and M. Metikos-Hukovic, Surf and Interf. Anal., 23 (1995) 529. 22 Wang P.S., Malghan S.G. and Hsu S.M., J. Mater. Res., 10 (2) (1995) 302. 23 L. Legendre, R. Marchand, Yiaurent, J.Eur.Ceram.Soc., 17 (1997) 1813. 24 N.I. II'Chenko, Russian Chemical Reviews, 45 (12) (1976) 1119. 25 N.Y. Topsoe, H. Topsoe and J.A. Dumesic, J. Catal., 151 (1995) 226. 26 O. Hendrickson, J. Hollander and W. Jolly, Inorg. Chem., 8 (1969) 2642. 27 L. Legendre, R. Marchand, B. Piriou, Eur. J. Solid State Inorg. Chem., 34 (1997) 973. 28 I--I.Wiame, M.A. Centeno, S. Picard, P. Bastians and P. Grange, J. Eur. Ceram. Soc., in press. 29 p. Goeuriot, P. Goursat and M. Billy, Mater.Chem., 1 (1976) 131. 3o P.Goursat, M.Billy, P.Goeuriot, J.C. Labbe, G.Roult, J.M. Villechenoux and J. Bardolle, Mater. Chem., 6 (1981) 81. 3~ J.B. Veyret and M. Van de Voorde andM. Billy, J.Am. Ceram. Soc., 75 [12] (1992) 3289.

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889

A novel p r e p a r a t i o n of cubic form of m e s o p o r o u s aluminosilicate J. Medina-Valtierra a, J. A. Montoya b and J. A. de los Reyes c aDepto. de Ing. Quimica, Instituto Tecnol6gico de Aguascalientes Av. A. L6pez Mateos y Av. Tecnol6gico s/n, Aguascalientes 20256, Ags. MEXICO Tel.: (0149)-700151 ext. 103, fax: (0149)-700423 E-mail: medinaj @cantera.reduaz.mx bInstituto Mexicano del Petr61eo, Eje Lfizaro Cfirdenas No. 152, D.F. 07730 MEXICO. cArea de Ingenieria Quimica, Universidad Aut6noma Metropolitana-Iztapalapa Apdo. Postal 55 534, D.F. 09340 MEXICO.

1. INTRODUCTION Crystalline aluminosilicates with hexagonal or cubic array of uniform mesopores (diameter size between 20 A and 100 A_) have been reported by researchers at Mobil Co. [1 ]. These solids designated as M41S are synthesized from hydrogels using quaternary ammonium components with different alkyl chain lengths. The MCM-41, with a hexagonal symmetry and pore size around 40 A is the most studied member of this family of novel materials [2]. This mesoporous solid is synthesized using hexadecyltrimethylammonium bromide (HDTMABr) as template. According to the literature [1,3], another member of this family, called MCM-48 with a cubic symmetry can be prepared by increasing the HDTMABr/Si ratio to values higher than one. However, Zhang and Pinnavaia [4] have recently reported the synthesis of MCM-48 metalosilicates using a HDTMABr/Si molar ratio of 0.5. There are few studies concerning the modification of this material and thus, the aim of this work was to modify and to characterize an Al-containing MCM-48 sample We report here the synthesis and characterization of the A1-MCM-48 mesoporous material, using sodium aluminate as the aluminum source and a HDTMABr / Si molar ratio < 0.1. This procedure leads to a cubic MCM-48 aluminosilicate showing high crystallinity and a very narrow pore size distribution. This mesoporous material could find applications as catalyst, separation membranes and sensor components.

2. EXPERIMENTAL METHODS

The MCM-48 sample was prepared using the following molar composition of the gel: 116.5SIO2-A1203 -14.9 Na20 -12.8(TMA)20 -10.6 HDTMABr - 1943H20. In a typical synthesis, a solution of sodium aluminosilicate ( 0.1 g A1 + 2.4g NaOH + 10g H:O ) was added

890 dropwise to a solution of 8g cetyltrimethylammonium (CTMABr; Aldrich ). This mixture was stirred for 30 min. To the above mixture, a solution of tetramethylammonium silicate (6.6g TMAOH + 14.5g SiO2 +22g H20 ) was added slowly. The resulting mixture was stirred for 30 min in order to get a homogeneous solution. Finally, a solution of 1.8 tetramethylammonium hydroxide pentahydrated ( TMAOH; Sigma ) was added and after stirring for 30 min, the final hydrogel formed was loaded into an autoclave and heated without stirring for 48 h at 150 ~ C. The resulting solid was filtered and washed with deionized water and dried at ambient temperature. The synthesized sample designated as A1-MCM-48/S was heated in a N2 flow at 550 ~ C for 1 h, then N2 was replaced by an air flow at 600~ for 6 h to obtain the calcined sample, A1-MCM-48/C. The samples were characterized by XRD ( Siemens, D500 model and CuKcz radiation ), Scanning Electron Microscopy ( Jeol 100CX), Transmission Electron Microscopy (Jeol 5200) and IR Spectroscopy (Nicolet 710 FTIR). Elemental analysis was performed using a Perkin Elmer 2380 Spectrometer. 27A1 Magic-Angle-Spinning NMR spectra ( 78.2 Mhz ) of the samples were recorded on a Bruker ASX 300 spectrometer at room temperature. Texture parameters were determined from the N2 adsorption data (Accusorb 2100 E instrument).

3. RESULTS AND DISCUSSION

In the synthesis of M41S materials, the surfactant concentration affects the formation of the mesoporous phases [2]. We report here a hydrothermal synthesis of MCM-48 material with the lowest surfactant concentration reported. In this preparation we used silica gel as the silicon source and a surfactant concentration of 15.8 wt % in the final gel. According to Myers [5], at this low surfactant concentration small spherical micelles are present in the aqueous solution. It appears that in our preparation the environmental parameters ( pH, temperature, the ionic strength and O H ions present ) lead to a cooperative assembly of inorganic ions in solution, charged surfactant-based-structures and inorganic counterions. Thus, the cubic mesophase is formed from a dilute solution. The X-ray diffraction patterns of the solids show a single intense peak at a low angle (d211) characteristic of mesoporous materials (Fig.l). The XRD scan does not appear to indicate a good resolution at higher angles. However, an expansion of the region 20 = 3-5 ~ shows that the patterns for both samples consist of seven Bragg peaks which can be indexed to different (hkl) reflections. These reflections confirm the presence of cubic phases as compared to data previously reported [ 1]. Unit cell dimensions for the A1-MCM-48/S and the A1-MCM-48/C samples, 83.2 and 78.4 A respectively, were calculated from the d211 reflection. This decrease of about 5 A in the d211 values upon exchange could be due to a decrease in the framework wall thickness [6]. SEM micrographs reveal that the particle morphology of the MCM samples consists essentially of plate-shaped particles (around 1.5 x 1.3 x 0.14 gm ). These particles are large enough to be easily observed by SEM ( Fig. 2a ). However, they can not be observed properly by TEM due to the thickness of the particles. As shown in Fig.2b, the electron diffraction pattern exhibits the well-defined cubic symmetry of the A1-MCM-48 sample.

891 i

[2111

|L

n

ii

hkl d(2)a d(2)b 0

OJ

Od,--~

211

I n

a)

t e

'~~b)

n

I

S

I

4

i t

Degrees 20

Y

34.00 32.00

220

31.50 29.82

321

28.10 26.83 ~

400

24.00 24.80

332

22.78 22.40

422

20.50 20.88

431

19.80 20.10

a) b) I

2

i

I

I

I

4

6

8

H

D e g r e e s 20

Figure 1. XRD pattems of MCM-48 samples: (a) As-synthesized sample; (b) calcined sample.

Figure 2. (a) Scanning electron micrograph (SEM) of the cubic shaped particles (A1-MCM48/S); (b) Electron diffraction pattern of A1-MCM-48/E.

892 From the AA spectrometry data, the SiO2/A1203 molar ratio for the exchanged sample was 90, which is consistent with the original reaction mixture composition. The 27A1 MAS NMR spectra of the as-synthesized and calcined samples clearly show the main peak around 50 ppm ( Figures 3a and b) that corresponds to tetrahedral A1 in the Si-OSi framework. However, in the calcined sample a minimal portion of octahedral A1 species can be observed as a shoulder around 0 ppm. According to the literature [7], octahedrally coordinated A13+ ions derived from amorphous alumina were observed as a small shoulder at 7 ppm. These results indicate that a transformation of the original tetrahedral A1 into extra framework octahedral A1 occured in a small amount after calcination at temperatures around 600 o C. For the sample without organic template in the framework ( AI-MCM-48/C ), the surface area SBET calculated from the desorption branch was 637 m z g-1 The pore size distribution for the exchanged sample indicated an average pore of 26 AI Similar results were found by Zhang [4] for a V-MCM-48 material. The narrow pore size distribution indicates that the mesopores in our material exhibit very uniform diameters.

i

i

i

Ill

48

49

I'

80

I

60

I

I

40

!

20

I

0

-20

ppm

"i .... i'

80

60

40

I

20

i

0

i

-20

Figure 3. 27A1 MAS NMR spectra of the A1-MCM-48 samples: (a) As-synthesized sample; (b) calcined sample.

4. CONCLUSIONS The A1-MCM-48 sample prepared using a small amount of sodium aluminate inhibited the transformation of the original tetrahedral A1 into octahedral A1 after calcination. These molecular sieves with a tight control over the pore size distribution could be used as adsorbents, supports or catalytic surfaces. Although the mesoporous materials are less active than the microporous zeolites [8], they have been found to be better catalysts than amorphous

893 aluminosilicates. Moreover, the MCM-41 aluminosilicates have exhibited shape selectivity in alkylation reactions [9,10] while the MCM-48 metalosilicates have demonstrated to be active catalysts for the selective oxidation of aromatic compounds [4].

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, J.S., Nature, 359 710(1992). 2. X.S. Zhao, G.Q. Lu and G.J. Millar, Ind. Eng. Chem. Res., 35 (1996) 2075. 3. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 4. W. Zhang, and T.J. Pinnavaia, Catal. Lett., 38 (1996) 261. 5. D. Myers, Surfactant Science and Technology, VCH, New York (1992) 6. C.Y. Chen, H.X. Li and M.E. Davis, Microscop. Mater., 2 (1993) 17. 7. P. Nortier, P. Fourre, A.B.M. Saad, O. Saur and J.C. Lavalley, Appl. Catal., 61 (1990) 141. 8. K.M. Reddy, and Ch. Song, Catal. Lett., 36 (1996) 103. 9. I.V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin, H. Van Bekkum, Catal. Lett., 30 (1995) 241. 10. J. Medina-Valtierra, M.A~ Sanchez, J.A., Montoya, J. Navarrete and J.A. de los Reyes, Appl. Catal. A:General 158 (1997) L 1.

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895

M e s o p o r o u s basic catalysts: comparison with alkaline exchange zeolites (basicity and porosity). Application to the selective etherification of glycerol to polyglycerols.

J-M. Clacens a, Y. Pouilloux a, J. Barrault a, C. Linares b and M. Goldwasserb ~Laboratoire de Catalyse en Chimie Organique. UMR 6503 ESIP, 40, avenue du recteur Pineau, 86022 Poitiers Cedex, France Tel.: +33/(0)5.49.45.40.24- Fax.: +33/(0)5.49.45.33.49 E-mail : [email protected] bFacultad de ciencias, escuela de quimica Apartado de correo 47102, Caracas, 1041-A, Los Chaguaramos, Caracas, Venezuela

The glycerol condensation (etherification) was studied in presence of alkaline exchange zeolites and mesoporous basic catalysts. It was observed that the diglycerol selectivity was increased when X zeolites were exchanged with cesium while the selectivity of Cs - ZSM5 was comparable to that of Na2CO3. Over mesoporous M La or M Mn materials, the formation of triglycerol and of tetraglycerol was more important. Such results could be due to changes of both the pore size and the basicity of the catalysts.

1. INTRODUCTION Glycerol is mainly a natural product issued from the methanolysis of vegetable oils. In Europe, due to the increasing use of methyl esters as fuel additives, one can expect an increase of glycerol production which could become a cheaper raw material for chemistry (1). For example, polyglycerols and specially polyglycerols-esters (PGEs) are gaining prominence in new products for tensioactive, lubricants, cosmetics, foods additives, .... Indeed PGEs exhibit multifunctional properties and a wide range of formulating options, if it is possible to control; (i) the length of the polyglycerols chain, (ii) the degree of esterification and (iii) the fatty acid molecular weight. These reactions are quite interesting goals for shape - selective catalytic processes. Previous works showed that the selectivity of the first step is not really controlled and that a mixture of di- to hexa-glycerol (linear or cyclic) is obtained (2). Then it is rather difficult to get a well-defined product and to predict the Hydrophilic Lipophilic Balance (HLB) after esterification. In our laboratory it was evidenced that the esterification of glycerol could be selective to ct monoglycerides over cationic resins (3). Nevertheless polyglycerols and

896 polyglycerols esters as well as acroleine were obtained as main by-products. Moreover we observed that the modification of the pseudo-pore size of these materials improved the selectivity to (PG + PGEs) but acroleine is always obtained over these acid catalysts. In order to obtain a selective formation of di-, tri-, tetra- or poly-glycerols, we investigated first the selective etherification of glycerol (figure 1) over solid bases and compared the results to those obtained with Na2CO3 catalysts. A first series of solid catalysts used for this reaction were mesoporous type materials (MCM-41) (4) or Metal-MCM-41. We also reported results obtained with microporous alkali exchanged zeolites and alkali supported over alumina. Moreover, the pore size was controlled in using different "templating systems" so as to know if there could exist a relationship between the polymerization degree and the size of the pores of the catalyst (5).

2

OH

HQ,v.~OH Glycerol

OH

OH

HO.v.,~ o V ~ o H

Diglycerol

OH

l

OH

+

H20

OH H20

Triglycerol

Poly[glycerols Figure 1. Etherification of glycerol 2. EXPERIMENTAL PART

2.1. Catalyst preparation Cesium-X zeolites are obtained from exchange experiment (CsX{i)) and exchangeimpregnation (CsX0,a)) at 60~ with a 0.4 M solution of cesium acetate (0.02 g of zeolite / mL of solution) so as to get Cs20 clusters close to exchange cesium ions. We made this exchange twice during 24 hours with a 13X zeolite coming from Prolabo; then the mixture is washed (CsX0)) or not (CsX(~a)) with distilled water and dried at 80~ for 12 hours. The resulting solid is calcined under air at 550~ for 5 hours at a heating rate of 1~ / min. Cesium ZSM-5 zeolites are obtained by exchanging two different H-ZSM-5 zeolites (Degussa) with Si / AI ratios of respectively 28 (CsZSM5(28)) and 1000 (CsZSM5(1000)); with a 0.4 M solution of cesium nitrate at 80~ for 24 hours (0.02 g of zeolite / mL of solution). Then the zeolite is dried at 80~ for 12 hours and calcined under air at 550~ for 5 hours at a heating rate of 2~ / min. Mesoporous (M) and promoted mesoporous (M Me(n), where n -- Si / Me ratio) materials were prepared according to a procedure developed in our laboratory (we add dropwise 0.2

897 mole of a solution of sodium silicate (27% SiO2) to a solution containing 0.022 mole of NaOH, 23.31 moles of 1-I20, 0.005 mole of the metal nitrate salt and 0.043 mole of template (cetyltrimethylamonium bromide). The pH is adjusted at 10.5 with diluted HC1; then the formed gel is placed in a autoclave at 100~ for 24 hours. The resulting solid is filtered, washed with water, dried at 100~ overnight and calcined under air at 550~ overnight at a heating rate of l~ / min. K/Alumina was prepared according to the following method : we add dropwise to 3 g of alumina (with particle size not greater than 0.15 mm), a solution of potassium acetate in order to have 1.5 mmoles of K / g of alumina. The catalyst is first dried under air at ambient temperature for 12 h, dried under air at 100~ for 12 h and finally calcined under air at 500~ for 12 h with an air flow of 30 mL / h.g, and a heating rate of 5~ / min.

2.2. Characterization Characterization of the solids is done using X-ray diffraction, surface BET and thermodesorption analysis. Basicity was deduced from carbon dioxide adsorptionthermodesorption pulsed experiments done using the following procedures, (i) for the modified zeolites, the calcined materials were first dried at 100~ for 8 hours under air, then they were activated at 500~ for 5 hours and cooled to 20~ under a helium flow. The activated materials were then saturated with dry gaseous carbon dioxide at 20~ Physisorbed carbon dioxide was removed by purging the sample under a helium flow at 20~ until a stable baseline was monitored. The TPD was performed by heating the sample from 20 to 500~ with a heating rate of 10~ / min., (ii) for the mesoporous materials, the calcined materials were first activated at 550~ for 8 hours and cooled to 20~ under a helium flow. The activated materials were then saturated with dry gaseous carbon dioxide at 20~ Physisorbed carbon dioxide was removed by purging the sample under a helium flow at 20~ until a stable baseline was monitored. The TPD was performed by heating the sample from 20 to 550~ with a heating rate of 10~ / min. In some cases the quantification of CO2 adsorbed over the solids was quite difficult due to a low or a strong CO2 adsorption. Then we only reported in the table 2 the ratio S sites / W sites, where strong (S) and weak (W) sites correspond respectively to CO2 desorption at 250~ and 100~

2.3. Catalytic testing Glycerol etherification was carried out in a glass-reactor equipped with a mechanical stirrer in the presence of 2 wt. of catalyst, water formed during the reaction being eliminated and collected using a dean-Stark system. The reaction was performed at 240~ or 260~ for 8 or/and 24 hours with 50 g of glycerol under nitrogen. Reagents and products were analyzed after a silylation procedure (6) with a GPC equipped with an on-column injector, a FID and a polar column (HT5) supplied by SGE (L = 25 m, ID = 0.22 mm, thickness of the film = 0.10 txm).The molar percentage of each compound was determined by using standardization methods with methyl laurate as an internal reference.

898 3. RESULTS AND DISCUSSION The results reported in the table 1 and figures 2-5 show that Na2CO 3 is the more active and the less selective catalyst; the distribution of polyglycerols being rather large. Cs exchanged zeolites are quite selective to diglycerol and triglycerol while exchanged ZSM-5 are less active and selective. This is the result of a change of the size of the channels for ZSM-5 solids whatever their Si/A1 ratio. In that particular case, the reaction could proceed on the surface of zeolite particles rather than inside the channels. Over M mesoporous materials, the reaction rate of glycerol was slow but rather similar to that of Cs exchanged ZSM-5(28). When these solids were promoted with basic elements (Mg or A1), rather than with acidic ones (A1), one observed an important increase of the reaction rate without change of the selectivity (diglycerol + triglycerol). But the comparison of the results for a glycerol isoconversion shows that : (i) the formation of triglycerol is more important over promoted M catalysts specially over the M La (20) sample, (ii) the formation of tetraglycerol is increased over Mn or La promoted M samples.

Table 1 Catalytic results Catalyst (Si/Me ratio)

Na2CO3 K/AI203 CsX(i) CsX(i,a) CsZSM5(28) CsZSM5(1000) M M AI (20) M Mg (20) M La (20) M Mn (20)

8 hours selectivity (%) (24 hours) 8 hours conversion (%) Diglycerol Triglycerol Tetraglycerol (24 hours) 96(-) 45(65) 36(82) 39(68) 16(21) 18(43) 12(31) 9(21) 15(65) 39(92) 12(91)

24(-) 81(59) 80(55) 85(68) 87(85) 92(79) 86(86) 74(91) 90(63) 82(26) 80(29)

35(-) 18(29) 18(29) 15(24) 13(15) 8(21) 14(14) 26(9) 10(27) 18(23) 14(27)

22(-) 1(10) 2(12) 0(7) 0(0) 0(0) 0(0) 0(0) 0(9) 0(27) 6(27)

others 29(-) 0(2) 0(4) 0(1) 0(0) 0(0) 0(0) 0(0) 0(1) 0(24) 0(17)

899 100 90 80

v

70

-e-Na2CO3

60

-.e- K/AI203

r-

--k- CsX(i)

Or) L_

5o

+

CsX(i,a)

> cO ~

4o

+

CsZSM5(28)

3o

--t-- CsZSM5(1000)

2O

10

0

0

5

10

15

20

25

Time (h) Figure 2. Conversion of glycerol over modified zeolites, K/A1203 and Na2CO3.

100

Diglycerol

70

--e- Na2CO3 - e - K/AI203

~o

.m

>

0

(l)

(!) or)

--A- CsX(i)

5o

--e- CsX(i,a)

4o

- ~ - CsZSM5(28)

Triglycerol

3O

0

0

1'0

2'0

30

- - ~ CsZSMS(1000)

40

5'0

6'0

Conversion (%)

7C)

8'0

9'0

100

Figure 3. Glycerol etherification over modified zeolites, K/A1203 and Na2CO3. Relationship between selectivity and conversion.

900

Na2CO3

7O v

60

"L~

50

cO

.-i-M - e - MAI(20) --&- MMg(20)

(!) E O r,J

40

MMn(20)

30

- 4 - MLa(20)

~'

5

10

Time (h)

1'5

20

25

Figure 4. Conversion of glycerol over modified mesoporous materials and Na2CO3.

100

=-.9, v

Diglycerol

7O

-,-Na2CO3 -a-M

60

- e - MAI(20)

5O

"6 1~ (D

- i - MMg(20) 4o

--4- MLa(20)

Triglycerol

30

0

10

20

30

--)K--MMn(20)

40

50

60

70

80

90

100

Conversion (%) Figure 5. Glycerol etherification over modified mesoporous materials and Na,2CO 3. Relationship between selectivity and conversion.

901 The results reported in the table 2 show that basic sites are stronger with the cesium exchange X zeolite than with the potassium over alumina. We observed no basicity for the mesoporous promoted samples probably due to the too low CO2 adsorption temperature used during the TPD that would not allow the activation of the basic sites.

Table 2 Characterization results Catalyst (Si/Me ratio)

BET surface (m2/g)

Basic sites ratio (%) (strong / weak)

K/A1203 CsX(i) CsX(i,a) CsZSM5(28) CsZSM5(1000) M M A1 (20) M M s (20) M La (20) M Mn (20)

205 352 338 251 264 960 923 789 736 972

20 / 80 62 / 38 56 / 44

0/0 0/ 0 0/ 0 8/ 0 0/0

X-ray diffraction characterization of the samples was also done and confirm the structure of the exchange zeolites as well as those of the mesoporous promoted materials. BET analysis don't show unexpected results except perhaps a quite lower crystallization level for the mesoporous materials promoted with Mg and La. 4. CONCLUSION The results presented in this paper show that some mesoporous materials containing a alkaline earth, a transition element or a rare earth (i.e. La) are as active as zeolites materials exchanged with alkaline. Acid solids such M and M A1 have a much lower activity. M La (20) solids are among the most active for the glycerol etherification. The product distribution obtained with all these catalysts seem rather similar, nevertheless the change of the heteroelement could modify the selectivity as for M La or M Mn samples. Up to now we have no clear interpretation of these results in relation with the first characterizations and work is in progress.

Acknowledgments : The authors gratefully acknowledged support from the European Community "FAIR program".

902 REFERENCES

1. A.J. Kaufman and R.J. Ruebush, Proceedings of the world conference on oleochemicals into 21 st century, T-H. Applewhite Ed., American Oil Chemist Society (1991) 10. 2. K. Cottin, DEPSUP, Poitiers, 1996. 3. S. Abro, Y. Pouilloux and J. Barrault, 4th Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, 1996. 4. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonovicz, C.T. Kresge, K.D Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCuller, J.B. Higgins, J.L. Schlenker, J. Amer. Chem. Soc., 114 (1992) 10834. 5. To be published 6. M.R. Sahasrabudhe, J.A.O.C.S., 44 (1996) 376.

91998 ElsevierScience B.V. All rights reserved. Preparation of CatalystsVII B. Delmon et al., editors.

903

Propane dehydrogenation on mesoporous chromium-containing silica catalysts P. Maireles-Torres, M. Alc~intara-Rodriguez, F. P6rez-Reina, E. Rodriguez-Castell6n, P. Olivera-Pastor and A. Jim4nez-L6pez Departamento de Quimica Inorg/mica, Cristalografia y Mineralogia, Facultad de Ciencias, Universidad de M/fiaga, Campus de Teatinos, 29071 M/tlaga, Spain

This paper reports the synthesis, characterisation and catalytic properties in the dehydrogenation of propane of a series of SiCr-MCM materials. The solids obtained under different synthesis conditions, using a surfactant-assisted sol-gel methodology, have been investigated by X-ray powder diffraction, thermogravimetric analyses, IR and diffuse reflectance spectroscopies. After surfactant removing, textural parameters obtained from nitrogen adsorption-desorption isotherms at 77 K reveal the mesoporous character of the SiCr-MCM materials. BET surface areas ranged between 550 and 790 m2 g-1 and pore size distributions exhibited a maximum pore diameter between 17 and 28 A. Catalytic properties have been evaluated by using the dehydrogenation of propane under non-oxidative and oxidative conditions.

1. INTRODUCTION Mesostructured inorganic solids have attracted a great deal of attention since their discovery at the beginning of this decade [ 1]. This family of solids (called M4 lS), which pore dimensions can be tuned between 20 and more than 100 .~ exhibits unique chemical and textural properties, finding many applications as catalysts, catalyst supports and sorbents of large molecules [2]. Although many kinds of mesoporous metal oxides can be prepared by a sol-gel method, the attention has been focused mainly in pure silica, B-, A1- Ti- and Vcontaining silica. Recently, the synthesis and characterisation of mesoporous chromium silicates have been reported [3]. These materials have demonstrated to be useful ascatalyst in redox reactions such as the oxidation of phenol, 1-naphthol and aniline with H202 in liquid phase [3 ], and the peroxide oxidation of styrene and methyl methacrylate to benzaldehyde and methyl pyruvate, respectively [4]. On the other hand, chromium(III) supported catalysts have been used for dehydrogenation of light alkanes and represent an interesting alternative for obtaining alkenes useful for polymerisation and other organic synthesis [5]. In the pure dehydrogenation of propane, the most efficient catalysts are those derived of chromic oxide supported on alumina, but spectroscopic data reveal that mononuclear Cr(III) ions with coordinative vacancies are the active sites for dehydrogenation of alkanes [6]. Because, the catalytic dehydrogenation of propane needs of very high reaction temperatures, it is important to use efficient catalysts with high thermal stability and refractory supports. Regarding the oxidative propane dehydrogenation, vanadium supported catalysts have been widely used, but Cr based catalysts

904 may be also applied. For instance, chromia pillared clays were found to be active in the oxidative dehydrogenation of ethane [7]. The present work was undertaken with the aim to evaluate the behaviour of mesoporous chromium-containing silica (SiCr-MCM) in the oxidative and non-oxidative dehydrogenation of propane. 2. E X P E R I M E N T A L SiCr-MCM materials were prepared by adding an aqueous solution of hexadecyltrimethylammonium bromide (25 wt.%) to a mixture of tetraethoxysilane and CrO3 previously dissolved in water at room temperature. The pH was then adjusted to 11 by addition of tetramethylammonium hydroxide. The resulting gels were stirred at room temperature or under hydrothermal conditions (130~ following the experimental conditions summarised in Table 1. The resulting solids were recovered by centrifugation, washed several times with water and ethanol, and air-dried at 60~ Calcined solids were obtained aRer thermal treatment of the precursor materials at 540~ in air. Cr was analysed colorimetrically using the chromate method (~,= 372 nm) in alkaline solutions, after treatment with NaOH-H202. Powder XRD patterns were recorded on a Siemens D501 diffractometer (graphite monochromator, Cu-Ka radiation). Differential and thermogravimetric analyses (TGA-DTA) were performed on a Rigaku Thermoflex TG 8110 instrument (heating rate of 10~ min-1). Adsorption-desorption of N2 was measured on a conventional volumetric apparatus (77 K, degassing at 200~ and 10 "4 mbar overnight). Pore size distributions were calculated with Cranston and Inkley method for cylindrical pores [8]. Diffuse reflectance electronic spectra were registered on a Shimadzu MPC 3100 spectrophotometer (BaSO4 reference) and IR spectra on a Perkin-Elmer 883 spectrometer as KBr disks. X-ray photoelectron spectra (XPS) were recorded with a Physical Electronics spectrometer equipped with a MgKet X-ray excitation source and multichannel hemispherical electron analyser. Catalysts were tested in the dehydrogenation of propane, under pure (DH) and oxidative (ODH) conditions. A fixed-bed quartz U-tube reactor working at atmospheric pressure and a catalyst charge of 0.120 g without dilution were used in all cases. Samples were pretreated at the reaction temperature under a He flow (30 ml min "1) for 60 minutes. The DH reaction was performed at 550~ using a gaseous mixture of 7.06 mol% propane in helium. In ODH, the reaction temperature was 400~ and the gaseous mixture formed by 7.06 mol% propane in O2/N2 (19.51 / 73.41, mol%). In both reactions, a total flow rate 28.3 ml min ~ and a spatial velocity of 25.7 g~at h/mol propane were used. The analysis of reactants and products was carried out on line with a gas Chromatograph Shimadzu GC-14B) with a column (7 m in length, 5 mm internal diameter) filled with sebaconitrile, and using a FID detector. 3.

RESULTS AND DISCUSSION

For a given Si/Cr ratio, the Cr content of the SiCr-MCM materials was invariable upon increasing the reaction time and temperature from 1 to 3 days and from r.t. to 130~ respectively. Increasing the Cr/Si ratio 10 times did not change significantly the amount of chromium incorporated to the MCM structure, although the total Cr content in this case raised up to 1.7% Cr. The TGA curves for the as-synthesised SiCr-MCM materials indicate a similar decomposition behaviour and show a continuous weight loss between room temperature and 450~ (41-57%) (Table 1), which corresponds to the water desorption at low temperature,

905 combustion and decomposition of the hexadecyltrimethylammonium ions (associated to three exothermic effects in the DTA curves at c a . 275, 315 and 420~ and water released from the condensation of silanol groups to form siloxane bonds. Table 1 Synthesis conditions and chemical analyses of SiCr-MCM materials: Reaction conditions Chemical analyses

Sample

Added Si/Cr temperature molar ratio

time

(~

(days)

% weight

%Cr

loss a (calcined (precursors) solids)

SiCr50rtl

50

25

1

41.1

1.23

SiCr50rt3

50

25

3

48.0

1.29

SiCr50hy3

50

130

3

45.6

1.30

SiCr5rt3

5

25

3

56.5

1.70

a From TGA. Temperature range: rt- 1000~

= = 36.8 A = -4-o

=

op~

J

30.4 A C I

1

3

5

7

9 20/~

Figure 1. XRD patterns of calcined a) SiCr50hy3, b) SiCr50rtl and c) SiCr5rt3 samples. Powder XRD patterns of calcined SiCr-MCM solids are dominated by an intense peak at low angle, corresponding to the 100 diffraction line supposing an hexagonal arrangement.

906 This peak became narrower and shifted to lower angle upon increasing the reaction time, and especially with the hydrothermal treatment (Table 1 and Figure 1). In this case, a broad band between 20 = 3.5-5.5 ~ is also visible. The absence of resolution of this higher angle diffraction peak has been previously attributed to the presence of small scattering domain sizes [6]. The increase in the Cr content gives rise to lower dl00 values. The incorporationof Cr into the siliceous framework may be deduced by the absence of any X R signal corresponding to segregated crystalline chromium oxide phase in the calcined materials. XPS spectra of Cr2p3/2 reveal the existence of Cr species in different oxidation states, including Cr(III) (BE at 577.1 eV) and Cr(VI) (579.6 eV), as corroborated by electronic spectroscopy. Thus, as-synthesised solids exhibited diffuse reflectance spectra composed of bands at 270, 344, 455 and 610 nm (bands I, II, III and IV, respectively, in Figure 2). The bands at 270 and 344 nm are due to the charge-transfer absorption of Cr(VI), whilst the other two bands are assigned to the presence of Cr(III). Upon calcination, the intensity of Cr(VI) bands increases whereas only a shoulder or a low intensity band at 440 nm associated to Cr(III) is observed. This implies that Cr(VI) are formed at high temperature, maybe due to the high degree of dispersion of Cr species in the siliceous matrix. -i|

"

~

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

,\ iv

\

r~3

1

250.0

350.0

~

450.0

!

550.0

i

650.0

J

750.0 L/nm

Figure 2. Diffuse reflectance spectra of (a) as-synthesised SiCr50rtl, (b)as-synthesised SiCr5rt3, (c) calcined SiCr50rtl, and (d) calcined SiCr5rt3 materials The IR spectra of calcined SiCr-MCM materials, in the region below 1300 cm1, have the characteristic absorption bands due to inorganic skeletal vibrations. The broad and intense band a t - 1 0 9 0 cm"1 with an associated shoulder at higher frequency are features of silica framework, and there are no changes with the Cr content. However, the ratio of the 949 cm1 band (Si-O stretching vibration of Si-OH or Si-OCr bonds) and the band at 807 cm1 (symmetric stretching vibrations of Si-O-Si groups) was found to be significantly higher for Cr-containing MCM samples than for pure mesoporous silica, as previously observed for Tiand Zr-doped analogues. Therefore, it can be inferred that the relative enhancement of the 949 cm"1band may be attributed to Cr incorporation.Calcined SiCrMCM materials exhibited almost reversible Type IV nitrogen adsorption-desorption isotherms, typical feature of the

907 members of this family of mesoporous solids (Figure 3a). Textural parameters of the studied catalysts are shown in Table 2. w~

k~ 300 ~

~

a

250

6o

b

200 150 ,g

100

"~20

[] SiCrSrt3 .... 9.... S i C r S O r t 3

50

Ii,~m_m roB_,,, am m ==_--.am m_mm_..

o

J

o.o

I

'

0.2

I

0.4

'

I

0.6

'

1

'

0.8

t

0 10 20 30 40 50 60 70 80 90100

1.0

average pore radii / A

relative pressure

Figure 3. Adsorption-desorption isotherms of Nz at 77 K (a) and pore size distributions (b) of SiCr-MCM sample BET surface areas ranged between 550 and 790 m2 g-a and pore volume between 0.340 and 0.864 cm3 g-X values being lower for SiCr50rtl and SiCr5rt3 samples, with short reaction time and higher Cr content, respectively. The pore size distributions are narrow, with a single maximum pore diameter between 17 and 28 A, depending on the synthesis conditions (Figure 3b). SiCrSrt3 material presented the lowest value (17/~), whereas the wall thickness obtained was barely affected (Table 2). Table 2 Textural parameters of calcined samples daoo

Ada

SBET

(~)

(z~)

m 2 g-1

Cm 3 ga

SiCr50rtl

36.8

19.5

547

0.340

SiCr50rt3

38.7

21.7

765

SiCr50hy3

40.5

18.8

791

SiCr5rt3

30.4

18.1

626

Muestra

Vp b

CBET

Sac c

~Vp

c

dpd

m 2 g-1

cm 3 g-1

46

555

0.345

23

0.740

49

821

0.729

23

0.864

124

1085

0.940

28

0.388

136

692

0.377

17

a Pore wall thickness: Ad= ao - dp; ao = 2d100/~3 b At P/Po = 0.95 c Calculated using the Cranston and Inkley method d Maximum pore diameter in the pore size distribution

908 Figure 4 shows the variation of the catalytic activity of SiCr-MCM catalysts in the dehydrogenation of propane as a function of time on stream. Conversion and selectivity towards the different reaction products are present in Table 3. In inert atmosphere (550~ the initial activity is maximum for the sample prepared under hydrothermal conditions, with 1.2 lamol propene g-1 s-l, but this sample displayed the most pronounced deactivation as well. However, in no case the presence of coke in the spent catalysts was detected. Thus initial variations in activity might be rather related to the presence of Cr(VI) ions on the catalyst surface which favours the non-catalytic oxidation of propane [6].

b 1.5-

9 SiCr50rtl 9 SiCr50rt3 o SiCr50hy3 x SiCr5rt3

3

t. 1.0X OX

~ 0.5| 0 XXXXXXXXXXXXxxxx X ~ ,~ O0 o ..~ ~ell 0.0. I I I 0 100 200 300

400

time on stream / min

Figure 4. Catalytic activity of SiCr-MCM materials as a function of time on stream After 200 minutes of reaction, the activity reach a steady value ranging between 0.08 and 0.29 ~tmol propene g-1 s-1. These data indicate that the most active catalyst is that with the highest Cr content (sample SiCr5rt3), in which the proportion of active surface Cr(III) ions is expected to be higher. Selectivity towards the formation of propene is found to be close to 92% for sample SiCr5rt3. The other reaction products are methane and ethane, produced by cracking reactions. Table 3 Catalytic activity in the dehydrogenation of propane of SiCr-MCM materials, after 200 min on stream. Sample Conversion Selectivity (%) Activity (%)

Methane

Ethane

Propene

~tmol propene.gas -1

SiCr50rtl

1.19

0.0

14.6

85.4

0.09

SiCr50rt3

0.99

13.2

27.3

59.5

0.08

SiCr50hy3

2.34

11.1

22.0

67

0.13

SiCr5rt3

1.81

1.2

6.4

92.4

0.29

At 400~ these catalysts are only active under oxidative conditions. In ODH the conversions ranged between 2.3 and 36%, and in this case, the deactivation is low. The most

909 active catalyst was again sample SiCr5rt3 (Figure 5 and Table 4). This catalyst shows aider 200 minutes of reaction an activity of 0.76 gmol propene gl s1. For the other three samples, no correlation between activity and surface area was found. In oxidative conditions, the selectivity toward propene is lesser than 20%. Similar experiments of ODH carried out at 400~ with pure siliceous MCM materials showed that this solid does not present activity in this reaction, confirming that chromium species, probably Cr(III), are the active sites or influence the acid properties of the siliceous support. Table 4 Catalytic activity in the oxidative dehydrogenation of propane of SiCr-MCM materials, after 200 rain on stream. Sample Conversion Act C1 a Act C2 a Act C3F16" Select. C3H6

(%) SiCr50rt 1

10.9

0.00

0.03

0.20

12.1

SiCr50rt3

2.5

0.00

0.02

0.07

18.1

S iCr50hy3

2.3

0.00

0.01

0.06

17.6

S iCr5 rt3

36.0

0.06

0.09

0.76

13.5

a Catalytic activity (~mol/g s) towards the formation of methane propene (C3).

(C1),ethane (C2) and

Under these conditions, SiCr5rt3 displays a higher activity than the catalyst VAPO-5 (0.14 ~mol g-X s-l), prepared according to reference [9]. Therefore, this preliminary study has shown that mesoporous Cr-based catalysts, where Cr species are highly dispersed into the silica matrix, show, at least, an oxidative propane dehydrogenation activity at 400~ comparable with that of V-based catalysts at T> 500~

1.0

0.8e

0 ::k

KXXXXXXXXXXxx XXXXxx X

0.6-

9 SiCr50rtl 9 SiCr50rt3 o SiCr50hy3 x SiCr5rt3

0.49

ilillililnmnniiinlnmi

i~ o.2"7.

l

eJul

0.0 0

I

I

50

100

I

I

I

I

-

150 200 250 300 350

time on stream / min Figure 5. Catalytic activity of SiCr-MCM materials as a function of time on stream

910 We thank the CICYT (Spain), project MAT97-906 and the University of Malaga for finantial support. REFERENCES

1. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L.Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. A. Sayari, Chem. Mater., 8 (1996) 1840. 3. N. Ulagappan and C.N.R. Rao, Chem. Commun., (1996) 1047. 4. W.Z. Zhang and T.J. Pinnavaia, Catalysis Letters, 38 (1996) 261. 5. K.K. Kearby in Catalysis, P.H. Emmett (eds.), Reinhold, New York, 1955, Vol. 3, p.453. 6. (a) S. De Rossi, G. Ferraris, S. Fremiotti, A. Cimino and V. Indovina, Applied Catal. A: General, 81 (1992) 113. (b)S. De Rossi, G. Ferraris, S. Fremiotti, E. Garrone, G. Ghiotti, M.C. Campa and V. Indovina, J. Catal., 148 (1994) 36. (c) O.F. Gorriz, V. Cort6sCorber~m and J.L.G. Fierro, Ind. Eng. Chem. Res., 31 (1992) 2670. 7. P. Olivera-Pastor, J. Maza-Rodriguez, A. Jim~nez-L6pez, I. Rodriguez-Ramos, A. Guerrero Ruiz and J.L.G. Fierro, in New Developments in Selective Oxidation II, V. Cort6s-Corber~m and S. Vic-Bell6n (eds.), Elsevier, 1994, p. 103. 8. R.W. Cranston and F.A. Inkley, Adv. Catal., 9 (1957) 143. 9. P. Concepci6n, J.M. L6pez-Nieto and, J. P6rez-Pariente, J. Molec. Catal., 99 (1995) 173.

91998ElsevierScienceB.V.All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

91l

I n f l u e n c e of v a n a d i u m a n d c e r i u m a d d i t i v e s in t h e d e v e l o p m e n t of porosity and surface acid c a t a l y t i c p r o p e r t i e s of m e s o p o r o u s aluminophosphates E.Galanos ~, K.Kolonia ~, D.Petrakis ~, M.Hudson b and P.Pomonis ~* a Department of Chemistry, University of Ioannina, Ioannina 45110, Greece b Department of Chemistry, University of Reading, Reading RG6 2AD, UK

Mesoporous materials promise to generate a new range of catalysts. Mesoporous aluminophosphates containing vanadium or cerium additives and having the general formula AllooPxMY (X,Y=0,5,10,20, M=V,Ce) have been prepared by precipitation with NH 3 and calcination at 600~ . XRD analysis of the resulting solids showed that vanadium is incorporated into the amorphous mesoporous aluminophosphate solids except for Y=20 where the phase of V2Q is apparent. On the contrary, cerium is not incorporated into the aluminophosphate structure and CeO 2 is apparent for Y=5 to 20 while in the last case some CePO 4 also appears in the XRD patterns. The calcined solids were examined for their surface area and porosity by N 2 adsorption. At zero or constant concentration of phosphorous, the addition of vanadium increases the specific surface area (ssa) of the parent AllooPoMo (201 m2/g) solid, its influence maximized at Al~ooP~V~0 (360m2/g) and at Allo0PloV5 (386 m2/g). On the contrary, the addition of cerium results in a gradual drop of ssa to -150 m2/g depending on the P content. All the materials AllooPxMY were essentially mesoporous in nature with mean pore diameters (dp)=5-12 nm. The surface acidity of each of the solids was determined by NH 3 adsorption in a flow system and a surface density between 0.6 to 2.4 acid sites.nm 2 was found for the vanadium containing solids while for the cerium containing ones it was about one order of magnitude lower, depending on the sample. The acid catalytic behaviour of these mesoporous materials was examined using the isopropanol decomposition as a probe reaction. The main products obtained were propene and di-iso-propyl-ether while for the vanadium containing solids some dehydrogenation was also observed towards acetone. These differences in catalysis between the vanadium and the cerium containing mesoporous aluminophsphates solids are discussed in relation to their surface acid density while the variations in selectivity are discussed in terms of apparent activation energies.

912

1. INTRODUCTION The addition of phosphate ions during the preparation stages of various aluminabased gels, results eventually in the formation of aluminophosphate materials with micro - and/or mesoporous texture. The resulting aluminophosphate solids, sometimes doped with a second and even a third cation, like such as nickel and molybdenum, are proven hydrotreating catalysts [1-6]. Such solids have been extensively studied by various workers [7-12] in order to understand the mode of incorporation of phosphorus in alumina [13] as well as the way this process modifies the electron donor and acceptor properties of the surface [5,14,15] and the structure and texture of the resulting solids [4,16]. Among the reasons put forward for the stabilising action of phosphorus is the inhibition of formation of spinel aluminates with a second added cation; the enhancement of tetrahedrally co-ordinated transition-metal cations doped into the system; the better dispersion of the active phase and the reduced coke formation - an effect apparently related to modified surface acidity. Perhaps the best known class of microporous phosphate solids are the SAPOs [17,18] which possess a zeolite-like structure. At the same time various research groups have studied the development of mesoporous aluminophosphates as adsorbents and catalysts [19-23]. The present work is a follow-up of previous studies [24-30] which were referred to the synthesis and the catalytic behaviour of aluminophosphate solids. Those studies were based on solids which were obtained via precipitation of nitrates salts with ammonia and eventual thermal decomposition of the ammonium nitrates and formation of extensive porosity and considerable surface area. Starting from 1992 the synthesis of mesoporous solids took a new turn after the ground breaking work by the Mobil group[31] for the development of the well known today as MSM and MCM mesoporous aluminosilicate materials possessing ordered structures. Various groups followed this initiative and made substantial progress in the synthesis of mesoporous solids like zirconia [32-33], zirconium oxophosphate [32], niobia [34], titania [35], and other oxides reviewed critically in a recent article [36]. In such studies the influence of phosphorous is considered to be of great importance for the development of specific surface area and porosity, which under controlled conditions obtains shapes mimicking the pores observed in microorganisms like radiolaria and echinoides [37]. The present study examines the above factors influencing the development of porous aluminophosphates, in which vanadium or cerium has been added. The choice of these elements, V and Ce, was influenced by the fact that they are components of proven catalysts in many cases [38,39]. 2. EXPERIMENTAL AND RESULTS 2.1 P r e p a r a t i o n of s p e c i m e n s The samples examined have the general formula Al~ooPxMy-600 where X,Y=0,5,10,20 and 600 the final firing temperature. The preparation took place as follows: The calculated amounts of Al(NO3)3.9H20 (Merck p.a.), H3PO 4 (Ferak

913 p.a.) and Ce(NO3)3.6H~O (Aldrich) were dissolved in 250 ml of distilled water for Al~ooPxCeY. For Al~ooPxVY V20 ~ was used which was dissolved in 10 ml of NH4OH and was finally added to the first solution. Then NH4OH (Ferak p.a.) was added gradually u n d e r stirring up to pH=9.5. The gel which formed was dried at ll0~ for 48 h. Since previous thermogravimetric studies had shown that such gels lose weight around 300-350~ and stabilise their weight above this temperature, the heating at this region took place very slowly in a tubular furnace under atmospheric conditions. The final firing temperature was set to 600~ for a 4h period for the samples with the formula Al~ooPxVY and 6h period for the samples with formula AI~ooPxCeY. The thirty-two samples were prepared, 16 containing vanadium and 16 containing cerium. These together with some of their properties are listed in Table 1. Table 1 Surface area, pore volume, mean pore diameter (4V/A) centre and FWHM (Full Width at Half Maximum) of the p.s.d. (pore size distribution) curve for the All ~176 . . . . . . . . . . . . . . . . . . . . . . .

Sample AllooPoMo AllooPoM5 AllooPoMlo AllooPoM2o AllooPsMo Al~ooP~M5 AllooP~M~o AllooP~M2o AllooPloMo AllooPloM5 AllooPloMlo AllooPloM2o AllooP2oMo AllooP2oMs AllooP2oMlo AI~ooP2oM~Q

Surface area

Pore Volume

(BET)/m2/g WCe ..... 201/234 270/193 302/177 177/168 245/386 353/261 359/228 386/209 320/390 386/319 321/231 305/210 239/322 336/235 257/255 200/183 .....

(BET)/cm3/g

Mean pore diameter (4V/A) nm

Centre of p.s.d, nm

FWHM of p.s.d. nm

WCe 0.43/0.39 0.45/0.32 0.50/0.28 0.46/0.27 0.52/0.73 0.70/0.39 0.83/0.46 0.74/0.29 1.19/0.91 1.15/0.93 1.11/0.60 1.02/0.33 0.93/0.85 0.47/0.92 0.36/1.01 0.24/0.38

WCe 8.5/6.7 6.6/6.6 6.6/6.4 10.3/6.4 8.4/7.6 8.0/6.0 9.3/8.0 10.4/5.6 14.0/9.3 11.9/11.7 13.8/11.0 13.4/6.2 15.5/10.5 5.7/15.6 5.6/15.9 4.9/8.2

WCe 6.5/5.7 5.4/5.5 5.2/5.3 8.1/5.3 6.3/6.5 6.2/5.7 6.9/7.4 7.3/4.6 9.1/8.1 9.2/11.3 10.2/11.7 9.2/6.0 10.7/9.0 10.3/13.6 12.4/14.0 12.6/8.7

WCe 2.5/2.0 2.1/2.5 2.1/2.5 5.2/2.7 2.4/3.1 2.9/4.4 3.4/3.7 4.1/3.9 4.4/3.8 4.6/6.0 5.4/6.9 6.0/5.8 6.3/7.4 5.9/9.7 10.2/9.2 10.3/7.9

2.2 XRD The XRD patterns of the dried samples were recorded in a Siemens automatic diaffractorr.eter. The results are shown in Fig. 1

914 2.3 S u r f a c e a r e a a n d P o r o s i t y The pore size distribution m e a s u r e m e n t s were carried out using a Fisons Sorptomatic 1900 instrument. The characterisation techniques included the determination of nitrogen adsorption-desorption isotherms from which the pore size distribution was also found. The calculated surface area and pore volumes are cited in Table 1 and shown as a function of P and M content in Fig. 2.

I +A1203, r

i

I

I

9CeP04

o CeO2,

Of le

I

I

J

9oo.

o

e _

I

I

4" A 1 2 0 3 ,

,

i

V AIPO4, t

'" '1

P2oCe2o

o

oo

I

9 V205 I

i

"

I

'

X2.5

0

P2oV2o

PoV2o P2oVlo r~ . I,,,w

PloVlo ,A

A

^

P2oCelo PoVlo P2oV5

<

P~oCe5

PoVo 20

40

60

80

20 2O

Figure 1. XRD p a t t e r n s for the AllooPxMYsolids

40

(60

8O

915

4~o

~ .

E

~=-.-

....

0%

_o- ~

~

,o~ § ~o%] . . . . . . .

AI:~176VY{. . . . . . .

AI'~176Cey

400 350 3OO 25O 2OO 150 ~

1,2

~

1,0

~

0,8 0,6

~

0,4

O

0,2 0,0 6,0 5,0 4,0 "~ 3,0 7~ 2,0 1,0

~

~.0 15

t

-I

5

~[..

'

0

5

10

15

20 0 5 i0 15 20 %P Figure 2. Variation of the specific surface area, the pore volume and the surface acidity as a function of P for various amounts Y of V and Ce.

916

2.4 Surface acidity The surface acidity of the prepared solids was determined by adsorption of NH 3 in a flow system. Each sample (0.3-0.4 g) was put in the perforated bed of a silica tube of 0.5 m m internal diameter heated externally by a tube furnace. A t e m p e r a t u r e p r o g r a m m e r enabled the temperature to be increased in a controlled way. The a d s o r p t i o n - desorption cell was connected to a Shimadzu gas chromatograph equipped with a thermal conductivity detector (TCD) in order to record and identify the evolving gases. The gases were m a d e to flow out through a trap containing a known amount of HC1 0.01M. The experiments took place as follows: the solids were initially degassed by heating t h e m for 89h at 500~ in a flow of 20 cm3/min of He. The temperature was dropped to 100~ and an a m m o n i a s t r e a m was forced slowly through the sample for 89h at 1.5 bar. Then the flow of ammonia was stopped and the system was switched again to He. The t e m p e r a t u r e was raised slightly to 105~ in order to remove a n y excess of NH~ t r a p p e d in the cell. This stripping process lasted for 2.5 h. Then the desorption was started by increasing the t e m p e r a t u r e of the furnace at rate of 10~ -1 up to 500~ The desorbed NH 3 was recorded in the Gas Chromatograph. The total a m o u n t of NH 3 trapped in the HC1 (0.01M) was calculated by titrating the excess of HC1 with NaOH (0.01M). The results calculated in terms of acid sites/gram of solid as well as acid sites/m 2 are shown as a function of P in Fig. 2.

2.5 Catalytic experiments The catalytic decomposition of isopropanol took place in a bench-scale flow microreactor. Briefly the reactor consisted of a silica tube, 1 cm in diameter, with a performed glass bed onto 0.5 g of the catalyst was placed. The system was h e a t e d with a t u b u l a r furnace with a control system to within _+2~ Analyses of r e a c t a n t s and products were carried out by sampling 1 cm 3 of the gases in a S h i m a d z u GC-15A gas Chromatograph equipped with a t h e r m a l conductivity detector and connected to Chromatopac C-R6 integrator. The column used for analysis was a 2 m stainless steel 0.125 in. tube (1/8") containing 10% Carbovax 20M-Chrom WR]N 0-100 mesh. Helium was used as the carrier gas in the gas chromatograph. Helium was bubbled through the reactor. U n d e r the experimental conditions, the partial pressure of IPA was 32.8 mm Hg. M e a s u r e m e n t s were t a k e n randomly between 200-300~ at 10~ intervals. Before any m e a s u r e m e n t s were made, the reaction was allowed to r u n for 30 min. No signs of catalyst 'die-off were observed during the experiments. From the percentance degree of conversion, the reaction rate per unit catalyst surface was calculated and the results are shown in Fig.3 as a function of t e m p e r a t u r e . In the same Fig. 3 the reaction rate per acid site is shown as a function of %P as well as the corresponding a p p a r e n t activation energies for the total conversion of isopropanol, found from the plots lnR=f(1/T).

917

'I

l

, ...... '

"'

I'"'

'''

' ....

'

I

'

'

'

'

'

-'-'

'

I

|"

AllooPxVy

10.0 ~ "-4

"

'

~

'

'

'

""'

I

'

~

'

'

'

I

!

il

|

,

I

hllooPx Coy T = 150 ~

T = 150 ~

"

8.0

.~~

6.0

~

,.0 2.0

m

6.0

"~

4.0 !

.

I

2.0

~

i

0.0

L_

140

! 0

120

~,

100

. "

o .

80 6O o

4o

~

~

0

J

_

J

5

l

l

|

l

10

l

,

l

l

15

l

i

l

20 0

.

.

.

.

5

.

.

.

10

15

20

%P Figure 3. Variation of reaction rate per m 2 and per acid site as a function of P for various amounts of V and Ce. The Eapp for the total conversion is also shown.

918

3.

D

I

S

C

U

S

S

I

O

N

The experimental results shown in figures 1, 2 and 3 can be summarized as follows: Addition of phosphorus gradually transforms the ~-A120~ into an amorphous material ( F i g . l ) w h i l e the addition of v a n a d i u m does not affect the crystallinity of alumina. At 20% addition of phosphorus, A1PO 4 also becomes apparent in the form of a poorly crystallized solid. Addition of v a n a d i u m up to 10% results to its incorporation into alumina and/or alttminophosphate solids and no other crystal phase is seen in XRD. Only at 20% addition of v a n a d i u m the phase of V20 ~ becomes apparent in X-rays. Addition of cemum into the aluminophosphate solids does not lead to its incorporation into the p a r e n t A1203 solid but it remains distinct as a CeO 2 phase for addition up to 10%, while for 20% addition of Ce and P some CePO 4 also becomes apparent. The surface area of the p a r e n t alumina increases substantially with 5-10% addition of P, while 20% addition of P results in a drop of ssa almost to its original values. The addition of v a n a d i u m leads to an increase of ssa and a kind of synergetic effect is a p p a r e n t between P and V in increasing the ssa. - The addition of cerium leads to a drop of ssa in a r a t h e r well defined way. - The pore volume for all the v a n a d i u m - containing solids is maximum at P -10% while for the cerium containing solids it shows a continuous increase with P addition, which is much less profound for the sample containing 20% Ce. - The addition of phosphorus increases the acid sites per g of the Al~ooP~V~ solids and the same effect is also seen with the addition of vanadium. On the contrary the addition of cerium results in a drop of acid sites g-~, as justified by its basic nature. - The density of the acid sites per m 2 also increases with the addition of phosphorous and vanadium, while the addition of Ce does not affect the surface density of the corresponding solids. - The catalytic activity for the isopropanol conversion of Al~ooPxVy calculated per m 2 is higher w h e n compared to Al~ooPxC% solids, a fact justified by the higher activity of the first group of solids. - The catalytic activity calculated per acidic site, appears almost similar for both classes of solids, as perhaps expected. An exception is the Al~ooPoVo material and to a lesser degree Al~ooPoV~ sample, which show much higher activity. The sample containing 20% of Ce is appreciably less acidic and to less active for the total isopropanol transformation. - The a p p a r e n t activation energies for the total conversion of isopropanol on Al~ooPxCey solids are almost constant and around 120_+10 kJ/mol. On the contrary, on the Al~ooPxVy solids the E pp drops for y = 5 and 10 for all the samples. The sample with V = 20% shows a r a t h e r unusual behaviour which might be due to poor dispersion of v a n a d i u m on the surface and the formation of V205 islands [29] which affect the activity in a different way as compared to the rest of the samples. -

-

-

-

-

-

919 4. CONCLUSION In conclusion, AllooPxMy (M = V, Ce) solids can be prepared in forms possessing high specific surface area and controlled pore size distribution. The acidity of such solids can be also controlled by the addition of P and V and/or Ce. Such solids can be proved useful catalysts in catalytic transformations when large pores and controlled acidities are needed. Aknowledgements: The authors wish to aknowledge financial support from progect TMR ERB FMRXRT 960084 and PENED '95,717 program. REFERENCES

,,

3. .

.

6. 7. 8. .

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

A. Morales, J.A.S. Guilen, M.M. Ramirez de Angelo, N.P. Martinez and A.R. Carasquel, U.S. Pat., 4 600 703, 198{}. A. Schmidtmeyer, J.B. Moffat, J. Catal., 1985, 96, 242. E.E. Santos, J.M. McRudio and M.P.M. Alvarez, React. Kinet. Catal. Lett., 1984, 24, 247. T.T.P. Cheyng, K.W. Willox, M.P. McPaniel and M.M. Johnson, J.Catal., 1986, 102, 10. S.M. Yang, M.J. Lee and S.T. Wu, J. Chin. Chem. Soc., 1987, 34, 99. J.M. Campelo, A. Garcia, D. Luna and J.M. Marinas, J. Catal., 1986, 102,447. C.W. Fitz and H.F. Rase, Ind. Eng. Chem., Prod. Res. Dev., 1983, 22, 40. R.E. Tischer, N.K. Narain, G.J. Stiegel and D.L. Cillo, Ind. Eng. Chem. Res., 1987, 26,422. A. Morales and M.M. Ramirez de Agudelo, Appl. Catal.,1986, 23,433. H. Topsoe, B.S. Clausen, R. Candia, C. Wivel, R. Nevald and S. Morup, J. Catal., 1981, 68,433. K. Gisti, A. Iannibello, S. Marengo, G. Moreli and P. Tittarelli, Appl. Catal., 1984, 12, 381. P.D. Hopkins and B.L. Meyers, Ind. Eng. Chem., Prod. Res. Dev., 1983, 22, 421. G. Buska, F. Gavini, G. Genii and F. Triffiro, J. Catal., 1986, 99, 400. J.S. Buchanan, J. Apostolakis and S. Sundaresan, Appl. Catal., 1985, 19, 65. B.K. Hodnett and B. Delmon, Appl. Catal., 1984, 9, 203. K. Segaw, Y. Kurusu, Y. Nakajima and M. Kinoshita, J. Catal., 1985, 94, 495. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 1982, 104, 1146. E.M. Flanigan, B.M. Lok, R.L. Paton and S.T. Wilson, Pure Appl. Chem., 198{}, 58, 1351. N.V.Grebenko, L.S.Eshchenko and V.V.Pechkovskii, Russ.J.Inorg.Chem., 1977, 22(9), 1275. A. Blanco, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, J. Catal., 1992, 137, 51.

920 21. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and M.S. Moreno, J. Chem. Soc. Faraday Trans., 1989, 85(1), 2535. 22. A. Blanco, J.M. C ampelo, A. Garcia, D. Luna, J.M. Marinas and M.S. Moreno, Appl. Catal., 1989, 53, 135. 23. J.M. Campelo, J.M. Marinas, S. Mendioroz and J.A. Pajares, J. Catal., 198{}, 101,484. 24. D.E. Petrakis, P.J. Pomonis and A.T. Sdoukos, J. Chem. Soc. Faraday Trans. 1,1989, 85, 3173. 25. D.E. Petrakis, P.J. Pomonis and A.T. Sdoukos, J. Chem. Soc. Faraday Trans.,1991, 87, 1439. 26. D.E. Petrakis, P.J. Pomonis and A.T. Sdoukos, J. Chem. Soc. Faraday Trans.,1991, 87,901. 27. D.E. Petrakis, M.J. Hudson, A.T. Sdoukos, P.J. Pomonis and T.B. Bakas, Colloids Surf. A: Physicochem. Eng. Aspects, 1994, 90, 191. 28. D.E. Petrakis, M.J. Hudson, A.T. Sdoukos, P.J. Pomonis and T.B. Bakas, J. Mater. Chem.,1995, 5(11), 1983. 29. K.M. Kolonia, D.E. Petrakis, T.N. Angelidis, P.N. Trikalitis and P.J. Pomonis, J. Mater. Chem.,1997, 7(9), 1925. 30. D. E. Petrakis, I. Pasxalidis, C. R. Theocharis, M. J. Hudson and P. J. Pomonis., J. Colloid Interface Sci., 1997, 185, 104. 31. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. T-W. Chu, D.H. Olsom, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlemker, J. Am. Chem. Soc., 1992, 114, 10834. 32. U. Ciesla, S. Schacht, G. Stucky, K. Unger and F. Sch~ith, Agnew. Chem. Int. Ed. Engl., 1996, 5(5), 54. 33. Knowles and M. Hudson, J. Chem. Soc. Chem. Commun., 1995, 2083. 34. D.M. Antonelli, A. Nakahira and J.Y. Ying, Inorg. Chem. 1996, 35, 3126. 35. D.M. Antonelli and J.Y. Ying, Agnew. Chem. Int. Ed. Engl.,1995, 34, 2014. 36. Q. Huo, D. Margolese, U. Ciesla, D. Demuth, P. Feng, T. Gier, P. Sieger, A. Fironzi, B. Chmelka, F. Schuth and G. Stucky, Chem. Mater. 1994, 6, 1176. 37. S. Oliver, A. Kuperman, N. Coombs, A. Lough and G. Ozin, Nature, 1995, 378, 47. 38. a) S. Albonetti, F. Cavani and F. Trifiro, Catalysis Reviews, 1996, 38 (4), 415. b) H.H. Kung, Adv. Catalysis, 1994, 40, 1. 39. A.Trovarelli, Catalysis Reviews, 1996, 38 (4), 439.

91998ElsevierScience B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

921

Group IV Phosphate - Cationic Surfactant Mesostructured Materials Carla Ferragina a Adriana De Stefanis b and Anthony A.G. Tomlinson b aIstituto di Metodologie Avanzate Inorganiche and blstituto di Chimica dei Materiali, Rome Research Area, CNR, C.P. 10 Monterotondo Staz., 00016 Rome, Italy

The hydrothermal intercalation of long-chain cationic surfactants C nH2n+lNMe3+OH" (n=12, 14, 16) into group IV phosphates has been investigated. With C12-14-16, a- and ~-zirconium phosphate and ~/-titanium phosphate give rise to organo-inorganic composite materials having mole ratio loadings ranging from 0.38 -0.57 (a) and 0.45-1.53 (~) and interlayer distances doo2 from 20.74A to 34.00A. The surfactants adopt differing conformations within the layers, the dominating factor being the presence of H-bonding between NMe 3+ end-group and interlamellar water molecules. Evidence for the presence of mono-, bi- and interdigitated (~/phases) is put forward on the basis of FT-IR spectra. Both amorphous and layered materials are given with titanium phosphate gels, the former again giving evidence for the presence of different interlamellar conformations. A new tetramethylammonium titanium phosphate, (Me4N)0.12 [Ti(HPO4)2]0.73H20(TMA.. TiP) is shown to differ structurally from a- and ~- structured analogues. This material also gives nanocomposites with varying gauche/trans conformations that are responsible for the changes in materials with water loss leading to interlamellar rearrangements.

1. I N T R O D U C T I O N Discovery of surfactant routes to mesoporous MCM zeolites [1] has led to a plethora of reports on surfactant-inorganic interactions with non-aluminosilicate-materials, including alumino-phosphates [2] and oxides, e.g. ZrO2, V205 [3 ]. Although group IV phosphates: aand "rM(HPO4)2-nH20 (M=Ti, Zr; n=l, 2) have been known for over 30 years [4], their interaction with surfactant templates has been little investigated. Hydrothermal methods have now been utilised with the series of cationic surfactant CnH2n+INMe3+OH- (n=12, 14, 16) as templates, to scope the existence of mesostructured materials, and whether intercalates with differing ordering are given. Recently, Giannelis et al pointed out that since long-chain amines can adopt variable trans/gauehe configurations in smectites [5] the straight, bent, mono- or bi-layer structures suggested in the past are too simple. An even more varied behaviour would be expected with the Gp. IV phosphates which differ greatly in surface structure from smectites and between a- and ~- forms. In addition, mesoporous materials are known to be formed hydrothermally via layered intermediates by folding over of silicate

922 sheets, as in FSM-16 materials [6]. Given this, and the fact that phosphate hydrolysis is believed to be common in these materials [7], it was hoped that phosphate sheets might fold over or phosphate fragments produced give rise to 'walls', leading to MCM analogues.

2. EXPERIMENTAL 2.1. Materials Titanium phosphate gel and the a- and )- group IV phosphates (TiPg, a-ZrP, ~-ZrP, ,/-TIP) were prepared by established methods and characterised by XRPD and TG-DTA [8]. To prepare TMA.TiP, a gel consisting of [85% H3PO4 : TiO2 : TMAOH : H20] [6.9 : 1 : 3.5 : 66] was reacted at 170~ for 5 days in a hydrothermal bomb. The white product was filtered, washed with water and dried in air. Water loading and order of addition of reagents were critical; precursor mixtures with higher water content or where phosphoric acid was added first to TiO2, gave amorphous solids. Differently than the TMA intercalate of ~Ti(HPO4)2-2H20 [3], the final solid from this reaction did not spontaneously re-disperse in water. C12, C14, C16 surfactants were Aldrich products (99.9 % pure) and were used exchanged with anionic resin. 2.2. Intercalation materials In a typical preparation, 100 mg of ,/-ZrP and 6 ml of C 16 solution (as C16H33N(CH3)3+OH ") were stirred for several minutes and the gel obtained treated hydrothermally at 100~ for 4 days. The white polycrystalline product was filtered off and dried at 45~ Preparations with other a-ZrP, ~-ZrP, ~-TiP and TiPg were analogous. Hydrothermal reaction of TMA .TiP with C 16 was carried out as before (100~ for 4 days) and the polycrystalline product was dried at 45~ for 3 days (washing with water removed C16, to give an amorphous material). Higher temperatures (> 100~ led to surfactant decomposition, lower ones gave double phases (precursor and precursor with surfactant). 2.3. Characterisation Manipulations were followed by XRPD on a Siefert 3000 diffractometer (Ni-filtered Cu-Ka radiation); slits being narrowed slightly in order to more clearly record peaks at 20 < 3, where necessary. TG-DTA analyses were performed on a Stanton Redcroff 1500 simultaneous thermoanalyser (10~ min -1 heating rate). FT-IR spectra (resolution 4 cm -1) recorded as KBr discs on a Perkin-Elmer 16F PC instrument. Atomic Force Microscope (AFM) images were collected on a Digital Instruments Nanoscope III in air and using tapping mode. Samples were cast gently pressing them onto freshly cut muscovite mica (001) supports.

923 3. RESULTS AND DISCUSSION Table 1 lists the stoichiometries of the materials obtained, together with their interlayer distances. In no case were peaks observed at 2.5-6.0 A, analogous to those in hexagonal surfactant-loaded MCMs, nor others characteristic of other (e.g. cubic) structures. The most obvious point to make is that although surfactant: matrix ratios used were at the maximum of uptake, there was no evidence for the formation of mesoporous phosphates analogous to those formed from aluminophosphates [2]. Table 1 Chemical composition and interlayer d!stances o f the obta'med materia! s. .... ~Material . . . . . . . . . . . . . . . . . . . . .

.d0,02/A. . . . . . . . . .

free.height/A,a

a-ZrP(C 12)0A4-57.70H20

20.74

14.14

a-ZrP(C 14)o.57"35.00H 20

33.10

26.50

a-ZrP(C 16)0.38"10.67H20

29.45

22.85

"~-ZrP(C12)0.58"69.15H20

32.40

22.60

~r

14)0.78"32.80H20

34.00

24.20

~/-ZrP(C 16)1.53.84.80H20

33.30

23.50

~-TiP(C 12)0.45"35.50H 20

32.24

22.94

~-TiP(C14)0.45.36.80H20

33.97

24.67

~/-TiP(C16)0.46"7.26H 20

30.00

20.70

TiPg(C 12)0.63-35.00H20

21.24

TiPg(C 14)0.59"14.54H 20

22.75

TiPg(C 16)0.36-6.00H20 TMA.TiP

amorph 11.35

TMA.TiP(C 12)o. lo 10-20.H2 O TMA'TiP(C 14)0.14-15.38H20

**20.22 **23.37

TMA'TiP(C16)l.OS"64.OOH20......

32.40 . . . . . . . . . . .

.....

.......

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

a As deduced from sheet thicknesses (a-ZrP = 6.6 A; z-TiP = 9.3 A; ~-ZrP = 9.8 A), TiPg = Titanium phosphate gel, ** Impurity of TMA.TiP Presumably this reflects the relative inflexibility of the phosphate layers compared to silicate sheet materials, and also suggests that hydrothermal conditions do not give rise to phosphate fragments, in contrast to previous reports of high hydrolysis in these substrates [9]. Instead, all are layered in structure, with a-Zr phosphate giving lower surfactant loadings than the 'tforms, e.g. as in the lowest-loaded c~-ZrP(C16)O.38.10.67H20, which also has the most ordered structure as judged from peak widths (see Fig. 1). However, although there are large variations in free heights (i.e. interlayer distance -layer thickness, often referred to as 'gallery

924 height'[9]) suggesting variable interlayer ordering of surfactant exist in the materials, these are distributed between a- and ~-forms. Thus, free heights of 22.85A and 23.50A for a-and ~ZrP with C16 agree with the presence of tilted, stretched monolayer C16 chains [5], whereas those of all C14-containing intercalates indicate that a bilayer-rather than fiat-arrangements present. However, free heights in a-ZrP/C12 and ~-TiP/C16 are anomalous, the former (14.14A) agreeing with a monolayer formulation.

d 2

I

32 38 2 0 deg (Cu-Kct) I

~ 5

A

20deg(Cu-Kct)

10 I ~ ~ ~ ~ ~ _ _ _ _ ~ ~ I Q _ ~ 30

35

f e

4O 20 deg (Cu-Kct) Figure 1. XRPD of the materials. Above : a) TMA'TiP(C16)l.08"64.00H20, b) ]TiP(C16)0.46"7.26H20, c) ~/-ZrP(C16)l.53"84.80H20, d) a-ZrP(C16)0.38"10.67H20, below: e) TMA.TiP; f) ~/-TiP (for comparison). Inset shows XRPD aider calcination at 900~

925 All the materials show a considerable increase of the interlayer distance with respect to the precursors, except for TiPg/C 16 which gives rise to an amorphous compound. In all, a single phase is present, except for, in this case, TMA-TiP/C 12 and TMA.TiP/C 14 for both of which mixed phases, with the precursor still being present were obtained throughout the uptake curve up to surfactant TiP 9 = 10. For the new form of titanium phosphate, tetramethylammonium/Ti(HPO4)2, (TMA.TiP), TG analysis provides the formulation (Me4N)o.12[Ti(HPO4)2].0.73H20 and the XRPD is typical of a layered material with basal spacing 11.35A. In addition, XRPD hk0 reflections in TMA.TiP and in the calcined (900~ product differ from a- and ~-Ti(HPO4)2.2H20, and cubic TiP207, respectively (Fig. 1) and the material clearly differs structurally from both. It also differs from the TMA.TiP (do02= 10A) reported recently [3]. The free height difference between ~-TiP/C 16 and ~-ZrP/C16 suggest that the C 16 molecules in the former are nested, i.e. the -----NMe3+ tail of C16 is lodged inside the hydrophobic PO43channels (as found previously for phenH2+). In agreement with this suggestion, the water content in ~-TiP/C 16 is very low and there are distinct differences between stand ~-ZrP/C16 in the FT-IR, especially in the presence of a strong N-H...O band at 1635 cm -1 and clearer N..H bands at 3506 and 3447 cm -1 (Fig. 2).

! A

.//I

'

A

-

a-ZrP/C16

B --- 7-ZrP/C16 C - 7-TiP/C16 D = TMA.TiP/C16 E=C16

r~

<

_

I

,.

I

I

.

I

,

3800 3700 3600 3500 3400 3300

1800

1600

1400

1200

1000

800

600

cm-1

Figure 2. FT-IR spectra of H-bonding (left) and Vl,3(PO43-) region (right).

More importantly, following Giannelis [5], account must also be taken of the only small differences (0.6 kcal/mol, 2.5 kJ/mol) between trans and gauche conformers of-(CH2)chains in such long-chain surfactants [9] Vas(CH2) shifts to higher frequency with respect to Vas(CH2)=2917.8 cm ~ in crystalline 2C16N+2C1Br having an all-trans structure as gauche/trans ratio and-(CH2)- packing density increase, which can be used to monitor alltrans/gauche configurational changes.

926 As seen in Fig. 3, there are indeed small shifts in Vas(CH2) indicating that mixed trans/gauche methylene chains are present in all the intercalates. However, the shifts are

closer to those in 2C16N+2C1Br - than those in smectites, particularly for tx-ZrP/C12_16, indicating presumably that a 'stiffer' (less liquid-like) environment is present in the phosphates. This may be ascribed to a higher directing influence of the more highly charged PO43" groups, which has been well documented elsewhere [10]. Naively, high positive charge would be expected to transmit better with short chains than with longer ones. ix'as CH 2 x ~

vsX,~aCH

w .,,,q

i

i

I

1

i

3000 2950 2900 2850 2800 em-1 Figure 3. FT-IR spectra of Vas(CH2) and vs(CH2) regions. Letters correspond to materials in Table 2.

Several other points also arise from the IR of Fig. 3. Firstly, the higher loaded ,/-phases give Vas(CH2) at lower frequencies than do the lower-loaded a-phase, indicating that Vas(CH2) shiRs are not attributable to packing density effects, and also that the P(OH)2 groups in the former have a more directing effect than the P-OH ones in the latter. Secondly in the a-series, Vas(CH2) frequencies follow the order C12>C 14>C16, and on dehydration shifts are higher in a- than in ~-phases and also higher in ~/-TiPs than in ~-ZrPs. Finally, there is evidence for the presence of non-phosphate associated surfactant, both in the Vas(CH2) (higher-frequency shoulders) and -more clearly- in the Vas(CH2) region. We conclude that configurations in asprepared materials differ, that changes in configuration are brought about by loss of water molecules followed by rearrangement, and that all are in fact solid solutions. This helps in rationalising the non-linear behaviour of the d002 vs C-chain length (see Table

2).

927 Table 2 Summary of XRpD, FTIR and TG data free height A Fig. 3 . . . . . . . . . . . . . . . a a-ZrP(C 16)o.38.10.67H20 22.85 b a-ZrP(C 14)0.57"35.00H20 26.50 c c~-ZrP(C12)o.44"57.70H20 14.14 d ~-TiP(C16)o.46-7.26H20 20.70 e ~-TiP(C14)0.45-36.80H20 24.67 f ~-TiP(C12)o.45-35.50H20 22.94 g ~-ZrP(C16)1.53"84.80H20 23.50 h TMA-TiP(C 16)1.o8.64.00H20 .... i TiPg(C 16)o.36.6.00H20 .... TiPB(C 14)0.59.14.54H20 * the fragment is not released first.

weight of the major fragment for complex thermal C 14-C16 decomposition

Vas CH2 Vs CH2 . 2918.3 2919.0 2921.5 2917.7 2921.5 2922.8 2919.0 2920.3 2921.5

2848.1 2849.4 2851.9 2848.1 2850.6 2851.9 2849.4 2850.6 2853.2

144 144'

200 158 112' .....................

TG-DTA analyses provide support for these conclusions, as shown in Fig. 4 for the aZrP/C12.16 and C16 series. , , , , , , . ,',

, .'l

,

,

, , a'

'

~

~o

o

~

d _

9 ~~ -

v<____~

~

i

1~

,

b

l

ENDO 80

801~

~NDO

, 200

, 400

, 600 T/~

, 800

L 1000 0 ....

~

g

40

80

200 400 660 860 ldoo T/~

Figure. 4. TG-DTA. right" a)a-ZrP(C 12)0.44"57.70H20; b) a-ZrP(C14)0.57-35.00H20; c) aZrP(C16)O.38-10.67H20; left" d) ~-ZrP(C16)l.53"84.80H20, e) '/-TiP(C16)0.46"7.26H20, f) TiPg(C 16)0.36"6.00H2O, g)TMA-TiP(C 16)1.08"64.00H20.

928 After the expected differentiated zeolitic water loss at < 200~ (derived from differences in inter-lamellar H bonding) all three show exothermic losses between 330~176 ascribable to surfactant loss. That of the higher-loaded a-ZrP/C14 is clearly more complex, the two losses at 440~ and 565~ being assignable to loss of entrapped C 14. Generally, we can say that C 12 surfactant is lost from all the matrices between 330 and 600~ with the same pattern: two exothermic peaks, the former high and sharp, the second as a shoulder probably related to the loss of water deriving from phosphate condensation to pyrophosphate. The C 14 and C 16 surfactants show a more complex behaviour. Although C14 in $-ZrP, ~-TiP and TMA.TiP follows the same pattern as C 12, in a-ZrP and TiPg that is a multistep loss between 330 and 600~ Conversely for C 16 the thermal behaviour is inverted.

Figure 5. AFM images. Above TiPg/C12, 9 ~,-TiP/C16, below

TMA.TiP/C 9 16.

A rationalisation of these pattems arises from Table 2 which shows the relationship between doo2 and the surfactant fragments in the TG/DTA curves. The dimension of C12 probably is too small for give evidence of different thermal behaviour in the matrices and the molecule is

929 lost without the necessity of multiple rearrangement during the heating. The C 14 and C 16 are fitted in the Q-type and ~-type matrices with relation with their length (and C number) and the inversion of the more simple loss is to ascribe to host matrix nature. It can be seen from Table 2 that the length of the fragments is directly correlated with the interlayer distance of the materials. The higher the distance the larger the size of the first fragment, probably the one bearing nitrogen. Surfactant-loaded materials from TiPg are also layered except for C 16- Nevertheless they show lower interlayer distances with respect to the other matrices. Their free height is anyway not calculated because the starting material is amorphous. Although all attempts to obtain atomic-scale information did not succeed, the morphology adopted, as monitored by AFM is also of some interest. Fig. 5 shows that TMA.TiP/C 16 has a lower degree of organisation, whereas ~t-TiP/CI6 gives rise to more ordered stacked platelets measuring c a 0.4 x 0.2 mm x 0.3 pm in thickness. These are "slipped" with respect to one another by c a 25nm. Turning to the TiPg/C12, this shows a well defmed morphology of the platelets with 119 ~ angles and the stackering is in fact closer than in the previous two due to the smaller size of the surfactant. In conclusion, lamellar Me(CH2)nNMe3+-Group IV phosphates, with mesostructures dependent on surface phosphate details, can be readily prepared hydrothermally, there is no direct correlation between the interlayer distance and the length of the carbon chain of the surfactant due to variable surfactant ordering. We thank the BRITE-Euram programme of the E.C. (contract No. BRE2-CT93-0450) for their continuing financial support of PLS research. We also thank Mr. R. Di Rocco for his technical assistance.

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, and J.C. Vartuli, USP, No. 5,098,684 (1992); No 5,102,643 (1992); C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and R. Beck, Nature, 359 (1992) 410; J.C. Vartuli, K.D. Schmitt, C.T. Kresge, W.J. Roth, M.E. Leonowiez et al., Chem. Mater., 6 (1994) 2317, and refs. therein. 2. A. Sarayi, V.R. Karra, J.S. Reddy, and I.L. Moudrakovski, Chem. Commun., (1996) 411. 3. M.J. Hudson and J.A. Knowles, J. Mater Chem., 6 (1996) 89. 4. M.S. Whittingham, J. Li, J.D. Guo, and P. Zavalij, Materials Science Forum, 152-153 (1994) 99.

930 5. R.A. Vaia, R.K. Teukolsky and E.P. Giannelis, Chem. Mater., 6 (1994) 1017. 6. S. Inagaki, Y. Fukushima, K. Kuroda, Stud. Surf. Sci. Catal., 84 (1994) 125. 7. C. Ferragina, M.A. Massucci, and A.A.G. Tomlinson, J. Mater. Chem., 6 (1996) 645.. 8. A. Clearfield, R.H. Blessing, J.A. Stynes, J. Inorg. Nucl. Chem., 30 (1968) 2249. 9. A. Alberti, Adv. Mater., (1996), and refs. therein; S.J. Price, D. O'Hare, R.J. France, A. Fogg, and S. O'Brien, Chem. Commun., (1996), 2453. 10.C.A.C. Sequeira and M.J. Hudson (eds.), Multifunctional Mesoporous Inorganic Solids, Kluwer Academic Pub., Netherlands, 1993.

91998ElsevierScienceB.V. All rightsreserved. Preparationof CatalystsVII B. Delmonet al., editors.

931

M e s o p o r o u s A 1 - F e - P - O solids p r e p a r e d in n o n - a q u e o u s m e d i u m : s t r u c t u r e a n d s u r f a c e acid c a t a l y t i c b e h a v i o u r . G. Aptel ~, D. Petrakis ~, D. Jones 2, J. Roziere 2 and P. Pomonis 1.. 1Department of Chemistry, University of Ioannina, Ioannina 45110, Greece. 2LAMMI, Universite' Montpellier II, Place Eugene Bataillon, 34095 Monpellier Cedex5-France. Mesoporous aluminum-iron-phosphate solids of general formula AllooFexPY with X=0, 5, 20 and Y=0, 15, 60 have been prepared in ethanolic medium in the presence of CTAB as precipitating precursor. The solids dried at 600~ are acidic and predominately mesoporous, with specific surface areas (BET) between 310460m2.g-1, pore volumes 0.20-0.35cc-g 1, pore diameters 3-4 nm and surface acid density 8-18-1017sites.m2, depending on the sample composition. The addition of phosphorous favors the development of microporosity while the presence of iron results in more mesoporous solids. The surface acid catalytic behaviour of these materials was tested by isopropanol decomposition in a flow system. The products were propene and diisopropylether. The reaction rate for the total isopropanol conversion, as well as the selectivity towards diisopropylether (SD~E=10-30%), increases with the density of surface acid sites, while that for propene drops with increasing acidity. A mechanism for the formation of the porous solids is proposed based on the increased interaction of iron with the phosphate species. 1. I N T R O D U C T I O N

The development of well modulated mesoporous inorganic structures has received a remarkable impetus [1-5] since the synthesis by the Mobil group of mesoporous molecular sieves (M41S) of which MCM-41 (hexagonal) and MCM-48 (cubic) are two typical members [6]. Such solids have attracted the interest of the scientific community for various reasons: first, the organized self-assemblied precursors usually involved in developing such mesoporous structures have been considered as paradigms mimicking the nucleation growth and final morphology of biomineralization processes [7]. Second, the preparation of mesoporous solids exhibiting controlled width openings, offers the possibility of amendment of the laws controlling the physical and chemical behaviour of adsorbents in such micro-vessel environment which differentiates from that in macro-systems [8]. Last but not least, such mesoporous systems appear promising as new catalysts

932 [9]. The present paper deals with this last subject, preparation and catalytic testing of some mesoporous A1-Fe-P-O solids 2. E X P E R I M E N T A L 2.1. P r e p a r a t i o n The preparation of the solids took place in an ethanolic environment as follows: a calculated a m o u n t of AI(NO~)3.9H20 and Fe(NO3)3.9H20 were dissolved in 100ml of absolute ethanol. To this 10ml of a solution of H~PO 4 of calculated concentration was added under stirring. Atom ratios were All0oFexPY with X=0, 15, 60 and Y=0, 5, 20. Then to this mixture a second solution containing CTAB dissolved in 100ml of absolute ethanol was added under stirring and slight heating until complete dissolution. The amount of CTAB added in each case was such t h a t (moles CTAB): moles (Al+Fe)=l. Then the samples were put in an oven and dried slowly at 60+5~ for 3-7 days, depending on the a m o u n t of phosphorous in the sample. The dried samples were subsequently fired at 220~ for 5h and then the t e m p e r a t u r e was raised slowly (2deg/min) up to and held constant for 4h. The samples were then cooled down in air and ground in a agate mortar. The catalysts thus prepared are listed in Table 1 together with some of their physical and catalytic properties. 2.2. S u r f a c e a r e a a n d p o r e s i z e m e a s u r e m e n t s The determination of specific surface area (ssa) and the pore size m e a s u r e m e n t s are performed using N 2 at 77K on a Sorptomatic - Fisons 1900 instrument. The characterization was based on the determination of adsorptiondesorption isotherms (Fig.l) from which we were able to determine the BET ssa (Sp) as well as the corresponding pore volumes (V) and pore diameters (d,). The results obtained are in Table 1. 2.3. S u r f a c e a c i d i t y d e t e r m i n a t i o n b y N H 3 T P D The surface acidity of the solids was determined by NH 3 TPD experiments in a flow system as follows [10]. Each sample -100mg was put in a perforated bed of silica tube of 0.5cm i.d. heated externally by a tube furnace connected to a temperature programmer. The adsorption-desorption cell was connected to a trap containing a known amount of 0.01M HC1. Initially the sample was degassed in situ by heating it at 500~ for 2h in a flow of 50cm3.min 1 of He. The temperature was lowered to 98-100~ and an ammonia stream was forced to flow slowly through the sample for l h at 1.5 bar. Then the flow of ammonia was stopped and the system was switched again to He. The temperature was raised to l l0~ and kept there for 0.5h to strip the sample of the excess ammonia. Then the desorption was started by increasing the temperature of the furnace at a rate of 10deg.min -1 up to 500~ The desorbed ammonia was trapped in the HC1 containing bottle and calculated by titrating the excess of HC1 by NaOH 0.01M. the results calculated as mmol NH3.g-1 or acid sites.m 2 are listed in Table 2.

933 Table 1: Surface area and Pore size characteristics of Al~ooFexPYsolids. S.~,T

Sample

Vp

(m2/g) (cc/g)

d.(BJH) (/k)

S

ZV.

Mean dp

( m 2 / g ) (cc/g)

(s

AllooPo Feo AllooPls Fe o AllooP6o Fe o

325 218 457

0.28 0.12 0.24

36 -33

395 154 490

0.32 0.09 0.25

33 24 20

AllooPoFe5 AI~ooP~F%

311 259

0.27 0.17

35 36

377 267

0.30 0.15

32 22

AllooP6oFe5

443

0.24

--

380

0.22

23

AllooPoF%o Al~ooP~sF%o AllooP6oF%o

295 289 433

0.34 0.23 0.31

38 34/36 36

375 320 517

0.38 0.25 0.38

40 31 28

The m e a n pore diameter, dp, the accumulated surface, Sac , and the sum of the pore volumes, zVp, were obtained by the C r a n s t o n - I n k l e y method based on the adsorption branch of the isotherm, SBET is the BET ssa and Vp the pore specific volume at P/Po=0.999. The dp(BJH) values are those obtained by direct reading on the B J H graph of p.s.d.. Table 2: Surface acidity and catalytic decomposition of the AllooFexPYsolids. Acid Sites Sample

behaviour

R~A conversion (moles.m-2.s-1.10-9)

m m o l g -1 1017.m-2 160~

190~

220~

for

Eapp

isopropanol

(IPA)

Sprop SDIPE Sacet

kJ/mol

(at x-5%)

AllooPo Fe o AllooP15 Fe o AllooPGo Fe o

0.46 0.50 1.27

8.58 13.82 18.08

0.18 2.44 4.87

2.01 16.49 high

12.48 high high

128 117 108

85 82 72

15 18 28

tr tr tr

AllooPoFe5 AllooP~F % AllooP6oFe5

0.42 0.10 0.04

8.12 2.84 0.56

0.11 0.08 0.06

0.99 0.62 0.60

7.27 3.93 3.44

136 120 110

87 90 90

13 10 10

tr tr tr

AllooPoFe2o AllooP~F%o

0.47 0.56

9.52 11.77

0.28 1.33

0.95 7.61

4.24 high

98 103

72 75

20 20

8 5

AllooP6oF%o

0.92

12.78

4.31

high

high

110

70

30

tr

h i g h - very high activity , tr- traces, Sp~op, SD~E a n d Sacet- selectivity towards propene, ~di-iso-propyl-ether and acetone.The E was calculated for the total app conversion of IPA.

934

I

2OO ,~

!

I

I

I

!

I

!

AllooPo Fe o

I

I

I

I

.__~

f

!

I

!

I

!

I

I

I

|

Alloo PlsFe 0

l..i

I

I

!

' I

!

I

!

'I

I

I ..... !

[.~

Alloo P6oFe o

....

150

100

]Vp=O.'279cc/g ~?

50 ~'l

, '

I I

i '

I I

i '

I I

i '

I I

i '

200 7.. AllooPoFe s

:

i

II 14

[ ,Vp=O.123cc/.~g -- / .... ._

I

, '

I I

I '

I I

I i

I I

i '

I I

i i

t

I Vp=O.239cc/gl "

l:i

II" 14

I l

, l

I I

, t

I I

, J

I I

i i

I I

! '

t

1

II I.

q

caq ,,~ 150 o 100 r162 ;> 50

S=311.0m2/g Vp=0.274cc/g

S=258"7m2/g [~I ~ •

AllooPoFe 2o

200

S=443"2m2/g Vp=0.242cc/g A! lOOP6oFe 20

AllooPlsFe2o

150

100 ;>

50

0

s=295.3n~/g

S=289.On~/g

Vp=O.341 cc/g

Vp=O.227cc/g

0.2 0.4 0.6 0.8

1 0

0.2 0.4 0.6 0.8

S=432.9m2/g I Vp=O.3O8cc/g[

1 0

0.2 0.4 0.6 0.8

P/Po P/Po P/Po Figure 1. Adsorption-desorption isotherms of N2(77K) on AllooPxFy solids. 2.4. C a t a l y t i c t e s t s

The acid catalytic behaviour of the solids was tested in a flow reactor system using isopropanol decomposition as a probe reaction [10]. The reactor consisted of a silica tube, lcm i.d. with a perforated glass bed onto which 250mg of the catalyst was placed. The reactor was heated externally with a tubular furnace controlled with a thermocouple and a feed-back mechanism providing precision >0.5~ Analyses of the reactants and products were carried out by sampling lcm 3 of gases in a Shimadzu GC-ISA Chromatograph equipped with a thermal conductivity detector and connected to a Chromatopac C-RGA

935 integrator. The column used for analysis was a 2m stainless steel tube (1/8 in) containing 10% Carbowax 20M - Chrom WR/N 100 mesh. Helium was used as the carrier gas in the GC and the same gas flowing (40_+2mYmin) through an isopropanol (IPA) s a t u r a t o r drived the alcohol vapours into the reactor at a partial pressure of 32,8 m m Hg. The products obtained after the suitable t r e a t m e n t are listed in Table 2. This table contains the total reaction rate ( R moles m-2.s-1) for the conversion of isopropyl alcohol (IPA) the apparent activation energies calculated from plots lnR=f(1/T) as well as the selectivities towards propene, diisopropylether (DIPE) and acetone. 3. R E S U L T S A N D D I S C U S S I O N In Figure 2 are represented the variation of the surface area and pore size characteristic results (Table 1), and the surface acid and catalytic data (Table 2), with Fe and P content. From this figure the following observations can be easily made:

936 9The addition of 15% P, for each iron content, results in a drop of ssa while 60% P addition increases the ssa again. This behaviour is exactly the inverse to that observed for similar aluminophosphate solids prepared in aqueous solutions [5b,c]. 9The addition of phosphorous into the predominately mesoporous All0oPoFey solids, introduces microporosity, the extent of which decreases in the samples containing increased (20%) amounts of iron. 9The surface density of acid sites increases with P content for 0% and 20% Fe, but drops with the P content in the samples containing 5% iron. This is a rather peculiar effect. 9The catalytic activity for the total conversion of IPA increases with the acidity of the samples as expected. 9The selectivity for the production of D IPE is directly related to the surface density of the acid sites as shown in Figure 3a. 9The selectivity for the production of acetone is favored in samples containing iron, but no phosphorous and, as shown in Figure 3b, it is directly related to the ratio Fe/(AI+P). This kind of dehydrating activity is related to the existence of reducible transition metal cations.

.0%Fe

30-

(a)

.

/

[] 5%Fe

lO

/

(b)

8

9

20

~-

lO

0

I

1

I

5

10

15

A c i d i c sites *10-17/m2

4

0 20

~

0

~*

0.05

I

I

0.1

0.15

I lw

0.2

Fe/(AI+P)

Figure 3. (a) Dependence of the selectivity for the production of diethylpropylether on the surface density of acid sites. (b) Dependence of the selectivity for the production of acetone on the iron content. Finally a reaction mechanism scheme is proposed in Figure 4 for the development of meso- and/or micro-porous AI-Fe-P-O solids. The reactions involved are: I. The reaction between hydroxymetalates (M=A1,Fe) and CTAB where hydroxymetalate-CTAB complexed salts are formed. II. The reaction between hydroxymetalates (M=A1,Fe) species and H3PO 4 to form metal-phosphate salts.

937

%c,

%c~

M-OH + Br- N+-C16

= HBr + ~M-O-N+-C1 s

I Ci

I

I

I

H20 +~M-O-P-0H

I

I

OH

~c, I

OH

( ni )

M=A1,Fe

(IV)

OH C1

o| % c ,

%.,,C, +

M=A1,Fe

1~ c, c,

C1

oI

(II)

OH

/~M-O-P-OH + Br- N+-C,6 "---" Hbr + >M-O-.P-O-N-C,6

HO-~-OH

M=A1,Fe

0

~ M - O H + HO-P-OH ~

OH

(I)

CI

0

?

M=AloFe

Br- 1~1+-C16

= HBr

+

HO-~-O-1]I-C,6

C1

OH C~

u

N t/ )M-O-N~

Mic,.eHeA M either AI o r

Fe

wwvw~ N- O- M/..

i.,. M I

.~M \o\

o ~ P\

OI

o

N

N

l /

)M- O-P-O-N~

/P

,o N

Micelle B M Mainly F e

~ N -

o P/

o"

n~ ,o

N, 0

~

O - P - O - M . .~

o, P\

o

M

Figure 4. For the mechanism of formation of mesoporous A1-Fe-P solids: Without addition of H3PO 4 reaction (I) take place with either A1 and/or Fe and micelles A are formed resulting, after drying, in mesoporous solids. When H3PO 4 is added, species according to reaction (III) can be formed only in the case of iron resulting in micelles of type B, which provide eventually mesoporous solids.

938 III. The reaction between the metal-phosphate salts and CTAB to form metalphosphoro-CTAB entities and IV. The reaction between H3PO 4 and CTAB to form phosphoro-CTAB species. During drying, micelles are formed when the concentration of the water reaches a limit depending on the system. Now one critical observation from figure i is that addition of phosphates into the samples containing only aluminum results in microporous solids while the addition of iron does not permit the total conversion of samples to microporous but favours the mesoporous structure. According to previous studies [11] the complexation of H3PO 4 with alumina (A1203) or geothite (F%O 3) surfaces is much stronger in the second case, the difference of relevant stability constants increasing with the degree of deprotonation of H3PO 4 as follows . First proton hlogK~e_~ = 1.19; second proton AlogKFo_A,- 2.79; third proton, AlogKFe_~=3.98. On the other hand the P-O-N bond formed in the course of reactions (IV) and/or (III), is quite strong. Therefore we reach the conclusion that reaction (III) takes place predominantly in case where M-Fe and to a much less degree when M=A1, according to the above. In other words the addition of H3PO 4 results in a "consumption" of CTAB according to reaction (IV), while in absence of H3PO 4 reaction (I) take place. The species (IV) formed during the corresponding reaction can keep bound >M-O groups, according to (III) only in the case where M=Fe, but not when M=A1. So micelles resulting in mesoporosity (see Fig.4) are formed either in case (I) for M=A1,Fe or in case (III) for M=Fe but not for M=A1. In this last case A1 precipitates as AI(OH) x species and since the growth of such crystals is inhibited by the surfactants and/or phosphate groups in the system, a microporous structure is developed. Acknowledgment: The authors wish to acknowledge financial support from EU projects: HMC contract CHRX-CT-93-0272 and TMR contract ERB FMRXRT 960084.

REFERENCES

(a) Q.Huo, D.I.Margolese, U.Ciesla, D.C.Demuth, P.Feng, T.Gier, P.Sieger, A.Firouzi, B.Chmelka, F.Schiith and G.Stucky, Chem. Mater., 6 (1994) 1176; (b) A.Monnier, F.Schiith, Q.Huo, D.Kumar, D.Margolese, R.S.Maxwell, G.D. Stucky, M.Krisnamurty, P.Petroff, A.Firouzi, M.Janicke and B.F.Chmelka, Science, 261 (1993) 3; (c) G.D.Stucky, A.Monnier, F.Schiith, Q.Huo, D. Kumar, D.Margolese, M.Krisnamurty, P.Petroff, A.Firouzi, M.Janicke and B.F.Chmelka, Mol. Cryst. Liq. Cryst., 240 (1994) 187; (d) U.Ciesla, D. Demuth, R.Leon, P.Petroff, G.D.Stucky, K.Unger and F.Schfith, J. Chem. Soc. Chem. Commun. (1994) 1387.

939 2. (a) P.Behrens and G.Stucky. Angew.Chem.Int.Ed.Engl. 32(1993) 696; (b) P.Behrens, Angew.Chem.Int.Ed.Engl. 33(1996) 515. 3. (a) D.M.Antonelli, A.Nakarita and J.Y.Ying, Inorg.Chem. 35(1996) 3126; (b) D.M.Antonelli and J.Y.Ying, Angew.Chem.Int.Ed.Engl. 34(1995) 2014; (c) D.M.Antonelli and J.Y.Ying, Angew.Chem.Int.Ed.Eng. 35(1996) 426; (d) D.M.Antonelli and J.Y.Ying, Chem.Mater., 8(1996) 874. 4. (a) P.T.Tanev, M.Chibwe, T.J.Pinnavaia, Nature, 368(1994) 321; (b) P.T.Tanev and T.J.Pinnavaia, Science 267(1995) 865;(c) S.A.Bagshaw, E.Prouzet and T.J.Pinnavia, 262(1995) 1242. 5. (a) K.M.Kolonia, D.E.Petrakis, T.M.Angelidis, P.N.Trikalitis and P.J.Pomonis, J.Mater.Chem. 7(1977) 1925; (b) D.E.Petrakis, M.J.Hudson, A.T.Sdoukos, P.J.Pomonis and T.V.Bakas, Colloids Surf A: Phys.Eng.Aspects, 90(1994) 191; (c) D.E.Petrakis, M.J.Hudson, P.J.Pomonis, A.T.Sdoukos and T.V.Bakas, J.Mater.Chem., 5(1995) 1975; (d) M.J.Hudson and J.A.Knowles, J.Mater.Chem., 6(1996) 89. 6. (a) C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartulli and J.S.Beck, Nature, 359 (1992) 710; (b) J.S.Beck, J.C.Vartulli, W.J.Roth, M.E.Leonowicz, C.T.Kresge, K.D.Schmitt, C.T.W.Chu, D.H.Olson, E.W.Shepard, S.B.McCullen, J.B.Higgins and S.L.Schlenker, J.Am.Chem.Soc., 114 (1992) 10834; (c) J.C.Vartulli, K.D.Schmitt, C.T.Kresge, J.S.Beck, J.L.Schlenker, D.H.Olson, E.W.Sheppard,. Chem.Mater., 6(1994) 2317; (d) J.S.Beck, J.C.Vartulli, G.J.Kennedy, C.T.Kresge, W.J.Roth and S.E.Schramm, Chem.Mater. 6(1994) 1816. 7. (a) S.Oliver, A.Kuperman, N.Coombs, A.Lough and G.A.Ozin, Nature, 378(1995) 47; (b) D.Walsh and S.Mann, Nature 377(1995) 320; (c) L.Addadi and S.Weiner, Angew. Chem.Int.Ed.Eng. 31(1992) 153; (d) S.Mann, Nature 365 (1993) 499. 8. (a) R.Evans, U.M.B.Marconi and P.Tarazon, J.Chem.Soc.Faraday Trans. 2, B2(1986) 1763; (b) P.C.Ball and R.Evans 5(1989), 714 (c) P.L.Llewellyn, Y.Grillet, F.Schuth, H.Reichert and K.Unger, Microporous Mater. 3(1994) 345, (d) M.Brun, A.Lallemand, J.M.Quinson and C.Eyrand, Thermochimica Acta 21 (1977) 59 (e) K.Morishige, H.Fujii, M.Uga and D.Kinnkawa Langmuir 13 (1977) 3494. 9. (a) A.Corma, A.Martinez, V.Martinez-Soria and J.B.Morton, J.Catal, 153(1995) 25; (b) A.Corma, V.Formes, H.Carcia, M.A.Miranda and M.S.Sabatier, J.Am.Chem.Soc. 116(1994) 9767; (c) E.Armengol, A.Sukal, O.Franke and G.Schulz-Ekloff, J.Chem.Soc.Faraday Trans. 91(1995) 937. 10. (a) G.Aptel, These Europeene, Universite Montpellier II, (1998); (b) P.N.Trikalitis and P.J.Pomonis, Applied Catalysis A: General 131 (1995) 309; (c) A.K.Ladavos, P.N.Trikalitis and P.J.Pomonis, J. Molecular Catalysis A: Chemical 106 (1996) 241. 11. (a) E.Laiti, P.Person, L.O.Ohman, Langmuir, 12(1996) 2969; (b) N.Nilson, L.Lougren, S.Sjoberg, Chem.Speciation Bioavailability 4(1992) 122.

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91998Elsevier Science B.V. All rightsreserved. Preparation of Catalysts VII B. Delmonet al., editors.

941

Synthesis and physicochemical properties of new hydrotalcite-like anionic clays containing Zr 4+ in the layers.

S.Velu*, A.Ramani, Veda Ramaswamy, Bhanu M.Chanda and S.Sivasanker National Chemical Laboratory, Pune-411 008, India.

New series of hydrotalcite (HT)-like anionic clays containing Zr 4+ in the brucite-like layer have been synthesised by a simple coprecipitation method at constant pH. The incorporation of Zr in the layer have been confirmed by various analytical methods such as powder X-ray diffraction (PXRD), chemical analysis, UV-Vis diffusereflectance (UV-Vis DR) spectra and thermogravimetric analysis. These Zr containing HT-like materials show very interesting properties as catalyst in the liquid phase hydroxylation of phenol to selectively catechol. 1. INTRODUCTION Hydrotalcite (HT)-like anionic clays are a new family of interesting materials with applications as catalysts, catalyst supports, ion exchangers and composite materials [1-3]. The structure of these compounds consists of brucite [Mg(OH)z] type octahedral layers in which a part of M(II) cations are isomorphously substituted by M(III) cations. The excess positive charge of the octahedral layers resulting from this substitution is compensated by interstitial layers built of anions such as CO32- and crystal water. These compounds are represented by the general formula; [M(II)I.xM(III)x(OH)2 ]x+ [(A'),~n y.H20] x', where, M(II) is the divalent cations such as Mg, Cu, Ni, Co, Mn, Zn; M(III) is the trivalent cations such as A1, Fe, Cr, Ga, V, Ru, Rh and Y [4-6]; An is the interlayer anions such as CO32, NO[, SO42, or organic anions such as terephtalate and the value of'x' is in the range from 0.1 to 0.33. A large number of HT-like compounds with a wide variety of M(II)-M(III) cation pairs as well as M(I)-M(III) cation pairs (eg., Li-A1), with different anions in the interlayer region have been reported [7]. However, HT-like compounds with M(II)-M(IV) cation pairs such as Mg-Zr or Mg-Ti are scarcely obtained. Taylor have reported the synthesis of a Co-Ti HT like compound employing an induced hydrolysis method [8]. In the present study we report a new series of ternary HT-like compounds containg Mg, A1 and Zr in the brucite-like layers, synthesised by employing a simple coprecipitation technique. The importance of these Zr containing HT-like compounds as a catalyst in the liquid phase hydroxylation of phenol has been investigated and the preliminary results are presented in this communication.

* Present address: Institute of Chemical Engineering-III, Swiss Federal Institute of Technology, CH- 1015 Lausanne, Switzerland.

942 2. EXPERIMENTAL

2.1. Synthesis of Zr-hydrotalcites Zr containing HT-like compounds with various Mg:AI:Zr atomic ratio were synthesised by coprecipitation method at room temperature by reacting aqueous solutions containing a mixture of Mg(NO3)z, AI(NO3)3 and ZrO(NO3)2, depending on the Mg:AI:Zr atomic ratio and a mixture of NaOH (~ 2M solution) and Na2CO3 (= 0.3M solution) at a contant pH (~ 10) [9]. The Mg/(AI+Zr) atomic ratio in the starting solution was kept as 3 while varying the AI:Zr atomic ratio from 0:1 to 1:0. The resulting precipitate was aged at 338 K for 30 min under stirring in a magnetic stirrer then filtered, washed with distilled water for several times until the pH of the filtrate was 7 and dried in an air oven at 373 K over night.

2.2. Characterisation The chemical composition of the samples were determined by X-ray fluorescence (XRF) spectroscopy (Rigaku 3070 X-ray wavelength dispersive spectrometer). The powder X-ray diffraction (PXRD) of the samples were obtained using an instrument (Rigaku, Model DMAX-III VC) equipped with a graphite crystal monochromator and Ni-filtered Cu I ~ radiation (~ = 1.5418 A). The data were collected in the 20 range 5-70 ~ with a scan rate of 0.5 ~ for calculation of lattice parameters while a speed of 4~ was used for obtaining the PXRD patterns. The observed interplanar 'd' spacing were corrected using elemental Si as an internal standard. The lattice parameters were refined using two different least square fitting programs (HOCT and PDP11). The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of all the samples were obtained using a Shimadzu UV-Vis spectrophotometer 2101 PC model in the range 200 to 400 nm. The thermogravimetry of these samples were recorded in a Setaram TG/DTA 92 model instrument at a scan rate of 10 K/rain under Ar atmosphere. The BET surface area of both uncalcined and calcined samples were determined employing BET N2 adsorption method at 77 K using an Omnisorp 100 CX (Coulter) instrument after degassing the sample at 423 K for 8h. 3. RESULTS AND DISCUSSIONS 3.1. Physicochemical Characterisation The results of elemental analysis for the samples prepared are included in Table-1. The Mg/(AI+Zr) atomic ratio as determined from X-ray fluorescence spectroscopy is coincident, within the experimental error, with that of the starting mixed aqueous solutions and it is similar (3.1 + 0.16) for all the samples indicating the completion of the precipitation. The PXRD patterns [9] of these compounds with Mg:AI:Zr atomic ratio ranging from 3 1"0 to 3:0"1 indicated that samples with Zr = 0 (Mg:AI:Zr - 3 1:0) to Zr = 0.8 (Mg:AI:Zr = 3:0.2:0.8) exhibited the diffraction patterns characteristic of HT-like structure. However, the crystallinity of the samples decreased with increasing Zr content. This is because of the introduction of large distortions in the brucite-like layers due to the substitution of AI 3+ (Shannon ionic radius 0.53 A) by Zr 4+ (Shannon ionic radius 0.72 A). A binary Zr containing HT without AI forms a poorly crystalline ZrO2 phase indicating that the presence of A1 favours the formation of pure HT-like phase. The crystallographic parameters 'a' and 'c' were calculated employing least square refinement assuming a hexagonal crystal

943 Table-1. C h e m i c a l compositions, molecular formula and surface area of M g A I Z r - H T

Sample

BET surface area

Mg:AI:Zr atomic ratio 1

carbon content 2 (%)

Chemical formula3

3:0.96:0.00 3:0.85:0.07 3:0.78:0.14 3:0.68:0.21 3:0.67:0.33 3:0.57:0.37 3:0.52:0.50

2.44 nd 2.71 2.91 3.01 nd 3.51

Mgo.75Alo.25Zro.oo(On)2(CO3)o.14 0.31.H20 Mgo.75Alo.2oZro.o4(OH)2(COs)o.ls0.34.H20 Mgo.75Alo.17Zro.05(on)2(CO3)o.19 0.33.H20

(mz ~-1)

723 K 4 ZrHT-0 ZrHT- 1 ZrHT-2 ZrHT-3 ZrHT-4 ZrHT-5 ZrHT-6

169 202 241 nd 199 nd 192

Mgo.75Alo.17Zro.08(OH)2(CO3)0.21 0.40.H20 -

Mgo.75Alo.asZro.13(OH)2(CO3)o.26 0.40.H20

1 Determined by X-ray fluorescence spectroscopy 2 Determined by CH microanalysis 3 Calculated based on the chemical analysis and the first weight loss in TG 4 Pretreatment temoerature I

I

I

,,

I

3"08

24"2

3 "07

v

"3"06

-

23-8

tJ

3"05 - 23-4

-23"0 I

0

I . . . . . . . .

0-18

I

0-54

....

l

1-22

Zr/AI ATOMIC RATIO

Fig.1

Variation of lattice parameters with Zr content for MgA1Zr-HT

944 system for samples ZrHT-0 to ZrHT-6, wherein the PXRD peaks were intense and sharp enough for the accurate calculation. It can be seen from Fig.1 that the lattice parameters both 'a' and 'c' increases with increase in Zr content and follows the Vegard's law in the region Zr/A1 = 0 to 1.5. The increase in 'a' parameter can be attributed to the isomorphouse substitution of A13+ in the octahedral coordination by Zr4+ in the brucite-like layer. The increase in 'c' parameter can be attributed to the weakening of the interaction between brucite-like layer and the interlayer due to the increase in the thickness of the brueite-like layer upon incorporation of Zr 4+. Furthermore, since a part of the trivalent cation A1 is being substituted by tetravalent cation Zr, the interlayer should accommodate more amounts of CO32 anions for the charge compensation. This argument is supported by an increase in carbon content from 2.44 % for ZrHT-0 (HT without Zr) to 3.55 % for ZrHT-6 (Table-l). A sharp increase in both 'a' and 'c' parameter is observed upto about 10 % of A13+ is substituted by Zr 4+. This is because the initial addition of Zr4+ might substitute A13+which are probably present in the corners of a hexagonal crystal lattice, thereby a sharp increase in the thickness of the brucite-like sheet.

v

I..iJ z

1313

no or)

1::13

,,~

g,

/

,

0

200

500 WAVELENGTH

400 (nm)

Fig.2 UV-Vis DR spectra ofMgAIZr-HT: (a) ZrHT-0, (b) ZrHT-1, 9ZrHT-3, (d) ZrHT3cal., (e)ZrHT-5, (f) ZrHT-8, (g) Pure ZrO2, ( .... ) ZrHT-0 + pure ZrO2 (physical mixture)

945 The dispersion of Zr in the HT matrix was studied by UV-Vis diffiasereflectance spectroscopy (UV-Vis DRS). It can be seen from Fig.2 that all these samples exhibited a ('"'In)] single narrow band around 210 nm which is attributed to charge transfer involving isolated Zr 4+ species [10, 11]. This electronic transition is clearly distinguishable from that of pure ZrO2 which showed strong absorption bands around 240 and 320 nm (Fig.2, curve-e). These results clearly demonstrates the absence of any ZrO2 specie within the sample and indicates that the Zr 4§ cations are well dispersed in the HT framework similar to that in Zr-silicalites [10, 11]. The thermogravimetric analysis (TGA) of hydrotalcite-like compounds generally exhibits two stages of weight loss .~ process. Fig.3 shows the differential curves of TGA (DTGA) of some of the o~ MgA1Zr-HT. All the samples except O -~ ZrHT-10 (without A1) showed two major weight loss process characteristic of HTlike compounds having layered structure [1,6]. The first weight loss (T1) occurring in the temperature range 423-523 K is attributed to the removal of interlayer water. The second weight loss (T2) observed around 623-723 K is due to the removal of structural water and CO2 from the decomposition of CO32- ions in the interlayer. It is interesting to note that incorporation of Zr in the HT lattice results in broadening of the T1 process (curves b-d), indicating that the weight loss is associated with some complicated process, probably due to the removal of large amount of physisorbed water and CO32 ions. In addition, the peak ,t,. I I 57:5 5~'5 7-f3 temperatures both T1 and T2 are shifted TEMPERATURE (K) towards lower temperature with Fig.3 DTG traces ofMgAIZr-HT; (a) ZrHT-0 increasing Zr content (eg., ZrHT-0; T1 = (b) ZrHT-1, 9ZrHT-3, (d) ZrHT-6, 514 K, T2 = 703 K: ZrHT-6; T1 = 436 K, (e) ZrHT-10 (without AI). T2 = 683 K). These results clearly indicates that the incorporation of Zr in

946 the HT lattice facilitates the decomposition process due to the weakening of the interaction between bmcite-like layer and the interlayer (Cf." PXRD results, Fig. 1). A net weight loss of around 40-50 % was recorded in TGA upto 873 K in these samples. From the weight loss recorded for T1 process (373-523 K) and the results of chemical compositions as determined from XRF and CH analysis, the chemical formula of the compounds are derived and they are presented in Table-1 along with the results of chemical compositions. The DTG diagram of ZrHT-10, without A1 shows (Curve-e) a single broad weight loss process probably due to the decomposition of some amorphous hydroxycarbonates since the PXRD of this sample shows a poorly crystalline ZrO2 phase (Fig.l, curve-k). These changes in the thermogravimetric study indicates the incorporation of Zr in the lattice.

9 M90 o MgAI203 V Zr02 HT Unidentified

v o

v

I/

o

d

>-

FO3 Z

lz =_=.

v

v

I

20

I

40 2e (DEGREE)

Fig.4 PXRD patterns of ZrHT-3 calcined at (a) 423, (b) 573,

I

60

9723, (d) 1073, (e) 1473 K

947 Thermal calcination of HT-like compounds results in decomposition of layered structure and forms various m e t a stable phases depending on the calcination temperature. The determination of crystalline phases formed during calcination at various temperatures is useful to understand the thermal stability of the compounds. Fig.4 shows the PXRD patterns of ZrHT-3 calcined at various temperatures for 4h in air. It can be seen that at 423 K, all the peaks corresponding to HT are retained with reduced intensities. At 573 K, the peak corresponding to (006) is completely disappeared while the (003) diffraction line is shined towards higher 20 value (lower interlayer spacing) due to the removal of interlayer water molecules. These results indicates that the HT-like structure in these compounds survive upto at least 573 K. However, at 723 K, the layered structure completely collapses due to the removal of structural water and CO2 from the interlayer. The evolution of these gases during calcination makes craters on the surface hence the surface area increases from around 40 m2 g-1 for the uncalcined samples to about 200 m2 g-1 for samples calcined at 723 K (Table-l). The material at this temperature is referred to as a poorly crystalline MgO, whose crystallinity increases with further increase in calcination temperature to 1073 K. At this temperature, in addition to MgO, weak diffraction lines are recorded which are attributed to the formation of MgAI204 spinel and ZrO2 phases. At 1473 K, the intensity of all these diffraction lines increased to form well defined crystalline phases. It has been reported [12,13] that in the case of MgA1-HT, calcination above 900 K yields a mixture of MgO and MgA1204 phases while a mixture of MgO, MgA1204 and RuO2 phases are reported to be formed in a ternary MgAIRu-HT. On the other hand, Fernandez et al [6] noticed a mixture of MgO, MgAI204 and A12Y409 phases during calcination of MgAIY-HT at 1273 K. However, in the present study we have not noticed any Zr containing phase other than ZrO2 even at 1473 K. Among the phases formed during calcination of HT-like materials, the one formed around 723 K is of particular interest since in many cases the product formed at this calcination temperature are used potentially as catalyst for many industrially important reactions [ 1-3 ]. Hence we have studied, in detail, the nature of product formed at 723 K by PXRD and UV-Vis DRS. Fig.5 shows the PXRD patterns of various MgAIZr-HT calcined at 723 K for 4h. It can be seen that a poorly crystalline MgO phase is obtained in all cases and the crystaUinity decreases further with increasing Zr content. In addition the formation of a separate ZrO2 phase (poorly crystalline) is evidenced in ZrHT-7. It has been reported by several authors [7,12,14] that thermal decomposition of MgA1-HT around 723 K yields a poorly crystalline phase corresponding to MgO whose lattice constant 'a' is less than that of pure MgO and has been attributed to the dissolution of a small amount of A13+in the MgO lattice to form solid solutions. The chemical composition of the resulting solid solution is given by the formula; Mg(1-x)/(2+x)Al2x/(Z+x)[]x/(2+x)O; where [ ] is a cation vacancy created due to the dissolution of part of A1 in the MgO lattice [12]. However, in the present study since the PXRD lines are very broad and less intense, the accurate determination of'a' parameter was not possible. Hence, we have calculated only the d(200) values of resulting MgO phase for all the samples and compared them. As expected, the d (200) value increases with increasing Zr content (Fig.6) and follows Vegard's law in the range Zr/A1 = 0 to 0.4, indicating that a part of Mg 2+(ionic radius 0.66 A) is being

948

9 MgO

.-.

0

~V Zr02_

~"0 (M (M II

"2. v

>.. I-CO Z

b

~

_

~ _ _

lz

V I

I

60 40 2 e (DEGREES) Fig.5 PXRD patterns of MgA1Zr-HT calcined at 723 K for 4h., (a) ZrHT-O, (b) ZrHT-1, 9ZrHT-3, (d) ZrHT-5, (e) ZrHT-8

20

I

2-12 -

/

2.11 -

/

/

/

/

/

I

/

/

/

2.10 -

t

2"0S 0

l

/

/

/

/

/

/

o/

! ....

0-2

I

0-4

I

0.6

Z r l A i ATOMIC RATIO

Fig.6 Variation of d(200) of MgO phase with Zr content for MgA1Zr-HT calcined at 723 K for 4 h.

949 isomorphousely substituted by Zr 4+ (ionic radius 0.72/~) in the MgO lattice. The decrease in d(200) values at higher Zr content can be attributed to the segregation of Zr 4+ at the surface to form a separate Zr02 phase as evidenced from PXRD (Fig.5, curve-e). The UV-Vis DR spectra of Zr0.3-HT calcined at 723 K is included in Fig.2 (curve d). It can be seen that the absorbance maxima is more intense and sharper than that of of the uncalcined samples (compare curves c and d) indicating that the Zr 4+ cations are well dispersed in calcined samples. Some of the Zr containing hydrotalcites have been tested as catalysts in the hydroxylation of phenol using 30 % aqueous solution of H202 as the oxidant. Preliminary studies with H202 ratios from 0.5 to 8 using solvents such as water, CCla or MeOH did not show any catalytic activity. However, appreciable activity (up to 10 % conversion of phenol) was observed when light petroleum (bp 333-343 K) was used as the solvent. Table 2 sumrnarises the results of hydroxylation of phenol over various Zr containing hydrotalcites. It is interesting to note from the Table that the turnover number (TON; moles of phenol converted per mole of Zr atom) increases four-fold (1.0 to 3.8) when I-/202 is added dropwise using a syringe pump. Similarly, a two-fold increase in catalytic activity (1.0-1.9) is noticed if the sample is calcined at 723 K for 5 h. In all these cases catechol is obtained as a unique product (100 % selectivity). Upon increasing the HzO2/phenol ratio, the TON also increases considerably, but the selectivity of catechol decreases at the expense of hydroquinone. The absence of catalytic activity in the ease of pure MgA1-HT without Zr (Zr0.0-HT) or pure ZrO2 clearly indicates that the Zr incorporated in the HT framework plays a pivotal role in catalytic activity. Surprisingly, under the similar experimental conditions, the Zr-containing silicalite (ZrS-1) which showed appreciable conversion in aqueous medium is found to be inactive in the hydroxylation of phenol. Further work is in progress in order to investigate the above differences and also to compare the catalytic performance with similar Ti-containing hydrotalcites.

Table 2 Catalyst

Hydroxylation of phenol with 11202 over MgAIZr-HT H202/Phenol TON 1 Product selectivity imass %.,) Catechol Hydroquinone

Zr0.0-HT 5.0 _ Zr0.1-HT 5.0 1.4 100 Zr0.3-HT 5.0 1.0 100 Zr0.3-HT 2 5.0 3.8 100 Zr0.3-CHT 3 5.0 1.9 100 Zr0.3-HT 7.0 29.1 73.8 26.2 Zr0.3-HT 10.0 40.5 65.1 34.9 ZrS-1 5.0 _ _ ZrO2 5.0 1 TON = Turnover number (moles of phenol converted per mole of Zr atom. 2 H202 was added drop wise using a syringe pump at a rate of 0.5 ml h-1 3 Sample calcined at 723 K for 5 h. Reaction conditions; solvent: light petroleum (bp 333-353 K) 30 ml. Phenol, 1 g. catalyst 100 mg, temperature 353 K, time 8 h.

950 4. CONCLUSIONS Based on the above experimental evidences, we report that pure and crystalline HT-like compounds containing Zr in the layer can be easily synthesised by the simple coprecipitation method. The crystallinity of the material depends on the Zr/AI atomic ratio. The Zr4+ cations are well dispersed in both unealcined as well as in calcined samples (at 723 K) and the resulting Zr-HT like compounds can be used as a potential catalyst in the liquid phase hydroxylation of phenol to selectively catechol.

REFERENCES 1. 2. 3. 4.

F.Cavani, F.Trifiro and A.Vaccari, Catal.Today., 11 (1991) 173. R.J.Davis and Derounce, Nature, 349 (1991) 313. S.M.Auer, R.Wandeler, U.Gobel and A.Baiker, J.Catal., 169 (1997) 1. A. de Roy, C.Forano, F.Ei Malld, J.P.Besse.: in Expanded Clays and other Microporous Solids, Ed. M.L.Occelli and H.F.Robson, Van Nostrand Reinhold, New York, 1992, p. 108. 5. F.M.Labojos, V.Rives, P.Malet, M.A.Centeno and M.A.Ulibarri, Inorg.Chem., 35 (1996) 1154. 6. J.M.Fernandez, C.Barriga, M.A.Ulibarri, F.M.Labajos and V.Rives, Chem.Mater., 9 (1997) 312. 7. M.Bellotto, B.Rebours, O.Clause, J.Lynch, D.Bazin and E.Elkaim, J.Phys.Chem., 100 (1996) 8527. 8. M.M.Taylor, Clay Miner., 19 (1984) 591. 9. S.Velu.S, Veda Ramasamy, A.Ramani, B.M.Chanda and S.Sivasanker, Chem. Commun., (1997) 2107. 10. B.Rakshe Veda Ramasamy, and A.V.Ramasamy, J.Catal., 163 (1996) 501. 11. A.Tuel, S.Gontier, R.Teissier, Chem.Commun., (1996) 651. 12. V.R.L.Constantino and T.J.Pinnavaia, Catal.Lett., 23 (1994)361. 13. S.Velu, D.Samuel and C.S.Swamy in: Catalysis: Modem Trends, Ed. N.M.Gupta and D.K.Chakrabarthy, Narosa, New Delhi, India, 1995, p. 470. 14. S.Velu and C.S.Swamy, Appl.Catal.A:General,, 119 (1994) 241. 15. M.K.Dongare, P.Singh, P.Moghe and P.Ratnasamy, Zeolites, 11 (1991) 690.

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

951

P r e p a r a t i o n a n d n a n o - s t r u c t u r e of p i l l a r e d l a y e r e d t i t a n a t e s a n d t a n t a l a t e s for p h o t o c a t a l y t i c m a t e r i a l s M. Machida, J. Yabunaka, H. Taniguchi, T. Kijima Department of Materials Science, Faculty of Engineering, Miyazaki University, 1-1 Gakuen-kibanadai-nishi, Miyazaki 889-2192 Japan

Layered titanates and tantalates pillared by metal oxide clusters (A1203 o r SiO 2) have been prepared through a stepwise exchange process. The pillared triand tetratitanates, which contained a number of cleavages running parallel with the TiO6 sheets, showed the 10-25 times higher specific surface areas (50-120 m2/g) than unpillared samples. On the other hand, the SiO2-pillared tantalate was a nonporous solid with low surface areas less than 20 m2/g. The photocatalytic activity of lwt%Pt/titanates for H 2 evolution from CH3OH]H20 mixtures under UV-irradiation was enhanced by pillaring in accord with an increase of the surface area. Partial substitution of various transition elements for the Ti site significantly affected the activity.

1. INTRODUCTION The synthesis of pillared structures from layered materials has led to the development of a new class of porous catalysts [1]. Pillared layered materials are prepared by intercalation of large polymeric inorganic cations into smectite clays, which are converted to oxide pillars after calcination. The resulting microporous structure is characterized by interlayer spaces with ca.1 nm wide and high specific surface areas. Another interesting feature of pillared materials is the variety of possible chemical combinations between layers and pillars. Apart from the conventional clays and related materials, several researchers have recently reported the pillaring of layered transition metal oxides, such as titanates and niobates [2-6]. Because of their non-swelling nature, the interlayer must be generally pre-expanded by using alkylamines to facilitate the pillaring process. The catalytic and electronic properties of transition metal oxides are expected to lead to a novel family of functional porous solids. It was already reported that pillared materials prepared from niobates exhibit the excellent photocatalytic activity for hydrogen evolution reactions [7]. In this study, preparation and characterization of titanates and tantalates

952 pillared by metal oxides ( S i O 2 o r A1203) were studied to use them as photocatalytic materials. An attempt was made to evaluate the effect of structural modification caused by ion-exchange and pillaring on the photocatalytic activity. In addition, chemical modification of pillared titanates was examined by introducing various different transition elements into the Ti site.

2. EXPERIMENTAL 2.1. Sample preparation Sodium trititanate, potassium tetratitanate, and their substituted samples (Na2Ti3.xMxO7, K2Ti4.xMxO9, M=Ti, Mn, Fe, Co, Ni, Cu) were prepared by calcining mixtures of carbonate/oxide powders at 800 ~ for 30 h. Layered tantalate (RbLaTa20 7) was also obtained by a solid state reaction at 1100 ~ and subsequently submitted to ion exchange in molten NaNO 3 at 673 K to convert into NaLaTa20 7. The pillared samples were prepared by a stepwise exchange process reported by several researchers [2-6]. The protonated derivatives of these samples, which were prepared by exchange in 1M HC1, were treated with 5M C6H13NH2. Pillar precursors, t e t r a e t h y l orthosilicate (TEOS) or a l u m i n u m hydroxide oligomers ([Al1304(OH)24(H20)1217+), were then absorbed into the expanded interlayer. The final microporous materials were obtained by calcination at elevated temperatures (300-500 ~ in air. 2.2. Characterization and photocatalytic reaction As prepared samples were characterized by means of XRD, FT-IR, EDX, FESEM, TEM, and UV-DRS. The BET surface area and pore size distribution were obtained from N 2 adsorption isotherm measurement at 77 K. The photocatalytic activity for H 2 evolution from aqueous solution of 4M CH3OH was conducted at room temperature in a conventional gas circulation system (Ar : 150 torr) under irradiation with light from a 400W high-pressure Hg lamp.

3. RESULTS AND DISCUSSION 3.1 Structure of pillared layered titanates X-ray diffraction patterns of tri- and tetratitanates were respectively in good agreement with the data already reported [8]. Part of Ti 4+ could be replaced by various transition metals (Mn, Fe, Co, Ni, Cu) without structural deterioration up to x=0.3 for Na2Ti3.xMxO7 and x=0.2 for K2Ti4.xMxO9. Precipitation of Na2Ti6013 in the Na2Ti30 7 sample could be avoided in the substituted products. Part of transition elements thus introduced was, however, eluted from the solid during protonation in 1M HC1. Their residual concentration in the solid after protonation was strongly dependent on M, e.g., 86 atom% of Mn remained whereas 78 atom% of

953 Cu dissolved out for the nominal Na2Ti2nMno.307 composition of Na2Ti2.7Mo.307 . H2Ti2.7~4_no.307 Figure 1 shows the change of XRD patterns during the pillaring process of Na2Ti2.7Mno207. The s t e p w i s e e x c h a n g e could be observed as a shift of the (100) reflection at 20=10.6 deg, which SiO2-Ti2.TMno.307 corresponds to the i n t e r l a y e r d i s t a n c e . P r o t o n a t i o n (H/ 0 10 20 30 (Na+H)>98%) d e c r e a s e d t h e 2 0 / deg interlayer distance from 0.83 to Fig. 1. Change of XRD p a t t e r n s of 0.78 n m . The TG a n a l y s i s Na2Ti2.7Mno.307 in protonation, intercaindicated that the interlayers of lation, and pillaring processes. pristine and protonated samples were not hydrated. The interlayer distance was then significantly expanded to 2.1 nm by the following intercalation of C6H13NH2 and TEOS. After calcination at 400 ~ in air, FT-IR measurement revealed the complete elimination of the organic components. The resultant pillared titanate possessed an interlayer distance of 1.6 nm, which corresponds to gaps of ca.1 nm wide between adjacent TiO 6 sheets. A substantial broadening and loss of the peak intensity of the (100) reflection mean lack of regularity of the layer stacking in the pillared sample. The SiO2- or A1203-pillared materials could be also prepared from other titanates and their substituted samples in the same manner. The pillared structure of these titanates was thermostable up to 500 ~ The layered titanates showed low surface areas less than 5 m2/g. In contrast, the pillared titanates showed much higher surface areas above 50 m2/g as shown in Fig.2. The largest value of 120 m2/g was attained for M=Mn (x=0.3), whereas Cu-substituted sample showed little increase of the surface area due to their structural disorder caused by the e l u t i o n of Cu ions in t h e a 'i I i t ! Na2Ti307 p r o t o n a t i o n step. Pore size . . . . . . . . . . . . . . . . , ~ x=0.15 M=Ti distribution calculated from N 2 0.30 Mn adsorption isotherms at 77 K ii I N Fe showed a broad peak centered at ~ Co ca.3 nm. Since SEM observation s h o w e d no m o r p h o l o g i c a l a.. Cu ,~ailigil / ~-~11 , I I I difference between pristine and 0 20 40 60 80 100 120 140 pillared microcrystals, the Surface area / m2g-1 reaction is topotactic and the increased surface area is due to Fig. 2. BET surface areas of Na2Ti307 the f o r m a t i o n of i n t e r l a y e r and SiO2-pillared Ti3.xMxO7. spaces accessible by gaseous l

|

,

|

9

l

I

|

954

Fig. 3. TEM images and selected area diffraction patterns of K2Ti409 and SiO 2pillared Ti409 after calcination at 400~ Arrows in a TEM image show the formation of cleavage between TiO 6 sheets. Schematic representation of the crystal structure shows K2Wi409projected along (010). molecules. As was evident from XRD patterns of Fig.l, the layered structure oftitanates is distorted by the pillaring process. To gain insight into the local structural inhomogeneity caused by pillaring, characterization by means of TEM was preformed on the pristine and A1203-pillared tetratitanate. Figure 3 indicates that as calcined K2Ti40 9 consisted a l m o s t e n t i r e l y of equal sized c o l u m n a r microcrystals of 1-3 ~m long and 0.1-0.4 ~m wide. Selected area electron diffraction (SAD) with the incident beam normal to the prism plane of several different crystals showed a single-crystal pattern consistent with the monoclinic lattice of

955

~

Na

SiO2-LaTa2OT(300~ I

. . . .

I

10

Fig. 4. Crystal structure of anhydrous NaLaTa207.

I

I

,,

I

30 2 0/deg

. . . .

I

50

I

70

Fig. 5. Change of XRD patterns of N aLaTa20 7 in protonation, intercalation, and pillaring processes.

K2Ti409 (Fig.3), which is composed of TiO 6 octahedra sheets stacking along the [100] direction. Since the diffraction spots can be indexed with the [011] zone axis, it was demonstrated that the direction of layer-stacking is just perpendicular to the fiber axis of crystals. In contrast, the TEM image of A1203-pillared samples after calcination at 400 ~ in air is characterized by many cleavages running parallel with the TiO 6 sheets. This is consistent with the formation of nano-spaces separated by the oxide pillars between layers. Oxide pillars could be observed as an amorphous contrast between layers but negligibly on the outer surface of the particles. In the SAD patterns with the [011] zone axis, all diffraction spots were elongated along the [100] direction, indicating lack of the periodicity of layerstacking. Actually, many cleavages with different spacings as well as interlayers remaining unpillared could be observed in the vicinity of the tail end surface. How to develop the pillared structure without such structural deterioration is the key to improvement of the surface area and the thermal stability. 3.2 Structure of pillared layered tantalates The layered tantalate, NaLaTa207 is a member of Dion-Jacobson series, which is based on the stacking of perovskite layers as shown in Fig. 4 [9]. Since the interlayer was easily protonated, it is considered that pre-swelling by using alkylamines is also applicable for the pillaring. Figure 5 shows the change of XRD patterns in the pillaring process of the NaLaTa20 7 sample (10 m2/g). After protonation (H/(Na+H)>72%) a shift of the (200) reflection suggested the decrease of interlayer distance from 1.23 to 1.08 nm. According to the TG measurement, the Na-type sample was a hydrous compound corresponding to the composition of NaLaTa207"I.9H20, whereas the protonated sample was composed of anhydrous interlayers. The hexylamine- and TEOS-intercalated tantalates showed much expanded interlayer distance, 2.59 and 2.38 nm, respectively. After calcination at

956

NaLaTa207 100

1O0

Na2Ti

80

80 /

40

a dgapo or

///

N~Ti~o~

/ //

20 o

/

I

300

I

..... H2Ti~O~ H2Ti307 SiO2-Ti307 I

I

I

I

i

400 500 600 Wavelength /nm

oV

3.59 3.17 3.18 I ......

I

700

i 6~ 40

~ - - " RbLaT~O~ /]

Band gap energy / eV

il

r.~o-76-7--

] ] ~ " TazO5

RbLaTa207 3.83

20 o

3

. ,

0

,

,.

.......

04000 50 600 Wavelength /nm

700

Fig. 6. UV-vis diffuse reflectance spectra of trititanates and tantalates.

400 ~ in air, the SiO2-pillared tantalate (SiVa=0.77) with an interlayer distance of 1.88 nm was produced. The corresponding interlayer spacing, 0.91 nm, was comparable to the pillared titanate case. However, it was apparent that the pillared interlayer structure of the tantalate is quite different from that of the titanates, because the surface area of SiO 2- pillared tantalates was still low (20 m2/ g). A possible reason for the low surface area of pillared tantalates is the nonporous structure in which oxide pillaring agents stuffed completely the interlayer space. A similar structure was reported for A1203-pillared zirconium phosphates with the surface area less than 40 m2/g [10]. Further attempts to produce porous pillared tantalates are in progress on the effect of intercalation conditions, pillaring agents, and deorganization conditions. 3.3 Photocatalytic property of titanates and tantalates From the UV-vis diffuse reflectance spectra shown in Fig.6, the absorption onset of Na2Ti307 was shifted to longer wavelength after the protonation and s u b s e q u e n t pillaring processes. This corresponds to the band gap energy decreasing from 3.6 to 3.2 eV. Since the same behavior was also observed for the K2Ti409 system, the band-gap transition of these layered titanates seems sensitive to the interlayer spacing or interlayer species. The substituted titanates showed no clear-cut absorption edges because of absorptions in the visible light region, which are associated with the presence of impurity states within the band gap. On the other hand, the band gap energy of NaLaTa207 (3.8 eV) was little changed by protonation. The pillared sample showed the continuous absorption in the visible light region due to the carbonous deposits in the nonporous interlayer. The photocatalytic H 2 evolution reaction from aqueous methanol solution was took place before and after photodeposition of Pt onto titanates and tantalates. Figure 7 showed the photocatalytic activity of u n s u b s t i t u t e d tri- and tetratitanates. The pristine sample of these titanates showed very low activity, which

957 w a s d e c r e a s e d by the SiO2pillaring. The remarkable e n h a n c e m e n t of activity was a t t a i n e d w h e n 1 w t % P t was d e p o s i t e d on t h e p i l l a r e d samples. As mentioned above, the pillared samples expose their i n t e r i o r surface, where Pt particles can deposit. Thus, the elevation of activity is considered to result from the high dispersion of Pt particles on the pillared structure. The effect of partial substitution on the catalytic activity of S i O 2 - p i l l a r e d t r i t i t a n a t e s is s u m m a r i z e d in Fig. 8. The activity of unplatinized samples w a s l i t t l e a f f e c t e d by t h e substitution up to x=0.15, but drastically increased at x=0.3. The activity of SiO2-Ti2.7Mo.307 d e c r e a s e d in a f o l l o w i n g sequence, M=Mn > Fe > Cu > Co > Ni > Ti. Since this order is different from that of the surface a r e a s h o w n in F i g . 2 , t h e s u b s t i t u e n t s t h e m s e l v e s are supposed to play a key role in the catalysis for the H 2 evolution. It is i n t e r e s t i n g t h a t a c t i v e catalysts s u c h as SiO 2Ti2.7Mno.30 7 s u f f e r e d f r o m serious d e a c t i v a t i o n w h e n 1 wt%Pt was loaded. In contrast, u n s u b s t i t u t e d sample, SiO 2TiaO 7, which itself showed very low activity, attained the highest H 2 evolution rate among supported Pt catalysts. These results suggest that the transition elements and Pt play the different roles in the H 2

Na2Ti307 PtJNa2Ti307

] m

H2Ti307 Pt/H2Ti307 SiO2-Ti307

m

~

m

~

]

Pt/SiO2-Ti307

I

0

i

I

i

i

0.5 1.0 1.5 2.0 2.5 H2 formationrate/mmol ffl h-1

3.0

K2Ti409 Pt/K2Ti409 H2Ti409 Pt/H2Ti409 SiO2-Ti409

none

Pt/SiO2-Ti409

i

0

!

i

!

i

0.5 1.0 1.5 2.0 2.5 I-I2 formation rate/retool ff i h-i

3.0

Fig. 7. Photocatalytic activity of layered triand tetratitanates for H 2 evolution. Pt loading was lwt%. a) SiO2-Ti3.xMxO7 M = T i ~ "

Mn.

r--I x=0.15

Fe

! !

0

x=0.30

I

1.0 2.0 3.0 H2 formation rate/mmol cat. g-1. h-1

b) lwt%Pt/SiO2-Ti3.xMxO7 M=TiL r-! x=0.15 m x=0.30

, 0

1.0 2.0 3.0 H2 formation rate/mmol cat. g-1 . h-1

Fig. 8. Photocatalytic activity of SiO 2pillared Ti3.xMxO 7 for H 2 evolution.

958 evolution reaction, but the [] unsupported detailed mechanism was not Ta205 [] 0. lwt%Pt been solved at this stage. [] lwt%Pt RbLaTazO 7 The l a y e r e d t a n t a l a t e s , which possessed the band gap NaLaTa207 energies larger than those of '1 HLaTa207 t i t a n a t e s (Fig. 6), were also 200 400 600 800 active for the photocatalytic H 2 H 2 evolution rate / # m o l g-cat." 9 h9 ~ evolution reaction. As was in the case of titanates, the ionFig. 9. The photocatalytic activity of layered exchange and pillaring greatly tatalates for H 2 evolution. affected the activity as shown in Fig. 9. In particular, protonated sample showed the higher activity as compared to the Na-type and Ta20 5. A further increase of the activity was attained after deposition of 0.1wt% Pt. Interestingly, the activity of the SiO2-pillared sample was negligibly low even after the Pt deposition. The deactivation in this case appears to be associated with not only the nonporous structure of this sample but also a possible change of the surface property of the tantalates caused by the SiO2-pillaring. I

I

I

I

I

I

I

I

REFERENCES 1. I.V.Mitchell(ed), Pillared Layered Structures: Current Trends and Applications; Elsevier Applied Science, London, 1990. 2. S.Cheng, T.C.Wang, Inorg. Chem., 28 (1989) 1283. 3. M.W.Anderson, J.Klinowski, J. Am. Chem. Soc.,29 (1990) 3260. 4. M.E.Landis, B.A.Aufdembrink, P.Chu, I.D.Johnson, G.W.Kirker, M.K.Rubin, J. Am. Chem. Soc., 113 (1991)3189. 5. W.Hou, Q.Yan, X.Fu, J. Chem. Soc. Chem. Commun., 1371 (1994). 6. W.Hou, B.Peng, Q.Yan, X.Fu, G.Shi, J. Chem. Soc. Chem. Commun., 253 (1994). 7. J.Kondo, S.Shibata, Y.Ebina, K.Domen, A.Tanaka, J. Phys. Chem., 99 (1995) 16043. 8. H.Izawa, S.Kikkawa, M.Koizumi, J. Phys. Chem., 86 (1982) 5023. 9. K.Toda, K.Uematsu, M.Sato, J. Ceram. Soc. Jpn., 105 (1997) 482. 10. A.Clearfield and B.D.Roberts, Inorg. Chem., 27(1988) 3237.

91998Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmonet al., editors.

959

Synthesis, characterization and catalytic activity of aluminum pillared synthetic Ti-, Fe- or Zn-substituted smectites. M.Lenarda *a, L. Storaro a, A.Talon a, R. Ganzerla a, V.Lucchini b. aDepartment of Chemistry and bDepartment of Environmental Sciences, University of Venice "Ca' Foscari", Calle Larga S.Marta 2137, 30123,Venice, Italy * Te1:++39415298562, Fax ++39415298517, E-mail : [email protected] ABSTRACT Synthetic smectites framework substituted with Ti, Fe, and Zn were synthetized in hydrothermal conditions. The materials were characterized by XRD, nitrogen adsorption, MAS-NMR. FT-IR spectroscopy of adsorbed pyridine and analysis of product distribution in the 1-butene isomerization was used to evaluate the surface acidity. Chemical properties were correlated to the nature and presumed lattice location of the framework substituents. 1. ~ T R O D U C T I O N There has been a considerable interest in the last years in the synthesis of new solid acid catalysts.[1,2] Smectite clays, containing predominantly Si IV and Almas the framework cations and their aluminum pillared derivatives, have been extensively used as acid catalysts [3-5]. It is well known on the other hand that framework substitutions can dramatically change the surface acidity of silicoaluminates. The synthesis of zeolites, smectite clays and mesoporous aluminosilicates in which the characteristic framework cations are replaced by metallic ions such as titanium or zinc have been reported [6-8]. In this work we report the preparation, characterization and catalytic activity of aluminum pillared clays derived from Ti IV , Fe Ill or Zn II lattice substituted synthetic smectites. 2. EXPERIMENTAL 2.1 Materials preparation A Si/A1 sodium beidellite was synthesized in hydrothermal conditions from a gel having a chemical composition of Nao.67A14.67Si7.33022, according to the procedure described by Schutz et al.[9]. The Ti-, Fe-, and Zn- substituted smectites were synthesized in hydrothermal conditions from a gel having a chemical composition respectively of Nao.67A14.67SiT.23Tio.1022, Nao.67A14.00Feo.67Si7.33022, Nao.67A13.soZno.43Si7.77022using a slight modification of the procedure followed to synthesize the Si/A1 beidellite [9]. All the pillared smectites were prepared from acetone clay suspension (50% w/w) following a synthetic procedure previously described [10, 11].

960

2.2 Materials characterization Elemental analyses were accomplished by Atomic Adsorption Spectroscopy with a Perkin-Elmer PE 3100 instrument. Thermogravimetric analyses were carried out on a Simultaneous Thermal Analyzer STA 429 Netzsh using samples weighting about 50 rag, a heathing rate of 10 K/min with air X-Ray diffraction spectra were measured with a PW 1310 Philips diffractometer using the Cu-Kot radiation. Nitrogen Adsorption/desorption experiments were carried out at 77 K on a C.Erba Sorptomatic 1900. IR spectral measurements spectra were recorded on a FT-IR Nicolet Magna-IR 750 spectrometer. Pressed wafers of the samples were allowed to adsorb pyridine vapour after which were evacuated respectively at 423,573 and 673K. The 29Si MAS NMN spectra were recorded on a Varian Unity 400 spectrometer operating at 79.4 MHz and the 27A1MAS spectra were recorded on the same instrument operating at 104.2 MHz as previously described [ 11 ]. 2.3 Catalytic activity evaluation Catalytic 1-butene isomerization tests were performed in a tubular glass flow microreactor. The catalyst samples were pretreated for 2 h in N2 flow at 673 K. The experiments were performed at ~ =2.4g(eat)'g(1-butene)- -h. The 1-butene was 5% in nitrogen and the time on stream was 120 min. The coke produced was measured by thermogravimetric analysis. 3. RESULTS AND DISCUSSION

3.1 Analytical and structural parameters In Table 1 we report some structural and analytical characteristics of the synthetic clays. Table 1 Unit cell formulas and some structural and analytical characteristics of the synthetic clays Sample Unit cell formulas CEC d001 S.A.BET Si/A1 (meq/g) (nm) (m2/g) 0.88

1.21

112.9

9.38

BeTi

Nao.77(Sir.23Alo.v7)(Ala) 020 (OH)4 Nao.69(Si7.20A10.69Tio.ll)(A14) O20(OH)4

0.60

1.23

132.4

9.0

BeFe

Nao.7o(Si7.3o Alo.vo)(A13.zoFeo.8o) O2o (OH)4

0.60

1.01

85.8

10.4

Nao.91Zno.lo(Siy.zsAlo.vz)(A13.61Zno.39)Ozo(OH)4 0.66

1.23

120.4

10.0

Be

BeZn

The composition of the unit cell of the synthetic clays can be calculated from the elemental analyses [12]. Analytical data suggest that Ti cations are inserted in the tetrahedral layer of the BeTi sample while Fe cations are most probably located in the octahedral layer of BeFe. In the case of the BeZn it was necessary to assume the presence of extraframework Zn species.

961 The XRD and the surface area data confirm the formation of a smectite-type clay. Scanning Electron Micrograph (SEM) of the synthetic smectites showed that the hydrotermal products consist of thin flakes formed by flexible layers folded like silk ribbons. This morphology is typical of a well formed smectite. An example is given in Figure 2.

Figure 2. Scanning Electron Micrograph (SEM)'of the BeZn synthetic smectite Structural data of the pillared derivatives of synthetic clays are reported in Table 2 Table 2 Basal spacings (d001), BET surface areas (S.A.) and amount of aluminum as pillar per O20(OH)4 up1."t of the pillared smectites. S.A.BET(m2/g) Sample A1 p dO0l(nm) BePil

1.47

1.85

279.5

BeTiPil

1.59

1.83

211.7

BeFePil

1.45

1.80

268.9

BeZnPil

1.34

1.90

256.8

Alp-- amount of aluminum as pillar per O2o(OH)4 unit

962

9

a.u.

I

'

I

'

I

'''

"

i

'

~ . _ .

t

k

a

b t2

5

10

15

20

25

30

20 Figure 1: X-Rays Diffraction spectra of the synthetic smectites pillared with ACH: a) BePil; b) BeZnPil; c) BeFePil; d) BeTiPil. Suitable pillaring was achieved for all the samples and the typical interlayer spacing of hydroxy-Al pillared smectites (dool>_l.8nm) was mantained after a prolunged treatment at 673 K. The BET surface area values were also in good agreement with the data available in the literature for the aluminum pillared smectites.

3.2 Magic Angle Spinning NMR 3.2.129Si NMR The 29 Si NMR spectra of all the beidellites consists of two peaks in the Q3 region and the data are reported in Table 3. Table 3 29Si Chemical shifts (8, ppm), full widths at half-height (fwhh, ppm) of Si-nAl components obtained from the deconvolution into Lorentzian functions. Sample fwhh (ppm) Q3Si-lAl(ppm) Q3Si-0Al(ppm) 3(I~+I0)/I~ Be

2.6

87.40

92.40

8.61

BeTi

2.8

87.20

92.54

8.97

BeZn

2.7

87.85

93.00

9.6

The signals can be respectively attributed to tetrahedral silicon with no metallic

'963 neighbours (Q3Si-0A1) and to tetrahedral silicon with one metallic neighbour (Q3Si-IA1). Deconvolution of the signal into Lorentzians gives the corresponding areas I0 and I1 from wich it is possible to calculate the Si/A1 ratio of the tetrahedral layer using the simplified formula Si/A1- 3( I0+11 )/11, [13]. The values are, within the experimental error, in good agreement with the Si/AI(Ti) ratios calculated from the analytical data.

3.2.2. 27A1 MAS-NMR The 27A1 MAS-NMR spectra are potentially very helpful for probing the quantity, coordination, and location of aluminum atoms in aluminosilicates. Nevertheless, the quadrupolar nature of the nucleus does not allow the observation of the structurally significant fine structure of the bands, limiting the applicability of 27A1MAS- MR to little more than the determination of the coordination number at the aluminum atom. As expected, the starting beidellite Be shows a 27A1 peak centered at 72.0 ppm, in the region corresponding to tetrahedrally coordinated aluminum ( AITM ) and a peak centered at 0.5 ppm attributable to aluminum atoms in the octahedral sheet. Chemical shifts and relative contribution of A1w to the 27A1 spectrum (25%) are in agreement with the data reported in literature previoulsly [14]. A similar pattern is shown by the other samples (Table 4). Table 4 27A1 Chemical shifts (8, ppm), full widths at half-height (fwhh, ppm) of tetrahedrally and octahedrally coordinated species. .. Sample AIrv A1vI 8 (ppm)

fwhh (ppm)

%

8 (ppm)

fwhh (ppm)

%

Be

72.0

13

25

0.5

20

75

BePil

69.0

16

16

0.0

14

84

BeZn

71.0

16

30

2.7

15

70

BeZnPil

71.0

18

16.5

2.7

15

83.5

BeTi

72.0

11

21.5

1.3

20

78.5

BeTiPil

72.0

15

17.5

2.7

15

82.5

Notwithstanding the difficulty of interpreting quantitatively the 27A1spectra, the trend of A1Iv and A1v~ ratios appears to reflect the isomorphous substitution in the BeZn and BeTi samples. In fact in the BeZn sample, where a part of the octahedral aluminum is replaced by Zn cations, the relative contribution of A1TMto the 27A1 spectrum resulted slightly higher in comparison with the unsubstituted beidellite, while in the BeTi sample, where the titanium cations are supposed to be in the tetrahedral layer, the relative contribution of AlIVwas found lower. The A1 ratios of the pillared materials confirm the Al13 species insertion in the interlayer space. More precise quantitative correlations between 27A1 NMR data and calculated chemical formulas were not attempted for the well known limits of both experimental approaches [ 14].

964 3.3 Evaluation of the surface acidity 3.3.1 FT-IR spectroscopy of adsorbed pyridine The concentration of the Bronsted (CB) and Lewis (CL) acid sites (Table 3) was calculated from the integration of the 1540 and 1450 cm- IR bands of the adsorbed pyridine as proposed by Datka [ 15]. The concentration values obtained after evacuation at 573 and 673 K can give also an evaluation of the strength of the acid sites. Since the pillared samples have quite different values of BET areas (SA) we judged more convenient to normalize the data dividing CB and CL for the specific surface area (SA). Table 3 Superficial concentration (~tmolg1) of Bronsted (CB) and Lewis (CL) acid sites from pyridine adsorption and normalized (~tmol/m2) superficial concentrations CB/SA and CL_/SA at 423K of the pillared clays. . . . . . . Sample

CB (~tmol/g)

CL(lamol/g)

CB/SA

CL/SA

423K

573K

673K

423K

573K

673K

423K

423K

BePil

47.6

20.8

6.4

73.8

45.4

35.6

0.17

0.26

BeTiPil

58.5

22.9

5.5

71.2

48.6

32.7

0.25

0.30

BeFePil

56.2

26.0

3.9

81.4

37.8

24.6

0.18

0.25

BeZnPil

50.0

13.7

1.5

104.0

60.4

30.5

0.17

0.36

Inspection of the data obtained at 423 K shows that octahedral substitution with Zn or Fe apparently does not induce strong modifications of Bronsted acid sites concentration on the clays. On the other hand titanium substitution in the tetrahedral layer appear to induce an important increase of the CB/SA value. The Lewis acid sites concentration was apparently not modified by Fe enrichment of the octahedral layer, while titanium substitution on the tetrahedral layer appeared to induce only a slight increase of these sites. This finding can be explained by the fact that the Lewis acid centres of aluminum pillared clays are in fact mainly located on the pillars [ 16]. The important increase of Lewis acid sites after Zn enrichment must be attributed mainly to the presence of extraframework zinc species on the clay surface [ 17, 18]. The intensity of the band related to the Bronsted acid sites decreased with the same trend for all the samples going from 423 to 673K. Whereas the intensity of the band assigned to the pyridine bonded to the Lewis acid sites of the BeZnPil clay seemed to decrease more rapidly with the increasing desorption temperature. This indicates that the Lewis centres generated on the clay surface by the Zn extraframework species are of medium strength. 3.2.2 Product distribution in the 1-butene isomerisation The catalytic 1-butene isomerisation was chosen as test reaction [ 19-21 ]. The product distribution of 1-butene conversion over the pillared smectites is shown in Table 4.

965 Table 4 Product distribution (w/w %) for the 1-butene conversion at 673Kon pillared clays Sample %iso% n%iso Z Z] % butane butane butene iso iso/SAa Coke BePil

3.7

1.0

20.2

23.9

0.08

2.20

BeTiPil

3.2

0.9

22.9

26.1

0.11

1.05

BeFePil

1.8

0.7

13.9

15.7

0.06

3.97

BeZnPil

1.1

0.4

13.5

14.5

0.05

2.90

a Total iso products / BET Specific Area Several were the products formed during the 1-butene conversion catalysed by the pillared clays (metane, ethane, ethylene, propane, propene, isobutane, n-butane, cis- and trans-2-butene and isobutene) but here we will discuss only the formation of isobutene that derives from n-butene by skeletal isomerization. This process necessitates of rather strong Bronsted acid sites [18-20] as it implies the transposition of a sec-butyl carbenium ion to a tert-butyl carbenium ion. The hydrogenated compounds, n-butane and isobutane, found among the other products, were probably formed, in our conditions, by the direct hydrogenation of the butene isomers through hydride transfer to the corresponding monomeric carbenium ions. In order to evaluate the skeletal isomerisation activity of the samples, the amount of isobutene produced was added to that of isobutane, assuming that isobutane originates from isobutene. The amount of coke was determined by TG on the exhaust catalyst samples in order to evaluate the strong acid sites. Analysis of the data confirm that iron and zinc structural substitution do not substantially alter the number of the medium strength Bronsted acid sites responsible of the formation iso products. Whereas the presence of titanium atoms in the tetrahedral layer appear to generate an increased number of medium strength protonic acid sites analogously to what is known for zeolitic materials [21]. All the examined samples produce a remarkable amount of coke, as is known for aluminum pillared clays [22] but interestingly the phenomenon is quite reduced in the case of the titanium substituted sample. Titanium substitution on the tetrahedral layer apparently generates not only medium strength acid sites, responsible of skeletal isomerization increase but also, as found for Ti substituted zeolites [7, 21 ] redox centers that protect the catalyst from deactivation slowing down coke deposition. 4. CONCLUSIONS It can be deduced from the results of this preliminary study that smectites with a certain extent of isomorphous substitution can be synthetized by hydrothermal methods. These materials differ from the simply ion exchanged pillared clays because the metal ions constitute permanent non-exchangeable sites of the smectite structure. It clearly appears that framework substitution modifies in various extent the surface acidity of the smectites and of their pillared derivatives. It was confirmed what was known about the preference of some cations for the octahedral coordination (Fe, Zn) while others (Ti) seeem to prefer the tetrahedral arrangement. Acidity ad catalytic activity evaluation carried out on the pillared

966 derivatives showed that isomorphous substitution in the tetrahedral layer is more effective. Introduction of titanium in the tetrahedral layer was found particularly interesting for the preparation of new catalysts because the material resulted not only more acid but showed new very useful redox properties.

REFERENCES 1. B.Imelik, C.Naccache, G.Coudurier, YT.Ben Taarit and J.C.Vedrine, (eds), Catalysis by acids and bases, Elsevier Pub., Amsterdam, 1985. 2. A.Corma, Chem.Rev., 95 (1995) 559. 3. R.Burch, Catal.Today, 2 (1988) 185. 4. F.Figueras, Catal.Rev., 30 (1988) 457. 5. A.Corma, Chem.Rev., 97 (1997) 2373. 6. a)V.Luca, L.Kevan, C.N.Rhodes and D.R.Brown, Clay Min., 27 (1992) 515; b)V.Luca, D.J. MacLachlan, R.F. Howe and R. Bramley, J.Mater.Chem., 5(4) (1995) 557. 7. B.Notari, in Stud.Surf.Sci.Catal., 37 (1988) 413. 8. A.Corma, M.T.Navarro and J.P.Pariente, J.Chem.Soc.Chem.Commun., (1994) 147. 9. A. Schutz, E.E.W.Stone, G.Poncelet and J.J.Fripiat, Clays and Clay Min., 35 (1987) 251. 10.L.Storaro, M.Lenarda, R.Ganzerla and A.Rinaldi, Microporous. Mater., 6 (1996) 55. 11.L.Storaro, M.Lenarda, M.Perissinotto, V.Lucchini, and R.Ganzerla, Microporous.and Mesop. Mater., (in press) (1998). 12.A.C.D.Newman and G.Brown in Chemistry of Clays and Clay Minerals, A.C.D. Newmann (Ed), The Mineralogical Soc. Monograph n.6, (1987) 10. 13.J-F.Lambert, S.Chevalier, R.Franck, H.Suquet and D.Barthomeuf, J.Chem.Soc. Faraday Trans.I, 90 (1994) 675. 14.D.Plee, F.Borg, L.Gatineau and J.J.Fripiat, J.Am.Chem.Soc., 107, (1985), 2362. 15.J.Dakta, A.M.Turek, J.M. Jehng and I.E.Wachs, J. Catal., 135 (1992) 186. 16.H.Ming-Yuan, L.Zhonghui, M.Enze, Catal.Today, 2 (1988) 321. 17.J.H.Clark, S.R.Cullen, S.J.Barlow and T.W.Bastock, J.Chem.Soc. Perkin Trans.2, (1994) 1117. 18.C.Cativiela, J.M.Fraile, J.I.Garcia, J.A.Mayoral, F.Figueras, L.C.de Menorval and P.J.Alonso, J.Catal., 137 (1992) 407. 19.A.La Ginestra, P.Patrono, M.L.Berardelli, P.Galli, C.Ferragina and M.A.Massucci, J. Catal., 103 (1987) 346. 20.P.Patrono, A.La Ginestra, G.Ramis and G.Busca, Appl.Catal.A, 107 (1994) 249. 21.J.P.Damon, B.Delmon and J.M.Monnier, J.Chem. Soc. Faraday Trans.I, 73 (1977) 372. 22.B. Sulikowsky, Heterogeneous Chem.Rev., 3 (1996) 203. 23.M.L.Occelli, R.J.Rennard, Catal.Today, 2 (1988) 319.

91998ElsevierScienceB.V.All rightsreserved, Preparationof CatalystsVII B. Delmonet al., editors.

Characterization and catalytic synthesized saponite-like m a t e r i a l s

967

activity

of

non- hydrothermally

M. Sychev and R. Prihod'ko Faculty of Chemical Technology, National Technical University of Ukraine, "Kiev Polytechnic Institute" 252056, Kiev, pr. Peremogy 37, Ukraine Saponite-like materials were synthesized at 1 atmosphere and 363 K from Si/A1, Si/Cr and Si/Fe-gels and a solution containing urea and M 2+ nitrate (M 2+ =Zn and Mg) or mixture of M 2+ nitrates (M 2+= Zn, Mg and Cu) during 24 hours. The products were characterized by variety of methods. Incorporation of Cr and Fe in the tetrahedral positions and Zn, Mg or their combination with copper cations in the octahedral sheet could be established. The materials prepared possess acidic and basic/redox active sites depending on the chemical composition of both tetrahedral and octahedral sheets of saponite, as found from 2-propanol and 2methyl-3-butyne-2-ol (MBOH) decomposition. Their Al-exchanged forms display a high activity in alcohol dehydration. 1. INTRODUCTION The use of natural clays as acid catalysts has been known for a long time. However, their wide range catalytic application is still hampered mainly due to relatively low thermal stability and difficult control of chemical composition and textural properties. The synthetic clays offer the possibility to avoid these problems. Unfortunately the usually demanding hydrothermal treatment and long duration of the preparation [1] have severely limited their use. Recently, Vogels et al. [2] proposed the non-hydrothermal approach allowing synthesis of saponite, a tetrahedrally charged trioctahedral smectite. Its structure built from two SiO4 tetrahedral sheets (T) and one MgO6 octahedral sheet (O) arranged in a TOT sandwich. The synthetic saponites prepared through the above mentioned way displayed interesting catalytic properties in the Friedel-Krafts alkylation of benzene and were thermally stable [2]. Moreover, because of a mild condition and relatively easy preparation mode this method can by sealed-up making industrial application viable. Incorporation of transition-metal ions into the framework of molecular sieves provides the materials with versatile catalytic properties. The iron, chromium and copper containing zeolites have been assumed to be catalysts for a range of transformations [3-5]. Synthesis of zeolites isomorphously substituted with two heteroatoms was already reported [6] that gives a possibility to create different active sites in the same silicate matrix. Therefore, the incorporation of transition-

968 metal ions into the lattice of clay-like materials offers potemially a promising route for the preparation of new heterogeneous catalysts. The aim of this study was characterization of non-hydrothermally synthesized saponite-like materials by physical methods and catalytic tests from the standpoint of incorporation of the transition-metal ions in the silicate lattice. 2. EXPERIMENTAL Saponite-like materials were synthesized at 363 K and 1 atmosphere. The starting gel containing Si and AI, Fe or Cr (M 3+) was prepared by mixing a Na2SiO3 (Merck, 27 wt % SiO2) with solution of NaOH and respective M 3+ nitrate under vigorous stirring at 273 K for 1 h. The Si/M 3+ ratio was varied within 3.0 and 12. Next, a solution containing nitrate of octahedral ions (M 2+) and urea was added to the diluted staging gel and then the mixture was kept under stirring at 363 K for 24 hours. After synthesis completion, the solid products were separated, washed and dried at 383 K for 12 h. The Al-containing saponite with ratio of Si/AI=8.0 and octahedral Zn cations was prepared under the same conditions and used as a reference material. Apparatus used and conditions of measurements. XRD" DRON-3 diffractometer, CuKcz-radiation. XRF: high-vacuum device, Si(Li) detector (180 eV at 5.9 keV), sample weight=200 rag. EPR: SE/X 2544 spectrometer, X-band, operating at 293 K and 77 K, 27A1 MAS NMR: Brucker MSL 400 spectrometer, ffequency=104 MHz, rotation ffequency=15 kHz, repetition time=0.5 s, pulse width=0.6 Vts, chemical shift reported relative to [AI(H20)6] 3+. IR: Specord M80 spectrometer. N2-adsorption: Sorptomatic 1990 (Carlo Erba Instrumems), degassing at 403 K, 10-4 mbar, 5h. The MBOH conversion was performed as described elsewhere [7]. The 2propanol decomposition was conducted at 473 K using pulse microreactor. Prior to the testing, catalysts were preheated at 523 K in an Ar flow for 2h. First order rate constants for alcohol transformation were calculated according to ref. [8] and related to the catalyst surface unit. The selectivity is defined as follow: 100 x (moles product formed/moles alcohol decomposed at the same overall conversion) [9]. The samples with A1, Cr or Fe in the tetrahedral and Zn in the octahedral sheets were denoted as AIZnSP, CrZnSP and FeZnSP, respectively. The materials with Zn-Cu and Mg-Cu in the octahedral and Cr, Fe in the tetrahedral positions were referred to CrZn/CuSP, CrMg/CuSP, FeZn/CuSP, respectively. 3. RESULTS AND DISCUSSION The examination of samples prepared by XRD revealed the crystal structure corresponding to 2:1 trioctahedral clays, among them saponite, as deduced from the interlayer spacing and the position of (006) reflection at about 1.54 A (Fig. 1 and Table 1). The obtained XRD patterns agree well with those of synthetic saponite reported previously [3]. However, when the Si/M 3+ ratio in starting gel is

969 lower than 5.0, the formation of considerable amount of poorly crystallized phase was observed.

(ool)

t(ool) (060)

0

20

40

60

0

20

40

60

2 Thcta Figure 1. XRD patterns of saponite-like materials with atomic ratios Si/M3+=8.0 and Zn(Mg)/Cu=5.0: (1) CrZnSP, (2) CrZn/CuSP, (3) CrMgSP, (4) FeZnSP and (5) FeZn/CuSP. Hence, the content of M 3+ in tetrahedral sheets influences structural regularity of the samples. A higher crystallynity was established for the materials with atomic ratios of Si/M3+=8.0 and Zn(Mg)/Cu=5.0 and, therefore they were subjected for the further investigations. Among the products synthesized, the Zn-containing ones exhibit the sharpest XRD peaks (Fig.l) indicating the formation of relatively large crystals. A combination of Zn, Mg and Cu in the octahedral sheet results in some broadening of (001) reflection most probably due to a decrease of stacking. In the case of the Mg-containing samples, this phenomenon is less pronounced. For the A1ZnSP sample, a preferable location of AI3+ in tetrahedral positions was evidenced by A127 MAS NMR (strong resonance peak at 62-64 ppm). Nevertheless, a small amount of A1 cations is placed in the octahedral sheets what was detected by weak N M R peak at approximately 10 ppm. To study swelling of the materials prepared, their saturation with ethylene glycol vapour has been performed. As can be seen from data collected in Table 1, all the samples display an ability to swell. However, the influence of the nature of lattice constituting cations on swelling properties of saponite-like materials is not clear enough. Most likely, this feature is related to the layer charge that can vary with the composition of tetrahedral and octahedral sheets. A high cation exchange capacity established for all the samples prepared (Table l) agrees well that of synthetic saponite [10].

970 For all solids obtained, the IR spectra contain the lattice vibration bands at about 1050-, 986- 820-, 690-, 457-440-cm -1 characteristic of saponite [10]. Table 1 Cation exchange capacity (CEC) and basal spacing (001) evolution upon ethylene glyco! saturation of saponite-like materials synthesyzed . . . . dq01 d001, Sample CEC dQ01 d001, Sample CEC meq/g A EG, A meq/g A EG, A A1ZnSP 91 12.90 16.83 CrMg/CuSP 102 15.80 16.25 CrZnSP 105 12.24 14.90 FeZnSP 95 12.87 14.87 CrMgSP 96 15.40 16.56 FeZn/CuSP 107 12.07 15.15 CrZn/CuSP 119 13.05 14.65 (d001 EG) basal spacing after ethylene glycol saturation for 24 h. The ESR spectra of Na-forms of CrSP and FeSP samples recorded at 293 and 77 K revealed a broad line at g=1.98 and g=2.0, respectively. In the case of Crcontaining samples, the observed line can be assigned to Cr ions in trigonal of tetragonal symmetry in the saponite lattice or to the exchanged Cr 3+ cations [ 11]. For Fe-saponites, this signal can be attributed to Fe ions in tetrahedral symmetry [12]. To examine the case that those cations are partially located in the interlayer space, an additional ion-exchange with NH4 + ions was performed. As appeared, such treatment does not affect significantly both the line intensity and g-values thus indicating the absence of the detectable amount of Cr and Fe ions in the exchangeable positions. However, due to the large width of ESR signals our results do not allow a more detailed discussion. The N2 adsorption isotherms of AIZnSP and CrZnSP samples are almost of Type IV that is characteristic of mesoporous adsorbents (Figure 2). 250

400

~, 200

300

r-"-i

"~ 150

200

100

i00

~ 50 0

0.0

,

0.5

1.0

0

0.0

0.5

1.0

0.5

1.0

Relative pressure [P/Po] Figure 2. N2 adsorption-desorption isotherms for saponite-like materials prepared: (a) CrZnSP, (b) AIZnSP, (c) CrZn/CuSP and (d) FeZnSP.

971 The hysteresis loop for those samples is of Type H2, representative of ink-bottle shaped pores [13]. The same implies the Mg-containing materials (not shown). The shape of the isotherm of the CrZn/CuSP and FeZnSP samples exhibit a combination of Type I and Type IV with hysteresis loop closely to Type H4 reflecting the presence of slit-like pores in structure of this sample. All samples exhibit a relatively narrow pore size distribution (Dollimore-Heal method) centred at about 40 A. From Table 2 showing the main textural properties of the materials prepared and adsorption isotherms (Fig. 2), can be concluded that they are mainly mesoporous irrespective of the chemical composition. The samples with copper exhibit some mieroporosity indicating that incorporation of Cu ions in the octahedral sheet influences their texture (Table 2). Table 2 Chemical composition of silicate layer and the main textural properties of saponite-like materials pre-heated at 403 K . . . . . . . . Sa* Wm* Vmeso* Sample Si/M 3+ Si/M 3+ SBET (EPMA). . . . . ( X R F ) (ma/g) (ma/~) (cc/8) (cc/g) AIZnSP 7.93 7.94 189 192 -0.16 CrZnSP 7.92 7.90 223 246 -0.30 CrZn/CuS P 7.93 7.91 331 336 0.063 0.37 CrMgSP 7.91 7.87 420 436 -0.55 CrMg/CuSP 7.95 7.96 465 470 0.043 0.49 FeZnSP 7.93 7.94 283 296 -0.26 FeZn/CuSP 7.92 7.93 350 360. 0.084 0.38 Note: S, Vm, Vmeso are stands for specific surface area, micropore and mesopore volumes, respectively, * calculated by ets-method. The catalyst acid-base properties can be characterized by different methods, in particular by test-reactions, among them alcohol decomposition. To study the nature of active sites originated by the lattice, Na-form of the materials prepared was tested by means of 2-propanol and 2-methyl-3-butyne-2-ol (MBOH) decomposition. These alcohols were chosen as test molecules because it is known that MBOH dehydrogenation into acetone and acetylene probes the basicity while hydrogen subtraction from 2-propanol requires participation of the redox active sites [ 14]. The 2-propanol decomposition at 473 K over the samples with transition-metal ions results mainly in the alcohol dehydrogenation yielding acetone and hydrogen as products (Fig.3). The overall alcohol conversion is not considerably dependent on the nature of transition metal (Cr or Fe), as follow from Fig. 3A. In the case of the A1ZnSP material, the alcohol dehydration was the unique reaction. Incorporation of Mg cations in the octahedral positions of Cr- and Fe-containing samples leads to some increase of the dehydration activity that was reflected by the propene appearance (Fig. 3B). For Cu-containing samples, the dehydrogenation activity is somewhat higher than that of the samples without copper. This can be explained by the action of octahedral Cu cations exposed at the crystal edges.

972 0.45,

aeetorte VJJJAproperte

A 80 ~

~'~ 0.4 "7, "~ 0.2 4

B

1

.........

60 401

r-q

*

0.10

20

O.05

"'---~. . . . . . i

ii

0

7

I

I

2

4

1

2

3

4

5

6

7

sample

n m time [h]

Figure 3. (A) 2-propanol decomposition versus rtm-time and (B) selectivity for acetone and propene formation at 40% conversion for samples: (1) A1ZnSP, (2) CrMgSP, (3) CrZn/CuSP, (4) CrMg/CuSP, (5) CrZnSP, (6) FeZnSP, (7) FeZn/CuSP. The MBOH transformation undergoes the acid and base catalyzed cleavage reaction over the catalysts studied giving 3-methyl-3-butene-2-one (MIPK), acetylene and acetone (Fig. 4). 0.45

~ ~ i acetone ~/'//~] M IP K

A 80

r

B

! ,--t

60

~, 0.30 r.ca

.

.

.

.

2

.\

40 6

20

0.15 0

I

......

I

1 2 run time [h]

I

3

1

2

3

4

5

6

7

sample

Figure 4. (A) MBOH decomposition versus run time and (B) selectivity for acetone and M I P K formation at 60% conversion for samples: (1) CrMgSP, (2) CrMg/CuSP, (3) CrZn/CuSP, (4) AIZnSP, (5) CrZnSP, (6) FeZn/CuSP, (7) FeZnSP.

973 Likewise 2-propanol decomposition, octahedral copper cations play a role in contributing the catalysts dehydrogenation activity (Fig. 4B). To elucidate a possible participation Na-exehangeable cations in the alcohol dehydrogenation, the catalysts containing transition-metal ions were ionexchanged with Cs +. For 2-propanol, no significant variations in the overall alcohol decomposition and selectivity were found. In the case of MBOH such treatment does not affected the activity whereas catalyst acidic function was completely suppressed resulting in the dehydrogenation activity increases. From the above, one can conclude that the 2-propanol dehydrogenation is linked to the lattice redox active sites such as Cr, Fe and Cu. By comparison, on basic catalysts without redox properties acetone formation from this alcohol only occur at high temperature (573 K) [15]. However, a participation of the acidic sites in 2-propanol dehydrogenation cannot be excluded. It was shown [15], that its dehydrogenation proceeds through a concerted mechanism involving the acid active centres action. The MBOH dehydrogenation seems to proceeds over redox and basic active sites originated by lattice constituting and exchangeable cations without participation of acidic sites. Due to this reasol~ the elucidation of surface acidity by means of MBOH dehydration appears to be realistic. For this alcohol, the dehydration activity (expressed as ratio between rate constants of the products formation) of the samples follows the order: A1SP> CrSP>_ FeSP. This sequence agrees well with the expected Bronsted acidity of bridging hydroxyls Si(OH)M 3+. It is known that incorporation of trivalent cations with increasing ion radius into the silicalite framework leads to decreasing acid Bronsted eentres [16]. Considering the similarity in symmetry of M 3+ ions in the silicalite framework and in the saponite lattice, it might be concluded that the introduced Fe and Cr cations occupy mainly the tetrahedral positions. The ion exchange of interlayer Na + cations with A13+ results in the increase of the total 2-propanol and MBOH conversion over the catalysts, dramatically decreasing their dehydrogenation activity. The achieved activity is remarkably higher than that of Al-montmorillonite and comparable with that of acid-leached commercial montmorillonite K10 (Fluka). 4. CONCLUSIONS It might be said that the results derived from XRD, IR, swelling tests, ionexchange and N2-adsorption indicate the formation of saponite-like structure thereby evidencing the preferable incorporation of Cr and Fe in the tetrahedral and combined insertion of Zn or Mg and Cu cations in the octahedral positions under synthesis procedure applied. Nevertheless, the presence of some M 3+ cations in the octahedral positions camlot be ruled out and additional study is necessary to elucidate this case. The non-hydrothermal synthesis approach enables combined introduction of Zn or Mg and Cu cations in the octahedral positions thus allowing isomorphous substitution of those cations by each other. The synthesized materials are mainly mesoporous with high surface area, differing in the pore shape. They possess both acidic and basie/redox active sites amount of which can be altered by

974 varying of the lattice composition and/or an additional ion exchange. Furthermore, all materials prepared being ion-exchanged with A13+cations displayed a high dehydration activity in alcohol decomposition. 5. ACKNOWLEDGEMENTS

The work was supported in part by the Ukrainian Ministry of Education. The authors gratefully thank Mrs. N. Kostoglod (NTUU, Kiev, Ukraine) for help and Mr. E.M. van Oers (TUE, Eindhoven, The Netherlands) for the adsorption measurements. REFERENCES

1. H. Sequet, J.T. Liyama, H. Kodama and H. Pezerat, Clays and Clay Min., 27 (1977) 231. 2. R.J.M.J. Vogels, M.J.H.V. Kerkhoffs and J.W. Geus, in Preparation of Catalysts VI, G. Poncelet et al. (eds.), Elsevier Publ., Amsterdam, 1995, 1153. 3. V.I. Sobolev, A.S. Kharitonov, O.V. Panna and G.I. Panov, Stud. Surf. Sci. Catal., 98 (1995) 159. 4. R. A. Sheldon and R.A. van Santen (eds.), Catalytic Oxidation, World Sci. Publ., Singapore, 1995. 5. D.J. ParriUo, D. Dolenec, R.J. Gorte and R.W. McCabe, J. Catal. 142 (1993) 708. 6. J. Kornatowski, M. Sychev, S. Kuzenkov, K. Strnadova, W. Pilz, D. Kassner, G. Pieper and W.H. Baur, J. Chem. Soc. Faraday Trans., 91 (14) (1995) 2217. 7. M. Sychev and R.A. van Santen, Proc. 3rd Polish-German Zeol. Col., M. Rozwadowski (ed.), N. Copernicus Univ. Press, Tortm, Poland, 1997, p.225. 8. D. Bassett and H.W. Habgood, J. Phys. Chem., 64 (1960) 769. 9. M.Y.H. Kwan, N.W. Cant, D.L. Trimm and M.S. Wainwright, Appl. Catal., 31 (1987) 25. 10. J. T. Kloprogge, Thesis, University of Utrecht, Utrecht, The Netherlands, 1992. 11. T. Chapus, A. Tuel, Y. Ben Taarit and C. Naccache, Zeolites 14 (1994) 349. 12. D. Goldfarb, M. Bernardo, K.G. Strohmaier, D.E.W. Vaughan and H. Thomann, Stud. Surf. Sci. Catal., 84 (1994) 403. 13. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure and Appl. Chem., 57 (1985) 6. 14. C.Lahousse, J. Bachelier, J.-C. Lavalley, H. Lauron-Pernot and A.-M. Le Govic, J. Mol. Catal., 87 (1994) 329. 15. A. Gervasini and A. Auroux, J. Catal. 131 (191) 190. 16. J. Janchen, G. Vorbek, H. Stach, B. Parlitz, J.H.C. van Hooff, Stud. Surf. Sci. Catal., 94 (1995) 108.

975 AUTHOR

INDEX

Abadjieva, N. Adam, S. Agashe, K. Airoldi, G. Alc~ntara-Rodriguez, M. Alekseeva, E.I. Alieva, E.D. Allain, J.F. Allan, E. Ananyin, V.N. Aptel, G. Ar~on, I. Atanasova, P.

295 109 147 119 903 255 255 725 305 797 931 127 73

Bacherikova, I.V. Baiardi, P. Bakos, I. Ballardini, D. Ballif, C. Bally, A.R. Baraket, L. Baron, G.V. Barrault, J. Basile, F. Basini, L. Batista, J. Bellido Gil, M. Belyaev, V.V. Bensch, W. Beretta, A. Berti, F. Bogdanov, S.V. Bogutskaya, L.V. Boote III, H. Borb~ith, I. Boutonnet Kizling, M. Brahmi, R. Bricker, M.L. Burgos, N. Buyanov, R.A.

385 541 269 787 485 485 657 717 895 31 31 127 167 797 109 541 787 15 385 773 195 495 403 147 157 185

Calafat, A.

837

469 827 699, 763 377 505, 879 577 403 941 321 469

Carrier, X. Castillo, R. Cauzzi, D. Cavani, F. Centeno, M.A. Centi, G. Cemak, J. Chanda, B.M. Chang, L. Che, M. Cheah, K.Y. Chen, H. Chen, J. Chen, Q. Chen, S.-Y. Chernetskii, V. Chiaro, S.S.X. Choi, J.S. Choplin, A. Chun, W. Chuvilin, A.L. Ciambelli, P. Clacens, J.M. Coman, S. Coote, C.F. Cot, L. Craciun, R. Cristiani, C. Croonenberghs, F. Cuperus, F.P.

431 321 147 651 237 633 477 91 411 15 349 421 691 773 205 691 119 517 55

Datye, A.K. Daum, J. de Back, R. Dekker, B.G. Delhez, P. Della Porta, G. Delmon, B. de los Reyes, J.A. Delsarte, S. Deltratti, M. Dessalces, G.

245 99 517 755 707 349 505, 827 889 869 699 735

1

976 De Stefanis, A. Devillers, M. Didillon, B. Dierich, A. Di Silvestri, F. Dobrynkin, N.M. dos Santos, S. Dubois, D. Dumitriu, D. Duplyakin, V.K. Duprez, D. Dykh, Zh.

921 699 41 137 219 213 91 63 485 213 403 237

Efendiev, A.A. Ehrburger-Dolle, F. Emerson, S.C. Ersson, A.G. Ertl, G. Estournes, C. Evans, L.R.

533 167 773 601 331 855 245

Fallbach, M. Fan, W. Farf~n-Torres, E.M. Faro, A.C., Jr Fazzini, F. Ferragina, C. Ferrari, M. Fierro, J.L.G. Fornasari, G. Forzatti, P. Fouilloux, P. Galanos, E. Ganzerla, R. Gao, Y.-B. Gardner, T.J. Gavrilov, V. Yu. Gazzano, M. Gerolin, F. Geus, J.W. Ghorbel, A. Gladden, L.F. Goldwasser, M. Gonz~lez, H.

Gottifredi, J.C. Graham, G.W. Grange, P. Groppi, G. Guizard, C.

669 411 505, 517, 691,869, 879 119 205

Haber, J. Hadjiivanov, K. H~ihnlein, M. Hampden-Smith, M. Hanaoka, T. Harada, T. Harlin, E.M. Haruta, M. Hayashi, H. Heinrich, B. Heinrichs, B. Hernandez, G.G. Hernfindez, R. Hudson, M.

385 295 99 73 265 313 855 83,277 265 855 707 679 807 911

73 617 669 633 577 921 505 577 31 119,541,787 63

Ilinitch, O.M. Isaeva, V. Ivanova, A.S. Iwai, A.

55 237 797 313

Jackson, N.B. J~iras, S.G. Jim6nez-L6pez, A. Johansson, E.M. Jones, D. Julbe, A.

305, 643 601 903 601 931 205

911 959 651 245 609 31 725 549,755 657

Kagawa, K. Kakuta, N. Kamphuis, A.J. Kapteijn, F. Kappenstein, C. Karki, K.S. Kawazoe, M. Keller, N. Kharlamov, A.I. Kiennemann, A. Kijima, T. Kirichenko, O.A.

83 745 451 175 403 557 83 855 385 285 951 411

1

895 807

977 Kishida, M. Kiviaho, J. Klimova, T. Klissurski, D. Koch, B. Kochubey, D.I. Kodas, T. Kodre, A. Kohler, S. Kotakowska, E. Kolenda, F. Kolesnichenko, N.V. Kolonia, K. Koningsberger, D.C. Kowalczyk, Z. Krawczyk, K. Krivomchkov, O.P. Kr6ger, M. Kryukova, G.N. Kucherov, A.V. Kuramoto, S. Kustova, G.N.

265 229 807 295 827 679 73 127 643 341 735 255 911 549 341 341 593 99 15,609 567 745 797

Lahousse, C. Lambert, J.-F. Lamprell, H. LaRocque, R. Laurent, Y. Ledoux, M.J. Lee, W.Y. Legendre, L. Lenarda, M. Levin, D. L6vy, F. Li, L. Li, Y. Lieftink, D.J. Lietti, L. Ligi, S. Likholobov, V.A. Lira, S.S. Lin, G.I. Linares, C. Lindblad, M. Lintz, H.-G.

5O5 469 855 773 869 855 477 879 959 359 485 431 321 755 395 377 15,213 477 797 895 817 421

Lipovich, V.G. Lisi, L. Litvak, G.S. Loktev, S.M. L6pez Granados, M. Lucchini, V. Lunin, V.V. Lunsford, J.H.

679 349 797 255 577 959 797 459

Machida, M. Maggi, R. Maireles-Torres, P. Marella, M. Margitfalvi, J.L. Markova, N.A. Masetti, S. Matsumura, Y. Matsuura, I. McCabe, R.W. McLaughlin, L.I. McLeod, A.S. Medina-Valtierra, J. Meregalli, L. Merlen, E. Mestl, G. Mihaylov, M. Mintchev, L. Miseo, S. Mita, S. Mizushima, T. Moggi, P. Mohan, D. Montes, M. Montoya, J.A. Morawsky, V. Moroz, E.M. Morselli, S. Moser, W.R. Moulijn, J.A. Miahler, M. Murgia, V.

951 505 903 625,725 195 255 377 83 313 411 245 889 625 41 109,459 295 295 359 313 745 219,763 557 157 889 99,137 15 763 773 175,845 331 669

Nakamura, S. Nakamura, T. Navada, A.

277 277 147

1

978 Navio, J.A. Nenoff, T.M. Niculae, C. Niemel~i, M. Nifant'ev, N. Noskov, A.S.

679 643 205 229 237 213

Odriozola, J.A. Ohkita, H. Okumura, M. Olivera-Pastor, P. Orsenigo, C. Osawa, T.

157 745 277 903 787 313

Pages, T. Paj onk, G.M. Pakhomov, N.A. Parera, J.M. Park, G.I. Parvulescu, V. Parvulescu, V.I. Paulis, M. Pavlova, S.N. Peltier, V. P6rez-Reina, F. Pesin, O. Yu. Petit, C. Petrakis, D. Petryk, J. Pham-Huu, C. Pieck, C.L. Pinna, F. Pintar, A. Pinto, N. Pirard, J.-P. Polukhina, I.A. Pomonis, P. Popescu, G. Potapova, Yu.A. Pouilloux, Y. Prasad, R. Predieri, G. Prihod'ko, R. Prina, D. Prosvirin, I.P.

41 167 185 369 477 205,691 205,485,691 157 797 869 903 255 285 911,931 341 855 369 625 127 167 707 213 911,931 205 797 895 557 219,699,763 967 541 15

Provendier, H. PriJsse, U. Quignard, F.

285 99, 137 91

Ramani, A. Ramaswamy, V. Ramirez, J. Regalbuto, J.R. Regali, F. Reinikainen, M. Reverchon, E. Reymond, J.P. Rodriguez-Castell6n, E. Roger, A.C. Romanenko, A.V. Root, A. Rosynek, M.P. Roziere, J. Rozovskii, A. Ya. Ruitenbeek, M. Ruiz, P.

941 941 807 147 495 229 349 63, 735 903 285 15 817 459 931 797 549 827

Sadykov, V.A. Sakurai, H. Salanov, A.N. Sambeth, J. Sanjin6s, R. Sannino, D. Sanz, R. Savelieva, G.G. Scarpa, M. Scattolin, R. Schl6gl, R. Schmid, P.E. Schnabel, M. Schoebrechts, J.-P. Schr6der, M. Serrano, D. Shakhtakhtinsky, T.N. Sham, E.L. Shao, Z. Sharf, V. Sheng, S. Shimizu, K.

797 83 797 157 485 349 577 213 625 625 109 485 99 707 99 577 533 669 431 237 431 369

979 Shitova, N.B. Signoretto, M. Simonov, P.A. Sinou, D. Sivasanker, S. Slinkin, A.A. Slivinski, Ye.V. Slivinsky, E.V. Soled, S. Song, I.K. Stoeh, J. Storaro, L. Strukul, G. Sun, J. Sun, Y. Sun, Y.-H. Syehev, M. Szab6, S.

213 625 15,55 91 941 567 679 255 359 477 385 959 625 617 617 651 967 269

Takayasu, O. Talon, A. Tang, Y. Taniguchi, H. Tashiro, S. Teleshev, A.T. Terekhova, G.V. Thoma, S.G. Tikhov, S.F. Timpe, O. Tirions, O. Tiripicchio, A. Tiu, F. Tomaselli, M. Tomlinson, A.A.G. Tompos, A. Trifir6, F. Truhmanova, N.I. Trusova, Ye.A. Tsodikov, M.V. Tsubota, S. Tsybulya, S.V. Tsyrulnikov, P.G. Tufts, J.C.

313 959 441 951 265 255 255 643 797 109 699 699 691 625,725 921 195 31,377 255 679 679 277 797 213 773

Usami, Y. Uzio, D.

83 41

Vaccari, A. Vaccaro, A. van Dillen, A.J. Velu, S. Venkateswara Rao, A. Vera, C.R. Vergunst, T. Villa, P.L. Vorlop, K.-D.

31 137 549 941 167 369 175 395 99,137

Wakabayashi, K. Walls, J.R. Wang, Y.Y. Webb, G. Wiame, H.M. Wild, U. Wise, J. Wu, D.

265 451 651 305 879 109 73 617

Xie, S. Xie, Y. Xiong, G.-X. Xiong, G. Xu, X.

459 441 717 431 845

Yabunaka, J. Ying, J. Young, N.C.

951 359 305

Zaikovskii, V.I. Zazhigalov, V.A. Zenkovets, G.A. Zhao, B. Zhao, H.-B. Zhao, J. Zhu, Y. Zolotovskii, B.P. Zong, B. Zotin, J.L. Ztihlke, S.

797 385 609 441 717 321 441 185 331 633 421

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981 STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Lo ~1vain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pitts )urgh, PA, U.S.A. Volume 1

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Preparation of Catalysts l.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control ofthe Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts il. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Dolmen, P.Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci~t~ de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon),September 9-11, 1980 edited by B. Imelik, C. Naccache, u Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Frornent New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.I. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Ldznieka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naecache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties- Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A.Jacobs, N.i. Jaeger, P.Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P~rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary ofthe Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, W~rzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A.Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings ofthe Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura

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Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by P. Ruiz and B. Dolmen Progress in Catalysis. Proceedings ofthe 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings ofthe 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and R Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and IVI.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings ofthe Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and IVI.Masai Heterogeneous Catalysis and Fine Chemicals II1. Proceedings ofthe 3rd International Symposium, Poitiers, April 5- 8, 1993 edited by iVi. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.E Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalm&dena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Beilon Zeolites and lVlicroporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by MI.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids I!1. Proceedings ofthe IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouqueroi, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger

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Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F. Vansant, P.Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution Control III. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J. Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dabrowski and V.A. Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995 edited by M. Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 1 lth International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 1lth ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J. W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon, S.I. Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzir~ski, W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 11th International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon, S.-K. Ihm and Y.S. Uh

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Hydrotreatment and Hydrocracking of Oil Fractions Proceedings ofthe 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.E Froment, B. Delmon and R Grange Natural Gas Conversion IV Proceedings of the 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoza, C.R Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12, 1996 edited by H.U. Blaser, A. Balker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium, Antwerp, Belgium, September 15-17,1997 edited by G.F. Froment and K.C. Waugh Third World Congress on Oxidation Catalysis. Proceedings ofthe Third World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings ofthe 7th International Symposium, Cancun, Mexico, October 5-8, 1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings ofthe 4th International Conference on Spiliover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4, 1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997 edited by T. Inui, M. Anpo, K. Izui, S. Yanagida and T. Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis and Automotive Pollution Control IV. Proceedings ofthe 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997 edited by N. Kruse, A. Frennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1st International Symposium, Baltimore, MD, U.S.A., July 10-12, 1998 edited by L.Bonneviot, F. B61and, C. Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4, 1998 edited by B. Delmon, P.A.Jacobs, R. Maggi, J.A. Martens, P. Grange and G. Poncelet

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