Comprehensive Chemical Kinetics: Complex Catalytic Processes (Journal of Chromatography Library)

COMPREHENSIVE CHEMICAL KINETICS

COMPREHENSIVE Section 1. THE PRACTICE AND THEORY OF KINETICS Volume 1 Volume 2 Volume 3

The Practice of Kinetics The Theory of Kinetics The Formation and Decay of Excited Species Section 2. HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS

Volume 4 Volume 5

Decomposition of Inorganic and Organometallic Compounds Decomposition and Isomerisation of Organic Compounds Section 3. INORGANIC REACTIONS

Volume 6 Volume 7

Reactions of Non-metallic Inorganic Compounds Reactions of Metallic Salts and Complexes, and Organometallic Compounds Section 4. ORGANIC REACTIONS (6 volumes)

Volume 8 Volume 9 Volume I0 Volume 12 Volume 13

Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)

Volume 14 Volume 14A Volume 15

Degradation of Polymers Free-radical Polymerisation Non-radical Polymerisation Section 6. OXIDATION AND COMBUSTION REACTIONS (2 volumes)

Volume 17

Gas-phase Combustion Section 7. SELECTED ELEMENTARY REACTIONS (1volume)

Volume 18

Selected Elementary Reactions Section 8. HETEROGENEOUS REACTIONS (4volumes)

Volume 19 Volume 20 Volume 21 Volume 22

Simple Processes a t the Gas-Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State Additional Section KINETICS AND TECHNOLOGICAL PROCESSES

CHEMICAL KINETIC: EDITED BY

C.H. BAMFORD M.A.,Ph.D., Sc.D. (Cantab.), F.R.I.C., F.R.S. Campbell-Brown Professor o f Industrial Chemistry, University of Liverpool AND

C.F.H. TIPPER Ph.D. (Bristol), D.Sc. (Edinburgh) Senior Lecturer in Physicaf Chemistry, University of Liverpool

VOLUME 20

COMPLEX CATALYTIC PROCESSES

ELSEVIER Amsterdam - Oxford - New York 1978

- Tokyo

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211,1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010,U.S.A. First edition 1978 Second impression 1991

Library of Congress CaIaloging in Publication Data

Barnford, C H 'Complex c a t a l y t i c processes. (Their Comprehensive chemical kinetics ; v. 20) Bibliography: p. Includes index. 1. Catalysis. I. Tipper, Charles Frank Howlett, j o i n t author. 11. T i t l e . QD501.€!242 vol.20 [QDSOSI 541l.39~ rs+1'.3951 78-4165 ISBN 0-W-41651-X

ISBN 0-444-41 651-X with 79 illustrations and 109 tables 0 Elsevier Science Publishers B.V., 1978

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 Publishers B.V./ Academic Publishing Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC 1, Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.

This book is printed on acid-free paper. Printed in The Netherlands

COMPREHENSIVE CHEMICAL KINETICS

ADVISORY BOARD Professor S.W. BENSON Professor SIR FREDERICK DAINTON Professor G. GEE the late Professor P. GOLDFINGER Professor G.S. HAMMOND Professor W. JOST Professor G.B. KISTIAKOWSKY Professor V.N. KONDRATIEV Professor K.J. LAIDLER Professor M. MAGAT Professor SIR HARRY MELVILLE Professor G. NATTA Professor R.G.W. NORRISH Professor S. OKAMURA the late Professor SIR ERIC RIDEAL Professor N.N. SEMENOV Professor Z.G. SZABO Professor 0. WICHTERLE

Contributors to Volume 20 L. BERANEK

Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 0 2 Praha 6 - Suchdol, Czechoslovakia

M. KRAUS

Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 0 2 Praha 6 - Suchdol, Czechoslovakia

P.J. VAN DEN BERG Department of Chemical Technology, Delft University of Technology, Delft, The Netherlands K. VAN DER WIELE

Department of Organic Products, Akzo Zout Chemie Nederland bv Research, Hengelo, The Netherlands

G. WEBB

Chemistry Department, The University, Glasgow, Scotland

Section 8 deals with reactions which occur at gassolid and solidsolid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reactions at the liquidsolid interface (and corrosion) involving electrochemical processes outside the coverage of this series are not considered. With respect to chemical processes at gassolid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. In Volume 20, complex processes catalysed by solids are covered. Chapter 1 deals with hydrogenation. After consideration of the nature of the metal catalysts, general aspects of the kinetics and alternative reaction pathways, the hydrogenation of olefins, alkynes, dienes and cyclic molecules are dealt with in detail. Finally, the relationship between catalyst structure and hydrogenation activity is discussed. Chapter 2 is concerned with heterogeneous oxidation processes. The oxidation of ethylene and propene, so important industrially, is considered at length and then higher olefins and aromatic hydrocarbons; the influence of ammonia (ammoxidation) is also discussed. There is a section on the oxidation of methanol, ammonia and sulphur dioxide and, to conclude, the role of the catalysts is considered. Elimination, addition and substitution processes occurring on solid acid-base catalysts are covered in the last chapter. These reactions include dehydration, deamination, dehydrohalogenation, dealkylation by cracking, dehydrosulphidation, hydration, hydrohalogenation, alkylation by olefins, aldol condensation, esterification and hydrolysis. The editors are very grateful for much invaluable advice from their colleague Professor D.A. King.

Liverpool January, 1978

C.H. Bamford C.F.H. Tipper

This Page Intentionally Left Blank

Contents ................................................. Chapter 1 (G . Webb) Catalytic hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . General principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Variables in the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface

2.1.1 Variables associated with the substrate . . . . . . . . . . . . . . . 2.1.2 Variables associated with the catalyst . . . . . . . . . . . . . . . . 2.2 Kinetics and the derivation of rate expressions . . . . . . . . . . . . . . . 2.2.1 Rate expressions for bimolecular surface reactions . . . . . . . 2.3 Selectivity and the concept of alternative reaction paths . . . . . . . . 2.4 Application of absolute rate theory t o bimolecular surface reactions 3. The hydrogenation of olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Adsorbed states of olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Possible reaction mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Treatment of experimental results . . . . . . . . . . . . . . . . . . . . . . . 3.5 Hydrogenation of ethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Hydrogenation of propene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Reactions of the n-butenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Reactions of higher aliphatic olefins . . . . . . . . . . . . . . . . . . . . . . 4 . The hydrogenation of alkynes and alkadienes . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Nature of the adsorbed state of alkynes and alkadienes . . . . . . . . . 4.3 Possible reaction mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Treatment of experimental results . . . . . . . . . . . . . . . . . . . . . . . 4.5 The hydrogenation of acetylene . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Hydrogenation of monoalkylacetylenes . . . . . . . . . . . . . . . . . . . 4.7 Hydrogenation of dialkylacetylenes . . . . . . . . . . . . . . . . . . . . . . 4.8 The hydrogenation of alka-l:2-dienes . . . . . . . . . . . . . . . . . . . . . 4.9 The hydrogenation of conjugated alkadienes . . . . . . . . . . . . . . . . 5 . The hydrogenation of cyclic molecules . . . . . . . . . . . . . . . . . . . . . . . . 5.1 The hydrogenation of alicyclio alkenes . . . . . . . . . . . . . . . . . . . . 5.2 The hydrogenation of cyclopropane . . . . . . . . . . . . . . . . . . . . . . 6 . Catalyst structure and hydrogenation activity . . . . . . . . . . . . . . . . . . . 6.1 Geometric factors in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Electronic factors in catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Surface migration and the influence of catalyst supports . . . . . . . . 7 . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

1 1 2 2 2 3 4 6 8 13 16 16 16 23 27 29 37 38 48 50 50 50 55 57 58 68 71 74 81 94 94 100 103 103 106 109 112 114

Chapter 2 (K . van der Wiele and P.J. van den Berg)

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

123

1. Scope of the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Oxidation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Ethylene oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123 124 126

Heterogeneous oxidation processes

2.1.1 Ethylene oxide production . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Acetaldehyde and acetic acid production . . . . . . . . . . . . . 2.2 Propene oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Propene oxide production . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Acrolein production . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Acrylic acid production . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Dimerization and aromatization . . . . . . . . . . . . . . . . . . . 2.2.5 Acetone production . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Ammoxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Butenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Isobutene to methacrolein and methacrylanitrile . . . . . . . . 2.3.2 Normal butenes to butadiene, furan and maleic anhydride . . 2.3.3 Dimerization and aromatization of iso- and n-butenes . . . . . 2.3.4 Oxyhydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Higher olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Toluene and xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Ortho-xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Naphthalene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Anthracene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.6 Other aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . 2.5.7 Ammoxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 The silver process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Iron molybdate and other metal oxide catalysts . . . . . . . . . 2.7 Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 The production of NO . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.2 The formation of N2 and NzO . . . . . . . . . . . . . . . . . . . . . 2.8 Sulphur dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Role of the catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Thermodynamic considerations . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Metal-oxygen bond strength . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Heat of formation of metal oxides, AHf . . . . . . . . . . . . . . 3.2.2 Heat of oxygen desorption . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 I8O2 isotope exchange . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Oxygen transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Participation of lattice oxygen . . . . . . . . . . . . . . . . . . . . 3.3.2 Role of Me=O type of oxygen . . . . . . . . . . . . . . . . . . . . . 3.3.3 Significance of 0; and 0- radicals . . . . . . . . . . . . . . . . . . 3.4 Aspects of charge transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Bulk electrical properties . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Charge transfer on an atomic scale . . . . . . . . . . . . . . . . . . 3.5 Nature of the active sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Acid-base properties . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Bifunctionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Adsorption and reaction complexes on the catalytic surface . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

126 133 135 136 137 159 160 162 164 174 175 179 194 195 195 196 197 204 210 217 218 219 221 224 224 225 227 228 228 230 231 231 233 233 234 234 235 236 239 241 242 243 244 247 248 250 251 253

Chapter 3 (L . Berinek and M . Kraus) Heterogeneous eliminations, additions and substitutions . . . . . . . . . . . . . . . . 263 1. General features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Correspondence between homogeneous and heterogeneous reactions

263 263

Nature of the catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Aluminosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Metal salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Ion exchange resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6 The working surface . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Type of kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Elimination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Common features of heterogeneous catalytic eliminations . . . . . . . 2.1.1 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Types of dehydration reactions . . . . . . . . . . . . . . . . . . . . 2.2.2 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Deamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Types of deamination reactions . . . . . . . . . . . . . . . . . . . . 2.3.2 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Dehydrohalogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Types of dehydrohalogenation reactions . . . . . . . . . . . . . . 2.4.2 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Dealkylation by cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Types of cracking reactions . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Dehydrosulphidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Types of dehydrosulphidation reactions and catalysts . . . . . 2.6.2 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 2.6.3 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Hydration of olefins to alcohols . . . . . . . . . . . . . . . . . . . 3.1.2 Hydration of acetylene to acetaldehyde . . . . . . . . . . . . . . 3.1.3 Hydration of alkene oxides to glycols . . . . . . . . . . . . . . . . 3.2 Hydrohalogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Types of hydrohalogenation reactions and catalysts . . . . . . 3.2.2 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Alkylation by olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Types of alkylation reactions and catalysts . . . . . . . . . . . . 3.3.2 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Addition of alcohols to alkenes . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Aldol condensation and related reactions . . . . . . . . . . . . . . . . . . 3.5.1 Types of reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2

264 264 266 268 269 270 271 272 274 275 275 277 280 281 281 282 282 290 295 295 296 296 298 300 300 300 301 308 309 309 310 311 315 318 318 319 319 320 321 321 327 329 332 332 332 333 334 334 335 336 336 337 337 340

3.5.3 Experimental kinetic results . . . . . . . . . . . . . . . . . . . . . . 3.5.4 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . Substitution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Esterification and transesterification . . . . . . . . . . . . . . . . . . . . . 4.1.1 Types of reactions and catalysts . . . . . . . . . . . . . . . . . . . 4.1.2 Reactions catalysed by inorganic catalysts . . . . . . . . . . . . . 4.1.3 Reactions catalysed by organic polymer-based cation exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Hydrolysis of esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Other hydrolyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index

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

342 345 348 348 348 351 356 371 371 380 385 399

1 Chapter 1

Catalytic Hydrogenation GEOFFREY WEBB

1.Introduction Since the early reports of Sabatier et al. [l] of the catalysed hydrogenation of unsaturated hydrocarbons and aldehydes, a vast number of studies of catalytic hydrogenation have been reported. The extent of this literature is a reflection, in part, of the rich variety of systems and the wealth of attainable detailed information and, in part, of the great complexities which face the catalytic chemist. Selwood [ 21 in 1962 remarked: “No problems in surface chemistry have been more hotly debated than the adsorption and hydrogenation mechanisms for ethylene; and few debates have resulted in such meagre conclusions”. Some 15 years later, the subject remains one in which there is still considerable dispute and debate regarding detailed mechanisms, although over the past few years some measure of order has begun to appear. It is against this background of complexity and uncertainty that this chapter sets out to review the present status of the problem of heterogeneous catalytic hydrogenation from the standpoint of (a) the kinetics and TABLE 1 Types of catalysed hydrogenation reactions Reactant a

Possible reaction products

a

>c=c
c=c=c< >C=CH-CH=C< >CH . (CH2)n+l . CH=C< ; >CH(CH?),+&H
10

1

2 3 Residence time ( h )

2

1 ____.c_

3

Residence time (h)

Fig. 1 . Product yields from ethylene oxidation for Pd/V205 and pure V205. - - - - - -, 0.2%Pd/V205 ;,vzo5 ' Fig. 2 . Ethylene conversion for Pd/V205,pure V z 0 5 and Pd/A1203.

135 V 2 0 5 [243]. The kinetics are diffusion-controlled. Seoane e t al. [287] confirmed this result using 0.2 wt. 5% Pd on V z 0 5 at 230°C. Figures 1 and 2 show some typical data. Cant and Hall [72] report that the reaction rate with Pd on SiOz shows a first-order dependence on oxygen. The rate is strongly suppressed by olefin. 2.2 PROPENE OXIDATION

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation t o acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. Although “allylic oxidation”, yielding products like acrolein and acrylonitrile, is the most important and successful partial oxidation reaction, several other processes are of interest. Table 3 represents a summary of the nature of the various processes and the main partial oxidation products. This section concentrates primarily on the processes and products listed in the table. It excludes processes that yield predominantly partial oxidation products with less than three carbon atoms and cannot be regarded as very selective processes. Before discussing the various reactions in more detail in the following sections, a short characterization of the processes and a description of the types of catalyst involved will be given. The epoxidation of propene is analogous t o that of ethylene catalyzed by silver. However, the selectivity is much lower. Due t o the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. Allylic oxidation constitutes the main group of oxidation processes

TABLE 3 Summary of propene oxidation processes .

_

_

_

_

_

_

_

~

~

Nature of the reaction .

Epoxidation (Amm)oxidation of the allyl CH3-group Oxyhydration Oxidative dimerisation

References p p . 2 5 3 - - 2 6 2

~

~

.

.

Main products ~

_

_

Propene oxide Acrolein, acrylic acid, acrylonitrile Acetone Hexadiene, benzene

~

136 which are of industrial importance for the synthesis of acrolein, acrylic acid and acrylonitrile. The commercial interest of the latter has markedly contributed t o the extraordinarily large number of metal oxide combinations that have been investigated and t o the complexity of the multicomponent catalysts applied in industry today. Some typical binary oxide combinations, which also form the basis of most multi-component catalysts, are Bi-Mo-0, U-Sb-0, Sn-Sb-0 and Fe-Sb-0. Characteristic features of the allylic oxidation reaction are the initiation by allylic-H abstraction, the formation of a symmetric allyl intermediate, and the role of the catalyst as supplier of oxygen according t o a redox mechanism. The oxidative dimerization has recently attracted attention, both from a fundamental viewpoint and as a means for synthesizing aromatics from lower olefins. The reaction is essentially a combination of allyl radicals, by which the oxidation is limited t o the abstraction of one hydrogen atom. Typically, the catalysts applied here do not contain MOO, or a similar component that promotes the selective incorporation of oxygen. The oxyhydration of propene t o acetone occurs at a much lower temperature than the allylic oxidation and demands, in principle, the presence of excess steam. The reaction is initiated by addition of a proton from the catalyst surface and the acetone formation involves oxygen originating from water.

2.2.1 Pro.Dene oxide production Attempts to produce propene oxide selectively by gas phase oxidation have been plentiful but not successful. One of the best results is reported by Zanderighi and Carra [ 3591, who investigated a number of tungstates in a pulse reactor at 250-350°C and with an oxygen/propene ratio of 3/2. Thallium tungstate appears to give propene oxide with a selectivity of almost 3096, besides considerable amounts of acetone and acrolein, at a temperature of 350°C. At lower temperatures, the last two products predominate. Tungstates were also studied in a batch reactor (autoclave) by Centola et al. [ 841. At 185°C with 10 atm propene and 12.5 atm oxygen, these authors also found thallium tungstate to be the most effective: a selectivity of 40% is reported at 37% conversion. Remarkably, even without catalysts, a considerable epoxide yield is observed under these conditions, The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [ 95,961 demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [ 841 are understandable, as the wall of the empty reactor may have been sufficiently active t o initiate the reaction.

137

2.2.2 Acrolein production The oxidation of propene t o acrolein has received much attention for several reasons. Firstly, the process is of industrial importance in itself, and it is also a suitable model reaction for the even more important, but a t the same time more complicated, ammoxidation. Secondly, propene oxidation is, in many aspects, representative of that of a class of olefins which possesses allylic methyl groups. Last, but not least, the allylic oxidation is a very successful example of selective catalysis, for which several effective metal oxide systems have been discovered. The subject has therefore attracted much interest from the fundamental point of view. Consequently, numerous metal oxide catalysts have been studied, ranging from single metal oxides t o complex multi-component mixtures, and accordingly the aim of the research has varied from purely fundamental aspects t o development and optimization of industrial catalysts. The flood of patent applications that started some years ago is still going on and today invariably concerns complex oxide compounds. Non-industrial research in the field of multi-component catalysts is still rather scarce at the moment, yet it is becoming more abundant as growing insight into the action of the simpler catalysts is gained. The oxidation process is carried out in the temperature range 300450°C, and generally studied at atmospheric pressure. Excess air is usually applied (with some exceptions) and substantial amounts of water vapour may be added t o the feed. High initial selectivities (>95%)are feasible, and, although some further oxidation (combustion) of the product is unavoidable, yields of 70-90% are reported in the patent literature. The main by-products are carbon oxides, in addition to minor amounts of acrylic acid, acetaldehyde and formaldehyde. Acrylic acid may be a main product depending on specific catalyst properties and reaction conditions, as described in more detail in Sect. 2.2.3. (a) Kinetics and mechanism in general

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. The reaction between propene and the catalyst is, in general, rate-determining, as catalyst reoxidation is a relatively fast reaction. This implies that the degree of catalyst reduction under steady state reaction conditions is fairly low (i.e. less than 10% with respect to the total amount of oxygen that can be removed with propene). Thus the observed kinetics Rcfercnces p p . 253--262

138 for these catalysts are principally the kinetics of the reaction between propene and the oxidized catalyst. For many cases, the kinetics are adequately represented by a simple first-order model, viz. = 'PC3H6

An inhibiting effect of the reaction product (acrolein) is reported for several catalysts, and usually accounted for by an adsorption term in the denominator, leading to equations of the form

R

= -hPC3H6

1+ K P C 3 H 4 0 The first-order dependence with respect t o propene may be explained by the assumption that the initial reaction step (dissociative adsorption) is ratedetermining, while the inhibiting effect of acrolein is obviously connected with its strong adsorption on the active sites of certain catalysts. For some catalysts, the contrary situation occurs and reoxidation is the rate-determining step. A typical example is cuprous oxide. The observed rates are, in this case, dependent on the oxygen instead of the propene pressure. Kinetic redox models, as formulated by Mars and van Krevelen [ 2041, have not been considered in any recent work. Although the combined dependence on both propene and oxygen pressures does arise in certain investigations, the authors seem t o ignore redox mechanisms completely and correlate their data with Langmuir-Hinshelwood type models. A detailed treatment of the kinetics of groups of catalysts, and comparison between them is hardly possible due t o the widely different experimental conditions (e.g. catalyst preparation and pretreatment, reactor type, reaction conditions and experimental methods). Results of kinetic studies will be individually reported in the section on catalysts [Sect. 2.2.2.(d)].

( b ) Mechanism of acrolein formation The formation of acrolein comprises several steps. The first and ratedetermining step is generally assumed t o be abstraction of an allylic hydrogen atom. Evidence is provided from several sources, e.g. the deuterium effect of the reaction (Adams [ 2 ] ) and the analogy between oxidation and DzO exchange (Christie et al. [86]), and for various catalysts. The hydrogen abstracted is taken up by a surface oxygen anion t o form a hydroxyl group; the allyl radical is bonded t o the catalyst as a symmetrical complex. Hence, the first reaction step is represented by CH,=:CH-CH, +Me"++ 02-=(CHzLCH%H,),d,,

+Me("-')+ + O H -

Since it is well established that the allyl complex is neutral or (weakly)

139 positive, the reduction of a metal ion of the catalyst must take place, as indicated in the above equation. Metal ion reduction effected by propene adsorption has, in fact, been demonstrated for several metal oxides. The exact nature of the bonding between the allyl species and the catalysts is not known. The initial formation of a complex that is n-bonded t o a metal ion is often presumed, although n-0-complexes, or complexes which include oxygen anions, are proposed as well. The occurrence of a symmetrical intermediate at an early stage of the reaction with a variety of catalysts however, is unmistakably indicated by experiments with l 3 C- and l 4 C-labelled propene. The lifetime of a radical type of intermediate (if it exists) must necessarily be very short, as otherwise radical combination reactions might become important, yielding products like hexadiene and benzene. These products and free allyl radicals have indeed been detected in some studies (Dolejsek and Novakova [ 1021, Seiyama et al. [ 285,2861,and Weiss et al. [345]). It must be emphasized, however, that the catalysts concerned differed from the usual acrolein-forming catalysts, being hardly selective a t all. The selective action of acrolein-producing catalysts is very probably due to the ability to oxidize the initially formed complex rapidly to an allyl carbocation [ 3451. This assumption is the more plausible as selective catalysts are characterized by a strong electron affinity combined with a good electron conductivity. Abstraction of a second hydrogen atom from the initially formed allyl radical, as proposed by Voge and Adams 13431, is much less likely to be the second reaction step: this second hydrogen abstraction presumably requires more energy than the first. Hence, it excludes the first as the ratedetermining step. The exact mechanism of lattice oxygen incorporation and second hydrogen abstraction, and the precise sequence of elementary events is still a subject of speculation. Several authors assume that two distinct active sites are involved in the acrolein formation. The first, presumably a cation, participates in the formation of the initial allyl complex, while the second, which may contain a different cation and reactive oxygen anions, is the place where further hydrogen abstraction and oxygen incorporation take place. An attractive reaction scheme, which does not require transfer of a reaction intermediate from one site t o another, is proposed by Weiss et al. [ 3451. The authors assume that propene is adsorbed at the site containing the cation with the highest valency and that the metal oxygen double bond plays an essential role. The initial adsorption may be represented by

References p p . 2 5 3 - 2 6 2

140 Depending on the nature of M (which is not necessarily a metal), the reaction may proceed via a a-ally1 or Ir-ally1 complex, eventually leading t o an ally1 carbocation

L

A

-e-

A'

M-OH M-OH r-ally1 This cation is attached t o a lattice oxygen atom which activates the abstraction of the second hydrogen atom, followed by desorption of acrolein. The complex role of the catalyst is separately discussed in Sect. 3 , where a great many of the references concern propene oxidation studies. (c) By-products

The main by-products of acrolein formation are carbon monoxide and carbon dioxide, as well as minor amounts of acrylic acid and lower aldehydes and acids. Combustion takes place both consecutive and parallel t o the main reaction. Acrylic acid (in free or adsorbed form) is a possible intermediate in the acrolein combustion. Including this product, the following simplified scheme applies.

,CH2=:H*Ho

C,Hz650°C).The 7'-phase which is then formed differs from the y-phase with respect to the molybdenum layer, which consists of separate Moo4 tetrahedra according t o the La2MoO4 structure [ 541. The structure of the 0-phase is not entirely clear. Moreover, this phase is said to be unstable and t o decompose into a and y. According to Erman et al. [105], it is related t o scheelite, but its unit cell is larger. Crystal phases that contain phosphorus in addition t o bismuth and molybdenum are not known. Phosphorus is probably present as bismuth phosphate. Remarkably, however, a fresh commercial catalyst seems to contain only a minor amount of the a-phase in spite of the Bi/Mo ratio which is close t o 2/3 (Schuit [Sl]). The phases (y' excluded) described above are all active and selective

143

catalysts for the propene oxidation. Several authors report that the highest activity is obtained at a Bi/Mo ratio of 1/1corresponding t o the 0-phase. As this phase is unstable, a correlation between activity and (bulk) structure seems doubtful. Arguments for both molybdenum tetrahedra and octahedra as the active species have been presented in the literature. A problem is the large difference in the conditions existing for the X-ray analysis and the condition of the steady state chemical reaction. Moreover, it is uncertain whether phases observed by X-ray analysis adequately represent the structure of the surface. There are quite a number of arguments in favour of the idea that the surface structure is, t o some extent, independent of the bulk structure. Studies concerning the effect of the method of preparation on the catalytic properties and comparison of catalysts prepared in different laboratories prove that large differences may occur a t the same Bi/Mo ratio and the same bulk structure [41,45,59,149,327], while, on the other hand, very similar properties may be displayed by catalysts with different bulk structures. Batist e t al. [42], for example, have shown that the bulk of a Bi9PMo,,052 catalyst initially consists mainly of the 0-phase, but gradually decomposes into 0 - and y-phases during its use in a reactor at 470"C, without a significant effect on the catalytic properties. Furthermore, it is noticeable that, although different activities are reported for distinct phases by various authors, no appreciable differences in the activation energy occur, indicating that it is not the quality but mainly the quantity of active sites that changes. Of particular interest is the study of Miura et al. [220] who investigated the existence of various types of oxygen by temperature-programmed reoxidation of partially reduced bismuth molybdates at 0-500°C. There was no relation between the recorded reoxidation peaks and the phases observed by X-ray analysis. However, the active and selective catalysts (Bi/Mo = 2/1,1/1 and 2/3) all showed a characteristic peak at 320"C, which also occurred in Moo3 at larger degrees of reduction (i.e. 4--10%). Addition of phosphorus t o the catalysts, P/(Mo + Bi) = 0.075, appeared t o intensify the 320°C peak; for the 1/1and 2/3 catalysts peaks other than the 320°C peak vanished with the addition of phosphorus; at the same time, the catalytic activity was increased, while the selectivity was invariably high (80%). The authors report a good reproducibility of the experiments and conclude that activity and selectivity are correlated with a specific type of oxygen, which is present irrespective of the bulk structure. Finally, Grzybowska e t al. [148] have studied structural changes in the surface layers of bismuth molybdate catalysts occurring on evacuation, reduction by hydrogen and interaction with propene/oxygen mixtures, by means of surface spectrography methods (XPS and UPS). They observed that, in a reducing atmosphere due t o either the action of high vacuum or the presence of reducing agents, rearrangements of the surface layers take place resulting in a lowering of the Bi/Mo ratio for the more Bi-rich catalysts and the formation of similar R e f e r e n c e s P P . 2 5 3-2 6 2

144 compositions on all three bismuth molybdates investigated. No rearrangements occurred by interaction with a reaction mixture (propene/oxygen/ nitrogen = 24/21/55) during 5 h at 440°C. It was found, however, that the surface became covered with a strongly bonded oxygenated hydrocarbon species. This species persisted at the surface even after outgassing at 430°C and could also be formed in situ in the spectrometer by adsorption of pr'opene at 500" C. Kinetics. The kinetics of the oxidation of propene over bismuth molybdate follow the general lines described above [Sect. 2.2.2(a)]. As acrylic acid, acetic acid and formaldehyde are minor by-products, a simple scheme well suited t o describe the acrolein production is

C3H40 C3H6 113) + T o , CO, The reactions in this scheme are first order with respect to the oxidized compound. Initial selectivities [k, /(k, + h , ) ] of 90% and more are possible at 400-500°C. The decrease in selectivity at higher conversions is mainly due to acrolein combustion (k,/k, = 0.2-0.3). The activation energy of acrolein formation is approximately equal for all bismuth molybdates (18-20 kcal mol-' ). The independence of the reaction rate on the oxygen partial pressure is confirmed by studies in which the reaction of the catalyst with propene and reoxidation of the catalyst are separately studied. Sancier et al. [ 2751 performed conductance measurements during reduction and reoxidation using a flow system at 327 and 387°C. The rate of change of the crystal voltage, dAV/dt, is a measure of the reaction rate and appears to satisfy the dependencies reduction:

dAV/dt

=

hrpCgH6

oxidation:

dAV/dt

=

kOp021/2

The calculated rate coefficients are listed in Table 4 and confirm the relative rapidity of the reoxidation for all compounds studied. The existence of a certain (albeit small) amount of reduction during steady state reaction is demonstrated; its level as expected, depends on the k , / k , and thepo,/pc,H6 ratios (see also ref. 251). Reduction and reoxidation rates were also measured by Brueckman et al. [61], who used a static circulation reactor at 220-460°C. The reduction with hydrogen or propene at 460°C proceeds t o Moo2 and Bio . The very fast reoxidation was studied at much lower temperatures. BiO is reoxidized first. The reduction process is rather complicated for the molybdenum-rich phases (Bi/Mo = 1/1and 2/3), which appear initially t o form a mixture of MOO, and the y-phase. Kinetic equations are presented by the authors, but do not seem relevant for catalysis in view of the too severe

145 TABLE 4 Rate coefficients for initial reduction and oxidation of catalysts a ~~

Catalyst

kr

( m V sec-’ tori--‘ 600K

x lo4)

660K

(mV sec-’ torr-ln)

krlko (torrln x

600K

600K

660K

0.35 1.7 0.16 0.06

4.2 5.0 1.1

k0 660K

lo3)

-

M003

Bi/Mo = 0.7 Bi/Mo = 6 Bi203 a

2.2 8.4 0.42 0.11

Estimated probable error Too small to estimate.

30 34 2.3 0.24 t

0.62 0.51 0.26 0.19

0.73 0.69 0.21 b

5%.

catalyst reduction. The same comment applies to some other catalyst reduction studies (Beres et al. [47]). A recent contribution with respect to the oxidation kinetics for a Biz03 * 2Mo03 catalyst is given by Cartlidge et al. [78,79], who used a well-stirred reactor. Contradictory t o the results of any other study, acrolein is reported t o accelerate its own formation, while,carbon oxides in turn accelerate the acrolein combustion. A check on these unusual effects by adding the products t o the feed is not reported. Misinterpretation of the data seems likely, e.g. by the fact that transfer limitations easily occur in this type of reactor. Mechanism. The mechanism outlined for the propene oxidation over metal oxides is, in general, fully applicable to bismuth molybdate. The occurrence of a symmetrical ally1 intermediate and the participation of lattice oxygen is well established (Hucknall [160], Voge and Adams [3431). With respect to the participation of lattice oxygen, some recent contributions concerning studies with labeled oxygen and experiments in the absence of gas phase oxygen must be mentioned. Results of investigations in a static circulation apparatus are reported by Gel’bshtein et al. [ 1231. An I6Oz atmosphere is replaced by a reaction mixture containing 1-2 torr propene and 3-6 torr ‘*OZ at 400°C, and circulated over a 1/1Bi/Mo catalyst. The results prove lattice oxygen participation and a large oxygen mobility in the catalyst. Carbon dioxide appears to be partially formed directly from the gase phase l80,presumably by an associative mechanism. Pulse experiments have been carried out by Sancier et al. [276], who tried to avoid the problem of non-stationary conditions by “seasoning” of the catalyst by I6O2 /propene pulses, followed by the “actual experiment” with l 8 0, /propene pulses. ESR measurements confirmed that the degree of catalyst reduction was indeed constant. The ‘ 8 0 / ’ 6 ratio 0 in the prodReferences PP. 253-262

146 ucts depended on the reaction conditions (temperature and O2/C3H6 ratio). This dependency proves that the kinetics of oxidation, reduction and diffusion through the catalyst determine the extent of gas phase l8O incorporation. Further, a good agreement was found with earlier work of Keulks [ 1741 and Wragg et al. [ 3521 performed with static systems. Experiments using a flow reactor under steady state reaction conditions were reported by Keulks and Krenzke [ 1751. A retarded breakthrough of l8O in the products was observed, after switching from I6O2 t o '*02in the feed which consisted of propene (9%), oxygen (10%)and helium (81%)at 1 atm. The 180/'60 similarly increased in both acrolein and C 0 2 , contrary to the results of Gel'bshtein et al. mentioned above (see also Sect. 3.2.2). The oxygen exchange between molybdates and water can be very rapid as appears from the work of Novakova and Jiru [238]. The authors suggest that this exchange might interfere with the I8O2 oxidation studies. However, as water is not actively participating in the oxidation process, its influence is probably of minor importance [ 3531. It is generally reported that propene oxidation can also occur in the absence of oyxgen. However, the activity rapidly decreases, even in the case of pulse experiments. This is in contrast t o the butene oxidation t o butadiene over the same catalyst, where the activity is remarkably constant. One might suppose that a special type of oxygen, which is readily consumed, is needed for the propene oxidation. A much more likely explanation is provided by Barannik et al. [38], who carried out pulse experiments with propene and acrolein over Bi203 3M003 at 350400°C. A gradual decrease in activity t o a final constant level occurs during the first five propene pulses, but the constant level is immediately attained by injection of an acrolein pulse. The authors conclude that, in absence of oxygen, the catalyst surface is saturated by strongly adsorbed acrolein. A strong adsorption of acrolein has indeed been observed by other authors (Forys and Grzybowska [117], Matsuura [209]). The former authors, moreover, report that the strong adsorption of acrolein is absent after preadsorption of water. This may also provide an explanation for the absence of a strong inhibition by acrolein in flow reactors, where water formed by the reaction is continuously present. As the main lines of the mechanism are well established, discussion in the literature at present is mainly focussed on the structure of the active sites and their method of participation. The hypothesis of a bifunctional mechanism involving ally1 radical formation and oxygen incorporation on distinct sites is advocated by Haber et al. [ 147,1521. This hypothesis is particularly based on experiments with M o o 3 , Bi203 and mechanical mixtures of these oxides, which are compared with bismuth molybdate catalysts. The reaction was carried out in cyclic operation (alternating feeds of oxygen and of propene diluted with nitrogen). The results are collected in Table 5. The authors con-

-

147 TABLE 5 Interaction of propene with mixtures of BizO, and Moo3 (C3H6 : NZ= 30 : 70 (vol.%); contact time = 2.5 sec.) Catalyst

Temp. ('C)

Conversion of C3H6

Selectivity (%)

1%)

Acrolein

1,5-Hexadiene

C02

~

MOO Bi203 9 Biz03 + 2 MOO, initial period after 1 h 4 Bi203 + 2 MOO, initial period after 1 h Bi20, + 2 MOO, initial period Bi2Mo2O9 a

480 480

0

10

75

18 a

480 480

10

70

30

480 480

10 0

30

10

60

480 44 0

8

79 90

Traces

20

0

8b 10

Other product: benzene. Other products: acetic acid, acrylic acid and propionic acid

clude from the surprising production of acrolein over the mechanical mixture that bismuth centres produce allyl radicals which may either combine to give hexadiene, o r further react to acrolein on molybdenum oxide. The significance of the experimental results may be questioned, however, in view of the instability of the mechanical mixtures, which the authors ascribe to migration of MOO, over the Bi203 surface and the formation of an inactive layer; solid state reactions between Moo3 and Bi203 leading to active bismuth molybdate are well known t o occur at such temperatures [246]. Important evidence in favour of this two-centre mechanism is provided by Gamid-Zade and Kisliev [ 1211, who carried out pulse experiments with allyl bromide as a model compound t o generate an allyl intermediate. Moo3 and BiAlly1 bromide and propene were oxidized over Bi-Mo-, S n 4 at 450°C. Very remarkably, the rate of oxidation of allyl bromide to acrolein over Moo3 approaches that over Bi-Mo-O, while for propene, the rates differ by three orders of magnitude. Over Bi-Sn--O (and other binary oxides containing Bi but no Mo), propene can be oxidized to diallyl and benzene. Similarly, no acrolein is formed in the oxidation of allyl bromide over Bi-Sn-P, although the rate of oxidation is higher. Bifunctionality of the catalyst is also assumed by Schuit [281] and is particularly based on the adsorption experiments carried out by Matsuura et al. [207-2121. Two sites are distinguished: (a) A-sites a t which activated and strong adsorption of butadiene, acrolein and ammonia takes place (20-30 kcal mol-' ) a t 25-200°C. The number of A-sites decreases linearly with increasing degree of reduction and each reduced site can References p p . 2 5 3 - 2 6 2

148 adsorb one molecule H 2 0 or a half molecule O 2 (reoxidation). These A-sites are supposed to be oxygen anions in between anion vacancies connected with bismuth ions and represented by “VBiOAVBi”. (b) B-sites which weakly adsorb olefins, acrolein, butadiene and ammonia (5-12 kcal mol-’). The sites are supposed to be anion vacancies connected to molybdenum in between oxygen anions, represented by “OBV M ~ O”,B viz .

@I = 02on top of M O ~ + @ = 0 2 -on top of Bi3’

B-site (@ 0 (@ A-site 0 @ @ B-site @

0

0

@

8

=Mo6‘ = Bi3’

The A- and B-sites are supposed to beintegrated in one reaction site, as depicted above. Based on the respective adsorption properties that are attributed to these sites, the mechanism proposed is (1)fast dissociative adsorption

p

F3HS

C3H6 + OBVM~OB + OBVMOOB (2) transfer of C3H5to VBi (rate-determining) 73HS

OB

V B ~ O A V+BVB~OAC~HS ~

(3) H-transfer and acrolein formation (fast) H VBiOAC3HS +- OB

I

--f

V B ~ V B ~+VOB B ~+ C3H40~

(4) H-migration to A-sites and water desorption

This mechanism implies that the ally1 complex is first bonded to Mo-0 and then to Bi, followed by the uptake of oxygen from the direct neighbourhood of the Bi-centre. These assumptions are in strong contrast with the ideas of Haber et al. [ 147,1521 and Gamid-Zade and Kisliev [ 1211 given above. Different types of oxygen are distinguished also by Mitchell and Trifiro [ 2191, who studied catalysts and model compounds by IR and UV reflectance spectroscopy. Activity appears to be correlated with MOO, species which contain three terminal (multi-bonded) oxygen atoms ( “Ot”)- Moreover, a M o - 0 bond was detected which is even weaker than Mo-0-Mo in Moo3 and assigned to Mo-0-Bi. Following the ideas of Schuit [ 2811,

149 above a Mo(O, ) ~species (-B-site) is ascribed the role of initial propene adsorption, while Mo-0-Bi (- A-site) contains the most loosely bonded oxygen and should serve for water desorption and reoxidation from the gas phase. A very attractive theory concerning the oxidation mechanism for scheelite-type molybdates containing bismuth is presented by Sleight and Linn [297] and is described below.

(ii) Multi-component catalysts containing Bi and M o . Defect scheelite systems were studied by Aykan e t al. [33] for catalysis of the oxidation and ammoxidation of propene at 400--500°C.The systems studied can be represented by A, - x & Moo4 in which A is a combination of metals including Bi and 4 denotes a cation vacancy (defect). The presence of defects appears essential for the catalytic activity. A typical example is presented Bi0.5+x@2xMo04. Some other complex phases and in Fig. 3 for Nao.5-3x their activity for the propene ammoxidation are listed in Table 13 (p. 169). The table shows that, in absence of defects, the activity is low. Moreover, it appears that the good catalytic properties are conserved when Mo is replaced by W , in contrast t o pure bismuth molybdate and tungstate

>

c 0

0

-

0

0 08

0.16

0.24

0.32

2 x In Na0,5-3x B10.5+x Moo4

Fig. 3 . Acrolein yield and propene conversion as a function of catalyst composition. References PP. 253-262

150 (the latter is not very active). The authors ascribe this fact t o the Mo-O and W - 0 coordination which is identical in the scheelites (tetrahedral), but may be different in bismuth tungstate (octahedral) and bismuth molybdate (tetrahedral or octahedral depending on the Bi/Mo ratio, but the authors assume that a tetrahedral coordination might prevail at the surface independent of the bulk structure). Finally, it is concluded that, besides defects, a (small) fraction of bismuth must be present for good activity and selectivity, as omission of bismuth or its substitution by Ce, Y or Ca yields rather inactive and non-selective catalysts. A very interesting hypothesis with respect t o the mechanism, particularly regarding scheelite-type catalysts, is given by Sleight and Linn [ 2971, Three essential elements are discerned at the catalyst surface: MOO, groups, cation vacancies and Bi3+ ions. The latter, due t o their surface position, are coordinated with only six oxygen anions of surrounding MOO, tetrahedra and possess a lone pair of electrons directed up out of the solid. Propene is dissociatively adsorbed on MOO, groups next t o a cation vacancy, forming an adsorbed allyl radical and a hydroxyl group stabilized by the cation vacancy, viz.

H o = oxygen anion c] = cation vacancy

C

H,C

8 ’A

CH, 0

0

Bi

0

Mo

0 0

0

0

Mo

0 0

Bi

This stabilization may also be interpreted in terms of oxygen anions, which, due, t o the vacancy, are initially double bonded t o Mo. One electron is transferred to the catalyst in this reaction step. To form acrolein, a second hydrogen atom is transferred (to form water) and an oxygen atom is bonded t o the allyl radical. In this (rather complex) process, another three electrons are transferred t o the catalysts and doubtless distributed over several Mo ions. Reoxidation takes place at the bismuth cations, where oxygen molecules are attracted by the free electron pair. The intermediate result is a surface bismuth with an oxygen coordination similar t o that in the bulk, viz.

The oxygen anions rapidly diffuse t o vacancies in the MOO, groups. The electron transfer certainly does not involve oxidation of Bi3+ t o higher oxidation states. On the contrary, the authors assume that the steady state situation will be something like leaving some electrons in the Bi6 p conduction band. This idea is the more attractive as it is well known

151 that bismuth molybdates are slightly reduced under steady state reaction conditions, while it is also known that the conductivity strongly increases on reduction (Sancier e t al. [275]). As this band is very close t o the Mo4d levels, a rapid transfer of electrons from reduced molybdenum ions to a (remote) Bi3' site is feasible. A number of Bi-Fe-Mo4 catalysts were investigated by Daniel and Keulks [94], using a flow reactor at 400°C. The catalysts were prepared by a slurry method from Bi2Mo3Ol2and Fe(OH), . At iron contents of 30-40 at.% with respect t o (Bi + Mo), a catalyst is obtained which is comparable with pure bismuth molybdate as to activity and selectivity, but remarkably resistant t o reduction and to heat treatment up t o 800°C. Regarding the kinetics, a first-order dependence with respect to oxygen is reported in contrast t o bismuth molybdate, but the experimental conditions are not given. The origin of the superior properties of the Bi-FeMo-0 catalysts is doubtlessly connected with the existence of a ternary phase. Several recent phase studies reveal the existence of a phase with (Bi + Fe)/Mo = 2 and a scheelite-like structure containing Moo4 and Fe04 tetrahedra. There is some discussion in the literature as t o the actual stoichiometry. According t o Lojacono e t al. [191] and Notermann e t al. [237], two compounds can form: Bi2Fe2Mo,Ol2 and- Bi3FeMo2012. Linn and Sleight [ 1881, however, argue that only the latter compound is formed, whereas the former consists of Bi3FeMozOI2mixed with known binary and single oxide phases.

(iii) Other Moo3 based catalysts. A large number of molybdates and compound oxides containing MOO, have been studied, but only a few of them approach the superior qualities of bismuth molybdates. Good selectivities can be obtained with Te-Mo-0, Sn-Mo-0 and probably with MnMo-0, but excessive combustion occurs on the major part of the studied combinations (e.g. MOO, with oxides of Cu, Fe, V, Zn, Co, Ni, etc.). A number of catalysts has been studied in more detail and will be briefly reviewed here. The system Te-Mo-0 has been investigated by Andrushkevich e t al. [ 271 and by Robin et al. [ 2671. A recirculation reactor was used by former (390°C; 1 atm; 3%C3H6;20% 0,; conversion 0-50%) while Robin e t al. used a pulse reactor (460-500°C; C3H6/O2 = 1/1.8; conversion 2%). Both studies reveal a strong promoting effect of TeO, . High selectivities (90-9576) are demonstrated over a large composition range (Te/Mo = 1/11-2/1). Andrushkevich e t al. [27] find amaximum activity a t 8 at. % Te, approximately coinciding with the maximum solubility of Te6+in Moo3 and with an observed maximum in the Mo5+ESR signal (Fig. 4). The Mo5+ signal intensity can be considered as a measure of the reducibility of the catalyst. Robin et al. [267] proved the existence of a Te,MOO, phase by X-ray and thermal analysis. They ascribe the catalytic activity t o this compound while Andrushkevich et al. propose the solid References p p . 253-262

152

0

b

c

X

.I

Y

iQ

N

E 3 3 u

-aJ

:

v

aJ

c

m

[r

1 ___le

Te Atomic ratio -

Mo

Fig. 4 . Correlation between catalyst activity and dation.

M05+ concentration

for propene oxi-

solution as the active phase. The latter also performed experiments which indicate a redox mechanism. Reoxidation of the catalyst is rapid and firstorder kinetics with respect t o propene are observed. The combination SnO2-MoO3 may also produce acrolein with reasonable selectivity. However, recent studies mainly concern acetone formation, which is favoured under appropriate conditions (excess of water and relatively low temperatures) (see Sect. 2.2.4). The properties of Mn-Mo-0 catalysts strongly depend on the Mn/Mo ratio. Machek and Tichy [193] report that at Mn/Mo > 1, the catalyst consists of MnMo0, and M n 2 0 3 , and is already very active a t 300°C. However, combustion is the only process. At Mn/Mo < 1, the catalyst consists of MnMoO, and M o o 3 , and may produce substantial amounts of partial oxidation products at 400-500°C. Machek and Tichy report, for example, that at 479°C and Mo/Mn = 7/1, a selectivity of 78% is obtained. However, formaldehyde is also formed and amounts t o 20-5096 of the total aldehyde yield. Viswanathan e t al. [342] have prepared a catalyst with a Mn/Mo ratio of 1/1which has catalytic qualities very similar to that of bismuth molybdates. At 400-450°C, a selectivity is 70% is obtained at

153 a 20% conversion level. Experiments in the absence of oxygen give evidence of a redox mechanism. Kinetics similar t o those with bismuth molybdates are observed. Mazzochia e t al. [213] report an acrylic acid selectivity of 30% a t 4 3 0 ° C and Mn/Mo = 1.01. Haber [151]studied solid state reactions occurring by reduction with hydrogen and noted a high mobility of Moo3 on the surface of Mn203. Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect t o acrolein. Acetone is the chief product at low temperature ( 200"C), whereas carbon oxides, besides some acrolein, predominate a t higher temperatures [ 182,2571. Firsova e t al. [ 112,1131 report that adsorption of propene on iron molybdate (Fe/Me = 1/2)a t 80--120°C causes cation reduction (Fe3++ Fez+) as revealed by y-resonance spectroscopy. Treatment with oxygen a t 4 0 0 ° C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related t o the low selectivity of iron molybdate. Cobalt molybdates have approximately the same activity as bismuth molybdates but the selectivity is much lower due t o combustion and acrylic acid formation as important side reactions. Vinogradova et al. [ 3411 report a kinetic study carried out with a flow-type gradientless reactor at 390°C Ssing excess water. The reaction orders with respect to propene and oxygen are 1 and 0. Experiments with propene/acrolein mixtures reveal complex kinetics and strong inhibition by acrolein. Grzybowska e t al. [ 1471 report equal activities for both CoMoOl modifications, and conclude that the rate-determining step must take place at C o sites, as Co has the same coordination in both modifications, whereas that of M o differs. Another conclusion of these authors concerns the low mobility of oxygen in these catalysts: apparently, a high mobility is not a necessary condition for a high activity. An iron-promoted cobalt molybdate catalyst (Fe,, 0 3 Coo., 7 M 0 0 4 ) was studied by Maksimov e t al. [195,196]with respect to the role of iron in the transfer of charge. Iron strongly enhances the catalytic activity and at the same time increases the conductivity by a factor of 100. Mossbauer spectroscopy reveals that 4% of the iron ions are present as Fe*+ "impurity". This fraction is doubled a t steady state reaction conditions, and indicates participation of iron in the charge transfer process. The interaction of propene and oxygen with sodium and potassium molybdates was studied by Burlamacchi et al. [ 661. ESR measurements reveal that Mo5+ is formed (at 380°C) although the catalytic activity is zero. The reoxidation appears t o be very difficult. The authors conclude that a cation like Bi3+or Fe3+is required to facilitate reoxidation. The activity of Moo3 supported on a high surface silica carrier was studied by Vaghi e t al. [331] using pulse and flow techniques at 400440°C. Oxidation activity and acrolein formation appear to be zero below 10 wt. % MOO,, but increase with the MOO, content above 10%.The References P P . 253-262

154 authors suppose that, below 10 wt. %, isolated tetrahedral M o - 0 species and silicomolybdic acid occur at the surface. Above 10 wt. 76 of MOO,, polymolybdates (octahedral coordination) and highly dispersed MOO, are formed, and the authors assume that these form the active sites for acrolein production; bulk MOO, is ruled out because of its low activity. Akimoto and Echigoya [ 161 studied Moo3 supported on SiOz , A l z 0 3 or TiO;, , modified by VA group or alkali metal oxides. These oxides were added to Moo3 by impregnation in amounts of 30 and 0.3 at. % (X/Mo), respectively. The activity of the modified oxides appears to increase in inverse proportion to the electronegativity of the modifying oxide. Thus the activity sequence is: Bi > Sb > As > P and Cs > Rb > K > Na > Li. The authors assume that the modifier affects the reactivity of the Mo=O double bond by decreasing the bond strength and giving the oxygen a more radical-like character. The addition of alkali metal must be confined to very small amounts to avoid the formation of alkali metal molybdates, in which case the level of activity is strongly decreased while the sequence is reversed.

(iu) Iron antimonates. The antimony-rich side of the Fe-Sb-0 system provides very selective and rather active catalysts for both the oxidation and ammoxidation of propene. Sramek and Tichy [ 3011 studied catalysts with Sb/Fe ratios ranging from 3/1 t o 9/1 in a flow reactor at 380-500°C The Sb/Fe ratio has only a small effect on the catalytic properties in this range. Some of the best results with respect to activity and selectivity are presented in Table 6. Regarding the kinetics, the authors report that the oxygen concentration and the presence or absence of water both hardly influence the rate (for the 3/1 catalyst). Gel'bshtein et al. [124] studied the same catalyst using flow and pulse techniques and reported a reaction order of 0.5 with respect to' propene, irrespective of the degree of reduction or the absence or presence of oxygen. As to the active components, little is known. Fattore et al. [lo91 ascribe the catalytic properties t o combination of FeSbO, and cu-Sb204 in a well dispersed mixture. Pure FeSbO, forms combustion products exclusively.

TABLE 6 Iron antimonates as catalysts for propene oxidation ~~~~

~~

~

Sb/Fe

Temp. ("C)

Conversion

Selectivity

311 31 1 311

385 425 496

12.1 19.1 31.2

92 84.4 69.2

155 (u) Tin antimonates. As with the Fe-Sb-0 system, very selective catalysts for acrolein and acrylonitrile formation are found over a large composition range (7-90 at. % Sb). Regarding the active phase, Godin et al. [ 1421 report that the catalyst consists of S b 2 0 4 and a saturated solution of Sb in SnOl, which contains 5 at. % Sb. The authors suppose that the solid solution is the active phase present at the catalyst surface. Christie et al. [86] proved that a redox mechanism is operative, by experiments with l80either in the gas or the catalyst phase. The authors confirmed the superior activity of the solid solution observed by Godin et al., and, moreover, observed a maximum C3H6-D20 exchange rate at an Sb/Sn ratio corresponding t o the saturated solid solution. The occurrence of this excharge reaction, which is confined to five H atoms and excludes the H atom of the central C atom of the propene molecule, is in agreement with the ideas of dissociative adsorption and formation of a symmetrical allyl complex as the initial step in the propene oxidation. Although the saturated solution demonstrates maximum catalytic activity, larger Sb contents are required for a high acrolein selectivity. As to the role of the two elements in the catalyst, Gamid-Zade and Kisliev [121] argue that Sn activates propene giving a reactive allyl complex, while the oxygen polyhedra of the anion-forming element of higher electronegativity (Sb) are the active centres for further oxidation t o acrolein. (A similar hypothesis is given for Bi and Mo in bismuth molybdate; see above.) Their arguments are based on a comparison of the results obtained with Bi,-Sb,., -Sn-0 and various binary oxides. Only binary oxides that contain Sb or M o are capable of producing acrolein (with 30-8096 selectivity), while Bi-Sn-0 and some other binary oxides containing Bi or Sn do not form acrolein but instead produce substantial amounts of diallyl and benzene (3-30%). The ternary oxide displays both properties depending on the Bi/Sb ratio. A kinetic study carried o u t with a well-stirred reactor was reported by Cartlidge et al. [78,79]. Temperatures below 420°C were used to avoid acrylic acid formation. At atmospheric pressure, the oxygen and propene concentrations were varied between 1 and 10, and 5 and 1576, respectively. Selectivities of 60-9096 and a maximum acrolein yield of 28% were reported at 400°C. The kinetic results were fitted t o a Langmuir-Hinshelwood type of rate equation

Although this model cannot correctly reflect a redox mechanism, it indicates that the reduction and reoxidation rates have the same order of magnitude, and hence both influence the kinetics. A commercial, iron-promoted ammoxidation catalyst (Fe/Sn/Sb = 0.25/1/4) was investigated by Crozat and Germain [93] using a flow reactor at 35O-48O0C, atmospheric pressure and a C3H6/02ratio of l/lO.The References p p . 253-262

156 initial selectivity is higher for oxidation than for ammoxidation, but the latter is better a t higher conversions due t o acrolein combustion. The kinetics are described by a simple parallel consecutive scheme. The ratio of reaction rates is almost independent of temperature. The initial selectivity is 9676, and propene oxidation and acrolein combustion have about the same apparent first-order rate coefficient. The overall activation energy is 20.4 ? 4.5 kcal mol-'. An overall reaction order of 1 is reported (approximately 0.5 with respect t o each of the reactants). The authors compared this catalyst with an unpromoted Sn-Sb-O catalyst and conclude that Fe has practically no effect on the oxidation reaction, although a substantial promoting effect on the ammoxidation is shown. Pulse experiments with a Sn/Sb = 2 / 1 catalyst in the absence of oxygen have been carried out by Barannik e t al. [38,39]. The activity rapidly decreases with increasing reduction, while the selectivity strongly increases. This is in contrast with bismuth molybdates, which demonstrate a similarly decreasing activity, but a constant (high) selectivity level.

(vi) Cuprous oxide. Cuprous oxide as an oxidation catalyst has been extensively studied in the past and amply reviewed (Hucknall [160], Margolis [203], Voge and Adams [343]). The active component is the cuprous oxide phase, and not cupric oxide, which only effects combustion. The selectivity is not very high (60-85% at'10-20% conversion). The conversion of propene is limited by the large excess of propene (e.g. C3H6/02 = 5) that is required to maintain the catalyst in the form of Cu20. The kinetics show a first-order dependence with respect t o oxygen and a zero-order for propene, which is different from the usual dependencies, partly because of the high propene/oxygen ratio. The mechanism is assumed t o follow the reduction-oxidation models and evidence has been provided for the occurrence of a symmetrical ally1 intermediate. There are only a few recent publications. Anshits et al. [29,30] have carried o u t adsorption studies with various C u - 0 phases and determined kinetics at low pressure in a static system. One of their conclusions is that the kinetics of partial and complete oxidation are very different. The mechanism of the latter is supposed to be of the associative type, contrary t o the redox mechanism of the partial oxidation. A kinetic study with a continuously stirred vessel (375-400°C, 1atm) was carried out by Lakshmanan and Rouleau [ 1851. In contrast t o the redox mechanism, a singlesite Langmuir-Hinshelwood model is proposed, for which the k values and activation energies are determined. The effect of methyl bromide, added as a modifier t o the feed, was studied by Holbrook and Wise [158]. The modifier appeared t o have a profound influence on the selectivity, even a t low concentrations (450 ppm CH,Br in propene). This resulted in a relatively high selectivity, which did not depend on the C3H6/02ratio. The action is explained as an effect on the Fermi level of the catalyst (see Sect. 3).

157

A CuO-MgO solid solution was investigated by Davydov and Budneva [97]. Propene adsorption complexes (n and Q) were detected a t room temperature and appeared t o react by heating t o 300°C.

(vii) Miscellaneous catalysts. A large number of Sb-containing binary oxides was studied by Sramek and Tichy [ 3011, i.e. combinations with the metals Fe, Ni, Cr, Sn, Pb, Cu, Ce, and Mn. At a 1/1 atomic ratio with Sb, the selectivities are generally low. Good results are obtained only with Fe-Sb-0 and Sn-Sb-0, which have already been mentioned. Zanderighi and Carra [359] investigated the tungstates of Cu, Mn, Pb, Bi, Fe and T1 in a pulse reactor (250--350"C, 02/C3H6 = 3/2). The tungstates of Cu (at 250"C), Bi, Pb and T1 are moderately selective and active. The latter is the most selective, producing acrolein, acetone and propene oxide. The main product at 250°C is acrolein, but a t 350°C propene oxide and acetone are the principal products. The epoxide formation indicates that peroxyradical species are produced by the catalyst, which may further react in the homogeneous gas phase. The reactivity data were correlated by the authors with the results of thermogravimetric analysis (reduction by hydrogen and propene). Zeolites of type X, containing various metal ions (Pd, Cu, Co, Zn, Ni, Mn, Cr, Fe) were investigated by Gentry e t al. [125]. Only very small amounts of partial oxidation products are found (acrolein, acetaldehyde, formaldehyde). Although V 2 0 5 catalysts are not very selective for the oxidation of propene, some studies were devoted to the investigation of the action of thk catalyst. Krylov [ 183,1841 studied V2OS catalysts supported on S O 2 , MgO, or A1203 by chemisorption of reactants and other techniques, and made assumptions about the types of oxygen and surface complexes involved from the results. ValdeliGvre e t al. [ 3331 investigated processes occurring in the surface layers of V z 0 5 and a 9/1 mixture of V z 0 4/ V 6 0 , , by a variety of techniques (adsorption, thermogravimetric analysis and analysis by IR, EPR and ESCA). The authors conclude that V5+is essential for oxidation activity. An oxidation-reduction mechanism is evidently operative. The selectivity of this catalyst is low. The fact that oxygen originates from different sites (V= 0 and V-0-V) is suggested to be the cause. Niwa and Murakami [ 235,2361 investigated various catalysts (Bi-Mo0, Bi-W-0, Sn-Sb-0, Sb-Mo-0, MOO, and Sn-P-0) with the periodic pulse technique. This method is distinguished from the conventional continuous flow reaction method by the alternate feeding of oxygen and propene. The reaction was carried out a t 386"C, and the products of the propene (P) and oxygen (0)pulse were separately analyzed. Acrolein is only formed during the P pulse, indicating reaction between propene and surface oxygen as the exclusive source of this aldehyde whereas carbon oxides are formed in both pulses. The data collected in Table 7 show that REfErenCes p p . 253-262

TABLE 7 Combustion products in pulse experiments for propene oxidation Catalyst

Production of COZ and CO -~

Bi-Mo (1/1) Bi-Mo/SiOZ Bi-W (1/1) Sn-P MOO, Sn-Sb (10/1) Sb-Mo (2/3)

Total amount (pmole per period per g cat.)

5% in 0 pulse

2.6 6.0 6.0 2.9 0.16 1.1 0.6

83 83 79 71 52 32 23

the most active catalysts (Bi-Mo-0 and Bi-W-) predominantly form the carbon oxides in the 0 pulse. (These catalysts are severely reduced by the P pulse.) Less active catalysts (Sn-Sb-0, Sb-Mo-0) maintain higher oxidation states and mainly form carbon oxides in the P pulse. MOO, and TABLE 8 Comparison between the periodic-pulse and continuous-flow reactions for propene oxidation Catalyst

Bi-Mo

(1/1)

Bi-Mo (1/1) Bi-W (1/1) Sb-Mo (2/3) Sn-Sb (10/1) Sn-Sb (4/1) Sn-P (10/1) Moo3

a

Technique

Pa fb P f P f P f P f P f P f P f

Flow

(mmol h - * )

coz

co

Acrolein

0.836 1.06 1.17 1.69 2.02 2.15 0.245 0.740 0.732 1.00 0.628 0.886 0.720 0.756 0.120 0.094

0.21 0.39 0.35 1.00 0.488 0.543 0.265 1.01 0.194 0.288 0.179 0.296 0.434 0.484 0.069 0.063

1.86 5.42 2.93 6.44 0.306 1.11 0.518 0.925 3.48 5.25 3.46 5.46 0.368 0.388 0.362 0.342

Pericjdic-pulse technique under t h e following conditions: period, 30 sec ( 1 5 0 , 15R); C3H6, 0.25 a t m ; 0 2 0.21 , atm. Continuous-flow technique a t the normalized pressure. 0.67 a t m of O2 in the 0 pulse.

159 Sn-P-O are intermediate. Comparisons with ordinary flow experiments (Table 8) reveals that much more acrolein is formed at normal flow conditions for Bi-Mo--O and Bi-W-0, while for the other catalysts the difference is small or similar for both acrolein formation and combustion. The system Te0,-SiOz was investigated by Castellan et al. [80] who found that mixtures containing more than 10% Te02 were catalysts at 440°C (air-propene ratio = 12). Selectivities of 30-7096 are reported. 2.2.3 Acrylic acid production

The one-stage conversion of propene to acrylic acid is much more difficult than the selective production of acrolein. The process is essentially a two-step process in which acrolein is the intermediate product. Further oxidation leads to acrylic acid. In fact, contrasting catalyst properties are required for these reaction steps. The acrylic acid production demands an acidic catalyst surface, while a basic, or only weakly acidic character is preferred for the selective acrolein formation. Therefore, enhanced combustion and by-product formation are unavoidable. It is doubtful whether a single-step process is at present competitive with the two-step process currently used in industry. In the latter, the oxidation of acrolein to acrylic acid is carried out with high selectivity over mixed-oxide catalysts based, for example, on Mo03-V205 or Moo3TeOz [160]. The catalysts that have been studied for the selective single-stage production of acrylic acid are all based on molybdates or modified MooJ.It appears that good yields can be achieved only with rather complex multicomponent catalysts. The quality of Moo3 combinations with iron, cobalt and manganese oxides has been investigated by Mazzocchia et al. [213]. Table 9 presents some results, obtained with a flow reactor at 300--430°C using a C3H6/02 ratio of 0.22-0.28. The best catalyst is MnMo04. The addition of small amounts of water (up to 2.5%) further increases the selectivity, but larger amounts cause rapid deactivation.

TABLE 9 Properties of molybdates in the conversion of propene to acrylic acid

X Fe-Mo-0 CO-Mo-0 Mn-Mo-0 Mn-Mo-0

a

a

X/Mo

Activity

Selectivity

112.3 111.02 111.01 116.03

Active Active Active Inactive

Small 15% 30%

~~

a

Contained some free MoO3.

References PP. 253-262

160 Cobalt molybdates are also the subject of the work of Alkhazov et al. [ 19,201, which includes extensive infrared spectrometry studies. The Mo/

Co ratio and preparation method were varied, while a standard feed consisting of 12% C3Ha, 15% Oz, 28% HzO and 45% Nz was supplied. The maximum yield of acrylic acid was found at Mo/Co = 2, i.e. with a mixture of equal amounts of CoMo04 and Moo3. This maximum was found to coincide with a maximum content of the P-CoMo04 modification, and the activity was ascribed to the terminal Mo=O double bonded oxygen present on the surface of this phase. The excess of Moo3 may also have a promoting effect due to its contribution to the acidity of the surface. More exotic combinations of Moo3 (20%) with AszO, (5--10%) and Nbz05 or TazO, (10%) were studied by Campbell et al. [71].A t 4OO0C, yields of about 50% can be obtained in the presence of water. Only very small amounts of acrolein are formed, and acetic acid is now the main byproduct. Unfortunately, the catalyst evaporates AszO3 during use. An interesting contribution with respect to the mechanism is given by Novakova et al. [ 2391. They studied the role of water in the formation of acrylic acid over a Mo-Te-W-Sn-0 catalyst. Water appears essential because, in its absence, acrolein is the only product. The use of HZ'*0 showed that one of the oxygen atoms incorporated in acrylic acid originates from water, while water does not participate in the formation of acrolein. The conclusion is that, in the oxidation of propene to acrylic acid, lattice oxygen is first introduced to form acrolein or an acrolein type of complex, followed by the introduction of an oxygen atom, or perhaps a hydroxyl group originating from water. This conclusion is in agreement with earlier work of Andrushkevich et al. (mentioned in ref. 239), who studied a similar type of catalyst (Mo-Te--Co--O). The latter authors, moreover, advance the idea of bifunctionality of these catalysts, by concluding that different active components of the catalyst are responsible for the axidation of propene to acrolein and of acrolein to acrylic acid. 2.2.4 Dimerization and aromatization Several single and binary oxides have a capacity to oxidize propene to dimerization products. The first compound formed is 1,5-hexadiene, which may undergo further dehydrogenation and cyclization leading to benzene. Many authors assume that the initial reaction step in the dimerization is identical with that in the acrolein production, namely hydrogen abstraction and formation of an allylic intermediate. Dimerization is then supposed t o occur because the ability to oxidize the ally1 radical to acrolein is absent. The best known dimerization catalysts are Biz03, bismuth salts and binary oxide mixtures containing Biz03. A very effective catalyst is BiZO3SnOz, in particular for the production of benzene.

161 Pure bismuth oxide has been investigated by several workers. It is generally established that high selectivities can be obtained in the absence of oxygen, up t o high degrees of reduction. According t o Swift et al. [311] and German et al. [136], hexadiene and benzene can be formed with selectivities of 54-76% and 30-4796, respectively, at a temperature of 475-500°C. Fattore et al. [lo81 measured the influence of temperature in the range 45O-60O0C, using a flow reactor and an oxygen-free reaction mixture. They report, for instance, that, at 450"C,the selectivities t o hexadiene and benzene are 60% and 2596, respectively, while at 550°C, the two selectivities are equal (45%), indicating that higher temperatures favour the dehydroaromatization reaction. Boersma [ 561 studied the kinetics with a differential flow reactor at 550°C. In the absence of oxygen, the reaction is first order with respect t o propene, with an activation energy of 20 kcal mol-' . It is concluded that the initial hydrogen abstraction is rate-controlling and, apparently, oxygen diffusion from the bulk t o the surface is fast. The kinetics of the formation of hexadiene are not affected by the presence of oxygen in a low concentration. However, carbon dioxide formation is strongly intensified, and predominates at higher oxygen concentrations. The author also carried out propene adsorption measurements on partially reduced catalysts and studied the relation between reaction rate and degree of reduction. The hexadiene production rate decreases t o zero as the reduction increases t o 10076,but initially the decrease is less than proportional. The propene adsorption capacity appears maximal on a partially reduced catalyst. The author hence concludes that the ally1 complex is adsorbed on an anion vacancy, while the abstracted hydrogen is taken up by an oxygen anion. Bismuth phosphates and various other bismuth salts (e.g. arsenate, basic sulfate, and titanate) are capable of producing benzene, as reported by Seiyama et al. [ 2831. A selectivity of 49% is reached with a combination 2Bi203. PzOs at 500°C. Sakamoto et al. [271] varied the Bi/P ratio and stated that a 2/1 ratio gives the maximum selectivity. Several other single oxides have been studied and compared with Biz0 3 .Fattore et al. [ 1081 report that S b 2 0 4 is very selective (75% t o hexadiene) but much less active than Bi2O3.The same applies t o pure SnO, . A high selectivity is obtained in the absence of oxygen, but reduction rapidly deactivates the catalyst [ 711. Other single oxides that remarkably demonstrate a dimerization or dehydroaromatization activity are ZnO, I n 2 0 3 and T1203 [ 286,3281. The Bi203-Sn02 combination was studied by Solymosi and Bozso [299] and by Seiyama et al. [284,285]. The former carried out pulse experiments in the absence of oxygen and report that even small amounts of SnOz added to Bi203 have a promoting effect and shift the product spectrum from hexadiene t o benzene. The best combination is a mechanical mixture of the two oxides in a 1/1ratio. With this catalyst, a selectivity of 80% (benzene) is reached at a 40% conversion level (at 500"C), R e f e r e n c r s PP. 2 5 3 - 2 6 2

162 which largely exceeds the capabilities of the individual oxides. Seiyama et al. studied the kinetics in a flow reactor at 500°C. At the low conversions applied ( Ti > V > Mo > Ni > Mn > Fe > CU from 71% to 33%. The relatively high initial selectivities demonstrated by the “deep oxidation” catalysts (e.g. Co, Ni, Mn) indicates that the primary activation is probably the same for all these catalysts, while the differences that actually determine the character of the catalyst are connected with the stability of intermediates and products.

2.5.4 Naphthalene The gas phase oxidation of naphthalene t o phthalic anhydride over V,O,-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V-K-S-0 catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80-85 mol. 5% (92-98 wt. 76) at 350--400°C are reported. By-products are naphthoquinone (2-5%), maleic anhydride (25%) and carbon oxides. Naphthalene oxidation is very similar t o benzene oxidation except for the much greater importance of naphthoquinone, compared with benzoquinone, as a reaction intermediate. Roughly equal amounts of phthalic anhydride and naphthoquinone are initially formed from naphthalene. A suitable simplified reaction scheme is

\

0 The kinetics of the naphthalene oxidation obviously depend on the properties of the catalyst used, but some general statements can be made for the majority of V,O,-based catalysts. Refcrcnces p p . 253-262

218 (i) The rates of reactions (1)+3) are of the same order of magnitude, while small fractions of both naphthalene and naphthoquinone are converted into CO, CO, and maleic anhydride. The relatively high stability of the anhydrides has been pointed out already in the case of the o-xylene oxidation, and implies that the phthalic anhydride decomposition is almost negligible until complete conversion of naphthalene is achieved. (ii) Regarding the form of the rate equations, the overall oxidation rate appears t o depend on the partial pressures of naphthalene, or oxygen or both, and to be best described by a redox model. Individual reaction steps have been amply investigated [ 1011, but disappointingly, no integral kinetic analysis, based on an extended redox model, has been reported. The initial selectivities, as well as the integral product distribution, are hardly dependent on temperature, which implies that the activation energy has approximately the same value (25-30 kcal mol-I) for all reaction steps involved. Extremely few new contributions have appeared in the literature. The participation of the lattice oxygen of a pure V 2 0 5catalyst was studied by pulse experiments (Andreikov e t al. [26]). Although the catalyst is capable of oxidizing naphthalene in the absence of gas phase oxygen, the latter was indispensable for achieving a good conversion and selectivity, and this was apparently related t o the strong adsorption of phthalic anhydride on the partially reduced catalyst. Butt and Kenney [68] have demonstrated the catalytic activity of a V205/K2S04melt. A naphthaleneair mixture was fed over the surface (15 cm') of a carefully stirred liquid consisting of 39% v205 and 61% K2S04 (m.p., 433°C) at 440-470°C. The same experiments were carried out at lower temperatures with the solid catalysts. The activity rises with increasing temperature up t o 380"C, then falls steeply (45°C below the melting point). At the same time, the selectivity falls t o values below 20% for both phthalic anhydride and naphthoquinone. Above the melting point, the activity increases again. A rough analysis of the kinetics indicate the validity of a redox model for both temperature regions, although the kinetic parameters differ. The ability of the melt to participate in oxidation-reduction processes was demonstrated. The melt appeared to release oxygen when the atmosphere of air was replaced by nitrogen, corresponding to the conversion of V204,,2 to V204.85.

2.5.5 Anthracene Anthraquinone is the primary product of the oxidation of anthracene over V,O,-based catalysts. The reaction is very selective and high yields of anthraquinone are possible due t o its relatively high stability. An iron vanadate catalyst is used in the industrial process and yields of 80-90 mol. % are reported at 320-370" C. Phthalic anhydride, maleic anhydride and carbon oxides are the by-products.

219 TABLE 37 Activation energies and pre-exponential factors for t h e first-order rate coefficients for anthracene oxidation

Activation energy (kcal mol-I) Pre-exponential factor (mole g-' min-' mm Hg-')

Reduction step

Reoxidation step

18.01

19.20

2.465

X

lo3

4.0119

The kinetics of this reaction have not been extensively studied. Redox kinetics are suggested by Mars and van Krevelen for V2OSand the same kinetics are recently reported by Subramanian and Murthy (307-309) for V205-K2S04 and CoMoO, catalysts, both supported on silica. The oxidation was carried o u t in a flow reactor a t 270--360"C, with negligible formation of by-products. Activation energies and pre-exponential factors for the cobalt molybdate catalyst are collected in Table 37, while the results for the V2OS-KZSO4 catalyst demonstrate a remarkable change in activation energy at 330°C. Above this temperature, the activation energies are more than twice the original values. Power rate equations are proposed by Andreikov and Rosyanova [25] for V20,-K2S04/Si02 at 330-370°C. These d o not seem very appropriate, as the coefficients depend on the temperature and the oxygen partial pressure. The negative order ( - 0 . 2 4 ) with respect to anthraquinone suggests a rather strong inhibiting effect of this product on its formation.

2.5.6 Other aromatic hydrocarbons Some aromatic hydrocarbons have not been mentioned in the previous sections and will be briefly discussed here. Aromatic hydrocarbons which d o not have side chains in general form p-quinones and acid anhydrides. Benzene, naphthalene and anthracene have been dealt with above. In the case of phenanthrene, no p-quinone is formed as the adjacent C-H groups of the central nucleus are the most reactive. Phthalic anhydride is the main partial oxidation product, in addition to minor products such as 9,lO-phenanthraquinone. Andreikov and TABLE 3 8 Relative oxidation rates of some aromatic hydrocarbons -~

_ -

~

Benzene Toluene Naphthalene Anthracene Phenanthrene

References p p . 253--262

1 24 1700 300-400 17,000

~~~

220 Rusyanova [ 251 describe the oxidation of phenanthrene t o various products (using the V,05 catalyst above) by power rate equations according to a parallel reaction scheme I

0

CH-C

It 11

0 \ 0 /

CH-C 0

1

Q

co

Product inhibition is reported for reactions (1)and (2) in this scheme. Of interest are the relative overall oxidation rates for some aromatic hydrocarbons reported by the authors (Table 38). Aromatic hydrocarbons which have methyl side chains mainly behave like toluene and form aldehydes, while combustion is stimulated and selective oxidation of the nucleus is repressed. The oxidation of methylnaphthalene, for example, exhibits a low selectivity with respect t o phthalic anhydride formation, combustion and maleic acid formation being the dominating reactions. Durene is a special case because it resembles 0-xycatalyst at 420°C is reported lene. The oxidation of durene over a V-W-0 t o produce pyromellitic dianhydride, phthalic and maleic anhydride, although combustion dominates (Geiman et al. [ 1221 ). 1,2,4-Trimethylbenzene yields dimethylbenzene and trimellitic acid if oxidized on a SnV-0 catalyst. Kinetic data have been measured by Balsubramanian and Viswanath [ 371. Aromatic hydrocarbons with ethyl and longer side chains are easily attacked at the side chain, which is either completely oxidized or reduced t o one C atom and converted into the aldehyde. In the case of ethylben-

221 zene, quite a number of investigations have been carried out to develop catalysts that would direct the oxidation t o styrene, which is analogous to the oxidative dehydrogenation of butene t o butadiene. A selectivity of 80% is reported by Cortes and Seoane [91] for Ni-W-0 catalysts with an atomic W/Ni ratio between 2 and 4. A very high selectivity (>95%) was found by Joseph et al. [166], using a cobalt molybdate catalyst in a flow reactor at 500-600°C. However, a low oxygenlethylbenzene ratio (below 0.5) is necessary t o achieve this high selectivity and coke formation problems are to be expected. Industrial alumina-supported cobalt molybdate catalysts were studied by Russo e t al. [269]. Selectivities of 6C-70% were obtained at a conversion level of 20--30%. Aluminasupported MOO, appears t o have the same qualities, while unsupported Moo3 exclusively produces benzaldehyde (beside carbon oxides). Alumina itself also has some activity and may be used as the catalyst. Lisovskii et al. 11891 suggest the addition of alkali metal oxides t o alumiria in order to reduce the surface acidity, and thus t o prevent poisoning of the catalyst by condensation and cracking products. Although some progress has been made, the oxidative dehydrogenation is far from competitive with the highly selective non-oxidative dehydrogenation process used in industry today.

2.5.7 Ammoxidation Methyl side chains of aromatic hydrocarbons can be selectively ammoxidized to nitrile groups. The process is very similar t o the ammoxidation of propene and the same catalysts are found to be effective. Identical mechanisms have been proposed, and will not be discussed here. The selectivity of the ammoxidation of molecules like toluene and xylene is much higher than that of the oxidation of these compounds to aldehydes. The selectivity difference is more pronounced here than in case of propene. The initial selectivities of the propene oxidation and ammoxidation are practically the same, and the selectivity difference is mainly due to the high stability of acrylonitrile compared with acrolein. For aromatic (amm)oxidation, however, the initial selectivities also differ. Apparently, ammonia interacts with the catalyst in such a way that the activity for oxidation of the aromatic nucleus is reduced. A few contributions with respect t o the ammoxidation of aromatic hydrocarbons that have appeared in the literature concern toluene and xylene.

( a ) Toluene Simon and Germain [293] investigated a number of molybdates at 450°C with a molar feed of toluene/ammonia/air = 1 : 5 : 50. The main results are presented in Table 39, in which selectivities to benzonitrile and Refewncc.s P P . 253-262

222 TABLE 39 Selectivities and activities with molybdenum-based catalysts for toluene ammoxidation Selectivity

Conversion

(%)

("/.I

Activity (mmol h-' m-2)

85 83 82 61 68 87 85

80 87 95 87 85 93 94

0.09 0.33 0.62 0.14 0.78 0.69 1.77

~~

Bi-Mo Sb-Mo Sn-Mo U-MO Fe-Mo Ti-Mo V-Mo

~~

~

activities are given. The authors conclude that U-Mo and Fe-Mo catalysts, which are the most selective in toluene oxidation, are the least selective in ammoxidation. Because the overall rates of oxidation and ammoxidation are equal, the rate-determining step occurs before formation of the C6HsCH: complex. It can very well be that imine is an intermediate, viz.

The selectivity t o nitrile is higher than the comparable selectivity t o benzaldehyde. This is probably due to the greater stability of the nitrile or a difference in desorption velocity of the imine compared with benzaldehyde. Nitriles are only weakly adsorbed. A combination of VzOs and SnO, (weight ratio 70 : 30) is a reasonable catalyst at 300-360°C in giving about 50% yield, as has been shown by Lodaya e t al. [ 1901.The yield was measured a t a 5-8% level of conversion and is hardly dependent on temperature in the given region. The optimal NHJtoluene ratio is 6.

TABLE 40 Selectivities and activities in the formation of nitriles from p-xylene _.

Catalyst

Temperature ("C)

Selectivities a (%) p-Toluylnitrile

Terephthalonitrile

_ _ _ _ _ _ _ _ ~ _ Sn-Mo Ti-Mo V-Mo V

460 415 430 430 -

a

__.-

64(52) 92156) 77(64) 78(66) -

Activity (mmol h-' m-2)

_ 30(78) 6(87 ) 19(88) 19(80) _ ~ _ _ _

The respective conversion levels are given in parentheses.

~ 1 0.5 0.6 12 -__

223 TABLE 4 1 Kinetic parameters for ammoxidation of p-xylene over V 2 0 5

kl

k2 12 3

k4

Rate coefficient EA (sec-' ) (kcal mol-') ___ ___.. ___________ 1.3 x 104 19.4 2.4 x 104 13.9 3.8 x 10-3 2.2 28.6 15.2

-_

( b ) Xylene

Simon and Germain [293] tested some Moo3-based catalysts and compared these with Vz05. Ammoxidation with a reactant ratio hydrocarbon/ NH3/air = 1 : 10 : 100 gives the results for p-xylene shown in Table 40. V 2 0 5 was also used by Novella e t al. as a catalyst in the ammoxidation of p-xylene [241]. These researchers carried out experiments at 390,400,410 and 420°C, with varying feed ratios (p-xylene/NH,/air = 1/3--5/60-80). They proposed the kinetic scheme

CN

\

CH,

CN

CN

The kinetic parameters are given in Table 41. m-Xylene can also be ammoxidized as was shown by Rizaev et al. [ 2651, who used a recirculation reactor with a V-Mo catalyst (6%V 2 0 5 , 2% Moo3 on Al,03). The kinetic scheme is

According t o the authors, the kinetics are zero order, provided that sufficient oxygen and ammonia are present. Referetices p p . 253-262

224 2.6 METHANOL

Selective oxidation of methanol is the industrial route t o formaldehyde. In practice, two types of process are used, differing with respect t o the catalyst and process conditions. Silver is a very active catalyst at 600700°C and requires a high methanol/oxygen ratio for a good selectivity, while iron molybdate catalysts are already active at 350°C and may be used with low methanol/oxygen ratios.

2.6.1 The silver process The silver process is the older one and is still used in many formaldehyde manufacturing plants today. Yields of about 90 mol. 5% are reported, and combustion to carbon dioxide and water is the main side reaction. As significant amounts of hydrogen are formed, it has long been assumed that formaldehyde is essentially formed by dehydrogenation of methanol, accelerated by the combustion of a large part of the liberated hydrogen. Recently, however, several authors explain the kinetics on the basis of direct interaction of methanol with oxygen. The reaction is carried out over a silver gauze or low surface supported catalyst at 600-7OO0C, indicating a very fast chemical reaction. This implies that determination of the intrinsic reaction rate in laboratory reactors is complicated by the interference of heat and mass transfer limitations. To avoid this problem, studies have been made at much lower temperatures, which in turn run the risk of being non-representative. A Langmuir-Hinshelwood type of model is suggested by Robb and Harriott [266] who studied the reaction at 420°C. They find that the intrinsic kinetics can be represented by

The equation reflects dual site reversible adsorption. Methanol and formaldehyde compete for sites, while oxygen is dissociatively adsorbed on different sites. At a not-too-low oxygen pressure (>0.01 atm) the coverage of the oxygen sites is complete and the equation reduces t o KCH30HPCH30H

R-k +

KCH30HPCH30H

+

Kprod.Pprod.

An Eley-Rideal model with dissociative adsorption of oxygen is proposed by Bhattacharyya et al. [48]. Because the oxygen adsorption is assumed t o be irreversible, the model is identical with a redox model and

225 The experiments were carried out a t a very low temperature (264-290°C) resulting in an unusually low conversion. The relevance of the calculated kinetic parameters is therefore doubtful. Recently, Hodges and Roselaar [157] used gold and platinum as catamixture was stoichiometrically lysts. At 400" C, a rich methanol-xygen converted to formaldehyde and water with a residence time of 270 psec. Larger partial pressures of oxygen and higher temperatures raised the degree of combustion t o carbon dioxide. With platinum, the maximum yield of formaldehyde was reached a t 210°C. The authors assumed that methanol was dissociatively chemisorbed and reacted with adsorbed oxygen atoms. 2.6.2 Iron molybdate and other metal oxide catalysts The use of iron molybdate in industrial plants started about 1960. Yields of about 90% are reported for this process, applying either excess air or excess methanol and recirculation of the latter. Carbon dioxide is the chief by-product. Kinetic investigations have appeared in the literature since 1965. A redox mechanism is generally accepted [254], and has been confirmed by pulse experiments which demonstrated the equal activity of the catalyst in the presence and absence of oxygen. The results of Pernicone [254] and Liberti et al. [187] seem t o indicate that the rate-determining step is either hydrogen abstraction from methanol or desorption of formaldehyde. The structure of the iron molybdate catalyst in relation to its oxidation properties has been studied by several authors. It is stated by Pernicone [254] that there is an excess of Mo6+ and 0'- ions in the Fe, ( h I 0 0 ~lattice )~ giving rise t o an enlargement of the unit cell in one direction. Two iron ions can be replaced by two molybdenum ions as the insertion of three 02-ions in the lattice is possible. The activity of such a structure is higher than with Moo3 and Fe2(Mo04)3in pure forms, although MOO, is very selective. Carbucicchio and Trifiro [75] have shown, however, that the specific activity is the same, when the different surface areas of the pure and the iron-deficient molybdate are accounted for. The selectivity to formaldehyde is also practically the same. Another property of the irondefective molybdate is the presence of Mo= 0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related t o Mo6+ contained in the Mo=O as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting t o note here that pure iron oxides accelerate combustion and that a W03-Fe2 (WO,), catalyst is practically inactive [254]. Replacement of iron by chromium is possible but leads t o a lower activity [ 2531. Baussart et al. [ 461 prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. References p p . 253-262

226 TABLE 42 Vanadium-chromium catalysts in methanol oxidation Cr (at.%)

Surface area (m2 g-l)

ko

(x

E (kcal mol-')

Order in O 2

5 8 9 14 14 19 20 15 6 28

1.4 0.81 0.59 0.14 0.18 0.001 0.0019 0.0001 0.0007 0.14

44-46 44 45 44 44 39 38 33 35 34

0.5

0.65 0.4

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due t o Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. Another group of binary oxides has been tested by Koval and Boreskov [ 1801. These authors studied 10 different compositions of VzOs--Cr2O3, starting with pure VzOs and adding increasing amounts of C r 2 0 3 .The rate data are given in Table 42 for a temperature of 300°C. The methanol concentration in the feed was 3.6-3.7 vol. 7%. Activities and selectivities are shown in Fig. 10. l80exchange rate measurements in the range 400-

-

Composition (mole % )

Fig. 1 0 . Selectivity and conversion in methanol oxidation on V z 0 5 - C r 2 0 3catalysts as a function o f composition at 30OoC.

227

D

m

1

I

I

20

A t o m i c ratio

I

I

40 (O/d-

I

I

60

v +Me

I 80

I

I 100

(Me=Fe.Co,Nl)

Fig. 11. Relation between selectivity (conversion) and catalyst composition for methanol oxidation at 31OoC.

500°C indicate that the oxygen in V-containing compositions, but not in C r 2 0 3 , is all exchangable. The selectivity decreases with decreasing strength of the oxygen bond on the surface, while the activity increases. Malinski et al. [199] combined vanadium pentoxide with oxides of nickel, iron and cobalt and reported that these mixed oxides have a much higher selectivity than the pure oxides. The results obtained a t 310°C are shown in Fig. 11, the methanol concentration in the gas phase being 44% and the C H 3 0 H / 0 , mole ratio 2.2. Selectivity and activity are given as a function of the atomic ratio V/Me. The ratio V/Me 2 1 gives the highest conversion to formaldehyde. A V-Ni catalyst gives the best results and does not show any activity for the side reaction which produces some hydrogen with other catalysts. The authors suggest that the latter group of oxides contain active oxygen centres which are not regenerated at a sufficient rate. Aldehyde molecules then get an opportunity t o decompose on the catalyst surface with simultaneous hydrogen evolution. 2.7 AMMONIA

The oxidation of ammonia can produce nitric oxide, nitrous oxide and nitrogen according to the stoichiometries References p p . 2 5 3 - 2 6 2

228

2 NH3 + 2; 0, = 2 N O + 3 H,O 2 NH3 + 2

0 2

= NZO

2 NH3 + If 0, = N,

+ 3 H,O + 3 HzO

The production of N O is of great industrial importance for the manufacture of nitric acid. The other two reactions do not have practical applications.

2.7.1 The production of NO The industrial process is carried out with platinum gauze as the catalyst at 750-900°C. Selectivities of 95-97% are reported for this extremely fast chemical reaction. The main by-product is N,, and only traces of N,O are formed. The kinetics were reviewed by Dixon and Longfield [ l o l l , since when the subject has not received much attention.

2.7.2 The formation of N, and N z O The conversion of ammonia t o N2 and N z O is catalyzed by metal oxides. Depending on the type of catalyst, N, or NzO may be the main product. The situation is analogous to the oxidation of hydrocarbons in that mild oxidation catalysts (e.g. MOO,, V 2 0 5 ) favour formation of nitrogen, while the more severe oxidation catalysts (e.g. Co304, MOO,) produce the largest amounts of NzO. Ilchenko et al. [ 161-1631 compared the oxides of Mn, Co, Cu, Fe and V, and found that MnO, gives a selectivity t o N z O of 42% at 155°C and p N H 3 = 0.1 atm at contact times, T , of 1.5-4 sec. Co304produces less N z O and more nitrogen at 143°C (selectivity = 18% at p N H 3 = 0.2 atm, T = 5-15 sec). At these low temperatures, the selectivity to N 2 0 was not very sensitive to variations in T , suggesting that the products are formed by parallel reactions, viz.

2 NH3 + oxygen

,N, + water - N 2 0 + water

If the temperatures are raised, catalytic N z O decomposition is observed, viz . N z O = N2 + 0, In principle, nitrogen can also be formed by the catalyzed reaction with ammonia.

3 NzO + 2 NH3 = 4 N, + 3 H,O The general rate equation for the oxidation of ammonia t o nitrogen is of the redox type (161-163).

TABLE 43 Rate coefficients (molecule cm-’ sec-’ atm-’) and activation energies (kcal mol-’) of ammonia to nitrogen Catalyst

Temp. ( ’ C )

kl x

k, x

El

EZ

MnOz CO304 CUO

145 143 240 250 290

0.66 0.26 5.13 0.38 6.50

1.21 0.61 16.13 0.55 2.57

30

17

20 16 23

20 21 20

Fe203

VZOS ~

in which n is a stoichiometric coefficient. Table 4 3 presents values of h l and k 2 and activation energies. The catalysts show a steadily increasing selectivity with increasing surface coverage of oxygen. It is clear that the formation of nitrogen is a “milder” oxidation than the one leading t o nitrous oxide. The major role of oxygen coverage has been confirmed by experiments in the absence of oxygen in which rate data have been determined for the reduction of the metal oxides with ammonia. Selectivities for the formation of N2 increase in the sequence MnOz < Co304< F e 2 0 3< CuO < MOO, < VzOs. The same pattern has been found in the mild oxidation of hydrocarbons and methanol. Ilchenko et al. [ 161-1631 relate the difference in selectivity t o the metal-oxygen bond strength; this is considered in Sect. 3. Holbrook and Wise [159] worked with crystalline Cu,O as a catalyst in the oxidation of ammonia at about 300°C. In this case, there is also a strong correlation between the amount of excess oxygen and selectivity. When the catalyst surface changes its defect structure from copper-rich t o oxygen-rich, the nitrogen concentration goes through a maximum. The rate of disappearance of NH3 is independent of the ammonia concentration and is first order in oxygen, comparable with the kinetics of acrolein production on Cu,O. The catalytic properties of Cu,O are controlled by the electronic properties (see Sect. 3). Another copper catalyst, prepared by treating a NaY zeolite with copper nitrate, for ammonia oxidation (160--185°C) has been studied by Williamson et al. [349]. The reaction is first order in NH3 and zero order in oxygen. The mechanism here is based on a Cu(II)(NH,):’ complex formed in the large cavities of the zeolite. The ratedetermining step is the reduction of Cu(I1) by ammonia. Wise [ 3501 investigated the parallel between ammoxidation and oxidation of ammonia over bismuth molybdates. It was shown that the rate of conversion t o nitrogen is first order in NH3 and independent of oxygen concentration, analogous to the selective oxidation of propene. Under conditions in which propene combusts, NH3 is converted t o nitrogen oxides. References p p . 253-262

230 Bismuth molybdate and other binary compositions (Fe-Mo, Sn-Sb and others) were tested by Germain and Perez [128) using a pulsed reactor. The authors demonstrate that a qualitative analogy may exist between ammonia and propene oxidation but if activities are compared, different sequences of catalytic efficiency arise. It must be noted, however, that these conclusions are based only on pulse experiments. These can be quite different from results in flow reactors, depending mainly on the nature of the steady state. From the different contributions, it may be concluded that, in the oxidation of ammonia, the same type of redox mechanism is operative for metal oxides as in the selective oxidation of hydrocarbons. As a consequence, the hydrogen atoms will be abstracted successively from the NH3 molecule by a stepwise mechanism. 2.8 SULPHUR DIOXIDE

Although there is only one oxidation reaction possible with sulphur dioxide and hence a selectivity problem does not exist, recent results from kinetic research are included in this chapter, since there is a close analogy with other oxidations, especially on V205-based catalysts. The oxidation of sulphur dioxide to trioxide is one of the oldest heterogeneous catalytic processes. The classic catalyst based on VzOs has therefore been the subject of numerous investigations which are amply reviewed by Weychert and Urbaneck [ 3461. These authors conclude that none of the 34 rate equations reported is applicable over a wide range of process conditions. Generally, these equations have the form of a power expression, in which the reverse reaction is taken into account within the limits imposed by chemical equilibrium, viz.

Also, Langmuir-Hinshelwood models have been proposed as well as models based on a redox mechanism. Recently, Happel et al. [154] using data from Kadlec et al. 1167,2171 conclude that a model based on the dissociative adsorption of oxygen, which is ratedetermining, fits the experimental results best, viz.

With h = A 1 exp(-E1/RT) and K = A z exp(E,/RT), the values of the parameters for the temperature region 380-480°C are

-

~

Parameter

Value

A1 El

7.34 X 1014 mol h-' atm-' (g cal)-' 4 . 7 1 X l o 4 cal mol-' 1.22 X mol h-' atm-' (g cal)-' 2.72 X l o 4 cal mol-' 8.20

_ -.

A2 E2

n

~~

__-

The industrial catalyst consists of a mixture of V z 0 5and KZSzO7supported on silica. Under technical reaction conditions (>440°C),this mixture forms a viscous molten phase on the surface of the porous silica structure. Apparently a redox model can also be applied to such a system [154]. Putanov et al. [ 2591 investigated K-V-S-0 catalysts carried by S O z . By different techniques, it was noted that compounds such as KV4010.4, K 2 V 5 0 1 3and K3V5014 are present. It was demonstrated that SiOz as a support plays an active role in transformations of the catalytic layer. In the binary system K2S04-VZ05, compounds with even higher K/V ratios were confirmed. Kato et al. [170] also drew attention t o the importance of the vanadium-potassium ratio. Working in the region 500-6OO0C, they found a simpler rate equation This is obviously valid for initial conditions only. The same comment applies t o the work of Herce e t al. [ 1551, who also d o not account for the effect of chemical equilibrium.

3. Role of the catalyst 3.1 THERMODYNAMIC CONSIDERATIONS

Thermodynamically, the oxidation of hydrocarbons t o carbon dioxide and water is preferred t o any partial oxidation reaction. The possibility of forming partial oxidation products is thus entirely dependent on the kinetics of the oxidation process. The oxidation of hydrocarbons, is in general, a stepwise process. One way to confine the depth of oxidation, therefore, is t o apply a low oxygen t o hydrocarbon ratio and a short reaction time. However, to avoid a multitude of products with different oxidation depths, the use of a catalyst is obviously required. In that case, the above two factors (oxygen deficient conditions and short reaction time) may loose their importance. Basically, the role of the catalyst can be twofold. (a) Activation of the hydrocarbon molecule by chemisorption in a specific way. The attack of oxygen may thus be selectively directed t o a particular site on the hydrocarbon molecule. References p p . 2 5 3 - 2 6 2

232 TABLE 44 Free energy for the transition of a higher to a lower oxide (kcal per mole of liberated oxygen), calculated from ref. 362 _ _ ~ ~ _ _ ~ ..___

Temperature (“C)

Ago Biz03 CUO C U 2 0

Fez03 Fe304 M003 SbzO, Sb2O4

SnOz Ti03

+Ag BiO

+

+cuzo +cu +Fe304 +FeO +Moo2 +Sbz04 +Sb203 +SnO +Ti203

uo3

+u,ox

u3ox

’U02 +V?04 +V203 +W02

v205

V204 W03

350

400

-3 6 I2 37 59 72 113 64 10 51 109 152 14 52 31 66 103

-4 4 65 30 54 62 105 51 3 44 102 145 9 46 30 61 98

~ _ _ _ _

(b) Reducing or “tempering” the activity of oxygen. The amount of energy liberated by the formation of C-0 or H-O bonds by reactions between hydrocarbons and molecular oxygen is roughly 100 kcal per mole of oxygen. This energy is so large that bonds within the hydrocarbon molecule can be broken and fragments result, which can be easily further oxidized. The catalyst can effect the distribution of the energy of oxidation over two partial reactions, i.e. the reaction between molecular oxygen and the catalyst and the reaction between the “loaded” or “oxidized” catalyst with the hydrocarbon. In the case of metal oxide catalysts, the degree of “tempering” can be derived from the free energy of the transition from a higher t o a lower oxide (Table 44). A value close t o zero means that the oxide has almost the same oxidation potential as molecular oxygen, while values of 100 kcal and more signify that the reactivity towards hydrocarbons is practically zero. The intermediate region, therefore, is of interest for catalysis by metal oxides. Quantitatively, the meaning of the figures in the table is very limited, however, because on a catalyst surface the situation is different from the bulk and the strength of the oxygen bonds is not uniform. It is also influenced by edges, corners and defects of lattices. With inorganic compounds, there can also be a selectivity problem, as illustrated by the oxidation of ammonia t o nitrogen. Deep oxidation leads t o nitrogen oxides. With sulphur dioxide, no selectivity problem rises.

233 In the following section, the metal-oxygen more detail. 3.2. METAL-OXYGEN

bonds will be treated in

BOND STRENGTH

Especially in those cases where 0’- is the active form of oxygen and the catalyst operates according to a redox mechanism, it is reasonable t o assume that the metal-oxygen bond plays an important role. It would be expected that the rate of oxidation should be inversely correlated with the bond strength, provided that the reduction of the catalyst by the hydrocarbon molecule is the rate-controlling step. Exceptions to such a correlation can easily occur, however, because of the heterogeneity of the surface. Indeed, it is found that the bond strength often depends on the degree of coverage. Another factor is the special geometry at the active site of the catalyst. Finally, it may be remarked that a concerted mechanism can occur in which the M e 4 bond strengths are only relevant in close connection with the complex to be oxidized. The most important properties used as a measure of the bonding strength are the heat of formation of the metal oxides, the heat of oxygen desorption, the reducibility of the metal oxide and the activation energy for isotope exchange between I8O2 in the gas phase and oxygen in the catalyst.

3.2.1 Heat of formation of metal oxides, AHf

AHf can be calculated, in principle, from thermochemical data. It is then necessary t o take into account the variable valency of most metals and t o fix the different oxidation states which occur during stationary or non-stationary reaction conditions. Some difficulties with this method are thy scarcity of data for mixed oxides, the difference in conditions between those on the surface of the catalyst and those in the bulk and the inaccuracy of a number of data obtained by measuring differences in AH. Attempts to correlate the activity with AHf have not been very successful. A fairly good inverse correlation was found by Moro-oka e t al. [223, 2241 but it concerns complete oxidation to carbon oxides. Some patterns of activity for various selective oxidation reactions, related t o LW,, are described by Germain [ 1341. With respect to the selectivity, the situation is even more complex. Only a rough classification into three groups can be made. The first one consists of metals which bind the oxygen loosely, e.g. noble metals, and generally promote complete oxidation. A second one has strongly bound oxygen but adsorbs oxygen loosely, which also favours combustion (e.g. Co, Mn, Ni, Cr). A third group is characterized by moderately bound oxygen, often coupled with variable oxidation states of the metal oxide. This group, in particular, effects a selective oxidation. Refere1ici.s P P . 2 5 3 - 2 6 2

234 3.2.2 Heat of oxygen desorption Measurements with a vacuum system of equilibrium oxygen partial pressures as a function of temperature indicate desorption energies. There is some difficulty in choosing a representative state of comparison. Generally, investigators evacuate a t increased temperature for a long time. Of interest is the new flash technique applied by Halpern and Germain [153]. This technique reveals that mobile oxygen generally occurs in discrete binding states. The authors compared V 2 0 5 and CuO with other catalysts mainly concerning total oxidation. Figueras et al. [ l l l ]emphasize the importance of the entropy of the oxygen bond, which can be considered as a measure of the surface mobility of oxygen. Unfortunately, their assumed positive correlation between entropy and selectivity is only based on two V 2 0 5 catalysts which differ with respect t o the carrier (Si02 and A1203). Portefaix e t al. [256] measured the equilibrium oxygen pressure as a function of temperature for iron molybdates. The authors demonstrated that, in the case of the system Fe-Mo-0, the bonding energy of oxygen increases with increasing degree of reduction if the composition is rich in iron. In the case of an iron-deficient combination, the bonding energy decreases with increasing degree of reduction. Only in the case of Fe,O, . 3Mo0, does the bonding energy remain constant. 3.2.3

isotope exchange

The activation energy for isotope exchange between "02in the gas phase and oxygen in the catalyst is a measure of the metal-oxygen bond strength. With selective catalysts, the exchange between gas phase and catalyst oxygen (hetero-exchange) is about as fast as the exchange between gas molecules via the catalyst (homo-exchange), implying that in both reactions the same oxygen species is involved, i.e. 02-.With nonselective catalysts, however, the homo-exchange rate may be considerably faster, and apparently involves a more loosely bonded, adsorbed form of oxygen. These principles are illustrated, for example, by Haber and Grzybowska [152], as shown in Table 45, in which a number of oxides are ordered according t o their homo-exchange activity. Indeed, the most selective catalyst is found at the t o p of the table, while a catalyst like Co304only effects combustion. Attempts t o correlate the exchange rate for selective catalysts with the activity for hydrocarbon oxidation have not been very successful, mainly due t o the fact that the oxidation activity of such catalysts is much greater than the exchange activity. The difference is often so large that the reactions must be studied in different temperature regions. The origin of this difference is obvious: liberation of oxygen from the catalyst is facilitated by the presence of a reducing agent (i.e. the hydrocarbon molecule),

235 TABLE 45 Activity of catalysts towards IRO2exchange Catalyst

Temperature

Rate (g O2 m-2 h-* 1

(“C)

Bi/Mo = 2 : 1 Bi/Mo = 1 : 1 Co/Mo = 1 : 1.7 MOO Fe/Mo = 1 : 1 Fe2°3 c o30 4

250-500 474-500 599-634 5 80-60 1 508-552 350-450 125-250 -__~____

None None 1.8 x 10-4 9 x 10-4 10-3 4 x 10-1 12.7

which may form an intermediate complex (transition state) involving the oxygen t o be transferred. The observed differences in activation energy between oxidation and oxygen exchange are considerable. Successful correlations may be found, however, within binary oxide systems, i.e. by comparing catalysts with different ratios of the same oxides. Blanchard e t al. [51,53], for example, studied the V2O5-MoO3 and V,Os -Ti02 systems and found a striking correspondence between the activation energy of isotopic exchange and the hydrocarbon oxidation selectivity, both as a function of the V/Mo and V/Ti ratios. Interesting reviews on the subject of isotopic oxygen exchange are those of Novakova [ 2401 and Parravano [ 2491. 3.3 OXYGEN TRANSFER

In heterogeneous catalytic oxidation, the reaction is always between a molecule t o be oxidized (in adsorbed form or not) and oxygen which is attached to, or is part of, the surface. A number of different oxygen species is possible, ranging from free oxygen molecules t o oxygen anions. The species in between can be represented by the scheme @2

0 (ads-)\

O,(gas) L_ O,(ads.) 0; (ads.)

@

2 0-(ads.) _7 2 0’- t--,2 0 2 surface

bulk

From left t o right, the oxidizing power will decrease. Which of the different oxygen species are active on a catalyst is determined by (a) the rates of the different steps in the scheme; these depend on temperature, oxygen pressure, state of the surface and type of catalyst; (b) the reactivity of the oxygen species with respect to the molecule to be oxidized. This depends on the oxygen bonding energy, the adsorbed References p p . 2 5 3 - 2 6 2

complex, the underlying geometry and the temperature. It is generally assumed that, in the group of transition metal oxides, the intermediates between 0, (gas) and 0’- (surface) do not come into the picture. 0’- (surface) is considered t o be the reactive oxygen species. In the case of a high oxygen anion mobility, surface 02-may rapidly exchange with bulk oxygen. Consequently, a large fraction or even all of the catalyst oxygen may appear t o participate in the oxidation reaction. Another consequence of a high mobility is that the sites at which oxygen reacts with the component t o be oxidized, and those at which oxygen is taken up, may be quite remote. Apart from the intrinsic properties of the catalyst lattice, the reaction conditions can also influence the oxygen mobility in the catalyst: as the transport of oxygen through the solid can be regarded as the diffusion of holes (anion vacancies), the number of these is an essential factor. This number obviously depends on the degree of reduction and thus on the rates of the reactions between the catalyst and oxygen, and between the catalyst and the compound t o be oxidized. These rates in turn depend on the reaction conditions. Several recent contributions concerning the participation of lattice oxygen in selective oxidation processes have appeared and fully agree with the above concepts. They will be discussed in more detail below. In a second group of metal oxides, which are not easily reduced, the oxygen is strongly bound and the catalyst is generally in a fully oxidized state. Thus 02-is not reactive, but an adsorbed form of oxygen, much more weakly bound, is active. This leads only to combustion. Quite a number of these metals are non-transition metal oxides. A third group contains those metal catalysts which d o not form specific crystal phases in an oxidized state. The common types of oxygen on the surface are then O2 (adsorbed) and 0 (adsorbed) which generally do not lead t o selective oxidation. One of the exceptions is silver, which very probably catalyses the selective oxidation of ethylene by providing 0; on the surface. However, an active role of surface oxides, which may be formed particularly by the action of promotors, is not excluded.

3.3.1 Participation of lattice oxygen The participation of lattice oxygen is inherent to the redox mechanism, which is operative in many of the oxidation processes that are catalyzed by metal oxides. Reviewing the processess described in Sect. 2, participation of lattice oxygen appears t o be the case for the majority of them, namely for the allylic (amm)oxidation of olefins, for the (amm)oxidation of aromatic hydrocarbons and for the oxidation of methanol, ammonia and sulphur dioxide. Two types of experiment are commonly used t o give evidence of participation of lattice oxygen: (a) experiments in the absence of gas phase oxygen and (b) experiments with labelled oxygen.

237 ( a ) Experiments in the absence of gas phase oxygen The activity of an oxide catalyst in the absence of gas phase oxygen provides direct evidence that lattice oxygen can perform the selective oxidation process, although it does not exclude the possibility that, in the presence of gas phase oxygen, other forms of oxygen also participate in some stage of the reaction. Pulse experiments are the most suitable for this purpose, because rapid catalyst reduction is then avoided. As pulse experiments have been amply reviewed in Sect. 2, only the conclusions will be discussed here. The activity of oxide catalysts in general declines as reduction proceeds. Characteristic of the processes that involve lattice oxygen is that the initial activity (i.e. that measured by the first pulse) approaches that in the presence of oxygen, while the selectivity is either identical in the presence or absence of oxygen, o r higher in the latter case, because side reactions due to adsorbed oxygen are excluded. The rate a t which the activity falls during reduction is dependent on both the nature of the catalyst and on the process studied. After a certain initial activity decrease, often a lower, but rather constant, activity level is reached. Different explanations are given for the fact that a part of the initial activity may be rapidly lost. Several authors suggest heterogeneity of the catalyst surface and conclude that more loosely bonded oxygen is consumed first. Another possible cause is the effect of the increasing of anion vacancies and reduced cations on the electronic properties of the solid, which in turn may affect the oxygen reactivity and the adsorption capacity for the reactant molecule. Finally, the irreversible adsorption of reaction products may be of importance. Barannik e t al. [ 38,391, for example, have shown that this is the predominating factor in the fall in activity during the pulse reduction of bismuth molybdates by propene. The occurrence of an almost constant, albeit rather low, activity level, which is reached after a number of pulses, signifies that a certain quasiequilibrium concentration of active sites is mzintained by transport of bulk oxygen anions t o the surface. Such a mobility of oxygen is particularly observed for bismuth molybdates and some related catalysts (see below). Typical examples of catalysts which completely loose their activity a t a low degree of reduction are the antimonates; this is primarily caused by the absence of anion mobility.

( b ) Experiments with labelled oxygen Most experiments concern the application of labelled gas phase oxygen in reaction mixtures, while only in a few studies has labelling of the solid phase been used. Catalysts that have received particular attention are the bismuth molybdates and the antimonates of U, Fe and Sn, all very selective catalysts for the oxidation of propene t o acrolein and similar allylic oxidations. References p p . 2 5 3 - 2 6 2

238

(i) Bismuth moly bdates. Bismuth molybdates have been extensively studied, mainly by using propene/”O, mixtures. The experiments have been performed in static systems [ 174,252,3521 or static recirculation systems [51,123] at rather low pressures, but also in a pulse reactor [276] and, very recently, in a flow reactor under atmospheric conditions [ 1751. It is clearly shown in all these studies that lattice oxygen is consumed in the selective oxidation, while the gas phase oxygen that reoxidises the catalyst diffuses into the solid. At temperatures of 400°C and higher, the mobility of oxygen anions appears t o be very large, and the oxygen introduced at the surface appears t o equilibrate with essentially all oxygen anions present in the lattice. The effect of temperature and oxygen partial pressure was studied by Sancier e t al. [276], They showed by pulse experiments that, at lower temperatures and higher oxygen t o hydrocarbon ratios, a certain amount of “short-circuiting” occurs between the catalyst reoxidation process and the transfer of oxygen from the cataiyst t o the reactant, as shown by a partial break-through of labelled oxygen in the reaction products. This short-circuiting is obviously caused by the decrease in ratio between the rate of diffusion into the lattice and the rate of reaction at the surface. Temperature primarily influences the diffusion rate, which has the highest activation energy, while the oxygen partial pressure may influence both: a higher pressure implies a higher oxidation state of the catalyst, i.e. it decreases the number of anion vacancies and thus the diffusion rate, while at the same time the reaction rate at the surface is increased. There is some uncertainty with respect t o the participation of lattice oxygen in the formation of carbon oxides parallel t o acrolein. Some authors report that an enhanced amount of l80is found in the COz produced, while others d o not observe any diEference between acrolein and carbon dioxide, with respect t o the l80/l6Oratio. The equal ratio in both products, however, may also be caused by the exchange of oxygen between C 0 2 and the catalyst. Gel’bshtein e t al. [ 1231 report that, in a static recirculation system, the amount of CI8O2formed is maximal in the beginning, and then decreases due t o exchange with the catalyst; Sancier et al. [276] find that, in pulse experiments, larger amounts of acrolein-”O are formed in the presence of C1802,and calculate an activation energy of only 4 kcal mol-‘ for the exchange reaction. On the other hand, carbon oxides are also formed in the absence of gas phase oxygen, while it is further known that, under the usual process conditions, the kinetics of acrolein formation and parallel combustion are the same, and both involve an allylic intermediate. One must conclude, therefore, that the initial reaction steps are very likely identical and involve lattice oxygen, but that, in the combustion of the ally1 intermediate, probably both lattice oxygen and adsorbed forms of oxygen can participate. Selective labelling of the catalyst is applied in an interesting study by Otsubo et al. [ 2461. Starting with labelled and unlabelled oxides of bis-

239 muth and molybdenum, y-Bi2’*03 . Moo3 and y-Biz03 - Mo1803 were prepared by solid state reaction between the oxides. Reduction by hydrogen was studied in a circulation reactor at 400°C. Initially significant differences occur between the H 2 ’ * 0 content of the produced water and the average “0 content of the catalyst, indicating that isotopic scrambling did not occur before the reduction took place. The results prove that the oxygen attached t o bismuth reacts with the hydrogen, while reoxidation proves that oxygen is introduced at the molybdenum sites. This with lS02 implies that oxygen transfer from molybdenum t o bismuth is a part of the redox cycle. The authors report that the same is indicated by experiments with propene. Details of this promising work have not been published a t the time of writing. ( i i ) Antimonates. The antimonates of tin, iron and uranium have been studied by using propene/l8O2 mixtures in static (circulation) systems [86,123,252]. As with bismuth molybdates, it has been shown that lattice oxygen is the only source of oxygen in the selective oxidation, while both lattice oxygen and adsorbed oxygen may be involved in the carbon dioxide formation. Compared with bismuth molybdate, however, a rapid break through of l 8 0 is observed, which proves that the exchange capacity of the antimonates is very small and is, in fact, restricted t o one or two surface layers, at least at the usual reaction temperatures (300-400” C). Apparently the mobility of anions in the antimonates is small, which also implies that reoxidation must take place practically on the reaction site, in contrast t o bismuth molybdates where reaction and reoxidation sites may be quite remote. This difference in mobility, therefore, may be one of the reasons why the kinetics of the selective propene oxidation differ for bismuth molybdates and antimonates.

(iii) Other catalysts. Vanadium pentoxide-based catalysts ( Vz05-Mo03 and V205-Ti02) have been studied by Blanchard and Louguet [ 511, using a butene/’*O, mixture in a static circulation apparatus. Labelled oxygen is immediately observed in the oxidation products, indicating that the mobility of oxygen is low. The authors d o not believe that adsorbed oxygen is involved, but assume short circuiting via a partially reduced catalyst surface that cannot receive oxygen anions from the bulk. 3.3.2 Role of Me=O type of oxygen Several workers correlate the catalytic activity of metal oxides with the presence and the nature of double bond type oxygen at the surface. This type of oxygen is coordinated with one cation and can be regarded as “terminal oxygen” in contrast to a-bonded oxygen (or “bridging oxygen”) that is coordinated with two cations (Me-O-Me). Oxygen anions in different coordination states can be detected by IR spectroscopy, while Refrrcriccs p p . 253-262

240 reflectance spectroscopy is particularly suitable for an investigation of the surface of a catalyst. One of the early studies in this field was that of Sachtler [270] concerning V 2 0 , in the oxidation of aromatic hydrocarbons. It was shown that the hydrocarbon interacts with V=O, which is abundantly present in the V 2 0 s structure. Much work concerning molybdates and the allylic oxidation of olefins was carried out by Trifiro et al. [ 219,318,3191. A strong correlation between the activity and the presence of oxygen double bonded t o molybdenum was observed particularly for bismuth molybdates. It was concluded, therefore, that Mo=O is the most reactive with respect t o the olefin molecule. Mitchell and Trifiro [219] specify the nature of the active sites more precisely as Mo(O,)~,i.e. a molybdenum ion with three terminal oxygen anions. The Mo(OJ3 configuration can be expected t o occur only at corners, edges and defects in the lattice. A remarkable parallel, therefore, exists with the observation of Sleight et al. [ 33,2971 that the activity of scheelite-type molybdates is strongly correlated with the presence of defects (cation vacancies) and the conclusion that olefins are adsorbed on M o - 0 polyhedra next t o these vacancies. With regard t o iron molybdates, the correlation is less clear. Trifiro and Pasquon [318]defend the view that Mo=O is also of importance for iron molybdates and state that pure F ~ , ( M o O ~is) ~inactive because of the absence of such oxygen species. However, Carbucicchio and Trifiro [75] have recently reported that no differences in selectivity and specific activity exist between iron-deficient molybdate and the pure compound, although Mo=O oxygen is only detected in the former. Double-bonded oxygen as the active oxygen species is also observed for tin antiinonate (Sb=O) by Sala and Trifiro [ 2721 and for Bi,WO, (W=O) by Villa et al. [340]. The fact that only Bi2W06is an active and selective propene (amm)oxidation catalyst, while other compositions ( B i 6 W 0 I 2 , Bi2W209,Bi2W30,,) primarily cause combustion, is ascribed t o the acidic W-0-W configuration, which is only absent in Bi,WO,. The W+-W sites are presumably responsible both for isomerization and combustion reactions. Several suggestions have been made with respect t o the particular acitivity of double-bonded oxygen. Trifiro [ 3191 assumes a parallel between gas phase oxidation and the oxidation of olefins in solution (at a low temperature), which is also catalysed by Me= 0-containing oxide compounds (e.g. Os04, R u 0 4 , SeO,), implying that similar complexes may be formed. The olefin oxidation mechanism proposed by Weiss et al. [ 3451, and presented in Sect. 2.2.2(b), is, in fact, based on this parallel. Trifiro even extends the parallel t o the positive effect of steam on the acrolein selectivity in the gas phase oxidation of propene, which might be analogous t o the solvolysis effect in solutions. Kazanskii [ 1711 suggests that the mechanism of selective oxidation possibly involves “electronic excitation” of a double bonded oxygen anion t o

241

an anion radical ( 0 - )induced , by the interaction of the olefin molecule with the metal cation, as presented in the scheme

CH 2 7CH-C H 3

Mt+

___L

= 0 2 -

CH2=CH-CH CH2-CH-CH2 + M(?l-l)+ -0M("-l)+ - O H -

The transition of Mo=O oxygen t o 0 - radicals is also assumed by Akimot0 and Echigoya [ 13,151, who investigated Moo3-based catalysts by ESR and IR spectroscopy. They distinguish Mo6'=0 from Mo5'=0 and state that the oxygen in the latter has the strongest radical character, in agreement with the observation that the maleic anhydride production from butene increases in parallel with the Mo" content of the catalyst. They further state that modifiers can influence the reactivity of Mo=O oxygen. The reactivity of this oxygen decreases as the electronegativity of the modifier increases, according t o the sequence Bi203> Sb203 > As203 > P 2 0 , . Correspondingly, the reducibility and activity for the propene oxidation is highest for the Mo03-Bi203 combination. Although this hypothesis has interesting aspects, an oversimplification of the nature of the binary compounds is obviously present. Finally, it must be noted that the assignment of catalytic activity t o Me=O type of oxygen does not imply that other types of oxygen are not involved. Assuming that the olefin molecule is indeed attacked by double bonded oxygen, o-bond oxygen may take part in other steps of the oxidation process. The alternation of double bonded and bridging oxygen in the reduction-oxidation mechanism is a possibility suggested by Akimoto and Echigoya [ 161 for Mo03-based catalysts, and represented by

reduction

-Mo6'=0

O=M06+-

by olefin

-MoS++-Mo5+-

reoxidation b y oxygen

In bismuth molybdates, the bridging Mo-0-Bi oxygen is the most weakly bound oxygen and therefore supposed t o participate in the transfer of oxygen [ 219,2811. 3.3.3 Significance of 0; and 0 - radicals From the large amount of work with labelled oxygen, it is clear that active oxygen in a selective reaction is a type of lattice oxygen. Neverthelrss, a number of publications, mainly of Russian origin, which investigate the presence of 0; and 0-radicals, are interesting. This is often done at rather low temperatures at which catalysis does not occur (pre-catalysis). Several authors state that such energetic radicals give rise t o combustion. Kazanskii 11711 observes 0; and 0 - radicals with ESR when oxygen is adsorbed on Mo03/Si02, V 2 0 5 and MgO. At -196"C, the presence of References p p . 2 5 3 - 2 6 2

only 0; is established, at room temperature 0 - and O;, at 160°C mainly 0 - and above 400°C the oxygen is present as 0 2 - The . oxygen is dissociated more easily if electrons are supplied. Transition metal oxides require small energies for electron transfer. With these n-type semi-conductors, the number of conducting electrons is so large that the reaction will be faster than the reaction of 0 - with the ccmpound to of 0- to 02be oxidized. In that case, a selective oxidation will take place instead of combustion. Yoshida et al. [356] show the presence of 0 - and 0 ; on V 2 0 5 supported by S O 2 . With strong reduction, it is mainly 0-.It was proved that reaction of 0; with propene gives rise t o aldehydes (propionaldehyde, acrolein and formaldehyde) at temperatures below 150°C. Yoshida et al. [357] confirm this and find that oxygen at room temperature is mainly adsorbed as molecular oxygen, only 10% is the sum of 0 - and 0;. 0 - is the oxygen species reactive towards carbon monoxide, 0; is not. Further spectroscopic research has been carried out by Krylov [183, 1841 on adsorption of oxygen on MOO,, WO,, V 2 0 5 and CuO, supported on A1203, MgO and BeO. In all cases, 0; radicals were formed. Extra stabilization occurs when the catalyst is reduced with hydrogen. On a number of active oxides, the 0; intensity is increased drastically by simultaneous adsorption of propene. It is suggested that 0 ; is attached t o the carrier cation. The electron transfer with simultaneous adsorption is then supposed t o be

0;

H+----C3Hs

ag-&-ho5+

Combustion sets in with a high coverage of the transition metal oxide, diminishing the number of Mg-O-Mo bonds. It is also increased by a high concentration of “mobile oxygen” (0; or split-off singlet oxygen). Burlamacchi e t al. [65,66] also used ESR techniques and found that CdMo04 differs from Bi2Mo0, in adsorbing molecular oxygen. The oxygen is activated through the formation of radicals on the surface and leads to deeper oxidation than bismuth molybdate. One has to be careful with the statement that combustion is exclusively caused by adsorbed radicals like 0 - (and 0;). Many oxides can oxidize hydrocarbons mainly t o carbon oxides in the absence of gas phase oxygen. Finally, it can be noted that Me=O type oxygen may be regarded as an intermediate between 02-anions and 0 - radicals, thus providing some relation between the respective theories. 3.4 ASPECTS OF CHARGE TRANSFER

The catalyst plays an important role in transporting electrons from the molecule t o be oxidized t o the reacting oxygen. It can be expected that

243 the capacity of the catalyst t o furnish electrons, to take these up and to transport them internally will influence the catalytic properties. Many authors accordingly have studied the electrical properties of catalysts; these are mainly semi-conductors. However, the correlation of the electrical properties of the bulk phase with the catalytic properties of the essentially heterogeneous catalyst surface is a classical difficulty. This may be one of the reasons why no general correlation between these properties is found when a variety of different metal oxide catalysts is compared. A close relationship is often shown, on the other hand, when a particular catalyst is modified or doped with minor amounts of an additional metal oxide. It is very likely that the correlation is successful in this case, because the nature of surface sites is not essentially changed. Studies have also been carried o u t which are more specifically aimed at charge transfer on an atomic scale and deal with the atomic situation within the lattice. This is especially so in the case of binary oxides. Many authors assume that, in these systems, both types of cation participate in electron transfer. The reactivity of the binary oxides is then explained by the hypothesis that the cation on the active site obtains an electron supply from the second type. 3.4.1 Bulk electrical properties

Holbrook and Wise [158,159]pay special attention to the electrical conductivity of copper oxide which catalyzes the selective oxidation and ammoxidation of propene. It was ascertained that selectivity is promoted by an oxygendeficient Cu,O in the case of propene conversion, as well as in the oxidation of ammonia t o nitrogen. The selectivity is lowered by increasing the oxygen-opper ratio and, with an oxygen-rich CuzO and CuO, complete combustion leading t o the formation of N 2 0 is the main reaction. There is a large change in the slope of a plot of conductivity as a function of oxygen pressure which coincides with a rapid selectivity change. The authors conclude that a charged oxygen species is responsible for this behaviour. The surface coverage with this species depends on the relative difference between the surface state energy level of oxygen and the Fermi level. The value of the activation energy for the NH3 oxidation ( < l o kcal mol-') is of the same order as the temperature coefficient of the conductivity. Sala and Trifiro [274] give evidence that dissolving antimony in SnO, increases and stabilizes the number of free electrons. Morrison [232,233] finds that the free energy of electrons in the bulk phase (Fermi energy) is about the same for different selective and active catalysts. He notes that this value is very near (or just above) the electron exchange level of oxygen and hence makes reduction of oxygen possible. References pp. 253-262

244 3.4.2 Charge transfer o n an atomic scale It is generally accepted that valency transitions of cations are connected with the redox mechanism. It is obvious therefore, that activity and selectivity demand that t h e cation in the active site has the right oxidation state before the hydrocarbon is adsorbed, and that it is effectively reoxidized afterwards. Accordingly, correlations are often found between activity, selectivity and the concentration of cations in specific oxidation states, e.g. V4+ in V z 0 5 . The improvement of selective catalytic qualities of metal oxides by addition of modifiers, or by combination in mixed oxides may hence be explained by stabilization of the essential cation in the proper oxidation state. In some cases, stabilization of a (partially) reduced cation appears to yield the most effective catalyst; however, more often it is the higher oxidation state that should be maintained, and accordingly the role attributed t o a second cation often concerns facilitating the reoxidation, for instance, by direct electron transfer between the cations or, in general, by increasing electron conductivity. Techniques that enable the observation of specific valencies of cations include E.S.R., y-resonance spectroscopy and ESCA, and have been considerably improved in the recent years. Bismuth molybdate (Bi/Mo = 0.7), MOO, and BiO, were investigated by Sancier et al. [275] who carried out ESR spectroscopy and conductivity measurements simultaneously. Reduction, reoxidation and steady state conditions were examined a t 325-380°C using propene and air. The kinetics of initial reduction and oxidation were treated in Sect. 2.2.2. As a measure of the conductivity, AV, the change in crystal voltage, was taken. Figure 1 2 demonstrates the relation between the degree of reduction with AV, on the one hand, and with the ESR signal strength for Mo5+ on the other. The highest degree of reduction in the steady state was observed for the bismuth molybdate sample. The reduction level depended o n the C3H6/02 molar ratio as illustrated in Fig. 13. It is remarkable that the signal strength of MoS+levels off in bismuth molybdate *. The authors explain this by supposing that “reduction by propene results in oxygen vacancies that form an impurity band and cause the conductivity t o increase. However, the concentration of Mo5+will reach a constant value because ionization of the vacancy levels t o the conduction band (to form Mo5+species) is limited by the pinned Fermi energy”. Various molybdates (Bi-Mo-O, Al-Mo-, Sn-Mo-, Fe-Mo-0) and a Vz0,-K2S04 catalyst were investigated by Maksimovskaya [ 1971 by ESR measurements. The Mo5+and V4+signals appeared t o increase by reduction with butene and t o disappear by successive reoxidation. The correlation between the ESR signal and the degree of reduction is good for

* Similar effects were reported earlier by Peacock et al. [250,251].

245

/

Bi /M0=0.7

.

7! BI/MO

=

o7

BI/Mo = 6 40 BipOq

20

Time ( s e c )

Fig. 1 2 . Dependence of ESR signal strength and change of crystal voltage o n degree of reduction of bismuth molyhdate catalysts during propene oxidation. , av;

-.

- . _ - -AI.

the vanadate catalyst, but less clear for the molybdates, in agreement with the results of Sancier et al. [ 2751. Iron moly bdates were investigated by several authors. I t is generally observed that iron is reduced first (Fe3+ Fez+),while deeper reduction is required to reduce the molybdenum ions as well. Both cations occur in partially reduced states during the reaction with butene. Pernicone [ 2541 concludes from his ESR work that under stationary reaction conditions the iron ions stay in the reduced state and that the redox process only involves Mo6+ and Mo". However, Trifiro and Pasquon [318]and Matsuura and Schuit [207] are of the opinion that reoxidation initially may lead to Fe3+which in turn (rapidly) oxidizes the Mo5+ ions at the hydrocarbon reaction sites of the catalyst. However, direct evidence is not provided. --f

246

-> E v

a,

m m

5 0

>

-

m +. Ul Ir L

U L

0 a, Is)

C

m

.c U

>‘ Q

t

-

C,H6/02

mole r a t i o

Fig. 13. Steady state value of crystal voltage as a function of C 3 H 6 / 0 2 mole ratio at constant flow (1 1 min-1 g cat-’) and constant P o 2 (0.12 atm).

An iron-modified Co-Mo--O catalyst was studied by Maksimov and Margolis [ 196,2031, using y-resonance spectroscopy. Replacing 3%of the cobalt in CoMo04 by Fe strongly increases the catalytic activity, while a hundredfold increase of the conductivity results. These effects are attributed to the occurrence of Fe”, the concentration of which doubles reversibly during reaction with propene air mixtures at 310-330°C. Strangely, the doubling is not observed with propene alone. The amount of M o 5 +on the surface of Mo-Ti-0 and Mo-Te-0 catalysts has been assessed with ESR techniques by Akimoto and Echigoya [13,15,17] and Andrushkevich et al. [27]. These workers find a strong correlation between the maximum intensity of the Mo5+signal with maximum activity in the oxidation of propene t o acrolein (at 8 at. % Te) and conversion of butadiene t o maleic anhydride (75 at. % Ti). Antimonates were also the subject of ESR investigations, and it was combinations only shown by Suzdalev et al. [310] that in Sn-Sb-0 reduction of Sb5+ takes place. The initial value of the Sb5+/Sb3+ratio These last two is 1 for S b z 0 4 , 2.3 for Sn-Sb-0 and 4.3 for Fe-Sb-. values are strongly reduced by chemisorption of acrolein. The Sb3+spectrum shows a change after formation of a complex with the adsorbate.

247 Margolis [203] confirms such results for antimonates and reports the existence of a surface compound containing Sb3+--O-C. Aykan and Sleight [34] examined the system U-Sb-0 in air up t o 1000°C by different techniques (e.g. ESR) and found the ternary components USb05 and USb3OI0. Since U S b 0 3 is paramagnetic, the formal oxidation state of U must be 5+, hence Sb must also be in the 5+ state. The authors conclude that USb3OI0 also contains pentavalent uranium. Finally, it may be noted that, although variable valency in binary oxides is important, it is not a sufficient requirement, as can be concluded from the fact that even in the systems Bi-Mo-0 and U-Sb-4 not every crystal phase is active and selective [294]. What matters is the configuration of the ions at the active site. Apparently, the character of the typical Me-0 bands is a function of the situation of oxygen in the lattice. 3.5 NATURE OF T H E ACTIVE SITES

It is generally accepted today that the oxidation activity of catalysts is not merely due t o the presence of a particular metal ion, but to the ensemble of metal and oxygen ions that forms the active site. The reactive properties of individual sites, where the interaction with molecules t o be oxidized takes place, and the determination of their geometry is the greatest challenge in catalytic research. With regard t o the geometry, a classical difficulty is the fact that the surface structure may differ considerably from the bulk. Only if surface and bulk structure are closely related may it be expected that specific crystal phases are responsible for active and selective oxidation. Otherwise these properties cannot be attributed t o a specific lattice structure. Although a discussion of the nature of active sites should, in fact, include all aspects of catalysis, attention will be focussed here on two aspects which receive considerable attention in the literature: the acidity or electron affinity of surface sites and the possible participation of different sites in one oxidation process, i.e. the bifunctional action of a catalyst. Some remarks must be made about the role of oxygen coordination. Several authors have remarked that the coordination in catalytic oxides is of major importance. Mitchell and Trifiro (e.g. ref. 219) concluded that a bismuth molybdate catalyst is most active if the amount of tetrahedrally coordinated molybdenum is large in comparison with octahedrally coordinated molybdenum. However, V , 0 5 and SbzO, are structures with specific octahedral coordination [ 1421 and often the coordination is changed by reduction of the catalyst or by the support [203]. In a - and 0-cobalt molybdates the coordination differs, but the catalytic behaviour is really the same. The low temperature Bi2Mo06(y phase) has an octahedral coordination but is an effective catalyst. It can be concluded from these and other investigations that the oxygen Rcfrrences P P . 253-262

248 coordination in the bulk is not a principal factor. It may very well be, however, that the type of coordination at the surface is important. Unfortunately, hardly any data are available. It may be expected that extension of electron spectroscopic techniques will throw light on this problem.

3.5.1 Acid -b use p ropert ies The interaction between selective metal oxides and molecules t o be oxidized is, of course, based on electron-accepting and electron-donating properties, respectively. In this way, Mo6+, V5+,etc. act as electron acceptors and molecules with 7r-bonds as donors. Ai et al. [5-121 have drawn attention t o the fact that this can also be described by acid-base properties. An electron donor molecule like butene is a basic entity interacting with acidic sites on the catalyst. Hence it follows that activity and selectivity depend on the relative acidity and basicity. MOO,, for example, is an acidic oxide, while Bi203is a basic oxide. Different compositions Bi: Mo have different acidities. The rate of oxidation depends on the number of acid sites (=acidity) and the acid strength, viz.

R

a

acidity X f (acid strength)

The same applies t o the rate of isomerization. The Ai and Suzuki [ 5,9] investigated the combination V,O,-P,O,. acidity was measured indirectly by the activity for dehydration of isopropanol and was shown t o decrease with increasing P 2 0 5 content. The activity for the oxidation of butene-1 and butadiene t o maleic acid anhydride decreased accordingly. It was shown that the adsorption equilibrium constant of the olefin on the catalyst also decreased in the same way. Ai [6,10,11] also reports work on Sn0,-based catalysts, i.e. Sn0,MOO,, Sn02-P20, and Sn0,-V,O,. SnO,, as such, does not have an acidic character but MOO, and V,Os change this effectively (more than P,O,). At 30-60 at. 3' 6 Mo, the acidity is highest and activity for isomerization and selective oxidation are a maximum. With tin vanadates, the selectivity for the formation of butadiene goes through a maximum at an atomic ratio Sn/V = 9. Below this ratio, the acidity is greater, leading t o more maleic acid anhydride in the reaction products. Butadiene will adsorb more with increasing acidity and will have a greater opportunity to be oxidized. The resulting acid anhydride will desorb relatively easily from an acid catalyst. A basic catalyst will result in more combustion products. Combinations of Bi203 and MOO,, promoted by P,O, at a constant P/Mo ratio (0.2) were studied over a full composition range by Ai and Ikawa [6]. Acidity (and basicity) were measured directly by adsorption of compounds like ammonia, pyridine and acetic acid. The effect of the Bi/Mo ratio on the acidity (Fig. 14) parallels the effect on the overall butene oxidation activity [presented in Fig. 5, Sect. 2.3.2(a)(i)].

249

,-. 0 c

-

X

0

-

E v

c

o_

+-

Cl L

0, D m

mK 0

E E Q

t

--

Fig. 1 4 . Acidity of 0.2).

I

I

0.2

0.4

I

0.6

I

0.8

, 3

BI

Atomic ratio ___ B I + Mo

Bi203-Mo03-P205

as a function of bismuth content (P/Mo =

With respect to the reaction products, the catalysts can be classified into three groups. The first group is very acidic in nature (Bi/Mo = 0 - 0 . 3 ) and converts olefins t o acidic products (e.g. butene t o maleic anhydride), the second group has medium acidity (Bi/Mo = 0.5-3) and provides the optimal conditions for the dehydrogenation of butene t o butadiene, while the third group (Bi/Mo > 3), which has a basic character, only forms combustion products. Pernicone et al. [ 253,2541 bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol t o formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ions. Ai finally notes that, with regard t o bismuth molybdates, such acid References p p . 253-262

250 sites can very well be equivalent t o the B-centers of Matsuura (p. 240) and the M o ( O ~ sites ) ~ of Trifiro (p. 240). Basic sites are then probably the oxidizing sites, equivalent t o Matsuura's A-centers. 3.5.2 Bifunctionality

Bifunctionality means that sites with different functions are present on the surface of a catalyst. In this general sense, two types of bifunctionality in hydrocarbon oxidation catalysis can be discerned.

(a) Bifunctionality connected with the redox mechanism Strong indications are present for some mixed oxide catalysts that the interaction with the molecule to be oxidized and the oxygen that reoxidizes the catalyst take place on different sites and involve different cations. These two sites may together form one ensemble that performs the complete reaction. However, they may also be actually separated and quite remote, provided that the transport of anions and the conduction of charge between such sites is sufficiently large. Bismuth molybdate-based catalysts are well known examples for which these conditions apply. Unfortunately, there is no agreement as to which function must be connected with either cation. For scheelite-type bismuth molybdates, Linn and Sleight [188] have advanced the theory that Bi cations with their free electron doublets at the surface are the favoured centres for reoxidation, while propene oxidation takes place at the Mo-0 tetrahedra. Schuit [ 2811 also assumes that, for bismuth molybdates, oxygen is introduced at the bismuth sites, but his mechanism is more complicated, as both Bi and Mo interact with the hydrocarbon substrate. Direct evidence to the contrary, i.e. oxygen introduction in the molybdenum layers of bismuth molybdate, has been provided by Otsubo et al. [ 2461. They proved that catalyst reduction by hydrogen and reoxidation with "02yields Bi203 MoI8O3, while hydrogen primarily consumes bismuth oxygen. For several other binary oxide catalysts, this type of bifunctionality is indicated by the fact that both cations are partially reduced under reaction conditions, as observed by ESCA and y resonance techniques. An example of a catalyst investigated is FeMo04 [ 751.

-

( b ) Bifunctionality related to different reaction steps Several authors have suggested that the allylic oxidation of olefins t o aldehydes requires a bifunctional catalyst. The two functions then concern the formation of an allylic radical and the coupling of such a radical with lattice oxygen. This idea is primarily based on the fact that several single oxides (e.g. Bi203, S n 0 2 , T1203)catalyze the formation of ally1

radicals, but lack the capability t o transfer oxygen; hence allyl dimers are formed. Molybdenum oxide, on the other hand, appears t o have a capacity t o oxidize allyl radicals t o acrolein, a capacity which largely exceeds that for the oxidation of propene [121]. Unfortunately, no other oxides have been investigated with respect t o their specific reactivity towards allyl radicals. The possible bifunctionality of bismuth molybdates is amply discussed in Sect. 2.2.2(d)(i). The fact that active and selective catalysts in general comprise two or more oxide components is certainly not a sufficient argument t o assume bifunctionality ; the combination of oxides may also cause modification of sites or formation of one type of new sites which combine the specific properties required for a sequence of reaction steps. Such properties may concern the geometry, the type of oxygen bonding, oxygen and charge mobility in the solid, acidity, etc. It can be concluded that the occurrence of a two-centre mechanism is not easily distinguished from that of a mechanism involving multi-function reaction sites, the more so as the separation of catalytic sites and of reaction steps is a practical difficulty. An example of real bifunctionality appears t o be the case of acrylic acid formation, because two reaction steps which can be individually studied, are involved, i.e. the formation of acrolein, in which lattice oxygen is incorporated, and the aldehyde t o acid conversion, which involves water as the oxygen source. The most effective catalysts are multi-component catalysts, which very likely possess different sites, probably on different catalyst phases (see Sect. 2.3.3). 3.6. ADSORPTION AND REACTION COMPLEXES ON T H E CATALYTIC SURFACE

In the foregoing discussion, the emphasis has been mainly on the properties of the catalyst, but it is evident that these must be regarded in close connection with the nature of the adsorbed hydrocarbons. Important information about this interaction can be gained from structure analysis of adsorption and reaction complexes, as well as adsorption measurements, Infrared spectra of propene and isobutene on different catalysts were measured by Gorokhovatskii [ 1431. Copper oxide, which converts olefins to butadiene and aldehydes, shows adsorption complexes different from structures on a V z 0 5 - P 2 0 5 catalyst which produces maleic acid anhydride. Differences also exist between selective oxidation catalysts and total oxidation catalysts. The latter show carbonate and formate bands, in contrast to selective oxides for which 7r-allylic species are indicated. A difficulty in this type of work is that only a few data are available under catalytic conditions; most of them refer t o a pre-catalysis situation. Therefore it is not certain that complexes observed are relevant for the catalytic action. References p p . 2 53-262

252

Sachtler [270] notes that the n-ally1 complex can be attached t o a metal ion or to an oxygen anion but doubts that a n-ally1 metal complex can be stable at the high temperatures normally used. He draws attention t o the fact that, in the case of aromatic oxidations, benzoates, maleinates, etc. are observed spectroscopically, indicating that a carbon-metal bond is not formed. 0 / Trifiro et al. [322], however, did not find (R-C=O)- groups in an investigation of Sn02-V205 catalyst by infrared spectroscopy. The spectra reveal the presence of MOO, on the surface. If propene is adsorbed, the Mo=O band of the oxide is influenced. The Mo=O band disappears when acrolein is adsorbed (at room temperature). Desorption at 225°C restores this peak. Electron spectroscopic studies were carried out by Haber et al. [150] on CoMo04, MOO, and Mooz at -200 t o +5OO0C. It was demonstrated that, during interaction with acrolein, a change in the spectra can be observed, indicating the change of a vinylic carbon atom t o a paraffinic one. Simultaneously, carbonyl peaks change into carboxyl. The conclusion is 0 //

that acrolein becomes bonded by a C

group. With desorption, decarboxy-

\

0 lation occurs leaving the hydrocarbon part at the surface. Studies with propionic acid showed that reduction of the surface favours decarboxylation of the acid molecules which does not occur on oxidized CoMo04. Haber suggests that the relation between active sites of different types is associated with the nature of the active complex (ref. in Butt [67]). The final products can be classified in the following manner. Active center

Active complex

Product

Cations, Bi3', Co2+, Sn4+ 0 2 -in polyhedra of Mo, W, Sb, Nb OH- of basic character OH- of acidic character

n-allylic complex

Dienes

o-bonded allylic species Carboxylate type of complex Carbonium ion

Unsaturated aldehydes and ketones Unsaturated acids

_ ___.

Saturated ketones

~ ~ _ _ _ _ _ _ _

Trifiro and Carra [ 3231 used the amount of c i s t r u n s isomerization and double bond shift as a method of investigating the type of intermediate. It is concluded that three groups of oxides exist. The first is a group of which Bi-Mo-O, Bi2W06 and Fe-Te-Mo-O are typical. These catalysts give only isomerization a t temperatures at which selective oxidation also occurs, probably via the same intermediate (allylic). A second group gives isomerization at much lower temperatures (Sn-Sb-0, Fe-Sb-0, for

253 example) and it is suggested that acidic Lewis centres are the cause of double bond isomerization. A third group (Co, Mn, Fe molybdates) carries the oxidation further and probably contains Bronstedt acid centers, which would act via carbocations according t o the authors. Finally, a classification of catalysts by Matsuura [212] may be mentioned, in which the relation of adsorption entropy t o heat of adsorption of butene-1 appears, surprisingly, t o be linear. The conclusion can be drawn that moderate heats of adsorption (about 40-50 kcal mol-’) characterize suitable catalysts. Only here is the right combination of surface mobility and adsorption intensity found. Apparently, the oxygen is then “tempered” sufficiently t o make a selective oxidation possible. Otherwise, the oxides are non-active (e.g. low heat of adsorption in FeP04 and low mobility) or active but non-selective because of high mobility coupled to a large heat of adsorption (e.g. Fe,04).

References 1 Abd. El-Salaam, E. Echigoya and M. Akimoto, Z. Phys. Chem. (Frankfurt am Main), 96 (1975) 311. 2 C.R. Adams, Proc. 3rd Int. Congr. Catalysis, Amsterdam, 1964, p. 240. 3 S.I. Ahmad, S.H. Ibrahim and N.R. Kuloor, Indian J. Technol., 8 (1970) 131. 4 S.I. Ahmad, S.H. Ibrahim and N.R. Kuloor, Indian J. Technol., 9 (1971) 251. 5 M. Ai and S. Suzuki, Bull. Chem. SOC.Jpn., 47 (1974) 3074. 6 M. Ai and T . Ikawa, J. Catal., 40 (1975) 203. 7 M. Ai, Bull. Chem. SOC.Jpn., 44 (1971) 761. 8 M. Ai and S. Suzuki, Bull. Jpn. Pet. Inst., 1 6 (1974) 118. 9 M. Ai, Bull. Chem. SOC.Jpn., 4 3 (1970) 3490. 10 M. Ai, J. Catal., 40 (1975) 318. 11 M. Ai, J. Catal., 40 (1975) 327. 12 M. Ai and S. Suzuki, J. Catal., 26 (1972) 202. 1 3 M. Akimoto and E. Echigoya, J. Catal., 31 (1973) 278. 14 M. Akimoto and E. Echigoya, Bull. Chem. SOC.Jpn., 48 (1975) 3518. 15 M. Akimoto and E. Echigoya, Bull. Jpn. Pet. Inst., 1 6 (1974) 8. 16 M. Akimoto and E. Echigoya, J . Catal., 35 (1974) 278. 1 7 M. Akimoto and E. Echigoya, J . Catal., 29 (1973) 191. 18 F. Alfani and 5.3. Carberry, Chim. Ind. (Milan), 52 (1970) 1192. 19 T.G. Alkhazov. K.Y. Adzhamov and N.K. Allakhverdova, Kinet. Katal., 1 5 (1974) 1492. 20 T.G. Alkhazov and K.Y. Adzhamov, Kinet Katal., 1 5 (1974) 201. 21 A.D. Ammosov and L.A. Sazonov, Kinet. Katal., 16 (1975) 1241. 22 A.D. Ammosov and L.A. Sazonov, React. Kinet. Catal. Lett., 3 (1975) 255. 23 A.D. Ammosov, L.A. Sazonov and S.A. Venyaminov, Kinet. Katal., 1 2 (1971) 608. 24 A.D. Ammosov and L.A. Sazonov, React. Kinet, Catal. Lett., 2 (1975) 403. 25 E.I. Andreikov and N.D. Rusyanova, Kinet. Katal., 10 (1969) 722. 26 E.I. Andreikov, Y.A. Sveshnikova and N.D. Rusyanova, Kinet. Katal., 1 6 (1975) 919. 27 T.V. Andrushkevich, G.K. Boreskov, L.L. Kuznetosova, L.M. Plyasova, Y.N. Tyurin and Y.M. Shchekochikhin, Kinet. Katal., 1 5 (1974) 424.

28 Anonymous, Hydrocarbon Process, Petrochemical Handbook Issue, 54 (1975) 97. 29 A.G. Anshits, V.D. Sokolovskii, G.K. Boreskov, Y.D. Pankrat’ev, V.F. Malakhov and A.M. Kolchin, Kinet. Katal., 1 4 (1973) 1209. 30 A.G. Anshits, V.D. Sokolovskii and G.K. Boreskov, Kinet. Katal., 15 (1974) 730. 31 A.G. Anshits, V.D. Sokolovskii and G.K. Boreskov, Kinet. Katal., 1 5 (1974) 1033. 32 A. Ayame, H. Kano, T . Kanazuka and H. Baba, Bull. Jpn. Pet. Inst., 1 5 (1973) 150. 33 K. Aykan, D. Halvorson, A.W. Sleight and D.B. Rogers, J. Catal., 35 (1974) 401. 34 K. Aykan and A.W. Sleight, d. Amer. Ceram. SOC.,53 (1970) 427. 35 Y.M. Bakshi, R.N. Gur’yanova and A.I. Gel’bshtein, Kinet Katal., 16 (1975) 443. 36 Y.M. Bakshi, N.K. Danilova, R.N. Gur’yanova and A.I. Gel’bshtein, Neftekhimiya, 11 (1971) 810, 817. 37 S . Balasubramanian and D.S. Viswanath, Ind. Eng. Chem. Fundam., 14 (1975) 158. 38 G.B. Barannik, S.A. Ven’yaminov, G.K. Boreskov and H. Wolf, React. Kinet. Catal. Lett., 2 (1975) 191. 39 G.B. Barannik and S.A. Yen’yaminov, React. Kinet. Catal. Lett., 4 (1976) 437. 40 Ph.A. Batist, P.C.M. van der Heijden and G.C.A. Schuit, J. Catal., 22 (1971) 411. 41 Ph.A. Batist, J.F.H. Bouwens and G.C.A. Schuit, J. Catal., 25 (1972) 1. 42 Ph.A. Batist, J.F.H. Bouwens and I . Matsuura, J. Catal., 32 (1974) 362. 43 Ph.A. Batist, H.J. Prette and G.C.A. Schuit, J. Catal., 1 5 (1969) 267. 44 Ph.A. Batist, L.G.M. van de Moesdijk, I. Matsuura and G.C.A. Schuit, J . Catal., 20 (1971) 40. 45 Ph.A. Batist and S.P. Lankhuijzen, J. Catal., 28 (1973) 496. 46 H. Baussart, R . Delobel, G. Francois and J.M Leroy, C.R. Acad. Sci. Ser. C, 281 (1975) 375. 47 J. Beres, K. Brueckman, J. Haber and J . Janas, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 20 (1972) 813. 48 S.K. Bhattacharyya, N.K. Nag and N.D. Ganguly, J. Catal., 2 3 (1971) 158. 49 A. Bielanski and A. Inglot, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 22 (1974) 785. 50 A. Bielanski and A. Inglot, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 22 (1974) 773. 51 M. Blanchard and G. Louguet, Kinet. Katal., 1 4 (1973) 30. 52 M. Blanchard and D. Vanhove, Bull. SOC.Chim. Fr., (1971) 4134. 5 3 M. Blanchard, G. Louguet, G.K. Boreskov, V.F. Muzykantov and G.I. Panov, Bull. S O C . Chim. Fr., (1971) 814. 54 G. Blasse, J . Inorg. Nucl. Chem., 28 (1966) 1124. 55 I.F. Boag. D.W. Bacon and J. Downie, J. Catal., 38 (1975) 375. 56 M.A.M. Boersma, Thesis, Eindhoven University of Technology, 1977. 57 G.K. Boreskov, A.A. Ivanov, O.M. Iluyich and V.G. Ponomareva, React. Kinet. Catal. Lett., 3 (1975) 1. 58 G.K. Boreskov, Kinet. Katal., 14 (1973) 7. 59 P. Boutry, R. Montarnal and J . Wrzyszcz, J. Catal., 1 3 (1969) 75. 60 A.M. Bradshaw, A. Engelhardt and D. Menzel, Ber Bunsenges. Phys. Chem., 76 (1972) 500. 61 K. Brueckman, J . Haber and J. Janas, Bull. Acad. Pol. Sci. Ser. Sci. Chim., 21 (1973) 763. 62 J . Buiten, J . Catal., 10 (1968) 188. 63 J. Buiten, J. Catal., 1 3 (1969) 373. 64 J. Buiten, J . Catal., 27 (1972) 232. 65 L. Burlamacchi, G. Martini and F . Trifiro, J. Catal., 33 (1974) 1.

66 L. Burlamacchi, G. Martini, F. Trifiro and G. Caputo, J. Chem. SOC., Faraday Trans. I, 7 1 (1975) 209. 67 J.B. Butt, AIChE J., 22 (1976) 1. 68 P.V. Butt and C.N. Kenney, Proc. 6th Int. Congr. Catal., The Chemical Society, London, 1977, p . 779. 69 P.H. Calderbank, Adv. Chem. Ser., 133 (1974) 646. 70 J.L. Callahan, R.K. Grasselli, E.C. Milberger and H.A. Strecker, Ind. Eng. Chem. Prod. Res. Dev., 9 (1970) 134. 7 1 W.E. Campbell, E.L. McDaniel, W.H. Reece, J.E. Williams and H.S. Young, Ind. Eng. Chem. Prod. Res. Dev., 9 (1970) 525. 72 N.W. Cant and W.K. Hall, J. Catal., 1 6 (1970) 220. 73 N.W. Cant and W.K. Hall, J . Catal., 22 (1971) 310. 7 4 J.J. Carberry, G.C. Kuczinski and E. Martinez, J. Catal., 26 (1972) 247. 75 M. Carbucicchio and F. Trifiro, J. Catal., 4 5 (1976) 77. 76 W.R. Cares and J.W. Hightower, J. Catal., 23 (1971) 193. 7 7 W.R. Cares and J.W. Hightower, J. Catal., 39 (1975) 36. 7 8 J. Cartlidge, L. McGrath and S.H. Wilson, Trans. Inst. Chem. Eng., 52 (1974) 222. 79 J . Cartlidge, L. McGrath and S.H. Wilson, Trans. Inst. Chem. Eng., 5 3 (1975)117. 80 A. Castellan, A. Vaghi, J.C.J. Bart and N. Giordano, J. Catal., 39 (1975) 213. 81 M. Cathala and J.E. Germain, Bull. SOC.Chim. Fr., (1970) 4114. 82 M. Cathala and J.E. Germain, Bull. SOC.Chim. Fr., (1971) 2174. 8 3 M. Cathala and J.E. Germain, Bull. SOC.Chim. Fr., (1971) 2167. 84 P. Centola, C. Mazzocchia, G. Terzaghi and I. Pasquon, Chim. Ind. (Milan), 54 (1972) 859. 85 M. Cesari, G. Perego, A. Zazzetta, G. Manari and B. Notari, J. Inorg. Nucl. Chem., 33 (1971) 3595. 86 J.R. Christie, D. Taylor and C.C. McCain, J. Chem. SOC., Faraday Trans. I , 72 (1976) 334. 87 R.B. Clarkson and A.C. Cirillo, J. Vac. Sci. Technol., 9 (1972) 1073. 8 8 R.B. Clarkson and A.C. Cirillo, J. Catal., 33 (1974) 392. 89 D.J. Cole, C.F. Cullis and D.J. Hucknall, J. Chem. SOC.,Faraday Trans. I, 72 (1976) 2185. 90 D. Cormack, D.H. Thomas and R.L. Moss, J. Catal., 32 (1974) 492. 9 1 A. Cortes and J.L. Seoane, J. Catal., 34 (1974) 7. 92 M. Crozat and J.E. Germain, Bull. SOC.Chim. Fr., (1972) 3526. 9 3 M. Crozat and J.E. Germain, Bull. SOC.Chirn. Fr., (1973) 2498. 94 C. Daniel and G.W. Keulks, J. Catal., 29 (1973) 475. 95 C. Daniel and G.W. Keulks, J. Catal., 24 (1972) 529. 96 C. Daniel, J.R. Monnier and G.W. Keulks, J. Catal., 3 1 (1973) 360. 97 A.A. Davydov and A.A. Budneva, Kinet. Katal., 15 (1974) 1557. 98 J. Dechelette and J.E. Germain, Bull. SOC.Chim. Fr., (1976) 874. 99 J.C. Delgrange and M. Blanchard, Bull. SOC.Chirn. Fr., (1971) 1093. 100 M. Dente, E. Ranzi, D.D. Quiroga and G. Biardi, Chim. Ind. (Milan), 55 (1973) 563. 101 J.K. Dixon and J.E. Longfield, in P.H. Emmett (Ed.), Catalysis, Vol. VII, Reinhold, New York, 1960, p. 183. 102 Z. Dolejsek and J. Novakova, J. Catal., 37 (1975) 540. 103 T. Dozono, D.W. Thomas and H. Wise, J. Chem. SOC., Faraday Trans. I, (1973) 620. 104 A.F. van den Elzen and G.D. Rieck, Acta Crystollogr. Sect. B, 29 (1973) 2433, 2436. LO5 L.Y. Erman, E.L. Galperin, I.K. Kolchin, G.F. Dobrzhanskii and K.S. Chernyshev, Zh. Neorg. Khim., 9 (1964) 2174.

106 107 108 109 110

A.B. Evnin, J.A. Rabo and P.H. Kasai, J . Catal., 30 (1973) 109. G. Fagherazzi and N. Pernicone, J. Catal., 1 5 6 (1970) 321. V. Fattore, Z.A. Fuhrman, G. Manara and B. Notari, J. Catal., 37 (1975) 215. V. Fattore, Z.A. Fuhrman, G. Manara and B. Notari, J . Catal., 37 (1975) 223. E.V. Fedevich, V.M. Zhiznevskii, M.V. Nikipanchuk, L.F. Yakubovskaya and I.M. Golub, Kinet. Katal., 1 5 (1974) 1288. 111 F. Figueras, M. Forissier, L. de Mourgues, J.L. Portefaiz and M. Rosa-Brussin, Kinet. Katal., 1 4 (1973) 25. 112 A.A. Kirsova, N.N. Khovanskaya, A.D. Tsyganov, I.P. Susdalev and L.Y. Margolis, Kinet. Katal., 1 2 (1971) 792. 113 A.A. Kirsova, I.P. Suzdalev and L.Y. Margolis, Kinet. Katal., 14 (1973) 1469. 114 E.L. Force and A.T. Bell, J . Catal., 38 (1975) 440. 115 E.L. Force and A.T. Bell, J . Catal., 44 (1976) 175. 116 E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356. 117 J. Forys and B. Grzybowska, Bull. Acad. Pol. Sci. Ser. Sci. Chim., 23 (1975) 269. 118 P. Forzatti, A. Klimasara, G.C. Kuczynski and J.J. Carberry, J . Catal., 29 (1973) 169. 119 K. Fujimoto and T. Kunugi, Proc. 5th Int. Congr. Catal. Miami Beach, 1972, North-Holland, Amsterdam, 1973, Vol. 1 , p. 445. 120 K. Fujimoto, H. Takeda and T. Kunugi, Ind. Eng. Chem. Prod. Res. Dev., 1 3 (1974) 237. 121 E.G. Gamid-Zade, A.R. Kuliyev, E.A. Mamedov, R.G. Rizayev and V.D. Sokolovski, React. Kinet. Catal. Lett., 3 (1975) 191. 122 1.1. Geiman, D.R. Kreile and V.A. Slavinskaya, Kinet. Katal., 1 6 (1975) 957. 123 A.I. Gel’bshtein, Y.A. Mishchenko, P.P. Nechiporuk and N.D. Gol’dshtein, Dokl. Akad. Nauk SSSR, 217 (1974) 377. 124 A.I. Gel’bshtein, S.S. Stroeva, A.N. Parfenov and T.V. Zinoveva, Kinet. Katal., 17 (1976) 412 (Engl. Transl. p. 357). 1 2 5 S.J. Gentry, R. Rudham and M.K. Sanders, J. Catal., 35 (1974) 376. 126 H.R. Gerberich, N.W. Cant and W.K. Hall, J. Catal., 1 6 (1970) 204. 127 J.E. Germain and J.C. Peuch, Bull. SOC.Chim. Fr., (1969) 1844. 128 J.E. Germain and R. Perez, Bull. SOC.Chim. Fr., (1975) 739. 129 J.E. Germain and R. Laugier, Bull. SOC.Chim. Fr., (1971) 541. 130 J.E. Germain and R. Laugier, Bull. SOC.Chim. Fr., (1971) 650. 131 J.E. Germain and R. Laugier, C.R. Acad. Sci., Ser. C, 276 (1973) 1349. 132 J.E. Germain and R. Laugier, C.R. Acad. Sci., Ser. C, 276 (1973) 371. 1 3 3 J.E. Germain and R. Laugier, Bull. SOC.Chim. Fr., (1972) 2910. 134 J.E. Germain, Intra-Sci. Chem. Rep., 6 (1972) 101. 1 3 5 J.E. Germain and R. Perez, Bull. SOC.Chim. Fr., (1975) 1216. 1 3 6 K. German, B. Grzybowska and J. Haber, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 21 (1973) 319. 137 E. Ghenassia and J.E. Germain, Bull. SOC.Chim. Fr., (1975) 731. 138 M.A. Gibson and J.W. Hightower, J. Catal., 4 1 (1976) 420. 139 M.A. Gibson and J.W. Hightower, J. Catal., 4 1 (1976) 431. 140 N. Giordano and J.C.J. Bart, J. Roy. Neth. Chem. SOC.,94 (1975) 28. 141 N. Giordano, A. Vaghi, J.C.J. Bart and A. Castellan, J. Catal., 38 (1975) 11. 142 G.W. Godin, C.C. McCain and E.A. Porter, Proc. 4 t h Int. Congr. Catal., Moscow, 1968, Akademiai Kiad6, Budapest, 1971, p. 271. 1 4 3 Y.B. Gorokhovatskii, Kinet. Katal., 1 4 (1973) 83. 144 A.P. Gorshkov, G.V. Isagulyants, Y.I. Derventesev, L.Y. Margolis and I.K. Kolchin, Dokl. Akad. Nauk SSSR, 1 8 6 (1969) 827. 145 R. Grabowski, B. Grzybowska, J. Haber and J. Sloczynski, React. Kinet. Catal. Lett., 2 (1975) 81.

146 R.K. Grasselli and D.D. Suresh, J. Catal., 25 (1972) 273. 1 4 7 B. Grzybowska, K. German, J . Haber, T. Kowalski and A. Mazurkiewicz, Proc. Symp. Mechan. Hydrocarbon React. Hungary, 1973, p. 265. 148 B. Grzybowska, J . Haber, W. Marczewski and L. Ungier, J . Catal., 42 (1976) 327. 149 B. Grzybowska, J . Haber and J . Komorek, J . Catal., 25 (1972) 25. 150 J . Haber, W. Marczewski, J. Stoch and L. Ungier, Proc, 6 t h Int. Congr. Catal., The Chemical Society, London, 1977, p. 827. 151 J. Haber, in P.C.H. Mitchell (Ed.), Proceedings of t h e Conference o n the Chemical Uses of Molybdenum, Climax Molybdenum Co. Ltd., London, (1973, p. 146. 152 J . Haber and B. Grzybowska, J. Catal., 28 (1973) 489. 153 B. Halpern and J.E. Germain, J. Catal., 37 (1975) 44. 154 J . Happel, R.E. Lief and R. Mezaki, J . Catal., 31 (1973) 118. 155 J.L. Herce, J . Besombes-Vailhe and R. Bugarel, Kinet. Katal., 14 (1973) 90. 156 W. Herzog, Ber. Bunsenges. Phys. Chem., 74 (1970) 216. 157 C.N. Hodges and L.C. Roselaar, J . Appl. Chem. Biotechnol., 25 (1975) 609. 158 L.L. Holbrook and H. Wise, J. Catal., 20 (1971) 367. 159 L.L. Holbrook and H. Wise, J. Catal., 27 (1972) 322. 160 D.J. Hucknall, Selective Oxidation of Hydrocarbons, Academic Press, New York, 1974. 161 N.I. Ilchenko and G.I. Golodets, J. Catal., 39 (1975) 57, 73. 162 N.I. Ilchenko and G.I. Golodets, Kinet. Katal., 1 5 (1974) 394. 163 N.I. Ilchenko, I.M. Avilova and G.I. Golodets, Kinet. Katal., 16 (1975) 679. 164 Y.V. Ionov, L.K. Zhidkova, N.V. Kulkova and M.I. Temkin, Kinet. Katal., 14 (1973) 642. 165 I.S. Jaswal, R.F. Mann, J.A. Juusola and J. Downie, Can. J. Chem. Eng., 47 (1969) 284. 166 A. Joseph, R.L. Mednick, L.M. Shorr, S.W. Weller and P. Rona, Isr. J. Chem., 12 (1974) 739. 167 B. Kadlec, J. Michalek and A. Simecek, Chem. Eng. Sci., 25 (2970) 319. 168 K. Kaneko, T. Hoshino and S. Wada, Bull. Jpn. Petrol. Inst., 1 6 (1974) 24. 169 K. Kaneko, T. Koyama and S. Wada, Bull. Jpn. Petrol. Inst., 1 6 (1974) 17. 170 A. Kato, K. Tomoda, I. Mochida and T. Seiyama, Bull. Chem. SOC. Jpn., 45 (1972) 690. 1 7 1 V.B. Kazanskii, Kinet. Katal., 1 4 (1973) 95. 172 K. Keizer, Ph.A. Batist and G.C.A. Schuit, J. Catal., 15 (1969) 256. 173 R.E. Kenson and M. Lapkin, J . Phys. Chem., 74 (1970) 1493. 174 G.W. Keulks, J . Catal., 19 (1970) 232. 175 G.W. Keulks and L.D. Krenzke, Proc. 6 t h Int. Congr. Catal., The Chemical Society, London, 1977, p. 806. 176 P.A.Kilty, N.C. Rol and W.M.H. Sachtler, Proc. 5th Int. Congr. Catal. Miami Beach, 1972, North-Holland, Amsterdam, 1973, p. 929. 1 7 7 P.A. Kilty and W.M.H. Sachtler, Catal. Rev., 1 0 (1974) 1. 178 P.D. Klugherz and P. Harriott, AIChE J., 17 (1971) 856. 179 N.A. Komarovskii, A.L. Tsailingol’d, M.E. Basner, F.S. Philipenko and M.G. Slin’ko, Kinet. Katal., 1 4 (1973) 1483. 180 L.M. Koval and G.K. Boreskov, Kinet. Katal., 1 5 (1974) 1193. 181 R. Krabetz, Chem. Ing. Tech., 46 (1974) 1029. 182 G. Kremenic and P. Perez Torio, Acta Cient. Venez., 34 (1973) 204. 183 O.V. Krylov, Kinet. Katal., 14 (1973) 35. 184 O.V. Krylov, Proc. 5th Int. Congr. Catal., Miami Beach, 1972, North-Holland, Amsterdam, 1973, p. 263. 185 R. Lakshmanan and D. Rouleau, Can. J. Chem. Eng., 47 (1969) 45. 186 S.P. Lankhuijzen, P.M. Florack and H.S. van der Baan, J . Catal., 42 (1976) 20. 187 G. Liberti, N . Pernicone and S. Soattini, J. Catal., 27 (1972) 52.

188 W.J. Linn and A.W. Sleight. J. Catal., 4 1 (1976) 134. 189 A.E. Lisovskii, T.G. Alkhazov, A.M. Dadasheva and S.A. Feizullaeva, Kinet. Katal., 1 6 (1975) 456. 190 K.D. Lodaya, A.H. Tijoriwala and M.R. Chivate, Indian J. Technol., 14 (1976) 36. 1 9 1 M. Lojacono, T. Notermann and G.W. Keulks, J. Catal., 40 (1975) 19. 192 A.G. Lyubarskii, A.G. Gorelik, V.P. Petoyan, E.V. Lyapin and V.S. Beskov, Kinet. Katal., 1 4 (1973) 410. 193 J. Machek and J. Tichy, Coll. Czech. Chem. Commun., 40 (1975) 3774. 194 J.M. Maissant, D. Vanhove and M. Blanchard, Bull. SOC.Chim. Fr., (1973) 2787. 195 Y.V. Maksimov, I.P. Suzdalev, V.I. Gol’danskii, O.V. Krylov, L.Y. Margolis and A.E. Nechitailo, Dokl. Akad. Nauk SSSR, 211 (1975) 880. 196 Y.V. Maksimov and L.Y. Margolis, Chem. Phys. Lett., 34 (1975) 172. 197 R.I. Maksimovskaya, S.A. Ven’yaminov and G.K. Boreskov, Dokl. Akad. Nauk SSSR, 211 (1973) 1389. 198 V.V. Malakhov and F.G. Abdikova, Kinet. Katal., 1 3 (1972) 196. 199 R. Malinski, M. Akimoto and E. Echigoya, J. Catal., 44 (1976) 101. 200 R.S. Mann and K.C. Yao, Ind. Eng. Chem. Prod. Res. Dev., 10 (1971) 25. 201 R.S. Mann, K.C. Yao and M.K. Desi, J . Appl. Chem. Biotechnol., 22 (1972) 915. 202 R.S. Mann and D.W. KO, J. Catal., 30 (1973) 276. 203 L.Y. Margolis, Catal. Rev., 8 (1973) 241. 204 P. Mars and D.W. van Krevelen, Chem. Eng. Sci., 3 (1954) 4 1 (spec. suppl.). 205 F.E. Massoth and D.A. Scarpiello, J. Catal., 2 1 (1971) 294. 206 B.C. Mathur and D.S. Viswanath, J. Catal., 32 (1974) 1. 207 I. Matsuura and G.C.A. Schuit, J. Catal., 20 (1971) 19. 208 I. Matsuura and G.C.A. Schuit, J. Catal., 25 (1972) 314. 209 I. Matsuura, J. Catal., 3 3 (1974) 420. 210 I. Matsuura, J. Catal., 35 (1974) 452. 211 I. Matsuura and M. Wolfs, J. Catal., 37 (1975) 174. 21 2 I. Matsuura, Proc. 6 t h Int. Congr. Catal., The Chemical Society, London, 1977, p. 819. 213 C. Mazzocchia, P. Centola, R. Del Rosso, G. Terzaghi and I. Pasquon, Chim. Ind. (Milan), 55 (1973) 687. 214 C.C. McCain, G. Gough and G.W. Godin, Nature (London), 198 (1963) 989. 215 E.A. Melnik, L.P. Shapovalova. I.Y. Tyuryaev and V.M. Belousov, Kinet. Katal., 1 4 (1973) 381. 216 P.L. Metcalf and P. Harriott, Ind. Eng. Chem. Proc. Res. Dev., 11 (1972) 478. 217 R. Mezaki and B. Kadlec, J. Catal., 25 (1972) 454. 218 J. Mikami, S. Satok and H. Kobayashi, J. Catal., 1 8 (1970) 265. 219 P.C.H. Mitchell and F. Trifiro, J. Chem. SOC.A, (1970) 3183. 2 20 H. Miura, Y. Morikawa and T. Shirasaki, J. Catal., 39 (1975) 22. 221 K. Miyake, H. Oka, H. Sakamoto, Y. Harano and T , Imoto, J. Chem. SOC.Jpn., (1972) 284. 222 Y. Morita, M. Miyazaki and E . Kikuchi, Bull. Jpn. Pet. Inst., 17 (1975) 71. 223 Y. Moro-oka and A. Ozaki, J. Catal., 5 (1966) 116. 224 Y. Moro-oka, Y. Morikawa and A. Ozaki, J. Catal., 7 (1967) 23. 225 Y. Moro-oka, S. Tan and A. Ozaki, J. Catal., 1 2 (1968) 291. 226 Y. Moro-oka, S. Tan, Y. Takita and A.Ozaki, Bull. Chem. SOC. Jpn., 4 1 (1968) 2820. 227 Y. Moro-oka, S. Tan and A. Ozaki, J. Catal., 17 (1970) 125. 2 28 Y. Moro-oka, Y. Takita and A. Ozaki, J. Catal., 23 (1971) 183. 2 29 Y. Moro-oka, Y. Takita and A. Ozaki, Bull. Chem. SOC.Jpn., 44 (1971) 293. 230 Y. Moro-oka, Y. Takita and A. Ozaki, J. Catal., 27 (1972) 177.

231 Y. Moroitka and Y. Takita, Proc. 5th Int. Congr. Catal., Miami Beach, 1972, North-Holland, Amsterdam, 1973, p. 1025. 232 S.R. Morrison, J. Catal., 20 (1971) 110. 233 S.R. Morrison, J. Catal., 34 (1974) 462. 234 W.J. Muizebelt and W.J. van Ooij, J. Catal., in press. 235 M. Niwa and Y. Murakami, J. Catal., 26 (1972) 359. 236 M. Niwa and Y. Murakami, J. Catal., 27 (1972) 26. 237 T. Notermann, G.W. Keulks, A. Skliarov, Y. Maximov, L.Y. Margolis and 0 . V . Krylov, J . Catal., 39 (1975) 286. 238 J. Novakova and P. Jiru, J. Catal., 27 (1972) 155. 239 J. Novakova, Z. Dolejsek and K. Habersberger, React. Kinet. Catal. Lett., 4 (1976) 389. 240 J. Novakova, Catal. Rev., 4 (1970) 77. 241 E.C. Novella, F.C. Rubio, J.L. Sotelo and R.A. Pedreira, An. Quim., 69 (1973) 1187. 242 H. Oka, K. Miyake, Y. Harano and T. Imoto, J. Appl. Chem. Biotechnol., 25 (1975) 663. 243 A. Omar, G. Djega-Mariadassou, F. Bozon-Verduraz and G. Pannetier, Bull. SOC. Chim. Fr., (1974) 2740. 244 I.Z. Onsan and D.L. Trimm, J. Catal., 38 (1875) 257. 245 W.J. van Ooij and W.J. Muizebelt, J. Catal., in press. 246 T. Otsubo, H. Miura, Y. Morikawa and T. Shirasaki, J. Catal., 36 (1975) 240. 247 G.S. Pant and M. Chanda, Can. J. Chem. Eng., 54 (1976) 305.. 248 N.S. Parera and D.L. Trimm, J . Catal., 30 (1973) 485. 249 G. Parravano, Catal. Rev., 4 (1970) 53. 250 J.M. Peacock, M.J. Sharp, A.J. Parker, P.G. Ashmore and J.A. Hockey, J:Catal., 1 5 (1969) 379. 251 J.M. Peacock, A.J. Parker, P.G. Ashmore and J.A. Hockey, J. Catal., 1 5 (1969) 387. 252 P. Pendleton and D. Taylor, J. Chem. SOC.,Faraday Trans. I , 72 (1976) 1114. 253 N. Pernicone, G. Liberti and L. Ersini, Roc. 4th Int. Congr. Catal., MOSCOW, 1968, Akademiai Kiad6, Budapest, 1971, p. 287. 254 N. Pernicone, J. Less-Common Met., 36 (1974) 289. 255 H. Pichler, H. Schulz and A.Z. El Deen, Erdoel Kohle Erdgas Petrochem., 29 (1976) 418. 256 J.L. Portefaix, M. Forissier, C. Pralus, F . Figueras and P. d e Montgolfier, C.R. Acad. Sci., Ser. C, 282 (1976) 523. 257 C. Pralus, F. Figueras and L. d e Mourgues, C.R. Acad. Sci., Ser. C, 27 (1970) 20. 258 G. Prauser, H. Kiemer and K. Dialer, Ber. Bunsenges. Phys. Chem., 8 0 (1976) 49. 259 P. Putanov, D. Smiljanec, B. Djukanovic, N. Jovanovic and R . Herak, Proc. 5th Int. Congr. Catal., Miami Beach, 1972, North-Holland, Amsterdam, 1973, P1061. 260 Y.I. Pyatnitskii and G.I. Golodets, React. Kinet. Catal. Lett., 2 (1975) 143. 261 S.K. Ray and M. Chanda, Ind. Eng. Chem. Process Des. Dev., 1 5 (1976) 234. 262 K.A. Reddy and L.K. Doraiswamy, Chem. Eng. Sci., 24 (1969) 1415. 263 R.J. Rennard, R.A. Innes and H.E. Swift, J. Catal., 30 (1973) 128. 264 R.J. Rennard and W.L. Kehl, J. Catal., 21 (1971) 282. 265 R.G. Rizaev, V.E. Sheinin and S.D. Mekhtiev, Kinet. Katal., 1 3 (1972) 1496. 266 D.A. Robb and P. Harriott, J. Catal., 35 (1974) 176. 267 J.Y. Robin, Y. Arnaud, J. Guidot and J.E. Germain, C.R. Acad. Sci. Ser. C, 280 (1975) 921. 268 E.V. Rozhkova, S.V. Gerei and Y.B. Gorokhovatskii, Kinet. Katal., 1 5 (1974) 694. 269 G. Russo, S. Crescitelli and P. Ciambelli, Chim. Ind. (Milan), 55 (1973) 634.

270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306

W.M.H. Sachtler, Catal. Rev., 4 ( 1 9 7 0 ) 27. T. Sakamoto, N. Egashira and T . Seiyama, J. Catal., 16 ( 1 9 7 0 ) 407. F. Sala and F. Trifiro, J . Catal., 4 1 ( 1 9 7 6 ) 1. F. Sala and F. Trifiro, Z. Phys. Chem. (Frankfurt a m Main), 9 5 ( 1 9 7 5 ) 279. F. Sala and F. Trifiro, Z. Catal., 3 4 ( 1 9 7 4 ) 68. K.M. Sancier, A. Aoshima and H. Wise, J. Catal., 3 4 ( 1 9 7 4 ) 257. K.M. Sancier, P.R. Wentrcek and H. Wise, J. Catal., 39 ( 1 9 7 5 ) 1 4 1 . N. Sato and M. Seo, J. Catal., 2 4 (1972) 224. N.N. Sazonova, S.A. Ven’yaminov and G.K. Boreskov, Kinet Katal., 15 ( 1 9 7 4 ) 419. N.N. Sazonova, S.A. Ven’yaminov and G.K. Boreskov, Kinet. Katal., 1 6 ( 1 9 7 5 ) 1518. N.N. Sazonova, S.A. Ven’yaminov and G.K. Boreskov, Kinet Katal., 11 ( 1 9 7 3 ) 1169. G.C.A. Schuit, J . Less-Common Met., 3 6 ( 1 9 7 4 ) 329. I.W. Schulz and J. Scheve, Proc. Symp. Mechanism Hydrocarbon React. Hungary, 1 9 7 3 , p. 283. T. Seiyama, M. Egashira, T. Sakamoto and I. Aso, J. Catal., 24 ( 1 9 7 1 ) 76. T. Seiyama, T. Uda, I. Mochida and M. Egashira, J . Catal., 3 4 ( 1 9 7 1 ) 29. T. Seiyama, M. Egashira and T. Sakamoto, J. Catal., 2 4 ( 1 9 7 1 ) 7 6 . T. Seiyama, N. Yamazoe and M. Egashira, Proc. 5 t h Int. Congr. Catal., Miami Beach, 1 9 7 2 , North-Holland, Amsterdam, 1 9 7 3 , p. 997. J.L. Seoane, P. Boutry and R. Montarnal, C.R. Acad. Sci., Ser. C., 278 ( 1 9 7 4 ) 565. V.P. Shchukin, S.A. Ven’yaminov and G.K. Boreskov, Kinet. Katal., 1 2 ( 1 9 7 1 ) 621. V.P. Shchukin, G.K. Boreskov, S.A. Ven’yaminov and D.V. Tarasova, Kinet. Katal., 11 ( 1 9 7 0 ) 153. V.P. Shchukin, S.A. Ven’yaminov and G.K. Boreskov, Kinet. Katal., 11 ( 1 9 7 0 ) 1236. N. Shimizu, K. Shimokoshi and I. Yasumori, Bull. Chem. SOC.Jpn., 4 6 ( 1 9 7 3 ) 2929. Y.M. Shulga, I.N. Ivleva, M.V. Shimanskaya, L.Y. Margolis and Y.G. Borodko, Russ. J . Phys. Chem., 4 9 ( 1 9 7 5 ) 2976. G. Simon and J.E. Germain, Bull. Soc. Chim. Fr., ( 1 9 7 5 ) 2617. Th.G.J. Simons, P.N. Houtman and G.C.A. Schuit, J. Catal., 2 3 ( 1 9 7 1 ) 1 . T.G. Skorbilina, Y.I. Pyatnitskii and G.I. Golodets, Kinet. Katal., 1 6 ( 1 9 7 5 ) 1207. V.A. Slavinskaya, D.R. Kreile, D.Y. Eglite and 1.1. Geiman, Kinet. Icatal., 13 ( 1 9 7 2 ) 1 4 8 2 (Engl. trans]. p. 1 3 2 1 ) . A.W. Sleight and W.J. Lonn, Ann. N.Y. Acad. Sci., 272 ( 1 9 7 6 ) 22. A.W. Sleight, K. Aykan and D.B. Rogers, J. Solid State Chem., 13 ( 1 9 7 5 ) 231. F. Solymosi and F. Rozso, Proc. 6th Int. Congr. Catal., The Chemical Society, London, 1 9 7 7 , p. 365. H.T. Spath and K.D. Handel, Adv. Chem. Ser., 1 3 3 ( 1 9 7 4 ) 395. B. Sramek and J. Tichy, Collect. Czech. Chem. Commun., 4 0 ( 1 9 7 5 ) 3500. J.G. Steenhof de Jong, C.H.E. Guffens and H.S. van der Baan, J. Catal., 3 1 (1973) 149. J.G. Steenhof de Jong, C.H.E. Guffens and H.S. van der Baan, J . Catal., 26 (1972) 401. J.G. Steenhof de Jong, Thesis, Eindhoven University of Technology, 1 9 7 2 . G. Stefani, Chim. Ind. (Milan), 5 4 (1972) 984. J.S. Sterrett and H.G. McIlvried, Ind. Eng. Chem. Process Des. Dev., 1 3 ( 1 9 7 1 ) 54.

307 P. Subramanian and M.S. Murthy, Chem. Eng. Sci., 29 (1974) 25. 308 P. Subramanian and M.S. Murthy, Ind. Eng. Chem. Porcess Des. Dev., 1 3 (1974) 112. 309 P. Subramanian and M.S. Murthy, Ind. Eng. Chem. Process Des. Dev., 11 (1972) 242. 310 I.P. Suzdalev, A.A. Firsova, A.U. Aleksandrov, L.Y. Margolis and D.A. Raltrunas, Dokl. Akad. Nauk SSSR, 204 (1972) 408. 311 H.E. Swift, J.E. Bozik and J.A. Ondrey, J. Catal., 2 1 (1971) 212. 312 Y. Takita, A. Ozaki and Y. Moro-oka, J. Catal., 27 (1972) 185. 313 S. Tan, Y. Moro-oka and A. Ozaki, J . Catal., 17 (1970) 132. 314 S. Tanaka and T. Yamashina, J. Catal., 40 (1975) 140. 315 M.I. Temkin and N.V. Kulkova, Kinet. Katal., 16 (1975) 1211. 316 J . Tichy and B. Sramek, Collect. Czech. Chem. Commun., 39 (1974) 2796 317 F. Trifiro, G. Caputo and P.L. Villa, J. Less-Common Met., 36 (1974) 305. 318 F. Trifiro and I. Pasquon, Chim. Ind. (Milan), 5 3 (1971) 577. 319 F. Trifiro, Chim. Ind. (Milan), 5 6 (1974) 835. 320 F. Trifiro, G. Caputo and P. Forzatti, Ind. Eng. Chem. Prod. Res. Dev., 1 4 (1975) 22. 321 F. Trifiro, Chim. Ind. (Milan), 56 (1974) 541. 322 F. Trifiro, L. Kubelkowa and I. Pasquon, J . Catal., 1 9 (1970) 121. 323 F. Trifiro and S. Carra, React. Kinet. Catal. Lett., 2 (1975) 411. 324 F. Trifiro and I. Pasquon, Chim. Ind. (Milan), 52 (1970) 228. 325 F. Trifiro, and C. Lambri and I. Pasquon, Chim. Ind. (Milan), 5 3 (1971) 339. 326 F. Trifiro, Ann. Chim. (Rome), 65 (1975) 113. 327 F. Trifiro, H. Hoser and R.D. Scarle, J . Catal., 25 (1972) 12. 328 D.L. Trimm and L.A. Doerr, J. Catal., 23 (1971) 49. 329 D.L. Trimm and D.S. Gabbay, Trans. Faraday Soc., 67 (1971) 2782. 330 D.L. Trimm and M. Irshad, J. Catal., 18 (1970) 142. 331 A. Vaghi, A. Castellan, J.C.J. Bart, N. Giordano and V. Ragaini, J. Catal., 42 (1976) 381. 332 K. Vaidyanathan and L.K. Doraiswamy, Chem. Eng. Sci., 23 (1968) 537. 333 M. ValdeliGvre, H. Baussart, R. Delobel and J.M. Leroy, Bull. SOC. Chim. Fr., (1975) 2467. 334 D. Vanhove and M. Blanchard, Bull. SOC.Chim. Fr., (1971) 3291. 335 D. Vanhove and M. Blanchard, J . Chim. Phys., Phys. Chim. Biol., 7 3 (1976) 51. 336 D. Vanhove and M. Blanchard, J. Catal., 36 (1975) 6. 337 S. Veleva and F. Trifiro, React. Kinet. Catal. Lett., 4 (1976) 19. 338 S.A. Ven’yaminov, N.N. Sazonova and N.I. Alferova, Kinet. Katal., 1 5 (1974) 1500. 339 A. Verma and S. Kaliaguine, J . Catal., 30 (1973) 430. 340 P.L. Villa, G. Caputo, F. Sala and F. Trifiro, J. Catal., 3 1 (1973) 200. 341 O.M. Vinogradova, G.F. Vytnov, I.V. Luiksaar and O.V. Al’tshuler, Kinet. Katal., 1 6 (1975) 671. 342 B. Viswanathan, V . Ramalingam and M.V.C. Sastri, Indian J. Chem., 1 2 (1974) 205. 343 H.H. Voge and C.R. Adams, Adv. Catal., 1 7 (1967) 151. 344 T. Watanabe and E . Echigoya, Bull. J p n . Pet. Inst., 1 4 (1972) 100. 345 F. Weiss, J. Marion, J. Metzger and J.M. Cognion, Kinet. Katal., 1 4 (1973) 45. 346 S. Weychert and A. Urbaneck, Int. Chem. Eng., 9 (1969) 396. 347 K. van der Wiele, Thesis, Delft University of Technology, 1976. 348 K. van der Wiele and P.J. van den Berg, J. Catal., 39 (1975) 437. 349 W.B. Williamson, D.R. Flentge and J.H. Lonsford, J. Catal., 37 (1975) 258. 350 H. Wise, F’roc. Symp. Mechanism Hydrocarbon React., Hungary, 1973, p. 297. 351 M.W.J. Wolfs and Ph.A. Batist, J . Catal., 32 (1974) 25.

352 353 354 355 356 357 358 359 360 361 362

R.D. Wragg, P.G. Ashmore and J.A. Hockey, J. Catal., 22 (1971) 49. R.D. Wragg, P.G. Ashmore and J.A. Hockey, J. Catal., 28 (1973) 337. R.D. Wragg, P.G. Ashmore and J.A. Hockey, J. Catal., 3 1 (1973) 293. A.A. Yabrov, G.K. Boreskov, A.A. Ivanov and O.M. Il’inich, Kinet. Katal., 1 6 (1975) 1534. S. Yoshida, T. Matsuzaki, T. Kashiwazaki, K. Mori and K. Tarama, Bull. Chem. Soc. Jpn., 47 (1974) 1564. S. Yoshida, T. Matsuzaki, S. Ishida and K. Tarama, Proc. 5th Int. Congr. Catal., Miami Beach, 1972, North-Holland, Amsterdam, 1973, p. 1049. L. Zanderighi, M.P. Faedda and S. Carra, J. Catal., 35 (1974) 427. L. Zanderighi and S. Carra, Chim. Ind. (Milan), 56 (1974) 815. V.M. Zhiznevskii, E.V. Fedevich, D.K. ToIopko, M.V. Nikipanchuk, L.F. Yakubovskaya, I.M. Golub and V.Y. Shipailo, Kinet. Katal., 1 5 (1974) 381. V.M. Zhiznevskii, E.V. Fedevich, D.K. Tolopko and I.M. Sulima, Kinet. Katal., 1 2 (1971) 431. Handbook of Chemistry and Physics, The Chemical Rubber Co. Press, Cleveland, Ohio, 49th edn., 1968.

263 Chapter 3

Heterogeneous Eliminations, Additions and Substitutions L. BERANEK and M. KRAUS

1. General features 1.1CORRESPONDENCE BETWEEN HOMOGENEOUS AND HETEROGENEOUS

REACTIONS

Elimination, addition and substitution reactions over solid catalysts are treated together in this chapter on the basis of some common features of their mechanisms and the acid-base nature of the catalysts. They behave in such an analogous way t o liquid phase reactions, both catalysed and uncatalysed, that electron shifts solely in pairs (heterolytic) have never been seriously doubted and free radical-like (homolytic) mechanisms have been considered only by few authors. The discovery of parallelism between acid-base reactions in solution and over solids helped t o advance the understanding of reaction mechanisms in this branch of heterogeneous catalysis much more than, for example, in catalysis over metals. The theory of organic reactions has been developed mostly with the help of experimental material concerning substitution and elimination in the liquid phase and the accumulated knowledge and proven research methods were utilised in interpretation of transformations over catalysts with acidic and basic properties. The first step in this approach was the recognition [l--31 that the cracking reactions of hydrocarbons over strongly acidic silica-alumina catalysts have patterns similar t o the reactions in the liquid phase catalysed by strong mineral Brplnsted or Lewis acids for which the carbonium ion mechanism has been suggested [4]. It took some time t o adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due t o the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid-base properties of the solids and their meaning for catalysis (for a detailed treatment see ref.

5). The analogy between homogeneous and heterogeneous eliminations and substitutions has been pursued further. Joint action of an acidic and a basic site, suggested quite early for the heterogeneous dehydration of alcohols [ 6 ] ,has been gradually accepted as a general mode of operation

264 in acid-base catalysis over solids (e.g. refs. 7-9). No basic difference is now seen between the action of a surface acid-base double-centre and heterolysis of the bonds in an organic molecule caused by an attack of a base (or an acid) assisted by the solvent acting as a conjugated acid (or base) [ 91. Also, the nomenclatures for homogeneous elimination and substitution mechanisms have been adopted for heterogeneous reactions with only a slightly modified meaning. Of course, steric requirements are more restrictive in surface processes than in solution because the surface sites are immobile. On solid acid-base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 1 . 2 NATURE OF THE CATALYSTS

From the point of view of chemical composition, the solid acid-base catalysts are oxides (like alumina, silica, thoria, magnesia), mixed oxides (like silicaalumina, silica-magnesia), crystalline aluminosilicates (zeolites), metal salts and ion exchange resins. The last type differs from the others in the character of reactant transport into the catalyst grain. With organic ion exchangers, which may or may not possess pores in the dry state, the important component of the reactant penetration into the grain is the diffusion through the more or less swollen macromolecular mass; then, in a favourable case, almost all acidic and basic functional groups may serve as active centres. With inorganic solid catalysts, the reactants reach the internal surface of a porous catalyst grain by means of diffusion through the pores; the bulk of the solid is not utilised for catalysis. Therefore, for understanding the ways in which a catalyst influences a reactant, the surface chemistry of the inorganic solids is important. In spite of much effort, the nature of the active sites on acid-base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface; the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced t o a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described.

1.2.1 Silica The surface of silica (for detailed description of results see refs. 5, 11 and 1 2 ) contains a variable amount of hydroxyl groups and adsorbed wa-

265 ter molecules. Even after heating t o 900°C in vacuum, it retains some OH groups (e.g. ref. 10). The absolute number of hydroxyl groups may differ from sample t o sample according t o the methods of preparation resulting in different participation of various crystal planes on the surface. Extensive research by means of IR spectroscopy and chemical reactions has shown (e.g. refs. 10-18) that two types of surface hydroxyl groups are present: single or free (A) and paired (B). Their relative proportions on partially dehydroxylated surfaces were estimated t o be 1 : 1 [14],1 : 2 [ 181, 1 : 9 [ 171. The nature of the paired hydroxyl groups is still a matter of discussion: vicinal ( B l ) and geminal (B2) structures are both possible. With vicincl OH groups, interhydroxyl hydrogen bonding is assumed H H I I 0 0 \ I

/St

/o\

0 0 0 I\ I \ I Si Si Si Si Si

0 0 0 I \ I\ Si Si Si B2

A

H I 0 I Si \ 0 \

H

H I 0 \

/si

0

I Si

B1

The surface of silica is highly reactive and hydroxyl groups exchange hydrogen for deuterium with DzO [ 14-16] but not with Dz. They can be replaced by C1 from Clz or CC14 [16] and they react with silanes and aluminium chloride [ 15,191. Surface alcoholates are formed when silica is contacted with primary or secondary alcohols [20] either by the reaction with hydroxyl groups \ \ -Si--OH + CH30H = S i - O - C H 3 + H,O l I

or by the rupture of a surface siloxane bond [21]

R f f r rc nccs u p . 38 5-3 98

266

A number of other substances react with surface hydroxyl groups forming surface compounds I221. However, for catalysis, the hydrogen bonding seems to be more important. With alcohols, the hydrogen bonds are formed in such a way that surface hydroxyl groups act as donors of hydrogen [ 231, viz. Si-O-H...O-R

I

H 1.2.2 Alumina

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and y-alumina but practical catalysts are seldom pure crystailographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08-0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [ 101. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. The surface hydroxyl groups and adsorbed water are important factors determining the surface properties of alumina (e.g. refs. 5, 11and 24). At present, we have at our disposal a model of the (100) plane which is probably exposed on the surface of spinel-type y-alumina (cf. ref. 24). The model is due to Peri [25] and is based on his detailed investigations by IR spectroscopy [ 261, by gravimetry [ 261, by ammonia adsorption [ 27 1 and by Monte Car10 modelling of surface dehydration [ 251. According t o Peri [ 2 5 ] , the (100) plane of alumina, fully hydrated at low temperatures, exposes a square lattice of OH groups [Fig. l ( a ) ] . If the dehydration were ideal, a regular surface of equally spaced 0 2 -ions would be formed [Fig. l(b)]. However, the splitting off of water molecules is a random process and, consequently, only two-thirds of the original OH groups can be removed without disturbing the original order. Further dehydration is possible only at the expense of some disorder. Ultimately, only isolated hydroxyl groups, which have no partner in the neighbourhood for the formation of water, remain on the surface. Five different types of these isolated hydroxyl groups can be distinguished according t o the number of neighbouring oxygen atoms in the surface layer (Fig. 2); their frequency depends on the degree of dehydration. The hydroxyl groups act as Br@nsted acidic sites and the exposed aluminium atoms in the second layer [Fig. l ( b ) ] as Lewis acidic sites. Rehydration of the surface changes the Lewis into Brqhsted sites. The Peri model of alumina also demonstrates that basic sites of various strength, consisting of oxygen atoms in various arrangements (isolated

267

0000 0000 000

(A)

Fig. 1. Ideal surface (100) plane of alumina after Peri [ 25 1. (A) T o p layer viewed perpendicualrly to the plane; (B) section through the three t o p layers. (a) Fully hydrated surface. ( b ) dehydroxylated surface. Open circles denote oxygen, filled circles hydroxyl, small black points aluminium,

0

8 00 (d)

0

00

o@o (e)

Fig. 2. Schematic representation of five different arrangements of oxygen atoms around the surface hydroxyl groups (filled circles) o n t h e (100) plane of alumina after Peri [25]. References p p . 385-398

268 atoms in the upper layer or two or three oxygen atoms on adjacent sites), are available on the surface. Experimental evidence for the presence of basic sites comes from adsorption of BF3 [ 281, titration with benzoic acid [29] and poisoning of the dehydration of alcohols over alumina by tetracyanoethylene [ 81 and by acetic acid [ 301. Different types of hydroxyl groups and oxygen atoms have different properties and the surface is therefore non-homogeneous. This heterogeneity manifests itself not only in the varying acid and base strengths of the sites but, and this might be more important for catalysis, in the frequency of suitably spaced pairs of acidic and basic sites. Strong evidence from mechanistic studies shows that such pairs are a prerequisite for the concerted elimination mechanism which predominates over alumina. The surface of alumina is highly reactive, not only t o water, ammonia or acetic acid, but also t o a number of other substances. Surface alcoholates are products of the interaction with alcohols [31] and carboxylate surface structures are formed from a fraction of adsorbed alcohol molecules [ 321. The action of hydrofluoric acid [ 33-35], as well as impregnation by BF3 [ 31,341, increases the acidity of alumina. 1.2.3 A lu m inosilica tes Aluminosilicates are the active components of amorphous silicaalumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica-alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according t o their strength have been found, e.g. by titration with t-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the Ho scale [ 381. The chemistry of silica-alumina catalysts has been reviewed several times (e.g. refs. 39-41) and the nature of acidic active sites has been discussed in numerous papers, very often from the point of view of whether Lewis or BrQlnsted sites are responsible for catalytic activity. The experimental methods for their separate determination are not very conclusive and in the actual catalytic process one type of centre may be converted t o another by the action of reagents, products or impurities. The experiments with various substances added t o the feed indicate (see the following sections dealing with individual reactions) that different types of reaction require sites of different strength. The great variety of Lewis and Brgnsted sites which may exist on the surface of silicaalumina has been demonstrated by Peri [42] on the basis of a simplified model of the reaction of AlCl, with a silica surface and subsequent hydrolysis. Peri has constructed eight different surface sites by combining possible groups on the surface of silica with possible aluminium ion structures; more arrange-

269 ments can probably be thought of. Some of the Peri sites are

0 Al’ I \ 0 0

‘si’

‘ A1 I \

0 0 \ I Si

do‘ lAf

I \ 0 0 I di si \ I 0

0 I

0 I si s i \ I 0

The original view, that in the reaction of silica with aluminium hydroxides a strong aluminosilicic acid, which possesses a dissociable proton (e.g. ref. 2), is formed has not been proved. H-aluminosilicates are unstable and spontaneously convert t o aluminium aluminosilicates [ 191. Crystalline aluminosilicates (zeolites, molecular sieves) catalyse a number of organic reactions [43] and the striking difference between them and amorphous silicaalumina is that they are active for cracking even in the form of Na’ of CaZ+salts [ 44,451 ; these cations are poisons for silicaalumina. However, metal salts of zeolites exhibit strong acidity [ 51. This acidity is of both the Lewis and Brq5nsted type and strong Lewis sites are converted t o Brgnsted by water [46]. The catalytic activity of zeolites depends on the nature of the cation but it seems (cf. refs. 47 and 48) that the active centres are not metal cations or hydroxyl groups attached t o such ions. As with amorphous silica-alumina, the active centres in zeolites are probably situated on the aluminosilicate surface. The function of metal cations is not clear; they might stabilise the structure and influence the degree of hydroxylation and hydration of the surface which are important factors for catalysis. In the section dealing with alumina and silica, the necessity of basic sites on the surface, which cooperate with acidic sites, has been stressed. Also, for both amorphous silica-alumina and zeolites, the simultaneous presence of acidic and basic sites has been proved and it has been suggested that OH groups act as amphiprotic centres according t o the nature of the adsorbed species [ 491. 1.2.4 Metal salts

Solid metal sulphates and phosphates also exhibit acid-base properties; their acid strength is lower than that of silicaalumina but they are stronger acids than some oxide catalysts [ 51. Correlation of activity with electronegativity of cations has been obtained for several reactions [ 9, 50,511.

270

1.2.5 Ion exchange resins Organic ion exchangers are macromolecular substances containing chemically welldefined acidic or basic functional groups, The macromolecular skeleton may be formed by polycondensation or, more frequently, by copolymerisation. The use of basic (anion) exchangers as catalysts (e.g. in aldol condensation) is rather rare; the main representatives of acidic sites in cation exchangers are sulphonic (-SO,H), phosphonic (-PO(OH),) and carboxylic (-COOH) groups. In the kinetic studies reported in this chapter, sulphonated styrenedivinylbenzene copolymers were used almost exclusively. They may be of two types: (i) non-porous (standard) ion exchangers whose grains do not possess internal porosity in the sense usual in catalysis, and (ii) porous (macroreticular) ion exchangers with artificially developed porous structure (pores of about 10-20 nm prevailing) and a large inner surface area. Ion exchangers can be used as catalysts both for liquid (standard ion exchangers are preferred) and vapour phase (macroreticular ion exchangers are more convenient) reactions. The main factors determining the catalytic activity of ion exchangers are: (i) the acid strength of the functional groups (sulphonated resins are much more active than the others), (ii) the concentration of functional groups in the protonated form (ion exchangers fully neutralised with cations are catalytically inactive) and (iii) the degree of crosslinking of the copolymer, i.e. the content of divinylbenznee (DVB). There is no doubt that the functional groups in ion exchangers are responsible for the catalytic activity. Although they are chemically defined, it is not clear in which form they participate in the catalytic reaction, since a certain amount of water is always present in the resin which cannot be removed easily. It has been proven by IR spectroscopy [52541 that in polystyrenesulphonic acid several hydrated forms of the --S03H groups may occur (mono-, di-, tri- and tetra-hydrates) and, in consequence, the mobility of the proton of a -S03H group and also the catalytic activity may change. Lower hydration states are also possible through hydrogen bonding of one water molecule to two or more sulphonic acid groups. Even fully dehydrated sulphonic groups may be hydrogen bridged, for example in the form

P-H***o o=\\

-S=O

ii 0.e.H-O

-

\\

The lack of information about relative activities of different forms and the unknown dependence of their relative concentrations on catalyst pretreatment and reaction conditions, and the influence of reactants, products (water) and solvents, introduce uncertainty into the interpretation of kinetic measurements.

271

It seems probable, in view of the idea presented in Sect. 1.1,that, in elimination, addition and substitution reactions over ion exchangers, also, two types of catalytic sites are involved, viz. acidic (protons of the functional groups) and basic, which are likely t o be represented by oxygen atoms of the functional groups. A typical property of ion exchange resins which distinguish them from inorganic catalysts is swelling; this is the more important factor the lower is the degree of crosslinking of the copolymer. Due t o swelling, a considerable amount of reactants, products and solvents can be retained (absorbed) by the resin and the functional groups inside the polymer mass may also be utilised for catalysis. Thus, the accessibility of the catalytically active groups can be facilitated, not only by a artificial porous structure (which increases only the number of the groups on the surface of the polymer mass), but also by swelling. In this situation, the rate of reactant transport (diffusion), not only through the pores (if their are present), but also through the more or less swollen polymer mass, may become important. If the rate of diffusion through the polymer is much larger than that of the chemical reaction, then, in the extreme case, all functional groups may be utilised for catalysing the reaction. In the opposite case, when the diffusion through the polymer mass is much slower than the chemical reaction, only the surface groups will act as catalytic sites. This latter was observed with highly crosslinked ion exchangers and large reactant molecules and the term “sieve effect” was used t o describe it. 1.2.6 The working surface

The surface structures outlined in the preceding sections have been determined under conditions very far from those of an actual catalytic reaction. At partial pressures of reactants used in flow reactors and in the steady state, the catalyst surface is very probably almost covered by starting substances and products. This is indicated by the type of kinetics found for various reactions (see following section); very often zero-order expressions or Langmuir-Hinshelwood type rate equations with high values of adsorption coefficients have been found. Some products of the catalytic reactions are of special interest in this connection. Water formed in elimination, esterification or condensation reactions is present in sufficient quantities to change almost all Lewis sites into Br@nsted sites. Much more fundamental changes can be caused by hydrogen halides produced in the decomposition of alkyl halides on oxides; it is well known that the catalytic activity of alumina can be enhanced by the action of hydrochloric or hydrofluoric acids. It is evident that the study of free surfaces and of surfaces covered only partially by various substances at temperatures much lower than those needed for a catalytic reaction to proceed can give only indirect inforReferences P P . 385-398

272 mation about possible states on working surfaces. Better evidence is obtained by observing the influence of substances added t o the feed which can interact with some surface sites a t reaction conditions. For example, in this way the importance of basic sites has been confirmed. Linear correlations of effects of reactant structure on rate and adsorptivity are also helpful and especially the interpretation of their slopes may yield valuable information (e.g. refs. 55 and 56). The transient-response technique, in which the changes in product composition after an abrupt stop or start of the feed flow are observed, is also promising. 1.3 TYPE OF KINETICS

The complex nature of heterogeneous catalytic reactions, which consist of a sequence of at least three steps (adsorption, surface reaction and desorption), the possible intervention of transport processes and the uncertainty about the actual state of the surface makes every attempt t o obtain a complete formal kinetic description without simplifying assumptions futile. In this situation, some authors prefer fully empirical equations of the type

r

=

kpi&

...

(1)

which bear no connection to the mechanism. With the exception of zero-, first- and second-order expressions, the interpretation of the constants h, a, b, ..., cannot be used as a basis for the elucidation of the laws governing catalytic reactions. However, simple kinetic models, especially of the Langmuir-Hinshelwood type, can serve with advantage for correlation of experimental data in spite of simplifying assumptions which are necessary for their derivation. Experience shows that heterogeneous acid-base catalysis is the very field where they fit best. Their most frequent general form

where Ki denotes the adsorption coefficient of the substance i, a, b, ..., = 1 o r $ and n = 1, 2, 3, .,., is well suited to the estimation of the competition of all substances present in the system for active centres. However, because the same equation may be obtained on the basis of various different assumptions (cf. ref. 57), its form cannot be used as a proof of a certain mechanism. Of the assumptions accepted for the generation of LangmuirHinshelwood type and related equations, the most controversial seems t o be that the surface is homogeneous. It has been shown in the preceding section that inorganic oxide catalysts and even ion exchangers contain a number of differing acidic and basic sites, i.e. they possess an inherent heterogeneity. The question is how this “static” non-homogeneity manifests itself

273 under the dynamic conditions of a catalytic reaction. Some sites may be ineffectual for steric reasons when they d o not find a basic (or acidic) partner site within a suitable distance. Out of the residual spectrum of sites differing in strength, some are probably too weak t o be able t o initiate bond reorganisation in adsorbed molecules. Other sites can bind the reactants or products too strongly and thus be blocked out. Working sites come, therefore, from a band which is narrower then the original one estimated on the basis of adsorption measurements (including determination of the number of acidic sites by titration with a base etc.). The position and width of this working band must depend on the chemical nature of the reagent (e.g. cracking of alkanes requires other sites than dehydration of alcohols) and on the form of the distribution curve of sites according t o their strength. Some experimental results are available which show the influence of surface heterogeneity on the kinetics and the contribution of sites of different strength t o the over-all rate. The surface of acidic catalysts has been divided into several fractions by acidimetric [ 581 or thermochemical [ 591 titrations and on the basis of group analysis [59]and partial poisoning [58,60] the contribution of these fractions has been calculated. It has been found that the over-all rate of dehydration is determined by the performance of a single narrow fraction, the contribution of the others being almost negligible. Another approach t o this problem involved modelling of acidic catalysts with different sites by mixing ion exchangers containing functional groups of different acidity [ 611. For dehydration; the over-all activity was again given by the activity of the strongest (-S03H) group. For re-esterification, the contribution of weaker centres (-PO(OH),) could not be neglected but the over-all kinetics could still be correlated by a single Langmuir-Hinshelwood rate equation. Summarising, it seems that the surface heterogeneity is not such a serious problem for the formal kinetic description of acid-base catalysis on solids as would be expected from the studies of the surface by non-kinetic methods. Moreover, the rate equations for non-homogeneous surfaces, developed by the Russian school (Temkin, Roginskii and Kiperman, see ref. 62) are similar t o eqn. (2); the term 1 is not present and n can have any value greater than 0 (cf. also ref. 63). Only their further drastic simplification leads to equations of type (1). The next problem of the LangmuirHinshelwood kinetics, the validity of the ratedetermining step approximation, has not been rigourously examined. However, as has been shown (e.g. refs. 57 and 63), the mathematical forms of the rate equations for the LangmuirHinshelwood model and for the steady-state models are very similar and sometimes indistinguishable. This makes the meaning of the constants in the denominators of the rate equations somewhat doubtful; in the Langmuir-Hinshelwood model, they stand for adsorption equilibrium constants and in the steady-state models, for rate coefficients or products and quotients of several rate coefficients. References P P . 385-398

274 The problem discussed in Sect. 1.2.6, i.e. what composition the working surface has, also has its kinetic counterpart. If the number of active sites of a certain type depends on the partial pressures of some reaction components, then the question arises whether rate equations of type (2) are sufficient for the description of such changes. All these facts and unsolved problems require that the rate equations of type (2) be taken as semi-empirical expressions. They may be directly utilised for engineering purposes with higher certainty than eqn. (l), but they reflect the actual react.ion mechanism only in general features. However, the constants are a good source of values for comparison of reactivities and adsorptivities of related reactants on the same catalyst. Such interpretations of experimental data are usually quite meaningful as is confirmed by successful correlations of the constants with other independent quantities. 2. Elimination reactions In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review t o the olefin-forming eliminations of the t Y Pe I I I I --(+?-C,=Cc =

X

+ HX

H

Although some observations (e.g. ref. 7) indicate that the process need not (Y, 0-(or 1 , 2-) elimination, practically all experimental results have been interpreted on the assumption that 1,3- and 1,4-eliminations d o not participate significantly. The substituents X may have very different structures but heterogeneous catalytic eliminations with X = halogen, OH, alkoxyl, NR2 (R = H or alkyl), SH, OCOCH3 and alkyl or aryl only have been described. The individual reactions are usually named according to the compound HX which is the product, i.e. dehydrohalogenation, dehydration etc. but some exceptions exist (e.g. cracking). The reverse reactions are additions t o the C--C multiple bonds which will be dealt with in Sect. 3 of this chapter. Homogeneous olefin-forming eliminations have been studied extensively, especially in the liquid phase and comprehensive treatments of the subject are available [ 64,651. The rules governing the course of homogeneous eliminations and their mechanisms are well established and the interpretation of the results obtained with heterogeneous catalytic sys-

to be always an

275 tems can obtain useful assistance from these. In this connection, a recent review on catalytic eliminations is especially valuable [9]. 2.1 COMMON FEATURES O F HETEROGENEOUS CATALYTIC ELIMINATIONS

2.1.1 Mechanism

In discussing the mechanism of eliminations over solids, the nomenclature which has been developed for homogeneous reactions will be used. Therefore the basic mechanisms of olefin formation have first to be outlined and their meaning in heterogeneous catalysis defined. The E2 mechanism is so called because the process is bimolecular and in solution consists of an attack by a base on the P-hydrogen atom with synchronous splitting of the substituent X in the form of an anion. In heterogeneous catalysis, the most important feature is the timing of the fission of the two bonds C,-X and CB-H: in the E2 or E2-like mechanism, these bonds are broken simultaneously. Because this can be achieved only by the action of two different centres, a basic one and an acidic one with both present on the sudace, the kinetic distinction of the mechanism loses its original sense under these circumstances. The E l mechanism has, as the ratedetermining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group

X. The ElcB mechanism is a two-step process beginning with the abstraction of a proton from the P-position by a base to give a carbanion. The second step is the loss of the group X as an anion. In heterogeneous catalysis, the corresponding mechanism consist of the primary action of a basic site assisted later by an acidic site which temporarily accomodates the group X-. It is evident that the simple model of heterogeneous catalytic eliminations assumes the same adsorption complex for all mechanisms, written schematically as

-c-c-

I I X. H.

. . 00 The only distinction between various mechanisms is the timing of the References p p . 385-398

276 fission of the bonds C,-X and C,-H. Usually, a continuous spectrum of mechanisms is assumed in which E l , E2 and ElcB are processes with a clearly defined character. This idea, which has been slowly developed during the last decade, has been discussed in detail in a recent review [ 91. It is certainly compatible with views on the nature of elimination catalysts. These solids are typically oxides o r metal salts which have positively and negatively charged atoms on their surface. In the array of electron-donating and electron-accepting centres, pairs of required acidic and basic sites with suitable spacings can be found. Because the strength of the sites is different on individual catalysts according t o their structure, the catalysts can be put into a sequence, from those where the basic character predominates through those where basic and acidic properties are in balance to those with prevailing acidic nature. It is d e a r that a catalyst wifl transform a reactant by means of the mechanism which corresponds t o the predominating acid o r base strength of the sites. It is well known from homogeneous reactions that the mechanism depends also on the strength of the C,-X and C,-H bonds and this applies also t o heterogeneous catalysis. The double influence on the “choice” of mechanism, i.e. of the nature of the catalyst and of the reagent, has been graphically represented by Mochida et al. [66] (Fig. 3). They have

-

-

Cd-H Bond strength Fig. 3 . Schematic representation of the influence of reactant structure, of catalyst nature and of temperature on the elimination mechanism. Numbers in parentheses denote the rate-determining steps on Scheme 1 .

Cp- H

277

*

xI

I

H .

A h

.

Br

H

cis

/c=c Br

\

trans

C6H,

The work with 1-bromo-2-chloroethane allowed the influence of the nature of the halogen on its reactivity to be observed as either vinyl bromide or vinyl chloride are formed. The ratio of the chloride t o the bromide in the products changed with the nature of the catalyst, being around 0.1 for sulphates of Ni, Co, Mn, Cu, Zn and for silica-alumina, 0.6 for alumina and 5 for KOH-Si02 [ 1791. (c) Direction o f elimination

A large amount of work has been devoted t o the problem of the rules governing the dehydrohalogenation of halogen derivatives which can form several olefins. Although some regularities may be observed, the general picture is clouded by the following facts. (a) The direction of elimination depends very strongly on the nature of the catalyst because on different catalyst types, different mechanisms operate. (b) The nature of a catalyst (and the mechanism) may be changed by the hydrogen halide which is produced by the reaction [ 1893. (c) The composition of the product (and the participation of different mechanisms) is temperaturedependent and, as various catalysts sometimes differ appreciably in activity, a comparison of selectivities under the same conditions is impossible in a broader series of catalysts. (d) The composition of the product may be changed by a secondary isomerisation of the olefins formed. The point (a) is demonstrated by the data in Table 11for 2-halobutanes which give three products: 1-butene, cis-2-butene and trans-2-butene. The differences in selectivities can be even larger than indicated by these data. The dehydrochlorination of l,l,Z-trichloroethane yields 1,2dichloroethylene (I) and trans- and cis-l,2-dichloroethylene(11). On silica-alumon alumina, 0.30 and on KOHina, the value of the ratio 1/11 was SiOz, 1 0 [66]. The data in Table 11show some facts, the mechanistic consequences of which will be discussed in Sect 2.4.4. The 2-alkene/l-alkene ratio for the catalytic reaction differs significantly from that for the homogeneous decomposition. On all catalysts, this ratio is higher for the 2-bromo- than for the 2-chloroderivative; therefore the orientation also depends on the nature of the halogen. On some catalysts, both ratios (the 241- and cis/ trans) are equal or approximately the same as the equilibrium values, but on other catalysts, significant differences appear. The influence of temperature on the ratio of the products of the dehydrochlorination of 2-chlorobutane is seen from Fig. 6 [ 1901. Other examples may be found in the literature [190,194,195]. Some ratios are almost temperature-independent while some show large changes. Moreover, the data from various sources differ sometimes appreciably (cf. refs. 190 and 914 for 2-chlorobutane and refs. 66 and 195 for 1,1,2-trichloroethane). This might be caused by secondary isomerisation on strongly acidic catalysts of the olefins first formed such a reaction was proved at References p p . 385-398

306 TABLE 11 Examples of product ratio obtained from 2-halobutanes on different catalysts Catalyst

Temp. (“C)

2-Chlorobutane MgS04 Bas04

2-Butene 1-Butene

cis-2-Butene trans-2-Butene

Ref.

Li2SO4 K2S04 CUO CaO None

200 240 24 0 270 345 200 150 350

6.7 8.1 6.1 5.7 8.1 2.8 9.0 1.5

0.58 0.43 0.69 2.15 1.07 1.8 2.0 0.53

190 190 190 190 190 19 1 191 192

2-Bromobutane MgS04 Bas04 A12 (so413 &SO4 KzS04 Si02 KO H-S i 02

200 180 210 230 300 150 160

11.5 10.1 8.1 6.7 9.0 5.3 3.2

0.53 0.58 0.62 2.0 1.04 1.00 0.94

190 190 190 190 190 193 193

Equilibrium composition [I 911 100 13.3 200 6.7 300 4.3 400 3.3

0.43 0.58 0.62 0.67

A12 (s04

)3

least for MgS04 [190]. The water content of metal salts as dehydrohalogenation catalysts also influences the selectivity [ 1961 and consequently the thermal history of the solids must play a role. Therefore, all selectivity data should be judged with caution, especially in cases where the over-all conversion of the haloalkane was high and the catalyst illdefined. Further information about the direction of dehydrohalogenation was obtained with threo- and erythro-2deutero-3-bromobutanes on Si02 and Si02-KOH catalysts [ 193,1951. The determination of deuterium content in the butenes formed allowed the estimation of the extent of the synand anti-eliminations. The values of the deuterium kinetic isotope effect showed that the C,-H (or C,-D) bond is split in the rate-determining step. Over KOH-Si02, the anti-elimination was preferred, but at 3OO0C, syn-elimination was the peferential reaction mode. With SiOz, synelimination was favoured under all conditions. Further stereochemical studies with di- or tri-haloalkanes corroborated the general picture of a strong dependence of the direction of elimination on the nature of the catalyst. The data in Table 1 2 may serve as an exam-

307 I

I

200

100

300 Temperature

400

("C)

Fig. 6. Dependence of the selectivity ratios of the dehydrochlorination of 2-chlorobutane on temperature for different catalysts [ 1901.

ple and additional results were reported for 2,3-dibromobutanes on alkalised silica [ 1981. Large changes of stereoselectivity with the catalyst nature were also demonstrated in the dehydrohalogenation of 1,1,2-trichloroethane t o cis- and trans-dichloroethylene [ 66,172,179,199,2001 and 1,2dihalopropanes [ 1901.

TABLE 12 Ratio of cis- to trans-2-chloro-2-butenes from the dehydrochlorination of meso- and dZ-2,3-dichlorobutanes on different catalysts [ 197 ] Catalyst

K2C03

BaC03 NiC03 LiCO3 CaC12 Ca3(P04)2

Temp.

cisltrans ratio from

("C)

meso

dl

250 330 250 330 350 220 220

11.5 49 6.7 3.0 0.90 0.28 0.20

0.075 0.53 0.23 0.67 0.90 0.30 0.16

References p p . 385-398

308 2.4.4 Mechanism

The hypothesis of a continuous transition of the elimination mechanism from the extreme E l through concerted E2 t o the other extreme E l c B with the change of reactant structure and catalyst nature, described in Sect. 2.1, can be easily adopted for dehalogenation also. The data summarised in Sect. 2.4.3 show some inconsistencies but the over-all picture is clear. This can be demonstrated for some selected examples. Typical basic catalysts are strontium oxide and alkalised silica (studied by the group of Mochida and Yoneda) o r potassium carbonate (studied by the Noller group). With SrO and NaOH-SO2, a large deuterium kinetic isotope effect was observed for 1,2-dibromoethane [ 1721, which shows that the C,-H bond is split in the ratedetermining step. The trans/cis olefin selectivities in the reaction of 1,2-dichloropropane and 1,1,2-trichloropropane [179,189] on SrO and alkalised silica give the greatest values indicating the high stereoselectivity of the elimination. The same results are obtained in experiments with meso- and dl-dihalobutanes on NaOHSiOl [ 1981 and K2C03 [ 1971. All this indicates an E2-type mechanism, very probably an E2cB mechanism in which the fission of the C,-X bond is slightly preceded by the fission of the C,-H bond. However, the structure of the reactant may shift the timing of the two steps of the elimination in the direction of the extreme E l c B mechanism. Evidence for this can be seen in the lower value of the deuterium kinetic isotope effect for the dehydrohalogenation of 1,1,2,2-tetrachloroethane(1.2) than of 1,2dibromoethane (1.6) on KOH-Si02 [66]. The cause of this transition is the increased acidity of hydrogen atoms in the compound with more halogen atoms. Typical acidic catalysts are silica-alumina, transition metal sulphates o r chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8, 11 and 12). These results correspond t o the E2cA or E l mechanisms, between which a transition may be observed due t o the influence of the structure of the reactants, i.e. according t o the polarity of the C,-X and CD-H bonds. Again, the reactions of 1,2dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica-alumina was 1.0 for the dibromo-derivative, which indicates a pure E l mechanism, whereas for the tetrachloroderivative, the value of 1.5 was found. Alumina is a catalyst which shows intermediate behaviour and over which the concerted E2 mechanism is accepted [66] with slight transition either t o the E2cA or E2cB mechanisms according t o the structure of the reactant. Salts of strong acids and bases also show similar intermediate properties. The concerted or partly concerted mechanisms require two-site adsorption and because the mechanisms are ionic, the active centre must consist of a pair of an acidic and a basic site. Metal salts fulfil this

309 condition; their relative activities should depend on factors like acid and base strength of the surface atoms, lattice spacing, concentration of suitable site pairs etc. Another important factor is the contamination of the surface by adsorbed species like catalytic poisons [ 1791, water [ 1961 or product hydrogen halide [ 1891. Therefore all studies which attempted to find correlation between some property of the catalysts and their activity ended with inconclusive results (cf. refs. 67, 172 and 194). The most reliable data concern dehydrochlorination on zeolites containing different monovalent and divalent cations. A good linear relationship was obtained between the activation energy of the first-order reaction and the electrostatic field of the cation [180] and between the cis/trans selectivity and acid strength [ 2001. Both correlations are in agreement with the proposed shifts in mechanism. The catalysts which operate by means of an E2 mechanism give a high proportion of reaction products which are formed by the anti-elimination. This fact has been discussed in Sect. 2.1 and only few remarks need t o be added here. Quantum chemical calculations [73] on the transition state of the dehydrochlorination of chloroethane, initiated by an attack of a basic species, confirmed the preference of the anti-elimination over the syn-mode. On the contrary, calculations on the transition state for non-catalytic (homogeneous) thermal elimination [ 201,2021 confirmed the syn-elimination path. 2.5 DEALKYLATION BY CRACKING

2.5.1 Types of cracking reactions

In the chemical and petroleum industries, the term cracking is used t o describe a chemically complex process in which the decomposition of larger hydrocarbon molecules into smaller fragments plays a dominant role but is accompanied by a number of other reactions (isomerisation, cyclisation, polymerisation, disproportionation etc.). In this section, under catalytic cracking, only the primary fission of a C--C bond, which yields an alkene and a fragment with a C-H bond in the place of the former C-C! bond

I

R-C-C-H

I

=

R-H

I I + C=C I I

(A)

will be considered. The group R must therefore be alkyl or aryl. Reaction (A) resembles other olefin-forming elimination, not only formally but also in all general features. Of course, there are some special characteristics of the cracking reaction which are due t o the nature of the eliminated groups, R. The general chemistry of the cracking over solid catalysts was studied Rcfcrenccs p p . 385-398

310 intensively in the short period during the forties which followed the introduction of the cracking process and of aluminosilicate catalysts into the production of petrol (for a review see ref. 203). Two types of cracking reactions are of interest for the present treatment: (i) dealkylation of alkylaromatic compounds and (ii) fission of a saturated hydrocarbon chain. The first reaction is characterised by the fission of the bond between an aromatic ring and an alkyl group, e.g. O ( f 4 - H

= O

I I H + C=C I I

(B)

The second type of cracking can take place at any C-C bond of a saturated hydrocarbon with the exception of the bonds t o the terminal CH3 groups, e.g. / CH3*HZ*H2*H3

+ CH2=CH2

CH3*H,*H2-CH,-CH,