Metal-Catalysed Reactions of Hydrocarbons FUNDAMENTAL AND APPLIED CATALYSIS Series Editors: M. V. Twigg Johnson Matthey Catalytic Systems Division Royston, Hertfordshire, United Kingdom M. S. Spencer Department of Chemistry Cardiff University Cardiff, United Kingdom CATALYST CHARACTERIZATION: Physical Techniques for Solid Materials Edited by Boris Imelik and Jacques C. Vedrine CATALYTIC AMMONIA SYNTHESIS: Fundamentals and Practice Edited by J. R. Jennings CHEMICAL KINETICS AND CATALYSIS R. A. van Santen and J. W. Niemantsverdriet DYNAMIC PROCESSES ON SOLID SURFACES Edited by Kenzi Tamaru ELEMENTARY PHYSICOCHEMICAL PROCESSES ON SOLID SURFACES V. P. Zhdanov HANDBOOK OF INDUSTRIAL CATALYSTS Lawrie Lloyd METAL-CATALYSED REACTIONS OF HYDROCARBONS Geoffrey C. Bond METAL–OXYGEN CLUSTERS: The Surface and Catalytic Properties of Heteropoly Oxometalates John B. Moffat SELECTIVE OXIDATION BY HETEROGENEOUS CATALYSIS Gabriele Centi, Fabrizio Cavani, and Ferrucio Trifir`o SURFACE CHEMISTRY AND CATALYSIS Edited by Albert F. Carley, Philip R. Davies, Graham J. Hutchings, and Michael S. Spencer A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher. Metal-Catalysed Reactions of Hydrocarbons Geoffrey C. Bond Emeritus Professor Brunel University Uxbridge, United Kingdom With 172 illustrations Geoffrey C. Bond 59 Nightingale Road Rickmansworth, WD3 7BU United Kingdom Library of Congress Cataloging-in-Publication Data Bond, G.C. (Geoffrey Colin) Metal-catalysed reactions of hydrocarbond/Geoffrey C. Bond. p. cm. — (Fundamental and applied catalysis) Includes bibliographical references and index. ISBN 0-387-24141-8 (acid-free paper) 1. Hydrocarbons. 2. Catalysis. 3. Metals—Surfaces. 4. Reaction mechanisms (Chemistry) I. Title. II. Series. QD305.H5B59 2005 547Ј.01—dc22 2004065818 ISBN-10: 0-387-24141-8 e-ISBN: 0-387-26111-7 Printed on acid-free paper. ISBN-13: 987-0387-24141-8 ᭧2005 Springer ScienceϩBusiness Media, Inc. All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer ScienceϩBusiness Media, Inc., 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed in the United States of America 987654321 springeronline.com P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 ACKNOWLEDGMENTS No work such as this can be contemplated without the promise of advice and assistance from one’s friends and colleagues, and I must first express my very deep sense of gratitude to Dr Martyn Twigg, who more than anyone else has been responsible for this book coming to completion. I am most grateful for his unfailing support and help in a variety of ways. I am also indebted to a number of my friends who have read and commented (sometimes extensively) on drafts of all fourteen chapters: they are Dr Eric Short, Professor Vladimir Ponec, Dr Adrian Taylor, Professor Norman Sheppard, Professor Zoltan Pa´al and Professor Peter Wells (who read no fewer than six of the chapters). Their advice has saved me from making a complete ass of myself on more than one occasion. As to the remaining errors, I must excuse myself in the words of Dr Samuel Johnson, who when accused by a lady of mis-defining a word in his dictionary gave as his reason: Ignorance, Madam; pure ignorance. One of the most pleasing aspects of my task has been the speed with which colleagues world-wide, some of whom I have never met, have responded promptly and fully to my queries about their work; Dr Andrzej Borodzi´nski and Professor Francisco Zaera deserve particular thanks for their extensive advice on respectively Chapters 9 and 4. Dr Eric Short has been especially helpful in teaching me some of the tricks that have made the use of my pc easier, and Mrs Wendy Smith has skillfully typed some of the more complex tables. Finally, I could not have completed this work without the patient and loving support of my wife Mary. v P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 PROLOGUE There must be a beginning of any good matter . . . SCOPE AND PURPOSE OF THE WORK It is important at the start to have a clear conception of what this book is about: I don’t want to raise false hopes or expectations. The science of heterogeneous catalysis is now so extensive that one person can only hope to write about a small part of it. I have tried to select a part of the field with which I am familiar, and which while significant in size is reasonably self-contained. Metal-catalysed reactions of hydrocarbons have been, and still are, central to my scientific work; they have provided a lifetime’s interest. Age cannot wither nor custom stale their infinite variety. Experience now extending over more than half a century enables me to see how the subject has developed, and how much more sophisticated is the language we now use to pose the same questions as those we asked when I started research in 1948. I can also remember papers that are becoming lost in the mists of time, and I shall refer to some of them, as they still have value. Age does not automatically disqualify scientific work; the earliest paper I cite is dated 1858. It is a complex field in which to work, and there are pitfalls for the unwary, into some of which I have fallen with the best. I shall therefore want to pass some value-judgements on published work, but in a general rather than a specific way. While there is little in the literature that is actually wrong, although some is, much is unsatisfactory, for reasons I shall try to explain later. I have always tried to adopt, and to foster in my students, a healthy scepticism of the written word, so that error may be recognised when met. Such error and confusion as there is arises partly from the complexity of the systems being studied, and the vii P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 viii PROLOGUE great number of variables, some uncontrolled and some even unrecognised, 1 that determine catalytic performance. Thus while in principle (as I have said before 2 ) all observations are valid within the context in which they are made, the degree of their validity is circumscribed by the care taken to define and describe that context. In this respect, heterogeneous catalysis differs from some other branches of physical chemistry, where fewer variables imply better reproducibility, and therefore more firmly grounded theory. Nevertheless it will be helpful to try to identify what constitutes the solid, permanent core of the subject, and to do this we need to think separately about observations and how to interpret them. Interpretation is fluid, and liable to be changed and improved as our knowledge and understanding of the relevant theory grows. Another source of confusion in the literature is the attempt to assign only a single cause to what is seen, whereas it is more likely that a number of factors contribute. A prize example of this was the debate, now largely forgotten, as to whether a metal’s ability in catalysis was located in geometric or in electronic character, whereas in fact they are opposite sides of the same coin. It was akin to asking whether one’s right leg is more important than one’s left. Similar miscon- ceived thinking still appears in other areas of catalysis. So in our discussion we must avoid the temptation to over-simplify; as Einstein said, We must make things as simple as possible – but not simpler. THE CATALYSED REACTIONS OF HYDROCARBONS This book is concerned with the reactions of hydrocarbons on metal catalysts under reducing conditions; many will involve hydrogen as co-reactant. This limi- tation spells the exclusion of such interesting subjects as the reactions of syngas, the selective hydrogenation of α,β-unsaturated aldehydes, enantioselective hydro- genation, and reactions of molecules analogous to hydrocarbons but containing a hetero-atom. For a recent survey of these areas, the reader is referred to another source of information 3 . There will be nothing about selective or non-selective oxidation of hydrocarbons, nor about the reforming of alkanes with steam or carbon dioxide. That still leaves us plenty to talk about; hydrogenation, hydrogenol- ysis, skeletal and positional isomerisation, and exchange reactions will keep us busy. Reactions of hydrocarbons by themselves, being of lesser importance, will receive only brief attention. Most of the work to be presented will have used supported metal catalysts, and a major theme is how their structure and composition determine the way in which reactions of hydrocarbons proceed. Relevant work on single crystals and polycrystalline materials will be covered, because of the impressive power of the physical techniques that are applicable to them. There are however important P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 PROLOGUE ix differences as well as similarities between the macroscopic and microscopic forms of metals. This may be an appropriate time to review the metal-catalysed reactions of hydrocarbons. The importance of several major industrial processes which depend on these reactions – petroleum reforming, fat hardening, removal of polyunsat- urated molecules from alkene-rich gas streams – has generated a great body of applied and fundamental research, the intensity of which is declining as new chal- lenges appear. This does not of course mean that we have a perfect understanding of hydrocarbon reactions: this is not possible, but the decline in the publication rate provides a window of opportunity to review past achievements and the present status of the field. I shall as far as possible use IUPAC-approved names, because although the writ of IUPAC does not yet apply universally I am sure that one day it will. Trivial names such as isoprene will however be used after proper definition; I shall try to steer a middle course between political correctness and readability. You must be warned of one other restriction; this book will not teach you to do anything. There will be little about apparatus or experimental methods, or how to process raw results; only when the method used bears strongly on the significance of the results obtained, or where doubt or uncertainty creeps in, may procedures be scrutinised. Some prior knowledge has to be assumed. Elementary concepts concerning chemisorption and the kinetics of catalysed reactions will not be described; only where the literature reveals ignorance and misunderstanding of basic concepts will discussion of them be included. Total linearity of presentation is impossible, but in the main I have tried to follow a logical progression from start to finish. UNDERSTANDING THE CAUSES OF THINGS I mentioned the strong feeling I have that there is much in the literature on catalysis that is unsatisfactory: let me try to explain what I mean. I should first attempt a general statement of what seems to me to be the objectives of research in this field. The motivation for fundamental research in heterogeneous catalysis is to de- velop the understanding of surface chemistry to the point where the physico- chemical characteristics of active centres for the reactions of interest can be identified, to learn how they can be modified or manipulated to improve the desired behaviour of the catalyst, and to recognise and control those aspects of the catalyst’s structure that limit its overall performance. If this statement is accepted, there is no need for a clear distinction to be made between pure and applied work: the contrast lies only in the strategy adopted to P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 x PROLOGUE reach the desired goals. In applied work, the required answer is often obtained by empirical experimentation, now sometimes aided by combinatorial techniques; in pure research, systematic studies may equally well lead to technically useful advances, even where this was not the primary objective. In the past, the work of academic scientists has concentrated on trying to understand known phenomena, although there has been a progressive change of emphasis, dictated directly or indirectly by funding agencies, towards the discovery of new effects or better catalyst formulations. I have no wish to debate whether or not this is a welcome move, so I will simply state my own view, which is that it is the task of academic scientists to uncover scientific concepts and principles, to rationalise and to unify, and generally to ensure that an adequate infrastructure of methodologies (the so-called ‘enabling technologies’) is available to support and sustain applied work. Industrial scientists must build on and use this corpus of knowledge so as to achieve the practical ends. The cost of scaling-up and developing promising processes is such that academic institutions can rarely afford to undertake it; this sometimes means that useful ideas are stillborn because the credibility gap between laboratory and factory cannot be bridged. The objective of the true academic scientist is therefore to understand, and the motivation is usually a strictly personal thing, sometimes amounting to a reli- gious fervour. It is no consolation to such a person that someone else understands, or thinks he understands: and although some scientists believe they are granted uniquely clear and divinely guided insights, many of us are continually plagued by doubts and uncertainties. In this respect the searches for religious and scientific truths resemble one another. With heterogeneous catalysis, perhaps more than with any other branch of physical chemistry, absolute certainty is hard to attain, and the sudden flash of inspiration that brings order out of chaos is rare. It says much for the subject that the last person to have heterogeneous catalysis mentioned in his citation for a Nobel Prize was F.W. Ostwald in 1909. For many of us, what we require is expressed as a reaction mechanism or as a statement of how physicochemical factors determine activity and/or product selectivity. What constitutes a reaction mechanism will be discussed later on. What is however so unsatisfactory about some of what one reads in the literature is that either no mechanistic analysis is attempted at all, or that the conclusions drawn often rest on a very insubstantial base of experimental observation; magnificent edifices of theoretical interpretation are sometimes supported by the flimsiest foun- dation of fact, and ignore either deliberately or accidentally much information from elsewhere that is germane to the argument. I particularly dislike those papers that devote an inordinate amount of space to the physical characterisation of catalysts and only a little to their catalytic properties. Obtaining information in excess of that required to answer the questions posed is a waste of time and effort: it is a work of supererogation. 4 Full characterisation should be reserved for catalysts P1: FBQ/FFX P2: FBQ/FFX QC: FBQ/FFX T1: FBQ KI196-FM.tex KI196/Geoffrey-v1.cls May 24, 2005 19:56 PROLOGUE xi that have interesting and worthwhile catalytic behaviour, and adequate time should be devoted to this. This book is not intended as an encyclopaedia, but I will try to cite as much detail and as many examples as are needed to make the points I wish to make. Three themes will pervade it. (1) The dependence of the chemical identity and physical state of the metal on its catalytic behaviour; integration of this behaviour for a given metal over a series of reactions constitutes its catalytic profile. (2) The effect of the structure of a hydrocarbon on its reactivity and the types of product it can give; this is predicated on the forms of adsorbed species it can give rise to. (3) The observations on which these themes are based will wherever pos- sible be expressed in quantitative form, and not merely as qualitative statements. Lord Kelvin said we know nothing about a scientific phenomenon until we can put numbers to it. However, with due respect to his memory, numbers are the raw material for understanding, and not the comprehension itself. We must chase the origin and significance of the numbers as far into the depths of theoretical chemistry as we can go without drowning. We shall want to see how far theoretical chemistry has been helpful to catalysis by metals. For most chemists there are however strict limits to the profundity of chemical theory that they can understand and usefully deploy,and it is chemists I wish to address. If however you wish to become better ac- quainted with the theoretical infrastructure of the subject, please read the first four chapters of a recently published book; 3 for these my co-author can claim full credit. The foregoing objectives do not require reference to all those studies that sim- ply show how the rate varies with some variable under a single set of experimental conditions, where the variable may for example be the addition of an inactive element or one of lesser activity, the particle size or dispersion, the addition of promoters, or an aspect of the preparation method. Such limited measurements rarely provide useful information concerning the mechanism, and many of the results and the derived conclusions have recently been reviewed elsewhere. 3 We look rather to the determination of kinetics and product distributions to show how the variable affects the reaction mechanism. To explore the catalytic chemistry of metal surfaces, and in particular of small metal particles, we shall have to seek the help of adjacent areas of science. These will include the study under UHV conditions of chemisorbed hydrocar- bons, concerning which much is now known; homogeneous catalysis by metal complexes, and catalysis by complexes adsorbed on surfaces (to a more limited extent); organometallic chemistry in general; and of course theoretical chemistry. [...]... Theories of the Metallic State 1.2 The Metallic Surface 1.2.1 Methods of Preparation 1.2.2 Structure of Metallic Surfaces 1.2.3 Theoretical Descriptions of the Metal Surface 1.3 Alloys 1.3.1 The Formation of Alloys 1.3.2 Electronic Properties of Alloys... catalysts for the reactions of hydrocarbons? The search will lead us from the bulk metallic state through the small supported metal particles whose greater area makes them more fit to catalyse in a useful way; and from the reactions of hydrogen and hydrocarbon molecules with both sorts of metal to their catalytic interactions The chain of cause and effect may not be straightforward In the metals of the Transition... 14.2 Reactions of Higher Alkanes with Hydrogen: Rates and Product Selectivities 14.2.1 Activities of Pure Metals 14.2.2 Effect of Varying Conversion 592 592 592 596 597 598 599 599 601 CONTENTS 14.2.3 Reactions of Linear Alkanes with Hydrogen 14.2.4 Reactions of Branched Alkanes with Hydrogen 14.2.5 Reactions of Cyclic... semi-metals (e.g beryllium, boron) The transition from non-metallic to semi-metallic to wholly metallic behaviour is most evident in Group 14, in which silicon and germanium are semi-metals, tin is ambivalent (the grey allotrope, α-Sn, is a semi -metal, while the much denser white form (β-Sn) is metallic), and lead is of course a metal In Group 15, arsenic and antimony are semi-metals, but bismuth is a metal; ... only partly a consequence of the Lanthanide Contraction (see below) Figure 1.3 Dependence of atomic density on the reciprocal of the cube of the radius for metals of the Third Transition Series; open points, close-packed structures; half-filled points, bcc structure; filled point, Hg 6 CHAPTER 1 Figure 1.4 Periodic variation of the reciprocal of the cube of the radius for metals of Groups 1 to 14; triangles,... needed to secure complete atomisation of a given mass of metal Division by the bulk co-ordination number gives the average bond strength Figure 1.5 shows the periodic variation of sublimation heat for metals in Groups 1 to 14: there Figure 1.5 Periodic variation of the heats of sublimation of metals of Groups 1 to 14 (see Figure 1.4 for meaning of points) METALS AND ALLOYS 7 are curious differences between... SURFACE10,33,46 1.2.1 Methods of Preparation Very many different forms of metals are used as catalysts: the size of the assembly of metal atoms varies from the single crystal, which may contain an appreciable fraction of a mole of the metal, to the tiniest particle containing only METALS AND ALLOYS 15 5 to 10 atoms For practical catalysis it is usually desirable for the metal to be in a such highly... scientists By conductive heating of a wire of the catalytic metal, or of a fragment of the metal attached to an inert wire, in an evacuated vessel, atoms of the metal evaporated and then condensed on the walls of the vessel, forming first islands and later a continuous film A major strength of the technique was the ability to apply a range of techniques to the study of chemisorption on the film; these included... 13.8 Hydrogenolysis of Alkanes on Bimetallic Catalysts 13.8.1 Introduction 13.8.2 Metals of Groups 8 to 10 plus Group 11 13.8.3 Metals of Groups 8 to 10 plus Groups 13 or 14 13.8.4 Platinum and Iridium plus Zirconium, Molybdenum, and Rhenium 13.8.5 Bimetallic Catalysts of Metals of Groups 8 to 10 13.9... between the behaviour of metals as shown by the chemical properties of their surfaces and the manner in which their valence electrons are used in bonding 4 CHAPTER 1 The metals of interest and use in catalysis are confined to a very small area of the Periodic Table, so that most of our attention will be given to the nine metals in Groups 8 to 10,8 with only occasional mention of neighbouring elements . SURFACES Edited by Kenzi Tamaru ELEMENTARY PHYSICOCHEMICAL PROCESSES ON SOLID SURFACES V. P. Zhdanov HANDBOOK OF INDUSTRIAL CATALYSTS Lawrie Lloyd METAL- CATALYSED REACTIONS OF HYDROCARBONS Geoffrey. microscopic forms of metals. This may be an appropriate time to review the metal- catalysed reactions of hydrocarbons. The importance of several major industrial processes which depend on these reactions. Metallic Surface 14 1.2.1. Methods of Preparation 14 1.2.2. Structure of Metallic Surfaces 16 1.2.3. Theoretical Descriptions of the Metal Surface . 22 1.3. Alloys 24 1.3.1. The Formation of Alloys