Metal-Catalysed Reactions of Hydrocarbons / 00-front-matter

Metal-Catalysed

Reactions

of Hydrocarbons

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 547 .01—dc22

2005 Springer Science Business 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 Science Business 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.

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great number of variables, some uncontrolled and some even unrecognised,1 that determine catalytic performance. Thus while in principle (as I have said before2) 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 misconceived 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 limitation spells the exclusion of such interesting subjects as the reactions of syngas, the selective hydrogenation of α,β-unsaturated aldehydes, enantioselective hydrogenation, 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 information3. 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, hydrogenolysis, 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

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 religious 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 foundation 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

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 possible 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 acquainted 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 simply 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 hydrocarbons, 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.