Catalysis – serie

Catalysis has made major contributions to many areas of chemical industry. Before embarking upon detailed considerations of catalytic science and technology, it is very helpful to look first at the nature of industrial catalysis, and how it has evolved and grown to meet demands imposed by changing industrial needs. Dr. H. Reinemann is uniquely qualified to place industrial catalysis in a historical perspective: in his distinguished industrial career he has been closely involved with many of the major innovations in industrial catalysis. Before a catalytic process is commercialized, the supporting research and development work is carried out in chemical reactors. It is obviously imperative that the behaviour of such reactor systems should be tho­ roughly understood by those who use them, and by those who may have to interpret their results, yet all too often this basic need is not met. Professor J. C. R. Turner provides a Straightforward yet thorough account of catalytic reactor theory which should make it impossi­ ble for any catalytic practitioner to plead ignorance. The catalytic hydrogenation of dinitrogen to ammonia is one of the world's great industrial processes and the catalytic activation of molecular dinitrogen is a key step in that process. The chapter by Professors A. Ozaki and K. Aika deals with the chemistry of dinitrogen activation at the catalyst surface, and shows how this relates to the synthesis of ammonia. The chapter also deals with the activation of dinitrogen by molecular complexes in homogeneaus systems.

NMR methods have for a considerable time been standard processes for the analysis of molecular structure: so much so that they are now universally regarded as indispensable for this purpose. Nevertheless, with the passage of time, NMR methodology has been elaborated to levels of ever increasing complexity and analytical sophistication so that the non­ specialist may now be readily excused for the belief that for anything beyond relatively elementary methods one would be well advised to work in collaboration with the specialist experts. The application of NMR methods to the field of catalysis occurred, in the main, relatively late in the day, mainly be­ cause those catalysts of greatest industrial importance, that is heterogeneous catalysts, are solids and so require special NMR methods if usefully narrow NMR lines are to be ob­ servable. Even so, magic-angle spinning NMR methodology is now thoroughly well established and is finding increasing use in the study of catalyst structure. Of course, conventional NMR methods have been used for a considerable time for the analysis of the products of catalytic reactions. Chapter 1 of the present volume by Professor Jacques Fraissard and his collaborators is designed to give an account of the application of NMR methods to the field of catalysis, but not including the conventional use of NMR for reaction product analysis, since this is already well covered in the existing NMR literature.

Hydrotreating processes in petroleum refining were introduced more than 50 years ago for the removal of sulfur and nitrogen. The sulfided cobalt-molybdenum catalyst, together with its near relatives, is still widely used. Two oil crises made it clear that pretroleum reserves are not inexhaustible and we shall be compelled to exploid less satisfactory sources with high sulfur and nitrogen making hydrotreating even more important. This review is particularly timely for the reason that only recently has a detailed understanding of process chemistry and catalyst structure been obtained. The authors concentrate on the catalytic chemistry of the processes, dealing in some detail with the structure of the most important types of catalysts and the relationship of structure to activity.

Our understanding of catalytic reactions exists at various levels which are mainly defined from detailed knowledge of reaction mechanism. When viewed in terms of the stoi­ chiometric reaction equation, most catalytic reactions are complex processes which occur via a sequence of elementary (i. e. irreducible) steps, and the elucidation ofthese elementary steps and the identification of a rate determining step (if one exists) constitutes the traditional approach to the problem of mechanism. The term "traditional" is not used here in a pejorative sense since mechanistic knowledge of this sort makes an important contribution to catalyst design, improvement, and optimization. This is the field which is discussed by Professor R. L. Burwell in Chapter 1 where the very wide range of useful approaches and techniques is made apparent, even when one is restricted to quasi-steady state conditions. Techniques which depend on observations under non­ steady state conditions (i. e. relaxation methods) have also been used in mechanistic studies, increasingly so in recent years. This topic is discussed in detail by Professor K. Tamaru in Chapter 2. At a deeper level of understanding, one may seek to enquire how an elementary reaction proceeds in terms of movement in a multicoordinate space where the variables define atomic positions and energy. This is a problem of great complexity even in relatively simple cases. Nevertheless, despite the problems some progress is being made, and this and allied topics are discussed in Chapter 3 by Professor G. L. Haller and Dr. G. W. Coulston.

Catalysis is a subject which draws upon many of the traditional scientific and technological disciplines, and its present structure has only been reached after many decades of detailed work. Nevertheless, in catalysis as in all other areas of science, experiments are carried out, new data are acquired, theories are proposed; and these things are done not in an intellectual vacuum, but in relation to previously established concepts and ideas. The history of how these guiding concepts have devel­ oped is nothing less than a skeletal history of the subject itself. In one respect at least, catalysis is similar to all other of mankind's endeavors: a failure to learn from history is a rejection of one's heritage. None of us should willingly plead guilty to this, and Professor O.-M. Schwab's opening chapter in this volume provides us with a ready means to avoid the need to re-invent what has been known for some time. Heterogeneous catalysis is dominated by the concept of a reactant molecule entering into some type of che­ mical interaction with the surface of a solid. Thus, to understand catalysis it is essential to understand as fully as possible the chemical nature of the solid. In the second chapter of this volume, Professor J. Haber provides a review of the majority of solid phases which are of catalytic interest. The framework for this review is chemical crystallography, and the author goes to some pains to draw attention to features that are of particular importance in catalysis.

A cursory examination of the current scientific and technological literature is sufficient to show the enormous interest in the possibility of producing liquid fuels from coal. There are, of course, a number of ways in which coal liquefaction may be effected. Many of the important steps are catalytic. The direct liquefaction route, that is, coal hydrogenation, has a long history with origins in the early years of this century. It also has the distinction of being a process which was once operated on a very large scale and which, having died, now shows every prospect of resurrection. The technology which finally emerges will doubtless differ significantly from the original practice, but it ·is sensible for those currently working in the field to be aware of the achievements of the past. Dr. E. Donath, who was personally involved during the heroic years of coal hydrogenation, has provid­ ed an historical account of the subject up to the time immediately following World War II, when the large scale process began its rapid decline to oblivion. Processes involving catalytic oxidation form a very large and important part of chemical industry. The reactions involved are very varied, ranging from the classical oxidation processes of heavy industry, such as the oxidation of sulfur dioxide or of ammonia, to selective oxidations designed to produce specific organic products from a range of possibilities. The chapter by Professor G. K.

A very wide range of catalytic conversions find industrial use in organic process chemistry. The scale of the ope­ rations varies enormously from very high volume pro­ cesses to specialty chemical preparations. Many of these processes are functional group conversions or class reac­ tions, and the more important of these will receive detailed treatment in specific chapters throughout this series. Nevertheless, the scope is very broad, and it is all too easy for the non-specialist to become lost in a large volume of detail. To try to avoid this, the first chapter in this volume, by Dr. Paul N. Rylander provides a working summary of the more important catalytic con­ versions of this type. In doing this, he also gives some valuable comments about catalyst selection, together with an indication of the reaction conditions used in practice, the more important of the problems usually encountered, and comments about the most important of the mechanistic features. It has long been recognized that an understanding of the chemical nature of solid surfaces is fundamental to an understanding of catalytic processes which may take place upon them. This question may be approached in two distinct ways. One is via surface crystallography which focuses attention upon long range order. The second concentrates upon the concept of the surface functional group where attention is mainly upon the chemistry characteristic of a particular localized atomic arrangement at the surface. In practice, of course, there exists a continuum between these idealized extremes.

Catalytic steam reforming has grown during the last two or three decades into one of the world's great catalytic processes. It is of major economic significance since the products from it form the feed for a number of other major processes. Nevertheless, catalytic steam reforming is a relatively difficult technology. It operates at high temperatures where problems of the maintenance of materials integrity and of catalyst stability and activity are severe, the establishment of high thermal efficiency of the plant is economically vital, and reactor operation is strongly influenced by mass and heat transport effects. The process is the subject of a thorough review by Dr. J. R. Rostrup-Nielsen who discusses both the basic cataly­ tic chemistry and the way in which this is interrelated with reactor and plant design. The use of catalytic converters for the purification of automotive exhaust gases is a relatively new technology which was brought into existence by social pressures for the preservation of acceptable environmental conditions. The majority of catalytic practitioners have been able to watch the growth of this technology from its inception to its current state of sophistication. Automotive catalytic converter technology is now in a mature state, and the chapter in this volume by Dr. K. C. Taylor provides a review which covers both the process chemistry and the most important converter design factors.

For catalytic practitioners who are concerned with laboratory studies of reaction mechanisins, as often as not catalyst deactivation is· treated as a nuisance to be ignored or factored out of the experimental results. How­ ever, the engineer concerned with the design and opera­ tion of real catalysts and processes cannot afford this luxury: for him deactivation and the need for regenera­ tion are inevitable facts of life which need to be treated as quantified design parameters. The first chapter in this volume by Prof. J. B. Butt deals with catalyst deactivation and regeneration as processes in their own right, and shows how they are to be approached from kinetic and design points of view. Catalytic olefin polymerization spans a very wide field in catalytic process chemistry and technology. Processes of this sort range from the generation of high volume products such as polyethylene and polypropylene, through more specialized commercial products, to con­ versions that still remain laboratory curiosities. The reaction chemistry is, in detail, often very complex. However, because of the insight provided by organo­ metallic reaction chemistry, many of the polymerization mechanisms are reasonably well understood, and the way in which product stereospecificity may be obtained is also understood in considerable detail. This highly complex subject is reviewed in detail in the second chapter of this volume by Prof. I. Pasquon and Dr. G. Giannini.

Catalytic oxidation processes are bf central importance to a substantial part of large-scale chemical industry. Indeed, this area of industrial catalysis has an extremely long history which stretches back well into the last century. The development and growth of catalytic oxi­ dation processes for the manufacture of commodities such as sulfuric acid and nitric acid can be viewed as indicators for the growth of the early and middle years of the entire inorganic chemical industry, and in an analogous fashion the manufacture of products such as phthalic anhydride, maleic anhydride and ethylene oxide has been central to the development of an organic chemical industry. We should all be able" to learn from history, and present-day scientists and technologists will find considerable benefit in following the account of the historical development of catalytic oxidation processes presented in Chapter I by Drs. G. Chinchen, P. Davies and R. J. Sampson. Alkenes are important intermediates in many processes in organic chemical industry. Being mostly petroleum­ derived, the alkene availability pattern does not necessar­ ily match consumption requirements and an alkene inter­ conversion process such as metathesis is clearly of in­ dustrial importance. In fact alkene metathesis, in addi­ tion to its industrial significance, poses an interesting mechanistic problem. upon which considerable effort has been expended in recent years and which is now fairly well understood.