Catalytic reactions classification

A rational classification of reactions based on mechanistic considerations is essential for the better understanding of such a broad research field as that of the organic chemistry of Pd. Therefore, as was done in my previous book, the organic reactions of Pd are classified into stoichiometric and catalytic reactions. It is essential to form a Pd—C cr-bond for a synthetic reaction. The Pd— C (T-bond is formed in two ways depending on the substrates. ir-Bond formation from "unoxidized forms [1] of alkenes and arenes (simple alkenes and arenes) leads to stoichiometric reactions, and that from oxidized forms of alkenes and arenes (typically halides) leads to catalytic reactions. We first consider how these two reactions differ. 

Reactions of another class are catalyzed by Pd(II) compounds which act as Lewis acids, and are treated in Chapter 5 and partly in Chapter 4. From the above-mentioned explanation, the reactions catalyzed by Pd(0) and Pd(II) are clearly different mechanistically. In this book the stoichiometric and catalytic reactions are classified further according to reacting substrates. However, this classification has some problems, viz. it leads to separate treatment of some unit reactions in different chapters. The carbonylation of alkenes is an example. Oxidative carbonylation of alkenes is treated in Chapter 3 and hydrocar-bonylation in Chapter 4. 

Table 4.1 lists all published electrochemical promotion studies of 58 catalytic reactions on the basis of the type of electrolyte used. Each of these reactions is discussed in Chapters 8 to 10 which follow the same reaction classification scheme. 

Table 4.2. Classification of electrochemical promotion studies on the basis of catalytic reaction.

Table 4.3. Classification of Electrochemical Promotion studies on the basis of catalytic reaction, showing the observed kinetic order with respect to the electron donor (D) and electron acceptor (A) reactant and the corresponding global rvs

The types of reactors used in industry for carrying out heterogeneous catalytic reactions may be classified in terms of a relatively small number of categories. One simple means of classification is in terms of the relative motion of the catalyst particles, or lack thereof. We consider ... 

Cutting across this classification is the catalytic reaction whose rate is altered by materials that are neither reactants nor products. These foreign materials, called catalysts, need not be present in large amounts. Catalysts act somehow as go-betweens, either hindering or accelerating the reaction process while being modified relatively slowly if at all. 

Since the classification is essentially based on rates of catalytic reactions relative to rates of diffusion of redox carriers, there are oxidation reactions that are intermediate between the two limiting cases. We note that neither the molecular size nor the polarity of reactant molecules is the principal characteristic determining the type of catalysis. Although oxide ions migrate rapidly in the bulk, bulk type II catalysis is not observed for oxidation catalyzed by Bi-Mo oxides. In this case the rate-limiting step is a surface reaction. 

Another possible electrocatalytic process is that related to a surface-bound molecule which can give rise to a two-electron reaction. In these conditions, the coupling of the catalytic reaction in the presence of an adequate species in solution can lead to different mechanistic schemes from which the elucidation of the global reaction path is not immediate. This situation matches the behavior of a great number of inorganic catalysts (such polyoxometallates or ion complexes) [86, 98] and biological molecules (enzymes, proteins, oligonucleotides, etc.) [79, 80], for which there is a lack of theoretical basis which enables a clear classification of the different possibilities that can be encountered. 

Classification of Catalytic Reactions Based on Mechanistic Consideration... 

Classification of catalytic reactions based on mechanistic consideration... 

To repeat the route of chemistry in the kinetic aspect , that was the formulation of the problem. To our mind, however, in the 1930s "the rational classification principle , whose appearance was predicted by Semenov, could not be realized. The possibility of solving this problem appeared only in recent times in terms of the concepts of the graph theory and the qualitative theory of differential equations. The analysis of the effect of the mechanism structure on the kinetic regularities of catalytic reactions is one of the connecting subjects in the present study. 

Problem 1. Define catalyst and catalysis. Mention the types and classification of catalysis. Discuss the characteristics of catalytic reactions. 

This statement corresponds to a new concept in classification of polyaromatic coke formed during catalytic reactions that provides a structural basis for the traditional classifications of hard coke and soft coke (d7-(5P). 

This chapter is intended to focus on catalysis in both thermal and photoinduced electron transfer reactions between electron donors and acceptors by investigating the effects of an appropriate substance that can reduce the activation barrier of electron transfer reactions. It is commonly believed that a catalyst affects the rate of reaction but not the point of equilibrium of the reaction. Thus, a substance is said to act as a catalyst in a reaction when it appears in the rate equation but not in the stoichiometric equation. However, autocatalysis involves a product acting as a catalyst. In this chapter, a catalyst is simply defined as a substance which affects the rate of reaction. This is an unambiguous classification, albeit not universally accepted, including a variety of terms such as catalyzed, sensitized, promoted, accelerated, enhanced, stimulated, induced, and assisted. Both thermal and photochemical redox reactions which would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis in the electron transfer steps. First, factors that accelerate rates of electron transfer are summarized and then each mechanistic viability is described by showing a number of examples of both thermal and photochemical reactions that involve catalyzed electron transfer processes as the rate-determining steps. Catalytic reactions which involve uncatalyzed electron transfer steps are described in other chapters in this section [66-68]. 

In what follows we discuss these criteria and present the main results. The number of vertices in a KG, JV, is not important for the classification, but it is convenient to introduce this criterion into the coding procedure immediately after the notation for the number of routes. The number of KG edges, E (the mechanism s elementary steps), is not regarded as a criterion because it is determined uniquely by the Horiuti rule M = E — J, where J is the number of linearly independent intermediates. In the case of non-catalytic reactions the number N includes the vertex with the so-called zero reagent. 

Nonlinear mechanisms are very common in heterogeneous catalytic reactions. They are also characteristic of chain reactions and, perhaps, of homogeneous catalysis involving metal complexes. Because of this, the classification of these mechanisms is of considerable interest. 

Surface reaction (sections 6-8). This is the largest section, concerned with the reaction event itself occurring between adsorbed species. It will include a brief description of reaction kinetics at surfaces, together with a classification of the kind of catalytic reactions which are important and a consideration of work mainly carried out on well-defined surfaces where the surface structure is well-characterised. It includes further sections on the effect of atomic number (electronic structure) on reactions on metals in the transition series, on the effects of surface structure on reaction rates, and on important aspects of catalysis, namely poisoning and promoter effects. 

This classification is important not only for kinetics and for the equilibrium of the heterogeneous catalytic reactions with a doublet mechanism, but for the equilibrium of homogeneous catalytic and non-catalytic reactions as well, because the equilibrium does not depend on the mechanism of the reaction. It is interesting to note that the cyclic activated 4- and 6-complexes, postulated by Syrkin 355), are nothing but doublet and triplet index groups, and consequently the multiplet classification must be proper for them as well hence it can also be applied to the kinetics of catalytic reactions that are not heterogeneous. 

The approaches to the creation of a theory of catalyst selection and of catalyst selectivity from the point of view of the multiplet theory are described in this review. First, the classification of reactions given in this review can be useful for this purpose. This calssification widely covers well-known catalytic reactions (and also enzymic reactions) and, in addition, can predict new reactions therefore it has heuristic value. 

Important catalytic reaction concepts include structure sensitivity and insensitivity of reactions, mechanistic classifications (Langmuir-Hinshelwood, Eley-Rideal), the compensation effect, the presence of strongly chemisorbed overlayer, and the roles of structure and bonding modifier additives (promoters). 

Figure 1. Classification of phase-transfer catalytic reaction.

These general relations have been taken to hold for catalytic actions as a result of the description of various reactions. They are therefore, used as definitions of catalytic reactions. If, by definition, a reaction follows these laws, it is catalytic, otherwise it is not. While it is necessary for purposes of classification, to have some definitions of this kind, the way in which the classification of catalytic reactions developed, that is, very often by the introduction of the term catalyst when unknown factors were involved, has confused the relation of these reactions to chemical reactions in general. 

The fact that intermediate complex compounds have been isolated in a number of catalytic reactions, naturally does not prove that they are formed (with the catalyst) in all such cases. Some evidence will now be presented to indicate that unsaturation of certain molecules plays an important part in catalytic reactions which apparently do not, at first sight, fall into the classification. 

---