Classification of catalysis

Although we confine our attention to mainly micellar catalysis (microheter-ogeneous catalysis) in this book, a brief mention of other kinds of catalysis is also included. 

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

Fig. 2.1-2 Classification of catalysis into the three important classes of heterogeneous, homogeneous, and biocatalysis.

In discussing base catalysis it will prove convenient to adopt, at the outset, a distinction first proposed by Bunnett and Garst22, who noted that the observed cases of catalysis in nucleophilic aromatic substitution could be broadly divided into two categories. The classification was in terms of the relative rates of the catalyzed and uncatalyzed reactions. Since all of the systems could be accommodated empirically by eqn. (4),... 

Figure 2.1 (Plate 2.1) shows a classification of the processes that we consider they aU involve interaction of the reactants both with the solvent and with the metal electrode. In simple outer sphere electron transfer, the reactant is separated from the electrode by at least one layer of solvent hence, the interaction with the metal is comparatively weak. This is the realm of the classical theories of Marcus [1956], Hush [1958], Levich [1970], and German and Dogonadze [1974]. Outer sphere transfer can also involve the breaking of a bond (Fig. 2. lb), although the reactant is not in direct contact with the metal. In inner sphere processes (Fig. 2. Ic, d) the reactant is in contact with the electrode depending on the electronic structure of the system, the electronic interaction can be weak or strong. Naturally, catalysis involves a strong...

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of carbocations. 

Classification of reactions 190 Nucleophilic vs. general acid-base catalysis 191... 

For films. Table 4.1 proposes an arbitrary classification of the polyethylene subfamilies (where m indicates metallocene catalysis). 

Classincation of the Proteases. The classification of the proteases is based on their mechanism of catalysis (4), The four primary classes of proteases are the serine, aspartic, cysteine, and metalloproteases (5). This classification is based on the primary functional group found in the en me s active site. There are likely to be other proteases eventually characterized which will not precisely fit into this categorization scheme and additional categories will be needed. One example of a potential new category is the ATP-dependent proteinases (6), a group of proteinases which require ATP for activity. 

Since its discovery about 30 years ago [1-3], shape selective catalysis in microporous crystalline materials has been the subject of countless investigations. Review articles are now available [4-10] in which the principles and classification of shape selectivity effects are discussed and numerous examples are given. 

Classification of some important defect structures and diffraction contrast in catalysis... 

Examples of this class of enzymes are glucose oxidase and D-amino acid oxidase The classification of the flavoproteins used here is that originally suggested which has been modified recently . In the author s own view the original classification has the advantage of being simple and yet quite useful whereas the new classification does not add to simplify and classify the rather complex picture of flavoprotein catalysis. Nevertheless, in flavoprotein oxidases, the 1,5-dihydroflavin is very reactive towards Oj. On the other hand, the two-electron reduced form of flavoprotein oxidases reacts slowly with pure one-electron acceptors, e.g. ferricyanide. That the two-electron transition is biologically favoured in these enzymes explains why they can react easily with sulfite... 

There is no generally accepted classification of elementary processes in heterogeneous catalysis. However, names for a few types of elementary processes are generally accepted and terminology for a partial classification [see M. Boudart, Kinetics of Chemical Processes, Chap. 2 (1968)] has received some currency. The particular reactions used below to exemplify this terminology are ones which have been proposed in the literature but some have not been securely established as occurring in nature at any important rate. 

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. 

Because there are many different ways to combine a catalyst with a membrane, there are numerous possible classifications of the CMRs. However, one of the most useful classifications is based on the role of the membrane in the catalytic process we have a catalytically active membrane if the membrane has itself catalytic properties (the membrane is functionalized with a catalyst inside or on the surface, or the material used to prepare the membrane is intrinsically catalytic) otherwise if the only function of the membrane is a separation process (retention of the catalyst in reactor and/or removal of products and/or dosing of reagents) we have a catalytically passive membrane. The process carried out with the second type of membrane is also known as membrane-assisted catalysis (a complete description of the different CMRs configurations will be presented in a specific chapter). 

The key NMR observations (i) that the proportion of homo- and heterochiral dimers is near-equal, and (ii) that their interconversion by a dissociative process is rapid compared to catalytic turnover, preclude the possibility of a monomer autocatalyst. In Kagan s classification, monomer catalysis with a positive NLE may only arise when there is an unequal concentration of homo- and heterochiral oligomers, in favour of the heterochiral form, which acts as a reservoir for the deficient enantiomer. NMR results show that the resting state for Soai s autocatalysis is an equal mixture of homo-and heterochiral species, predominantly dimeric. The lack of ground-state stereo-discrimination requires that the number of chiral entities in the resting state must be less than or equal to the number in the enantioselectivity-determining transition state, else there is no possibility of the vital non-linear effect. Even after the publication of these results in late 2004, their consequences are not always applied. For recent discussions where a monomeric catalyst for Soai s system is permitted or promoted, see [91-93]. 

The first more comprehensive classification of the photocatalytic processes was proposed by Salomon, who divided the processes into two main classes (1) photogenerated catalysis, which is catalytic in photons, and (2) catalyzed photolysis, which is non-catalytic in photons [130] (Figure 6.11). 

Post-synthesis methods (pore-size engineering) allow an existing shape-selectivity effect to be intensified, and also a new one to be established. However, normally not only the pore size will be influenced by most of these methods, but also the catalytic activity. Vansant [104] gives a classification of post-synthesis modification methods which covers the entire range of zeolite applications (gas separation, gas purification, encapsulation of gases and catalysis). 

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