Heterogeneous catalysis classification

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. 

In the next sections, solid catalysts are classified on the basis of their type of material, the most important types comprising metals, metal oxides, and metal sulfides. It is emphasized, however, that a more fundamental classification is also possible, although it can be developed to only a small degree because the science of catalysis is so complex. The next few paragraphs summarize this alternative classification based on the reactions catalyzed and the structures of intermediates this classification also is a start toward unifying homogeneous and heterogeneous catalysis, which have traditionally been considered separately. 

Solving the problem on the interaction of a solid particle, drop, or bubble with the surrounding continuous phase underlies the design and analysis of many technological processes. The industrial applications of such interaction include classification of suspensions in hydrocyclones, sedimentation of colloids, pneumatic conveyers, fluidization, heterogeneous catalysis in suspension, dissolving solid particles, extraction from drops, absorption, and evaporation into bubbles [69, 107, 111, 122,137,478,505],... 

Table 8.2. Typical responses (M. Kobayashi, Characterization of transient response curves in heterogeneous catalysis—I. Classification of the curves. Chemical Engineering Science, 37 (1982) 393).

Classification of Applications of Ionic Liquids in Heterogeneous Catalysis... 

A further classification is based on the number of phases in the system homogeneous (1 phase) and heterogeneous (more than 1 phase) catalysis. 

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. 

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

Asymmetric catalysis can be homogeneous or heterogeneous and can take place either in the organic phase or (as recently found) in the aqueous phase. Table 9.5 presents a broad classification of the major categories of asymmetric catalysis. The most common is homogeneous catalysis in the aqueous phase, and the most recent and novel is the use of aqueous phase catalysts immobilized on solid supports. The various categories are considered in the following sections. 

II. Another classification cf catalysis is based on the number cf phases present in the catalytic system homogeneous (single phase) and heterogeneous (multiphase) catalysis. 

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