Mechanism of Catalytic Cracking

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

Wojdechowski, B.W. (1998) The reaction mechanism of catalytic cracking quantifying activity, selectivity, and catalyst decay. Catal. Rev. — Sci. Eng., 40, 209. 

K. A. Cumming and B. W. Wojciechowski, Hydrogen transfer, coke formation, and catalyst decay and their role in the chain mechanism of catalytic cracking, Catal. Rev. Sci. Eng., 38, 101-157 (1996). 

Figure 3.4 Mechanism of catalytic cracking of polyolefins over acid solid catalysts [28]. (Reproduced by permission of the American Chemical Society)...

Corma, A. Orchilles, A.V. Current views on the mechanism of catalytic cracking. Micropor. Mesopor. Mater. 2000, 35-36, 21-30. 

Before we look at the mechanism of catalytic cracking on catalysts, it is appropriate to give a definition of a catalyst. Catalysis is derived from the Greek word katalysis meaning destruction or weakening. A catalyst is a substance that changes the rate of a chemical reaction but remains chemically unchanged at the end of the reaction. 

The writer (42) published the first attempt to explain the mechanism of catalytic cracking in terms of an ionic mechanism, later modifying the initial concept, which involved both cationic and anionic complexes, to include only a cationic mechanism (47). Thomas (18) arrived at essentially the same conclusions regarding a cationic (carbonium ion) mechanism somewhat earlier, although his very comprehensive paper on the subject was not published until 1949. Similarly, Greensfelder and his co-workers (48) came to substantially the same general conclusions. 

In recapitulating the preceding discussion on the chemistry of catalytic cracking, the following conclusions are offered by the writer as to the present status of the mechanism of catalytic cracking, the role played by the catalyst, and the chemical nature of the catalyst. 

Development of the carbonium ion mechanism of catalytic cracking in 1940-1950 is reviewed. Studies of the thermal and catalytic cracking of pure hydrocarbons played a key role, and were complemented by earlier hypotheses of Whitmore regarding the probable carbonium ion intermediates in... 

The carbonium ion theory originally proposed by Frank Whitmore (11) is thus as applicable to the mechanism of catalytic cracking as it is to other acid-catalyzed reactions like isomerization or alkylation. 

This chapter deals with the catalytic cracking of pure hydrocarbons. The discussion covers the cracking of compounds representative of different hydrocarbon classes, the mechanism of catalytic cracking, and the nature of cracking catalysts. 

It should be remembered that the above reaction is also the first step in the mechanism for the alkylation of paraffins by olefins as proposed by Schmerling (9). As a matter of fact, the mechanism proposed by Schmerling for the alkylation reaction applies equally well, in reverse, for the mechanism of catalytic cracking of paraffins. 

The discussion presented above is based almost entirely upon the concept of the carbonium-ion mechanism of catalytic cracking. Since such a mechanism requires the presence of an acidic medium, it remains to be shown that cracking catalysts are acidic. 

Short chain alkanes (C4 and C5), especially isobutane and neopentane, were frequently used to elucidate the reaction mechanism of catalytic cracking, e.g., in papers by Hall et al. [110,114] and Kranilla et al. [115]. In these publications, however, few comparisons were made to distinguish the intensive and extensive factors of the zeolite acidity. 

Theory. Porous adsorbent catalysts of the silica alumina type are widely used. The fact that one or more hydrous metallic oxides are present in all successful catalysts suggests that water in some way is important in this type of catalysis. Nevertheless, completely satisfactory explanations of the mechanism of catalytic cracking have not been presented. Greensfelder and associates have studied the cracking of dozens of paraffinic, naphthenic, and aromatic hydrocarbons. They find that... 

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