Cracking catalysts chain propagation

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Returning now to the question of the initiation step of the cationic hydrocarbon reactions, there are essentially two schools of thought on the subject. One is that the catalyst, itself, has the intrinsic ability to extract a hydride ion from the hydrocarbon or to distort a carbon-hydrogen bond to such an extent that the net result is the same (Milliken et al., 39). The other mechanism is the formation of a cationic complex by the simple addition of a proton to olefinic impurities, either initially present or formed in small amounts by thermal cracking or oxidation. This complex, however small its concentration on the surface of the catalyst, can then start a chain reaction through the hydrogen transfer mechanism, i.e., it can extract a hydride ion from a saturated hydrocarbon, which then becomes the propagating complex (Hansford, 43, 47 Thomas, 18). 

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