Chemical Mechanism of Hydrocarbon Cracking over Solid Acidic Catalysts

Abstract

The article analyzes cracking reactions of isoalkanes and isoolefins over zeolite-based catalysts and discusses formation mechanisms of light reaction products under very mild conditions, at 150–250°C. Cracking patterns of 28 methyl- and ethyl-branched isoalkanes show that the compositions of light products can be described by an empirical rule: (1) the reaction site is formed at the tertiary carbon atom in an isoalkane molecule, (2) the predominant fission reaction involves the weakest C–C bond in the α-position to the reaction site, (3) the primary fission products are olefins. None of the cracking mechanisms described in the literature and involving reactions of carbenium and carbonium ions can adequately predict the observed product structures. A new cracking mechanism of isoalkanes which includes reactions between isoalkanes and Brønsted centers on the catalyst surface with the formation of transient hydrosiloxonium ions >Si–O+(H)–C< is proposed. The ions undergo the scission of the C–C bond in their alkyl groups in the β-position to O+with the formation of olefin molecules (which rapidly isomerize) and smaller hydrosiloxonium ions. Comparison of cracked products from olefins and alkanes with the same skeletons and the same expected carbocations shows that the respective products are drastically different when they are formed under very mild conditions, i.e., that the cracking mechanisms of olefins and alkanes are also different. Studies of olefins with low oligomerization abilities (to prevent scrambling of the product structures) show that the olefin cracking can indeed be explained by fragmentation of carbenium ions via the β–C–C bond scission mechanism.