Study of the mechanism of the cracking of small alkane molecules on HY Zeolites

Abstract

Cracking of i-butane and n-pentane was studied on HY zeolites. These reactions were initiated by the protonation of C-H and C-C bonds by the Brønsted acid sites. The pentacoordinated carbonium ions thus formed decomposed into carbenium ions and the products of the initiation reactions, viz., H 2, CH 4, and C 2H 6. These carbenium ions propagated chain reactions, mainly by isomerization followed by hydride transfer. Disproportionation occurred concomitantly. For these small alkanes the contribution of /gb-scission to product formation was negligible. “Chain length” (the number of chain cycles per initiation) was defined as the ratio of reactant molecules consumed by hydride transfer to those reacted by protonation, i.e., the ratio of rates of bimolecular propagation to unimolecular initiation. Thus, chain length reflected the average lifetime of the carbenium ions. The extent of protonation was found to increase with the strength of acid sites while the chain length remained relatively constant for preparations of a similar nature. The product distribution obtained was therefore critically dependent on the steady-state population of carbonium ions. Finally the chain reactions were terminated by decomposition of carbenium ions into corresponding alkenes. Mass balances derived from these initiation, propagation, and termination steps were in agreement for both the substrates. The product distributions obtained for i-butane and n-pentane cracking were satisfactorily explained on this basis.