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

• Published in: Journal of catalysis 1992-08, Vol.136 (2), p.446-462
• Main Authors: Shertukde, Prasad V., Marcelin, George, Sill, Gustave A., Keith Hall, W.
• Format: Article
• Language: English
• Subjects: 02 PETROLEUM; 020400 - Petroleum- Processing; ALKANES; ALKENES; Applied sciences; BROENSTED ACIDS; BUTANE; CATALYSIS; CATALYTIC CRACKING; CATALYTIC EFFECTS; CHEMICAL REACTION KINETICS; CHEMICAL REACTIONS; CRACKING; Crude oil, natural gas and petroleum products; DECOMPOSITION; ELEMENTS; Energy; ETHANE; Exact sciences and technology; Fuels; HYDROCARBONS; HYDROGEN; HYDROGEN COMPOUNDS; HYDROGEN TRANSFER; INORGANIC ACIDS; INORGANIC ION EXCHANGERS; ION EXCHANGE MATERIALS; ISOMERIZATION; KINETICS; MASS BALANCE; MATERIALS; METHANE; MINERALS; NONMETALS; ORGANIC COMPOUNDS; PENTANE; Processing of crude oil and oils from shales and tar sands. Processes. Equipment. Refinery and treatment units; PYROLYSIS; RADICALS; REACTION INTERMEDIATES; REACTION KINETICS; SILICATE MINERALS; SYNTHESIS; THERMOCHEMICAL PROCESSES; ZEOLITES
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(92)90075-S

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.