Evidence for alkylcarbenium ion reaction intermediates from intrinsic reaction kinetics of C6-C9 n-alkane hydroisomerization and hydrocracking on Pt/H-Y and Pt/USY zeolites

A quaternary mixture of hexane, heptane, octane, and nonane was hydroconverted on Pt/H-Y with a Si/Al ratio of 2.7 and two Pt/USY catalysts with Si/Al ratios of 13 and 30, respectively. Although these zeolite catalysts have largely different activities, they show unique yield curves of monobranched isomers, multibranched isomers, and cracked products versus conversion. Intrinsic reaction rate constants were obtained from the modeling of experimental conversions with a combined adsorption-reaction model assuming alkylcarbenium ion reactions as rate-determining steps, and independently determined multicomponent adsorption equilibrium expressions. On the three catalysts, the relative intrinsic reactivities of the different n-alkanes are constant. The relative intrinsic reaction rates of skeletal branching and cracking reaction steps are also catalyst independent. With respect to the reaction mechanism, this independence of relative reaction rates of the nature of the zeolite favors the occurrence of the classic reaction mechanism with alkylcarbenium ions as true reaction intermediates rather than as transition states not completely separated from the acid site. The dependence on carbon number of the intrinsic rate constants is consistent with branching reactions of alkylcarbenium ions via protonated cyclopropanes. (C) 2000 Academic Press.