Evidence for alkylcarbenium ion reaction intermediates from intrinsic reaction kinetics of C-6-C-9 n-alkane hydroisomerization and hydrocracking on Pt/H-Y and Pt/USY zeolites

A quaternary mixture of hexane, heptane, octane, and nonane was hydroconver ted 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 l argely different activities, they show unique yield curves of monobranched isomers, multibranched isomers, and cracked products versus conversion. Int rinsic reaction rate constants were obtained from the modeling of experimen tal conversions with a combined adsorption-reaction model assuming alkylcar benium ion reactions as rate-determining steps, and independently determine d multicomponent adsorption equilibrium expressions. On the three catalysts , the relative intrinsic reactivities of the different n-alkanes are consta nt. The relative intrinsic reaction rates of skeletal branching and crackin g reaction steps are also catalyst independent. With respect to the reactio n mechanism, this independence of relative reaction rates of the nature of the zeolite favors the occurrence of the classic reaction mechanism with al kylcarbenium ions as true reaction intermediates rather than as transition states not completely separated from the acid site. The dependence on carbo n number of the intrinsic rate constants is consistent with branching react ions of alkylcarbenium ions via protonated cyclopropanes. (C) 2000 Academic Press.