Alkene protonation enthalpy determination from fundamental kinetic modeling of alkane hydroconversion on Pt/H-(US)Y-zeolite

Alkane, c.q., C-5 to C-12, hydrocracking was performed on Pt/HY-zeolite and on Pt/H-USY-zeolites with Si/Al ratio of 13 and 30 at temperatures of 506-563 K, pressures of 0.45-1.5 MPa, and molar hydrogen to hydrocarbon ratio's in the 4.23-250 range. The catalytic conversion was described with a fundamental molecular model, relying on experimentally determined physisorption equilibria and on a network of elementary reactions according to the bifunctional reaction scheme. The three zeolite samples showed substantial differences in activity, but not in selectivity. The activity differences among the zeolites mainly resulted from differences in both the number of acid sites and the average acid strength, while differences in physisorption effects for these zeolite samples were of minor importance. On each catalyst, the reactivity of alkanes increased with carbon number. This tendency was related to three phenomena: (1) physisorption of heavier molecules was more favorable; (2) the reaction network and the number of parallel reactions became larger with larger molecules, and (3) in the range of carbon numbers from C-5 to C-8, the stabilization of alkylcarbenium ions and, hence, their concentration increased with increasing size and electron donating property of alkyl-substituents. The differences in average acid strength between the three catalysts were quantified with alkene protonation enthalpy values extracted from the model. The kinetic parameters obtained for a reference hydrocarbon component and a reference Pt/H-(US)Y-type zeolite are adaptable to any other hydrocarbon and any other Pt/H-(US)Y-type catalyst by adjusting the standard protonation enthalpy. (C) 2001 Academic Press.