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 funda mental molecular model, relying on experimentally determined physisorption equilibria and on a network of elementary reactions according to the bifunc tional reaction scheme. The three zeolite samples showed substantial differ ences in activity, but not in selectivity. The activity differences among t he zeolites mainly resulted from differences in both the number of acid sit es and the average acid strength, while differences in physisorption effect s for these zeolite samples were of minor importance. On each catalyst, the reactivity of alkanes increased with carbon number. This tendency was rela ted to three phenomena: (1) physisorption of heavier molecules was more fav orable; (2) the reaction network and the number of parallel reactions becam e 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 con centration increased with increasing size and electron donating property of alkyl-substituents. The differences in average acid strength between the t hree catalysts were quantified with alkene protonation enthalpy values extr acted from the model. The kinetic parameters obtained for a reference hydro carbon component and a reference Pt/H-(US)Y-type zeolite are adaptable to a ny other hydrocarbon and any other Pt/H-(US)Y-type catalyst by adjusting th e standard protonation enthalpy. (C) 2001 Academic Press.