Ab initio and density-functional theory study of zeolite-catalyzed hydrocarbon reactions: hydride transfer, alkylation and disproportionation

The mechanism of some zeolite-catalyzed reactions of hydrocarbons involving the (C2H5-H-C2H5)(+) carbonium ion as a reaction intermediate has been theoretically investigated by means of the ab initio correlated second-order Moller-Plesset perturbation theory and the density-functional B3PW91 methods. It has been found that while in the homogeneous phase the mechanism of the acid-catalyzed hydride transfer, alkylation, dehydrogenation and disproportionation reactions can be explained in terms of different intramolecular rearrangements of a common intermediate, over a zeolite only those carbonium ions in which the positive charge is delocalized and sterically inaccessible to framework oxygens can exist as reaction intermediates, and the transformations they undergo are completely different from those occurring in homogeneous phase. In particular, any transformation leading to a localized charge distribution results in a transfer of an H atom to the catalyst surface and therefore on a neutral molecule adsorbed on the Bronsted acid site as the reaction intermediate.