Quantum-chemical modeling of the hydrocarbon transformations in acid zeolite catalysts

Results of quantum-chemical modeling of a number of elementary steps involved in the acid zeolite-catalyzed conversion of hydrocarbons are collected together and compared. The elementary steps considered are protolytic cracking, protolytic dehydrogenation, hydride transfer, skeletal isomerization, and beta-scission. The hydrocarbon parts of transition states (TS) for these steps represent carbocations specific for each reaction. Geometry parameters of the TS and activation energies depend on the relative stability of these carbocations. The reactions considered can proceed via several alternative routes dependent on the species involved and on the details of the interaction of the hydrocarbon portion of the activated complex with the zeolite oxygen atoms. Variation of the acid strength of zeolite cluster models can be employed for studies of the acid strength sensitivity of the activation energies and other quantities of interest as well as for extrapolation of these quantities computed on small clusters towards zeolitic values.