Quantum-chemistry calculations of surface complex and orbital control in para/ortho toluene alkylation catalyzed by big pore zeolites

By means of MNDO-PM3 semiempirical quantum-chemistry calculations, the mechanism for alkylation of toluene by methanol on two zeolite clusters with Al and Ga as TIII has been studied. The process variables controlled through the hypersurface of the reaction were: the distances of methyl group to the cluster of zeolite and to the toluene molecule; and the distances of the hydrogen atom of the toluene to be transfered to the zeolite cluster and to the toluene. The energetic factor associated with the electronic interaction between the molecular orbitals (LUMO of the zeolite-methoxy and HOMO of toluene) have been calculated by a PMO analysis for two geometries: the transition state and a geometry closer to reactants. On these bases, influence of orbital control and influence of the softness-hardness of the zeolite on regioselectivity are discussed.