A quantum-chemical study of para/ortho-toluene alkylation by adsorbed methoxy species on zeolites

MNDO and PM3 semiempirical methods have been employed to study the mechanism of toluene alkylation by methanol on zeolites, by means of the cluster (H)₃SiO(CH₃)T(H)₂OSi(H)₃ in were T = Al, B, Ga. Two possible reaction mechanisms have been found. The first one corresponds to the classical process of aromatic electrophilic substitution, in which the transition state of the reaction shows distances and angles according to the precursor of the Wheland intermediate. The second mechanism proceeds in one concerted step, in which the transfer of the proton from the toluene to the basic oxygen of the zeolite is simultaneous to the transfer of the methyl cation from the zeolite to the toluene. The effect of chemical composition of the cluster on regioselectivity has been studied, and perturbed molecular orbital (PMO) analysis of the system at the ab initio HF/STO-3G level has shown that covalent interactions are important. The energy of the covalent interaction between the molecular orbitals of the toluene and the cluster mainly HOMO-LUMO, are more important in para than in ortho positions.