A QUANTUM-CHEMICAL STUDY OF PARA-TOLUENE ORTHO-TOLUENE ALKYLATION BY ADSORBED METHOXY SPECIES ON ZEOLITES

MNDO and PM3 semiempirical methods have been employed to study the mec hanism of toluene alkylation by methanol on zeolites, by means of the cluster (H)(3)SiO(CH3)T(H)(2)OSi(H)(3) in were T = Al, B, Ga. Two poss ible reaction mechanisms have been found. The first one corresponds to the classical process of aromatic electrophilic substitution, in whic h the transition state of the reaction shows distances and angles acco rding to the precursor of the Wheland intermediate. The second mechani sm proceeds in one concerted step, in which the transfer of the proton from the toluene to the basic oxygen of the zeolite is simultaneous t o the transfer of the methyl cation from the zeolite to the toluene. T he effect of chemical composition of the cluster on regioselectivity h as been studied, and perturbed molecular orbital (PMO) analysis of the system at the ab initio HF/STO-3G level has shown that covalent inter actions 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 pam than in ortho positions.