Molecular modeling studies on zeolite catalysts for shape-selective electrophilic substitution: formation of xylenes

Abstract

Energy minimization methodology has been used to study the interaction of isomers of xylene with ZSM-5, mordenite and MCM-22 zeolites. Adsorption sites and the mechanism of diffusion of the reactant molecules in the alkylation of toluene and xylene isomers inside these zeolites were studied. When molecules diffuse through 10-member and 12-member ring channels, they pass through favorable and unfavorable adsorption sites. ZSM-5 shows significant selectivity for p-xylene over the o- and m-xylenes. In large pore mordenite, the difference between favorable and unfavorable adsorption sites is not significant for xylene isomers. In the case of MCM-22, there are significant energy barriers, and the diffusivity is not high in spite of the presence of large cages. Results of this study emphasize the importance of the shape and the size of molecules as well as the pore dimensions and architecture of zeolites in effectively controlling molecular diffusion characteristics. The diffusion characteristics of the molecules inside a zeolite are sensitive to its pore architecture. The results offer the reasons based on molecular level interactions, for the experimentally observed shape selectivities in the above three zeolites.