Theoretical Investigation of the Isomerization and Disproportionation of m-Xylene over Medium-Pore Zeolites with Different Framework Topologies

Abstract

The strain energies of three trimethylated diphenylmethane (3mDPM) and six tetramethylated diphenylmethane (4mDPM) isomers serving as the main reaction intermediates of m-xylene isomerization and disproportionation over eight medium-pore zeolites with different framework topologies have been determined theoretically in order to elucidate the effects of zeolite pore structure on this aromatic transformation. Although the strain energies of 3mDPM and 4mDPM derivatives in MCM-22, TNU-9, and NU-87, all of which have large 12-ring cavities/channels, are always lower than 40 kJ mol–1, some of them in cavity-free ZSM-5, ZSM-57, and TNU-10 are characterized by the strain energies higher than 40 kJ mol–1. In particular, all the species in ZSM-22 and ZSM-23 with narrower one-dimensional 10-ring channels have the strain energies much higher than 40 kJ mol–1. On the other hand, the energy difference (<30 kJ mol–1) between the (dimethylphenyl)methylium ion and the transition state for formation of the tetramethylated benzenium-type carbenium ions was calculated to be much lower than the energy barrier (183 kJ mol–1) to the hydride transfer from the reactant molecule. The overall results of this study clearly show that transition-state shape selectivity is responsible for the formation of 3mDPM derivatives, as well as of slightly larger 4mDPM ones, in medium-pore zeolites.