Effects of zeolite pore sizes on the mechanism and selectivity of xylene disproportionation—a DFT study

The disproportionation reaction of xylenes, catalyzed by acidic zeolites, is investigated by means of periodic ab initio calculations. Two relevant 10- and 12-membered ring (MR) frameworks are chosen: theta-1 (TON) and mordenite (MOR). As a first step, two bimolecular mechanisms proposed for this reaction are investigated: the first one is assumed to occur through a one-step methyl shift and involves a highly unstable methyl cation as a transition state. The second and energetically favored mechanism implies the formation of a benzylic-like carbocation, subsequently converted into a trimethyldiphenylmethane, also called DPM intermediate. The energetics of this reaction path are mainly governed by the different sterical constraints induced by the 10-MR or 12-MR zeolites on the DPM intermediates. Therefore, the relative stability of the DPM inside the pores appears as the key parameter for understanding the selectivity observed experimentally.