Topology-Dependent Free-Energy Barriers Underlying Activity Differences in Toluene Disproportionation over HZSM-5 and HMCM-22 Zeolites

Abstract Toluene disproportionation is an important industrial route to para-xylene, in which HZSM-5 (MFI topology) and HMCM-22 (MWW topology) are two representative zeolite catalysts that exhibit pronounced differences in their catalytic activity. To understand the molecular origin of this activity difference, we employ ab initio molecular dynamics combined with enhanced free-energy sampling techniques to investigate the complete reaction network of toluene disproportionation over representative HZSM-5 and HMCM-22 models. The calculated free energy profiles show that HZSM-5 favors a stepwise methyl-transfer route, whereas HMCM-22 favors a concerted route. Toluene protonation barriers are comparable in the two zeolites, and transition-state confinement alone, as captured by radial distribution functions around the methyl-transfer transition states, does not provide a uniform explanation for the barrier trends across the two mechanisms. Decomposition of the concerted methyl-transfer barriers further reveals that the near-attack barriers are comparable in both frameworks, while the substantially higher overall barriers in HZSM-5 are mainly associated with the free-energy cost of bringing the two aromatics from their initially separated sites into a near-attack configuration. A geometric descriptor based on the distance change between the initial and transition states is further introduced and shown to correlate approximately linearly with the calculated barriers across both zeolites and both mechanisms.