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

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.