Mechanistic investigation of methanol to propene conversion catalyzed by H-beta zeolite: a two-layer ONIOM study.

Two-layer ONIOM calculations have been carried out to study methanol to propene (MTP) conversion reactions catalyzed by H-beta zeolite. On the basis of the so-called side-chain hydrocarbon pool (HCP) mechanism, this work proposes the complete catalytic cycle pathway for the MTP reaction. The cycle starts from the methylation of pentamethylbenzene (PMB), which leads to the formation of hexamethylbenzenium ion (hexaMB(+)). Subsequent steps involving deprotonation, methylation, an internal H-shift, and a unimolecular CH₃-shift are required to produce propene and ethene. The calculated activation barriers and reaction energy data indicate that propene is the more favored product, rather than ethene, from both kinetic and thermodynamic perspectives, which is consistent with experimental observations. In addition, the calculations suggest that the activation barriers of the reaction steps decrease in the order: internal H-shift > methylation > unimolecular CH₃-shift ≥ deprotonation. In the methylation step, methylation of the exocyclic double bond is easier than methylation of the ring carbons on the aromatic benzene derivative.