Methanol to Propene over Gallium-Modified FAU Zeolite: Theoretical Study on the Polymethylbenzene and Alkene Cycles

The catalytic performance of gallium-modified acidic FAU (Ga-FAU) zeolite in the methanol to propene (MTP) process was theoretically investigated by a two-layer ONIOM (our own N-layered integrated molecular orbital and molecular Mechanics) method. The whole MTP mechanism includes two cycles: the polymethylbenzene (polyMB) cycle and the alkene cycle. The polyMB cycle consists of the direct internal H-shift pathway, spiro pathway, methyl-transfer pathway, and paring pathway. Compared with the previous theoretical results on the MTP process on the pure acidic Si/Al FAU, the addition of Ga atoms into the Si/Al H-FAU could reduce the contribution of the polyMB cycle and increase the reactivity of the alkene cycle on the MTP process. A careful analysis of activation free energy barriers on the transition states (TSs) shows that the internal H-shift step is the rate-determining step for the direct internal H-shift pathway and the paring pathway. The rate-determining step for the spiro and methyl-transfer pathways is the methylation of the PMB molecule. The methylation of the propene molecule is the rate-determining step for the alkene cycle. The polyMB and alkene cycles have almost the same reactivity on the Ga-FAU zeolite. For the polyMB cycle, the different elementary steps are in the following order of reactivity: internal methyl transfer > deprotonation > C–C bond cracking > ring contraction > methylation > internal H-shift. The order in the alkene cycle is different: deprotonation > proton transfer > beta-scission > methylation. The differential charge density (DCD), local orbital locator (LOL), and reduced density gradient (RDG) revealed the direction of electron flow in different fragments in the TS structures and the nature of interactions between the fragments in the TSs.