New Theoretical Insights into the Contributions of Poly(methylbenzene) and Alkene Cycles to the Methanol to Propene Process in H-FAU Zeolite

The contributions of the poly(methylbenzene) (polyMB) and alkene cycles to the methanol to propene (MTP) process in H-FAU zeolite have been investigated by a two-layer ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method, which is important to understand the nature of formation of propene in zeolite with large pore sizes. The calculated results demonstrate that the different pathways in the polyMB cycle occur in the following order of reactivity: methyl-transfer pathway > spiro pathway > direct internal H-shift > paring pathway. The polyMB cycle is more competitive than the alkene cycle for the MTP process in H-FAU, which is different from the previous results on H-ZSM-S. Introduction of Li+ and Ag+ cations into FAU zeolite does not reduce the free energy barriers of the methylation steps involved in polyMB and alkene cycles, indicating that the experimental efforts to improve propene selectivity probably should focus on the physical effect of Li+ and Ag+ cations. For the step of formation of propene in both cycles, the difference in charge densities suggests a clear electron transfer between the propene fragment and the aromatic ring or propoxy group. Decomposing ONIOM energy barriers into quantum mechanics and molecular mechanics contributions suggests that the stabilizing effect of the zeolite environment on transition states mainly originates from the van der Waals interactions for the spiro and methyl-transfer pathways in the polyMB cycle, but from the electrostatic interactions for the alkene cycle. Generally speaking, the formation step of propene is entropy-increased. The direct internal H-shift and paring pathways are entropy-decreased. The entropy effect in the alkene cycle is larger than that in the polyMB cycle due to the larger entropic barriers.