A theoretical study on the methanol to propene mechanism catalyzed by a phosphorus-modified acidic FAU zeolite

Abstract

The reaction mechanisms of methanol to propene (MTP) on a phosphorus-modified acidic FAU (H-FAU) zeolite have been theoretically investigated by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. The MTP mechanisms include the polyMB cycle and the alkene cycle. A careful analysis of the potential energy surface (PES) suggests that the rate-determining steps for different pathways are the internal H-shift step of INT-direct-5 (the direct internal H-shift pathway), the internal H-shift step of INT-paring-1 (the paring pathway), the deprotonation step of INT-direct-5 (the spiro pathway), the methylation step of INT-direct-2 (the methyl transfer pathway), and the methylation step of 3-methyl-1-butene (alkene cycle). The different reaction steps in the polyMB cycle occur in the following order of reactivity: protonation > internal CH₃-shift > methylation > ring contraction/expansion > internal H-shift > deprotonation. In the alkene cycle, the order of reactivity is different: beta scission > internal H-shift > deprotonation > methylation. The addition of phosphorus atoms to the FAU zeolite decreases the apparent free energy barriers of the rate-determining steps. The differential charge density (DCD), local orbital locator (LOL) and reduced density gradient (RDG) plots reveal the direction of electron migration and the TS nature. The complicated van der Waals (VDW) interactions and weak covalent interactions between the atoms in the forming or breaking of chemical bonds are found in the different TS structures.