Reaction Mechanism of Benzene Methylation with Methanol over H-ZSM-5 Catalyst

The stepwise and concerted mechanisms of benzene methylation with methanol were studied with the 5T, 12T, 104T(9), and 104T(12) H-ZSM-5 models using the "our own-N-layered integrated molecular orbital + molecular mechanics"(ONIOM) in combination with density functional theory (DFT) methods. The structures of intermediate species and transition states were described. The effect of the Bronsted (B) acid strength of H-ZSM-5 catalyst on the reaction mechanism of benzene methylation with methanol was considered. The reaction activation energy results indicate that benzene methylation with methanol preferentially occurs over H-ZSM-5 catalyst with greater B acid strength, and a lowering of the activation barrier was observed. With increasing B acid strength, the reaction activation energy of the stepwise mechanism decreases more than that of the concerted mechanism. Increasing the B acidic strength is more beneficial to the stepwise mechanism. When the stepwise mechanism becomes the dominant reaction path, the secondary reaction arising from further formation of bulky hydrocarbons through the methoxide intermediate produced in the methanol dehydration step of the stepwise mechanism might lead to the inactivation of the H-ZSM-5 catalyst owing to coke formation. Reasonable modulation the acid strength of the H-ZSM-5 catalyst is important in improving its catalytic activity and stability of the catalyst.