Xin, Xin; Yang, Chengguang; Fang, Dong; Liu, Jia; Li, Shenggang

JOURNAL OF CATALYSIS

2025, ,

Xin, Xin; Yang, Chengguang; Fang, Dong; Liu, Jia; Li, Shenggang

Zeolite-based catalysts found successful applications for the methanol-to-olefins (MTO) reaction, which offers a non-petroleum route for olefin production. For the small pore H-SSZ-13 zeolites, recent studies revealed cooperative effects of the active sites in that direct participation of the second acid site has a substantial influence on the side-chain mechanism of the aromatic cycle. However, it is unclear whether such cooperative effects are specific to some particular zeolites, for instance whether the double acid sites in the medium pore H-ZSM-5 zeolites will have similar effects on the side-chain mechanism. Herein, we performed periodic density functional theory (DFT) calculations to investigate the potential influence of a proximate second acid site on the side-chain mechanism of the aromatic cycle for the H-ZSM-5 zeolites. Our calculations show that the formation of the 1,1-dimethyl-4-ethylbenzenium cation (diMEB+) with an ethyl group is the key step of the side-chain mechanism for both single and double acid site models, while the elimination of the ethyl side chain is facile. Introducing a second acid site in proximity should thus greatly reduce the free energy barrier of the rate-determining step for the side-chain mechanism from 1.73 to 1.41 eV. Moreover, this mechanism occurs by indirect rather than direct participation of the second acid site. Further analysis indicates that the promoting effect of the second acid site on the side-chain mechanism results from stabilization of the transition states of the methylation reaction. Our work provides valuable atomic level insights into the MTO reaction, and should benefit the rational design of more efficient zeolite-based catalysts for product distribution regulation.