Methanol conversion on ZSM-22, ZSM-35 and ZSM-5 zeolites: effects of 10-membered ring zeolite structures on methylcyclopentenyl cations and dual cycle mechanism

Abstract

ZSM-22, ZSM-35 and ZSM-5, aluminosilicate zeolites possessing 10-membered ring channels, have been used in the present study as the catalysts of the MTO reaction. The diversities in dimensions and connection types of the 10-membered ring channels of the three zeolite catalysts make their performances in the MTO reaction quite different. As the key active species involved in the hydrocarbon-pool mechanism in the MTO reaction, methylcyclopentenyl cations (MCP⁺) and methylbenzenes have been captured by ¹³C MAS NMR and GC-MS over the three zeolite catalysts during methanol conversion. The comparative studies of the retained organics generation over the zeolite catalysts indicate that due to the spatial confinement effects of the inorganic frameworks, the retained organic species generated in the catalysts during the MTO reaction are influenced by both their sizes and amounts. A detailed analysis of the confined organic species showed the formation of MCP with varied methyl substitutions over the three zeolites. ¹²C/¹³C-methanol switch experiments were employed to investigate the reaction route for product generation. The differences in the participation levels of the methylbenzene and methylcyclopentadiene over the three zeolite catalysts imply that the formation and function of the organic species formed in the 10-membered ring channel were impacted by the chemical environment of the zeolites, and the methanol conversion that occurred in the 10-membered ring channels of the three zeolites also followed different reaction routes.