Simultaneously enhanced aromatics selectivity and catalyst lifetime in methanol aromatization over [Zn, Al]-ZSM-5 via isomorphous substitution with optimized acidic properties and pore structure

Abstract

Zinc-modified HZSM-5 zeolites as efficient catalysts have been widely applied in methanol to aromatics (MTA) reactions, however, the relationship between aromatics selectivity and catalytic stability in traditionally loaded catalysts seems to be a stubborn trade-off. To improve both catalytic indicators simultaneously and reveal the relation of the states of introduced Zn species to the consequential influence in physiochemical properties, a series of [Zn, Al]-Z5 zeolites with different Zn contents were prepared via isomorphous substitution under a fixed Si/Me (Me = Al or Al and Zn) ratio. Results revealed that there were three possible states of the introduced Zn species, namely the highly-dispersed ZnO phase, the Zn(OH)+ occupying the ion-exchange sites, and the tetra-coordinated ZnO42- integrated into the zeolite framework. A portion of Zn was successfully integrated into the five-membered rings of zeolite in the form of Si-O(H)-Zn, which played the role of strong Brönsted acid sites and led to the formation of hierarchical structures aside from the intrinsic micropores. Besides, extra-framework Zn(OH)+ was formed by the interaction between dissociate Zn species and the Si-O(H)-Al structure, giving rise to larger proportions of Lewis acid sites. Benefitting from the optimized acidic properties and pore structure, 0.5[Zn, Al]-Z5 exhibited higher aromatics selectivity and longer catalyst lifetime (26.0% and 132 h) than [Al]-Z5 without Zn addition (19.6% and 52 h) in methanol aromatization. Overall, our findings opened a new way to improve the efficiency of MTA reaction and broadened the method to obtain metal-modified zeolite catalysts.