Influence of preparation method on the performance of Zn-containing HZSM-5 catalysts in methanol-to-aromatics

Abstract

Zn-containing HZSM-5 zeolites (Zn/ZSM-5) were prepared by four methods including impregnation (IM), ion exchange (IE), physical mixing with ZnO (PM), and direct synthesis (DS); the influence of preparation method on the catalytic performance of Zn/ZSM-5 in the process of methanol-to-aromatics (MTA) was investigated. The results indicated that Lewis acid sites of zinc species (ZnOH+) are formed by introducing zinc into HZSM-5, at the expense of the silanol hydroxyl and proton acid sites. The distribution of acid sites and the nature of zinc species as well as the subsequent catalytic performance of Zn/ZSM-5 in MTA are significantly influenced by the preparation method for introducing zinc. In Zn(PM)/ZSM-5, zinc is mainly present as macro ZnO particles and trace ZnOH+ is formed by solid state reaction; in Zn(IM)/ZSM-5, ZnOH+ is the main ingredient, together with nano ZnO particles dispersed in the zeolite channel; in Zn(IE)/ZSM-5 and Zn(DS)/ZSM-5, however, only ZnOH+ species are observed. There is a linear correlation between the amount of ZnOH+ species and the selectivity to aromatics for MTA over the Zn/ZSM-5 catalysts prepared by different methods; ZnOH+ species may promote the dehydrogenation of light hydrocarbons to aromatics and suppress the hydrogen transfer reaction and the formation of alkanes by depressing the Brønsted acidity. Zn(DS)/ZSM-5 with small particle size and high mesoporous volume exhibits the longest catalytic lifetime, whereas Zn(IE)/ZSM-5 with high fraction of surface ZnOH+ species gives the highest selectivity to aromatics in MTA.