Insight into the Formation of COX By-Products in Methanol-to-Aromatics Reaction over Zn/HZSM-5: Significantly Affected by the Chemical State of Surface Zn Species

摘要

Several Zn-modified HZSM-5 catalysts prepared by impregnation or ion exchange were characterized, and tested in methanol-to-aromatics (MTA) reaction. The obviously enhanced output of COX by-products (CO & CO2) was observed over these modified catalysts, although they showed the significantly improved selectivity of aromatics. It was found COX formation is actually determined by the chemical state of surface Zn species. Concretely, both surface ZnO and surface metal-zinc are the reactive sites for the conversion of methanol to COX and H-2. At the initial period of reaction, surface ZnO (no reduction) causes COX (mainly CO2) formation may almost-exclusively through methanol steam-reforming. With the extending of reaction-time, while it ' s partially reduced to metal-zinc which more violently catalyzes the conversion of methanol to COX (both CO and CO2) may through the pathway of first dehydrogenation-decomposition and subsequent Water-Gas-Shift reaction. However, the surface ZnOH+ species as Lewis acid hardly results in COX formation, but significantly promotes aromatization due to its interaction-dehydrogenation effect. As high as possible ratio of the ZnOH+ in surface Zn species is conducive to both suppressing the side-reaction which produces COX and incubating the high aromatic selectivity even in the time-on-stream reaction. Our work may provide a theoretical guidance for developing the elegant catalyst for the MTA process with high carbon atom utilization.