Finding the active species: The conversion of methanol to aromatics over Zn-ZSM-5/alumina shaped catalysts

Highlights

• A shaped Zn-ZSM-5/alumina catalyst displays improved performance in the conversion of methanol to aromatics.

• The MTA mechanism is shifted from paraffin to hydrogen formation upon Zn incorporation.

• By X-ray absorption spectroscopy, it is possible to distinguish between Zn in the zeolite and Zn in the alumina binder.

• Calcination causes migration of Zn from the alumina binder to the exchange positions in the zeolite.

• At high Zn loadings, ZnAl₂O₄ species are detected by X-ray absorption spectroscopy.

Abstract

The Zn-loaded zeolite ZSM-5 is an active and promising catalyst for the conversion of methanol to aromatics with high yield. In this work, we have investigated the catalytic performance of shaped Zn-ZSM-5/alumina catalysts in this reaction at industrially relevant pressures. This has been combined with extensive characterization to identify the active Zn species. It is clear that the introduction of Zn leads to an improved catalyst lifetime when the reaction is carried out at elevated pressure. A clear shift from reaction pathways leading to alkane formation to pathways leading to molecular hydrogen formation during the production of aromatics is observed. For industrial catalysts, Zn incorporation is conveniently carried out by impregnation after shaping. A significant migration of Zn into the zeolite ion exchange positions is observed upon calcination of impregnated catalysts, and it is these species that are active for the reaction. At high Zn loadings, ZnAl₂O₄ species observable by X-ray absorption spectroscopy, but not detectable by diffraction, are formed in the alumina binder.