Isomorphous Substitution in ZSM-5 in Tandem Methanol/Zeolite Catalysts for the Hydrogenation of CO 2 to Aromatics

Intensified reactors for conversion of CO to methanol (via hydrogenation) using metal oxide catalysts coupled with methanol conversion to aromatics in the presence of zeolites (e.g., H-ZSM-5) in a single step are investigated. Brønsted acid sites (BAS) in H-ZSM-5 are important sites in methanol aromatization reactions, and correlations of the reactivity with zeolite acid properties can guide reaction optimization. A classical way of tuning the acidity of zeolites is via the effect of the isomorphous substitution of the heteroatom in the framework. In this work, H-[Al/Ga/Fe]-ZSM-5 zeolites are synthesized with Si/ ratios = 80, 300, affecting the acid site strength as well as distribution of Brønsted and Lewis acid sites. On catalytic testing of the H-[Al/Ga/Fe]-ZSM-5/ZnO-ZrO samples for tandem CO hydrogenation and methanol conversion, the presence of weaker Brønsted acid sites improves the aromatics selectivity (CO to aromatics selectivity ranging from 13 to 47%); however, this effect of acid strength was not observed at low atom content. Catalytic testing of H-[B]-ZSM-5/ZnO-ZrO provides no conversion of CO to hydrocarbons, showing that there is a minimum acid site strength needed for measurable aromatization reactivity. The H-[Fe]-ZSM-5-80/ZnO-ZrO catalyst shows the best catalytic activity with a CO conversion of ∼10% with a CO to aromatics selectivity of ∼51%. The catalyst is shown to provide stable activity and selectivity over more than 250 h on stream.