(654d) Synthesis of Aromatic Compounds Via CO2 hydrogenation on H-Fe-ZSM-5 / ZnO-ZrO2 Tandem Catalysts

Intensified reactors for conversion of CO₂ 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 have been introduced as a single step approach for chemical synthesis from CO₂and H₂. However, obtaining high selectivity to aromatics remains a challenge, with CO from the RWGS reaction and alkanes from olefin hydrogenation being significant co-products. To this end, we are interested in tuning the catalyst properties to favor aromatics production.

Brønsted acid sites (BAS) in H-ZSM-5 are important sites in methanol aromatization reactions. Prior reports have evaluated the modification of acid sites in H-ZSM-5 to promote aromatization. However, the effect of isomorphous substitution has not been investigated in these tandem catalytic reactions. In this work, H-[Al/Ga/Fe]-ZSM-5 zeolites are synthesized with Si/T ratios = 80, 300, affecting acid site strength and distribution of Brønsted and Lewis acid sites across these materials. These catalysts are combined in tandem with ZnO-ZrO₂ (methanol synthesis catalyst) for CO₂hydrogenation, seeking to better understand the reactivity and improve aromatics selectivity. Of these zeolites, H-[Fe]-ZSM-5/ZnO-ZrO₂ gave the highest selectivity for aromatics at Si/Fe = 80 (~45%), with good aromatics selectivity even at Si/Fe = 300 (~36%).

Interestingly, at Si/Fe = 300, the aromatization reactivity seems to be mainly controlled by Lewis acid sites, as evidenced by its competitive conversion rate despite no measurable zeolite Brønsted acid site density. To further study this family of zeolites, H-[Fe]-ZSM-5 materials over a broader range of different Si/Fe ratios are synthesized to further explore the compositional space. Under similar conditions, at Si/Fe = 160, H-[Fe]-ZSM-5/ZnO-ZrO₂ shows the highest selectivity for aromatics (~50%). Interestingly, there are no measurable BAS even at this Si/Fe ratio, implying the potential for extraframework Fe species being involved in the aromatization reaction pathways.