Direct hydrogenation of CO₂ into valuable aromatics over K/Fe-Cu-Al @HZSM-5 tandem catalysts: Effects of zeolite surface acidity on aromatics formation

Direct hydrogenation of carbon dioxide (CO₂) to value-added aromatics can not only provide a sustainable aromatics synthesis route but also realize CO₂ mitigation. However, it remains a great challenge to identify the optimal surface acidity and regulate the selective formation of aromatics during CO₂ hydrogenation. Herein, we designed and prepared a series of K/Fe-Cu-Al@HZSM-5 tandem catalysts, and deeply investigated the effects of zeolite surface acidity on the catalytic performance, by changing the SiO₂/Al₂O₃ ratios of HZSM-5 from 25 to 400. It was found that the surface acidity, especially for the Brønsted acidity, plays a crucial role in the aromatics formation. With an increasing Brønsted acidity from 0 to 290 μmol/g, the CO₂ conversion keeps relatively stable (around 44%), however, the aromatics yield monotonously increases from 0.9 to 12.8%, indicating a distinct correlation with the Brønsted acidity. Among the obtained aromatics, the majority ones are C_6–8 light aromatics, accounting for c.a. 30–60% within time on stream (TOS) of 24 h, but their proportion tends to gradually decrease with TOS, probably due to the catalyst deactivation by coking. Moreover, the possible reaction pathways for aromatics formation over the prepared K/Fe-Cu-Al@HZSM-5 tandem catalysts were also proposed.