CO₂-Assisted propane aromatization over phosphorus-modified Ga/ZSM-5 catalysts

Modification of HZSM-5 with phosphorus (P) and gallium (Ga) was found to enhance the activity of CO₂-assisted oxidative dehydrogenation and aromatization (CO₂-ODA) of propane to liquid aromatics. Flow reactor studies measuring the simultaneous CO₂ reduction and the CO₂-ODA of propane were performed at 873 K at atmospheric pressure over unmodified HZSM-5 and P/Ga/ZSM-5 catalysts. At comparable propane conversion, HZSM-5 showed a high selectivity toward the undesired methane product (41.4%) with a relatively low selectivity toward aromatics (16.5%). In comparison, P/Ga/ZSM-5 demonstrated a significantly higher aromatics selectivity (42.3%) with a simultaneous reduction in methane selectivity (13.5%). Density functional theory (DFT) calculations identified the mechanisms for propane dehydrogenation and aromatization over P/Ga/ZSM-5 and the role of CO₂. Cooperative with P, CO₂ addition avoided the formation of molecular H₂ and reduced the barriers for benzene formation, leading to an enhanced activity and selectivity to aromatics from CO₂-ODA of propane over P/Ga/ZSM-5.