Direct Conversion of CO₂ to Olefins over a Cr₂O₃/ZSM-5@CaO Cooperative and Bifunctional Material Under Isothermal Conditions

Direct conversion of point-source CO₂ into fine chemicals over cooperative and bifunctional materials (BFMs) - composed of adsorbents and catalysts - has emerged as a promising approach to improve the energy efficiency of the carbon capture and conversion processes. In this study, a bifunctional material consisting of Cr₂O₃/ZSM-5 catalyst and CaO adsorbent was developed and tested in the CO₂-oxidative dehydrogenation of propane (CO₂-ODHP) for reactive capture of CO₂ in a fixed bed reactor. First, CaO was prepared using two distinct methods: solid-state and citrate sol-gel. The citrate sol-gel method resulted in small and finely-distributed CaO particles, allowing more accessible sites for CO₂ adsorption. Consequently, a high CO₂ adsorption capacity of ∼14 mmol/g was achieved with fast adsorption kinetics compared to CaO prepared by the solid-state method. The CaO adsorbent was then combined with the Cr₂O₃/ZSM-5 catalyst for BFM synthesis and tested in the CO₂-ODHP process, targeting propylene production. The BFM was extensively characterized to provide insights into the BFM's surface chemistry, morphology, and reaction mechanism in the reactive capture process of CO₂-ODHP. The results revealed that under isothermal adsorption-reaction conditions at 600 °C, a propane conversion of 22.5%, a propylene selectivity of 55.3%, and an olefin selectivity of 67.3% were achieved. The excellent propylene selectivity was attributed to the catalyst acidity and redox property of the Cr₂O₃/ZSM-5 catalyst, which facilitated the reaction pathway of propane dehydrogenation in the process of CO₂-ODHP. Overall, this study renders Cr₂O₃/ZSM-5@CaO as promising BFMs with high CO₂ capture capacity and catalytic activity for integrated CO₂ capture and conversion in the ODHP reaction.