(431b) Synergistic Conversion of Captured CO2 and Green H2 to Olefins and SAF By Microwave Catalytic Processing

The escalation of atmospheric carbon dioxide (CO₂) concentrations linked directly to human activities presents a formidable environmental problem, encompassing oceanic acidification, augmented sea levels, and the intensified occurrence of heat waves. Through the strategic utilization of CO₂, harmful emissions can be mitigated by transforming CO₂ into valuable chemicals. A distinguished methodology entails the conversion of CO₂ into C2+ hydrocarbons, notably light olefins such as ethylene, propylene, and butylene, which are pivotal for the chemical industry. The olefins can be oligomerized to from sustainable aviation fuel (SAF) to benefit aviation fuel industry. However, the utilization of CO₂ is impeded by its chemical inactivity and the elevated energy barriers associated with the formation of carbon-carbon bonds, rendering the efficient and selective transformation of CO₂ to C₂+ derivatives a challenging yet immensely promising venture. CO₂ is located at the bottom of the energetic ladder in thermodynamics, and the reaction of CO₂ requires considerable Gibbs energy input (the Gibbs formation energy, ΔG= −394.4 kJ mol^−1). This research is pertinent to the processes of transforming CO₂ into methane and the non-oxidative coupling of methane to form ethylene. The primary challenges encountered include the formation of carbon deposits and inadequate selectivity favoring ethylene synthesis.

In this study, to overcome the activation energy barrier of CO₂ we have utilized microwave heating to energize the reaction. The application of electromagnetic energy, exemplified by microwave irradiation, can preferentially elevate the temperature of active sites on catalytic surfaces, thereby facilitating the activation of carbon-oxygen double bonds in CO₂, a crucial step in the hydrogenation of carbon dioxide. The incorporation of microwave energy has the potential to modulate reaction mechanisms, reduce the required thermal threshold for reaction initiation, and attenuate the kinetics of carbonaceous deposit formation. Consequently, these alterations can lead to a significant improvement in both the yield and selectivity of the desired chemical products.

In the present investigation, we have engineered a ruthenium-supported cerium dioxide (Ru/CeO₂) catalyst that demonstrates a notable selectivity for methane across a broad temperature spectrum, alongside a molybdenum catalyst supported on cerium dioxide (Mo/CeO₂), which catalyzes the conversion of methane into olefins. Our strategy encompasses the synthesis of lower olefins through a dual catalytic approach: firstly, the hydrogenation of CO₂ to methane facilitated by Ru/CeO₂, followed by the selective transformation of methane into lower olefins utilizing Mo/CeO₂. A single reactor consisted of two separate zones (thermal-microwave) was configured to allow methanation to take place in thermal zone followed by microwave-driven methane coupling zone. The resultant gaseous products are subsequently analyzed using an Inficon Fusion Micro-GC to elucidate the composition and efficiency of the catalytic processes. The catalysts used in either zone are characterized by XRD, XPS, TEM and in-situ Raman to correlate the catalytic structural changes with CO₂ conversion and selectivity to ethylene. Our approach achieved high CO₂ conversion of 70% , methane selectivity of 99.5%, and methane production up to 11445.8 umol CH₄/g.cat hr. This research shows that microwave-driven CO₂ transformation to ethylene can enable decarbonization of several industrial sectors by electrification. Efforts to mitigate CO₂ emissions in the chemical industry could catalyze a transition towards carbon-negative production, leveraging advanced technologies to transform environmental liabilities into sustainable assets.