Thermochemical processes for CO2 hydrogenation to fuels and chemicals: Challenges and opportunities

This manuscript discusses the potential use of CO2 as a carbon and oxygen carrier to return it to the carbon life cycle in the form of fuels or chemicals via thermochemical catalytic pathways. Theoretically, CO2 hydrogenation can form a variety of fuels and chemicals. Practically, however, the selectivity, conversion, operating range, and kinetic limitations and operational cost determine the end product. Because of their high value and versatility as a chemical or fuel, small olefins (i.e., C2H4) and methanol are often considered as target species. Whether alcohol or olefin formation, CH4 and CO are the nature's primary choices of CO2 conversion. They can be formed over a wide operating temperature, pressure, and CO2:H2 ratios. They are often competitive carbon species in olefin or methanol formation steps. Although they are less valuable as end products, they can be used as intermediate species. Secondary processes of CO and CH4 to form chemicals and fuels can increase the overall CO2 hydrogenation yield. Independent of the choice of end product, CO2 hydrogenation requires hydrogen. Pure hydrogen derived mostly from fossil fuels adds to the overall process cost and also increases the CO2 emissions. Hydrogen produced from water electrolysis is a renewable green pathway but is limited by the process efficiency and cost. As an alternative to pure hydrogen, low-value, small chain paraffins are suggested as the hydrogen carriers. Carbon dioxide-assisted oxidative dehydrogenation of paraffins to olefins process is a direct pathway to olefin formation. The paper discusses these potential pathways for CO2 utilization based on theoretical analysis and recent advances in academia and industry.