The paper in Nature Communications can be found here.
One of the most challenging scientific issues of our time is to effectively convert the very stable CO2 molecule into value-added products. High atmospheric concentrations of CO2 contribute to ocean acidification and other detrimental effects on the climate that impact human health. The need to reduce CO2 emissions is evident, and the International Panel on Climate Change (IPCC) stated that climate stabilization (no more than a 2oC rise from pre-industrial levels) will require rigorous combinations of mitigation, utilization, and even negative emission technologies. Many of the storage and sequestration technologies under development today would still require a price tag on carbon emissions to become economically viable. Since our group specializes in heterogeneous thermocatalysis with bimetallic catalysts, we entertained this leading question:
How can we effectively utilize CO2 to help close the carbon cycle, but with a chemical process that would be economically incentivizing in today’s market?
The reverse water gas shift reaction (RWGS) and the dry reforming of methane (DRM) are the most widely investigated catalytic CO2 reduction reactions. However, the feasibility of RWGS depends on large scale CO2-free renewable hydrogen sources, which are still under development. We realized that a viable option was to utilize other light hydrocarbons, such as ethane or propane, as feedstocks for CO2 reduction. Unlike CH4, these light alkanes contain a C-C bond that could be preserved and used to build valuable olefins. Particularly, it is of interest to use propane due to its increasing abundance, competitive pricing, and highly marketable corresponding olefin. Propylene is an important petrochemical building block that is used in the production of polypropylene, propylene oxide, and acrylonitrile. Currently, propylene is co-produced by steam cracking of naphtha and fluidized cracking of both heavy and light oils. In recent years, the cracking of lighter feeds has caused a significant gap in propylene production. As the demand for propylene continues to rise globally, industry has turned to investigate direct, “on-purpose” propylene technologies. Utilizing CO2 as a soft oxidant has several advantages compared to conventional direct propane dehydrogenation or even O2 assisted oxidative dehydrogenation. Thus, CO2 reduction by propane is a viable chemical process that is economically incentivizing since it has the potential to fill the propylene gap while consuming a greenhouse gas.
In the present study, Fe-Ni, Fe-Pt, and Ni-Pt supported on CeO2 were evaluated for the reduction of CO2 with propane at atmospheric pressure and 823 K. At this temperature the CO2+C3H8 chemistry can proceed via two distinct reaction pathways simultaneously. The dry reforming of propane (DRP) can produce synthesis gas in a favorable ratio for downstream Fischer Tropsch reactions, while the CO2 oxidative dehydrogenation of propane (CO2-ODHP) can produce propylene. We were interested in identifying catalytic systems that either can effectively break the C-C bond to produce reforming products or selectively break the C-H bonds to produce propylene.
Through flow reactor studies and Density Functional Theory calculations, we found that Fe-Ni favors the production of propylene via the CO2-ODHP reaction pathway whereas the Ni-Pt catalyst favors DRP. In situ X-ray absorption near edge and extended X-ray absorption fine structure spectroscopy identified that the Fe-Ni catalyst consists of metallic Ni and oxidized Fe. This finding is key for future efforts geared toward the design of more active and selective catalytic materials for the CO2-ODHP reaction.