Successful oxidative upgrading of methane to methanol would simultaneously utilize abundant shale gas reserves, reduce human reliance on traditional fossil fuels, and mitigate greenhouse gas emissions in a thermodynamically favorable manner. As the preferred oxidation target, methanol is quite versatile since it can be converted to a variety of hydrocarbons and retain much of the energy density of traditional fossil fuels. Unfortunately, the key issue with the oxidative conversion of methane is related to selectivity, rather than reactivity; it is difficult to avoid total oxidation to carbon monoxide or carbon dioxide. To partially oxidize methane, catalysts must be designed that either disfavor the elementary steps comprising the total oxidation process, or selectively release only a requisite amount of oxygen to produce the desired products.
Prof. Cynthia Lo from Washington University, St. Louis, MO hypothesizes that copper oxide/graphene oxide composites will be effective catalysts for the selective methane oxidation to methanol. The active sites in the proposed catalyst design mimic the active sites in the MMO enzyme, which contains oxo-bridged copper (II) complexes that stabilize methane derivatives. n-type Graphene oxide serves as the source of oxygen for this reaction, with p-type cupric oxide used to stabilize the composites. Photochemical activation of the catalyst composite will be used to achieve selective methane oxidation to methanol. The product distribution should be a monotonic function of the oxygen content on the graphene surface. The catalyst should be regenerated by oxidation in air.
With this EAGER award made by the Catalysis & Biocatalysis Program of the National Science Foundation, the PI plans to experimentally validate the catalyst design in a flow-through reactor and compare performance to the predictions generated from first-principles calculations. This requires a unique reactor design that facilitates selective methane oxidation to value-added chemicals and fuels. The new design is for a flow-through, monolithic optical fiber photoreactor necessary to accommodate the catalyst system proposed here. The logic of the catalyst design rests on utilizing the cupric oxide/graphene oxide composite as a mild oxidant should reduce the propensity of methane to completely oxidize to carbon dioxide. Using photochemical activation should facilitate the breaking of only one C-H bond in methane, compared to the multiple C-H bond dissociations observed at elevated temperatures in standard oxidation reactions. The integration of these concepts represents a potentially transformative approach to catalyst design and reaction engineering, where the catalyst provides one of the reactants (i.e., oxygen) for use in the process; indeed, this reaction may ultimately be incorporated into a chemical looping technology.
The broader impact of this proposal is to advance desired societal outcomes in two areas: the mitigation of greenhouse gas emissions, and the creation of an economically-viable means of transitioning from traditional fossil fuels to alternative energy technologies. As the global warming potential of methane is approximately 25 times higher than that of carbon dioxide, pursuit of basic scientific research on methane conversion technologies that are carbon neutral and do not produce carbon dioxide as byproducts is logical. More academic and industrial effort should be devoted to upgrading methane to value-added chemicals and fuels, and closing the carbon cycle. In addition, all materials used to synthesize the catalysts and construct the reactor are composed of inexpensive and earth-abundant elements, in the best example of sustainablility.
This project is appropriate for the EAGER funding mechanism, in that it involves a radically different approach to the current research efforts aimed at methane conversion. By developing a more complete understanding of catalyst structure-property-reactivity relationships across multiple length and time scales, a start will be made on achieving effective utilization of the abundant and inexpensive methane reserves, and lay the path towards energy independence. The skills of Prof. Lo will be utilized in testing the hypotheses in this EAGER. Over the past six years at Washington University, she has developed significant expertise in computational catalysis elucidating structure property relationships in metal oxide catalysts, and demonstrating that methane activation and dehydrogenation proceeds at the interface between catalyst phases (e.g., metal-support) particularly when site defects are present. This supports the catalyst hypothesis presented in this study.
The PI plans to continue existing efforts in science outreach to the local K-12 community, by working with underserved middle school students at KIPP: Inspire Academy in St. Louis City that are preparing for the Missouri Science Olympiad Competition; this effort has been ongoing since 2011, and has successfully promoted STEM learning. The participating middle school students will be invited to visit the PIs laboratory at Washington University and observe a demonstration of the reactor.
