(298d) Understanding the Synergistic Effects of Feed Composition and Metal Oxide Formulation in a Chemical Looping Oxidative Coupling of Methane System

The abundance of methane in the form of shale gas greatly incentivizes the direct conversion of methane to value added products such as olefins. Chemical looping oxidative coupling of methane (CL-OCM) is a promising pathway for converting methane to hydrocarbons such as ethylene. The novelty of the chemical looping mode lies in methane being activated by a metal oxide, instead of molecular oxygen as done in the conventional co-feed OCM to produce ethylene as the desired product. The metal oxide thus serves as a catalytic oxygen carrier (COC), which provides lattice oxygen to oxidize hydrogen and dopant-induced oxygen vacancies serve as active sites for methyl radical formation and desorption. Previous studies have highlighted the mechanism of CL-OCM and the key reactions responsible for the production of ethylene on Mg₆MnO₈ COC. This information aided in a rational modification of the base COC resulting in the overall hydrocarbon yield of 28.6% for a modified Mg₆MnO₈ COC.

The objective of this study is to further optimize the modified Mg₆MnO₈ COC and investigate the synergistic effect of different feed compositions on the performance of CL-OCM by a combined computational-experimental approach. The effect of different supports for the COC on methane conversion and hydrocarbon selectivity has been investigated experimentally in a fixed bed reactor. These results are corroborated with solid characterization tools such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) to look at the bulk and surface changes. Thermogravimetric analysis (TGA) helps in characterizing the kinetics of lattice oxygen consumption on different supports. Ab initio atomistic simulations and density functional theory calculations were carried out to explore to the role of supports in enhancing the activity of COC. Further, the effect of co-feeding ethane with methane has also been investigated on the supported and un-supported COC. The trade-off between over-oxidation to carbon oxides versus OCM reaction temperature due to ethane in the feed has also been explored. This study provides a comprehensive understanding of the CL-OCM system and enables further development of this technology.