Chemical looping oxidative coupling of methane (CL-OCM) is a promising process for direct methane conversion to C2 products. 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. 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. Co-doping is a promising strategy that can be used for effectively tuning the oxygen vacancy populations. This study performs redox experiments that probe the C2 selectivity enhancement of composite oxygen carrier through the use of lithium and tungsten dopants. The Li and W doubly doped oxide material showed a significant improvement in the reactivity towards chemical looping oxidative coupling of methane as compared to Li or W doped oxygen carriers. This improvement was seen to be directly related to the relative amounts of doping in the Mg6MnO8 unit cell. The optimum composition for the Li-W doubly doped oxygen carrier was identified to be 2.65% of Li and 4% of W, giving a C2+ yield of ~30% at 850°C. The atomistic thermodynamics methods and density functional theory calculations are carried out to investigate the co-doping effect. It reveals that Li and W dopants have a synergistic effect on the surface termination geometry and oxygen vacancy formation, which significantly influences the correlation between the C−H bond activation and the CHx radical adsorption. This study provides guidance on catalytic dopant screening for chemical looping oxidative coupling of methane and will have an impact on catalytic oxygen carrier design.

Chemical looping oxidative coupling of methane over W-Li bimetal-doped Mg-Mn composite redox catalysts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.