Binary Li/Na tungstates modified Mg6MnO8 as redox catalysts for enhanced performance of chemical looping oxidative dehydrogenation of ethane

Abstract

The present investigation delves into the utilization of Mg6MnO8 modified with binary Li2WO4 and Na2WO4 tungstates as oxygen carriers for the chemical looping oxidative dehydrogenation (CL-ODH) of ethane. The Mg6MnO8-LiNaW2 oxygen carrier demonstrates remarkable performance, achieving up to 77.7 % ethane conversion, 86.5 % ethylene selectivity, and 67.2 % ethylene yield. HRTEM-EDS reveals the formation of a molten binary tungstates layer covering the solid Mg6MnO8 substrate. The molten binary tungstates layer effectively modulates the release of lattice oxygen in a step-wise manner during ODH reactions. XPS analysis reveals that the doping of Li/Na binary tungstates inhibits the formation of OH−/CO32− and Mn4+ species, thus preventing the deep oxidation of ethylene. The Mg6MnO8-LiNaW2 sample maintains stable reactivity and surface morphology throughout 20 redox cycles. DFT calculations further indicate that the modification with Li/Na binary tungstates promotes electron accumulation around the local environment of the Me-O bond, thereby reducing the formation energy (Eo) of oxygen vacancy in Mg6MnO8 and improving the oxygen vacancy concentration within the bulk of the oxygen carriers. The superior ethylene selectivity of the Mg6MnO8-LiNaW2 oxygen carrier can be attributed to the higher oxygen diffusion capability in the bulk of the Mg6MnO8 core, coupled with the lower oxygen transport rate through the molten tungstates layer.