Effective CO_<i>x</i> Suppressionby a Na₄Mg(WO₄)₃ Promoter in ChemicalLooping Oxidative Dehydrogenation of Ethane

Chemical looping oxidative dehydrogenation (CL-ODH) of ethane represents a promising intensification strategy to produce ethylene. A key aspect to improving CL-ODH performance revolves around mitigating the formation of CO_<i>x</i> from the ethane feedstock and ethylene product. This work reports Na₄Mg­(WO₄)₃ as a highly effective promoter for the Mg₆MnO₈-based redox catalyst for ethane ODH through the enrichment on the oxide surface by Na₄Mg­(WO₄)₃ to suppress CO_<i>x</i> formation. Compared to the state-of-the-art Na₂WO₄-promoted Mg₆MnO₈ catalyst, the Na₄Mg­(WO₄)₃ promoter lowers CO_<i>x</i> selectivity by up to 88% on a relative basis while achieving up to 70% C₂₊ olefin yield. This represents some of the highest olefin selectivity and yield values among previously reported CL-ODH catalysts. The role of the promoter in suppressing nonselective oxidation was investigated using <i>in situ</i> X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (H₂-TPR), C₂H₆-temperature-programmed surface reaction (TPSR), methanol-TPSR with <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and <i>in situ</i> Raman spectroscopy. Compared with the Na₂WO₄-promoted Mg₆MnO₈, the Na₄Mg­(WO₄)₃ promoter exhibits improved dispersion of the promoter on Mg₆MnO₈ and inhibits the release of lattice oxygen from the Mg₆MnO₈ phase. Meanwhile, a higher average Mn oxidation state is maintained for the Mg₆MnO₈ phase. This suggests that the uniformity of promoter dispersion over the Mg₆MnO₈ phase, coupled with reduced lattice oxygen transport kinetics, is crucial in suppressing the formation of CO_<i>x</i> formation.