Effective COₓ Suppression by a Na₄Mg(WO₄)₃ Promoter in Chemical Looping 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_x 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_x formation. Compared to the state-of-the-art Na₂WO₄-promoted Mg₆MnO₈ catalyst, the Na₄Mg(WO₄)₃ promoter lowers CO_x 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 in situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (H₂-TPR), C₂H₆-temperature-programmed surface reaction (TPSR), methanol-TPSR with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and in situ 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_x formation.