Effective CO_<i>x</i> 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 COx 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 COx formation. Compared to the state-of-the-art Na2WO4-promoted Mg6MnO8 catalyst, the Na4Mg(WO4)3 promoter lowers COx 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 COx formation.