Unveiling the role of oxygen species in surface promoted Fe–Mn oxides for chemical looping oxidative dehydrogenation of ethane

Abstract

In this study, redox catalysts composed of Fe–Mn mixed oxides with Na₂WO₄ and Na₄Mg(WO₄)₃ promoters are investigated for chemical looping oxidative dehydrogenation (CL-ODH) of ethane. Although Na₄Mg(WO₄)₃ was shown to be more effective in inhibiting CO_x formation when used to promote a Mg–Mn oxide, both promoters performed similarly for Fe–Mn mixed oxides during CL-ODH. Due to their tendency to spontaneously release lattice oxygen at the CL-ODH operating temperature range, a number of reoxidation conditions were investigated for a Na₄Mg(WO₄)₃ promoted (Fe_0.1Mn_0.9)_xO_y redox catalyst to determine the effect of the cation oxidation state. Tuning the oxidation conditions significantly improved ethylene (from 59 to 63%) and C₂₊ high-value products (from 67 to 70%) yields, which can be primarily attributed to lowered CO_x formation. From in situ XRD, XPS, and pulsed experiments, the inhibition of CO_x formation resulted from the lower proportion of Mn³⁺ cations, which decreased the amount of highly active lattice oxygen species responsible for CO_x formation.