Abstract
The present study investigates the effect of sodium and tungsten promoters on Mg6MnO8-based redox catalysts in a chemical looping oxidative dehydrogenation (CL-ODH) scheme. CL-ODH has the potential to significantly lower energy consumption and CO2/NOx emissions for ethylene production compared with conventional steam cracking. Sodium tungstate (Na2WO4) was previously shown to be an effective promoter for Mg6MnO8-based redox catalysts. Overall, the CL-ODH reaction proceeds via parallel gas-phase cracking of ethane and selective combustion of H2 on the surface of the Na2WO4-promoted redox catalyst. Reaction testing indicates that both Na and W are necessary to form Na2WO4 and to achieve high ethylene selectivity. A Na:W ratio lower than 2:1 lead to significant formation of additional mixed tungsten oxide phases and decreases ethylene selectivity. Further characterizations based on low-energy ion scattering (LEIS) and differential scanning calorimetry (DSC) indicate that the NaW promoter forms a molten shell around the Mg6MnO8 redox catalyst. Methanol TPSR and in situ DRIFTS experiments indicate that the promoter significantly suppresses the number of basic sites on Mg6MnO8. 18O–16O exchange experiments reveal that the promoter decreases the rate of oxygen exchange. O2 cofeed studies indicate that below the melting temperature of Na2WO4, H2 and CO conversions are both inhibited, but above the melting temperature, H2 combustion significantly increased while CO combustion is still inhibited. On the basis of extensive characterizations, it was determined that H2 is primarily combusted at the gas–Na2WO4 molten shell interface via redox reactions of the tungsten salt, likely between the WO42– (tungstate) and WO3– (tungsten bronze).
