Effects of Sodium and Tungsten Promoters on Mg 6 MnO 8 -Based Core–Shell Redox Catalysts for Chemical Looping—Oxidative Dehydrogenation of Ethane
The present study investigates the effect of sodium and tungsten promoters on Mg 6 MnO 8 -based redox catalysts in a chemical looping oxidative dehydrogenation (CL-ODH) scheme. CL-ODH has the potential to significantly lower energy consumption and CO 2 /NO x emissions for ethylene production compared with conventional steam cracking. Sodium tungstate (Na 2 WO 4 ) was previously shown to be an effective promoter for Mg 6 MnO 8 -based redox catalysts. Overall, the CL-ODH reaction proceeds via parallel gas-phase cracking of ethane and selective combustion of H 2 on the surface of the Na 2 WO 4 -promoted redox catalyst. Reaction testing indicates that both Na and W are necessary to form Na 2 WO 4 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 Mg 6 MnO 8 . 18 O– 16 O exchange experiments reveal that the promoter decreases the rate of oxygen exchange. O 2 cofeed studies indicate that below the melting temperature of Na 2 WO 4 , H 2 and CO conversions are both inhibited, but above the melting temperature, H 2 combustion significantly increased while CO combustion is still inhibited. On the basis of extensive characterizations, it was determined that H 2 is primarily combusted at the gas–Na 2 WO 4 molten shell interface via redox reactions of the tungsten salt, likely between the WO 4 2– (tungstate) and WO 3 – (tungsten bronze).