Effect of Sodium Tungstate Promoter on the Reduction Kinetics of CaMn0.9Fe0.1O3 for Chemical Looping - Oxidative Dehydrogenation of Ethane

Reduction kinetics of unpromoted and Na2WO4-promoted CaMn0.9Fe0.1O3 redox catalysts are measured in the context of chemical looping-oxidative dehydrogenation (CL-ODH). CL-ODH is a promising alternative for ethylene production compared to steam cracking, as it reduces the energy demand and increases the single-pass ethane conversion. The ability of a redox catalyst for selective hydrogen combustion (SHC), i.e. selectively oxidizing hydrogen co-product from ethane dehydrogenation, represents an effective strategy for CL-ODH because it can shift the reaction equilibrium and facilitate exothermic overall reaction. In this work, kinetic models and parameters of unpromoted and Na2WO4-promoted, Fe-doped CaMnO3 (CaMn0.9Fe0.1O3) under H-2 and C2H4 were investigated. The reduction of unpromoted CaMn0.9Fe0.1O3 follows reaction-order models. C2H4 reduction has a higher energy barrier and a greater dependence on active lattice oxygen concentration, resulting in an order-of-magnitude decrease in the reduction rate. In comparison, the reduction of Na2WO4-promoted CaMn0.9Fe0.1O3 follows an Avrami-Erofe'ev nucleation and nuclei growth model. The addition of Na2WO4 more significantly suppressed C2H4 combustion relative to H-2 oxidation. As a result, the reduction rate of Na2WO4-promoted CaMn0.9Fe0.1O3 under H-2 was three orders of magnitude greater than that under C2H4, demonstrating its excellent SHC properties. The resulting redox catalyst was shown to be effective for ethane CL-ODH with measured 90.5% ethylene selectivity and 41.6% ethylene yield at 750 degrees C. The kinetics models and parameters provide useful information for CL-ODH reactor design and further development of the redox catalyst.