Reduction Kinetics of Perovskite Oxides for Selective Hydrogen Combustion in the Context of Olefin Production

Abstract

Chemical looping represents a novel approach for generating light olefins in which thermal cracking or catalytic dehydrogenation is coupled with selective hydrogen combustion (SHC) by a metal oxide redox catalyst, which enables autothermal operation, increased per-pass conversion, and greater-than-equilibrium yields. Recent studies indicate that Na2WO4-promoted perovskite oxides are effective redox catalysts with high olefin selectivity. Herein, kinetic parameters, rates, and reaction models for the reduction of unpromoted and Na2WO4-promoted CaMnO3 redox catalysts by H2, C2H4, and C2H6, is reported. Reduction rates of CaMnO3 under ethylene and ethane are significantly lower than under H2. Model fitting of reduction kinetics show good agreement with reaction order–controlled models for CaMnO3 reduction and predict greater oxygen site dependence and higher activation energy for CaMnO3 reduction by C2H4 as compared with H2. Avrami–Erofe'ev nucleation and growth models provide the best fit to the reduction of Na2WO4/CaMnO3 in H2 and in C2H4. After Na2WO4 promotion, the reduction rate of CaMnO3 is three orders of magnitude lower in ethylene in comparison to hydrogen, consistent with its superior selectivity to hydrogen combustion. The models developed can be applied toward reactor design and optimization in the context of enhanced olefin production via SHC under a cyclic redox scheme.