Chemical looping oxidative dehydrogenation (CL-ODH) represents a novel approach for upgrading natural gas liquids to light olefins via the use of a metal oxide redox catalyst. In the CL-ODH scheme, thermal cracking or catalytic dehydrogenation of ethane and higher alkanes is coupled with selective hydrogen combustion (SHC) by a redox catalyst (such as CaMnO3), enabling autothermal operation, increased per-pass conversion, and greater-than-equilibrium yields. CaMnO3 promoted with 20 wt.% sodium tungstate (Na2WO4/CaMnO3) has recently emerged as an effective redox catalyst for ethane CL-ODH with high olefin selectivity; moreover, the introduction of B-site dopants to the perovskite structure can enhance the redox kinetics and material stability. In this presentation, we report a rigorous investigation of the redox reaction kinetics for the reduction of unpromoted and Na2WO4-promoted CaMnO3 redox catalysts by H2, C2H4, and C2H6 in the context of CL-ODH, including the determination of kinetic parameters, rates, and reaction models. Reduction rates of CaMnO3 under ethylene and ethane were significantly lower than under H2. Model fitting of reduction kinetics (dimensionless solid conversion X vs. t, and dX/dt vs. X) showed good agreement with reaction order-controlled models for CaMnO3 reduction and predicted greater oxygen site dependence and higher activation energy for CaMnO3 reduction by C2H4 as compared to H2. Avrami-Erofe’ev nucleation and growth models provided the best statistical fit to the reduction of Na2WO4/CaMnO3 in H2 and in C2H4. After Na2WO4 promotion, the reduction rate of CaMnO3 was three orders of magnitude lower in ethylene in comparison to hydrogen, consistent with its superior selectivity to hydrogen combustion. We continue by demonstrating the influence of Fe, Ti, and other transition metals as B-site dopants on the reduction kinetics and stability of unpromoted and Na2WO4-promoted CaMnO3. Finally, we share additional kinetics data and structural characterization results describing the phase behavior of Na2WO4 near its melting point and the kinetic regimes on either side of the transition to form a complete kinetic modeling description of CaMnO3-based redox catalysts for enhanced olefin production.

Reduction kinetics of perovskite oxide CaMnO3 for selective hydrogen combustion: Effects of B-site doping and promotion with Na2WO4

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.