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(706e) A Molten Carbonate Shell Modified Perovskite Redox Catalyst for Anaerobic Oxidative Dehydrogenation of Light Alkanes

Gao, Y. - Presenter, North Carolina State University
Wang, X., North Carolina State University
Liu, J., North Carolina State University
Li, F., North Carolina State University
Acceptor doped, redox-active perovskite oxides such as La0.8Sr0.2FeO3 (LSF), La0.8Sr0.2MnO3 (LSM) and La0.8Sr0.2Co0.2Fe0.8O3 (LSCF) are active for light alkane oxidation to COx. However, they show poor selectivity to olefins. This study investigates molten alkali metal salts as effective “promoters” to modify the surface of perovskite oxide for chemical looping–oxidative dehydrogenation (CL-ODH) of light alkanes including ethane, propane and butane. Li2CO3 coated LSF (LSF@Li2CO3) is an efficient redox catalyst for CL-ODH of ethane [1]. Unlike previously reported redox catalysts which are typically endothermic during the lattice oxygen assisted ethane ODH step, the labile oxygen species in LSF@Li2CO3 facilitate exothermic operation in both redox steps as confirmed by TGA-DSC measurements (-69.5 kJ/mol [O] during the ODH step and -40 kJ/mol [O] during the re-oxidation step). Up to 92.2% ethylene selectivity and 63.6% ethane conversion were obtained. TEM, XPS, XRD characterizations and DSC measurement indicate that the redox catalyst is composed of a layer of molten Li2CO3 covering the solid LSF substrate. 18O2-exchange experiments and electrochemical impedance spectroscopy indicate that the molten Li2CO3 layer facilitates oxygen shuttling from LSF bulk to the molten carbonate layer surface while blocking the non-selective sites for ethane oxidation. Further investigations of the potential reaction pathways indicates that peroxide species (O22-) are the most likely active species for CL-ODH. TGA measurements, in-situ XRD and Mössbauer spectroscopy indicate that Fe4+ species reduction to Fe3+ is responsible for the formation of the active peroxide, which are subsequently transported to the outer surface of the molten Li2CO3 layer for the ODH reaction (Fig. 1a). The formation of active peroxide via Fe4+ to Fe3+ transition is further supported by DFT calculation. With other molten alkali metal salt promoters, the perovskite oxide can also be modified for CL-ODH of propane and butane. More than 50% of C2+ olefin yield (ethylene + propylene) at 600 °C for CL-ODH of propane was obtained. In a similar manner, >40% C4 olefin yield at 500 °C for CL-ODH of butane was obtained (Fig. 1b). With high olefin selectivity, high olefin yield and favorable heat of reactions, the core-shell redox catalyst has excellent potential to be effective for intensified light alkane conversion. The mechanistic findings also provide a generalized approach for designing CL-ODH redox catalysts.


[1] Gao et al., Science Advanced, Accepted in 2020, In Press