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

Acceptor doped, redox-active perovskite oxides such as La_0.8Sr_0.2FeO₃ (LSF), La_0.8Sr_0.2MnO₃ (LSM) and La_0.8Sr_0.2Co_0.2Fe_0.8O₃ (LSCF) are active for light alkane oxidation to CO_x. 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 (O₂^2-) 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.

Reference

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