Li-Promoted LaxSr2-xFeO4-δ Core-Shell Redox Catalysts for Oxidative Dehydrogenation of Ethane under a Cyclic Redox Scheme

Chemical looping oxidative dehydrogenation (CL-ODH) of ethane utilizes a transition metal oxide based oxygen carrier, also known as a redox catalyst, to convert ethane into ethylene under an autothermal cyclic redox scheme. The current study investigates a Li-promoted LaxSr2-xFeO4-delta (LaSrFe) redox catalyst for CL-ODH reactions. While LaSrFe without Li promoter exhibits low ethylene selectivity, addition of Li leads to high selectivity/yield and good regenerability. Up to 61% ethane conversion and 90% ethylene selectivity are achieved with Li-promoted LaSrFe. Further characterization indicates that the Li-promoted LaSrFe redox catalyst consists of LiFeO2 (disordered rock salt) and LaSrFe (Ruddlesden-Popper) phases. Moreover, the surface of the redox catalysts is enriched with Li cations. It is also determined the LaSrFe phase contributes to oxygen storage and donation, whereas the activity and selectivity of the redox catalysts are modified by the Li promoter: while oxygen for the CL-ODH reaction is supplied from the lattice of the LaSrFe phase, the enrichment of Li cations on the surface increases the resistance for O2- diffusion from the bulk and its subsequent evolution into electrophilic oxygen species on the surface. The nonselective nature of the surface oxygen species and the inhibition effects of Li promoter on O2- diffusion are further confirmed by pulse experiments. On the basis of such findings, it is concluded that Li-promoted LaxSr2-xFeO4-delta is an effective redox catalyst for ethane ODH in the absence of gaseous oxygen. Moreover, the selectivity of the redox catalysts can be enhanced by the alkali metal oxide promoters.