3 years ago

Alkali Metal-Promoted LaxSr2–xFeO4−δ Redox Catalysts for Chemical Looping Oxidative Dehydrogenation of Ethane

Alkali Metal-Promoted LaxSr2–xFeO4−δ Redox Catalysts for Chemical Looping Oxidative Dehydrogenation of
Fang He, Farrah Haeri, Fanxing Li, Yunfei Gao
Chemical looping oxidative dehydrogenation (CL-ODH) represents a redox approach to convert ethane into ethylene under an autothermal scheme. Instead of using gaseous oxygen, CL-ODH utilizes lattice oxygen in transition metal oxides, which acts as an oxygen carrier or redox catalyst, to facilitate the ODH reaction. The oxygen-deprived redox catalyst is subsequently regenerated with air and releases heat. The current study investigated alkali metal (Li, Na, and/or K)-promoted LaxSr2–xFeO4−δ (LaSrFe) as redox catalysts for CL-ODH of ethane. While unpromoted LaSrFe exhibited poor ethylene selectivity, addition of Na or K promoter achieved up to 61% ethane conversion and 68% ethylene selectivity at 700 °C. The promotional effect of K on LaSrFe was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEIS), transmission electron microscopy (TEM), O2-temperature-programmed desorption (TPD), H2-temperature-programmed reduction (TPR), and 18O2 surface exchange. XPS and XRD showed that K incorporates into the mixed-oxide structure at low loading levels (e.g., 0.1K-LaSrFe), whereas the surface of LaSrFe was enriched with K cation at high loading levels. LEIS indicates that the outermost surface layer was covered by potassium oxide. This surface layer was characterized to be amorphous under TEM. It was further determined that the surface layer increased the resistance for O2– diffusion from the bulk and its subsequent evolution into electrophilic oxygen species on the surface. As such, nonselective oxidation of ethane is inhibited. The synergistic effect of copromoting LaSrFe with Li and K was also investigated. Li and K copromotion improved the redox catalyst performance to 86% ethylene selectivity and 60% ethane conversion while maintaining an oxygen capacity of ca. 0.65 wt %, making it a promising candidate for CL-ODH.

Publisher URL: http://dx.doi.org/10.1021/acscatal.7b03928

DOI: 10.1021/acscatal.7b03928

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