The performances and characterization of BaO- and BaX₂ (X = F, Cl, and Br)-promoted Y₂O₃ catalysts for the selective oxidation of ethane to ethene

The 30 mol% MO (M=Mg, Ca, Sr, Ba)-, 30 mol% BaCO₃-, and 30 mol% BaX₂ (X = F, Cl, and Br)-promoted Y₂O₃ catalysts have been investigated for the oxidative dehydrogenation of ethane reaction. Adding BaO or BaX₂ to Y₂O₃ could significantly enhance the C₂H₄ selectivity. We also found that the doping of BaX₂ into Y₂O₃ could considerably reduce C₂H₄ deep oxidation. Among these catalysts, 30 mol% BaCl₂/Y₂O₃ performed the best. It was stable within a reaction period of 40 h, giving a C₂H₆ conversion, a C₂H₄ selectivity, and a corresponding C₂H₄ yield of ca. 72, 74, and 53%, respectively, at 640°C and 6000 mL h^-1 g^-1 space velocity. X-ray photoelectron spectroscopy and chemical analysis of halides indicated that the Cl^- ions were uniformly distributed in 30 mol% BaCl₂/Y₂O₃ whereas the halide ions in 30 mol% BaF₂/Y₂O₃ and 30 mol% BaBr₂/Y₂O₃ were not. With the increase of space velocity, the C₂H₆ conversion decreased and the C₂H₄ selectivity increased at 640°C over the 30 mol% BaCl₂/Y₂O₃ catalyst. We observed that Cl leaching was not significant in 30 mol% BaCl₂/Y₂O₃. However, gradual Br leaching was observed over 30 mol% BaBr₂/Y₂O₃. X-ray powder diffraction and CO₂ temperature-programmed desorption (CO₂-TPD) results demonstrated that the 30 mol% BaCl₂/Y₂O₃ catalyst is durable and is resistant to CO₂ poisoning whereas the 30 mol% BaO/Y₂O₃ and BaX₂ (X= F and Br)/Y₂O₃ catalysts are readily poisoned by CO₂ due to BaCO₃ formation. O₂-TPD studies showed that the addition of BaO (or BaX₂) to Y₂O₃ could obviously enhance the adsorption of oxygen molecules. We consider that such enhancement is closely associated with the defects generated due to ionic exchanges between the BaO (or BaX₂) and the Y₂O₃ phases. Among the three 30 mol% BaX₂/Y₂O₃ catalysts calcined at 900°C, 30 mol% BaCl₂/Y₂O₃ showed a cubic Y₂O₃ lattice most significantly enlarged and a BaX₂ lattice most pronouncedly contracted. In situ laser raman results indicated that there were dioxygen adspecies such as O₂^2-, O₂^n- (1 < n < 2), O₂^- and O₂^δ- (0 < δ < 1) on the 30 mol% BaO/Y₂O₃ and 30 mol% BaX₂/Y₂O₃ catalysts. Electron paramagnetic resonance results indicated that there were monoxygen O^- and dioxygen O₂^- species on Y₂O₃, 30 mol% BaO/Y₂O₃, and 30 mol% BaX₂/Y₂O₃. We suggest that the O₂^- O₂^n-, O₂^δ-, and O₂^2- species participate in the selective oxidation of ethane to ethene whereas the O^- species were responsible for the deep oxidation of ethane.