The Performances and Characterization of BaO- and BaX2 (X=F, Cl, and Br)-Promoted Y2O3 Catalysts for the Selective Oxidation of Ethane to Ethene

Abstract

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 O22−, O2n− (1<n<2), O−2, and O2δ− (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− O2n−, O2δ−, and O22− species participate in the selective oxidation of ethane to ethene whereas the O− species were responsible for the deep oxidation of ethane.