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 oxid ative dehydrogenation of ethane reaction. Adding BaO or BaX2 to Y2O3 could significantly enhance the C2H4 selectivity. We also found that the doping o f BaX2 into Y2O3 could considerably reduce C2H4 deep oxidation. Among these catalysts, 30 mol% BaCl2/Y2O3 performed the best. It was stable within a r eaction 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 degr ees C and 6000 mL h(-1) g(-1) space velocity. X-ray photoelectron spectrosc opy and chemical analysis of halides indicated that the Cl- ions were unifo rmly distributed in 30 mol% BaCl2/Y2O3 whereas the halide ions in 30 mol% B aF2/Y2O3 and 30 mol% BaBr2/Y2O3 were not. With the increase of space veloci ty, the C2H6 conversion decreased and the C2H4 selectivity increased at 640 degrees C over the 30 mol% BaCl2/Y2O3 catalyst. We observed that Cl leachi ng 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 temp erature-programmed desorption (CO2-TPD) results demonstrated that the 30 mo l% 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 po isoned by CO2 due to BaCO3 formation. O-2-TPD studies showed that the addit ion of BaO (or BaX2) to Y2O3 could obviously enhance the adsorption of oxyg en 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 90 0 degrees C, 30 mol% BaCl2/Y2O3 showed a cubic Y2O3 lattice most significan tly enlarged and a BaX2 lattice most pronouncedly contracted. In situ laser raman results indicated that there were dioxygen adspecies such as O-2(2-) , O-2(n-) (1 < n < 2), O-2(-), and O-2(delta-) (0 < delta < 1) on the 30 mo l% BaO/Y2O3 and 30 mol% BaX2/Y2O3 catalysts. Electron paramagnetic resonanc e results indicated that there were monoxygen O- and dioxygen O-2(-) specie s on Y2O3, 30 mol% BaO/Y2O3, and 30 mol% BaX2/Y2O3. We suggest that the O-2 (-) O-2(n-), O-2(delta-), and O-2(2-) species participate in the selective oxidation of ethane to ethene whereas the O- species were responsible for t he deep oxidation of ethane. (C) 1999 Academic Press.