The SrCl2-promoted Ln2O3 (Ln=Sm and Nd) catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. With the doping of SrCl2 into Ln2O3, the C2H4 selectivity and C2H6 conversion were enhanced considerably. We also found that the addition of SrCl2 to Ln2O3 could markedly reduce the deep oxidation of C2H4. The 40 mol% SrCl2/Ln2O3 catalysts were stable for 60 h of on-stream ODE reaction. Under the reaction conditions of temperature=640°C and space velocity=6000mlh-1 g-1, 40 mol% SrCl2/Sm2O3 showed 80.3% C2H6 conversion, 70.9% C2H4 selectivity, and 56.9% C2H4 yield while 40 mol% SrCl2/Nd2O3 gave 63.8% C2H6 conversion, 74.3% C2H4 selectivity, and 47.4% C2H4 yield. X-ray photoelectron spectroscopic and chemical analysis of chloride indicated that the Cl- anions were evenly distributed in the 40 mol% SrCl2/Ln2O3 catalysts. We observed that Cl leaching was insignificant. The results of temperature-programmed desorption of oxygen and temperature-programmed reduction studies demonstrated that the addition of SrCl2 to Ln2O3 enhanced the activation of oxygen molecules. We believe that such improvement is closely associated with the defects formed during the exchanges of ions between the SrCl2 and Ln2O3 phases. X-ray powder diffraction results revealed that the Ln2O3 lattices were enlarged, whereas the SrCl2 lattices contracted in the 40 mol% SrCl2/Ln2O3 catalysts. In situ Raman results indicated that there were dioxygen adspecies such as O22-, O2n- (1<n<2), O2-, and O2δ- (0<δ<1) on the 40 mol% SrCl2/Ln2O3 catalysts. Electron paramagnetic resonance (EPR) results indicated that there were dioxygen O2- and mono-oxygen O- adspecies present on the SrCl2-doped catalysts. Based on the results of in situ Raman and EPR studies as well as the catalytic activity data, we suggest that the O22-, O2n-, O2-, and O2δ- adspecies favor the selective oxidation of C2H6 to C2H4, whereas the O- adspecies is responsible for the deep oxidation of C2H6.
Scopus Subject Areas
- Process Chemistry and Technology
- Oxidative dehydrogenation
- Oxygen species
- Raman and EPR characterization
- Rare earth oxides
- SrCl-promoted SmO and NdO catalysts