Comparison of Praseodymium Oxide and SrF2-Promoted Praseodymium Oxide Catalysts for Oxidative Coupling of Methane

Abstract

The performance and characterization of the SrCl2-promoted REOx (RE=Ce, Pr, Tb) catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) reaction. The doping of SrCl2 to REOx significantly reduced C2H4 deep oxidation and enhanced C2H4 selectivity and C2H6 conversion. It has been shown that the catalytic performance increases in the order of 30 mol% SrCl2/CeO2<30 mol% SrCl2/PrO1.83<40 mol% SrCl2/TbO1.75. We observed that Cl leaching was modest in the latter two catalysts but gradual Cl loss was observed over the first catalyst. Within a reaction period of 60 h, the first catalyst degraded, whereas the latter two catalysts were stable. The C2H6 conversion, C2H4 selectivity, and C2H4 yield measured 1 h after the start of the ODE reaction were, respectively, 72.6, 68.8, and 49.9% for 30 mol% SrCl2/CeO2, 79.1, 71.4, and 56.5% for 30 mol% SrCl2/PrO1.83, and 82.6, 75.8, and 62.6% for 40 mol% SrCl2/TbO1.75 at 660°C and 1.67×10−4 h g mL−1 contact time. The results of X-ray photoelectron spectroscopy (XPS) and chemical analyses of chloride indicated that Cl− ions were uniformly distributed in 30 mol% SrCl2/PrO1.83 and 40 mol% SrCl2/TbO1.75, but were not so in 30 mol% SrCl2/CeO2. The Ce 3d, Pr 3d, and Tb 4d spectra obtained in XPS studies demonstrated that there are RE3+ and RE4+ ions present in the SrCl2-doped catalysts and SrCl2 doping facilitates the redox cycle of the RE3+/RE4+ couple via RE3+ generation. The results of O2 temperature-programmed desorption (TPD) studies showed that the addition of SrCl2 to REOx could obviously lower the desorption temperature of lattice oxygen. Temperature-programmed reduction (TPR) results revealed that SrCl2 doping causes the reduction temperatures of lattice O2− in REOx to decrease; in other words, the activity of lattice O2− was promoted. We consider that such behaviors are closely associated with the defect structures formed in ionic exchanges between the SrCl2 and the REOx phases. X-ray diffraction (XRD) results indicated that, among the three SrCl2-doped catalysts, 40 mol% SrCl2/TbO1.75 showed a cubic TbO1.75 lattice most significantly enlarged and a SrCl2 lattice most pronouncedly shrunk. In situ laser Raman results indicated that there were dioxygen adspecies such as O2−2 and O−2 on the 30 mol% SrCl2/CeO2 catalyst. XPS results indicated that there were O−, O2−2, and/or O−2 species on REOx, 30 mol% SrCl2/CeO2, 30 mol% SrCl2/PrO1.83, and 40 mol% SrCl2/PrO1.75. On the basis of the results of in situ Raman, O2-TPD, TPR, 18O2- and C2H6-pulsing, and XPS studies, we suggest that O2−2 and O−2 as well as surface lattice O2− species participate in the selective oxidation of ethane to ethene, whereas in excessive amount, the O− species tend to induce the deep oxidation of ethane.