Oxidative coupling of methane over (Na 2WO 4+Mn or Ce)/SiO 2 catalysts: In situ measurement of electrical conductivity

Abstract

The effects of manganese oxide or ceria promoters on the performance of Na 2WO 4/SiO 2 catalysts for oxidative coupling of methane (OCM) are reported. The OCM reaction was performed in a continuous-flow microreactor at 800 °C, atmospheric pressure and under GHSV = 13200 ml·g Cat −1·h −1. Catalysts were characterized by in situ conductivity measurement, FT-IR spectroscopy, XRD, SEM and temperature programmed reduction analysis. Manganese oxide promoted Na 2WO 4/SiO 2 is considered as one of the active and selective catalysts for OCM reaction. Ceria with high oxygen storage capacity is selected as a proper oxygen activator, providing a higher concentration of the oxy-anion species which is suitable for OCM reaction and compared with manganese oxide. Electrical conductivity of the catalysts was measured in OCM reaction under oxidizing atmosphere, i.e. in the absence of methane. It was found that the trimetallic catalysts, i.e. the catalysts having sodium, tungsten and Mn or Ce species, exhibited similar crystalline structures and morphologies, which lead to suitable bulk properties for the formation of an active and selective catalyst. However, tungsten had significant effect on the texture and redox properties of the catalysts. It was also shown that the crystalline structure of the bimetallic (Na+Mn or Ce)/SiO 2 samples was quite different. This reveals that the metal oxides have significant effect on the extent of crystallization, taking place in the course of interaction of sodium with silica support. Similar conductivities and catalytic performances of (Na 2WO 4+Mn or Ce)/SiO 2 catalysts propose that the ability of Na 2WO 4/SiO 2 for utilizing oxy-anions formed in presence of different metal oxides is limited.