Critical influence of the amorphous silica-to-cristobalite phase transition on the performance of Mn/Na2WO4/SiO2 catalysts for the oxidative coupling of methane

XRD, XPS/XAES, TPR analysis and catalytic testing have been applied to Na/W/Mn/SiO2 methane coupling catalysts and to corresponding formulations without one or more of Na, Mn, and W. We find a clear correlation between catalyst performance and support structure in the final calcined material. Amorphous silica yields active but very unselective catalysts. Crystalline SiO2 (alpha-cristobalite) generates active and highly selective catalysts-especially with respect to the formation of ethylene. We demonstrate that the presence of Na is essential for the anomalous low temperature silica --> cristobalite support phase transition to occur. The structural, catalytic, and spectroscopic results indicate that Na plays a dual role as both structural and chemical promoter. (C) 1998 Academic Press.