The relationship between the structure and the performance of Na-W-Mn/SiO2 catalysts for the oxidative coupling of methane

A series of Na-W-Mn/SiO2 catalysts with different sodium, tungsten, and manganese contents (wt.%) were prepared and their catalytic performance for the oxidative coupling of methane (OCM) was evaluated in a continuous micro-reactor. The structural properties of the catalysts have been characterised using XPS, laser Raman spectroscopy (LRS), and X-ray diffraction (XRD). The results show that the catalysts containing Na, W, and Mn between the range of 0.4-2.3, 2.2-8.9, and 0.5-3.0 wt.%, respectively, had high CH4 Conversion and high selectivity to C-2 hydrocarbons in the OCM reaction. The addition of sodium to the catalyst is believed to bring about the migration of manganese to the catalyst surface, as both elements are found in high concentrations in this area. CH4 conversion and the selectivity to C2H4 are closely related to the surface Mn concentration of the catalysts. Both Na-O-Mn and Na-O-W act as the active centres of the catalysts for the oxidative coupling of methane. The catalyst surface is mainly covered with Na and O, while the Mn and W are present in the sub-surface of the catalyst. There is a synergic effect of sodium, tungsten, and manganese components, and the Na2W2O7 crystalline phase is active for the oxidative coupling of methane. (C) 2002 Elsevier Science B.V. All rights reserved.