The relationship between the structure and the performance of Na-W-Mn/SiO_2 catalysts for the oxidative coupling of methane

摘要

A series of Na-W/Mn/SiO_2 catalysts with different sodium,tungsten,and manganese contents (wt.precent)were prepared and their catlytic performance for the oxidative coupling of methane (OCM)was evaluated in a continuous micro-rector.The structural properties of the catlysts 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 0.5-3.0wt.precent,respectiviely,had high CH_4 conversion and high selectivity to C_2 hydrocarbons in the OCM reation.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 concentration in this area.CH_4 conversion and the selectivity to C_2H_4 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 catlysts for the oxidative coupling of methane.The catalyst surface in aminly covered with Na and O,while the Mn and W are present in the sub-surface of the catalyst.These is synergic effect of sodium,tungsten,and manganese components,and the Na_2W_2O_7 crystalline phase is active for the oxidative coupling of methane.