Structural features of Na2WO4–MOx/SiO2 catalysts in oxidative coupling of methane reaction

Oxidative coupling of methane conversion (OCM) and C2 selectivity on Na2WO4–MOx/SiO2 catalysts, where M is V, Cr, Mn, Fe, Co or Zn are related to surface morphology, textural properties and reduction behavior of the catalysts. The catalysts are prepared by sequential impregnation of aqueous solution of metal salts and then sodium tungstate on the silica followed by drying and calcination at 800 °C for 8 h. Methane conversion increases with the transition metal compound of the catalysts in the order of V ∼ Cr ∼ Zn < Fe ∼ Co < Mn while the C2 selectivity enhances in the order of V ∼ Cr ≪ Fe ∼ Co ∼ Zn < Mn. XRD and SEM results reveal some interactions between the V and Cr oxides and silica support, decreasing the tungstate peaks intensities and deteriorating catalyst bulk properties and both OCM conversion and selectivity. Both silica and Mn, Fe, Co and Zn transition metals, however, interact with sodium tungstate, forming an OCM selective catalyst. TPR analyses combined with the XRD and SEM results show that the metal–metal and metal–support interactions, which take place due to the presence of sodium ion, depend on the transition metal that in turn affect catalyst performance. Moderation of the transition metal–tungstate interaction is proposed as the main chemical role of sodium ion in these series of catalysts. Results reveal that this interaction improves the redox properties of catalyst, which are related to the transition metals, forming a selective catalyst.