Metal effects in Mn-Na2WO4/SiO2 upon the conversion of methane to higher hydrocarbons

Abstract

The roles of Na, Mn, W and silica, and the synergistic effects between each metal in the <TEX>$MnNa_2WO_4/SiO_2$</TEX> catalyst have been investigated for oxidative coupling of methane (OCM). The crystallisation of amorphous silica during calcination at <TEX>$900^{\circ}C$</TEX> was promoted primarily by Na, but Mn and W also facilitated this process. The interaction between Na and Mn tended to increase the extent of conversion of <TEX>$Mn_3O_4$</TEX> to <TEX>$Mn_2O_3$</TEX>. The formation of <TEX>$Na_2WO_4$</TEX> was dependent on the order in which Na and W were introduced to the catalyst. The impregnation of W before Na resulted in the formation of <TEX>$Na_2WO_4$</TEX>, but this did not occur when the impregnation order was reversed. <TEX>$MnWO_4$</TEX> formed in all cases where Mn and W were introduced into the silica support, regardless of the impregnation order; however, the formation of <TEX>$MnWO_4$</TEX> was inhibited in the presence of Na. Of the prepared samples in which a single metal oxide was introduced to silica, only <TEX>$Mn/SiO_2$</TEX> showed OCM activity with significant oxygen conversion, thus demonstrating the important role that Mn plays in promoting oxygen transfer in the reaction. The impregnation order of W and Na is critical for catalyst performance. The active site, which involves a combination of Na-Si-W-O, can be formed in situ when distorted <TEX>$WO_4^{2-}$</TEX> interacts with silica during the crystallisation process facilitated by Na. This can only occur if the impregnation of W occurs before Na addition, or if the two components are introduced simultaneously.