Site requirements for the oxidative coupling of methane on SiO2-supported Mn catalysts

The oxidative coupling of methane ( OCM) to C-2 hydrocarbons ( C2H4 and C2H6) was examined on Mn/SiO2 and sodium salt-modified Mn/SiO2 catalysts containing different oxo anions, i.e., WO42-, MoO42-, SO42-, PO43-, P2O74-, CO32-, and SiO32-. The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and temperature-programmed reduction with H-2. The structural and catalytic properties of the catalysts largely depend on the presence of the Na+ ions and the identity of the oxo anions of the salts. Mn/SiO2 consisted of Mn3O4 and amorphous SiO2 phases. The addition of the sodium salts to Mn/SiO2 led to the transformation of amorphous SiO2 exclusively to alpha-cristobalite, and the concurrent oxidation of Mn3O4 to different Mn species. Mn2O3 was the predominant species as the salts contained oxo anions of WO42-, MoO42-, SO42-, PO43-, and P2O74-, whereas the basic sodium salts of CO32- and SiO32- led to the preferential formation of Mn4+ species. These effects on the formation of the Mn species were demonstrated indeed to require the coexistence of Na+ and the oxo anions. Compared to Mn/SiO2, the sodium salt-modified samples with the formation of Mn2O3 showed much higher reducibility, activities, and selectivities to C-2 products. However, the samples with the formation of Mn4+ species exhibited very low OCM activities, as a result of the strong basicity of Na2CO3 and Na2SiO3 inhibiting the partial reduction of the Mn4+ species. The observed effects on the structures and catalytic performances suggest that Mn2O3 species act as the active sites responsible for the methane activation, which may provide the rationale for the design of new efficient catalysts for the OCM reaction.