Site Requirements for the Oxidative Coupling of Methane on SiO₂-Supported Mn Catalysts

The oxidative coupling of methane (OCM) to C 2 hydrocarbons (C 2 H 4 and C 2 H 6 ) was examined on Mn/SiO 2 and sodium salt-modified Mn/SiO 2 catalysts containing different oxo anions, i.e., WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, P 2 O 7 4-, CO 3 2-, and SiO 3 2- . 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/SiO 2 consisted of Mn 3 O 4 and amorphous SiO 2 phases. The addition of the sodium salts to Mn/SiO 2 led to the transformation of amorphous SiO 2 exclusively to α-cristobalite, and the concurrent oxidation of Mn 3 O 4 to different Mn species. Mn 2 O 3 was the predominant species as the salts contained oxo anions of WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, and P 2 O 7 4-, whereas the basic sodium salts of CO 3 2- and SiO 3 2- led to the preferential formation of Mn 4+ 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/SiO 2, the sodium salt-modified samples with the formation of Mn 2 O 3 showed much higher reducibility, activities, and selectivities to C 2 products. However, the samples with the formation of Mn 4+ species exhibited very low OCM activities, as a result of the strong basicity of Na 2 CO 3 and Na 2 SiO 3 inhibiting the partial reduction of the Mn 4+ species. The observed effects on the structures and catalytic performances suggest that Mn 2 O 3 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.