Effect of Additives on Mn/SiO2 Based Catalysts on Oxidative Coupling of Methane

The Oxidative Coupling of Methane (OCM) over M-Na-Mn/SiO2 catalysts (M=W, Cr, Nb and V) was investigated using a continuous-flow quartz reactor at 775°C, 1 atm and 100 cm3min-1 gas flow rates, and correlated with the observed structure and redox properties.The interaction effects of the metal-metal and metal-support on the methane conversion and C2+ yield were investigated using X-Ray Diffraction (XRD), laser Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), and Temperature Programmed Reduction with H2 (TPR). The results revealed that the improvement of C2+ selectivity (or C2+ yield) follows the order W>Cr>Nb>V, while the catalytic conversion did not change significantly. XRD data indicated that Mn is well dispersed on the SiO2 support and also show that Mn2O3 and α-cristobalite were the predominant species in the surface catalysts. TPR data show that most of the Mn is present as Mn3+ and Mn2+. FT-IR analyses combined with the Raman results show that terminal M=O and bridging M–O–M species and the metal–metal and metal–support interactions, which take place due to the presence of sodium ion, depend on the transition metal that affect the catalyst performance. Results reveal that the interaction between metal oxide and sodium is required for high selectivity and control redox mechanism in transition metal oxide in OCM reaction.