Effect of Additives on Mn/SiO۲ Based Catalysts on Oxidative Coupling of Methane

The Oxidative Coupling of Methane (OCM) over M-Na-Mn/SiO۲ catalysts (M=W, Cr, Nb and V) was investigated using a continuous-flow quartz reactor at ۷۷۵°C, ۱ atm and ۱۰۰ cm۳min-۱ 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 C۲+ yield were investigated using X-Ray Diffraction (XRD), laser Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), and Temperature Programmed Reduction with H۲ (TPR). The results revealed that the improvement of C۲+ selectivity (or C۲+ 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 SiO۲ support and also show that Mn۲O۳ and α-cristobalite were the predominant species in the surface catalysts. TPR data show that most of the Mn is present as Mn۳+ and Mn۲+. 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.

کلیدواژه ها:

Oxidative Coupling of Methane (OCM) ، structural properties ، Transition metal oxide ، Redox mechanism