Identification of active site in oxidative coupling of methane on silica support manganese catalysts

فاقد چكيده

The catalytic performance of oxidative coupling of methane (OCM) over sodium salt-modified Mn/SiO2 catalysts containing different oxo anions, i.e., W, Cr, Nb and V has been investigated in a continuous-flow micro reactor. The structural properties of the catalysts have been studied using X-ray diffraction (XRD), laser Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), 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. FT-IR and XRD characterization experiments show that Mn2O3 and α-cristobalite were the predominant species in the surface catalysts and both Na2WO4 crystalline phases contributed to achieving high selectivity of C2 + products whereas the Mn2V2O7 phase was very unselective. TPR 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.