Oxidative coupling of methane (OCM): Effect of noble metal (M = Pt, Ir, Rh) doping on the performance of mesoporous silica MCF-17 supported MnxOy-Na2WO4 catalysts

Abstract

Noble metal doped M-MnxOy-Na2WO4/SiO2 (M=Pt, Ir, Rh) catalysts were prepared via a wet impregnation method using mesoporous silica MCF-17 as support, and their performance in the oxidative coupling of methane (OCM) was studied in a fixed-bed flow reactor. The reaction was carried out at 750°C under atmospheric pressure with a gas feed composition of CH4:O2=4:1. The incorporation of the noble metals yielded an enhanced selectivity towards both C2 and C3 hydrocarbons as compared to the undoped MnxOy-Na2WO4/MCF-17 catalyst in the order of Rh-doped > Ir-doped > Pt-doped samples together with a lower olefin to paraffin ratio. On the other hand, the Ir-doped catalyst showed the highest overall yield for C2 production. Elemental analysis, N2-adsorption, and X-ray powder diffraction (XRD) measurements confirmed similar metal loading, surface area, and phase composition for both the undoped and doped catalysts. Electron microscopy analysis showed a near-homogeneous distribution of Na and W but a higher tendency to form aggregates for Mn, with the Rh-doped catalyst being the best in overall elemental dispersion. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H2-TPR) results indicate a more reduced nature of the surface oxide species in the noble metal doped catalysts as compared to the undoped one. They also suggest a more optimized strength of interaction between the carbon intermediates and the surface of the noble metal doped catalysts, which in combination with the improved reducibility of Mn and W species and metal dispersion, accounted for the enhanced C2 production on the noble metal doped MnxOy-Na2WO4/MCF-17 catalysts.