Study on the Synergistic Effect of SiO₂ and H₂O on Oxidative Coupling of Methane over Mn-Na₂WO₄/SiO₂ Catalyst

Mn-Na₂WO₄-based catalysts with different supports were prepared using the incipient wetness impregnation method and evaluated for their oxidative coupling of methane (OCM) reaction performance. The results demonstrated that the SiO₂- supported catalyst exhibited the best catalytic performance, and the introduction of H₂O further enhanced its activity. Under the conditions of a feed gas mixture of CH₄/O₂/H₂O = 6:1:24 at 800 °C and atmospheric pressure, the CH₄ conversion and C₂₊ selectivity over the Mn-Na₂WO₄/SiO₂ catalyst increased from 28.4% and 77.4% (without H₂O) to 33.2% and 84.9%, respectively. In contrast, the catalytic activity using TiO₂ and MgO supports drastically declined. Characterizations using X-ray diffraction (XRD), in situ infrared spectroscopy (In-situ IR), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H₂-TPR), and oxygen temperature-programmed desorption (O₂-TPD) revealed that the introduction of steam not only served as a diluent to decrease the partial pressures of the reactants CH₄ and O₂, thereby reducing deep oxidation reactions in the gas phase, but also interacted with surface oxygen species (O₂ ^2-) and silica to form adjacent surface-bonded disilanol free radicals Si-O^IH. This interaction facilitated the dehydrogenation and coupling of two methane molecules on the catalyst surface, resulting in the generation of C₂₊ products, significantly enhancing the catalytic activity and selectivity of OCM.