Molecular structure and catalytic promotional effect of Mn on supported Na₂WO₄/SiO₂ catalysts for oxidative coupling of methane (OCM) reaction

The structure and promotional effect of Mn in supported Mn-Na₂WO₄/SiO₂ catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current investigation, with the aid of multiple in-situ characterization studies, we show that the freshly calcined supported 1.2Mn-5Na₂WO₄/SiO₂ catalyst possesses crystalline Na₂WO₄, Mn₂O₃ and SiO₂ (cristobalite phase) along with surface MnO_x and Na-WO_x sites at low temperature and oxidizing environments. Under the OCM reaction environment (T > 800 °C), the crystalline Na₂WO₄ phase melts and Mn₂O₃ phase reduces. In contrast, the surface MnO_x and Na-WO_x sites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na₂WO₄/SiO₂ catalyst to the OCM reaction environment redisperses the molten Na₂WO₄ phase on the SiO₂ support to form new surface WO_x sites. Interestingly, the stable MnO_x species interacts with both molten Na₂WO₄ phase and surface Na-WO_x sites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na₂WO₄/SiO₂ catalyst. The W-oxides (both molten Na₂WO₄ and surface Na-WO_x sites) are the active sites for the catalytic OCM reaction and the MnO_x species only function as promoters. The promotion of MnO_x strongly depends on the gas phase O₂ partial pressure and the MnO_x species act as mediators for oxygen exchange between the gas phase molecular O₂ and catalyst lattice oxygen. The temperature dependent MnO_x promotion reveals that the MnO_x species selectively promote the molten Na₂WO₄ phase at lower reaction temperature and the surface Na-WO_x sites at higher temperature.