Effect of preparation mode on the properties of Mn-Na-W/ Sio₂ catalysts for oxidative coupling of methane: conventional methods vs. POSS nanotechnology

Using XPS, BET, XRD, TG-DTA, HRTEM-EDX, TPR and UV-Vis Diffuse Reflectance spectroscopic methods the electronic, redox and structural properties of Mn-Na-W/ SiO₂ catalysts prepared by the incipient wetness impregnation method and mixture slurry method were studied in detail. Since POSS nanotechnology (POSS = polyhedral oligomeric silsesquioxanes) has attracted attention as tooling for synthesis of catalysts with novel properties and functionalities, we expanded this method for the preparation of Mn-Na-W/ SiO₂ catalyst. The physicochemical and catalytic properties of Mn-Na-W/ SiO₂ catalysts prepared by conventional methods and POSS nanotechnology were examined comparatively. In all studied Mn-Na-W/ SiO₂ catalysts both individual oxides (MnO_x, WO₃) and bimetal oxide phases (Na₂WO₄, MnWO₄) are found in addition to oxide particles of high dispersion. The UV-Vis Diffuse Reflectance indicates that Na⁺ cations facilitates stabilization of octahedrally coordinated Mn³⁺ _Oh cations in the isolated state, while Mn³⁺ _Oh promote the disordering of W⁶⁺ cations in the supported system. The Mn-Na-W/ SiO₂ prepared using metal-POSS precursors marks out presence of unglobular SiO₂ particles, higher dispersion of MnO_x and MnWO₄ particles and more easily reducible metal-oxide species. The catalysts prepared by incipient impregnation method and mixture slurry method have practically similar catalytic performance while the catalyst prepared by POSS nanotechnology method shows lower activity and selectivity. At 800−850 °C the increase of C2 hydrocarbons yield from 4 to 15% and the rise of molar ratio C₂H₄/C₂H₆ from 0.2 to 1 are observed when impregnation or mixture slurry method are used for catalyst preparation instead of POSS nanotechnology method.