Formation of the Phase Composition of Crystalline Silica in the Processing of Amorphous Precursors in Super- and Subcritical Water Media

Abstract

The regularities of the formation of the phase composition of crystalline silica during the processing of amorphous precursors in the aqueous fluid media below and above the critical point of water aimed at the formation of optimal support for the methane oxidative coupling (OCM) catalyst were studied. It was shown that the phase composition of SiO2 and the rate of phase formation strongly depend on the processing conditions (temperature, time, phase state of the water fluid) and the presence of trace amounts of impurities in the initial amorphous material. Nevertheless, for different precursors, the phase formation occurs, apparently, via the formation of the same bulk-hydrated structures. Optimization of the processing in the water fluid and subsequent heat treatment made it possible to obtain an OCM catalyst that is significantly more efficient than the one obtained by the conventional procedure using an amorphous support. It was concluded that the catalytic properties are entirely determined by chemical and phase transformations occurring in the active component (Na2WO4–Mn2O3) on the support surface and do not depend on the doping of the support with the ions composing the active phase.