The effect of the density of water fluid in the range of ~10–3–0.25 g cm–3 on the structuring of amorphous silica gel was studied at 380°C that exceeds the temperature of critical point of water. It was shown that a decrease in the specific surface area (Ssp) is observed already at the lowest density. As the latter increases, Ssp decreases further, and starting from the density of ~0.01 g cm–3, the formation of crystalline silica phases (cristobalite, keatit) was observed in the sample. Based on the observed regularities in the change in the morphology and crystallinity of SiO2, as well as on the data on the variations in the properties of the water fluid with temperature and pressure below and above the critical point, it was concluded that the increase in the structuring rate with an increase in the density of the water fluid is more likely due to the kinetic factor (mass action law) than with a change in the physical state of water (intermolecular interaction forces action). Using the obtained samples of treated silica gel as a support for the NaWMn/SiO2 catalysts it was shown that their efficiency in the oxidative coupling of methane decreases with increasing degree of crystallinity of the support. However, when supports that have undergone processing in the water fluid of relatively low densities (<0.05 g cm3) were used, the catalysts were more active and selective than the one prepared using the untreated silica gel.
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Authors are deeply grateful to Prof. S.Ya. Umanskii and Dr. Yu.A. Chaikina (FC “Chemical Physics” RAS) for the fruitful discussion of the obtained results and their theoretical interpretation.
This work was financially supported by the Russian Foundation for Basic Research (project no. 18-29-06055).
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Lagunova, E.A., Ivakin, Y.D., Sinev, M.Y. et al. Structuring and Phase Formation in Silica Gel in Water Fluids of Different Densities. Russ. J. Phys. Chem. B 14, 1163–1171 (2020). https://doi.org/10.1134/S199079312007009X
- water fluid
- catalyst support
- oxidative coupling of methane