Сверхкритические флюиды. Теория и практика
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2019, N4, pp. 49-62

E.A. Lagunova, Yu.D. Ivakin, M.Yu. Sinev, D.P. Shashkin, Z.T. Fattakhova, Yu.A. Gordienko

Structuring and Phase Formation in Silica Gel in Water Fluids of Different Densities

The effect of the density of water fluid in the range of ~10-3—0.25 g/cm3 on the structuring of amorphous silica gel was studied at 380 °C that exceeds the temperature of critical point of water. It is shown that a decrease in the specific surface area (Ssp) is already observed at the lowest density; as the latter increases, Ssp decreases further, and starting from a density of ~0.01 g/cm3, the formation of crystalline silica phases (cristobalite, keatit) is observed in the sample. Based on the observed regularities in the change in the morphology and crystallinity of SiO2, as well as the analysis of 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 aqueous 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 catalyst for the oxidative coupling of methane it was shown that the efficiency of the catalyst decreases with increasing degree of crystallinity of the support. However, when using supports that have undergone processing at relatively low densities of water fluid (< 0.05 g/cm3), the catalysts are more active and selective than the one supported on the initial silica gel.

Key words: water fluid, density, silica, morphology, structuring, crystallization, catalyst support, oxidative coupling of methane

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