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

• Published in: Russian journal of physical chemistry. B 2020-12, Vol.14 (7), p.1163-1171
• Main Authors: Lagunova, E. A., Ivakin, Yu. D., Sinev, M. Yu, Shashkin, D. P., Fattakhova, Z. T., Gordienko, Yu. A.
• Format: Article
• Language: English
• Subjects: Catalysts; Chemistry; Chemistry and Materials Science; Cristobalite; Critical point; Crystal structure; Crystallinity; Degree of crystallinity; Density; Morphology; Physical Chemistry; Silica gel; Silicon dioxide
• ISSN: 1990-7931; 1990-7923
• DOI: 10.1134/S199079312007009X

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 ( S sp ) is observed already at the lowest density. As the latter increases, S sp 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 SiO 2 , 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/SiO 2 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 (