Effect of aerosil on the hydrothermal modification of porous structure of silica gel adsorbents

The effect of pyrogeneous silica (aerosil) on hydrothermal treatment (HTT) of narrow and mesoporous silica gels possessing Various sizes of primary globules was studied. The HTT process was carried out in the autoclave at 200 degrees C (6 h) in the liquid and gaseous phases. In the systems studied, aerosil was in the form of mechanical mixture with silica gel or constituted a separate phase in the aqueous suspension form. It was stated that the presence of pyrogeneous silica during the hydrothermal modification of silica gels affects the size of their specific surface area, average pores size and curves of pores volume distribution based on their radii. The extent of observed changes depends on the porous structure of initial adsorbents. The investigations show that it is possible to transfer the silica mass through the gaseous phase during the hydrothermal treatment. It was also pointed out that intensity of silica mass transfer observed during the hydrothermal modification in the gaseous phase does not differ from the mass transfer in the liquid phase systems. It was observed that for both modified silica gels relative efficiencies of the hydrothermal treatment are similar. This makes it possible to predict the extent of structural changes in the silica matrix, for silica gels of different porous structure, under the similar hydrothermal treatment conditions. (C) 2000 Elsevier Science B.V. All rights reserved.