The Ageing of Silica Gels Affected by Hydrothermal Treatment

Commercial silica gels of the pyrogenic and precipitated types were hydrothermally treated for 72 hours at temperatures between 90 °C and 210 °C in water of autogenous pressure. The samples were characterised by IR spectroscopy, water and nitrogen adsorption. Stability and sorption capacity run similarly. The more Q4 (Si(OSi)4) building units the framework of a silica gel contained, the lower was its sorption capacity. Sorption capacity increased with rising number of structural defects characterised by the presence of Q3 (Si(OSi)3OH) and Q2 (Si(OSi)2(OH)2) groups. Simultaneously, the hydrothermal stability of the silica material dropped off.