Hybrid materials and silica: drastic control of surfaces and porosity of xerogels via ageing temperature, and influence of drying step on polycondensation at silicon

Abstract

The effect of drying and ageing conditions on both the degree of polycondensation at silicon and the texture connected with the surface properties (BET, porosity) has been investigated in the case of silica and nanostructured hybrid organic–inorganic solids, obtained by hydrolytic sol–gel performed from a molecular precursor. Various molecular geometries and structural features were investigated using acid or nucleophilic catalysts in MeOH or THF. The aim of this work is first to study the effect of the temperature on the surface properties occurring during the ageing alone which was studied independently of gel formation. The second aim was to consider whether the polycondensation at silicon is influenced by the drying step. The main trend that emerges from these results is the drastic influence of ageing temperature on the texture and porosity of the xerogels. Whereas the level of condensation at silicon is not modified, we observed that higher ageing temperatures favor an increase of the specific surface area and an evolution towards a mesoporosity with narrow pore size distribution, whatever is the level of polycondensation at silicon. In contrast, the drying procedure (transformation of gel into xerogel) induces a significant increase of the level of condensation at silicon (10–14%) when the leaving group of the precursor is OEt, whereas in the case of the OMe group no evolution is observed. The same experimental facts have also been observed in the case of SiO₂.