EFFECT OF ACIDIC, BASIC AND FLUORIDE-CATALYZED SOL-GEL TRANSITIONS ONTHE PREPARATION OF SUB-NANOSTRUCTURED SILICA

Amorphous porous silica can be prepared under acid, base and neutral p olycondensation conditions from tetraethoxysilane by the sol-gel proce ss. After drying and calcination of the resulting gels the glasses pre pared under acidic or fluoride-catalyzed conditions are microporous wi th a narrow pore size distribution and high surface area, while the gl ass prepared under basic conditions shows a broad distribution of meso pores and a low surface area. Rheological studies of the sol-gel proce ss under acidic conditions show a simultaneous increase of viscosity a nd elasticity with time, indicative of a linear polymerisation mechani sm. The same is observed under the fluoride-catalyzed conditions. In t he base-catalyzed sol-gel there is no elasticity component with increa sing viscosity typical for particle growth and a lack of a three-dimen sional network. High-resolution transmission electron microscopy (HR-T EM) studies show the acid and fluoride metal oxide to consist of amorp hous continuous glass-like material even at atomic resolution, while t he basic metal oxide consists of agglomerations of particles, suggesti ng that the observed mesoporosity of the material can be attributed to inter-particle voids. We have shown, that the final microstructure of the materials is mainly a result of the initial polycondensation cond itions.