Micro to mesoporous SiO2xerogels: the effect of acid catalyst type in sol–gel process

Silicon dioxide (SiO2) obtained by Sol–Gel methods is widely used as adsorbents, catalytic supports, filter membranes and in drugs delivery, among others. For most of the applications, surface area and porosity are key parameters that should be controlled, depending on the purpose of the material. These characteristics depend on the chemistry of the precursors in solution. Silicon alkoxides are commonly used as precursors, where the chemical pathway to produce sols and then gels depends on several factors such as water/alcohol ratio, pH, type of catalyst, temperature, etc. In order to control the microstructural characteristics of SiO2, it is necessary to understand the effect of the different chemical components on the hydrolysis-condensation reactions. In this work, we explored the acid-catalyzed hydrolysis and condensation reactions of silicon tetra-ethyl-alkoxide (TEOS) employing three common acids: HF, HCl, and HNO3. Gel formation kinetics was studied by low field nuclear magnetic resonance. Structural evolution of gels and xerogels at the nanoscale was determined by small angle X-ray scattering (SAXS). The microstructure of xerogels was determined by nitrogen adsorption (BET method), and by scanning and transmission electron microscopy (SEM and TEM, respectively). The final SiO2 products revealed different porosity type and texture depending on the acid employed, which are related to the chemical pathway during the sol–gel transition.