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

• Published in: Journal of sol-gel science and technology 2022-04, Vol.102 (1), p.197-207
• Main Authors: Huck-Iriart, Cristián, Morales, Noé J., Herrera, María Lidia, Candal, Roberto J.
• Format: Article
• Language: English
• Subjects: Acids; Alkoxides; Catalysts; Ceramics; Chemical reactions; Chemistry and Materials Science; Composites; Condensates; Glass; Hydrolysis; Inorganic Chemistry; Materials Science; Microstructure; Nanotechnology; Natural Materials; NMR; Nuclear magnetic resonance; Optical and Electronic Materials; Original Paper: Fundamentals of sol–gel and hybrid materials processing; Porosity; Precursors; Silicon dioxide; Sol-gel processes; Sol-Gel Research in Latin America; Sols; X-ray scattering; Xerogels
• ISSN: 0928-0707; 1573-4846
• DOI: 10.1007/s10971-021-05601-2

Silicon dioxide (SiO 2 ) 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 SiO 2 , 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 HNO 3 . 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 SiO 2 products revealed different porosity type and texture depending on the acid employed, which are related to the chemical pathway during the sol–gel transition. Highlights SiO 2 gelation time determined by LF- 1 H-NMR decreases with nucleophilicity of acid catalyst anion. SiO 2 gels microstructure change slightly with aging for HF catalyst but quite remarkably with HNO 3 /HCl. SiO 2 xerogel microstructure is defined during drying for HF catalyst but during aging for HNO 3 /HCl. HF as catalyst lead to mesoporous xerogels, while HNO 3 /HCl lead to microporous ones. Catalyst selection allows tailoring SiO 2 morphologies and microstructure for different applications.