Ammonia-catalyzed silica xerogels: Simultaneous effects of pH, synthesis temperature, and ethanol:TEOS and water:TEOS molar ratios on textural and structural properties

Preparation of sol–gel derived materials with tailored porosity is a goal in fields such as catalysis, adsorbents, separation media and sensors. The aim of this work was to study the simultaneous effects of pH (8–10), temperature (295–314 K), and ethanol:TEOS (6:1–12:1) and water:TEOS (2:1–6:1) molar ratios on the textural and structural properties of xerogels synthesized in alkaline media. We applied a 24 factorial design with five replicates of the central point to evaluate the effect of each independent variable and their interactions. Samples were characterized by XRD, SEM, TEM, FTIR, mercury porosimetry and gas adsorption (N2 at 77 K and CO2 at 273 K). The structural bonds in xerogels were similar; the siloxane microstructure did not change as a function of concentration of reagents and temperature. All xerogels were amorphous and mesoporous. Higher ethanol:TEOS molar ratios yielded xerogels with a higher specific surface area, lower pore volume and average pore size, and narrower pore size distribution. On the other hand, increasing the water:TEOS molar ratio produced xerogels with a lower specific surface area and higher meso, macro and total pore volumes.