Synthesis and characterization of ultramicroporous silica xerogels

Tailoring the porous texture of silica xerogels is of great interest in catalysis, adsorption and sensors. We report the preparation and characterization of xerogels with pore size distribution lower than 0.5 nm, synthesized by hydrolysis and condensation of tetraethylorthosilicate (TEOS) at pH 2.5. N2 (77 K) and CO2 (273 K) adsorption isotherms and immersion calorimetry into liquids with different molecular size (dichloromethane, 0.33 nm; trichloromethane, 0.40 nm; n-hexane, 0.43 nm; cyclohexane, 0.48 nm; isooctane, 0.59 nm; and carbon tetrachloride, 0.66 nm) were used to characterize ultramicroporous xerogels. All the xerogel samples adsorbed CO2 (273 K) but not N2 (77 K). Pore size distributions from immersion calorimetry showed that hydrolysis and condensation of TEOS at pH 2.5 produced silica xerogels with pore size distributions in the 0.33–0.48 nm range. Xerogels synthesized at 333 K and a 5.5:1 water:TEOS molar ratio produced xerogels with higher specific surface areas and wider pore size distributions than xerogels obtained at 293 K and at 2.5:1 water:TEOS molar ratio.