Titania aerogels prepared by low-temperature supercritical drying. Influence of extraction conditions

Abstract

Titania aerogels with meso- to macroporosity and high specific surface area were prepared by varying the conditions of semicontinuous extraction of methanolic titania gels with CO₂. The conditions varied were extraction temperature, extraction duration, and CO₂ in liquid or supercritical state. The resulting titania aerogels were characterised by means of nitrogen physisorption, X-ray diffraction, thermal analysis and transmission electron microscopy. All uncalcined aerogels contained significant amounts of organic residues (12–14 wt% elemental carbon), and remained X-ray amorphous during calcination in air up to 673 K. Thermoanalytical studies showed that crystallization generally occurred in the range 730–745 K. The variation of the extraction temperature at either constant density or pressure of CO₂, the use of either liquid or supercritical CO₂, and the duration of extraction greatly influenced surface area, pore size distribution, and pore volume. The highest specific surface area (623 m² g⁻¹) and nitrogen pore volume (4.0 cm³ g⁻¹) were obtained, if the density of supercritical CO₂ corresponded to that of methanol at the lowest temperature applied (313 K). The studies indicate that textural properties can be varied over a wide range by choosing appropriate extraction conditions.