Journalof PorousMaterials, 1,165-174 (1995) © 1995Kluwer Academic Publishers,Boston. Manufactured in The Netherlands. Titania Aerogels Prepared by Low-temperature Supercritical Drying. Influence of Extraction Conditions D.C.M. DUTOIT, M. SCHNEIDER AND A. BAIKER Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092, Ziirich, Switzerland Received August 29, 1994; Accepted October 25, 1994 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 m 2 g-l) and nitrogen pore volume (4.0 cm 3 g-l) 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. Keywords: titania aerogels, carbon dioxide, extraction conditions, supercritical drying, sol-gel 1 Introduction The potential of aerogels for material science resides in their unique morphological and chemical properties. These properties originate from their wet-chemical preparation by the solution-sol-gel (SSG) method and the subsequent removal of the solvent by supercriti- cal drying (SCD). As this "gentle" drying procedure is capable of "preserving" the structural properties of an open gel-network, aerogels are usually solids of high porosity. This pronounced porosity leads to interesting physico-chemical properties, such as low thermal con- ductivity, low longitudinal sound velocity, small index of refraction, or large internal surface area. SCD of gels is a suitable procedure for reducing the differential capillary stresses in the gel-network, which usually result in drastic structural rearrange- ments. The capillary stress is circumvented either by transferring the solvent into the supercritical state [1, 2] (high-temperature method) or by replacing the solvent with either liquid [3, 4] or supercritical [5] CO2 (low- temperature methods), so eliminating any liquid-vapor interface inside of the gel-network during the solvent removal. The important difference between these two basic routes resides in the markedly different operating temperature, which originates from the rather high crit- ical temperature of alcoholic solvents (Tc > ca. 510 K) (high-temperature method) compared to the low criti- cal temperature of CO2 (To = 304 K) (low-temperature methods). With titania, high-temperature SCD at 538 K resulted in crystallization [6], whereas samples extracted with supercritical CO2 at 343 K remained X- ray amorphous [7]. In the case of the low-temperature methods based on CO2, different procedures have been applied, including semicontinuous [5] or batchwise [3, 4] operation, CO2 in liquid [8] or supercritical [7, 9] state, different extraction times [7, 8] and CO2 density similar or different to that of the SSG solvent [9], Con- cerning the latter procedure, Ayen and Iacobucci [9] studied the effect of pressure and CO2 density at "con- stant" temperature (313-318 K). They found that BET surface area increased with a rise in pressure and thus CO2 density. Brodsky and Ko [10] investigated the in- fluence of the extraction temperature between 343 and