Zirconia aerogels: effect of acid-to-alkoxide ratio, alcoholic solvent and supercritical drying method on structural properties

Abstract

Zirconia aerogels have been prepared by the sol–gel method using tetra- n-butoxy zirconium(IV) and acid catalysis. The nitric acid-to-alkoxide ratio and the alcoholic solvent, as important sol–gel parameters, were varied. After aging, the solvent entrapped in the sol–gel products was removed applying two different methods: supercritical drying at high-temperature and low-temperature extraction with supercritical CO 2. Finally, for gel stabilization and removing of organic residues the aerogels were calcined at 573 and 773 K, respectively. The effects of the varied sol–gel parameters, different supercritical drying methods, and calcination temperature on the structural and textural properties were investigated. The aerogels were characterized by means of N 2 physisorption, X-ray diffraction, thermal analysis, scanning and transmission electron microscopy. Depending on the kind of alcoholic solvent, nitric acid-to-alkoxide ratio and drying method, the specific surface areas of the mesoporous aerogels varied from 55 to 205 m 2 g −1 after calcination in air at 773 K. The aerogels supercritically dried at high-temperature possessed larger pores (17–65 nm) and BET surface areas (143–205 m 2 g −1) compared to the aerogels dried by low temperature extraction (<20 nm and 55–112 m 2 g −1, respectively). The width of the pore size distribution of all high-temperature supercritically dried gels became smaller with increasing amount of acid. All aerogels dried by low temperature extraction with supercritical CO 2 were X-ray amorphous. Upon calcination in air at 773 K, the amorphous aerogels crystallized to tetragonal zirconia. The high-temperature supercritically dried aerogels contained predominantly tetragonal zirconia. The fraction of monoclinic ZrO 2 shares increased with increasing nitric acid-to-alkoxide ratio, the use of a long-chained branched alcoholic solvent, such as t-butanol, and increasing calcination temperature.