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

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 CO2. 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 CO2 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 ZrO2 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. (C) 1998 Elsevier Science B.V.