Zirconia aerogels and xerogels: Influence of solvent and acid on structural properties

Abstract

High-surface-area zirconia aerogels with meso- to macroporosity have been prepared by an acid-catalyzed alkoxide-sol-gel route with tetrabutoxyzirconium(IV) and subsequent high-temperature supercritical drying at 578 K. The effect of solvent (ethanol, propanol, butanol, t-amylalcohol), amount of nitric acid, calcination temperature, and drying method was studied by nitrogen physisorption, X-ray diffraction, Fourier transform Raman and diffuse reflectance infrared Fourier transform spectrosopy, scanning electron microscopy, thermal analysis, and temperature-programmed desorption of NH3. After calcination in air at 573 or 773 K, the aerogels possess specific surface areas of up to 270 or 180 m2 · g−1, respectively. The use of ethanol as solvent resulted in the highest specific surface areas and pore volumes (up to 1.5 cm3 · g−1) among all samples studied, whereas bulky t-amylalcohol caused a shift of the maxima of the broad pore size distributions from 30 to 70 nm. With the corresponding xerogels, prepared via the same wet-chemical procedure but evaporatively dried at ambient temperature, butanol resulted in a maximum at 3 nm and t-amylalcohol in a bimodal pore size distribution with maxima at 3 and 15 nm. The variation of the acid-to-alkoxide ratio in the range 0.08–0.12 at a hydrolysis level of 4 did not significantly influence the structural properties of aerogels and related xerogels. In contrast to the aerogels, the xerogels had significantly lower specific surface areas and prominent microporosity. All uncalcined aerogels contained crystalline ZrO2, whereas the corresponding uncalcined xerogels were X-ray amorphous and crystallized only during calcination at 573 K. Both aerogels and xerogels possessed Bronsted-type and Lewis-type acid sites. With the xerogels, the density of acid sites on the surface was significantly lower. This behaviour is attributed to the higher amounts of organic residues which persisted in and on the xerogels up to 773 K and thus blocked the acid sites partially.