Zirconia Aerogels: Effect of the Use of Mono- and Dicarboxylic Acids in the Sol-Gel Process on Structural Properties

Zirconia aerogels were prepared from tetra-n-butoxy zirconium(IV) using acid catalysis with different types of mono- and dicarboxylic acids and ensuing supercritical drying of the gels either at high temperature (578 K, HT-aerogels) or by semicontinuous extraction with supercritical CO₂ at low temperature (318 K, LT-aerogels). The resulting HT- and LT-aerogels were calcined in air at 573 and 773 K, respectively, and characterized by nitrogen physisorption, X-ray diffraction, Raman spectroscopy, thermal analysis, and thermal desorption of ammonia. All HT-aerogels were mesoporous and possessed a high specific surface area between ca. 90 and 170 m²·g^-1 after calcination at 773 K. With oxalic acid the pore size distributions were either bimodal (R = 1) or unimodal R = 2), depending on the acid-to-alkoxide ratio, R. In contrast, the use of acetic and adipic acid, respectively, afforded bimodal pore size distributions. Generally, the crystalline part of the raw HT-aerogels prepared with acetic and adipic acid was made up of tetragonal ZrO₂ with traces of monoclinic zirconia, whereas with oxalic acid at higher acid-to-alkoxide ratio (R = 2) the monoclinic phase became dominant. Calcination in air at 773 K had no influence on the phase distribution and crystallite size. With LT-aerogels, the sample prepared with the largest acetic acid-to-alkoxide ratio (R = 2) afforded the highest BET surface area (ca. 90 m²·g^-1). All LT-aerogels were X-ray amorphous after extraction and calcination in air at 573 K. Crystallization resulted in the formation of tetragonal zirconia. The crystallization temperature increased with higher acetic acid-to-alkoxide ratio, whereas the crystallite size decreased.