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) usin g acid catalysis with different types of mono-and dicarboxylic acids a nd ensuing supercritical drying of the gels either at high temperature (578 K, HT-aerogels) or by semicontinuous extraction with supercritic al 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 c haracterized by nitrogen physisorption, X-ray diffraction, Raman spect roscopy, thermal analysis, and thermal desorption of ammonia. All HT-a erogels were mesoporous and possessed a high specific surface area bet ween ca. 90 and 170 m(2) . g(-1) after calcination at 773 K. With oxal ic acid the pore size distributions were either bimodal (R = 1) or uni modal (R = 2), depending on the acid-to-alkoxide ratio, R. In contrast , the use of acetic and adipic acid, respectively, afforded bimodal po re size distributions. Generally, the crystalline part of the raw HT-a erogels 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 domi nant. Calcination in air at 773 K had no influence on the phase distri bution and crystallite size. With LT-aerogels, the sample prepared wit h the largest acetic acid-to-alkoxide ratio (R = 2) afforded the highe st BET Surface area (ca. 90 m(2) . g(-1)). All LT-aerogels were X-ray amorphous after extraction and calcination in air at 573 K. Crystalliz ation resulted in the formation of tetragonal zirconia. The crystalliz ation temperature increased with higher acetic acid-to-alkoxide ratio, whereas the crystallite size decreased.