EFFECT OF SUPERCRITICAL DRYING TEMPERATURE ON THE PROPERTIES OF ZIRCONIA, NIOBIA AND TITANIA SILICA AEROGELS

Aerogels of zirconia, and titania-silica were prepared by supercritical drying of the corresponding alcogels with carbon dioxide at 343 and 473 K. The higher drying temperature of 473 K increased the pore volume of all three oxides (by 30-70% relative to samples dried at 343 K) after calcination at 773 K; but affected their pore size distributions differently. There was an increase in average pore diameter for zirconia and titania-silica but not for niobia. There was also a significant effect on crystallization behavior. A higher drying temperature facilitated crystallization of tetragonal zirconia and anatase titania, but a lower drying temperature facilitated the anatase-to-rutile transformation of titania and crystallization of a low-temperature modification of niobia, the TT phase. The structural evolution of niobia and titania-silica altered their acidic properties as shown by kinetic results of 1-butene isomerization.