Structural characterization of intermediate species during synthesis of Al2O3-aerogels

Alumina sols in the system (Al(OC4H9sec)(3)/H2O/HNO3) were synthesized based on a well known method. Structural changes during sol-gel transition, aging of the gel and water-acetone exchange were studied, and the effect of these parameters on the structure and morphology of the aerogels was investigated. The alumina aerogels were prepared by supercritical drying with CO2. Structural changes during the various synthesis and processing steps were followed by Raman and Fourier transform infrared spectroscopy, Al-27 nuclear magnetic resonance, and rheological measurements. Additionally, pore morphology of the aerogels was investigated by BET analysis and scanning electron microscopy observations. In the sol alumina polycations with four- and sixfold coordination were detected by Al-27 NMR. During sol-gel transition at 403 K a growth of the boehmite crystallites as well as a growth of the network was observed. The gel consists of an amorphous network with tetra- and octahedrally coordinated Al-atoms and boehmite crystallites which are embedded in an amorphous phase. During aging the number of network points in the gel increases by reactions between the amorphous phase and the crystallites or between the crystallites. Additionally, the crystallite size increases at the expense of the amorphous phase. For the calcination residue of 13.7 wt% Al2O3 the pore structure of the aerogels is controlled by two effects: at the beginning of aging by the growth of the network, for long aging times by the irregular growth of boehmite crystallites.