Synthesis of High-Surface-Area Alumina Aerogels without the Use of Alkoxide Precursors

Alumina aerogels were prepared through the addition of propylene oxide to aqueous or ethanolic solutions of hydrated aluminum salts, AlCl 3 ·6H 2 O or Al(NO 3 ) 3 ·9H 2 O, followed by drying with supercritical CO 2 . This technique affords low-density (60−130 kg/m 3 ), high-surface-area (600−700 m 2 /g) alumina aerogel monoliths without the use of alkoxide precursors. The dried alumina aerogels were characterized using elemental analysis, high-resolution transmission electron microscopy, powder X-ray diffraction, solid-state NMR, acoustic measurements, and nitrogen adsorption/desorption analysis. Powder X-ray diffraction and TEM analysis indicated that the aerogel prepared from hydrated AlCl 3 in water or ethanol possessed microstructures containing highly reticulated networks of pseudoboehmite fibers, 2−5 nm in diameter and of varying lengths, whereas the aerogels prepared from hydrated Al(NO 3 ) 3 in ethanol were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5−15 nm range. The difference in microstructure resulted in each type of aerogel displaying distinct physical and mechanical properties. In particular, the alumina aerogels with the weblike microstructure were far more mechanically robust than those with the colloidal network, based on acoustic measurements. Both types of alumina aerogels can be transformed to γ-Al 2 O 3 through calcination at 800 °C without a significant loss in surface area or monolithicity.