Preparation of alumina through a sol-gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite

Hydrated boehmite with nano-sized crystallites AlO(OH).0.8H(2)O are obtained through a very reproducible sol-gel procedure and characterized using powder X-ray diffraction, in-situ IR spectroscopy, thermal analysis, BET surface area and photoelectron spectroscopy. The unit cell parameters, obtained after deleting the first shifted (020) diffraction peak, are a = 3.686 Angstrom, b = 12.179 Angstrom and c = 2.855 Angstrom. They are consistent with well-crystallized boehmite and with the position of the harmonic (080) diffraction peak. For the (020) peak, the correlation between position and peak width is confirmed for nano-crystal lites. The average shape of the crystallites. determined from three peak widths, corresponds to slabs with dimensions 7.9 x 2.7 x 8.7 nm. Based on this crystallite shape, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH).0.55H(2)O. The thermal evolution of hydrated boehmite leads first to hydrated gamma-alumina Al2O3.0.33H(2)O = Al2O2.67(OH)(0.66) and the number of remaining hydroxyl groups is critical for the porosity. Between Al2O3.0.33H(2)O and Al2O3.0.2H(2)O the surface area remains approximately constant (300 m(2) g(-1)) and the hydroxyl density decreases deeply. For calcination temperature higher than 800 K, the loss of the remainder of the water leads to a strong decrease of this area. The limiting value corresponds to about 9-10 OH-groups nm(-2) and can be related to the hydrogen spinel HAl5O8. The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm(-1)). A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters.