Hydrated boehmite with nano-sized crystallites AlO(OH)Á0.8H 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 ¼ 3.686 A ˚, b ¼ 12.179 A ˚and c ¼ 2.855 A ˚. 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-crystallites. The average shape of the crystallites, determined from three peak widths, corresponds to slabs with dimensions 7.9 Â 2.7 Â 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)Á0.55H 2 O.The thermal evolution of hydrated boehmite leads first to hydrated g-alumina Al 2 O 3 Á0.33H 2 O ¼ Al 2 O 2.67 (OH) 0.66 and the number of remaining hydroxyl groups is critical for the porosity. Between Al 2 O 3 Á0.33H 2 O and Al 2 O 3 Á0.2H 2 O the surface area remains approximately constant (300 m 2 g À1 ) 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 À2 and can be related to the hydrogen spinel HAl 5 O 8 . The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm À1 ). A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters.

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