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

Abstract

Hydrated boehmite with nano-sized crystallites AlO(OH)·0.8H₂O 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 Å, b = 12.179 Å and c = 2.855 Å. 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 × 2.7 × 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)·0.55H₂O.

The thermal evolution of hydrated boehmite leads first to hydrated γ-alumina Al₂O₃·0.33H₂O = Al₂O_2.67(OH)_0.66 and the number of remaining hydroxyl groups is critical for the porosity. Between Al₂O₃·0.33H₂O and Al₂O₃·0.2H₂O the surface area remains approximately constant (300 m² 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.