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

TLDR: In this article, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH)·055H2O.

Abstract: Hydrated boehmite with nano-sized crystallites AlO(OH)·08H2O 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 = 3686 A, b = 12179 A and c = 2855 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 79 × 27 × 87 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)·055H2OThe thermal evolution of hydrated boehmite leads first to hydrated γ-alumina Al2O3·033H2O = Al2O267(OH)066 and the number of remaining hydroxyl groups is critical for the porosity Between Al2O3·033H2O and Al2O3·02H2O 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