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.