The topotactic transformation of boehmite (AlOOH) taking place under calcination and leading to γ-alumina (γ-Al2O3) has been investigated theoretically and a step-by-step mechanism for the structural transformation is proposed This mechanism reveals two major steps First, the structural collapse of boehmite occurs, after hydrogen transfers and water extraction Then, through an aluminum migration process, the γ-alumina characteristics appear The proposed mechanism demonstrates the existence of an equilibrium structure for γ-alumina containing 25−31% of tetrahedral aluminum sites in agreement with nuclear magnetic resonance (NMR) results Temperature effects on the thermodynamic stability of the different structures involved in the mechanism has been investigated Theoretically simulated X-ray diffraction (XRD) patterns confirm the validity of our model structures According to this mechanism, a coherent skeleton of γ-alumina inherited from the original AlOOH network is constructed This skeleton can be

Theoretical Study of the Dehydration Process of Boehmite to γ-Alumina

High pressure/low temperature sintering of nanocrystalline alumina

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

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

The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET), X-ray powder diffraction (XRD), N2 adsorption–desorption isotherms, thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600–1150°C. Although powder La2O3, which is mechanically mixed with γ-Al2O3 or pseudo boehmite, can effectively retard the α phase transformation by solid phase interaction with Al2O3, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of γ-Al2O3, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600°C and for atomic ratio of La/Al up to 0.1 or at 1150°C and La/Al≤0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La2O3, LaAlO3 and crystalline La2O3. At T≤1000°C the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000°C results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO3 mainly retards the surface area loss resulting from the α phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from α phase transformation at high temperature ≥1000°C has been proposed in this paper.

High temperature thermal stabilization of alumina modified by lanthanum species

Growth characteristics of boehmite-derived ultrafine theta and alpha-alumina particles during phase transformation

Sintering of pseudo-boehmite, acicularγ-Al2O3 produced by dehydration of pseudo-boehmite, andγ-Al2O3 ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction toα-Al2O3 causes a steep drop of the surface area to less than 10 m2g−1. The acicular pseudo-boehmite andγ-Al2O3 supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m2g−1. It was established that reaction to α-Al2O3 and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of α-Al2O3. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite andγ-Al2O3 studied leads to the reaction to α-Al2O3 to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.

P. A. Elberse

Papers

Sintering of pseudo-boehmite and γ-Al2O3