Aluminium boehmite

The mechanism of sealing has been shown to involve an initial dissolution and reprecipitation of hydrated aluminium oxide on the pore walls, pseudo-boehmite gel formation within the pores, and conversion of this to crystalline boehmite at the film surface. The presence of an intermediate layer close to the film surface, in which the identity of the original pores has been lost, has also been recognised . 

Lastly, the positions of hydrogen atoms have been located in a series of low-surface-area aluminium oxide-hydroxides. Hence, the long known solid-state structures of diaspore (a-AlOOH) and boehmite (y-AlOOH) have been recently probed using H CRAMPS [45] along with X-ray and neutron diffraction techniques [46]. For diaspore results point to the presence of Alaf/Xa-OH) groups with 6-fold and 4-fold coordination at aluminium and oxygen, respectively, while Al2(/x-OH) moieties with 6-coordinate aluminium and 3-coordinate oxygen are revealed in boehmite [46]. 

Lastly, the previously-mentioned study into the low-surface-area aluminium oxide-hydroxides diaspore and boehmite by CRAMPS (Section 2.2.1) [46] has also been extended to the aluminium hydroxides gibbsite and bayerite, revealing three distinct resonances in a 3 2 1 ratio in these systems and suggesting the presence of Al2(/r-OH) groups with 6-coordinate metal centres. 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

Boehmite is of considerable interest to the surface scientist. It was pointed out by Lippens and Steggerda (1970) that a clear distinction should be made between crystalline boehmite and the gelatinous forms of pseudoboehmite, which always contains some non-stochiometric, interlamellar water. Pseudoboehmite is the main constituent of European bauxites and can be easily prepared by the neutralization of aluminium salts, but hydrothermal conditions are required for the formation of crystalline boehmite. 

Bacteria associated with bauxite ore have been shown to reductively mobilize significant amounts of the Fe(III) oxides in the bauxite under anaerobic conditions without mobilizing significant amounts of the Al(III) oxides like gibbsite or boehmite as long as the pH of the reaction system was kept above 4.5. The bauxite residue was thus enriched in aluminium (Ehrlich et al., 1995 Ehrlich Wickert, 1997). 

AIHO2 (s) AlOOH (s) Aluminium Oxide Hydroxide Boehmite AIHO2 (s) AlOOH (s)... 

Alhydrogeh, aluminium hydroxide adjuvant aluminium oxy-hydroxide poorly crystalline boehmite pseudoboehmite Rehydragel. 

In addition, there Is pressure to reduce the cost of the catalyst, Vashcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropDxlde or boehmite have been suggested to be useful materials to calcine [42]. Both of these precursors are not cheap, and less expensive raw materials would be desired,... 

Several authors [1, 18-20] have surveyed the chemical and crystallographic changes which occur during the dehydroxylation of the three forms of aluminium hydroxide, (Al(OH)3 gibbsite (y), bayerite (a) and norstrandite) and the two forms of aluminium oxyhydroxide, (AlOOH diaspore (a) and boehmite (y)). These phases... 

The development of stress during calcination is shown in Fig. 8.20 for boehmite membranes calcined at 600°C (thickness after calcination is 5 pm). Curve c in Fig. 8.20 represents the curve which is corrected for support effects (see the preceding section on this subject). Three heating and cooling cycles are shown. During the first heating the Al-hydroxide particles of the gel are transformed to boehmite and subsequently to (hydrated) y-aluminium oxide particles and the shape of the first peak of curve c differs from the subsequent peaks. The maximum tensile stress calculated from the deflection amounts about 30 MPa. 

Lovrecek, B., Bolanca, Z., and Korelic, O.. Surface charge and interaction with sulphate ions at the boehmitized aluminium. Surf. Coat. Technol., 31, 351, 1987. 

An amorphous layer was observed on the surface of the particles (refer Fig. 5) as analysed by TEM. The possible origin of this amorphous layer can be described by the following hypothesis During the aqueous phase cobalt nitrate catalyst preparation step, the alumina support dissolves and the dissolved aluminium ions can precipitate either as boehmite or may, in combination with cobalt ions, precipitate as a cobalt aluminium hydrotalcite-like layer [16], producing a physically amorphous layer, uniformly covering the sur ce of the bulk support material [15]. It was, however, not possible to characteri2e the observed layer. 

Different phases of alumina may be prepared by calcination of aluminium tri-hydroxides (gibbslte and bayerlte) or oxy-hydroxides (boehmite and... 

Aluminium hydroxyoxides. These include gibbsite (Al(OH)3), boehmite and diaspore (AlOOH), and allophane. The loss of silicon from soils leaves an Aland Fe-rich residue in soil clays. 

This preliminary study focuses on the ability of organic compounds such as polyols to tune size and morphology of boehmite nanoparticles by aluminium precipitation in aqueous medium. Results show a notable effect of both the synthesis pH and the carbon chain length of polyols, ie C4 and C5, on the size of particles and their thickness to width ratio. The specific surface area can be increased up to 382 m. g In addition, the morphology of particles is modified by C5 polyol. 

Boehmite nanoparticles were synthesized in aqueous medium by precipitation of aluminium nitrate A1(N03)3 (0.1 mol.L ) in the presence of polyols in various concentrations (0.01 or 0.1 mol.L ). Protocol used has been adapted from procedures described by Froidefond [9, 22]. The pH was adjusted at a selected value by addition of NaOH (1 mol.L ). Final concentrations were 0.07 mol.L in aluminium, and respectively 0.007 and 0.07 mol.L in polyol. The suspensions were aged at 95°C for one week. Precipitation and growth of boehmite nanoparticles occurred in presence of polyols. After ageing, the pH values of the suspensions were equal to 4.5, 6.5 and 11.5. Reference boehmite nanoparticles were prepared using the same conditions (pH, temperature, ageing time) without polyols. In every case, boehmite was the only phase observed. 

Washcoat alumina is generally produced from boehmite, which is relatively expensive but gives a more thermally stable product. The present studies were initiated in order to study the possibihty of producing thermally stable alumina from gibbsite, via the intermediate production of sulphates. It is known that alumina can be produced by the dehydration and decomposition of aluminium sulphate [4] and it seemed possible that the conversion of cheaper precursors to the sulphate followed by decomposition to the oxide could offer some economic advantage in the production of washcoat material. 

Although aluminium in combinations as alumino-silicate is one of the most abundant constituents of the earth s crust, free alumina is comparatively rare. It is usually found in a hydrated form as bauxite rock, which is actually a mixture of the minerals gibbsite, A1(0H)3, diaspore and boehmite, (both of formula HAIO2). Bauxite is found in Jamaica, British Guiana, Europe and Russia and else-... 

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