Boehmite synthesis

Boehmite (OC-Aluminum Oxide-Hydroxide). Boehmite, the main constituent of bauxite deposits in Europe, is also found associated with gibbsite in tropical bauxites in Africa, Asia, and Austraha. Hydrothemial transformation of gibbsite at temperatures above 150 °C is a common method for the synthesis of weU-cry stalhzed boehmite. Higher temperatures and the presence of alkali increase the rate of transfomiation. Boehmite ciy stals of 5—10 ]liii size (Fig. 3) are produced by tliis method. Fibrous (acicular) boehmite is obtained under acidic hydrothemial conditions (6). Excess water, about 1% to 2% higher than the stoichiometric 15%, is usually found in hydrothemiaHy produced boehmite. 

Diaspore (P-Aluminum Oxide Hydroxide). Diaspore, found in bauxites of Greece, Cliina, and the USSR, can also be obtained by hydrothemial transfomiation of gibbsite and boehmite. Higher (>200°) temperatures and pressure (>15 AlPa-150bar) are needed for synthesis and the presence of diaspore seed cry stals helps to avoid boehmite fomiation. 

The mode of synthesis of alumina membranes through the colloidal suspension route is given in Figure 2.6. The first step involves the preparation of a slip consisting of boehmite particles. These arc plate-shaped in the form of pennies with a diameter of 25-50 nm and a thickness of 3.5-5.5nm (Leenaars ct al. 1984,1985). The synthesis chemistry of the colloidal boehmite (y-AlOOH) solution is described in detail by Leenaars and Yoldas (1975) and to some extent by Anderson, Gieselman and Xu (1988) and by Larbot et al. (1987). 

Only one paper that we are aware of explores a combined synthesis and processing route to aluminosilicates. Kemmitt and Milestone use precursors made by reaction of sodium hydroxide, boehmite [Al(0)OH] and silica in ethylene glycol in a 4 3 1 ratio160. The precursor structures are related to those shown above. On removal of solvent (ethylene glycol) a glycolate precursor is obtained that contains a pentacoordinated... 

The mixture was subsequently cooled down to 60°C and peptised with 1M HNO3 at a pH of about 2.5. During the synthesis, the sol was stirred vigorously. The peptised mixture was refluxed for 20 hours at 90°C, resulting in a very stable 0.5 molar boehmite sol with a clear white/blue appearance. 

The starting material is a state-of-the-art flat y-alumina membrane prepared by dipcoating of a boehmite solution on a macroporous a-alumina support and subsequent firing at 600°C as described in [4], The a-alumina support is prepared from AKP-30 powder by making a colloidal suspension of this powder in diluted nitric acid and subsequent filtration. After filtration the wet cake is dried overnight and sintered for 1 hour at 1100°C. The resulting flat a-alumina supports have a mean pore diameter of 80 nm. A detailed description of the support synthesis is provided in chapter 4. 

Hydrothermal synthesis of a-alumina has been well studied. Since the hydro-thermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C), transformation of boehmite was examined and it was reported that more than 10 hours is required for complete conversion into a-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in the presence of seed crystals. On the other hand, under glycothermal conditions, a-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore (another polymorph of AlOOH) and a-alumina under the saturated vapor pressure of water was determined to be 360°C. However, near the equilibrium point, the transformation rate is very sluggish, and only a small conversion of diaspore is observed. Therefore complete conversion of diaspore into a-alumina requires a much higher temperature. Since boehmite is slightly less stable than diaspore, the hypothetical equilibrium point between boehmite and a-alumina would be lower than that for diaspore-alumina. However, a-alumina would not be formed by a hydrothermal reaction at such a low temperature as has been achieved in the glycothermal reaction. 

Landry CC, Pappe N, Mason M, Apblett AW, Tyler AN, Macirmes AnN, Barron AR (1995) From minerals to materials-synthesis of alumoxanes from the reaction of boehmite with carboxyhc acids. 1 Mater Chem 5 331-341... 

Using a different gel composition, with pseudo-boehmite instead of AIP, the effects of gel metal concentration and crystallization temperature on MAP0-34 synthesis with TEAOH can be illustrated (Figures 4 and 5). Reaction mixtures were prepared differing only in Mg concentration (x) ... 

Commonly used aluminum sources in the synthesis of zeolites include sodium aluminate (NaA102, Na20 54%), pseudo-boehmite, home-made aluminum hydroxide, and aluminum isopropoxide, which is the mostly used in the synthesis of microporous aluminophosphates. The structures of the two most important aluminum sources are shown in Figure 5.11 and Figure 5.12, respectively. 

X.L. Pan et al., Mesoporous spinel MgA1204 prepared by in situ modification of boehmite sol particle surface I Synthesis and characterization of the unsupported membranes. Colloids Surf. A-Physicochem. Eng. Asp. 179(2-3), 163-169 (2001). 

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. 

Four acyclic polyols of various chain lengths (C2 to C5) were selected for boehmite synthesis in complexing conditions (fig. 2). The effect of stereochemistry was not considered. 

Whatever the pH of synthesis, no significant effect of C2 and C3 polyols is observed upon nanocrystallite morphologies in comparison with standard boehmite elaborated in the same pH conditions. At pH 4-5 and pH 7, weak effects of C4 and C5 polyols are observed on particle size, whereas morphologies seem to be unchanged. However, major effects are obtained with these polyols in alkaline conditions. C4 and C5 polyols influence the size of the particles, as it is shown by XRD powder patterns (fig. 3, table 1) that exhibit a peak broadening relatively to reference pattern. 

The first observations clearly show that polyols modify the equilibrium morphology of boehmite particles, as it has been reported in the case of gibbsite crystallization [20, 21]. The use of polyols allows to obtain departures of the diamond shaped morphology observed at pH ll. If boehmite particles synthesized in presence of C2 to C4 polyols are always diamond shaped, the proportion of (101) and (010) planes is modified (table 1). The highest proportion of (101) face is reached for boehmite synthesis in presence of mesoerythritol (C4). Xylitol (C5) causes much important changes as the particle morphology is isotropic in this case. 

We have reported a novel formation of nanosized boehmite powder in flaky morphology by a modified sol-gel route. " The sol-gel method is a well-known chemical synthesis route with high purity, high chemical homogeneity, lower calcination temperatures and good control of particle size. It is a versatile method not only to synthesize nanocrystalline powder but also nanostnictured... 

S. C. Kuiry, E. Megen, S. D. Patil, S. A. Deshpande, S. Seal, Solution-Based Chemical Synthesis of Boehmite Nanofibers and Alumina Nanorods, J. Phys. Chem. B 109, 3868-72 (2005). 

The second example is the synthesis of -y-alumina nanorods from boehmite nanofibers using a modified sol-gel process (42). First, a solution of aluminum iso-propoxide in anhydrous ethanol is prepared. To this, ethanol with 4% water is added leading to a viscous liquid after 15 h. The viscous hquid heated at 600°C leads to y-alumina nanorods (diameter < 10 nm length 50 to 200 nm). Since in the synthesis, less water is taken, only partial hydrolysis takes place. The removal of one water molecule from two A10(0H) octahedra leads to the formation of AI2O3 nanorods, (CH3CH2CH20)3A1 + 2H2O A10(0H) + 3CH3CH2CH2OH. 

Diaspore, which is thermodynamically more stable than boehmite, can also be obtained by hydrothermal transformation of gibbsite or boehmite, but its formation is slow. Higher temperatures (i.e., >200 °C) and pressures (>15 MPa) are required for the synthesis, and the presence of diaspore seed crystals helps to avoid boehmite formation. Methods to produce weU-crystaUized diaspore have been reported these include hydrothermal synthesis at 300 °C and 3.45 x 10 Pa (5000 psi) over a 72-h period (67) or high-pressure calcination of boehmite (68). 

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