Synthesis gibbsite

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. 

Figsch V., Bunanova E. Synthesis of spinel-magnesia ceramics from finely milled mixtures of magnesite MgCOs and gibbsite Al(OH)j. In Book of Abstract I Int. Conf. INCOME, Kosice, 23-26 March,1993... 

Cordierite synthesis method based on mechanical activation of mixtures of hydrated oxides of calcium, aluminium and silicon, as well as natural hydrated compounds (talc, kaolinite and gibbsite), has been developed in [2, 3]. Mechanical activation of these mixtures does not lead to the formation of new phases but provides good mixing at the cluster level giving aggregates that form cordierite during the subsequent thermal treatment. 

H. Reveron, et al., Chemical synthesis and thermal evolution of MgA1204 spinel precursor prepared from industrial gibbsite and magnesia powder. Mater. Lett. 56(1-2), 97-101 (2002). 

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. 

Gibbsite of high purity is needed to prepare aluminas for catalytic appfi-cations. A number of synthesis protocols are reported, aU characterized by very long aging times for the crystallization to avoid high sodium concentrations, and with the final steps sometimes at acidic pH (44). Nanometersized crystals of gibbsite samples can be prepared by neutralization of sodium aluminate solutions (45) or at acidic pH (46). 

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). 

Inoue, M. Kitamura, K., and Inui, T., Synthesis of wide-pore alumina support from Gibbsite, J. Chem. Technol. Biotechnol., 46(3), 233-248 (1989). 

Finally, gibbsite can be envisaged as a template for developing new nanostructured materials such as imogolite-like NTs. The mild conditions for the synthesis in aqueous solutions make them very attractive for technological and environmental applications. 

Fujiyoshi K, Ishida S, Takeuchi N, Nanri H. Synthesis of whisker-like MgAl204 spinel from gibbsite and magnesium acetate. Taikabutsu Overseas 2002 ... 

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