Boehmite

One model for rod-like colloids is tire tobacco mosaic vims (TM V), which consists of rods of diameter D about 18 nm and lengtli L of 300 nm [17,18]. These colloids have tire advantage of being quite monodisperse, but are hard to obtain in large amounts. The fd vims gives longer, semi-flexible rods (L = 880 nm, D = 9 nm) [18,19]. Inorganic boehmite rods have also been prepared successfully [20]. 

Buining P A, Veidhuizen Y S J, Pathmamanoharan C and Lekkerkerker FI N W 1992 Preparation of a non-aqueous dispersion of sterioally stabilized boehmite rods Coiioid. Surf. 64 47-55... 

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

The CAS registry Hsts 5,037 aluminum-containing compounds exclusive of alloys and intermetaUics. Some of these are Hsted in Table 1. Except for nepheline and alunite in the USSR and Poland, bauxite is the raw material for all manufactured aluminum compounds. The term bauxite is used for ores that contain economically recoverable quantities of the aluminum hydroxide mineral gibbsite or the oxide—hydroxide forms boehmite and diaspore. 

Fig. 3. Surface area of boehmite samples after activation, where A and B are experimental boehmite and C is Catapal SB.

Stmctural order varies from x-ray indifferent (amorphous) to some degree of crystallinity. The latter product has been named pseudoboehmite or gelatinous boehmite. Its x-ray diffraction pattern shows broad bands that coincide with the strong reflections of the weU-crystallized boehmite. 

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. 

Fig. 3. Aluminum oxide—hydroxide hydrothemiaHy prepared boehmite, x2,000.

Tlie structure of boehmite consists of double layers in wliich the oxygen ions exliibit cubic packing. Hydroxyl ions of one double layer are located over the depression between OH ions in the adjacent layer such that the double layers are linked by hydrogen bonds between hydroxyls in neighboring planes. Tliere is some technical production and use of synthetically 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. 

Gelatinous boehmite, called alumina gel in commeicial use, is used in the piepaiation of adsorbents, desiccants (qv), catalysts, and catalyst supports (see Catalysts, SUPPORTBd). A significant amount is used in pharmaceutical preparations. 

Under equiUbrium vapor pressure of water, the crystalline tfihydroxides, Al(OH)2 convert to oxide—hydroxides at above 100°C (9,10). Below 280°—300°C, boehmite is the prevailing phase, unless diaspore seed is present. Although spontaneous nucleation of diaspore requires temperatures in excess of 300 °C and 20 MPa (200 bar) pressure, growth on seed crystals occurs at temperatures as low as 180 °C. For this reason it has been suggested that boehmite is the metastable phase although its formation is kinetically favored at lower temperatures and pressures. The ultimate conversion of the hydroxides to comndum [1302-74-5] AI2O2, the final oxide form, occurs above 360°C and 20 MPa. 

The possible content of hydrated alumina and iron. Hydrated alumina minerals like gibbsite [14762-49-3] Al(OH)2, boehmite [1318-23-6] AlOOH, and diaspore [14457-84-2] AlOOH, occur ia bauxitic clays. Bauxites grade chemically iato hydrated fermgiaous and manganiferous laterites. Hence, finely divided M2O2, usually hydrated, may be a significant constituent of a clay where M may be A1 or Fe. Hydrated colloidal s ica may play a role ia the sHppery and sticky properties of certain clays. 

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