Aluminum oxide 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.

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. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

Dehydration of gibbsite under pressure in moist air produces boehmite (aluminum oxide mono-hydrate). An infrared spectrum of boehmite (Kaiser substrate grade alumina) is shown in Figure 3c. 

The results obtained for the various aluminum oxides and hydroxides indicate that infrared photoacoustic spectroscopy may be useful in characterizing structural transformations in these species. Very clear differences between a-alumina and y-alumina were noted in the region of the lattice vibrations. The monohydrate, boehmite, showed a very distinct Al-OH stretching feature at 1070... 

Boehmeria nivea, 11 294 Boehmite (aluminum oxide monohydrate), 1 6 2 345t, 347, 421, 425-426 classification, 2 422... 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

There are two main types of alumina (bauxite) ores used as the primary sources for aluminum metal and aluminum chemicals aluminum hydroxide [Al(OH)3] (gibbsite) and a mixed aluminum oxide hydroxide [AIO(OH)] (boehmite). Thus, bauxite is a term for a family of ores rather than a substance of one definite composition. An average composition of the ores used by industry today would be alumina (A1203), 35 to 60% silica (Si02), 1 to 15% ferric oxide (Fe203), 5 to 40% and titanium dioxide (Ti02), 1 to 4%. 

In the aluminum oxide system the precipitation pH is one of the variables which controls the nature of the phase eventually obtained. However, aging conditions of the initially formed amorphous precipitate are at least equally important. In general, it can be stated that precipitation above pH = 8 leads to the formation of bayerite, while precipitation under more acidic conditions favors the subsequent formation of boehmite. Hydrargillite is formed as the product of the Bayer process by seeding a supersaturated alkali containing aluminum solution. The formation of bayerite is strongly facilitated by the presence of alkali cations which stabilize the structure. 

At our laboratory we have studied aluminum-oxide montmorillonite complexes prepared from ACH-solutions hydrothermally treated at temperatures up to 160 0 (9). Hydrothermal treatment of ACH at temperatures above about 120 0 yields positively charged, fibrillar boehmite in colloidal suspension (1 ). The size of the boehmite fibrils increases with increasing temperature and time of hydrothermal treatment. Ion-exchange of montmorillonite with these positively charged fibrils resulted in AMCs with... 

Aluminum oxide occurs naturally as the minerals bauxite, bayerite, boehmite, corundum, diaspore, and gibbsite. 

The industrially most important minerals for aluminum manufacture are the bauxites (laterite). Bauxite is not a uniform material, but comprises a mixture of different aluminum oxide hydrates and hydroxides [e.g. boehmite, y-AIO(OH), diaspore, a-AlO(OH)] with aluminum silicates, iron and titanium oxides etc. with between 35 and 70% (mostly 50 to 65%) aluminum oxide. 

The initial step of the process is the formation of aluminum triethyl from aluminum metal, ethylene and hydrogen. In a second step ethylene is added to the aluminum triethyl causing the carbon chains to grow in increments of two carbon atoms. After the chain growth reaction the aluminum alkyl is oxidized to an aluminum alkoxide. The alkoxide is then hydrolyzed with water, forming fatty alcohols and alumina slurry. The alcohols and the alumina slurry can be separated from each other and processed into the final products. After drying of the slurry the alumina is obtained in the form of a high purity aluminum oxide monohydrate of boehmite or pseudoboehmite structure. 

Aluminum oxide (uh-LOO-min-um OK-side) is white crystalline powder that occurs in nature in a variety of minerals, including boehmite, bayerite, corundum, diaspore, and gibb-site. Corundum is second hardest naturally occurring mineral. Only diamond is harder. Aluminum oxide occurs in a variety of chemical forms in a variety of gemstones, including chryso-beryl, ruby, sapphire, and spinel. The color of these gemstones is a result of impurities, such as chromium (in the case of ruby) and iron and titanium (in the case of sapphire). The colors may also vary depending on the kind and amount of each impurity. 

The term alumina hydrates or hydrated aluminas is used in industry and commerce to designate aluminum hydroxides. These compounds are tme hydroxides and do not contain water of hydration. Several forms are known a general classification is shown in Figure 1. The most well-defined crystalline forms ate the trihydroxides, Al(OH)3 gibbsite [14762-49-3], bayerite [20257-20-9], and nordstrandite [13840-05-6], In addition, two aluminum oxide—hydroxides, AIO(OH), boehmite [1318-23-6] and diaspore [14457-84-2], have been clearly defined. The existence of several other forms of aluminum hydroxides have been claimed. However, there is controversy as to whether they ate truly new phases or structures having distorted lattices containing adsorbed or intedameUar water and impurities. 

Al/AljOj- based Pseudo- boehmite AlPOs (SAPOs, MeAPSOs) Anodic oxidized Aluminum oxide gels Foams... 

Chemical Names Boehmite alumina, alumina monohydrate, aluminum oxide hydroxide Trade Name Dispal Alumina Function in Formula Thickener... 

Recently, a fume pyrolysis with the sol-gel technique was examined to prepare aluminum oxide powders from boehmite sol suspension derived from Al-alkoxide. Aluminum oxide was spherical shape and crude structure. In addition, the aluminum oxide possessed a high thermal resistance at 1473K as high as 50 m g- l in surface area[5]. The fume pyrolysis method was also an effective preparation for thin films composed of mixed oxides because one of the advantages is that the atomic composition in source solution is the same as the formed oxide particles. A Yi Ba2Cu307.x film was successfully prepared[6]. These suggest that the combination of the fume pyrolysis and the sol-gel method seem to be an interesting preparation method. 

---