alumina or aluminum oxide, Al2O3, chemical compound with m.p. about 2,000°C; and sp. gr. about 4.0. It is insoluble in water and organic liquids and very slightly soluble in strong acids and alkalies. Alumina occurs in two crystalline forms. Alpha alumina is composed of colorless hexagonal crystals with the properties given above; gamma alumina is composed of minute colorless cubic crystals with sp. gr. about 3.6 that are transformed to the alpha form at high temperatures. Alumina powder is formed by crushing crystalline alumina; it is white when pure. Alumina is widely distributed in nature. Combined with silica and other minerals it occurs in clays, feldspars, and micas. It is the major component of bauxite and occurs in an almost pure form as corundum. Alumina is commercially important. A major use is in the production of aluminum metal. It is also used for abrasives; corundum and emery are widely used, as are artificially prepared alumina abrasives. Trade names for alumina abrasives include Alundum and Aloxite. Alumina is also used in ceramics, in pigments, and in the manufacture of chemicals. Clays containing alumina are used in porcelain, pottery, and bricks. Pure alumina is used in making crucibles and other refractory apparatus. Hydrated alumina is used in mordant dyeing to make lake pigments; it is also used in glassmaking, in cosmetics, and in medicine as an antacid.
Silica-alumina is also known as alumino-silicate(s). It is an oxide-like combination of aluminium, silicon and oxygen, and it has a major component of kaolin and clays.

When amorphous silica-alumina is treated with strong acids such as hydrochloric and hydrofluoric acids, the material acts as a solid acid and becomes capable to promote reactions that require acid catalysis. Under other circumstances, when trivalent aluminum is forced by surrounding tetravalent silicon to crystallize in a tetrahedral symmetry, a partial electron deficiency occurs at the aluminum site allowing it to act as a Lewis acid, or to react with a water molecule and form a conjugated Brønsted. Natural and synthetic zeolites can be such crystalline alumino-silicates if aluminum precursors are added in the synthesis process.

See also: acid site, catalysis, catalyst, active site.

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