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.