The classical representatives of solid acids are aluminosilicates used as catalysts since the 1920s. Using Tanabe's procedure one can calculate that substitution of Al atom for the Si atom in the tetrahedron of the silica structure results in the appearance of one excess negative charge at the Al atom, which must be compensated by adding the proton to one of the oxygen ions (Fig. 1.1Oa) . A center containing a loosely bonded proton is thus formed with properties of a Bronsted acid center. On outgassing at higher temperatures the surface becomes dehydroxylated and the Bronsted acid center is transformed into a Lewis center (Fig. 1. lOb ) . AI04 tetrahedra may be substituted into any form of silica, resulting in the formation of amorphous aluminosilicates, those with layer structure like in montmorillonite or a three-dimensional framework as in zeolites.