Molecular Sieve 5A is an alkali metal aluminosilicate; Molecular sieve 5A is the calcium form of the type A crystal structure. Molecular sieve 5A adsorbs molecules with effective diameters small than 5 angstrom (0.5nm).
Molecular sieves are crystalline metal aluminosilicates having a three-dimensional interconnecting network of silica and alumina tetrahedra. Natural water of hydration is removed from this network by heating to produce uniform cavities which selectively adsorb molecules of a specific size. The 5A form has divalent calcium ions in place of sodium cations give apertures of ~5Å, which exclude molecules of effective diameter >5Å