MOLECULAR SIEVES

white powder
Porous crystallinesubstances, especially aluminosilicates(see zeolite), that can be dehydratedwith little change in crystalstructure. As they form regularlyspaced cavities, they provide a highsurface area for the adsorption ofsmaller molecules.The general formula of these substancesis MnO.Al2O3.xSiO2.yH2O,where M is a metal ion and n is twicethe reciprocal of its valency. Molecularsieves are used as drying agentsand in the separation and purificationof fluids. They can also be loadedwith chemical substances, which remainseparated from any reactionthat is taking place around them,until they are released by heating orby displacement with a morestrongly adsorbed substance. Theycan thus be used as cation exchangemediums and as catalysts and catalystsupports. They are also used asthe stationary phase in certain typesof chromatography (molecular-sievechromatography).
Molecular sieves are crystalline metal aluminosilicates having a 3-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 3A form is made by substituting potassium cations for the inherent sodium ions of the 4A structure, reducing the effective pore size to ~3, excluding diameter >3, e.g., ethane. The 4 to 8-mesh type is normally used in gas-phase applications. Molecular sieves are considered a universal drying agent in polar and nonpolar media.