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Molecular sieves are a group of adsorptive desiccants which are crystalline aluminosilicates, chemically similar to clays and feldspars, belonging to a class of minerals known as zeolites. The most important characteristic of these materials is their ability to undergo dehydration with little or no change in crystal structure. The dehydrated crystals are interlaced with regularly spaced channels of molecular dimensions, comprising almost 50% of the total volume of crystals.
The empty cavities in activated molecular sieve crystals have a strong tendency to recapture the water molecules that have been driven out. This tendency is so strong that if no water is present, they will accept any material that can enter or filter into them. However, only those molecules that are small enough to pass through the pores of the crystal can enter the cavities and be adsorbed on the interior surface. This sieving or screening action, which makes it possible to separate smaller molecules from the larger ones, is the most unusual characteristic of molecular sieves.
Molecular sieves are used in many fields of technology to dry gases and liquids. They are also used for selective molecular separations based on such size and polar properties as ion-exchangers, as catalysts and in petroleum industry to remove normal paraffins from distillates.