A microporous structure composed of either crystalline aluminosilicates, chemically similar to clays and feldspars and belonging to a class of materials known as zeolites, or crystalline aluminophosphates derived from mixtures containing an organic amine or quaternary ammonium salt. Pore sizes range from 5 to 10Å. The outstanding characteristic of these materials is their ability to undergo dehydration with little or no change in crystalline structure. The dehydrated crystals are interlaced with regularly spaced channels of molecular dimensions, that compose almost 50% of the total volume of the crystals.The empty cavities in activated “molecular sieve” crystals have a strong tendency to recapture the water molecules that have been driven off. This tendency is so strong that if no water is present they will accept any material that can get into the cavity. However, only those molecules that are small enough to pass through the pores of the crystals 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 larger ones, is the most unusual characteristic of molecular sieves. They are used in many fields of technology; to dry gases and liquids; for selective molecular separations based on size and polar properties; as ion-exchangers; as catalysts; as chemical carriers; in gas chromatography; and in the petroleum industry to remove normal paraffins from distillates. See Zeolite; Gel Filtration; Pore.
