Separation of Closely Related Systems by Molecular Sieve Zeolites

Molecular sieve zeolites are robust, thermally stable selective absorbents. They are chemically pure crystalline aluminosilicates with up to 50% of their crystal volume available for adsorption. Zeolites are widely used for industrial drying, purification, and separation, as well as catalysts and ion exchangers. They can be synthesized, on a plant scale, from silica (or sodium silicate) and sodium aluminate. The product is a powder which is pellitized and “activated” before use. Their structures are regular arrangements of channels and cavities entered through “windows” of molecular dimensions which aid their molecular sieving action. Further control of the property comes from their cation content, temperature, and, in some cases, by the preabsorption of small polar molecules. Industrial separations commonly achieved on zeolites include: (1) n-paraffins from isoparaffins, aromatics, and naphthenes (e.g., to produce high octane petrol); (2) p-xylene from other xylene isomers and ethyl benzenes; and (3) cryogenic separation of permanent gases. Zeolites are also useful for a wide range of analytical separations by gas–solid chromatography and more recently they have been used as high-pressure chromatographic media. Some naturally occurring zeolite minerals may find a future use as selective adsorbents