Molecular Sieves

sorbents that selectively absorb from the surrounding medium substances whose molecules do not exceed certain dimensions. Such sorbents “sift out” larger molecules. Both mineral (inorganic) and organic molecular sieves are known.

Inorganic molecular sieves have a rigid crystal structure in which there are cavities connected to one another by thin channels called pores or windows. The small size of the pores hinders the diffusion of large molecules into the internal cavities of the sorbent. Certain aluminosilicates (natural and synthetic zeolites) are characteristic representatives of inorganic molecular sieves.

Organic molecular sieves are gel-like sorbents obtained from macromolecular compounds. The structure of such sorbents is a spatial network of chain macromolecules that are linked at certain points by chemical bonds. The most common types of industrial gel-like molecular sieves are various types of Sephadex, which is a dextran-based sorbent (dextran is a macromolecular polysaccharide).

Molecular sieves that contain ionogenic groups (that is, groups that dissociate into ions) and are suitable for ion exchange are called ion-exchange sieves. Contrary to ordinary ion exchangers, they selectively absorb only small ions, excluding from the ion-exchange process the large ions, whose diffusion through the structural network of the sorbent is hindered.

Molecular sieves are produced in the form of powder or irregular or spherical granules. They are used for removing undesirable impurities from substances, for fractionation of synthetic polymers, and for chromatographic separation of proteins, carbohydrates, hormones, and antibiotics.