Adsorbents

artificial and naturally occurring solids with a highly developed surface which readily absorbs (adsorbs) matter from gases and solutions surrounding the adsorbent bed.

The adsorptive properties of an adsorbent depend on its chemical composition, its physical surface condition, the degree of its porosity, and its specific surface area (surface area per gram of material). Nonporous adsorbents—such as pulverized crystals, fine crystalline sediments, particles of smoke, soot, and Aerosil—present a specific surface area ranging from 1 to 500 square meters per gram (m²/g). The specific surface area presented by porous adsorbents—such as silica gels, alumogels, aluminosilicate catalysts, and active charcoal—may be as great as 1,000 m²/g.

Nonporous highly dispersed adsorbents are produced primarily by thermal decomposition or incomplete combustion of hydrocarbon, as in the production of soot, and in combustion of heteroorganic compounds or halides, as in the production of highly dispersed silica—Aerosil. Porous adsorbents are obtained by the following methods: (1) A network of pores is created in coarsely dispersed solids through chemical action. (2) Gels are produced from colloidal solutions, or sols. The solvent is eliminated from the interstices between the colloidal particles when these gels dry out, so that the resulting material exhibits a highly developed system of pores. (3) Porous crystals of the zeolite type are synthesized; these have acquired special importance as catalysts, adsorbents, and molecular sieves. Adsorbents are also produced by thermal decomposition of carbonates, oxalates, hydroxides, and some polymers and by the molecular sublimation of solids in a vacuum.

Adsorbents are also used as carriers in catalysis, as fillers in polymers, for chromatographic separation of mixtures, in gas masks, in medicine, in petroleum chemistry for purification of petrochemicals and gases, and in high-vacuum engineering for sorption pumps.