Zeolites

(from the Greek zeo, “I boil,” and lithos, “rock,” because of the intumescence of zeolites upon heating), a family of aluminosilicates, the crystal structure of which is composed of tetrahedrons of [SiO₄]^4– and [AlO₄]^5– joined at common vertices to form a three-dimensional framework permeated with cavities and channels. The latter contain molecules of water and cations of metals (from Groups I and II of Mendeleev’s periodic system), as well as ammonium, hydronium, tetraalkyl ammonium, and other polyvalent ions introduced by cation exchange.

Zeolites are found in nature and produced synthetically. Their general formula is Me_x/n [Al_xSi_y,O_{2(x + y)}]·zH₂O, where Me is a metal, n is the degree of oxidation of the metal, x is the number of aluminum atoms, y is the number of silicon atoms, and z is the number of water molecules.

About 30 minerals are included in the family of natural zeolites. The most important are given in Table 1.

Nine crystallochemical groups of zeolites are distinguished on the basis of general structural features and the presence of channeis.

The frameworks of zeolites of the analcime group are constructed in 4-fold rings of [Si, AlO₄] tetrahedrons. The frameworks of zeolites of the laumontite and phillipsite group are also constructed from different combinations of four-member rings. The structures of the natrolite group consist of small chains composed of four-member rings connected to one another by a fifth tetrahedron. The typical structures of zeolites of the mordenite and heulandite-clinoptilolite group are represented by 5-fold loops of tetrahedrons [Si, AlO₄]. Single 6-fold rings are the basic framework of zeolites of the erionite group, while two 6-fold rings indicate the chabazite and faujasite group.

The natural zeolites crystallize in all crystal systems, forming colorless transparent crystals, ranging in size from a few centimeters to a few microns. Their hardness ranges from 3 to 5 on Mohs’ scale; the density varies between 1,800 and 2,250 kg/m³ (between 2,500 and 2,700 for barium zeolites).

Natural zeolites form primarily under conditions of relatively low temperatures (to 250°–300°C) and pressures (to several thousand atmospheres) during the last stage of the hydrothermal process. They are confined to volcanic strata of basaltic, andesitic, and rhyolitic composition, in which they fill cavities and cracks or form a cement of tuffs. The origin of natural zeolites is also linked with the diagenesis of marine deposits and alkaline salt lakes and with the interaction of tuffs with interstitial waters. In this case, industrially useful accumulations are formed and are worked as natural zeolite deposits. As the temperature rises, the zeolites that form are relatively less hydrated. The appearance of laumontite in deposits subjected to subsidence is typical of zeolite metamorphic facies; analcime, which can crystallize as a postmagmatic mineral from low-silica alkalic magmas at temperatures above 600°C, occupies a special place.

In the USSR, deposits of zeolites are found in Transcaucasia, Transcarpathia, and Kamchatka. Elsewhere deposits occur in New Zealand, Japan, the United States, and Iceland.

Artificial zeolites. There are approximately 100 artificial zeolites. Three of these find the broadest practical use: A—Na[AlSi0₄]·(2÷3)H₂O; X—Na[AlSi_1–1.5O_4–5]·3H₂O Y—Na[AlSi_1.5–3O_5–8]·(3÷4)H₂O.

Zeolite A has no natural analogs, while X and Y resemble faujasite. All three are made by heating to 100°C the gels that form when solutions of sodium aluminate and liquid glass or the sol of silicic acid are mixed or by heating a mixture of caustic soda (sodium hydroxide) and calcined kaolin, also to 100°C. The crystals obtained, which are several microns in size, are subjected to granulation.

Natural and artificial zeolites exhibit ion-exchange properties and, after water molecules are removed from their cavities during heating, adsorption properties, which, combined with the rigid dimensions of entrances to the cavities and channels, give them the properties of molecular sieves and selective ion exchangers. In cases where the cations are polyvalent cations, primarily La, Ce, and other rare-earth elements, as well as hydronium or hydrogen, the zeolites exhibit the properties of catalysts.

The specific features of the various zeolites are linked to the size of the entrances to their cavities (3–10 angstroms), the volume of the cavities, the nature and arrangement of the cations, and the chemical stability of the zeolites in different media.

Zeolites are used to separate and purify petroleum hydrocarbons. They are also used as catalysts. In addition, they are used for purifying, drying, and separating gases (including air), drying Freons, extracting radioactive elements, and creating a deep vacuum.