Zeolites are a large group of natural and synthetic hydrated aluminum silicates. They are characterized by complex three-dimensional structures with large, cagelike cavities that can accommodate sodium, calcium, or other cations (positively charged atoms or atomic clusters); water molecules; and even small organic molecules. Ions and molecules in the cages can be removed or exchanged without destroying the aluminosilicate framework. Zeolites find wide use as ion-exchange agents, catalysts, and molecular filters in a range of industrial processes. The word "zeolite" comes from the Greek for "boiling stone," because of the early observation that zeolites release water when heated. As their compositions are not fixed, they are examples of nonstoichiometric compounds.
The Structures of Zeolites
The atomic structures of zeolites are based on three-dimensional frameworks of silica and alumina tetrahedra, that is, silicon or aluminum ions surrounded by four oxygen ions in a tetrahedral configuration. Each oxygen is bonded to two adjacent silicon or aluminum ions, linking them together. Clusters of tetrahedra form boxlike polyhedral units that are further linked to build up the entire framework. In different zeolites the polyhedral units may be equidimensional, sheetlike, or chainlike. The aluminosilicate framework of a zeolite has a negative charge, which is balanced by the cations housed in the cagelike cavities. Zeolites have much more open, less dense structures than other silicates; between 20 and 50 percent of the volume of a zeolite structure is voids. Silicates such as zeolites that have three-dimensional frameworks of tetrahedra are termed tectosilicates. Besides the zeolites, other tectosilicates include quartz and feldspars.
There are about forty-five natural zeolites. They form in a number of relatively low temperature geologic environments. Gas pockets in basalt and other volcanic rocks may contain dramatic crystal groups of zeolites. Economically more important are the fine-grained zeolites such as clinoptilolite (Na, K)AlSi 5 O 12 · 3H 2 O formed by the alteration of fine-grained volcanic deposits by underground water. These are mined in the western United States and Mexico. Zeolites also form in alkaline desert lake sediments, in alkaline soils in deserts, and in marine sediments. Zeolites occur in low-temperature metamorphic rocks in geologically young regions of mountain building, such as South Island, New Zealand.
Although some natural zeolites occur in large amounts, they offer only a limited range of atomic structures and properties. Synthetic zeolites have a wider range of properties and larger cavities than their natural counterparts. They were first produced in the 1950s. Today more than 100 different zeolites have been made, and the annual production of synthetic zeolites exceeds 12,000 tons. Zeolites are manufactured in a number of ways; one important technique involves mixing sodium, aluminum, and silica chemicals with steam to create a gel (an amorphous, noncrystalline, water-rich solid). The gel is aged, then heated to about 90°C (194°F). Another technique uses kaolin clay that has been heated in a furnace until it begins to melt, then chilled and ground to powder. This powder is mixed with sodium salts and water, aged, and heated. In all the synthesis methods, the zeolite produced depends on the compositions of the starting materials and the conditions of reaction, including acidity, temperature, and water pressure.
Uses of Zeolites
The uses of zeolites derive from their special properties: They can interact with water to absorb or release ions (ion exchange); they can selectively absorb ions that fit the cavities in their structures (molecular sieves); they can hold large molecules and help them break into smaller pieces ( catalytic cracking). Zeolites are used as water softeners, to remove calcium ions, which react with soap to form scum. The water is filtered through a sodium-bearing zeolite, which absorbs the calcium and releases sodium ions into the water. When the zeolite can absorb no more calcium, it may be recharged by flushing it with brine (a saturated sodium chloride solution), which forces out the calcium ions and replaces them with sodium. At the Hanford Nuclear Facility in Richland, Washington, radioactive strontium-90 (Sr 90 ) and cesium-137 (Cs 137 ) have been removed from radioactive waste solutions by passing them through tanks packed with the natural zeolite clinoptilolite. Zeolites have also been used to clean radioactive wastes from the Three Mile Island nuclear power plant site and elsewhere. In addition, clinoptilolite is used to clean ammonium ions (NH 4 + ) from sewage and agricultural wastewater.
Sulfur dioxide (SO 2 ) is a pollutant produced by burning high-sulfur coal. It is a major cause of acid rain . Natural zeolites are the most effective filters yet found for absorbing sulfur dioxide from waste gases. As efforts to improve air quality continue, zeolites can be used to help purify the gases from power plants that burn high-sulfur coal from the Ohio River Valley and other regions.
Industrial applications make use of synthetic zeolites of high purity, which have larger cavities than the natural zeolites. These larger cavities enable synthetic zeolites to absorb or hold molecules that the natural zeolites do not. Some zeolites are used as molecular sieves to remove water and ni trogen impurities from natural gas. Because of their ability to interact with organic molecules, zeolites are important in refining and purifying natural gas and petroleum chemicals. The zeolites are not affected by these processes, so they are acting as catalysts. Zeolites are used to help break down large organic molecules found in petroleum into the smaller molecules that make up gasoline, a process called catalytic cracking. Zeolites are also used in hydrogenating vegetable oils and in many other industrial processes involving organic compounds.
SEE ALSO Green Chemistry; Petroleum; Stoichiometry .
Peter B. Leavens
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