Introduction to Zeolite Science & Practice
The term “zeolite” describes a family of aluminosillicate that share a unique microporous structure. Because of this highly regular structure, zeolites can be used as molecular sieves, selectively sorting molecules according to their size. Zeolites have multiple industrial, medical and domestic applications.
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Structure
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Zeolites have a crystalline three-dimensional framework structure with uniformly sized pores of molecular dimensions. Those pores will adsorb molecules that fit inside the pores while excluding molecules that are too large, acting as miniature sieves. Note that the process of adsorption differs from absorption. In adsorption, the molecules of an adjacent fluid are attracted and aligned into layers.
Catalysis
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Because zeolites confine molecules into very tight spaces, they can act as catalysts, accelerating the rate of chemical reactions. Some zeolite forms can act as powerful solid-state acids. This is exploited in organic acid-catalyzed reactions, including crude oil cracking, isomerisation and fuel synthesis. Some of the reactions that would occur naturally in the absence of zeolytes fail to occur because the resultant molecule would not fit within the porous structure. Thus, zeolites are said to be shape-selective catalysts.
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Applications
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Zeloites have multiple applications in industry, medicine, and farming among many other fields. Zeolite is used for water purification and softening, for oil refining, as a substitute for phosphates in detergents, as a molecular trap for fission products in nuclear waste, as a filter for removal of carbon and sulfur dioxide in natural gas streams, as medical disintoxicant, and as an ingredient in the manufacture of clump-free kitty litter.
Sources
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Natural zeolites are a product of the reaction of volcanic rock and ash with alkaline groundwater. Natural zeolite can be readily extracted by conventional open pit mining techniques. However, naturally occurring varieties are usually contaminated by other minerals, metals, quartz. For applications where a higher degree of purity is required, zeolites can be synthesized by a process of slow crystallization of a silica-alumina gel in the presence of alkalis. Of the 175 unique zeolite frameworks identified so far, approximately 40 occur naturally.
Zeolites and the Environment
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Many of the applications of zeolites are directly beneficial to the environment. For instance, zeolite replaces harmful phosphate builders in detergent, reducing water pollution. Because of their selective adsorption capabilities, zeolites can also remove athmospheric pollutants such as exhaust gases, as well as heavy metal ions from water. Nuclear waste can be trapped within blocks of zeolite, providing a safer medium for storage.
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References
- Photo Credit radioactive image by red2000 from Fotolia.com
