Properties of Zeolite

Zeolites are found in washing powder.

Zeolites, a class of microporous materials, are able to selectively screen certain molecules, allowing them to act as molecular sieves, a property that make zeolites ideal for a number of applications in industry and domestic settings.

History

Natural zeolites are often contaminated with silica.

The first known reference to a zeolite came from Swedish mineralogist Cronstedt, who discovered the substance in 1756. This new type of material consisted of hydrated aluminosilicates containing alkali and alkaline earth metals. The term zeolite was coined from the Greek words "zeo" and "lithos" meaning to "to boil" and "a stone." Water can appear to boil out of the rock when it is heated. Naturally occurring zeolites tend not to be pure and are contaminated by other minerals. Synthetic zeolites were not successfully synthesised until the 1930s, and the first definitive synthesis was not achieved until 1948 by Richard Barrer. There are currently 176 unique zeolite framework types, and each is assigned a three-letter code by the Structure Commission of the International Zeolite Association. Zeolites are microporous aluminosilicates with unique 3D lattices of Si, Al and O that make up silica and alumina tetrahedra. The framework structure is relatively open and characterized by the presence of channels and cavities. The framework is negatively charged and is balanced by interchangeable cations.

Properties

Porous nature of zeolites

Zeolites are porous in nature, and their structure can accommodate a wide variety of cations. These positive ions can be easily exchanged for others.

Zeolites belong to the "molecular sieves" family of microporous solids. This term refers to a particular property of these materials, i.e., the ability to sort molecules based on size. This property arises due to the regular pore structure. These dimensions are on a molecular level and can be used to control the molecules that can enter/leave the zeolite framework.

Uses

Zeolites have many industrial uses.

Zeolites have many uses both commercially and in research. They are widely used as ion-exchange beds for water purification and softening. As their structures are size specific, they act as molecular traps and can also be used for the separation of gases, including the removal of water, carbon dioxide and sulfur dioxide from low-grade natural gas streams. They are also employed in the separation of other gases, such as noble gases, nitrogen and oxygen.

The largest single use for zeolite is the global laundry detergent market, and it is used in non-clumping cat litter. Synthetic zeolites are widely used as catalysts in the petrochemical industry, for instance in fluid catalytic cracking and hydro-cracking. When hydrogen is the balancing cation, the zeolites are termed acidic zeolites and become useful in a wide variety of catalytic reactions. Zeolites have also found a use in the nuclear industry in connection with fuel reprocessing. Their structures are resistant to radiation. Certain radioactive ions can be trapped whilst others pass through the zeolite, which allows for permanent removal of radioactive waste. Zeolites can also be used in soil treatment. Clinoptilolite acts as a chemical filter due to its high cation exchange capacity.

References

• British Zeolite Association
• IZA Structure Commission: Database of Zeolite Structures
• Zeolyst
• EUZEPA: Zeolites and Detergents

Resources

• International Zeolite Association