Properties and Applications

Zeolites were discovered in 1756 by Axel Frederick Cronstedt. They consist of the basic building blocks silicate, aluminate and phosphate. They belong to the group of aluminosilicates. The aluminium, silicon and phosphorus atoms are connected via oxygen atoms in all three spatial directions. The result is a highly porous structure with many pores and channels[1]. Depending on the type of channel system, zeolites are divided into fiber, leaf and cube zeolites. The applications of zeolites are very often based on their ability to absorb other substances inside. Either to release previously introduced substances (fertilizing plants with potassium) or to absorb – in most cases unwanted - substances as in cat litter, where odour molecules are bound.

 

Structure of ZeolithA ©natros/fotolia.comStructure of ZeolithA ©natros/fotolia.comDue to the micro to nanoscale cavities and channels, zeolites have a large internal surface area, which is why they are important in industry as catalysts in chemical processes. Since the zeolite lattice framework is composed of negatively charged anions, zeolites can absorb positively charged ions. Zeolites are therefore also used as ion exchangers, filters for the separation of chemical substances or as water softeners in detergents (as polyphosphate substitutes) and for flue gas desulphurization. Zeolites are suitable for removing ammonia from soil and water and for purifying radioactively contaminated water. Zeolites serve as filters for dirt in swimming pools and as odour filters in cat litter.


Nanoscale zeolite powders are offered as part of commercial products to keep the water of aquariums and fish ponds clean. In Fukushima, zeolites are used to decontaminate the sea and soil from radioactive contaminants. Due to their low density of 2-2.5 g/cm³ they are used as fillers for plastics, rubber, paper or asphalt. They are also used as additives for adhesives.


In agriculture, zeolites are used as additives to fertilizers to supply the plant with nutrients such as potassium and nitrogen as required.
Zeolites are used in cosmetics to bind moisture or neutralize odors. It is claimed that nano-zeolites as dietary supplements should improve the absorption of minerals in the body or remove toxins such as heavy metals or radicals. However, there are no medical studies available and thus the application of zeolites is discussed very controversially.

 

Although zeolites have been known for a long time, they have only been used intensively in recent decades, and new applications are increasingly being developed.


Zeolites are nanometer sized powders; they are not self-igniting. Even when igniting a fine dispersed mixture of zeolites with air, there is no possibility of a dust explosion. Zeolites are not flammable.

 

 

Occurence and Production

Zeolites are found in nature mainly in volcanic rocks and in the surroundings of hot springs. Of the approximately 60 zeolites occurring on earth as minerals, nine are mined and technically usable. They have different pore sizes and shapes and are loaded with different alkali or alkaline earth ions depending on their area of origin. The water content also varies.


One example of naturally occurring zeolites is the sponge-like faujasite. The industrially produced zeolites X and Y have a comparable structure. Faujasite is not toxic, but is even used in medicine. However, the naturally occurring, rare, fibrous erionite is known to be carcinogenic - for understandable reasons no applications are known in Europe.

 

Naturally occurring zeolite in Iceland (Photo: C. Steinbach)Naturally occurring zeolite in Iceland (Photo: C. Steinbach)Zeolite structures that do not occur naturally are chemically produced. The starting materials used are scaffolding materials such as silica, boric acid or aluminium hydroxide. These reaction mixtures are heated in a furnace until they crystallize. Depending on the composition of the reaction mixture, stirring speed and crystallization temperature, different types of zeolites can be produced. In the manufacturing process, organic molecules are often used as "templates" around which the zeolite framework crystallizes. Usually particles or flakes with dimensions in the range of a few micrometers are produced.


Particles with dimensions of 200 nm - 500 nm are offered as "nano-zeolites". The production of even finer and therefore even more active zeolites is an issue worldwide. Particles with a diameter in the range of 50 nm can already be generated. The Korean KAIST Institute reports on the production of thin flakes with a thickness of a few nm that form larger, very porous particles.

 

 

 

Literature

  1. Osterhoff, C (200). Untersuchung der Kristallinität oberflächennaher Bereiche mikroporöser Materialien mittels NMR-Spektroskopie, Dissertation, Ruhr-University Bochum. (PDF-Document, in German).

 

 

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