Properties and Applications
Due to the micro- to nanoscaled pores and channels, zeolites have a large inner surface area, which makes them well suited as a catalyst in chemical processes. Because of the negative matrix loading zeolites absorb positively charged ions. Therefore, zeolites are used as ion exchangers, as a filter for separating chemical substances or as a water softener in detergents (substituting polyphosphate) and used for flue gas desulfurization . Zeolites are used as odour absorbers in cat litter, to remove ammonia from soil and water and for the cleaning of radioactive waste water. In swimming pools they act as water cleaning filters.
Nanosized zeolite powders are applied for keeping the water of aquariums and fish ponds clean. In Fukushima tests are underway to decontaminate sea water and soil by means of zeolites. Because of their low density of 2-2.5 g / cm ³ they are used as filler for plastics, rubber, paper or asphalt and as an additive in adhesives.
In agriculture, zeolites are added to fertilizers to deliver nutrients such as potassium and nitrogen to the plant as necessary.
In cosmetics zeolites were used to bind moisture or to odour neutralization. As dietary supplement nano-zeolites are said to enhance absorption of minerals in the body or to absorb toxins such as heavy metals or radicals. But, there are no medical studies and the application is discussed very controversially .
Although having been known for a long time, zeolites have been used intensive only in recent decades, with more new applications being developed. An overview of properties and applications of zeolites is given in " Innere Werte - Zeolithe: nanoporöse Materialien für vielfältige Anwendungsfelder ” . Possible, yet speculative applications of nanoscaled zeolites are discussed in the chemistry-blog .
Appearance and production
Zeolites are found in nature mainly in volcanic rock and in the surrounding of hot springs. Natural zeolites are composed of different structural types. They usually have foreign atoms in their crystal lattice and contain water and impurities in the pores and channels. About 60 different zeolit types are found on Earth, with nine of them being degradable and technically exploited.
Pure zeolites and zeolite structures, which do not occur naturally, are synthesized. As structure builders silicic acid, boric acid or aluminium hydroxide are used. The starting materials are heated in an oven until crystallization occurs. Depending on the composition of the raw materials mixture, stirring rate and crystallization temperature, different kinds of zeolites can be synthesized. In the manufacturing process of zeolites often organic molecules are used as "template" to initiate the formation of the zeolite framework. Usually isometric particles or flakes with dimensions in the range of a few micrometers are formed. Zeolite particles with dimensions of 200 nm - 500 nm are called "nano-zeolites". Attempts to produce finer and more active zeolites are underway worldwide. By a higher nucleation rate particles in the range of 50 nm can already be produced. KAIST, the Korean Institute reported the production of thin sheets with a few nm thickness, which aggregate to form larger, highly porous particles. In the pores or channels, different materials are introduced in a targeted manner. Examples of pore fillers are water (H2O) or calcium hydroxide (Ca (OH)2).