Rare, beautiful, complex – just some of the names given to describe zeolites. This month Anna Ploszajski takes a closer look at their properties.
The zeolites are a family of microporous aluminosilicate minerals. Their basic framework structure is a 3D lattice, made up of interlinked alumina (AlO4)5- and silica (SiO4)4- tetrahedra in a plethora of different patterns, that cages with interconnecting channels to form a microporous framework. Since the aluminosilicate framework is overall negatively charged, there exist cations, such as Na+, H+ , K+, Ca+ or Mg+, which charge neutralise the framework. These cations tend to sit in well-defined sites on the lattice, and are loosely bound so they can be easily exchanged with others in a contact solution. The number of cations required is dictated by the number of alumina tetrahedra compared to silica tetrahedra most commonly referred to as the silicon to aluminium ratio. The more alumina tetrahedra there are, the more cations are needed.
Zeolites are characterised using two key features. Firstly, the framework structure, or the atomic formation, adopted by the silica and alumina tetrahedra, of which there have been more than 200 frameworks identified. The framework geometry governs the size and arrangement of the cages and channels in the structure. The second feature is the silicon to aluminium ratio already discussed, which dictates the surface chemistry. Each framework can tolerate a range of different silicon to aluminium ratios, from as little as one to high ratios of around 80.
In 1756, Swedish mineralogist Axel Fredrik Cronstedt, better known for the discovery of nickel (see MOTM, September 2015), named zeolites. He perceived that when he heated the mineral stilbite, a large amount of steam was produced, which appeared to come straight from the rock itself. He called it a zeolite, from the Greek zeo, to boil, and lithos, meaning stone. Unbeknown to him, the origin of the steam was the atmospheric water, held within the mineral’s porous interior.
Naturally formed zeolites are made when volcanic rocks and ash layers react with alkaline groundwater, or crystallise over many thousands of years in shallow marine basins. There are more than 40 known zeolite minerals including heulandite, clinoptilolite, stilbite, mordenite, analcime and chabazite. They can be extracted by conventional open pit mining and processed by crushing, drying and milling.
The commercial potential of natural zeolites was only realised in 1957, after the discovery of large deposits of relatively high purity zeolite minerals in volcanic tuffs in the USA. Today, annual global production of natural zeolites amounts to around three million tonnes, a third of which comes from China with most of the rest produced by South Korea, Japan, Jordan, Turkey, Slovakia and the USA.
In 1948 New Zealand born chemist Richard Barrer succeeded in synthesising mordenite in the laboratory, launching the production of synthetic zeolites. During the following decade, research duo Milton and Breck synthesised zeolite types A, X and Y in New York. The method typically involves creating a gel from silica, alumina and sodium hydroxide in an aqueous solution and heating it, causing the slow crystallisation of the zeolite material. This sol gel processing route allows the easy incorporation of metals and metal oxides into the framework. Zeolites have the intrinsic advantage that their synthesis process is easy to scale up and the alumina and silica are the most abundant mineral components on Earth.
In the beginning
The first synthetic zeolite to be commercialised was Linde Type A, named after Milton and Breck’s laboratory, in 1953. It was used as an adsorbent to remove oxygen impurity from argon gas, an application which made use of the molecular sieving properties of zeolites. The small and regular pore size of zeolites is of the order of some simple gaseous molecules, which lends them to molecular sieving applications, selectively separating molecules by size exclusion and allowing for small molecules to pass through, hindering the larger molecules. Natural zeolites chabazite, erionite and mordenite were commercialised for this purpose in 1962. Today, zeolites are employed to remove water, CO2 and SO2 from natural gas. Oxygen-generation systems use zeolites combined with pressure swing adsorption to remove nitrogen from compressed air to provide a supply of oxygen in hospitals and aircraft.
The high porosity of zeolites gives them high internal surface areas of hundreds of square metres per gram. Combined with the exchangeability of the cations, this makes them excellent candidate materials for catalytic applications. For example, zeolites containing H+ cations are powerful solid state acids, so can catalyse a range of interactions such as isomerisation and alkylation. In 1959, X- and Y-type zeolites were introduced as hydrocarbon-conversion catalysts, cracking hydrocarbons from longer chains into gasoline, diesel, kerosene and other petroleum by-products. During the 1980s and 1990s, the industrial possibilities of zeolites exploded, and they are now used for all sorts of catalytic applications, for example those with metal cations make excellent oxidation or reduction catalysts, such as Cu2+-doped zeolites to decompose NOx in vehicle exhaust emissions or Pd2+-doped zeolites to oxidise NH3 into N2 and to catalyse CH4 combustion.
While catalytic applications use zeolites, which are ion exchanged prior to use, the exchanging of cations during the applications themselves is also advantageous. This is exploited in water purification and softening systems, where the host Na+ or K+ cations are exchanged with Ca2+ and Mg+ and in aquarium filters to adsorb ammonia and other harmful nitrogenous compounds. The use of zeolites to purify water dates back to the ancient Greeks and Romans, who were renowned for their ingenuity when it came
to water systems and sanitation.
Safety and softeners
Zeolites are employed in the safe storage of nuclear waste from nuclear fission reactors. The aluminosilicate framework is inherently resistant to radiation, and zeolites have high selectivity for the residually radioactive fission by-products such as 90Sr and 137Cs. These materials must be safely stored indefinitely. Once loaded with fission products, the zeolites can be hot-pressed into an extremely durable ceramic, trapping the harmful waste inside. Furthermore, the selectivity for strontium and caesium makes zeolites integral to clean-up operations in the event of nuclear fallout. For example, the natural zeolite clinoptilolite was used to counteract the fallout from the 1986 Chernobyl disaster. It was added to soil and reduced the uptake of radioactive caesium by plants in the vicinity, and zeolites were even ingested by the people affected to nullify the effect of radioactivity in the gastro-intestinal tract.
In the 1960s, the laundry detergent industry was seeking an alternative to phosphate water softeners, as these resulted in the eutrophication of natural waters, killing plant and animal life. The solution was found in Na+-exchanged zeolite A. This zeolite contains a low silicon to aluminium ratio, and a high proportion of Na+ cations on its framework, which allows the zeolites to effectively act as water softeners by exchanging for Ca2+ in the water, making the stain-removal by the surfactants in the detergent more effective. Today, the largest single use of synthetic zeolites globally is in laundry detergent, exceeding one million tonnes per year.
Keeping with the domestic theme, the zeolite diatomite is also used in cat litter. The zeolites adsorb ammonia and moisture, significantly reducing unpleasant odours and making animal litter trays more hygienic. Similarly, zeolites have also been added to chicken feed, rendering the droppings drier and less malodorous. In agriculture, the natural zeolite clinoptilolite can be blended with ammonium fertilisers and used as a soil treatment, extending the efficiency and performance of the fertiliser as well as moderating moisture to prevent root rot and tempering drought cycles.
An added element
Zeolites also have an important role to play in the built environment. Since the 1990s, they have been used as an additive to asphalt concrete mix on the roads. The zeolites have the effect of decreasing the temperature of the mix necessary for laying – reducing fossil fuel consumption. As additions to Portland cement, they reduce chlorine permeability and weight, improve workability, and moderate the water content to slower drying – increasing the break strength of the product material.
From the kitchen to nuclear power, this little-known class of materials touches our lives in myriad applications. With ongoing zeolite research into solid state hydrogen storage, templating of novel porous materials and carbon capture and sequestration, to name just a few areas, zeolites are sure to turn heads in the coming years.