Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents and catalysts. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that rapidly heating a material, believed to have been stilbite, produced large amounts of steam from water that had been adsorbed by the material. Based on this, he called the material zeolite, from the Greek ζέω (zéō), meaning "to boil" and λίθος (líthos), meaning "stone". The classic reference for the field has been Breck's book Zeolite Molecular Sieves: Structure, Chemistry, And Use.
Zeolites occur naturally but are also produced industrially on a large scale. (As of December 2018), 253 unique zeolite frameworks have been identified, and over 40 naturally occurring zeolite frameworks are known. Every new zeolite structure that is obtained is examined by the International Zeolite Association Structure Commission and receives a three letter designation.
Properties and occurrence
Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite. An example of the mineral formula of a zeolite is: Na
10·2H2O, the formula for natrolite. Cation exchanged zeolites possess different acidity and catalyse different reactions.
Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater. Zeolites also crystallize in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins. Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz, or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential.
Zeolites are the aluminosilicate members of the family of microporous solids known as "molecular sieves", and mainly consist of silicon, aluminum, oxygen, and have the general formula MxAlxSi1-xO2·yH2O where M is either a metal ion or H+. The value of x is between 0 and 1 and y is the number of water molecules in the formula unit. Although there are a wide range of possible structures a commonality is that they are formed by the linking of the corner oxygen atoms of AlO4 and SiO4 tetrahedra to form covalent network structures. 
The term molecular sieve refers to a particular property of these materials, i.e., the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels. These are conventionally defined by the ring size of the aperture, where, for example, the term "eight-ring" refers to a closed-loop that is built from eight tetrahedrally coordinated silicon (or aluminum) atoms and eight oxygen atoms. These rings are not always perfectly symmetrical due to a variety of causes, including strain induced by the bonding between units that are needed to produce the overall structure or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical.
- The sequence of silica-rich volcanic rocks commonly progresses from:
- The sequence of silica-poor volcanic rocks commonly progresses from:
Industrially important zeolites are produced synthetically. Typical procedures entail heating aqueous solutions of alumina and silica with sodium hydroxide. Equivalent reagents include sodium aluminate and sodium silicate. Further variations include the use of structure directing agents (SDA) such as quaternary ammonium cations.
Synthetic zeolites hold some key advantages over their natural analogs. The synthetic materials are manufactured in a uniform, phase-pure state. It is also possible to produce zeolite structures that do not appear in nature. Zeolite A is a well-known example. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are among the most abundant mineral components on earth, the potential to supply zeolites is virtually unlimited.
Conventional open-pit mining techniques are used to mine natural zeolites. The overburden is removed to allow access to the ore. The ore may be blasted or stripped for processing by using tractors equipped with ripper blades and front-end loaders. In processing, the ore is crushed, dried, and milled. The milled ore may be air-classified as to particle size and shipped in bags or bulk. The crushed product may be screened to remove fine material when a granular product is required, and some pelletized products are produced from fine material.
(As of 2016) the world's annual production of natural zeolite approximates 3 million tonnes. Major producers in 2010 included China (2 million tonnes), South Korea (210,000 t), Japan (150,000 t), Jordan (140,000 t), Turkey (100,000 t) Slovakia (85,000 t) and the United States (59,000 t). The ready availability of zeolite-rich rock at low cost and the shortage of competing minerals and rocks are probably the most important factors for its large-scale use. According to the United States Geological Survey, it is likely that a significant percentage of the material sold as zeolites in some countries is ground or sawn volcanic tuff that contains only a small amount of zeolites. Some examples of such usage include dimension stone (as an altered volcanic tuff), lightweight aggregate, pozzolanic cement, and soil conditioners.
There are over 200 synthetic zeolites that have been synthesized by a process of slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates. Many more such structures could theoretically be made. In addition to variations in structures, zeolites can also be made with a variety of other atoms in them to make them chemically interesting and active. Some examples of the so-called heteroatoms that have been incorporated include germanium, iron, gallium, boron, zinc, tin, and titanium. One of the important processes used to carry out zeolite synthesis is sol-gel processing. The product properties depend on reaction mixture composition, pH of the system, operating temperature, pre-reaction 'seeding' time, reaction time as well as the templates used. In the sol-gel process, other elements (metals, metal oxides) can be easily incorporated. The silicalite sol formed by the hydrothermal method is very stable. The ease of scaling up this process makes it a favored route for zeolite synthesis.
The zeolite conundrum
Computer calculations have predicted that millions of hypothetical zeolite structures are possible. However, only 232 of these structures have been discovered and synthesized so far, so many zeolite scientists question why only this small fraction of possibilities are being observed. This problem is often referred to as "the bottleneck problem". Currently, several theories are attempting to explain the reasoning behind this question.
- Zeolite synthesis research has primarily been concentrating on hydrothermal methods; however, new zeolites may be synthesized using alternative methods. Synthesis methods that have started to gain use include microwave-assisted, post-synthetic modification, steam.
- Geometric computer simulations have shown that the discovered zeolite frameworks possess a behavior known as "the flexibility window". This shows that there is a range in which the zeolite structure is "flexible" and can be compressed but retain the framework structure. It is suggested that if a framework does not possess this property then it cannot be feasibly synthesized.
- As zeolites are metastable, certain frameworks may be inaccessible as nucleation cannot occur because more stable and energetically favorable zeolites will form. Post-synthetic modification has been used to combat this issue with the ADOR method, whereby frameworks can be cut apart into layers and bonded back together by either removing silica bonds or including them.
Uses of zeolites
Zeolites are widely used as ion-exchange beds in domestic and commercial water purification, softening, and other applications. Earlier, polyphosphates were used to soften hard water. The polyphosphates forms complex with metal ions like Ca2+ and Mg2+ to bind them up so that they could not interfere in cleaning process. However, when this phosphate rich water goes in main stream water, it results in eutrophication of water bodies and hence use of polyphosphate was replaced with use of a synthetic zeolite. In chemistry, zeolites are used to separate molecules (only molecules of certain sizes and shapes can pass through), and as traps for molecules so they can be analyzed.
Zeolites are also widely used as catalysts and sorbents. Their well-defined pore structure and adjustable acidity make them highly active in a large variety of reactions.
Zeolites have the potential of providing precise and specific separation of gases, including the removal of H2O, CO2, and SO2 from low-grade natural gas streams. Other separations include noble gases, N2, O2, freon, and formaldehyde.
On-board oxygen generating systems (OBOGS) and oxygen concentrators use zeolites in conjunction with pressure swing adsorption to remove nitrogen from compressed air to supply oxygen for aircrews at high altitudes, as well as home and portable oxygen supplies.
Synthetic zeolites, like other mesoporous materials (e.g. MCM-41), are widely used as catalysts in the petrochemical industry, such as in fluid catalytic cracking and hydrocracking. Zeolites confine molecules into small spaces, which causes changes in their structure and reactivity. The acidic forms of zeolites prepared are often powerful solid-state solid acids, facilitating a host of acid-catalyzed reactions, such as isomerization, alkylation, and cracking.
Catalytic cracking uses a reactor and a regenerator. Feed is injected onto a hot, fluidized catalyst where large gasoil molecules are broken into smaller gasoline molecules and olefins. The vapor-phase products are separated from the catalyst and distilled into various products. The catalyst is circulated to a regenerator, where the air is used to burn coke off the surface of the catalyst that was formed as a byproduct in the cracking process. The hot, regenerated catalyst is then circulated back to the reactor to complete its cycle.
Zeolites have used advanced nuclear-waste reprocessing methods, where their micro-porous ability to capture some ions while allowing others to pass freely allows many fission products to be efficiently removed from the waste and permanently trapped. Equally important are the mineral properties of zeolites. Their alumino-silicate construction is extremely durable and resistant to radiation, even in porous form. Additionally, once they are loaded with trapped fission products, the zeolite-waste combination can be hot-pressed into an extremely durable ceramic form, closing the pores and trapping the waste in a solid stone block. This is a waste form factor that greatly reduces its hazard, compared to conventional reprocessing systems. Zeolites are also used in the management of leaks of radioactive materials. For example, in the aftermath of the Fukushima Daiichi nuclear disaster, sandbags of zeolite were dropped into the seawater near the power plant to adsorb the radioactive cesium that was present in high levels.
The German group Fraunhofer e.V. announced that they had developed a zeolite substance for use in the biogas industry for long-term storage of energy at a density four times greater than water. Ultimately, the goal is to store heat both in industrial installations and in small combined heat and power plants such as those used in larger residential buildings.
Solar energy storage and use
Zeolites can be used to thermochemically store solar heat harvested from solar thermal collectors as first demonstrated by Guerra in 1978 and for adsorption refrigeration, as first demonstrated by Tchernev in 1974. In these applications, their high heat of adsorption and ability to hydrate and dehydrate while maintaining structural stability is exploited. This hygroscopic property coupled with an inherent exothermic (energy releasing) reaction when transitioning from a dehydrated form to a hydrated form make natural zeolites useful in harvesting waste heat and solar heat energy.
Commercial and domestic
The largest single use for zeolite is the global laundry detergent market. Zeolites are used in laundry detergent as water softeners, removing Ca2+ and Mg2+ ions which would otherwise precipitate from the solution. The ions are retained by the zeolites which releases Na+ ions into the solution, allowing the laundry detergent to be effective in areas with hard water.
Non-clumping cat litter is often made of zeolite or diatomite.
Synthetic zeolites are used as an additive in the production process of warm mix asphalt concrete. The development of this application started in Germany in the 1990s. They help by decreasing the temperature level during manufacture and laying of asphalt concrete, resulting in lower consumption of fossil fuels, thus releasing less carbon dioxide, aerosols, and vapors. The use of synthetic zeolites in hot mixed asphalt leads to easier compaction and, to a certain degree, allows cold weather paving and longer hauls.
When added to Portland cement as a pozzolan, they can reduce chloride permeability and improve workability. They reduce weight and help moderate water content while allowing for slower drying, which improves break strength. When added to lime mortars and lime-metakaolin mortars, synthetic zeolite pellets can act simultaneously as a pozzolanic material and a water reservoir.
Debbie Meyer Green Bags, a produce storage and preservation product, uses a form of zeolite as its active ingredient. The bags are lined with zeolite to adsorb ethylene, which is intended to slow the ripening process and extend the shelf life of produce stored in the bags.
Thomsonites, one of the rarer zeolite minerals, have been collected as gemstones from a series of lava flows along Lake Superior in Minnesota and, to a lesser degree, in Michigan. Thomsonite nodules from these areas have eroded from basalt lava flows and are collected on beaches and by scuba divers in Lake Superior.
These thomsonite nodules have concentric rings in combinations of colors: black, white, orange, pink, purple, red, and many shades of green. Some nodules have copper inclusions and rarely will be found with copper "eyes". When polished by a lapidary, the thomsonites sometimes displays a "cat's eye" effect (chatoyancy).
Research into and development of the many biochemical and biomedical applications of zeolites, particularly the naturally occurring species heulandite, clinoptilolite, and chabazite has been ongoing.
Zeolite-based oxygen concentrator systems are widely used to produce medical-grade oxygen. The zeolite is used as a molecular sieve to create purified oxygen from air using its ability to trap impurities, in a process involving the adsorption of nitrogen, leaving highly purified oxygen and up to 5% argon.
In agriculture, clinoptilolite (a naturally occurring zeolite) is used as a soil treatment. It provides a source of slowly released potassium. If previously loaded with ammonium, the zeolite can serve a similar function in the slow release of nitrogen. Zeolites can also act as water moderators, in which they will absorb up to 55% of their weight in water and slowly release it under the plant's demand. This property can prevent root rot and moderate drought cycles.
Clinoptilolite has also been added to chicken food: the absorption of water and ammonia by the zeolite made the birds' droppings drier and less odoriferous, hence easier to handle.
Pet stores market zeolites for use as filter additives in aquaria, where they can be used to adsorb ammonia and other nitrogenous compounds. They must be used with some care, especially with delicate tropical corals that are sensitive to water chemistry and temperature. Due to the high affinity of some zeolites for calcium, they may be less effective in hard water and may deplete calcium. Zeolite filtration is also used in some marine aquaria to keep nutrient concentrations low for the benefit of corals adapted to nutrient-depleted waters.
Where and how the zeolite was formed is an important consideration for aquarium applications. Most Northern hemisphere, natural zeolites were formed when molten lava came into contact with sea water, thereby "loading" the zeolite with Na (sodium) sacrificial ions. The mechanism is well known to chemists as ion exchange. These sodium ions can be replaced by other ions in solution, thus the take-up of nitrogen in ammonia, with the release of the sodium. A deposit near Bear River in southern Idaho is a fresh water variety (Na < 0.05%). Southern hemisphere zeolites are typically formed in freshwater and have a high calcium content.
Zeolite mineral species
- 09.GA. - Zeolites with T5O10 units (T = combined Si and Al) – the fibrous zeolites
- 09.GB. - Chains of single connected 4-membered rings
- 09.GC. - Chains of doubly connected 4-membered rings
- 09.GD. - Chains of 6-membered rings – tabular zeolites
- Chabazite framework (CHA): chabazite-series, herschelite, willhendersonite and SSZ-13
- Faujasite framework (FAU): faujasite-series, Linde type X (zeolite X, X zeolites), Linde type Y (zeolite Y, Y zeolites)
- Mordenite framework (MOR): maricopaite, mordenite
- Offretite–wenkite subgroup 09.GD.25 (Nickel–Strunz, 10 ed): offretite (OFF), wenkite (WEN)
- Bellbergite (TMA-E, Aiello and Barrer; framework type EAB), bikitaite (BIK), erionite-series (ERI), ferrierite (FER), gmelinite (GME), levyne-series (LEV), dachiardite-series (DAC), epistilbite (EPI)
- 09.GE. - Chains of T10O20 tetrahedra (T = combined Si and Al)
- Chemistry:Geopolymer – Polymeric Si–O–Al framework similar to zeolites but amorphous, the amorphous alumino-silicate equivalent of zeolite
- Earth:List of minerals – List of minerals for which there are articles on Wikipedia
- Chemistry:Hypothetical zeolite
- Chemistry:Adsorption – Science of surface adhesion
- Chemistry:Pyrolysis – Thermal decomposition of materials at elevated temperatures in an inert atmosphere
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- International Zeolite Association
- Database of zeolite pore characterizations
- The Synthesis Commission of the International Zeolite Association
- Federation of European Zeolite Associations
- British Zeolite Association
- Database of Zeolite Structures
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