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Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial
adsorbents Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These positive ions can be exchanged for others in a contacting
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
solution. exchanged zeolites are particularly useful as solid
acid catalysts In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
. The term ''zeolite'' was originally coined in 1756 by
Swedish Swedish or ' may refer to: Anything from or related to Sweden, a country in Northern Europe. Or, specifically: * Swedish language, a North Germanic language spoken primarily in Sweden and Finland ** Swedish alphabet, the official alphabet used by ...
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 , meaning "to boil" and , meaning "stone". Zeolites occur naturally but are also produced industrially on a large scale. , 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.


Characteristics


Properties

Zeolites are the aluminosilicate members of the family of
microporous material A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks. Porous materials are classified into several kinds by their size. The recom ...
s, and mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. The value of x (Si/Al molar ratio) is greater than 1 and y is the number of water molecules in the formula unit. Zeolites have microporous structures with a typical diameter of 0.3–0.8 nm and accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are often loosely held and can readily be exchanged for others in a contacting
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
solution. Cation exchanged zeolites possess different acidity and catalyze different reactions.. The Si/Al ratio is greater than 1 because zeolites have no Al-O-Al bond (Löwenstein rule). They are formed by the linking of the corner oxygen atoms of AlO4 and SiO4 tetrahedra to form covalent network structures. The general formula of zeolite, , where the part is ionic bond-like and the part is
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
-like. Zeolites therefore have both ionic crystal and covalent crystal properties, and the balance of these properties depends on the Si/Al ratio (x). Si/Al ratios below about 3 correspond to natural zeolites and some synthetic zeolites such as A-type and X-type zeolites. They are useful as ion-exchange agents because of their high ion-exchange capacity. Commercially available molecular sieve adsorbents often belong to this group. Zeolites with a Si/Al ratios higher about 3 are classified as high-silica zeolites, which are rarely found in nature and are synthesized industrially. They possess high physical and chemical stability due to the large colvalent bonding contribution. They have excellent hydrophobicity and are suited for adsorption of builky, hydrophobic molecules such as hydrocarbons. In addition to that, high-silica zeolites are exchangeable, unlike natural zeolites, and are used as
solid acid catalyst In acid catalysis and base catalysis, a chemical reaction is catalyzed by an acid or a base. By Brønsted–Lowry acid–base theory, the acid is the proton (hydrogen ion, H+) donor and the base is the proton acceptor. Typical reactions catalyzed ...
s. The acidity is strong enough to protonate hydrocarbons and high-silica zeolites are used in acid catalysis processes such as
fluid catalytic cracking Fluid Catalytic Cracking (FCC) is the conversion process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, olefinic gases, and other petroleum prod ...
in petrochemical industry. Apart from the adsorption, catalytic, and other properties of zeolites, it also shows several interesting properties, e.g. window effect, single-file diffusion, and Levitation Effect. The Levitation Effect is an anomalous diffusion phenomenon observed in all systems with van der Waals interactions.


Framework structure

As of December 2018, the framework structures of 253 different zeolites or their analogues are known, nearly 200 of which can only be synthesized artificially. For each structure, the International Zeolite Association (IZA) gives a three-letter code called framework type code (FTC). For example, the major molecular sieves, 3A, 4A and 5A, are all LTA (Linde Type A). Most commercially available natural zeolites are of the MOR, HEU or ANA-types. An example of the notation of the ring structure of zeolite and other silicate materials is shown in the upper right figure. The middle figure shows a common notation using
structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...
. The left figure emphasizes the SiO tetrahedral structure. Connecting oxygen atoms together creates a four-membered ring of oxygen (blue bold line). In fact, such a ring substructure is called four membered ring or simply four-ring. The figure on the right shows a 4-ring with Si atoms connected to each other, which is the most common way to express the topology of the framework. The figure on the right compares the typical framework structures of LTA (left) and
FAU FAU or Fau may refer to: Education * Florida Atlantic University, in Boca Raton, Florida, U.S. * University of Erlangen–Nuremberg (German: ), in Bavaria, Germany People * Felix Anudike-Uzomah (born 2002), American football player * André ...
(right). Both zeolites share the truncated octahedral structure (
sodalite Sodalite ( ) is a tectosilicate mineral with the formula , with royal blue varieties widely used as an wikt:ornamental, ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite i ...
cage) (purple line). However, the way they are connected (yellow line) is different: in LTA, the four-membered rings of the cage are connected to each other to form a skeleton, while in FAU, the six-membered rings are connected to each other. As a result, the pore entrance of LTA is an 8-ring (0.41 nm) and belongs to the small pore zeolite, while the pore entrance of FAU is a 12-ring (0.74 nm) and belongs to the large pore zeolite, respectively. Materials with a 10-ring are called medium pore zeolites, a typical example being
ZSM-5 ZSM-5, Zeolite Socony Mobil–5 (framework type MFI from ZSM-5 (five)), is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96–nO192·16H2O (0
(MFI). Although more than 200 types of zeolites are known, only about 100 types of aluminosilicate are available. In addition, there are only a few types that can be synthesized in industrially feasible way and have sufficient thermal stability to meet the requirements for industrial use. In particular, the FAU (faujasite, USY), *BEA (beta), MOR (high-silica mordenite), MFI (ZSM-5), and FER (high-silica ferrierite) types are called the big five of high silica zeolites, and industrial production methods have been established.


Porosity

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 aluminium) 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.


Isomorphous substitution

Isomorphous substitution In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word is ...
of Si in zeolites can be possible for some heteroatoms such as titanium,} zinc and
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
. Al atoms in zeolites can be also structurally replaced with
boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
and
gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
. The silicoaluminophosphate type (AlPO molecular sieve), in which Si is isomorphous with Al and P and Al is isomorphous with Si, and the gallogermanate and others are known.


Natural occurrence

Some of the more common mineral zeolites are analcime, chabazite,
clinoptilolite Clinoptilolite is a natural zeolite composed of a microporous arrangement of silica and alumina tetrahedra. It has the complex formula . It forms as white, green to reddish tabular monoclinic tectosilicate crystals with a Mohs hardness of 3.5 to ...
, heulandite, natrolite,
phillipsite Phillipsite is a mineral series of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to . The members of the series are phillipsite-K, phillipsite-Na and phillipsite-Ca. The crystals are monoclinic, but only c ...
, and stilbite. An example of the mineral formula of a zeolite is: ·2H2O, the formula for natrolite. Natural zeolites form where volcanic rocks and
ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...
layers react with
alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
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 transform to other minerals under weathering, hydrothermal alteration or metamorphic conditions. Some examples: * The sequence of silica-rich volcanic rocks commonly progresses from: ** Clay → quartz → mordeniteheulanditeepistilbitestilbitethomsonite
mesolite Mesolite is a tectosilicate mineral with formula Na2 Ca2( Al2 Si3 O10)3·8 H2O. It is a member of the zeolite group and is closely related to natrolite which it also resembles in appearance. Mesolite crystallizes in the orthorhombic system and t ...
scolecitechabazite
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
. * The sequence of silica-poor volcanic rocks commonly progresses from: **
Cowlesite Cowlesite is a mineral named after American mineralogist John Cowles. It was first described by W.S. Wise and Rudy W. Tschernich in material from roadcuts along Neer Road, Goble, Oregon, United States. The description also incorporated data fr ...
levyne
offretite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
analcimethomsonite
mesolite Mesolite is a tectosilicate mineral with formula Na2 Ca2( Al2 Si3 O10)3·8 H2O. It is a member of the zeolite group and is closely related to natrolite which it also resembles in appearance. Mesolite crystallizes in the orthorhombic system and t ...
scolecitechabazite
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
.


Gemstones

Thomsonites, one of the rarer zeolite minerals, have been collected as
gemstone A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...
s 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).


Production

Industrially important zeolites are produced synthetically. Typical procedures entail heating aqueous solutions of alumina and silica with
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
. Equivalent reagents include sodium aluminate and
sodium silicate Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...
. Further variations include the use of structure directing agents (SDA) such as
quaternary ammonium cation In chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cations ...
s. 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.


Ore mining

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 A projection screen is an installation consisting of a surface and a support structure used for displaying a projected image for the view of an audience. Projection screens may be permanently installed, as in a movie theater; painted on the ...
to remove fine material when a granular product is required, and some pelletized products are produced from fine material. , the world's annual production of natural zeolite approximates 3 million tonnes. Major producers in 2010 included
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
(2 million tonnes), South Korea (210,000 t),
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
(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 Dimension stone is natural stone or rock that has been selected and finished (e.g., trimmed, cut, drilled, ground, or other) to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements. A ...
(as an altered volcanic tuff), lightweight
aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
, pozzolanic cement, and
soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...
s.


Synthesis

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 An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
, 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 Silicalite is an inorganic compound with the formula SiO2. It is one of several forms ( polymorphs) of silicon dioxide. It is a white solid. It consists of tetrahedral silicon centers and two-coordinate oxides. It is prepared by hydrothermal rea ...
sol formed by the hydrothermal method is very stable. The ease of scaling up this process makes it a favored route for zeolite synthesis.


Applications

Zeolites are widely used as catalysts and sorbents. Their well-defined pore structure and adjustable acidity make them highly active in a large variety of reactions. 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. Research into and development of the many
biochemical Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
and
biomedical Biomedicine (also referred to as Western medicine, mainstream medicine or conventional medicine)
applications of zeolites, particularly the naturally occurring species heulandite,
clinoptilolite Clinoptilolite is a natural zeolite composed of a microporous arrangement of silica and alumina tetrahedra. It has the complex formula . It forms as white, green to reddish tabular monoclinic tectosilicate crystals with a Mohs hardness of 3.5 to ...
, and chabazite has been ongoing.


In organic synthesis

In synthetic chemistry, homogeneous catalysts are preferred because of availability, low cost, and excellent catalytic activity as all the catalytic sites are readily available. But these homogeneous catalysts have several disadvantageous, such as being non-reusable, and require more than the stoichiometric amount. Also, some other drawbacks in its use include the potential dangers in handling, toxicity, corrosive nature, difficulty in separation and recovery, and disposal problems due to the acidic effluent. In addition to that, hydrolysis and purification of the resultant complex results in corrosive by-products. So, the basic idea is to find alternative heterogeneous solid catalysts which are stable, reusable, and nature-friendly, and there has been much attention to finding new ones which will also allow a better work up of reaction products. Among these different solid catalysts, zeolites were found to be superior due to their shape selectivity, thermal stability, and reusability . Freidel-Crafts alkylation and acylations using zeolites as catalyst are common in organic synthesis.


Ion exchange and softeners

Zeolites are widely used as
ion-exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
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. The largest single use for zeolite is the global laundry
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...
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.


Catalysis

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 In petrochemistry, petroleum geology and organic chemistry, cracking is the process whereby complex organic molecules such as kerogens or long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of ...
. 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 Fluid Catalytic Cracking (FCC) is the conversion process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, olefinic gases, and other petroleum prod ...
uses a reactor and a regenerator. Feed is injected onto a hot, fluidized catalyst where large
gasoil Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil, marine fuel oil (MFO), bun ...
molecules are broken into smaller gasoline molecules and
olefins In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
. 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.


Nuclear waste reprocessing

Zeolites have been used in advanced
nuclear reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...
methods, where their micro-porous ability to capture some ions while allowing others to pass freely allows many
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
s 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 The was a nuclear accident in 2011 at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima, Japan. The proximate cause of the disaster was the 2011 Tōhoku earthquake and tsunami, which occurred on the afternoon of 11 March 2011 and ...
, sandbags of zeolite were dropped into the seawater near the power plant to adsorb the radioactive cesium-137 that was present in high levels.


Gas separation and storage

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. 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. 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.
Debbie Meyer Green Bags The Debbie Meyer Green Bag are food storage bags sold and licensed by Housewares America, Inc. Independent laboratory results (Nova Biologicals, Inc - Conroe, Texas) and television commercials for the product claim it will keep fruits and vegetabl ...
, a produce storage and preservation product, uses a form of zeolite as its active ingredient. The bags are lined with zeolite to adsorb
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
, which is intended to slow the ripening process and extend the shelf life of produce stored in the bags. 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. Zeolites are also used as a molecular sieve in cryosorption style vacuum pumps.


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 Adsorption refrigeration was invented by Michael Faraday in 1821, even though the basis of artificial modern refrigeration dates back to 1748 with William Cullen's experiments. Adsorption is sometimes referred to as solid sorption. It is very s ...
, as first demonstrated by Tchernev in 1974. In these applications, their high heat of adsorption and ability to
hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
and dehydrate while maintaining structural stability is exploited. This hygroscopic property coupled with an inherent
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
(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.


Light emission

Zeolites were discovered to help silver naturally emit light, which may compete with fluorescent lights or
LED A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s.


Building materials

Synthetic zeolites are used as an additive in the production process of warm mix
asphalt concrete Asphalt concrete (commonly called asphalt, blacktop, or pavement in North America, and tarmac, bitumen macadam, or rolled asphalt in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface roads, parkin ...
. 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.


Cat litter

Non-clumping cat litter is often made of zeolite (or diatomite), one form of which, invented at MIT, can sequester the
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
methane from the atmosphere.


Hemostatic agent

QuikClot brand hemostatic agent, which is used to stop severe bleeding, contains a calcium-loaded form of zeolite found in kaolin clay.


Soil treatment

In agriculture,
clinoptilolite Clinoptilolite is a natural zeolite composed of a microporous arrangement of silica and alumina tetrahedra. It has the complex formula . It forms as white, green to reddish tabular monoclinic tectosilicate crystals with a Mohs hardness of 3.5 to ...
(a naturally occurring zeolite) is used as a soil treatment. It provides a source of slowly released potassium. If previously loaded with
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
, 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.


Aquaria

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

The zeolite structural group ( Nickel-Strunz classification) includes: * 09.GA. - Zeolites with T5O10 units (T = combined Si and Al) – the fibrous zeolites ** Natrolite framework (NAT):
gonnardite Gonnardite is a comparatively rare, fibrous zeolite, natrolite subgroup. Older papers claim that a complete solid solution exists between tetranatrolite and gonnardite, but tetranatrolite was discredited as a separate species in 1999.American Mi ...
, natrolite,
mesolite Mesolite is a tectosilicate mineral with formula Na2 Ca2( Al2 Si3 O10)3·8 H2O. It is a member of the zeolite group and is closely related to natrolite which it also resembles in appearance. Mesolite crystallizes in the orthorhombic system and t ...
, paranatrolite, scolecite, tetranatrolite ** Edingtonite framework (EDI):
edingtonite Edingtonite is a white, gray, brown, colorless, pink or yellow zeolite mineral. Its chemical formula is Ba Al2 Si3 O10·4 H2O. It has varieties with tetragonal, orthorhombic or triclinic crystals. The mineral occurs within cavities in nephelin ...
, kalborsite ** Thomsonite framework (THO): thomsonite-series * 09.GB. - Chains of single connected 4-membered rings ** Analcime framework (ANA): analcime, leucite, pollucite,
wairakite Wairakite is a zeolite mineral with an analcime structure but containing a calcium ion. The chemical composition is Ca8(Al16Si32O96)•16H2O. It is named for the location of its discovery in Wairakei, North Island, New Zealand, by Alfred Steiner ...
** Laumontite (LAU),
yugawaralite Yugawaralite is a clear or pinkish mineral of the Zeolite group. It was first described by Sakurai and Hayashi (1952) near a waterfall by some hot springs near Yugawara. Etymology Yugawaralite is named after the town Yugawara, where it was fou ...
(YUG), goosecreekite (GOO), montesommaite (MON) * 09.GC. - Chains of doubly connected 4-membered rings ** Phillipsite framework (PHI): harmotome,
phillipsite Phillipsite is a mineral series of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to . The members of the series are phillipsite-K, phillipsite-Na and phillipsite-Ca. The crystals are monoclinic, but only c ...
-series ** Gismondine framework (GIS):
amicite Amicite is a silicate mineral of the zeolite family. It has a general formula of K2Na2Al4Si4O16·5(H2O). Amicite was described in 1979 from specimens obtained at the Höwenegg quarry in Immendingen, Hegau, in the German state of Baden-Württembe ...
,
gismondine Gismondine is a mineral with the chemical formula CaAl2Si2O8·4(H2O). It is a zeolite or hydrated alumino-silicate. It forms colorless, bipyramidal crystals of orthorhombic symmetry. Gismondine was named for Italian mineralogist Carlo Giuseppe ...
, garronite, gobbinsite ** Boggsite (BOG), merlinoite (MER),
mazzite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
-series (MAZ),
paulingite Paulingite or paulingite-K is a rare zeolite mineral that is found in vesicles in the basaltic rocks from the Columbia River near Rock Island Dam, Washington. Paulingite was named for Linus Carl Pauling (1901–1994), professor of chemistry, C ...
-series (PAU), perlialite (Linde type L framework, zeolite L, LTL) * 09.GD. - Chains of 6-membered rings – tabular zeolites ** Chabazite framework (CHA): chabazite-series, herschelite, willhendersonite and
SSZ-13 SSZ-13 (framework type code CHA) is a high-silica aluminosilicate zeolite possessing 0.38 × 0.38 nm micropores.Baerlocher, Ch.; McCusker, L.B..; Olson, D.H. Atlas of Zeolite framework Types, Elsevier, Amsterdam, 6th edn., 2007, see also: http ...
** 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 Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
(OFF),
wenkite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
(WEN) ** Bellbergite (TMA-E, Aiello and Barrer; framework type EAB), bikitaite (BIK),
erionite Erionite is a naturally occurring fibrous mineral that belongs to a group of minerals called zeolites. It usually is found in volcanic ash that has been altered by weathering and ground water. Erionite forms brittle, wool-like fibrous masses in t ...
-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) ** Heulandite framework (HEU):
clinoptilolite Clinoptilolite is a natural zeolite composed of a microporous arrangement of silica and alumina tetrahedra. It has the complex formula . It forms as white, green to reddish tabular monoclinic tectosilicate crystals with a Mohs hardness of 3.5 to ...
, heulandite-series ** Stilbite framework (STI):
barrerite Barrerite is a tectosilicate mineral and a member of the zeolite family. It is one of the rarer zeolites. It was named for Richard Barrer, a New Zealand-born chemist. Barrerite crystal are white to pinkish, with a vitreous-glassy luster. The cry ...
,
stellerite Stellerite is a rare mineral discovered by and named after Georg Wilhelm Steller, a German explorer and zoologist. The mineral has a general formula of Ca Al2Silicon.html"_;"title="Aluminum.html"_;"title="/nowiki>Aluminum">Al2Silicon">Si7Oxyge ...
, stilbite-series ** Brewsterite framework (BRE): brewsterite-series * Others **
Cowlesite Cowlesite is a mineral named after American mineralogist John Cowles. It was first described by W.S. Wise and Rudy W. Tschernich in material from roadcuts along Neer Road, Goble, Oregon, United States. The description also incorporated data fr ...
, pentasil (also known as
ZSM-5 ZSM-5, Zeolite Socony Mobil–5 (framework type MFI from ZSM-5 (five)), is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96–nO192·16H2O (0
, framework type MFI), tschernichite (beta polymorph A, disordered framework, BEA), Linde type A framework (zeolite A, LTA)


Computational study

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 observed. This problem is often referred to as "the bottleneck problem". Currently, several theories attempt to explain the reasoning behind this question. # Zeolite synthesis research has primarily concentrated 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, and 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 retains 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. # Based on dense crystal model systems, the theory of crystallization via solute pre-nucleation clusters was developed. Investigation of zeolite crystallization in hydrated silicate ionic liquids (HSIL) has shown that zeolites can nucleate via the condensation of ion-paired pre-nucleation clusters. This line of research identified several connections between the synthesis medium liquid chemistry and important properties of zeolite crystals, such as the role of inorganic structure-directing agents in zeolite framework selection, the role of ion-pairing on the zeolite molecular composition and topology, and the role of liquid cation mobility on the zeolite crystal size and morphology. Consequently, complex relations exist between the properties of zeolite synthesis media and the crystallizing zeolite, potentially explaining why only a small fraction of the hypothetical zeolite frameworks can be synthesized. While these relations are not yet fully understood, HSIL zeolite synthesis is an exceptional model system for zeolite science, providing opportunities to advance current understanding of the zeolite conundrum.


See also

* , the amorphous alumino-silicate equivalent of zeolite * * * * *


References

*


Further reading

* The classic reference for the field has been Breck's book ''Zeolite Molecular Sieves: Structure, Chemistry, And Use''. * * * * * *


External links


International Zeolite Association

Database of zeolite pore characterizations

The Synthesis Commission of the International Zeolite Association

Federation of European Zeolite Associations

British Zeolite Association

Bulk Zeolite

Database of Zeolite Structures
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