Zeolite

Microscopic structure of a zeolite (mordenite) framework, assembled from corner-sharing SiO
₄ tetrahedra. Sodium is present as an extra-framework cation (in green). Si atoms can be partially replaced by Al or other tetravalent metals.

Zeolites are white solids with ordinary handling properties, like many routine aluminosilicate minerals, e.g. feldspar. They have the general formula MAlO₂)(SiO₂)_x(H₂O)_y where M⁺ is usually H⁺ and Na⁺. The Si/Al ratio is variable, which provides a means to tune the properties. Zeolites with a Si/Al ratios higher about 3 are classified as high-silica zeolites, which tend to be more hydrophobic. The H⁺ and Na⁺ can be replaced by diverse cations, because zeolites have ion exchange properties. The nature of the cations influences the porosity of zeolites.

Zeolites have microporous structures with a typical diameter of 0.3–0.8 nm. Like most aluminosilicates, the framework is formed by linking of aluminum and silicon atoms by oxides. This linking leads to a 3-dimensional network of Si-O-Al, Si-O-Si, and Al-O-Al linkages. The aluminum centers are negatively charged, which requires an accompanying cation. These cations are hydrated during the formation of the materials. The hydrated cations interrupt the otherwise dense network of Si-O-Al, Si-O-Si, and Al-O-Al linkage, leading to regular water-filled cavities. Because of the porosity of the zeolite, the water can exit the material through channels. Because of the rigidity of the zeolite framework, the loss of water does not result in collapse of the cavities and channels. This aspect - the ability to generate voids within the solid material - underpins the ability of zeolites to function as catalysts. They possess high physical and chemical stability due to the large covalent bonding contribution. They have excellent hydrophobicity and are suited for adsorption of bulky, hydrophobic molecules such as hydrocarbons. In addition to that, high-silica zeolites are H⁺
exchangeable, unlike natural zeolites, and are used as solid acid catalysts. The acidity is strong enough to protonate hydrocarbons and high-silica zeolites are used in acid catalysis processes such as fluid catalytic cracking in petrochemical industry.[7]

Zeolite Mordenite with some Si atoms substituted with Al atoms.

Three ways to represent the oxygen 4-membered ring structure of silicate compounds.

Comparison of framework structures of LTA-type zeolite (left) and FAU-type zeolite (right)

The structures of hundreds of zeolites have been determined. Most do not occur naturally. For each structure, the International Zeolite Association (IZA) gives a three-letter code called framework type code (FTC).[4] 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 left figure emphasizes the SiO₄ 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 (right). Both zeolites share the truncated octahedral structure (sodalite 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[4]) and belongs to the small pore zeolite, while the pore entrance of FAU is a 12-ring (0.74 nm[4]) and belongs to the large pore zeolite, respectively. Materials with a 10-ring are called medium pore zeolites, a typical example being ZSM-5 (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,[8] and industrial production methods have been established.

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 of Si in zeolites can be possible for some heteroatoms such as titanium,[9] zinc[10] and germanium.[11] Al atoms in zeolites can be also structurally replaced with boron[12] and gallium.[13]

The silicoaluminophosphate type (AlPO molecular sieve),[14] in which Si is isomorphous with Al and P and Al is isomorphous with Si, and the gallogermanate[15] and others are known.

Polished thomsonite

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).[17]

Natrolite from Poland

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).[19] 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. These materials are used for construction, e.g. dimension stone (as an altered volcanic tuff), lightweight aggregate, pozzolanic cement, and soil conditioners.[20]

Synthetic zeolite

Over 200 synthetic zeolites have been reported.[21] Most zeolites have aluminosilicate frameworks but some incorporate germanium, iron, gallium, boron, zinc, tin, and titanium.[22] Zeolite synthesis involves sol-gel-like processes. 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.

In synthetic chemistry, homogeneous catalysts are preferred because of availability, low cost, and excellent catalytic activity as all the catalytic sites are readily available. However, these homogeneous catalysts have several disadvantages, such as being non-reusable, requiring more than a stoichiometric amount, and difficulty in separation and recovery. Some other drawbacks in its use include the potential dangers in handling, toxicity, corrosive nature, and disposal problems due to the acidic effluent. In addition to that, hydrolysis and purification of the resultant complex results in corrosive by-products. Research is ongoing into alternative heterogeneous solid catalysts which are stable, reusable, and nature-friendly, and 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.

Friedel–Crafts alkylation and acylations using zeolites as catalyst are common in organic synthesis.[2]

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.

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.[25]

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 containing cobalt nanoparticles have applications in the recycling industry as a catalyst to break down polyethylene and polypropylene, two widely used plastics, into propane.[26]

A researcher at Sandia National Laboratories examines vials of SOMS (Sandia Octahedral Molecular Sieve), a zeolite that shows potential for radioactive waste and industrial metals cleanup.

Zeolites have been used in advanced nuclear 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-137 that was present in high levels.[27]

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.[28]

Animation of pressure swing adsorption, (1) and (2) showing alternating adsorption and desorption

I	compressed air input		A	adsorption
O	oxygen output		D	desorption
E	exhaust

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.[29] 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, 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.

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.[30]

Zeolites are also used as a molecular sieve in cryosorption style vacuum pumps.[31]

Zeolites can be used to thermochemically store solar heat harvested from solar thermal collectors as first demonstrated by Guerra in 1978[32] and for adsorption refrigeration, as first demonstrated by Tchernev in 1974.[33] 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.

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.[34] When added to lime mortars and lime-metakaolin mortars, synthetic zeolite pellets can act simultaneously as a pozzolanic material and a water reservoir.[35][36]

Non-clumping cat litter is often made of zeolite (or diatomite), one form of which, invented at MIT, can sequester the greenhouse gas methane from the atmosphere.[37]

The original formulation of QuikClot brand hemostatic agent, which is used to stop severe bleeding,[38] contained zeolite granules. When in contact with blood, the granules would rapidly absorb water from the blood plasma, creating an exothermic reaction which generated heat. The absorption of water would also concentrate clotting factors present within the blood, causing the clot formation process to occur much faster than under normal circumstances, as shown in vitro.[39]

The 2022 formulation of QuikClot uses a nonwoven material impregnated with kaolin, an inorganic mineral activating Factor XII, in turn accelerating natural clotting.[40] Unlike the original zeolite formulation, kaolin does not exhibit any thermogenic properties.

Experimental use of zeolite as soil stabilizer in San Miniato, Tuscany.

Mixing composted waste matter from wine production with zeolites.

The microporous structure of the zeolites put into ground stabilizes water release and pH.

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.

Pet stores market zeolites for use as filter additives in aquaria,[20] 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%).[41] Southern hemisphere zeolites are typically formed in freshwater and have a high calcium content.[42]