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Zeolite
Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, and oxygen, and have the general formula Mn+
1/n(AlO
2)−
(SiO
2)
x･yH
2O where Mn+
1/n is either a metal ion or H+.
The term 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".
Zeolites occur naturally, but are also produced industrially on a large scale. As of December 2018[update], 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 (IZA-SC) and receives a three-letter designation.
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 than 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.
Their ability to generate voids within a rigid solid – underpins the use of zeolites as catalysts and molecular sieves. They possess high physical and chemical stability due to the covalent bonding within their framework. Some are hydrophobic and thus are suited for adsorption of 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 high silica zeolites especially are sufficiently acidic to protonate hydrocarbons. These are used for fluid catalytic cracking in petrochemical industry. Cracking catalysts become poisoned by carbonaceous residues. These residues can be combusted without major damage to the host zeolite, a further testament to their robustness. For some applications, such regenerated zeolites are called equilibrium catalysts.
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). 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.
Zeolite
Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, and oxygen, and have the general formula Mn+
1/n(AlO
2)−
(SiO
2)
x･yH
2O where Mn+
1/n is either a metal ion or H+.
The term 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".
Zeolites occur naturally, but are also produced industrially on a large scale. As of December 2018[update], 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 (IZA-SC) and receives a three-letter designation.
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 than 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.
Their ability to generate voids within a rigid solid – underpins the use of zeolites as catalysts and molecular sieves. They possess high physical and chemical stability due to the covalent bonding within their framework. Some are hydrophobic and thus are suited for adsorption of 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 high silica zeolites especially are sufficiently acidic to protonate hydrocarbons. These are used for fluid catalytic cracking in petrochemical industry. Cracking catalysts become poisoned by carbonaceous residues. These residues can be combusted without major damage to the host zeolite, a further testament to their robustness. For some applications, such regenerated zeolites are called equilibrium catalysts.
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). 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.
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