Editing Zeolite (section)

==Characteristics==
===Properties===
[[File:Zeolite structure as an assembly of tetrahedra.png|thumb|458x458px|Microscopic structure of a zeolite ([[mordenite]]) framework, assembled from corner-sharing {{chem|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 {{chem2|(MAlO2)(SiO2)_{x}(H2O)_{y}|}} where M<sup>+</sup> is usually H<sup>+</sup> and Na<sup>+</sup>. 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<sup>+</sup> and Na<sup>+</sup> 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&nbsp;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 {{Chem|H|+}} exchangeable, unlike natural zeolites, and are used as [[solid acid catalyst]]s. 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.<ref>{{Greenwood&Earnshaw2nd}} </ref>
[[File:Zeolite Mordenite (with Al substitution).png|thumb|Zeolite Mordenite with some Si atoms substituted with Al atoms. ]]
===Framework structure===
[[File:Zeolite4ring.svg|350px|thumb|Three ways to represent the oxygen 4-membered ring structure of silicate compounds.]]
[[File:FAU and LTA.png|350px|thumb|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).<ref name="IZA-SC" /> 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 SiO{{sub|4}} 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-type zeolite|LTA]] (left) and [[Faujasite|FAU]] (right). Both zeolites share the [[Truncated octahedron|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&nbsp;nm<ref name="IZA-SC" />) and belongs to the '''small pore zeolite''', while the pore entrance of FAU is a 12-ring (0.74&nbsp;nm<ref name="IZA-SC" />) 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), <sup>*</sup>BEA (beta), MOR (high-silica mordenite), MFI (ZSM-5), and FER (high-silica ferrierite) types are called the '''big five''' of high silica zeolites,<ref>{{Cite journal|title=An Overview on Zeolite Shaping Technology and Solutions to Overcome Diffusion Limitations|journal=Catalysts|issue=8|pages=163|year=2018}}</ref> 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]] of Si in zeolites can be possible for some heteroatoms such as [[titanium]],<ref name="ref12">{{Ref patent|country=US|number=4410501A|status=patent|gdate=1979-12-21|title = Preparation of porous crystalline synthetic material {{sic|comprised |hide=y|of}}} silicon and titanium oxides}}</ref> [[zinc]]<ref name="ref13">{{Ref patent|country=US|number=2016243531A1|status=patent|gdate=2015-02-24|title = Processes for preparing zincoaluminosilicates with aei, cha, and gme topologies and compositions derived therefrom}}</ref> and [[germanium]].<ref name="ref14">{{Cite journal|title=Post-Synthesis Stabilization of Germanosilicate Zeolites ITH, IWW, and UTL by Substitution of Ge for Al|url=https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/chem.201603434|journal=Chemistry: A European Journal|volume=22|issue=48|pages=17377–17386|year=2016|doi=10.1002/chem.201603434 |last1=Shamzhy |first1=Mariya V. |last2=Eliašová |first2=Pavla |last3=Vitvarová |first3=Dana |last4=Opanasenko |first4=Maksym V. |last5=Firth |first5=Daniel S. |last6=Morris |first6=Russell E. |pmid=27754569 |hdl=10023/11880 |hdl-access=free }}</ref> Al atoms in zeolites can be also structurally replaced with [[boron]]<ref name="ref15">{{Ref patent|country=US|number=5187132A|status=patent|gdate=1993-02-16|title = Preparation of borosilicate zeolites}}</ref> and [[gallium]].<ref name="ref16">{{Cite journal|title=Incorporation of Gallium into Zeolites: Syntheses, Properties and Catalytic Application|journal=Chem. Rev.|issue=100|pages=2303–2405|year=2000}}</ref>

The [[silicoaluminophosphate]] type (AlPO molecular sieve),<ref name="ref17">{{Cite journal|title=Crystal Structure of Tetrapropylammonium Hydroxide-Aluminium Phosphate Number 5|journal=ACS Sym. Ser.|issue=218|pages=109–118|year=1983}}</ref> in which Si is isomorphous with Al and P and Al is isomorphous with Si, and the gallogermanate<ref name="ref18">{{Cite journal|title=Hydrothermal synthesis and structural characterization of zeolite-like structures based on gallium and aluminium germanates|journal=J. Am. Chem. Soc.|issue=120|pages=13389–13397|year=1998}}</ref> and others are known.