Wikipedia

Clathrate compound

A clathrate or clathrate compound is a type of inclusion compound in which a guest atom, ion, or molecule is enclosed in a cage-like cavity formed by a host molecule or by an extended host lattice. The term is derived from the Latin clatratus, meaning 'with bars' or 'latticed'. According to the International Union of Pure and Applied Chemistry (IUPAC), clathrates are inclusion compounds "in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules".[1]

Clathrates occur in several areas of chemistry and materials science. In clathrate hydrates, water molecules form hydrogen-bonded cages that can trap small gases such as methane, carbon dioxide, nitrogen, or hydrogen sulfide. In organic and supramolecular chemistry, hosts such as hydroquinone, urea, thiourea, cyclodextrins, calixarenes, and Dianin's compound can form inclusion compounds with suitable guest molecules.[2] In solid-state chemistry, inorganic clathrates are extended covalent frameworks, commonly based on group 14 elements such as silicon, germanium, or tin, that enclose guest atoms in polyhedral cages.[3]

The properties of a clathrate depend on both the host framework and the guest species. Guest atoms or molecules may stabilize a framework, occupy only some cages, influence phase stability, scatter phonons, or provide charge balance. Clathrates are therefore relevant to host–guest chemistry, methane hydrates, gas storage and separation, thermoelectric materials, superconductivity, and synthetic phases formed under unusual or extreme conditions.[4][3]

Terminology and history

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IUPAC definition

clathrates: Inclusion compounds in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules.

Early investigators found or synthesized substances now understood as clathrate hydrates, before their structures were established. Chlorine hydrate, for example, had been described by Humphry Davy and Michael Faraday in the 19th century, and was later recognized as a gas hydrate.[5][6]

Later investigators discovered that certain inclusion compounds are special, in that the host-chemical forms a crystal, while this is not so for other inclusion compounds. The word "clathrate" was coined by Herbert Marcus Powell in 1948.[7]

Structure and types

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Clathrate hydrate cages. The 512 dodecahedral and 51262 tetrakaidecahedral cages occur in structure I hydrates.[4]

Clathrates consist of a host framework and guest species. The host framework defines cavities or cages, while the guest species occupy those cavities. In many clathrates the guest is not strongly bonded to the framework, but interacts through van der Waals forces, hydrogen bonding, electrostatic interactions, or weak covalent interactions. The presence, size, and occupancy of guest species can determine whether a clathrate framework is stable.[2][8]

The cages of many clathrate structures are described by the number and type of faces in their coordination polyhedra. Common cage types include 512, a cage with twelve pentagonal faces; 51262, with twelve pentagonal and two hexagonal faces; and 51264, with twelve pentagonal and four hexagonal faces.[4][3]

Clathrate hydrates

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Methane clathrate embedded in sediment from Hydrate Ridge off the coast of Oregon, United States.

Clathrate hydrates, also called gas hydrates, are water-based crystalline solids in which guest molecules occupy cages formed by hydrogen-bonded water molecules. The guests are commonly small gases or volatile molecules, including methane, carbon dioxide, nitrogen, oxygen, hydrogen sulfide, noble gases, and light hydrocarbons. Some polar molecules with large hydrophobic groups can also form hydrates. Without suitable guest molecules, most hydrate frameworks are unstable relative to ordinary ice or liquid water.[9][8]

Clathrate hydrates are commonly classified into three structure types: structure I (sI), structure II (sII), and structure H (sH). Structure I hydrates contain 46 water molecules per unit cell, arranged as two small 512 cages and six larger 51262 cages. Structure II hydrates contain 136 water molecules per unit cell, with sixteen 512 cages and eight 51264 cages. Structure H hydrates contain 34 water molecules per unit cell and three cage types; they usually require both small guest molecules and larger helper guests for stability.[10][8]

Methane hydrates occur naturally in marine sediments, deep-lake sediments, and permafrost regions, where low temperature and high pressure favour hydrate stability. They are of interest as a potential energy resource, as part of the carbon cycle, and as a geohazard because hydrate dissociation can affect sediment stability and release methane.[11][8]

Inorganic clathrates

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Crystal structure of Na8Si46, a type-I inorganic clathrate with sodium atoms in silicon cages.[4]

Inorganic clathrates are crystalline solids with covalently bonded frameworks that enclose guest atoms or ions. Many contain silicon, germanium, or tin frameworks with alkali-metal, alkaline-earth-metal, or rare-earth guests.[12]

Many inorganic clathrates are Zintl or Zintl-like phases. A common type-I inorganic clathrate has the idealized formula A8E46, where A is a guest atom and E is a framework element such as silicon, germanium, or tin.[13]

A notable feature of many inorganic clathrates is low lattice thermal conductivity. This is often associated with the motion of guest atoms inside oversized framework cages, sometimes described as "rattling". Such motion can scatter phonons while leaving electronic transport through the framework relatively less affected, making some inorganic clathrates candidates for thermoelectric materials.[14]

Formation and stability

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The conditions under which a clathrate forms depend on both the thermodynamics of the host–guest system and the kinetics of nucleation and crystal growth. In clathrate hydrates, stability is commonly represented by pressure–temperature phase boundaries that vary with guest composition, salinity, and the presence of thermodynamic promoters or inhibitors.[8][15]

Natural and extreme-condition occurrence

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Clathrate phases are discussed in planetary science and astrochemistry because water ice and volatile molecules are common in the outer Solar System and in cold astrophysical environments.[16]

A calcium–copper–silicon type-I inorganic clathrate has been identified in red trinitite formed during the 1945 Trinity nuclear test.[17]

Applications

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Clathrates have been explored for gas storage, gas separation, carbon dioxide capture and sequestration, desalination, refrigeration and cooling, thermoelectric energy conversion, photovoltaics, batteries, and superconducting materials.[4][3]

Examples

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See also

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References

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  1. ↑ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "clathrates". doi:10.1351/goldbook.C01097
  2. 1 2 Atwood, J. L. (2012). "Inclusion Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_119.
  3. 1 2 3 4 Dolyniuk, Juli-Anna; Owens-Baird, Bryan; Wang, Jian; Zaikina, Julia V.; Kovnir, Kirill (2016). "Clathrate thermoelectrics". Materials Science and Engineering: R: Reports. 108: 1–46. doi:10.1016/j.mser.2016.08.001.
  4. 1 2 3 4 5 Krishna, Lakshmi; Koh, Carolyn A. (February 2015). "Inorganic and methane clathrates: Versatility of guest–host compounds for energy harvesting". MRS Energy & Sustainability. 2 (1): 8. doi:10.1557/mre.2015.9. ISSN 2329-2229.
  5. ↑ Davy, Humphry (1811). "On a Combination of Oxymuriatic Gas and Oxygene Gas". Philosophical Transactions of the Royal Society of London. 101: 155–162. doi:10.1098/rstl.1811.0008.
  6. ↑ Faraday, Michael (1823). "On Hydrate of Chlorine". Quarterly Journal of Science, Literature, and the Arts. 15: 71–74.
  7. ↑ Powell, H. M. (1948). "The structure of molecular compounds. Part IV. Clathrate compounds". Journal of the Chemical Society: 61–73. doi:10.1039/JR9480000061.
  8. 1 2 3 4 5 Sloan, E. Dendy; Koh, Carolyn A. (2007). Clathrate Hydrates of Natural Gases (3rd ed.). CRC Press. doi:10.1201/9781420008494. ISBN 978-1-4200-0849-4.
  9. ↑ Englezos, Peter (1993). "Clathrate hydrates". Industrial & Engineering Chemistry Research. 32 (7): 1251–1274. doi:10.1021/ie00019a001.
  10. ↑ von Stackelberg, M.; Müller, H. R. (1954). "Feste Gashydrate II. Struktur und Raumchemie" [Solid gas hydrates II. Structure and space chemistry]. Zeitschrift für Elektrochemie, Berichte der Bunsengesellschaft für physikalische Chemie (in German). 58 (1): 25–39. doi:10.1002/bbpc.19540580105.
  11. ↑ Kvenvolden, Keith A.; McMenamin, Mark A. (1980). Hydrates of natural gas; a review of their geologic occurrence (Report). U.S. Geological Survey Circular. doi:10.3133/cir825.
  12. ↑ Kovnir, Kirill A.; Shevelkov, Andrei V. (2004). "Semiconducting clathrates: synthesis, structure and properties". Russian Chemical Reviews. 73 (9): 923–938. doi:10.1070/RC2004v073n09ABEH000916.
  13. ↑ Shevelkov, Andrei V.; Kovnir, Kirill (2011). "Zintl Clathrates". In Fässler, Thomas F. (ed.). Zintl Phases: Principles and Recent Developments. Structure and Bonding. Vol. 139. Springer. pp. 97–142. doi:10.1007/430_2010_25. ISBN 978-3-642-21182-9.
  14. ↑ Nolas, G. S.; Cohn, J. L.; Slack, G. A.; Schujman, S. B. (13 July 1998). "Semiconducting Ge clathrates: Promising candidates for thermoelectric applications". Applied Physics Letters. 73 (2): 178–180. Bibcode:1998ApPhL..73..178N. doi:10.1063/1.121747.
  15. ↑ Sum, Amadeu K.; Koh, Carolyn A.; Sloan, E. Dendy (2009). "Clathrate Hydrates: From Laboratory Science to Engineering Practice". Industrial & Engineering Chemistry Research. 48 (16): 7457–7465. doi:10.1021/ie900679m.
  16. ↑ Ghosh, Jyotirmoy; Methikkalam, Rabin Rajan J.; Bhuin, Radha Gobinda; Ragupathy, Gopi; Choudhary, Nilesh; Kumar, Rajnish; Pradeep, Thalappil (2019). "Clathrate hydrates in interstellar environment". Proceedings of the National Academy of Sciences of the United States of America. 116 (5): 1526–1531. doi:10.1073/pnas.1814293116. PMC 6358667. PMID 30630945.
  17. ↑ Bindi, Luca; Mihalkovič, Marek; Widom, Michael; Steinhardt, Paul J. (2026). "Extreme nonequilibrium synthesis of a Ca–Cu–Si clathrate during the Trinity nuclear test". Proceedings of the National Academy of Sciences of the United States of America. 123 (21) e2604165123. doi:10.1073/pnas.2604165123.
  18. ↑ Hoppe, R. (August 1964). "Valence Compounds of the Inert Gases". Angewandte Chemie International Edition in English. 3 (8): 538–545. doi:10.1002/anie.196405381. ISSN 0570-0833.
  19. ↑ Lim, Sol Geo; Lee, Jong-Won; Fujihisa, Hiroshi; Oh, Chang Yeop; Jang, Jiyeong; Moon, Dohyun; Takeya, Satoshi; Muraoka, Michihiro; Yamamoto, Yoshitaka; Yoon, Ji-Ho (2023). "Neon encapsulation by a hydroquinone organic crystalline clathrate under ambient conditions". Communications Materials. 4 (1): 51. Bibcode:2023CoMat...4...51L. doi:10.1038/s43246-023-00378-z. S2CID 259583357.
  20. ↑ Bindi, Luca; Mihalkovič, Marek; Widom, Michael; Steinhardt, Paul J. (2026-05-26). "Extreme nonequilibrium synthesis of a Ca–Cu–Si clathrate during the Trinity nuclear test". Proceedings of the National Academy of Sciences. 123 (21). doi:10.1073/pnas.2604165123. ISSN 0027-8424. PMC 13213991. PMID 42114011.
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