Clathrate hydrate

Clathrate hydrates, or gas hydrates, clathrates, or hydrates, are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen-bonded, frozen water molecules. In other words, clathrate hydrates are clathrate compounds in which the host molecule is water and the guest molecule is typically a gas or liquid. Without the support of the trapped molecules, the lattice structure of hydrate clathrates would collapse into conventional ice crystal structure or liquid water. Most low molecular weight gases, including O₂, H₂, N₂, CO₂, CH₄, H₂S, Ar, Kr, Xe, and Cl₂ as well as some higher hydrocarbons and freons, will form hydrates at suitable temperatures and pressures. Clathrate hydrates are not officially chemical compounds, as the enclathrated guest molecules are never bonded to the lattice. The formation and decomposition of clathrate hydrates are first order phase transitions, not chemical reactions. Their detailed formation and decomposition mechanisms on a molecular level are still not well understood. Clathrate hydrates were first documented in 1810 by Sir Humphry Davy who found that water was a primary component of what was earlier thought to be solidified chlorine.

Clathrates have been found to occur naturally in large quantities. Around 6.4 trillion (6.4×10¹²) tonnes of methane is trapped in deposits of methane clathrate on the deep ocean floor. Such deposits can be found on the Norwegian continental shelf in the northern headwall flank of the Storegga Slide. Clathrates can also exist as permafrost, as at the Mallik gas hydrate site in the Mackenzie Delta of northwestern Canadian Arctic. These natural gas hydrates are seen as a potentially vast energy resource and several countries have dedicated national programs to develop this energy resource. Clathrate hydrate has also been of great interest as technology enabler for many applications like seawater desalination, gas storage, carbon dioxide capture & storage, cooling medium for data centre and district cooling etc. Hydrocarbon clathrates cause problems for the petroleum industry, because they can form inside gas pipelines, often resulting in obstructions. Deep sea deposition of carbon dioxide clathrate has been proposed as a method to remove this greenhouse gas from the atmosphere and control climate change. Clathrates are suspected to occur in large quantities on some outer planets, moons and trans-Neptunian objects, binding gas at fairly high temperatures.

Clathrate hydrates were discovered in 1810 by Humphry Davy. Clathrates were studied by P. Pfeiffer in 1927 and in 1930, E. Hertel defined "molecular compounds" as substances decomposed into individual components following the mass action law in solution or gas state. Clathrate hydrates were discovered to form blockages in gas pipelines in 1934 by Hammerschmidt that led to increase in research to avoid hydrate formation. In 1945, H. M. Powell analyzed the crystal structure of these compounds and named them clathrates. Gas production through methane hydrates has since been realized and has been tested for energy production in Japan and China.

The word clathrate is derived from the Latin clathratus (clatratus), meaning 'with bars, latticed'.

Gas hydrates usually form two crystallographic cubic structures: structure (Type) I (named sI) and structure (Type) II (named sII) of space groups ${\displaystyle Pm{\overline {3}}n}$ and ${\displaystyle Fd{\overline {3}}m}$ respectively. A third hexagonal structure of space group ${\displaystyle P6/mmm}$ may also be observed (Type H).

The unit cell of Type I consists of 46 water molecules, forming two types of cages – small and large. The unit cell contains two small cages and six large ones. The small cage has the shape of a pentagonal dodecahedron (5¹²) (which is not a regular dodecahedron) and the large one that of a tetradecahedron, specifically a hexagonal truncated trapezohedron (5¹²6²). Together, they form a version of the Weaire–Phelan structure. Typical guests forming Type I hydrates are CO₂ in carbon dioxide clathrate and CH₄ in methane clathrate.

The unit cell of Type II consists of 136 water molecules, again forming two types of cages – small and large. In this case there are sixteen small cages and eight large ones in the unit cell. The small cage again has the shape of a pentagonal dodecahedron (5¹²), but the large one is a hexadecahedron (5¹²6⁴). Type II hydrates are formed by gases like O₂ and N₂.

The unit cell of Type H consists of 34 water molecules, forming three types of cages – two small ones of different types, and one "huge". In this case, the unit cell consists of three small cages of type 5¹², two small ones of type 4³5⁶6³ and one huge of type 5¹²6⁸. The formation of Type H requires the cooperation of two guest gases (large and small) to be stable. It is the large cavity that allows structure H hydrates to fit in large molecules (e.g. butane, hydrocarbons), given the presence of other smaller help gases to fill and support the remaining cavities. Structure H hydrates were suggested to exist in the Gulf of Mexico. Thermogenically produced supplies of heavy hydrocarbons are common there.