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Methane clathrate hydrate I

Methane clathrate hydrate I

Clathrate hydrates

Clathrate hydrates are solid cages of water containing small non-polar guest molecules like carbon dioxide and methane.

 

V CS-I clathrate hydrate
V CS-II clathrate hydrate
V HS-III clathrate hydrate
V Other structures


    "The solution of oxymuriatic gas "(chlorine) in                     water freezes more readily than pure water"

Humphrey Davy 1811; the first reference to a clathrate a

 

Clathrate hydrate crystalline ices b  form from water and non-stoichiometric amounts of small non-polar molecules (hence usually gaseous) under moderate pressure (typically of a few MPa) and at cold temperatures (typically close to 0 °C, but increased pressure raises the melting point). They generally involve higher solubilities in the solid state relative to the liquid state. Clathrates are notable due to their hazardous formation in natural gas pipeline blockages, their occasional hazardous release of large volumes of gas from underwater natural reservoirs, their potential for the production of natural gas from deep-sea sources and their potential in the removal of the global warming gas, carbon dioxide.

 

Clathrate hydrates are in equilibrium with their free guest species with their occupancy dependant on the temperature
and pressure. Clathrate hydrates are unstable, at positive pressure [520], in the absence of the solute. Every water molecule forms a vertex of four cages, which may, or may not, contain a small guest molecule. Their structures require a minimum amount of these small molecules to fit into and stabilize the cavities (usually one or none in each cavity) without forming any covalent or hydrogen bonds to the water molecules. The cavities obey Euler's convex polyhedra theorem:

 

Faces - Edges+ Vertices = 2

 

There are van der Waals interactions between the guest molecules and the surrounding cages, but the important factor in the cages' stabilities is that the guest molecules prevent their collapse. Without these interstitial molecules the clathrate cavities, shown below, would collapse at positive pressures and they have been shown to dissipate, if surprisingly slowly, after the clathrate ice melts [897]. During formation and dissociation, the solid clathrates interact significantly with the structure of the neighboring aqueous solution [904]. Gas hydrates may slowly dissociate below their melting point with the formation of gas and surface ice [2690]. The hydrogen bondings in the clathrate ices are oriented in an imperfectly disorderly manner obeying the 'ice rules' [1990]. The relationships between the molecular properties of various hydrate-forming gas species and the thermodynamic stability of the associated clathrate hydrate systems have been investigated [2823].

 

CS-I and CS-II are the most stable structures and no other hydrate structure with a single guest component has been found at an ambient condition (except for bromine clathrate) [1732]. CS-I has 46 H2O per unit cell in three non-equivalent sites with 6, 16 and 24 H2O, average O····O distance of 2.793 Å and average departure of the O··O··O angle from tetrahedral of 3.7°. CS-II has 136 H2O per unit cell in three non-equivalent sites with 8, 32 and 96 H2O, average O····O distance of 2.790 Å and average departure of the O··O··O angle from tetrahedral of 3.0°.

 

It has been proposed that the HS-IQ clathrate (maximum 0.175 guest/H2O) structure may be an intermediate in the gas pressure transition of CS-I (maximum 0.174 guests/H2O) to CS-II (maximum 0.176 guest/H2O) [1870].


Characteristic properties of the clathrates
Type
Lattice
Space group
Unit cell
 Unit cell formula a
H2O/guest, min
Clathrate I, CS-I  Cubic 
Pm3n
 a=1.203 nm
 (S)2·(L)6·46H2O
46/8 = 5.75
Clathrate II, CS-II  Face-centered  cubic 
Fd3m
 a=1.731 nm
 (S)16(L+)8·136H2O
136/24 = 5.67
Clathrate H, HS-III  Hexagonal
P6/mmm
 a=1.23 nm
 c=1.02 nm
 (S)5(L++)·34H2O
34/6 = 5.67
Clathrate TS-I, [1734]  Tetragonal
P42/mnm
  a=2.318 nm
  c=1.215 nm
 (L)12-16 ·(L+)4
172/20 = 8.6
Clathrate HS-IQ [1869]  Hexagonal
P6/mmm
 a=1.1987 nm
 c=1.1509 nm
 (S)(L)2(L+)2·40H2O
40/7 = 5.71
Clathrate S-III [2507]  Cubic
p-4 3n
 a=1.3431 nm
 (L++)2·48H2O
48/2 = 24

 

Cavity
512
51262
51263
51264
51268
435663
H2O
20
24
26
28
36
20
Mean cavity radius, Å
3.95
4.33
4.53
4.73
5.71
4.06
Major axis, Å
4.84
6.12
-
6.29
8.44
4.96
Minor axis, Å
4.84
5.07
-
6.29
6.80
4.26
free volume, Å3
51
77
98
120
213
44
Faces
12
14
15
16
20
12
Edges
30
36
39
42
54
30
Vertices
20
24
26
28
36
20
CS-I, /unit cell
2
6
-
-
-
-
CS-II, /unit cell
16
-
-
8
-
-
HS-III, /unit cell
3
-
-
-
1
2
TS-I, /unit cell
10
16
4
-
-
-
HS-IQ, /unit cell
3
2
2
-
-
-
Guest molecules,
for example;   approximate radius, Å
Ar, H2, O2, N2, CO2, CH4
CO2, C2H6
Br2
C3H8, (CH3)3CH, (H2)4
(CH3)3CC2H5, Xe
CH4
1.8-2.2
1.8-2.7
~2.4
2.8-3.1
3.5-4.3
1.8

Cage structuresCavities as given in table, above

Cavities as given in table, above

a  Not all cavities would normally be filled; S = small guest; L = large guest; L+ = larger guest; L++ = largest guest

 

Connectivity maps for the clathrate cages

 

Connectivity maps for the clathrate cages

 

 

Some clathrate hydrates can form, at atmospheric pressure, at the interface between a liquid of suitable guest molecules and water (for example, CH3CCl2F in clathrate CS-II hydrate [408]). At low pressures (e.g. atmospheric) most clathrate hydrates decompose to release the guest molecules, except at low temperatures (for example, < 270 K) where they may remain in a metastable state, for several hours. At very high pressures, clathrate hydrates show complex phase behavior, often giving filled hexagonal ice [1144 ] with the smaller guest molecules/atoms, then at higher pressures they break down to give high-density ice and a solid phase formed by the guest molecules (for example, see [898]. Gas hydrates have been recently reviewed [395]. Water itself cannot be contained in the cavities of solid clathrates [1114].

 

The relative content of the cavities can be determined by techniques such as Raman spectroscopy or NMR as the different cavities present differing environments. [Back to Top to top of page]

 

Clathrate-I crystal structure

Clathrate-I crystal structure

For interactive Figures, see Jmol

CS-I clathrate

Shown opposite is the cubic clathrate CS-I network formed by small non-polar (gaseous) molecules, such as CH4 and CO2, in aqueous solution (for example, (CO2)8-y·46H2O) under pressure and at low but not necessarily (normally) freezing temperatures (only the oxygen atoms of water are shown.). The included molecules randomly occupy many of the cavities dependent on their size. Linear tetrakaidecahedral (51262) cavities form three orthogonal axes holding a dodecahedral cavity wherever they cross (ratio 6:2 respectively per unit cell); each dodecahedral cavity sitting (in a body-centered cubic arrangement) within a cube formed by six tetrakaidecahedral (51262) cavities. These (51262) cavities join at their hexagonal faces to form columns, going away from the viewer in the figure.

 

This clathrate contains small rings of hydrogen-bonded water molecules containing 5, 6, and 10 molecules in the ratio of 24:3:6 respectively.

 

Acids such as HClO4 also form CS-I clathrates with the anion in the cages and the excess protons causing occasional vacancies in the clathrate structure [2684].

 

Burning methane clathrate,

from USGS

Burning methane clathrate, courtesy of the U.S. Geological Survey

 

Methane hydrate resources, from USGS

Methane hydrate resources, from USGS

About 6.4 trillion (that is, 6.4x1012) metric tons of methane lies at the bottom of the oceans in the form of its clathrate hydrate [899]. Methane is mostly of microbial origin forming in sediments at the edges of continental shelves. They form when enough methane is present to fill ~90% of the cages. Each kilogram of maximal occupied hydrate (only a maximum of about 96% occupancy is found) holds about 190 liters of methane (at atmospheric pressure and ambient temperature). As shown left, this ice will burn and could provide energy equal to over twice the world reserves of natural gas. Exploitation, however, presents a number of challenges [2557], with China leading the way [2988].

 

Investigation of the formation of methane clathrate has shown that their growth starts with one pentagonal ring of water molecules with one methane molecule for the small cages, and one hexagonal ring of water molecules with one methane molecule for the large cages [2813].

 

Phase diagram for methane hydrate, showing ocean depths

phase diagram for methane hydrate, showing ocean depths

The phase diagram for methane hydrate is shown right [2283] but note that the salts present shifts the phase boundary to the left and CO2 shifts the curve to the right. The main problems facing commercial exploitation is the development of a safe and commercially viable technology for extracting methane gas from the hydrate deposits. One possible method is to exchange the CH4 with CO2. However, if this methane is not successfully mined then global warming may melt this methane hydrate and vast amounts of methane will be released into the atmosphere. There is an area of the phase diagram above the hydrate-gas-phase boundary shown opposite (i.e. at lower pressure) where the clathrate is metastable but persists for anomalously long periods reversibly along with gas and supercooled water (where hexagonal ice is absent but if eventually present causes dissociation) [2814]. This is the so-called 'self-preservation' effect.

 

Under higher pressure, CS-I methane hydrate transforms to the CS-II phase at about 120 MPa, and then to the HS-III phase at about 600 MPa before decomposing at pressures above 3 GPa to form solid methane and ice VII [2380]. The CS-II phase is likely the dominant phase within the deep hydrate-bearing sediments underlying continental margins [2380].

 

Conversion of CO2 to its clathrate has been investigated as a way of removing the global warming gas CO2 from the atmosphere. It is formed on seabeds and may be created relatively easily. It is highly unusual in that it not only decomposes at less-cold temperatures (> 5 °C) but also if its temperature is lowered below -150 °C. The stability range is thought due to the rotational freedom of CO2 above -150 °C with fixed orientation causing clathrate shell distortion below -150 °C [2901].

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CS-II clathrate

Clathrate-II crystal structure

Clathrate-II crystal structure

For interactive Figures, see Jmol

 

Opposite is shown the CS-II hydrate structure (cubic crystals containing sixteen 512 cavities, eight larger 51264 cavities and 136 H2O molecules per unit cell, and containing larger molecules such as 2-methylpropane in the larger cavities only). The tetrahedral 51264 cavities form an open tetrahedral network, with their centers arranged reminiscent to the cubic ice structure and separated by groups of three 512 cavities. This structure can be thought of as a layered structure with layers of 512 cavities between layers formed of a mixture of 512 and 51264 cavities. The large proportion of 512 cavities is thought responsible for the similarities in the Raman spectra to gas saturated water [831].

 

This clathrate contains small rings of hydrogen-bonded water molecules containing 5, 6, and 10 molecules in the ratio of 18:2:12 respectively.

 

Alkali hydroxide doping can lead to a low-temperature orientational ordering of the hydrogen bonds [2865].

 

Rather surprisingly the CS-II clathrate forms with molecular hydrogen (H2), four molecules sitting in the large cages and one [1257b] or two [1257a] in the small cages, that is, (2H2)16.(4H2)8.136H2O. [1257a].

[Back to Top to top of page]

 

 

Clathrate-H crystal structure

Clathrate-H crystal structure

For interactive Figures, see Jmol

HS-III clathrate

Opposite is shown the HS-III hydrate structure. It has hexagonal crystals containing three 512 cavities, two small 435663 cavities, one large 51268 cavity and 34 H2O molecules per unit cell, and containing even larger molecules such as 2,2-dimethylbutane in the larger cavities only). Each 51268 barrel-shaped cavity is surrounded by six 435663 cavities around its central ring of 6 hexagons. These (51268) cavities join at their top and bottom hexagonal faces to form columns, going away from the viewer in the figure. This structure can be thought of as a layered structure with layers of 512 cavities between layers formed of a mixture of 435663 and 51268 cavities.

 

This clathrate contains small rings of hydrogen-bonded water molecules containing 4, 5, 6, and 10 molecules in the ratio of 3:30:7:18 respectively.

 

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Other structures

Connectivity maps for the clathrate cages

of SIII clathrate

Connectivity maps for the clathrate cages of SIII clathrate

Many other tiled three-dimensional structures are possible (see in JavaView) and other clathrate structures are being discovered; some related to the Frank-Kasper (FK) structures [1733]. Thus there is a tetragonal S-T clathrate (made up of 425864 cavities with 12 water molecules per unit cell), a tetragonal S-K clathrate (made up of 6 ˣ 512, 4 ˣ 51262 and 4 ˣ 51263 cavities with 80 water molecules per unit cell) and a cubic S-III clathrate (made up of 2 ˣ 4126886 and 6 ˣ 4882 cavities with 48 water molecules per unit cell, see also elsewhere) [25. 07]. The connectivity maps for the clathrate cages in the cubic S-III clathrate are shown left.

 

Some materials can form a range of clathrate hydrates dependent on their hydration ratio. For example, tetramethylammonium hydroxide (TMA+OH-) forms a range of solid hydrates TMA+OH-.(H2O)n with n = 2 (α and β) , 4, 5 (α and β), 7.5 (α and β) and 10 [97b]. The β-pentahydrate has the structure 2[4668].10 H20.2 TMA+OH2, the β-7.5 hydrate 8[51263].4[4258].60 H20.8 TMA+OH- (the [51263]occupied by the TMA+ with the [4258] unoccupied) and the decahydrate, 4[4151066].4[4356].40 H20.4 TMA+OH- (the [4151066] occupied by the TMA+ with the [4356] unoccupied); all four-connected throughout, in spite of their proton deficiencies and with the rare weak hydrogen bond of the type OH-···OH2. The α-7.5 hydrate has the depleted structure 2[4159(5)164(6)1].2[4256(5)263(6)2].2[42(4)155(5)1].30 H20.4 TMA+OH- with some of the oxygen atoms being three-connected only (incomplete faces are shown bracketed) [97b].

 

Molecular hydrogen (H2) forms a number of different clathrates as the H2 molecules can fit in most cages with multiple H2 in the larger cages (see above). At high pressures (~400 MPa, 280 K) a different structure (a filled ice rather than a clathrate) forms with the composition (H2O)2H2 and consisting of interpenetrating spiral chains of water molecules showing topological similarity to the mineral quartz [2773].

 

Compression of CO2-clathrate hydrates gives rise to a range of products [3059] including solid H2CO3 and dry ice (solid CO2). A CO2-filled ice is formed from the CS-I hydrate [3059c], whereas CS-II gives a chiral structure [3059b], similar to ice-XVII, which is created with large open spiral channels containing the freely-moving CO2 guest molecules (1:3.5 CO2: H2O, compared with 1:2 H2: H2O for the pre-ice-XVII structure). It has been suggested that ice XVII may be used as a cheap, useful, and environmentally friendly, microporous material for the storage of CO2.

 

4(1) 5(10) 6(2) cavity

4(1) 5(10) 6(2) cavity

 

 

Conversion between clathrate structures may make use of such structures as intermediate with the 4151062 (H2O)22 cluster (see right) being proposed in the interconversion of CS-I and HS-III clathrates [2384].

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Footnotes

a H. Davy, The Bakerian Lecture: On some of the combinations of oxymuriatic gas and oxygene, and on the chemical relations of these principles, to inflammable bodies, Philosophical Transactions of the Royal Society, 101 (1811) 1-35. [Back]

 

b 'Clathrate' term (clathratus; grated, latticed) was first introduced by H. M. Powell, The structure of molecular compounds. Part,VI. Clathrate compounds, J . Chem. Soc. (1948) 61-73, [Back]

 

 

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