Other Clathrate Hydrates

While si, sll, and sH are the most common clathrate hydrates, a few other clathrate hydrate phases have been identified. These other clathrate hydrates include new phases found at very high pressure conditions (i.e., at pressures of around 1 GPa and higher at ambient temperature conditions). Dyadin et al. (1997) first reported the existence of a new methane hydrate phase at very high pressures (500 MPa). This discovery was followed by a proliferation in molecular-level studies to identify the structure of the high pressure phases of methane hydrate (Chou et al., 2000 Hirai et al., 2001 Kurnosov et al., 2001 Loveday et al., 2001, 2003). 

At high pressures, different crystalline forms appear. One of the intriguing forms is so-called filled ice, which has a structure of known crystalline polymorphs of ice such as ice Ic and ice II. These ices have vacant spaces in which small molecules are incorporated. So far, each water molecule in clathrate hydrates is hydrogen-bonded with four neighbors as in ice. Other clathrate hydrate is also known, called semiclathrate hydrate in which guest molecules such as tetra-n-butylammonium bromide replace several water molecules so that some of the cages are broken [11]. Hereafter, we will not refer to those semiclathrate hydrates, which are beyond the scope of the present statistical mechanical theory. 

The history of iaclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations iaclude the preparation of graphite iatercalates ia 1841, the P-hydroquiaone H2S clathrate ia 1849, the choleic acids ia 1885, the cyclodexthn iaclusion compounds ia 1891, and the Hofmann s clathrate ia 1897. Later milestones of the development of iaclusion compounds refer to the tri-(9-thymotide benzene iaclusion compound ia 1914, pheaol clathrates ia 1935, and urea adducts ia 1940. 

In the next section we shall give a brief account of the crystal structure of the hydroquinone clathrates and of the gas hydrates, as far as is needed for a proper understanding of the subsequent parts. The reader who is interested in the phenomenology of other clathrate compounds should consult one of the many review articles7,8 39 on inclusion compounds. 

As follows from his laboratory notes, the first discovered clathrate hydrate (of chlorine) was observed, but not recognized, by Davy in 1810. Then Cl2, Br2, so2) co2, ch3ci, ch4, c2h, and numerous other gases were shown to form clathrate hydrates [22, 23]. Contrary to inorganic stoichiometric hydrates, those involving hydrocarbons are both non-stoichiometric and crystalline. In addition, gas hydrate composition was found to depend on temperature, pressure, and some... 

Before proceeding, it is important to recall the significant feature which appears to distinguish the cluster model from the two other prominent mixture models—i.e., the broken-down ice lattice and the clathrate hydrate cage structures. The latter two theories allow for the existence of discrete sites in water, owing to the cavities present either in the ice... 

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

The above (l)-(3) exceptions do not involve natural gas compounds and therefore are not described in any detail, but rather just mentioned in passing. The reader is referred to reviews by Davidson (1973), Davidson and Ripmeester (1978), and the papers of Jeffrey (1984), Dyadin et al. (1991), Udachin and Ripmeester (1999) and Udachin et al. (2001a) for further details on the less common hydrate structures of other compounds. The relevance of other compound structures is becoming increasingly of interest in areas of refrigeration, gas storage, and gas separation using clathrate hydrates. 

The initial limitations of the book are still largely present in the third edition. First the book applies primarily to clathrate hydrates of components in natural gases. Although other hydrate formers (such as olefins, hydrogen, and components larger than 9 A) are largely excluded, the principles of crystal structure, thermodynamics, and kinetics in Chapters 2 through 5 will still apply. 

In addition to the various forms of ice, there are a large number of ice-like structures with four-coordinated water molecules that constitute a host framework, which are only stable when voids in the host framework are occupied by other guest molecules. Such compounds are known as clathrate hydrates. 

Many gases, such as Ar, Kr, Xe, N2, O2, CI2, CH4 and CO, can be crystallized with water to form ice-like clathrate hydrates. The basic structural components of these hydrates are the (H2O)20 pentagonal dodecahedron and other larger polyhedra bounded by five- and six-membered hydrogen-bonded rings, which can accommodate the small neutral molecules. The inclusion properties of water imply that such polyhedra are likely to be present in liquid water as its structural components. 

A key configuration in these water clusters, in ice and in clathrate hydrates is the pentamer (Fig. 4) in which one water molecule at the centre of a tetrahedron is hydrogen-bonded to four other water molecules at the vertices. Here two hydrogen bonds are formed by H-transfer and two by electron transfer . The calculated average bond energy is similar to that for the dimer (Kollmann and Allen, 1970 Hoyland and Kier, 1969). More sophisticated calculations on... 

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