Clathrate Hydrates: Occurrence, Uses, and Problems

Clathrate hydrates are supramolecular framework materials in which guest molecules are physically trapped inside cages made of hydrogen-bonded water molecules. Naturally occurring hydrocarbon gas hydrates constitute a vast untapped energy resource, generally little known by the layman, though receiving increasing attention in the media. Ice-like in appearance, methane hydrate can generate about 160 times its own volume of gas at standard temperature and pressure (STP). Gas hydrates already have a huge impact on industry, as the cause of numerous problems for the oil and gas industry, but they also have a few specialized beneficial applications, and they have potential for use in a number of other areas. This review will be rather eclectic, because the study of hydrates cuts across many sciences, from the basic physics and chemistry of hydrates to their involvement in biological systems, in geological processes, in astronomy, and in climatology. They even have a place as a source of entertainment. The sources for much of the current information, especially regarding natural gas hydrates, are a number of books, reviews, and conference proceedings, to which the interested reader may refer.1-151, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 First, we will present a little history and basic science. Although gas hydrates have been known to scientists since the early nineteenth century, with the work of Davy (in 1811) and Faraday (in 1823) on chlorine hydrate, their true nature as clathrates was not demonstrated until the advent of x-ray crystallographic studies in the 1950s. Gas hydrates began to gain more attention when their potential for causing blockages in natural gas pipelines was first noted in 1934. Their existence as natural deposits in permafrost regions of the Earth was recognized in Siberia in 1965 and in Canada in 1974. Off-shore deposits were found with the advent of the Deep Sea Drilling Project, with the first indications of hydrate recorded around 1972 and the first samples recovered around 1983. The detailed physical science of hydrates can be found in a number of reviews.2, 4-94, 5, 6, 7, 8, 9 Three principal crystal structures are known: Structures I and II, which are cubic, and Structure H, which is hexagonal. All have small cages together with cages of increasing size (in the order I, II, H) that can accommodate larger guest molecules. They are nonstoichiometric, and their stabilities depend on the particular guest molecules and the pressure (P) and temperature (T) conditions. Stability models are based on the statistical thermodynamic description formulated by van der Waals and Platteeuw.16 Many hydrates can exist above the melting point of ice, some up to 28\ub0C under pressure. Because the guest does not have any chemical bonds to the host, it has considerable translational and rotational freedom within its cage. Resonant coupling between these guest motions and the low-frequency lattice vibrational modes results in a thermal conductivity for hydrates that is considerably lower than that in ice. Many different techniques have been applied to study hydrate compositions and physical properties. The most reliable methods for determining structure type are x-ray diffraction, solid-state nuclear magnetic resonance (NMR), and (to a lesser extent) Raman spectroscopy.NRC publication: Ye