Gas Hydrates (Clathrate Hydrates)

Clathrate hydrates constitute a class of solids in which the guest molecules occupy, fully or partially, cages in host structures made up of H-bonded water molecules. The usually unstable empty clathrate is stabilised by inclusion of the guest species. In case of guest molecules which are gaseous at ambient conditions the resulting clathrate hydrate is often called a gas hydrate. These compounds are interesting for several reasons: much could be learned about water-water interactions in these topologically rather complex systems, especially if one could follow the pressure dependency of the host structure over a wide pressure range. Likewise, much could be learned about the guest-host interactions in a wide range of guest species from noble gas atoms to large (and polar) organic molecules. Clathrates are believed to occur in large quantities on some outer planets binding gas at fairly high temperatures, which is an interesting issue for planetologists.
This is, however, far from being only of academic interest: The petrol industry suffers from the nuisance of hydrocarbon-hydrates blocking gas pipelines in arctic regions, yet are beginning to show interest in the giant natural methane hydrate deposits on the deep ocean floor and in permafrost regions. Rapid methane hydrate decomposition triggered e.g. by earthquakes may lead to catastrophic submarine landslides producing tsunamis of a giant size. Moreover, the decomposition of methane hydrate may well affect the world's climate; there are indications for climatic changes provoked by gas hydrate decomposition in geological records. Yet this is not the only area of interest in the field of gas hydrates: They may be used as a cheaper alternative of gas storage and transport as compared to liquefied gas, and gas hydrates may be used in the desalination of sea water. Moreover, Japanese companies have suggested in the early 1990s a deep sea deposition of CO2-clathrate to remove this greenhouse gas from the atmospheric cycles and, recently, large CO2 sequestering programmes were started in the US.
The cage filling of gas hydrates is governed by thermodynamics. In the late 1950s a statistical thermodynamic theory was developed which allowed the prediction of stability and gas filling for gas hydrates. The theory is based on the following main assumptions: 1. The free energy of water structure is independent of the guest occupation; i.e. no lattice distortion. 2. Each cavity contains only one guest entity. 3. No interaction between guest entities, i.e. enclathration is described as Langmuir adsorption. 4. No quantum effects.

Our early work on gas hydrates

Early on, our interest in the high pressure phases of ice brought us into contact with gas-filled ices as well as clathrate hydrates. In 1988 we were the first to describe "stuffed" ice, a helium-filled high pressure ice with a water topology identical to ice II. In 1992 we succeeded for the first time worldwide to produce pure argon clathrate hydrate starting from ice Ih close to the melting point, a method which was reinvented a few years later by other groups. Using this material, we were again the first to determine the crystal structure of a gas hydrate under in situ conditions.

References:

• Londono, D., W. F. Kuhs, and J. L. Finney (1988)
  Enclathration of helium in ice II: The first helium-hydrate
  Nature 332 (6160), 141-142.
• Londono, D., J. Finney and W. F. Kuhs (1992)
  Formation, stability, and structure of helium hydrate at high pressure
  Journal of Chemical Physics 97, 547-552.
• Kuhs, W. F., R. Dorwarth, R. Londono and J. L. Finney (1992)
  In-situ study on composition and structure of Ar-clathrate
  in Physics and Chemistry of Ice (eds. N. Maeno and T. Hondoh, Hokkaido University Press, Sapporo), 126-130.

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Air hydrates in ice sheets

Snow is a material with very low density. Upon densification some air is contained in firn and eventually closed off in the final glacier ice. In polar ice sheets these air bubbles experience an increasing mechanical pressure by the overburdening snow and ice. Upon snow accumulation the underlying ice is pushed into deeper and deeper parts eventually reaching pressures where ice and air will transform into a solid crystalline compound, air hydrate. In cooperation with the Alfred-Wegener-Institut in Bremerhaven, our group has investigated the fractionation processes of oxygen and nitrogen during this transition and has revealed the complicated crystallisation process taking place in several steps.

References:

• Pauer, F., J. Kipfstuhl and W. F. Kuhs (1995)
  Raman spectroscopic study on the nitrogen/oxygen ratio in natural ice clathrates in the GRIP Ice Core
  Geophys. Res. Lett. 22, 969-971.
• Pauer, F., J. Kipfstuhl, W. F. Kuhs and H. Shoji (1996)
  Classification of air clathrates found in polar ice sheets
  Z. Polarforsch. 66, 31-38.
• Pauer, F., J. Kipfstuhl and W. F. Kuhs (1996)
  Raman spectroscopic study on the spatial distribution of nitrogen and oxygen in natural ice clathrates and their decomposition to air bubbles
  Geophys. Res. Lett. 23(2), 177-180.
• Pauer, F., J. Kipfstuhl and W. F. Kuhs (1997)
  Raman spectroscopic and statistical studies on natural clathrates from the GRIP ice core, and neutron diffraction studies on synthetic nitrogen clathrates
  J. Geophys. Res. 102(C12), 26519-26526.
• Chazallon, B., B. Champagnon, G. Panczer, F. Pauer, A. Klapproth and W. F. Kuhs (1998)
  Micro-Raman analysis of synthetic air-clathrates
  Eur. J. Miner. 10, 1125-1134.
• Pauer, F., J. Kipfstuhl, W. F. Kuhs and H. Shoji (1999)
  Air clathrate crystals from the GRIP deep ice core – a number, size, and shape distribution study
  J. Glaciology 45(149), 22-30.
• Kuhs, W. F., A. Klapproth and B. Chazallon (2000)
  Chemical physics of air clathrate hydrates
  in Physics of Ice Core Research (ed. T. Hondoh, Sapporo, Hokkaido University Press), 373-392.
• Kipfstuhl, J., F. Pauer, W. F. Kuhs and H. Shoji (2001)
  Air bubbles and clathrate hydrates in the transition zone of the NGRIP deep ice core
  Geophys. Res. Lett. 28, 591-594.

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Crystallographic structures

Two main distinct structure types exist for gas hydrates, both with cubic symmetry: type I with spacegroup Pm3n and type II with spacegroup Fd3m. The nomenclature is from von Stackelberg who studied gas hydrates in the 1940s and 1950s.
Our group has a long-standing record in the structural determination of gas hydrates under in situ conditions. Such work is very suitable to the check assumptions underlying thermodynamic theories mentioned above. These theories are the basis for computer programs widely used in chemical engineering and geosciences for predicting gas hydrate composition and stability. Our work (1997) has established for the first time that certain gas hydrates may contain more than one molecule in their cages. Likewise, we have found evidence for a pressure-dependent distortion of the water host-lattice which was assumed in earlier thermodynamic theories to be negligible.

The crystal structure of air hydrate. The red-white wire-fram shows the water host lattice forming two types of cages in which the oxygen (blue) and nitrogen (green) atoms are located. The resulting structure has cubic symmetry and belongs to von Stackelberg's type II.

References:

• Kuhs, W. F., B. Chazallon, G. Radaelli and F. Pauer (1997)
  Cage occupancy and compressibility of deuterated N₂-hydrate by neutron diffraction
  J. Incl. Phenom. 29, 65-77.
• Kuhs, W. F., B. Chazallon, A. Klapproth and F. Pauer (1998)
  Filling isotherms in clathrate hydrates
  The Review of High Pressure Science and Technology 7, 1147-1149.
• Chazallon, B., and W. F. Kuhs (2002)
  In situ structural properties of N₂-, O₂-, and air-clathrates by neutron diffraction
  J. Chem. Phys. 117(2), 308-320.
• Lobban, C., J. L. Finney and W. F. Kuhs (2002)
  The p-T dependency of the ice II crystal structure and the effect of helium inclusion
  J. Chem. Phys. 117(8), 3928-3934.
• Klapproth, A., E. Goreshnik, D. K. Staykova, H. Klein and W. F. Kuhs (2003)
  Structural studies of gas hydrates
  Can. J. Phys. 81(1/2), 503-518.

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Microstructure

Gas hydrates have a number of unusual properties. Among them is their remarkable microstructure. We have found in 2000, using field-emission scanning electron microscopy, that all investigated gas hydrates may form single crystals sized from a few microns to a few tens of a micron and showing a nanometric porous microstructure. Gas hydrates are apparently the only known material in which regular pore structures are formed spontaneously within quite perfect single crystals. Dense (i.e. non-porous) hydrate crystals also exist. Whether dense or porous hydrates are formed depends on the formation conditions. Meanwhile we have, partly in collaboration with GEOMAR and the University of Bremen, investigated the microstructure of a large number of natural gas hydrates from the ocean sea floor and sub-permafrost regions.

Single crystal of methane hydrate with facettes and a nanometric porous microstructure (left: overview, right: detail). The pore diameter in this example typically is 200 nm.

References:

• Kuhs, W. F., A. Klapproth, F. Gotthardt, K. Techmer and T. Heinrichs (2000)
  The formation of meso- and macroporous gas hydrates
  Geophys. Res. Lett. 27, 2929-2932.
• Suess, E., G. Bohrmann, D. Rickert, W. F. Kuhs, M. E. Torres, A. Trehu and P. Linke (2002)
  Properties and fabric of near-surface methane hydrates at Hydrate Ridge, Cascadia Margin
  in Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002, 740-744.
  Download paper as pdf-file
• Salamatin, A. N., and W. F. Kuhs (2002)
  Formation of porous gas hydrates
  in Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002, 766-770.
  Download paper as pdf-file
• Klapproth, A., E. Goreshnik, D. K. Staykova, H. Klein and W. F. Kuhs (2003)
  Structural studies of gas hydrates
  Can. J. Phys. 81(1/2), 503-518.
• Techmer, K., T. Heinrichs and W. F. Kuhs (2004)
  Cryo-electron microscopic studies on the structures and composition of Mallik gas-hydrate-bearing samples
  in Scientific Results from JAPEX/JNOC/GSC et al., Mallik as Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada (eds. S. R. Dallimore and T. S. Collett)
  Geological Survey of Canada Bulletin 544 (in press).

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Formation kinetics

The formation process of gas hydrates is quite complicated and not understood in detail. The gas molecules have to be built into the host lattice during the growth process. Starting from water with its well-known low-solubility for most gases, the growth process is quite slow. Starting from ice can considerably speed up the process. Quantitative studies of the growth kinetics can give detailed insights into the growth mechanism. We have undertaken a large number of in-situ diffraction experiments to study the growth kinetics. In combination with formation runs interrupted at various stages of the process where samples were recovered and investigated by electron microscopy we were able to establish a multi-stage model for the gas hydrate growth. Each stage has its characteristic time-dependency with an overall reaction rate slowing down as the formation proceeds.

References:

• Staykova, D. K., T. Hansen, A. N. Salamatin and W. F. Kuhs (2002)
  Kinetic diffraction experiments on the formation of porous gas hydrates
  in Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002, Vol. 2, 537-542.
  Download paper as pdf-file
• Staykova, D. K., W. F. Kuhs, A. N. Salamatin and T. Hansen (2003)
  Formation of porous gas hydrates from ice powders: Diffraction experiments and multi-stage model
  J. Phys. Chem. B 107, 10299-10311.

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Decomposition kinetics

Gas hydrate decomposition can be as complicated as the formation process. In particular, in a temperature window below 0°C the gas hydrate decomposition could be slowed down by several orders of magnitude, the so-called „anomalous preservation“. This effect is not well understood. By a detailed diffraction study of the decomposition kinetics and an analysis of the formed defective ice we can suggest that the onset of „anomalous preservation“ is caused by the annealing of defective ice. The annealed ice forms an effective diffusion barrier for gases hindering the out-diffusion of the gases released upon decomposition and providing the chemical activity at the gas hydrate–ice interface to stabilize the hydrate.

Reference:

• Kuhs, W. F., G. Genov, D. K. Staykova, and T. Hansen (2004)
  Ice perfection and onset of anomalous preservation of gas hydrates
  Phys. Chem. Chem. Phys. 6, 4917-4920

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Inelastic neutron scattering and molecular dynamics

Gas hydrates show unusually small thermal conductivities with temperature dependencies more closely resembling glassy materials than crystalline ice. These properties can be understood by the interaction of localized low-frequency guest modes with acoustic phonons of the host-lattice. We have studied these interactions by means of inelastic neutron scattering and molecular dynamics simulations. On varying the guest entities, we found that the guest-host coupling varied remarkably in strength. Likewise, pronounced differences were found in the guest modes of molecules quite similar in size and mass (like N2 and O2). This led us to the conclusion that "gas hydrates are individuals".

References:

• Klapproth, A., B. Chazallon and W. F. Kuhs (1999)
  Monte-Carlo sorption and neutron diffraction study of the filling isotherm in clathrate hydrates
  AIP Conference Proceedings 479, 70-73.
• Chazallon, B., A. Klapproth and W. F. Kuhs (1999)
  Molecular-dynamics modelling and neutron powder diffraction study of the site disorder in air clathrate hydrates
  AIP Conference Proceedings 479, 74-77.
• Chazallon, B., H. Itoh, M. Koza, W. F. Kuhs and H. Schober (2002)
  Anharmonicity and guest-host coupling in clathrate hydrates
  Phys. Chem. Chem. Phys. 4, 4809-4816.
• Itoh, H., B. Chazallon,H. Schober,K. Kawamura and W. F. Kuhs (2002)
  Low frequency modes of gas hydrates - inelastic neutron scattering and molecular dynamics studies
  in Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002, Vol. 2, 692-696.
  Download paper as pdf-file
• Itoh, H., B. Chazallon, H. Schober, K. Kawamura and W. F. Kuhs (2003)
  Inelastic neutron scattering and molecular-dynamics studies on low-frequency modes of clathrate hydrates
  Can. J. Phys. 81, 493-501.
• Schober, H., H. Itoh, A. Klapproth, V. Chihaia and W. F. Kuhs (2003)
  Guest-host coupling and anharmonicity in clathrate hydrates
  European Physical Journal E – Soft Matter 12, 41-50.

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Ab-initio work

Despite the simple chemical composition of water, the interactions of its molecules are quite complicated and still far from being fully understood. Water molecules can form a large number of clusters in which they are hydrogen-bonded. In these clusters, molecules can either accept or donate a H-bond. We have undertaken ab-initio studies to better understand the structure and stability of water clusters, in particular the so-called Buckyball-clusters which topologically resemble the cages in clathrate hydrates. Interestingly, we have found a new classification scheme for H-bonds, which could help in predicting the relative stability of water clusters without the necessity for expensive ab-initio calculations. This classification scheme is presently used to analyse the molecular rearrangements of water molecules at the surface of gas hydrates.

a)		b)
c)
Certain Buckyball water clusters are topologically identical to cages in clathrate hydrates. The three types of cages forming cubic clathrate hydrates of type I and II consisting of 5- and 6- membered rings of H-bonded water molecules are shown here. (a) 512 cage or pentagondodecahedron – small cage of both the type I and type II clathrate structure, (b) 51262 cage or tetrakaidecahedron – large cage in a type I structure, (c) 51264 cage or hexakaidecahedron – large cage in a type II structure.

Reference:

• Chihaia, V., S. Adams and W. F. Kuhs (2004)
  Influence of water molecules arrangement on the structure and stability of 5¹² and 5¹²6² Buckyball water clusters. A theoretical study
  Chem. Phys. 297, 271-287.