for the application of clathrate‐based gas separation studies with water dispersed in stationary beds of either microporous or low‐porosity silica gel. As well spatial resolution shows that nucleation and growth are simultaneous processes rather than time‐separated events and are spatially highly heterogeneous. A number of experiments showed that spatially resolved kinetics are needed to develop an understanding of mechanisms. Results from spatially averaged experiment like gas uptake and NMR spectroscopy may be useful for measuring process parameters but cannot give the molecular scale details required for development of mechanisms. Molecular dynamics simulations and consideration of classical nucleation theory led to reconsideration of some well‐established kinetic models. For instance, the assumption of hydrate processes being isothermal was shown to be incorrect, and taking hydrate processes to be phase changes are more consistent with observations than the often assumed chemical reaction – activated process model.
The ability of methane hydrate to exist outside its usual thermodynamic range of stability was first reported in a calorimetric study of hydrate decomposition (Davidson et al. studies on the naturally occurring hydrate from Gulf of Mexico, 1986). Later this property became an important feature (self‐preservation or anomalous preservation) of the concept to use methane hydrate as a medium for the storage and transport of natural gas. Many laboratories contributed to the development of this concept, including contributions from NRC (Takeya 2001) on the guest dependence of the preservation process and the nature of the ice formed during the preservation process. A recent study showed that THF hydrate could be superheated by coating it with cyclopentane hydrate that has a higher melting point than THF hydrate.
As described earlier, a combination of powder diffraction and NMR spectroscopic results led to the characterization of HS‐III as a previously unknown clathrate hydrate family. Another Xe hydrate structure, previously known only hypothetically, was characterized in a similar way as HS‐III. The hydrate, known as HS‐I, is of similar composition as the Xe hydrates CS‐I and HS‐III and demonstrates that the synthetic pathway is important in defining the structure of the product.
It is well known that ice Ih and ammonium fluoride are isostructural and form a solid solution with a maximum NH4F concentration of ∼22%. The NH4+ and F– ions replace two water molecules in the ice lattice. It was likely that clathrate hydrate lattices could be built with some water sites substituted with NH4+ and F– ions, although it is not possible to build pentagonal rings from only NH4F. The NMR spectrum of the xenon guest in the NH4F‐substituted clathrate shows up to five distinguishable D cages because of different NH4F distribution patterns, and the unit cell parameters shrink with increasing NH4F content (2012). The CS‐I version of the NH4F‐substituted hydrate lattice was shown to be a viable host lattice for methanol guests, an impossibility for the pure CS‐I hydrate lattice. A new help‐gas role for methanol was discovered so that unconventional guests such as alcohols and diols could be incorporated in the large cages of NH4F substituted lattices of CS‐I and HS‐III hydrates.
Physical aspects of clathrate hydrates as solid‐state materials have been studied at the NRC. In 1986, Don Davidson and coworkers measured the index of refraction of hydrate for the first time. They measured the index of refractive of water–THF solutions and THF clathrate hydrate crystals that formed upon cooling the solution up to −20 °C. The refractive index of the THF hydrate, which was greater than ice, was reproduced fairly accurately using a reactive field model where the THF molecule was assumed to lie in a cavity with the radius of the CS‐II 51264 cage (Davidson et al. 1986). The elastic constants of ice in the high‐pressure range of clathrate hydrate formation were measured by H. Kiefte and coworkers (Memorial University of Newfoundland) and E. Whalley of the NRC in 1988. Inelastic neutron scattering studies were performed on methane hydrate under high‐pressure conditions in 2000 as a collaboration between the University of Edinburgh and the NRC. At room temperature and high pressure (0.9 GPa), methane hydrate was found to form a hexagonal HS‐III (MH‐II) phase (Loveday et al. 2001, 2003). Further work on methane hydrate at even higher pressures was carried out by the same group in 2001. The structure of a new methane hydrate is solved from neutron and X‐ray powder diffraction at pressures of 2.0 GPa and higher. A transition from a clathrate to a filled ice structure was observed and the structure of methane hydrate III was finally uncovered (Loveday 2001). In a follow‐up paper published in Nature that year, neutron and synchrotron X‐ray diffraction studies determined the thermodynamic nature of methane hydrate which probably exists on Saturn's moon Titan, suggesting that the hydrate phases are a plausible source for the continuing replenishment of Titan's methane atmosphere.
High‐pressure inelastic X‐ray scattering was used to study the elastic properties of the high‐pressure methane hydrate MH‐II and MH‐III structures in 2005 by the NRC staff and coworkers from Germany and France. These studies revealed how guest molecules interact with the cages in clathrates and filled ice structures and how under high pressures, the water–methane guest repulsive interactions lead to the elastic properties of the methane hydrate phases becoming significantly different from the ice phases at the same pressure.
A 2005 Nature Materials paper entitled, “Anharmonic motions of Kr in the clathrate hydrate,” determined the origins of the low thermal conductivity in clathrates using incoherent inelastic neutron scattering, nuclear resonant inelastic X‐ray scattering (NRIXS) – a powerful new technique, and molecular dynamics simulations. The low thermal conductivity in the hydrate phase was related to the coupling of the local anharmonic guest rattling motions in the cages with the host lattice vibrations. This coupling leads to the scattering of the heat‐carrying lattice phonons resulting in a glass‐like anomaly in the clathrate phase thermal conductivity.
1.4 Contributors to NRC Clathrate Hydrate Research
NRC scientific staff
Don Davidson, Surendra K. Garg, Roger Gough, John Ripmeester, John Tse, Paul Handa, Chris Ratcliffe, Igor Moudrakovski, Gary Enright, Konstantin Udachin, Hailong Lu, Dennis Klug.
NRC technical staff
Ron Hawkins, Graham McLaurin, Régent Dutrisac, Jamie Bennett, Steve Lang, Jeff Farnand.
Other collaborators from the NRC
Edward Whalley, B. Morris, Derek Leaist, Michael Collins, Ian Cameron, Mike Desando, Litao Chen, Xu Zhu, Anivis Sanchez, Darren Brouwer, Marek Zakrzewski, Chris Tulk, Lee Wilson, Yong Ba, Luy Ding, Eric Brouwer, Andreas Brinkmann, Erik van Klaveren, Hidenotsuke Itoh, Serguei Patchkovskii, Vladimir P. Shpakov, Saman Alavi, D. Yu. Stupin.
Scientific collaborators on clathrate hydrates
Alan Judge, Scott Dallimore, Fred Wright, Michael Riedel (Geologic Survey of Canada).
George Spence, Ross Chapman (University of Victoria).
Peter Englezos, Robin Susilo, Rajnish Kumar, Praveen Linga, Nagu Daraboina, Alireza Bagherzadeh, Adebola Adeyemo, Hassan Sharifi (University of British Columbia) Virginia Walker, Huang Zeng, Hiroshi Ohno, Raimond Gordienko (Queens University), Harry Kiefte, Maynard J. Clouter, Robert E. Gagnon (Memorial University of Newfoundland), Tom K. Woo (University of Ottawa).
Richard Coffin, John Pohlman (Naval Research Laboratory, Washington), Jamie J. Molaison, António M. dos Santos, Neelam Pradhan, Bryan C. Chakoumakos, Ling Yang (Oak Ridge National Laboratory), Wolfgang Sturhahn, Esen E. Alp, Jiyong Zhao, Charles D. Martin (Argonne National Laboratory) W. F. Lawson (US Department of Energy, Morgantown), Peter Brewer (Monterey Bay Aquarium Research
