Читать книгу Clathrate Hydrates - Группа авторов - Страница 15

For about 50 years, gas hydrate research was a supported project at the NRC in Ottawa. It had its start in the early 1960s when Don Davidson, working in the Colloid Section of the Division of Applied Chemistry (Figure 1.1), took the initiative to follow up on a fundamental question that arose during his investigation of the dielectric properties of liquids: in the dielectric properties of liquids, is it possible to separate the contributions from molecular reorientation and diffusion? This led to the first studies on clathrate hydrates where molecules are trapped in pseudo‐spherical molecule‐sized cages so that one may well expect that contributions from diffusion should be reduced or eliminated.

Clathrate hydrate science was at a stage of development where two hydrate structures were known, confirming their clathrate nature. Statistical thermodynamics had provided a model of clathrate hydrates based on weak guest–host interactions; however, many of the features of the model remained untested.

Early work on guest dynamics in clathrates at the University of British Columbia (by Charles McDowell) convinced Don, Figure 1.2a, that NMR spectroscopy would be another useful technique for the study of guest motion, as it provided the means to study the effect of polar versus non‐polar guests. It was an opportune time to initiate an NMR capability, as a 12″ NMR electromagnet became available from William G. Schneider's lab. Dr. Schneider, FRS (Figure 1.2b), was an internationally known pioneer in NMR spectroscopy who had taken on the Directorship of the Division of Pure Chemistry in 1963 and the Presidency of NRC in 1967.

Figure 1.1 National Research Council of Canada Building M‐12 on the NRC Montreal Road campus in Ottawa, home of the Colloid Chemistry (later Colloid and Clathrate Chemistry) group until 1990. Source: Reproduced with permission from the National Research Council.

Figure 1.2 (a) Donald Davidson observing a clathrate hydrate sample; (b) William Schneider in the official portrait as president of the NRC. Source: Photographs by the authors.

Figure 1.3 (a) S. K. Garg at the controls of the Bruker 1.4 T SXP spectrometer as modified for broadline and pulsed NMR experiments at low temperatures (2 K). (b) J. A. Ripmeester at the console of the Bruker CXP180 NMR spectrometer. Source: Photographs by the authors.

With his associates S. R. Gough and S. K. Garg, post‐doctoral, and technical staff, Don Davidson designed and built equipment to carry out dielectric and broadline NMR measurements over a temperature range from 2 to 295 K.

John Ripmeester joined the group (1972), arriving at the same time as a commercial pulsed NMR spectrometer (Bruker Bkr). The instrument was modified and upgraded (Bruker SXP, see Figure 1.3) so that both broadline and pulsed NMR experiments could be carried out down to 2 K.

During this period, Don Davidson published “Clathrate Hydrates” an overview of clathrate hydrate science from 1810 to 1972, which appeared in Water, A Comprehensive Series, Vol. 2, edited by Felix Franks. This chapter summarized much of the knowledge on these substances up to that time and is still cited as a source of fundamental information on clathrate hydrates.

As NMR and dielectric measurements were extended down to 2 K to characterize guest molecule motional dynamics, by 1980, dielectric and NMR properties of clathrate hydrates had been thoroughly investigated and resulted in a good overall understanding. With these techniques, a considerable number of new guests forming structure I and structure II hydrates were identified.

The first direct measurements of hydrate cage occupancies (¹²⁹Xe NMR and calorimetry) were made, confirming the general correctness of the van der Waals–Platteeuw solid solution theory. This pioneering development of ¹²⁹Xe NMR spectroscopy which illustrated the sensitivity of NMR chemical shift parameters to the size and shape of the clathrate cages became a widely used method to characterize porous materials. A review summarizing the new results “NMR, NQR and dielectric properties” was published in Inclusion Compounds, Vol. 3, edited by J. Atwood, J.E.D. Davies, and D. D. MacNicol in 1984.

As natural gas hydrate research advanced, the period of 1970–1980 saw publication of initial reports with results from both offshore and under permafrost locations. Davidson was invited to join a committee with members from the Canadian Federal Departments of Energy, Mines and Resources (EMR) and Indian and Northern Affairs (DINA) to learn about the gas hydrate potential of the Canadian Arctic. At the NRC, this new interest sparked a dielectric and NMR study of a number of hydrocarbon clathrate hydrates over a broad temperature range. These developing interests gave the NRC group an entry into the energy research field and the clathrate group benefited by being able to expand their clathrate hydrate program with funding from the National Energy Program. In recognition of the importance of the clathrate hydrate work, the Colloid Section was renamed the Colloid and Clathrate Section of the Division of Applied Chemistry.

Other members joining the group were John Tse (computation and diffraction, 1980), Chris Ratcliffe (NMR spectroscopy, 1982), and Paul Handa (calorimetry, 1982), see Figure 1.4. Funds for equipment were also received, leading to acquisitions of a Tian–Calvet calorimeter, a powder X‐ray diffractometer, and a multinuclear FT NMR instrument dedicated to solid‐state experiments. A reorganization of the Chemistry Division (1984) brought Dennis Klug, Ted Whalley, and Graham McLaurin to the Clathrate Group, bringing with them expertise in high‐pressure techniques and Raman spectroscopy. Yuri Makogon, who documented the Messoyakha natural gas hydrate deposit, and a delegation from the USSR became regular visitors to Ottawa to visit the NRC, EMR, and other hydrate labs in Canada to share new findings on natural gas hydrates. In 1986, Don Davidson passed away after a lengthy illness and John Ripmeester became section head in his place.

Figure 1.4 Clathrate group, Division of Chemistry, National Research Council Canada circa 1984. Back row, left to right, Tony Antoniou, Ron Hawkins, Gerry McIntyre, John Ripmeester, Paul Handa, Chris Ratcliffe; front row, left to right, John Tse, Roger Gough, Don Davidson, Surendra Garg, Michael Collins. Source: Reproduced with permission from the National Research Council.

With the new group members and capabilities, a number of important contributions to understanding of hydrate solid‐state properties were made, including the confirmation of the anomalously low thermal conductivity as a general property of clathrate hydrates. Also, the first lattice dynamics calculations on hydrates, including the hydrate of methane, were conducted. To measure thermodynamic properties of hydrocarbon hydrates, it became necessary to learn how to synthesize the hydrates under carefully controlled three‐phase equilibrium conditions.

In 1987, a new hexagonal clathrate hydrate, structure H (HS‐III or sH), was reported. It was characterized with an early version of an approach now known as NMR crystallography. It was the first new family of hydrate structures since the CS‐I and CS‐II hydrates were recognized. Among the pentagonal and hexagonal rings that form the sH cages, it also features square faces constructed from four water molecules with highly strained hydrogen bonds. In 1990, the first inventory of structure H hydrate formers was created, also extending the number of guests suitable for sII hydrate and noting guests which did not form ternary hydrates.

The synthesis and characterization of structure I carbon monoxide hydrate were reported and its clathrate hydrate nature was demonstrated from dielectric and ¹³C NMR measurements. Much later, a CS‐II hydrate of CO was reported.

The 1980s saw some further advances in the science of natural gas hydrates. The success of the multi‐technique approach to hydrate characterization led to collaboration with the US Geological Survey (USGS), Morgantown WV, to characterize natural gas hydrate recovered from the Gulf of Mexico. X‐ray diffraction confirmed the existence of structure II hydrate for the natural gas; the calorimetry revealed delayed melting/decomposition as an example of a self‐preservation effect, and ¹³C NMR techniques showed the distribution of methane over large and small cages in CS‐II hydrate. ¹³C Magic Angle Spinning (MAS) NMR was used as a means of identification of structure I and structure II hydrates. Knowledge of cage occupancies proved to be the key to the determination of better parameters for the guest–host potential.

In 1990, NRC underwent a major reorganization that saw the disappearance of the familiar Division and Section structure. Most of the Clathrate Group members joined a larger group entitled Molecular Structure and Dynamics in the newly minted Steacie Institute for Molecular Sciences at the Sussex Drive location of the NRC (Figure 1.5). This group, led by Keith Preston, had a broader outlook on materials and also brought new expertise and capabilities.

For the hydrate work, the most significant changes were the inheritance of a single‐crystal X‐ray diffractometer. Gary Enright (1990) and Konstantin Udachin (1995) joined the group and developed a high level of expertise in solving highly disordered crystal structures, including clathrate hydrates. Structures of note included those of structure H hydrate, confirming the initial structural features determined from powder diffraction experiments, and that of CS‐I CO₂ hydrate, giving the distribution of CO₂ over the large and small cages. In 1998, the single‐crystal structure of tetragonal bromine hydrate was reported. Four different crystal morphologies were observed depending on synthesis conditions. All crystals had the same microscopic crystal structure, but with different cage occupancies. This work was seen as a further vindication of the “solid solution” model. In a paper published in Nature entitled, “A complex clathrate hydrate structure showing bimodal hydration,” Udachin and Ripmeester showed that a new hydrate structure formed which consists of alternating stacks of structure H and structure II hydrates. This structure is just one example of the many possible hydrate structures that can be formed from stacked layers.

Figure 1.5 National Research Council Canada building at 100 Sussex Drive, Ottawa, the home of much of the clathrate hydrate research 1990–2015. Source: Reproduced with permission from the National Research Council.

From phase equilibrium studies, dimethyl ether was known to form two hydrates, one having the CS‐II structure, the second was thought to be isostructural with bromine hydrate. However, it, was discovered that the second hydrate had a novel, dense trigonal structure. Unique among the simple hydrate structures, it did not have any pentagonal dodecahedral cages, although some novel cage geometries were observed.

In collaboration with Satoshi Takeya, a major advance was made in the diffraction of powdered hydrate samples (2009). Direct methods for the analysis of powder X‐ray diffraction (PXRD) patterns, dealing with molecular fragments rather than atoms, reduced the number of parameters to be refined and were able to give detailed structural information, including cage occupancies.

The NMR facilities were renewed and improved and Igor Moudrakovski, joining initially as a post‐doctoral fellow (1992), was able to develop a number of unique capabilities suitable for clathrate hydrate work. One of these, MRI, was used to image hydrate layers on ice particles, which, on melting leave intact water droplets inside a hydrate coat. Another new technique, NMR spectroscopy with hyperpolarized ¹²⁹Xe was developed to give unprecedented signal enhancement to allow the time‐dependent studies of structural and compositional changes. Applications to hydrate formation kinetics showed evidence for a precursor structure before nucleation of crystalline hydrate with the primacy of a small cage environment. Also, a transient structure II hydrate of Xe was identified. Time‐resolved MRI studies of hydrate formation kinetics revealed that the formation process is locally very inhomogeneous even though the spatially averaged kinetics in the bulk system may look homogeneous. Nucleation and crystal growth are simultaneous rather than sequential processes, with multiple nucleation sites forming during the crystal growth process.

The Clathrate Group joined a Canada–Japan project (JAPEX/JNOC/GSC) on the exploration of natural gas hydrate deposits in the Mallik site in the Canadian Arctic. The NRC group contributed laboratory analysis of recovered hydrate samples from the Mallik 2L‐38 site: ¹³C NMR, Raman, diffraction, calorimetry, gas, and saturation analyses were conducted on samples. Hailong Lu joined the group, first as a visiting scientist (2002), bringing his expertise on the geochemical analysis of natural gas hydrates. One outcome was the establishment of a protocol for the characterization of hydrate from natural sources, including the determination of structure and composition. Collaboration with members of a number of hydrate cruises followed. Samples from many worldwide hydrate locations were shipped to NRC in liquid nitrogen containers. One highlight, published in 2007, was the discovery of naturally occurring structure H hydrate on the Cascadia margin, offshore Vancouver Island. The synthetic version was first reported 20 years before by the hydrate group. The structure is capable of trapping large hydrocarbon molecules and is far more stable than CS‐I hydrate. The findings were published in Nature entitled, “Complex gas hydrate from the Cascadia Margin.”

The van der Waals–Platteeuw theory was developed to describe hydrate formation from water and small non‐polar guests. Potentials to describe guest–host interactions in this case tend to be of the Lennard–Jones or Kihara type. Differences due to guest chemistry tend to be hidden because of the limited number of potential parameters which ultimately are obtained by fitting to experimental observables. This procedure effectively captures all guest–host interactions (e.g. van der Waals interactions, dipolar interactions, hydrogen bonding interaction, etc.) even though only the non‐directional van der Waals interactions are explicitly taken into account when developing the approach. At the NRC, efforts to look for explicit effects of guest chemistry on hydrate properties were spread over many years and involved a number of techniques and contributions from Davidson, Alavi, Udachin, Ratcliffe, Moudrakovski, and Ripmeester, among others.

A most evident guest observable is its water solubility, a property enabled by hydrogen bonding between the water and the guest species. One can then ask: Does hydrogen bonding just disappear once the guest is incorporated from the bulk aqueous solution into the hydrate lattice? In the 1960s Don Davidson, using dielectric relaxation to study water motions in hydrates noted that water reorientational dynamics for hydrates of non‐polar guests were very similar to those for ice Ih. On the other hand, water dynamics for hydrates of water‐soluble guests were much faster with smaller activation energies. He attributed this to transient host–guest hydrogen bonding. For THF, this would be hydrogen bonding of the oxygen atom in the rapidly rotating guest with the water molecules forming the cage wall. Bjerrum defect injection was proposed as a suitable mechanism for this process, as discussed previously by Onsager and Runnels for water molecule reorientation in ice. From the relatively small differences in guest reorientational activation energies for hydrogen‐bonded versus non‐hydrogen‐bonded guests, it can be seen that the interactions tend to be weak, e.g. for THF ∼3.9 kJ mol⁻¹ as compared to cyclopentane with ∼2.8 kJ mol⁻¹. Insight from molecular dynamics computational studies gave the dependence of the defect concentration for different guest types on temperature. One effect of guest–host hydrogen bonding is the transfer of electron density from the host lattice to guest–host units, thus affecting lattice stability. Another is that in a guest–host O⋯HO hydrogen bond, the O⋯O distance is less than the sum of the van der Waals radii. This makes some of the guests appear to be too large for the cage in which they reside, according to calculations of the free van der Waals volume in the cage, e.g. CO₂ in the small cage of CS‐I CO₂ hydrate. Whereas the oxygen in guests with ether or ketone functions are hydrogen bond acceptors, guests with OH, NH, or NH₂ functions can also be hydrogen bond donors, giving rise to complex guest–host hydrogen‐bonding geometries.

Stronger hydrogen bond formation, such as for methanol in the double CS‐II THF‐methanol hydrate and for t‐butylamine in the CS‐II t‐butylamine + H₂S binary hydrate, can lead to displaced or vacant water positions in the hydrate lattice. It can be surmised that a sufficient number of such defects can destabilize the hydrate lattice.

Since water‐soluble molecules affect the activity of aqueous solutions, it is clear that water‐soluble guests will also act as inhibitors, as accounted for by a modified van der Waals–Platteeuw equation. Thus, it is important to recognize that hydrate instability may have two origins – one from strong liquid water–guest interactions accounted for by the altered activity, the other by insufficiently strong guest–hydrate cage interactions as accounted for by the magnitude of potential function parameters. Note that the first of these effects can be “turned off” by eliminating the liquid aqueous phase and producing hydrate from an ice–guest molecule reaction. Clathrate hydrates of formaldehyde and ammonia were made this way. On the other hand, so far it has not been possible to produce a binary methanol hydrate. Another interesting observation was the catalyst‐like behavior when small quantities of methanol or ammonia were added to the reaction of methane and ice. These molecules, while being excluded from bulk ice, function as catalysts by disrupting the ice surface by hydrogen bonding to surface water molecules. This greatly enhances the rate of clathrate hydrate formation. This is not a true “catalytic” effect since a small amount of the methanol or ammonia may be incorporated into the hydrate phase.

Halogen–water interactions have proven to give chlorine and bromine hydrates unusual properties. Although the chlorine van der Waals diameter is far too large to fit into the CS‐I hydrate small D cage, cage occupancies of ∼30% have been measured. Compositional analysis by Cady has shown that chlorine hydrate is more stable than expected from applications of the van der Waals–Platteeuw equation.

Bromine by forming a unique hydrate structure illustrates the strong structure‐directing effect of the halogen–water interaction. One feature that stands out is the bromine guest in the tetrakaidecahedral (T) cage of the structure where, considering the accepted van der Waals diameter of Br₂, this guest is far too large to fit into the T cage. Another feature is the presence of the pentrakaidecahedral (P) cage in Br₂ hydrate, rarely observed in other hydrate structures. The halogen–water interaction in bromine hydrate is reminiscent of the electrophilic electron acceptor Br₂ in the bromine‐p‐dioxane complex reported by Hassel many years ago.

Guest–guest interactions in hydrates with a single guest per cage rarely manifest themselves directly. The best‐known example is the guest dipole ordering of trimethylene oxide in the large CS‐I cage. From diffraction, the guest molecules are sterically constrained so that the guest dipoles lie along the symmetry axis of the T cages, but in disordered directions. Below ∼105 K the dipole directions order, as evident from dielectric relaxation, NMR, and calorimetry.

Computational work has shown that longer range interactions exist between THF guests in the large CS‐II cage and small guests in neighboring small cages. The number of Bjerrum defects generated by THF–water hydrogen‐bonding appears to depend on the electron donating properties and size of the small molecule in binary THF+small gas CS‐II hydrates. Most likely this interaction between guests is mediated by the intervening cage wall.

In the early 2000s, the US Department of Energy (DOE) set a target of 5 mass % as a target for onboard hydrogen storage in vehicles. In 2002, Mao and coworkers determined that multiple cage occupancies of the CS‐II clathrate hydrate cages with H₂ molecules were possible at low‐temperature and high‐pressure conditions. In 2003, a paper published in the Proceedings of the National Academy of Sciences (PNAS) at the NRC by Patchkovskii and Tse examined the stability of the type II hydrogen clathrate with respect to hydrogen occupancy with a statistical mechanical model in conjunction with first‐principles quantum calculations. These works suggested that the required mass % of H₂ gas in the hydrate phase was possible to attain, but at pressures and temperatures which would not be easily accessible to vehicles. To lower the pressure required to incorporate substantial amounts of H₂ gas into a clathrate hydrate phase, a joint Korean Advanced Institute of Science and Technology (KAIST) and NRC study was carried out and published in Nature in 2005. This study showed hydrogen storage capacity is enhanced by composition tuning with THF. When aqueous solutions of THF with less than stoichiometric amounts of THF relative to the pure CS‐II phase were exposed to pressures of H₂ gas, it was found that the H₂ guests enter both the large and the small cages under milder conditions than the pure CS‐II hydrate. These experimental studies were followed by a series of molecular dynamics simulations where the energies of different H₂ guest occupancies in pure and binary clathrate hydrates were determined.

A further development in this area was that it was experimentally realized that once pressure is relieved from an H₂ containing clathrate hydrate, the hydrogen content gradually decreases, even though the crystal structure of the hydrate phase remained intact. In 2007, a quantum chemical study at the NRC, the energy barriers and diffusion rates of H₂ molecules diffusing through the hexagonal and pentagonal faces of the CS‐II cages were determined. The barriers to diffusion from the hexagonal faces can be overcome at lower temperatures, making the CS‐II phase somewhat porous to the diffusion of this gas.

A unique method was described for producing high‐occupancy hydrogen hydrate in 2012. Replacement of the nitrogen guest in CS‐II nitrogen clathrate by hydrogen at high pressures was found to result in hydrogen clathrate where small cages are doubly occupied.

Following earlier work on the observation that organisms in cold climates use AFPs for protection against destructive freezing by a non‐colligative mechanism, A. R. Edwards and coworkers suggested that such proteins may well have a similar action on solid hydrate formation. Because of the difficulty in obtaining significant quantities of AFPs, much effort was expended in searching for polymers that might mimic the antifreeze behavior of AFPs. This led to a burgeoning research area focused on discovering and testing of low‐dosage kinetic inhibitors (LDKIs) and resulted in some early successes such as polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) by E. D. Sloan. With the greater availability of AFPs and related materials from V. Walker's (2005) group, a collaborative effort with the NRC group and later with P. Englezos' group was initiated to characterize the AFP function as a LDKI. This was carried out on a scale from the size of droplets to that of a stirred reactor with multiple techniques, including gas uptake and release, calorimetry, PXRD, solid‐state NMR spectroscopy and micro‐imaging, quartz crystal microbalance, and Raman spectroscopy. Some findings include there is no correlation between AFP function on ice or hydrate; some AFPs are as effective as polymers for antifreeze function; elimination of the memory effect for strong AFPs; and the presence of multiple decomposition temperatures for hydrate made in the presence of AFPs in calorimetric measurements. As explained in the chapter on kinetics of hydrate formation (Chapter 14), it is expected that the supercooling of hydrate forming solutions with inhibitor is mirrored by a superheating effect of the solid hydrate coated with inhibitor. What these effects have in common is that the processes are limited by the local radius of curvature of the advancing or retreating solid–liquid interface.

Molecular dynamics simulations were performed at the NRC and University of British Columbia to determine the mechanism of action of the winter flounder AFP as an inhibitor of methane hydrate nucleation and growth. These studies showed that the properly oriented dangling methyl groups on the amino acids of the AFP are incorporated into the half‐formed hydrate cages on the hydrate surface, thus inhibiting local growth of the hydrate and inhibiting global growth of the hydrate through pinning to the hydrate surface and the action of the Kelvin effect.

In the early part of the twenty‐first century, environmental concerns brought about an interest in the utilization of clathrate hydrates as a working medium for gas storage and transportation and separation of flue and fuel gases. The groups of P. Englezos and H. Lee had been active in the engineering research of the aforementioned processes and linked up with the NRC group to provide information on the molecular aspects using a multi‐technique approach, in some cases with in situ time resolution. Some early results were obtained on “guest‐swapping” where methane was recovered from methane hydrate by exposing the hydrate to CO₂ and “composition tuning” where the composition of binary hydrates can be varied over a wide range.

Studies reveal that vastly improved kinetics occur 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.