Clathrate Hydrates. Группа авторов

      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.