Thesis (Ph. D.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 204-220). Also available via the World Wide Web.

When ice is compressed alongside a gas, crystalline 'host - guest' inclusion compounds known as gas clathrate hydrates form. These compounds are of interest not only for their environmental and possible technological impact as gas storage and separation materials, but also for their ability to probe networks not readily adopted by the pure `host' water molecules, and to study the interactions between water and gas molecules. Despite the pressure dependent crystal structures being fully determined for a large variety of `guest' gas species there is still relatively little known about the crystal structures in small guest gas systems such as H2 hydrate. The majority of structural studies have been done with x-ray diffraction and report a number of conflicting structures or hydrogen contents for the four known stable phases (sII, C0, C1 and C2). As this is a very hydrogen rich system the most ideal method to study the structure is neutron diffraction, which is able to fully determine the location of the hydrogen atoms within the structure and would allow a direct measurement of any hydrogen ordering within the host structure and the H2 content. In this work the phase diagram of the deuterated analogue of the H2-H2O system is explored at low pressures (below 0.3 GPa) with neutron diffraction. In the pressure/temperature region where the sII phase is known to be stable, two metastable phases were observed between the formation of sII from ice Ih and that this transition sequence occurred in line with Ostwald's Rule of Stages. One of these metastable phases was the C0 phase known to be stable in the H2-H2O system above 0.5 GPa, and the other is a new structure not previously observed in this system and is dubbed in this work as C-1 . Prior to this work the C0 phase has been reported with various structures that were determined with x-ray diffraction, and here the crystal structure and H2 content at low pressure are determined with neutron diffraction. The C0 phase was found to form a similar host structure to those of the previous studies with spiral guest sites but is best described with highly mobile H2 guests and a higher symmetry space group which make it the same structure as the spiral hydrate structure (s-Sp) recently observed in the CO2 hydrate system. In addition to this structure being determined at pressure a sample of C0 was also recovered to ambient pressure at low temperature and its structure/H2 content is presented as it was warmed to decomposition. The crystal structure of the C-1 phase was determined to be similar to ice Ih and a sample was recovered to ambient pressure to study its decomposition behaviour. Evidence for a similar structure in the helium hydrate system at low pressure is also reported here. This work was then extended to higher pressures with the recent developments of a hydrogen-compatible gas loader and large-volume diamond anvil cells. Several test experiments on gas-loaded Paris-Edinburgh presses are described on systems that are similar to hydrogen-water like urea-hydrogen and neon-water. And a further preliminary high pressure study on the deuterated analogue of the H2- H2O system in a diamond anvil cell between 3.6 and 28 GPa shows decomposition behaviour as pressure was increased.

Les clathrates organiques, particulièrement ceux formés entre l’hydroquinone (HQ) et les gaz, sont des entités supramoléculaires montrant un potentiel intéressant comme matériau alternatif pour les applications de stockage et de séparation de gaz. Cette étude traite de l’évaluation du clathrate d’HQ pour la séparation du CO2 contenu dans les mélanges CO2/CH4 par réaction gaz-solide. D’un point de vue fondamental, différentes propriétés des clathrates d’HQ-CO2, -CO2/CH4 et -CH4 ont été analysées: signatures spectroscopiques, structures cristallines, morphologies, capacités de stockage de gaz, températures de relargage de gaz et températures de transition structurales. Ce travail offre aussi de nouveaux éléments de compréhension des mécanismes de formation et de dissociation des clathrates d’HQ. Il est montré que, pour capturer efficacement et sélectivement le CO2, la réaction d’enclathration doit être faite en utilisant l’intermédiaire « clathrate vide » formé à partir du clathrate d’HQ-CO2. D’un point de vue pratique, les courbes d’équilibre, les enthalpies de dissociation, et les occupations dans les conditions d’équilibre ont été déterminées pour les clathrates d’HQ-CO2 et -CH4 dans une gamme étendue de température allant de 288 à 354 K. De plus, la cinétique de la réaction d’enclathration a été étudiée expérimentalement et modélisée. Dans cette optique, un matériau composite à base d’hydroquinone a été développé, et permet de capter et stocker le gaz de manière réversible, et d’améliorer significativement la cinétique d’enclathration. Le procédé de séparation de gaz basé sur la formation du clathrate d’hydroquinone a aussi été étudié. L’influence des paramètres opératoires (i.e. temps de réaction, pression, température et composition du gaz d’alimentation) sur la cinétique de capture, la sélectivité et la capacité de stockage de gaz ont été évaluées à travers des expériences menées à l’échelle pilote. / Organic clathrate compounds, particularly those formed between hydroquinone (HQ) and gases, are supramolecular entities recently highlighted as promising alternatives for applications such as gas storage and separation processes. This study deals with an evaluation of the HQ clathrates to separate CO2 from CO2/CH4 gas mixtures through direct gas-solid reaction. On the fundamental point of view, new insights into several properties of the CO2-, CO2/CH4-, and CH4-HQ clathrates were studied: spectroscopic signatures, crystal structures, morphologies, gas storage capacities, guest release temperatures and structural transition temperatures. This work also offers new elements of understanding HQ clathrate formation and dissociation mechanisms. It is shown that, for capturing CO2 the most selectively and efficiently, the enclathration reaction has to be done with the “guest-free intermediate” derived from the CO2−HQ clathrates. On a practical point of view, the equilibrium curves, the dissociation enthalpies, and the occupancies at the equilibrium clathrate forming conditions, were determined for the CO2- and CH4-HQ clathrates in an extended range of temperature from about 288 to 354 K. Moreover, the kinetics of the gas-solid enclathration reaction were studied experimentally and modelled. In this way, HQ-based composite materials were developed and allows to reversibly capture and store gases, and to significantly improve the enclathration kinetics. The hydroquinone clathrate based gas separation (HCBGS) process was also investigated. The influence of the process operating parameters (i.e. reaction time, pressure, temperature and feed gas composition) on the CO2 capture kinetics, the selectivity toward CO2, and the storage capacity were assessed through experiments performed at pilot scale.