The class of water-ice compound known as gas hydrate has been of interest to science for sometime where, for instance, gas hydrates make excellent candidates for studying the interactions of water and gas molecules. They are also of relevance to industry, where they present an interesting material for the separation, transport, and storage of different gases, and also due to the vast quantities of methane gas that are trapped in natural gas hydrate formations. While much is known about the behaviour of many gas hydrate systems at high-pressure, the CO2 hydrate system is less well studied, with apparent hydrate dissociation at just 10 kbar, and (prior to this work) an unsolved crystalline phase in the pressure range 6-10 kbar. In this work the CO2-H2O system has been studied at high-pressure and, by heating samples to the liquid state and observing their behaviour on refreezing, it has been confirmed that there are indeed no hydrate phases in the system above 10 kbar (up to at least 40 kbar). While performing this investigation, an interesting effect of CO2 on the behaviour of water crystallisation was also observed, and additionally, a simple yet effective technique for making solubility measurements in the system at high-pressure has been discovered. Using a combination of neutron and x-ray diffraction techniques, the crystal structure of the previously unsolved ‘HP’ CO2 hydrate phase has been determined by ab-initio methods. It has been found to be a new gas hydrate structure, but is shared by a small number of Zintl compounds, and may also be common to the unsolved C0 phase of H2 hydrate. The structure has a characteristic spiral of guest molecule sites, leading to its suggested label as the spiral hydrate structure (s-Sp). Its composition has been measured as a tri-hydrate, and the compressibility of s-Sp and the low-pressure s-I CO2 hydrate phases have also been measured. On cooling to 77 K it has been discovered that a third CO2 hydrate phase is formed with a significantly larger unit cell, which is thought to possess a structure similar to that of s-Sp, but with an ordered arrangement of CO2 molecules. Finally, a pilot study of the high-pressure behaviour of the binary H2-CO2 hydrate system has been performed. Using Raman spectroscopy it has been found that a new mixed hydrate phase exists in the pressure range 5-15 kbar, and it is speculated that this could exhibit a freely tunable H2/CO2 content, based on suspicion that it forms the s-Sp structure. Additionally, it has been found that H2 and CO2 chemically react at room temperature, when compressed to ~5 kbar in a rhenium gasket. From the Raman spectrum this reaction product has been identified to be aqueous-methanol.

Synthesis of polycrystalline, vs. single-crystalline porous materials, such as metal-organic frameworks (MOFs), is usually beneficial due to shorter synthetic time and higher yields. However, the structural characterization of these materials by X-ray powder diffraction can be complicated. Even more difficult, is to track structural changes of MOFs by in situ experiments. Hence, we designed several successful techniques for the structural investigation of porous MOFs. These methods utilize the Structure Envelope (SE) density maps. SEs are surfaces which describing the pore system with the framework. It was shown that these maps can be easily generated from the structure factors of a few (1 to 10) of the most intense low index reflections. Application of SE in Charge Flipping calculations shortens and simplifies structure determination of MOF materials. This method provides excellent MOF models which can be used as a good starting point for their refinement. However, the most interesting results have been found by using Difference Envelope Density (DED) analysis. DED plots are made by taking the difference between observed and calculated SE densities. This allows us to study guest related issues of MOFs such as, location of guest molecules in the pores, tracking activation of MOFs and gas loading, etc. We also have shown that, DED created from routine powder diffraction patterns might provide very important information about MOF structure itself. In fact DED can be used for study of interpenetration, substituents locations and effects conformational changes in the MOF ligands. Generation and analysis of SEs and DEDs are easy and straightforward. It provides the information needed to explain major deviations in structure-property relationship in MOFs. In our opinion, this method might become one of the important and routine techniques for MOFs structural analysis.