Structural studies of ionic liquids and ionothermally-prepared materials
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The aim of this thesis was to examine materials using high resolution X-ray diffraction techniques. Initial work involved the synthesis of various metal phosphates to investigate their suitability for charge density work. Many of these were discovered to be of insufficient quality for further study. Much of the phosphate synthesis work performed at the moment utilises an ionic liquid both as a solvent and structure directing agent which dictates the topology of the structure due to its size and charge density. As such the ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate used in the synthesis process was examined with high resolution X-ray diffraction as it was possible to produce large pure crystals which could be examined further. A high resolution data set was also collected from the metal organic framework SIMOF-1 which produced a preliminary multipole model however further data collections are required to improve the quality of the model. A multi-technique investigation involving X-ray diffraction, solid state NMR and first principles calculations was carried out on the aluminophosphate material AlPO₄-15. A synchrotron X-ray single crystal diffraction study was carried out on the same sample as that used in solid state NMR studies. The model from the single crystal study, together with a model from a literature high resolution study of the same material, were used as starting points for the first-principles calculations of the NMR parameters. This enabled the ³¹P and ²⁷Al NMR spectra to be unambiguously assigned and all the NMR parameters calculated agreed well with the experimental spectra even without relaxing the X-ray derived structural models. Highlighting that as long as a good data set has been collected in the first place the atomic positions would not change too drastically. Other aspects of this thesis involved investigations into other ionothermally prepared systems such as the use of different phosphonate sources to provide functionality to the materials. This work resulted in some interesting findings such as the ionic liquid breaking down and being incorporated into the framework via the metal. Many of the structures produced were of a layered nature however a molecular structure was also synthesised which is unlike the vast majority of hydrothermally prepared phosphonates, which are layered. The negligible vapour pressure provided by the ionic liquid has enabled synthesis reactions to be investigated with glass vessels on an energy dispersive beam line. This work highlighted how it is possible to study the synthesis process in-situ and compare microwave assisted reactions against a conventional heating method, the results indicate that two different types of reactions are occurring resulting in different intermediates which is due to the way the reagents are heated. The microwave assisted reactions also result in larger purer crystals which highlights the importance of the method in materials synthesis. The use of a specially designed environmental gas cell was used to investigate the adsorption properties of the metal organic framework CPO-27-Co in-situ. Using the cell it was possible to locate sulfur dioxide physisorbed and chemisorbed sites with in the framework which could be removed by the application of a vacuum and heat. It was also possible to locate the chemisorbed sites for nitric oxide within the metal organic framework however due to the low scattering factor and disorder from the gas it was not possible to locate the physisorbed sites.
Thesis, PhD Doctor of Philosophy
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