Abstract
The initial motivation behind this work was inspired by the reports on metal oxide dissolution by deep eutectic solvents. The original aim was to understand the underpinning mechanism of dissolution and, building on this, to derive improved systems for metal recovery from waste streams.Review of the literature led to a re-examination of thermal properties of the archetypical system of choline chlorideiurea, focusing on careful moisture control. The key outcomes, reported in Chapter 2, include an updated phase diagram and conclusive proof that the eutectic composition is thermally unstable even at relatively low temperature of 90 °C. This is consistent with recent literature reports suggesting that oxide dissolution mechanism relies on decomposition products, rather than on the inherent properties of the eutectic mixture.
In parallel, choline chloride:urea and choline chloride:oxalic acid deep eutectic solvents were studied by neutron scattering (Chapter 3), to elucidate the difference in hydrogen bonding between an amide and a carboxylic acid at 338 K. In either case, the -OH group on the choline chloride was found to have less prominent influence on the overall hydrogen bonding within the system, than previously suggested, indicating that the structure of the cholinium cation is not pivotal in the design of deep eutectic solvents. The chapter is concluded with multi-technique studies on zinc oxide speciation in deep eutectic solvents, with the aim to elucidate the dissolution mechanism. Two plausible mechanisms are proposed, but unfortunately these studies remained inconclusive.
Studies described in Chapters 2 and 3 led to the conclusion that the components of the archetypical deep eutectic system, choline chloride and urea, are neither uniquely suited for metal oxide dissolution, nor particularly inclined to form deep eutectic solvents, which motivated the search for alternatives (Chapter 4). Seeking a more thermally stable alternative to urea, with improved affinity to metals, several approaches were investigated, combining choline chloride with various hydrogen bond donors: imidazole (N-donor), thioureas (5-donors) and various carboxylic acids. However, the most interesting systems given their low viscosity and hydrophobic nature were prepared by completely abandoning choline chloride, and moving onto a non-ionic hydrogen bond acceptor, trioctylphosphine oxide (TOPO).
An in-depth study into the first TOPO-based eutectics is presented in Chapter 5, which combines a physical chemistry investigation into the TOPO-phenol system and preliminary studies into its applications for extraction of uranyl from aqueous media.
| Date of Award | Dec 2019 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Queen's University Ionic Liquid Laboratories (QUILL) |
| Supervisor | Gosia Swadzba-Kwasny (Supervisor) & John Holbrey (Supervisor) |