Ionic Liquids and Electrolytes for Cellulose Dissolution

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Title: Ionic Liquids and Electrolytes for Cellulose Dissolution
Author: Holding, Ashley John
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Publisher: Helsingin yliopisto
Date: 2016-12-02
Thesis level: Doctoral dissertation (article-based)
Abstract: In this thesis, the synthesis and application of tetraalkylphosphonium-based ionic liquids towards the dissolution of cellulose (and lignocellulose) is explored. Ionic liquids were synthesised from trialkylphosphines by quaternisation with alkyl halides or dimethylcarbonate and subsequent anion exchange reactions. The ionic liquids were used to dissolve lignin, and were found only to dissolve cellulose upon addition of a polar aprotic molecular solvent, such as DMSO (dimethylsulfoxide). The cellulose dissolution capabilities of a range of these phosphonium ionic liquids in combination with DMSO was studied. It was found that these organic electrolyte solutions were very effective solvents for cellulose, with a high molar dissolution capacity. At the greatest extent, only one mole of ionic liquid per glucose units in cellulose is needed to dissolve cellulose. The role of the cation and anion in the dissolution process is explored, with the aid of solvent parametisation techniques and NMR studies. Other solvents, including GVL (gamma-valerolactone), were explored as greener replacements for DMSO. For the shorter chain phosphonium ionic liquids with DMSO and GVL, upper critical solution temperature behaviour was observed and explored in more detail. In these solutions, cellulose is only soluble at high temperatures, and reforms at low temperatures to form a gel with a spherical micro-particle morphology. The phase behaviour of hydrophobic phosphonium ionic liquids, DMSO, and water was studied and applied to the recovery of the ionic liquid after cellulose dissolution in the electrolyte solutions. Ternary phase diagrams of three of the hydrophobic ionic liquids in combination with DMSO and water were constructed. Finally, phosphonium ionic liquid and deuterated DMSO electrolytes were studied and successfully used for the NMR analysis of high molecular weight cellulose materials. Future work in this area is expected to focus further on the theoretical understanding of cellulose dissolution in phosphonium ionic liquid-based organic electrolyte solutions - with expanded NMR measurements, and other experimental techniques, in tandem with molecular dynamics modelling. Additionally, it is expected that techniques for the solution-state NMR of cellulose will be applied at extended range of analytes, including but not limited to, whole biomass, modified and unmodified nano-celluloses, and high molecular weight cellulose derivatives. The thermo-responsive behaviour (UCST-type) phase-separation of cellulose will continue to be explored especially in its application to new materials, included fibres and shaped spherical particles.
Subject: chemistry
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