Developing and understanding lignin dissolving ionic liquids for sustainable carbon fibre production from lignin

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Ionic liquids (ILs) are increasingly popular alternative solvents for many processes, due to their tuneable physicochemical properties.1 They are utilised in catalysis,2 electrodeposition3,4 and biomass valorisation, in particular, in the high yielding dissolution and recovery of lignin from wood biomass. In order to enable a larger numbers of viable industrial processes utilising ionic liquids, both cost and safety need to be assessed for each ionic liquid that shows promising performance. While certain ionic liquids are known to be able to withstand high-temperature processes, resulting in high recyclability in principle, they are often expensive to synthesise. A number of ILs have been shown to have high solubility for lignin, with water being an inexpensive antisolvent, which makes these ILs interesting solvents for isolating lignin from biomass and for use in lignin purification and in fibre formation. Recently reported protic hydrogen sulfate ionic liquids have shown particular promising potential as industrial candidates for wood fractionation, with bulk solvent cost as low as $1.87 kg-1.6,7 However, the thermal properties of these ILs are poorly understood. Hence, one aim of this project is to produce comprehensive kinetic and thermal data regarding the stability and decomposition mechanisms of lignin dissolving low-cost ILs, and the vapour phase composition upon degradation of these ionic liquids will be studied. A second aim of the project is to produce lignin dissolving ionic liquids that display the anisotropic behaviour of liquid crystals phases at temperatures of interest. It is the hypothesis that the unsatisfactory performance of lignin based carbon fibres to date is the disordered arrangement of the lignin polymers in the precursor fibres, which appears to limit the formation of ordered graphitic domains during carbonisation and hence the strength and stiffness of resulting carbon fibres. Using isotropic hydrogen sulfate ionic liquids as well as the novel ionic liquid crystals, the project will investigate whether either type of ionic liquid can help produce more ordered precursor fibres, which should result in stronger, cheaper, more sustainable carbon fibres. The ILs will be used to modify the lignin either as part of the extraction or as a post-extraction purification step. As a third aim, to enable fibre production, the project will develop a heated microextruder, which can be used for melt-spinning lignin fibres or for hot wet-sinning of lignin fibres at a very small (<1 g) scale, with the heat helping to reduce the viscosity of the spinning dope, which increases polymer loading in the ionic liquid dopes and hence improving processing economics.