Ionic liquids are seeing use as solvents and catalysts towards sustainable chemical processes. A challenge for implementation is their relatively high solvent cost which can be reduced by designing lower cost ionic liquids and ensuring they are recyclable. Here, low-cost protic and aprotic ionic liquids, particularly those with the hydrogen sulfate anion, are investigated for their thermal stability. These are then demonstrated in two applications.
The thermal stability of hydrogen sulfate ionic liquids was investigated showing thermal stability to be independent of proticity. Mass loss contributions for hydrogen sulfate ionic liquids were monitored for protic and aprotic analogues and Arrhenius kinetic analysis used to distinguish between contributions from evaporation and chemical decomposition. The vapour phase over the intact and decomposing ionic liquids was investigated with several mass spectrometric techniques. The vapour for protic ionic liquids was mostly composed of molecular amines and degradation products via SN2 decomposition whilst aprotic analogues only formed decomposition products, also via SN2 decomposition. Both protic and aprotic analogues showed vaporisation of neutral ion pairs when heated under vacuum.
Hydrogen sulfate ionic liquids were included in a screening of ionic liquids to investigate the impact of anion and cation selection in the chemical recycling of poly(ethylene terephthalate) via hydrolysis. Ionic liquids with the acetate anion showed greatest efficiency towards depolymerisation and even potential for reuse of both the protic and aprotic acetate ionic liquids.
Hydrogen sulfate ionic liquids were instead employed as a dissolution solvent for production of scalable lignin-derived carbon fibres. Formulated solutions of an industrial Kraft lignin and poly(vinyl alcohol) showed complete dissolution of lignin in the protic ionic liquid with a consistent rheology forming a high-quality lignin fibre which was collected as a single long fibre on a take-up swift. The lignin fibres were thermostabilised and carbonised. The lignin-derived carbon fibre mechanical performance remains uncompetitive against existing commercial carbon fibres, however the developed continuous wet-spinning methodology can be tuned to improve end fibre performance.