During the course of the past twenty years, the applications of contemporary ionic liquids have become increasingly widespread and varied; as solvents or catalysts for sustainable synthetic processes, as battery electrolytes, and for the dissolution and deconstruction of lignocellulosic biomass. These ionic liquid-assisted procedures frequently operate at high temperatures, therefore a comprehensive understanding of ionic liquid thermal stability is of great practical value.
It has been demonstrated that ionic liquids incorporating carboxylate anions are capable of dissolving a substantial quantity of cellulose, the world’s most abundant bio-renewable resource. The thermal stabilities of carboxylate ionic liquids are thoroughly characterised in this contribution, employing a broad range of experimental and computational methods. The impact of structural modification of the cation/anion on thermal stability is evaluated, and the prevailing thermal decomposition mechanisms are elucidated. Subsequently, the reactivity and decomposition pathways of cellulose and carbohydrate model compounds, dissolved in carboxylate ionic liquids, are uncovered.
Thermal stabilities of carboxylate ionic liquids are found to be highly sensitive to relatively modest changes in the ion chemical structure (e.g. via fluorination or substitution of sulfur into the carboxylate group), and the experimental conditions. Furthermore, the prevailing thermal decomposition mechanisms are dependant on the electronic and steric properties of the ions. Crucially, the prototypical carboxylate ionic liquid 1-ethyl-3-methylimidazolium acetate is susceptible to transient and reversible formation of an N-Heterocyclic Carbene species through abstraction of the ring C2 proton, enabled by the basicity of the anion.
This ‘non-innocent’ behaviour of carboxylate ionic liquids is critical in initiating a sequence of undesirable degradation pathways with dissolved carbohydrates, yielding imidazolium-derived adducts bearing hydroxyalkyl substituents. Strikingly, the analogous ionic liquid 1-butyl-3-methylimidazolium chloride does not initiate the unwanted series of reactions, yet is capable of dissolving a significant quantity of cellulose.