Relative and inherent reactivities of imidazolium-based ionic liquids: the implications for lignocellulose processing applications

Novel methods for the fractionation of wood, as a major renewable chemical and material feedstock, are in demand. Ionic liquids, such as 1-ethyl-3-methylimidazolium acetate ([emim][OAc]), are promoted as potential media for these processes. However, the chemical stabilities of such ionic liquids are in question as they may have an effect on process sustainability or efficiency. With anion nucleophilicity and basicity being implicated more in ionic liquid reactivity, a rough scale of the relative reactivities for [emim]-based ionic liquids is demonstrated, based upon their TGA decomposition temperatures. These values are compared to the proton affinities for the anions of those ionic liquids, as a crude measure of nucleophilicity or basicity. The implications for the temperature-dependent chemical stability of imidazolium-based ionic liquids are discussed, in regard to their interactions with wood biopolymers. It is observed that for ionic liquids with less diffuse anions (more nucleophilic or basic), such as [emim][OAc], they unfortunately become more unstable. This is exhibited by a decrease in the thermal stability and an increase in the degree of interaction with the biomass, to the point of better solvation and even covalent interactions with dissolved components. The ab initio proton affinities, dipole moments, van der Waals surface area, and volumes, are presented for an extended series of anions, commonly used in ionic liquids.