Enhancing the stability of ionic liquid media for cellulose processing: Acetal protection or carbene suppression?

Abstract

Although excellent candidate solvents for cellulose, capable of dissolving ≥20 wt% of the carbohydrate for electrospinning processes, dialkylimidazolium carboxylate ionic liquids undergo undesirable side reactions with the reducing end of saccharides, terminating in an equilibrium concentration of a 2-(hydroxymethyl)-substituted imidazolium 'adduct'. The addition of small molar quantities of a benign, non-toxic and inexpensive co-solvent, e.g. glycerol, reduces the rate of adduct accumulation, thereby enhancing the long-term thermal stability and recyclability of the expensive ionic liquid component. NMR, UV-vis and mass spectrometry experiments reveal that the improved stability is likely attributable to suppression of the transient dialkylimidazol-2-ylidene carbene, via hydrogen-donation by the protic co-solvent, rather than by cyclic acetal protection of the carbohydrate. The incorporation of (up to) 10 wt% of glycerol into the solvent mixture does not exacerbate the rate of cellulose depolymerisation compared to in the neat ionic liquid, and high solubility of cellulose is maintained. Furthermore, a colourimetric comparison of the recovered solvents, following cellulose re-precipitation, demonstrates that glycerol does not increase the concentration of contaminant reducing sugars in the organic electrolyte.