Aprotic vs Protic Ionic Liquids for Lignocellulosic Biomass Pretreatment: Anion Effects, Enzymatic Hydrolysis, Solid-State NMR, Distillation, and Recycle

Imidazolium-based ionic liquids (ILs) with acetate, formate, and chloride anions have been investigated for lignocellulosic biomass pretreatment in both their (conventional) aprotic and (largely unexplored) protic forms. Notably, the protic IL 1-ethylimidazolium chloride has been demonstrated to reversibly dissolve whole wood (Pinus radiata) and significantly enhance enzymatic hydrolysability of the pretreated biomass. The acetate-based and formate-based protic IL could only extract lignin. Solid-state 13C NMR and thermogravimetric analysis demonstrate that the protic IL physically pretreats the biomass in a different manner to the aprotic ILs; however, the extent of lignin side-reactions such as methoxy group loss was dominated by the nature of the anion. The protic IL 1-ethylimidazolium chloride could be recycled by vacuum distillation and undergo several rounds of wood pretreatment followed by distillation recycle without a decrease in effectiveness.