Levulinate amidinium protic ionic liquids (PILs) as suitable media for the dissolution and levulination of cellulose

Abstract

Two new levulinate-based protic ionic liquids (Lev PILs) have been synthesized through an easy, scalable neutralization reaction between levulinic acid (LA), which is one of the most promising renewable building blocks obtainable from cellulose, and amidine superbases (either DBU or DBN). The prepared PILs have been characterized (by means of NMR, FT-IR, TGA and viscosity measurements) and studied as potential cellulose dissolution media. These species display a dissolving ability (weight_cellulose/weight_PIL) comparable with the popular, and to date best performing, acetate-based PILs. In contrast with these systems, Lev PILs are composed of a larger renewable anion (levulinate vs. acetate): this allows for a reduced amount of the expensive non-renewable superbase cation component while only slightly affecting the cellulose dissolution performance. An application of the proposed Lev PILs, namely the levulination of cellulose, has also been studied. The impact of various reaction parameters (i.e. temperature, amount of anhydride, amount of co-solvent) on the reaction outcome and on the functionalization degree (up to 1.87) has been studied. γ-Valerolactone, a renewable green solvent which is in turn prepared from LA, has been found as an effective DMSO replacement when used as co-solvent for the reaction, allowing for satisfactory functionalization degrees.