Efficient and sustainable solvents for lignin dissolution: aqueous choline carboxylate solutions

Abstract

Lignin is a promising alternative to fossil resources for the production of fuels, biocomposites and value-added chemicals. To better utilize this kind of natural resource, it is still necessary to develop efficient lignin solvents. As such, 13 kinds of novel lignin solvents have been developed, in this work, by the addition of water into bio-derived choline carboxylate ionic liquids (ILs). The solubility of lignin has been determined at 25 °C as a function of IL contents. The effect of the anionic structure of the ILs on lignin solubility is systematically examined. It is found that among these solvents, six types of ILs exhibit highly efficient capacity for lignin dissolution at room temperature. The solubility of lignin increases with increasing alkyl chain length in the carboxylate anions, and a substitution of H in carboxylate anions by the OH or NH₂ group as well as the use of choline di-/tri-carboxylates leads to the decrease of lignin solubility, or even makes the lignin insoluble in the solvents. From NMR measurements, these results have been explained by the strong hydration of the carboxyl group of the anions and the possible interaction of alkyl chains of the anions and cations with lignin. In addition, the dissolved lignin can be regenerated by addition of additional water, and no chemical reaction takes place during the dissolution and regeneration processes.