pH-Responsive Lignin Hydrogel for Lignin Fractionation

The efficient strategy for lignin fractionation is of great significance for its valorization into aromatics, fine chemicals, and functional materials. In this work, inspired by the traditional gel filtration and pH-responding solubility of lignin, a pH-responsive lignin-based hydrogel was fabricated via a simple cross-linking reaction and used for specific fractionation of lignin. The fraction with a higher molecular weight, more flexible linkages, aliphatic hydroxyl and carboxyl groups, and a high S/G ratio tends to entangle or aggregate with the network of the hydrogel. The pore size and water absorbency of the hydrogel can be tuned in different pH environments. As the solution switches to an acid environment, the unabsorbed fraction was precipitated completely, while the absorbed fraction was held by the shrunken pores of the hydrogel and then released to the fresh alkaline solution. This new strategy will find a new green and smart pathway for the development of not only the pH-responsive hydrogel but also lignin valorization.