A Simple Regeneration Process Using a CO2-switchable Solvent for Cellulose Hydrogels

Cellulose is one of the main components of plant cell walls, abundant on earth, and is a non-edible material that can be acquired at a low cost. Furthermore, there has been increasing interest in its use in environmentally friendly, carbon-neutral, sustainable materials. It is expected that the applications of cellulose will expand with the development of a simple processing method. Previously, it was demonstrated that cellulose can be dissolved in a non-heated, short-duration process using an aqueous pyrrolidinium hydroxide solution. In this study, we dissolved cellulose in aqueous N-butyl-N-methylpyrrolidinium hydroxide solution ([C4mpyr][OH]/H2O) and investigated the cellulose regeneration process based on changes in solubility upon application of CO2 gas. We investigated the effect of transformation of the anion chemical structure on cellulose solubility by flowing CO2 gas into [C4mpyr][OH]/H2O and conducted pH, FT-IR, and 13C NMR measurements. We observed that the changes in anion structure allowed for the modulation of cellulose solubility in [C4mpyr][OH]/H2O, thus establishing a simple and safe cellulose regeneration process. This regeneration process was also applied to enable the production of cellulose hydrogels. The hydrogel formed using this approach was revealed to be of a higher mechanical strength than that of an analogous hydrogel produced using the same dissolution solvent with addition of a cross-linker. The ability to produce cellulose-based hydrogels of different mechanical properties is expected to expand the possible applications.