Molecular insight into cellulose regeneration from a cellulose/ionic liquid mixture: effects of water concentration and temperature

Cellulose regeneration from a cellulose/ionic liquid (IL) mixture is investigated using molecular simulation. The IL considered is 1-n-butyl-3-methylimidazolium acetate ([BMIM][Ac]). Water is added as an anti-solvent into the cellulose/[BMIM][Ac] mixture to regenerate cellulose. The simulated structural properties demonstrate that cellulose interacts more strongly with the anion [Ac](-) than with the cation [BMIM](+). With increasing water concentration, the cellulose-[Ac] interaction strength diminishes. The addition of water leads to the destruction of the cellulose-[Ac](-) hydrogen-bonds (H-bonds), and the subsequent formation of cellulose-cellulose and [Ac](-)-water H-bonds. On this basis, a mechanism for cellulose regeneration is proposed. The torsional angle distributions of hydroxymethyl groups in regenerated cellulose chains are substantially different from those in cellulose crystals, implying that the regenerated cellulose is amorphous, as is also observed in the experiment. Furthermore, the effect of temperature on regeneration is investigated. At a higher temperature, the cellulose-cellulose interaction is enhanced and regeneration is increased. On a microscopic level, this simulation study provides a useful insight into the structural and energetic properties in cellulose/[BMIM][Ac]/water mixtures, and reveals that H-bonding is the key factor governing cellulose regeneration.