Swelling and dissolution of cellulose in binary systems of three ionic liquids and three co-solvents

The dissolution of cellulose is a critical step for the efficient utilization of this renewable resource as a starting material for high value-added chemical and biofuel production. In this study, three aprotic solvents were chosen to couple with three ionic liquids (ILs) as binary solvent systems for cellulose dissolution. The percentage of dissolved cellulose of each IL/co-solvent was evaluated and the crystallinity index (CrI) of the undissolved cellulose residues after dissolution were investigated by XRD. Dimethyl sulfoxide exhibited the most effective synergistic interaction with the ILs among three co-solvents for cellulose dissolution. In general, the higher percentage of dissolved cellulose of the IL/co-solvent binary system, the lower CrI value of the undissolved cellulose residue after dissolution. In addition, the dissolution of cellulosic materials with different crystallinity and degree of polymerization indicated that the crystallinity of a cellulosic material played a more dominating role than degree of polymerization in its dissolution process. The hydrogen bond basicity (β value) of selected IL/co-solvent binary system was also calculated. The results showed that the β value of a binary IL/co-solvent system exhibited different trend than neat IL in terms of the cellulose dissolution ability, which should be attributed to the co-solvent effect.