Enhanced Dissolution of Cotton Cellulose in 1-Allyl-3-methylimidazolium Chloride by the Addition of Metal Chlorides

Effective dissolution of cellulose with ionic liquids (ILs) allows the high-value utilization of cellulose. However, cotton cellulose (CC) with a high degree of polymerization inherently has a low solubility, and long-term heating of ILs may adversely influence their structure. Herein, a series of metal chlorides (ZnCl2, LiCl, or NaCl) were mixed with AmimCl to improve the dissolution capacity for CC and avoid thermal degradation of ILs. The dissolution mechanism was explored by density functional theory calculations and multiple spectroscopic techniques. The addition of ZnCl2 effectively reduced the dissolution time of 2 wt % CC (degree of polymerization (DP) = 796) from 52 to 5 min under magnetic stirring, and increased the maximum solubility of CC in AmimCl by 1.37 times at 80 °C. Systematic investigations revealed that the reason for enhanced dissolution was that in addition to the more significant role of Cl–, interaction of metal cations with the hydroxyl oxygen (O3) of cellulose also destroyed the intermolecular hydrogen bond (O6H···O3). Furthermore, the regenerated cellulose (RC) maintained core structure and sufficient thermostability, and the RC films exhibited smooth morphology and high mechanical strength. The findings in this work are expected to open up new avenues in the development of cellulose solvents for a broad variety of applications in conversion of cellulose products.