Effects of Cationic Structure on Cellulose Dissolution in Ionic Liquids: A Molecular Dynamics Study

In recent years, great progress has been made in the dissolution of cellulose with ionic liquids (ILs). However, the mechanism of cellulose dissolution, especially the role the IL cation played in the dissolution process, has not been clearly understood. Herein, the mixtures of cellulose with a series of imidazolium-based chloride ionic liquids and 1-butyl-3-methyl pyridinium chloride ([C4mpy]Cl) were simulated to study the effect that varying the heterocyclic structure and alkyl chain length of the IL cation has on the dissolution of cellulose. It was shown that the dissolution of cellulose in [C4mpy]Cl is better than that in [C4mim]Cl. For imidazolium-based ILs, the shorter the alkyl chain is, the higher the solubility will be. In addition, an all-atom force field for 1-allyl-3-methyl imidazolium cation ([Amim]+) was developed, for the first time, to investigate the effect the electron-withdrawing group within the alkyl chain of the IL cation has on the dissolution of cellulose. It was found that the interaction energy between [Amim]+ and cellulose was greater than that between [C3mim]+ and cellulose, indicating that the presence of electron-withdrawing group in alkyl chain of the cation enhanced the interaction between the cation and cellulose due to the increase of electronegativity of the cations. These findings are used to assess the cationic effect on the dissolution of cellulose in ILs. They are also expected to be important for rational design of novel ILs for efficient dissolution of cellulose.