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Cellulose is the most abundant biorenewable and biodegradable resource on the earth. However, the extent of its application is limited due to its inefficient dissolution in solvents. Thus, the development of new cellulose solvents continues to be an active area of investigation. In this work, a series of ionic liquids (ILs) have been synthesized by coupling the 1-N-butyl-3-methylimidazolium cation [C 4 mim] + with the Brønsted basic anionsThe solubilities of microcrystalline cellulose (MCC) in these ionic liquids were determined as a function of temperature. The effect of the anion structure on the solubility of cellulose has been estimated, and investigated by 1 H NMR and a solvatochromic UV/vis probe. It was found that the solubility of cellulose increases almost linearly with increasing hydrogen bond accepting ability of anions in the ionic liquids. At the same time, novel [C 4 mim][CH 3 COO]/lithium salt (LiCl, LiBr, LiAc, LiNO 3 , or LiClO 4 ) solvent systems have been developed by adding 1.0 wt% of lithium salt into [C 4 mim][CH 3 COO]. It was shown that the addition of lithium salts significantly increased the solubility of the cellulose. This observation was studied by 13 C NMR spectra, and the results suggested that the enhanced solubility of cellulose originated from the disruption of the intermolecular hydrogen bond, O(6)H ◊ ◊ ◊ O(3) owing to the interaction of Li + with the hydroxyl oxygen O(3) of cellulose. Furthermore, the cellulose materials regenerated from the ionic liquids were characterized by scanning electron micrograph, thermogravimetric analysis and Fourier transform infrared spectroscopy, and the degree of polymerization of the original and regenerated cellulose materials was also determined. Good thermal stability was found for the regenerated cellulose. It is expected that the above information is useful for the design of novel ionic liquids and ionic liquid-based solvent systems for cellulose.

As a kind of promising solvents, ionic liquids (ILs) have been used to dissolve cellulose and great progress has been made in recent years. However, the dissolution mechanism, especially the role of cations of ILs in the dissolution of cellulose, is still in debate. In this work, 13 kinds of ILs with a fixed anion [CH 3 COO] − but varied cationic backbones and alkyl chains have been prepared and characterized. The solubilities of cellulose in these ILs were measured at different temperatures. This allowed us to systematically study the effect of cationic structures on the cellulose dissolution at a given temperature. In order to investigate the dissolution mechanism, Kamlet-Taft parameters of these ILs in the temperature range from 25 to 65 °C and 13 C NMR spectra of 1-benzyl-3-methylimidazolium acetate ([phC 1 mim][CH 3 COO]) + cellulose systems at 90 °C were also determined. It was found that acidic protons on the heterocyclic rings of the cations are essential for the dissolution of cellulose in the ILs, but the van der Waals interaction of cation with cellulose is not important. These protons may form C-H⋯O hydrogen bonds with hydroxyl and ether oxygen of cellulose to increase cellulose solubility. Cations of the ILs may also decrease cellulose solubility by strong interaction with anions or steric hindrance effect of large size group in their alkyl chains. These interactions together with strong O-H⋯O hydrogen bonds between the anion and hydroxyl protons of cellulose resulted in the disruption of the inter-and intra-molecular hydrogen bonds and thus effective dissolution of cellulose. † Electronic supplementary information (ESI) available. See

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