Effects of anionic structure and lithium salts addition on the dissolution of cellulose in 1-butyl-3-methylimidazolium-based ionic liquid solvent systems

Abstract

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 [C4mim]+ with the Bronsted basic anions [CH3COO]−, [HSCH2COO]−, [HCOO]−, [(C6H5]COO]−, [H2NCH2COO]−, [HOCH2COO]−, [CH3CHOHCOO]− and [N(CN)2]−. The 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 1H 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 [C4mim][CH3COO]/lithium salt (LiCl, LiBr, LiAc, LiNO3, or LiClO4) solvent systems have been developed by adding 1.0 wt% of lithium salt into [C4mim][CH3COO]. It was shown that the addition of lithium salts significantly increased the solubility of the cellulose. This observation was studied by 13C 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.