Substituent effects on cellulose dissolution in imidazolium-based ionic liquids

Abstract

The dissolution of cotton cellulose with ionic liquids (ILs) has been extensively studied. However, the mechanism of cellulose dissolution, especially the role of the IL cation in the dissolution process, is not well understood. This paper describes a systematic study of the effects of the substituent groups on the cation in imidazolium-based ILs on cellulose dissolution. A series of imidazolium-based ILs with acetate as the anion, 1-hepyl-3-methylimidazolium acetate ([C7C1im][OAc]), 1-(cyclohexylmethyl)-3-methylimidazolium acetate ([CyhmC1im][OAc]), 1-benzyl-3-methylimidazolium acetate ([BnzC1im][OAc]), 1,3-dibenzylimidazolium acetate ([(Bnz)2im][OAc]), and 1-(2-napthylmethyl)-3-methylimidazolium acetate ([NapmC1im][OAc]) were synthesized. In each dissolution experiment, 5% (w/w) ground cotton fiber was dissolved in the ILs at 90 °C. The progress of the dissolution was monitored periodically with a polarized light microscope. This study revealed that [BnzC1im][OAc] dissolved cotton cellulose more efficiently than the other four ILs. The results are discussed within the context of previous published theoretical and experimental studies on cellulose dissolution in ILs. For the five ILs that were investigated, we find that the effect of the cation can be rationalized on the basis of both the size and shape of the cation. In addition to the dissolution, cellulose was regenerated and characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM).