Cation does matter: how cationic structure affects the dissolution of cellulose in ionic liquids

Abstract

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 [CH₃COO]⁻ 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 ¹³C NMR spectra of 1-benzyl-3-methylimidazolium acetate ([phC₁mim][CH₃COO]) + 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.