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Experimental and theoretical studies on dissolution and regeneration of microcrystalline cellulose with dihydroxyl ionic liquids

Fahui Li,Yiyue Shen,Yupei Deng,Jingchun Zhang,Qiqi Wang,Haoyuan Chen,Mai Ouyang,Qianwen Jiang,Guowei Wang,Linghua Zhuang

Ionic liquids (ILs) have emerged as a promising class of green solvents suitable for cellulose dissolution. In this study, two dihydroxyl-functionalized ionic liquids, 1-methyl-3-(2,3-dihydroxypropyl)-imidazolium chloride ([DPmim]Cl) and 1-butyl-3-(2,3-dihydroxypropyl)-imidazolium chloride ([DPbim]Cl), were prepared for microcrystalline cellulose (MCC) dissolution. The physical properties of [DPmim]Cl and [DPbim]Cl, such as relative viscosity, density, electrical conductivity, and Kamlet-Taft parameters (β and π*), were evaluated. The dissolution and regeneration properties of MCC in [DPmim]Cl and [DPbim]Cl were investigated. The regenerated MCC was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Density functional theory (DFT) simulations were used to investigate the interactions between ionic liquid and cellobiose. [DPmim]Cl and [DPbim]Cl, with higher β-values (0.975 and 1.098) respectively, exhibited stronger hydrogen bonding alkalinity. This increased alkalinity enhanced their ability to accept hydrogen bonds from cellulose, which in turn weakened the inter- and intra-molecular hydrogen bonds within cellulose. As a result, the dissolution of MCC was promoted. The dissolution of MCC in [DPmim]Cl was faster than [DPbim]Cl at same temperature. FTIR, TGA, and XRD characterizations showed that the hydrogen bonding network of microcrystalline cellulose was disrupted during ionic liquid dissolution. DFT simulations unveiled the formation of strong hydrogen bonds between the hydroxyl groups of ILs and the hydroxyl or ether linkages in cellulose. These hydrogen bonds have been identified as a potential crucial factor in facilitating the dissolution of cellulose.