Dissolution feature of microcrystalline cellulose with hydroxyl-containing ionic liquids: experimental and theoretical studies

Two hydroxyl-containing ionic liquids (HCILs) were synthesized and used for dissolving microcrystalline cellulose (MCC). Two HCILs were [MPmim][Cl] (1-methyl-3-(3-hydroxypropyl)-imidazolium chloride) and [MPbim][Cl] (1-butyl-3-(3-hydroxypropyl)-imidazolium chloride). Two hydroxyl-containing ionic liquids were studied with nuclear magnetic resonance ( 1 H NMR and 13 C NMR), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and elemental analysis. Solvatochromic parameters ( β and π * values) of two HCILs were studied. MCC dissolution performance was evaluated and the regenerated MCC was characterized. Density functional theory (DFT) calculations were employed to gain insights into dissolution mechanism of MCC in [MPmim][Cl] or [MPbim][Cl]. [MPmim][Cl] and MPbim][Cl] demonstrated approximately same β value (0.72, or 0.73), while π * value of [MPmim][Cl] ( π * = 1.11) was slightly higher than that of [MPbim][Cl] ( π * = 1.02). At same temperature, MCC dissolution time in HCILs shortened from butyl group ([MPbim][Cl]) to methyl group ([MPmim][Cl]). DFT calculations revealed that hydroxyl group in [MPmim][Cl] or [MPbim][Cl] formed strong hydrogen bonds with hydroxyl groups of cellobiose. The strong hydrogen bonds were formed between cellobiose and imidazolium cation, as O44-H45…O67 (1.777 Å) and O67-H68…O42 (2.022 Å), which weakened the intermolecular hydrogen bonding network of MCC, thus improving the MCC dissolution process. Characterization of regenerated MCC confirmed that the highly crystalline cellulose transformed into more amorphous form during the ionic liquid dissolution process. The results will shed light on preparation and selection of functional ionic liquids for cellulose dissolution and broaden industrial application of cellulose materials.