Ionic Liquids and Their Double Salts for Dissolution and Modification of Cellulose

Abstract

Ionic liquids (ILs) are highly regarded in the scientific community due to their unique characteristics, including low vapor pressure, non-volatility, wide electrochemical potential window, and relatively low melting points below 100 ℃. Their diverse applications in physics, chemistry, and engineering, make them a prominent research focus in recent decades. Currently, double salts ionic liquids (DSILs), which are mixtures of ILs with varying cations, anions, or both, are gaining increasing attention among researchers. DSILs offer the ability to tailor their physicochemical properties and provide diverse molecular interactions, guiding the synthesis of task-specific ILs for specific applications. These properties result from various interactions, including hydrogen bonding, dipole-dipole interactions, coulombic interactions, and electron pair donor-acceptor interactions. Understanding the structural and physicochemical properties of DSILs is crucial for their potential use as solvents. In this study, 1-butyl-3-methyl imidazolium chloride ([C4mim]Cl) and 1-butyl-3-methyl imidazolium acetate ([C4mim]CH3CO2), were used for the preparation of DSILs, while maintaining a constant cation [C4mim]+ and varying the anion composition of [C4mim](CH3CO2)1-xClx (where x is the mole fraction of ILs). The structure of DSILs was investigated by utilizing ATR-FTIR, Raman, and NMR spectroscopic techniques. Furthermore, the study assesses the impact of ionic interactions on the liquid structure of DSILs through FTIR and Raman spectroscopy. To understand the adjustable characteristics of DSILs and differentiate them from conventional ILs, an investigation into their physicochemical properties is essential. In this study, temperature-dependent assessments of density, viscosity, refractive index, and conductivity were conducted from entire composition range, spanning from 30 to 70 ℃. Excess properties, including excess molar volume, excess viscosity, and excess refractive index, using appropriate models and equations were also evaluated. These analyses provide a deeper understanding of the interactions among the constituent ions [C4mim]+, Cl-, and CH3CO2- within the DSILs. For the evaluation of DSILs as material, ILs and DSILs were subjected to UV-visible absorption spectroscopy, fluorescence spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry for investigating the optical and thermal properties of the DSILs and ILs. UV-Visible absorption spectroscopy reveals that DSILs showed UV-shielding behavior. i Thermal stabilities of the DSILs are in the range of 220 to 280 ℃ and isotherm TGA reveals the enhanced stability regarding the DSILs compared to ILs at 120 ℃. Heat capacities of the DSILs increase linearly with the operation temperature and DSILs have slightly lower heat capacities compared to ILs. Finally, the glass transition temperature (Tg) of the DSILs exhibits similar behavior as the single ILs. However, Tg decreased in case of DSILs compared to ILs. In this research, for the first time DSILs was used over the whole mole fractions for the dissolution of cellulose. Due to the synergistic effect of the Cl-, and CH3CO2- ions in DSILs, the dissolution enhanced upto 32.8 wt% in [C4mim](CH3CO2)0.6Cl0.4 at 100 ℃. Cellulose was isolated from jute using kraft pulping process. It was found that [C4mim](CH3CO2)0.6Cl0.4 is able dissolve 30.5 wt % of pre-hydrolysed kraft pulp (PHKP). Cellulose was regenerated from cellulose –DSILs solution using anti-solvent water. Effect of pre-hydrolysis pulping of jute fiber was investigated for the dissolution of cellulose. PHKP showed better solubility than KP due to higher purity of PHKP. As ILs are much more expensive than other common solvents involved in the cellulose dissolution and derivatization process, recovery operations of ILs must be highly efficient to make it economically viable and sustainable. In this study, the attempts that were taken for recycling a DSIL of [C4mim](CH3CO2)0.6Cl0.4 after the dissolution of PHKP are discussed. The recycled DSIL was repeatedly use for the dissolution of fresh PHKP and regeneration of PHKP up to five times. A novel process was also developed to produce cellulose acetated (CA) from PHKP using [C4mim](CH3CO2)0.6Cl0.4. The successful acetylation was confirmed by FTIR and NMR spectroscopy. A strong band for stretching vibration of –OH of cellulose was absent and a strong band for carbonyl (C=O) group was observed in FTIR spectra suggesting successful esterification of PHKP. All cellulose, regenerated cellulose and synthesized cellulose acetate were characterized by FTIR, XRD, TGA, and SEM analysis. Finally, correlations were established with the obtained physicochemical properties of ILs and DSILs with the dissolution of cellulose.