The Synthesis Of Functionalized Ionic Liquids And Mechanisms In Dissolution Of Cellulose

Cellulose,as a natural polymer,has the high-crystallinity supramolecular structure.In the cellulose processing,the poor-accessibility,weak-reactivity and low-selectivity of raw material usually resulted in decreased productivity,and generated by-products.The effective dissolution of cellulose prepares for its successful utilization.In this paper,three series of functionalized ionic liquids?ILs?were synthesized to improve the hydrophobic interaction,as well as the reactivity of hydroxyl group and hydrogen bond network structure of cellulose.Polyether-functionalized ILs,morpholinium-based ILs and choline chloride-based deep eutectic solvents?DESs?were successfully synthesized as green and cost-efficient solvents.The ILs were analized and investigated for their behaviors in dissolution of cellulose.Additionally,insights into the dissolution mechanism were investigated.?1?Polyethylene glycol?PEG?-group had stronger ability to accept hydrogen bonds owing to the alkoxy?–?CH₂–CH₂–O?–?in comparison with alkyl-group,carboxylic acid-group,ethyl acetate-group and allyl-group.The hydrogen on methine in PEG was polarized under the influence of binuclear-imidazoles structure,thus stronger polarity and hydrogen-bond donating acidity of ILs.The ILs improved the hydrophobicity of cellulose under appropriate chain lengths of PEG.Above-mentioned characteristics enabled [PEG400-?Et-Im?₂][Cl]₂ a good solvent for cellulose?13.26 m%?.In the screening of anions,OAc-contributed more hydrogen bond basicity??,1.04?than HSO₄^-,HCOO^-,Br-and Cl^-.The low viscosity of [PEG400-?Et-Im?₂][OAc]₂?63 cP,120 °C?increased the accessibility between ILs and cellulose.The highest solubility of cellulose in [PEG400-?Et-Im?₂][OAc]₂ was 14.76 m%.?2?The allyl-functionlized morpholinium-based ILs has good solubility for cellulose in comparison with alkyl-,carboxylic-,ethyl acetate-and ethermorpholinium-based ILs as well as allyl-functionlized imidazolium-and pyridinebased ILs.N-methyl-N-allyl-morpholinium ILs?[AMMor][Cl]?showed high Hammett acidity?H₀,1.73?and dipolarity/polarizability effects??*,1.09?owing to the electron-rich center of oxygen in morpholinium ring.[AMMor][Cl] was prone to act with the oxygen on hydroxyl of cellulose in the formation of C–H···O structure,thus promoting the dissolution process.OAc-contributed more hydrogen bond basicity???for allyl-functionlized morpholinium-based ILs compared to Cl^-?HCOO^- and HSO₄^-.The [AMMor][OAc] owned high ? value?0.88?,low viscosity?47 cP,110 °C?and high-conductivity?6390 ?s/cm,110 °C?,and was active to hydrogen on hydroxyl groups of cellulose to obtain high solubility?11.05 wt%?.?3?Imidazolium was the optimized hydrogen-bond donor for choline chloride-based DESs in comparison with urea,ammonium thiocyanate,caprolactam and acetamide.Choline chloride-imidazolium?ChCl-Im?showed the highest H₀?1.58?,??0.864?and ?* values,as well as highest pseudo activation energy for conduction(31.81 kJ mol^-1)and lowest pseudo activation energy for viscosity(31.76 kJ mol^-1),among the choline chloride-based DESs.The properties of ChCl-Im strengthened the ability to react with hydrogen in hydrogen bond network of cellulose.PEG,as a co-solvent,significantly enhanced the accessibility between ChCl-Im and cellulose,and broken the supramolecular structure of cellulose.The solubility of ChCl-Im coupled PEG increased by more than 80%.The compectition of protic hydroxyl group on the cation of chloride-based DES with hydron bonds in cellulose was harmful to dissolution.The substitution of the protic hydroxy group in choline cation potentially promoted the solubility.Allyl triethyl ammonia chloride-oxalic acid?[ATEAm][Cl]-Oxa?exhibited good solubility owing to the ?-? conjugative effect of allyl group.The highest solubility?8.62 wt%?for cellulose was obtained under the solubilization of dimethyl sulfoxide.?4?The morphology of cellulose showed the same change during the dissolution in [PEG400-?Et-Im?₂][OAc]₂,[AMMor][OAc],ChCl-Im and [ATEAm][Cl]-Oxa.The original cellulose was dense and ordered,and then was rough and disordered with the dissolution.The crystal form was changed from ? to ?.There was no other derivatives generated in the dissolution,and cellulose was directly dissolved.Polyether-functionalized ILs was hydrophilic,and was easy to accept the hydrogen of cellulose.The amphiphilicity of cellulose was improved by Polyether-functionalized ILs,and the accessibility between cellulose and ILs was strengthened,thus higher solubility was obtained.For the bipolar structure of morpholinium?the electron-poor center of nitrogen and electron-rich center of oxygen?,[AMMor]⁺ exhibited strong ability to donate hydrogen bonds and increased reactivity with hydroxyl of cellulose,resulting in enhanced solubility for cellulose.The well designed DESs had the appropriate hydrogen-bond network structure,and could dissolve cellulose through recombination of hydrogen bond in the ILs/cellulose system.