Synthesis Of Pyridinium-based Ionic Liquid And Its Dissolubility Of Cellulose

The development and application of cellulose, as the most abundant biorenewableand biodegradable organic polymer on the earth, is significant to resolve the environmentpollution and energy crisis problems. Nonetheless, cellulose is neither meltable norsoluble in conventional solvents because of its molecular structural characteristic, whichgreatly limits the utilization of cellulose. Ionic liquid is called one of the three greensolvents together with water and supercritical carbon dioxide. As the cellulose solvent, ithas the advantage of feasible to be used repeatedly and less pollution to the environmentand becomes the concern of scientists in many countries. Some ionic liquids have beenreported, but most are imidazolium^-based ILs, which exist some disadvantages, such asunendurable viscosity, difficult in processing. In this work, a series of pyridinium^-basedILs are synthesized and the solubility properties of cellulose in these solvents are studied.Pyridinium^-based ILs, containing N^-ethylpyridinium cations （[EPy]+） andN^-butylpyridinium cations （[BPy]+） are synthesized by two steps with a range of anions,including HCOO^-, CH3COO^-, H₂PO₄^-, HSO₄^-, SCN^-, CF₃CO₃^<sup>-and BF4^-.Their structuresare characterized by FT^-IR and MS analyzers and used to dissolve cellulose. The resultsshow that a little cotton pulp can be dissolved in the HSO₄^-anion at70℃. The solubilityof cotton pulp is increased and the degree of polymerization （DP） of cellulose is decreaseddramatically with increasing temperature. So, they can be used as catalyst in cellulosehydrolysis reaction or the preprocessing solvent in the cellulase hydrolysis.Five types of functionalized pyridinium^-based ILs are synthesized by one^-stepmethod and characterized by FT-IR, MS and¹H NMR analyzers. The solubility propertyand degradation mechanism of cellulose are studied. The results show that1^-allyl^-3^-methylpyridinium chloride ionic liquid （[3MAPy]Cl） and N^-allylpyridiniumchloride ionic liquid （[APy]Cl） are found to be novel excellent solvents for cellulose. Thesolubilities of cotton pulp （DP=556） are11.66%and19.71%, and of absorbent cotton withhigh DP value （DP=1971） are up to7.57%and15.29%at120℃, respectively. Thethermal cracking reactions, mainly including the homolytic reaction, oxidative degradationreaction and peeling reaction, cause the decrease of DP value of the regenerated cellulosein those solvents and their reaction rates increase with increasing temperature.A co^-solvent is added into the ILs at a specific mass ratio and the absorbent cotton isactivated to resolve the aggravated thermal degradation of regenerated cellulose and thepoor dissolving ability for high DP cellulose respectively, which are the universal phenomenon for pyridinium^-based ILs as cellulose solvents. Results indicate that thedissolution temperature is reduced and the degradation behavior is alleviated and thesolubility of cellulose of high DP value is increased. The solubility of cotton pulp is8.96%and the DP value of regenerated cellulose is516with little degradation in[APy]Cl/[AMIM]Cl. The solubility of the absorbent cotton, which is activated byethylenediamine, rise from4.56%to10.06%.The FT^-IR, XRD and TGA spectrograms show that these solvents arenon^-derivatizing solvents for cellulose, the cellulose is changed from cellulose I tocellulose II during dissolution and regeneration and the thermal stability of the regeneratedcellulose decreases a little. The photographs, polarizing microscope and SEM imagesindicate that the regenerated cellulose membrane is homogeneous and dense. The possibledissolution mechanism of cellulose in ionic liquid is discussed.