Studies On Lignin Depolymerization In Double Salt Ionic Liquids

With the increase in the demand for non-renewable resources such as coal and oil,the problem of environmental pollution has become more serious.To prevent the depletion of natural resources and improve the natural environment,it has been a pretty hot topic in social research to find the sustainable alternatives to coal and petroleum to meet the needs of today’s rapid development of society.As an recyclable aromatic polymer compound rich in content and the only biomass resource which has abundant benzene ring,lignin is particularly important for its high value utilization.The“environmentally friendly solvent”functionalized ionic liquid has the advantages of structural designable,good solubility of lignin,and recyclability.Therefore,it has attracted more and more attention in the process of lignin separation and depolymerization.The double salt ionic liquids is a composite system composed of two or more ionic liquids,which can finely control the physicochemical properties of the system and further expand the application of ionic liquids.In this work,based on the ionic liquid（neutral or acidic）containing different anions and cations,a variety of double salt ionic liquids systems were constructed.Using it as a solvent and catalysts,we studied the lignin depolymerization process and then explored the mechanism of interaction from macro and micro levels.The main results are as follows:（1）Lignin depolymerization:Using enzymatic lignin as raw material,we investigated the effects of ionic liquid species,ionic liquid molar ratio,liquid-solid ratio,temperature,time and reaction atmosphere on the process of lignin depolymerization.The results shows that the lignin liquefaction rate is the highest（48.9%）in the[BMIm]Cl/[BSO₃HMIm][HSO₄]with a molar ratio of 5:1 at a liquid-solid ratio of 20:1,150 ^oC and a treatment time of 3 h.From the infrared and nuclear magnetic characterization results of lignin,it can be seen that the liquefaction rate is significantly reduced when the content of[BSO₃HMIm][HSO₄]is large（for example,the molar ratio of n（IL1）:n（IL2）=1:1 or 0:1）due to the large degree of lignin polycondensation.When H₂O₂ was used as the oxidizing agent,even 3wt%of H₂O₂ caused the ionic liquid to be oxidized.The liquefaction rate of lignin did not change significantly when air was used as the reaction atmosphere.In general,functionalized double salt ionic liquids exist synergistic effects during lignin depolymerization.The liquefaction rate of lignin in the double salt ionic liquids is significantly higher than that in the single ionic liquid,which is related to the interaction between anions and cations.（2）The physicochemical properties of the double salt ionic liquids:With the decrease in the molar ratio of 1-butyl-3-methylimidazolium chloride（[BMIm]Cl）/1-butylsulfonate-3-methylimidazolium hydrogen sulfate,the thermal stability of the double salt ionic liquids first increases and then decreases.The double salt ionic liquids has best thermal stability(T_onset=340 ^oC)when the molar ratio of[BMIm]Cl/[BSO₃HMIm][HSO₄]is 1/1.This is mainly due to the interaction between anions and cations of ionic liquids（hydrogen bonds,coulomb forces,etc.）.As the amount of[BSO₃HMIm][HSO₄]increases,the Hammett acid value of the double salt ionic liquid decreases,and the acidity of the double salt ionic liquid increases.Attenuated total reflection infrared spectroscopy（ATR-IR）and nuclear magnetic resonance（NMR）were used to analyze the interaction between anions and cations.The molar ratio of two ionic liquids affects the ability of anions to form hydrogen bonds with C2,C4 and C5-H in double salt ionic liquids systems.The nuclear magnetic characterization analysis shows that the microscopic environment of the system changes with different amounts of[BSO₃HMIm][HSO₄].The changes in the interaction and microenvironment between ionic liquid and lignin may affect the mechanism of linkage bond cleavage in the macromolecular structure of lignin during degradation reactions.