Investigation On Lignin Degradation Based On Ionic Liquids

Lignin is the only rich and renewable aromatic resource on the planet.It is difficult for conventional solvents to effectively dissolve/degrade due to its complex structure and strong interactions between basic units,leading to its underutilization.Acetate based ionic liquid（IL）is widely used in the process of lignin dissolution and conversion,nevertheless it always has poorer thermal stability.It is also very crucial for the lignin conversion to better understand the interactions of solvents between each other as well as the interactions between solvent and solute.In this paper,it was aimed at the issues of the poor thermal stability of acetate-based ILs,unclear mechanism of lignin dissolution and conversion and the efficient conversion of lignin in ILs.The interactions between two ILs each other as well as the interactions between PILs andβ-O-4 lignin dimer was mainly investigated,and the lignin degradation in ILs was also studied.The main points of this work are as follows:（1）In terms of the problem of the poor thermal stability of acetate-based ILs,the effect of the interactions between ILs on the thermal stability of acetate ILs was investigated.The thermal stability of[C₈C₁Im][OAc]was significantly improved by adding the[C₄C₁Im][NTf₂]to[C₈C₁Im][OAc].And it is found that the temperature of mass loss at 5%(T_5%)increased by43°C when the molar fraction was 0.3（where the molar ration of[C₈C₁Im][OAc]to[C₄C₁Im][NTf₂]is around 1:2）in the binary IL system.Then the interactions between the two ILs with different proportions were investigated by spectroscopic methods.And the results results suggest that the[C₄C₁Im][NTf₂]IL makes the anion-π⁺structure formed by the hydrogen bond interaction between the two ILs.And the improved thermal stability of[C₈C₁Im][OAc]is due to the synergistic effect between the hydrogen bond and electrostatic interaction in the binary system.（2）Aiming at the unclear mechanism of lignin dissolution,acids（acetate acid（HOAc）,lactic acid（HLA）and oxalic acid（HOA））and bases(monoethanolamine（EOA）,Ethanolamine（DEOA）and triethanolamine（TEOA）were used as precursors,and a series of different types of low-cost protic ionic liquids（PILs）were designed and synthesized.And the solubility of enzymatic lignin was tested at different temperatures in different PILs.The results indicate that the monoethanolamine acetate（[EOA][OAc]）IL has the best solubility for lignin,which can dissolve 35 wt%of the enzymatic lignin at 90°C;Then,the interaction between the PIL and lignin dimer（2-phenoxy-1-phenylethanol,pp-ol）at different molar ratios were studied by spectroscopic methods.The results showed that the effects of anions and cations of PILs on the solubility of lignin follow the order:[OAc]^->[LA]^->[OA]^-;[EOA]⁺>[DEOA]⁺>[TEOA]⁺.It also revealed that the hydrogen bond interaction between the cation of PIL and lignin and the electrostatic interaction between the anion of PIL and benzene ring synergistically activate lignin and promoted the dissolution of lignin.（3）Lignin degradation were performed in the synthetic acetate PILs（[EOA][OAc],[DEOA][OAc],and[TEOA][OAc]）.The degradation of lignin with different catalysts were investigated at different temperatures,and the degradation products were analyzed and tested.The results indicated that phenol,styrylphenol,dibutyl phthalate and other aromatic compounds were formed under the above conditions.