Study On The Depolymerization Of Lignin In Deep Eutectic Solvents And The Application Of Degradation Products

Lignin is the most abundant aromatic biomass resource in nature and the main by-product of paper industry and biorefinery industry,which is renewable,low cost and abundant source.Industrial lignin is mostly treated as fuel,mainly because of the heterogeneity of lignin molecular structure and differences in properties,which affect the further high-value conversion and utilization of lignin.Lignin degradation is one of the effective means to achieve high value lignin applications,but degradation usually requires the use of high temperatures and harsh reaction conditions.Deep eutectic solvent（DES）is a safe,efficient and green catalyst and reaction system,and the treatment process does not produce excessive harmful substances.Through degradation,oligomers or small molecule compounds with reactive groups can be obtained,and the preparation of high value-added products is one of the effective ways to utilize lignin.In this study,we selected a series of DES degradation systems based on alkali lignin,selected DES systems for efficient degradation of alkali lignin,prepared lignin-based phenolic resins using degraded small molecules,deciphered the reaction mechanism of lignin-based phenolic resins,and carried out research on the development and application of lignin in the field of electrode materials.The specific research contents and results are:（1）Different kinds of DES degraded lignin were prepared from alkaline lignin with choline chloride as the hydrogen bond acceptor and lactic acid,urea,ethylene glycol,glycerol,formic acid,acetic acid,mixed acid of formic acid and acetic acid,oxalic acid and p-toluenesulfonic acid as the hydrogen bond donors.The phenolic hydroxyl content of lignin before and after degradation was compared by ¹H NMR spectroscopy,the changes of lignin functional groups during DES treatment were observed by Fourier transform infrared spectroscopy（FT-IR）,and the molecular weight of lignin before and after degradation was analyzed by gel permeation chromatography（GPC）.The results showed that the molecular weight M_Wof regenerated lignin decreased significantly（2478-4330 g/mol）,the phenolic hydroxyl content increased significantly,the acidic p-toluenesulfonic acid-choline chloride DES system was more effective for alkaline lignin depolymerization,and the lignin degradation was mainly based on ether bond breaking.The solvent recovery of lignin DES of the nine treatments reached more than 80%.（2）The depolymerized lignin TC-lignin and LC-lignin obtained from two different acidic DES treatments of lignin,and the structures of lignin were characterized by GPC and FT-IR.The results showed that the acidic DES-treated alkaline lignin products had relatively low and homogeneous molecular weight,high phenolic hydroxyl content,and TC-lignin was 6.81 mmol/g.The conformational relationship between the solubilized lignin and antioxidant activity was investigated.The lignin scavenging rate of DPPH radicals was ranked as follows:TC-lignin,90.35%>LC-lignin,89.12%>commercially available antioxidant BHT,88.79%.TC-lignin with low molecular weight and high phenolic hydroxyl content showed the highest antioxidant activity,and the possible mechanism of lignin antioxidant response was analyzed with the help of lignin model substances.Compared with the raw alkali lignin,LC-lignin and TC-lignin both possessed efficient UV-blocking ability.The lignin-based phenolic resin was prepared by replacing part of phenol with the depolymerized lignin monomer,and the structure of phenolic resin was characterized by FTIR,GPC,TG,DSC,¹H NMR,¹³C NMR,¹H-¹³C NMR,etc.The possible reactions in the intermediate process of phenolic resin reaction were simulated by using density flooding theory（DFT）with the help of lignin model substances and the reaction of phenol with formaldehyde.The reaction mechanism of lignin-based phenolic resins was proposed,and the results of the study provided a theoretical basis for the optimization of the structure of lignin-based resins.（3）Porous carbon materials were prepared by two-step carbonization using lignin-based phenolic resin as the carbon source.The morphological composition of the carbon was examined by SEM,TEM,BET,XRD,TGA,XPS and Raman spectroscopy,and the electrochemical properties of the samples were tested.The results showed that the specific surface area of the lignin-based phenolic resin was significantly higher compared to that of the pure phenolic resin carbon,and the resulting porous carbon had a specific surface area of up to 1025.8 m²/g.Applied to supercapacitor electrodes,the porous carbon exhibited a specific capacitance of 112.4 F/g at0.5 A/g.In addition,it exhibits high cycling stability with a capacitance retention of 98.8%over5000 charge/discharge times.The assembled symmetrical supercapacitor has high energy and power densities of 3.9 Wh/kg and 125 W/kg.The results show that the lignin-derived phenolic resin provides a new idea of porous carbon for high-performance supercapacitor electrodes.（4）Lignin-based phenolic resin was used as the carbon source and silicon nanopowder as the silicon source.The lignin-based phenolic resin was coated on the surface of silicon nanoparticles,and the carbon layer was obtained by charring the outer layer of the resin through high temperature calcination.The morphological composition of carbon and silicon was examined by SEM,TEM,XRD,TGA,XPS and Raman spectroscopy,and the carbon layer was found to be uniformly encapsulated on the silicon nanoparticle surface,and the electrochemical properties of the samples were tested.The cycling performance of the lignin-based phenolic resin-coated silicon nanoparticles was significantly improved compared to that of pure phenolic resin-coated silicon,with the best electrochemical performance of Si/C-LPR with an initial specific capacity of 782.45 m Ah/g and a specific capacity of 605.43 m Ah/g after 100 cycles（0.5A/g）.（5）The depolymerized product lignin/polyaniline composites（LDP/PANI）were prepared by polymerization of DES depolymerized lignin products with aniline as raw material.The morphological and structural characteristics of the depolymerization product/polyaniline composites were characterized by FTIR,SEM,TEM,TGA and elemental analysis.The LDP_3.0/PANI nanocomposite had a specific capacitance of 416.6 F/g at 1 A/g and could be used as a lignin-based supercapacitor with good electrical conductivity.Assembled in a symmetrical supercapacitor device,it provided an exceptionally good energy density of 57.86 Wh/kg,an excellent power density of 952.43 W/kg,notably,a sustained cycling stability.The combination of polyaniline and lignin degradate is therefore environmentally friendly and it enhances the capacitive function on the basis of polyaniline.