Preparation And Performance Study Of Schiff Base And Their Metal Complexes For The Catalytic Depolymerization Of Lignin

Lignin is the second largest component in nature found in lignocellulosic plants that is second only to cellulose.Lignocellulosic plants are one of the most abundant biomass resources on the planet today.They consist of a large amount of carbohydrates,including cellulose,hemicellulose,and lignin.The global pulp and paper industry produces large amounts of lignin each year.For these produced lignin,most of it is used as fuel to burn off,and only a small part（about 2%）is converted to chemicals and other commercially valuable products.At the same time,lignin can be used for applications with high added value,such as the preparation of bio-based materials and phenolic platform chemicals.Because lignin is the only material in the lignocellulosic raw material that has a natural aromatic structure.Therefore,in the future,lignin is one of the substances that have the potential to replace the existing fossil resources.The full use of lignin is of great significance in reducing dependence on petroleum.In recent years,there have been many researches on the catalyzed depolymerization of lignin.The transformation of lignin into valuable bio-oil and platform chemicals through low-temperature catalytic depolymerization has become a research focus.By se lecting suitable catalysts,oriented depolymerization of lignin can be achieved.This dissertation focuses on the following aspects of the preparation of lignin catalysts and lignin depolymerization into platform chemicals:（1）Preparation of autocatalytic poplar ethanol pulping lignin（2）Preparation of Schiff base ligands and Schiff base metal complexes.Preparation of Schiff base ligands and Schiff base metal complexes（C u-complex Schiff base,Mn-complex Schiff base,Co-complex Schiff base）.It was characterized by FTIR,SEM,EDS,TEM and TGA.（3）Lignin catalytic depolymerization at low temperature.The effects of different solvent systems,different reaction conditions（reaction time,Na OH content,H₂O₂ content）and different catalysts on the depolymerization products of poplar ethanol lignin were studied.It was characterized by FTIR,GPC and GC-MS.Experimental results show:（1）Successful synthesis of target Schiff bases and their metal complexes（2）The main monomer structure of poplar ethanol lignin is syringyl.It can be found through the infrared spectra before and after the contrast reaction that it is mainly the syringyl involved in the reaction.（3）The lignin depolymerization reaction is carried out in a 50 ml hydrothermal synthesis reactor.The optimum reaction conditions are:temperature 100°C,lignin 0.16g,catalyst 0.03g,H₂O₂ 2ml,methanol 4ml,deionized water 4ml,Na OH 0.5g and reaction time 48h.（4）Under the optimal reaction conditions,the effect of different catalysts on the depolymerization performance of lignin is explored.By comparing the catalytic results of different Schiff base catalysts,it can be found that the weight average molecular weight,number average molecular weight,and polydispersity coefficient of lignin residue were the lowest,when Cu-complexed Schiff base is used as a catalyst.This shows that the effect of catalyzed oxidation of lignin is better,and the degree of depolymerization of lignin is deeper.（5）The lignin depolymerized water phase products are mostly phenolic substances.According to their different structures,the lignin depolymerized aqueous phenolic products are classified.Because the lignin macromolecules are composed of three basic structures（H type,G Type and S type）monomer compositio n,so after depolymerization its aqueous phenolic products are divided into H type（p-hydroxyphenyl）,G type（guaiacyl）and S type（syringol）.（6）Different catalysts have a significant influence on the depolymerization effect of lignin.The depolymerized phenol products contain three types of lignin structural monomers（H-type G and S-type）.When there is no catalyst and Schiff base as catalyst the monomer total phenol content obtained is 3.78 wt%and 3.16 wt%,respectively.When the catalyst is Co-complex Schiff base,the monomer total phenol content obtained is the highest（24.15wt%）.The catalyst was C u-complex Schiff base followed by（9.99wt%）,and the catalyst was Mn-complex Schiff base with a total monomer phenol content of 7.64 wt%.（7）Through various tests on the catalyst,the results showed that the target Schiff base and its metal complexes were successfully synthesized.At the same time,the optimal reaction conditions for depolymerization were found by designing a series of control variables experiments.Under these conditions,different catalysts were used to depolymerize lignin.The results showed that the weight average molecular weight,number average molecular weight,and polydispersity coefficient of lignin residue were the lowest,when Cu-complexed Schiff base is used as a catalyst.This shows that the effect of catalyzed oxidation of lignin is better,and the degree of depolymerization of lignin is deeper.Q uantitative analysis of the depolymerized phenolic monomer product by GC-MS showed that the total phenolic content of the monomer was the highest（24.15 wt%）when the catalyst was Co-complex Schiff base.