Study On The Preparation Of Aromatic Compounds By Stepwise Depolymerization Of Lignin

Lignin is the only renewable resource in nature with an aromatic structure,and it has great potential in the depolymerization and preparation of high-value aromatic compounds.Pyrolysis is one of the commonly used methods for lignin depolymerization,which follows the free radical reaction mechanism.That is,the lignin’s bridge and side chains break to form free radicals under heat,and the free radicals recombine to form liquid and gas-solid products.Previous research by the research group showed that adding hydrogen-donating solvents during the pyrolysis process can effectively capture free radicals and reduce free radical coupling,significantly improving the liquid product yield under conditions without hydrogen gas and catalysts.Enzymatic extraction of lignin has mild extraction conditions,which essentially preserves the original structure and theoretically has higher reactivity than other lignins.In addition,stepwise depolymerization is an effective way to achieve lignin depolymerization and obtain high yield monomer products.This paper focuses on enzymatic extraction of lignin from corn cobs.9,10-dihydroanthracene is used as the hydrogen donating solvent,and the stepwise depolymerization method is employed.First,lignin is treated with tetrahydrofuran to obtain tetrahydrofuran-soluble lignin and tetrahydrofuran-insoluble lignin.Then,lignin depolymerization is conducted in a high-temperature and high-pressure reaction kettle,and the effects of reaction temperature（300-440℃）,reaction time（0-120 min）,and the mass ratio of lignin to hydrogen-donating solvent（1:2-1:8）on the depolymerization products are investigated.The lignin before and after treatment is characterized and analyzed,and the influence of tetrahydrofuran treatment on the lignin structure and functional groups is examined.The following are the primary findings obtained from this research paper:（1）Increasing the reaction temperature,extending the reaction time,and increasing the mass ratio of lignin to hydrogen donor solvent can increase the yields of gas and solid products,while the yield of monomer product first increases and then decreases.The lignin depolymerization reaction by enzymatic hydrolysis can achieve the maximum monomer yield of 22.6 wt.%at a reaction temperature of 350℃and a reaction time of 60 min,with gas yield of 6.2 wt.%,liquid yield of 83.7 wt.%,and solid yield of 10.1 wt.%.The gas products mainly contain CO₂,CO,and CH₄,while the monomers in the liquid products are mainly phenol and guaiacol compounds.However,the methoxy side chains of guaiacol compounds are prone to break under high temperature and long reaction time,leading to the formation of smaller compounds.（2）Dissolving lignin with tetrahydrofuran at room temperature does not destroy the aromatic structure of lignin.Instead,individual molecules and small aggregates are dissolved,and some aryl ether bonds of the soluble components are broken.The tetrahydrofuran treatment reduces the weight-average molecular weight of lignin from 4373 g/mol to 2326 g/mol and decreases the polydispersity index from 4.6 to 2.1,resulting in about72.7%of soluble lignin with lower molecular weight and more concentrated distribution.（3）The depolymerization of tetrahydrofuran-soluble lignin shows that increasing the reaction temperature and extending the reaction time is conducive to the conversion of products to gas and solid,while the liquid yield decreases accordingly.At 300℃,extending the reaction time is conducive to increasing the monomer yield.The maximum monomer yield of 19.0 wt.%was obtained at 120 min.At 350℃and 400℃,with the increase of reaction time,the monomer yield first increased and then decreased.The maximum monomer yields of 24.4 wt.%and 25.4 wt.%were obtained at 60 min,respectively.Under the selected reaction conditions,tetrahydrofuran treatment can increase the maximum monomer yield of lignin depolymerization from22.6 wt.%to 25.4 wt.%,an increase of 12.4%.（4）At 400℃and with a reaction time of 30 minutes,the maximum monomer yield of 20.9 wt.%can be obtained from insoluble lignin in tetrahydrofuran.In the reaction at 350℃,the monomer yields obtained from insoluble and soluble lignin after 60 minutes were 18.4 wt.%and 24.4 wt.%,respectively.The solid product yields were 12.6 wt.%and 5.7 wt.%,respectively.Compared with soluble lignin,the solid yield of insoluble lignin increased significantly,while the monomer yield decreased significantly.Therefore,under the same reaction conditions,tetrahydrofuran-soluble lignin is more suitable for preparing high-yield aromatic compounds.