Preparation Of Spinel Oxide Composite Photocatalyst For The Depolymerization Of Lignin

Lignin is the second largest biomass resource in nature after cellulose,with a global annual production of nearly 50 million tons.Its structure is rich in aromatic groups with huge development potential,and may replace fossil resources to produce high value-added aromatic chemicals.Only by efficiently depolymerizing lignin can high value utilization of lignin be achieved.Photocatalytic reactions have the advantages of low cost,mild conditions,high efficiency,and low pollution.Currently,they are widely used in the degradation of pollutants in water,hydrogen production through photodegradation of water,and organic synthesis.In this thesis,a series of spinel oxide composite photocatalysts Co Fe₂O₄/g-C₃N₄（Co Fe/CN）and Cu Fe₂O₄/g-C₃N₄（Cu Fe/CN）were synthesized by physical and chemical methods for the photocatalytic depolymerization of lignin.The composite photocatalyst has stable properties,great repeatability,efficiency,high photogenerated carrier transport efficiency and electron-hole separation.The strong magnetism of the catalyst can effectively separate the catalyst from the system,thereby easy to recycle and reducing the pollution of metal ions to the environment.The main research content as follows:（1）The composition and morphology of the catalyst were characterized using Fourier transform infrared spectroscopy,X-ray diffraction,X-ray photoelectron spectroscopy,field emission scanning electron microscopy,and transmission electron microscopy;By using UV visible spectrophotometer,photoluminescence spectrometer,the transient photocurrent response spectrum,AC impedance spectrum,and Mott-Schottky photoelectric properties of the catalyst were tested,and the band structure of single catalyst was calculated.The results show that Co Fe₂O₄ and Cu Fe₂O₄nanoparticles are evenly dispersed on the surface of g-C₃N₄,effectively surmounting the disadvantage of spinel iron oxide nanoparticles that are easy to agglomerate.Co Fe/CN and Cu Fe/CN broaden the light absorption range of g-C₃N₄ from 440 nm to470 nm.Compared to the single catalysts Co Fe₂O₄,Cu Fe₂O₄ and g-C₃N₄,the recombination rate of photogenerated electron hole pairs from Co Fe/CN and Cu Fe/CN is significantly reduced and the charge transport efficiency is significantly improved.（2）The prepared catalyst was used for the photocatalytic depolymerization of lignin,and the effects of different reaction atmospheres,types of catalysts,solvent ratios,and irradiation time on the yield of the products vanillin and vanillic acid obtained from the photocatalytic depolymerization of lignin were explored.Quantitative and qualitative analysis of depolymerized products was carried out with using high-performance liquid chromatography gas chromatography/mass spectrometry（GC/MS）.Both Co Fe/CN and Cu Fe/CN exhibit good photocatalytic depolymerization effects.When the reaction system is in an air atmosphere,the reaction lasts for 4 hours,and 50 m L（water:acetonitrile=1:4）of solvent is used,the highest yields of vanillin and vanillic acid obtained over 80Co Fe/CN(m _{Co Fe2O4}:m_g-C3N4=4:1)and 80Cu Fe/CN(m _{Cu Fe2O4}:m_g-C3N4=4:1)were 23.9 mg/g and 53.4 mg/g lignin,respectively.After five cycles of recycling,80Cu Fe/CN exhibited great stability,and the product yield still reached 80.9%of the first.The analysis results of GC/MS show that the catalysts help to depolymerize lignin,with most of the products being aromatic compounds.（3）The effects of different photocatalytic active substances on the photocatalytic depolymerization of lignin over 80Cu Fe/CN were exploed,by adding trace amounts of active substance capture agents to the reaction system,and the possible photocatalytic mechanisms were also proposed.The results showed that photogenerated hole,hydroxyl radical and superoxide anion radical participated in the depolymerization of lignin.There are 42 figures,11 tables and 112 references in this thesis.