Study On Degradation Of Lignin Model Compounds By Light-driven Laccase/mediator System

Laccase?EC 1.10.3.2?is a copper-containing polyphenol oxidase that catalyzes the oxidation of various phenolic compounds.Laccase-mediator system?LMS?broaden the substrate specificity of laccase,which mediates the laccase oxidation of some compounds above the laccase redox potential and polymers that do not directly enter the laccase active center.At present,laccase/mediator systems have been widely used in the fields of delignification,pulp bleaching,dye decolorization and bioremediation.In this paper,the degradation efficiency of lignin/mediator system?LMS?on lignin and its model compounds was studied.It not only optimized the oxidation conditions of phenolic compound guaiacol and non-phenolic compound resveratrol,but also explored the effects of mediator?ABTS,HBT,ACE?,ionic liquids and carbon-based materials?graphene,carbon nanotubes,graphene oxide?on the degradation of resveratrol and lignin by LMS system.In this paper,different photoreactions?photoexcitation sensitizing dye methylene blue and photoexcited photosynthetic pigment chlorophyll?were studied for the degradation of lignin and its model compounds,and photoactive oxygen species?single-line oxygen,hydroxyl radicals and superoxide?were explored for different substrates.Finally,the effect of light-driven chlorophyll on the degradation of alkali lignin by laccase/mediator system was studied,and the strengthening mechanism of light-driven chlorophyll was further explored.The main research contents and results of this thesis are as follows:?1?The laccase alone can achieve high-efficiency conversion of phenolic lignin compound guaiacol.When the concentration of laccase is 0.5 U/mL,the optimal pH value is 5.5,the conversion rate of guaiacol can reach 84.4%.The optimal reaction conditions for the degradation of non-phenolic lignin model compounds,VA and lignin by laccase/mediator?LMS?system are:laccase final concentration 0.5 U/mL,ABTS final concentration 5.0??,reaction pH 5.5,after 24 h of reaction,the conversion rate of VA can reach about 44%,and the conversion rate of lignin can reach about 55%.?2?Combining the mediators of three mediators for the degradation of VA and lignin and the electrochemical analysis of the mediator,the catalytic activity was ABTS>HBT>ACE.The 15%ionic liquid 1-ethyl-3-methylimidazolium sulfate?[EMIM]EtOSO₃?exhibited the best catalytic activity for lignin in the LMS system.Comparing the effects of three carbon-based materials on the degradation of lignin in LMS system,the catalytic activity was graphene?GR?>carbon nanotubes?CNT?>graphene oxide?GO?.?3?Visible light excitation of methylene blue Degradation of phenolic compound guaiacol,when the concentration of methylene blue is 16.6?M,the degradation rate of guaiacol was highest about 85%in 4h.This is mainly due to the fact that the active oxygen species produced by photoexcitation--single-line oxygen exhibits higher catalytic activity for guaiacol,while the action of superoxide radicals is second,and the contribution of hydroxyl radicals is minimal.In the process of photoexcited chlorophyll degradation of non-phenolic compound resveratrol,the pH value of the solution was 7.0under full-wavelength irradiation,and the conversion rate of VA was the highest when the concentration of chlorophyll was 1.66?M,which was 68.7%in 4 hours.This is mainly due to the high catalytic activity of the reactive oxygen species produced by photoexcitation--superoxide radicals for VA,while the action of singlet oxygen is second,and the contribution of hydroxyl radicals is minimal.?4?Photoexcited photosynthetic pigment chlorophyll degradation mechanism of VA reaction is that chlorophyll is involved in photochemical reactions as a photosensitizer.The light excites chlorophyll to transfer electrons to the resveratrol molecule in an excited state to form an intermediate radical.The photosensitizer contains a hole to react with oxygen to form a superoxide radical to act on the oxidized intermediate,thereby obtaining an oxidation product VAD;while the excited chlorophyll is reduced and participates in the new excitation cycle.?5?Photosensitive reaction had no significant effect on lignin degradation.When the concentration of methylene blue was 16.6?M,the conversion rate was only 14.8%,and the conversion rate was lower in the process of photoexcited chlorophyll.Mainly the singlet oxygen produced by photoexcitation shows certain catalytic activity to lignin,while the contribution of superoxide radical and hydroxyl radical is almost zero.?6?Methylene blue and chlorophyll cannot function as redox mediators in LMS system under dark conditions;in photoexcitation system,methylene blue as a special mediator competes with ABTS for the active center of laccase,leading to oxidation efficiency greatly reduced.Light-excited chlorophyll enhanced the oxidative ability of laccase-ABTS to lignin,and the conversion rate of lignin was significantly increased by2 times,and the conversion rate reached 68.6%after 4 hours of reaction.?7?The strengthening mechanism of chlorophyll in the laccase-ABTS system is that light-driven chlorophyll makes it reach the excited state to generate more photoelectrons,accelerating the electron transfer between the excited state chlorophyll and laccase,and promoting the reaction of oxygen and laccase.The accelerated electron transfer rate indirectly increased the catalytic activity of laccase;on the other hand,photoexcited chlorophyll accelerated the conversion process of ABTS^+·to ABTS²⁺in the laccase-ABTS system,and increased the rate of electron transport from ABTS^+·to ABTS²⁺.Therefore,the catalytic degradation ability of the laccase-ABTS system to alkali lignin was enhanced.