Study On The Structure Regulation Of Photocatalytic Materials And The Depolymerization Of Lignin

Lignin is a renewable aromatic polymer with abundant sources in nature.It has the potential to prepare high-value chemicals.However,due to the complexity and stubbornness of the lignin structure,most of the lignin is directly discharged and burned as waste,which pollutes the environment and wastes resources.Photocatalytic technology is considered to be a green and sustainable technology due to its relatively mild reaction conditions and the ability to directly use sunlight as energy.Using photocatalytic technology for depolymerization of lignin to obtain high-value chemicals can not only solve the problem of harsh reaction conditions faced by traditional methods of depolymerizing lignin,but also accurately break the C-O linkage existed in lignin.However,photocatalytic performance was severely restricted due to the fast photogenerated electron/hole recombination rate,insufficient redox capacity and poor stability.It is believed that photocatalytic ability to depolymerize lignin can be improved significantly by adjusting the structure of the photocatalyst,changing the energy band position of the photocatalyst,controlling the redox ability of the photo-generated electrons-holes and changing the transport path of the photo-generated electrons-holes.In this work,photocatalytst for effective depolymerization of lignin was designed by constructing type II heterojunction,Ni-doping Zn O and regulating the concentration of oxygen vacancies,changing the energy band position of photocatalyst Zn_xCd_1-xS.In order to study the cleavage process of the C-O bonds existed in the model compound,the photocatalytic mechanism of the photocatalyst was proposed.It is believed that the energy band position and the electron transmission path of the photocatalyst have a very important influence on effectively depolgymeriztion of lignin.The paper provides a certain theoretical basis for the subsequent use of photocatalytic technology to depolymerize the lignin into the high-value compounds.(1).Polyethylene glycol-based eutectic solvent(PEGylated)was synthesized by mixing polyethylene glycol 200 and thiourea.The PEGylated eutectic solvent was used to synthesize Cd S@Ce O₂ composite for the first time.In order to investigate the ability of the photocatalyst to decomposition the organic pollutants,tetracycline was used as a model pollutant to evaluate the photocatalytic performance.The results found that the amount of added Ce O₂ can significantly affect the photocatalytic performance.When the molar ratio of Cd S to Ce O₂ is 1:1,the photocatalyst has the best photocatalytic performance.Under the optimal conditions,the degradation rate of tetracycline is 91.5%,which is 1.57 times higher that of pure Cd S.The optimal photocatalytic performance possessed by the Cd S@Ce O₂-1 photocatalyst can be attributed to the faster photo-generated electron-hole separation ability and stronger sunlight trapping ability in the photocatalytic process.(2).The structure of the Zn O photocatalyst was successfully adjusted by the combination of Ni²⁺doping and oxygen vacancies.Interestingly,the doping of Ni²⁺can promote the formation of oxygen vacancies.Meanwhile,the concentration of oxygen vacancies in Zn O can be controlled by adjusting the doping amount of Ni²⁺.When the mass fraction of Ni²⁺is 3%,the 3Zn O photocatalyst has the best ability to depolymerize sodium lignosulfonate.Under the optimal conditions,the depolymerization rate of sodium lignosulfonate is 94%.The yield of the obtained vanillic acid and guaiacol were 9.3%and 1.5%,respectively.The formation of oxygen vacancies promoted the formation of new energy levels below the conduction band of the Zn O photocatalyst.The formed new energy level changes the electron transmission path in the photocatalyst process,so that the3Zn O photocatalyst can combine with oxygen to generate a higher concentration of superoxy radicals and suppress the concentration of hydroxyl radicals.Appropriate superoxy radical and hydroxyl radical concentrations have a very important effect on the depolymerization of sodium lignosulfonate to obtain high-value chemical.(3).Graphene oxide(GO)has good conductivity and lower Feimi energy.These characteristics can effectively reduce the conduction band potential of the photocatalyst,enhancing the separation ability of photogenerated electrons-holes,and adjusting the reduction ability of photogenerated electrons.In the experiment,photocatalyst Zn₄In₂S₇/GO was successfully prepared by combining Zn₄In₂S₇and graphene oxide using the hydrothermal reaction.Under optimal conditions,the conversion of organosolv lignin is 56.7%.The depolymerization products are mainly phenols and ketones.Mechanism studies found that the thiol group on the surface of the photocatalyst ZIS-100 can first combine with the H in C_?-H to form thiol radicals.Subsequently,C_?H-OH removes one proton H to form C_?radicals and reduces the bond energy of the adjacent C_?-O bond.Then the photogenerated electrons and the removed proton H break the C-O linkage and finally form high-value chemicals.(4).Photocatalyst Cd_xZn_1-xS with a controllable band structure was synthesized by using 3-mercaptopropionic acid as a structural regulator.Photocatalysts with different energy band positions have great differences to the ability of photocatalytic depolymerizating alkali lignin into high-value chemicals.Under optimal conditions,ZCS-70 can effectively catalyze the conversion of alkali lignin into vanillin and other high-value chemicals.Each gram of alkali lignin can produce46.5 mg of vanillin.The study for depolymerization of lignin model compounds showed that the thiol groups existing on the surface of photocatalyst ZCS-70 can effectively converse C_?H-OH into C_?radicals and reduces the bond energy of adjacent C-O bonds.Then the photogenerated electrons and the removed proton hydrogen break the C-O bond and finally form high-value chemicals.The main reason for the excellent photocatalytic performance possed by ZCS-70 should be attributed to the suitable energy band position,the excellent photoelectron-hole separation efficiency and the good sunlight trapping ability.