Study On The Oxidation Of Kraft Lignin Catalyzed By Single-site And Dual-site Enzyme-mimicking Catalyst

In recent years,due to the shortage of resources and the improvement of environmental protection requirements,the environmentally friendly conversion technology of lignin has attracted more and more attention.It is the key field in lignin utilization to find high efficiency and high activity catalytic conversion for lignin.The effect of lignin biodegradation is limited,and the microbial and biological enzyme is difficult to prepare,sensitive to environmental conditions.Biomimetic catalysis has the advantages of both enzymatic and chemical catalysis.It is another direct and effective way to realize the green conversion of biomass.The biomimetic process is the fact that mimic biological process with non-bio compounds.Salen complexes have high catalytic efficiency and porphyrin complexes have high selectivity.Salen-porphyrin double center complex used in this paper has the excellent properties of easy synthesis of salen and high selectivity of porphyrin.According to this characteristics,the salen-porphyrin double center complex was used to promote the lignin degradation,so as to promote the effective utilization of lignin.The mechanism of lignin degradation by the salen-porphyrin complex was studied,which provide new ideas and theoretical basis for the lignin degradation and efficient application.Because salen-porphyrin complex has the structure of salen and porphyrin,it has a double catalytic sites.There are much better catalytic delignification and bleaching effects than the salen complex and porphyrin complex.At the same time,it has the advantages of easy synthesis of salen and high selective catalytic oxidation of porphyrin.The lignin was catalyzed by the salen-porphyrin complex and efficiently transformed.The degradation products and the structure change of diphenyl lignin models and kraft lignin catalyzed by salen-porphyrin complex were studied.The results were obtained as following:1)By comparing TON and TOF,the catalyst was found to accelerate the conversion of the reaction substrate,increasing the reaction rate and the yield of the products;the catalytic ability of ZnPSC6 to catalyze the diphenyl ether lignin model was stronger than that of Co(salen).The degradation product of the diphenyl ether lignin model was phenol in the oxidation by 02 and H202.2)ZnPSC6 and Co(salen)can accelerate the transformation of the substrate.The catalytic ability of ZnPSC6 to catalyze the diphenylethane lignin model was stronger than that of Co(salen).The degradation product of the diphenylethane lignin model was benzoic acid in the oxidation by 02 and H2O2.3)The catalytic ability of ZnPSC6 to catalyze the diphenylmethane lignin model was stronger than that of Co(salen).The degradation product of the diphenylmethane lignin model was benzoic acid in the oxidation by O2 and H2O3.4)The Cβ-Cγ and Cα-Cβ in the diphenyl lignin model were cleaved in the catalytic oxidation of diphenyl lignin models by salen-porphyrin complex.5)In the catalytic oxidation of Kraft lignin by catalyst,C=0,benzene ring,-OCH3,ether bond structure changed.The catalytic reaction promoted the oxidative degradation of lignin side chains.The catalytic reaction promoted the decomposition of lignin benzene ring,demethylation,and cleavage of ether bond.Compared with Co(salen)catalysis,under the action of the supported catalyst,C=O peak absorption intensity increased;benzene ring,-OCH3,ether bond decreased,indicating that the immobilized catalyst can enhance the further oxidative degradation of kraft lignin.After catalytic oxidation of the kraft lignin,the syringyl and guaicyl structures decreased,p-hydroxyphenyl units increased.The catalytic oxidation of lignin resulted in the degradation of β-0-4,β-1,β-5.The methoxy content was reduced.The phenolic hydroxyl content increased and the alcoholic hydroxyls were reduced.The immobilized Co(salen)was relatively good for the catalytic effect due to the the immobilization.Kraft lignin was catalytically treated by salen-porphyrin complex,and the p-hydroxyphenyl units increased,the syringyl and guaicyl structures were reduced.The methoxy content was reduced.The phenolic hydroxyl and alcoholic hydroxyl were also reduced,and the carboxylic and aldehyde groups increased.The β-0-4,erythro β-0-4 and threo β-0-4,β-1,β-5,β-β,5-CH2-5’ linkages were cleaved in lignin.In the absence of catalyst,the oxidation capacity of hydrogen peroxide on kraft lignin was stronger than oxygen.Under the condition of the same oxidants,the catalytic ability of ZnPSC6 to bamboo kraft lignin was stronger than that of Co(salen).The β-0-4,β-β,β-5 and 5-CH2-5’ bonds were degraded more,and methoxyl content was reduced more,which was the advantage of ZnPSC8.The catalytic effect of ZnPSC6 complex was much better than that of the immobilized Co(salen).6)The structural units of lignin were transformed to obtain 4-hydroxybenzaldehyde,2-methoxyphenol,4-hydroxy-3,5-dimethoxyphenylethanone,Others such as vanillin,4-hydroxy-3-methoxybenzoic acid,2,6-dimethoxybenzoquinone.The presence of 1,2-butanediol and dimethyl malonate indicated that the degradation products were secondarily degraded in the catalytic oxidation of kraft lignin.The double center complex enhances the efficiency and selectivity of the reaction,this is mainly due to the fact that it combines the high catalytic efficiency of Co(salen)and the excellent selectivity of porphyrin catalysis.