Preparation And Properties Of Lignin-based Polyphenol Antioxidants

Under the background of green and sustainable development,industrial production urgently needs to develop bio-based phenolic functional chemicals and materials.Lignin is a renewable plant phenolic polymer abundant in nature and a natural antioxidant with low toxicity,good biocompatibility,and easy degradation.It is considered as a potential substitute for petroleum-based phenolic antioxidants.The development of lignin-based natural antioxidants is one of the promising platforms for high-value utilization and transformation of lignin.At present,the researches on lignin as an antioxidant have some common problems,such as low phenolic hydroxyl content,high molecular weight,fixed type of lignin,poor antioxidant performance of lignin raw materials without modification,and complex modification process,which restrict its large-scale application in chemical industry.Aiming at these problems,a facile,economical and feasible preparation method of lignin-based polyphenol antioxidants was developed in this thesis.The preparation of lignin-based polyphenols from different industrial sources of lignin and different treatment methods were studied respectively,and the relationship between lignin structure changes and antioxidant properties was studied.Compared with commercial antioxidants,the technical and economic feasibility of lignin-based polyphenols was evaluated.Firstly,the method of preparing lignin-based polyphenols by demethylation and the relationship between the antioxidant properties of lignin-based polyphenols and the content of phenolic hydroxyl groups were investigated.Using beech wood flour as raw material and iodocyclohexane（ICH）as demethylation reagent,an alcoholysis-demethylation strategy was developed to prepare lignin-based polyphenols（DMOL）from native wood fibers.The yield of DMOL-3 was 8.6 wt.%,Mw was 2267 g/mol,the selectivity conversion of Ar-OCH₃was95%,phenolic hydroxyl content was 6.8 mmol/g,and IC₅₀value（DPPH·free radical scavenging activity）was 6.4μg/m L.Compared with the alcoholysis raw material lignin OL（1.0 mmol/g;24.7μg/m L）,the content of phenolic hydroxyl groups increased by 5.8 times,and the antioxidant activity was increased by 3.8 times.And its antioxidant activity is 1.8times that of the commercial antioxidant 2,6-di-tert-butyl-4-methylphenol BHT（11.5μg/m L）and 1.5 times that of the plant polyphenol ferulic acid（9.3μg/m L）.Combined with different phenolic hydroxyl content phenolic standards,different sources of Alkali Lignin（KL）,enzymatic hydrolyzed lignin（EL）and its demethyl lignin-based polyphenols（DMKL and DMEL）to analyze the relationship between the structure and antioxidant properties.Studies have shown that phenolic hydroxyl is one of the necessary structures for phenolic compounds to have antioxidant capacity,and the content of phenolic hydroxyl is positively correlated with antioxidant activity.During the reaction,the attack of ICH breaks the linkage bonds between lignin units;the O-Csp3 bond in the Ar-O-CH₃structure of lignin is broken to form Ar-OH and CH₃I;the C-O-C and C-C bonds in theβ-O-4,β-5,andβ-βconnecting structural units are broken to form Ar-OH and Ar-CH₂-,etc..And with the increase of ICH treatment time,the molecular weight of lignin decreased,the content of phenolic hydroxyl group increased,and the antioxidant activity increased.Then,based on the understanding of the positive correlation between phenolic hydroxyl content and antioxidant activity in the above studies,this chapter reported for the preparation of lignin-based polyphenols with catechol structure unit by ethanolysis of tung nut shell.Here,ethanol/water was used to extract C-type lignin（OCL）with catechol skeleton from the tung nut shell.The isolated yield was 14.7 wt.%,the content of catechol structural units was as high as 57%,the Mw was 2186 g/mol,the phenolic hydroxyl content was 4.7 mmol/g,and the IC₅₀value was 9.7μg/m L.And its antioxidant performance is 1.2 times higher than that of BHT,reaching 95.8%of ferulic acid,and 2.5,3.7,and 3.1 times higher than that of industrial lignin OL,KL,and EL.Technical and economic analysis shows that the direct preparation of lignin-based polyphenol antioxidants by alcoholysis of oil paulowna shells is about US$130/ton,which is more competitive than BHT（market price:US$5000/ton）or ferulic acid（market price:US$60,000/ton）.The tung nut husk is not limited by source,and is one of the ideal lignins for the development of lignin-based antioxidants.Finally,a hydrogenolysis strategy was developed for the above-mentioned tung nut shell to prepare high phenolic hydroxyl content and low molecular weight lignin-based polyphenols（HOCL）.The hydrolyzed products were light yellow and oily,including dihydroeugenol,dihydroconicol,propyl catechol,3,4-dihydroxyphenylpropanol and other phenolic monomers,dimers and oligomers,etc..And the separation yield of hydrogenolysis oil was 20.3%,the molecular weight was 583 g/mol,the phenolic hydroxyl content was 8.4mmol/g,and the IC₅₀value reached 5.3μg/m L.Compared with OCL extracted from ethanol,the molecular weight of OCL decreased by 2.7 times,phenolic hydroxyl group increased by78.7%,and antioxidant activity increased by 76.4%,which was 2.2 times of BHT and 1.8times of ferulic acid.At the same time,it was better than the alcoholysis-demethylation lignin polyphenol（DMOL-3）,and had lower molecular weight and higher yield.In addition,the MTT cytotoxicity evaluation showed that the low concentration（<120μg/m L）hydrogenolysis oil product used as a lignin-based natural antioxidant(IC₅₀=5.3μg/m L)showed no cytotoxicity or little toxicity.According to the identification of phenolic monomer compounds in oily products by GC-MS,the possible pathways of hydrogenolysis of C-type lignin were studied.Studies have shown that C_α-O,C_β-O,and C-C linkages in lignin molecules are catalytically cleaved during hydrogenolysis.Among them,C_β-O and C-C in the connection ofβ-O-4,β-5 andβ-βof G-type structure are cleaved and reduced to generate guaiacol monomers dihydroeugenol and dihydroconiferyl alcohol;C_α-O and C_β-O in the benzodioxane linkage of C-type structure are cleaved and reduced to generate catechol monomers propyl catechol and 3,4-dihydroxyphenylpropanol.The phenolic hydroxyl group increases,the molecular weight decreases,and the antioxidant activity increases.In summary,three simple and efficient strategies were developed for the preparation of lignin-based polyphenols.Among them,a lignin-based polyphenol with high phenolic hydroxyl content,low molecular weight,high antioxidant activity and low cytotoxicity was prepared from one-step hydrogenolysis of tung nut shell.It is expected to replace commercial synthetic antioxidants in materials,daily chemicals,ecological agriculture and other products,with potential great economic value and broad application prospects.