Study On Determination Of Phenolic Hydroxyl Group Content In Protolignin As Well As In Isolated Lignins From Spruce And Eucalyptus

Phenolic hydroxyl group occupies an important role in lignin structure and applicational reaearch.Its content affects the physical and chemical properties of lignin macromoLecules.Therefore,accurate and fast determination of phenolic hydroxyl group content is of great significance for lignin valorization research.In this paper,phenolic hydroxyl group content in wood protolignin and isolated lignins from spruce and eucalyptus were determined by applying 31P-NMR method,modified periodate oxidation method,and Folin-Ciocalteu reagents method.In addition,phenolic hydroxyl group content in twelve lignin model compounds were determined by the methods mentioned above.The accuracy,advantages and disadvantages of various methods were verified and evaluated by comparing the results from model compounds.In the 31P-NMR method,N-hydroxysuccinamide was used as theinternal standard to quantify phenolic hydroxyl group content.Phenolic hydroxyl group content in milled wood lignin from spruce and eucalyptus were 1.3596 mmoL/g and 1.3599 mmoL/g,and in kraft lignin were 4.2731’mmoL/g and 4.7231 mmoL/g,respectively.The results showed that different methods for separating and extracting lignin was of great influence on its phenolic hydroxyl group content.The β-O-4 ether bond in the lignin structure would be broken up during the cooking process,making phenolic hydroxyl group content would be greatly increased in kraft lignin.The experimental design of determination of phenolic hydroxyl group in lignin by periodate oxidation method was improved and optimized by means of headspace gas chromatography.According to the difference in the area of the methanol peak provided by the guaiacyl and syringly structures,a new algorithm was established based on the response coefficients of the phenolic hydroxyl group of vanillyl alcohol and 4-hydroxy-3,5-dimethoxybenzyl alcohol model compounds.It is t important that periodate oxidation method can be used to determine phenolic hydroxyl group content in wood protolignin from spruce and eucalyptus,which were 0.8864 mmoL/g and 0.5269 mmoL/g(Based on Klason lignin in raw materials).Phenolic hydroxyl group content in milled wood lignin were 1.3819 mmoL/g and 1.3245 mmoL/g,and in kraft lignin were 3.6070 mmoL/g and 3.8377 mmoL/g from spruce and eucalyptus,respectively.The results showed that phenolic hydroxyl group content in wood protolignin was the lowest.And phenolic hydroxyl group content in milled wood lignin were increased because of mechanical degradation.The kraft cooking process can break β-O-4 ether bonds in protolignin effectively,making the phenolic hydroxyl group content in kraft lignin the highest.Comparison of phenolic hydroxyl group content of model compounds and isolated lignins determined by 31P-NMR method,modified periodate oxidation method and FC reagents method.The results showed FC reagents method produced large errors.It can only be used to estimate phenolic hydroxyl group content in lignin quickly.Phenolic hydroxyl group content will be greatly overestimated when the structure contains a ring-conjugated double bond.31P-NMR method and modified periodate oxidation method could determine phenolic hydroxyl group content in model compounds accurately.Two methods for determination of phenolic hydroxyl group content in milled wood lignin were relatively close.Phenolic hydroxyl group content determined by periodate oxidation method was lower than that of 31P-NMR method in kraft lignin because of the removal of partial methoxy groups during cooking process.