Structural Changes Of Lignin In Autohydrolysis Of Aspen Wood And Its Influence On Subsequent Formic Acid Pulping

The autohydrolysis of woody biomass not only could produce a large anount of hemicellulose and a small amount of lignin,but also could cause the structural change of lignin.The structural change of lignin in solid phase will affect the performance of follow-up pulp,while the structural characteristics of lignin in liquid phase relate to the subsequent extraction and utilization of lignin.Therefore,the clarity of the relationship between the lignin chemical and lignin structure change with the performance of pulping is one of the urgent problems establishing biomass refining platform.This paper studied the structural change of lignin in autohydrolysis of aspen wood,and its influence on subsequent formic acid pulping,and systematically analysed the structural characteristics of dissolved lignin during formic acid pulping.Aspen wood was subjected to autohydrolysis and milled wood lignin(MWL)and dissolved lignin were isolated from residual lignin in the solid phase and liquid phase,and their structural features were characterized by Fourier transform infrared(FT-IR),nuclear magnetic resonance(1H NMR,13 C NMR,2D HSQC,31 P NMR),gel permeation chromatography/multi-angle laser light scattering(GPC-MALLS),and thermal analysis(TGA,DSC).The results showed that after autohydrolysis,the inter-unit linkages of ?-O-4 of lignin were cleaved,-OCH3 groups were removed and the condensed structures in the lignin formed.The lignin remaining in the hydrolyzed wood chips and dissolved lignin in liquid phase revealed more phenolic OH groups,fewer aliphatic OH groups,higher syringyl/guaiacyl ratios(S/G),higher molecular weights,narrower polydispersities and higher thermal stability.The condensation degree of dissolved lignin in liquid phase was higher than MWL.Then the unpretreated aspen wood and the hydrolyzed wood chips were subjected to formic acid pulping.Compared with unpretreated aspen wood,the hydrolyzed wood chips after formic acid pulping,had lower yield of fine pulp,lower content of fiber and holocellulose.With the increase of autohydrolysis intensity,the whitness,the degree of cellulose polymerization,average length of fiber,curl index,kink index,and burst into paper,tensile strength and tear strength of pulp after formic acid pulping had varying degrees of decline.And the condesation of lignin in autohydrolysis would affect the remove of lignin,which made the pulp after formic acid pulping have higher kappa number.At last,extract and purify the lignin in formic acid pulping black liquor and its structural features were characterized by FT-IR,1H NMR,13 C NMR,2D HSQC,31 P NMR,GPC-MALLS,and TGA.The results showed that during formic acid pulping,the aryl ether bond was mostly fractured,methoxy groups were partly removed,thus the lignin would occur condensation reaction at C3 and C5 of G-unit.What's more,the amount of aliphatic hydroxyl showed a trend of decline,while the content of the type of condensation phenolic hydroxyl and uncondensation phenol hydroxyl were significantly improved,the lignin had high molecular weights and the thermal stability of lignin was further improved.In addition,with the increase of pre-treatment temperature,the amount of phenol hydroxyl and aliphatic hydroxyl showed a trend of decline,the molecular weights were gradually increased and the polydispersities were gradually decreased in the black liquor lignin.