Elucidation on Structural Changes of Lignin-Carbohydrate Complexes during Kraft Cooking

摘要

To understand the structure of lignin-carbohydrate complexes(LCCs)in the process of kraft cooking,we injected 13C-enriched lignin precursor into internode cavities of the stalk of growing rice plant to obtain 13C-enriched rice stalks.According to the procedure of Bjorkman et al,labeled lignin-carbohydrate complexes were separated and determined by 13C-NMR.The result of analysis by 13C-NMR spectra of 13C-enriched LCC fractions from rice plant showed that lignin was linked to carbohydrate by benzyl ether bond,ester bond and α-ketal linkage.It was also proved that β-O-4was main substructures of lignin in LCC.Then,the obtained LCC and labeled LCC were cooked by kraft method respectively.Lignin-carbohydrate complex-kraft lignins(LCC-KLs)were isolated from black liquor.The overall structures of the obtained LCC-KLs were determined by 13C-NMR and FT-IR spectra.Marked increase in the signal intensities of α-carbons was observed when α-13C-labelled sample was applied.The result showed that most of benzyl ether bonds and ketai linkage between lignin and carbohydrate were broken in pulping,but minor content of ketal linkage was observed.Main substructures of LCC-KL were found to be β-O-4 units,with low content of phenylcoumaran units and pinoresinol units.