Understanding the structural changes of lignin in poplar following steam explosion pretreatment

To understand the modifications of lignin in the steam explosion process (209 degrees C, 7 min), lignin samples in native poplar (L-P), steam explosion solid residue (L-S) and steam explosion liquid (L-L) were separated and studied. The lignin samples (L-P, L-S and L-L) were characterized and analyzed by size exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance [carbon 13 numclear magnetic resonance (C-13-NMR) and heteronuclear single quantum coherence or heteronuclear single quantum correlation experiment NMR (HSQC) NMR] analysis. The results revealed that the pretreatment induced reductions in amounts of beta-O-4', beta-beta', and spirodienone structure, and increases in the syringyl/guaiacyl (S/G) ratio from 1.14 (L-P) to 1.70 (L-S) and 1.86 (L-L). The HSQC NMR spectra also gave more information about the predominance of G and S units, and only small amounts of p-hydroxyphenyl (H) units. Moreover, SEC demonstrated the depolymerization and repolymerization of lignin, which were the main reasons for the increase in the average molecular weight of L-S and the decrease in average molecular weight of L-L, respectively. In brief, after steam explosion treatment, the lignin structure changed, but the backbone structure was not noticeably modified.