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

To understand the modifications of lignin in the steam explosion process (209°C, 7 min), lignin samples in native poplar (LP), steam explosion solid residue (LS) and steam explosion liquid (LL) were separated and studied. The lignin samples (LP, LS and LL) were characterized and analyzed by size exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance [carbon 13 numclear magnetic resonance (¹³C-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 β-O-4′, β-β′, and spirodienone structure, and increases in the syringyl/guaiacyl (S/G) ratio from 1.14 (LP) to 1.70 (LS) and 1.86 (LL). 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 LS and the decrease in average molecular weight of LL, respectively. In brief, after steam explosion treatment, the lignin structure changed, but the backbone structure was not noticeably modified.