Changes in Lignin Structure after Autohydrolysis Pretreatment Analyzed By Quantitative NMR Techniques

The economic success of a lignocellulosic biorefinery is highly sensitive to the final price at which the lignin residue can be sold for at market.  To identify possible pathways for lignin valorization, it is first necessary to understand the changes to the chemical structure of lignin during the conversion process.  It is known that autohydrolysis pretreatment will alter lignin structure through both depolymerization and repolymerization reactions.  These reactions were tracked by comparing the chemical structures of isolated lignin fractions from both raw and autohydrolyzed biomasses using quantitative 13C and 2D-HSQC NMR.  Three hardwood and three non-wood lignocellulosic biomasses were subjected to identical autohydrolysis pretreatments at 180 °C for 40 minutes.  A lignin isolation scheme involving the generation of milled-wood lignin (MWL) and cellulolytic enzyme lignin (CEL) was applied to both the raw and autohydrolyzed biomasses, which were then analyzed by NMR.  For hardwood E. nitens, it was found that the amount of β-O-4 ether bonds decreased from 53 per 100 aromatic units in the raw MWL to 34 per 100 aromatic units in the autohydrolyzed CEL, showing evidence of acidolysis depolymerization reaction occurring in the lignin.  Additionally, the amount of cinnamyl alcohol end groups increased after autohydrolysis from 1 to 4 per 100 aromatic units, suggesting additional depolymerization reactions involving liberation of these moieties.  The quantification of major inter-lignin linkages in the remaining hardwood and non-wood lignins will be presented.  In addition, we will discuss the challenges and our approach of non-wood lignin characterization induced by the para-coumaroyl and feruloyl linkages within lignin as well as between lignin and hemicellulose.