Unraveling the Fate of Lignin from Eucalyptus and Poplar during Integrated Delignification and Bleaching

Efficient deconstruction of lignocellulose is vitally important for the biorefinery industry because lignin structures play a crucial role in the high value-added conversion of lignin. In this study, an integrated process based on hydrothermal pretreatment (HTP) and Kraft delignification was proposed to deconstruct lignocellulosic biomass. It was found that the HTP not only facilitated the production of xylo-oligosaccharides but also reduced the chemicals dosage of the following delignification. The structural characteristics of lignin obtained from the integrated process were investigated by NMR spectroscopy and gel-permeation chromatography. Additionally, double enzymatic lignins (DELs) isolated from different feedstocks were used as model lignin to delineate the structural transformations of lignin during H2O2, ClO2, and O-3 bleaching. Significant changes of the lignin structure were observed during the ClO2 bleaching process, including degradation of aromatic rings, enrichment in p-hydroxyphenyl units, and increase of carboxylic groups. A comparison of the structural characteristics of the bleached lignins indicated that HTP benefited the subsequent bleaching process. Enhanced knowledge of lignin chemistry during deconstruction and delignification could provide valuable insight into the current lignocellulose biorefinery.