Insights of Ethanol Organosolv Pretreatment on Lignin Properties of <i>Broussonetia papyrifera</i>

Delignification of Broussonetia papyrifera during ethanol organosolv pretreatment was studied. Milled wood lignin (MWL), the ethanol organosolv lignin recovered at 5 min (PL5), 10 min (PL10), and 60 min (PL60) during the pretreatment, and the residual lignin (RL60) after the pretreatment were characterized and compared. GPC results demonstrated that the weight-average molecular weights (Mw) of the ethanol organosolv lignin fractions increased as the pretreatment time extended from 5 to 60 min. 31P NMR analysis revealed that the content of aliphatic OH decreased in the following order: RL60 < PL60 < PL5 < PL10 < MWL. HSQC NMR results suggested that the contents of syringyl, guaiacyl, p-hydroxybenzoate unit, phenylcoumaran, and resinol in the solubilized lignin fractions were significantly changed during the pretreatment. In addition, a significant portion of β-O-4 interunit linkages of lignin was cleaved during the pretreatment, leading to the formation of stilbene structures in lignin samples such as PL60