Structural transformations of triploid of Populus tomentosa Carr. lignin during auto-catalyzed ethanol organosolv pretreatment

Abstract

Auto-catalyzed ethanol organosolv pretreatment (AEOP) has attracted considerable attention because it can effectively remove the recalcitrance of biomass and therefore benefit the downstream process, e.g., enzymatic hydrolysis. Triploid of Populus tomentosa Carr. chips were subjected to AEOP process. Three lignin preparations, including lignin dissolved in the pretreatment liquor (DL), lignin reprecipitated onto the pretreated residue (PL) and residual lignin in the ethanol-water washing residue (RL) were sequentially isolated. To delineate the structural transformations of lignin during this process, these three fractions were systematically characterized by gel permeation chromatography (GPC), quantitative 13C NMR, 31P NMR, heteronuclear single quantum coherence spectroscopy (2D-HSQC-NMR) and thermogravimetry analysis (TGA), and the results were compared with those of enzymatic hydrolysis lignin (EHL) from raw material. The results demonstrated that certain amounts of aryl-β ether (β-O-4') linkages were cleaved and stilbene units were formed during AEOP process, whereas the resinol (β-β') and phenylcoumaran (β-5') units were intact under the same reaction condition. In addition, DL, PL and RL fractions contained less aliphatic hydroxyl group and more phenolic hydroxyl group than EHL. Moreover, thermal stability of lignin samples was associated with their inherent structures. In short, understanding of fundamental lignin chemistry during AEOP process was beneficial to optimize, control delignification process, and obtain high-purity lignin and digestible substrates for bio-ethanol industry.