Chemical transformations of Populus trichocarpa during dilute acid pretreatment

In this study, Populus trichocarpa was subjected to dilute acid pretreatment at varying pretreatment times. The three major components of lignocellulosic biomass, namely cellulose, hemicellulose and lignin, were isolated from the starting and dilute acid pretreated poplar. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) techniques were utilized to elucidate structural transformations of poplar during dilute acid pretreatment. The results demonstrated that the pretreatment dissolved hemicelluloses and disrupted structural features of lignin and polysaccharides. As revealed by NMR, the aryl-O-ether linkage (β-O-4) of lignin was extensively cleaved and lignin repolymerization occurred during pretreatment. The lignin was also observed to have a decrease in S/G ratio and methoxyl group content and these changes were accompanied with an increase in condensed lignin. The dilute acid pretreatment resulted in a reduction in molecular weight of cellulose and hemicellulose, while no prominent change of molecular weight was observed for lignin. The polydispersity index of cellulose appeared to increase initially within a short time of pretreatment (0.3-1 min) and start to decrease with longer pretreatment time during the bulk phase of chain scission (5.4-26.8 min). The DA pretreatment demonstrated no significant impact on the crystalline index (CrI) of cellulose particularly within the short time range of pretreatments examined in this study, with CrI remaining almost unchanged during the pretreatment time of 0.3-5.4 min and a slight increase observed as the pretreatment time extended to 8.5 and 26.8 min.