Structure and integrity of sequentially extracted lignin during poplar (alkaline) pretreatment

Abstract

Lignocellulosic biomass (LCB) utilization demands pretreatment to disrupt the compact structure and reduce the recalcitrance originating from LCB constituents interaction. Alkaline pretreatment (AP) selectively solubilizes lignin, enhances the accessible surface area, and eventually alters the pretreated residue structure. During pretreatment, heterogeneous susceptibility/recalcitrance of LCB constituents towards pretreatments and various phenomena such as phase transition, de/repolymerization, and lignin solubility are primarily responsible for structure alteration. Lignin structure comprises various kinds of bonds/linkages (β-O-4′, β-β′, and β-5′), and each one might be differently susceptible to a specific pretreatment. Differently susceptible lignin bonds/linkages and the relationship between AP and lignin bond degradation are not fully understood. Sequential alkaline pretreatment (SAP) was applied at constant pretreatment severity to investigate the lignin degradation, structure of residual lignin in the pretreated residue, and the heterogeneous susceptibility/recalcitrance of lignin bonds to categorize the labile/recalcitrant lignin moieties/bonds during SAP. Cellulolytic enzyme lignin (CEL) was isolated from un/pretreated residues and characterized by FTIR, GPC, and 2D NMR (HSQC). Semi-quantification of 2D NMR spectra anticipated the heterogeneous degradation of lignin ether bonds/C-C linkages as β-5′ degraded completely than β-O-4′ and β-β′. It might be due to the accessibility of cellulose/hemicellulose-lignin matrix as each pretreatment step degraded LCB and exposed embedded structure.