Structural Transformations of Hybrid Pennisetum Lignin: Effect of Microwave-Assisted Hydrothermal Pretreatment

To understand the structural features and transformations of hybrid Pennisetum (HP) lignin, an integrated treatment on the basis of microwave-assisted hydrothermal pretreatment (MHTP) (at 180–200 °C for 10–20 min) followed by 2% NaOH aqueous post-treatment (at 90 °C for 2.0 h) was proposed in this study. The isolated alkali lignins (ALs) were investigated by GPC and FT-IR and comparatively studied with residual lignins (DELs, prepared by the combination of MHTP and following double ball-milling and enzymatic hydrolysis) by HPAEC and advanced NMR techniques. Results demonstrated that the yields of ALs (35.2%–82.3%) after MHTP increased significantly, and the content of the associated polysaccharides (0.1%–1.8%) was much lower than those of the DELs (2.6%–11.1%). NMR characterization revealed that the two classes of lignin fractions were SGH-type, and the content of various linkages (e.g., β-O-4, BE, and α-O-4/β-O-4) and the ratios of S/G and pCE/FA varied regularly with increasing MHTP intensity. Meanwhile, the depolymerization and repolymerization reactions of lignin were found to be simultaneous during the MHTP process, in which the depolymerization was the dominate reaction. The efforts focused on the HP lignin in this study will maximize the utilization of energy crop resources for biorefinery industries.