Revealing the topochemical and structural changes of poplar lignin during a two-step hydrothermal pretreatment combined with alkali extraction

Abstract Lignin is an underutilized renewable resource in nature because of its complex structure and special distribution in plant cell walls. The proposal of an efficient treatment and the reveal of the structural characteristics of the isolated lignin are considerably important for the sustainable valorization of lignin. In this work, a two-step of hydrothermal pretreatment combined with alkali extraction was assembled into an integrated treatment and performed to extract lignin from poplar. The yield, purity, and structural changes of the lignin isolated from different treatment conditions were comparatively investigated. Results showed that 71.8 % of lignin with ultra-high purity was isolated during the integrated treatment at 180 °C. Confocal Raman microscopy demonstrated that the delignification occurred preferentially in the secondary wall (SW) and compound middle lamella (CML), and then in the whole morphological regions of the cell wall during the integrated treatment. NMR results revealed that the cleavage of β-O-4 linkages was dominant among the various reactions, which resulted in the increase of phenolic hydroxyl groups, the decrease of molecular weight, and the heterogeneity of the isolated lignin. However, the condensation of lignin became more serious with the increase of hydrothermal pretreatment temperature. These findings provide new insights for the separation and sustainable utilization of lignin for industries.