Structural Changes of Poplar Wood Lignin in Hydrothermal pretreatment in Acetic Acid-Sodium Acetate system

Hydrothermal pretreatment of poplar wood chips was performed in an acetic acid/sodium acetate buffer medium. To characterize the structural changes of lignin in the pretreatment process, milled wood lignin (MWL) was isolated from both the un-treated and treated poplar wood chips, and analyzed by ¹H-NMR, ¹³C-NMR, ³¹P-NMR, FT-IR, and GPC-MALLS. Results showed that the lignin in the pretreated wood chips had more phenolic OH groups, less aliphatic OH groups, higher molecular weight, and narrower polydispersity, in comparison with the lignin in the un-treated wood. The acetic acid-sodium acetate buffer pretreatment facilitated the removal of -OCH₃ groups and the cleavage of the inter-unit linkages of β-O-4 bond in the lignin. Furthermore, the re-polymerization of lignin in the acetic acid-sodium acetate buffer pretreatment was found to be less than that in autohydrolysis.