Comprehensive Study on the Chemical Structure of Dioxane Lignin from Plantation Eucalyptus globulus Wood

Abstract

Results of a comprehensive study on the chemical structure of lignin from plantation Eucalyptus globulus Labill are presented. Lignin has been isolated by a modified mild acidolysis method and thoroughly characterized by functional group analysis, by a series of degradation techniques (nitrobenzene oxidation, permanganate oxidation, thioacidolysis, and Py−GC−MS), and ¹H and ¹³C NMR spectroscopy. Plantation Eucalyptus globulus lignin was found to be of the S/G type with an extremely high proportion of syringyl (S) units (82−86%) and a minor proportion of p-hydrophenyl propane (H) units (roughly 2−3 mol %). Unknown C-6 substituted and 4-O-5‘ type syringyl substructures represent about 65% of lignin “condensed” structures. Eucalypt lignin showed high abundance of β-O-4 (0.56/C₆) structures and units linked by α-O-4 bonds (0.23/C₆). The proportion of phenylcoumaran structures was relatively low (0.03/C₆). Different kinds of β-β substructures (pino-/syringaresinol and isotaxiresinol types) in a total amount of 0.13/C₆ were detected. ESI-MS analysis revealed a wide molecular weight distribution of lignin with the center of gravity of mass distribution around 2500 u. Keywords: Lignin;Eucalyptus globulus; functional group analysis; nitrobenzene oxidation; permanganate oxidation; thioacidolysis; analytical pyrolysis; ESIMS; ¹H NMR spectroscopy; ¹³C NMR spectroscopy