Lignin Structural Variation in Hardwood Species

TLDR

The study presents a comprehensive lignin structure analysis of ten hardwood species and introduces a new method to predict the total lignin S/G ratio using a calibration line based on the syringaldehyde/vanillin ratio and 13C NMR data. Milled wood lignin was isolated from each species by a modified protocol and characterized by quantitative 13C NMR, elemental, methoxyl, sugar analyses, and nitrobenzene oxidation and ozonation of extractive‑free wood, alkali‑extracted wood, MWL, and alkali‑extracted lignin. The isolated MWL accurately represents total lignin in alkali‑extracted wood, and lignin structures vary markedly among species—differences in S/G ratio, β‑O‑4 linkages, condensation, elemental and methoxyl contents—correlating with the syringyl/guaiacyl ratio.

Abstract

A comprehensive lignin structure analysis of ten industrially relevant hardwood species is presented. Milled wood lignin (MWL) was isolated from each species using a modified protocol and all milled wood lignin preparations were analyzed through quantitative 13C NMR spectroscopy, elemental analysis, methoxyl analysis, sugar analysis, and nitrobenzene oxidation. Nitrobenzene oxidation and ozonation were carried out on extractive-free wood, alkali-extracted wood, milled wood lignin, and alkali-extracted lignin. Milled wood lignin isolated by the modified protocol was found to be representative of the total lignin in alkali-extracted wood. Significant variations in lignin structures, such as syringylpropane/guaiacylpropane ratio (S/G ratio), arylglycerol-β-aryl ether (β-O-4), degree of condensation, and elemental and methoxyl contents, were found among the hardwood species studied. These structural variations among species appear to be correlated to a single factor, the syringyl/guaiacyl ratio. A new method to predict the S/G ratio of total lignin in wood was developed, using a calibration line established by the syringaldehyde/vanillin (S/V) ratio (nitrobenzene oxidation) and the S/G ratio (13C NMR) of milled wood lignin (MWL).