Chemical Elucidation of Structurally Diverse Willow Lignins

Abstract

A new fast-growing wood raw material, willow (Salix matsudana cv. Zhuliu), was subjected to pulping to identify the structure of its lignin. Thus, the black liquor lignin (AL) and enzymatic mild acidolysis lignin (EMAL) were prepared, and their molecular structure and molecular weight of the isolated lignin polymers were comprehensively investigated by Fourier transform infrared spectroscopy (FT-IR), two-dimensional nuclear magnetic resonance (2D-NMR HSQC), 13C nuclear magnetic resonance (13C-NMR), and gel permeation chromatography (GPC). The NMR results showed that syringyl (S) unit was the predominant structural monomeric unit in willow lignin, as opposed to guaiacyl (G) and p-hydroxyphenyl (H) units. The S/G ratio for the EMAL was found to be 2.02, whereas that for the AL was 0.94. The lignin in the black liquor (AL) fraction was modified during pulping, as shown by its reduced molecular weight. The two isolated lignin polymers, EMAL and AL showed low weight-average molecular weight: 4127 g/mol and 3522.5 g/mol, and in addition they exhibited low polydispersity index (Mw/Mn < 2.0).