Fractional isolation and physico-chemical characterization of alkali-soluble lignins from fast-growing poplar wood

This paper examines the physico-chemical properties and structural features of six alkali-soluble lignin preparations extracted with 5, 7.5, and 10% NaOH at 50°C for 4–12 h from fast-growing poplar wood. The pure lignin (PL) preparations were characterized using UV, FT-IR, 13C-NMR, GPC, and alkaline nitrobenzene oxidation methods. The results showed that all the PL fractions are relatively free of associated polysaccharides and are composed of large amounts of syringyl units together with noticeable quantities of guaiacyl and fewer p-hydroxyphenyl units. Their weight-average molecular weights ranged from 4520 to 6900 g mol−1. Noticeable amounts of esterified p-hydroxybenzoic acids, minor quantities of esterified p-coumaric acid, and traces of both ester and ether linked ferulic acids were identified in the isolated lignin preparations. The lignin fraction, extracted with 5% NaOH at 50°C for 12 h from the dewaxed fast-growing poplar wood, is composed mainly of β-O-4 ether bonds together with small amounts of β–β′ and β-5 carbon–carbon linkages between the lignin structural units.