Physico-Chemical and Structural Characterization of Alkali Lignins from Abaca Fibre

Abstract

Abaca fibre was treated with 1, 2.5, and 5% sodium hydroxide at 25, 35, and 50° C for 0.5-5 h. The dissolved alkali lignins LA were separated from the solubilized polysaccharides using a two step precipitation method. The chemical and structural compositions of the pure lignin fractions LA were determined by UV, GPC, FT-IR, ¹³C-NMR spectroscopy and nitrobenzene oxidation. All of the lignin fractions LA are free of associated polysaccharides. Their molecular-average weights ranged from 1960 to 2640. The results obtained by alkaline nitrobenzene oxidation showed that all the lignin fractions LA contain large amounts of non-condensed syringyl units, together with fewer non-condensed guaiacyl and p-hydroxyphenyl units. β-O-4 and β-5 ether bonds are found to be the major linkages between the lignin structural units. The less common β-β, β-5, and 5-5' carbon-carbon linkages are also identified to be present between the lignin structural units. p-Coumaric acid and ferulic acid are linked to lignin molecules by ester and ether bonds, respectively.