Chemical Analysis and Structural Characterization of Oil Palm Lignins from Black Liquor of Empty Fruit Bunch Fiber Pulping

Abstract

Oil palm empty fruit bunch (OPEFB) fiber was pulped with 20% KOH-0.1% anthra-quinone at 170°C for 3h. The black liquors were acidified to pH 7.0, 5.5, 5.0, 3.5, and 2.0 with 9.68 N H3PO4, respectively. Polysaccharide degradation products were precipitated in ethanol. Lignin fractions were then recovered from the corresponding super-natants of the black liquor by precipitation at pH 2.0 after evaporation of the ethanol. The physicochemical and structural features of the isolated lignins were characterized by UV, FT-IR. 13C-NMR spectroscopy, alkaline nitrobenzene oxidation, and gel permeation chromatography. The results showed that the major components of the alkaline nitrobenzene oxidation products were syringaldehyde and vanillin, together with a small amount of p-hydroxybenzaldehyde. The molar ratio of S (moles of syringaldehyde and syringic acid): V (moles of vanillin, acetovanillone. and vanillic acid): H (moles of p-hydroxybenzaldehyde and p-hydroxybenzoic acid) in the five lignin fractions had a range of 9–11:6–8:1. Optimum conditions for recovery of the lignin fraction with a relatively high yield and purity can be obtained from the supernatant of the black liquor after isolation of the polysaccharide degradation products at pH 5.0.