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Physicochemical and Structural Characterization of Alkali Soluble Lignins from Oil Palm Trunk and Empty Fruit-Bunch Fibers

1999; American Chemical Society; Volume: 47; Issue: 7 Linguagem: Inglês

10.1021/jf9810463

1520-5118

Sun, Jiaxin Fang, J. Tomkinson, J. Bolton,

Biochemical and biochemical processes

Six alkali soluble lignin fractions were extracted from the cell wall materials of oil palm trunk and empty fruit-bunch (EFB) fibers with 5% NaOH, 10% NaOH, and 24% KOH/2% H3BO3. All of the lignin fractions contained rather low amounts of associated neutral sugars (0.8−1.2%) and uronic acids (1.1−2.0%). The lignin fractions isolated with 5% NaOH from the lignified palm trunk and EFB fibers gave a relatively higher degree of polymerization as shown by weight-average molecular weights ranging between 2620 and 2840, whereas the lignin fractions isolated with 10% NaOH and 24% KOH/2% H3BO3 from the partially delignified palm trunk and EFB fibers showed a relatively lower degree of polymerization, as shown by weight-average molecular weights ranging between 1750 and 1980. The results obtained by alkaline nitrobenzene oxidation showed that all of the lignin preparations contained a high proportion of noncondensed syringyl units with small amounts of noncondensed guaiacyl and fewer p-hydroxyphenyl units. The lignin fraction extracted with 5% NaOH from the lignified EFB fiber was mainly composed of β-O-4 ether-linked units. Small amounts of 5-5', β-5, and β-β' carbon−carbon linkages were also found to be present between the lignin structural units. Further studies showed that uronic, p-hydroxybenzoic, and ferulic acids in the cell walls of palm fibers were esterified to lignin. Keywords: Oil palm trunk fiber; empty fruit-bunch fiber; alkaline lignin; alkaline nitrobenzene oxidation; molecular weight; FT-IR; 13C NMR spectroscopy