Xylan accessibility of bleached eucalypt pulp in alkaline solutions

Abstract

The accessibility of glucuronoxylan from a refined, bleached Eucalyptus globulus industrial kraft pulp in 0-18% aqueous NaOH solution at room temperature (25°C) was studied. The extraction profile revealed a maximum extraction of xylan in the pulp at about 10-12% NaOH concentration and was explained by the maximum swelling of the pulp according to the Gibbs-Donnan equilibrium. The kinetics of xylan removal and the monitoring of its structural features were performed at 5 and 10% NaOH concentrations. The maximum yields of xylans with 10% and 5% NaOH were as high as 90% and 60% for 2 h extraction, respectively. The structural features of xylan were assessed by acid methanolysis and one-dimensional (1D)/two-dimensional (2D) nuclear magnetic resonance (NMR), and the molecular weight by size exclusion chromatography (SEC). The xylan extracted with 10% NaOH had a slightly higher molecular weight and a lower branching with uronic moieties than the xylan extracted with 5% NaOH. The former was less pure (contained more β-cellulose) than the latter. Structural studies by NMR resulted in the conclusion that there are at least two types of xylans removed from the pulp: One xylan with relatively high 4-O-methyl-α-D-glucuronosyl [MeGlcA-(1→] and [→2)-MeGlcA-(1→] substituents and another xylan with a much lower substitution with uronic residues.