Fractional Separation and Physicochemical Characterization of Polysaccharides from Poplar Chips

Abstract

The polysaccharides in the cell walls of dewaxed poplar chips were separated into hemicellulose–lignin complexes (HLC), residual hemicelluloses, and cellulose by extraction with sodium hydroxide at various concentrations. Celluloses (43.9–44.5%), HLC (17.4–21.9%), and residual hemicelluloses (4.3–9.0%) were the major polysaccharide components. Initial extraction of the dewaxed poplar chips with 5% NaOH at 50°C for 4, 6, 8, and 12 hr, and 7.5 and 10% NaOH at 50°C for 6 hr yielded 17.4, 18.2, 18.4, 18.6, 20.3, and 21.9% HLC, respectively. Further extraction of the corresponding alkali-treated and delignified poplar chips with 17.5% NaOH–1% Na₂B₄O₇·10H₂O at 20°C for 2 hr released 9.0, 8.2, 7.9, 7.2, 5.5, and 4.3% residual hemicelluloses, respectively. Xylose was found to be a predominant sugar component (84.3–90.5%) in all of the more linear, acidic, and larger molecular size HLC preparations ([Mbar]_w , 26,090–40,500 g·mol⁻¹), in addition to 4.0–4.8% bound lignin. These lignin molecules were found to be chemically linked to hemicelluloses mainly via syringyl units. On the other hand, all of the residual hemicellulosic fractions contained less xylose (57.3–59.1%) and less uronic acids (4.5–8.9%), but had a higher content of mannose (24.9–26.5%) and glucose (13.3–16.0%). In comparison with the HLC, the residual hemicellulosic fractions had a small molecular size ([Mbar]_w , 18,300–29,730 g·mol⁻¹), and had less than half the amount of bound lignin (1.5–1.7%). These lignin molecules were associated with residual hemicelluloses mainly via guaiacyl units. All of the cellulose fractions were relatively free of noncellulosic sugars (contained more than 99.6% glucose), and no considerable degradation of the cellulose was observed under the various separation conditions given.

Keywords:

• Poplar chips
• Polysaccharides
• Separation
• Hemicellulose-lignin complexes
• Residual hemicelluloses
• Cellulose
• Lignin

ACKNOWLEDGMENTS

The authors are grateful for the financial support of this research from the European Community under the Industrial & Materials Technologies Programme (Brite-EuRam III)—Depolymerisation, Polymerisation and Applications of Biosustainable Raw Materials for Industrial End Uses.