Fractionation and characterization of xylan rich extracts from birch

Abstract There is a need for efficient separation processes that provide clean fractions of individual wood polymers. The purpose of this study was to remove lignin from a xylan rich fraction from birch by solid-liquid extraction followed by a liquid-liquid extraction. By this means, it was possible to diminish the lignin concentration (12%) of mildly extracted xylans from birch to a total lignin concentration of 3%, as determined as a sum of Klason lignin and acid soluble lignin. The molecular masses of the fraction were determined by size exclusion chromatography (SEC). The two xylan rich fractions obtained by the removal of lignin have molecular masses (M W) of 15 300 and 27 900 Da in comparison to 11 900 Da for the lignin rich starting material. NMR and IR analysis revealed that the lignin removed is mostly unbound to the polysaccharides. The lignin was water soluble in the presence of xylan (as part of the xylan rich fraction), but not in isolated form in water alone. The remaining lignin in the xylan fractions has obviously a closer interaction with the xylan, and thereby its separation by liquid-liquid extraction is more difficult. It is also observed with NMR that the extractable lignin is built up mainly of syringyl units, chiefly bonded by β-O-4 linkages.