Structural and Molecular Properties of Residual Birch Kraft Lignins

Abstract

Abstract The residual lignins from unbleached and oxygen-bleached birch kraft pulps were isolated by acid hydrolysis and characterized by elemental analysis, methoxyl content, permanganate degradation, size exclusion chromatography and quantitative 13-C NMR. Calculations based on the reduction in kappa number after the acid hydrolysis revealed that only about one third or less of the material assumed to be lignin could be isolated as polymeric lignin. This was shown to be a syringyl-guajacyl type of lignin with a syringyl-to-guaiacyl ratio of approximately 1:1. The rest of the material found in the acid solution after hydrolysis and identified by GC-MS and by pyrolysis-GC-MS was to a large extent low molecular weight furanoid structures probably originating from polysaccharide degradation products, with only traces of lignin-derived compounds present.