Structural purification of technical lignins via fractional dissolution using non-azeotropic solvent mixtures

Two technical lignins, a softwood kraft lignin (SKL) and a wheat straw organosolv lignin (WSOSL) were fractionated using a Soxhlet extractor that was connected to a piston pump for solvent movement such that Soxhlet extraction using non-azeotropic solvent mixtures was feasible. Fractionation of the lignins using such solvent mixtures that could be tuned in terms of hydrogen-bond acceptor and donor characteristics and polarities yielded novel fractions not accessible in standard Soxhlet-based fractionations. Two SKL fractions could be obtained applying aqueous acetone that displayed homogeneous structural characteristics while differing significantly in molecular weights. WSOSL could be gradually purified, allowing for the generation of a rather pure lignin carbohydrate complex (LCC) fraction and a purified high molecular weight lignin fraction.