Chemical Properties of Softwood Soda-Anthraquinone Lignin

Soda-anthraquinone (soda-AQ) cooking, focused on obtaining lignin rather than cellulose, was performed under various cooking conditions. The lignin obtained by acidification of the spent liquor to pH 5 was chemically characterized and the correlation between the cooking conditions and the chemical properties of lignin was discussed. The lignin yield ranged from 6.7 to 13.3% on wood, depending on the cooking conditions. The phenolic hydroxyl group content of the lignin was about twice that of wood lignin and similar to that of commercial pine kraft lignin. The cooking time of the soda-AQ cooking significantly influenced the vanillin yield of the lignin by nitrobenzene oxidation. The vanillin yield of the lignin was less than half that of wood lignin and MWL. The non-phenolic α-O-4 linkage of the lignin was more stable than the non-phenolic β-O-4 linkage of the lignin under soda-AQ cooking.