Condensation Reactions of Lignin During Oxygen Delignification Under Acidic Conditions

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Abstract
Spruce wood (Picea abies) has been subjected to delignification by oxygen under acidic conditions using different solvent media. The residual and removed lignins were submitted to permanganate oxidation and the products analyzed by GC and GC-MS. Results from the analysis of the residual lignins indicate that lignin “condensation” depends on the nature of oxidation medium. It was found that the addition of acetone to the aqueous liquor decreases both acid catalyzed and radical side reactions. Lignin “condensation” in water containing media is dominated by radical reactions. Results from the analysis of the lignin removed during oxygen delignification in an acetone/water medium indicate that significant amounts of “condensed” structures are present, which are relatively stable towards oxidation. The presence of diphenylmethane, diarylether and biphenyl type structures in the removed lignin was confirmed by CP-MAS 13C NMR spectroscopy.