Role and Fate of Lignin's Condensed Structures During Oxygen Delignification, 1998 Pulping Conference Proceedings

The fate and role of the condensed structures present in softwood residual kraft lignin during oxygen delignification was investigated by isolating it and systematically oxidizing it as a function of time and temperature under homogeneous conditions. Detailed quantitative 31 P NMR measurements provided for the first time three-dimensional plots describing the actual kinetics of formation and/or elimination of the condensed phenolic units present in residual kraft lignin. A large portion of the free phenolic units (40-50%) present in residual kraft lignin was found to be fairly resistant to degradation by oxygen. As oxygen delignification intensifies, the rate of elimination of uncondensed guaiacyl phenolic units present in residual kraft lignin was significantly more rapid than the rate of elimination of the condensed phenolic units. Consequently, the condensed phenolic units were found to accumulate in the residual lignin after oxidation. 5-5’ biphenolic condensed moieties that originate from protolignin, or formed via radical coupling reactions during oxygen delignification, were found to be fairly resistant to oxygen delignification conditions and dominated (>55%) the condensed phenolic units after extensive oxygen treatment. Diphenylmethane structures, formed during kraft pulping, were also seen to partially survive oxygen delignification conditions. Therefore, despite their phenolic nature, diphenylmethanes and 5-5’ biphenols may be key inert units contributing to the resistance of residual kraft lignin toward oxygen delignification.