The role and fate of lignin's condensed structures during oxygen delignification

The changes imparted to the condensed phenolic structures of softwood resid ual kraft lignin by oxygen delignification were investigated by isolating a nd systematically oxidizing the lignin as a function of time and temperatur e. The rates of formation and/or elimination of the condensed phenolic unit s were evaluated through quantitative P-31 NMR measurements. A large portio n of the free phenolic units (40-50%) in the residual kraft lignin is fairl y resistant to degradation under oxygen delignification conditions. As oxyg en delignification intensified the rate of elimination of uncondensed pheno lic units was more rapid than the rate of elimination of the condensed phen olic units. Consequently, the condensed phenolic units accumulated after ox idation. 5-5' biphenolic condensed moieties originating from protolignin, o r formed via radical coupling reactions during oxygen delignification, were fairly resistant to oxygen-delignification conditions, comprising >55% of the condensed phenolic units after extensive oxygen treatment. Therefore, d espite their phenolic nature, 5-5' biphenols may be key inert units contrib uting to the resistance of residual kraft lignin toward oxygen delignificat ion.