Abstract To possibly help in understanding the chemistry of the D process using E. urophylla × E. grandis
LH 107 as the pulping material, residual lignin in kraft-oxygen pulp bleached by chlorine dioxide was
extracted by the dioxane/water acidolysis method. The content of functional groups in the extracted lignin
was determined by 13C and 31P NMR spectroscopy. A relatively significant difference could be seen in the
chemical properties of residual lignin between oxygen delignified (O) and chlorine dioxide treated (OD)
pulp. The results suggest a decreased function group content in the residual lignin of chlorine dioxide
treated kraft-oxygen pulps. It is quite interesting that the depletion of phenolic hydroxyl and methoxyl
group present in residual lignin is due to the quinone formation during the oxidation of chlorine dioxide,
based on the data obtained in HSQC and 13C NMR spectra of non-cyclic and cyclic MATS, and the content
in condensed and unsubstituted phenolics is reduced to the same degree, which is contrary to what we
generally perceive