Structural changes in residual lignin of Eucalyptus urophylla ××Eucalyptus grandis LH 107 oxygen delignified kraft pulp upon chlorine dioxide bleaching

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 1313C and 3131P 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 1313C 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.