On the delignification mechanism in kraft pulping

摘要

The successive retardation of lignin dissolution towards the end of a kraft pulping process can be explained by the predominance of biphenyl and biphenyl ether structures being present in the lignin. As a possible mode of formation of such structures during the cook, a one-electron transfer reaction involving elemental sulfur/polysulfide has been suggested. Consequently, reactions between dissolved phenolic lignin fragments and pulp lignin might give rise to the formation of stable biphenyl (or biphenyl ether/thioether) structures. In order to find further support for such reactions, kraft pulps (isolated lignins) and the corresponding black liquor lignins were subjected to analysis using thioacidolysis in combination with GC-MS as well as quantitative 1D and 2D NMR techniques. The results obtained so far support the hypothesis that radical coupling reactions may play an essential role for the presence of residual lignin in kraft pulps.