Soda-AQ delignification of poplar wood. Part 2: Further degradation of initially dissolved lignins

Abstract

Abstract Degraded lignin fragments dissolved in pulping liquor during the soda AQ pulping of poplar (Populus deltoides) wood with a flow through reactor were divided in two fractions. The first one (LF-1) was collected during the heating-up period (100°C until the final temperature t max=170°C); the second one (LF-2) was collected at t max with 60 min cooking time. These two liquors were then treated at 170°C for 70 min (leading to a liquor LF-1A) and 10 min (leading to a liquor LF-2A). By these post-treatments, pulping liquors of batch pulpings were simulated. From the four liquors, lignins were isolated and after purification the chemical and physical properties were characterized by Gel Permeation Chromatography (GPC: to obtain their molecular weight distribution), nitrobenzene oxidation, permanganate oxidation, and 13C NMR spectroscopy. By these means, the reaction mechanisms of dissolved and degraded lignin fragments in pulping liquors was elucidated and insight was gained concerning the effectiveness of the anthraquinone-anthrahydroquinone (AQ-AHQ) redox system using the corresponding batch lignin preparation as reference. The simulated batch fractions LF-1A, LF-2A, and true batch lignin preparations are similar. They have a high content of condensed arylpropane units. Among them, syringylpropane units are most severely degraded, in particular, etherified β-aryl ether bonds are cleaved to a great extent after dissolution in pulping liquor. The structure of the remaining degraded lignin fragments was changed seriously, particularly on their side chains. Oxidation and disproportionation reactions are typical. In the presence of fibers, xylans tend preferentially to simple deposition on fibers instead of grafting with the degraded lignin fragments. Dissolution of carbohydrates is not affected by higher alkali concentration at the end of the pulping, and the yield losses are caused by a better lignin removal.