Behavior of Eucalyptus globulus lignin during kraft pulping. II. Analysis by NMR, ESI/MS, and GPC


Residual and dissolved lignin from different phases of kraft delignification of Eucalyptus globulus wood were isolated and characterized by ID and 2D H-1 NMR, C-13 NMR, Electrospray Ionization Mass Spectrometry (ESI/MS), and gel permeation chromatography (GPC). During the temperature rise period, below 70degreesC, about 20% of the lignin was dissolved without significant structural changes. Above 70degreesC, the lignin suffered significant degradation/fragmentation in the cell wall prior to dissolution. The lignin ether-linked syringyl units were the most susceptible to alkaline degradation. Through the course of pulping, the residual lignin (RL) revealed a gradual increase of aliphatic moieties of unknown structure, as well as a decrease of native structures such as phenylcoumaran and pino-/syringaresinol lignin units. A significant decrease of the beta-O-4 structures content in RL was detected only at the Final cooking temperature. The lignin dissolved in the black liquor (BL) consisted of highly branched oligomers with rather low molecular weight (average mass 800-1000u). A part of BL (about 30%) was chemically linked to carbohydrates and possessed a large molecular weight distribution (500-4000u). BL showed a progressive decrease in beta-O-4 and pino-/syringaresinol structures and formation of enol ether and stilbene structures. The GPC analyses showed a continuous decrease of the Molecular weight of both the residual and dissolved lignins during the pulping process, particularly in the residual stage.




Materials Science


Pinto, PC; Evtuguin, DV; Neto, CP; Silvestre, AJD; Amado, FML

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