Study On The Lignin Structural Changes During NaOH-AQ Cooking And TCF Bleaching Of Wheat Straw

Although the structure of raw materials has being adjusted through increasing the proportion of wood pulp and by effectively utilizing the waste paper, non-wood species (especially the wheat straw) will still play an important part. So it is significant for Chinese paper industry to study the changes of lignin structure during cooking and bleaching of wheat straw.The effect of enzyme dosage on the two-stage method of enzyme-mild acidic hydrolysis solution to extract residual lignin from wheat straw pulp was studied in this paper. The result showed that the yield and purity of lignin is highest when the dosage is 3 mL/g (dry slurry). The sample of isolated lignin by the two-stage method was representative, and can represent the protolignin well.The wheat straw lignin and residual lignin samples were isolated from wheat straw, pulps from all stages and OQP pulps, by the two-stage method: enzyme-mild acidic hydrolysis methods. The crude lignin isolated from black liquor by acidic precipitation was purified by mild acidic hydrolysis and dissolved lignin samples with high purity and yield were obtained.The lignin molecular weight distribution was studied through GPC analysis. The results indicated that: during the NaOH-AQ cooking process of wheat straw, the average molecular weight of residual lignin increased at first and then decreased with the deepening of the cooking, and the molecular weight decreasing mainly happened at the last stage of rising temperature and the first stage of keeping temperature. The average molecular weights of dissolved lignin increased gradually and had a rapid increased at the last time. The average molecular weight of residual lignin decreased during OQP bleaching. Compared with hydrogen peroxide bleaching, oxygen delignification is the main factor to decrease the molecular weight. The molecular weight of residual lignin is much larger than the dissolved at each stage of cooking and bleaching, but the dispersivity of lignin keeps at the similar level.The functional groups in all lignin samples in the cooking and OQP bleaching were qualitative and quantitatively analyzed with infrared spectroscopy and 31P-NMR techniques, and were compared to one another.At the first stage of increasing temperature of NaOH-AQ cooking, the contents of aliphaticOH (A-OH) and COOH increase, but the content of total phenolic OH (T-OH) decreases. With the deepening of cooking,the content of A-OH decrease rapidly, and the content of COOH had almost unchanged, while the content of T-OH increased a lot. The contents of condensed phenolic OH (C-OH) and syringyl phenolic (S-OH) increased, but guaiacyl and dementhylated OH (G-OH) and p-hydroxy-phenyl OH (p-OH) decreased in the residual lignin.In NaOH-AQ cooking process, the content of A-OH decreases rapidly during the initial stage of cooking, and then changed a little. At the first stage of cooking, a large number of COOH was generated, as the deepening of cooking, the content was reduced. Along with the deepening of cooking, the content of T-OH,S-OH and C-OH increased, while the contents of G-OH and p-OH were not changed so much in the dissolved lignin.After oxygen delignification lignin, the content of A-OH and COOH increased, while the content of T-OH had almost unchanged in the residual lignin. Compared with unbleached pulp, the contents of A-OH in dissolved lignin were lower, and the contents of T-OH and COOH were higher. The contents of C-OH,S-OH,G-OH and p-OH changes a little in the residual lignin, and the contents of all kind phenolic hydroxyl groups in dissolved lignin were higher than residual lignin.After hydrogen peroxide bleaching, the content of total phenolic hydroxyl and other phenolic hydroxyl were significantly decreased, while the contents of aliphatic hydroxyl and carboxyl increased a little in the residual Lignin.