Kinetic Study of Hexenuronic and Methylglucuronic Acid Reactions in Pulp and in Dissolved Xylan during Kraft Pulping of Hardwood

During kraft pulping, the side group in the xylan backbone, 4-O-methyl-d-glucuronic acid, is partly converted to hexenuronic acid. Simultaneously, degradation reactions of these side groups take place. The rates of these reactions were studied during the kraft pulping of hardwood and were shown to be strongly affected by the location of the xylan; dissolved xylan had markedly higher methylglucuronic acid and hexenuronic acid contents than pulp xylan did. The degree of substitution of methylglucuronic acid in dissolved xylan was found to be higher at reduced cooking temperatures; no such change was seen for pulp xylan. A kinetic model was developed that included the energies of activation for formation (129 kJ/mol) and degradation (143 kJ/mol) of hexenuronic acid and degradation (141 kJ/mol) of methylglucuronic acid and bulk delignification (118 kJ/mol, in accordance with earlier studies). Decreased cooking temperatures thus increase the number of acidic charged groups in the pulp and in dissolved xylan.