Mechanism and Kinetics Studies of Oxidative Ammonolysis of Technical Lignins, 2000 Pulping / Process & Product Quality Conference Proceedings

E. A. Capanema, M. Y. Balakshin, C-L. Chen, J. S. Gratzl, A. G. Kirkman--The effect of temperature, oxygen pressure and ammonium hydroxide concentration on the rate of oxidative ammonolysis of organosolv lignin (Repap) was studied. The kinetic experiments were conducted in 300 mL Parr’s reactor in ammonium hydroxide solutions of 0.4 - 1.6 M at the reaction temperature of 70 - 130 degrees C under oxygen pressure of 0.5 - 1.2 MPa. The reaction mechanism of nitrogen incorporation does not change during entire course of oxidative ammonolysis. Most of nitrogen is incorporated into the lignin via cleavage of aromatic rings. The kinetics of nitrogen incorporation has two phases due to different reactivity of phenolic and non-phenolic lignin moieties and follows a pseudo-first order rate law in the both phases. The activation energy were found to be rather low, about 35-40 kJ/mole. The rate of nitrogen incorporation is directly proportional to oxygen pressure indicating the reaction is first order with respect to oxygen concentration. The reaction order with respect to ammonium hydroxide concentration is 0.5 suggesting possible participation of NH4OH in reactions inhibiting nitrogen incorporation. The reactivity of technical lignins of different origin (hardwood organosolv lignin, wheat straw organosolv and soda lignins, Eucalyptus globulus (EGL), Eucalyptus grandis (EGR) and pine kraft lignins) is proportional to the S/G ratio of C9-units. Linear relationships between nitrogen incorporation and oxygen uptake and incorporation, CO2 formation, decreases in methoxyl group content and carbon loss were found. Except for CO2 formation, these relationships do not depend on the reaction conditions in the range studied. The mechanism of nitrogen incorporation is generally the same under the conditions studied. However, minor changes in the relation between different reaction pathways are possible.