Oxidative Ammonolysis of Technical Lignins Part 3. Effect of Temperature on the Reaction Rate

Summary The effect of the reaction temperature on the kinetics and the reaction mechanism of oxidative ammonolysis of Repap organosolv lignin have been studied. The reaction was conducted in 0.8 M NH4OH solution under oxygen pressure of 12 bar and at three different temperatures, 70 °C, 100 °C and 130 °C. The resulting N-modified lignins were analyzed by elemental and methoxyl group. About 20‐25% of maximum nitrogen content is incorporated into the lignin very fast, in 1‐2 min of the reaction. The reaction kinetics then follows a pseudo-first order reaction law and consists of two phases. The activation energies for nitrogen incorporation and lignin solubilization are rather low, in the range of 33‐34 kJ/mol. Linear correlation between nitrogen incorporated into the lignin and molecular oxygen uptake, oxygen incorporation, CO2 formation, O-demethylation and total carbon loss was analyzed at the different reaction temperatures. On the basis of kinetic data obtained so far, we have postulated that the reaction temperature affects the reaction rate, but not the reaction pathways. The reaction temperature also affects the ratios between different reaction pathways, though the effect is not very strong. The results obtained are discussed in the terms of competitive reactions of lignin oxidation followed by nitrogen incorporation and lignin deactivation involving nitrogen-containing species.