Kinetics of the oxidative ammonolysis of lignin

The effect of reaction variables on the oxidative ammonolysis of technical lignins was studied in the range of 0.4-0.8 M NH₄OH, reaction temperature of 70 - 130°C, oxygen pressure of 0.5 - 1.2 MPa and pH 9- 12.7. The kinetics of nitrogen incorporation consists of two phases, both of which follow a pseudo-first order reaction law. The reaction is 1^st and 0.5 order with respect to oxygen and NH₄OH concentration, respectively. The effective activation energy of nitrogen incorporation is rather low, 33-43 kJ/mol. The dependence of the reaction rate on pH of the reaction solution goes through a maximum. Linear correlation between nitrogen incorporation and O-demethylation, CO₂ formation, oxygen uptake as well as oxygen incorporation were observed. Structural analyses of the soluble N-modified lignins by FTIR and ¹H NMR spectroscopic techniques showed only qualitative differences of the spectra obtained under different reaction conditions indicating that the reaction conditions do not affect the reaction pathways. A scheme of possible reaction mechanisms is postulated based on the experimental results.