Oxidative Ammonolysis of Technical Lignins. Part 2. Effect of Oxygen Pressure

Summary Investigations were conducted on the effects of oxygen pressure on the oxidative ammonolysis of REPAP organosolv lignin at 130 °C under oxygen pressure of 5, 8 and 12 bar. The rates of reactions monitored, such as nitrogen incorporation, lignin solubilization, oxygen uptake and CO2 formation, increase with increasing oxygen pressure. Kinetics of nitrogen incorporation under different oxygen pressure consists of two phases and follows a first order law in each phase. Linear correlation between the rate of nitrogen incorporation and oxygen pressure implies that the reaction is first order with respect to oxygen concentration. This indicates that oxygen participates directly in the rate-determining step of nitrogen incorporation. The rate of lignin solubilization also linearly increases with increasing oxygen pressure, implying that the rate of lignin degradation directly depends on oxygen pressure. The nitrogen incorporation is linearly correlated with the oxygen uptake, CO2 formation, oxygen incorporation into lignin, loss of carbon and methoxyl group content under all values of oxygen pressure and during the entire reaction period. This suggests that the reactions in the oxidative ammonolysis of lignin proceed via the same pathways in the different kinetic phases. In addition, the changes in the oxygen pressure were found to have only minor effect upon the coefficients of these linear correlations. This is in good agreement with the structures of N-modified lignin elucidated from FTIR and indicates that oxygen pressure affects only the reaction rate, but not the reaction mechanism.