Kinetics of Low-Temperature Oxidation of Enzymatic Lignin from Pine Wood (Pinus silvestris) in an Aqueous Alkaline Medium

The kinetics of oxidation of the enzymatic lignin from pine wood (brown rotted wood) by oxygen in an aqueous alkaline medium at 90–160°C was investigated. It was established that the vanillin yield increased gradually with the temperature increase in this range from 3.4 to 5.6 wt % with respect to lignin. The observed activation energy of the process of oxygen consumption varies in the range 6–19 kJ/mol depending on the process conditions. The process order with respect to oxygen pressure calculated from the rates of O₂ consumption during the first two hours of oxidation at 90–120°C is 1.02 ± 0.05. The low value of activation energy of the process as well as the first order in oxygen indicates a diffusion-controlled process under these conditions. The kinetic data demonstrate that the role of degenerate chain branching processes increases with the increase in the degree of oxidation as a result of decomposition of the hydroperoxides formed. The hydroperoxide decomposition during oxidation in quantity of more than 16 mol % with respect to the initial lignin was registered using a volumetric method based on the oxygen release. The main causes and features determining the monotonically increasing function of the dependence of the selectivity of lignin oxidation to aromatic aldehydes on temperature were discussed.