Transition metal-catalysed oxidation of lignin model compounds for oxygen delignification of pulp

摘要

Molecular oxygen (O2) is widely used as a delignification agent in pulp bleaching. Unfortunately, carbohydrates are attached to a greater extent during oxygen delignification than during treatment with chlorine-containing chemicals, and the delignification effectiveness of oxygen is limited to about 50% of the residual lignin in kraft pulp. This study was part of a larger project targeted at increasing the use of oxygen in delignification through application of water-soluble transition metal catalysts. The underlying idea of catalytic oxygen delignification is that the oxygen should be fixed reversibly within compounds that are capable of transferring it selectively to lignin under reaction conditions in which carbohydrates remain intact.In this work, the catalytic performance of a cobalt-Schiff base catalyst, Co-sulphosalen, and complexes generated in situ in the presence of copper and diimine ligands such as o-phenanthroline and 2,2'-bipyridine was elucidated with various model compounds. Oxidation experiments were carried out in alkaline water (pH 9–12) with use of oxygen (p(O2) 1–8 bar) as oxidant. Both Co-sulphosalen and Cu-diimines were active catalysts, increasing the oxidation rate of the phenolic model compounds 4-ethylguaiacol, guaiacol and 2,2'-biphenol. They also catalysed the oxidation of a non-phenolic model compound, veratryl alcohol.The Co-sulphosalen-catalysed oxidation rate of the lignin model compounds was maximal around pH 11. In studies with a UV-vis spectrometer, Co-sulphosalen was found to form a new complex with an axial ligand, pyridine. Pyridine had almost no effect on the activity of Co-sulphosalen, however, and it was concluded that Co-sulphosalen forms 5-coordinated complexes, Co-sulphosalen(OH−) and Co-sulphosalen(pyridine), without and with pyridine, respectively. With a new HPLC method, Co-sulphosalen was shown gradually to decompose in the alkaline reaction medium. The rate of the decomposition was independent of the catalytic oxidation reaction but increased with pH (9–12).The concentrations of hydroxide ion (i.e., pH) and ligand also had an effect on the structure and activity of in situ formed Cu-diimines. In active forms of Cu-diimine complexes, the Cu2+ ion was coordinated with two ligands and one hydroxide ion. The catalytic activity of Cu-diimines could be enhanced by suitable substitution of the ligand.Unfortunately, Co-sulphosalen, and still more so the Cu-diimines, also catalysed the oxidative depolymerisation of the carbohydrate model compound dextran. As a result of the depolymerisation, the relative viscosity of the reaction mixture decreased. Irrespective of the transition metal catalyst, both the oxidation of the lignin model compounds and the depolymerisation of dextran generated hydrogen peroxide as by-product. However, the reactions of the generated hydrogen peroxide depended on the catalyst. In the presence of Co-sulphosalen, oxygen was selectively transferred to the lignin model compounds and dextran was depolymerised only by the generated hydrogen peroxide. In the presence of Cu-diimines, the generated hydrogen peroxide was essential for the oxidation of the lignin model compounds, and the depolymerisation of dextran proceeded with oxygen as well as hydrogen peroxide.The activities of Co-sulphosalen and Cu-diimines in the oxidation of 2,2'-biphenol and veratryl alcohol correlated well with their ability to catalyse the oxygen delignification of softwood pulp. In addition, their strong tendency to increase the depolymerisation rate of dextran was in accordance with the loss in viscosity detected in delignification experiments. The model compound studies were confirmed to be a useful way to obtain information about the reactions occurring during catalytic oxygen delignification.