An efficient method to investigate metal?ligand combinations for oxygen bleaching

Catalytic activities of 189 metal–ligand combinations in aqueous alkaline solution were studied in the oxidation of veratryl alcohol with molecular oxygen. The parallel screening revealed high oxidation activity of copper ions with dimethylglyoxime, 2,2′-bipyridine and 1,10-phenanthroline ligands. Further study with these in situ prepared catalysts indicated that ionic complexes with two ligands are probably the most active. The maximum activities of the three catalysts were achieved at different pH and metal–ligand molar ratio. In 3 h reaction with catalyst–substrate molar ratio 1:1000, the highest turnover numbers obtained were 420 for the Cu-dimethylglyoxime catalyst, 830 for Cu-bipyridine and 760 for Cu-phenanthroline. These most effective Cu-complexes catalysed not only the oxidation of lignin model compounds but also the depolymerisation of the carbohydrate model compound dextran. Results achieved in oxygen delignification of softwood pulp were in accordance with the model compound studies; the catalysts effectively lowered the kappa number, but they also tended to damage the cellulose fibres.