Oxidation activity and stability of homogeneous cobalt-sulphosalen catalyst: Studies with a phenolic and a non-phenolic lignin model compound in aqueous alkaline medium

Abstract

Oxidation activity and stability of cobalt-Schiff base complex catalyst, Co-sulphosalen, were studied in aqueous alkaline medium. The 4-coordinated Co-sulphosalen was shown to be an active catalyst, with molecular oxygen increasing the initial oxidation rates of both a phenolic (guaiacol) and a non-phenolic (veratryl alcohol) lignin model compounds. Studies with a visible absorption spectrometer showed that 4-coordinated Co-sulphosalen forms a new complex with pyridine, which attaches as axial ligand. The activity of Co-sulphosalen complex was not increased, however, but instead was slightly decreased by the addition of pyridine. With a newly developed HPLC method, it was shown that Co-sulphosalen is not stable but decomposes as a function of time. The decomposition of the catalyst through hydrolysis of its imine structures was independent of the oxidation of the model compound. The decomposition rate of Co-sulphosalen increased with increasing pH and was higher in the absence of oxygen. The presence of pyridine had virtually no effect on the stability of the catalyst.