Abstract
The heterogenization of an active homogeneous ruthenium oxidation catalyst on silica has been studied. The ligand 2-(phenylazo)pyridine has been covalently linked onto silica and a ruthenium complex with this anchored ligand has been characterized by elemental analysis, CP MAS 13C NMR and DRIFT infrared. The obtained catalysts appeared to be active for the epoxidation of trans-stilbene, and the catalytic activity and the selectivity were found to be dependent on the cooxidant. The best results were obtained with a catalyst having a ruthenium to ligand ratio of 1.1 to 1.0 and when dioxygen/isobutyraldehyde was used as cooxidant; in that case, after 24 hours at 40°C, 96% conversion of trans-stilbene was found, with 95% selectivity for the epoxide. The catalysts were also found to be active for alcohol oxidation. Cyclobutanol was oxidized to cyclobutanone and acyclic products, indicating both a two- and a one-electron oxidation pathway. 1,2-Cyclohexanediol could almost quantitatively be converted to adipic acid. The immobilized catalysts have also been used for the oxidation of octyl α- d-glucopyranoside to octyl α- d-glucuronic acid. However, for this reaction the corresponding homogeneous ruthenium catalyst was found to be more selective than the heterogenized catalyst. Recycling of the immobilized catalyst for the oxidation of n-butanol showed that a catalyst in which the amine function is methylated is more stable than a catalyst with a secondary amine; in that case a coordinated ruthenium is lost easier. The catalyst with the methylated amine function retains its high oxidation reactivity over at least 4 oxidation cycles.
Published Version
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