Manganese Porphyrin-catalysed Heterogeneous and Unusually Selective Oxidation of Sulfides by Monopersulfate in Supercritical C02

The oxidation of sulfides by Oxone® in the presence of catalytic amount of various manganese porphyrins in supercritical CO2 (scCO2) has been studied. The reactions have been carried out in an anhydrous two phase-system (solid Oxone®, solid catalyst/scCO2). In the experimental conditions employed (40 oC, 20 MPa), only the new synthesised 5,10,15,20-tetrakis(heptafluoropropyl)porphyrinate-manganese(III) chloride showed a slight solubility in scCO2. In the absence of manganese porphyrin and/or of an aromatic nitrogen base a slow stoichiometric sulfide oxidation leading mainly to sulfoxide is observed in spite of the extremely low solubility of Oxone® in scCO2. Conversely, in the presence of 4-tert.-butylpyridine and catalytic amount of manganese porphyrins (0.6% with respect to sulfide) enhanced reaction rates are observed. Furthermore, the selectivity of sulfide oxidation reverses and sulfone is formed in larger or comparable amount than sulfoxide even when an excess of sulfide over oxidant is employed. The results collected suggest that the local concentration of the intermediate sulfoxide around the catalyst is higher than that of the bulk supercritical phase thus determining unusual high reaction rates for sulfone formation.