A Mechanistic Study on Alcohol Oxidations with Oxygen Catalysed by TPA-doped Ormosils in Supercritical Carbon Dioxide

Abstract: The heterogeneous oxidation of various alcohols with oxygen catalysed by TPAP doped ormosils in scCO2 at 75 oC and 22.0 MPa has been studied in detail. Sol-gel segregation of TPAP into the inner porosity of organically modified silica along with the use of a reaction medium which does not dissolve the catalyst, prevents aggregation of oxidation-inactive ruthenium derivatives without the need of chemical tethering. Thus, at least 140 TONs may be obtained in the oxidation of primary alcohols with the formation of aldehydes as sole reaction product. Investigation of the oxidation mechanism shows that the catalytic process exhibits a first-order dependence on the amount of catalyst, a fractional order on the alcohol concentration and a negative order for oxygen pressures higher than 0.2 Pa. Evidence is reported for an associative oxidation mechanism simultaneously involving TPAP, organic substrate and oxygen.