A mechanistic study on alcohol oxidations with oxygen catalysed by TPAP-doped ormosils in supercritical carbon dioxide

The heterogeneous oxidation of various alcohols with oxygen catalysed by TPAP-doped ormosils in scCO(2) at 75 degrees C and 22.0 MPa has been studied in detail. Sol-gel segregation of TPAP into the inner porosity of an organically modified silica (ormosil) xerogel 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 products. 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 bar. Evidence is presented for an associative oxidation mechanism simultaneously involving TPAP, organic substrate and oxygen