Photosensitized degradation of cyclohexanol by Fe(III) species in alkaline aqueous media

The degradation of cyclohexanol that can be photoinduced by Fe(III) derivatives (e.g. nitrate, pyrophosphate and sulfate) has been investigated in aqueous media at room temperature and under neutral to alkaline conditions with continuous irradiation at 254, 366 and around 500 nm for 24 h. The conditions chosen are those putative to the Martian soil, and the experiments were relevant in view of the Viking experiments. The conventional homogenous photocatalytic process that is based on the reduction of Fe(III) into Fe(II) and that is responsible for the continuous production of hydroxyl radicals also occurs under the alkaline conditions used. During the oxidation of cyclohexanol, cyclohexanone was formed in reasonable amounts in the presence of the nitrate salt, while with the other two iron salts it was present in negligible amounts, and carbon dioxide was instead the dominant reaction product. Together with cyclohexanone, some unusual dihydroxylated and ketohydroxylated cyclic products were also formed. The full product distribution allows us to infer the mechanism involved: the nitrate salt produces a large amount of HO that leads to an unselective reaction pathway, with the formation of a several cyclohexyl reaction products, while with iron pyrophosphate and iron sulfate, mineralization to CO2 is the dominant pathway, although there remains a minor conversion pathway to cyclohexanol (35% versus 85% of the aqueous cyclohexanol nitrate salt).