Photocatalytic Processes with Polyoxotungstates: Oxidation of CyclohexylaminE

Photochemical excitation of (nBu4N)4W10O32 in the presence of cyclohexylamine leads to the monoelectronic reduction of the decatungstate (φ = 8 × 10-2) with the formation of cyclohexylamine radical species which can be detected by ESR spin-trapping investigations. Using this technique we could also reveal the presence of OH radicals coordinated to the polytungstate "surface". In the presence of oxygen, catalytic cycles are realized with the formation of the following cyclohexylamine oxidation products: C6H10=NOH (44%), C6H10=O (18%), C6H11NO2 (22%), C6H10=NC6H11 (0.7%), C6H11NHOH (14%). The overall photooxidation quantum yield is 1.5 × 10-1. Under prolonged irradiation, we observe only a slow degradation of the photocatalyst with the oxidation of about 104 mol of cyclohexylamine/mol of consumed decatungstate. Under aerobic conditions, WO3 and TiO2 photoexcited powder suspensions present a very similar behavior, confirming that polyoxometalates can be considered models of transition metal oxides able of mimicking the interactions between the organic substrates and solid surfaces of interest in heterogeneous catalysis.