Investigation of the nature of the active oxygen intermediate at graphite-supported SmCl3 and FeCl3 working as a cathode for the partial oxidation of alkanes and aromatics

Abstract

The nature of the active oxygen species on SmCl₃-supported and FeCl₃-supported graphite cathodes (Sm/Gr and Fe/Gr) and the reaction mechanism of the oxidation of cyclohexane initiated by these active oxygens have been studied. The active oxygen species on Sm/Gr and Fe/Gr exhibited a similar regioselectivity to that of HO˙ for the oxidation of alkanes (hexane, adamantane, cyclohexane) rather than those of the metal–prophyrin or the Gif(IV) systems. The active oxygen on Sm/Gr exhibited lower electrophilicity for the oxidation of alkanes than that on Fe/Gr. The product selectivities in the oxidation of toluene over Fe/Gr were those expected from Fenton chemistry. However, the selectivities over Sm/Gr could not be explained in terms of Fenton chemistry. The product yield of Fe/Gr decreased appreciably upon addition of CCl₄ and (CH₃)₂S as radical scavengers; however, that for Sm/Gr was affected only slightly. These results suggest that the active oxygen on Sm/Gr is not of radical character in contrast with that on Fe/Gr. The kinetic isotope effect in the oxidation of cyclohexane over Sm/Gr (1.7) was larger than that over Fe/Gr (1.3) and that for HO˙(1.3). This result suggests that the active oxygen species on Fe/Gr is HO˙. On the other hand, the active oxygen species on Sm/Gr was suggested to be Sm^(3–δ)(O₂H)^δ+ from the results of cyclic voltammetry experiments. It was suggested that the cyclohexyl radical was the common reaction intermediate for formation of cyclohexanol (CyOH) and of cyclohexanone (CyO) in the oxidation of cyclohexane on Sm/Gr. Because cyclohexyl chloride (CyCl) was produced by addition of CCl₄ and the CyOH : CyO ratio did not change during oxidation with or without addition of CCl₄, the cyclohexyl radical was suggested to be the reaction intermediate for CyO formation but not for CyOH formation over Fe/Gr. The oxygen isotope experiments for the reaction over Sm/Gr indicated that 93% of the oxygen in the products came from the gaseous oxygen and 7% from electrolyte solutions. In the case of Fe/Gr, all of the oxygen in the products came from the gas phase. Reasonable reaction mechanisms on the basis of the results described above have been proposed for the oxidation of cyclohexane on Sm/Gr and Fe/Gr.