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

The nature of the active oxygen species on SmCl3-supported and FeCl3-s upported graphite cathodes (Sm/Gr and Fe/Gr) and the reaction mechanis m of the oxidation of cyclohexane initiated by these active oxygens ha ve been studied. The active oxygen species on Sm/Gr and Fe/Gr exhibite d a similar regioselectivity to that of HO. for the oxidation of alkan es (hexane, adamantane, cyclohexane) rather than those of the metal-po rphyrin or the Gif(IV) systems. The active oxygen on Sm/Gr exhibited l ower 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 produc t yield of Fe/Gr decreased appreciably upon addition of CCl4 and (CH3) (2)S 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 isot ope 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 sugges ts 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-delta)(O2 H)(delta+) from the results of cyclic voltammetry experiments. It was suggested that the cyclohexyl radical was the common reaction intermed iate for formation of cyclohexanol (CyOH) and of cyclohexanone (CyO) i n 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 cyclohex yl radical was suggested to be the reaction intermediate for CyO forma tion but not for CyOH formation over Fe/Gr. The oxygen isotope experim ents for the reaction over Sm/Gr indicated that 93% of the oxygen in t he products came from the gaseous oxygen and 7% from electrolyte solut ions. In the case of Fe/Gr, all of the oxygen in the products came fro m the gas phase. Reasonable reaction mechanisms on the basis of the re sults described above have been proposed for the oxidation of cyclohex ane on Sm/Gr and Fe/Gr.