Oxidation of toluene and nitrobenzene with 30% aqueous hydrogen peroxide catalyzed by vanadium(V)-substituted polyoxometalates

Product distribution in the oxidation of toluene, as well as nitrobenzene, with 30% aqueous hydrogen peroxide catalyzed by Keggin- and Dawson-type selectively site-substituted vanadium(V) polyoxometalates (POMs) was carefully examined with GC and GC/MS measurements in order to clarify the mechanism of benzene oxidation catalyzed by them. The POMs used here as catalyst precursors were Y4[PMo11VO40] (Y=Bu4N, 1a; K, 1b; Na, 1c), Y4K[PW10V2O40] (Y=Bu4N, 2a; K, 2b) and K7[P2W15Mo2VO62] 3b.1 As control experiments, radical reactions with [VO(O2)(pic)(H2O)2]·2H2O 4 (pic, picolinate) and Fenton reagent 5 (FeIISO4/30% aqueous H2O2 plus 0.05 ml of 60% aqueous HClO4) for the toluene and nitrobenzene oxidations were also examined under our conditions; the hydroxylation of benzene by 4 has been proposed to be promoted by the radical species formed on the vanadium complexes, while that by 5 has been considered to be promoted by the hydroxy radical species OH. The GC/MS spectral patterns of toluene oxidation catalyzed by the POMs have suggested that there are four different reactions by the POM groups and at least three of them, i.e. (1), (3) and (4), proceed via different radical mechanisms: (1) the reaction by 2a resembled that by Fenton reagent 5 closely in the preferential side-chain oxidation; (2) the reactions by 2b and 3b showed preferential ring-oxidation, in particular, the selective production of o-cresol, which were quite different from the radical reactions by 4 and 5; (3) the reactions by 1b and 1c were very similar to the radical reaction by 4 showing comparably both the side-chain oxidation and the ring-oxidation; and (4) the reaction by 1a showed low selectivity in side-chain oxidation, although the ring-oxidation was very similar to that by 1b and 1c. These facts will be also the case for benzene oxidation. The unique catalytic behavior of 2b and 3b has been also observed in nitrobenzene oxidation.