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 sel ectively 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 prec ursors were Y-4[PMo11 VO40] (Y = Bu4N, 1a; K, 1b; Na,: 1c), Y4K[PW10V2O40] (Y = Bu4N, 2a; K, 2b) and K-7[P2W15Mo2VO62] 3b.(1) As control experiments, radical reactions with [VO(O-2)(pic)(H2O)(2)].2H(2)O 4 (pic, picolinate) an d Fenton reagent 5 ((FeSO4)-S-II/30% aqueous H2O2 plus 0.05 ml of 60% aqueo us HClO4) for the toluene and nitrobenzene oxidations were also examined un der 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)-O-.. The GC/MS spectral patterns of toluene oxidation catalyzed by th e POMs have suggested that there are four different reactions by the POM gr oups and at least three of them, i.e. (1), (3) and (4), proceed via differe nt radical mechanisms: (1) the reaction by 2a resembled that by Fenton reag ent 5 closely in the preferential side-chain oxidation, (2) the reactions b y 2b and 3b showed preferential ring-oxidation, in particular, the selectiv e production of o-cresol, which were quite different from the radical react ions by 4 and 5; (3) the reactions by lb and 1c were very similar to the ra dical reaction by 4 showing comparably both the side-chain oxidation and th e ring-oxidation; and (4) the reaction by la showed low selectivity in side -chain oxidation, although the ring-oxidation was very similar to that by l b and 1c. These facts will be also the case for benzene oxidation. The uniq ue catalytic behavior of 2b and 3b has been also observed in nitrobenzene o xidation. (C) 2001 Elsevier Science BN. All rights reserved.