Kinetics and mechanism of selective hydroxylation of benzene catalysed by vanadium substituted heteropolymolybdates

Hydroxylation of benzene to phenol was catalysed by vanadium substituted he teropolymolybdates of the general formula H3+x [PMo12-xVxO40]. nH(2)O where , x = 1, 2, 3. The optimum catalytic activities (TON = 50-90) were found, w hen the reaction was performed with equimolar ratio of substrate to oxidant (hydrogen peroxide) at 338 K. Kinetic investigations of the reaction catal ysed by mono vanadium substituted heteropolymolybdates, revealed a first-or der dependence with respect to the concentrations of the substrate and the catalyst and a half-order dependence with respect to concentration of the o xidant. Temperature dependence on the rate of hydroxylation was also studie d and from Arrhenius plot, the activation energy was evaluated to be 21.5 k cal mol(-1). In situ UV-VIS and electron paramagnetic resonance (EPR) spect roscopic studies were carried out to establish the active species. A mechan ism involving vanadium (V) peroxo intermediate species has been proposed fo r the hydroxylation of benzene to phenol. (C) 2000 Elsevier Science B.V. Al l rights reserved.