FUNCTIONAL ACTION OF KEGGIN-TYPE MONO-VANADIUM(V)-SUBSTITUTED HETEROPOLYMOLYBDATE AS A SINGLE-SPECIES ON CATALYTIC HYDROXYLATION OF BENZENEIN THE PRESENCE OF HYDROGEN-PEROXIDE

Benzene hydroxylation in the presence of hydrogen peroxide has been st udied using tetrabutylammonium salts of three types of mono-vanadium(V )-substituted heteropolyanions (HPA) as catalyst precursors: (i) [PMo1 1VO40](4-) (PMo11 V, 1), which has been first prepared herein and iden tified as a single species, (ii) '[PMo11VO40](4-)' ('PMo11V', 4), whic h has been obtained traditionally by the so-called ether-extraction me thod and known as a mixed species only with an averaged composition P: Mo:V = 1:11:1 and (iii) [PW11VO40](4-) (PW11V, 2), which has been prep ared as a single species. The PMo11 V, 1, catalytically oxidized benze ne to phenol under the conditions: 0.1 mmol catalyst, 10 ml (113 mmol) benzene, 5 or 10 mi of CH3CN and 2 ml of 30% aqueous H2O2. The cataly tic turnover by 1 for phenol production was dependent on the amount of CH3CN: it was 1.3 after 120 h reaction with 10 ml CH3CN and 1.7 after 192 h reaction with 5 ml CH3CN. On the other hand, the PW11 V, 2, did not show catalytic activity even after 240 h reaction under the same conditions. In comparison, it was also confirmed there was no catalyti c effect by [PMo12O40](3-) (PMo12, 3) without a vanadium(V) center. Th e 'PMo11V', 4, has apparently shown a more enhanced effect, e.g. with catalytic turnover of 7.5 after 144 h reaction with 10 ml CH3CN. These observations significantly suggest that the catalysis by 1 for benzen e hydroxylation is due to cooperative action of the molybdenum framewo rk with one vanadium(V) center. Such action of the polyoxoanion framew ork has not been observed in the selectively site-substituted vanadium (V) heteropolytungstates such as [alpha-1,2-PW10V2O40](5-) and [alpha- 1,2,3-PW9V3O40](6-), the catalyses by which were due to the dinuclear vanadium centers with corner-shared octahedra. Furthermore, the marked stability of the mono-vanadium(V)-substituted species 1 of several va nadium(V)-substituted heteropolymolybdates as catalyst precursors was clarified in the benzene hydroxylation. (C) 1997 Elsevier Science B.V.