Catalytic properties of heteropoly complexes containing Fe(III) ions in benzene oxidation by hydrogen peroxide

Elemental analysis, magnetic measurements, IR, P-31 NMR, and UV-VIS spectroscopy were used to study heteropoly complexes (HPC), containing Fe(III) ions and heteropoly anion [PW11O39](7-), isolated from aqueous solutions as tetrabutylammonium (TEA) salts and dissolved in acetonitrile. The complexes identified are: Fe(III)-substituted complexes [PW11O39Fe(H2O)](4-) (1) and [PW11O39Fe(SO4)](6-) (1'); Fe(III)-substituted hydroxo complex [PW11O39Fe(OH)](5-) (2') formed during the precipitation of binuclear mu-oxo complex [(PW11O39Fe)(2)O](10-) (2) from aqueous solutions (pH = 3 divided by 5) by TBA cations; polynuclear Fe(III)- hydroxo complexes [PW11O39FenOxHy](m-) (3) (n similar to 8). The catalytic activity of complexes both in the hydrogen peroxide decomposition and benzene oxidation in a one-phase system HPC + CH3CN + H2O2(aq) + C6H6, with [HPC] = 6 . 10(-3),[H2O2] = 0.175 divided by 1.6, and [C6H6] = 1.4 divided by 5.6 M at 70 degrees C have been studied. In the presence of 1 or 1', the molar ratio between phenol formed and H2O2 decomposed equals 10-20%. HPC containing SO42- ions shows a far lower activity in both reactions. The kinetics of PhOH accumulation conforms to the chain mechanism of H2O2 decomposition. Benzene is oxidized by OH radicals coordinated to HPC. In the presence of complex 2' the rate of H2O2 decomposition is considerably lower. Thus the molar ratio of phenol formed and H2O2 decomposed is almost 60%. Tne mechanism of H2O2 activation by 2' includes most likely the initial formation of a peroxo complex which was observed in absorption spectra. The state of HPC affected by the reaction mixture components was studied by W-VIS spectra. The products of one-electron phenol (or pyrocatechol) oxidation inhibit the catalytic activity due to the complex formation with Fe(III) ions in HPC.