Catalytic properties of Cr-containing heteropolytungstates in H2O2 participated reactions: H2O2 decomposition and oxidation of unsaturated hydrocarbons with H2O2

Cr-containing derivatives of PW11O397- heteropolyanion (PW11Cr) were synthesized under variation of precursors and procedure (increasing temperature, prolonged keeping of solution, varying molar ratio of starting reagents and acidity) and characterized by elemental analysis, UV-Vis and IR spectroscopy. Tetrabutylammonium salts of PW11Cr were examined in catalysis of H2O2 decomposition and oxygen transfer from H2O2 to the hydrocarbon substrates, cyclohexene and benzene. With all the other conditions of synthesis identical, increasing the pH gave samples more active in H2O2 decomposition. In the H2O2/PW11Cr system, benzene formed phenol and cyclohexene formed a mixture of cyclohexenone, cyclohexenol and cyclohexene oxide. The activity of PW11Cr in hydrocarbon oxidation was sensitive to the procedure used for sample synthesis. Variable spectral characteristics and catalytic properties of PW11Cr samples were suggested to depend on the nature of bond between Cr(III) ion and PW11O397- anion. As distinguished from typical M-substituted polyoxometalates, the samples obtained contain Cr(III) ions which are probably connected with surface oxygen atoms of PW11O397- heteropolyanion near the vacancy of the Keggin structure. Surface location of Cr(III) ions is supposed to favor oxygen transfer from H2O2 to hydrocarbon. In the presence of H2O2, PW11Cr solutions showed the increase of visible absorption and a shift of its maximum from 15200-16300 to 15500-18000 cm(-1), that arose from the reaction of PW11Cr with intermediate products of the H2O2 decomposition. The resulting oxidized species PW11Cr[O] are active oxidants in the reaction with unsaturated hydrocarbons.