Liquid-phase oxidation of benzene to phenol by molecular oxygen over transition metal substituted polyoxometalate compounds

Abstract

Transition metal substituted polyoxometalate (TMSP) compounds were used as catalysts for the liquid-phase oxidation of benzene to phenol by molecular oxygen with ascorbic acid as a reducing agent in an acetone/sulfolane/water mixed solvent. [(C 4H 9) 4N] 5[PW 11CuO 39(H 2O)] is the best catalyst tested in this study. It showed 9.2% benzene conversion (TON = 25.8) and 91.8% selectivity to phenol for the oxidation of benzene at 323 K for 12 h. The effect of the substituted transition metal in the TMSP compounds on the benzene conversion is in the order: Cu > V > Fe ≫ Mn > Ti > Cr > Co > Ni > Zn. The effect of the central atom in the TMSP compounds on the benzene conversion is in the order: P ≈ Si > Ge > B. [SiW 11O 39] 8− is a good polyoxometalate ligand for transition metal ions as [PW 11O 39] 7− for the oxidation of benzene. The selectivity to phenol was dramatically improved by adding the sulfolane solvent, but the benzene conversion decreased when a large amount of sulfolane was used in the mixed solvent. Ascorbic acid is indispensable for forming phenol from benzene oxidation by O 2 over TMSP compounds. Higher O 2 pressure in the catalytic system ensured more oxygen molecules solving in the solvent, and promoted the benzene conversion.