Preparation and characterization of vanadium oxide species supported on mesoporous silica for the hydroxylation of benzene

The homogeneous liquid-phase direct catalytic oxidation of benzene to phenol was performed at 60°C in acetonitrile solvent using various catalysts of VO_x supported on aluminum-substituted mesoporous silica (Al-MPS) and hydrogen peroxide as the oxidant. We employed the molecular designed dispersion method to prepare the VO_x catalyst supported on Al-MPS. The deposit of monolayer VO(acac)₂ complexes (acac: acetylacetonate) could be achieved by hydrogen bonding or a ligand-exchange mechanism and subsequently decomposition of organic acac ligands in an oxygen atmosphere at elevated temperature, yielding the six coordination of V^v center. The octahedral vanadium oxide samples possess much mobile ligands (i.e., H ₂O or solvent) around the catalytic center that could be easily substituted by hydrogen peroxide and yielded the active peroxo intermediate for the hydroxylation of benzene. From the studies of powder X-ray diffraction, N₂ adsorption and desorption isotherms, diffuse reflectance UV-vis, and electron paramagnetic resonance, we show that highly dispersed isolated vanadium oxide catalyst centers could be formed. For comparison, we also prepared the tetrahedral vanadium oxide catalyst by the coordination of V ^v center with mono-, di-, and triamine-modified SiO₂. The tetrahedral coordination of vanadium oxide catalysts lack mobile ligands with good leaving groups in the catalytic medium and yield a lower activity toward the hydroxylation of benzene.