Spectroscopic investigation of the molybdenum active sites on MoVI heterogeneous catalysts for alkene epoxidation

Spectroscopic and spectromagnetic investigations on molybdenum centres in MoO3·H2O and its polymer-supported form have been carried out. Both were tested as catalysts for cyclohexene epoxidation in a heterogeneous phase; both needed activating pretreatment before the reaction. XPS and EPR data show that the metal is mainly present as MoVI. The activation process increases the acidic character of MoVI and its efficiency depends both on the oxidizing power of the activating reactant and on the accessibility of the metal to the reactant. The steric hindrance of MoVI lowers the catalytic activity owing to the replacement of H2O by carboxylic groups of the functionalized resin.