Supported dodecaphosphotungstate and undecaphosphotungstate: a study on the kinetic behavior for the oxidation of styrene

Zirconia supported dodecaphosphotungstate and undecaphosphotungstate were successfully used as heterogeneous catalysts for the liquid-phase solvent-free oxidation of styrene. A detailed kinetic study was carried out in order to study the effect of removal of one addenda atom from the parent Keggin unit. The supported catalysts were also systematically characterized by various physicochemical techniques to show that the Keggin unit remains intact even after supporting on the support. Thermal studies also indicate that the supported catalysts are stable up to 450 A degrees C. Maximum conversion with a single selective product was obtained after 30 h for the undecaphosphotungstate and after 20 h for dodecaphosphotungstate. The rate constants as well as the activation energy for the reaction were determined. From the values of activation energy derived from the Arrhenius plot, different thermodynamic parameters such as enthalpy and Gibbs free energy were also calculated. From the above studies, supported dodecaphosphotungstate was found to be more active as compared to undecaphosphotungstate.