A novel manganese tungstate nanorod catalyst for the oxidation of styrene with tert-butyl hydroperoxide as oxidant

Catalyst, solvent and process parameters are the keys for the selective oxidation of styrene and other olefins to value added intermediates. Hence MnWO₄ was synthesized by the one-pot hydrothermal method using Pluronic F127 as a surfactant, characterized and evaluated for the oxidation of styrene and different olefins, using tert-butyl hydroperoxide (TBHP) as oxidant. X-ray diffractogram confirmed the phase purity of MnWO₄ which became further evident from the near surface equimolar proposition of Mn and W in + 2 and + 6 oxidation states, respectively, as inferred from X-ray photoelectron spectra. N₂ sorption followed type IVa isotherm with H₃ hysteresis loop, starting at the relative pressure (P/P_o ) of 0.9, typically of mesoporous materials containing macropores as further confirmed by pore-size distribution. Transmission electron microscope images of MnWO₄ revealed its nanorod morphology. In the absence of catalyst, styrene conversion was 35% with benzaldehyde as the sole product and addition of 75 g of MnWO₄ increased the styrene conversion to 97.3%, and the styrene oxide selectivity from 0% to 85% demonstrated its activity and selectivity. For the maximum conversion and styrene oxide selectivity optimum temperature, styrene: TBHP molar ratio, catalyst amount and solvent were found to be 70 °C, 1:2, 75 mg, and acetonitrile, respectively. MnWO₄ catalyst retained its activity and selectivity for five recycles, confirming its recoverability and reusability. From the kinetic studies, pre-exponential factor and apparent activation energy were found to be 0.206 s⁻¹ and 31.5 kJ·mol⁻¹, respectively, and a reaction mechanism was proposed. MnWO₄ catalyst was active for oxidation of alkenes, cycloolefins and alkenyl aromatics with high selectivity for epoxides.