The effect of redox properties of ceria-supported vanadium oxides in liquid phase cyclohexene oxidation

Abstract

Several VO₂/CeO₂ based catalysts, with V loadings between 5 and 20 wt%, were prepared by impregnation and subsequently dried, calcined and reduced at 673 K. The structural characterization of these materials was carried out using X-ray diffraction (XRD), N₂ adsorption–desorption at 77 K, UV-vis diffuse reflectance spectroscopy (DR UV-vis) and Fourier-transform infrared spectroscopy (FTIR). Their catalytic activities in the epoxidation of cyclohexene, with TBHP as oxidant and heptane as solvent, were also examined. XRD results revealed a good dispersion of VO₂ species on ceria. FTIR and DR UV-visible spectroscopy techniques indicated that polymeric vanadyl species are present on all catalysts and a crystalline VO₂ phase appears on the 20 wt% vanadium sample. Experimental results revealed that catalyst activity decreases with increased vanadium loading, whereas the epoxide selectivity increased up to 90% when the vanadia loading reached 15 wt%. In the case of 20 wt% VO₂/CeO₂, the reaction switched towards a 100% enol, enone selectivity. The presence of CeVO₄ was shown to influence the conversion and selectivity of the reaction.