Investigation of the effect of VOx/ZrO2 structure on the catalytic activity in cyclohexene epoxidation

Abstract

Vanadium oxide catalysts were prepared by impregnation of ammonium metavanadate on monoclinic zirconia with a metal loading equal to 5 wt%. Further thermal treatment led to V₂O₅/ZrO₂ upon calcination at 400 °C and VO₂/ZrO₂ upon calcination at 400 °C, followed by reduction at the same temperature. The third compound underwent further heat treatment at 800 °C, under argon. The effect of high temperature treatment on the structure of the materials as well as on the catalytic properties of the vanadate species was investigated in the epoxidation of cyclohexene, with TBHP as the oxidant and heptane as the solvent. Structural characterization of these materials was carried out using ICP-OES, X-ray diffraction (XRD), N₂ adsorption–desorption at 77 K, UV-Vis diffuse reflectance spectroscopy (DR UV-Vis), Fourier-Transform Infrared Spectroscopy (FTIR) and pyridine adsorption followed by FTIR. The coexistence of a solid solution with polymeric vanadium species in the case of 5VZr-2 and 5VZr-3 with the formation of ZrV₂O₇ in the case of 5VZr-3, resulted in good catalytic performance, with up to 80% epoxide selectivity and 32% cyclohexene conversion. In the case of 5VZr-1, the appearance of the crystalline V₂O₅ phase decreased the catalytic activity in the cyclohexene reaction.