The effect of zirconia polymorphs on the structure and catalytic properties of V2O5/ZrO2 catalysts

The effect of zirconia polymorphs on the activity and selectivity of V2O5/ZrO2 for the alkylation of phenol with methanol has been investigated. Relatively pure monoclinic zirconia (m-ZrO2) and tetragonal zirconia (t-ZrO2) were prepared. A series of vanadium oxide supported on m- and t-ZrO2 with varying vanadium oxide content from 2.5 to 15 wt% were prepared by wet impregnation method. The catalysts were characterized by X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (UV-DRS), temperature-programmed reduction of H2 (TPR), temperature-programmed desorption (TPD) of NH3, specific surface area and pore size distribution measurements. The XRD results suggest that highly dispersed or amorphous state of vanadium oxide is present at lower loadings. X-ray diffraction patterns also indicate the presence of crystalline V2O5 phase from 7.5 wt% of V2O5 on m-ZrO2 support and there is no indication for the formation of V2O5 crystalline phase up to 15 wt% of V2O5 on t-ZrO2 support. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V5+ species at lower loadings in all catalysts. The temperature-programmed reduction of H2 shows a single reduction peak corresponding to V5+ to V3+ in all the samples. TPD of ammonia suggests that surface acid density of m-ZrO2 phase is found to be more than t-ZrO2 phase. Acidity of the supported catalysts is increased with increase in V2O5 loading up to monolayer formation and decreased with the appearance of crystalline phase of V2O5. At monolayer coverage, V2O5/m-ZrO2 is fourfold more active than V2O5/t-ZrO2 for alkylation of phenol at 673 K. Selectively C-alkylated products were formed on all the catalysts. A linear correlation is observed between catalytic activity and surface acidity of catalysts.