The effect of zirconia polymorphs on the structure and catalytic properties of V 2O 5/ZrO 2 catalysts

Abstract

The effect of zirconia polymorphs on the activity and selectivity of V 2O 5/ZrO 2 for the alkylation of phenol with methanol has been investigated. Relatively pure monoclinic zirconia (m-ZrO 2) and tetragonal zirconia (t-ZrO 2) were prepared. A series of vanadium oxide supported on m- and t-ZrO 2 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 H 2 (TPR), temperature-programmed desorption (TPD) of NH 3, 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 V 2O 5 phase from 7.5 wt% of V 2O 5 on m-ZrO 2 support and there is no indication for the formation of V 2O 5 crystalline phase up to 15 wt% of V 2O 5 on t-ZrO 2 support. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V 5+ species at lower loadings in all catalysts. The temperature-programmed reduction of H 2 shows a single reduction peak corresponding to V 5+ to V 3+ in all the samples. TPD of ammonia suggests that surface acid density of m-ZrO 2 phase is found to be more than t-ZrO 2 phase. Acidity of the supported catalysts is increased with increase in V 2O 5 loading up to monolayer formation and decreased with the appearance of crystalline phase of V 2O 5. At monolayer coverage, V 2O 5/m-ZrO 2 is fourfold more active than V 2O 5/t-ZrO 2 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.