Dispersion and reactivity of V 2O 5 catalysts supported on Al 2O 3–ZrO 2

Abstract

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt% supported on Al 2O 3–ZrO 2 (1:1 wt%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction, BET surface area, pulse oxygen chemisorption, and temperature programmed desorption (TPD) of NH 3. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. At lower loadings, vanadia is found to be present in a highly dispersed state and dispersion decreases steadily with increase in vanadia loadings. X-ray diffraction results also suggest that vanadium oxide exists in a highly dispersed amorphous state at lower loadings and in a crystalline V 2O 5 phase at higher vanadia loadings. The effect of vanadia loading has been studied on the ammoxidation of toluene to benzonitrile over V 2O 5/Al 2O 3–ZrO 2 catalysts. The catalytic activity for the ammoxidation of toluene increases with vanadia loading up to 12.5 wt%, which corresponds to monolayer coverage and levels off at higher vanadia loadings. The correlation of catalytic activity with oxygen chemisorption sites and ammonia TPD measurements was discussed.