Microcalorimetric adsorption characterizations of supported vanadia catalysts for the selective oxidation of propylene to acetone

Structural characterizations and chemisorption of O-2 revealed that vanadium species in our 10% V2O5/TiO2 and 10% V2O5/gamma-Al2O3 catalysts were highly dispersed polyvanadates, while that in the 10% V2O5/SiO2 was small-crystalline V2O5. Although the 10% V2O5/H-ZSM-5 exhibits strong Bronsted acidity that favors the adsorption of propylene, the poorly dispersed vanadium species makes it a poor catalyst for the conversion of propylene to acetone. The key factors determining the activity of the reaction are the number and strength of Bronsted acid sites of V-OH groups and the oxidation ability of oxidized vanadium species in the catalysts. The V2O5/TiO2 catalyst exhibits the strongest Bronsted acidity of V-OH groups in the catalysts studied, and therefore it adsorbs propylene and shows the high activity for the conversion of propylene to acetone. In fact, Fourier transform infrared indicates that the adsorption of propylene on the V2O5/TiO2 catalyst at room temperature leads to the formation of surface isopropoxy groups and adsorbed acetone. Both the 10% V2O5/gamma-Al2O3 and 10% V2O5/SiO2 catalysts possess weak Bronsted acidity of V-OH groups and therefore exhibit the low activity for the conversion of propylene to acetone. Microcalorimetric adsorption of reactants and products on the catalysts provides the information about the surface bonding strengths that may be used for the microkinetic analysis of the corresponding surface reactions. The initial heat for water adsorption on the V2O5/TiO2 catalyst is lower than 90 kJ/mol, indicating that water is reversibly adsorbed at temperatures higher than 360 K. The heat of adsorption of acetone on the reduced V2O5/TiO2 catalyst is about 100 kJ/mol, which can be used to estimate the activation energy for desorption of acetone from the catalyst. O-2 adsorption on the reduced catalysts produces high heat, indicating that the reoxidation of reduced vanadium species by O-2 is an irreversible reaction Step. (C) 2002 Elsevier Science.