Surface Acidic and Redox Properties of V–Ag–O/TiO₂ Catalysts for the Selective Oxidation of Toluene to Benzaldehyde

Abstract

Previous work showed that the V–Ag–O complex oxides exhibited quite good catalytic behavior for the selective oxidation of toluene to benzaldehyde. In this work, TiO₂ was added into V–Ag–O by co-precipitation with a sol–gel method. Structural characterizations using X-ray diffraction and Fourier transform infrared spectroscopy indicated the phases of Ag₂V₄O₁₁, Ag_1.2V₃O₈ and TiO₂ in the V–Ag–O/TiO₂ before the reaction. No complex oxide phases involving titanium were observed. Thus, the addition of TiO₂ seemed to generate the interfaces between TiO₂ and the silver vanadates. The Ag₂V₄O₁₁ and part of Ag_1.2V₃O₈ were converted into Ag_0.68V₂O₅ and metallic Ag during the reaction. The results of temperature programmed reduction, microcalorimetric adsorption of NH₃ and isopropanol probe reaction in air revealed that the addition of TiO₂ might increase both the surface acidity and redox ability of the catalysts. The increased redox ability seemed to improve the activity for the oxidation of toluene, but the increased surface acidity might lead to the decrease of selectivity to benzaldehyde. The V–Ag–O/TiO₂ with 20% TiO₂ exhibited significantly improved catalytic behavior for the selective oxidation of toluene to benzaldehyde, as compared to the un-promoted V–Ag–O catalyst. The conversion of toluene reached 7.3% over the V–Ag–O/20%TiO₂ at 613 K with 95% selectivity to benzaldehyde.