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

摘要

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, Ag1.2V3O8 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 Ag1.2V3O8 were converted into Ag0.68V2O5 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.