Surface acidic and redox properties of V-Ag-Ni-O catalysts for the selective oxidation of toluene to benzaldehyde

V-Ag-O and V-Ag-Ni-O complex oxides were found to exhibit good catalytic reactivity for the selective oxidation of toluene to benzaldehyde. In order to understand the effects of each metal in the two catalysts, a series of single and complex oxides were prepared including V2O5, Ag2O, NiO, V-Ni-O, Ag-Ni-O, V-Ag-O and V-Ag-Ni-O with the previously optimized compositions for the binary and tertiary oxides. Data from the X-ray diffraction (XRD), infrared spectroscopy and laser Raman spectroscopy showed the formation of complex oxides with phases of silver and nickel vanadates as well as the broadening and red shift of the band of VO bonds, suggesting the formation of VOAg and VONi bonds and the increased redox ability of vanadium species. The results from temperature programmed reduction (TPR) and the combined technique of TPR-XRD revealed that the addition of Ag and Ni promoted the reduction of vanadium species in the complex catalysts. Microcalorimetric adsorption of ammonia and the probe reaction of isopropanol in air demonstrated that the surface acidity was greatly decreased while the redox cycles of vanadium sites were significantly enhanced upon the addition of Ag and Ni. The reactivity for the selective oxidation of toluene was evaluated by using a fixed-bed microreactor, and was correlated with the surface acidity and redox property of the catalysts. Silver seemed to be a special promoter that not only decreased the surface acidity but also enhanced the redox cycles of vanadium sites, leading to the increased conversion of toluene and the selectivity to benzaldehyde. Such effect was even more pronounced when Ag and Ni were added simultaneously. Accordingly, the V-Ag-Ni-O catalyst studied in this work exhibited excellent performance for the selective oxidation of toluene to benzaldehyde. The conversion of toluene was found to be 8% with 95% selectivity to benzaldehyde on the V-Ag-Ni-O at 613 K.