Involvement of active sites of promoted vanadyl pyrophosphate in selective oxidation of propane

Involvement of active sites in the selective oxidation of propane was investigated using (VO)2P2O7 catalysts promoted by Sb, Ti, Zr, Hf, Cu, and Ce. Based on X-ray diffraction patterns, infrared spectra, and Raman spectra, the formation of a (VO)2P2O7 phase was confirmed for all of the promoted catalysts; no other phases were detected. After the addition of Sb, Ti, and Zr, the selectivity to acrylic acid increased, whereas Cu-, Ce-, and Hf-promoted catalysts showed lower selectivity than pure (VO)2P2O7. The effect of promoters on the activity and selectivity for the propane oxidation was investigated on the basis of the nature of surface active sites. The surface VO species, evaluated by the nitric oxide-ammonia rectangular pulse technique, decreased with the addition of the promoters. A proportional correlation between the number of surface VO species and reaction rate of propane clarified that the surface VO species is the controlling factor for the catalytic activity of propane oxidation. In the case of Sb-, Ti-, and Zr-promoted catalysts, the selectivity to acrylic acid increased significantly. The selectivity was compared with the number and strength of Brønsted and Lewis acid sites estimated from dimethylpyridine temperature-programmed desorption. A good correlation was observed between the selectivity to acrylic acid and the surface concentration of Lewis acid sites. It was experimentally demonstrated that Lewis acid sites represent the key factor in selective oxidation.