Molecular Level Insights into the Structure of Active Sites of VAIO Mixed Oxides in Propane Ammoxidation
Résumé
Several techniques such as Al-27 800-MHz NMR, X-ray diffraction (XRD; powder), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), Raman spectroscopy, and electrical conductivity measurements have been applied to characterize VAlO mixed oxides used in the selective ammoxidation of propane to acrylonitrile. The results confirmed that the catalysts were nitrided when exposed to ammoxidation conditions and that the nitridation occurred only for mixed oxides. The solids corresponding to poorly crystallized x-alumina contained usual six-, five-, and four-coordinated aluminum and after testing, AlO3N species in significant amount. These results are in line with those of XPS showing besides adsorbed nitrogenous species, the presence of NH2- species allowing to propose AlO3(NH)VO5,6 as active site for catalytic nitridation. On another side, the samples prepared by a new method allowing to better disperse vanadium in the mixed oxide as shown by EPR, were shown to be more active and selective. Based on these observations, it is proposed that most selective sites for propane activation correspond to monomeric isolated species or to low-size oligomeric vanadium oxide species, which is in line with any other study of vanadium containing catalysts used for light alkanes oxidation or ammoxidation. Finally, nitrogen substitution in the bulk was shown to greatly enhance the electrical conductivity, which should facilitate the complete catalytic redox cycling.