Antimony oxide-modified vanadia-based catalysts - Physical characterization and catalytic properties

Antimony-modified vanadia-on-titania catalysts were prepared for the select ive oxidation of o-xylene to phthalic anhydride by ball milling of powder m ixtures followed by calcination. A binary Sb2O3-V2O5 system was also prepar ed for comparison purposes. The resulting materials were physically charact erized by surface area measurements, X-ray diffraction analysis (XRD), lase r Raman spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron spin resonance (ESR), magnetic susceptibility determination, and V -15 solid-state NMR. The catalytic performance of the TiO2-supported materi als was tested for o-xylene oxidation. After calcination of the Sb2O3-V2O5 binary mixture at 673 K, Sb3+ is almost quantitatively oxidized to Sb5+, wh ile both V3+ and V4+ are detected. V3+ and some V4+ are most likely located in a nonstoichiometric VSbO4-like structure, while the majority of V4+ pre ferentially concentrates within shear domains in oxygen-deficient V2O5-x pa rticles. In the titania-supported catalyst system, both Sb2O3 and V2O5 spre ad on the anatase surface. Sb3+ is oxidized to Sb5+, and V3+, V4+, and V5are detected. VSbO4-like structures are not observed. The presence of antim ony leads to the formation of presumably V3+-O-V5+ redox couples. The param agnetic centers-in contrast to the binary mixture-are largely isolated. Ant imony preferentially migrates to the surface and appears to exhibit a dual function catalytically. It is inferred from the experimental data that the addition of antimony leads to site isolation and to a reduction of surface acidity. We suggest that V-O-V-O-V domains or clusters are interrupted by i ncorporation of Sb to form V-O-Sb-O-V species. As a consequence of this sit e isolation and a reduction of surface acidity, overoxidation of o-xylene i s reduced. These two effects are therefore most probably responsible for th e improved selectivity of the ternary catalyst system over the binary one t oward phthalic anhydride.