Comparative study of the vanadium species in VAPO-5 and VAPSO-5 molecular sieves

By using XRD, TG-DTA, NMR, and UV resonance Raman spectroscopy, the nature and location of vanadium species between VAPO-5 and VAPSO-5 molecular sieves were comparatively studied. It was found that different vanadium species exist in VAPO-5 and VAPSO-5 molecular sieves. Polymerized vanadium oxides and bulk V₂O₅ crystallites are the main vanadium species in VAPO-5 molecular sieves. However, in VAPSO-5 molecular sieves, tetrahedrally coordinated vanadium species are dominant. Namely, most of the vanadium species in VAPO-5 exist outside the framework, whereas most of the vanadium species in VAPSO-5 are incorporated into the framework. The formation of framework vanadium in VAPSO-5 molecular sieves is found to be associated essentially with the presence of framework silicon atoms.