Chemistry and spectroscopy of vanadium in VAPO-5 molecular sieves

Abstract

VAPO-5 molecular sieves were synthesized hydrothermally starting with different vanadium, phosphorus, and template contents and with different templates as a function of time. The crystallization process was followed by X-ray diffraction and scanning electron microscopy; the coordination of vanadium was investigated by diffuse reflectance spectroscopy (d.r.s.) and electron spin resonance (e.s.r.). In the gels, vanadium is present as an octahedral ion and can be washed off easily. The crystallization process is retarded by increasing the vanadium content and gel (V + Al)/P ratios different from 1. Good crystalline materials were obtained by using tripropylamine, triethylamine, and tetrapropylammoniumhydroxide as template and synthesis times between 48 and 120 h. In the as-synthesized samples, isolated (pseudo-)octahedral V 4+ is detected by e.s.r. and d.r.s. This vanadium is accessible by ammonia and comprises only 10% of the total vanadium content. Upon calcination, V 4+ ions are oxidized to tetrahedral V 5+, which can only be reduced partially to V 4+. On this basis, no unambiguous evidence is found for isomorphous substituted vanadium in AlPO-5 molecular sieves.