Comparative study of vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31 Original Research Article

Vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31 were synthesized hydrothermally and fully characterized using XRD, A.A, SEM, N2 and n-butane adsorption, diffuse reflectance UV-Vis, XPS, 27Al, 31P, 51V NMR and ESR techniques. XRD data of VAPOs were consistent with their corresponding AlPOs structural patterns and indicated that all VAPOs were very well crystallized. The 51V NMR spectra of calcined air-exposed (hydrated) VAPOs samples revealed that vanadium cations are present as V5+ both in distorted octahedral and tetrahedral coordination and quite probably in a framework position. DRS UV-Visible spectroscopy shows that in low vanadium content calcined samples, V5+ ions are mostly in tetrahedral coordination without any presence of polymeric extraframework oxy-vanadium phase. ESR and XPS spectroscopies indicated the presence of residual V4+ in calcined samples. VAPOs were found to be active in vapor phase oxidation of toluene with molecular oxygen. VAPO-11 and -31 showed high oxidation catalytic activity and selectivity for benzaldehyde. The order of activity were VAPO-11 > VAPO-31 > VAPO-5 ≥ VAPO-17. VAPO-11 better selectivity is related to its unique structural properties compared to those of the other VAPOs synthesized in this study. Article Outline