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

Abstract

Vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31 were synthesized hydrothermally and fully characterized using XRD, A.A, SEM, N 2 and n-butane adsorption, diffuse reflectance UV-Vis, XPS, 27 Al , 31 P , 51 V 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 51 V NMR spectra of calcined air-exposed (hydrated) VAPOs samples revealed that vanadium cations are present as V 5+ 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, V 5+ ions are mostly in tetrahedral coordination without any presence of polymeric extraframework oxy-vanadium phase. ESR and XPS spectroscopies indicated the presence of residual V 4+ 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.