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

Vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31 were sy nthesized hydrothermally and fully characterized using XRD, A.A, SEM, N-2 a nd n-butane adsorption, diffuse reflectance W-Vis, XPS, Al-27, P-31, V-51 N MR 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 V-5 1 NMR spectra of calcined air-exposed (hydrated) VAPOs samples revealed tha t vanadium cations are present as V5+ both in distorted octahedral and tetr ahedral coordination and quite probably in a framework position. DRS UV-Vis ible spectroscopy shows that in low vanadium content calcined samples, V5ions are mostly in tetrahedral coordination without any presence of polymer ic 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 mole cular 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 greater than or equal to VAPO-17. VAPO-11 better selectivity is re lated to its unique structural properties compared to those of the other VA POs synthesized in this study. (C) 2000 Elsevier Science B.V. All rights re served.