SYNTHESIS AND CHARACTERIZATION OF VAPSO-44 AND VAPSO-5

Aluminophosphate molecular sieves of structures 5 (AFI) and 44 (CHA) have been prepared with different vanadium contents. The crystallization was performed by means of a mcirowave power system. The crystallinity of the samples was checked by X-ray diffraction (XRD), thermal analysis and adsorption measurements, and compared with the data for the corresponding SAPO molecular sieves. The acidity was studied by calorimetric measurements of the differential molar heat of NH3 sorption and by temperature-programmed desorption (TPD) of ammonia. Electron paramagnetic resonance (EPR) spectra indicate the presence of isolated immobile VO2+ species in square-pyramidal symmetry. VO2 clusters were also found in samples with higher vanadium content. On calcination V4+ is partially oxidized to V2O5. Only a small amount of VO2+ (<1 atom%) may be stabilized on the inner surface of the pore system. The incorporation of V4+ into tetrahedrally coordinated framework positions can be excluded from the EPR results. It follows from IR measurements that VO2+ ions are fixed on the inner surface via a condensation process with hydroxy groups.