Synthesis and characterization of VAPSO-44 and VAPSO-5

Abstract

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 microwave 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.