A1PO-5 molecular sieves were synthesized hydrothermally in
presence of 2 - 20% of vanadium in the primary gel composition.
Triethylamine was used as a structure directing agent. Aluminum oxy
hydroxide and phosphoric acid were used as sources of aluminum and
phosphorous of the gel, respectively. Crystallization was performed by
heating the samples at 19(fC for 18-72 hours to obtain highly crystalline
materials. The samples containing 2-16% vanadium preserve their
structure upon calcination at 550°C but structure of the sample containg
20% of it colaps at this temperature and tridymite is the produced
phase. Methods of X -ray diffraction, energy dispersive X-ray
spectrometry associated with scanning electron microscope and diffuse
reflectance spectroscopy were used for structural and chemical
characterization of the materials.
The evidences acquired from above methods show that some of
the V(V) in the as-synthesized materials have been reduced to V(IV)
during synthesis procedures. The DR spectra of the as-synthesized
samples show three distinct 280, 580 and 750nm bands which the first
one was attributed to the tetrahedral V(V) and the two later ones to
tetrahedral V(IV). The samples containing higher amounts of vanadium
(10% , 16%) show a 350 nm band on the shoulder of 280nm which was
assigned to V(V) surface- grafted species.