On the synthesis of vanadium containing molecular sieves by experimental design from a VOSO4.5H2O.Al(iPrO)3.Pr2NH.H2O gel: occurrence of VAPO-41 as a secondary structure in the synthesis of VAPO-11

An experimental design was applied to the hydrothermal synthesis of VAPO-11 molecular sieve (AEL structure)from a VOSO4.5H2O.Al(iPrO)3.Pr2NH.H2O gel. The influence of five synthesis parameters (synthesis temperature, synthesis time, vanadium content, water content and template content) on the crystallinity and physicochemical properties of the as-synthesized materials was studied. The obtained materials were characterized by electron spin resonance (ESR), diffuse reflectance spectroscopy (DRS), X-ray diffraction, thermal analysis measurements, scanning electron microscopy and in situ Fourier transform infrared (FTIR) spectroscopy. The AEL structure appears either as a single phase or as a physical mixture together with the AFO structure (VAPO-41). A statistical model is proposed, which relates the synthesis variables with the crystallinity allowing to indicate the optimal synthesis conditions for VAPO-11. Highly crystalline single-phase VAPO-11 can be best prepared at 170°C from a synthesis gel with a low vanadium content (0.02 mol) and a high water content (25 mol). The co-ordination of vanadium in these materials is discussed on the basis of the ESR and DRS spectra. The spectroscopic data are explained by the presence of a vanadium species in distorted octahedral co-ordination, composed of two V-O-T bonds with the framework, one bond with the vanadyl oxygen and three water molecules as ligands