On the synthesis of vanadium containing molecular sieves by experimental design from a VOSO4 center dot 5H(2)O center dot Al(iPrO)(3)center dot Pr2NHcenter dot 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. 5H(2)O . Al(iPrO)(3). Pr2NH . H2O gel. The influence of five synthesis parameters (synthesis temperatur e, synthesis time, vanadium content, water content and template content) on the crystallinity and physicochemical properties of the as-synthesized mat erials was studied. The obtained materials were characterized by electron s pin resonance (ESR), diffuse reflectance spectroscopy (DRS), X-ray diffract ion, thermal analysis measurements, scanning electron microscopy and in sit u Fourier transform infrared (FTIR) spectroscopy. The AEL structure appears either as a single phase or as a physical mixture together with the AFO st ructure (VAPO-41). A statistical model is proposed, which relates the synth esis variables with the crystallinity allowing to indicate the optimal synt hesis conditions for VAPO-11. Highly crystalline single-phase VAPO-11 can b e best prepared at 170 degrees C from a synthesis gel with a low vanadium c ontent (0.02 mel) and a high water content (25 mol). The co-ordination of v anadium in these materials is discussed on the basis of the ESR and DRS spe ctra. The spectroscopic data are explained by the presence of a vanadium sp ecies in distorted octahedral co-ordination, composed of two V-O-T bonds wi th the framework, one bond with the vanadyl oxygen and three water molecule s as ligands. (C) 2000 Elsevier Science B.V. All rights reserved.