CHARACTERIZATION OF VANADIUM-CONTAINING MESOPOROUS SILICAS

Vanadium-containing mesoporous silicas (V-MS) have been synthesized with primary amines as templates and characterized using spectroscopic techniques. Vanadium could be incorporated into the silica framework up to a content of about 2.5 wt.-% without formation of significant amounts of vanadium oxide after calcination. The dimensions of the mesopores of the structure increased from 25 to 35 Angstrom with the alkyl chain length of the amine. Several techniques have shown that vanadium species were highly dispersed in the materials. In the as-synthesized form, V4+ cations are probably occluded in the mesopores in a coordination similar to that observed in aqueous solutions, without direct chemical bonding to the framework. However, the observation of an anisotropic hyperfine electron paramagnetic resonance (EPR) signal at room temperature indicates that the species are relatively immobile. V-O-Si bonds were more likely created upon calcination in air at 650 degrees C. V(V) species are isolated and tetrahedrally coordinated in dried samples and rapidly change their coordination in the presence of water vapor. This was confirmed by ultraviolet-visible (W-VIS) and V-51 solid-state nuclear magnetic resonance (NMR) spectroscopies.