Vanadium hydrolytic reactions

It is still not apparent whether vanadium is an essential element in humans. However, there are marine organisms, ascidians and worms, which accumulate vanadium (Crans et al, 2004). These organisms accumulate the vanadium(V) present in seawater and convert it into either vanadium(IV) or vanadium(III) for storage within the organisms, and consequently, hydrolytic reactions involved in the mechanism are important. The function of vanadium in these organisms remains elusive even though many credible hypotheses have been postulated (Crans etal., 2004). 

Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbVOs, SbV04 and Nb2Mo30n are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500 C, propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 

In acidic solution, vanadium(V) exists as the univalent cation, V02. The hydrolytic behaviour of this ion is quite complex, forming a large number of monomeric and polymeric hydrolysis species. In this review, these species are related to reaction (2.5) (M = V02 ). Species with 1, 2, 4, 5 and 10 V02" molecules are believed to form, although other species have been reported. Once the univalent cation is hydrolysed as V020H(aq), or equivalently HjVOjfaq), the chemistry it displays is quite similar to that of the phosphate anion, in terms of its ability to form polymeric species and the strength of the various steps of dissociation of phosphoric acid. 

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