Vanadium hydrolysis species

A second pathway which also contributes significantly to the adipic acid production, involves formation of 1,2-cyclohexanedione, perhaps by hydrolysis of the ketoxime tautomer of 2-nitrosocyclohexanone. Conversion of the ketoxime and of the diketone to adipic acid requires a vanadium(V) catalyst (Figure 10). The resulting vanadium(III) species, VO, is eventually reoxidized by nitric acid. The copper(II) apparently helps to reduce multiple nitrosation of cyclohexanone, as that eventually affords glutaric acid. 

Vanadium has divalent, trivalent, tetravalent and pentavalent cation states and hydrolysis species form with respect to all of these four states. The tetravalent state exists as the oxoanion, and similarly the pentavalent state as V02 . ... 

The ionic radii of the vanadium cations are 0.79 and 0.64 A for vanadium(II) and vanadium(III), respectively (Shannon, 1976), and with apparent ionic radii of 0.56 and 0.50 A for the hydrolysis species of vanadium(IV) and vanadium(V), respectively (see discussion in Brown and Wanner (1987) in relation to the derivation of the apparent ionic radii for oxo-cations). 

Vanadium(II) is unstable in water (Baes and Mesmer, 1976) being oxidised to vanadium(III), the reaction being accompanied by the production of hydrogen gas. However, hydrogen can be utilised to maintain the vanadium(II) oxidation state in solution. Data are only available for the first monomeric hydrolysis species ofvana-dium(II), VOH", which forms according to reaction (2.5) (M = p = l, q=l). 

The formation of vanadium(III) hydrolysis species is described by reaction (2.5) (M = Evidence has been given in the literature that vanadium(III) forms the species, VOH +, V(OH)2 and V2(OH)2 . Although it is probable that higher monomeric hydrolysis species would form, their detection is likely hindered by oxidation of vanadium(III) to higher oxidation states in the pH region where these species would exist. As is the case with some other trivalent transition metal cations, the formation of V3(OH)4 may also occur but no data for this species have been reported. 

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. 

Hydrolysis species that form for vanadium(III) are similar to those that have been postulated for titanium(III), with hydrolysis being dominated by the formation of and V2(OH)2. In addition, data are also available for the second... 

There are less data available for the stability constant of V2(OH)2 than were available for those of the monomeric vanadium(III) hydrolysis species, but nevertheless, the majority of the data still come from the work of Pajdowski and co-workers (see Table 11.4). As was the case with V(OH)2, the data have been acquired in chloride media across the temperature range of 20-25 0. It is believed that utilisation of these data without correction will not significantly impact calculations for obtaining the stability constant at zero ionic strength since the change in each constant is likely to be within the uncertainty assigned to each constant. 

Table 11.4 Data for the stability constants of vanadium(lll) hydrolysis species (reaction (2.5), M = V +). 

The determination of the maj ority of the stability constants for the vanadium( V) hydrolysis species has not been related to reaction (2.5) (M = V02 ). This is because other reactions better facilitate the calculation of the relevant stability constants at zero ionic strength, as is illustrated in the following. For consistency with data given for other cations. Table 11.9 contains the stability constants for all vanadium(V) species that relate to reaction (2.5) (M=V02" ). To undertake these calculations, the derived stability constants at zero ionic strength have been combined with the relevant stability constant of a monomeric vanadium) V) species (also given in the following) and that for water, as given in Chapter 5. 

The stability constants for the formation of the monomeric hydrolysis species of vanadium(V) are given in Table 11.11. The table contains data for the reaction of V02" with water to form the species V02(0H)(aq) (or HVOjCaq)), V02(OH)2-(or VO3-), V02(0H)32- (or HVO -) and V02(0H)/- (or VO/-). The data from Borgen, Mahmoud and Skauvik (1977) or Schiller and Thilo (1961) are not included in Table 11.11 as the complete experimental conditions used are not clear in either study. Moreover, the stability constants derived in these studies appear to be inconsistent with those of other studies. 

There is some conjecture over whether trimeric or tetrameric vanadium(V) species form and, indeed, whether both may form simultaneously. Cruywagen, Heyns and Visagie (1989) studied the hydrolysis of vanadium(V) using both potentiometry and spectrophotometry. Their results indicated that a model with trimers gave a slightly better fit than did that with tetramers however, only the model with the tetramers could be used to fit the spectrophotometric... 

Hydrolysis species that form for chromium(III) are similar to those that have been postulated for titanium(III) and vanadium(III), with hydrolysis being dominated by the formation of CrOH and Cr2(OH)2 - In addition, data are also available for the second monomeric hydrolysis species Cr(OH)2 and a higher polymeric species, CrjfOH), has ako been postulated. Moreover, data for the higher monomeric species, Cr(OH)3(aq) and Cr(OH)4, are ako available. Data that have been given in the hterature for CrOH are hsted in Table 11.15. 

The phosphorus(V) oxide written in simplest form is P205, so it should be expected that there would be considerable similarity between the various "phosphates" and the "vanadates." This is precisely the case, and the various forms of "vanadate" include V043-, V2074, V3()<,3, HV042, ll2V()4, H3V04, V ()0286, and others. These species illustrate the fact that vanadium is similar in some ways to phosphorus, which is also a group V element. The numerous vanadate species can be seen to result from reactions such as the hydrolysis reaction... 

VO(acac)2 < VO(Et-acac)2 VO(Me-acac)2 BMOV. Conversion rates for all hydrolysis products were faster than for the original species. Both EPR and visible spectroscopic studies of solutions prepared for administration to diabetic rats ocumented both a salt effect on the species formed and formation of a new halogen-containing complex. The authors concluded that vanadium compound efficacy with respect to long-term lowering of plasma glucose levels in diabetic rats traced the concentration of the hydrolysis product in the administration solution. 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

Tables 1 and 2 gives the numerical data for a series of vanadium (II), chromium (III), manganese (IV), molybdenum (III), rhenium (IV), iridium (VI), cobalt (II), and nickel (II) complexes. The first spin-allowed absorption band, caused by an internal transition in the partly filled shell, has the wavenumber equal to A. If spin-forbidden transitions are superposed on this band, a certain distortion from the usual shape of Gaussian error curve can be observed, and one takes the centre of gravity of intensity as the corrected wavenumber ai. One has to be careful not to confuse electron transfer or other strong bands with the internal transitions discussed here. Obviously, one has also to watch for absorption due to other coloured species, produced e. g. by oxidation or hydrolysis of the solutions. In the case of certain octahedral nickel (II), and nearly all tetrahedral cobalt (II) complexes, the first band has not actually been...

The amount of grafted metal is usually low, for several possible reasons low support OH reactivity, chlorination of the surface, and release of metal species during hydrolysis. Bond and Bruckman [30] mentioned that it is difficult to obtain a monolayer of vanadium species on Ti02 because of the chlorination of the support surface by the HC1 evolved during the anchoring of VOCI3 at 313 K. 

The nature of the species formed when V-contaminated FCC are exposed to steam remains somewhat controversial. When immersed in water (at room temperature), vanadium (supported on solids) undergoes complex hydrolysis-condensation-polymerization reactions that form H2V207 , HV207 and H2Vio02g ions [22,26]. V concentration, surface composition, and liquid pH control the nature of the polyanions formed and their degree of protonation. Different reactions and reaction products are expected to occur when the same V-contaminated materials are exposed to steam. However, it is believed that the same parameters (such as surface compositions, V-levels, and residence times) that influence the nature of the polyanions formed when V-contaminated solids are exposed to water will also affect the nature of the volatile V-compound formed when the same catalyst is exposed to steam. 

Overbeek, R.A. et al., The hydrolysis behavior of vanadium species in aqueous solutions and their adsorption on alumina, silica and titania surfaces, Appl. Catal. A, 163, 129, 1997. 

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