Vanadium structure

Sjbberg B-M (1997) Ribonucleotide Reductases - A Group of Enzymes with Different Metallosites and a Similar Reaction Mechanism. 88 139-174 Slebodnick C, Hamstra BJ, Pecoraro VL (1997) Modeling the Biological Chemistry of Vanadium Structural and Reactivity Studies Elucidating Biological Function. 89 51-108 Smit HHA, see Thiel RC (1993) 81 1-40... 

Nunes, G.G., G.R. Friedermann, M.H. Herbst, R.B. Barthem, N.V. Vugman, J.E. Barclay, DJ. Evans, P.B. Hitchcock, G.J. Leigh, E.L. Sa, and others. 2003. The first hetero-binuclear alkoxide of iron and vanadium Structural and spectroscopic features. Inorg. Chem. Commun. 6 1278-1281. 

These findings prompted us to reconsider the results of EPR spectra and to make more profound analysis of kinetic data for N2 reduction by catecholate vanadiu-m(II) complexes. The conclusion was that a tetra-vanadium structure for the complexes is in a better agreement with the EPR spectra than a trinuclear structure kinetic results also confirmed the tetranuclear structure of the complex [19], We therefore regard the tetranuclear structure as confirmed for unsubstituted and substituted catecholate complexes, at least for those so far investigated. 

Pig. 19. Vanadium structural changes during oxidation/reduction. 

Figure B 1.11.5 is an example of how relative integrals can detennine structure even if the peak positions are not adequately understood. The decavanadate anion has the structure shown, where oxygens lie at each vertex and vanadiums at the centre of each octaliedron. An aqueous solution of decavanadate was mixed with about 8 mol% of molybdate, and the tiiree peaks from the remaining decavanadate were then computer-subtracted...

Iseda M, Nishio T, Han S Y, Yoshida H, Terasaki A and Kondow T 1997 Electronic structure of vanadium cluster anions as studied by photoeieotron spectroscopy J. Chem. Phys. 106 2182... 

Wu H, Desai S R and Wang L S 1996 Evolution of the electronic structure of small vanadium clusters from molecular to bulk-like Phys. Rev. Lett. 77 2436... 

The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene (1,2,4,5-tetramethylbenzene), using a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-amino-phenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown in Figure 18.36. 

Figure 22.4 Alternative representations of (a) infinite chains of vanadium atoms in VF5, (b) tetrameric structures of NbFs and TaFs, and (c) dimeric structure of MX5 (M = Mb, Ta X = Cl, Br).

A number of nitrogen-fixing bacteria contain vanadium and it has been shown that in one of these, Azotobacter, there are three distinct nitrogenase systems based in turn on Mo, V and Fe, each of which has an underlying functional and structural similarity.This discovery has prompted a search for models and the brown compound [Na(thf)]+[V(N2)2(dppe)2] (dppe = Pli2PCH2CH2PPh2) has recently been prepared by reduction of VCI3 by sodium naphthalenide... 

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

There are several theories about the chemistry of vanadium poisoning. The most prominent involves conversion of VjOj to vanadic acid (H-iVO ) under regenerator conditions. Vanadic acid, through hydrolysis, extracts the tetrahedral alumina in the zeolite crystal structure, causing it to collapse. 

The metals in the FCC feed have many deleterious effects. Nickel causes excess hydrogen production, forcing eventual loss in the conversion or thruput. Both vanadium and sodium destroy catalyst structure, causing losses in activity and selectivity. Solving the undesirable effects of metal poisoning involves several approaches ... 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

Vanadium and sodium neutralize catalyst acid sites and can cause collapse of the zeolite structure. Figure 10-5 shows the deactivation of the catalyst activity as a function of vanadium concentration. Destruction of the zeolite by vanadium takes place in the regenerator where the combination of oxygen, steam, and high temperature forms vanadic acid according to the following equations ... 

The produced vanadic acid, VO (OH)3, is mobile. Sodium tends to accelerate the migration of vanadium into the zeolite. This acid attacks the catalyst, causing collapse of the zeolite pore structure. 

This dement is important mainly because of its use as an additive to iron in the manufacture of steel. A few percent of vanadium stabilizes a high-temperature crystal structure of iron so that it persists at room temperature. This form is tougher, stronger, and more resistant to corrosion than ordinary iron. Automobile springs, for example, are often made of vanadium steel. 

The process of burning out the impurities is slowest in the open-hearth furnace. This implies there is plenty of time to analyze the melt and add whatever is needed to obtain the desired chemical composition. Manganese, vanadium, and chromium are frequent additives. The properties of the finished steel depend upon the amount of carbon left in and upon the identity and the quantity of other added elements. Soft steel, for example, contains 0.08-0.18 weight percent carbon structural steel, 0.15-0.25% hard steel ox toot steel, 1-1.2%. 

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

Figure 6.5 Proposed structure for the vanadium complex prior to the oxygen transfer from peroxide to the allylic olefin.

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