Selectivity of vanadium

Vanadium, in different oxidation states, has been used in conjunction with dipicolinic acid and its analogues to produce coordination complexes. A selection of vanadium-containing complexes is discussed below. 

The model of Schiott and Jorgensen [98] shows that the proximity of vanadium atoms in a dimer present on the (200) plane of (VOjaPaOy and ti -superoxo or r -peroxo species chemisorbed on coordinatively unsaturated vanadium site are required for the selective oxidation of hydrocarbons. In fact, such activated molecular oxygen species are responsible for the activity and selectivity of vanadium catalysts in homogeneous oxidations [99-101]. Such oxidations are one-electron processes and involve free radicals. Although Schiott and Jorgensen [98] did not provide the complete mechanism of n-butane oxidation to maleic anhydride on vanadyl dimers, their model indicated that the superoxo species and one-electron, free radical processes may be involved in such oxidation. 

Table 1.1 Selection of vanadium minerals with information on the nature of vanadium.

Selection of vanadium complexes with multifunctional ligands including sulfur. See refs 57b-61. 

Selection of vanadium complexes with organic ligands, for which in vitro and/or in vivo insulin-mimetic (or insulin-enhancing) activity has been reported. See Table 5.1 for specification and references. 

Selectivity of vanadium monolayer on silica and titama and titania-silica... 

Sananes-Schulz, M.T., Tuel, A., Hutchings, G.J., and Volta, J.C. The V " "A + balance as a criterion of selection of vanadium phosphorus oxide catalysts for M-butane oxidation to maleic anhydride a proposal to explain the role of Co and Fe dopants. 7. Catal 1997,166, 388-392. 

Butler, A. and Tschirret-Guth, R.A. (1996) On the selectivity of vanadium bromoperoxidase, in Mechanisms of Biohalogenation and Dehalogenation, vol. 25F (eds D.B. Janssen, K. Soda, and R. Wever), Royal Netherlands Academy of Arts and Sciences, Amsterdam, pp. 55-68. 

Some properties of selected vanadium compounds are Hsted in Table 1. Detailed solubiUty data are available (3), as are physical constants of other vanadium compounds (4). Included are the lattice energy of several metavanadates and the magnetic susceptibiUty of vanadium bromides, chlorides, fluorides, oxides, and sulfides (5). 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

Neither the oxidation of methanol to make formaldehyde nor that of ethanol to make acetaldehyde is sensitive to the adjacency of redox sites in the catalysts the number of exposed vanadium serves well to normalize the reaction rate in both cases. This conclusion does not contradict a statement by other authors [23-25] that the selectivity of alcohol... 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

The conventional selective reduction of NOx for car passengers still competes but the efficient SCR with ammonia on V205/Ti02 for stationary sources is not available for mobile sources due to the toxicity of vanadium and its lower intrinsic activity than that of noble metals, which may imply higher amount of active phase for compensation. As illustrated in Figure 10.9, such a solution does not seem relevant because a subsequent increase in vanadium enhances the formation of undesirable nitrous oxide at low temperature. Presently, various attempts for the replacement of vanadium did not succeed regarding the complete conversion of NO into N2 at low temperature. Suarez et al. [87] who investigated the reduction of NO with NH3 on CuO-supported monolithic catalysts... 

We refrain here from giving an extensive overview of studies on the surface structure of vanadium oxide nanolayers, as this has already been done for up to year 2003 in our recent review [97]. Instead, we would like to focus on prototypical examples, selected from the V-oxide-Rh(l 1 1) phase diagram, which demonstrate the power of STM measurements, when combined with state-of-the-art DFT calculations, to resolve complex oxide nanostructures. Other examples will highlight the usefulness of combining STM and STS data on a local scale, as well as data from STM measurements, and sample area-averaging spectroscopic techniques, such as XPS and NEXAFS, to derive as complete a picture as possible of the investigated system. 

Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 g. i.s.ml 1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (A1203, HMS, MFI). 

Thus there is considerable incentive to find extractants that could tolerate higher quantities of solids in H2SO4 leach liquors. Stripping of uranium from the Amex process extractant and subsequent regeneration of the amine solvent also consume considerable quantities of acid and base. Recovery of uranium from H2SO4 solutions would be simplified if a convenient neutral extractant could be found. An extractant with better selectivity for vanadium and molybdenum than HDEHP and long-chain amines is also desirable. 

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