Vanadium species

Under similar reaction-conditions, the vanadium species V(N2)6 (139) has been isolated. In addition, a species V2(Na) (n probably = 12) was observed (139). The metal nuclearity was established by the standard, metal-concentration techniques. A comparison of the optical spectra of V(N2) and V(CO)e 128) suggested that these molecules have very similar, electronic properties, and the data clearly established that N2 is a strong, field ligand in its bonding properties. Interestingly, atomic V could be isolated in N2 matrices from 8- 12K co-... 

For ail samples, both a.p. and s.o., irrespective of the preparation method, the experimental intensity ratios, V2p/Zr3d, increased proportionally to the V-content up to 3 atoms nm 2 (pjg 2). The ratio approaches those calculated with the spherical model proposed recently by Cimino et al. [27] (full line in Fig. 2). For ZV samples with V-content < 3 atoms nm 2, this finding shows that vanadium species are uniformly spread on the Zr02 surface. On ZV catalysts with a larger V content (not shown in Fig. 2), the intensity ratios were markedly larger than the corresponding values yielded by the spherical model. The results obtained on samples with V-content > 3 atoms nm 2 point therefore to a V surface enrichment. 

An inductively coupled plasma formed by passing argon through a quartz torch is widely used for the mass spectroscopic analysis of metal compounds separated by online HPLC.6 Samples are nebulized on introduction into the interface. Plasma impact evaporates solvent, and atomizes and ionizes the analyte. Applications include separation of organoarsenic compounds on ion-pairing F4PLC and vanadium species on cation exchange. 

A vanadium catalyst is essential although the combination of Zn and MejSiCl is capable of reductive dimerization of aldehydes [20]. A reversible redox cycle for the in situ generated low-valent vanadium species mediating the electron transfer is achieved in the presence of Zn as the stoichiometric co-reductant (Scheme 4). 

Vanadium compounds in a low oxidation state are known to be effective for inducing one-electron reduction. The highly stereoselective monodebromination of gem-dibromocyclopropanes proceeds with the help of a low-valent vanadium species generated from vanadium(III) chloride and zinc in dimethoxyethane in cooperation with diethyl phosphonate or triethyl phosphite... 

In addition, a recent report details a very efficient nonenzymatic method for the asymmetric oxidation of sulfides this employs an organo-vanadium species featuring the imine (38) (Scheme 25)[111]. A second, complementary strategy for the preparation of optically active sulfoxides involves the enantioselective oxidation of racemic sulfoxides. ... 

In this study, the structure of the vanadium species supported on AIPO -S molecular sieve has been studied by X-ray diffraction, infrared, diffused reflectance, and EPR spectroscopy, temperature programmed desorption, and their properties compared with those of VAPO -S. 

In the Infrared spectrum of the sample after calcination at 550°C, a new band was observed at 923 cm (Fig. 4 (b)). AIPO4-5 and crystalline VjO, did not show any band around 920 - 930 cm . The band at 923 cm was detectable for the sample of 0.7 % V O, content and its intensity increased with increasing VjOj content supported on AIPO4-5 (Fig. 5). This indicates strong interaction between the adsorbed vanadium species and the AIPO4-5 surface during the calcination step. 

Diffused reflectance spectra in the UV-VIS region were examined to characterize the structure of vanadium species supported on the AIPO -S surface. The value of the election-charge-transfer energy was reported to be strongly influenced by the number of ligands of the central vanadium ions and give information on the symmetry of the vanadium ions in the clusters [21,22]. 

Diffused reflectance spectra of the vPs/AIPO -S samples of different contents are shown in Fig. 7. By increasing vp, content, the absorption band appeared in the range between 400 and 550 nm, which gave rise to the formation of crystalline vp. Therefore, as evidenced by infrared measurements, it is concluded that the crystalline phase is formed on the AiP04-5 surface after the formatbn of surface vanadium species containing V in a tetrahedral symmetry. The diffused reflectance spectmm of VAPO -5 [7,8] was similar to that of calcined VjOj/AIPO -S. 

Infrared spectra of VjO, supported catalysts after calcination at 550°C showed the V=0 stretching vibration band at 923 cm. This indicates that surface vanadium species is located on the basic sites within the AIPO,-5 surface. According to the diffused reflectance spectra of the samples with low VjO, contents, the vanadium species supported on AIPO4-5 surface was mainly in a distorted tetrahedral environment. However, upon increasing the V Oj coment, crystalline VjOj was formed on the AIPO -S surface prior to the formation of approximately a monolayer of the surface vanadium species. The vanadium species measured by EPR and DRS and catalytic properties of vanadium loaded VjOg/AIPO -S were similar to those of vanadium substituted VAPO -S. 

Nature of Vanadium Species in Vanadium-Containing Silicalite and Their Behavior in Oxidative Dehydrogenation of Propane... 

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

Characterization Data. A complete characterization of V- containing silicalite samples has been reported in a previous paper (7), but the main relevant aspects useful for a better understanding of the nature of vanadium species in V- silicalite samples and for the correlation with the catalytic behavior will be briefly summarized. 

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

BELLUSSI ET AL. Vanadium Species in Vanadium-Containing SUicalite 293... 

Vanadium species in V-containing silicalite in propane oxidative dehydrogenation— Continued... 

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