Vanadyl site nature

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

The nature of the 0(s) species for ethane activation probably varies depending on the catalyst. On reduced Mo/Si02, the surface O species generated by the decomposition of N20 has been shown to react readily with ethane (15), and O has also been suggested as the active species for the Li-Mg oxide catalyst (17). Oxygen vacancies or surface vanadyl groups may also be active sites. 

A range of coordination complexes of has been prepared in which the vanadyl group is absent, a number of these compounds being synthesized as part of an effort to understand the nature of biological vanadium sites, such as amavadin," which contains an octadentate, hydroxyiminodipropionate ligand bound to vanadium A number of model complexes of this type have been prepared and discussion of this area can be found in the article Vanadium in Biology Further discussion in this section follows the main ligand donor atom to vanadium. 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

The nature of the binding sites for vanadyl and vanadate in Tf has been inferred from EPR and NMR spectroscopy, respectively. Figure 5.22 provides selected spectral... 

Stable structures such as the naturally occurring ferric porphyrin complexes or porphyrins substituted with copper, cobalt, silver or vanadyl probe the active site of heme-containing enzyme [227]. Complexes of copper not associated with heme are also common. They are frequently formed at an amino terminus because the amino group provides a good primary amine donor atom. Two or three amino acid residues beginning at the N-terminus are often flexible until a more rigid portion of the polypeptide, such as the a helix, is encountered. A peptide nitrogen is available to... 

The combination of acidic and oxidizing properties of vanadyl pyrophosphate makes several different transformations possible over paraffins, as illustrated by the scheme in Figure I for the reactions which may occur on n-pentane. The relative contribution of the different pathways (i.e., cyclization of intermediate olefin or dienic compound vs. O-insertion, or dimerization vs. cyclization) is a function of the nature of the reactant and of the availability of surface oxidizing centres or of sites which can favour the dimerization or condensation reactions. 

Analysis of the principal values of the chemical shift tensor derived from the solid-phase P n.m.r. spectrum of 5 -AMP adsorbed on Zn +-exchanged bentonite clay shows that the zinc ion is co-ordinated directly to an oxygen atom of the phosphate. Multinuclear n.m.r. studies, including P n.m.r., have been used to investigate the nature and co-ordination pattern of the complexes formed by ATP with aluminium(m) ions and with vanadyl ions at different pH values. At least four different complexes could be distinguished with Al + ions, and three with VO + ions. The perturbation of the P n.m.r. spectrum of ATP, bound in the high-affinity ATP-binding site of nitrated... 

The first structural elucidation of a naturally occurring vanadium compound has been achieved. Amavadine, which has been isolated from the fly Amanita muscaria, has been shown by degradative, synthetic, and spectroscopic studies to have the structure (32) (p.54). The use of the vanadyl group as a new spectroscopic probe of metal binding sites in proteins has been demonstrated by its incorporation into insulin. E.s.r., optical, and i.r. spectra of the protein-bound have indicated that it is held on two types of site the co-ordination geometry about the metal was determined in each case. This approach should be valuable in the study of other metalloproteins. ... 

Bordes, E. (1993). Nature of the Active and Selective Sites in Vanadyl Pyrophosphate, Catalyst of Oxidation of n-Butane, Butene and Pentane to Maleic Anhydride, Catal. Today, 16, pp. 27-38. 

Bordes, E. Nature of the active and selective sites in vanadyl pyrophosphate, catalyst of oxidation of -butane, butene and -pentane in maleic anhydride. Catal. Today 1993,16, 27-38. 

The combination of spectroscopic tools and the use of the actual reactants allowed identifying for the first time, the nature of the surface active sites and the intermediate reactive species of vanadium-based catalysts during DeNO c reaction. In fact, the authors observed that the catalytic activity was related to the ammonia adsorbed as V +-ONH3H" species over oxidized V" -OH that further interact with vanadyl species V " =0 to form a V -0NH3H+-0-V complex that reacts with NO. 

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