Vanadium phosphates cations

For solvent extraction of a tetravalent vanadium oxyvanadium cation, the leach solution is acidified to ca pH 1.6—2.0 by addition of sulfuric acid, and the redox potential is adjusted to —250 mV by heating and reaction with iron powder. Vanadium is extracted from the blue solution in ca six countercurrent mixer—settler stages by a kerosene solution of 5—6 wt % di-2-ethyIhexyl phosphoric acid (EHPA) and 3 wt % tributyl phosphate (TBP). The organic solvent is stripped by a 15 wt % sulfuric acid solution. The rich strip Hquor containing ca 50—65 g V20 /L is oxidized batchwise initially at pH 0.3 by addition of sodium chlorate then it is heated to 70°C and agitated during the addition of NH to raise the pH to 0.6. Vanadium pentoxide of 98—99% grade precipitates, is removed by filtration, and then is fused and flaked. 

The products of such reactions depend upon the pressure, temperature, pH, phosphate, and cation concentrations, and may be difficult to predict or rationalize. For example, the equation (3) produces an acid phosphate and a phosphate hydroxide. Microporous aluminophosphates and related phases (see Section 5.1.2) are prepared in hydrothermal bombs using hydrated cations or molecular templates such as organic amines or ammonium cations to direct the porous framework. Many new structures with metal phosphate chains, layers, or three-dimensional networks have been prepared hydrothermally in recent years, for example, templated vanadium phosphates and iron phosphates. ... 

Vanadium phosphate (VPO) compounds are important industrial catalysts used in the conversion of n-butane to maleic anhydride[l]. Although it is now generally accepted that the reaction requires the presence of both V and V cations in close proximity, the precise nature of the active site in this catalyst is still a matter for... 

M. I. Khan, R. Haushalter, C. J. O Connor, C. Tao, and Zubieta, An octahedral-tetrahedral vanadium phosphate one-dimensional chain incorporating l-aminoethane-2-ammonium cations as ligands, Chem. Mater. 7 593 (1995). 

VANADIUM PHOSPHATE PHASES WITH CHARGE-COMPENSATING ORGANIC CATIONS... 

Selected Structural Information for Vanadium Phosphates with Negatively Charged V/P/O Motifs and Charge-Compensating Inorganic Cations... 

S. Boudin, A. Guesdon, A. Leclaire, M. Borel, Review on vanadium phosphates with mono and divalent metallic cations syntheses, structural relationships and classification, properties, Int. J. Inorg. Mater. 2 (2000) 561-579. 

Vanadium borophosphates 107-109 that contain cluster anions with different ring sizes can be synthesized by hydrothermal reactions of vanadium(III) oxide, boric acid and an appropriate phosphate salt [145,146]. The ring size of the cluster anion with the composition [(VO)2BP20iol 4-6 is determined by the size of the cation that is, however, included only in the interior of the cages with n = 5 and 6 (Fig. 28). 

Insulin binding to the extracellular side of cell membranes initiates the insulin cascade , a series of phosphorylation/dephosphorylation steps. A postulated mechanism for vanadium is substitution of vanadate for phosphate in the transition state structure of protein tyrosine phosphatases (PTP).267,268 In normal physiological conditions, the attainable oxidation states of vanadium are V111, Viv and Vv. Relevant species in solution are vanadate, (a mixture of HV042-/ H2VOO and vanadyl V02+. Vanadyl is not a strong inhibitor of PTPs, suggesting other potential mechanisms for insulin mimesis for this cation. 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

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