Tantalum oxide reaction with

Mesoporous Metal Oxide Solid Acids Three-dimensional porous metal oxides have been recently synthesized and applied to acid-catalyzed reactions. The use of mesoporous metal oxides is an interesting approach to develop a solid acid catalyst with enhanced activity. The mesopores in the oxide allow the reactants to access additional active acid sites in the pores, resulting in improved rates of acid catalysis. Mesoporous niobium oxides and tantalum oxides treated with phosphoric acid or sulfuric acid have been examined as solid acid catalysts [57-59]. These mesoporous oxides exhibited remarkable activity in Friedel-Crafts alkylation and 1-hexene isomerization in the liquid phase. For sulfated mesoporous tantalum oxides /m-TsL O ), the effect of pore size has been investigated using... 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

TaCl593 reacted with metallic sodium in neat trimethylphosphine to give the phosphinocarbene tantalum complexes 100 and 101, respectively. These reactions are the first examples of double activation of coordinated trimethylphosphine via oxidative cleavage of a substituent methyl C-H bond. A similar process was also observed in the reduction of tantalum pentabrom-ide with magnesium turnings in the presence of dimethylphenylphosphine.94... 

Reaction with amorphous silicon at 900°C, catalyzed by steam produces cadmium orthosilicate, Cd2Si04. The same product also is obtained by reaction with sdica. Finely divided oxide reacts with dimethyl sulfate forming cadmium sulfate. Cadmium oxide, upon rapid heating with oxides of many other metals, such as iron, molybdenum, tungsten, titanium, tantalum, niobium, antimony, and arsenic, forms mixed oxides. For example, rapid heating with ferric oxide at 750°C produces cadmium ferrite, CdFe204 ... 

Above 300°C in air, fine tantalum powder can ignite because of the rapid oxygen diffusion through the oxide layer into the substrate. Once burning starts, a dramatic temperature rise usually occurs and the oxidation reaction can accelerate rapidly. Tantalum powder can be ignited by the localized heating associated with a static electrical discharge or contact with a hot surface. 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

This transformation is carried out by intimately mixing metal oxide powders with carbon, again as with the pure metals, at temperatures between 1500-2300 K, with or without the presence of a hydrocarbon gas. The reactions of oxides with carbon are thermodynamically favored, but high temperatures are again needed because the transformations are limited by diffusion. The direct transformation of oxides to carbides is economically advantageous over the use of metals since the need to separately reduce the oxide phases is avoided. Wide application is found for the commercial production of carbides of molybdenum, tungsten, and tantalum. 

After 2 hours of reaction with oxygen, tantalum shows a formation of gray films at 250° and 350°C. and of blue gray films at 400° and 450°C. It must be concluded that at these temperatures the rate of solution of the oxide is smaller than the rate of surface oxidation. 

One of the prototypes of this behavior is the low-temperature form of tantalum pentoxide, L-Ta20s. The structure of this phase is ill defined, even though it is a stoichiometric oxide, and the room temperature stracture is a function of the prior thermal treatment temperature of the material. Reaction with other oxides, notably WO3, Zr02 and AI2O3, gives rise to solids with a broad oxygen nonstoichiometry. However, each composition appears to generate a uniquely ordered structure, which have been termed infinitely adaptive structures. 

Niobium and tantalum halides also fonn adducts with numerous N-donors. Their reactions with pyridine and related ligands (bipyridine, phenanthroline, 7-azaindole ) depend critically on the reaction conditions. Indeed, aromatic amines have a tendency to reduce the metal to oxidation state IV especially for niobium but the reduction can be prevented, even at rt, by an appropriate choice of the solvent (equations 2a-c). Imide adducts M(NR)Cl3L2 are obtained with primary or secondary amines. ... 

Oxidizer, Poison, Corrosive SAFETY PROFILE Poisonous and corrosive. Very reactive, a powerful oxidizer. Explosive or violent reaction with organic materials, water, acetone, ammonium halides, antimony, antimony trichloride oxide, arsenic, benzene, boron, bromine, carbon, carbon monoxide, carbon tetrachloride, carbon tetraiodide, chloromethane, cobalt, ether, halogens, iodine, powdered molybdenum, niobium, 2-pentanone, phosphoms, potassium hexachloroplatinate, pyridine, silicon, silicone grease, sulfur, tantalum, tin dichloride, titanium, toluene, vanadium, uranium, uranium hexafluoride. 

Zemski KA, Justes DR, Bell RC, Castleman AW Jr (2001) Reactions of niobium and tantalum oxide cluster cations and anions with n-butane. J Phys Chem A 105 4410 Bell RC, Castleman AW Jr (2002) Reactions of vanadium oxide cluster ions with 1, 3-butadiene and isomers of butane. J Phys Chem A 106 9893... 

White et al. studied electron transfer reactions at a tantalum surface covered by 2.5 nm of native tantalum-(V)-oxide (42). The SECM detected microscopic electroactive sites with a diameter between 4 and 100 /rm. Interestingly, some sites turned out to be active only for the reduction of Ru(NI lit, while others were also capable of oxidizing iodide. The authors studied the kinetics of mediator interaction under various sample potentials and locations in detail to determine parameters relevant for the growth of such tantalum oxide films. An example is given in Figure 17. 

Silica-supported Ta oxide catalyst prepared by the reaction between Ta alk-oxide and surface hydroxy groups of Si02 had higher catalytic efficiency than catalysts prepared by impregnation with TaClj or hydrated tantalum oxide [13]. Pt/ sulfated Zr02 is reported in a patent to be active and durable [14],... 

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