Tantalum-based catalysts

Similar observations were reported by Wallace and Schrock18a) who utilized a tantalum based catalyst in their studies. In fact, studies of other catalyst systems are presently being persued vigorously 18 b). 

Synthesis and characterization of several catalyst precursors of [Ta] and [W] on different supports like silica [44], alumina [45], or silica-alumina [70] have been discussed in the previous section. These catalyst precursors were tested mainly in propane metathesis under similar condition in batch reactor at 150°C for 120 h. Catalytic performances revealed that the [Si02-Al203.5oo-W-H] and [AI2O3.500-W-H] have similar and better activity (TON) than the corresponding silica-supported tantalum-based catalyst precursors (Table 2). It was also observed that the product selectivity for tungsten hydrides is narrower than for tantalum hydrides [71]. 

Although hydroamination of allenes can be easily achieved with group 4 and group 5 metal catalysts, the stereoselectivity of these systems is rather limited. Several attempts to perform asymmetric hydroamination/cyclization of aminoallenes employing chiral aminoalcohols [260, 261] and sulfonamide alcohols [262] as chiral proligands for titanium- and tantalum-based catalyst systems have produced vinyl pyrrolidines with low selectivities only. While the titanium catalysts were... 

Terminal to interior double olefins catalyst cyclopentadienyl-tantalum. Isomerisation catalyst based on Al-silicate 55... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

Le Roux, E. (2004) New Generation of Tantalum and Tungsten Based Catalysts Supported on Oxides for Alkane Metathesis. PhD thesis. University Claude Bemard-Lyon I (Lyon). 

A conceptual process for ethylene dimerization in the presence of tantalum or niobium based catalysts has been developed by MIT researchers [6-8]. The technology is based on a metal hydride-based homogeneous catalyst that selectively dimerizes ethylene to butene-1. The particular catalyst is neopentylidene complex of tantalum or niobium. The preparation of the homogeneous catalyst is rather a complex process the tantalum complex is prepared by reacting tri-neopentyl tantalum dichloride, Ta(CH2CMe3)3Cl2 and neopentyl lithium LiCH2CMe3 in octane solvent to yield thermally stable neopentylidene tantalum catalyst in quantitative yield. 

MIT Tantalum based 80 100 20 95 - Catalyst preparation is a complex method. - Catalyst recovery is required. 

With increase in the acidity of the catalyst, methyl alcohol or dimethyl ether undergo condensation to saturated hydrocarbons or aromatics, with no olefin by-products. With tantalum or niobium pentafluoride based catalyst the studies at 300 °C resulted in conversion to gasoline range branched hydrocarbons and some aromatics (30% of the product) This composition is similar to that reported with H-ZSM-5 zeolite catalyst. 

Ushikubo, T. (2003). Activation of Propane and Butanes over Niobium- and Tantalum-Based Oxide Catalysts, Catal. Today, 78, pp. 79-84. 

Ushikubo, T. Activation of propane and butanes over niobium- and tantalum-based oxide catalysts. Catal. Today 2003, 78, 79-84. 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

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