Tungsten-based catalysts

Among the various catalysts tungsten-based catalysts show better activity. In 1981, they reported detailed study of the Cr, Mo and W catalysts for homogeneous WGS reaction [23],... 

Dicyclopentadiene is also polymerized with tungsten-based catalysts. Because the polymerization reaction produces heavily cross-Unked resins, the polymers are manufactured in a reaction injection mol ding (RIM) process, in which all catalyst components and resin modifiers are slurried in two batches of the monomer. The first batch contains the catalyst (a mixture of WCl and WOCl, nonylphenol, acetylacetone, additives, and fillers the second batch contains the co-catalyst (a combination of an alkyl aluminum compound and a Lewis base such as ether), antioxidants, and elastomeric fillers (qv) for better moldabihty (50). Mixing two Uquids in a mold results in a rapid polymerization reaction. Its rate is controlled by the ratio between the co-catalyst and the Lewis base. Depending on the catalyst composition, solidification time of the reaction mixture can vary from two seconds to an hour. Similar catalyst systems are used for polymerization of norbomene and for norbomene copolymerization with ethyhdenenorbomene. 

Sodium tungstate is used in the manufacture of heteropolyacid color lakes, which are used in printing inks, plants, waxes, glasses, and textiles. It is also used as a fuel-ceU electrode material and in cigarette filters. Other uses include the manufacture of tungsten-based catalysts, for fireproofing of textiles, and as an analytical reagent for the deterrnination of uric acid. 

H risson and Chauvin (88) examined the metathesis between acyclic alkenes and cycloalkenes (telomerization) in the presence of two other tungsten-based catalysts, namely WOCl4-Sn(n-C4He)4 and WOCI4-... 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

Tungsten catalysts are also known, and are of considerable interest because they are mimics for tungsten-based oxotransferase enzymes found in thermochemical bacteria. Studies on such compounds have been reported (16,17). 

Tungsten-based catalysts including POMs show high efficiency of H202 utilization [17,18,78-100]. Ishii and coworkers [18] have reported effective H202-based epoxidation of alkenes catalyzed by H3PW12O40 combined with cetyl pyridinium chloride (CPC) as a phase-transfer agent ... 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

The M(C0)6 (M = Cr, Mo, W) stable carbonyls have been used to prepare metal supported catalysts of elements of group 6 that have been used as catalysts in several reactions, such as metathesis, water-gas shift, CO hydrogenation and olefin hydrogenation and polymerization [15-24]. Table 8.2 compiles several examples in which M(CO)s (M = Cr, Mo, W) compounds are used as an alternative for preparing chromium-molybdenum or tungsten-based catalysts. 

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). 

Numerous catalyst systems have been developed. Most common catalysts are based on tungsten of molybdenum. Transition metals ranging from group IV to group VIII have been found to be suitable. The catalysts are commonly classified as given in Table 1.4. 

Tungsten-based catalysts with a Lewis base promoter are used in the polymerization of dicyclopentadiene also isolated from the C5 stream-cracker fraction.150,155 The product poly(dicyclopentadiene) or Metton resin has unusual physical and chemical properties. It is processed together with elastomers to increase viscosity of the end product by using reaction injection molding (RIM) technology.155... 

There is a wide variety of transition metal compounds, ranging from group 4 (Ti) to group 8 metals (Ir), that can be applied as catalysts or catalyst precursors for the ring-opening metathesis polymerisation of cycloolefins. However, the most commonly used are W, Mo, Re and Ru compounds tungsten-based catalysts appeared to be the most effective. Other transition metal compounds such as Nb and Ta compounds have also often been used as catalysts, but especially for mechanistic studies [45]. 

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