W-based catalyst systems

Among the large variety of catalytic methods available for the epoxidation of alkenes, those based on tungstate catalysts hold a prominent place when it comes to industrial relevance. Several positive features of W-based catalyst systems explain their popularity in industry ... 

FIGURE 16.2 Conversion versus time for the epoxidation of cyclooctene catalyzed by various W-based catalyst systems (0.1 mol% W) at 60 °C in toluene in the presence of 1.5 equiv 50% H2O2. Reprinted with permission from [10]. Copyright 2004 American Chemical Society. 

Table 2.4 Examples of W-based catalyst systems" for productive olefin metathesis (see...

In addition to the W and Mo carbonyl complexes that have most commonly been used for the cycloisomerization of alkynols, an Rh-based catalyst system has recently been developed which uses substantially lower catalyst loadings (1.5-2.5 mol%) than have typically been required for the W and Mo systems (10-50 mol%).369 Among the various ligands studied, P(/>-F-C6H4)3 proved to be particularly effective. Interestingly, this ligand has also been found to be optimal for an Ru system that catalyzes the same type of cycloisomerization (Equation (104)).370,371... 

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... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

We have carried out an experimental comparison of the epoxidation activity of various W-based catalysts in order to assess the performance of the [WZn3 (ZnW9034)2] sandwich POM relative to other W-based epoxidation catalysts [10]. The 11 catalyst systems used for this experimental study are listed below. The catalysts were either added as an isolated tungsten compound or formed without isolation in situ ... 

There are perhaps more known W-based catalysts for olefin metathesis than all others together. Many studies have been made on the catalyst systems themselves in an attempt to elucidate the nature of the active species and its mode of formation. They are especially effective for internal and cyclic olefins. Many examples will be found throughout this book. Here we will illustrate some of the main types. 

W-based catalysts for the metathesis of terminal olefins are comparatively few in number. However, this is partly an illusion because systems such as WClg/EtAlCla/ EtOH, although not effective in the sense of yielding ethene and an internal olefin, cause rapid non-productive metathesis in which the products can only be distinguished from the reactants by isotopic labelling see Ch. 5. 

An example of this type of forward reaction has already been given in Seheme 2.4. Further examples are provided by (i) the use of norbomene epoxide with W-based catalysts to enhance the ROMP of norbomene derivatives (Devine 1982) (ii) the use of oxygen with norbomene in Ru-based systems to generate such an epoxide (Ivin 1981b) (iii) the use of cyclopropane derivatives with Re207/Al203 (Anisimov 1991) see Ch. 2. 

Using reference substances, either comercially available or synthesized and purified by known methods like 5-ethinyl-2-norbornene (ET-NBE, Scheme 3), and adding these separately to the solution of 1 in highly purified DCPD, we found that ET-NBE activates the system. This was rather unexpected, since acetylenes are known as catalyst poisons in the RIM of DCPD with Mo- and W-based catalyst and are therefore carefully removed during the purification of DCPD. If the monomer contained very low levels of ethinyl-norbornene, a very bad curing behaviour with 1 as catalyst was observed. Best results were obtained with approximately equimolar amounts of ET-NBE and 1. It turned out that crude DCPD (e.g. 94% from Shell) contained - by chance -approximately the appropriate amount of this acetylene and therefore was nicely cured with 1. 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

Demonceau, A., Stumpf, A.W., Saive, E., Noels, A.F., Novel ruihenium-based catalyst systems for the ring-opening metathesis polymerization of low-strain cyclic olefins. Macromolecules 1997,30 3127-3136. 

Recent developments in gold based catalysts systems for automotive applications, G. Pattrick, E. van der Lingen, C. W. Corti, R. J. Holliday, and D. T. Thompson, Preprints CAPoC 6, Brussels, October 2003, 014. 

Major differences were noted between the systems derived from Fe(CO)c and M(CO) (M = Cr, Mo, and W) with respect to the effect of the base concentration on the reaction rate. Thus in the case of the catalyst system derived from Fe(CO)5 tripling the amount of KOH while keeping constant the amounts of the other reactants had no significant effect on the rate of H2 production (11). However, in the case of the catalyst system derived from W(CO)g the rate of production increased as the amount of base was increased regardless of whether the base was KOH, sodium formate, or triethylamine (12). This increase may be interpreted as a first order dependence on base concentration provided some solution non-ideality is assumed at high base concentrations. Similar observations were made for the base dependence of H2 production in catalyst systems derived from the other metal hexacarbonyls Cr(CO) and Mo(CO) (12). Thus the water gas shift catalyst system derived from Fe(CO)5 has an apparent zero order base dependence whereas the water gas shift catalyst systems derived from M(CO)g (M - Cr, Mo, and W) have an approximate first order base dependence. Any serious mechanistic proposals must accommodate these observations. 

Langanke J, Leitner W (2008) Regulated Systems for Catalyst Immobilisation Based on Supercritical Carbon Dioxide. 23 91-108 Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

Steric interaction was also clearly an issue for all of the systems investigated. Most notable is the large difference in the reactivity between ThrePHOX 6b- and PHOX 5c-based catalysts with respect to the substituents bound to the nitrogen atom. ThrePHOX consistently performed best with A-methyl-A-benzyl enamines while PHOX preferred a W-methyl-A-phenyl derivative. Lowering the temperature had a favorable outcome on enantioselectivity but slowed the reaction and caused drops in conversion for 5c and 6b. 

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

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