Rhenium-based catalysts

Besides good activity and high selectivity, the lifetime, or stability, of a catalyst 

Unpromoted supported rhenium oxide catalysts can be regenerated many times without any loss of activity. However, the activity of a deactivated tetraalkyltin- 

Re207/Si02 is not active below 120°C (Aldag 1977). This is perhaps because Si02 does not stabilize the necessary intermediate oxidation state of rhenium. ReOs is catalytically active, either unsupported or on Si02, at 200°C. In contrast to WO3, ReOs is a conductor, which favours metathesis relative to dimerization (Tsuda 1981, 1985). 

A remarkable observation is that Me4Sn deposited on dehydroxylated alumina brings about propene metathesis at 25°C in a pulse reactor with an activity comparable to that of Rc207/Al203 (Ahn, H-G. 1992). This is one of the rare cases where metathesis appears to proceed without the mediation of a transition metal complex. 

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The metathetic reaction occurs in the gas phase at relatively high temperatures (150°-350°C) with molybdenum or tungsten supported catalysts or at low temperature (=50°C) with rhenium-based catalyst in either liquid or gas-phase. The liquid-phase process gives a better conversion. Equilibrium conversion in the range of 55-65% could be realized, depending on the reaction temperature. ... 

The process calls for feeding ethylene and mixed butylenes to the bottom of a reactor. The mixed butylenes consist of both butene-1 and butene-2. (Refer to Figure 1—10 to refresh your memory about the difference.) A slurry of rhenium-based catalyst is introduced at the top. As the ethylene and butylenes bubble past the catalyst, the ethylene and butene-2 will react to form propylene (the carbon count is right). Simultaneously, as the butene-2 is consumed, butene-1 isomerizes to create more. 

The use of organometallic rhenium complexes has found a very broad scope as oxidation catalysts as described in the previous section, making MTO the catalyst of choice for many oxidation reactions of olefins. Interestingly, MTO and related rhenium compounds have also found application in the reverse reaction, the deoxygenation of alcohols and diols. Especially in recent years, this reaction has attracted much attention due to the increased interest in the use of biomass as feedstock for the chemical industry. This section provides an overview of the use of rhenium-based catalysts in the deoxygenation reaction of renewables. 

Reactions of Phosphonium Ylides. - 2.3.1 Reactions with Carbonyl Compounds. This year we are able to report several variations of the traditional Wittig olefination which employ the addition of catalysts to effect the reaction. For example, Lebel et al. have reported a new salt-free process for the methyl-enation of aldehydes, in which the phosphorane is generated in situ from triphenylphosphine and a diazo precursor with either a rhodium- or rhenium-based catalyst (Scheme 6). It was found that the most effective combination of catalyst and diazo-compound were Wilkinson s catalyst [RhCl(PPh3)3] and... 

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

Bromopentacarbonylrhenium(I) [ReBr(CO)5] promotes the Friedel-Crafts acylation of arenes with acyl chlorides. Toluene rmdergoes benzoy-lation with BC in the presence of the rhenium-based catalyst (0.1% mol), affording a mixture of ortho-, meta-, and para-methylbenzophenones in 91% yield (11 4 85 molar ratio). The yield decreases to 72% when using the same catalyst in a lower amount (0.01% mol). The process can be applied to different acyl chlorides, giving the corresponding ketones a satisfactory to high yield (Table 3.17). 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state (ii) adsorption of (polymeric) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

There is a broad agreement that carbene species developed on partly oxidized transition metal ions are necessary to bring about the metathesis reaction. The data reported earlier provide support that the environment of the active site does possess acid properties. It has been proposed that there is a relationship between the Brpnsted acidity of the rhenium-based catalysts and the metathesis activity [52], and that the activity is not related to their Lewis acidity. Dynamic infrared spectroscopic experiments were conducted by poisoning a Re207/Al203 catalyst (8.0 wt% Re) under a propene flow [53], Ammonia adsorption gave rise to coordinated NH3 species, characterized by the bands at 1618, 1326, and... 

When highly pure propene is not available commercially, it can be prepared by the reverse metathesis reaction of ethene and 2-butene [Eq. (1)]. The process is performed either at high temperatures (150-350°C) in the gas phase, over molybdenum or tungsten catalysts (Phillips triolefin process) [4], or at low temperatures (50°C) in the liquid phase, in the presence of rhenium-based catalysts (IFP-CPC process) [13], The raw material may be either ethene and the C4 fraction available from the hydroisomerization unit (previously submitted to an isomerization step to maximize its 2-butene content) or ethene alone, which, before admission to the metathesis unit, is partly dimerized to 1-butene, then isomerized to 2-butene in separate units. The process is useful in the event of a high demand for propene, since the C4 fraction is readily available from a cracking unit. 

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