Flow metathesis catalysts

The butene-1 is mixed with butene recycle from the autometathesis recovery section and is vaporized, preheated and fed to the autometathesis reactor (3) where butene-1 reacts with itself to form hexene-3 and ethylene over a fixed bed of proprietary metathesis catalyst. Some propylene and pentene are also formed from the reaction of butene-2 in the butene-1 feed. Reactor effluent is cooled and flows to the autometathesis recovery section (4), where two fractionation columns separate it into a hexene-3 product that flows to the hexene isomerization unit (5), an ethylene/propylene mix, and butene-1 that is recycled to the butene... 

Attachment to the Support via Adsorption onto Inorganic Oxide Supports In 2008, Sels, Jacobs, and coworkers [89] described a simple process, where catalyst 5 was adhered onto silica, and the resulting material was employed in batch and continuous-flow metathesis applications. When both the substrates and reaction media were nonpolar, the system worked effectively however, when polar substrates were introduced, Ru was leached from the material. This observation was further supported when the catalytic system passed a cyclooctene, ROMP-based split-test performed in hexane, yet failed the same experiment in diethyl ether. Likewise, the adhered catalyst performed well in continuous-flow processing of cyclooctene in hexane however, no continuous-flow results were reported using polar substrates. 

The results described herewith demonstrate that the activity of Ru metathesis catalysts can be enhanced by introduction of EWGs without detriment to catalysts stability. This principle can be used not only to increase the catalyst activity, but also to alter its physical-chemical properties, such as solubility in given medium or affinity to silica gel. An example of novel immobilization strategy, based on this concept is presented. In fact, the possibility of reversibly binding catalysts to a solid phase is of major importance for industrial applications, particularly when continuous flow processes with immobilised homogeneous catalysts are pursued. 

Table 8-5 indicates the wide variety of catalysts that can effect this type of disproportionation reaction, and Figure 8-7 is a flow diagram for the Phillips Co. triolefm process for the metathesis of propylene to produce 2-butene and ethylene. Anderson and Brown have discussed in depth this type of reaction and its general utilization. The utility with respect to propylene is to convert excess propylene to olefins of greater economic value. More discussion regarding olefin metathesis is noted in Chapter 9. 

Moulijn et al. (33) studied the reactions of some linear alkynes over a W08-Si02 catalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. 

Very recently, Luckner et al. (116) obtained initial rate data for the metathesis of propene using the W0r-Si02 catalyst at flow rates where mass transfer effects were found to be negligible. Their experimental data referring to measurements at 0.1 to 0.9 MNm-2 and 672 to 727 K could be correlated by Eq. (53). 

Olefin metathesis is a useful reaction for the production of propylene from ethylene and butenes using certain transition-metal compound catalysts. The two main equilibrium reactions that take place simultaneously are metathesis and isomerization. Metathesis transforms the carbon-carbon double bond, a functional group that is unreactive toward many reagents that react with many other functional groups. New carbon-carbon double bonds are formed at or near room temperature even in aqueous media from starting materials. Because olefin metathesis is a reversible reaction, propylene can be produced from ethylene and butene-2. Metathesis can be added to steam crackers to enhance the production of propylene by the transformation of ethylene and the cracking of mixed butenes. Fig. 3 shows a schematic flow diagram of a typical metathesis process. Examples of metathesis... 

Other reactions evaluated by the authors included Wittig-Horner olefination and a series of ring-closing metathesis (RCM) reactions, employing Grubb s II catalyst. In all cases, a reduction in power consumption, increase in yield, and reduction in reaction time were obtained as a result of employing microwave-assisted continuous flow reactions [33],... 

Fig. 5.1 Initial rate of propene metathesis, ro, on a 5.8 wt% Re207/Al203 catalyst in a flow system, as a function of temperature and propene pressure P. The lines represent the rates calculated from eqn. (5) (Kapteijn 1981).

Fig. 5.5 Rate of propene metathesis on (a) W03/Si02 at 375°C and 425°C squares 0.4 g, circles 0.8 g catalyst, (b) M0O3/C0O/AI2O3 at 150°C squares 0.3 g, circles 0.6 g catalyst. Note that (a) shows diffiisional effects at low flow rates whereas (b) does not (Moffat 1971).

A kinetic study has been made of the liquid-phase metathesis of oct-l-ene over Re207/Al203 in a flow reactor Fig. 5.8 shows the fractional conversion X at four temperatures as a function of the contact time expressed in terms of W/F as defined in the caption. The data are best interpreted in terms of a model in which either product desorption or interconversion of the alkene/carbene complex is rate-determining. This model leads to eqn. (11), where r is the reaction rate per unit weight of catalyst, and from which the curves in Fig. 5.8 have been calculated to give the best fit to the data (Spronk 1992). 

SCCO2 has also been used as a solvent with a silica-immobilized catalyst in metathesis reactions [25]. A heterogeneous catalytic process is developed, in which catalyst leaching is avoided but the reactivity is lower than when using a homogeneous catalyst This application has also been extended to continuous-flow processes for hydrogenation [26], Friedel-Crafts alkylations [27], etherification [28], and hydroformylation [29] reactions. 

Grubbs-Hoveyda catalyst evaporated on to silica can be used for metathesis, but only with nonpolar substrates and products [59]. More polar flowing chemicals cause extensive leaching. 

Recently, the metathesis reaction of -butane employing supported W-polyhydrides on silica-alumina was reported using a continuous-flow reactor at 150 °C under pressure (P=20bar) [61]. In these harsh conditions, -butane was converted into a mixture of Uquid, linear alkanes (Cj to C12) as the major products. Unfortunately, the catalyst was rapidly deactivated (<40 h). Using similar reaction conditions, supported Ta-polyhydrides on siUca-alumina were also active, but less selective, for the heavier alkanes. Ultimately, it follows a general trend that supported, W-polyhydride catalysts on alumina oxide outperform supported, Ta-polyhydrides on silica. This has been attributed to better catalyst stability and the possible formation of novel, W-oxo polyhydrides. 

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