Catalysts homogeneous single-component

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

The known catalyst systems for olefin metathesis can be grouped into heterogeneous catalysts, homogeneous multi-component catalysts, and homogeneous single-component catalysts. 

Figure 1. Classification of homogeneous single-component catalysts of the lanthanides.

Dahlmann, M. Erker, G Nissinen, M. Frdhlich, R. Direct experimental observation of the stereochemistry of the first propene insertion step at an active homogeneous single-component metallocene Ziegler catalyst. J. Am. Chem. Soc. 1999,121, 2820-2828. 

Table 3.16. Alkylidene and carbyne complexes as single-component catalysts for homogeneous-phase alkene metathesis.

To a certain extent the expression multicomponent catalysts is an arbitrary one. There is no doubt that the pure chemical elements and pure chemical compounds have to be called single component catalysts. It is, however, questionable whether a material such as steel should be classified as a single component system or as a multicomponent system. Some of the multicomponent catalysts, for instance, the iron-alumina catalyst consist of two separate solid phases but it would be misleading to accept the presence of more than one phase as the decisive criterion for multicomponent catalysts. The more than additive catalytic action of Cu-ions and Fe-ions in an homogeneous aqueous medium represents obviously a case of multicomponent catalysis, although it occurs in a single-phase system. As to solid multicomponent catalysts, they usually consist of more than one single phase, but there are exceptions to this rule, such as in cases in which mixed crystals or solid solutions are formed from the components. 

During this process, the layer thickness of three catalyst components on a titer-plate was simultaneously varied in such a way that 48 single catalyst mixtures were obtained in a one-step process. Thus, the amount of each catalyst component in the mixture could either be decreased or increased, resulting in a homogeneous multi-component layer. 

The metathesis polymerisation of dicyclopentadiene (endo, exo) yields a linear polymer when carried out with single-component homogeneous metathesis catalysts of the metallacyclobutane or metal alkylidene type in diluted toluene solutions [142, 143]. 

For the cross-metathesis reactions we used two different homogeneous ruthenium catalysts one multicomponent catalyst prepared in situ, and the other catalyst consisting of one single component. Both are based on ruthenium(II) and the latter can be isolated in pure form. Figure 1 shows the ruthenium catalysts that were used in cross-metathesis reactions to synthesize silicon-containing ot,(o-dienes. 

The recent development of single-component "cationic" metallocene catalysts is an extraordinary new development which eliminates many of the problems associated with the methyalumoxane cocatalysts. A particular advantage of the cationic metallocene catalysts is that they are considerably more tolerant of functional groups than the conventional hetereogeneous catalysts or homogeneous catalysts based on methylaluminoxane. 

Binder is the material used in the FCC catalyst to bind the matrix and zeolite components into a single homogeneous particle. 

It may be recalled that in homogeneous reactions all reacting materials are found within a single phase, be it gas, liquid or solid if the reaction is catalytic, then the catalyst must also be present within the phase. Thus, there are a number of means of defining the rate of a reaction the intensive measure based on unit volume of the reacting volume (V) is used practically exclusively for homogeneous systems. The rate of reaction of any component i is defined as... 

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