Tailor-made Homogeneous Catalysts

1 University of Amsterdam,van t Hoff Institute for Molecular Sciences, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands. 2Separation Technology Group, Faculty of Science and Technology, University of Twente PO Box 217, 7500 AE Enschede, The Netherlands 

J Cole-Hamilton andR. P. Tooze (eds.), Catalyst Separation, Recovery and Recycling, 39—72. 2006 Springer. Printed in the Netherlands. 

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Reek, J.N.H., van Leeuwen, P.W.N.M., van der Ham, A.G.J., and de Haan, A.B. (2006) Supported catalysts - immobilization of tailor-made homogeneous catalysts, in Catalyst Separatum, Recovery and Recycling - Chemistry and Process Design (eds D,-(, Cole-Hamilton and R.P. Tooze), Springer, Dordrecht, pp. 39-72. 

Tailor-made surface catalysts are required to improve the technical performance of known homogeneous processes as in alkene dimerization, alcohol homologation, and methanol carbonylation. 

When comparing chemical and biocatalytic methods, one could say that, especially for asymmetric oxidations, enzymatic methods enter the scene. This is most evident in the area of asymmetric Baeyer-Villiger oxidation, where biocatalysts take the lead and homogeneous chiral catalysts lag far behind in terms of ee values. Significant progress can be expected in the area of biocatalysis due to the advancement in enzyme production technologies and the possibility of tailor-made enzymes. 

Table 2 shows to what extent homogeneous catalysts are tailor-made and how variable and adaptable they are to the problem concerned by suitable reaction and unit processes, taking as examples the modem hydroformylation processes and catalysts. It clearly illustrates that a variety of different solutions in terms of reaction conditions and product separation technologies are available to meet any... 

Nowadays homogeneous metallocene catalysts activated with oligomeric methylalumoxanes or other co-catalysts [16, 20, 46-54] open new prospects. These systems have an excellent activity, they have the ability to form random copolymers in combination with a narrow molecular mass and comonomer distribution. Further important advantages are that a broad variety of structures can be synthesized to obtain tailor-made catalysts [49, 53], and that zirconium compounds are scarcely reduced with the co-catalyst [54]. It is further reported that metallocenes have been used in combination with methylalumoxanes for EPDM production at temperatures below 100 °C in liquid propylene [55]. 

As a result of this discussion it has to be concluded that the preparation of useful multicomponent as well as multifunctional catalysts requires, first of all, highly selective catalytic sites which reach their optimal activity at nearly the same temperatures. Since, already, this requirement can hardly to be met with the usual heterogeneous contacts, multifunctional catalysts are best prepared as homogeneous catalysts, e. g., with transition metal complexes. Being well-defined chemical species, they can be tailor-made for a given reaction. Experience shows that these compounds accelerate the reaction by opening a relatively complex catalytic... 

Useful multicomponent catalyst systems as well as multifunctional catalysts both offer new possibilities for the performance of catalytic processes this potential, however, can hardly be used as yet. One of the reasons for this difficulty stems from the fact that the preparation of such catalytic systems requires highly selective as well as sufficiently active catalytic components which, in addition, all reach their optimal catalytic properties for the same reaction conditions. This demand can be fulfilled by the use of tailor-made, catalytically active, transition metal complexes. The problem, however, is that these catalysts normally work via a relatively complex catalytic cycle. In a one-pot reaction system, therefore, a large number of different chemical species must be expected. Such a complex structured system can lead to several problems since it cannot be assumed that in a homogeneously catalyzed reaction system all components do not negatively interact. Even if a sufficiently stable catalyst system can be found by applying one or more of the different heterogenization techniques, this type of problem is hard to solve be-... 

This concept allows the important advantages of homogeneous over heterogeneous catalysts to be utilized for the first time. Particular mention should be made here of the significantly greater variability of homogeneous catalysts which enables them to be tailored to the requirements of the desired reaction by means of steric and/or electronic modification (cf. Table 1). 

Although all of these fields are summarized in the literature, time after time there are different opinions on the subject of the results that may be achieved in catalysis by using MX immobilized on a tailor-made polymer. Some researchers believe this approach to be a panacea to eliminate all disadvantages of traditional homogeneous catalysts others emphasize shortcomings of such systems in comparison with typical heterogeneous catalysts. Here we try to analyze in detail the possibilities and perspectives of polymer-immobilized metal complex catalysts for a wide variety of reactions in organic synthesis. 

Interfaces are tailored with molecules. Heterogeneous catalysts can be made from molecules that are known to be homogeneous catalysts by immobilizing them on the surfaces of solids. Surfaces can be made hydrophobic with halogenized silane derivatives. Molecules can act as antenna dyes in novel types of solar cells or in nanoelectronic materials for optical devices. Surface chemistry is molecular chemistry. 

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