Catalysts immobilized metal complexes

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Immobilization is a popular means of simplifying separation of a catalyst from the reaction mixture. In contrast with immobilized metal complexes (via a solid-support-bound ligand) leaching problems are a less critical issue when using organocatalysts immobilized by covalent bonding to the solid support. 

Pomogailo, A.D. Polymeric Immobilized Metal-Complex Catalysts. Science Moscow, 1988, 303 pp. 

Pomogailo AD (1988) Polymeric immobilized metal complex catalysts. Nauka, Moscow... 

Catalytic tests in sc CO2 were run continuously in an oil heated flow reactor (200°C, 20 MPa) with supported precious metal fixed bed catalysts on activated carbon and polysiloxane (DELOXAN ). We also investigated immobilized metal complex fixed bed catalysts supported on DELOXAN . DELOXAN is used because of its unique chemical and physical properties (e. g. high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions). The effects of reaction conditions (temperature, pressure, H2 flow, CO2 flow, LHSV) and catalyst design on reaction rates and selectivites were determined. Comparative studies were performed either continuously with precious metal fixed bed catalysts in a trickle bed reactor, or discontinuously in stirred tank reactors with powdered nickel on kieselguhr or precious metal on activated carbon catalysts. Reaction products were analyzed off-line with capillary gas chromatography. 

Up to now a broad variety of common organic and inorganic polymer systems have been used as a solid support for immobilized metal complex catalysts. During the first period of the development work the need for a tailor- made support to meet the requirements of this application became apparent, e. g., with respect to general and structural stability, nature and degree of functionalization, functional group distribution and density, and accessibility of the functional sites [17]. 

In spite of the initial enthusiasm and the large amount of work that has been carried out in the meantime, any important practical application of an immobilized metal complex catalyst is not known so far. Promising and really satisfying results could only be obtained to a small extent in selective hydrogenation reactions in the synthesis of chemical specialities and pharmaceutical products and in polymerization reactions with immobilized metallocene catalysts [21]. The situation for the latter type of reactions, however, is different. 

The original idea to apply insoluble polymers as supports for immobilized metal complex catalysts was abandoned by Bayer [57] and Bergbreiter [22]. In... 

Inorganic polymers have been used less often than organic polymers as supports for immobilized metal complex catalysts, although the better physical properties of the former in general more than compensate for the better chemical properties... 

The term "gel immobilized metal complex catalysts" was introduced by Kabanov and Smetanyuk [4,5], and then research continued by the authors of this paper in collaboration with laboratory of late Prof. Dr. V.I. Smetanyuk. 

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