Heterogenized metal complex catalyst

Some of the supported (heterogenized) metal complex catalysts discussed above show good performance in the selective semihydrogenation of alkynes.446-449... 

Available vast information in the scientific literature on synthesis and mechanism of heterogeneous metal-complex catalysts (HMC) action points to combination of advantages in both homogeneous and heterogeneous catalysts. The catalytic activity of HMC has been investigated in a series of processes, including oligomerization of lower olefins [1]. However, in connection with limitations of physicochemical methods of research of HMC, the data on formation of catalytic active centers on the surface of the support and on mechanism of their action are not numerous and are contradictory. 

Abstract Recent progresses in molecularly imprinted metal-complex catalysts are highlighted in this chapter. Molecular imprinting is a technique to produce a cavity with a similar shape to a particular molecule (template), and the molecularly imprinted cavity acts as shape-selective reaction space for the particular reactant. The application of the molecular-imprinting technique to heterogeneous metal-complex catalysts is focused in the viewpoint of a novel approach in the design of shape-selective catalysis mimicking enzymatic catalysis. 

Heterogeneous metal complex catalysts, synthesized by a new photoimmobilization method, for low-temperature oxidation of olefins C3-C4 to partial oxidation products are proposed. The process selectivity is about 100 %. 

The heterogeneous metal complex catalysts containing Cu, Mo, W, V, Ti, Ag were examined. 

The nature of the active centers of heterogeneous metal complex catalysts. 

C.V, Lisichkin, A.Ya. Yuffa, Geterogennye Metallokompleksnye Katalizatory (Heterogeneous Metal Complex Catalysts). Nauka, Moscow 1981 (in Russian)... 

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

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. 

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