Catalyst Concepts in Homogeneous Catalysis

A catalytic process can be depicted as a reaction cycle in which substrates are converted to products with regeneration of the catal5dically active species. At the end of the process, the catalyst is present in its original form. The cyclic depiction of catalytic processes is particularly clear and is also helpfiil in developing new processes. 

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The last two decades have seen enormous developments in catalyst discovery and optimization tools, notably in the area of high-throughput experimentation (HTE) and process optimization (5). However, the basic concept used for exploring the catalyst space in homogeneous catalysis has not changed Once an active catalyst complex is discovered, small modifications are made on the structure to try and screen the activity of neighboring complexes, covering the space much like an ink drop spreads on a sheet of paper. This is not a bad method, but can we do better with the new tools that are available today ... 

Herrmann WA (2002) N-Heterocyclic carbenes a new concept in organometal-lic catalysis. Angew Chem Int Ed 41 1290-1309 Herrmann WA, Elison M, Fischer J, Kocher C, Artus GRJ (1995) Metal complexes of N-heterocyclic carbenes—a new structural principle for catalysts in homogeneous catalysis. Angew Chem Int Ed 34 2371-2374 Herrmann WA, Kocher C (1997) N-Heterocyclic carbenes. Angew Chem Int Ed 36 2162-2187... 

In addition to the synthesis of specific ligands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, like catalysis under supercritical conditions, colloidal catalysis , organometallic electrocatalysis and multi-metallic catalysis " ( cooperative catalysis ). 

Beyond these methodical developments which will prove their utility in the next decade catalyst recycling is of crucial importance in homogeneous catalysis. In contrast to heterogeneous catalysis recycling of the expensive metal is usually difficult. To facilitate catalyst/product separation often the attachment of a catalyst to an organic polymeric resin has been used. Although this concept workes nicely on a laboratory scale decreased activity and... 

We obtained some preliminary results in the isomerization of the benzyUm-ine derivative 39 to benzylidene derivative 40. The catalyst solution was prepared by reducing Co(acac)2 with DIBALH in the presence of DIOP or BINAP. This isomerization may open a new access to the enantiopure a-amino acids 41 by introducing the modern concept of homogeneous catalysis, (Scheme 6) [38]. 

Liquid-liquid biphasic systems where the catalyst is designed to stay immobilized in one of the two immiscible phases is obviously a very elegant solution to the problem of product separation in homogeneous catalysis. Moreover, the concept has been demonstrated to work on an industrial scale in the aqueous hydroformylation Ruhrchemie/Rhbne-Poulenc process as described in detail above. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

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]. 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

One of the most important difficulties in the use of homogeneous catalysis for technical applications is the recovery and reuse of the expensive metal catalyst. Various concepts for catalyst recychng are apphed to overcome this problem in current technical applications. A simple method is to perform the catalytic reaction in a single phase and to separate the product and the catalyst afterwards by distillation of the product, as reahzed, for... 

The basic concept is the intuition that, whether homogeneous or heterogeneous, catalysis is primarily a process controlled by a molecular phenomenon since it implies the catalyzed transformation of molecules into other molecules. It follows that on the surface of metals or metal oxides, sulfides, carbides, nitrides usually involved as heterogeneous catalysts, the relevant surface species and the mechanism of their mutual reactions must be of molecular character, as occurs in homogeneous or enzymatic catalysis. 

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