Catalysis catalyst immobilization

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Oehme G (1999) Catalyst immobilization two-phase systems. In Jacobsen EN, Pfaltz A, Yamamoto H (eds) Comprehensive asymmetric catalysis, vol III. Springer, Berlin Heidelberg New York, p 1377... 

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

Catalysis of supported metal ions is an area of interest. There are a number of advantages in depositing catalytically active metal ions on a support. The ion exchange method of catalyst immobilization is simple and the attractiveness of this method is further increased by providing stable inorganic ion exchangers of known structures as supports. 

Immobilization of catalysts is an important process design feature (see Chapter 9.9). A recent example of catalyst immobilization is the biphasic approach which seems superior to immobilization on solids, as successfully proven in the Ruhrchemie/Rhone Poulenc process for the hydro-formylation of olefins.286 Supported liquid phase catalysis was devised as a method for the immobilization of homogeneous catalysts on solids. When the liquid phase is water, a water-soluble catalyst may be physically bound to the solid. 

Reetz, M.T., Quaiser, S.A., Breinbauer, R., and Tesche, B., A New Strategy in Heterogeneous catalysis the design of cortex catalysts/catalysis/clusters/ immobilization/ surface chemistry, Angew. Chem. Int. Ed. Engl., 34, 2728,1995. 

Other important aspects of modem catalysis including bio- and homogeneous catalysis are beyond the scope of the current book and are themselves the themes of several excellent books. Further, the emerging areas of computational catalysis and immobilized catalysts are not included here but are covered dedicated texts in the literature. 

Many different soluble polymers have been used as supports for catalyst immobilization. Since solvation of otherwise insoluble catalysts can frequently be accom-pHshed by attachment to a soluble polymer, these supports have found significant use in the immobihzation of classical solution phase catalysts. Here, we will only survey polyethylene glycol (PEG) as a soluble polymeric support for catalysis. The use of other types of soluble polymers (e.g., polyethylene, non-cross-linked polystyrene) has been reviewed elsewhere [49]. 

Bianchini, C. Barbaro, P. (2002) Recent aspects of asymmetric catalysis by immobilized chiral metal catalysts.. Topics in Catalysis, 19 17-32. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

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. 

H. U. Blaser, B. Pugin, M. Studer, Enantioselective heterogeneous catalysis academic and industrial challenges, in D. E. de Vos, I. F. J. Vankelecom, P. A. Jacobs (Eds.), Chiral Catalyst Immobilization and Recycling, Wiley-VCFI, Weinheim, 2000, p. 1. 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

Concentration modulation experiments have been reported for applications to heterogeneous catalysis (48). The experimental implementation was accomplished by periodically flowing solutions with different (reactant) concentrations over the catalyst immobilized on the IRE. Fast concentration modulation in the liquid phase is limited by mass transport (diffusion and convection), and an appropriately designed cell is essential. The cell depicted in Fig. 12 has two tubes ending at the same inlet (65). This has the advantage that backmixing in the tubing upstream of the cell can be avoided. With this cell, concentration modulation periods of about 10 s were achieved (45,65). 

Asymmetric Catalysis with Immobilized Phosphine-Based Catalysts. 64... 

In microporous supports or zeolites, catalyst immobilization is possible by steric inclusion or entrapment of the active transition metal complex. As catalyst retention requires the encapsulation of a relatively large complex into cages only accessible through windows of molecular dimensions, the term ship-in-a-bottle has been coined for this methodology. Intrinsically, the size of the window not only determines the retention of the complex, but also limits the substrate size that can be used. The sensitivity to diffusion limitations of zeolite-based catalysis remains unchanged with the ship-in-a-bottle approach. In many cases, complex deformation upon heterogenization may occur. 

The research reviewed here reflects the intense activity of the preceding 30 years in the field of oxidation catalyst immobilization. Obviously, the literature contains many erroneous and unreliable results, particularly with respect to leaching of active metal components. Examples are the reactions with silica- or alumina-supported Mo, W, or Cr and organic peroxides as the oxidants. The majority of these reports simply deal with homogeneous catalysis. Nevertheless, many concepts have been proposed in which truly heterogeneous catalysis has been obtained. Examples include the Mo-polybenzimidazole epoxidation catalyst (243), the Os-tetrasubstituted dio-late catalyst for dx-dihydroxylation (391), the heteronuclear P-W epoxidation catalysts (359, 377), and the dioxirane systems (406, 407). 

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