Carbon-Anchored Metal Complex Catalysts

Improvement in the catalytic properties derived from the confinement concept results from imprisonment of the substrate within the pores of the support, which leads to enhanced interactions between the active catalyst and the substrate [11]. However, this is a two-edged sword, as tight entrapment of the immobilized catalyst molecules can induce lower activity or chemo- and enantioselectivity as a consequence of a compulsory conformation. The lower activities can also be due to reduced or even blocked mass transport in the narrow pores, imposing limits on the effective range of substrates that can be used. On the other hand, the reduced enantioselectivity can be due either to a strong physisorption of the complex on the walls of the support or to a constrained environment that prevents 

Carbon Materials for Catalysis, Edited by PhUippe Serp and Jos6 Luts Figueiredo Copyright 2009 John Wiley Sons, Inc. 

The improved catalytic activity can also be a consequence of the fact that when metal complexes are attached to a support, the immobilized catalysts can no longer interact with each other as in homogeneous media, preventing the active forms of the catalysts from reacting with each other. This is a phenomenon that is usually observed in homogeneous media and is responsible to a large extent for the deactivation of homogeneous catalysts [l,2,4-8]. 

This chapter begins with a general description of the several strategies to het-erogenize transition-metal complexes onto solid supports, with a special emphasis on those methodologies that have been used for complex grafting onto carbon materials. It will include sections that will focus on the various transition-metal complexes that have been immobilized onto several carbon materials activated carbons, black carbons, carbons xerogels, and carbon nanotubes the specific catalytic reactions with these carbon-based systems are also discussed in some detail. 

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C. Freire, 2009, Carbon-anchored metal complex catalysts in Carbon Materials for Catalysis , ed. P. Serp and J.L. Figueiredo, Wiley. 

In other reactions, particularly where strongly complexing reactants, e. g., carbon monoxide, are involved, leaching of the immobilized metal center may take place. Generally, the parameters to be considered in a polymer-anchored metal complex catalyst are of a manifold nature. It is still an unsolved problem and an incompatible situation that, on the one hand, a leaching process should be avoided while, on the other hand, sufficient activity and the selectivity necessary for industrial applications are to be maintained. As a consequence it has become... 

A Rh diamine complex has been anchored on two carbon xerogels of different porous textme. The reduction of the metal complex takes place under reaction conditions and the used catalysts contain the anchored metal complex and Rh particles. The catalysts are very active and fully recyclable for cyclohexene hydrogenation. The supports favor the reduction of the active phase and dispersed metallic particles have been obtained. 

Chapter 6 provides an extensive review of the uses of carbon as a catalyst, with particnlar emphasis being placed on cases in which active sites have been properly identified and activity correlations established. The special case of nitrogen-doped carbons and their catalytic activity in oxidation reactions is discussed in Chapter 7, and Chapter 8 covers the heterogenization of homogeneous catalysts by anchoring transition-metal complexes onto the snrface of suitable carbon materials. 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

De Vos, Sels, and Jacobs illustrate strategies of immobilizing molecular oxidation catalysts on supports. The catalysts include complexes of numerous metals (e.g., V, Cr, Mn, Fe, Co, and Mo), and the supports include oxides, zeolites, organic polymers, and activated carbons. Retention of the catalyt-ically active metal species on the support requires stable bonding of the metal to the support at every step in the catalytic cycle, even as the metal assumes different oxidation states. Examples show that catalysts that are stably anchored and do not leach sometimes outperform their soluble analogs in terms of lifetimes, activities, and selectivities. 

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