Immobilized metal catalyst

T. Matsumoto, M. Ueno, N. Wang, S. Kobayashi, Recent Advances in Immobilized Metal Catalysts for Environmentally Benign Oxidation of Alcohols, Chem. Asian J. 3 (2008) 196-214. 

Keywords Immobilized metal catalysts Heterogenized homogeneous catalysts Solid supported catalysts Industrial catalysis ... 

Figure 11 Generation of immobilized metal catalysts with a substrate pocket by imprinting with a pseudo-substrate (a) attachment of the pseudo-substrate (b) polymerization (c) selective cleavage of the pseudo-substrate (d) removal of the pseudo-substrate and the metal ion (e) addition of the catalytically active metal ion.

The first example of a combination of a metal-catalyzed substrate synthesis with a biotransformation conducted in a one-pot maimer proceeding according to Scheme 19.23 was reported by Hanefeld, Maschmeyer, Sheldon, and coworkers in 2006 [58]. In this pioneering work, enantioselective hydrogenation of methyl N-acetyl amino acrylate (72) with a heterogenized rhodium-diphosphane complex as catalyst gave the N-acetyl alanine (S)-73 with 100% conversion and 95% ee. This intermediate was then directly converted in situ after separation of the immobilized metal catalyst by means of an L-amino acylase (Scheme 19.23). This enzymatic resolution then led to the formation of the desired amino acid L-alanine (l-74 (S)-74) with 98% conversion and with an excellent enantiomeric excess of >98%. 

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. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

Ambient-temperature ionic liquids have received much attention in both academia and industry, due to their potential as replacements for volatile organic compounds (VOCs) [1-3]. These studies have utilized the ionic liquids as direct replacements for conventional solvents and as a method to immobilize transition metal catalysts in biphasic processes. 

Fontecave M, Hamelin O, Menage S (2005) Chiral-at-Metal Complexes as Asymmetric Catalysts. 15 271-288 FraUe JM, Garcia JI, Mayoral JA (2005) Non-covalent Immobilization of Catalysts Based on Chiral Diazaligands. 15 149-190 Frenking G, see Deubel D (2005) 12 109-144 Fu GC, see Netherton M (2005) 14 85-108... 

The immobilization of metal catalysts onto sohd supports has become an important research area, as catalyst recovery, recycling as well as product separation is easier under heterogeneous conditions. In this respect, the iron complex of the Schiff base HPPn 15 (HPPn = iVA -bis(o-hydroxyacetophenone) propylene diamine) was supported onto cross-linked chloromethylated polystyrene beads. Interestingly, the supported catalyst showed higher catalytic activity than the free metal complex (Scheme 8) [50, 51]. In terms of chemical stability, particularly with... 

The most spectacular results, in terms of comparison between CFPs- and carbon-supported metal catalysts, were likely provided by Toshima and co-workers [33,34]. As illustrated in Section 3.3.3, they were able to produce platinum and rhodium catalysts by the covalent immobilization of pre-formed, stabilized metal nanoclusters into an amine functionalized acrylamide gel (Scheme 5). To this purpose, the metal nanopartides were stabilized by a linear co-polymer of MMA and VPYR. The reaction between its ester functions and the amine groups of the gel produced the covalent link between the support and the... 

Another recent patent (22) and related patent application (31) cover incorporation and use of many active metals into Si-TUD-1. Some active materials were incorporated simultaneously (e.g., NiW, NiMo, and Ga/Zn/Sn). The various catalysts have been used for many organic reactions [TUD-1 variants are shown in brackets] Alkylation of naphthalene with 1-hexadecene [Al-Si] Friedel-Crafts benzylation of benzene [Fe-Si, Ga-Si, Sn-Si and Ti-Si, see apphcation 2 above] oligomerization of 1-decene [Al-Si] selective oxidation of ethylbenzene to acetophenone [Cr-Si, Mo-Si] and selective oxidation of cyclohexanol to cyclohexanone [Mo-Si], A dehydrogenation process (32) has been described using an immobilized pincer catalyst on a TUD-1 substrate. Previously these catalysts were homogeneous, which often caused problems in separation and recycle. Several other reactions were described, including acylation, hydrogenation, and ammoxidation. 

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. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Since many metallic catalysts have high adsorption affinities, we often find that certain poison molecules are adsorbed in an immobile form after only a very few collisions with the catalyst surface. In this situation, the outer periphery of the catalyst particle will be completely poisoned while the inner shell will be completely free of poison. The thickness of the poisoned shell grows with prolonged exposure to poison molecules until the pellet is completely deactivated. During the poisoning process, the boundary between active and deactivated regions is relatively sharp. 

Several technical arrangements have been used successfully to immobilize this catalyst on an electrode surface as thin films.80-85 In such arrangements the metal sites in films show a dramatic increase in reactivity and stability toward C02 reduction into CO. Moreover, this kind of modified electrode (for instance [Re(bpy)(CO)3Br] incorporated in Nafion membrane) appeared as a good electrocatalyst in pure aqueous electrolyte.86 However, in such systems both CO and HCOO are also produced, and the total current yield of C02 reduction is lowered by the concurrent H+ reduction into H2. 

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