Encapsulated homogeneous catalyst

In homogeneous catalysis ligand systems may define the medium for the reaction. Water-soluble ligands have been developed, but in other instances two phases have to be used. Ultrafiltration may become a useful technique for catalyst removal if one uses encapsulated homogeneous catalysts or larger oligomeric catalysts. New catalytic systems involve the use of easily functionalizable den-drimers. In all these systems a combination of catalysis and separation can be envisaged that also enhances rates and selectivity. 

Although zeolite-encapsulated metal complexes were known for some time, the principle that such complexes could act as a new type of immobilized homogeneous catalysts was probably first demonstrated only in 1985 [37]. This achievement opened a new and fruitful area of research in immobilizing homogeneous catalysts. These catalysts are named appropriately ship-in-the-bottle (SIB) catalysts (see Fig. 42.5). General overviews of zeolite-encapsulated metal complexes were given [38, 39]. 

Oxidation of alcohols to carbonyl compounds is an important reaction. Stoichiometric oxidants such as chromates, permanganates and MO4 (M = Ru, Os) are the commonly used reagents [19a,59,60]. However, they are going out of favour increasingly because they create heavy metal wastes . In view of this, development of environmentally friendly heterogeneous catalysts for alcohol oxidation is very important. In the use of catalytic amounts of transition metal salts or complexes as homogeneous catalysts for the oxidation of alcohols [61-64], separation of the catalyst from the reaction mixture and its subsequent recovery in active form is cumbersome. Heterogeneous catalysts for this kind of reaction are therefore necessary [65]. Clearly, encapsulation and/or immobilization of known... 

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

When a homogeneous catalyst, that is an enzyme, is immobilized there are in principle three different approaches [3]. The enzyme can be adsorbed onto a carrier, it can be encapsulated in the carrier, or it can be covalently attached to the carrier. In an extreme case of covalent attachment the enzyme can be cross-linked, making a carrier uimecessary. The choice of immobiUzation method and carrier can greatly influence the properties of the immobiUzed enzyme, ideally improving them. To date many excellent reviews [5-13] and even comprehensive books [14] on the topic of enzyme immobiUzation have been written. Many of these reviews and books use their own division of approaches they range from three to eight and can also include membrane techniques. In order to ease the comparison with chemical catalysts, a division according to reference [3] was chosen. This chapter aims to demonstrate, with examples from the research performed within the COST D25 Action, the importance of the area under discussion. 

One of the most smdied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225], The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. 

Metal macrocycles encapsulated in zeolites seem to be a solution to overcome the above mentioned problem because they combine successfully the advantages of homogeneous catalysts, especially their selectivity and controllability, with the ease of the separation of heterogeneous catalysts. In these catalysts the large, electroneutral metal macrocycle species is held in the zeolite cavities topologically rather than chemically. 

As mentioned earlier, research on creating hybrid catalytic materials has been an area of interest for some time, and thus, there are a variety of methods that are used to immobilize catalytic species onto support materials. Some common examples are physisorption of the catalyst to the surface, electrostatic catalyst/surface interactions, and encapsulation of the catalyst into the pores of microporous materials [2], These methods can suffer from leaching in many solvents, competitive binding with charged or polar substrates, and limited usable substrate size and diffusion due to the support s small pore size, respectively. The method that offers the most promise of stability of attachment as well as flexibility in synthesis is covalent reaction between the catalyst and the support. This is the method employed in our research. By approaching the tethering process in a controlled and defined way, the surface catalytic species can be more uniform and behave more similarly to very well-defined homogeneous catalysts. 

During the last decade a variety of mctluxloiogies have been developed to immobili/e molecular homogeneous catalysts on the surface of heterogeneous organic as well as inorganic polymeric materials (a) adsorption of the catalyst into the pores of the support, (b) encapsulation of the catalyst within the confines of cavities of the support, the Sii-callcd ship in the bottle technique", (c) attachment of the catalyst to the support by covalent bond formation and (d) direct synthesis into the final composite material [7. ... 

From the viewpoint of molecular encapsulation, enzymes have evolved to accommodate and orient their substrate within a well-defined enviromnent provided by the protein. Figure 13.1a. In many cases, the substrate is embedded within the enzyme. In contrast, homogeneous catalysts typically possess a (chelating) ligand which occupies one hemisphere around the metal, while the substrate occupies the other hemisphere. The main effort in catalyst optimization has thus been to extend the reach of the selectivity-orienting ligand. Figure 13.1b. 

There have been numerous attempts to link phosphines or their corresponding homogeneous catalysts to solid supports. Heterogenization can be achieved via covalent bonding or ionic interactions. Moreover, conventional small metal phosphine catalyst can be encapsulated in chaimels or cavities of a heterogeneous... 

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