Immobilization of Metal Complexes

Successful applications of the oxygen-modified CNFs are reported on immobilization of metal complexes ]95], incorporation of small Rh particles [96], supported Pt and Ru CNFs by adsorption and homogeneous deposition precipitation ]97, 98], Co CNFs for Fischer-Tropsch synthesis ]99], and Pt CNFs for PEM fuel cells [100]. 

THE DIFFERENT PROCEDURES OF IMMOBILIZATION OF METAL COMPLEXES ON SOLID SUPPORTS... 

The chemical modification of the surface of solids has led to increased possibilities in a number of fields on laboratory as well as on industrial scale. Applications of modified silicas may be classified according to the field in which they are of interest. In each field the interaction with a specific type of molecules is effectuated. In the analytical field organic compounds and metal ions are selectively adsorbed. The chemical field aims at the immobilization of metal complexes for use as catalyst... 

The imprinting of metal oxides is a relatively new method for the immobilization of metal complexes. The case of Rh2(CO)4Cl2 on silica is illustrative. The dimer is first adsorbed onto the surface of silica, most likely through a surface hydroxyl group, and then reacted with trimethylphosphite, P(OMe)3. Amorphous silica is then... 

Polymers can also be used for the physical immobilization of metal complexes. For example, the degree of swelling observed for 1% crosslinked polystyrene varies with solvents more swelling is observed in THF than in methanol. Thus, the cationic complex [(diphos)Rh(NBD)]PF6 can be adsorbed onto polystyrene in THF. When dried and extracted with methanol, the complex is bound within the polystyrene matrix. The immobilized complex is a catalyst for the hydrogenation of 1-hexene in methanol. ... 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

The results obtained show that immobilization of metal complexes in polymer gels allows to prepare physically heterogeneous and chemically homogeneous catalysts and leads to an important increase in their activity, selectivity and stability in the reactions of dimerization of lower olefins. The immobilization of the complexes opens new possibilities of macromolecular design of the catalysts with desired structural organization and will contribute to the development of general principles of synthesis of highly efficient and environmentally friendly catalytic systems for liquid phase processes. 

The other two methodologies for immobilization of metal complexes, physical adsorption and electrostatic interaction, belong to the group of noncovalent interactions between the support and the metal complex [1-10]. In the former case it includes n-n interactions, van der Waals and hydrogen bonds, and hydrophobic-hydrophilic interactions between the support and the complex. In the second method, there is an electrostatic interaction between the support and the complex, and consequently, there must be charges of opposite signals between the support and complex. The noncovalent interactions can also occur directly between the support and the complex or via the use of spacers in the latter case the interaction between the spacer and the complex can be of different in nature, covalent or noncovalent. 

Research on the immobilization of metal complexes using carbon materials is scarce compared with inorganic supports, such as zeolites, silicas, and clay-based materials [1-10]. Nevertheless, carbon materials are unique supports, as they can provide a variety of surface groups at the edges and/or defects of graphene sheets that can be tailored by adequate thermal or chemical treatments, besides the inherent chemical-physical reactivity associated with the graphene sheets themselves, which are hydrophobic, have low polarity, and have a rich n-electron density [13-15]. This can lead to a huge diversity of methods for immobilization of molecular species. 

Polymer Supports for the Immobilization of Metal Complexes 12.14.2.1 Introduction... 

Polymer-supported triphenylphosphine (Ps-PPh2) has attracted much attention as a ligand for immobilization of metal complexes, as it is the supported analog of the ubiquitous simple tertiary phosphine ligand PPhs- Also, it is commercially available from a range of suppliers. 

Immobilization of metal complexes within, or on, a silicate material can be accomplished using one of four basic methods, which differ with respect to the order in which the various components are formed or introduced (Fig. 18) (15) ... 

Immobilization of metal complexes usually increases their efficiency because their concentration on the polymer support is not limited by solubility (as a rule the set of suitable solvents is restricted by the solubility of MX ). 

A.D. Pomogailo, Immobilization of Metal Complexes on Macromolecular Supports. Catalytic Properties of Immobilized Systems in Polymerization Processes, Doctoral Dissertation, Institute of Chemical Physics, Moscow, 1981. 

In contrast to these post-synthetic modifications, it is also possible to functionalize the pore walls directly during the synthesis, as was first shown by Mann and co-workers [7,8] and Stucky and coworkers [9], who used trialkoxysilanes R-Si(OR )3. In our approach, such R Si(OR )3 molecules substitute for part of the TEOS. After hydrolysis, they serve as additional framework components during the hydrothermally induced condensation. An essential condition for this approach is that the trialkoxysilane does not destroy the micellar arrangement of the surfactant, which gives rise to the mesostructure. In mesostructures produced in this way, the R residues should be covalently linked to the silica walls. After the synthesis, the organic surfactant molecules can be removed by extraction so that a modified mesoporous material should remain. For example, when using phenyltrimethoxysilane (PTMOS), phenyl groups may become attached to the walls of the mesopores these can be utilized for further modifications, e.g. the immobilization of metal complexes. 

---