Dendrimer bimetallic

As shown by Scheme 1 in Fig. 4.2, the first method for the synthesis of bimetallic DENs is called co-complexation, in which two metal ions are mixed with dendrimers at the same time and are reduced simultaneously after both of them form complexes with the dendrimers. Bimetallic DENs, such as Pd/Pt and Pd/Rh, have been synthesized by this method and have shown enhanced activity in the catalytic hydrogenation of 1,3-cyclooctadiene and allyl alcohol [69-72]. 

There is currently considerable interest in processing polymeric composite materials filled with nanosized rigid particles. This class of material called "nanocomposites" describes two-phase materials where one of the phases has at least one dimension lower than 100 nm [13]. Because the building blocks of nanocomposites are of nanoscale, they have an enormous interface area. Due to this there are a lot of interfaces between two intermixed phases compared to usual microcomposites. In addition to this, the mean distance between the particles is also smaller due to their small size which favors filler-filler interactions [14]. Nanomaterials not only include metallic, bimetallic and metal oxide but also polymeric nanoparticles as well as advanced materials like carbon nanotubes and dendrimers. However considering environmetal hazards, research has been focused on various means which form the basis of green nanotechnology. 

The hexanuclear compound 21 is a particular case of dendrimer in that a [Rh(ju-Cl)2Rh]4+ moiety can be identified as the core. The photophysical properties of this compound [40] indicate that the bimetallic core plays the role of insulator with respect to the peripheral luminescent Ru(II)-based units. 

The team of Crooks is involved in the synthesis and the use of dendrimers and, more particularly, poly(amidoamine) dendrimers (PAMAM), for the preparation of dendrimer-encapsulated mono- or bimetallic nanoparticles of various metals (Pt, Pd, Cu, Au, Ag, Ni, etc.) [55, 56]. The dendrimers were used as nanocatalysts for the hydrogenation of allyl alcohol and N-isopropylacrylamide or other alkenes under different reaction conditions (water, organic solvents, biphasic fluorous/or-ganic solvents or supercritical COz). The hydrogenation reaction rate is dependent on dendrimer generation, as higher-generation dendrimers are more sterically... 

Rhee and coworkers published the synthesis of bimetallic Pt-Pd nanoparticles [57] or Pd-Rh nanoparticles [58] within dendrimers as nanoreactors. These nanocatalysts showed a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene. The reaction was carried out in an ethanol/water mixture at 20 °C under dihydrogen at atmospheric pressure. The dendrimer-encapsulated nanoclusters could be reused, without significant loss of activity. 

The dendrimer-assisted method can be extended to bimetallic clusters. The second metal ion can be introduced by partial displacement of the first metal, or sequentially after the first, or together with the first by cocomplexation. Although these methods succeed in making bimetallic particles, it is not obvious that they can generate uniform composition particles. Thus, new methods or variations of the existing method have to be developed. 

In the typical nomenclature for DENs, the dendrimer is designated by Gx-R where x is the dendrimer generation and R is the surface group (typically -OH or -NH2, see Fig. 7.1). The stoichiometry between the dendrimer and complexed ions or reduced encapsulated nanoparticles is denoted in parentheses after the dendrimer description, e.g. (M )n or (M ). For bimetallic DENs, the metahmetal stoichiometry is typically included, e.g. G5-OH (PtigAuig). 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

Y. M. Chung and H. K. Rhee, Partial hydrogenation of 1,3-cyclooctadiene using dendrimer-encapsulated Pd-Rh bimetallic nanoparticles, J. Mol. Catal. A—Chem. 206, 291-298 (2003). 

Bimetallic metal particles are important materials because their characteristics, especially their catalytic properties, are often quite different from those of pure metal particles. Dendrimer-encapsulated bimetallic clusters can be synthesized by any of three methods (Fig. 17) (1) partial displacement of the dendrimer-encapsidated cluster, (2) simultaneous co-complexation of two different metal ions followed by reduction, or (3) sequential loading and reduction of two different metal ions. 

Preparation of mixed-metal intradendrimer clusters by partial displacement is a straightforward extension of the complete displacement approach for forming single-metal clusters described in the previous section. If less than a stoichiometric amount of Ag+, Au +, Pd +, or Pt + is added to a Gh-OHlCujj) solution, or if less than a stoichiometric amount of Au +,Pd +, or Pt + is added to G6-OH(Agiio) solution, it is possible to form Ag/Cu,Au/Cu (Au/Ag),Pd/Cu (Pd/Ag), and Pt/Cu (Pt/Ag) bimetallic clusters inside dendrimers. 

Dendrimer-encapsulated bimetallic clusters can also be prepared by simultaneous co-complexation of two different metal ions, followed by a single reduction step. For example, the absorption spectrum of a solution containing G6-OH, PtCl , and PdCl is essentially the sum of the spectra of a solution containing G6-OH + PtCl and a second solution containing G6-OH -i- PdCl, which strongly suggests co-complexation of Pt + and Pd + within individual dendrimers. After reduction of these co-complexed materials, a new interband transition, which has an intensity different from that of either a pure Pt or Pd cluster, is observed. 

Fig. 17. Schematic illustration of the preparation of dendrimer-encapsulated bimetallic clusters by three different methods. Displacement reaction, co-complexation, and sequential loading...

Presumably these three methods for preparing bimetallic, dendrimer-encap-sulated nanoparticles can be extended to trimetallics,bi- and trimetallics having unique structures (such as core/shell materials), and interesting combinations of two (or more) zero-valent metals plus intradendrimer ions. However, analysis of such materials await more sophisticated analytical methods than are currently at OUT disposal. 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

In core- (and focal point-) functionalized dendrimers, the catalyst may benefit from the site isolation created by the environment of the dendritic structure. Site-isolation effects in dendrimers can also be beneficial for other functionalities (a review of this topic has appeared in Reference (10)). When reactions are deactivated by excess ligand and when a bimetallic deactivation mechanism is operative, core-functionalized dendrimers can minimize the deactivation. 

Chandler BD, Gilbertson JD (2006) Dendrimer-Encapsulated Bimetallic Nanoparticles Synthesis, Characterization, and Applications to Homogeneous and Heterogeneous Catalysis. 20 97-120... 

Abstract We review the preparation, characterization, and properties of dendrimer-templated bimetallic nanoparticles. Polyamidoamine (PAMAM) dendrimers can be used to template and stabilize a wide variety of mono- and bimetallic nanoparticles. Depending on the specific requirements of the metal system, a variety of synthetic methodologies are available for preparing nanoparticles with diameters on the order of 1-3 nm with narrow particle size distributions. The resulting dendrimer-encapsulated nanoparticles, or DENs, have been physically characterized with electron microscopy techniques, as well as UV-visible and X-ray photoelectron spectroscopies. 

For certain metal systems, the chemical properties of bimetallic DENs include selective extraction from the dendrimer interior into organic solvents. Catalytic properties include homogeneous hydrogenation catalysis heterogeneous hydrogenation and oxidation catalysis have also been examined. Homogeneous hydrogenation studies indicate that... 

Since the first report of dendrimer-encapsulated Cu nanoparticles [15], several types of mono and bi-metallic DENs have been prepared. DEN synthesis has been recently reviewed [9,16], so only the synthesis of bimetallic DENs is described here. Bimetallic DENs can be prepared by one of three methods co-complexation of metal salts, galvanic displacement, and sequential reduction. Several bimetallic systems have already been prepared inside PAMAM dendrimers Table 1 summarizes the current literature and synthetic methods employed. 

Co-complexation has been used to prepare a variety of bimetallic DENs, including PdPt [19,20], PdAu [21], and PdRh [22] (Table 1). In a typical study, for example the PdPt system studied by Scott, Datye, and Crooks [ 19], K2PdCl4 and K2PtCl4 are added to a dilute aqueous solution of G4-OH in a fixed metal-ion to dendrimer ratio of 40 1. The solution is stirred for four days to allow complete complexation of the metal ions with the interior ter-... 

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