Catalyst dendrimer-encapsulated

Some conclusions can be drawn from comparison of the TOF values obtained using biphasic catalysis with literature data for the same reactions catalyzed by a polymer-supported Pd(0) catalyst (Table 2) [161]. First, although the TOFs for the dendrimer-encapsulated catalysts are consistently lower than for the polymer-supported catalysts, the values are in some cases comparable. Second, the selectivity pattern exhibited by the two types of catalysts is somewhat different. Specifically, the range of TOF numbers for biphasic catalysis is far greater than for conventional polymer catalysis, which suggests the possibility of the type of... 

To demonstrate further the powerful utiHty of the fluorous/organic biphasic approach to catalyst recycling, a dendrimer-encapsulated catalyst (DEC) with covalently attached perfluorinated polyether chains was synthesized [17] and metallic nanoparticles were introduced into the interior. 

Allylic alcohols were hydrogenated using f to 3 nm diameter bimetallic Pd-Au dendrimer-encapsulated catalysts (DECs) [fi8]. Both alloy and core/shell Pd-Au nanoparticles were prepared. The catalytic hydrogenation of allyl alcohol was significantly enhanced in the presence of the alloy and core/shell Pd-Au nanoparticles as compared to mixtures of single-metal nanoparticles [fi8]. 

Scott RW, Datye AK, Crooks RM. Bimetallic palladium-platinum dendrimer encapsulated catalysts. J Am Chem Soc 2003 125 3708-3709. 

Dendrimer-encapstdated catalysts are another area of active research for polymer-supported catalysts. The nanoparticles are stabilized by the dendrimers preventing precipitation and a omeration. Bimetallic nanoparticles with encapsulated metals (dendrimer-encapsulated catalyst DEC) from commercially available fourth-generation PAMAM dendrimers and palladium and platinum metal salts were prepared via reduction by Crooks and co-workers [34], following previous work in this area [35], The simultaneous incorporation of Pt and Pd reflects the concentrations in solution. The bimetallic DECs are more active than the physical mixture of single-metal DEC [35, 36] in the case of the hydrogenahon of allyl alcohol in water, with a maximum TOP of 230 h compared to TOP = 190 h obtained for monometallic palladium nanoparticles (platinum TOP = 50 h ). 

R.W.J. Scott, A.K. Datye, R.M. Crooks, Bimetallic Palladium-Platinum Dendrimer-Encapsulated Catalysts, Journal of the American Chemical Society 125, 3708, 2003. 

Crooks group prepared monodisperse (1.7 0.2nm) palladium nanoparh-cles within the interiors of three different generations of hydroxyl-terminated PAMAM dendrimers [41]. This process involved encapsulation of the nonselec-tive catalyst (the Pd nanoparticle) within a selective nanoporous cage (the dendr-imer). These dendrimer-encapsulated palladium nanoparticles were used as catalysts to hydrogenate allyl alcohol and four R-substituted derivatives in a metha-nol/water mixture. The results showed that higher-generation dendrimer encapsulated catalysts (DECs) or larger substrates resulted in lower turnover frequencies. 

Activation of Dendrimer Encapsulated Pt Nanoparticles for Heterogeneous Catalysts... 

Dendrimer encapsulated Pt nanoparticles (DENs) were prepared via literature methods (1, 11). PtCl42 and dendrimer solutions (20 1 Pt2+ dendrimer molar ratio) were mixed and stirred under N2 at room temperature for 3 days. After reduction with 30 equivalents of BH4 overnight, dialysis of the resulted light brown solution (2 days) yielded Pt2o nanoparticle stock solution. The stock solution was filtered through a fine frit and Pt concentration was determined with Atomic Absorption Spectroscopy (11). Details on catalyst characterization and activity measurements have been published previously (11). 

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. 

We are developing a new method for preparing heterogeneous catalysts utilizing polyamidoamine (PAMAM) dendrimers to template metal nanoparticles. (1) In this study, generation 4 PAMAM dendrimers were used to template Pt or Au Dendrimer Encapsulated Nanoparticles (DENs) in solution. For Au nanoparticles prepared by this route, particle sizes and distributions are particularly small and narrow, with average sizes of 1.3 + 0.3 nm.(2) For Pt DENs, particle sizes were around 2 nm.(3) The DENs were deposited onto silica and Degussa P-25 titania, and conditions for dendrimer removal were examined. 

Fig. 2 Schematic representations of metallodendritic architectures according to the metal (catalyst) location A at the periphery of a dendrimer or of a dendron B at the core of a dendrimer or at the focal point of a dendron C at branching points of a dendrimer or of a dendron D dendrimer-encapsulated metal nanoparticles (DEMNs)...

Chandler BD, Gilbertson JD (2006) Dendrimer-Encapsulated Bimetallic Nanoparticles Synthesis, Characterization, and Applications to Homogeneous and Heterogeneous Catalysis. 20 97-120 ChataniN (2004) Selective Carbonylations with Ruthenium Catalysts. 11 173-195 Chatani N, see Kakiuchi F (2004) 11 45-79... 

This chapter will only deal with catalytic systems covalently attached to the support. Dendrimer [96-101], hyperbranched polymer [102, 103], or other polymer [100] encapsulated catalysts, micellar catalysis [104] and non-cova-lently bound catalysts (via ionic [105,106], fluorous, etc. intercations) are not being treated. Also catalysis with colloidal polymers [ 107,108] and biocatalysts, such as enzymes and RNA, will not be reviewed here. 

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