Dendrimer oxide support

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. 

Dendrimers can be used as templates for synthesis of monodisperse gold particles, which can then be deposited onto oxide supports metal particles are encapsulated within dendrimers, and the size of the particles can be tuned by varying the metal-to-dendrimer ratio prior to reduction. 

Mixed-metal dendrimers have also been prepared. In compound 5, ferrocenyl units are present at the periphery, while platinum units play the role of connectors along the branches [56]. Electrochemical oxidation in acetonitrile involves only the ferrocenyl units and confirms the presence of multiple noninteracting sites. By combining different electrochemical techniques it was possible to measure the number of ferrocene units contained in the dendrimer, thus supporting the proposed structure. 

Dendritic catalysis have been used in various chemical reactions, including the Suzuki-Miyaura reaction, Mizoroki-Heck reaction, hydrogenation reaction, carbonylation and hydroformylation reactions, oxidation reaction, polymerization and oligomerization reactions, arylation reaction, alkylation reaction, and asymmetric synthesis [6]. Recently, dendritic catalysts have been reviewed by Astmc et al. [6], In another review article. Reek et al. reviewed the applications of dendrimers as support for recoverable catalysts and reagents [58]. The authors believed that catalytic performance in these systems depends on used dendritic architecture. 

Catalysts with high activity and selectivity could dramatically reduce the energy required for many energy intensive catalytic processes. Many factors could affect the catalytic activity and selectivity of metal nanoparticles, such as particle size, shape, capping agent, and oxide support. With the help of dendrimers, monodis-perse nanoparticles <1 nm in diameter can be synthesized repeatedly, which extends the study of size-induced activity and selectivity changes of metal nanoparticles to the sub-nm regime. 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

Mixed-metal dendrimers containing up to 6 Pt(IV)-based organometallic species in the branches and 12 peripheral ferrocene units (8) have recently been synthesized and their electrochemical behavior investigated [13]. As in the previously discussed examples, multi-electron reversible oxidation processes, assigned to the equivalent, non-interacting ferrocene units, have been observed. The authors point out that cyclic voltammetry is a powerful tool to support the structure of the dendrimers containing ferrocene units. 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

B. Auten, H. Lang, and B. D. Chandler, Support effects on dendrimer templated Pt—Au catalysts II CO oxidation catalysis, manuscript in preparation (2004). 

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). 

Physical or electrochemical adsorption uses non-covalent forces to affix the nucleic acid to the solid support and represents a relatively simple mechanism for attachment that is easy to automate. Adsorption was favoured and described in some chapters as suitable immobilization technique when multisite attachment of DNA is needed to exploit the intrinsic DNA oxidation signal in hybridization reactions. Dendrimers such as polyamidoamine with a high density of terminal amino groups have been reported to increase the surface coverage of physically adsorbed DNA to the surface. Furthermore, electrochemical adsorption is described as a useful immobihzation strategy for electrochemical genosensor fabrication. 

Similar 0—4 generations silica-supported Pd-PAMAM dendrimers with various spacer lengths were used by Alper et al. as recyclable catalysts for the hydroesterification reaction of alkenes (55) and the oxidation of terminal alkenes to methyl ketones (56). The hydroesterification experiments (Scheme 16) showed that (PPh3)2Pd-PPh2-PAMAM-Si02 complexes were highly active catalysts for styrene derivatives and linear long-chain alkenes (numbers of turnovers up to 1200). 

In a subsequent paper, the authors developed another type of silica-supported dendritic chiral catalyst that was anticipated to suppress the background racemic reaction caused by the surface silanol groups, and to diminish the multiple interactions between chiral groups at the periphery of the dendrimer 91). The silica-supported chiral dendrimers were synthesized in four steps (1) grafting of an epoxide linker on a silica support, (2) immobilization of the nth generation PAMAM dendrimer, (3) introduction of a long alkyl spacer, and (4) introduction of chiral auxiliaries at the periphery of the dendrimer with (IR, 2R)-( + )-l-phenyl-propene oxide. Two families of dendritic chiral catalysts with different spacer lengths were prepared (nG-104 and nG-105). 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. 

The persistence of the dendrimer decomposition products is the likely cause of the catalyst deactivation over time. The presence of dendrimer and dendrimer byproducts indicates that even the more active catalysts are not particularly clean. It is difficult to distinguish between species adsorbed on the NPs from those primarily on the support however, it is likely that the location of the dendrimer decomposition varies widely along the surface of the catalyst. The dendrimer fragments present on the support could migrate over time and poison the metal active sites, resulting in the lower catalytic activity over time. It is also possible that the residual dendrimer undergoes some slower oxidation processes that result in a stronger, unobservable poison. 

In the dendritic [Co(salen)] complexes prepared by Breinbauer and Jacobsen the dendrimer again serves as - covalent - support material for the catalytic entities attached to the periphery [62]. These dendritic Jacobsen catalysts were obtained by reaction of the corresponding PAMAM dendrimers with active ester derivates of chiral ]Co(II)-(salen)] units according to standard peptide coupling methods. In hydrolytic kinetic resolution of vinylcyclohexane oxide the dendrimer 14 (Fig. 6.40) showed a dramatically increased reactivity compared to the commercially available monomeric Jacobsen catalyst [63-67]. Whereas the latter merely gave a conversion of less than 1% with an indeterminable ee, 14 afforded a conversion of 50% with an ee of 98 2. 

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