Colloid-bonded metal

Colloid-bonded metals (Mota and Simaes-Goncalves 1996), e.g., Fe-oxides, ftilvic acid-metal complexes. 

Several investigators have attempted to identify the nature of the metal species that catalyze these coupling reactions in solution. Due to the harsh conditions of these reactions and the thermodynamic instability of colloidal particles, it is highly possible and probably likely that multiple catalytic mechanisms occur simultaneously. A few review articles on carbon-carbon bond formation reactions catalyzed with colloidal transition metal nanoparticles are available [7, 11]. 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The combination of CuS04-5H20 and sodium borohydride is useful for the reduction of oximes to primary and secondary amines (eq 35), of aryl or alkyl azides to primary amines, and of acyl azides to amides (eqs 36 and 37). In all these cases, in situ generated, finely divided colloidal copper metal with a large active metal surface area is expected to be responsible for reduction of the double bonds. For the reduction of azides to primary amines, the CuS04-5H20 and sodium borohydride combination appears to be milder than H2/Pd-C or H2/Raney Ni, as reactive groups such has NO2 survive the reaction conditions. 

Many metal ions react with water to produce hydrolysis products that are multiply charged inorganic polymers. These may react specifically with negative sites on the colloidal particles to form relatively strong chemical bonds, or they may be adsorbed at the interface. In either case, the charge on the particle is reduced. 

Graphite lubricants include the dry powder, admixtures with liquid lubricants or greases, volatile liquids compounded with film-forming substances to produce bonded dry films, synthetic resins and powder metal compositions containing graphite for bearings, and finely divided suspensions in liquids (colloidal graphite). 

Colloidal systems of particles are different from molecular clusters in having a small distribution in sizes. Also, colloidal systems may be stabilized by organic molecules, resulting in chemical bonds between these and the outer monolayer of metal atoms. The Mossbauer study of such systems includes rather diverse fields ... 

One way to overcome the problem of chirality existing only at the metal-matrix interface is to encase the metal particle inside the chiral matrix. In that case, all of the metal surface atoms should be close to a chiral center however, this approach has some problems too. For example, access to the metal surface may be inhibited by the encasing matrix. In spite of this, several attempts have produced moderately successful catalysts by creating metal—polymer catalysts. Pd has been deposited on poly-(5)-leucine (Scheme 3.4) and Pd and Pt colloids have been encased in a polysaccharide to produce catalysts that enanti-oselectively hydrogenated prochiral C=C and C=N bonds (Scheme 3.5).7... 

Partial hydrogenation of acetylenic compounds bearing a functional group such as a double bond has also been studied in relation to the preparation of important vitamins and fragrances. For example, selective hydrogenation of the triple bond of acetylenic alcohols and the double bond of olefin alcohols (linalol, isophytol) was performed with Pd colloids, as well as with bimetallic nanoparticles Pd/Au, Pd/Pt or Pd/Zn stabilized by a block copolymer (polystyrene-poly-4-vinylpyridine) (Scheme 9.8). The best activity (TOF 49.2 s 1) and selectivity (>99.5%) were obtained in toluene with Pd/Pt bimetallic catalyst due to the influence of the modifying metal [87, 88]. 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

D.T. Ray and R. Hogg, Bonding of Ceramics Using Polymers at Low Concentration Levels in Innovations in Materials Processing Using Aqueous, Colloid Surface Chemistry, F.M. Doyle, S. Raghavan, P. Somasundaran and G.W. Warren (eds.). The Minerals, Metals Materials Soeiety, Warrendale PA, 1989, pp. 165-180. 

A new method to synthesis nanoparticles of group VIII-X elements has been developed by Choukroun et al. [178] by reduction of metalUc precursors with CP2V. Mono- and bimetallic colloids of different metals (stabilized by polymers) have been prepared in this way (Fe, Pd, Rh, Rh/Pd). These colloids are then used as catalysts in various reactions such as hydrogenation of CC, CO, NO or CN multiple bonds, hydroformylation, carbonylation, etc. 

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