Nanoparticle catalysts, polymer-protected

The use of polymer-protected nanoparticles can contribute new features to catalysis. A platimun sol as well as Pt on AI2O3 are well-known enantioselective hydrogenation catalysts. The possible uses of hybrid nanocomposites in catalysis are far from all realized. ... 

Catalytic Hydrogenations. Table IV shows some results for the catalytic hydrogenation of cyclohexene, and a selection of results obtained for the hydrogenation of cis-cyclooctene and 1-hexene is given in Table V. The results show that a variety of polymer-protected platinum nanoparticles are catalytically active, and conversions of 100 % were obtained in most cases. The catalysts could be employed either directly as a colloidal dispersion or as a solid after the evaporation of the solvent and the subsequent redissolving in the mixture for liquid-phase hydrogoiation. Unlike most catalysts systems, they could be stored in air for several weeks/months and still showed very good catalytic activity. 

It was recently reported that the colloidal Pt nanoparticles may be protected by glycol, which serves as both a solvent and the protecting agent [71, 102]. For example, Kongkanand and co-workers [102] used ethylene glycol as a reducing agent to prepare well-dispersed Pt catalysts supported by polymer-wrapped CNTs. In their procedure, the CNTs were sonicated in A,7V-dimethyIformamide for 15 h to break up the nanotube bundles into individual CNTs. However, polymer such as... 

Prior to inclusion of PVP-protected Pt nanoparticles the SBA-15 silica is calcined at 823K for 12h to remove residual templating polymer. Removal of PVP is required for catalyst activation. Due to the decomposition profile of PVP (Figure 6), temperatures > 623 K were chosen for ex situ calcination of Pt/SBA-15 catalysts. Ex-situ refers to calcination of 300-500 mg of catalyst in a tube furnace in pure oxygen for 12-24 h at temperatures ranging from 623 to 723 K (particle size dependent) [13]. Catalysts were activated in He for 1 h and reduced at 673 K in H2 for 1 h. After removal, the particle size was determined by chemisorption. Table 2 is a summary of chemisorption data for Cl catalysts as well as nanoparticle encapsulation (NE) catalysts (see description of these samples in proceeding section). 

In particular, highly stable Pd nanoparticles, protected by an imidazolium based ionic polymer in a functionalized IL, can be easily prepared. These Pd nanoparticles are excellent pre-catalysts for Suzuki, Heck and Stille coupling reactions and can be stored without undergoing degradation for at least two years. 

Ti-HMS modified Pd-C was found to accelerate the hydrogenolysis of simple benzyl ethers in the presence of acid-sensitive functional groups. The use of benzyl protection for polymer-supported syntheses has been a problem because of trapping of the catalyst by the polymer. This problem is partially solved by the use of Pd nanoparticles which result in efficient benzyl group hydrogenolysis from polymer supports. ... 

Coram et al. [6] have described the polymer support as a soluble macromolecule or a micellar aggregate that wraps the metal nanoparticle in solution, thus preventing metal sintering and precipitation. It can also be a resin, that is an insoluble material consisting in a bundle of physically and/or chemically cross-linked polymer chains in which the metal nanoparticles are embedded (Figure 11.2). Thus, soluble cross-linked polymers ( microgels ) that can stabilize metal nanoparticles can be prepared in addition, metal colloids protected by soluble linear polymers have been grafted onto insoluble resin supports to yield insoluble catalysts. This chapter is devoted mainly to metal nanoparticles on insoluble resin supports [8]. 

In contrast, colloid chemistry has provided the colloidal dispersion of metal fine particles in water.As early as the 1950 s, colloidal dispersion of fine particles of precious metals was conducted and applied to catalyses. Although they contain fine metal particles, the size of the particles was not so uniform. In addition they were not very stable when used in solution. Thus, there still remained many problems in the reproducibility of the preparation and catalysis using metal nanoparticles. In 1976, the author s group prepared colloidal dispersions of rhodium particles protected by water-soluble polymers by reduction of rhodium(III) ions under mild conditions, i.e., reduction with refluxing alcohol in the presence of water-soluble polymers. They were applied to the catalyst for hydrogenation of olefins. In 1989, we developed colloidal dispersions of Pd/Pt bimetallic nanoclusters by the simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP). 

Mayer, A. B. R., Mark, J. E. Transition metal nanoparticles protected by amphiphilic block copolymers as tailored catalyst systems. Coll Polym Sci 1997, 275, 333-340. 

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