Palladium, encapsulated, 19, supported

Hara T, Kaneta T, Mori K, Mitsudome T, MizugaM T, Ebitani K, Kaneda K (2007) Magnetically recoverable heterogeneous catalyst palladium nanocluster supported on hydroxyapatite-encapsulated c-Fe203... 

Entrapment or intercalation of metal species in pores and cavities of solid supports has frequently been used for the immobilization of catalysts in inorganic materials such as zeolites, clays, charcoals, silicas, aluminas, and other solids. Though this review article focuses on the immobilization of palladium complexes on polymer supports via covalent and/or coordination bonds, recent novel approaches to polymer-supported palladium species (including palladium nanoparticles) via nonbonding immobilization, such as encapsulation and incarceration, are intriguing because of their high potential for utility. In this section, several representatives are introduced. 

Polymer-stabilized palladium nanoparticles (or nanoclusters) [125-127] have recently received increasing attention in the field of synthetic organic chemistry [128, 129]. Thus, for example, the poly(iV-vinyl-2-pyrrolidone) (PVP)-supported Pd particle catalyzed the Suzuki-Miyaura coupling in water [130]. Poly(amidoamine) (PAMAM) dendrimer-encapsulated palladium nanoparticles were designed and prepared to provide highly selective catalysts for hydrogenation of olefins [131-133]. Hyperbranched aromatic amides (aramids) and PS-DVB-methacryloylethylenesulfonic acid resin have also been... 

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 ). 

Supported catalysts involving palladium on carbon and dendrimer-encapsulated palladium and a polymer-supported phosphine palladium catalyst have facilitated C-C coupling reactions in SCCO2. Polymer-tethered substrates or amine bases have also been successfully used for the Mizoroki-Heck and Suzuki-Miyaura reactions in SCCO2. For example, REM resin underwent a Mizoroki-Heck reaction with iodobenzene to yield, after cleavage, ( )-methyl cinnamate 48 (74%) (Scheme 88). It is assumed that SCCO2 acts as a good solvent that swells the polymers and exposes reactive sites. 

Chemical interactions between metal and support are also observed on main group metal oxides such as Si02, AI2O3, and MgO, which can normally be regarded as chemically highly inert. Strong interactions have also been found between various metals of Groups 8-10 and carbon supports. Palladium and nickel form carbide phases, and the transformation of carbon and the encapsulation of metal crystallites have been proven [18],... 

One example is the pellistor-type sensor first described by Baker (2) which is shown in Figure 12.2. This sensor uses palladium supported on thoria as the catalyst. This is deposited on the surface of a refractory bead of c. 1 mm diameter encapsulating the platinum coil. Palladium is more active than platinum for hydrocarbon oxidation and readily oxidizes methane at temperatures of about 500 °C. Encapsulation of the coil within a spherical bead in this way produces a device which is insensitive to orientation and also resistant to shock. This type of sensor is widely used in all types of flammable gas detection instruments. 

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