Nanoparticulate Transition Metal Catalysts

The scientific interest in catalysis by transition metal nanoparticles has seen a dramatic increase in recent years and significant progress has been made in improving selectivity, efficiency and recyclability of the catalytic systems [267]. Usually nanoparticulate catalysts are prepared from a metal salt, a reducing agent and a stabilizer and are supported on oxides, charcoal or zeolites. 

Ionic liquids are quite unique media for the synthesis of nanoparticles. This perception has rapidly developed over the last three years and Section 6.3 of this book is devoted to the synthetic aspects of particle preparation in ionic liquids. Thus ionic liquids represent both innovative liquid support materials and stabilizers for catalytic reactions using transition metal nanoparticles. 

In the context of this chapter we aim to illustrate the state-of-the-art in this rapidly progressing field of ionic liquid catalysis, exemplified for selective hydrogenation and Heck reactions. Other applications of nanoparticulate catalyst systems have been reported in hydrosilylation reactions [268], Suzuki [269] and Stille coupling [270]. 

Hydrogenation reactions catalyzed by nanoparticulate transition metal catalysts 

Catalytic hydrogenation reactions have to date been explored using nanopartides of palladium, platinum, ruthenium, iridium and rhodium. 

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Transition metal carbides and nitrides find broad interest in chemistry and technology. In the form of nanopowders they can be used in electronics and for catalysis. As catalysts they have similar properties to those of the expensive noble metals. They are extremely hard and therefore are often used in cutting tools or in harsh abrasive conditions. However, nanoparticulate nitrides and carbides are more reactive due to heir higher specific surface area. Some of them oxidize rapidly in air and are typically passivated with a thin layer of oxide before exposure to ambient conditions. 

Hydroxyapatite (CajQ(P04)g(0H)2) has also attracted considerable interest as a catalyst support. In these materials, wherein Ca sites are surrounded by P04 tetrahedra, the introduction of transition metal cations such as Pd into the apatite framework can generate stable monomeric phosphate complexes that are efficient for aerobic selox catalysis [99]. Carbon-derived supports have also been utihzed for this chemistry, and are particularly interesting because of the ease of precious metal recovery from spent catalysts simply by combustion of the support. Carbon nanotubes (CNTs) have received considerable attention in this latter regard because of their superior gas adsorption capacity. Palladium nanoparticles anchored on multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) show better selectivity and activity for aerobic selox of benzyl and cinnamyl alcohols [100, 101] compared to activated carbon. Interestingly, Pd supported on MWCNTs showed higher selectivity toward benzaldehyde, whereas activated carbon was found to be a better support in cinnamyl alcohol oxidation. Functionalized polyethylene glycol (PEG) has also been employed successfully as a water-soluble, low-cost, recoverable, non-toxic, and non-volatile support with which to anchor nanoparticulate Pd for selox catalysis of benzyl/cinnamyl alcohols and 2-octanol [102-104]. 

Hydrogen oxidation catalysis happens to be more difficult to obtain than hydrogen production, if noble metals are excluded. In particular, several nanoparticulate catalysts such as transition metal oxides/sulfides-based nanoparticles catalyze H2 evolution [29-34], while only tungsten carbide has been demonstrated to be active for H2 oxidation [35]. Even in the case of organometallic catalysts, only few complexes have proved to be able to catalyze H2 oxidation rather than evolution (see below). 

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