Cluster biphasic catalysts

The hydrogenation of arenes is industrially important, but so far is dominated by the use of heterogeneous catalysts. Ionic liquids offer, in principle, the chance to use a liquid-liquid biphasic system where the homogeneous catalyst is immobilized and therefore recyclable. Dyson et al. applied ruthenium clusters as catalyst for... 

Ru3(CO)12(117)3] and [H4Ru4(CO)11(117)] as catalyst precursors in the hydrogenation of non-activated alkenes under biphasic conditions. Each cluster displays activity under moderate conditions, ca. 60 atm. H2 at 60 °C with catalytic turnovers up to ca. 500. The trinuclear clusters undergo transformations during reaction but can be used repeatedly without loss of activity.325... 

A series of tri- and tetra-nuclear Ru clusters previously reported by the groups of Siiss-Fink [60] and of Dyson [61] as catalysts for the hydrogenation of benzene and other simple aromatics in biphasic media have later been shown to consist predominantly of active metallic particles [9, 10, 62]. 

The dicationic cluster catalyst H4Ru4(r/ -C6H6) [BF4]2, originally developed for biphasic aqueous-organic arene hydrogenation reactions (see Section 8.2.1), has proven to be more effective in ionic liquids presumably due to increased... 

More recently, the ruthenium-catalyzed hydrogenation of sorbic acid to cis-hex-3-enoic acid. Scheme 16, was achieved in a biphasic bmim-PF6-methyl tert- miy ether (MTBE) system. The ruthenium cluster [H4Ru(q -C6H6)4] [Bp4]4, in [bmim][BF4], was shown to be an effective catalyst for the hydrogenation of arenes to the corresponding cycloalkanes at 90 °C and 60 bar. The cycloalkane product formed a separate phase, which was decanted and the IL phase, containing the catalyst, could be repeatedly recycled. 

Attempts have also been made to develop biphasic methodologies for the hydrogenation of aromatics. The hydrogenation of benzene derivatives was studied using various Ru complexes.468,469 A trinuclear cluster cationic species was isolated as the tetrafluoroborate salt and showed increased activity in hydrogenation.470 The air/ moisture-stable [bmim][BF4] ionic liquid and water with [Rh r -CgHg).,] [BF4] as the catalyst precursor is an effective system under usual conditions (90°C, 60 atm)471 A system composed of stabilized Rh(0) nanoparticles proved to be an efficient catalyst in the hydrogenation of alkylbenzenes 472... 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

The transitions of supported liquid-phase catalysts (SLPC) and supported aqueous-phase catalysts (SAPC) are dealt with in Section 3.1.1.3, while special aspects of clusters and colloids are discussed in Sections 3.1.1.4 and 3.1.1.5 and those of aqueous-phase, re-immobilized catalysts in Section 3.1.1.6. The combination of heterogeneous catalysis with aqueous (biphasic) techniques is also under investigation, e. g., [209]. 

The proposed mechanisms, however, have not yet been clearly established and more detailed studies under catalytic conditions [46-47] would be welcome. These ruthenium clusters have also been shown to be active for arene hydrogenation in a biphasic medium using ionic liquids as the solvent, where the catalysts is retained, allowing its easy recovery and recycling [48]. 

More recently, catalytic hydrogenations of alkenes by other catalysts in water have been explored. For example, water-soluble ruthenium complex RuCl2(TPPTS)3 has been used for the catalytic hydrogenation of unsaturated alkenes (and benzene). Hydrogenation of nonactivated alkenes catalyzed by water-soluble ruthenium carbonyl clusters was reported in a biphasic system. The tri-nuclear clusters undergo transformation during reaction but can be reused repeatedly without loss of activity. The organometallic aqua complex [Cp Ir (H20)3] " ... 

Hydridoarene clusters of Rh and Ru are moderately active catalysts of hydrogenation of simple olefins [20]. Conversely, benzene and monosubstituted benzenes can be efficiently hydrogenated in aqueous biphasic systems with hydridoareneruthe-nium cluster catalysts, such as [Ru3(p2-H)2(p2 OH) (pj-O) (ri -C5Hg)(ri "-C5Me5)2] [21]. 

Other biphasic reductions using molecular hydrogen or an H-donor molecule include the selective hydrogenation of dienes to monoenes by K3[HCo(CN)5] (4) and reduction of arenes by hydridoruthenium clusters, such as [H4Ru4( -C6H6)4][BF4]2 (106). These reactions represent potentially important chemical transformations however, owing to catalyst instability or low productivity they have not been developed to practical biphasic processes. 

Bimetallic phase-transfer-catalysis is a process whereby a reaction that occurs using two different metal complexes, does not proceed in the absence of either metal species, or proceeds only at reduced rate. An apparent system of this class has been reported, in which Co2(CO)g and [RhCl(l,5-hexadiene)]2 mutually increased their reactivity when used as catalysts for the conversion of nitrobenzene to aniline in a biphasic system (benzene, aqueous NaOH, dodecyltrimethylammonium chloride) in a carbon monoxide atmosphere [73]. However, another member of the same research group later showed [74] that the apparent bimetallic promotion was due to the fact that the alkylammonium salt used as a phase-transfer agent actually inhibited the activity of the active rhodium complex (apparently a cluster, which is active in the absence of both the alkylammonium salt and the cobalt compound) by rendering it insoluble. The added Co2(CO)g reacts with the alkylammonium salt to generate... 

Gas-soUd catalytic reaction is the most common chemical process in the industry. However, owing to the limitation of this book, only three chapters were devoted to this theme. Chapter 11 describes the preparation of gold clusters and its application on the solid-gas biphasic catalytic reaction. The clarification of catalytic mechanism and reactive sites is very important for designing more efficient catalysts. So the identification of binding and reactive sites in metal cluster catalysis through imaging technique, kinetic study, and other methods are introduced in Chapter 9. To reflect the importance of theoretical calculation on catalysis, the molecular kinetics of the Fischer-Tropsch reaction by computational chemistry is introduced in Chapter 16. 

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