Rhodium catalysts colloidal

The most common catalyst for low- and medium-pressure hydrogenation is platinum. Platinum oxide is available from a number of suppliers and is converted to colloidal platinum in situ by hydrogenation. Palladium is another commonly used catalyst and is usually prepared on some inert support such as charcoal, barium sulfate, or calcium carbonate. The procedure for the preparation of these catalysts is given in Organic Syntheses. - A rhodium catalyst appears to be particularly effective in reducing aromatic compounds at low pressure and is available on an alumina support. ... 

The catalytic hydrogenation of arenes with an arene ruthenium catalyst precursor has also been performed in ionic liquids and the products separated from the ionic solution by distillation under high vacuum, allowing the same batch of ionic liquid to be used repeatedly for the catalytic hydrogenation of several different arenes.Interestingly, colloidal rhodium catalysts can hydrogenate arenes in aqueous/sc fluid biphasic media whereas the reactions do not work in [BMIM][BF4]. ... 

The addition of transition metal salts, such as Fe(N03)3 -9H20, Cu(N03)2 -2.5H20, and Ni(N03)2 6H20, to colloidal rhodium catalyst was reported by Hanaoka et al. (1999) to be very effective to enhance the selectivity of the system as shown in Figure 20.5. In particular, addition of Fe(N03)3-9H20, enhances the selectivity to 94% after 6 h of irradiation and the hydrogenation rate for both steps (COD to COE, and COE to COA), whereas the hydrogenation rate of COE to COA decreased at a lower amount of Fe salt added in the solution. 

Rafaeloff, R., Tricot, Y.-M., Nome, F., and Fendler, J.H., Colloidal catalyst coated semiconductors in surfactant vesicles In situ generation of rhodium-coated cadmium sulfide particles in diocta-decyldimethylammonium halide surfactant vesicles and their utilization for photosensitized charge separation and hydrogen generation, J. Phys. Chem., 89, 533,1985. 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

Hydrogenation of unsaturated carboxylic acids, such as acrylic, methacryUc, maleic, fumaric, cinnamic etc. acids was studied in aqueous solutions with a RhCU/TPPTS catalyst in the presence of p-CD and permethylated P-cyclodextrin [7]. In general, cyclodextrins caused an acceleration of these reactions. It is hard to make firm conclusions with regard the nature of this effect, since the catalyst itself is rather undefined (probably a phosphine-stabilized colloidal rhodium suspension, see 3.1.2) moreover the interaction of the substrates with the cyclodextrins was not studied separately. 

Biologically important substrates can be hydrogenated in aqueous solution by using sulfonated triarylphosphine ligands that confer water solubility upon the catalyst. However, it was later reported that rhodium metal was formed from these complexes in the presence of hydrogen. It was claimed that the role of the ligands is merely to prevent aggregation of the rhodium colloid produced. 

In some other cases, the noble metals were deposited onto ceria from colloidal suspensions. Tlius, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(i%)/Ce02 catalyst (182). Likewise, a series of Pd/Ce02 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofiiran (78). 

The newer type of colloidal catalysts have been prepared containing palladium (4), platinum (4), rhodium (5), and iridium (6). A variety of synthetic polymers has been applied. Among those tested were polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), polymethyl methacrylate (PMMA), and polymethyl acrylate (PAMA). In general, polyvinyl alcohol (4a) has been found most satisfactory. 

---