Olefin hydrogenation, supercritical

Catalytic Hydrogenation of Olefins in Supercritical Carbon Dioxide Using Rhodium Catalysts Supported on Fluoroacrylate Copolymers... 

The hydrogenation of olefins in supercritical carbon dioxide catalyzed by palladium nanoparticles synthesized in a water-in-C02 microemulsion was reported by Ohde et al (27). The Pd nanoparticles were prepared by hydrogen reduction of Pd ions (a PdCl2 solution) dissolved in the water core of the microemulsion. Effective hydrogenation of both C02-soluble olefins (4-methoxycinnamic acid and trans-stilbene) and a water-soluble olefin (maleic acid) catalyzed by the palladium nanoparticles in the microemulsion was demonstrated. 

P. G. Jessop, T. Ikariya, R. Noyori, Selectivity for Hydrogenation or Hydroformylation of Olefins by Hydrido-pentacarbonylmanganese(I) in Supercritical Carbon Dioxide , Organometallics 1995,14,1510-1513. 

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

Bianchini, C. Glendenning, L. Catalytic production of dimethylformamide from supercritical carbon dioxide. Methyl formate synthesis by hydrogenation of supercritical carbon dioxide in the presence of methanol. Selectivity for hydrogenation or hydroformylation of olefins by hydridopentacarbonylmanganese(I) in supercritical carbon dioxide. Chemtracts Org. Chem. 1996, 9 (6), 318-321. 

Water-in-C02 microemulsions with diameters in the order of several nanometers are prepared by a mixture of AOT and a Pn E-P04 co-surfactant. The CO2 microemulsions allow metal species to be dispersed in the nonpolar supercritical CO2 phase. By chemical reduction, metal ions dissolved in the water core of the microemulsion can be reduced to the elemental state forming nanoparticles with narrow size distribution. The palladium and rhodium nanoparticles produced by hydrogen reduction of Pd and Rh ions dissolved in the water core are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. 

The Pd nanoparticles synthesized in the CO2 microemulsion are effective for hydrogenation of C02-soluble and water-soluble olefins but are not effective for hydrogenation of aromatic compounds. Hydrogenation of arenes is conventionally carried out with heterogeneous catalysts. Bonilla et al. recently reported a Rh catalyzed hydrogenation of arenes in a water/supercritical ethane biphasic system (35). Hydrogenation occurred well in this biphasic system with excellent results obtained for a number of arenes after 62 hours of reaction... 

Our initial studies have demonstrated that the Pd and Rh nanoparticles formed in the CO2 microemulsions are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. Dispersing metal nanoparticles in supercritical CO2 utilizing the microemulsion is a new approach for homogenization of heterogeneous catalysis. This approach may have important applications for chemical synthesis in supercritical fluids. 

The National Laboratory at Los Alamos has been actively engaged over the past several years in research in the applications of SC-CO2 as a synthetic solvent. The work of Tumas (61-66) and co-workers as detailed in a later chapter of this volume profiles the performance of reactions such as polymerization of epoxides, oxidation of olefins and asymmetric hydrogenations in supercritical systems. In each of these cases the reactions proceeded without compromise when compared to conventional solvent systems, and superior performance was reported in the asymmetric hydrogenation reactions. 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

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