Supercritical fluids , catalysis hydrogenation

Minder, B., Mallat, T., Pickel, K.H., Steiner, K., Baiker, A. Enantioselective Hydrogenation of Pyruvate in Supercritical Fluids. Catalysis Letters. 1995, 34, 1-9. 

P. G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, Homogeneous Catalysis in Supercritical Fluids Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides ,J. Am Chem Soc 1996,118, 344-355. 

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. 

Jessop, P.G. Hsiao, Y. Ikariya, T. Noyori, R. Homogeneous catalysis in supercritical fluids hydrogenation of supercritical carbon dioxide to formic acid, alkyl formates, and formamides. J. Am. Chem. Soc. 1996, 118 (2), 344-55. 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

In a recent communication, Ohde et al. showed the synthesis of palladium nanoparticles by hydrogen reduction of Pd ions dissolved in the water core of a CO2 microemulsion 18, 21). The Pd nanoparticles so produced are uniformly dispersed in the supercritical fluid phase and are stable over an extended period of time long enough for catalysis experiments. Reduction of a... 

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. 

Grunwddt, J.D., Wandeler, R., Baiker, A. (2003) Supercritical fluids in catalysis opportunities of in situ spectroscopic studies and monitoring phase behavior, Catal. Rev. Sci. Eng. 45, Nl, 1-96 Minder, R, Mallat, T., Pickel, K.H., Steiner, K., Baiker, A. (1995) Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids, Catal. Lett. 34, 1-9. 

T Tacke, C Rehren, S Wieland, P Panster, SK Ross, J Toler, MG Hitzler, F Small, M Poliakoff. Continuous hydrogenation in supercritical fluids. In FE Herkes, ed. Catalysis of Organic Reactions. New York Marcel Dekker, 1998, pp 345-356. 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

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