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Platinum Metals Rev., 2002, 46, (4), 176

Polymer-Supported Rhodium Catalysts Soluble in SC-CO2


In organic syntheses utilising homogeneous catalysts, the catalysts are dissolved in a suitable solvent which also acts as the reaction medium. These solvents are often toxic organic liquids, so there is a growing need to replace them with envi-ronmentally benign solvents, such as water or supercritical carbon dioxide (sc-CO2). At present, the majority of organic syntheses are solvent-sensitive and most homogeneous catalysts are not soluble in either water or Sc-CO2.

Separating and recovering the homogeneous catalysts at the end of the reaction is also a major problem. To overcome this, increasing attention is being directed at developing polymer-supported ligands for complexing with homogeneous metal catalysts for straightforward membrane separation.

Researchers at Texas A & M University, U.S.A., have now succeeded in developing a homogeneous catalyst that is soluble in Sc-CO2 solvent (Z. K. Lopez-Castillo, R. Flores, I. Kani, J. P. Fackler and A. Akgerman, lnd. Eng. Chem. Res., 2002, 41, (13), 3075-3080). They did this by attaching a homogeneous rhodium (Rh) catalyst to the backbone of a fluoroacrylate copolymer.

The polymer was prepared by polymerisation of the monomer 1H, 1H, 2H, 2H-heptadecafluorode-cyl acrylate (zonyl TAN) and N-acrylosuccirdmide (NASI); zonyl TAN increases the solubility in sc-CO2 While NASI provides attachment sites for the catalyst. NH2(CH2)3PPh2 (DPPA) was then used to exchange the NASI groups in the polymer. Finally, this was reacted with [RhCl(COD)]2 to obtain the Sc-CO2 soluble, polymer-supported Rh catalyst. As the polymer is a very large molecule it was easily separated by a membrane.

Catalyst hydrogenation activity was evaluated using l-octene and cyclohexene at different molar ratios of substrate:Rh and different temperatures. Most reactions were performed at 173.4 bar pressure for 12 hours. Conversion of l-octene to n-octane was nearly 100%. Conversion of cyclohexene increased with temperature: at 368 and 393 K, the maximum conversions were 39 and 51%, respectively. For this hydrogenation, the catalyst was Rh(TAN15DPPA)Cl with a Rh dimer:polymer ratio of 1:3 and a Rh content of 1.95 mg of Rh/g of catalyst. The synthesis route for this Rh-polymer catalyst is reproducible.