Membrane reactor for homogeneous catalysis in supercritical carbon dioxide

A membrane reactor is presented for homogeneous catalysis in supercritical carbon dioxide with in situ catalyst separation. This concept offers the advantages of benign high-density gases, i.e., the possibility of achieving a high concentration of gaseous reactants in the same phase as the substrates and catalyst as well as easy catalyst localization by means of a membrane. For the separation of the homogeneous catalyst from the products an inorganic microporous membrane is used. The concept is demonstrated for the hydrogenation of 1-butene using a fluorous derivative of Wilkinsonʹs catalyst [RhCl{P–(C6H4-p-SiMe2CH2CH2C8F17)3}3]. The size of Wilkinsonʹs catalyst, 2–4 nm, is clearly larger than the pore diameter, 0.5–0.8 nm, of the silica membrane. The membrane will, therefore, retain the catalyst, while the substrates and products diffuse through the membrane. Stable operation and continuous production of n-butane has been achieved at a temperature of 353 K and a pressure of 20 MPa. A turnover number of 1.2×105 has been obtained during 32 h of reaction. The retention of the catalyst was checked using UV–vis spectroscopy and ICP-AAS; no rhodium or phosphorous species were detected at the permeate side of the membrane.