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 Wilkinson's catalyst [RhCl{P-(C 6 H 4 -p-SiMe 2 CH 2 CH 2 C 8 F 17 ) 3 } 3 ]. The size of Wilkinson's 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 × 10 5 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.
Membrane separation technology is successfully applied for the immobilization of a homogeneous catalyst (a (1H,1H,2H,2H‐perfluoroalkyl)dimethylsilyl‐substituted derivative of Wilkinson's catalyst) in a continuous process that uses supercritical carbon dioxide as solvent. The catalyst is separated from the products by a microporous silica membrane (see scheme).
The one-component transient permeation of carbon dioxide through an alumina-supported silica membrane is studied for pressures up to 2.0 × 10 7 Pa, at temperatures of 296, 313, and 358 K. The permeation is obtained for gaseous, liquid, and supercritical carbon dioxide. For all conditions high carbon dioxide fluxes were obtained. The membrane has been tested with a maximum pressure difference between the feed and the permeate side of about 2.5 × 10 6 Pa. At a feed-side pressure of 2.0 × 10 7 Pa and at 358 K the permeance is equal to 8.0 × 10 -8 mol m -2 s -1 Pa -1 . The transient transport through the microporous membrane can be described with a single mass-transfer coefficient. For a feed-side pressure between 1.0 × 10 5 and 200 × 10 5 Pa and a temperature of 296 K the mass-transfer coefficient increases by a factor of 80. The fact that carbon dioxide has a high flux through this type of silica membrane opens the way for regeneration of carbon dioxide at supercritical conditions.
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