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Abstract: The kinetics of 1-octene hydroformylation catalyzed by triphenylphosphine modified rhodium in carbon dioxide have been explored at 90°C and pressures up to 48MPa. The apparent catalyst solubility was determined by evaluating the reaction rate for different rhodium amounts. The kinetics follow a first order in 1-octene, a negative order of −1.2 in carbon monoxide, an order of 0.25 in hydrogen, and a negative order of −0.2 in triphenylphosphine, which is to a great extent in agreement with studies on the hydroformylation of linear 1-alkenes in organic solvents. The observed apparent turnover frequencies range from 1900 to 7000molaldehyde molRh −1 h−1. These turnover frequencies are of the same order of magnitude as observed for hydroformylations in organic solvents, indicating that rhodium modified with triphenylphosphine can be used with high efficiency in supercritical carbon dioxide rich mixtures. [Copyright &y& Elsevier]

Abstract: The kinetics of the hydroformylation of 1-octene in a supercritical carbon dioxide medium, catalyzed by a tris(3,5-bis[trifluoromethyl]phenyl)phosphine-modified rhodium catalyst, have been investigated. The influence of the concentration of carbon dioxide, reactants, catalyst precursors, and the reaction temperature has been determined. A kinetic model was developed, which describes the concentration-time profiles of the reactants, the linear and branched aldehydes, and the internal alkenes. Using the kinetic model activation energies for hydroformylation of 1-octene to nonanal and 2-methyloctanal were determined. Throughout the concentration ranges studied an approximate first order dependence of the hydroformylation rate on the hydrogen and catalyst concentration was found which indicated that oxidative addition of hydrogen was the rate limiting step. The increase in reaction rate and regioselectivity with an increase in ligand concentration is a striking feature of the catalyst investigated here. At higher concentrations the reaction rate was found to have a strong negative order dependence on the carbon monoxide concentration. The reaction rate had a positive order in 1-octene at a concentration lower than 0.5molL−1 while saturation kinetics were observed at a higher concentration. The results were explained by invoking the contribution of both monophosphine and diphosphine rhodium species to the hydroformylation catalysis. [Copyright &y& Elsevier]

Abstract: The permeation of supercritical CO2 and SF6 across two different types of microporous membranes has been studied as a function of temperature and feed pressure. A general picture appears where for both species the permeance as a function of the feed pressure has a maximum. For both the polymeric and the silica membranes close to the temperature where the different species become supercritical the maximum in the permeance is observed at the critical pressure. In the supercritical regime the main mechanism for the mass transport is viscous flow, and there is almost no contribution from surface diffusion through the micropores. The permeance of the fluids obtained for different condition can be described by a single mobility constant, which is independent of temperature and pressure. [Copyright &y& Elsevier]

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–(C6H4-p-SiMe2CH2CH2C8F17)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 1.2×105 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. [Copyright &y& Elsevier]