Continuous flow homogeneous hydroformylation of alkenes using supercritical fluids

The hydroformylation of long chain alkenes, carried out as a continuous flow process by using a catalyst made in situ from [Rh(CO)(2)(2,4-pentanedioate)] and [1-propyl-3-methylimidazolium][Ph2P(3-C6H4SO3)] dissolved in 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonamide), is reviewed. The substrates are introduced into and the products are removed from the reactor dissolved in continuously flowing supercritical carbon dioxide. Optimisation studies allow long term operation with rates up to 500 catalyst turnovers h(-1) and rhodium leaching into the product as low as 0.012 ppm. Using a different ligand based on the xantphos skeleton, linear : branched ratios in the product aldehydes can be as high as 40 : 1, but the rate is somewhat lower (272 h(-1)) and the rhodium retention is slightly less efficient. By removing the ionic liquid and dissolving the catalyst in the mixture of products and substrates that develops during the reaction, it is possible to carry out the reaction at much lower pressures (125 bar). 1-alkyl-3-methylimidazolium salts of [Ph2P(3-C6H4SO3)] are used as ligands and the success of the reaction depends crucially on the alkyl chain employed in the imidazolium cation. Too long a chain causes large amounts of rhodium leaching, whilst too small a chain causes the solubility of the ligand to be too low so that the reaction is very poor and unliganded rhodium is rapidly extracted from the system. The best results are obtained using a pentyl chain, which gives a rate of 162 h(-1) and rhodium leaching of 0.1-0.5 ppm. Comparisons of these systems with current commercial systems are made and consideration is given to the design of a plant suitable for operating these reactions in a totally emissionless fashion.