Continuous Flow Hydroformylation of Alkenes in Supercritical Fluid−Ionic Liquid Biphasic Systems

A process for the hydroformylation of relatively low volatility alkenes (demonstrated for
1-dodecene) in a continuous flow system is described. The catalyst is dissolved in an ionic liquid while the
substrate and gaseous reagents are transported into the reactor dissolved in supercritical CO2, which
simultaneously acts as a transport vector for aldehyde products. Decompression of the fluid mixture
downstream yields products which are free of both reaction solvent and catalyst. The use of rhodium
complexes of triaryl phosphites leads to ligand degradation through reaction of the ionic liquid with water
and subsequent attack of the released HF on the phosphite. Sodium salts of sulfonated phosphines are
insufficiently soluble in the ionic liquids to obtain acceptable rates, but replacing the sodium by a cation
similar to that derived from the ionic liquid, allows good solubility and activity to be obtained. The nature of
the ionic liquid is very important in achieving high rates, with 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amides giving the best activity if the alkyl chain is at least C8. Catalyst turnover frequencies
as high as 500 h-1 have been observed, with the better rates at higher substrate flow rates. Rhodium
leaching into the product stream can be as low as 0.012 ppm, except at low partial pressures of CO/H2,
when it is significantly higher. Oxygen impurities in the CO2 feed can lead to oxidation of the phosphine
giving higher rates, lower selectivities to the linear aldehyde, increased alkene isomerization and greater
leaching of rhodium. However, it is found that under certain process conditions, the supercritical fluid-ionic
liquid (SCF−IL) system can be operated continuously for several weeks without any visible sign of catalyst
degradation. Comparisons with commercial hydroformylation processes are provided.