Supercritical carbon dioxide as an alternative reaction medium for hydroformylation with integrated catalyst recycling

The cobalt-catalyzed hydroformylation of 1-octene using the complex bis{tri(3-fluorophenyl)phosphine}hexacarbonyldicobalt (1) and Co2(CO)8 as pre-catalysts in supercritical carbon dioxide (scCO2) as a reaction medium was investigated. The catalytic performance in scCO2 was compared to the one in toluene as a conventional solvent. Similar activities and selectivities were obtained in both reaction media. In scCO2, a substantial improvement of the selectivity for aldehydes was found by using (1) (P:Co = 1:1) in comparison to the unmodified catalyst Co2(CO)8 (P:Co = 0:1). Surprisingly, further addition of (m-FC6H4)3P (P:Co = 6:1 and 11:1, respectively) resulted in a little enhancement of the aldehydes selectivity only, whereas the conversion and, hence, the aldehydes yield were reduced. A concept for catalyst recycling by using scCO2 was introduced. It was found that (1) was insoluble in the cold reaction mixture and was completely soluble in the supercritical reaction medium. By cooling the reactor content after the olefin conversion, the catalyst was regenerated as a solid, separated by filtration and could be recycled several times.