“Solventless” continuous flow homogeneous hydroformylation of 1-octene

The hydroformylation of 1-octene under continuous flow conditions is described. The system involves dissolving the catalyst, made in situ from [Rh(acac)(CO)₂] (acacH = 2,4-pentanedione) and [RMIM][TPPMS] (RMIM = 1-propyl (Pr), 1-pentyl (Pn) or 1-octyl (O) -3-methyl imidazolium, TPPMS = Ph₂P(3-C₆H₄SO₃)), in a mixture of nonanal and 1-octene and passing the substrate, 1-octene, together with CO and H₂ through the system dissolved in supercritical CO₂ (scCO₂). [PrMIM][TPPMS] is poorly soluble in the medium so heavy rhodium leaching (as complexes not containing phosphine) occurs in the early part of the reaction. [PnMIM][TPPMS] affords good rates at relatively low catalyst loadings and relatively low overall pressure (125 bar) with rhodium losses <1 ppm, but the catalyst precipitates at higher catalyst loadings, leading to lower reaction rates. [OMIM][TPPMS] is the most soluble ligand and promotes high reaction rates, although preliminary experiments suggested that rhodium leaching was high at 5–10 ppm. Optimisation aimed at balancing flows so that the level within the reactor remained constant involved a reactor set up based around a reactor fitted with a sight glass and sparging stirrer with the CO₂ being fed by a cooled head HPLC pump, 1-octene by a standard HPLC pump and CO/H₂ through a mass flow controller. The pressure was controlled by a back pressure regulator. Using this set up, [OMIM][TPPMS] as the ligand and a total pressure of 140 bar, it was possible to control the level within the reactor and obtain a turnover frequency of ca. 180 h⁻¹. Rhodium losses in the optimised system were 100 ppb. Transport studies showed that 1-octene is preferentially transported over the aldehydes at all pressures, although the difference in mol fraction in the mobile phase was less at lower pressures. Nonanal in the mobile phase suppresses the extraction of 1-octene to some extent, so it is better to operate at high conversion and low pressure to optimise the extraction of the products relative to the substrate. CO and H₂ in the mobile phase also suppress the extraction efficiency by as much as 80%.