Hydroformylation of 1-hexene catalyzed with rhodium fluorinated phosphine complexes in supercritical carbon dioxide and in conventional organic solvents: effects of ligands and pressures

Abstract

Rhodium-catalyzed hydroformylation of 1-hexene was investigated in compressed CO₂ and organic solvents using different fluorinated phosphine compounds as ligands at a temperature of 333 K. The reaction runs were conducted under conditions where the reaction mixtures were homogeneous in order to examine the activity of the rhodium complexes in different media. The effects of phosphine ligand, CO₂ pressure, syngas (H₂/CO) pressure and solvent on the hydroformylation activity were studied, along with FTIR examination of reaction mixtures. Such phosphine compounds as diphenyl(pentafluorophenyl)phosphine (II), bis(pentafluorophenyl)phenylphosphine (III), and tris(p-trifluoromethylphenyl)phosphine (VI) are effective ligands in scCO₂. Compound II is better since with it the undesirable isomerization side reaction is avoided. The n/iso ratio (heptanal/2-methylhexanal) does not change so much with the phosphine ligand used. It is interesting that the aldehyde yield goes through a minimum at about 9 MPa with increasing CO₂ pressure, and it tends to increase with an increase in the syngas pressure. The catalytic activities in scCO₂ are comparable with those in toluene and it is suggested that scCO₂ has some positive effects in promoting the hydroformylation.