Intensification of catalytic olefin hydroformylation in CO 2 ‐expanded media

CO 2 ‐expanded liquids (CXLs) are demonstrated to be effective reaction media for the catalytic hydroformylation of 1‐octene. The performance of several rhodium catalysts, Rh(acac)(CO) 2 , Rh(acac)[P(OPh) 3 ] 2 , Rh(acac)(CO)[P(OAr) 3 ], and two phosphorous ligands, PPh 3 and biphephos, was compared in neat organic solvents and in CXLs wherein more than 50% of the solvent volume is replaced with dense CO 2 at relatively mild temperatures (30–90°C) and pressures (<12 MPa). For all catalysts, enhanced turnover frequencies (TOFs) were observed in CXLs. For the most active catalyst, Rh(acac)(CO) 2 modified by biphephos ligand, the selectivity to aldehyde products was improved from approximately 70% in neat solvent to nearly 95% in CXL media. The enhanced rates and selectivity are attributed to increased syngas availability in the CXL phase. In experiments performed without added solvent, a TOF maximum was observed at an optimum CO 2 content. It appears that at higher than optimum CO 2 content, the TOF decreases as a result of dilution of substrate by CO 2 , whereas at lower than optimum values the TOF is limited by reduced syngas availability. The observed TOF (∼300 h −1 ), n/i ratio (>10), and aldehyde selectivity (∼90%) at the optimum CO 2 content were either comparable to or better than values reported with other media and catalysts. Furthermore, the operating pressure (3.8 MPa) and temperature (60°C) for the CXL process are significantly milder than those reported for industrial hydroformylation processes. © 2006 American Institute of Chemical Engineers AIChE J, 2006