| Catalytic hydroformylation of higher olefins faces several challenges, including difficulty in catalyst recovery, and limited solubility of the gaseous reagent, syngas (H2/CO), in the liquid phase. Current processes are operated at harsh conditions (140-200°C, 5-20 MPa) with inexpensive albeit low-activity cobalt catalysts. Significant quantities of acid, base, and aqueous solutions are involved in the complex catalyst recovery step. An environmentally-friendlier yet economically viable technology is desired with features such as highly active and stable catalyst, facile catalyst recovery, and enhanced syngas solubility at mild conditions.; The present work successfully demonstrates the use of CO2-expanded liquids (CXLs) for performing higher-olefin hydroformylation. Advantages of employing CXLs as solvent media include: significant replacement of organic solvents by benign CO2, enhanced syngas solubility in the reaction phase, and milder operating pressures than those required in conventional processes. Further, it is possible to precipitate and recover the polar transition metal catalyst by CO2 addition post reaction.; The reaction benefits of CXLs are demonstrated using 1-octene substrate and high-activity rhodium catalysts. In catalyst activity screening studies, turnover frequencies in CO2-expanded acetone are up to four-fold higher than in either neat acetone or compressed CO2; the chemoselectivity to hydroformylation products (aldehydes) and regioselectivity to desired linear aldehyde are improved with CO2 addition. The increased rates and selectivities are explained by the enhanced syngas solubility measured in CO2-expanded acetone, compared to that in neat acetone.; Hydroformylation of 1-octene is performed with an industrial catalyst, HRh(CO)(PPh3)3, in the absence of an added solvent. CO2 addition to the liquid phase enhances syngas solubility but dilutes substrate concentration, resulting in an optimal turnover frequency at an intermediate CO2 content in the liquid phase. The turnover frequency and selectivity at optimal CO2 content, obtained under mild conditions (60°C, 3.8 MPa), are either comparable to or better than those reported in industrial and other proposed processes. Effects of mass transfer limitations on observed induction periods and reaction kinetics, investigated using a stirred high-pressure ReactIR apparatus, are elucidated though complementary experimental and modeling studies. These results pave the way for further studies aimed at rational design and scale-up of CXL-based hydroformylation processes. |
