(239e) Catalysis and Reactor Engineering with Carbon Dioxide-Expanded Liquids

In recent years, CO₂-expanded liquids (CXLs) have been shown to be promising alternative media for performing catalytic reactions (Wei et al., 2002; Jessop et al., 2003; Rajagopalan et al., 2003). A CXL is a mixed solvent composed of dense CO₂ dissolved in an organic solvent, typically at mild pressures (tens of bars). By varying the CO₂ composition, a continuum of liquid media with tunable properties may be generated; e.g., a large amount of CO₂ favors gas solubility and the presence of polar organic solvents enhances metal catalyst solubility. Demonstrated advantages of CXLs include enhanced gas (oxygen, syngas, etc.) solubilities in the liquid phase relative to neat solvents, facile separation of homogeneous catalysts, reduced viscosities of reaction mixtures, and enhanced safety due to significant replacement of pure gases with CO₂. The application of CXLs in catalytic hydrofomylation will be presented in detail. For 1-octene hydroformylation, the performance of several rhodium catalysts is compared in neat organic solvents and in CXLs. 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 observed TOF (~300 h-1), n/i ratio (>10) and aldehydes selectivity (~90%) at the optimum CO₂ content were either comparable or better than values reported with other media and catalysts. The enhanced rates and selectivity are shown to be due to increased syngas availability in the CXL phase. Furthermore, the operating pressure (~40 bar) and temperature (60 °C) for the CXL process are significantly milder than those reported for industrial hydroformylation processes. In other words, the use of CXLs intensifies the process while simulataneously reducing the environmental burden. Mass transfer and kinetic studies using an in situ ReactIR probe, reactor modeling, economic and environmental impact analyses, aimed at systematic development of practically viable systems, will be presented.

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