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148 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 two-phase mixture) may be such that the rate in such a situation is higher than in the neat substrate case, despite the presence of a diluent (CO2). Such compar- isons would be useful for the purposes of determining the viability of such CO2-based processes. 2.7.3. Industrial activity: hydroformylation in CO2 Only one industrial patent of note [111], assigned to Mitsubishi Chemical Co. was identified during our patent search. No scale-up work seems to have fol- lowed. 2.7.4. Summary: hydroformylation in CO2 In summary, one could report many of the same conclusions regarding hydroformylation in CO2 as for hydrogenation in CO2 . In hydroformylation, however, process conditions for the industrial route are rather severe and hence, if one could obtain the high yields and selectivities of the industrial process but at mod- erate conditions (p, T) via use of CO2 as a solvent, the process would be both greener and less expensive. A rich area for further work is in hydroformylation in two-phase systems where CO2 acts as the ‘reversible diluent’. 2.8. Oxidation in CO2 At first glance, CO2 appears to be an ideal solvent for use in oxidations. Unlike most any organic solvent, CO2 will not oxidize further in the presence of oxygen and catalysts, and hence use of CO2 as the solvent eliminates the solvent byproduct waste stream that is usually expected in oxidations. Many of the conclusions found from recent research on hydrogenation and hydroformylation in CO2 can also be applied to oxidations conducted in CO2. However, while hydrogenation and hydroformylation focused exclusively on H2 (and H2/CO) as reagents, oxidations conducted in CO2 have been pursued using a variety of oxidants. The use of O2 as a benign oxi- dant has naturally received the most attention, as it is ultimately the least expensive and most atom-efficient route. Research on oxidation of substrates using O2 in CO2 has targeted the elimination of transport re- sistance (as for hydrogenation and hydroformylation) through the elimination of the gas-liquid interface. This is then proposed to enhance the efficiency of the reaction, leading to fewer byproducts. As in the preceding cases, it would be extremely interesting to examine oxidation in a single-phase system where CO2 is the minor component (a diluent for the sub- strate or swelling agent) or in a two-phase system where the substrate resides primarily in the lower phase. Here the role of the CO2 is simply to enhance the solubility of oxygen in the substrate-rich phase, where we assume that the dilution effect owing to CO2’s presence is more than offset by the enhanced oxygen concentration. This would allow lower pres- sure operation and might eliminate the need for fluo- rinated catalyst ligands (for homogeneous processes) in that the catalyst need be soluble in a concen- trated substrate–CO2 mixture, rather than a mixture that is primarily CO2. Indeed, Wu et al. [112] ex- amined precisely this type of system, although it is not clear from the paper whether they recognized all of the ramifications of the work. Wu studied the oxidation of cyclohexane with oxygen in the pres- ence of an iron porphyrin catalyst and acetaldehyde where CO2 was the solvent. The yield (of cyclohex- anol/cyclohexanone) increased with pressure up to ≈100 bar, then decreased sharply at higher pressures. Phase behavior measurements were not made, but qualitative observations (via sapphire windows in the reactor) suggested that the drop in yield coincided with a transformation from two- to one-phase. In this system, the presence of significant quantities of CO2 in the lower phase of a two-phase mixture allows for solubilization of substantial quantities of oxygen, providing for a high rate of reaction. Transformation to a one-phase mixture merely produced a dilution effect, lowering the rate. An additional consideration that recommends the use of CO2 as ‘diluent’ rather than major component (‘solvent’) is that oxidations using O2 are typically carried out using air (O2 /N2 ). Air is superior from an economic standpoint, as use of O2 mandates some- what energy-intensive O2 –N2 separation (and hence inadvisable from a green perspective). However, if one were to use O2/N2 in a single-phase system where CO2 is the primary solvent, nitrogen would build up in the system unless a concerted effort (pressure re- duction) were made to continuously remove it. In a two-phase mixture where CO2 is the minor compo- nent, the nitrogen concentration in the lower phase would quickly saturate (equilibrium would be estab- lished with the upper phase) and hence, this additionalPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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