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with CO2 itself—such side reactions can be inhibited through proper catalyst design or choice of operating conditions. 2.4.2. Engineering rationale for homogeneous versus heterogeneous catalysis In homogeneous hydrogenation, the catalyst has been designed such that it is soluble in the liquid phase; the ligands of the catalyst are usually con- structed to produce high selectivity to product. The rationale for conducting homogeneous hydrogenation reactions in CO2 has three primary thrusts, (1) that operation in CO2 eliminates the need for organic sol- vent; (2) operation in CO2 eliminates the gas–liquid interface and hence allows for kinetic control over the reaction; and (3) use of CO2 will alter the selectivity of the reaction (hopefully for the better). Much of the recent work on homogeneous hydrogenation has been directed at asymmetric synthesis, with the general hypothesis that use of CO2 could possibly alter the enantioselectivity of the reactions concerned. The rate of a homogeneous hydrogenation reac- tion conducted in an organic solvent or water is likely to be governed by the rate at which hydrogen diffuses across the vapor–liquid interface. As such, elimination of this interface (via operation in CO2) eliminates this transport resistance. Indeed, because the catalyst in this case is soluble, elimination of the interface entirely eliminates transport resistance. To allow direct replacement of the organic solvent in a homogeneous hydrogenation reaction with CO2 , both the catalyst and the substrate must be soluble in CO2. Consequently, the majority of the scientific effort in literature works on homogeneous hydrogenation in CO2 is directed at synthesis of CO2-soluble analogs of conventional catalysts. Substrates must be chosen that are CO2-soluble and hence one observes pre- dominantly ‘model’ compounds employed rather than necessarily compounds of industrial interest. One could pose the question, ‘if a liquid substrate is being employed, why not simply run the reaction using the homogeneous catalyst neat, in the absence of any solvent?’ The solubility of hydrogen in organic liquids is typically quite low, and hence running the hydrogenation of a neat substrate will encounter sig- nificant transport resistance (of hydrogen across the in- terface) to reaction. If carbon dioxide readily dissolves or swells the liquid phase (catalyst and substrate), the rate of reaction can increase owing to enhance hydro- gen concentration at the locus of reaction, despite the presence of CO2 , a diluent. An example of the use of homogeneous catalysis to achieve an engineering goal was shown by Hancu and Beckman [14], who examined the generation of H2O2 in CO2 directly from H2 and O2 in a single step using a CO2-soluble palladium catalyst. This process has been examined in industry for over two decades, as elimination of the anthraquinone from the process eliminates several unit operations and greatly reduces raw material input. If one examines Gelbein’s numbers for the economics of H2O2 production [73], one would estimate that the using the direct route would reduce the cost of production by over 50%, a significant amount for a commodity process. Hancu proposed that one could generate H2O2 in CO2 (from H2 and O2) using a soluble palladium catalyst, where the H2O2 is then rapidly stripped into water. The green aspects of this process include elimination of solvent waste and anthraquinone input/byproducts, elimination of the distillation train and the associated energy input, and elimination of several unit oper- ations and the associated energy input. The process could be run continuously and the product recovered from CO2 without a large pressure drop, rendering the process economics more favorable. Previous work on the direct route to H2O2 has focused on the bal- ance between safety and productivity, where most of the patented processes employ water as the reaction medium to maintain safety. However, because the solubility of H2 and O2 in water is so low, the pro- ductivity of these processes is not sufficient to merit scale-up. In addition, the Pd catalysts employed tend to catalyze degradation of H2O2 as well as forma- tion, and hence running the reaction in water does not lead to the desired productivity. Hancu showed that one could employ a CO2-soluble catalyst, and hence run the reaction in CO2 without transport lim- itations and in a non-explosive concentration regime where rates are high. Future work is needed in this area with respect to optimizing catalyst performance and lifetime, yet this is a good example of the use of homogeneous hydrogenation in carbon dioxide to ac- complish what are normally perceived to be process goals. Unlike in the previous example, in cases where a separate aqueous phase is not present, we may be able E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 141PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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