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was observed. While this work does not suggest as to how or why such reactions could be considered ‘green’, it does demonstrate that stable (with respect to temperature and pressure) catalytic hydrogenation in a continuous reactor using CO2 as solvent is read- ily achievable. Again, the assumption here is that use of CO2 will eliminate the gas–liquid interface, rendering the reaction more efficient and potentially less wasteful. Subramaniam has authored a compre- hensive review on process design issues inherent to catalytic processes performed in carbon dioxide [59]; interested readers should consult this paper. Hancu and Beckman [72] examined the hydrogena- tion of oxygen (production of H2 O2 ) in CO2 under both liquid and supercritical conditions. Hydrogen peroxide is currently produced via hydrogenation (over a Pd supported catalyst), then oxidation of a 2-alkyl anthraquinone (AQ) in an organic solvent (see Fig. 4). Whereas H2O2 is widely accepted as a green oxidant, the process by which it is manufactured ex- hibits a number of less-than-green attributes. First, use of the organic solvent (coupled with the liquid–liquid extraction against water used to recover the product) creates a significant contamination issue, one that is currently remedied using energy-intensive distillation. Further, because each of the reactions are transport controlled (again, by the rate of diffusion of H2 or O2 from the gas to liquid phase), CSTRs (continuous stirred tank reactors) are used, allowing for a range of anthraquinone residence times and hence over hydrogenation of the AQ to form waste byproducts. Fig. 4. Schematic of the anthraquinone route to hydrogen peroxide [15]. Gelbein [73] has estimated that one-third of the cost of H2O2 can be tied directly to anthraquinone and solvent make-up/regeneration; ≈1.5 million pounds of anthraquinone and 15 million pounds of solvent are produced each year simply to support consumption in the AQ process for producing hydrogen peroxide. Hancu first examined the use of CO2 as the or- ganic solvent in the anthraquinone process by gen- erating a highly CO2-soluble analog to conventional alkyl anthraquinones (alkyl AQs exhibit solubilities in CO2 that are three orders of magnitude below what is employed in the commercial process). These fluoroether-functional AQs exhibited complete misci- bility with CO2; maximum miscibility pressures were sensitive functions of anthraquinone composition and topology. Hancu showed that kinetic control could be obtained in both the hydrogenation and oxidation reactions using CO2 as the solvent. Here, use of CO2 eliminates the need for the distillation train, as con- tamination of the aqueous phase by solvent and other byproducts is not an issue. Further, while the solvent in the conventional process is prone to both hydro- genation and oxidation, this is not the case for the CO2 analog. Despite the promising laboratory results, Hancu’s process in its original state exhibited a critical economic flaw, yet one that could be corrected given recent results. The fluoroether-functional AQ will be significantly more expensive than an alkyl AQ and pressures required to maintain a homogeneous mix- ture will be high, despite the use of the CO2-philic AQ. If, however, we examine the results of Bertucco, Chouchi and Devetta [65,67,68], it is clear that an al- ternative route exists where one could take advantage of the green aspects of CO2 use while minimizing the AQ cost issues and reducing the operating pressure. The works cited in the previous sentence show that it is quite possible that one does not need to achieve a single phase of hydrogen, CO2 and substrate to elim- inate gas–liquid diffusional limitations to reaction. In gas–liquid reaction systems, often the primary resis- tance to transport is the low solubility of the reactant gases in the liquid phase and slow diffusion across the interface. The high degree of swelling of a sub- strate by CO2 can allow for significant increases in hydrogen solubility in the liquid phase, while the low viscosity of carbon dioxide enhances diffusion rates. Thus, it is quite likely that one could derivatize an E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 139PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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