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138 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 at relatively low pressures would enhance solubility of H2 in the substrate, raising rates while not impact- ing process costs precipitously. Even safety could be improved, as previous work has shown that addition of CO2 to a mixture of hydrogen and air expands the non-explosive regime more so than addition of nitro- gen [9]. As such, a sudden leak in the reactor, lead- ing to a mixture of CO2, air and hydrogen would still be safer than the same case where nitrogen was being used as the pressure-transmitting fluid. Use of CO2 in such reactions could thus be green, safe and practical. Bertucco [67] and later Devetta [68], also showed the advantages of using a multi-phase system in their work on the hydrogenation of an unsaturated ketone over a Pd/alumina catalyst. These researchers found that one could eliminate transport resistance while op- erating in the three-phase (solid catalyst plus liquid plus gas) regime. Here again, the fact that CO2’s pres- ence in the lower liquid phase greatly enhances the sol- ubility of hydrogen in the liquid (substrate plus CO2) allows one to eliminate transport resistance without the need to apply pressure high enough to create one phase. Consequently, one could conceivably render the reaction more efficient (and hence less wasteful) and economically practical by using moderate pressures. Arai et al. examined the hydrogenation of unsat- urated aldehydes in both CO2 and ethanol over a Pt/Al2O3 catalyst [69]. The selectivity of the reac- tion towards unsaturated alcohol in CO2 was signif- icantly better than that in ethanol; while increasing the pressure in the CO2 case improved selectivity, the opposite occurred when increasing the hydrogen pressure in the ethanol analog. Indeed, here is a case where the use of CO2 appears to enhance selectiv- ity, and thus reduce waste in a reaction versus the ‘liquid’ analog. It is not clear from the discussion by Arai whether this improvement in selectivity is enough to offset the difficulties involved in scaling up a high-pressure process and whether the energy input to the CO2-based analog is more or less than the liquid case. Interestingly, Arai did not observe the rapid catalyst deactivation formerly observed by Minder et al. [70] during hydrogenation in CO2 over a platinum catalyst. Minder’s results were readily explained by formation of CO and other poison- ing species owing to the hydrogenation of CO2 it- self; it is unclear why Arai was able to avoid this problem. Poliakoff et al. [71] have evaluated the efficiency of hydrogenation of a wide variety of substrates in su- percritical fluids (propane and CO2) over a Pd catalyst in a continuous flow reactor. Substrates included aro- matic alcohols, aldehydes, ketones, unsaturated cyclic ethers, nitro compounds, oximes and Schiff bases. Re- actions were conducted at temperatures ranging from 360 to 670 K at pressures between 80 and 120 bar. All of the substrates examined could be hydrogenated to some extent, with measured space-time yields ex- ceeding 2×105 kg h−1 m−3 for the hydrogenation of cyclohexene. Given the high temperatures employed, the relatively low pressure, the presence of significant amounts of hydrogen and the low volatility of some of the substrates employed, it is highly likely that two or more phases existed in the reactor during the initial phases of the process. CO2’s density will not be ‘liquid-like’ at these pressures and temperatures, while hydrogen will act as a non-solvent owing to its low critical temperature (and hence low reduced den- sity at the reaction conditions). Poliakoff examined the phase behavior in the cyclohexene-to-cyclohexane system and indeed found that multiple phases exist initially, while a single phase forms near the end of the reaction. Single-phase behavior results because the temperature increases to a point above the critical temperatures of both cyclohexene and cyclohexane. Whereas Poliakoff demonstrated the breadth of con- tinuous hydrogenation in CO2, lack of comparisons with traditional hydrogenation reactions make it dif- ficult to judge whether the technology will ultimately be deemed ‘green’. Catalyst lifetime, for example, is not mentioned—rapid loss in activity could render this technology less than adequate from both green and financial perspectives. If CO2 -based hydrogena- tion allows for elimination of significant volumes of solvent without greatly increasing energy or catalyst demand, then this technology could ultimately be both economically successful and green. Subramanian et al. [26] also examined the hydro- genation of cyclohexene to cyclohexane (over Pd/C) in supercritical CO2, although under conditions where the system remained single phase throughout the re- action and the temperature was held at a constant 343 K. The reaction remained stable over periods exceed- ing 20 h and catalyst activity was maintained at a high level by pretreating the cyclohexene feed to remove deleterious peroxides. No CO or formate formationPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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