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172 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 catalysts [255]—in this process one must push the equilibrium towards product via the removal of alco- hol. Meanwhile, the commercial process operates very effectively from CO and alcohol over relatively inex- pensive copper catalysts [256]. Despite the negative results described above, it is important to note that ≈110 megatons of CO2 are consumed each year to produce low molecular weight products [254]. Most of this is consumed to gen- erate urea; in addition salicylic acid is synthesized (Kolbe-Schmitt reaction) from CO2 and a phenolic salt while alkylene carbonates are generated from the analogous alkylene oxides and CO2. The alkylene carbonates are considered relatively benign solvents (they exhibit low toxicity and low vapor pressure), and hence their synthesis from CO2 is an example of green chemistry. Monsanto, as well as academic researchers, have studied the synthesis of isocyanates from CO2 [183]. While the traditional route reacts amines with phosgene, creating the isocyanate plus salt, the CO2-based routes react the amine with CO2 in the presence of strong dehydrating agent. The yields of such CO2-based reactions are excellent, yet the cost of the dehydrating agent (or rather, its regeneration) has inhibited commercialization of such chemistry. Behr, among others, has reviewed a range of small molecule reactions that employ CO2 as a reactant [257]. In summary, CO2 has the potential to be a useful C1 synthon but recent work, while scientifically in- teresting, has not led to processes that can effectively compete with existing routes/plants. Further, when considering CO2 as a green reactant, one must always be cognizant of any energy differences required to em- ploy CO2 in a synthetic scheme versus a conventional reactant (such as CO). If use of CO2 is more energy intensive, then one might create a situation where more CO2 is created than chemically ‘sequestered’. 4.5. Other organic reactions As was mentioned previously, volatile metal car- bonyls (for example) exhibit sufficient solubility (or sufficiently low miscibility pressures) to support catalysis in CO2 without catalyst modification. As such, there are a number of examples in the literature where CO2 has been used as a ‘drop-in’ replace- ment for catalytic reactions ordinarily carried out in organic solvents. Nevertheless, once Leitner and Tu- mas demonstrated in 1997 that one could perform homogeneous catalysis in CO2 if the catalyst ligands were properly designed, a number of researchers have extended this work, examining a wide range of name reactions in CO2. The importance of the Leitner and Tumas papers was perhaps to demonstrate that effec- tively any catalyst could be rendered CO2-soluble, if the fluorination of the ligands could be accomplished synthetically. Consequently carbonylation [258], Heck and Stille couplings [259], vinylic substitution [260], hydrosilation [261], isocyanate trimerization [262], dechlorination [263], Pauson-Khand cyclization [264] and others have been successfully performed in car- bon dioxide. The use of fluorinated catalyst ligands is common, providing the solubility needed for the reaction to proceed smoothly. While these papers demonstrate the scope of ‘chem- istry in CO2, it is not clear as to the impact of such work on the overall aims of green chemistry. Granted, such reactions would ordinarily be performed in an organic solvent, and hence use of CO2 replaces such solvent use. On the other hand, the reactions described above are typically used for small volume, batch re- actions and hence, the overall impact of this work on the greening of industrial chemistry will be small. Per- haps the most significant impact of this work on green chemistry is in its ability to show chemists that CO2 is a viable solvent for a variety of reactions, and hence the greatest value of the work may be to educate the next generation of chemists. 4.6. Industrial activity: Friedel-Crafts chemistry and other name reactions Both Poliakoff [265] and Subramaniam [266] have patented alkylations in supercritical fluids, albeit using different types of catalysts. Each of these academic groups is/was working with an industrial partner (Thomas Swan and Engelhard, respectively [267]) and hence the work may ultimately be transferred to industry. Schiraldi et al., as well as Harris et al. [268] have patented the esterification of specific substrates in car- bon dioxide. Finally, a group at BASF has patented the generation of -tocopherol (and derivatives) in carbon dioxide [269]. It is not clear at this time if these in- ventions are being pursued further by the companies involved.PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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