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many of the newer Friedel-Crafts ‘catalysts’ are flu- orinated, and hence highly CO2-soluble. Chateauneuf and Nie [245] examined the alkyla- tion reaction between methoxy benzene and triph- enyl methanol using trifluoroacetic acid as catalyst. Kobayashi et al. [246] found that rhenium triflate pro- moted the acylation of aromatic compounds (as in Chateauneuf’s work, if electron donating substituents were present on the aryl compound) with an anhy- dride. The reaction proceeded smoothly in either or- ganic solvents or CO2. Finally, Poliakoff’s group first examined the Friedel-Crafts alkylation of various ac- tivated aryl compounds using a supported (Deloxan) acid catalyst in CO2 [247]. Although not large, the literature on Friedel-Crafts chemistry in CO2 demon- strates that this reaction is indeed feasible, and that many of the Lewis acids proposed as catalysts are readily CO2-soluble. Olah et al. [248] examined the acid catalyzed isobutene-isobutylene reaction in carbon dioxide; they found that CO2 acted as a weak base and use of CO2 as solvent lowered the acidity of the system and hence the alkylate quality. However, in cases where the acidity was increased to counteract this effect, the use of CO2 decreased the amount of acid needed to perform the alkylation. Further, use of CO2 increased the octane number of the product. In a final intriguing note, Pernecker and Kennedy [249], during an investigation into the Lewis acid cat- alyzed polymerization of isobutylene in CO2, found that addition of only the Lewis acid to carbon dioxide formed a product, either a solid precipitate or a second liquid. Removal of the CO2 regenerated the original Lewis acid. On the other hand, incubation of a Lewis acid with the polymerization initiator, followed by addition to CO2 , resulted in no ‘CO2 -product’ for- mation. Pernecker’s results suggest that one might activate CO2 itself for further reaction using a Lewis acid, but if the Lewis acid is presented with a more reactive substrate, it will preferentially bind to this substrate. In summary, Friedel-Crafts chemistry is (in fine chemical synthesis) performed in solvent, and hence CO2 represents a potentially useful and green substi- tute. Catalysts that one would ordinarily use to per- form such reactions are soluble in CO2 without further modification. The effective use of CO2 then depends upon substrate solubility. 4.4. CO2 as reactant and solvent In this section, those reactions where CO2 is em- ployed as reactant and solvent, yet where small molecules (rather than polymers, see Section 3) are formed as products, will be discussed. A large number of reactions using CO2 as a raw material have been demonstrated in the laboratory, but very few such reactions are practiced commercially. For example, it has been shown in the literature that one can generate formic acid [250], dimethyl formamide [251], car- boxylic acids [252] and methanol [253] using CO2 as reactant (and in many cases the solvent as well). To date, however, the economics of such processes have not been sufficiently favorable to warrant significant industrial attention. Part of the problem is that use of CO2 to create commodities, such as those listed above competes directly with use of highly reactive CO to create the same molecules. For example, methanol is produced from CO and hydrogen (synthesis gas, or syngas) in an atom-efficient process [13]. Further, one can readily generate the needed synthesis gas from coal, natural gas or petroleum. To form methanol from CO2, one would need an additional clean and inexpensive source of hydrogen. Further, the ther- modynamics of the two routes are such that one can obtain twice the yield of methanol from the syngas route (e.g. at 470 K) than the CO2 route [254]. At present, CO2 is only used to supplement syngas dur- ing methanol production if the ratio of hydrogen to CO is significantly higher than 2.0 (which can occur when natural gas is used as the syngas source). Other small molecules such as formic acid, formates, and formamides are then generated from methanol (plus CO, ammonia, alkyl amines)—this chemistry is also atom-efficient and hence alternative routes using CO2 as a starting material have been unable to compete. In general, it is presumed that CO2-based routes for basic commodity chemicals would be competitive if a relatively inexpensive, non CO2 -producing source of hydrogen can be developed [254]. Granted, CO is a much more toxic material than CO2, yet syngas has been used successfully for decades in chemical processes, so this factor carries little weight currently. The generation of dialkyl carbonates presents a sim- ilar example to those described above—a number of researchers have investigated the synthesis of dialkyl carbonates from CO2 and alcohols using alkoxy tin E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 171PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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