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126 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 1.5.3. CO2 is a Lewis acid Carbon dioxide will react with strong bases (amines, phosphines, alkyl anions) [23]. When attempting to use amines as reactants, this can be a serious disadvan- tage, in that carbamate formation can slow the rate of the intended reaction and can also alter the solubility characteristics of the substrate. While alkyl-functional primary and secondary amines react readily with CO2, tertiary amines are non-reactive. Further, the pres- ence of electron-withdrawing groups in close prox- imity to the nitrogen atom (as in anilines) prevents formation of carbamates between CO2 and such com- pounds. Carbon dioxide will also react (not surpris- ingly) with metal alkoxides, metal alkyls, and metal hydrides. CO2 has been shown to react reversibly with a number of enzymes (lysine residues, specifically), leading to low activity in the presence of CO2 (al- though activity returns to normal following removal of the enzyme from the CO2-rich environment) [24]. Because carbamate formation is reversible, even at high pressure, researchers have employed CO2 as a protecting group for amines [25] and hence, CO2’s re- activity with amines can be an advantage as well as a disadvantage. Finally, because CO2 reacts readily with carbanions to form relatively unreactive carboxylates, anionic polymerization cannot be conducted in carbon dioxide. 1.5.4. CO2 can be hydrogenated in the presence of noble metal catalysts to produce CO If one is trying to hydrogenate a substrate in CO2 over a heterogeneous platinum catalyst, production of CO will poison the catalyst and produce toxic byprod- ucts. Unfortunately, this reaction takes place at rela- tively mild temperatures [6]. There has been a certain degree of controversy recently as to whether the same reaction occurs over palladium catalysts. For exam- ple, Hancu and Beckman [14] demonstrated that hy- drogenations could be carried out successfully in CO2 (over palladium), although it should be noted that the hydrogenation in question was very fast and was con- ducted at 298 K. Subramaniam et al. [26] was able to successfully conduct a hydrogenation reaction over palladium in a continuous reactor; no loss in catalyst activity was observed over a period of 1–2 days. By contrast, Brennecke and Hutchensen [27] found that a palladium catalyst de-activated rapidly during batch hydrogenations in CO2. Subramanian [28] recently in- vestigated these apparent contradictions and found that higher temperatures (>343 K) and greater residence times (such as would be found in batch reactions) do lead to the formation of CO which ultimately poisons’ the catalyst. This is an area where further research work is certainly merited, given the potential impor- tance of hydrogenation reactions. In addition to CO, it is likely that some formate could be created through hydrogenation of CO2 over noble metals; formate has been observed during ho- mogeneous catalysis [29] and could theoretically form under heterogeneous conditions as well. 1.5.5. Dense CO2 produces low pH (2.85) upon contact with water Carbon dioxide dissolves in water at molar con- centrations [30] at moderate pressures (<100 bar), rapidly forming H2 CO3 . This can render some bio- catalytic reactions problematic, in that many enzymes are denatured (unfolded and/or de-activated) by low pH. Johnston et al. has shown that buffering is possi- ble but that impractically high ionic strength (for en- zymatic reactions) is needed [31]. On the other hand, one could employ carbonic acid as a reagent, in which case CO2 could be treated as a very low cost, sus- tainable acid that does not require addition of base for neutralization. Enick [32], for example, has em- ployed carbonic acid, formed from CO2/water, to ex- tract contaminants from steel waste into water, where depressurization results in a rapid increase in pH and precipitation of the extracted materials. Carbonic acid formed from CO2 and water reacts with hydrogen peroxide under basic conditions to produce a per- carbonate species, which can then epoxidize alkenes [33]. In summary, the low pH of water in contact with liq- uid CO2 can be an advantage or disadvantage, depend- ing upon circumstances. Hancu and Beckman [14], for example, have investigated the generation of H2O2 in CO2, where the product is stripped into water follow- ing synthesis in CO2 . The optimum pH for H2 O2 sta- bility is 2–4, so the low pH of water/CO2 mixtures is an advantage for this process. The low pH of water in contact with CO2 also enhances the back-extraction of caffeine in the decaffeination process for coffee. Clearly, however, the low pH of CO2–water systems is a detriment to the processing of biomolecules.PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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