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Improving Carbon Dioxide Solubility in Ionic Liquids Essay

Previously we showed that CO2 could be used to extract organic molecules from ionic liquids without contamination of the ionic liquid. Consequently a number of other groups demonstrated that ionic liquid/CO2 biphasic systems could be used for homogeneously catalyzed reactions. Large differences in the solubility of various gases in ionic liquids present the possibility of using them for gas separations. More recently we and others have shown that the presence of CO2 increases the solubility of other gases that are poorly soluble in the ionic liquid phase. Therefore, a knowledge and understanding of the phase behavior of these ionic liquid/ CO2 systems is important.

With the aim of finding ionic liquids that improve CO2 solubility and gaining more information to help us understand how to design CO2-philic ionic liquids, we present the low- and high-pressure measurements of CO2 solubility in a range of ionic liquids possessing structures likely to increase the solubility of CO2. We examined the CO2 solubility in a number of ionic liquids with systematic increases in fluorination.

We also studied nonfluorinated ionic liquids that have structural features known to improve CO2 solubility in other compounds such as polymers, for example, carbonyl groups and long alkyl chains with branching or ether linkages. Results show that ionic liquids containing increased fluoroalkyl chains on either the cation or anion do improve CO2 solubility when compared to less fluorinated ionic liquids previously studied. It was also found that it was possible to obtain similar, high levels of CO2 solubility in nonfluorous ionic liquids. In agreement with our previous results, we found that the anion frequently plays a key role in determining CO2 solubility in ionic liquids.

Introduction

The phase behavior of CO2 with ionic liquids (ILs) is important for the development of several potential IL applications. Since we first showed that CO2 was soluble in ILs and could be used to extract organic solutes,1-3 IL/CO2 systems have been demonstrated for various catalytic reactions.4-17 In some cases, CO2 is used to extract the product from the IL, leaving behind the catalyst and the IL for reuse. Others have adopted IL/CO2 continuous flow systems, where CO2 is used to deliver the substrates to the IL/catalyst phase and extract the products.

Recently we also confirmed that CO2 could increase the solubility of gases normally not very soluble on their own in ILs.18 We found that the solubility of CH4 and O2 increased in ILs even at low partial pressures of CO2. Others studying the enantioselective hydrogenation of imines using a cationic iridium catalyst in an IL/CO2 biphasic system found that the catalyst performance was increased dramatically in the IL when CO2pressure was added.19 High-pressure NMR showed that the addition of CO2 increased the amount of H2 dissolved in the IL. The authors believed that the catalyst, which was known to be sensitive to H2 availability, could operate more effectively due to an increase in H2 concentration in the IL along with a decrease in viscosity of the IL with added CO2. Another

* Corresponding author. Telephone: (574) 631-5847. Fax: (574) 6318366. E-mail: [email protected] † Current address: EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K. E-mail: [email protected]

‡ Current address: Intermediates R&D, Invista S.a.r.l, Sabine River Laboratory B568, P. O. Box 1003, Orange, TX 77631-1003.

potential application of ILs that several research groups along with our group have examined is the separation of CO2 from
gas mixtures.20-22
To design and optimize processes using ILs, it is essential
that we understand the important factors that determine CO2
solubility. It is possible to design many potential ILs; therefore, the systematic study of different cation/anion combinations will allow us to decipher the key properties in making CO2-philic ILs.

To date we have studied the solubility of CO2 in a range ofcommonly used ILs.2,23-29 We found that there were a number of factors that controlled the CO2 solubility in the chosen ILs, both cation and anion related.26 Of the ionic liquids studied, the anion played the biggest role in CO2 solubility, a fact that was supported by a recent X-ray diffraction study by Kanakubo et al.30 Anions that contain fluoroalkyl groups were found to have some of the highest CO2 solubilities, and as the quantity of fluoroalkyl groups increased, the CO2 solubility also increased. The previous studies showed CO2 solubility for 1-butyl3-methylimidazolium ([bmim]+) based ILs at 60 °C increased in the order nitrate ([NO3]-) < tetrafluoroborate ([BF4]-) < dicyanamide ([DCA]-) ? hexafluorophosphate ([PF6]-) ? trifluoromethanesulfonate ([TfO]-) < bis[(trifluoromethyl)sulfonyl]imide ([Tf2N]-) < tris(trifluoromethylsulfonyl)methide ([methide]-). For the cations, there were two factors thatinfluenced the CO2 solubility. The biggest effect was seen in increasing alkyl chain length on the cation. For a given cation, the CO2 solubility increased with increasing chain length, and these results are consistent with those reported elsewhere.31-35


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