Improving Carbon Dioxide Solubility in Ionic Liquids

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J. Phys. Chem. B 2007, 111, 9001-9009

9001

Improving Carbon Dioxide Solubility in Ionic Liquids
Mark J. Muldoon,† Sudhir N. V. K. Aki,‡ Jessica L. Anderson, JaNeille K. Dixon, and Joan F. Brennecke*
Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556
ReceiVed: March 8, 2007; In Final Form: May 4, 2007

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 CO2

pressure 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: jfb@nd.edu. † Current address: EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K. E-mail: mjm30@st-andrews.ac.uk.

‡ 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...
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