Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-Based Ionic Liquids

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• 作者：Matthew S. Shannon ; Jason M. Tedstone ; Scott P. O. Danielsen ; Michelle S. Hindman ; A. Christopher Irvin ; Jason E. Bara
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2012
• 出版时间：April 18, 2012
• 年：2012
• 卷：51
• 期：15
• 页码：5565-5576
• 全文大小：529K
• 年卷期：v.51,no.15(April 18, 2012)
• ISSN：1520-5045

文摘

While molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be shown to be the underlying property driving gas solubility and selecitivity. Previously published observations as to the distinct differences in solubility trends for gases such as CH₄ and N₂ relative to CO₂ in systematically varied ILs can be attributed to positive and negative effects arising from increasing free volume with increasing alkyl chain length. Through the use of COSMOtherm as a powerful and rapid tool to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium ([C_nmim][X]) ILs, a previously unreported, yet speculated, critical underlying relationship between gas solubility in ILs is herein described. These results build upon previous assertions that Regular Solution Theory is applicable to imidazolium-based ILs, which appeared to indicate that a global maximum had already been observed for CO₂ solubility in imidazolium-based ILs. However, the findings of this computational study suggest that the perceived maximum in CO₂ solubility might be exceeded through rational design of ILs. We observe that although Henry鈥檚 constants in ILs are dependent on the inverse of molar volume and free volume, the volume-normalized solubility of CH₄ and N₂ are proportional to free volume, while CO₂ is inversely proportional to the square root of free volume. Our free volume model is compared to experimental data for CO₂/CH₄ and CO₂/N₂ selectivity, and a nearly identical plot of selectivity relative to IL molar volume can be generated from the computational method alone. The overall implication is that large, highly delocalized anions paired with imidazolium cations that have minimally sized alkyl chains may hold the key to achieving greater CO₂ solubility and selectivity in ILs.