Ethane and Ethylene Solubility in an Imidazolium-Based Lipidic Ionic

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Ethane and Ethylene Solubility in an Imidazolium-based Lipidic Ionic Liquid Blane D. Green, Richard A O'Brien, James H. Davis, and Kevin Neal West Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie505071t • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 17, 2015

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Industrial & Engineering Chemistry Research

Ethane and Ethylene Solubility in an Imidazoliumbased Lipidic Ionic Liquid Blane D. Green,† Richard A. O’Brien,‡ James H. Davis, Jr.‡ and Kevin N. West†,* †

Department of Chemical & Biomolecular Engineering, University of South Alabama, Mobile,

Alabama, 36688, United States ‡

Department of Chemistry, University of South Alabama, Mobile, Alabama, 36688, United

States KEYWORDS: ionic liquids, nonpolar gas, ethane, ethylene, solubility

ABSTRACT: The solubilities of ethane and ethylene in the lipidic ionic liquid 1-(Z-octadec-9enyl)-3-methylimidazolium bistriflimide are measured at 298, 313 and 333 K from 0.1 MPa to about 2 MPa and are correlated and well reproduced with the Krichevsky-Kasarnovsky equation. Ethane is shown to have slightly higher solubility than ethylene, in agreement with results from other ionic liquids with significant nonpolar structural content, although fit Henry’s constants are the same within experimental error. Additionally, this is the first example of an imidazoliumbased ionic liquid with Henry’s constants for ethane and ethylene that are below 5 MPa at room temperature. This supports the idea that these classes of solvents have “nonpolar-like” solvent properties and may be used as alternatives for reaction and separations processes that currently require volatile, nonpolar solvents, even though these species are ionic in nature. Experimental

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solubility data are compared to COSMO-RS predictions, which give good qualitative predictions of solubility but significantly over-predict the Henry’s constant for both species.


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Introduction In recent years, ionic liquids have attracted considerable industrial and academic interest as alternatives for volatile organic solvents. The combination of vanishingly low vapor pressures and the ability to tune their properties at the molecular-level through structural changes make ionic liquids attractive potential replacements for hazardous organics.

Unlike molecular

solvents, which can volatilize in industrial processes leading to fugitive emissions and are a source of air pollution, the low volatility of ionic liquids makes their emission potential very low. While the strong coulombic interactions of the cations and anions are responsible for the low vapor pressures, they also result in many ionic liquids being polar to moderately polar in solvent character, having high solubilities for salts and polar organic compounds, but low solubilities for nonpolar species such as alkanes. These beneficial characteristics and solvent properties are exemplified in much of the work that has been done examining infinite dilution activity coefficients of alkanes in ionic liquids.1-10 Our group has recently described a class of low melting point ionic liquids designed to have increased solubilities for nonpolar species.

These ionic liquids are comprised of a central

imidazolium cation appended with a long chain (16-20 carbons), added to impart nonpolar character. However, it is well known that as the alkyl chain length is increased past about 9 carbon atoms on a 1-alkyl-3-methylimidzolium salt, the melting point begins to increase with chain length11-14 as the alkyl chain becomes the dominant structural feature of the ion. Taking a cue from nature, where long alkyl chains are required to create lipid bilayers, but high fluidity must still be maintained, we synthesized a series of long chain imidazolium salts with unsaturations (double bonds) near the midpoint of the alkyl chain.15 This resulted in species with significantly lower melting points than their saturated analogues, and well below room


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temperature.

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For the lipidic ionic liquid studied in this work, 1-(Z-octadec-9-enyl)-3-

methylimidazolium bistriflimide, the presence of the cis 9,10 double bond results in a melting point 70 °C lower than its saturated counterpart. The Z-octadec-9-enyl chain is equivalent to an oleyl substituent in the nomenclature of fatty acid chemistry, and is abbreviated hereafter as [oleyl-mim][Tf2N]; the salt is shown along with several of its properties in Figure 1. Like natural lipids, the melting point depression is induced through the inclusion of asymmetry (unsaturations) in the long alkyl chain, disrupting packing efficiency in the solid phase. As these ionic liquids were inspired by and synthesized from natural fatty acids (a type of lipid), we have called these species lipidic ionic liquids. A broad set of these compounds were first described at the International Symposium on Ionic Liquids and Life Science in Yokohama, Japan in 2007,16 including ammonium ions and carboxylate anions with lipidic structures. Recently, our group was part of a collaboration that described the first known naturally occurring protic ionic liquid, produced from reaction of venom from two ant species;17 the cation formed from this reaction is very similar to the one studied in this work, including the presence of an oleyl chain. In addition to the original series of unsaturated lipidic ionic liquids, we have demonstrated that including other symmetry breaking moieties, such as cyclopropyl rings18 and chain branches,19 can have a similar effect on melting points. Of particular interest are saturated, long chain appendages with sulfur substituted for a carbon atom in the chain. These thioether appended ionic liquids, synthesized through thiol-ene “click” chemistry, have low melting points which are a function of the position of the sulfur atom in the chain.20 Their facile synthesis and oxidative stability, relative to unsaturations, make them particularly attractive candidates for both solvent and lubrication applications.


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Having described the pure component thermophysical properties of several of the unsaturated species of lipidic ionic liquids,21 we now turn our attention to describing their solvent properties and begin by examining the solubilities of the small, nonpolar gases, ethane and ethylene, in a representative lipidic ionic liquid. Recently, Liu et al. have investigated the solubilities of small hydrocarbons in phosphonium-based bis(2,4,4-trimethylpentyl) phosphinate (TMPP) salts22,

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and their mixtures with lower viscosity ionic liquids24 and found the solubilities in these species to be the highest reported for ionic liquids. Like the lipidic ionic liquid described here, the high solubility is a direct result of the significant nonpolar content of the ions. Particularly interesting was the finding that ethane has a higher solubility than ethylene in the phosphonium TMPP salts at the same temperature and pressure, a reversal of the trend found for most ionic liquids. In more polar ionic liquids, the interaction between the π electrons in ethylene leads to higher solubility than for ethane, however, Liu et al. demonstrated that with enough nonpolar content, the trend can be reversed. In this work we have measured the solubility of ethane and ethylene in [oleyl-mim][Tf2N] at three temperatures (298, 313 and 333 K) from 0.1 MPa to 2 MPa using a gravimetric microbalance and we correlate the data with the Krichevsky-Kasarnovsky equation and compare the results with COSMO-RS predictions. The pure component properties of the three previously studied lipidic ionic liquids were very similar and the “oleyl” appended imidazolium salt was chosen as a representative of the first generation lipidic ionic liquids for study here because the oleyl chain is among the most common in nature, and it is synthetically accessible and less prone to oxidation than the “linoleyl” chain, which contains two double bonds. One motivation for selecting ethane and ethylene as solubility probes in this work is their industrial relevance. Steam cracking of ethane to produce olefins is the single most energy


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intensive operation in the chemical process industry with ~8% of total industry energy consumption and ~1-2% of overall industry energy consumption attributable to separations alone.25 While a comprehensive evaluation of these types of ionic liquids as solvents for nonpolar solutes will involve a larger survey of their phase equilibria, and evaluation of their potential as separations agents for nonpolar gas mixtures will require examining the effect of varying the chain length and its constitution, this work represents an initial step towards a broader description of the solvent properties of these species.

Experimental Section Materials The ionic liquid, [oleyl-mim][Tf2N], was synthesized by methods previously described15 and verified to have ≥99% purity via NMR (see SI). Samples were placed in high vacuum (

Ethane and Ethylene Solubility in an Imidazolium-Based Lipidic Ionic

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