Joan F. Brennecke

Green processing using ionic liquids and CO2

Many organic solvents evaporate into the atmosphere with detrimental effects on the environment and human health. But room-temperature ionic liquids, with low viscosity and no measurable vapour pressure, can be used as environmentally benign media for a range of industrially important chemical processes, despite uncertainties about thermal stability and sensitivity to oxygen and water. It is difficult to recover products, however, as extraction with water works only for hydrophilic products, distillation is not suitable for poorly volatile or thermally labile products, and liquid-liquid extraction using organic solvents results in cross-contamination. We find that non-volatile organic compounds can be extracted from ionic liquids using supercritical carbon dioxide, which is widely used to extract large organic compounds with minimal pollution. Carbon dioxide dissolves in the liquid to facilitate extraction, but the ionic liquid does not dissolve in carbon dioxide, so pure product can be recovered.

Equimolar CO2 Absorption by Anion-Functionalized Ionic Liquids

Amino acid ionic liquid trihexyl(tetradecyl)phosphonium methioninate [P(66614)][Met] and prolinate [P(66614)][Pro] absorb CO(2) in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO(2) capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO(2). Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.

Octanol-water partition coefficients of imidazolium-based ionic liquids

Ionic liquids (ILs) are low melting organic salts that are being vigorously investigated as possible replacements for volatile organic solvents. While they cannot contribute to air pollution due to their negligible vapor pressure, they do have significant solubility in water. As a result, this is the most likely medium through which ILs will enter the environment. Therefore, it is important to understand how ILs will influence aquatic ecosystems. A simple thermodynamic measurement that has been extremely useful in estimating effects of chemical pollutants on aquatic environments is the octanol–water partition coefficient (KOW). It is an extremely important quantity because it describes the hydrophobicity or hydrophilicity of a compound and has been correlated with bioaccumulation and toxicity in fish, as well as sorption to soils and sediments. Here we present measurements of the KOW of twelve imidazolium-based ILs at room temperature, using the slow-stirring method. For the butylmethylimidazolium cation, KOW values range from 0.003 to 11.1, depending on the choice of anion. In addition, we find that the KOW values increase with increasing alkyl chain length on the cation and that replacing the acidic hydrogen on the carbon between the two nitrogens in the imidazolium ring with a methyl group has negligible effect on the KOW. However, all of the KOW values measured, even for the most “hydrophobic” imidazolium-based ILs, are less than 15 so these ILs will not accumulate or concentrate in the environment.

Effect of water and temperature on absorption of CO2 by amine-functionalized anion-tethered ionic liquids.

Amine-functionalized anion-tethered ionic liquids (ILs) trihexyl(tetradecyl)phosphonium asparaginate [P(66614)][Asn], glutaminate [P(66614)][Gln], lysinate [P(66614)][Lys], methioninate [P(66614)][Met], prolinate [P(66614)][Pro], taurinate [P(66614)][Tau], and threoninate [P(66614)][Thr] were synthesized and investigated as potential absorbents for CO(2) capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature were determined. Furthermore, the CO(2) absorption isotherms of [P(66614)][Lys], [P(66614)][Tau], [P(66614)][Pro], and [P(66614)][Met] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. The most important result of this study is that the viscosity of [P(66614)][Pro] only increased by a factor of 2 when fully complexed with 1 bar of CO(2) at room temperature. This is in stark contrast to the other chemically reacted ILs investigated here and all other amino acid-based ILs reported in the literature, which dramatically increase in viscosity, typically by 2 orders of magnitude, when complexed with CO(2). The unique behavior of [P(66614)][Pro] is likely due to its ring structure, which limits the number and availability of hydrogen atoms that can participate in a hydrogen bonding network. We found that water can be used to further reduce the viscosity of the CO(2)-complexed IL, while only slightly decreasing the CO(2) capacity. Finally, from temperature-dependent isotherms, we estimate a heat of absorption of -63 kJ/mol of CO(2) for the 1:1 reaction of CO(2) with [P(66614)][Pro], when we use the two-reaction model.

Thermodynamic properties of the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate from monte Carlo simulations

We report results from the first molecular simulation study of 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6], a widely studied ionic liquid. Monte Carlo simulations are carried out in the isothermal–isobaric (NPT) ensemble to calculate the molar volume, cohesive energy density, isothermal compressibility, cubic expansion coefficient and liquid structure as a function of temperature and pressure. A united atom forcefield is developed using a combination of ab initio calculations and literature parameter values. Calculated molar volumes are within 5% of experimental values, and a reasonable agreement is obtained between calculated and experimental values of the isothermal compressibility and cubic expansion coefficient. PF6 anions are found to preferentially cluster in two favorable regions near the cation.

Interactions of Ionic Liquids and Water

Experimental excess enthalpies of ionic liquid and water mixtures in combination with calculated CHELPG atomic charges were used to investigate the interactions between the species in solution. The excess enthalpies of ionic liquids in water were obtained by calorimetry, using a Setaram C80 calorimeter, including temperatures from (313.15 to 348.15) K and the entire range of composition. The ionic liquids investigated all contain the 1-ethyl-3-methylimidazolium cation except one, which has an added hydroxyl group on the cation (1-(2-hydroxyethyl)-3-methylimidazolium cation). The anions investigated are ethylsulfate, methylsulfate, hydrogensulfate, trifluoromethanesulfonate, methanesulfonate, and trifluoroacetate, and these will demonstrate the effect of systematically varying the substituents on the anion. The CHELPG atomic charges on the cations and anions were calculated using the Gaussian 03 program. The CHELPG atomic charges are consistent with the observed trends in excess enthalpy and provide insight into cation/water, anion/water, and cation/anion interactions.

Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO2 Capture

Ionic liquids (ILs) with aprotic heterocyclic anions, or AHAs, can bind CO2 with reaction enthalpies that are suitable for gas separations and without suffering large viscosity increases. In the present work, we have synthesized ILs bearing an alkyl-phosphonium cation with indazolide, imidazolide, pyrrolide, pyrazolide and triazolide-based anions that span a wide range of predicted reaction enthalpies with CO2. Each AHA-based IL was characterized by NMR spectroscopy and their physical properties (viscosity, glass transition, and thermal decomposition temperature) determined. In addition, the influence of substituent groups on the reaction enthalpy was investigated by measuring the CO2 solubility in each IL at pressures between 0 and 1 bar at 22 °C using a volumetric method. The isotherm-derived enthalpies range between -37 and -54 kJ mol(-1) of CO2, and these values are in good agreement with computed enthalpies of gas-phase IL-CO2 reaction products from molecular electronic structure calculations. The AHA ILs show no substantial increase in viscosity when fully saturated with CO2 at 1 bar. Phase splitting and compositional analysis of one of the IL/H2O and IL/H2O/CO2 systems conclude that protonation of the 2-cyanopyrrolide anion is improbable, and this result was confirmed by the equimolar CO2 absorption in the presence of water. Taking advantage of the tunable binding energy and absence of viscosity increase after the reaction with CO2, AHA ILs are promising candidates for efficient and environmental-friendly absorbents in postcombustion CO2 capture.

Predicting melting points of quaternary ammonium ionic liquids

A melting point at or below ambient temperature is an essential property of ionic liquids being considered as non-volatile replacement solvents. Here we use the Quantitative Structure-Property Relationship (QSPR) method to correlate and predict the melting points of organic salts based on the quaternary ammonium cation. For a set of 75 tetraalkyl-ammonium bromides, a correlation with R2 = 0.790 is created, and for a set of 34 (n-hydroxyalkyl)-trialkyl-ammonium bromides, two correlations are created with R2 = 0.716 and R2 = 0.766. Descriptors used in the correlations are analyzed to determine structural features that lower melting point, and melting points are predicted for salts that incorporate these features.

Thermodynamic and thermophysical properties of the reference ionic liquid: 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (including mixtures). Part 1. Experimental methods and results (IUPAC Technical Report)

This article summarizes the results of IUPAC Project 2002-005-1-100 (Thermodynamics of ionic liquids, ionic liquid mixtures, and the development of standardized systems). The methods used by the various contributors to measure the thermophysical and phase equilibrium properties of the reference sample of the ionic liquid 1-hexyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl]amide and its mixtures are summarized along with the uncertainties estimated by the contributors. Some results not previously published are presented. Properties of the pure ionic liquid included thermal properties (triple-point temperature, glass-transition temperature, enthalpy of fusion, heat capacities of condensed states), volumetric properties, speeds of sound, viscosities, electrolytic conductivities, and relative permittivities. Properties for mixtures included gas solubilities, solute activity coefficients at infinite dilution, liquid-liquid equilibrium temperatures, and excess volumes. The companion article (Part 2) provides a critical evaluation of the data and recommended values with estimated combined expanded uncertainties.

Extraction of alcohols from water with 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Ethanol production in the U. S. has increased 36% between 2006 and 2007 (J. M. Urbanchuk, Contribution of the Ethanol Industry to the Economy of the United States, LECG, LLC, Renewable Fuels Association, 2008) in response to a growing demand for its use as a commercial transportation fuel. 1-Butanol also shows potential as a liquid fuel but both alcohols require high energy consumption in separating them from water. 1-Butanol, in particular, is considered an excellent intermediate for making other chemical compounds from renewable resources, as well as being widely used as a solvent in the pharmaceutical industry. These alcohols can be synthesized from bio-feedstocks by fermentation, which results in low concentrations of the alcohol in water. To separate alcohol from water, conventional distillation is used, which is energetically intensive. The goal of this study is to show that, using an ionic liquid, extraction of the alcohol from water is possible. Through the development of ternary diagrams, separation coefficients are determined. The systems studied are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/ethanol/water, which exhibits Type 1 liquid–liquid equilibrium (LLE) behavior, and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/1-butanol/water, which exhibits Type 2 LLE behavior. Based on the phase diagrams, this ionic liquid (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) can easily separate 1-butanol from water. It can also separate ethanol from water, but only when unreasonably high solvent/feed ratios are used. In addition, we use four excess Gibbs free energy (gE) models (NRTL, eNRTL, UNIQUAC and UNIFAC), with parameters estimated solely using binary data and/or pure component properties, to predict the behavior of the ternary LLE systems. None of the models adequately predicts the Type 1 system, but both UNIQUAC and eNRTL aptly predict the Type 2 system.