Selection of Ionic Liquids for Enhancing the Gas Solubility of Volatile Organic Compounds

A systematic thermodynamic analysis has been carried out for selecting cations and anions to enhance the absorption of volatile organic compounds (VOCs) at low concentration in gaseous streams by ionic liquids (ILs), using COSMO-RS methodology. The predictability of computational procedure was validated by comparing experimental and COSMO-RS calculated Henry’s law constant data over a sample of 125 gaseous solute–IL systems. For more than 2400 solute–IL mixtures evaluated, including 9 solutes and 270 ILs, it was found that the lower the activity coefficient at infinite dilution (γ^∞) of solutes in the ILs, the more the exothermic excess enthalpy (<i>H</i>^E) of the equimolar IL–solute mixtures. Then, the solubility of a representative sample of VOC solutes, with very different chemical nature, was screened in a wide number of ILs using COSMO-RS methodology by means of γ^∞ and <i>H</i>^E parameters, establishing criteria to select the IL structures that promote favorable solute–solvent intermolecular interactions. As a result of this analysis, an attempt of classification of VOCs respect to their potential solubility in ILs was proposed, providing insights to rationally select the cationic and anionic species for a possible development of absorption treatments of VOC pollutants based on IL systems

Solubility and Diffusivity of CO₂ in [hxmim][NTf₂], [omim][NTf₂], and [dcmim][NTf₂] at <i>T</i> = (298.15, 308.15, and 323.15) K and Pressures up to 20 bar

Solubilities and diffusion coefficients of CO₂ absorption in the ionic liquids (ILs) [hxmim]­[NTf₂], [omim]­[NTf₂], and [dcmim]­[NTf₂] at temperatures of (298.15, 308.15, and 323.15) K and pressures up to 20 bar were obtained by thermogravimetric measurements using a high pressure sorption analyzer with magnetic suspension balance operating in dynamic mode. The effect of the length of the alkyl side chain of the imidazolium cation and the operating conditions on the thermodynamics and kinetics of the CO₂ absorption process in ILs were evaluated. Absorption data confirmed that the CO₂ solubility in ILs increases with increasing length of the alkyl side chain of the cation and with decreasing temperatures and increasing pressures. The diffusion coefficients of CO₂, calculated by applying a mass diffusion model, decrease with increasing lengths of the alkyl side chain of the cation and increase with both temperature and pressure of absorption. These results illustrate the importance of considering both thermodynamic and kinetic aspects in the selection of an IL as absorbent and the operating conditions for developing absorption processes based on ILs. In addition, the empirical correlation of Wilke–Chang was successfully applied as an alternative to estimate the diffusion coefficients of the systems

Interactions of Ionic Liquids and Acetone: Thermodynamic Properties, Quantum-Chemical Calculations, and NMR Analysis

The interactions between ionic liquids (ILs) and acetone have been studied to obtain a further understanding of the behavior of their mixtures, which generally give place to an exothermic process, mutual miscibility, and negative deviation of Raoult’s law. COSMO-RS was used as a suitable computational method to systematically analyze the excess enthalpy of IL–acetone systems (>300), in terms of the intermolecular interactions contributing to the mixture behavior. Spectroscopic and COSMO-RS results indicated that acetone, as a polar compound with strong hydrogen bond acceptor character, in most cases, establishes favorable hydrogen bonding with ILs. This interaction is strengthened by the presence of an acidic cation and an anion with dispersed charge and non-HB acceptor character in the IL. COSMO-RS predictions indicated that gas–liquid and vapor–liquid equilibrium data for IL–acetone systems can be finely tuned by the IL selection, that is, acting on the intermolecular interactions between the molecular and ionic species in the liquid phase. NMR measurements for IL–acetone mixtures at different concentrations were also carried out. Quantum-chemical calculations by using molecular clusters of acetone and IL species were finally performed. These results provided additional evidence of the main role played by hydrogen bonding in the behavior of systems containing ILs and HB acceptor compounds, such as acetone

On the Kinetics of Ionic Liquid Adsorption onto Activated Carbons from Aqueous Solution

Adsorption with activated carbons (ACs) has been recently proposed as a thermodynamically favored treatment to remove and/or recover ionic liquids (ILs) from aqueous streams. In this work, a kinetic analysis of the adsorption of a hydrophobic IL (1-methyl-3-octylimidazolium hexafluorophosphate, OmimPF₆) by commercial ACs was performed. The results indicated that adsorption kinetics is remarkably slower for the IL than for phenol, used as a reference solute. Then, the effects of the main operating conditions (stirring, AC particle size, temperature, and initial concentration of IL) on the adsorption rate were investigated. For the purpose of developing criteria to improve the kinetics of IL adsorption with ACs, different empirical and phenomenological kinetic models were applied to describe the experimental adsorption data. The kinetic analysis indicated that the mechanism of IL adsorption onto ACs is mainly controlled by the mass transfer into the pores. Therefore, the selection of adequate particle size of the adsorbent plays a major role in the development of feasible IL adsorption. Increasing the temperature led to significantly faster adsorption, which was found to be of interest for removing and/or recovering IL from aqueous solution in spite of the associated decrease of equilibrium capacity

Diffusion Coefficients of CO₂ in Ionic Liquids Estimated by Gravimetry

The estimation of diffusion coefficients of CO₂ in ionic liquids by gravimetry is analyzed with the aim of establishing a measurement method that provides consistent values of diffusivity. Absorption kinetic curves of CO₂ in three common ILs were measured at different temperatures (293–323 K) and pressures (1–20 atm) by using a high pressure sorption analyzer with magnetic suspension balance operating in dynamic mode. A mass diffusion model widely used in the literature was applied to estimate effective diffusion coefficients for CO₂–IL systems from time-dependent absorption data. The measuring conditions (IL mass, dimension of sample container, gas flow) in the dynamic absorption experiments were modified to verify the assumptions of the diffusion model. Obtained results were compared to available data. In addition, the suitability of theoretical methods commonly used for estimating diffusion coefficients of CO₂ in ILs was analyzed, in order to select a computational approach for preliminary selection of ILs with favorable transport properties for CO₂ capture

Anion Effects on Kinetics and Thermodynamics of CO₂ Absorption in Ionic Liquids

A thermogravimetric technique based on a magnetic suspension balance operating in dynamic mode was used to study the thermodynamics (in terms of solubility and Henry’s law constants) and kinetics (i.e., diffusion coefficients) of CO₂ in the ionic liquids [bmim]­[PF₆], [bmim]­[NTf₂], and [bmim]­[FAP] at temperatures of 298.15, 308.15, and 323.15 K and pressures up to 20 bar. The experimental technique employed was shown to be a fast, accurate, and low-solvent-consuming method to evaluate the suitability of the ionic liquids (ILs) to be used as CO₂ absorbents. Thermodynamic results confirmed that the solubility of CO₂ in the ILs followed the order [bmim]­[FAP] > [bmim]­[NTf₂] > [bmim]­[PF₆], increasing with decreasing temperatures and increasing pressures. Kinetic data showed that the diffusion coefficients of CO₂ in the ILs followed a different order, [bmim]­[NTf₂] > [bmim]­[FAP] > [bmim]­[PF₆], increasing with increasing temperatures and pressures. These results evidenced the different influence of the IL structure and operating conditions on the solubility and absorption rate of CO₂, illustrating the importance of considering both thermodynamic and kinetic aspects to select adequate ILs for CO₂ absorption. On the other hand, the empirical Wilke–Chang correlation was successfully applied to estimate the diffusion coefficients of the systems, with results indicating the suitability of this approach to foresee the kinetic performance of ILs to absorb CO₂. The research methodology proposed herein might be helpful in the selection of efficient absorption solvents based on ILs for postcombustion CO₂ capture

Ionic Liquid MixturesAn Analysis of Their Mutual Miscibility

The use of ionic liquid mixtures (IL–IL mixtures) is being investigated for fine solvent properties tuning of the IL-based systems. The scarce available studies, however, evidence a wide variety of mixing behaviors (from almost ideal to strongly nonideal), depending on both the structure of the IL components and the property considered. In fact, the adequate selection of the cations and anions involved in IL–IL mixtures may ensure the absence or presence of two immiscible liquid phases. In this work, a systematic computational study of the mixing behavior of IL–IL systems is developed by means of COSMO-RS methodology. Liquid–liquid equilibrium (LLE) and excess enthalpy (<i>H</i>^E) data of more than 200 binary IL–IL mixtures (including imidazolium-, pyridinium-, pyrrolidinium-, ammonium-, and phosphonium-based ILs) are calculated at different temperatures, comparing to literature data when available. The role of the interactions between unlike cations and anions on the mutual miscibility/immiscibility of IL–IL mixtures was analyzed. On the basis of proposed guidelines, a new class of immiscible IL–IL mixtures was reported, which only is formed by imidazolium-based compounds

Excess Enthalpy of Monoethanolamine + Ionic Liquid Mixtures: How Good are COSMO-RS Predictions?

Mixtures of ionic liquids (ILs) and molecular amines have been suggested for CO₂ capture applications. The basic idea is to replace water, which volatilizes in the amine regeneration step and increases the parasitic energy load, with a nonvolatile ionic liquid solvent. To fully understand the thermodynamics of these systems, here experimental excess enthalpies for binary mixtures of monoethanolamine (MEA) and two ILs: 1-hexyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide, [hmim]­[NTf₂], and 1-(2-hydroxyethyl)-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide, [OHemim]­[NTf₂], were obtained by calorimetry, using a Setaram C80 calorimeter, over the whole range of compositions at 313.15 K. Since it is the temperature derivative of the Gibbs energy, enthalpy is a sensitive measure of intermolecular interactions. MEA + [hmim]­[NTf₂] is endothermic and MEA + [OHemim]­[NTf₂] is exothermic. The reliability of COSMO-RS to predict the excess enthalpy of the (MEA+IL) systems was tested based on the implementation of two different molecular models to define the structure of the IL: the IL as separate cation and anion [C+A] and the IL as a bonded single specie [CA]. Quantum-chemical calculations were performed to gain additional insight into the intermolecular interactions between the components of the mixture. For MEA + [hmim]­[NTf₂] both the [C+A] and [CA] models predict endothermic behavior, but the [CA] model is in better agreement with the experimental results. For MEA + [OHemim]­[NTf₂] the [C+A] model provides the best match to the experimental exothermic results. However, what is really surprising is that two different conformations of the cation–anion pair with nearly identical energies in the [CA] model result in completely different (exothermic vs endothermic) predictions of the excess enthalpy. Nonetheless, the results do show that the influence of the structure of the IL on the thermodynamic behavior of the mixture (endothermic vs exothermic) can be attributed to hydrogen bonding between the cation and the MEA molecule. However, this study highlights the importance of carefully selecting the molecular model and conformation in order to obtain even qualitatively correct predictions with COSMO-RS. The fact that even very slightly different conformations of the IL can drastically change the thermodynamic estimations using COSMO-RS is of significant concern. Overall, we believe the present work provides a better understanding of the behavior of mixtures involving amines and ILs, which is an important aspect to consider when evaluating the use of such solvent mixtures in CO₂ capture technologies

Statistical Refinement and Fitting of Experimental Viscosity-to-Temperature Data in Ionic Liquids

The viscosity-to-temperature experimental data available in the open literature sources for 134 ionic liquids (ILs) was refined using a statistical procedure based on the confidence bands formalism. 1860 data points of 143 different references, among more than 2600 raw data points found in literature, were processed. As a result, 21% of the data set was rejected. The refined viscosity-to-temperature experimental data were successfully fitted to the η = <i>f</i>(<i>T</i>) Arrhenius-type equation with an <i>R</i>² correlation coefficient higher than 0.99 in all cases. Parameters <i>A</i> and <i>B</i> of the Arrhenius function for 134 ILs were given for the accurate estimation of viscosity in potential uses and compared to <i>A</i> and<i> B</i> parameters for 134 organic solvents. It was found that the obtained <i>A</i> and <i>B</i> values correlate linearly for the wide sample of 134 ionic liquids. As a consequence, it is concluded that ionic liquids having high viscosities at relatively low temperatures also exhibit an abrupt decay of the viscosity with the temperature. This <i>A</i>–<i>B</i> Arrhenius’ parameter relationship was also found in organic solvents, obtaining a regression line with a nearly identical slope but different intercept than that in the case of ILs. It indicated similar temperature dependence in the viscosity of ILs and organic compounds, but a differential higher viscosity of ionic fluids. In addition, it is observed that, for temperatures over 330–373 K, the viscosities of most ILs studied here are moderate, providing a potential range to manage this kind of solvent in practical applications with less transport property limitations

Encapsulation of Ionic Liquids with an Aprotic Heterocyclic Anion (AHA-IL) for CO₂ Capture: Preserving the Favorable Thermodynamics and Enhancing the Kinetics of Absorption

The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl­(tetradecyl)­phosphonium 2-cyanopyrrolide ([P₆₆₆₁₄]­[2-CNPyr]), for CO₂ capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas–liquid equilibrium isotherms of CO₂–AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO₂ solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO₂ chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO₂ absorption rate is mass-transfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO₂ capture. The newly synthesized AHA-ENIL material was evaluated as a CO₂ sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO₂ absorption capacity of the AHA-IL but drastically enhance the CO₂ absorption rate because of the increased gas–liquid surface contact area achieved by solvent encapsulation