High pressure CO₂ absorption studies on imidazolium-based ionic liquids: Experimental and simulation approaches

Abstract

A combined experimental–computational study on the CO₂ absorption on 1-butyl-3-methylimidazolium hexafluophosphate, 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, and 1-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide ionic liquids is reported. The reported results allowed to infer a detailed nanoscopic vision of the absorption phenomena as a function of pressure and temperature. Absorption isotherms were measured at 318 and 338 K for pressures up to 20 MPa for ultrapure samples using a state-of-the-art magnetic suspension densimeter, for which measurement procedures are developed. A remarkable swelling effect upon CO₂ absorption was observed for pressures higher than 10 MPa, which was corrected using a method based on experimental volumetric data. The experimental data reported in this work are in good agreement with available literature isotherms. Soave–Redlich–Kwong and Peng–Robinson equations of state coupled with bi-parametric van der Waals mixing rule were used for successful correlations of experimental high pressure absorption data. Molecular dynamics results allowed to infer structural, energetic and dynamic properties of the studied CO₂ + ionic liquids mixed fluids, showing the relevant role of the strength of anion–cation interactions on fluid volumetric properties and CO₂ absorption.

Introduction

Ionic liquids (ILs) are salts existing in the liquid state at room temperature, consisting of large asymmetric organic cations and anions, which are resulted in low lattice energy and moderate coulombic forces [1], [2]. The variability of cations and anions leading to ILs allows tailor-made ILs with desired physical and chemical properties of interest [3], [4]. Most major characteristics of ILs are their low volatility and high ionic conductivity together with their good thermal and electrochemical stability [5], [6], [7], for which they have attracted academic and industrial interest [8], [9], [10], [11], [12] in applications such as solvents and catalysts for chemical reactions, and flue gas separation agents for chemical and other industrial processes [13], [14].

Compared to traditional organic solvents, the non-volatility nature of ILs makes them benign solvents for gas treatment and separation processes [15]. ILs are reported for high solubilities of water and CO₂ compared to conventional organic solvents [16], [17], [18], [19]. Due to solubility differences between CO₂ and other gases such as N₂, O₂, and CH₄, ILs have started to gain great academic and industrial interest for separation of CO₂ from flue gas or natural gas at pre or post combustion process streams [14], [20], [21]. Literature contains a large number of articles on the solubility of CO₂ in ILs [18], [22], [23], [24], [25], [26], [27], [28], [29], [30], which show the interest, both in industry and academia, on this ILs application. Experimental studies on determining CO₂ solubilities in selected ILs have been widely investigated in recent years [27], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. In particular, classic imidazolium-based ILs have been studied comprehensively for their CO₂ capture performances [41], [42], [43]. Blanchard et al. [44] reported for the very first time the properties of imidazolium-based ionic liquids as CO₂ absorbents, showing the null solubility of ionic liquids in CO₂ in comparison with the remarkable solubility of CO₂ in the ionic liquid phase. Zhang and Chan [45] reviewed the available literature on the use of imidazolium-based ionic liquids in sustainable chemistry, including CO₂ capture. Huang et al. [46] analyzed the CO₂ absorption mechanism in imidazolium-based ionic liquids from molecular dynamics simulations results through the spatial rearrangement of available free volume to accommodate CO₂ molecules but without remarkable changes in anion–cation interactions. The CO₂ absorption is strongly dependent on anion type, and in a minor extension in alkyl chain lengths in imidazolium rings [46]. Brennecke and coworkers [47] analyzed the anion effect on CO₂ solubility, showing increasing absorption with anion fluorination. Nevertheless, the available results show low CO₂ capture ability for imidazolium-based ILs, i.e. up to 3.5 mol% at ambient temperature and pressure [45], which is clearly insufficient for industrial purposes, especially for flue gases treatment. Therefore, new efforts have been developed in the literature to improve CO₂ capturing ability using imidazolium-based ionic liquids. Several authors have proposed the use of amine-functionalized imidazolium-based ionic liquids, which leads to chemical absorption, and thus showing remarkable increase in CO₂ solubility but also important problems such as their very high viscosity and high energy penalties for the desorption processes [48], [49], [50], [51].

Experimental studies have showed that the CO₂ solubility in ILs increases with pressure and decreases with temperature [39], [52]. Nevertheless, most of the data available are at atmospheric or low-pressure conditions (e.g. lower than 10 MPa) [14]. The main reason for this scarcity in high pressure absorption data rises from the technical difficulties to carry out these measurements with acceptable uncertainties. High pressure data on CO₂ capture is also important to test possible sorbent materials on both pre and post combustion CO₂ capture purposes for industrial scale applications. Pre-combustion CO₂ conditions are set at higher pressures, e.g. a water–gas shift reaction produces a mixture of H₂ (61.5%) and CO₂ (35.5%) at 30 bar [53]. Pipeline compression of crude natural gas reaches [54] up to pressures in excess of 175 bar while CO₂ transport for sequestration demands [55] pressures of 150 bar. Liquefied natural gas (LNG) stack temperatures have to be limited to a maximum of 180 °C by regulations [56]. Ideally, a sorbent should tolerate all these conditions (0–175 bar and 40–180 °C), while retaining its CO₂ capacity. Most gas adsorption and absorption (sorption) experiments are measured with a volumetric, a gravimetric or combined volumetric/gravimetric equipment. There are also some other experimental techniques, which do not get serious interest due to limitations, accuracy and repeatability problems [57]. For measurements at conditions similar to process conditions the volumetric method has some limitations, because only atmospheric conditions are experimented. Recently, Rubotherm^® state-of-the-art magnetic suspension sorption equipment has been used for ILs and CO₂ solubility measurements [24], [58], [59], [60]. With this apparatus, measurements can be performed from 253 to 523 K for pressures up to 35 MPa, with suitable accuracy in the whole pressure–temperature ranges.

An important problem rising in the study of thermophysical behavior of ILs stands on the purity of the used samples, which has a strong effect on ILs properties [13]. Many available studies on CO₂ absorption in ILs were carried out with samples of not clarified purity. Moreover, Freire et al. [61] reported the hydrolysis of the common hexafluorophosphate and tetrafluoroborate anions, which were used in many CO₂ absorption studies paired with imidazolium-based cations. Therefore the use of ultrapure and properly characterized ILs samples is required to obtain reliable results [62].

Molecular dynamics simulations is a powerful tool to study nanoscopic behavior of complex systems such as those containing ILs + CO₂. Deschamps et al. [63] studied the CO₂ + [bmim][PF₆] system, pointing to CO₂ molecules non-interacting remarkably with C2 position in imidazolium ring and placed preferentially near the anion. Cadena et al. [29] analyzed the CO₂ + [bmim][PF₆] system showing the small volume expansion and the negligible changes in the ionic liquid structure upon CO₂ absorption. Huang et al. [46] studied the CO₂ + [bmim][PF₆] system analyzing the molecular level reasons of the low values for CO₂ partial molar volume, and showing that CO₂ molecules occupy cavities rising from small angular rearrangements of the anions without remarkable expansion. Bhargava et al. [64] studied the CO₂ + [bmim][PF₆] system showing a remarkable volume expansion and specific interaction between CO₂ molecules and [PF₆]⁻ anions; these authors also reported that the CO₂ solvation in [bmim][PF₆] is primary controlled by the anion [65]. Kerle et al. [66] simulated the temperature dependence of CO₂ solubility in [emim][Tf₂N] and [bmim][Tf₂N], showing that the solvation free energy of CO₂ is almost insensitive to the alkyl chain length on the imidazolium cation. Shim et al. [67] studied the solvation structure of CO₂ in [bmim][PF₆] showing the appearance of preferential solvation for a diatomic probe. Yue et al. [68] carried out molecular dynamics studies of several CO₂ + imidazolium-based ionic liquids including [bmim][PF₆] and [emim][Tf₂N], showing that CO₂ molecules overlap with [PF₆]⁻ anions around the imidazolium cation, whereas in the case of [Tf₂N]⁻ systems no overlapping is inferred. Ghobadi et al. [69] carried out NPT Monte Carlo simulations for the CO₂ absorption in [bmim][PF₆] and [bmim][Tf₂N] ILs; the authors proposed the use of a new solubility index obtained from calculated intermolecular interaction energies to analyze gas solubility. Zhang et al. [15] published a review work recently on the use of ionic liquids for CO₂ capturing purposes in which available results from molecular modeling are also analyzed.

The enormous amount of possible anion–cation combinations leading to ILs [70] make necessary to carry out systematic studies on their properties and molecular level structure, which is especially relevant to find ILs with suitable CO₂ properties for industrial purposes between the plethora of possible candidates. This objective can be fulfilled using a combined experimental–computational approach, which on one side would provide with the required CO₂ absorption data, and on the other side would lead to the nanoscopic vision of the absorption process, and thus, allowing to obtain a vision of the relationships between sorption abilities and fluids’ structuring and properties. Therefore, the results of the combined high pressure CO₂ absorption and molecular dynamics studies on three selected imidazolium-based ILs: butyl-3-methylimidazolium hexafluophosphate, [bmim][PF₆], 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [emim][Tf₂N], and 1-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [bmim][Tf₂N], are reported in this work. The objectives of the work were: (i) to extend available information of CO₂ absorption data for imidazolium-based ionic liquids to the high pressure region using reliable and accurate state-of-the-art apparatus, (ii) to use samples synthesized in our laboratories, being ultrapure and well-characterized, and (iii) to infer molecular level information from simulation results that combined with the experimental measurements will lead to valuable information on the absorption process.