Theoretical and Experimental Studies of CO2 and H2 Separation Using the 1-Ethyl-3-methylimidazolium Acetate ([emim][CH3COO]) Ionic Liquid

New Functionalized Di-substituent Imidazolium Ionic Liquids as Superior Faster Absorbents for Carbon Dioxide Gas

Functionalization of room temperature liquids based on disubstituted imidazolium ionic liquids represents a promising avenue for tailoring their tunable physicochemical properties and expanding their potential application as green solvents to capture carbon dioxide as a greenhouse gas. In this work, new hydroxyl functionalized imidazolium ionic liquids were prepared from condensation of ethanolamine with glyoxal and formalin in the presence of acetic acid as catalyst. The chemical modification of the hydroxyl groups with epichlorohydrine added new hydroxylpropanoxychloride groups on the imidazolium cation that were quaternized with N-methylimidazolium chloride to produce new imidazolium acetate ionic liquids. The chemical structures, thermal stability, and thermal characteristics of the prepared imidazolium ionic liquids were evaluated. The incorporation of functionalized 1-chloro-2-hydroxypropanoxy and N-methylimidazolium chloride groups into the chemical structure of the imidazolium cations improved the thermal properties of the prepared ionic liquids. The application of the prepared ionic liquids as pure or mixed solvents with saline water to capture CO2 under atmospheric and 55.2 bar pressures was evaluated at room temperature. The data indicate that the prepared ionic liquids have superior CO2 adsorption/desorption rate in short time during 30 and 15 min and that their CO2 capture efficiency increased from 6.2 to 16.8 molCO2/kgIIL and from 9.1 to 20.0 molCO2/kgIIL at atmospheric and 55.2 bar pressures, respectively.

Size Matters: A Computational Study of Hydrogen Absorption in Ionic Liquids

We combined both density functional theory and classical molecular dynamics simulations to investigate the molecular mechanisms governing hydrogen solvation in a total of 12 ionic liquids. Overall, the analysis of the structural properties under high temperature and pressure conditions revealed weak interactions between hydrogen and the ionic liquids, with a slight preference of this gas to be placed at the apolar domains. Interestingly, those ionic liquids comprising nitrate anions allow the accommodation of hydrogen molecules also in the polar areas. The study of the hydrogen velocity autocorrelation functions supports this observation. In addition, the structure of all of the tested ionic liquids was almost insensitive to the addition of hydrogen, so the available free volume and cavity formation are presumably the most important factors affecting solubility.

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

A brief account is given of highlights of our computational efforts - often in collaboration with experimental groups - to understand spectroscopic and chemical properties of ionic liquids (ILs). Molecular dynamics, including their inhomogeneous character, responsible for key spectral features observed in dielectric absorption, infra-red (IR) and fluorescence correlation spectroscopy (FCS) measurements are elucidated. Mechanisms of chemical processes involving imidazolium-based ILs are illustrated for CO2 capture and related reactions, transesterification of cellulose, and Au nanocluster-catalyzed Suzuki cross-coupling reaction with attention paid to differing roles of IL ions. A comparison with experiments is also made.

Structure-Property Relationships of CO2 Absorbing Core-Shell Microparticles with Encapsulated Ionic Liquid

The demand for new ionic liquid (IL)-based systems to selectively sequester carbon dioxide from gas mixtures has prompted the development of individual components involving the tailored design of IL themselves or solid-supported materials that provide excellent gas permeability of the overall material as well as the ability to incorporate large amounts of ionic liquid. In this work, novel IL-encapsulated microparticles comprising a cross-linked copolymer shell of β-myrcene and styrene and a hydrophilic core of the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) are proposed as viable materials for CO2 capture. Water-in-oil (w/o) emulsion polymerization of different mass ratios of β-myrcene to styrene (i.e. 100/0, 70/30, 50/50, 0/100) yielded IL-encapsulated microparticles, where the encapsulation efficiency of [EMIM][DCA] was dependent on the copolymer shell composition. Thermal analysis using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) revealed that both thermal stability and glass transition temperatures depend on the mass ratio of β-myrcene to styrene. Images from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microparticle shell morphology as well as measure the particle size perimeter. Particle sizes were found to be between 5 and 44 μm. CO2 sorption experiments were conducted gravimetrically using TGA instrumentation. Interestingly, a trade-off between CO2 absorption capacity and ionic liquid encapsulation was observed. While increasing the β-myrcene content within the microparticle shell increases the amount of encapsulated [EMIM][DCA], the observed CO2 absorption capacity did not increase as expected due to reduced porosity compared to microparticles with higher styrene content in the microparticle shell. [EMIM][DCA] microcapsules with a 50/50 weight ratio of β-myrcene/styrene showed the best synergistic effect between spherical particle diameter (32.2 μm), pore size (0.75 μm), and high CO2 sorption capacity of ∼0.5 mmol CO2/g sample within a short absorption period of 20 min. Therefore, core-shell microcapsules composed of β-myrcene and styrene are envisioned as a promising material for CO2 sequestration applications.

Fast and non-destructive determination of water content in ionic liquids at varying temperatures by Raman spectroscopy and multivariate regression analysis

Imidazolium acetate ionic liquids (ILs) have been utilized as promising solvents in many applications that involve varying water content and temperature. These experimental variables affect the anion-cation intermolecular interactions, which in turn influence the performance of the ILs in these applications. This paper shows Raman spectroscopy can be used as an operando method to measure water content in IL solvents when simultaneous temperature changes may occur. The Raman spectra of 1-alkyl-3-methylimidazolium acetate ILs (alkyl chain length n = 2, 4, 6, 8) with varying water content (from 0.028 to 0.899 water mole fraction) and temperature (from 78.1 K to 423.1 K) were measured. Increasing the water content or decreasing the temperature of the tested ILs weakens the anion-cation intermolecular interactions. The water content of these ILs can be quantified even in conditions when the temperature is changing using Raman spectroscopy combined with multivariate regression analysis, including principal component regression (PCR), partial-least-squares regression (PLSR), and artificial neural networks (ANNs). The ANN model combined with partial-least-squares (PLS) achieved the highest prediction accuracy of water content in ILs at varying temperatures (RMSECV = 0.017, R²_CV = 99.1%, RMSEP = 0.019, R²_P = 98.8%, RPD = 8.93). Raman spectroscopy provides a potential fast non-destructive operando method to monitor the water content of ILs even in applications when the temperature may be simultaneously altered; this information can lead to the optimized use of these ILs in many applications.

Computational Screening of Physical Solvents for CO2 Pre-combustion Capture

A computational scheme was used to screen physical solvents for CO₂ pre-combustion capture by integrating the commercial NIST database, an in-house computational database, chem-informatics, and molecular modeling. A commercially available screened hydrophobic solvent, diethyl sebacate, was identified from the screening with favorable physical properties and promising absorption performance. The promising performance to use diethyl sebacate in CO₂ pre-combustion capture has also been confirmed from experiments. Water loading in diethyl sebacate is very low, and therefore, water is kept with H₂ in the gas stream. The favorable CO₂ interaction with diethyl sebacate and the intermediate solvent free volume fraction leads to both high CO₂ solubility and high CO₂/H₂ solubility selectivity in diethyl sebacate. An in-house NETL computational database was built to characterize CO₂, H₂, N₂, and H₂O interactions with 202 different chemical functional groups. It was found that 13% of the functional groups belong to the strong interaction category with the CO₂ interaction energy between -15 and -21 kJ/mol; 62% of the functional groups interact intermediately with CO₂ (-8 to -15 kJ/mol). The remaining 25% of functional groups interact weakly with CO₂ (below -8 kJ/mol). In addition, calculations show that CO₂ interactions with the functional groups are stronger than N₂ and H₂ interactions but are weaker than H₂O interactions. The CO₂ and H₂O interactions with the same functional groups exhibit a very strong linear positive correlation coefficient of 0.92. The relationship between CO₂ and H₂ gas solubilities and solvent fractional free volume (FFV) has been systematically studied for seven solvents at 298.2 K. A skewed bell-shaped relation was obtained between CO₂ solubility and solvent FFV. When an organic compound has a density approximately 10% lower than its density at 298.2 K and 1 bar, it exhibits the highest CO₂ loading at that specific solvent density and FFV. Note that the solvent densities were varied using simulations, which are difficult to be obtained from the experiment. In contrast, H₂ solubility results exhibit an almost perfect positive linear correlation with the solvent FFV. The theoretical maximum and minimum physical CO₂ solubilities in any organic compound at 298.2 K were estimated to be 11 and 0.4 mol/MPa L, respectively. An examination of 182 experimental CO₂ physical solubility data and 29 simulated CO₂ physical solubilities shows that all the CO₂ physical solubility data are within the maximum and minimum with only a few exceptions. Finally, simulations suggest that in order to develop physical solvents with both high CO₂ solubility and high CO₂/H₂ solubility selectivity, the solvents should contain functional groups which are available to interact strongly with CO₂ while minimizing FFV.

Fast Track to Acetate-Based Ionic Liquids: Preparation, Properties and Application in Energy and Petrochemical Fields

Acetate-based ionic liquids (AcILs), as a kind of typical carboxylate-based ILs, display excellent structure tunability, non-volatility, good solubility to biomass, and favorable adsorption capacity, etc. These unique characteristics of AcILs make them important candidates for a range of applications in the field of energy and in the petrochemical industry. This paper intends to provide a comprehensive overview of recent advances in AcILs, including pure AcILs, AcIL-based multi-solvents, and AcIL-based composites, etc. Preparation methods, with one- and two-step synthesis, are reviewed. The relationship between properties and temperature is discussed, and some physical and thermodynamic properties of different AcILs are summarized and further calculated. The applications of AcILs in the fields of biomass processing, organic synthesis, separation, electrochemistry, and other fields are reviewed based on their prominent properties. Thereinto, the dual functions of AcILs as solvents and activators for biomass dissolution are discussed, and the roles of AcILs as catalysts and reaction mediums in clean organic synthesis are highlighted. Meanwhile, the reaction mechanisms of AcILs with acid gases are posed by means of molecular simulation and experimental characterization. Moreover, AcILs as electrolytes for zinc batteries, supercapacitors, and electrodeposition are particularly introduced. Finally, the future research challenges and prospects of AcILs are presented.

Synthesis of Low-Viscosity Ionic Liquids for Application in Dye-Sensitized Solar Cells

Two types of ionic liquids (ILs), 1-(3-hexenyl)-3-methyl imidazolium iodide and 1-(3-butenyl)-3-methyl imidazolium iodide, are synthesized by introducing an unsaturated bond into the side alkyl chain of the imidazolium cation. These new ionic liquids exhibit high thermal stability and low viscosity (104 cP and 80 cP, respectively). The molecular dynamics simulation shows that the double bond introduced in the alkane chain greatly changes the molecular system space arrangement and diminishes the packing efficiency, leading to low viscosity. The low viscosity of the synthesized ionic liquids would enhance the diffusion of redox couples. This enhancement is detected by fabricating dye-sensitized solar cells (DSSCs) with electrolytes containing the two ILs and I₂ . The highest efficiency of DSSCs is 6.85 % for 1-(3-hexenyl)-3-methyl imidazolium iodide and 5.93 % for 1-(3-butenyl)-3-methyl imidazolium iodide electrolyte, which is much higher than that of 5.17 % with the counterpart 1-hexyl-3-methyl imidazolium iodide electrolyte.

CS2 capture in the ionic liquid 1-alkyl-3-methylimidazolium acetate: reaction mechanism and free energetics

Reaction pathways for CS2 and COS in the ionic liquid, 1-ethyl-3-methylimidazolium (EMI+) acetate (OAc-), are studied using the ab initio self-consistent reaction field theory (SCRF) and molecular dynamics (MD) computer simulations. It is found that while CS2 converts to COS nearly at the 100% level through S/O exchange with acetate, both conversion and capture processes are kinetically possible for COS, yielding CO2/thioacetate and 1-ethyl-3-methylimidazole-2-thiocarboxylate (EMI-COS)/acetic acid as reaction products, respectively. These findings are in excellent agreement with recent experimental observations in the closely related 1-butyl-3-methylimidazolium acetate (BMI+OAc-) ionic liquid system. Constrained ab initio MD indicates that the capture reaction of COS (and CS2 if allowed) proceeds in a concerted fashion; viz., proton transfer from EMI+ to OAc- and carboxylation of EMI+ by COS (and CS2) occur concurrently, analogous to the concerted pathway proposed recently for CO2 capture in the imidazolium acetate ionic liquid family. As N-heterocyclic carbene (NHC) is not required, the concerted mechanism is fully consistent with the experimental fact that NHC has not been detected directly in this ionic liquid family. Computational analysis further predicts that if NHC would be present in the ionic liquid, it would react with CS2 and produce 1-ethyl-3-imidazole-2-dithiocarboxylate, prior to the conversion of CS2 to COS. Since such a dithiocarboxylate compound was not detected experimentally, the present analysis lends support to the view that NHC is not formed in the pure imidazolium acetate ionic liquid family.

CO2 capture in ionic liquid 1-alkyl-3-methylimidazolium acetate: a concerted mechanism without carbene

Ionic liquids (ILs) provide a promising medium for CO₂ capture. Recently, the family of ILs comprising imidazolium-based cations and acetate anions, such as 1-ethyl-3-methylimidazolium acetate (EMI⁺OAc^-), has been found to react with CO₂ and form carboxylate compounds. N-Heterocyclic carbene (NHC) is widely assumed to be responsible by directly reacting with CO₂ though NHC has not been detected in these ILs. Herein, a computational analysis of CO₂ capture in EMI⁺OAc^- is presented. Quantum chemistry calculations predict that NHC is unstable in a polar environment, suggesting that NHC is not formed in EMI⁺OAc^-. Ab initio molecular dynamics simulations indicate that an EMI⁺ ion "activated" by the approach of a CO₂ molecule can donate its acidic proton to a neighboring OAc^- anion and form a carboxylate compound with the CO₂ molecule. Analysis of this termolecular process indicates that the EMI⁺-to-OAc^- proton transfer and the formation of 1-ethyl-3-methylimidazolium-2-carboxylate occur essentially concurrently. Based on these findings, a novel concerted mechanism that does not involve NHC is proposed for CO₂ capture.

Proton transfer and esterification reactions in EMIMOAc-based acidic ionic liquids

Proton transfer and esterification reactions are observed when formic acid and ionic liquid EMIMOAc are mixed.

Simultaneous CO2 and SO2 capture by using ionic liquids: a theoretical approach

Density functional theory (DFT) methods were used to analyze the mechanism of interaction between acidic gases and ionic liquids based on the 1-ethyl-3-methylimidazolium cation coupled with five different anions.

Understanding the mechanism of CO2 capture by 1,3 di-substituted imidazolium acetate based ionic liquids

Efficient CO₂ capture by ionic liquids needs a thorough understanding of underlying mechanisms of the CO₂ interaction with ionic liquids, especially when it involves chemisorption.

Molecular mechanism of CO2 absorption in phosphonium amino acid ionic liquid

The time-scale and site preferential interaction of CO₂ absorption in tetra-butylphosphonium lysinate amino acid ionic liquid is examined using molecular dynamics simulations.

Water Effect on Acid-Gas Capture Using Choline Lactate: A DFT Insight beyond Molecule-Molecule Pair Simulations

The suitability of CO2 and SO2 capture by using choline lactate ionic liquid as a sorbent and the effect of water content for acid-gas absorption were investigated through density functional theory (DFT) simulations in this work. Simulations that contain model systems considering up to four molecules (cholinium, lactate, water, and CO2/SO2) have been analyzed, and compositional effects on small cluster(s) formed by four ionic pairs and variable number of water molecules have been studied in this work. Assessment of the effect of water content on acid-gas capture that uses exotic ionic liquids is a rare study, and our results showed that water presence hinders CO2/SO2 affinity and solubility dramatically, mainly due to the dominated affinity between the ionic pair and water molecule rather than the CO2/SO2 molecule. Moreover, our studies also showed that affinity between ionic liquid and CO2 is hindered by more than ionic liquid and SO2 rich system with the presence of water in the environment.

Branched isomeric 1,2,3-triazolium-based ionic liquids: new insight into structure–property relationships

Series of branched isomeric 1,2,3-triazolium-based ionic liquids (ILs) were synthesized and characterized. The effect of branching on thermal and physical properties is investigated.

Cation-assisted interactions between N-heterocycles and CO2

Interactions between CO₂ and N-heterocycles are greatly enhanced by divalent cations.

Density Functional Theory Study on the Mechanism of Absorption of CO2 by the Ionic Liquid 1-Butyl-3-Methylimidazolium Acetate

The absorption of CO2 on 1-butyl-3-methylimidazolium acetate ([Bmim] [Ac]) with different substituents are calculated systematically at the GGA/PW91 level. Three hydrogen bonds are formed between [Ac]– and the cations of 1-n-[Bmim][Ac] ([NBmim]+) and 1-tert-[Bmim][Ac] ([TBmim]+). The interaction between CO2 and [NBmim][Ac] via a C – O bond is much weaker than that with [TBmim][Ac] by its forming an O…O…C…C four-membered ring. The chemisorption of CO2 on the ion pair [NBmim][Ac] is much weaker than that on [TBmim][Ac], as indicated from an absorption energy analysis. The frontier molecular orbitals show the electron density overlap between absorbed CO2 and the [Ac]– in CO2 – [NBmim][Ac] is much weaker than that in [TBmim][Ac]. As a result, the chemisorption of CO2 on [NBmim][Ac] is much weaker than that on [TBmim][Ac]. Ionic liquids based on [NBmim]+ can be used repeatedly, and the adsorbed CO2 would be more easily desorbed.

Chemically tunable ionic liquids with aprotic heterocyclic anion (AHA) for CO(2) 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.

Carbene formation in ionic liquids: spontaneous, induced, or prohibited?

We present a theoretical study of carbene formation from the 1-ethyl-3-methylimidazolium acetate ionic liquid in the absence and presence of CO2 in gas and liquid phase. Although CO2 physisorption constitutes a precursory step of chemisorption (the CO2's reaction with carbenes, which forms from cations via proton abstraction by anions), it also enables a very stable CO2-anion associate. However, this counteracts the chemical absorption by reducing the basicity of the anion and the electrophilicity of the CO2, which is reflected by charge transfer. Accordingly, the observable carbene formation in the gas phase is hindered in the presence of CO2. In the neat liquid, the carbene formation is also suppressed by the charge screening compared to the case of the gas phase; nevertheless, indications for carbene incidents appear. Interestingly, in the CO2-containing liquid we detect more carbene-like incidents than in the neat one, which is caused by the way CO2 is solvated. Despite the weakness of the CO2-cation interaction, the CO2-anion associate is distorted by cations, which can be seen in longer associate distances and reduced "binding" energies. While the single solvating anion is shifted away from CO2, many more solvating cations approach it compared to the case of the gas phase. This leads to the conclusion that while the ionic liquid effect stabilizes charged species, introducing neutral species such as CO2 provides an opposite trend, leading to an inverse ionic liquid effect with the facilitation of carbene formation and thus of chemical absorption.