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

Reactive capture and electrochemical conversion of CO2 with ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode-electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation.

Tailored carbon dioxide capacity in carboxylate-based ionic liquids

We have used a library of thermally stable tetraalkylphosphonium carboxylate ionic liquids that were easily prepared from available carboxylic acids. Depending on the pKa in water of the precursor acids, the resulting ionic liquids either dissolve or reversibly chemically absorb CO2, with some exhibiting notable gas capacities, reaching a CO2 mole fraction of 0.2 at 1 bar and 343 K. While equilibrium constants and ionic liquid capacities generally correlate with the pKa of the acids, certain exceptions underscore the influence of liquid structure and physical properties of the ionic liquids, elucidated through molecular dynamics simulations and density functional theory calculations. Unlike the trends observed in other CO2-absorbing ILs, phosphonium carboxylates do not experience increased viscosity upon gas absorption; instead, enhanced diffusivities are observed, facilitating efficient gas-liquid transfer.

Key Experimental Considerations When Evaluating Functional Ionic Liquids for Combined Capture and Electrochemical Conversion of CO2

Ionic liquids (ILs) are considered functional electrolytes for the electrocatalytic reduction of CO2 (ECO2R) due to their role in the double-layer structure formation and increased CO2 availability at the electrode surface, which reduces the voltage requirement. However, not all ILs are the same, considering the purity and degree of the functionality of the IL. Further, there are critical experimental factors that impact the evaluation of ILs for ECO2R including the reference electrode, working electrode construction, cosolvent selection, cell geometry, and whether the electrochemical cell is a single compartment or a divided cell. Here, we describe improved synthesis methods of imidazolium cyanopyrrolide IL for electrochemical studies in consideration of precursor composition and reaction time. We explored how IL with cosolvents (i.e. acetonitrile, dimethylformamide, dimethyl sulfoxide, propylene carbonate, and n-methyl-2-pyrrolidone) affects conductivity, CO2 mass transport, and ECO2R activation overpotential together with the effects of electrode materials (Sn, Ag, Au, and glassy carbon). Acetonitrile was found to be the best solvent for lowering the onset potential and increasing the catalytic current density for the production of CO owing to the enhanced ion mobility in combination with the silver electrode. Further, the ECO2R activity of molecular catalysts Ni(cyclam)Cl2 and iron tetraphenylsulfonato porphyrin (FeTPPS) on the carbon cloth electrode maintained high Faradaic efficiencies for CO in the presence of the IL. This study presents best practices for examining nontraditional multifunctional electrolytes amenable to integrated CO2 capture and conversion technologies for homogeneous and heterogeneous ECO2R.

Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review

CO2 absorption has been driven by the need for efficient and environmentally sustainable CO2 capture technologies. The development in the synthesis of ionic liquids (ILs) has attracted immense attention due to the possibility of obtaining compounds with designated properties. This allows ILs to be used in various applications including, but not limited to, biomass pretreatment, catalysis, additive in lubricants and dye-sensitive solar cell (DSSC). The utilization of ILs to capture carbon dioxide (CO2) is one of the most well-known processes in an effort to improve the quality of natural gas and to reduce the green gases emission. One of the key advantages of ILs relies on their low vapor pressure and high thermal stability properties. Unlike any other traditional solvents, ILs exhibit high solubility and selectivity towards CO2. Frequently studied ILs for CO2 absorption include imidazolium-based ILs such as [HMIM][Tf2N] and [BMIM][OAc], as well as ILs containing amine groups such as [Cho][Gly] and [C1ImPA][Gly]. Though ILs are being considered as alternative solvents for CO2 capture, their full potential is limited by their main drawback, namely, high viscosity. Therefore, the hybridization of ILs has been introduced as a means of optimizing the performance of ILs, given their promising potential in capturing CO2. The resulting hybrid materials are expected to exhibit various ranges of chemical and physical characteristics. This review presents the works on the hybridization of ILs with numerous materials including activated carbon (AC), cellulose, metal-organic framework (MOF) and commercial amines. The primary focus of this review is to present the latest innovative solutions aimed at tackling the challenges associated with IL viscosity and to explore the influences of ILs hybridization toward CO2 capture. In addition, the development and performance of ILs for CO2 capture were explored and discussed. Lastly, the challenges in ILs hybridization were also being addressed.

Tuning Functionalized Ionic Liquids for CO2 Capture

The increasing concentration of CO₂ in the atmosphere is related to global climate change. Carbon capture, utilization, and storage (CCUS) is an important technology to reduce CO₂ emissions and to deal with global climate change. The development of new materials and technologies for efficient CO₂ capture has received increasing attention among global researchers. Ionic liquids (ILs), especially functionalized ILs, with such unique properties as almost no vapor pressure, thermal- and chemical-stability, non-flammability, and tunable properties, have been used in CCUS with great interest. This paper focuses on the development of functionalized ILs for CO₂ capture in the past decade (2012~2022). Functionalized ILs, or task-specific ILs, are ILs with active sites on cations or/and anions. The main contents include three parts: cation-functionalized ILs, anion-functionalized ILs, and cation-anion dual-functionalized ILs for CO₂ capture. In addition, classification, structures, and synthesis of functionalized ILs are also summarized. Finally, future directions, concerns, and prospects for functionalized ILs in CCUS are discussed. This review is beneficial for researchers to obtain an overall understanding of CO₂-philic ILs. This work will open a door to develop novel IL-based solvents and materials for the capture and separation of other gases, such as SO₂, H₂S, NOx, NH₃, and so on.

Computational Insights into Malononitrile-Based Carbanions for CO2 Capture

Although anionic N and O sites have been widely used in chemisorption of CO₂, carbanions are much less explored for CO₂ capture. Here we employ ab initio calculations and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations to examine the interaction between CO₂ and the malononitrile carbanion, [CH(CN)₂]^-. We have explored the potential energy surface of CO₂ binding by scanning the C-C distance between CO₂ and the central C site of the carbanion. We find that CO₂ prefers to bind to the nitrile group physically rather than to form a C-C bond via the carboxylation reaction at the sp² C site. Moreover, the two -CN groups can attract two CO₂ molecules at equal strength. The presence of an alkali metal ion enhances both physical and chemical interactions of CO₂ with the malononitrile carbanion. QM/MM MD simulations further confirm the preference of physical interaction in the condensed ionic liquid phase with a phosphonium cation. Our findings suggest that ionic liquids based on the malononitrile carbanion may have a high CO₂ solubility for carbon capture.

Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P₆₆₆₁₄][Benzim]) or tetrazolide ([P₆₆₆₁₄][Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO₂ and SO₂, respectively, under realistic gas concentrations. The results show that [P₆₆₆₁₄][Tetz] is capable of efficiently capturing SO₂ in preference to CO₂ and thus, in a stepwise separation process, protects [P₆₆₆₁₄][Benzim] from the negative effects of the highly acidic contaminant. This results in [P₆₆₆₁₄][Benzim] maintaining >53% of its original CO₂ uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P₆₆₆₁₄][Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO₂ were irreversibly absorbed to the [Benzim]^- anion responsible for the decrease in CO₂ capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P₆₆₆₁₄][Tetz] is a suitable sorbent for reversibly capturing SO₂ and significantly extending the lifetime of [P₆₆₆₁₄][Benzim] for CO₂ uptake.

Solvents for Membrane-Based Post-Combustion CO2 Capture for Potential Application in the Marine Environment

Carbon capture on-board ships represents a powerful technological measure in order for the shipping industry to meet the very stringent GHG emission reduction requirements. Operation within the ship environment introduces a number of constraints associated mainly with space, energy supply, and safety which have to be addressed using compact yet efficient solutions. To this end, solvent-based membrane CO2 capture offers several advantages and has the necessary technological maturity for on-board installation. Solvent choice remains a critical issue both for reasons associated with process efficiency as well as on-board safety. In this paper, we present an up-to-date comprehensive review of the different solvents that can be used for post-combustion CO2 capture. Furthermore, we investigated the solvents’ performance as determined by their inherent characteristics, properties, and behavior for a range of operating conditions against the strict shipping requirements. A preliminary qualitative comparative assessment was carried out based on appropriately selected key performance indicators (KPIs) pertinent to the requirements of the shipping industry. The identified solvent classes were compared using the most critical KPIs for system integration with the ship. It was concluded that at present, no solvent category can efficiently address all the requirements of the ship. However, widely used solvents such as secondary amines showed relatively good compatibility with the majority of the introduced KPIs. On the other hand, more recently developed molecules, such as phase change solvents and ionic liquids, can easily prevail over the vast majority of the identified solvents as long as they are brought to the same level of technological maturity with benchmark solvents. Such a conclusion points toward the need for accelerating research on more tailor-made and performance-targeted solvents.

Effectiveness of ionic liquid-supported membranes for carbon dioxide capture: a review

The world's population explosion creates a need for natural resources for energy, which will become a significant contributor to global climate change. As we all know, carbon dioxide (CO₂) is one of the most critical elements of the global greenhouse gas effect. CO₂ capture and storage innovations have piqued researchers' attention in recent decades. Compared to other methods, membrane separation has some positive performance in CO₂ capture. CO₂ capture with membrane separation using enhanced ionic liquids (ILs) is described in this review. ILs have made an appearance in CO₂ capture work as the potential additive, and companies and academics have been interested in CO₂ separation for the past two decades. This article comprehensively analyzes the current modern approach in ILs and IL-based membranes for gas separation processes. Based on the latest literature and performance data, this work provides a complete compressive examination of types of ILs and IL-supported membrane performances. ILs for CO₂ capture were also explored, and IL-based membranes for different ILs were also studied. This study emphasizes the supremacy of novel ILs for CO₂ capture in membrane separation.

Advanced Theory and Simulation to Guide the Development of CO2 Capture Solvents

Increasing atmospheric concentrations of greenhouse gases due to industrial activity have led to concerning levels of global warming. Reducing carbon dioxide (CO₂) emissions, one of the main contributors to the greenhouse effect, is key to mitigating further warming and its negative effects on the planet. CO₂ capture solvent systems are currently the only available technology deployable at scales commensurate with industrial processes. Nonetheless, designing these solvents for a given application is a daunting task requiring the optimization of both thermodynamic and transport properties. Here, we discuss the use of atomic scale modeling for computing reaction energetics and transport properties of these chemically complex solvents. Theoretical studies have shown that in many cases, one is dealing with a rich ensemble of chemical species in a coupled equilibrium that is often difficult to characterize and quantify by experiment alone. As a result, solvent design is a balancing act between multiple parameters which have optimal zones of effectiveness depending on the operating conditions of the application. Simulation of reaction mechanisms has shown that CO₂ binding and proton transfer reactions create chemical equilibrium between multiple species and that the agglomeration of resulting ions and zwitterions can have profound effects on bulk solvent properties such as viscosity. This is balanced against the solvent systems needing to perform different functions (e.g., CO₂ uptake and release) depending on the thermodynamic conditions (e.g., temperature and pressure swings). The latter constraint imposes a "Goldilocks" range of effective parameters, such as binding enthalpy and pK _a, which need to be tuned at the molecular level. The resulting picture is that solvent development requires an integrated approach where theory and simulation can provide the necessary ingredients to balance competing factors.

Quantification of Ylide Formation in Phosphonium-Based Ionic Liquids Reacted with CO2

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) are found to react with CO₂ to form both a carbamate product and a carboxyl product. The carboxyl product is formed primarily at elevated temperatures through the formation of a phosphonium ylide intermediate. The formation of the carboxyl product leads to the formation of the neutral azole, which can lead to an irreversible process if the neutral azole is sufficiently volatile. To understand how the ILs would behave in a CO₂ capture process operated at elevated temperatures, it was necessary to quantify the two reaction products. CO₂ was reacted with seven different AHA ILs to determine the equilibrium amounts of carbamate and carboxyl, the equilibrium constants for both reactions, and the rate of CO₂ absorption by each reaction path. The reactions were tracked and quantified in situ by using ATR-FTIR spectroscopy, while NMR spectroscopy was used after equilibrium was reached to determine the extent of each reaction at multiple temperatures and pressures. It was found that both the basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide. In the extreme case of [P₆₆₆₁₄][4-Triaz] only half of the reacted product was the desired carbamate at 60 °C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.

Combined Experimental and Theoretical Study of the Competitive Absorption of CO2 and NO2 by a Superbase Ionic Liquid

A superbase ionic liquid (IL), trihexyltetradecylphosphonium benzimidazolide ([P66614][Benzim]), is investigated for the capture of CO2 in the presence of NO2 impurities. The effect of the waste gas stream contaminant on the ability of the IL to absorb simultaneously CO2 is demonstrated using novel measurement techniques, including a mass spectrometry breakthrough method and in situ infrared spectroscopy. The findings show that the presence of an industrially relevant concentration of NO2 in a combined feed with CO2 has the effect of reducing the capacity of the IL to absorb CO2 efficiently by ∼60% after 10 absorption-desorption cycles. This finding is supported by physical property analysis (viscosity, 1H and 13C NMR, and X-ray photoelectron spectroscopy) and spectroscopic infrared characterization, in addition to density functional theory (DFT) calculations, to determine the structure of the IL-NO2 complex. The results are presented in comparison with another flue gas component, NO, demonstrating that the absorption of NO2 is more favorable, thereby hindering the ability of the IL to absorb CO2. Significantly, this work aids understanding of the effects that individual components of flue gas have on CO2 capture sorbents, through studying a contaminant that has received limited interest previously.

Proton transfer from water to aromatic N-heterocyclic anions from DFT-MD simulations

The phenomenon of proton transfer from water to six N-heterocyclic anions and free energy landscapes of this process are studied using both electronic structure calculations and first principles molecular metadynamics simulations. Our investigation involves microhydrated and aqueous phase interaction of water with six aromatic heterocyclic anions relevant to chemistry and biology: imidazolide, pyrrolide, benzimidazolide, 2-cyanopyrrolide, indolide, and indazolide. The basic structures of all these heterocyclic anions differ by substituted functional groups as well as fused rings. We study the proton transfer reaction and the minimum number of required water molecules for the reaction in hydrated microclusters. We find out that at least four water molecules are necessary for hydrated clusters to facilitate the intracluster proton transfer reaction from water to anions except for pyrrolide, for which this magic number is 3. To obtain the reaction free energy and activation barrier of the proton transfer process in an aqueous solution, the metadynamics method based first principles molecular dynamics simulations were performed. The complete proton transfer was observed in aqueous solutions for all the anions. The water molecule directly involved in proton transfer becomes acidic due to the cooperative effect of neighboring water molecules. From the metadynamics simulation, we obtain the values of activation barrier for the proton transfer processes from neutral water to anions, and the highest activation barrier is obtained for benzimidazolide, whereas the lowest activation barrier is obtained for pyrrolide. The structures and free energy profiles of the process for all the anions are discussed, and a comparative outlook of the study is presented here.

Cation-Anion and Anion-CO2 Interactions in Triethyl(octyl)phosphonium Ionic Liquids with Aprotic Heterocyclic Anions (AHAs)

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO₂ capture applications. The anions of AHA ILs play a significant role in tuning anion-CO₂ complexation. In addition, AHAs are able to trigger the abstraction of acidic protons located at the α position of phosphonium cations by forming hydrogen bonds between cations and anions, eventually leading to cation-driven CO₂ complexation. Here we investigate the role of the anion in cation-anion hydrogen bonding and ylide formation. Using CO₂ uptake measurements, ³¹P nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) deuterium exchange equilibrium and rates, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and density functional theory calculations, we show that the key is the proximity of the negatively charged nitrogen atoms on the anion to the α protons, which is governed not just by anion basicity but by sterics. Thus, we show that triethyl(octyl)phosphonium 3-methyl-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P₂₂₂₈] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.

Capsules of Reactive Ionic Liquids for Selective Capture of Carbon Dioxide at Low Concentrations

The task-specific ionic liquid (IL), 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]), was encapsulated with polyurea (PU) and graphene oxide (GO) sheets via a one-pot Pickering emulsion, and these capsules were used to scrub CO₂ (0-5000 ppm) from moist air. Up to 60 wt % of IL was achieved in the synthesized capsules, and we demonstrated comparable gravimetric CO₂ capacities to zeolites and enhanced absorption rates compared to those of bulk IL due to the increased gas/liquid surface-to-volume area. The reactive IL capsules show recyclability upon mild temperature increase compared to zeolites that are the conventional absorber materials for CO₂ scrubbing. The measured breakthrough curves in a fixed bed under 100% relative humidity establish the utility of reactive IL capsules as moisture-stable scrubber materials to separate CO₂ from air, outperforming zeolites owing to their higher selectivity. It is shown that thermal stability, CO₂ absorption capacity, and rate of uptake by IL capsules can be further modulated by incorporating low-viscosity and nonreactive ILs to the capsule core. This study demonstrates an alternative and facile approach for CO₂ scrubbing, where separation from gas mixtures with extremely low partial pressures of CO₂ is required.

Cooperative CO2 absorption by amino acid-based ionic liquids with balanced dual sites

In this study, a variety of functionalized ILs with dual sites including amino acid group (AA) and basic anion (R) were synthesized to investigate the suppression and cooperation between the sites in CO₂ absorption. The basic anions selected in this study with different basicity include sulfonate (Su), carboxylate (Ac), imidazolium (Im), and indolium (Ind). These ILs ([P₆₆₆₁₄]₂[AA-R]) were applied to CO₂ absorption. The results present that CO₂ capacity increases first and then decreases later with the continuous increase in the activity of the anion site. Combined with CO₂ absorption experiments, IR and NMR spectroscopic analyses and DFT calculation demonstrate that the ability of one site to capture CO₂ would be suppressed when the activity of another site is much stronger. Thus, the cooperation of dual site-functionalized ILs and high CO₂ capacity might be achieved through balancing the two sites to be equivalent. Based on this point, [P₆₆₆₁₄]₂[5Am-iPA] was further synthesized by taking the advantage of the conjugated benzene ring. As expected, [P₆₆₆₁₄]₂[5Am-iPA] showed capacity as high as 2.38 mol CO₂ per mol IL at 30 °C and 1 bar without capacity decrease even after 10 times recycling performance of CO₂ absorption and desorption.

Reaction Mechanism and Free Energy Barriers for the Chemisorption of CO2 by Ionic Entities

Ionic liquids, a class of alternative solvents, are known for their ability to capture carbon dioxide (CO₂). The understanding of the role of the individual ionic entity of the ionic liquid (IL) and the involved mechanism is essential to design a better solvent for the capture process. In the present study, we employed density functional theory based electronic structure calculations and metadynamics method based first-principles molecular dynamics (FPMD) simulations to investigate the roles of the cation and anion of the IL by analyzing the energetics and free energy profile of the involved chemical reactions. The mechanism of chemisorption of CO₂ by the aprotic N-heterocyclic and phenolate anions paired with tetrapropyl phosphonium cation [P₃₃₃₃] were studied to understand the reaction mechanism of the initial capture process. The process of uptaking of CO₂ by the [P₃₃₃₃][1,2,4-Triz] was studied by the first-principles calculations. The transition states in the reaction pathways were computed by the synchronous transit-guided quasi-Newton method and confirmed by the intrinsic reaction coordinate calculations using first-principles simulations. The dynamics of the energetics of the chemisorption process were studied by constructing the free energy surface using metadynamics-based FPMD simulations. First, the nucleophilic center was generated at the α-carbon of the cation by transferring a proton to the anion with the formation of the phosphorus ylide. The formed cation ylide chemisorbs CO₂ through the formation of a bond between the α-carbon of ylide and the carbon of CO₂. The direct addition of CO₂ to the anion of the ionic pair was studied as the second pathway. We find that the chemisorption of CO₂ by the anion is more favorable than that by the cation. By comparing the chemisorption of CO₂ by the ions, we observe that the deprotonation of the alkyl chain is the more deciding factor, which depends on the basicity of anion and the length of the alkyl chain. We computed the free energy landscapes for the ionic pairs by varying another four anions like cyclohexanolate, 2,4,6-trifluorophenolate, imidazolate, and benzotriazolide paired with tetrapropyl phosphonium cation. The effect of the alkyl chain on the proton transfer was studied by tetrabutyl and tetrapentyl phosphonium cations paired with 1,2,4-triazolide anion. The carbonated product, formed from the anion, is thermodynamically controlled, while the carboxylated product (formed from cation) is kinetically controlled. We hope that our findings will enhance the knowledge of the selectivity of ionic entities for designing IL-based solvents for the capture process of CO₂.

Gas-Phase and Ionic Liquid Experimental and Computational Studies of Imidazole Acidity and Carbon Dioxide Capture

The capture and storage of carbon dioxide are pressing environmental concerns. Nucleophilic capture by anions in ionic liquids, such as imidazolates, is a promising strategy. Herein, the gas-phase acidity of a series of imidazoles is examined both experimentally and computationally. The intrinsic acidity of these imidazoles has not heretofore been measured; these experimental data provide a benchmark for the computational values. The relationship between imidazole acidity and carbon dioxide capture is explored computationally, both in the gas phase and in ionic liquid. The improved understanding of imidazolate properties provided herein is important for the design and development of improved systems for carbon dioxide capture.

Dynamics and Vibrational Spectroscopy of Alcohols in Ionic Liquids: Methanol and Ethanol

The structure, dynamics, and vibrational spectroscopy of dilute HOD, methanol, and ethanol in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf₂], ionic liquid (IL) are investigated with molecular dynamics (MD) simulations. The structure of the ILs around the solutes is qualitatively similar, where the OD bond of the deuterated alcohols donates an interaction to an [NTf₂] anion and the [emim] cations interact with the oxygen atom of the OD group. The slowest time scale for the reorientational dynamics of the OD bond varied considerably for HOD, methanol, and ethanol (27, 71, and 87 ps, respectively). In contrast, the slowest time scales for spectral diffusion of the OD vibrational frequency were 11 ps for each of the three solutes, which indicates that the dynamics of the IL is relatively unchanged by the presence of the alcohols at dilute concentration. The theoretical results for the reorientational and spectral diffusion dynamics compare favorably with prior two-dimensional infrared (2D IR) spectroscopic measurements.

Cation-Anion Interactions in 1-Ethyl-3-methylimidazolium-Based Ionic Liquids with Aprotic Heterocyclic Anions (AHAs)

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for CO₂ capture but have been considered for other applications as well. Previously, we have shown that AHA ILs, where the only site for reaction with CO₂ is the anion, the CO₂ capacity correlates with anion basicity. Moreover, we have shown that 1-ethyl-3-methylimidazolium ([Emim]⁺)AHA ILs can react with CO₂ in two ways. The first is with the anion to form a carbamate. The second is by reaction of the deprotonated C2 of the imidazolium cation to form a carboxylate. Here we show that the amount of carboxylate formed when [Emim]⁺ AHA ILs are exposed to CO₂ is not proportional to the anion basicity, contrary to expectations. Rather, it is roughly the same for all AHA ILs investigated, as long as the anion is able to readily deprotonate the C2. Moreover, the strength of the hydrogen bond between C2-H and the anion is not proportional to the anion basicity, once again contrary to expectations. In addition, we discovered that deuterium exchange occurs not only with the acidic proton on the C2 of the cation but also with the less acidic C4 and C5 protons of the imidazolium cation, when the anions are sufficiently basic. These conclusions were reached based on quantification of cation-CO₂ and anion-CO₂ complex formation, IR spectra, ¹H NMR chemical shifts, and deuterium exchange equilibrium and kinetics.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: IV. Temperature Dependence

In previous papers in the series, the vibrational spectroscopy of CO₂ in ionic liquids (ILs) was investigated at ambient conditions. Here, we extend these studies to understand the temperature dependence of the structure, dynamics, and thermodynamics of CO₂ in the 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF₆], IL. Using spectroscopic mapping techniques, the infrared absorption spectrum of the CO₂ asymmetric stretch mode is simulated at a number of temperatures, and the results are found to be consistent with similar experimental studies. Structural correlation functions are used to reveal the thermodynamics of complete CO₂ solvent cage breakdown. The enthalpy and entropy of activation for solvent cage reorganization are found to be 6.9 and 7.6 (kcal/mol)/K, respectively, and these values are similar to the those for spectral, orientational, and translational diffusion. Caging times for CO₂ are calculated, and it is shown that the short time dynamics of CO₂ are unaffected by temperature, even though the long-time dynamics are highly sensitive to temperature.

On the Performance of Confined Deep Eutectic Solvents and Ionic Liquids for Separations of Carbon Dioxide from Methane: Molecular Dynamics Simulations

Classical molecular dynamics simulations were used to investigate the performance of slit graphite and titania (rutile) pores of 5.2 nm in width, partially and completely filled with deep eutectic solvents (DESs) or ionic liquids (ILs), for gas separations of a carbon dioxide-methane mixture of 5:95 molar ratio and temperatures and pressures on the order of 318 K and 100 bar, respectively. The DESs studied were ethaline and levuline (1:2 molar mixtures of choline chloride with ethylene glycol or levulinic acid), and the IL considered was 1- n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim⁺][NTf₂^-]. The performance of these systems in terms of solubility selectivity, diffusion selectivity, and permselectivity was compared against the performance of the bulk solvents (which could also be viewed as a model system for the micrometer-sized pores of a supported IL or DES membrane) and against carbon and rutile pores without preadsorbed solvent. The best performance in terms of permselectivity is obtained for bulk levuline and by rutile pores fully filled by ethaline, followed by graphite pores filled by ethaline and the IL. Empty rutile pores have the largest value of solubility selectivity, followed by bulk ethaline and rutile pores completely filled by the IL. The largest values of diffusivity selectivity were observed for bulk levuline, followed by ethaline completely filling a rutile nanopore and a graphite nanopore completely filled with the IL. These observations are rationalized by examining local density profiles and interaction energies among the different entities in our systems. In general, systems of nanopores fully filled by solvents, as well as the bulk solvents, have larger permselectivities than pores partially filled by the IL or the DESs. Drops of 2-3 orders of magnitude are observed in the gas diffusivity in pores filled with solvents with respect to systems of empty pores, which may be problematic if gas permeation is mainly controlled by diffusion. However, if adsorption dominates the gas permeation within the membrane, our results suggest that systems of levuline in the micrometer-sized pores of a supported DES membrane or ethaline confined in the rutile nanopores of a supported DES phase material might represent promising systems for gas separation.

Evaluating the Performance of Micro-Encapsulated CO2 Sorbents during CO2 Absorption and Regeneration Cycling

We encapsulated six solvents with novel physical and chemical properties for CO₂ sorption within gas-permeable polymer shells, creating Micro-Encapsulated CO₂ Sorbents (MECS), to improve the CO₂ absorption kinetics and handling of the solvents for postcombustion CO₂ capture from flue gas. The solvents were sodium carbonate (Na₂CO₃) solution, uncatalyzed and with two different promoters, two ionic liquid (IL) solvents, and one CO₂-binding organic liquid (CO₂BOL). We subjected each of the six MECS to multiple CO₂ absorption and regeneration cycles and measured the working CO₂ absorption capacity as a function of time to identify promising candidate MECS for large-scale carbon capture. We discovered that the uncatalyzed Na₂CO₃ and Na₂CO₃-sarcosine MECS had lower CO₂ absorption rates relative to Na₂CO₃-cyclen MECS over 30 min of absorption, while the CO₂BOL Koechanol appeared to permeate through the capsule shell and is thus unsuitable. We rigorously tested the most promising three MECS (Na₂CO₃-cyclen, IL NDIL0309, and IL NDIL0230) by subjecting each of them to a series of 10 absorption/stripping cycles. The CO₂ absorption curves were highly reproducible for these three MECS across 10 cycles, demonstrating successful absorption/regeneration without degradation. As the CO₂ absorption rate is dynamic in time and the CO₂ loading per mass varies among the three most promising MECS, the process design parameters will ultimately dictate the selection of MECS solvent.

Absorption and thermodynamic properties of CO2 by amido-containing anion-functionalized ionic liquids

In this contribution, two kinds of amido-containing anion-functionalized ionic liquids (ILs) were designed and synthesized, where the anions of these ILs were selected from deprotonated succinimide (H-Suc) and o-phthalimide (Ph-Suc). Then, these functionalized ILs were used to capture CO2. Towards to this end, solubility of CO2 in the ILs was determined at different temperatures and different CO2 partial pressures. Based on these data, chemical equilibrium constants of CO2 with the ILs were derived at different temperatures from the "deactivated IL" model. The other thermodynamic properties such as reaction Gibbs energy, reaction enthalpy, and reaction entropy in the absorption were also calculated from the corresponding equilibrium constant data at different temperatures. It was shown that these anion-functionalized ILs exhibited high CO2 solubility (up to 0.95 mol CO2 mol-1 IL) and low energy desorption, and enthalpy change was the main driving force for CO2 capture by using such ILs as absorbents. In addition, the interactions of CO2 with the ILs were also investigated by 1H NMR, 13C NMR, and FT-IR spectroscopy.

Reply to the Correspondence on "Preorganization and Cooperation for Highly Efficient and Reversible Capture of Low-Concentration CO2 by Ionic Liquids"

The water content is crucial to the preparation of [P₄₄₄₂ ][Suc] and its capture of CO₂ . The use of a large amount of water in the preparation of this ionic liquid results in the significant formation of the byproduct succinamate anions and difficulties in water removal, which strongly reduces the capacity of CO₂ absorption through a bicarbonate mechanism. By contrast, the addition of a small amount of water maintains a high absorption capacity through cooperation.