New Insights into CO2 Absorption Mechanisms with Amino-Acid Ionic Liquids

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.

Unveiling protic amino acid ionic liquids for the efficient capture of carbon dioxide

A series of novel protic amino acid ionic liquids (PAAILs) are designed and synthesized for the first time through acid-base neutralization and an ion exchange reaction. Among the synthesised PAAILs, the [DBNH][Maba] PAAIL has the largest CO2 absorption capacity of 0.78 mol mol-1 (0.142 g g-1) at 313.2 K. The PAAILs are found to be efficient, reversible, and selective CO2 absorbents.

Efficient Absorption of CO2 by Protic-Ionic-Liquid Based Deep Eutectic Solvents

Carbon capture, utilization, and storage (CCUS) are among the key technologies to achieve large-scale carbon emission reduction globally. Deep eutectic solvents (DESs) are considered as designable solvents, which has attracted intensive attention for CO2 capture. Here, a series of binary DESs are synthesized through one-step mixing with the starting materials of protic ionic liquid (PIL) and amine. The eutectic behavior was investigated by measuring the melting point of PILs and amine. The saturated vapor of these DESs and industrial MDEA solution were measured and compared. These DESs are investigated to have high absorption capacity (0.1 g ⋅ g-1 at 1.0 bar and 25 °C), superior apparent absorption rate constant (0.381 min-1 vs 0.012 min-1 of 70 wt.% MDEA), moderate interaction with CO2 (the enthalpy change is as low as -34.8 kJ ⋅ mol-1). The absorption mechanism is also investigated by NMR analysis. Eight absorption/desorption regeneration experiments are carried out to show their reversibility. Considering the advantages, including convenience of synthesis, large absorption capacity, fast absorption rate, and moderate interaction energy as well as good regeneration, these DESs are believed to be as potential CO2 absorbent in practical applications.

On the Key Influence of Amino Acid Ionic Liquid Anions on CO2 Capture

Amino acid ionic liquids (AAILs) are promising green materials for CO2 capture and conversion due to their large chemical structural tunability. However, the structural understanding of the AAILs underlying the CO2 reaction dynamics remains uncertain. Herein, we examine the steric effects of AAIL anions with various chemical structures on CO2 capture behavior. Based on ab initio free-energy sampling, we assess reaction mechanisms for carbamate formation via a two-step reaction pathway with a zwitterion intermediate undergoing dynamic proton transfer. Our results show that free-energy barriers for carbamate formation can be significantly reduced as the degree of steric hindrance of the anions decreases. Further analyses reveal that reduced steric hindrance of anions causes markedly stronger intermolecular interactions between zwitterion and anions, leading to an increased kinetically favorable intermolecular proton transfer for carbamate production. We also describe the correlation strength between intramolecular interactions within the zwitterion and intermolecular interactions between the zwitterion and anions. We conclude that the favored structural flexibility due to the less steric hindrance of the zwitterion leads to enhanced intermolecular interactions, facilitating proton transfer to nearby AAIL anions for carbamate formation. Our study provides invaluable insight into the influence of various degrees of steric hindrance of the AAIL anions governing CO2 chemisorption. These findings may aid in the design of optimal AAIL solvents for the CO2 capture process.

Adsorption of Choline Phenylalanilate on Polyaromatic Hydrocarbon-Shaped Graphene and Reaction Mechanism with CO2: A Computational Study

The interaction of ionic liquids (ILs) with carbon materials is of fundamental importance in several areas of materials science, physics, and chemistry. Their adsorption on pristine and N-doped graphene surfaces is discussed here on the basis of results of density functional theory calculations. The nature of adsorption was investigated for an amino acid (AA)-based IL consisting of the choline cation [Ch] and the l-phenylalanilate anion [Phe] that interacts with a sheet of N-doped graphene. The interaction mechanism, binding energy, electron density, and non-covalent interaction analysis were evaluated by considering the cation, anion, and ion pair adsorbed on graphene separately. The distribution of cations and anions in the liquid bulk and on the graphene surface was then analyzed by molecular dynamics simulations. Since AA-based ILs are efficient absorbents for capture of CO2 due to the pronounced affinity of carbon dioxide to react with amino groups, we investigated the capacity of [Ch][Phe] to react with CO2 under various conditions. We considered the multistep mechanism of the reaction of [Phe] with CO2 first for the anion in the liquid bulk and then for the [Phe] anion adsorbed on the graphene surface. The initial step, the formation of the zwitterionic addition product, is followed by its structural rearrangement through intramolecular proton transfer and conformational isomerization processes to form carboxylic acid derivatives. The entire mechanism was evaluated for the [Phe] anion before and after adsorption on graphene to investigate how interactions with surfaces of carbon materials can affect the CO2 capture capacity of an AA-based IL such as [Ch][Phe].

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.

Reaction Mechanism of CO2 with Choline-Amino Acid Ionic Liquids: A Computational Study

Carbon capture and sequestration are the major applied techniques for mitigating CO2 emission. The marked affinity of carbon dioxide to react with amino groups is well known, and the amine scrubbing process is the most widespread technology. Among various compounds and solutions containing amine groups, in biodegradability and biocompatibility perspectives, amino acid ionic liquids (AAILs) are a very promising class of materials having good CO2 absorption capacity. The reaction of amines with CO2 follows a multi-step mechanism where the initial pathway is the formation of the C-N bond between the NH2 group and CO2. The added product has a zwitterionic character and can rearrange to give a carbamic derivative. These steps of the mechanism have been investigated in the present study by quantum mechanical methods by considering three ILs where amino acid anions are coupled with choline cations. Glycinate, L-phenylalanilate and L-prolinate anions have been compared with the aim of examining if different local structural properties of the amine group can affect some fundamental steps of the CO2 absorption mechanism. All reaction pathways have been studied by DFT methods considering, first, isolated anions in a vacuum as well as in a liquid continuum environment. Subsequently, the role of specific interactions of the anion with a choline cation has been investigated, analyzing the mechanism of the amine-CO2 reaction, including different coupling anion-cation structures. The overall reaction is exothermic for the three anions in all models adopted; however, the presence of the solvent, described by a continuum medium as well as by models, including specific cation- -anion interactions, modifies the values of the reaction energies of each step. In particular, both reaction steps, the addition of CO2 to form the zwitterionic complex and its subsequent rearrangement, are affected by the presence of the solvent. The reaction enthalpies for the three systems are indeed found comparable in the models, including solvent effects.

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.

Molecular screening of ionic liquids for CO2 absorption and molecular dynamic simulation

Though ionic liquids (ILs) are considered potential materials for CO2 capture because of their unique properties, it is time-consuming and costly to choose task-specific and suitable IL using the traditional “try-and-error” method. From the point of molecular design view, 25 cations and 20 anions are combined and screened using COSMOtherm to predict CO2 solubility at 298 K and 100 kPa. The prediction result showed that ILs with bFAP(tris(nonafluorobutyl)trifluorophosphate) anion could dissolve more CO2 than any others. To further understand the absorption performance of CO2 in ILs, molecular dynamic simulations are carried out to explore the interactions between CO2 and the four selected ILs, namely, [EMPyr][bFAP](1-ethyl-2-methylpyrazolium tris(nonafluorobutyl)trifluorophosphate), [B(Hex)3P][bFAP](butyl-trihexyl-phosphonium tris(nonafluorobutyl) trif-luorobutyl trifluorophosphate), [(Me)5isobuGua][bFAP](n,n,n,n,n-pentamethyl-n-isopropylguanidinium tris(nona-fluorobutyl)-trifluorophosphate), and [BEIM][bFAP] (1-butyl-3-ethyl-imidazolium tris(nonafluorobutyl)trifluo-rophosphate), at the atomic and molecular levels.

Extraction Behavior of Indole from Simulated Wash Oil Using Halogen-Free Ionic Liquids

Indole is an important raw material in the chemical industry, and more than 1 wt % indole is contained in wash oil. Therefore, the extraction of indole from wash oil is of much importance. The conventional separation methods generally cost much money, pollute the environment, and corrode the metallic devices due to the use of large amounts of inorganic acid and alkali solutions, and therefore, new methods should be proposed. In this work, a solvent extraction process for separating indole from simulated wash oil by five halogen-free ionic liquids (HFILs) has been designed, and the extraction behavior of indole has been evaluated. All the studied HFILs presented excellent extraction behavior for indole, and the whole separation process took no more than 5 min. For the same HFIL, the minimum residual indole contents remained the same, even if the initial indole contents changed. Among the HFILs, 1-butyl-3-methylimidazolium dimethyl phosphate ([Bmim][DMP]) has attracted more attention than other HFILs. The results showed that [Bmim][DMP] could extract over 96.9 wt % indole from the simulated wash oil, and the minimum residual indole content was as low as 2.1 g/dm³. For indole, [Bmim][DMP] presented a maximum distribution coefficient of 201, which was much improved compared to other methods. The HFILs could be regenerated by using diethyl ether with ease. The regenerated HFILs could be reused, and the extraction behavior remained the same as the original HFILs. Based on FT-IR results, a mechanism of hydrogen bonds forming between HFILs and indole was proposed. In addition, the superiorities of HFILs over other separation agents in reusability, amounts needed, distribution coefficient for indole, and chemical structure were proved by comparison.

CO2 Absorption Mechanism by Diamino Protic Ionic Liquids (DPILs) Containing Azolide Anions

Protic ionic liquids have been regarded as promising materials to capture CO2, because they can be easily synthesized with an attractive capacity. In this work, we studied the CO2 absorption mechanism by protic ionic liquids (ILs) composed of diamino protic cations and azolide anions. Results of 1H nuclear magnetic resonance (NMR), 13C NMR, 2-D NMR and fourier-transform infrared (FTIR) spectroscopy tests indicated that CO2 reacted with the cations rather than with the anions. The possible reaction pathway between CO2 and azolide-based protic ILs is proposed, in which CO2 reacts with the primary amine group generated from the deprotonation of the cation by the azolide anion.

Halogen-free ionic liquids as high performance extractants for phenols separation

As precious chemical raw materials, phenols can be applied to produce pharmaceuticals, new materials, engineering products, and so on. The separation of phenols from oil mixtures shows great economic value. In this work, five halogen-free ionic liquids (HFILs) were designed and employed to separate phenols from simulated oils, and all of them showed excellent separation performance. Among the HFILs, 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]) showed the highest separation efficiency of 98.6% for phenol, and achieved a minimum ultimate content of 1.96 g dm⁻³. The calculated distribution coefficient of phenol reached a high value of 431.8. The separation process could be finished within 3 min, and could be performed at normal temperature. It was also found that the HFILs could separate different types of phenols effectively. During separation, toluene was entrained in the HFIL, and an n-hexane treatment was used. After treatment, the toluene entrained in the HFIL after separation was largely removed, and the purity of the phenol was greatly improved. In addition, the HFILs could be easily regenerated by diethyl ether and reused 6 times without a decrease in separation efficiency. Meanwhile, the separation mechanism was explored by using FT-IR spectroscopy, and the FT-IR results indicated the existence of hydrogen bonds.

In this work, five halogen-free ionic liquids (HFILs) were designed and employed to separate phenols from simulated oils, and all of them showed excellent separation performance.

New chemistry for enhanced carbon capture: beyond ammonium carbamates

Carbon capture and sequestration is necessary to tackle one of the biggest problems facing society: global climate change resulting from anthropogenic carbon dioxide (CO2) emissions. Despite this pressing need, we still rely on century-old technology-aqueous amine scrubbers-to selectively remove CO2 from emission streams. Amine scrubbers are effective due to their exquisite chemoselectivity towards CO2 to form ammonium carbamates and (bi)carbonates, but suffer from several unavoidable limitations. In this perspective, we highlight the need for CO2 capture via new chemistry that goes beyond the traditional formation of ammonium carbamates. In particular, we demonstrate how ionic liquid and metal-organic framework sorbents can give rise to capture products that are not favourable for aqueous amines, including carbamic acids, carbamate-carbamic acid adducts, metal bicarbonates, alkyl carbonates, and carbonic acids. These new CO2 binding modes may offer advantages including higher sorption capacities and lower regeneration energies, though additional research is needed to fully explore their utility for practical applications. Overall, we outline the unique challenges and opportunities involved in engineering new CO2 capture chemistry into next-generation technologies.

The Nature of Carbon Dioxide in Bare Ionic Liquids

Ionic liquids (ILs) are among the most studied and promising materials for selective CO₂ capture and transformation. The high CO₂ sorption capacity associated with the possibility to activate this rather stable molecule through stabilization of ionic/radical species or covalent interactions either with the cation or anion has opened new avenues for CO₂ functionalization. However, recent reports have demonstrated that another simpler and plausible pathway is also involved in the sorption/activation of CO₂ by ILs associated with basic anions. Bare ILs or IL solutions contain almost invariable significant amounts of water and through interaction with CO₂ generate carbonates/bicarbonates rather than carbamic acids or amidates. In these cases, the IL acts as a base and not a nucleophile and yields buffer-like solutions that can be used to shift the equilibrium toward acid products in different CO₂ reutilization reactions. In this Minireview, the emergence of IL buffer-like solutions as a new reactivity paradigm in CO₂ capture and activation is described and analyzed critically, mainly through the evaluation of NMR data.

Is it Always Chemical When Amino Groups Come Across CO2? Anion-Anion-Interaction-Induced Inhibition of Chemical Adsorption

Amino-functionalized ionic liquids (IL) are often applied to fix CO₂. However, as far as we know, none of them have ever been reported to exhibit considerable physical CO₂ capture. Herein, we describe an amino-functionalized room-temperature ionic liquid, 1-butyl-3-methylimidazolium 3-amino-1H-1,2,4-triazolate ([Bmim][ATZ]), with an unusual ultrafast physical CO₂ capture at room temperature and atmospheric pressure. Within the time needed for a chemisorbent to reach an equilibrium, 15 adsorption and desorption cycles are finished for [Bmim][ATZ], with an accumulative molar ratio of up to 2.04. The CO₂/IL ratio for one adsorption process reaches 0.14, which is 4 times the highest recorded physical CO₂ solubility by [thtdp][Cl] (trihexyltetradecylphosphonium chloride). The first theoretical study on anion-anion interactions of ionic liquids is reported, which rationalizes the inhibition of chemical adsorption. These results provide a new perspective on the aspect of CO₂ capture, as well as designing of ionic liquids.

Protein-Rich Biomass Waste as a Resource for Future Biorefineries: State of the Art, Challenges, and Opportunities

Protein-rich biomass provides a valuable feedstock for the chemical industry. This Review describes every process step in the value chain from protein waste to chemicals. The first part deals with the physicochemical extraction of proteins from biomass, hydrolytic degradation to peptides and amino acids, and separation of amino acid mixtures. The second part provides an overview of physical and (bio)chemical technologies for the production of polymers, commodity chemicals, pharmaceuticals, and other fine chemicals. This can be achieved by incorporation of oligopeptides into polymers, or by modification and defunctionalization of amino acids, for example, their reduction to amino alcohols, decarboxylation to amines, (cyclic) amides and nitriles, deamination to (di)carboxylic acids, and synthesis of fine chemicals and ionic liquids. Bio- and chemocatalytic approaches are compared in terms of scope, efficiency, and sustainability.

CO2 Absorption by DBU-Based Protic Ionic Liquids: Basicity of Anion Dictates the Absorption Capacity and Mechanism

PILs are promising solvent systems for CO₂ absorption and transformations. Although previously tremendous work has been paid to synthesize functionalized PILs to achieve a high-performance absorption, the underlying mechanisms are far less investigated and still not clear. In this work, a series of DBU-based PILs, i.e., [DBUH][X], with anions of various basicities were synthesized. The basicities of the anions were accurately measured in [DBUH][OTf] or extrapolated from the known linear correlations. The apparent kinetics as well as the capacities for CO₂ absorption in these PILs were studied systematically. The results show that the absorption rate and capacity in [DBUH][X] are in proportional to the basicity of PIL, i.e., a more basic PIL leads to a faster absorption rate and a higher absorption capacity. In addition, the spectroscopic evidences and correlation analysis indicate that the capacity and mechanism of CO₂ absorption in [DBUH][X] are essentially dictated by the basicities of anions of these PILs.

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

The preorganization and cooperation mechanism of imide-based ionic liquids reported in a recent Communication was evocated to rationalize the extremely high gravimetric CO₂ capture displayed by these fluids. An analysis of the reported spectroscopic evidences together with additional experiments led to the proposition of an alternative, simpler, and feasible mechanism involving the formation of bicarbonate.

Unexpected solvation-stabilisation of ions in a protic ionic liquid: insights disclosed by a bond energetic study

Equilibrium acidities (pK_as) of 42 organic acids were precisely determined in protic ionic liquid (PIL) [DBUH][OTf]. Surprisingly, the often seen homoassociation complication during the pK_a measurement of O-H acids in DMSO was not detected in [DBUH][OTf], implying that the incipient oxanion should be better solvation-stabilized by the PIL, although its "apparent" dielectric constant is much lower than that of the conventional molecular solvent DMSO. Evidence showing that the solute ions in the PIL are also free from other specific ion associations like ion-pairing is further demonstrated by the identical pK_as of protic amine salts bearing largely different counter-anions. Correlations between the RO-H, N-H, N⁺-H and RCOO-H bond acidities in [DBUH][OTf] and in water revealed different slopes and intercepts for each individual series, suggesting far superior properties of the DBUH⁺-based PIL for differentiating the solvation effect of various species in structural analysis to the well applied EAN that is known for leveling out differential solvation.

DFT investigation on the selective complexation of ionic liquids based on α-amino acid anion and N7,N9-dimethyladeninium cation with CO2

A series of aliphatic amino acid ionic liquids (AAILs) composed of N7,N9-dimethyladeninium cation with an amino acid anion as the functionalized ILs, with dual amine group, have been designed for CO₂ capture.

Insights into the effect of CO2 absorption on the ionic mobility of ionic liquids

Comparative physico-chemical and diffusion NMR studies of two bio-renewable ionic liquids upon CO₂ absorption.