Mechanism Of CO2 Absorption Research Articles

Structure stability and CO2 absorption mechanism on surfaces of B-site doped SrFeO3-δ perovskite ceramic membrane

Perovskite oxides have huge potential applications in carbon capture and utilization as oxygen transport membranes (OTM) and solid oxide fuel cell (SOFC) electrodes. SrFeO3-δ-based perovskite material, as the promising candidate, possesses high mixed ionic-electronic conductivity, but its partial application is impeded by poor stability under CO2 conditions. In this work, SrFe0.9M0.1O3-δ (SFM, M = Fe, Al, Zr, Nb, W) ceramic membranes were synthesized to study the effects of different valence B-site ions doping on the stability and CO2 adsorption process of perovskite oxides. The phase structure, thermal expansion property, CO2 tolerance, and CO2 absorption mechanism on surfaces of SFM were investigated systematically. B-site doping stables the pure cubic structure, improves the high-temperature structural stability, and reduces the thermal expansion coefficient (TEC) by increasing the lattice parameters and metal-oxygen average binging energy (ABE) of SFM. Moreover, density functional theory (DFT) calculations indicate that although B-site ions doping promotes the adsorption of CO2 on the FeO2-terminated, it inhibits CO2 adsorption on the Sr-O-Sr site after Zr4+, Nb5+, and W6+ doping, causing the reduction of the CO2 desorption. The results offer new insights for designing perovskite oxides in the carbon neutralization field.

Deciphering the transfer mechanisms for CO2 absorption into binary blended solutions

A thorough understanding of the mechanisms involved in CO2 chemical absorption is crucial for the development of efficient CO2 capture technologies. Due to the insufficient insight into the interaction mechanism of different components in blended solutions, CO2 absorption reactions were often categorized as unidirectional transfer between absorption products. In this study, the CO2 absorption performance of binary blended solutions, including piperazine (PZ)/n-methyldiethanolamine (MDEA), ethanolamine (MEA)/MDEA, ammonia (NH3)/MDEA, PZ/potassium carbonate (K2CO3), and NH3/K2CO3, was investigated using a combination of experimental and computational methods. The CO2 absorption mechanisms of “competitive reaction”, “transfer reaction” and “parallel reaction” in the binary blended solution system were proposed. Kinetic experiments revealed that different blended solutions had varying impacts on the process of CO2 absorption. Among them, PZ/MDEA and MEA/MDEA solutions reduced the absorption rates by an average of 8% and 25%, respectively, compared to PZ or MEA component solutions. NH3/MDEA and PZ/K2CO3 solutions had absorption rates similar to those of single NH3/PZ component solutions. NH3/K2CO3 solutions, on the other hand, exhibited an average increase of 17% in absorption rates compared to NH3 solutions. Quantum mechanical (QM) methods were employed to evaluate of the absorption products and key processes in terms of kinetics and thermodynamics. Quantitative 13C NMR analyses were conducted to further investigate the interactions between components and the pathways of mass transport in blended solutions, which demonstrated proton transfer and CO2/-COO transfer between adsorption products. This study highlights an accurate description of the transfer mechanisms of various blended systems for the enhanced CO2 capture.

Study on CO2 absorption by EmimCl-MEA deep eutectic solvent in microchannel

Carbon dioxide (CO2) is widely acknowledged as the primary contributor to the greenhouse effect. Microchannel reactors can be used for enhancing chemical processes, solving the problems of limited gas-liquid mass transfer, and have significant advantages in CO2 capture. In this study, the deep eutectic solvent (DES) prepared by mixing ionic liquid (1-ethyl-3-methylimidazolium chloride, EmimCl) and Ethanolamine (MEA) in a molar ratio of 1:1 was selected for CO2 capture in a microchannel. The structure and physical properties of the DES were investigated, and the effects of gas and liquid flow rates, water content, and CO2 concentration on the CO2 absorption properties were analyzed. The increase of the water content not only significantly reduces the viscosity of DES, but also improves the CO2 absorption performance. In addition, the CO2 absorption mechanism of this DES has been obtained by the NMR spectrum, showing that the absorption process is a rapid chemical absorption process between CO2 and MEA. The Cl- of EmimCl could stabilize the protonated amine and prevent it from further reacting with MEACO2-, furthermore increasing the CO2 absorption rate.

Computational study on CO2 absorption mechanism and structure–activity relationship of biphasic solvent

The biphasic solvents are conducive to improving the efficiency of CO2 capture and reducing the energy consumption caused by regeneration. This study explored their reaction process at the molecular level based on density functional theory (DFT): selection of diamines such as active amines with distinct amino groups, 1,4-butanediamine (BDA) and N-(2-aminoethyl)ethanolamine (AEEA), and with typical phase separation promoters, 2-(diethylamino)ethanolamine (DEEA), and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA). This study systematically explores the potential reaction paths in CO2 capture to quantitatively assesse the structure–activity relationships among different amines in different stages of absorption, the influence of reaction sequence on reaction rates, and further elucidates the characteristics of product formation and its impact on the phase separation process. The results indicated that the zwitterion mechanism is widely applicable and remains the dominant explanation for the biphasic solvent CO2 capture mechanism. The intramolecular proton transfer reaction is the key to influencing the absorption rate of CO2, but in the case of interproton transfer, the rate-determining step needs to be judged in conjunction with the type of amino groups on it. And the amine groups on AEEA react with CO2 in different orders, forming distinct zwitterions that have similar activity in later intramolecular proton transfers. Under the zwitterion mechanism, primary amino groups exhibit stronger proton affinity than secondary ones. In solvents approaching saturation stage, DEEA reacts with AEEA and CO2 earlier than PMDETA. This study reveals the reaction mechanism of biphasic solvents and the key paths in different phases, providing insights for developing new solvents.

CO2 absorption using two morpholine protic ionic liquids: measurement, model, and quantum chemical calculation

Two kinds of protic ionic liquids, N-methylmorpholinium formate ([NMMH][For]) and N-ethylmorpholinium formate ([NEMH][For]), were synthesized by the acid-base neutralization method. The solubility of carbon dioxide (CO2) in the two ionic liquids was measured at temperatures of 298.15 to 338.15 K and pressures of up to 900 kPa. The solubility increases linearly with pressure, indicating that CO2 is physically absorbed in ionic liquids. The solubility of CO2 in [NEMH][For] is higher than that of [NMMH][For]. Absorption behavior was investigated by thermodynamic properties, such as the Henry's law constant, partial molar Gibbs free energy, partial molar enthalpy, and partial molar entropy. The Pitzer's model and the Soave-Redlich-Kwong (SRK) cubic equation of state were used to fit the solubility data, respectively. The Pitzer's model is found to have better prediction accuracy. The interaction of two protic ionic liquids with CO2 was analyzed using quantum chemistry. The molecular mechanism of CO2 absorption by two protic ionic liquids was explained from the microscopic point of view. CO2 + [NEMH][For] has more hydrogen bonds and a higher interaction energy. The cation–anion interaction of [NEMH][For] diminishes in the presence of CO2, resulting in an increase in the cation–anion distance. These factors result in higher solubility of CO2 in [NEMH][For].

Water effect on CO2 absorption mechanism and phase change behavior in [N1111][Gly]/EtOH anhydrous biphasic absorbent: In density functional theory and molecular dynamics view

Ionic liquids-based anhydrous biphasic absorbents have the potential to lower the energy consumption for CO2 capture. Recent experimental studies have revealed that the presence of water in [N1111][Gly]/EtOH system leads to generation of a new CO2-product – bicarbonate and affects the phase change behavior. However, the reaction pathway for bicarbonate formation remains unclear. In this work, the reaction pathways were investigated in depth through quantum chemical calculations employing density functional theory, with reaction rate constants determined via transition state theory. Among the three possible reaction pathways, the base-catalyzed CO2 hydration reaction prevailed, i.e. a one-step reaction involving [Gly]-, H2O, and CO2. An H atom in H2O was transferred to [Gly]-’s amino group, forming protonated [Gly]-, and hydroxide combined with CO2 to generate bicarbonate. Additionally, molecular dynamics simulations were conducted to understand the phase change behaviors with varying water content. When water content exceeded 5 wt%, an increased water content led to reduced hydrogen bonding among products and enhanced hydrogen bonding of products-solvent with the solvent, diminishing product self-aggregation. No phase change behavior were observed at water contents up to 80 wt%. This study could well explain the experimental phenomena of water effect on CO2 absorption in [N1111][Gly]/EtOH, providing theoretical references for application of ILs-based anhydrous biphasic absorbent.

Numerical simulations of CO2 absorption by MgO-based sorbent in a gas–solid fluidized bed

Given that carbon dioxide (CO2) constitutes the predominant proportion of greenhouse gas emissions, its capture, utilization and storage have always been a subject of great concern. Gas-solid fluidized bed has become one of the commonly applied equipment for the removal of CO2 from flue gases owing to its advantages such as exceptional mixing effect and substantial interfacial area between the phases. However, there is a lack of attention to the mechanism of CO2 absorption by individually-tracked moving solid sorbents as well as their effects on the decarbonization performance in a gas–solid fluidized bed. In this study, CO2 removal model by solid particles based on MgO adsorbent was established with the implementation of the shrinking-core model and each particle was tracked individually. Results validated the accuracy of the absorption model and revealed that smaller sizes of the solid absorbent, lower inlet gas velocities and larger inlet CO2 mass fractions exhibit relatively high CO2 removal efficiency. In addition, the decarbonization performance of fluidized and packed beds was compared. The packed bed exhibits more uniform gas flow and higher CO2 removal efficiency, whereas the fluidized bed provides larger interphase contact area, which facilitates the heat transfer process. Through the analysis of a series of parameters, the results provide recommendations for improving the CO2 removal efficiency and help to explore the optimal protocol for the design of bed reactors for CO2 removal.

Deep eutectic solvents formed by novel metal-based amino acid salt and dihydric alcohol for highly efficient capture of CO2

Deep eutectic solvents (DESs) are considered to be potential absorbents for CO2 capture compared to traditional amine solutions, because they have many advantages such as lower volatility and better thermal stability. In this study, a series of novel γ− and ε− m−based amino acid salts (AASs) were prepared through one-step hydrolysis reaction of accessible and cheap lactams as hydrogen bond acceptor (HBA). The chemical structures of these AASs were characterized by ESI-MS and NMR. Dihydric alcohol was selected as hydrogen bond donor (HBD) to prepare AAS-based DESs because of its low specific heat capacity and high boiling point. The physical properties of AAS-based DESs, such as density at 313.2–353.2 K, viscosity at 313.2–353.2 K, the melting temperature from 173.2 to 273.2 K, and thermal decomposition temperature from 300 to 750 K, were also determined. The gas absorption experiments were carried out on a dual-chamber at temperatures from 313.2 to 333.2 K and pressures up to 1.1 bar. It is found that K[Gaba]-EG has the largest CO2 uptake capacity with 0.12 g·g−1 at 1.0 bar and 313.2 K, better than almost all absorbent with AASs reported in literatures (0.02 g·g−1 ∼ 0.07 g·g−1). The CO2 absorption mechanisms were proved to be a combination of 2:1 and 1:1 stoichiometric reaction through thermomechanical analysis and NMR. This paper will present distinctive insights into the preparation of CO2 absorbent for effective carbon capture.

Inhibiting effect of 2-amino-2-methyl-1-propanol on gelatinous product formation in non-aqueous CO2 absorbents: Experimental study and molecular understanding

Diamines or polyamines with two or more amino groups are suitable for formulating non-aqueous absorbents (NAAs) with high CO2 loading capacity and low regeneration energy consumption. However, these two types of amines in NAAs are prone to produce insoluble gelatinous products after absorbing CO2, resulting in difficult operation of the CO2 capture system. In this study, by using 2-amino-2-methyl-1-propanol (AMP) as an inhibitor, the insoluble gelatinous products of the aminoethylethanolamine (AEEA)-N-methyl-2-pyrrolidone (NMP) (A-A-N) and 3-(methylamino)propylamine (MAPA)-NMP (M-A-N) NAAs were successfully eliminated. The experimental results showed that the AMP-regulated NAAs possessed a high absorption rate, high CO2 loading capacity, excellent desorption efficiency, and stable recyclability. The reaction mechanism of CO2 absorption and the inhibitory mechanism of AMP on the formation of gelatinous products were comprehensively elucidated. Taking the A-A-N system as a representative, AEEA reacted with CO2 to form zwitterionic protonated carbamate species (AEEACO2−(P)H+(S)), which tended to self-aggregate via hydrogen-bond interaction, resulting in the formation of insoluble gelatinous products. With the introduction of AMP, the AMP-derived products could combine easily with AEEACO2−(P)H+(S) via strong electrostatic attraction to form ion pairs, preventing the AEEACO2−(P)H+(S) molecules from self-aggregating to form insoluble gelatinous products. The regeneration heat duty of the A-A-N system was 1.89 GJ∙ton−1 CO2, which was 49.9 % lower than that of the benchmark 30 wt% MEA. Overall, introducing AMP as a gelatinous product inhibitor was beneficial for the development of NNAs with high CO2 absorption capacity and low-energy consumption.

State-of-the-art of cold energy storage, release and transport using CO2 double hydrate slurry

CO2 hydrate slurry is a promising cold storage and transport medium due to the large latent heat, favorable fluidity and environmental friendliness, and the CO2 utilization can also be simultaneously achieved. However, the phase change pressure of CO2 hydrate is too high for applications in refrigeration system, thus the thermodynamic promoters are used to moderate the phase change conditions by forming CO2 double hydrate. In this review, a comprehensive summarization of research progresses of CO2 double hydrate is presented, encompassing the aspects of thermodynamic and kinetic characteristics, cold storage and release, cold transport and system operation. Quaternary salts, tetrahydrofuran, and cyclopentane are concluded to be the main thermodynamic promoters used to alleviate hydrate formation conditions. The cold storage capacity of CO2 double hydrate slurry highly depends on CO2 absorptivity which can be enhanced by increasing pressure, adding kinetic promoters and external disturbance. The non-Newtonian behaviors of most CO2 double hydrate slurries exhibit shear-thinning characteristics and addition of anti-agglomerates can effectively reduce their apparent viscosity. There is an optimal hydrate mass fraction and phase change temperature that make the system energy efficiency the highest due to the reduced pumping power consumption and improved primary refrigeration system efficiency. On the way to practical applications, the research challenges remain in elucidation of underlying mechanism of CO2 absorption and release with the presence of kinetic promoters, stable control of cold storage and release processes, and the detailed heat and mass transfer characteristics. In addition, operation strategy, optimization techniques, economic analysis and CO2 management method of the entire system are potential research topics.

Molecular insights into the CO2 absorption mechanism by superbase protic ionic liquids by a combined density functional theory and molecular dynamics approach

Protic ionic liquids (PILs) are emerging as a new class of sustainable and efficient solvents for CO2 capture, requiring a fundamental understanding of their properties for their optimal design. To obtain a molecular-level understanding of the mechanism behind CO2 absorption in this class of absorbents, we selected four novel and high-efficient PILs prepared from superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]-5-nonene (DBN) as cations, with imidazole (Im) and pyrazole (Pyr) as anions. Density functional theory (DFT) and molecular dynamics (MD) simulations were used to quantify their interactions and reaction mechanisms as well as the dynamics of the CO2 absorption process. Results indicate that CO2 primarily interacts with the anions of PILs through van der Waals forces, while the cations and anions of PILs mainly engage in strong hydrogen-bonding interactions. Additionally, the anions primarily serve as the absorption reaction sites for CO2, with their molecule centers of mass being the closest. Meanwhile, reaction with CO2 requires overcoming a relatively low energy barrier (i.e., ∼35–40 kJ·mol−1), making them more favorable for regeneration than benchmark solvents. Notably, MD simulations have also shown that CO2 molecules are preferentially accumulating at the gas/PILs interfaces and that chemisorption is leading the CO2 capture at low pressures in these PILs. Among the studied systems [DBUH][Pyr] is the most reacting system with CO2, while [DBUH][Pyr] shows the lower regeneration energy. The findings would shed more light on understanding and designing PILs for CO2 capture.

Understanding the CO2 capture potential of tetrapropylammonium-based multifunctional deep eutectic solvent via molecular simulation

A multiscale theoretical study on the suitability of the bifunctional hydrophobic Deep Eutectic Solvent based on tetrapropylammonium chloride and acetic acid with ethanolamine is reported. Density Functional Theory quantum calculations along with classical Molecular Dynamics simulations provide a nanoscopic characterization of the properties of the studied sorbent in terms of intermolecular forces and liquid phase structuring and the characterization of the mechanism of CO2 absorption considering gas – liquid phase interfaces for neat CO2 and a realistic flue gas mixture. Results in this paper characterize the evolution of CO2 into liquid phases as well as the changes produced in the fluid upon gas capture through hydrogen bonding, void occupation and molecular clustering. The possible effect of having two different types of functionalities in the sorbent, amine and hydroxyl groups, which may lead to possible chemical and/or physical sorption is analyzed, considering their possible competing in CO2 capture. The reported study provides the first characterization of multifunctional Deep Eutectic Solvents for carbon capture from complex realistic gas mixtures.

The Effect of DBU on Microstructure and CO2 Absorption of EmimCl‐TEG and BmimCl‐TEG Deep Eutectic Solvents

AbstractMicrostructure of deep eutectic solvents (DESs) based on EmimCl or BmimCl ionic liquid (IL) and triethylene glycol (TEG) was studied by FT‐IR spectra. The influence of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) on CO2 absorption behavior and mechanism of DESs was investigated. FT‐IR spectra of pure EmimCl‐TEG and BmimCl‐TEG DESs show that O−H⋅⋅⋅Cl− hydrogen bond interaction formed by IL and TEG gradually strengths with the increase in the molar ratio of IL to TEG, resulting in a corresponding increase in viscosity. Due to the lack of a chemical interaction site, the CO2 absorption capacity of pure DESs at 313.2 K and 1.0 bar is only between 0.15 and 0.56 mol kg−1. Surprisingly, the addition of equimolar DBU to DESs can significantly improve the CO2 absorption capacity, which is up to 2.94 mol kg−1 at the maximum and similar to that of amine‐functionalized DESs. CO2 absorption mechanism characterized by FT‐IR spectra indicates that DBU can form hydrogen bond with O−H of TEG, which leads to deprotonation of O−H of TEG into basic anion under the synergistic effect of O−H⋅⋅⋅Cl− hydrogen bond to achieve efficient CO2 chemisorption via carbonate formation. The obtained results provide a strategy for effectively improving the CO2 capture capacity of EmimCl‐TEG and BmimCl‐TEG DESs.

Mass transfer mechanism and model of CO2 absorption into a promising DEEA-HMDA solvent in a packed column

Mass transfer performance plays an important role in solvent evaluation as well as design and scale-up of CO2 absorption process. The mass transfer process of CO2 absorption into blended DEEA -HMDA solution was studied over different operating conditions in a lab-scale absorption column packed with Sulzer DX structured packing. The effect of those operating parameters on KGav was fully investigated and discussed in this work. Meanwhile, mass transfer mechanism of CO2 absorption into blended DEEA-HMDA solution was comprehensively presented by identifying the rate-controlling step of mass transfer and the reaction zone of CO2 with amines, which could provide the operation guideline for running plans. In addition, a new KGav model was proposed and developed on basis of the observed experimental value of KGav, which gave a much better predication performance in comparison with empirical Kohl-Risenfield-Astarita model and Sheng model with respect to AAD.

Solid base LDH-catalyzed ultrafast and efficient CO2 absorption into a tertiary amine solution

Due to its high capacity, low energy consumption, and low corrosiveness, chemical absorption using aqueous tertiary amine solutions is a potential approach for CO2 capture. The fundamental limitation of this technology is its low CO2 absorption kinetics (rate), which impedes its large-scale use. Here, five typical solid-base catalysts, MgAl-layered double hydroxide (LDH) and its corresponding layered double oxide (LDO), BaCO3, MgCO3, and CaCO3, were adopted to improve the rate of CO2 absorption in an aqueous methyldiethanolamine solution (MDEA). The results indicated that LDH and LDO promoted CO2 absorption, with LDH exhibiting significantly superior catalytic activity. In particular, compared to the non-catalytic absorption, using LDH considerably improved the CO2 absorption rate by 92.7%. The catalytic effects of LDH on CO2 absorption were demonstrated by 13C NMR and FT-IR techniques, and theoretical calculation. A possible catalytic CO2 absorption mechanism over LDH was suggested. Additionally, the stability of LDH was certified with 10 cyclic CO2 absorption–desorption experiments. The excellent catalytic CO2 absorption performance of LDH may be primarily owing to the chemical absorption improvement mechanism of basic sites. Therefore, the LDH-catalyzed CO2 absorption process is a promising option for increasing the CO2 absorption rate in tertiary amine solution. This work may open up a new approach to developing plentiful and affordable solid-base catalysts for CO2 capture technologies with faster absorption kinetics and lower energy requirement.

Study on CO2 absorption by novel choline chloride-diethylenetriamine-water deep eutectic solvents in a rotor-stator reactor

Excessive CO2 emissions from flue gas lead to the greenhouse effect and catastrophic climate change and have become a global concern. To address this problem, novel choline chloride-diethylenetriamine-water (ChCl-DETA-H2O) deep eutectic solvents (DESs) with high CO2 absorption capacity was synthesized in this study. It was found that the DES with a composition of 1ChCl-5DETA-2H2O had the highest CO2 absorption capacity, which was up to 0.250 gCO2/gDES. The as-synthesized DESs were adopted for CO2 absorption in a rotor-stator reactor (RSR), and the effects of ChCl-DETA-H2O molar ratio, high gravity factor, liquid flow rate, gas flow rate, temperature, and inlet CO2 concentration on the CO2 absorption efficiency, overall gas-phase volumetric mass-transfer coefficient (KGa) and height of a transfer unit in the RSR were investigated. Analyses on the mechanism of CO2 absorption in the DES suggest that carbamate formed after CO2 absorption by 1ChCl-5DETA-2H2O. A comparison with the traditional packed columns indicates that the KGa in the 1ChCl-5DETA-2H2O/RSR process was about one order of magnitude larger than that in the DETA/packed column process. Also, the 1ChCl-5DETA-2H2O/RSR process achieved a higher CO2 absorption efficiency and KGa than the DETA/rotating packed bed process. This work demonstrates that the RSR enhanced the absorption of CO2 by the DES and the combination of RSR and DES can provide an efficient process for CO2 capture.

A new look to the old solvent: Mass transfer performance and mechanism of CO2 absorption into pure monoethanolamine in a spray column

The most mature CO2 capture technology is absorption of monoethanolamine (MEA) in packed or spray columns. Typically, an aqueous 30 wt.% MEA solution is used. High MEA concentrations are believed to hinder mass transfer rate due to high viscosity of MEA. We worked, for the first time, on the effects of using pure MEA on overall mass transfer coefficient (KGɑ) and absorption efficiency in a spray column by varying several operational parameters. Image analysis results suggested that interfacial area increased with increasing liquid flow rate. As a result, KGɑ increased. As opposed the belief in literature, KGɑ also increased with increasing MEA concentrations. The highest KGɑ was obtained in this work (11.7 kmol·m−³∙kPa−1∙h−1) is around one order of magnitude higher than most literature studies. Having more MEA molecules on the surface of the droplets led to higher KGɑ. In addition, it was shown that absorption efficiency was largely determined by inlet CO2 to MEA molar ratio. 13C NMR spectra results revealed that similar levels of carbamate were formed for MEA concentrations up to 70 wt.%. A simplified analysis on regeneration heat duty showed that decreasing water amount can lead to 3–10 fold decrease in reboiler energy duty.

Mechanism Study of Imidazole-Type Deep Eutectic Solvents for Efficient Absorption of CO2.

Deep eutectic solvents (DESs) are a new class of green solvents that exhibit unique properties in various process applications. In this regard, this study evaluated imidazole-type DESs as solvents for carbon dioxide (CO2) capture. A series of imidazole-type DESs with different ratios was prepared through one-step synthesis. The absorption capacity of CO2 in imidazole-type DESs was measured through weighing, and the effects of temperature, hydrogen bond acceptors, hydrogen bond donors, and water content were discussed. DESs absorbed the effects of CO2. Im-MEA (1:2) was selected to linearly fit lnη and 1/T using the Arrhenius equation under variable temperature conditions, and a good linear relationship was found. The results show the best absorption effect for Im-MEA (1:4). At 303.15 K and 0.1 MPa, the absorption capacity of Im-MEA (1:4) was as high as 0.323 g CO2/g DES; through five times of absorption-desorption after the cycle, the absorption capacity of DES was almost unchanged. Finally, the mechanism of CO2 absorption was studied using Fourier transform infrared and nuclear magnetic resonance spectroscopy. The absorption mechanism of imidazole-type DESs synthesized using imidazole salt and an amine-based solution was chemical absorption, and the reaction formed carbamate (-NHCOO) to absorb CO2.

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.

Design of novel dual functional ionic liquids and DFT study on their CO2 absorption mechanism

Dual functional ionic liquids with the combinations of diethylenetriamine cation ([DETAH]+) or 1-ethanolamine-ethylenediamine cation([1-AOEt-EDAH]+) and 4-fluorobenzoate anion ([4-F-PhO]−) were explored for efficient CO2 capture in this work, and the absorption mechanisms of isolated ions and ion pairs were investigated using the dispersion corrected density functional theory (DFT) method. Above all, the possible reaction sites and reaction activity of TSILs were inferred by electrostatic potential (ESP) and Frontier molecular orbital analysis, then the reaction pathways for absorbing CO2 were proposed, and the transition state search of each elementary reaction by cation, anion and ion pair absorption was performed. Furthermore, the Gibbs free energy and reaction heat of each path were separately calculated, and the Gibbs free energy barrier and enthalpy barrier diagrams were drawn. In addition, the change of structural parameter, bond order and ADCH charge of reactants (R), transition states (TS), intermediates (IM), and products (P) were compared. The results showed that the absorption reaction of [DETAH]+ was the most likely to occur in kinetics, while that of [1-AOEt-EDAH]+ was favorable in thermodynamics. For the [DETAH][4-F-PhO], ion pair binding competed strongly with the CO2 chemisorption, while for [1-AOEt-EDAH][4-F-PhO], anions improved the stability of the whole structure, leading to the decrease of the Gibbs free energy barrier. Hopefully, the results of this work are helpful to provide a theoretical basis for designing of novel functional ionic liquids.