Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes

Exploring the stability and catalytic activity of monoethanolamine functionalized CuO electrode in electrochemical CO2 reduction

Electrochemical carbon dioxide reduction reactions (eCO2RR) have emerged as promising strategies for both mitigating CO2 emissions and converting them into valuable products. Despite the promise, challenges such as stability, efficiency, and availability of CO2 on the electrode surface, especially at high current densities, still need to be overcome. Herein, this study explores the precipitation of CuO nanoparticles with monoethanolamine to preserve nitrogen groups on the surface of the material. These groups can act by adsorbing the CO2 and stabilizing its catalytic performance during the electroreduction procedure. The incorporation of monoethanolamine as functionalization on the surface of the CuO catalyst was confirmed by XPS measurements. Electrodes utilizing the S-MEA catalyst demonstrated enhanced electrochemical activity, achieving a current density of -187 mA cm-2 at a half-cell potential of -1.2 V versus RHE. Furthermore, long-term stability tests confirmed consistent activity for at least 100 hours in both flow cell and zero gap cell configurations. These results indicate that electrodes featuring the S-MEA catalyst display notably superior electrochemical activity and stability compared with the non-functionalized CuO (S-KOH) and commercial CuO nanopowder (c-CuO). The S-MEA enhancement is attributed to the introduction of amine functional groups that serve as CO2 adducts, facilitating CO2 adsorption and fostering electrode activation. It was evidenced by higher current densities and improved structural integrity during prolonged tests. The insights gained from the comparative performance of these electrodes provide valuable directions for future research in developing more robust and efficient catalysts for environmental remediation technologies.

CO2 induced phase transition on a self-standing droplet studied by X-ray scattering and magnetic resonance

HYPOTHESIS

Acoustic levitation is a suitable approach for studying processes occurring at the gas-liquid interfaces, as it allows its investigation in a contact-free manner while providing control over the gas phase. Here, we hypothesize that phase transitions induced by a CO2 rich atmosphere can be examined, at different length scales, in a contact-free manner.

EXPERIMENTAL

A system consisting of 12-hydroxysteric acid (HSA) soaps mixed with different ratios of monoethanolamine (MEA) and choline hydroxide, was prepared. Microliter droplets of the samples were acoustically levitated and monitored with a camera, while exposed to CO2 to modify the pH through diffusion at the air-liquid interface and inside the droplet. The phase transition and water mobility in the levitated droplets were evaluated through X-ray scattering (SAXS/WAXS) and magnetic resonance studies, in real-time. Finally, the droplets were collected and examined under the microscope.

FINDINGS

The introduction of CO2 gas induced a phase transition from micelles to multi-lamellar tubes, resulting in a gel-like behavior both in the bulk and at the interface. The high stability of the acoustic levitator allowed the investigation of this dynamic phenomenon, in real-time, in a contact-free environment. This study showcases the suitability of acoustic levitation as a tool to investigate complex chemical processes at interfaces.

Photoinduced Carbon Dioxide Release via a Metastable Photoacid in a Nonaqueous Environment

Capturing carbon dioxide (CO2) from the atmosphere is a scientific and technological challenge. CO2 can be captured by forming carbamate bonds with amines, most notably monoethanolamine (MEA). Regenerating MEA by releasing captured CO2 requires that the carbamate solution be heated. Recently, photoacids were used to induce a pH change to release CO2 from aqueous carbonate solutions. We report a merocyanine photoacid that releases CO2 from nonaqueous carbamate solutions of MEA, which has a CO2 loading capacity that is higher than that of water. On the basis of the absorption spectra of the photoacid in the presence of acids and CO2, we show that the photoacid cycle and the CO2 capture of MEA are two separate equilibria coupled to each other via protons. We demonstrate that irradiating the sample with 405 nm light induces the release of CO2, which we detect using an in-line mass spectrometer. This work highlights an alternative path for optimizing a photoinduced CO2 capture and release system.

Enhancing microbial CO2 electrocatalysis for multicarbon reduction in a wet amine-based catholyte

Microbial CO2 electroreduction (mCO2ER) offers a promising approach for producing high-value multicarbon reductants from CO2 by combining CO2 fixing microorganisms with conducting materials (i. e., cathodes). However, the solubility and availability of CO2 in an aqueous electrolyte pose significant limitations in this system. This study demonstrates the efficient production of long-chain multicarbon reductants, specifically carotenoids (~C40), within a wet amine-based catholyte medium during mCO2ER. Optimizing the concentration of the biocompatible CO2 absorbent, monoethanolamine (MEA), led to enhanced CO2 fixation in the electroautotroph bacteria. Molecular biological analyses revealed that MEA in the catholyte medium redirected the carbon flux towards carotenoid biosynthesis during mCO2ER. The faradaic efficiency of mCO2ER with MEA for carotenoid production was 4.5-fold higher than that of the control condition. These results suggest the mass transport bottleneck in bioelectrochemical systems could be effectively addressed by MEA-assissted mCO2ER, enabling highly efficient production of valuable products from CO2.

Insights into Aerosol Emission Control in the Postcombustion CO2 Capture Process: From Cluster Formation to Aerosol Growth

Aerosols produced in the amine carbon capture process can lead to secondary environmental pollution. This study employs molecular dynamics (MD) simulations to investigate cluster formation, amine behavior, and aerosol growth of amines, essential for reducing amine aerosol emissions. Results showed that the cluster evolution process can be divided into cluster formation and growth in terms of molecular content, and the nucleation rate for the present systems was estimated in the order of 1028 cm-3 s-1. CO2 absorption was observed alongside successful nucleation, with CO2 predominantly localizing in the cluster's outer layer postabsorption. Monoethanolamine (MEA) exhibited robust electrostatic interactions with other components via hydrogen bonding, leading to its migration toward regions where CO2 and H2O coexisted within the cluster. While MEA presence markedly spurred cluster formation, its concentration had a marginal effect on the final cluster size. Elevating water content can augment the aerosol growth rate. However, altering the gas saturation is possible only within narrow confines by introducing vapor. Contrarily, gas cooling introduced dual, opposing effects on aerosol growth. These findings, including diffusion coefficients and growth rates, enhance theoretical frameworks for predicting aerosol formation in absorbers, aiding in mitigating environmental impacts of amine-based carbon capture.

Carbon capture in polymer-based electrolytes

Nanoparticle organic hybrid materials (NOHMs) have been proposed as excellent electrolytes for combined CO2 capture and electrochemical conversion due to their conductive nature and chemical tunability. However, CO2 capture behavior and transport properties of these electrolytes after CO2 capture have not yet been studied. Here, we use a variety of nuclear magnetic resonance (NMR) techniques to explore the carbon speciation and transport properties of branched polyethylenimine (PEI) and PEI-grafted silica nanoparticles (denoted as NOHM-I-PEI) after CO2 capture. Quantitative 13C NMR spectra collected at variable temperatures reveal that absorbed CO2 exists as carbamates (RHNCOO- or RR'NCOO-) and carbonate/bicarbonate (CO32-/HCO3-). The transport properties of PEI and NOHM-I-PEI studied using 1H pulsed-field-gradient NMR, combined with molecular dynamics simulations, demonstrate that coulombic interactions between negatively and positively charged chains dominate in PEI, while the self-diffusion in NOHM-I-PEI is dominated by silica nanoparticles. These results provide strategies for selecting adsorbed forms of carbon for electrochemical reduction.

Electricity generation from carbon dioxide adsorption by spatially nanoconfined ion separation

Selective ion transport underpins fundamental biological processes for efficient energy conversion and signal propagation. Mimicking these 'ionics' in synthetic nanofluidic channels has been increasingly promising for realizing self-sustained systems by harvesting clean energy from diverse environments, such as light, moisture, salinity gradient, etc. Here, we report a spatially nanoconfined ion separation strategy that enables harvesting electricity from CO2 adsorption. This breakthrough relies on the development of Nanosheet-Agarose Hydrogel (NAH) composite-based generators, wherein the oppositely charged ions are released in water-filled hydrogel channels upon adsorbing CO2. By tuning the ion size and ion-channel interactions, the released cations at the hundred-nanometer scale are spatially confined within the hydrogel network, while ångström-scale anions pass through unhindered. This leads to near-perfect anion/cation separation across the generator with a selectivity (D-/D+) of up to 1.8 × 106, allowing conversion into external electricity. With amplification by connecting multiple as-designed generators, the ion separation-induced electricity reaching 5 V is used to power electronic devices. This study introduces an effective spatial nanoconfinement strategy for widely demanded high-precision ion separation, encouraging a carbon-negative technique with simultaneous CO2 adsorption and energy generation.

Developing semi-empirical water model for efficiently simulating temperature-dependent chemisorption of CO2 in amine solvents

Classical molecular dynamics (MD) simulations without bond forming/breaking cannot be used to model chemical reactions (CRs) among small molecules. Although the first-principle MD simulation can adequately describe CRs with explicit water molecules, such simulation is normally too costly for most researchers to afford. Generally, water molecules in a solvent can exert hydrophobic forces on reacting molecules, which yields a so-called caging effect that cannot be ignored when constructing a free energy landscape for reacting molecules. Many recently developed semi-empirical methods (such as DFTB, PM6 and xTB) are highly efficient for modeling CRs, however none of them can be directly used to model bulk water properly. Here, we developed a modified xTB approach that enables the simulation of CRs in explicit water. Using the chemisorption of CO2 by amines in water as an example application, we demonstrate that our approach yielded results comparable with the first-principle ones, while only using a limited computing resource. Potentially, our proposed semi-empirical water model can be utilized for the computational study of any CR in water.

A High-Throughput Experimental Approach to Screening Gas Sorption by Liquids and Solids

High-precision measurement of gas uptake from single or mixed feeds in solid and liquid sorbents traditionally requires time-consuming experimental procedures and/or complex and costly equipment. A simple and cost-effective headspace gas chromatography (HS-GC) approach for the fast, universal experimental screening of sorbents for gas uptake and/or determination of their real gas separation selectivity has been developed and is demonstrated for pressures up to 2500 mbar and temperatures above 30 °C. This method allows screening of solids and both volatile and nonvolatile liquid materials, physisorbents, and chemisorbents using both single and mixed permanent gases that can include CO2, CH4, H2, and NH3, for gas uptakes as low as 0.04 mmol or 1.8 mg of CO2. We estimate that this method allows for the screening of at least 30-96 sorbents (in triplicate) or 90-264 sorbents (singles) per day, representing at least a 90-3000 times reduction in the time required for equivalent analysis.

Towards green carbon capture and storage using waste concrete based seawater: A microfluidic analysis

Carbon capture and utilization technology is the research stream dedicated to mitigating the pressing effect of rising atmospheric carbon dioxide (CO2). The present study investigates a potential environmentally conscious solvent to capture and utilize CO2 using waste concrete and seawater under reactor conditions. Although seawater's CO2 soubility is low due to salinity, waste concrete raises seawater's pH and alkalinity, acting as a feedstock for CO2 dissolution and offsetting the adverse effects of salinity. To evaluate the performance of the novel natural seawater-concrete solutions for CO2 capture, time-dependent pH changes of solutions exposed to CO2 were measured in a microchannel using fluorescence microscopy. The concentration of dissolved CO2 in the solution was derived from pH change, revealing a 4-fold increase in the total dissolved carbon from 0.034 to 0.13 M and a 57.54% increase in the CO2 dissolution coefficient from 530 to 835 μm2/s in seawater upon concrete addition. Electrolysis further enhanced the CO2 capture capacity of the seawater-concrete solution by increasing the pH, enabling the solid precipitation of carbonate minerals. Raman spectroscopy and scanning electron microscopy showed that electrolysis-driven precipitates are mainly amorphous calcium carbonates, useful building blocks for seashells and coral reefs.

Carbon Capture Using Porous Silica Materials

As the primary greenhouse gas, CO₂ emission has noticeably increased over the past decades resulting in global warming and climate change. Surprisingly, anthropogenic activities have increased atmospheric CO₂ by 50% in less than 200 years, causing more frequent and severe rainfall, snowstorms, flash floods, droughts, heat waves, and rising sea levels in recent times. Hence, reducing the excess CO₂ in the atmosphere is imperative to keep the global average temperature rise below 2 °C. Among many CO₂ mitigation approaches, CO₂ capture using porous materials is considered one of the most promising technologies. Porous solid materials such as carbons, silica, zeolites, hollow fibers, and alumina have been widely investigated in CO₂ capture technologies. Interestingly, porous silica-based materials have recently emerged as excellent candidates for CO₂ capture technologies due to their unique properties, including high surface area, pore volume, easy surface functionalization, excellent thermal, and mechanical stability, and low cost. Therefore, this review comprehensively covers major CO₂ capture processes and their pros and cons, selecting a suitable sorbent, use of liquid amines, and highlights the recent progress of various porous silica materials, including amine-functionalized silica, their reaction mechanisms and synthesis processes. Moreover, CO₂ adsorption capacities, gas selectivity, reusability, current challenges, and future directions of porous silica materials have also been discussed.

Amine-Assisted Simultaneous CO2 Absorption and Mineral Carbonation: Effect of Different Categories of Amines

The hybrid technology of CO₂ capture-mineral carbonation (CCMC) using alkaline streams has emerged in recent years. However, thus far, there has been no comprehensive study revealing the mechanisms of the simultaneous CCMC process regarding the choice of amine types and sensitivity of parameters. Combining with the analysis of multistep reaction mechanisms for different amines, we investigated a representative from each category in CCMC using calcium chloride to simulate the alkaline resource after leaching, i.e., primary (ethanolamine, MEA), secondary (diisopropanolamine, DIPA), tertiary (diethylethanolamine, DEAE), and triamine (diethylenetriamine, DETA), respectively. In the adsorption step, increasing the amine concentration beyond 2 mol/L reduced the absorption efficiency of DEAE due to the hydration mechanism, motivating a rational choice of concentration. In CCMC sections, when the amine concentration increased, only DEAE exhibited an increased carbonation efficiency of up to 100%, while DETA showed the lowest conversion. The carbonation of DEAE demonstrated the least sensitivity to temperature. The crystal transformation experiments suggested that over time, the produced vaterite could completely transform to calcite or aragonite, except those from DETA. Thus, with rationally chosen conditions, DEAE was demonstrated ideal for CCMC. These findings obtained in this work provided a theoretical foundation for designing future CCMC processes.

Experimental and Computational Mechanisms that Govern Long-Term Stability of CO2-Adsorbed ZIF-8-Based Porous Liquids

Porous liquids (PLs) based on the zeolitic imidazole framework ZIF-8 are attractive systems for carbon capture since the hydrophobic ZIF framework can be solvated in aqueous solvent systems without porous host degradation. However, solid ZIF-8 is known to degrade when exposed to CO₂ in wet environments, and therefore the long-term stability of ZIF-8-based PLs is unknown. Through aging experiments, the long-term stability of a ZIF-8 PL formed using the water, ethylene glycol, and 2-methylimidazole solvent system was systematically examined, and the mechanisms of degradation were elucidated. The PL was found to be stable for several weeks, with no ZIF framework degradation observed after aging in N₂ or air. However, for PLs aged in a CO₂ atmosphere, formation of a secondary phase occurred within 1 day from the degradation of the ZIF-8 framework. From the computational and structural evaluation of the effects of CO₂ on the PL solvent mixture, it was identified that the basic environment of the PL caused ethylene glycol to react with CO₂ forming carbonate species. These carbonate species further react within the PL to degrade ZIF-8. The mechanisms governing this process involves a multistep pathway for PL degradation and lays out a long-term evaluation strategy of PLs for carbon capture. Additionally, it clearly demonstrates the need to examine the reactivity and aging properties of all components in these complex PL systems in order to fully assess their stabilities and lifetimes.

Experimental investigation of novel ternary amine-based deep eutectic solvents for CO2 capture

This study investigates the effect of using water as a low-viscosity component in ternary amine-based deep eutectic solvents (DESs) on the physicochemical properties, thermal stability, and CO2 absorption capacity of the resulting DESs. It should be emphasized that water is a component of the ternary DES. The effect of water content in the DES, type of hydrogen bond acceptors (HBAs), hydrogen bond donors (HBDs), and HBA:HBD ratio on the above parameters was investigated. Moreover, the effect of temperature and pressure on the CO2 absorption capacity of DESs was predicted using the predictive model COSMO-RS. This model was also used to predict the CO2 solubility in the DESs and the results were compared with the experimental values. The results showed that the addition of small amounts of water, e.g., 5 and 10 wt% during preparation, can significantly decrease the viscosity of the resulting DESs, up to 25% at room temperature, while maintaining the high CO2 absorption capacity and high thermal stability. The ternary DESs based on MEA exhibited a high CO2 absorption capacity of 0.155-0.170 g CO2 / g DES. The ternary DESs were found to be thermally stable with a decomposition temperature of 125°C, which promotes the use of such solvents in post-combustion capture processes. Finally, COSMO-RS proved to be a suitable tool for qualitative prediction of CO2 solubility in DESs and demonstration of trends related to the effects of temperature, pressure, molar ratio, water content, HBD and HBA on CO2 solubility.

A Strategical Improvement in the Performance of CO2/N2 Gas Permeation via Conjugation of L-Tyrosine onto Chitosan Membrane

Rubbery polymeric membranes, containing amine carriers, have received much attention in CO₂ separation because of their easy fabrication, low cost, and excellent separation performance. The present study focuses on the versatile aspects of covalent conjugation of L-tyrosine (Tyr) onto the high molecular weight chitosan (CS) accomplished by using carbodiimide as a coupling agent for CO₂/N₂ separation. The fabricated membrane was subjected to FTIR, XRD, TGA, AFM, FESEM, and moisture retention tests to examine the thermal and physicochemical properties. The defect-free dense layer of tyrosine-conjugated-chitosan, with active layer thickness within the range of ~600 nm, was cast and employed for mixed gas (CO₂/N₂) separation study in the temperature range of 25-115 °C in both dry and swollen conditions and compared to that of a neat CS membrane. An enhancement in the thermal stability and amorphousness was displayed by TGA and XRD spectra, respectively, for the prepared membranes. The fabricated membrane showed reasonably good CO₂ permeance of around 103 GPU and CO₂/N₂ selectivity of 32 by maintaining a sweep/feed moisture flow rate of 0.05/0.03 mL/min, respectively, an operating temperature of 85 °C, and a feed pressure of 32 psi. The composite membrane demonstrated high permeance because of the chemical grafting compared to the bare chitosan. Additionally, the excellent moisture retention capacity of the fabricated membrane accelerates high CO₂ uptake by amine carriers, owing to the reversible zwitterion reaction. All the features make this membrane a potential membrane material for CO₂ capture.

CO2 Aggregation on Monoethanolamine: Observations from Rotational Spectroscopy

The initial stages of the gas-phase nucleation between CO₂ and monoethanolamine were investigated via broadband rotational spectroscopy with the aid of extensive theoretical structure sampling. Sub-nanometer-scale aggregation patterns of monoethanolamine-(CO₂ )_n , n=1-4, were identified. An interesting competition between the monoethanolamine intramolecular hydrogen bond and the intermolecular interactions between monoethanolamine and CO₂ upon cluster growth was discovered, revealing an intriguing CO₂ binding priority to the hydroxyl group over the amine group. These findings are in sharp contrast to the general results for aqueous solutions. In the quinary complex, a cap-like CO₂ tetramer was observed cooperatively surrounding the monoethanolamine. As the cluster approaches the critical size of new particle formation, the contribution of CO₂ self-assembly to the overall stability increases.

Structural Features of Monoethanolamine Aqueous Solutions with Various Compositions: A Combined Experimental and Theoretical Study Using Vibrational Spectroscopy

In this work, we studied aqueous solutions of monoethanolamine (MEA), which are widely used to remove CO₂ from flue and oil gases. This study combined experimental and theoretical methods of vibrational spectroscopy, using high-temperature infrared spectroscopy, quantum-chemical calculations of theoretical vibrational spectra, and structural electronic and energy characteristics of model structures. MEA has a propensity to form associations between various compositions and structures with water molecules, as well as those composed solely of water molecules. The structural and energy characteristics of such associates were analyzed in terms of their ability to interact and retain carbon dioxide. The influence of elevated temperatures and concentration of aqueous MEA solution on change in the structure of associates has also been investigated. An analysis of theoretical and experimental vibrational spectra allowed us to examine the IR spectra of MEA solutions, and identify the bands responsible for the formation of associates that would sorb CO₂ well, but would delay its desorption from the solution.

[EMmim][NTf2 ]-a Novel Ionic Liquid (IL) in Catalytic CO2 Capture and ILs' Applications

Ionic liquids (ILs) have been used for carbon dioxide (CO₂ ) capture, however, which have never been used as catalysts to accelerate CO₂ capture. The record is broken by a uniquely designed IL, [EMmim][NTf₂ ]. The IL can universally catalyze both CO₂ sorption and desorption of all the chemisorption-based technologies. As demonstrated in monoethanolamine (MEA) based CO₂ capture, even with the addition of only 2000 ppm IL catalyst, the rate of CO₂ desorption-the key to reducing the overall CO₂ capture energy consumption or breaking the bottleneck of the state-of-the-art technologies and Paris Agreement implementation-can be increased by 791% at 85 °C, which makes use of low-temperature waste heat and avoids secondary pollution during CO₂ capture feasible. Furthermore, the catalytic CO₂ capture mechanism is experimentally and theoretically revealed.

Functionalized Ionic Liquids for CO2 Capture under Ambient Pressure

Ionic liquids (ILs) have been widely explored as alternative solvents for carbon dioxide (CO2) capture and utilization. However, most of these processes are under pressures significantly higher than atmospheric level, which not only levies additional equipment and operation costs, but also makes the large-scale CO2 capture and conversion less practical. In this study, we rationally designed glycol ether-functionalized imidazolium, phosphonium and ammonium ILs containing acetate (OAc-) or Tf2N- anions, and found these task-specific ILs could solubilize up to 0.55 mol CO2 per mole of IL (or 5.9 wt% CO2) at room temperature and atmospheric pressure. Although acetate anions enabled a better capture of CO2, Tf2N- anions are more compatible with alcohol dehydrogenase (ADH), which is a key enzyme involved in the cascade enzymatic conversion of CO2 to methanol. Our promising results indicate the possibility of CO2 capture under ambient pressure and its enzymatic conversion to valuable commodity.

MOF-Derived Robust and Synergetic Acid Sites Inducing C-N Bond Disruption for Energy-Efficient CO2 Desorption

Amine-based scrubbing technique is recognized as a promising method of capturing CO₂ to alleviate climate change. However, the less stability and poor acidity of solid acid catalysts (SACs) limit their potential to further improve amine regeneration activity and reduce the energy penalty. To address these challenges, here, we introduce two-dimensional (2D) cobalt-nitrogen-doped carbon nanoflakes (Co-N-C NSs) driven by a layered metal-organic framework that work as SACs. The designed 2D Co-N-C SACs can exhibit promising stability, superhydrophilic surface, and acidity. Such 2D structure also contains well-confined Co-N₄ Lewis acid sites and -OH Brønsted acid sites to have a synergetic effect on C-N bond disruption and significantly increase CO₂ desorption rate by 281% and reduce the reaction temperatures to 88 °C, minimizing water evaporation by 20.3% and subsequent regeneration energy penalty by 71.7% compared to the noncatalysis.

Preventing Aerosol Emissions in a CO2 Capture System: Combining Aerosol Formation Inhibition and Wet Electrostatic Precipitation

Aerosol emission from the CO₂ capture system has raised great concern for causing solvent loss and serious environmental issues. Here, we propose a comprehensive method for reducing aerosol emissions in a CO₂ capture system under the synergy of aerosol formation inhibition and wet electrostatic precipitation. The gas-solvent temperature difference plays a vital role in aerosol formation, with aerosol emissions of 740.80 mg/m³ at 50 K and 119.36 mg/m³ at 0 K. Different effects of SO₂ and SO₃ on aerosol formation are also found in this research; the aerosol mass concentration could reach 2341.25 mg/m³ at 20 ppm SO₃ and 681.01 mg/m³ at 50 ppm SO₂ with different aerosol size distributions. After the CO₂ capture process, an aerosol removal efficiency of 98% can be realized by electrostatic precipitation under different CO₂ concentrations. Due to the high concentration of aerosols and aerosol space charge generated by SO₂ and SO₃, the removal performance of the wet electrostatic precipitator decreases, resulting in a high aerosol emission concentration (up to 130.26 mg/m³). Thus, a heat exchanger is installed before the electrostatic precipitation section to enhance aerosol growth and increase aerosol removal efficiency. Under the synergy of aerosol formation inhibition and electrostatic precipitation, an aerosol removal efficiency of 99% and emission concentrations lower than 5 mg/m³ are achieved, contributing to global warming mitigation and environmental protection.

Covalent Organic Frameworks with Ionic Liquid-Moieties (ILCOFs): Structures, Synthesis, and CO2 Conversion

CO₂, an acidic gas, is usually emitted from the combustion of fossil fuels and leads to the formation of acid rain and greenhouse effects. CO₂ can be used to produce kinds of value-added chemicals from a viewpoint based on carbon capture, utilization, and storage (CCUS). With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO₂ capture and conversion, including the reduction and cycloaddition of CO₂. Finally, future directions and prospects for ILCOFs are outlined. This review is beneficial for academic researchers in obtaining an overall understanding of ILCOFs and their application of CO₂ conversion. This work will open a door to develop novel ILCOFs materials for the capture, separation, and utilization of other typical acid, basic, or neutral gases such as SO₂, H₂S, NOx, NH₃, and so on.

Energy-Efficient Biphasic Solvents for Industrial Carbon Capture: Role of Physical Solvents on CO2 Absorption and Phase Splitting

Physical solvent is a promising alternative for the phase splitting of solvent to drastically reduce the regeneration energy during CO₂ capture. Here, an aqueous biphasic solvent, optimally composed of 30 wt % polyamine (N, N-dimethylpropylamine, DMPA) and 50 wt % physical solvent (polyethyleneglycol dimethyl ether, NHD), is prepared, which presents high cyclic loading, low regeneration energy, and good stability. L₁₆(4⁵) orthogonal tests are performed to comprehensively evaluate the mass-transfer kinetics and the effect of crucial conditions, verifying the weak effect of NHD solvent on mass transfer. The solvent effect of NHD could decrease the energy barrier of carbamate generation from zwitterions (DMPA⁺COO^-) to enhance chemical absorption. The low polarity of the NHD solvent provides source motivation and accelerates phase splitting. Time-space resolution distribution of CO₂ capacity is established based on a scale-up separator with 5 L solvent, which supports multiscale force analysis for the various stages during phase splitting. The drag force of the homogeneous cluster was first introduced into separation dynamics, referred to as an important reason for the various splitting behaviors of a scale-up separator.

Energy comparison of sequential and integrated CO2 capture and electrochemical conversion

Integrating carbon dioxide (CO2) electrolysis with CO2 capture provides exciting new opportunities for energy reductions by simultaneously removing the energy-demanding regeneration step in CO2 capture and avoiding critical issues faced by CO2 gas-fed electrolysers. However, understanding the potential energy advantages of an integrated process is not straightforward due to the interconnected processes which require knowledge of both capture and electrochemical conversion processes. Here, we identify the upper limits of the integrated process from an energy perspective by comparing the working principles and performance of integrated and sequential approaches. Our high-level energy analyses unveil that an integrated electrolyser must show similar performance to the gas-fed electrolyser to ensure an energy benefit of up to 44% versus the sequential route. However, such energy benefits diminish if future gas-fed electrolysers resolve the CO2 utilisation issue and if an integrated electrolyser shows lower conversion efficiencies than the gas-fed system.

Advancing integrated CO2 electrochemical conversion with amine-based CO2 capture: a review

Carbon dioxide (CO₂) electrolysis is a promising route to utilise captured CO₂ as a building block to produce valuable feedstocks and fuels such as carbon monoxide and ethylene. Very recently, CO₂ electrolysis has been proposed as an alternative process to replace the amine recovery unit of the commercially available amine-based CO₂ capture process. This process would replace the most energy-intensive unit operation in amine scrubbing while providing a route for CO₂ conversion. The key enabler for such process integration is to develop an efficient integrated electrolyser that can convert CO₂ and recover the amine simultaneously. Herein, this review provides an overview of the fundamentals and recent progress in advancing integrated CO₂ conversion in amine-based capture media. This review first discusses the mechanisms for both CO₂ absorption in the capture medium and electrochemical conversion of the absorbed CO₂. We then summarise recent advances in improving the efficiency of integrated electrolysis via innovating electrodes, tailoring the local reaction environment, optimising operation conditions (e.g., temperatures and pressures), and modifying cell configurations. This review is concluded with future research directions for understanding and developing integrated CO₂ electrolysers.

Azo-Linked Porous Organic Polymers for Selective Carbon Dioxide Capture and Metal Ion Removal

The facile and environmentally friendly synthesis of porous organic polymers with designed polar functionalities decorating the interior frameworks as an excellent adsorbent for selective carbon dioxide capture and metal ion removal is a target worth pursuing for environmental applications. In this regard, two azo-linked porous organic polymers denoted man-Azo-P1 and man-Azo-P2 were synthesized in water by the azo-linking of 4,4'-diaminobiphenyl (benzidine) and 4,4'-methylenedianiline, respectively, with 1,3,5-trihydroxybenzene. The resulting polymers showed good BET surface areas of 290 and 78 m² g^-1 for man-Azo-P1 and man-Azo-P2, respectively. Due to the enriched core functionality of the azo (-N=N-) and hydroxyl groups along with the porous frameworks, man-Azo-P1 exhibited a good CO₂ uptake capacity of 32 cm³ g^-1 at 273 K and 1 bar, in addition to the remarkable removal of lead (Pd), chromium (Cr), arsenic (As), nickel (Ni), copper (Cu), and mercury (Hg) ions. This performance of the synthesized man-Azo-P1 and man-Azo-P2 in the dual application of CO₂ capture and heavy metal ion removal highlights the unique properties of azo-linked POPs as excellent and stable sorbent materials for the current challenging environmental applications.

Engineered assembly of water-dispersible nanocatalysts enables low-cost and green CO2 capture

Catalytic solvent regeneration has attracted broad interest owing to its potential to reduce energy consumption in CO₂ separation, enabling industry to achieve emission reduction targets of the Paris Climate Accord. Despite recent advances, the development of engineered acidic nanocatalysts with unique characteristics remains a challenge. Herein, we establish a strategy to tailor the physicochemical properties of metal-organic frameworks (MOFs) for the synthesis of water-dispersible core-shell nanocatalysts with ease of use. We demonstrate that functionalized nanoclusters (Fe₃O₄-COOH) effectively induce missing-linker deficiencies and fabricate mesoporosity during the self-assembly of MOFs. Superacid sites are created by introducing chelating sulfates on the uncoordinated metal clusters, providing high proton donation capability. The obtained nanomaterials drastically reduce the energy consumption of CO₂ capture by 44.7% using only 0.1 wt.% nanocatalyst, which is a ∽10-fold improvement in efficiency compared to heterogeneous catalysts. This research represents a new avenue for the next generation of advanced nanomaterials in catalytic solvent regeneration.

Catalytic solvent regeneration is of interest to reduce energy consumption in CO2 separation, however, the development of engineered nanocatalysts remains a challenge. Here, a new avenue is presented for the next generation of advanced metal-organic frameworks (MOFs) in energy-efficient CO2 capture.

Innovative method for CO2 fixation and storage

The concentration of CO₂ in Earth’s atmosphere has been gradually increasing since the Industrial Revolution, primarily as a result of the use of fossil fuels as energy sources. Although coal and oil have been vital to the development of modern civilization, it is now recognized that atmospheric CO₂ levels must be reduced to avoid the serious effects of climate change, including natural disasters. Consequently, there is currently significant interest in developing suitable methods for the fixation of CO₂ in the air and in exhaust gases. The present work demonstrates a simple yet innovative approach to the chemical fixation of extremely low and very high CO₂ concentrations in air, such as might result from industrial sources. This process is based on the use of aqueous solutions of the water-soluble compounds NaOH and CaCl₂, which react with CO₂ to produce the harmless solids CaCO₃ (limestone) and NaCl (salt) via intermediates such as NaHCO₃ and Na₂CO₃. The NaCl generated in this process can be converted back to NaOH via electrolysis, during which H₂ (which can be used as a clean energy source) and Cl₂ are produced simultaneously. Additionally, sea water contains both NaCl and CaCl₂ and so could provide a ready supply of these two compounds. This system provides a safe, inexpensive approach to simultaneous CO₂ fixation and storage.

Hierarchical porous polystyrene-based activated carbon spheres for CO2 capture

It is rather essential to design porous carbon adsorbents with high CO2 capture performance for improving global warming and climate change. Activated carbon spheres with high specific surface area and hierarchical porous texture were prepared from polystyrene-based macroreticular resin spheres due to their low ash and mechanical stability by air pre-oxidization and steam activation. The as-prepared carbon spheres had a specific surface area of 1274.95 m2 g-1, total pore volume of 1.09 cm3 g-1 and micropore volume of 0.47 cm3 g-1. Moreover, these carbon spheres showed a hierarchical porous texture composed of ultrafine micropores (0.5-1 nm), micropores (1-2 nm), mesopores (10-50 nm) and macropores (50-100 nm). A CO2 adsorption capacity of 2.82 mmol g-1 for carbon spheres can be obtained at 30 °C and 1 atm. Further, after introducing nitrogen-containing functional groups by gaseous ammonia at 600 °C, these carbon spheres (NPSRCSs) exhibited a high CO2 adsorption capacity of 3.2 mmol g-1. In addition, excellent cyclic stability, low hygroscopicity and regenerability temperature suggested these carbon spheres were favorable for CO2 capture.

Presence of activated carbon particles from waste walnut shell as a biosorbent in monoethanolamine (MEA) solution to enhance carbon dioxide absorption

Greenhouse effects are a natural phenomenon that plays a high role in shaping the climate system. In this research, MEA solution was used for CO₂ capture in presence of activated carbon particles from waste walnut shells as a biosorbent. The process parameters including temperature, pressure, MEA concentration, and activated carbon were used in the central composite design (CCD) model. The absorption experiments were carried out in a laboratory setup at operational conditions including temperature in range of 20-60 °C, pressure in range of 3.5-9.5 bar, MEA concentration in range of 2.5-8.5 wt%, and active carbon amount in range of 0.3-0.9 g/L. The process responses including CO₂ loading, the amounts of CO₂ absorption, and absorption percentage were obtained in the range of 0.444-0.720 mol_CO2/mol_MEA, 0.294-0.687 mol/L, and 19.32-52.25%, respectively. The optimal value of CO₂ loading was obtained at temperature of 30 °C, pressure of 5.19 bar, activated carbon of 0.75 g, and MEA concentration of 7.00 wt%. The optimum values of responses were obtained 0.531, 0.609 mol/L and 50.04% for maximum loading, absorption amount, and absorption percentage, respectively. From the results, carbon dioxide loading in MEA solution increases in presence of activated carbon particles.

Highly Efficient Absorption of CO2 by Protic Ionic Liquids-Amine Blends at High Temperatures

In view of the increasingly serious harm of CO₂ to the environment, it is highly desirable to develop effective CO₂ absorbents. In this work, we demonstrated an efficient absorption of CO₂ by blends of protic ionic liquids (PILs) plus amines. The density and viscosity of investigative four PILs-amine mixtures were measured. By systematically studying the effects of the solution ratio, temperature, CO₂ partial pressure, and water content on the absorption of CO₂, it is found that the 3-dimethylamino-1-propylamine acetate ([DMAPAH][OAc]) plus ethanediamine (EDA) mixture shows the highest CO₂ uptake of 0.295 g CO₂ per g absorbent at 50 °C and 1 bar and a further increase in the absorption of CO₂ to 0.299 g/g by adding water with a mass fraction of 20%. Furthermore, the absorption mechanism of CO₂ in the presence and absence of water has also been investigated by FTIR and NMR spectra.

Experimental investigation on CO2 absorption and physicochemical characteristics of different carbon-loaded aqueous solvents

The anthropogenic carbon dioxide (CO₂) denseness in the earth's atmosphere is increasing day-to-day by combusting fossil fuels for power generation. And, it is the most important greenhouse gas (GHG) responsible for 64% of global warming. Solvent-based carbon capture gained more attention towards researchers because of its easiness to integrate with the coal-fired power plant without significant modifications. During CO₂ absorption, the physical property of the solvent gets changed. A change in the solvent's physicochemical property affects further CO₂ absorption, thereby increasing the carbon-capture energy demand. The present experimental study encompasses CO₂ absorption studies using 30 wt% aqueous monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ) followed by the detailed analysis of physicochemical properties (pH, carbon loading (α), viscosity (μ), density (ρ) and surface tension (σ)) of various CO₂-loaded solutions. The results revealed that these properties are exhibiting interdependent eccentrics. Furthermore, an empirical model was developed to predict the carbon loading of the tested solvents. This model includes the tested physicochemical properties, reaction mixture temperature, diffusivity and change in the mass of solvent during carbon loading. In addition, an empirical model for viscosity as a function of temperature, carbon loading and molecular weight of solvents was developed. These models appear to predict the carbon loading and the viscosity well with greater accuracy. Graphical abstract.

A Novel Dual-Stage Phase Separation Process for CO2 Absorption into a Biphasic Solvent with Low Energy Penalty

An amine-based biphasic solvent is promising to cut down the energy penalty of CO₂ capture. However, the high viscosity of the CO₂-enriched solvent retards its industrial application. This work proposed a novel dual-stage phase separation process using a triethylenetetramine and 2-(diethylamino)ethanol blend as a biphasic solvent, which separates a certain proportion of CO₂-enriched phase during CO₂ absorption to reduce its viscosity. Experimental results showed that the proposed dual-stage phase separation process improved the phase separation behavior and effectively enhanced the absorption rate by 49% at 50 °C, when 50 vol % CO₂-enriched phase was separated at 0.3 mol mol^-1. Kinetic analysis showed that the absorption rate was mainly controlled by liquid-side mass transfer. The regeneration heat of the dual-stage phase separation process cut down the energy penalty by 33% compared with the monoethanolamine-based process. Compared with the conventional biphasic solvent-based process, the heat duty was further declined by 8%. The ¹H nuclear magnetic resonance analysis showed that the dual-stage phase separation process could effectively control the generation of absorption products and intensify the interphase migration of tertiary amines.

CO2 capturing methods: Chemical looping combustion (CLC) as a promising technique

This reports recent advances on CO₂ capturing methods, focusing on chemical looping combustion (CLC) as a promising technology to achieve this goal. Generally, there are three main methods to capture CO₂ resulting from fossil fuel combustion: post-combustion, oxy-combustion, and pre-combustion. In CLC, which is either classified as a pre-combustion method or as the fourth capturing method, the solid oxygen carrier provides the oxygen needed for combustion. This technique helps to avoid diluting the combustion effluent stream with the N₂ released from air and therefore, minimizes the requirement of CO₂ separation, a major cost of CO₂ capture. In addition, it minimizes the formation of NO_x that results when N₂ comes in contact with oxygen and fuels at high temperatures. The desired properties of oxygen carrier candidates for CLC are high reduction and re-oxidation rates, high oxygen capacity, good stability and fludiziability at high temperatures, friendly to the environment, and low cost. Transition metal oxides are common candidates for CLC. Most investigations in this field have examined the reactivity and stability of oxygen carriers but few investigations have focused on their reduction and re-oxidation reaction mechanisms. Researchers have proposed two mechanisms for these reactions, the nucleation-nuclei growth and unreacted shrinking core models. Despite numerous investigations of CLC, there is still a lack of knowledge in some of its aspects such as the underlying surface chemistry and the economic impact. This work critically reviewed all capturing methods of CO₂ with focusing on CLC process as a promising technology due to its ability to concentrate the resulted CO₂ and minimizes the separation cost. This work provides essential insight information into CLC technology and highlights its status and needs.

CO2 Activation Within a Superalkali-Doped Fullerene

With the aim of finding a suitable synthesizable superalkali species, using the B3LYP/6-31G* density functional level of theory we provide results for the interaction between the buckminsterfullerene C60 and the superalkali Li3F2. We show that this endofullerene is stable and provides a closed environment in which the superalkali can exist and interact with CO2. It is worthwhile to mention that the optimized Li3F2 structure inside C60 is not the most stable C2v isomer found for the "free" superalkali but the D3h geometry. The binding energy at 0 K between C60 and Li3F2 (D3h) is computed to be 119 kJ mol-1. Once CO2 is introduced in the endofullerene, it is activated, and theO C O ^ angle is bent to 132°. This activation does not follow the previously studied CO2 reduction by an electron transfer process from the superalkali, but it is rather an actual reaction where a F (from Li3F2) atom is bonded to the CO2. From a thermodynamic analysis, both CO2 and the encapsulated [Li3F2⋅CO2] are destabilized in C60 with solvation energies at 0 K of 147 and < -965 kJ mol-1, respectively.

Stability of a Monoethanolamine-CO2 Zwitterion at the Vapor/Liquid Water Interface: Implications for Low Partial Pressure Carbon Capture Technologies

The need to chemically convert CO₂ at the interface of aqueous amine solutions has become particularly relevant for the development and the broad distribution of cost-effective and near-future devices for direct air capture working at low (e.g., ambient) partial pressure. Here, we have determined the stability of a CO₂-monoethanolamine zwitterion and its chemical conversion into carbamate at the vapor/liquid water interface by first-principles molecular dynamics simulations coupled with a recently introduced enhanced sampling technique. Contrary to the bulk water case, our results show that both the zwitterion and carbamate ions are poorly stable at the vapor/amine aqueous interface, further stating the differences between the homogeneous and heterogeneous CO₂ chemical conversion. The design of novel and cost-effective capture systems, such as those offered by amine-based scrubbing solutions, working at low (e.g., ambient) CO₂ partial pressure should explore the use of novel solvents, different from aqueous mixtures, to overcome the limits of the current absorbents.

Chemisorption of CO2 by diamine-tetraamido macrocyclic motifs: a theoretical study

Although alkanolamines have been systematically utilized for CO2 capture, intensive research efforts are still required to ultimately design more efficient CO2 sorbents with appropriate sorption characteristics. In this article, we have explored a series of diamine-tetraamido macrocyclic molecules with different organic linkers, namely, pyridine, phenylene, pyrrole, furan, and thiophene, for the titled purpose using quantum chemical calculations. The optimized structures of the sequestration reaction revealed the formation of a carbamate anion within the macrocyclic cavity that was stabilized through several intramolecular interactions compared to parent amines. The reaction thermodynamics indicated that the macrocyclic compounds with pyridine, pyrrole and furan can effectively capture CO2. The results highlight the potential application of macrocyclic structures as efficient CO2 capturing agents.