The "Missing" Bicarbonate in CO2 Chemisorption Reactions on Solid Amine Sorbents

Understanding the Key Role of Cations in Water Tolerance during the CO2/CO Separation Process under Low-Humidity Conditions

The efficient separation of CO2 and CO under low-humidity conditions is crucial for ensuring the long-term operational stability of industrial applications. While the number of adsorbent cations plays a key role in separation, their influence on purification under low humidity remains insufficiently understood. The breakthrough results indicate that, even under extremely low humidity, the adsorption capacity of CO2 and CO can decrease by up to 6 and 24%, respectively. It is found that the presence of water could increases the CO2/CO separation factor from 6.91 to 9. This enhancement occurs because CO, with its lower quadrupole moment, experiences a more significant reduction in adsorption capacity than CO2. The quantity and accessibility of cations significantly influence the water tolerance in adsorption processrs. As the number of cations decreases, CO adsorption stabilizes due to the associated hydrophobicity. However, for CO2, the high accessibility of cations at the S3 site in NaX(88) facilitates its conversion to stable carbonates and bicarbonates in the presence of water, enabling exceptional water resistance. These findings offer valuable insights into designing high-performance adsorbents for efficient CO2 capture and separation from industrial flue gas under low-humidity conditions.

Probing the Distribution and Mobility of Aminopolymers after Multiple Sorption-Regeneration Cycles: Neutron Scattering Studies

Solid-supported amines are effective CO2 adsorbents capable of capturing CO2 from flue gas streams (10-15 vol % CO2) and from ultradilute streams, such as ambient air (∼400 ppm CO2). Amine sorbents have demonstrated promising performance (e.g., high CO2 uptake and uptake rates) with stable characteristics under repeated, idealized thermal swing conditions, enabling multicycle application. Literature studies suggest that solid-supported amines such as PEI/SBA-15 generally exhibit slowly reducing CO2 uptake rates or capacities over repeated thermal swing capture-regeneration cycles under simulated DAC conditions. While there are experimental reports describing changes in supported amine mass, degradation of amine sites, and changes in support structures over cycling, there is limited knowledge about the structure and mobility of the amine domains in the support pores over extended use. Furthermore, little is known about the effects of H2O on cyclic applications of PEI/SBA-15 despite the inevitable presence of H2O in ambient air. Here, we present a series of neutron scattering studies exploring the distribution and mobility of PEI in mesoporous silica SBA-15 as a function of thermal cycling and cyclic conditions. Small-angle neutron scattering (SANS) and quasielastic neutron scattering (QENS) are used to study the amine and H2O distributions and amine mobility, respectively. Applying repeated thermal swings under dry conditions leads to the thorough removal of water from the sorbent, causing thinner and more rigid wall-coating PEI layers that eventually lead to slower CO2 uptake rates. On the other hand, wet cyclic conditions led to the sorption of atmospheric water at the wall-PEI interfaces. When PEI remains hydrated, the amine distribution (i.e., wall-coating PEI layer thickness) is retained over cycling, while lubrication effects of water yield improved PEI mobility, in turn leading to faster CO2 uptake rates.

Extended MOF-74-Type Variant with an Azine Linkage: Efficient Direct Air Capture and One-Pot Synthesis

Direct air capture (DAC) shows considerable promise for the effective removal of CO2; however, materials applicable to DAC are lacking. Among metal-organic framework (MOF) adsorbents, diamine-Mg2(dobpdc) (dobpdc4- = 4,4-dioxidobiphenyl-3,3'-dicarboxylate) effectively removes low-pressure CO2, but the synthesis of the organic ligand requires high temperature, high pressure, and a toxic solvent. Besides, it is necessary to isolate the ligand for utilization in the synthesis of the framework. In this study, we synthesized a new variant of extended MOF-74-type frameworks, M2(hob) (M = Mg2+, Co2+, Ni2+, and Zn2+; hob4- = 5,5'-(hydrazine-1,2-diylidenebis(methanylylidene))bis(2-oxidobenzoate)), constructed from an azine-bonded organic ligand obtained through a facile condensation reaction at room temperature. Functionalization of Mg2(hob) with N-methylethylenediamine, N-ethylethylenediamine, and N,N'-dimethylethylenediamine (mmen) enables strong interactions with low-pressure CO2, resulting in top-tier adsorption capacities of 2.60, 2.49, and 2.91 mmol g-1 at 400 ppm of CO2, respectively. Under humid conditions, the CO2 capacity was higher than under dry conditions due to the presence of water molecules that aid in the formation of bicarbonate species. A composite material combining mmen-Mg2(hob) and polyvinylidene fluoride, a hydrophobic polymer, retained its excellent adsorption performance even after 7 days of exposure to 40% relative humidity. In addition, the one-pot synthesis of Mg2(hob) from a mixture of the corresponding monomers is achieved without separate ligand synthesis steps; thus, this framework is suitable for facile large-scale production. This work underscores that the newly synthesized Mg2(hob) and its composites demonstrate significant potential for DAC applications.

Efficient Selective Carbon Dioxide Separation via Task-Specific Ionic Liquids Incorporated in ZIF-8

Owing to the rapid increase in anthropogenic emission of carbon dioxide (CO2) in the atmosphere, which has resulted in a number of global climate challenges, a decrease in CO2 emissions is urgently needed in the current scenario. This study focuses on the development and characterization of composites for carbon dioxide (CO2) separation. The composites consist of two task-specific ionic liquids (TSILs), namely, tetramethylgunidinium imidazole [TMGHIM] and tetramethylgunidinium phenol [TMGHPhO], impregnated in ZIF-8. The performance of CO2 separation, including sorption capacity and selectivity, was evaluated for pristine ZIF-8 and composites of TMGHIM@ZIF-8 and TMGHPhO@ZIF-8. To demonstrate the thermal stability of the material, thermogravimetric analysis (TGA) was performed. Additionally, powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to showcase the crystal structures and morphology. Fourier transform infrared spectroscopy (FTIR) and BET were also utilized to confirm the successful incorporation of TSILs into ZIF-8. The composite synthesized with TMGHIM@ZIF-8 demonstrated superior CO2 sorption performance as compared with TMGHPhO@ZIF-8. This is attributed to its strong attraction toward CO2, resulting in a higher CO2/CH4 selectivity of 110 while pristine MOFs showed 12 that is 9 times higher than that of the pristine ZIF-8. These TSILs@ZIF-8 composites have significant potential in designing sorbent materials for efficient acid gas separation applications.

Cold Temperature Direct Air CO2 Capture with Amine-Loaded Metal-Organic Framework Monoliths

Zeolites, silica-supported amines, and metal-organic frameworks (MOFs) have been demonstrated as promising adsorbents for direct air CO2 capture (DAC), but the shaping and structuring of these materials into sorbent modules for practical processes have been inadequately investigated compared to the extensive research on powder materials. Furthermore, there have been relatively few studies reporting the DAC performance of sorbent contactors under cold, subambient conditions (temperatures below 20 °C). In this work, we demonstrate the successful fabrication of adsorbent monoliths composed of cellulose acetate (CA) and adsorbent particles such as zeolite 13X and MOF MIL-101(Cr) by a 3D printing technique: solution-based additive manufacturing (SBAM). These monoliths feature interpenetrated macroporous polymeric frameworks in which microcrystals of zeolite 13X or MIL-101(Cr) are evenly distributed, highlighting the versatility of SBAM in fabricating monoliths containing sorbents with different particle sizes and density. Branched poly(ethylenimine) (PEI) is successfully loaded into the CA/MIL-101(Cr) monoliths to impart CO2 uptakes of 1.05 mmol gmonolith-1 at -20 °C and 400 ppm of CO2. Kinetic analysis shows that the CO2 sorption kinetics of PEI-loaded MIL-101(Cr) sorbents are not compromised in the monoliths compared to the powder sorbents. Importantly, these monoliths exhibit promising working capacities (0.95 mmol gmonolith-1) over 14 temperature swing cycles with a moderate regeneration temperature of 60 °C. Dynamic breakthrough experiments at 25 °C under dry conditions reveal a CO2 uptake capacity of 0.60 mmol gmonolith-1, which further increases to 1.05 and 1.43 mmol gmonolith-1 at -20 °C under dry and humid (70% relative humidity) conditions, respectively. Our work showcases the successful implementation of SBAM in making DAC sorbent monoliths with notable CO2 capture performance over a wide range of sorption temperatures, suggesting that SBAM can enable the preparation of efficient sorbent contactors in various form factors for other important chemical separations.

Direct Air Capture of CO2 Using Amine/Alumina Sorbents at Cold Temperature

Rising CO2 emissions are responsible for increasing global temperatures causing climate change. Significant efforts are underway to develop amine-based sorbents to directly capture CO2 from air (called direct air capture (DAC)) to combat the effects of climate change. However, the sorbents' performances have usually been evaluated at ambient temperatures (25 °C) or higher, most often under dry conditions. A significant portion of the natural environment where DAC plants can be deployed experiences temperatures below 25 °C, and ambient air always contains some humidity. In this study, we assess the CO2 adsorption behavior of amine (poly(ethyleneimine) (PEI) and tetraethylenepentamine (TEPA)) impregnated into porous alumina at ambient (25 °C) and cold temperatures (-20 °C) under dry and humid conditions. CO2 adsorption capacities at 25 °C and 400 ppm CO2 are highest for 40 wt% TEPA-incorporated γ-Al2O3 samples (1.8 mmol CO2/g sorbent), while 40 wt % PEI-impregnated γ-Al2O3 samples exhibit moderate uptakes (0.9 mmol g-1). CO2 capacities for both PEI- and TEPA-incorporated γ-Al2O3 samples decrease with decreasing amine content and temperatures. The 40 and 20 wt % TEPA sorbents show the best performance at -20 °C under dry conditions (1.6 and 1.1 mmol g-1, respectively). Both the TEPA samples also exhibit stable and high working capacities (0.9 and 1.2 mmol g-1) across 10 cycles of adsorption-desorption (adsorption at -20 °C and desorption conducted at 60 °C). Introducing moisture (70% RH at -20 and 25 °C) improves the CO2 capacity of the amine-impregnated sorbents at both temperatures. The 40 wt% PEI, 40 wt % TEPA, and 20 wt% TEPA samples show good CO2 uptakes at both temperatures. The results presented here indicate that γ-Al2O3 impregnated with PEI and TEPA are potential materials for DAC at ambient and cold conditions, with further opportunities to optimize these materials for the scalable deployment of DAC plants at different environmental conditions.

Distribution and Transport of CO2 in Hyperbranched Poly(ethylenimine)-Loaded MCM-41: A Molecular Dynamics Simulation Approach

Fossil fuel use is accelerating climate change, driving the need for efficient CO2 capture technologies. Solid adsorption-based direct air capture (DAC) of CO2 has emerged as a promising mode for CO2 removal from the atmosphere due to its potential for scalability. Sorbents based on porous supports incorporating oligomeric amines in their pore spaces are widely studied. In this study, we investigate the intermolecular interactions and adsorption of CO2 and H2O molecules in hyperbranched poly(ethylenimine) (HB-PEI) functionalized MCM-41 systems to understand the distribution and transport of CO2 and H2O molecules. Density Functional Theory (DFT) is employed to compute the binding energies of CO2 and H2O molecules with HB-PEI and MCM-41 and to develop force field parameters for molecular dynamics (MD) simulations. The MD simulations are performed to examine the distribution and transport of CO2 and H2O molecules as a function of the HB-PEI content. The study finds that an HB-PEI content of approximately 34 wt % is thermodynamically favorable, with an upper limit of HB-PEI loading between 45 and 50 wt %. The distribution of CO2 and H2O molecules is primarily determined by their adsorptive binding energies, for which H2O molecules dominate the occupation of binding sites due to their strong affinity with silanol groups on MCM-41 and amine groups of HB-PEI. The HB-PEI content has a considerable impact on the diffusion of CO2 and H2O molecules. Furthermore, a larger number of water molecules (higher relative humidity) reduces the correlation of CO2 with the MCM-41 pore surface while enhancing the correlation of CO2 with the amine groups of the HB-PEI. Overall, the presence of H2O molecules increases the CO2 correlation with the amine groups and also the CO2 transport within HB-PEI-loaded MCM-41, meaning that the presence of H2O enhances the CO2 capture in the HB-PEI-loaded MCM-41. These findings are consistent with experimental observations of the impact of increasing humidity on CO2 capture while providing new, molecular-level explanations for the macroscopic experimental findings.

Developing Versatile Contactors for Direct Air Capture of CO2 through Amine Grafting onto Alumina Pellets and Alumina Wash-Coated Monoliths

The optimization of the air-solid contactor is critical to improve the efficiency of the direct air capture (DAC) process. To enable comparison of contactors and therefore a step toward optimization, two contactors are prepared in the form of pellets and wash-coated honeycomb monoliths. The desired amine functionalities are successfully incorporated onto these industrially relevant pellets by means of a procedure developed for powders, providing materials with a CO2 uptake not influenced by the morphology and the structure of the materials according to the sorption measurements. Furthermore, the amine functionalities are incorporated onto alumina wash-coated monoliths that provide a similar CO2 uptake compared to the pellets. Using breakthrough measurements, dry CO2 uptakes of 0.44 and 0.4 mmol gsorbent-1 are measured for pellets and for a monolith, respectively. NMR and IR studies of CO2 uptake show that the CO2 adsorbs mainly in the form of ammonium carbamate. Both contactors are characterized by estimated Toth isotherm parameters and linear driving force (LDF) coefficients to enable an initial comparison and provide information for further studies of the two contactors. LDF coefficients of 1.5 × 10-4 and of 1.2 × 10-3 s-1 are estimated for the pellets and for a monolith, respectively. In comparison to the pellets, the monolith therefore exhibits particularly promising results in terms of adsorption kinetics due to its hierarchical pore structure. This is reflected in the productivity of the adsorption step of 6.48 mol m-3 h-1 for the pellets compared to 7.56 mol m-3 h-1 for the monolith at a pressure drop approximately 1 order of magnitude lower, making the monoliths prime candidates to enhance the efficiency of DAC processes.

Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines

A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO2 have been developed, yet the effect of amine-solid support interactions on the CO2 adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, commercial γ-Al2O3 and MIL-101(Cr), they show different trends in CO2 sorption when the temperature (-20 to 25 °C) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO2 sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al2O3-supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement observed at -20 °C. However, while equilibrium H2O sorption is high at cold temperatures (e.g., -20 °C), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H2O uptake kinetics. This work suggests that the CO2 capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H2O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20 °C) or ambient temperature (e.g., 25 °C) operations.

Single-Walled Zeolitic Nanotubes: Advantaged Supports for Poly(ethylenimine) in CO2 Separation from Simulated Air and Flue Gas

Previous research has demonstrated that amine polymers rich in primary and secondary amines supported on mesoporous substrates are effective, selective sorbent materials for removal of CO₂ from simulated flue gas and air. Common substrates used include mesoporous alumina and silica (such as SBA-15 and MCM-41). Conventional microporous materials are generally less effective, since the pores are too small to support low volatility amines. Here, we deploy our newly discovered zeolite nanotubes, a first-of-their-kind quasi-1D hierarchical zeolite, as a substrate for poly(ethylenimine) (PEI) for CO₂ capture from dilute feeds. PEI is impregnated into the zeolite at specific organic loadings. Thermogravimetric analysis and porosity measurements are obtained to determine organic loading, pore filling, and surface area of the supported PEI prior to CO₂ capture studies. MCM-41 with comparable pore size and surface area is also impregnated with PEI to provide a benchmark material that allows for insight into the role of the zeolite nanotube intrawall micropores on CO₂ uptake rates and capacities. Over a range of PEI loadings, from 20 to 70 w/w%, the zeolite allows for increased CO₂ capture capacity over the mesoporous silica by ∼25%. Additionally, uptake kinetics for nanotube-supported PEI are roughly 4 times faster than that of a comparable PEI impregnated in SBA-15. It is anticipated that this new zeolite will offer numerous opportunities for engineering additional advantaged reaction and separation processes.

Nuclear magnetic resonance studies of carbon dioxide capture

Carbon dioxide capture is an important greenhouse gas mitigation technology that can help limit climate change. The design of improved capture materials requires a detailed understanding of the mechanisms by which carbon dioxide is bound. Nuclear magnetic resonance (NMR) spectroscopy methods have emerged as a powerful probe of CO2 sorption and diffusion in carbon capture materials. In this article, we first review the practical considerations for carrying out NMR measurements on capture materials dosed with CO2 and we then present three case studies that review our recent work on NMR studies of CO2 binding in metal-organic framework materials. We show that simple 13C NMR experiments are often inadequate to determine CO2 binding modes, but that more advanced experiments such as multidimensional NMR experiments and 17O NMR experiments can lead to more conclusive structural assignments. We further discuss how pulsed field gradient (PFG) NMR can be used to explore diffusion of adsorbed CO2 through the porous framework. Finally, we provide an outlook on the challenges and opportunities for the further development of NMR methodologies that can improve our understanding of carbon capture.

Revealing carbon capture chemistry with 17-oxygen NMR spectroscopy

Carbon dioxide capture is essential to achieve net-zero emissions. A hurdle to the design of improved capture materials is the lack of adequate tools to characterise how CO2 adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a promising probe of CO2 capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that 17O NMR is a powerful probe of CO2 capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by 17O NMR experiments on a series of CO2-loaded frameworks that clearly identify ammonium carbamate chain formation and provide evidence for a mixed carbamic acid - ammonium carbamate adsorption mode. We further find that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Our work paves the way for investigations of carbon capture chemistry that can enable materials design.

Review on CO2 Capture Using Amine-Functionalized Materials

CO₂ capture from industry sectors or directly from the atmosphere is drawing much attention on a global scale because of the drastic changes in the climate and ecosystem which pose a potential threat to human health and life on Earth. In the past decades, CO₂ capture technology relied on classical liquid amine scrubbing. Due to its high energy consumption and corrosive property, CO₂ capture using solid materials has recently come under the spotlight. A variety of porous solid materials were reported such as zeolites and metal-organic frameworks. However, amine-functionalized porous materials outperform all others in terms of CO₂ adsorption capacity and regeneration efficiency. This review provides a brief overview of CO₂ capture by various amines and mechanistic aspects for newcomers entering into this field. This review also covers a state-of-the-art regeneration method, visible/UV light-triggered CO₂ desorption at room temperature. In the last section, the current issues and future perspectives are summarized.

A scalable solid-state nanoporous network with atomic-level interaction design for carbon dioxide capture

Carbon capture and sequestration reduces carbon dioxide emissions and is critical in accomplishing carbon neutrality targets. Here, we demonstrate new sustainable, solid-state, polyamine-appended, cyanuric acid-stabilized melamine nanoporous networks (MNNs) via dynamic combinatorial chemistry (DCC) at the kilogram scale toward effective and high-capacity carbon dioxide capture. Polyamine-appended MNNs reaction mechanisms with carbon dioxide were elucidated with double-level DCC where two-dimensional heteronuclear chemical shift correlation nuclear magnetic resonance spectroscopy was performed to demonstrate the interatomic interactions. We distinguished ammonium carbamate pairs and a mix of ammonium carbamate and carbamic acid during carbon dioxide chemisorption. The coordination of polyamine and cyanuric acid modification endows MNNs with high adsorption capacity (1.82 millimoles per gram at 1 bar), fast adsorption time (less than 1 minute), low price, and extraordinary stability to cycling by flue gas. This work creates a general industrialization method toward carbon dioxide capture via DCC atomic-level design strategies.

Recent advances in direct air capture by adsorption

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO₂ capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO₂ adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO₂ concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO₂ adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

Carbon Dioxide Capture at Nucleophilic Hydroxide Sites in Oxidation-Resistant Cyclodextrin-Based Metal-Organic Frameworks

Carbon capture and sequestration (CCS) from industrial point sources and direct air capture are necessary to combat global climate change. A particular challenge faced by amine-based sorbents-the current leading technology-is poor stability towards O₂ . Here, we demonstrate that CO₂ chemisorption in γ-cylodextrin-based metal-organic frameworks (CD-MOFs) occurs via HCO₃ ^- formation at nucleophilic OH^- sites within the framework pores, rather than via previously proposed pathways. The new framework KHCO₃ CD-MOF possesses rapid and high-capacity CO₂ uptake, good thermal, oxidative, and cycling stabilities, and selective CO₂ capture under mixed gas conditions. Because of its low cost and performance under realistic conditions, KHCO₃ CD-MOF is a promising new platform for CCS. More broadly, our work demonstrates that the encapsulation of reactive OH^- sites within a porous framework represents a potentially general strategy for the design of oxidation-resistant adsorbents for CO₂ capture.

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O

An extensive correlated molecular orbital theory study of the reactions of CO₂ with a range of substituted amines and H₂O in the gas phase and aqueous solution was performed at the G3(MP2) level with a self-consistent reaction field approach. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH₃ reactions. A catalytic NH₃ reduces the energy barrier more than a catalytic H₂O for the formation of H₂NCOOH and H₂CO₃. In aqueous solution, the barriers to form both H₂NCOOH and H₂CO₃ are reduced, with HCO₃^- formation possible with one amine present and H₂NCOO^- formation possible only with two amines. Further reactions of H₂NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH₃NH₂, CH₃CH₂NH₂, (CH₃)₂NH, CH₃CH₂CH₂NH₂, (CH₃)₃N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (ΔG^‡_aq < 15 kcal/mol) and alkylammonium bicarbonate (ΔG^‡_aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO₂ capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

CO2 Adsorption on Modified Mesoporous Silicas: The Role of the Adsorption Sites

The post-synthesis procedure for cyclic amine (morpholine and 1-methylpiperazine) modified mesoporous MCM-48 and SBA-15 silicas was developed. The procedure for preparation of the modified mesoporous materials does not affect the structural characteristics of the initial mesoporous silicas strongly. The initial and modified materials were characterized by XRD, N₂ physisorption, thermal analysis, and solid-state NMR. The CO₂ adsorption of the obtained materials was tested under dynamic and equilibrium conditions. The NMR data revealed the formation of different CO₂ adsorbed forms. The materials exhibited high CO₂ absorption capacity lying above the benchmark value of 2 mmol/g and stretching out to the outstanding 4.4 mmol/g in the case of 1-methylpiperazin modified MCM-48. The materials are reusable, and their CO₂ adsorption capacities are slightly lower in three adsorption/desorption cycles.

Selectivity control in hydrogenation through adaptive catalysis using ruthenium nanoparticles on a CO2-responsive support

With the advent of renewable carbon resources, multifunctional catalysts are becoming essential to hydrogenate selectively biomass-derived substrates and intermediates. However, the development of adaptive catalytic systems, that is, with reversibly adjustable reactivity, able to cope with the intermittence of renewable resources remains a challenge. Here, we report the preparation of a catalytic system designed to respond adaptively to feed gas composition in hydrogenation reactions. Ruthenium nanoparticles immobilized on amine-functionalized polymer-grafted silica act as active and stable catalysts for the hydrogenation of biomass-derived furfural acetone and related substrates. Hydrogenation of the carbonyl group is selectively switched on or off if pure H₂ or a H₂/CO₂ mixture is used, respectively. The formation of alkylammonium formate species by the catalytic reaction of CO₂ and H₂ at the amine-functionalized support has been identified as the most likely molecular trigger for the selectivity switch. As this reaction is fully reversible, the catalyst performance responds almost in real time to the feed gas composition.

A multifunctional catalytic system composed of ruthenium nanoparticles immobilized on a silica surface decorated with an amine-functionalized polymer is used for the hydrogenation of biomass-derived furfural acetone and related substrates. The presence or absence of CO₂ in the gas feed alters the selectivity of the hydrogenation—producing either a ketone or a saturated alcohol, respectively—in a fully reversible manner.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Unravelling moisture-induced CO2 chemisorption mechanisms in amine-modified sorbents at the molecular scale

This work entails a comprehensive solid-state NMR and computational study of the influence of water and CO₂ partial pressures on the CO₂-adducts formed in amine-grafted silica sorbents. Our approach provides atomic level insights on hypothesised mechanisms for CO₂ capture under dry and wet conditions in a tightly controlled atmosphere. The method used for sample preparation avoids the use of liquid water slurries, as performed in previous studies, enabling a molecular level understanding, by NMR, of the influence of controlled amounts of water vapor (down to ca. 0.7 kPa) in CO₂ chemisorption processes. Details on the formation mechanism of moisture-induced CO₂ species are provided aiming to study CO₂ : H₂O binary mixtures in amine-grafted silica sorbents. The interconversion between distinct chemisorbed CO₂ species was quantitatively monitored by NMR under wet and dry conditions in silica sorbents grafted with amines possessing distinct bulkiness (primary and tertiary). Particular attention was given to two distinct carbonyl environments resonating at δ _C ∼161 and 155 ppm, as their presence and relative intensities are greatly affected by moisture depending on the experimental conditions. 1D and 2D NMR spectral assignments of both these ¹³C resonances were assisted by density functional theory calculations of ¹H and ¹³C chemical shifts on model structures of alkylamines grafted onto the silica surface that validated various hydrogen-bonded CO₂ species that may occur upon formation of bicarbonate, carbamic acid and alkylammonium carbamate ion pairs. Water is a key component in flue gas streams, playing a major role in CO₂ speciation, and this work extends the current knowledge on chemisorbed CO₂ structures and their stabilities under dry/wet conditions, on amine-modified solid surfaces.

Distribution and Transport of CO2 in Hydrated Hyperbranched Poly(ethylenimine) Membranes: A Molecular Dynamics Simulation Approach

Hyperbranched poly(ethylenimine) (HB-PEI) has been distinguished as a promising candidate for carbon dioxide (CO2) capture. In this study, we investigate the distribution and transport of CO2 molecules in a HB-PEI membrane at various hydration levels using molecular dynamics (MD) simulations. For this, model structures consisting of amorphous HB-PEI membranes with CO2 molecules are equilibrated at various hydration levels. Under dry conditions, the primary and secondary amines are highly associated with CO2, indicating that they would participate in CO2 capture via the carbamate formation mechanism. Under hydrated conditions, the pair correlations of CO2 with the primary and secondary amines are reduced. This result suggests that the carbamate formation mechanism is less prevalent compared to dry conditions, which is also supported by CO2 residence time analysis. However, in the presence of water molecules, it is found that the CO2 molecules can be associated with both amine groups and water molecules, which would enable the tertiary amine as well as the primary and secondary amines to capture CO2 molecules via the bicarbonate formation mechanism. Through our MD simulation results, the feasibilities of different CO2 capture pathways in HB-PEI membranes are demonstrated at the molecular level.

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.

A Unified Approach to CO2-Amine Reaction Mechanisms

A unified CO₂-amine reaction mechanism applicable to absorption in aqueous or nonaqueous solutions and to adsorption on immobilized amines in the presence of both dry and humid conditions is proposed. Key findings supported by theoretical calculations and experimental evidence are as follows: (1) The formation of the 1,3-zwitterion, RH₂N⁺-COO^-, is highly unlikely because not only the associated four-membered mechanism has a high energy barrier, but also it is not consistent with the orbital symmetry requirements for chemical reactions. (2) The nucleophilic attack of CO₂ by amines requires the catalytic assistance of a Bro̷nsted base through a six-membered mechanism to achieve proton transfer/exchange. An important consequence of this concerted mechanism is that the N and H atoms added to the C=O double bond do not originate from a single amine group. Using ethylenediamine for illustration, detailed description of the reaction pathway is reported using the reactive internal reaction coordinate as a new tool to visualize the reaction path. (3) In the presence of protic amines, the formation of ammonium bicarbonate/carbonate does not take place through the widely accepted hydration of carbamate/carbamic acid. Instead, water behaves as a nucleophile that attacks CO₂ with catalytic assistance by amine groups, and carbamate/carbamic acid decomposes back to amine and CO₂. (4) Generalization of the catalytic assistance concept to any Bro̷nsted base established through theoretical calculations was supported by infrared measurements. A unified six-membered mechanism was proposed to describe all possible interactions of CO₂ with amines and water, each playing the role of a nucleophile and/or Bro̷nsted base, depending on the actual conditions.

Sequential Ammonia and Carbon Dioxide Adsorption on Pyrolyzed Biomass to Recover Waste Stream Nutrients

The amine-rich surfaces of pyrolyzed human solid waste (py-HSW) can be "primed" or "regenerated" with carbon dioxide (CO₂) to enhance their adsorption of ammonia (NH₃) for use as a soil amendment. To better understand the mechanism by which CO₂ exposure facilitates NH₃ adsorption to py-HSW, we artificially enriched a model sorbent, pyrolyzed, oxidized wood (py-ox wood) with amine functional groups through exposure to NH₃. We then exposed these N-enriched materials to CO₂ and then resorbed NH₃. The high heat of CO₂ adsorption (Q _st) on py-HSW, 49 kJ mol^-1, at low surface coverage, 0.4 mmol CO₂ g^-1, showed that the naturally occurring N compounds in py-HSW have a high affinity for CO₂. The Q _st of CO₂ on py-ox wood also increased after exposure to NH₃, reaching 50 kJ mol^-1 at 0.7 mmol CO₂ g^-1, demonstrating that the incorporation of N-rich functional groups by NH₃ adsorption is favorable for CO₂ uptake. Adsorption kinetics of py-ox wood revealed continued, albeit diminishing NH₃ uptake after each CO₂ treatment, averaging 5.9 mmol NH₃ g^-1 for the first NH₃ exposure event and 3.5 and 2.9 mmol NH₃ g^-1 for the second and third; the electrophilic character of CO₂ serves as a Lewis acid, enhancing surface affinity for NH₃ uptake. Furthermore, penetration of ¹⁵NH₃ and ¹³CO₂ measured by NanoSIMS reached over 7 μm deep into both materials, explaining the large NH₃ capture. We expected similar NH₃ uptake in py-HSW sorbed with CO₂ and py-ox wood because both materials, py-HSW and py-ox wood sorbed with NH₃, had similar N contents and similarly high CO₂ uptake. Yet NH₃ sorption in py-HSW was unexpectedly low, apparently from potassium (K) bicarbonate precipitation, reducing interactions between NH₃ and sorbed CO₂; 2-fold greater surface K in py-HSW was detected after exposure to CO₂ and NH₃ than before gas exposure. We show that amine-rich pyrolyzed waste materials have high CO₂ affinity, which facilitates NH₃ uptake. However, high ash contents as found in py-HSW hinder this mechanism.

Boosting C2 products in electrochemical CO2 reduction over highly dense copper nanoplates

Exclusive C2 selectivity of Cu-Nplates over C1 during electrocatalytic CO₂ reduction offers opportunities for large scale, long-term renewable energy storage and lessens carbon emissions.

Water Enables Efficient CO2 Capture from Natural Gas Flue Emissions in an Oxidation-Resistant Diamine-Appended Metal-Organic Framework

Supported by increasingly available reserves, natural gas is achieving greater adoption as a cleaner-burning alternative to coal in the power sector. As a result, carbon capture and sequestration from natural gas-fired power plants is an attractive strategy to mitigate global anthropogenic CO₂ emissions. However, the separation of CO₂ from other components in the flue streams of gas-fired power plants is particularly challenging due to the low CO₂ partial pressure (∼40 mbar), which necessitates that candidate separation materials bind CO₂ strongly at low partial pressures (≤4 mbar) to capture ≥90% of the emitted CO₂. High partial pressures of O₂ (120 mbar) and water (80 mbar) in these flue streams have also presented significant barriers to the deployment of new technologies for CO₂ capture from gas-fired power plants. Here, we demonstrate that functionalization of the metal-organic framework Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) with the cyclic diamine 2-(aminomethyl)piperidine (2-ampd) produces an adsorbent that is capable of ≥90% CO₂ capture from a humid natural gas flue emission stream, as confirmed by breakthrough measurements. This material captures CO₂ by a cooperative mechanism that enables access to a large CO₂ cycling capacity with a small temperature swing (2.4 mmol CO₂/g with ΔT = 100 °C). Significantly, multicomponent adsorption experiments, infrared spectroscopy, magic angle spinning solid-state NMR spectroscopy, and van der Waals-corrected density functional theory studies suggest that water enhances CO₂ capture in 2-ampd-Mg₂(dobpdc) through hydrogen-bonding interactions with the carbamate groups of the ammonium carbamate chains formed upon CO₂ adsorption, thereby increasing the thermodynamic driving force for CO₂ binding. In light of the exceptional thermal and oxidative stability of 2-ampd-Mg₂(dobpdc), its high CO₂ adsorption capacity, and its high CO₂ capture rate from a simulated natural gas flue emission stream, this material is one of the most promising adsorbents to date for this important separation.

Carbonate-Promoted Drift of Alkali Cations in Small Pore Zeolites: Ab Initio Molecular Dynamics Study of CO2 in NaKA Zeolite

An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K⁺ cation and withdraw it from the 8R window, so that the probability of CO₂ diffusion through 8R increases. For the first time, correlations between cationic and HCO₃^-/CO₃^2- positions are demonstrated in favor of their significant interaction leading to the cationic drift from 8R windows. This phenomenon explains a nonzero CO₂ adsorption in narrow pore zeolites upon high Na/K exchange. In a gas mixture, such deblocking effect reduces the separation factor because of the possible passage of both components through the plane of partly open 8R windows.

Oxidation-Resistant, Cost-Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air

CO₂ adsorbents based on the reaction of pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) with propylene oxide (PO) were easily prepared in "one pot" by impregnation on a silica support in water. The starting materials were readily available and inexpensive, facilitating the production of the adsorbents on a large scale. The prepared polyamine/epoxide adsorbents were efficient in capturing CO₂ and could be regenerated under mild conditions (50-85 °C). They displayed a much-improved stability compared with their unmodified amine counterparts, especially under oxidative conditions. Leaching of the active organic amine became minimal or nonexistent after treatment with the epoxide. The adsorption as well as desorption kinetics were also greatly improved. The polyamine/epoxide adsorbents were able to capture CO₂ from various sources including ambient air and indoor air with CO₂ concentrations of only 400-1000 ppm. The presence of water, far from being detrimental, increased the adsorption capacity. Their use for indoor air quality purposes was explored.

Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle

This review focuses on important stability issues facing amine-functionalized CO2 adsorbents, including amine-grafted and amine-impregnated silicas, zeolites, metal-organic frameworks and carbons. During the past couple of decades, major advances were achieved in understanding and improving the performance of such materials, particularly in terms of CO2 adsorptive properties such as adsorption capacity, selectivity and kinetics. Nonetheless, to pave the way toward commercialization of adsorption-based CO2 capture technologies, in addition to other attributes, adsorbent materials should be stable over many thousands of adsorption-desorption cycles. Adsorbent stability, which is of utmost importance as it determines adsorbent lifetime and operational costs of CO2 capture, is a multifaceted issue involving thermal, hydrothermal, and chemical stability. Here we discuss the impact of the adsorbent physical and chemical properties, the feed gas composition and characteristics, and the adsorption-desorption operational parameters on the long-term stability of amine-functionalized CO2 adsorbents. We also review important insights associated with the underlying deactivation pathways of the adsorbents upon exposure to high temperature, oxygen, dry CO2, sulfur-containing compounds, nitrogen oxides, oxygen and steam. Finally, specific recommendations are provided to address outstanding stability issues.

Elucidating CO2 Chemisorption in Diamine-Appended Metal-Organic Frameworks

The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal-organic frameworks of the type diamine-M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO₂ uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO₂ chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M₂(dobpdc) variants, we demonstrate that the canonical CO₂ chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg₂(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves the formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO₂ uptake in diamine-M₂(dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO₂ adsorption within diamine-M₂(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.

Silica-Supported Sterically Hindered Amines for CO2 Capture

Most studies exploring the capture of CO₂ on solid-supported amines have focused on unhindered amines or alkylimine polymers. It has been observed in extensive solution studies that another class of amines, namely sterically hindered amines, can exhibit enhanced CO₂ capacity when compared to their unhindered counterparts. In contrast to solution studies, there has been limited research conducted on sterically hindered amines on solid supports. In this work, one hindered primary amine and two hindered secondary amines are grafted onto mesoporous silica at similar amine coverages, and their adsorption performances are investigated through fixed bed breakthrough experiments and thermogravimetric analysis. Furthermore, chemisorbed CO₂ species formed on the sorbents under dry and humid conditions are elucidated using in situ Fourier-transform infrared spectroscopy. Ammonium bicarbonate formation and enhancement of CO₂ adsorption capacity is observed for all supported hindered amines under humid conditions. Our experiments in this study also suggest that chemisorbed CO₂ species formed on supported hindered amines are weakly bound, which may lead to reduced energy costs associated with regeneration if such materials were deployed in a practical separation process. However, overall CO₂ uptake capacities of the solid supported hindered amines are modest compared to their solution counterparts. The oxidative and thermal stabilities of the supported hindered amine sorbents are also assessed to give insight into their operational lifetimes.