Reversible Chemisorbents for Carbon Dioxide and Their Potential Applications

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.

New Porous Silicon-Containing Organic Polymers: Synthesis and Carbon Dioxide Uptake

The design and synthesis of new multifunctional organic porous polymers has attracted significant attention over the years due to their favorable properties, which make them suitable for carbon dioxide storage. In this study, 2-, 3-, and 4-hydroxybenzaldehyde reacted with phenyltrichlorosilane in the presence of a base, affording the corresponding organosilicons 1–3, which further reacted with benzidine in the presence of glacial acetic acid, yielding the organic polymers 4–6. The synthesized polymers exhibited microporous structures with a surface area of 8.174–18.012 m2 g−1, while their pore volume and total average pore diameter ranged from 0.015–0.035 cm3 g−1 and 1.947–1.952 nm, respectively. In addition, among the synthesized organic polymers, the one with the meta-arrangement structure 5 showed the highest carbon dioxide adsorption capacity at 323 K and 40 bar due to its relatively high surface area and pore volume.

Unusually Low Heat of Adsorption of CO2 on AlPO and SAPO Molecular Sieves

The capture of CO₂ from post-combustion streams or from other mixtures, such as natural gas, is an effective way of reducing CO₂ emissions, which contribute to the greenhouse effect in the atmosphere. One of the developing technologies for this purpose is physisorption on selective solid adsorbents. The ideal adsorbents are selective toward CO₂, have a large adsorption capacity at atmospheric pressure and are easily regenerated, resulting in high working capacity. Therefore, adsorbents combining molecular sieving properties and low heats of adsorption of CO₂ are of clear interest as they will provide high selectivities and regenerabilities in CO₂ separation process. Here we report that some aluminophosphate (AlPO) and silicoaluminophosphate (SAPO) materials with LTA, CHA and AFI structures present lower heats of adsorption of CO₂ (13-25 kJ/mol) than their structurally analogous zeolites at comparable framework charges. In some cases, their heats of adsorption are even lower than those of pure silica composition (20-25 kJ/mol). This could mean a great improvement in the regeneration process compared to the most frequently used zeolitic adsorbents for this application while maintaining most of their adsorption capacity, if materials with the right stability and pore size and topology are found.

MOF-Based Membranes for Gas Separations

Metal-organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

Multiphase carbon mineralization for the reactive separation of CO2 and directed synthesis of H2

There is a need to capture, convert and store CO₂ by atom-efficient and energy-efficient pathways that use as few process configurations as possible. This need has motivated studies into multiphase reaction chemistries and this Review describes two such approaches in the context of carbon mineralization. The first approach uses aqueous alkaline solutions containing amine nucleophiles that capture CO₂ and eventually convert it into calcium and magnesium carbonates, thereby regenerating the nucleophiles. Gas-liquid-solid and liquid-solid configurations of these reactions are explored. The second approach combines silicates such as CaSiO₃ or Mg₂SiO₄ with CO and H₂O from the water-gas shift reaction to give H₂ and calcium or magnesium carbonates. Coupling carbonate formation to the water-gas shift reaction shifts the latter equilibrium to afford more H₂ as part of a single-step catalytic approach to carbon mineralization. These pathways exploit the vast abundance of alkaline resources, including naturally occurring silicates and alkaline industrial residues. However, simple stoichiometries belie the complex, multiphase nature of the reactions, predictive control of which presents a scientific opportunity and challenge. This Review describes this multiphase chemistry and the knowledge gaps that need to be addressed to achieve 'step-change' advancements in the reactive separation of CO₂ by carbon mineralization.

Double Phosphinoboration of CO2 : A Facile Route to Diphospha-Ureas

The reactions of CO2 with a series of phosphinoboranes, including R2 PBpin (R=Ph, tBu; pin=pinacol), R2 PBMes2 (R=Ph, tBu; Mes=2,4,6-Me3 -C6 H2 ), and R2 PBcat (R=Ph, tBu, Mes; cat=catechol) are described. Although R2 PBpin and R2 PBMes2 afford products of the form R2 PCO2 Bpin (R=Ph 1, tBu 4) and R2 PCO2 BMes2 (R=Ph 2, tBu 3), respectively, R2 PBcat lead to further reaction affording the diphospha-ureas, (R2 P)2 CO (R=Ph 5, tBu 6, Mes 7), together with O(Bcat)2 . Computational studies provide insight into the mechanism, revealing an intermediate derived from double phosphinoboration of CO2 .

Efficient Separation of Ethanol-Methanol and Ethanol-Water Mixtures Using ZIF-8 Supported on a Hierarchical Porous Mixed-Oxide Substrate

This work reports a new approach for the synthesis of a zeolitic imidazolate framework (ZIF-8) composite. It employs the direct growth of the crystalline ZIF-8 on a mixed-metal oxide support TiO₂-SiO₂ (TSO), which mimics the porous structure of Populus nigra. Using the natural leaf as a template, the TSO support was prepared using a sol-gel method. The growth of the ZIF-8 layer on the TSO support was carried out by the seeds and second growth method. This method facilitates the homogeneous dispersion of ZIF-8 crystals at the surface of the TSO composite. The ZIF-8@TSO composite adsorbs methanol selectively, mainly due to the hierarchical porous structure of the mixed oxide support. As compared with the as-synthesized ZIF-8, a 50% methanol uptake is achieved in the ZIF-8@TSO composite, with only 25 wt % ZIF-8 loading. IAST simulations show that the ZIF-8@TSO composite has a preferential adsorption toward methanol when using an equimolar methanol-ethanol mixture. An opposite behavior is observed for the as-synthesized ZIF-8. The results show that combining MOFs and mixed-oxide supports with bioinspired structures opens opportunities for synthesizing new materials with unique and enhanced adsorption and separation properties.

Fixation of atmospheric CO2 as novel carbonate-(water)2-carbonate cluster and entrapment of double sulfate within a linear tetrameric barrel of a neutral bis-urea scaffold

A meta-phenylenediamine-based disubstituted bis-urea receptor L1 with electron-withdrawing 3-chloro and electron-donating 4-methylphenyl terminals has been established as a potential system to fix and efficiently capture atmospheric CO₂ as air-stable entrapment of an unprecedented {CO₃^2--(H₂O)₂-CO₃^2-} cluster (complex 1a) within its tetrameric long straight pillar-like assembly entirely sealed by n-TBA cations via formation of a barrel-type architecture. L1 and its isomeric 4-bromo-3-methyl disubstituted bis-urea receptor L2 have been found to entrap similar kinds of water-free naked sulfate-sulfate double anion (complexes 1b and 2a) by cooperative binding of urea moieties inside the two pairs of the inversion-symmetric linear tetrameric barrel of L1 and L2, respectively. On the other hand, in the presence of excess halides, L1 self-assembles to form hexa-coordinated fluoride complex 1c and tetra-coordinated bromide complex 1d, while L2 self-assembles to form penta-coordinated fluoride complex 2b in the solid state via semicircular receptor architectures and non-cooperative H-bonding interactions of urea moieties.

Critical review of existing nanomaterial adsorbents to capture carbon dioxide and methane

Innovative gas capture technologies with the objective to mitigate CO₂ and CH₄ emissions are discussed in this review. Emphasis is given on the use of nanoparticles (NP) as sorbents of CO₂ and CH₄, which are the two most important global warming gases. The existing NP sorption processes must overcome certain challenges before their implementation to the industrial scale. These are: i) the utilization of the concentrated gas stream generated by the capture and gas purification technologies, ii) the reduction of the effects of impurities on the operating system, iii) the scale up of the relevant materials, and iv) the retrofitting of technologies in existing facilities. Thus, an innovative design of adsorbents could possibly address those issues. Biogas purification and CH₄ storage would become a new motivation for the development of new sorbent materials, such as nanomaterials. This review discusses the current state of the art on the use of novel nanomaterials as adsorbents for CO₂ and CH₄. The review shows that materials based on porous supports that are modified with amine or metals are currently providing the most promising results. The Fe₃O₄-graphene and the MOF-117 based NPs show the greatest CO₂ sorption capacities, due to their high thermal stability and high porosity. Conclusively, one of the main challenges would be to decrease the cost of capture and to scale-up the technologies to minimize large-scale power plant CO₂ emissions.

Predictive Guide for Collective CO2 Adsorption Properties of Mg-Al Mixed Oxides

Although solid adsorption processes offer attractive benefits, such as reduced energy demands and penalties compared with liquid absorption processes, there are still pressing needs for solid adsorbents with high adsorption capacities, thermal efficiencies, and energy-intensive regeneration in gas-treatment processes. The CO₂ adsorption capacities of layered double oxides (LDOs), which are attractive solid adsorbents, have an asymmetric volcano-type correlation with their relative crystallinities. Furthermore, new collective adsorption properties (adsorption capacity, adsorptive energy and charge-transfer amount based on the adsorbent weight) are proposed based on density functional theory (DFT) calculations and measured surface areas. The correlation of these collective properties with their crystallinities is in good agreement with the experimentally measured CO₂ adsorptive capacity trend, providing a predictive guide for the development of solid adsorbents for gas-adsorption processes.

Quantifying the efficiency of CO2 capture by Lewis pairs

A microfluidic strategy is used to assess the relative efficiency and thermodynamic parameters of CO₂ binding by three Lewis acid/base combinations.

A microfluidic strategy has been used for the time- and labour-efficient evaluation of the relative efficiency and thermodynamic parameters of CO₂ binding by three Lewis acid/base combinations, where efficiency is based on the amount of CO₂ taken up per binding unit in solution. Neither tBu₃P nor B(C₆F₅)₃ were independently effective at CO₂ capture, and the combination of the imidazolin-2-ylidenamino-substituted phosphine (NIiPr)₃P and B(C₆F₅)₃ was equally ineffective. Nonetheless, an archetypal frustrated Lewis pair (FLP) comprised of tBu₃P and B(C₆F₅)₃ was shown to bind CO₂ more efficiently than either the FLP derived from tetramethylpiperidine (TMP) and B(C₆F₅)₃ or the highly basic phosphine (NIiPr)₃P. Moreover, the proposed microfluidic platform was used to elucidate the thermodynamic parameters for these reactions.

Alkali Metal CO2 Sorbents and the Resulting Metal Carbonates: Potential for Process Intensification of Sorption-Enhanced Steam Reforming

Sorption-enhanced steam reforming (SESR) is an energy and cost efficient approach to produce hydrogen with high purity. SESR makes it economically feasible to use a wide range of feedstocks for hydrogen production such as methane, ethanol, and biomass. Selection of catalysts and sorbents plays a vital role in SESR. This article reviews the recent research aimed at process intensification by the integration of catalysis and chemisorption functions into a single material. Alkali metal ceramic powders, including Li₂ZrO₃, Li₄SiO₄ and Na₂ZrO₃ display characteristics suitable for capturing CO₂ at low concentrations (<15% CO₂) and high temperatures (>500 °C), and thus are applicable to precombustion technologies such as SESR, as well as postcombustion capture of CO₂ from flue gases. This paper reviews the progress made in improving the operational performance of alkali metal ceramics under conditions that simulate power plant and SESR operation, by adopting new methods of sorbent synthesis and doping with additional elements. The paper also discusses the role of carbonates formed after in situ CO₂ chemisorption during a steam reforming process in respect of catalysts for tar cracking.

Theoretical design and mechanistic study of the metal-free reduction of CO2 to CO

The metal-free silylborane Me₂BSi(CH₂F)₃, screened based on the rules of thumb coming from the summary of a hierarchy of silylboranes, can be used to catalyze the reduction of CO₂ to CO.

Material Exhibiting Efficient CO2 Adsorption at Room Temperature for Concentrations Lower Than 1000 ppm: Elucidation of the State of Barium Ion Exchanged in an MFI-Type Zeolite

Carbon dioxide (CO2) gas is well-known as a greenhouse gas that leads to global warming. Many efforts have been made to capture CO2 from coal-fired power plants, as well as to reduce the amounts of excess CO2 in the atmosphere to around 400 ppm. However, this is not a simple task, particularly in the lower pressure region than 1000 ppm. This is because the CO2 molecule is chemically stable and has a relatively low reactivity. In the present study, the CO2 adsorption at room temperature on MFI-type zeolites exchanged with alkaline-earth-metal ions, with focus on CO2 concentrations <1000 ppm, was investigated both experimentally and by calculation. These materials exhibited a particularly efficient adsorption capability for CO2, compared with other presented samples, such as the sodium-form and transition-metal ion-exchanged MFI-type zeolites. Ethyne (C2H2) was used as a probe molecule. Analyses were carried out with IR spectroscopy and X-ray absorption, and provided significant information regarding the presence of the M(2+)-O(2-)-M(2+) (M(2+): alkaline-earth-metal ion) species formed in the samples. It was subsequently determined that this species acts as a highly efficient site for CO2 adsorption at room temperature under very low pressure, compared to a single M(2+) species. A further advantage is that this material can be easily regenerated by a treatment, e.g., through the application of the temperature swing adsorption process, at relatively low temperatures (300-473 K).

High-Temperature CO2 Sorption on Hydrotalcite Having a High Mg/Al Molar Ratio

Hydrotalcites having a Mg/Al molar ratio between 3 and 30 have been synthesized as promising high-temperature CO2 sorbents. The existence of NaNO3 in the hydrotalcite structure, which originates from excess magnesium nitrate in the precursor, markedly increases CO2 sorption uptake by hydrotalcite up to the record high value of 9.27 mol kg(-1) at 240 °C and 1 atm CO2.

A homochiral vanadium–salen based cadmium bpdc MOF with permanent porosity as an asymmetric catalyst in solvent-free cyanosilylation

This chiral framework shows the highest H₂ adsorption and CO₂ capacity for currently known salen-based MOFs and shows an excellent performance as an asymmetric catalyst in solvent-free cyanosilylation.

Design rationale of thermally responsive microgel particle films that reversibly absorb large amounts of CO2: fine tuning the pKa of ammonium ions in the particles

Fine-tuning of pK_a value of ammonium ions at both CO₂ capture and release temperature is found to be crucial for the design of the thermally responsive gel particle films that reversibly capture large amounts of CO₂.

Herein we revealed the design rationale of thermally responsive gel particle (GP) films that reversibly capture and release large amounts of CO₂ over a narrow temperature range (30–75 °C). The pK_a value of ammonium ions in the GPs at both the CO₂ capture temperature (30 °C) and release temperature (75 °C) is found to be the primary factor responsible for the stoichiometry of reversible CO₂ capture by the amines in the GP films. The pK_a values can be tuned by the properties of GPs such as volume phase transition temperature (VPTT), size, swelling ratio, and the imprinted microenvironment surrounding the amines. The optimal GP obtained according to the design rationale showed high capture capacity (68 mL CO₂ per g dry GPs, 3.0 mmol CO₂ per g dry GPs), although the regeneration temperature was as low as 75 °C. We anticipate that GP films that reversibly capture other acidic and basic gases in large amounts can also be achieved by the pK_a tuning procedures.

Hybrid aminopolymer–silica materials for efficient CO2adsorption

Experimental polymerization method of hyperbranched PEI based on the use of compressed CO₂.

Monitoring solid oxide CO2 capture sorbents in action

The separation, capture, and storage of CO2 , the major greenhouse gas, from industrial gas streams has received considerable attention in recent years because of concerns about environmental effects of increasing CO2 concentration in the atmosphere. An emerging area of research utilizes reversible CO2 sorbents to increase conversion and rate of forward reactions for equilibrium-controlled reactions (sorption-enhanced reactions). Little fundamental information, however, is known about the nature of the sorbent surface sites, sorbent surface-CO2 complexes, and the CO2 adsorption/desorption mechanisms. The present study directly spectroscopically monitors Na2 O/Al2 O3 sorbent-CO2 surface complexes during adsorption/desorption with simultaneous analysis of desorbed CO2 gas, allowing establishment of molecular level structure-sorption relationships between individual surface carbonate complexes and the CO2 working capacity of sorbents at different temperatures.

Hydrothermal synthesis of K2CO3-promoted hydrotalcite from hydroxide-form precursors for novel high-temperature CO2 sorbent

In many materials for CO2 sorption, hydrotalcite is attracting substantial attention as a high temperature (200-500 °C) CO2 sorbent because of its fast sorption/desorption kinetics and easy regenerability. However, the CO2-sorption capacity of conventional hydrotalcite is relatively low for large-scale commercial use. To enhance CO2-sorption capacity, hydrotalcite is conventionally impregnated with alkali metals such as K2CO3. Although K2CO3-impregnated hydrotalcite has high CO2-sorption capacity, the preparation method takes long time and is inconvenient because hydrotalcite synthesis step and alkali metal impregnation step are separated. In this study, K2CO3-promoted hydrotalcite was newly synthesized from hydroxide-form percursors by a simple and eco-friendly method without a solvent-consuming washing step. Analysis based on X-ray diffraction indicated that the prepared samples had structures of well-defined hydrotalcite crystalline and un-reacted Mg(OH)2 precursor. Moreover, K2CO3 was successfully incorporated in hydrotalcite during the synthesis step. The prepared K2CO3-promoted hydrotalcite showed high CO2-sorption capacity and had potential for use as a high-temperature CO2 sorbent.

Temperature-responsive microgel films as reversible carbon dioxide absorbents in wet environment

Hydrogel films composed of temperature-responsive microgel particles (GPs) containing amine groups work as stimuli-responsive carbon dioxide absorbent with a high capacity of approximately 1.7 mmol g(-1). Although the dried films did not show significant absorption, the reversible absorption capacity dramatically increased by adding a small amount of water (1 mL g(-1)). The absorption capacity was independent of the amount of added water beyond 1 mL g(-1), demonstrating that the GP films can readily be used under wet conditions. The amount of CO2 absorbed by the GP films was proportional to their thickness up to 200-300 μm (maximum capacity of about 2 L m(-2) . Furthermore, the films consisting of GPs showed faster and greater absorption and desorption of CO2 than that of monolithic hydrogel films. These results indicated the importance of a fast stimulus response rate of the films that are composed of GPs in order to achieve long-range and fast diffusion of bicarbonate ions. Our study revealed the potential of stimuli-responsive GP films as energy-efficient absorbents to sequester CO2 from high-humidity exhaust gases.