Metal-Organic Frameworks for Water Harvesting and Concurrent Carbon Capture: A Review for Hygroscopic Materials

As water scarcity becomes a pending global issue, hygroscopic materials prove a significant solution. Thus, there is a good cause following the structure-performance relationship to review the recent development of hygroscopic materials and provide inspirational insight into creative materials. Herein, traditional hygroscopic materials, crystalline frameworks, polymers, and composite materials are reviewed. The similarity in working conditions of water harvesting and carbon capture makes simultaneously addressing water shortages and reduction of greenhouse effects possible. Concurrent water harvesting and carbon capture is likely to become a future challenge. Therefore, an emphasis is laid on metal-organic frameworks (MOFs) for their excellent performance in water and CO2 adsorption, and representative role of micro- and mesoporous materials. Herein, the water adsorption mechanisms of MOFs are summarized, followed by a review of MOF's water stability, with a highlight on the emerging machine learning (ML) technique to predict MOF water stability and water uptake. Recent advances in the mechanistic elaboration of moisture's effects on CO2 adsorption are reviewed. This review summarizes recent advances in water-harvesting porous materials with special attention on MOFs and expects to direct researchers' attention into the topic of concurrent water harvesting and carbon capture as a future challenge.

The Challenge of Water Competition in Physical Adsorption of CO2 by Porous Solids for Carbon Capture Applications - A Short Perspective

With ever-increasing efforts to design sorbent materials to capture carbon dioxide from flue gas and air, this perspective article is provided based on nearly a decade of collaboration across science, engineering, and industry partners. A key point learned is that a holistic view of the carbon capture problem is critical. While researchers can be inclined to value their own fields and associated metrics, often, key parameters are those that enable synergy between materials and processes. While the role of water in the chemisorption of CO2 is well-studied, in this perspective, it is hoped to highlight the often-overlooked but critical role of water in assessing the potential of a physical adsorbent for CO2 capture. This is a challenge that requires interdisciplinarity. As such, this document is written for a general audience rather than experts in any specific discipline.

Evaluation of the Hydrophilic/Hydrophobic Balance of 13X Zeolite by Adsorption of Water, Methanol, and Cyclohexane as Pure Vapors or as Mixtures

Adsorption isotherms of pure vapors and vapor mixtures of water, methanol, and cyclohexane were studied using a synthesized 13X zeolite (FAU topology), by means of a DVS gravimetric vapor analyzer. These results were validated by GCMC calculations. The surface chemistry of the adsorbent was characterized by the thermodesorption of ammonia, and its textural properties were studied using nitrogen physisorption. The 13X zeolite was found to be strongly acidic (BrØnsted acid sites, Si/Al = 1.3) and its specific surface area around 1100 m2·g-1. Water was found to be able to diffuse within both the supercages and the sodalite cavities of the FAU structure, whereas methanol and cyclohexane were confined in the supercages only. The water/methanol sorption selectivity of the 13X zeolite was demonstrated by co-adsorption measurements. The composition of the water/methanol adsorbed phase could be calculated by assuming IAST hypotheses. This model failed in the case of the water/cyclohexane co-adsorption system, which is in line with the non-miscibility of the components in the adsorbed state. The sorption isotherms could be successfully simulated, confirming the robustness of the forcefields used. The 13X zeolite confirmed its a priori expected hydrophilic nature, which is useful for the selective adsorption of water in a methanol-water vapor mixture.

MFM-300(Sc): a chemically stable Sc(III)-based MOF material for multiple applications

Developing robust multifunctional metal-organic frameworks (MOFs) is the key to advancing the further deployment of MOFs into relevant applications. Since the first report of MFM-300(Sc) (MFM = Manchester Framework Material, formerly known as NOTT-400), the development of applications of this robust microporous MOF has only grown. In this review, a summary of the applications of MFM-300(Sc), as well as some emerging advanced applications, have been discussed. The adsorption properties of MFM-300(Sc) are presented systematically. Particularly, this contribution is focused on acid and corrosive gas adsorption. In addition, recent applications for catalysis based on the outstanding hemilabile Sc-O bond character are highlighted. Finally, some new research areas are introduced, such as host-guest chemistry and biomedical applications. This highlight aims to showcase the recent advances and the potential for developing new applications of this promising material.

Impedance-Based Detection of NO2 Using Ni-MOF-74: Influence of Competitive Gas Adsorption

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO2 detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N2, NO2, SO2, CO2, and H2O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 103, observed only when NO2 was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO2. Without NO2, H2O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO2 by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H2O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO2 adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO2. Together, this work points to the unique interaction of NO2 and other specific gases (e.g., H2O, SO2) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2 detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.

Visual detection of fluoride in water by a dual-emitting, Eu-doped Sc-based metal organic framework

The structure of (Eu/Sc)-EBTC and its turn-off optical sensing mechanism due to the high affinity between F− and Eu3+ in (Eu/Sc)-EBTC.

Sc(iii)-Based metal–organic frameworks

In the universe of MOFs, their construction with Sc(iii) is rather limited. This highlight shows the exciting chronological development of Sc(iii)-MOFs which have afforded promising applications due to their exceptional chemical stability.

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.

CO2 capture enhancement in MOFs via the confinement of molecules

This review focuses on exploring a new approach to improve the CO₂ adsorption properties of MOFs by confining small amounts of molecules with different nature, such as: water, alcohols, amines, and even aromatic molecules.

Partially Reversible H2S Adsorption by MFM-300(Sc): Formation of Polysulfides

The metal-organic framework (MOF)-type MFM-300(Sc) exhibits a combined physisorption and chemisorption capture of H₂S, leading to a high uptake (16.55 mmol g^-1) associated with high structural stability. The irreversible chemisorbed sulfur species were identified as low-order polysulfide (n = 2) species. The isostructural MFM-300(In) was demonstrated to promote the formation of different polysulfide species, paving the way toward a new methodology to incorporate polysulfides within MOFs for the generation of novel MOF-lithium/sulfur batteries.

Ethylenediamine loading into a manganese-based metal-organic framework enhances water stability and carbon dioxide uptake of the framework

Metal-organic frameworks (MOFs) based on 2,5-dihydroxyterepthalic acid (DOBDC) as the linker show very high CO2 uptake capacities at low to moderate CO2 pressures; however, these MOFs often require expensive solvent for synthesis and are difficult to regenerate. We have synthesized a Mn-DOBDC MOF and modified it to introduce amine groups into the structure by functionalizing its metal coordination sites with ethylenediamine (EDA). Repeat framework synthesis was then also successfully performed using recycled dimethylformamide (DMF) solvent. Characterization by elemental analysis, FTIR and thermogravimetric studies suggest that EDA molecules are successfully substituting the original metal-bound DMF. This modification not only enhances the material's carbon dioxide sorption capacity, increasing stability to repeated CO2 sorption cycles, but also improves the framework's stability to moisture. Moreover, this is one of the first amine-modified MOFs that can demonstrably be synthesized using recycled solvent, potentially reducing the future costs of production at larger scales.

CO2 capture enhancement for InOF-1: confinement of 2-propanol

The confinement of small amounts of i-PrOH demonstrated and enhanced CO₂ capture for InOF-1 as a result of the bottleneck effect and the formation of essential hydrogen bonds.

Unusual Moisture-Enhanced CO2 Capture within Microporous PCN-250 Frameworks

Developing metal-organic frameworks (MOFs) with moisture-resistant feature or moisture-enhanced adsorption is challenging for the practical CO₂ capture under humid conditions. In this work, under humid conditions, the CO₂ adsorption behaviors of two iron-based MOF materials, PCN-250(Fe₃) and PCN-250(Fe₂Co), were investigated. An interesting phenomenon is observed that the two materials demonstrate an unusual moisture-enhanced adsorption of CO₂. For PCN-250 frameworks, H₂O molecule induces a remarkable increase in the CO₂ uptake for the dynamic CO₂ capture from CO₂/N₂ (15:85) mixture. For PCN-250(Fe₃), its CO₂ adsorption capacity increases by 54.2% under the 50% RH humid condition, compared with that under dry conditions (from 1.18 to 1.82 mmol/g). Similarly, the CO₂ adsorption uptake of PCN-250(Fe₂Co) increases from 1.32 to 2.23 mmol/g, exhibiting a 68.9% increase. Even up to 90% RH, for PCN-250(Fe₃) and PCN-250(Fe₂Co), obvious increases of 43.7 and 70.2% in the CO₂ adsorption capacities are observed in comparison with those under dry conditions, respectively. Molecular simulations indicate that the hydroxo functional groups (μ₃-O) within the framework play a crucial role in improving CO₂ uptake in the presence of water vapor. Besides, partial substitution of Fe³⁺ by Co²⁺ ions in the PCN-250 framework gives rise to a great improvement in CO₂ adsorption capacity and selectivity. The excellent moisture stability (stable even after exposure to 90% RH humid air for 30 days), superior recyclability, as well as moisture-enhanced feature make PCN-250 as an excellent MOF adsorbent for CO₂ capture under humid conditions. This study provides a new paradigm that PCN-250 frameworks can not only be moisture resistant but can also subtly convert the common negative effect of moisture to a positive impact on improving CO₂ capture performance.

Structure stability of HKUST-1 towards water and ethanol and their effect on its CO2 capture properties

Water and ethanol stabilities of the crystal structure of the Cu-based metal-organic framework (MOF) HKUST-1 have been investigated. Vapour (water and ethanol) sorption isotherms and cyclability were measured by a dynamic strategy. The ethanol sorption capacity of HKUST-1 at 303 K remained unchanged contrasting water sorption (which decreased along with the sorption experiment time). Considering the binding energy of each sorbate with the open Cu(ii) sites, obtained by the use of diffusion coefficients, we showed the superior crystal stability of the HKUST-1 framework towards ethanol. Finally, a small quantity of ethanol (pre-adsorbed) slightly enhanced CO₂ capture without crystal structure degradation.

Confinement of H2O and EtOH to enhance CO2 capture in MIL-53(Al)-TDC

EtOH adsorption–desorption properties of MIL-53(Al)-TDC along with the confinement of small amounts of water to enhance its CO₂ capture, and the reversible capture of iodine are presented in this article.

Humidity-induced CO2 capture enhancement in Mg-CUK-1

Mg-CUK-1 showed a 1.8-fold increase in CO₂ capture (from 4.6 wt% to 8.5 wt%) in the presence of 18% RH.

Theoretical and experimental studies on three water-stable, isostructural, paddlewheel based semiconducting metal–organic frameworks

Theoretical and experimental studies were carried out on three isostructural, water stable, visible light absorbing MOFs, [M₂(TCS)(BPY)] (M = Co(1), Ni(2) and Cu(3); H₄TCS = tetrakis(4-carboxyphenyl) silane, BPY = 4,4′-bipyridine).

Confined methanol within InOF-1: CO2 capture enhancement

The CO₂ capture in InOF-1 was enhanced by confining small amounts of MeOH. DFT calculations coupled with forcefield based-MC simulations revealed that such an enhancement is due to an increase of the degree of confinement.

A chiral salen-based MOF catalytic material with high thermal, aqueous and chemical stabilities

A highly stable chiral Ni(salen)-based MOF material possessing a 1D open channel can efficiently catalyze the cycloaddition of simulated industrial CO₂ with epoxides, as well as the cycloaddition of epoxides with azides and alkynes under mild conditions.

Confinement of alcohols to enhance CO2 capture in MIL-53(Al)

CO₂ capture of MIL-53(Al) was enhanced by confining small amounts of MeOH and i-PrOH within its micropores.

Water Adsorption Properties of NOTT-401 and CO2 Capture under Humid Conditions

The water-stable material NOTT-401 was investigated for CO₂ capture under humid conditions. Water adsorption properties of NOTT-401 were studied, and their correlation with CO₂ sequestration at different relative humidities (RHs) showed that the CO₂ capture increased from 1.2 wt % (anhydrous conditions) to 3.9 wt % under 5% RH at 30 °C, representing a 3.2-fold improvement.

CO2 capture under humid conditions in NH2-MIL-53(Al): the influence of the amine functional group

NH₂-MIL-53(Al) exhibited a considerable stronger affinity to water than MIL-53(Al). Thus, the hydrophobicity (shown by in situ FTIR) of the pores within MIL-53(Al) enhanced the CO₂ adsorption.

CO2 capture enhancement in InOF-1 via the bottleneck effect of confined ethanol

Partial loading of the pores in InOF-1 with EtOH creates wide sections separated by “bottlenecks” and leads to 2.7-fold enhanced, kinetic experiment, CO₂ capture.