Water and Metal-Organic Frameworks: From Interaction toward Utilization

Prospects for Simultaneously Capturing Carbon Dioxide and Harvesting Water from Air

Mitigation of anthropogenic climate change is expected to require large-scale deployment of carbon dioxide removal strategies. Prominent among these strategies is direct air capture with sequestration (DACS), which encompasses the removal and long-term storage of atmospheric CO₂  by purely engineered means. Because it does not require arable land or copious amounts of freshwater, DACS is already attractive in the context of sustainable development, but opportunities to improve its sustainability still exist. Leveraging differences in the chemistry of CO₂  and water adsorption within porous solids, here, the prospect of simultaneously removing water alongside CO₂  in direct air capture operations is investigated. In many cases, the co-adsorbed water can be desorbed separately from chemisorbed CO₂  molecules, enabling efficient harvesting of water from air. Depending upon the material employed and process conditions, the desorbed water can be of sufficiently high purity for industrial, agricultural, or potable use and can thus improve regional water security. Additionally, the recovered water can offset a portion of the costs associated with DACS. In this Perspective, molecular- and process-level insights are combined to identify routes toward realizing this nascent yet enticing concept.

Flexible Coordination Network Exhibiting Water Vapor-Induced Reversible Switching between Closed and Open Phases

That physisorbents can reduce the energy footprint of water vapor capture and release has attracted interest because of potential applications such as moisture harvesting, dehumidification, and heat pumps. In this context, sorbents exhibiting an S-shaped single-step water sorption isotherm are desirable, most of which are structurally rigid sorbents that undergo pore-filling at low relative humidity (RH), ideally below 30% RH. Here, we report that a new flexible one-dimensional (1D) coordination network, [Cu(HQS)(TMBP)] (H₂HQS = 8-hydroxyquinoline-5-sulfonic acid and TMBP = 4,4'-trimethylenedipyridine), exhibits at least five phases: two as-synthesized open phases, α ⊃ H₂O and β ⊃ MeOH; an activated closed phase (γ); CO₂ (δ ⊃ CO₂) and C₂H₂ (ϵ ⊃ C₂H₂) loaded phases. The γ phase underwent a reversible structural transformation to α ⊃ H₂O with a stepped sorption profile (Type F-IV) when exposed to water vapor at <30% RH at 300 K. The hydrolytic stability of [Cu(HQS)(TMBP)] was confirmed by powder X-ray diffraction (PXRD) after immersion in boiling water for 6 months. Temperature-humidity swing cycling measurements demonstrated that working capacity is retained for >100 cycles and only mild heating (<323 K) is required for regeneration. Unexpectedly, the kinetics of loading and unloading of [Cu(HQS)(TMBP)] compares favorably with well-studied rigid water sorbents such as Al-fumarate, MOF-303, and CAU-10-H. Furthermore, a polymer composite of [Cu(HQS)(TMBP)] was prepared and its water sorption retained its stepped profile and uptake capacity over multiple cycles.

Fluorinated metal-organic frameworks for gas separation

Fluorinated metal-organic frameworks (F-MOFs) as fast-growing porous materials have revolutionized the field of gas separation due to their tunable pore apertures, appealing chemical features, and excellent stability. A deep understanding of their structure-performance relationships is critical for the synthesis and development of new F-MOFs. This critical review has focused on several strategies for the precise design and synthesis of new F-MOFs with structures tuned for specific gas separation purposes. First, the basic principles and concepts of F-MOFs as well as their structure, synthesis and modification and their structure to property relationships are studied. Then, applications of F-MOFs in adsorption and membrane gas separation are discussed. A detailed account of the design and capabilities of F-MOFs for the adsorption of various gases and the governing principles is provided. In addition, the exceptional characteristics of highly stable F-MOFs with engineered pore size and tuned structures are put into perspective to fabricate selective membranes for gas separation. Systematic analysis of the position of F-MOFs in gas separation revealed that F-MOFs are benchmark materials in most of the challenging gas separations. The outlook and future directions of the science and engineering of F-MOFs and their challenges are highlighted to tackle the issues of overcoming the trade-off between capacity/permeability and selectivity for a serious move towards industrialization.

Synthesis of Hierarchical-Porous Fluorinated Metal-Organic Frameworks with Superior Toluene Adsorption Properties

Constructing metal-organic frameworks (MOFs) with high volatile organic compounds (VOCs) adsorption capacity and excellent water resistance remain challenging. Herein, a monocarboxylic acid-assisted mixed ligands strategy was designed to synthesize a novel fluorinated MOFs, MIL-53 (Al). The monocarboxylic acid promoted crystallization and produced abundant crystal defects, which increased pore volume. Moreover, the competitive coordination between tetrafluoroterephthalic acid and 1,4-dicarboxybenzene was moderated by monocarboxylic modulators, significantly improving the hydrophobicity. The toluene uptake of the optimal sample reached 254.85 mg g^-1 under humid conditions, increased by 33.56 % of MIL-53(Al), and the Q_Wet /Q_Dry (the ratio of adsorption quality under wet to adsorption quality under dry) was 0.92, remarkably surpassing that of origin MIL-53 (0.72). The recycle experiment showed superior reusability with no performance degradation after 10 recycle under RH=50 % (relative humidity). The adsorptive kinetic and thermodynamic analysis proves that the adsorption process is controlled by surface mono-layer adsorption and pore diffusion. The fluorine group affects the internal diffusion, which weakens the transfer rate. This strategy opens a new prospect of obtaining hierarchical functional MOFs for meeting the VOCs uptake under the practical application.

Topologically Driven Pore/Surface Engineering in a Recyclable Microporous Metal-Organic Vessel Decorated with Hydrogen-Bond Acceptors for Solvent-Free Heterogeneous Catalysis

The use of metal-organic frameworks (MOFs) comprising custom-designed linkers/ligands as efficient and recyclable heterogeneous catalysts is on the rise. However, the topologically driven bifunctional porous MOFs for showcasing a synergistic effect of two distinct activation pathways of substrates (e.g., involving hydrogen bonding and a Lewis acid) in multicomponent organic transformations are very challenging. In particular, the novelty of such studies lies in the proper pore and/or surface engineering in MOFs for bringing the substrates in close proximity to understand the mechanistic aspects at the molecular level. This work represents the topological design, solid-state structural characterization, and catalytic behavior of an oxadiazole tetracarboxylate-based microporous three-dimensional (3D) metal-organic framework (MOF), {[Zn₂(oxdia)(4,4'-bpy)₂]·8.5H₂O}_n (1), where the tetrapodal (4-connected) 5,5'-(1,3,4-oxadiazole-2,5-diyl)diisophthalate (oxdia^4-), the tetrahedral metal vertex (Zn(II)), and a 2-connected pillar linker 4,4'-bipyridine (4,4'-bpy) are unique in their roles for the formation, stability, and function. As a proof of concept, the efficient utilization of both the oxadiazole moiety with an ability to provide H-bond acceptors and the coordinatively unsaturated Zn(II) centers in 1 is demonstrated for the catalytic process of the one-pot multicomponent Biginelli reaction under mild conditions and without a solvent. The key steps of substrate binding with the oxadiazole moiety are ascertained by a fluorescence experiment, demonstrating a decrease or increase in the emission intensity upon interaction with the substrates. Furthermore, the inherent polarizability of the oxadiazole moiety is exploited for CO₂ capture and its size-selective chemical fixation to cyclic carbonates at room temperature and under solvent-free conditions. For both catalytic processes, the chemical stability, structural integrity, heterogeneity, versatility in terms of substrate scope, and mechanistic insights are discussed. Interestingly, the first catalytic process occurs on the surface, while the second reaction occurs inside the pore. This study opens new ways to catalyze different organic transformation reactions by utilizing this docking strategy to bring the multiple components close together by a microporous MOF.

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C₂H₄) from C2 hydrocarbon mixtures such as ethane (C₂H₆), acetylene (C₂H₂), and other natural gas elements (CO₂, CH₄) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m² g⁻¹) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C₂H₂ > C₂H₆ > CO₂ > C₂H₄ > CH₄ under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C₂H₄) not only for a binary mixture (C₂H₆/C₂H₄) but also for a quaternary combination (C₂H₄/C₂H₆/C₂H₂/CO₂ and C₂H₄/C₂H₆/C₂H₂/CH₄) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

Industrially important C₂H₄ purification from multi-component hydrocarbon mixtures.

Construction and application of base-stable MOFs: a critical review

Metal-organic frameworks (MOFs) are a new class of porous crystalline materials constructed from organic ligands and metal ions/clusters. Owing to their unique advantages, they have attracted more and more attention in recent years and numerous studies have revealed their great potential in various applications. Many important applications of MOFs inevitably involve harsh alkaline operational environments. To achieve high performance and long cycling life in these applications, high stability of MOFs against bases is necessary. Therefore, the construction of base-stable MOFs has become a critical research direction in the MOF field. This review gives a historic summary of the development of base-stable MOFs in the last few years. The key factors that can determine the robustness of MOFs under basic conditions are analyzed. We also demonstrate the exciting achievements that have been made by utilizing base-stable MOFs in different applications. In the end, we discuss major challenges for the further development of base-stable MOFs. Some possible methods to address these problems are presented.

Trace removal of benzene vapour using double-walled metal-dipyrazolate frameworks

In principle, porous physisorbents are attractive candidates for the removal of volatile organic compounds such as benzene by virtue of their low energy for the capture and release of this pollutant. Unfortunately, many physisorbents exhibit weak sorbate-sorbent interactions, resulting in poor selectivity and low uptake when volatile organic compounds are present at trace concentrations. Herein, we report that a family of double-walled metal-dipyrazolate frameworks, BUT-53 to BUT-58, exhibit benzene uptakes at 298 K of 2.47-3.28 mmol g^-1 at <10 Pa. Breakthrough experiments revealed that BUT-55, a supramolecular isomer of the metal-organic framework Co(BDP) (H₂BDP = 1,4-di(1H-pyrazol-4-yl)benzene), captures trace levels of benzene, producing an air stream with benzene content below acceptable limits. Furthermore, BUT-55 can be regenerated with mild heating. Insight into the performance of BUT-55 comes from the crystal structure of the benzene-loaded phase (C₆H₆@BUT-55) and density functional theory calculations, which reveal that C-H···X interactions drive the tight binding of benzene. Our results demonstrate that BUT-55 is a recyclable physisorbent that exhibits high affinity and adsorption capacity towards benzene, making it a candidate for environmental remediation of benzene-contaminated gas mixtures.

Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Accessing 14-Connected Nets: Continuous Breathing, Hydrophobic Rare-Earth Metal Organic Frameworks Based on 14-c Hexanuclear Clusters with High Affinity for Non-Polar Vapors

Highly connected metal organic frameworks (MOFs) in which at least one building block has connectivity higher than twelve are very rare and much desirable. We report here the first examples of isostructural 14-connected MOFs, RE-frt-MOF-1, constructed from the assembly of 14-c hexanuclear rare-earth clusters, [RE₆(μ₃-X)₈(COO)₁₂]^2- (RE: Y³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Yb³⁺ and X: OH^-/F^-) with a tritopic carboxylate-based organic linker. This linker serves as a 3-c and 4-c organic node resulting in the formation of a unique, trinodal (3,4,14)-c framework. RE-frt-MOF-1 are stable in air and alkaline aqueous solutions and show an intriguingly continuous, reversible breathing behavior, between a wide and a narrow-pore phase, upon guest removal. Crystallinity is retained during breathing, and single-crystal X-ray diffraction shed light into the associated structural transformation. Vapor sorption studies performed on Y-frt-MOF-1 revealed a high affinity for non-polar vapors such as n-hexane, cyclohexane, and benzene, displaying type I isotherms with high uptake at low relative pressures (<10^-3 p/p₀), associated with the hydrophobic nature of the 1D channels and also with their rhombic shape. In contrast, polar vapors such as acetonitrile and ethanol show type V isotherms due to favorable vapor-vapor interactions. Notably these vapors, except cyclohexane, trigger the transition from the narrow to the wide pore phase, accompanied by a remarkable increase in uptake, reaching 70.6, 109, 100.4, and 87.7% for n-hexane, benzene, acetonitrile, and ethanol, respectively.

Metal organic framework/polyelectrolyte composites for water vapor sorption applications

Metal-organic framework (MOF) core particles of MIL-101(Cr), aluminum fumarate (Basolite® A520), MIL-53-TDC, zirconium fumarate, and UiO-66 were modified by adsorption of thin polyelectrolyte (PE)-based shells without deterioration of their crystal structure. By applying different PEs and depositing a single layer (MOF/PE) or one to three layer-by-layer assembled bilayers (MOF/LbL), the mass percent of shell material in the composite was varied from 0.6-2.5% to 50%. Under a constant relative pressure of water vapor, the moisture uptake by a MOF/PE and a MOF/LbL is rather comparable with its S-shaped curvature to that of pristine MOFs. The relevant differences, such as a shift of the ascending adsorption part to lower/higher relative pressure or an increase/decrease in water uptake in selected regions, are associated with the core-shell structure and related to the morphological changes of the MOF powders. The hydrophilic surface promotes the formation of liquid menisci at the points of contact between particles and accelerates the moisture uptake and loss. A decrease in water sorption under an atmosphere with high humidity by some composites can be associated with the inhibition of liquid water condensation by the more hydrophobic shells.

Linker Functionalization Strategy for Water Adsorption in Metal-Organic Frameworks

Water adsorption in metal-organic frameworks has gained a lot of scientific attention recently due to the potential to be used in adsorption-based water capture. Functionalization of their organic linkers can tune water adsorption properties by increasing the hydrophilicity, thus altering the shape of the water adsorption isotherms and the overall water uptake. In this work, a large set of functional groups is screened for their interaction with water using ab initio calculations. The functional groups with the highest water affinities form two hydrogen bonds with the water molecule, acting as H-bond donor and H-bond acceptor simultaneously. Notably, the highest binding energy was calculated to be -12.7 Kcal/mol for the -OSO₃H group at the RI-MP2/def2-TZVPP-level of theory, which is three times larger than the reference value. Subsequently, the effect of the functionalization strategy on the water uptake is examined on a selected set of functionalized MOF-74-III by performing Monte Carlo simulations. It was found that the specific groups can increase the hydrophilicity of the MOF and enhance the water uptake with respect to the parent MOF-74-III for relative humidity (RH) values up to 30%. The saturation water uptake exceeded 800 cm³/cm³ for all candidates, classifying them among the top performing materials for water harvesting.

Immobilization of Lewis Basic Nitrogen Sites into a Chemically Stable Metal-Organic Framework for Benchmark Water-Sorption-Driven Heat Allocations

Developing efficient and stable water adsorbents for adsorption-driven heat transfer technology still remains a challenge due to the lack of efficient strategies to enhance low-pressure water uptakes. The authors herein demonstrate that the immobilization of Lewis basic nitrogen sites into metal-organic frameworks (MOFs) can improve water uptake and target benchmark coefficient of performances (COPs) for cooling and heating. They present the water sorption properties of a chemically stable MOF (termed as Zr-adip), designed by incorporating hydrophilic nitrogen sites into the adsorbent MIP-200. Zr-adip exhibits S-shaped sorption isotherms with an extremely high water uptake of 0.43 g g-1  at 303 K and P/P0  = 0.25, higher than MIP-200 (0.39 g g-1 ), KMF-1 (0.39 g g-1 ) and MOF-303 (0.38 g g-1 ). Theoretical calculations reveal that the incorporated N sites can serve as secondary adsorption sites to moderately interact with water, providing more binding sites to strengthen the water binding affinity. Zr-adip achieves exceptionally high COPs of 0.79 (cooling) and 1.75 (heating) with a low driving temperature of 70 °C, outperforming MIP-200 (0.78 and 1.53) and KMF-1 (0.75 and 1.74). Combined with its ultrahigh stability, excellent cycling performance, and easy regeneration, Zr-adip represents one of the best water adsorbents for adsorption-driven cooling and heating.

Sandwich-Structured Carbon Paper/Metal-Organic Framework Monoliths for Flexible Solar-Powered Atmospheric Water Harvesting On Demand

Solar-powered atmospheric water harvesting (AWH) with metal-organic frameworks (MOFs) has been recognized as an attractive way to alleviate water shortage stress in rural arid areas given the naturally abundant solar energy. However, the existing solar-powered AWH technologies only allow a singular water production mode: either solar heating-driven AWH which usually results in rather poor water productivity due to the limited availability of sufficient sunlight or conductive heating-driven all-day AWH with significantly improved water productivity but requiring additional electricity provided with a photovoltaic module. This greatly limits the flexibility in managing AWH based on climate conditions, water productivity, and energy cost. Herein, a sandwich-structured MOF monolith (denoted as CACS) with dual heating capacity, localized solar heating (LSH) and electrical heating (LEH), is presented. Compared with LSH, the use of LEH leads to more rapid and uniform heating of CACS monoliths, thereby driving a significantly enhanced water desorption efficiency with faster kinetics. Using the CACS monolith as an AWH sorbent, a new type of atmospheric water harvester is developed and able to produce water in multiple working modes: LSH-, LEH-, and LSH-/LEH-driven AWH, thereby enabling flexible AWH on demand: direct use of sunlight for LSH-driven AWH during the sunlight-sufficient day and/or LEH-driven all-day AWH powered by a photovoltaic module particularly during the sunlight-absent/-insufficient time (night or cloudy day). When working at the LSH-/LEH-driven AWH mode, the resulting prototype delivers 1.4 L_H2O kg_MOF^-1 day^-1 of water productivity with 2.3 kW·h L^-1_H2O of energy consumption, corresponding to 5.4 times higher water productivity than the LSH-driven AWH working mode alone and 17.9% of energy saving at the cost of 22.2% of water productivity reduction compared with the LEH-driven AWH working mode alone. The current work, therefore, demonstrates a novel solar-powered AWH strategy that enables all-day water production with flexible choices on AWH working modes in terms of climate conditions, desired water productivity, and energy cost.

Kinetics of Sorption in Hygroscopic Hydrogels

Hygroscopic hydrogels hold significant promise for high-performance atmospheric water harvesting, passive cooling, and thermal management. However, a mechanistic understanding of the sorption kinetics of hygroscopic hydrogels remains elusive, impeding an optimized design and broad adoption. Here, we develop a generalized two-concentration model (TCM) to describe the sorption kinetics of hygroscopic hydrogels, where vapor transport in hydrogel micropores and liquid transport in polymer nanopores are coupled through the sorption at the interface. We show that the liquid transport due to the chemical potential gradient in the hydrogel plays an important role in the fast kinetics. The high water uptake is attributed to the expansion of hydrogel during liquid transport. Moreover, we identify key design parameters governing the kinetics, including the initial porosity, hydrogel thickness, and shear modulus. This work provides a generic framework of sorption kinetics, which bridges the knowledge gap between the fundamental transport and practical design of hygroscopic hydrogels.

Significance of an Environmental Gas Cell to Obtain a Fully Dehydrated Form and CO2-Pressurized Structure of a Metal-Organic Framework Using In Situ Single-Crystal X-ray Diffraction at 298 K

The single-crystal X-ray diffraction method was employed to characterize a rigid hydrated metal-organic framework (MOF), [Co₂(MA)(INA)·2H₂O]_n, that displays an affinity toward water molecules under ambient conditions after dehydration. The fully dehydrated form was obtained using an environmental gas cell technique in a stepwise manner followed by its CO₂-pressurized structure at 298 K using in situ crystallography.

Confined Water Vapor in ZIF-8 Nanopores

Metal-organic frameworks (MOFs) possess an ordered and size-controllable porous structure, making them an interesting heterogeneous confining environment for water. Herein, molecular dynamics simulations are employed to investigate the structure of confined water vapor in zeolitic imidazolate framework-8 (ZIF-8) nanopores. Water dimers, which are rarely observed in liquid or water vapor, can form in ZIF-8 at room temperature. The six-ring-member gate is the main location of a water dimer in ZIF-8. The terminal methyl and CH groups of the imidazole linker interact with the water dimer by relatively weak hydrogen bonding. The above-presented findings provide a foundation for the elucidation of water confined in ZIF-8 and demonstrate the potential of obtaining low-order clusters of water by MOFs.

Ultralow-temperature-driven water-based sorption refrigeration enabled by low-cost zeolite-like porous aluminophosphate

Thermally driven water-based sorption refrigeration is considered a promising strategy to realize near-zero-carbon cooling applications by addressing the urgent global climate challenge caused by conventional chlorofluorocarbon (CFC) refrigerants. However, developing cost-effective and high-performance water-sorption porous materials driven by low-temperature thermal energy is still a significant challenge. Here, we propose a zeolite-like aluminophosphate with SFO topology (EMM-8) for water-sorption-driven refrigeration. The EMM-8 is characterized by 12-membered ring channels with large accessible pore volume and exhibits high water uptake of 0.28 g·g-1 at P/P0 = 0.2, low-temperature regeneration of 65 °C, fast adsorption kinetics, remarkable hydrothermal stability, and scalable fabrication. Importantly, the water-sorption-based chiller with EMM-8 shows the potential of achieving a record coefficient of performance (COP) of 0.85 at an ultralow-driven temperature of 63 °C. The working performance makes EMM-8 a practical alternative to realize high-efficient ultra-low-temperature-driven refrigeration.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Selective luminescent sensing of metal ions and nitroaromatics over a porous mixed-linker cadmium(ii) based metal–organic framework

A potential luminescent sensor based on porous metal organic framework for the detection of metal ions (Al3+, Fe3+ or Cr3+) and nitro-explosive, 2,4,6-tri-nitrophenol has been discovered. MOF is capable of detecting aqueous phase analyte through luminescent sensing.

Fabrication of 3D Amino-Functionalized Metal-Organic Framework on Porous Nickel Foam Skeleton to Combinate Follicle Stimulating Hormone Antibody for Specific Recognition of Follicle-Stimulating Hormone

In this study, a superficial and highly efficient hydrothermal synthesis method was developed for the in situ growth of amine-functionalized iron containing metal-organic frameworks (H2N-Fe-MIL-101 MOFs) on porous nickel foam (NicF) skeletons (H2N-Fe-MIL-101/NicF). The uniform decoration of the H2N-Fe-MIL-101 nanosheets thus generated on NicF was immobilized with follicle-stimulating hormone (FSH) antibody (Ab-FSH) to detect FSH antigen. In the present work, the Ab-FSH tagged H2N-Fe-MIL-101/NicF electrode was first applied as an immunosensor for the recognition of FSH, electrochemically. With all of the special characteristics, this material demonstrated superior specific recognition and sensitivity for FSH with an estimated detection limit (LOD) of 11.6 and 11.5 fg/mL for buffered and serum solutions, respectively. The availability of specific functional groups on MOFs makes them an interesting choice for exploring molecular sensing applications utilizing Ab-FSH tagged biomolecules.

Water Capture Mechanisms at Zeolitic Imidazolate Framework Interfaces

Water capture mechanisms of zeolitic imidazolate framework ZIF-90 are revealed by differentiating the water clustering and the center pore filling step, using vibrational sum-frequency generation spectroscopy (VSFG) at a one-micron spatial resolution and state-of-the-art molecular dynamics (MD) simulations. Through spectral line shape comparison between VSFG and IR spectra, the relative humidity dependence of VSFG intensity, and MD simulations, based on MB-pol, we found water clustering and center pore filling happen nearly simultaneously within each pore, with water filling the other pores sequentially. The integration of nonlinear optics with MD simulations provides critical mechanistic insights into the pore filling mechanism and suggests that the relative strength of the hydrogen bonds governs the water uptake mechanisms. This molecular-level detailed mechanism can inform the rational optimization of metal-organic frameworks for water harvesting.

Highly Water-Permeable Metal-Organic Framework MOF-303 Membranes for Desalination

New membrane materials with excellent water permeability and high ion rejection are needed. Metal-organic frameworks (MOFs) are promising candidates by virtue of their diversity in chemistry and topology. In this work, continuous aluminum MOF-303 membranes were prepared on α-Al₂O₃ substrates via an in situ hydrothermal synthesis method. The membranes exhibit satisfying rejection of divalent ions (e.g., 93.5% for MgCl₂ and 96.0% for Na₂SO₄) on the basis of a size-sieving and electrostatic-repulsion mechanism and unprecedented permeability (3.0 L·m^-2·h^-1·bar^-1·μm). The water permeability outperforms typical zirconium MOF, zeolite, and commercial polymeric reverse osmosis and nanofiltration membranes. Additionally, the membrane material exhibits good stability and low production costs. These merits recommend MOF-303 as a next-generation membrane material for water softening.

Uranyl phosphonates: crystalline materials and nanosheets for temperature sensing

Ultrathin nanosheets of luminescent metal-organic frameworks or coordination polymers have been widely used for sensing ions, solvents and biomolecules but, as far as we are aware, not yet used for temperature sensing. Herein we report two luminescent uranyl phosphonates based on 2-(phosphonomethyl)benzoic acid (2-pmbH₃), namely (UO₂)(2-pmbH₂)₂ (1) and (H₃O)[(UO₂)₂(2-pmb)(2-pmbH)] (2). The former has a supramolecular layer structure, composed of chains of corner-sharing {UO₆} octahedra and {PO₃C} tetrahedra which are connected by hydrogen bonds between phosphonate and carboxylic groups. Compound 2 possesses a unique 2D anionic framework structure, where the inorganic uranyl phosphonate chains made up of {UO₇} and {PO₃C} polyhedra are cross-linked by 2-pmb^3- ligands. The carboxylic groups of 2-pmbH^2- ligands are pendant on the two sides of the layers and form hydrogen bonds between the layers. Both compounds can be exfoliated in acetone via a top-down freeze-thaw method, resulting in nanosheets of two-layer thickness. Interestingly, the photoluminescence (PL) of 1 and 2 is highly temperature sensitive. Variable temperature PL studies revealed that compounds 1 and 2 can be used as thermometers in the temperature ranges 120-300 K and 100-280 K, respectively. By doping the nanosheets into polymer matrix, 1-ns@PMMA and 2-ns@PMMA were prepared. The PL intensity of 1-ns@PMMA is insensitive to temperature, unlike that of the bulk sample. While 2-ns@PMMA exhibits similar temperature-dependent luminescence behaviour to its bulk counterpart, thereby enabling its potential application as a thermometer in the temperature range 100-280 K.

Water cluster in hydrophobic crystalline porous covalent organic frameworks

Progress over the past decades in water confinement has generated a variety of polymers and porous materials. However, most studies are based on a preconception that small hydrophobic pores eventually repulse water molecules, which precludes the exploration of hydrophobic microporous materials for water confinement. Here, we demonstrate water confinement across hydrophobic microporous channels in crystalline covalent organic frameworks. The frameworks are designed to constitute dense, aligned and one-dimensional polygonal channels that are open and accessible to water molecules. The hydrophobic microporous frameworks achieve full occupation of pores by water via synergistic nucleation and capillary condensation and deliver quick water exchange at low pressures. Water confinement experiments with large-pore frameworks pinpoint thresholds of pore size where confinement becomes dominated by high uptake pressure and large exchange hysteresis. Our results reveal a platform based on microporous hydrophobic covalent organic frameworks for water confinement.

Research on water confinement in small hydrophobic pores remains scarce because of a preconception that small hydrophobic pores repulse water molecules. Here, the authors demonstrate water confinement across hydrophobic microporous channels in crystalline covalent organic frameworks.

Structure-Property Correlation of Hierarchically Porous Carbons for Fluorocarbon Adsorption

Although traditional commercially available porous carbon-fluorocarbon working pairs have shown promising applicability for adsorption cooling, advancements in engineered carbons may further improve the performance. Moreover, insights into structure-property relationships that target higher sorption capacities within these synthesized carbons may guide such materials' future design. We utilized hierarchically porous carbons (HPCs), synthesized with colossal microporous and mesoporous content characterized by high surface areas (up to 2689 m²/g) and pore volume values (up to 10.31 cm³/g) toward fluorocarbon R134a adsorption. This unique pore topology leads to exceptional R134a uptake, ∼250 wt %, outperforming the highest uptake carbon material to date, Maxsorb III (∼220 wt %). Material characterizations reveal that the outstanding R134a capacity may be attributed to textural properties and oxygen-terminated functional groups more than graphitization of the material. Most importantly, HPCs are efficiently utilized in a two-bed model chiller device, where the performance shows excellent working capacity (105 wt %, ∼2 times the value of reported carbon materials/R134a). Fluorocarbon adsorption on HPCs also displays fast kinetics (equilibrium time: ∼2 min) mainly driven by physical adsorption (Qst: ∼27 kJ/mol), characteristic of swiftly reversible behavior adsorption-desorption behaviors. This work provides a fundamental understanding of the applicability of HPCs/R134a working pair for adsorption cooling.

High Water Adsorption MOFs with Optimized Pore-Nanospaces for Autonomous Indoor Humidity Control and Pollutants Removal

The indoor air quality is of prime importance for human daily life and health, for which the adsorbents like zeolites and silica-gels are widely used for air dehumidification and harmful gases capture. Herein, we develop a pore-nanospace post-engineering strategy to optimize the hydrophilicity, water-uptake capacity and air-purifying ability of metal-organic frameworks (MOFs) with long-term stability, offering an ideal candidate with autonomous multi-functionality of moisture control and pollutants sequestration. Through variant tuning of organic-linkers carrying hydrophobic and hydrophilic groups in the pore-nanospaces of prototypical UiO-67, a moderately hydrophilic MOF (UiO-67-4Me-NH₂ -38 %) with high thermal, hydrolytic and acid-base stability is screened out, featuring S-shaped water sorption isotherms exactly located in the recommended comfortable and healthy ranges of relative humidity for indoor ventilation (45 %-65 % RH) and adverse health effects minimization (40-60 % RH). Its exceptional attributes of water-uptake working capacity/efficiency, contaminants removal, recyclability and regeneration promise a great potential in confined indoor environment application.

Chemically Stable Carbazole-Based Imine Covalent Organic Frameworks with Acidochromic Response for Humidity Control Applications

Isoreticular chemically stable two-dimensional imine covalent organic frameworks (COFs), further denoted as DUT-175 and DUT-176, are obtained in a reaction of 4,4′-bis(9H-carbazol-9-yl)biphenyl tetraaldehyde with phenyldiamine and benzidine. The crystal structures, solved and refined from the powder X-ray diffraction data and confirmed by high-resolution transmission electron microscopy, indicate AA-stacked layer structures. Both structures feature distorted hexagonal channel pores, assuring remarkable porosity (S_BET = 1071 m² g^–1 for DUT-175 and S_BET = 1062 m² g^–1 for DUT-176), as confirmed by adsorption of gases and vapors. The complex conjugated π system of the COFs involves electron-rich carbazole building units, which in combination with the imine groups allow reversible pH-dependent protonation of the frameworks, accompanied by charge transfer and shift of the absorption bands in the UV–vis spectrum. The sigmoidal shape of the water vapor adsorption and desorption isotherms with a steep adsorption step at p/p₀ = 0.4–0.6 in combination with excellent stability over dozens of adsorption and desorption cycles ranks these COFs among the best materials for indoor humidity control applications.

A switchable sensor and scavenger: detection and removal of fluorinated chemical species by a luminescent metal-organic framework

Fluorosis has been regarded as a worldwide disease that seriously diminishes the quality of life through skeletal embrittlement and hepatic damage. Effective detection and removal of fluorinated chemical species such as fluoride ions (F⁻) and perfluorooctanoic acid (PFOA) from drinking water are of great importance for the sake of human health. Aiming to develop water-stable, highly selective and sensitive fluorine sensors, we have designed a new luminescent MOF In(tcpp) using a chromophore ligand 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine (H₄tcpp). In(tcpp) exhibits high sensitivity and selectivity for turn-on detection of F⁻ and turn-off detection of PFOA with a detection limit of 1.3 μg L⁻¹ and 19 μg L⁻¹, respectively. In(tcpp) also shows high recyclability and can be reused multiple times for F⁻ detection. The mechanisms of interaction between In(tcpp) and the analytes are investigated by several experiments and DFT calculations. These studies reveal insightful information concerning the nature of F⁻ and PFOA binding within the MOF structure. In addition, In(tcpp) also acts as an efficient adsorbent for the removal of F⁻ (36.7 mg g⁻¹) and PFOA (980.0 mg g⁻¹). It is the first material that is not only capable of switchable sensing of F⁻ and PFOA but also competent for removing the pollutants via different functional groups.

A robust In-MOF, In(tcpp), demonstrates sensitive detection of the fluorinated chemical species F⁻ and PFOA via distinctly different luminescence signal change, and effective adsorption and removal of both species from aqueous solution.

An improved water-harvesting cycle

[Figure: see text].

Evolution of water structures in metal-organic frameworks for improved atmospheric water harvesting

[Figure: see text].

A Hydrolytically Stable Cu(II)-Based Metal-Organic Framework with Easily Accessible Ligands for Water Harvesting

Water scarcity is a critical issue in desert and arid regions, and atmospheric water harvesting is a potential solution. The challenge is lacking ideal adsorbents that can efficiently capture water from low-humidity air and be regenerated readily. Herein, we report a hydrolytically stable metal-organic framework (MOF), [Cu₂(AD)₂(SA)] (Cu-AD-SA), with excellent performance in water harvesting. More importantly, this material can be facilely prepared from two easily accessible ligands adenine (HAD) and succinic acid (H₂SA). Cu-AD-SA has a three-dimensional (3D) framework structure with the crs topology and intersecting channels of ∼5 Å in diameter. The channel surface is decorated by uncoordinated aromatic N atoms, amine groups, and alkyl moieties. Interestingly, Cu-AD-SA shows a high water adsorption capacity of 0.16 g g^-1 at low pressure of 0.2 P/P₀ and 25 °C. Furthermore, dynamic water adsorption-desorption cycling experiments demonstrated a stable working capacity of 0.13 g g^-1 for uptaking water from a low-humidity air (water partial pressure: 0.85 kPa, 20% RH at 30 °C, 5.3% RH at 55 °C) at 30 °C and desorption at 55 °C. The water adsorption mechanism was also studied by analyzing its single-crystal structure after water loading. The results indicated the existence of strong H-bonding interactions between water molecules and uncoordinated N atoms and amine groups on the framework, which should play an important role in the high adsorption at low pressure. All the above features suggest great potential of Cu-AD-SA for water harvesting in arid regions.

Performance-Based Screening of Porous Materials for Carbon Capture

Computational screening methods have changed the way new materials and processes are discovered and designed. For adsorption-based gas separations and carbon capture, recent efforts have been directed toward the development of multiscale and performance-based screening workflows where we can go from the atomistic structure of an adsorbent to its equilibrium and transport properties at different scales, and eventually to its separation performance at the process level. The objective of this work is to review the current status of this new approach, discuss its potential and impact on the field of materials screening, and highlight the challenges that limit its application. We compile and introduce all the elements required for the development, implementation, and operation of multiscale workflows, hence providing a useful practical guide and a comprehensive source of reference to the scientific communities who work in this area. Our review includes information about available materials databases, state-of-the-art molecular simulation and process modeling tools, and a complete catalogue of data and parameters that are required at each stage of the multiscale screening. We thoroughly discuss the challenges associated with data availability, consistency of the models, and reproducibility of the data and, finally, propose new directions for the future of the field.

Fine-Tuning Window Apertures in ZIF-8/67 Frameworks by Metal Ions and Temperature for High-Efficiency Molecular Sieving of Xylenes

Separation of structurally similar components from their mixtures is one of the most promising applications of metal-organic frameworks (MOFs). A high efficiency of such molecular sieving requires fine tuning of the MOF structure. In this work, we investigate subtle metal- and temperature-induced changes in window dimensions of zeolitic imidazolate frameworks (ZIF-8(Zn) and ZIF-67(Co)) and apply such structural tuning for efficient separation of xylene isomers (p-, m-, and o-xylenes). The use of Co instead of Zn favorably modifies window geometry: it accelerates the diffusion of all components by a factor of 2-3 while maintaining closely the same separation efficiency as that of ZIF-8(Zn). Outstanding selectivity above 18:1 and faster isolation of demanded p-xylene from the ternary mixture using ZIF-67(Co) have been demonstrated at room temperature, opening new horizons for its energy-efficient xylene separation. More generally, our findings suggest the prospective ways to tune various MOFs for target liquid-state separations.

Insights into Catalytic Hydrolysis of Organophosphonates at M-OH Sites of Azolate-Based Metal Organic Frameworks

Organophosphorus nerve agents, a class of extremely toxic chemical warfare agents (CWAs), have remained a threat to humanity because of their continued use against civilian populations. To date, Zr(IV)-based metal organic framework (MOFs) are the most prevalent nerve agent hydrolysis catalysts, and relatively few reports disclose MOFs containing nodes with other Lewis acidic transition metals. In this work, we leveraged this synthetic tunability to explore how the identity of the transition metal node in the M-MFU-4l series of MOFs (M = Zn, Cu, Ni, Co) influences the catalytic performance toward the hydrolysis of the nerve agent simulant dimethyl (4-nitrophenyl)phosphate (DMNP). Experimental studies reveal that Cu-MFU-4l exhibits the best performance in this series with a half-life for hydrolysis of ∼2 min under these conditions. In contrast, both Ni- and Co-MFU-4l demonstrate significantly slower reactivity toward DMNP, as they both fail to surpass 30% conversion of DMNP after 1 h under analogous conditions. Further modification of the active site within Cu-MFU-4l is possible, and we found that although the identity of the anion coordinated to the Cu(II)-X (X = Cl^-, HCOO^-, ClO₄^-, NO₃^-) active site has little influence on the catalytic performance, reduction of the Cu(II) sites yields nodes that contain Cu(I) ions in a trigonal geometry with open metal sites, leading to remarkable catalytic activity with a half-life for hydrolysis less than 2 min. Computational studies indicate the Cu(I) sites exhibit stronger binding affinities than Cu(II) to both water and DMNP, which corroborates the experimental results.

Ultrahigh water sorption on highly nitrogen doped carbonaceous materials derived from uric acid

HYPOTHESIS

Developing materials for thermally driven adsorption chillers and adsorption heat pumps is a growing research field due to the potential of these technologies to address up to 50% of the world's total energy demand. These materials must be abundant, easy to synthesize, hydrophilic, and low in cost. Bare carbon materials are hydrophobic and therefore usually not considered for these applications. However, by introducing heteroatoms and tuning their porosity, the hydrophilicity of carbonaceous networks can be increased significantly.

EXPERIMENTAL

Herein, a series of highly nitrogen doped carbonaceous materials (CNs) have been synthesized by submitting uric acid to heat treatment at different temperatures in the presence of an inorganic salt mix as solvent and pore template. The effect of the thermal treatment on the materials composition, pore network, and water sorption capability has been studied.

FINDINGS

At 800 °C, a nitrogen depleted carbonaceous material with a maximal water uptake of 1.38g_H2O g^-1 is obtained. Condensation at 750 °C creates an ultra-hydrophilic CN with a water uptake of 0.8 g_H2O g^-1 at already much lower partial pressures. While the maximum uptake is mainly ascribed to the mesopore volume of the material, the differences in hydrophilicity can be controlled by functionality.