How Can a Hydrophobic MOF be Water-Unstable? Insight into the Hydration Mechanism of IRMOFs

Metal-Organic Framework Stability in Water and Harsh Environments from Data-Driven Models Trained on the Diverse WS24 Data Set

Metal-organic frameworks (MOFs) are porous materials with applications in gas separations and catalysis, but a lack of water stability often limits their practical use given the ubiquity of water. Consequently, it is useful to predict whether a MOF is water-stable before investing time and resources into synthesis. Existing heuristics for designing water-stable MOFs lack generality and limit the diversity of explored chemistry due to narrowly defined criteria. Machine learning (ML) models offer the promise to improve the generality of predictions but require data. In an improvement on previous efforts, we enlarge the available training data for MOF water stability prediction by over 400%, adding 911 MOFs with water stability labels assigned through semiautomated manuscript analysis to curate the new data set WS24. The additional data are shown to improve ML model performance (test ROC-AUC > 0.8) over diverse chemistry for the prediction of both water stability and stability in harsher acidic conditions. We illustrate how the expanded data set and models can be used with a previously developed activation stability model in combination with genetic algorithms to quickly screen ∼10,000 MOFs from a space of hundreds of thousands for candidates with multivariate stability (upon activation, in water, and in acid). We uncover metal- and geometry-specific design rules for robust MOFs. The data set and ML models developed in this work, which we disseminate through an easy-to-use web interface, are expected to contribute toward the accelerated discovery of novel, water-stable MOFs for applications such as direct air gas capture and water treatment.

Flat-Histogram Monte Carlo Simulation of Water Adsorption in Metal-Organic Frameworks

Molecular simulations of water adsorption in porous materials often converge slowly due to sampling bottlenecks that follow from hydrogen bonding and, in many cases, the formation of water clusters. These effects may be exacerbated in metal-organic framework (MOF) adsorbents, due to the presence of pore spaces (cages) that promote the formation of discrete-size clusters and hydrophobic effects (if present), among other reasons. In Grand Canonical Monte Carlo (MC) simulations, these sampling challenges are typically manifested by low MC acceptance ratios, a tendency for the simulation to become stuck in a particular loading state (i.e., macrostates), and the persistence of specific clusters for long periods of the simulation. We present simulation strategies to address these sampling challenges, by applying flat-histogram MC (FHMC) methods and specialized MC move types to simulations of water adsorption. FHMC, in both Transition-matrix and Wang-Landau forms, drives the simulation to sample relevant macrostates by incorporating weights that are self-consistently adjusted throughout the simulation and generate the macrostate probability distribution (MPD). Specialized MC moves, based on aggregation-volume bias and configurational bias methods, separately address low acceptance ratios for basic MC trial moves and specifically target water molecules in clusters; in turn, the specialized MC moves improve the efficiency of generating new configurations which is ultimately reflected in improved statistics collected by FHMC. The combined strategies are applied to study the adsorption of water in CuBTC and ZIF-8 at 300 K, through examination of the MPD and the adsorption isotherm generated by histogram reweighting. A key result is the appearance of nontrivial oscillations in the MPD, which we show to be associated with water clusters in the adsorption system. Additionally, we show that the probabilities of certain clusters become similar in value near the boundaries of the isotherm hysteresis loop, indicating a strong connection between cluster formation/destruction and the thermodynamic limits of stability. For a hydrophobic MOF, the FHMC results show that the phase transition from low density to high density is suppressed to water pressure far above the bulk-fluid saturation pressure; this is consistent with results presented elsewhere. We also compare our FHMC simulation isotherm to one measured by a different technique but with ostensibly the same molecular interactions and comment on observed differences and the need for follow-up work. The simulation strategies presented here can be applied to the simulation of water in other MOFs using heuristic guidelines laid out in our text, which should facilitate the more consistent and efficient simulation of water adsorption in porous materials in future applications.

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.

Ethanol-mediated synthesis of γ-cyclodextrin-based metal-organic framework as edible microcarrier: performance and mechanism

Here, an ethanol-mediated method was introduced to fabricate γ-cyclodextrin-based metal-organic frameworks (γ-CD-MOFs) as microcarriers for epigallocatechin-3-gallate (EGCG). Through adjusting ethanol gas diffusion temperature and ethanol liquid feed speed, we achieved control of crystallization efficiency and crystals size without extra surfactants. Under the sequential regulatory by ethanol in two phases, the obtained γ-CD-MOFs with cubic shape exhibited excellent crystallinity, high surface area, and uniform size distribution. Through the interplay of hydrogen bonding, hydrophobic interactions and π stacking, EGCG molecules could be stored efficiently within cavities and tunnels of the γ-CD-MOFs with high load capability of 334 mg g^-1. More importantly, the incorporation of EGCG within frameworks wouldn't disintegrate the unique body-centered cubic structure of γ-CD-MOFs, in turn, would improve the thermostability and antioxidative activity of EGCG. Significantly, all food-grade materials ensured the γ-CD-MOFs high acceptance and applicability for food and biomedical applications.

Aqueous Stability of Metal-Organic Frameworks Using ReaxFF-Based Metadynamics Simulations

Aqueous stability is a critical property for the application of metal-organic framework (MOF) materials in humid conditions. The sampling of the free energy surface for a water reaction is challenging due to a lack of a reactive force field. Here, we developed a ReaxFF force field for simulating the reaction of zeolitic imidazole frameworks (ZIFs) with water. We carried out metadynamics simulations based on ReaxFF to study the reaction of water with a few different types of MOFs. We also conducted an experimental water immersion test and characterized the XRD, TG, and gas adsorption properties of the MOFs before and after the immersion test. By considering the energy barrier for a hydrolysis reaction, the simulation results are in good agreement with the experiments. MOFs with open structures and large pores are found to be unstable in metadynamics simulations, where the water molecule can attack or bond with the metallic node relatively easily. In contrast, it is more difficult for water to attack the Zn atom in the ZnN₄ tetrahedral structure of ZIFs. We also found that ZIFs with the -NO₂ functional groups have higher water stability. Discrepancies between the metadynamics simulation and gas adsorption experiments have been accounted for from the phase/crystallinity change of the structure reflected in the X-ray diffraction and thermogravimetry analysis of the MOF samples.

Exploring the Potential of a Highly Scalable Metal-Organic Framework CALF-20 for Selective Gas Adsorption at Low Pressure

In this study, the ability of the highly scalable metal-organic framework (MOF) CALF-20 to adsorb polar and non-polar gases at low pressure was investigated using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results from the simulated adsorption isotherms revealed that the highest loading was achieved for SO2 and Cl2, while the lowest loading was found for F2 molecules. The analysis of interaction energies indicated that SO2 molecules were able to form the strongest adsorbent-adsorbate interactions and had a tight molecular packing due to their polarity and angular structure. Additionally, Cl2 gas was found to be highly adsorbed due to its large van der Waals surface and strong chemical affinity in CALF-20 pores. MD simulations showed that SO2 and Cl2 had the lowest mobility inside CALF-20 pores. The values of the Henry coefficient and isosteric heat of adsorption confirmed that CALF-20 could selectively adsorb SO2 and Cl2. Based on the results, it was concluded that CALF-20 is a suitable adsorbent for SO2 and Cl2 but not for F2. This research emphasizes the importance of molecular size, geometry, and polarity in determining the suitability of a porous material as an adsorbent for specific adsorbates.

State of the art on the ultrasonic-assisted removal of environmental pollutants using metal-organic frameworks

The environmental and health issues of drinking water and effluents released into nature are among the major area of contention in the past few decades. With the growth of ultrasound-based approaches in water and wastewater treatment, promising materials have also been considered to employ their advantages. Metal-organic frameworks (MOFs) are among the porous materials that have received great attention from researchers in recent years. Features such as high porosity, large specific surface area, electronic properties like semi-conductivity, and the capacity to coordinate with the organic matter have resulted in a substantial increase in scientific researches. This work deals with a comprehensive review of the application of MOFs for ultrasonic-assisted pollutant removal from wastewater. In this regard, after considering features and synthesis methods of MOFs, the mechanisms of several ultrasound-based approaches including sonocatalysis, sonophotocatalysis, and sono-adsorption are well assessed for removal of different organic compounds by MOFs. These methods are compared with some other water treatment processes with the application of MOFs in the absence of ultrasound. Also, the main concern about MOFs including environmental hazards and water stability is fully discussed and some techniques are proposed to reduce hazardous effects of MOFs and improve stability in humid/aqueous environments. Economic aspects for the preparation of MOFs are evaluated and cost estimates for ultrasonic-assisted AOP approaches were provided. Finally, the future outlooks and the new frontiers of ultrasonic-assisted methods with the help of MOFs in global environmental pollutant removal are presented.

Iron-based metal-organic framework: Synthesis, structure and current technologies for water reclamation with deep insight into framework integrity

Water is a supreme requirement for the existence of life, the contamination from the point and non-point sources are creating a great threat to the water ecosystem. Advance tools and techniques are required to restore the water quality and metal-organic framework (MOFs) with a tunable porous structure, striking physical and chemical properties are an excellent candidate for it. Fe-based MOFs, which developed rapidly in recent years, are foreseen as most promising to overcome the disadvantages of traditional water depolluting practices. Fe-MOFs with low toxicity and preferable stability possess excellent performance potential for almost all water remedying techniques in contrast to other MOF structures, especially visible light photocatalysis, Fenton, and Fenton-like heterogeneous catalysis. Fe-MOFs become essential tool for water treatment due to their high catalytic activity, abundant active site and pollutant-specific adsorption. However, the structural degradation under external chemical, photolytic, mechanical, and thermal stimuli is impeding Fe-MOFs from further improvement in activity and their commercialization. Understanding the shortcomings of structural integrity is crucial for large-scale synthesis and commercial implementation of Fe-MOFs-based water treatment techniques. Herein we summarize the synthesis, structure and recent advancements in water remediation methods using Fe-MOFs in particular more attention is paid for adsorption, heterogeneous catalysis and photocatalysis with clear insight into the mechanisms involved. For ease of analysis, the pollutants have been classified into two major classes; inorganic pollutants and organic pollutants. In this review, we present for the first time a detailed insight into the challenges in employing Fe-MOFs for water remediation due to structural instability.

Crystal violet-modified HKUST-1 framework with improved hydrostability as an efficient adsorbent for direct solid-phase microextraction

Metal-organic frameworks (MOFs) have received extensive attention in adsorption applications owing to their high surface area. However, some MOFs do not perform well as the extraction medium when used under aqueous conditions. The low hydrostability of MOFs limits the practical application of these materials in solid-phase microextraction (SPME). Here, the fabrication of a water resistance SPME fiber coating is introduced based on the crystal violet (CV)-modified HKUST-1 framework on copper (Cu@HKUST-1@CV). The HKUST-1 was prepared by the in situ growth method, followed by post-synthetic modification of HKUST-1 with the CV layer. The preparation of the modified HKUST-1 was characterized by FESEM, XRD, FTIR, and DFT approaches. The prepared SPME coating was successfully employed for the quantification of anthracene (AN), as a model analyte, in water samples. The limit of detection was 0.8 ng mL^-1. The developed method will open up a new door towards searching for promising materials in SPME applications.

Trisequential Postsynthetic Modification of a Tagged IRMOF-9 Framework

Tailoring the pore environments of metal-organic frameworks (MOFs) is key to improving their performance and expanding their applicability. Postsynthetic methods, wherein an already synthesized MOF undergoes further chemical reactions, present many advantages for such tailoring and lead to much interesting new chemistry. However, this method has seldom been pushed farther than two reaction steps on the organic component. Here we report a three-step sequence starting from an alkenyl group on the biphenyl backbone of an IRMOF-9 analogue. The alkene is converted to an oxirane group and subsequently to a 1,2-azidoalcohol. The ultimate product is a framework functionalized with an aziridine ring. The reaction efficiency of each step is high, which suppresses the formation of undesired functional groups and the buildup of unintended multivariate frameworks. The synthesis of each framework was attempted via a direct synthetic method employing the appropriately functionalized biphenyldicarboxylate ligand. In general, this met with failure, which demonstrates the power and utility of postsynthetic methods for preparing new materials.

Effect of ionic liquids in carbon nanotube bundles on CO2, H2S, and N2 separation from CH4: A computational study

Carbon nanotube (CNT) bundles are being explored as a support structure for four ionic liquids (ILs) in gas separation. Grand canonical Monte Carlo simulations were performed to investigate the CO₂/CH₄, H₂S/CH₄, and N₂/CH₄ separation performance in CNT bundles and CNT-supported ILs (CNT-ILs) as a function of pressure and IL loading. The results show that by adding ILs to the CNT bundles, the gas separation performance can be significantly increased. Increasing the number of IL molecules in the composites increases the separation performance. Such a phenomenon is more evident for the CO₂/CH₄ mixture in comparison to H₂S/CH₄ and N₂/CH₄. Calculations of isosteric heat of adsorption and selectivities in gas mixtures as a function of pressure show promising gas separation performance for CNT-ILs. Due to the excellent mechanical properties of CNTs, it has been shown that this structure may be used as a strong mechanical support for structures containing ILs with excellent CO₂/CH₄ separation performance.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.

Quantum Mechanical Calculations for Biomass Valorization over Metal-Organic Frameworks (MOFs)

Metal-organic framework (MOF) in biomass valorization is a promising technology developed in recent decades. By tailoring both the metal nodes and organic ligands, MOFs exhibit multiple functionalities, which not only extend their applicability in biomass conversion but also increase the complexity of material designs. To address this issue, quantum mechanical simulations have been used to provide mechanistic insights into the catalysis of biomass-derived molecules, which could potentially facilitate the development of novel MOF-based materials for biomass valorization. The aim of this review is to survey recent quantum mechanical simulations on biomass reactions occurring in MOF catalysts, with the emphasis on the studies of the catalytic activity of active sites and the effects of organic ligand and porous structures on the kinetics. Moreover, different model systems and computational methods used for MOF simulations are also surveyed and discussed in this review.

Effect of diol isomer/water mixtures on the stability of Zn-MOF-74

The stability of metal-organic frameworks is a key factor in many applications in some fields that require working under harsh conditions. It is known that a large number of MOFs are vulnerable to humid air. It means that when they are exposed to water, a structural collapse of the crystal happens. In this work, Molecular Dynamics simulations using a reactive force field have been performed to study the stability of MOF-74 against the adsorption of catechol, resorcinol and hydroquinone in the presence of water. We reproduced the water instability of Zn-MOF-74 and we studied the resistance of the structure. Our simulations showed that the three isomers generate a volume change in the framework but the structural collapse does not happen. In contrast, for water-isomer mixtures, there is structural collapse. Not only do catechol, resorcinol and hydroquinone not behave as stabilizing agents but they do enhance the hydration effect on the structure.

A comparative study of the physical and chemical properties of pelletized HKUST-1, ZIF-8, ZIF-67 and UiO-66 powders

The physical and chemical properties of four metal organic frameworks were examined before and after the pelletization process with pressure (tablet) and binders (pellet) for their possible use in industrial or other commercial processes. Due to the uniqueness of their crystal structure, each MOF behaved differently under the same treatment. ZIF-8 proved to be very robust in the presence of binding materials (pellet) as well as after the application of pressure (tablet). The presence of water resulted in the reversible partial loss of their crystalline morphology in the case of HKUST-1 and UiO-66 (pellet). The crystal structure of ZIF-67 was irreversibly lost in the case of the pellet but it was well maintained under pressure (tablet).

Water Adsorption in Soft and Heterogeneous Nanopores

ConspectusLiquids under confinement differ in behavior from their bulk counterparts and can acquire properties that are specific to the confined phase and linked to the nature and structure of the host matrix. While confined liquid water is not a new topic of research, the past few years have seen a series of intriguing novel features for water inside nanoscale pores. These unusual properties arise from the very specific nature of nanoporous materials, termed "soft porous crystals"; they combine large-scale flexibility with a heterogeneous internal surface. This creates a rich diversity of behavior for the adsorbed water, and the combination of different experimental characterization techniques along with computational chemistry at various scales is necessary to understand the phenomena observed and their microscopic origins. The range of systems of interest span the whole chemical range, from the inorganic (zeolites, imogolites) to the organic (microporous carbons, graphene, and its derivatives), and even encompass the hybrid organic-inorganic systems (such as metal-organic frameworks).The combination of large scale flexibility with the strong physisorption (or even chemisorption) of water can lead to unusual properties (belonging to the "metamaterials" category) and to novel phenomena. One striking example is the recent elucidation of the mechanism of negative hydration expansion in ZrW₂O₈, by which adsorption of ∼10 wt % water in the inorganic nonporous framework leads to large shrinkage of its volume. Another eye-catching case is the occurrence of multiple water adsorption-driven structural transitions in the MIL-53 family of materials: the specific interactions between water guest molecules and the host framework create behavior that has not been observed with any other adsorbate. Both are counterintuitive phenomena that have been elucidated by a combination of experimental in situ techniques and molecular simulation.Another important direction of research is the shift in the systems and phenomena studied, from physical adsorption toward studies of reactivity, hydrothermal stability, and the effect of confinement on aqueous phases more complex than pure water. There have been examples of water adsorption in highly flexible metal-organic frameworks being able to compete with the materials' coordination bonds, thereby limiting its hydrothermal stability, while tweaking the functional groups of the same framework can lead to increased stability while retaining the flexibility of the material. However, this additional complexity and tunability in the macroscopic behavior can occur from changes in the confined fluid rather than the material. Very recent studies have shown that aqueous solutions of high concentration (such as LiCl up to 20 mol L^-1) confined in flexible nanoporous materials can have specific properties different from pure water and not entirely explained by osmotic effects. There, the strong ordering of the confined electrolyte competes with the structural flexibility of the framework to create an entirely new behavior for the {host, guest} system.

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.

Cationic metal-organic frameworks as an efficient adsorbent for the removal of 2,4-dichlorophenoxyacetic acid from aqueous solutions

Metal-organic frameworks (MOFs) material with high surface area, good chemical stability and multi-functionality, has become an emerging adsorbent for water treatment. A novel kind of quaternary amine anionic-exchange MOFs UiO-66 namely UiO-66-NMe₃⁺ was firstly synthesized for adsorptive removal of a widely used toxic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solutions. The well-prepared UiO-66-NMe₃⁺ MOFs were fully characterized, and then the main parameters affecting the adsorption process including solution pH, adsorbent dosage and coexisting anions were systematically investigated. The maximum adsorption capacity of UiO-66-NMe₃⁺ toward 2,4-D reached as high as 279 mg g^-1, much higher than that of pristine UiO-66 and aminated UiO-66. The adsorption mechanism could be attributed to the electrostatic interactions efficiently enhanced by the functionalization of quaternary amine groups, combining with the π-π conjugations between the linkers in MOFs and 2,4-D molecules, leading to the better adsorption performance of UiO-66-NMe₃⁺. Additionally, the UiO-66-NMe₃⁺ could be well regenerated by simple solvent washing and exhibited a slight decline of adsorption capacity after seven successive recycle. Furthermore, satisfactory adsorption capacity and reusability of the MOFs in environmental water samples were attained. Comparing with reported activated carbon and resin materials, the UiO-66-NMe₃⁺ MOFs possessed higher adsorption capacity and shorter equilibrium time, as well as good reusability and practicality. The developed ion-exchange functionalized MOFs provided an ideal alternative for efficient adsorptive-removal of 2,4-D from complicated aqueous environment.

Coordination tailoring of water-labile 3D MOFs to fabricate ultrathin 2D MOF nanosheets

A novel coordination tailoring top-down delamination strategy involving the controllable partial disassembly of water-labile 3D MOFs has been explored to fabricate ultrathin 2D MOF nanosheets, and it exhibits many advantages such as green, convenience and high efficiency. Accordingly, 2D ultrathin MOF nanosheets with a two-fold interpenetrated architecture have been rapidly obtained from a 3D pillar-layered MOF, and they show distinguished fluorescence responses to different solvents.

Thermal Post-Treatments to Enhance the Water Stability of NH2-MIL-125(Ti)

NH2-MIL-125(Ti) is a metal organic framework (MOF) based on Ti-oxo-clusters widely investigated in water-related applications. Such applications require MOFs with an excellent stability in the aqueous phase, but, despite this, the extent of MOFs’ degradation in water is still not yet fully understood. In this study, we report a quantitative study of the water stability of NH2-MIL-125(Ti), analyzing the ligand release along the contact time in water. This study demonstrates that NH2-MIL-125(Ti) easily leached out over time while maintaining its structure. The effect of different thermal treatments applied to NH2-MIL-125(Ti) was investigated to enhance its water stability. The structural and textural properties of those modified MOFs were studied in detail and those maintaining the NH2-MIL-125(Ti) properties were exposed to aqueous medium. The analysis of the released ligand concentration in the filtrate can provide information on the water stability of this material.

Precise Control of Molecular Self-Diffusion in Isoreticular and Multivariate Metal-Organic Frameworks

Understanding the factors that affect self-diffusion in isoreticular and multivariate (MTV) MOFs is key to their application in drug delivery, separations, and heterogeneous catalysis. Here, we measure the apparent self-diffusion of solvents saturated within the pores of large single crystals of MOF-5, IRMOF-3 (amino-functionalized MOF-5), and 17 MTV-MOF-5/IRMOF-3 materials at various mole fractions. We find that the apparent self-diffusion coefficient of N,N-dimethylformamide (DMF) may be tuned linearly between the diffusion coefficients of MOF-5 and IRMOF-3 as a function of the linker mole fraction. We compare a series of solvents at saturation in MOF-5 and IRMOF-3 to elucidate the mechanism by which the linker amino groups tune molecular diffusion. The ratio of the self-diffusion coefficients for solvents in MOF-5 to those in IRMOF-3 is similar across all solvents tested, regardless of solvent polarity. We conclude that average pore aperture, not solvent-linker chemical interactions, is the primary factor responsible for the different diffusion dynamics upon introduction of an amino group to the linker.

Facile stabilization of a cyclodextrin metal-organic framework under humid environment via hydrogen sulfide treatment

Increasing resistance to humid environments is a major challenge for the application of γ-CD-K-MOF (a green MOF) in real-world utilisation. γ-CD-K-MOF-H2S with enhanced moisture tolerance was obtained by simply treating MOF with H2S gas. XPS, Raman and TGA characterizations indicated that the H2S molecules coordinated with the metal centers in the framework. H2S acting as a newly available water adsorption potential well near the potassium centers protects the metal-ligand coordination bond from attack by water molecules and thus improves the moisture stability of MOF. After 7 days exposure in 60% relative humidity, γ-CD-K-MOF-H2S retained its crystal structure and morphology, while γ-CD-K-MOF had nearly collapsed. In addition, the formaldehyde uptake tests indicated that γ-CD-K-MOF retain their permanent porosity after interaction with H2S. This simple and facile one-step strategy would open a new avenue for preparation of moisture stable MOFs for practical applications.

Transformation of Metal-Organic Frameworks into Stable Organic Frameworks with Inherited Skeletons and Catalytic Properties

Metal-organic frameworks (MOFs) are an emerging class of porous materials with attractive properties, however, their practical applications are heavily hindered by their fragile nature. We report herein an effective strategy to transform fragile coordination bonds in MOFs into stable covalent organic bonds under mild annealing decarboxylative coupling reaction conditions, which results in highly stable organic framework materials. This strategy successfully endows intrinsic framework skeletons, porosity and properties of the parent MOFs in the daughter organic framework materials, which exhibit excellent chemical stability under harsh catalytic conditions. Therefore, this work opens a new avenue to synthesize stable organic framework materials derived from MOFs for applications in different fields.

Metal-Organic Frameworks for Water Harvesting from Air

Water harvesting from air in passive, adsorption-based devices holds great potential for delivering drinking water to arid regions of the world. This technology requires adsorbents that can be tailored for a maximum working capacity, temperature response, and the relative pressure range in which reversible adsorption occurs. In this respect, metal-organic frameworks (MOFs) are promising, owing to their structural diversity and the precision of their functionalization for adjusting both pore size and hydrophilicity, thereby facilitating the rational design of their water-sorption characteristics. Here, chemical and structural factors crucial for the design of hydrolytically stable MOFs for water adsorption are discussed. Prevalent water adsorption mechanisms in micro- and mesoporous MOFs alongside strategies for fine-tuning of their adsorption behavior by means of reticular chemistry are presented. Finally, an approach for the selection of promising MOFs with respect to water harvesting from air is proposed and design concepts for next-generation MOFs for application in passive adsorption-based water-harvesting devices are outlined.

The synergistic effect of oxygen and water on the stability of the isostructural family of metal-organic frameworks [Cr3(BTC)2] and [Cu3(BTC)2]

The instability of most prototype metal-organic frameworks (MOFs) in the presence of water has limited their industrial scale development. The sensitivity of certain MOFs to humid conditions has been vigorously studied and most of the carrier gas used to make the humid conditions is inert gas. However, a large amount of industrial activities are carried out in air containing 21% oxygen, so it is important to study the effect of oxygen on the hydrostability of MOFs for future industrial applications. In this work, we have studied the stability of M₃(BTC)₂ (M = Cu, Cr; BTC = 1,3,5-benzenetricarboxylate) under controlled environments (pure oxygen environment; water vapor environment and mixed O₂ and H₂O environment). The stability was evaluated using water vapor and oxygen adsorption isotherms combined with powder X-ray diffraction (PXRD) experiments and surface area analysis. Our research shows that Cr₃(BTC)₂ has a relatively high stability under a single atmosphere of either oxygen or water vapor (with Ar). Interestingly, when it was placed under the mixed O₂ and H₂O environment, it rapidly lost approximately 96% of its original surface area. Cu₃(BTC)₂ is more stable; however, it was also degraded, especially under a mixed O₂ and H₂O environment. The experiments show that water molecules and oxygen molecules have a synergistic effect on the stability of MOFs. Computational simulations were used to provide insight into the mechanism governing these trends in the stability of the materials studied.

Water Adsorption and Insertion in MOF-5

The high surface areas and tunable properties of metal-organic frameworks (MOFs) make them attractive materials for applications in catalysis and the capture, storage, and separation of gases. Nevertheless, the limited stability of some MOFs under humid conditions remains a point of concern. Understanding the atomic-scale mechanisms associated with MOF hydrolysis will aid in the design of new compounds that are stable against water and other reactive species. Toward revealing these mechanisms, the present study employs van der Waals-augmented density functional theory, transition-state finding techniques, and thermodynamic integration to predict the thermodynamics and kinetics of water adsorption/insertion into the prototype compound, MOF-5. Adsorption and insertion energetics were evaluated as a function of water coverage, while accounting for the full periodicity of the MOF-5 crystal structure, that is, without resorting to cluster approximations or structural simplifications. The calculations suggest that the thermodynamics of MOF hydrolysis are coverage-dependent: water insertion into the framework becomes exothermic only after a sufficient number of H₂O molecules are coadsorbed in close proximity on a Zn-O cluster. Above this coverage threshold, the adsorbed water clusters facilitate facile water insertion via breaking of Zn-O bonds: the calculated free-energy barrier for insertion is very low, 0.17 eV at 0 K and 0.04 eV at 300 K. Our calculations provide a highly realistic description of the mechanisms underlying the hydrolysis of MOFs under humid working conditions.

Screening the Effect of Water Vapour on Gas Adsorption Performance: Application to CO2 Capture from Flue Gas in Metal-Organic Frameworks

A simple laboratory-scale protocol that enables the evaluation of the effect of adsorbed water on CO₂ uptake is proposed. 45 metal-organic frameworks (MOFs) were compared against reference zeolites and active carbons. It is possible to classify materials with different trends in CO₂ uptake with varying amounts of pre-adsorbed water, including cases in which an increase in CO₂ uptake is observed for samples with a given amount of pre-adsorbed water. Comparing loss in CO₂ uptake between "wet" and "dry" samples with the Henry constant calculated from the water adsorption isotherm results in a semi-logarithmic trend for the majority of samples allowing predictions to be made. Outliers from this trend may be of particular interest and an explanation for the behaviour for each of the outliers is proposed. This thus leads to propositions for designing or choosing MOFs for CO₂ capture in applications where humidity is present.

Ab initio molecular dynamics determination of competitive O₂ vs. N₂ adsorption at open metal sites of M₂(dobdc)

The separation of oxygen from nitrogen using metal-organic frameworks (MOFs) is of great interest for potential pressure-swing adsorption processes for the generation of purified O2 on industrial scales. This study uses ab initio molecular dynamics (AIMD) simulations to examine for the first time the pure-gas and competitive gas adsorption of O2 and N2 in the M2(dobdc) (M = Cr, Mn, Fe) MOF series with coordinatively unsaturated metal centers. Effects of metal, temperature, and gas composition are explored. This unique application of AIMD allows us to study in detail the adsorption/desorption processes and to visualize the process of multiple guests competitively binding to coordinatively unsaturated metal sites of a MOF.

Recent advances in the computational chemistry of soft porous crystals

We highlight recent progress in the field of computational chemistry of nanoporous materials, focusing on methods and studies that address the extraordinary dynamic nature of these systems: the high flexibility of their frameworks, the large-scale structural changes upon external physical or chemical stimulation, and the presence of defects and disorder.

Emerging adsorptive removal of azo dye by metal-organic frameworks

Adsorptive removal of toxic compounds using advanced porous materials is one of the most attractive approaches. In recent years, the metal-organic frameworks (MOFs), a subset of advanced porous nano-structured materials, due to their unique characteristics are showing great promise for better adsorption/separation of various water contaminants. Given the importance of azo dye removal, as an important class of pollutants, this paper aims to review and summarize the recently published research on the effectiveness of various MOFs adsorbents under different physico-chemical process parameters in dyes adsorption. The effect of pH, the adsorption mechanism and the applicability of various adsorption kinetic and thermodynamic models are briefly discussed. Most of the results observed showed that the adsorption kinetic and isotherm of azo dyes onto the MOFs mostly followed the pseudo-second order and Langmuir models respectively. Also, the optimum pH value for the removal of majority of azo dyes by MOFs was observed to be in the range of ∼5-7.

Dynamic acidity in defective UiO-66

The metal-organic framework (MOF) material UiO-66 has emerged as one of the most promising MOF materials due to its thermal and chemical stability and its potential for catalytic applications. Typically, as-synthesised UiO-66 has a relatively high concentration of missing linker defects. The presence of these defects has been correlated with catalytic activity but characterisation of defect structure has proved elusive. We refine a recent experimental determination of defect structure using static and dynamic first principles approaches, which reveals a dynamic and labile acid centre that could be tailored for functional applications in catalysis.