Flexible Metal-organic Frameworks Research Articles

Simulating the structural phase transitions of metal-organic frameworks with control over the volume of nanocrystallites

Flexible metal-organic frameworks (MOFs) can undergo structural transitions with significant pore volume changes upon guest adsorption or other external triggers while maintaining their porosity. In computational studies of this breathing behavior, molecular dynamics (MD) simulations within periodic boundary conditions (PBCs) are commonly performed. However, to account for the finite size and surface effects affecting the phase transition mechanism, the simulation of non-periodic nanocrystallite (NC) models without the constraint of PBCs is an important alternative. In this study, we present an approach allowing the analysis and control of the volume of finite-size structures during MD simulations by a tetrahedral tessellation of the (deformed) NC’s volume. The method allows for defining the current NC’s volume during the simulation and manipulating it regarding a particular reference volume to compute free energies for the phase transformation via umbrella sampling. The application on differently sized DMOF-1 and DUT-128 NCs reveals flexible pore closing mechanisms without significant biasing of the transition pathway. The concept provides the theoretical foundation for further research on flexible materials regarding targeted initialization of the structural phase behavior to elucidate the underlying mechanism, which can be used to improve the applications of flexible materials by targeted controlling of the phase transition.

Investigating the mechanical stability of flexible metal–organic frameworks

As we continue to develop metal–organic frameworks (MOFs) for potential industrial applications, it becomes increasingly imperative to understand their mechanical stability. Notably, amongst flexible MOFs, structure-property relationships regarding their compressibility under pressure remain unclear. In this work, we conducted in situ variable pressure powder X-ray diffraction (PXRD) measurements up to moderate pressures (<1 GPa) using a synchrotron source on two families of flexible MOFs: (i) NU-1400 and NU-1401, and (ii) MIL-88B, MIL-88B-(CH3)2, and MIL-88B-(CH3)4. In this project scope, we found a positive correlation between bulk moduli and degree of flexibility, where increased rigidity (e.g., smaller swelling or breathing amplitude) arising from steric hindrance was deleterious, and observed reversibility in the unit cell compression of these MOFs. This study serves as a primer for the community to begin to untangle the factors that engender flexible frameworks with mechanical resilience.

Multi-faceted dynamic behaviour of a flexible metal–organic framework through post-synthetic modification

Multi-faceted dynamic behaviour of a flexible metal–organic framework through post-synthetic modification

Flexible nanofibrous membranes of dual metallic metal–organic framework with enhanced Lewis basic sites and high loading mass for efficient CO2 capture

Excessive CO2 emissions and the resultant global warming present significant environmental challenges, posing threats to human health and public safety. Metal–organic frameworks (MOFs), known for their high specific area and large porosity, hold the promise for CO2 capture. However, a major obstacle is the low loading mass of MOFs and the limited interface affinity and compatibility between MOFs and substrates. In this study, we present an electrospinning-assisted in-situ synthesis dual metallic framework strategy for preparing flexible Zn/Co-ZIF nanofibrous membranes (NFMs). This method achieves the high loading mass of MOFs and introduces abundant Lewis basic sites, thereby enhancing the CO2 adsorption. The dual metallic Zn/Co-ZIF NFMs exhibit remarkable features, including high MOF loading mass (70.23 wt%), high specific surface area (379.63 m2g−1), large porosity (92.34 %), high CO2 adsorption capacity (4.43 mmol/g), high CO2/N2 adsorption selectivity (37), and high CO2/CH4 adsorption selectivity (31). Moreover, the dual metallic Zn/Co-ZIF NFMs demonstrate robust structural stability and durability attributed to the excellent interface affinity between MOFs and NFMs, retaining 96.56 % of their initial capacity after 10 adsorption–desorption cycles. This work presents a prospective direction for developing flexible dual metallic MOF NFMs for the efficient capture of CO2.

Reversible Thermal Conductivity Switching Using Flexible Metal-Organic Frameworks.

The ability to control thermal transport is critical for the design of thermal rectifiers, logic gates, and transistors, although it remains a challenge to design materials that exhibit large changes in thermal conductivity with switching ratios suitable for practical applications. Here, we propose the use of flexible metal-organic frameworks, which can undergo significant structural changes in response to various stimuli, to achieve tunable switchable thermal conductivity. In particular, we use molecular dynamics simulations to show that the thermal conductivity of the flexible framework Fe(bdp) (bdp2- = 1,4-benzenedipyrazolate) becomes highly anisotropic upon transitioning from the expanded to the collapsed phase, with the conductivity decreasing by nearly an order of magnitude along the direction of compression. Our results add to a small but growing number of studies investigating metal-organic frameworks for thermal transport.

Theoretical isotherm equation for adsorption-induced structural transition on flexible metal–organic frameworks

Flexible metal-organic frameworks (MOFs) exhibit an adsorption-induced structural transition known as "gate opening" or "breathing," resulting in an S-shaped adsorption isotherm. This unique feature of flexible MOFs offers significant advantages, such as a large working capacity, high selectivity, and intrinsic thermal management capability, positioning them as crucial candidates for revolutionizing adsorption separation processes. Therefore, the interest in the industrial applications of flexible MOFs is increasing, and the adsorption engineering for flexible MOFs is becoming important. However, despite the establishment of the theoretical background for adsorption-induced structural transitions, no theoretical equation is available to describe S-shaped adsorption isotherms of flexible MOFs. Researchers rely on various empirical equations for process simulations that can lead to unreliable outcomes or may overlook insights into improving material performance owing to parameters without physical meaning. In this study, we derive a theoretical equation based on statistical mechanics that could be a standard for the structural transition type adsorption isotherms, as the Langmuir equation represents type I isotherms. The versatility of the derived equation is shown through four examples of flexible MOFs that exhibit gate opening and breathing. The consistency of the formula with existing theories, including the osmotic free energy analysis and intrinsic thermal management capabilities, is also discussed. The developed theoretical equation may lead to more reliable and insightful outcomes in adsorption separation processes, further advancing the direction of industrial applications of flexible MOFs.

Validating the Mechanism Underlying the Slacking of the Gate-Opening Behavior in Flexible Metal-Organic Frameworks Arising from the Application of External Force.

Flexible metal-organic frameworks (MOFs) are innovative adsorbents expected to revolutionize conventional separation systems as they exhibit stepwise adsorption arising from structural transitions, commonly known as "gate opening." However, because MOFs are typically obtained in powder form, they require shaping for industrial applications. In our previous study, we reported that the stepwise uptake observed in the CO2 gate opening of ELM-11 ([Cu(BF4)2(4,4'-bipyridine)2]) became less distinct when molded with polymer binders and found that this slacking phenomenon could be caused by the polymer binder inhibiting the structural change of the ELM-11 particles. In this study, we aimed to fully validate and generalize the mechanism behind the slacking of gate adsorption from both theoretical and experimental perspectives. First, we conducted grand canonical molecular dynamics simulations for a simplified MOF model to directly calculate free energy profiles of the particle to validate the slacking theory without any assumptions. The results confirmed the fundamental assumption made in our previous study that the deformation of the flexible motifs within the MOF particles occurs sequentially, which is a key factor contributing to the slacking phenomenon. The second part of the study focused on the relationship between the volume expansion ratio of MOFs and the degree of slacking. The relationship predicted by the theory was experimentally validated by comparing ELM-11, which exhibits 30% volume expansion, to another MOF with a mutually interpenetrating jungle-gym structure, which exhibits 10% volume expansion. These findings strengthened and generalized the understanding of the mechanism underlying the slacking of gate adsorption induced upon the application of external force, which could guide the fabrication of molded MOFs while maintaining a high adsorption efficiency for various industrial applications.

Strategies for designing &amp; improving flexible MOFs

Even today, the problem of greenhouse gas emissions and recycling endanger human health continuously. The dynamic structure of the flexible MOFs (metal-organic frameworks) allows them to change their structure in response to external stimuli. And a fresh approach to reducing greenhouse gas emissions comes from using flexible frames for gas absorption, notably in the area of lower energy and highly selective gas separation. This review starting from the design and improvement of flexible MOFs summarizes some excellent cases in carbon dioxide capture. Finally, the potential applications of flexible MOFs are classified and briefly analyzed.

Breathing porous liquids based on responsive metal-organic framework particles

Responsive metal-organic frameworks (MOFs) that display sigmoidal gas sorption isotherms triggered by discrete gas pressure-induced structural transformations are highly promising materials for energy related applications. However, their lack of transportability via continuous flow hinders their application in systems and designs that rely on liquid agents. We herein present examples of responsive liquid systems which exhibit a breathing behaviour and show step-shaped gas sorption isotherms, akin to the distinct oxygen saturation curve of haemoglobin in blood. Dispersions of flexible MOF nanocrystals in a size-excluded silicone oil form stable porous liquids exhibiting gated uptake for CO2, propane and propylene, as characterized by sigmoidal gas sorption isotherms with distinct transition steps. In situ X-ray diffraction studies show that the sigmoidal gas sorption curve is caused by a narrow pore to large pore phase transformation of the flexible MOF nanocrystals, which respond to gas pressure despite being dispersed in silicone oil. Given the established flexible nature and tunability of a range of MOFs, these results herald the advent of breathing porous liquids whose sorption properties can be tuned rationally for a variety of technological applications.

Green Synthesis of MIL-88B(Cr) with the Co-Modulator of Nitric Acid and Acetic Acid

MIL-88B(Cr) is a prototypical flexible chromium-based metal-organic framework (MOF), which possesses extremely strong water/thermal stability and excellent “swelling/breathing” ability. However, in previous studies, there have been very few reports on MIL-88B(Cr) due to unclear synthesis details. Here, we found that the pure MIL-88B(Cr) can be facile synthesized through a hydrothermal method with the co-use of nitric acid and acetic acid (molar ratio = 1:15). The obtained MIL-88B(Cr) was sufficiently characterized by diverse techniques to assure its high-level quality. This work emphasizes a future valuable approach to expanding the production of flexible Cr-based MOF.

Decoding polymer chains via gated inclusion into flexible nanoporous crystals

•Vinyl co-polymers are inserted in nanopores of a flexible MOF crystal •The flexible nanopores enable recognition of polymer microstructures •Monomer structure, composition, sequence, and block architecture are identified •Separation of polymers by decoding their microstructures is achieved Recognizing the microstructures of synthetic polymers remains a challenging task because there is currently no rational approach for accessing the microstructures of each polymer chain. Conversely, during protein biosynthesis, the microstructure of an RNA string is read with precision at the molecular level such that the nanospace within the ribosome serves as the workplace for highly specific molecular recognition. Herein, we report the microstructure recognition of polymers using nanopores of a flexible metal-organic framework (MOF) crystal, in which well-ordered coordination sites (open metal sites) developed in the MOF pores enable the identification of monomer species, composition, block segments, and local monomer sequence of vinyl polymers via coordination-driven impregnation. This allows for the selective adsorption of polymer chains with specific monomer structure, composition, and arrangements from a mixture of structural analogs, facilitating polymer separation and characterization for material chemistry, as well as information chemistry, by using synthetic polymers as data sources. Recognizing the microstructures of synthetic polymers remains a challenging task because there is currently no rational approach for accessing the microstructures of each polymer chain. Conversely, during protein biosynthesis, the microstructure of an RNA string is read with precision at the molecular level such that the nanospace within the ribosome serves as the workplace for highly specific molecular recognition. Herein, we report the microstructure recognition of polymers using nanopores of a flexible metal-organic framework (MOF) crystal, in which well-ordered coordination sites (open metal sites) developed in the MOF pores enable the identification of monomer species, composition, block segments, and local monomer sequence of vinyl polymers via coordination-driven impregnation. This allows for the selective adsorption of polymer chains with specific monomer structure, composition, and arrangements from a mixture of structural analogs, facilitating polymer separation and characterization for material chemistry, as well as information chemistry, by using synthetic polymers as data sources.

On the role of history-dependent adsorbate distribution and metastable states in switchable mesoporous metal-organic frameworks

A unique feature of metal-organic frameworks (MOFs) in contrast to rigid nanoporous materials is their structural switchabilty offering a wide range of functionality for sustainable energy storage, separation and sensing applications. This has initiated a series of experimental and theoretical studies predominantly aiming at understanding the thermodynamic conditions to transform and release gas, but the nature of sorption-induced switching transitions remains poorly understood. Here we report experimental evidence for fluid metastability and history-dependent states during sorption triggering the structural change of the framework and leading to the counterintuitive phenomenon of negative gas adsorption (NGA) in flexible MOFs. Preparation of two isoreticular MOFs differing by structural flexibility and performing direct in situ diffusion studies aided by in situ X-ray diffraction, scanning electron microscopy and computational modelling, allowed assessment of n-butane molecular dynamics, phase state, and the framework response to obtain a microscopic picture for each step of the sorption process.

Beyond the Conventional Limitation of Photocycloaddition Reaction in the Roomy Nanospace of a Metal-Organic Framework.

Topochemical reactions provide selective products based on the molecular position; however, they generally require molecules to be placed in strictly limited orientations and distances, making them less versatile. In this study, we found that by confining trans-4-styrylpyridine (4-spy) as a reactive substrate in a flexible metal-organic framework (MOF) nanospace, [2+2] cycloadducts can be selectively obtained, even when the distance between two C═C bonds of 4-spy in the crystal is 5.9 Å, which is much larger than the conventionally observed upper limit (4.2 Å). Such an unusual cyclization reaction is suggested to occur due to the transient proximity of the 4-spy due to the "swing" motion in the nanospace. The MOF nanospace, with its high degree of molecular structural freedom, can be applied to different platforms that do not require the fine constraints of reactive distances for solid-phase reactions.

Discriminatory Gate-Opening Effect in a Flexible Metal-Organic Framework for Inverse CO2 /C2 H2 Separation.

Considering the significant application of acetylene (C2 H2 ) in the manufacturing and petrochemical industries, the selective capture of impurity carbon dioxide (CO2 ) is a crucial task and an enduring challenge. Here, a flexible metal-organic framework (Zn-DPNA) accompanied by a conformation change of the Me2 NH2 + ions in the framework is reported. The solvate-free framework provides a stepped adsorption isotherm and large hysteresis for C2 H2 , but type-I adsorption for CO2 . Owing to their uptakes difference before gate-opening pressure, Zn-DPNA demonstrated favorable inverse CO2 /C2 H2 separation. According to molecular simulation, the higher adsorption enthalpy of CO2 (43.1kJmol-1 ) is due to strong electrostatic interactions with Me2 NH2 + ions, which lock the hydrogen-bond network and narrow pores. Furthermore, the density contours and electrostatic potential verifies the middle of the cage in the large pore favors C2 H2 and repels CO2 , leading to the expansion of the narrow pore and further diffusion of C2 H2 . These results provide a new strategy that optimizes the desired dynamic behavior for one-step purification of C2 H2 .

Pore Size Modulation in Flexible Metal-Organic Framework Enabling High Performance Gas Sensing.

Pore size plays a critical role in determining the performance of metal-organic frameworks (MOFs) in catalysis, sensing, and gas storage or separation. However, revealing the pore-size/property relationship remains extremely challenging because ideal structure models possessing different pore sizes but having the same components are lacking. In this work, a solvent-coordination directed structure swelling method was developed for modulating the ratio between the large and narrow pore phases of a flexible MOF, MIL-88B. Pore-size-dependent gas sensitivity and selectivity were studied for the first time in the MIL-88B samples. The optimized MIL-88B-20 % sample showed one of the best sensing performances among all the reported MOF-based H2 S-sensing materials. This work not only provides a method to synthesize ideal structure models for revealing the relationship between pore-size and properties, but also may inspire the development of high-performance gas sensing materials.

Pore Size Modulation in Flexible Metal‐Organic Framework Enabling High Performance Gas Sensing

AbstractPore size plays a critical role in determining the performance of metal‐organic frameworks (MOFs) in catalysis, sensing, and gas storage or separation. However, revealing the pore‐size/property relationship remains extremely challenging because ideal structure models possessing different pore sizes but having the same components are lacking. In this work, a solvent‐coordination directed structure swelling method was developed for modulating the ratio between the large and narrow pore phases of a flexible MOF, MIL‐88B. Pore‐size‐dependent gas sensitivity and selectivity were studied for the first time in the MIL‐88B samples. The optimized MIL‐88B‐20 % sample showed one of the best sensing performances among all the reported MOF‐based H2S‐sensing materials. This work not only provides a method to synthesize ideal structure models for revealing the relationship between pore‐size and properties, but also may inspire the development of high‐performance gas sensing materials.

Preparation and Lithium-Ion Separation Property of ZIF-8 Membrane with Excellent Flexibility.

Metal-organic framework (MOF) membranes exhibit immense potential for separation applications due to their regular pore channels and scalable pore sizes. However, structuring a flexible and high-quality MOF membrane remains a challenge due to its brittleness, which severely restricts its practical application. This paper presents a simple and effective method in which continuous, uniform, defect-free ZIF-8 film layers of tunable thickness are constructed on the surface of inert microporous polypropylene membranes (MPPM). To provide heterogeneous nucleation sites for ZIF-8 growth, an extensive amount of hydroxyl and amine groups were introduced on the MPPM surface using the dopamine-assisted co-deposition technique. Subsequently, ZIF-8 crystals were grown in-situ on the MPPM surface using the solvothermal method. The resultant ZIF-8/MPPM exhibited a lithium-ion permeation flux of 0.151 mol m-2 h-1 and a high selectivity of Li+/Na+ = 1.93, Li+/Mg2+ = 11.50. Notably, ZIF-8/MPPM has good flexibility, and the lithium-ion permeation flux and selectivity remain unchanged at a bending curvature of 348 m-1. These excellent mechanical characteristics are crucial for the practical applications of MOF membranes.

Fluorescence Enhancement of a Metal‐Organic Framework for Ultra‐Efficient Detection of Trace Benzene Vapor

AbstractIndoor detection of volatile organic compounds (VOCs) concentration is necessary due to the serious toxicity hazards even at trace level. However, physisorbents usually exhibit weak interactions especially in the presence of trace concentrations of VOCs, thus exhibiting poor responsive signal. Herein, we report a new flexible metal–organic framework (MOF) that exhibits interesting pore‐opening behavior after immersing in H2O. The pore‐opening phase shows significant (≈116 folds) and extremely fast (&lt;1 minute) fluorescence enhancement after being exposed to saturated benzene vapor. The limit of detection concentration for benzene vapor can be calculated as 0.133 mg L−1. Thus this material represents the first MOF to achieve visual detection of trace benzene vapor by the naked eyes. Theoretical calculations and single‐crystal structure reveal that the special “bilateral π–π stacking” interactions between the host and guest, which facilitate electron transfer and greatly enhance the intensity of fluorescence.

Fluorescence Enhancement of a Metal-Organic Framework for Ultra-Efficient Detection of Trace Benzene Vapor.

Indoor detection of volatile organic compounds (VOCs) concentration is necessary due to the serious toxicity hazards even at trace level. However, physisorbents usually exhibit weak interactions especially in the presence of trace concentrations of VOCs, thus exhibiting poor responsive signal. Herein, we report a new flexible metal-organic framework (MOF) that exhibits interesting pore-opening behavior after immersing in H2 O. The pore-opening phase shows significant (≈116 folds) and extremely fast (<1 minute) fluorescence enhancement after being exposed to saturated benzene vapor. The limit of detection concentration for benzene vapor can be calculated as 0.133 mg L-1 . Thus this material represents the first MOF to achieve visual detection of trace benzene vapor by the naked eyes. Theoretical calculations and single-crystal structure reveal that the special "bilateral π-π stacking" interactions between the host and guest, which facilitate electron transfer and greatly enhance the intensity of fluorescence.

Super Proton Conductivity Through Control of Hydrogen-Bonding Networks in Flexible Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have received much attention as a solid-state electrolyte in proton exchange membrane fuel cells. The introduction of proton carriers and functional groups into MOFs can improve the proton conductivity attributed to the formation of hydrogen-bonding networks, while the underlying synergistic mechanism is still unclear. Here, a series of flexible MOFs (MIL-88B, [Fe3 O(OH)(H2 O)2 (O2 C-C6 H4 -CO2 )3 ] with imidazole) is designed to modify the hydrogen-bonding networks and investigate the resulting proton-conducting characteristics by controlling the breathing behaviors. The breathing behavior is tuned by varying the amount of adsorbed imidazole into pore (small breathing (SB) and large breathing (LB)) and introducing functional groups onto ligands (-NH2 , -SO3 H), resulting in four kinds of imidazole-loaded MOFs-Im@MIL-88B-SB, Im@MIL-88B-LB, Im@MIL-88B-NH2 , and Im@MIL-88B-SO3 H. Im@MIL-88B-LB without functional groups exhibits the highest proton conductivity of 8.93×10-2 Scm-1 at 60°C and 95% relative humidity among imidazole-loaded proton conductors despite the mild condition, indicating that functional groups may not be always required to enhance proton conductivity. The elaborately controlled pore size and host-guest interaction in flexible MOFs through imidazole-dependent structural transformation are translated into the high proton concentration without the limitation of proton mobility, contributing to the formation of effective hydrogen-bonding networks in imidazole conducting media.