Flexible Metal-organic Frameworks Research Articles

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.

Ligand Torsion Engineering in Three Flexible Metal-Organic-Frameworks Exhibiting Distinct Breathing Behavior for Enhancing C2H2 Capture.

Acetylene is an important industrial gas for the production of vinyl chloride and 1,4-butynediol, but its storage remains a major challenge because it is highly explosive. Flexible metal-organic frameworks (FMOFs) are always at the forefront of porous materials due to the transformation of the structure under the external stimuli. In this work, divalent metal ions and multifunctional aromatic N,O-donor ligands were chosen, and three FMOFs [M(DTTA)2]·guest [M = Mn (1), Cd (2), and Cu (3)] (H2DTTA = 2,5-bis(1H-1,2,4-trazol-1-yl) terephthalic acid) have been successfully constructed. Single-crystal X-ray diffractions show that these compounds are isostructural and feature a three-dimensional framework. Topological analysis shows a (4, 6)-connected network with a Schläfli symbol of {44.610.8}{44.62}. All three compounds exhibit breathing behavior on N2 adsorption at 77 K, and due to the difference of ligand torsion angles, compounds 2 and 3 exhibit extraordinary adsorptions for C2H2 of 101 and 122 cm3 g-1 at 273 K under 1 bar, respectively. Compared with previous work, successfully obtaining compound 3 with an innovative structure can be attributed to the solvent-induced effect in the process of crystal synthesis, leading to the structure transformation promoting the significantly increased C2H2 adsorption performance. This study provides a platform for improvement of synthetic structures, which can effectively boost gas adsorption performance.

Entropy driven disorder–order transition of a metal–organic framework with frustrated flexibility

Flexible metal–organic frameworks (MOFs), showing a reversible phase change behavior in response to guest adsorption or temperature, provide unique opportunities for molecular separation or energy storage applications. Herein, we investigate the complex guest- and temperature-responsive behavior of a functionalized MOF-5 derivative. The material is characterized by a geometrically rigid network structure that is decorated with dispersion energy donating hexyloxy substituents. Distinguished by the phenomenon of frustrated flexibility, the functionalized MOF-5 derivative switches between a highly crystalline, cubic structure and a semi-crystalline, aperiodically distorted structure depending on guest adsorption and temperature. Via a combination of several variable temperature global and local structure techniques (x-ray diffraction, x-ray total scattering, and Fourier-transform IR spectroscopy), detailed insights into the complementary disorder–order transitions of the framework backbone and the dangling hexyloxy substituents are provided. Our results set the stage for the discovery of new responsive MOFs exhibiting a more complex phase change behavior interfacing periodic and aperiodic structural changes.

Multivariate Flexible Framework with High Usable Hydrogen Capacity in a Reduced Pressure Swing Process

Step-shaped adsorption-desorption of gaseous payloads by flexible metal-organic frameworks can facilitate the delivery of large usable capacities with significantly reduced energetic penalties. This is desirable for the storage, transport, and delivery of H2, as prototypical adsorbents require large swings in pressure and temperature to achieve usable capacities approaching their total capacities. However, the weak physisorption of H2 typically necessitates undesirably high pressures to induce the framework phase change. As de novo design of flexible frameworks is exceedingly challenging, the ability to intuitively adapt known frameworks is required. We demonstrate that the multivariate linker approach is a powerful tool for tuning the phase change behavior of flexible frameworks. In this work, 2-methyl-5,6-difluorobenzimidazolate was solvothermally incorporated into the known framework CdIF-13 (sod-Cd(benzimidazolate)2), resulting in the multivariate framework sod-Cd(benzimidazolate)1.87(2-methyl-5,6-difluorobenzimidazolate)0.13 (ratio = 14:1), which exhibited a considerably reduced stepped adsorption threshold pressure while maintaining the desirable adsorption-desorption profile and capacity of CdIF-13. At 77 K, the multivariate framework exhibits stepped H2 adsorption with saturation below 50 bar and minimal desorption hysteresis at 5 bar. At 87 K, saturation of step-shaped adsorption occurs by 90 bar, with hysteresis closing at 30 bar. These adsorption-desorption profiles enable usable capacities in a mild pressure swing process above 1 mass %, representing 85-92% of the total capacities. This work demonstrates that the desirable performance of flexible frameworks can be readily adapted through the multivariate approach to enable efficient storage and delivery of weakly physisorbing species.

Surprising Use of the Business Innovation Bass Diffusion Model To Accurately Describe Adsorption Isotherm Types I, III, and V

Using adsorption isotherm data to determine heats of adsorption or predict mixture adsorption using the ideal adsorbed solution theory (IAST) relies on accurate fits of the data with continuous, mathematical models. Here, we derive an empirical two-parameter model to fit isotherm data of IUPAC types I, III, and V in a descriptive way based on the Bass model for innovation diffusion. We report 31 isotherm fits to existing literature data covering all six types of isotherms, various adsorbents, such as carbons, zeolites, and metal-organic frameworks (MOFs), as well as different adsorbing gases (water, carbon dioxide, methane, and nitrogen). We find several cases, especially for flexible MOFs, where previously reported isotherm models reached their limits and either failed to fit the data or could not sufficiently be fitted due to stepped type V isotherms. Moreover, in two instances, models specifically developed for distinct systems are fitted with a higher R2 value compared to the models in the original reports. Using these fits, it is demonstrated how the new Bingel-Walton isotherm can be used to qualitatively assess the hydrophilic or hydrophobic behavior of porous materials from the relative magnitude of the two fitting parameters. The model can also be employed to find matching heats of adsorption values for systems with isotherm steps using one, continuous fit instead of partial, stepwise fits or interpolation. Additionally, using our single, continuous fit to model stepped isotherms in IAST mixture adsorption predictions leads to good agreement with the results from the osmotic framework adsorbed solution theory that was specifically developed for these systems using a stepwise, approximate fitting, which is yet far more complex. Our new isotherm equation accomplishes all of these tasks with only two fitted parameters, providing a simple, accurate method for modeling a variety of adsorption behavior.

One-dimensional alignment of defects in a flexible metal-organic framework.

Crystalline materials are often considered to have rigid periodic lattices, while soft materials are associated with flexibility and nonperiodicity. The continuous evolution of metal-organic frameworks (MOFs) has erased the boundaries between these two distinct conceptions. Flexibility, disorder, and defects have been found to be abundant in MOF materials with imperfect crystallinity, and their intricate interplay is poorly understood because of the limited strategies for characterizing disordered structures. Here, we apply advanced nuclear magnetic resonance spectroscopy to elucidate the mesoscale structures in a defective MOF with a semicrystalline lattice. We show that engineered defects can tune the degree of lattice flexibility by combining both ordered and disordered compartments. The one-dimensional alignment of correlated defects is the key for the reversible topological transition. The unique matrix is featured with both rigid framework of nanoporosity and flexible linkage of high swellability.

Breakthrough analysis for parameter estimation of CO2 adsorption on pelletized flexible metal–organic framework

Pellets of elastic layer-structured metal–organic framework-11 (ELM-11), one of the flexible metal–organic frameworks (MOFs), were prepared by mixing ELM-11 powder with a cellulose binder, and the CO2 adsorption behavior of ELM-11 pellets in a fixed-bed adsorber was analyzed to estimate equilibrium and kinetic parameters. Adsorption breakthrough tests were performed under various conditions for the simulated flue gas (20 % CO2–80 % N2) of a blast furnace gas in addition to CO2 isotherm measurements. Experimental CO2 adsorption isotherms on ELM-11 followed a weighted dual-site Langmuir equilibrium model well. The breakthrough analysis results by numerical calculation using the equilibrium model suggested that the gate-opening pressure of ELM-11 shifted to a lower pressure through repeated breakthrough tests. The CO2 adsorption kinetics on ELM-11 pellets agreed well with the linear-driving-force model. Furthermore, CO2 adsorption behavior under various conditions was described well by a mathematical model using a constant value of the mass transfer coefficient under the condition of this study, which was validated against additional experimental data. In addition, it was found that the mass transfer becomes slow under certain conditions that the driving force of the phase transition of ELM-11 is smaller than a threshold, where the amount of CO2 adsorbed on the breakthrough tests did not reach the equilibrium capacity of CO2, which may because the rate-limiting step may be dependent on the CO2 partial pressure and temperature.

Hydrocarbon Sorption in Flexible MOFs-Part II: Understanding Adsorption Kinetics.

The rate of sorption of n-butane on the structurally flexible metal-organic framework [Cu2(H-Me-trz-ia)2], including its complete structural transition between a narrow-pore phase and a large-pore phase, was studied by sorption gravimetry, IR spectroscopy, and powder X-ray diffraction at close to ambient temperature (283, 298, and 313 K). The uptake curves reveal complex interactions of adsorption on the outer surface of MOF particles, structural transition, of which the overall rate depends on several factors, including pressure step, temperature, as well as particle size, and the subsequent diffusion into newly opened pores. With the aid of a kinetic model based on the linear driving force (LDF) approach, both rates of diffusion and structural transition were studied independently of each other. It is shown that temperature and applied pressure steps have a strong effect on the rate of structural transition and thus, the overall velocity of gas uptake. For pressure steps close to the upper boundary of the gate-opening, the rate of structural transition is drastically reduced. This feature enables a fine-tuning of the overall velocity of sorption, which can even turn into anti-Arrhenius behavior.

Flexing of a Metal–Organic Framework upon Hydrocarbon Adsorption: Atomic Level Insights from Neutron Scattering

Metal–organic frameworks (MOFs) offer considerable opportunities for gas uptake, storage, and separation due to their porosity, chemical tunability, and flexibility. Flexible MOFs undergo reversible structural transformations triggered by external stimuli such as adsorption of specific guest molecules. The MUF-16 family of materials has exceptional gas adsorption properties including selective uptake of carbon dioxide over other gases. We observed one member of this family, MUF-16(Mn), to be flexible upon the adsorption of hydrocarbon gases. We used a combination of in situ synchrotron X-ray and neutron diffraction to identify the framework–gas interactions that underlie the structural flexibility. Inelastic neutron scattering, along with calculations, also enables an understanding of the dynamics of the flexibility. In essence, C3 hydrocarbons effectively bridge across hydrogen-bonded carboxyl dimers in the framework, triggering pore expansion and inhibiting certain types of motion in the framework.

Strengthening Intraframework Interaction within Flexible MOFs Demonstrates Simultaneous Sieving Acetylene from Ethylene and Carbon Dioxide.

Efficient separation of acetylene (C2 H2 )/ethylene (C2 H4 ) and acetylene/carbon dioxide (CO2 ) by adsorption is an industrially promising process, but adsorbents capable of simultaneously capturing trace acetylene from ethylene and carbon dioxide are scarce. Herein, a gate-opening effect on three isomorphous flexible metal-organic frameworks (MOFs) named Co(4-DPDS)2 MO4 (M = Cr, Mo, W; 4-DPDS =4,4-dipyridyldisulfide) is modulated by anion pillars substitution. The shortest CrO4 2- strengthens intraframework hydrogen bonding and thus blocks structural transformation after activation, striking a good balance among working capacity, separation selectivity, and trace impurity removal of flexible MOFs out of nearly C2 H2 /C2 H4 and C2 H2 /CO2 molecular sieving. The exceptional separation performance of Co(4-DPDS)2 CrO4 is confirmed by dynamic breakthrough experiments. It reveals the specific threshold pressures control in anion-pillared flexible materials enabled elimination of the impurity leakage to realize high purity products through precise control of the intraframework interaction. The adsorption mechanism and multimode structural transformation property are revealed by both calculations and crystallography studies. This work demonstrates the feasibility of modulating flexibility for controlling gate-opening effect, especially for some cases of significant aperture shrinkage after activation.

Investigation of Guest-Induced Flexibility in Pyrazine Derivative of ALFFIVE MOF via Molecular Simulation.

One of the important understandings of porous solids like metal-organic frameworks (MOFs) is their flexibility. Therefore, there are certain computational studies on flexible MOFs in the literature, primarily concentrating on MIL-53, UiO-66, and DUT-49. Here, investigation of another class of MOF, that is, [Ni(1,4-pyrazine)2(AlF5)]n, was shown to have guest-induced flexible characteristics; nevertheless, the mechanism for the emergence of flexibility is uncertain. We simulated the structural flexibility of [Ni(1,4-pyrazine)2(AlF5)]n, named ALFFIVE-Ni-pyr-TBP, upon adsorption of a guest molecule based on force fields using the molecular dynamics (MD) method and Monte Carlo (MC) simulations. As the first step towards understanding guest-induced flexibility, the MC simulations were performed by relaxing the framework and then further comparing it with the rigid framework. Subsequently, MD simulations were executed on the ALFFIVE-Ni-pyr-TBP framework with and without the guest molecules. In the case of moisture adsorption, the MOF system was identified to undergo a geometric transformation from trigonal bipyramidal to square bipyramidal geometry due to the strong interaction of oxygen of the water with the metal aluminum. However, some tilting in the pyrazine ligand was observed in the presence of all the guest molecules. Therefore, the detailed guest-induced flexibility described in this work could support the ALFFIVE series to be explored for future adsorption applications.

Propagating MOF flexibility at the macroscale: the case of MOF-based mechanical actuators.

Shapeshifting materials have captured the imagination of researchers for their myriad potential applications, yet their practical development remains challenging. These materials operate by mechanical actuation: their structural responses to external stimuli generate mechanical work. Here, we review progress on the use of flexible metal-organic frameworks (MOFs) in composite actuators that shapeshift in a controlled fashion. We highlight the dynamic behaviour of flexible MOFs, which are unique among materials, even other porous ones, and introduce the concept of propagation, which involves the efficient transmission of flexible MOF deformations to the macroscale. Furthermore, we explain how researchers can observe, measure, and induce such effects in MOF composites. Next, we review pioneering first-generation MOF-composite actuators that shapeshift in response to changes in humidity, temperature, pressure, or to other stimuli. Finally, we allude to recent developments, identify remaining R & D hurdles, and suggest future directions in this field.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions.

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.

Disclosing gate-opening/closing events inside a flexible metal-organic framework loaded with CO2 by reactive and essential dynamics.

We have combined reactive molecular dynamics simulations with principal component analysis to provide a clearer view of the interactions and motion of the CO2 molecules inside a metal-organic framework and the movements of the MOF components that regulate storage, adsorption, and diffusion of the guest species. The tens-of-nanometer size of the simulated model, the capability of the reactive force field tuned to reproduce the inorganic-organic material confidently, and the unconventional use of essential dynamics have effectively disclosed the gate-opening/closing phenomenon, possible coordinations of CO2 at the metal centers, all the diffusion steps inside the MOF channels, the primary motions of the linkers, and the effects of their concerted rearrangements on local CO2 relocations.

Particle size-dependent flexibility in DUT-8(Cu) pillared layer metal-organic framework.

The nature of metal in the isomorphous flexible metal-organic frameworks is often reported to influence flexibility and responsivity. A prominent example of such behaviour is the DUT-8(M) family ([M2(2,6-ndc)2(dabco)]n, 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane), where the isostructural compounds with Ni, Zn, Co, and Cu in the paddle wheel cluster are known. The macro-sized crystals of Ni, Co, and Zn based compounds transform to the closed pore (cp) phase under desolvation and show typical gate opening behaviour upon adsorption. The choice of metal, in this case, allows the adjustment of switching kinetics, selectivity in adsorption, and gate-opening pressures. The submicron-sized crystals of of Ni, Co, and Zn based compounds remain in the open pore (op) phase after desolvation. In this contribution, we demonstrate that the presence of Cu in the paddle wheel leads to fundamentally different flexible behaviour. The DUT-8(Cu) desolvation does not lead to the formation of the cp phase, independent of the particle size regime. However, according to in situ powder diffraction analysis, the desolvated, macro-sized crystals of DUT-8(Cu)_op show breathing upon adsorption of CO2 at 195 K. The submicron-sized particles show rigid, nonresponsive behaviour.

Design of Flexible Metal–Organic Framework-Based Superprotonic Conductors and Their Fabrication with a Polymer into Proton Exchange Membranes

In recent times, the deployment of metal–organic frameworks (MOFs) to develop efficient proton conductors has gained immense popularity in the arena of sustainable energy research due to the ease of structural and functional tunability in MOFs. In this work, we have focused on developing “flexible MOF”-based proton conductors with Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs using postsynthetic modification (PSM) as the tool. Taking advantage of the porous nature of these frameworks, we have carried out PSM on the primary amine groups present on the MOFs and converted them to −NH(CH2CH2CH2SO3H) groups. The PSM increased the number of labile protons in the channels of the modified MOFs as well as the extent of H-bonded networks inside the framework. The modified Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs, named hereafter as 53-S and 88B-S, respectively, showed proton conductivity of 1.298 × 10–2 and 1.687 × 10–2 S cm–1 at ∼80 °C and 98% relative humidity (RH), respectively. This reflects ∼10-fold and ∼5-fold increases in their proton conductivity than their respective parent MOFs. Since MOFs as such are difficult to make directly into flexible membranes, and these are essential for practical applications as proton conductors, we have incorporated 53-S and 88B-S as fillers into a robust imidazole-based polymer matrix, namely, OPBI [poly(4,4′-diphenylether-5,5′-bibenzimidazole)]. The resulting polymer–MOF mixed matrix membranes (MMMs) after doping with phosphoric acid (PA) performed as flexible proton exchange membranes (PEMs) above 100 °C under anhydrous conditions and were found to be much more efficient and stable than the pristine OPBI membrane (devoid of any filler loading). By optimizing the amount of filler loading in the membrane, we obtained the highest proton conductivity of 0.304 S cm–1 at 160 °C under anhydrous conditions.

Flexible 2D Boron Imidazolate Framework for Polysulfide Adsorption in Lithium–Sulfur Batteries

The “polysulfide shuttle,” a process initiated by the dissolution of polysulfides, is recognized to be one of the major failure mechanisms of lithium–sulfur (Li–S) batteries. Much research effort has been dedicated toward efficient cathode additives and host materials to suppress the leaching of polysulfide species. Herein, we report a new 2D metal–organic framework constituted by a tritopic ligand, boron imidazolate ([BH(Im)3]−, Im = imidazole), and Co2+ ions for lithium polysulfide adsorption. The cobalt imidazolate framework (CoN6-BIF) contains octahedrally coordinated Co centers that form two-dimensional layers in the a,b plane. Composite cathodes containing CoN6-BIF exhibited high sulfur utilization and capacity retention, resulting in improved specific capacity and cycle life compared to sulfur/carbon controls. Density functional theory (DFT) calculations suggest that CoN6-BIF linkers are rotationally flexible, allowing the framework to accommodate polysulfide in the expanded pores. This unusual property of BIFs opens up new avenues for exploring flexible metal–organic frameworks (MOFs) and their applications to energy storage.

The Dilemma of Reproducibility of Gating Isotherms for Flexible MOFs.

Porous materials receive a high level of scientific and technological interest due to their applications in various fields such as adsorption, separation and storage, catalysis, ion exchange, nanotechnology, etc. Gas adsorption is a well-established tool for the characterization of the texture of porous solids. Physisorption isotherms are generally expected to be well reproducible for rigid adsorbents, but this is not always the case for nonrigid (flexible) materials. The presence of a metastability region and sensitivity of the activation barriers to the material's texture often influence the isotherms' run. Here, we address the complexity that arises in terms of reproducibility and sample handling for flexible metal-organic frameworks, with the example of DUT-8(Ni). It belongs to the group of "gate opening" metal-organic frameworks and is a typical representative of the pillared layer compounds. We propose characteristic parameters for the analysis and comparison of adsorption isotherms, showing the "gate opening" step, associated with the adsorption-induced solid-state phase transition. A set of 50 nitrogen physisorption isotherms measured at 77 K were analyzed and correlated with the synthetic and outgassing conditions. The study highlights the importance of accurate descriptions and record-keeping of experimental details and their role in the replication of scientific results.

Atomic-level structural responsiveness to environmental conditions from 3D electron diffraction

Electron microscopy has been widely used in the structural analysis of proteins, pharmaceutical products, and various functional materials in the past decades. However, one fact is often overlooked that the crystal structure might be sensitive to external environments and response manners, which will bring uncertainty to the structure determination and structure-property correlation. Here, we report the atomic-level ab initio structure determinations of microcrystals by combining 3D electron diffraction (3D ED) and environmental transmission electron microscope (TEM). Environmental conditions, including cryo, heating, gas and liquid, have been successfully achieved using in situ holders to reveal the simuli-responsive structures of crystals. Remarkable structural changes have been directly resolved by 3D ED in one flexible metal-organic framework, MIL-53, owing to the response of framework to pressures, temperatures, guest molecules, etc.

Model-Based Analysis of a Highly Efficient CO2 Separation Process Using Flexible Metal–Organic Frameworks with Isotherm Hysteresis

To realize large-scale CO2 separation to prevent global warming, energy and space saving separation technologies are required. Flexible metal–organic frameworks (flexible MOFs) with gate-opening properties have been found to be promising adsorbents, but the unconventional sigmoidal shapes and hysteresis of the isotherms have hindered quantitative evaluation of their applications in separation processes. This paper is the first to evaluate ELM-11, a flexible MOF that exhibits sigmoidal isotherms with hysteresis, in a process model and quantify its advantages over a conventional adsorbent. Based on the experimental uptake data, isotherm models for adsorption and desorption were developed, and their parameters were estimated. The resulting isotherm models are incorporated into a rigorous dynamic model of partial differential algebraic equations (PDAEs) to simulate a vacuum pressure swing adsorption (VPSA) process to evaluate the process performance. Numerical challenges to solve the PDAEs, including sigmoidal and hysteresis isotherm models, were resolved with the proposed numerical approaches. Sensitivity analysis for feed pressure and temperature was performed to identify the optimal operating strategy. A comparison with a conventional adsorbent, zeolite 13X, showed that high selectivity and sigmoidal isotherms of ELM-11 give higher productivity and CO2 product purity exceeding 99% without rinse and purge operations as well as lower power consumption for the compressor and vacuum pump.