Flexible Metal-organic Frameworks Research Articles

Hierarchically structural PAN/UiO-66-(COOH)2 nanofibrous membranes for effective recovery of Terbium(III) and Europium(III) ions and their photoluminescence performances

Metal organic frameworks (MOFs) as adsorbents are attracting wide attention due to their high porosity and rich functionalities. However, their intrinsic fragility and poor separability limited their practical applications. Herein, an intriguing MOF UiO-66-(COOH)2 nanoparticles with abundant carboxyl functional groups were embedded in polyacrylonitrile (PAN) nanofibers by colloid-electrospinning technique to construct flexible MOF nanofibrous membranes (NFMs) with hierarchical structure for effective recovery of lanthanide (Ln) ions and sustainable reutilization in photoluminescent applications. The MOF UiO-66-(COOH)2 contained twenty-four free carboxyl functional groups per crystal which could endow it with powerful affinity to Ln3+ ions. The resultant PAN/UiO-66-(COOH)2 NFMs with quite high UiO-66-(COOH)2 loading (up to 60 wt%) exhibited outstanding adsorption capacities and good recyclability for Ln3+ ions. The adsorption data could be better fitted by the Langmuir isotherm and pseudo-second-order kinetic models, and the maximum adsorption capacities of Terbium(III) (Tb3+) and Europium(III) (Eu3+) ions calculated from the Langmuir isotherm model were 214.1 mg/g and 191.9 mg/g, respectively. Moreover, it was found that the adsorbent could be well regenerated without obvious loss of adsorption capacities after three adsorption-desorption cycles. In addition, benefiting from the carboxyl ligands acting as excellent sensitizers for Tb3+ and Eu3+ ions, the flexible Ln3+-loaded PAN/UiO-66-(COOH)2-60 NFMs showed a series of characteristics including good photoluminescence properties and tunable emission colors, indicating their use as promising luminescent materials for luminescent patterning and optoelectronics.

Thermodynamically Controlled Linker Installation in Flexible Zirconium Metal–Organic Frameworks

A series of metal–organic frameworks (MOFs) (PCN-606-OH-TPDC, PCN-606-OMe-TPDC, PCN-606-OH-EDDB, and PCN-606-OMe-EDDB) are synthesized through linker installation onto open sites of 8-connected Zr6 clusters in PCN-606-R (where R = −OH or −OMe) parent frameworks of differing flexibilities. The two postsynthetically installed linear linkers possess slight differences in length and bulk, which result in a noticeable difference in the installation temperatures, reflective of a different thermodynamic barrier to incorporation into PCN-606-R. The X-ray crystallographic data as well as the N2 adsorption properties of these four newly produced MOFs are explored and compared to gain a more comprehensive understanding of the implications that this difference in linker size and bulk have during insertion into flexible MOFs.

Structural Basis of CO₂ Adsorption in a Flexible Metal-Organic Framework Material.

This paper reports on the structural basis of CO2 adsorption in a representative model of flexible metal-organic framework (MOF) material, Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)4] (NiBpene or PICNIC-60). NiBpene exhibits a CO2 sorption isotherm with characteristic hysteresis and features on the desorption branch that can be associated with discrete structural changes. Various gas adsorption effects on the structure are demonstrated for CO2 with respect to N2, CH4 and H2 under static and flowing gas pressure conditions. For this complex material, a combination of crystal structure determination and density functional theory (DFT) is needed to make any real progress in explaining the observed structural transitions during adsorption/desorption. Possible enhancements of CO2 gas adsorption under supercritical pressure conditions are considered, together with the implications for future exploitation. In situ operando small-angle neutron and X-ray scattering, neutron diffraction and X-ray diffraction under relevant gas pressure and flow conditions are discussed with respect to previous studies, including ex situ, a priori single-crystal X-ray diffraction structure determination. The results show how this flexible MOF material responds structurally during CO2 adsorption; single or dual gas flow results for structural change remain similar to the static (Sieverts) adsorption case, and supercritical CO2 adsorption results in enhanced gas uptake. Insights are drawn for this representative flexible MOF with implications for future flexible MOF sorbent design.

Functionalization-Induced Breathing Control in Metal–Organic Frameworks for Methane Storage with High Deliverable Capacity

Metal–organic frameworks (MOFs) that can undergo structural flexibility upon gas sorption cycle bear enormous potential particularly as adsorbents for methane storage. A molecular level control over such flexibility can be advantageous for the development of smart adsorbents with increased deliverable capacity. Herein, we report the flexible and stable MIL-53(Al) type MOFs, whose breathing behavior can be controlled by systematic installation of hydrogen bonding sites into the frameworks. Such a control has been fine-tuned to induce high deliverable capacity in MOFs by changing gas pressure, temperature, and density. The establishment of a structure–property relationship for methane storage and successful pelletization of MOFs pave the way for MOF-based onboard natural gas storage.

Structural dynamics of a metal\u2013organic framework induced by CO2 migration in its non-uniform porous structure

Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO2-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.

The impact of lattice vibrations on the macroscopic breathing behavior of MIL-53(Al)

Abstract The mechanism inducing the breathing in flexible metal-organic frameworks, such as MIL-53(Al), is still not fully understood. Herein, the influence of lattice vibrations on the breathing transition in MIL-53(Al) is investigated to gain insight in this phenomenon. Through solid-state density-functional theory calculations, the volume-dependent IR spectrum is computed together with the volume-frequency relations of all vibrational modes. Furthermore, important thermodynamic properties such as the Helmholtz free energy, the specific heat capacity, the bulk modulus, and the volumetric thermal expansion coefficient are derived via these volume-frequency relations using the quasi-harmonic approximation. The simulations expose a general volume-dependency of the vibrations with wavenumbers above 300 cm−1 due to their localized nature. In contrast, a diverse set of volume-frequency relations are observed for vibrations in the terahertz region (<300 cm−1) containing the vibrations exhibiting collective behavior. Some terahertz vibrations display large frequency differences over the computed volume range, induced by either repulsion or strain effects, potentially triggering the phase transformation. Finally, the impact of the lattice vibrations on the thermodynamic properties is investigated. This reveals that the closed pore to large pore phase transformation in MIL-53(Al) is mainly facilitated by terahertz vibrations inducing rotations of the organic linker, while the large pore to closed pore phase transformation relies on two framework-specific soft modes.

A facile one-step synthesis of porous N-doped carbon from MOF for efficient thermal energy storage capacity of shape-stabilized phase change materials

N-doped porous carbon (NPC-Al) synthesized using one step process of NH2-MIL-53(Al) metal organic framework was used to prepare polyethylene glycol (PEG)-based composite phase change materials (PCMs). NPC-Al exhibited large and regular pore dimension, large specific surface area (2193.5 m2/g), high mesopore proportion, high nitrogen content and chemically tunable material, which are difficult for post-synthesis driven N-doped porous carbons and raw carbons, and taken as a potential candidate for storage applications. Importantly, the effect of nitrogen on PCM loading, thermal conductivity, energy storage and efficiency was systematically studied. NPC-Al exhibited homogeneous PEG loading capacity (up to 90 wt%), improved thermal conductivity up to 52% and thermal storage capability (reach to 100.3%) in a melting enthalpy of 168.3 J/g, 47.2% higher than that of un-doped carbon derived from the same process due to porous characteristics and additional interaction presented between nitrogen atoms along with flexible original metal organic framework material. In addition, the composites exhibited excellent durability up to 99.5% retention after 50-times thermal cycling performance without leakage, better thermal effusivity and specific heat capacity than pristine PEG, which can realize to use for efficient building energy applications.

Coating of 2D Flexible Metal–Organic Frameworks on Metal Nanocrystals

We report, for the first time, metal (Pd) nanocrystals (NCs) covered with a 2D flexible metal–organic framework (MOF) of [Zn(NO2-ip)(bpy)]n (NO2-ip: 5-nitro-isophthalate, bpy: 4,4′-bipyridine). The...

Combining Organic and Inorganic Redox-Activity: Porphyrin-Based Metal-Organic Frameworks As Electrode Material

Active cathode materials in lithium ion batteries are often based on transition metals like cobalt or nickel. High cost and toxicity as well as the mining and fabrication under ethically questionable conditions are serious drawbacks of these metals used in state-of-the-art materials. On the route towards a “greener” and more sustainable lithium ion technology, alternatives such as organic and hybrid inorganic/organic active materials obtained increasing attraction in the last years. [1] One promising hybrid material class are metal-organic frameworks (MOFs). Due to their high surface area and well-defined porosity as well as their flexible and designable structure MOFs have received increasing attention over the last decades for different potential applications like gas storage or catalysis. More recently, this promising class of materials has also been investigated for energy storage devices as electrode active material. [2] MOFs consist of inorganic metal-oxo clusters (called secondary building unit, SBU in short) coordinated to multivalent rigid organic molecules (often referred as “linker”), which can be modified with a variety of functional groups forming a crystalline porous structure. The well-defined pore size is tailorable by the length of the linker molecules leading to a high flexibility accessible for reversible insertion and removal of guest molecules. The highly porous crystalline structure, especially their large pore size, make them interesting for reversible cation or anion storage, e.g. in the dual-ion battery concept. [3] Furthermore, multivalent metal ions in the SBU as well as organic linker molecules can act as redox-active sites, leading to a promising active material. [4] Porphyrin-based organic derivates, which occur. e.g. in human blood and vitamin B12, are well known for their catalytic- and redox-activity. In the present work, we synthesize various porphyrin-based MOFs with different coordinated metals, which are successfully applied as an energy storage material in a lithium metal cell and characterized with respect to the structural and surface properties. Combining the redox-active porphyrin derivate, Tetrakis(4-carboxyphenyl)porphyrin (TCPP), and a redox-active metal(oxo-)cluster, a non-toxic and environmentally friendly cathode material was achieved. Constant current cycling and cyclic voltammetry studies reveal a high and reversible redox activity. Using suitable methods such as X-ray diffraction (XRD), the redox reaction behavior of the metal-organic framework and the structural properties of the MOFs were investigated upon charge/discharge operation. Furthermore, the influence of different conductive salts and solvents on the electrochemical performance were analyzed.

Modeling Gas Adsorption in Flexible Metal–Organic Frameworks via Hybrid Monte Carlo/Molecular Dynamics Schemes

AbstractHerein, a hybrid Monte Carlo (MC)/molecular dynamics (MD) simulation protocol that properly accounts for the extraordinary structural flexibility of metal–organic frameworks (MOFs) is developed and validated. This is vital to accurately predict gas adsorption isotherms and guest‐induced flexibility of these materials. First, the performance of three recent models to predict adsorption isotherms and flexibility in MOFs is critically investigated. While these methods succeed in providing qualitative insight in the gas adsorption process in MOFs, their accuracy remains limited as the intrinsic flexibility of these materials is very hard to account for. To overcome this challenge, a hybrid MC/MD simulation protocol that is specifically designed to handle the flexibility of the adsorbent, including the shape flexibility, is introduced, thereby unifying the strengths of the previous models. It is demonstrated that the application of this new protocol to the adsorption of neon, argon, xenon, methane, and carbon dioxide in MIL‐53(Al), a prototypical flexible MOF, substantially decreases the inaccuracy of the obtained adsorption isotherms and predicted guest‐induced flexibility. As a result, this method is ideally suited to rationalize the adsorption performance of flexible nanoporous materials at the molecular level, paving the way for the conscious design of MOFs as industrial adsorbents.

Control of structural flexibility of layered-pillared metal-organic frameworks anchored at surfaces

Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.

Unraveling the Interfacial Structure-Performance Correlation of Flexible Metal-Organic Framework Membranes on Polymeric Substrates.

Pure metal-organic framework (MOF) layers deposited on porous supports are important candidates for molecular sieving membranes, but their performance usually deviates from theoretical estimations. Here, we combine step-wise scanning electron microscopy imaging, time-resolved synchrotron X-ray scattering, terahertz infrared spectroscopy, and density functional theory calculation to investigate the ZIF-8 membrane formation on two types (polydopamine and TiO2) of functionalized porous supports. Though molecular sieving of ZIF-8 membranes for smaller gases (He, H2, and CO2) can be achieved with both types of functionalized supports, we unravel that the strong interaction between MOF and polydopamine can disrupt the formation of "perfect" MOF crystals at the interface, leading to a "contracted" MOF structure with partially uncoordinated imidazolate ligands. This further affects the low-frequency dynamical parameters of the framework and inhibits the effective seeded growth. Eventually, it leads to an unexpected loss of selectivity for the bulkier gases (N2 and CH4) for ZIF-8 on polydopamine-functionalized supports. This work links the dynamical aspects of MOFs with their gas transport behavior and highlights the importance of regulating the interfacial weak forces to preserve the ideal molecular sieving efficiency of MOF membranes, which also provides guidance for defect engineering of MOF film fabrication for sensing and electronic devices beyond membranes.

Common but differentiated flexible MIL-53(Al): role of metal sources in synthetic protocol for tuning the adsorption characteristics

The property tuning of metal–organic frameworks (MOFs) has been an active pursuit in both academia and industry. In this work, structural properties of a promising flexible MOF, MIL-53(Al), were finely tuned via a metal source-based synthetic protocol. Varying degrees of framework flexibility and hydrophilicity have been achieved using water-insoluble metal sources, such as alumina, aluminum hydroxide, boehmite, and traditional aluminum nitrate for synthesis. MIL-53(Al) prepared from alumina was the most rigid and hydrophilic as is confirmed by powder X-ray diffraction, vapor adsorption, and diffuse reflectance infrared Fourier transform spectroscopy. Magic-angle spinning nuclear magnetic resonance results revealed that utilizing insoluble metal sources entailed different reaction mechanisms during MOF synthesis and introduced uncoordinated carboxyl into the framework. Through selection of metal sources, the adsorption characteristics of MIL-53(Al) were successfully tuned. The samples prepared from insoluble metal sources showed increased adsorption capacities toward iodine and bisphenol A. The maximum capacity toward iodine in water and n-hexane was one and six times higher than that of conventional MIL-53(Al), respectively. This finding offers excellent prospects for the structural regulation and property tuning of MOFs.

Intermediate states approach for adsorption studies in flexible metal–organic frameworks

Adsorption studies in flexible metal-organic frameworks are challenging and time-consuming. It is mainly because the mechanism of adsorption, defined by structural framework properties, is constantly modified during the process, as the framework transformation depends on the adsorption uptake. We propose here a new approach to investigate adsorption in such complex systems, in which the simulations of adsorption in a deforming framework are replaced by the analysis of adsorption in intermediate rigid structures. As a proof of concept we analyze carbon dioxide, hexane, and methane adsorption in MIL-53. 19 intermediate structures were generated using geometrical interpolation between the open and the closed MOF forms and optimized with quantum DFT calculations. The grand canonical Monte Carlo method was applied to calculate adsorption isotherms in all intermediate structures. The comparison with experimental results enabled the identification of the intermediate adsorption states. The analysis of the microscopic configurations of the adsorbed molecules in these structures allowed us to propose a new mechanism of adsorbate evolution over the entire process.

Desolvation process in the flexible metal–organic framework [Cu(Me-4py-trz-ia)], adsorption of dihydrogen and related structure responses

Structural changes and the unusual H2 adsorption behaviour of a Cu2+-based MOF were studied by X-ray diffraction in combination with DFT modelling and by inelastic neutron scattering.

Cobalt substitution in a flexible metal-organic framework: modulating a soft paddle-wheel unit for tunable gate-opening adsorption.

Developing feasible ways to achieve tunable gate-opening pressure (Pgo) while minimizing the side effects on the adsorption capacity and enthalpy is greatly desired for flexible MOFs. In this work, we focused on solving this issue by cobalt substitution. We showed the successful modulation of the energy required for the reversible transformation of a soft paddle-wheel so that the whole framework presented a substitution-dependent Pgo for CO2 adsorption.

Chemical control of structure and guest uptake by a conformationally mobile porous material.

Metal-organic frameworks (MOFs) are crystalline synthetic porous materials formed by binding organic linkers to metal nodes: they can be either rigid1,2 or flexible3. Zeolites and rigid MOFs have widespread applications in sorption, separation and catalysis that arise from their ability to control the arrangement and chemistry of guest molecules in their pores via the shape and functionality of their internal surface, defined by their chemistry and structure4,5. Their structures correspond to an energy landscape with a single, albeit highly functional, energy minimum. By contrast, proteins function by navigating between multiple metastable structures using bond rotations of the polypeptide6,7, where each structure lies in one of the minima of a conformational energy landscape and can be selected according to the chemistry of the molecules that interact with the protein. These structural changes are realized through the mechanisms of conformational selection (where a higher-energy minimum characteristic of the protein is stabilized by small-molecule binding) and induced fit (where a small molecule imposes a structure on the protein that is not a minimum in the absence of that molecule)8. Here we show that rotation about covalent bonds in a peptide linker can change a flexible MOF to afford nine distinct crystal structures, revealing a conformational energy landscape that is characterized by multiple structural minima. The uptake of small-molecule guests by the MOF can be chemically triggered by inducing peptide conformational change. This change transforms the material from a minimum on the landscape that is inactive for guest sorption to an active one. Chemical control of the conformation of a flexible organic linker offers a route to modifying the pore geometry and internal surface chemistry and thus the function of open-framework materials.

Ultrathin MOF nanosheet assembled highly oriented microporous membrane as an interlayer for lithium-sulfur batteries

Lithium sulfur (Li-S) batteries are attracting increasing attentions as promising next-generation rechargeable batteries. However, the rapid capacity fading of sulfur cathodes caused by the shuttling of polysulfide intermediates between the cathodes and anodes restricts the application of Li-S batteries. In this work, a facile wet-chemistry method is developed for the direct synthesis of few-molecular-layer thin metal-organic framework (MOF) nanosheets without using surfactant. By assembling these ultrathin MOF nanosheets with a facile vacuum filtration method, a highly oriented and flexible MOF membrane with favorable mechanical properties is achieved for the first time. The excellent features make the as-prepared MOF nanosheets ideal to fabricate lightweight interlayer modified separators for suppressing the polysulfide shuttling of Li-S batteries. When using the MOF membrane modified separator, the Li-S batteries made from commercial carbon materials exhibits the significantly enhanced cycling stabilities. This work brings new opportunities for the synthesis and application of MOF materials.

Tuning the balance between dispersion and entropy to design temperature-responsive flexible metal-organic frameworks

Temperature-responsive flexibility in metal-organic frameworks (MOFs) appeals to the imagination. The ability to transform upon thermal stimuli while retaining a given crystalline topology is desired for specialized sensors and actuators. However, rational design of such shape-memory nanopores is hampered by a lack of knowledge on the nanoscopic interactions governing the observed behavior. Using the prototypical MIL-53(Al) as a starting point, we show that the phase transformation between a narrow-pore and large-pore phase is determined by a delicate balance between dispersion stabilization at low temperatures and entropic effects at higher ones. We present an accurate theoretical framework that allows designing breathing thermo-responsive MOFs, based on many-electron data for the dispersion interactions and density-functional theory entropy contributions. Within an isoreticular series of materials, MIL-53(Al), MIL-53(Al)-FA, DUT-4, DUT-5 and MIL-53(Ga), only MIL-53(Al) and MIL-53(Ga) are proven to switch phases within a realistic temperature range.

A Self-Folding Polymer Film Based on Swelling Metal-Organic Frameworks.

Herein, we exploit the well-known swelling behaviour of metal-organic frameworks (MOFs) to create a self-folding polymer film. Namely, we show that incorporating crystals of the flexible MOF MIL-88A into a polyvinylidene difluoride (PVDF) matrix affords a polymer composite film that undergoes reversible shape transformations upon exposure to polar solvents and vapours. Since the self-folding properties of this film correlate directly with the swelling properties of the MIL-88A crystals, it selectively bends to certain solvents and its degree of folding can be controlled by controlling the relative humidity. Moreover, it shows a shape-memory effect at relative humidity values from 60 % to 90 %. As proof of concept, we demonstrate that these composite films can lift cargo and can be used to assemble 3D structures from 2D patterns. Our strategy is a straightforward method for designing autonomous soft materials with folding properties that can be tuned by judicious choice of the constituent flexible MOF.