Liquid metal-organic frameworks

Insights into Solid-To-Solid Transformation of MOF Amorphous Phases

Metal-organic frameworks (MOFs) have been recently explored as crystalline solids for conversion into amorphous phases demonstrating non-specific mechanical, catalytic, and optical properties. The real-time control of such structural transformations and their outcomes still remain a challenge. Here, we use in situ high-resolution transmission electron microscopy with 0.01 s time resolution to explore non-thermal (electron induced) amorphization of a MOF single crystal, followed by transformation into an amorphous nanomaterial. By comparing a series of M-BTC (M: Fe³⁺, Co³⁺, Co²⁺, Ni²⁺, and Cu²⁺; BTC: 1,3,5-benzentricarboxylic acid), we demonstrate that the topology of a metal cluster of the parent MOFs determines the rate of formation and the chemistry of the resulting phases containing an intact ligand and metal or metal oxide nanoparticles. Confocal Raman and photoluminescence spectroscopies further confirm the integrity of the BTC ligand and coordination bond breaking, while high-resolution imaging with chemical and structural analysis over time allows for tracking the dynamics of solid-to-solid transformations. The revealed relationship between the initial and resulting structures and the stability of the obtained phase and its photoluminescence over time contribute to the design of new amorphous MOF-based optical nanomaterials.

Liquid and Glass Phases of an Alkylguanidinium Sulfonate Hydrogen-Bonded Organic Framework

Glassy phases of framework materials feature unique and tunable properties that are advantageous for gas separation membranes, solid electrolytes, and phase-change memory applications. Here, we report a new guanidinium organosulfonate hydrogen-bonded organic framework (HOF) that melts and vitrifies below 100 °C. In this low-temperature regime, non-covalent interactions between guest molecules and the porous framework become a dominant contributor to the overall stability of the structure, resulting in guest-dependent melting, glass, and recrystallization transitions. Through simulations and X-ray scattering, we show that the local structures of the amorphous liquid and glass phases resemble those of the parent crystalline framework.

Synergistic Stimulation of Metal-Organic Frameworks for Stable Super-cooled Liquid and Quenched Glass

Metal-organic framework (MOF) glasses are a fascinating new class of materials, yet their prosperity has been impeded by the scarcity of known examples and limited vitrification methods. In the work described in this report, we applied synergistic stimuli of vapor hydration and thermal dehydration to introduce structural disorders in interpenetrated dia-net MOF, which facilitate the formation of stable super-cooled liquid and quenched glass. The material after stimulus has a glass transition temperature (T_g) of 560 K, far below the decomposition temperature of 695 K. When heated, the perturbed MOF enters a super-cooled liquid phase that is stable for a long period of time (>10⁴ s), across a broad temperature range (26 K), and has a large fragility index of 83. Quenching the super-cooled liquid gives rise to porous MOF glass with maintained framework connectivity, confirmed by EXAFS and PDF analysis. This method provides a fundamentally new route to obtain glassy materials from MOFs that cannot be melted without causing decomposition.

The deformation of short-range order leading to rearrangement of topological network structure in zeolitic imidazolate framework glasses

In recent years, the study of the glassy structure of zeolitic imidazolate frameworks (ZIFs) has been a key breakthrough in glass science. Yet the theoretical understanding of the structure of these complex materials is still in its infancy, especially the short-range structure. The short-structural disorder of two ZIFs and their corresponding molten structure, namely, ZIF-4 and ZIF-62 are studied, using ab initio simulations. Changes in short-range order are investigated, particularly the changes in bond length, bond angle, and tetrahedral unit volume. Furthermore, the asymmetric distribution of organic groups caused by the benzimidazole functional group leads to the difference in short-range disorder between ZIF-4 and ZIF-62 glasses, which contribute to the glass-forming ability difference.

Morphologically Controlled Metal-Organic Framework-Derived FeNi Oxides for Efficient Water Oxidation

The complex oxygen evolution reaction (OER) is recognized as the most studied and explored electrochemical conversion, which plays a crucial role in energy-related applications. In this work, a series of metal-organic framework (MOF)-derived FeNi oxides from a barrel-shaped Ni-based BMM-10 precursor are conveniently obtained to show an excellent OER performance. Under mild Fe(III) etching, a type of core-shell Fe_0.5-BMM-10 can be well preserved and the coordination bond of the middle frame structure is decomposed. Furthermore, the Fe_x-BMM-10-T series is successfully synthesized with a well-preserved morphology compared to precursors after direct oxidation. Finally, followed by initial electrochemical activation, the decomposition of FeNi oxides generates active Fe-doped nickel oxyhydroxides for efficient water oxidation. The improved OER performance stems from the high specific surface area and abundant exposed active centers, as well as the significant synergistic effect between iron and nickel, which is further verified by the theoretical calculation. This approach can be extended to precisely adjust the morphology of MOFs and their derivatives that can result in superior electrocatalytic properties in terms of energy conversion and storage applications.

X‐ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal–Organic Frameworks**

Glass‐forming metal–organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X‐ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass‐forming Zn‐ZIF‐zni and the isostructural thermally decomposing Co‐ZIF‐zni. Electron density analysis shows that the Zn−N bonds are more ionic than the Co−N bonds, which have distinct covalent features. Variable‐temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co‐ZIF‐zni above 673 K. Melting can be controlled by tuning the metal–ligand and imidazole bonding strength as shown from thermal analysis of nine solid‐solution Co_xZn_1−x‐ZIF‐zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co‐doping into Zn‐ZIF‐zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal–ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.

Porous liquids - the future is looking emptier

The development of microporosity in the liquid state is leading to an inherent change in the way we approach applications of functional porosity, potentially allowing access to new processes by exploiting the fluidity of these new materials. By engineering permanent porosity into a liquid, over the transient intermolecular porosity in all liquids, it is possible to design and form a porous liquid. Since the concept was proposed in 2007, and the first examples realised in 2015, the field has seen rapid advances among the types and numbers of porous liquids developed, our understanding of the structure and properties, as well as improvements in gas uptake and molecular separations. However, despite these recent advances, the field is still young, and with only a few applications reported to date, the potential that porous liquids have to transform the field of microporous materials remains largely untapped. In this review, we will explore the theory and conception of porous liquids and cover major advances in the area, key experimental characterisation techniques and computational approaches that have been employed to understand these systems, and summarise the investigated applications of porous liquids that have been presented to date. We also outline an emerging discovery workflow with recommendations for the characterisation required at each stage to both confirm permanent porosity and fully understand the physical properties of the porous liquid.

Type II porous ionic liquid based on metal-organic cages that enables L-tryptophan identification

Porous liquids with chemical separation properties are quite well-studied in general, but there is only a handful of reports in the context of identification and separation of non-gaseous molecules. Herein, we report a Type II porous ionic liquid composed of coordination cages that exhibits exceptional selectivity towards L-tryptophan (L-Trp) over other aromatic amino acids. A previously known class of anionic organic-inorganic hybrid doughnut-like cage (HD) is dissolved in trihexyltetradecylphosphonium chloride (THTP_Cl). The resulting liquid, HD/THTP_Cl, is thereby composed of common components, facile to prepare, and exhibit room temperature fluidity. The permanent porosity is manifested by the high-pressure isotherm for CH4 and modeling studies. With evidence from time-dependent amino acid uptake, competitive extraction studies and molecular dynamic simulations, HD/THTP_Cl exhibit better selectivity towards L-Trp than other solid state sorbents, and we attribute it to not only the intrinsic porosity of HD but also the host-guest interactions between HD and L-Trp. Specifically, each HD unit is filled with nearly 5 L-Trp molecules, which is higher than the L-Trp occupation in the structure unit of other benchmark metal-organic frameworks.

Multivariate analysis of disorder in metal-organic frameworks

The rational design of disordered frameworks is an appealing route to target functional materials. However, intentional realisation of such materials relies on our ability to readily characterise and quantify structural disorder. Here, we use multivariate analysis of pair distribution functions to fingerprint and quantify the disorder within a series of compositionally identical metal–organic frameworks, possessing different crystalline, disordered, and amorphous structures. We find this approach can provide powerful insight into the kinetics and mechanism of structural collapse that links these materials. Our methodology is also extended to a very different system, namely the melting of a zeolitic imidazolate framework, to demonstrate the potential generality of this approach across many areas of disordered structural chemistry.

Structural disorder in materials is challenging to characterise. Here, the authors use multivariate analysis of atomic pair distribution functions to study structural collapse and melting of metal–organic frameworks, revealing powerful mechanistic and kinetic insight.

Advancing the Mechanical Performance of Glasses: Perspectives and Challenges

Glasses are materials that lack a crystalline microstructure and long-range atomic order. Instead, they feature heterogeneity and disorder on superstructural scales, which have profound consequences for their elastic response, material strength, fracture toughness, and the characteristics of dynamic fracture. These structure-property relations present a rich field of study in fundamental glass physics and are also becoming increasingly important in the design of modern materials with improved mechanical performance. A first step in this direction involves glass-like materials that retain optical transparency and the haptics of classical glass products, while overcoming the limitations of brittleness. Among these, novel types of oxide glasses, hybrid glasses, phase-separated glasses, and bioinspired glass-polymer composites hold significant promise. Such materials are designed from the bottom-up, building on structure-property relations, modeling of stresses and strains at relevant length scales, and machine learning predictions. Their fabrication requires a more scientifically driven approach to materials design and processing, building on the physics of structural disorder and its consequences for structural rearrangements, defect initiation, and dynamic fracture in response to mechanical load. In this article, a perspective is provided on this highly interdisciplinary field of research in terms of its most recent challenges and opportunities.

Mixed metal node effect in zeolitic imidazolate frameworks

We synthesized two series of bimetallic (zinc and cobalt) zeolitic imidazolate frameworks (ZIF-62) under different solvothermal conditions. It is found that the structure of the derived ZIF crystals is highly sensitive to synthesis conditions. One series possesses the standard ZIF-62 structure, whereas the other has a mixed structure composed of both the standard structure and an unknown one. The standard series exhibits a slight negative deviation from linearity of melting temperature (T _m) and glass transition temperature (T _g) with the substitution of Co for Zn. In contrast, the new series displays a stronger negative deviation. These negative deviations from linearity indicate the mixed metal node effect in bimetallic ZIF-62 due to the structural mismatch between Co²⁺ and Zn²⁺ and to the difference in their electronic configurations. The new series involves both cobalt-rich and zinc-rich phases, whereas the standard one shows one homogeneous phase. Density functional theory calculations predict that the substitution of Co for Zn increases the bulk modulus of the ZIF crystals. This work indicates that the structure, melting behaviour, and mechanical properties of ZIFs can be tuned by metal node substitution and by varying the synthetic conditions. Both series of ZIFs have higher glass forming abilities due to their higher T _g/T _m ratios (0.77-0.84) compared to most good glass formers.

Post-Synthetic Modification of a Metal-Organic Framework Glass

Melt-quenched metal-organic framework (MOF) glasses have gained significant interest as the first new category of glass reported in 50 years. In this work, an amine-functionalized zeolitic imidazolate framework (ZIF), denoted ZIF-UC-6, was prepared and demonstrated to undergo both melting and glass formation. The presence of an amine group resulted in a lower melting temperature compared to other ZIFs, while also allowing material properties to be tuned by post-synthetic modification (PSM). As a prototypical example, the ZIF glass surface was functionalized with octyl isocyanate, changing its behavior from hydrophilic to hydrophobic. PSM therefore provides a promising strategy for tuning the surface properties of MOF glasses.

Large Cages of Zeolitic Imidazolate Frameworks

The design and synthesis of permanently porous materials with extended cage structures is a long-standing challenge in chemistry. In this Account, we highlight the unique role of zeolitic imidazolate frameworks (ZIFs), a class of framework materials built from tetrahedral nodes connected through imidazolate linkers, in meeting this challenge and illustrate specific features that set ZIFs apart from other porous materials. The structures of ZIFs are characteristic of a variety of large, zeolite-like cages that are covalently connected with neighboring cages and fused in three-dimensional space. In contrast to molecular cages, the fusion of cages results in extraordinary architectural and chemical stability for the passage of gases and molecules through cages and for carrying out chemical reactions within these cages while keeping the cages intact. The combination of the advantages from both cage chemistry and extended structures allows uniquely interconnected yet compartmentalized void spaces inside ZIF solids, rendering their wide range of applications in catalysis, gas storage, and gas separation.While the field of ZIFs has seen rapid development over the past decade, with hundreds of ZIF structures built from dozens of different cages of varying composition, size, and shapes reported, rational approaches to their design are largely unknown. In this Account, we summarize a vast number of cages formed in reported ZIFs and then review how the thermodynamic factors and traditional guest-templating strategies from zeolites influence the formation of cages. We highlight how the link-link interactions perform in the ZIF formation mechanism and serve as a means to target the formation of frameworks containing cages of specific sizes with structures exhibiting a level of complexity as yet unachieved in discrete coordination cages. For example, the giant ucb cage features a dimension of 46 Å and the complex moz cage is constructed from as many as 660 components.With the finding of these large and complex cages in ZIFs, we envision that the collection of cage structures will further be diversified by a mixed-linker approach utilizing a more complex combination of link-link interactions or by creating multivariant (MTV) systems that have been realized in other framework materials yet not widely employed in ZIFs. The more complicated cage structures can provide extra variations in chemical environments, and in addition to that, MTV systems can generate inhomogeneity inside each type of cage structure. The fused cages at such complexity that are difficult to be realized in solution environments will potentially enable more complex materials for smart applications.

Principles of melting in hybrid organic-inorganic perovskite and polymorphic ABX3 structures

Four novel dicyanamide-containing hybrid organic-inorganic ABX3 structures are reported, and the thermal behaviour of a series of nine perovskite and non-perovskite [AB(N(CN)2)3] (A = (C3H7)4N, (C4H9)4N, (C5H11)4N; B = Co, Fe, Mn) is analyzed. Structure-property relationships are investigated by varying both A-site organic and B-site transition metal cations. In particular, increasing the size of the A-site cation from (C3H7)4N → (C4H9)4N → (C5H11)4N was observed to result in a decrease in T m through an increase in ΔS f. Consistent trends in T m with metal replacement are observed with each A-site cation, with Co < Fe < Mn. The majority of the melts formed were found to recrystallise partially upon cooling, though glasses could be formed through a small degree of organic linker decomposition. Total scattering methods are used to provide a greater understanding of the melting mechanism.

Structural Analysis of Molecular Materials Using the Pair Distribution Function

This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.

Coordination polymer-forming liquid Cu(2-isopropylimidazolate)

The structure of the melt state of one-dimensional (1D) coordination polymer crystal Cu(isopropylimidazolate) (melting temperature T_m = 143 °C) was characterized by DSC, variable temperature PXRD, solid-state NMR (SSNMR), viscoelastic measurements, XAS, and DFT-AIMD calculations. These analyses suggested “coordination polymer-forming liquid” formation with preserved coordination bonds above T_m. Variable chain configurations and moderate cohesive interaction in adjacent chains are the keys to the rarely observed polymer-forming liquid. The melt structure is reminiscent of the common 1D organic polymer melts such as entanglement or random coil structures.

The melt state of coordination polymer crystals is composed of macromolecular-chain assemblies without coordination bond breaking. The coordination-polymer-forming liquid provides various morphologies, including spun fibers.

Sensitivity of the glass transition and melting in a metal-organic framework to ligand chemistry

The effect of substituting linkers with electron-donating moieties for part of the conventional ones on the melting and glass transition behaviours of ZIF-62 was investigated by calorimetry and X-ray diffraction. Specifically, substituting 5,6-dimethylbenzimidazole for benzimidazole in ZIF-62 increases both T_m and T_g. The structural origin of this effect was explained.

Properties of Single-Component Metal-Organic Framework Crystal-Glass Composites

The formation, and subsequent structural, thermal and adsorptive properties of single-component metal-organic framework crystal-glass composites (MOF-CGCs) are investigated. A series of novel materials exhibiting chemically identical glassy and crystalline phases within the same material were produced, where crystalline ZIF-62(Zn) was incorporated within an a_g ZIF-62(Zn) matrix. X-ray diffraction showed that the crystalline phase was still present after heating to above the glass transition temperature of a_g ZIF-62(Zn), and interfacial compatibility between the crystalline and glassy phases was investigated using a mixed-metal (ZIF-62(Co))_0.5 (a_g ZIF-62(Zn))_0.5 analogue. CO₂ gas adsorption measurements showed that the CO₂ uptakes of the MOF-CGCs were between those of the crystalline and glassy phases.

ResponZIF Structures: Zeolitic Imidazolate Frameworks as Stimuli-Responsive Materials

Zeolitic imidazolate frameworks (ZIFs) have long been recognized as a prominent subset of the metal-organic framework (MOF) family, in part because of their ease of synthesis and good thermal and chemical stability, alongside attractive properties for diverse potential applications. Prototypical ZIFs like ZIF-8 have become embodiments of the significant promise held by porous coordination polymers as next-generation designer materials. At the same time, their intriguing property of experiencing significant structural changes upon the application of external stimuli such as temperature, mechanical pressure, guest adsorption, or electromagnetic fields, among others, has placed this family of MOFs squarely under the umbrella of stimuli-responsive materials. In this review, we provide an overview of the current understanding of the triggered structural and electronic responses observed in ZIFs (linker and bond dynamics, crystalline and amorphous phase changes, luminescence, etc.). We then describe the state-of-the-art experimental and computational methodology capable of shedding light on these complex phenomena, followed by a comprehensive summary of the stimuli-responsive nature of four prototypical ZIFs: ZIF-8, ZIF-7, ZIF-4, and ZIF-zni. We further expose the relevant challenges for the characterization and fundamental understanding of responsive ZIFs, including how to take advantage of their flexible properties for new application avenues.

Mechanochemically Synthesised Flexible Electrodes Based on Bimetallic Metal-Organic Framework Glasses for the Oxygen Evolution Reaction

The melting behaviour of metal-organic frameworks (MOFs) has aroused significant research interest in the areas of materials science, condensed matter physics and chemical engineering. This work first introduces a novel method to fabricate a bimetallic MOF glass, through melt-quenching of the cobalt-based zeolitic imidazolate framework (ZIF) [ZIF-62(Co)] with an adsorbed ferric coordination complex. The high-temperature chemically reactive ZIF-62(Co) liquid facilitates the formation of coordinative bonds between Fe and imidazolate ligands, incorporating Fe nodes into the framework after quenching. The resultant Co-Fe bimetallic MOF glass therefore shows a significantly enhanced oxygen evolution reaction performance. The novel bimetallic MOF glass, when combined with the facile and scalable mechanochemical synthesis technique for both discrete powders and surface coatings on flexible substrates, enables significant opportunities for catalytic device assembly.

Concluding remarks: current and next generation MOFs

This paper describes the content of my "Concluding remarks" talk at the Faraday Discussion meeting on "MOFs for energy and the environment" (online, 23-25 June 2021). The panel consisted of sessions on the design of MOFs and MOF hybrids (synthetic chemistry), their applications (e.g., capture, storage, separation, electrical devices, photocatalysis), advanced characterization (e.g., transmission electron microscopy, solid-state nuclear magnetic resonance), theory and modeling, and commercialization. MOF chemistry is undergoing a significant evolution from simply network chemistry to the chemistry of synergistic integration with heterogeneous materials involving other disciplines (we call this the fourth generation type). As reflected in the papers of the invited speakers and discussions with the participants, the present and future of this field will be described in detail.

Ionic liquid facilitated melting of the metal-organic framework ZIF-8

Hybrid glasses from melt-quenched metal-organic frameworks (MOFs) have been emerging as a new class of materials, which combine the functional properties of crystalline MOFs with the processability of glasses. However, only a handful of the crystalline MOFs are meltable. Porosity and metal-linker interaction strength have both been identified as crucial parameters in the trade-off between thermal decomposition of the organic linker and, more desirably, melting. For example, the inability of the prototypical zeolitic imidazolate framework (ZIF) ZIF-8 to melt, is ascribed to the instability of the organic linker upon dissociation from the metal center. Here, we demonstrate that the incorporation of an ionic liquid (IL) into the porous interior of ZIF-8 provides a means to reduce its melting temperature to below its thermal decomposition temperature. Our structural studies show that the prevention of decomposition, and successful melting, is due to the IL interactions stabilizing the rapidly dissociating ZIF-8 linkers upon heating. This understanding may act as a general guide for extending the range of meltable MOF materials and, hence, the chemical and structural variety of MOF-derived glasses.

The changing state of porous materials

Porous materials contain regions of empty space into which guest molecules can be selectively adsorbed and sometimes chemically transformed. This has made them useful in both industrial and domestic applications, ranging from gas separation, energy storage and ion exchange to heterogeneous catalysis and green chemistry. Porous materials are often ordered (crystalline) solids. Order-or uniformity-is frequently held to be advantageous, or even pivotal, to our ability to engineer useful properties in a rational way. Here we highlight the growing evidence that topological disorder can be useful in creating alternative properties in porous materials. In particular, we highlight here several concepts for the creation of novel porous liquids, rationalize routes to porous glasses and provide perspectives on applications for porous liquids and glasses.

Composite Porous Liquid for Recyclable Sequestration, Storage and In Situ Catalytic Conversion of Carbon Dioxide at Room Temperature

Permanent pores combined with fluidity renders flow processability to porous liquids otherwise not seen in porous solids. Although porous liquids have been utilized for sequestration of different gases and their separation, there is still a dearth of studies for deploying in situ chemical reactions to convert adsorbed gases into utility chemicals. Here, we show the design and development of a new type of solvent-less and hybrid (meso-)porous liquid composite, which, as demonstrated for the first time, can be used for in situ carbon mineralization of adsorbed CO₂ . The recyclable porous liquid composite comprising polymer-surfactant modified hollow silica nanorods and carbonic anhydrase enzyme not only sequesters (5.5 cm³  g^-1 at 273 K and 1 atm) and stores CO₂ but is also capable of driving an in situ enzymatic reaction for hydration of CO₂ to HCO₃ ^- ion, subsequently converting it to CaCO₃ due to reaction with pre-dissolved Ca²⁺ . Light and electron microscopy combined with X-ray diffraction reveals the nucleation and growth of calcite and aragonite crystals. Moreover, the liquid-like property of the porous composite material can be harnessed by executing the same reaction via diffusion of complimentary Ca²⁺ and HCO₃ ^- ions through different compartments separated by an interfacial channel. These studies provide a proof of concept of deploying chemical reactions within porous liquids for developing utility chemical from adsorbed molecules.

Interwrapping Distinct Metal-Organic Frameworks in Dual-MOFs for the Creation of Unique Composite Catalysts

Incorporating metal nanoparticles (MNPs) inside metal-organic frameworks (MOFs) demonstrates superior catalytic properties in numerous reactions; however, the size and distribution of MNPs could not be well controlled, resulting in low product selectivity in catalysis by undergoing different catalytic reaction pathways. We report herein a facile strategy for integrating lattice-mismatched MOFs together to fabricate homogeneously distributed “dual-MOFs,” which are the ideal precursors for the preparation of MNPs@MOFs with unique catalytic properties. As a proof of concept, we successfully synthesize a dual-MOF HKUST-1/ZIF-8 for in situ creation of redox-active Cu NPs inside hierarchical porous ZIF-8 under controlled pyrolytic conditions. Combining the advantages of size-tunable Cu NPs in the molecular sieving matrix of ZIF-8, Cu@ZIF-8 demonstrates high activity and selectivity for transformation of alkynes into alkenes without overhydrogenation, which surpasses most of the catalysts in the literature. Therefore, this work paves a new pathway for developing highly efficient and selective heterogeneous catalysts to produce highly value-added chemicals.

Acidic open-cage solution containing basic cage-confined nanospaces for multipurpose catalysis

The nanoscale chemical spaces inherent in porous organic/coordination cages or solid/liquid materials have been continuously explored for their nanoconfinement effect on selective adsorption and reaction of small gas or organic molecules. Herein, we aim to rationalize the unconventional chemical reactivities motivated by the cage-confined nanospaces in aqueous solutions, where the robust yet permeable nanospaces defined by the open cages facilitate dynamic guest exchange and unusual chemical reactions. The high positive charges on [(Pd/Pt)₆(RuL₃)₈]²⁸⁺ nanocages drive imidazole–proton equilibrium to display a significantly perturbed pK_a shift, creating cage-defined nanospaces in solution with distinct intrinsic basicity and extrinsic acidity. The supramolecular cage effect plays pivotal roles in elaborating robust solution nanospaces, controlling ingress-and-egress molecular processes through open-cage portals and endowing nanocages with transition-state stabilization, amphoteric reactivities and the phase transfer of insoluble molecules, thus promoting chemical transformations in unconventional ways. Consequently, a wide range of application of cage-confined catalysis with anomalous reactivities may be expected based on this kind of open-cage solution medium, which combines cage nanocavity, solution heterogeneity and liquid-phase fluidity to benefit various potential mass transfer and molecular process options.

Emergence of Coupled Rotor Dynamics in Metal-Organic Frameworks via Tuned Steric Interactions

The organic components in metal–organic frameworks (MOFs) are unique: they are embedded in a crystalline lattice, yet, as they are separated from each other by tunable free space, a large variety of dynamic behavior can emerge. These rotational dynamics of the organic linkers are especially important due to their influence over properties such as gas adsorption and kinetics of guest release. To fully exploit linker rotation, such as in the form of molecular machines, it is necessary to engineer correlated linker dynamics to achieve their cooperative functional motion. Here, we show that for MIL-53, a topology with closely spaced rotors, the phenylene functionalization allows researchers to tune the rotors’ steric environment, shifting linker rotation from completely static to rapid motions at frequencies above 100 MHz. For steric interactions that start to inhibit independent rotor motion, we identify for the first time the emergence of coupled rotation modes in linker dynamics. These findings pave the way for function-specific engineering of gear-like cooperative motion in MOFs.

Porous Metal-Organic Framework Liquids for Enhanced CO2 Adsorption and Catalytic Conversion

The unique applications of porous metal-organic framework (MOF) liquids with permanent porosity and fluidity have attracted significant attention. However, fabrication of porous MOF liquids remains challenging because of the easy intermolecular self-filling of the cavity or the rapid settlement of porous hosts in hindered solvents that cannot enter their pores. Herein, we report a facile strategy for the fabrication of a MOF liquid (Im-UiO-PL) by surface ionization of an imidazolium-functionalized framework with a sterically hindered poly(ethylene glycol) sulfonate (PEGS) canopy. The Im-UiO-PL obtained in this way has a CO₂ adsorption approximately 14 times larger than that of pure PEGS. Distinct from a porous MOF solid counterpart, the stored CO₂ in Im-UiO-PL can be slowly released and efficiently utilized to synthesize cyclic carbonates in the atmosphere. This is the first example of the use of a porous MOF liquid as a CO₂ storage material for catalysis. It offers a new method for the fabrication of unique porous liquid MOFs with functional behaviors in various fields of gas adsorption and catalysis.

Mechanics, Ionics, and Optics of Metal-Organic Framework and Coordination Polymer Glasses

Melt and glassy states of coordination polymers (CPs)/metal-organic frameworks (MOFs) have gained attention as a new class of amorphous materials. Many bridging ligands such as azolate, nitrile, thiocyanide, thiolate, pyridine, sulfonate, and amide are available to construct crystals with melting temperatures in the range of 60-593 °C. Here, we discuss the mechanism of crystal melting, glass structures, and mechanical properties by considering both experimental and theoretical studies. High and exclusive H⁺ or Li⁺ conductivities in moldable CP glasses have been proven in the all-solid-state devices such as fuel cells or secondary batteries. Transparent glasses with wide composition and available dopants are also attractive for nonlinear optics, photoconductivity, emission, and light-harvesting. The ongoing challenge in the field is to develop the design principles of CP/MOF melts and glasses, corresponding functions of mass (ion, electron, photon, phonon, and so forth). transport and conversion, and the integration of devices with the use of their tunable mechanical properties.

Cellulose acetate-based membranes by interfacial engineering and integration of ZIF-62 glass nanoparticles for CO2 separation

Composite membranes typically used for gas separation are susceptible to interfacial voids and CO₂ plasticization which adversely affects the gas permeation performance. This paper evaluates routes towards the enhancement of CO₂ permeation performance and CO₂ plasticization resistance of composite membranes using non-stoichiometric ZIF-62 MOF glass and cellulose acetate (CA). Single and mixed gas permeation results, obtained with CO₂ and CH₄, demonstrate that the presence of ZIF-62 glass in CA polymer enhanced the CO₂ permeability and CO₂/CH₄ ideal selectivity from 15.8 to 84.8 Barrer and 12.2-35.3, respectively. The composite membrane loaded with 8 wt% of ZIF-62 glass showed the highest CO₂ permeability and CO₂/CH₄ ideal selectivity of 84.8 Barrer and 35.3, which were 436.7% and 189.3% higher compared to the pristine CA membrane, respectively. A CO₂ plasticization pressure of 26 bar was achieved for the composite membranes, which is 160% higher compared to the pristine CA membranes, at about 10 bar. The mechanisms for the materials stabilization and greater separation performance were attributed to higher pore size (7.3 Å) and significant CO₂ adsorption on the unsaturated metal nodes followed by metal cites electrostatic interaction with CO₂. These findings confirm the potential of ZIF-62 glass materials as promising materials solutions towards the design of composite membranes for CO₂ separation at industrial scale.

Melting of hybrid organic-inorganic perovskites

Several organic-inorganic hybrid materials from the metal-organic framework (MOF) family have been shown to form stable liquids at high temperatures. Quenching then results in the formation of melt-quenched MOF glasses that retain the three-dimensional coordination bonding of the crystalline phase. These hybrid glasses have intriguing properties and could find practical applications, yet the melt-quench phenomenon has so far remained limited to a few MOF structures. Here we turn to hybrid organic-inorganic perovskites-which occupy a prominent position within materials chemistry owing to their functional properties such as ion transport, photoconductivity, ferroelectricity and multiferroicity-and show that a series of dicyanamide-based hybrid organic-inorganic perovskites undergo melting. Our combined experimental-computational approach demonstrates that, on quenching, they form glasses that largely retain their solid-state inorganic-organic connectivity. The resulting materials show very low thermal conductivities (~0.2 W m^-1 K^-1), moderate electrical conductivities (10^-3-10^-5 S m^-1) and polymer-like thermomechanical properties.

Current Trends in Metal-Organic and Covalent Organic Framework Membrane Materials

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been thoroughly investigated with regards to applications in gas separation membranes in the past years. More recently, new preparation methods for MOFs and COFs as particles and thin-film membranes, as well as for mixed-matrix membranes (MMMs) have been developed. We will highlight novel processes and highly functional materials: Zeolitic imidazolate frameworks (ZIFs) can be transformed into glasses and we will give an insight into their use for membranes. In addition, liquids with permanent porosity offer solution processability for the manufacture of extremely potent MMMs. Also, MOF materials influenced by external stimuli give new directions for the enhancement of performance by in situ techniques. Presently, COFs with their large pores are useful in quantum sieving applications, and by exploiting the stacking behavior also molecular sieving COF membranes are possible. Similarly, porous polymers can be constructed using MOF templates, which then find use in gas separation membranes.

Frustrated flexibility in metal-organic frameworks

Stimuli-responsive flexible metal-organic frameworks (MOFs) remain at the forefront of porous materials research due to their enormous potential for various technological applications. Here, we introduce the concept of frustrated flexibility in MOFs, which arises from an incompatibility of intra-framework dispersion forces with the geometrical constraints of the inorganic building units. Controlled by appropriate linker functionalization with dispersion energy donating alkoxy groups, this approach results in a series of MOFs exhibiting a new type of guest- and temperature-responsive structural flexibility characterized by reversible loss and recovery of crystalline order under full retention of framework connectivity and topology. The stimuli-dependent phase change of the frustrated MOFs involves non-correlated deformations of their inorganic building unit, as probed by a combination of global and local structure techniques together with computer simulations. Frustrated flexibility may be a common phenomenon in MOF structures, which are commonly regarded as rigid, and thus may be of crucial importance for the performance of these materials in various applications.

Solution-Processable Metal-Organic Framework Nanosheets with Variable Functionalities

Metal-organic frameworks (MOFs) intrinsically lack fluidity and thus solution processability. Direct synthesis of MOFs exhibiting solution processability like polymers remains challenging but highly sought-after for multitudinous applications. Herein, a one-pot, surfactant-free, and scalable synthesis of highly stable MOF suspensions composed of exceptionally large (average area > 15 000 µm² ) NUS-8 nanosheets with variable functionalities and excellent solution processability is presented. This is achieved by adding capping molecules during the synthesis, and by judicious controls of precursor concentration and MOF nanosheet-solvent interactions. The resulting 2D NUS-8 nanosheets with variable functionalities exhibit excellent solution processability. As such, relevant monoliths, aero- and xerogels, and large-area textured films with a great homogeneity, controllable thickness, and appreciable mechanical properties can be facilely fabricated. Additionally, from both the molecular- and chip-level it is demonstrated that capacitive sensors integrated with NUS-8 films functionalized with different terminal groups exhibit distinguishable sensing behaviors toward acetone due to their disparate host-guest interactions. It is envisioned that this simple approach will greatly facilitate the integration of MOFs in miniaturized electronic devices and benefit their mass production.

What Lies beneath a Metal-Organic Framework Crystal Structure? New Design Principles from Unexpected Behaviors

The rational design principles established for metal-organic frameworks (MOFs) allow clear structure-property relationships, fueling expansive growth for energy storage and conversion, catalysis, and beyond. However, these design principles are based on the assumption of compositional and structural rigidity, as measured crystallographically. Such idealization of MOF structures overlooks subtle chemical aspects that can lead to departures from structure-based chemical intuition. In this Perspective, we identify unexpected behavior of MOFs through literature examples. Based on this analysis, we conclude that departures from ideality are not uncommon. Whereas linker topology and metal coordination geometry are useful starting points for understanding MOF properties, we anticipate that deviations from the idealized crystal representation will be necessary to explain important and unexpected behaviors. Although this realization reinforces the notion that MOFs are highly complex materials, it should also stimulate a broader reexamination of the literature to identify corollaries to existing design rules and reveal new structure-property relationships.

Mixed hierarchical local structure in a disordered metal-organic framework

Amorphous metal-organic frameworks (MOFs) are an emerging class of materials. However, their structural characterisation represents a significant challenge. Fe-BTC, and the commercial equivalent Basolite® F300, are MOFs with incredibly diverse catalytic ability, yet their disordered structures remain poorly understood. Here, we use advanced electron microscopy to identify a nanocomposite structure of Fe-BTC where nanocrystalline domains are embedded within an amorphous matrix, whilst synchrotron total scattering measurements reveal the extent of local atomic order within Fe-BTC. We use a polymerisation-based algorithm to generate an atomistic structure for Fe-BTC, the first example of this methodology applied to the amorphous MOF field outside the well-studied zeolitic imidazolate framework family. This demonstrates the applicability of this computational approach towards the modelling of other amorphous MOF systems with potential generality towards all MOF chemistries and connectivities. We find that the structures of Fe-BTC and Basolite® F300 can be represented by models containing a mixture of short- and medium-range order with a greater proportion of medium-range order in Basolite® F300 than in Fe-BTC. We conclude by discussing how our approach may allow for high-throughput computational discovery of functional, amorphous MOFs.