Heterogeneity within Order in Crystals of a Porous Metal–Organic Framework

Structural features of proton-conducting metal organic and covalent organic frameworks

Proton conductivity in MOFs and COFs have been attracted due to their applicability as electrolytes in proton exchange membrane fuel cells. A short overview with recent updates on the structural features of MOFs and COFs for proton conduction are presented here.

Defect-engineering a metal–organic framework for CO2 fixation in the synthesis of bioactive oxazolidinones

Three-component, solvent-free cycloaddition of epoxides with aromatic amines and CO₂.

Construction of Hierarchical Metal-Organic Frameworks by Competitive Coordination Strategy for Highly Efficient CO2 Conversion

Hierarchical porosity and functionalization help to fully make use of metal-organic frameworks (MOFs) for their diverse applications. Herein, a simple strategy is reported to construct hierarchically porous MOFs through a competitive coordination method using tetrafluoroborate (M(BF₄ )_x , where M is metal site) as both functional sites and etching agents. The resulting MOFs have in situ formed defect-mesopores and functional sites without sacrificing their structure stability. The formation mechanism of the defect-mesopores is elucidated by a combination of experimental and first-principles calculation method, indicating the general feasibility of this new approach. Compared with the original microporous counterparts, the new hierarchical MOFs exhibit superior adsorption for the bulky dye molecules and catalytic performance for the CO₂ conversion attributed to their specific hierarchical pore structures.

Hierarchically porous metal–organic frameworks: synthetic strategies and applications

Despite numerous advantages, applications of conventional microporous metal–organic frameworks (MOFs) are hampered by their limited pore sizes, such as in heterogeneous catalysis and guest delivery, which usually involve large molecules. Construction of hierarchically porous MOFs (HP-MOFs) is vital to achieve the controllable augmentation of MOF pore size to mesopores or even macropores, which can enhance the diffusion kinetics of guests and improve the storage capacity. This review article focuses on recent advances in the methodology of HP-MOF synthesis, covering preparation of HP-MOFs with intrinsic hierarchical pores, and modulated, templated and template-free synthetic strategies for HP-MOFs. The key factors which affect the formation of HP-MOF architectures are summarized and discussed, followed by a brief review of their applications in heterogeneous catalysis and guest encapsulation. Overall, this review presents a roadmap that will guide the future design and development of HP-MOF materials with molecular precision and mesoscopic complexity.

Dependence of Dye Molecules Adsorption Behaviors on Pore Characteristics of Mesostructured MOFs Fabricated by Surfactant Template

In this work, mesostructured metal-organic frameworks (MOFs) of MIL-101-Crs with different specific surface areas were synthesized successfully under solvothermal conditions using cationic surfactant cetyltrimethyl ammonium bromide (CTAB) as a structural template. It was found that crystallinity degrees, specific surface areas, and pore size distributions strongly depended on the loading of CTAB. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) results showed that the mean mesopore size increased with loading more CTAB due to the formation of larger templated mesopores. Although Langmuir adsorption of both methylene blue (MB) and methyl orange (MO) was confirmed in MIL-101-Crs, the experimental results showed different adsorption behaviors for them depending on the dye molecular size, pore structure, and charge properties of dye molecules/MOFs in solution. The MB molecules were found to be mainly adsorbed in the interspaces between grains and the templated mesopores, whereas the MO molecules were adsorbed in the inherent pores as well as the templated ones in MOFs due to the unsaturated metal sites' electrostatic attraction on them. Remarkably, MO adsorption capacity was observed to be proportional to the specific surface area, which allowed one to get a good linear fitting of experimental data. Interestingly, the good consistence between the fitting experimental parameter, that is, the number of adsorbed MO^-s per unit specific surface area, and the calculated one according to our rough estimation strongly suggests that MO^-s are electrostatically attracted and rotating around the unsaturated metal sites on MOFs' inner surfaces, which exclude other MO^-s to be adsorbed around due to the "hindering effect" of the rotating motion.

Crystallographic Visualization of Postsynthetic Nickel Clusters into Metal-Organic Framework

Postsynthetic metalation (PSM) has been employed as a robust method for the postsynthetic modification of metal-organic frameworks (MOFs). However, the lack of relevant information that can be obtained for the postsynthetically introduced metallic ions has hindered the development of PSM applications. Thanks to the advancement in single-crystal X-ray diffraction (SCXRD) technology, there have been a few recent examples in which successful postsynthetic introduction of single metal ions into MOFs occurred at the defined chelating sites. These works have provided useful explanations about the complicated host-guest chemistry involved in PSMs. On the other hand, there are only limited examples with crystallographic snapshots of the postsynthetic installation of metal clusters into the pores of MOFs using an ordinary SCXRD due to the loss of crystallinity of parent matrix during the PSM process. Herein, by the careful selection of starting materials and controlling the reaction conditions, we report the first crystallographic visualization of metal clusters inserted into Zr-based MOFs via PSM. The structural advantages of the parent Zr-MOF, which are inherited from the stable Zr₆ cluster and triazole-containing dicarboxylate ligand, ensure both the preservation of high crystallinity and the presence of flexible coordination sites for PSM. Furthermore, PSM of metal clusters in a MOF pore space enhances stability of the final samples while also imparting the functionality of a successful catalyst toward ethylene dimerization reaction. The related construction ideas and structural information detailed in this work can help lay the foundation for further advancements using the postmodification of MOFs as well as open new doors for the utilization of SCXRD technology in the field of MOFs.

Metal-Organic Frameworks for Food Safety

Food safety is a prevalent concern around the world. As such, detection, removal, and control of risks and hazardous substances present from harvest to consumption will always be necessary. Metal-organic frameworks (MOFs), a class of functional materials, possess unique physical and chemical properties, demonstrating promise in food safety applications. In this review, the synthesis and porosity of MOFs are first introduced by some representative examples that pertain to the field of food safety. Following that, the application of MOFs and MOF-based materials in food safety monitoring, food processing, covering preservation, sanitation, and packaging is overviewed. Future perspectives, as well as potential opportunities and challenges faced by MOFs in this field will also be discussed. This review aims to promote the development and progress of MOF chemistry and application research in the field of food safety, potentially leading to novel solutions.

Hierarchical Metal-Organic Frameworks with Macroporosity: Synthesis, Achievements, and Challenges

Introduction of multiple pore size regimes into metal-organic frameworks (MOFs) to form hierarchical porous structures can lead to improved performance of the material in various applications. In many cases, where interactions with bulky molecules are involved, enlarging the pore size of typically microporous MOF adsorbents or MOF catalysts is crucial for enhancing both mass transfer and molecular accessibility. In this review, we examine the range of synthetic strategies which have been reported thus far to prepare hierarchical MOFs or MOF composites with added macroporosity. These fabrication techniques can be either pre- or post-synthetic and include using hard or soft structural template agents, defect formation, routes involving supercritical CO2, and 3D printing. We also discuss potential applications and some of the challenges involved with current techniques, which must be addressed if any of these approaches are to be taken forward for industrial applications.

Liesegang Banding for Controlled Size and Growth of Zeolitic-Imidazolate Frameworks

Here, the formation of periodic precipitation (Liesegang bands) from hybrid (organic-inorganic) components is reported, namely ZIF-8, ZIF-67, and their mixed metal derivatives. The spacing and width laws that characterize the Liesegang system are determined and the resulting pattern is exploited to crystal engineer various sizes and doping of the ZIF material. Several key parameters that govern the crystallization process of ZIF-8 are investigated in each band and the growth of the particles in each band is found to follow an Ostwald ripening mechanism.

Novel Brönsted Acidic Ionic Liquids Confined in UiO-66 Nanocages for the Synthesis of Dihydropyrido[2,3-d]Pyrimidine Derivatives under Solvent-Free Conditions

The effective and simple one-pot, three-component protocol for the synthesis of dihydropyrido[2,3-d]pyrimidine derivatives is presented using a triethylenediamine or imidazole Brönsted acidic, ionic-liquid-supported Zr metal-organic framework (TEDA/IMIZ-BAIL@UiO-66) as a green, novel, and retrievable heterogeneous catalyst under mild conditions. The multicomponent reactions of 6-amino-1,3-dimethyl uracil, various aromatic aldehydes, and acetyl acetone were conducted under solvent-free conditions so that dihydropyrido[2,3-d]pyrimidine derivatives can be obtained. It is possible to separate and purify the respective products easily using crystallization. We can recycle the catalysts six times without losing any major activity. Also, the characterization of the catalyst was done by energy-dispersive X-ray, field emission scanning electron microscopy, Fourier transform infrared, Brunauer-Emmett-Teller, X-ray diffraction, and thermogravimetric analysis analyses.

Formation of trigons in a metal-organic framework: The role of metal-organic polyhedron subunits as meta-atoms

Shape control of metal-organic materials on the meso- and macroscale has been an important theme due to emerging properties. Particularly, chemical etching has been useful to create various forms such as core-shells and hollow crystals in metal-organic frameworks. Here we present a unique chemical etching strategy to create trigonal patterned surfaces in metal-organic frameworks. The mechanism suggests that metal-organic polyhedron subunits serve as meta-atoms, playing a crucial role in the formation of trigons on the surface. Such a patterned surface in porous solids can be utilized in meta-surface applications in the foreseeable future.

Incorporating Heavy Alkanes in Metal-Organic Frameworks for Optimizing Adsorbed Natural Gas Capacity

Metal-organic frameworks (MOFs) as methane adsorbents are highly promising materials for applications such as methane-powered vehicles, flare gas capture, and field natural gas separation. Pre- and post-synthetic modification of MOFs have been known to help improve both the overall methane uptake as well as the working capacity. Here, a post-synthetic modification strategy to non-covalently modify MOF adsorbents for the enhancement of the natural gas uptake for the MOF material is introduced. In this study, PCN-250 adsorbents were doped with C₁₀ alkane and C₁₄ fatty acid and their impact on the methane uptake capabilities was investigated. It was found that even trace amounts of heavy hydrocarbons could considerably enhance the raw methane uptake of the MOF while still being regenerable. The doped hydrocarbons are presumably located at the mesoporous defects of PCN-250, thus optimizing the framework-methane interactions. These findings reveal a general approach that can be used to modify the MOF absorbents, improving their ability to be sustainable and renewable natural gas adsorption platforms.

Electrodeposited metal-organic framework films as self-assembled hierarchically superstructured supports for stable omniphobic surface coatings

Superhierarchically rough films are rapidly synthesised on metal substrates via electrochemically triggered self-assembly of meso/macroporous-structured metal-organic framework (MOF) crystals. These coatings are applied to immobilise a functional oil with low surface energy to provide stable coatings repellent to a wide range of hydrophobic as well as hydrophilic fluids. Such omniphobic surfaces are highly interesting for several applications such as anti-fouling, anti-icing, and dropwise condensation, and become easily scalable with the presented bottom-up fabrication approach. As investigated by environmental scanning electron microscopy (ESEM), the presented perfluorinated oil-infused Cu-BTC coating constitutes of a flat liquid-covered surface with protruding edges of octahedral superstructured MOF crystals. Water and non-polar diiodomethane droplets form considerably high contact angles and even low-surface-tension fluids, e.g. acetone, form droplets on the infused coating. The repellent properties towards the test fluids do not change upon extended water spraying in contrast to oil-infused porous copper oxide or native copper surfaces. It is discussed in detail, how the presented electrodeposited MOF films grow and provide a proficient surface morphology to stabilise the functional oil film due to hemiwicking.

Rational Design of Mimic Multienzyme Systems in Hierarchically Porous Biomimetic Metal-Organic Frameworks

A facile approach was reported to establish mimic multienzyme systems with hierarchically porous (HP) biomimetic metal-organic frameworks (MOFs) and natural enzymes for tandem catalysis. The hierarchically porous MOF HP-PCN-224(Fe) with peroxidase-like activity and tunable hierarchical porosity was synthesized via a modulator-induced strategy. HP-PCN-224(Fe) not only acts as the enzyme-immobilization matrix but also as an effective enzyme mimic, which could cooperate with the immobilized natural enzyme to catalyze the cascade reactions. The mimic multienzyme systems were used for the efficient colorimetric detection of a series of biomolecules, including glucose and uric acid. This work displays the great potential to construct highly functional biocatalysts by integrating the merits of both natural enzymes and MOF mimics, which are promising for applications in biosensing and biomimetic catalysis.

Mesoporous Metal-Organic Frameworks: Synthetic Strategies and Emerging Applications

Metal-organic frameworks (MOFs) have attracted much attention over the past two decades due to their highly promising applications not only in the fields of gas storage, separation, catalysis, drug delivery, and sensors, but also in relatively new fields such as electric, magnetic, and optical materials resulting from their extremely high surface areas, open channels and large pore cavities compared with traditional porous materials like carbon and inorganic zeolites. Particularly, MOFs involving pores within the mesoscopic scale possess unique textural properties, leading to a series of research in the design and applications of mesoporous MOFs. Unlike previous Reviews, apart from focusing on recent advances in the synthetic routes, unique characteristics and applications of mesoporous MOFs, this Review also mentions the derivatives, composites, and hierarchical MOF-based systems that contain mesoporosity, and technical boundaries and challenges brought by the drawbacks of mesoporosity. Moreover, this Review subsequently reveals promising perspectives of how recently discovered approaches to different morphologies of MOFs (not necessarily entirely mesoporous) and their corresponding performances can be extended to minimize the shortcomings of mesoporosity, thus providing a wider and brighter scope of future research into mesoporous MOFs, but not just limited to the finite progress in the target substances alone.

Controllable formation of meso- and macropores within metal-organic framework crystals via a citric acid modulator

Citric acid (CA) is introduced to modulate the synthesis of octahedral HKUST-1 (also known as Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate) particles with hierarchical micro, meso- and/or macropores (noted as HP-HKUST-1 particles) under mild conditions. The particle size of the resultant product could be tuned from 300 nm to 2.4 μm by varying the amount of CA added, solvents, reaction time and so on. The intrinsic microporous structure of HKUST-1 is maintained despite the strong chelation capability of CA towards Cu2+ while the larger pores' size is in the range of 30 to 100 nm. The possible formation mechanism of these octahedral HP-HKUST-1 particles is proposed on the basis of our systematic investigations. The resulting micrometer HP-HKUST-1 particle is employed to encapsulate phosphomolybdic acid hydrate (HPMo), a catalyst for the methanolysis of styrene oxide. After encapsulation, a superior catalytic activity with a high conversion rate (99.6%) is achieved when the reaction is carried out at 40 °C for 20 min.

PVBA-UiO-66 using a flexible PVBA with multi-coordination groups as mixed ligands and their super adsorption towards methylene blue

A series of poly-vinyl benzoic acid (PVBA) and UiO-66 materials (PVBA-UiO-66) were prepared by a mixed-linker approach. Using a flexible PVBA with multi-coordination groups as mixed ligands, mesopores and uncoordinated benzoic groups were introduced into the UiO-66 crystal structures, thus leading to a special structure and functionality. The structure of the PVBA-UiO-66 was characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and nitrogen gas adsorption measurements. The adsorption property toward methylene blue (MB) of PVBA-UiO-66 was studied. The adsorption of MB by UiO-66 and PVBA (43%)-UiO-66 fit the Langmuir model and Freundlich model well. The result showed that PVBA (43%)-UiO-66 has super adsorption capacity as high as 909 mg g-1 for MB owing to the mesopores and the uncomplexed carboxyl groups which were caused by the long PVBA involved in coordination. The adsorption kinetics of MB onto PVBA (43%)-UiO-66 can be well fitted to the pseudo-second-order model. The process of adsorption MB onto PVBA-UiO-66 is spontaneous and thermodynamically favorable at 303, 313 and 323 K.

Impact of Higher-Order Structuralization on the Adsorptive Properties of Metal-Organic Frameworks

The structural processing of metal-organic frameworks (MOFs) over multiple length scales is critical for their successful use as adsorbents in a variety of emerging applications. Although significant advances in molecular-scale design have provided strategies to boost the adsorptive capacities of MOFs, relatively little attention has been directed toward understanding the influence of higher-order structuralization on the material performance. Herein, we present the main strategies that are currently available for the structural processing of MOFs and discuss the influence these processes can impart on the adsorptive properties of the materials. In all, this intriguing area of research is expected to provide significant opportunities to enhance the properties of MOFs further, which will ultimately aid in their optimization in the context of specific real-world applications.

Stable Metal-Organic Frameworks with Group 4 Metals: Current Status and Trends

Group 4 metal-based metal-organic frameworks (M^IV-MOFs), including Ti-, Zr-, and Hf-based MOFs, are one of the most attractive classes of MOF materials owing to their superior chemical stability and structural tunability. Despite being a relatively new field, M^IV-MOFs have attracted significant research attention in the past few years, leading to exciting advances in syntheses and applications. In this outlook, we start with a brief overview of the history and current status of M^IV-MOFs, emphasizing the challenges encountered in their syntheses. The unique properties of M^IV-MOFs are discussed, including their high chemical stability and strong tolerance toward defects. Particular emphasis is placed on defect engineering in Zr-MOFs which offers additional routes to tailor their functions. Photocatalysis of M^IV-MOF is introduced as a representative example of their emerging applications. Finally, we conclude with the perspective of new opportunities in synthesis and defect engineering.

Electroactive Ferrocene at or near the Surface of Metal-Organic Framework UiO-66

Here, we describe the installation of a ferrocene derivative on and within the archetypal metal-organic framework (MOF), UiO-66, by solvent-assisted ligand incorporation. Thin films of the resulting material show a redox peak characteristic of the Fc/Fc⁺ couple, as measured by cyclic voltammetry. Consistent with restriction of redox reactivity solely to Fc molecules sited at or near the external surfaces of MOF crystallites, chronoamperometry measurements indicate that less than 20% of the installed Fc molecules are electrochemically active. Charge-transport diffusion coefficients, D_CT, of 6.1 ± 0.8 × 10^-11 and 2.6 ± 0.2 × 10^-9 cm²/s were determined from potential step measurements, stepping oxidatively and reductively, respectively. The 40-fold difference in D_CT values contrasts with the expectation, for simple systems, of identical values for oxidation-driven versus reduction-driven charge transport. The findings have implications for the design of MOFs suitable for delivery of redox equivalents to framework-immobilized electrocatalysts and/or delivery of charges from a chromophoric MOF film to an underlying electrode, processes that may be central to MOF-facilitated conversion of solar energy to chemical or electrical energy.

A facile strategy for enzyme immobilization with highly stable hierarchically porous metal-organic frameworks

Metal-organic frameworks (MOFs) have drawn extensive research interest as candidates for enzyme immobilization owing to their tunable porosity, high surface area, and excellent chemical/thermal stability. Herein, we report a facile and universal strategy for enzyme immobilization using highly stable hierarchically porous metal-organic frameworks (HP-MOFs). The HP-MOFs were stable over a wide pH range (pH = 2-11 for HP-DUT-5) and met the catalysis conditions of most enzymes. The as-prepared hierarchical micro/mesoporous MOFs with mesoporous defects showed a superior adsorption capacity towards enzymes. The maximum adsorption capacity of HP-DUT-5 for glucose oxidase (GOx) and uricase was 208 mg g^-1 and 225 mg g^-1, respectively. Furthermore, we constructed two multi-enzyme biosensors for glucose and uric acid (UA) by immobilizing GOx and uricase with horseradish peroxidase (HRP) on HP-DUT-5, respectively. These sensors were efficiently applied in the colorimetric detection of glucose and UA and showed good sensitivity, selectivity, and recyclability.

Compositional nanodomain formation in hybrid formate perovskites

We report the synthesis and structural characterisation of three mixed-metal formate perovskite families [C(NH₂)₃]Cu_xM_1-x(HCOO)₃ (M = Mn, Zn, Mg). Using a combination of infrared spectroscopy, non-negative matrix factorization, and reverse Monte Carlo refinement, we show that the Mn- and Zn-containing compounds support compositional nanodomains resembling the polar nanoregions of conventional relaxor ferroelectrics. The M = Mg family exhibits a miscibility gap that we suggest reflects the limiting behaviour of nanodomain formation.

Construction of hierarchically porous metal-organic frameworks through linker labilization

A major goal of metal–organic framework (MOF) research is the expansion of pore size and volume. Although many approaches have been attempted to increase the pore size of MOF materials, it is still a challenge to construct MOFs with precisely customized pore apertures for specific applications. Herein, we present a new method, namely linker labilization, to increase the MOF porosity and pore size, giving rise to hierarchical-pore architectures. Microporous MOFs with robust metal nodes and pro-labile linkers were initially synthesized. The mesopores were subsequently created as crystal defects through the splitting of a pro-labile-linker and the removal of the linker fragments by acid treatment. We demonstrate that linker labilization method can create controllable hierarchical porous structures in stable MOFs, which facilitates the diffusion and adsorption process of guest molecules to improve the performances of MOFs in adsorption and catalysis.

Expanding pore sizes and volumes in metal-organic frameworks is challenging, but crucial for the encapsulation of larger guest molecules. Here, Zhou and colleagues report a linker labilization strategy to construct MOFs containing hierarchical pore architectures with dimensions ranging from 1.5 to 18 nm.

Curved crystal morphology, photoreactivity and photosalient behaviour of mononuclear Zn(II) complexes

A dramatic effect of crystal morphology, photoreactivity and photosalient property is observed in a zinc(II) complex due to solvent effects and fluorine substitution at the backbone of the ligand. Of the two crystal forms with a 3-fluoro derivative, one yielded a curved morphology of single crystals and the second form shows photoreactivity in the solid state, whereas crystals of the 2-fluoro derivative pop during the [2 + 2] photocycloaddition reaction. This is the first report documenting curved single crystals of metal complexes obtained naturally during crystallization, although such bent crystals have been observed in extended solids naturally, or bent by mechanical force or by UV irradiation.

Template synthesis of hierarchical porous metal–organic frameworks with tunable porosity

A versatile method to synthesize various stable hierarchical porous metal–organic frameworks, such as Cu–BTC and ZIF–8, by using an organic amine as the template.

Hierarchically porous materials: synthesis strategies and structure design

This review addresses recent advances in synthesis strategies of hierarchically porous materials and their structural design from micro-, meso- to macro-length scale.

Regulation of NO Uptake in Flexible Ru Dimer Chain Compounds with Highly Electron Donating Dopants

On-demand design of porous frameworks for selective capture of specific gas molecules, including toxic gas molecules such as nitric oxide (NO), is a very important theme in the research field of molecular porous materials. Herein, we report the achievement of highly selective NO adsorption through chemical doping in a framework (i.e., solid solution approach): the highly electron donating unit [Ru₂(o-OMePhCO₂)₄] (o-OMePhCO₂^- = o-anisate) was transplanted into the structurally flexible chain framework [Ru₂(4-Cl-2-OMePhCO₂)₄(phz)] (0; 4-Cl-2-OMePhCO₂^- = 4-chloro-o-anisate and phz = phenazine) to obtain a series of doped compounds, [{Ru₂(4-Cl-2-OMePhCO₂)₄}_1-x{Ru₂(o-OMePhCO₂)₄}_x(phz)] (x = 0.34, 0.44, 0.52, 0.70, 0.81, 0.87), with [Ru₂(o-OMePhCO₂)₄(phz)] (1) as x = 1. The original compound 1 was made purely from a "highly electron donating unit" but had no adsorption capability for gases because of its nonporosity. Meanwhile, the partial transplant of the electronically advantageous [Ru₂(o-OMePhCO₂)₄] unit with x = 0.34-0.52 in 0 successfully enhanced the selective adsorption capability of NO in an identical structurally flexible framework; an uptake at 95 kPa that was 1.7-3 mol/[Ru₂] unit higher than that of the original 0 compound was achieved (121 K). The solid solution approach is an efficient means of designing purposeful porous frameworks.