Defect Termination in the UiO-66 Family of Metal-Organic Frameworks: The Role of Water and Modulator

Morphology Regulation of UiO-66-2I Supporting Systematic Investigations of Shape-Dependent Catalytic Activity for Degradation of an Organophosphate Nerve Agent Simulant

Phosphonate-based nerve agents, as a kind of deadly chemical warfare agent, are a persistent and evolving threat to humanity. Zirconium-based metal-organic frameworks (Zr-MOFs) are a kind of highly porous crystalline material that includes Zr-OH-Zr sites and imitates the active sites of the phosphotriesterase enzyme, representing significant potential for the adsorption and catalytic hydrolysis of phosphonate-based nerve agents. In this work, we present a new Zr-MOF, UiO-66-2I, which attaches two iodine atoms in the micropore of the MOF and exhibits excellent catalytic activity on the degradation of a nerve agent simulant, dimethyl 4-nitrophenyl phosphate (DMNP), as the result of the formation of halogen bonds between the phosphate ester bonds and iodine groups. Furthermore, various morphologies of UiO-66-2I, such as blocky-shaped nanoparticles (NPs), two-dimensional (2D) nanosheets, hexahedral NPs, stick-like NPs, colloidal microspheres, and colloidal NPs, have been obtained by adding acetic acid (AA), formic acid (FA), propionic acid (PA), valeric acid (VA), benzoic acid (BA), and trifluoroacetic acid (TFA) as modulators, respectively, and show different catalytic hydrolysis activities. Specifically, the catalytic activities follow the trend UiO-66-2I-FA (t1/2 = 1 min) > UiO-66-2I-AA-NP (t1/2 = 4 min) ≈ UiO-66-2I-VA (t1/2 = 4 min) > UiO-66-2I-BA (t1/2 = 5 min) > UiO-66-2I-PA (t1/2 = 15 min) > UiO-66-2I-TFA (t1/2 = 18 min). The experimental results show that the catalytic hydrolysis activity of Zr-MOF is regulated by the crystallinity, defect quantity, morphologies, and hydrophilicity of these samples, which synergistically affect the accessibility of catalytic sites and the diffusion of phosphate in the pores of Zr-MOFs.

Rare-earth acetates as alternative precursors for rare-earth cluster-based metal-organic frameworks

RE-UiO-66 analogues are synthesized using RE acetates as precursors for the first time. These MOFs are fully characterized and the influence of the precursor on the materials obtained is studied. Additionally, the influence of water on the yield of the syntheses and the quality of the materials is explored.

Improving the removal efficiency of oxygenated volatile organic compounds by defective UiO-66 regulated with water

The purification of oxygenated volatile organic compounds VOCs (OVOCs), important precursors of ozone and particulate matters, has triggered intensive research interests. UiO-66 with high photocatalytic activity have shown great potential. However, the lack of active sites severely limited the trapping and degradation of OVOCs. Herein, reo-UiO-66 with increased specific surface area, hierarchical porous structure and tunable acidic/basic sites was synthesized by simply adding water as the modulator. XRD, TGA and FTIR results confirmed the formation of reo-defects, which significantly affected the surface hydrophilicity and active sites of UiO-66. The adsorption of Lewis acidic acetaldehyde was enhanced by 265 times with coordinative unsaturated Zr acting as the dominant adsorption sites. The degradation efficiency of typical OVOCs (acetaldehyde and acetone) increased from 0% and 25% to 50% and 73%, respectively. This work provided a facile method to modulate the micro-environment in MOFs for the efficient capture and catalytic purification of OVOCs.

Molecular Insights into Sequence-Specific Protein Hydrolysis by a Soluble Zirconium-Oxo Cluster Catalyst

The development of catalysts for controlled fragmentation of proteins is a critical undertaking in modern proteomics and biotechnology. {Zr6O8}-based metal-organic frameworks (MOFs) have emerged as promising candidates for catalysis of peptide bond hydrolysis due to their high reactivity, stability, and recyclability. However, emerging evidence suggests that protein hydrolysis mainly occurs on the MOF surface, thereby questioning the need for their highly porous 3D nature. In this work, we show that the discrete and water-soluble [Zr6O4(OH)4(CH3CO2)8(H2O)2Cl3]+ (Zr6) metal-oxo cluster (MOC), which is based on the same hexamer motif found in various {Zr6O8}-based MOFs, shows excellent activity toward selective hydrolysis of equine skeletal muscle myoglobin. Compared to related Zr-MOFs, Zr6 exhibits superior reactivity, with near-complete protein hydrolysis after 24 h of incubation at 60 °C, producing seven selective fragments with a molecular weight in the range of 3-15 kDa, which are of ideal size for middle-down proteomics. The high solubility and molecular nature of Zr6 allow detailed solution-based mechanistic/interaction studies, which revealed that cluster-induced protein unfolding is a key step that facilitates hydrolysis. A combination of multinuclear nuclear magnetic resonance spectroscopy and pair distribution function analysis provided insight into the speciation of Zr6 and the ligand exchange processes occurring on the surface of the cluster, which results in the dimerization of two Zr6 clusters via bridging oxygen atoms. Considering the relevance of discrete Zr-oxo clusters as building blocks of MOFs, the molecular-level understanding reported in this work contributes to the further development of novel catalysts based on Zr-MOFs.

Designed Synthesis of Fe/Zr Bimetallic Organic Framework to Enhance the Selective Conversion of H2S to Sulfur

The development of stable and effective catalysts to convert toxic H2S into high value-added sulfur is essential for production safety and environmental protection. However, the inherent defects of traditional iron- and zirconium-based catalysts, such as poor activity, high oxygen consumption, and low sulfur selectivity, limit their further developments and applications. Herein, the Fe-Zr bimetallic organic framework FeUIO-66(x) with different cubic morphologies was synthesized via a facile solvothermal method. The results indicate that the introduction of Fe not only increases the specific surface area and weak L-sites of the catalyst without changing its crystal structure, which provides enough reaction space and more active sites for the adsorption and activation of H2S, but also reduces the activation energy of the reaction, significantly promoting the selective oxidation of H2S. As a result, the as-obtained FeUIO-66(1) catalyst exhibits the highest desulfurization activity and superior durability and water resistance stability, and its H2S conversion and sulfur selectivity within 50 h are 100 and 88%, respectively. More importantly, the structure of the catalyst after the desulfurization reaction is consistent with that of the fresh counterpart. The study offers new insights into the development of effective and stable bimetallic catalysts to eliminate H2S and recycle sulfur.

Regulating luminescence thermal enhancement in negative thermal expansion metal-organic frameworks

Overcoming thermal quenching is generally essential for the practical application of luminescent materials. It has been recently found that frameworks with negative thermal expansion (NTE) could be a promising candidate to engineer unconventional luminescence thermal enhancement. However, the mechanism through which luminescence thermal enhancement can be well tuned remains an open issue. In this work, enabled by altering ligands in a series of UiO-66 derived Eu-based metal-organic frameworks, it was revealed that the changes in the thermal expansion are closely related to luminescence thermal enhancement. The NTE of the aromatic ring part favors luminescence thermal enhancement, while contraction of the carboxylic acid part plays the opposite role. Modulation of functional groups in ligands can change the thermal vibration of aromatic rings and then achieve luminescence thermal enhancement in a wide temperature window. Our findings pave the way to manipulate the NTE and luminescence thermal enhancement based on ligand engineering.

Disorder and Sorption Preferences in a Highly Stable Fluoride-Containing Rare-Earth fcu-Type Metal-Organic Framework

Rare-earth (RE) metal-organic frameworks (MOFs) synthesized in the presence of fluorine-donating modulators or linkers are an important new subset of functional MOFs. However, the exact nature of the REaXb core of the molecular building block (MBB) of the MOF, where X is a μ2 or 3-bridging group, remains unclear. Investigation of one of the archetypal members of this family with the stable fcu framework topology, Y-fum-fcu-MOF (1), using a combination of experimental techniques, including high-field (20 T) solid-state nuclear magnetic resonance spectroscopy, has determined two sources of framework disorder involving the μ3-X face-capping group of the MBB and the fumarate (fum) linker. The core of the MBB of 1 is shown to contain a mixture of μ3-F- and (OH)- groups with preferential occupation at the crystallographically different face-capping sites that result in different internally lined framework tetrahedral cages. The fum linker is also found to display a disordered arrangement involving bridging- or chelating-bridging bis-bidentate modes over the fum linker positions without influencing the MBB orientation. This linker disorder will, upon activation, result in the creation of Y3+ ions with potentially one or two additional uncoordinated sites possessing differing degrees of Lewis acidity. Crystallographically determined host-guest relationships for simple sorbates demonstrate the favored sorption sites for N2, CO2, and CS2 molecules that reflect the chemical nature of both the framework and the sorbate species with the structural partitioning of the μ3-groups apparent in determining the favored sorption site of CS2. The two types of disorder found within 1 demonstrate the complexity of fluoride-containing RE-MOFs and highlight the possibility to tune this and other frameworks to contain different proportions and segregations of μ3-face-capping groups and degrees of linker disorder for specifically tailored applications.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired µ2 -OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated µ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Tuning the Fluorometric Sensing of Phosphate on UiO-66-NH2(Zr, Ce, Hf) Metal Nodes

Metal-organic frameworks (MOFs) with intrinsic luminescent properties, modular structure, and tunable electronic properties, provide unique opportunities for designing target-specific molecular sensors by systematically choosing their constituent building blocks. We report a simple one-step MOF-based sensing platform for phosphate (P) detection that combines the luminescent properties of 2-aminoterephthalic acid (ATA) with the affinity of rationally selected nodes in UiO-66-NH2 to bind with P. This MOF possesses an electron-donating amine group that controls the light-harvesting characteristics of the linkers. Substituting Zr6 node with Ce6 or Hf6 results in a series of isostructural MOFs with distinct optical properties that are nonexistent in the unsubstituted MOF. We have utilized these MOFs to quantitatively measure P, using its ability to bind strongly to metal nodes inhibiting the LMCT process and altering the linker's photon emission. Using this system, detection limits of 4.5, 7.2 and 10.5 μM were obtained for the UiO-66-NH2(Ce), UiO-66-NH2, and UiO-66-NH2(Hf) respectively, adopting a straightforward single step procedure. These results demonstrate that the selection of metal nodes in a series of isostructural MOFs can be used to modulate their electronic properties and create sensing probes possessing the desired characteristics needed for the detection of environmental contaminants.

Modulated Ultrathin NiCo-LDH Nanosheet-Decorated Zr3+-Rich Defective NH2-UiO-66 Nanostructure for Efficient Photocatalytic Hydrogen Evolution

Defect engineering through modification of their surface linkage is found to be an effective pathway to escalate the solar energy conversion efficiency of metal-organic frameworks (MOFs). Herein, defect engineering using controlled decarboxylation on the NH2-UiO-66 surface and integration of ultrathin NiCo-LDH nanosheets synergizes the hydrogen evolution reaction (HER) under a broad visible light regime. Diversified analytical methods including positron annihilation lifetime spectroscopy were employed to investigate the role of Zr3+-rich defects by analyzing the annihilation characteristics of positrons in NH2-UiO-66, which provides a deep insight into the effects of structural defects on the electronic properties. The progressively tuned photophysical properties of the NiCo-LDH@NH2-UiO-66-D-heterostructured nanocatalyst led to an impressive rate of HER (∼2458 μmol h-1 g-1), with an apparent quantum yield of ∼6.02%. The ultrathin NiCo-LDH nanosheet structure was found to be highly favored toward electrostatic self-assembly in the heterostructure for efficient charge separation. Coordination of Zr3+ on the surface of the NiCo-LDH nanosheet support through NH2-UiO-66 was confirmed by X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy techniques. Femtosecond transient absorption spectroscopy studies unveiled a photoexcited charge migration process from MOF to NiCo-LDH which favorably occurred on a picosecond time scale to boost the catalytic activity of the composite system. Furthermore, the experimental finding and HER activity are validated by density functional theory studies and evaluation of the free energy pathway which reveals the strong hydrogen binding over the surface and infers the anchoring effect of the ultrathin layered double hydroxide (LDH) in the vicinity of the Zr cluster with a strong host-guest interaction. This work provided a novel insight into efficient photocatalysis via defect engineering at the linker modulation in MOFs.

Controllable Synthesis of Defective UiO-66 for Efficient Degradation and Detection of Ozone

Metal-organic framework (MOF) structures have gained significant attention for their exceptional catalytic performance in ozone degradation, even under high humidity conditions, which is attributed to the presence of unsaturated metal sites (MOF defects). However, the correlation between MOF defects and catalytic ozone remains ambiguous, and a general approach for the controllable synthesis of high-performance MOF structures is currently lacking. Herein, different defective UiO-66 materials with cluster or ligand defects were obtained by precisely controlling small molecular acid modulators. Their catalytic performance can be analyzed in real time through the specific cataluminescence (CTL) signal of ozone at the interface. The presence of ligand defects was found to be crucial for both catalytic degradation and luminescence of ozone, and the CTL signal exhibited a positive correlation with the endogenous hydroxyl group content in the material (R2 = 0.982), while external humidity further supplemented internal water molecules within the material. Furthermore, theoretical calculations were conducted to compare the adsorption behaviors of ozone on the defective UiO-66 under dry/wet conditions, leading to the proposal of two potential reaction pathways. Subsequently, UiO-66-DA with superior catalytic performance was employed to develop a highly efficient CTL sensor capable of accurately detecting ozone (LOD = 23.3 ppb). This study held significant value in elucidating the reaction site of ozone on MOFs and achieving optimal catalytic effects through the careful selection of modulators and humidity levels.

Creating Order in Ultrastable Phosphonate Metal-Organic Frameworks via Isolable Hydrogen-Bonded Intermediates

The stability presented by trivalent metal-organic frameworks (MOFs) makes them an attractive class of materials. With phosphonate-based ligands, crystallization is a challenge, as there are significantly more binding motifs that can be adopted due to the extra oxygen tether compared to carboxylate counterparts and the self-assembly processes are less reversible. Despite this, we have reported charge-assisted hydrogen-bonded metal-organic frameworks (HMOFs) consisting of [Cr(H2O)6]3+ and phosphonate ligands, which were crystallographically characterized. We sought to use these HMOFs as a crystalline intermediate to synthesize ordered Cr(III)-phosphonate MOFs. This can be done by dehydrating the HMOF to remove the aquo ligands around the Cr(III) center, forcing metal-phosphonate coordination. Herein, a new porous HMOF, H-CALF-50, is synthesized and then dehydrated to yield the MOF CALF-50. CALF-50 is ordered, although it is not single crystalline. It does, however, have exceptional stability, maintaining crystallinity and surface area after boiling in water for 3 weeks and soaking in 14.5 M H3PO4 for 24 h and 9 M HCl for 72 h. Computational methods are used to study the HMOF to MOF transformation and give insight into the nature of the structure and the degree of heterogeneity.

A nanoscale MOF-based heterogeneous catalytic system for the polymerization of N-carboxyanhydrides enables direct routes toward both polypeptides and related hybrid materials

Synthetic polypeptides have emerged as versatile tools in both materials science and biomedical engineering due to their tunable properties and biodegradability. While the advancements of N-carboxyanhydride (NCA) ring-opening polymerization (ROP) techniques have aimed to expedite polymerization and reduce environment sensitivity, the broader implications of such methods remain underexplored, and the integration of ROP products with other materials remains a challenge. Here, we show an approach inspired by the success of many heterogeneous catalysts, using nanoscale metal-organic frameworks (MOFs) as co-catalysts for NCA-ROP accelerated also by peptide helices in proximity. This heterogeneous approach offers multiple advantages, including fast kinetics, low environment sensitivity, catalyst recyclability, and seamless integration with hybrid materials preparation. The catalytic system not only streamlines the preparation of polypeptides and polypeptide-coated MOF complexes (MOF@polypeptide hybrids) but also preserves and enhances their homogeneity, processibility, and overall functionalities inherited from the constituting MOFs and polypeptides.

Coordination Defect-Induced Frustrated Lewis Pairs in Polyoxo-metalate-Based Metal-Organic Frameworks for Efficient Catalytic Hydrogenation

Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)-based metal-organic framework (MOF) by introducing coordination-defect metal nodes (LA) and surface-basic POM with abundant oxygen (LB). The well-defined and unique spatial conformation of the defective POM-based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H2 . This FLP catalyst can heterolytically dissociate H2 into active Hδ- , thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect-free POM-based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi-site catalyst to achieve specific activation of target substrate for synergistic catalysis.

Tailoring the Nanoporosity and Photoactivity of Metal-Organic Frameworks With Rigid Dye Modulators for Toluene Purification

Facile synthesis of hierarchically porous metal-organic frameworks (MOFs) with adjustable porosity and high crystallinity attracts great attention yet remains challenging. Herein, a micromolar amount of dye-based modulator (Rhodamine B (RhB)) is employed to easily and controllably tailor the pore size of a Ti-based metal-organic framework (MIL-125-NH2 ). The RhB used in this method is easily removed by washing or photodegradation, avoiding secondary posttreatment. It is demonstrated that the carboxyl functional group and the steric effects of RhB are indispensable for enlarging the pore size of the MIL-125-NH2 . The resulting hierarchically porous MIL-125-NH2 (RH-MIL-125-NH2 ) exhibits optimized adsorption and photocatalytic activity because the newly formed mesopore with defects concurrently facilitates mass transport of guest molecules (toluene) and photogenerated charge separation. This work offers a meaningful basis for the construction of hierarchically porous MOFs and demonstrates the superiority of the hierarchical pore structure for adsorption and heterogeneous catalysis.

Efficient Generation of Large Collections of Metal-Organic Framework Structures Containing Well-Defined Point Defects

High-throughput molecular simulations of metal-organic frameworks (MOFs) are a useful complement to experiments to identify candidates for chemical separation and storage. All previous efforts of this kind have used simulations in which MOFs are approximated as defect-free. We introduce a tool to readily generate missing-linker defects in MOFs and demonstrate this tool with a collection of 507 defective MOFs. We introduce the concept of the maximum possible defect concentration; at higher defect concentrations, deviations from the defect-free crystal structure would be readily evident experimentally. We studied the impact of defects on molecular adsorption as a function of defect concentrations. Defects have a slightly negative or negligible influence on adsorption at low pressures for ethene, ethane, and CO₂ but a strong positive influence for methanol due to hydrogen bonding with defects. Defective structures tend to have loadings slightly higher than those of defect-free structures for all adsorbates at elevated pressures.

Synergy of Paired Brønsted-Lewis Acid Sites on Defects of Zr-MIL-140A for Methanol Dehydration

As a common defect-capping ligand in metal-organic frameworks (MOFs), the hydroxyl group normally exhibits Brønsted acidity or basicity, but the presence of inherent hydroxyl groups in the MOF structure makes it a great challenge to identify the exact role of defect-capping hydroxyl groups in catalysis. Herein, we used hydroxyl-free MIL-140A as the platform to generate terminal hydroxyl groups on defect sites via a continuous post-synthetic treatment. The structure and acidity of MIL-140A were properly characterized. The hydroxyl-contained MIL-140A-OH exhibited 4.6-fold higher activity than the pristine MIL-140A in methanol dehydration. Spectroscopic and computational investigations demonstrated that the reaction was initiated by the respective adsorption of two methanol molecules on the terminal-OH and the adjacent Zr vacancy. The dehydration of the adsorbed methanol molecules then occurred in the Brønsted-Lewis acid site co-participated associative pathway with the lowest energy barrier.

Solvent-dependent strategy to construct mesoporous Zr-based metal-organic frameworks for high-efficient adsorption of tetracycline

The accumulated antibiotics in the aquatic environment pose great threat to human and ecological health, boosting the development of porous materials for antibiotic removal. Mesoporous metal-organic frameworks (MOFs) have shown great promise in adsorption, which, however, usually need supramolecular design or cooperative template strategy for synthesis. Here we report the successful construction of mesoporous zirconium based metal-organic frameworks (Zr-MOFs) via a simple solvent-dependent strategy. Regulation of the ratio of water to N, N-dimethylacetamide during synthesis determined the porous structure of the synthesized MOFs. Systematic characterizations including SEM, FTIR, XRD and nitrogen sorption isotherm were carried out for structure analysis of the MOFs. With water fraction of 20% (v/v), the obtained Zr-MOF exhibited the highest adsorption capacity (Q_max of 337.0 mg⋅g^-1) towards tetracycline (TC). The adsorption kinetics fitted the pseudo-second-order kinetics, and the adsorption isotherms fitted the Freundlich model well. Adsorption mechanism investigation revealed that the abundant Zr-OH groups stemming from coordination defects mainly accounted for TC adsorption. The hydrogen bonding interaction between TC and Zr-MOF and the generated mesopores contributed to the satisfactory adsorption capacity. This work is anticipated to provide insights on facile synthesis of mesoporous MOFs and application in environmental remediation.

Unveiling Unexpected Modulator-CO2 Dynamics within a Zirconium Metal-Organic Framework

Carbon capture, storage, and utilization (CCSU) represents an opportunity to mitigate carbon emissions that drive global anthropogenic climate change. Promising materials for CCSU through gas adsorption have been developed by leveraging the porosity, stability, and tunability of extended crystalline coordination polymers called metal-organic frameworks (MOFs). While the development of these frameworks has yielded highly effective CO₂ sorbents, an in-depth understanding of the properties of MOF pores that lead to the most efficient uptake during sorption would benefit the rational design of more efficient CCSU materials. Though previous investigations of gas-pore interactions often assumed that the internal pore environment was static, discovery of more dynamic behavior represents an opportunity for precise sorbent engineering. Herein, we report a multifaceted in situ analysis following the adsorption of CO₂ in MOF-808 variants with different capping agents (formate, acetate, and trifluoroacetate: FA, AA, and TFA, respectively). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis paired with multivariate analysis tools and in situ powder X-ray diffraction revealed unexpected CO₂ interactions at the node associated with dynamic behavior of node-capping modulators in the pores of MOF-808, which had previously been assumed to be static. MOF-808-TFA displays two binding modes, resulting in higher binding affinity for CO₂. Computational analyses further support these dynamic observations. The beneficial role of these structural dynamics could play an essential role in building a deeper understanding of CO₂ binding in MOFs.

Solvent-derived defects suppress adsorption in MOF-74

Defects in metal-organic frameworks (MOFs) have great impact on their nano-scale structure and physiochemical properties. However, isolated defects are easily concealed when the frameworks are interrogated by typical characterization methods. In this work, we unveil the presence of solvent-derived formate defects in MOF-74, an important class of MOFs with open metal sites. With multi-dimensional solid-state nuclear magnetic resonance (NMR) investigations, we uncover the ligand substitution role of formate and its chemical origin from decomposed N,N-dimethylformamide (DMF) solvent. The placement and coordination structure of formate defects are determined by ¹³C NMR and density functional theory (DFT) calculations. The extra metal-oxygen bonds with formates partially eliminate open metal sites and lead to a quantitative decrease of N₂ and CO₂ adsorption with respect to the defect concentration. In-situ NMR analysis and molecular simulations of CO₂ dynamics elaborate the adsorption mechanisms in defective MOF-74. Our study establishes comprehensive strategies to search, elucidate and manipulate defects in MOFs.

Spatially confined protein assembly in hierarchical mesoporous metal-organic framework

Immobilization of biomolecules into porous materials could lead to significantly enhanced performance in terms of stability towards harsh reaction conditions and easier separation for their reuse. Metal-Organic Frameworks (MOFs), offering unique structural features, have emerged as a promising platform for immobilizing large biomolecules. Although many indirect methods have been used to investigate the immobilized biomolecules for diverse applications, understanding their spatial arrangement in the pores of MOFs is still preliminary due to the difficulties in directly monitoring their conformations. To gain insights into the spatial arrangement of biomolecules within the nanopores. We used in situ small-angle neutron scattering (SANS) to probe deuterated green fluorescent protein (d-GFP) entrapped in a mesoporous MOF. Our work revealed that GFP molecules are spatially arranged in adjacent nanosized cavities of MOF-919 to form "assembly" through adsorbate-adsorbate interactions across pore apertures. Our findings, therefore, lay a crucial foundation for the identification of proteins structural basics under confinement environment of MOFs.

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

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

Node Distortion as a Tunable Mechanism for Negative Thermal Expansion in Metal-Organic Frameworks

Chemically functionalized series of metal-organic frameworks (MOFs), with subtle differences in local structure but divergent properties, provide a valuable opportunity to explore how local chemistry can be coupled to long-range structure and functionality. Using in situ synchrotron X-ray total scattering, with powder diffraction and pair distribution function (PDF) analysis, we investigate the temperature dependence of the local- and long-range structure of MOFs based on NU-1000, in which Zr₆O₈ nodes are coordinated by different capping ligands (H₂O/OH, Cl^- ions, formate, acetylacetonate, and hexafluoroacetylacetonate). We show that the local distortion of the Zr₆ nodes depends on the lability of the ligand and contributes to a negative thermal expansion (NTE) of the extended framework. Using multivariate data analyses, involving non-negative matrix factorization (NMF), we demonstrate a new mechanism for NTE: progressive increase in the population of a smaller, distorted node state with increasing temperature leads to global contraction of the framework. The transformation between discrete node states is noncooperative and not ordered within the lattice, i.e., a solid solution of regular and distorted nodes. Density functional theory calculations show that removal of ligands from the node can lead to distortions consistent with the Zr···Zr distances observed in the experiment PDF data. Control of the node distortion imparted by the nonlinker ligand in turn controls the NTE behavior. These results reveal a mechanism to control the dynamic structure of MOFs based on local chemistry.

Elucidating correlated defects in metal organic frameworks using theory-guided inelastic neutron scattering spectroscopy

Metal organic frameworks (MOFs) that incorporate metal oxide cluster nodes, exemplified by UiO-66, have been widely studied, especially in terms of their deviations from the ideal, defect-free crystalline structures. Although defects such as missing linkers, missing nodes, and the presence of adventitious synthesis-derived node ligands (such as acetates and formates) have been proposed, their exact structures remain unknown. Previously, it was demonstrated that defects are correlated and span multiple unit cells. The highly specialized techniques used in these studies are not easily applicable to other MOFs. Thus, there is a need to develop new experimental and computational approaches to understand the structure and properties of defects in a wider variety of MOFs. Here, we show how low-frequency phonon modes measured by inelastic neutron scattering (INS) spectroscopy can be combined with density functional theory (DFT) simulations to provide unprecedented insights into the defect structure of UiO-66. We are able to identify and assign peaks in the fingerprint region (<100 cm^-1) which correspond to phonon modes only present in certain defective topologies. Specifically, this analysis suggests that our sample of UiO-66 consists of predominantly defect-free fcu regions with smaller domains corresponding to a defective bcu topology with 4 and 2 acetate ligands bound to the Zr₆O₈ nodes. Importantly, the INS/DFT approach provides detailed structural insights (e.g., relative positions and numbers of acetate ligands) that are not accessible with microscopy-based techniques. The quantitative agreement between DFT simulations and the experimental INS spectrum combined with the relative simplicity of sample preparation, suggests that this methodology may become part of the standard and preferred protocol for the characterization of MOFs, and, in particular, for elucidating the structure defects in these materials.

CO2 coordination-driven top-down synthesis of a 2D non-layered metal-organic framework

Combining the physical advantages of two-dimensional (2D) inorganic nanosheets and the modular design and programmed structure of metal-organic frameworks (MOFs), 2D MOFs remain at the forefront of functional material research. Despite tremendous efforts, precise control in the synthesis of 2D nonlayered MOFs with predesigned topology for desired applications remains challenging. Success in the bottom-up synthesis of 2D nonlayered MOFs via ligand exchange motivated us to incorporate partial BTC (BTC = 1,3,5-benzenetricarboxylate) ligand dissociation and CO2 capped coordination into the top-down treatment of bulk Cu-BTC MOF, leading to successful conversion of a 3D nonlayered network to a 2D Cu-based topological structure. Notably, a supercritical CO2-containing solvent mixture is employed to provide the desired defect and coordination engineering. Thus, our work introduces a new top-down concept based on modulated synthesis to fabricate high-quality 2D nonlayered MOFs for the first time.

Exploring the Defect Sites in UiO-66 by Decorating Platinum Nanoparticles for an Efficient Hydrogen Evolution Reaction

UiO-66 has been tailored using defect engineering methodology to introduce thiol functionalities into the MOF skeletal structure. The thiolated UiO-66 serves as a scaffold to support the platinum nanoparticles with a size of ∼2 nm through a soft-soft interaction. This Pt@UiO-66-SH, utilized as an HER catalyst, exhibited an overpotential of 57 mV at a current density of 10 mA cm^-2 in an acidic medium with a Tafel slope of 75 mV/dec and a high TOF value (389.37 s^-1). This catalyst showed long-term durability for 30 h, specifying the potential of the material to produce neat hydrogen.

Hydroxylation of UiO-66 Metal-Organic Frameworks for High Arsenic(III) Removal Efficiency

Zirconium clusters of UiO-66 have been hydroxylated with NaOH to generate strong binding sites for As(III) species in wastewater treatment. Hydroxylated UiO-66 provides high adsorption capacity over a wide range of pH from 1 to 10 with a maximum uptake of 204 mg g^-1, which is significantly enhanced compared to those of pristine UiO-66, acid-modulated UiO-66, and other adsorbents for use in a wide pH range of treatment processes. The local structure of hydroxylated sites and As(III) adsorption mechanism are determined by extended X-ray absorption fine structure combined with density functional theory calculations.

Crystalline hydrogen bonding of water molecules confined in a metal-organic framework

Hydrogen bonding (H-bonding) of water molecules confined in nanopores is of particular interest because it is expected to exhibit chemical features different from bulk water molecules due to their interaction with the wall lining the pores. Herein, we show a crystalline behavior of H-bonded water molecules residing in the nanocages of a paddlewheel metal-organic framework, providing in situ and ex situ synchrotron single-crystal X-ray diffraction and Raman spectroscopy studies. The crystalline H-bond is demonstrated by proving the vibrational chain connectivity arising between hydrogen bond and paddlewheel Cu-Cu bond in sequentially connected Cu-Cu·····coordinating H₂O·····H-bonded H₂O and by proving the spatial ordering of H-bonded water molecules at room temperature, where they are anticipated to be disordered. Additionally, we show a substantial distortion of the paddlewheel Cu²⁺-centers that arises with water coordination simultaneously. Also, we suggest the dynamic coordination bond character of the H-bond of the confined water, by which an H-bond transitions to a coordination-bond at the Cu²⁺-center instantaneously after dissociating a previously coordinated H₂O.

Superprotonic Conductivity of UiO-66 with Missing-Linker Defects in Aqua-Ammonia Vapor

The design and preparation of proton-conducting metal-organic frameworks (MOFs) with superconductivity are of significance for the proton-exchange membrane fuel cell (PEMFC). Introducing functional structural defects to enhance proton conductivity is a good approach. Here, we synthesized a series of UiO-66 (first synthesized in the University of Oslo) with missing-linker defects and investigated the effect of defect numbers on the proton conductivity of the samples. Among them, 60-UiO-66-1.8 (60 represents the synthesis temperature and 1.8 the number of defects) prepared with 3-mercaptopropionic acid as a modulator has the best proton conductivity, which is 3 × 10^-2 S cm^-1 at 100 °C and under 98% relative humidity (RH). The acidic sites induced by missing-linker defects further promote the chemisorption of ammonia molecules, resulting in the formation of a richer hydrogen-bond network and hence boosting the proton conductivity to 1.04 × 10^-1 S cm^-1 at 80 °C, which is one of the highest values among the reported MOF-based proton conductor. Therefore, this work provides a new strategy for enhancing proton conduction in MOF-based materials.

Identifying UiO-67 Metal-Organic Framework Defects and Binding Sites through Ammonia Adsorption

Ammonia is a widely used toxic industrial chemical that can cause severe respiratory ailments. Therefore, understanding and developing materials for its efficient capture and controlled release is necessary. One such class of materials is 3D porous metal-organic frameworks (MOFs) with exceptional surface areas and robust structures, ideal for gas storage/transport applications. Herein, interactions between ammonia and UiO-67-X (X: H, NH₂ , CH₃ ) zirconium MOFs were studied under cryogenic, ultrahigh vacuum (UHV) conditions using temperature-programmed desorption mass spectrometry (TPD-MS) and in-situ temperature-programmed infrared (TP-IR) spectroscopy. Ammonia was observed to interact with μ₃ -OH groups present on the secondary building unit of UiO-67-X MOFs via hydrogen bonding. TP-IR studies revealed that under cryogenic UHV conditions, UiO-67-X MOFs are stable towards ammonia sorption. Interestingly, an increase in the intensity of the C-H stretching mode of the MOF linkers was detected upon ammonia exposure, attributed to NH-π interactions with linkers. These same binding interactions were observed in grand canonical Monte Carlo simulations. Based on TPD-MS, binding strength of ammonia to three MOFs was determined to be approximately 60 kJ mol^-1 , suggesting physisorption of ammonia to UiO-67-X. In addition, missing linker defect sites, consisting of H₂ O coordinated to Zr⁴⁺ sites, were detected through the formation of nNH₃ ⋅H₂ O clusters, characterized through in-situ IR spectroscopy. Structures consistent with these assignments were identified through density functional theory calculations. Tracking these bands through adsorption on thermally activated MOFs gave insight into the dehydroxylation process of UiO-67 MOFs. This highlights an advantage of using NH₃ for the structural analysis of MOFs and developing an understanding of interactions between ammonia and UiO-67-X zirconium MOFs, while also providing directions for the development of stable materials for efficient toxic gas sorption.

Elimination of Grain Boundary Defects in Zeolitic Imidazolate Framework ZIF-95 Membrane via Solvent-Free Secondary Growth

Metal-organic framework membranes are usually prepared by in situ or secondary growth in a solution/hydrogel. The use of organic solvents may cause safety and environmental problems and produce solvent-induced defects. Here, highly oriented and permselective ZIF-95 membranes are prepared for the first time via a solvent-free secondary growth method. The solvent-free growth is not only helpful to control the membrane microstructure and thickness, but also to reduce the intercrystalline defects. In case of solvent-free growth, a perfectly oriented structure leads to an outstanding reduction of intercrystalline defects and transport resistances. For the separation of equimolar binary gas mixtures by using the highly oriented ZIF-95 membrane at 25 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ are 184 and 140, respectively, with H₂ permeance of over 1.9×10^-7  mol m^-2  s^-1  Pa^-1 which are much higher than those of the randomly oriented ZIF-95 membrane.

Defect Level and Particle Size Effects on the Hydrolysis of a Chemical Warfare Agent Simulant by UiO-66

Defect engineering in metal-organic frameworks (MOFs) has recently become an area of significant research due to the possibility of enhancing material properties such as internal surface area and catalytic activity while maintaining stable 3D structures. Through a modulator screening study, the model Zr⁴⁺ MOF, UiO-66, has been synthesized with control of particle sizes (100-1900 nm) and defect levels (2-24%). By relating these properties, two series were identified where one property remained constant, allowing for independent analysis of the defect level or particle size, which frequently change coincident with the modulator choice. The series were used to compare UiO-66 reactivity for the hydrolysis of a chemical warfare agent simulant, dimethyl 4-nitrophenylphosphate (DMNP). The rate of DMNP hydrolysis displayed high dependence on the external surface area, supporting a reaction dominated by surface interactions. Moderate to high concentrations of defects (14-24%) allow for the accessibility of some interior MOF nodes but do not substantially promote diffusion into the framework. Individual control of defect levels and particle sizes through modulator selection may provide useful materials for small molecular catalysis and provide a roadmap for similar engineering of other zirconium frameworks.

Metal-Organic-Framework Based Functional Materials for Uranium Recovery: Performance Optimization and Structure/Functionality-Activity Relationships

Uranium recovery has profound significance in both uranium resource acquisition and pollution treatment. In recent years, metal-organic frameworks (MOFs) have attracted much attention as potential uranium adsorbents owing to their tunable structural topology and designable functionalities. This review explores the research progress in representative classic MOFs (MIL-101, UiO-66, ZIF-8/ZIF-67) and other advanced MOF-based materials for efficient uranium extraction in aqueous or seawater environments. The uranium uptake mechanism of the MOF-based materials is refined, and the structure/functionality-property relationship is further systematically elucidated. By summarizing the typical functionalization and structure design methods, the performance improvement strategies for MOF-based adsorbents are emphasized. Finally, the present challenges and potential opportunities are proposed for the breakthrough of high-performance MOF-based materials in uranium extraction.