Defective Metal-organic Frameworks Research Articles

Application and performance Enhancement of acetic acid-Regulated ligand defect engineering in NiMOF electrocatalysts

Introducing organic ligands into metal-organic frameworks (MOFs) is an effective method for preparing defective MOFs. This approach enables the fabrication of cost-effective, efficient, highly conductive, and richly active-site electrocatalysts. Herein, the defective NiMOF is synthesized via a straightforward one-pot solvothermal method by partially substituting phthalic acid (PTA) ligands with acetic acid (HOAc), which effectively regulates the micro-morphology and electronic structure of the NiMOF nanoflowers, thus creating abundant electrochemical active sites, significantly improving electronic conductivity and promoting rapid charge transfer. The resulting DE-NiMOF-0.5 nanoflowers, prepared with HOAc substitution, demonstrate excellent electrochemical performance at a current density of 10 mA cm−2, the hydrogen evolution reaction (HER) overpotential is 188 mV (Tafel slope of 175 mV dec−1), while the oxygen evolution reaction (OER) overpotential is 205 mV (Tafel slope of 37 mV dec−1). The introduction of acetic acid ligands in DE-NiMOF-0.5 not only constructs the ligand defects within the catalyst, but also increases the abundant active sites, enhancing the hydrophilicity of the catalyst and facilitating electronic transfer between the catalyst surface and the electrolyte. This study explores a strategy for preparing defective MOF catalysts through introducing modulators, providing an economically viable material pathway for electrocatalysis and opening new possibilities for designing and synthesizing efficient electrocatalysts in future research endeavors.

Defect strategies in Mn-doped metal–organic frameworks to enhance adsorption-based NOx gas sensing performance

Post-synthetic metalation (PSM) represents a powerful toolkit for modifying the chemical composition of pre-formed metal-organic frameworks (MOFs). However, accessing defective MOFs with exchanged metal nodes via this approach remains challenging. Herein, we report a transmetalation strategy to induce desired metal exchange frustrated flexibility defects in a prototypical MOF platform. Specifically, the pristine Zn-based MOF undergoes selective Zn-to-Mn exchange through a carefully controlled solvothermal transmetalation process. This protocol generates a series of Mn-doped MOFs with increasing Mn integration and the concomitant formation of defect sites within the framework. Detailed structural characterization confirms the successful Mn doping and illuminates the generation of vacancies and distortions around the exchanged Mn nodes. The transmetallated Mn-doped MOF materials exhibit altered chemical environments, as probed by spectroscopic techniques, and display promising sensing activity for NOx through the MOF matrix membrane. The proposed application of the MOF samples as an adsorbent-based chemical sensor includes sensing mechanisms for NO2 gas.

Improving the Photoactivity of Porphyrin-Based Metal-Organic Frameworks via Constructing [Ce-O] Active Site and Vacancy.

Defect engineering of metal-organic frameworks (MOFs) is a versatile approach to tailoring their electronic structures and photocatalytic performance. Herein, Ce-based porphyrin MOFs (CMFs) featuring controlled structural defects were successfully prepared using a simple acid modulation strategy to drive the photocatalytic H2 generation. The [Ce-O] unit serves as the active site via a ligand-to-metal charge transfer process, which has been confirmed by in situ XPS analysis. Abundant exposed coordinatively unsaturated Ce-O centers are beneficial for the adsorption and activation of water molecules, which is an important factor for improving the photocatalytic performance of the synthesized defective MOFs. In addition, optical and electrochemical experiments indicate that CMFs with more oxygen vacancies possess higher charge separation efficiency. As a result, the optimized CMF(Zn)-200 sample afforded high stability and activity in the H2 generation (up to 1603.3 μmol·g-1·h-1 under cocatalyst-free conditions) and tetracycline hydrochloride removal efficiency (97%), which was 8.45 and 97 times higher than that of pure meso-tetra(4-carboxyphenyl)porphine. This study demonstrates that effective structural modulation and defect introduction can improve the activity and stability of PMOF-based photocatalysts.

Defect Engineering in Ce-Based Metal-Organic Frameworks toward Enhanced Catalytic Performance for Hydrogenation of Dicyclopentadiene.

Defective metal-organic frameworks (MOFs) have shown great potential for catalysis due to abundant active sites and adjustable physical and chemical properties. A series of Ce-based MOFs with different defect contents were synthesized via a modulator-induced defect engineering strategy with the aid of the cell pulverization technique. The effects of modulators on the pore structure, morphology, valence distribution of Ce, and Lewis acidity of Ce-MOF-801 were systematically investigated. Among the different samples studied, the optimal Ce-MOF-801-50eq sample exhibited remarkable catalytic activity for DCPD hydrogenation, achieving a conversion rate of 100%, which is significantly higher compared to other Ce-MOF-801-neq samples as well as the Zr-MOF-801-50eq and Hf-MOF-801-50eq samples. The enhanced catalytic performance of Ce-MOF-801-50eq can be attributed to advantages provided by defect engineering, such as the high specific surface area, proper pore size distribution, abundant unsaturated metal sites, and Ce3+/Ce4+ atom ratio, which have been supported by various characterizations. This study provides important insights into the rational design of Ce-MOFs in the field of catalytic DCPD hydrogenation.

Defective MOFs as nano carrier for drug loading with controlled release

Metal-organic frameworks (MOFs) are considered highly promising nanomedicine carriers due to their well-defined structures, high specific surface area and porosity, adjustable pore size, and ease of chemical functionalization. However, traditional MOFs face limitations in lattice size and strong non-covalent interactions between the host and guest, posing significant challenges for high and responsive drug loading. In this study, defect-engineered MOFs (UiO-66-NH2) with uniform mesopores were successfully prepared by reducing ligands and adding modulators. Compared to perfect UiO-66-NH2, the loading capacity of the defective UiO-66-NH2 increased more than twofold. Utilizing phase change material as a trigger for drug release, precise temperature-responsive drug release was accomplished. This offers a novel strategy for MOFs materials in drug delivery and controlled release. This study will aid future efforts in using MOFs for selective drug delivery to cells, particularly in cancer treatment and related fields.

Defects materials of Institut Lavoisier-125(Ti) materials enhanced photocatalytic activity for toluene and chlorobenzene mixtures degradation: Mechanism study

In this paper, the effect of three monocarboxylic acids on MIL-125 synthesis was systematically investigated and the results were discussed in detail. X-ray diffractometry (XRD) and nitrogen adsorption–desorption curves indicated that small molecule acids (acetic acid, propionic acid and butyric acid) affected the morphology of MIL-125 and induced lamellar pores and structural defects in the crystals. Thermogravimetric measurements confirmed the presence of acid-regulated defective metal–organic frameworks (MOFs). Electrochemical tests and density function theory calculations indicated that acid modulation could change the forbidden bandwidth of the material. The acid modification strategy effectively promoted the transfer of photogenerated electrons and enhanced the adsorption and activation of O2 and H2O molecules, generating reactive radicals. The modified MOFs also showed excellent performance in the removal of mixed toluene and chlorobenzene. The degradation pathways of the mixture were analyzed by in situ infrared (IR) and gas chromatography-mass spectrometry (GC–MS). The mixture was converted to chlorophenolic intermediates in the presence of reactive oxygen species, further decomposed to form ethers and ethanol, and finally formed small molecules such as carbon dioxide and water. A feasible method was provided for the preparation of photocatalysts for the treatment of mixed VOCs.

Defect Engineered Metal–Organic Framework with Accelerated Structural Transformation for Efficient Oxygen Evolution Reaction

AbstractMetal–organic frameworks (MOFs) have been increasingly applied in oxygen evolution reaction (OER), and the surface of MOFs usually undergoes structural transformation to form metal oxyhydroxides to serve as catalytically active sites. However, the controllable regulation of the reconstruction process of MOFs remains as a great challenge. Here we report a defect engineering strategy to facilitate the structural transformation of MOFs to metal oxyhydroxides during OER with enhanced activity. Defective MOFs (denoted as NiFc′xFc1‐x) with abundant unsaturated metal sites are constructed by mixing ligands of 1,1′‐ferrocene dicarboxylic acid (Fc′) and defective ferrocene carboxylic acid (Fc). NiFc′xFc1‐x series are more prone to be transformed to metal oxyhydroxides compared with the non‐defective MOFs (NiFc′). Moreover, the as‐formed metal oxyhydroxides derived from defective MOFs contain more oxygen vacancies. NiFc′Fc grown on nickel foam exhibits excellent OER catalytic activity with an overpotential of 213 mV at the current density of 100 mA cm−2, superior to that of undefective NiFc′. Experimental results and theoretical calculations suggest that the abundant oxygen vacancies in the derived metal oxyhydroxides facilitate the adsorption of oxygen‐containing intermediates on active centers, thus significantly improving the OER activity.

Defect Engineered Metal-Organic Framework with Accelerated Structural Transformation for Efficient Oxygen Evolution Reaction.

Metal-organic frameworks (MOFs) have been increasingly applied in oxygen evolution reaction (OER), and the surface of MOFs usually undergoes structural transformation to form metal oxyhydroxides to serve as catalytically active sites. However, the controllable regulation of the reconstruction process of MOFs remains as a great challenge. Here we report a defect engineering strategy to facilitate the structural transformation of MOFs to metal oxyhydroxides during OER with enhanced activity. Defective MOFs (denoted as NiFc'x Fc1-x ) with abundant unsaturated metal sites are constructed by mixing ligands of 1,1'-ferrocene dicarboxylic acid (Fc') and defective ferrocene carboxylic acid (Fc). NiFc'x Fc1-x series are more prone to be transformed to metal oxyhydroxides compared with the non-defective MOFs (NiFc'). Moreover, the as-formed metal oxyhydroxides derived from defective MOFs contain more oxygen vacancies. NiFc'Fc grown on nickel foam exhibits excellent OER catalytic activity with an overpotential of 213 mV at the current density of 100 mA cm-2 , superior to that of undefective NiFc'. Experimental results and theoretical calculations suggest that the abundant oxygen vacancies in the derived metal oxyhydroxides facilitate the adsorption of oxygen-containing intermediates on active centers, thus significantly improving the OER activity.

Efficient structure tuning over the defective modulated zirconium metal organic framework with active coordinate surface for photocatalyst CO2 reduction

Structure engineering of zirconium-based metal organic frameworks (MOFs) aims to develop efficient catalysts for transforming intermittent renewable energy into value-added chemical fuels. In order to have a deeper understanding of industrial scaling, it is vital to ascertain the favourable operational parameters that are necessary for projecting at the atomic level. The proposed paradigm provides a robust basis for the efficient design of MOFs based heterogeneous photocatalysts. In this study, set of defective MOF (D-NUiO66) was effectively produced using a modular acidic method. Afterwards, the D-NUiO66 was combined with CeO2 to form the D-CeNUiO66 heterojunction for the purpose of carbon dioxide reduction. The morphological aspect of the composite investigation suggested that D-CeNUiO66 had a mesoporous structure with favourable adsorption properties. The optimized D-CeNUiO66 photocatalyst showed the high activity for the reduction of CO2 to CO, with a rate of 38.6 µmolg−1h−1 and demonstrated remarkable repeatability in terms of CO production. The incorporation of defect sites in the D-NUiO66 enhanced the light response to visible light, resulting in reduced band gap of 2.9 eV. The photoelectrochemical tests indicated that the introduction of defects in the UiO66 and coupling CeO2 in the D-CeNUiO66 composite induced fast charge transfer, therefore suppressing the charge recombination rate. This study provides valuable insights into the use of defective engineering and heterojunction approaches to metal–organic frameworks for photocatalytic applications.

Defective metal–organic frameworks by linker fragmentation and its effects on organosulfur removal

Defective metal–organic frameworks by linker fragmentation and its effects on organosulfur removal

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.

A robust ultra-microporous cationic aluminum-based metal-organic framework with a flexible tetra-carboxylate linker

Al-based cationic metal-organic frameworks (MOFs) are uncommon. Here, we report a cationic Al-MOF, MIP-213(Al) ([Al18(μ2-OH)24(OH2)12(mdip)6]6Cl·6H2O) constructed from flexible tetra-carboxylate ligand (5,5'-Methylenediisophthalic acid; H4mdip). Its crystal structure was determined by the combination of three-dimensional electron diffraction (3DED) and high-resolution powder X-ray diffraction. The structure is built from infinite corner-sharing chains of AlO4(OH)2 and AlO2(OH)3(H2O) octahedra forming an 18-membered rings honeycomb lattice, similar to that of MIL-96(Al), a scarce Al-polycarboxylate defective MOF. Despite sharing these structural similarities, MIP-213(Al), unlike MIL-96(Al), lacks the isolated μ3-oxo-bridged Al-clusters. This leads to an ordered defective cationic framework whose charge is balanced by Cl- sandwiched between two Al-trimers at the corner of the honeycomb, showing strong interaction with terminal H2O coordinated to the Al-trimers. The overall structure is endowed by a narrow quasi-1D channel of dimension ~4.7 Å. The Cl- in the framework restrains the accessibility of the channels, while the MOF selectively adsorbs CO2 over N2 and possesses high hydrolytic stability.

Specific separation of flavonoids isomers by defect modulated metal-organic framework HKUST-1: Experiment and density functional theory analysis

Discovering and separating active monomers from natural products is an efficient way to develop new drugs, but faces the difficulty in complex and time-consuming separation process. Metal-organic frameworks (MOFs) are considered a promising separation strategy due to their molecular recognition properties, but their microporous nature poses challenges for capturing most natural compounds with molecule sizes larger than 20 Å. For the first time, this study proposed a simple and efficient approach to separate active monomeric components with larger diameters from structurally similar natural compounds by defect modulation strategy. In this study, defective HKUST-1 with hierarchical microporous/ mesoporous structures and Cu1+/Cu2+ mixed-valence sites was rapidly synthesized. The defective MOF demonstrated specific loading of silibinin A&B from silymarin containing seven structurally similar flavonoids within 5 min, and high separation capacity with the purity of ∼91.2% after primary separation. FTIR, XPS, and DFT calculation revealed the site competition mechanism for specific recognition and loading, where silibinin A&B preferentially acted on Cu2+ and Cu1+ clusters with the highest binding energy via hydrogen and coordination bonds, respectively. Compared to traditional separation techniques, the separation approach via defective MOF shows multiple advantages including high specificity, high separation efficiency, and easy to operate.

Nearly zero peroxydisulfate consumption for persistent aqueous organic pollutants degradation via nonradical processes supported by in-situ sulfate radical regeneration in defective MIL-88B(Fe)

The porous defective MIL-88B(Fe) with abundant oxygen vacancies and Fe-N sites was fabricated to accomplish nearly zero peroxydisulfate (PDS) consumption for persistent bisphenol A (BPA) degradation via electron-transfer pathway (ETP). Interestingly, the generated sulfates during ETP were oxidized to yield the confined sulfate radicals and to accomplish the peroxydisulfate regeneration in the fine-tuned MIL-88B(Fe), which was verified by series experiments and DFT calculations. Further studies suggested that the optimal De-MIL-88B(Fe)-1.25 catalyst achieved the persistent nonradical reactions for BPA decomposition under visible light irradiation with both low input and low consumption of PDS. It was the first case to achieve nearly zero PDS consumption for emerging pollutants elimination, which provided new strategy to design and tune defective metal-organic frameworks for the purpose of reducing the stoichiometry between PDS and contaminants for nearly zero PDS consumption.

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.

Recent advances in the type, synthesis and electrochemical application of defective metal-organic frameworks

Recent advances in the type, synthesis and electrochemical application of defective metal-organic frameworks

Hierarchically porous zirconium-based metal–organic frameworks for rapid adsorption and enrichment of sulfonamide antibiotics

Due to serious risk to human health and eco-environmental security, antibiotics in water and foods received considerable concern. Herein, a series of stable hierarchically porous zirconium-based metal–organic frameworks (MOFs) namely HP-NU-902-X were prepared via modulator-induced defect-guided formation strategy for the first time and functioned as sorbents to removal and preconcentration of sulfonamide antibiotics (SAs). The pore size and specific surface area of HP-NU-902-X were systematically adjusted by changing concentration of monocarboxylic acid modulator. The adsorption performance of HP-NU-902-X were greatly improved by forming defects. Experiments demonstrated that HP-NU-902-80 possessed the highest adsorption efficiency to SAs. The limits of detection (S/N = 3) were achieved ranging 0.08 to 0.25 ng/mL. The spiked recoveries of water and milk samples were obtained in the range of 70.34–103.4 % and 73.83–100.5 %. Enrichment factors were calculated between 203 and 358. The SAs can be completely removed within 10 min, and adsorption kinetic data was best consistent with pseudo-second-order model. Using Langmuir isotherm model, maximum adsorption capacities for sulfadiazine, sulfapyridine, sulfametoxydiazine, sulfachloropyridazine, sulfabenzamide and sulfamethazine were calculated as 279.9, 467.7, 414.4, 433.1, 415.5 and 336.6 mg/g, respectively. The improved adsorption capacity was mainly owing to high surface area and extra defect sites. This synthetic strategy provides a way to prepare stable defective MOFs and enhance their potential applications in removal and extraction of environmental pollutants.

Au nanoparticles-anchored defective metal–organic frameworks for photocatalytic transformation of amines to imines under visible light

Designing efficient metal organic frameworks (MOF)-based photocatalysts has recently attracted wide attention. In this work, Au nanoparticles(NPs)-decorated defective DUT-67(Zr) (Au@DUT-67(Zr)) with enlarged channels was successfully fabricated and achieved 7.5 times higher conversion than Au NPs-decorated UiO-66(Zr) (Au@UiO-66(Zr)) for photocatalytic selective oxidation of amines to imines driven by visible light. Au NPs are more effectively fixed on DUT-67(Zr) due to its partially hollow structure. Au@DUT-67(Zr) possesses more active sites including unsaturated Zr atoms and oxygen vacancies (VO) than Au@UiO-66(Zr). In situ Fourier transform infrared (FTIR) spectrum reveals that benzylamine is activated on unsaturated Zr sites via HN…Zr species, facilitating deprotonation of –CH2 in benzylamine. VO can not only adsorb and activate oxygen (O2) but also capture plasmonic hot electrons, enhancing the forming of superoxide radical (O2−). Plasmonic hot holes assisted with O2− effectively achieve the selective oxidation of benzylamine. Finally, a possible synergetic mechanism combining the plasmon with the molecular activation is presented to illustrate the photocatalytic pathway at the molecular level.

Microfluidic synthesis of metal-organic framework crystals with surface defects for enhanced molecular loading

Inherent properties of metal–organic frameworks (MOFs) such as porosity and chemical diversity allows utilizing them as drug delivery systems which are not toxic at 25 MOFs per B16-F10 cell. Herein, the MOF developed surface due to intrinsic defects enables exploration of a new form of efficient delivery with remote control. Here we experimentally demonstrate the directed synthesis of MOF microcrystals (HKUST-1) with developed surface defects by a single-step microfluidics method. The synthesis process has been optimized to obtain the desired MOFs that ensure the enhanced loading of molecules (fluorescence dye) on their surface as compared with MOFs synthesized by solvothermal approach. Structural analysis and optical spectroscopy confirmed the defective MOF surface. Dye loading and remote light-induced release has been demonstrated under continuous wave laser irradiation (532 nm). Toxicity of the obtained MOFs and their interaction with murine melanoma cells have been additionally tested. Therefore, the ability to design and synthesize MOFs with the desired physicochemical properties and 2 times higher loading capacities than a solvothermally synthesized one due to the use of microfluidic approach opens up new prospects for the development of improved drug delivery platforms.

Defective UiO-66-NH2 Functionalized with Stable Superoxide Radicals toward Electrocatalytic Nitrogen Reduction with High Faradaic Efficiency.

The electrocatalytic nitrogen reduction reaction (NRR) to NH3 is limited by low Faradaic efficiency (FE). Herein, defective UiO-66-NH2 functionalized with quite stable superoxide radicals (O2•) is developed as a highly active NRR catalyst. The experimental and computational results show that one linker per Zr6 node is missed and two Zr atoms are exposed in the defective UiO-66-NH2. One of the two exposed Zr atoms can stably adsorb O2•, and thus, a Zr-OO• site forms during the preparations without light excitation or postoxidation, while the other Zr atom is activated as an active site. The synergistic effects of the two Zr sites in the defective UiO-66-NH2 suppress hydrogen and hydrazine evolutions considerably. They are as follows: (i) due to repulsion of the proton on the active Zr site and stabilization of the proton on the Zr-OO• site, the active Zr site is unfavorable for the adsorption of the proton with a high energy barrier, which is the HER rate-determining step (RDS); (ii) under the assistance of the OO• of the Zr-OO• site, the first hydrogenation step of *N2 (i.e., NRR RDS) on the active Zr site is promoted; and (iii) relying on the assistance of the OO• of the Zr-OO• site, the continuous hydrogenation of *NH2NH2 to produce NH3 on the active Zr site is spontaneously exothermic, whereas its desorption to hydrazine is blocked. Accordingly, an extremely high FE of ∼85.21% has been realized along with a high yield rate of NH3 (∼52.81 μg h-1 mgcat-1). To the best of our knowledge, it is the highest FE that has been achieved in recent years. Radical scavenging treatment of the defective UiO-66-NH2 and detailed investigations of two categories of control samples further verify the favorable effects of the O2• that closely correlates with the missed linkers on the performance of the NRR to NH3. This work opens a new way toward highly efficient NRR catalysts, i.e., stable radical-activating defective metal-organic frameworks.