Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

Synthesis and Mechanical Properties of sub 5-µm PolyUiO-66 Thin Films on Gold Surfaces

Metal-organic framework (MOF) thin films currently lack the mechanical stability needed for electronic device applications. Polymer-based metal-organic frameworks (polyMOFs) have been suggested to provide mechanical advantages over MOFs, however, the mechanical properties of polyMOFs have not yet been characterized. In this work, we developed a method to synthesize continuous sub-5 µm polyUiO-66(Zr) films on Au substrates, which allowed us to undertake initial mechanical property investigations.  Comparisons between polyUiO-66 and UiO-66 thin films determined polyUiO-66 thin films exhibit a lower modulus but similar hardness to UiO-66 thin films. The initial mechanical characterization indicates that further development is needed to leverage the mechanical property advantages of polyMOFs over MOFs. Additionally, the demonstration in this work of a continuous surface-supported polyUiO-66 thin film also enables utilization of the emerging class of polyMOF materials in sensors and devices applications.

Molecular Cleavage of Metal-Organic Frameworks and Application to Energy Storage and Conversion

The physicochemical properties of metal-organic frameworks (MOFs) significantly depend on composition, topology, and porosity, which can be tuned via synthesis. In addition to a classic direct synthesis, postsynthesis modulations of MOFs, including ion exchange, installation, and destruction, can significantly expand the application. Because of a limitation of the qualitative hard and soft acids and bases (HSAB) theory, posttreatment permits regulation of MOF structure by cleaving chemical bonds at the molecular level. Here, methods of coordination bond scission to tailor the structure are critically appraised and the application to energy storage and conversion is assessed. MOF structures synthesized by molecular-level coordination bond cleavage are described and the corresponding MOFs for electrocatalysis and renewable battery applications are evaluated. Significant emphasis is placed on various coordination bond cleavage to tune properties, including chemical groups, electronic structures, and morphologies. The review concludes with a critical perspective on practical application, together with challenges and future outlook for this emerging field.

Exploring the Role of Cluster Formation in UiO Family Hf Metal-Organic Frameworks with in Situ X-ray Pair Distribution Function Analysis

The structures of Zr and Hf metal-organic frameworks (MOFs) are very sensitive to small changes in synthetic conditions. One key difference affecting the structure of UiO MOF phases is the shape and nuclearity of Zr or Hf metal clusters acting as nodes in the framework; although these clusters are crucial, their evolution during MOF synthesis is not fully understood. In this paper, we explore the nature of Hf metal clusters that form in different reaction solutions, including in a mixture of DMF, formic acid, and water. We show that the choice of solvent and reaction temperature in UiO MOF syntheses determines the cluster identity and hence the MOF structure. Using in situ X-ray pair distribution function measurements, we demonstrate that the evolution of different Hf cluster species can be tracked during UiO MOF synthesis, from solution stages to the full crystalline framework, and use our understanding to propose a formation mechanism for the hcp UiO-66(Hf) MOF, in which first the metal clusters aggregate from the M₆ cluster (as in fcu UiO-66) to the hcp-characteristic M₁₂ double cluster and, following this, the crystalline hcp framework forms. These insights pave the way toward rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.

Two-step preparation of Keggin-PW12@UIO-66 composite showing high-activity and long-life conversion of soybean oil into biodiesel

A polyoxometalate acid can be encapsulated into a metal-organic framework to construct a novel kind of solid-acid catalyst. In this work, the two-step method -high-temperature preparation of Zr₆O₄(OH)₄(CH₃COO)₁₂ and low-temperature self-assembly-has been adopted to synthesize the PW₁₂@UIO-66 composite (PW₁₂ = H₃PW₁₂O₄₀; UIO-66 = Zr₆O₄(OH)₄(OOC-C₆H₄-COO)₁₂). The as-synthesized PW₁₂@UIO-66 composite exhibits highly crystalline, good octahedron morphology, large specific surface area (1960 m² g^-1) and high thermal stability (>500 °C), which clearly demonstrates the potential as a solid-acid catalyst. Additionally, the PW₁₂@UIO-66 composite may be accomplished with 85% utilization of H₃PW₁₂O₄₀ and 95% yield through this synthetic procedure. The performances of the PW₁₂@UIO-66 composite are investigated by catalyzing the simultaneous transesterification and esterification of soybean oil into biodiesel. Under the optimal conditions, the conversion of the soybean oil into biodiesel would exceed 90% over the as-synthesized PW₁₂@UIO-66 composite. As the crucial indexes for industrial prospects, the recycling and life experiments were surveyed. After 10 times recycling and 4 weeks, the structure and performance of the PW₁₂@UIO-66 composite remained unchanged and in the meantime the PW₁₂@UIO-66 composite still maintained a high activity to convert soybean oil into biodiesel.

Tailoring Defect Density in UiO-66 Frameworks for Enhanced Pb(II) Adsorption

Defect engineering of metal organic frameworks offers potential prospects for tuning their features toward particular applications. Herein, two series of defective UiO-66 frameworks were synthesized via changing the concentration of the linker and synthetic temperature of the reaction. These defective materials showed a significant improvement in the capability of Pb(II) removal from wastewater. This strategy for defect engineering not only created additional active sites, more open framework, and enhanced porosity but also exposed more oxygen groups, which served as the adsorption sites to improve Pb(II) adsorption. A relationship among degree of defects, texture features, and performances for Pb(II) removal was successfully developed as a proof-of-concept, highlighting the importance of defect engineering in heavy metal remediation. To investigate the kinetic and adsorption isotherms, we performed adsorption experiments influenced by the time and concentration of the adsorbate, respectively. For the practicality of the materials, the most significant parameters such as pH, temperature, adsorbent concentration, selectivity, and recyclability as well as simulated natural surface water were also examined. This study provides a clue for the researchers to design other advanced defective materials for the enhancement of adsorption performance by tuning the defect engineering.

Thiol-functionalized UiO-66 anchored atomically dispersed metal ions for the photocatalytic selective oxidation of benzyl alcohol

Thiol-functionalized UiO-66-anchored atomically dispersed metal ions, denoted as UiO-66(SM)₂ (M = Pd, Pt, or Au), were prepared as photocatalysts for the selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD) under visible light irradiation. The introduction of the thiol group shifted the light absorption edge of UiO-66 from the ultraviolet (UV) to the visible light region, minimized the bandgap, and improved the selectivity of producing BAD from 26.8% to 99.9%. The conversion of BA was greatly boosted from 8.9% to 33.4% after atomically dispersed metal ions were anchored.

Advanced strategies in Metal-Organic Frameworks for CO2 Capture and Separation

The continuous carbon dioxide (CO₂ ) gas emissions associated with fossil fuel production, valorization, and utilization are serious challenges to the global environment. Therefore, several developments of CO₂ capture, separation, transportation, storage, and valorization have been explored. Consequently, we documented a comprehensive review of the most advanced strategies adopted in metal-organic frameworks (MOFs) for CO₂ capture and separation. The enhancements in CO₂ capture and separation are generally achieved due to the chemistry of MOFs by controlling pore window, pore size, open-metal sites, acidity, chemical doping, post or pre-synthetic modifications. The chemistry of defects engineering, breathing in MOFs, functionalization in MOFs, hydrophobicity, and topology are the salient advanced strategies, recently reported in MOFs for CO₂ capture and separation. Therefore, this review summarizes MOF materials' advancement explaining different strategies and their role in the CO₂ mitigations. The study also provided useful insights into key areas for further investigations.

Monitoring a MOF Catalyzed Reaction Directly in Blood Plasma

Herein, we establish a method to quantitatively monitor a metal-organic framework (MOF)-catalyzed, biomedically relevant reaction directly in blood plasma, specifically, the generation of nitric oxide (NO) from the endogenous substrate S-nitrosoglutathione (GSNO) catalyzed by H3[(Cu4Cl)3-(BTTri)8] (CuBTTri). The reaction monitoring method uses UV-vis and 1H NMR spectroscopies along with a nitric oxide analyzer (NOA) to yield the reaction stoichiometry and catalytic rate for GSNO to NO conversion catalyzed by CuBTTri in blood plasma. The results show 100% loss of GSNO within 16 h and production of 1 equiv. of glutathione disulfide (GSSG) per 2 equiv. of GSNO. Only 78 ± 10% recovery of NO(g) was observed, indicating that blood plasma can scavenge the generated NO before it can escape the reaction vessel. Significantly, to best apply and understand reaction systems with biomedical importance, such as NO release catalyzed by CuBTTri, methods to study the reaction directly in biological solvents must be developed.

Construction of hierarchical-porous metal-organic frameworks through esterification reaction for efficient catalysis

A solvent-assisted strategy was proposed by controlling the coordination equilibrium to fabricate hierarchical-porous metal-organic frameworks (HP-MOFs). The obtained HP-MOFs showed remarkable enhancement in catalytic efficiency in Lewis acid catalysis resulting from the joint efforts of the hierarchical pores and the exposed metal clusters.

Adsorptive removal of pharmaceutical pollutants by defective metal organic framework UiO-66: Insight into the contribution of defects

The development of defective structure in MOFs can offer a novel approach to tailor the properties of MOFs-type adsorbent for better adsorption performance. In this study, the contribution of defective structure in UiO-66 to the adsorptive removal of pharmaceutical pollutants from aqueous solution was investigated. The results showed that the controlled defects in UiO-66 greatly affected adsorption equilibrium time, adsorption capacity and adsorption selectivity. Slightly defected UiO-66 contained more open frameworks, and it exhibited faster adsorption equilibrium. However, a high degree of destruction to the amorphous state resulted in a longer equilibrium time, due to the interference in the diffusion process as the result of severe structural collapse and interpenetration. Moreover, a higher degree of structural damage of UiO-66 led to a higher adsorption capacity because of the increased active sites. The maximum adsorption capacity was 321 mg/g for the as-prepared defective UiO-66, which was much higher than that of perfective UiO-66 (54.5 mg/g). Furthermore, defective UiO-66 had a higher adsorption affinity for diclofenac sodium than other studied pharmaceutical pollutants. This study could provide insight into the relationship between defective property and adsorption performance. The results will deepen the understanding of the adsorption mechanism of MOFs-type adsorbents, and help the design of MOFs-type adsorbents with fast adsorption equilibrium, higher adsorption capacity and adsorption selectivity.

The impact of framework flexibility and defects on the water adsorption in CAU-10-H

Aluminum-based metal-organic framework (MOF) CAU-10-H is a promising candidate for heat transformation and water harvesting applications due to its hydrothermal stability, beneficial step-wise water adsorption isotherm and low toxicity. In this study, the effects of the framework flexibility and structural defects on the mechanism of water sorption in CAU-10-H were studied by grand canonical Monte Carlo (GCMC) methods. It was shown by the simulations that the rigid ideal MOF framework is hydrophobic. The account of the linker "flapping" motion during the simulations made the framework more hydrophilic due to unblocking of hydroxyl groups that are inaccessible to water molecules for the rigid structure model. However, this model cannot predict the experimental pressure, at which the step on the adsorption isotherm is observed. Based on this result, we suggested that the presence of structural defects could increase the MOF hydrophilicity. The investigation of the water adsorption using several models of defective structures demonstrated that even a small number of defects shift the calculated position of the step on the adsorption isotherm towards the experimental values. The results obtained in this study emphasize that the controlled synthesis of defective structures is one of the most efficient methods of regulating the MOF adsorption properties.

Elaboration of Porous Salts

A large library of novel porous salts based on charged coordination cages was synthesized via straightforward salt metathesis reactions. For these, solutions of salts of oppositely charged coordination cages are mixed to precipitate MOF-like permanently porous products where metal identity, pore size, ligand functional groups, and surface area are highly tunable. For most of these materials, the constituent cages combine in the ratios expected based on their charge. Additional studies focused on the rate of salt metathesis or reaction stoichiometry as variables to tune particle size or product composition, respectively. It is expected that the design principles outlined here will be widely applicable for the synthesis of new porous salts based on a variety of charged porous molecular precursors.

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.

Visualizing the {110} surface structure of equilibrium-form ZIF-8 crystals by low-dose Cs-corrected TEM

The properties of zeolitic imidazolate framework (ZIF) crystals highly depend on the structures of the low-energy surfaces, such as {110} of ZIF-8. However, the atomic/molecular configurations of the ZIF-8 {110} surfaces remain debated. In this study, the near-atomic-scale characterization of {110} surfaces of ZIF-8 is conducted by low-dose aberration-corrected transmission electron microscopy (TEM). The real-space images with mitigated surface delocalization by minimized spherical aberration of TEM, together with the solvent corrected surface energy calculations, demonstrate that the {110} surfaces of ZIF-8 crystals with an equilibrium-form rhombic morphology have a zigzag-type termination. This study provides experimental evidence to clarify the debated structure of {110} ZIF-8 surfaces and has important implications in understanding the crystal growth and surface related properties of ZIF-8.

Defect Engineering in Metal‒Organic Frameworks as Futuristic Options for Purification of Pollutants in an Aqueous Environment

Clean water scarcity is becoming an increasingly important worldwide issue. The water treatment industry is demanding the development of novel effective materials. Defect engineering in nanoparticles is among the most revolutionary of technologies. Because of their high surface area, structural diversity, and tailorable ability, Metal‒Organic Frameworks (MOFs) can be used for a variety of purposes including separation, storage, sensing, drug delivery, and many other issues. The application in wastewater treatment associated with water stable MOF‒based materials has been an emerging research topic in recent decades. Defect engineering is a sophisticated technique used to manufacture defects and to change the geometric framework of target compounds. Since MOFs have a series of designable structures and active sites, tailoring properties in MOFs by defect engineering is a novel concept. Defect engineering can excavate hidden active sites in MOFs, which can lead to better performance in many fields. Therefore, this technology will open new opportunities in water purification processes. However, there has been little effort to comprehensively discuss this topic. In this review, we provide an overview of the development of defect engineered MOFs for water purification processes. Furthermore, we discuss the potential applications of defect engineered materials.

Atomically Dispersed Copper Sites in a Metal-Organic Framework for Reduction of Nitrogen Dioxide

Metal–organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity, and designable functionality. However, the unambiguous identification of atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically dispersed copper sites in UiO-66 for the catalytic reduction of NO₂ at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO₂ at 25 °C without the use of reductants. A selectivity of 88% for the formation of N₂ at a 97% conversion of NO₂ with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h^–1 is achieved under nonthermal plasma activation. In situ and operando infrared, solid-state NMR, and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO₂ molecules, with the formation of {Cu(I)···NO} and {Cu···NO₂} adducts promoting the conversion of NO₂ to N₂. This study will inspire the further design and study of new efficient single-atom catalysts for NO₂ abatement via detailed unravelling of their role in catalysis.

UIO-66 as Nucleating Agent on the Crystallization Behavior and Properties of Poly(Ethylene Terephthalate)

In this study, not only was the similar terephthalate structure between UIO-66 and PET utilized to improve compatibility, but the Zr4+ exposed by defects of UIO-66 was also utilized to improve the interaction between PET and UIO-66. Furthermore, PET nanocomposites with different contents of UIO-66 were also fabricated. Due to the high specific surface area and coordination of Zr4+, UIO-66 has high nucleation efficiency in the PET matrix. Compared with pure PET, the crystallization rate of PET/UIO-66 nanocomposite is significantly increased, and the crystallization temperature of PET-UIO66-1 is significantly increased from 194.3 °C to 211.6 °C. In addition, the tensile strength of nanocomposites has also been improved due to coordination.

Experimental Study on Activated Carbon-MIL-101(Cr) Composites for Ethanol Vapor Adsorption

In this study, the hydrothermal method was used to synthesize MIL-101(Cr), and activated carbon (AC) with different content was incorporated in to MIL-101(Cr), thereby obtaining AC-MIL-101(Cr) composite material with a huge specific surface area. The physical properties of MIL-101(Cr) and AC-MIL-101(Cr) were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), nitrogen adsorption and desorption and specific surface area testing, and ethanol vapor adsorption performance testing. The results show that with the increase of activated carbon content, the thermal stability of AC-MIL-101(Cr) is improved. Compared with the pure sample, the BET specific surface area and pore volume of AC-MIL-101(Cr) have increased. In the relative pressure range of 0–0.4, the saturated adsorption capacity of AC-MIL-101(Cr) to ethanol vapor decreases slightly. It is lower than MIL-101(Cr), but its adsorption rate is improved. Therefore, AC-MIL-101(Cr)/ethanol vapor has a good application prospect in adsorption refrigeration systems. The exploration of AC-MIL-101(Cr) composite materials in this paper provides a reference for the future application of carbon-based/MOFS composite adsorbent/ethanol vapor working fluid in adsorption refrigeration.

Processable UiO-66 Metal-Organic Framework Fluid Gel and Electrical Conductivity of Its Nanofilm with Sub-100 nm Thickness

Zr-based UiO-66 metal-organic framework (MOF) is one of the most studied MOFs with a wide range of potential applications. While UiO-66 is typically synthesized as a microcrystalline solid, we employ a particle downsizing strategy to synthesize UiO-66 as fluid gel with unique rheological properties, which allows the solution-based processing as sub-100 nm films and enhances the electrical conductivity of its pristine structure. Film thicknesses ranging from 40 to 150 nm could be achieved by controlling the spin-coating parameters. The generality of the method is also demonstrated for other Zr-based MOFs including MOF-801 and MOF-808. The impact of particle size and film thickness at the nanoscale on electrical properties of UiO-66 is shown to realize new features that are distinct from those of the bulk powder phase. An electrical insulator UiO-66 shows a significant increase in the electrical conductivity (10^-5 S cm^-1 compared to 10^-7 S cm^-1 in the bulk powder phase) when the 10 nm particles are distributed on the substrate with a thickness less than 100 nm. The findings establish a new route for processing of MOF materials as thin films with fine-tuned thickness and offer a new perspective for transport properties of Zr-based MOFs without structural modification.

Ligand-Conformer-Induced Formation of Zirconium-Organic Framework for Methane Storage and MTO Product Separation

In pursuit of novel adsorbents with efficient adsorptive gas storage and separation capabilities remains highly desired and challenging. Although the documented zirconium-tricarboxylate-based metal-organic frameworks (MOFs) have displayed a variety of topologies encompassing underlying and geometry mismatch ones, the employed organic linkers are exclusively rigid and poorly presenting one type of conformation in the resultant structures. Herein, a used and semirigid tricarboxylate ligand of H₃ TATAB was judiciously selected to isolate a zirconium-based spe-MOF after the preliminary discovery of srl-MOF. Single-crystal X-ray diffraction reveals that the fully deprotonated TATAB linker in spe-MOF exhibits two distinct conformers, concomitant with popular O_h and rare S₆ symmetrical Zr₆ molecular building blocks, generating an unprecedented (3,3,12,12)-c nondefault topology. Specifically, the spe-MOF exhibits structurally higher complexity, hierarchical micropores, open metal sites free and rich electronegative groups on the pore surfaces, leading to relatively high methane storage capacity without considering the missing-linker defects and efficient MTO product separation performance.

Synthesis of macroscopic monolithic metal-organic gels for ultra-fast destruction of chemical warfare agents

The potential threat that has originated from chemical warfare agents (CWAs) has promoted the development of advanced materials to enhance the protection of civilian and military personnel. Zr-based metal–organic frameworks (Zr-MOFs) have recently been demonstrated as excellent catalysts for decomposing CWAs, but challenges of integrating the microcrystalline powders of Zr-MOFs into monoliths still remain. Herein, we report hierarchically porous monolithic UiO-66-X xerogels for the destruction of CWAs. We found that the UiO-66-NH₂ xerogel with a larger pore size and a higher surface area than the UiO-66-NH₂ powder possessed better degradability of 2-chloroethyl ethyl sulfide (2-CEES), which is a sulfur mustard simulant. These UiO-66-X xerogels exhibit outstanding performance for decomposing CWAs. The half-lives of vesicant agent sulfur mustard (HD) and nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) are as short as 14.4 min and 1.5 min, respectively. This work is, to the best of our knowledge, the first report on macroscopic monolithic UiO-66-X xerogels for ultrafast decomposition of CWAs.

For the first time, we report hierarchically porous monolithic UiO-66-X xerogels for ultra-fast destruction of chemical warfare agents. The half-lives of the vesicant agent sulfur mustard (HD) and of the nerve agent VX are as short as 14.4 min and 1.5 min, respectively.

Defect Engineering of Nanoscale Hf-Based Metal-Organic Frameworks for Highly Efficient Iodine Capture

With the rapid development of the nuclear industry, how to deal with radioactive iodine waste in a timely and effective manner has become an important issue to be solved urgently. Herein, the defect-engineering strategy has been applied to develop a metal-organic framework (MOF)-based solid adsorbent by using the classical UiO-type Hf-UiO-66 as an example. After simple acid treatment, the produced defect-containing Hf-UiO-66 (DHUN) not only retains its topological structure, high crystallization, and regular shape but also shows a great increase in the Brunauer-Emmett-Teller value and pore size in comparison with the original Hf-UiO (HUN). These formed defects within DHUN have been demonstrated to be important for the great enhancement of the iodine capture and following application in computed tomography imaging in vitro. This present work gives a new insight into the control and formation of defect sites, and this simple and efficient defect-engineering strategy also shows great promise for the development of novel solid adsorbents and other functional MOF materials.

Neighboring Zn-Zr Sites in a Metal-Organic Framework for CO2 Hydrogenation

ZrZnO_x is active in catalyzing carbon dioxide (CO₂) hydrogenation to methanol (MeOH) via a synergy between ZnO_x and ZrO_x. Here we report the construction of Zn²⁺-O-Zr⁴⁺ sites in a metal-organic framework (MOF) to reveal insights into the structural requirement for MeOH production. The Zn²⁺-O-Zr⁴⁺ sites are obtained by postsynthetic treatment of Zr₆(μ₃-O)₄(μ₃-OH)₄ nodes of MOF-808 by ZnEt₂ and a mild thermal treatment to remove capping ligands and afford exposed metal sites for catalysis. The resultant MOF-808-Zn catalyst exhibits >99% MeOH selectivity in CO₂ hydrogenation at 250 °C and a high space-time yield of up to 190.7 mg_MeOH g_Zn^-1 h^-1. The catalytic activity is stable for at least 100 h. X-ray absorption spectroscopy (XAS) analyses indicate the presence of Zn²⁺-O-Zr⁴⁺ centers instead of Zn_mO_n clusters. Temperature-programmed desorption (TPD) of hydrogen and H/D exchange tests show the activation of H₂ by Zn²⁺ centers. Open Zr⁴⁺ sites are also critical, as Zn²⁺ centers supported on Zr-based nodes of other MOFs without open Zr⁴⁺ sites fail to produce MeOH. TPD of CO₂ reveals the importance of bicarbonate decomposition under reaction conditions in generating open Zr⁴⁺ sites for CO₂ activation. The well-defined local structures of metal-oxo nodes in MOFs provide a unique opportunity to elucidate structural details of bifunctional catalytic centers.

Dual-Selective Catalysis in Dephosphorylation Tuned by Hf6-Containing Metal-Organic Frameworks Mimicking Phosphatase

Selective dephosphorylation is full of great challenges in the field of biomimetic catalysis. To mimic the active sites of protein phosphatase, Hf-OH-Hf motif-containing metal-organic frameworks (MOFs) were obtained and structurally characterized, which are assembled from [Hf48Ni6] cubic nanocages and exhibit good stability in various solvents and acid/base solutions. Catalytic investigations suggest as-synthesized Hf-Ni and Hf-Ni-NH 2 display accurate type-selectivity (selectively catalyzed P-O rather than S-O or C-O bonds) and position-selectivity (selectively catalyzed phosphomonoesters over phosphodiesters) for the hydrolysis of phosphoesters. Reaction kinetic studies further revealed the high activity of the catalytic sites in these catalysts, and the unique catalytic selectivity and high activity are comparable to phosphatase. Additionally, these MOF catalysts possess good recursivity and hypotoxicity. Control experiments (including Hf- and Zr-based isomorphous MOFs) and theoretical calculations indicate that both triplet nickel and Hf6 clusters play significant roles in the unique binding site and favorable binding energy. To our knowledge, this is the first example of selective dephosphorylation through MOF catalysts as mimic enzymes, which paves a potential way for the development of specific therapeutic MOFs.

Understanding disorder and linker deficiency in porphyrinic zirconium-based metal-organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221

Porphyrin-based metal–organic frameworks (MOFs), exemplified by MOF-525, PCN-221, and PCN-224, are promising systems for catalysis, optoelectronics, and solar energy conversion. However, subtle differences between synthetic protocols for these three MOFs give rise to vast discrepancies in purported product outcomes and description of framework topologies. Here, based on a comprehensive synthetic and structural analysis spanning local and long-range length scales, we show that PCN-221 consists of Zr₆O₄(OH)₄ clusters in four distinct orientations within the unit cell, rather than Zr₈O₆ clusters as originally published, and linker vacancies at levels of around 50%, which may form in a locally correlated manner. We propose disordered PCN-224 (dPCN-224) as a unified model to understand PCN-221, MOF-525, and PCN-224 by varying the degree of orientational cluster disorder, linker conformation and vacancies, and cluster–linker binding. Our work thus introduces a new perspective on network topology and disorder in Zr-MOFs and pinpoints the structural variables that direct their functional properties.

Zirconium-based metal–organic frameworks have defective structures that are useful in catalysis and gas storage. Here, the authors study the interplay between cluster disorder and linker vacancies in PCN-221 and propose a new structure model with tilted Zr₆O₄(OH)₄ clusters rather than Zr₈O₆ clusters.

A hafnium-based metal-organic framework for the entrapment of molybdenum hexacarbonyl and the light-responsive release of the gasotransmitter carbon monoxide

A carbon monoxide-releasing material (CORMA) has been prepared by inclusion of molybdenum hexacarbonyl in a hafnium-based metal-organic framework (MOF) with the UiO-66 architecture. Mo(CO)₆ was adsorbed from solution to give supported materials containing 6.0-6.6 wt% Mo. As confirmed by powder X-ray diffraction (PXRD) and SEM coupled with energy dispersive X-ray spectroscopy, neither the crystallinity nor the morphology of the porous host was affected by the loading process. While the general shape of the N₂ physisorption isotherms (77 K) did not change significantly after encapsulation of Mo(CO)₆, the micropore volume decreased by ca. 20%. Thermogravimetric analysis of the as-prepared materials revealed a weight loss step around 160 °C associated with the decomposition of Mo(CO)₆ to subcarbonyl species. Confirmation for the presence of encapsulated Mo(CO)₆ complexes was provided by FT-IR and ¹³C{¹H} cross-polarization magic-angle spinning NMR spectroscopies. To test the capability of these materials to behave as CORMAs and transfer CO to heme proteins, the standard myoglobin (Mb) assay was used. While stable in the dark, photoactivation with low-power UV light (365 nm) liberated CO from the encapsulated hexacarbonyl molecules in Mo(6.0)/UiO-66(Hf), leading to a maximum amount of 0.26 mmol CO released per gram of material. Under the simulated physiological conditions of the Mb assay (37 °C, pH 7.4 buffer), minimal leaching of molybdenum occurred, PXRD showed only slight amorphization, and FT-IR spectroscopy confirmed the high chemical stability of the MOF host.

Trends and Prospects in UiO-66 Metal-Organic Framework for CO2 Capture, Separation, and Conversion

Among thousands of known metal-organic frameworks (MOFs), the University of Oslo's MOF (UiO-66) exhibits unique structure topology, chemical and thermal stability, and intriguing tunable properties, that have gained incredible research interest. This paper summarizes the structural advancement of UiO-66 and its role in CO₂ capture, separation, and transformation into chemicals. The first part of the review summarizes the fast-growing literature related to the CO₂ capture reported by UiO-66 during the past ten years. The second part provides an overview of various advancements in UiO-66 membranes in CO₂ purification. The third part describes the role of UiO-66 and its composites as catalysts for CO₂ conversion into useful products. Despite many achievements, significant challenges associated with UiO-66 are addressed, and future perspectives are comprehensively presented to forecast how UiO-66 might be used further for CO₂ management.

Controllable Microporous Framework Isomerism within Continuous Mesoporous Channels: Hierarchically Porous Structure for Capture of Bulky Molecules

To date, some attempts have been made to synthesize hierarchically porous metal-organic frameworks (HPMOFs), and in most cases, mesopores are formed in microporous frameworks. However, mass transfer and diffusion are still limited in such HPMOFs since micropores connect mesopores and mesopores are noncontinuous. Here, we fabricate a new hierarchical structure through the formation of microporous MOFs within continuous mesoporous channels. Confined space in the as-prepared mesoporous silica-containing template was used to prepare well-dispersed metal precursor of ZnO. The strategy of ligand vapor-induced crystallization was then designed to construct MOFs inside mesoporous channels, in which vapored ligand at elevated temperature diffuses and reacts with metal precursor. Our results indicate that framework isomerism is controllable by adjusting the crystallization conditions. In comparison to their microporous and mesoporous counterparts, the hierarchically porous materials show obviously enhanced adsorption performance on a series of bulky molecules including dye, enzyme, and metal-organic polyhedron.

E. coli@UiO-67 composites as a recyclable adsorbent for bisphenol A removal

E. coli@UiO-67 composites were obtained using an effective and simple self-assembly method. The composites showed unique properties as a remarkable and recyclable adsorbent for the efficient removal of bisphenol A (BPA) from water with a high adsorption capacity (402.930 mg g^-1). The increase in pore size is a key factor why E. coli@UiO-67 composites maintained high capacity. The reason might be due to that the composites with large pore sizes and defects could effectively improve mass transport and active molecular metal sites. The adsorption of BPA is a chemisorption process due to the Zr-OH groups in UiO-67 exhibit affinity toward BPA molecules, π-π interaction, and electrostatic attraction. The adsorption efficiency remained at 82.5% after 15 cycles without any remarkable changes in the PXRD patterns of E. coli@UiO-67. Moreover, the use of microorganism-loading MOFs could reduce the cost to at least 50% and minimize secondary pollution through nanoscale MOFs usage reduction. The developed composites have advantages, including low-cost, high adsorption capacity, easy to be separated and regenerated from aqueous solution, a large number of cycles, short adsorption equilibrium time, and stability, showing excellent application prospects. The presented strategy would be a potentially promising way to produce novel MOFs-based adsorbents with high-performance to control environmental pollution from wastewater.

Photocatalytic nitrogen fixation of metal-organic frameworks (MOFs) excited by ultraviolet light: insights into the nitrogen fixation mechanism of missing metal cluster or linker defects

The vacancies of semiconductors have proven to be effective active sites for photocatalytic nitrogen fixation, but what about the role of defects in MOF materials? Herein, we report the first UiO-66 with photo-excited cluster defects and linker defects for photocatalytic nitrogen fixation. It was determined through the post-synthetic ligand exchange (PSE) process that the linker defects, rather than cluster defects, can greatly improve the performance, which is due to linker defects forming unsaturated metal nodes such as the vacancy in a semiconductor. Specifically, for photo-activated UiO-66, the NH4+ production rate was 196 and 68 μmol g-1 h-1 in air atmosphere under ultraviolet-visible (UV-Vis) and visible light, respectively. This report provides a new effective strategy to design efficient nitrogen fixation photocatalysts.

Rational incorporation of defects within metal-organic frameworks generates highly active electrocatalytic sites

The allure of metal–organic frameworks (MOFs) in heterogeneous electrocatalysis is that catalytically active sites may be designed a priori with an unparalleled degree of control. An emerging strategy to generate coordinatively-unsaturated active sites is through the use of organic linkers that lack a functional group that would usually bind with the metal nodes. To execute this strategy, we synthesize a model MOF, Ni-MOF-74 and incorporate a fraction of 2-hydroxyterephthalic acid in place of 2,5-dihydroxyterephthalic acid. The defective MOF, Ni-MOF-74D, is evaluated vs. the nominally defect-free Ni-MOF-74 with a host of ex situ and in situ spectroscopic and electroanalytical techniques, using the oxidation of hydroxymethylfurtural (HMF) as a model reaction. The data indicates that Ni-MOF-74D features a set of 4-coordinate Ni–O₄ sites that exhibit unique vibrational signatures, redox potentials, binding motifs to HMF, and consequently superior electrocatalytic activity relative to the original Ni-MOF-74 MOF, being able to convert HMF to the desired 2,5-furandicarboxylic acid at 95% yield and 80% faradaic efficiency. Furthermore, having such rationally well-defined catalytic sites coupled with in situ Raman and infrared spectroelectrochemical measurements enabled the deduction of the reaction mechanism in which co-adsorbed *OH functions as a proton acceptor in the alcohol oxidation step and carries implications for catalyst design for heterogeneous electrosynthetic reactions en route to the electrification of the chemical industry.

The allure of metal–organic frameworks (MOFs) in heterogeneous electrocatalysis is that catalytically active sites may be designed a priori with an unparalleled degree of control.

Metal-Organic Framework Membranes Encapsulating Gold Nanoparticles for Direct Plasmonic Photocatalytic Nitrogen Fixation

Photocatalytic nitrogen fixation reaction can harvest the solar energy to convert the abundant but inert N₂ into NH₃. Here, utilizing metal-organic framework (MOF) membranes as the ideal assembly of nanoreactors to disperse and confine gold nanoparticles (AuNPs), we realize the direct plasmonic photocatalytic nitrogen fixation under ambient conditions. Upon visible irradiation, the hot electrons generated on the AuNPs can be directly injected into the N₂ molecules adsorbed on Au surfaces. Such N₂ molecules can be additionally activated by the strong but evanescently localized surface plasmon resonance field, resulting in a supralinear intensity dependence of the ammonia evolution rate with much higher apparent quantum efficiency and lower apparent activation energy under stronger irradiation. Moreover, the gas-permeable Au@MOF membranes, consisting of numerous interconnected nanoreactors, can ensure the dispersity and stability of AuNPs, further facilitate the mass transfer of N₂ molecules and (hydrated) protons, and boost the plasmonic photocatalytic reactions at the designed gas-membrane-solution interface. As a result, an ammonia evolution rate of 18.9 mmol g_Au^-1 h^-1 was achieved under visible light (>400 nm, 100 mW cm^-2) with an apparent quantum efficiency of 1.54% at 520 nm.