Missing Linker Defects Research Articles

An investigation on the influence of highly acidic media on the microstructural stability and dye adsorption performance of UiO-66

Stability of UiO-66 in 1 M HCl for different exposure times up to 15 days was investigated by employing various characterization techniques such as XRD, FESEM, TEM, TGA, and nitrogen adsorption/desorption. The lower surface area, smaller pore size, and slightly lower thermal stability indicated that the UiO-66 structure experienced a partial breakdown, but could maintain its main structure and showed acceptable stability. In addition, the exposure to the HCl solution added to the missing-linker defects in the structure of UiO-66. The adsorptive performance of HCl-aged UiO-66 nanoparticles towards anionic methyl orange (MO) dye was then evaluated. Compared to as-synthesized UiO-66, maximum dye adsorption capacity of HCl-aged UiO-66 towards MO exhibited over 60 % loss for 15 days aged UiO-66. Moreover, negative charges took over the positive surface charge of as-synthesized UiO-66 upon HCl aging. Therefore, the selective adsorption analysis showed that while neat UiO-66 adsorbs MO selectively over cationic dyes, acid-aged samples have a higher affinity towards cationic dyes. The adsorption kinetics of pristine and acid-aged UiO-66 particles were well outlined by pseudo-second-order, and isotherm studies demonstrated that the adsorption could be delineated well by the Langmuir and Freundlich model, inferring that adsorption takes place both multilayer and monolayer in parts.

Adsorption and degradation of multiple micropollutants with trace concentrations by porous Zr-porphyrin metal-organic frameworks with missing linker defects in water

Metal-organic frameworks (MOFs) are effective photocatalysts with high surface area and excellent visible light absorption. In this study, a kind of Zr-porphyrin (PCN-134) and its derivatives PCN-134(Fe) and PCN-134 (Cu) were prepared for removing six structurally diverse micropollutants with extremely low concentrations (100 μg/L). The initial adsorption rates for five selected pharmaceuticals (antipyrine (AT), sulfadimethoxypyrimidine (SSM), clofibric acid (CA), diclofenac (DF), and ibuprofen (IBU)) ranged from 174.5 μg/(g • min) to 2256.5 μg/(g • min) over PCN-134 (Cu), which is 1.2–2.5 times higher than those of PCN-134. A series of optical and photoelectric properties were studied using Mott−Schottky plots, UV–Vis diffuse reflectance spectrum (UV-DRS), and transient photocurrent intensity analysis. PCN-134 (Fe) successfully extended the visible-light absorption edges at around 750 nm. PCN-134 and PCN-134 (Fe) were favorable for generating •O2− with ECB values calculated at −1.04 eV and - 0.55 eV by Mott−Schottky plots and UV–Vis diffuse reflectance spectrum (UV-DRS). The incorporation of metals enhanced the charge separation properties of PCN-134. The transient photocurrent intensity of PCN-134 (Fe) was 3.1 times higher than that of PCN-134 (Cu). The degradation efficiency for naproxen (NPX) over PCN-134 (Fe) for antipyrine (AT) and clofibric acid (CA) was 2.4 times and 11.3 times higher than those over the pristine PCN-134. •O2− and 1O2 could be the major ROS produced, confirmed by quenching experiments and electron paramagnetic resonance (EPR) technologies. This study offers useful guidance to improve the current materials for water purification.

Repair of missing linker defects in UiO-66 by a "molecular patch" boosting the 1-hexene hydroformylation reaction.

Fabrication of MOFs with missing linker defects has become a common means to improve catalytic performances. However, the stability of the defects deserves to be investigated first. In this work, we found that 3-phenylpropionaldehyde (3-PPA) could coordinate with the missing linker defects of UiO-66, which highlighted the instability of the missing linker defects. 3-PPA acted as a molecular patch for the modification of the Rh/UiO-66 catalyst, which repaired the open Zr6 sites and resulted in a remarkable improvement of aldehyde selectivity (from 50.0% to 89.6%) in 1-hexene hydroformylation.

Correlated missing linker defects increase thermal conductivity in metal-organic framework UiO-66.

Thermal transport in metal-organic frameworks (MOFs) is an essential but frequently overlooked property. Among the small number of existing studies on thermal transport in MOFs, even fewer have considered explicitly the influence of defects. However, defects naturally exist in MOF crystals and are known to influence many of their material properties. In this work, we investigate the influence of both randomly and symmetrically distributed defects on the thermal conductivity of the MOF UiO-66. Two types of defects were examined: missing linker and missing cluster defects. For symmetrically distributed (i.e., spatially correlated) defects, we considered three experimentally resolved defect nanodomains of UiO-66 with underlying topologies of bcu, reo, and scu. We observed that both randomly distributed missing linker and missing cluster defects typically decrease thermal conductivity, as expected. However, we found that the spatial arrangement of defects can significantly impact thermal conductivity. In particular, the spatially correlated missing linker defect nanodomain (bcu topology) displayed an intriguing anisotropy, with the thermal conductivity along a particular direction being higher than that of the defect-free UiO-66. We attribute this unusual defect-induced increase in thermal conductivity to the removal of the linkers perpendicular to the primary direction of heat transport. These perpendicular linkers act as phonon scattering sources such that removing them increases thermal conductivity in that direction. Moreover, we also observed an increase in phonon group velocity, which might also contribute to the unusual increase. Overall, we show that structural defects could be an additional lever to tune the thermal conductivity of MOFs.

Real-Space Imaging of the Node-Linker Coordination on the Interfaces between Self-Assembled Metal-Organic Frameworks.

Surface and interface, with unique local characteristics different from bulk structure, are of great significance in various applications of metal-organic frameworks (MOFs), which should be studied by real-space imaging methods, such as electron microscopy. However, it is still challenging to atomically resolve these local structures in MOFs, because they are even more sensitive to electron irradiation. Here, we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to achieve the atomic imaging of both the metal nodes and organic linkers in UiO-66 (Zr) nanocrystals and their assembly. After adding acetic acid, we modulate the whole process of MOF assembly and observe the organic linkers at both the surfaces and twin interfaces in the chemically assembled UiO-66 (Zr) crystals by the iDPC-STEM. These results bring us a deeper understanding on the role of acid modulators that promote the MOF assembly by generating the missing-linker defects on the crystal surface.

Boosting Photocatalytic Hydrogen Peroxide Production from Oxygen and Water Using a Hafnium-Based Metal–Organic Framework with Missing-Linker Defects and Nickel Single Atoms

Boosting Photocatalytic Hydrogen Peroxide Production from Oxygen and Water Using a Hafnium-Based Metal–Organic Framework with Missing-Linker Defects and Nickel Single Atoms

Controlling the Adsorption and Release of Ocular Drugs in Metal-Organic Frameworks: Effect of Polar Functional Groups.

A series of UiO-66 materials with different functional groups (-H, -NH2, and -NO2) have been evaluated for the adsorption and release of a common ocular drug such as brimonidine tartrate. UiO-66 samples were synthesized under solvothermal conditions and activated by solvent exchange with ethanol. Experimental results suggest that the incorporation of surface functionalities gives rise to the development of structural defects (missing linker defects) but without altering the basic topology of the UiO-66 framework. These defects improve the adsorption performance of the parent metal-organic framework (MOF), while the bulkier functionalities infer slower release kinetics, with the associated benefits for prolonged delivery of brimonidine. Among the evaluated MOFs, defective UiO-66-NO2 can be proposed as the most promising candidate due to the combination of a larger brimonidine volumetric uptake (680 mg/cm3), a prolonged delivery (period of up to 25 days), a small particle size, and a larger instability. Contrariwise, at high concentrations UiO-66-NO2 has higher toxicity toward human retinal pigment epithelium cells (ARPE-19) compared to the pure and NH2-functionalized UiO-66.

Ultrafast and selective adsorption of pharmaceuticals from wastewater by precisely designed metal organic framework with missing linker defects

Pharmaceuticals and personal care products (PPCPs) are widely used and frequently detected in the environment. A series of novel, precisely designed metal-organic framework (MOF) materials with long-range ordered defective structure was proposed by tuning the node connectivity and missing linkers/clusters. The adsorption abilities of anti-inflammatory drugs were significantly improved (79.87 mg/g, 561.37 mg/g·min for ibuprofen (IBU)) over the defective UiO-66 (node connectivity = 7.8), which were 2.5 and 59.8 times higher than that over the defect-free one (32.16 mg/g, 9.39 mg/g • min). The adsorption rate constant of IBU was accelerated 1.12 × 104 times greater than that of the activated carbon. For the first time, the performance was evaluated under the environmentally relevant concentration (200 μg/L to 0.025 μg/L) with removal percentages ∼ 90% within 1 min. Theoretical calculations were conducted to study the mechanism. Furthermore, 15 micropollutants with various functional groups (sulfonamides, quinolones, macrolides, etc.) with high environmental risks proved our concept that electrostatic interaction could be the primary mechanism. Real wastewater in Beijing was sampled and evaluated the performance of removing the pharmaceuticals (concentration: 199.21 ± 3.65 ng/L ∼ 0.10 ± 0.01 ng/L), a complete removal can be achieved in only 3 min with concentrations below instrumental detection limits. This finding provides a promising strategy for designing materials for rapidly removing pharmaceuticals in water.

Defect controlled MOF-808 for seawater uranium capture with high capacity and selectivity

Highly effective extraction of uranium from seawater calls for the development of adsorbents with both high capacity and selectivity. Defective Zr-based metal–organic frameworks (MOFs) are potential candidates for uranium sequestration due to their high surface area and tunable active sites. In this work, MOF-808 samples with adjustable missing linker defects were successfully prepared by manipulating the concentration of the carboxylic acid modulator. Impressively, MOF-808 with the highest defect density, namely M808-4, showed excellent selectivity towards uranium in comparison with coexisting competitive ions in seawater. Moreover, M808-4 exhibited remarkable uranium extraction performance in real seawater with an uptake capacity of 5.83 mg/g in 28 d, which is 10.41 times higher than that for vanadium. The results indicated that the abundant Zr unsaturated active sites formed by defect inducing strategy are the key to the efficient capture of uranyl ions. This study highlighted the advantages of defect engineering in MOF based materials for highly selective uranium extraction from seawater.

Missing-linker Defects in Covalent Organic Framework Membranes for Efficient CO2 Separation.

Covalent organic framework (COF) membranes with tunable ordered channels and free organic groups hold great promise in molecular separations owing to the synergy of physical and chemical microenvironments. Herein, we develop a defect engineering strategy to fabricate COF membranes for efficient CO2 separation. Abundant amino groups are in situ generated on the COF nanosheets arising from the missing-linker defects during the reactive assembly of amine monomer and mixed aldehyde monomers. The COF nanosheets are assembled to fabricate COF membranes. Amino groups, as the CO2 facilitated transport carriers, along with ordered channels endow COF membrane with high CO2 permeances exceeding 300 GPU and excellent separation selectivity of 80 for CO2 /N2 , and 54 for CO2 /CH4 mixed gas under humidified state. Our defect engineering strategy offers a facile approach to generating free organic functional groups in COF membranes and other organic framework membranes for diverse chemical separations.

Effect of Missing-Linker Defects on CO2Hydrogenation to Methanol by Cu Nanoparticles in UiO-66

Effect of Missing-Linker Defects on CO₂Hydrogenation to Methanol by Cu Nanoparticles in UiO-66

Influence of the defects on selective hydrogenation of cinnamaldehyde to cinnamyl alcohol over UiO-66 supported Pt catalysts

UiO-66 supported metal catalysts have shown high catalytic activity and selectivity to cinnamyl alcohol (COL) in the selective hydrogenation (SHYD) of cinnamaldehyde (CAL), but the roles of missing-linker defects (MLDs) in UiO-66 on the catalytic performance of the UiO-66 supported metal catalysts have less been reported. Here, UiO-66 supports with the different amount of MLDs were used as the supports, and three series of UiO-66 supported Pt (Pt/U) catalysts were prepared by sol-impregnation, wet-impregnation as well as in-situ encapsulation methods, respectively. The influences of the MLDs in UiO-66 on the catalytic properties of the prepared Pt/U catalysts in the SHYD of CAL were investigated. The series of Pt/U-720 catalysts with the more MLDs showed higher CAL conversion and selectivity to COL than the corresponding Pt/U-F catalysts with fewer MLDs. The different preparing methods affect the location and dispersion of Pt species on UiO-66 supports and the metal-support interaction, but the location of the supported Pt species on UiO-66 is the critical factor affecting the catalytic properties of Pt/U catalysts. The presence of the MLDs in UiO-66 is the key to preparing UiO-66 supported Pt catalysts with high SHYD performance.

Defect-engineered metal-organic framework with enhanced photoreduction activity toward uranium extraction from seawater

Photoreduction conversion of soluble U(VI) to insoluble U(IV) is considered a promising strategy for uranium capture from radioactive wastewater and seawater. Metal-organic frameworks (MOFs) are emerging as potential photocatalysts due to their inherent large surface areas, tunable cavities, and tailorable photo response. Herein, the electronic structure of UiO-66-NH2 (Zr-MOF) is modulated by missing-linker defects and node metal substitution, allowing for the reduced energy needed to transfer the excited linker’s electrons to the node’s unoccupied d orbitals, which contributes to enhanced photocatalytic uranium extraction capacity. Moreover, the missing-linker defects create more open frameworks, which facilitate fast diffusion of uranyl ions (UO22+) into inner pores and hence increase utilization of the hidden adsorption binding sites. Consequently, light-irradiated Zr/Ti-MOF-25 delivers a high uranium sorption capacity (8.66 ± 0.29 mg-U/g-Ads) with excellent antimicrobial ability in natural seawater. Creation of defects in MOFs opens novel opportunities for enhancing uranium capture through combined complexation and photoreduction.

Defect Engineering in Metal-Organic Framework Nanocrystals: Implications for Mechanical Properties and Performance.

The growth process of metal–organic framework (MOF) nanocrystals defines their properties and functions. However, defects may be prevalent during the crystallization of even seemingly perfect MOFs, such as zeolitic imidazolate framework-8 (ZIF-8), and yet direct probing of such structural defects has been challenging because of the lack of nanoscale techniques to locally examine individual nanocrystals. Here, we directly study local defects, such as missing linkers or metal vacancies, in ZIF-8 nano- and microcrystals with near-field IR nanospectroscopy combined with density functional theory calculations. We track the chemical changes during crystallization and show that structural defects like zinc cations that are bound to molecules of the reactant gradually disappear with ripening of the crystals, while dangling and missing linker defects prevail. The resulting defect-terminating groups or open-metal sites produce mechanical anisotropy and reduce the Young’s modulus, as measured via tip force microscopy with nanoscale resolution and supported by theoretical modeling. However, these structural defects also open the door for defect engineering to tune the performance of ZIF-8 by offering additional adsorption sites for targeted catalytic reactions, chemical sensing, or gas capture.

Missing-linker engineering of Eu (III)-doped UiO-MOF for enhanced detection of heavy metal ions

A modulator-induced defect formation strategy was proposed to engineer luminescent lanthanide-based metal–organic framework (Eu@UIO-MOFs) and the missing-linker defects shows a positive regulation effect on the detection properties. By elaborately tailoring the defective numbers, Eu@UIO-MOFs-X with tunable structure is developed with remarkably enhanced response slope in the tiny detection window (0–10 ppm), acting as a powerful signal amplifier for trace detection of Cd 2+ . • A modulator-induced defect formation strategy was proposed. • The elaborately tailored defect numbers are compared and quantitively interpretated. • The positive regulation was realized by structural defects. • The dual signal with reverse trend provides multiple insurance for precise detection. • The defective Eu@UiO-MOFs-2 with 0.53 missing linkers shows a LOD of 5.67 × 10 −7 M. The creation of defects into luminescent MOFs allows for manipulation of fluorescent properties and thus leads to the enhanced detection performance for technological applications. Herein, a modulator-induced defect formation strategy was proposed to introduce missing-linker defect into lanthanide-based metal–organic framework (Eu@UiO-MOFs) and the enhanced detection properties was investigated. Notably, the missing-linker numbers in per Zr 6 cluster were calculated to facilitate comparison and quantitively interpretation. By elaborately tailoring defect numbers, the resultant Eu@UiO-MOFs-2 sensor with 0.53 missing linkers per Zr-O cluster showed a remarkably enhanced response slope and realized excellent trace detection under the range of 0–10 ppm with ultralow detection limit to 5.67 × 10 −7 M (114 ppb), acting as a powerful signal amplifier for trace detection of Cd 2+ . Moreover, the relationship between structural change and defect numbers of Eu@UiO-MOFs-X has been studied to understand the positive effect of the improved enrichment for the optical behavior. As far as we know, this is the first experimental demonstration of missing-linker defects in luminescent MOFs for regulation of optical sensing, offering a unique approach to explore innovative technologies for precisely detecting of heavy metal ion pollutants.

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.

Zr‐Containing UiO‐66 Metal‐Organic Frameworks as Highly Selective Heterogeneous Acid Catalysts for the Direct Ketalization of Levulinic Acid

AbstractZr‐containing UiO‐66 materials are active and reusable heterogeneous catalysts for the selective ketalization of levulinic acid (LA) with 1,2‐propanediol, affording selectivities of up to 91–93% at full LA conversion, with very low levels of ester or ketal–ester byproducts. This allows the preparation of the target ketal directly from LA and avoiding intermediate esterification steps of LA to levulinate esters to minimize the formation of unwanted side products. The catalytic activity of UiO‐66 is found to depend critically on the hydration degree of the solid and the amount of missing linker defects. The most likely active sites for ketalization in (defective) UiO‐66 are Brønsted‐induced acid sites arising from the strong coordination and polarization of H2O molecules onto accessible Zr4+ associated with missing linker defects. A progressive deactivation is observed upon catalytic reuse, which is attributed to adsorbed reaction products poisoning the catalytic sites. These adsorbed products are easily removed by washing the spent catalyst with a dilute 2% HCl ethanolic solution, which completely restores the initial catalytic activity while maintaining the crystallinity of the solid intact.

Robust ultrathin nanoporous MOF membrane with intra-crystalline defects for fast water transport

Rational design of high-performance stable metal–organic framework (MOF) membranes is challenging, especially for the sustainable treatment of hypersaline waters to address critical global environmental issues. Herein, a molecular-level intra-crystalline defect strategy combined with a selective layer thinning protocol is proposed to fabricate robust ultrathin missing-linker UiO-66 (ML-UiO-66) membrane to enable fast water permeation. Besides almost complete salt rejection, high and stable water flux is achieved even under long-term pervaporation operation in hash environments, which effectively addresses challenging stability issues. Then, detailed structural characterizations are employed to identify the type, chemical functionality, and density of intra-crystalline missing-linker defects. Moreover, molecular dynamics simulations shed light on the positive atomistic role of these defects, which are responsible for substantially enhancing structural hydrophilicity and enlarging pore window, consequently allowing ultra-fast water transport via a lower-energy-barrier pathway across three-dimensional sub-nanochannels during pervaporation. Unlike common unfavorable defect effects, the present positive intra-crystalline defect engineering concept at the molecular level is expected to pave a promising way toward not only rational design of next-generation MOF membranes with enhanced permeation performance, but additional water treatment applications.

Ultrafast and Selective Extraction of Pharmaceuticals from Natural Wastewater by Precisely Designed Mofs with Missing Linker Defects

Ultrafast and Selective Extraction of Pharmaceuticals from Natural Wastewater by Precisely Designed Mofs with Missing Linker Defects

A diamantane-4,9-dicarboxylate based UiO-66 analogue: challenging larger hydrocarbon cage platforms

HHUD-3, based on a bulky barrel shaped ligand, possesses 35%+ missing linker defects, essential for accessible porosity. With SBET= 890 m2g−1(N2), HHUD-3 features higher CH4but lower CO2and H2adsorption compared to UiO-66.