Missing Linker Research Articles

Tuning Size and Properties of Zinc Ascorbate Metal-Organic Framework via Acid Modulation.

One of the biggest advantages of MOFs is the possibility of modifying their properties and tuning their inherent activity (i.e., sorption, storage, catalytic activity etc.). Textural properties can be tuned by manipulating process and compositional parameters, among which, the effect of additives can be even further distinguished among them based on the way they affect these properties. Beyond the effect that additives have on the size and morphology of nanoMOFs, there is also an effect on properties via creating point defects-missing linker and missing node defects. In this study, we investigated the effect of four monotopic acid modulators-formic, acetic, dichloroacetic and propionic acid, their concentration and the heating type (conventional and microwave-MW) on the size, morphology and textural properties of a recently discovered bioNICS1. It was confirmed that the proposed seesaw model for the controlled size of nanoMOF crystals is less applicable in the case of MW-assisted synthesis, in comparison to conventional heating. In the case of formic acid- and propionic acid-modified materials, we demonstrated that the type of additive plays a different role in crystal growth and generation of defects, implying high tunability being crucial for a material's structure-property performance optimization.

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.

Histidine-engineered metal-organic frameworks with enhanced peroxidase-like activity for sensitive detection of metallothioneins

The elaborate design and fabrication of nanomaterials to mimic natural enzyme-catalyzed active sites is a promising approach to significantly improve catalytic performances, which is beneficial to the signal amplification for biosensing applications. Metal–organic frameworks (MOFs) with tailorable components, structures and well-distributed metal sites are supposed to tackle the challenges in traditional nanomaterials of vague structure and imprecise coordination. Herein, defect-engineered MOFs with histidine-functionalized active sites (His-MIL-101) has been constructed by the missing linker strategy. Bearing a histidine unit in the iron active site, the resultant His-MIL-101 can vividly mimic the natural enzyme catalytic pocket and exhibits double the peroxidase-like activity of parent MOFs. Notably, the introduction of histidine can not only increase the specific surface area for boosting the metal active site exposure but also provide an optimized electronic structure of active sites to promote the generation of active intermediate (hydroxyl radical). By virtue of the synergistic effects of metallothioneins on metal active site-blocking impact and free radicals scavenging, His-MIL-101-based colorimetric biosensing platform was constructed to accurately detect metallothioneins with a wide detection range from 20 nM to 50 μM, as well as a low detection limit of 10.49 nM. This approach paves the way for the design of advanced biomimetic nanomaterials to efficiently mimic the active center of natural enzymes and further extend the application in biosensing.

Efficient photo-Fenton degradation performance, mechanism, and pathways of tetracycline hydrochloride over missing-linker metal–organic framework with mix-valence coordinatively unsaturated metal sites

Photo-Fenton performance of Fe-based metal–organic frameworks needs to be further improved owing to little coordinatively unsaturated metal sites (Lewis acid sites) and poor photogenerated carrier separation and migration efficiency. In view of this problem, this work has developed missing-linker metal–organic framework with mix-valence coordinatively unsaturated metal site (abbreviated as CUS-Pac-MIL-100 (Fe)) for photo-Fenton degradation by thermal activation and introducing missing linker. The structure of catalysts was characterized by SEM, XRD, XPS, in-situ DRIFT, TG, BET, and EPR, which proves abundant mixed-valence coordinatively unsaturated metal sites are introduced by vacuum thermal activation and plenty of ligand vacancies are introduced by missing-linker. The conditions for TC-HCl degradation were optimized. The results indicate that CUS-Pac-MIL-100 (Fe) exhibits remarkable removal rate of TC-HCl within 80 min at 10 mL/L H2O2 dosage, 0.2 g/L catalyst dosage, and a wide pH range (4.0–7.0). The total organic carbon (TOC) removal rate also reaches 52.3% within 80 min. Such remarkable improvement in the photo-Fenton activity is attributed to the mix-valence coordinatively unsaturated metal sites improving the adsorption and activation ability of H2O2, and the ligand vacancies promoting the separation efficiency of carriers to accelerate Fe2+ regeneration. The catalyst also exhibits excellent cyclability and stability after multiple cycles. Further, the degradation mechanism and degradation pathways of TC-HCl in the photo-Fenton reaction has been proposed on the basis of radical quenching, electron paramagnetic resonance, and LC-MS tests. This work provides a worthy insight for the construction of high-efficiency MOF-based photo-Fenton 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.

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.

Creation of Exclusive Artificial Cluster Defects by Selective Metal Removal in the (Zn, Zr) Mixed-Metal UiO-66.

The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO2 sorption and heterogeneous catalysis.

Defective Metal–Organic Framework Assisted with Nitrogen Doping Enhances the Photoelectrochemical Performance of BiVO4

BiVO4 is a promising n-type semiconductor for photoelectrochemical (PEC) water splitting, which can serve as a photoanode. However, severe surface recombination and slow water oxidation kinetics hinder the realization of its highly theoretical PEC performance. Single-atom catalyst-like metal–organic frameworks (MOFs) and their derived metal oxides have been broadly investigated to enhance the kinetics of BiVO4 photoanodes. According to the principle of catalysis, only coordinatively unsaturated atoms can participate in the catalytic reaction. Herein, a defective cobalt-based MOF (d-CoMOF) with missing linker defects is modified onto the surface of N-doped BiVO4 (N:BVO) photoanodes. The photocurrent density of d-CoMOF/N:BVO is 3.59 times higher than that of pristine BVO (0.56 mA/cm2) at 1.23 VRHE. The onset potential of d-CoMOF/N:BVO shows a cathodic shift of 300 mV relative to BVO. The roles of the d-CoMOF overlayer and N doping are experimentally revealed. The d-CoMOF modification and N doping could increase the electron density, passivate the surface states, and promote the catalysis kinetics of BVO. Thus, the kinetic parameters, such as the charge separation/injection efficiency, onset potential, and photocurrent density, are significantly enhanced. This work provides a way to use MOFs for PEC water splitting by the introduction of defects.

The missing linker between SUN5 and PMFBP1 in sperm head-tail coupling apparatus

The sperm head-to-tail coupling apparatus (HTCA) ensures sperm head-tail integrity while defective HTCA causes acephalic spermatozoa, rendering males infertile. Here, we show that CENTLEIN is indispensable for HTCA integrity and function, and that inactivation of CENTLEIN in mice leads to sperm decapitation and male sterility. We demonstrate that CENTLEIN directly interacts with both SUN5 and PMFBP1, two proteins localized in the HTCA and related with acephalic spermatozoa syndrome. We find that the absence of Centlein sets SUN5 and PMFBP1 apart, the former close to the sperm head and the latter in the decapitated tail. We show that lack of Sun5 results in CENTLEIN and PMFBP1 left in the decapitated tail, while disruption of Pmfbp1 results in SUN5 and CENTLEIN left on the detached sperm head. These results demonstrate that CENTLEIN cooperating with SUN5 and PMFBP1 participates in the HTCA assembly and integration of sperm head to the tail, indicating that impairments of CENTLEIN might be associated with acephalic spermatozoa syndrome in humans.

A Computational Study of Isopropyl Alcohol Adsorption and Diffusion in UiO-66 Metal-Organic Framework: The Role of Missing Linker Defect.

Defect engineering leads to an effective manipulation of the physical and chemical properties of metal-organic frameworks (MOFs). Taking the common missing linker defect as an example, the defective MOF generally possesses larger pores and a greater surface area/volume ratio, both of which favor an increased amount of adsorption. When it comes to the self-diffusion of adsorbates in MOFs, however, the missing linker is a double-edged sword: the unsaturated metal sites, due to missing linkers, could interact more strongly with adsorbates and result in a slower self-diffusion. Therefore, it is of fundamental importance to evaluate the two competing factors and reveal which one is dominating, a faster self-diffusion due to larger volume or a slower self-diffusion owing to strong interactions at unsaturated sites. In this work, via Monte Carlo and molecular dynamics simulations, we investigate the behavior of isopropyl alcohol (IPA) in the Zr-based UiO-66 MOFs, with a specific focus on the missing linker effects. The results reveal that unsaturated Zr sites bind strongly with IPA molecules, which in return would significantly reduce the self-diffusion coefficient of IPA. Besides this, for the same level of missing linkers, the location of defective sites also makes a difference. We expect such a theoretical study will provide an in-depth understanding of self-diffusion under confinement, inspire better defect engineering strategics, and promote MOF based materials toward challenging real-life applications.

Interaction of metal organic framework with fluorinated polymer on ceramic hollow fiber

Zirconium-based metal–organic framework (Zr-based MOF) often appears as a highly stable material in terms of thermal and chemical aspects. However, intrinsic defects that occur in its framework lead to poor membrane performance, which hinders its amazing properties. Therefore, a new approach in preparing UiO-66-NDC (UiO: University of Oslo, NDC: naphthalenedicarboxylic acid) membrane supported on ceramic hollow fibre was designed to tackle such problems. Polymerising the MOF active layer using ultraviolet curable resin (UCR) improved the original UiO-66-NDC framework, resulting in perfect octahedron crystal morphology and smoother and densely packed integrated layers, as shown through FESEM and AFM. XPS results reveal that the photopolymerisation process lengthened the UiO-66-NDC chain during the reaction, which then supplied additional atoms from UCR to the missing linker position inside UiO-66-NDC framework. The negative water flux turned to positive water flux (from −718.820 to 16.189 L/m2∙h), with a sharp decrement in reverse solute flux (from 34.194 to 0.007 kg/m2∙h) in the forward osmosis (FO) test.

Tuning the Properties of MOF-808 via Defect Engineering and Metal Nanoparticle Encapsulation.

Defect engineering and metal encapsulation are considered as valuable approaches to fine‐tune the reactivity of metal–organic frameworks. In this work, various MOF‐808 (Zr) samples are synthesized and characterized with the final aim to understand how defects and/or platinum nanoparticle encapsulation act on the intrinsic and reactive properties of these MOFs. The reactivity of the pristine, defective and Pt encapsulated MOF‐808 is quantified with water adsorption and CO2 adsorption calorimetry. The results reveal strong competitive effects between crystal morphology and missing linker defects which in turn affect the crystal morphology, porosity, stability, and reactivity. In spite of leading to a loss in porosity, the introduction of defects (missing linkers or Pt nanoparticles) is beneficial to the stability of the MOF‐808 towards water and could also be advantageously used to tune adsorption properties of this MOF family.

Impact of defects on the decomposition of chemical warfare agent simulants in Zr‐based metal organic frameworks

AbstractMetal organic frameworks (MOFs) containing zirconium secondary building units (SBUs) in UiO‐67 and related MOFs, are highly active for neutralizing both the chemical warfare agents and simulants, such as dimethyl methylphosphonate (DMMP). However, two recent publications gave conflicting reports of DMMP reaction with UiO‐67 under ultra high vacuum (UHV) conditions, with one reporting chemisorption and reaction (Wang et al., J Phys Chem C, 2017, 121, 11261–11272) and the other reporting only physisorption and reversible desorption (Ruffley et al., J Phys Chem C, 2019, 123, 19748–19758) from very similar temperature programmed desorption experiments. We show that the discrepancy between these experiments may be explained by different levels of missing linker defects in the UiO‐67 samples. We present density functional theory calculations showing that SBU sites having two‐adjacent missing linkers exhibit reaction barriers that are about 30 kJ/mol lower than SBU sites having a single missing linker. We also show that topology of the undercoordinated sites plays an important role in the reaction barrier under UHV conditions.

The Effects of Functional Groups and Missing Linkers on the Adsorption Capacity of Aromatic Hydrocarbons in UiO-66 Thin Films

The adsorption of benzene, toluene, ethylbenzene, and xylene isomers, also known as BTEX, from the gas phase into porous thin films of the metal–organic framework UiO-66-X, where X = H, NH2, and NO2, was measured to quantify adsorption capacity. The thin films were grown by a vapor-conversion method onto Au-coated quartz microbalance crystals. The MOF thin films were characterized by IR and Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The thin films were activated by heating under high vacuum and exposed to each gas to calculate the Henry’s constant. The results demonstrate that the functional groups in the organic linker and missing-linkers both play important roles in the adsorption capacity. Several trends can be observed in the data. First, all the compounds in the BTEX family have lower Henry’s constants in the UiO-66-H films compared to the UiO-66-NH2 and UiO-66-NO2 films, which can largely be attributed to the absence of a functional group on the linker. Second, at 25 °C, the Henry’s constants for all the BTEX compounds in UiO-66-NO2 films are larger than UiO-66-NH2 films. Third, the role of missing linkers is addressed by comparing the measured adsorption capacity to ideal pore filling. The results show that the UiO-66-H films are the most defect-free and the UiO-66-NO2 films have the most missing linker defects.

Ultrafast Removal of Phosphate from Eutrophic Waters Using a Cerium-Based Metal-Organic Framework.

Phosphate removal has become a critical need to mitigate the negative effect of water eutrophication, which is responsible for the overgrowth of toxic algal blooms and the significant ecological harm generated to aquatic ecosystems. However, some of the currently available adsorbents have low removal capacity and function optimally at specific pH ranges. Here, we present an example of a cerium-based metal-organic framework (MOF) as a high-capacity sorbent for phosphate removal from eutrophic waters. Specifically, a Ce(IV)-based UiO-66 analogue, Ce 1,4-benzenedicarboxylate (Ce-BDC), was selected due to its water stability, high surface area, microporous structure, and the high binding affinity of phosphate with its open metal sites. Mechanistic studies supported by density functional theory (DFT) calculations indicate the formation of a Ce-O-P bond through ion exchange between the terminal (nonbridging) hydroxyl groups at the missing linker sites and the phosphate adducts. Experimental results demonstrate that Ce-BDC is highly selective for phosphates over other common anions (Cl-, Br-, I-, NO3-, HCO3-, SO42-) and stable in a broad pH range of (2-12), covering the relevant range for the treatment of contaminants in aquatic systems. The sorbent shows a fast removal rate, capturing significant amounts of phosphate within 4 min with a maximum adsorption capacity of 179 mg·g-1, outperforming other porous materials. These results show a remarkable adsorption capacity and fast kinetics compared with the current state-of-the-art crystalline porous materials. This study may advance the design of new microporous materials with high adsorption capabilities, good stability, and make a significant contribution to the development of future generation technology to mitigate the negative effects of water eutrophication.

Imaging defects and their evolution in a metal-organic framework at sub-unit-cell resolution.

Defect engineering of metal-organic frameworks (MOFs) offers promising opportunities for tailoring their properties to specific functions and applications. However, determining the structures of defects in MOFs-either point defects or extended ones-has proved challenging owing to the difficulty of directly probing local structures in these typically fragile crystals. Here we report the real-space observation, with sub-unit-cell resolution, of structural defects in the catalytic MOF UiO-66 using a combination of low-dose transmission electron microscopy and electron crystallography. Ordered 'missing linker' and 'missing cluster' defects were found to coexist. The missing-linker defects, reconstructed three-dimensionally with high precision, were attributed to terminating formate groups. The crystallization of the MOF was found to undergo an Ostwald ripening process, during which the defects also evolve: on prolonged crystallization, only the missing-linker defects remained. These observations were rationalized through density functional theory calculations. Finally, the missing-cluster defects were shown to be more catalytically active than their missing-linker counterparts for the isomerization of glucose to fructose.

100 MeV O7+ ion irradiation induced electrochemical enhancement in NiBTC metal-organic framework based composite polymer electrolytes incorporated with ionic liquid

Nickel 1,3,5-benzenetricarboxylate (NiBTC) metal-organic framework (MOF), has been synthesized by solvothermal method and dispersed in Poly (vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) polymer. The composite polymer electrolyte membranes were irradiated with 100 MeV O7+ swift heavy ion (SHI) at different fluences and ionic liquid (IL) BMIMBF4 has been incorporated after irradiation. Increasing % uptake of IL has been observed upon IL incorporation as SHI irradiation can enlarge the micropore volumes of NiBTC-MOF by inducing missing linker defects. FTIR spectra depict shifting of carboxylate vibrational modes with increasing fluence suggesting elongation of metal-linker molecular bonds in NiBTC-MOF due to interaction of BF4- anion of IL with unsaturated Ni metal nodes with increasing uptake of IL. Non-Debye relaxation ion dynamics has been revealed from dielectric modulus spectra and optimum ionic conductivity of 9.9 × 10−3 Scm−1 is achieved for the nanocomposite membrane at the fluence of 3.3 × 1012 ions cm−2 at 380 K. The frequency dependent AC conductivity increases with increasing fluence due to higher IL uptake as well as chain scission of the polymer segments. The electrochemical stability of the nanocomposite membrane increases with increasing fluence and attains a maximum value of 6.1 V at 3.3 × 1012 ions cm−2.

Zr-Metal-Organic Frameworks Featuring TEMPO Radicals: Synergistic Effect between TEMPO and Hydrophilic Zr-Node Defects Boosting Aerobic Oxidation of Alcohols.

Metal-organic frameworks (MOFs) featuring multiple catalytic units are excellent platforms for heterogeneous catalysis. However, the synergism between multiple catalytic units for catalysis is far from being well understood. Herein, we reported the synthesis of a robust 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) radical-functionalized Zr-MOF (UiO-68-TEMPO) in the form of single-crystalline and microsized crystals with varied missing linker defects. Detailed catalytic studies and theoretical calculations reveal that the synergistic effect between the TEMPO radicals and hydrophilic and defective Zr-nodes endows UiO-68-TEMPO with superior catalytic activity toward aerobic oxidation of alcohols. Our work not only offers a new route to design and synthesize highly effective MOF catalysts but also provides insights into the synergism between multiple catalytic sites.

Integrational analysis of miRNAs data sets as a plausible missing linker between Epstein-Barr virus and vitamin D in relapsing remitting MS patients

Given the multifactorial state of autoimmune complex diseases such as multiple sclerosis (MS), it is not clear if different risk factors act jointly or independently. Despite intensive studies investigating multi aspects of MS risk factors, findings with regards to potential biomarkers that may link these risk factors remained largely inconclusive.System biology or data integration utilizes different validated datasets to extract meaningful information and map the plausible biological pathways and networks. As such, we integrated eight transcriptome datasets to find the differentially expressed miRNAs in peripheral blood (PB) between relapsing remitting MS patients (RRMS) and normal group. After identification the targeted genes of miRNAs, the hub genes were used to construct the underlying protein–protein interaction network and signaling pathways.As results, 9 miRNAs were best exemplified by significant dysregulation including hsa-mir-15a, hsa-mir-484, hsa-mir-30d, hsa-mir-145, hsa-mir-363, has-let-7e, hsa-mir-30a, hsa-let-7b, and hsa-mir-146a. System biology analysis of miRNAs in PB of RRMS patients clearly indicates the involvement of miRNAs in many vital pathways and highlighted the possibility of an association between miRNAs with EBV and vitamin D in MS pathogenesis.Described novel pathways and genes related to miRNAs such as Transient receptor potential channels and Acid sphingomyelinase may provide a potential target for therapeutic approaches although further functional studies are warranted to test these candidates.