Design Of Metal-organic Frameworks Research Articles

Molecular identification and quantification of defect sites in metal-organic frameworks with NMR probe molecules

The defects in metal-organic frameworks (MOFs) can dramatically alter their pore structure and chemical properties. However, it has been a great challenge to characterize the molecular structure of defects, especially when the defects are distributed irregularly in the lattice. In this work, we applied a characterization strategy based on solid-state nuclear magnetic resonance (NMR) to assess the chemistry of defects. This strategy takes advantage of the coordination-sensitive phosphorus probe molecules, e.g., trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO), that can distinguish the subtle differences in the acidity of defects. A variety of local chemical environments have been identified in defective and ideal MOF lattices. The geometric dimension of defects can also be evaluated by using the homologs of probe molecules with different sizes. In addition, our method provides a reliable way to quantify the density of defect sites, which comes together with the molecular details of local pore environments. The comprehensive solid-state NMR strategy can be of great value for a better understanding of MOF structures and for guiding the design of MOFs with desired catalytic or adsorption properties.

Size-dependent acute toxicity and oxidative damage caused by cobalt-based framework (ZIF-67) to Photobacterium phosphoreum

Metal-organic frameworks (MOFs) are emerging nanomaterials with widespread applications for their superior properties. However, the potential health and environmental risks of MOFs still need further understanding. In this work, we investigated the toxicity of a typical cobalt-based MOF (ZIF-67) with varied primary particle sizes (100, 200, 400, 700 and 1200 nm) to Photobacterium Phosphoreum T3 strain, a kind of luminescent bacteria. The luminescence inhibition rate of all ZIF-67 nanoparticles (NPs) reached 40 % and higher at the concentration of 5 mg/L, exhibiting strong toxicity. Combined cellular assays and gene expression analysis confirmed that the general bioactivity inhibition and oxidative damage were induced mainly by ZIF-67 NPs, rather than Co2+ released from the ZIF-67 NPs. Additionally, the toxicity of ZIF-67 NPs demonstrated an evident size-dependent effect. For ZIF-67 smaller than 400 nm, the toxicity increased with the particle size decreased, while the trend was not significant when the particle size was larger than 400 nm. A potential explanation for this phenomenon is the smaller NPs (100 and 200 nm) may enter the cytoplasm, accumulating in the cytoplasm and causing more severe toxicity. Furthermore, Co2+ released from the ZIF-67 NPs was not the primary contributor to the toxic effect of ZIF-67 NPs which was verified by the toxicity results and the variation of toxicity-related indicators. These findings provided insight into the better design and safer use of MOFs, and it also implied the potential environmental risk of the MOF's cannot be ignored, especially for the bioapplication.

Precise Introduction of Single Vanadium Site into Indium-Organic Framework for CO2 Capture and Photocatalytic Fixation.

The capture and fixation of CO2 under mild conditions is a cost-effective route to reduce greenhouse gases, but it is challenging because of the low conversion and selectivity issues. Metal-organic frameworks (MOFs) are promising in the fields of adsorption and catalysis because of their structural tunability and variability. However, the precise structural design of MOFs is always pursued and elusive. In this work, a metal-mixed MOF (SNNU-97-InV) was designed by precisely introducing single vanadium site into the isostructural In-MOF (SNNU-97-In). The single V sites clearly change the interactions between the MOF framework and CO2 molecules, leading to a 71.3% improvement in the CO2 adsorption capacity. At the same time, the enhanced light absorption enables SNNU-97-InV to efficiently convert CO2 into cyclic carbonates (CCs) with epoxides under illumination. Controlled experiments showed that the promoted performance of SNNU-97-InV may be that the V═O site can more easily combine with CO2 and convert them into an intermediate state under illumination, and the possible mechanism was thus speculated.

Morphology Control Strategy of Bimetallic MOF Nanosheets for Upgrading the Sensitivity of Noninvasive Glucose Detection.

The precise measurement of glucose level is significant for the health management of the human body. However, the existing sensitive materials and detection methods for glucose are less satisfying for practical applications. Herein, an ultrathin reticular two-dimensional nanosheets array composed of trimesic acid (H3BTC)-based bimetal metal-organic frameworks (MOFs) and carbon cloth (CC), which is constructed through a morphology control strategy, is reported for glucose sensing. Meanwhile, this nonmoving sweat glucose sensor based on a NiCo-BTC/CC electrode has been successfully prepared by a screen printing method. Benefiting from the regular and ultrathin nanosheets array, the NiCo-BTC/CC electrode has an excellent sensitivity of 2701.29 μA mM-1 cm-2, which is about 2.4 times that of its unregulated counterpart (1127.85 μA mM-1 cm-2) in the linear range 5-205 μM. In addition, an ultralow detection limit (0.09 μM, S/N = 3) and good selectivity of NiCo-BTC/CC were also obtained. The high sensitivity of the glucose sensor based on NiCo-BTC/CC electrode is 0.174 μA μM-1 (50-1000 μM). Remarkably, the preciously designed sensor is used to detect glucose concentration in sweat with a noninvasive mode, and the results are basically consistent with those of a commercial glucose device with an invasive mode. This research exhibits potential methodology for the morphology design of bimetallic MOFs nanosheets to achieve a high accuracy rate and noninvasive and timeless measurement of a glucose sensor.

Approaches to Enhancing Electrical Conductivity of Pristine Metal-Organic Frameworks for Supercapacitor Applications.

Metal-organic frameworks (MOFs), known as porous coordination polymers, have attracted intense interest as electrode materials for supercapacitors (SCs) owing to their advantageous features including high surface area, tunable porous structure, structural diversity, etc. However, the insulating nature of most MOFs has impeded their further electrochemical applications. A common solution for this issue is to transform pristine MOFs into more stable and conductive metal compounds/porous carbon materials through pyrolysis, which however losses the inherent merits of MOFs. To find a consummate solution, recently a surge of research devoted to improving the electrical conductivity of pristine MOFs for SCs has been carried out. In this review, the most related research work on pristine MOF-based materials is reviewed and three effective strategies (chemical structure design of conductive MOFs (c-MOFs), composite design, and binder-free structure design) which can significantly increase their conductivity and consequently the electrochemical performance in SCs are proposed. The conductivity enhancement mechanism in each approach is well analyzed. The representative research works on using pristine MOFs for SCs are also critically discussed. It is hoped that the new insights can provide guidance for developing high-performance electrode materials based on pristine MOFs with high conductivity for SCs in the future.

Metal‐Organic Frameworks Functionalized Separators for Lithium‐Sulfur Batteries

Lithium sulfur batteries (LSBs) have attracted tremendous attention owing to their high theoretical specific capacity and specific energy. However, their practical applications are hindered by poor cyclic life, mainly caused by polysulfide shuttling. The development of advanced materials to mitigate the polysulfide shuttling effect is urgently demanded. Metal-organic frameworks (MOFs) have been exploited as multifunctional materials for the decoration of separators owing to their high surface area, structural diversity, tunable pore size, and easy tailor ability. In this review, we aim to present the state-of-the-art MOF-based separators for LSBs. Particular attention is paid to the rational design (pore aperture, metal node, functionality, and dimension) of MOFs with enhanced ability for anchoring polysulfides and facilitating Li+ transportation. Finally, the challenges and perspectives are provided regarding to the future design MOF-based separators for high-performance LSBs.

Conductive Metal-Organic Frameworks for Supercapacitors.

As a class of porous materials with crystal lattices, metal-organic frameworks (MOFs), featuring outstanding specific surface area, tunable functionality, and versatile structures, have attracted huge attention in the past two decades. Since the first conductive MOFissuccessfully synthesized in 2009, considerable progress has been achieved for the development of conductive MOFs, allowing their use in diverse applications for electrochemical energy storage. Among those applications, supercapacitors have received great interest because of their high power density, fast charging ability, and excellent cycling stability. Here, the efforts hitherto devoted to the synthesis and design of conductive MOFs and their auspicious capacitive performance are summarized. Using conductive MOFs as a unique platform medium, the electronic and molecular aspects of the energy storage mechanism in supercapacitors with MOF electrodes are discussed, highlighting the advantages and limitations to inspire new ideas for the development of conductive MOFs for supercapacitors.

Magnetically recyclable nanocomposites via lanthanide-based MOFs grown on natural sea sponge: Screening hydrogenation of nitrophenol to aminophenol

Catalysts have played a significant role in chemical industries and received significant attention as a result. Unfortunately, many common catalytic systems are comprised of a small number of elemental species or use similar synthetic strategies, which potentially limit innovation in areas such as the discovery of new transformations. Metal-organic frameworks (MOFs) have been established as versatile materials employed in various applications that include catalysis; however, the majority of MOFs have been synthesized using transition metal building blocks, possibly hindering advances towards innovative materials with unique properties. This work reports the synthesis of MOFs based on lanthanide elements (Ln = Ce, Gd, La, and Sm), which demonstrate great potential yet are often overlooked. The Ln-based MOFs were grown in situ on natural sea sponge supports and Fe ion containing solution, followed by a thermal carbonization treatment to generate FeLn metal nanoparticles (NPs) embedded on carbon support (FeLn@C). These nanostructured products exhibited good crystallinity and well-confined particle sizes. We then demonstrate that these hybrid materials can be used as effective heterogeneous nanocatalysts for the hydrogenation of nitrophenol to aminophenol in aqueous media. Besides, FeLn@C catalysts had a magnetic property due to Fe atoms in the NPs, which can be mechanically separated out from the aqueous solution since the catalysts are magnetically attracted to the external magnets. This magnetic recyclability combined with the usage of the natural materials highlight the importance of sustainability in the design of Ln-based MOFs and their catalytic applications.

Molecular-fingerprint machine-learning-assisted design and prediction for high-performance MOFs for capture of NMHCs from air

The capture of trace amounts of non-methane hydrocarbons (NMHCs) from air due to the toxicity of volatile organic compounds is a significant challenge. A total of 31399 hydrophobic metal–organic frameworks (MOFs) were first screened from 137953 hypothetical MOFs using high-throughput computational screening (HTCS), and their performance indices (adsorption capacity and selectivity) for the adsorption of NMHCs (C 3 –C 6 ) were obtained by molecular simulations. The discovery of a “second peak” near twice the kinetic diameter of the corresponding NMHC provided more choices for excellent MOFs that adsorb NMHCs. Four machine learning (ML) classification and regression algorithms predicted the performance of MOFs, and the relative importance values of the six descriptors were determined. The combination of the Random Forests algorithm and Molecular ACCess Systems molecular fingerprint (MF) had an excellent predictive ability for MOFs. According to the performance, the fingerprint commonalities of the 100 top-performing MOFs were counted, and the excellent bits (EBs) that could promote the performance were defined. Finally, new substructures containing all of the EBs were designed for each NMHC to build a new MOF database. This work combined the HTCS, ML, and MF to provide a detailed insight into the design of efficient MOFs for adsorbing NMHCs.

Layer-by-layer growth of ferrocene decorated metal–organic framework thin films and studies of their electrochemical properties

As electroactive metal–organic frameworks (MOFs) become the centerpiece of many emerging applications in electrocatalysis, energy storage/conversion and electronics, the ability to grow high-quality of electroactive MOF thin films and understanding the mechanism of charge transport in the frameworks are crucial. Herein, we demonstrate that the use of ferrocene pendant biphenyl dicarboxylate (bpdc-Fc) ligand allows the successful growth of redox-active surface-attached MOFs (SURMOFs) [Cu2(bpdc-Fc)2dabco] SURMOF, on self-assembled monolayers (SAMs) functionalized surfaces by layer-by-layer (LBL) deposition method. The nature of the SAMs significantly affects the morphologies and electrochemical properties of [Cu2(bpdc-Fc)2dabco] SURMOFs owing to their different ability to coordinate Cu2 dimmer. The purine-based SAMs offer excellent anchoring sites for the nucleation of MOF crystals leading to dense films with good electrochemical properties. On the contrary, sparse MOF nanocrystals with poor electrochemical properties were obtained on pyridine-based SAMs. A redox hopping mechanism accompanied by anion diffusion was proposed to explain the different redox properties of [Cu2(bpdc-Fc)2dabco] SURMOFs on SAMs. This work has potential to contribute to rational design and growth of novel MOFs for the applications in ionic conductors, electrocatalysts, energy storage devices and electrochemical sensors.

The Growth of Metal-Organic Frameworks in the Presence of Graphene Oxide: A Mini Review.

Integrated metal–organic frameworks (MOFs) with graphene oxide (GO) have aroused huge interest in recent years due to their unique properties and excellent performance compared to MOFs or GO alone. While a lot of attention has been focused on the synthesis methodologies and the performance analysis of the composite materials in recent years, the fundamental formation/crystallization mechanism(s) is (are) still not fully understood. Ascribed to the distinctive structural and functional properties of GO, the nucleation and crystallization process of MOFs could be altered/promoted, forming MOF/GO composite materials with different nanostructures. Furthermore, the MOF’s parental structure could also influence how the GO and MOF bond together. Thus, this short review attempted to provide critical and indepth discussions of recent research results with a particular focus on the factors that influence the directional growth of parent MOFs in the presence of graphene oxide. Due to the unique structure and enhanced properties, the derived MOF/GO composites have a wide range of applications including gas separation, electrochemistry, and photocatalysis. We hope this review will be of interest to researchers working on MOF design, crystal structure control (e.g., orientation), and composite materials development.

Screening of hierarchical porous UiO-67 for efficient removal of glyphosate from aqueous solution

Defect engineering is essential for improving the adsorption performance of metal-organic frameworks (MOFs). Herein, hierarchical porous UiO-67(Zr) and analogs with controlled levels of defects were successfully obtained and applied to chemoselectively capture glyphosate (GP) from water. The obtained defective UiO-67 was stable at different pH values. The removal performance of defective UiO-67(Zr) was evaluated systematically as a function of different modulators, water chemistry and interfering ions. The maximum adsorption capacity of defective UiO-67(Zr) reached 322.58 mg/g, superior to most MOFs-based adsorbents. The defective UiO-67(Zr) also showed high selectivity for GP adsorption at high concentrations of humic acids (3 times) and interfering anions (100 times). In addition, the defective UiO-67(Zr) has shown good recovery performance and can effectively remove GP from a variety of contaminated water samples and multi-pollutant systems. The results of batch experiments and microscopic spectral characterization (XPS and FTIR) revealed that GP adsorption was mainly guided by electrostatic action, coordination, pore-filling and hydrogen bonding. These findings verified that the fabrication of defect sites played an important role in promoting the adsorption performance of MOFs. Our work provides a clue to the design of defective MOFs with high adsorption performance.

Switched Proton Conduction in Metal-Organic Frameworks.

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal–organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

High-performance Hf/Ti-doped defective Zr-MOFs for cefoperazone adsorption: Behavior and mechanisms

Due to the excessive use, cephalosporin antibiotics have been detected in various aquatic environments, which may result in environmental and health concern. In this study, bimetallic metal–organic frameworks (MOFs), Hf-UiO-66 and Ti-UiO-66, were successfully designed and obtained via post-synthetic method from UiO-66 for the adsorption of cefoperazone. A serious of adsorption experiments were investigated. The adsorption kinetics and isotherms demonstrated that the adsorption process followed pseudo-second-order kinetic model and Langmuir isotherm model. Hf/Ti-UiO-66 exhibited much better adsorption performance than the undoped UiO-66. Particularly, Hf-UiO-66 had the highest adsorption capacity (346.0 mg g−1). With the introduction of Hf or Ti, the chemical coordination environment changed, which resulted in defective structures of MOFs. Such defective structures increased the adsorption active sites, changed the surface charges and enlarged pores. Based on the characterization by FT-IR, XPS, and zeta potential, it was identified that the electrostatic attraction, π-π stacking, hydrogen bonding and metal complexation were the driving force for the high adsorption capacity of the bimetallic MOFs. The as-prepared MOFs displayed good stability and reusability. This work provides a great prospect for the design and application of bimetallic defective MOFs in cephalosporins removal in environmental water.

Metal‐Organic Frameworks: Synthesis, Structures, and Applications

Metal‐Organic Frameworks: Synthesis, Structures, and Applications

Boosting the electrochemical energy storage and conversion performance by structural distortion in metal–organic frameworks

The engineering of multifunctional materials applied to energy storage and conversion remains an important challenge in addressing the intermittency issues associated with natural energy resources. Herein, we have achieved an overall improvement in the electrochemical performance of metal–organic frameworks (MOFs) through an elaborate lattice distortion strategy. Compared to the ordered lattice in conventional materials, the partially distorted lattice in the newly synthesized MOFs contributes a completely new kinetics feature: a surface-controlled and high-rate Faradaic reaction. In-situ water splitting can be achieved by employing the lattice-distorted MOF as both the anode of an asymmetric supercapacitor and electrocatalyst, thereby converting intermittent clean energy into electrochemical energy. The lattice distortion strategy in this work provides new insight into the functional design of MOFs, and may provide a novel and economically sound strategy for practical energy sustainability.

Smart Tetraphenylethene-Based Luminescent Metal-Organic Frameworks with Amide-Assisted Thermofluorochromics and Piezofluorochromics.

Luminescent metal–organic frameworks (MOFs) are appealing for the design of smart responsive materials, whereas aggregation‐induced emission (AIE) fluorophores with twisted molecular rotor structure provide exciting opportunities to construct MOFs with new topology and responsiveness. Herein, it is reported that elongating AIE rotor ligands can render the newly formed AIE MOF (ZnETTB) (ETTB = 4',4''',4''''',4'''''''‐(ethene‐1,1,2,2‐tetrayl)tetrakis(([1,1'‐biphenyl]‐3,5‐dicarboxylic acid))) with more elasticity, more control for intramolecular motion, and specific amide‐sensing capability. ZnETTB shows specific host–guest interaction with amide, where N,N‐diethylformamide (DEF), as an example, is anchored through C—H···O and C—H···π bonds with Zn cluster and ETTB8− ligand, respectively. DEF anchoring reduces both the distortion level and the intramolecular motions of ETTB8− ligand to lead a blueshifted and intensified emission for DEF ∈ ZnETTB. Moreover, amide anchoring also affords the DEF ∈ ZnETTB with the excellent thermofluorochromic behavior, and further increases the piezofluorochromic sensitivity at low‐pressure ranges on the basis of its elastic framework. This work is one of the rare examples of amide‐responsive smart materials, which shall shed new lights on design of smart MOFs with twisted AIE rotors for further sensing and detection applications.

Engineering Hierarchical Architecture of Metal-Organic Frameworks for Highly Efficient Overall CO2 Photoreduction.

Previous studies on syntheses of metal-organic frameworks (MOFs) for photocatalytic CO2 reduction are mainly focused on the exquisite control over the net topology and the functionality of metal clusters/organic building blocks. This contribution demonstrates that the rational design of MOF-based photocatalyst can be further extended to the hierarchical structure at micrometer scales well beyond the conventional MOF design at the molecular level. By taking advantage of the disparity of two selective MOFs in nucleation kinetics, a hierarchical core-shell MOF@MOF structure is successfully constructed through a simple one-pot synthesis. Besides inheriting the high porosity, crystallinity, and robustness of parent MOFs, the obtained heterojunction exhibits extended photoresponse, optimized band alignment with large overpotential, and greatly enhanced photogenerated charge separation, which would be hardly realized by the merely molecular-level assembly. As a result, the challenging overall CO2 photoreduction is achieved, which generates a record high HCOOH production (146.0µmol/g/h) without using any sacrificial reagents. Moreover, the core-shell structure exhibits a more effective use of photogenerated electrons than the individual MOFs. This work shows that harnessing the hierarchical architecture of MOFs present a new and effective alternative to tuning the photocatalytic performance at a mesoscopic level.

Computational Screening of Metal-Organic Frameworks for Ethylene Purification from Ethane/Ethylene/Acetylene Mixture.

Identification of high-performing sorbent materials is the key step in developing energy-efficient adsorptive separation processes for ethylene production. In this work, a computational screening of metal-organic frameworks (MOFs) for the purification of ethylene from the ternary ethane/ethylene/acetylene mixture under thermodynamic equilibrium conditions is conducted. Modified evaluation metrics are proposed for an efficient description of the performance of MOFs for the ternary mixture separation. Two different separation schemes are proposed and potential MOF adsorbents are identified accordingly. Finally, the relationships between the MOF structural characteristics and its adsorption properties are discussed, which can provide valuable information for optimal MOF design.

Hypothetical yet effective: Computational identification of high-performing MOFs for CO2 capture

With the advances in computational resources and algorithms, computer simulations are being increasingly used to tackle the most challenging problems of the world. Among them, CO2 capture is a topic that needs imminent attention as the presence of high levels of CO2 in the air can lead to drastic shifts in global climate. Here, a recently developed hypothetical metal-organic framework (MOF) database comprised of anion-pillared (AP) MOFs is computationally screened for the separation of CO2/CO, CO2/H2, and CO2/N2 gas mixtures at room temperature. The best performing MOFs are identified using three performance metrics, adsorption selectivity, working capacity, and regenerability, in conjunction. In these top materials, the preferential adsorption sites are illustrated, which will be useful in guiding the experimental design of new MOFs with extraordinarily high CO2 selectivities. The favorable separation performances of AP MOFs suggest that efficient gas separations can be conducted using MOFs without open metal sites.