Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

On the mechanism of intermolecular nitrogen-atom transfer from a lattice-isolated diruthenium nitride intermediate

Catalyst confinement within microporous media provides the opportunity to site isolate reactive intermediates, enforce intermolecular functionalization chemistry by co-localizing reactive intermediates and substrates in molecular-scale interstices, and harness non-covalent host-guest interactions to achieve selectivities that are complementary to those accessible in solution. As part of an ongoing program to develop synthetically useful nitrogen-atom transfer (NAT) catalysts, we have demonstrated intermolecular benzylic amination of toluene at a Ru2 nitride intermediate confined within the interstices of a Ru2-based metal-organic framework (MOF), Ru3(btc)2X3 (btc = 1,3,5-benzenetricarboxylate, i.e., Ru-HKUST-1 for X = Cl). Nitride confinement within the extended MOF lattice enabled intermolecular C-H functionalization of benzylic C-H bonds in preference to nitride dimerization, which was encountered with soluble molecular analogues. Detailed study of the kinetic isotope effects (KIEs, i.e., kH/kD) of C-H amination, assayed both as intramolecular effects using partially labeled toluene and as intermolecular effects using a mixture of per-labeled and unlabeled toluene, provided evidence for restricted substrate mobility on the time scale of interstitial NAT. Analysis of these KIEs as a function of material mesoporosity provided approximate experimental values for functionalization in the absence of mass transport barriers. Here, we disclose a combined experimental and computational investigation of the mechanism of NAT from a Ru2 nitride to the C-H bond of toluene. Computed kinetic isotope effects for a H-atom abstraction (HAA)/radical rebound (RR) mechanism are in good agreement with experimental data obtained for C-H amination at the rapid diffusion limit. These results provide the first detailed analysis of the mechanism of intermolecular NAT to a C-H bond, bolster the use of KIEs as a probe of confinement effects on NAT within MOF lattices, and provide mechanistic insights unavailable by experiment because rate-determining mass transport obscured the underlying chemical kinetics.

Degradation of contaminants of emerging concern in a secondary effluent using synthesized MOF-derived photoanodes: A comparative study between photo-, electro- and photoelectrocatalysis

Three metal-organic framework (MOF)-based photoanodes were prepared by deposition on TiO₂ nanotubes using Ti as substrate (Ti/TiO₂NT): i) Ti/TiO₂NT-Au@ZIF-8, ii) Ti/TiO₂NT-Ru₃(BTC)₂, iii) Ti/TiO₂NT-UiO-66(Zr)NH₂. These photoanodes were characterized by FEG-SEM, EDX and DRX. The analyses showed a successful modification and a high homogeneity of the different MOFs on the Ti/TiO₂NT surface. The photoanodes were studied in the degradation of Contaminants of Emerging Concern (CECs) in a spiked secondary effluent from a Municipal Wastewater Treatment Plant (MWWTP). Sodium diclofenac (DCF), sulfamethazine (SMT) and carbamazepine (CBZ) were used as CECs at low concentration (200 μg/L each CEC). The samples were preconcentrated using Solid Phase Extraction (SPE) and analyzed by a HPLC-DAD system. The MOF-based photoanodes exhibited a high photoelectrochemical (PEC) activity towards the oxidation of CECs, achieving up to 50%, 70% and 80% of removal using Ti/TiO₂NT-Au@ZIF-8, Ti/TiO₂NT-UiO-66(Zr)NH₂, Ti/TiO₂NT-Ru₃(BTC), respectively. The influence of the generation of hydroxyl radical was then studied. The results indicate that PEC degradation using Ti/TiO₂NT-Ru₃(BTC)₂ and Ti/TiO₂NT-UiO-66(Zr)NH₂ is more affected by the concentration of the radical.

Metal-organic frameworks as O2-selective adsorbents for air separations

Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N2-selective adsorbents, although the low capacities, poor selectivities, and high regeneration energies associated with these materials limit the extent of their usage. In contrast, the realization of O2-selective adsorbents may facilitate more widespread adoption of adsorptive air separations, which could enable the decentralization of O2 production and utilization and advance new uses for O2. Here, we present a detailed evaluation of the potential of metal-organic frameworks (MOFs) to serve as O2-selective adsorbents for air separations. Drawing insights from biological and molecular systems that selectively bind O2, we survey the field of O2-selective MOFs, highlighting progress and identifying promising areas for future exploration. As a guide for further research, the importance of moving beyond the traditional evaluation of O2 adsorption enthalpy, ΔH, is emphasized, and the free energy of O2 adsorption, ΔG, is discussed as the key metric for understanding and predicting MOF performance under practical conditions. Based on a proof-of-concept assessment of O2 binding carried out for eight different MOFs using experimentally derived capacities and thermodynamic parameters, we identify two existing materials and one proposed framework with nearly optimal ΔG values for operation under user-defined conditions. While enhancements are still needed in other material properties, the insights from the assessments herein serve as a guide for future materials design and evaluation. Computational approaches based on density functional theory with periodic boundary conditions are also discussed as complementary to experimental efforts, and new predictions enable identification of additional promising MOF systems for investigation.

Electrochemiluminescence resonance energy transfer system between ruthenium-based nanosheets and CdS quantum dots for detection of chlorogenic acid

A new strategy is proposed for ultrasensitive detection of chlorogenic acid (CGA) by fabricating an electrochemiluminescence resonance energy transfer (ECL-RET) sensing platform. The novel system designed by introducing ruthenium-based 2D metal-organic framework nanosheets (Ru@Zn-MOF) as ECL acceptor and L-cysteine capped CdS quantum dots (L-CdS QDs) as ECL donor, exhibited good ECL response. The possible mechanism of the modified electrode surface reaction was discussed. Modifying of the electrode surface by application of L-CdS QDs directly on ultrathin MOF nanosheets greatly shortened the electron-transfer distance and reduce energy loss, therefore significantly improving the ECL efficiency. The prepared sensor demonstrated good stability and highly selective detection of the target molecule. Under optimal conditions, the constructed sensor for the detection of CGA exhibited a wide linear range from 1.0 × 10^-10 to 1.0 × 10^-4 mol·L^-1 and a low detection limit of 3.2 × 10^-11 mol·L^-1 with a correction coefficient of 0.995. The recovery for spiked samples was calculated to be 94.4-109% and the RSD was 1.07-1.72% in real samples. The obtained sensor is considered to be a promising platform for CGA detection. Electrochemiluminescence resonance energy transfer (ECL-RET) sensing platform is used for the detection for chlorogenic acid.

In Situ Simultaneous Cavitation-Doping Approach for Constructing Bimetallic Metal-Organic Framework Hollow Nanospheres with Enhanced Electrocatalytic Hydrogen Production

This Communication demonstrates a novel and in situ simultaneous cavitation-doping (SCD) approach to construct bimetallic metal-doped cobalt metal-organic framework hollow nanospheres (CoM-MOF HNSs, with M = Ru or Fe). The key point of the SCD approach is the careful balance between the kinetics of Co-MOF being etched and the coordinative growth of a more stable CoM-MOF shell induced by Lewis acid (MCl₃, with M = Ru or Fe). Our work provides a new method to synthesize bimetallic hollow MOFs and benefits the development of electrocatalysts.

Dynamic Restructuring of Coordinatively Unsaturated Copper Paddle Wheel Clusters to Boost Electrochemical CO2 Reduction to Hydrocarbons*

The electrochemical reduction of CO₂ to hydrocarbons involves a multistep proton-coupled electron transfer (PCET) reaction. Second coordination sphere engineering is reported to be effective in the PCET process; however, little is known about the actual catalytic active sites under realistic operating conditions. We have designed a defect-containing metal-organic framework, HKUST-1, through a facile "atomized trimesic acid" strategy, in which Cu atoms are modified by unsaturated carboxylate ligands, producing coordinatively unsaturated Cu paddle wheel (CU-CPW) clusters. We investigate the dynamic behavior of the CU-CPW during electrochemical reconstruction through the comprehensive analysis of in situ characterization results. It is demonstrated that Cu₂ (HCOO)₃ is maintained after electrochemical reconstruction and that is behaves as an active site. Mechanistic studies reveal that CU-CPW accelerates the proton-coupled multi-electron transfer (PCMET) reaction, resulting in a deep CO₂ reduction reaction.

A more effective catalysis of the CO2 fixation with aziridines: computational screening of metal-substituted HKUST-1

A vital issue for the fixation and conversion of CO2 into useful chemical products is to find effective catalysts. In this work, in order to develop more effective and diverse catalysts, we implemented the first computational screening study (at M06-2X//B3LYP level) on the cycloaddition of CO2 with aziridines under eighteen metal-substituted HKUST-1 MOFs and tetrabutylammonium bromide (TBAB) as a co-catalyst. For all considered catalytic systems, the ring-opening of aziridine is calculated to be the rate-determining step. Up to 11 M-HKUST-1 systems, i.e., Rh (31.87 kcal mol-1), Y (31.02), Sc (30.50), V (30.02), Tc (29.90), Cd (29.80), Ti (29.32), Mn (29.05), Zn (28.29), Fe (27.85) and Zr (25.09), possess lower ring-opening barrier heights than the original Cu-HKUST-1 (32.90), indicative of their superior catalytic ability to the original Cu-HKUST-1 in theory. With the lowest ring-opening barrier, Zr-HKUST-1 is strongly advocated for future synthetic and catalytic studies.

Influence of carbohydrate polymer shaping on organic dye adsorption by a metal-organic framework in water

A number of studies have been conducted to develop new metal-organic frameworks (MOFs) as adsorbents for the removal of contaminants from polluted water. However, few reports exist describing detailed and thorough examinations of the effects of shaping on the adsorption properties of MOFs. In this study, a thorough analysis and comparison was conducted of the Orange II and Rhodamine B dye adsorption properties of unshaped MIL-100(Fe) (MIL) particles and alginate polymer-shaped MIL beads (MIL-alg). The adsorption affinities of Orange II and Rhodamine B for unshaped MIL were observed to be higher than those for shaped MIL-alg because partial coating of the surface of MIL particles by alginate polymer weakens adsorption forces. Kinetic analysis using a two-compartment model indicates that the contribution of the slow step in the mechanistic pathway for adsorption is more pronounced in MIL-alg compared to MIL because slow dye diffusion takes place in the alginate polymer. We believe that these fundamental findings will have a beneficial impact on approaches to design shaped MOFs that display improved dye removal performance.

Conversion of bimetallic MOF to Ru-doped Cu electrocatalysts for efficient hydrogen evolution in alkaline media

The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction (HER) is of great importance for water splitting. Herein, we develop a facile approach for preparation of porous carbon-confined Ru-doped Cu nanoparticles (denoted as Ru-Cu@C) by direct pyrolysis of the Ru-exchanged Cu-BTC metal-organic framework. When served as the electrocatalyst for HER, strikingly, the obtained Ru-Cu@C catalyst exhibits an ultralow overpotential (only 20 mV at 10 mA cm^-2) with a small Tafel slope of 37 mV dec^-1 in alkaline electrolyte. The excellent performance is comparable or even superior to that of commercial Pt/C catalyst. Density functional theory (DFT) calculations confirm that introducing Ru atoms into Cu nanocrystals can significantly alter the desorption of H₂ to achieve a close-to-zero hydrogen adsorption energy and thereby boost the HER process. This strategy gives a fresh impetus to explore low-cost and high-performance catalysts for HER in alkaline media.

Enhanced stability and activity of Cu–BTC by trace Ru3+ substitution in water photolysis for hydrogen evolution

Constructing stable, efficient and cost-effective cocatalysts is of great significance for photocatalytic H2 evolution in a dye-sensitization system.

Synthesis of atomically precise single-crystalline Ru2-based coordination polymers

Methods to incorporate kinetically inert metal nodes and highly basic ligands into single-crystalline metal-organic frameworks (MOFs) are scarce, which prevents synthesis and systematic variation of many potential heterogeneous catalyst materials. Here we demonstrate that metallopolymerization of kinetically inert Ru2 metallomonomers via labile Ag-N bonds provides access to a family of atomically precise single-crystalline Ru2-based coordination polymers with varied network topology and primary coordination sphere.

Thermal Defect Engineering of Precious Group Metal-Organic Frameworks: A Case Study on Ru/Rh-HKUST-1 Analogues

A methodology is introduced for controlled postsynthetic thermal defect engineering (TDE) of precious group metal-organic frameworks (PGM-MOFs). The case study is based on the Ru/Rh analogues of the archetypical structure [Cu₃(BTC)₂] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate). Quantitative monitoring of the TDE process and extensive characterization of the samples employing a complementary set of analytical and spectroscopic techniques reveal that the compositionally very complex TDE-MOF materials result from the elimination and/or fragmentation of ancillary ligands and/or linkers. TDE involves the preferential secession of acetate ligands, intrinsically introduced via coordination modulation during synthesis, and the gradual decarboxylation of ligator sites of the framework linker BTC. Both processes lead to modified Ru/Rh paddlewheel nodes. These nodes exhibit a lowered average oxidation state and more accessible open metal centers, as deduced from surface-ligand IR spectroscopy using CO as a probe and supported by density functional theory (DFT)-based computations. The monometallic and the mixed-metal PGM-MOFs systematically differ in their TDE properties and, in particular in the hydride generation ability (HGA). This latter property is an important indicator for the catalytic activity of PGM-MOFs, as demonstrated by the ethylene dimerization reaction to 1-butene.

Two-dimensional, conductive niobium and molybdenum metal-organic frameworks

The incorporation of second-row transition metals into metal-organic frameworks could greatly improve the performance of these materials across a wide variety of applications due to the enhanced covalency, redox activity, and spin-orbit coupling of late-row metals relative to their first-row analogues. Thus far, however, the synthesis of such materials has been limited to a small number of metals and structural motifs. Here, we report the syntheses of the two-dimensional metal-organic framework materials (H2NMe2)2Nb2(Cl2dhbq)3 and Mo2(Cl2dhbq)3 (H2Cl2dhbq = 3,6-dichloro-2,5-dihydroxybenzoquinone), which feature mononuclear niobium or molybdenum metal nodes and are formed through reactions driven by metal-to-ligand electron transfer. Characterization of these materials via X-ray absorption spectroscopy suggests a local trigonal prismatic coordination geometry for both niobium and molybdenum, consistent with their increased covalency relative to related first-row transition metal compounds. A combination of vibrational spectroscopy, magnetic susceptibility, and electronic conductivity measurements reveal that these two frameworks possess distinct electronic structures. In particular, while the niobium compound displays evidence for redox-trapping and strong magnetic interactions, the molybdenum phase is valence-delocalized with evidence of large polaron formation. Weak interlayer interactions in the neutral molybdenum phase enable solvent-assisted exfoliation to yield few-layer hexagonal nanosheets. Together, these results represent the first syntheses of metal-organic frameworks containing mononuclear niobium and molybdenum nodes, establishing a route to frameworks incorporating a more diverse range of second- and third-row transition metals with increased covalency and the potential for improved charge transport and stronger magnetic coupling.

Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal-Organic Frameworks

Metal-organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal-organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe-Cu paddlewheels as well as monometallic Cu-Cu and Fe-Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal-organic framework materials and the thorough characterization thereof is particularly important to derive structure-property or structure-activity correlations.

Osteochondral and bone tissue engineering scaffold prepared from Gallus var domesticus derived demineralized bone powder combined with gellan gum for medical application

Osteochondral (OC) lesions can occur in the knee and ankle. Such lesions induce a fracture in the cartilage protecting the bone joints. Cartilage tissue shows limited self-regeneration ability, hence the tissue is avascular and lack of vascular innervation, while the bone is a unique organ with the capacity to self-repair of small defects. In this present study, we have prepared a scaffold using demineralized bone powder (DBP) extracted from Gallus gallus var domesticus (GD), and Gellan gum (GG) for OC tissue regeneration. They were characterized for their chemical, physical, mechanical and biological properties using different available techniques, in vitro bioactivity was performed in simulated body fluid for 14 days confirming the formation of bone-like apatite. The in vitro biocompatibility was analyzed using chondrocyte cells and osteogenic and chondrogenic marker gene expression using RT-PCR, in vivo experiments performed by implanting scaffold in rabbit and characterized by histology and immunofluorescent stainings. The obtained results indicated that the prepared pores scaffold was biocompatible, and promote OC regeneration and integration of newly formed tissues with the host tissues in a rabbit. The prepared 1% DBP/GG scaffold can be used as a potential and promising alternate material for OC regeneration.

HKUST-1-Supported Cerium Catalysts for CO Oxidation

The synthesis method of metal–organic frameworks (MOFs) has an important impact on their properties, including their performance in catalytic reactions. In this work we report on how the performance of [Cu3(TMA)2(H2O)3]n (HKUST-1) and Ce@HKUST-1 in the reaction of CO oxidation depends on the synthesis method of HKUST-1 and the way the cerium active phase is introduced to it. The HKUST-1 is synthesised in two ways: via the conventional solvothermal method and in the presence of a cationic surfactant (hexadecyltrimethylammonium bromide (CTAB)). Obtained MOFs are used as supports for cerium oxide, which is deposited on their surfaces by applying wet and incipient wetness impregnation methods. To determine textural properties, structure, morphology, and thermal stability, the HKUST-1 supports and Ce@HKUST-1 catalysts are characterised using X-ray diffraction (XRD), N2 sorption, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). It is proven that the synthesis method of HKUST-1 has a significant impact on its morphology, surface area, and thermal stability. The synthesis method also influences the dispersion and the morphology of the deposited cerium oxide. Last but not least, the synthesis method affects the catalytic activity of the obtained material.

Thermal defect engineering of precious group metal–organic frameworks: impact on the catalytic cyclopropanation reaction

This work highlights the catalytic cyclopropanation and its characteristics as a novel analytical tool to investigate complex MOF structures.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Elucidating the Structure of the Metal-Organic Framework Ru-HKUST-1

Ru-HKUST-1 (Ru 3 ( btc ) 2 X 1.5 ; btc 3 - = 1 , 3 , 5 -benzenetricarboxylate ; X - = chloride , acetate , trimesate , hydroxide ) has received considerable attention as a result of its structure type, tunability, and the redox-active nature of its constituent metal paddlewheel building units. As compared to some of the other members of the HKUST-1 family, its surface area is typically reported as ~25% lower than expected. In contrast to this, a related ruthenium-based porous coordination cage, Ru 24 ( t Bu-bdc ) 24 Cl 12 , displays the expected surface area when compared to Cr 2 + and Mo 2 + analogs. Here, we examine the factors that result in this decreased surface area for the MOF. We show that with appropriate solvent exchange and activation conditions, Ru-HKUST-1 can display a B.E.T. surface areas as high as 1439 m2/g. We utilize a combination of spectroscopic and diffraction techniques to accurately determine the structure of the MOF, which is most accurately described here as Ru 3 ( btc ) 2 ( OAc ) 1.07 Cl 0.43 , as prepared under our conditions. Further, by simply treating the sample as air-sensitive upon isolation, adsorption selectivities toward unsaturated molecu les greatly improve.

Ultralong Cycle Life Li-O2 Battery Enabled by a MOF-Derived Ruthenium-Carbon Composite Catalyst with a Durable Regenerative Surface

The cycling performance of Li-O₂ batteries (LOBs), which is an important parameter determining the practical use of this advanced energy technology with ultrahigh energy density, is strongly affected by the nature of the oxygen electrocatalyst. As a good oxygen electrode, it should possess good activity for both the oxygen evolution reaction and the oxygen reduction reaction and superior stability under operating conditions. During the past, oxygen electrodes for LOBs were generally fabricated by loading noble metal nanoparticles on the surface of a porous carbon support. However, the nanoparticles could easily lose contact with the carbon support during the reversible liquid-gas-solid reactions that involve lithium ions, oxygen gas, and Li₂O₂. Herein, we reported a novel Ru-metal-organic framework (MOF)-derived carbon composite, characterized by stereoscopic Ru nanoparticle distribution within the carbon matrix, as an alternative oxygen catalyst of LOBs, enabling superior operational stability and favorable activity. More specifically, the battery demonstrated stable charge-discharge cycling for up to 800 times (∼107 days) at a current density of 500 mA g^-1 with low discharge/charge overpotentials (∼0.2/0.7 V vs Li). A mechanism of regenerative surface was further proposed to explain the excellent cycling stability of the LOBs through the use of the Ru-MOF-C catalyst. These encouraging results imply an accessible solution to address issues related to the oxygen catalyst for the realization of practical LOBs.

Computational study of the carbonyl-ene reaction between formaldehyde and propylene encapsulated in coordinatively unsaturated metal-organic frameworks M3(btc)2 (M = Fe, Co, Ni, Cu and Zn)

The carbonyl-ene reaction between encapsulated formaldehyde and propylene over the coordinatively unsaturated metal-organic frameworks M3(btc)2 (M = Fe, Co, Ni, Cu and Zn) has been investigated by means of density functional calculations. Zn3(btc)2 adsorbs formaldehyde strongest due to electron delocalization between Zn and the oxygen atom of the reactant molecule. The reaction is proposed to proceed in a single step involving proton transfer and carbon-carbon bond formation. We find the relative catalytic activity to be Zn3(btc)2 > Fe3(btc)2 ≥ Co3(btc)2 > Ni3(btc)2 > Cu3(btc)2, based on activation energy and turnover frequencies (TOF). The low activation energy for Zn3(btc)2 compared to the others can be explained by the delocalization of electron density between the carbonyl bond and the catalyst active sites, leading to a more stable transition state. The five MOFs are used to propose a descriptor for the relationship between activation energy on one side and electronic properties or adsorption energies on the other side in order to allow a quick screening of other catalytic materials for this reaction.

Mixed precious-group metal–organic frameworks: a case study of the HKUST-1 analogue [RuxRh3−x(BTC)2]

Mixed precious-group metal–organic frameworks [Ru_xRh_3−x(BTC)₂] of the HKUST-1-type were synthesized and characterized (PXRD, BET, IR, Raman, XPS, TGA, SS-UV/VIS, EA, and HR-TEM).

Design and synthesis of capped-paddlewheel-based porous coordination cages

A novel cluster capping strategy is employed to leverage the structural diversity of metal–organic cages toward the isolation of porous cages.

Metal–Organic Framework Membranes: From Fabrication to Gas Separation

Gas membrane-based separation is considered one of the most effective technologies to address energy efficiency and large footprint challenges. Various classes of advanced materials, including polymers, zeolites, porous carbons, and metal–organic frameworks (MOFs) have been investigated as potential suitable candidates for gas membrane-based separations. MOFs possess a uniquely tunable nature in which the pore size and environment can be controlled by connecting metal ions (or metal ion clusters) with organic linkers of various functionalities. This unique characteristic makes them attractive for the fabrication of thin membranes, as both the diffusion and solubility components of permeability can be altered. Numerous studies have been published on the synthesis and applications of MOFs, as well as the fabrication of MOF-based thin films. However, few studies have addressed their gas separation properties for potential applications in membrane-based separation technologies. Here, we present a synopsis of the different types of MOF-based membranes that have been fabricated over the past decade. In this review, we start with a short introduction touching on the gas separation membrane technology. We also shed light on the various techniques developed for the fabrication of MOF as membranes, and the key challenges that still need to be tackled before MOF-based membranes can successfully be used in gas separation and implemented in an industrial setting.

Lewis basicity generated by localised charge imbalance in noble metal nanoparticle-embedded defective metal-organic frameworks

Interactions between metal nanoparticles (NPs) and metal–organic frameworks (MOFs) in their composite forms have proven to exhibit beneficial properties, such as enhanced catalytic performance through synergistic effects. Herein, we show that Lewis basic sites can be created within an anionic defective MOF by engineering the electronic state of the pendant carboxylate groups situated at the defect sites. This is achieved from the concerted interactions between the pendant carboxylate groups, embedded Pd NPs and charge-balancing cations (Mⁿ⁺ = Ce³⁺, Co²⁺, Ni²⁺, Cu²⁺, Mg²⁺, Li⁺, Na⁺ or K⁺). This work is the first example of generating a new collective property, i.e. Lewis basicity, in metal-carboxylate MOFs. Importantly, the choice of Mⁿ⁺, used during cation exchange, acts as a convenient parameter to tune the Lewis basicity of the MOF-based nanocomposites. It also provides a facile way to incorporate active metal sites and basic sites within carboxylate-based MOFs to engineer multifunctional nanocatalysts.

Interactions between metal nanoparticles (NPs) and metal–organic frameworks (MOFs) in their composite forms have proven to exhibit beneficial properties. Here the authors present a unique approach to immobilise Pd NPs and, more importantly, to generate tunable basic sites within an anionic defective MOF.

Dissolving uptake-hindering surface defects in metal-organic frameworks

Metal-organic frameworks (MOFs) have unique properties which make them perfectly suited for various adsorption and separation applications; however, their uses and efficiencies are often hindered by their limited stability. When most MOFs are exposed to water or humid air, the MOF structure, in particular at the surface, is destroyed, creating surface defects. These surface defects are surface barriers which tremendously hinder the uptake and release of guest molecules and, thus, massively decrease the performance in any application of MOFs. Here, the destruction by exposure to water vapor is investigated by using well-defined MOF films of type HKUST-1 as a model system for uptake experiments with different-sized probe molecules as well as for spectroscopic investigations, complemented by density functional theory calculations of the defect structure. In addition to the characterization of the surface defects, it is found that the pristine MOF structure can be regenerated. We show that the surface defects can be dissolved by exposure to the synthesis solvent, here ethanol, enabling fast uptake and release of guest molecules. These findings show that the storage of MOF materials in a synthesis solvent results in healing of surface defects and enables ideal performance of MOF materials.

An experimental and computational study of CO2 adsorption in the sodalite-type M-BTT (M = Cr, Mn, Fe, Cu) metal-organic frameworks featuring open metal sites

We present a comprehensive investigation of the CO2 adsorption properties of an isostructural series of metal-organic frameworks, M-BTT (M = Cr, Mn, Fe, Cu; BTT3- = 1,3,5-benzenetristetrazolate), which exhibit a high density of open metal sites capable of polarizing and binding guest molecules. Coupling gas adsorption measurements with in situ neutron and X-ray diffraction experiments provides molecular-level insight into the adsorption process and enables rationalization of the observed adsorption isotherms. In particular, structural data confirms that the high initial isosteric heats of CO2 adsorption for the series are directly correlated with the presence of open metal sites and further reveals the positions and orientations of as many as three additional adsorption sites. Density functional theory calculations that include van der Waals dispersion corrections quantitatively support the observed structural features associated with the primary and secondary CO2 binding sites, including CO2 positions and orientations, as well as the experimentally determined isosteric heats of CO2 adsorption.

Synthesis of heterometallic metal–organic frameworks and their performance as electrocatalyst for CO2 reduction

The solventless synthesis of heterometallic metal–organic frameworks and their proficient behavior as electrocatalysts in the CO₂ reduction to alcohols is presented.

Fabrication of [Cu2(bdc)2(bpy)]n thin films using coordination modulation-assisted layer-by-layer growth

We take the advantage of the layer-by-layer process to adapt it to the coordination modulation method to fabricate highly oriented [Cu₂(bdc)₂(bpy)]_n films.

Defects as Color Centers: The Apparent Color of Metal-Organic Frameworks Containing Cu2+-Based Paddle-Wheel Units

As in the case of other semiconducting materials, optical and electronic properties of metal-organic frameworks (MOFs) depend critically on defect densities and defect types. We demonstrate here that, in addition to the influence of imperfections on MOF chemical properties like guest binding energies and catalytic activity, the optical properties of these crystalline molecular solids also crucially depend on deviations from the perfect crystalline structure. By recording UV-vis absorption spectra for MOF thin films of particularly high quality, we demonstrate that low-defect samples of an important MOF, HKUST-1, are virtually colorless. Electronic structure calculations of the excited states by employing complete active space self-consistent field (CASSCF) calculations show that the d-d excitations in defects result in the typical green color of the MOF material synthesized by conventional methods.