Enhanced H2 Adsorption in Isostructural Metal−Organic Frameworks with Open Metal Sites: Strong Dependence of the Binding Strength on Metal Ions

The Impact of Metal Centers in the M-MOF-74 Series on Formic Acid Production

The confinement effect of porous materials on the thermodynamical equilibrium of the CO2 hydrogenation reaction presents a cost-effective alternative to transition metal catalysts. In metal-organic frameworks, the type of metal center has a greater impact on the enhancement of formic acid production than the scale of confinement resulting from the pore size. The M-MOF-74 series enables a comprehensive study of how different metal centers affect HCOOH production, minimizing the effect of pore size. In this work, molecular simulations were used to analyze the adsorption of HCOOH and the CO2 hydrogenation reaction in M-MOF-74, where M = Ni, Cu, Co, Fe, Mn, Zn. We combine classical simulations and density functional theory calculations to gain insights into the mechanisms that govern the low coverage adsorption of HCOOH in the surrounding of the metal centers of M-MOF-74. The impact of metal centers on the HCOOH yield was assessed by Monte Carlo simulations in the grand-canonical ensemble, using gas-phase compositions of CO2, H2, and HCOOH at chemical equilibrium at 298.15-800 K, 1-60 bar. The performance of M-MOF-74 in HCOOH production follows the same order as the uptake and the heat of HCOOH adsorption: Ni > Co > Fe > Mn > Zn > Cu. Ni-MOF-74 increases the mole fraction of HCOOH by ca. 105 times compared to the gas phase at 298.15 K, 60 bar. Ni-MOF-74 has the potential to be more economically attractive for CO2 conversion than transition metal catalysts, achieving HCOOH production at concentrations comparable to the highest formate levels reported for transition metal catalysts and offering a more valuable molecular form of the product.

Geometric Tuning of Coordinatively Unsaturated Copper(I) Sites in Metal-Organic Frameworks for Ambient-Temperature Hydrogen Storage

Porous solids can accommodate and release molecular hydrogen readily, making them attractive for minimizing the energy requirements for hydrogen storage relative to physical storage systems. However, H2 adsorption enthalpies in such materials are generally weak (-3 to -7 kJ/mol), lowering capacities at ambient temperature. Metal-organic frameworks with well-defined structures and synthetic modularity could allow for tuning adsorbent-H2 interactions for ambient-temperature storage. Recently, Cu2.2Zn2.8Cl1.8(btdd)3 (H2btdd = bis(1H-1,2,3-triazolo-[4,5-b],[4',5'-i])dibenzo[1,4]dioxin; CuI-MFU-4l) was reported to show a large H2 adsorption enthalpy of -32 kJ/mol owing to π-backbonding from CuI to H2, exceeding the optimal binding strength for ambient-temperature storage (-15 to -25 kJ/mol). Toward realizing optimal H2 binding, we sought to modulate the π-backbonding interactions by tuning the pyramidal geometry of the trigonal CuI sites. A series of isostructural frameworks, Cu2.7M2.3X1.3(btdd)3 (M = Mn, Cd; X = Cl, I; CuIM-MFU-4l), was synthesized through postsynthetic modification of the corresponding materials M5X4(btdd)3 (M = Mn, Cd; X = CH3CO2, I). This strategy adjusts the H2 adsorption enthalpy at the CuI sites according to the ionic radius of the central metal ion of the pentanuclear cluster node, leading to -33 kJ/mol for M = ZnII (0.74 Å), -27 kJ/mol for M = MnII (0.83 Å), and -23 kJ/mol for M = CdII (0.95 Å). Thus, CuICd-MFU-4l provides a second, more stable example of optimal H2 binding energy for ambient-temperature storage among reported metal-organic frameworks. Structural, computational, and spectroscopic studies indicate that a larger central metal planarizes trigonal CuI sites, weakening the π-backbonding to H2.

Extended MOF-74-Type Variant with an Azine Linkage: Efficient Direct Air Capture and One-Pot Synthesis

Direct air capture (DAC) shows considerable promise for the effective removal of CO2; however, materials applicable to DAC are lacking. Among metal-organic framework (MOF) adsorbents, diamine-Mg2(dobpdc) (dobpdc4- = 4,4-dioxidobiphenyl-3,3'-dicarboxylate) effectively removes low-pressure CO2, but the synthesis of the organic ligand requires high temperature, high pressure, and a toxic solvent. Besides, it is necessary to isolate the ligand for utilization in the synthesis of the framework. In this study, we synthesized a new variant of extended MOF-74-type frameworks, M2(hob) (M = Mg2+, Co2+, Ni2+, and Zn2+; hob4- = 5,5'-(hydrazine-1,2-diylidenebis(methanylylidene))bis(2-oxidobenzoate)), constructed from an azine-bonded organic ligand obtained through a facile condensation reaction at room temperature. Functionalization of Mg2(hob) with N-methylethylenediamine, N-ethylethylenediamine, and N,N'-dimethylethylenediamine (mmen) enables strong interactions with low-pressure CO2, resulting in top-tier adsorption capacities of 2.60, 2.49, and 2.91 mmol g-1 at 400 ppm of CO2, respectively. Under humid conditions, the CO2 capacity was higher than under dry conditions due to the presence of water molecules that aid in the formation of bicarbonate species. A composite material combining mmen-Mg2(hob) and polyvinylidene fluoride, a hydrophobic polymer, retained its excellent adsorption performance even after 7 days of exposure to 40% relative humidity. In addition, the one-pot synthesis of Mg2(hob) from a mixture of the corresponding monomers is achieved without separate ligand synthesis steps; thus, this framework is suitable for facile large-scale production. This work underscores that the newly synthesized Mg2(hob) and its composites demonstrate significant potential for DAC applications.

Solvent Exchange Dynamics in M2(dobdc): An Interplay among Binding Strength, Exchange Kinetics, and Cooperativity

Solvent exchange is a crucial step in ensuring the complete activation of metal-organic frameworks (MOFs); however, the conditions for solvent exchange vary among MOFs, even within the isostructural variants. This study examines the factors contributing to solvent exchange by investigating the isostructural M2(dobdc) (M═Mg, Co, Zn) series. Common solvents N,N-dimethylformamide (DMF), ethanol (EtOH), and methanol (MeOH) are employed to assess the solvent exchange at coordinatively unsaturated sites (CUS) of M2(dobdc). By monitoring both solvents released from the MOF during solvent exchange and the coordination environment of metals within the MOF, a picture is constructed of exchange rates during early stages of solvent exchange as well as expulsion of the last traces of bound solvents. This differentiation is achieved by a combination of bulk monitoring of solvent phase composition and microscopic application of Raman spectroscopy on the single-crystal level. The kinetics of solvent replacement is revealed to have a substantial contribution from cooperativity; this phenomenon is observed in both the forward and reverse directions. Thermogravimetric analysis coupled with IR spectroscopy and density functional theory (DFT) calculations are employed to elucidate the relationship between solvent exchange rates and solvent binding energy. The solvent exchange rates are determined by the kinetic barriers of solvent exchange that do not follow the order of the solvent binding affinity. This work contributes to understanding the solvent exchange of MOFs by examining the interplay among the binding strength, exchange kinetics, and cooperativity. It further provides valuable insights for scrutinizing MOF activation protocols.

Efficient improvement in the electrochemical performance of petal-like lamellar NiMn-LDHs with affluent oxygen vacancies derived from Mn MOF-74

Supercapacitors (SCs) as a kind of novel energy storage devices have emerged to meet the urgent requirement of environmentally friendly clean energy storage equipment. However, unsatisfactory energy density and low operating voltage tremendously restrict their practical application. Herein, petal-like lamellar NiMn-layered double hydroxide (NiMn-LDH) was successfully fabricated through a simple Ni(NO3)2 etching method with Mn MOF-74 as a sacrificial template. This NiMn-LDH 3/NF electrode exhibited an improved specific capacitance of 1410.2 F g-1 at a current density of 1 A g-1 (Mn MOF-74/NF: 172.2) owing to its high redox activity, compositional flexibility and intercalating capability. Importantly, NiMn-LDH was further optimized via a facile hydroperoxide treatment to harvest NiMn-LDH (O-LDH) with abundant oxygen vacancies, exhibiting remarkable improvement in specific capacitance (990%) compared to original MOF-74 before modification. The preparation of O-LDH enriches the electrode material engineering strategy and achieves improved electrochemical performance for application in new-generation SCs.

Understanding Carbon Monoxide Binding and Interactions in M-MOF-74 (M = Mg, Mn, Ni, Zn)

Small molecules may adsorb strongly in metal-organic frameworks (MOFs) through interactions with under-coordinated open metal sites (OMS) that often exist within these structures. Among adsorbates, CO is attractive to study both for its relevance in energy-related applications and for its ability to engage in both σ-donation and π-backbonding interactions with the OMS in MOFs. Concomitant with strong adsorption, structural changes arise due to modifications of the electronic structure of both the adsorbate and adsorbent. These structural changes affect the separation performance of materials, and accurately capturing these changes and the resulting energetics is critical for accurate predictive modeling of adsorption. Traditional approaches to modeling using classical force fields typically do not capture or account for changes at the electronic level. To characterize the structural and energetic effects of the local structural changes, we employed density functional theory (DFT) to study CO adsorption in M-MOF-74s. M-MOF-74s feature OMS at which CO is known to adsorb strongly and can be synthesized with a variety of divalent metal cations with distinct performance in adsorption. We considered M-MOF-74s with a range of metals of varied d-band occupations (Mg (3d0), Mn (3d5), Ni (3d8), and Zn (3d10)) with various structural constraints ranging from geometrically constrained adsorbent and adsorbate ions to fully optimized geometries to deconvolute the relative contributions of various structural effects to the adsorption energetics and binding distances observed. Our data indicate that the most significant structural changes during adsorption correlate with the greatest π-backbonding behaviors and commensurately result in a sizable binding energy change observed for CO adsorption. The insights built from this work are relevant to two longstanding research challenges within the MOF community: rational design of materials for separations and the design of force fields capable of accurately modeling adsorption.

Thiol and thioether-based metal-organic frameworks: synthesis, structure, and multifaceted applications

Metal-organic frameworks (MOFs) are unique hybrid porous materials formed by combining metal ions or clusters with organic ligands. Thiol and thioether-based MOFs belong to a specific category of MOFs where one or many thiols or thioether groups are present in organic linkers. Depending on the linkers, thiol-thioether MOFs can be divided into three categories: (i) MOFs where both thiol or thioether groups are part of the carboxylic acid ligands, (ii) MOFs where only thiol or thioether groups are present in the organic linker, and (iii) MOFs where both thiol or thioether groups are part of azolate-containing linkers. MOFs containing thiol-thioether-based acid ligands are synthesized through two primary approaches; one is by utilizing thiol and thioether-based carboxylic acid ligands where the bonding pattern of ligands with metal ions plays a vital role in MOF formation (HSAB principle). MOFs synthesized by this approach can be structurally differentiated into two categories: structures without common structural motifs and structures with common structural motifs (related to UiO-66, UiO-67, UiO-68, MIL-53, NU-1100, etc.). The second approach to synthesize thiol and thioether-based MOFs is indirect methods, where thiol or thioether functionality is introduced in MOFs by techniques like post-synthetic modifications (PSM), post-synthetic exchange (PSE) and by forming composite materials. Generally, MOFs containing only thiol-thioether-based ligands are synthesized by interfacial assisted synthesis, forming two-dimensional sheet frameworks, and show significantly high conductivity. A limited study has been done on MOFs containing thiol-thioether-based azolate ligands where both nitrogen- and sulfur-containing functionality are present in the MOF frameworks. These materials exhibit intriguing properties stemming from the interplay between metal centres, organic ligands, and sulfur functionality. As a result, they offer great potential for multifaceted applications, ranging from catalysis, sensing, and conductivity, to adsorption. This perspective is organised through an introduction, schematic representations, and tabular data of the reported thiol and thioether MOFs and concluded with future directions.

MRI Contrasting Agent Based on Mn-MOF-74 Nanoparticles with Coordinatively Unsaturated Sites

PURPOSE

The development of magnetic resonance imaging (MRI) contrasting agents (CAs) that are safer and have a higher relaxivity than Gd(III)-based agents is a significant research topic. Herein, we propose the use of a Mn-based metal organic framework (MOF), Mn-MOF-74, characterized by a safe paramagnetic center, a coordinatively unsaturated site (CUS) for aquation, and a long rotational correlation time, endowing high relaxivity. Furthermore, biocompatibility and delivery to the tumor are generally expected for MOFs that are obtainable in the nanometer size range.

PROCEDURE

Drop-wise mixing of 2,5-dihydroxyterephthalic acid (DHTP) and Mn(II) acetate yielded Mn-MOF-74 with a diameter of < 150 nm, which was then modified with 1-fivefold higher amounts of poly(ethylene glycol) (M.W. = 5000) to afford MOFs stably dispersed in water for at least 24 h.

RESULTS

The longitudinal and transverse relaxivity of the PEG-modified MOF was in the range of r1 = 8.08-13.5 and r2 = 32.7-46.8 mM-1 s-1, respectively (1.0 T, 23.7-23.9 °C), being larger than those of typical Gd(III)- and Mn(II)-based CAs of single-nuclear metal complexes. The in vivo imaging of a tumor-bearing mouse clearly showed that the tumor could be readily recognized due to signal enhancement (117%) in T1-weighted images, whereas other tissues showed small signal changes.

CONCLUSIONS

These results suggest that PEG-Mn-MOF-74 can be passively delivered to tumors and can act as a high-relaxivity T1 agent.

Effect of modulator ligands on the growth of Co2(dobdc) nanorods

Control over the size, shape, uniformity, and external surface chemistry of metal-organic framework nanocrystals is important for a wide range of applications. Here, we investigate how monotopic modulators that mimic the coordination mode of native bridging ligands affect the growth of anisotropic Co2(dobdc) (dobdc4- = 2,5-dihydroxy-1,4-benzenedicarboxylic acid) nanorods. Through a combination of transmission electron microscopy (TEM) and nuclear magnetic resonance spectroscopy (NMR) studies, nanorod diameter was found to be strongly correlated to the acidity of the modulator and to the degree of modulator incorporation into the nanorod structure. Notably, highly acidic modulators allowed for the preparation of sub-10 nm nanorods, a previously elusive size regime for the M2(dobdc) family. More broadly, this study provides new insights into the mechanism of modulated growth of metal-organic framework nanoparticles.

Design of a MOF-based nano-trap for the efficient separation of propane from propylene

A parallel Cu paddle wheel structure was developed to form an efficient C₃H₆ nano-trap. Benefiting from having this trap, ATC-Cu showed a very high capacity for binding C₃H₆ and high C₃H₆/C₃H₈ selectivity at 298 K.

Sediment quality and environmental risk assessment in a Mediterranean coastal system using geochemical and multivariate statistical analyses: the case of Boughrara Lagoon (southeastern Tunisia)

Coastal ecosystems are the most vulnerable to natural and anthropogenic pressures which should be assessed using various chemical and ecological indicators. Our study aims to provide practical monitoring of anthropogenic pressures related to metal discharges in coastal waters for identifying potential ecological deterioration. The spatial variability of various chemical elements concentrations and their main sources were determined in the surficial sediments of a Mediterranean coastal area submitted to high anthropogenic pressure, the semi-enclosed Boughrara Lagoon located in southeastern Tunisia, by conducting several geochemical and multi-elemental analyses. The grain size and the geochemical analyses both suggested a marine influence of the sediment inputs in the north of the area (near Ajim channel), whereas continental and aeolian features dominated the sedimentary inputs in the southwestern lagoon. This last area was also characterized by the highest concentrations of metals, in particular Pb (4.45-173.33 ppm), Mn (68.45-1469.27 ppm), Cu (7.64-134.26 ppm), Zn (28.74-244.79 ppm), Cd (0.11-2.23 ppm), Fe (0.5-4.9%), and Al (0.7-3.2%). By referring to background crustal values and the contamination factor calculations (CF), the lagoon is considered as highly polluted for Cd, Pb, and Fe (3 < CF < 6). Three possible sources of pollution were identified: phosphogypsum effluents (P, Al, Cu, and Cd), the ex-Pb mine (Pb and Zn), and the cliff weathering and streams input from the red clay quarry (Fe). Furthermore, pyrite precipitation was identified for the first time in the Boughrara lagoon, suggesting the occurrence of anoxic conditions in this lagoon.

Enhanced sorption in an indium-acetylenedicarboxylate metal-organic framework with unexpected chains of cis-μ-OH-connected {InO6} octahedra

Single crystals of the new metal-organic framework (MOF) In-adc (HHUD-4) were obtained through the reaction of linear acetylenedicarboxylic acid (H₂adc) with In(NO₃)₃·xH₂O as a racemic conglomerate in the chiral tetragonal space groups P4₃22 and P4₁22. Fundamentally different from other MOFs with linear linkers and trans-μ-OH-connected infinite {MO₆} secondary building units as in the MIL-53-type, the linear adc^2- linker leads to the formation of cis-μ-OH connected {InO₆} polyhedra, which have otherwise only been found before for V-shaped ligands, as in CAU-10-H. A far-reaching implication of this finding is the possibility that trans-μ-OH/straight MIL-53-type MOFs will have polymorphs of CAU-10-H cis-μ-OH/helical topology and vice versa. HHUD-4 is a microporous MOF with a BET surface area of up to 940 m² g^-1 and a micropore volume of up to 0.39 cm³ g^-1. Additionally, HHUD-4 features good adsorption uptakes of 3.77 mmol g^-1 for CO₂ and 1.25 mmol g^-1 for CH₄ at 273 K and 1 bar, respectively, and a high isosteric heat of adsorption of 11.4 kJ mol^-1 for H₂ with a maximum uptake of 6.36 mmol g^-1 at 77 K and 1 bar. Vapor sorption experiments for water and volatile organic compounds (VOCs) such as benzene, cyclohexane and n-hexane yielded uptake values of 135, 269, 116 and 205 mg g^-1, respectively, at 293 K. While HHUD-4 showed unremarkable results for water uptake and low stability for water, it exhibited good stability with steep VOC uptake steps at low relative pressures and a high selectivity of 17 for benzene/cyclohexane mixtures.

Cu-BDC and Cu2O Derived from Cu-BDC for the Removal and Oxidation of Asphaltenes: A Comparative Study

Asphaltenes have been associated with a number of problems in the petroleum industry with regard to the storage, exploration, and transportation of petroleum crude. In the current work, Copper-BenzeneDiCarboxylic acid (Cu-BDC) and Cu-BDC derived metal oxide has been used in the removal and oxidation of the asphaltenes. The MOF derived metal oxide was confirmed to be Cu2O. Though adsorption of asphaltenes followed a Langmuir adsorption isotherm in both cases, Cu-BDC was superior to Cu2O with an adsorption capacity four times that of the adsorption capacity of Cu2O. Also, the kinetic studies showed that the adsorption kinetics followed pseudo second order adsorption kinetics in both cases. From the oxidation studies, it was found that Cu-BDC was unstable beyond 350 °C and had no role in catalyzing the oxidation reaction. The Cu2O, however, was successful at catalyzing the asphaltene oxidation reaction and a reduction of 50 °C in oxidation temperature was observed. Hence comparing Cu-BDC with Cu2O, MOF was successful in the adsorption reaction but the MOF derived metal oxide had the upper hand in the oxidation reaction.

Dipole Switching by Intramolecular Electron Transfer in Single-Molecule Magnetic Complex [Mn12O12(O2CR)16(H2O)4]

We study intramolecular electron transfer in the single-molecule magnetic complex [Mn₁₂O₁₂(O₂CR)₁₆ (H₂O)₄] for R = -H, -CH₃, -CHCl₂, -C₆H₅, and -C₆H₄F ligands as a mechanism for switching of the molecular dipole moment. Energetics is obtained using the density functional theory (DFT) with onsite Coulomb energy correction (DFT + U). Lattice distortions are found to be critical for localizing an extra electron on one of the easy sites on the outer ring in which localized states can be stabilized. We find that the lowest-energy path for charge transfer is for the electron to go through the center via superexchange-mediated tunneling. The energy barrier for such a path ranges from 0.4 to 54 meV depending on the ligands and the isomeric form of the complex. The electric field strength needed to move the charge from one end to the other, thus reversing the dipole moment, is 0.01-0.04 V/Å.

A Theoretical Investigation of Cu+, Ni2+ and Co2+-exchanged Zeolites for Hydrogen Storage

This study investigates the H 2 adsorption on Cu + , Ni 2+ and Co 2+ -exchanged SSZ-13 (CHA) and SSZ-39 (AEI) using periodic DFT computations. Most stable Cu + position was found to be the 6-membered-ring window for both zeolites. Similarly, for investigated Ni 2+ and Co 2+ loadings on 6-membered-ring windows, the third nearest neighbor Al positions, i.e., Al-O-Si-O-Si-O-Al coordination, was found to be the most stable position. H 2 adsorption was investigated for all the Cu + , Ni 2+ and Co 2+ centers. AEI and CHA resulted in similar H 2 -Cu interactions for the Al and B substituted structures. H 2 adsorption on Cu + located in the 8-membered-ring gave the highest adsorption energy for both frameworks. Replacing Al with B in the framework increased the electron back donation from Cu + (3d) orbitals to H 2 antibonding orbital (s H2 * ). The H 2 adsorption energies on the Ni 2+ and Co 2+ -exchanged zeolites were found to be between -15 and -44 kJ/mol. Higher energy values were observed on the AEI framework, especially when two Al atoms have the Al-O-Si-O-Al configuration. Lesser interaction of the d-orbitals in the case of the Co 2+ and Ni 2+ cations resulted in heat of H 2 adsorption close to optimum values required for H 2 storage on porous materials.

Tough ion gels composed of coordinatively crosslinked polymer networks using ZIF-8 nanoparticles as multifunctional crosslinkers

Constructing crosslinked polymer networks via reversible interactions is a promising approach to recover the mechanical strength of damaged gels. In addition, by designing effective reversible crosslinks, the mechanical strength of the gel can be enhanced through energy dissipation based on the destruction of the crosslinks by an applied force. In this study, we introduced zeolitic imidazole framework-8 nanoparticles (ZIF-8 NPs), which acted as multifunctional crosslinkers, to provide multipoint coordination bonding with a poly(N,N-dimethylacrylamide)-based polymer network in a gel containing an ionic liquid. The mechanical strength of the gel increased with an increase in the content of ZIF-8 NPs up to 6 wt%. It was confirmed that the energy loaded onto the gel was dissipated through the desorption of the polymer network from the surface of the ZIF-8 NPs. Owing to the reversible destruction and reconstruction of the coordinative crosslinking between the polymer network and ZIF-8 NPs, the mechanical strength of the damaged gel was almost fully recovered through annealing.

Electrocatalytic methane oxidation to formate on magnesium based metal-organic frameworks

The electrochemical methane oxidation reaction is a potential approach for upgrading the nature-abundant methane (CH₄) into value-added chemicals, while the activity and selectivity have remained substantially low due to the extremely inert chemical property of CH₄. Inspired by the methane mono-oxygenase in nature, we demonstrated Mg-substituted metal-organic frameworks (Mg-MOF-74) containing a uniform distribution of Mg-oxo-Mg nodes as efficient catalytic sites. Compared to MgNi-MOF-74 and Mg(OH)₂ without the Mg-oxo-Mg nodes, the Mg-MOF-74 presented a much enhanced CH₄ electrooxidation performance, with a unique selectivity of producing formate. The maximum Faradaic efficiency of all liquid products reached 10.9% at 1.60 V versus reversible hydrogen electrode (RHE), corresponding to the peak production rate of 126.6 μmol·h^-1·g^-1.

Ab Initio Study of Hydrostable Metal-Organic Frameworks for Postsynthetic Modification and Tuning toward Practical Applications

Metal-organic frameworks (MOFs), a subclass of nanoporous coordination polymers, have emerged as one of the most promising next-generation materials. The postsynthetic modification method, a strategy that provides tunability and control of these materials, plays an important role in enhancing its properties and functionalities. However, knowing adjustments which leads to a desired structure-function a priori remains a challenge. In this comprehensive study, the intermolecular interactions between 21 industrially important gases and a hydrostable STAM-17-OEt MOF were investigated using density functional theory. Substitutions on its 5-ethoxy isophthalate linker included two classes of chemical groups, electron-donating (-NH₂, -OH, and -CH₃) and electron-withdrawing (-CN, -COOH, and -F), as well as the effect of mono-, di-, and tri-substitutions. This resulted in 651 unique MOF-gas complexes. The adsorption energies at the ground state and room temperature, bond lengths, adsorption geometry, natural bond orbital analysis of the electric structure, HOMO-LUMO interactions, and the predicted zwitterionic properties are presented and discussed. This study provides a viable strategy for the functionalization, which leads to the strongest affinity for each gas, an insight into the role of different chemical groups in adsorbing various gas molecules, and identifies synthetic routes for moderating the gas adsorption capacity and reducing water adsorption. Recommendations for various applications are discussed. A custom Python script to assess and visualize the hypothetical separation of two equal gas mixtures of interest is provided. The methodology presented here provides new opportunities to expand the chemical space and physical properties of STAM-17-OEt and advances the development of other hydrostable MOFs.

Manipulating Pore Topology and Functionality to Promote Fluorocarbon-Based Adsorption Cooling

ConspectusWith the worldwide demand for refrigeration and cooling expected to triple, it is increasingly important to search for alternative energy resources to drive refrigeration cycles with reduced electricity consumption. Recently, adsorption cooling has gained increased attention since energy reallocation in such systems is based on gas adsorption/desorption, which can be driven by waste/natural heat sources. Eco-friendly sorption-based cooling relies on the cyclic transfer of refrigerant gas from a high to low energy state by the pseudocompression effect resulting from adsorption and desorption. The driving force for energy transfer relies on heat rather than electricity. The performance of a sorption chiller is primarily influenced by this cyclic sorption behavior, which is characterized as the working capacity of the porous sorbent. Thus, increases in this working capacity directly translate to a more compact and efficient cooling system. However, a lack of highly effective sorbent/refrigerant pairs lowers cooling performance and therefore has limited applicability. To this end, synthetic metal-organic frameworks (MOFs) and covalent organic polymers (COPs) possess higher porosity and greater tunability leading to more substantial potential benefits for adsorption, compared to traditional sorbent materials. Similarly, hydrofluorocarbon refrigerants have more favorable applicability given the ease of operation above atmospheric pressures due to suitable saturated vapor pressures and boiling points. For these reasons, our work focuses on an ongoing strategy to promote sorption cooling via improvements in the sorbent/refrigerant pair. Specifically, we target the interaction of hydrofluorocarbon refrigerants with MOF/COP materials at a molecular level by interpreting the host-guest chemistry and the role of framework pore topology. These molecular-level differences translate to cooling performance, which is described herein. These strategies include engineering framework porosity (i.e., pore size, pore volume) by using elongated organic linkers and stereochemistry control during synthesis; manipulating the sorbate/sorbent interaction by introducing functional moieties or unsaturated metal centers to enhance working capacities in narrow pressure ranges; varying pore topology/morphology to impact adsorption isotherm behavior; and leveraging defective sites within the frameworks to further enhance adsorption capability. This atomic level understanding of sorbate-sorbent interactions is conducted using various in situ experimental techniques such as synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, in situ Fourier transform infrared spectroscopy, and direct sorption energies determinization with calorimetry. Moreover, the experimentally studied interactions and the corresponding adsorption mechanism are corroborated by computational studies using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations. Using this approach, we have made strides toward engineering designed frameworks with precise molecular control to target refrigerant molecules and thereby enhance the performance of desired working pairs for sorption-based cooling.

The Effect of Pore Sizes on D2/H2 Separation Conducted by MOF-74 Analogue

Four stable MOF-74 analogue Ni2(dobdc), Ni2(dobpdc), Ni2(olz) and Ni2(dotpdc) possess abundant open metal sites (OMSs) and honeycomb channels with pore sizes ranging from 1.0 to 2.6 nm, are used to...

Porous Metal-Organic Frameworks for Hydrogen Storage

The high gravimetric energy density and environmental benefit place hydrogen as a promising alternative to the widely used fossil fuel, which is however impeded by the lack of safe, energy-saving...

MOF-derivated MnO@C nanocomposite with bidirectional electrocatalytic ability as signal amplification for dual-signal electrochemical sensing of cancer biomarker

Dual-signal strategy has great potential in improving the accuracy and sensitivity of cancer biomarker determination. However, most sensors based on nanomaterials as signal amplification usually output single detectable signal. It is still a challenge to achieve dual-signal sensing of biomarkers with nanomaterials as signal amplification. Herein, MnO@C nanocomposite was prepared with Mn-MOF-74 as precursor by pyrolysis. It possesses bidirectional electrocatalytic ability toward both oxidation and reduction of H₂O₂ for its fully exposed crystal facets. After loading AuNPs, MnO@C@AuNPs can connect aptamer (Apt) via Au-S and then as a signal amplification for the construction of sandwich-type aptasensor for dual-signal electrochemical sensing of cancer biomarker. Thus, taking mucin 1 (MUC1) as a model system. The aptasensor has the parallel output of differential pulse voltammetry (DPV) and chronoamperometry responses based on oxidation and reduction of H₂O₂, respectively, which implemented sensitive and accurate measurements to avoid false results. The linear response ranges of 0.001 nM-100 nM (detection limit of 0.31 pM) for DPV technique and 0.001 nM-10 nM (detection limit of 0.25 pM) for chronoamperometry technique were obtained. It opens up a new way to design elegant dual-signal aptasensors with potential applications in early disease diagnosis.

Rapid mechanochemical synthesis of metal-organic frameworks using exogenous organic base

Metal-organic frameworks (MOFs) bearing coordinatively unsaturated metal centers, exemplified by the MOF-74 family of frameworks, are promising for applications ranging from gas separations and storage to Lewis acid catalysis. However, the scalable synthesis of MOF-74 analogues remains a significant challenge. Recently, mechanochemistry has emerged as a sustainable strategy for the preparation of MOFs in the solid state with minimal solvent waste. Mechanochemical methods typically rely on metal salts bearing basic anions to deprotonate the conjugate acid of the organic linker and a small amount of organic solvent or water to facilitate liquid assisted grinding. Here, we demonstrate that the liquid exogenous organic base Hünig's base (N,N-diisopropylethylamine) can fulfill both roles, enabling the mechanochemical synthesis of M2(dobdc) analogues (M = Mg, Mn, Co, Ni, Cu, Zn; dobdc4- = 2,5-dioxidobenzene-1,4-dicarboxylate) using metal nitrate salts in only 5 minutes at room temperature. Importantly, we demonstrate that this straightforward method can be generalized to prepare the isomeric framework Mg2(m-dobdc) (m-dobdc4- = 2,4-dioxidobenzene-1,5-dicarboxylate) and the expanded framework Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) under solvent-free conditions for the first time. The MOFs prepared using this method possess high crystallinities and surface areas, with the Mg2(m-dobdc) prepared herein representing the first reported permanently porous variant of this framework. This new sustainable mechanochemical synthesis of MOF-74 analogues should enable their preparation on a large scale for industrial applications.

Conductive Metal-Organic Frameworks: Electronic Structure and Electrochemical Applications

Smarter and minimization of devices are consistently substantial to shape the energy landscape. Significant amounts of endeavours have come forward as promising steps to surmount this formidable challenge. It is undeniable that material scientists were contemplating smarter material beyond purely inorganic or organic materials. To our delight, metal-organic frameworks (MOFs), an inorganic-organic hybrid scaffold with unprecedented tunability and smart functionalities, have recently started their journey as an alternative. In this review, we focus on such propitious potential of MOFs that was untapped over a long time. We cover the synthetic strategies and (or) post-synthetic modifications towards the formation of conductive MOFs and their underlying concepts of charge transfer with structural aspects. We addressed theoretical calculations with the experimental outcomes and spectroelectrochemistry, which will trigger vigorous impetus about intrinsic electronic behaviour of the conductive frameworks. Finally, we discussed electrocatalysts and energy storage devices stemming from conductive MOFs to meet energy demand in the near future.

Molecular Insights into the Correlation between Microstructure and Thermal Conductivity of Zeolitic Imidazolate Frameworks

The thermal conductivity of metal-organic frameworks (MOFs) imposes significant impacts on the thermal transfer performance of related adsorption systems in engineering applications. However, how the structural properties of MOFs affect their thermal conductivities has yet to be unraveled. In this work, the thermal conductivities of 18 zeolitic imidazolate frameworks (ZIFs) were calculated by equilibrium molecular dynamics (MD) simulations. It was revealed that the thermal conductivities of ZIFs were not directly correlated with the commonly investigated structural properties. Thus, two parameters including alignment tensor (A_i) and pathway factor (P_f) were proposed to quantitatively evaluate the orientation and distribution of heat transfer pathways within frameworks, which was demonstrated to correlate better with the thermal conductivities of ZIFs. This study provides new insights into the thermal transfer mechanism within framework-based nanoporous materials, which may also facilitate fundamental understanding and guide the rational design of porous crystals with the thermal conductivity of interest.

Engineering Nanoparticles toward the Modulation of Emerging Cancer Immunotherapy

Cancer immunotherapy is a new therapeutic strategy to fight cancer by activating the patients' own immune system. At present, immunotherapy approaches such as cancer vaccines, immune checkpoint blockade (ICB), adoptive cell transfer (ACT), monoclonal antibodies (mAbs) therapy, and cytokines therapy have therapeutic potential in preclinical and clinical applications. However, the intrinsic limitations of conventional immunotherapy are difficulty of precise dosage control, insufficient enrichment in tumor tissues, partial immune response silencing or hyperactivity, and high cost. Engineering nanoparticles (NPs) have been emerging as a promising multifunctional platform to enhance conventional immunotherapy due to their intrinsic immunogenicity, convenient delivery function, controlled surface chemistry activity, multifunctional modifying potential, and intelligent targeting. This review presents the recent progress reflected by engineering NPs, including the diversified selection of functionalized NPs, the superiority of engineering NPs for enhancing conventional immunotherapy, and NP-mediated multiscale strategies for synergistic therapy consisting of compositions and their mechanism. Finally, the perspective on multifunctional NP-based cancer immunotherapy for boosting immunomodulation is discussed, which reveals the expanding landscape of engineering NPs in clinical translation.

Properties and Detailed Adsorption of CO2 by M2(dobpdc) with N,N-Dimethylethylenediamine Functionalization

We have systematically investigated the CO₂ adsorption performance and microscopic mechanism of N,N-dimethylethylenediamine (mm-2) appended M₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn) with density functional theory. These calculations show that the mm-2 has strong interactions with the open metal site of these structures via the first amine, and the mm-2 binding energies are generally between 123 and 172 kJ/mol. After the CO₂ is attached, the ammonium carbamate molecule is created by insertion. The CO₂ adsorption energies (31-81 kJ/mol) depend on the metal used (Mg; Sc-Zn). The microscopic mechanism of the CO₂ adsorption process is presented at the atomic level, and the detailed potential energy surface and reaction path information are provided. The CO₂ molecule and mm-2 grafted M₂(dobpdc) are firstly combined via physical interactions, and then, the complex is converted into an N-coordinated zwitterion intermediate over a large energy barrier (1.02-1.51 eV). Finally, the structure is rearranged into a stable ammonium carbamate configuration through a small energy barrier (0.05-0.25 eV). We hope that this research will contribute to the understanding and production of real-world carbon capture materials.

The investigation of methane storage at the Ni-MOF-74 material: a periodic DFT calculation

To develop a high-performance methane storage material, an understanding of the mechanism and electronic interactions between methane and the material is essential. In this study, we performed detailed theoretical analyses to investigate the methane storage capacity of Ni-MOF-74 using a large-scale periodic DFT code CONQUEST. In a single pore of the unit cell, we considered three possible sites, iSBU, L, and P sites, where iSBU is the inorganic secondary building unit with a metal center, and L is the linker consisting of the organic building unit, while the P site is the vacuum site in the center of the pore. It shows that the methane molecule adsorption possesses the largest methane molecule adsorption energy on the iSBU site. Our calculations indicate that both C-HO and weak agostic interactions exist between the methane molecule and the iSBU site. The adsorption energy of one methane molecule on the iSBU site is in good agreement with previous experimental and theoretical studies. The calculation of the stepwise methane molecule adsorption shows that the first six methane molecules can first occupy the iSBU sites via C-HO and weak agostic interactions. The second six methane molecules are adsorbed on the remaining L sites, where the C-Hπ interaction becomes important, leading to the synergistic effect together with the C-HO interaction to enhance the adsorption energy of the methane molecule. Finally, it can adsorb up to sixteen CH4 molecules in a single pore of a unit cell at Ni-MOF-74. Moreover, we conducted DOS and EDD analyses, which clearly show that the interactions play a vital role in the adsorption of a methane molecule on Ni-MOF-74, especially the C-HO interaction.

MOF-74-type frameworks: tunable pore environment and functionality through metal and ligand modification

This highlight demonstrates a comprehensive overview of MOF-74-type frameworks in terms of synthetic approaches and pre- or post-synthetic modification approaches.

Impact of Small Adsorbates in the Vibrational Spectra of Mg- and Zn-MOF-74 Revealed by First-Principles Calculations

In this work, we analyze the influence of small adsorbates on the vibrational spectra of Mg- and Zn-metal-organic framework MOF-74 by means of first-principles calculations. In particular, we consider the adsorption of four representative species of different interaction strengths: Ar, CO₂, H₂O, and NH₃. Apart from a comprehensive characterization of the structural and energetic aspects of empty and loaded MOFs, we use a fully quantum ab initio approach to evaluate the Raman and IR activities of the normal modes, leading to the construction of the whole vibrational spectra. Under this approach, not only are we able to proceed with the complete assignment of the spectra in terms of the usual internal coordinates but also we can discern the most relevant vibrational fingerprints of the adsorbates and their impact on the whole MOF spectra. On the one hand, some of the typical vibrational modes of the small molecules are slightly shifted but still visible when adsorbed on the MOFs, especially those appearing at high wavenumbers where the empty MOFs lack IR/Raman signals. On the other hand, some bands arising from the organic ligands are affected by the presence of the absorbates, displaying non-negligible frequency shifts, in agreement with recent experiments. We find a strong correlation between all of these frequency shifts and the interaction strength of the adsorbate with the hosting framework. The findings presented in this work expand the capabilities of vibrational spectroscopy techniques to analyze porous materials and can be useful for the design of sensors and new devices based on MOF technology.

Elucidation of the Underlying Mechanism of CO2 Capture by Ethylenediamine-Functionalized M2(dobpdc) (M = Mg, Sc-Zn)

We, for the first time, systematically investigated the crystal structures, adsorption properties, and microscopic mechanism of CO₂ capture with ethylenediamine (en)-appended isostructural M₂(dobpdc) materials (M = Mg, Sc-Zn), using spin polarized density functional theory (DFT) calculations. The binding energies of en range from 142 to 210 kJ/mol. The weakest binding materials are en-Cr₂(dobpdc) and en-Cu₂(dobpdc). Two typical models, the pair model and the chain model, have been considered for CO₂ adsorption. Generally, the chain model is more stable than the pair model. The CO₂ adsorption energies of the chain model are in the range of 30-96 kJ/mol, with a strong metal dependence. Among these, the en-Sc₂(dobpdc) and en-Cu₂(dobpdc) have the highest and lowest CO₂ adsorption energies, respectively. Moreover, the dynamic progress of CO₂ adsorption has been unveiled via exploration of the full reaction pathway, including transition states and intermediates. First, the CO₂ molecule interacts with en-MOFs to form a physisorbed complex with a shallow potential well. This is followed by overcoming a relatively large energy barrier to form a chemisorbed complex. Finally, ammonium carbamate is formed along the one-dimensional channels within the pore with a small energy barrier for configuration transformation. These results agree well with the experimental observations. Understanding the detailed microscopic mechanism of CO₂ capture is quite crucial for improving our fundamental knowledge base and potential future applications. This work will improve our understanding of CO₂ adsorption with amine functionalized MOFs. We expect our results to stimulate future experimental and theoretical research and advance the development of this field.

Disclosing the microscopic mechanism and adsorption properties of CO2 capture in N-isopropylethylenediamine appended M2(dobpdc) series

The detailed picture of the microscopic mechanism for CO₂ capture in N-isopropylethylenediamine (i-2) functionalized M₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn) has been determined for the first time via systematic computations with van der Waals (vdW) corrected density functional theory (DFT) methods. The results show that acting as a Lewis base, the i-2 molecule can strongly interact with the acidic open metal sites of M₂(dobpdc) via its primary amine with binding energies of 132 to 178 kJ mol^-1 for different metals. After exposure to gaseous CO₂, CO₂ is captured by inserting into the metal-N bond. The corresponding CO₂ binding energies (43-69 kJ mol^-1) vary depending on the metal centers. i-2-Sc₂(dobpdc) and i-2-Mg₂(dobpdc) with high CO₂ binding energies have promising potential for CO₂ capture. Moreover, the results demonstrate that the CO₂ capture process involves two steps, consisting of simultaneous nucleophilic attack of the CO₂ onto the metal-bound N atom with proton transfer. This results in the formation of a zwitterion intermediate (step1), and then rearrangement of the zwitterion intermediate into the final product ammonium carbamate (step2). The first step with relatively high barriers (0.99-1.49 eV) is rate-determining. The second step with low barriers (less than 0.50 eV) can easily occur and will promote the reaction. This work uncovers the complicated microscopic mechanism of CO₂ capture with i-2 functionalized MOFs at the molecular level. This study provides fundamental understanding of the adsorption process and insights into the design and synthesis of highly efficient CO₂ capture materials.

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal-Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2 , H2 , CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

Crystallographic characterization of the metal-organic framework Fe2(bdp)3 upon reductive cation insertion

Precisely locating extra-framework cations in anionic metal–organic framework compounds remains a long-standing, yet crucial, challenge for elucidating structure–performance relationships in functional materials. Single-crystal X-ray diffraction is one of the most powerful approaches for this task, but single crystals of frameworks often degrade when subjected to post-synthetic metalation or reduction. Here, we demonstrate the growth of sizable single crystals of the robust metal–organic framework Fe₂(bdp)₃ (bdp²⁻ = benzene-1,4-dipyrazolate) and employ single-crystal-to-single-crystal chemical reductions to access the solvated framework materials A₂Fe₂(bdp)₃·yTHF (A = Li⁺, Na⁺, K⁺). X-ray diffraction analysis of the sodium and potassium congeners reveals that the cations are located near the center of the triangular framework channels and are stabilized by weak cation–π interactions with the framework ligands. Freeze-drying with benzene enables isolation of activated single crystals of Na_0.5Fe₂(bdp)₃ and Li₂Fe₂(bdp)₃ and the first structural characterization of activated metal–organic frameworks wherein extra-framework alkali metal cations are also structurally located. Comparison of the solvated and activated sodium-containing structures reveals that the cation positions differ in the two materials, likely due to cation migration that occurs upon solvent removal to maximize stabilizing cation–π interactions. Hydrogen adsorption data indicate that these cation–framework interactions are sufficient to diminish the effective cationic charge, leading to little or no enhancement in gas uptake relative to Fe₂(bdp)₃. In contrast, Mg_0.85Fe₂(bdp)₃ exhibits enhanced H₂ affinity and capacity over the non-reduced parent material. This observation shows that increasing the charge density of the pore-residing cation serves to compensate for charge dampening effects resulting from cation–framework interactions and thereby promotes stronger cation–H₂ interactions.

Single-crystal X-ray diffraction reveals structural influences on gas adsorption properties in anionic metal–organic frameworks.

A Scandium MOF with an Unprecedented Inorganic Building Unit, Delimiting the Micropore Windows

A new scandium metal-organic framework (Sc-MOF) with the composition of [Sc(OH)(OBA)], denoted as Sc-CAU-21, was prepared under solvothermal reaction conditions using 4,4'-oxidibenzoic acid (H₂OBA) as the ligand. Single-crystal structure determination revealed the presence of the new inorganic building unit (IBU) {Sc₈(μ-OH)₈(O₂C)₁₆}. It is composed of cis-connected ScO₆ polyhedra forming an eight-membered ring through bridging μ-OH groups. The connection of the IBUs leads to a 3D framework, containing 1D pores with a diameter between 4.2 and 5.6 Å. Pore access is limited by the size of the IBU, and in contrast to the isoreticular aluminum compound Al-CAU-21 [Al(OH)(OBA)], which is nonporous toward nitrogen at 77 K, Sc-CAU-21 exhibits a specific surface area of 610 m² g^-1. The title compound is thermally stable in air up to 350 °C and can be employed as a host for photoluminescent ions. Sc-CAU-21 exhibits a ligand-based blue emission, and (co)substituting Sc³⁺ ions with Ln³⁺ ions (Eu³⁺, Tb³⁺, and Dy³⁺) allows the tuning of the emitting color of the phosphor from red to green. Single-phase white-light emission with CIE color coordinates close to the ideal for white-light emission was also achieved. The luminescence property was utilized in combination with powder X-ray diffraction to study in situ the crystallization process of Sc-CAU-21:Tb and Sc-CAU-21:Eu. Both studies indicate a two-step crystallization process, with a crystalline intermediate, prior to the formation of Sc-CAU-21:Ln.

Electrically Conductive Metal-Organic Frameworks

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.