A spin transition mechanism for cooperative adsorption in metal-organic frameworks

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.

Morphology Engineering of Metal-Organic Frameworks by Facet-Selective Protection and Etching

Anisotropic etching is a novel and effective means to modulate facets of metal-organic frameworks (MOFs) which deserves continuous exploration. Herein, we developed a facet-selective protection and etching method to achieve morphology control of MOFs. Our approach exploits the compositional differences between the facets of zeolitic imidazolate framework-67 and the moderate coordinating and etching properties of ethylenediaminetetraacetic acid disodium salt (EdtaH2Na2). The selected chelator, EdtaH22-, can specifically coordinate with unsaturated metal sites on the {100} crystal planes, protecting them from proton etching and meanwhile releasing protons. Moreover, the released protons with locally high concentration led to the etching of the unprotected {110} facets, ultimately forming nanocrystals with selectively exposed surfaces. This anisotropic etching strategy facilitates the precise modification of MOF surfaces, which is anticipated to play a crucial role in enhancing their properties in different application areas.

Adaptive healing of stress-induced dynamic cracks in a metal-organic framework membrane using nanoparticles

Dewatering of aqueous azeotropes is crucial and pervasive in raw chemical refineries and solvent recovery in the chemical industry but is recognized as one of the most energy-intensive processes. Pervaporation using crystalline molecular sieve membranes provides an energy-efficient solution, but stress loads stemming from thermal and mechanical risks of pervaporation are most likely to cause membrane cracks, which greatly reduces reliability of membranes in real-world applications. Here, we propose adaptive healing of stress-induced dynamic cracks (AHSDC) in the membrane in a risk-responding manner before separation by using in situ-formed nanoparticles in the same chemical environment. These nanoparticles naturally filled in fissure gaps once cracks formed in the membrane, forming adaptive healing zones. Without loss of dewatering capacity, the separation durability of the membrane after AHSDC was improved by at least two orders of magnitude. The membrane also exhibited tolerance to industrial-grade azeotropes that epitomize industrial multisource nature and complexity.

Modulating the Moderate d-Band Center of Indium in InVO4 Nanobelts by Synergizing MnOx and Oxygen Vacancies for High-Efficiency CO2 Photoreduction

Modulating the electronic properties of transition metal sites in photocatalysts at the atomic level is essential for achieving high-activity carbon dioxide photoreduction (CO2PR). An electronic strategy is herein proposed to engineer In-d-band center of InVO4 by incorporating MnOx nanoparticles and oxygen vacancies (VO) into holey InVO4 nanobelts (MnOx/VO-InVO4), which synergistically modulates the In-d-band center to a moderate level and consequently leads to high-efficiency CO2PR. The MnOx/VO-InVO4 catalyst with optimized electronic property exhibits a single carbon evolution rate of up to 145.3 µmol g-1 h-1 and a carbon monoxide (CO) product selectivity of 92.6%, coming out in front of reported InVO4-based materials. It is discovered that the modulated electronic property favors the interaction between the In sites and their intermediates, which thereby improves the thermodynamics and kinetics of the CO2PR-to-CO reaction. This work not only demonstrates the effective engineering of the d orbital of the low-coordination In atoms to promote CO2PR, but also paves the way for the application of tuning d-band center to develop high-efficiency catalysts.

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.

Isomeric Porous Cu(I) Triazolate Frameworks Showing Periodic and Aperiodic Flexibility for Efficient CO Separation

Guest-induced (crystal-to-crystal) transformation, i.e., periodic flexibility, is a typical feature of molecule-based crystalline porous materials, but its role for adsorptive separation is controversial. On the other hand, aperiodic flexibility is rarely studied. This work reports a pair of isomeric Cu(I) triazolate frameworks, namely, α-[Cu(fetz)] (MAF-2Fa) and β-[Cu(fetz)] (MAF-2Fb), which show typical periodic and aperiodic flexibility for CO chemical adsorption, respectively. Quantitative mixture breakthrough experiments show that, while MAF-2Fa exhibits high adsorption capacity at high pressures but negligible adsorption below the threshold pressure and with leakage concentrations of 3-8%, MAF-2Fb exhibits relatively low adsorption capacity at high pressures but no leakage (residual CO concentration <1 ppb). Tandem connection of MAF-2Fa and MAF-2Fb can combine their advantages of high CO adsorption capacities at high and low pressures, respectively. MAF-2Fa and MAF-2Fb can both keep the separation performances unchanged at high relative humidities, but only MAF-2Fb shows a unique coadsorption behavior at a relative humidity of 82%, which can be used to improve purification performances.

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Revealing the Origin of Cooperative Adsorption of Chains on Nanoparticle Surfaces through Coarse-Grained Simulations

This work aims to deepen our understanding of the molecular origin of the recently observed phenomenon of polymer cooperative adsorption onto faceted nanoparticle (NP) surfaces. By exploring a large parameter space for polymer/NP interactions through coarse-grained (CG) molecular dynamics (MD) simulations, it is found that consistent with experiments the presence or absence of cooperativity is related to solvent quality and relative interaction strengths between the polymer and the adsorbent. Specifically, positive cooperativity is associated with stronger polymer-polymer interaction than polymer-surface interactions and vice versa for negative cooperativity. This contrast in interaction energies manifests in positive cooperativity (i.e., increased affinity) and negative cooperativity (i.e., decreased affinity) as concentration increases. It is also found that increasing chain length strengthens cooperativity effects and that the nanoscale confinement of polymer chains to the adsorbing facet (due to weaker affinity to corners and edges) enhances positive cooperativity but weakens negative cooperativity. Moreover, adsorption onto a spherical NP shows stronger positive cooperativity but weaker negative cooperativity compared with adsorption onto a cubic NP of equal surface area. It was further found that as polymer bulk concentration increases, the free energy of adsorption decreases in positive cooperativity, increases in negative cooperativity, and is independent of concentration in noncooperative systems consistent with the phenomenological explanation of cooperativity. We further found that positive cooperativity is associated with growing fluctuations in the adsorption density at critical bulk polymer concentrations. This behavior can be attributed to the competition between enthalpic gains and entropic losses upon adsorption. Overall, our results shed light on the microscopic origin of cooperative adsorption and the role of solvent quality, which can be leveraged in, for example, controlling NP growth into target shapes and designing NP catalysts with improved performance.

Universal Strategy for Metal-Organic Framework Growth: From Cascading-Functional Films to MOF-on-MOFs

Transformation of metal-organic framework (MOF) particles into thin films is urgently needed for the persistent development of well-applicable devices, and recently emerging functional-integrated hybrid frameworks. Although some flexible polymers and exclusive modification approaches have been proposed, the additive-free and widely applicable strategy has not been reported, hampering the deep investigation of the structure-performance relationship. A universal strategy for the in situ growth of large-area and continuous MOF films with controllable microstructures is introduced, through the modification of multi-scale and multi-structure substrates with poly(4-vinylpyridine) as the anchor to capture metal ions via Coulomb attraction. Based on the clarified structure-adsorption-separation mechanisms, the customized devices fabricated by in situ growth can achieve highly selective adsorption and excellently synergetic separation of various industrially relevant isomers. In addition, this strategy is also feasible for the construction of MOF-on-MOFs with varied lattice parameters. This strategy is easy to implement and will be widely applicable to the surface growth of diverse MOFs on desired substrates, and provides a new concept for developing hybrid MOFs integrating with customized functionalities.

Molecular-scale Insights into Cooperativity Switching of xTAB Adsorption on Gold Nanoparticles

Quantifying adsorption behaviors is crucial for various applications such as catalysis, separation, and sensing, yet it is generally challenging to access in solution. Here, we report a combined experimental and computational study of the adsorption behaviors of alkyl-trimethylammonium bromides (xTAB), a class of ligands important for colloidal nanoparticle stabilization and shape control, with various alkyl chain lengths x on Au nanoparticles. We use density functional theory (DFT) to calculate xTAB binding energies on Au{111} and Au{110} surfaces with standing-up and lying-down configurations, which provides insights into the adsorption affinity and cooperativity differences of xTAB on these two facets. We demonstrate the key role of van der Waals interactions in determining the xTAB adsorption behavior. These computational results predict and explain the experimental discovery of xTAB's adsorption behavior switch from stronger affinity, negative cooperativity to weaker affinity, positive cooperativity when the concentration of xTAB increases in solution. We also show that in the standing-up configuration, bilayer adsorption may occur on both facets, which can lead to different differential binding energies and consequently adsorption crossover between the two facets when the ligand concentration increases. Our combined experimental and computational approaches demonstrate a paradigm for gaining molecular-scale insights into adsorbate-surface interactions.

Quasi-open Cu(I) sites for efficient CO separation with high O2/H2O tolerance

Carbon monoxide (CO) separation relies on chemical adsorption but suffers from the difficulty of desorption and instability of open metal sites against O2, H2O and so on. Here we demonstrate quasi-open metal sites with hidden or shielded coordination sites as a promising solution. Possessing the trigonal coordination geometry (sp2), Cu(I) ions in porous frameworks show weak physical adsorption for non-target guests. Rational regulation of framework flexibility enables geometry transformation to tetrahedral geometry (sp3), generating a fourth coordination site for the chemical adsorption of CO. Quantitative breakthrough experiments at ambient conditions show CO uptakes up to 4.1 mmol g-1 and CO selectivity up to 347 against CO2, CH4, O2, N2 and H2. The adsorbents can be completely regenerated at 333-373 K to recover CO with a purity of >99.99%, and the separation performances are stable in high-concentration O2 and H2O. Although CO leakage concentration generally follows the structural transition pressure, large amounts (>3 mmol g-1) of ultrahigh-purity (99.9999999%, 9N; CO concentration < 1 part per billion) gases can be produced in a single adsorption process, demonstrating the usefulness of this approach for separation applications.

The AM-4 Family of Layered Titanosilicates: Single-Crystal-to-Single-Crystal Transformation, Synthesis and Ionic Conductivity

Flexible crystal() structures, which exhibit() single-crystal()-to-single-crystal() (SCSC) transformations(), are attracting attention() in many applied aspects: magnetic() switches, catalysis, ferroelectrics and sorption. Acid treatment() for titanosilicate material() AM-4 and natural() compounds with the same structures led to SCSC transformation() by loss() Na+, Li+ and Zn2+ cations with large structural() changes (20% of the unit()-cell() volume()). The conservation() of crystallinity through complex() transformation() is possible due() to the formation() of a strong hydrogen bonding() system(). The mechanism() of transformation() has been characterized using single-crystal() X-ray() diffraction analysis(), powder() diffraction, Rietvield refinement, Raman spectroscopy and electron microscopy. The low migration() energy() of cations in the considered materials() is confirmed using bond()-valence and density() functional() theory() calculations, and the ion conductivity of the AM-4 family's materials() has been experimentally verified.

CO2-Sensitive Porous Magnet: Antiferromagnet Creation from a Paramagnetic Charge-Transfer Layered Metal-Organic Framework

Porous magnets that undergo a magnetic phase transition in response to gaseous adsorbates are desirable for the development of sustainable sensing and memory devices. Familiar gases such as O2 and CO2 are one class of target adsorbates because of their close association with life sciences and environmental issues; however, it is not easy to develop magnetic devices that respond to these ubiquitous gases. To date, only three examples of gas-responsive magnetic phase transitions have been demonstrated: (i) from a ferrimagnet to an antiferromagnet, (ii) its vice versa (i.e., change of magnetic phase), and (iii) from a ferrimagnet to a paramagnet (i.e., erasure of the magnetic phase). However, the creation of a magnet, meaning the change from a nonmagnet to a magnet by O2 or CO2 gas adsorption and magnetic switching by this phenomenon have not yet been explored. Herein, we report a CO2-induced antiferromagnet modified from a paramagnetic charge-flexible layered compound, [{Ru2(2,4-F2PhCO2)4}2TCNQ(OEt)2] (1; 2,4-F2PhCO2- = 2,4-difluorobenzoate; TCNQ(OEt)2 = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane), where three molar equivalents of CO2 was accommodated at a CO2 pressure of 100 kPa. The magnetic change originates from charge fluctuation due to the transfer of electrons moving from the electron-donor to the electron-acceptor unit or vice versa, resulting in a change in the electron distribution induced by CO2 adsorption/desorption in the donor-acceptor-type charge transfer framework. Owing to the reversible electronic state change upon CO2 adsorption/desorption, these magnetic phases are switched, accompanied by modification of the electrical conductivity, which is boosted by the CO2 accommodation. This is the first example of the creation of a CO2-responsive magnet, which is promising for novel molecular multifunctional devices.

Predicting Gas Adsorption without the Knowledge of Pore Structures: A Machine Learning Method Based on Classical Density Functional Theory

Predicting gas adsorption from the pore structure is an intuitive and widely used methodology in adsorption. However, in real-world systems, the structural information is usually very complicated and can only be approximately obtained from the characterization data. In this work, we developed a machine learning (ML) method to predict gas adsorption form the raw characterization data of N2 adsorption. The ML method is modeled by a convolutional neural network and trained by a large number of data that are generated from a classical density functional theory, and the model gives a very accurate prediction of Ar adsorption. Though the training set is limited to modeling slit pores, the model can be applied to three-dimensional structured pores and real-world materials. The good agreements suggest that there is a universal relationship among adsorption isotherms of different adsorbates, which could be captured by the ML model.

Perspective from a Hubbard U-density corrected scheme towards a spin crossover-mediated change in gas affinity

By employing a recently proposed Hubbard U density-corrected scheme within density functional theory, we provide design principles towards the design of materials exhibiting a spin crossover-assisted gas release. Small molecular fragments are used as case study to identify two main mechanisms behind the change in binding energy upon spin transitions. The feasibility of the proposed mechanism in porous crystals is assessed by correlating the change in binding energy of CO2, CO, N2, and H2, upon spin crossover, with the adiabatic energy difference associated with the spin state change of the square-planar metal in Hofmann-type clathrates (M = Fe, Mn, Ni). A few promising cases are identified for the adsorption of intermediate ligand field strength molecules such as N2 and H2. The latter stands out as the most original result as the strong interaction in low spin, as expected from a Kubas mechanism, results in a large change in binding energy. This work provides a general perspective towards the engineering of open-metal site frameworks exhibiting local environments designed to have a spin crossover upon adsorption of specific gas molecules.

Air-Stable Cu(I) Metal-Organic Framework for Hydrogen Storage

Metal-organic frameworks (MOFs) that contain open metal sites have the potential for storing hydrogen (H2) at ambient temperatures. In particular, Cu(I)-based MOFs demonstrate very high isosteric heats of adsorption for hydrogen relative to other reported MOFs with open metal sites. However, most of these Cu(I)-based MOFs are not stable in ambient conditions since the Cu(I) species display sensitivity toward moisture and can rapidly oxidize in air. As a result, researchers have focused on the synthesis of new air-stable Cu(I)-based materials for H2 storage. Here, we have developed a de novo synthetic strategy to generate a robust Cu(I)-based MOF, denoted as NU-2100, using a mixture of Cu/Zn precursors in which zinc acts as a catalyst to transform an intermediate MOF into NU-2100 without getting incorporated into the final MOF structure. NU-2100 is air-stable and displays one of the initial highest isosteric heats of adsorption (32 kJ/mol) with good hydrogen storage capability under ambient conditions (10.4 g/L, 233 K/100 bar to 296 K/5 bar). We further elucidated the H2 storage performance of NU-2100 using a combination of spectroscopic analysis and computational modeling studies. Overall, this new synthetic route may enable the design of additional stable Cu(I)-MOFs for next-generation hydrogen storage adsorbents at ambient temperatures.

Unraveling the origin of the cooperative adsorption of carbon monoxide in an Fe(II) metal-organic framework

Using periodic DFT calculations, we have established the mechanism of the unusual cooperative adsorption of CO gas in an Fe-bistriazolate MOF observed previously. The binding of one CO molecule to FeII triggers structural alteration of the neighbouring Fe centres, reducing the steric energy penalty and aiding cooperative adsorption. This is similar to the entatic state concept proposed for metalloenzymes, and offers novel strategies for selective gas adsorption.

A Novel Platform of MOF for Sonodynamic Therapy Advanced Therapies

Metal-organic frameworks (MOFs) combined with sonodynamic therapy (SDT) have been introduced as a new and efficient treatment method. The critical advantage of SDT is its ability to penetrate deep tissues and concentrate energy on the tumor site to achieve a non-invasive or minimally invasive effect. Using a sonosensitizer to generate reactive oxygen species (ROS) under ultrasound is the primary SDT-related method of killing tumor cells. In the presence of a sonosensitizer, SDT exhibits a more lethal effect on tumors. The fast development of micro/nanotechnology has effectively improved the efficiency of SDT, and MOFs have been broadly evaluated in SDT due to their easy synthesis, easy surface functionalization, high porosity, and high biocompatibility. This article reviews the main mechanism of action of sonodynamic therapy in cancer treatment, and also reviews the applications of MOFs in recent years. The application of MOFs in sonodynamic therapy can effectively improve the targeting ability of SDT and the conversion ability of reactive oxygen species, thus improving their killing ability on cancer cells. This provides new ideas for the application of micro/nano particles in SDT and cancer therapy.

Photocatalytic CO2 Reduction with Dissolved Carbonates and Near-Zero CO2(aq) by Employing Long-Range Proton Transport

Photocatalytic CO₂ reduction (CO₂R) in ∼0 mM CO₂(aq) concentration is challenging but is relevant for capturing CO₂ and achieving a circular carbon economy. Despite recent advances, the interplay between the CO₂ catalytic reduction and the oxidative redox processes that are arranged on photocatalyst surfaces with nanometer-scale distances is less studied. Specifically, mechanistic investigation on interdependent processes, including CO₂ adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO₂R in ∼0 mM CO₂(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO₃ (aq) of pH 7 but without continuously bubbling CO₂, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrO_x nanoparticles that are supported on a coating-protected GaInP₂ photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H₂, even with copious protons co-generated nearby. CO₂ flux to the Ag@CrO_x CO₂R sites enhances CO₂ adsorption, probed by in situ Raman spectroscopy. CO is produced with local protonation of dissolved inorganic carbon species in a pH as high as 11.5 when using fast electron donors such as ethanol. Isotopic labeling using KH¹³CO₃ was used to confirm the origin of CO from the bicarbonate solution. We then employed COMSOL Multiphysics modeling to simulate the spatial and temporal pH variation and the local concentrations of bicarbonates and CO₂(aq). We found that light-driven CO₂R and CO₂ reactive transport are mutually dependent, which is important for further understanding and manipulating CO₂R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO₂, thereby achieving CO₂ capture and conversion without purifying and feeding gaseous CO₂.

Carbon monoxide separation: past, present and future

Large amounts of carbon monoxide are produced by industrial processes such as biomass gasification and steel manufacturing. The CO present in vent streams is often burnt, this produces a large amount of CO2, e.g., oxidation of CO from metallurgic flue gasses is solely responsible for 2.7% of manmade CO2 emissions. The separation of N2 from CO due to their very similar physical properties is very challenging, meaning that numerous energy-intensive steps are required for CO separation, making the CO separation from many process streams uneconomical in spite of CO being a valuable building block in the production of major chemicals through C1 chemistry and the production of linear hydrocarbons by the Fischer-Tropsch process. The development of suitable processes for the separation of carbon monoxide has both industrial and environmental significance. Especially since CO is a main product of electrocatalytic CO2 reduction, an emerging sustainable technology to enable carbon neutrality. This technology also requires an energy-efficient separation process. Therefore, there is a great need to develop energy efficient CO separation processes adequate for these different process streams. As such the urgency of separating carbon monoxide is gaining greater recognition, with research in the field becoming more and more crucial. This review details the principles on which CO separation is based and provides an overview of currently commercialised CO separation processes and their limitations. Adsorption is identified as a technology with the potential for CO separation with high selectivity and energy efficiency. We review the research efforts, mainly seen in the last decades, in developing new materials for CO separation via ad/bsorption and membrane technology. We have geared our review to both traditional CO sources and emerging CO sources, including CO production from CO2 conversion. To that end, a variety of emerging processes as potential CO2-to-CO technologies are discussed and, specifically, the need for CO capture after electrochemical CO2 reduction is highlighted, which is still underexposed in the available literature. Altogether, we aim to highlight the knowledge gaps that could guide future research to improve CO separation performance for industrial implementation.

Cooperative CO2 adsorption mechanism in a perfluorinated CeIV-based metal organic framework

Adsorbents able to uptake large amounts of gases within a narrow range of pressure, i.e., phase-change adsorbents, are emerging as highly interesting systems to achieve excellent gas separation performances with little energy input for regeneration. A recently discovered phase-change metal-organic framework (MOF) adsorbent is F4_MIL-140A(Ce), based on Ce^IV and tetrafluoroterephthalate. This MOF displays a non-hysteretic step-shaped CO₂ adsorption isotherm, reaching saturation in conditions of temperature and pressure compatible with real life application in post-combustion carbon capture, biogas upgrading and acetylene purification. Such peculiar behaviour is responsible for the exceptional CO₂/N₂ selectivity and reverse CO₂/C₂H₂ selectivity of F4_MIL-140A(Ce). Here, we combine data obtained from a wide pool of characterisation techniques - namely gas sorption analysis, in situ infrared spectroscopy, in situ powder X-ray diffraction, in situ X-ray absorption spectroscopy, multinuclear solid state nuclear magnetic resonance spectroscopy and adsorption microcalorimetry - with periodic density functional theory simulations to provide evidence for the existence of a unique cooperative CO₂ adsorption mechanism in F4_MIL-140A(Ce). Such mechanism involves the concerted rotation of perfluorinated aromatic rings when a threshold partial pressure of CO₂ is reached, opening the gate towards an adsorption site where CO₂ interacts with both open metal sites and the fluorine atoms of the linker.

Inter-layer magnetic tuning by gas adsorption in π-stacked pillared-layer framework magnets

Magnetism of layered magnets depends on the inter-layer through-space magnetic interactions (J NNNI). Using guest sorption to address inter-layer pores in bulk-layered magnets is an efficient approach to magnetism control because the guest-delicate inter-layer distance (l trans) is a variable parameter for modulating J NNNI. Herein, we demonstrated magnetic changes induced by the adsorption of CO2, N2, and O2 gases in various isostructural layered magnets with a π-stacked pillared-layer framework, , (M = Co, 1, Fe, 2, Cr, 3; Cp* = η5-C5Me5; 2,3,5,6-F4PhCO2 - = 2,3,5,6-tetrafluorobenzoate; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane). Each compound had almost identical adsorption capability for the three types of gases; only CO2 adsorption was found to have a gated profile. A breathing-like structural modulation involving the extension of l trans occurred after the insertion of gases into the isolated pores between the [Ru2]2-TCNQ ferrimagnetic layers, which is more significant for CO2 than for O2 and N2, due to the CO2-gated transition. While adsorbent 1 with M = Co (S = 0) was an antiferromagnet with T N = 75 K, 1⊃CO2 was a ferrimagnet with T C = 76 K, whereas 1⊃N2 and 1⊃O2 were antiferromagnets with T N = 68 K. The guest-insertion effect was similarly confirmed in 2 and 3, and was characteristically dependent on the type of sandwiched spin in as M = Fe (S = 1/2) and Cr (S = 3/2), respectively. This study reveals that common gases such as CO2, O2, and N2 can serve as crucial triggers for the change in magnetism as a function of variable parameter l trans.

Metal Organic Framework Glasses: a New Platform for Electrocatalysis?

Metal organic framework (MOF) glasses are a coordination network of metal nodes and organic ligands as an undercooled frozen-in liquid, and have therefore broadened the potential of MOF materials in the fundamental research and application scenarios. On the road to deploying MOF glasses as electrocatalysts, it remains several basic scientific hurdles although MOF glasses not only inherit the structural merits of MOFs but also endow with active catalytic features including concentrated defects, metal centers and disorder structure etc. The research on the ionic conductivity, catalytic stability and reactivity of MOF glasses has yielded scientific insights towards its electrocatalytic applications. Here, we first comb the history, definition and basic properties of MOF glasses. Then, we identify the main synthetic methods and characterization techniques. Finally, we advance the potentials and challenges of MOF glasses as electrocatalysts in furthering the understanding of these themes.

A ligand insertion mechanism for cooperative NH3 capture in metal-organic frameworks

Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber-Bosch process¹. The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production^2-5. However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation^6-9. Given their high surface areas and structural and chemical versatility, metal-organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas^10,11. Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal-carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management¹², and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents.

Metal-Organic Frameworks and Their Composites for Environmental Applications

From the point of view of the ecological environment, contaminants such as heavy metal ions or toxic gases have caused harmful impacts on the environment and human health, and overcoming these adverse effects remains a serious and important task. Very recent, highly crystalline porous metal-organic frameworks (MOFs), with tailorable chemistry and excellent chemical stability, have shown promising properties in the field of removing various hazardous pollutants. This review concentrates on the recent progress of MOFs and MOF-based materials and their exploit in environmental applications, mainly including water treatment and gas storage and separation. Finally, challenges and trends of MOFs and MOF-based materials for future developments are discussed and explored.

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.

Prospects for Simultaneously Capturing Carbon Dioxide and Harvesting Water from Air

Mitigation of anthropogenic climate change is expected to require large-scale deployment of carbon dioxide removal strategies. Prominent among these strategies is direct air capture with sequestration (DACS), which encompasses the removal and long-term storage of atmospheric CO₂  by purely engineered means. Because it does not require arable land or copious amounts of freshwater, DACS is already attractive in the context of sustainable development, but opportunities to improve its sustainability still exist. Leveraging differences in the chemistry of CO₂  and water adsorption within porous solids, here, the prospect of simultaneously removing water alongside CO₂  in direct air capture operations is investigated. In many cases, the co-adsorbed water can be desorbed separately from chemisorbed CO₂  molecules, enabling efficient harvesting of water from air. Depending upon the material employed and process conditions, the desorbed water can be of sufficiently high purity for industrial, agricultural, or potable use and can thus improve regional water security. Additionally, the recovered water can offset a portion of the costs associated with DACS. In this Perspective, molecular- and process-level insights are combined to identify routes toward realizing this nascent yet enticing concept.

Highly Selective Adsorption of Perfluorinated Greenhouse Gases by Porous Organic Cages

Anthropogenic greenhouse gases contribute to global warming. Among those gases, perfluorocarbons (PFCs) are thousands to tens of thousands of times more harmful to the environment than comparable amounts of carbon dioxide. To date, materials that selectively adsorb perfluorocarbons in favor of other less harmful gases have not been reported. Here, a series of porous organic cage compounds with alkyl-, fluoroalkyl-, and partially fluorinated alkyl groups is presented. Their isomorphic crystalline states allow the study of the structure-property relationship between the degree of fluorination of the alkyl chains and the gas sorption properties for PFCs and their selective uptakes in comparison to other, nonfluorinated gases. By this approach, one compound having superior selectivities of PFCs versus N₂ or CO₂ under ambient conditions is identified.

Flexible Cuprous Triazolate Frameworks as Highly Stable and Efficient Electrocatalysts for CO2 Reduction with Tunable C2 H4 /CH4 Selectivity

Cu-based metal-organic frameworks have attracted much attention for electrocatalytic CO₂ reduction, but they are generally instable and difficult to control the product selectivity. We report flexible Cu(I) triazolate frameworks as efficient, stable, and tunable electrocatalysts for CO₂ reduction to C₂ H₄ /CH₄ . By changing the size of ligand side groups, the C₂ H₄ /CH₄ selectivity ratio can be gradually tuned and inversed from 11.8 : 1 to 1 : 2.6, giving C₂ H₄ , CH₄ , and hydrocarbon selectivities up to 51 %, 56 %, and 77 %, respectively. After long-term electrocatalysis, they can retain the structures/morphologies without formation of Cu-based inorganic species. Computational simulations showed that the coordination geometry of Cu(I) changed from triangular to tetrahedral to bind the reaction intermediates, and two adjacent Cu(I) cooperated for C-C coupling to form C₂ H₄ . Importantly, the ligand side groups controlled the catalyst flexibility by the steric hindrance mechanism, and the C₂ H₄ pathway is more sensitive than the CH₄ one.

A spin-crossover framework endowed with pore-adjustable behavior by slow structural dynamics

Host-guest interactions play critical roles in achieving switchable structures and functionalities in porous materials, but design and control remain challenging. Here, we report a two-dimensional porous magnetic compound, [Fe^II(prentrz)₂Pd^II(CN)₄] (prentrz = (1E,2E)−3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine), which exhibits an atypical pore transformation that directly entangles with a spin state transition in response to water adsorption. In this material, the adsorption-induced, non-uniform pedal motion of the axial prentrz ligands and the crumpling/unfolding of the layer structure actuate a reversible narrow quasi-discrete pore (nqp) to large channel-type pore (lcp) change that leads to a pore rearrangement associated with simultaneous pore opening and closing. The unusual pore transformation results in programmable adsorption in which the lcp structure type must be achieved first by the long-time exposure of the nqp structure type in a steam-saturated atmosphere to accomplish the gate-opening adsorption. The structural transformation is accompanied by a variation in the spin-crossover (SCO) property of Fe^II, i.e., two-step SCO with a large plateau for the lcp phase and two-step SCO with no plateau for the nqp phase. The unusual adsorption-induced pore rearrangement and the related SCO property offer a way to design and control the pore structure and physical properties of dynamic frameworks.

Host-guest interactions can play a critical role in achieving switchable porous materials, but controlling them remains challenging. Here the authors report an atypical pore rearrangement in a magnetic 2D porous framework upon water adsorption; the structural transformation affects the magnetic properties of the material.

Pore Size Dictates Anisotropic Thermal Conductivity of Two-Dimensional Covalent Organic Frameworks with Adsorbed Gases

Two-dimensional covalent organic frameworks (2D COFs) are a class of modular polymeric crystals with high porosities and large surface areas, which position them as ideal candidates for applications in gas storage and separation technologies. In this work, we study the influence of pore geometry on the anisotropic heat transfer mechanisms in 2D COFs through systematic atomistic simulations. More specifically, by studying COFs with varying pore sizes and gas densities, we demonstrate that the cross-plane thermal conductivity along the direction of the laminar pores can either be decreased due to solid-gas scattering (for COFs with relatively smaller pores that are ≲2 nm) or increased due to additional heat transfer pathways introduced by the gas adsorbates (for COFs with relatively larger pores). Our simulations on COF/methane systems reveal the intricate relationship among gas diffusivities, pore geometries, and solid-gas interactions dictating the modular thermal conductivities in these materials. Along with the understanding of the fundamental nature of gas diffusion and heat conduction in the porous framework crystals, our results can also help guide the design of efficient 2D polymeric crystals for applications with improved gas storage, catalysis, and separation capabilities.

Reconstructed covalent organic frameworks

Covalent organic frameworks (COFs) are distinguished from other organic polymers by their crystallinity^1-3, but it remains challenging to obtain robust, highly crystalline COFs because the framework-forming reactions are poorly reversible^4,5. More reversible chemistry can improve crystallinity^6-9, but this typically yields COFs with poor physicochemical stability and limited application scope⁵. Here we report a general and scalable protocol to prepare robust, highly crystalline imine COFs, based on an unexpected framework reconstruction. In contrast to standard approaches in which monomers are initially randomly aligned, our method involves the pre-organization of monomers using a reversible and removable covalent tether, followed by confined polymerization. This reconstruction route produces reconstructed COFs with greatly enhanced crystallinity and much higher porosity by means of a simple vacuum-free synthetic procedure. The increased crystallinity in the reconstructed COFs improves charge carrier transport, leading to sacrificial photocatalytic hydrogen evolution rates of up to 27.98 mmol h^-1 g^-1. This nanoconfinement-assisted reconstruction strategy is a step towards programming function in organic materials through atomistic structural control.

Exploring mechanistic routes for light alkane oxidation with an iron-triazolate metal-organic framework

In this work, we computationally explore the formation and subsequent reactivity of various iron-oxo species in the iron-triazolate framework Fe₂(μ-OH)₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) for the catalytic activation of strong C-H bonds. With the direct conversion of methane to methanol as the probe reaction of interest, we use density functional theory (DFT) calculations to evaluate multiple mechanistic pathways in the presence of either N₂O or H₂O₂ oxidants. These calculations reveal that a wide range of transition metal-oxo sites - both terminal and bridging - are plausible in this family of metal-organic frameworks, making it a unique platform for comparing the electronic structure and reactivity of different proposed active site motifs. Based on the DFT calculations, we predict that Fe₂(μ-OH)₂(bbta) would exhibit a relatively low barrier for N₂O activation and energetically favorable formation of an [Fe(O)]²⁺ species that is capable of oxidizing C-H bonds. In contrast, the use of H₂O₂ as the oxidant is predicted to yield an assortment of bridging iron-oxo sites that are less reactive. We also find that abstracting oxo ligands can exhibit a complex mixture of both positive and negative spin density, which may have broader implications for relating the degree of radical character to catalytic activity. In general, we consider the coordinatively unsaturated iron sites to be promising for oxidation catalysis, and we provide several recommendations on how to further tune the catalytic properties of this family of metal-triazolate frameworks.

Meniscus-Guided 3D Microprinting of Pure Metal-Organic Frameworks with High Gas-Uptake Performance

Metal-organic frameworks (MOFs) are a promising nanoporous functional material system; however, the practicality of shaping freeform MOF monoliths, while retaining their porosity, remains a challenge. Here, we demonstrate that meniscus-guided three-dimensional (3D) printing can produce pure MOF monoliths with high gas-uptake performance. The method exploits a femtoliter precursor ink meniscus to highly confine and guide supersaturation-driven crystallization in a layer-by-layer manner to print a pure HKUST-1 micro-monolith with a high spatial resolution of <3 μm. The proposed 3D printing technique does not involve rheological additives, binders, or mechanical forces. Thus, the resulting HKUST-1 monolith displays a prominently high Brunauer-Emmett-Teller surface area of 1192 m²/g, which is superior to monoliths produced using other 3D printing approaches. This technique enables both structural design freedom and high material performance in the manufacturing of MOFs for practical use.

Hidden ordered structure in the archetypical Fe(pyrazine)[Pt(CN)4] spin-crossover porous coordination compound

The actual low-spin state of Fe(pyrazine)Pt(CN)4 implies an ordered arrangement of the pyrazine rings, which significantly affects the spin transition.

Room-Temperature On-Spin-Switching and Tuning in a Porphyrin-Based Multifunctional Interface

Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II), S = 0] to [Ni(I), S = 1/2]. The chemically active Ni(I), even in a buried multilayer system, can be functionalized with nitrogen dioxide, allowing a selective tuning of the electronic properties of the Ni center that is switched to a [Ni(II), S = 1] state. While Ni acts as a reversible spin switch, it is found that the electronic structure of the macrocycle backbone, where the frontier orbitals are mainly localized, remains unaffected. These findings pave the way for using the present porphyrin-based system as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli.

Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin (ls)-[Co^III (nEGEspy)₂ (3,6-DTBSQ)(3,6-DTBCat)] (ls-[Co^III ], 3,6-DTBSQ = 3,6-di-tert-butyl semiquinonato, 3,6-DTBCat = 3,6-di-tert-butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls-[Co^III ] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high-spin (hs)-[Co^II (nEGEspy)₂ (3,6-DTBSQ)₂ ] (hs-[Co^II ]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls-[Co^III ] exhibited partial VT (33 %) and only a 9.3 % VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.

Giant single-crystal-to-single-crystal transformations associated with chiral interconversion induced by elimination of chelating ligands

Numerous single crystals that exhibit single-crystal-to-single-crystal (SCSC) transformations have been reported, and some of them show great promise for application to advanced adsorption materials, magnetic switches, and smart actuators. However, the development of single crystals with super-adaptive crystal lattices capable of huge and reversible structural change remains a great challenge. In this study, we report a Zn^II complex that undergoes giant SCSC transformation induced by a two-step thermal elimination of ethylene glycol chelating ligands. Although the structural change is exceptionally large (50% volume shrinkage and 36% weight loss), the single-crystal nature of the complex persists because of the multiple strong hydrogen bonds between the constituent molecules. This allows the reversible zero-dimensional to one-dimension and further to three-dimensional structural changes to be fully characterized by single-crystal X-ray diffraction analyses. The elimination of chelating ligands induces a chiral interconversion in the molecules that manifests as a centric-chiral-polar symmetric variation of the single crystal. The study not only presents a unique material, featuring both a periodic crystal lattice and gel-like super-ductility, but also reveals a possible solid-state reaction method for preparing chiral compounds via the elimination of chelating ligands.

Revisiting molecular adsorption: unconventional uptake of polymer chains from solution into sub-nanoporous media

Adsorption of polymers from the solution phase has been extensively studied to cope with many demands not only for separation technologies, but also for the development of coatings, adhesives, and biocompatible materials. Most studies hitherto focus on adsorption on flat surfaces and mesoporous adsorbents with open frameworks, plausibly because of the preconceived notion that it is unlikely for polymers to enter a pore with a diameter that is smaller than the gyration diameter of the polymer in solution; therefore, sub-nanoporous materials are rarely considered as a polymer adsorption medium. Here we report that polyethylene glycols (PEGs) are adsorbed into sub-nanometer one-dimensional (1D) pores of metal-organic frameworks (MOFs) from various solvents. Isothermal adsorption experiments reveal a unique solvent dependence, which is explained by the balance between polymer solvation propensity for each solvent and enthalpic contributions that compensate for potential entropic losses from uncoiling upon pore admission. In addition, adsorption kinetics identify a peculiar molecular weight (MW) dependence. While short PEGs are adsorbed faster than long ones in single-component adsorption experiments, the opposite trend was observed in double-component competitive experiments. A two-step insertion process consisting of (1) an enthalpy-driven recognition step followed by (2) diffusion regulated infiltration in the restricted 1D channels explains the intriguing selectivity of polymer uptake. Furthermore, liquid chromatography using the MOFs as the stationary phase resulted in significant PEG retention that depends on the MW and temperature. This study provides further insights into the mechanism and thermodynamics behind the present polymer adsorption system, rendering it as a promising method for polymer analysis and separation.