Modulating supramolecular binding of carbon dioxide in a redox-active porous metal-organic framework

Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

Efficient structure tuning over the defective modulated zirconium metal organic framework with active coordinate surface for photocatalyst CO2 reduction

Structure engineering of zirconium-based metal organic frameworks (MOFs) aims to develop efficient catalysts for transforming intermittent renewable energy into value-added chemical fuels. In order to have a deeper understanding of industrial scaling, it is vital to ascertain the favourable operational parameters that are necessary for projecting at the atomic level. The proposed paradigm provides a robust basis for the efficient design of MOFs based heterogeneous photocatalysts. In this study, set of defective MOF (D-NUiO66) was effectively produced using a modular acidic method. Afterwards, the D-NUiO66 was combined with CeO2 to form the D-CeNUiO66 heterojunction for the purpose of carbon dioxide reduction. The morphological aspect of the composite investigation suggested that D-CeNUiO66 had a mesoporous structure with favourable adsorption properties. The optimized D-CeNUiO66 photocatalyst showed the high activity for the reduction of CO2 to CO, with a rate of 38.6 µmolg-1h-1 and demonstrated remarkable repeatability in terms of CO production. The incorporation of defect sites in the D-NUiO66 enhanced the light response to visible light, resulting in reduced band gap of 2.9 eV. The photoelectrochemical tests indicated that the introduction of defects in the UiO66 and coupling CeO2 in the D-CeNUiO66 composite induced fast charge transfer, therefore suppressing the charge recombination rate. This study provides valuable insights into the use of defective engineering and heterojunction approaches to metal-organic frameworks for photocatalytic applications.

Analysis and Refinement of Host-Guest Interactions in Metal-Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) are a class of hybrid porous materials characterized by their periodic assembly using metal ions and organic ligands through coordination bonds. Their high crystallinity, extensive surface area, and adjustable pore sizes make them promising candidates for a wide array of applications. These include gas adsorption and separation, substrate binding, and catalysis, of relevance to tackling pressing global issues such as climate change, energy challenges, and pollution. In comparison to traditional porous materials such as zeolites and activated carbons, the design flexibility of organic ligands in MOFs, coupled with their orderly arrangement with associated metal centers, allows for the precise engineering of uniform pore environments. This unique feature enables a rich variety of interactions between the MOF host and adsorbed gas molecules, which are fundamental to understanding the observed uptake capacity and selectivity for target gas molecules and thus the overall performance of the material.In this Account, a data set for three-dimensional MOFs has been constructed based upon the structural analysis of host-guest interactions using the largest experimental database, the Cambridge Structural Database (CSD). A full screening was performed on structures with guest molecules of H2, C2H2, CO2, and SO2, and the relationship between the primary binding site, the isosteric heats of adsorption (Qst), and the adsorption uptake was extracted and established. We review the methodologies to refine host-guest interactions based primarily on our studies on the host-guest chemistry of MOFs. The methods include ligand functionalization, variation of metal centers, formation of defects, addition of single atom sites, and control of pore size and structure. In situ structural and dynamic investigations using diffraction and spectroscopic techniques are powerful tools to visualize the details of host-guest interactions upon the above modifications, affording key insights into functional performance at a molecular level. Finally, we give an outlook of future research priorities in the study of host-guest chemistry in MOF materials. We hope this Account will encourage the rational development and improvement of future MOF-based sorbents for applications in challenging gas adsorption, separations, and catalysis.

Photo-Induced Construction and Recovery of Cu+ Sites in Metal-Organic Frameworks

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu+ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu2+ doesn't have such ability. Meanwhile, Cu+ sites are easily oxidized to Cu2+ , leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu+ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu+ sites in metal-organic frameworks (MOFs) and recover Cu+ after oxidation. The Cu2+ precursor was loaded on NH2 -MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti3+ from Ti4+ in framework, which reduces the supported Cu2+ in the absence of any additional reducing agent, thus simplifying the process for Cu+ generation significantly. Due to π-complexation interaction, the presence of Cu+ results in remarkably enhanced CO capture capacity (1.16 mmol g-1 ) compared to NH2 -MIL-125 (0.49 mmol g-1 ). More importantly, Cu+ can be recovered conveniently via re-irradiation when it is oxidized to Cu2+ , and the oxidation-recovery process is reversible.

Self-Reconstructed Two-Dimensional Cluster-Based Co-Organic Layer Heterojunctions for Enhanced Oxygen Evolution Reactions

When it comes to an efficient catalytic oxygen evolution reaction (OER) in the production of renewable energy and chemicals, the construction of heterogeneous structures is crucial to break the linear scalar relationship of a single catalyst. This heterogeneous structure construction helps creatively achieve high activity and stability. However, the synthesis process of heterogeneous crystalline materials is often complex and challenging to capture and reproduce, which limits their application. Here, the dynamic process of structural changes in Co-MOFs in alkali was captured by in situ powder X-ray diffraction, FT-IR spectroscopy, and Raman spectroscopy, and several self-reconfigured MOF heterogeneous materials with different structures were stably isolated. The created β-Co(OH)2/Co-MOF heterojunction structure facilitates rapid mass-charge transfer and exposure of active sites, which significantly enhanced OER activity. Experimental results show that this heterogeneous structure achieves a low overpotential of 333 mV at 10 mA cm-2. The findings provide new insights and directions for the search for highly reactive cobalt-based MOFs for sustainable energy technologies.

MFM-300(Sc): a chemically stable Sc(III)-based MOF material for multiple applications

Developing robust multifunctional metal-organic frameworks (MOFs) is the key to advancing the further deployment of MOFs into relevant applications. Since the first report of MFM-300(Sc) (MFM = Manchester Framework Material, formerly known as NOTT-400), the development of applications of this robust microporous MOF has only grown. In this review, a summary of the applications of MFM-300(Sc), as well as some emerging advanced applications, have been discussed. The adsorption properties of MFM-300(Sc) are presented systematically. Particularly, this contribution is focused on acid and corrosive gas adsorption. In addition, recent applications for catalysis based on the outstanding hemilabile Sc-O bond character are highlighted. Finally, some new research areas are introduced, such as host-guest chemistry and biomedical applications. This highlight aims to showcase the recent advances and the potential for developing new applications of this promising material.

Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH₄)₂K₂[V₄(μ₂-OH)₄(ox)₄(pz)₄]·9H₂O (1, ox = oxalate and pz = pyrazole) and (NH₄)₂Na₂[V₄(μ₂-OH)₄(ox)₄(4-mpz)₄]·7H₂O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K₂[V₆(μ₂-OH)₆(ox)₆(Hdatrz)₆]Cl₂·29.5H₂O (3) and [V₈(μ₂-OH)₈(SO₃)₈(Hdatrz)₈]·38H₂O (4, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V₄(μ₂-OH)₄} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V₆(μ₂-OH)₆} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V₈(μ₂-OH)₈(SO₃)₈} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO₂ and O₂ more favorably than N₂, CH₄ and H₂ gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO₂ adsorption.

Predicting Spontaneous Orientational Self-Assembly: In Silico Design of Materials with Quantum Mechanically Derived Force Fields

De novo design of self-assembled materials hinges upon our ability to relate macroscopic properties to individual building blocks, thus characterizing in such supramolecular architectures a wide range of observables at varied time/length scales. This work demonstrates that quantum mechanical derived force fields (QMD-FFs) do satisfy this requisite and, most importantly, do so in a predictive manner. To this end, a specific FF, built solely based on the knowledge of the target molecular structure, is employed to reproduce the spontaneous transition to an ordered liquid crystal phase. The simulations deliver a multiscale portrait of such self-assembly processes, where conformational changes within the individual building blocks are intertwined with a 200 ns ensemble reorganization. The extensive characterization provided not only is in quantitative agreement with the experiment but also connects the time/length scales at which it was performed. Realizing QMD-FF predictive power and unmatched accuracy stands as an important leap forward for the bottom-up design of advanced supramolecular materials.

Advances in Oxidative Desulfurization of Fuel Oils over MOFs-Based Heterogeneous Catalysts

Catalytic oxidative desulfurization (ODS) of fuel oils is considered one of the most promising non-hydrodesulfurization technologies due to the advantages of mild reaction conditions, low cost and easy removal of aromatic sulfur compounds. Based on this reason, the preparation of highly efficient ODS catalysts has been a hot research topic in this field. Recently, metal-organic frameworks (MOFs) have attracted extensive attention due to the advantages involving abundant metal centers, high surface area, rich porosity and varied pore structures. For this, the synthesis and catalytic performance of the ODS catalysts based on MOFs materials have been widely studied. Until now, many research achievements have been obtained along this direction. In this article, we will review the advances in oxidative desulfurization of fuel oils over MOFs-based heterogeneous catalysts. The catalytic ODS performance over various types of catalysts is compared and discussed. The perspectives for future work are proposed in this field.

Covalent Organic Frameworks for Simultaneous CO2 Capture and Selective Catalytic Transformation

Combination of capture and simultaneous conversion of CO2 into valuable chemicals is a fascinating strategy for reducing CO2 emissions. Therefore, searching for heterogeneous catalysts for efficient catalytic conversion of CO2 is of great importance for carbon capture and utilization. Herein, we report a metalloporphyrin-based covalent organic framework (Co(II)@TA-TF COF) that can capture CO2 and simultaneously convert it into cyclic carbonates under mild conditions. The COF was designed to possess micropores for the adsorption of CO2 and integrated with cobalt(II) porphyrin (Co(II)@TAPP) units as catalytic sites into the vertices of the layered tetragonal networks. The structure of the Co(II)@TA-TF COF is unique where Co(II)@TAPP units are alternately stacked along the z direction with a slipped distance of 1.7 Å, which gives an accessible space to accommodate small molecules, making it possible to expose catalytic sites to substrates within the adjacent stacked layers. As a result, this COF is found to be highly effective for the addition of CO2 and epoxides. Importantly, the Co(II)@TA-TF COF exhibited a dramatic size selectivity for substrates. In conjunction with its reusability, our results highlight the development of a new function of COFs for targeting simultaneous CO2 absorption and utilization upon complementary exploration of the structural features of skeletons and pores. Such promising catalytic performance of the COF makes it possible for its potential practical application.

Construction of C-C bonds via photoreductive coupling of ketones and aldehydes in the metal-organic-framework MFM-300(Cr)

Construction of C-C bonds via reductive coupling of aldehydes and ketones is hindered by the highly negative reduction potential of these carbonyl substrates, particularly ketones, and this renders the formation of ketyl radicals extremely endergonic. Here, we report the efficient activation of carbonyl compounds by the formation of specific host-guest interactions in a hydroxyl-decorated porous photocatalyst. MFM-300(Cr) exhibits a band gap of 1.75 eV and shows excellent catalytic activity and stability towards the photoreductive coupling of 30 different aldehydes and ketones to the corresponding 1,2-diols at room temperature. Synchrotron X-ray diffraction and electron paramagnetic resonance spectroscopy confirm the generation of ketyl radicals via confinement within MFM-300(Cr). This protocol removes simultaneously the need for a precious metal-based photocatalyst or for amine-based sacrificial agents for the photochemical synthesis.

Multipoint Hydrogen Bonding-Based Molecular Recognition of Amino Acids and Peptide Derivatives in a Porous Metal-Macrocycle Framework: Residue-Specificity, Diastereoselectivity, and Conformational Control

Porous crystals have great potential to exert space-specific functions such as multipoint molecular recognition. In order to rationally enhance the porous function, it is necessary to precisely control molecular recognition event in the pores. Hydrogen bonding is an effective tool for controlling molecular recognition. However, multiple hydrogen bonds, which are essentially the origin of high complementarity and specificity, remain difficult to innovate in porous crystals in an intelligent way. This paper demonstrates molecular recognition of amino acid and peptide derivatives by multipoint hydrogen bonding in a porous metal-macrocycle framework revealed by single-crystal X-ray diffraction analysis. l-Serine residues are site-selectively and residue-specifically adsorbed on the pore surface via multiple hydrogen bonds. A serine derivative is diastereoselectively recognized on the (P)- or (M)-side of the enantiomeric pore surface. Moreover, the conformation of the peptide is highly regulated, incorporating a poly-l-proline type I helix-like structure into the pore. These findings will bring deep scientific knowledge to the design of new porous crystals and functions.

Molecular engineering in a family of pillared-layered metal-organic frameworks for tuning gas adsorption behavior

In this work, inspired by a water-assisted three-dimensional supramolecular structure 1, we use a mixed-ligand strategy to form a 3D pillared-layered matrix by the introduction of linear ligands to compete against the water molecules. The resulting analogue microporous MOFs of 2-H, 2-F and 2-N, decorated with different functional groups, similarly show the CO2 uptake. Thanks to the negligible N2 adsorption capacity, enhanced selective adsorption towards CO2 is achieved in compound 2-N. That is, we present here an alternative plan for the high CO2 selective adsorption performance. In addition, the structure stability and moderate affinity for CO2 of these microporous MOFs endow them with excellent reusability.

Visualizing defects and pore connectivity within metal-organic frameworks by X-ray transmission tomography

Metal-Organic Frameworks (MOFs) have the potential to change the landscape of molecular separations in chemical processes owing to their ability of selectively binding molecules. Their molecular sorting properties generally rely on the micro- and meso-pore structure, as well as on the presence of coordinatively unsaturated sites that interact with the different chemical species present in the feed. In this work, we show a first-of-its-kind tomographic imaging of the crystal morphology of a metal-organic framework by means of transmission X-ray microscopy (TXM), including a detailed data reconstruction and processing approach. Corroboration with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) images shows the potential of this strategy for further (non-destructively) assessing the inner architecture of MOF crystals. By doing this, we have unraveled the presence of large voids in the internal structure of a MIL-47(V) crystal, which are typically thought of as rather homogeneous lattices. This challenges the established opinion that hydrothermal syntheses yield relatively defect-free material and sheds further light on the internal morphology of crystals.

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.

High Ammonia Adsorption in MFM-300 Materials: Dynamics and Charge Transfer in Host-Guest Binding

Ammonia (NH₃) is a promising energy resource owing to its high hydrogen density. However, its widespread application is restricted by the lack of efficient and corrosion-resistant storage materials. Here, we report high NH₃ adsorption in a series of robust metal-organic framework (MOF) materials, MFM-300(M) (M = Fe, V, Cr, In). MFM-300(M) (M = Fe, V^III, Cr) show fully reversible capacity for >20 cycles, reaching capacities of 16.1, 15.6, and 14.0 mmol g^-1, respectively, at 273 K and 1 bar. Under the same conditions, MFM-300(V^IV) exhibits the highest uptake among this series of MOFs of 17.3 mmol g^-1. In situ neutron powder diffraction, single-crystal X-ray diffraction, and electron paramagnetic resonance spectroscopy confirm that the redox-active V center enables host-guest charge transfer, with V^IV being reduced to V^III and NH₃ being oxidized to hydrazine (N₂H₄). A combination of in situ inelastic neutron scattering and DFT modeling has revealed the binding dynamics of adsorbed NH₃ within these MOFs to afford a comprehensive insight into the application of MOF materials to the adsorption and conversion of NH₃.

A "Thermodynamically Stable" 2D Nickel Metal-Organic Framework over a Wide pH Range with Scalable Preparation for Efficient C2 s over C1 Hydrocarbon Separations

The design and construction of "thermodynamically stable" metal-organic frameworks (MOFs) that can survive in liquid water, boiling water, and acidic/basic solutions over a wide pH range is highly desirable for many practical applications, especially adsorption-based gas separations with obvious scalable preparations. Herein, a new thermodynamically stable Ni MOF, {[Ni(L)(1,4-NDC)(H₂ O)₂ ]}_n (IITKGP-20; L=4,4'-azobispyridine; 1,4-NDC=1,4-naphthalene dicarboxylic acid; IITKGP stands for the Indian Institute of Technology Kharagpur), has been designed that displays moderate porosity with a BET surface area of 218 m²  g^-1 and micropores along the [10-1] direction. As an alternative to a cost-intensive, cryogenic, high-pressure distillation process for the separation of hydrocarbons, MOFs have recently shown promise for such separations. Thus, towards an application standpoint, this MOF exhibits a higher uptake of C₂ hydrocarbons over that of C₁ hydrocarbon under ambient conditions, with one of the highest selectivities based on the ideal adsorbed solution theory (IAST) method. A combination of two strategies (the presence of stronger metal-N coordination of the spacer and the hydrophobicity of the aromatic moiety of the organic ligand) possibly makes the framework highly robust, even stable in boiling water and over a wide range of pH 2-10, and represents the first example of a thermodynamically stable MOF displaying a 2D structural network. Moreover, this material is easily scalable by heating the reaction mixture at reflux overnight. Because such separations are performed in the presence of water vapor and acidic gases, there is a great need to explore thermodynamically stable MOFs that retain not only structural integrity, but also the porosity of the frameworks.

Adsorption of Nitrogen Dioxide in a Redox-Active Vanadium Metal-Organic Framework Material

Nitrogen dioxide (NO₂) is a toxic air pollutant, and efficient abatement technologies are important to mitigate the many associated health and environmental problems. Here, we report the reactive adsorption of NO₂ in a redox-active metal-organic framework (MOF), MFM-300(V). Adsorption of NO₂ induces the oxidation of V(III) to V(IV) centers in MFM-300(V), and this is accompanied by the reduction of adsorbed NO₂ to NO and the release of water via deprotonation of the framework hydroxyl groups, as confirmed by synchrotron X-ray diffraction and various experimental techniques. The efficient packing of {NO₂·N₂O₄}_∞ chains in the pores of MFM-300(V^IV) results in a high isothermal NO₂ uptake of 13.0 mmol g^-1 at 298 K and 1.0 bar and is retained for multiple adsorption-desorption cycles. This work will inspire the design of redox-active sorbents that exhibit reductive adsorption of NO₂ for the elimination of air pollutants.

Refinement of pore size at sub-angstrom precision in robust metal-organic frameworks for separation of xylenes

The demand for xylenes is projected to increase over the coming decades. The separation of xylene isomers, particularly p- and m-xylenes, is vital for the production of numerous polymers and materials. However, current state-of-the-art separation is based upon fractional crystallisation at 220 K which is highly energy intensive. Here, we report the discrimination of xylene isomers via refinement of the pore size in a series of porous metal-organic frameworks, MFM-300, at sub-angstrom precision leading to the optimal kinetic separation of all three xylene isomers at room temperature. The exceptional performance of MFM-300 for xylene separation is confirmed by dynamic ternary breakthrough experiments. In-depth structural and vibrational investigations using synchrotron X-ray diffraction and terahertz spectroscopy define the underlying host-guest interactions that give rise to the observed selectivity (p-xylene < o-xylene < m-xylene) and separation factors of 4.6-18 for p- and m-xylenes.

Polymorphous Indium Metal-Organic Frameworks Based on a Ferrocene Linker: Redox Activity, Porosity, and Structural Diversity

The metallocene-based linker molecule 1,1'-ferrocenedicarboxylic acid (H₂FcDC) was used to synthesize four different polymorphs of composition [In(OH)(FeC₁₂H₈O₄)]. Using conventional solvent-based synthesis methods and varying the synthetic parameters such as metal source, reaction temperature, and solvent, two different MOFs and one 1D-coordination polymer denoted as CAU-43 (1), In-MIL-53-FcDC_a (2), and In-FcDC (3) were obtained. Furthermore, thermal treatment of CAU-43 (1) at 190 °C under vacuum yielded a new polymorph of 2, In-MIL-53-FcDC_b (4). Both MOFs 2 and 4 crystallize in a MIL-53 type structure, but in different space groups C2/m for 2 and P1̅ for 4. The structures of the four title compounds were determined by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), or a combination of three-dimensional electron diffraction measurements (3D ED) and PXRD. N₂ sorption experiments of 1, 2, and 4 showed specific surface areas of 355 m² g^-1, 110 m² g^-1, and 140 m² g^-1, respectively. Furthermore, the electronic properties of the title compounds were characterized via Mössbauer and EPR spectroscopy. All Mössbauer spectra showed the characteristic doublet, proving the persistence of the ferrocene moiety. In the cases of 1, 3, and 4, appreciable impurities of ferrocenium ions could be detected by electron paramagnetic resonance spectroscopy. Cyclovoltammetric experiments were performed to demonstrate the accessible redox activity of the linker molecule of the title compounds. A redox process of FcDC^2- with oxidation (between 0.86 and 0.97 V) and reduction wave (between 0.69 and 0.80 V) was observed.

Role of hydrogen bond capacity of solvents in reactions of amines with CO2: A computational study

Various computational methods were employed to investigate the zwitterion formation, a critical step for the reaction of monoethanolamine with CO₂, in five solvents (water, monoethanolamine, propylamine, methanol and chloroform) to probe the effect of hydrogen bond capacity of solvents on the reaction of amine with CO₂ occurring in the amine-based CO₂ capture process. The results indicate that the zwitterion can be formed in all considered solvents except chloroform. For two pairs of solvents (methanol and monoethanolamine, propylamine and chloroform) with similar dielectric constant but different hydrogen bond capacity, the solvents with higher hydrogen bond capacity (monoethanolamine and propylamine) facilitate the zwitterion formation. More importantly, kinetics parameters such as activation free energy for the zwitterion formation are more relevant to the hydrogen bond capacity than to dielectric constant of the considered solvents, clarifying the hydrogen bond capacity could be more important than dielectric constant in determining the kinetics of monoethanolamine with CO₂.

Ethylenediamine loading into a manganese-based metal-organic framework enhances water stability and carbon dioxide uptake of the framework

Metal-organic frameworks (MOFs) based on 2,5-dihydroxyterepthalic acid (DOBDC) as the linker show very high CO2 uptake capacities at low to moderate CO2 pressures; however, these MOFs often require expensive solvent for synthesis and are difficult to regenerate. We have synthesized a Mn-DOBDC MOF and modified it to introduce amine groups into the structure by functionalizing its metal coordination sites with ethylenediamine (EDA). Repeat framework synthesis was then also successfully performed using recycled dimethylformamide (DMF) solvent. Characterization by elemental analysis, FTIR and thermogravimetric studies suggest that EDA molecules are successfully substituting the original metal-bound DMF. This modification not only enhances the material's carbon dioxide sorption capacity, increasing stability to repeated CO2 sorption cycles, but also improves the framework's stability to moisture. Moreover, this is one of the first amine-modified MOFs that can demonstrably be synthesized using recycled solvent, potentially reducing the future costs of production at larger scales.

Alterations to secondary building units of metal–organic frameworks for the development of new functions

Post-synthetic modification methods for the secondary building units in MOFs facilitate unique structures and properties that are impossible to access via direct syntheses, which can be classified as four categories.

Tetrazole-based porous metal–organic frameworks for selective CO2 adsorption and isomerization studies

Tetrazole-based porous MOFs and isomers were synthesized for adsorbing carbon dioxide, showing high selectivity.

An ultrasensitive and selective fluorescent nanosensor based on porphyrinic metal–organic framework nanoparticles for Cu2+ detection

A fluorescent nanosensor based on ultrasmall MOF-525 NPs was proposed for the monitoring of Cu²⁺ in aqueous solution and living cells.

Understanding How Ligand Functionalization Influences CO2 and N2 Adsorption in a Sodalite Metal-Organic Framework

In this work, a detailed study is conducted to understand how ligand substitution influences the CO₂ and N₂ adsorption properties of two highly crystalline sodalite metal-organic frameworks (MOFs) known as Cu-BTT (BTT^-3 = 1,3,5-benzenetristetrazolate) and Cu-BTTri (BTTri^-3 = 1,3,5-benzenetristriazolate). The enthalpy of adsorption and observed adsorption capacities at a given pressure are significantly lower for Cu-BTTri compared to its tetrazole counterpart, Cu-BTT. In situ X-ray and neutron diffraction, which allow visualization of the CO₂ and N₂ binding sites on the internal surface of Cu-BTTri, provide insights into understanding the subtle differences. As expected, slightly elongated distances between the open Cu²⁺ sites and surface-bound CO₂ in Cu-BTTri can be explained by the fact that the triazolate ligand is a better electron donor than the tetrazolate. The more pronounced Jahn-Teller effect in Cu-BTTri leads to weaker guest binding. The results of the aforementioned structural analysis were complemented by the prediction of the binding energies at each CO₂ and N₂ adsorption site by density functional theory calculations. In addition, variable temperature in situ diffraction measurements shed light on the fine structural changes of the framework and CO₂ occupancies at different adsorption sites as a function of temperature. Finally, simulated breakthrough curves obtained for both sodalite MOFs demonstrate the materials' potential performance in dry postcombustion CO₂ capture. The simulation, which considers both framework uptake capacity and selectivity, predicts better separation performance for Cu-BTT. The information obtained in this work highlights how ligand substitution can influence adsorption properties and hence provides further insights into the material optimization for important separations.

Three metal–organic framework isomers of different pore sizes for selective CO2 adsorption and isomerization studies

Three MOF isomers including framework-catenation and framework-topological isomers were synthesized for adsorbing carbon dioxide with high selectivity.

Iodine Adsorption in a Redox-Active Metal-Organic Framework: Electrical Conductivity Induced by Host-Guest Charge-Transfer

We report a comparative study of the binding of I2 (iodine) in a pair of redox-active metal-organic framework (MOF) materials, MFM-300(VIII) and its oxidized, deprotonated analogue, MFM-300(VIV). Adsorption of I2 in MFM-300(VIII) triggers a host-to-guest charge-transfer, accompanied by a partial (∼30%) oxidation of the VIII centers in the host framework and formation of I3- species residing in the MOF channels. Importantly, this charge-transfer induces a significant enhancement in the electrical conductivity (Δσ = 700000) of I2@MFM-300(VIII/IV) in comparison to MFM-300(VIII). In contrast, no host-guest charge-transfer or apparent change in the conductivity was observed upon adsorption of I2 in MFM-300(VIV). High-resolution synchrotron X-ray diffraction of I2@MFM-300(VIII/IV) confirms the first example of self-aggregation of adsorbed iodine species (I2 and I3-) into infinite helical chains within a MOF.

Porous metal-organic frameworks for gas storage and separation: Status and challenges

Gases are widely used as energy resources for industry and our daily life. Developing energy cost efficient porous materials for gas storage and separation is of fundamentally and industrially important, and is one of the most important aspects of energy chemistry and materials. Metal-organic frameworks (MOFs), representing a novel class of porous materials, feature unique pore structure, such as exceptional porosity, tunable pore structures, ready functionalization, which not only enables high density energy storage of clean fuel gas in MOF adsorbents, but also facilitates distinct host-guest interactions and/or sieving effects to differentiate different molecules for energy-efficient separation economy. In this review, we summarize and highlight the recent advances in the arena of gas storage and separation using MOFs as adsorbents, including progresses in MOF-based membranes for gas separation, which could afford broader concepts to the current status and challenges in this field.

Structural dynamics of a metal-organic framework induced by CO2 migration in its non-uniform porous structure

Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO₂-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO₂ migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.

Metal–organic frameworks that undergo structural transitions in response to external stimuli are promising for gas storage, but the mechanisms of such dynamics are poorly understood. Here the authors show that the structural transformation of ZIF-7 is induced by CO₂ migration through its non-uniform porous structure.

State-of-the-Art Advances and Challenges of Iron-Based Metal Organic Frameworks from Attractive Features, Synthesis to Multifunctional Applications

Metal organic frameworks (MOFs), as an original kind of organic-inorganic porous material, are constructed with metal centers and organic linkers via a coordination complexation reaction. Among uncountable MOF materials, iron-containing metal organic frameworks (Fe-MOFs) have excellent potential in practical applications owing to their many fascinating properties, such as diverse structure types, low toxicity, preferable stability, and tailored functionality. Here, recent research progresses of Fe-MOFs in attractive features, synthesis, and multifunctional applications are described. Fe-MOFs with porosity and tailored functionality are discussed according to the design of building blocks. Four types of synthetic methods including solvothermal, hydrothermal, microwave, and dry gel conversion synthesis are illustrated. Finally, the applications of Fe-MOFs in Li-ion batteries, sensors, gas storage, separation in gas and liquid phases, and catalysis are elucidated, focusing on the mechanism. The aim is to provide prospects for extending Fe-MOFs in more practical applications.

Comparing Geometry and Chemistry When Confined Molecules Diffuse in Monodisperse Metal-Organic Framework Pores

The monodisperse pore structure of MOFs (metal-organic frameworks) is advantageous for investigating how porosity influences diffusion. Here we report translational and rotational diffusion using fluorescence correlation spectroscopy and time-correlated single-photon counting, using the three-dimensional pores of the zeolitic-like metal-organic framework family. We compare the influence of size and electric charge as well as dependence on pore size that we controlled through postsynthetic cation-exchange modifications. Charge-charge interactions with the MOF appeared to produce transient adsorption, manifested as a relatively fast and a slower diffusion process, but diffusants without net electric charge displayed a single diffusion process. Obtained from this family of guest molecules selected to be fluorescent, these findings suggest potentially useful general design rules to predict how pore size, guest size, and host-guest interaction control guest mobility within nanopores. With striking fidelity, diffusion coefficient scales with the ratio of cross-sectional areas of diffusant and host pores when charge is taken into account.

Post-synthetic modulation of the charge distribution in a metal-organic framework for optimal binding of carbon dioxide and sulfur dioxide

Modulation of pore environment is an effective strategy to optimize guest binding in porous materials. We report the post-synthetic modification of the charge distribution in a charged metal-organic framework, MFM-305-CH₃, [Al(OH)(L)]Cl, [(H₂L)Cl = 3,5-dicarboxy-1-methylpyridinium chloride] and its effect on guest binding. MFM-305-CH₃ shows a distribution of cationic (methylpyridinium) and anionic (chloride) centers and can be modified to release free pyridyl N-centres by thermal demethylation of the 1-methylpyridinium moiety to give the neutral isostructural MFM-305. This leads simultaneously to enhanced adsorption capacities and selectivities (two parameters that often change in opposite directions) for CO₂ and SO₂ in MFM-305. The host-guest binding has been comprehensively investigated by in situ synchrotron X-ray and neutron powder diffraction, inelastic neutron scattering, synchrotron infrared and ²H NMR spectroscopy and theoretical modelling to reveal the binding domains of CO₂ and SO₂ in these materials. CO₂ and SO₂ binding in MFM-305-CH₃ is shown to occur via hydrogen bonding to the methyl and aromatic-CH groups, with a long range interaction to chloride for CO₂. In MFM-305 the hydroxyl, pyridyl and aromatic C-H groups bind CO₂ and SO₂ more effectively via hydrogen bonds and dipole interactions. Post-synthetic modification via dealkylation of the as-synthesised metal-organic framework is a powerful route to the synthesis of materials incorporating active polar groups that cannot be prepared directly.

Porous Zr6L3 Metallocage with Synergetic Binding Centers for CO2

Coordination-driven assembly has been widely successful in the synthesis of metallocages and used for many applications, such as catalysis. However, studies on CO₂ adsorption with metallocages have been rarely conducted, compared to other well-known cage-type materials, such as porous organic cages. In this study, a rational choice of ligand and metal led to the synthesis of a Zr₆L₃-type metallocage, exhibiting exceptional CO₂ adsorption properties. CO₂ adsorption experiments revealed that the metallocage shows highly selective adsorption of CO₂ over N₂ with high CO₂ binding energy. Density functional theory calculations uncovered the origin of this exceptional affinity for CO₂ over N₂.

Direct observation of supramolecular binding of light hydrocarbons in vanadium(iii) and (iv) metal-organic framework materials

Binding of C₂H₂ in MFM-300(V^III) showing interactions to O–H, carboxylate O-centres and intermolecular packing.

Fine tuning of host–guest supramolecular interactions in porous systems enables direct control over the properties of functional materials. We report here a modification of hydrogen bonding and its effect on guest binding in a pair of redox-active metal–organic frameworks (MOFs). Oxidation of MFM-300(V^III) {[VIII2(OH)₂(L)], LH₄ = biphenyl-3,3′,5,5′-tetracarboxylic acid} is accompanied by deprotonation of the bridging hydroxyl groups to afford isostructural MFM-300(V^IV), [VIV2O₂(L)]. The precise role of the hydroxyl groups, O-carboxylate centres and π–π interactions in the supramolecular binding of C₂ hydrocarbons in these materials has been determined using neutron diffraction and inelastic neutron scattering, coupled with DFT modelling. The hydroxyl protons are observed to bind to adsorbed unsaturated hydrocarbons preferentially in MFM-300(V^III), particularly to C₂H₂, which is in a sharp contrast to MFM-300(V^IV) where interactions with O-carboxylate centres and π–π interactions predominate. This variation in structure and redox leads to notably higher separation selectivity for C₂H₂/CH₄ and C₂H₄/CH₄ in MFM-300(V^III) than in MFM-300(V^IV). Significantly, owing to the specific host–guest interactions, MFM-300(V^III) shows a record packing density for adsorbed C₂H₂ at 303 K and 1 bar, demonstrating its potential for use in portable acetylene stores.

Multifunctional Metal-Organic Frameworks Based on Redox-Active Rhenium Octahedral Clusters

The redox-active rhenium octahedral cluster unit [Re₆Se₈(CN)₆]^4- was combined with Gd³⁺ ions and dicarboxylate linkers in novel types of metal-organic frameworks (MOFs) that display a set of functional properties. The hydrolytically stable complexes [{Gd(H₂O)₃}₂(L)Re₆Se₈(CN)₆]·nH₂O (1, L = furan-2,5-dicarboxylate, fdc; 2, L = thiophene-2,5-dicarboxylate, tdc) exhibit a 3D framework of trigonal symmetry where 1D chains of [{Gd(H₂O)₃}₂(L)]⁴⁺ are connected by [Re₆Se₈(CN)₆]^4- clusters. Frameworks contain spacious channels filled with H₂O. Solvent molecules can be easily removed under vacuum to produce permanently porous solids with high volumetric CO₂ uptake and remarkable CO₂/N₂ selectivity at room temperature. The frameworks demonstrate an ability for reversible redox transformations of the cluster fragment. The orange powders of compounds 1 and 2 react with Br₂, yielding dark-green powders of [{Gd(H₂O)₃}₂(L)Re₆Se₈(CN)₆]Br·nH₂O (3, L = fdc; 4, L = tdc). Compounds 3 and 4 are isostructural with 1 and 2 and also have permanently porous frameworks but display different optical, magnetic, and sorption properties. In particular, oxidation of the cluster fragment "switches off" its luminescence in the red region, and the incorporation of Br^- leads to a decrease of the solvent-accessible volume in the channels of 3 and 4. Finally, the green powders of 3 and 4 can be reduced back to the orange powders of 1 and 2 by reaction with hydrazine, thus displaying a rare ability for fully reversible chemical redox transitions. Compounds 1-4 are mentioned as a new class of redox-active cluster-based MOFs with potential usage as multifunctional materials for gas separation and chemical contamination sensors.

Microporous mixed-metal mixed-ligand metal organic framework for selective CO2 capture

Layered-pillared framework of the CO₂-loaded MOF developed using this mixed-metal mixed-ligand approach showing the multiple-adsorption sites within the MOF.

Potential of ultramicroporous metal–organic frameworks in CO2 clean-up

This article explains the need for energy-efficient large-scale CO₂ capture and briefly mentions the requirements for optimal solid sorbents for this application.