Carbon Dioxide Sensitivity of Zeolitic Imidazolate Frameworks

Exploitation of Pore Structure for Increased CO2 Selectivity in Type 3 Porous Liquids

CO2 capture requires materials with high adsorption selectivity and an industrial ease of implementation. To address these needs, a new class of porous materials was recently developed that combines the fluidity of solvents with the porosity of solids. Type 3 porous liquids (PLs) composed of solvents and metal-organic frameworks (MOFs) offer a promising alternative to current liquid carbon capture methods due to the inherent tunability of the nanoporous MOFs. However, the effects of MOF structural features and solvent properties on CO2-MOF interactions within PLs are not well understood. Herein experimental and computational data of CO2 gas adsorption isotherms were used to elucidate both solvent and pore structure influences on ZIF-based PLs. The roles of the pore structure including solvent size exclusion, structural environment, and MOF porosity on PL CO2 uptake were examined. A comparison of the pore structure and pore aperture was performed using ZIF-8, ZIF-L, and amorphous-ZIF-8. Adsorption experiments here have verified our previously proposed solvent size design principle for ZIF-based PLs (1.8× ZIF pore aperture). Furthermore, the CO2 adsorption isotherms of the ZIF-based PLs indicated that judicious selection of the pore environment allows for an increase in CO2 selectivity greater than expected from the individual PL components or their combination. This nonlinear increase in the CO2 selectivity is an emergent behavior resulting from the complex mixture of components specific to the ZIF-L + 2'-hydroxyacetophenone-based PL.

Adsorptive removal of heavy metals from aqueous solutions: Progress of adsorbents development and their effectiveness

Increased levels of heavy metals (HMs) in aquatic environments poses serious health and ecological concerns. Hence, several approaches have been proposed to eliminate/reduce the levels of HMs before the discharge/reuse of HMs-contaminated waters. Adsorption is one of the most attractive processes for water decontamination; however, the efficiency of this process greatly depends on the choice of adsorbent. Therefore, the key aim of this article is to review the progress in the development and application of different classes of conventional and emerging adsorbents for the abatement of HMs from contaminated waters. Adsorbents that are based on activated carbon, natural materials, microbial, clay minerals, layered double hydroxides (LDHs), nano-zerovalent iron (nZVI), graphene, carbon nanotubes (CNTs), metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) are critically reviewed, with more emphasis on the last four adsorbents and their nanocomposites since they have the potential to significantly boost the HMs removal efficiency from contaminated waters. Furthermore, the optimal process conditions to achieve efficient performance are discussed. Additionally, adsorption isotherm, kinetics, thermodynamics, mechanisms, and effects of varying adsorption process parameters have been introduced. Moreover, heavy metal removal driven by other processes such as oxidation, reduction, and precipitation that might concurrently occur in parallel with adsorption have been reviewed. The application of adsorption for the treatment of real wastewater has been also reviewed. Finally, challenges, limitations and potential areas for improvements in the adsorptive removal of HMs from contaminated waters are identified and discussed. Thus, this article serves as a comprehensive reference for the recent developments in the field of adsorptive removal of heavy metals from wastewater. The proposed future research work at the end of this review could help in addressing some of the key limitations facing this technology, and create a platform for boosting the efficiency of the adsorptive removal of heavy metals.

Shaking Things from the Ground-Up: A Systematic Overview of the Mechanochemistry of Hard and High-Melting Inorganic Materials

We provide a systematic overview of the mechanochemical reactions of inorganic solids, notably simple binary compounds, such as oxides, nitrides, carbides, sulphides, phosphides, hydrides, borides, borane derivatives, and related systems. Whereas the solid state has been traditionally considered to be of little synthetic value by the broader community of synthetic chemists, the solid-state community, and in particular researchers focusing on the reactions of inorganic materials, have thrived in building a rich and dynamic research field based on mechanically-driven transformations of inorganic substances typically seen as inert and high-melting. This review provides an insight into the chemical richness of such mechanochemical reactions and, at the same time, offers their tentative categorisation based on transformation type, resulting in seven distinct groupings: (i) the formation of adducts, (ii) the reactions of dehydration; (iii) oxidation-reduction (redox) reactions; (iv) metathesis (or exchange) reactions; (v) doping and structural rearrangements, including reactions involving the reaction vessel (the milling jar); (vi) acid-base reactions, and (vii) other, mixed type reactions. At the same time, we offer a parallel description of inorganic mechanochemical reactions depending on the reaction conditions, as those that: (i) take place under mild conditions (e.g., manual grinding using a mortar and a pestle); (ii) proceed gradually under mechanical milling; (iii) are self-sustained and initiated by mechanical milling, i.e., mechanically induced self-propagating reactions (MSRs); and (iv) proceed only via harsh grinding and are a result of chemical reactivity under strongly non-equilibrium conditions. By elaborating on typical examples and general principles in the mechanochemistry of hard and high-melting substances, this review provides a suitable complement to the existing literature, focusing on the properties and mechanochemical reactions of inorganic solids, such as nanomaterials and catalysts.

The Effect of Surface Composition on the Selective Capture of Atmospheric CO2 by ZIF Nanoparticles: The Case of ZIF-8

We performed theoretical studies of CO₂ capture in atmospheric conditions by the zeolitic imidazolate framework-8 (ZIF-8) via classical Monte Carlo (MC) simulations with Metropolis sampling and classical molecular dynamics (MD) simulations in the NVT and NPT ensembles and different thermodynamic conditions. The ZIF-8 framework was described by varying unit cell dimensions in the presence of pure gases of CO₂, N₂, O₂, Ar, and H₂O steam as well as binary mixtures of CO₂:N₂ and CO₂:H₂O in s 1:1 concentration. Different chemical compositions of the framework surface was considered to provide an accurate treatment of charge and charge distribution in the nanoparticle. Hence, surface groups were represented as unsaturated zinc atom (Zn⁺²), 2-methylimidazole (mImH), and deprotonated 2-methylimidazole (mIm^-). Force field reparameterization of the surface sites was required to reproduce the interactions of the gas molecules with the ZIF-8 surface consistent with quantum mechanics (QM) calculations and Born-Oppenheimer molecular dynamics (BOMD). It was observed that ZIF-8 selectively captures CO₂ due to the negligible concentrations of N₂, O₂, Ar, and H₂O. These molecules spontaneously migrate to the inner pores of the framework. At the surface, there is a competitive interaction between H₂O, CO₂, and N₂, for the positively charged ZIF-8 nanoparticle with a large binding energy advantage for water molecules (on average -62, -15, and -8 kcal/mol respectively). For the neutral ZIF-8 nanoparticle, the water molecules dominate the interactions due to the occurrence of hydrogen bond with the imidazolate groups at the surface. Simulations of binary mixtures of CO₂/water steam and CO₂/N₂ were performed to investigate binding competition between these molecules for the framework positively charged and neutral surfaces. It was found that water molecules drastically block the interaction between CO₂ molecules and the framework surface, decreasing CO₂ capture in the central pore, and CO₂ molecules fully block the interaction between N₂ molecules and the framework. These findings show that CO₂ capture by ZIF-8 is possible in atmospheric environments only upon dehydration of the atmospheric gas. It further shows that ZIF-8 capture of CO₂ from the atmospheric environment is dependent on thermodynamic conditions and can be increased by decreasing temperature and/or increasing pressure.

Large Cages of Zeolitic Imidazolate Frameworks

The design and synthesis of permanently porous materials with extended cage structures is a long-standing challenge in chemistry. In this Account, we highlight the unique role of zeolitic imidazolate frameworks (ZIFs), a class of framework materials built from tetrahedral nodes connected through imidazolate linkers, in meeting this challenge and illustrate specific features that set ZIFs apart from other porous materials. The structures of ZIFs are characteristic of a variety of large, zeolite-like cages that are covalently connected with neighboring cages and fused in three-dimensional space. In contrast to molecular cages, the fusion of cages results in extraordinary architectural and chemical stability for the passage of gases and molecules through cages and for carrying out chemical reactions within these cages while keeping the cages intact. The combination of the advantages from both cage chemistry and extended structures allows uniquely interconnected yet compartmentalized void spaces inside ZIF solids, rendering their wide range of applications in catalysis, gas storage, and gas separation.While the field of ZIFs has seen rapid development over the past decade, with hundreds of ZIF structures built from dozens of different cages of varying composition, size, and shapes reported, rational approaches to their design are largely unknown. In this Account, we summarize a vast number of cages formed in reported ZIFs and then review how the thermodynamic factors and traditional guest-templating strategies from zeolites influence the formation of cages. We highlight how the link-link interactions perform in the ZIF formation mechanism and serve as a means to target the formation of frameworks containing cages of specific sizes with structures exhibiting a level of complexity as yet unachieved in discrete coordination cages. For example, the giant ucb cage features a dimension of 46 Å and the complex moz cage is constructed from as many as 660 components.With the finding of these large and complex cages in ZIFs, we envision that the collection of cage structures will further be diversified by a mixed-linker approach utilizing a more complex combination of link-link interactions or by creating multivariant (MTV) systems that have been realized in other framework materials yet not widely employed in ZIFs. The more complicated cage structures can provide extra variations in chemical environments, and in addition to that, MTV systems can generate inhomogeneity inside each type of cage structure. The fused cages at such complexity that are difficult to be realized in solution environments will potentially enable more complex materials for smart applications.

Integration of a Metal-Organic Framework Film with a Tubular Whispering-Gallery-Mode Microcavity for Effective CO2 Sensing

Carbon dioxide (CO₂) sensing using an optical technique is of great importance in the environment and industrial emission monitoring. However, limited by the poor specific adsorption of gas molecules as well as insufficient coupling efficiency, there is still a long way to go toward realizing a highly sensitive optical CO₂ gas sensor. Herein, by combining the advantages of a whispering-gallery-mode microcavity and a metal-organic framework (MOF) film, a porous functional microcavity (PF-MC) was fabricated with the assistance of the atomic layer deposition technique and was applied to CO₂ sensing. In this functional composite, the rolled-up microcavity provides the ability to tune the propagation of light waves and the electromagnetic coupling with the surroundings via an evanescent field, while the nanoporous MOF film contributes to the specific adsorption of CO₂. The composite demonstrates a high sensitivity of 188 nm RIU^-1 (7.4 pm/% with respect to the CO₂ concentration) and a low detection limit of ∼5.85 × 10^-5 RIU. Furthermore, the PF-MC exhibits great selectivity to CO₂ and outstanding reproducibility, which is promising for the next-generation optical gas sensing devices.

Origins of Acid-Gas Stability Behavior in Zeolitic Imidazolate Frameworks: The Unique High Stability of ZIF-71

Zeolitic imidazolate frameworks (ZIFs) are promising materials for industrial process separations, but recent literature reports have highlighted their vulnerability to acid gases (e.g., SO₂, CO₂, NO₂, H₂S), often present in practical applications. While previous work has documented the widely varying stability behavior of many ZIFs under varying (humid and dry) acid gas environments, efforts to explain or correlate these experimental observations via empirical descriptors have not succeeded. A key observation is that ZIF-71 (RHO topology) is an extraordinarily stable ZIF material, retaining both structure and porosity under prolonged humid SO₂ exposure whereas many other well-known ZIFs with different linkers and topologies (such as ZIF-8) were shown to degrade. Through a combination of hybrid quantum mechanics/molecular mechanics (QM/MM) based methods and statistical mechanical models, we successfully explain this important experimental observation via atomistic investigations of the reaction mechanism. Our holistic approach reveals an ∼9 times lower average defect formation rate in ZIF-71 RHO compared to ZIF-8 SOD, leading to the conclusion that the observed experimental stability of this material rises from kinetic effects. Moreover, our analysis reveals that differing stability of the two materials is determined by the distributions of acid gas molecules, which is difficult to capture using empirical descriptors. Our results suggest wider applicability of the present approach, toward identifying tuned functional groups and topologies that move the acid gas distributions away from more reactive sites and thus allow enhanced kinetic stability.

Carbon supports on preparing iron-nitrogen dual-doped carbon (Fe-N/C) electrocatalysts for microbial fuel cells: mini-review

Microbial fuel cells (MFCs) are devices that treat sewage and generate electricity. Recent researches have demonstrated that the characteristics of carbon precursors can tremendously influence the performance of the MFC cathode. Carbon nanomaterials with good crystallinity as well as high specific surface area (e.x., graphene and carbon nanotube) can not only accelerate charge transport but also afford a good dispersion of catalytic active components, leading to high MFC performance. On these bases, the preparation of highly-active Fe-N/C catalysts using different carbon substrates are mainly discussed in this review. It is pointed out that increasing the surface area and conductivity as well as elevating the density of active sites to reduce the oxygen reduction overpotential is still the emphasis of the current works. At present, although the researchers have made some progress, the output power density is far from meeting the actual application needs.

Understanding the opportunities of metal–organic frameworks (MOFs) for CO2 capture and gas-phase CO2 conversion processes: a comprehensive overview

The rapid increase in the concentration of atmospheric carbon dioxide is one of the most pressing problems facing our planet.

Synthesis of ZIF-8 Nanocrystals Mediated by CO2 Gas Bubbling: Dissolution and Recrystallization

Crystal size and morphology of zeolitic imidazolate frameworks (ZIFs) can be generally controlled based on the classical theory of nucleation and growth. Herein, we have developed an alternative method to adjust the nucleation and growth kinetics of microporous ZIF-8 nanocrystals mediated by continuous CO₂ gas bubbling. In particular, CO₂ bubbling led to the dissolution of ZIF-8 slurry, while the evacuation of CO₂ bubbling resulted in the formation of new ZIF-8 nanoparticles with a considerably smaller size. A plausible mechanism of the CO₂-mediated synthesis of ZIF-8 nanoparticles was proposed based on comprehensive characterizations and analyses, which indicated that the dissolved CO₂ in methanol was able to perturb the pre-equilibrium states of crystallization intermediates and led to a comparatively fast nucleation rate due to a low number of overcoordinated species between the metal ion and the ligand. Both methanol and the base were critically important to the dissolution-recrystallization of ZIF-8, wherein the methyl carbonate linker might be reversibly produced by CO₂ insertion into the methoxide group (Zn-OCH₃). Also, the CO₂-mediated synthesis led to the small particle size, high crystallinity, good thermal stability, and high purity of ZIF-8, as compared to the conventional ZIF-8 prepared without CO₂ gas bubbling. As proof of workability, the prepared monodispersed ZIF-8 nanoparticles showed a much higher photocatalytic activity toward various organic dyes' decomposition than the conventional ZIF-8. Also, the CO₂ bubbling-mediated method could be further extended to prepare other ZIFs (e.g., ZIF-67).

Kinetics and Mechanisms of ZnO to ZIF-8 Transformations in Supercritical CO2 Revealed by In Situ X-ray Diffraction

ZIF-8 was synthesized in supercritical carbon dioxide (scCO₂ ). In situ powder X-ray diffraction, ex situ microscopy, and simulations provide an encompassing view of the formation of ZIF-8 and intermediary ZnO@ZIF-8 composites in this nontraditional solvent. Time-resolved imaging exposed divergent physicochemical reaction pathways from previous studies of the growth of anisotropic ZIF-8 core@shell structures in traditional solvents. Synthetically relevant physiochemical properties of scCO₂ were integrated into classical nucleation theory, relating interfacial forces, calculated through DFTB+ based molecular dynamics (MD), with 3D nucleation outcomes. The kinetics of crystallization were examined and displayed a characteristic signature of time- and temperature-dependent mechanisms over the extent of the reaction. Lastly, it is shown that subtle factors, such as the extent of reaction and the size/shape of sacrificial templates can tailor ZIF-8 composition and size, eliciting control over hierarchical porosity in a nonconventional green solvent.

Thermodynamic Evidence of Structural Transformations in CO2-Loaded Metal-Organic Framework Zn(MeIm)2 from Heat Capacity Measurements

Metal-organic frameworks are a class of porous compounds with potential applications in molecular sieving, gas sequestration, and catalysis. One family of MOFs, zeolitic imidizolate frameworks (ZIFs), is of particular interest for carbon dioxide sequestration. We have previously reported the heat capacity of the sodalite topology of the zinc 2-methylimidazolate framework (ZIF-8), and in this Article we present the first low-temperature heat capacity measurements of ZIF-8 with various amounts of sorbed CO₂. Molar heat capacities from 1.8 to 300 K are presented for samples containing up to 0.99 mol of CO₂ per mol of ZIF-8. Samples with at least 0.56 mol of CO₂ per mol of ZIF-8 display a large, broad anomaly from 70 to 220 K with a shoulder on the low-temperature side, suggesting sorption-induced structural transitions. We attribute the broad anomaly partially to a gate-opening transition, with the remainder resulting from CO₂ rearrangement and/or lattice expansion. The measurements also reveal a subtle anomaly from 0 to 70 K in all samples that does not exist in the sorbate-free material, which likely reflects new vibrational modes resulting from sorbate/ZIF-8 interactions. These results provide the first thermodynamic evidence of structural transitions induced by CO₂ sorption in the ZIF-8 framework.

Hydrophobic Metal-Organic Frameworks: Assessment, Construction, and Diverse Applications

Tens of thousands of metal-organic frameworks (MOFs) have been developed in the past two decades, and only ≈100 of them have been demonstrated as porous and hydrophobic. These hydrophobic MOFs feature not only a rich structural variety, highly crystalline frameworks, and uniform micropores, but also a low affinity toward water and superior hydrolytic stability, which make them promising adsorbents for diverse applications, including humid CO2 capture, alcohol/water separation, pollutant removal from air or water, substrate-selective catalysis, energy storage, anticorrosion, and self-cleaning. Herein, the recent research advancements in hydrophobic MOFs are presented. The existing techniques for qualitatively or quantitatively assessing the hydrophobicity of MOFs are first introduced. The reported experimental methods for the preparation of hydrophobic MOFs are then categorized. The concept that hydrophobic MOFs normally synthesized from predesigned organic ligands can also be prepared by the postsynthetic modification of the internal pore surface and/or external crystal surface of hydrophilic or less hydrophobic MOFs is highlighted. Finally, an overview of the recent studies on hydrophobic MOFs for various applications is provided and suggests the high versatility of this unique class of materials for practical use as either adsorbents or nanomaterials.

Manometric real-time studies of the mechanochemical synthesis of zeolitic imidazolate frameworks

We demonstrate a simple method for real-time monitoring of mechanochemical synthesis of metal–organic frameworks, by measuring changes in pressure of gas produced in the reaction. Using this manometric method to monitor the mechanosynthesis of the zeolitic imidazolate framework ZIF-8 from basic zinc carbonate reveals an intriguing feedback mechanism in which the initially formed ZIF-8 reacts with the CO₂ byproduct to produce a complex metal carbonate phase, the structure of which is determined directly from powder X-ray diffraction data. We also show that the formation of the carbonate phase may be prevented by addition of excess ligand. The excess ligand can subsequently be removed by sublimation, and reused. This enables not only the synthesis but also the purification, as well as the activation of the MOF to be performed entirely without solvent.

We demonstrate a simple method for real-time monitoring of mechanochemical synthesis of metal–organic frameworks, by measuring changes in pressure of gas produced in the reaction.

Diversity-Oriented Metal Decoration on UiO-Type Metal-Organic Frameworks: an Efficient Approach to Increase CO2 Uptake and Catalytic Conversion to Cyclic Carbonates

A library of metallo-bipyridine UiO-type metal-organic frameworks (MOFs) has been successfully synthesized by postmetallation of a wide range of metal complexes into bidentate bipyridine moieties. Then, a systematic investigation is devoted to a catalytic evaluation of the resultant MOFs containing a binary Lewis acid function for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The result indicated that the metal-grafted MOFs exhibit improvement in terms of CO2 uptake capacity and catalytic activity in comparison with their nonmetallated counterparts. The comprehensive investigation provides a valuable insight into the synergetic effects of MOF functionalities including metal node, grafted metal, and its counterion in the cycloaddition reaction. Furthermore, the metal coordination modulation due to its benefits such as being a solvent-free process, nearly full conversion to cyclic carbonates, high selectivity and high CO2 uptake, applying atmospheric CO2 pressure, and excellent stability and easy recyclability of the catalyst demonstrates them as promising candidates for practical utilization of CO2 conversion into value-added chemicals.

Heterogenization of Photochemical Molecular Devices: Embedding a Metal-Organic Cage into a ZIF-8-Derived Matrix To Promote Proton and Electron Transfer

Application of a molecular catalyst in artificial photosynthesis is confronted with challenges such as rapid deactivation due to photodegradation or detrimental aggregation in harsh conditions. In this work, a metal-organic cage [Pd₆(RuL₃)₈]²⁸⁺ (MOC-16), characteristic of a photochemical molecular device (PMD) concurrently integrating eight Ru²⁺ light-harvesting centers and six Pd²⁺ catalytic centers for efficient homogeneous H₂ production, is successfully heterogenized through incorporation into a metal-organic framework (MOF) of ZIF-8 and then transformed into a carbonate matrix of Zn_x(MeIm)_x(CO₃)_x (CZIF), leading to hybridized MOC-16@CZIF. This MOC@MOF integrated photocatalyst inherits a highly efficient and directional electron transfer in the picosecond domain of MOC-16 and possesses one order increased microsecond magnitude of the triplet excited-state electron in comparison to that of the primitive MOC-16. The carbonate CZIF matrix endows MOC-16@CZIF with water wettability, serving as a proton relay to facilitate proton delivery by virtue of H₂O as proton carriers. Electron transfer during the photocatalytic process is also enhanced by infiltration of a sacrificial agent of BIH into the CZIF matrix to promote conductivity, owing to its strong reducing ability to induce free charge carriers. These synergistic effects contribute to the extra high activity for H₂ generation, making the turnover frequency of this heterogeneous MOC-16@CZIF photocatalyst maintain a level of ∼0.4 H₂·s^-1, increased by 50-fold over that of a homogeneous PMD. Meanwhile, it is robust enough to tolerate harsh reaction conditions, presenting an unprecedented heterogenization example of homogeneous PMD with a MOF-derived matrix to mimic catalytic features of a natural photosystem, which may shed light on the design of multifunctional PMD@MOF materials to expand the number of molecular catalysts for practical application in artificial photosynthesis.

Torsion Angle Effect on the Activation of UiO Metal-Organic Frameworks

We present a systematic investigation of the factors influencing the surface area of zirconium-based UiO-type metal-organic frameworks (MOFs), revealing an important relationship between factors including the conformation of the organic linker in the MOF, surface tension of the guest molecules (solvent), and the stability of MOFs toward activation (removal of guest molecules). The results obtained demonstrate how the structure of the linkers forming the isostructural series of UiO MOFs with fcu topology could alter the resistance and stability of the MOF frameworks toward capillary force-driven structural degradation governed by the solvent during activation.

Controlled Manipulation of Metal-Organic Framework Layers to Nanometer Precision Inside Large Mesochannels of Ordered Mesoporous Silica for Enhanced Removal of Bisphenol A from Water

Considerable attention has been paid on the design of hierarchical porous metal-organic framework (MOF) composites, which not only enhances the performance but also broadens the applications of MOFs. So far, controlled manipulation of nanometer-thick MOF layers in ordered mesochannels, while retaining their respective intrinsic properties, is still a main challenge because of the difficulty of growing MOFs in confined space. Herein, using a step-by-step coordination method, the formation of a hierarchical micro-mesoporous hybrid with a wall (channel wall and coating layer) thickness of up to 8.0 nm and open pore size down to 7.7 nm has been achieved based on large mesoporous SBA-15, and the wall thickness with nanometer precision can be controlled by adjusting the growth cycles of zeolite imidazolate framework-8 (ZIF-8) coating layers. Compared to pure ZIF-8, the obtained ZIF-8@SBA-15 composites showed more than 2-fold enhancement in adsorption capacity and approximately 20-fold improvement in the adsorption rate constant for bisphenol A in water, which could be ascribed to the synergistic effects of the high adsorption ability from ZIF-8 and the fast diffusion property from SBA-15. More importantly, the degraded ZIF-8@SBA-15 composite can be completely restored by a simple immersion into 2-methylimidazole solution. The easy restorability and good reusability further enable ZIF-8@SBA-15 as a promising adsorbent for effectively removing organic contaminants from water.

Loading across the Periodic Table: Introducing 14 Different Metal Ions To Enhance Metal-Organic Framework Performance

Loading metal guests within metal-organic frameworks (MOFs) via secondary functional groups is a promising route for introducing or enhancing MOF performance in various applications. In this work, 14 metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Co²⁺, Mn²⁺, Ag⁺, Cd²⁺, La³⁺, In³⁺, and Pb²⁺) have been successfully introduced within the MIL-121 MOF using a cost-efficient route involving free carboxylic groups on the linker. The local and long-range structure of the metal-loaded MOFs is characterized using multinuclear solid-state NMR and X-ray diffraction methods. Li/Mg/Ca-loaded MIL-121 and Ag nanoparticle-loaded MIL-121 exhibit enhanced H₂ and CO₂ adsorption; Ag nanoparticle-loaded MIL-121 also demonstrates remarkable catalytic activity in the reduction of 4-nitrophenol.

Hierarchically structured metal–organic frameworks assembled by hydroxy double salt–template synergy with high space–time yields

A simple, green, and versatile method to rapidly synthesize hierarchically structured metal–organic frameworks at room temperature using a cooperative template strategy.

Carbon dioxide capture and conversion by an acid-base resistant metal-organic framework

Considering the rapid increase of CO₂ emission, especially from power plants, there is a constant need for materials which can effectively eliminate post-combustion CO₂ (the main component: CO₂/N₂ = 15/85). Here, we show the design and synthesis of a Cu(II) metal-organic framework (FJI-H14) with a high density of active sites, which displays unusual acid and base stability and high volumetric uptake (171 cm³ cm⁻³) of CO₂ under ambient conditions (298 K, 1 atm), making it a potential adsorbing agent for post-combustion CO₂. Moreover, CO₂ from simulated post-combustion flue gas can be smoothly converted into corresponding cyclic carbonates by the FJI-H14 catalyst. Such high CO₂ adsorption capacity and moderate catalytic activity may result from the synergistic effect of multiple active sites.

Increasing CO₂ emissions pose serious environmental issues. Here, the authors report the synthesis of a robust metal-organic framework which displays high volumetric uptake of post-combustion CO₂ under ambient conditions and can catalyze CO₂ fixation into cyclic carbonates.

Converting Metal-Organic Framework Particles from Hydrophilic to Hydrophobic by an Interfacial Assembling Route

Here, we propose to modify the hydrophilicity of metal-organic framework (MOF) particles by an interfacial assembling route, which is based on the surface-active nature of MOF particles. It was found that hydrophilic UiO-66-NH₂ particles can be converted to hydrophobic particles through an oil-water interfacial assembling route. The underlying mechanism for the conversion of UiO-66-NH₂ was investigated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was revealed that the close assembly of UiO-66-NH₂ particles at the oil-water interface strengthens the coordination between organic ligands and metal ions, which results in a decrease in the proportion of hydrophilic groups on UiO-66-NH₂ particle surfaces. Hydrophobic UiO-66-NH₂ particles show improved adsorption capacity for dyes in organic solvents compared with pristine UiO-66-NH₂ particles. It is expected that the interfacial assembling route can be applied to the synthesis of different kinds of MOF materials with tunable hydrophilicity or hydrophobicity required for diverse applications.

Pickering emulsions stabilized by a metal-organic framework (MOF) and graphene oxide (GO) for producing MOF/GO composites

Herein we demonstrate the formation of a novel kind of Pickering emulsion that is stabilized by a Zr-based metal-organic framework (Zr-MOF) and graphene oxide (GO). It was found that the Zr-BDC-NO₂ and GO solids assembling at the oil/water interface can effectively stabilize the oil droplets that are dispersed in the water phase. Such a Pickering emulsion offers a facile route for fabricating Zr-MOF/GO composite materials. After removing water and oil by freeze drying from Pickering emulsions, the Zr-MOF/GO composites were obtained and their morphologies, structures and interaction properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry, respectively. The influences of the concentration of GO and Zr-MOF on the emulsion microstructures and the properties of the MOF/GO composites were studied. Based on experimental results, the mechanisms for the emulsion formation by Zr-MOF and GO and the as-synthesized superstructures of the Zr-MOF/GO composite were proposed. It is expected that this facile and tunable route can be applied to the synthesis of different kinds of MOF-based or GO-based composite materials.

CO2 Capture and Separations Using MOFs: Computational and Experimental Studies

This Review focuses on research oriented toward elucidation of the various aspects that determine adsorption of CO₂ in metal-organic frameworks and its separation from gas mixtures found in industrial processes. It includes theoretical, experimental, and combined approaches able to characterize the materials, investigate the adsorption/desorption/reaction properties of the adsorbates inside such environments, screen and design new materials, and analyze additional factors such as material regenerability, stability, effects of impurities, and cost among several factors that influence the effectiveness of the separations. CO₂ adsorption, separations, and membranes are reviewed followed by an analysis of the effects of stability, impurities, and process operation conditions on practical applications.

Stepwise crystallographic visualization of dynamic guest binding in a nanoporous framework

Binding sites are at the heart of all host-guest systems, whether biological or chemical. When considering binding sites that form covalent bonds with the guest, we generally envision a single, highly specific binding motif. Through single-crystal X-ray crystallography, the dynamic binding of a guest that displays a variety of covalent binding motifs in a single site of adsorption is directly observed for the first time. The stepwise crystallographic visualization of the incorporation of I2 within a porous MOF is presented, wherein the preferred binding motifs throughout the uptake process are identified. The guest I2 molecules initially bind with terminal iodide atoms of the framework to form [I4]2- units. However, as the adsorption progresses, the I2 molecules are observed to form less energetically favorable I3- groups with the same framework iodide atoms, thereby allowing for more guest molecules to be chemisorbed. At near saturation, even more binding motifs are observed in the same pores, including both physisorbed and chemisorbed guest molecules. Herein, we present the successful identification of a unique set of host-guest interactions which will drive the improvement of high capacity iodine capture materials.

A Hierarchical Bipyridine-Constructed Framework for Highly Efficient Carbon Dioxide Capture and Catalytic Conversion

As a C₁ feedstock, CO₂ has the potential to be uniquely highly economical in both a chemical and a financial sense. Porous materials bearing particular binding and active sites that can capture and convert CO₂ simultaneously are promising candidates for CO₂ utilization. In this work, a bipyridine-constructed polymer featuring a high surface area, a hierarchical porous structure, and excellent stability was synthesized through free-radical polymerization. After metalation, the resultant catalysts exhibited superior activities in comparison with those of their homogeneous counterparts in the cycloaddition of CO₂ to epoxides. The high performance of the heterogeneous catalysts originates from cooperative effects between the CO₂ -philic polymer and the embedded metal species. In addition, the catalysts showed excellent stabilities and are readily recyclable; thus, they are promising for practical utilization for the conversion of CO₂ into value-added chemicals.

Advances in Solid-State Transformations of Coordination Bonds: From the Ball Mill to the Aging Chamber

Controlling the formation of coordination bonds is pivotal to the development of a plethora of functional metal-organic materials, ranging from coordination polymers, metal-organic frameworks (MOFs) to metallodrugs. The interest in and commercialization of such materials has created a need for more efficient, environmentally-friendly routes for making coordination bonds. Solid-state coordination chemistry is a versatile greener alternative to conventional synthesis, offering quantitative yields, enhanced stoichiometric and topological selectivity, access to a wider range of precursors, as well as to molecules and materials not readily accessible in solution or solvothermally. With a focus on mechanochemical, thermochemical and “accelerated aging” approaches to coordination polymers, including pharmaceutically-relevant materials and microporous MOFs, this review highlights the recent advances in solid-state coordination chemistry and techniques for understanding the underlying reaction mechanisms.

Synthesis of a partially fluorinated ZIF-8 analog for ethane/ethene separation

ZIF-318, isostructural to ZIF-8 but built from the mixed linkers of 2-methylimidazole and 2-trifluoromethylimidazole can be activated for gases sorption and the separation of ethane/ethene mixtures.

Strategies for the design of porous polymers as efficient heterogeneous catalysts: from co-polymerization to self-polymerization

Porous organic polymers serve as a versatile platform for the development of highly efficient heterogeneous catalysts.

Imparting amphiphobicity on single-crystalline porous materials

The sophisticated control of surface wettability for target-specific applications has attracted widespread interest for use in a plethora of applications. Despite the recent advances in modification of non-porous materials, surface wettability control of porous materials, particularly single crystalline, remains undeveloped. Here we contribute a general method to impart amphiphobicity on single-crystalline porous materials as demonstrated by chemically coating the exterior of metal-organic framework (MOF) crystals with an amphiphobic surface. As amphiphobic porous materials, the resultant MOF crystals exhibit both superhydrophobicity and oleophobicity in addition to retaining high crystallinity and intact porosity. The chemical shielding effect resulting from the amphiphobicity of the MOFs is illustrated by their performances in water/organic vapour adsorption, as well as long-term ultrastability under highly humidified CO₂ environments and exceptional chemical stability in acid/base aqueous solutions. Our work thereby pioneers a perspective to protect crystalline porous materials under various chemical environments for numerous applications.

Cu,N-codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction

It remains a huge challenge to develop nonprecious electrocatalysts with high activity to substitute commercial Pt catalysts for oxygen reduction reactions (ORR). Here, the Cu,N-codoped hierarchical porous carbon (Cu-N-C) with a high content of pyridinic N was synthesized by carbonizing Cu-containing ZIF-8. Results indicate that Cu-N-C shows excellent ORR electrocatalyst properties. First of all, it nearly follows the four-electron route, and its electron transfer number reaches 3.92 at -0.4 V. Second, both the onset potential and limited current density of Cu-N-C are almost equal to those of a commercial Pt/C catalyst. Third, it exhibits a better half-wave potential (∼16 mV) than a commercial Pt/C catalyst. More importantly, the Cu-N-C displays better stability and methanol tolerance than the Pt/C catalyst. All of these good properties are attributed to hierarchical structure, high pyridinic N content, and the synergism of Cu and N dopants. The metal-N codoping strategy can significantly enhance the activity of electrocatalysts, and it will provide reference for the design of novel N-doped porous carbon ORR catalysts.

Mechanochemical and solvent-free assembly of zirconium-based metal-organic frameworks

We develop the first mechanochemical and solvent-free routes for zirconium metal-organic frameworks, making the frameworks UiO-66 and UiO-66-NH2 accessible on the gram scale without strong acids, high temperatures or excess reactants. The frameworks form either by milling, or spontaneous self-assembly by simply exposing solid mixtures of reactants to organic vapour. The generated frameworks exhibit high porosity and catalytic activity in the hydrolysis of model nerve agents, on par with their solvothermally generated counterparts.

Recent development on the effect of water/moisture on the performance of zeolite membrane and MMMs containing zeolite for gas separation; review

Understanding the effects of water vapour on gas permeation and separation properties of zeolite membranes especially at lower temperatures is important for the applications of these zeolite membranes for gas separations involving water vapour.

Yolk–shell nanospheres with soluble amino-polystyrene as a reservoir for Pd NPs

Pd NPs immobilized in yolk–shell nanospheres confined with soluble amino-polystyrene could efficiently catalyze the selective hydrogenation of acetophenone.