Zeolitic imidazolate frameworks: next-generation materials for energy-efficient gas separations

Tuning Coordination in ZIF-67 Through the Solid-State Thermal Synthesis for Balancing Structural Stability and Catalytic Reactivity

Tailor-made unsaturated coordination of metal ions or organic linkers in zeolitic imidazole frameworks (ZIFs) has great potential in tuning the ZIFs' properties and reactivity for their applications. Taking advantage of the solid-state thermal (SST) method as a facile and eco-friendly synthesis method, the rational coordination of metal ions with imidazole ligands in ZIF-67 through the SST method is investigated. The rational precursor ratio (metal-to-ligand source) under the solvent-free SST method emerges as a perfect strategy to tune the coordinately unsaturated sites within the ZIF-67 frameworks. Different analysis techniques, computational methods (DFT), and catalytic model reactions examine unsaturated coordination in ZIF-67 materials (defect structures). The unsaturated coordination provides unique characteristic properties on materials with excellent catalytic performance. However, the higher reactive properties are negotiated with weaker structural stability on materials. In addition, the post-SST approach is applied to enable rational coordination and modify the pristine ZIF-67 materials. The post-SST method rearranges and modifies coordination in the framework of materials. These findings are crucial to understanding the role of the uncoordinated degree to balance with structural stability based on ZIF-67, which is critical for effective heterogeneous catalysts.

Hierarchically Micro- and Mesoporous Zeolitic Imidazolate Frameworks Through Selective Ligand Removal

A new method to engineer hierarchically porous zeolitic imidazolate frameworks (ZIFs) through selective ligand removal (SeLiRe) is presented. This innovative approach involves crafting mixed-ligand ZIFs (ML-ZIFs) with varying proportions of 2-aminobenzimidazole (NH2-bIm) and 2-methylimidazole (2-mIm), followed by controlled thermal treatments. This process creates a dual-pore system, incorporating both micropores and additional mesopores, suggesting selective cleavage of metal-ligand coordination bonds. Achieving this delicate balance requires adjustment of heating conditions for each mixed-ligand ratio, enabling the targeted removal of NH2-bIm from a variety of ML-ZIFs while preserving their inherent microporous framework. Furthermore, the distribution of the initial thermolabile ligand plays a pivotal role in determining the resulting mesopore architecture. The efficacy of this methodology is aptly demonstrated through the assessment of hierarchically porous ZIFs for their potential in adsorbing diverse organic dyes in aqueous environments. Particularly striking is the performance of the 10%NH2-ZIF-2 h, which showcases an astonishing 40-fold increase in methylene blue adsorption capacity compared to ZIF-8, attributed to larger pore volumes that accelerate the diffusion of dye molecules to adsorption sites. This versatile technique opens new avenues for designing micro/mesoporous ZIFs, particularly suited for liquid media scenarios necessitating efficient active site access and optimal diffusion kinetics, such as purification, catalysis, and sensing.

Zeolitic imidazolate frameworks (ZIFs): Advanced nanostructured materials to enhance the functional performance of food packaging materials

Zeolite imidazole framework (ZIF) materials are a class of metallic organic framework (MOF) materials that have several potential applications in the food and other industries. They consist of metal ions or clusters of metal ions coordinated with imidazole-based organic linkers, creating a three-dimensional solid structure with well-defined pores and channels. ZIFs possess several important features, including high porosity, tunable pore sizes, high surface areas, adjustable surface chemistries, and good stabilities. These characteristics make them highly versatile materials that can be used in a variety of applications, including smart and active food packaging. Based on their controllable compositions, dimensions, and pore sizes, the properties of ZIFs can be tailored for a diverse range of applications, including energy storage, sensing, separation, encapsulation, and catalysis. In this article, we focus on recent progress and potential applications of ZIFs in food packaging materials. Previous studies have shown that ZIFs can significantly improve the optical, mechanical, barrier, thermal, sustainability, and preservative properties of packaging materials. Moreover, ZIFs can be used as carriers to encapsulate, protect, and control the release of bioactive agents in packaging materials. ZIFs are capable of selectively adsorbing and releasing molecules based on their size, shape, and surface properties. These unique characteristics make them particularly suitable for smart or active food packaging applications. By selectively removing gases (such as oxygen, carbon dioxide, water, or ethylene) ZIFs can improve the shelf life and quality of packaged foods. In addition, they can be employed to control the growth of spoilage microorganisms and minimize oxidation reactions, thereby enhancing the freshness and extending the shelf life of foods. They may also be used to create sensors capable of detecting and indicating food spoilage. For instance, ZIFs that change color or release specific compounds when spoilage products are present can provide visual or chemical indications of food deterioration. This feature is especially valuable in ensuring the safety and quality of packaged food, as it enables consumers and retailers to easily identify spoiled products. ZIFs can be functionalized using various additives, including antioxidants, antimicrobials, pigments, and flavors, which can improve the preservative and sensory properties of packaged foods. Moreover, ZIF-based packaging materials offer sustainability benefits. Unlike traditional plastic packaging, ZIFs are biodegradable and can easily be disposed of without causing harm to the environment, thereby reducing the adverse effects of plastic waste materials. The application of ZIFs in smart/active food packaging offers exciting possibilities for enhancing the shelf life, quality, and safety of foods. With further research and development, ZIF-based packaging could become a sustainable alternative to plastic-based packaging in the food industry. An important aim of this review article is to stimulate further research on the development and application of ZIFs within food packaging materials.

Structural Chemistry of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a subclass of reticular structures based on tetrahedral four-connected networks of zeolites and minerals. They are composed of transition-metal ions and imidazolate-type linkers, and their pore size and shape, surface area, and functionality can be precisely controlled. Despite their potential, two questions remain unanswered: how to synthesize more diverse ZIF structures and how ZIFs differentiate from other crystalline solids. In other words, how can we use our understanding of their unique structures to better design and synthesize ZIFs? In this Review, we first summarize the methods for synthesizing a wide range of ZIFs. We then review the crystal structure of ZIFs and describe the relationship between their structure and properties using an in-depth analysis. We also discuss several important and intrinsic features that make ZIFs stand out from MOFs and discrete molecular cages. Finally, we outline the future direction for this class of porous crystals.

Three in One: Superoleophilic, Chemically and Mechanically Resistant ZIF-7 and ZIF-11 Percolation Networks for Selective Permeation of Oils and Chlorinated Solvents

Imbuing superwetting functions to organic-inorganic hybrid networks displaying chemical resistance, self-cleaning ability, and selective permeation of liquids has received increasing attention in recent years. Here we report superhydrophobic ZIF-7 and ZIF-11 on multilayer fluorinated graphene (FG) nanosheets with long-lasting water-repellent features. By exploring the solution processing of these chemically resistant dispersions, superoleophilic FG-ZIF-7 stainless steel mesh (FG-ZIF-7-SSM) and FG-ZIF-11 over cotton cloth (FG-ZIF-11-CC) possessing superior adhesion were fabricated. These permselective oil-liking prototypes were explored toward mesitylene and crude oil pickup from chemically harsh marine conditions such as seawater, acidic water, and alkaline water, with a separation efficiency of 96-94% up to 10 cycles. Furthermore, using an FG-ZIF-11-CC-wrapped glass pipet, upward diffusion of chloroform from sea, acidic, and alkaline water in 45 s was demonstrated with a separation efficacy of 94% up to 20 cycles. In addition to the chemical resistance and reusability, the mechanical stability of FG-ZIF-7-SSM and FG-ZIF-11-CC was investigated through folding, tape peeling, and dragging through sandpaper up to 250 cycles, showing no signs of changes in the hydrophobic responses. This research sheds light on the application of physiochemically resistant percolation coatings based on fluorinated graphene multilayers supporting ZIF-7 and ZIF-11 toward oil/water separation.

Superior Selective CO2 Adsorption and Separation over N2 and CH4 of Porous Carbon Nitride Nanosheets: Insights from GCMC and DFT Simulations

Development of high-performance materials for the capture and separation of CO₂ from the gas mixture is significant to alleviate carbon emission and mitigate the greenhouse effect. In this work, a novel structure of C₉N₇ slit was developed to explore its CO₂ adsorption capacity and selectivity using Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) calculations. Among varying slit widths, C₉N₇ with the slit width of 0.7 nm exhibited remarkable CO₂ uptake with superior CO₂/N₂ and CO₂/CH₄ selectivity. At 1 bar and 298 K, a maximum CO₂ adsorption capacity can be obtained as high as 7.06 mmol/g, and the selectivity of CO₂/N₂ and CO₂/CH₄ was 41.43 and 18.67, respectively. In the presence of H₂O, the CO₂ uptake of C₉N₇ slit decreased slightly as the water content increased, showing better water tolerance. Furthermore, the underlying mechanism of highly selective CO₂ adsorption and separation on the C₉N₇ surface was revealed. The closer the adsorption distance, the stronger the interaction energy between the gas molecule and the C₉N₇ surface. The strong interaction between the C₉N₇ nanosheet and the CO₂ molecule contributes to its impressive CO₂ uptake and selectivity performance, suggesting that the C₉N₇ slit could be a promising candidate for CO₂ capture and separation.

H-CoNiSe2/NC dodecahedral hollow structures for high-performance supercapacitors

The synergistic effect between metal ions and increasing the surface area leads to the fabrication of supercapacitor materials with high capacities. It is predicted that transition metal selenide compounds will be ideal electrode materials for supercapacitors. However, the defects of poor conductivity and volume expansion of the compounds are fundamental problems that must be solved. In this work, we successfully synthesized the cobalt-nickel selenide nitrogen-doped carbon (H-CoNiSe₂/NC) hollow polyhedral composite structure using ZIF-67 as a precursor. The CoSe₂ and NiSe₂ nanoparticles embedded in the NC polyhedral framework offer a wealth of active sites for the whole electrode. Moreover, the presence of the NC structure in the proposed composite can simultaneously lead to improved conductivity and reduce the volume effect created during the cycling procedure. The H-CoNiSe₂/NC electrode provides high specific capacity (1131 C/g at 1.0 A/g) and outstanding cyclic stability (90.2% retention after 6000 cycles). In addition, the H-CoNiSe₂/NC//AC hybrid supercapacitor delivers ultrahigh energy density and power density (81.9 Wh/kg at 900 W/kg) and excellent cyclic stability (92.1% of the initial capacitance after 6000 cycles). This study will provide a supercapacitor electrode material with a high specific capacity for energy storage devices.Please confirm the corresponding affiliation for the 'Ali A. Ensafi' author is correctly identified.Error during converting author query response. Please check the eproofing link or feedback pdf for details.

Leaching in Specific Facets of ZIF-67 and ZIF-L Zeolitic Imidazolate Frameworks During the CO2 Cycloaddition with Epichlorohydrin

Zeolitic imidazolate frameworks (ZIFs) have been profusely used as catalysts for inserting CO₂ into organic epoxides (i.e., epichlorohydrin) through cycloaddition. Here, we demonstrate that these materials suffer from irreversible degradation by leaching. To prove this, we performed the reactions and analyzed the final reaction mixtures by elemental analysis and the resulting materials by different microscopies. We found that the difference in catalytic activity between three ZIF-67 and one ZIF-L catalysts was related to the rate at which the materials degraded. Particularly, the {100} facet leaches faster than the others, regardless of the material used. The catalytic activity strongly depended on the amount of leached elements in the liquid phase since these species are extremely active. Our work points to the instability of these materials under relevant reaction conditions and the necessity of additional treatments to improve their stability.

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.

Postsynthetic Treatment of ZIF-67 with 5-Methyltetrazole: Evolution from Pseudo-Td to Pseudo-Oh Symmetry and Collapse of Magnetic Ordering

Reaction of Co(II) nitrate with 2-methylimidazole (2mIm) yields ZIF-67, the structure of which features Co(II) ions in pseudo-tetrahedral coordination geometry. Strong antiferromagnetic interactions between Co(II) ions mediated by the 2mIm ligands lead to antiferromagnetic ordering at 22 K. Postsynthetic treatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) results in the loss of crystallinity and magnetic order. The local structure of the Co(II) ions was probed by a combination of diffuse-reflectance electronic absorption spectroscopy and Co K-edge X-ray absorption spectroscopy (in the XANES and EXAFS regions). Upon reaction with 5mT, the ⁴A₂(F)-⁴T₁(F) and ⁴A₂(F)-⁴T₁(P) transitions at 1140 and 585 nm, respectively, of the pseudo-tetrahedral Co(II) center in ZIF-67 become less prominent and are replaced by transitions at 990 and 475 nm attributable to the ⁴T_1g(F)-⁴T_2g(F) and ⁴T_1g(F)-⁴T_1g(P) transitions of a pseudo-octahedral Co(II) center, respectively. Furthermore, the 1s-3d pre-edge absorption feature in the Co K-edge XANES spectrum loses intensity during this reaction, and the edge feature becomes more sharp, consistent with a change from pseudo-T_d to pseudo-O_h geometry. EXAFS analysis further supports the proposed change in geometry: EXAFS data for ZIF-67 are well fitted to four Co-N scatterers at 1.99 Å, whereas the data for the 5mT-substituted compound are best fitted with 6 Co-N scatterers at 2.14 Å. Our results support the conclusion that a six-coordinate, pseudo-O_h geometry is adopted upon ligand substitution. The increase in coordination number directly increases the Co-N bond distances, which in turn weakens magnetic exchange interactions. No magnetic ordering is found in the 5mT-substituted materials.

Mixed metal node effect in zeolitic imidazolate frameworks

We synthesized two series of bimetallic (zinc and cobalt) zeolitic imidazolate frameworks (ZIF-62) under different solvothermal conditions. It is found that the structure of the derived ZIF crystals is highly sensitive to synthesis conditions. One series possesses the standard ZIF-62 structure, whereas the other has a mixed structure composed of both the standard structure and an unknown one. The standard series exhibits a slight negative deviation from linearity of melting temperature (T _m) and glass transition temperature (T _g) with the substitution of Co for Zn. In contrast, the new series displays a stronger negative deviation. These negative deviations from linearity indicate the mixed metal node effect in bimetallic ZIF-62 due to the structural mismatch between Co²⁺ and Zn²⁺ and to the difference in their electronic configurations. The new series involves both cobalt-rich and zinc-rich phases, whereas the standard one shows one homogeneous phase. Density functional theory calculations predict that the substitution of Co for Zn increases the bulk modulus of the ZIF crystals. This work indicates that the structure, melting behaviour, and mechanical properties of ZIFs can be tuned by metal node substitution and by varying the synthetic conditions. Both series of ZIFs have higher glass forming abilities due to their higher T _g/T _m ratios (0.77-0.84) compared to most good glass formers.

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.

Recent Advances in Mixed-Matrix Membranes for Light Hydrocarbon (C1–C3) Separation

Light hydrocarbons, obtained through the petroleum refining process, are used in numerous applications. The separation of the various light hydrocarbons is challenging and expensive due to their similar melting and boiling points. Alternative methods have been investigated to supplement cryogenic distillation, which is energy intensive. Membrane technology, on the other hand, can be an attractive alternative in light hydrocarbon separation as a phase change that is known to be energy-intensive is not required during the separation. In this regard, this study focuses on recent advances in mixed-matrix membranes (MMMs) for light hydrocarbon (C1–C3) separation based on gas permeability and selectivity. Moreover, the future research and development direction of MMMs in light hydrocarbon separation is discussed, considering the low intrinsic gas permeability of polymeric membranes.

Covalent-Linking-Enabled Superior Compatibility of ZIF-8 Hybrid Membrane for Efficient Propylene Separation

The interface is a critical issue for metal-organic-framework hybrid membranes in propylene separation. Here, a covalent-linking strategy is reported for strikingly reinforcing the interfacial compatibility in a ZIF-8-based membrane. A functionalized ZIF-8 material named ZIF-8-CN is synthesized using the mixed-ligand approach. ZIF-8-CN has an identical crystalline structure to ZIF-8, and the 4,5-dicyanoimidazole ligand is available for further functionalization. Covalent linkage of ZIF-8-CN with PIM-1 is driven by the thermal reaction of the cyano groups on both entities, which strengthens the filler-polymer connection in the ZIF-8-CN@tPIM-1 membrane. ZIF-8-CN@tPIM-1 exhibits remarkably enhanced propylene permeation property with C₃ H₆ /C₃ H₈ selectivity of ≈28, which is 350% and 180% higher than those on non-treated ZIF-8-CN/PIM-1 and non-functionalized ZIF-8@tPIM-1, respectively. Additionally, ZIF-8-CN@tPIM-1 shows the highest C₃ H₆ permeability of ≈370 Barrer among all relevant ZIF-8 membranes. This strategy opens an avenue for precise interface engineering in membranes and the resultant high performance is appealing in the propylene separation industry.

Hybrid Metal-Organic Framework-Cellulose Materials Retaining High Porosity: ZIF-8@Cellulose Nanofibrils

Metal-organic frameworks have attracted a great deal of attention for future applications in numerous areas, including gas adsorption. However, in order for them to reach their full potential a substrate to provide an anchor may be needed. Ideally, this substrate should be environmentally friendly and renewable. Cellulose nanofibrils show potential in this area. Here we present a hybrid material created from the self-assembly of zeolitic imidazolate framework (ZIF-8) nanocrystals on cellulose nanofibrils (CNF) in aqueous medium. The CNF/ZIF-8 was freeze dried and formed free standing materials suitable for gas adsorption. A BET area of 1014 m2 g−1 was achieved for the CNF/ZIF-8 hybrid materials ZIF-8@cellulose which is comparable with reported values for free standing ZIF-8 materials, 1600 m2 g−1, considering the dilution with cellulose, and a considerable enhancement compared to CNF on its own, 32 m2 g−1.

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.

Performance-Based Screening of Porous Materials for Carbon Capture

Computational screening methods have changed the way new materials and processes are discovered and designed. For adsorption-based gas separations and carbon capture, recent efforts have been directed toward the development of multiscale and performance-based screening workflows where we can go from the atomistic structure of an adsorbent to its equilibrium and transport properties at different scales, and eventually to its separation performance at the process level. The objective of this work is to review the current status of this new approach, discuss its potential and impact on the field of materials screening, and highlight the challenges that limit its application. We compile and introduce all the elements required for the development, implementation, and operation of multiscale workflows, hence providing a useful practical guide and a comprehensive source of reference to the scientific communities who work in this area. Our review includes information about available materials databases, state-of-the-art molecular simulation and process modeling tools, and a complete catalogue of data and parameters that are required at each stage of the multiscale screening. We thoroughly discuss the challenges associated with data availability, consistency of the models, and reproducibility of the data and, finally, propose new directions for the future of the field.

Zeolitic imidazolate frameworks as intrinsic light harvesting and charge separation materials for photocatalysis

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal organic frameworks that have attracted considerable attention in the past years and have found many applications including heterogeneous catalysis due to their highly ordered porous structure, large surface area, and structural flexibility. However, ZIFs are largely utilized as simple hosts or passive media for dispersing other catalytically active species, resembling the roles of zeolites in catalysis. In contrast, our recent findings show that ZIFs not only have broad absorption across the UV-visible and near IR spectral region but also have an exceptionally long-lived excited charge separated state, suggesting that ZIFs may be used as intrinsic light harvesting and photocatalytic materials rather than as inert hosts. This Perspective will focus on the recent progress on the fundamental studies of the intrinsic light absorption, charge separation, and photocatalytic properties of ZIFs and will discuss the outlook for future development.

Multifunctional metal-organic frameworks in oil spills and associated organic pollutant remediation

The release of toxic organic compounds into the environment in an event of oil spillage is a global menace due to the potential impacts on the ecosystem. Several approaches have been employed for oil spills clean-up, with adsorption technique proven to be more promising for the total reclamation of a polluted site. Of the several adsorbents so far reported, adsorbent-based porous materials have gained attention for the reduction/total removal of different compounds in environmental remediation applications. The superior potential of mesoporous materials based on metal-organic frameworks (MOFs) against conventional adsorbents is due to their intriguing and enhanced properties. Therefore, this review presents recent development in MOF composites; methods of preparation; and their practical applications towards remediating oil spill, organic pollutants, and toxic gases in different environmental media, as well as potential materials in the possible deployment in reclaiming the polluted Niger Delta due to unabated oil spillage and gas flaring.

Synthesis and Characterization of Hybrid Metal Zeolitic Imidazolate Framework Membrane for Efficient H2/CO2 Gas Separation

In this paper, we propose mixed metal ions in the node of the zeolitic imidazolate framework (ZIF) structure. The hybrid metal ZIF is formed for the gas separation of hydrogen and carbon dioxide. In the first stage, the nanoparticles were prepared as a coating on a substrate, and acting as secondary growing nuclei. The hybrid metal ZIF structures were characterized by X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). N₂ adsorption–desorption isotherms determined surface area, and scanning electron microscopy (SEM) was used to observe the microstructure and surface morphology. The hybrid metal ZIF-8-67 powder had the largest surface area (1260.40 m² g⁻¹), and the nanoparticles (100 nm) could be fully dense-coated on the substrate to benefit the subsequent membrane growth. In the second stage, we prepared the hybrid metal ZIF-8-67 membrane on the pre-seeding substrate with mixed metal nanoparticles of cobalt and zinc, by the microwave hydrothermal method. Cobalt ions were identified in the tetrahedral coordination through UV–Vis, and the membrane structure and morphology were determined by XRD and SEM. Finally, a gas permeation analyzer (GPA) was used to determine the gas separation performance of the hybrid metal ZIF-8-67 membrane. We successfully introduced zinc ions and cobalt ions into the ZIF structure, where cobalt had a strong interaction with CO₂. Therefore, GPA analysis showed an excellent H₂/CO₂ separation factor due to lower CO₂ permeability. The CO₂ permeance was ~0.65 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹, and the separation factors for H₂/CO₂ and H₂/N₂ were 9.2 and 2.9, respectively. Our results demonstrate that the hybrid metal ZIF-8-67 membrane has a superior H₂/CO₂ separation factor, which can be attributed to its very high specific surface area and structure. Based on the above, hybrid metal ZIF-8-67 membranes are expected to be applied in hydrogen or carbon dioxide gas separation and purification.

Magnetically Aligned and Enriched Pathways of Zeolitic Imidazolate Framework 8 in Matrimid Mixed Matrix Membranes for Enhanced CO2 Permeability

Metal-organic frameworks (MOFs) as additives in mixed matrix membranes (MMMs) for gas separation have gained significant attention over the past decades. Many design parameters have been investigated for MOF based MMMs, but the spatial distribution of the MOF throughout MMMs lacks investigation. Therefore, magnetically aligned and enriched pathways of zeolitic imidazolate framework 8 (ZIF−8) in Matrimid MMMs were synthesized and investigated by means of their N₂ and CO₂ permeability. Magnetic ZIF−8 (m–ZIF−8) was synthesized by incorporating Fe₃O₄ in the ZIF−8 structure. The presence of Fe₃O₄ in m–ZIF−8 showed a decrease in surface area and N₂ and CO₂ uptake, with respect to pure ZIF−8. Alignment of m–ZIF−8 in Matrimid showed the presence of enriched pathways of m–ZIF−8 through the MMMs. At 10 wt.% m–ZIF−8 incorporation, no effect of alignment was observed for the N₂ and CO₂ permeability, which was ascribed anon-ideal tortuous alignment. However, alignment of 20 wt.% m–ZIF−8 in Matrimid showed to increase the CO₂ diffusivity and permeability (19%) at 7 bar, while no loss in ideal selectivity was observed, with respect to homogeneously dispersed m–ZIF−8 membranes. Thus, the alignment of MOF particles throughout the matrix was shown to enhance the CO₂ permeability at a certain weight content of MOF.

Separation Selectivity of CH4/CO2 Gas Mixtures in the ZIF-8 Membrane Explored by Dynamic Monte Carlo Simulations

Here we report a series of nonequilibrium dynamic Monte Carlo simulations combined with dual control volume (DCV-DMC) to explore the separation selectivity of CH₄/CO₂ gas mixtures in the ZIF-8 membrane with a thickness of up to about 20 nm. Meanwhile, an improved DCV-DMC approach coupled with the corresponding potential map (PM-DCV-DMC) is further developed to speed up the computational efficiency of conventional DCV-DMC simulations. Our simulation results provide the molecular-level density and selectivity profiles along the permeation direction of both CH₄ and CO₂ molecules in the ZIF-8 membrane, indicating that the parts near membrane surfaces at both ends play a key role in determining the separation selectivity. All densities initially show a sharp increase in the individual maximum within the first outermost unit cell at the feed side and follow a long fluctuating decrease process. Accordingly, the corresponding selectivity profiles initially display a long fluctuating increase in the individual maximum and follow a sharp decrease near the membrane surface at the permeation side. Furthermore, the effects of feed composition, temperature, and pressure on the relevant separation selectivity are also discussed in detail, where the temperature has a greater influence on the separation selectivity than the feed composition and pressure. More importantly, the predicted separation selectivities from our PM-DCV-DMC simulations are well consistent with previous experimental results.

Probing the bent bonds in cyclopropane systems for gas storage and separation process: A computational study

The hydrogen, carbon dioxide, and carbon monoxide gas adsorption and storage capacity of lithium-decorated cyclopropane ring systems were examined with quantum chemical calculations at density functional theory, DFT M06-2X functional using 6-31G(d) and cc-pVDZ basis sets. To examine the reliability of M06-2X DFT functional, a few representative systems are also examined with complete basis set CBS-QB3 method and CCSD-aug-cc-pVTZ level of theory. The cyclopropane systems can bind to one Li⁺ ion; however, the corresponding the methylated systems can bind with two Li⁺ ions. The cyclopropane systems can adsorb six hydrogen molecules with an average binding energy of 3.8 kcal/mol. The binding free energy (ΔG) values suggest that the hydrogen adsorption process is feasible at 273.15 K. The calculation of desorption energies indicates the recyclable property of gas adsorbed complexes. The same number of CO₂ and CO gas molecules can also be adsorbed with an average binding energy of -14.4 kcal/mol and -10.7 kcal/mol, respectively. The carbon dioxide showed ~3-4 kcal/mol better binding energy as compared to carbon monoxide and hence such designed systems can function as a potential candidate for the separation of these flue gas molecules. The nature of interactions in complexes was examined with atoms in molecules analysis revealed the electrostatic nature for the interaction of Li⁺ ion with cyclopropane rings. The chemical hardness and electrophilicity calculations showed that the gas adsorbed complexes are rigid and therefore robust as gas storage materials.

Postsynthetic Modification of ZIF-8 Membranes via Membrane Surface Ligand Exchange for Light Hydrocarbon Gas Separation Enhancement

The ability to tailor the pore structure of metal-organic framework (MOF) membranes enables synthesis of new or modified MOF membranes with enhanced separation characteristics. This work employs a modified version of solvent-assisted ligand exchange, termed membrane surface ligand exchange (MSLE), to modify the pore structure of zeolitic imidazolate framework-8 (ZIF-8) membranes. This paper is the first to perform a time-based, ex situ characterization and gas permeation study of ZIF-8 MSLE with 5,6-DBIM (DBIM, dimethylbenzimidazole) to effectively narrow the ZIF-8 pores, enhance light hydrocarbon gas-phase separations, and give insight into the exchange mechanism with respect to time and temperature. The results show that relatively fast exchange kinetics occur mainly at the outer surface of the ZIF-8 membrane during the initial 30 min of exchange and enables significant (40-70%) increases in propylene/propane selectivity with minimal (10-20%) propylene permeance losses for the modified ZIF-8 membranes. We postulate as the reaction time proceeds, the ligand-exchange rate slows as the DBIM linker diffuses into the ZIF-8 membrane beyond the external surface, exchanges with the original linker, disrupts the original framework's crystallinity, and then increases long-range order/crystallinity as the reaction proceeds. The H₂/C₂ separation factor increases with increased 5,6-DBIM content in the ZIF-8 framework which is facilitated by increased MSLE time and reaction temperature.

MOF-Based Photonic Crystal Film toward Separation of Organic Dyes

Metal-organic framework (MOF)-directed photonic structure materials have inspired great attention for extended and enhanced functions. However, the direct construction of photonic crystals (PCs) with MOF particles as building blocks still remains a challenge. Herein, we designed and synthesized monodisperse polyamidoamine (PAMAM) dendrimer-modified zeolitic imidazolate framework (ZIF-8) particles (PAMAM@ZIF-8) via a postsynthetic method, rendering ZIF-8 with hydrophilicity. It was found that the PAMAM@ZIF-8 particles could directly assemble into a uniform photonic structure and effectively suppressed the coffee-ring effect, forming homogeneous PC films with different structural colors. A PC pattern with angle-dependent colors was also achieved, which might have potential applications in the field of anticounterfeiting printing. More importantly, by taking advantages of a membrane separation-assisted assembly process, a colorful and robust PC film was accomplished on the surface of reduced graphene oxide (rGO). The hierarchal PAMAM@ZIF-8/rGO film demonstrates a superior separation ability toward organic dye solutions, which enriches the function of PC materials. This work gives a new insight into the fabrication of MOF-based functional PC materials, which will extend the application of PCs in the high selective and effective separation field.