Eliminating lattice defects in metal-organic framework molecular-sieving membranes

Non-van-der-Waals Oriented Two-Dimensional UiO-66 Films by Rapid Aqueous Synthesis at Room Temperature

The synthesis of MOFs in a two-dimensional (2D) film morphology is attractive for several applications including molecular and ionic separation. However, 2D MOFs have only been reported from structures that crystallize in lamellar morphology, where layers are held together by van der Waals (vdW) interaction. By comparison, UiO-66, one of the most studied MOFs because of its exceptional chemical stability, has only been reported in three-dimensional (3D) morphology. 2D UiO-66 is challenging to obtain given the robust isotropic bonds in its cubic crystal structure. Herein, we report the first synthesis of non-vdW 2D UiO-66-NH2 by developing crystal growth conditions that promote in-plane growth over out-of-plane growth. Continuous, oriented UiO-66-NH2 film with thickness tunable in the range of 0.5 to 2 unit cells could be obtained by sustainable, scalable chemistry, which yielded attractive ion-ion selectivity. The preparation of non-vdW 2D MOF is highly attractive to advance the field of MOF films for diverse applications.

Repairing Lattice Defects by an Orienting Strategy in a Porous Crystal: Boosting Inverse C2H6/C2H4 Separation

Defects are a common occurrence in various materials, making their repair a topic of widespread attention. However, the repair of atomic-scale lattice defects in porous crystals presents a substantial challenge. Herein, the repair of a linker-defective framework (NTU-70D) is presented through an orienting strategy to obtain a perfect framework (NTU-70P). Caused by steric hindrance from adjacent carboxylates of isonicotinic acid (INA), the lattice linker defect in NTU-70D is repaired by formic acid-assistant (pushed to the opposite direction by steric hindrance) installation of additional INAs in a defined direction, an unprecedented example in PCP chemistry. The resulting NTU-70P exhibits regular and smooth nano-channels that are adorned with more OINA sites, leading to a significant increase in C2H6 uptake (51.0 to 90.2 cm3 g-1) and C2H6/C2H4 selectivity (1.6 to 2.5), as evidenced by modeling calculations and in situ IR analysis. Furthermore, it demonstrates a notable ability to produce poly-grade C2H4 (with a record value of 46.4 mL g-1) from C2H6-containing mixtures. This work presents the first example of repaired porous crystals for boosted inverse C2H6/C2H4 separation, and the insights gained into the lattice repair offer avenues for the development of rich defective systems in practical applications.

Regularly Arranged Heterogeneous Pores in Gas Separation Membranes Constructed by Cocrystallization of Porous Organic Molecules

Integrating two or more materials to construct membranes with heterogeneous pore structures is an effective strategy for enhancing separation performance. Regularly arranging these heterogeneous pores can significantly optimize the combined effect of the introduced components. Porous Organic Cages (POCs), an emerging subclass of porous materials composed of discrete molecules, assemble to form interconnected pores and exhibit permanent porosity in the solid state. A unique feature of POCs is their modularity, enabling a "mix and match" approach to create co-crystal structures driven by the intermolecular interactions. Herein, we adopted the cocrystallization strategy for fabricating gas separation membranes. We prepared membranes from a series of [4+6] imine cage pairs, which vary in cavity sizes and include cages with fluorescence properties. By optimizing the cocrystalization conditions, we successfully fabricated continuous, defect-free gas membranes, benefiting from chiral recognition interactions. By introducing regularly alternating small and large pores, we addressed challenges such as the trade-off between permeability and selectivity in gas separation membrane. Moreover, the cocrystallization strategy has been proven effective for other molecular systems besides POCs, such as macrocycles, for the preparation of co-crystal membranes. This method broadens the scope for fabricating high-performance gas separation membranes with ordered heterogeneous pores.

UiO-66 with missing cluster defects for high-efficient extraction and enrichment of benzoylurea insecticides

The creation of defects in crystalline structures can tune metal-organic frameworks (MOFs) properties, such as improving their adsorptive and catalytic performance with producing more porosity and active sites. In this work, the bimetallic UiO-66 containing Zn and Zr was prepared. And then UiO-66 with missing cluster defects (UiO-66-1/3) were obtained by acid washing to remove the Zn nodes. UiO-66-1/3 was used as sorbent of dispersive solid-phase extraction (dSPE) to extract and enrich (BUs). Combination with high-performance liquid chromatography-diode array detector (HPLC-DAD) was developed to detect trace BUs in soil samples. Adsorption equilibrium can be reached in 3 min. The method possesses high enrichment factor (202-325), low detection limit (0.005-0.04 ng·mL-1), and wide linear range (0.02-200 ng·mL-1). In addition, the recovery rate of UiO-66-1/3 as an adsorbent was still higher than 95% after reused for 16 times. This work provides a new material for the enrichment and detection of benzoylurea insecticides in the environment.

Directly Gel-Thermal Processing of Linker-Mixed Crystal-Glass Composite Membranes for Sorption-Preferential Gas Separation

Membrane processes are promising for energy-saving industrial applications. However, efficient separation for some valuable gas mixtures with similar characteristics, such as CH4/N2 and O2/N2, remains extremely challenging. Metal-organic framework (MOF) membranes have been attracting intensive attention for gas sieving, but it is difficult to manufacture MOF membranes in scalability and precisely tune their transport property. In this study, Gel-thermal processing of linker-mixed MOF crystal-glass composite membranes is reported directly, with the mechanism of adjusting metal-linker bond strengths and angles to disrupt long-range periodicity of MOFs and promote glass phase formation, for sharply sorption-preferential gas separation. This strategy can be realized by using a simple, solvent/precursor-less, and cost-effective gel-thermal approach with two steps of gel coating and thermal conversion, thereby constructing crystal-glass composite membranes in a controllable, processable, versatile, and environmentally friendly route. Moreover, the mixed linkers enable preferential gas affinities and the ultramicroporous glasses can eliminate any membrane defects. The membranes exhibit outstanding gas separation performance for the challenging systems of CH4/N2 and O2/N2, with mixture selectivities up to 9.3 and 9.6, respectively, far exceeding those of polymer, MOF, and mixed-matrix membranes. The study provides an available route for architecting high-performance membranes for gas separations.

Angstrom-Scale Defect-Free Crystalline Membrane for Sieving Small Organic Molecules

Crystalline membranes, represented by the metal-organic framework (MOF) with well-defined angstrom-sized apertures, have shown great potential for molecular separation. Nevertheless, it remains a challenge to separate small molecules with very similar molecular size differences due to angstrom-scale defects during membrane formation. Herein, a stepwise assembling strategy is reported for constructing MOF membranes with intrinsic angstrom-sized lattice aperture lattice to separate organic azeotropic mixtures separation. The membrane is synthesized by redesigning the metal source, which reduces the coordination reaction rate to avoid cluster-missing defects. Then, extra ligands are introduced to overcome the coordination steric hindrance to heal the linker-missing defects. Ultralow-dose transmission electron microscopy is used to realize a direct observation of the angstrom-scale defects. For separating the challenging methanol-containing ester or ether azeotropic mixtures with molecular size difference as small as <1 Å, the angstrom-scale defect-free MOF membrane exhibits an outstanding flux of ≈3700 g·m-2 h-1 and separation factor of ≈247-524, far beyond the upper-bound of state-of-the-arts membranes. This study offers a feasible strategy for precisely constructing angstrom-confined spaces for diverse applications (e.g., separation, catalysis, and storage).

Efficient photocatalytic hydrogen production via single-atom Pt anchored hydrogen-bonded organic frameworks

Constructing single-atom catalysts (SACs) using organic porous framework materials as supports presents a promising approach for developing highly efficient photocatalysts for hydrogen evolution. However, the fabrication of SACs that are both highly stable and active poses a significant challenge, particularly in the precise anchoring of metal single atoms. In this study, we utilized 1,3,6,8-tetra (p-methyl benzoate) pyrene as a ligand to synthesize pyrene-based hydrogen-bonded organic frameworks (denoted as PFC-1) through a self-assembly approach. Subsequently, a liquid-phase photoreduction process was employed to deposit noble metal platinum (Pt) onto PFC-1, resulting in the fabrication of SACs (PFC-1@Pt). Characterization results confirmed that Pt existed in a monatomic state, anchored through PtC and PtO coordination bonds with PFC-1. Serving as electron capture and separation centers, the Pt single atoms effectively suppressed electron-hole recombination, thereby prolonging carrier lifetimes. Consequently, the PFC-1@Pt SAC exhibited efficient hydrogen evolution performance with a rate of 2202.5 μmol g-1 h-1 and maintained photocatalytic activity for over 40 h. Our findings provide a systematic approach for developing efficient and stable SACs based on HOFs, expanding the potential applications of HOF materials in photocatalysis.

Scalable and Low-Energy Synthesis of Metal-Organic Frameworks by a Seed-Mediated Approach

The synthesis of metal-organic frameworks (MOFs) by low energy input has been a long-term target for practical applications yet remains a great challenge. Herein, we developed a low-energy MOF growth strategy at a temperature down to 50 °C by simply introducing seeds into the reaction system. The MOFs are continuously grown on the surface of the seeds at a growth rate dozens of times higher than that of conventional solvothermal synthesis at low temperature, while the resulting MOFs possess high crystallinity, porosity, and stability similar to solvothermal seeds. Remarkably, the obtained MOFs feature high-density structural defects with Lewis acidity, thereby displaying more than one order of magnitude higher activity than the MOFs obtained by the conventional solvothermal method in the iodination reaction of substituted arenes. This low-energy synthetic approach is readily scaled up, which would be a significant step forward in the dream of the MOF industry.

Engineering grain boundaries in monolayer molybdenum disulfide for efficient water-ion separation

Two-dimensional (2D) materials have long been considered as ideal platforms for developing separation membranes. However, it is difficult to generate uniform subnanometer pores over large areas on 2D materials. We report that the well-defined eight-membered ring (8-MR) pores, typically formed at the boundaries of two antiparallel grains of monolayer molybdenum disulfide (MoS2), can serve as molecular sieves for efficient water-ion separation. The density of grain boundaries and, consequently, the number of 8-MR pores can be tuned by regulating the grain size. Optimized MoS2 membranes outperformed the state-of-the-art membranes in forward osmosis tests by demonstrating both ultrahigh water/sodium chloride selectivity and exceptional water permeance. Creating precise pore structures on atomically thin films through grain boundary engineering presents a promising route for producing membranes suitable for various applications.

Oriented Titanium-MOF Membrane for Hydrogen Purification

Precise hydrogen sorting from purge gas (H2/N2) and coke gas (H2/CH4), commonly carried out by cryogenic distillation, still suffers from low separation efficiency, high energy consumption, and considerable capital cost. Though still in its infancy, membrane technology offers a potential to achieve more efficient hydrogen purification. In this study, an optimum separation of hydrogen towards both methane and nitrogen via a kinetically-driven mechanism is realized through preferred orientation control of a MOF membrane. Relying on the 0.3 nm-sized window aligned vertical to the substrate, b-oriented Ti-MOF membrane exhibits ultra-high hydrogen selectivity, surpassing the upper bound limit of separating H2/N2 and H2/CH4 gas pairs attained so far by inorganic membranes. This spectacular selectivity is combined with a high H2 permeability owing to the synergistic effect of the 1 nm-sized MOF channel.

Solution Processable Metal-Organic Frameworks: Synthesis Strategy and Applications

Metal-organic frameworks (MOFs), constructed by inorganic secondary building units with organic linkers via reticular chemistry, inherently suffer from poor solution processability due to their insoluble nature, resulting from their extensive crystalline networks and structural rigidity. The ubiquitous occurrence of precipitation and agglomeration of MOFs upon formation poses a significant obstacle to the scale-up production of MOF-based monolith, aerogels, membranes, and electronic devices, thus restricting their practical applications in various scenarios. To address the previously mentioned challenge, significant strides have been achieved over the past decade in the development of various strategies aimed at preparing solution-processable MOF systems. In this review, the latest advance in the synthetic strategies for the construction of solution-processable MOFs, including direct dispersion in ionic liquids, surface modification, controllable calcination, and bottom-up synthesis, is comprehensively summarized. The respective advantages and disadvantages of each method are discussed. Additionally, the intriguing applications of solution-processable MOF systems in the fields of liquid adsorbent, molecular capture, sensing, and separation are systematically discussed. Finally, the challenges and opportunities about the continued advancement of solution-processable MOFs and their potential applications are outlooked.

UiO-66 Metal-Organic Framework Membranes: Structural Engineering for Separation Applications

Metal-organic frameworks (MOFs) have been recognized as promising materials for membrane-based separation technologies due to their exceptional porosity, structural tunability, and chemical stability. This review presents a comprehensive discussion of the advancements in structure engineering and design strategies that have been employed to optimize UiO-66 membranes for enhanced separation performance. Various synthesis methods for UiO-66 membranes are explored, with a focus on modulated approaches that incorporate different modulators to fine-tune nucleation rates and crystallization processes. The influence of preferred orientation, membrane thickness, pore size, pore surface chemistry, and hierarchical structures on the separation performance is concluded. By providing a consolidated overview of current research efforts and future directions in UiO-66 membrane development, this review aims to inspire further advancements in the field of separation technologies.

Accurate stacking engineering of MOF nanosheets as membranes for precise H2 sieving

Two-dimensional (2D) metal-organic framework (MOF) nanosheet membranes hold promise for exact molecular transfer due to their structural diversity and well-defined in-plane nanochannels. However, achieving precise regulation of stacking modes between neighboring nanosheets in membrane applications and understanding its influence on separation performance remains unrevealed and challenging. Here, we propose a strategy for accurately controlling the stacking modes of MOF nanosheets via linker polarity regulation. Both theoretical calculations and experimental results demonstrate that a high linker polarity promotes neighboring nanosheets to a maximum AB stacking due to steric hindrance effects, leading to a controlled effective pore size of the membrane and consequently to improved molecular sieving. Among series of CuBDC-based 2D MOFs with different linkers, the CuBDC-NO2 nanosheet membranes exhibit a reduced effective stacking aperture of 0.372 nm, yielding H2 permeance of 4.44 × 10-7 mol m-2 s-1 Pa-1 with a high H2/CO2 and H2/CH4 selectivity of 266 and 536, respectively. This work represents the in-depth investigation of the accurate tuning of MOF nanosheet stacking in the field of 2D materials, offering more perspectives for broader applications with universality for various 2D materials.

Strongly coupled and highly-compacted zirconium aminobenzenedicarboxylate crystal membranes for accelerating carbon dioxide capture

This study presents the fabrication of a UiO-66-NH2 composite membrane on a porous carbon cloth-polypyrrole substrate (CC-PPy@UiO-66-NH2) that demonstrates strongly coupled and highly ordered features to enhance CO2 capture efficiency. This free-standing configuration exhibits a higher adsorption kinetics with a slope of 0.48 mg g-1 min-1 for CO2 and superior adsorption selectivity, demonstrating that the adsorption kinetics rate of the CC-PPy@UiO-66-NH2 composite membrane for CO2 is approximately twice that of bulk UiO-66-NH2 (0.27 mg g-1 min-1) crystals at a bar.

Preprocessed Monomer Interfacial Polymerization for Scalable Fabrication of High-Valent Cluster-Based Metal-Organic Framework Membranes

Current research on emergent membrane materials with ordered and stable nanoporous structures often overlooks the vital facet of manufacturing scalability. We propose the preprocessed monomer interfacial polymerization (PMIP) strategy for the scalable fabrication of high-valent cluster-based metal-organic framework (MOF) membranes with robust structures. Using a roll-to-roll device on commercial polymer supports, Zr-fum-MOF membranes are continuously processed at room temperature through the PMIP approach. These large-area membranes demonstrate the preeminent hydrogen separation capabilities, boasting an order of magnitude of permeance and a thrice-enhanced selectivity when juxtaposed with conventional polymeric membranes. The obtained PMIP-Zr-fum-MOF membranes possess superior stability in water compared with interfacial polymerization (IP)-processed low-valent metal-ion-based ZIF-8 membranes. Moreover, we have implemented the PMIP strategy's universality to process the other four diverse MOF membranes. The proposal of PMIP significantly advances the scalable fabrication of water-stable high-valent cluster MOF membranes.

Enhanced Propylene/Propane Separation via Aniline-Decorated ZIF-8 Membrane: Lattice Rigidity Adjustment and Adsorption Site Introduction

Metal-organic framework (MOF)-based membranes excel in molecular separation, attracting significant research interest. The crystallographic microstructure and selective adsorption capacity of MOFs closely correlate with their gas separation performance. Here, aniline was added to the ZIF-8 synthesis in varying concentrations. Aniline, encapsulated within ZIF-8 cavities, interacts strongly with the 2-methylimidazole linker, resulting in both a shift in crystallographic phase from I_43 m to Cm in Rietveld refinement of X-ray diffraction (XRD) patterns and the selective adsorption behavior between propylene and propane. Consequently, an aniline decorative ZIF-8 (Anix-ZIF-8) membrane was prepared using a fast current-driven synthesis method, which exhibits good propylene/propane separation selectivity of up to 85. Calculation of the interaction energy between aniline and the various crystallographic phases of ZIF-8 using density functional theory (DFT) further verifies that aniline not only promotes the formation of crystallographic Cm phase, but also enhances the adsorption selectivity of propylene over propane. Aniline modification effectively tunes the crystallographic microstructure of ZIF-8, thereby, improving molecular sieving capabilities.

Rapid extraction of trace benzene by a crown-ether-based metal-organic framework

Due to almost identical boiling points of benzene and cyclohexane, the extraction of trace benzene from cyclohexane is currently performed via the energy-intensive extractive distillation method. Their adsorptive separation by porous materials is hampered by their similar dimensions. Metal-organic frameworks (MOFs) with versatile pore environments are capable of molecular discrimination, but the separation of trace substrates in liquid-phase remains extremely challenging. Herein, we report a robust MOF (NKU-300) with triangular channels decorated with crown ether that can discriminate trace benzene from cyclohexane, exhibiting an unprecedented selectivity of 8615(10) for the mixture of benzene/cyclohexane (v/v = 1/1000). Remarkably, NKU-300 demonstrates exceptional selectivities for the extraction of benzene from cyclohexane over a wide range of concentrations of 0.1%-50% with ultrafast sorption kinetics and excellent stability. Single-crystal X-ray diffraction and computational modelling reveal that multiple supramolecular interactions cooperatively immobilise benzene molecules in the triangular channel, enabling superior separation performance. This study will promote the application of advanced sorbents with tailored binding sites for challenging industrial separations.

Asymmetric Pore Windows in Pillar-Layered Metal-Organic Framework Membranes for H2/CO2 Separation

In this study, a novel ultramicroporous pillar-layered Ni-LAP-NH2 [Ni2(l-asp)2(Pz-NH2)] (l-asp = l-aspartic acid, Pz-NH2 = aminopyrazine) membranes on porous α-Al2O3 tubes with high performance and good thermal stability was first fabricated using isostructural Ni-LAP[Ni2(l-asp)2(Pz)] (Pz = pyrazine) crystals as seeds. Utilizing the principle of reticular chemistry, here, we introduced the active amino side group into the Ni-LAP frameworks by replacing the pillar-layered ligand Pz with Pz -NH2 while maintaining the original Ni-LAP small pore size, and the amino side group induced a "steric hindrance" effect and the physical adsorption affinity, which synergistically delayed CO2 penetration. It was found that the preferential (111) orientation Ni-LAP-NH2 membrane (Z10) exhibited a high H2/CO2 separation performance with a separation factor of 41.7 and H2 permeance of 9.08 × 10-8 mol·m-2·s-1·Pa-1 under optimal conditions. These MOF materials demonstrated potential for industrial H2 purification due to their tunable pore structure and remarkable stability. Moreover, this strategy offers an effective approach to tailoring pillar-layered MOF membranes with targeted molecular sieving ability.

The Key to MOF Membrane Fabrication and Application: the Trade-off between Crystallization and Film Formation

Metal-organic frameworks (MOFs), owing the merits of ordered and tailored channel structures in the burgeoning crystalline porous materials, have demonstrated significant promise in construction of high-performance separation membranes. However, precisely because this crystal structure with strong molecular interaction in their lattice provides robust structural integrity and resistance to chemical and thermal degradation, crystalline MOFs typically exhibit insolubility, infusibility, stiffness and brittleness, and therefore their membrane-processing properties are far inferior to the flexible amorphous polymers and hinder their subsequent storage, transportation, and utilization. Hence, focusing on film-formation and crystallization is the foundation for exploring the fabrication and application of MOF membranes. In this review, the film-forming properties of crystalline MOFs are fundamentally analyzed from their inherent characteristics and compared with those of amorphous polymers, influencing factors of polycrystalline MOF membrane formation are summarized, the trade-off relationship between crystallization and membrane formation is discussed, and the strategy solving the film formation of crystalline MOFs in recent years are systematically reviewed, in anticipation of realizing the goal of preparing crystalline membranes with optimized processability and excellent performance.

Steering diffusion selectivity of chemical isomers within aligned nanochannels of metal-organic framework thin film

The movement of molecules (i.e. diffusion) within angstrom-scale pores of porous materials such as metal-organic frameworks (MOFs) and zeolites is influenced by multiple complex factors that can be challenging to assess and manipulate. Nevertheless, understanding and controlling this diffusion phenomenon is crucial for advancing energy-economic membrane-based chemical separation technologies, as well as for heterogeneous catalysis and sensing applications. Through precise assessment of the factors influencing diffusion within a porous metal-organic framework (MOF) thin film, we have developed a chemical strategy to manipulate and reverse chemical isomer diffusion selectivity. In the process of cognizing the molecular diffusion within oriented, angstrom-scale channels of MOF thin film, we have unveiled a dynamic chemical interaction between the adsorbate (chemical isomers) and the MOF using a combination of kinetic mass uptake experiments and molecular simulation. Leveraging the dynamic chemical interactions, we have reversed the haloalkane (positional) isomer diffusion selectivity, forging a chemical pathway to elevate the overall efficacy of membrane-based chemical separation and selective catalytic reactions.

Selective ion transport through hydrated micropores in polymer membranes

Ion-conducting polymer membranes are essential in many separation processes and electrochemical devices, including electrodialysis1, redox flow batteries2, fuel cells3 and electrolysers4,5. Controlling ion transport and selectivity in these membranes largely hinges on the manipulation of pore size. Although membrane pore structures can be designed in the dry state6, they are redefined upon hydration owing to swelling in electrolyte solutions. Strategies to control pore hydration and a deeper understanding of pore structure evolution are vital for accurate pore size tuning. Here we report polymer membranes containing pendant groups of varying hydrophobicity, strategically positioned near charged groups to regulate their hydration capacity and pore swelling. Modulation of the hydrated micropore size (less than two nanometres) enables direct control over water and ion transport across broad length scales, as quantified by spectroscopic and computational methods. Ion selectivity improves in hydration-restrained pores created by more hydrophobic pendant groups. These highly interconnected ion transport channels, with tuned pore gate sizes, show higher ionic conductivity and orders-of-magnitude lower permeation rates of redox-active species compared with conventional membranes, enabling stable cycling of energy-dense aqueous organic redox flow batteries. This pore size tailoring approach provides a promising avenue to membranes with precisely controlled ionic and molecular transport functions.

Yttrium-based metal-organic frameworks built on hexanuclear clusters

Yttrium-based metal-organic frameworks built on hexanuclear clusters (Y6-MOFs) represent an important subgroup of MOFs that are assembled from Y6 clusters and diverse organic linkers, featuring a variety of topologies. Due to the robust Y-O bonds and high connectivity of hexanuclear SBUs, Y6-MOFs are generally thermally stable and resistant to water. Additionally, their pore structures are highly tunable through the practice of the reticular chemistry strategy, resulting in excellent performance in gas adsorption and separation related applications. Y6-MOFs are structurally analogous to Zr6-MOFs; however, the existence of charge-balancing cations in Y6-MOFs serves as an additional pore structure regulator, enhancing their tailorability with respect to pore shape and dimensions. In this Frontier article, we summarize the main advances in the design and synthesis of Y6-MOFs, with a particular focus on the precise engineering of their pore structure for gas separation. Future directions of research efforts in this field are also discussed.

A Bioinspired Membrane with Ultrahigh Li+/Na+ and Li+/K+ Separations Enables Direct Lithium Extraction from Brine

Membranes with precise Li+/Na+ and Li+/K+ separations are imperative for lithium extraction from brine to address the lithium supply shortage. However, achieving this goal remains a daunting challenge due to the similar valence, chemical properties, and subtle atomic-scale distinctions among these monovalent cations. Herein, inspired by the strict size-sieving effect of biological ion channels, a membrane is presented based on nonporous crystalline materials featuring structurally rigid, dimensionally confined, and long-range ordered ion channels that exclusively permeate naked Li+ but block Na+ and K+. This naked-Li+-sieving behavior not only enables unprecedented Li+/Na+ and Li+/K+ selectivities up to 2707.4 and 5109.8, respectively, even surpassing the state-of-the-art membranes by at least two orders of magnitude, but also demonstrates impressive Li+/Mg2+ and Li+/Ca2+ separation capabilities. Moreover, this bioinspired membrane has to be utilized for creating a one-step lithium extraction strategy from natural brines rich in Na+, K+, and Mg2+ without utilizing chemicals or creating solid waste, and it simultaneously produces hydrogen. This research has proposed a new type of ion-sieving membrane and also provides an envisioning of the design paradigm and development of advanced membranes, ion separation, and lithium extraction.

Ion-Ion Selectivity of Synthetic Membranes with Confined Nanostructures

Synthetic membranes featuring confined nanostructures have emerged as a prominent category of leading materials that can selectively separate target ions from complex water matrices. Further advancements in these membranes will pressingly rely on the ability to elucidate the inherent connection between transmembrane ion permeation behaviors and the ion-selective nanostructures. In this review, we first abstract state-of-the-art nanostructures with a diversity of spatial confinements in current synthetic membranes. Next, the underlying mechanisms that govern ion permeation under the spatial nanoconfinement are analyzed. We then proceed to assess ion-selective membrane materials with a focus on their structural merits that allow ultrahigh selectivity for a wide range of monovalent and divalent ions. We also highlight recent advancements in experimental methodologies for measuring ionic permeability, hydration numbers, and energy barriers to transport. We conclude by putting forth the future research prospects and challenges in the realm of high-performance ion-selective membranes.

Covalent Organic Framework Membranes with Patterned High-Density Through-Pores for Ultrafast Molecular Sieving

The creation of uniformly molecular-sized through-pores within polymeric membranes and the direct evidence of these pores are essential for fundamentally understanding the transport mechanism and improving separation efficiency. Herein, we report an electric-field-assisted interface synthesis approach to fabricating large-area covalent organic framework (COF) membranes that consist of preferentially oriented single-crystalline COF domains. These single-crystalline frameworks were translated into high-density, vertically aligned through-pores across the entire membrane, enabling the direct visualization of membrane pores with an ultrahigh resolution of 2 Å using the low-dose high-resolution transmission electron microscopy technique (HRTEM). The density of directly visualized through-pores was quantified to be 1.2 × 1017 m-2, approaching theoretical predictions. These COF membranes demonstrate ultrahigh solvent permeability, which is 10 times higher than that of state-of-the-art organic solvent nanofiltration membranes. When applied to high-value pharmaceutical separations, their COF membranes exhibit 2 orders of magnitude higher methanol permeance and 20-fold greater enrichment efficiency than their commercial counterparts.

Confined-Coordination Induced Intergrowth of Metal-Organic Frameworks into Precise Molecular Sieving Membranes

Metal-organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect-free high-connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined-coordination induced intergrowth strategy to fabricate lattice-defect-free Zr-MOF membrane towards precise molecular separation. The confined-coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter-diffusion of MOF precursors (metal cluster and ligand), thereby inter-growing MOF crystals into integrated membrane. The resulting Zr-MOF membrane with angstrom-sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m-2 ⋅ h-1 ⋅ bar-1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state-of-the-art membranes. The molecular transport mechanism related to size-sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

Molecular-Sieving Separation of Methanol/Benzene Azeotrope by a Flexible Metal-Organic Framework

Separation of methanol/benzene azeotrope mixtures is very challenging not only by the conventional distillation technique but also by adsorbents. In this work, we design and synthesize a flexible Ca-based metal-organic framework MAF-58 consisting of cheap raw materials. MAF-58 shows selective methanol-induced pore-opening flexibility. Although the opened pores are large enough to accommodate benzene molecules, MAF-58 shows methanol/benzene molecular sieving with ultrahigh experimental selectivity, giving 5.1 mmol g-1 high-purity (99.99%+) methanol and 2.0 mmol g-1 high-purity (99.97%+) benzene in a single adsorption/desorption cycle. Computational simulations reveal that the preferentially adsorbed, coordinated methanol molecules act as the gating component to selectively block the diffusion of benzene, offering a new gating adsorption mechanism.

Phase Engineering of Zirconium MOFs Enables Efficient Osmotic Energy Conversion: Structural Evolution Unveiled by Direct Imaging

Creating structural defects in a controlled manner within metal-organic frameworks (MOFs) poses a significant challenge for synthesis, and concurrently, identifying the types and distributions of these defects is also a formidable task for characterization. In this study, we demonstrate that by employing 2-sulfonylterephthalic acid as the ligand for synthesizing Zr (or Hf)-based MOFs, a crystal phase transformation from the common fcu topology to the rare jmt topology can be easily facilitated using a straightforward mixed-solvent strategy. The jmt phase, characterized by an extensively open framework, can be considered a derivative of the fcu phase, generated through the introduction of missing-cluster defects. We have explicitly identified both MOF phases, their intermediate states, and the novel core-shell structures they form using ultralow-dose high-resolution transmission electron microscopy. In addition to facilitating phase engineering, the incorporation of sulfonic groups in MOFs imparts ionic selectivity, making them applicable for osmotic energy harvesting through mixed matrix membrane fabrication. The membrane containing the jmt-phase MOF exhibits an exceptionally high peak power density of 10.08 W m-2 under a 50-fold salinity gradient (NaCl: 0.5 M|0.01 M), which surpasses the threshold of 5 W m-2 for commercial applications and can be attributed to the combination of large pore size, extensive porosity, and abundant sulfonic groups in this novel MOF material.

Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and "ion-channel" interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

Hydrogen-bonded organic frameworks for membrane separation

Hydrogen-bonded organic frameworks (HOFs) are a new class of crystalline porous materials that are formed through the interconnection of organic or metal-organic building units via intermolecular hydrogen bonds. The remarkable flexibility and reversibility of hydrogen bonds, coupled with the customizable nature of organic units, endow HOFs with mild synthesis conditions, high crystallinity, solvent processability, and facile self-healing and regeneration properties. Consequently, these features have garnered significant attention across various fields, particularly in the realm of membrane separation. Herein, we present an overview of the recent advances in HOF-based membranes, including their advanced fabrication strategies and fascinating applications in membrane separation. To attain the desired HOF-based membranes, careful consideration is dedicated to crucial factors such as pore size, stability, hydrophilicity/hydrophobicity, and surface charge of the HOFs. Additionally, diverse preparation methods for HOF-based membranes, including blending, in situ growth, solution-processing, and electrophoretic deposition, have been analyzed. Furthermore, applications of HOF-based membranes in gas separation, water treatment, fuel cells, and other emerging application areas are presented. Finally, the challenges and prospects of HOF-based membranes are critically pointed out.

Cation-Intercalated Clay-Based Two-Dimensional Membranes for Effective Desalination and Molecule Sieving

Montmorillonite (MMT) is known as an ion-exchangeable material, and cations between MMT nanosheets are easily exchanged by other cations. In this work, Ca2+, Fe3+, and Al3+ intercalated two-dimensional MMT membranes were developed by ion exchange of pristine MMT membranes (Na+-MMT), and their ion and dye removal abilities were investigated. The d-spacings of hydrated Fe3+ intercalated MMT membrane (Fe3+-MMT) and Al3+ intercalated MMT membrane (Al3+-MMT) were decreased compared with hydrated Na+-MMT membrane due to the stronger electrostatic attraction between Fe3+/Al3+ and negatively charged MMT nanosheets. Ion and dye sieving performances were improved significantly after the intercalation of Ca2+, Fe3+, and Al3+ into MMT membranes. Al3+-MMT membrane with a thickness of 1.17 μm could exclude 94% of Na+, and its ion sieving performance remained stable during a 120-h ion sieving experiment. Moreover, the rejection rate for rhodamine B (RB) reached 94% using an Al3+-MMT membrane with a thickness of 500 nm, and a water permeance of 73 L m-2 h-1 bar-1 was achieved. The excellent ion and dye sieving performances make it promising in the application of desalination and nanofiltration.

Radiation-Induced De Novo Defects in Metal-Organic Frameworks Boost CO2 Sorption

Defects in metal-organic frameworks (MOFs) can significantly change their local microstructures, thus notably leading to an alteration-induced performance in sorption or catalysis. However, achieving de novo defect engineering in MOFs under ambient conditions without the scarification of their crystallinity remains a challenge. Herein, we successfully synthesize defective ZIF-7 through 60Co gamma ray radiation under ambient conditions. The obtained ZIF-7 is defect-rich but also has excellent crystallinity, enhanced BET surface area, and hierarchical pore structure. Moreover, the amount and structure of these defects within ZIF-7 were determined from the two-dimensional (2D) 13C-1H frequency-switched Lee-Goldburg heteronuclear correlation (FSLG-HETCOR) spectra, continuous rotation electron diffraction (cRED), and high-resolution transmission electron microscopy (HRTEM). Interestingly, the defects in ZIF-7 all strongly bind to CO2, leading to a remarkable enhancement of the CO2 sorption capability compared with that synthesized by the solvothermal method.

Solid-solvent processing of ultrathin, highly loaded mixed-matrix membrane for gas separation

Mixed-matrix membranes (MMMs) that combine processable polymer with more permeable and selective filler have potential for molecular separation, but it remains difficult to control their interfacial compatibility and achieve ultrathin selective layers during processing, particularly at high filler loading. We present a solid-solvent processing strategy to fabricate an ultrathin MMM (thickness less than 100 nanometers) with filler loading up to 80 volume %. We used polymer as a solid solvent to dissolve metal salts to form an ultrathin precursor layer, which immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF) and provides adhesion to the MOF in the matrix. The resultant membrane exhibits fast gas-sieving properties, with hydrogen permeance and/or hydrogen-carbon dioxide selectivity one to two orders of magnitude higher than that of state-of-the-art membranes.

Anisotropic flexibility and rigidification in a TPE-based Zr-MOFs with scu topology

Tetraphenylethylene (TPE)-based ligands are appealing for constructing metal-organic frameworks (MOFs) with new functions and responsiveness. Here, we report a non-interpenetrated TPE-based scu Zr-MOF with anisotropic flexibility, that is, Zr-TCPE (H4TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene), remaining two anisotropic pockets. The framework flexibility is further anisotropically rigidified by installing linkers individually at specific pockets. By individually installing dicarboxylic acid L1 or L2 at pocket A or B, the framework flexibility along the b-axis or c-axis is rigidified, and the intermolecular or intramolecular motions of organic ligands are restricted, respectively. Synergistically, with dual linker installation, the flexibility is completely rigidified with the restriction of ligand motion, resulting in MOFs with enhanced stability and improved separation ability. Furthermore, in situ observation of the flipping of the phenyl ring and its rigidification process is made by 2H solid-state NMR. The anisotropic rigidification of flexibility in scu Zr-MOFs guides the directional control of ligand motion for designing stimuli-responsive emitting or efficient separation materials.

Enhancing Nanofiltration Selectivity of Metal-Organic Framework Membranes via a Confined Interfacial Polymerization Strategy

Development of well-constructed metal-organic framework (MOF) membranes can bring about breakthroughs in nanofiltration (NF) performance for water treatment applications, while the relatively loose structures and inevitable defects usually cause low rejection capacity of MOF membranes. Herein, a confined interfacial polymerization (CIP) method is showcased to synthesize polyamide (PA)-modified NF membranes with MOF nanosheets as the building blocks, yielding a stepwise transition from two-dimensional (2D) MOF membranes to polyamide NF membranes. The CIP process was regulated by adjusting the loading amount of piperazine (PIP)-grafted MOF nanosheets on substrates and the additional content of free PIP monomers distributed among the nanosheets, followed by the reaction with trimesoyl chloride in the organic phase. The prepared optimal membrane exhibited a high Na2SO4 rejection of 98.4% with a satisfactory water permeance of 37.4 L·m-2·h-1·bar-1, which could be achieved by neither the pristine 2D MOF membranes nor the PA membranes containing the MOF nanosheets as the conventional interlayer. The PA-modified MOF membrane also displayed superior stability and enhanced antifouling ability. This CIP strategy provides a novel avenue to develop efficient MOF-based NF membranes with high ion-sieving separation performance for water treatment.

A Mini Review of Ceramic-Based MOF Membranes for Water Treatment

Ceramic membranes have been increasingly employed in water treatment owing to their merits such as high-stability, anti-oxidation, long lifespan and environmental friendliness. The application of ceramic membranes mainly focuses on microfiltration and ultrafiltration processes, and some precise separation can be achieved by introducing novel porous materials with superior selectivity. Recently, metal-organic frameworks (MOFs) have developed a wide spectrum of applications in the fields of the environment, energy, water treatment and gas separation due to the diversity and tunable advantages of metal clusters and organic ligands. Although the issue of water stability in MOF materials inhibits the development of MOF membranes in water treatment, researchers still overcome many obstacles to advance the application of MOF membranes in water treatment processes. To the best of our knowledge, there is still a lack of a reviews on the development process and prospects of ceramic-based MOF membranes for water treatment. Therefore, in this review, we mainly summarize the fabrication method for ceramic-based MOF membranes and their application in water treatment, such as water/salt separation, pollutant separation, heavy metal separation, etc. Following this, based on the high structural, thermal and chemical stability of ceramic substrates, and the high controllability of MOF materials, the superiority and insufficient use of ceramic-based MOF membranes in the field of water treatment are critically discussed.