Recent progress in 2D and 3D metal–organic framework-based membranes for water sustainability

Abstract

Metal–organic frameworks (MOFs) have emerged as promising candidates for high-performance separation processes due to their desirable porous structure and highly tunable properties. However, the efficient separation of dyes from industrial wastewater, salts from seawater, and oil/water mixtures remains a critical challenge in many industrial processes. This review focuses on synthesizing two-dimensional (2D) and three-dimensional (3D) MOF-based membranes. Then, we discuss the physicochemical properties, such as tunable pore sizes, surface modifications, and selectivity enhancement of both 2D and 3D MOF-based membranes that make them capable of achieving superior separation performance. Further, the applications of 2D and 3D MOF-based membranes for salt, dye, and oil–water separations are discussed in detail. Furthermore, we examine the separation mechanisms, performance evaluation metrics, and case studies that demonstrate the effectiveness of MOF-based membranes. Additionally, we summarize the current challenges associated with both 2D and 3D MOF-based membranes, including their stability under harsh conditions, fouling, scalability, and cost-effectiveness.

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Water impact

Water is critical to human health, economic development, and agricultural productivity. Globally, billions of people do not have access to safe drinking water. All of these issues are becoming more severe as the Earth's temperature rises and population increases. As a result, water shortages are a critical concern for both developing and developed countries. To address this issue, different purifying procedures have been investigated. Recently, the use of two-dimensional metal organic frameworks (MOFs) has emerged as a promising approach for the development of efficient separation membranes in many applications, such as water desalination, treatment of industrial effluent, and solvent/water separation. This is primarily attributed to the hydrophilic nature of their surface, together with their notable mechanical and chemical durability, as well as their inherent flexibility. This article provides a comprehensive overview of the current fabrication techniques employed for the production of MOF-based lamellar membrane. Furthermore, it explores the many uses of these membranes in the field of water desalination.

1. Introduction

MOFs are a fascinating class of porous materials that are composed of metal ions (nodes) or clusters coordinated with organic ligands (secondary building blocks), forming a highly porous crystalline framework.^1–3 A controllable porosity (0.5 nm to 2.0 nm), high stability, and well-defined pores provide significant advantages in energy storage and conversion, biomedicines, chemical sensors, photocatalysis, gas storage and separation, and water desalination applications.^4–6 There are many ways to classify MOFs, e.g., Isoreticular metal–organic frameworks (IRMOFs), zeolitic imidazolate framework materials (ZIFs), and porous coordination networks (PCNs), depending on the source of the configuration (Fig. 1). Some of the time, MOFs can also be named according to the name of the researcher, place or university that discovered them.

Fig. 1 Representative MOF types. Schematic illustrations of the metal centers (secondary building blocks), organic linkers, unit cells, and frameworks of IRMOF-1 (a), ZIF-8 (b), MIL-101(Cr) (c), PCN-222(Fe) (d), and UiO-66 (e). Reproduced from ref. 7 with permission from Nature-Springer, copyright 2021.

In recent years, the integration of MOFs into membranes has provided a highly tailorable structure with precise pore sizes and diverse functionalities.^8,9 The obtained materials have unique pore sizes and surface areas by adding different metals with different valences. This could depend on the type of metal, its valency, nature of ligand and the final structure of the MOFs.⁷ Therefore, these methods offer highly selective and efficient separations, including salt, dye, and oil–water separations, and other fields.^10–12 In the petrochemical industry, MOF-based membranes have shown great potential for separating different hydrocarbon gases, such as ethylene and propylene, which are crucial for the production of plastics and other industrial chemicals. Furthermore, the tunable porosity of MOFs allows for precise control of the size, shape, and functionality of the pores, enabling selective separation of molecules based on size, shape, or chemical affinity and enabling the separation of closely related compounds with high precision.^13–15 The ability to customize the pore size and surface functionality of MOFs allows for enhanced selectivity and efficient separation processes. The large surface area provides a significant number of active sites for molecular adsorption, facilitating high adsorption capacities and efficient mass transfer across the membrane. This property is particularly advantageous in separation processes where high flux rates are desired. The high surface area also allows interactions with a wide range of molecules, enabling the tailoring of MOFs for specific applications.^16,17 MOF-based membranes also exhibit high chemical stability, making them suitable for various water-based separations. The strong metal–ligand bonds in MOFs contribute to their robustness and resistance to harsh chemical environments. Unlike conventional polymeric membranes, which may degrade or lose performance under aggressive conditions, MOF-based membranes can maintain their structural integrity and separation performance. This chemical stability endows MOF-based membranes with strong resistance to withstand corrosive substances, extreme pH levels, and high temperatures, leading to extended operational lifetimes and reduced maintenance costs.^18,19

In this review, we comprehensively summarize the synthesis methods employed to fabricate 2D and 3D MOF-based membranes and highlight the diverse approaches used to prepare well-defined and defect-free membranes. Then, we discussed the applications of these materials in salt, oil/water, and dye separations. Furthermore, we discuss the current challenges associated with these membranes, especially focus in their stability under harsh conditions, fouling, scalability, and cost-effectiveness. We hope this review will open new research directions for the scientific community to utilize MOFs-based membranes in various energy, environmental, and medical applications.

2. Synthetic methods for MOF-based membranes

2.1. Synthesis of 3D MOF membranes

MOF-based membranes are typically synthesized using various methods that allow for the controlled growth and formation of MOF layers on a suitable substrate. The choice of synthetic method depends on the desired MOF material, substrate compatibility, and targeted membrane properties.²⁰ This section discusses common synthetic methods employed for MOF-based membranes.

2.1.1. Solvothermal method. The solvothermal method²¹ is one of the most commonly used technique for the preparation of MOF membranes (Fig. 2a). However, this process involves the interaction of metal ions or clusters with organic ligands within a high-pressure autoclave system operating at elevated temperature. An autoclave contains a solvent or mixture of solvents, which act as both the reaction medium and the source of the organic ligands. Under solvothermal conditions, the nucleation and subsequent growth of MOF crystals occur on the substrate, ultimately leading to the creation of continuous MOF-based membranes. This method offers several advantages, including the ability to obtain high-quality, defect-free MOF membranes with good adhesion to the substrate (Fig. 2b–d). Moreover, the reaction parameters, including temperature, pressure, solvent composition, and reaction duration, may be systematically fine-tuned to exert precise control over the size, morphological characteristics, and orientation of the resulting MOF-based membrane. However, this method may require careful selection of solvents and reaction conditions to prevent the formation of undesired MOF phases or the occurrence of secondary reactions.^18,22,23

Fig. 2 (a) A schematic illustration of the surface modification and subsequent fabrication of ZIF-8 MOF (https://www.sciencedirect.com/topics/chemistry/metal-organic-framework) membranes across porous metal substrates by the solvothermal method. SEM images showing (b) the porous stainless-steel substrate, (c) the surface of the solvothermal-ZIF-8 membrane, (d) a view of the interface between the stainless-steel substrate and solvothermal-ZIF-8 crystals, (d) the surface of the RTD-ZIF-8 membrane, (f) a higher magnification image for (e), and (g) a view of the interface between the stainless-steel substrate and the RTD ZIF-8 crystals. Reproduced from ref. 24 with permission from Elsevier, copyright 2022.

It is very challenging to fabricate MOF-based membranes on microporous metal substrates due to the lack of reactive functional groups on the surface, poor wettability, and large pore sizes across the substrate, which prevent high-density nucleation of MOF crystals on the substrate. Maina et al. fabricated ZIF-8 crystals on a substrate made up of stainless steel (44 × 40 μm pore size) and functionalized them with amine-based functional groups through solvothermal and rapid thermal deposition methods. In this work, the authors analyzed the effect of the synthesis method on the microstructure of the membrane and its performance. In comparison with the non-modified substrate, the modified substrate produced highly dense, well-interconnected crystals via the solvothermal method, as shown in Fig. 2.²⁴ This method provides completely different morphologies, as shown in the above SEM images (Fig. 2b–g). The microstructure-based membranes obtained through this method were mainly composed of blocks ranging in size from 36 to 46 μm. Interestingly, Li et al.²⁵ carried out a study in which they investigated the controllable synthesis of MOFs and the manipulation of MOF films through solvothermal synthesis of microstructure-based membranes. The authors introduced a solvothermal methodology that offers a means to achieve precise tailoring of the crystal dimensions and structural morphology of MOF materials. Additionally, this approach allows the manipulation of the orientation of MOF films through an evolutionary selection process facilitated by randomly oriented seed layers. This study also highlights the importance of highly aligned MOF thin films in the context of molecular sieve membrane applications for wastewater treatment. Notably, the authors observed a correlation between the microstructure and the membrane performance of ZIF-7 membranes, highlighting its importance in developing high-quality MOF membranes. However, the study lacked a detailed analysis of the MOF membrane's performance in practical applications.

2.1.2. Secondary growth method. The secondary growth technique involves the process of promoting MOF crystals upon the deposition of a pre-established seed layer onto the substrate. The seed layer can be prepared using various techniques, such as physical vapor deposition, liquid-phase deposition, or solution-based methods. Once the seed layer is formed, it serves as a template for the growth of MOF crystals in a subsequent growth step. Moreover, the secondary growth approach affords superior command over both the nucleation and crystal growth phases, thereby yielding improved membrane quality and uniformity over the pristine substrate. By manipulating the growth parameters, such as precursor concentration, reaction time, and temperature, it is possible to tune the crystal size, morphology, and thickness of the MOF-based membranes. This method also allows for the use of different seed layers and sequential growth steps to achieve more complex MOF architectures and composite membranes.^26,27 Liu et al. focused on the synthesis of thin MOF membranes on porous inorganic or polymer supports. The authors investigated two methods: seeded secondary growth and in situ synthesis using counter-diffusion. These methods are considered relatively economical for MOF synthesis. However, the authors suggested the need to develop more scalable and cost-effective strategies to achieve the synthesis of pure MOFs.²⁸Fig. 2 shows the results of SEM investigations conducted on a membrane synthesized over a 24 hour period. A continuous film was successfully produced, with noticeable intergrowth between individual grains evident in the results. When examining the cross-sectional view, it becomes apparent that the membrane comprises interlocked grains, with each individual grain representing a single-crystal structure. The membrane thickness is approximately 25 μm, and the defect-free nature was also confirmed by taking high-resolution images with SEM. The main drawback of secondary growth synthesis is the time-consuming nature of the process. An extension of the synthesis time hampers the scalability and practicality of the method, especially when considering the large-scale production of MOF membranes for industrial applications. Additionally, secondary growth synthesis techniques for MOF-based membranes are limited primarily to a few specific MOF materials because of defects in the membrane. While successful examples have been reported for certain MOFs, Zhao et al. introduced seeding-size-reduced MOF-5 crystals followed by secondary growth synthesis.^29–31 In addition, the quality of the membrane was assessed utilizing a molecular probe, which revealed an insignificant pervaporation flux, indicating that the absence of defects exceeded the dimensions of the MOF-5 crystalline pores. It remains challenging to extend this method to a broader range of MOF structures and compositions.

2.1.3. Seeding method. The seeding method involves the direct deposition of MOF crystals onto the substrate using a preformed MOF seed suspension (Fig. 3a–c). The seed suspension is typically prepared by synthesizing MOF particles in solution and dispersing them onto the substrate surface. The seeds act as nucleation sites, facilitating the growth of MOF crystals and the formation of a continuous membrane. The seeding method offers advantages in terms of simplicity and scalability. This allows for the rapid formation of MOF-based membranes on large-area substrates, and these membranes can be easily adapted to different MOF materials. The seed size, concentration, and suspension conditions can be optimized to control crystal growth and membrane thickness. However, careful attention must be given to the uniformity and coverage of the seed layer to ensure the formation of a defect-free membrane.^32–34 A novel approach termed reactive seeding (RS) was introduced to facilitate the fabrication of continuous MOF-based membranes on porous alumina supports (Fig. 3a).³⁵ In this approach, the porous support acts as an inorganic source that engages in a reaction with the organic precursor to generate a seeding layer. The integrity of the resulting MIL-53 membrane was demonstrated through single-gas permeation experiments. Remarkably, the prepared MIL-53 membrane exhibited exceptional selectivity for dehydrating an azeotropic mixture of ethyl acetate and water through pervaporation. These significant findings open up new prospects for utilizing MOF-based membranes in liquid mixture separation applications.

Fig. 3 (a) Schematic diagram of the synthesis of MOFs by the secondary growth method. (b and c) Top and cross-sectional views of MOF-based membranes, respectively. Reproduced from ref. 28 with permission from Elsevier, copyright 2009.

2.1.4. In situ growth method. The in situ growth approach encompasses the direct synthesis of 3D MOF-based membranes on the substrate surface, eliminating the necessity for preformed seed layers. In this approach, the substrate is immersed in a reaction solution that comprises metal ions, organic ligands, and solvents. Under appropriate conditions, MOF crystals nucleate and grow directly on the substrate, forming a continuous membrane. The in situ growth method is simple and versatile because it eliminates the need for seed layers and additional processing steps. This allows for the direct integration of MOF membranes onto various substrate materials and geometries. Furthermore, in situ, the synthesis is followed by either layer-by-layer growth or template-assisted growth, as depicted in Fig. 4a and b. Growth parameters, such as reactant concentration, solvent composition, and reaction time, can be adjusted to control the membrane.^36,37 Depending on the specific substrates employed, in situ growth methods can be classified into two categories: direct growth methods and modified substrate methods.

Fig. 4 (a) Schematic diagram of the preparation of the MIL-53 membrane on an alumina support via the RS method (1–3). (b and c) Surface and cross-sectional SEM images of the MIL-53 membrane. Reproduced from ref. 35 with permission from Royal Society of Chemistry, copyright 2011.

(i) The direct growth method. The direct growth process pertains to the synthesis of 3D MOF-based membranes employing unaltered or unmodified substrates. Typically, hydrothermal or solvothermal reactions provide the necessary heat for this preparation method. In this approach, the substrate is placed in direct contact with the precursor solution, which facilitates MOF crystal growth and nucleation on the surface of the substrate (Fig. 5c). Previous studies have successfully prepared MOF-5 continuous membranes on unmodified alumina substrates using hydrothermal or solvothermal methods.²⁸ In another study, a porous TiO₂ substrate was incorporated in zinc chloride, sodium formate, and 2-methylimidazole solutions. After heating and washing, ZIF-8 membranes were obtained.³⁸ Despite its simplicity, the direct synthesis method is not widely adopted because of its limitation in providing an adequate number of nucleation sites on the substrate. This limitation may cause weaker bonding between the substrate and MOF crystals.³⁹

Fig. 5 Schematic diagram of the in situ method. (a) In situ layer-by-layer growth of MOFs. (b) Template-assisted in situ growth of MOFs. (c) Schematic diagram of the synthesis of MOF-based membranes by direct growth. (d) Schematic diagram of the modified substrate method for the fabrication of MOF membranes.

(ii) Modified substrate method. To address the issue of weak bonding between a MOF and a substrate, researchers have explored substrate modification as a more effective method. Organic and inorganic compound modifications are commonly employed. Polydopamine is frequently used for organic modification because it enhances the compatibility and interaction between the MOF filler and the substrate, resulting in improved membrane continuity.⁴⁰ Using PDA-modified α-alumina as the substrate, a continuous ZIF-100 membrane with denser and thicker materials was obtained through a thermal reaction and repeated washing.⁴¹ Zinc (Zn) ion-doped PDA-modified substrates have also been used to fabricate ZIF-8-based membranes, which are more crystalline and more ordered than conventional ZIF-based membranes and have shorter synthesis times.⁴² In the case of inorganic compound modification, the same metal compound as that present in the MOF-based membrane is typically selected. Zn-based sol was applied to the substrate to obtain ZnO, which serves as a metal source and provides nucleation and anchoring sites for the growth of ZIF nanosheet membranes, resulting in stable and continuous membranes.⁴³ Likewise, by employing zinc oxide as the metal source and ammonium oxide as a modifier, ultrathin ZIF-based membranes were effectively fabricated, demonstrating favorable permeability and selectivity for hydrogen gas.⁴⁴ By utilizing the same metal oxides as modification materials, greater binding between the membrane and the substrate is achieved, leading to denser membranes. The use of organic molecules in substrate modification provides nucleation sites and enhances the binding force with MOFs, while inorganic molecules facilitate membrane growth on the substrate surface due to their nanostructure and increased surface area.^30,45 In addition to the previously mentioned in situ growth methods, novel approaches, such as the freeze-assisted growth technique, have emerged. In this method, the substrate is submerged in a metal solution by rapid freezing in liquid nitrogen and subsequently immersed in a solution containing 2-methylimidazole to facilitate MOF membrane formation. The freezing process facilitates the development of ZIF-8 particles within the pores of the substrate, obviating the necessity for a densely packed separation layer on the surface of the membrane.⁴⁶ Electrochemically assisted in situ growth is another approach in which metal plates replace the solvent, and the metal ions required for the reaction are continuously deposited on the substrate surface, resulting in a continuous film. The simple approach for modified substrate synthesis is explained in the scheme given in Fig. 5d.

2.1.5. Electrochemical methods. The electrochemical method offers several advantages over other synthetic methods to make 3D MOFs, making it a preferred choice due to its room temperature operation, low energy consumption, and environmentally friendly characteristics.⁴⁷ This method also boasts a short reaction time and requires simple and convenient equipment. In recent years, electrochemical methods have focused primarily on anode and cathode synthesis.⁴⁵ Moreover, the anode precipitation method involves the application of a high positive voltage to dissolve the metal located at the anode, whereby the resultant metal ions subsequently react with the ligand, culminating in the formation of 3D MOF membranes.^48,49 However, when high-valent cations are used in MOF-based membranes, a higher reaction temperature may be needed. In a recent scientific investigation, the authors employed a high-temperature and high-pressure battery setup to effectively fabricate MOF-based membranes incorporating cations with a high valence state. The use of an environmentally friendly and noncorrosive solution in this method has yielded promising results.⁵⁰ In the cathodic deposition method, metal sources for cathodic deposition are typically obtained by adding metal salts. However, the cathodic precipitation method can lead to impurities and hinder the formation of MOF particles. To address this issue, researchers proposed a method using hydrogen peroxide to prepare MOF membranes. The oxidation of hydrogen peroxide to superoxide aids in the deprotonation of the OH⁻ ligand, effectively preventing coprecipitation of metals and yielding high-purity MOF-based membranes.^51,52 Additionally, traditional electrochemical methods involve the use of various solvents, which can result in environmental damage. To mitigate this, a cathodic precipitation method utilizing water as the sole solvent was introduced. This approach yields a ZIF-8 membrane with low defect density after only one hour, presenting a simple and pollution-free technique for MOF membrane preparation.⁵³ He et al. synthesized 2D Cu-CAT-1, and a unique MOF membrane was directly grown on a copper mesh by a one-step electrochemical method, as shown in Fig. 6 for the synthetic scheme.⁵⁴ This membrane shows excellent separation efficiency in crude oil/water separation due to its high wettability and superhydrophilicity. Moreover, such a high-performance membrane can be readily synthesized through electrochemical methods for 20 min. In summary, the primary difference between the cathode synthesis method and the anode synthesis method lies in the origin of the metal ions utilized in the process. The cathode synthesis method involves the addition of metal ions directly, while the anode synthesis method relies on the dissolution of the electrode. The hydroxide ions (OH⁻) produced at the cathode deprotonate the ligand, thereby promoting self-assembly of the ligand with metal ions present on the electrode surface, ultimately resulting in the formation of 3D MOF membranes.^55,56 Membranes produced using the anodic deposition method tend to exhibit a greater propensity for direct formation on the substrate, while the cathodic deposition method predominantly promotes membrane formation on the electrodes themselves (Table 1).⁵⁷

Fig. 6 Scheme of the preparation of Cu-CAT-1@CM and the separation process for oil–water extraction. Reproduced with permission from ref. 54.

Table 1 Comparison of synthesis methods for MOF-based membranes

Synthesis methods	Merits	Demerits	Ref.
In situ growth	Easy to prepare	High energy consumption	58
Secondary growth method	Can produce a more dense and continuous membrane	A complex process, difficult to produce on a large scale	59
Electrochemical method	Easy and precise control membrane structure by altering the voltage	It is necessary to ensure continuous contact between the metallic pattern and the power	48, 58
Solvo-thermal method	High crystalline and pure MOF	Harsh condition required with longer time	60
Secondary growth method	Particle size control, can modify	Complex experimental setup	61
Seeding method	Nucleation control so MOFs with several size can be obtained	Sensitive to quality of seed crystal so chance of contamination	62
Direct growth	One step synthesis	Limited control over morphology	63
Modified substrate	Functionalize substrate targeted application	Effects MOF growth	26

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2.1.6. Post-synthetic modifications (PSM). The PSM technique is employed to enhance the properties and functionalities of 3D MOF-based membranes after synthesis. PSM involves the introduction of additional functional groups to the MOF structure or modify it through chemical reactions or physical processes. This approach allows for the fine-tuning of membrane properties, such as surface chemistry, pore size, and hydrophobicity/hydrophilicity, to achieve improved separation performance. PSM offers a versatile and flexible approach to optimize the performance of MOF-based membranes.²¹ These methods enable tailoring of membrane properties to specific separation requirements, enhance stability, improve compatibility with different solvents or environments, and extend the range of separable species. Studies focused on modifying MOFs by introducing functional groups into materials, such as IRMOF-1 and UiO-66. Functional groups were incorporated into the MOFs' linkers to tailor their properties.⁶⁴

Covalent and coordinative modifications are main two main types of post modification of MOFs. The covalent PSM has emerged as a versatile approach for introducing functional groups into MOFs. Amines, aldehydes, and azides are commonly used as reactive moieties. PSM led to the discovery of MOFs with desirable properties, such as catalytic and gas separation capabilities. PSM was also applied to create hybrid MOF–polymer materials, combining the advantages of MOFs with the flexibility of polymers. Various methods, including mixed matrix membrane preparation and post synthetic polymerization, have been employed to synthesize MOF–polymer hybrid materials. These approaches improve the performance and separation capabilities of hybrid materials. MOF-templated polymeric gels were also developed using click reactions and acid treatment. Furthermore, three common post synthetic strategies are shown in Fig. 7a.⁶⁵ Covalent PSMs play a significant role in developing hierarchical MOFs with diverse pore sizes and environments. Different strategies have been pursued, including selective etching of MOF SBUs or linkers, labialization, and photolytic techniques. A modular programming approach was adopted to synthesize hierarchical MOF–polymer hybrid materials by sequentially growing MOFs and cross-linking them with polymers.^66–68 Coordination chemistry offers an alternative approach to PSM by modifying the secondary building units of MOFs. Chelating molecules such as carboxylates and phosphates were used to selectively decorate MOF surfaces, resulting in core–shell structures. Polymer grafting methods involve densely coating MOFs with polymer chains, altering their bulk properties. These approaches were also applied to develop MOF-based porous liquids by combining the stability of MOFs with the fluidity of liquids, as shown in Fig. 7b. While a coordinative PSM was successfully employed to attach biomolecules, such as lipids and nucleic acids, to the surface of MOFs. Phosphate groups are used to immobilize biomolecules on MOF surfaces. Lipid coatings facilitate targeted drug delivery, while nucleic acids enable self-assembly and cellular entry of MOFs. These advancements in coordinative PSMs open up new possibilities for functional MOFs and biomedical applications.⁶⁹ An example of a coordinative PSM is the selective decoration of MOF surfaces with chelating molecules to create core–shell structures. This approach was demonstrated by Nguyen and coworkers, who utilized carboxylate ligands to modify the surface of a Zr-based MOF. The carboxylate groups are selectively coordinated with the metal nodes of the MOF, resulting in a core–shell structure with a modified shell layer. This approach allows for the controlled functionalization of MOFs, providing enhanced properties and potential applications in areas such as catalysis and gas separation.^70,71

Fig. 7 (a) Representation of covalent (left), coordinate covalent (middle), and a combination of PSM strategies (right) for the PSM of MOFs. Reproduced from ref. 65 with permission from Royal Society of Chemistry, copyright 2011. (b) Illustrative schematic for the synthesis of phospholipid MOFs. A phospholipid coordinative molecule is first linked at the MOF SBU. Subsequently, the favorable interactions at the MOF result in the formation of colloidally stable MOFs as a function of hydrophobic lipid assembly. Reproduced from ref. 64 with permission from American Chemical Society, copyright 2011.

In summary, covalent and coordinative PSMs have revolutionized the field of MOFs by enabling the introduction of functional groups, tailoring properties, creating hybrid materials, synthesizing hierarchical structures, and attaching biomolecules. These approaches offer versatile strategies for developing MOFs with enhanced functionalities and applications in various fields. However, it is important to note that PSMs may introduce additional challenges, such as changes in membrane morphology, loss of selectivity, or decreased permeability. Therefore, careful optimization and characterization of the modified membranes are necessary to ensure the desired improvements without compromising their overall performance.

2.2. Synthesis of 2D MOFs nanosheet membranes

In the architecture of 3D MOFs, the metal or metal cluster nodes coordinate with organic ligands in all orientations due to the directionality of coordination bonds. However, in certain MOFs, particularly UiO MOFs, linker or node deficiencies may be observed.^39,40 Consequently, MOFs crystals tend to grow in all directions from the cores, forming bulky polyhedrons or other crystals with irregular shapes, as observed in various natural or artificial crystals.^41–43 Nevertheless, the growth of MOFs crystals can be constrained under specific conditions, where ligands are unable to coordinate with the metal nodes in all directions. For example, when the structure of MOFs crystals can only grow along the a and b axes, and the growth along the c axis is suppressed through amenable methods, ultrathin MOF nanosheets (MNS) with a high aspect ratio can be achieved.^44,45 Based on this concept, two primary strategies, designated as “top-down” and “bottom-up”, are commonly employed to obtain MNS.^46,47 The former strategy involves exfoliating MNS from pristine bulky MOFs using chemical or physical assistance, such as sonication and small molecular compounds. On the other hand, the latter strategy entails the direct synthesis of MNS from the outset with the aid of surfactants or phase interfaces.⁴⁸

2.2.1. Top-down synthesis. Studies showed that certain MOFs exhibit layered crystal structures, where ligands and nodes are interconnected by strong coordination bonds, while adjacent layers are held together by weak van der Waals forces.^72–74 These weak interactions can be disrupted through suitable chemical or physical methods, such as heating or ultrasonication in specific solvents, leading to the exfoliation of MOF layers from the bulk material. This process enables the acquisition of ultrathin MNS with nanoscale dimensions, while preserving their ordered pores and arrangements for subsequent applications. The initial report of 2D MNS exfoliation from the bulky [Cu₂Br(IN)₂]_n (IN = isonicotinato) MOFs was presented.⁷⁵ In this instance, layers along the a-axis were connected through π–π stacking facilitated by the aromatic ring. Their work demonstrated that ultrasonication in acetone could effectively exfoliate the bulky layered MOFs, yielding [Cu₂Br(IN)₂]_n MNS with a thickness of 5 ± 0.15 Å, consistent with the theoretical thickness of a single layer. The layered MOF Zn₂(Bim)₄ (Zn₂(benzimidazole)₄) serves as a typical precursor for MNS production through a physical exfoliation process. In the structure of Zn₂(Bim)₄, Zn nodes coordinate with four benzimidazole (Bim) ligands, forming a distorted tetrahedral geometry. Each Bim ligand bridges two Zn atoms through a bismonodentate linkage, while adjacent layers are connected by van der Waals forces. To address potential in-plane structural damage during physical exfoliation, Peng and co-workers introduced a novel soft physical method.⁷⁶ Pristine Zn₂(Bim)₄ crystals were first crushed through a wet-ball milling process at a low speed (60 rpm) and subsequently exfoliated in a mixture solvent of methanol and propanol under ultrasonication. The process aimed to allow small methanol molecules to penetrate the galleries of layered Zn₂(Bim)₄, breaking interlayer interactions and facilitating MNS exfoliation. Propanol was then employed to stabilize the exfoliated MNS by adsorbing onto the nanosheets with hydrophobic alkane tails. The resulting Zn₂(Bim)₄ MNS, dispersed in the solvent, exhibited a Tyndall effect, and the thickness, determined by AFM, was approximately ∼1.12 nm, featuring a flat, smooth surface. TEM images revealed ultrathin MNS with a side length of ∼600 nm, including some folds or curled edges. Subsequently, the same research group utilized a layered Zn₂(bim)₃ precursor,⁷⁷ an isomer of Zn₂(Bim)₄. In the structure of Zn₂(bim)₃, each Zn atom bridged three Bim ligands, forming a six-membered-ring pore-like structure in the layers. Employing a modified top-down method by extending the milling time (from 60 min to 70 min), Zn₂(bim)₃ MNS were prepared. AFM images and height profiles indicated that the as-prepared Zn₂(bim)₃ MNS had a thickness of 1.6 nm, corresponding to a bi-layered nanostructure. Apart from the Zn₂(Bim)₄ and Zn₂(bim)₃ precursors, another layered MOFs precursor, Zn₂PdTCPP (PdTCPP = palladium tetrakis(4-carboxyphenyl)porphyrin), was utilized for obtaining 2D MNS by Ding and his colleagues.⁷⁸ In this approach, a chemically labile intercalating agent, 4,4′-dipyridyl disulfide (DPDS), replaced the role of methanol as the intercalating agent. When DPDS was blended with the as-synthesized Zn₂PdTCPP in a mixture solution of N,N-diethyl formamide and ethanol, it coordinated to the unsaturated Zn sites with one pyridinic N, as confirmed by PXRD patterns. DPDS then weakened the interlayer interaction between adjacent layers through a scissoring effect via the chemical reduction of disulfide bonds, using trimethyl phosphine (TMP) as the reducing agent. This resulted in the production of ultrathin MNS with a high yield (∼57%), surpassing conventional top-down methods.⁷⁹ Excess TMP (20-fold) was added during the exfoliation process to ensure maximum yield, and the height of the obtained MNS, measured from AFM and TEM images, was ∼1.0 nm, corresponding to the theoretical thickness of a single PdTCPP layer. Control experiments verified that the exfoliation process initiated with the reduction of DPDS ligands. When only TMP was added to Zn₂PdTCPP, no spontaneous exfoliation occurred. Additionally, the exfoliation process was shown to be controllable by adjusting the amount of TMP. When TMP was used in a quantity 10-fold greater than that of disulfide groups in the pristine MOFs, most of the obtained MNS featured a multilayered structure with a thickness of ∼4 nm. This control method may fulfill the requirements for obtaining 2D MNS with desired thickness. UiO MOFs, a series consisting of zirconium (Zr) or hafnium (Hf) clusters as nodes and various binary aromatic carboxylic acid ligands, such as 1,4-dicarboxylic acid and biphenyl-4,4′-dicarboxylic acid.^80,81 They reported on hcp UiO-67(Hf) MOFs, a variant of the face-centered cubic fcu UiO-67, which could further convert into 2D MOFs, hxl UiO-67. Both the 3D hcp and 2D hxl UiO-67 could exfoliate into ultrathin MNS through ultrasonication or grinding processes. Hcp UiO-67, comprising a double-cluster (Hf1₂O₈(OH)₁₄) and 4,4′-biphenyldicarboxylate, could be synthesized at a higher temperature and in the presence of a substantial amount of formic acid. The clusters in hcp UiO-67 adopt an “ABBA” stacking, distinct from the typical “ABC” stacking observed in fcu UiO-67. Conversely, hxl UiO-67 would form when hcp UiO-67 was left in ambient conditions for one week, resulting in a complete transformation. The structure of hxl UiO-67 closely resembled that of hcp UiO-67 but exhibited a considerable contraction along the c-axis (reduction of 44%), as indexed from the PXRD data. The layers in hxl UiO-67 were not covalently bonded along the c-direction. Through grinding or ultrasonication in methanol, both hcp and hxl UiO-67 could yield identical MNS with a thickness of 11 nm, equivalent to about 4-unit cells of hxl UiO-67. Comparing the PDFs of hcp UiO-67-derived samples and fcu UiO-67, a characteristic peak of the two Hf6 octahedra in a double-cluster was identified, indicating the well-maintained crystal structure after exfoliation. Importantly, the successful exfoliation implied that the metal–ligand bonds in hcp and hxl UiO-67, under suitable conditions, could be selectively cleaved to form 2D materials. This ability could be attributed to the control of defects to introduce “weak links” with a monotopic acid modulator.

2.2.2. Bottom-up synthesis. It is crucial to acknowledge that the top-down method often yields MNS with low efficiency due to the challenging exfoliation process, and controlling the thickness of MNS remains a significant challenge. MNS exfoliated from bulky MOFs are prone to aggregation, leading to the formation of multilayers.^82,83 Additionally, the majority of layered MOFs suitable for the exfoliation process are M–bim and M–BDC MOFs, displaying limited universality.^84,85 As discussed earlier, achieving direct synthesis of 2D MNS becomes feasible when the growth of MOF crystals is controlled, allowing extension along the a and b axes while restricting growth along the c axis. Various approaches have been developed for the direct synthesis of 2D MNS, including surfactant assistance and liquid/liquid or liquid/air interfaces, all proven to be practical methods for constructing 2D MNS. Despite the certain directionality of coordination bonds of metal nodes, the introduction of small carboxylic or solvent molecules connecting with the nodes can lead to ligands bridging nodes in the same plane due to spatial effects.⁸⁶ Cao and co-workers reported a highly scalable bottom-up strategy for the direct assembly of 2D metal–organic layers from molecular building blocks in a one-pot solvothermal reaction.⁸⁷ They utilized Hf⁴⁺ clusters [Hf₆(μ₃-O)₄(μ₃-OH)₄(carboxylate)₁₂] as nodes, known for forming stable coordination bonds with BTB (benzene-1,3,5-tribenzoate) ligands. Typically, one Hf cluster could coordinate 12 ligands in various directions, forming a 3D crystal structure that does not meet the geometric requirement for 2D layers.⁸⁸ To address this geometric mismatch, the authors proposed a capping method, wherein six coordination sites of the cluster were protected by formic acid groups, while the remaining six were connected to the BTB moieties in the same plane. To validate their hypothesis, a mixed solution consisting of HfCl₄/BTB/HCO₂H/H₂O/DMF with a molecular ratio of 1.5 : 1 : 830 : 290 : 2280 was heated to 120 °C for 48 h. Water was added to utilize the partial hydrolysis of Hf⁴⁺: 6Hf⁴⁺ + 8H₂O + 6HCO₂H → [Hf6(μ₃-O)₄(μ₃-OH)₄(HCO₂)₆]₆₊ + 18H⁺, and DMF as the solvent could decompose to generate dimethylamine in the presence of water, neutralizing the HCl generated during the reaction. TEM images revealed ultrathin wrinkled films with an average lateral size of approximately 4 × 4 μm², and AFM images indicated a thickness of 1.2 ± 0.2 nm, corresponding to the van der Waals size of the Hf cluster. Additionally, MNS with thicknesses of 2.2 ± 0.2 nm and 3.2 ± 0.2 nm, corresponding to bilayer and trilayer structures, respectively, were observed (Fig. 7). High-resolution TEM images displayed clear lattice fringes, with Hf6 clusters appearing as dark spots against the background and non-metallic atoms, consistent with positive contrast. Furthermore, high-angle annular dark field images in STEM mode depicted clear 2D kgd nets where white spots represented the Hf6 clusters, aligning well with the expected infinite 3,6-connected 2D network of Hf6(μ3-O)₄(μ3-OH)₄-(HCO₂)₆(BTB)₂. Since traditional solvothermal synthesis utilizing Zr/Hf and BTB as nodes and ligands in a DMF/acetic acid solution typically resulted in 3D MOFs.⁸⁹ Hu and his colleagues reported a novel kinetically controlled synthetic approach for 2D MOF nanosheets (MNS).⁶⁰ They discovered that in a typical 3D MOF, designated as NUS-16(Zr/Hf), the 2D porous grids tended to interpenetrate to compensate for the energy penalty, reaching a thermodynamically stable state. This interpenetration could be avoided in heterogeneous conditions through fast precipitation at high temperatures (120 °C). A mixed solvent consisting of deionized water and acetic acid (AA) (v/v = 3/2) was employed in the synthesis, resulting in successfully obtaining layered 2D MNS with this modulated hydrothermal approach, named NUS-8(Zr/Hf). The kinetically controlled synthesis was confirmed by a control experiment, where the as-synthesized 2D NUS-8 was placed in a DMF/acetic acid solution at 120 °C for 24 h, and no transition from 2D NUS-8 to 3D NUS-16 was observed. Compared with 3D NUS-16, the calculated distances between interlayers shrank to 16.82 and 17.27 Å for NUS-8(Zr) and NUS-8(Hf), respectively, further confirming a 2D layered structure (Fig. 8). The 2D NUS-8 exhibited a nanosheet morphology with a thickness of approximately 10–20 nm and a lateral size up to 500–1000 nm, as observed in the FE-SEM and HRTEM images. Furthermore, NUS-8 MNS with a thickness as thin as ∼4 nm could be observed according to the AFM images, suggesting a two or three-layer stacking. These results leave no doubt that anticipated 2D layered MNS could be obtained under heterogeneous synthetic conditions, locking desired MOFs products in the kinetically favorable intermediate stage. Surfactants also play a significant role in the direct synthesis of 2D MNS due to their capability to control the growth of MOF crystals. Particularly, polyvinylpyrrolidone (PVP) has emerged as a potential surfactant in the direct synthesis of 2D MNS.⁹⁰ Cao and co-workers utilized PVP (average molecular weight: 40 000) in the synthesis of layered 2D PPF-3.⁹¹ In this structure, each paddlewheel Co₂(COO)₄ metal node bridged four TCPP (5,10,15,20-tetrakis(4-carboxyl-phenyl)-porphyrin) in the same plane, and the adjacent layers were further pillared by BPY (4,4′-bipyridine) in an “AB” stacking. During MOF crystal growth, PVP molecules were expected to selectively attach to the crystal's surface and control the vertical growth of MOF crystals, resulting in the formation of 2D PPF MNS. The lateral size of the square-like PPF nanosheets was ∼1.5 μm, as observed in SEM and TEM images, and an average thickness of 42.7 ± 8.4 nm was determined according to AFM images. The derived thick PPF-3 nanosheets were attributed to the strong connections between layers through the BPY moieties. The authors also proposed that the thickness of PPF-3 nanosheets could be easily tuned by changing the concentration of starting materials; the thickness of as-obtained ultrathin PPF-3 nanosheets would decrease to 11.9 ± 4.2 nm when a low concentration of starting materials was applied in the synthesis. In addition to their work using Co(NO₃)₂ and TCPP to synthesize 2D MNS in the presence of PVP, Jian and co-workers chose Zn as the metal node, and the resulting few-layer MNS, zinc tetra-(4-carboxyphenyl)porphyrin complexes (PPF-1),⁹² displayed a 2D square-grid layer motif. In their research, temperature was demonstrated as a significant factor in the growth of PPF-1 crystals. As the crystallization temperature increased from 60 °C to 150 °C, the layers of PPF-1 MNS decreased to 2–3 layers, while the lateral size tended to increase (up to ∼10 μm). The yield also increased from 7.4% to 73.2% with a higher temperature. Different from bulky PPF-1 with a thickness of more than 30 nm, the PPF1 MNS prepared at 120 °C exhibited ultrathin morphology with a thickness of 2.5 nm, corresponding to a two-layer MNS according to the theoretical thickness of 1.47 nm. The crystallization temperature played a crucial role in the synthesis of ultrathin 2D MNS. The average thickness of PPF-1 MNS prepared at 65 °C was about 10.8 nm, and no PPF-1 MNS could be found when the temperature was lower than 65 °C. Additionally, the yield of PPF-1 MNS synthesized at various temperatures increased when PVP was added, especially at high temperatures (above 120 °C). To understand the nucleation and growth mechanism of the PPF-1 MNS, UV-vis spectroscopy was carried out to analyze and track the formation of PPF1 crystals. Based on the analysis, the author proposed that the PPF-1 MNS would be more likely to grow heterogeneously on the parent phase at low temperatures due to the small interfacial energy and strong hydrogen bonds between adjacent layers. These factors greatly restricted the long chains of PVP from accessing the surface of 2D MNS, ultimately affecting the exfoliation process. On the contrary, when the temperature increased, these strong interactions would weaken sharply, followed by an expanded interspace allowing more PVP molecules to access the interlayer space, accelerating the exfoliation progress. Moreover, PVP made a magnificent contribution to the interface, as metal salts as nodes and organic linkers were dissolved in chosen aqueous and organic solvents, respectively. Then, the two solutions were mixed together, and the reaction taking place in the interface of two immiscible phases greatly helped the formation of ultrathin 2D MNS, which may be ascribed to the well-controlled ability of interfacial interactions in the nucleation and growth of MOFs.^93,94 Cheng and colleagues have successfully synthesized ultrathin CuBDC MNS through a liquid/liquid interface method.⁹⁵ In their work, two solutions containing different ratios of DMF and CH₃CN were prepared to dissolve Cu(NO₃)₂ and H₂BDC (1,4-benzenedicarboxylic acid). After pouring the solution of H₂BDC into a vial, a mixed solution of DMF and CH₃CN was added to form the transitional layer, and finally, the solution of Cu(NO₃)₂ was added to the vial as the top layer. In this three-layer system, Cu²⁺ cations and H₂BDC molecules would slowly diffuse into the transitional layer, where the crystal would form and precipitate at the bottom. A relatively limited amount of Cu²⁺ was used in this reaction to prevent further crystal growth and maintain the nanosheet structure. The average thickness of the as-obtained CuBDC MNS was approximately 15–50 nm, as determined by the AFM image. A rectangular shape with a lateral size of about 2–5 μm was also evident in TEM and SEM images, indicating a well-controlled nucleation process during the reaction. Additionally, a periodic lattice interval of 0.34 nm was clearly observed in HRTEM, corresponding to the crystal plane.^96,97 Since the yield of MNS depends significantly on the interfacial area, the introduction of a transitional layer in this reaction aimed to improve the MNS yield by offering a larger interface area. In a typical liquid/liquid system without a transitional layer, the reaction occurs only in the limited area of the interface. However, the transitional layer, consisting of two kinds of solvents, allows more cations and ligands to react with each other, accelerating the formation of the desired crystal.⁷³ Taking advantage of this three-layer interfacial method, other types of 2D MNS, such as ZnBDC, CoBDC, and Cu(1,4-NDC) (NDC = naphthalenedicarboxylate), have also been successfully synthesized. Nevertheless, this method still presents some challenges because the as-obtained 2D MNS tend to aggregate or restack. Overcoming these obstacles is crucial to obtaining well-dispersed MNS, which is essential for the practical application of 2D MNS.

Fig. 8 (a) Preparation of microemulsion droplets that serve as confined nanoreactors for the synthesis of small-sized MOFs; (b and c) structural morphology of Ln₂(BDC)₃(H₂O)₄ particles at different water/surfactant ratios. Reproduced from ref. 98 with permission from American Chemical Society, copyright 2006. (d and e) Structural morphology of Mn(BTC)₂(H₂O)₆ synthesized at various temperatures. Reproduced from ref. 99 with permission from Elsevier, copyright 2016. (f) Synthesis of Fe-soc-MOF cubes. Reproduced from ref. 100 with permission from American Chemical Society, copyright 2013.

3. Factors affecting the properties of MOF-based membranes

The properties, structures, and observed morphologies of MOFs are influenced not only by building block formats but also by various factors, such as solvent selection, compositional parameters (e.g., pH, salt type, and molar ratio), and additives. However, for each system, fine-tuning through trial and error is necessary to obtain specific knowledge regarding the uniqueness and properties of each MOF.

3.1. Solvents

The choice of solvent plays a pivotal role, as it can directly or indirectly impact the coordinating ability and the reaction dynamics between ligands and metal species in chemical processes. Solvent systems have been proven to influence metal ion coordination and act as guests in final lattice systems.¹⁰¹ Although solvents are not typically incorporated into the synthesis of MOFs, they often serve as structure-directing agents during the crystal growth of MOF membranes in various fabrication processes. The most commonly used solvents in the preparation of MOFs include diethyl formamide, dimethyl formamide, ethanol, water, methanol, NMP, and other mixed solvents.¹⁰² Several different solvents may form MOF crystals of various sizes. Ghorbani et al.¹⁰³ investigated the influence of various solvents, including water, DMF, ammonia, acetone, and certain ionic liquids, on the synthesis of ZIF-8 at room temperature. Their findings demonstrated that the polarity of the solvent plays a critical role in regulating the size of the MOF crystals. Specifically, they observed that as the solvent polarity increased, the resulting MOF crystals decreased in size. Therefore, this phenomenon can be renowned for the thermal disintegration of the solvent system, which further corresponds to the amine-based functional groups present in the solvent, ultimately leading to proton removal from organic ligands.¹⁰⁴ Although several MOFs can be synthesized using organic solvents, the toxic secondary side products obtained during their use raise serious environmental issues. Furthermore, it is important to note that traces of these solvents may remain within the small pore networks of MOFs and must be thoroughly eliminated before employing MOF membranes in practical applications. Efforts have been directed toward minimizing or eliminating the use of environmentally harmful solvent systems in the preparation of MOFs. This focus on more sustainable and eco-friendly synthesis methods aligns with the broader goals of green chemistry and sustainable material development. To address environmental concerns and reduce the reliance on harmful solvents, alternative solvent-free synthesis methods have been explored. Some of these methods include ultrasound-assisted processes, aerosol-based techniques, and microwave-based irradiation. These approaches offer more environmentally friendly alternatives for MOF synthesis while potentially enhancing the efficiency and control of the reaction conditions.¹⁰⁵ However, given that methods are often limited in their less common usage, they can be complex. Therefore, recent attempts have focused on synthesizing MOFs using water as a replacement for organic solvents. Several studies have successfully synthesized MOF crystals in water systems or water/organic binary solvent systems, including MOF, UiO-66, CuBTC, CAU-1, ZIF-8, HKUST, and MIL-53(Al).¹⁰⁶

3.2. Parameters related to composition

The reported literature extensively acknowledges the influence of composition-related parameters, such as pH, molar ratios of reactants, and the type of salt used, on the properties and characteristics of prepared MOFs. These factors play a crucial role in tailoring the structure, morphology, and properties of MOFs, allowing for precise control and customization of their properties for various applications. The acidity or basicity of the reaction mixture during MOF synthesis is known to impact the growth and crystallization of the synthesized MOF complex. Here, pH plays a role in several important aspects: (i) the formation of specific ligands depends on the degree of protonation, (ii) in aqueous systems, the formation of hydroxo ligands is pH dependent, and (iii) the pH controls the kinetics of the reaction while synthesizing MOFs.¹⁰⁷ Regarding (i) and (ii) during the process of deprotonation, some of the given ligands may coordinate with –OH groups, leading to metal–ligand complexes and building blocks during crystallization. For (iii), varying pH favors the connection of ligands to metal ions, which results in the crystallization of the bulk in solid form. Schejn et al.¹⁰⁸ examined the effect of pH on the preparation of ZnBTC MOFs and reported that changing the pH of the reaction medium to basic conditions favors deprotonation, leading to changes in the architectural dimensions of the resulting MOFs. The type of metal salt precursor also affects the synthesis of MOFs. Zhang et al.¹⁰⁹ investigated different kinds of Zn-based precursors for the preparation of ZIF-8 membranes. They found that faster nucleation and smaller-sized particles are achieved with more reactive metal precursors. The obtained MOF sizes ranged from 211 nm (ZnSO₄) to 45 nm (Zn(NO₃)₂). Moreover, the molar ratio of reactants is a main factor in MOF preparation, as it affects the resulting topological pattern of the framework. Several studies have reported that the molar ratio of metal to ligand also affects the structure of the developed framework. Ma et al. reported that increasing the molar ratio of Hmim/Zn²⁺ results in a smaller size of ZIF-8 MOFs.¹¹⁰ In this case, excess 2-methylimidazole, a stabilizing molecule, was also found to decrease the growth rate of ZIF-8 crystals on the supporting system.

3.3. Temperature

Temperature can significantly affect the properties of MOFs prepared through solvothermal, hydrothermal, and microwave-assisted methods. Most MOFs are prepared in closed-type systems at temperatures between 81 and 165 °C to achieve high-quality crystal growth. At optimized temperatures, nanosized MOFs can be developed through quick nucleation and slower growth of crystals. Khoshhal et al. studied nanocrystals of HKUST-1, which were synthesized through three temperature phases: (i) reactant incubation, (ii) nucleation temperature, and (iii) growth of crystals at low temperature.¹¹¹ They further examined the impact of variable temperatures (80 °C, 110 °C, and 140 °C) during the synthesis of given CuBTC MOFs. Their results suggest that higher temperatures result in reduced crystallinity, microspore volume, and specific surface area, as well as in the presence of identified impurities (Cu₂O). However, excessively high temperatures can oxidize metal ions.¹⁰⁴ Therefore, the temperature during synthesis should be controlled and optimized because the ideal synthesis temperature varies depending on the type of MOF being developed and synthesized.

3.4. Additives

The additives have been shown to exert a significant influence on the properties of synthesized MOFs. Numerous studies have investigated the impact of additives with varying ionic strengths, acidities, and polarities when introduced into reaction solutions. Some of these additives include acetic acid, alkylamines, pyridine, benzoic acid, glycerol, and sodium acetate. The inclusion or modulation of these materials during synthesis can lead to tailored MOF structures and properties, making them key for MOF development and customization for specific applications.¹¹² These additives are incorporated into the reaction to mediate crystal growth and coordination by adjusting the kinetics of the reaction.¹¹³ Drache et al.¹¹⁴ investigated the impact of trifluoroacetic acid (HTFA) polymorphism behavior on Zr-MOFs. They revealed that by adding HTFA, a new Zr-based MOF (ZIF-8) with an 8,8-a bimodal attachment framework was obtained with CTAB as a capping agent for controlling crystal growth.¹¹⁵ In addition, the structure of Fe-MOF was controlled by adding glycerol as an additive to change the material from octahedral to pyramidal hexagonal shapes in the reaction medium. The presence of hydroxyl groups in glycerol limited crystal growth, resulting in enhanced monodispersity and uniformity of the MOFs. Additionally, the hydroxyl groups prevented unfavorable heterogeneous particle sizes and structures from forming by slowing the diffusivity of the nucleated solution.

4. Characteristics of the synthesized MOF-based membrane for water treatments

4.1. Stability of 2D/3D MOFs in water

Water stability is a critical factor in the performance of MOFs, especially for applications in humid environments. By functionalizing MOFs to block water entry, their stability and performance can be significantly enhanced, and hydrophobic MOF crystals can be obtained. For this purpose, some hydrophobic functional groups, such as alkyl groups or encapsulating agents, can be added to the MOF membrane to prevent water from passing through the channels. Zhang et al. developed an MOF with hydrophobic functional groups that effectively blocks water molecules from entering the framework, resulting in improved hydro-stability of 2D membranes and improved gas separation performance.^116–118 Additionally, optimized pore size and surface area also play key roles in determining the performance and stability of membranes.

The extremely porous and thermally stable MOF-5 and HKUST-1 are vulnerable to moisture, which restricts their usage in water-related treatments. Using high-valence metal ions, isolate-based organic linkers, functionalizing MOFs with blocked metal ions or hydrophobic pore surfaces, and other methods, Wang et al. suggested methods to create water-stable MOFs. For MOFs used in water desalination procedures, the first two methods are frequently used.¹¹⁹ The stability of MOFs can be evaluated by exposing the materials to water or moisture and comparing the XRD pattern with that of the pristine sample. As one example of a groundbreaking study¹²⁰ demonstrated that the ultrathin nanoporous membranes constructed from 2D MOF nanosheets have emerged as champions in the realm of ion separation from water. These meticulously engineered laminar membranes showcase an extraordinary feat—their impeccable long-term stability in water—and are resilient against the notorious challenge of swelling that often plagues other 2D-based membranes. The longevity of the integrity of the Al-MOF membrane was rigorously scrutinized, with continuous testing focusing on the Na⁺ permeation rate and water flux. These results were unremarkable: a steadfast plot of the Na⁺ concentration on the feed side and an unwavering water flux persisted over an impressive 30 day period, attesting to the exceptional durability of the membrane.

The reason behind the robust stability of Al-MOF membranes lies in the ingenious use of parallel π–π interactions, which are locking mechanisms employed between adjacent nanosheets. Notably, XRD analysis further validated the stability of the material, with the (0k0) peak at 2θ = 7.6° for the Al-MOF membrane remaining unchanged after rigorous 1 month continuous testing. Moreover, the antiswelling ability of the Al-MOF membranes was tested throughout the entire experimental duration and was unsatisfactory because of the lack of scattering.¹²¹ Alemayehu et al. innovatively created a 2D composite membrane by integrating highly porous 2D Al-MOF nanosheets into a GO membrane, enhancing both the stability and solvent permeability. Strong interactions, such as Al–O coordination and π–π stacking between Al-MOFs and GO nanosheets, enhance the structural stability of the composite membrane. Chemical and mechanical stability tests revealed the exceptional resilience of the GO@Al-MOF membrane. After two months of exposure to water or basic or acidic solutions, the membrane exhibited no swelling or detachment of the GO layers, in stark contrast to what was observed for the pristine GO membrane. Even under intensive ultrasonication treatment, the GO@Al-MOF membrane showed negligible damage, retaining a separation performance identical to that of the untreated membrane. This breakthrough underscores the remarkable stability and durability of the novel 2D MOF-based composite membrane.¹²² Calculations of the specific surface area and nitrogen adsorption potential can also be used to estimate the stability of MOFs in aqueous systems.

4.2. Role of metal ion with multiple valence

The use of metal ions with higher valency in MOFs play a crucial role in enhancing their stability in aqueous media and catalytic properties. Some of the established high-valence metal ions originate from 3p metals (Al³⁺, Ga³⁺, and In³⁺) or transition metal cations (Fe³⁺, Cr³⁺, Zr⁴⁺, and Ti⁴⁺).⁹⁹ The charge density of these metals is correlated with their ionic radius. High-charge-density metal ions generally exhibit a small effective ionic radius, resulting in enhanced stability of the obtained MOFs. While some divalent cations readily dissolve under basic conditions at room temperature, synthesis reactions involving trivalent or tetravalent cations need to be conducted under acidic conditions or, in some cases, at higher temperatures.¹²³

Nevertheless, due to these characteristics, the resulting MOFs typically exhibit high stability over a wide range of pH values attributed to the slow potential dissociation of the metal ions from the linker. MIL is among the highly stable MOFs that incorporate high-valence metal ions and showcasing high tolerance toward aqueous media, humid atmospheres, and aqueous solutions over a broad pH and temperature range.¹²⁴ Like Cr³⁺ metal-based BDC linkers, MIL-101 exhibits high stability in water, organic solvents, and humid atmospheres. Zirconium is another well-established high-valence metal used in the construction of highly stable MOFs.³⁷ Some of the highly stable Zr-based MOFs reported in studies include UiO-66, which consists of an inner Zr₆O₄(OH)₄ octahedron with 4,4-biphenyl-dicarboxylate as the linker.¹²⁵ This MOF has been reported to be stable in water, acetone, benzene, ethanol, strong acid solutions, and strong basic solutions. Additionally, the integration of metal nanoparticles (MNPs) with high atomic numbers can enhance membrane stability and assist in the preparation of membranes with optimized pore sizes.¹²⁶

4.3. Utilization of azolate-based organic linkers

Azolate-based organic linkers have shown promise for improving the stability and functionality of MOFs. In this context, the pK_a values of azolate organic ligands, such as imidazolates, pyrazolates, triazolates, and tetrazolates, play a crucial role due to their higher pK_a values. Results in MOFs with enhanced stability, even when high-valence metal ions are not utilized.¹²⁴ The ZIF-8 framework of zeolitic imidazolates serves as an excellent example of this strategy. It is constructed from Zn²⁺ or Co²⁺ linked with an imidazolate linker and has a pK_a of approximately 18.6.¹²⁴ Additionally, imidazolates also maintain strong metal–nitrogen bond structures, contributing to increased stability not only in water but also under chemical and thermal conditions. Park et al.¹²⁷ reported that ZIF-8 demonstrated outstanding water stability, with its structure remaining unchanged even after exposure to boiling water for 7 days. Besides this, Colombo et al.¹²⁸ constructed pyrazolate MOFs based on Ni, Cu, Zn, and Co metals with high stability. All these MOFs exhibited no loss of crystallinity, and their hydrothermal stability could withstand boiling in an acidic solution (pH 2.0) for 2 weeks.^119,124 Yuan et al. demonstrated the preparation of a MOF with azolate-based linkers, which exhibited excellent CO₂ adsorption capacity, making it a potential candidate for carbon capture applications.¹²⁹

4.4. Appropriate size of the pores and surface area

The pore size and surface area of MOFs are crucial for several applications, such as the separation and storage of gas. MOFs with well-defined and appropriate pore sizes can selectively adsorb specific gas molecules.¹³⁰ Zhao et al.¹³¹ synthesized an MOF with precisely controlled pore sizes, enabling efficient separation of CO₂ and CH₄. Furthermore, Haque and colleagues conducted a comparison analysis of MIL-Fe-53 Fe(OH)BDC synthesis using conventional electric heating, a microwave-assisted technique, and ultrasound or sono-irradiation.¹³² This study's observation of the influence of reaction time on crystallization demonstrates that all three methods result in equivalent crystal growth rates but that the size of the MOFs decreases in the following order: only ultrasonication irradiation and microwave heating produce MOFs that are uniform and well defined, as opposed to conventional heating, microwave heating, and conventional heating.

4.5. Ion selectivity

MOFs can exhibit high selectivity towards specific ions, making them valuable for applications such as ion sieving and ion exchange. The choice of metal ions and organic linkers in MOF synthesis can influence the affinity of the membrane for specific ions. In another study, an MOF with tailored pores demonstrated exceptional selectivity for lithium ions, suggesting the potential for efficient lithium extraction from brine solutions.¹³³

4.6. Mechanical stability

To ensure the long-term performance of MOFs, their mechanical and chemical stability is crucial. MOFs should withstand harsh conditions without compromising their structure or functionality. Furthermore, by changing ligands and incorporating nanoparticles, the stability of MOFs can be further enhanced as needed.

4.7. Rapid nucleation

Controlling the nucleation process is important for the synthesis of MOFs with desirable properties. Rapid nucleation can lead to the formation of highly crystalline MOFs. To attain highly crystalline frameworks, alternative synthetic approaches combining rapid nucleation processes with conventional heating, microwave-assisted heating, or ultrasonication irradiation heating can be employed. Among these methods, microwave-assisted synthesis is commonly applied because it enables fast, direct, and uniform energy generation within pressurized vessels, resulting in the formation of high-crystallinity frameworks due to solvent boiling. Gupta et al. developed a method that achieves rapid nucleation of MOFs, resulting in improved adsorption capacity for volatile organic compounds.¹³⁴

4.8. Nanoreactor confinement

MOFs with confined spaces can serve as nanoreactors for catalytic reactions, allowing enhanced control over reaction kinetics and selectivity. Pang et al.¹⁰⁰ synthesized MOFs with confined nano spaces, enabling efficient catalytic conversion of biomass-derived compounds into valuable chemicals. Fig. 8 shows the confinement process for nanoreactors. This method produces monodisperse nanoscale droplets by mixing non-mixing liquids (such as oil and water). The size of emulsified droplets can be tuned by varying the proportion of additives, such as amphiphilic surfactants. The kinetics of the particle nucleation process can be controlled by adding these droplets to MOF precursors. Using this method, lower MOF particle sizes have been effectively obtained in several investigations. Lin and coworkers synthesized Ln-based MOFs (Ln₂(BDC)₃(H₂O)₄) by treating LnCl₃ and bis(methylammonium)-BDC in an emulsified system of isooctane/1-hexanol/water. Where the quantity of CTAB surfactant used determines the size of the nanorods. Emulsions with higher water/surfactant ratios produce nanorods with smaller diameters, resulting in nanorods with condensed sizes ranging from 2 μm × 100 nm to 125 nm × 40 nm (Fig. 8a and b).⁷⁷ The reaction temperature also plays a role in controlling the size of the resulting MOFs, with lower temperatures producing smaller nanorods (Fig. 8c–e). The microemulsion approach effectively regulates and controls the desired nanosize of MOFs but has several disadvantages, such as poor reproducibility due to complex micelle formation, low yields, the use of harsh chemicals, and the requirement for multiple washing steps.⁷⁸

4.9. Coordination modulation

The modulation of metal–ligand coordination in MOFs can lead to unique properties and functionalities. By altering the coordination environment, MOFs can exhibit different catalytic activities. Fischer et al.¹³⁷ studied the coordination modulation in an MOF, which resulted in enhanced photocatalytic performance for hydrogen production. By regulating the number of nucleation sites and binding to crystal faces, modulators can change the size of the produced MOFs (Fig. 9a). The steric profile, concentration, and pK_a of modulators are important factors in controlling particle size.¹³⁸ Diring et al.¹³⁵ controlled the shape of HKUST crystals by combining two modulators, acetic acid and dodecanoic acid. The correct transition between nanoscale and microscale MOF NPs was achieved by adjusting the concentration of these acids (Fig. 9b). Schaate et al. published the first investigation on the use of modulators on Zr-MOFs.¹³⁶ To modulate the size of UiO-66 and UiO-67 in their investigations, benzoic acid and acetic acid were mixed and utilized as modulators (Fig. 9c). To create frameworks, these monotopic acids gradually take the place of ditopic linker molecules. This equilibrium exchange controls the nucleation rate and prevents the aggregation of intergrown crystallites by allowing all NPs. Overall, coordination modulation has proven to be the most successful method for producing small MOFs of all those described.

Fig. 9 In the coordination modulation process, (a) added modulators usually act as capping agents and control the number of nucleation sites; (b) the structural morphology of HKUST-1 MOFs when the concentration of dodecanoic acid increases. Reproduced from ref. 135 with permission from American Chemical Society, copyright 2010. (c) Structural morphology of UiO-660 MOFs as the concentration of acetic acid increases. Reproduced from ref. 136 with permission from Wiley-VCH, copyright 2011.

4.10. Fouling resistance and scalability

Fouling, the accumulation of contaminants on the membrane surface, can reduce desalination membrane performance.¹³⁹ MOF-based membranes with tailored surface properties minimize fouling. The hydrophilic surfaces are less prone to fouling. Zhao et al.¹⁴⁰ developed a hydrophilic MOF-based membrane with reduced fouling. Surface modifications with zwitterionic or charged groups create repulsive forces that hinder foulant attachment. Fang et al.¹⁴¹ modified MOF-based membranes with zwitterionic groups, enhancing fouling resistance. Like MOF-based nanocomposites, hierarchical MOFs enhance fouling resistance. Zhao et al. incorporated GO into a MOF-based nanocomposite membrane, which exhibited excellent fouling resistance.¹⁴⁰ Furthermore, scalable and cost-effective MOF-based membranes are essential for widespread adoption. They developed a cost-effective MOF synthesis method using readily available precursors. Durability and long-term stability minimize replacement and maintenance costs. Integration with existing desalination systems improves cost-effectiveness.¹⁴² Rego et al. integrated MOF-based membranes with reverse osmosis systems, resulting in higher water recovery rates and reduced energy requirements.¹⁴³ In summary, the optimization of MOFs for various applications, including desalination, necessitates comprehensive consideration of several key factors. However, these factors encompass the use of high-valence metal ions, azolate-based organic linkers, strategies for blocking water ingress, tailored pore sizes and surface areas, ion selectivity, mechanical and chemical stability, efficient nucleation kinetics, the utilization of nanoreactor confinement, modulation of coordination chemistry, and the crucial evaluation of water permeability and flux. Combining and fine-tuning these elements play a critical role in tailoring MOFs to meet specific performance requirements and application needs. By understanding and manipulating these factors, researchers can design and develop MOFs with enhanced properties and functionalities, opening up new opportunities for their application in diverse fields (Table 2).

Table 2 Different MOF-based membranes for water desalination applications

Feed (salts)	Feed concentration	Type of membrane	Type of MOF	Metal	Organic linker	Fabrication method	Thickness	Pressure	Permeance	Rejection	Ref.
NaCl	0.5 M	Freestanding membrane	ZIF-8	Zn	Metylimidazolate	Solvothermal	29.43 A	60	—	99	144
Mixed of saline water	2000	Grown on alumina support	UiO-66	Zr	1,4-BenzenedicarboXylic acid	In situ solvothermal	2.0 μm	10	0.28	98	145
NaCl	2000	TFN (PA)	ZIF-8 (200 nm)	Zn	Metylimidazolate	IP	100 nm	15.5	3.35	98.5	146
NaCl	2000	TFN (PA)	ZIF-8 (80 nm)	Zn	Metylimidazolate	IP	150 μm	15	1.68	99.4	138
NaCl	2000	TFN (PA)	Mil-101(Cr)	Cr	Benzene-1,4-dicarboxylic acid	IP	100–300 Mm	16	2.2	99	147
NaCl	2000	TFN (PA)	MIL-125 (100 nm)	Ti	Benzene-1,4-dicarboxylic acid	IP	NA	20.6	3.64	98.4	148
NaCl	2000		UiO-66 (100 nm)	Zr	Benzene-1,4-dicarboxylic acid	IP	NA	20.6	4.13	98.6
NaCl	2000	TFN (PA)	PCN-222 treated myristic acid	Zr	meso-Tetrakis (4-carboxyphenyl)	IP	300 nm	17.6	5.8	96	149
NaCl	2000	TFN (substrate)	HKUST-treated sulfuric acid	Cu	1,3,5-Benzene-tricarboxylic acid	IP	0.029 μm	15.5	3.03	96	150
NaCl	2000	TFN (PA)	ZIF-8 (150)	Zn	Metylimidazolate	IP	250 nm	15.5	3.95	99.2	151
NaCl	1000	TFN (PA)	UiO-66 (30 nm)	Zr	1,3,5-Benzene-	IP	50–100 nm	10	11.5	38	152
NaCl	1000	TFN (PA)	UiO-66-NH2 treated palmitoyl chloride	Zr	Amino-benzenedicarboxylic acid	IP	380 nm	4	12.4	28	153
MgSO4	1000									92
Na2SO4	1000									98
Sea water	∼35 000	Grown on PDA modified alpha	ZIF-8	Zn	2-Imidazolate-2-carboxaldehyde	In situ solvothermal	20 μm	—	—	99.8	154
NaCl	1000	Vacuum filtration of UiO-66-NH2 in PIP solution	UiO-66-NH2	Zr	Amino-benzenedicarboxylic acid	Solvothermal	93 nm	6	30.8	21.3	155
MgCl2									—	58
MgSO4									—	91.2
Na2SO4										97.3
NaCl	1000	TFN (substrate)	CuBDC	Cu	Benzene-1,3,5-tricarboxylate	Blending in substrate	100	8	31	44.9	156
MgSO4	1000									97.3
CaCl2		TFN	UiO-66-(NH2)2			Solvothermal reaction			19.44	96.07%	157

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5. 2D/3D MOFs for water purification and desalination applications

MOF-based membranes have attracted increased amounts of attention in the modern scientific world because of their potential for use in desalination and purification processes. Several crucial parameters and requirements, such as high-water permeability, high salt rejection, and durability, are required for the desalination of MOF-based membranes.¹⁵⁸ Compared to other desalination technologies,^159–162 MOF-based membranes offer several advantages, such as high desalination efficiency. They are also flexible in terms of design, allowing for tailoring of the pore size and surface chemistry to optimize performance. MOF-based membranes also have the potential to be more environmentally friendly and energy efficient than other desalination methods, as they can be operated at lower pressures and temperatures. However, MOF-based membranes are still in the early stages of development and face challenges such as scalability and cost-effectiveness. This review has focused mainly on the advancement and development of 3D/2D MOF-based membranes for water treatment applications. In this context, various strategies for designing 2D and 3D MOF-based membranes have been assessed, and further focus on recent advancements in the realm of water desalination. This discussion delves into the details of these MOF-based membrane technologies, encompassing different membrane methods and processes employed in the pursuit of efficient and sustainable water desalination solutions.

5.1. Pressure-driven desalination processes and their recent advancements

5.1.1. 3D MOF based membrane. The RO membrane of the selective layer is dense and lacks a distinct pore structure, while the obtained NF membranes with a selective layer are relatively less dense and possess proper pore sizes in the range of 1–10 nm.^118,163 The key properties of both the RO and NF membranes include dense layers capable of effectively retaining dissolved ions and a much greater degree of hydrophilicity, which makes water transport feasible.^27,164 However, due to the dense nature of the prepared thin layer, RO, NF, and TFC membranes can face challenges in achieving a balance between water flux and selectivity, which can result in decreased permeability at high salt ion rejection. Consequently, rather than modifying the substrate layer, researchers typically focus on tailoring thin-film layer-based NF and RO membranes. Initially, Hu et al.¹⁴⁴ conducted a simulation study using a ZIF-8-based membrane to study the applicability of MOF-based membranes for desalination applications. The results showed minimal transport of Na⁺ and Cl⁻ ions from the absence of defects and from the rigid ZIF-8 membrane. However, the presence of defects in the frame must be considered when scaling up the membrane. Moreover, the sieving effect of MOFs, which is affected by the size of the aperture and window cage-like structure, is found to be crucial. ZIF-8, with its zeolite-like topology and small aperture size (3.4 Å), effectively retains hydrated sodium ions (7 Å). Another simulation study by Gupta et al.¹³⁴ investigated several other types of ZIF membranes, such as ZIF-(25, 71, 93, 96, and 97), and all these membranes exhibited the same zeolitic topology with different functionalities on the imidazolate linker. The results suggest that functional groups and aperture size influence the desalination process. Moving from simulation studies to experimental work, in another work, Aghili and coworkers prepared UiO-66 via in situ solvothermal synthesis. The resulting composite membrane displays outstanding desalination performance, with high rejection rates for Ca²⁺ (86.5%), Mg²⁺ (98%), and Al³⁺ (99.4%) and a permeability of approximately 0.3 L m⁻² h⁻¹ bar⁻¹. Optimizing the process conditions is essential for obtaining UiO-66 layers free of flaws.¹⁶⁵ Duan et al.¹⁴⁶ studied the impact of adding ZIF-8 nanoparticles into a polyamide layer. The addition of a small amount of ZIF-8 (0.05 wt%) improved the water permeability by 88% and maintained high NaCl rejection (98%) during filtration tests with brackish water. The presence of a ZIF-8 membrane enables freely moving water molecules, leading to increased permeability. Regis et al.¹⁶⁶ explaining the effect of pH on the separation of boron (B)- and arsenic (As)-based samples using a pressure-driven membrane, it was also observed that in the presence of B, the As separation efficiency increased. Furthermore, he concluded that the developed membrane can be used for seawater desalination by optimizing the pressure and pH of a solution. In this study, several unique synergetic effects on the rejection of As and B were observed through membrane-based treatment. Regis et al. explained the effect of pH on the separation of B- and As-based samples using pressure-driven membranes. Interestingly, smaller molecules separated at lower pH levels, while larger ions separated at higher pH levels. Additionally, the presence of B increased the separation efficiency. The researchers concluded that by optimizing the pressure and pH of the tested solution, the developed membrane can be used for seawater desalination. Furthermore, Park et al. developed a thin-film composite RO membrane by modifying the support layer polyether sulfone, which contains HKUST-1 [Cu₃(BCT₂)] MOF treated with sulfuric acid. Moreover, a membrane was fabricated through interfacial polymerization involving the reaction of m-phenylene diamine (MPD) in an aqueous solution with trimesoyl chloride (TMC) in an organic solvent applied onto the MOF/PSF support layer. An acidified MOF enhances the water flux and permeability of the membrane through the addition of hydrophilic functional groups and the creation of strong bonds between copper ions and clusters. Therefore, as the water flux of the membrane increased from 21 to 28 GDF, which is 33% greater than that of the pure RO membrane, a salt rejection of 96% was observed with 20 ppm NaCl feed solution, clearly revealing the influence of the modified layer on the properties of the prepared membrane.¹⁵⁰

These preliminary studies demonstrated that UiO-66 and ZIF-8, which have small window cage structures, achieve excellent rejection of salt when compared to commercially available RO membranes. Furthermore, MOF-integrated membranes exhibit more than twice the permeability of the current membranes available on the market. This improvement in water permeability is due to the hydrophilic functionalities on the surface of the MOFs. In this way, water molecules are attracted to form a compact hydration layer and decrease the intrinsic resistance of the implanted MOFs; as a result, water molecules move faster across the whole membrane and its substrate.¹⁶⁷ In addition, different studies have explored the fouling propensity of MOF-integrated membranes. With the addition of ZIF-8 to the PA layer, Aljundi et al.¹³⁸ reported increased membrane hydrophilicity and an approximately 107.5% increase in permeate flux. With 99.5% salt rejection, a commercial membrane was outperformed by 50% in terms of permeate flux. The inclusion of ZIF-8 decreases membrane fouling, resulting in a decreased permeate flux (13.1% for the ZIF-8 membrane vs. 53.5% for the neat membrane), according to fouling studies employing bovine serum albumin (BSA) as a model foulant. The addition of ZIF-8 causes physicochemical modifications that improve the anti-fouling capabilities of the RO membrane. The desalination capabilities of additional MOFs, including MIL-100(Cr), MIL-125, UiO-66, and PCN 222, were also examined. MIL-100(Cr) or UiO-66 increases the water flux by 45% or 33.1%, respectively, and in another study, Zirehpour et al.¹⁵⁵ developed a porous hydrophilic TFC-RO membrane by modifying the polysulphone surface with acidified HOKUST-1 MOF in the PA layer. The performance of the developed membrane was evaluated with different salts, surfaces and film thicknesses. As shown in Fig. 10, the flux of the R-1 membrane is 21 GFD with a salt rejection of 94%, while when acidic MOF is added, the flux of the MR-I membrane increases to 28 GFD with decreasing salt rejection. Using acidic MOF-based TFC-RO membranes can significantly improve the rejection and flux performance due to the extra porosity of the substrate and hydrophilicity of the modified MOF, as shown in Fig. 10a.¹⁵⁰ With more constant permeance over a longer operating period, separation performance shows that an increase in water flux of 33.1% could be achieved without affecting salt rejection or membrane fouling resistance against BSA or artificial contaminants.¹⁵⁴ Similarly, the sizes of 60 nm, 150 nm, and 250 nm ZIF-8 nanoparticles also affect the interfacial area between ZIF-8 and the PA matrix (Fig. 10b), influencing the performance of thin-film nanocomposite (TFN) membranes.^147,149,168 Therefore, it can be concluded that the current progress in the development of MOF-based membranes for desalination applications has shown promising results. By optimizing MOF selection, synthesis techniques, and membrane architecture, researchers have achieved improved permeability of water, a high rejection rate, and enhanced anti-fouling properties. These advancements contributed to the development of more efficient and commercially viable desalination processes.

Fig. 10 (a) Water permeability and salt rejection of the TFC RO membrane on the PES and acid-MOF/PSf membranes: (a) R-I, (b) R-II, (c) MR-I, and (d) MR-II. Reproduced from ref. 150 with permission from Elsevier, copyright 2017. (b) ZIF-8-deposited NPs with 60, 150, and 250 nm membrane support layers prior to another interfacial polymerization process. Reproduced from ref. 154 with permission from American Chemical Society, copyright 2016.

5.1.2. 2D MOF based membrane. Water desalination^67,169 can also be conducted with 2D MOF-based membranes, which are more unique in their properties. Hong et al.¹⁷⁰ evaluated the performance of the 2D MOF membranes using CuBDC and IRMOF-1 in the RO desalination process through dynamic molecular simulation. Both membranes are the same in structure because they contain the same benzene dicarboxylate linkers but different metal nodes are used. The effects of the different metal nodes used were examined in terms of their water flux and ion rejection rate, and both MOFs exhibited 100% salt/NaCl rejection and high-water flux compared to those of other 3D MOFs due to the metallic effect. 2D MOFs have drawn significant attention in the fabrication of TFN membranes for the desalination of water because of their compatibility and proper molecular sieving performance. Liu et al. developed an ultrathin Ni-based 2D MOF-based TFN membrane via interfacial polymerization, and applied it for water desalination at different ratios. They observed that a very low amount (0.015 wt%) of 2D MOF-modified membrane (M-3) can increase the water permeance by up to 150% while maintaining a salt rejection of 99% compared to that of other 3D MOF membranes, as indicated in Fig. 11a–c These synthesized membranes also exhibit excellent durability and antifouling properties, and their effect is also confirmed through MD simulations. However, these developments are still lab-based, so it is necessary to observe their performance in wider areas, such as water treatment plants, gas separation, and industry.¹⁷¹

Fig. 11 Schematic illustration of the (a) fabrication of the 2D-MOF-modified TFN membrane via the interfacial polymerization reaction, and their (b) permeance, and (c) salt rejection. Reproduced from ref. 171 with permission from Elsevier, copyright 2022.

5.2. MOF-based nanofiltration (NF) membranes

The NF membrane is commonly utilized to remove several salt ions, such as MgCl₂, Na₂SO₄, and MgSO₄.^172–175 These membranes are amenable to nano size-based separation and can be prepared through several methods on a porous polyether sulfone (PES) substrate, a continuous ZIF-8 layer was created by Dai et al.¹⁷⁶ using a liquid–liquid interfacial coordination technique. An aqueous Zn²⁺ solution and a 2-methyl imidazole ligand solution were applied to the substrate to completely saturate it during the production process. A ZIF-8 layer eventually grows on the substrate after a particular amount of time. Through the IP procedure, He et al.¹⁵² investigated the impact of UiO-66 MOF on the physiochemical properties of thin film nanocomposite (TFN) membranes. Fig. 12a shows that different membranes were used for the removal of Se ions, but the TFN membrane efficiently removed both ions with high rejection. For example, the rejection of SeO₃²⁻ increases from 82.4 to 96.5%, and for HAsO₄^2–, it increases from 91.3 to 98.6%. This increase is due to the aperture of the UiO-66 particles, which allows water to permeate smoothly while selectively repelling the solutes. In this study, a TFN-30 membrane was used, which indicates that the MOF added to this membrane is 30 nm in size because of the reasonable flux of 8.0LMH/bar with favorable rejection of the mentioned salts. Fig. 12b shows the NF performance of the TFN-30 membrane as a function of the nanofiller. The pure water permeance increased from 5.5 to 11.5 L m⁻² h⁻¹ bar⁻¹, while the rejection of the abovementioned four salts remained the same. This difference may be due to a change in the microstructure of the polyamide resulting from the addition of MOF nanoparticles. Fig. 12c shows the optimized dose at which the amount of MOF added to the membrane results in the maximum amount of reaction. Here, it was observed by the author that 0.025% of the material provides the best results, with approximately 96% rejection. This is mainly due to the nanoparticle apertures produced, which are smaller than the hydrated diameters of As and Se anions. Furthermore, Donnan exclusion plays a role because the TFN-30 membrane is negatively charged in the pH range of 2–11. Fig. 12d shows the experimental study of the operational stability of the membrane with respect to time. In this study, the authors used a TFN-30 membrane for 160 h in salt solutions. The results showed that the membrane is stable in terms of separation performance for up to a long time but shows a slight decrease in the water permeance in the initial stage, possibly due to chain packing. The membrane has promising stability due to the addition of MOF UiO-66 nanoparticles in aqueous solution, which enhances the compactness of the polymer chain. However, despite serving as an efficient water-transport channel, UiO-66 is 500 nm in length, which results in a decrease in the membrane water permeability. This decrease can be attributed to two main factors: (i) increase in thickness of polyamide layer due to larger size of nanoparticle which results in high mass transport resistance,^118,155 while (ii) the presence of overlapped pores inside produces rough dispersion and distribution of the nanoparticles on the surface of PA layer.⁹⁵

Fig. 12 (a) Effects of UiO-66 particle size on the removal of Se and As from NF membranes and comparison with the results for other membranes (the loading of UiO-66 particles was 0.025 wt% for all the TFN membranes). The pH values of the SeO₃²⁻, SeO₄²⁻ and HAsO₄²⁻ solutions were 7.5, 8.0 and 8.6, respectively. (b) Effects of 30 nm UiO-66 loading on the PWP and salt rejection of the TFN membranes. (c) Effects of 30 nm UiO-66 loading on Se and As removal by the TFN membranes. The pH values of the SeO₃²⁻, SeO₄²⁻ and HAsO₄²⁻ solutions were 7.5, 8.0 and 8.6, respectively. (d) Evolution of the pure water permeability and SeO₃²⁻ rejection of the TFN-30–0.15 wt% membrane (the pH of the SeO₃²⁻ solutions was 7.5). Reproduced from ref. 152 with permission from Elsevier, copyright 2017.

Liu et al.¹⁵³ modified UiO-66-NH₂ with palmitoyl chloride to address the problem of nanofiller dispersion. The dispersity of the nanofillers within the PA layer is improved by this alteration. According to the study, there is 95% Na₂SO₄ rejection and an increase in the pure water flux from 8.1 to 12.4 L m⁻² h⁻¹ bar⁻¹. The improved UiO-66-NH₂ material achieved excellent dispersity, enabling consistent membrane performance with good durability after continuous filtration for 80 h, despite the higher permeability of the nonmodified nanofillers.^113,117,175 A fascinating study was published on the incorporation of MOFs into the PA layer using the Langmuir–Schaefer method as opposed to traditional mixing in the organic phase.^173,177,178 The authors created a MOF thin film by Langmuir–Schaefer, transferred it to a cross-linked polyimide (P84) support, placed it there, and then performed the standard IP procedure. In contrast to the control approach, where the MOF is disseminated in the organic phase, the resulting TFN membrane displays a homogenous MOF coating and high permeability and selectivity, which prevent potential membrane flaws.^179,180 Similarly, Zhu et al.¹⁸¹ employed a vacuum filtering technique to place a UiO-66-NH₂ dispersion in an aqueous solution before continuing with the standard IP process with an organic solution. This method prevents any loss of the loaded MOFs and enables regulated loading of UiO-66-NH₂. This process produces a rough and crumpled PA surface, similar to a fishnet, as opposed to the usual techniques of dispersing MOFs in aqueous or organic solutions, which leads to random and unpredictable MOF dispersion (Fig. 15). Despite the covalent reaction of the amine groups of UiO-66-NH₂ with the PA matrices, the presence of MOFs causes a looser PA layer to develop. By decreasing the interlocking effect between the PA layer and the substrate, this fishnet-like structure improved water movement throughout the separation membrane.^30,181 Moreover, the presence of Cu within the Cu–BTC structure considerably decreased the fouling of the membrane by up to 40% compared with that of BSA. The decrease in membrane fouling potential can be attributed to the improved hydrophilicity, surface hindrance caused by the addition of CuBTC and surface negativity. Wang et al.¹⁸² synthesized a ZIF-8 MOF, crumpled PA layer on a sacrificial template. In their method, the IP technique was used to introduce ZIF-8 with single-walled carbon nanotubes attached to the PA layer. Water was used to cleave the coordination bonds in the MOFs, which caused the MOF structures to collapse due to the limited hydro stability of the MOFs. With a superior water flux of 1830 L m⁻² h⁻¹ bar⁻¹ and a Na₂SO₄ rejection of 97.3%, the geometric structure of ZIF-67 allows for a larger effective surface area to hold more water molecules. In a further thorough investigation, Zhao et al. used the blending IP and preloading IP methodologies to assess how three different kinds of water-stable MOFs such as MIL-53(Al), ZIF-8, and UiO-66-NH₂—interact with the PA layer. In comparison to those of the control TFC membrane, all the MOF-integrated TFN membranes had a thicker PA layer, greater surface negativity, and rougher surface. Among the other TFN membranes introduced with various MOFs, the TFN membrane containing UiO-66-NH₂ had the highest membrane permeability; this membrane is almost 1.3 times more permeable than the TFN membranes incorporated with the other two MOFs. The characterization results showed that each MOF has a varied capacity for binding during the IP process, with MIL-53(Al) displaying stronger PA-MOF interactions than UiO-66-NH₂ and ZIF-8.¹⁵⁷

5.3. Osmotic pressure-driven MOF membrane-based desalination process

Despite its long history of being well established, forward osmosis (FO) has attracted much attention in the last ten years. The draw solution is diluted via osmosis, and then pure water permeates through the diluted draw solution to complete the FO process.^21,183,184 The fact that FO does not require any hydraulic pressure is one of its main advantages over pressure-driven membranes. This approach capitalizes on the disparity in osmotic pressure between the feed solution and the draw solution, causing the permeate to flow from an area of lower osmotic pressure (feed solution) to an area of higher osmotic pressure (draw solution), thereby diluting the draw solution.¹⁸⁵ Unlike RO, the FO process does not necessitate the application of external pressure. Like NF and RO membranes, most FO membranes are composed of TFC membranes, which possess a dense layer without distinct pores. This design ensures effective ion separation and allows the permeation of the draw solution. Thus, the desired properties of FO membranes align with those employed in NF and RO processes. On the other hand, due to the accumulation of solute in the membrane structure, the FO is more prone to internal concentration polarization (ICP). Consequently, to alleviate the effect of ICP, the use of a porous substrate with high porosity, less tortuosity, and reduced thickness is suggested.¹⁸⁶

5.3.1. 3D MOF based membrane. Yuan et al. developed a microbial desalination cell forward osmosis membrane for the treatment of saline water. In this process, the anode effluent was used as the FO feed.¹⁸⁷ The efficiency of MDC-FO in reducing the conductivity of saline water to half that of the other methods mentioned in the study. The draw solution used in the experiment consisted of a 100 mL NaCl solution with a concentration of 35 g L⁻¹, which was recirculated at the same velocity. The results demonstrated the potential of the MDC-FO system for the treatment of brackish water and desalination of high-salinity water while concurrently addressing wastewater treatment requirements.¹⁸⁷ Zirehpour and colleagues successfully synthesized a TFC/FO membrane incorporating nanosized MOF particles that are composed of Ag(I) and 1,3,5-benzene tricarboxylic acid (BTC). This innovative membrane was designed not only for desalination purposes but also for enhancing the structural properties of the membrane. In this study, MOF nanocrystals were added to a polyamide layered membrane through interfacial polymerization. Incorporating MOF crystals leads to a change in the active layer of a membrane in terms of transport properties and hydrophilicity without affecting the selectivity of the membrane. This process caused an increase in permeability of up to 129% when 0.04% MOF was added to the organic phase during polymerization. As a result, an enhancement in FO seawater desalination was observed through this modified membrane. Consequently, this study elucidated the potential of MOF particles to augment the desalination performance of TFC membranes within FO systems.¹⁵⁵ Water flux in FO membranes typically follows patterns similar to those of other membrane processes. However, due to the utilization of both sides of the membrane, there is a propensity for reverse solute flux (RSF), also referred to as RSD. RSF involves the diffusion of draw solution from the draw side to the feed side across the membrane. Several factors can influence RSF, including the difference in solute concentration, the specific type of draw solution employed, and the physicochemical properties of the membrane material being used. These factors play a critical role in determining the overall performance and efficiency of FO processes. To date, most studies on MOF-integrated membranes have focused on RSFs, and these studies have incorporated MOFs into either the PA layer or the substrate layer of TFC membranes. Currently, there are no reported studies on standalone MOF membranes for FO processes.¹⁸⁸ In the context of FO, Shaffer et al.¹⁸⁹ created a novel TFC membrane containing UiO-66 nanoparticles that exhibited a 25% increase in maximum water flux and a decrease in RSF compared to those of a normal TFC membrane. To create a suitable TFC membrane, Dai et al.¹⁷⁶ filled the active layer of PA with copper 1,4-benzene-dicarboxylate nanosheets (CuBDC-NSs). The novel TFC membrane demonstrated an approximately 51% increase in water flux and a 50% reduction in RSF when utilizing a 1.0 M solution of NaCl as the extraction solution in active layer-oriented feed solution (AL-FS) mode, according to the experimental results. Additionally, the improved hydrophilicity and biocidal capacity of the new TFC membrane are attributed to its improved antifouling ability. For FO procedures, Zirehpour et al.¹⁵⁵ added MOFs to the PA layer of a TFC membrane. The organic component of their investigation contained rod-shaped MOFs made of Ag and 3HBTC. Because of the dangling functionalities of carboxylic acid groups on the TFN membrane containing MOFs, the membrane hydrophilicity increases. Furthermore, the membrane exhibited a higher B/A value, indicating a reduced level of transport resistance within the thicker active layer, while maintaining a desirable level of permeation selectivity. Because of the disruption of polymer chain packing caused by the addition of MOFs during the interfacial polymerization process, the transport resistance is found to be reduced. Compared to that of the TFC membrane (27 L m⁻² h⁻¹ bar), the water flux of the TFN membrane was greater (34 L m⁻² h⁻¹ bar⁻¹) in tests using actual seawater samples from the Caspian Sea. Furthermore, to lessen ZIF-8 dissolution, Abdullah et al. coated nanoparticles with poly(sodium-4-styrene sulfonate). The water stability of ZIF-8 nanocrystals is improved by the electrostatic stabilizing effects of this anionic polymer. In comparison to the TFN membrane created without triethanolamine (TEA), the TFN membrane containing PSS-coated ZIF-8 has 116.4% greater pure water permeability while preserving equivalent NaCl ion rejection. The creation of a thinner, rougher PA coating and the presence of hydrophilic, less convoluted ZIF-8 protrusions on the membrane surface are both credited with this improvement.¹⁷⁷

5.3.2. 2D MOF based membrane. Wang et al. developed 2DZIF-8 on a flexible polysulfide surface with modified polydopamine via a facile immersion strategy.¹⁹⁰ The ZIF-8/PDA/PS membrane exhibited distinctive characteristics, notably an increased surface roughness ranging from 17.2 nm to 154 nm. Consequently, this membrane has demonstrated a notable water flux of 9.6 L m⁻² h⁻¹ bar⁻¹ and a low solute reverse flux of 3.8 g m⁻² h⁻¹ (Fig. 13a). These performance metrics were achieved by employing a 1 M MgCl₂ solution as the draw solution and DI water as the feed solution under forward osmosis mode. Furthermore, the fabricated membrane also showed satisfactory antimicrobial properties against Escherichia coli (E. coli), with an antibacterial performance of up to 99%, which was greater than that of the PDA/PS membrane (64%) during the desalination process. Therefore, the authors declared that the membrane has good antibacterial activity as well as efficient FO performance in the desalination of seawater. However, more attention is required to study the stability of these membranes in different media to make them available from the laboratory to the commercial market. He and his coworkers introduced an innovative PDA/MOF thin-film nanocomposite (PDA/MOF-TFN) FO membrane. This membrane was synthesized through an interfacial polymerization process involving an aqueous phase consisting of m-phenylene diamine (MPD) and PDA and an organic phase incorporating TMC and MOFs. The synthesized PDA/MOF-TFN membrane was used for salt rejection and heavy metal removal from polluted water. This MOF-801 membrane strongly attached to the deposited PDA, which caused an increase in the uniformity of the distribution of the deposited PDA into the PA layer without agglomeration. As a result, additional water channels were introduced. Therefore, an increase in water permeability and water flux was observed. According to recent research, the PDA/MOF-TFN membrane shows significant improvement over the conventional TFC membrane. The PDA/MOF-TFN membrane exhibited an increase in water flux of up to 30% while showing a 56% increase in specific salt flux. Moreover, there was a decrease in the reverse salt flux of approximately 44%. The prepared PDA/MOF-TFN composite membrane have good ability for removal of heavy metal ion rejection (2000 ppm of Cd²⁺, Ni²⁺, and Pb²⁺ > 94%), as shown in Fig. 13b. The high efficiency of these membranes is because of its greater adsorption capability. The obtained results indicate the potential of these developed membranes for salt rejection and heavy metal ion removal in wastewater treatment. However, for commercialization, additional research and attention are required in terms of durability and antifouling properties.¹⁹¹

Fig. 13 (a) Water permeance of (Jv) and reverse water flux (Js) of the ZIF-8/PDA/PS and PDA/PS membranes under FO mode. Reproduced from ref. 190 with permission from Elsevier, copyright 2020. (b) Separation performance of the M1 (PDA/TFN) and M-3 (PDA/MOF-TFN) membrane. Reproduced from ref. 191 with permission from Elsevier, copyright 2020.

5.4. Thermally driven desalination process

MD can be categorized as a thermally driven desalination process because it relies on thermal heat for operation. In the MD method, heat is applied to generate a vapor pressure gradient that drives the mass transfer flux.¹⁹² The membrane utilized in this process must possess hydrophobic (non-wetting) characteristics and exhibit microporosity, with pore sizes and porosities typically in the range of 0.1 to 1 μm and from 40% to 90%, respectively, in order to achieve its required efficiency.¹⁹³ Furthermore, an ideal MD-based membrane should demonstrate low resistance to mass flow to enable efficient vapor transport and minimal thermo-conductivity to reduce heat loss. The optimal heat transfer can be achieved by employing a membrane with a thickness between 31 and 60 μm. Additionally, MD-based membranes should possess good thermostability, a narrow distribution of pore sizes, and favorable liquid entry pressures (LEPs).¹⁹⁴

5.4.1. 3D MOF based membrane. To ensure optimal performance, the membranes used in MD must meet specific criteria, including hydrophobic nature, excellent thermal stability, low resistance to solute transport, and thermal conductivity in nature on the feeding solution side. One approach for achieving these desired properties is through the addition of hydrophobic MOFs. Zuo et al.¹⁹⁵ fabricated a novel MOF-based membrane by surface functionalization of a porous type of alumina support membrane along with NH₂-MIL-53(Al). Porous alumina acts as an active site for the growth of MOF MIL-3; in this way, it eliminates the delamination issues associated with MOF-based layers. Moreover, the ligand used in MIL-53, amino-terephthalic acid, enhances the surface hydrophobicity significantly by reacting with perfluorooctanoyl chloride. This modification results in a membrane with water flux, a stable response to high pressure and temperature, and a hydrophobic surface. Thus, compared with the commercially available membrane polytetrafluoroethylene (PTFE), which is 12.0 L m⁻² h⁻¹ bar⁻¹, this modified membrane has a permeability of 32.4 L m⁻² h⁻¹ bar⁻¹, as reported by Alsaadi et al.,¹⁹⁶ while maintaining adequate desalination by rejection of ions. In another study by Yang et al.,¹⁹⁷ a superhydrophobic membrane containing 5% MOFs was prepared from polyvinylidene fluoride (PVDF) nanofibers. This composite type of membrane results in the super rejection of NaCl of up to 99.9% and an increase in hydrophobicity. A contact angle of 138.06° was observed when the material was used in direct contact membrane desalination (DCMD). Kebria et al.¹⁹⁸ developed another interesting TFN membrane for MD treatment by coating a hydrophobic material, PVDF, with an ultrathin layer of ZIF-8/chitosan. Here, surface modification improved vapor permeability without affecting membrane hydrophobicity. This improvement is achieved by decreasing the mean surface pore size and enhancing the porosity of the surface. Because of this, the permeability of water increased to 350%, while 99% rejection of NaCl was observed. In conclusion, the added chitosan prevents fouling of the membrane and acts as an antifouling agent, stabilizing the membrane during sea water desalination. Therefore, this modified membrane has shown excellent properties in sea water desalination, as a flux recovery of 90% is achieved, which is more than 30% better than that of the neat PVDF membrane.

5.4.2. 2D MOF based membrane. Traditionally membrane distillation-based membrane method is very less used for water treatment in commercial market due to reluctant scalability of synthesis, stability and non-smooth pore size of prepared membrane. So as to tackle this 2D MOF based membrane have been fabricated which made MD process more feasible currently. In addition to possessing pores small enough to reject ions, the ideal material for water desalination must also feature pores large enough to facilitate rapid water transfer. Hexaaminobenzene (HAB)-derived 2D MOFs, including other 2D conductive MOFs⁹⁷ have been extensively explored for energy storage and separation applications.¹⁹⁹ The nanopores within conductive MOF layers exhibit an area of approximately 43.32 and a diameter of 8 nm. These membranes stand out as superior options for water desalination due to the size of their pores and the experimental capability to manufacture a few layers of 2D MOFs.^200,201 Studies showed that a major global challenge revolves around providing access to clean drinking water. The utilization of energy-efficient nanoporous materials²⁰² for water desalination may become feasible as advancements in nanomaterials research unfold. In their work, they demonstrated how extremely thin conductive MOFs can effectively reject ions while permitting significant water flux. The optimal ion rejection rate was identified using two-dimensional multi-layer MOFs through molecular dynamic modeling. The naturally porous structure of 2D MOFs enables water permeability that is 3 to 6 orders of magnitude greater than that of conventional membranes. When compared to single-layer nanoporous graphene or molybdenum disulfide (MoS₂), few layers of MOF membranes exhibit an order of magnitude higher water flux without the need for pore drilling. Water permeation simulations, water density/velocity profiles at the pore, and water interfacial diffusion near the pore all validate the excellent performance of 2D MOF membranes. The water desalination capabilities of MOFs present a potential solution for energy-efficient water desalination. Recently several researchers are investigating the synthesis feasibility as well as performance of 2D MOFs membrane in water desalination. In this regard different membranes mixed composite containing 2D MOF based materials have been assessed. In connection, a hydrophobic membrane imitating nature and exhibiting remarkable resistance to wetting have been developed for the process of seawater desalination using vacuum membrane distillation (VMD). The key innovation lies in the molecular engineering applied to the surfaces of alumina through the integration of MOF. To create these hydrophobic membranes, a two-step synthetic technique was devised: firstly, MOF crystals were intergrown on the alumina tube substrate, and secondly, perfluoro molecules were introduced onto the MOF-functionalized membrane surface. The initial step allowed for precise control over the surface morphology, particularly the hierarchical roughness, by adjusting the MOF crystal structure. Following the second step, the perfluoro molecules acted as an ultrathin layer of hydrophobic material, reducing the surface energy. Consequently, the resulting membranes not only retained the inherent advantages of alumina supports, such as exceptional stability and high water permeability, but also featured a hydrophobic surface achieved through MOF functionalization. Under optimal conditions, the prepared membrane demonstrated a commendable VMD flux of 32.3 L m⁻² h⁻¹ at 60 °C. This exploration introduced a groundbreaking approach for the design of next-generation, high-performance membrane distillation systems tailored for seawater desalination.¹⁹⁵

5.5. Removal of oil and micro-pollutant from water

The issue of oil-based pollution has become a global environmental concern, primarily due to its origins in industries such as textiles, steel, and petroleum. Accidents involving oil leakage during exploration or transportation further contribute to this problem.^203–205 To address these challenges, several researchers have comprehensively used MOF-containing membranes for the removal of oily wastewater. These membranes, which can be easily fixed onto surfaces, offer excellent properties. By incorporating MOFs, the membranes achieve different affinities for the oil/water phases, enabling the selective separation of oil–water mixtures. The combined effect of super wettability and high porosity synergistically enhances the superhydrophobic and super lipophilic characteristics of the membranes, making them highly effective for the treatment of oily wastewater.²⁰⁶ A superhydrophobic/super-lipophilic membrane consisting of ZIF-8 incorporated within rGO nanosheets was designed using a high-temperature reduction self-assembly process.^27,118 This innovative method results in a membrane with exceptional water-repellent and oil-attracting properties, making it well suited for applications involving the treatment of oily substances.²⁰⁷ The ZIF-8@rGO@Sponge membrane not only provides a useful solution for treating polluted water and separating oil/water but also provides an innovative idea for the advancement of membranes with incorporated MOF materials. This development represents a significant step in the exploration of MOF-based membranes for various applications beyond traditional separation processes. The ZIF-8@rGO@Sponge membrane not only provides promising materials for water–oil separation and wastewater disposal but also introduces a new concept for the development of MOF-containing membranes. Furthermore, this innovative approach combines the properties of PLA and ZIF-8 to develop efficient membranes suitable for oil–water separation applications. As the quantity of ZIF-8 nanomaterials within the membranes increased, the surface roughness of the membrane increased, and the diameter of the blended fibers decreased. This adjustment in material composition results in altered membrane properties, potentially affecting its performance in oil–water separation applications. Additionally, the contact angle test confirmed that the oil wettability of the PLA/ZIF-8 composite membrane substantially improved as the content of ZIF-8 nanoparticles increased. This indicates that a higher ZIF-8 content leads to enhanced oil-repelling properties, which can be advantageous for effective oil–water separation processes. However, by enhancing the roughness, wetting ability, and adsorptive characteristics of polyporous ZIF-8 nanomaterials, the PLA/ZIF-8 membranes effectively facilitate the separation of diesel and water mixtures. This combination of properties makes these membranes promising candidates for efficient diesel–water separation applications. Therefore, these membranes rapidly absorb oil droplets while permitting the passage of water molecules, resulting in exceptionally high separation efficiency. This selective permeability makes them highly effective at separating oil from water mixtures.²⁰⁸ In another study, Dai et al.²⁰⁸ developed a multifunctional porphyrinic Zr-MOF composite membrane with PCN-224/PVDF/TA via a facile in situ deposition method under vacuum. This prepared membrane was applied to separate water/oil mixtures with a vacuum pumping system at 0.1 MPa. Different mixtures, such as heptane in water, petroleum ether in water, diesel oil/water, and toluene/water, were chosen as feed emulsions to evaluate the separation performance of the super wetting membrane. Fig. 14a shows a 50 mm membrane that allows water to penetrate while retaining a particular oil at the surface. This membrane exhibited a high separation efficiency of more than 99% for all the mentioned oil-containing analytes. The filtrate also contained a very low amount of remaining oil, ranging from 7.5 to 11.4 ppm, suggesting successful operation. The fluxes of various substances, including heptane (829), hexane (830), petroleum ether (1542), diesel in water (894), and toluene/water (994 L m⁻² h⁻¹ bar⁻¹), are shown in Fig. 14b. This super wettable hydrophilic/underwater super oleophobic membrane shows high antifouling properties due to the nonadherence of oil droplets on the membrane surface and is stable under certain conditions. However, the mechanism of the formation of these membrane composites is still intermediate and is not fully understood; thus, additional research is needed.²⁰⁹

Fig. 14 (a) Separation efficiencies and oil contents in the filtrates. (b) Fluxes of different oil-in-water emulsions over the PCN-224/TA/PVDF membrane. Reproduced from ref. 209 with permission from Elsevier, copyright 2021.

In addition, MOF-containing membranes are superior to other conventional membranes in terms of selectivity, permeability, stability and flux. Indeed, the inherent advantages of MOF membranes, including their exceptional selectivity, permeability, and chemical stability, render them highly appealing options for a wide range of separation processes. These devices are particularly well suited for situations where these performance attributes are of paramount importance, offering innovative solutions in diverse applications that demand precise and efficient separation.^39,173 The principal mechanisms underlying the separation of oil from water facilitated by MOF-based membranes primarily involve size exclusion effects and hydrophilic interactions. Nevertheless, practical challenges persist, including low membrane flux rates, elevated preparation costs, and the potential for membrane fouling. Addressing these challenges is essential for making MOF membranes more viable and efficient for real-world applications in oil–water separation processes.^210,211 In future research endeavors concerning MOF membranes for the removal of oily wastewater, the primary emphasis will be on developing and producing cost-effective MOF-based membranes that exhibit exceptional separation efficiency, permeability, and robust antifouling properties. These objectives align with the broader aim of advancing MOF-based membrane technology for practical and sustainable applications in the field of oil separation from water.²¹²

In addition to oil-containing pollutants, micro pollutants including pharmaceutical contaminants, endocrine disruptors, pesticides, cosmetics, and poly-aromatic hydrocarbon (PAHs), are significant contributors to water pollution. These micro pollutants can have adverse environmental and health effects, even at low concentrations. Addressing the removal and mitigation of these micro-contaminants is a critical aspect of water treatment and pollution control efforts.⁵⁴ Incorporating MOFs into polymer-based substrates to develop membranes with multifunctionality is a promising strategy for the efficient removal of these micro contaminants from samples. This approach leverages the robust ability of MOFs to form complexes and allows them to effectively capture and remove micro-contaminants from water sources, contributing to enhanced water treatment and purification processes. For example, Zhu et al.²¹³ functionalized a zeolitic imidazolate bilayer filter membrane with ZIF-8 to eliminate progesterone. Compared to the original membrane, the MOF-incorporated composite membrane exhibited an approximately 41% greater adsorption capacity. Moreover, even after three regenerative cycles using high concentrations of hormones, the MOF composites exhibited 95% high ion removal efficiency, which was attributed to the hydrogen bonding and effective contact area interactions between ZIF-8 and the H-donor and H-acceptor agents. Similarly, Dai et al.²¹⁴ investigated the incorporation of MIL-101(Cr) into the chain of a polyamide layer to create a TFC membrane with 0.20 wt/v% MOF content (MOF0.20). The membrane was used for the removal of endocrine disrupting compounds (EDCs). In particular, the rejection rates of (EDCs), propyl paraben, methyl paraben, and BPA were significantly greater when the MOF0.20 membrane was used than when the standard NF membrane was used. This finding underscores the effectiveness of MOF membranes for removing EDCs from water sources. Incorporating hydrophilic MOFs creates water/EDCs-selective channels, leading to increased rejection of EDCs. In this study, the authors illustrated the mechanism of removal of EDCs from the MIL-101 TFN membrane. The EDC-based compounds from the membrane are controlled by adsorption and diffusion. The introduction of MIL-101 decreases the sorption rate of EDCs due to its hydrophilic nature and results in the generation of a polyamide layer with hydrophilic character, thereby mitigating the issue of EDCs migration from the compound type of the membrane they identified. The enhanced rejection of EDCs results from the negative relationship between adsorption and rejection, which is primarily caused by the effect of dilution.

5.5.1. 2D MOF based membrane for oil/water separation. The search for membranes capable of achieving rapid oil–water separation has been on the rise to meet the demand in applications generating substantial volumes of oil–water mixtures, such as mining operations.²¹⁵ Recently, a breakthrough was achieved with the development of an ultrafast oil–water separation membrane through the growth of a 2D Cu triphenylene catecholate MOF on a copper mesh (Fig. 15). This membrane exhibited a distinctive hierarchical structure, coupled with a polar nano-cavity within the MOF crystal, resulting in effective superhydrophilic and underwater superoleophobic wettability. Consequently, the membrane showcased gravity-driven separation of various oils, including crude oil, achieving an ultrahigh flux of up to 329 kL m⁻² h⁻¹ during the separation process. Moreover, the membrane's unique hierarchical structure contributed to enhanced separation efficiency, with permeates exhibiting less than 24.6 mg L⁻¹ of oil contents (Fig. 15a–c).⁵⁴ Despite the progress in developing high-flux MOF-based membranes for oil–water separation, a significant limitation lies in their ability to demonstrate only a single type of surface wettability, which may not meet the complex requirements for sustainable wastewater treatment. This challenge can be addressed, in part, by creating membranes with the capability to separate oil–water mixtures on demand. One promising approach involves incorporating switchable wettability, allowing alternation between hydrophobic and oleophobic wettability under the influence of external triggers such as pH change,^216,217 light irradiation,^218,219 temperature change,²²⁰ and electricity.²²¹ In a particular study, a membrane with switchable superwettability was developed by growing CAU-10-H MOF on a stainless-steel mesh surface using the solvothermal method. The interpenetrating hierarchical structure of the CAU-10-H crystals enabled dual superoleophobicity in submerged conditions. The prewetting of the membrane with water formed a trapped layer preventing oil permeation, while initial wetting by oil created a trapped oil layer resisting water permeation. This property allowed the membrane to demonstrate on-demand separation of stabilized oil–water emulsions with a flux exceeding 1.85 × 10⁵ L m⁻² h⁻¹ and a separation efficiency exceeding 99.9%. Another emerging line of research focuses on developing sustainable MOF-based membranes using biodegradable and renewable materials, referred to as Bio-MOFs. These are synthesized using saccharides, peptides, and amino acids.²²² In one study, a superhydrophobic Bio-MOF coating was prepared on fabric for oil–water separation.²²³ The Bio-MOF, synthesized through an electrochemical process utilizing aspartic acid as the ligand and copper cores, underwent treatment with stearic acid to reduce the solid surface energy.¹³⁴ The resulting membrane, when sprayed onto the fabric surface, demonstrated separation efficiency ranging from 95% to 99.4% and flux rates between 15 400 and 15 700 L m⁻² h⁻¹ for different oil–water mixtures.¹³⁴ MOF-based membranes exhibit outstanding efficiency in terms of micro contaminant removal because of the use of hydrogen bonding, hydrophobic interactions, and π–π interactions. However, most studies on the removal and remediation of micro-contaminants through MOF-based membranes have been conducted at the laboratory scale. Although the potential of these research findings should not be underestimated, the effective utilization of MOF-based membranes in real polluted wastewater treatment applications requires further investigation. Additional studies are necessary to synthesize MOF-based membranes that can be successfully applied for cleaning micro-pollutants from sewage disposal plants.

Fig. 15 (a) The schematic diagram of the preparation of Cu-CAT-1@CM and the separation process for oil–water. (b) Flux and residual oil content of Cu-CAT-1@CM separation for different oil/water mixtures. (c) The underwater OCA for seven types of oils on the surface of Cu-CAT-1@CM. Reproduced from ref. 54 with permission from Elsevier, copyright 2022.

5.6. Separation of dyes from wastewater

The decontamination of dyes present in water holds paramount importance owing to their highly toxic nature and potential to cause different types of cancers.^224,225 Dyes found in water represent a key source of pollution, particularly in industries such as textiles, paints, rubber, leather, plastics, and paper production.²²⁶ Effectively treating and purifying dye wastewater is essential for mitigating the environmental and health risks associated with the discharge of these pollutants into water bodies.^22,227 In this section we would like to discuss the utilization of 3D MOFs and 2D MOFs for the separation of different dyes present in water system and the will assess the features of each one with critical eye.

5.6.1. 3D MOF based membranes. To effectively treat organic dye wastewater, membranes containing 3D-MOFs can be constructed by selectively choosing appropriate MOF types based on size exclusion. Guo et al.²²⁸ developed a ZIF-8 membrane for the separation of dyes, which demonstrated high retention rates of 91% to 97.5% for Rhodamine B (RhB) and 92.5% to 98.9% for rose red (RR) due to the efficient retention of organic dyes by ZIF-8 MOF particles containing pores between the dyes and water molecules. Li et al.²²⁹ has also demonstrated ZIP-8 membranes with high separation efficiency for the dyes molecules. Numerous research efforts have focused on the factors that affect the efficiency and removal capacity of synthesized MOF membranes, including the space between layers of the prepared membrane, the membrane layer thickness and the hydrophobic/hydrophilic nature of the prepared membrane. Zhao et al.²³⁰ developed a composite mixed PVDF/Cu–BTC membrane on a hollow fiber support, which exhibited a permeability of 65 kg m⁻² h⁻¹ and a rejection rate >99.5% for Congo red (CR) because of the presence of multiple Cu–BTC layers. Yang et al.²³¹ utilized a vacuum filtration approach to prepare a variety of Sm/MOF/GO mixed nanocomposite membranes to study the effect of metal functionality on the performance of MOF/graphene oxide membranes and on pure Sm/MOF materials. The M-0.31 composition demonstrated excellent permeance (26 L m⁻² h⁻¹ bar⁻¹) and a high rejection rate of more than 91% for RhB when more Sm/MOF was added, which resulted in greater interlamellar spacing in the membrane. Nevertheless, the additional GO sheets deposited result in irregular structures, which leads to decreased rejection of dyes after a few cycles. Therefore, it is crucial to maintain an appropriate interlayer spacing in fabricated membranes to make these membranes promising candidates for real-time dye removal from wastewater. Functionalized ZIF-8 (mZIF) was developed by Zhu et al.²³² through the mixing of ZIF-8 and PSS in a reaction mixture, and a TFN/mZIF composite membrane was subsequently constructed on polyamide-based layers. The addition of mZIF nanomaterials improved the membrane hydrophilicity and permeability up to 2-fold and resulted in excellent retention of more than 99% for the reactive dyes blue 2 and reactive black 5. In another study,¹⁷⁷ the authors used a freezing-assisted method to prepare F-ZIF-8 membrane composites to remove different dyes from water. In the membrane evaluation of separation performance, the chrome black T dye was removed, as shown in Fig. 16a. Three different materials were used as highly porous alumina substrates that exhibit high permeability but very low rejection. On the other hand, R-ZIF-8 exhibited a rejection rate of 72% with a permeance of 626 kg m⁻² h⁻¹ MPa. A comparison of F-ZIF-8 with F-ZIF-8 revealed that the rejection of dye increased to 99% when the permeance was maintained at 450 kg m⁻² h⁻¹ MPa. This excellent performance of the F-ZIF-8 membrane is due to the nanoconfined structure of this composite membrane, which contains large defects on the substrate. Therefore, the prepared F-ZIF-8 membrane was further used by the author for the separation of several dyes from water, such as Evans blue (EB, 960.8), Congo red (CR, 696.6), Eriochrome black T (EBT, 461.4), and methyl orange (MO, 327.3). Fig. 16b shows that the membrane has good separation performance for EB and CR, with a rejection rate greater than 97%. However, the rejection of MO is poor because the molecular weight of MO is low.⁴⁶ Similarly, Li et al.²³³ fabricated composite membranes known as PDA/ZIF-67@PP membranes. They achieved this by depositing biomimetic poly-dopamine (PDA) and ZIF-67 layers onto a substrate made up of polypropylene. This innovative approach combines the characteristics of PDA and ZIF-67 to create membranes suitable for various applications, including separation and purification processes. The incorporation of functional groups that are hydrophilic, such as hydroxyl- and amine-containing groups, into PDA materials plays an important role in increasing the performance of developed composite membranes for dye separation. As a result, the fabricated mixed membrane achieved a high retention of 92% for methylene blue dye (MLB) and methyl orange (MO) while simultaneously exhibiting a substantially greater flux of water (approximately 217 kg m⁻² h⁻¹ MPa⁻¹) than pure polypropylene (PP) membranes. This combination of improved retention and water flux makes the composite membrane a promising candidate for applications in separation and water treatment processes. The size screening process is instrumental for efficiently intercepting contaminants using a separation membrane. Additionally, the surface functionalization of MOF particles enhances their hydrophilicity, increasing the likelihood of entry of water molecules. In addition to size exclusion, various other mechanisms, including the removal of organic dyes from water sources, are mostly dependent on Lewis acid–base interactions, hydrogen bonds, and electrostatic interactions. These multifaceted interactions contribute to the effectiveness of the separation and purification processes employed by the membrane. By adjusting the charge characteristics of MOFs, the affinity of these membranes for several dyes can be enhanced. Guo et al.²³⁴ and Yang et al.¹¹⁶ used an interfacial reaction and self-assembly method to create a thin and uniform ZIF-8/PEI composite membrane. Due to the powerful interactions between negatively charged dyes and positively charged membranes, the membrane showed high retention rates for MLB (99.6%, 33 kg m⁻² h⁻¹), CR (99.2%, 37.4 kg m⁻² h⁻¹ MPa⁻¹), acid magenta (AM, 94.4%, 45.6 kg m⁻² h⁻¹ MPa⁻¹), and MO (81.2%, 51 kg m⁻² h⁻¹). A lower MLB permeation is the result of thicker dye layers, principally as a result of the stronger interaction of MLB with the membrane surface (which has a more negative charge). The ability of a Zr MOF developed from a polyurethane foam (Zr-MOFs-PUF) membrane to intercept variously charged dyes, such as anionic CR, cationic MB, and neutral RB, was examined.²³⁵ Due to the synergistic effects of Lewis acid–base interactions, hydrogen bond interactions, and electrostatic interactions, the membranes concurrently eliminate these model pollutants from the CR/MB, CR/RB, and MB/RB systems as well as from the CR/MB/RB systems. Filtration trials revealed excellent removal capabilities of approximately 95.73% for CR, 97.67% for MB, and 97.95% for RB. These studies demonstrate that MOF-containing membranes outperform single substrates and materials in terms of elimination capacity. The removal of the majority of dyes by MOF-containing membranes is significantly influenced by electrostatic interactions.¹¹⁸ However, challenges regarding stability in wastewater and robustness in the presence of complex pollutants in the water environment hinder the practical application of MOF-containing membranes in dye elimination. Additionally, limited research has been conducted on the influence of coexisting ions and operating modes on dye elimination from wastewater. Therefore, in-depth studies on the factors influencing dye removal and reaction mechanisms are essential for strengthening the continuous use of MOF-based membranes for dye decontamination.

Fig. 16 (a) Comparison of the separation performances of different membranes. (b) Separation performance of the F-ZIF-8 membrane for removing different dyes (EB, CR, EBT, MO) from water. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article. Reproduced from ref. 46 with permission from Elsevier, copyright 2021.

5.6.2. 2D MOF based membrane. Now a days researchers are getting more interest in 2D MOFs because these material are more feasible to use in different applications as they have more active sites, surface area and easily modifiable. 2D MOFs based membrane along with other applications also used in selective dye separation in water treatments system. Li et al.²³⁶ have developed a potable membrane containing device for the selective removal of particular dyes from water. In this self-made portable device, the 2D Cu-MOF was used as filter while syringe used as filter holder. The results revealed from experiments were extraordinary as more than 99% rejection of MLB, MB, MO, RB, copper phthalocyanine and phthalocyanine green was observed. Therefore, author claimed as the developed indigenous portable 2D MOF based device is economical, easy to operate and has capability of removing dyes in any emergency situation at anywhere from water system. Solid contaminants, such as colorants, pose a significant challenge in treating textile wastewater. The remediation of this type of wastewater is particularly challenging. NF membranes have emerged as effective tools for extracting dyes and organic pollutants from wastewater, thereby safeguarding water resources. In a study by Jafarian et al.,²³⁷ innovative NF membranes were developed using a thin layer of Zn-based MOF (ZIF-7) and GO nanocomposites on chitosan (CS)-coated polyether sulfone (PES) substrates. These membranes demonstrated efficiency in removing DR16 dyes and humic acid from synthetic wastewater. In another approach, Peng et al.²³⁸ enhanced MOF nanosheet-based complex membranes by attaching PAMAM nanoparticles to the nanosheets and employing a one-step self-crosslinking process for water purification. This novel nanosheet membrane exhibited a significant improvement in rejecting dye molecules, along with remarkable structural stability in aqueous solutions. A variety of MOFs have been developed and utilized for wastewater treatment, Notably, 2D MOFs have shown superiority over other dimensional MOFs. The nanosheets of 2D MOFs offer increased exposure of metal sites, enhancing catalytic activity.^239–241 Moreover, these 2D MOF nanosheets possess high porosity and specific surface area, making them more easily accessible.^242–244

6. Conclusion

In conclusion, MOFs, with their tunable and highly porous structures, offer great promise as separation materials. This review has provided an extensive overview of MOF-based membranes, highlighting their unique properties that enable efficient separation in diverse processes, such as salt separation, oil/water separation, and dye separation. Despite the current challenges in terms of membrane design and scalability, MOF-based membranes have demonstrated their ability to overcome obstacles and exhibit superiority over traditional separation materials, paving the way for sustainable and efficient separation solutions in various industrial and environmental applications. Continued research and innovation in MOF technology are expected to further optimize their performance and drive increased integration into practical separation processes, solidifying their position as cutting-edge separation materials. This review emphasizes that MOF-based membranes have the ability to overcome existing obstacles in separation processes, leading to improved efficiency and sustainability in industrial and environmental applications. However, it is crucial to acknowledge the current challenges in membrane design, scalability, and long-term stability. These aspects necessitate further research and development efforts to optimize the performance and real-world implementation of these methods.

Abbreviations

2D	Two-dimensional
3D	Three-dimensional
AFM	Atomic force microscopy
CR	Congo red
DPDS	Dipyridyl disulfide
EB	Evans blue
FO	Forward osmosis
GO	Graphene oxide
HTFA	Trifluoroacetic acid
MO	Methyl orange
MLB	Methylene blue
MB	Methyl blue
NF	Nanofiltration
NMP	N-Methyl-pyrrolidone
TFN	Thin film nanocomposite
MOF	Metal organic framework
ZIF-8	Zeolitic imidazolate framework-8
MIL-53	Materials of the Institute Lavoisier-53
PVDF	Polyvinylidene fluoride
PdTCPP	Palladium tetrakis(4-carboxyphenyl)porphyrin
PSM	Post-synthetic modifications
RB	Rhodamine B
RR	Rose red
PVP	Polyvinylpyrrolidone
rGO	Reduced graphene oxide
RO	Reverse osmosis
SEM	Scanning electron microscopy
TEM	Transmission electron microscopy
TMP	Trimethyl phosphine
XRD	X-ray diffraction

Conflicts of interest

We have no conflicts of interest with any author or research organization.

Acknowledgements

We gratefully acknowledge the financial supports from the State Key Laboratory of Mesoscience and Engineering, Institute of Process Engineering, Chinese Academy of Sciences (MESO-23-A06). Ayaz Ali Memon also acknowledges the grant from Higher Education Commission Islamabad Pakistan (Project 20-LCF-69/RGM/R&G/HEC/2020 and Project No. 2014470/NRPU/R&D/HEC/2021) for financial support to this work.

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