Metal–Organic Frameworks-Based Membranes with Special Wettability for Oil–Water Separation: A Review

Abstract

The presence of oily wastewater poses a significant threat to both the ecological environment and public health. In order to solve this problem, the design and preparation of an efficient oil–water separation membrane is very important. Metal–organic frameworks (MOFs) are currently a promising material for oil–water separation due to their tunable wettability, adjustable pore size and also low density, high porosity, and high surface area. Therefore, MOFs-based membranes show great potential in the field of oil–water separation. In this paper, we first introduce the oil–water separation mechanism and then comprehensively summarize the common preparation methods of MOFs-based oil–water separation membranes and the research progress of different MOFs-based membranes, including the ZIF series, UiO series, MIL series, etc. Finally, we also analyze the challenges faced by MOFs-based membranes in oil–water separation and provide an outlook on their future development and application.

1. Introduction

With the progression of industrialization and the increasing incidents of crude oil spills, the issue of pollution resulting from oily sewage has become increasingly pressing [1,2]. The presence of oil and other contaminants, including toxic organic compounds, in water not only undermines the ecological system and depletes resources [3,4] but also poses a major threat to human health [5,6]. To solve this problem, promote economic efficiency and sustainably advance. It is crucial to treat and effectively reuse oily wastewater [7]. Oil–water mixtures can typically be classified into three categories: free oil–water mixtures, oil dissolved in water, and oil–water emulsions [8]. The presence of oil dissolved in water is generally not deemed necessary for separation. Free oil–water mixtures, which contain larger dispersed droplets (>20 μm), can separate in a static state as a result of thermodynamic instability, with the denser phase settling and the lighter phase floating to the surface. Conversely, oil–water emulsions are thermodynamically stable colloidal dispersions composed of micro/nanoscale droplets (<20 μm) of the dispersed phase in a continuous phase [9]. The stability of the emulsion droplets is further perpetuated by the presence of surfactants, making them difficult to separate naturally over time. Conventional approaches for treating oily wastewater include physical precipitation, chemical flotation, mechanical centrifugation, etc. [10]. However, these methods are associated with high costs, low efficiency, and the potential for secondary pollution, limiting their practical application [2,11,12].

Membrane filtration technology is widely regarded as an efficient method for treating oil–water mixtures, due to its low cost, ease of operation, low energy consumption, and limited potential for secondary pollution. This makes it a popular choice for industrial applications [13]. In recent years, the use of special wettability materials in membrane separation technology has become more appeal to oil–water separation due to its high efficiency, suitable recoverability, and high oil–water selectivity [14,15]. These special wettabilities, including superhydrophilicity, superoleophilicity, superhydrophobicity, and superoleophobicity, can be generated by adjusting the surface free energy and micro-nano structure of materials.

Metal–organic frameworks (MOFs), a novel class of porous materials composed of metal ions and organic ligands [16,17,18], hold huge potential in the field of oil–water separation. MOFs show significant advantages over zeolites, activated carbon, and other porous materials in terms of adjusted pore structure and properties [19,20,21]. Firstly, MOFs have the advantage of easy microstructure construction and pore-size adjustment [22]. Secondly, their surface wettability can be easily tuned [23]. Additionally, due to their low density, high porosity, and high surface area, MOFs possess suitable adsorption properties, enabling them to effectively address complex pollutants such as heavy metal ions, dyes, drugs, antibiotics, and other pollutants in wastewater [24,25,26,27,28,29]. The versatility of MOFs also confers multifunctionality to oil–water separation materials, including photocatalysis [30] and anti-biological pollution [31]. As shown in Figure 1, a retrieval of publications about MOFs* and oil–water separation in the Web of Science core collection between 2015 and 2022 reveals that MOFs have been increasingly utilized in the field of oil–water separation in recent years.

There are limited studies that provide a comprehensive review of MOFs-based membranes for oil–water separation. A recent review in the field aims to assess the current progress and potential of MOF-based membranes in oil–water separation [32]. In contrast, this paper distinguishes itself by focusing on the classification and synthesis methods of MOFs-based membranes. The current progress of MOFs-based membranes with special wettability in oil–water separation is reviewed in detail. The fundamental theory of surface wettability and the mechanism of oil–water separation are introduced, followed by a summary of the common methods for preparing MOFs-based membranes. The classification and properties of MOFs-based oil–water separation membranes are analyzed in depth, and finally, strategies for improving these membranes are proposed.

2. The Mechanism of Oil–Water Separation

2.1. Basic Theory of Wettability

Wettability, a macroscopic representation of the interaction between a liquid and a solid material, is primarily determined by the surface morphology and chemical composition of the solid material. The contact angle between a liquid droplet’s edge and the surface of the material is used to calculate the wettability of a liquid on a solid surface. Traditionally, materials were considered lyophobic when the contact angle was greater than 90° and lyophilic when the contact angle was less than 90°. However, recent research has suggested that the intrinsic wetting threshold to differentiate between hydrophobic and hydrophilic surfaces should be 65° [33]. Additionally, materials are considered superlyophobic when the contact angle is more than 150° and superlyophilic when the contact angle is less than 5°. These materials with superlyophilic or superlyophobic properties are referred to as special wetting materials [34,35,36,37]. Materials with selectivity for water and oil, as determined by their opposite wettability properties, can be used to separate oil–water mixtures.

In air, the contact angle of a droplet on an ideal smooth solid surface can be expressed by Young model (Figure 2A), and the contact angle (θ) can be calculated by Young Equation (1):

$$\cos \theta =\frac{{\gamma }_{SV}-{\gamma }_{SL}}{{\gamma }_{LV}}$$

(1)

where γ_SV, γ_SL, and γ_LV denote the surface tension between the solid/gas, solid/liquid, and liquid/gas interfaces, respectively.

In reality, the material surface is often rough. According to Wenzel, a rough surface’s actual contact area is substantially bigger than its apparent contact area (Figure 2B). He put out an enhanced model to describe the effect of roughness on the wettability of rough surfaces:

$$\cos {\theta }^{\prime }=r\cos \theta$$

(2)

Among them, θ′ is the contact angle of droplets on the rough surface, and r is the roughness factor of the solid surface, which is defined as the ratio of the actual solid–liquid contact area to the apparent contact area of the rough surface. For the rough surface, r > 1, when θ < 90°, that is, when the material is lyophilic, the rough structure can make the material more lyophilic; when θ > 90°, that is, when the material is lyophobic, the rough structure makes the material more lyophobic. The contact angle of the droplets on this surface, however, may be determined using the Cassie model when air is trapped in the space between the droplets and the rough structure, forming a solid/liquid/gas three-phase interface (Figure 2C).

$$\cos {\theta }^{″}=f\cos \theta +f-1$$

(3)

where θ″ is the contact angle of droplets on the solid–gas multiphase surface, and f is the ratio of the solid–liquid interface contact area to the total contact area (solid–liquid contact area and gas–liquid contact area).

The Young model, Wenzel model, and Cassie model can not only analyze the wettability of droplets on solid surfaces in the air but also effectively describe the oil–water–solid system (Figure 2D–F). The oil contact angle (θ_OW) on an ideal underwater smooth surface for the oil–water–solid three-phase system can be expressed as:

$$\cos {\theta }_{OW}=\frac{{\gamma }_{OV}\cos {\theta }_O-{\gamma }_{WV}\cos {\theta }_W}{{\gamma }_{OW}}$$

(4)

where θ_O and θ_W are the contact angles of oil and water droplets in air, respectively; γ_OV, γ_WV, and γ_OW are the interfacial tensions at the oil–vapor, water–vapor, and oil–water interfaces, respectively. Therefore, if γ_OVcosθ_O < γ_WVcosθ_W, an underwater oleophobic surface can be obtained; when γ_OVcosθ_O > γ_WVcosθ_W, an underwater oleophilic surface can be obtained. Most hydrophilic surfaces are underwater oleophobic because the surface tension of water is significantly greater than that of oils or other organic liquids. Hydrophobic/oleophilic surfaces at the solid–air–liquid interface always show lipophilicity at the solid–water–oil interface. Similar to the Wenzel and Cassie models in air, the rough surface can enhance the wetting property of materials underwater environment. Based on the aforementioned research, it is possible to construct the superhydrophobic-superoleophilic and superhydrophilic-underwater superoleophobic materials utilized for oil–water separation by controlling their surface chemical composition and creating the materials’ microstructures.

2.2. Separation of Free Oil–Water Mixture

The force of a liquid (water or oil) on the material surface is shown in Figure 3. One of the crucial factors influencing the effectiveness of materials for oil–water separation is the penetration pressure (∆P_c), which is the highest pressure applied to the surface before the liquid penetrates the membrane pore. For pores with cylindrical geometry, ∆P_c can be determined by the Young–Laplace Equation (5):

$$\Delta P_c=-\frac{2{\gamma }_L\cos \theta }{r_p}$$

(5)

where γ_L denotes the surface tension of the liquid, θ is the contact angle of the liquid in a plane, and r_p denotes the pore radius. In air, if θ > 90°, ∆P_c > 0, indicating that a certain external force is generated on the material surface to prevent the liquid from passing through the porous material, and an external pressure must be applied to pass through (Figure 3a). On the contrary, if θ < 90°, ∆P_c < 0, indicating that the liquid may spontaneously flow through the pore without the need of external force (Figure 3b).

The penetration pressure of oil-in-water ∆P_c^W can be calculated from Equation (6) for a three-phase solid–water–oil contact in an aqueous environment as follows:

$$\Delta P_c{}^W=-\frac{2{\gamma }_{OW}\cos {\theta }_{OW}}{r_p}$$

(6)

where γ_OW denotes the water/oil interfacial tension, and θ_OW denotes the underwater oil contact angle. When θ_OW > 90°, ∆P_c^W > 0, indicating that a certain external force is generated on the material surface to prevent the oil phase from entering the pore space, and the larger θ_OW is, the larger the value of ∆P_c^W is (Figure 3c). On the contrary, when θ_OW < 90°, ∆P_c^W < 0, indicating that the oil phase may spontaneously flow through the pores without the need for external force (Figure 3d).

For oil–water separation membranes, the closer θ to 180° or 0°, the higher the upward penetration pressure and downward penetration pressure, the better the oil–water two-phase selection separation performance would be obtained. The smaller the ∆P_c, the larger the ∆P_c^W, i.e., the more hydrophilic in air and more oleophobic underwater, the better the material makes water pass through.

2.3. Separation of Oil–Water Emulsion

For emulsion separation, there are two main effects currently known: the “pore-size sieving” effect and the emulsion-breaking effect, which are shown in Figure 4. The “pore-size sieving” effect refers to the separation of oil-in-water emulsions using hydrophilic membranes with pore sizes smaller than the particle size of the emulsion. Under the influence of gravity, the oil droplets are intercepted, allowing the water phase to pass through. Similarly, water-in-oil emulsions can be separated using oleophilic membranes with pore that are smaller than the emulsion’s particle size [39]. However, the need for small pore sizes in the “pore-size sieving” effect often results in reduced separation flux, leading to the development of the emulsion-breaking effect. This process involves three main steps: (1) the emulsified droplets are captured by an agglomerated medium under hydrodynamic or other external forces; (2) the droplets combine to form larger droplets through wetting and shear collisions; and (3) the larger droplets are separated from the surface of the agglomerated medium through gravity and buoyancy [40]. By combining the “pore-size sieving” effect and emulsion breaking, emulsions can be separated with relatively larger membrane pore sizes, thus improving the separation flux and efficiency.

3. Methods to Prepare MOFs-Based Materials for Oil–Water Separation

3.1. In Situ Growth Method

3.1.1. Substrate Pre-Modification Method

The substrate pre-modification method involves modifying the surface of the substrate prior to immersing it in a configured precursor solution for the self-growth of the film. The role of surface modifiers is mainly to anchor metal ions or organic ligands used to generate MOFs. Mussel-inspired polydopamine (PDA) is a widely used surface modifier. It mainly acts as a binder to improve the substrate and is not involved in subsequent reactions. Song et al. used PDA coating to grow Cu-MOF on stainless steel mesh (SSM), which can continuously separate heavy oil–water mixture for more than 160 min with an efficiency of >98.5% [41]. Tannic acid (TA) and Ti⁴⁺ were combined by Wang et al. to create metal phenolic networks (TTN), which were applied to the PVDF membrane’s surface. Then, the as-modified membrane was immersed in Zn²⁺ and 2-methylimidazole solution to synthesize ZIF-8 by self-loading. TTN offers general metal chelating affinity (TA-Zn²⁺) and thereby can manipulate the even dispersion of ZIF-8 nanocrystals. Finally, the hydrophilic multi-layer TTN-ZIF-8 composite coating was obtained on the membrane [42].

3.1.2. Precursor Sacrifice Method

Utilizing some metal resources on the membrane as the precursor, MOFs could be obtained on the membrane directly with the subsequent conversion reaction. Metal oxides and hydroxides are the most used metal precursors in this case because they are easy to react with some acidic organic ligands to form the target MOF. For example, Ma et al. first grew ZnO nanorods on SSM by hydrothermal method and then placed SSM in a Teflon autoclave lined with a solution containing Zn²⁺ and 2-methylimidazole. The obtained composite SSM has a 300 °C thermal stability and can be utilized to separate water-in-oil emulsion [43]. Zhu et al. used an alkaline oxide solution to chemically etch and grow Cu(OH)₂ nanowires on the copper mesh, then the mesh was placed in the solution of phthalic acid to grow Cu-MOF and finally modified with PDMS to obtain the composite mesh, which has strong wear resistance and peel resistance [44]. The substrate pre-modification method can provide nucleation sites for MOF on the substrate and guide the raw materials in the reaction solution to crystallize on the substrate. At the same time, the bonding force between MOF and substrate is also enhanced.

3.1.3. Crystal Seed Growth

The crystal seed growth method is a process in which MOF crystals are nucleated and grown separately. The substrate is first covered with a crystal seed layer in this procedure. Then, the substrate with the seed layer is placed in a solution containing metal ions and organic ligands, allowing for the secondary growth of MOFs. As an example, a ZIF-8 seed solution was synthesized, and the substrate was immersed in it. This resulted in the growth and crystallization of ZIF-8 on the substrate through a secondary growth process under high-temperature conditions. Finally, further modification with PVA resulted in the formation of a PVA/ZIF-8-coated substrate, which was capable of separating both oil-in-water and water-in-oil emulsions [45]. The crystal seed growth method enhances the growth quality of MOFs by avoiding heterogeneous nucleation and elevating the nucleation rate on the substrate surface. Despite these benefits, this approach also has several drawbacks, including an extended duration, stringent reaction conditions, and a complicated operational process.

3.2. Deposition Method

3.2.1. Direct Deposition

The direct deposition method involves the direct synthesis of MOF composite membranes by placing the substrate into a mixture of metal ions and organic ligands. For example, a PP/ZIF-8 composite membrane can be prepared by simply adding polypropylene (PP) non-woven fabric into a mixed solution of zinc nitrate and 2-methylimidazole at room temperature, and it has the ability to separate free oil–water mixtures [46]. Yang et al. prepared a PDMS/Ti-MOFs composite cotton fabric by placing cotton fabric in a blend of titanium isopropanol and 2-amino-terephthalic acid for 24 h and modifying it with PDMS after drying [47]. In both cases, PP non-woven fabric and cotton fabric are used as substrates, zinc nitrate and titanium isopropanol provide zinc and titanium ions, and 2-methylimidazole and 2-amino-terephthalic acid are organic ligands that are mixed together to form composite membranes of MOFs on the substrate. The direct deposition method is simple in its approach, but due to a deficiency of sufficient nucleation sites on the pristine substrate to interact with MOFs, ensuring suitable dispersity of the MOFs and strong bonding with the substrate can be difficult.

3.2.2. Electrochemical Deposition

The electrochemical deposition method, similar to the direct deposition method, involves the rapid growth of MOFs on the substrate through the use of metal cations and organic ligands in the presence of an applied voltage. For example, after only 200 s of applied voltage, Co²⁺ and 2-methylimidazole react and accelerate to produce electrochemical deposition that aggregates onto the copper mesh, resulting in the formation of a uniform and dense ZIF-67 film [48]. This method offers advantages such as a short process time, uniform growth, and a high deposition rate.

3.2.3. Layer-by-Layer Self-Assembly

The layer-by-layer (LBL) self-assembly method is a distinctive approach that involves alternating the deposition of metal ions and organic ligands instead of directly mixing them. The substrate can either be modified or left unmodified. The key factors that influence the membrane’s performance are the number of alternating depositions and the concentration of precursors. In a study by Gao et al., UiO-66-NH₂-coated PP membranes with varying thicknesses were prepared using the LBL method. The PP membrane was first soaked in a solution of Zr⁴⁺ ions at high temperature, followed by immersion in a solution of 2-aminoterephthalic acid. By repeating this alternating process, UiO-66-NH₂-coated PP membranes with different thicknesses were obtained [49]. Although the LBL method allows for the controlled synthesis of MOFs membranes, it is a time-consuming process.

3.2.4. Filtration Deposition

The filtration deposition method is a technique for creating functional membranes by depositing solid suspensions on a substrate using vacuum filtration. For example, Zhu et al. first synthesized UiO-66-NH₂ powder using Zr⁴⁺ and 2-NH₂-benzenedicarboxylate by the solvent thermal method, added it to chitosan solution, and attached it to a cellulose membrane through vacuum filtration to obtain a superhydrophilic and underwater superoleophobic membrane, which can effectively separate oil-in-water emulsions and has excellent corrosion resistance [50]. Similarly, You et al. synthesized micro- and nano-sized ZIF-8 particles, which were then deposited onto a cellulose membrane by vacuum filtration. Both water-in-oil and oil-in-water emulsions can be separated by the resultant membrane [51]. The presence of voids between large particles enhances the separation flux, while the presence of small particles ensures the separation efficiency. Despite its simplicity of operation, the filtration deposition method may struggle with ensuring the bonding and uniformity of MOFs on the substrate.

3.2.5. Spin-Coating

The spin-coating method involves coating a substrate with a suspension of MOFs using a spinning instrument. For instance, ZIF-8 nanoparticles were evenly deposited on a polyacrylonitrile (PAN) fiber membrane using the spin-coating method to create a biomimetic inverse desert beetle ZIF-8/PAN composite nanofiber membrane that can effectively separate oil-in-water emulsions [52]. The spin-coating method is simple to operate, but it can be challenging to achieve a uniform distribution of MOFs with a strong attachment to the substrate.

3.3. Blending Membrane Method

In contrast to the above methods of coating MOFs onto the membrane surface, the blending membrane method focuses on creating a composite membrane in which the MOFs are dispersed within the membrane materials. This can be achieved through two main approaches: electrospinning and phase inversion.

3.3.1. Electrospinning

Electrospinning is a process in which droplets are charged and transformed into nanometer-sized fibers through stretching and solidification. The precursor solution is extruded by adjusting the applied voltage and the receiving distance, and the solution is continuously ejected from the tip of the droplet and gathers on the receiver to form a film when the electrostatic repulsion force overcomes the surface tension. Li et al. synthesized ZIF-8 nanoparticles through the solvothermal method and then combined them with PES and PSA to create a PES@ZIF-8-PSA/PES bilayer fiber membrane for PM2.5 adsorption and oil–water separation [53]. The nanofiber membrane produced through electrospinning has high flexibility, porosity, and mechanical strength. However, it is a time-consuming process that requires extreme standardization to avoid safety hazards like fire.

3.3.2. Phase Inversion

Phase inversion is a method of preparing homogeneous polymer solutions, which are then transformed into a three-dimensional macromolecular network gel structure by adding a non-solvent. The resulting structure is solidified into a film, making it a widely used method for the preparation of mixed matrix membranes (MMMs). For example, Cui et al. used the delayed phase inversion method to prepare UiO-66-NH₂-modified PVDF membranes, which exhibit superhydrophilic and underwater superoleophobic properties and may be used for oil-in-water emulsion separation and tetracycline removal [54]. Similarly, Wang et al. synthesized MIL-101 by a solvothermal method and then prepared a MIL-101 modified PVDF membrane by an immersion precipitation phase inversion method, which can be utilized for the separation of oil-in-water emulsions and resist contamination from bovine albumin [55].

4. The Classification of MOFs Used for Oil–Water Separation

The variations in MOFs are determined by the choice of central metal ions/clusters and organic ligands [56]. The organic ligands, mostly composed of oxygen- or nitrogen-containing donor compounds, serve as bridges between the metal ions. The synthesis of MOFs is accomplished through coordinative covalent bonds between the metal ions/clusters and the organic ligands, leading to the formation of 3D crystal structures [57]. Since 2016, the use of MOFs-based materials in oil–water separation has gained significant attention, with the frequently utilized MOFs being the ZIF series, UiO series, MIL series, etc.

4.1. ZIF Series

ZIFs, also known as zeolite imidazolate frameworks, are constructed from transition metals (zinc, cobalt, indium, etc.) with tetrahedral coordination geometry and imidazole-based organic ligands. ZIF series is one of the most extensively used MOFs in the field of oil–water separation.

The effectiveness of oil–water separation using ZIFs-based membranes is found in the pore-size sieving effect, where different substrate sizes are suitable for separating different types of oil–water mixtures.

Table 1. A summary and comparison of ZIF series membranes in oil/water separation.

Material	Preparation Method	Liquids of Separation	O/W Volume Ratio	Separation Efficiency (%)	Permeation Flux (L·m−2·h−1)	Ref.
ZIF-8/GO	LBL	Toluene in water	-	-	110 ± 6	[58]
ZIF-67@Cu(OH)2	In-suit growth	Oil–water mixture	1:1	99	23,854	[59]
ZIF-71/PVDF-HFP	Electrospinning and in-suit growth	Water-in-oil emulsions	99:1	>99	6577.68	[60]
ZIF-9-III@PVDF	Phase inversion	Water-in-oil emulsions	99:1	99.8	14.3	[61]
ZIF-90	Solvothermal method	Water-in-oil emulsions	-	99.98	1260	[62]
CPE-ZIF-7-coated SSM	Hydrothermal growth and dip-coating	Water-in-oil emulsion	9:1	99.9	1056	[63]
ZIF-8/PAN	Coprecipitation and electrospinning	Oil-in-water emulsions	1:50	99.92	2514	[52]
ZIF-L(Co)@SSM	One-pot method	Polar/nonpolar liquids	1:1	>96	-	[64]
ZIF-L(Zn)@SSM	Pre-seeding and secondary growth	Oil–water mixture	-	99.98	1.24 × 105	[65]

showcases the success of using ZIFs for efficient oil–water separation. For instance, Yue et al. utilized a photo-induced wettability composite membrane composed of ZIF-8 and graphene oxide (GO) loaded onto a PDA-modified polyvinylidene fluoride (PVDF) membrane through a layer-by-layer self-assembly method. This membrane has the ability to separate free oil–water mixtures and can even degrade toluene under visible light [58]. Similarly, by loading ZIF-67@Cu(OH)₂ nanowire arrays onto a metal mesh, the membrane exhibits underwater superoleophobicity and demonstrates a high separation flux of 23,854 L·m⁻²·h⁻¹ in the separation of free oil–water mixtures, maintaining its underwater superoleophobicity in varying pH environments [59]. The ZIF-based membranes can also be utilized to separate water-in-oil emulsions after being hydrophobically modified. For example, ZIF-71/PVDF membrane was created by coordinating zinc acetate with 4,5-dichloroimidazole on a pretreated PVDF electrospinning membrane. The membrane is superhydrophobic and capable of separating dissolved water-in-oil emulsions under gravity (Figure 5A) [60]. Additionally, ZIF-9-III nanosheets were synthesized with cobalt nitrate and benzimidazole, and the ZIF-9-III@PVDF membrane was made using a non-solvent-induced phase inversion method, which exhibits superhydrophobic-oleophilic properties and the ability to separate water-in-oil emulsions (Figure 5B) [61]. The ZIFs-based membranes also have suitable separation performance for both oil-in-water and water-in-oil emulsions. For example, a ZIF-90 composite membrane modified with Zn²⁺ and 2-imidazolaldehyde by solvothermal method can separate water-in-oil emulsions (Figure 5C) [62]. Moreover, ZIF-7 coating synthesized using zinc nitrate and benzimidazole on SSM and further modified with coordinated polysiloxane elastomer is superhydrophobic and capable of separating water-in-oil emulsions with self-healing ability [63]. Nanofibrous membranes composed of underwater superoleophobic polyacrylonitrile (PAN) and hydrophobic/oleophilic ZIF-8 bumps have a high separation efficiency of 99.92% for oil-in-water emulsions [52]. In addition to oil removal or water removal, the ZIF series can also separate free oil–water mixtures as required, with the separation achieved through the change in liquid prewetting. For example, Flake ZIF-L(Co)- and ZIF-L(Zn)-modified SSM can achieve on-demand separation of free oil–water mixtures (Figure 5D) [64,65].

4.2. UiO Series

The UiO (Universitetet I Oslo) series of MOFs are known for their versatility and widespread use in various fields. Among the UiO series, UiO-66 and its derivatives have gained popularity for their effectiveness in oil–water separation. These MOFs have a three-dimensional microporous structure composed of a regular octahedron (Zr₆O₄(OH)₄) containing Zr ions that are connected to 12 organic ligands of terephthalic acid (BDC). The structure consists of central pore cages in the shape of octahedrons and eight smaller tetrahedral corner cages.

UiO-66-based membranes can not only separate free oil–water mixture and water-in-oil emulsion but also absorb dyes and heavy metal ions. Wei et al. first synthesized UiO-66 by the solvothermal method using ZrCl₄ and terephthalic acid as raw materials and then integrated oxidized poly arylene sulfide sulfone (OPASS) and UiO-66 to prepare nanofiber membrane by electrospinning technology (Figure 6A) [66]. The membrane is capable of separating water-in-oil emulsion and adsorbing dyes and heavy metal ions. Zhu et al. successfully modified the organic ligand of UiO-66 to 2-aminoterephthalic acid, resulting in the synthesis of the UiO-66-NH₂ composite metal mesh. This material demonstrated not only the effective separation of free oil–water mixtures but also the ability to perform photocatalytic degradation of dyes (Figure 6B) [67]. To improve the corrosion resistance and wear resistance of UiO-66, UiO-66-F₄ was synthesized from tetrafluoroterephthalic acid and coated with reduced graphene oxide (rGO) on filter paper to obtain superhydrophobic-superoleophilic filter paper, which can be utilized for the separation of various water-in-oil emulsion (Figure 6C). The composite exhibits stable superhydrophobic/superoleophilic properties under high/low temperatures, corrosive acid/base solutions, and harsh physical conditions [68]. Moreover, UiO-66-(COOH)₂ was prepared by reflux method using pyromellitic acid as an organic ligand, and MXene@UiO-66-(COOH)₂ membrane was constructed for oil-in-water emulsion and dye-synergistic separation with high chemical stability (Figure 6D) [69]. The addition of UiO-66-(COOH)₂ enhanced the material’s hydrophilicity, MXene nanosheets improved the mechanical properties of the filter membrane, and UiO-66-(COOH)₂ and MXene nanosheet had a synergistic effect to improve the separation effect of the membrane. A comparison of the above-mentioned membranes’ performance has been listed in

Table 2. A summary and comparisons of UiO series membranes in oil/water separation (pollutant concentrations are shown in brackets for other functions).

Material	Preparation Method	Liquids of Separation	O/W Volume Ratio	Separation Efficiency (%)	Permeation Flux (L·m−2·h−1)	Other Functions	Ref.
OPASS/UiO-66	Electrospinning	Water-in-oil emulsions	40:1	98.44	636	Adsorption of dyes (10 mg/L)and heavy metal ions (4 mg/L)	[66]
SSM/UiO-66-NH2/CMn	Hydrothermal	Oil–water mixtures	1:1	>99	-	Degradation of dyes (10mg/L)	[67]
UiO-66-F4@rGO	Dip-coating	Water-in-oil emulsions	-	99.73	990.45	-	[68]
MXene@UiO-66-(COOH)2	Vacuum-assisted self-assembly	Toluene-in-water emulsion	-	99.54	498.91	-	[69]

.

4.3. MIL Series

MIL series, also known as Lavahir skeleton series materials, are generally composed of trivalent metal ions (such as aluminum, chromium, iron, etc.) and carboxylic acid ligands. In addition to the extensive adsorption properties of MOFs, Fe-based MOFs can often endow the materials with photo-Fenton catalytic properties.

A comparison of the MIL series membrane was summarized in

Table 3. A summary and comparisons of MIL series membranes in oil/water separation (pollutant concentrations are shown in brackets for other functions).

Material	Preparation Method	Liquids of Separation	O/W Volume Ratio	Separation Efficiency (%)	Permeation Flux (L·m−2·h−1)	Other Functions	Ref.
SPAN@GO/M88A	Hydrothermal	Oil-in-water emulsions	1:100	>99	920–7083(20–100 kpa)	Photo-Fenton self-cleaning properties	[70]
MIL-53(Fe)	Solvothermal and electrospinning	Oil-in-water emulsions	1:100; 1:20; 1:10	>90	380	Dye degradation (100 ppm)	[71]
NH2-MIL-125@PAA	Hydrothermal and vacuum-assisted self-assembly process	Oil-in-water emulsions	1:100	99.5	500	Dyes separation (20 ppm)	[72]
PAN/PEI/MIL	Electrospinning and hydrothermal	Oil-in-water emulsions	1:1	>99	4000	Dyes adsorption (10 ppm)	[73]
GO/NH2-MIL-101(Fe)	One-pot method	Gasoline–water mixtures	1:1408	-	120	Tetracycline degradation (100 mg/L)	[74]
Ag NPs@MIL-100(Fe)/GG	Blending and self-crosslinking	Oil–water mixtures	1:1	>97.82	-	Photocatalytic performance (40 mg/L)	[75]

. The SPAN@GO/M88A composite membrane was obtained by loading MIL-88A(Fe) and GO onto the PAN fiber membrane by co-blending method, which has underwater superoleophobicity and can separate oil-in-water emulsion [70]. The existence of GO nanosheets ensures high separation flux, and the introduction of MIL-88A(Fe) gives the membrane underwater oil resistance and photo-Fenton catalytic ability to remove pollutants on the membrane. MIL-53(Fe) was also reported in oil-in-water emulsion separation (Figure 7A) [71], in which MIL-53(Fe) and PAN were introduced onto fiber membrane by electrospinning technology, and the membrane showed superamphiphilic in air and underwater superoleophobicity. In addition to oil–water separation, the MIL series have versatilities in wastewater treatment. Zhu et al. modified the PVDF membrane with NH₂-MIL-125(Ti) and polyacrylic acid (PAA). The prepared membrane showed superhydrophilic and underwater superoleophobic properties, which can be utilized to separate oil-in-water emulsion and dye at the same time [72]. MIL-53-OH(Al) was coated on PAN/PEI electrospinning membrane, demonstrating the dual function of the composite membrane by simultaneously separating an oil-in-water emulsion containing dyes (Figure 7B) [73]. Li et al. prepared a PVDF hybrid membrane by adding hydrophilic GO and NH₂-MIL-101(Fe) particles, which can separate the free oil–water mixture and degrade tetracycline (Figure 7C) [74]. To improve the antifouling performance of the membrane, Duan et al. prepared a bio-based hydrogel by blending and self-crosslinking Ag NPs, MIL-100(Fe), and guar gum (Figure 7D). The hydrogel was coated on filter paper to separate immiscible oil and had antibacterial and photocatalytic self-healing properties. The emulsion-breaking performance of MIL-100(Fe) can be enhanced by fine-tuning the surface charge [75]. Wang et al. studied the demulsification of cationic surfactant stabilized emulsion by MIL-100(Fe). The surface charge of MIL-100(Fe) can be adjusted by changing the pH value of the phosphate precursor, which was captured by the emulsion droplet. The change will cause the release of surfactant and heterogeneity of interfacial film, a decrease in emulsion elasticity, and irreversible deformation of droplets, resulting in demulsification [76].

4.4. Other MOFs

In addition to the ZIF, UiO, and MIL series, other MOFs such as HKUST-1, PCN series materials (containing multiple cub octahedral nanopore cages and forming a pore cage-pore channel-like topology in space), and Cu-MOFs have recently been developed for use in oil–water separation application.

A comparison of other MOFs membranes was summarized in

Table 4. A summary and comparisons of other MOFs membranes in oil/water separation (Pollutant concentrations are shown in brackets for other functions).

Material	Preparation Method	Liquids of Separation	O/W Volume Ratio	Separation Efficiency (%)	Permeation Flux (L·m−2·h−1)	Other Functions	Ref.
PDMS/(Cu-MOFs)5@paper	LBL and in-suit growth	Oil–water mixtures	1:1	>98	55.8	Antibacterial activity	[77]
prGO@cHKUST-1	Hydrothermal	Oil–water emulsion	-	99.6	28.6	Dye adsorption (20 mg/L–100 mg/L)	[78]
PCN-224/TA/PVDF	In-suit deposition	Oil-in-water emulsions	1:99	>99	1542	Dye adsorption (10–50 ppm)	[79]
Ce/Cu-MOF	Dip-coating/in-suit growth	Oil–water mixtures	1:4	>98	-	-	[80]
Co-CAT-1	Vapor-assisted conversion	Oil–water mixtures	1:1	99.98	8.4 × 105	-	[81]
Cr-soc-MOF	Vacuum-assisted self-assembly method	Oil-in-water emulsions	1:100	98.7	7040.1	-	[82]
MOF-808-EDTA	Solvothermal and electrospinning	Oil-in-water emulsions	1:10	99.97	379.3	Heavy metal ions adsorption (Cu2+: 50 ppm)	[83]
HKUST-1/PDA@SM	Step-by-step deposition and hydrothermal synthesis	Oil-in-water emulsions	1:9	95	200	-	[84]
SSM@HKUST-1	Electrochemical and in-suit growth method	Oil-in-water and water-in-oil emulsions	1:100; 100:1	97.46; 99.63	569.7; 88.1	-	[85]

. Cu-MOFs (Cu²⁺, 1,4-benzenedicarboxylic acid) were grown in situ on filter paper by layer-by-layer assembly method and modified by PDMS to obtain composite filter paper with superhydrophobic-superoleophilic and antibacterial properties, which can be used to separate water-in-oil emulsion (Figure 8A) [77]. PDA, GO, and HKUST-1 were deposited onto the cellulose acetate membrane by the vacuum-assisted deposition method (Figure 8B) [78]. The cubic HKUST-1 improved the layer spacing of GO, improved the hydrophilicity and flux of the modified membrane, and was able to adsorb water-soluble dyes. Similarly, the PCN-224/TA/PVDF membrane obtained by coating PCN-224 (Zr₆, tetrakis(4-carboxyphenyl) porphyrin) on PVDF membrane with tannic acid under vacuum condition, which could separate oil-in-water emulsion (Figure 8C) [79]. Due to the photosensitive porphyrin ligand in PCN-224, the composite membrane has a visible light response and can photodegrade water-soluble organic dyes. Liu et al. directly deposited bimetal Ce/Cu-MOF on a polyimide membrane with cerium nitrate, copper chloride, and trimesic acid as raw materials to obtain superhydrophilic and underwater superoleophobic composite membrane, which is highly stable and capable of separating the free oil–water mixture (Figure 8D) [80]. Mahringer et al. used 2,3,6,7,10,11-hexahydroxytriphenylene and cobalt catecholate as raw materials to grow columnar Co-CAT-1 on a gold-plated SSM in a directional manner to obtain a modified mesh with superhydrophilic and underwater superoleophobic properties. It has a strong stain resistance to crude oil, and under gravity, the separation flux of free oil–water mixture can reach 10⁶ L·m⁻²·h⁻¹ [81]. In addition, MOF-808 has adsorption properties for copper ions, and there is some recent research, such as Cr-soc-MOF and HKUST-1, which can be applied to oil–water separation, especially SSM@HKUST-1 can separate oil-in-water and water-in-oil emulsions on demand [82,83,84,85].

5. Conclusions and Outlook

In this review, the mechanisms of oil–water separation are described, and the preparation and recent progress of MOFs-based oil–water separation membranes are summarized. There are diverse methods for the preparation of MOFs, many of which have the potential for large-scale industrialization, opening up new avenues for future practical engineering applications. Furthermore, this paper focuses on various MOF-based membranes for oil–water separation, such as the ZIF series, the UiO series, and the MIL series, and presents the functional properties and their related applications. The versatility of MOFs gives these oil–water separation membranes superior performances, such as antifouling properties, self-healing capabilities, high adsorption of heavy metal ions and dyes, and photocatalytic degradation of organic pollutants. To sum up, it can be concluded that MOFs-based membranes have a broader application in the treatment of oily wastewater.

However, in the existing literature, most studies using MOF-based oil–water separation membranes are somewhat flawed, and the following efforts are still needed to solve the environmental problems of wastewater: (1) The long-term performance of MOFs-based membranes has been neglected, especially in continuous oil–water separation processes. Researchers can therefore focus on ways to improve the long-term durability of the membranes; (2) Most MOFs materials are unstable in wet environments due to weak coordination between their own metals and organic ligands. How to improve the stability of MOFs without affecting their structural properties remains to be investigated. (3) The specific wastewater environment is very complex and may contain organic molecules, heavy metals, microorganisms, etc. MOFs-based membranes need to be more stable and fouling resistant in the complex components. Based on this, there is a need for researchers to create materials with better stability and self-cleaning properties. (4) MOFs-based membranes should be used in a wider range of applications, and they must also be able to treat actual wastewater, from municipal wastewater to industrial wastewater, broadening their practical applications. (5) In addition, there is still a lack of longitudinal comparisons of data on the separation efficiency, mechanical and chemical stability, and pollution resistance of MOF-based membranes based on the separation of complex oil–water mixtures, which will need to be reviewed by subsequent researchers as this research progresses.