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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 8: 1154 - 1167

DOI: https://doi.org/10.7536/PC230111

Review

Preparation and Modification of MOF-Polymer Mixed Matrix Membrane and its Application in Pervaporation

  • Hao Zhang 1 ,
  • Yanhui Wu , 1, 2, *
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  • 1 School of Chemical Science and Engineering, Tongji University,Shanghai 200092, China
  • 2 Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092, China
*Corresponding author email:

Received date: 2023-01-30

  Revised date: 2023-03-12

  Online published: 2023-04-20

Supported by

National Key Research and Development Program of China(2019YFC0408200)

National Natural Science Foundation of China(22078249)

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Abstract

Pervaporation is a membrane separation technology with the advantages of low energy consumption and easy operation. At present, the traditional polymer pervaporation membrane still lacks in separation performance and stability. Metal-organic framework (MOF) is a crystalline porous material formed by self-assembly of metal ions and organic ligands. It has unique properties such as selective adsorption of target molecules and molecular sieving effect. In recent years, many studies have shown that the introduction of MOF as a filler into the polymer matrix to construct mixed matrix membranes (MMMs) has a good effect on its pervaporation performance. Starting from different series of MOF, this paper discusses the types of MOF suitable for pervaporation mixed matrix membrane, analyzes the preparation methods and modification strategies of MOF-polymer mixed matrix membrane, and reviews the application progress of this kind of mixed matrix membrane in pervaporation (dehydration of organic solvent, recovery of organic matter from dilute solution, separation of organic mixture). The challenges in the research of MOF-polymer mixed matrix membrane for pervaporation are summarized, and its future development is prospected.

Contents

1 Introduction

2 Different series of MOFs for pervaporation

2.1 Introduction of different series of MOFs

2.2 Selection of MOF fillers

3 Preparation and modification strategies of MOF based MMMs

3.1 Preparation methods of MOF based MMMs

3.2 Modification strategies of MOF based MMMs

4 Application of MOF based MMMs in pervaporation

4.1 Solvent dehydration

4.2 Recovery of organic compounds from diluted aqueous solutions

4.3 organic-organic mixture separation

5 Conclusion and outlook

Key words： pervaporation; metal-organic framework; mixed matrix membrane; modification

Cite this article

Hao Zhang , Yanhui Wu . Preparation and Modification of MOF-Polymer Mixed Matrix Membrane and its Application in Pervaporation[J]. Progress in Chemistry, 2023 , 35(8) : 1154 -1167 . DOI: 10.7536/PC230111

1 Introduction

Separation process plays an important role in chemical industry and related industries[1]. Although many mixture systems can be effectively separated by traditional separation technologies such as distillation, absorption and extraction, the high energy consumption in the actual separation process can not be ignored[2⇓~4]. In the mid-20th century, membrane separation technology appeared and was gradually applied to industrial separation. Various membrane processes were used for purification, concentration and separation of gas or liquid mixtures. After decades of rapid development, membrane separation technology has been considered to be very promising in solving energy and environmental challenges[5][6].
Pervaporation is an important branch of membrane separation technology, which is considered to be one of the most promising liquid separation technologies in bio-oil refining, petrochemical and pharmaceutical industries[7]. The principle of pervaporation is that the liquid component is adsorbed on one side of the membrane and then diffused to the other side of the membrane to be released in the form of vapor. The separation process is mainly realized by the difference of adsorption (dissolution) and diffusion of different components in the membrane (Figure 1). Compared with traditional evaporation and distillation processes, pervaporation has lower energy consumption, higher cost performance, simpler operation and higher efficiency, and shows great advantages in the separation of azeotropes and systems with close boiling points[8]. The application of pervaporation can be divided into dehydration of organic solvent, recovery of organic matter from aqueous solution, and separation of organic-organic mixture according to the different systems to be separated[9].
图1 渗透汽化的原理

Fig.1 The principle of pervaporation

At present, the membranes used for pervaporation are mainly polymer membranes due to their low cost, high flux and structural diversity. However, the polymer membrane has a short lifetime, low thermal and chemical stability, and is prone to swelling during use, resulting in a decrease in selectivity[10]. In order to overcome these limitations and further improve the efficiency of polymer membranes, inorganic porous materials can be introduced into polymers to construct mixed matrix membranes (MMMs) to adjust the structure and performance of membranes, so as to make use of the advantages of polymers and inorganic fillers, so that the membranes have both high permeability and high selectivity[11]. However, there are still some problems to be solved in the preparation of MMMs: (1) the compatibility between the filler and the polymer; (2) The stability and mechanical properties of the membrane are poor[12]. In order to solve the above problems, the preparation of pervaporation membrane with excellent separation performance can be considered from the following three aspects: first, modifying the surface of the membrane to improve its hydrophobicity/hydrophilicity; The second is to modify the filler to enhance the interfacial compatibility and the stability of the film; The third is to design new membrane materials to make up for the defects.
Metal-organic framework (MOF), also known as porous coordination polymer, is a kind of porous material composed of metal ions and organic linkers, which has many advantages, such as diverse structure, large specific surface area, high stability and low cost, and has been widely used in catalysis, separation, sensing and drug delivery[13,14]. In recent years, MOF and many inorganic fillers have been selected for the preparation of MMMs. Compared with molecular sieves, MOF materials have more diverse types and structures, are easy to design and modify, and can replace and modify ligands according to actual needs to regulate pore channels. The structure of molecular sieve is relatively fixed, and its controllability is not as good as that of MOF. Moreover, compared with molecular sieves, MOF has higher pore volume and specific surface area, and the modification of MOF can improve the separation selectivity while maintaining high throughput. In addition, because the MOF structure contains organic ligands, it has better compatibility with the polymer matrix, which can solve the problem of poor polymer-filler compatibility in the mixed matrix membrane, and can further improve the membrane performance by increasing the MOF loading, while the molecular sieve loading in the molecular sieve mixed matrix membrane is generally low due to the poor compatibility between the molecular sieve and the polymer.
In 2005, Takamizawa et al. Found that ethanol vapor could be selectively adsorbed by MOF([Cu2(bza)4(pyz)]n) due to the molecular synergy between the host and guest[15]. In 2007, the research group used [Cu2(bza)4(pyz)]n as a filler to compound with polydimethylsiloxane (PDMS) to obtain new MMMs, which showed good separation ability in alcohol/water separation and opened the application of MOF-polymer mixed matrix membrane in the field of pervaporation[16]. In 2010, Basu et Al. Prepared MMMs with [Cu3(BTC)2], MIL-47, MIL-53 (Al) and ZIF-8 as dispersed phases, which were applied to separate rose Bengal (RB) and isopropyl alcohol, and the permeability of the mixed matrix membrane was increased compared with the unfilled membrane[17]. Although MMMs for pervaporation have been proposed and gradually applied, the types of MOF fillers that can be used for pervaporation are limited because many MOFs cannot maintain good stability or do not have suitable pore sizes[18]. In this paper, the hydrophobic/hydrophilic modification methods of different MOFs and the feasibility of pervaporation were introduced, and the design strategies, preparation and modification methods of different types of MOF-based pervaporation MMMs were discussed.The application progress of MOF-polymer mixed matrix membrane in organic solvent dehydration, organic recovery from aqueous solution and organic-organic mixture separation was analyzed, and the application prospect of MOF-polymer mixed matrix membrane in pervaporation was prospected.

2 MOF materials for pervaporation

2.1 Introduction to different series of MOF

The types of MOFs currently available for pervaporation include: IRMOF series, ZIF series, HKUST series, MIL series, PCN series, and UiO series (Figure 2).
图2 不同类型MOF的结构图

Fig.2 Structure diagram of different types of MOFs

2.1.1 IRMOF Series

IRMOF is a microporous crystalline material self-assembled by bridging a series of aromatic carboxylic acid ligands in the form of octahedra with separated secondary building units [Zn4O]6+ inorganic groups[19]. IRMOF-1 is one of the first MOF packings for pervaporation MMMs. However, the IRMOF-1 framework is sensitive to water due to its weak metal oxygen bond, and its structure will collapse when exposed to moisture[20]. In order to improve the water stability of IRMOF-1, researchers have developed many strategies, one of which is to add transition metals to the structure of IRMOF-1, which can improve both the water stability and the pervaporation performance[21]. Other members of the IRMOF family of MOFs can be synthesized under suitable conditions by replacing the organic ligand[22]. Table 1 collates the main characteristics of the partial IRMOF.
表1 IRMOF系列的主要特征

Table 1 Main features of IRMOF series

IRMOF series Center atom Pore size
(Å)		 Hydrophilicity/
hydrophobicity		 ref
IRMOF-1 Zn 15 Hydrophilicity 23
IRMOF-3 Zn 15 Hydrophilicity 24
IRMOF-14 Zn 20 Hydrophilicity 25
IRMOF-16 Zn 25 Hydrophilicity 25

2.1.2 ZIF series

ZIFs are crystalline porous materials with high specific surface area, microporous structure, high thermal and chemical stability. The nanopores in the ZIF structure are four-, six-, eight-, and twelve-membered rings of ZnN4, CuN4, and CoN4 tetrahedral clusters formed by connecting metal ions such as Zn, Cu, and Co through functionalized imidazolate (Im) or Im through nitrogen atoms[26]. Due to the molecular sieve effect, good compatibility with polymer matrix, and facile synthesis process, ZIF series MOF has attracted increasing attention as a promising filler in MMMs[27,28].
Common ZIF series MOFs have small pore sizes, such as ZIF-7 (3.00Å), ZIF-8 (3.44Å), ZIF-22 (3.22Å) and ZIF-90 (3.55Å). ZIF-8 is the most widely used material in the ZIF family due to its stable tetrahedral ZnN4structure, hydrophobic pores, high chemical and thermal stability, and resistance to water and organic solvents. The narrow window size, high specific surface area (1400 m2/g) and good compatibility of ZIF-8 make it widely used in pervaporation research. In order to improve the hydrophobicity of ZIF-8, the auxiliary ligand modification method is often used to replace 2-methylimidazole with ligands with hydrophobic functional groups such as 2,5-dimethylbenzimidazole[29]. Similarly, the replacement of 2-methylimidazole with imidazole with hydroxyl group will make the modified ZIF-8 hydrophilic. ZIF-90 contains aldehyde groups, which can be modified by 3-aminopropyltriethoxysilane to greatly enhance its hydrophobicity[30]. Table 2 collates the main characteristics of common ZIF series MOFs.
表2 ZIF系列的主要特征

Table 2 Main features of ZIF series

ZIF series Center atom Pore size
(Å)		 Hydrophilicity/
hydrophobicity		 ref
ZIF-7 Zn 3.0 hydrophobicity 31
ZIF-8 Zn 3.4 hydrophobicity 32
ZIF-90 Zn 3.5 hydrophilicity 33
ZIF-71 Zn 13 hydrophobicity 34
ZIF-68 Zn 7.5 hydrophobicity 35
ZIF-67 Co 3.4 hydrophobicity 36

2.1.3 HKUST-1(Cu-BTC)

HKUST-1([Cu3(BTC)2(H2O)3])(BTC= benzene-1,3,5-tricarboxylic acid) was one of the first MOFs used as fillers for the synthesis of MMMs. Cu-BTC has a pore size of about 5.22 Å, a large square channel of 9 9 Å × 9 Å along the a axis, which facilitates the penetration of solvent molecules, and a triangular window with a size of 3.55 Å, which is suitable for molecular sieving[37]. The thermal stability temperature of Cu-BTC is 240 ℃, which makes the preparation methods of mixed matrix membrane containing HKUST-1 more diverse and the applicable conditions wider. Previous studies have shown that HKUST-1 nanosheets show good compatibility with different polymer matrices and can promote the permeability of guests through the membrane.

2.1.4 MIL series

The MIL-n materials studied by the Lavoisier Institute (MIL) are trivalent metal-based porous carboxylates, in which the metals are Cr (Ⅲ), V (Ⅲ), Fe (Ⅲ], Al (Ⅲ) and Ga (Ⅲ][38]. They have very large channels/cages with topology similar to that of zeolites. These MOFs have an open framework structure, and their pore size and shape strongly depend on host-guest interactions[39]. At present, MIL-53 (Al, Cr, Fe), MIL-101 (Al, Cr, Fe) and MIL-125 (Ti) are widely used in mixed matrix MOF[40,41]. Table 3 summarizes the main characteristics of the common MIL series.
表3 MIL系列的主要特征

Table 3 Main features of MIL series

MIL series Center atom Pore size
(Å)		 Hydrophilicity/
hydrophobicity		 ref
MIL-53 Al 8.6 hydrophobicity 42
MIL-68 Al 6 Modifiable 43
MIL-88 Fe 12 Modifiable 44
MIL-100 Cr 25 hydrophobicity 45
MIL-100 Fe 25 Modifiable 46
MIL-101 Fe 23 Modifiable 47
MIL-101 Cr 29 Modifiable 48

2.1.5 PCN series

PCN series materials contain multiple cuboctahedral nanopore cages and form a pore-cage-pore-channel-like topology in space. Common PCN-333 (Al), which is bonded using triazine ligands and Al metal clusters, has a large specific surface area (3500 m2/g) and is able to remain stable in aqueous and organic solvents[49]. The azo ligand in PCN-250 (Fe) increases the framework conjugation, allowing the MOF to remain stable in aqueous solution and even in alkaline conditions[50]. PCN-222 (Fe) has a large pore size and a large specific surface area, and is stable in boiling water and strong acid environment[51]. PCN-777 is stable in weak acid-weak base aqueous solution (pH 3-11), has organic solvent stability, and has a thermal decomposition temperature of more than 450 deg C. However, up to now, there are few examples of MMMs synthesized based on PCN series MOF for pervaporation, which need further study. Table 4 shows the characteristics of the PCN series.
表4 PCN系列的主要特征

Table 4 Main features of PCN series

PCN series Center atom Pore size
(Å)		 Hydrophilicity/
hydrophobicity		 ref
PCN-222 Zr 13 hydrophobicity 52
PCN-250 Fe 9.6 hydrophobicity 53
PCN-333 Al 11 hydrophobicity 54
PCN-777 Zr 38 hydrophobicity 55
PCN-222 Fe 13 hydrophobicity 56

2.1.6 UiO series

UiO series MOF has ultra-high hydrothermal stability and can remain stable under strong acid and alkali. Where UiO-66 is a cubic crystal structure built from a Zr6(μ3-O)4(μ-OH)4(COO)12 cluster and a linear dicarboxylate linker[57]. It has a central octahedral cage (diameter: 11 11 Å) interconnected with a tetrahedral cage by a triangular window (size: 6 6 Å). UiO-66 has a high specific surface area (1600 m2·g-1) due to its strong Zr — O bond, which makes it have remarkable chemical stability, excellent water stability, and remain stable at 550 ° C[58]. The UiO-66 pore size also provides preferential water channels[59]. The main characteristics of common UiO series are listed in Table 5. UiO-67 is assembled by Zr ions and 4,4 '-biphenyldicarboxylic acid, and there are few studies on its application in pervaporation separation, but UiO-67 has good research prospects because of its high thermal and chemical stability.
表5 UiO系列的主要特征

Table 5 Main features of UiO series

UiO series Center atom Pore size (Å) Hydrophilicity/
hydrophobicity		 ref
UiO-66 Zr 6 hydrophilicity 60
UiO-68 Zr 17.7 Modifiable 61
UiO-67 Zr 11 Modifiable 62
NH2-UiO-66 Zr 8 Modifiable 63
UiO-66-SO3H Zr 8 Modifiable 64
UiO-66-(OH)2 Zr 7 Modifiable 65

2.1.7 Two-dimensional MOF

Two-dimensional MOF (2D-MOF) materials with sheet-like structure, atomic thickness, micrometer lateral size, large specific surface area, and properly stacked two-dimensional transport channels can be used in hybrid matrix membranes to limit the flexibility of polymer chains, and the MOF nanosheets combined with the polymer membranes can effectively improve the membrane permeability by providing additional channels[66][67]. Some 2D-MOFs have different affinities for different components, which helps to further improve the selectivity of the membrane[68]. In recent years, more and more studies have been carried out on 2D-MOF-based MMMs for pervaporation, which is a hot topic in the field of membrane separation in the future[69,70].

2.2 Selection of MOF filler

2.2.1 Hydrophobicity/hydrophilicity

The hydrophobicity/hydrophilicity of MOF is mainly determined by the ligand and the open metal sites. Common hydrophilic MOFs include MOF-74, Cu-BTC, ZIF-90, HKUST-1, etc. MMMs prepared by introducing them into polymer membranes such as PVA, PAN, PVP, etc. Have strong hydrophilicity, which is beneficial to the removal of water from organic solvents by pervaporation and oil-water separation. Common hydrophobic MOFs include ZIF-71, MAF-2, MAF-6, FMOF-1, etc. MMMs with strong hydrophobicity can be prepared by introducing them into polymer membranes such as PEBA and PU, which show significant adsorption selectivity for organic matter in water and are usually used as membrane materials for pervaporation separation of organic matter in water.
Hydrophobic MOF, the hydrophobicity of ZIF-8 causes it not to adsorb water before the onset of capillary condensation. ZIF-71 has a RHO topology and is interconnected with a larger cage (1.68 nm) through a pore window of 0.42 nm, which is more hydrophobic than ZIF-8. Dong et al. First prepared ZIF-71 membranes for the pervaporation separation of alcohol-water mixtures and dimethyl carbonate-methanol mixtures. The results show that ZIF- 71 can be used not only for the separation of organic matter and water, but also for the separation of mixed organic matter, which opens up a new application field for ZIF membranes[71]. Wang et al. Constructed two new MOF materials (MAF-9 and MAF-2F) with fluorinated triazole ligands. Experiments showed that both MAF-9 and MAF-2F had superhydrophobicity and high stability, and could adsorb a large number of organic molecules without water[72]. The common strategies for the design and synthesis of hydrophobic MOF include: (1) using hydrophobic ligands such as fluorinated ligands as chains to construct MOF; (2) modifying the metal/ligand center of MOF by post-synthesis to improve its hydrophobicity; (3) Surface synergy.
In the aspect of hydrophilicity adjustment of MOF, Wu et al. Synthesized UiO-66-OH and UiO-66-(OH)2 with hydroxyl groups by replacing 1,4-phthalic acid with 2-hydroxyterephthalic acid and 2,5-dihydroxyterephthalic acid, respectively, and added them into polyvinyl alcohol membrane for pervaporation dehydration of ethanol solution. The UiO-66-(OH)2 with two hydroxyl groups showed better hydrophilicity and separation performance[73]. Zheng et al. Synthesized a Cu-MOF-74 at room temperature and coated it on PVDF membrane, and the results showed that the introduction of Cu-MOF-74 greatly improved the hydrophilicity and roughness of PVDF membrane surface[74].

2.2.2 Stability

Partial MOF has low chemical stability and cannot be in contact with water even at room temperature, otherwise framework collapse will occur. The development of MOF with superior water stability is very important for the development of its pervaporation applications. MOF structures with good water stability usually have strong coordination bonds (thermodynamic stability) or significant steric hindrance (kinetic stability) to prevent the disruption of metal-ligand bonds. At present, there are three main types of water-stable MOF: (1) MOF assembled by high-valence metal ions and metal carboxylates. This is because high-valence metal ions with high charge density form stronger coordination bonds with ligands, and high-valence metal units with higher coordination numbers usually form rigid structures, making the metal sites less susceptible to water molecules. (2) Metal frameworks containing azo ligands. Because of the flexibility of azo ligands, they can form MOF with strong force when they interact with soft divalent metal ions. The most representative example of this class is the ZIF series, which uses Zn2+/Co2+ and nitrogen-containing ligands to build a variety of water-stable structures similar to zeolite topology. (3) Post-modification MOF. The water stability of MOF can also be improved by post-synthesis methods, such as ion exchange, ligand modification, or metal/ligand exchange reactions.
The thermodynamic stability of MOF in water is determined by the free energy of the hydrolysis reaction and is conditioned by the following aspects. (1) Metal type. Metal properties (metal oxidation state, ionic radius, polarizability, etc.) will directly determine the strength of the metal-ligand bond. Generally speaking, transition metals have abundant coordination sites, which are beneficial to reinforce the rigidity of secondary structural units. (2) Ligand. The use of ligands with high pKa values helps to improve water resistance. (3) HSAB theory. One of the reasons for the instability of most MOF nanocrystals is that soft metal cations (Co2+, Zn2+, Ni2+, Cu2+, etc.) coordinate with hard carboxylate ligands to form soft and hard compounds, which do not conform to the soft and hard acid-base principle. Therefore, choosing relatively "soft" ligands instead of hard carboxylic acid ligands can make the material obtain better thermodynamic stability.
The kinetic stability of a MOF depends on the activation energy barrier, which depends on the product and reactant States as well as the specific reaction pathways and transition States involved. Kinetic factors such as hydrophobicity and ligand steric hindrance can increase the activation energy of hydrolysis.

2.2.3 Dispersibility

Filler agglomeration, which often occurs in the synthesis of MMMs, has a serious impact on the performance of the membrane. If the dispersion of MOF is not good, MOF will produce interface defects in the polymer that can not be repaired and ultimately affect the performance of MMMs. The uniform dispersion of MOF in polymer is the key to design MOF-based MMMs with excellent pervaporation performance. The commonly used methods to improve the dispersion of MOF are as follows:
(1) Preprocessing. After MOF synthesis, it is often directly filtered and vacuum-dried to remove the organic solvent. It is found that the specific surface area of MOF can be increased by activating MOF with low boiling point solvent on the premise of ensuring the stability of MOF. More importantly, the removal of the low boiling point solvent does not cause significant agglomeration of the MOF. For some viscous MOF solutions, they need to be centrifuged and washed many times, and finally sonicated in the solvent to disperse evenly. However, it should be noted that the more washing times and the longer washing time, the better. For example, the surface potential of newly prepared ZIF-8 particles decreases with the increase of washing time, resulting in the increase of particle size. The problem of poor dispersion of MOF can also be improved by directly blending wet MOF with polymer solution.
(2) Surface modification and pre-coating of MOF. Functional group grafting can enhance the dispersion and hydrophilicity of MOF and change the charge properties of MOF. Pre-coating generally refers to priming a wet MOF with a small amount of polymer solution to introduce a thin coating, and then adding and mixing the remaining polymer. With this method, the MOF particles can be uniformly distributed in the polymer without agglomeration.
(3) New synthesis technology. In situ self-assembly is a novel design strategy for MOF-based membranes for liquid separation. In this process, metal ions and organic ligands are assembled on the surface of the carrier through interfacial reaction, and then the polymer solution is mixed with the MOF layer, which can effectively inhibit the aggregation of MOF in MMMs while increasing the loading of fillers.
(4) Control of MOF particle size. The size of MOF particles can be adjusted by improving the synthesis conditions, such as adjusting the ratio of metal ions to organic ligands, acidity, temperature, etc. For example, for the common Fe-MOF and Zr-MOF series, the agglomeration of particles can be avoided by properly increasing the particle size.

3 Preparation Methods and Modification Strategies of MOF-based MMMs

3.1 Preparation of MOF-based MMMs

3.1.1 Blending method

Blending is one of the most commonly used methods to prepare MOF-based MMMs. In this method, the polymer is first dissolved in a suitable solvent and a homogeneous solution is formed. Then the MOF particles are added into the solvent and dispersed, and then the two suspensions are mixed and stirred, and the mixture obtained after uniform stirring is used for preparing a film by means of impregnation, spin coating, casting or pressure assistance, wherein the solvent is evaporated and dried in the medium to obtain the mixed matrix film. The method is simple and feasible, does not need to consider the original MOF synthesis conditions, is suitable for different types of MOF fillers, and the thickness of the prepared MMMs film can be controlled according to the concentration of the filler and the polymer, and has been widely applied to the preparation of MOF-based MMMs films.
Rajati et al. Mixed a certain amount of MIL-101 (Cr) particles dispersed in N-methylpyrrolidone, and then added Matrimid@5218 solution and polyvinylidene fluoride (PVDF) solution prepared in advance, respectively, to obtain MIL-101/Matrimid/PVDF mixed matrix film after blending and scraping[75]. S Sánchez-La Laínez et al. Dispersed ZIF-8 in chloroform and sonicated it into a suspension, then dissolved PIM-1 in a certain amount of anhydrous chloroform to obtain a homogeneous solution and added 6FDA (4,4 ′- (hexafluoroisopropylidene) dibenzoic anhydride) -DAM (1,3,5-trimethyl-2,4-phenylenediamine). The obtained solution and ZIF-8 suspension were further mixed, stirred and degassed, and then cast on a glass Petri dish and dried to obtain MMMs[76]. Chen et al. Prepared polyimide mixed matrix films with different KAUST-7 loadings by blending method. KAUST-7 nanocrystals were dispersed in chloroform solution, and then blended with polyimide solution formed by the reaction of 6FDA and Durene (2,3,5,6-tetramethyl-1,4-phenylenediamine) at a molar ratio of 1:1. After a series of subsequent treatments, the KAUST-7/6FDA-Durene mixed matrix films were obtained[77]. Knebel et al. Obtained ZIF-67 by blending method and ZIF-67 modified by two different N-heterocyclic carbenes (ZIF-67-IDip, ZIF-67-IMes) on the outer surface, and formed a mixed matrix membrane with 6FDA-DAM[78].
The blending method has the advantage of simple operation, but the binding force between the polymer phase and MOF is mainly van der Waals force or hydrogen bonding, which is not strong enough, so it is necessary to consider the combination of physical blending and other methods to obtain better and more stable MOF-based mixed matrix films.

3.1.2 Combination of blending and crosslinking

In the process of membrane preparation, the expected performance of the mixed matrix membrane is greatly reduced due to the poor compatibility between the filler and the matrix and the poor stability of the membrane. The particle/polymer interface can be improved by chemical crosslinking[79]. Chemical crosslinking refers to the process in which macromolecules are crosslinked through chemical bonds to form macromolecular structures under the action of light, heat, mechanical force, ultrasonic wave and crosslinking agent. Many examples have proved that MMMs prepared by chemical crosslinking are more compact, more stable and more dispersed. However, this method also has limitations, such as the high reaction temperature usually required for the thermal crosslinking reaction, which can lead to the brittleness of the film, making the film easy to break during operation. In addition, many MOFs can only remain stable below 200 ℃, which may lead to the decomposition of MOF fillers during heat treatment and the failure to maintain their inherent microporous structure.
ZIF-8 material can maintain high stability at 400 ℃. Therefore, in 2013, Askari et al. Obtained ZIF-8/6FDA-Durene/DABA mixed matrix membrane by solution casting method, and then heated the membrane to 350 ℃/400 ℃ in a vacuum furnace to produce thermal crosslinking. After heat treatment, the membrane has better elasticity and mechanical strength, and the separation performance is greatly improved[80]. Similarly, MIL-53 (Al) has high thermal and chemical stability. MIL-53 (Al) and NH2-MIL-53(Al) were chemically crosslinked with polyimide 6FDA-ODA-DAM (ODA: DAM = 1:1, 2% crosslinking agent APTMDS) to prepare mixed matrix membranes (MMMs). Compared with uncrosslinked MMMs, the MMMs prepared by the crosslinking method have better separation performance[81]. Wang et al. Added N, N-dimethylformamide (DMF) suspension of NH2-UiO-66 to polymer polyimide/polyfluoroethylene solution, and the mixture was stirred at room temperature and then cast and evaporated to dryness to obtain MMMs[82]. Then, the membrane was transferred to a vacuum oven at 150 ° C for heat treatment to remove the residual solvent, and the amide bond was formed between the amino-modified UiO-66 and the carboxylic acid-functionalized polyimide, which could effectively improve the compatibility between the filler and the polymer. Han et al. Used (3-aminopropyl) triethoxysilane (APTES) to modify ZIF-90 through Schiff base reaction, which enhanced the interaction force between ZIF-90 and PDMS, and the cross-linking of alkoxy groups on the surface of the modified APTES-ZIF-90 nanoparticles and PDMS chain hydroxyl groups minimized the interfacial defects[83]. Chemical crosslinking can enhance the interaction of different components in the mixed matrix membrane, resulting in higher interfacial compatibility and better separation performance.

3.1.3 Combination of UV-induced polymerization and blending

In the process of film formation, there are many problems, such as slow curing speed, easy agglomeration, and uncontrollable crosslinking process. Ultraviolet-induced polymerization is considered to be an effective means to solve these problems. The research of Si et al. Shows that under ultraviolet irradiation, some polymerization reactions can be completed within 30 s to achieve second-level curing, and this method greatly reduces the degree of particle agglomeration, and can prepare MMMs with high loading, which has a good development prospect[84].
In 2013, Hao et al. Added ZIF-8/methanol suspension to a mixture of ionic liquid with double bonds (RTIL) and photoinitiator, and removed the excess methanol by rotary evaporation until a gel-like substance was obtained[85]. Then the RTIL/ZIF-8 mixture was placed on a silicon wafer and subjected to UV polymerization for 60 min to prepare poly (RTIL)/ZIF-8 MMMs with excellent separation properties. UV-induced polymerization can not only solve the problem of particle agglomeration, but also has the advantages of eliminating defects and improving the hydrophobicity of MMMs. Zhang et al. Explored nanoporous carbon (P-ZNC) derived from PDMS functionalized ZIF-8 as a nanofiller, which was blended with PDMS and TEOS on PVDF matrix membrane, and defect-free P-ZNC/PDMS mixed matrix membrane was prepared by dip coating and thermal crosslinking method[86]. ZIF-derived porous carbon (ZNC) modified by PDMS under UV irradiation can effectively eliminate the interfacial defects between the polymer and the filler particles in the mixed matrix film, and also solve the problem of particle agglomeration. Ding et al. Added ZIF-8 to the mixture of monomer-polyethylene glycol methyl ether acrylate (PEGMEA) and crosslinking agent pentaerythritol triacrylate (PETA), and obtained ZIF-8/PEO mixed matrix film by solvent-free in situ polymerization of PEGMEA and PETA induced by ultraviolet light with 1-hydroxycyclohexyl phenyl ketone as photoinitiator, which provided a new idea for the preparation of MMMs film[87].
UV irradiation can lead to in situ polymerization of well-mixed MOF nanoparticles with organic monomers, and some functional groups (such as hydroxyl and carboxyl groups) on the surface of filler particles produce free radicals, cations, or anions under high radiation energy.It can initiate the polymerization of monomers on different surfaces, and in situ polymerization makes the MOF particles containing functional groups combine with polymer chains more effectively.

3.2 Modification Strategy of MOF-based MMMs

3.2.1 Ligand modification

There are two main methods for ligand modification: one is to directly graft functional groups on the original ligand to modify MOF. The common hydrophobic groups are alkyl, halogen atom, ester group, etc., and the hydrophilic groups are amino, hydroxyl, aldehyde, carboxyl, etc. Another method is MOF modification by ligand exchange. Both methods can improve the compatibility of MOF fillers with polymers and enhance the performance of MMMs. For example, Penkova et al. Modified polyisophthalamide (PA) with newly synthesized UiO-66(NH2)-EDTA particles, and then developed a composite membrane with a UiO-66(NH2)-EDTA/PA selective layer on a regenerated cellulose-based membrane, which showed high permeation flux and high selectivity for methanol in a methanol/toluene mixture[88]. Yin et al. Modified ZIF-71 particles with four different imidazole ligands by Solvent assisted linker exchange (SALE)[89]. The crystal structure and morphology of ZIF-71 remained unchanged after ligand exchange modification, and the adsorption experiment showed that the modified ZIF-71 had a greater adsorption capacity for ethanol and n-butanol at low pressure. The modified ZIF-70/PDMS composite membrane was prepared, and the modified ZIF- 71 could be more uniformly dispersed in the PDMS polymer, and had better ethanol and n-butanol permeability and alcohol/water selectivity.

3.2.2 Modification of ionic liquids (ILs)

Room temperature ionic liquids (ILs) are generally composed of organic cations and inorganic or organic anions, which have good solubility for many organic and inorganic substances, and their physicochemical properties can be adjusted by changing the structure or combination of anions and cations, so they can be designed and widely studied in recent years[90]. ILs have been used in a variety of separation processes, such as liquid-liquid extraction (LLE), extractive distillation, and membrane separation[91]. However, the high cost and recycling issues of ILs are still a challenge for extraction applications. In recent years, researchers have combined ILs with membrane technology to develop ionic liquid supported liquid membranes, poly (ionic liquid) membranes, polymer-ionic liquid composite membranes, ionic liquid gel membranes and mixed matrix membranes, which have been applied in gas separation and pervaporation[92].
Zhang et al. Designed and synthesized MIL-101 modified by coordination covalent grafting of ILs containing hydrophobic ditrifluoromethanesulfonimide anion (Tf2N-), and filled the ILs modified MIL-101 into PEBA polymer to prepare MMMs for selective pervaporation of ethyl acetate. The results showed that the grafted ILs not only successfully regulated the pore structure and surface properties of MIL-101, but also inhibited the formation of defects in MMMs[93]. The modified MIL-101 has better interfacial compatibility with PEBA, and the separation performance of ethyl acetate is improved. The research group also designed and synthesized ionic liquid modified ZIF-8 nanoparticles, and then introduced them into PDMS matrix to prepare MMMs. Molecular dynamics simulation showed that IL-PTES had preferential adsorption performance for ethyl acetate. IL-PTES can be cross-linked with PDMS chains, which is beneficial to enhance the interfacial compatibility between ZIF-8 and PDMS (Fig. 3), resulting in a relatively continuous and defect-free interface. The pervaporation experiment showed that the ILs modified ZIF-8/PDMS mixed matrix membrane showed better ethyl acetate separation performance[94]. These studies provide an important basis for the development of high-performance MMMs through the synergistic effect of MOF and ionic liquid.
图3 ILs修饰MIL-101制备MMMs[94]

Fig.3 Preparation of MMMs by ILs modified MIL-101[94]. Copyright 2021, J. Membr. Sci.

3.2.3 Other modification strategies

Li et al. Coated ZIF-8 particles with polydopamine (PDA) to form a chemically reactive surface, and then modified ZIF-8 @ PDA with silane coupling agents N-propyltrimethoxysilane and N-octyltriethoxysilane to obtain P-ZIF-8 @ PDA and O-ZIF-8 = PDA with significantly enhanced hydrophobicity, and the silane-modified ZIF-8 particles were introduced into PDMS to prepare MMMs with improved adsorption selectivity for butanol[95]. Space confinement method is considered to be an effective strategy to control the size of MOF. Xu et al. Synthesized two-dimensional Cu-BDC sheets between graphene oxide (GO) and 1-bromodecane by space confinement method at room temperature, and introduced them into PDMS to prepare MMMs, which significantly improved the pervaporation performance of PDMS, because the hierarchical pore structure of two-dimensional MOF has ultra-high permeability and selectivity to ethyl acetate[96].

4 Application of MOF-based MMMs in pervaporation

4.1 Solvent dehydration

When pervaporation is used to dehydrate organic solvents such as alcohols, ethers, esters, etc., the membrane is usually prepared from hydrophilic polymers, so the corresponding hybrid matrix membrane is prepared by combining hydrophilic polymers (such as polyvinyl alcohol, chitosan, etc.) With various types of MOFs[97].
Fazlifard et al. Successfully prepared ZIF-8/CS mixed matrix membranes to evaluate the performance of MMMs by pervaporation dehydration of isopropyl alcohol (IPA)[98]. When ZIF-8 loading was 5 wt%, the total flux increased by 25% and the separation factor decreased by 10%. Benzaqui et al. Also used ZIF-8 as a filler to prepare MMMs for pervaporation dehydration of isopropanol, but they first modified ZIF-8 with acryl polyethylene glycol (PEG) to improve its hydrophilicity[99]. The experimental results showed that the permeation flux of PEG @ ZIF-8 membrane was 11 times higher than that of pure PEG membrane, and the separation factor was high. Zhang et al. Used poly (4-styrenesulfonic acid) (PSS) to modify ZIF-8 nanoparticles, and designed and synthesized MZIF-8/PA mixed matrix membrane for ethanol dehydration. The total flux was up to 4.47 kg/(m2·h) when separating 80 wt% ethanol aqueous solution, and the ethanol/water separation factor was 127. It can be seen that the modification of MOF can improve its hydrophilicity and stability[100]. In order to obtain thinner ZIF-8-containing mixed matrix membranes, Lin et al. Coated ZIF-8/polyvinyl alcohol (PVA) on α-alumina (α-Al2O3) hollow fibers for ethanol dehydration[101]. The hollow fiber supported ZIF-8/PVA mixed matrix membrane has a thinner coating thickness, and the separation factor reaches 4821. The application of coating method can not only improve the performance of MMMs, but also be used for the quantitative production of membranes. Shi et al. Prepared ZIF-8/polybenzimidazole (PBI) mixed matrix membranes with uniform morphology for pervaporation dehydration of ethanol, isopropanol and butanol, and the content of ZIF-8 in MMMs could be as high as 58 wt%[102]. The test results show that the water permeability of the mixed matrix membrane is about one order of magnitude higher than that of the original PBI membrane. The improvement of membrane performance was mainly attributed to the high thermal stability of ZIF-8 nanoparticles and the good compatibility with PBI. In addition to the widespread use of ZIF-8, ZIF-90 can also be used to prepare solvent-dehydrated mixed matrix membranes. Hua et al. Synthesized ZIF-90 particles with an average particle size of 55 nm by modifying the synthesis route, and introduced them into P84 polymer with good dispersion to obtain a mixed matrix membrane for isopropanol dehydration. The results showed that when the loading of ZIF-90 was less than 20%, the separation factor of water/isopropanol could reach 5432[103].
In addition to the ZIF series, there are also other MOFs used as fillers to prepare mixed matrix membranes for pervaporation dehydration. Sorribas et al. Used polyimide (PI) as the polymer matrix to prepare HKUST-1 based MMMs with a loading of 20% – 40% for water/ethanol separation[104]. With the addition of this hydrophilic MOF, the water flux increased from 0.24 kg/(m2·h) for blank PI to 0.43 kg/(m2·h), and the separation factor reached 4200. Xu et al. Used UiO-66 nanoparticles and 6FDA-HAB/DABA polyimide copolymer to form MMMs for pervaporation dehydration of ethanol, isopropanol and n-butanol. The particle size of the UiO-66 nanoparticles used was about 100 nm, and they could be uniformly dispersed even at up to 30 wt% of UiO66 loading. The water permeability of the resulting mixed matrix membrane to isopropanol-water and n-butanol-water systems was 0.329 and 0.292 mg/ (m · H · kPa), respectively, and the alcohol/water separation[105].
The study shows that using MOF modified by hydrophilic functional groups as filler to prepare MMMs will have better compatibility and high selectivity. Su et al. Selected NH2-MIL-125(Ti) nanoparticles as a filler to prepare a mixed matrix membrane with sodium alginate (NaAlg) for the separation of acetic acid and water mixtures[106]. It was found that the NH2-MIL-125(Ti) nanoparticles had good compatibility with the NaAlg matrix and were well dispersed. The pervaporation dehydration separation factor for 90 wt% MOF in the membrane is 328 for acetic acid aqueous solution, and the flux is 190.7 g/(m2·h). Zhang et al. Prepared MMMs based on hydrophilic polymer polyvinyl alcohol (PVA) and MOF(SO3H-MIL-101-Cr) with hydrophilic sulfonic acid groups for pervaporation dehydration of ethylene glycol[107]. Dopamine layer was introduced to control the thickness of the SO3H-MIL-101-Cr surface during the preparation process, and hydrogen bonds were formed between the abundant amino groups in dopamine and the hydroxyl groups in PVA, which enhanced the compatibility between MOF and PVA. The results showed that the water permeability and selectivity of the SO3H-MIL-101-Cr@PD-PVA mixed matrix membrane were increased by 483% and 567%, respectively, compared with those of the pure PVA membrane. Xu et al. Blended porous UiO-66-NH2 nanoparticles and 6FDA-HAB/DABA polyimide to prepare MMMs for C1 – C3 alcohol dehydration. The amino-functionalized MOF can promote water transport, and due to the bonding of amino and imide groups, the UiO-66-NH2 particles and polyimide 6FDA-HAB/DABA have good compatibility. In a suitable MOF loading range, the addition of UiO-66-NH2 nanoparticles can improve the permeability and separation factor at the same time[108].
Some defect-rich MOF-based MMMs can also show better compatibility, higher pervaporation performance and separation factor. Vinu et Al. Prepared 2D Al-MOF materials CYCU-7 and CAU11 by hydrothermal and solvothermal methods, respectively, and then combined them with chitosan (CS) to prepare CYCU-7/CS and CAU-11/CS mixed matrix membranes for pervaporation dehydration of ethanol aqueous solution.The effects of structural properties (such as crystallinity and defects) of CYCU-7 and CAU-11 on the separation performance were studied. CAU-11 prepared by solvothermal method will lose part of the ligand in the preparation process.The mixed matrix membrane obtained by the method has a hierarchical microporous structure and rich defects, and the mixed matrix membrane obtained by the method shows higher flux, while the CAU-11 prepared by the hydrothermal method has higher crystallinity, which is beneficial to improving the separation factor of the membrane[109]. This work provides a new idea for the subsequent structural regulation of 2D MOF for hybrid matrix membranes.

4.2 Recovery of organic compounds from dilute aqueous solution

The applications of MOF-based MMMs in the separation of organic compounds from aqueous solution include the removal of volatile organic compounds from aqueous solution and the production of biofuels from fermentation broth. Hydrophobic MOFs have been widely used in this field and show broad prospects.
Alcohols are important chemical raw materials, which are often used as solvents. A large amount of alcohol-containing wastewater will be produced in chemical and related industries, resulting in the loss of alcohols. The production cost of biofuel (bioethanol or biobutanol, etc.) is mainly focused on the recovery of ethanol or butanol from fermentation broth (containing 1 wt% – 5 wt% bioalcohol). Pervaporation has been developed for more than ten years as a key technology for the recovery of alcohols from dilute aqueous solutions. In recent years, MOF-based MMMs have been widely studied for pervaporation recovery of alcohols. Mao et al. Synthesized ZIF-8 nanoparticles in PDMS matrix in situ with PVDF as the supporting membrane, and prepared a defect-free mixed matrix membrane with a thickness of about 1 μm[110]. The introduction of ZIF-8 nanoparticles can enhance the ethanol affinity, hydrophobicity, and thermal stability of MMMs. The results showed that the ZIF-8/PDMS mixed matrix membrane exhibited excellent pervaporation performance, and the permeation flux for separation of 5. 0 wt% ethanol aqueous solution reached 1778 g/(m2·h) at 40 ℃, and the ethanol/water separation factor was 12. 1. In order to further improve the compatibility between ZIF-8 and polymers, ZIF-8 was directly carbonized to obtain nanoporous carbon (ZNC), which was used to prepare hybrid matrix membranes. Compared with ZIF-8/PDMS membrane, the n-butanol permeability of ZNC/PDMS membrane with particle loading of 3 wt% increased by 68.7%. Because of the good compatibility between ZNC and PDMS, ZNC was dispersed more uniformly in the PDMS matrix[111]. ZIF-8 was also grown on polypyrrole (PPy) nanotubes, and then ZIF-8 @ PPy/PDMS mixed matrix membrane was prepared for the separation of n-butanol aqueous solution. The separation factor of the membrane for 1 wt% n-butanol aqueous solution increased from 36.4 to 70.2, and the total flux increased from 312.4 to 564.8 g/(m2·h). The PPy nanotube can effectively inhibit the movement of water molecules without changing the internal structure of ZIF-8[112]. To further improve the hydrophobicity of MMMs, hydrophobic groups such as —CF3 and —CH3 can also be selected to modify the MOF. Li et al. Used organosilane with terminal group of —CF3 to self-assemble and deposit on the surface of ZIF-8/PDMS, and treated PDMS with UV/ozone to produce hydroxyl groups to react with fluorine-containing silane, resulting in a ZIF-8/PDMS-CF3 mixed matrix film with a superhydrophobic surface (water contact angle of 152.4 °), which has a high separation factor of 95.8 and excellent total flux (1041 g/(m2·h) for 1 wt% n-butanol/water solution at 60 ° C[113].
ZIF-71 is another hydrophobic MOF material, which has also been used in the pervaporation separation of aqueous organics. Li et al. Prepared MMMs based on PDMS and ZIF-71 for the separation of alcohols (methanol, ethanol, isopropanol, or sec-butanol) from aqueous solution[114]. The experimental results showed that the pervaporation performance of the mixed matrix membrane with ZIF-71 was improved, and the separation factor was twice that of the PDMS membrane without ZIF-71. Similarly, some researchers have prepared ZIF-71/PEBA mixed matrix membranes for the pervaporation recovery of butanol from acetone-butanol-ethanol (ABE) fermentation broth, and the introduction of hydrophobic ZIF-71 improved the pervaporation performance of the membrane. When the mixed matrix membrane containing 20 wt% ZIF- 71 was used for the separation of simulated ABE fermentation broth, the permeation flux was 520.2 g/(m2·h) and the butanol separation factor was 18.8 at 37 ℃[115]. Naik et al. Grown ZIF-71 nanocrystals on the surface of mesoporous silica spheres (MSS), and then uniformly dispersed these MSS-ZIFs with a particle size of 2 ~ 3 μm into PDMS matrix to prepare MMMs, and carried out pervaporation separation experiments of ethanol in water.Due to the specific pore structure, hydrophobicity and surface chemistry of the hydrophobic ZIF coating, which improves the ethanol selectivity, and the mesoporous structure of silica, which provides a fast channel for the permeate to pass through, the mixed matrix membrane filled with MSS-ZIF has a significant improvement in both flux and separation factor[26].
The development of other new MOFs as fillers to prepare MMMs for pervaporation separation of organic matter in aqueous solution has also attracted attention. The researchers designed and synthesized Zn(BDC)(TED)0.5 particles, which were used as fillers to prepare mixed matrix membranes with PEBA polymer for pervaporation recovery of butanol. Due to the rich pore structure and strong hydrophobicity of Zn(BDC)(TED)0.5, the MMMs showed good butanol pervaporation performance, and the total flux of MMMs was up to 630.2 g/(m2·h) and the separation factor of n-butanol/water was 17. 4 at 40 ℃ with simulated ABE solution as feed solution[116]. Another researcher introduced superhydrophobic alkyl-modified RHO-[Zn(eim)2](MAF-6) into PDMS to prepare mixed matrix membrane. Due to the hydrophobicity and high porosity of MAF-6, the pervaporation separation performance of MAF-6/PDMS membrane for ethanol aqueous solution was improved[117].
As a representative of two-dimensional MOF, ZIF-L has a unique leaf-like morphology, semi-SOD topology, high hydrophobicity, and the same building blocks as ZIF-8, with a 3.44 Å diameter pore perpendicular to the 2D crystal layer, which is comparable to the kinetic diameter of butanol, and has potential for biobutanol pervaporation. Mao et al. Filled ZIF-L nanosheets into a PDMS matrix to prepare a mixed matrix membrane[118]. Due to the high aspect ratio and anisotropy, ZIF-L can be orderly distributed in the polymer matrix, and the ZIF-L nanosheets are interconnected in PDMS to construct the path of permeants, thus optimizing the selectivity and permeability of the membrane, so the introduction of ZIF- L nanosheets makes the membrane have excellent pervaporation performance, while the mechanical properties are improved (Fig. 4). The ZIF-L/PDMS membrane was used for pervaporation separation of 1. 0 wt% n-butanol aqueous solution, and the separation factor reached 57. 6 at 40 ℃, and the permeation flux was 402 g/(m2·h). Similarly, Liu et al. Doped ZIF-L nanosheets into poly-R-siloxane to prepare a mixed matrix membrane for simultaneous enrichment of ABE (acetone, butanol and ethanol, mass ratio of 3:6:1) from simulated fermentation broth, and also achieved good pervaporation separation effect. Compared with the blank membrane, the flux and butanol/water separation factor of the mixed matrix membrane containing 5.0 wt% ZIF-L were increased by 72.3% and 106%, respectively[119].
图4 ZIF-L基MMMs用于渗透汽化[118]

Fig.4 ZIF-L based MMMs for pervaporation[118]. Copyright 2019, J. Membr. Sci.

In addition to the recovery of ethanol and butanol, organic pervaporation is also used in the separation of other organic-water systems, such as furfural, which is an important chemical widely used.It is usually obtained by hydrolysis of cellulose followed by dehydration over an acidic catalyst. Due to the low concentration of furfural in the hydrolysate, it is urgent to develop an energy-saving and environmentally friendly separation technology. Jin et al. Used Zn2(bim)4 as inorganic filler and polymethylphenylsiloxane (PMPS) to prepare a mixed matrix membrane by blending method, which was used in the membrane reactor for xylose dehydration to furfural. The introduction of the Zn2(bim)4-PMPS membrane realized the in-situ separation of the reaction product furfural, which greatly increased the yield of furfural compared with the traditional reactor, up to 41.1%[120]. Mao et Al. Added MIL-53 (Al) particles (MAPs) to PEBA matrix to prepare a series of mixed matrix membranes for the separation of furfural from aqueous solution by pervaporation[121]. The total flux of MAPs-PEBA MMMs under the optimal conditions was as high as 3800 g/(m2·h), and the furfural/water separation factor reached 50. 2. The continuous experiment in furfural/acetic acid/water ternary simulated hydrolysate for 200 H showed that the MIL-53 (Al)/PEBA membrane had good stability.

4.3 Separation of organic-organic mixture

The separation of common organic-organic mixtures, such as methyl tert-butyl ether (MTBE)/methanol, dimethyl carbonate/methanol, benzene/cyclohexane, benzene/hexane, ethylbenzene/xylene, gasoline/organic sulfur, is very important in the chemical industry. Because the properties of the organic compounds to be separated are close to or even form azeotropes, the separation of organic compounds is difficult and challenging in chemical separation. If the MOF material shows molecular sieving properties for certain organic liquids, it can be considered to use this characteristic of MOF to construct a mixed matrix membrane for pervaporation separation of liquid organic mixtures. At present, there are few studies on MOF-based mixed matrix membranes for organic-organic mixture separation, which deserves further attention.
Xu et al. Uniformly filled nanosized ZIF-8 particles into chitosan (CS) matrix to prepare ZIF-8/CS mixed matrix membrane for pervaporation separation of methanol-dimethyl carbonate (DMC) mixture[122]. Benefiting from the flexible framework, high surface area of ZIF-8 crystal and its high affinity with CS matrix, the methanol permeability of ZIF-8/CS membrane was significantly increased, and the methanol permeability and methanol/DMC selectivity of the mixed matrix membrane loaded with 15 wt% ZIF-8 were improved by 361.3% and 9.4%, respectively, compared with the blank CS membrane. Similarly, some researchers introduced UiO-66 into CS membrane to prepare mixed matrix membrane to separate methanol-DMC mixture. Pervaporation experiments on methanol-DMC show that the introduction of UiO-66(ZrCl4) nanofillers can simultaneously improve the permeation flux and separation factor of CS membrane, and the total flux and methanol/DMC separation factor of UiO-66(ZrCl4)/CS membrane for the separation of 10 wt% methanol-DMC mixture are 355 g/(m2·h) and 337 (50 ℃), respectively, due to the good interaction forces (hydrogen bonds and van der Waals forces) between UiO-66 and CS[123].
Knozowska et al. Used fluorine-containing ligands trifluoroacetic acid (TFA) and pentafluoropropionic acid (PFPA) in the synthesis of MOF-808 to obtain fluorinated MOF-808-TFA or MOF-808-PFPA. The introduction of fluorinated MOF into PDMS membranes effectively enhanced the hydrophobicity of the membranes and the separation performance of organic-organic mixtures[124]. The membrane exhibited excellent transport and separation characteristics for the pervaporation separation of ethanol/ethyl tert-butyl ether, and the PSI value of MMMs loaded with 5 wt% MOF-808-TFA could reach 1540μm·kg/(m2·h), which was 97. 3% higher than that of the blank PDMS membrane. Msahel et al. Used Fe-MOF (MIL-100) as filler to introduce polylactic acid (PLA) membrane to prepare new MMMs for the separation of methanol/methyl tert-butyl ether azeotropic mixture[125]. Spherical MOF particles with a pore volume of 0.96 cm3/g and a pore size distribution centered at 5.55 Å were obtained by microwave synthesis within 20 min. The window size of the MOF is beneficial to the improvement of methanol/MTBE selectivity, and the activation energy calculation also shows that methanol molecules are more easily permeated through the membrane, and the methanol flux of MMMs loaded with 0. 5 wt% Fe-MOF is 22% higher than that of the blank membrane.
The trace organic sulfur in gasoline will turn into SO2 during combustion, which will cause serious pollution and harm to the environment. In recent years, the removal of organic sulfur in gasoline by pervaporation has attracted much attention, and MOF-based MMMs have also been applied in this direction. For example, Cu-BTC nanoparticles were introduced into PEBA for thiophene removal by pervaporation of simulated gasoline (n-octane-thiophene). The membrane containing 2 wt% Cu-BTC showed the best separation performance, with permeation flux of 4.4 kg/(m2·h) and enrichment factor of 6. 0, which increased by 36% and 11%, respectively, compared with the blank membrane[126]. Some researchers also introduced MOF-505 into polyethylene glycol (PEG) matrix to prepare mixed matrix membrane for thiophene removal from simulated gasoline by pervaporation. The mixed matrix membrane containing 3 wt% MOF-505 showed the best desulfurization performance, with a permeation flux of 2.66 kg/(m2·h) and a thiophene enrichment factor of 8.15, which was 158% and 25% higher than that of the blank PE G membrane, respectively[127].

5 Conclusion and prospect

Pervaporation has been industrially applied in the field of solvent dehydration, and has a good application prospect in the recovery of organic compounds from dilute solution and the separation of organic-organic mixtures, so the development of high-performance pervaporation membranes is a hot topic in the field of membrane separation. In recent years, MOF-polymer mixed matrix membrane has attracted much attention in the field of pervaporation, and has achieved good separation results. According to many literature data, the introduction of MOF is expected to solve the Trade-off effect between the permeability and selectivity of polymer membranes. However, thousands of MOFs have been developed and synthesized in the past decade, but not many MOFs can be used as MMMs fillers, which is related to the stability, pore size, adsorption of MOF, and the compatibility between MOF and polymer phase. In addition, the actual effect of MOF introduction is different from the theoretical simulation data. Therefore, there are still many challenges to obtain MOF-polymer mixed matrix membranes with excellent pervaporation separation performance and high stability: finding or designing MOFs with higher stability; Think deeply about the combination of MOF and polymer in mixed matrix membrane and its relationship with the target separation system; The compatibility of MOF and polymer in the hybrid matrix membrane needs to be further improved because the improper interfacial interaction between MOF and polymer phases may lead to the defects of the hybrid matrix membrane, such as pore blockage of MOF, polymer hardening around MOF particles, and interfacial voids. In addition, in order to obtain practical application of MOF-polymer mixed matrix membrane, it is necessary to solve the problem of production scale-up of MOF and its mixed matrix membrane. Gao et al. Directly obtained MOF-polymer mixed matrix membrane by in-situ heat-assisted solvent evaporation method after mixing polymer solution, metal salt and ligand. This method of simultaneous MOF growth and membrane formation is worthy of attention[128].
In order to solve the above problems, future research can focus on the following aspects:
(1) According to the solution-diffusion mechanism of pervaporation separation, new MOF materials should be further designed, developed or modified to optimize the adsorption performance and pore structure of the target separator. In addition, due to the affinity of the new two-dimensional MOF to the components and the easy adjustment of its stacking channel size, the future research on the introduction of two-dimensional MOF into pervaporation MMMs deserves attention.
(2) With the help of theoretical methods (such as molecular simulation), the properties of MOF and polymer are calculated respectively, and the matching of MOF and polymer is selected and optimized according to the target separation system. At the same time, the calculation of MOF properties is also an important link in selecting MOF with good compatibility for a specific polymer matrix and then constructing a high-performance mixed matrix membrane.
(3) In order to improve the compatibility between MOF and polymer matrix, attention should be paid to the modification of MOF surface while controlling the size of MOF particles. The decrease of particle size is beneficial to increase the interfacial area between polymer and MOF, which provides more selective paths for component penetration, but too small particle size can also lead to agglomeration, so the particle size of MOF should be properly optimized when constructing mixed matrix membrane.
(4) Reasonably design the preparation method of MOF-polymer hybrid matrix membrane which is easy to scale up, and conduct in-depth research on it to help the future high-performance MOF-polymer hybrid matrix membrane to practical application.

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