Recent Progress and Challenges of Metal-Organic Framework-Based Membranes for Gas Separation

1. Introduction

Research, development and demonstration tests for the practical application of metal-organic frameworks (MOFs) are underway involving companies and universities in various fields [1,2,3,4,5]. MOFs are porous materials consisting of coordination bonds between metal ions and multifunctional organic ligands, which can exhibit unparalleled properties and functions (e.g., storage, adsorption, separation, catalytic, electromagnetic, and optical properties by tuning their framework composition and pore structure. As companies begin to produce and market MOFs, products are being created that exploit their properties. Queen’s University Belfast start-ups MOF Technologies and DECCO have applied MOFs to a product that keeps fruit and vegetables fresh [6]. The role of MOFs is to store and release 1-methylcyclopropene, which inhibits the action of ethylene that ripens fruit and vegetables, as required. NuMat Technologies, a start-up company from Northwestern University, has commercialized MOFs as a gas cylinder that can store and safely transport toxic gases for the semiconductor industry [7]. Atomis, a start-up company from Kyoto University, is in the process of gaining approval for commercial use of MOF-based high-pressure gas container, CubiTan^®. SyncMOF, a start-up company from Nagoya University, is in the process of commercializing MOF-based gas separation systems, MOFclean. Transaera, a start-up company from Massachusetts Institute of Technology, is in the process of commercializing dehumidifying air conditioning systems using MOFs. Svante and Crimeworks are also piloting the application of MOFs in direct air capture, which captures CO₂ directly from the atmosphere. Thus, large-scale applications of MOFs are expected to expand.

2. Characteristics of MOFs

2.1. Structural Flexibility

Some MOFs have flexible pore structures. It is known that the pore structure changes when gas is adsorbed. Some of these MOFs exhibit unique adsorption behavior in that they behave as nonporous materials under low gas pressure conditions and show no adsorption performance. On the other hand, when the gas pressure reaches a certain threshold pressure (so-called gate-opening pressure), they change to a porous structure, resulting in a rapid increase in adsorption. The gate-opening type adsorption behavior, which is not observed in conventional porous materials, depends on the combination of metal ions and ligands constituting the framework. Various types of structural flexibility have been reported [8]. For example, (1) changes in pore shape from a rhombic structure to a square structure and vice versa, (2) changes in the relative position of interpenetrating structures, (3) stretching and shrinking of lattice layers, and (4) rotation of ligands at the pore aperture. and is caused by various factors, such as the pore shape changing from a rhombic structure to a square structure or vice versa, the relative position of interpenetrating structures changing, the lattice layers expanding and contracting, and ligand rotation at the pore aperture, and so on. Furthermore, adsorption behavior has been reported to vary with crystal size and shape. For example, [Cu₂(bdc)₂(bpy)]n (bdc = benzene-1,4-dicarboxylic acid, bpy = 4,4’-dipyridyl) [9] and ZIF-8 [10] have been reported to exhibit higher gate-opening pressure with smaller crystals.

2.2. Structural Stability

Thermal and chemical stability of materials is one of the most important properties for not only membrane separation but also for many industrial applications. Due to the instability of the metal-ligand coordination bond, the structure of many MOFs is degraded by moisture in the air. In order to prevent the collapse of the network structure due to hydrolysis reactions of the metal-ligand coordination bonds or ligand substitution reactions, it is effective to have either a strong coordination bond that is thermodynamically stable or a kinetically stable structure using large steric hindrance. Basically, when the coordination environment with the ligand is the same, metal ions with higher valence and charge density form a more stable framework. This tendency is explained according to the HSAB theory and supported by many findings in MOF studies [11]. According to the HSAB theory, carboxylic acid ligands can be regarded as hard bases that form stable complexes with hard acid metal ions such as Al³⁺, Cr³⁺, Fe³⁺, Ti⁴⁺, and Zr⁴⁺. MIL series and UiO-66 are well-known MOFs with high structural stability synthesized by such a combination (Figure 1). Imidazolate and azolate ligands of soft bases form relatively stable structures together with divalent metal ions of soft acids such as Zn²⁺, Co²⁺, and Cu²⁺. The most representative example is ZIF series, which is composed of Zn²⁺ and imidazolate [12].

3. Hydrocarbon Adsorption on MOFs

3.1. Olefins and Paraffins

The first MOF investigated for potential application to olefin/paraffin separation was HKUST-1, which consists of a paddle-wheel Cu(II) dimer and 1,3,5-benzenetricarboxylate as building blocks. Wang et al. measured the adsorption isotherms of C₂H₄ and C₂H₆ on HKUST-1 at 295 K and showed that C₂H₄ is preferentially adsorbed [13]. Water molecules are coordinated to the metal site of HKUST-1 and dehydration forms coordinatively unsaturated open metal sites [14]. Lamia et al. found that C₂H₄ is adsorbed due to the interaction between the π-electrons of C₂H₄ and the partially positively charged open metal site, whereas C₂H₆, which has no C=C double bond, has a low binding affinity to the open metal site, resulting in a selective separation function [15].

MOFs with open metal sites include the MIL series such as MIL-53, MIL-96 and MIL-100 and MOF-74. The MIL series, consisting of trivalent transition metals such as Fe(III), Cr(III), Al(III) and V(III), has been widely studied as MOFs for gas separation. Compared to divalent metals, trivalent transition metals have stronger bonds to ligands and can form more chemically stable structures [16]. However, the strong bonding between the metal and the ligand makes it difficult to synthesize MOFs with high crystallinity, and synthetic methods that satisfy the conditions for spontaneous self-assembly by reversible "weak bonding" are required. For example, MOFs have been synthesized under strongly acidic conditions using HF or HCl [17,18,19,20] or by a solvothermal method at high temperatures (100~ °C) [21,22,23].

The MIL series has trivalent metal sites with high electrophilicity and is excellent for adsorption of electron-rich olefins. Yoon et al. reported that MIL-100(Fe) can be applied to C₃H₆/C₃H₈ separation [24]. Lee et al. reported that MIL-101(Cr), from which terephthalate anions were removed by treatment with NH₄F solution, showed C₂H₄/C₂H₆ selectivity ~4 [25]. In addition, attempts to improve the selectivity by using the interaction between Cu(I) or Ag(I) sites and C=C bonds of C₂H₄ have been reported by depositing Cu nanoparticles on the pore surface of MIL-101(Cr) [26] or by introducing a functional group -SO₃Ag as a building block ligand [27]. Similarly, Kim et al. obtained C₃H₆/C₃H₈ selectivity ~13 by modifying MIL-100(Fe) with Cu(I) [28].

MOF-74 is a honeycomb structure composed of Mg(II), Mn(II), Ni(II), Co(II), Zn(II), Cu(II) or Fe(II) and 2,5-dihydroxyterephthalate as building blocks. Bao et al. first investigated Mg-MOF-74 for the separation of C₂H₄/C₂H₆ and C₃H₆/C₃H₈ (Figure 2) [29]. Bae et al. compared the influence of metal sites on the adsorption selectivity of C₃H₆/C₃H₈ using Mg-, Mn-, and Co-MOF-74. The effect of metal sites on the adsorption selectivity of C₃H₆/C₃H₈ was compared, and it was reported that the selectivity was higher for Mg (selectivity 4.5) < Mn (24) < Co (46) [30]. The influence of the type of phthalate ligand of MOF-74 on the olefin/paraffin separation was also studied, and the replacement of 2,5-dihydroxyterephthalate with 4,6-dihydroxyisophthalate resulted in higher C₂H₄/C₂H₆ (>259 and C₃H₆/C₃H₈ selectivity (>55) of Fe-MOF-74 [31].

Olefin selective adsorption using open metal sites of MOFs is enhanced by increasing the charge density of coordinatively unsaturated open metal sites. However, these MOFs exhibit very high enthalpies of adsorption (> tens of kJ/mol) and suffer a significant energy penalty in adsorbent regeneration. Furthermore, such MOFs may decrease the adsorption capacity in the presence of water.

Most MOFs without open metal sites do not show selective adsorption of olefins/paraffins, with the notable exception of NOTT-300, which is composed of [AlO₄(OH)₂] and biphenyl-3,3’,5,5’-tetracarboxylate as building blocks. NOTT-300 exhibits a very high C₂H₄/C₂H₆ selectivity of 48.7, while its low enthalpy of adsorption, approximately 16 kJ/mol, reduces the energy penalty for regeneration. The energy penalty for regeneration is also reduced [32].

The use of adsorbents that selectively adsorb paraffins saves energy by eliminating the adsorption-desorption cycle required for olefin recovery. However, C₂H₆ has a smaller quadrupole moment and larger dynamic molecular size than C₂H₄, making selective adsorption generally more difficult. On the other hand, selective adsorption of C₂H₆ has been reported in several MOFs. ZIF-7, composed of Zn(II) and benzimidazolate, has been reported to adsorb C₂H₆ (and C₃H₈ compared to C₃H₆) at lower pressures than C₂H₄, although there is no large difference in saturation adsorption capacity for olefins and paraffins [33,34].

MAF-49, which is composed of Zn(II) and the triazole ligand bis(5-amino-1H-1,2,4-triazol-3-yl)methane and has one-dimensional zigzag channels, is also known to preferentially adsorb C₂H₆ [35]. The enthalpy of C₂H₆ adsorption by MAF-49 (60 kJ/mol) is higher than that of C₂H₄ (48 kJ/mol), and it preferentially adsorbs C₂H₆ in the low-pressure region, where C-H⋯N hydrogen bonds and electrostatic interactions occur between electronegative nitrogen atoms and C₂H₆ (Figure 3). On the other hand, for C₂H₄, it was concluded that steric hindrance and electrostatic repulsion occur between the C-H of C₂H₄ and the methylene group of the ligand. Therefore, the placement of multiple polar functional groups at appropriate positions in the framework may be effective in achieving the desired selective separation.

3.2. Other Hydrocarbons

Separation of 1,3-butadiene from C4 hydrocarbon mixtures is essential to produce synthetic rubber. However, the C4 isomers have close boiling points, and some components form azeotropic mixtures. Kishida et al. discussed the possibility of separating 1,3-butadiene from C4 hydrocarbons by MOF [36]. The synthesized MOF is called SD-65 and has an interpenetrating structure in which Zn(II) is coordinated to two components, 5-nitroisophthalate and 1,2-di(4-pyridyl)ethylene. SD-65 adsorbed almost no n-C₄H₁₀, i-C₄H₁₀, 1-butene, isobutene, trans-2-butene and cis-2-butene (adsorption capacity ~2.5 cm³/g at approximately 1 bar), while it adsorbed 40 cm³/g of 1,3-butadiene. The pore structure remains closed until the pressure of 1,3-butadiene is about 0.6 bar, at which point the pore structure rapidly transitions to an open pore structure and butadiene is adsorbed. Other MOFs have been investigated for 1,3-butadiene separation, all of which have potential, but there are still many issues to be solved to meet the separation selectivity requirements [36,37,38,39,40].

Separation of linear/branched hydrocarbons using MOFs has also been studied. Pan et al. reported that MOF composed of paddlewheel Cu(II) dimer and 4,4’-(hexafluoroisopropylidene)bis-(benzoic acid) adsorbs C₃H₈, C₃H₆ and n-C₄H₁₀, while i-C₄H₁₀, n-pentane, i-pentane, n-Hexane and 3-methylpentane are not adsorbed [41]. Peralta et al. reported the separation of linear/branched hydrocarbons by ZIF-8 [42]. ZIF-8 adsorbs n-hexane, and 3-methylpentane, but not 2,2-dimethylbutane.

The MIL series, including MIL-47 and MIL-53, has also been studied for xylene isomer separation [43,44,45,46,47]. MIL-47 and MIL-53 have the same crystal topology consisting of [MO₄(OH)₂] and phthalic acid. MIL-47, which is composed of V(III) has a rigid structure, whereas MIL-53, which is composed of Al(III), Cr(III) and Fe(III), shows a unique flexibility called the breathing effect. The p-xylene/m-xylene separation by MIL-47 showed a selectivity of 2.9. On the other hand, MIL-53(Al) could not separate p-xylene and m-xylene.

UiO-66, composed of zirconium and terephthalic acid, is well-known for its excellent chemical and thermal stability. UiO-66 preferentially adsorbs branched hydrocarbons, 2,2-dimethylbutane and 2,3-dimethylbutane, over linear hydrocarbons, n-hexane [48]. This unique adsorption behavior is attributed to the 6-7 Å triangular lattice of the channel pores of UiO-66, which is believed to be responsible for the preferential adsorption of o-xylene over p-xylene.

4. CO₂ Separation and Capture

Since global CO₂ emissions from energy conversion such as power generation account for more than 40% of total global CO₂ emissions, decarbonization of energy conversion is crucial to reducing emissions. CO₂ separation and capture processes in the power generation sector can be classified into pre-combustion, post-combustion, and oxy-fuel combustion. The most mature technology for capturing CO₂ after combustion is chemical absorption using monoethanolamine (MEA). However, the energy cost of CO₂ separation and capture is high, even for power plants that use the captured CO₂ for enhanced oil recovery (EOR) [49]. Carbon pricing through "carbon taxes" and "emissions trading" has been introduced as a measure to reduce CO₂ emissions. The cap-and-trade European Union Emissions Trading Scheme (EU-ETS) has become the most recognized carbon market in the world, with the EU-ETS price exceeding 50€/t-CO₂ in May 2021. Many international organizations, including the International Energy Agency (IEA) and the International Renewable Energy Agency (IRENA), have stated that carbon pricing will spur innovation in low-carbon technologies and increase the potential for new technologies to replace existing technologies [50]. Membrane separation is considered a promising next-generation separation technology because it can operate continuously (no need to regenerate separators), consumes less energy than other separation methods, and can be easily integrated into existing technologies due to its compact equipment [51]. Membrane gas separation was commercialized in the late 1970s for hydrogen separation and has since been applied to carbon dioxide separation from natural gas, biogas, and landfill gas, air separation (nitrogen-enriched gas and oxygen-enriched gas production), and air dehumidification. However, membrane separation as a CO₂ separation and recovery technology for CO₂ Capture, Utilization and Storage (CCUS) has only been studied up to bench scale with a few exceptions.

Polymers such as silicone rubber, cellulose acetate, polysulfone, and polyimide have been mainly used as membrane materials. Recently, porous membranes with sub-nanometer sized pores have been extensively studied, with silica and zeolite membranes receiving much attention. Mixed matrix membranes (MMMs), in which MOFs are mixed with polymer matrix as filler, have also been actively studied. Pre-combustion is primarily intended for use in integrated gasification combined cycle (IGCC), a process in which coal and natural gas are partially oxidized to produce natural gas vapor. Fuel gas is purified by separating and recovering CO₂ from synthesis gas (consisting primarily of H₂ and CO) produced by partial oxidation of coal and natural gas or by steam reforming of natural gas to produce H₂ and CO₂ by reacting CO with aqueous gas shift. Since high-pressure gas is the separation target (mainly CO₂/H₂) in pre-combustion, equipment such as vacuum pumps are not required, saving energy and cost. However, the separation membrane must be durable under high temperature and high pressure. In addition, since H₂ has a smaller molecular size than CO₂, H₂ selective permeation membranes have been mainly studied. On the other hand, post-conversion targets the separation of combustion exhaust gas generated from boilers in power plants at relatively low pressure, which requires the installation of vacuum pumps and compressors, making it difficult to achieve significant energy conservation and cost reduction compared to existing technologies. For energy conservation and cost reduction, high permeability is required for separation membranes from the viewpoint of reducing the required membrane area.

5. MOF-Based Membranes

5.1. Types of Membranes

Separation membranes based on MOF can be broadly classified into two categories. One is a polycrystalline membrane composed of MOF alone, and the other is a mixed matrix membrane (MMM) in which MOF is mixed with a polymer membrane as a filler. Similar to porous inorganic membranes such as silica and zeolite membranes, MOF polycrystalline membranes are often formed on porous ceramic supports to ensure the mechanical strength of the membrane. MOFs are often compared and discussed with zeolites because of their similarities with zeolites in terms of crystalline porous structure. MMM, on the other hand, is a strategy to improve membrane performance by synergistically combining the excellent processability of polymers with the porous properties of MOF fillers.

MOF polycrystalline membranes exhibit high separation performance by selecting the optimum structure for the separation target because the only membrane permeation pathway for gas molecules is through the pores of the MOFs. However, nonselective permeation often occurs due to the formation of grain boundaries between crystals, pinholes, and intracrystalline defects. In order to fabricate membranes with dense grain boundaries, polycrystalline membranes are generally prepared by using seed crystals via secondary growth method [52,53,54,55]. Although pioneering studies of MOF membrane formation reported in the late 2000s did not lead to the reporting of gas permeation results, these studies stimulated research on polycrystalline MOF membranes and various membrane preparation methods have been reported.

MMM is a membrane in which MOF fillers are dispersed in a polymer matrix. The dispersion state of the polymer and filler greatly affects the performance of the membrane [56]. MMMs may be prepared on supports, but they differ from MOF polycrystalline membranes in that the processability of polymers can be used to fabricate freestanding membranes. Since MOFs contains organic ligands, it is expected to interact well with the polymer matrix and inhibit microvoid formation between filler/polymer. The use of highly porous MOFs as fillers is expected to improve membrane permeability. However, to improve permeability, it is necessary to increase the MOF filler content. However, as filler content increases, mechanical properties and processability of polymers decrease. In general, the smaller the fillers, the more likely they are to aggregate. If interface defects are formed in the MMM due to non-uniform dispersion caused by aggregation of fillers and/or poor interaction between filler and polymer, gas molecules will preferentially diffuse through the defects and separation performance will be degraded. In order to suppress filler agglomeration and poor dispersion in the polymer matrix, a technique to control the filler/polymer heterointerface at the molecular level is required.

5.2. MOF Membrane Preparation Method and Points to Consider

If MOFs can be thinned so that there are no voids between crystals, they can be applied as separation membranes. However, fabricating polycrystalline membranes is not so easy. It must be noted that cracks, pinholes, and intra-crystal defects between crystals cause non-selective permeation, and that large areas must be achieved with thin membranes. Various methods have been proposed for preparing MOF membranes (Figure 4).

To fabricate continuous polycrystalline membranes on a support, a dense heterogeneous nucleation field must exist on the support surface. The secondary growth method is often used, in which pre-prepared seed crystals are loaded on the support surface and grown to form continuous films. Seeding techniques such as dip coating [57], slip coating [58] and rubbing [59] are used, followed by solvothermal or hydrothermal synthesis. In general, it is important to uniformly load seed crystals on support surface, and to make thin membranes (<1 μm), seed crystals of about 100 nm are required to allow sufficient crystal intergrowth [60]. The secondary growth method is an effective way to promote the formation of dense heterogeneous nuclei, which is important for thin membrane growth, but it still poses a challenge in terms of adhesion between membrane and support.

To address the issue of adhesion between membrane and support, modification of the support surface with compounds that bind the MOF crystals and the support has been used [61,62,63,64]. These compounds have one end that can coordinate with the nodes constituting the MOF and the other end that can covalently bond with the support. The functional groups immobilized on the support cause heterogeneous nucleation of MOFs and promote crystal growth, resulting in continuous MOF membranes with a high degree of crystallinity and relatively thin membrane thickness. The chemical modification method is also effective when using polymers as supports in addition to ceramic supports [65,66].

Another method has been proposed to solve the problem of adhesion between the membrane and the support by growing and immobilizing MOFs in the pores of the porous support. The counter-diffusion method is used to deposit MOFs in the pores of the support [67,68,69]. In the counter-diffusion method, the solutions of metal ions and organic ligands are supplied from opposite sides of the support, and the MOF layer is formed at the interface where the diffusing raw materials come into contact by chemical potential gradient.

Grain boundary defects are one of the most important problems in continuous polycrystalline membranes. The difference in the coefficient of thermal expansion between MOF crystals and support is a source of stress and causes defects in the membrane. Membrane defects can also occur during the activation process of MOF. ZIF-78 membrane synthesized using N,N-dimethylformamide as the reaction solvent easily forms membrane defects when activated at 100°C under vacuum [70]. Therefore, it is effective to bring the coefficients of thermal expansion of the two materials closer, but such a combination is not always possible. On the other hand, it has been demonstrated that membrane defects can be reduced by optimizing the cooling rate after membrane formation at high temperature [71]. To solve this problem, it is effective to replace the remaining reaction solvent with a solvent with low boiling point and surface tension, such as methanol or ethanol, before heating MOF under vacuum. In addition, various post-synthetic modifications have been investigated to suppress the generation of membrane defects and to repair defects that have occurred.

5.3. Olefin/Paraffin Separation

MOFs have potential for a wide range of separation targets due to their excellent pore structure and composition, as well as the diversity of their synthesis and membrane production methods. Although MOFs appear promising for olefin/paraffin separation, only a few MOF membranes are currently available. While they have been demonstrated to be effective for the separation of C₃H₆/C₃H₈, few have been reported to be able to efficiently separate C₂H₄/C₂H₆.

ZIF-8, which is composed of Zn(II) and 2-methylimidazolate as building blocks and has a SOD structure, has been the most studied for C₃H₆/C₃H₈ separation. The effective pore size of ZIF-8 is 4.0-4.2 Å, but even 1,2,4-trimethylbenzene of approximately 7.6 Å enters the pores [72], suggesting a lack of sharp molecular sieving. Indeed, the selectivity of CO₂/CH₄ separation by the ZIF-8 membrane is only about 5 [73]. On the other hand, the structural flexibility of ZIF-8 works effectively in C₃H₆/C₃H₈ separation, showing a sharp cut-off between C₃H₆ and C₃H₈ molecular sizes. The diffusion selectivity of C₃H₆/C₃H₈ in ZIF-8 is theoretically estimated to be approximately 125 [74], and various studies on ZIF-8 membranes have been conducted with this value as a benchmark. Pan et al. first reported the separation of C2/C3 hydrocarbons (C₂H₆/C₃H₈, C₂H₄/C₃H₆ and C₂H₄/C₃H₈) using a ZIF-8 membrane prepared on a porous alumina disc [75]. Meanwhile, at the same time, Bux et al. reported a selectivity of only 2.8 for C₂H₄/C₂H₆ separation [76]. Subsequently, intensive research on ZIF-8 membranes was undertaken after Zhang et al. showed that the pore size of ZIF-8 was effective for C₃H₆/C₃H₈ separation by estimating the diffusion coefficient [77] (Table 1). Various improvements have been made to meet the separation performance requirements, such as optimizing secondary growth and activation conditions, and devising unique membrane preparation methods.

Brown et al. devised an interfacial microfluidic membrane processing (IMMP) method to fabricate ZIF-8 membranes on polyamide-imide hollow fibers (Torlon^®) [78]. In the IMMP method, an aqueous solution of 2-methylimidazole is fed to one side of the hollow fiber and a 1-octanol solution of zinc nitrate is continuously fed to the opposite side for counter-diffusion to form ZIF-8 membrane at an incompatible interface. This method is promising for scale-up and mass production of membranes, as it is low-cost and can process several hollow fibers with high specific surface area simultaneously. In their initial report, the C₃H₆/C₃H₈ selectivity was only 12 due to the presence of membrane defects. Thereafter, the C₃H₆/C₃H₈ selectivity reached 180 (at a feed gas pressure of 1 bar) [79] by controlling the membrane formation and optimizing the membrane growth process and the microstructure of the hollow fibers. It was also confirmed that the selectivity of 90 was maintained even when the feed gas pressure was 9.5 bar.

Besides controlling the microstructure between neighboring crystals, it is also important to control the structural flexibility of MOFs to improve the separation selectivity of polycrystalline membranes. For ZIFs, the framework flexibility caused by the rotation of the ligands allows larger molecules to permeate, resulting in reduced molecular sieving effect. In contrast, Tanaka et al. showed that the structural flexibility of ZIF-8 varies depending on the crystal size [10] and proposed that the membrane performance can be tuned by the size of the primary particles constituting the polycrystalline membrane [64]. On the other hand, the kinetic properties of the ZIF ligand are altered by substituting the metal nodes. ZIF-67 has the same crystal topology as ZIF-8, with Co(II) as a node instead of Zn. It is known that the bonding of Co(II)-2-methylimidazolate is stronger than that of Zn(II)-2-methylimidazolate, which makes ZIF-67 more rigid than ZIF-8 and limits the rotation of the ligand. Kwon et al. grew ZIF-67 heteroepitaxially on a ZIF-8 seed layer and then ZIF-8 on a ZIF-67 layer to prepare a membrane with a trilayer ZIF-8/ZIF-67/ZIF-8 structure (Figure 5) and demonstrated that extremely high C₃H₆/C₃H₈ selectivity is achieved [80]. Zhou et al. devised a fast current-driven synthesis (FCDS) method and fabricated ZIF-8 membranes on anodic alumina oxide (AAO) [81]. In the FCDS method, it was found that the formation of ZIF-8 was promoted by the DC current, resulting in the formation of lattice-distorted ZIF-8_Cm as a crystalline polymorph (Figure 6). ZIF-8_Cm, which accounts for 60-70% of the membranes formed, shows higher rigidity and better C₃H₆/C₃H₈ separation than the common cubic ZIF-8_I$\overline{4}$3m, resulting in highest C₃H₆/C₃H₈ selectivity among ZIF-8 membrane reported so far. The methods have been reported to provide sharp molecular sieving ability by suppressing the structural flexibility characteristic of MOFs.

Although the selectivity of ZIF membranes for C₃H₆/C₃H₈ separation has improved significantly, from tens up to about 300, the permeability is still on the order of 10⁻⁸ mol m⁻² Pa⁻¹ s⁻¹ (Table 1). The main reason for this is the membrane thickness, which for most ZIF-8 membranes is several to tens of μm. In contrast, Li et al. developed a gel vapor deposition (GVD) method that combines the sol-gel and chemical vapor deposition methods to fabricate extremely thin ZIF-8 membranes (17~ nm) in PVDF hollow fiber (Figure 7) [82]. The ZIF-8 membranes prepared by the GVD method showed relatively high C₃H₆/C₃H₈ selectivity and one to three orders of magnitude higher permeability than conventional membranes. Ma et al. developed an all-gas phase process for ZIF-8 membrane production [83]. In this method, an ultrathin ZnO layer is deposited on a support by atomic layer deposition (ALD), and then the ZnO layer is converted to ZIF-8 by 2-methylimidazole vapor treatment. The membrane thickness and microstructure are controlled by the number of ALD cycles.

5.4. Other Hydrocarbons Separation

Eum et al. applied the IMMP method, which produced ZIF-8 membranes on polyamide-imide hollow fibers, to carbon hollow fibers to produce ZIF-90 membranes [84]. In general, polymer supports have poor chemical resistance and swell when exposed to organic compounds. In contrast, ZIF-90 membranes fabricated on chemically inert carbon hollow fibers exhibited high chemical resistance. ZIF-90, which is composed of Zn(II) and 2-imidazolecarboxaldehyde, has the same crystal topology as ZIF-8 and its crystallographic pore size (3.5 Å) is not much different from that of ZIF-8. On the other hand, its effective pore size (5.0 Å) is larger than ZIF-8 due to its structural flexibility. The ZIF-90 membrane showed n-C₄H₁₀/i-C₄H₁₀ selectivity of 12 and n-C₄H₁₀ permeability of 6.0×10⁻⁸ mol m⁻² Pa⁻¹ s⁻¹, indicating its potential for separation of butane isomers.

Huang et al. prepared MIL-160 membranes on porous alumina discs modified with polydopamine and applied them to the separation of xylene isomers [85]. The MIL-160 membranes showed p-xylene/o-xylene selectivity of 38.5 and p-xylene permeation flux of 467 g m⁻² h⁻¹. MIL-160, which is composed of [AlO₄(OH)₂] and 2,5-furan-dicarboxylate as building blocks, has an effective pore size of 5 to 6 Å and shows higher adsorption enthalpy and diffusivity for p-xylene than for o-xylene. Therefore, MIL-160 membranes are effective for xylene isomer separation and are promising candidates for thermal and chemical stability. In addition, high thermal and chemical stability of MIL-160 membranes is effective for separation of xylene isomers.

5.5. CO₂ Separation

MOF-based membrane research targeting CO₂ separation has been actively investigated [86]. HKUST-1, MIL-53, MIL-100, and MIL-101 are candidates for combustion flue gas, natural gas purification, and hydrogen purification due to their higher CO₂ adsorption capacity than typical zeolites (Table 2) [23,87,88,89,90,91,92,93,94,95,96,97,98].

Since H₂/CO₂ is the main separation target for pre-combustion and the molecular size of H₂ is smaller than that of CO₂, research has focused on H₂ selective permeation membranes. Table 3 shows the top data for H₂/CO₂ separation using MOF polycrystalline membranes. MOFs with suitable pore size and high CO₂ affinity can be candidates for CO₂/N₂ separation. CAU-1 with amino groups is one of them (Table 4). The structure of CAU-1 consists of distorted octahedral and tetrahedral cages, which are connected by a triangular window with an opening diameter of 3-4 Å. The amino groups in the CAU-1 framework interacts with CO₂ through acid-base interactions, resulting in improved CO₂/N₂ separation performance. Efficient CO₂/CH₄ separation is very important in natural gas and biogas refining. Corrosion control is important in pipeline transportation, and CO₂ is corrosive in the presence of water vapor, thus it must be kept at low concentrations. Currently, membrane separation accounts for only 10% of the natural gas refining market. If membranes with high permeability and selectivity can be developed, membrane separation may be superior to chemical absorption in natural gas and biogas purification; polycrystalline membranes such as ZIF-8, IRMOF-1, MIL-53-NH₂, and UiO-66 have been reported for CO₂/CH₄ separation applications (Table 5). However, it has been noted that many MOF polycrystalline membranes have low CO₂/CH₄ ideal separation factors. ZIF-8 and MIL-96 have been considered suitable for CO₂/CH₄ separation because their pore entrance diameters are between the molecular sizes of CO₂ and CH₄. However, it should be noted that some MOFs have flexible structures and exhibit dynamic pore characteristics. For example, the effective pore size of ZIF-8 is 4.0-4.2 Å, which is larger than the molecular sizes of CO₂, N₂, and CH₄, and thus does not allow for sharp molecular sieving for CO₂/N₂ and CO₂/CH₄. Ligands with polarizable functional groups and metal nodes with high valence, such as Zr⁴⁺, Al³⁺, Cr³⁺, and Fe³⁺, show high adsorption to gas molecules with large quadrupole moments, such as CO₂. On the other hand, strong adsorption may result in low diffusion coefficients. So far, the separation performance of MOF polycrystalline membranes often falls within the trade-off range of higher permeability, but lower selectivity compared to zeolite membranes.

On the other hand, separation membranes that exceed the upper limit of polymer membrane performance have been reported by using MOF as a filler in a mixed matrix membrane. The combination of polymer matrix and filler is very important. Note that the introduction of fillers can alter the arrangement and free volume of the polymer chains and cause interfacial defects between filler/filler and filler/matrix (Figure 8). Since the affinity between the filler and the polymer matrix plays an important role in the processability and performance of the membrane, the compatibility of both components must also be considered. It has been reported that dispersing MOF fillers in the polymer matrix without interfacial defects improves the separation performance of MMMs due to the molecular sieving effect derived from the uniform pores of the filler (Table 6) [44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61]. Recently, it has also been reported that the synergistic effects of different fillers can be obtained by adding MOF fillers together with graphene oxide (GO) [51] and ionic liquids (ILs) [52] to polymer matrices [53].

6. Conclusions

The development of membrane separation using MOFs has been active due to the rapid increase in the number of studies on MOFs, from synthesis and structural design to application. Relatively long-term durability tests have also been conducted at the laboratory level. Although various MOF-based membranes have been fabricated, the common issue is how to achieve thin membrane formation without generating defects such as pinholes, cracks, and grain boundaries. To this end, it is important to understand the formation mechanism of MOFs based on complexation reactions between metal ions and ligands, and to develop the elementary processes of membrane formation, which can control nucleation and crystal growth. Such fundamental understanding will be the driving force for the next step toward the practical application of separation membranes based on MOFs.