Porous Membranes with Special Wettabilities: Designed Fabrication and Emerging Application

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Mar 2021Porous Membranes with Special Wettabilities: Designed Fabrication and Emerging Application Jiancheng Di, Li Li, Qifei Wang and Jihong Yu Jiancheng Di State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Li Li State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Qifei Wang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 and Jihong Yu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.020.202000457 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Porous materials have become a burgeoning research interest in materials science because of their intrinsic porous characteristics, versatile chemical compositions, and abundant functionalities. Recently, inspired by natural superwetting surfaces originating from the cooperation of surface energy and surface geometry, porous membranes with special wettabilities are finding emerging opportunities associated with a wide variety of environmental and energy-related applications. This review will present an overview of the state-of-the-art research on the designed fabrications and applications of superwetting porous membranes based on zeolites, metal–organic frameworks (MOFs), porous organic materials (POMs), and mesoporous materials. General synthetic strategies for the fabrication of porous membranes (e.g., hydrothermal/solvothermal crystallization, interfacial polymerization, electrospinning, etc.), and principles for tuning the wettability of porous membranes through surface energy modulation are introduced. Furthermore, their emerging applications as oil–water separation membranes, lithium-ionbattery separators, self-cleaning layers, and anticorrosion coatings are demonstrated. Finally, we emphasize on future perspectives regarding the development of superwetting porous membranes for practical applications. Download figure Download PowerPoint Introduction Inspired by the fascinating features of natural plants or animals such as the self-cleaning property of lotus leaves, the lubrication effect of pitcher plants, the underwater oil resistance of fish scales, and others, a great deal of efforts have been devoted to engineering surface wettability of solid materials, which hold promising potentials to address the problems related to energy sources, environmental protection, and human health.1–6 As a fundamental property, the wettability of material surfaces depends on the interfacial interaction in solid–liquid–gas or solid–water–oil system. For a smooth substrate, wettability only relies on the surface chemical composition, which can be elucidated typically by the classic Young's equation.5 However, for rough surfaces, the influence of surface topography on the wettability should be considered; therefore, there are two possible models: Wenzel and Cassie−Baxter models. The Wenzel model theorizes that the solid surfaces are fully wetted by air or liquid, giving rise to the maximum air–solid or liquid–solid contact area,7 while solid surfaces are partially wetted or fully unwetted, the Cassie−Baxter model explains the wetting phenomena.8 According to these wetting models, surface wettability can be modulated by manipulating the surface geometry or controlling the surface energy by coating the existing substrates with chemical layers. In principle, the material surfaces exhibit more affinity to oil along with the successive decrease of surface energy, and the surface wettability changes in turns such as amphiphilicity, hydrophobicity/oleophilicity, and dual lyophobicity in solid–liquid–gas system. Moreover, the lyophilicity or lyophobicity of surfaces is enhanced dramatically by increasing the surface roughness, even achieving superlyophilicity or superlyophobicity. Thus far, most of the reported superwetting membranes are constructed by solid materials (e.g., oxides and polymers), which cannot afford the membranes' additional properties of large surface area and open structure, as well as adsorption ability, ion-exchange performance, ion conductivity, and so forth. Porous materials, including zeolites, mesoporous materials, metal–organic frameworks (MOFs), and porous organic materials (POMs), are an important class of solid materials with large surface areas, regular and well-defined channels, and high chemical/thermal stability (Figure 1a).9–15 They have widespread applications in the fields of petrochemical industry, hazardous substance cleaning, energy storage, and others, and are emerging as ideal hosts for loading ultrasmall metal catalysts in high-efficiency catalytic systems, as well as various drugs or proteins in the biological medicine.16–18 Recently, porous membranes have been fabricated through growing porous layers on rigid substrates or blending porous particles in polymeric matrixes, followed by a shaping process (Figure 1b). Based on the anisotropic morphology of porous materials that are randomly overlaid on top of the membrane, there is no need for sophisticated molding processes such as laser ablation, electrolytic deposition, chemical etching, and so forth, to increase the surface roughness further. More importantly, diverse functional groups can be incorporated into porous membranes during either in situ fabrication or the posttreatment process, which can change the surface energy of the porous membranes effectively, giving rise to modulated wettabilities, including hydrophobicity/oleophilicity in air, hydrophilicity/underwater oleophobicity, underliquid dual lyophobicity, and so forth (Figure 1c). The superwetting porous membranes are finding emerging opportunities in applications such as oil–water membranes, lithium-ion battery separators, self-cleaning layers, and anticorrosion coatings (Figure 1d). Figure 1 | (a) Schematic illustration of the classification of superwetting porous membranes and their advantageous features. (b) General fabrication methods of the porous membranes. (c) Chemical modification approaches for modulating the wettabilities of superwetting porous membranes. (d) Environment and energy-related applications of superwetting porous membranes. Download figure Download PowerPoint In this minireview, we will summarize the recent progress in the designed fabrication of superwetting porous membranes and their emerging applications triggered by the synergistic effect between inherent characteristics of porous materials and their unique surface wettabilities. These porous membranes are classified as zeolite membranes, MOF membranes, POM-based membranes, and mesoporous membranes. In each section, we will outline the fabrication methods based on the type of porous material, displaying the wetting features, and present the emerging applications of such membranes in the environmental and energy-related fields. Finally, we will highlight on the current challenges and prospects for the designed fabrication of superwetting porous membranes to meet the increasing demand of practical applications. Zeolite membranes Zeolites are a type of microporous aluminosilicates composed of interconnected TO4 tetrahedra (T = Si or Al), which have become one of the most important heterogeneous catalysts and adsorbents in the chemical industry because of their open-framework structures with molecule-sized entrance, high surface area, and abundant catalytic active sites.19 The pure zeolite coatings are traditionally prepared on inorganic porous substrates (i.e., metal meshes and alumina disks) via an in situ or seeding-assistant hydrothermal process.20 Owning to the high surface roughness coupled with the multiple surface hydroxyl groups, most zeolite membranes without further modification exhibit superamphiphilicity in air.21–26 For instance, our group fabricated the silicalite-1 [Mobil-type five ( MFI) topological structure = all-silica zeolite containing Si, O, and H in the framework] zeolite membranes on stainless steel meshes known as zeolite-coated mesh films (ZCMFs; Figures 2a and 2b) via a seeding-assistant hydrothermal crystallization, and first extended the application of zeolite membrane in the oil–water separation based on superior wettability. The ZCMFs exhibited superamphiphilicity in air, but they showed strong repulsion to various oils when immersed in water, indicating their underwater superoleophobic property (Figures 2c and 2d). Therefore, water could quickly pass through the pinholes that deliberately remained in ZCMFs; but the trapped water layers blocked oils. The residual oil content in the filtrate was <6 ppm, suggesting a highly efficient separation performance (Figure 2e). Meanwhile, the sizes of the pinholes that governed the water flux and intrusion pressure of ZCMFs could be modulated simply by changing the hydrothermal crystallization time.21 An optimal crystallization time of 12 h endowed ZCMF with a pinhole size of ∼14 μm, which gave rise to the water flux of 25 L m−2 s−1 and the intrusion pressure of 729 Pa. Owing to the high thermal/chemical stability of silicalite-1 zeolite, the ZCMFs were corrosion resistant in a strong corrosive media, and the adsorbed oils could be removed by calcination, exhibiting a stable separation performance.22 Besides, the introduction of Al or B atoms into the silicalite-1 zeolite skeleton resulted in the anionic frameworks, which not only increased the surface hydrophilicity further, leading to higher underwater oleophobicity but also afforded the zeolite membranes excellent ion-exchange capability.23–26,29 For example, the Ag+ ion-exchanged Al- MFI zeolite membranes ([email protected]) remained the underwater superoleophobicity, exhibited a stable separation efficiency of 99%, and high water flux of 54,720 L m−2 h−1 for oil-in-water mixtures. Furthermore, [email protected] demonstrated superior biofouling activity because of the broad-spectrum antibacterial performance of the exchanged Ag+ ions in zeolite crystals.26 Figure 2 | (a and b) SEM images of ZCMFs. (c) Photographs of water and oil contact angles of ZCMFs in air. (d) Underwater oil contact angles of ZCMFs for a selection of oils. (e) Residual oil contents in the filtrates after the oil–water separation process by ZCMFs.21 (f and g) SEM images of pure-silica beta zeolite membranes. (h) Photographs of contact angles in air. Left to right correspond to the water contact angles on bare stainless steel mesh and pure-silica beta zeolite membrane, and the oil contact angle on pure-silica beta zeolite membrane. (i) Variations of water contact angles and oil–water separation efficiencies of pure-silica beta zeolite membrane after different abrasion cycles.27,28 Reprinted with permission from ref 21. Copyright 2013 Royal Society of Chemistry; ref 27. Copyright 2018 American Chemical Society; ref 28. Copyright 2019 American Chemical Society. SEM, scanning electron microscopy. Download figure Download PowerPoint Recently, pure zeolite membranes and zeolite-mixed polymer membranes have been employed as the separator in Li-ion battery due to the massive electrolyte uptake arising from the superhydrophilicity and large surface area, high ion conductivity, and high thermal stability of zeolite crystals.30–32 For instance, when the ZSM-5-coated polypropylene (PP) membrane was used as the separator, this combination resulted in better charge and discharge capacities and lowered the inner resistance of Li4Ti5O12/Li cell at a relatively high current rate than that separated by PP membrane alone.30 Wang et al.32 reported the fabrication of Li+ ion-exchange LiX [Faujasite ( FAU)-type topological structure] zeolite layer on the Celgard substrate, which was used as the separator in the lithium–sulfur (Li–S) battery. The high surface energy enabled membrane-improved electrolyte wettability. Notably, Li+ ions could migrate freely through the membrane, but polysulfide (PS) with size larger than the pore diameter of LiX zeolite was blocked; therefore, the LiX zeolite membrane that acted as the ion-selective barrier could achieve considerable cycling stability. When evaluating the performance at a C-rate of 0.2 C, the Li–S battery-specific capacity with LiX separator reached ∼1083 mAh g−1, 70% of which remained after 400 cycles. In addition to the hydrophilic zeolite membranes, several hydrophobic zeolite membranes have been prepared by decreasing the surface energy. For instance, hydrofluoric acid (HF) is a commonly used mineralizer that promotes crystallinity and reduces the framework defects of zeolites, thereby enabling them to attain intrinsic hydrophobic properties via the elimination of surface hydroxyl groups. Zhang's group27 demonstrated the fabrication of the pure-silica beta ( BEA-type topological structure) zeolite membranes on stainless steel meshes in the fluoride-containing near-neutral medium via secondary hydrothermal crystallization (Figures 2f and 2g). The pure-silica beta zeolite membranes showed superhydrophobicity/superoleophilicity in air with water and oil contact angles of 150.4° and 0°, respectively (Figure 2h). Owning to the strong adhesion of the beta zeolite coating on the substrate, the membranes had satisfactory mechanical durability and maintained high hydrophobicity along with separation efficiency for chloroform–water mixture after severe abrasion test (Figure 2i).28 The hydrophobic zeolite membranes can also be fabricated by modifying hydrophobic chemicals on the as-prepared membranes. In this method, zeolite coatings or particles are employed to increase the surface roughness, grown on hydrophilic substrates, followed by the modification of hydrophobic silane agents,33,34 or physically adhered onto the hydrophobic sponges,35 or embedded in the hydrophobic polymer membranes.36,37 These composite zeolite membranes showed excellent superhydrophobicity/superoleophilicity, resulting in a superior oil-removal ability from oil–water mixtures. MOF membranes MOFs are a class of porous inorganic–organic hybrid crystals in which metal cations (or clusters) and organic ligands are combined via coordination bonds.38 Due to their promising features such as large surface areas, targeted pore size, controllable active sites, and network modularities, MOFs have become promising candidate as heterogeneous catalysts, drug carriers, gas storage materials, and others.11,12,39,40 In addition, MOF membranes can be fabricated on various substrates via the solvothermal crystallization process,41–43 or by embedding the as-prepared MOF particles into graphite oxides,44–46 fabrics,47,48 and polymer-based matrixes.49–55 For example, ZIF-8 is a well-known zeolite imidazole framework (ZIF) with exceptional hydrothermal and chemical stability, composed of Zn metal centers coordinated by four 2-methylimidazolate (MIM) molecules.56 The nanosized ZIF-8 crystals can be inserted between fluorinated graphene oxide sheets,44,46 or grafted onto the melamine sponges53,54 or carbon nitride foams,57 achieving excellent superhydrophobicity along with superoleophilicity, which could adsorb various organic pollutants from oily wastewater. Also, ZIF-8 crystals can act as functional units blended with polymeric matrixes, followed by various shaping processes.51,53,58–60 Among them, the electrospinning technique is a practical approach to mold materials (e.g., solid polymers, inorganics, and porous substances) into fibrous morphology.61,62 The electrospun fibrous membranes have microstring nonwoven networks composed of intertwined fibers that can trap air efficiently and hold the infused liquids in solid–liquid–gas and solid–water–oil systems, respectively, forming the superwetting composite interfaces.1 Xu et al.51 presented a facile approach for growing ZIF-8 crystals on the membrane composed of the electrospun poly(vinylidene fluoride) (PVDF)@ZnO nanofibers (Figure 3a). The evenly distributed nanosized ZnO particles in PVDF fibers acted as seeds to ensure firm grafting of the ZIF-8 crystals around the nanofibers. Notably, the grafted ZIF-8 crystals could simultaneously increase the surface roughness and energy, and eventually, tune the PVDF membrane's wettability from amphiphobicity to hydrophobicity/oleophilicity (Figures 3b and 3c). The improved oil removal capability endowed the [email protected] membranes with a separation efficiency of 92.93% for water-in-oil emulsions (Figure 3d). Further, when changing the polymer matrix into polyacrylonitrile (PAN), the resultant membrane ([email protected]) exhibited superamphiphilicity in air and extraordinary underliquid dual lyophobicity (Figures 3e and 3f).59 Therefore, [email protected] nanofibrous membrane is suitable for the separation of various oil-water mixtures with efficiencies >99%, even achieving the separation of surfactant-stabilized emulsions (Figure 3g). Figure 3 | (a) Schematic diagram of the fabrication of [email protected] nanofibrous membrane, and the oil–water separation mechanism. (b and c) Water and oil contact angles on a PVDF nanofibrous membrane without (left) and (right) the growth of ZIF-8 crystals. (d) Optical photos of the water-in-oil emulsion before and after filtration (left), and the corresponding water rejection and membrane flux of [email protected] nanofibrous membranes (right).51 (e) Illustration of the fabrication of [email protected] membrane for oil–water separation. (f) Water and oil contact angles in air and underliquid contact angles on [email protected] nanofibrous membrane, respectively. (g) Separation process of various oil–water mixtures by [email protected] nanofibrous membrane.59 (h) Preparation procedure of HKUST-1 membrane. (i) Underwater crude oil contact angle, sliding angle, and dynamic adhesion of HKUST-1 membrane. (j) Antifouling test of the HKUST-1 membrane. (k) Illustration of the fabrication of HKUST-1 membrane on a copper substrate, and the corresponding SEM images before and after the growth of HKUST-1 crystals.63 (l–n) Illustration of the oil–water separation processes and the purification efficiencies of HKUST-1 membrane for different oil–water mixtures during five separation-regeneration cycles.42 Reprinted with permission from ref 42. Copyright 2019 Elsevier; ref 51. Copyright 2018 Wiley-VCH; ref 59. Copyright 2017 Elsevier; ref 63. Copyright 2018 Royal Society of Chemistry. SEM, scanning electron microscopy. Download figure Download PowerPoint The Hong Kong University of Science and Technology (HKUST)-1 is a copper-based MOF built up of dimeric metal units, connected by benzene-1,3,5-tri-carboxylate linker molecules to form repeating coordination motives [Cu3(BTC)2] that extend in three-dimensions (3Ds) channel structure, which has high-water stability and unsaturated abundant active sites.64 A recent research indicated that the HKUST-1 powder could selectively capture oil droplets, and the oil removal capacity was about six times that of activated carbon.65 Meanwhile, the oil-adsorbed HKUST-1 crystals would aggregate spontaneously and precipitate when added to the oil–water mixtures; thereby realizing the effective removal of oils. Based on these outstanding characteristics, the as-prepared HKUST-1 crystals were adhered onto the stainless steel meshes by a mussel-inspired method in which dopamine (PDA) not only acted as the organic binder to gather the crystals but also increased the hydrophilicity of the membrane due to the existence of large polar groups (Figure 3h).63 The HKUST-1 membrane with an architecture similar to Chinese yam, has an underwater superoleophobicity/underoil superhydrophilicity, as well as a small sliding angle and low adhesion force to crude oil when immersed in water, hence, it displays excellent self-cleaning ability (Figures 3i and 3j). Consequently, for the separation of surfactant-stabilized oil-in-water emulsions, the water fluxes of the HKUST-1 membrane were >300 L m−2 h−1, and the chemical oxygen demand (COD) values in the filtrates were <400 mg L−1, which were comparable with those of reported PDA-based surface-modified membranes.66,67 Moreover, the HKUST-1 membrane could adsorb water from water-in-oil emulsions with a separation efficiency up to 99.99%. Hereafter, Du et al.42 reported an economical and green synthetic approach to transform the Cu2(OH)2CO3 coating directly into the HKUST-1 membrane at room temperature (Figure 3k). By elaborately patterning the sacrificial template of Cu2(OH)2CO3, HKUST-1 crystals were coated onto various copper-based substrates, including copper sheet, tube, nail, mesh, and others. Taking the HKUST-1-decorated copper mesh as an example, the underliquid dual superlyophobicity enabled the membrane high-separation efficiency (97%) for any layered oil–water mixture (Figures 3l–3n). Except for ZIF-8 and HKUST-1, other water-stable MOFs have also been adopted to prepare superwetting porous membranes. For example, a physically mixed [email protected] composite was used as the separator in Li-ion battery, which was directly coated on the LiNi0.5Co0.2Mn0.3O2 electrode using a blade-coating method.68 In terms of the high hydrophilicity and large surface area, the ZIF-67-based separator demonstrated higher electrolyte uptake, better thermal stability, and improved ion conductivity, compared with the commercial microporous PP separator, leading to a nearly 2-times retention capacity at 55 °C. Kang's group69 demonstrated the influence of surface wettability on the pervaporation efficiency of Ni2(l-asp)2bipy MOF for water/ethanol mixtures. The MOF membrane was prepared on the porous silica substrates through a seeding-assistant solvothermal process. Regarding the hydrophilic property, the water flux and the separation factor of this membrane for water/ethanol mixture reached as high as 27.6 kg·m−2 h−1 and 73.6, respectively. The posttreatment by coating polydimethylsiloxane (PDMS) would result in the hydrophobic surface, which weakened the affinity of the membrane to water, thereby decreasing both the water flux and separation factor, but this membrane could further application in membrane is a MOF commonly by using acid as the from the unique growth the layer can be prepared a such as room temperature and 1 which has been demonstrated by et The hydrophilic chemical and the rough surface arising from the of gave rise to the underwater superoleophobic performance of the membrane, leading to a high water flux of L m−2 h−1 and separation efficiency of Meanwhile, the enabled the fabrication of membranes to practical such as oil–water separation. the groups could be in situ incorporated into the crystals using acid as the organic which improved the water adsorption performance and further modification by other functional groups. reported the of which were in the membrane form via a process. The containing abundant groups not change the wettability of membranes but improved the stability of the membrane both acid and due to the excellent and Therefore, performance and high-separation efficiency of for oil-in-water different of water-stable MOFs have been most of the as-prepared MOFs from Therefore, are to increasing the hydrophobicity of MOFs to the of the coordination between metal and typically in two of the MOFs and in situ during the MOF The process can be by coating or chemical grafting of various layers (e.g., hydrophobic and organic molecules with or on the as-prepared MOF membranes to decrease the surface energy. Among them, is a commonly used hydrophobic coating to increase the water repulsion of MOF membranes. For example, the hydrophilic crystals could be in situ on the via a solvothermal crystallization process, which stability when in the corrosive and temperature Also, it could even the and abrasion and exhibited excellent mechanical and environmental The membrane hydrophobic after coating by displaying to water, and Meanwhile, the membranes had a high factor of therefore, these could be used for in Besides, the removal efficiencies for layered oil–water mixtures and oil-in-water emulsions were and respectively. Based on unique features such as abundant unsaturated metal and the the chemical of membrane surface can be modulated by grafting low surface energy organic thereby changing the surface wettability into For example, the ZIF-8 membrane was hydrophilic because of the unsaturated ions and the on the surface that with water The surface process using molecules could the surface molecules without the ZIF-8 and hence, enhanced the hydrophobicity of the membrane due to the of (Figures Therefore, the ZIF-8 membranes with and without processes had the wettability that the of oil and water through the membrane, respectively, and oil–water separation (Figures and et a to molecules onto the activated MOFs such as and HKUST-1 that abundant unsaturated metal (Figures and The molecules with the surface energy without the inherent and decreasing crystallinity of Taking the as an example, this MOF composite the superhydrophobicity in air with a water contact angle of which the high adsorption capacities for various oils The separation was fabricated by loading on the commercial which of oil through the membrane, giving rise to a high-separation efficiency of Figure | (a) Illustration of the ZIF-8 membrane's wettability through a surface process. Water contact SEM and oil–water separation process of ZIF-8 membranes and before and and g) after the (h) Illustration of the fabrication of

Topics & Concepts