High-performance Mixed Matrix Membranes Research Articles

Pearl-necklace-structured hollow MOF filled in membrane for molecular separation

Membrane technology is important for saving energy in the semiconductor industry through purifying isopropanol (IPA). The strategic incorporation of efficient molecular transport pathways into mixed matrix membranes (MMMs), known for their exceptional molecular selectivity and minimal diffusion resistance, presents challenges but holds promising potential for enhancing the progress of high-performance membrane production. This study proposes a novel surface coating-assisted etching strategy to synthesize a highly continuous metal-organic framework with a hydrophilic surface and distinct hollow structure (HZIF-8@PPy). When integrated into the polyvinyl alcohol (PVA) matrix, HZIF-8@PPy demonstrates exceptional compatibility at the interface and significantly enhances the membrane's capacity for selective water molecule transport. The pervaporation dehydration efficiency from an IPA aqueous solution shows significant enhancement, with selectivity and permeability reaching 5.9 and 3.7 times higher than those of the pristine PVA membrane, respectively. The effectiveness of HZIF-8@PPy's structure and its separation mechanism was validated by resolving the five types of molecular channels in MMMs during permselective pervaporation. This research holds significant implications for the purification of organic compounds and also presents new opportunities for developing robust and high-performance MMMs with effective fillers, thereby contributing to advancements in membrane technology.

Polyethylene Glycol-Mediated High-Performance Mixed Matrix Membranes via a “Nesting Effect” for CO2 Separation

Polyethylene Glycol-Mediated High-Performance Mixed Matrix Membranes via a “Nesting Effect” for CO₂ Separation

Covalently assembling multi-dimensional carbon-based nanoclay composites via ionic liquid linkers into PVA membrane for molecular separation

The application of membrane-based separation technology in purifying azeotropic solutions plays a crucial role in minimizing energy consumption during reactive distillation. The strategic integration of multi-dimensional structural composites within mixed matrix membranes (MMMs), characterized by a robust framework and efficient molecular transport channels, poses challenges but demonstrates significant potential for advancing the development of high-performance membrane fabrication. Herein, we proposed a novel strategy to covalently assemble halloysite nanotubes (HNTs) and graphene oxide (GO) using ionic liquids (ILs) as linkers. When incorporated into the polyvinyl alcohol (PVA) matrix, the ILs@HNTs@GO (IHGO) composites exhibit excellent interfacial compatibility, and ILs also enhanced the composite’s selective transfer ability to water molecules. The optimal separation performance for pervaporation dehydration from a ternary solution was respectively 2.57 (selectivity) and 1.74 (permeability) times higher than those of the pristine PVA membrane. The structural stability of the MMMs and their separation mechanism has been comprehensively validated by adjusting the content of GO in IHGO composites and conducting simulations. This study holds significant theoretical and practical implications for organic compounds purification, while simultaneously paving new avenues for the fabrication of structurally robust and high-performance MMMs with optimal filler effectiveness.

Nanocomposite membranes comprising covalent organic framework and polymer of intrinsic microporosity for efficient CO2 separation

The fabrication of new mixed-matrix membranes (MMMs) with superior gas permeability and selectivity are highly sought for industrial development of membrane separation technology for CO2 capture. This study reports improvement in CO2 permeability and selectivity by employing porous covalent organic framework (COF-TpPa-1). Herein, we report a novel PIM-1 doped with COF-TpPa-1 in various concentrations ranging from 0 to 10 wt% to prepare COF-TpPa-1/PIM-1 MMMs. Good compatibility between COF-TpPa-1 particles and PIM-1 matrix rendered an ideal interfacial morphology. The MMMs composed of 7 wt% COF-TpPa-1 exhibit a CO2 permeability of 6336 Barrer with an enhancement of 104 % compared to pristine sample and CO2/N2 ideal selectivity 20.1 among five tested compositions. The CO2 permeability of COF-TpPa-1 7 wt% doped membrane was 88 % more than pristine PIM-1 after 30 days aging, while CO2/N2 selectivity was well maintained, coupled with excellent anti-plasticization and mixed-gas separation properties. The overall gas separation performance of MMMs exceeds the Robeson Upper bound (2008) for CO2/N2 and O2/N2. The measured experimental permeability exhibited excellent agreement with the Singh thermal conductivity model at low loading (<5 wt%). This work provides a potential approach to fabricate PIM-1-based high performance MMMs with COF fillers for efficient CO2 separation.

High-Performance Mixed-Matrix Membranes Using a Zeolite@MOF Core-Shell Structure Synthesized via Ion-Exchange-Induced Crystallization and Post-Synthetic Conversion.

Strategic design of nanostructures, such as the core-shell configuration, offers a promising avenue to harness the desired properties while mitigating the inherent limitations of individual materials. In our pursuit of synergizing the advantages of two distinct porous materials, namely, zeolites and metal-organic frameworks (MOFs), we aimed to develop the zeolite@MOF core-shell structures. To synthesize this targeted material while minimizing undesirable side reactions, we devised an innovative approach involving ion-exchange-induced crystallization and post-synthetic conversion. This method enabled the exclusive growth of a MOF on the zeolite surface. Specifically, we successfully crafted a CaA@ZIF-8 core-shell structure, employing it in the fabrication of mixed-matrix membranes for CO2 separation. Within this core-shell configuration, the ZIF-8 in the shell played a crucial role in enhancing the filler-polymer interfaces, leading to the development of defect-free membranes. Simultaneously, the CaA zeolite core exhibited a highly selective transport of CO2. The synergistic effects resulted in a membrane incorporating a CaA@ZIF-8 core-shell filler, which demonstrated a high CO2 permeability of 1142 Barrer and a CO2/CH4 selectivity of 43.3, significantly surpassing the established upper limits for polymeric membranes. Our findings underscore the potential of core-shell structures composed of microporous materials for achieving the coveted properties necessary for high-performance gas separation membranes.

Dormancy and double-activation strategy for construction of high-performance mixed-matrix membranes.

Mixed-matrix membranes (MMMs) have the potential for energy-efficient gas separation by matching the superior mass transfer and anti-plasticization properties of the fillers with processability and scaling up features of the polymers. However, construction of high-performance MMMs has been prohibited due to low filler-loading and the existence of interfacial defects. Here, high MOF-loaded, i.e., 55 wt %, MMMs are developed by a 'dormancy and double-activation' (DDA) strategy. High MOF precursor concentration suppresses crystallization in the membrane casting solution, realizing molecular level mixing of all components. Then, the polymeric matrix was formed with uniform encapsulation of MOF nutrients. Subsequently, double-activation was employed to induce MOF crystallization: the alkali promotes MOFs nucleation to harvest small porous nanocrystals while excessive ligands activate the metal ions to enhance the MOFs conversion. As such, quasi-semi-continuous mass transfer channels can be formed in the MMMs by the connected MOFs nanocrystals to boost the gas permeability. The optimized MMM shows significantly ameliorated CO2 permeability, i.e., 2841 Barrer, five-fold enhancement compared with pristine polymer membrane, with a good CO2 /N2 selectivity of 36. Besides, the nanosized MOFs intensify their interaction with polymer chains, endowing the MMMs with good anti-plasticization behaviour and stability, which advances practical application of MMMs in carbon capture.

Reconstruction of mixed matrix membranes by in situ vapor aminolysis for CO2/N2 and CH4/N2 separations

Membrane technology has great potential for gas separation. Incorporation of nanoporous fillers in polymers is attractive for constructing high-performance mixed matrix membranes (MMMs). However, for some mixtures, especially those molecules with similar properties, efficient membrane separation remains extremely challenging. In this study, we report post-synthetic reconstruction of MMMs by in situ aminolysis via chemical vapor deposition for enhancing gas separation performance. By utilization of amine substitution to functional groups of polymers of intrinsic microporosity and chelation to metal centers of metal-organic frameworks (MOFs), the MMMs are modified under mild conditions. Since aminolysis promotes amino grafting, boosts preferential sorption, regulates pathway, and enhances filler-polymer adhesion, the MMMs have substantially improved antiaging property and CO2/N2 separation performance, with permeability of 6986 Barrer and selectivity of 30.7. Importantly, for CH4/N2 mixture separation, that is critical for natural gas purification but difficult to separate, the MMMs also exhibit competitive selectivity and permeability of 5.0 and 897 Barrer, respectively. The performances for CO2/N2 and CH4/N2 separations can be close to and surpass the least trade-off upper bounds, respectively. Moreover, in situ aminolysis by vapor deposition has universal applicability for reconstructing various MMMs. This strategy is an alternative route to fabricate membranes for efficient separations.

Nanocomposite polymer blend membrane molecularly re-engineered with 2D metal-organic framework nanosheets for efficient membrane CO2 capture

Membrane gas separation technology is an energy-efficient alternative to the traditional industrial processes for CO2 capture towards the mitigation of global warming. To overcome the inherent performance trade-off behavior of polymeric membranes, many nanoarchitectures have been incorporated for developing high-performance mixed matrix membranes. Yet, a successful nanocomposite membrane design remains challenging, especially for CO2 capture, because of polymer-filler incompatibility and interfacial defects. Herein, 2D Cu-TCPP nanosheets were synthesized and incorporated into Pebax/Poly (ethylene glycol) methyl ether acrylate (PEGMEA) blends to design MMMs with exceptional CO2/N2 separation performance. This synergistic polymer blend after crosslinking, offers a highly accommodating and robust matrix environment for the homogenous distribution of nanosheets. Cu-TCPP particles were also produced as 3D nanoflowers to probe their morphological effect on gas transport. The finely dispersed 2D nanosheets expanded the free volume of the resultant nanocomposite membranes, leading to a significantly enhanced CO2 permeability of up to 1183 Barrer which was 93% higher than the neat polymers, while they also increased the transport pathway tortuosity that contributes to a very high CO2/N2 selectivity of 57.6 at a loading of merely 0.1 wt%. Not only do such separation performances surpassed the Robeson upper bound by a large margin and outcompeted many existing advanced MMM designs, but they also remained stable after long-term operations. By carefully elucidating the gas transport tuning effect of 2D metal-organic framework nanosheets in nanocomposite membranes, this work could help broaden their applicability for other environmentally important molecular separations.

Cellulose acetate mixed-matrix membranes doped with high CO2 affinity zeolitic tetrazolate-imidazolate framework additives

Introducing CO2 affinity additives into membrane is one of significant approaches to improve the CO2 permeability of membranes for natural gas sweetening. In current work, high performance mixed-matrix membranes were successfully obtained using the cellulose acetate (CA) and the zeolitic tetrazolate-imidazolate frameworks (ZTIFs) with highly CO2 affinity as matrix and additive respectively. The interaction between CA matrix and ZTIFs was verified by the FT-IR. The SEM, EDS and XRD were used to evaluate the morphology properties of these membranes. The CO2 permeability of CA/ZTIFs mixed-matrix membranes which is closely correlated to the loading of high CO2 affinity ZTIF additive were in a range of 9.59–15.85 Barrer, implying the indispensable role of the ZTIF additive in changing the CO2 dissolution and diffusion behavior inside the membranes. The ideal compatibility and gas selectivity proved the great potential of the ZTIFs additives for the gas separation application.

Enriching Fe3O4@MoS2 composites in surface layer to fabricate polyethersulfone (PES) composite membrane: The improved performance and mechanisms

Transition metal dichalcogenides (TMDs) have been regarded as prominent nanofiller to manufacture high-performance mixed-matrix membranes. However, their efficient application still faces the challenge of reasonable nanostructure regulation and controllable distribution of TMDs nanofillers. In this work, magnetic Fe3O4@MoS2 composites with hierarchical flower-like structure were firstly synthesized and then mixed in polyethersulfone (PES) casting solution. A composite membrane was fabricated by enriching Fe3O4@MoS2 in the membrane's upper layer with the assistance of a magnetic field. The Fe3O4@MoS2 composites endowed the composite membrane with enhanced permeability, rejection and self-cleaning performance for filtration of typical foulant (bovine serum albumin (BSA) and humic acid (HA)) solutions. The optimum membrane possessed a pure water flux of 550.7 ± 22.7 L·m−2·h−1·bar−1, which is about 1.85 times higher than that of the bare PES membrane. Meanwhile, the modified membrane had a rejection to BSA and HA higher than 90 % as compared with only about 60 % for the bare membrane. Furthermore, the cyclic filtrations revealed that the membrane flux can be recovered 90 % under visible light irradiation, indicating the excellent photocatalytic self-cleaning ability introduced by Fe3O4@MoS2. Mechanisms underlying the improved antifouling performance and self-cleaning ability of the composite membrane were proposed. The facile strategy, together with the improved membrane performance clearly demonstrates broad application prospect of the Fe3O4@MoS2 PES composite membrane in water treatment.

Preparation of high performance mixed matrix membranes by one-pot synthesis of ZIF-8 nanoparticles into Pebax-2533 for CO2 separation

Herein, an interesting strategy, viz. one-pot synthesis, is proposed to incorporate ZIF-8 nanoparticles into the Pebax-2533 for facile fabrication of MMMs with high permeability and selectivity. Based on characterization analyses, incorporating 8 wt% of ZIF-8 particles via this route increased the fractional free volume (FFV) of the membranes, which contributed to the improvement in the CO2 permeability, avoided the aggregation of the nanoparticles in the polymer matrix, increased the CO2 sorption property due to imidazole linkers, boosted the screening properties owing to intrinsic pore size, thereby improved CO2/CH4 and CO2/N2 selectivities, and in addition, resulted in membranes with desirable thermal properties. As a result, MMM with 8 wt% of ZIF-8 by providing an admirable performance, improved the CO2 permeability and CO2/CH4 and CO2/N2 selectivities of the neat Pebax-2533 membrane from 62.20 Barrer, 11.35, and 24.98 to 158.43 Barrer (+155% enhancement), 27.73 (+144.32%) and 50.76 (+103.21%), respectively. This membrane at an operating pressure of 8 bar overcame Robeson’s upper bound (1991) in the case of CO2/CH4 separation.

Upgrading CO2/CH4 separation performances of Pebax-based mixed-matrix membranes incorporated with core/shell-structured ZIF-L(Co)@ZIF-8 composite nanosheets

The development of high-performance mixed matrix membranes (MMMs) with satisfactory structure stability has attracted much attention for the past few years. The rational design and synthesis of fillers to well coordinate themselves in the polymeric matrix is still challenging. The core-shell structured fillers were synthesized via the heteroepitaxial growth of ZIF-8 on ZIF-L(Co) surface (ZIF-L(Co)@ZIF-8), which were then embedded into Pebax to fabricate MMMs for enhanced CO2/CH4 separation. The synthesized fillers and MMMs were characterized via SEM with EDS mapping, TEM, XRD, FIIR and nanoindentation measurements. The effects of ZIF-L(Co)@ZIF-8 loading and filler components on gas separation performance were studied with the evaluation of membrane stability. The experimental results indicated that the filled ZIF-L(Co)@ZIF-8 could be evenly distributed within the bulky Pebax. The synergistic effect of the core-shell structured fillers in adsorption and molecular sieving endowed the synthesized MMMs with superior gas separation performances. The optimal separation performance is achieved for the MMM containing 3 wt% of ZIF-L(Co)@ZIF-8 with a CO2 permeability of 145.7 Barrer and a CO2/CH4 selectivity of 40.9, corresponding to 79.1% and 110.1% increases over pure Pebax membrane.

Heterostructured ZIF-8/lamellar talc composites incorporated polydimethylsiloxane membrane with enhanced separation performance for butanol recovery

In this study, heterostructured MOF/lamellar talc composites was successfully synthesized by in-situ growth of zeolitic imidazolate framework-8 (ZIF-8) nanoparticles on acidified black talc (ABT) and introduced into polydimethylsiloxane (PDMS) matrix to prepare high-performance mixed matrix membranes (MMMs) via spin coating and thermal cross-linking. The submicron-size natural lamellar black talc (BT) was first explored as stable carrier to synthesize laminated composite with continuous MOF phase. Owing to its silica-oxygen tetrahedral structure, the resultant laminated composite presented good compatibility with PDMS matrix, avoiding the formation of interfacial defect between polymer and nanoparticles. Moreover, well-dispersed and continuous ZIF-8 nanoparticles on lamellar nanosheets had preferential adsorption effect for alcohol, enhancing the selectivity of membranes. The coupling effects of graphene-like lamellar structure from talc and sequential transport channels from ZIF-8 attributed to greatly enhance permeation flux. Various techniques such as FESEM, TEM, FTIR, XRD and EDS were applied to investigate the morphology and structure of composites and membranes, and pervaporation experiments were applied to probe the behavior of polymer-based composite membranes. High performance defect-free ZIF-8@ABT/PDMS MMMs exhibited excellent total permeate flux of 1711 g m−2 h−1 with very competitive separation factor of 41 for low concentration n-butanol recovery.

Mixed-matrix membranes consisting of Pebax and novel nitrogen-doped porous carbons for CO2 separation

Fabricating high-performance mixed matrix membranes (MMMs) for CO 2 separation is of great importance for industrial application, but the poor compatibility between polymer and filler hinders their further development. Herein, novel nitrogen-doped porous carbons (NPC) were gained by a facile hydrothermal reaction between glucose and melamine, subsequently KOH activation. Highly micropore NPC with the BET surface area of 1229.36 m 2 g −1 and Langmuir surface area of 1868.46 m 2 g −1 accompanied with the BJH pore volume of 0.68 cm 3 g −1 was achieved by scanning electron microscopy (SEM) and N 2 adsorption. After blending with polyether-block-amide (Pebax), the as-obtained MMMs showed excellent mechanical properties probably due to the homogeneous dispersion of NPC in the Pebax as shown in SEM results. The nitrogen in MMMs was expected to be significantly increased upon incorporation with NPC, as verified by attenuated total reflectance Fourier-transform infrared spectroscopy, thus increased the CO 2 transport. MMM with 5 wt% NPC exhibited a CO 2 permeability of 553 Barrer. This value is much higher than that of the pristine Pebax membrane. Surprisingly, a significantly improved CO 2 /N 2 selectivity of as high as 56.4 was observed for MMMs. Thus, MMM having a 5 wt% NPC had an excellent gas separation, approaching Robeson's upper bound revised in 2019, which was superior to the one incorporated with other activated carbons prepared from similar reactants. The enhanced separation performance is mainly ascribed to the presence of nitrogen-containing groups for high CO 2 affinity and also the micropore structure to promote CO 2 diffusion in NPC. These results indicated that the improvement in gas permeability and ideal selectivity simultaneously could be achieved by MMMs using the microporous and functional nanofillers. • Novel nitrogen-doped porous carbons were introduced to Pebax to prepare MMMs for high performance CO 2 separation. • The nitrogen in porous carbons can provide more affinity sites for CO 2 transport. • Thus, a CO 2 permeability of as high as 553 Barrer with a CO 2 /N 2 selectivity of 56.4 was observed.

Novel Cellulose Triacetate (CTA)/Cellulose Diacetate (CDA) Blend Membranes Enhanced by Amine Functionalized ZIF-8 for CO2 Separation.

Currently, cellulose acetate (CA) membranes dominate membrane-based CO2 separation for natural gas purification due to their economical and green nature. However, their lower CO2 permeability and ease of plasticization are the drawbacks. To overcome these weaknesses, we have developed high-performance mixed matrix membranes (MMMs) consisting of cellulose triacetate (CTA), cellulose diacetate (CDA), and amine functionalized zeolitic imidazolate frameworks (NH2-ZIF-8) for CO2 separation. The NH2-ZIF-8 was chosen as a filler because (1) its pore size is between the kinetic diameters of CO2 and CH4 and (2) the NH2 groups attached on the surface of NH2-ZIF-8 have good affinity with CO2 molecules. The incorporation of NH2-ZIF-8 in the CTA/CDA blend matrix improved both the gas separation performance and plasticization resistance. The optimized membrane containing 15 wt.% of NH2-ZIF-8 had a CO2 permeability of 11.33 Barrer at 35 °C under the trans-membrane pressure of 5 bar. This is 2-fold higher than the pristine membrane, while showing a superior CO2/CH4 selectivity of 33. In addition, the former had 106% higher CO2 plasticization resistance of up to about 21 bar and an impressive mixed gas CO2/CH4 selectivity of about 40. Therefore, the newly fabricated MMMs based on the CTA/CDA blend may have great potential for CO2 separation in the natural gas industry.

Fabrication of high-performance mixed-matrix membranes via constructing an in-situ crosslinked polymer matrix for gas separations

• A binaphthyl-ether tetramine was designed using to form an in-situ crosslinked polymer matrix. • Desirable interfacial morphology and uniform dispersion of TiO 2 were obtained in MMMs. • Overall gas separation properties of MMMs containing 5–23 wt% of TiO 2 were maintained. In this work, a b i n aphthyl-ether t etr a mine (BNTA) monomer was designed, synthesized, and reacted with 6FDA to form an in-situ crosslinked polyimide (6FDA-BNTA). Then, novel mixed-matrix membranes (MMMs) were prepared by incorporating the titanium dioxide (TiO 2 ) nanoparticles into the 6FDA-BNTA polymer matrix to fabricate TiO 2 /6FDA-BNTA MMMs for gas separations. The desirable interfacial morphology and uniform dispersion of TiO 2 were obtained due to the nano-confinement effect and stronger hydrogen bond interactions between TiO 2 and 6FDA-BNTA when compared with these of their linear analogues as polymer matrixes. The pristine 6FDA-BNTA membrane and MMMs containing 5–23 wt% of TiO 2 had good mechanical properties, which possessed tensile strengths in the range of 65.2–116.6 MPa and elongation at break of 4.8–7.8%. Gas transport results further revealed that the overall performances of TiO 2 /6FDA-BNTA MMMs were maintained with the increasing of the TiO 2 loading content. Specifically, 6FDA-BNTA MMMs with TiO 2 loading of 23 wt% exhibited the highest gas permeability (e.g. PCO 2 = 376.2 Barrer, PO 2 = 76.3 Barrer) and moderate selectivity (e.g. αCO 2 /N 2 = 24.3, αO 2 /N 2 = 4.9), locating near the Robeson 2008 upper bounds very closely for O 2 /N 2 , H 2 /N 2 , and CO 2 /CH 4 gas pairs. This facile approach, aiming at improving the incompatibility between two phases via constructing an in-situ crosslinked polymer matrix, provides a new insight for rational design of high-performance MMMs.

Mixed-Matrix Membranes Comprising of Polysulfone and Porous UiO-66, Zeolite 4A, and Their Combination: Preparation, Removal of Humic Acid, and Antifouling Properties.

High-performance Mixed-Matrix Membranes (MMMs) comprising of two kinds of porous fillers UiO-66 and Zeolite 4Aand their combination were fabricated with polysulfone (PSf) polymer matrix. For the very first time, UiO-66 and Zeolite 4A were jointly used as nanofillers in MMMs with the objective of complimenting synergistic effects. The individual and complimentary effects of nanofillers were investigated on membrane morphology and performance, pure water flux, humic acid rejection, static humic acid adsorption, and antifouling properties of membranes. Scanning Electron Microscopy (SEM) analysis of membranes confirmed that all MMMs possessed wider macrovoids with higher nanofiller loadings than neat PSf membranes and the MMMs (PSf/UiO-66 and PSf/Zeolite 4A-UiO-66) showed tendency of agglomeration with high nanofiller loadings (1 wt% and 2 wt%). All MMMs exhibited better hydrophilicity and lower static humic acid adsorption than neat PSf membranes. Pure water flux of MMMs was higher than neat PSf membranes but the tradeoff between permeability and selectivity was witnessed in the MMMs with single nanofiller. However, MMMs with combined nanofillers (PSf/Zeolite 4A-UiO-66) showed no such tradeoff, and an increase in both permeability and selectivity was achieved. All MMMs with lower nanofiller loadings (0.5 wt% and 1 wt%) showed improved flux recovery. PSf/Zeolite 4A-UiO-66 (0.5 wt%) membranes showed the superior antifouling properties without sacrificing permeability and selectivity.

Interfacial manipulation of MOFs/polymer mixed matrix membranes for gas separations: A review

Membrane-based separation technology holds great promise to effectively address separation needs in chemical industries due to advantages in energy efficiency, scalability, and small footprint. Mixed matrix membranes based on polymer/metal organic frameworks (MOFs), which combine the ease of processability of polymer materials with the excellent adsorption characteristics of MOFs, have been widely investigated for gas separations. The preparation of gas separation membrane materials with both high flux and gas selectivity is the key to achieving efficient gas separation. Recent research results show that the permeability coefficient and gas pair selectivity of mixed matrix membrane materials are hardly improved simultaneously by adding second phase materials such as MOFs. When the loading of MOFs increases, most membranes have common problems such as particle agglomeration and non-selective interfacial defective structures, which lead to a significant decrease in separation performance. To resolve these issues, research scientists are devoted to tailoring the interfacial microstructure of mixed matrix membranes with an aim to exceed the Robeson upper limit. Many novel characterization techniques coupled with simulation studies were carried out to probe the filler dispersion behavior and interfacial structure within membranes in order to provide rules for rationally designing mixed matrix membranes. A variety of approaches have been proposed to finely tune the interfacial interaction by constructing hydrogen bonding and covalent bonding in the membranes. For example, synthesizing functionalized MOFs via mixed-linker or ligand exchange methods provides a good option to enhance interfacial adhesion within membranes. The amino groups, hydroxyl groups and other organic functional groups in MOFs have the capability to create hydrogen bonding. Moreover, these groups can preferentially adsorb specific gas molecules (such as CO2 molecules) to improve gas selectivity. Some researchers post-functionalized MOFs to graft reactive groups on their surface, incorporating them with cross-linkable polymers. Then the light/heat induced crosslinking reaction occurred within the membrane to seal its defective structure, thereby achieving a substantial improvement in membrane separation performance. The past publications on mixed matrix membranes in the decade demonstrated that the researchers have much more in-depth understanding and insights on the fabrication process, membrane morphology, and structure-property relationship of mixed matrix membrane materials. With respect to fundamental research, research scientists are committed to investigating the MOFs properties including crystal morphology, coordinated metal ion, functional groups on membrane performance. To yield high performance mixed matrix membranes, highly permeable polymers were rationally designed as the polymer matrix. However, to complement their widespread industrial application, scale-up of hollow fiber mixed matrix membranes needs to be further investigated. The membrane stability in humid gas feeds is required to be improved. It is also of importance to develop stable, cost-effective and scalable MOFs fillers. With a specific focus on interfacial manipulation of MOFs/polymer mixed matrix membranes for gas separations, this paper mainly reviewed the research progress in the fields of MOFs/polymer-based hybrid matrix membranes for gas separations. It focuses on three aspects: (1) Research progress on optimizing and characterizing the interfacial structure; (2) fundamental understanding of the influences of MOFs and polymers properties on membrane performance; (3) research progress on scale-up of mixed matrix membranes. Finally, the future development direction of mixed matrix membrane materials is proposed in order to provide research ideas for the rational design of high-performance mixed matrix membranes.

Enhancing the pervaporation performance of PEBA/PVDF membrane by incorporating MAF-6 for the separation of phenol from its aqueous solution

Phenol is highly toxic and carcinogenic even at low concentrations. This study focused on preparing high-performance mixed matrix membranes (MMMs) for the pervaporation separation of phenol from its aqueous solution. A highly-efficient zeolitic metal azolate framework, RHO-[Zn(Heim)2] (MAF-6, Heim = 2-ethylimidazole) was incorporated into the polyether block amide (PEBA-2533) supported on the polyvinylidene fluoride (PVDF) ultrafiltration membrane. The MAF-6 was chosen as the filler due to its inherent hydrophobic surfaces and aperture size (7.6 Å), which is larger than the molecular kinetic diameter of phenol (6.6 Å). Effects of the casting solution concentration, active layer thickness, MAF-6 loading, operating temperature, and phenol concentration were systematically investigated. With the increase of the MAF-6 loading ratio X from 0 to 7, the phenol flux of the membrane was gradually enhanced, but the water flux was reduced. For 1000 ppm phenol aqueous solution at 80 °C, MAF-6–7@MMM displayed the optimal separation factor of 25.9 with a phenol flux of 89.2 g∙m−2∙h−1. The pervaporation separation index (PSI) of the MAF-6–7@MMM was 1317.7 kg∙µm∙m−2∙h−1 at 80 °C, which was much higher than the PEBA/PVDF membrane (787 kg∙µm∙m−2∙h−1). Impressively, an increasing feed temperature enhanced the phenol flux more prominently than water, owing to the hydrophobicity and porosity of MAF-6. Consequently, the resultant MAF-6-X@MMMs broke the trade-off limitation between phenol permeability and selectivity. The as-prepared membrane also exhibited exceptional long-term stability, suggesting its prospect of industrial application.

High-performance mixed-matrix membranes enabled by organically/inorganic modified montmorillonite for the treatment of hazardous textile wastewater

In this study, a low pressure-driven ultrafiltration (UF) membrane has been developed using cellulose acetate (CA), α-aminophosphonate modified montmorillonite (MMT) and Ag-TiO2 nanoparticles. The characteristics of the developed materials and the performance of the fabricated membranes were exhaustively evaluated. FTIR and TGA results proved the successful incorporation of α-aminophosphonate in the montmorillonite (MT), while the XRD results showed that it was intercalated into the MT layers. XRD results also showed that only silver doped anatase TiO2 (Ag-TiO2 NPs) was obtained in nanosized form as inferred from the TEM and XRD micrographs. Compared to neat CA membrane, the composite membranes (CMs) exhibited a tremendous high flux (up to 400 LMH bar), almost 6-fold increment, and a substantially reduced fouling rate. The CMs were durable, behaved as an ultrafiltration membrane, and their performance in terms of water and protein (BSA) solution permeance, and rejection were comparable to commercial UF membranes reported in the literature. The optimum CM (CM-6) exhibited a satisfactory efficiency in treating textile wastewater and showed an adequate rejection for hazardous contaminants (chromium, nickel, lead and fluoride). To sum up, fillers enhanced the hydrophilicity, mechanical stability and performance of the fabricated membranes. The fabricated membranes in this work showed potentiality for hazardous textile wastewater treatment.