Advanced Porous Materials in Mixed Matrix Membranes

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

Unexpectedly High Propylene/Propane Separation Performance of Asymmetric Mixed-Matrix Membranes through Additive-Assisted In Situ ZIF-8 Filler Formation: Experimental and Computational Studies

Zeolitic-imidazolate framework-8 (ZIF-8), composed of a zinc center tetrahedrally coordinated with 2-methylimidazolate linkers, has garnered extensive attention as a selective filler for propylene-selective mixed-matrix membranes (MMMs). Recently, we reported an innovative and scalable MMM fabrication approach, termed "phase-inversion in sync with in situ MOF formation" (PIMOF), aimed at addressing the prevailing challenges in MMM processing. In this study, we intend to investigate the effect of additives, specifically sodium formate and 1,4-butanediol, on the modification of ZIF-8 filler formation within the polymer matrix in order to further improve the separation performance of the asymmetric MMMs prepared by the PIMOF. Remarkably, MMMs prepared with sodium formate as an additive in the coagulation bath exhibited an unprecedented C3H6/C3H8 separation factor of 222.5 ± 1.8 with a C3H6 permeance of 10.1 ± 0.3 GPU, surpassing that of MMMs prepared without additives (a C3 separation factor of 57.7 ± 11.2 with a C3 permeance of 22.5 ± 4.5 GPU). Our computational work complements the experimental investigation by studying the effect of ZIF-8 nanoparticle size on the specific surface interaction energy and apertures of ZIF-8. Calculations indicate that by having smaller ZIF-8 nanoparticles, stronger interactions are present with the polymer affecting the aperture of ZIF-8 nanoparticles. This reduction in aperture size is expected to improve selectivity toward propylene by reducing the permeability of propylene. These results represent a significant advancement, surpassing the performance of all previously reported propylene-selective MMMs and most high-quality polycrystalline ZIF-8 membranes. The notably enhanced separation performance primarily arises from the formation of exceedingly small ZIF-8-like particles with an amorphous or poorly crystalline structure, corroborated by our computational work.

Entanglement-Stabilized Nanoporous Polymer Films Made by Mechanical Deformation

We present a new simulation-guided process to create nanoporous materials, which does not require specific chemical treatment and solely relies on mechanical deformation of pure highly entangled homopolymer films. Starting from fully equilibrated freestanding thick polymer melt films, we apply a simple "biaxial expansion" deformation. Upon expansion holes form, which are prevented from growing and coalescing beyond a characteristic size due to the entanglement structure of the melt. We investigate the local morphology, the void formation upon expansion, and their stabilization. The dependence of the average void (pore) size and void fraction (porosity) on the total strain and subsequent relaxation is investigated. Furthermore, the stabilization of the porous structure of the thin expanded films through cooling below the glass transition temperature Tg is discussed.

2D MOFs and Zeolites for Composite Membrane and Gas Separation Applications: A Brief Review

Commercial membranes have predominantly been fabricated from polymers due to their economic viability and processability. This choice offers significant advantages in energy efficiency, cost-effectiveness, and operational simplicity compared to conventional separation techniques like distillation. However, polymeric membranes inherently exhibit a trade-off between their permeability and selectivity, which is summarized in the Robeson upper bound. To potentially surpass these limitations, mixed-matrix membranes (MMMs) can be an alternative solution, which can be constructed by combining polymers with inorganic additives such as metal-organic frameworks (MOFs) and zeolites. Incorporating high-aspect-ratio fillers like MOF nanosheets and zeolite nanosheets is of significant importance. This incorporation not only enhances the efficiency of separation processes but also reinforces the mechanical robustness of the membranes. We outline synthesis techniques for producing two-dimensional (2D) crystals (including nanocrystals with high aspect ratio) and provide examples of their integration into membranes to customize separation performances. Moreover, we propose a potential trajectory for research in the area of high-aspect-ratio materials-based MMMs, supported by a mathematical-model-based performance prediction.

Hydrogen-bonded organic frameworks for membrane separation

Hydrogen-bonded organic frameworks (HOFs) are a new class of crystalline porous materials that are formed through the interconnection of organic or metal-organic building units via intermolecular hydrogen bonds. The remarkable flexibility and reversibility of hydrogen bonds, coupled with the customizable nature of organic units, endow HOFs with mild synthesis conditions, high crystallinity, solvent processability, and facile self-healing and regeneration properties. Consequently, these features have garnered significant attention across various fields, particularly in the realm of membrane separation. Herein, we present an overview of the recent advances in HOF-based membranes, including their advanced fabrication strategies and fascinating applications in membrane separation. To attain the desired HOF-based membranes, careful consideration is dedicated to crucial factors such as pore size, stability, hydrophilicity/hydrophobicity, and surface charge of the HOFs. Additionally, diverse preparation methods for HOF-based membranes, including blending, in situ growth, solution-processing, and electrophoretic deposition, have been analyzed. Furthermore, applications of HOF-based membranes in gas separation, water treatment, fuel cells, and other emerging application areas are presented. Finally, the challenges and prospects of HOF-based membranes are critically pointed out.

Fabrication of Oriented MOF-Based Mixed Matrix Membrane via Ion-Induced Synchronous Synthesis

Developing a facile strategy for constructing oriented mixed matrix membranes (MMMs) with uniformly dispersed and high-loading metal-organic frameworks (MOFs) is a crucial scientific challenge in probing the enhanced capability and potential applications of MOF-polymer MMMs. Herein, a novel synchronous synthetic method for constructing oriented CuBDC/poly(m-phenylenediamine) (CuBDC/PmPD) MMM with uniform MOF dispersion at high loading at the air-solution interface via the dual function of metal ions is reported. The resulting MMM exhibits excellent separation performance in ion sieving and seawater desalination due to the structural integrity of the proposed membrane and the highly interconnected channels created through the oriented distribution of MOF in a polymer matrix. Such a cutting-edge approach may provide promising insights into the development of advanced MMMs with optimized structure and superior performances.

Removing polyfluoroalkyl substances (PFAS) from wastewater with mixed matrix membranes

Polyfluoroalkyl and perfluoroalkyl (PFAS) chemicals are fluorinated and exhibit complicated behavior. They are determined and highly resistant to ecological modifications that render plants ecologically robust. Thermal stability and water and oil resistance are examples of material qualities. Their adverse consequences are causing increasing worry due to their bioaccumulative nature in humans and other creatures. Direct data indicates that PFAS exposure in humans causes endocrine system disruption, immune system suppression, obesity, increased cholesterol, and cancer. Several PFASs are present in drinking water at low doses and may harm people. These cancer-causing PFAS have caused concern for water bodies all around the globe. Analytical techniques are used to identify and measure PFAS in an aqueous medium (membrane). Furthermore, a deeper explanation is provided for PFAS removal methods, including mixed matrix membrane (MMM) technology. By removing over 99 % of the PFAS from wastewater, MMMs may effectively remove PFAS from sewage when the support matrix contains adsorbing components. Furthermore, we consider several factors affecting the removal of PFAS and practical sorption methods for PFAS onto various adsorbents.

Non-CO2 greenhouse gas separation using advanced porous materials

Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

Fabrication of Metal-Organic Framework-Based Mixed-Matrix Membranes by "Soft Spray" Technique

Metal-organic framework (MOF)-based mixed-matrix membranes (MMMs) represent a class of composite membranes that seamlessly integrate the properties of MOF fillers and polymer matrix into a hybrid system and have been widely used in countless advanced technologies. However, there remains a need for scalable and simple manufacturing techniques that can fabricate a MOF-based MMM with uniform dispersion. Herein, a series of MMMs with well-dispersed MOFs are constructed by a soft spray technique. In brief, by uniformly spraying metal ions onto the surface of a mixed solution containing polyvinylpyrrolidone (PVP) and organic ligands, a free-standing MMM is synthesized at the miscible liquid-liquid interface, facilitated by the dual function of metal ions. Moreover, soft spray technology can also introduce multifunctional materials into the MMM to customize performance. We have successfully introduced carbon black into a MOF-based MMM by soft spray, resulting in MMMs with excellent photothermal effects. The resulted MOF-based MMM exhibits favorable catalytic performance in the condensation reaction of benzaldehyde with primary amines, and the MOF-based MMM modified with carbon black significantly boosts the endothermic CO2 conversion. The work opens a new avenue for the development of MOF-based MMMs with a promising future.

Modification of AgNP-Decorated PET: A Promising Strategy for Preparation of AgNP-Filled Nuclear Pores in Polymer Membranes

Polymer-based membranes represent an irreplaceable group of materials that can be applied in a wide range of key industrial areas, from packaging to high-end technologies. Increased selectivity to transport properties or the possibility of controlling membrane permeability by external stimuli represents a key issue in current material research. In this work, we present an unconventional approach with the introduction of silver nanoparticles (AgNPs) into membrane pores, by immobilising them onto the surface of polyethyleneterephthalate (PET) foil with subsequent physical modification by means of laser and plasma radiation prior to membrane preparation. Our results showed that the surface characteristics of AgNP-decorated PET (surface morphology, AgNP content, and depth profile) affected the distribution and concentration of AgNPs in subsequent ion-track membranes. We believe that the presented approach affecting the redistribution of AgNPs in the polymer volume may open up new possibilities for the preparation of metal nanoparticle-filled polymeric membranes. The presence of AgNPs on the pore walls can facilitate the grafting of stimuli-responsive molecules onto these active sites and may contribute to the development of intelligent membranes with controllable transport properties.

Modular Customization and Regulation of Metal-Organic Frameworks for Efficient Membrane Separations

Metal-organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H2 /CO2 separation performance sequence is achieved (separation factor of 1656-5.4, H2 permeance of 964-2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

A roadmap to enhance gas permselectivity in metal-organic framework-based mixed-matrix membranes

Performing gas separation at high efficiency with minimum energy input and reduced carbon footprint is a major challenge. While several separation methods exist at various technology readiness levels, porous membrane-based separation is considered as a disruptive technology. To attain sustainability and required efficiency, different approaches of membrane design have been explored. However, the selectivity-permeation trade-off and membrane aging have restricted further advancement. In this regard, a new generation composite made of organic polymers and metal-organic framework (MOF) fillers shows substantial promise. Organic polymer matrix allows easy processibility, but it has poor permselectivity for gas molecules. Metal-organic frameworks are excellent sieving materials; however, they suffer from poor processibility issues. A combination of these two components makes an ideal sieving membrane, which can potentially outnumber the existing energy intensive distillation strategies. In this perspective, we have discussed key indices that regulate gas permselectivity by a careful selection of the existing literature. While the target gas flux and selectivity values have been a part of many previous reviews and articles, we have presented a concise discussion on the interface design of the MOF-polymer membrane, morphology, and orientation control of MOF fillers in the matrix. Following this, a future roadmap to overcome challenges related to MOF-polymer interfacial defects is outlined.

Covalent organic frameworks for CO2 capture: from laboratory curiosity to industry implementation

CO2 concentration in the atmosphere has increased by about 40% since the 1960s. Among various technologies available for carbon capture, adsorption and membrane processes have been receiving tremendous attention due to their potential to capture CO2 at low costs. The kernel for such processes is the sorbent and membrane materials, and tremendous progress has been made in designing and fabricating novel porous materials for carbon capture. Covalent organic frameworks (COFs), a class of porous crystalline materials, are promising sorbents for CO2 capture due to their high surface area, low density, controllable pore size and structure, and preferable stabilities. However, the absence of synergistic developments between materials and engineering processes hinders achieving the qualitative leap for net-zero emissions. Considering the lack of a timely review on the combination of state-of-the-art COFs and engineering processes, in this Tutorial Review, we emphasize the developments of COFs for meeting the challenges of carbon capture and disclose the strategies of fabricating COFs for realizing industrial implementation. Moreover, this review presents a detailed and basic description of the engineering processes and industrial status of carbon capture. It highlights the importance of machine learning in integrating simulations of molecular and engineering levels. We aim to stimulate both academia and industry communities for joined efforts in bringing COFs to practical carbon capture.

Metal-Organic Framework/Polyvinyl Alcohol Composite Films for Multiple Applications Prepared by Different Methods

The incorporation of different functional fillers has been widely used to improve the properties of polymeric materials. The polyhydroxy structure of PVA with excellent film-forming ability can be easily combined with organic/inorganic multifunctional compounds, and such an interesting combining phenomenon can create a variety of functional materials in the field of materials science. The composite membrane material obtained by combining MOF material with high porosity, specific surface area, and adjustable structure with PVA, a non-toxic and low-cost polymer material with good solubility and biodegradability, can combine the processability of PVA with the excellent performance of porous filler MOFs, solving the problem that the poor machinability of MOFs and the difficulty of recycling limit the practical application of powdered MOFs and improving the physicochemical properties of PVA, maximizing the advantages of the material to develop a wider range of applications. Firstly, we systematically summarize the preparation of MOF/PVA composite membrane materials using solution casting, electrostatic spinning, and other different methods for such excellent properties, in addition to discussing in detail the various applications of MOF/PVA composite membranes in water treatment, sensing, air purification, separation, antibacterials, and so on. Finally, we conclude with a discussion of the difficulties that need to be overcome during the film formation process to affect the performance of the composite film and offer encouraging solutions.

The Difference in Performance and Compatibility between Crystalline and Amorphous Fillers in Mixed Matrix Membranes for Gas Separation (MMMs)

An increasing number of high-performing gas separation membranes is reported almost on a daily basis, yet only a few of them have reached commercialisation while the rest are still considered pure research outcomes. This is often attributable to a rapid change in the performance of these separation systems over a relatively short time. A common approach to address this issue is the development of mixed matrix membranes (MMMs). These hybrid systems typically utilise either crystalline or amorphous additives, so-called fillers, which are incorporated into polymeric membranes at different loadings, with the aim to improve and stabilise the final gas separation performance. After a general introduction to the most relevant models to describe the transport properties in MMMs, this review intends to investigate and discuss the main advantages and disadvantages derived from the inclusion of fillers of different morphologies. Particular emphasis will be given to the study of the compatibility at the interface between the filler and the matrix created by the two different classes of additives, the inorganic and crystalline fillers vs. their organic and amorphous counterparts. It will conclude with a brief summary of the main findings.

Novel Core/Shell Nylon 6,6/La-TMA MOF Electrospun Nanocomposite Membrane and CO2 Capture Assessments of the Membrane and Pure La-TMA MOF

Membrane technology plays a vital, applicable, and essential role in human life and industry. The high adsorption capacity of membranes can be employed for capturing air pollutants and greenhouse gases. In this work, we tried to develop a shaped industrial form of a metal-organic framework as an adsorbent material with the ability to capture CO₂ in the laboratory phase. To do so, a core/shell Nylon 6,6/La-TMA MOF nanofiber composite membrane was synthesized. This organic/inorganic nanomembrane is a kind of nonwoven electrospun fiber that was prepared using the coaxial electrospinning approach. FE-SEM, surface area calculations, nitrogen adsorption/desorption, XRD grazing incidence on thin films, and histogram diagrams were applied to assess the quality of the membrane. This composite membrane as well as pure La-TMA MOF were assessed as CO₂ adsorbent materials. The CO₂ adsorption abilities of the core/shell Nylon 6,6/La-TMA MOF membrane and pure La-TMA MOF were as high as 0.219 and 0.277 mmol/g, respectively. As a result of preparing the nanocomposite membrane from microtubes of La-TMA MOF, the %A of the micro La-TMA MOF (% 43.060) increased to % 48.524 for Nylon 6,6/La-TMA MOF.

Rapid Fabrication of High-Permeability Mixed Matrix Membranes at Mild Condition for CO2 Capture

Mixed matrix membranes (MMMs), conjugating the advantages of flexible processing-ability of polymers and high-speed mass transfer of porous fillers, are recognized as the next-generation high-performance CO₂ capture membranes for solving the current global climate challenge. However, controlling the crystallization of porous metal-organic frameworks (MOFs) and thus the close stacking of MOF nanocrystals in the confined polymer matrix is still undoable, which thus cannot fully utilize the superior transport attribute of MOF channels. In this study, the "confined swelling coupled solvent-controlled crystallization" strategy is employed for well-tailoring the in-situ crystallization of MOF nanocrystals, realizing rapid (<5 min) construction of defect-free freeway channels for CO₂ transportation in MMMs due to the close stacking of MOF nanocrystals. Consequently, the fabricated MMMs exhibit approximately fourfold enhancement in CO₂ permeability, i.e., 2490 Barrer with a CO₂ /N₂ selectivity of 37, distinctive antiplasticization merit, as well as long-term running stability, which is at top-tier level, enabling the large-scale manufacture of high-performance MMMs for gas separation.

Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation

A new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters is presented. The method is based on fast mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench far below the glass transition temperature T_g . The process is first designed generally by simulation and then verified for the example of polystyrene films. The methodology does not need any chemical processing, supporting substrate, or self assembly process and is solely based on polymer inherent entanglement effects. Pore diameters are of the order of ten polymer reptation tube diameters. The resulting membranes are stable over months at ambient conditions and display remarkable elastic properties.

Review of Synthesis and Separation Application of Metal-Organic Framework-Based Mixed-Matrix Membranes

Metal-organic frameworks (MOFs) are porous crystalline materials assembled from organic ligands and metallic secondary building blocks. Their special structural composition gives them the advantages of high porosity, high specific surface area, adjustable pore size, and good stability. MOF membranes and MOF-based mixed-matrix membranes prepared from MOF crystals have ultra-high porosity, uniform pore size, excellent adsorption properties, high selectivity, and high throughput, which contribute to their being widely used in separation fields. This review summarizes the synthesis methods of MOF membranes, including in situ growth, secondary growth, and electrochemical methods. Mixed-matrix membranes composed of Zeolite Imidazolate Frameworks (ZIF), University of Oslo (UIO), and Materials of Institute Lavoisier (MIL) frameworks are introduced. In addition, the main applications of MOF membranes in lithium-sulfur battery separators, wastewater purification, seawater desalination, and gas separation are reviewed. Finally, we review the development prospects of MOF membranes for the large-scale application of MOF membranes in factories.

Mixed Matrix Membranes with Surface Functionalized Metal-Organic Framework Sieves for Efficient Propylene/Propane Separation

Membrane technology, regarded as an environmentally friendly and sustainable approach, offers great potential to address the large energy penalty associated with the energy-intensive propylene/propane separation. Quest for molecular sieving membranes for this important separation is of tremendous interest. Here, a fluorinated metal-organic framework (MOF) material, known as KAUST-7 (KAUST: King Abdullah University of Science and Technology) with well-defined narrow 1D channels that can effectively discriminate propylene from propane based on a size-sieving mechanism, is successfully incorporated into a polyimide matrix to fabricate molecular sieving mixed matrix membranes (MMMs). Markedly, the surface functionalization of KAUST-7 nanoparticles with carbene moieties affords the requisite interfacial compatibility, with minimal nonselective defects at polymer-filler interfaces, for the fabrication of a molecular sieving MMM. The optimal membrane with a high MOF loading (up to 45 wt.%) displays a propylene permeability of ≈95 barrer and a mixed propylene/propane selectivity of ≈20, far exceeding the state-of-the-art upper bound limits. Moreover, the resultant membrane exhibits robust structural stability under practical conditions, including high pressures (up to 8 bar) and temperatures (up to 100 °C). The observed outstanding performance attests to the importance of surface engineering for the preparation and plausible deployment of high-performance MMMs for industrial applications.

A Bipyridyl Covalent Organic Framework with Coordinated Cu(I) for Membrane C3 H6 /C3 H8 Separation

Covalent organic frameworks (COFs) mixed matrix membranes (MMMs) combining individual attributes of COFs and polymers are promising for gas separation. However, applying COF MMMs for propylene/propane (C₃ H₆ /C₃ H₈ ) separation remains a big challenge due to COF inert pores and C₃ H₆ /C₃ H₈ similar molecular sizes. Herein, the designed synthesis of a Cu(I) coordinated COF for membrane C₃ H₆ /C₃ H₈ separation is reported. A platform COF is synthesized from 5,5'-diamino-2,2'-bipyridine and 2-hydroxybenzene-1,3,5-tricarbaldehyde. This COF possesses a porous 2D structure with high crystallinity. Cu(I) is coordinated to bipyridyl moieties in the COF framework, acting as recognizable sites for C₃ H₆ gas, as shown by the adsorption measurements. Cu(I) COF is blended with 6FDA-DAM polymer to yield MMMs. This COF MMM exhibits selective and permeable separation of C₃ H₆ from C₃ H₈ (C₃ H₆ permeability of 44.7 barrer, C₃ H₆ /C₃ H₈ selectivity of 28.1). The high porosity and Cu(I) species contribute to the great improvement of separation performance by virtue of 2.3-fold increase in permeability and 2.2-fold increase in selectivity compared to pure 6FDA-DAM. The superior performance to those of most relevant reported MMMs demonstrates that the Cu(I) coordinated COF is an excellent candidate material for C₃ H₆ separation membranes.

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Palladium-Percolated Networks Enabled by Low Loadings of Branched Nanorods for Enhanced H2 Separations

Nanoparticles (NPs) at high loadings are often used in mixed matrix membranes (MMMs) to improve gas separation properties, but they can lead to defects and poor processability that impede membrane fabrication. Herein, it is demonstrated that branched nanorods (NRs) with controlled aspect ratios can significantly reduce the required loading to achieve superior gas separation properties while maintaining excellent processability, as demonstrated by the dispersion of palladium (Pd) NRs in polybenzimidazole for H₂ /CO₂ separation. Increasing the aspect ratio from 1 for NPs to 40 for NRs decreases the percolation threshold volume fraction by a factor of 30, from 0.35 to 0.011. An MMM with percolated networks formed by Pd NRs at a volume fraction of 0.039 exhibits H₂ permeability of 110 Barrer and H₂ /CO₂ selectivity of 31 when challenged with simulated syngas at 200 °C, surpassing Robeson's upper bound. This work highlights the advantage of NRs over NPs and nanowires and shows that right-sizing nanofillers in MMMs is critical to construct highly sieving pathways at minimal loadings. This work paves the way for this general feature to be applied across materials systems for a variety of chemical separations.

Polymer-Infiltrated Metal-Organic Frameworks for Thin-Film Composite Mixed-Matrix Membranes with High Gas Separation Properties

Thin-film composite mixed-matrix membranes (TFC-MMMs) have potential applications in practical gas separation processes because of their high permeance (gas flux) and gas selectivity. In this study, we fabricated a high-performance TFC-MMM based on a rubbery comb copolymer, i.e., poly(2-[3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate)-co-poly(oxyethylene methacrylate) (PBE), and metal-organic framework MOF-808 nanoparticles. The rubbery copolymer penetrates through the pores of MOF-808, thereby tuning the pore size. In addition, the rubbery copolymer forms a defect-free interfacial morphology with polymer-infiltrated MOF-808 nanoparticles. Consequently, TFC-MMMs (thickness = 350 nm) can be successfully prepared even with a high loading of MOF-808. As polymer-infiltrated MOF is incorporated into the polymer matrix, the PBE/MOF-808 membrane exhibits a significantly higher CO₂ permeance (1069 GPU) and CO₂/N₂ selectivity (52.7) than that of the pristine PBE membrane (CO₂ permeance = 431 GPU and CO₂/N₂ selectivity = 36.2). Therefore, the approach considered in this study is suitable for fabricating high-performance thin-film composite membranes via polymer infiltration into MOF pores.

Polyamide membranes with nanoscale ordered structures for fast permeation and highly selective ion-ion separation

Fast permeation and effective solute-solute separation provide the opportunities for sustainable water treatment, but they are hindered by ineffective membranes. We present here the construction of a nanofiltration membrane with fast permeation, high rejection, and precise Cl^-/SO₄^2- separation by spatial and temporal control of interfacial polymerization via graphitic carbon nitride (g-C₃N₄). The g-C₃N₄ nanosheet binds preferentially with piperazine and tiles the water-hexane interface as revealed by molecular dynamics studies, thus lowering the diffusion rate of PIP by one order of magnitude and restricting its diffusion pathways towards the hexane phase. As a result, membranes with nanoscale ordered hollow structure are created. Transport mechanism across the structure is clarified using computational fluid dynamics simulation. Increased surface area, lower thickness, and a hollow ordered structure are identified as the key contributors to the water permeance of 105 L m²·h^-1·bar^-1 with a Na₂SO₄ rejection of 99.4% and a Cl^-/SO₄^2- selectivity of 130, which is superior to state-of-the-art NF membranes. Our approach for tuning the membrane microstructure enables the development of ultra-permeability and excellent selectivity for ion-ion separation, water purification, desalination, and organics removal.

Propagating MOF flexibility at the macroscale: the case of MOF-based mechanical actuators

Shapeshifting materials have captured the imagination of researchers for their myriad potential applications, yet their practical development remains challenging. These materials operate by mechanical actuation: their structural responses to external stimuli generate mechanical work. Here, we review progress on the use of flexible metal-organic frameworks (MOFs) in composite actuators that shapeshift in a controlled fashion. We highlight the dynamic behaviour of flexible MOFs, which are unique among materials, even other porous ones, and introduce the concept of propagation, which involves the efficient transmission of flexible MOF deformations to the macroscale. Furthermore, we explain how researchers can observe, measure, and induce such effects in MOF composites. Next, we review pioneering first-generation MOF-composite actuators that shapeshift in response to changes in humidity, temperature, pressure, or to other stimuli. Finally, we allude to recent developments, identify remaining R & D hurdles, and suggest future directions in this field.

Constructing Mechanical Shuttles in a Three-dimensional (3D) Porous Architecture for Selective Transport of Lithium Ions

Lithium (Li) extraction from brines is a major barrier to the sustainable development of batteries and alloys; however, current separation technology suffers from a trade-off between ion selectivity and permeability. Herein, a crown ether mechanically interlocked 3D porous organic framework (Crown-POF) was prepared as the porous filler of thin-film nanocomposite membranes. Crown-POF with penta-coordinated (four O_crown atoms and one N_tert-amine atom) adsorption sites enables a special recognition for Li⁺ ion. Moreover, the four N_tert-amine atoms on each POF branch facilitate the flipping motion of Li⁺ ion along the skeletal thread, while retaining the specified binding pattern. Accordingly, the crown ether interlocked POF network displays an ultrafast ion transfer rate, over 10 times that of the conventional porous materials. Notably, the nanocomposite membrane gives high speed and selectivity for Li⁺ ion transport as compared with other porous solid-based mixed-matrix membranes.

Amphiphilically Modified Porous Polymeric Nanosandwich-Based Membranes for Rapid and Efficient Water Treatment

Low removal efficiency, long treatment time, and high energy consumption hinder advanced and eco-friendly use of traditional adsorbents and separation membranes. Here, a class of amphiphilically modified 2D porous polymeric nanosandwich is designed and is subsequently assembled into adsorptive membranes. The 2D nanosandwich is gifted with high porosity and excellent pore accessibility, demonstrating rapid adsorption kinetics. The as-assembled membrane integrates unimpeded interlayer channels and well-developed, amphiphilic, and highly accessible intralayer nanopores, leading to ultrafast water permeation (1.2 × 10⁴  L m^-2  h^-1  bar^-1 ), high removal efficiency, and easy regeneration. The family of the membrane can be expanded by changing amphiphilic functional groups, further providing treatment of a wide-spectrum of pollutants, including aromatic compounds, pesticide, and pharmaceuticals. It is believed that the novel amphiphilically modified adsorptive membrane offers a distinct water treatment strategy with ultrahigh water permeation and efficient pollutants removal performances, and provides a multiple-in-one solution to the detection and elimination of pollutants.

A reverse-selective ion exchange membrane for the selective transport of phosphates via an outer-sphere complexation-diffusion pathway

Specific-ion selectivity is a highly desirable feature for the next generation of membranes. However, existing membranes rely on differences in charge, size and hydration energy, which limits their ability to target individual ion species. Here we demonstrate a nanocomposite ion-exchange membrane material that enables a reverse-selective transport mechanism that can selectively pass a single ion species. We demonstrate this transport mechanism with phosphate ions selectively transporting across negatively charged cation exchange membranes. Selective transport is enabled by the in situ growth of hydrous manganese oxide nanoparticles throughout a cation exchange membrane that provide a diffusion pathway via phosphate-specific, reversible outer-sphere interactions. On incorporating the hydrous manganese oxide nanoparticles, the membrane's phosphate flux increased by a factor of 27 over an unmodified cation exchange membrane, and the selectivity of phosphorous over sulfate, nitrate and chloride reaches 47, 100 and 20, respectively. By pairing ion-specific outer-sphere interactions between the target ions and appropriate nanoparticles, these nanocomposite ion-exchange materials can, in principle, achieve selective transport for a range of ions.

Metal-Organic Cage as Single-Molecule Carrier for Solid-State Nanopore Analysis

The ability to detect biomolecules at the single-molecule level is at the forefront of biological research, precision medicine, and early diagnosis. Recently, solid-state nanopore sensors have emerged as a promising technique for label-free and precise diagnosis assay. However, insufficient sensitivity and selectivity for small analytes are a great challenge for clinical diagnosis applications via solid-state nanopores. Here, for the first time, a metal-organic cage, PCC-57, is employed as a carrier to increase the sensitivity and selectivity of solid-state nanopores based on the intrinsic interaction of the nanocage with biomolecules. Firstly, it is found that the carrier itself is undetectable unless bound with the target analytes and used oligonucleotides as linkers to attach PCC-57 and target analytes. Secondly, two small analytes, oligonucleotide conjugated angiopep-2 and polyphosphoric acid, are successfully distinguished using the molecular carrier. Finally, selectivity of nanopore detection is achieved by attaching PCC-57 to oligonucleotide-tailed aptamers, and the human alpha-thrombin sample is successfully detected. It is believed that the highly designable metal-organic cage could serve as a rich carrier repository for a variety of biomolecules, facilitating single-molecule screening of clinically relevant biomolecules based on solid-state nanopores in the future.

Collagen Fiber-Based Advanced Separation Materials: Recent Developments and Future Perspectives

Separation plays a critical role in a broad range of industrial applications. Developing advanced separation materials is of great significance for the future development of separation technology. Collagen fibers (CFs), the typical structural proteins, exhibit unique structural hierarchy, amphiphilic wettability, and versatile chemical reactivity. These distinctive properties provide infinite possibilities for the rational design of advanced separation materials. During the past 2 decades, many progressive achievements in the development of CFs-derived advanced separation materials have been witnessed already. Herein, the CFs-based separation materials are focused on and the recent progresses in this topic are reviewed. CFs widely existing in animal skins display unique hierarchically fibrous structure, amphiphilicity-enabled surface wetting behaviors, multi-functionality guaranteed covalent/non-covalent reaction versatility. These outstanding merits of CFs bring great opportunities for realizing rational design of a variety of advanced separation materials that were capable of achieving high-performance separations to diverse specific targets, including oily pollutants, natural products, metal ions, anionic contaminants and proteins, etc. Besides, the important issues for the further development of CFs-based advanced separation materials are also discussed.

Recent Advances in Membrane-Based Biogas and Biohydrogen Upgrading

Biogas and biohydrogen, due to their renewable nature and zero carbon footprint, are considered two of the gaseous biofuels that will replace conventional fossil fuels. Biogas from anaerobic digestion must be purified and converted into high-quality biomethane prior to use as a vehicle fuel or injection into natural gas networks. Likewise, the enrichment of biohydrogen from dark fermentation requires the removal of CO2, which is the main pollutant of this new gaseous biofuel. Currently, the removal of CO2 from both biogas and biohydrogen is carried out by means of physical/chemical technologies, which exhibit high operating costs and corrosion problems. Biological technologies for CO2 removal from biogas, such as photosynthetic enrichment and hydrogenotrophic enrichment, are still in an experimental development phase. In this context, membrane separation has emerged as the only physical/chemical technology with the potential to improve the performance of CO2 separation from both biogas and biohydrogen, and to reduce investment and operating costs, as a result of the recent advances in the field of nanotechnology and materials science. This review will focus on the fundamentals, potential and limitations of CO2 and H2 membrane separation technologies. The latest advances on membrane materials for biogas and biohydrogen purification will be systematically reviewed.

Modelling Sorption and Transport of Gases in Polymeric Membranes across Different Scales: A Review

Professor Giulio C. Sarti has provided outstanding contributions to the modelling of fluid sorption and transport in polymeric materials, with a special eye on industrial applications such as membrane separation, due to his Chemical Engineering background. He was the co-creator of innovative theories such as the Non-Equilibrium Theory for Glassy Polymers (NET-GP), a flexible tool to estimate the solubility of pure and mixed fluids in a wide range of polymers, and of the Standard Transport Model (STM) for estimating membrane permeability and selectivity. In this review, inspired by his rigorous and original approach to representing membrane fundamentals, we provide an overview of the most significant and up-to-date modeling tools available to estimate the main properties governing polymeric membranes in fluid separation, namely solubility and diffusivity. The paper is not meant to be comprehensive, but it focuses on those contributions that are most relevant or that show the potential to be relevant in the future. We do not restrict our view to the field of macroscopic modelling, which was the main playground of professor Sarti, but also devote our attention to Molecular and Multiscale Hierarchical Modeling. This work proposes a critical evaluation of the different approaches considered, along with their limitations and potentiality.

Tailoring the Solvation Sheath of Cations by Constructing Electrode Front-Faces for Rechargeable Batteries

Solvent molecules within the solvation sheath of cations (e.g., Li⁺ , Na⁺ , Zn²⁺ ) are easily to be dehydrogenated especially when coupled with high-voltage cathodes, and lead to detrimental electrolytes decompositions which finally accelerate capacity decays of rechargeable batteries. Tremendous efforts are devoted to tackle with this long-lasting issue. Among them, salt-concentrated strategies are frequently employed to tailor the solvation sheath of cations and improve the stabilities of electrolytes. However, the cost challenges caused by adding extra dose of expensive salts, additives/cosolvents in preparing highly concentrated electrolytes, hinder their further utilizations to some extent. Introducing porous materials-based electrode front-faces on the surface of electrodes even within dilute electrolytes can transfer the high-energy-state desolvated solvents from the reactive electrodes to the nonconductive porous material surfaces, thus eliminate the contact chances between desolvated solvents and electrode materials, and greatly reduce solvents-related decomposition issues. Herein, recent advances in using electrode front-faces to tailor the solvation sheath of metal ions for rechargeable batteries are discussed. Finally, perspectives to the future challenges and opportunities of constructing electrode front-faces to tailor the solvation sheath of cations by constructing electrode front-face for rechargeable batteries are provided.