Optimizing Sieving Effect for CO2 Capture from Humid Air Using an Adaptive Ultramicroporous Framework

Excessive CO2 in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal-organic framework (FJI-H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO2 from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO2 from N2 , and the distinguishable adsorption sites for H2 O and CO2 enable them to be co-adsorbed. Notably, the adsorbed-CO2 -driven pore shrinkage can further promote CO2 capture while the adsorbed-H2 O-induced phase transitions in turn inhibit H2 O adsorption. Moreover, FJI-H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO2 adsorbent. This will provide a new strategy for developing practical adsorbents for CO2 capture from the air.

Rational Design of MXene Hollow Fiber Membranes for Gas Separations

One scalable and facile dip-coating approach was utilized to construct a thin CO₂-selection layer of Pebax/PEGDA-MXene on a hollow fiber PVDF substrate. An interlayer spacing of 3.59 Å was rationally designed and precisely controlled for the MXene stacks in the coated layer, allowing efficient separation of the CO₂ (3.3 Å) from N₂ (3.6 Å) and CH₄ (3.8 Å). In addition, CO₂-philic nanodomains in the separation layer were constructed by grafting PEGDA into MXene interlayers, which enhanced the CO₂ affinity through the MXene interlayers, while non-CO₂-philic nanodomains could promote CO₂ transport due to the low resistance. The membrane could exhibit optimal separation performance with a CO₂ permeance of 765.5 GPU, a CO₂/N₂ selectivity of 54.5, and a CO₂/CH₄ selectivity of 66.2, overcoming the 2008 Robeson upper bounds limitation. Overall, this facile approach endows a precise controlled molecular sieving MXene membrane for superior CO₂ separation, which could be applied for interlayer spacing control of other 2D materials during membrane construction.

Electrically Modulated Nanofiltration Membrane Based on an Arch-Bridged Graphene Structure for Multicomponent Molecular Separation

Tunable regulation of molecular penetration through porous membranes is highly desirable for membrane applications in the pharmaceutical and medical fields. However, in most previous reports additional reagents or components are usually needed to provide the graphene-based membranes with responsiveness. Herein, we report tunable arch-bridged reduced graphene oxide (rGO) nanofiltration membranes modulated by the applied voltage. Under a finite voltage of 5 V, the rGO membrane could completely reject organic/anionic molecules. With assistance of the voltage, the positive-charge-modified rGO membrane realized the universal rejection of both cationic and anionic dyes, also showing the valid modulation in harsh organic solvents. The efficient electrical modulation depended on the synergetic effects of Donnan repulsion and size exclusion, benefiting from the electric field enhancement in arch-bridged rGO structures. Furthermore, multicomponent separation was achieved by our electrically modulated rGO-based membranes, demonstrating their potential in practical applications such as pharmaceutical industries.

Water-Stable Metal Azolate Frameworks Showing Interesting Flexibilities for Highly Effective Bioethanol Dehydration

The ethanol/water separation challenge highlights the adsorption capacity/selectivity trade-off problem. We show that the target guest can serve as a gating component of the host to block the undesired guest, giving molecular sieving effect for the adsorbent possessing large pores. Two hydrophilic/water-stable metal azolate frameworks were designed to compare the effects of gating and pore-opening flexibility. Large amounts (up to 28.7 mmol g-1) of ethanol with fuel-grade (99.5%+) and even higher purities (99.9999%+) can be produced in a single adsorption process from not only 95:5 but also 10:90 ethanol/water mixtures. More interestingly, the pore-opening adsorbent possessing large pore apertures showed not only high water adsorption capacity but also exceptionally high water/ethanol selectivity characteristic of molecular sieving. Computational simulations demonstrated the critical role of guest-anchoring aperture for the guest-dominated gating process.

A 2D Soft Covalent Organic Framework Membrane Prepared via a Molecular Bridge

Highly flexible and robust self-standing covalent organic framework (COF) membranes with rapid preparation are important but technically challenging for achieving precise separation. Herein , a novel imine-based 2D soft covalent organic framework (SCOF) membrane with a large area of 226.9 cm² , via ingeniously selecting an aldehyde flexible linker and a trigonal building block, is reported. The soft 2D covalent organic framework membrane is rapidly formed (≈5 min) based on the sodium dodecyl sulfate (SDS) molecular channel constructed at the water/dichloromethane (DCM) interface, which is the record-fast SCOF membrane formation and 72 times faster than that in the reported literature. MD simulation and DFT calculation elucidate that the dynamic, self-assembled SDS molecular channel facilitates faster and more homogeneous transfer of amine monomers in the bulk, thereby forming a soft 2D self-standing COF membrane with more uniform pores. The formed SCOF membrane exhibits superb sieving capability for small molecules, robustness in strong alkaline (5 mol L^-1 NaOH), acid (0.1 mol L^-1 HCl), and various organic solutions, and sufficient flexibility with a large curvature of 2000 m^-1 for membrane-based separation science and technology.

Transformation of Imine Cage to Covalent Organic Framework Film at the Liquid-Liquid Interface

Dynamic covalent chemistry (DCC) opens up a fascinating route for the construction of well-organized supramolecular architectures, starting from organic molecular cages to crystalline macromolecular covalent organic frameworks (COFs). Herein, for the first time, we have manifested a facile room-temperature DCC-directed transformation of discrete organic imine cage-to-COF film at the liquid-liquid interface. The unfolding of the cage leading to the generation of imine intermediates, followed by their interface-assisted preorganization and subsequent growth of the COF film, are elucidated through detailed spectroscopic and microscopic investigations. The interfacial cage-to-COF transformation provides a facile route for the faster fabrication of free-standing COF films with high porosity and crystallinity, demonstrating excellent performance towards molecular sieving and high solvent permeance. Thus, the current study opens up a new route for structural interconversion between two crystalline entities with diverse dimensionality employing DCC at the confined interface.

Molecular Sieving of Propylene from Propane in Metal-Organic Framework-Derived Ultramicroporous Carbon Adsorbents

The development of adsorption-based separation processes alternative to the energy-intensive cryogenic distillation for a mixture of propylene and propane remains essential but challenging in gas industries. Molecular sieving separation of C₃H₆/C₃H₈ on stable carbon adsorbents appeals to be promising, while it is quite challenging to realize due to the random distributions and arrangements of the internal pores in common carbons. Herein, a series of polysaccharide-based CD-MOF-derived ultramicroporous carbon adsorbents with their pore size tuned at a subangstrom level were prepared. Molecular sieving separation of C₃H₆/C₃H₈ was realized on the optimal C-CDMOF-2-700 owing to the delicate structure with an appropriate pore size (5.0 Å). Besides, C-CDMOF-2-700 exhibited a high C₃H₆ uptake of 1.97 mmol g^-1 under ambient conditions. An ultrahigh uptake ratio of C₃H₆/C₃H₈ at 1.0 kPa (403) was also achieved, outperforming all reported adsorbents. Kinetic adsorption tests and breakthrough experiments further demonstrate this well-designed carbon adsorbent to be promising in industrial C₃H₆/C₃H₈ separation.

Metal-Organic Framework Composite Photothermal Membrane for Removal of High-Concentration Volatile Organic Compounds from Water via Molecular Sieving

Water-soluble volatile organic compounds (VOCs) are among the most difficult-to-treat species during wastewater treatment. The current purification and removal of high-concentration VOCs still rely on the energy-consuming distillation and high-pressure driven reverse osmosis technology. There is an urgent need for an advanced technology that can effectively remove high-concentration VOCs from water. Here, we report a metal-organic framework (MOF)/polyaniline (PANI) nanofiber array composite photothermal membrane for removal of high-concentration VOCs from water via molecular sieving during a solar-driven evaporation process. The modified zeolitic imidazole framework-8 (ZIF-8) layer grown on a PANI nanofiber array acts as a molecular sieving layer to evaporate water but intercept VOCs. The composite membrane exhibits high VOCs rejection and a high-water evaporation rate for water containing different concentrations of VOCs. When treating water containing VOCs with a concentration of up to 400 mg L^-1, the VOCs rejection rate is up to 99% and the water evaporation rate is 1.0 kg m^-2 h^-1 under 1 sun irradiation (1 kW m^-2). Our work effectively combines the molecular sieve effect with a solar-driven evaporation process, which provides an effective strategy for the treatment of water containing VOCs.

Flexible Zn-MOF with Rare Underlying scu Topology for Effective Separation of C6 Alkane Isomers

Adsorptive separation by porous solids provides an energy-efficient alternative for the purification of important chemical species compared to energy-intensive distillations. Particularly, the separation of linear hexane isomers from its branched counterparts is crucial to produce premium grade gasoline with high research octane number (RON). Herein, we report the synthesis of a new, flexible zinc-based metal-organic framework, [Zn5(μ3-OH)2(adtb)2(H2O)5·5 DMA] (Zn-adtb), constructed from a butterfly shaped carboxylate linker with underlying (4,8)-connected scu topology capable of separating the C6 isomers nHEX, 3MP, and 23DMB. The sorbate-sorbent interactions and separation mechanisms were investigated and analyzed through in situ FTIR, solid state NMR measurements and computational modeling. These studies reveal that Zn-adtb discriminates the nHEX/3MP isomer pair through a kinetic separation mechanism and the nHEX/23DMB isomer pair through a molecular sieving mechanism. Column breakthrough measurements further demonstrate the efficient separation of linear nHEX from the mono- and dibranched isomers.

Coordination Polymer Glasses with Lava and Healing Ability for High-Performance Gas Sieving

Coordination polymer (CP) glasses offer a way to tackle the fabrication challenges encountered by inorganic porous membranes and show great potentials for size-exclusive gas separation. However, their processability and performance still cannot simultaneously meet the requirements for high-performance membrane separation. Herein, we have developed a series of CP glasses (M-P-dmbIm, M=Zn, Cd, Cu, and Mn), which possess low vitrification temperature as well as low viscosity (η) and lave capability above the transition temperatures. The derived glass (a_g M-P-dmbIm) membranes show outstanding performances for H₂ /CO₂ , H₂ /N₂ , and H₂ /CH₄ separation, which all far surpass the Robeson upper bound and even rival against the best of the state-of-the-art gas separation membranes. The low viscosities not only allow us to hot-cast or hot-press the CP glasses into thin membranes within 5 min without sacrificing their selectivity and permeability, but also endow the resulted glass membranes with healing ability.

Ab Initio Energetic Barriers of Gas Permeation across Nanoporous Graphene

Realizing membranes of atomic thickness functioning reliably constitutes a giant leap forward for a plethora of applications where the efficient separation of fluid constituents at the molecular level is critical. Here, by employing density functional theory, we explore the energy landscape of typical gas molecules attempting permeation through graphene nanopores and determine the minimum energy permeation pathways, based on the precise knowledge of the related molecular level interactions. With this approach we investigate two basic permeation routes: direct permeation and surface-based transport. We find that for subnanometer pores, the diffusion barrier of direct and surface transport depends on the pore chemical functionalization, while the molecule pore permeation barrier is independent of the gas-pore approach due to the overlap of surface and direct diffusion paths over the pore center. The overall minimum energy permeation pathway of He, H₂, CO₂, and CH₄ molecules, across nanopores of different dimensions and chemical functionalization, defines the pore diameter (∼1.2 nm) below which effusion theory is inaccurate, as well as the critical pore diameter (∼0.8 nm) required to achieve positive permeation barriers driving molecular sieving. We determine that achieving positive permeation barriers required for high selectivity gas separation is inseparably combined with postpermeation desorption barriers due to attractive van der Waals interactions. The discovered permeation energetics are pore-molecule-specific and are incorporated into an analytical model extending existing theory. Our results provide a scientific background for rational pore design in graphene membranes, which can lead to gas separation at a commercially relevant performance level.

Molecular Sieving of Acetylene from Ethylene in a Rigid Ultra-microporous Metal Organic Framework

Rigid molecular sieving materials are the ideal candidates for gas separation (e. g., C₂ H₂ /C₂ H₄ ) due to their ultrahigh adsorption selectivity and the absence of gas co-adsorption. However, the absolute molecular sieving effect for C₂ H₂ /C₂ H₄ separation has rarely been realized because of their similar physicochemical properties. Herein, we demonstrate the absolute molecular sieving of C₂ H₂ from C₂ H₄ by a rigid ultra-microporous metal-organic framework (F-PYMO-Cu) with 1D regular channels (pore size of ca. 3.4 Å). F-PYMO-Cu exhibited moderate acetylene uptake (35.5 cm³ /cm³ ), but very low ethylene uptake (0.55 cm³ /cm³ ) at 298 K and 1 bar, yielding the second highest C₂ H₂ /C₂ H₄ uptake ratio of 63.6 up to now. One-step C₂ H₄ production from a binary mixture of C₂ H₂ /C₂ H₄ and a ternary mixture of C₂ H₂ /CO₂ /C₂ H₄ at 298 K was achieved and verified by dynamic breakthrough experiments. Coupled with excellent thermal and water stability, F-PYMO-Cu could be a promising candidate for industrial C₂ separation tasks.

S-Layer Ultrafiltration Membranes

Monomolecular arrays of protein subunits forming surface layers (S-layers) are the most common outermost cell envelope components of prokaryotic organisms (bacteria and archaea). Since S-layers are periodic structures, they exhibit identical physicochemical properties for each constituent molecular unit down to the sub-nanometer level. Pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes. The functional groups on the surface and in the pores of the S-layer protein lattice are accessible for chemical modifications and for binding functional molecules in very precise fashion. S-layer ultrafiltration membranes (SUMs) can be produced by depositing S-layer fragments as a coherent (multi)layer on microfiltration membranes. After inter- and intramolecular crosslinking of the composite structure, the chemical and thermal resistance of these membranes was shown to be comparable to polyamide membranes. Chemical modification and/or specific binding of differently sized molecules allow the tuning of the surface properties and molecular sieving characteristics of SUMs. SUMs can be utilized as matrices for the controlled immobilization of functional biomolecules (e.g., ligands, enzymes, antibodies, and antigens) as required for many applications (e.g., biosensors, diagnostics, enzyme- and affinity-membranes). Finally, SUM represent unique supporting structures for stabilizing functional lipid membranes at meso- and macroscopic scale.

Insights into the Role of Polymer Conformation on the Cutoff Size of Carbon Molecular Sieving Membranes for Hydrogen Separation and Its Novel Pore Size Detection Technology

In this study, three polymer precursor conformations, dilute, semi-dilute, and concentrated, were used to fabricate carbon molecular sieving (CMS) membranes via a fixed carbonization protocol. The effects of the precursor conformation on the microstructure of the resultant CMS membranes were characterized by Raman analysis. Their ability to separate light gases, such as H₂/CH₄ and H₂/N₂, was assessed with a single-gas system. Additionally, a novel method was proposed to detect the cutoff size of the CMS membranes created in this study. The method combined high-resolution transmission electron microscopy (HR-TEM) and a focused ion beam (FIB) system. Finally, due to the semi-dilute solution's denser polymer chains and lack of severe polymer entanglement, highly graphited CMS membranes with excellent gas separation performance were successfully synthesized using a semi-dilute polyetherimide dope solution. Interlayer distances in the carbon matrix were visualized and measured using our novel probing tool (HR-TEM and FIB) and software. The CMS membrane fabricated with a semi-dilute dope exhibited the best gas separation performance of the tested membranes. It had the most ordered carbon sheet orientation and exhibited a superior selectivity of H₂/CH₄ = 293 with a hydrogen permeability of 1138.7 Barrer, far surpassing the reported permselectivity of other membranes. We believe that the high H₂/CH₄ selectivity presented here is unprecedented for CMS membranes reported in the literature.

Hierarchical Metal-Organic Framework Films with Controllable Meso/Macroporosity

The structuring of the metal-organic framework material ZIF-8 as films and membranes through the vapor-phase conversion of ZnO fractal nanoparticle networks is reported. The extrinsic porosity of the resulting materials can be tuned from 4% to 66%, and the film thickness can be controlled from 80 nm to 0.23 mm, for areas >100 cm2. Freestanding and pure metal-organic frameworks (MOF) membranes prepared this way are showcased as separators that minimize capacity fading in model Li-S batteries.

Pathways and Challenges for Biomimetic Desalination Membranes with Sub-Nanometer Channels

Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrices on a robust support. While biological components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers. However, the experimental performance of biomimetic membranes remains far below that of biological membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection. We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examination of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m^-2 h^-1 bar^-1, which is comparable to that of current state-of-the-art polymeric desalination membranes. Through a quantitative assessment of biomimetic membrane formats, we analytically show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains observed subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.

Selective Etching of Graphene Membrane Nanopores: From Molecular Sieving to Extreme Permeance

Two-dimensional materials are the essential building blocks of breakthrough membrane technologies due to minimal permeation barriers across atomically thin pores. Tunable pore size fabrication combined with independently controlled pore number density is necessary for outstanding performance but remains a challenge. There is a great need for parallel, upscalable methods that can control pore size from sub-nm to >5 nm, a pore size range required for membranes with effective molecular separation. Here we report a dry, facile, and scalable process introducing atomic defects by design, followed by selective etching of graphene edge atoms able to controllably expand the nanopore dimensions from sub-nm to 5 nm. The attainable average pore sizes at 10¹⁵ m^-2 pore density promise applicability to various separation applications. We investigate the gas permeation and separation mechanisms, finding that these membranes display molecular sieving (H₂/CH₄ separation factor = 9.3; H₂ permeance = 3370 gas permeation units (GPU)) and reveal the presence of interweaved transport phenomena of pore chemistry, surface flow, and gas molecule momentum transfer. We observe the smooth transition from molecular sieving to effusion at unprecedented permeance (H₂/CH₄ separation factor = 3.7; H₂ permeance = 10⁷ GPU). Our scalable graphene membrane fabrication approach in combination with sub-5 nm pores opens a new route employing 2D membranes to study gas transport and effectively paving the way to industrial applications.

Ultrapermeable Organic Solvent Nanofiltration Membranes with Precisely Tailored Support Layers Fabricated Using Thin-Film Liftoff

Thin-film composite (TFC) membranes are favored for precise molecular sieving in liquid-phase separations; they possess high permeability due to the minimal thickness of the active layer and the high porosity of the support layer. However, current TFC membrane fabrication techniques are limited by the available materials for the selective layer and do not demonstrate the level of structural control needed to substantially advance organic solvent nanofiltration (OSN) membrane technology. In this work, we employ the newly developed thin-film lift-off (T-FLO) technique to fabricate polybenzimidazole (PBI) TFC membranes with porous support layers uniquely tailored to OSN. The drop-cast dense PBI selective layers endow the membranes with an almost complete rejection of common small dye molecules. The polymeric support layer is optimized by a combinatorial approach using four different monomers that alter the cross-linking density and polymer chain flexibility of the final composite. These two properties substantially affect the porogen holding capacity of the reticular polymer network, leading to the formation of different macropore structures. With a 150 nm thick PBI selective layer and fine-tuning of the support layer, the resulting membrane achieves stable and superior permeance of 14.0, 11.7, 16.4, 11.4, 17.1, and 19.7 L m^-2 h^-1 bar^-1 for water, ethanol, methanol, isopropanol, tetrahydrofuran (THF), and acetonitrile, respectively.

MXene Materials for Designing Advanced Separation Membranes

MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance evaluation of MXene-based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene-based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer-based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer-by-layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene-based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.

Nanostructured Graphene Oxide Composite Membranes with Ultrapermeability and Mechanical Robustness

Graphene oxide (GO) membranes have great potential for separation applications due to their low-friction water permeation combined with unique molecular sieving ability. However, the practical use of deposited GO membranes is limited by the inferior mechanical robustness of the membrane composite structure derived from conventional deposition methods. Here, we report a nanostructured GO membrane that possesses great permeability and mechanical robustness. This composite membrane consists of an ultrathin selective GO nanofilm (as low as 32 nm thick) and a postsynthesized macroporous support layer that exhibits excellent stability in water and under practical permeability testing. By utilizing thin-film lift off (T-FLO) to fabricate membranes with precise optimizations in both selective and support layers, unprecedented water permeability (47 L·m^-2·hr^-1·bar^-1) and high retention (>98% of solutes with hydrated radii larger than 4.9 Å) were obtained.

Effective Separation of CO2 Using Metal-Incorporated rGO Membranes

Graphene-based materials, primarily graphene oxide (GO), have shown excellent separation and purification characteristics. Precise molecular sieving is potentially possible using graphene oxide-based membranes, if the porosity can be matched with the kinetic diameters of the gas molecules, which is possible via the tuning of graphene oxide interlayer spacing to take advantage of gas species interactions with graphene oxide channels. Here, highly effective separation of gases from their mixtures by using uniquely tailored porosity in mildly reduced graphene oxide (rGO) based membranes is reported. The gas permeation experiments, adsorption measurement, and density functional theory calculations show that this membrane preparation method allows tuning the selectivity for targeted molecules via the intercalation of specific transition metal ions. In particular, rGO membranes intercalated with Fe ions that offer ordered porosity, show excellent reproducible N₂ /CO₂ selectivity of ≈97 at 110 mbar, which is an unprecedented value for graphene-based membranes. By exploring the impact of Fe intercalated rGO membranes, it is revealed that the increasing transmembrane pressure leads to a transition of N₂ diffusion mode from Maxwell-Stefan type to Knudsen type. This study will lead to new avenues for the applications of graphene for efficiently separating CO₂ from N₂ and other gases.

Ultrafast, Stable Ionic and Molecular Sieving through Functionalized Boron Nitride Membranes

Porous membranes play an important role in the separation technologies such as gas purification, solute nanofiltration, and desalination. An ideal membrane should be thin to maximize permeation speed, have optimum pore sizes to maximize selectivity, and be stable in various harsh conditions. Here, we show that the nanometer-thick membrane prepared by means of filtration of functionalized boron nitride (FBN) water suspensions can block solutes with hydrated radii larger than 4.3 Å in water. The FBN membranes with abundant nanochannels reduce the path length of ions. As molecular sieves, the FBN membrane can permeate small ions at an ultrahigh rate-a 25-fold enhancement compared with that of its theoretical diffusion rate and much higher than the graphene oxide membrane. Importantly, the FBN membrane exhibits excellent permeability even when it is immersed in acidic, alkaline, and basic salts solutions because of its intrinsic chemical stability. The molecular dynamics simulations further confirmed that the nanocapillaries formed within the FBN membrane in the hydrated state were responsible for high permeation performance. The simple vacuum filtration fabricated FBN membrane with angstrom-sized channels and ultrafast permeation of ions promises great potential applications in the areas of barrier separation and water purification.

Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membranes for Carbon Capture

Although polycrystalline metal-organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO₂ separation performance, crucial for energy-efficient carbon capture. ZIF-8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO₂ /N₂ and CO₂ /CH₄ separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF-8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH₄ > CO₂ > H₂ ). Unprecedented CO₂ /CH₄ , CO₂ /N₂ , and H₂ /CH₄ selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C₃ H₆ , are achieved. Spectroscopic and X-ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF-8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas-separation performance of the MOF membranes.

Thin Composite Carbon Molecular Sieve Membranes from a Polymer of Intrinsic Microporosity Precursor

Ultra-thin composite carbon molecular sieve (CMS) membranes were fabricated on well-defined inorganic alumina substrates using a polymer of intrinsic microporosity (PIM) as a precursor. Details of the pyrolysis-related structural development were elucidated using focused-beam, interference-enhanced spectroscopic ellipsometry (both in the UV-vis and IR range), which allowed accurate determination of the film thickness, optical properties as well as following the chemical transformations. The pyrolysis-induced collapse of thin and bulk PIM-derived CMS membranes was compared with CMS made from a well-known non-PIM precursor 6FDA-DABA. Significant differences between the PIM and non-PIM precursors were discovered and explained by a much larger possible volume contraction in the PIM. In spite of the differences, surprisingly, the gas separation properties did not fundamentally differ. The high-temperature collapse of the initially amorphous and isotropic precursor structure was accompanied by a significant molecular orientation within the formed turbostratic carbon network guided by the laterally constraining presence of the substrate. This manifested itself in the development of uniaxial optical anisotropy, which was shown to correlate with increases in gas separation selectivity for multiple technologically important gas pairs. Reduction of CMS skin thickness significantly below ∼1 μm induced large losses in permeability coefficients with only small to moderate effects on selectivity. Remarkably, skin thickness reduction and physical aging seemed to superimpose onto the same trend, which explains and strengthens some of the earlier fundamental insights.

A Robust Zeolitic Imidazolate Framework Membrane with High H2/CO2 Separation Performance under Hydrothermal Conditions

An upsurge in searching membranes with high selectivity, permeability, and stability, which have been considered to be promising in the membrane-based gas separation process, has received huge attentions in academia and industry. In this work, we demonstrated a new molecular sieving ZIF membrane, which has a unique mixed-ligand ZIF structure constructed by two bulk imidazolate linkers within the zeolite GIS topology, resulting in appropriate aperture size, strong affinity to CO₂, exceptional thermal and chemical stabilities, and superhydrophobic properties. Benefited from these features, the resulting JUC-160 membrane indeed exhibited remarkable separation efficiency and stability, with excellent H₂/CO₂ selectivity of 26.3 and considerable H₂ permeance of 9.75 × 10^-7 mol m^-2 s^-1 Pa^-1 under high temperature up to 200 °C. Furthermore, owing to its superhydrophobicity, the JUC-160 membrane could maintain its prominent separation performance even with the presence of steam.

Unraveling the Interfacial Structure-Performance Correlation of Flexible Metal-Organic Framework Membranes on Polymeric Substrates

Pure metal-organic framework (MOF) layers deposited on porous supports are important candidates for molecular sieving membranes, but their performance usually deviates from theoretical estimations. Here, we combine step-wise scanning electron microscopy imaging, time-resolved synchrotron X-ray scattering, terahertz infrared spectroscopy, and density functional theory calculation to investigate the ZIF-8 membrane formation on two types (polydopamine and TiO₂) of functionalized porous supports. Though molecular sieving of ZIF-8 membranes for smaller gases (He, H₂, and CO₂) can be achieved with both types of functionalized supports, we unravel that the strong interaction between MOF and polydopamine can disrupt the formation of "perfect" MOF crystals at the interface, leading to a "contracted" MOF structure with partially uncoordinated imidazolate ligands. This further affects the low-frequency dynamical parameters of the framework and inhibits the effective seeded growth. Eventually, it leads to an unexpected loss of selectivity for the bulkier gases (N₂ and CH₄) for ZIF-8 on polydopamine-functionalized supports. This work links the dynamical aspects of MOFs with their gas transport behavior and highlights the importance of regulating the interfacial weak forces to preserve the ideal molecular sieving efficiency of MOF membranes, which also provides guidance for defect engineering of MOF film fabrication for sensing and electronic devices beyond membranes.

Tailor-Made Microporous Metal-Organic Frameworks for the Full Separation of Propane from Propylene Through Selective Size Exclusion

Adsorptive separation of olefin/paraffin mixtures by porous solids can greatly reduce the energy consumption associated with the currently employed cryogenic distillation technique. Here, the complete separation of propane and propylene by a designer microporous metal-organic framework material is reported. The compound, Y₆ (OH)₈ (abtc)₃ (H₂ O)₆ (DMA)₂ (Y-abtc, abtc = 3,3',5,5'-azobenzene-tetracarboxylates; DMA = dimethylammonium), is rationally designed through topology-guided replacement of inorganic building units. Y-abtc is both thermally and hydrothermally robust, and possesses optimal pore window size for propane/propylene separation. It adsorbs propylene with fast kinetics under ambient temperature and pressure, but fully excludes propane, as a result of selective size exclusion. Multicomponent column breakthrough experiments confirm that polymer-grade propylene (99.5%) can be obtained by this process, demonstrating its true potential as an alternative sorbent for efficient separation of propane/propylene mixtures.

High-Performance Nanowire Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and Metal-Organic Framework Membranes

Herein, we report the fabrication of hydrogen gas sensors with enhanced sensitivity and excellent selectivity. The sensor device is based on the strategic combination of ZnO nanowires (NWs) decorated with palladium nanoparticles (Pd NPs) and a molecular sieve metal-organic framework (MOF) nanomembrane (ZIF-8). The Pd NPs permit the sensors to reach maximal signal responses, whereas the ZIF-8 overcoat enables for an excellent selectivity. Three steps were employed for the fabrication: (i) coating of a miniaturized sensor with vapor-grown ZnO NWs, (ii) decoration of these NWs with Pd NPs by atomic layer deposition, and (iii) partial solvothermal conversion of the tuned NWs surface to ZIF-8 nanomembrane. The microstructure and composition investigations of the ZIF-8/Pd/ZnO nanostructured materials confirmed the presence of both metallic Pd NPs and uniform ZIF-8 thin membrane layer. The integration of these nanomaterials within a miniaturized sensor device enabled the assessment of their performance for H₂ detection at concentrations as low as 10 ppm in the presence of various gases such as C₆H₆, C₇H₈, C₂H₅OH, and CH₃COCH₃. Remarkably high-response signals of 3.2, 4.7, and 6.7 ( R_a/ R_g) have been measured for H₂ detection at only 10, 30, and 50 ppm, whereas no noticeable response toward other tested gases was detected, thus confirming the excellent H₂ selectivity obtained with such a sensor design. The results obtained showed that the performance of gas sensors toward H₂ gas can be greatly increased by both the addition of Pd NPs and the use of ZIF-8 coating, acting as a molecular sieve membrane. Furthermore, the presented strategy could be extended toward the sensing of other species by a judicious choice of both the metallic NPs and MOF materials with tuned properties for specific molecule detection, thus opening a new avenue for the preparation of highly selective sensing devices.

Micropatterned Ultrathin MOF Membranes with Enhanced Molecular Sieving Property

Metal-organic frameworks (MOFs) are attractive crystalline materials for membranes due to their diverse crystalline pore structures and molecular separation properties. However, the fabrication cost is relatively high compared to conventional polymeric membranes. The concern of the cost could be eased if they are part of a value-added device, for example, as the key separation unit in a lab-on-a-chip device. This study demonstrates the feasibility of miniaturization of MOF membranes by patterning the membrane surface, a necessary step for MOF membranes to be used in compact devices. Water-stable ultrathin UiO-66 membranes with a thickness down to 250 nm on a substrate with a complex pattern were grown. The patterned membranes showed a 100 % improvement in the apparent permeation flux over conventional flat-UiO-66 membranes without compromising the molecular separation property, indicating the complexity of a surface would not be a formidable obstacle to the MOF membrane fabrication.

Chiral Separation via Molecular Sieving: A Computational Screening of Suitable Functionalizations for Nanoporous Graphene

In a recent study [Angew. Chem. Int. Ed., 2014, 53, 9957–9960] a new concept of chiral separation has been suggested, which is based on functionalized, nanoporous sheets of graphene. In this follow‐up article we discuss the underlying principle in greater detail and make suggestions for suitable pore functionalizations with respect to a selection of chiral prototype molecules. Considering drug molecules as future targets for a chiral separation via membranes, the necessary pore sizes represent a big challenge for standard methods of computational chemistry. Therefore, we test two common force fields (GAFF, CGenFF) as well as a semiempirical tight‐binding approach recently developed by the Grimme group (GFN‐xTB) against the computationally much more expensive density functional theory. We identify the GFN‐xTB method as the most suitable approach for future simulations of functionalized pores for the given purpose, as it is able to produce reaction pathways in very good agreement with density functional theory, even in cases where force fields tend to an extreme overestimation of barrier heights.

Controlling Pore Shape and Size of Interpenetrated Anion-Pillared Ultramicroporous Materials Enables Molecular Sieving of CO2 Combined with Ultrahigh Uptake Capacity

The separation of carbon dioxide (CO₂) from hydrocarbons is a critical process for the production of clean energy and high-purity chemicals. Adsorption based on molecular sieving is an energy-saving separation process; however, most of molecular sieves with narrow and straight pore channels exhibit low CO₂ uptake capacity. Here, we report that a twofold interpenetrated copper coordination network with a consecutive pocket-like pore structure, namely, SIFSIX-14-Cu-i (SIFSIX = hexafluorosilicate, 14 = 4,4'-azopyridine, i = interpenetrated) is a remarkable CO₂/CH₄ molecular sieving adsorbent which completely blocks the larger CH₄ molecule with unprecedented selectivity, whereas it has excellent CO₂ uptake (172.7 cm³/cm³) under the ambient condition. The exceptional separation performance of SIFSIX-14-Cu-i is attributed to its unique pore shape and functional pore surface, which combine a contracted pore window (3.4 Å) and a relatively large pore cavity decorated with high density of inorganic anions. Dispersion-corrected density functional theory calculation and neutron powder diffraction were performed to understand the CO₂ binding sites. The practical feasibility of SIFSIX-14-Cu-i for CO₂/CH₄ mixtures separation was validated by experimental breakthrough tests. This study not only demonstrates the great potential of SIFSIX-14-Cu-i for CO₂ separation but also provides important clues for other gas separations.

Preparation and Gas Permeation Properties of Fluorine-Silica Membranes with Controlled Amorphous Silica Structures: Effect of Fluorine Source and Calcination Temperature on Network Size

Triethoxyfluorosilane (TEFS), which is a pendant-type alkoxysilane with a Si-F bond, was utilized for the development of a molecular sieving membrane. The effect that a source of fluorine and calcination temperature exerted on gas permeation properties and network pore size was evaluated via single-gas permeation properties across a wide range of temperatures. A TEFS membrane calcined at 350 °C showed high H₂ permeance (2.0 × 10^-6 mol m^-2 s^-1 Pa^-1) and high selectivity for H₂ over larger molecules (H₂/CF₄: >300; H₂/SF₆: >18 000), indicating that this network pore size would be suitable for a H₂ permselective membrane that could promote the process of methylcyclohexane (MCH) dehydrogenation to produce toluene (TOL). Based on the gas permeation properties and the results of XPS and FTIR, network pore size depended on the fluorine concentration incorporated in SiO₂ that existed as Si-F bonds, irrespective of the fluorine source. A TEFS membrane showed approximately the same pore size distribution and level of gas permeance, irrespective of calcination temperature (350 and 550 °C), due to the low Si-OH density in the networks as suggested by the result of FTIR, which can prevent the densification caused by the condensation of Si-OH groups. The pair distribution function also suggested that densification of the network structure for TEFS was apparently suppressed compared with that of a tetraethoxysilane (TEOS)-derived structure.

Stability, Molecular Sieving, and Ion Diffusion Selectivity of a Lamellar Membrane from Two-Dimensional Molybdenum Disulfide

Two-dimensional (2D) subnanometer channels allow unique mass transport promising for molecular sieving. New 2D channels of MoS₂ nanosheets allow one to understand molecular transmission and separation, unlike the graphene oxide counterpart containing various defects and cationic metal contaminants. Membranes from layered MoS₂ platelets show extraordinary stability in an aqueous environment and compatibility with polymer filters, both beneficial to efficient manufacturing. Sharing gas-tightness and unimpeded water vapor permeation with a graphene oxide membrane, our lamellar MoS₂ membrane demonstrates a molecular sieving property for organic vapor for the first time. The MoS₂ membrane also reveals diffusion selectivity of aqueous ions, attributable to the energy penalty in bulk-to-2D dimensional transition. These newly revealed properties of the lamellar membrane full of angstrom-sized 2D channels point to membrane technology applications for energy and environment.

Ultrafast Nanofiltration through Large-Area Single-Layered Graphene Membranes

Perforated single-layered graphene has demonstrated selectivity and flux that is orders of magnitude greater than state-of-the-art polymer membranes. However, only individual graphene sheets with sizes up to tens of micrometers have been successfully fabricated for pressurized permeation studies. Scaling-up and reinforcement of these atomic membranes with minimum cracks and pinholes remains a major hurdle for practical applications. We develop a large-area in situ, phase-inversion casting technique to create 63 cm² high-quality single-layered perforated graphene membranes for ultrafast nanofiltration that can operate at pressures up to 50 bar. This result demonstrates the feasibility of our technique for creating robust large-area, high quality, single-layered graphene and its potential use as a pressurized nanofiltration membrane.

Magnetoresponsive Poly(ether sulfone)-Based Iron Oxide cum Hydrogel Mixed Matrix Composite Membranes for Switchable Molecular Sieving

Stimuli-responsive membranes that can adjust mass transfer and interfacial properties "on demand" have drawn large interest over the last few decades. Here, we designed and prepared a novel magnetoresponsive separation membrane with remote switchable molecular sieving effect by simple one-step and scalable nonsolvent induced phase separation (NIPS) process. Specifically, poly(ether sulfone) (PES) as matrix for an anisotropic membrane, prefabricated poly(N-isopropylacrylamide) (PNIPAAm) nanogel (NG) particles as functional gates, and iron oxide magnetic nanoparticles (MNP) as localized heaters were combined in a synergistic way. Before membrane casting, the properties of the building blocks, including swelling property and size distribution for NG, and magnetic property and heating efficiency for MNP, were investigated. Further, to identify optimal film casting conditions for membrane preparation by NIPS, in-depth rheological study of the effects of composition and temperature on blend dope solutions was performed. At last, a composite membrane with 10% MNP and 10% NG blended in a porous PES matrix was obtained, which showed a large, reversible, and stable magneto-responsivity. It had 9 times higher water permeability at the "on" state of alternating magnetic field (AMF) than at the "off"-state. Moreover, the molecular weight cutoff of such membrane could be reversibly shifted from ∼70 to 1750 kDa by switching off or on the external AMF, as demonstrated in dextran ultrafiltration tests. Overall, it has been proved that the molecular sieving performance of the novel mixed matrix composite membrane can be controlled by the swollen/shrunken state of PNIPAAm NG embedded in the nanoporous barrier layer of a PES-based anisotropic porous matrix, via the heat generation of nearby MNP. And the structure of such membrane can be tailored by the NIPS process conditions. Such membrane has potential as enabling material for remote-controlled drug release systems or devices for tunable fractionations of biomacromolecule/-particle mixtures.

Insights into Surface Interactions between Metal Organic Frameworks and Gases during Transient Adsorption and Diffusion by In-Situ Small Angle X-ray Scattering

The fabrication of molecular gas sieving materials with specific affinities for a single gas species and able to store large quantities of materials at a low or atmospheric pressure is desperately required to reduce the adverse effects of coal and oil usage in carbon capture. Fundamental understanding of the dynamic adsorption of gas, the diffusion mechanisms across thin film membranes, and the impact of interfaces play a vital role in developing these materials. In this work, single gas permeation tests across micro-porous membrane materials, based on metal organic framework crystals grown on the surface of carbon nanotubes (ZiF-8@CNT), were performed for the first time in-situ at the Australian Synchrotron on the small angle X-ray scattering beamline in order to reveal molecular sieving mechanisms and gas adsorption within the material. The results show that specific chemi-sorption of CO₂ across the ZiF-8 crystal lattices affected the morphology and unit cell parameters, while the sieving of other noble or noble like gases across the ZiF-8@CNT membranes was found to largely follow Knudsen diffusion. This work demonstrates for the first time a novel and effective technique to assess molecular diffusion at the nano-scale across sub-nano-porous materials by probing molecular flexibility across crystal lattice and single cell units.

Subnanometer Two-Dimensional Graphene Oxide Channels for Ultrafast Gas Sieving

Two-dimensional (2D) materials with atomic thickness and extraordinary physicochemical properties exhibit unique mass transport behaviors, enabling them as emerging nanobuilding blocks for separation membranes. Engineering 2D materials into membrane with subnanometer apertures for precise molecular sieving remains a great challenge. Here, we report rational-designing external forces to precisely manipulate nanoarchitecture of graphene oxide (GO)-assembled 2D channels with interlayer height of ∼0.4 nm for fast transporting and selective sieving gases. The external forces are synergistic to direct the GO nanosheets stacking so as to realize delicate size-tailoring of in-plane slit-like pores and plane-to-plane interlayer-galleries. The 2D channels endow GO membrane with excellent molecular-sieving characteristics that offer 2-3 orders of magnitude higher H2 permeability and 3-fold enhancement in H2/CO2 selectivity compared with commercial membranes. Formation mechanism of 2D channels is proposed on the basis of the driving forces, nanostructures, and transport behaviors.

Permeability properties of ENaC selectivity filter mutants

The epithelial Na(+) channel (ENaC), located in the apical membrane of tight epithelia, allows vectorial Na(+) absorption. The amiloride-sensitive ENaC is highly selective for Na(+) and Li(+) ions. There is growing evidence that the short stretch of amino acid residues (preM2) preceding the putative second transmembrane domain M2 forms the outer channel pore with the amiloride binding site and the narrow ion-selective region of the pore. We have shown previously that mutations of the alphaS589 residue in the preM2 segment change the ion selectivity, making the channel permeant to K(+) ions. To understand the molecular basis of this important change in ionic selectivity, we have substituted alphaS589 with amino acids of different sizes and physicochemical properties. Here, we show that the molecular cutoff of the channel pore for inorganic and organic cations increases with the size of the amino acid residue at position alpha589, indicating that alphaS589 mutations enlarge the pore at the selectivity filter. Mutants with an increased permeability to large cations show a decrease in the ENaC unitary conductance of small cations such as Na(+) and Li(+). These findings demonstrate the critical role of the pore size at the alphaS589 residue for the selectivity properties of ENaC. Our data are consistent with the main chain carbonyl oxygens of the alphaS589 residues lining the channel pore at the selectivity filter with their side chain pointing away from the pore lumen. We propose that the alphaS589 side chain is oriented toward the subunit-subunit interface and that substitution of alphaS589 by larger residues increases the pore diameter by adding extra volume at the subunit-subunit interface.