Robust Homochiral Polycrystalline Metal-Organic Framework Membranes for High-Performance Enantioselective Separation.

Homochiral MOF membranes offer a promising route to efficient chiral separation, but their fabrication remains challenging. Here, we report for the first time the design and preparation of homochiral polycrystalline MOF-808 membranes for the first time. The membrane exhibits a high integrity and thin membrane thickness. Achieving homochirality through chiral amino acid postsynthetic modification, MOF-808 membranes demonstrate remarkable solvent stability. Notably, they successfully separated racemic naproxen enantiomers, achieving enantiomeric excess (ee) values of up to ∼95.0%. This work paves the way for turning achiral polycrystalline MOF membranes into high-performance chiral membranes for enantioselective separation.

Chemically Asymmetric Polymers Manipulate the Crystallization of Two-Dimensional Covalent Organic Frameworks to Synthesize Processable Nanosheets.

Nanosheets derived from two-dimensional covalent organic frameworks (2D COFs) are increasingly desirable in various fields. While breakthroughs in the chemical and physical delamination of 2D COFs are rising, precisely regulating the growth of the COF nanosheets has not been realized yet. Herein, we report an effective strategy of polymer-manipulated crystallization to accurately control the growth of COF nanosheets. Chemically asymmetric polyvinylpyrrolidone (PVP) is developed as the manipulator that selectively interacts with the aldehydes and (100) facet to induce anisotropic growth of COFs. The number of PVP constitutional units determines this specific interaction, leading to molecularly thin but thickness-controllable nanosheets with excellent dispersity. We process these nanosheets into robust A4-sized membranes for ultraselective molecular separation. The membrane intercalated with long-chain PVP demonstrates largely improved performance, surpassing the reported COF membranes. This work reports a strategy for anisotropically crystallizing 2D COFs to yield processable nanosheets toward practical applications.

Temperature-dependent rearrangement of gas molecules in ultramicroporous materials for tunable adsorption of CO2 and C2H2.

The interactions between adsorbed gas molecules within porous metal-organic frameworks are crucial to gas selectivity but remain poorly explored. Here, we report the modulation of packing geometries of CO2 and C2H2 clusters within the ultramicroporous CUK-1 material as a function of temperature. In-situ synchrotron X-ray diffraction reveals a unique temperature-dependent reversal of CO2 and C2H2 adsorption affinities on CUK-1, which is validated by gas sorption and dynamic breakthrough experiments, affording high-purity C2H2 (99.95%) from the equimolar mixture of C2H2/CO2 via a one-step purification process. At low temperatures (<253 K), CUK-1 preferentially adsorbs CO2 with both high selectivity (>10) and capacity (170 cm3 g-1) owing to the formation of CO2 tetramers that simultaneously maximize the guest-guest and host-guest interactions. At room temperature, conventionally selective adsorption of C2H2 is observed. The selectivity reversal, structural robustness, and facile regeneration of CUK-1 suggest its potential for producing high-purity C2H2 by temperature-swing sorption.

Perpendicular Alignment of Covalent Organic Framework (COF) Pore Channels by Solvent Vapor Annealing.

Covalent organic frameworks (COFs) have showcased great potential in diverse applications such as separation and catalysis, where mass transfer confined in their pore channels plays a significant role. However, anisotropic orientation usually occurs in polycrystalline COFs, and perpendicular alignment of COF pore channels is ultimately desired to maximize their performance. Herein, we demonstrate a strategy, solvent vapor annealing, to reorient COF pore channels from anisotropic orientation to perpendicular alignment. COF thin films are first synthesized to have flexible N-H bonds in their skeletons, thus having structural mobility to enable molecular rearrangement. A solvent with low relative permittivity and a conjugated structure is then identified to have a strong affinity toward the COFs, allowing its vapor to easily penetrate into the COF interlayers. The solvent vapor weakens the π-π interaction and consequently allows the COF monolayers to dissociate. The COF monolayers undergo a reorientation process that converts from random stacking into the face-on stacking fashion, in which the through COF pores are perpendicularly aligned. The aligned COF film exhibits high separation precision toward ions featuring a size difference down to 2 Å, which is 8 times higher than that of the anisotropically oriented counterpart. This work opens up an avenue for COF orientation regulation by solvent vapor annealing and reveals the essential role of the perpendicular alignment of COF pore channels to enable precision separations.

Design of Three-Dimensional Covalent Organic Framework Membranes for Fast and Robust Organic Solvent Nanofiltration.

Organic solvent nanofiltration (OSN) has become increasingly important in petrochemical and pharmaceutical industries, demanding superior and robust membranes. Herein, we report advanced OSN processes by designing three-dimensional covalent organic framework (3D COF) membranes through moderated interfacial crystallization. Nanoporous supports work as the moderator allowing the crystallization of 3D COF membranes. The 3D COF features sub-nanometer and anti-swelling channels, affording a sharp selectivity to fine targets with an exceptionally high and stable methanol permeance. Thus-synthesized membrane exhibits a record stability against high-concentration feeds and long-term operation for ≈1000 h. Moreover, we unambiguously demonstrate that our membrane holds excellent practicality in purifying active pharmaceutical ingredients from organic liquids. This work reveals the great potential of distinctive 3D COFs in producing prominent OSN membranes for industrial applications.

Heterostructured two-dimensional covalent organic framework membranes for enhanced ion separation.

A heterostructured covalent organic framework (COF) membrane is synthesized via in situ linker exchange. Narrowed pores can be formed at the interface between two types of COFs by adjusting the linker exchange duration. The resultant COF membrane demonstrates a high rejection rate toward Na2SO4 of up to 97%.

Electrosynthesis of Ionic Covalent Organic Frameworks for Charge-Selective Separation of Molecules.

Covalent organic frameworks (COFs) have emerged as potent material platforms for engineering advanced membranes to tackle challenging separation demands. However, the synthesis of COF membranes is currently hampered by suboptimal productivity and harsh synthesis conditions, especially for ionic COFs with perdurable charges. Herein, ionic COFs with charged nanochannels are electrically synthesized on conductive supports to rapidly construct composite membranes for charge-selective separations of small molecules. The intrinsic charging nature and strong charge intensity of ionic COFs are demonstrated to collectively dominate the membrane growth. Spontaneous repairing to diminish defects under the applied electric field is observed, in favor of generating well-grown COF membranes. Altering electrosynthetic conditions realizes the precise control over the membrane thickness and thus the separation ability. Electrically synthesized ionic COF membranes exhibit remarkable molecular separation performances due to their relatively ordered and charged nanochannels. With these charge-selective pathways, the membranes enable the efficient sieving of charged and neutral molecules with analogous structures. This study reveals an electrical route to synthesizing COF thin films, and showcases the great potential of ionic nanochannels in precise separation based on charge selectivity.

Flexible and Robust Three-Dimensional Covalent Organic Framework Membranes for Precise Separations under Extreme Conditions.

Membranes based on covalent organic frameworks (COFs) have demonstrated huge potential to resolve the long-standing bottlenecks in separation fields due to their structural and functional attributes. Herein, a three-dimensional COF featuring interpenetrated apertures, 3D-OH-COF, is rationally synthesized on polyimide supports to generate flexible, robust membranes. The resultant 3D-OH-COF presents excellent crystallinity, prominent porosity, and exceptional solvent resistance, enabling the produced membrane a sharp and durable selectivity to small molecules in water and organic solvents. Impressively, the membrane also exhibits excellent flexibility and robustness as verified by the well-maintained performances after serious bending and solvent soaking under elevated temperatures. We further chemically convert 3D-OH-COF into the carboxyl-decorated 3D-COOH-COF by a postsynthetic strategy. The 3D-COOH-COF retains high crystallinity, and the converted membrane receives a remarkable capture ability for targeted multivalent ions over other competing ions. This study exploits a viable avenue to produce practical 3D COF membranes toward ultimate separations under extreme conditions.

Fast Evaporation Enabled Ultrathin Polymer Coatings on Nanoporous Substrates for Highly Permeable Membranes.

Thin polymer coatings covering on porous substrates are a common composite structure required in numerous applications, including membrane separation, and there is a strong need to push the coating thicknesses down to the nanometer scale to maximize the performances. However, producing such ultrathin polymer coatings in a facile and efficient way remains a big challenge. Here, uniform ultrathin polymer covering films (UPCFs) are realized by a facile and general approach based on rapid solvent evaporation. By fast evaporating dilute polymer solutions spread on the surface of porous substrates, we obtain ultrathin coatings (down to ∼30 nm) exclusively on the top surface of porous substrates, forming UPCFs with a block copolymer of polystyrene-block-poly(2-vinyl pyridine) at room temperature or a homopolymer of poly(vinyl alcohol) (PVA) at elevated temperatures. Upon selective swelling of the block copolymer and crosslinking of PVA, we obtain highly permeable membranes delivering ∼2-10 times higher permeance in ultrafiltration and pervaporation than state-of-the-art membranes with comparable selectivities. We have invented a very convenient but highly efficient process for the direct preparation of defective-free ultrathin coatings on porous substrates, which is extremely desired in different fields in addition to membrane separation.

Design of Block-Copolymer Nanoporous Membranes for Robust and Safer Lithium-Ion Battery Separators.

Lithium-ion batteries (LIBs) suffer from unsatisfied performance and safety risks mainly because of the separators. Herein, a block copolymer (BCP) composed of robust and electrolyte-affinitive polysulfone (PSF) and Li+-affinitive polyethylene glycol (PEG) is rationally designed to prepare a new type of LIB separator. The copolymer is subjected to selective swelling, producing nanoporous membranes with PEG chains enriched along the pore walls. Intriguingly, when used as LIB separators, thus-produced BCP membranes efficiently integrate the merits of both PSF and PEG chains, endowing the separators thermal resistance as high as 150 °C and excellent wettability. Importantly, the nanoporous separator is able to close the pores with a temperature of 125 °C, offering the battery a thermal shutdown function. The membrane exhibits ultrahigh electrolyte uptake up to 501% and a prominent ionic conductivity of 10.1 mS cm-1 at room temperature. Batteries assembled with these membranes show excellent discharge capacity and C-rate performance, outperforming batteries assembled from other separators including the extensively used Celgard 2400. This study demonstrates a facile strategy, selective swelling of block copolymer, to engineer high-performance and safer LIB separators, which is also applicable to produce advanced copolymer-based separators for other types of batteries.

In Situ Growth of Cationic Covalent Organic Frameworks (COFs) for Mixed Matrix Membranes with Enhanced Performances.

Covalent organic frameworks (COFs) are increasingly utilized as doping agents for the design of advanced ultrafiltration mixed matrix membranes, thanks to their prominent nanoporosity and excellent polymer compatibility. However, current strategies are largely limited in the complicated postaddition of neutral COF particulates. Herein, cationic COFs, namely, TpEB, with sizes down to ∼39 nm are in situ synthesized in polyacrylonitrile (PAN) solution as crystalline fillers for the production of highly permeable TpEB-PAN ultrafiltration membranes. After the condensation of monomer pairs, the growth of cationic TpEB crystallites is restrained due to the electrostatic interaction with negatively charged PAN chains, leading to the formation of a homogeneous TpEB-incorporated casting solution. During the subsequent nonsolvent-induced phase separation process, TpEB crystallites facilitate exchange between the solvent and the nonsolvent because of their hydrophilic and nanoporous nature, accelerating the rate of phase inversion to form a highly porous membrane surface. Thus-prepared TpEB-PAN membranes deliver a tight rejection of BSA with water permeance of up to 380 L m-2 h-1 bar-1, which is 35.6% higher than that of the original PAN membranes prepared without TpEB. The TpEB-PAN membranes also exhibit enhanced operation stabilities and fouling resistances. This in situ growth strategy suggests a new avenue for the preparation of advanced mixed matrix membranes.

Ultra-permeable polyamide membranes harvested by covalent organic framework nanofiber scaffolds: a two-in-one strategy.

Polyamide-based thin-film composite nanofiltration membranes are widely used for potable water decontamination, brackish water desalination and wastewater reutilization. However, enhancing the water permeance of the polyamide layer within the thin-film composite nanofiltration membrane still remains a great challenge, because of the relatively large thickness and high transport resistance of the polyamide layer produced by intractable interfacial polymerization. Herein, we reported a two-in-one strategy to prepare ultra-permeable nanofiltration membranes via covalent organic framework (COF) nanofiber scaffold mediated interfacial polymerization. The highly porous and hydrophilic COF nanofiber scaffolds enhanced the controlled release of amine monomers, leading to ultrathin polyamide layers. Also, the relatively smooth COF nanofiber scaffolds can be spontaneously evolved into rugged and uneven architectures during interfacial polymerization, providing rough substrates for enlarging the actual areas of polyamide layers. Therefore, the increased areas of polyamide layers were employed as additional water permeable domains. Arising from the synergetic effect of the ultrathin and increased water permeation domains, the produced membranes exhibit exceptional nanofiltration performance with a water permeance of up to 31.1 L m-2 h-1 bar-1 and a Na2SO4 rejection rate of about 95%, outperforming most other nanofiltration membranes. This highly accessible technique opens a new avenue for the design and engineering of ultra-permeable thin-film composite nanofiltration membranes, highlighting its great potential in providing clean water.

Enabling Covalent Organic Framework Nanofilms for Molecular Separation: Perforated Polymer-Assisted Transfer.

Covalent organic frameworks (COFs) with ordered arrays of sub-2 nm regular pores are drawing increasing attention in membrane separation, and it remains highly desirable for effective and controllable strategies to fabricate COF-based membranes. Herein, we demonstrate a perforated polymer-assisted transfer strategy enabling COF nanofilms for molecular separation. Solvothermal synthesis is used for the confined growth of TpPa, a stable, imine-linked COF, on the smooth surfaces of silicon substrates. Continuous, crystalline COF nanofilms are obtained, and their thicknesses are tunable in the range from a few tens to several hundred nanometers depending on monomer concentrations and reaction time. A block copolymer layer is coated on the COF nanofilms, which is then perforated to produce interconnected mesopores by the mechanism of selective swelling-induced pore generation. The perforated polymer coating functions as a protective but permeable layer enabling the easy transfer of the COF nanofilm onto porous substrates. Thus, we obtain a new type of composite membranes with the microporous COF nanofilm as the selective layer, sandwiched between the macroporous substrate and the mesoporous protective layer. The composite membranes exhibit good separation performances with water permeance up to ∼51 L m-2 h-1 bar-1 and high rejection rates to various dyes. This work demonstrates a new method to prepare COF-based membranes for molecular separation, and the invented perforated polymer-assisted transfer technology is expected to find applications in transferring other ultrathin materials to demanded substrates.

Table-salt enabled interface-confined synthesis of covalent organic framework (COF) nanosheets.

Two-dimensional covalent organic frameworks (COFs) are gaining tremendous interest for their potential applications in a diversity of fields. However, synthesis of COF nanosheets (CONs) usually suffers from tedious exfoliation processes and low yields. Herein, we present an exfoliation-free and scalable strategy to prepare few-layered CONs based on interface-confined synthesis, in which cheap and recyclable table salt (NaCl) is used as the sacrificial substrate. Salt particles are introduced into the reaction system, creating billions of solid-liquid interfaces. Oligomers formed upon the reaction between monomers are immediately adsorbed on salt surfaces, and the following polymerization leading to crystalline CONs is exclusively confined to salt surfaces. Salts can be easily removed by water washing, producing CONs with the thickness down to a few nanometers and lateral sizes up to hundreds of micrometers depending on the size of salt particles and the concentration of monomers. Four different kinds of CONs, both imine-linked and boron-containing, are synthesized from this generic method. As a demonstration, we prepare highly permeable and selective membranes using resultant CONs as building blocks. Thanks to the defect-free stacking of CONs with thin thicknesses and large lateral sizes on porous substrates, the membranes precisely separate similarly sized dyes while allowing ultrafast water permeation. This interface-confined strategy opens a new platform for the controllable and scalable synthesis of COF nanosheets and is essential for the burgeoning real-world applications of COFs in various fields.