Efficient Molecular Separation Research Articles

Computational Multiscale Study of the Interaction Between the PDMS Polymer and Sunscreen-Related Pollutant Molecules

Sunscreen molecules play a critical role in protecting skin from ultraviolet radiation, yet their efficient detection and separation pose challenges in environmental and analytical contexts. In this work, we employ a multilevel modeling approach to investigate the molecular interactions between representative sunscreen molecules and the polydimethylsiloxane (PDMS) polymer, a material widely recognized for its sorbent properties. Our goal is to explore how these interactions can be fine-tuned to facilitate the effective separation of sunscreen molecules in portable membrane inlet mass spectrometry (MIMS) systems, potentially leading to the development of new membrane materials. Using a combination of advanced computational techniques—force field molecular dynamics simulations, semiempirical GFN2-xTB, and density functional theory calculations—we assess the interaction strength and noncovalent interactions of sunscreen molecules, namely oxybenzone, naphthalene, benzo[a]anthracene, avobenzone, and 1,3,5-trichlorobenzene, with PDMS. Additionally, the effect of temperature on the interaction dynamics is evaluated, with the aim of extending the sorbent capacities of PDMS beyond light polar molecules to larger, polar sunscreen compounds. This study provides critical insights into the molecular-level interactions that may guide the design of novel membrane materials for efficient molecular separation.

Construction of an efficient molecular enrichment and magnetic separation flow with Fe3O4@Ag@COF for ratiometric SERS detection of methamidophos

As a typical organophosphorus pesticide widely utilized in agricultural production with its residue as enormous threat to human health and environment, methamidophos (MTD) has gained increasing attention in recent years, so, it is necessary to develop an effective strategy to detect MTD from complex matrix. Here, an efficient molecular enrichment and magnetic separation flow was constructed through Fe3O4@Ag@COF (covalent organic framework) for ratiometric SERS detection of MTD. Specifically, Fe3O4@Ag@COF is shown to have a high-density hot-spot region and optimal SERS performance by adjusting the particle size and addition volume of Fe3O4@Ag. COF endows Fe3O4@Ag@COF with a platform for enriching MTD molecules and isolating impurities, allowing the separation of Fe3O4@Ag@COF with MTD molecules from complex matrix by magnetism. More importantly, due to its natural core–shell structure, the Raman peaks of COF can be employed as an internal standard for ratiometric SERS sensing, thereby increasing its analysis accuracy and reliability. Under the optimal extraction method and solvent conditions, the developed detection flow exhibited a wide linear range from 10-4 M to 10-9 M and a low MTD detection limit of 8.3 × 10-5 mg/kg. Furthermore, the SERS sensor had excellent reproducibility and long-term stability with a low relative standard deviation (RSD). In practical application, this detection flow was tested for quantifying trace MTD in green tea samples, with recoveries of 89.54 % to 101.89 % and RSDs below 5 % (n = 5), comparable to the performance of gas chromatography mass spectrometry (GC–MS).

Ultramicroporous cardo-pendant polyamide nanofilms for enhanced organic solvent nanofiltration

Permeable, selective, and stable polymeric membranes are essential for efficient molecular separation in organic solvents. Polymers of intrinsic microporosity (PIMs) with rigid and twisted chain structures offer abundant pores for high solvent permeability and are emerging as an ideal membrane material. However, guaranteeing high separation selectivity and stability is challenging owing to the wide pore size distribution and low swelling resistance in organic solvents. In this work, we developed intrinsically ultramicroporous polyamides (PAs) with cardo-pendant groups to create crosslinking, ultrathin, and defect-free nanofilms for organic solvent nanofiltration. The molecular simulation confirmed the pivotal role of cardo-pendant group structures in the amine monomers and exhibited their effect in creating pore channel structures within the PAs. Distinctive rigid fluorenyl-based cardo-pendant groups were introduced in amine monomers to finely tailor the packing of PA chains for high ultramicroporosity and a finely tuned pore size. We further achieved the growth of such PAs using a cyclic deposition strategy, in-situ generating highly crosslinking nanofilms with an ultralow thickness of 23–36 nm without structure relaxation. As such, the typically-designed PA membranes demonstrated high and stable solvent permeance (e.g., 18 L m−2 h−1 bar−1 for methanol) and high solute rejection (up to 95 % rejection of 306 Da Azure I), which is superior to its counterparts and current PIM membranes, presenting a new perspective for efficient organic solvent nanofiltration.

How to transform microporous organic polymers for membrane-based separation: A review

Microporous organic polymers (MOPs) are ideal candidates for designing highly efficient and precise molecular sieving separation membranes due to their exceptional porosity, rigid pore structure, pore size designability, and ease of functionalization. These materials encompass COFs, POCs, PAFs, PIMs, CMPs, and HCPs. However, the path to utilizing MOPs as separation membrane applications has encountered obstacles. Several studies have been conducted to fully exploit the exceptional properties of these polymers. The better application of MOPs materials should be considered in terms of material design and synthesis, membrane formation, and membrane task-specific applications. This review objectively reclassifies MOPs materials based on the latest research and material ordering and provides a systematic summary of MOPs material synthesis and design, MOPs-based membrane fabrication difficulties, and current novel MOPs-based membrane design. Additionally, it introduces in detail the preparation of different types of MOPs-based membranes. The applications of MOPs-based membranes in separation applications are also thoroughly summarized. Finally, this review summarizes the challenges of current MOPs-based membrane development, proposes future opportunities and potential applications in the separation and purification field.

Intrinsically microporous polyimide membranes with synergistic aromatic and alicyclic linking groups for efficient molecular separation

Polymers of intrinsic microporosity (PIMs) are widely recognized for having great potential in reforming membrane technology for molecular separation within organic solvents, due to their rich and appropriate pore structures with a size < 2 nm. However, the swelling in organic solvents has a significant effect on their pore structures due to the amorphous and linear nature, resulting in the permeability and selectivity performances of developed PIM membranes being far from ideal. In this work, we synthesized an advanced polyimide of intrinsic microporosity by integrating a Tröger’s base (TB) structure with an synergistic structure of aromatic (naphthalenediimide, NDI) and alicyclic (bicyclooctenediimide, BCODI) linking groups. The TB with a rigid and twisted structure was used to furnish a basis for inhibiting chain packing for high overall free volume. The ultra-rigid NDI units maintained such a tailored high free volume and provided superior solvent resistance. The relatively flexible and polar BCODI units promoted local chain mobility to tailor uniform pore size distribution and increased affinity for polar solvents. The innovative design fostered the formation of stable, uniform, and highly solvent-accessible intrinsic micropores. The prepared TFC membrane exhibited good organic solvent separation capabilities, achieving an ethanol permeance of 7.6 L m-2 h−1 bar−1 and a low molecular weight cut-off (MWCO) of approximately 470 Da. This work contributes significantly to the strategic design of PIMs for preparing high-performance membranes for realistic chemical-related separation application scenarios.

A water-stable two-dimensional Zn(BDC) lamellar membranes for efficient molecular separation with enhanced water permeability

Although 2D Zn(BDC) MOFs exhibit exceptional water stability, the synthesis of ultrathin 2D Zn(BDC) nanosheets has proven to be a formidable challenge. Conventional bottom-up methods suffer from limitations such as low yield and difficulties in eliminating surfactant molecules or end ligands, which are introduced to control the directional growth of 2D Zn(BDC). In this work, 2D Zn(BDC) nanosheets featuring a two-nuclear paddlewheel zinc structure were successfully synthesized through a three-phase synthesis method, exhibiting a thickness of about 2 nm. The 2D Zn(BDC) lamellar membranes were prepared through simple vacuum filtration. By adjusting the colloidal concentration of the 2D Zn(BDC) nanosheets, fine control over the resulting membrane thickness was achieved. Moreover, the incorporation of polyvinyl alcohol (PVA) as a cross-linking agent significantly enhanced the stability of the 2D Zn(BDC) lamellar membranes. The addition of PVA demonstrated effective regulation of pore size in the membrane, facilitating well-ordered stacking of nanosheets for superior screening efficiency in dye/ion separation applications. The permeability of 2D Zn(BDC) lamellar membranes was observed to be 372.9 L m−2 h−1 bar−1, while exhibiting a rejection rate exceeding 99.7 % for dye molecules, such as direct red (DR23), reactive black (RB5), chromium black T (CBT), Congo red (CR). The remarkable performance is ascribed to a synergistic effect between molecular sieving and electrostatic repulsion. 2D Zn(BDC) lamellar membranes exhibit exceptional long-term stability, and resistance to fouling and chlorine, thereby showcasing their immense potential for diverse applications in the realm of dye wastewater treatment.

Tailoring the pore size distribution of nanofiltration membranes via surfactants with different alkyl chain lengths: Towards efficient molecular separation

The diffusion of amine monomer during interfacial polymerization (IP) directly affects the structure and properties of the polyamide (PA) separation layer. Surfactants can regulate the diffusion rate of monomers and change the microstructure of the PA layer. In this study, a PA nanofiltration (NF) membrane with the tailor-made pore size is fabricated by modulating the diffusion rate of 1, 4, 7, 10-tetraazacyclododecane (Cyclen). Surfactants with different alkyl chain lengths are used to precisely regulate the diffusion rate of Cyclen monomer. The longer the alkyl chain length of the surfactant is, the faster the diffusion rate of the Cyclen monomer is, resulting in the formation of a PA layer with a narrow pore size distribution. The prepared composite membrane is qualified for on-demand desalting and antibiotic/salt separation performance by changing the length of the alkyl chain. For example, surfactants with short alkane chains (C4) endow the NF membranes with a high water permeance of 30.7 L−1 m−2h−1 bar−1 for tetracycline separation. Surfactants with long alkane chains (C12) endow the NF membranes with a high selectivity of 84.8 for tetracycline/NaCl separation. This study provides further evidence for surfactant-regulated IP reactions and offers new ideas for the tailor-made regulation of NF membrane properties.

Cleanable natural-sugar-alcohol-based polyester membrane for highly efficient molecular separation

Achieving zero discharge of salt-containing dye wastewater is essential for sustainable development, which requires precise separation of organic dyes and inorganic salts. This work describes a new strategy for the preparation of cleanable polyester loose nanofiltration (LNF) membranes using inexpensive natural-sugar-alcohols (erythritol, sorbitol, and maltitol) as aqueous monomers. The resulting polyester membrane has a loose structure with hydrophilic properties and negative charge density. The optimized sorbitol/trimesoyl chloride (TMC) membranes exhibited high pure water permeability (80.63 ± 1.72 L·m−2·h−1·bar−1), a high selectivity factor for salt/dye separation at 229.07(dye rejection of 99.6 ± 0.48%, salt rejection of 8.17 ± 0.8%). Moreover, Sorbitol/TMC polyester membranes exhibit excellent compatibility with dye-fouling detergents, antifouling ability, and long-term stability. The unoptimized erythritol/TMC and maltitol/TMC membranes also exhibit high water permeability and accurate dye/salt sieving ability. The results reveal that natural-sugar-alcohol-based polyester membranes are expected to stand out in the future of salt-containing dye wastewater treatment.

Rapid synthesis of ultrathin covalent organic polymer membranes with subnanometer pores for efficient organic solvent nanofiltration

Covalent organic polymers (COPs) with abundant micropores are deemed as promising materials for building molecular separation membranes. In spite of this potential, fabricating COP separation membranes through an efficient and scalable method remains a significant challenge. Herein, we report a simple and efficient strategy for the synthesis of COP membranes with three-dimensionally interconnected micropores. Utilizing transition-metal nitrates as a catalyst, we achieve fast polymerization of amorphous COPs with relatively uniform micropores (0.7 nm in diameter). The rational design of organic-aqueous interface allows for the direct fabrication of robust COP membranes with an ultrathin thickness of ∼20 nm on porous polyacrylonitrile substrates. The resulting hydrophobic but pore-uniform frameworks in these membranes permit fast permeation of organic liquids with a notable molecular weight cutoff of 388 g mol−1 in ethanol. To demonstrate scalability, we show that our strategy can produce large-size COP membranes with a prominent area of 200 cm2, which exhibit performances similar to that of small membrane coupons. The present study offers a potentially scalable method for producing highly microporous COP membranes toward efficient molecular separation in organic liquids.

Constructing low-friction mass transfer channel in laminar graphene oxide membrane for efficient molecular separation with enhanced permeability

In recent years, graphene oxide (GO) has attracted increasing interest in fabricating next-generation separation membranes. The perm-selectivity of laminar GO membrane is generally determined by their interlayer nanochannel structure. Herein, we used a new molecule called 5, 5′-diamine-2, 2′-biphenol (BIPOL-NH2) as the intercalator of laminar GO-based membrane to construct low-friction interlayer mass transfer channels. The aromatic BIPOL-NH2 molecules act as hydrophobic side walls of the mass transfer channels between adjacent GO nanosheets, reducing the interaction between water molecules and channel structure, thereby enhancing water permeability of GO membranes. This was confirmed by the further molecular dynamics simulation. Compared with pristine GO membrane, the BIPOL-GO membrane showed a water permeability increase by a factor of about 3 (up to 72.4 L m-2h−1 bar−1), which was consistent with our simulation results. Also, it represented excellent dye/salt separation performance (the selectivity of NaCl/CR is more than 117.2) and good structural stability. Overall, our work provides a novel and efficient approach to modify GO-based membranes for enhanced permeability.

Constructing reduced porous graphene oxide for tailoring mass-transfer channels in ultrathin MXene (Ti3C2Tx) membranes for efficient dye/salt separation

Two-dimensional (2D) membranes such as graphene oxide (GO) and MXene have attracted increasing interest in water purification. Critical challenges that impede their performance in separation applications include poor stability (swelling), low mass-transfer rate (high tortuosity), and an unavoidable trade-off between permeability and selectivity. Herein, we propose an effective multidimensional channel design strategy for preparing high-performance 2D composite membranes, featuring robust and abundant mass transfer channels. Specifically, reduced porous GO (rPGO) with high-density nanopores (size: ca. 14.7 nm, density: ca. 2.2 × 1014 m−2) and a few oxygen-containing functional groups were rationally designed and then deployed as multifunctional intercalators in MXene interlayers (rPGO-MXene, rPGM) to construct highly permeable, in-plane nanochannels and stable and tunable interlayer sieve channels. The intercalation of tailorable rPGO provides additional transport channels and weakens the interlayer repulsive hydration force, yielding composite membranes with remarkably enhanced water permeability and anti-swelling properties. As a result, the optimized membrane (10 % rPGM) exhibited an outstanding water permeability of 198.8 L m−2h−1 bar−1, 100 % rejection of Congo red, 5.3 % rejection of NaCl, and satisfactory stability (30 h) under cross-flow separation conditions. This study provides an innovative and facile approach for designing robust 2D membranes with abundant permeable nanochannels for highly efficient and precise molecular separation.

Construction the hierarchical architecture of molybdenum disulfide/MOF composite membrane via electrostatic self-assembly strategy for efficient molecular separation

• Electrostatic LBL self-assembly strategy is firstly employed to integrate MoS 2 with CMOF. • Superior rejection rates are increased above 27–55% more than that of monomer membranes for FQs. • Synergy of size exclusion, electrostatic repulsion, and hydrophilic interaction is explained. • High stability in harsh environments for 72 h and over 90% rejection against FQs in the real sample. Developing a novel nanofiltration membrane for effectively purifying antibiotic wastewater has been a daunting challenge in recent years. However, a convenient and controllable strategy for constructing membranes with two-dimensional nanomaterials (2DNMs) and metal–organic frameworks (MOFs) needs to be further studied. Herein, a loosely hierarchical architecture composite membrane has been fabricated, utilizing an electrostatic layer-by-layer (LBL) self-assembly method to intercalate chitosan crosslinked UiO-66-NH 2 MOFs (CMOF) into MoS 2 laminates. Through regulating the CMOF content, lower hydraulic-resisted channels and precise separation regions have been formed in the hierarchical architecture. The well-designed MoS 2 /CMOF composite membrane exhibited enhanced water permeance (78.98 ± 1.45 L·m -2 h −1 bar −1 ) and superior antibiotic rejection capability (98.01 ± 1.12% for ciprofloxacin and 94.31 ± 0.68% for ofloxacin), which are increased by 27–55% more than monomer membranes. Detailed analysis results indicate the nanofiltration performance of the MoS 2 /CMOF composite membrane depends heavily on the synergistic mechanism, including the molecular sieving effect (size exclusion), electrostatic repulsion, and hydrophilic-hydrophobic interaction. Besides, the MoS 2 /CMOF composite membrane has emerged with excellent antifouling ability and long-term stability, which could maintain favorable rejection efficiency in continuous nanofiltration for 72 h under an alkaline and acidic environment. Furthermore, it presents over 90% antibiotic rejection rates for real water samples obtained from the municipal domestic sewage and livestock wastewater along the Xiangjiang River in China, indicating its good availability. In general, this work provides a new vision and a reasonable design strategy to construct 2DNMs/MOFs-based nanofiltration membranes with hierarchical architecture, further expanding their application in wastewater treatment and reclamation, in harmony with the global trend of green and low-carbon development.

Loosely Sandwich-Structured Membranes Decorated with UiO-66-NH2 for Efficient Antibiotic Separation and Organic Solvent Resistance

Thin-film nanocomposite (TFN) membranes with efficient molecular separation and organic solvent resistance are active in demand in wastewater treatment and resource reclamation, meeting the goal of emission peaks and carbon neutrality. In this work, a simple and rational design strategy has been employed to construct a sandwich-structured membrane for removing fluoroquinolone antibiotics and recycling organic solvents. The sandwich-structured membrane is composed of a porous substrate, a hydrophilic tannic acid-polyethyleneimine (TA-PEI) interlayer, and a polyamide (PA) selective layer decorated with metal-organic framework (PA-MOF). Results manifest that the hydrophilic TA-PEI interlayer played a bridging and gutter effect to achieve effective control in amide storage, amine diffusion, and nanomaterial downward leakage at the immiscible interface. The PA-MOF selective layer has been changed to a loosely crumpled surface, endowing functionalities on the sandwich-structured membrane that included limited pores, strengthened electronegativity, and stronger hydrophilicity. Thus, an enhanced water flux of 87.23 ± 7.43 LMH was achieved by the TFN-2 membrane (0.04 mg·mL-1 UiO-66-NH2), which is more than five times that of the thin-film composite membrane (17.46 ± 3.88 LMH). The rejection against norfloxacin, ciprofloxacin, and levofloxacin is 92.94 ± 1.60%, 94.62 ± 1.29%, and 96.92 ± 1.05%, respectively, effectively breaking through the "trade-off" effect between membrane permeability and rejection efficiency. Further antifouling results showed that the sandwich-structured membrane had lower flux decay ratios (3.36∼7.07%) and higher flux recovery ratios (93.40∼98.40%), as well as superior long-term stability after 30 days of filtration. Moreover, organic solvent resistance testing confirms that the sandwich-structured membrane maintained stable solvent flux and better recovery rates in ethanol, acetone, isopropanol, and N,N-dimethylformamide. Detailed nanofiltration mechanism studies revealed that these outstanding performances are based on the joint effect of the TA-PEI interlayer and PA-MOF selective layer, proposing a new perspective to break through the bottleneck of nanofiltration application in a complex environment.

Chemically tailored microporous nanocomposite membranes with multi-channels for intensified solvent permeation

Membrane technology is of great significance to realize efficient and energy-saving molecular separation in the petrochemicals, pharmaceuticals, and food industries. However, current membrane materials are subject to an insurmountable trade-off between permeability and selectivity. Herein, we report on a microporous nanocomposite membrane with multiple sophisticated transport channels to intensify solvent permeation. To achieve this goal, we designed polymers of intrinsic microporosity (PIMs) with precise contorted rejection pores as the matrix and covalent organic frameworks (COFs) with uniform one-dimensional (1D) channels as porous nanomaterials. Notably, the selected PIMs and COFs have similar chemical components and structures, ensuring that the interfacial compatibility between them was perfectly addressed. The as-prepared PIMs/COFs nanocomposite membranes showed a significant boost in solvent permeance (e.g., 18.1 and 4.2 L m−2 h−1 bar−1 for acetonitrile and ethanol), while leading to high rejection (>90%) toward solute molecules larger than 450 Da. Therefore, this work paves a promising avenue for the development of advanced membranes to achieve efficient organic solvent nanofiltration.

Fabrication of b-oriented MFI membranes from MFI nanosheet layers by ammonium sulfate modifier for the separation of butane isomers

Fabrication of thin and oriented zeolite membrane is highly preferable and challenging, providing the advantage of less resistance and high permeation for efficient molecular separation. MFI zeolite nanosheets serve as the desirable building block for fabricating thin and preferably b-oriented MFI membranes. However, the secondary growth of deposited MFI zeolite nanosheet layer is an indispensable aspect that comes with the challenges of sealing the nanoscale/interlayer gaps while preserving the orientation and controlling the thickness of fabricated zeolite membrane for efficient gas separation. Herein, ammonium sulfate assisted hydrothermal secondary growth of MFI nanosheet slip coated on the γ-Al2O3 disc was carried out to seal the nanoscale and interlayer gaps for the separation of butane isomers. The addition of ammonium sulfate helps to preserve the b-orientation of the deposited nanosheet layer and plays a critical role in tuning the thickness of the MFI membrane. Ammonium sulfate assisted thin MFI membrane was obtained with preserved b-orientation having a thickness of about 630 nm while exhibiting a separation factor and n-butane permeance of 41 and 1.10 × 10−7 mol m−2 s−1.Pa−1, respectively for the separation of butane isomers.

In-situ fabricated covalent organic frameworks-polyamide hybrid membrane for highly efficient molecular separation

Covalent organic frameworks (COFs) membranes fabricated via interfacial polymerization typically present high permeance but relatively low selectivity for small molecules. To address this issue, COFs-polyamide hybrid membranes were in-situ fabricated onto polyacrylonitrile (PAN) substrate via interfacial polymerization for highly efficient molecular separation. Specifically, P-phenylenediamine (Pa) or benzidine (BD) as the aqueous monomer was reacted with a mixture of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Tp) and trimesoyl chloride (TMC) to generate COFs-polyamide hybrid structure, comprising the imide-linked COFs and the amide-linked polyamide. Since the TMC could react with the residual unreacted amine groups after COFs formation, the COFs-polyamide hybrid structure exhibited few defects and excellent stability. The as-prepared TpPaTMC/PAN or TpBDTMC/PAN hybrid membranes presented excellent performance for dye rejection and dye desalination with pure water permeances of 84∼95 L·m-2·h-1·bar-1, congo red rejection of ∼99%, acid red 66 rejection of 85%∼92%, and Na2SO4/CR separation factor of 65∼86. Moreover, the separation performance of COFs-polyamide hybrid membranes could be easily tunable by adjusting the TMC contents in the organic phase during the membrane fabrication, and the optimized membranes could be applied for dye rejection or dye desalination. The COFs-polyamide hybrid membranes also showed excellent anti-fouling performance and chemical stability. Therefore, the convenient and efficient in-situ fabrication of high-performance COFs-polyamide hybrid membrane would provide novel insight into the structure, property and application prospects of COFs membrane.

Interfacial synthesis of large-area ultrathin polyimine nanofilms as molecular separation membrane

SummaryThin film membranes of covalent organic frameworks are promising for high-permeance molecular separation. However, their synthesis needs a high temperature or longer reaction time, unsuitable for large-scale fabrication of thin film composite membranes. The ultrathin film of porous organic polymers as a separation layer of the composite membrane could be a close alternative to COF membranes. Here we report transition metal ion-catalyzed room temperature fabrication of the ultrathin (≈12 nm) polyimine nanofilms via interfacial polymerization of melamine and triformylphloroglucinol onto hydrolyzed polyacrylonitrile support within a short reaction time. Composite membranes exhibit high water permeance (≈78 L m−2 h−1 bar−1), high rejection (99.6%) of brilliant blue R (825.9 g mol−1), low rejection of NaCl (≈1.8%) and Na2SO4 (≈17%), and enable efficient molecular separation. The role of metal ion catalysts for large-area fabrication of the ultrathin polyimine nanofilm membranes used for molecular separation is demonstrated.

Polyarylester thin films with narrowed pore size distribution via metal-phenolic network modulated interfacial polymerization for precise separation

Traditional nanofiltration (NF) membranes meet a bottleneck in throughput due to the lack of flexibility in pore structure control of the polyamide selective layer. This work developed a highly-permselective NF membrane by creating a controlled polyarylester selective layer via a facile method of metal-phenolic network (MPN) modulated interfacial polymerization between 5,5,6′,6′-tetrahydroxy-3,3,3′,3′-tetramethyl spirobisindane (TTSBI) and trimesoyl chloride (TMC). It was hypothesized that in the presence of FeCl3, the catechol groups of TTSBI first formed a Fe3+/TTSBI complex to construct the MPN and subsequently reacted with TMC, eliminating structural defects in the selective layer. The solution chemistry studies and membrane optimization results verified the significant role of Fe3+ in forming metal-phenolic coordination and thus defectless pore structure. The resulted Fe-TTSBI-TMC membrane exhibited greatly reduced pore size of 0.68 nm and narrow pore size distribution. The rigid and twisted feature of TTSBI endowed the polyarylester layer with high porosity. This ingeniously designed synergistic effect rendered the resulted membrane with a high pure water permeability of 15 L m−2 h−1 bar−1, about 3 times higher than typical polyamide NF membrane prepared by piperazine and TMC; while achieved high salt and molecular rejections (e.g., 98% rejection of Na2SO4, 100% rejection of vitamin B12), exhibiting superior perm-selective properties as compared to most NF membranes incorporating microporous materials. Accurate separation between molecules of different sizes of vitamin B2 and vitamin B12 was achieved with 100% efficiency. Hence, this work provides a promising alternative for skillful control of NF membrane pore structure towards efficient and precise molecular separation.

Covalent Organic Framework-Mediated Thin-Film Composite Polyamide Membranes toward Precise Ion Sieving.

Covalent organic frameworks (COFs) have evinced a potential solution that promises for fast and efficient molecular separation due to the presence of orderly arranged pores and regulable pore apertures. Herein, the synthesized COF (TPB-DMTP-COF) with the pore aperture matching the pore size of the nanofiltration (NF) membrane was utilized to modulate the physicochemical characters of the polyamide (PA) membranes. It is demonstrated that COFs with superior polymer affinity and hydrophilicity not only circumvent the nonselective interfacial cavities but also improve the hydrophilicity of the resultant thin-film nanocomposite (TFN) membranes. Furthermore, the predeposited COF layer is able to slow down the diffusion rate toward the reaction boundary through hydrogen bonding, which is consistent with the results of molecular dynamic (MD) and dissipative particle dynamic (DPD) simulations. In this context, COF-modulated TFN membranes show a roughened and thickened surface with bubble-shaped structures in contrast to the nodular structure of original polyamide membranes. Combined with the introduced in-plane pores of COFs, the resultant TFN membranes display a significantly elevated water permeance of 35.7 L m2 h-1 bar-1, almost 4-fold that of unmodified polyamide membranes. Furthermore, the selectivity coefficient of Cl-/SO42- for COF-modulated TFN membranes achieves a high value of 84 mainly related to the enhanced charge density, far exceeding the traditional NF membranes. This work is considered to provide a guideline of exploring hydrophilic COFs as an interlayer for constructing highly permeable membranes with precise ion-sieving ability.

A prebiotic chemistry inspired one-step functionalization of zwitterionic nanofiltration membranes for efficient molecular separation

Loose nanofiltration (LNF) membranes have shown promises in achieving efficient separation on molecular level at relatively low pressure. In this study, a zwitterionic PES/AMN/PSBAE LNF membrane was designed by co-depositing the AMN and an in-house made zwitterionic copolymer of poly(sulfobetaine methacrylate-co-2-aminoethyl methacrylate hydrochloride) (PSBAE), in which a bio-inspired molecule aminomalononitrile (AMN) with prebiotic chemistry was used to replace the commonly applied polydopamine for rapid membrane coating, significantly reducing the surface modification time from more than 20 h to less than 1 h. The AMN-based LNF, namely PES/AMN/PSBAE membrane, was demonstrated in the separation of dye/salt from synthetic textile wastewater, exhibiting superior performance with a high water permeance of 62.9 LMH bar−1 and nearly complete dye/salt fractionation, i.e., simultaneously a high rejection of all dyes tested up to 99.9% and almost complete transport of salts. Combining with material and membrane characterization, the separation performance of the membrane was evaluated, demonstrating the advantages of the prebiotic chemistry coating and zwitterionic functionality. This study provides a new and facile strategy based on the AMN chemistry for LNF membrane surface functionalization to achieve efficient molecular separation.