Superior Permeance Research Articles

Conjugated Microporous Polymers-Based Catalytic Membranes with Hierarchical Channels for High-Throughput Removal of Micropollutants.

Engineering a catalytic membrane capable of efficiently removing emerging organic microcontaminants under ultrahigh flux conditions is of significance for water purification. Herein, drawing inspiration from the functional attributes of lymphatic vessels involved in immunosurveillance and fluid transport with minimal energy consumption, a novel hierarchical porous catalytic membrane is engineered. This membrane, based on an innovative nitrogen-rich conjugated microporous polymer (polytripheneamine, PTPA), is synthesized using an electrospinning coupled in situ polymerization approach. The resulting bioinspired membrane with hierarchical channels comprises a thin layer (≈1.7µm) of crosslinked PTPA nanoparticles covering the interconnected electrospun nanofibers. This unique design creates an intrinsic microporous angstrom-confined system capable of activating peroxymonosulfate (PMS) to generate 98.7% singlet oxygen (1O2), enabling durable and highly efficient degradation of microcontaminants. Additionally, the presence of a thin layer of mesoporous structure between PTPA nanoparticles and macroporous channels within the interwoven nanofibers enhances mass transfer efficiency and facilitates high flux rates. Notably, the prepared hierarchical porous organic catalytic membrane demonstrates enduring high-efficiency degradation performance with a superior permeance (>95% and >2500 L m-2 h-1 bar-1) sustained over 100 h. This work introduces an innovative pathway for the design of high-performance catalytic membranes for the removal of emerging organic microcontaminants.

Covalent organic framework membranes reconstructed pore channels through metal-phenolic complexation for dye/salt separation

Covalent organic frameworks (COFs) characterized by high porosity, ordered channels, and remarkable chemical stability are potential candidates for the fabrication of nanofiltration separation membranes. In general, COFs membranes, generated through the process of interfacial polymerization, exhibit superior permeance without comparatively poor selectivity toward small molecules. Hereby, it was addressed with a simplified route to construct an extra selective layer on the COFs membranes. Concretely, COFs (TpPa/HPAN) membranes were first fabricated via interfacial polymerization on the hydrolyzed polyacrylonitrile (HPAN) membranes by utilizing monomers of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa). Then, a selective layer of metal-phenolic complex was constructed on the TpPa/HPAN membranes by electrostatic adsorption and metal-phenolic complexation. The introduction of a metal-phenolic complex selective layer masked the intrinsic pore size of TpPa and enhanced the hydrophilicity and electronegativity of the membrane, which improved the rejection property of the membranes for dyes larger than 350 Da. The fabricated M-TpPa/HPAN membranes showed exceptional performance in dye desalination with a water permeance of 92.1 L‧m−2‧h−1‧bar−1, rejections of Congo red and Eriochrome black T (EBT) over 99 %, 78.6 % of acid fuchsin rejection, 85.3 % of acid orange 7 rejection, low rejection of salts (e.g., 6.2 % for NaCl rejection), and the NaCl/EBT separation factor up to 243. Additionally, the M-TpPa/HPAN membranes maintained excellent stability during long-term filtration. The strategy of building an extra selection layer with eco-friendly, cost-effective, and scaled-up methods is also extensible for the other COFs and framework-based membranes.

Investigating the impact of metal ion variations in terephthalate metal–organic frameworks on the organic solvent nanofiltration performance of mixed matrix membranes

Nano-fillers integrated into mixed matrix membranes (MMMs) present promising alternatives for organic solvent nanofiltration (OSN). In this work, our objective is to enhance OSN performance by incorporating terephthalate metal-organic frameworks (M-BDC, where M = Zr or Cu) into a P84 matrix to fabricate M-BDC/P84 composite MMMs. This work focuses on the influence of metal cluster variation on particle size, pore size distribution, and specific surface area of M-BDC, while systematically investigating the impact of M-BDC addition on OSN performance and underlying separation mechanisms. The synthesized membranes underwent comprehensive characterization using SEM, XRD, ATR-FTIR, contact angle, zeta potential, and thermogravimetric analysis. Significantly, all MMMs exhibited superior permeance and methyl blue (MB) rejection compared to P84 membranes. This enhancement is attributed to the improved interfacial contact between M-BDC and P84, as well as their effective dispersion within the membrane matrix. Interestingly, comparative analysis between Cu-BDC/P84 and Zr-BDC/P84 MMMs revealed the latter's superior permeance, attributed to its larger pore size and surface area. The resulting MMMs demonstrated outstanding MB rejection (>96.7%) with ethanol permeance of 21.5 and 19.0 LMH/Bar for Zr-BDC/P84 and Cu-BDC/P84 MMMs, respectively. Furthermore, the stable performance of M-BDC-based MMMs during prolonged MB/ethanol operation underscores their potential significance in the field of OSN, providing valuable insights for future membrane design and application.

Bioinspired Gradient Covalent Organic Framework Membranes for Ultrafast and Asymmetric Solvent Transport.

Gradients play a pivotal role in membrane technologies, e.g., osmotic energy conversion, desalination, biomimetic actuation, selective separation, and more. In these applications, the compositional gradients are of great relevance for successful function implementation, ranging from solvent separation to smart devices; However, the construction of functional gradient in membranes is still challenging both in scale and directions. Inspired by the specific function-related, graded porous structures in glomerular filtration membranes, a general approach for constructing gradient covalent organic framework membranes (GCOMx) applying poly (ionic liquid)s (PILs)as template is reported here. With graded distribution of highly porous covalent organic framework (COF) crystals along the membrane, GCOMx exhibts an unprecedented asymmetric solvent transport when applying different membrane sides as the solvent feed surface during filtration, leading to a much-enhanced flux (10-18 times) of the "large-to-small" pore flow comparing to the reverse direction, verified by hydromechanical theoretical calculations. Upon systematic experiments, GCOMx achieves superior permeance in nonpolar (hexane≈260.45 LMH bar-1) and polar (methanol≈175.93 LMH bar-1) solvents, together with narrow molecular weight cut-off (MWCO, 472g mol-1) and molecular weight retention onset (MWRO, <182g mol-1). Interestingly, GCOMx shows significant filtration performance in simulated kidney dialysis, revealing great potential of GCOMx in bionic applications.

Polyelectrolyte-based antifouling and pH-responsive multilayer coatings for reverse osmosis membrane

Membrane fouling, the accumulation of material on the membrane surface, has been a challenge that limits the application of reverse osmosis technology. Ordinary cleaning processes do not remove contaminants from the membrane surface. To effectively clean a membrane, we fabricated a sacrificial multilayer polyelectrolyte coating based on a layer-by-layer assembly approach using chitosan (CS) and tannic acid (TA) on the membrane surface. The CS/TA coated membranes represent a superior permeance flux of 64 L m−2 h−1 with a clear NaCl rejection of 98.89 %. Meanwhile, the membrane surface hydrophilicity was enhanced due to the abundant amino and hydroxyl groups, which shows superior anti-fouling properties against proteins, polysaccharides, and surfactant. Moreover, the coatings were unstable, which would decompose at pH 8.5. The degradation endows the coated membranes approving long-term anti-fouling performance, which the permeance recovery reaching up to 85 %. Constructing pH-responsive smart coatings provides a promising route to address membrane fouling. It can be upscaled in numerous practical applications like sensors, medical devices, and drug delivery.

Precisely regulated in-plane pore sizes of Co-MOF nanosheet membranes for efficient dye recovery

Nanofiltration (NF) is widely used to treat highly saline textile waters, but its efficiency in dye recovery is limited by low permeance. This study presents a novel class of Co-based metal-organic framework (Co-MOF) nanosheet membranes for efficient and selective dye recovery. The Co-MOF membranes have precisely regulated in-plane pore sizes and exhibit superior permeance and selectivity compared to non-porous nanosheet membranes. By adjusting the length of the ligand, the in-plane pore size was precisely tuned from 1.01 × 0.63 to 1.43 × 0.64 nm2. The Co-MOF membranes exhibited high selectivity for salts over dye in both diffusion and pressure-driven filtration modes, along with excellent and tunable pure water permeance and high rejection of the dye OII. The remarkable permeability and selectivity of the Co-MOF membranes were attributed to the in-plane pores on the nanosheets, which serve as extra fast “lifts” for water and salts while exhibiting high rejection to the dye molecules. Long-term filtration performance and Co leaching tests demonstrated the stability of the Co-MOF membranes, making them promising candidates for practical dye recovery applications. Overall, this work provides a new approach for the development of high-performance membranes for textile wastewater treatment.

Design of hybrid g-C3N4/GO/MCE photocatalytic membranes with enhanced separation performance under visible-light irradiation

Photocatalytic membranes with concurrent catalytic oxidation and separating functions possess promising potentials for the degradation of organic pollutants in wastewater treatment. Herein, we designed a hybrid photocatalytic membrane (viz., g-C3N4/GO/MCE) by vacuum-filtration-assisted assembling method, consisting of graphitic carbon nitride (g-C3N4) nanosheets as a top layer, graphene oxide (GO) nanosheets as an interlayer, and commercial mixed cellulose ester (MCE) as a substrate layer. The microstructures, chemical compositions, and physicochemical properties of the designed g-C3N4/GO/MCE membranes were systematically characterized by a series of characterization techniques. Particularly, photocatalysis-filtration reactor cells were designed for the evaluation of the simultaneous separation and photocatalytic degradation performance in a single unit using both organic dyes and antibiotics as modeling pollutants in wastewater. Four parameters were adopted to represent the integrated photocatalysis-filtration performance of our designed photocatalytic membranes, including permeance (P), rejection (R’, filtration alone), removal (R, combined photocatalysis-filtration under light-irradiation), and degradation rate (D). The hybrid membranes showed superior permeance and solute removal under visible-light irradiation (300 W, λ > 420 nm), in which the permeance was 20% higher than that of no-irradiation conditions. In addition, 10 consecutive cycling tests and light-on/off cycles of the as-prepared g-C3N4/GO/MCE membrane toward RhB showed quite stable permeance in the range of 268.9 to 352.9 L h−1 m−2 bar−1, and the removal was maintained in the range of 88.4% to 95.4%, indicating excellent long-term stability. Remarkably, it also exhibited good removal performance of antibiotics (e.g., ofloxacin, norfloxacin, sulfamethoxazole, erythromycin, and roxithromycin). Accordingly, the designed hybrid g-C3N4/GO/MCE photocatalytic membranes with excellent photocatalytic activity and photocatalysis-filtration performance are promising for wastewater treatment and water purification applications.

Preparation of highly permeable and selective nanofiltration membranes with antifouling properties by introducing the capsaicin derivative into polyamide thin selective layer by bidirectional interfacial polymerization

A novel thin film composite (TFC) nanofiltration (NF) membrane was prepared by introducing the capsaicin derivative AMTHBA into polyamide (PA) layer by bidirectional interfacial polymerization. Ultrathin, smooth and defect-free PA layers can be prepared by bidirectional interface polymerization. Meanwhile, the surface properties of NF membranes were precisely regulated by adjusting the concentration of AMTHBA in an aqueous solution. As a result, the AMTHBA-PAR/PAF/PS membrane displayed superior permeance owing to its hydrophilic and negatively charged surface. The optimized NF membrane exhibited an ultrahigh permeance of 128.95 L m−2 h−1 (almost two-fold higher permeance than that of the nanofiltration membrane prepared by traditional interfacial polymerization), together with a steady efficiency above 98.43% for rejection of Na2SO4. In addition, the AMTHBA-PAR/PAF/PS membrane exhibited excellent antifouling performance and therefore has broad application prospects in the field of water treatment.

Development of Ultrahigh Permeance Hollow Fiber Membranes via Simple Surface Coating for CO2/CH4 Separation

Most researchers focused on developing highly selective membranes for CO2/CH4 separation, but their developed membranes often suffered from low permeance. In this present work, we aimed to develop an ultrahigh permeance membrane using a simple coating technique to overcome the trade-off between membrane permeance and selectivity. A commercial silicone membrane with superior permeance but low CO2/CH4 selectivity (in the range of 2-3) was selected as the host for surface modification. Our results revealed that out of the three silane agents tested, only tetraethyl orthosilicate (TEOS) improved the control membrane's permeance and selectivity. This can be due to its short structural chain and better compatibility with the silicone substrate. Further investigation revealed that higher CO2 permeance and selectivity could be attained by coating the membrane with two layers of TEOS. The surface integrity of the TEOS-coated membrane was further improved when an additional polyether block amide (Pebax) layer was established atop the TEOS layer. This additional layer sealed the pin holes of the TEOS layer and enhanced the resultant membrane's performance, achieving CO2/CH4 selectivity of ~19 at CO2 permeance of ~2.3 × 105 barrer. This performance placed our developed membrane to surpass the 2008 Robeson Upper Boundary.

Anchoring the TiO2@crumpled graphene oxide core–shell sphere onto electrospun polymer fibrous membrane for the fast separation of multi-component pollutant-oil–water emulsion

Membrane separation technology offers the unique advantages in oily wastewater treatment. However, the oily wastewater in petrochemical industry often contains oil emulsions and soluble organic pollutants, which pose great obstacles to membrane materials. Herein, to address the aforementioned challenges, a multifunctional fibrous composite membrane (FCM) was prepared by simple spraying of three-dimensional (3D) TiO2@crumpled graphene oxide (GO) core–shell sphere onto electrospun poly (arylene ether nitrile) (PEN) porous support. In the hierarchical skin layer of composite membrane, the TiO2 nanoparticles were anchored onto 3D crumpled graphene oxide sphere surface assisted by the mussel-inspired dopamine coating, which further triggered the formation of chemical cross-linking networks and hydrogen bond interaction with tannic acid. Such rational design not only ensured the structure stability of the functional layer, but also achieved the extraordinary versatility and high separation efficiency compared the conventional two-dimensional GO stacked lamellar membranes. Due to the super-hydrophilicity/underwater hydrophobic feature, low oil adhesion, and well-regulated water channels, the GO@TiO2/PEN FCM exhibited superior permeance for various surfactant free (4830–5160 L·m−2·h−1) and stabilized oil-in-water emulsions (3142–3514 L·m−2·h−1) while keeping stable rejection rate over 99%. Moreover, the combination of TiO2 nanoparticles and GO endowed the FCM with synergistically enhanced photocatalytic degradation performance for the soluble organics. Under visible light irradiation, the degradation rate of methylene blue (MeB) and crystal violet (CV) could reach 90.8% and 92.5% in 60 min, respectively. Therefore, combination of high permeability and efficient photocatalytic degradation of the soluble organics enable the fibrous composite membrane to realize the fast separation of multi-component pollutant-oil–water emulsion.

Triethanolamine modification produces ultra-permeable nanofiltration membrane with enhanced removal efficiency of heavy metal ions

Nanofiltration membranes are attractive for the removal of the heavy metal ions but limited by the trade-off between permeance and solute rejection. Membrane permeance is closely correlated to hydrophilicity and solute rejection is closely linked to size exclusion and Donnan electrostatic repulsion. Here, triethanolamine was employed as a crosslinking agent to prepare modified polyethyleneimine/trimesoyl chloride nanofiltration (PEI/TMC NF) membranes with excellent separation performance for the removal of heavy metal ions from contaminated water. The permeance was more than 2 times increased from 6.4 Lm −2 h −1 bar −1 to 13.6 Lm −2 h −1 bar −1 due to the increase in membrane hydrophilicity resulting from the incorporation of hydrophilic hydroxyl groups on the membrane surface; the rejection of metals was high (around 97% rejection for Zn 2+ , Cd 2+ , Ni 2+ , Cu 2+ ; around 92% rejection for Pb 2+ ) due to the enhancement of positive charges on the membrane surface resulting from lone pair electrons of N elements on triethanolamine and the decrease in pore size of the membrane (MWCO from 280 Da to 220 Da). The short-term filtration test (72 h) of the simulated wastewater with various metal ions shows that the prepared membranes have excellent stability for industrial application. Unlike the traditional superior permeance achieved by ultrathin membranes with weakness and defects, the triethanolamine modification endows the pristine PEI-TMC NF membranes with superior performance without the complicated methods for thickness reduction. • Two times water permeance increased without decreasing membrane thickness. • Improved hydrophilicity due to the incorporation of hydroxyl groups on membrane surface. • Enhanced charges density due to the grafting quaternary ammonium (N+) on triethanolamine. • Fabricated NFM-1 membranes with outstanding rejection performance of heavy metal ions.

MXene Nanosheet Templated Nanofiltration Membranes toward Ultrahigh Water Transport.

The demand for thin-film composite (TFC) nanofiltration membranes with superior permeance and high rejection is gradually increasing for seawater desalination and brackish water softening. However, improving the membrane permeance remains a great challenge due to the formation of excrescent polyamide in the substrate pores and thick polyamide film. Herein, we fabricated a high-performance TFC nanofiltration membrane via a classical interfacial polymerization reaction on a two-dimensional lamellar layer of transition-metal carbides (MXene). The MXene layer promoted the absorption of the reactive monomer, and higher amine monomer concentration facilitated the self-sealing and self-termination of interfacial polymerization to generate a thinner outer polyamide film from 68 to 20 nm. The almost nonporous lamellar interface inhibited the formation of inner polyamide in the substrate pores. In addition, the MXene lamellar layer could be eliminated by mild oxidation after interfacial polymerization to avoid imparted additional hydraulic resistance. The resulting TFC membrane conferred a high rejection above 96% for Na2SO4 and excellent permeance of 45.7 L·m-2·h-1·bar-1, which was almost 4.5 times higher than that of the control membrane (10.2 L·m-2·h-1·bar-1). This research provides a feasible strategy for fabricating a high-performance nanofiltration membrane using two-dimensional nanosheets as a templated interface.

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.

Phosphonium Modification Leads to Ultrapermeable Antibacterial Polyamide Composite Membranes with Unreduced Thickness.

Water transport rate in network membranes is inversely correlated to thickness, thus superior permeance is achievable with ultrathin membranes prepared by complicated methods circumventing nanofilm weakness and defects. Conferring ultrahigh permeance to easily prepared thicker membranes remains challenging. Here, a tetrakis(hydroxymethyl) phosphonium chloride (THPC) monomer is discovered that enables straightforward modification of polyamide composite membranes. Water permeance of the modified membrane is ≈6 times improved, give rising to permeability (permeance × thickness) one magnitude higher than state-of-the-art polymer nanofiltration membranes. Meanwhile, the membrane exhibits good rejection (RNa2SO4 = 98%) and antibacterial properties under crossflow conditions. THPC modification not only improves membrane hydrophilicity, but also creates additional angstrom-scale channels in polyamide membranes for unimpeded transport of water. This unique mechanism provides a paradigm shift in facile preparation of ultrapermeable membranes with unreduced thickness for clean water and desalination.

Ideal two-dimensional molecular sieves for gas separation: Metal trihalides MX3 with precise atomic pores

Membrane technology has been widely applied in gas separation field. In this work, a series of two-dimensional (2D) metal trihalides MX3 (AsI3, ScI3, SbI3, YI3, BiI3, ScCl3, ScBr3 and YBr3) are expected to be served as realistic 2D molecular sieves in gas separation applications due to their intrinsically and uniformly well-defined atomic pores. The gas separation performance of these 2D MX3 molecular sieves has been systematically explored by using first-principles calculations. Different from the conventional membranes, the atomic pores of 2D MX3 molecular sieves behave like breathing mode via flexible change of M-X bond during the gas molecules transport process. These 2D MX3 molecular sieves exhibit excellent H2/CO and O2/N2 separation performance, which surpasses the state-of-the-art upper bound. Remarkably, the O2/N2 selectivity of YBr3, SbI3, YI3 and BiI3 molecular sieves is within the range of 102-103 under superior permeance, which exceeds that of common membranes (~101) to a large extent. The extremely high selectivity and superior permeance indicate that these 2D MX3 molecular sieves possess great potentials in gas separation applications.

Ultrathin fluorinated self-cleaning membranes via coordination-driven metal-bridging assembly for water purification

Coordination-driven metal-bridging assembly between polyphosphate ligands and perfluorosulfonate polymeric ligands generates an ultrathin fluorine-based membrane with superior permeance and self-cleaning properties.

Influence of coating conditions on the H2 separation performance from H2/CH4 gas mixtures by the PDMS/PEI composite membrane

In the present study, the composite polyetherimide (PEI) membrane coated with poly dimethyl siloxane (PDMS) was synthesized and optimum conditions of coating were obtained for separation of hydrogen from methane. Three coating techniques “pouring solution inclined by 45°”, “film casting” and “dip-coating” were used. The effect of sequential coating for different methods on permselectivity of the membranes was investigated. In addition, the influences of coating conditions including coating solution concentration, coating and curing temperatures were examined. The results showed that when the concentration of PDMS coating solution was increased; the permeance of H2 was initially declined rapidly and was then gradually leveled off. The optimum concentration of coating solution was 15 wt.%. The examination of the curing and coating temperatures showed no significant effect on H2 permeance and selectivity. In the “dip coating” method, two times coating showed superior permeance and selectivity and in “film casting”, the performance of triple coating was promising. Higher selectivities for the composite membrane prepared by “dip-coating” introduced this method as the best method. The sequential dip-coating with different PDMS concentrations was applied and the selectivity was enhanced significantly from 26 to 96 for pure gases and from 22 to 70 for the binary gas mixture. Finally, the influence of pressure on the separation performance of the fabricated membrane was investigated.