Permeability-selectivity Trade-off Research Articles

In situ grown cyclodextrin metal-organic framework nanoparticles templated stripe nano-wrinkled polyamide nanofiltration membranes for efficient desalination and antibiotic removal

Nanofiltration membranes composed of polyamide (PA) and characterized by a refined, periodic stripe pattern morphology are identified as promising solutions to address the permeability-selectivity trade-off in membrane technology. Modifying the morphology of PA membranes by utilizing pre-deposited soluble nanoparticles as templates has been investigated, yet the attainment of a periodic stripe pattern using this method is still a considerable challenge. This study successfully employed cyclodextrin-based metal-organic framework (CD-MOF) nanoparticles, grown in-situ, to create periodic stripe patterns on PA membranes. Morphological characterization revealed that in-situ grown CD-MOF nanoparticles, unlike those pre-deposited through vacuum filtration which formed noticeable aggregations and large flake-like PA structures, displayed a more evenly distributed pattern with reduced aggregation, aiding in the development of a striped PA morphology. The resulting membrane, with enhanced surface area, reduced thickness, increased hydrophilicity, and lower cross-linking degree, demonstrated an impressive 617 % augmentation in pure water permeance relative to the control PA membrane, achieving 29.4 L m-2 h-1 bar-1. Furthermore, this membrane displays a 99.0 % Na2SO4 rejection and maintained over 95 % efficacy in rejecting antibiotics. This research sheds light on the mechanisms by which nanoparticle templates influence polyamide morphology and introduces an innovative approach for precisely tailoring high-performance PA membranes.

Overcoming the permeability-selectivity challenge in water purification using two-dimensional cobalt-functionalized vermiculite membrane

Clean water and sanitation are major global challenges highlighted by the UN Sustainable Development Goals. Water treatment using energy-efficient membrane technologies is one of the most promising solutions. Despite decades of research, the membrane permeability-selectivity trade-off remains the major challenge for synthetic membranes. To overcome this challenge, here we develop a two-dimensional cobalt-functionalized vermiculite membrane (Co@VMT), which innovatively combines the properties of membrane filtration and nanoconfinement catalysis. The Co@VMT membrane demonstrates a high water permeance of 122.4 L·m−2·h−1·bar−1, which is two orders of magnitude higher than that of the VMT membrane (1.1 L·m−2·h−1·bar−1). Moreover, the Co@VMT membrane is applied as a nanofluidic advanced oxidation process platform to activate peroxymonosulfate (PMS) for degradation of several organic pollutants (dyes, pharmaceuticals, and phenols) and shows excellent degradation performance (~100%) and stability (for over 107 h) even in real-world water matrices. Importantly, safe and non-toxic effluent water quality is ensured by the Co@VMT membrane/PMS system without brine, which is totally different from the molecular sieving-based VMT membrane with the concentrated pollutants remaining in the brine. This work can serve as a generic design blueprint for the development of diverse nanofluidic catalytic membranes to overcome the persistent membrane permeability-selectivity issue in water purification.

Breaking the Permeability-Selectivity Trade-Off: Advanced carbon molecular sieve membranes derived from thermally rearranged Mixed-Matrix membrane precursors

A crucial problem in membrane-based gas separation is to fabricate membranes with excellent permeability, selectivity, and stability. Here, a high-performance carbon molecular sieves membrane of TR-CMS/SSZ-13 was successfully produced by pyrolyzing a thermally rearranged (TR) mixed matrix membrane (MMM) that obtained from a copolyimide precursor (6FDA/ODA/6FAP(3:2)) and SSZ-13 zeolite as filler at 450 °C. The effect of SSZ-13 zeolite content and structures of precursors on the transport behavior of derived CMS membranes was thoroughly studied. After TR process, the as formed polybenzoxazole (PBO) chain resulted in an enhanced rigidity, higher fractional free volume (0.15 vs 0.17), and a significantly increased (1898 times) BET surface area. Further enhancing the treatment temperature to 650 °C, the formed TR-CMS membranes exhibited a huge improved BET surface area from 228.2 to 639.3 m2/g, delivering a remarkable gas separation performance with H2 permeability of 7089 Barrer and H2/CH4 selectivity of 136. When doping with SSZ-13 till 15 wt%, the TR-CMS/SSZ-13–15 % showed an even higher H2 (9726 Barrer) and CO2 permeability (6932 Barrer), as well as H2/CH4 (150 vs 136) and CO2/CH4 (107 vs 57) selectivity than the pure TR-CMS, which surpassed the newest 2019 Robeson Upper bound. This can associate with the strong interaction between the modified SSZ-13 and the polymer matrix that prevent the collapse of the aromatic strand during the pyrolysis process, thereby create more ultra-micropore volume and concentration. Conclusively, pyrolyzing TR-MMM precursor is an effective way to form high-performance membranes with good potential for CO2 separation and H2 purification.

Phytic acid and ferric chloride compound additives-regulated interfacial polymerization for high-performance nanofiltration membrane

To balance the permeability-selectivity trade-off effect of the polyamide nanofiltration membrane, interfacial polymerization regulation strategy is widely used to optimize the physicochemical structure of the polyamide film. In the present work, the compound additives of phytic acid and ferric chloride were added into the aqueous phase to regulate the interfacial polymerization process. On the one hand, a lot of piperazine monomers will be adsorbed around the phytic acid molecules due to the electrostatic interaction between phosphoric acid groups and amino groups. Meanwhile, the phytic acid molecules and ferric ions can form a complex in the piperazine solution, which results in a low diffusion rate of the other piperazine monomers without adsorbed on the phytic acid and ferric chloride complex due to steric hindrance. The synergistic effect of electrostatic interaction and steric hindrance leads to the optimized nanofiltration membrane with a thinner and looser polyamide structure. The best-performing nanofiltration membrane exhibits an approximately four-fold water permeance (28.4 L m−2 h−1 bar−1) compared with the controlled M0 nanofiltration membrane (6.5 L m−2 h−1 bar−1) without obviously sacrificing the Na2SO4 rejection (96.9 %). This simple interfacial polymerization regulation strategy provides a novel insight for the preparation of high-performance NF membrane.

Enhancing the perm-selectivity of thin-film nanocomposite membranes intercalated with cyclodextrin-chelated Metal-Organic Framework via modulated interfacial polymerization

Nanofiltration (NF), an exceedingly promising technology, has been extensively implemented within the domain of desalination. The pursuit of optimizing water permeance, while upholding outstanding rejection rates to ameliorate the vexing permeability-selectivity trade-off effect, has raised tremendous concerns. Herein, a novel approach has been proposed, involving the construction of a cyclodextrin-chelated Metal-Organic Framework (MOF) interlayer through vacuum-assisted filtration, aimed at orchestrating the interfacial polymerization (IP) process and sculpting the membrane morphologies of polyamide membranes. The results demonstrated that cyclodextrin-chelated MOF interlayer produced a “gutter” effect for faster water transport and enhanced the interaction of the substrate and PA layer. Simultaneously, the interlayer helped to alleviate the diffusion rate of piperazine (PIP) via the interactions (e.g., hydrogen bonds, electrostatic attraction and steric hindrance) between interlayer and PIP, leading to a lower cross-linking degree and a defect-free thinner active layer. Meanwhile, cyclodextrin with intrinsic nano-cavity created more free volume for water transport inside the polymer. Furthermore, the crumpled interlayer established a hydrophilic undulated interface, followed by the generation of micro-wrinkled polyamide layer with larger filtration areas. The optimal membrane exhibited a high permeability of 39.59 L/m2 h bar, nearly 6.5 times than that of the original membrane, while maintaining a remarkable Na2SO4 rejection of 95.71 %. The strategic deployment of cyclodextrin-chelated MOF interlayers in membrane modification held the potential to efficaciously mitigate the trade-off effect, thereby presenting a promising avenue for the realization of highly-efficient desalination processes.

Dual 2D nanosheets with tunable interlayer spacing enable high-performance self-cleaning thin-film composite membrane

Interlayered thin-film composite (TFC) membranes are promising in overcoming the permeability-selectivity tradeoff of conventional membranes. However, fabricating high-performance TFC membranes with tunable interlayer spacing and antifouling properties remains challenging. Here we develop a high-performance self-cleaning TFC graphene oxide (GO) membrane by employing multifunctional bismuthyl bromide (BiOBr) nanosheets to construct a unique photocatalytic GO@BiOBr interlayer. BiOBr has multiple functions, including regulating the GO nanosheet spacing (i.e., the channel size), mediating the surface properties of the interlayer to enable a desirable selective layer during interfacial polymerization, and imparting photocatalytic properties to the TFC membrane. Because of the dimensional difference between GO and BiOBr nanosheets, the spacing of the dual 2D nanosheets (GO and BiOBr) can be finely tailored by adjusting BiOBr loading. Coupling of the 2D nanosheets enables tunable interlayer properties and a highly thin and selective rejection layer, leading to significantly enhanced separation performance of the TFC membrane. The GO@BiOBr interlayered TFC membrane shows a water permeability of 29.9 L·m−2·h−1·bar−1, which is six times higher than the TFC membrane without an interlayer. The GO@BiOBr interlayered TFC membrane displays 100% rejections to various small molecules (e.g., Rhodamine B, Basic Blue 26 and Ofloxacin). The rise of BiOBr loading in the GO@BiOBr composite increases the interlayer spacing of the dual 2D nanosheets, likely leading to more monomer diffusion and eruptive interfacial polymerization. As a result, thicker and rougher polyamide layers are formed, resulting in decreased water permeability of the TFC membrane. This study presents a viable approach for engineering the next generation of high-performance self-cleaning TFC membranes for reverse osmosis, forward osmosis and nanofiltration within the realm of water treatment.

Chiral metal-organic cage modified polyvinylidene fluoride membrane via metal phenolic networks for enantioseparation

High-performance chiral membrane materials hold great promise for the separation of pharmacological enantiomers. The key to achieving high-enantioselective chiral membrane separation is the development of suitable chiral membrane materials. However, traditional chiral separation membranes still face several limitations, including the main problem of the permeability-selectivity trade-off. In this study, a kind of novel chiral metal–organic cage (MOC) modified polyvinylidene fluoride (PVDF) membrane, namely PVDF@TA-Zn@MOC, was fabricated successfully by growing the chiral MOC [Zn3L2] onto the surface of PVDF substrate membranes induced by tannic acid (TA)-Zn2+ (TA-Zn) networks. In brief, ultrathin TA-Zn2+ metal-phenolic network coating was first adhered onto the surface of PVDF substrate membranes, followed with the in-situ growth of chiral MOC [Zn3L2] that was triggered by the coordination-enabled adhesion of Zn2+. A series of chiral PVDF@TA-Zn@MOC membranes were prepared with different hydrophilic properties and MOC loading by adjusting the concentrations of TA and metal/ligand. The transport and separation performances of these membranes were investigated for the mandelic acid (MA) enantiomers, and as a result, when the addition of TA was 1 mg and the metal ion concentration was 3.125 × 10-3 mol·L-1, the chiral membrane PVDF@TA-Zn@MOC-2 exhibited the most extraordinary entrapment ability in both permeability and selectivity for D-MA with an enantiomeric excess value of up to 71 % at a feed solution concentration of 300 ppm for 6 h under the neutral conditions. This method allows for the construction of the chiral hydrophilic MOC nanocoating membranes, which has the potential to broaden the applications of chiral MOC.

Highly enantioseparation in membrane-supported one-dimensional metal–organic framework hollow nanotube array prepared via mussel-inspired chemistry

High-enantioselective chiral metal–organic framework (CMOF) nanochannel membranes are of critical importance for chiral enantiomeric separations by taking advantage of their inherent high porosity and intrinsic chirality. However, traditional chiral membranes are still constrained by the permeability-selectivity trade-off, making it challenging to improve both simultaneously. This study described the structure of a new class of membrane-supported one-dimensional MOF hollow nanotube for transport and separation of chiral 1-phenylethanol (PE), utilizing a biomimetic polydopamine (PDA)-mediated counter-diffusion synthesis strategy. The inner cylindrical pore channel surface of polycarbonate track-etched membranes (PCTM) was modified by PDA chemistry to control the nucleation and interfacial growth of CMOF crystals, leading to the development of integrated chromatographic micro-column array membranes PCTM@PDA@Cu2C2D and PCTM@PDA@Cu2C2B with one-dimensional (1D) CMOF hollow nanochannels. Compared to PCTM@PDA@Cu2C2B, it was highlighted that the chiral membrane PCTM@PDA@Cu2C2D possessed the higher loading of Cu2C2D functional layer and more suitable steric chiral microenvironment, thus exhibited more outstanding permeability and selectivity for S-PE enantiomer with an enantiomeric excess value up to 90 % under an optimal guest concentration of 250 ppm at 30 °C for 1 h. The chiral membrane with 1D MOF hollow nanotubes, created by applying a gentle and simple method to introduce CMOF into the surface and pore channels of the substrate membrane, was expected to be utilized for the large-scale production of chiral separation membranes for industrial applications.

MXene-based membranes in water treatment: Current status and future prospects

Emerging highly selective MXene-based membranes have drawn a greater scientific interest in the field of water treatment due to their ion-selective and controlled molecular transport characteristics. However, MXene-based membranes have limitations in water treatment due to the difficulties including permeability-selectivity trade-off, swelling, fouling, oxidation, etc. Thus, a better understanding of the rational structural design principles of the membranes and their separation mechanisms are essential to further their development in water treatment. This review provides a concise overview of the key timeline in the fabrication of MXene-based membranes, the membrane characterization approaches used to understand their structure-property correlations, and the present difficulties in synthesizing high-performance MXene-based membranes. To overcome these challenges, new designing approaches are comprehensively reviewed to enhance the permeability, selectivity, stability, and antifouling properties of MXene-based membranes. These approaches include physical intercalation, chemical cross-linking, membrane surface modification, etc. Then, the separation mechanisms involved in the membranes in water treatment are presented, followed by the related applications. Looking forward, the perspective is provided on the future research directions for further advancing the membranes. This timely review may help to bridge the gap between academic research and industrial needs of MXene-based membranes for water treatment.

Innovative construction of nano-wrinkled polyamide membranes using covalent organic framework nanoflowers for efficient desalination and antibiotic removal

In recent years, the scientific community has identified the intriguing potential of polyamide (PA) nanofiltration (NF) membranes with nano-wrinkled structures, promising to transcend the longstanding permeability-selectivity trade-off inherent in conventional membranes. The present study advances this frontier by masterfully employing covalent organic framework (COF) nanoflowers, meticulously constructed on substrate surfaces using a sophisticated layer-by-layer self-assembly technique. This innovation mediates the interfacial polymerization (IP) process, integral to PA NF membrane fabrication. Through rigorous analysis employing scanning electron microscopy and atomic force microscopy, the derived PA membrane was found to exhibit a distinctive nano-wrinkled morphology. Remarkably, this novel structure yielded a pure water permeance nearly threefold that of control membranes with conventional nodular PA layers. This enhancement is attributed to a synergistic combination of factors that includes increased surface hydrophilicity, greater surface area, thinner PA layer, and an optimized water transport pathway. Furthermore, this membrane demonstrated an impressive 98.7 % Na2SO4 rejection rate, greater than 96.4 % rejections for four critical antibiotics, and commendable stability in flux with superior anti-fouling characteristics. These groundbreaking insights open new avenues for the development of high-performance nano-wrinkled PA membranes, with far-reaching implications for various environmental applications.

Modulation of ion transport through nanopores in water desalination: a molecular dynamics study

ABSTRACT A good understanding of ion transport mechanisms through nanopores is an important issue for the development of advanced water desalination technologies. We use the molecular dynamics simulation method to systematically investigate the translation dynamics of ions through nanopores in the water desalination process by designing four kinds of nano-membranes based on carbon nanomaterials. Results indicate that circular-shaped pore exhibits better water permeability, nevertheless, the slit pore has a lower resistance due to the larger pore area; nanochannel membranes increase the residence time of ions. Fluorination induces more ordered ionic hydration structures, and enhances Na + -Cl- ion pair association. -OH groups replace partial ionic hydration water molecules and facilitate ions transport into membranes. The -NH3 +, -COO- groups can strongly adsorb the oppositely charged ions, and substantially slow down ion dynamics. Functionalisation within nanochannel interior can further enhance interfacial friction and transport resistance, even causing pore blocking by charged groups. The fluorinated nanochannel membrane demonstrates complete rejection of ions with a water permeability coefficient of 1.88 × 104 L·m−2·h−1·bar−1, breaking the permeability-selectivity trade-off. This study indicates that ion transport in nanopores could be finely modulated to obtain enhanced performance in water desalination.

Effect of Mixing Technique on Physico-Chemical Characteristics of Blended Membranes for Gas Separation

Polymer blending has attracted considerable attention because of its ability to overcome the permeability–selectivity trade-off in gas separation applications. In this study, polysulfone (PSU)-modified cellulose acetate (CA) membranes were prepared using N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF) using a dry–wet phase inversion technique. The membranes were characterized using scanning electron microscopy (SEM) for morphological analysis, thermogravimetric analysis (TGA) for thermal stability, and Fourier transform infrared spectroscopy (FTIR) to identify the chemical changes on the surface of the membranes. Our analyses confirmed that the mixing method (the route chosen for preparing the casting solution for the blended membranes) significantly influences the morphological and thermal properties of the resultant membranes. The blended membranes exhibited a transition from a finger-like pore structure to a dense substructure in the presence of macrovoids. Similarly, thermal analysis confirmed the improved residual weight (up to 7%) and higher onset degradation temperature (up to 10 °C) of the synthesized membranes. Finally, spectral analysis confirmed that the blending of both polymers was physical only.

Breaking the trade-off between selectivity and permeability of nanocomposite membrane modified by UIO66@PDA through nonsolvent thermally induced phase separation method

Developing polymer membranes with high permeability, fouling resistance, and rejection capability is a challenging task that requires the use of novel materials to enhance these properties. In this study, composite ultrafiltration membranes were fabricated by incorporating two types of metal–organic frameworks (MOFs), UIO66 and UIO66@PDA, into polyvinylidene fluoride (PVDF) using the nonsolvent thermally induced phase separation (NTIPS) method. The results demonstrated that the NTIPS method combined the characteristics of nonsolvent-induced phase separation (NIPS) and thermally induced phase separation (TIPS) to fabricate asymmetric membranes with a dense ultra-thin top layer and a bicontinuous network structure. Moreover, the introduction of 1.0 wt% UIO66@PDA into PVDF membranes yielded notable improvements in mechanical strength, thermal stability, and porosity. Concurrently, this led to a water flux rise from 182.5 to 464.7 L·m−2·h−1, and enhanced BSA and HA rejection to 96% and 83%, respectively. Additionally, there was a significant improvement in the membrane's antifouling capacity, achieving a water flux recovery ratio of 93%. This was facilitated by the increased hydrophilicity of UIO66@PDA, which attracted more water molecules and reduced membrane fouling. This work offers a solution to overcome the permeability-selectivity trade-off in membrane separation by hydrophilic modification of hydrophobic membranes, while also addressing fouling issues.

Design of a novel TA-MoS2 nanosheets interlayer-based loose nanofiltration membrane for enhanced dye/salt separation

From the view of sustainable development, the loose nanofiltration (LNF) membrane with high permeability and excellent separation of dye/salt mixture becomes more attractive for dye-containing wastewater. However, the previous LNF membranes inevitably suffered from the permeability-selectivity trade-off due to the inherent nature of polymer-based selective layer. This study reported a tannic acid (TA)-MoS2 nanosheets (NSs) interlayered nanocomposite polyester (PE) LNF membrane (iLNF-3) that exhibited a remarkably enhanced water permeance of 55.8 LMH·bar−1 compared to the control LNF membrane of 27.3 LMH·bar−1, and a higher separation selectivity of dye/salt (rejection rate: 99.6 % for congo red, 98.0 % for methyl blue, 5.6 % for Na2SO4, respectively). Furthermore, the iLNF-3 showed high ionic interference resistance, long-term stability, and superior antifouling performance. The enhanced permeance and dye/salt separation were mainly attributed to the introduction of a high-permeability TA-MoS2 NSs interlayer. In particular, the permeability was improved due to the “gutter” effect and nanochannels and the formed thinner PE selective layer caused by TA-MoS2 NSs interlayer. Notably, the selectivity was enhanced owing to the looser porous structure of PE layer resulting from the slow diffusion rate of aqueous monomer. This study can provide a foundational framework for the development of iLNF membranes for treating dye containing wastewater.

Two-Dimensional Sodium Channels with High Selectivity and Conductivity for Osmotic Power Generation from Wastewater.

Conducting target ions rapidly while rejecting rival ions efficiently is challenging yet highly demanded for ion separation related applications. Two-dimensional (2D) channels are widely used for ion separation, but highly selective 2D channels generally suffer from a relatively low ionic conductivity. Here we report that the 2D vermiculite channels have a Na+ conductivity higher than bulk and at the same time reject heavy metal ions with a selectivity of a few hundreds. Such performance is attributed to the highly electronegative crystal surface and the extremely narrow channel (0.2 nm high), as also supported by the ab initio molecular dynamics simulation. We demonstrate that the highly selective and conductive sodium channels can be utilized to harvest osmotic power from industrial wastewater, achieving a power density of more than 20 W m-2 while preventing pollution from waste heavy metal ions. This work provides a strategy for wastewater utilization as well as treatment. Moreover, the investigation suggests the possibility to break the ionic permeability-selectivity trade-off by combining Ångstrom-scale confinement with proper surface engineering, which could lead to applications that are challenging for previous materials.

Side-Chain-Dependent Functional Intercalations in Graphene Oxide Membranes for Selective Water and Ion Transport.

Subnanometer interlayer space in graphene oxide (GO) laminates is desirable for use as permselective membrane nanochannels. Although the facile modification of the local structure of GO enables various nanochannel functionalizations, precisely controlling nanochannel space is still a challenge, and the roles of confined nanochannel chemistry in selective water/ion separation have not been clearly defined. In this study, macrocyclic molecules with consistent basal plane but varying side groups were used to conjunct with GO for modified nanochannels in laminates. We demonstrated the side-group dependence of both the angstrom-precision tunability for channel free space and the energy barrier setting for ion transport, which challenges the permeability-selectivity trade-off with a slightly decreased permeance from 1.1 to 0.9 L m-2 h-1 bar-1 but an increased salt rejection from 85% to 95%. This study provides insights into the functional-group-dependent intercalation modifications of GO laminates for understanding laminate structural control and nanochannel design.

Innovative role of polyvinylpyrrolidone in tailoring polyamide layer for high-performance nanofiltration membranes

While being the preferred choice in the desalination industry, polyamide (PA) nanofiltration (NF) membranes still suffer from the permeability-selectivity trade-off challenge. This study successfully fabricated a high-performing PA NF membrane by regulating the role of polyvinylpyrrolidone (PVP) in the PA layer, which is contrary to the traditional view of PVP as a simple pore-forming agent. The study demonstrated that PVP can partially persist in the polyethersulfone (PES) substrate and impact the interfacial polymerization (IP) process, resulting in a thinner PA layer with a higher cross-linking degree and a more uniform pore structure compared to a PA layer formed on a control substrate without PVP. Molecular dynamic simulations revealed that the van der Waals force between PVP and piperazine (PIP) slowed down the diffusion of PIP at the PVP-PES substrate surface, facilitating the formation of a PA layer with a uniform pore size distribution and reduced thickness. By adjusting the concentrations of PVP in the dope solution and PIP during interfacial polymerization, a PA membrane with pure water permeability of 30.2 ± 0.9 L·m−2·h−1·bar−1 and a Na2SO4 rejection rate of 97.7 ± 0.1 % was obtained. This study provides new insights and presents a novel approach to tailoring the PA layer structure of NF membranes.

Modelling the Performance of Electrically Conductive Nanofiltration Membranes

Electrically conductive membranes are a class of stimuli-responsive materials, which allow the adjustment of selectivity for and the rejection of charged species by varying the surface potential. The electrical assistance provides a powerful tool for overcoming the selectivity–permeability trade-off due to its interaction with charged solutes, allowing the passage of neutral solvent molecules. In this work, a mathematical model for the nanofiltration of binary aqueous electrolytes by an electrically conductive membrane is proposed. The model takes into account the steric as well as Donnan exclusion of charged species due to the simultaneous presence of chemical and electronic surface charges. It is shown that the rejection reaches its minimum at the potential of zero charge (PZC), where the electronic and chemical charges compensate for each other. The rejection increases when the surface potential varies in positive and negative directions with respect to the PZC. The proposed model is successfully applied to a description of experimental data on the rejection of salts and anionic dyes by PANi–PSS/CNT and MXene/CNT nanofiltration membranes. The results provide new insights into the selectivity mechanisms of conductive membranes and can be employed to describe electrically enhanced nanofiltration processes.

Solvent-activated thin-film nanocomposite membranes molecularly tuned with macrocyclic cavities for efficient water desalination and boron removal

Tuning the permeation properties of thin-film nanocomposite (TFN) membranes through the molecular redesign of their underlying nanoarchitecture is a challenging but important task for realizing reverse osmosis (RO) desalination performance above the trade-off limit. Activation by organic solvents, as one of the most facile methods to rearrange the microstructure of RO membranes, can effectively boost the water permeability but is constrained by the significant rejection loss. Here, we innovated an integrative approach whereby soluble organic macrocyclic cavitands (OMCs) with molecular-sieving open cavities were simultaneously dissolved in the activating solvent during membrane post-treatment such that their infiltration into the interfacially polymerized (IP) polyamide (PA) layer enabled the formation of a nanocomposite structure with recovered or even enhanced salt rejections. In this work, prefabricated polysulfone-supported pristine PA membranes were transformed into cavity-bearing nanocomposites after being treated with methanol solutions containing 4-sulfocalix[4]arene (SCA4), an OMC structure with a strongly size-sieving ∼ 3.0 Å cavity opening. This integrative solvent-cavitand treatment allowed their water permeability to increase by up to 129% from 1.98 to 4.53 Lm−2h−1bar−1 with uncompromised or even enhanced NaCl rejection, leading to overall separation performances that well surpassed the permeability-selectivity trade-off line. Meanwhile, these SCA4-infiltrated TFN membranes also demonstrated promising boron removal efficiency with an increase in both the water permeability and boron rejection from 1.95 to 4.17 Lm−2h−1bar−1 and 96.9% to 97.4% at pH = 10, respectively. With the wide array of solvent and soluble nanoarchitecture options, this study could provide a new and versatile direction in designing high-performance TFN membranes for environmentally important separations.

Enhanced CO2 separation performance of polysulfone membranes via incorporation of pyrazole modified MCM-41 mesoporous silica as a nano-filler

Polymeric membranes are one of the widely used materials of choice for CO2 capture from major industrial sources. However, the current generation of polymeric materials are unable to keep up with the increasing separation needs of CO2 from the air (CO2/N2) or natural gas (CO2/CH4) on an industrial scale due to their so-called permeability-selectivity tradeoff. In an effort to improve the overall gas separation performance of polymeric membranes, mixed matrix membranes (MMMs) have been suggested as a substitute strategy. In this study, Polysulfone (PSf) based MMMs were prepared by incorporating pyrazole modified MCM-41 mesoporous silica (MP) as a nano-filler in the polymer matrix to enhance the gas separation properties of membranes. MCM-41 nano-filler was synthesized and functionalized by post-grafting technique to prepare the MMMs of three different loadings (10, 20, and 30 wt%). All the membranes were characterized for physical, chemical, and thermal analysis. The CO2 gas separation performance of membranes was assessed, and the results concluded that the MCM-41-based MMMs showed the tradeoff between permeability and selectivity in the case of ideal and mixed gases. This permeability-selectivity trade-off is solved by the MP-based MMMs due to the presence of CO2-philic functional groups present on the surface of membranes. The PSf-MP membrane with the highest nanofiller loading (30 wt%) showed 79% more permeability than the pure PSf membrane. Additionally, it showed a significant enhancement of 33% and 51% in ideal selectivity for CO2/CH4 and CO2/N2, respectively, compared to pure PSf membrane. These findings support the lack of membrane faults, the enhanced filler/polymer interface, and the remarkable nano-filler dispersion in the polymer matrix. The excellent outcomes demonstrated the potential of these membranes for more industrial gas separation applications.