Selective Skin Layer Research Articles

Carbon Molecular Sieve Membranes with Ultra‐Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers (Small 20/2024)

Carbon Molecular Sieve Membranes with Ultra‐Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers (Small 20/2024)

Carbon Molecular Sieve Membranes with Ultra-Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers.

Translating carbon molecular sieve (CMS) membranes into highly scalable hollow fiber geometry with ultra-thin selective layer (<1µm) for gas separation remains as great challenge. The porous support layer of precursor hollow fiber membranes is prone to collapse during pyrolysis, which induces thick skin layer (15-50µm) of CMS hollow fiber membranes. Here, a novel strategy is present to obtain an ultra-thin selective skin layer by carbonization of hollow fiber membranes with porous skin. P84-based defect-free CMS hollow fiber membranes with ultra-thin selective skin layer (0.9µm) for gas separation are prepared without any coating or complex chemical pretreatment. Compared with the carbon membranes derived from defect-free fibers, the H2 permeance (93.9 GPU) of CMS membranes derived from the porous fibers increases ≈1353% with comparable selectivity of H2/CH4 (143) and higher H2/N2 (120). Furthermore, the porous fibers are pre-aged near the Tg in N2 conditions before carbonization, and the H2 permeance of the derived CMS hollow fiber membranes reached 147 GPU (increased 2180%). It is a new facile way to prepare CMS hollow fiber membranes with ultra-thin selective layer by porous fibers, demonstrating its versatile potential in gas separation or organic liquids separation.

Antifouling and Chemical-Resistant Nanofiltration Membrane Modulated by a Functionalized MWCNT Interlayer for Efficient Dye/Salt Separation at High Salinity.

Dye/salt separation in textile wastewater is of great importance. Membrane filtration technology is an environmentally friendly and effective approach to solve this issue. In this study, a thin-film composite membrane with a tannic acid (TA)-modified carboxylic multiwalled carbon nanotube (MWCNT) interlayer (M-TA) was prepared by interfacial polymerization with amino-functionalized graphene quantum dots (NGQDs) acting as aqueous monomers. The addition of the M-TA interlayer favored the formation of a thinner, more hydrophilic, and smoother selective skin layer for the composite membrane. The pure water permeability of the M-TA-NGQDs membrane was ∼9.32 L m-2 h-1 bar-1, which was higher than that of the NGQDs membrane without the interlayer. Meanwhile, the M-TA-NGQDs membrane presented better methyl orange (MO) rejection (97.79%) than the NGQDs membrane (87.51%). The optimal M-TA-NGQDs membrane exhibited excellent dye rejection (Congo red (CR): 99.61%; brilliant green (BG): 96.04%) and low salt rejection (NaCl < 15%). Noticeably, the M-TA-NGQDs membrane displayed effective selective separation performance (CR and BG > 99%) for dye/NaCl mixed solutions even at a high NaCl concentration of 50,000 mg/L. Furthermore, the M-TA-NGQDs membrane presented high water permeability recovery ratio values (91.02-98.20%). Importantly, the M-TA-NGQDs membrane showed excellent chemical stability (acid/alkali resistance). Generally, the fabricated M-TA-NGQDs membrane exhibited a great prospect for applications in dye wastewater treatment and water recycling, especially for the effective selective separation of dye/salt mixtures for high-salinity textile dyeing wastewater.

Sulfonated polyaniline interlayer with controllable doping conditions for high-performance nanofiltration

Thin-film composite (TFC) membranes have been widely used in water purification, seawater desalination and waste treatment, etc. The interface between a support layer and a selective skin layer not only plays a crucial role in the separation performance of membrane but also regulate the formation of the selective layer. Herein, we report an interlayer enhanced TFC membrane with an interlayer composed of self-doping sulfonated polyaniline (SPANI) nanofibers which possess both excellent hydrophilicity and unique chemical properties. The surface properties prior to the interfacial polymerization (IP) process can be readily tuned by adding different kinds of alkaline solutions to the SPANI interlayer, forming skin layers with various morphology in the subsequent IP process. By simply adding NaHCO3 to the SPANI interlayer, a thin and defect-free polyamide (PA) selective layer with dual morphology of crumple and nanobubble structures can be formed. The optimal i-TFC-NaHCO3-3 membrane shows excellent permeate flux of 35.35 ± 1.35 L m−2 h−1 bar−1 with good capacities of retention for both Na2SO4 (98.95 ± 0.29%) and MgSO4 (95.37 ± 0.43%). This work provides a facile route to enhance the membrane performance by simply adjusting the chemical properties of membrane interlayers.

Isoporous Polyvinylidene Fluoride Membranes with Selective Skin Layers via a Thermal-Vapor Assisted Phase Separation Method for Industrial Purification Applications.

The membrane filtration process is the most widely used purification process in various industries due to its high separation efficiency, process simplicity, and low cost. Although there is a wide range of membrane products with diverse materials and pore sizes on the market, there is a technological gap between microfiltration and ultrafiltration membranes. Here we developed highly porous polyvinylidene fluoride (PVDF) membranes with a selective skin layer with a pore size range of 20 to 80 nm by using a thermal-vapor assisted phase separation method. Porous and bi-continuous sublayers were generated from spinodal decomposition induced by cooling. The overall membrane structure and pore size changed with the dope composition, while the pore size and thickness of the selective skin layer were effectively controlled by water vapor exposure. The excellent nanoparticle removal efficiencies of the prepared PVDF membranes were confirmed, indicating their potential application in high-level purification processes to remove small trace organic or inorganic impurities from various industrial fluids.

Highly selective porous separator with thin skin layer for alkaline water electrolysis

Advanced porous separators with thin selective skin layers to reduce the hydrogen permeation are developed for applications in alkaline water electrolysis. A thin skin layer based on crosslinked polyvinyl alcohol (cPVA) is fabricated on a porous substrate by a facile and scalable ultrasonic spray coating process. As the number of ultrasonic spraying cycles increases, the resulting separator demonstrates a decrease in the large-diameter pore fraction, an increase in the bubble-point pressure, and a reduction in the hydrogen permeability without a significant increase in the areal resistance. As a result, the optimized separator with a cPVA skin layer combines a low ionic resistance of 0.267 Ω cm2, a high bubble point pressure of 2.71 bar, and a low hydrogen permeability of 1.12 × 10−11 mol cm−2 s−1 bar−1. The electrolytic cell assembled with cPVAZ-30 achieves current densities of 861 mA cm−2 and 1890 mA cm−2 at 2.0 V and 2.6 V, respectively, in a 30 wt% KOH electrolyte solution at 80 °C.

Phase inversion method for the preparation of Pebax® 3533 thin film membranes for CO2/N2 separation

Thin film composite membranes of poly(ether-block-amide) copolymer Pebax® 3533 were prepared for the first time on asymmetric polysulfone supports by a phase inversion method. The casting solution concentration and the number of layers were varied to study their influence on the selective layer thickness and the gas separation performance. The casting solution concentrations of polymer dissolved in the 1-propanol/1-butanol mixture were 0.25, 0.5, 1.0 and 1.5 wt%. These conditions produced membranes with selective skin layers with thicknesses from 0.2 to 1.8 µm. All the membranes were characterized by scanning electron microscopy, thermogravimetric analysis and infrared spectroscopy. Furthermore, the intrinsic viscosity of all the casting solutions was studied to understand the effect of the polymer concentration on the homogeneity and the gas separation properties of the obtained membranes. In general, lower viscosity of casting solutions rendered to more defective skin layers, resulting in a higher number of layers required to obtain selective membranes. The gas separation performance was tested for the post-combustion 15/85 CO2/N2 mixture at 25–50 °C and under a feed pressure of 3 bar. The best separation performance was achieved with the 0.5 wt% casting solution membranes after the deposition of four polymer layers, obtaining a CO2 permeance of 127 GPU and a CO2/N2 selectivity of 21.4 at 35 °C, the same selectivity of the corresponding dense membrane but with much higher permeance.

Novel gelatin/polyacrylonitrile thin film nanofibrous composite membranes with high filtration performance

High performance thin film nanofibrous composite (TFNC) membrane consisting of a thin gelatin (GE) selective skin layer and a polyacrylonitrile (PAN) nanofibrous substrate was fabricated in this work. Ultrathin GE nanofibers were electrospun and deposited onto the PAN nanofibrous supporting membranes and followed by moistening treatment and hot-pressing treatment. The moistened electrospun GE nanofibers would be melted and pressed into an integrated selective skin layer on the porous PAN nanofibrous membranes. The treated GE/PAN double-layer membranes were then chemically cross-linked by glutaraldehyde (GA). The depositing time of GE nanofibers was optimized to achieve a thin and integrated GE separating layer. The optimized GE/PAN TFNC membrane exhibited high flux (182.1 L m−2 h−1) and rejection (98.3%) in BSA test at 0.3 MPa. In addition, the GE/PAN composite membrane possessed great mechanical stabilities.

Membranes with Well‐Defined Selective Layer Regulated by Controlled Solvent Diffusion for High Power Density Flow Battery

AbstractMembranes with precise control of selective layer are designed and prepared by adjusting diffusion of solvents. Combining experiments and theoretical calculations, the formation mechanism of ion conductive membranes prepared by a non‐solvent induced phase separation (NIPS) method is found to be related to internal diffusion flux of solvent to the non‐solvent bath and external diffusion flux of non‐solvent to the casting solution. By regulating the internal and external diffusion rates via a two‐step NIPS method, a series of polybenzimidazole (PBI) porous membranes with independently controlled thin selective skin layers and highly porous support layers are fabricated, which achieve a simultaneous improvement in ion selectivity and proton conductivity. A vanadium flow battery assembled with a PBI membrane demonstrates an energy efficiency of 80% at a current density of 220 mA cm−2, which is the highest value among the reported PBI membranes. This provides a simple and effective way to fabricate membranes with well‐defined morphologies.

Highly Propylene-Selective Mixed-Matrix Membranes by in Situ Metal–Organic Framework Formation Using a Polymer-Modification Strategy

Despite the potential of C3H6/C3H8 separation, there have been no industrial applications of zeolitic-imidazole framework-8 (ZIF-8) mixed-matrix membranes (MMMs) because of the moderate separation performances and several challenging processing issues. Herein, we present a new paradigm of MMM fabrication, named polymer-modification-enabled in situ metal-organic framework formation (PMMOF), enabling in situ formation of ZIF-8 fillers inside the 4,4-(hexafluoroisopropylidene)diphthalic anhydride 2,4,6-trimethyl-1,3-phenylenediamine polymer. PMMOF consists of four steps including hydrolysis of a polymer, ion-exchange, ligand treatment, and imidization. Each step was thoroughly analyzed and important processing parameters were identified, enabling the structural control of MMMs by PMMOF. The binary C3H6/C3H8 separation performance of the MMMs showed much higher separation factors than conventionally prepared MMMs at similar filler loadings, satisfying the commercial C3H6/C3H8 separation performance criteria. PMMOF was successfully applied for other MOFs, demonstrating that the process could be general. Finally, as a proof of concept, asymmetric mixed-matrix hollow fiber membranes (i.d. of 0.45 mm and o.d. of 0.63 mm) with ultrathin selective skin layers were prepared by PMMOF, showing C3H6 permeance of 2.17 GPU and C3H6/C3H8 separation factor of ∼20.

Asymmetric polybenzimidazole membranes with thin selective skin layer containing ZIF-8 for H2/CO2 separation at pre-combustion capture conditions

This work addresses an optimization in the fabrication of flat PBI membranes containing ZIF-8 nanoparticles for gas separation purposes. The PBI membranes were prepared in an asymmetric configuration on P84® supports, representing a new way of preparing flat PBI membranes. An optimization of the conditions for the PBI phase inversion preparation method, including the dope composition (in the 15–26 wt% range), has been carried out to obtain PBI membranes with a 1 µm selective skin layer. The asymmetric membranes showed an unprecedented gas separation capacity in pre-combustion CO2 capture, much superior to that of dense membranes, under harsh operating conditions (250 °C and 6 bar feed), performing up to 20.3 GPU of H2 and a H2/CO2 selectivity of 35.6. Their much thinner selective layer made possible the increase in selectivity because of the saturation of the CO2 flow at high pressures. The reduction in the amount of ZIF-8 for obtaining a membrane with the same filler loading (3.7 vs. 9.3 g/m2) was also possible; the performance of this ZIF-8 containig membrane was 22.4 GPU of H2 and a H2/CO2 selectivity of 22.3. The activation energy of the membranes, as well as the flow resistances, were calculated, providing a resistance in series model to understand the flow inside the membrane.

Integrated polyamide thin-film nanofibrous composite membrane regulated by functionalized interlayer for efficient water/isopropanol separation

A novel polyamide thin film nanofibrous composite (TFNC) membrane was fabricated by interfacial polymerization (IP) with a modified graphene oxide interlayer on highly porous electrospun polyacrylonitrile (PAN) nanofibrous substrate for isopropanol dehydration by pervaporation. Benefiting from the utilization of ethylenediamine modified graphene oxide (eGO) lamellae, which provided thin and relatively smooth surface but also reaction sites (amine groups) for interfacial polymerization, the resultant TFNC membranes exhibited an integrated structure with a compacter and thinner polyamide (PA) selective skin layer. Significantly, the skillful regulation of interfacial polymerization by the control of deposition amount of eGO on nanofibrous substrate enabled the strong interfacial interaction and the formation of compact selective layer, determining the augment of selectivity of TFNC membranes. The optimized PA/eGO/PAN integrated TFNC membranes exhibited an excellent separation performance for dehydrating 90 wt% aqueous isopropanol solution with remarkably enhanced separation factor (1866) and high permeate flux (4150 g/m2 h) at 70 °C. This result was markedly superior to those of TFC membranes prepared by IP reported so far, suggesting an efficient and facile approach to fabricate high performance pervaporation membrane with unique structures.

High-performance sulfosuccinic acid cross-linked PVA composite pervaporation membrane for desalination

ABSTRACTPervaperation (PV), as a novel technology, has shown great promise in fresh water production from salty water. However, the low water flux of the present membranes hinders their practical applications. Here, a new type of PV composite membrane, consisting of a selective skin layer fabricated from poly(vinyl alcohol) (PVA) cross-linked by sulfosuccinic acid and a porous support layer using a commercial polyacrylonitrile (PAN) ultrafiltration membrane, was developed for applications in desalination. The separation performance of S-PVA/PAN composite PV membranes with different S-PVA layer thicknesses was tested in detail. The best result showed a water flux of 27.9 kg m−2 h−1 with a salt rejection of 99.8%, which was obtained at a vacuum of 100 Pa and temperature of 70°C when separating a 35,000 ppm NaCl solution. The S-PVA/PAN composite membranes could also be used for the desalination of high-concentration (100,000 ppm) NaCl solutions with a water flux of 11.2 kg m−2 h−1 with a salt rejection of 99.8%. Moreover, a stable desalination performance was obtained for a 120 h operation time. This study shows the possibility of using PV in desalination applications for seawater, brackish water and reverse osmosis concentrate treatment.

Effect of humidity on surface structure and permeation of triblock terpolymer derived SNIPS membranes

Block copolymer (BCP) ultrafiltration (UF) membranes derived from a hybrid process of BCP self-assembly and non-solvent induced phase separation (SNIPS) exhibit an asymmetric structure consisting of ordered pores in a selective skin layer above a highly permeable substructure. In this work, we investigate relative humidity (RH) as a casting condition that influences poly(isoprene-b-styrene-b-4-vinylpyridine) (ISV) membrane surface structure in terms of order and uniformity. An optimum RH of 40–45% is determined to produce membranes characterized by a high density of square packed pores with a narrow pore size distribution. Membranes cast at lower and higher RH show a drop off in order and uniformity. The RH dependent structural changes correlate well with small molar mass dye solute (methyl orange; molar mass of 327 g/mol) diffusion rates. Membranes cast at 40% RH reveal higher diffusivity than when cast at 75% RH. The rate of permeation is further controlled via pH dependent pore closure as well as through ISV terpolymer molecular architecture with increasing solute diffusivity observed in membranes cast from increasing terpolymer molar mass. Experimental findings on small molar mass solute permeability can only be accounted for when compared with theoretical predictions from a hydrodynamic theory combined with the effects of tortuosity when a simplified membrane structure is assumed. Results suggest that SNIPS derived BCP membranes may have potential for applications in drug delivery.

Enhanced both perm-selectivity and fouling resistance of poly(piperazine-amide) nanofiltration membrane by incorporating sericin as a co-reactant of aqueous phase

This study focuses on the fabrication of flat-sheet thin-film composite poly(piperazine-amide)-based nanofiltration membrane with enhanced both perm-selectivity and fouling resistance. Sericin was used as the co-reactant of piperazine (PIP) to form the selective skin layer through the interfacial reaction with trimesoyl chloride (TMC) on the surface of polysulfone porous support. Membranes prepared with different contents of sericin were rigorously characterized to investigate the effects of the incorporation of sericin on membrane physico-chemical and permeation properties. ATR-FTIR and EDX analyses confirmed the incorporation of sericin in the formed active layer, which was found to significantly enhance membrane water flux through enlarging membrane pore size and increasing surface hydrophilicity while maintaining Na2SO4 rejection through intensifying electrostatic repulsion effect. Membranes with both enhanced water permeability and solute selectivity could be prepared through controlling the content of sericin. For instance, by adding 0.06%(w/v) of sericin into the PIP-aqueous solution, the pure water permeability was enhanced by 36.7%, the Na2SO4 rejection was remained as high as 97.5% of the PIP-TMC membrane, and the selectivity to mixed NaCl/Na2SO4 solution was increased from 21.2 to 25.2. Moreover, the incorporation of sericin was also found to endow the membrane with improved fouling resistance to bovine serum albumin.

Membranes with selective laminar nanochannels of modified reduced graphene oxide for water purification

Developing graphene-based membranes is important for a wide range of applications such as in desalination, wastewater treatment, separation and purification. Herein, a novel graphene-based nanofiltration (NF) membrane was fabricated by depositing the magnesium silicate modified reduced graphene oxide (MgSi@RGO) nanosheets on a polyacrylonitrile (PAN) ultrafiltration membrane via a vacuum filtration method. The MgSi@RGO/PAN composite membrane exhibits special selectivities for separation of organic molecules. Molecule permeation through the MgSi@RGO selective skin layer was observed with the broadened nanochannels propped up by MgSi nanosheets. The rejections of different PEG molecules (PEG200, PEG400, PEG600 and PEG1000) were tested, and the experimental results and theoretical calculations showed high consistency, which means the theoretical model can be effectively used for calculating the real diameter of “pore” in NF membrane. Additionally, three dye molecules were selected to investigate the NF membrane performance, and the MgSi@RGO/PAN composite membrane showed different separation characteristics of charged molecules.

On individual resistances of selective skin, porous support and diffusion boundary layer in water vapor permeation

The efficiency of gas permeation processes is typically determined by two main parameters: selectivity and permeance. While selectivity depends to a large extent on chemical nature of the selective polymer layer, permeance is often dominated by fluid dynamics next to the membrane interface. A well-known phenomenon in terms of diffusive transport limitation is concentration polarization. It occurs in both the stagnant boundary layer and the porous support. The latter is called internal concentration polarization. The more selective and permeable a material is, the more severe the impact of concentration polarization can be. As a result process optimization is an issue far beyond the mere optimization of the selective membrane layer. The accurate identification and quantification of the limiting resistances other than the selective layer is a major challenge. Hence we introduce a systematic characterization approach to deconvolute the total mass transfer resistance. By combining single and mixed gas measurements, we quantify contributions of the selective skin, the porous support and the stagnant boundary layer. All tests were performed with a system comprising water vapor and air, with water vapor as the preferentially permeating component. Depending on process parameters, boundary layer resistance was found to be larger or in the same order of magnitude as the one of the selective layer. The influence of the porous substrate varied with the materials used. The methodology developed is important for humidification as well as dehydration processes, in particular for enthalpy exchangers in building ventilation systems.

Hydroxyhexafluoroisopropylnorbornene Block and Random Copolymers via Vinyl Addition Polymerization and Their Application as Biobutanol Pervaporation Membranes

Vinyl addition polymers of substituted norbornene (NB) monomers possess very high glass-transition temperatures, making them useful in diverse applications; however, until very recently, the lack of an applicable living polymerization chemistry has precluded the synthesis of such polymers with controlled architecture, or copolymers with controlled sequence distribution. In the present work, block and random copolymers of NB monomers bearing hydroxyhexafluoroisopropyl and n-butyl substituents (HFANB and BuNB) are synthesized via living vinyl addition polymerization, using (η3-allyl)Pd(i-Pr3P)Cl activated by [Li(OEt2)2.5]B(C6F5)4 as the initiator. Both series of polymers are cast into the selective skin layers of thin film composite (TFC) membranes, and these organophilic membranes are investigated for the concentration of n-butanol from dilute aqueous solution via pervaporation. The block copolymers show well-defined microphase-separated morphologies, both in bulk and as the selective skin layers on TFC membranes, while the random copolymers are homogeneous. Both block and random vinyl addition copolymers are effective as n-butanol pervaporation membranes, with the block copolymers showing a better flux-selectivity balance; the optimal block copolymer, containing 19 wt % BuNB, showed a process separation factor of 21 and a flux of 4300 g m–2 h–1 with a 1.00 wt % aqueous n-butanol feed, at a selective layer thickness of 1.3 μm. While polyHFANB has much higher permeability and selectivity than polyBuNB, incorporating BuNB units into the polymer (in either a block or random sequence) limits the swelling of the polyHFANB and thereby improves the n-butanol pervaporation selectivity. An analogous block copolymer derived from ring-opening metathesis polymerization, which shows much greater swelling than the vinyl addition polymers, shows a correspondingly higher flux and lower selectivity.

Estimations of effective diffusivity of hollow fiber mixed matrix membranes

Mixed matrix membranes (MMMs) are composed of a polymeric matrix, which gives them good processability, and crystalline microporous fillers, which give them high separation performance. The need to predict the separation performance of MMMs has led to the development of a number of analytical and numerical methods for the estimation of diffusivity in MMMs, based on the intrinsic diffusivity of the matrix and filler phases. However, current reports on the estimations of diffusivity in MMMs are only for flat dense MMMs (FDMMMs), rather than hollow fiber MMMs (HFMMMs), a scalable membrane configuration used in industrial membrane separation applications. In this paper, we presented the first numerical method for the estimation of effective diffusivity in HFMMMs. We determined that the curved geometry of HFMMMs results in mass transport pathways different from those found in FDMMMs, which produces higher effective diffusivity. We also investigated how the diffusivity and size of the filler as well as the interior diameter and thickness of the selective skin layer of HFMMMs affect diffusivity. The proposed methodology provides an effective tool for the design of high-performance HFMMMs applicable to cases involving separation.

Composite forward osmosis hollow fiber membranes: Integration of RO- and NF-like selective layers for enhanced organic fouling resistance

Novel composite forward osmosis (FO) hollow fiber membranes with two selective skin layers were developed in the present work. The polyethersulfone (PES) hollow fiber substrate was firstly fabricated by a phase inversion method. The RO-like polyamide skin layer was then prepared on the inner surface of the hollow fiber substrate via interfacial polymerization (IP), while the NF-like outer skin layer was prepared via polyelectrolyte layer-by-layer (LBL) assembly. Three double-skinned membranes with negative (DS#1.5), positive (DS#2.0) and minimal (DS#2.5) outer surface charges under neutral pH environment were evaluated for their capabilities against organic fouling in the FO process.The newly developed double-skinned membranes possess water permeability greater than 2l/m2hbar and NaCl rejection higher than 95% under 2bar pressure, and have superior water flux and less salt leakage than the commercial HTI FO membranes as well as all the reported double-skinned FO membranes with RO-like selective layers. Most importantly, the double-skinned DS#2.5 hollow fiber membrane exhibits exciting organic fouling resistance because of its highly hydrophilic NF-like secondary layer with minimal surface charge and tightened surface structure to mitigate possible substrate pore clogging and fouling layer formation. A stable water flux of around 25l/m2h was attained for DS#2.5 membrane using <0.5M NaCl as the draw solution and a water containing 200ppm dextran (DEX) or Aldrich humic acid (AHA), or lysozyme (LYS) and 10mM NaCl as the model feed in the active layer against the draw solution (AL-DS) configuration, suggesting its great potential for practical FO applications.