Defect-free Membranes Research Articles

High Efficient CO2 Separation at High Pressure by Grain-Boundary-Controlled CHA Zeolite Membrane Investigated by Non-Equilibrium Molecular Dynamics

The CO2 permeability and selectivity of CHA-type zeolite membranes in the separation of a CO2/CH4 mixture gas at high pressure were evaluated using non-equilibrium molecular dynamics (NEMD). It was found that in a perfectly crystalline, defect-free CHA membrane, the adsorption of CH4, which diffuses slowly in the pores, hinders CO2 permeation. Therefore, an increase in the amount of CH4 adsorbed at high pressure decreases the CO2 permeability and significantly reduces the CO2 selectivity of the CHA membrane. CHA membranes with grain boundaries parallel to the permeation direction were found to show higher CO2 selectivity than perfectly crystalline CHA membranes at high pressure, as the blocking effect of CH4 on CO2 permeation occurring within the grain boundary is not significant. This paper is the first to show that the CO2 permeability of CHA membranes with controlled grain boundaries can exceed the intrinsic performance of fully crystalline zeolite membranes at high pressure.

Metal ion-catalyzed interfacial polymerization of functionalized covalent organic framework films for efficient separation

Covalent organic frameworks (COFs), with high porosity, uniform and tunable pore size, which are advantageous to prepare defect-free membranes with high separation performance and high permeability, and have achieved efficient applications in the field of separation membranes in numerous directions. According to our knowledge, the transition metal ion (Cu2+) which is used in Schiff base condensation reaction has been rarely described. Herein, we report the interfacial polymerization of ternary amines and binary aldehyde monomers via the catalyst Cu(ClO4)2·6H2O. In this work, we report the fabrication of covalent organic framework films from four different functional structure monomers by using the transition metal ion-catalyzed interfacial polymerization at room temperature. The four as-synthesized films are flexible, continuous, defect-free and have large areas, which have excellent performance in the direction of dye separation as well as solvent penetration. After the COF films were transferred to the polyethersulfone support, the composite films exhibited enhance the rejection of methylene blue (98.43%), high-water flux (81.46 L m−2 h−1 bar−1) and solvent flux (ethanol (13.03 L m−2 h−1 bar−1), acetonitrile (50.38 L m−2 h−1 bar−1)). The large area, tunable pore size, and tailored molecular composite membranes show potential for nanofiltration applications.

Synthesis of Proton Conducting and Highly Stable PWA-ZRP-Doped Composite Membrane for Proton Exchange Membrane Fuel Cell

Abstract Mechanically stable, proton-conducting, and very cost-effective nanocomposite membrane was synthesized successfully using a simple and scalable phase-inversion approach. Phosphotungstic acid (PWA) and zirconium phosphate (ZRP) were synthesized using sol–gel and co-precipitation method, respectively. PWA-ZrP nanoparticles showed remarkable compatibility with cross-linked poly(vinyl alcohol) (c-PVA) and thus forming uniform and defect-free composite membrane of thickness ∼100–120µm. Doped PWA-ZRP nanoparticles into c-PVA membrane led to introduced bronsted acidic sites, and thereby, drastic improvement in proton conductivity of membrane was observed. Composite membrane revealed excellent water-holding capabilities with proton conductivity of 5.2 × 10−5 Scm−1 under fully hydrated conditions (i.e., 98% relative humidity). The synthesized proton-conducting nanocomposite membrane was demonstrated as a potential advanced functional solid electrolyte for possible application in proton exchange membrane fuel cell.

Beta-crystallin interacts with phospholipid membrane forming semi-transmembrane defects: A study using atomic force microscopy.

Highly concentrated lens proteins, mostly crystallins, are responsible for maintaining the structure and refractivity of the eye lens. These crystallins alter the refractivity of the lens due to self-modifications and might change the mechanical properties by interacting with the membrane. However, the interaction between a crucial lens protein, β-crystallin, with the membrane is still ambiguous. In this study, we explore the interaction of native β-crystallin extracted and purified from the cortex of the bovine lens with the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane using atomic force microscopy (AFM). We prepared a supported lipid membrane by incubating small unilamellar vesicles (SUVs) on top of a freshly cleaved mica surface. Incubating β-crystallin solution on a defect-free membrane introduced locally modified semi-transmembrane defects. Rather than developing in multiple locations, such defects expand in the area with increased incubation time. Interestingly, the thickness mismatch between the β-crystallin-induced defects and the unaffected membrane is about half of the bilayer membrane thickness, suggesting partial removal of the distal leaflet. Force curves were obtained using force spectroscopy to access the membrane's mechanical properties. Typical membrane force curves were obtained in a defect-free region. However, the force curves obtained inside semi-transmembrane defects deviate from the characteristic membrane force curves. Membrane elasticity in the defect-free region is not significantly different from the membrane before incubating with the β-crystallin. We speculate that the defect region consists of a combination of three structural possibilities: the presence of proximal monolayer with free acyl chain, β-crystallin combined with lipids forming lipoprotein, and β-crystallin binding with lower leaflet acyl chains monolayer. Our results confirm that the interaction of β-crystallin with the POPC membrane significantly modulates membrane structure by creating semi-transmembrane defects in the supported membrane.

Thermal treated amidoxime modified polymer of intrinsic microporosity (AOPIM-1) membranes for high permselectivity reverse osmosis desalination

Despite the commercial success of polyamide reverse osmosis (RO) membranes for desalination, shortcomings of the material intrinsic properties hinder the further improvement of RO membrane permselectivity towards more energy-efficient desalination. Exploring new polymer materials is thus important to overcome the limitation of existing RO membrane materials. This work reports an investigation on an amidoxime modified polymer of intrinsic microporosity (AOPIM-1) as a membrane material for reverse osmosis desalination. AOPIM-1 is casted into a defect-free dense membrane, and followed by a thermal treatment to finetune its structural feature for enhanced salt rejection. As a polymer with stiff and distorted backbone structure, thermal treated AOPIM-1 possesses a high free volume and highly connected 0.3–0.8 nm micropores that provide fast water transport routes while regulating the permeation of ions. The rapid transport through the microporous channels gives the AOPIM-1 membrane a water permeability of 1.92 × 10−7 m·kg/m2·h·bar with NaCl rejection as high as 98.5 %. The thickness-normalized water permeability of AOPIM-1 membrane exhibits order-of-magnitude improvement comparing to conventional polyamide-based desalination membranes, and the permselectivity of the membrane is also found to be on the high side. This study confirms the possibility of utilizing microporous polymers as reverse osmosis membrane materials for high performance desalination.

Vacuum treated amorphous MOF mixed matrix membrane for methane/nitrogen separation

CH4/N2 separation is considered a feasible method for reducing the greenhouse effect, but its utility in industrial applications has limited by the low permselective performance. In this work, a novel method for enhancing CH4/N2 separation performance using amorphous UIO-66-NH2 with a carbonized structure (ACU) via high vacuum resistance calcination is proposed. Owing to the presence of carbonized framework that contains rich Zr4+ active sites, these vacuum-treated ACU nanoparticles showed high IAST CH4/N2 selectivity, good alkaline stability and satisfactory compatibility with polymers. Then, ACU were uniformly dispersed in polyvinyl alcohol (PVA) to obtain defect-free mixed matrix membranes (MMMs), the resulting ACU-based MMMs exhibited an enhanced CH4 separation performance. Remarkably, the PVA/ACU-120A/MPSf MMM that consisted of 80 ​wt% ACU-120A exhibited the outstanding CH4 permselectivity, achieving a CH4 permeance of 5584 GPU and CH4/N2 selectivity of 3.43.

Effect of phase disparity of MoS2 nanosheets on the performance of PES membranes for dual industrial oil-in-water emulsion separation and dyes adsorption

Water shortage is a global challenge due to increased demand and hence, reliable wastewater treatment avenues must be created to solve this problem. In this work, we report about the fabrication of dual performance polyethersulfone (PES) membranes modified with molybdenum disulfide (MoS2) nanosheets of different phases for wastewater remediation. We optimised process parameters to obtain large area, defect free membrane sheets. Comprehensive characterization revealed the phase disparity of MoS2, and changes introduced in the modified PES membranes. Blending MoS2 with PES membranes enhanced the mechanical properties as well as water uptake differently. The optimum PES membrane was modified with 0.2 wt% of 70% 1 T/2 H MoS2 and possessed superior pure water permeance of 778 kg/m2.h.bar. It also showed superior performance towards oil removal with rejection efficiency of 99% for 1 g/L oil concentration and it revealed excellent antifouling properties. The optimum membrane showed appreciable performance in organic dye rejection, even though it was outpaced by the 0.5 wt% of 70% 1 T/2 H MoS2 modified PES with a dye rejection of 98% towards methylene blue and 92% towards Rhodamine B. This work revealed that the phase of MoS2 strongly influences the structure and performance of the resultant PES-based nanocomposite membranes in ultrafiltration applications.

Hyperlooping Carbon Nanotube-Graphene Oxide Nanoarchitectonics as Membranes for Ultrafast Organic Solvent Nanofiltration

Membrane technology is a key enabler for a circular pharmaceutical industry, but chemically resistant polymeric membranes for organic solvent nanofiltration (OSN) often suffer from lower-than-required performances. Recently, graphene-based laminated membranes using small-flake graphene oxide (SFGO) nanosheets open up new avenues for high-performance OSN, but their permeance toward high viscosity solvents is below expectation. To address this issue, we design hyperlooping channels using multiwalled carbon nanotubes (MWCNTs) intercalated within lanthanum(III) (La3+)-cross-linked SFGO nanochannels to form a ternary nanoarchitecture for low-resistant transport toward high viscosity solvents. At optimized MWCNT loading, the defect-free membrane exhibits 138 L m–2 h–1 bar–1 ethanol permeance at >99% rejections toward organic dyes, outperforming state-of-the-art graphene oxide (GO)-based membranes to date. Even butanol─with twice the viscosity of ethanol─exhibits a permeance no less than 60 L m–2 h–1 bar–1 at comparable rejection rates. Theoretical simulation suggests that La3+ cross-linking is critical and can create an intact architecture that brings size exclusion into play as the dominant separation mechanism. Also, MWCNT nanochannel offers at least 1.5-fold lower ethanol transport resistance than that of the GO nanochannel, owing to greater bulk freedom in orientating ethanol molecules. Overall, the hyperlooping architecture demonstrates ∼3-fold higher permeance than neat SFGO membrane for elevating OSN performances.

Smart Integration of Mofs and Cqds to Fabricate Defect-Free and Self-Cleaning Tfn Membranes for Dye Removal

Smart Integration of Mofs and Cqds to Fabricate Defect-Free and Self-Cleaning Tfn Membranes for Dye Removal

Enhanced CO2 separation performance by incorporating KAUST-8 nanosheets into crosslinked poly(ethylene oxide) membrane

The trade-off relationship between permeability and selectivity as well as plasticization issues have largely impeded the advancement of membrane technology. Mixed-matrix-membranes (MMMs) with the incorporation of fillers into the polymer matrix is one promising method to deal with these challenges by carefully tuning their chemical properties. Compared with nanoparticles, the counterpart of nanosheet has a larger external surface area with a tendency to have closer contact with a polymer matrix to form a defect-free membrane at higher loading. This work successfully synthesized a high aspect ratio (30/1) of KAUST-8 nanosheets with high CO2 uptake (60 cm3/g). This enables an optimal loading of 30 wt.% in the CO2-attractive crosslinked poly(ethylene oxide) (XLPEO) membrane and results in the simultaneously enhanced CO2 permeability and CO2/N2 selectivity with 280 % and 30 % up, respectively in the mixed gas permeation measurements (50/50 vol.). The resultant membrane performance surpassed the 2008 Robeson upper bound. Meanwhile, the stabilized MMM performance of up to 6 bar and high thermal stability of up to 350 °C bring extra benefits to the KAUST-8-based XLPEO membrane for post-combustion flue gas treatment.

Thin film composite nanofiltration membrane with nanocluster structure mediated by graphene oxide/metal-polyphenol nanonetwork scaffold interlayer

In this study, novel thin film composite (TFC) nanofiltration membrane with nanocluster structure was successfully synthesized by confining interfacial polymerization (IP) on graphene oxide/metal-polyphenol (GO/TA-Mn+) nanonetwork scaffold interlayer for the first time. By metal-polyphenol network coordination cross-linking, very stable GO/TA-Mn + nanonetwork scaffold was constructed via brush coating, which well-regulated the pore structure and distribution of the substrate, greatly restricting the invasion of polyamide (PA) matrix to the pore of substrate. Moreover, the GO/TA-Mn + modified substrate effectively mediated the amine monomers diffusion during interfacial polymerization, contributing to form an ultrathin PA layer (∼19.2 nm) with nanocluster structure. The separation performance therefore can feasibly be tuned by the metal ions of different valence states. The strategy also gives appropriate level of generality for synthesis of versatile defect-free TFC membranes on various substrates. The prepared TFC-GO/TA-Al3+ membrane held outstanding separation performance with water permeability of 16.87 L m−2 h−1 bar−1 and competitive rejection of 97.2% for Na2SO4, displaying the permeability increment of about 4.5 times without sacrificing salt rejection.

Synthesis and Characterization of PES/Pebax-MWCNTs Mixed Matrix Membranes for Gas Separation

The development of MWCNT mixed matrix membranes (MWCNT-MMMs) has shown good performance for gas separation. Interfacial interactions between the CNTs and the polymer and the dispersion of the MWCNTs in the polymeric matrix affects the overall performance of CNT mixed matrix membranes. Although CNTs properties are superior to other inorganic fillers but due to their chemically inert properties and inability to disperse in typical organic solvents, their applicability in industry is still uncertain. Typically, nanotubes are bound together by strong van der Waals interactions and form tight bundles. Therefore, MWCNTs tend to accumulate to each other in polymer matrix. This has been the biggest challenge in development of MWCNT-MMMs in producing defect free membranes. In this research, PES/Pebax-MWCNTs mixed matrix membraness were fabricated to capture carbon for natural gas. Sorbitol has been utilized in the fabrication of membranes to break the agglomeration of MWCNTs. In this study, the effect of MWCNTs loading and addition of sorbitol on the structure of synthesized membranes was analyzed using FESEM, DSC and FTIR. The incorporation of MWCNTs in the polymer matrix allows more active path for small molecules to flow across the membranes. The agglomeration of MWCNTs is more significant with the increase of fillers loading. Addition of sorbitol improved the morphology of membranes. Smooth surface of membranes with no agglomeration are observed for all MWCNTs loading (2-6 wt%). Glass transition temperature (Tg) was shifted to lower temperature with the addition of both MWCNTs and sorbitol.

Solvent induced effect on morphology and properties of disulfonated polyarylene ether sulfone block copolymer membranes for PEMFC applications

A systematic investigation to ascertain a fundamental understanding of the residual solvent effect on the bonding characteristics, thermal and mechanical properties of poly(arylene ether sulfone) (PAES) membranes was carried out. Transparent PAES membranes were synthesized using different solvents, which include dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), and N-methyl-2pyrrolidone (NMP). A correlation was established between polymer-solvent interaction using Hansen solubility parameters (HSP) and Flory-Huggins interaction parameters (χP-S). Proton conductivity, thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and FTIR were correlated with χP-S parameter. PAES membrane derived using DMF solvent exhibited the highest proton conductivity (0.133 ± 0.01 S∙cm−1). Scanning electron microscopy (SEM) confirmed the formation of defect-free membranes. The presence of residual solvent in the membranes resulted in a plasticizer effect which was confirmed through storage modulus behavior from DMA. This study will act as a standard protocol for the development of high-performance PAES membranes with desired properties for polymer fuel cells applications.

N-Oxide Zwitterion Functionalized Positively Charged Polyamide Composite Membranes for Nanofiltration.

N-Oxide zwitterionic polyethyleneimine (ZPEI), a new kind of aqueous phase monomer synthesized by commercially branched polyethyleneimine (PEI) via oxidation reaction, was prepared for fabrication of thin-film composite (TFC) polyamide membranes via interfacial polymerization. The main factors, including the monomer concentration and immersion time of the aqueous phase and organic phase, were investigated. Compared with PEI-TFC membranes, the obtained optimal defect-free ZPEI-TFC membranes exhibited a lower roughness (3.3 ± 0.3 nm), a better surface hydrophilicity, and a smaller pore size (238 Da of MWCO). The positively charged ZPEI-TFC membranes (isoelectric point at pH 8.05) showed higher rejections toward both divalent cationic (MgCl2, 93.0%) and anionic (Na2SO4, 96.1%) salts with a water permeation flux of up to 81.0 L·m-2·h-1 at 6 bar, which surpassed currently reported membranes. More importantly, mainly owing to N-oxide zwitterion with strong hydration capability, ZPEI-TFC membranes displayed a high flux recovery ratio (97.0%) toward a model protein contaminant (bovine serum albumin), indicating good anti-fouling properties. Therefore, the novel N-oxide zwitterion functionalized positively charged nanofiltration membranes provide an alternative for water desalination and sewage reclamation.

Establishing a high-performance anti-fouling PEI-ZIF-PAA membrane with improved Lewis acid-base interactions and hydrophilicity

Membrane fouling and the trade-off between membrane permeability and selectivity restrict the potential applications of membrane filtration for water treatment. ZIF-8 was found having great permeability and antibiofouling performance, but with issue on particle aggregation makes it difficult to achieve high ZIFs loading and fabricate a defect-free molecular sieving membrane in previous research. In this study, we formed a scalable antibiofouling surface with improved permeability and fouling resistance on a PEI-ZIF-PAA membrane using a layer-by-layer assembly technique. The synergistic effects of being sandwiched between two different polyelectrolyte layers with opposite charges endowed the ZIF nanoparticles with improved stability and scalability for membrane modification. The PEI-ZIF-PAA membrane exhibited a satisfactory water flux of 120.78 LMH, which was 46.97% higher than that of the pristine PES membrane. The normalized water flux loss was serious in the absence of ZIF-8, and the flux increased with the ZIF-8 concentration. Antifouling tests suggested that the PEI-ZIF-PAA membrane possessed good antifouling performance due to the much higher surface hydrophilicity and positive Lewis acid-base interactions with foulants. The HA rejection increased with the ZIF-8 concentration and reached a maximum of 92.1% in the presence of 1.00% (w/v) ZIF-8. The membrane regeneration was tested under physical and chemical cleaning with flux recovery rates of about 85% and 95%. XDLVO analysis showed that the total interaction energy between HA and the PEI-ZIF-8-PAA membrane was 26.45 mJ/m2, and the superior antifouling performance was mainly attributed to Lewis acid-base interactions. This study indicates that ZIF-8 nanocrystals are promising materials for fabricating novel membranes for sewage treatment.

Hetero-metal solution to construct high permselective zeolite membranes

Zeolite membranes possess great application prospects in purifying chemical substances, however, the current synthetic conditions are insufficient to eliminate defects, restricting the industrial applications of zeolite membranes. In this study, compact X-type zeolite membranes were synthesized in hetero-alkali-metal cation synthesis solutions via in-situ heating method. The obtained X-type zeolite membrane possessed a methanol/methyl tert-butyl ether (MTBE) separation factor greater than 10,000 at a permeation flux of 1.3 kg m−2 h−1. A possible mechanism for the influence of hetero alkali meatal cations on the formation of zeolite membranes was proposed, which involved structural induction and regulation of the rates of crystal nucleation and growth. The hetero-alkali-metal cations entering the zeolite crystal caused distortion of the crystal structure, which promoted transformation of the zeolite crystal structure and accelerated crystal growth. Meanwhile, the rates of crystal nucleation and growth on the surface of the support were regulated by the cooperative action of the hetero-alkali-metal cations and Na+ in the synthesis solution through their solubility for the aluminosilicate gel, leading to the formation of compact zeolite membranes. This method controlled the zeolite crystal structure and compactness of the zeolite membrane by simply adding hetero-alkali-metal cations to the synthesis solution, providing a new idea for designing defect-free membranes with high permselectivity.

Oxygen plasma-assisted contra-diffusion self-assembly of covalent organic framework pervaporation membranes for organic-solvent dehydration

Pervaporation is considered to have great potential for application in the field of organic-solvent dehydration. In this study, a type of keto-enamine-linked covalent organic framework TpPa (synthesized using the Schiff-base reaction of 1,3,5-triformylphloroglucinol [Tp] with p-phenylenediamine [Pa]), was fabricated on commercial polyacrylonitrile (PAN) ultrafiltration membranes through an oxygen-plasma assisted contra-diffusion self-assembly strategy for organic-solvent dehydration. The PAN membranes with a glycerol exhibited a better separation factor for dehydration of organic solvents than those without a glycerol layer. The influence of oxygen plasma and glycerol on the formation of the TpPa membrane was investigated. The best plasma treatment condition and optimum glycerol impregnation concentration were determined as a device power of 60 W and 30 wt%, respectively. Defect-free TpPa membrane formation took only 24 h. The as-prepared TpPa membrane exhibited a separation factor of 1491 and a high permeation flux of 2530 g m−2 h−1 for a 90 wt% isopropanol–water mixture. The separation performance of the TpPa membrane was stable for >100 h, exhibiting great potential in organic-solvent dehydration.

One-Step Synthesis of Ultrathin Zeolitic Imidazole Framework-8 (ZIF-8) Membrane on Unmodified Porous Support via Electrophoretic Deposition.

Metal-organic frameworks (MOFs) are regarded as the next-generation, disruptive membrane materials, yet the straightforward fabrication of ultrathin MOF membranes on an unmodified porous support remains a critical challenge. In this work, we proposed a facile, one-step electrophoretic deposition (EPD) method for the growth of ultrathin zeolitic imidazole framework-8 (ZIF-8) membranes on a bare porous support. The crystallinity, morphology and coverage of ZIF-8 particles on support surface can be optimized via regulating EPD parameters, yet it is still difficult to ensure the integrity of a ZIF-8 membrane with the constant voltage mode. In contrast, the constant current mode is more beneficial to the growth of a defect-free ZIF-8 membrane due to the steady migration rate of colloid particles toward the electrode. With a current of 0.65 mA/cm2 and deposition time of 60 min, a 300 nm thick ZIF-8 membrane was obtained, which exhibits a CO2 permeance of 334 GPU and a CO2/CH4 separation factor of 8.8, evidencing the defect-free structure.

Highly flexible and superhydrophobic MOF nanosheet membrane for ultrafast alcohol-water separation.

High-performance pervaporation membranes have potential in industrial separation applications, but overcoming the permeability-selectivity trade-off is a challenge. We report a strategy to create highly flexible metal-organic framework nanosheet (MOF-NS) membranes with a faveolate structure on polymer substrates for alcohol-water separation. The controlled growth followed by a surface-coating method effectively produced flexible and defect-free superhydrophobic MOF-NS membranes. The reversible deformation of the flexible MOF-NS and the vertical interlamellar pathways were captured with electron microscopy. Molecular simulations confirmed the structure and revealed transport mechanism. The ultrafast transport channels in MOF-NS exhibited an ultrahigh flux and a separation factor of 8.9 in the pervaporation of 5 weight % ethanol-water at 40°C, which can be used for biofuel recovery. MOF-NS and polydimethylsiloxane synergistically contribute to the separation performance.

Preparation and gas separation performance of seeding-free aqueous synthesis ZIF-8 membrane by homologous-like ligands method

Due to the poor heterogeneous nucleation properties of MOFs crystal on ceramic substrates resulting from the absence of interaction of MOFs with support substrate surface, it is usually difficult to prepare well-intergrown MOF membranes on substrate by simple in situ growth. In this work, a novel synthesis strategy was used to prepare defect-free ZIF-8 membranes on α-Al2O3 supports by depositing gold particles modified with 2-mercaptoimidazole which creates pseudo-surfaces to bridge the metal centers with support. The end groups of the 2-mercaptoimidazole guides the heterogeneous nuclei to the support surface as homologous-like ligands. Furthermore, investigation of the metal sources revealed that the ZIF-8 membranes prepared with Zn(OAc)2 as the zinc source possessed more continuous and well-intergrown ZIF-8 membranes with a thickness of about 1 μm compared to other zinc sources. The results of gas separation performance show that the optimized ZIF-8 membranes possess high hydrogen permeance and thermal stability. The gas separation selectivity for binary H2/CH4 mixture reached 20.8 at 150 °C and 1 bar, with a high H2 permeance of about 6.15 × 10−7 mol m−2 s−1Pa−1, which is promising for hydrogen separation and purification. Moreover, free-organic solvents and room temperature growth by our strategy dramatically reduces the difficulty of preparation and environmental destruction, thus offering a new avenue to prepare MOFs membrane with good separation performance.