Selective Top Layer Research Articles

Catalytic membrane with dual-layer structure for ultrafast degradation of emerging contaminants in surface water treatment

The catalytic membrane-based oxidation-filtration process integrates physical separation and chemical oxidation, offering a highly efficient water purification strategy. However, the oxidation-filtration process is limited in practical applications due to the short residence time of milliseconds within the catalytic layer and the interference of coexisting organic pollutants in real water. Herein, a dual-layer membrane containing a top selective layer and a bottom catalytic layer was fabricated using an in situ co-casting method with a double-blade knife. Experimental results demonstrated that the selective layer rejected macromolecular organic pollutants, thereby alleviating their interference with bisphenol A (BPA) degradation. Concurrently, the catalytic layer activated peracetic acid oxidant and achieved a high BPA degradation exceeding 90 % in milliseconds with reactive oxygen species (especially •OH). The finite-element analysis confirmed a high-concentration reaction field occupying the pore cavity of the catalytic layer, enhancing collision probability between reactive oxygen species and BPA, i.e., the nano-confinement effect. Additionally, the dual-layer membrane achieved a long-term stable performance for emerging contaminant degradation in surface water treatment. This work underscores a novel catalytic membrane structure design for high-performance oxidation-filtration processes and elucidates its mechanisms underlying ultrafast degradation.

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives.

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Towards fully epoxy-based thin film composite membranes for solvent-resistant and solvent-tolerant nanofiltration

Epoxy curing is a versatile chemistry that can be applied in membrane preparation, both via phase inversion and interfacial polymerization. Here, fully epoxy-based TFC membranes are demonstrated for the first time and tested for solvent-tolerant nanofiltration (STNF) applications. First, UF membranes with a retention for bovine serum albumin (BSA, ∼66000 g mol−1) of ∼40% and a water permeance of 171 L m−2 h−1 bar−1 were prepared via phase inversion by pre-curing epoxide resins (EPON™ resin 1009F and EPON™ resin SU-8) with 1,6-hexanediamine (HDA) prior to phase inversion in water. Subsequently, selective top layers were synthesized on these supports layers via interfacial polymerization (IP) or interfacial initiation of polymerization (IIP) of the same epoxy resins, resulting in a thin selective top layer of the same material. By using different amines, different epoxide ring-opening polymerizations (ROP) could be targeted. These TFC membranes were then tested for use in water-solvent mixtures and achieved a 92.8% methyl orange (MO, 327 g mol−1) retention and a permeance of 0.21 L m−2 h−1 bar−1 in a 20/80 DMF/water feed solution. Cured epoxy chemistry is known for its excellent chemical stability, making these membranes suitable for a wide variety of solvent-resistant NF (SRNF) and STNF applications.

Comparison of hydrogen permeation and structural properties of a microporous silica membrane and a dense BaCe0.9Y0.1O3-δ (BCY) perovskite membrane

Silica composite membranes and BaCe0.9Y0.1O3-δ (BCY) perovskite membranes were successfully synthesized to separate hydrogen in an equimolar mixture of H2, CH4, CO, and CO2 at temperature range of 500–900 °C and pressure difference of 1 bar. The phase structure of both membranes was characterized by X-ray diffraction (XRD). Thermogravimetric analysis (TGA) was used to evaluate phase stability of perovskite membrane. FESEM images confirmed graded structure of silica membrane and uniform, dense structure of perovskite membrane. H2 permeation in semi-dense silica layer deposited on alumina substrate indicated that permeation in top selective layer follows a diffusion mechanism which is based on jumps between solubility sites. On the other hand, low permeation rates of around 10−8 mol m−2 s−1 Pa−1 in perovskite membranes revealed a proton-electron conductivity mechanism which occurs through dense structures. Increasing hydrogen separation factor (SF) in gas mixture by increasing deposition time from 3.5 h to 6 h in silica composite membrane confirms formation of crack-free selective layer; however, this factor is still lower than SF of 0.97–0.99 in perovskite membranes. A set of gas-permeability data is collected at the laboratory scale for the statistical characterization of both membrane types (P01, S01) to provide a dataset from which one can assess statistical scaling features displayed by the data and their scaling increments.

Enhancing interfacial interaction of PDMS matrix with ZIF-8 via embedding TiO2@ZIF-8 composites for phenol extraction in aqueous-aqueous membrane extractive process

The presence of phenol as an organic pollutant in industrial effluents is the biggest threat to the aquatic environment and human health. The aqueous-aqueous extraction process is an attractive method to treat saline phenolic wastewater (feed solution) via diffusion through a semi-permeable polydimethylsiloxane (PDMS) to the receiving solution. To improve the value of mass transfer coefficient, thinning the PDMS layer and embedding it with nanoparticles/metal-organic frameworks (MOFs) can lead to membrane defects or weakened mechanical strength, which is not promising for large-scale practical applications. Therefore, herein, an interfacial design strategy is demonstrated to disperse TiO2@ZIF-8 composites as hybrid particles into the top selective layer (PDMS) of TFC (thin-film composite) membrane to enhance polymer-filler compatibility and mass transfer coefficient. The TiO2@ZIF-8 composites enhanced the affinity to the polymer matrix, thereby increasing the mechanical stability and preventing interfacial voids and agglomeration. Moreover, the TiO2@ZIF-8 incorporated TFC membranes exhibited an enhanced mass transfer coefficient (ko) of (25.28 ± 0.5) × 10−7 and a reduced reverse salt flux of 9.5 mg/m2.h. The enhanced mechanical stability and mass transfer coefficient were attributed to the synergistic effect provided by TiO2@ZIF-8 composites, as ZIF-8 particles provided continuous pathways (pore-flow mode) and TiO2 nanoparticles provided compactivity with PDMS. The synergistic effects of hybrid particles could provide a new strategy to develop PDMS for the extraction of various organic compounds.

Effect of graphene oxide in the formation of polymeric asymmetric membranes via phase inversion

Membrane morphology of integrally skinned asymmetric membranes made by phase inversion can be tailored by precise control of the thermodynamic properties of the casting solution. In this work, the morphology of polyethersulfone (PES) and polyvinylidene fluoride (PVDF) membranes containing graphene oxide (GO) and their phase separation behavior have been studied. The Flory-Huggins theory and the Hansen solubility parameters have been used to construct theoretical binodal lines for ternary and quaternary systems, which have been validated by conducting cloud titration experiments. Moreover, the effect of different solvents on the morphology of PES membranes is discussed through the thermodynamic and kinetic properties of the casting solutions. Nontoxic solvents dimethyl sulfoxide (DMSO) and dihydrolevoglucosenone (Cyrene™) induce PES/GO membranes with finger-like pores and cellular voids, respectively. The addition of GO in the PES and the PVDF casting solutions increases their thermodynamic instability, leading to thinner selective top layers and higher porosities, up to a point where the high viscosity of the GO-containing solutions hinders the solvent-nonsolvent demixing. The highest porosity and pure water flux (PWF) occurs at 0.1 wt% GO for PES/GO/DMSO and at a loading of 0.3 wt% GO for systems PES/GO/Cyrene and PVDF/GO.

Preparation and modification of thin film composite membrane using a bulky dianhydride monomer

AbstractOne of the most effective methods to modify thin film composite (TFC) membranes is changing the chemistry of top selective layer by different monomers and different monomer concentrations. Herein, we report the preparation of modified TFC membranes using a pyromellitic dianhydride (PMDA) mixed with organic phase (trimesoyl chloride) and meta phenylene diamine (MPD). By manipulating the PMDA amount in organic phase, the structures and chemical compositions of polyamide selective layer could be modified. It was realized that the presence of PMDA could result in a modified membrane with higher surface roughness, less dense selective layer, more surface charge density, and better hydrophilic properties and consequently less fouling. The optimum PMDA concentration was found 0.05 wt%, such that the obtained membrane had 35.6 L m−2 h−1 pure water flux, about 1.6‐fold higher than the reference membrane with similar salt rejection. Fouling intensity for the reference membrane was 38.1%, while for the modified membranes it decreased to 16.7%.

Amine-based vapor-phase crosslinking processes of polyimide-based membranes for energy efficient separations

Membrane processes are crucial in the industrial and engineering chemistry, for separation and purification of not only chemicals but also gases and wastewater treatment. In order to obtain high quality products from membrane processes, the membranes must be engineered to cater such performance, namely by chemical crosslinking process. One of the emerging crosslinking methods is the vapor phase crosslinking (VPC). The advantages of VPC for enhancing membranes are usage of fewer amounts of chemicals, with specific crosslinking of the top selective layer only (no unnecessary crosslinking of bulk layer). Therefore, membranes can boost their quality without sacrificing much of their productivity (quantity of flux). In this paper, several utilization of amine molecules for VPC of polyimide membranes for gas separations, pervaporation (gas-liquid separation), and organic solvent nanofiltration (OSN) are covered. The performance enhancements of the polymeric membranes for the aforementioned processes are summarized. Furthermore, the molecular mechanisms of the VPC of the amine-based vapors are reviewed, based on X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared characterizations. It was found that there are several possible scenarios of interaction between amines and polyimides, which are not only involving crosslinking (combination of several polymeric chains), but also grafting, and sometimes chain scissions. The investigation of molecular mechanism of the amine-based VPC process provides insights to the relationship between structural configurations towards the separation performance of various types of membrane processes, which paves way for the development of membranes of the future.

Ultrapermeable Composite Membranes Enhanced Via Doping with Amorphous MOF Nanosheets.

Thin-film composite (TFC) polymeric membranes have attracted increasing interest to meet the demands of industrial gas separation. However, the development of high-performance TFC membranes within their current configuration faces two key challenges: (i) the thickness-dependent gas permeability of polymeric materials (mainly poly(dimethylsiloxane) (PDMS)) and (ii) the geometric restriction effect due to the limited pore accessibility of the underlying porous substrate. Here we demonstrate that the incorporation of trace amounts (∼1.8 wt %) of amorphous metal–organic framework (MOF) nanosheets into the gutter layer of TFC assemblies can simultaneously address these two limitations by the creation of rapid, transmembrane gas diffusion pathways. The resultant PDMS&MOF membrane displayed excellent CO2 permeance of 10450 GPU and CO2/N2 selectivity of 9.1. Leveraging this strategy, we successfully fabricate a novel TFC membrane, consisting of a PDMS&MOF gutter and an ultrathin (∼54 nm) poly(ethylene glycol) top selective layer via surface-initiated atom transfer radical polymerization. The complete TFC membrane exhibits excellent processability and remarkable CO2/N2 separation performance (1990 GPU with a CO2/N2 ideal selectivity of 39). This study reveals a strategy for the design and fabrication of a new TFC membrane system with unprecedented gas-separation performance.

Hierarchy Control of MFI Zeolite Membrane towards Superior Butane Isomer Separation Performance

AbstractMicrostructural optimization (such as thickness and preferred orientation) is a major concern for performance enhancement of zeolite membranes. In this study, we demonstrated that the introduction of hierarchy easily enabled concurrent thickness reduction and orientation control of zeolite membranes. Specifically, hierarchical MFI zeolite membranes comprising higher degree of (h0h) preferentially oriented ultrathin (ca. 390 nm) selective top layers and porous intermediate layers on porous α‐Al2O3substrates were fabricated. The use of hollow‐structured MFI nanoseeds and the employment of single‐mode microwave heating during membrane processing were found indispensable for the preparation of MFI zeolite membranes with superior butane isomer separation performance, thereby surpassing the currentn‐/i‐butane selectivity versusn‐butane permeance trade‐off limits of MFI zeolite membranes prepared via solution‐based synthetic protocols.

Hierarchy Control of MFI Zeolite Membrane towards Superior Butane Isomer Separation Performance.

Microstructural optimization (such as thickness and preferred orientation) is a major concern for performance enhancement of zeolite membranes. In this study, we demonstrated that the introduction of hierarchy easily enabled concurrent thickness reduction and orientation control of zeolite membranes. Specifically, hierarchical MFI zeolite membranes comprising higher degree of (h0h) preferentially oriented ultrathin (ca. 390 nm) selective top layers and porous intermediate layers on porous α-Al2 O3 substrates were fabricated. The use of hollow-structured MFI nanoseeds and the employment of single-mode microwave heating during membrane processing were found indispensable for the preparation of MFI zeolite membranes with superior butane isomer separation performance, thereby surpassing the current n-/i-butane selectivity versus n-butane permeance trade-off limits of MFI zeolite membranes prepared via solution-based synthetic protocols.

Preparation of amorphous polyphenylsulfone nanofiltration membrane via thermally-induced lamination

A novel approach to the fabrication of thin-film nanofibrous (TFN) membranes with high productivity was proposed, and a polyphenylsulfone (PPSU) TFN membrane was fabricated to address the low water permeability of PPSU membranes. The fusion of fibers with each other and with the selective top layer, which was evident in the scanning electron microscopy (SEM) images, gave rise to the structural stability of the membrane and enhanced permeability. Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), thermogravimetric analysis (TGA), and tensile test were applied to study the membranes in terms of their properties. The efficiency of the PPSU TFN membranes was assessed in the filtration of an industrial wastewater and also a model foulant. Compared with a conventional PPSU membrane, the TFN membrane provided almost 2 times higher pure water flux (PWF) as well as about 4 times higher steady permeate flux without sacrificing the rejection percent.

Influence of providing a three-layer spectrally selective floating absorber on passive single slope solar still productivity under tropical conditions

Solar still with a novel floating absorber made of three layers is experimentally investigated under the climatic conditions of South India. The three-component floating absorber comprises of high absorptivity-low emissivity plasmonic micro-nano sized copper coated spectrally selective top layer, bio-derived carbon foam as insulating bottom layer and cotton fabric wrapped around the Carbon foam as the middle layer. In comparison to an Aluminium floating absorber based solar still, the three-layer floating absorber still displayed excellent solar thermal conversion with a maximum efficiency of 88.6%. It also showed the potential to demonstrate high sensitivity and quick response. The proposed absorber attained its maximum temperature in 15 min from the time of recording maximum solar intensity as against 155 min for Aluminium absorber. The experiments reveal and validate for the first time, the significance of vapour space temperature and internal relative humidity on the productivity of solar stills operating with a floating absorber. Vapour space temperature relatively increased up to three times for the three-layer floating absorber based still which adversely affected its productivity and daily still efficiency despite its better evaporation potential. The cost of water production increased by 14.7% as a consequence of high vapour space temperature.

Recent Development in Composite Membranes for Flow Batteries.

Flow batteries (FBs) are one of the most attractive candidates for stationary energy storage and vital in realizing the wide application of renewable energies. Membranes play an important role in isolating redox couples while transporting ions to close the internal electrical circuit. Therefore, membranes with high selectivity and conductivity are highly important. Among different membranes, a composite membrane with independent design of support layer and thin selective top layer becomes one of the most promising candidates to break the trade-off between selectivity and conductivity. In this Review, recent studies on composite membranes for FBs and the principles of membrane design in different systems are discussed and summarized. Finally, the future direction on membrane design for different FBs is presented, which will provide an extensive, comprehensive reference to design and construct high-performance composite membranes for FBs.

Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures.

Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to determine the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300 °C) temperatures and bottom layer replacement for complete conversion to WSe2. Atomic-resolution electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Molecular dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable composition.

Atomic scale manipulation of sublayer with functional TiO2 nanofilm toward high-performance reverse osmosis membrane

Recently, engineering the sublayer of polyamide thin film composite (PA-TFC) membrane by functional nanomaterials at nanoscale has been developed as an effective strategy to control interfacial polymerization reaction to fabricate high-performance reverse osmosis (RO) membrane. However, a reliable and practical manipulation technique is still needed to overcome the limitation of conventional sublayers. In this work, an approach of using atomic layer deposited (ALD) monomer-affinitive titanium dioxide (TiO2) nanofilm to modify the sublayer of TFC was proposed to form a thin, smooth, and highly cross-linked PA selective top layer. The functional TiO2 nanofilm increases the affinity between modified sublayer and amine monomer in favor of the efficient and subtle tuning of the adsorption and diffusion of amine monomer during the interfacial polymerization process. The obtained TFC membrane with optimal ALD TiO2 coverage demonstrates well-improved RO performance in terms of high permeance of 1.8 L.m−2.h−1.bar−1 and high salt rejection rate of 96.1% in a dead-end process. This work reveals that coating functional nanomaterials by ALD is a practical manipulation technique toward the controllable fabrication of promising TFC membrane and the optimization of sublayer materials.

CuBTC metal organic framework incorporation for enhancing separation and antifouling properties of nanofiltration membrane

Novel thin film nanocomposite (TFN) nanofiltration membrane with tunable physico-chemical properties and separation performances was fabricated by incorporating the copper benzene-1,3,5-tricarboxylate (CuBTC) nanoparticles with different concentrations (ranging from 0 to 0.75wt.%) in the polysulfone (PSf) substrates, followed by the interfacial polymerization process of trimesoyl chloride (TMC) and piperazine (PIP) to establish top selective layer. Charaterization results show that both chemical and physical properties of poly(piperazineamide) selective layer was altered when PSf substrate was modified by CuBTC. The introduction of CuBTC nanoparticles improved the hydrophilicity of the TFN membranes (from 70.25° to 59.02°) and promoted formation of more linear structure of poly(piperazineamide) entangled with −COOH pendant groups. By incorporating 0.25wt.% of CuBTC into the PSf substrate, the resultant membrane flux was enhanced by 22% with MgSO4 rejection remained at 97.31%. Furthermore, a notable increment of rejection against NaCl could be attained by increasing the CuBTC content in the substrate. This could be explained by the Donnan potential effect occurred on the more linear structures of poly(piperazineamide) surfaces, which results in an increase in the selectivity of monovalent salts. Moreover, the incorporation of CuBTC rendered the TFN membranes to exhibit good anti-fouling property against bovine serum albumin.

A theoretical study of permeability enhancement for ultrafiltration ceramic membranes with conical pores and slippage

Ceramic membranes are currently favourable in membrane filtration applications due to their excellent mechanical strength, thermal and chemical resistance, backflush capability, and thus a long-service cycle. Coated on top of a mesoporous support, the selective top layer of ultrafiltration ceramic membranes has pore size not exceeding a few tens of nanometers and thickness in the order of O10 μm. In fact, the permeability of an ultrafiltration ceramic membrane can be estimated by the permeability of the top layer due to its smallest pore size. Without impairing the filtration function but still improving the permeability, a gradient conical pore shape is proposed. Two formulae for the filtrate flow rate versus pressure drop relationship through a conical pore exhibiting surface slippage are established here by extending the Hagen-Poiseuille law and an analytical solution for the axisymmetric creeping flow. It is analytically proved that the surface slip length in a conical flow is proportional to a local pore radius by a slip coefficient that is unique for a given pore configuration at a prescribed flow rate. The permeability of a conical-pore membrane is enhanced for radius ratio not exceeding 6.5. The optimum configuration, achieved at a ratio of 2.3, produces an enhancement factor for a membrane permeability of 1.5 for a no-slip surface; this enhancement increases linearly with the slip coefficient if a surface slippage exists.

Hyperfast Water Transport through Biomimetic Nanochannels from Peptide-Attached (pR)-pillar[5]arene.

Synthetic water channels offer great promise to replace natural aquaporins (AQPs) for making new-generation biomimetic membranes for water treatment. However, the water permeability of the current synthetic water channels is still far below that of AQPs. Here, peptide-attached (pR)-pillar[5]arene (pR-PH) channels are reported to mimic the high permeability of AQPs. It is demonstrated that the pR-PH channels with an open pore can transport water smoothly and efficiently. The pR-PH channels are competitive with AQPs in terms of water permeability and are much superior to diastereomer peptide-attached (pS)-pillar[5]arene (pS-PH) and other reported synthetic water channels. The exceptional water-transport properties of the pR-PH channels are further demonstrated in a composite polymeric membrane that incorporates the nanochannels into the top selective layer. This membrane gives a significantly improved water flux while retaining high salt rejection. The results establish a tangible foundation for developing highly efficient artificial water channel-based biomimetic membrane for water purification applications.

Improved operational stability of Pebax-based gas separation membranes with ZIF-8: A comparative study of flat sheet and composite hollow fibre membranes

Polymer-based hollow fibre composite membranes are composed of a porous support coated with a highly permeable gutter layer, and a thin selective top layer. They are promising candidates for industrial gas separation, but there are challenges in maintaining their performance under high pressure or during long-term operation. To tackle this, we incorporated ZIF-8 into Pebax-1657 (polyamide-polyether copolymer) for gas separation membrane fabrication. To fully understand the behaviour of hollow fibre composite membranes, special attention was paid to the effect of ZIF-8 on the Pebax chain structure using dense film flat sheet membranes. The presence of porous ZIF-8 disrupted the original well-packed polymer chain structure, leading to higher fractional free volume and gas permeability. However, the introduced microvoids were non-continuous and the gas selectivity was only slightly reduced. On the other hand, the stable structure of ZIF-8, together with the hydrogen bonds between ZIF-8 organic ligands and polyamide chains, significantly improved the linear glassy polymer chain stiffness, ensuring good operational stability under elevated pressure for both flat sheet and hollow fibre membranes. Finally, good long-term operational stability was observed with the hollow fibre membrane, indicating the ZIF-8/Pebax coating overcame the aging previously observed for the poly[1-(trimethylsilyl)-1-propyne] (PTMSP) gutter layer.