Ultrathin Selective Layer Research Articles

Solvent-Less Vapor-Phase Fabrication of Membranes for Sustainable Separation Processes

Abstract Sustainable processes for purifying water, capturing carbon, producing biofuels, operating fuel cells, and performing energy-efficient industrial separations will require next-generation membranes. Solvent-less fabrication for membranes not only eliminates potential environmental issues with organic solvents, but also solves the swelling problems that occur with delicate polymer substrates. Furthermore, the activation procedures often required for synthesizing microporous materials such as metal–organic frameworks (MOFs) can be reduced when solvent-less vapor-phase approaches are employed. This perspective covers several vacuum deposition processes, including initiated chemical vapor deposition (iCVD), initiated plasma-enhanced chemical vapor deposition (iPECVD), solvent-less vapor deposition followed by in situ polymerization (SLIP), atomic layer deposition (ALD), and molecular layer deposition (MLD). These solvent-less vapor-phase methods are powerful in creating ultrathin selective layers for thin-film composite membranes and advantageous in conformally coating nanoscale pores for the precise modification of pore size and internal functionalities. The resulting membranes have shown promising performance for gas separation, nanofiltration, desalination, and water/oil separation. Further development of novel membrane materials and the scaling up of high-throughput reactors for solvent-less vapor-phase processes are necessary in order to make a real impact on the chemical industry in the future.

Synthesis, Transfer, and Gas Separation Characteristics of MOF-Templated Polymer Membranes.

This paper discusses the potential of polymer networks, templated by crystalline metal–organic framework (MOF), as novel selective layer material in thin film composite membranes. The ability to create mechanically stable membranes with an ultra-thin selective layer of advanced polymer materials is highly desirable in membrane technology. Here, we describe a novel polymeric membrane, which is synthesized via the conversion of a surface anchored metal–organic framework (SURMOF) into a surface anchored gel (SURGEL). The SURGEL membranes combine the high variability in the building blocks and the possibility to control the network topology and membrane thickness of the SURMOF synthesis with high mechanical and chemical stability of polymers. Next to the material design, the transfer of membranes to suitable supports is also usually a challenging task, due to the fragile nature of the ultra-thin films. To overcome this issue, we utilized a porous support on top of the membrane, which is mechanically stable enough to allow for the easy membrane transfer from the synthesis substrate to the final membrane support. To demonstrate the potential for gas separation of the synthesized SURGEL membranes, as well as the suitability of the transfer method, we determined the permeance for eight gases with different kinetic diameters.

Fine-tuning selective layer architecture of hydrogel membrane towards high separation performances for engineered osmosis

Ultrathin and/or ultrasmooth selective layer is one of the paramount goals in membrane realm for maximizing separation efficiency and/or minimizing fouling tendency. Towards this goal, the architecture of hydrogel selective layer is finely tuned for the first time for improving engineered osmosis (EO) membrane performance. Through delicately controlling synthesis parameters, ultrathin selective layer as thin as 30 nm, and ultrasmooth selective layer with sub-1 nm roughness (the smoothest EO membrane in literature) are successfully synthesized respectively. Analysis of reverse osmosis (RO) experimental results reveals hydrogel layer resistance to water permeation is linearly reduced by 1.40 × 1013 m−1 as the layer is tailored thinner per 10 nm, which leads to the remarkable enhancement of water permeability by ~10 times from 0.49 L m−2 h−1 bar−1 of 500 nm thickness to 4.75 L m−2 h−1 bar−1 of 30 nm thickness. Pressure-retarded osmosis (PRO) and forward osmosis (FO) tests indicate 45-nm-thick hydrogel layer achieves the maximum separation efficiency in terms of specific water flux (JW/JS). Moreover, the mechanism for tuning hydrogel layer architecture is discussed on the basis of microscopic characterizations. This study sheds new light on ultrathin and ultrasmooth selective layer for promoting EO membrane to smartly tackle different kinds of wastewater.

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.

High‐Performance CO2 Capture through Polymer‐Based Ultrathin Membranes

AbstractThin film composite (TFC) membranes have attracted great research interest for a wide range of separation processes owing to their potential to achieve excellent permeance. However, it still remains challenging to fully exploit the superiority of thin selective layers when mitigating the pore intrusion phenomenon. Herein, a facile and generic interface‐decoration‐layer strategy collaborating with molecular‐scale organic–inorganic hybridization in the selective layer to obtain a high‐performance ultrathin film composite (UTFC) membrane for CO2 capture is reported. The interface‐decoration layer of copper hydroxide nanofibers (CHNs) enables the formation of an ultrathin selective layer (≈100 nm), achieving a 2.5‐fold increase in gas permeance. The organic part in the molecular‐scale hybrid material contributes to facilitating CO2‐selective adsorption while the inorganic part assists in maintaining robust membrane structure, thus remarkably improving the selectivity toward CO2. As a result, the as‐prepared membrane shows a high CO2 permeance of 2860 GPU, superior to state‐of‐the‐art polymer membranes, with a CO2/N2 selectivity of 28.2. The synergistic strategy proposed here can be extended to a wide range of polymers, holding great potential to produce high‐efficiency ultrathin membranes for molecular separation.

Metal-induced ordered microporous polymers for fabricating large-area gas separation membranes.

Metal-induced ordered microporous polymers (MMPs), a class of porous polymer, are synthesized from amine-bearing polymers, small organic linkers and divalent metal ions using a polymer-directed chemical synthesis process. Specifically, small organic linkers first coordinate to metal ions, with the resulting unit cells then self-assembling along the extension of polymer chains to construct three-dimensional frameworks. The MMPs demonstrate good controllability of crystal and framework size, as well as hydrolytic stability. MMP dispersions were coated on a modified polysulfone substrate to fabricate MMP/mPSf membranes with an ultrathin selective layer (below 50 nm) and surface areas of >100 cm2. The MMPs are readily fabricated into defect-free thin selective-layered membranes with high CO2 permeance (3,000 GPU) and stable CO2/N2 selectivity (78) under both humid and dry gas feed conditions, demonstrating promising CO2 membrane separation performance. This synthetic methodology could be extended to other polymers, potentially enabling facile synthesis of membrane materials.

Ultrathin gutter layer for high-performance thin-film composite membranes for CO2 separation

Ultrathin film composite (TFC) membranes with selective layers less than 100 nm thick are highly desired to maximize the permeance of gas separation membranes for high energy efficiency. For membranes with ultrathin selective layers, a gutter layer is usually required to prevent pore penetration in the selective layers. Also, according to a recent model, the introduction of a gutter layer strongly improves TFC membrane performance by increasing the overall membrane performance up to an order of magnitude. Unfortunately, this improvement comes with an undesired decrease in selectivity unless the gutter layer is properly designed. This study found that the gutter layer permeability should be five- to tenfold that of the selective layer to minimize the decrease in selectivity. However, the most commonly used material for the gutter layer, polydimethylsiloxane, does not meet the requirements for high-performance membrane materials. Thus, here we report a gutter material with CO2 permeance sixfold that of polydimethylsiloxane, and prepare a TFC membrane with ultrathin gutter (75 nm) and selective layers (70 nm) for CO2/N2 separation. This work represents a guideline for developing next-generation TFC membranes and provides a comprehensive understanding of the impact of the gutter layer.

Ultralarge Single-Layer Porous Protein Nanosheet for Precise Nanosize Separation.

Highly permeable and precisely size-selective membranes are the subject of continuous pursuit for energy-efficient separation of fine chemicals. However, challenges remain in the fabrication of an ultrathin selective layer with homogeneous pores, in particular, with the pore sizes in the 1-10 nm range. We report the design of a free-standing porous nanosheet assembled with a single layer of proteins. Tobacco mosaic virus mutant (TMVm), a cylinder-shaped protein containing an inner pore of 4 nm in diameter, was cross-linked via a Cu2+-catalyzed disulfide-bond-forming reaction along the 2D orientation. By such a design, ultralarge single-layer TMVm nanosheets extending over tens of micrometers in width and with well-defined nanopores were successfully developed. A ∼40 nm thick ultrafiltration membrane laminated by the single-layer TMVm nanosheets through simple vacuum filtration accomplished the precise separation of ∼4 nm sized substances. Meanwhile, the membrane exhibited water permeance up to ∼7000 L m-2 h-1 bar-1, which is an order of magnitude improvement compared with traditional ultrafiltration membranes with a similar rejection profile.

Pebax/ionic liquid modified graphene oxide mixed matrix membranes for enhanced CO2 capture

The development of mixed matrix membranes (MMMs) is mostly challenged by the filler dispersion and the fabrication of defect-free membranes with an ultra-thin selective layer. Graphene oxide based MMMs are promising materials for gas separation application. The low filler content and preference of extended GO lamella to align perpendicular to a membrane surface, and hence the gas flow direction permits the development of thin film composite membrane (TFC). Here, facilitated transport MMMs were fabricated by incorporating ionic liquid functionalized graphene oxide (GO-IL) into poly(ether-block-amide) (Pebax 1657). The 1-(3-aminopropyl)-3-methylimidazolium bromide ionic liquid was reacted to graphene oxide sheets, enhancing the CO2 solubility and CO2/gas selectivity of the MMMs. Moreover, hydrogen bonding interactions between the ionic liquid and amide moieties in Pebax provide a homogeneous dispersion of GO-IL. The pure (H2, CO2, O2, N2, CH4) and mixed (CO2/H2, CO2/N2) gas permeability of the membranes were performed at 25 °C and 4 bar. The gas permeability measurements indicate an improvement of over 90% in CO2/N2 selectivity and 50% in CO2 permeability for the GO-IL MMMs compared to the pure Pebax membrane. The resulting TFC membranes showed high CO2 permeance up to 900 GPU (10−6 cm3 (STP) cm−2 s−1 cm Hg−1) and the CO2/N2 and CO2/H2 selectivities of about 45 and 5.8, respectively. Our finding underlines the importance of GO-IL nanosheet to design high selective thin film membranes, providing a direction to fulfil the concept of mixed matrix membranes for practical applications.

Ultrathin graphene oxide membranes on freestanding carbon nanotube supports for enhanced selective permeation in organic solvents

Among the various factors required for membranes in organic solvent separations, the stability of membrane supports is critical in the preparation of membranes with universal chemical stability, mechanical flexibility, and high flux. In this study, nanoporous freestanding carbon nanotube (CNT) films were fabricated and utilized as supports for enhanced permeation in organic solvents. The excellent chemical stability of the CNT support allowed it to withstand various organic solvents such as toluene, acetone, and dimethylformamide. In addition, the structural stability and high pore density of CNT supports allowed the deposition of an ultrathin selective layer for an enhanced-flux membrane. Membrane performance was demonstrated by depositing a thin graphene oxide (GO) layer on the CNT support; GO was selected because of its high chemical stability. CNT-supported GO membranes effectively blocked molecules with molecular weight larger than ~800 g mol−1 while allowing the fast permeation of small molecules such as naphthalene (permeation was 50 times faster than that through thick GO membranes) and maintaining selective permeation in harsh solvents even after 72 hours of operation. We believe that the developed CNT support can provide fundamental insights in utilizing selective materials toward organic solvent membranes.

Highly permeable thermally rearranged polymer composite membranes with a graphene oxide scaffold for gas separation

A 2D scaffold of graphene oxide is formed inside a polymer to assist the fabrication of a defect-free and ultrathin (<40 nm) selective layer of thermally rearranged polybenzoxazole-co-imide membrane for energy-efficient CO2 separation.

Polyethersulfone/Graphene Oxide Ultrafiltration Membranes from Solutions in Ionic Liquid

ABSTRACTNovel high flux polyethersulfone (PES) ultrafiltration membranes were fabricated by incorporating different amounts of graphene oxide (GO) sheets to PES as nanofillers. The membranes were prepared from solutions in 50/50 1-ethyl-3-methylimidazolium-diethylphosphate/N,N-dimethyl formamide. It was observed that the water permeance increased from 550 to 800 L m-2h-1bar-1, with incorporation of 1 wt% GO, keeping a molecular weight cut-off (MWCO) of approximately 32-34 kg mol-1. Cross-sectional scanning electron microscopy images of GO/PES membranes showed the formation of ultrathin selective layer unlike pristine membranes. Contact angle measurements confirmed the increase of hydrophilicity, by increasing the GO concentration. The rejection of humic acid and bovine serum albumin was demonstrated. The mechanical properties were improved, compared with the pristine membranes. The performance was just above the trade-off relationship between permeance and separation factor for PES membranes reported in the literature.

Cellulose nanofiber intermediary to fabricate highly-permeable ultrathin nanofiltration membranes for fast water purification

Highly-permeable nanofiltration membranes are highly desired in water production and dissolved solutes removal due to environmental and energy concerns. Here a facile approach is presented to prepare ultrathin polymeric nanofiltration membrane using surface modification of ultrafine cellulose nanofiber (UCN) membrane via interfacial polymerization. The hydrophilic UCN membrane endows an interconnected nanoporous microstructure containing free spaces for the growth of the crosslinked-PEI layer creating narrow permeation channels that are responsible for the transport of water moleucles. The resultant membranes comprising an ultrathin selective layer intertwined with cellulose nanofiber matrix are smooth and allow fast permeation of water. Typically, the 77.4nm-thick membrane with a mean pore size of about 0.45nm and molecular weight cut-off of 824gmol−1 has a high pure water flux of 32.7Lm−2h−1bar−1 that is an order of magnitude higher than those of previously reported similar nanofiltration membranes. The membranes are also positively charged and display high permeation flux and sufficient rejections for inorganic salts and organic dyes in the water purification. Further performance evaluation using model synthetic wastewater demonstrates the membranes have a great potential in the wastewater treatment. This approach presents a promising strategy for the development of highly-permeable nanofiltration membranes for fast water purification and high-efficient separation of small molecules.

Asymmetrically porous anion exchange membranes with an ultrathin selective layer for rapid acid recovery

Diffusion dialysis for acid recovery from acidic waste solution suffers from the low process capacity due to low proton permeability of current dense anion exchange membranes. In this work, asymmetrically porous ultrafiltration membranes with a thin skin layer (e.g., <1 micrometer) were converted to diffusion dialysis membranes with a hierarchically porous structure after in situ crosslinking and quaternization. Except for good thermal and chemical stabilities, the resulting membranes exhibited an acid permeability coefficient as high as 0.041–0.062mh−1 and a separation factor of 30.4–84.4, which are 4.1–6.3 times and 0.6–3.6 times higher than the commercial DF-120 membrane, respectively. The acid recovery capacity can be enhanced from 11.3 to 82.4Lm−2d−1, with a 64% increase in acid purity when our new membrane was used to replace the commercial DF-120 membrane. Moreover, these results are also superior to most of the reported membranes. Therefore, the excellent performance and simple preparation process endow our new diffusion dialysis membranes with great potential for high-efficiency industrial acid recovery applications.

High-performance nanofiltration membrane prepared by dopamine-assisted interfacial polymerization on PES nanofibrous scaffolds

Thin-film nanofiltration composite (TFNC) membrane with high performance was demonstrated in this work. The membrane consisted of dopamine-modified polyethersulfone (PES) nanofibrous supporting layer and interfacially polymerized polyamide selective barrier layer. PES nanofibrous scaffold was modified with dopamine through self-polymerization. Here, dopamine was introduced to facilitate the formation of ultra-thin TFNC membrane on PES substrate with high performance and enhance the interfacial compatibility and structural stability of the composite membrane. An ultra-thin selective layer was generated by interfacial polymerization reaction between solutions of piperazine and trimesoyl chloride on the dopamine-modified porous PES membrane. Various parameters in interfacial polymerization, including monomer concentration, curing temperature, and curing time, were discussed and optimized to achieve high-performance composite nanofiltration membrane. The resulting TFNC membrane possessed relative high salt rejection (~99.4%) and high flux (~63.0 L/m2 h) to Na2SO4 solutions at a low pressure of 0.6 MPa, doubling the flux of TFNC membranes prepared at the same conditions, but without dopamine modification. It is believed that dopamine modification offers an efficient route for the fabrication of composite nanofiltration membranes with high filtration performance.

Hydrophilic nylon 6,6 nanofibers supported thin film composite membranes for engineered osmosis

Previous studies have concluded that an ideal thin film composite (TFC) membrane specially designed for Engineered Osmosis (EO) should have an ultra-thin selective layer with excellent permselectivity supported by a hydrophilic, highly porous, non-tortuous and thin support structure. In this study, an emerging TFC supporting material, electrospun nanofibers, were used to fabricate a TFC-EO membrane where the support structure and the selective layer properties were individually optimized. Specifically, nylon 6,6 nanofibers fabricated via electrospinning were used for the first time to form the support structure due to its intrinsic hydrophilicity and superior strength compared to other nanofiber materials. The resulting membrane exhibited half of the structural parameter of a regularly used commercial FO membrane. Furthermore, the selective layer permselectivity could be adjusted using a co-solvent during the interfacial polymerization processes. Adding acetone to the organic phase (hexane) was found to increase permeance and decrease selectivity and hence affect the osmotic flux performance of our membranes. Our best membrane outperformed the standard commercial FO membrane by exhibiting a 1.5 to 2 fold enhanced water flux and an equal or lower specific salt flux.

Deswelling of ultrathin molecular layer-by-layer polyamide water desalination membranes

The selective layer of pressure-induced water desalination membranes is an ultrathin and highly crosslinked aromatic polyamide (PA) film that separates salt from water based on differences in permeability, which is a product of diffusivity and solubility. Characterizing the transport properties of the selective layer is necessary in understanding its permselective performance. However, measuring transport of ultrathin films in general is nontrivial. Here, Poroelastic Relaxation Indentation (PRI) is employed as a simple deswelling technique for measuring the transport properties of these ultrathin selective layers.

Composite porous membranes with an ultrathin selective layer for vanadium flow batteries

An ultrathin Nafion film was introduced onto the poly(ether sulfone) (PES)-sulfonated poly(ether ether ketone) (SPEEK) porous membranes for vanadium flow battery (VFB) application. The composite membranes showed very high ion selectivity, good stability and excellent performance (energy efficiency 86.5% at 80 mA cm(-2)) in VFB.

Bioenabled Interfacial Polymerized on Nanofibrous Scaffold as Composite Nanofiltration Membrane

Thin-film nanofiltration composite (TFNFC) membrane consisting of polyethersulfone (PES) nanofibrous support layer modified by 3, 4-dihydroxy-phenethylamine (dopamine) and interfacial polymerization (IFP) polyamide selective barrier layer was obtained in this study. The hydrophilicity of PES nanofibrous membrane was tremendously improved as the water static contact angle changed from 81.6° to 26.83° by dopamine modification. An ultrathin selective layer was produced by IFP reaction between solutions of piperazine (PIP) and trimesoyl chloride (TMC) on the dopamine modified porous PES membrane. The TFNFC membrane presented relatively high permeate flux (~59.9 L/m2h) and high salt rejection (~98.9%) to divalent anion solutions (1000mg/L, Na2SO4) at a low pressure of 0.6 MPa. It could be believed that dopamine modification would be very efficient to fabricate the composite membranes with stable structure and high filtration performance.

Modification of ceramic membranes for pore structure tailoring: The atomic layer deposition route

The technique of atomic layer deposition (ALD) is applied to the structural modification and pore tailoring of ceramic membranes. Ceramic microfiltration membranes with an average pore size of 50nm are used as substrates, on which Al2O3 is deposited by ALD with the purpose to tailor their pore size. The thickness of the deposited Al2O3 layer increases with the number of ALD cycles, and it is confirmed by scanning electron microscopy that the pore size of the ceramic membrane decreases with increasing ALD cycles until the pores are completely sealed. It is found that an ultrathin selective layer with a gradient porous structure is formed, whose thickness can be tuned by varying the exposure time of percursors. With the increased ALD cycles, the Al2O3-deposited membranes have a decreasing water flux, and an increasing retention to bovine serum albumin. The membrane subjected to 600 ALD cycles has a reduced pure water flux from 1698L (m2hbar)−1 for an undeposited membrane to 118L (m2hbar)−1, while its BSA retention increases significantly from 2.9% to 97.1%.