Ultrathin Skin Layer Research Articles

Hierarchical MFI Zeolite Membranes for Superior Xylene Separation

AbstractThe separation of p‐xylene from its xylene isomers (o‐xylene, m‐xylene) is a significant concern in the petrochemical industry. MFI zeolite membranes offer great potential for energy‐efficient and high‐selective xylene separation. Synthesizing ultrathin (< 1 µm) MFI zeolite membrane for high‐flux separation is highly desired but remains challenging. Herein, a novel and facile strategy for synthesizing hierarchical MFI zeolite membranes on ceramic hollow fibers from multi‐dimensionally assembled (2D@0D) seed layers is demonstrated. Owing to the rapid growth of 0D MFI seeds and the voids‐preserved growth of stacked 2D MFI seeds, the hierarchical MFI membrane synthesized for 2 h yielded an ultrathin skin layer (≈255 nm thick) and many macro‐voids within the sublayer (void fraction: 14.86%), which facilitated molecular permeation and simultaneously maintained high separation selectivity of zeolitic pores. The membrane exhibited over sevenfold higher p‐xylene permeance (2.81 × 10−7 mol m−2 s−1 Pa−1) and over threefold improved p‐/o‐xylene separation factor (1228) than the conventional MFI membrane.

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.

Novel polyamide forward osmosis membrane prepared via synergistically substrate-floating interfacial polymerization and salt regulation for efficient solvent recovery

Obtaining an ultrathin and high-quality skin layer on a macro-porous substrate is considered as an obstacle for the desalted membrane fabrication, although macro-porous substrate seems nearly ideal for the forward osmosis (FO) membranes. In this work, a novel substrate-floating interfacial polymerization (SFIP) method was proposed to realize the fabrication of a defect-free polyamide skin layer on a macro-porous substrate. Meanwhile, inorganic salt such as NaCl was added into aqueous phase to cause local differences in interface tension during SFIP process, promoting the MPD transfer from the bulk to surface of aqueous solution via Marangoni convection so as to diffuse more MPD molecules. The resulting FO membrane exhibited an excellent ethanol flux (JV)/reverse LiCl flux (JS) value of 277.17 g L−1, which was 2–10 times higher than the data reported in previous literature. Besides, our SFIP method was also employed to fabricate the polyamide skin layer on the hydrophobic PVDF substrate, and found that the hydrophilicity/hydrophobicity of substrate could control the forming position of polyamide skin layer via SFIP method. This work provides a feasible strategy for fabricating high-quality desalted membranes, and demonstrates their potential in organic solvent recovery for energy conservation and emission reduction.

Plasticization-enhanced trimethylbenzene functionalized polyethersulfone hollow fiber membranes for propylene and propane separation

Polymeric membranes for gas separations often suffer from plasticization, which leads to the swelling of polymer matrices and thus the loss of the separation selectivity and performance. Such intrinsic and detrimental issues remain challenging and unsolved. Here, we report a newly designed polymer, poly trimethyl phenylene ethersulfone (PTPES) that is a trimethylbenzene-functionalized polyethersulfone and beneficial from the plasticization. When the polymer is fabricated into defect-free hollow fiber membranes, both propylene permeance and propylene/propane selectivity increase exponentially with the feed pressure after plasticization. The PTPES hollow fibers have a propylene permeability and a propylene/propane selectivity about two orders of magnitude higher than their intrinsic values of dense films. The exceptionally plasticization-enhanced phenomena of PTPES hollow fibers might be ascribed to the unique plasticization-enlarged steric-hindrance molecular sieving effect, which happens in the ultra-thin PTPES skin layer and is caused by the trimethylbenzene moieties in the well-oriented polymer chains. Our work provides new insights to the membrane gas transport mechanisms and may inspire the development of new-generation polymeric materials and membranes for propylene/propane and other gas separations.

Hierarchically Ordered Perovskites with High Photo‐Electronic and Environmental Stability via Nanoimprinting Guided Block Copolymer Self‐Assembly

AbstractThe development of a sub‐50 nm nanopattern of a thin perovskite film is of significant interest with its excellent photo‐electronic properties and environmental stability. By employing nanoimprinting of a moldable perovskite precursor, self‐assembled with a block copolymer, a thin perovskite nanopattern with excellent photo‐electronic stability is developed over a large area. Approximately 40 nm nanodomains consisting of perovskite crystals, which are 10 nm in diameter, are templated in the block copolymer, and the registry of the perovskite nanodomains is further guided by the topological pattern with a periodicity of a few hundred nanometers. This method can produce hierarchically controlled nanostructures of the perovskites from molecular crystals to self‐assembled domains with tens of nanometer scale and patterned structures of a few hundred nanometers. With an ultrathin top skin layer on a patterned perovskite/block copolymer film developed during the nanoimprinting, the hierarchically ordered perovskite is environmentally stable in ambient conditions over 100 days with its photoluminescence quantum yield of ≈28%. Furthermore, the enhanced photoconduction of the perovskite nanocrystals hierarchically structured in the nanopatterns allows for developing the arrays of photodetectors with long‐term environmental stability over 90 days.

In Situ Electrodeposition of Gold Nanostructures in 3D Ultra‐Thin Hydrogel Skins for Direct Molecular Detection in Complex Mixtures with High Sensitivity (Laser Photonics Rev. 15(12)/2021)

Au–Hydrogel Nanocomposites In article number 2100316, Sung-Gyu Park and co-workers develop a new Auhydrogel nanocomposite structure and in situ Raman measurements system. The ultra-thin hydrogel skin layer on the Au nanopillar arrays selectively allows the infusion of small target molecules. The new nanocomposite materials and in situ Raman monitoring techniques enable rapid and affordable SERS-based on-site analysis and point-of-care testing.

Combing three-dimensional water channels and ultra-thin skin layer enable high flux and stability of loose polyimide/SiO2 nanofiltration membranes at low operating pressure via one step in-situ modification

Desalination of brackish water and purification of drinking water still pose several challenges. Combination of in-situ hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in polyamic acid (PAA) solution along with imidization reaction provide a novel polyimide/SiO2 (PI/SiO2) membrane. This is the first synthesis of a PI nanofiltration (NF) membrane with three-dimensional (3D) water channels, ultra-thin skin layer (30 ± 2 nm), and high flux at low operating pressure, by one-step phase inversion method. There was no additional requirement of an acid, since PAA served as a catalyst. The SiO2 cross-linked network provided the skeletal support, due to hydrogen bonding between PAA-PI and silicon hydroxyl groups. The fabricated PI/SiO2 membrane demonstrated a water contact angle of 45°, pore size of 1.05 ± 0.15 nm, Zeta potential of −30 mV, and tensile strength of 2.24 MPa when under optimized conditions. The loose PI/SiO2 NF membrane exhibited salt rejection in the sequence of Na2SO4 (87%)>MgSO4 (67%)>NaCl (41%)>MgCl2 (37%), which was consistent with size exclusion and Donnan exclusion effects. In particular, the water flux of 20.4 L/(m2 h bar) was about 4–15 times higher than that of universal PI-NF membranes. The membrane also exhibited excellent stability for over six days when used for separation. Furthermore, the membrane showed excellent dye (Rose Bengal (RB), Brilliant Blue G (BBG), Amido Black (AB)) rejection of over 90%. To conclude, this work paves a new pathway for the fabrication of PI-NF membranes for efficient NF separation.

Amine-functionalized ZIF-8 nanoparticles as interlayer for the improvement of the separation performance of organic solvent nanofiltration (OSN) membrane

We successfully fabricated a kind of novel sandwich-like thin-film nanocomposite (TFN) polyamide (PA) membranes via interfacial polymerization (IP) method on polyimide (PI) support surface which was modified by a layer of nanosized amine-modified zeolitic imidazolate framework - 8 (mZIF-8) nanoparticles for organic solvent nanofiltration (OSN). Uniform distribution of the nanoparticles was obtained via directly immersing the PI support membrane in aqueous mZIF-8 suspension prior to the IP. A post-IP cross-linking procedure was conducted to enhance further the solvent resistance of the fabricated TFN OSN membranes. Ultra-smooth (average surface roughness of 13.7 nm) and ultra-thin (average thickness of about 33 nm) skin layer was achieved, together with high Rhodamine B (RDB, 479 Da) rejection (99.1%) and reasonable ethanol permeance (~28 L m−2 h−1 MPa−1), as well as nearly unchanged RDB rejection after being submerged in DMF at high temperature (80 °C) for 120 h, indicating their very good resistance in very harsh strong polar solvent circumstance and the promising aspect of their industrial application.

Porous heat exchange tube with ultra-thin dense skin layer via NIPS for AGMD process

In this paper, a kind of poly(vinylidene fluoride) (PVDF) porous heat exchange tube (HET) was prepared via non-solvent induced phase separation (NIPS) method. This porous HET possessed an ultra-thin dense skin layer of 2 μm. Since the thermal conductivity of water is higher than that of PVDF, filling the pores with water can significantly improve the heat performance of the porous HET. A full plastic double-pipe air gap membrane distillation (AGMD) module was fabricated with the porous HETs and PVDF hollow fiber membranes. In the AGMD process, the wetting treatment can make the membrane distillation flux (J) and the gained output ratio (GOR) increased by 30%. During the long-term stability test, the maximum conductivity of distillate water was only 17.2 μS/cm. The experimental results demonstrated that the porous HET can be applied for the AGMD process to treat high saline brine, and the ultra-thin dense layer can prevent the leakage of saltwater.

Dimensionally-controlled densification in crosslinked thermally rearranged (XTR) hollow fiber membranes for CO2 capture

Thermal densification in asymmetric hollow fibers fabricated using thermally rearranged (TR) polymers has been regarded as a challenging issue due to productivity reduction by severe permeance loss. However, it has recently been reported from our group that the densification phenomenon could be exploited to induce ultrathin skin layer from highly porous precursor fibers. We successfully prepared densification-induced crosslinked thermally rearranged (diXTR) fibers from porous XHPI precursor fibers. The proposed hollow fiber fabrication method using the XTR material effectively enhanced CO2 permeance by 2-fold, without any loss of CO2/N2 selectivity, compared to a traditional method. Extending from our previous work, in this study it was found that the densification during TR process can be dimensionally restricted in order to maximize the gas permeance. Generally, the thermal densification induces omnidirectional shrinkage of heat treated fibers above Tg. The longitudinal shrinkage can be prevented by physically holding both ends of hollow fibers during thermal treatment. This approach was applied to diXTR hollow fibers, which allowed a remarkable CO2 permeance exhibiting around 4,600 GPU with 18 CO2/N2 selectivity. It was discovered that an effective suppression of the thermal densification occurred at the vicinity of Tg. A skin layer thickness of 52 nm was achieved (calculated using O2 permeance). Evaluation of mechanical properties resulted in no evidence of mechanical weakness in dimensionally-controlled diXTR (2D-diXTR) fibers. Additionally, a direct methanol treatment was adopted to restore CO2 permeance of 30 day elapsed 2D-diXTR fiber modules. The method effectively recovered CO2 permeance up to 92% for an original permeance.

Preparation of Polymer Membranes by In Situ Interfacial Polymerization

Membranes currently have a wide application in sewage treatment and water purification processes, in seawater desalination, and in various technological processes where high product purity is required. Deposition of an ultrathin skin layer of TFC (thin-film composite) and TFN (thin-film nanocomposite) onto the surface of membranes is discussed in this article. Their presence improves membrane properties such as retention of impurities and permeability. The aim of this paper is to present the current state of knowledge about the methods of preparing composite and nanocomposite membranes. The properties of the prepared TFC membranes can be modified by changing the type and concentration of the reacting monomers, and the physical conditions in which the membrane preparation process itself is carried out are also significant. Additionally, the properties of TFN membranes can be further modified with nanocomposites. The membranes are characterized by different properties not only because they have nanoparticles in their structure but also because their concentration and the way they are blended into the membrane structure were changed. This paper provides information on modifications of TFN membranes with nanoparticles, as well as modification by changes in polymerization reaction conditions and monomer concentration. Examples of the use of TFN and TFC membranes are also presented.

Densification-induced hollow fiber membranes using crosslinked thermally rearranged (XTR) polymer for CO2 capture

Since thermally rearranged (TR) polymers were known as high gas permeable and processable materials, fabricating high performance hollow fiber (HF) membranes have been tried using them. However, an unexpected drawback emerged which is the gas productivity loss by thermal densification of skin layers during thermal treatment above their glass transition temperature (Tg). In this work, we used a recently reported crosslinked-TR (XTR) polybenzoxazole to develop a new class of high-flux TR hollow fibers by directly exploiting the thermal densification phenomenon. The TR temperature range (320–460 °C) and Tg (394 °C) were determined by thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The chain rigidity of the XTR polymer increased during an isotherm treatment at its Tg, suggesting a restricted densification. Surprisingly, the undesired pinhole-defects (pore diameter < 5 nm) on precursor fibers were perfectly healed after thermal treatment (>400 °C), forming an ultrathin defect-free skin layer on thermally-densified XTR hollow fiber membranes. The pore-healed XTR hollow fibers exhibited an outstanding CO2 permeance of ~2300 GPU and a CO2/N2 selectivity of 17.4 with a skin thickness of 103 nm.

Enhanced pervaporation performance of polyamide membrane with synergistic effect of porous nanofibrous support and trace graphene oxide lamellae

Electrospun nanofibrous membrane as a supporting layer has attracted great interests in fabricating high performance composite membranes due to its interconnected porous structure. However, it’s still a challenge to prepare an ultrathin and integrated skin layer on porous and undulating nanofibrous support, thereby restricting its application in pervaporation. Here, graphene oxide (GO) was introduced into amine aqueous phase to form an integrated and compact polyamide (PA) layer on nanofibrous substrate via interfacial polymerization (IP). Under the synergistic effect of high permeability of the nanofibrous scaffold and flexibility of GO lamellae, tiled GO lamellae acting as a skeleton got involved in the initial reaction for the construction of selective PA layer. By loading trace amount of GO in aqueous phase during IP, the selectivity of PA layer was enhanced significantly compared with that of pristine PA layer on nanofibrous scaffold. High permeate flux of 6593 g/m2h with a desirable separation factor of 1491 was achieved using the hybrid membrane with just 0.001 wt% of GO loading for pervaporation dehydrating 90 wt% isopropanol aqueous solution at 70 °C. These results indicated the feasibility of constructing a single homogeneous selective layer on nanofibrous support for high-performance pervaporation process.

A dual-layer micro/nanostructured fibrous membrane with enhanced ionic conductivity for lithium-ion battery

Electrospun fiber-based composite membrane with interconnected porous structure has drawn significant attention for application in lithium-ion battery (LIB) owing to its high ion transportation. Here, we report a facile approach to fabricate the hierarchically micro/nanostructured ultrathin SiO2-PAN skin layer on the PAN-PU fibrous membrane surface via electrospinning and sequential electrospraying techniques. Benefiting from the synergistic effect of the micro/nanostructure and the superior electrolyte affinity of the SiO2-PAN skin layer, the as-prepared SiO2-PAN@PAN-PU membrane demonstrates the decreased pore size (1.18 μm), enhanced ionic conductivity (1.4 mS cm−1) and robust electrochemical stability up to 5.08 V. Moreover, the introduction of skin layer endows the as-prepared SiO2-PAN@PAN-PU membrane with improved stress strength (15.7 MPa) and excellent thermostability of 200 °C. Compared with the PAN-PU and Celgard membranes, the dual-layer micro/nanostructured SiO2-PAN@PAN-PU fibrous membrane based Li/LiFePO4 cell assembled with high mass-loading LiFePO4 demonstrates a more stable cycling lifetime with a high discharge capacity (107.4 mAh g−1 at the 105th cycles) and higher rate capability (75.2 mAh g−1 at 1 C). The strategy using electrospraying technique demonstrated in this work will open the door to narrow fibrous membrane pore size and enhance ionic conductivity for potential application in the high-power LIB.

Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation

A highly breathable membrane integrating an asymmetrically superwettable Janus skin and a hydrophobic nanofibrous membrane (NFM) was developed via sequential electrospinning and electrospraying for application in membrane distillation (MD). The electrosprayed asymmetrically superwettable Janus skin composed of lotus-leaf-like nano/microstructured nanofilaments exhibited an intriguing underwater superoleophobicity of 164° and an in-air superhydrophobicity of 166°, thereby providing a robust resistance to membrane fouling with high flux. The newly developed membrane with an ultrathin Janus skin layer, high porosity and interconnected pore structure displayed a high water flux of over 25 L m−2 h−1, a robust oil resistance and an excellent durability in direct contact MD (ΔT = 40 °C) during the treatment of oil-in-saline water emulsion (1000 ppm oil). The present development can thus endow MD with the ability to desalinate challenging water matrices with complex compositions. Significantly, the sequential electrospraying process utilized in the construction of the Janus skin is expected to be applicable to a wide range of other selective separations.

A morphology strategy to disentangle conductivity–selectivity dilemma in proton exchange membranes for vanadium flow batteries

A novel integrally thin skinned asymmetric proton exchange membrane (ITSA-PEM) is proposed to disentangle the typical conductivity–selectivity dilemma in PEMs for vanadium flow batteries (VFBs). The membrane is successfully fabricated by a porogen-leaching-out method. It consists of a porous sublayer and an ultrathin skin layer, which is defect-free verified by the high H2/N2 separation factor of 64.9. The degree of sulfonation (DS) of PEM is reduced to extremely low (DS = 36.3%) to suppress swelling, and numerous interconnected pores are introduced to facilitate proton transfer. Low swelling ratio and defect-free skin layer lead to undetectable vanadium permeation. Meanwhile, the area resistance of ITSA-PEM is dramatically lowered to 2.1 Ω cm2 from 5.4 Ω cm2 of the dense PEM. Therefore a membrane with both improved proton conductivity and ion selectivity is obtained. Low DS also equips the membrane with sufficient mechanical strength and enhanced thermal stability. The VFB assembled with ITSA-PEM displays high energy efficiencies (EE: 75.6–90.2%) over a current density of 20–80 mA cm−2, much superior to those of Nafion 211 (EE: 55.9–73.4%). It also shows favorable stability and slow capacity decay rate during cycling test over 50 cycles.

CO2 Absorption with NaOH Solution through the Porous PVDF Hollow Fibre Membrane Contactor

In this study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase inversion process. In order to improve the phase inversion rate and provide porous membranes, 4 wt.% lithium chloride (LiCl) was used in the spinning dope. The prepared membrane morphology was studied using field emission scanning electron microscopy (FESEM). Chemical CO2absorption by NaOH solution (1M) was conducted through the PVDF hollow fiber membrane contactor. The effect of the main operating condition such as absorbent temperature, CO2 pressure and absorbent flow rate on the performance of CO2 absorption was investigated. From FESEM examination, the membrane possesses an almost sponge–like structure with ultra thin skin layer. Results of CO2absorption test showed that by increasing the absorbent flow rate the CO2 flux increased which confirmed the existence of liquid side mass transfer resistance. It was found that by increasing the absorbent temperature the CO2 flux considerably improved. Meanwhile, the effect of CO2 pressure on the absorption rate was insignificant. Therefore, it can be concluded that by applying a porous hydrophobic membrane with improved structure and optimizing the operating conditions, high CO2 removal efficiency can be achieved through gas–liquid membrane contactors

Polymer and Membrane Design for Low Temperature Catalytic Reactions.

Catalytically active asymmetric membranes have been developed with high loadings of palladium nanoparticles located solely in the membrane's ultrathin skin layer. The manufacturing of these membranes requires polymers with functional groups, which can form insoluble complexes with palladium ions. Three polymers have been synthesized for this purpose and a complexation/nonsolvent induced phase separation followed by a palladium reduction step is carried out to prepare such membranes. Parameters to optimize the skin layer thickness and porosity, the palladium loading in this layer, and the palladium nanoparticles size are determined. The catalytic activity of the membranes is verified with the reduction of a nitro-compound and with a liquid phase Suzuki-Miyaura coupling reaction. Very low reaction times are observed.

Fabrication of thermally rearranged (TR) polybenzoxazole hollow fiber membranes with superior CO2/N2 separation performance

Thermally rearranged polybenzoxazole (TR-PBO) hollow fiber membranes were fabricated from a poly(amic acid) (HPAAc) precursor through a non-solvent induced phase separation technique (NIPS). All the major fabrication conditions (e.g. dope composition, the use of additional inorganic salt, dope and bore flow rates, and coagulation bath temperature) were systematically evaluated and optimized, in order to produce defect-free hollow fiber membranes with an ultra-thin skin layer. The hollow fiber membranes fabricated with the optimized spinning conditions exhibited superior pure gas permeation behavior (CO2 permeance of 2500GPU and CO2/N2 ideal selectivity of 16). Slow beam positron annihilation lifetime spectroscopy (slow beam PALs) measurements revealed that such an exceptional separation performance was mainly attributed to the ideal cavity radius (3.584Å) and ultra-thin skin layer thickness (193nm) obtained using the optimal fabrication conditions. In addition, mixed-gas permeation tests were also performed to demonstrate the feasibility of using such membranes for post-combustion CO2 capture.

Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications

Nonsolvent-induced phase separation (NIPS) and thermally induced phase separation (TIPS) are widely adopted membrane fabrication methods with both advantages and drawbacks. In an attempt to combine the benefits of both processes, in this work, a novel hybrid method, i.e., nonsolvent thermally induced phase separation (NTIPS), was proposed to fabricate polyvinylidene difluoride (PVDF) membranes by careful choice of a water-soluble diluent, ε-caprolactam (CPL), for membrane distillation (MD) applications. Combined with polymer/diluent compatibility analysis, a series of PVDF/CPL binary solutions was studied by differential scanning calorimetry (DSC) and polarized light microscopy (PLM) to identify an appropriate PVDF concentration range (<30wt% PVDF) for the fabrication of membranes with desirable pore structures in the NTIPS process. The SEM images showed an asymmetric structure of NTIPS membranes with an ultrathin top skin layer of 0.5μm and a highly porous support layer with a bicontinuous network of pores. Based on the X-ray diffractometry (XRD) examination and attenuated total reflection-Fourier transform infrared spectroscopic (ATR-FTIR) analysis for the NTIPS membranes, combined NIPS and TIPS mechanisms were identified through the formation of different PVDF crystalline phases at different locations (surface and bulk layers). Further characterization confirmed that, compared to the currently reported MD membranes, the novel NTIPS membrane exhibited a much higher overall porosity of 86% and a higher liquid entry pressure (LEP) above 3.5bar, as well as exceptional mechanical strength, which are desirable characteristics for MD applications to avoid membrane pore wetting and to ensure high salt rejection. Through the performance evaluation of NTIPS membranes using synthetic seawater (3.5wt% sodium chloride solutions) in direct contact membrane distillation (DCMD), superior permeation flux of 85.6kgm−2h−1 at a feed temperature of 80°C and a salt rejection rate greater than 99.99% were achieved. This study is expected to have profound implications in the development of PVDF membrane fabrication methods not only for MD applications but also for other membrane processes.