Molecular-Sieving Membrane by Partitioning the Channels in Ultrafiltration Membrane by In Situ Polymerization

Commercial ultrafiltration membranes have proliferated globally for water treatment. However, their pore sizes are too large to sieve gases. Conjugated microporous polymers (CMPs) feature well-developed microporosity yet are difficult to be fabricated into membranes. Herein, we report a strategy to prepare molecular-sieving membranes by partitioning the mesoscopic channels in water ultrafiltration membrane (PSU) into ultra-micropores by space-confined polymerization of multi-functionalized rigid building units. Nine CMP@PSU membranes were obtained, and their separation performance for H₂/CO₂, H₂/N₂, and H₂/CH₄ pairs surpass the Robeson upper bound and rival against the best of those reported membranes. Furthermore, highly crosslinked skeletons inside the channels result in the structural robustness and transfer into the excellent aging resistance of the CMP@PSU. This strategy may shed light on the design and fabrication of high-performance polymeric gas separation membranes.     

Continuous Porous Aromatic Framework Membranes with Modifiable Sites for Optimized Gas Separation

Continuous microporous membranes are widely studied for gas separation, due to their low energy premium and strong molecular specificity. Porous aromatic frameworks (PAFs) with their exceptional stability and structural flexibility are suited to a wide range of separations. Main-stream PAF-based membranes are usually prepared with polymeric matrices, but their discrete entities and boundary defects weaken their selectivity and permeability. The synthesis of continuous PAF membranes is still a major challenge because PAFs are insoluble. Herein, we successfully synthesized a continuous PAF membrane for gas separation. Both pore size and chemistry of the PAF membrane were modified by ion-exchange, resulting in good selectivity and permeance for the gas mixtures H₂/N₂ and CO₂/N₂. The membrane with Br^? as a counter ion in the framework exhibited a H₂/N₂ selectivity of 72.7 with a H₂ permeance of 51844 gas permeation units (GPU). When the counter ions were replaced by BF₄^?, the membrane showed a CO₂ permeance of 23058 GPU, and an optimized CO₂/N₂ selectivity of 60.0. Our results show that continuous PAF membranes with modifiable pores are promising for various gas separation situations.     

Fabrication of a Hydrogen-Bonded Organic Framework Membrane through Solution Processing for Pressure-Regulated Gas Separation

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large-scale membranes for the efficient gas separation. Herein, the first microporous hydrogen-bonded organic framework (HOF) membrane has been fabricated by an optimized solution-processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π–π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure-responsive H₂/N₂ separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.     

Fabrication of a Hydrogen‐Bonded Organic Framework Membrane through Solution Processing for Pressure‐Regulated Gas Separation

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large‐scale membranes for the efficient gas separation. Herein, the first microporous hydrogen‐bonded organic framework (HOF) membrane has been fabricated by an optimized solution‐processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π–π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure‐responsive H₂/N₂ separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.     

Formation of porous flat membrane by phase separation with supercritical CO2

Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO₂ as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO₂ was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO₂ pressure was diminished. Effects of the experimental conditions such as the CO₂ pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.     

Fabrication of multi-layer composite hollow fiber membranes for gas separation

Using multilayer composite hollow fiber membranes consisting of a sealing layer (silicone rubber), a selective layer (poly(4-vinylpyridine)), and a support substrate (polysulfone), we have determined the key parameters for fabricating high-performance multilayer hollow fiber composite membranes for gas separation. Surface roughness and surface porosity of the support substrate play two crucial roles in successful membrane fabrication. Substrates with smooth surfaces tend to reduce defects in the selective layer to yield composite membranes of better separation performance. Substrates with a high surface porosity can enhance the permeance of composite membranes. However, SEM micrographs show that, when preparing an asymmetric microporous membrane substrate using a phase-inversion process, the higher the surface porosity, the greater the surface roughness. How to optimize and compromise the effect of both factors with respect to permselectivity is a critical issue for the selection of support substrates to fabricate high-performance multilayer composite membranes. For a highly permeable support substrate, pre-wetting shows no significant improvement in membrane performance. Composite hollow fiber membranes made from a composition of silicone rubber/0.1–0.5 wt% poly(4-vinylpyridine)/25 wt% polysulfone show impressive separation performance. Gas permeances of around 100 GPU for H₂, 40 GPU for CO₂, and 8 GPU for O₂ with selectivities of around 100 for H₂/N₂, 50 for CO₂/CH₄, and 7 for O₂/N₂ were obtained.     

Molecular‐Sieving Membrane by Partitioning the Channels in Ultrafiltration Membrane by In Situ Polymerization

Commercial ultrafiltration membranes have proliferated globally for water treatment. However, their pore sizes are too large to sieve gases. Conjugated microporous polymers (CMPs) feature well‐developed microporosity yet are difficult to be fabricated into membranes. Herein, we report a strategy to prepare molecular‐sieving membranes by partitioning the mesoscopic channels in water ultrafiltration membrane (PSU) into ultra‐micropores by space‐confined polymerization of multi‐functionalized rigid building units. Nine CMP@PSU membranes were obtained, and their separation performance for H₂/CO₂, H₂/N₂, and H₂/CH₄ pairs surpass the Robeson upper bound and rival against the best of those reported membranes. Furthermore, highly crosslinked skeletons inside the channels result in the structural robustness and transfer into the excellent aging resistance of the CMP@PSU. This strategy may shed light on the design and fabrication of high‐performance polymeric gas separation membranes.     

Permeation properties to hydrocarbons,perfluorocarbons and chlorofluorocarbons of cross-linked membranes of polymethacrylates with poly(ethylene oxide) and perfluorononyl moieties

Two kinds of cross-linked polymer membranes were prepared by photo polymerization. One of them (PEO membrane) is a co-polymer of methoxy-terminated poly(ethylene glycol) methacrylate (MEMA) and poly(ethylene glycol) dimethacrylate (EDMA) in feed ratio of 70/30 in wt%. The other (PF/PEO membrane) is a co-polymer of 1H,1H,9H-hexadecafluorononyl methacrylate and EDMA in feed ratio of 70/30 in wt%. The block lengths of poly(ethylene oxide) (PEO) are 9 and 14 for MEMA and EDMA, respectively. Permeation properties of inorganic gases, hydrocarbons, perfluorocarbons and chlorofluorocarbons (CFCs) were investigated for these membranes compared with a silicone rubber (SR) membrane. The PF/PEO membrane is inferior to the SR membrane as for CFCs/N₂ separation because the former has lower permeabilities. The PEO membrane has good performance for separation of hydrocarbons and CFCs from N₂ or perfluorocarbons.     

The role of additives on dope rheology and membrane formation of defect-free Torlon hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon^® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon^® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon^® hollow fiber membranes with an O₂/N₂ selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O₂/N₂ selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O₂/N₂ selectivity for compressed air varies from 7.12 to 9.00.     

Fabrication of supported Si3N4 membranes using the pyrolysis of liquid polysilazane precursor

The fabrication process is described of supported microporous Si₃N₄ membranes, prepared by pyrolytically decomposing organo-substituted polysilazane precursor. The membrane had a composite asymmetric structure consisting of a mechanically strong porous Si₃N₄ support which had 42 vol% pores between 0.4 and 0.52 μm, coated with an intermediate and one or two thin active top layers. The individual layers were fabricated by the conventional dip-coating technique.Permeation experiments with He, N₂ and CO₂ have been performed to determine the gas transport characteristics and separation performance of the processed membranes. The permeation is pressure-independent, indicating no viscous flow in the supported top layer. The proposed process has made it possible to prepare membranes with He permeation rates of ≥5.3×10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ and He/N₂ permselectivities of ≥2.0, even in the membrane with one top layer. It is also demonstrated from separation experiments, that the membrane with high quality top layer has the separation factors of 4.7 for He/N₂ and of the theoretical of Knudsen flow for CO₂/N₂.     

Anodic alumina supported dual-layer microporous silica membranes

We present a new processing scheme for the deposition of microporous, sol–gel derived silica membranes on inexpensive, commercially available anodic alumina (Anodisk™) supports. In a first step, a surfactant-templated mesoporous silica sublayer (pore size 2–6 nm) is deposited on the Anodisk support by dip-coating, in order to provide a smooth transition from the pore size of the support (20 or 100 nm) to that of the membrane (3–4 Å). Subsequently, the microporous gas separation membrane layer is deposited by spin-coating, resulting in a defect-free dual-layer micro-/mesoporous silica membrane exhibiting high permeance and high selectivity for size selective gas separations. For example, in the case of CO₂:N₂ separation, the CO₂ permeance reached 3.0 MPU (1 MPU = 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) coupled with a CO₂:N₂ separation factor in excess of 80 at 25 °C. This processing scheme can be utilized for laboratory-scale development of other types of microporous or dense inorganic membranes, taking advantage of the availability, low cost and low permeation resistance of anodic alumina (or other metal oxide) meso- and macroporous supports.     

Thermally Rearranged Polymer Membranes Containing Tröger's Base Units Have Exceptional Performance for Air Separations

The influence of segmental chain motion on the gas separation performance of thermally rearranged (TR) polymer membranes is established for TR polybenzoxazoles featuring Tröger's base (TB) monomer subunits as exceptionally rigid sites of contortion along the polymer backbone. These polymers are accessed from solution‐processable ortho‐acetate functionalized polyimides, which are readily synthesized as high‐molecular‐weight polymers for membrane casting. We find that thermal rearrangement leads to a small increase in d‐spacing between polymer chains and a dramatic pore‐network reconfiguration that increases both membrane permeability and O₂/N₂ selectivity, putting its performance above the 2015 upper bound.     

Designing GO Channels with High Selectivity for CO2/N2 Separation via Incorporating Metal Ions

Graphene oxide (GO) membranes holds great potential for high-performance CO₂ capture. Aiming at enhancing the CO₂ separation performance and structural stability of GO membranes, functionalizing GO channels with metal ions confers a promising strategy. In this study, we reported the fabrication of metal ion-incorporated GO membranes with remarkably improved CO₂/N₂ separation performance. The metal ions within GO channels contribute to facilitating CO₂ transport, decreasing N₂ solubility, hindering N₂ diffusion, and form multiple interactions with GO nanosheets. After introducing Mg²⁺ ions, the CO₂/N₂ separation factor of GO membrane is remarkably increased from 4 to 48.8 with the CO₂ permeance increases 1.5 times. Moreover, the separation performance of the GO-Mg²⁺ membranes shows an excellent long-term stability owing to the structural robustness. This study could provide insights into the regulation of the microstructure of metal ion-functionalized GO membranes for highly selective transport of specific molecules.     

Selective permeation of CO2 through poly 2-(N,N-dimethyl)aminoethyl methacrylate membrane prepared by plasma-graft polymerization technique

A membrane having an amine moiety was prepared by plasma-grafting 2-(N,N-dimethyl)aminoethyl methacrylate (DAMA) onto a microporous polyethylene substrate. Permselectivity of the membrane for CO₂ over N₂ was achieved in both dry and water swollen conditions. When the CO₂ partial pressure in the feed gas was 0.047 atm, the selectivity of CO₂ over N₂ reached 130 for the highly swollen water containing membrane. This value was found to agree with that obtained with a mobile carrier membrane (supported liquid membrane) using DAMA as the carrier. The effects of several experimental conditions such as degree of grafting, feed partial pressure and temperature on the membrane performance were studied. It was suggested that the membrane acted as a fixed carrier membrane for CO₂ facilitated transport in under the dry condition and acted as a fixed reaction site membrane in the water swollen condition. The carrier transport mechanism is discussed for dry and aqueous membranes.     

Two‐Dimensional Microporous Material‐based Mixed Matrix Membranes for Gas Separation

Since the discovery of graphene and its derivatives, the development and application of two‐dimensional (2D) materials have attracted enormous attention. 2D microporous materials, such as metal‐organic frameworks (MOFs), covalent organic frameworks (COFs), graphitic carbon nitride (g‐C₃N₄) and so on, hold great potential to be used in gas separation membranes because of their high aspect ratio and homogeneously distributed nanometer pores, which are beneficial for improving gas permeability and selectivity. This review briefly summarizes the recent design and fabrication of 2D microporous materials, as well as their applications in mixed matrix membranes (MMMs) for gas separation. The enhanced separation performances of the membranes and their long‐term stability are also introduced. Challenges and the latest development of newly synthesized 2D microporous materials are finally discussed to foresee the potential opportunities for 2D microporous material‐based MMMs.     

PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications

A new application for used reverse osmosis (RO) membranes as gas separation membranes was studied. In this regard, firstly, three pretreatment procedures were used to remove the foulants from the surface of used membrane and then they were coated with polydimethylsiloxane (PDMS). The results indicated that PDMS-coated used RO membranes were capable of separating O₂/N₂ and CO₂/N₂. The maximum O₂/N₂ and CO₂/N₂ selectivities of coated membranes were 5.9 and 32.5, respectively. The O₂/N₂ and CO₂/N₂ selectivities of PDMS membrane were reported in the range of 2.1–2.2 and 11–12, respectively. Finally, an economic assessment was carried out to compare prepared PDMS coated RO membranes with commercial PPO membrane. This showed that coated membranes are less expensive than PPO membrane for CO₂/N₂ gas separation. The outcome of the research was a simple method for converting used RO membranes to cost effective gas separation membranes.     

Evaluating the impact of functional groups on membrane-mediated CO2/N2 gas separations using a common polymer backbone

Polymeric membranes have shown tremendous promise for the separation of CO₂ from flue gas streams. However, few systematic studies have been conducted to better understand the impact that chemical functionalities have on membrane-based gas separation performance. To address this gap, we herein describe the synthesis and gas separation performance of a series of vinyl-addition polynorbornenes bearing various CO₂-philic functional groups. To facilitate direct comparison between functional groups, each material was designed to maintain a common polymer backbone. Though the incorporation of CO₂-philic moieties within a dense polymeric membrane is frequently hypothesized to enhance CO₂ solubility, and thereby increase CO₂/N₂ selectivity, our results demonstrate that the incorporation of CO₂-philic groups onto a common polymer backbone do not necessarily result in increased gas separation performance. Experimental and computational results demonstrate that the incorporation of amidoxime groups onto a polynorbornene backbone increase CO₂/N₂ selectivity, whereas commonly employed ethereal side chains only increased permeability.     

Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF‐8 for Membrane‐Based CO2 Capture

Fine‐tuning of effective pore size of microporous materials is necessary to achieve precise molecular sieving properties. Herein, we demonstrate that room temperature ionic liquids can be used as cavity occupants for modification of the microenvironment of MOF nanocages. Targeting CO₂ capture applications, we tailored the effective cage size of ZIF‐8 to be between CO₂ and N₂ by confining an imidazolium‐based ionic liquid [bmim][Tf₂N] into ZIF‐8’s SOD cages by in‐situ ionothermal synthesis. Mixed matrix membranes derived from ionic liquid‐modified ZIF‐8 exhibited remarkable combinations of permeability and selectivity that transcend the upper bound of polymer membranes for CO₂/N₂ and CO₂/CH₄ separation. We observed an unusual response of the membranes to varying pressure, that is, an increase in the CO₂/CH₄ separation factor with pressure, which is highly desirable for practical applications in natural gas upgrading.     

Etched ZIF-8 as a Filler in Mixed-Matrix Membranes for Enhanced CO2/N2 Separation

Zeolite ZIF-8 has been etched with acid to form microporous ZIF-8-E crystals. These were then introduced into a polyethersulfone (PES) membrane matrix to enhance its CO₂/N₂ separation performance. Open through pores of size about 100 nm formed in the ZIF-8 crystals allow the ingrowth of polyethersulfone chains, ensuring a reduction in the number of nonselective voids, thereby achieving better interaction between ZIF-8-E and PES. As a result, the CO₂/N₂ separation performance of the ZIF-8-E/PES membrane increased significantly, showing a CO₂ permeability of 15.7 Barrer and a CO₂/N₂ ideal selectivity of 6.5.     

High performance of PES-GNs MMMs for gas separation and selectivity

In this study, graphene nanosheets (GNs) were incorporated into polyethersulfone (PES) by phase inversion approach for preparing PES-GNs mixed matrix membranes (MMMs). To investigate the impact of filler content on membrane surface morphology, thermal stability, chemical composition, porosity and mechanical properties, MMMs were constructed with various GNs loadings (0.01, 0.02, 0.03, and 0.04 wt%). ?The performance of prepared MMMs was tested for separation and selectivity of CO₂, N₂, H₂ and CH₄ gases at various pressures from 1 to 6 bar and temperature varying from 20 to 60 °C. It was observed that, compared to the pristine PES membrane, the prepared MMMs significantly improved the gas separation and selectivity performance with adequate mechanical stability. The permeability of CO₂, N₂, H₂ and CH₄ for the PES + 0.04 wt% GNs increases from 9 to 2246, 11 to 2235, 9 to 7151, and 3 to 4176 Barrer respectively, as compared with pure PES membrane at 1 bar and 20 °C due to improving the membrane absorption and porosity. In addition, by increasing the pressure, the permeability and selectivity of CO₂, N₂, H₂ and CH₄ are increased due to the increased driving force for the transport of gas via membranes. Furthermore, the permeability of CO₂, N₂, H₂ and CH₄ increased by increasing the temperature from 20 to 60 °C due to the plasticization in the membranes and the improvement in polymer chain movement. This result proved that the prepared membranes can be used for gas separation applications.