High Gas Permeability in Aged Superglassy Membranes with Nanosized UiO-66−NH2/cPIM-1 Network Fillers

Superglassy membranes synthesised by polymers of intrinsic microporosity (PIMs) suffer from physical aging and show poor gas permeance over time, especially thin membranes, due to the fast rearrangement of nonequilibrium polymer chains. Herein, we constructed a novel PIM-1 thin film nanocomposite membrane (TFN) using nanosized UiO-66−NH₂ (≈10 nm)/carboxylated PIM-1 (cPIM-1) as the composite filler. Unlike conventional fillers, which interact with the polymer only via the surface, the UiO-66−NH₂/cPIM-1 forms a stable three-dimensional (3D) network intertwining with the polymer chains, being very effective to impede chain relaxation, and thus physical aging. Nanosizing of UiO-66−NH₂ was achieved by regulating the nucleation kinetics using carbon quantum dots (CQD) during the synthesis. This led to increased surface area, and hence more functional groups to bond with cPIM-1 (via hydrogen bonding between −NH₂ and −COOH groups), which also improved interfacial compatibility between the 3D network and polymer chains avoiding defect formation. As a result, the novel TFN showed significantly improved performance in gas separation along with reduced aging (i.e. ≈6 % loss in CO₂ permeability over 63 days); the aged membranes had a CO₂ permeance of 2504 GPU and ideal selectivity values of 37.2 and 23.8 for CO₂/N₂ and CO₂/CH₄, respectively.     

Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: Polybenzodioxane PIM-1

A detailed study of gas permeation, thermodynamic properties and free volume was performed for a novel polymer of intrinsic microporosity (PIM-1). Gas permeability was measured using both gas chromatographic and barometric methods. Sorption of vapors was studied by means of inverse gas chromatography (IGC). In addition, positron annihilation lifetime spectroscopy (PALS) was employed for investigation of free volume in this polymer. An unusual property of PIM-1 is a very strong sensitivity of gas permeability and free volume to the film casting protocol. Contact with water in the process of film preparation resulted in relatively low gas permeability (P(O₂) = 120 Barrer), while soaking with methanol led to a strong increase in gas permeability (P(O₂) = 1600 Barrer) with virtually no evidence of fast aging (decrease in permeability) that is typical for highly permeable polymers. For various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂) the data points on the Robeson diagrams are located above the upper bound lines. Hence, a very attractive combination of permeability and selectivity is observed. IGC indicated that this polymer is distinguished by the largest solubility coefficients among all the polymers so far studied. Free volume of PIM-1 includes relatively large microcavities (R = 5 Å), and the results of the PALS and IGC methods are in reasonable agreement.     

Impact of operating pressure on the permeance of hollow fiber gas separation membranes

Asymmetric and slightly pressure dependent permeation properties of commercial air separation hollow fibers have been observed – phenomena that have not been mentioned in the literature before. In bore-side feed the permeance of oxygen, nitrogen and helium slightly increases with pressure while in shell-side feed the permeance decreases with pressure. At a feed pressure of 12.8 bar_a the permeance in bore-side feed is about 10–15% higher than in shell-side feed mode. Interestingly, the effect was more or less similar for different gases and different fibers. The results suggest that mechanical stress induced by the pressure difference across the membrane affects the permeability of the skin layer.     

Modulation of membrane permeability by carbon dioxide

Promoting drug delivery across the biological membrane is a common strategy to improve bioavailability. Inspired by the observation that carbonated alcoholic beverages can increase the absorption rate of ethanol, we speculate that carbon dioxide (CO₂) molecules could also enhance membrane permeability to drugs. In the present work, we have investigated the effect of CO₂ on the permeability of a model membrane formed by 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipids to three drug-like molecules, namely, ethanol, 2′,3′-dideoxyadenosine, and trimethoprim. The free-energy and fractional-diffusivity profiles underlying membrane translocation were obtained from μs-timescale simulations and combined in the framework of the fractional solubility-diffusion model. We find that addition of CO₂ in the lipid environment results in an increase of the membrane permeability to the three substrates. Further analysis of the permeation events reveals that CO₂ expands and loosens the membrane, which, in turn, facilitates permeation of the drug-like molecules. © 2019 Wiley Periodicals, Inc.     

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.     

Metal–Organic Framework Crystal–Glass Composite Membranes with Preferential Permeation of Ethane

Metal–organic framework (MOF) glass is an easy to process and self-supported amorphous material that is suitable for fabricating gas separation membranes. However, MOF glasses, such as ZIF-62 and ZIF-4 have low porosity, which makes it difficult to obtain membranes with high permeance. Here, a self-supported MOF crystal–glass composite (CGC) membrane was prepared by melt quenching a mixture of ZIF-62 as the membrane matrix and ZIF-8 as the filler. The conversion of ZIF-62 from crystal to glass and the simultaneous partial melting of ZIF-8 facilitated by the melt state of ZIF-62 make the CGC membrane monolithic, eliminating non-selective grain boundaries and improving selectivity. The thickness of CGC membrane can be adjusted to fabricate a membrane without the need of a support substrate. CGC membranes exhibit a C₂H₆ permeance of 41 569 gas permeation units (GPU) and a C₂H₆/C₂H₄ selectivity of 7.16. The CGC membrane has abundant pores from the glassy state of ZIF-62 and the crystalline ZIF-8, which enables high gas permeance. ZIF-8 has preferential adsorption for C₂H₆ and promotes C₂H₆ transport in the membrane, and thus the GCG membrane exhibits ultrahigh C₂H₆ permeance and good C₂H₆/C₂H₄ selectivity.     

MIL-101(Cr) Microporous Nanocrystals Intercalating Graphene Oxide Membrane for Efficient Hydrogen Purification

Graphene oxide (GO) is a promising two-dimensional building block for fabricating high-performance gas separation membranes. Whereas the tortuous transport pathway may increase the transport distance and lead to a low gas permeation rate, introducing spacers into GO laminates is an effective strategy to enlarge the interlayer channel for enhanced gas permeance. Herein, we propose to intercalate CO₂-philic MIL-101(Cr) metal-organic framework nanocrystals into the GO laminates to construct a 2D/3D hybrid structure for gas separation. The interlayer channels were partially opened up to accelerate gas permeation. Meanwhile, the intrinsic pores of MIL-101 provided additional transport pathways, and the affinity of MIL-101 to CO₂ molecules resulted in higher H₂/CO₂ diffusion selectivity, leading to a simultaneous enhancement in gas permeance and separation selectivity. The MIL-101(Cr)/GO membrane with optimal structures exhibited outstanding and stable mixed-gas separation performance with H₂ permeance of 67.5 GPU and H₂/CO₂ selectivity of 30.3 during the 120-h continuous test, demonstrating its potential in H₂ purification application.     

Characterization of SAPO-34 membranes by water adsorption

The effects of humidity on gas permeation were studied for five SAPO-34 membranes with different fractions of permeation through non-SAPO pores. Membranes with high CO₂/CH₄ separation selectivities (>20) were stable in humidified gases, but degradation was seen for some membranes after months of exposure to the laboratory atmosphere. Once the membranes started to degrade, the rate of degradation appeared to accelerate. The degradation created non-SAPO pores that were larger than the SAPO-34 pores, as indicated by i-C₄H₁₀ permeance, CO₂/CH₄ selectivity, and CO₂ flux dependence on pressure. The effect of humidity on gas permeance correlated with these indicators of non-SAPO pores. Adsorbed water appeared to completely block the SAPO pores, but permeation through non-SAPO pores increased with humidity. Therefore, water adsorption can be used to determine membrane quality and the fraction of transport through non-SAPO pores.     

Sub‐10 nm Polyamide Nanofiltration Membrane for Molecular Separation

To separate small molecules from the solvent with high permeability and selectivity, the membrane process is thought to be highly effective with much lower energy consumption when compared to the traditional thermal‐based separation process. To achieve high solvent permeance, a sub‐10 nm thick polyamide nanofiltration membrane was synthesized through interfacial polymerization of ethidium bromide (EtBr) and trimesoyl chloride (TMC). Thanks to the extremely low solubility of the EtBr monomer in the organic phase, the polymerization process was strictly limited at the interface of the water and hexane, leading to an ultrathin polyamide membrane with a thickness down to sub‐10 nm. When used in nanofiltration, these ultrathin membranes display ultrafast water permeation of 40 liter per square meter per hour per bar (L m^?2 h^?1 bar^?1), and a high Congo red rejection rate of 93 %. This work demonstrates a new route to synthesize ultrathin polyamide membranes by the traditional interfacial polymerization.     

Preparation and characterization of silica—polyimide composite membranes coated on porous tubes for CO2 separation

Silica-polyimide microcomposite membranes were prepared on γ-alumina-coated α-alumina support tubes, and their gas permeation properties were evaluated with He, N₂ and CO₂. Smoothing of the substrate surface and hybridization of silica and polyamic acid were both effective to form defect-free thin composite membranes. The CO₂ permeance of a membrane with a silica content of 68 wt% was one order of magnitude higher than that of a polyimide membrane having the same thickness. The permselectivity of CO₂ to N₂ was 30 at 30°C and 13 at 100°C. Contributions of the silica and polyimide phases to permeance of the composite membrane were analyzed with a two-phase permeation model. The effective thickness of the rate-controlling polyimide phase was less than one-tenth of the total thickness of the silica-polyimide membrane.     

Toward predictive permeabilities: Experimental measurements and multiscale simulation of methanol transport in Nafion

A polymer membrane's permeability to solutes determines its suitability for various applications: a permeability value is essential for predicting performance in diverse contexts. Using aqueous methanol permeation through Nafion as an example, we describe a methodology for determining membrane permeability that accounts for boundary layer effects and the possibility of swelling. For the materials and apparatus used herein, analysis of a permeance measurement and computational fluid dynamics simulations show that the mass transfer boundary layer is on the order of ones to tens of microns. The data are used to develop and validate a multiscale model describing solute permeation through a hydrated membrane as a series of physical mechanistic steps: reversible adsorption from solution at the membrane interface, diffusion driven by a concentration gradient within the membrane, and reversible desorption into solution at the opposite membrane interface. The validated model is used to predict methanol transport across a solar-driven CO₂ reduction device and to assess the impact of polymer changes on the measured value. The approach of combining experimental data, computational fluid dynamics, and the mechanistic multiscale model is expected to provide more accurate analysis of membrane permeation data in cases with polymer swelling or unusual device geometries, among others.     

Gas permeability and stability of poly(1-trimethylsilyl-1-propyne-co-1-phenyl-1-propyne) membranes

A significant reduction in the gas permeability of the poly(1-trimethylsilyl-1-propyne) (PMSP) membrane was investigated in terms of the membrane thickness and the storage environment. The effects of physical aging were observed with thinner membranes and under vacuum conditions compared with storage in air. The decrease in the permeability coefficient was dependent on the decrease in the hole saturation constant of Langmuir adsorption (C'_H), which is related to the volume of the microvoids. Physical aging in the PMSP membrane affected not only the glassy domain but also the rubbery one. To stabilize the permeability of the PMSP membrane, a poly(1-trimethylsilyl-1-propyne-co-1-phenyl-1-propyne) [poly(TMSP-co-PP)] membrane was prepared. Poly(TMSP-co-PP) has the same unit of poly(1-phenyl-1-propyne), which membrane has stable permeability. The poly(TMSP-co-PP) with less than 20 mol % PP content was estimated to be a random copolymer based on theoretical gas permeation analysis. In the poly(TMSP-co-PP) membrane, the relation between the PP content and C'_H was similar to the relation between the PP content and the gas permeability. The stability of the permeability was dependent on the PP content. The poly(TMSP-co-PP) membrane containing 10 mol % PP had both high permeability and good stability under some of the aging conditions performed in this work. © 1995 John Wiley & Sons, Inc.     

Cross-linking of Polymer of Intrinsic Microporosity (PIM-1) via nitrene reaction and its effect on gas transport property

Polymer of Intrinsic Microporosity (i.e. PIM-1) has been crosslinked thermally via nitrene reaction using polyethylene glycol biazide (PEG-biazide) as a crosslinker. The crosslinking temperature was optimized using TGA coupled with FT-IR spectroscopy. The dense membranes containing different ratios of PIM-1 to PEG-biazide were cast from chloroform solution. Crosslinking of PIM-1 renders it insoluble even in excellent solvents for the uncrosslinked polymer. The resulting crosslinked membranes were characterized by FT-IR spectroscopy, TGA and gel content analysis. The influence of crosslinker content on the gas transport properties of PIM-1, its density and fractional free volume (FFV) were investigated. Compared to the pure PIM-1 membrane, the crosslinked PIM-1 membranes showed better gas separation performance especially for CO₂/N₂, CO₂/CH₄ and propylene/propane (C₃H₆/C₃H₈) gas pairs and as well as suppressed penetrant-induced plasticization under high CO₂ pressure.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

Enhancement on the permeation performance of polyimide mixed matrix membranes by incorporation of graphene oxide with different oxidation degrees

Graphene oxide (GO) with different oxidation degrees were synthesized by harsh oxidation of graphite using the improved Hummers method. The GO/polyimide (PI) mixed matrix membrane was successfully fabricated by in situ polymerization of PI monomers (3,3′,4,4′‐biphenyltetracarboxylic dianhydride and 4,4′‐diaminodiphenyl ether) with GO. The structure of GO was characterized by Fourier transform infrared, transmission electron microscopy, atomic force microscopy, X‐ray diffraction, and thermal gravimetric analysis–differential thermal analysis. The performance of different GO/PI mixed matrix membranes was evaluated by permeation experiments of CO₂/N₂ gas mixture (volume ratio, 1:9). Results showed that more polar functional groups were introduced to GO with the increase in oxidation degree of GO in the preparation process, producing fewer layers and more translucent structures. GO with higher oxidation degree has significant effect on its dispersion in the N,N‐dimethylacetamide solvent and polymer matrix materials. The permeability of GO/PI hybrid membranes for CO₂ and N₂ increased. The CO₂/N₂ permeation selectivity of membranes exhibited a trend of initial increase, followed by a decrease, with the increase in oxidation degree, when the same amount of GO was added. For GO with the same oxidation degree, the permeability and permeation selectivity of hybrid membrane initially increased, and then decreased with the addition content of GO. In the case of hybrid membrane containing 1 wt% monolayer GO, the maximum permeability and permeation selectivity of hybrid membranes for CO₂ were 14.3 and 4.2 times more than that of PI membrane without GO, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Gas permeation properties of blend and copolymer membranes composed of 1-trimethylsilyl-1-propyne and 1-phenyl-1-propyne structures

The poly(1-trimethylsilyl-1-propyne) (PMSP) has the potential to be an important membrane gas separation material due to the fact that it has the highest gas permeability of all polymeric membranes. One problem with PMSP is a decrease in the gas permeability with age. In order to understand the aging processes, we studied the change in free volume and the molecular motions of the PMSP and its membranes modified with 1-phenyl-1-propyne (PP) structures; that is, a copolymer and a blend of PMSP and PPP. During aging, the unrelaxed volume of the PMSP membrane was relaxed, and the molecular motion of carbons dropped, suggesting that the decrease in the microvoids caused a tighter chain packing. The copolymer and blend membrane had stable permeability compared to the PMSP. In particular, the addition of a small amount of the PP structure provided excellent stability with high gas permeability. A decrease in the unrelaxed volume of modified membranes was hardly observed with age; however, the molecular motion of some carbons slightly changed. This change did not affect the gas permeability. In this case, a larger unrelaxed volume was probably a dominant factor in the gas permeation of the PMSP rich membranes relative to the molecular motion in the T₁ measurement. © 1997 John Wiley & Sons, Inc.     

聚二苯基乙炔耐溶剂纳滤膜的制备与性能

聚二苯基乙炔(PDPA)不溶于有机溶剂,是理想的新型耐溶剂纳滤膜材料。采用溶液浇铸法制备聚[1-(4-三甲基硅基)苯基-2-苯乙炔](PTMSDPA)均质膜,经脱硅反应得到PDPA膜,研究其乙醇渗透性能和染料截留性能。结果表明,乙醇渗透通量与压力呈正相关性,传质机理可能介于孔流机理和溶解扩散机理之间的过渡区。染料的分子尺寸、电荷性质以及染料和膜之间的相互作用共同影响PDPA膜的截留性能。     

Pebax/TSIL blend thin film composite membranes for CO_2 separation

In this study a thin film composite (TFC) membrane with a Pebax/Task-specific ionic liquid (TSIL) blend selective layer was prepared. Defect-free Pebax/TSIL layers were coated successfully on a polysulfone ultrafiltration porous support with a polydimethylsiloxane (PDMS) gutter layer. Different parameters in the membrane preparation (e.g. concentration, coating time) were investigated and optimized. The morphology of the membranes was studied by scanning electron microscopy (SEM), while the thermal properties and chemical structures of the membrane materials were investigated by thermo-gravimetric analyzer (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The CO₂ separation performance of the membrane was evaluated using a mixed gas permeation test. Experimental results show that the incorporation of TSIL into the Pebax matrix can significantly increase both CO₂ permeance and CO₂/N₂ selectivity. With the presence of water vapor, the membrane exhibits the best CO₂/N₂ selectivity at a relative humidity of around 75%, where a CO₂ permeance of around 500 GPU and a CO₂/N₂ selectivity of 46 were documented. A further increase in the relative humidity resulted in higher CO₂ permeance but decreased CO₂/N₂ selectivity. Experiments also show that CO₂ permeance decreases with a CO₂ partial pressure increase, which is considered a characteristic in facilitated transport membranes.     

Pervaporation of alcohols through highly permeable PIM-1 polymer films

Polycondensation material PIM-1 has attracted the attention of researchers owing to its high transport parameters in gas separation and a high free volume. The pervaporation characteristics of PIM-1 have been systematically studied. Lower aliphatic alcohols (CH₃OH, C₂H₅OH, and n-C₄H₉OH) and water were selected as objects of research. The rates of mass transfer for individual components at various temperatures have been estimated, and for the ethanol-water binary mixture, the process of separation has been examined. The films based on the polymer under study exhibit the properties of organophilic membranes and are characterized by high permeability with respect to alcohols. The apparent activation energy of permeability is low. This behavior is common for pervaporation membranes based on glassy polymers with a high free volume. The parameters of separation for the water-alcohol mixture surpass corresponding values for typical organophilic membranes based on PDMS. These parameters are commensurable with the values observed for membranes based on poly(trimethylsilylpropyne). At the same time, PIM-1 does not demonstrate a rapid decrease in permeability in the course of time.     

Thin Pd membrane on α-Al2O3 hollow fiber substrate without any interlayer by electroless plating combined with embedding Pd catalyst in polymer template

Palladium acetate and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were dissolved in chloroform to form a homogeneous solution. Using this solution, thin polymer template film with embedded Pd catalyst was coated on a porous α-Al₂O₃ hollow fiber substrate. The Pd in the template film was used as the catalyst for electroless plating of Pd membranes. After the template was removed by heat treatment, the thin Pd membranes without any intermediate layers and substrate penetration were synthesized successfully. The as-synthesized Pd composite membranes of thickness less than 5 μm not only have a very high hydrogen permeance/permeability but also have a good hydrogen selectivity. Moreover, the good membrane stability was verified by the long-term operation under the condition of hydrogen permeation and the gas exchange cycles between pure hydrogen and pure helium. The good membrane stability was interpreted by estimating the shear stress of the special membrane configuration with small gap between Pd membrane and porous substrate layer.