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 Crystalline Microporous Membrane from 2D MOF Nanosheets for Gas Separation

Metal‐organic frameworks (MOFs)‐based membranes have shown great potentials as applications in gas separation. In this work, a uniform membrane based on 2D MOF Ni₃(HITP)₂ (HITP=2,3,6,7,10,11‐hexaaminotriphenylene) was fabricated on ordered macroporous AAO via the filtration method. To fabricate the membrane, we obtained the Ni₃(HITP)₂ nanosheets as building blocks via a soft‐physical exfoliation method successfully that were confirmed by AFM and TEM. We also studied the H₂, CO₂ and N₂ adsorption isotherms of Ni₃(HITP)₂ powder at room temperature, which shows Ni₃(HITP)₂ has high heats of adsorption for CO₂ and high selectivity of CO₂ over N₂. Gas permeation tests indicate that the Ni₃(HITP)₂ membrane shows high permeance and selectivity of CO₂ over N₂, as well as good selectivity of H₂ over N₂. The ideal separation factors of CO₂/N₂ and H₂/N₂ from sing‐gas permeances are 13.6 and 7.8 respectively, with CO₂ permeance of 3.15×10^?6 mol?m^?2?s^?1?Pa^?1. The membrane also showed good stability, durability and reproducibility, which are of potential interest for practical applications in the CO₂ separations.     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO₂ from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO₂‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO₂/H₂ mixtures and a CO₂ permeance of 51×10^?7 mol m^?2 s^?1 Pa^?1 at ?35 °C, making it promising for practical CO₂ separation from mixtures.     

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.     

A Highly Permeable Aligned Montmorillonite Mixed‐Matrix Membrane for CO2 Separation

Highly permeable montmorillonite layers bonded and aligned with the chain stretching orientation of polyvinylamineacid are immobilized onto a porous polysulfone substrate to fabricate aligned montmorillonite/polysulfone mixed‐matrix membranes for CO₂ separation. High‐speed gas‐transport channels are formed by the aligned interlayer gaps of the modified montmorillonite, through which CO₂ transport primarily occurs. High CO₂ permeance of about 800 GPU is achieved combined with a high mixed‐gas selectivity for CO₂ that is stable over a period of 600 h and independent of the water content in the feed.     

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.     

Remarkably Enhanced Gas Separation by Partial Self‐Conversion of a Laminated Membrane to Metal–Organic Frameworks

Separation methods based on 2D interlayer galleries are currently gaining widespread attention. The potential of such galleries as high‐performance gas‐separation membranes is however still rarely explored. Besides, it is well recognized that gas permeance and separation factor are often inversely correlated in membrane‐based gas separation. Therefore, breaking this trade‐off becomes highly desirable. Here, the gas‐separation performance of a 2D laminated membrane was improved by its partial self‐conversion to metal–organic frameworks. A ZIF‐8‐ZnAl‐NO₃ layered double hydroxide (LDH) composite membrane was thus successfully prepared in one step by partial conversion of the ZnAl‐NO₃ LDH membrane, ultimately leading to a remarkably enhanced H₂/CH₄ separation factor and H₂ permeance.     

Using PDMS coated TFC-RO membranes for CO2/N2 gas separation: Experimental study,modeling and optimization

Thin film composite (TFC) reverse osmosis (RO) membranes are semipermeable membranes that are utilized in water purification or water desalination systems. Discarding these membranes after end-of-life leads to environmental problems. Reusing old TFC-RO membranes is one way to solve this problem. For this reason, in this study, used TFC-RO membranes were coated with polydimethylsiloxane (PDMS) for CO₂/N₂ gas separation application. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was utilized to confirm the crosslinking of coated PDMS. The morphology of PDMS/TFC-RO membranes was characterized using scanning electron microscopy (SEM). The parameters that can affect performance of prepared membranes (N₂ permeance and CO₂/N₂ selectivity) are concentration of PDMS solution, coating time, solvent evaporation time and curing temperature and time. Given that the used membranes don't have uniform surfaces, the first step of this study was to investigate the effect of the above mentioned factors on virgin membranes using fractional factorial design (FFD) of experiments. The results obtained showed that PDMS concentration is the most significant factor that has a negative effect on N₂ permeance and positive effect on CO₂/N₂ selectivity. The reported CO₂/N₂ selectivity of PDMS membranes was 11–12, but this selectivity for prepared PDMS/TFC-RO membranes was in the range of 6.7–22.5. After determining optimum conditions, the gas separation performance of PDMS coated used TFC-RO membrane under these conditions was finally determined. The results showed that the used membranes had a better performance than virgin membranes.     

Ending Aging in Super Glassy Polymer Membranes

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4‐methyl‐2‐pentyne) (PMP), and polymers with intrinsic microporosity (PIM‐1) reduces gas permeabilities and limits their application as gas‐separation membranes. While super glassy polymers are initially very porous, and ultra‐permeable, they quickly pack into a denser phase becoming less porous and permeable. This age‐old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO₂ permeability for one year and improving CO₂/N₂ selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.     

Transforming 3D CAU-10-H into 2D Materials with High Base Stability for Membrane Separation

Two-dimensional (2D) nanomaterials have received a significant research attention owing to their unique chemical and physical properties. These materials not only provide the chemically active sites and exposed surface atoms, but also display the porous nature suitable for their use as membranes for gas separation. In this study, 3D CAU-10-H has been transformed into a novel alkali stabilized 2D CACl-10 (180). Though CACl-10 (180) is similar to AlOOH, it is a novel 2D nanomaterial synthesized by using 4-chloroisophthalic acid and aluminum nitrate nonahydrate, with thermal decomposition at 300 °C. Further, CACl-10 (180) is noted to retain its framework structure in strong alkali solutions, attributed to the alkali-resistant aluminum hydroxide. At the same time, it has been demonstrated that 3D CAU-10-H can also transform into 3D CACl-10 (140) and 3D CACl-10 (130), and the halogen atoms of the ligands (−Cl) affect the alkali stability of the materials. Subsequently, the PVAm-CACl-10 (180)/MPSf mixed matrix membranes were prepared and applied for CH₄/N₂ separation. The developed membrane exhibits the CH₄ permeance of 1647.99 GPU with a CH₄/N₂ selectivity of 3.1. As a result, 2D CACl-10 (180), with a strong alkali stability and an acceptable CH₄/N₂ membrane separation performance, represents a high potential of application in the membrane separation process.     

Tuning the Stacking Modes of Ultrathin Two-Dimensional Metal–Organic Framework Nanosheet Membranes for Highly Efficient Hydrogen Separation

Two-dimensional (2D) metal–organic framework (MOF) membranes are considered potential gas separation membranes of the next generation due to their structural diversity and geometrical functionality. However, achieving a rational structure design for a 2D MOF membrane and understanding the impact of MOF nanosheet stacking modes on membrane separation performance remain challenging tasks. Here, we report a novel kind of 2D MOF membrane based on [Cu₂Br(IN)₂]_n (IN=isonicotinato) nanosheets and propose that synergetic stacking modes of nanosheets have a significant influence on gas separation performance. The stacking of the 2D MOF nanosheets is controlled by solvent droplet dynamic behaviors at different temperatures of drop coating. Our 2D MOF nanosheet membranes exhibit high gas separation performances for H₂/CH₄ (selectivity >290 with H₂ permeance >520 GPU) and H₂/CO₂ (selectivity >190 with H₂ permeance >590 GPU) surpassing the Robeson upper bounds, paving a potential way for eco-friendly H₂ separation.     

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.     

Radical copolymerization of alkyl 2‐norbornene‐2‐carboxylate with alkyl acrylates: Facile incorporation of norbornane framework into poly(alkyl acrylate)s

Radical copolymerization of alkyl 2‐norbornene‐2‐carboxylates (alkyl = Me 1a , nBu 1b ) with alkyl acrylates (alkyl = ethyl, methyl, and n‐butyl) was investigated. Copolymerization of 1a,b with the alkyl acrylates initiated by 1,1′‐azobis (cyclohexane‐1‐carbonitrile) at 85 °C proceeded to give random copolymers, although the homopolymerization of 1a,b did not proceed efficiently under the same conditions. Typically, bulk copolymerization of 1a with ethyl acrylate in a feed ratio of 1:3 ([ 1a ]:[EA]) afforded a copolymer with M_n = 33,300 containing 19.4 mol % of 1a unit in the composition. An increase of T_g derived from the incorporation of the rigid norbornane framework was observed, although the extent of the temperature rise was rather moderate. The ternary radical copolymerization of 1a,b /alkyl acrylate/N‐phenylmaleimide proceeded to give copolymers with the three repeating units in the main chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4597–4605, 2007     

Imidazolium‐based poly(ionic liquid)s with poly(ethylene oxide) main chains: Effects of spacer and tail structures on ionic conductivity

Ten types of cationic glycidyl triazole polymers (GTPs) are prepared from combinations of five alkyl‐imidazolium units (methyl‐, ethyl‐, n‐propyl‐, iso‐propyl‐, and n‐butyl‐imidazoliums) and two spacers [di‐ and tri(ethylene glycol)s]. Since these poly(ionic liquid)s are prepared from the same sample of glycidyl azide polymer by postfunctionalization method, they have the same degree of polymerization. Therefore, the structure–property relationship can be discussed without influence of molecular weight difference. The samples are characterized by NMR, differential scanning calorimetry, and thermogravimetric analysis. The ionic conductivity data are obtained by impedance measurements. The GTPs with the tri(ethylene glycol) spacer and ethyl‐ and n‐butyl‐imidazolium units afford the highest anhydrous conductivity of 1.5 × 10^?5 S cm^?1 at 30 °C. Based on electrode polarization (EP) analysis, we calculate the conducting ion (carrier) concentration and mobility. We discuss the effect of the spacer and N‐alkyl tail structures on the ionic conductivity using the data obtained by EP analysis and X‐ray diffraction. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2896–2906     

Polyurethane‐mesoporous silica gas separation membranes

The concept of mixed matrix membrane comprising dispersed inorganic fillers into a polymer media has revealed appealing to tune the gas separation performance. In this work, the membranes were prepared by incorporation of mesoporous silica into polyurethane (PU). Mesoporous silica particles with different pore size and structures, MCM‐41, cubic MCM‐48 and SBA‐16, were synthesized by templating method and functionalized with 3‐aminopropyltriethoxysilane (APTES). High porosity and aminated surface of the mesoporous silica enhance the adhesion of the particles to the PU matrix. The SEM and FTIR results showed strong interactions between the particles and the PU chains. Moreover, the thermal stability of the hybrid PUs improved compared to the pure polymer. Gas transport properties of the membranes were measured for pure CO₂, CH₄, O₂, and N₂ gases at 10 bar and 25°C. The results showed that the gas permeabilities enhanced with increasing in the loading of modified mesoporous silica particles. High porosity and amine‐functionalized particles render opportunities to enhance the gas diffusivity and solubility through the membranes. The enhanced gas transport properties of the mixed matrix membranes reveal the advantages of mesoporous silica to improve the gas permeability (CO₂ permeability up to ~70) without scarifying the gas selectivity (α(CO₂/N₂)~ 30 for 5 wt% SBA‐16 content).     

Ultrathin Films of Porous Metal–Organic Polyhedra for Gas Separation

Ultrathin films of a robust Rh^II-based porous metal–organic polyhedra (MOP) have been obtained. Homogeneous and compact monolayer films (ca. 2.5 nm thick) were first formed at the air–water interface, deposited onto different substrates and characterized using spectroscopic methods, scanning transmission electron microscopy and atomic force microscopy. As a proof of concept, the gas separation performance of MOP-supported membranes has also been evaluated. Selective MOP ultrathin films (thickness ca. 60 nm) exhibit remarkable CO₂ permeance and CO₂/N₂ selectivity, demonstrating the great combined potential of MOP and Langmuir-based techniques in separation technologies.     

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.     

Structural Optimization of Interpenetrated Pillared‐Layer Coordination Polymers for Ethylene/Ethane Separation

With the goal of achieving effective ethylene/ethane separation, we evaluated the gas sorption properties of four pillared‐layer‐type porous coordination polymers with double interpenetration, [Zn₂(tp)₂(bpy)]_n ( 1 ), [Zn₂(fm)₂(bpe)]_n ( 2 ), [Zn₂(fm)₂(bpa)]_n ( 3 ), and [Zn₂(fm)₂(bpy)]_n ( 4 ) (tp=terephthalate, bpy=4,4′‐bipyridyl, fm=fumarate, bpe=1,2‐di(4‐pyridyl)ethylene and bpa=1,2‐di(4‐pyridyl)ethane). It was found that 4 , which contains the narrowest pores of all of these compounds, exhibited ethylene‐selective sorption profiles. The ethylene selectivity of 4 was estimated to be 4.6 at 298 K based on breakthrough experiments using ethylene/ethane gas mixtures. In addition, 4 exhibited a good regeneration ability compared with a conventional porous material.     

Pebax‐Based Membrane Filled with Two‐Dimensional Mxene Nanosheets for Efficient CO2 Capture

Mixed matrix membranes (MMMs) made from inorganic fillers and polymers is a kind of promising candidate for gas separation. In this work, two‐dimensional MXene nanosheets were synthesized and incorporated into a polyether‐polyamide block copolymer (Pebax) matrix to fabricate MMM for CO₂ capture. The physicochemical properties of MXene nanosheets and MXene/Pebax membranes were studied systematically. The introduction of MXene nanosheets provided additional molecular transport channels and meanwhile enhanced the CO₂ adsorption capacity, thereby enhancing both the CO₂ peremance and CO₂/N₂ selectivity of Pebax membrane. The optimized MXene/Pebax membrane with a MXene loading of 0.15 wt % displayed a high separation performance with a CO₂ permeance of 21.6 GPU and a CO₂/N₂ selectivity of 72.5, showing potential application in CO₂ capture.     

Endowing g‐C3N4 Membranes with Superior Permeability and Stability by Using Acid Spacers

g‐C₃N₄ membranes were modulated by intercalating molecules with SO₃H and benzene moieties between layers. The intercalation molecules break up the tightly stacking structure of g‐C₃N₄ laminates successfully and accordingly the modified g‐C₃N₄ membranes give rise to two orders magnitude higher water permeances without sacrificing the separation efficiency. The sulfonated poly(2,6‐dimethyl‐1,4‐phenylene oxide) (SPPO)/g‐C₃N₄ with a thickness of 350 nm presents an exceptionally high water permeance of 8867 L h^?1 m^?2 bar^?1 and 100 % rejection towards methyl blue, while the original g‐C₃N₄ membrane with a thickness of 226 nm only exhibits a permeance of 60 L h^?1 m^?2 bar^?1. Simultaneously, SO₃H sites firmly anchor nitrogen with base functionality distributing onto g‐C₃N₄ through acid–base interactions. This enables the nanochannels of g‐C₃N₄ based membranes to be stabilized in acid, basic, and also high‐pressure environments for long periods.