High-performance Gas Separation Membranes Research Articles

Graphene in gas separation membranes—State-of-the-art and potential spoors

Graphene and derivatives have been frequently used to form advanced nanocomposites. A very significant utilization of polymer/graphene nanocomposite was found in the membrane sector. The up-to-date overview essentially highpoints the design, features, and advanced functions of graphene nanocomposite membranes towards gas separations. In this concern, pristine thin layer graphene as well as graphene nanocomposites with poly(dimethyl siloxane), polysulfone, poly(methyl methacrylate), polyimide, and other matrices have been perceived as gas separation membranes. In these membranes, the graphene dispersion and interaction with polymers through applying the appropriate processing techniques have led to optimum porosity, pore sizes, and pore distribution, i.e., suitable for selective separation of gaseous molecules. Consequently, the graphene derived nanocomposites brought about numerous revolutions in high performance gas separation membranes. The structural diversity of polymer/graphene nanocomposites has facilitated the membrane selective separation, permeation, and barrier processes especially in the separation of desired gaseous molecules, ions, and contaminants. Future research on the innovative nanoporous graphene-based membrane can overcome design/performance related challenging factors for technical utilizations.

Nanoporous Ti3C2O2 monolayer towards high permeance and selectivity for CO2/N2 separation: CO2-MXene interaction tuning

MXene, as a new gas separation membrane, has good stability and suitable pore topological structures to mitigate the greenhouse effect caused by excessive CO2 emission. However, the large quadrupole and polarizability of CO2 make it a strong interaction with MXene, thus leading to a strong solubility and weak diffusivity of CO2. Here, we design nanoporous Ti3C2O2 monolayers (YmXn) with exposed C atoms that could tune CO2-Ti3C2O2 interaction so as to improve gas separation performance by density functional theory and molecular dynamics simulation. The scope of “Cut-off” size is obtained by analyzing the gas separation state from un-permeation to permeation. Results reveal that nanoporous Ti3C2O2 membranes achieve both outstanding CO2 selectivity and permeance. The best-performance Y3X3-Ti possesses CO2 permeance of 1.01 × 10-4 and 2.56 × 10-5 mol s−1 m−2 Pa−1 for equimolar gas mixtures and flue gases, respectively, and both the CO2/N2 selectivity reach 100 %. The gas separation mechanism is dominated by diffusion selectivity rather than solution selectivity in nanoporous Ti3C2O2. The dynamic processes of gas permeation, including the center of mass, mean square displacement, and radial distribution function of the gases, are precisely investigated. The electrostatic repulsion between the C atoms in the channel of the membrane and CO2 follows the sequence Y3X3-C > Y2X4 > Y3X3-Ti, which is the key to regulating gas diffusion. Results of this work highlight that tuning gas-membrane interaction and designing “Cut-off” size are effective strategies to achieve excellent CO2 separation performance, and shed new light on the intrinsic mechanism in high-performance gas separation membranes.

Predicting and screening high-performance polyimide membranes using negative correlation based deep ensemble methods.

Polyimide polymer membranes have become critical materials in gas separation and storage applications due to their high selectivity and excellent permeability. However, with over 107 known types of polyimides, relying solely on experimental research means potential high-performance candidates are likely to be overlooked. This study employs a deep learning method optimized by negative correlation ensemble techniques to predict the gas permeability and selectivity of polyimide structures, enabling rapid and efficient material screening. We propose a deep neural network model based on negative correlation deep ensemble methods (DNN-NCL), using Morgan molecular fingerprints as input. The DNN-NCL model achieves an R2 value of approximately 0.95 on the test set, which is a 4% improvement over recent model performance, and effectively mitigates overfitting with a maximum discrepancy of less than 0.03 between the training and test sets. High-throughput screening of over 8 million hypothetical polymers identified hundreds of promising candidates for gas separation membranes, with 14 structures exceeding the Robeson upper bound for CO2/N2 separation. Visualization of high-throughput predictions shows that although the Robeson upper bound was never explicitly used as a model constraint, the majority of predictions are compressed below this limit, demonstrating the deep learning model's ability to reflect real-world physical conditions. Reverse analysis of model predictions using SHAP analysis achieved interpretability of the deep learning model's predictions and identified three key functional groups deemed important by the deep neural network for gas permeability: carbonyl, thiophene, and ester groups. This established a bridge between the structure and properties of polyimide materials. Additionally, we confirmed that two polyimide structures predicted by the model to have excellent CO2/N2 selectivity, namely 6-methylpyrimidin-5-amine and 1,4,5,6-tetrahydropyrimidin-2-amine, have been experimentally validated in previous studies. This research demonstrates the feasibility of using deep learning methods to explore the vast chemical space of polyimides, providing a powerful tool for discovering high-performance gas separation membranes.

Challenges and recent advances in MOF-based gas separation membranes.

Membrane-based gas separation, characterized by a small footprint, low energy consumption and no pollution, has gained widespread attention as an environmentally friendly alternative to traditional gas separation. Metal-organic-frameworks (MOFs) are considered to be one of the most promising membrane-based gas separation materials because of their large specific surface area and high porosity. One of the hottest studies at the moment is how to utilize the characteristics of MOFs to prepare higher performance gas separation membranes. This paper provides a review of gas separation membranes used in recent years. Firstly, the synthesis methods of MOFs and MOF membranes are briefly introduced. Then, methods to improve the membrane properties of MOFs are described in detail, and include applications of lamellar MOFs, ionic liquid (IL) spin coating, functionalization of MOFs, defect engineering and mixed fillers. In addition, the challenges of MOF-based gas separation membranes are presented, including pore size, environmental disturbances, plasticization, interfacial compatibility, and so on. Finally, based on the current development status of the MOF membranes, the development prospects of MOF gas separation membranes are discussed. It is hoped to provide reliable and complete ideas for researchers to prepare high-performance gas separation membranes in the future.

Effect of uniaxial stretching on gas separation performance of polyimide membranes

Fine-tuning of the physical stacking structure for microporous polyimide (PI) membranes via the stretching processing is a practical way of achieving advanced membrane materials with improved aging resistance as well as high gas separation performance. Here, a fluorinated polyimide 6FDA:BPDA-TFDB(1:1:2) is used as a prototype and a series of stretched membranes X%-PI are fabricated by uniaxially stretching the membrane to different elongation ratio and then annealing. Based on birefringence and wide-angle X-ray diffraction measurements, it's found that regular orientation of polymer chains along the stretching direction and fine-tuning of d-spaces from interchain packing and π-π stacking happen during the uniaxial stretching process. As such, compared to pristine PI membrane, the stretched membranes show better mechanical and thermal stability, especially for the one with higher stretching ratio. In addition, owing to the increased packing density of polymer chains, the highly stretched membrane of 40%-PI shows significantly improved gas perm-selectivity by 32.8%, 24.3%, and 12.6% for He/CH4, H2/CH4, and CO2/CH4 gas pairs, respectively, in comparison to pristine membrane. Furthermore, 40%-PI exhibits much higher gas permeability retention rate than that of other membranes after three months' aging. We expect that this work can offer polymer materials processing guidance for the preparation of high-performance gas separation membrane in future.

Delaminated or multilayer Ti3C2TX-MXene-incorporated polydimethylsiloxane mixed-matrix membrane for enhancing CO2/N2 separation

Membrane-based gas separation is currently attracting tremendous attention as this unit operation has been considered as one of the available strategies that can contribute toward carbon neutrality. Nevertheless, polymeric membranes suffer from material-related limitation, namely the permeability/selectivity trade-off as exemplified through the Robeson plot due to the intrinsic properties of polymers. Recently, a new rising star in 2D nanomaterial, MXenes, shows a new route to produce high-performance gas separation membranes. This study investigates the potential utility of 2D MXenes as nanofillers to enhance the gas separation performance of polydimethylsiloxane (PDMS) membrane, which generally possesses high intrinsic permeability but low selectivity. Thus, we fabricated mixed-matrix membranes by incorporating multilayer MXene (ML-MXene) and single-layer MXene (D-MXene) under different loading (1–5 wt%) into PDMS matrices. The addition of MXene improved the separation capacity of the rubbery dense layers. Based on the optimization study performed in this work, D-MXene-loaded mixed-matrix membrane at 1 wt% loading demonstrated an increment in both CO2 permeability and CO2/N2 selectivity by 64.1% and 21.3%, respectively, with respect to pure PDMS membrane.

Ionic Microporous Polymer Membranes for Advanced Gas Separations

Microporous polymers are uniquely attractive for membrane-mediated gas separations; however, conventional microporous polymers suffer a ubiquitous trade-off between gas permeability and selectivity, leading to bottlenecks in their practical applications. Functionalization of microporous polymers via molecule engineering is an effective way to enhance their gas separation performance and processability. This review outlines the research progress of ionization to improve the gas separation performance of typical microporous polymers (e.g., polymers of intrinsic microporosity (PIM), perfluorinated polymers, microporous polyimides, etc.) and summarizes the different ionization methods, including carboxylation, sulfonation, quaternization, and other ionization processes. Additionally, the review also explores the research progress of ionization to regulate the processability, microporosity, and gas separation properties of microporous polymers. Specifically, ionization can effectively tailor the microporosity, improve the solubility coefficients of gas molecules, especially CO2, and enhance gas selectivities. In addition, ionization can improve the processability of PIMs and enhance the membrane plasticization resistance. Ionic microporous polymers provide an essential platform for developing energy-efficient and high-performance gas separation membranes.

Two-dimensional basic cobalt carbonate supported ZIF-67 composites towards mixed matrix membranes for efficient CO2/N2 separation

Two-dimensional (2D) porous materials have great potential in high-performance gas separation membranes. Herein, we demonstrate a facile synthesis of basic cobalt carbonate supported zeolitic imidazolate framework-67 (BCoC-ZIF) nanoplates via a template conversion method in aqueous solutions. Basic cobalt carbonate nanosheets were used as the metal source and BCoC-ZIF composites with various ZIF-67 loadings were obtained through adjusting the reaction time in an aqueous solution of 2-methylimidazole. Numerous mixed matrix membranes (MMMs) are prepared with different loadings of BCoC-ZIF composite nanoplates in the polymer of intrinsic porosity-1 (PIM-1), ranging from 1 to 20 wt% of filler loadings. The typical MMM with 10 wt% loading shows a CO2 permeability of 7326 Barrer and CO2/N2 separation factor of 32.5, exceeding the latest 2019 upper bound. The improved separation performance greatly relies on the multiply synergistic effect of high CO2 adsorption capacity, alignment of 2D morphology and molecular sieving ability of BCoC-ZIF. This study may offer a new direction for the rational design of MMMs for gas separation applications.

Carbonic Anhydrase-Mimicking Supramolecular Nanoassemblies for Developing Carbon Capture Membranes.

As a ubiquitous family of enzymes with high performance in converting carbon dioxide (CO2) into bicarbonate, carbonic anhydrases (CAs) sparked enormous attention for carbon capture. Nevertheless, the high cost and operational instability of CAs hamper their practical relevance, and the utility of CAs is mainly limited to aqueous applications where CO2-to-bicarbonate conversion is possible. Taking advantage of the chemical motif that endows CA-like active sites (metal-coordinated histidine), here we introduce a new line of high-performance gas separation membranes with CO2-philic behavior. We first self-assembled a histidine-based bolaamphiphile (His-Bola) molecule in the aqueous phase and coordinated the resulting entities with divalent zinc. Optimizing the supramolecular synthesis conditions ensured that the resultant nanoparticles (His-NPs) exhibit high CO2 affinity and catalytic activity. We then exploited the His-NPs as nanofillers to enhance the separation performance of Pebax MH 1657. The hydrogen-bonding interactions allowed the dispersion of His-NPs within the polymer matrix uniformly, as confirmed by microscopic, spectroscopic, and thermal analyses. The imidazole and amine functionalities of His-NPs enhanced the solubility of CO2 molecules in the polymer matrix. The CA-mimic active sites of His-NPs nanozymes, on the other hand, catalyzed the reversible hydration of CO2 molecules in humid conditions, facilitating their transport across the membranes. The resulting nanocomposite membranes displayed excellent CO2 separation performance, with a high level of stability. At a filling ratio as low as 3 wt %, we achieved a CO2 permeability of >145 Barrer and a CO2/N2 selectivity of >95 with retained performance under humid continuous gas feeds. The bio-inspired approach presented in this work offers a promising platform for designing durable and highly selective CO2 capture membranes.

Sensitivity of Material, Microstructure and Operational Parameters on the Performance of Asymmetric Oxygen Transport Membranes: Guidance from Modeling.

Oxygen transport membranes can enable a wide range of efficient energy and industrial applications. One goal of development is to maximize the performance by the improvement of the material, microstructural properties and operational conditions. However, the complexity of the transportation processes taking place in such commonly asymmetric membranes impedes the identification of the parameters to improve them. In this work, we present a sensitivity study that allows identification of these parameters. It is based on a 1D transport model that includes surface exchange, ionic and electronic transport inside the dense membrane, as well as binary diffusion, Knudsen diffusion and viscous flux inside the porous support. A support limitation factor is defined and its dependency on the membrane conductivity is shown. For materials with very high ambipolar conductivity the transport is limited by the porous support (in particular the pore tortuosity), whereas for materials with low ambipolar conductivity the transport is limited by the dense membrane. Moreover, the influence of total pressure and related oxygen partial pressures in the gas phase at the membrane’s surfaces was revealed to be significant, which has been neglected so far in permeation test setups reported in the literature. In addition, the accuracy of each parameter’s experimental determination is discussed. The model is well-suited to guiding experimentalists in developing high-performance gas separation membranes.

High-performance gas separation membranes derived from thermal-oxidative block poly(benzoxazole-co-imide)

Thermally rearranged(TR) polymers are usually prepared at an insert atmosphere and display high gas permeability, but their low gas selectivity results in high energy consumption. Thermal oxidation is an effective method to enhance the gas selectivity of microporous polymers. In this work, the effect of oxygen in the purge environment on imide-to-benzoxazole conversion and membrane structures was investigated. 1% oxygen content of the purge environment could restrain imide-to-benzoxazole conversion of 6FDA-DETDA/bisAPAF hydroxyl block polyimide(bHPI) membranes effectively, and oxygen induced the oxygen bridge formation, improving the gas selectivities of the resulting thermal-oxidative membranes with the increased oxygen content. Thermal-oxidative PBOCI membranes (TO-PBOCI) based on block poly(benzoxazole-co-imide)(PBOCI) precursor membranes were prepared to enhance gas separation performance of microporous polymers. The stable polybenzoxazole structures and microphase separation of polybenzoxazole blocks maintained membrane structures and the gas transport channels in the thermal-oxidative process at high temperature. Significantly, the remaining hydroxyl groups in PBOCI precursor membranes were beneficial for oxygen bridge formations of TO-PBOCI membranes, enhancing H2/CH4 selectivity. As a result, the TO-PBOCI membranes treated at a high temperature kept high H2 permeability and H2/CH4 selectivity, surpassing the 2008 Robeson upper bound. The study about TO-PBOCI membranes promotes a better understanding of the polymeric materials and process development of microporous polymer membranes for hydrogen separation applications.

Hydrogen bonding-induced 6FDA-DABA/TB polymer blends for high performance gas separation membranes

To promote polyimide gas transport properties, we propose a facile technique through adjustable hydrogen bonding energy through blending polyimide (6FDA-DABA) containing –COOH groups and Tröger's base (TB) polymer without damaging polymer chemical structures. The hydrogen bonding strength between –OH groups in PI and the N atoms in TB (O–H⋯N) is well tailored via controlled thermal treatment which has been confirmed by molecular simulations. Hydrogen bonding amounts and strength reach maximum within membranes annealed at 120 °C for 16 h, and exhibit CO2/CH4 and H2/CH4 selectivities of 115.0 and 481.8 respectively, surpassing the 2008 upper bound. Contributed by the enhanced hydrogen bonding strength, the membranes annealed at 120 °C for 16 h display excellent CO2 plasticization resistance and physical aging in comparison with pristine PI and TB membranes. Such a approach potentially renders a synthetic membrane applicable for CO2 separation and hydrogen purification at high temperatures.

Microporous Pentiptycene-Based Polymers with Heterocyclic Rings for High-Performance Gas Separation Membranes

Microporous Pentiptycene-Based Polymers with Heterocyclic Rings for High-Performance Gas Separation Membranes

High-aspect ratio zeolitic imidazolate framework (ZIF) nanoplates for hydrocarbon separation membranes.

Metal-organic frameworks with high aspect ratios have the potential to yield high-performance gas separation membranes. We demonstrate the scalable synthesis of high–aspect ratio zeolitic imidazolate framework (ZIF)–8 nanoplates via a direct template conversion method in which high aspect ratio–layered Zn hydroxide sheets [Zn5(NO3)2(OH)8] were used as the sacrificial precursor. Successful phase conversion occurs as a result of the collaboration of low template stability and delayed delivery of 2-methylimidazole in weakly interacting solvents, particularly using acetone. When the ZIF-8 nanoplates with an average aspect ratio of 20 were shear aligned in the 6FDA-DAM polymer matrix by bar coating, the separation performance for propylene/propane far surpassed that of the previously reported mixed matrix and polymeric membranes, showing a propylene permeability of 164 Barrer and selectivity of 33.4 at 40 weight % loadings.

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.

Tailoring the Microporosity of Polymers of Intrinsic Microporosity for Advanced Gas Separation by Atomic Layer Deposition

Tailoring the microporosity of intrinsically microporous polymers at the atomic level is one of the biggest challenges in achieving high-performance polymeric gas separation membranes. In this study , for the first time, the Al 2 O 3 atomic layer deposition (ALD) technique was used to modify the microporosity of a typical polymer of intrinsic microporosity (PIM-1) at the atomic level. PIM-1 with six ALD cycles (PIM-1-Al 2 O 3 -6) exhibited simultaneous high thermal, mechanical, pure- and mixed-gas separation, and anti-aging properties. The O 2 /N 2 , H 2 /N 2 , and H 2 /CH 4 separation performances were adequate above the latest trade-off lines. PIM-1-Al 2 O 3 -6 showed CO 2 and O 2 permeabilities of 624 and 188 Barrer, combined with CO 2 /CH 4 and O 2 /N 2 selectivities of 56.2 and 8.8, respectively. This significantly enhanced performance was attributed to the strong size sieving effect induced by the Al 2 O 3 deposition.

Rational Design of Halloysite Surface Chemistry for High Performance Nanotube-Thin Film Nanocomposite Gas Separation Membranes.

The interfacial region has a critical role in determining the gas separation properties of nanofiller-containing membranes. However, the effects of surface chemistry of nanofillers on gas separation performance of thin film nanocomposite (TFN) membranes, prepared by the interfacial polymerization method, have been rarely studied in depth. In this work, pristine and three differently surface-modified halloysite nanotubes (HNTs), by non- (SHNT), moderately (ASHNT), or highly CO2-philic (SFHNT) agents, are embedded in the polyamide top layer of thin film nanocomposite (TFN) membranes for CO2/N2 and CO2/CH4 separations. Trimethoxyoctyl silane, 3-(2-aminoethylaminopropyl)trimethoxysilane, and poly(styrenesulfonic acid) are used as modifying agents to quantitatively investigate the effects of interfacial interactions between the polyamide and HNTs on the gas permeation of TFNs. This allows us to provide an interfacial design strategy to fabricate high-performance gas separation membranes. Pure gas permeations conducted on the TFNs at the feed gas pressure of 10 bar showed that CO2 permeance and CO2/N2 and CO2/CH4 selectivities were increased by 145%, 130%, and 108%, respectively, after addition of 0.05 w/v% of sulfonated HNTs. The experimental gas permeations through all TFNs/HNTs, except TFNs/SFHNTs, agree well with predictions of a recently developed model, which suggests the importance of considering the neglected role of CO2 interactions with the HNT/polyamide interface in the model. These results unambiguously proved that designing the interfacial layer thickness in the nanotube-containing membranes is an effective approach to tuning the gas separation properties. The results show that the dispersion of HNTs in the polyamide top layer and the experimental CO2/gas selectivity was increased with increasing interfacial thickness, aint, upon surface modification. Moreover, it is quantitatively demonstrated that the thickness of the interfacial layer between the filler and polymer matrix is a function of gas pressure applied on the membrane.

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.

Macromolecular design strategies toward tailoring free volume in glassy polymers for high performance gas separation membranes

This review highlights recently reported novel macromolecular design strategies providing tailorable free volume for high performance gas separation membranes.

Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations

Finely tailoring the interfacial interaction to minimize the defective structure in the hybrid membranes is a key to yield a mixed matrix membrane with enhanced gas separation performance. Here, a highly selective mixed matrix membrane based on imidazole containing polyimides and ZIF-8 fillers is reported. The ZIF-8 with imidazole linker offers a more compatible interface with imidazole containing polyimides. At a MOF loading of 20 wt. %, the membranes have a 2.2-fold increase in the gas permeabilities over unfilled polymers with exceptional H2/CH4 and CO2/CH4 gas selectivities of 224 and 58, respectively. The H2 and CO2 gas permeabilities are 78.5 and 20.3 Barrer, respectively. The Zn2+ post-modification enables the formation of metal coordination crosslinking with enhanced polymer/ZIF-8 interaction, as indicated by increased glass transition temperature, improved thermal stability and insoluble properties in organic solvents. With the proper control of Zn2+ treatment conditions, gas permeabilities of H2 and CO2 can be increased to 110.1 and 27.4 Barrer, respectively, with the constant H2/CH4 and CO2/CH4 gas selectivity. For membranes treated at higher Zn2+ concentration, an enhancement in H2/CH4 gas selectivity was observed with the selectivity increasing from 223.9 to 318.3 and H2 permeability around 72.3 Barrer. The separation performance of H2/CH4 for all Zn2+ modified membranes exceeds the 2008 Robeson upper bound. This facile approach to tune polymer/MOF interaction via metal ion modification promotes the rational design of high-performance gas separation membranes.