Membrane-based Gas Separation Research Articles

Maleimide-induced crosslinking of polyimide membranes for high gas separation performance and plasticization resistance

Trade-off effect and plasticization are the key issues confronted in the area of membrane separation when dealing with applications involving high-pressure, soluble gases like CO2. In this study, a novel diamine (TAPA-MAA) was synthesized derived from tris(4-aminophenyl)amine (TAPA) and maleic anhydride (MA). This diamine was copolymerized with DAM (2,4,6-trimethylbenzene-1,3-diamine) and 6FDA (4,4′-(hexafluoroisopropylidene)diphthalic anhydride) to form the polyimide (PI) membrane containing maleimide (MI) cross-linking sites via chemical imidization way. Then, under the action of heat treatment, the crosslinked PI (cPI) membrane was constructed since the MI crosslinking initiates a radical cycloaddition reaction to form a succinimide-linked structure, and named as cPSI membrane for short. Further, an increase in the ratio of TAPA-MAA gives rise to the resultant cPSI membrane with elevated cross-linking density. The obtained cPSI membranes showed significantly enhanced gas selectivities and superior anti-plasticization property compared to the un-crosslinked PI membranes due to the MI-induced crosslinking reaction in the PI main chain, resulting in a marked increment (24 %) in ultra-microporosity (<7 Å) and a reduction in the d-spacing value (6.10 Å vs 5.80 Å), generating a strong molecular sieving effect. Meanwhile, the highly interconnected covalent structure formed by the MI-induced crosslinking will suppress the plasticizing phenomenon induced by high-pressure soluble gases. The typical membrane (cPSI-3:7-350) possessed great CO2/CH4 separation performance, CO2 permeability of 334.4barrer and a CO2/CH4 selectivity of 61.9 that is exceeded the 2008Roberson upper bound, along withCO2 plasticization pressures above 44 bar. Besides, its mixed CO2/CH4 separation performance exceeded the 2018 upper gas mixture limit when the upstream pressure changed from 2-20 bar. This study provides valuable insights for the design of PIs containing MI cross-linking sites, which can enhance the performance of membrane-based gas separation.

Hydrogen Separation Membranes: A Material Perspective

The global energy market is shifting toward renewable, sustainable, and low-carbon hydrogen energy due to global environmental issues, such as rising carbon dioxide emissions, climate change, and global warming. Currently, a majority of hydrogen demands are achieved by steam methane reforming and other conventional processes, which, again, are very carbon-intensive methods, and the hydrogen produced by them needs to be purified prior to their application. Hence, researchers are continuously endeavoring to develop sustainable and efficient methods for hydrogen generation and purification. Membrane-based gas-separation technologies were proven to be more efficient than conventional technologies. This review explores the transition from conventional separation techniques, such as pressure swing adsorption and cryogenic distillation, to advanced membrane-based technologies with high selectivity and efficiency for hydrogen purification. Major emphasis is placed on various membrane materials and their corresponding membrane performance. First, we discuss various metal membranes, including dense, alloyed, and amorphous metal membranes, which exhibit high hydrogen solubility and selectivity. Further, various inorganic membranes, such as zeolites, silica, and CMSMs, are also discussed. Major emphasis is placed on the development of polymeric materials and membranes for the selective separation of hydrogen from CH4, CO2, and N2. In addition, cutting-edge mixed-matrix membranes are also delineated, which involve the incorporation of inorganic fillers to improve performance. This review provides a comprehensive overview of advancements in gas-separation membranes and membrane materials in terms of hydrogen selectivity, permeability, and durability in practical applications. By analyzing various conventional and advanced technologies, this review provides a comprehensive material perspective on hydrogen separation membranes, thereby endorsing hydrogen energy for a sustainable future.

Hydroxyl-Functionalized Polymers of Intrinsic Microporosity and Dual-Functionalized Blends for High-Performance Membrane-Based Gas Separations.

Membrane technology has shown significant growth during the past two decades in the gas separation industry due to its energy-savings, compact and modular design, continuous operation, and environmentally benign nature. Robust materials with higher permeability and selectivity are key to reduce capital and operational cost, pushing it forward to replace or debottleneck conventional energy-intensive unit operations such as distillation. Recently designed ladder polymers of intrinsic microporosity (PIM) and polyimides of intrinsic microporosity (PIM-PI) with pores <20 Å have demonstrated excellent gas permeation performance. Here, a series of plasticization-resistant PIM-based membrane materials is reported, including the first example of a hydroxyl-functionalized triptycene- and Tröger's base-derived ladder PIM and two PIM-PI homopolymers and a series of dual-functionalized polyimide blends containing hydroxyl- and carboxyl-functionalized groups. Specifically, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based PIM-PI blends demonstrated extremely high selectivity for a variety of industrially important applications. An optimized polyimide blend containing ─OH and ─COOH groups showed permselectivity values of 136 for CO2/CH4, 11.4 for O2/N2 and 636 for H2/CH4. Such extreme size-sieving capabilities are attributed to physical crosslinking induced by strong hydrogen bonding forming tightly structured polymer networks. The study provides a new general strategy for developing plasticization resistant, robust, and highly-selective PIM-based membrane materials.

Unveiling the Mystery: How TR precursors lead to exceptional gas separation performance in CMSMs

One of the main issues with membrane-based gas separation lies in the unexplored relationship between the precursors and their derived carbon molecular sieve membranes (CMSMs). Herein, we designed a series of TR polymers with gradient PBO content as precursors to investigate the carbonization mechanisms of PBO structure. The process of transforming the polymer precursor into carbon structure throughout pyrolysis involves utilizing an array of characterization techniques along with pyrolysis simulation. Results indicated that a higher PBO content leads to stronger molecular sieving effect of the resulting CMSM. The TR-CMS-50 membrane demonstrated exceptional gas separation performance with high permeability (PH2 = 3154 and PCO2 = 1495 Barrer) as well as ultra-high H2/CH4 (384) and CO2/CH4 (166) selectivity, which far exceeded the newest Upper bound line, showing significant potential for practical applications. This may can be attributed to “delayed effect” of the PBO structure which requires pyrolysis at higher temperatures, leading to more intense pyrolysis and the release of nitrogen-containing substances. Meanwhile, PBO has a cyclic system with a strong aromatic structure that closely resembles a carbon strand, which probably facilitates aromatization and enables it to more easily integrate into CMS structure, resulting in a more orderly arrangement in the Langmuir phase and generating higher concentration of pyrrolic-N (58.7 % vs. 44.1 %) as well as graphitic-N (31.6 % vs. 17.2 %). Thus, increasing the PBO content makes the derived CMS denser and significantly enhances its molecular sieving capacity. This study offers new insights into the development of precursor structures that creates high-performance CMSMs.

Regulation of MOF-74 nanosheet channels for precise H2 purification

Mixed matrix membranes (MMMs), composed of functional fillers and a polymer matrix, are considered effective demonstrations for overcoming the trade-off limitation of polymeric membrane-based gas separations. MOF-74, with its high density of open metal sites and one-dimensional straight honeycomb channels, is an extremely attractive filler material. However, the intrinsically large pore aperture and micron-sized, spindle-shaped grain morphology hinder the use of high-performance, interfacial defect-free MMMs for small gas separations. Herein, we report novel mixed matrix membranes integrated with dual-metallic MOF-74 nanosheets and poly(vinylamine) for precise molecular sieving for H2 purification applications. The nanosheets of parallel orientation within the membrane maximize the exposure of channels for polymeric chain penetration. The amino side groups anchored on the polymeric chains coordinate with the open metal sites on the channel walls of MOF-74 nanosheets, exposing the hydrophobic alkyl backbones to shrink the originally large channel apertures and simultaneously enhancing the interfacial compatibility between MOF-74 and the polymer matrix. Benefiting from the merits of these elegant channel regulations, MMMs exhibit superior H2/CH4 and H2/CO2 separation performances toward binary, ternary and even quaternary feedstocks, along with remarkable long-term separation stability. This MMM is expected to constitute a new membrane paradigm that holds great promise for practical H2 purification and separation.

Defect-Engineered Metal-Organic Framework/Polyimide Mixed Matrix Membrane for CO2 Separation.

Defect-engineered metal-organic frameworks (MOFs) with outstanding structural and chemical features have become excellent candidates for specific separation applications. The introduction of structural defects in MOFs as an efficient approach to manipulate their functionality provides excellent opportunities for the preparation of MOF-based mixed matrix membranes (MMMs). However, the use of this strategy to adjust the properties and develop the separation performance of gas separation membranes is still in its early stages. Here, a novel defect-engineered MOF (quasi ZrFum or Q-ZrFum) was synthesized via a controlled thermal deligandation process and incorporated into a CO2-philic 6FDA-durene polyimide (PI) matrix to form Q-ZrFum loaded MMMs. Defect-engineered MOFs and fabricated MMMs were investigated regarding their characteristic properties and separation performance. The incorporation of defects into the MOF structure increases the pore size and provides unsaturated active metal sites that positively affect CO2 molecule transport. The interfacial compatibility between the Q-ZrFum particles and the PI matrix increases via the deligandation process, which improves the mechanical strength of Q-ZrFum loaded membranes. MMM containing 5 wt.% of defect-engineered Q-ZrFum exhibits excellent CO2 permeability of 1308 Barrer, which increased by 99 % compared to the pure PI membrane (656 Barrer) at a feed pressure of 2 bar. CO2/CH4 and CO2/N2 selectivity reached 44 and 26.6 which increased by about 70 and 16 %, respectively. This study emphasizes that defect-engineered MOFs can be promising candidates for use as fillers in the preparation of MMMs for the future development of membrane-based gas separation applications.

Highly Selective Soluble Polyimides Simultaneously Containing Benzimidazole and Hydroxyl Groups for Membrane-Based Gas Separation

Highly Selective Soluble Polyimides Simultaneously Containing Benzimidazole and Hydroxyl Groups for Membrane-Based Gas Separation

Synergistic effect of combining UiO-66 nanoparticles and MXene nanosheets in Pebax mixed-matrix membranes for CO2 capture

The use of membrane-based gas separation has garnered significant interest owing to its cost-effectiveness, outstanding performance and positive environmental impact. The performance of gas separation membranes can be considerably enhanced by mixed-matrix membranes (MMMs), but poor interactions between additives and membrane matrix and the agglomeration of highly incorporated fillers in MMMs limit the benefits of overcoming tradeoff effect. Therefore, the regulation of the state of fillers in membrane matrix is a crucial indicator in the development of MMMs. In this study, MXene (Ti3C2Tx) nanosheets and UiO-66 nanoparticles were prepared and used as fillers, in combination with a Pepax-1657 matrix, to synthesize MMMs for CO2-gas separation. As-prepared MMMs were used for the separation; the large pores (0.5–0.6 nm) of UiO-66 served as a highway to enhance CO2 permeability, and MXene nanosheets were used as selective channels to achieve high selectivity via the terminal polar surface groups of MXene. The combination of UiO-66 and MXene not only enhanced the selectivity, but also increased the solubility and diffusivity via selective CO2 adsorption and the tortuous pathway provided by MXene nanosheets. As a result, adding MXene and UiO-66 to Pebax helped MMMs overcome the tradeoff effect. MMMs loaded with 10 wt% MXene/UiO-66 exhibited a CO2 permeability coefficient of 214.6 Barrer and an ideal CO2/N2 selectivity coefficient of 102. Dual filler-containing MMMs showed enhanced CO2 permeability coefficient and CO2/N2 selectivity in comparison to pristine Pebax and individual nanofiller-added membranes. In this study, the combination of 2D MXene and UiO-66 nanoparticles exerted a synergistic effect in improving the CO2 separation efficiency of MMMs. This research provides a novel route for fabricating high-performance MMMs in the separation of CO2.

Nanocellulose membrane with double-salt deep eutectic solvent for efficient carbon capture

Membrane-based gas separation technology is becoming increasingly attractive as a feasible alternative for carbon capture to alleviate global warming. Here, a double-salt deep eutectic solvent (DES) composed of organic salt (choline chloride) and inorganic salt (ZnCl2) is designed. The as-prepared DES exhibits a superb affinity and compatibility with nanocellulose because of the interactions between salt ions and cellulose carboxyl/hydroxyl groups. Through a solvent impregnation process, the DES was integrated into the nanocellulose gel precursor and used as a facilitated transport agent to form the functionalized nanocellulose membrane. The DES is tightly attached to the cellulose fibers with a high uniformity in the membrane matrix. Gas separation tests demonstrated that the DES@nanocellulose membranes presented a maximum CO2 permeability of 155.8 Barrer, with CO2/N2 and CO2/CH4 ideal selectivities of 43.6 and 48.4, respectively. Mixed gas separation tests were performed and the optimal membrane exhibited separation factors of 48.1 and 52.3 for CO2/N2 and CO2/CH4, respectively. The membranes provide a high solubility for CO2 due to the presence of abundant zinc ions, choline cations and carboxyl groups in the network, which can establish reversible coordination with CO2 and promote CO2 transport.

Amidoxime-functionalized tetraphenylethylene ladder polymer for efficient membrane-based gas separations

Recent advances in solution-processible polymers of intrinsic microporosity (PIM) have attracted extensive research efforts in membrane-based gas separations due to their significantly enhanced gas permeability originating from inefficient chain packing. However, PIM-based membrane materials exhibit typically only moderate gas-pair selectivity, which is often insufficient especially for challenging gas separations containing highly sorbing feed components such as CO2. More importantly, they suffer from CO2-induced polymer chain dilation commonly referred as plasticization, which leads to poor separation performance under aggressive CO2/CH4 mixed-gas testing conditions. In this work, we conducted amidoxime (AO)-functionalization on a microporous tetraphenylethylene-based ladder polymer (TPE-PIM) by reacting its nitrile groups with hydroxyl amine under reflux conditions. Because of strong inter-chain hydrogen bonding, AO-functionalized TPE polymer (AO-TPE) exhibited a tightened and rigidified microstructure with 64 % reduction in Brunauer-Emmett-Teller (BET) surface area (175 m2/g) as compared to TPE-PIM (485 m2/g) derived from N2 adsorption isotherms at 77 K. As a result, fresh AO-TPE showed decreased gas permeability concurrent with a two-fold increase in CO2/CH4 selectivity (from 18 to 40) compared to TPE-PIM. Under aggressive 50:50 CO2:CH4 mixed-gas testing conditions, AO-TPE showed a strong plasticization resistance with decreased CH4 permeability over total feed pressure ranging from 4 to 30 bar. At 20 bar total pressure, AO-TPE demonstrated a mixed-gas CO2/CH4 selectivity of 37.7 as compared to 14.5 for TPE-PIM. The simple AO functionalization strategy holds promise to be an effective post-modification method to increase gas selectivity and alleviate the detrimental effects from CO2-induced plasticization.

Parameters estimation of fabricated polysulfone membrane for CO2/CH4 separation

The emission of CO2 into the climate due to factory operations is a major health and environmental concern. The simplicity and cost-effectiveness of membrane-based gas separation make it a competitive industrial gas separation technology. However, further research is necessary to study fabrication and theoretical simulations membrane performance. There is also a research gap in analyzing different membrane designs and operational factors that affect gas separation performance. Therefore, this study employed computational fluid dynamics (CFD) to assess the polysulfone (PSF) fabricated membrane, considering four proposed operational and design factors. The CFD predicts the concentration and velocity distribution of the components. Fick's law is employed to represent the gas transport process over the membrane, while the Navier-Stokes equation is utilized to drive the flow of gases in both the inlet and permeate sides of the permeation unit. The outcomes of gas flow rate, temperature, pressure, and diameter of the membrane module on the CO2 mole fraction were investigated. Furthermore, at a high gas flow rate, the CO2 mole fraction increase in permeate. Increasing the feed temperature from 313 to 393 K results in a decrease in the CO2 mole fraction at the permeate side from 39% to 36%. The CO2 mole fraction at the site of permeation slightly decreases as the pressure increases. However, increasing the inside diameter of the cell membrane improves the mole fraction of CO2. According to the findings, the CFD model provides a valuable study on the influence of operating and design factors on a gas separation unit.

A Microporous Poly(Arylene Ether) Platform for Membrane-Based Gas Separation.

Membrane-based gas separations are crucial for an energy-efficient future. However, it is difficult to develop membrane materials that are high-performing, scalable, and processable. Microporous organic polymers (MOPs) combine benefits for gas sieving and solution processability. Herein, we report membrane performance for a new family of microporous poly(arylene ether)s (PAEs) synthesized via Pd-catalyzed C-O coupling reactions. The scaffold of these microporous polymers consists of rigid three-dimensional triptycene and stereocontorted spirobifluorene, endowing these polymers with micropore dimensions attractive for gas separations. This robust PAE synthesis method allows for the facile incorporation of functionalities and branched linkers for control of permeation and mechanical properties. A solution-processable branched polymer was formed into a submicron film and characterized for permeance and selectivity, revealing lab data that rivals property sets of commercially available membranes already optimized for much thinner configurations. Moreover, the branching motif endows these materials with outstanding plasticization resistance, and their microporous structure and stability enables benefits from competitive sorption, increasing CO2 /CH4 and (H2 S+CO2 )/CH4 selectivity in mixture tests as predicted by the dual-mode sorption model. The structural tunability, stability, and ease-of-processing suggest that this new platform of microporous polymers provides generalizable design strategies to form MOPs at scale for demanding gas separations in industry.

Advancements in defect engineering of two-dimensional nanomaterial-based membranes for enhanced gas separation.

The advent of two-dimensional nanomaterials, a revolutionary class of materials, is marked by their atomic-scale thickness, superior aspect ratios, robust mechanical attributes, and exceptional chemical stability. These materials, producible on a large scale, are emerging as the forefront candidates in the domain of membrane-based gas separation. The concept of defect engineering in 2D nanomaterials has introduced a novel approach in their application for membrane separation, offering an effective technique to augment the performance of these membranes. Nonetheless, the development of customized microstructures in gas separation membranes via defect engineering remains nascent. Hence, this review is designed to serve as a comprehensive guide for the application of defect engineering in 2D nanomaterial-based membranes. It delves into the most recent developments in this field, encompassing the synthesis methodologies of defective 2D nanomaterials and the mechanisms underlying gas transport. Special emphasis is placed on the utilization of defect-engineered 2D nanomaterial-based membranes in gas capture applications. Furthermore, the paper encapsulates the burgeoning challenges and prospective advancements in this area. In essence, defect engineering emerges as a promising avenue for enhancing the efficacy of 2D nanomaterial-based membranes in gas separation, offering significant potential for advancements in membrane-based gas separation technologies.

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.

Penetrant-induced plasticization in microporous polymer membranes.

Penetrant-induced plasticization has prevented the industrial deployment of many polymers for membrane-based gas separations. With the advent of microporous polymers, new structural design features and unprecedented property sets are now accessible under controlled laboratory conditions, but property sets can often deteriorate due to plasticization. Therefore, a critical understanding of the origins of plasticization in microporous polymers and the development of strategies to mitigate this effect are needed to advance this area of research. Herein, an integrative discussion is provided on seminal plasticization theory and gas transport models, and these theories and models are compared to an exhaustive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas permeation tests and mixed-gas permeation tests for pure polymers and composite films. Finally, an evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided along with suggestions for future directions of fundamental and applied research on the topic.

Breaking the Permeability-Selectivity Trade-Off: Advanced carbon molecular sieve membranes derived from thermally rearranged Mixed-Matrix membrane precursors

A crucial problem in membrane-based gas separation is to fabricate membranes with excellent permeability, selectivity, and stability. Here, a high-performance carbon molecular sieves membrane of TR-CMS/SSZ-13 was successfully produced by pyrolyzing a thermally rearranged (TR) mixed matrix membrane (MMM) that obtained from a copolyimide precursor (6FDA/ODA/6FAP(3:2)) and SSZ-13 zeolite as filler at 450 °C. The effect of SSZ-13 zeolite content and structures of precursors on the transport behavior of derived CMS membranes was thoroughly studied. After TR process, the as formed polybenzoxazole (PBO) chain resulted in an enhanced rigidity, higher fractional free volume (0.15 vs 0.17), and a significantly increased (1898 times) BET surface area. Further enhancing the treatment temperature to 650 °C, the formed TR-CMS membranes exhibited a huge improved BET surface area from 228.2 to 639.3 m2/g, delivering a remarkable gas separation performance with H2 permeability of 7089 Barrer and H2/CH4 selectivity of 136. When doping with SSZ-13 till 15 wt%, the TR-CMS/SSZ-13–15 % showed an even higher H2 (9726 Barrer) and CO2 permeability (6932 Barrer), as well as H2/CH4 (150 vs 136) and CO2/CH4 (107 vs 57) selectivity than the pure TR-CMS, which surpassed the newest 2019 Robeson Upper bound. This can associate with the strong interaction between the modified SSZ-13 and the polymer matrix that prevent the collapse of the aromatic strand during the pyrolysis process, thereby create more ultra-micropore volume and concentration. Conclusively, pyrolyzing TR-MMM precursor is an effective way to form high-performance membranes with good potential for CO2 separation and H2 purification.

A perspective on data-driven screening and discovery of polymer membranes for gas separation, from the molecular structure to the industrial performance

Abstract In the portfolio of technologies available for net zero-enabling solutions, such as carbon capture and low-carbon production of hydrogen, membrane-based gas separation is a sustainable alternative to energy-intensive processes, such as solvent-based absorption or cryogenic distillation. Detailed knowledge of membrane materials performance in wide operative ranges is a necessary prerequisite for the design of efficient membrane processes. With the increasing popularization of data-driven methods in natural sciences and engineering, the investigation of their potential to support materials and process design for gas separation with membranes has received increasing attention, as it can help compact the lab-to-market cycle. In this work we review several machine learning (ML) strategies for the estimation of the gas separation performance of polymer membranes. New hybrid modelling strategies, in which ML complements physics-based models and simulation methods, are also discussed. Such strategies can enable the fast screening of large databases of existing materials for a specific separation, as well as assist in de-novo materials design. We conclude by highlighting the challenges and future directions envisioned for the ML-assisted design and optimization of membrane materials and processes for traditional, as well as new, membrane separations.

Understanding the performance of RHO type zeolite membrane for CH4/N2 separation based on molecular dynamics and deep neural network methods

This study shows a molecular dynamics (MD) simulation study on the performance of the RHO zeolite membrane for separating nitrogen from methane/nitrogen gas mixtures. The contamination of natural gas, predominantly composed of methane, with nitrogen diminishes its value. Zeolite membranes offer promising prospects for gas separation due to their stability, rigid pore structure, and molecular sieving properties. The study investigates the impact of pressure difference (up to 30 MPa), feed composition, and membrane thickness on the separation rate at a system temperature of 298 K. Results demonstrate that the RHO zeolite membrane exhibits high permeability and selectivity for N2 separation, surpassing the upper limit defined by Robson with a maximum permeability of 2.14 × 105 GPU (Gas Permeation Units). Exceptional selectivity of N2 over CH4 molecules is observed. Additionally, altering the feed composition and membrane thickness positively influences the membrane's separation performance, thereby enhancing its efficiency. The findings contribute to the advancement of separation technologies, providing valuable insights into the potential application of zeolite membranes for efficient N2 separation from CH4/N2 gas mixtures in natural gas processing. Furthermore, the study explores the use of Deep Neural Network (DNN) models to predict the membrane's performance under diverse operating conditions. The DNN models, trained using simulation data from MD simulations, exhibit high accuracy with a coefficient of determination (R2) exceeding 0.9, ensuring reliable predictions. The integration of DNN models facilitates the optimization of zeolite membrane-based gas separation systems, improving their design and operation.

Excellent gas separation performance of hybrid carbon molecular sieve membrane derived from polyimide/10X zeolite for hydrogen purification

Membrane-based gas separation is attracting more and more attention in hydrogen purification and CO2 separation. One critical challenging remaining for large-scale production is fabricating a high-performance membrane with both inexpensive and excellent separation performance. In this work, a novel hybrid-carbon molecular sieve (CMS) membranes were reported for hydrogen purification by pyrolysis of commercial polyimide precursor (PMDA-ODA) with incorporation of modified 10X zeolite. The addition of modified 10X zeolite provided additional gas transport channels to the CMS mixed matrix membrane (MMM), and contained a significant fraction of micropores within the 0.6–0.8 nm range, which greatly enhance hydrogen adsorption capacity. Interestingly, despite this, the selectivity for H2/N2 and H2/CH4 also increases with higher zeolite content. This unexpected phenomenon may be attributed to the partial blocking of zeolite pores by CMS strand, which narrows the 10X pore size to a dimension smaller than CH4 and N2, enabling differentiation between H2 and CH4 gas pairs. Therefore, the permeability and selectivity of H2 was significantly enhanced. The CMS/10X-7.5 (with 7.5 wt % 10X zeolite) exhibited an advanced H2 permeability (1709 Barrer) and high gas selectivity of 551 for H2/CH4 and 105 for H2/N2, respectively, which is higher than other samples and exceed the 2015 trade-off lines. Furthermore, the CMS/10X-7.5 delivered excellent stability after 20 cycles. Therefore, this hybrid CMS/10X-7.5 obtained by carbonization of zeolite-doped polymer precursors has great potential for larger industrial applications.

Ultra-stable fully-aromatic microporous polyamide membrane for molecular sieving of nitrogen over volatile organic compound

Microporous polymer membranes are promising candidates for industrial membrane-based gas separation because of their high separation performance. However, their relatively low stability due to the local rearrangement of polymer chains during usage remains a problem. Hence, we propose the construction of a fully aromatic polymer structure in a microporous polymer membrane to enhance membrane stability. Four triptycene-based microporous polyamides were synthesized via the polymerization of 2,6,14-triaminotriptycene with aromatic acyl chloride and/or aliphatic acyl chlorides. Their properties were characterized and compared by using nuclear magnetic resonance (NMR) and Brunauer–Emmett–Teller analyses. The synthesized polyamides were fabricated into composite membranes by employing a solution process; their stability was evaluated for the molecular sieving of nitrogen over volatile organic compounds such as cyclohexane. Low-field NMR and X-ray photoelectron spectroscopy were used to investigate the differences in the properties of membranes with different structures at different times. The results showed that the fully aromatic polyamide membrane made from 2,6,14-triaminotriptycene and aromatic acyl chloride displayed constant rejection (99 %) and nitrogen permeability (approximately 50 Barrer) for the molecular sieving of nitrogen over cyclohexane during 100-d experiments, indicating good stability. This approach paves the way for the industrialization of microporous polymer membranes from a theoretical perspective.