Gas Separation Membranes Research Articles

Interdependent factors influencing the carbon yield, structure, and CO_{2} adsorption capacity of lignocellulose-derived carbon fibers using multiple linear regression

Cellulose has experienced a renaissance as a precursor for carbon fibers (CFs). However, cellulose possesses intrinsic challenges as precursor substrate such as typically low carbon yield. This study examines the interplay of strategies to increase the carbonization yield of (ligno-) cellulosic fibers manufactured via a coagulation process. Using Design of Experiments, this article assesses the individual and combined effects of diammonium hydrogen phosphate (DAP), lignin, and CO_{2} activation on the carbonization yield and properties of cellulose-based carbon fibers. Synergistic effects are identified using the response surface methodology. This paper evidences that DAP and lignin could affect cellulose pyrolysis positively in terms of carbonization yield. Nevertheless, DAP and lignin do not have an additive effect on increasing the yield. In fact, combined DAP and lignin can affect negatively the carbonization yield within a certain composition range. Further, the thermogravimetric CO_{2} adsorption of the respective CFs was measured, showing relatively high values (ca. 2 mmol/g) at unsaturated pressure conditions. The CFs were microporous materials with potential applications in gas separation membranes and CO_{2} storage systems.Graphical abstract

An Overview of Practical Aspects of the Design and Application of Polymeric/Ceramic Supports in Supported Liquid Membranes for Gas Separation

An Overview of Practical Aspects of the Design and Application of Polymeric/Ceramic Supports in Supported Liquid Membranes for Gas Separation

Balancing the Crystallinity and Film Formation of Metal-Organic Framework Membranes through In Situ Modulation for Efficient Gas Separation.

Polycrystalline metal-organic framework (MOF) layers hold great promise as molecular sieve membranes for efficient gas separation. Nevertheless, the high crystallinity tends to cause inter-crystalline defects/cracks in the nearby crystals, which makes crystalline porous materials face a great challenge in the fabrication of defect-free membranes. Herein, for the first time, we demonstrate the balance between crystallinity and film formation of MOF membrane through a facile in situ modulation strategy. Monocarboxylic acid was introduced as a modulator to regulate the crystallinity via competitive complexation and thus concomitantly control the film-forming state during membrane growth. Through adjusting the ratio of modulator acid/linker acid, an appropriate balance between this structural "trade-off" was achieved. The resulting MOF membrane with moderate crystallinity and coherent morphology exhibits molecular sieving for H2 /CO2 separation with selectivity up to 82.5.

Balancing the Crystallinity and Film Formation of Metal–Organic Framework Membranes through In Situ Modulation for Efficient Gas Separation

AbstractPolycrystalline metal–organic framework (MOF) layers hold great promise as molecular sieve membranes for efficient gas separation. Nevertheless, the high crystallinity tends to cause inter‐crystalline defects/cracks in the nearby crystals, which makes crystalline porous materials face a great challenge in the fabrication of defect‐free membranes. Herein, for the first time, we demonstrate the balance between crystallinity and film formation of MOF membrane through a facile in situ modulation strategy. Monocarboxylic acid was introduced as a modulator to regulate the crystallinity via competitive complexation and thus concomitantly control the film‐forming state during membrane growth. Through adjusting the ratio of modulator acid/linker acid, an appropriate balance between this structural “trade‐off” was achieved. The resulting MOF membrane with moderate crystallinity and coherent morphology exhibits molecular sieving for H2/CO2 separation with selectivity up to 82.5.

Metal-Organic Framework Membrane Hybridized with Graphitic Materials for Gas Separation.

Metal-organic frameworks (MOFs) are an exceptional class of crystalline materials that have been extensively used to fabricate membranes for various applications such as gas separation, ion transport, and desalination due to their well-defined pore structure, chemical features, and simple synthesis process. The incorporation of graphitic carbon materials in MOFs has garnered significant attention as it can provide abundant nucleation sites and modulate gas transport by influencing the orientation or rigidity of MOF crystals without changing their porous structure. This review insights of previous studies utilizing graphene, graphene oxide, carbon nanotubes, and graphene nanoribbons for MOF-based gas separation membranes, particularly focusing on polycrystalline MOF membrane hybridization with graphitic materials. We also briefly discuss the use of carbon/MOF hybrid materials for preparing mixed matrix membranes.

Metal-organic framework-based mixed matrix membranes for gas separation: Recent advances and opportunities

The demand for carbon dioxide (CO2) emission mitigation has driven the scientific community to develop a viable strategy. Gas separation technology are considered as the most promising and effective carbon capture approach. The utility of mixed-matrix membranes (MMMs) for selective gas separation has prompted a paradigm shift from conventional methods towards more efficient and sustainable processes. Metal-organic frameworks (MOFs), with high porosity, surface area, and structural tunability, are identified as suitable fillers in MMMs. Their application in carbon capture and gas separation processes could significantly reduce greenhouse gas emissions while maintain cost-effectiveness. Herein, this review presents the latest scientific and technological advancements in several types of representative MOF fillers, including Zeolitic Imidazolate Frameworks (ZIFs), Materials of Institute Lavoisier (MILs), University of Oslo (UiO), and Hong Kong University of Science and Technology (HKUST-1). It also highlights the current challenges associated with MOF fillers and outlines potential research opportunities for future improvements in the performance of MOF-based MMMs.

Polymeric Membranes for H2S and CO2 Removal from Natural Gas for Hydrogen Production: A Review

Natural gas, an important source of hydrogen, is expected to be crucial in the transition to a hydrogen-based economy. The landscape of the gas processing industry is set to change in the near future with the development of highly acidic sour gas wells. Natural gas purification constitutes a major share of the gas separation membrane market, and the shift to low-quality sour gas wells has been mirrored in the trends of membrane material research. Purification also constitutes the major portion of the cost of natural gas, posing implications for the cost of hydrogen production. This review provides an update on the current state of research regarding polymeric membranes for H2S removal, along with CO2 separation, from natural gas that is used for hydrogen production via steam methane reforming. The challenges of adapting polymeric membranes to ternary H2S/CO2/CH4 separations are discussed in detail. Key polymeric materials are highlighted, and the prospects for their application in H2S removal from natural gas are evaluated. Finally, the growing interest in H2 production from H2S is discussed. Advances in the membrane industry and the emergence of new membrane materials may significantly improve the commercial viability of such processes.

Electrochemical bulk and film-type oxygen sensors: Strategies for detecting extremely low concentration in hydrogen environments

Potentiometric oxygen sensors with excellent sensitivity in a low oxygen concentration range are designed based on intrinsic logarithmic response characteristics, and an asymmetric electrode structure, differentiated from conventional oxygen sensors with reference oxygen gases or parts exposed to air, is implemented. Electrolytes and electrode materials that formulate oxygen sensor devices are evaluated by comprehensively considering their reactivity to trace oxygen, oxygen ion formation, and ease of movement. The sensor using an yttria-stabilized zirconia bulk ceramic electrolyte measures the oxygen concentration in an oxygen-hydrogen mixture down to 0.5%, with a response time of 7.8 s. The sensor with a Nafion proton conductor film and a polyimide gas separation membrane allows room-temperature sensing and measures the oxygen concentration to a minimum of 2%.

Energy Application of Graphene Based Membrane: Hydrogen Separation.

Hydrogen gas (H2 ) is a viable energy carrier that has the potential to replace the traditional fossil fuels and contribute to achieving zero net emissions, making it an attractive option for a hydrogen-based society. However, current H2 purification technologies are often limited by high energy consumption, and as a result, there is a growing demand for alternative techniques that offer higher H2 purity and energy efficiency. Membrane separation has emerged as a promising approach for obtaining high-purity H2 gas with low energy consumption. Nevertheless, despite years of development, commercial polymeric membranes have limited performance, prompting researchers to explore alternative materials. In this context, carbon-based membranes, specifically graphene-based nanomaterials, have gained significant attention as potential membrane materials due to their unique properties. In this review, we provide a comprehensive overview of carbon-based membranes for H2 gas separation, fabrication of the membrane, and its characterization, including their advantages and limitations. We also explore the current technological challenges and suggest insights into future research directions, highlighting potential ways to improve graphene-based membranes performance for H2 separations.

Simulation of the Membrane Process of CO2 Capture from Flue Gas via Commercial Membranes While Accounting for the Presence of Water Vapor.

Carbon capture and storage is one of the potential options for reducing CO2 emissions from coal-fired power plants while preserving their operation. Mathematical modeling was carried out for a one-stage membrane process of carbon dioxide capture from the flue gases of coal-fired power plants using commercial gas separation membranes. Our calculations show that highly CO2-permeable membranes provide similar characteristics with respect to the separation process (e.g., a specific area of membrane and a specific level of electrical energy consumption) despite the significant variation in CO2/N2 and H2O/CO2 selectivity. Regarding the development of processes for the recovery of CO2 from flue gas using membrane technology, ensuring high CO2 permeance of a membrane is more important than ensuring high CO2/N2 selectivity. The presence of water vapor in flue gas provides a higher driving force of CO2 transfer through the membrane due to the dilution of CO2 in the permeate. A cross-flow membrane module operation provides better recovery of CO2 in the presence of water vapor than a counter-current operation.

Impact of concentration polarization on membrane gas separation processes: From 1D modelling to detailed CFD simulations

Membranes are one key technology for gas separation. Current design methodologies make use of a 1D set of mass balance equations with membrane taken as the only mass transfer resistance (no concentration polarization hypothesis). This strategy has however to be reconsidered with emerging high-performance membrane materials. Accordingly, a rigorous CFD simulation approach is developed in order to correctly compute their separation efficiency. The results reveal that, compared to CFD, in the presence of concentration polarization, 1D models largely overestimate the separation performance of high-performing membranes. Based on a generic modelling of mass transfer in the gas phase under concentration polarization, a novel 1D modified approach is reported, showing similar predictions compared to CFD simulations, but offering very fast computations. This strategy is of interest for the simple and systematic design of membrane gas separation processes, including process synthesis approaches.

Nanocomposite polymer blend membrane molecularly re-engineered with 2D metal-organic framework nanosheets for efficient membrane CO2 capture

Membrane gas separation technology is an energy-efficient alternative to the traditional industrial processes for CO2 capture towards the mitigation of global warming. To overcome the inherent performance trade-off behavior of polymeric membranes, many nanoarchitectures have been incorporated for developing high-performance mixed matrix membranes. Yet, a successful nanocomposite membrane design remains challenging, especially for CO2 capture, because of polymer-filler incompatibility and interfacial defects. Herein, 2D Cu-TCPP nanosheets were synthesized and incorporated into Pebax/Poly (ethylene glycol) methyl ether acrylate (PEGMEA) blends to design MMMs with exceptional CO2/N2 separation performance. This synergistic polymer blend after crosslinking, offers a highly accommodating and robust matrix environment for the homogenous distribution of nanosheets. Cu-TCPP particles were also produced as 3D nanoflowers to probe their morphological effect on gas transport. The finely dispersed 2D nanosheets expanded the free volume of the resultant nanocomposite membranes, leading to a significantly enhanced CO2 permeability of up to 1183 Barrer which was 93% higher than the neat polymers, while they also increased the transport pathway tortuosity that contributes to a very high CO2/N2 selectivity of 57.6 at a loading of merely 0.1 wt%. Not only do such separation performances surpassed the Robeson upper bound by a large margin and outcompeted many existing advanced MMM designs, but they also remained stable after long-term operations. By carefully elucidating the gas transport tuning effect of 2D metal-organic framework nanosheets in nanocomposite membranes, this work could help broaden their applicability for other environmentally important molecular separations.

Fabrication of PES MMMs with Improved Separation Performances Using Two-Dimensional rGO/ZIF-8 and MoS<sub>2</sub>/ZIF-8 Nanofillers

Modifying polymeric membranes using nanofiller is a promising method to enhance gas permeability and selectivity performance. This work used two types of ZIF-8 functionalized-2D nanofillers to fabricate polyethersulfone mixed matrix membranes. The rGO/ZIF-8 and MoS<sub>2</sub>/ZIF-8 nanofillers were first synthesised and characterised using FTIR and XRD. Then, 10 wt% of each nanofillers was added to the PES solution. TGA analysis indicates that MMMs containing rGO/ZIF-8 and MoS<sub>2</sub> /ZIF-8 exhibit improved thermal stability. No additional peaks in FTIR and XRD were observed in the MMMs, indicating that the 2D nanofillers were compatible with the PES matrix. The MMMs show significantly enhanced gas separation properties where the highest selectivity was observed for 10 wt%PRG/Pebax membrane of 35.71 with CO<sub>2</sub> permeability of 611 barrer and CH4 permeability of 17.11 barrer. These results confirm the possibility of using 2D nanofillers to develop high-performance membranes for gas separation.

Autocatalytic one-step high-temperature synthesis of carboxylated polyimides for in-situ high performance applications

Carboxylated polyimides (CxPIs) represent a great potential for gas separation membranes, highly thermostable coatings and fibers, ion-conducting materials, aerogels, (nano)composites, etc. However a prolonged and expensive two-stage method proceeding through the formation of an unstable poly(amic acid) and subsequent thermal or chemical imidization is traditionally used for the synthesis of CxPIs. Herein a new strategy consisting in one-step high-temperature synthesis of CxPIs based on 3,5-diaminobenzoic acid (DABA), the carboxyl groups of which provide an autocatalytic effect, in N-methyl-2-pyrrolidone (NMP) is presented. The effect of DABA content on the formation and properties of CxPIs is studied. It is established that high-molecular-weight CxPIs (Mw = 158–356 kDa) with five- and more stable six-membered imide rings from various dianhydrides of aromatic tetracarboxylic acids and diamines as comonomers are synthesized within 1–4 h of the reaction even with 25 mol% of DABA content. Additional catalyst-free synthesis of CxPIs in NMP allows to use the solutions obtained in situ for the production of various materials. In particular, CxPI-varnishes form the primary protective coating of the optical fiber superior to the commercial analogue in terms of its manufacturing technology, thermal and hydrolytic stability, which provides more reliable and long-term operation.

Utilization of plastic bottle waste of polyethylene terephthalate as a low-cost membrane and its modifications for gas separation

Polyethylene terephthalate (PET) is a widely used polymer in packaging products, leading to the daily disposal of millions of PET bottles as waste. 1.1 to 8.8 million tonnes of plastic waste enter the sea each year. The environmental challenge of non-biodegradable PET waste can be addressed by utilizing it as a thin-layer membrane for gas separation. This study modified the PET membrane by blending it with Pebax polymer and adding zeolite as a filler to enhance its performance. Characterization techniques, including FTIR, SEM, TGA, tensile strength testing, and contact angle measurements, were performed on all modified membranes. The membranes were prepared using phase inversion via immersion precipitation. The results showed that the PET waste membrane had a denser surface pore morphology and asymmetrical cross-sectional pores than other membranes. Adding Pebax and zeolite resulted in a more regular sponge-like pore structure. The PET, PET-Pebax, and PET-Zeolite NaY-Pebax membranes exhibited hydrophilic properties, as indicated by contact angle values ranging from 48-78°. Regarding CO2/CH4 separation, the 9% PET-Pebax membrane had the highest CO2 permeability, a 21% increase from the original PET waste membrane. Adding zeolite to the 9% PET-Pebax membrane increased CO2 permeability to 1044%.

Chemical topology molecular engineering of CO2-philic membranes toward highly efficient carbon capture

Membrane gas separation processes represent a crucial platform for achieving carbon neutral climate goals. CO2-philic membranes offer excellent solubility selectivity due to their high affinity, thereby allowing the separation of gas fractions with similar molecular sizes or smaller than CO2 molecules. This level of separation is difficult to achieve with conventional diffusion-dominated separation membranes. However, the gas permeability of PEO membranes is often hampered by dense chain packing and crystallization behavior. To overcome this challenge, we employed a molecular engineering approach to develop a unique PEO membrane with a hyperbranched chemical topology connection. The packing density of the polymer chains was effectively regulated by employing cross-linker molecules with distinct spatial configurations, while the free volume fraction was successfully reinforced. This unique molecular engineering strategy led to the creation of a PEO membrane exhibiting superior performance that surpassed the limit of the 2019 upper-bound line. This work demonstrated a molecular design strategy for the precise control of structural attributes in crosslinked PEO networks, offering an alternative solution for developing CO2 capture membranes and enabling further opportunities for rational membrane material design.

Vacuum-Assisted Self-Healing Amphiphilic Copolymer Membranes for Gas Separation.

Membrane gas separation provides a multitude of benefits over alternative separation techniques, especially in terms of energy efficiency and environmental sustainability. While polymeric membranes have been extensively investigated for gas separations, their self-healing capabilities have often been neglected. In this work, we have developed innovative self-healing amphiphilic copolymers by strategically incorporating three functional segments: n-butyl acrylate (BA), N-(hydroxymethyl)acrylamide (NMA), and methacrylic acid (MAA). Utilizing these three functional components, we have synthesized two distinct amphiphilic copolymers, namely, APNMA (PBAx-co-PNMAy) and APMAA (PBAx-co-PMAAy). These copolymers have been meticulously designed for gas separation applications. During the creation of these amphiphilic copolymers, BA and NMA segments were selected due to their vital role in the ease of tuning mechanical and self-healing properties. The functional groups (-OH and -NH) present on the NMA segment interact with CO2 through hydrogen bonding, thereby boosting CO2/N2 separation and achieving superior selectivity. We assessed the self-healing potential of these amphiphilic copolymer membranes using two distinct strategies: conventional and vacuum-assisted self-healing. In the vacuum-assisted approach, a robust vacuum pump generates a suction force, leading to the formation of a cone-like shape in the membrane. This formation allows common fracture sites to adhere and trigger the self-healing process. As a result, APNMA maintains its high gas permeability and CO2/N2 selectivity even after the vacuum-assisted self-healing operation. The ideal CO2/N2 selectivity of the APNMA membrane aligns closely with the commercially available PEBAX-1657 membrane (17.54 vs 20.09). Notably, the gas selectivity of the APNMA membrane can be readily restored after damage, in contrast to the PEBAX-1657 membrane, which loses its selectivity upon damage.

Preparation and characterization of asymmetric Kapton membranes for gas separation

It is a challenge to simultaneously achieve a high selectivity and permeance for the glassy polymeric membranes. Herein, the asymmetric Kapton membrane fabricated by pyromellitic dianhydride and 4,4′-oxydiphenylenediamine was prepared by the phase inversion method and characterized by rheometer, SEM, TGA, FT-IR and tensile testing. The aim of this work is to enhance gas separation performance of the membrane and its application value via optimizing the concentration of acetone additive, coagulation bath composition and evaporation time. After optimizing, results showed that the transition from the instantaneous coagulation to the delayed coagulation, and the inner membrane structure is gradually transferred from the finger-like shape to the sponge-like shape. The optimized gas separation performance of the resulting Kapton membrane has the permeance of 2.49 GPU and 1.10 GPU for H2 and CO2, respectively, as well as selectivity of 57.07 and 25.27 for H2/CH4 and CO2/CH4, respectively.

A sustainable methodology to produce open-cell porous membranes with control on the dense layer thickness

A new approach to produce porous membranes with dense or porous top layer is proposed in this work by employing a solvent-free method. PMMA/MAM formulations were selected as a base material in order to create open-cell or close-cell structures by gas dissolution foaming employing CO2 as a blowing agent. Furthermore, by introducing the gas diffusion barrier approach to CO2 dissolution foaming, it is possible to control the thickness of the dense layer in both edges, obtaining defect-free membranes (i.e., completely dense without pin-holes). The effectiveness of nanocellular polymers as gas separation membranes was evaluated. In this way, the permeability, selectivity, and permeance were correlated to the cellular structure (open or close-cell) as well as to the dense layer thickness. Furthermore, the effective thickness of the selective layer has been calculated from gas permeability measurements, obtaining an accurate control of that parameter from the tunable cellular structure. Therefore, membranes composed of desired selective layer and a porous structure as a mechanical support are produced by a solvent-free methodology.

Studies on application of Petcoke-based graphene membranes for CO2 separation

ABSTRACT Graphene-based separation membranes exhibit high separation potential to separate ions and molecules by the interlayer spacing with high surface area, porosity and tunable nanosized channels. The large-scale manufacturing of graphene-based membranes for gas separation is limited and challenging. Deposition of uniform, and selective ultrathin films as membrane is a big challenge for scalable separation with existing methods. In present study, solvent-ink-jet printing technique is applied for dispersing and uniform coating of graphene layers onto the membrane support. Preparation of graphene-based ink is the core objective of the operation for evaluating stability, coating and followed by separation studies. As prepared, graphene ink is filled in the cartridge of HP Desk-Jet-1110 series, which is available and slightly modified for the printing on the RO-membrane as support. The prepared membranes are characterized and analyzed for gas separation performance study. The decrease in gas composition of CO2 in permeate is observed and around 93% ± 0.5 of CO2 is separated from the gaseous mixtures.