CO2 Permeability Research Articles

Preparation and Gas Separation of Amorphous Silicon Oxycarbide Membrane Supported on Silicon Nitride Membrane.

An amorphous silicon oxycarbide membrane supported on a silicon nitride membrane substrate was prepared. A starting suspension containing polyhydromethylsiloxane (PHMS), tetramethyltetravinyl-cyclotetrasiloxane (TMTVS) and a platinum catalyst was first prepared and spin-coated on a silicon nitride membrane, and then the suspension was cross-linked and cured, followed by pyrolyzing at 1000 °C under a flowing Ar atmosphere. A dense amorphous silicon oxycarbon ceramic membrane with a thickness of about 1.8 µm was strongly bonded to the Si3N4 membrane substrate. The single gas permeation of H2 and CO2 indicated that the ideal permeation selectivity of H2/CO2 was up to 20 at 25 °C and 0.5 MPa with good long-term stability, indicating the potential application of the obtained membrane for hydrogen purification.

Merging polymers of intrinsic microporosity and porous carbon-based zinc oxide composites in novel mixed matrix membranes for efficient gas separation

Mixed matrix membranes (MMMs) have demonstrated significant promise in energy-intensive gas separations by amalgamating the unique properties of fillers with the facile processability of polymers. However, achieving a simultaneous enhancement of permeability and selectivity remains a formidable challenge, due to the difficulty of achieving an optimal match between polymers and fillers. In this study, we incorporate a porous carbon-based zinc oxide composite (C@ZnO) into high-permeability polymers of intrinsic microporosity (PIMs) to fabricate MMMs. The dipole–dipole interaction between C@ZnO and PIMs ensures their exceptional compatibility, mitigating the formation of non-selective voids in the resulting MMMs. Concurrently, C@ZnO with abundant interconnected pores can provide additional low-resistance pathways for gas transport in MMMs. As a result, the CO2 permeability of the optimized C@ZnO/PIM-1 MMMs is elevated to 13,215 barrer, while the CO2/N2 and CO2/CH4 selectivity reached 21.5 and 14.4, respectively, substantially surpassing the 2008 Robeson upper bound. Additionally, molecular simulation results further corroborate that the augmented membrane gas selectivity is attributed to the superior CO2 affinity of C@ZnO. In summary, we believe that this work not only expands the application of MMMs for gas separation but also heralds a paradigm shift in the application of porous carbon materials.

Macromolecular insight into the adsorption and migration properties of CH4/CO2/N2 in bituminous coal matrix under uniaxial strain loading

Gas adsorption–migration in coal is of crucial importance for coalbed methane (CBM) recovery; however, the effect of coal deformation on it is not yet very clear, especially at the molecular level. In this study, the effects of uniaxial tension–compression strains on the CH4/CO2/N2 adsorption–migration characteristics in bituminous coal matrix were investigated by integrating the grand canonical Monte Carlo and molecular dynamics methods. The results show that the dual-mode equation fits the isothermal adsorption results, and the adsorption concentration and Langmuir volume are positively correlated with strain. Tension strain has a small effect on a thermodynamic factor but a large effect on Henry constant. In addition, the swelling resistance of coal matrix is positively correlated with tension strain which has a greater impact on the shear resistance of coal matrix containing CO2. The average mass density of the gas is linearly positively correlated with strain that has a large impact on N2 stability. More significantly, the self-diffusion coefficient (Ds) of CH4 is larger than that of CO2/N2, and the relationship between Fick diffusion coefficient and strain is roughly similar to that between Ds and strain. Also, the mass transfer of CO2 permeation is more significant compared to N2, especially for tension strain. These research results provide a basis for the optimization design of CBM recovery in deformed coal reservoirs.

Tungsten-doped high-silica CHA zeolite membranes with improved hydrophobicity for CO2 separation

High-silica CHA (SSZ-13) zeolites membranes, with pore size between the molecular size of CO2 and N2 or CH4, are promising for some industrial CO2 separation process. Considering water is typically present in these industrial gas feed and the negative impact of moisture on the membranes’ gas permeation, it is highly desirable to increase the surface hydrophobicity of membranes to reduce the adsorption of water. In this paper, a W-doping strategy was proposed to increase the hydrophobicity of SSZ-13 membranes by eliminating the silanols on the surface, and the W-doped high-silica CHA (W-SSZ-13) zeolite membranes were prepared for the first time. Characterizations such as SEM, XRD, and XPS confirmed the successful incorporation of W into CHA membrane. The incorporation of W reduced the pore mouth size, and the W-SSZ-13 zeolite membranes showed improved CO2/N2 and CO2/CH4 separation performance. After W-doping, the maximum separation factors (αmax) for the CO2/N2 mixture with equimolar composition under dry conditions increased from 8.8 to 16.9, and for the equimolar CO2/CH4 mixture, the value increased from 105.9 to 176. The W-SSZ-13 membranes also showed better separation performance under wet conditions. For both systems, the W-SSZ-13 membrane showed higher αmax and less CO2 permeance reductions, indicating a more resistance in wet conditions.

Effect of ZIF-78 metal-organic framework crystal morphology and defect in resulting polysulfone mixed matrix membranes

BackgroundMixed matrix membranes (MMMs) provide a new possibility for membranes to exceed the tradeoff between permeability and selectivity found in pure polymeric membranes. Factors that enhance gas separation performance in MMMs include filler morphology and aspect ratio. MethodsHigh aspect ratio microporous ZIF-78, a metal-organic framework (MOF) demonstrated in literature to have one of the highest carbon dioxide uptake capacities, was successfully synthesized using a solvothermal method. These particles were incorporated into MMMs in non-defective and systematically defective forms by using two common solvents – chloroform and DMF. Detailed characterization of the various membranes was performed using BET, XRD, SEM, EDS, DSC, TGA, FTIR, and a custom-built gas permeability measurement apparatus. Significant findingsIntroduction of crystals with no apparent defect results in a more tortuous pathway for permeating gas molecules, demonstrated by nitrogen permeation results. Conversely, for carbon dioxide, strong dipole-quadrupole interaction with ZIF-78 creates excellent permeation pathways through the crystal themselves. The defective system possesses more inter-filler free volume which allow all gas components to permeate more easily, in addition to the original mechanism. The gas separation performance of the resulting MMMs in this work displayed a 140 % improvement of CO2 permeability and 129 % improvement of CO2/N2 ideal selectivity over pure polysulfone membranes. This work shows the potential of using deliberately designed fillers to increase performance via special pathways for gas molecules.

Ultrathin PEI-functionalized carboxyl covalent organic framework membranes for efficient CO2/N2 separation

Covalent organic frameworks (COF) are deemed as disruptive membrane materials owing to their versatile functionalities, high stability and ordered nanochannels. However, the size mismatch between the COF intrinsic pore and gas molecules becomes a grand challenge when using COF membrane for carbon capture. Herein, we propose a new type of polymer-functionalized COF laminar membranes through grafting CO2-phlic polyethyleneimine (PEI) polymer onto carboxylic acid COF (cCOF) laminar membranes. The cCOF nanosheets with high density of carboxyl groups are prepared via modified single phase solution method and then assembled to fabricate membranes. Further, the cCOF membranes are functionalized with PEI using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-Hydroxysuccinimide (EDC/NHS) coupling chemistry. The introduction of abundant amino groups from PEI afford the affinity of membranes toward CO2. Meanwhile, the graft of the PEI polymers reduces the defects or pinholes on the membrane surface. As a result, the PEI functionalized cCOF membranes exhibit a high CO2 permeance of 1004 GPU and a CO2/N2 selectivity of 33.7, which reach the carbon capture target performance and show great prospect for treating real flue gas. Our approach of functionalizing cCOF membranes with tailored functionalities may inspire the design of molecule-selective COF membranes.

Room temperature synthesis, characterization and enhanced gas transport properties of novel poly(oxindolylidene arylene)s with dibenzothiophene, dibenzothiophene-S-oxide and dibenzothiophene-S,S-dioxide fragments in the main chain

Aromatic sulfur-containing polymers poly(arylene ether sulfone)s and poly(arylene sulfide sulfone)s are widely used in membrane-based gas- and liquid separations and purification technologies. In this work, we report a facile, robust, room temperature synthesis of a series of novel, solution processable aromatic polymers containing dibenzothiophene, dibenzothiophene-S-oxide, and dibenzothiophene-S,S-dioxide fragments in the main chain, alternating with bulky, torsion resistant oxindolylidene groups. The NMR studies of the polymers obtained revealed ladderized, rigid kink-structured backbones. This combination provides membranes with higher permeability and selectivity with gas pairs CO2/CH4, H2/CH4 and H2/N2, close to or even surpassing the 2008 Robeson’s trade-off line, that compare favorably with polysulfones and poly(ether sulfone)s reported in the literature that lie below the 1991 upper bond. Polymer P2-SO2 containing dibenzothiophene-S,S-dioxide fragments showed impressive CO2 and H2 permeabilities (243. 3 and 315.1 Barrer, respectively); on the other hand, P1-SO containing dibenzothiophene-S-oxide exhibited remarkable H2/CH4 and H2/N2 selectivity (127.26 and 99.34, respectively). This work offers a novel and facile route to fabricate various polysulfones and polysulfoxides with reversible-tunable gas separation properties inspired by the concept of rigid kink-structured backbones that shift their gas transport properties towards higher permeability and higher selectivity.

Amidation-Reaction Strategy Constructs Versatile Mixed Matrix Composite Membranes towards Efficient Volatile Organic Compounds Adsorption and CO2 Separation.

Mixed matrix composite membranes (MMCMs) have shown advantages in reducing VOCs and CO2 emissions. Suitable composite layer, substrate, and good compatibility between the filler and the matrix in the composite layer are critical issues in designing MMCMs. This work develops a high-performance UiO-66-NA@PDMS/MCE for VOCs adsorption and CO2 permea-selectivity, based on a simple and facile fabrication of composite layer using amidation-reaction approach on the substrate. The composite layer shows a continuous morphological appearance without interface voids. This outstanding compatibility interaction between UiO-66-NH2 and PDMS is confirmed by molecular simulations. The Si─O functional group and UiO-66-NH2 in the layer leads to improved VOCs adsorption via active sites, skeleton interaction, electrostatic interaction, and van der Waals force. The layer and ─CONH─ also facilitate CO2 transport. The MMCMs show strong four VOCs adsorption and high CO2 permeance of 276.5 GPU with a selectivity of 36.2. The existence of VOCs in UiO-66-NA@PDMS/MCE increases the polarity and fine-tunes the pore size of UiO-66-NH2, improving the affinity towards CO2 and thus promoting the permea-selectivity for CO2, which is further verified by GCMC and EMD methods. This work is expected to offer a facile composite layer manufacturing method for MMCMs with high VOC adsorption and CO2 permea-selectivity.

Thermally rearranged OH-containing polyimide composite membranes with enhanced gas separation performance and physical aging resistance

Gas separation membranes play a pivotal role in various industrial processes, demanding continuous improvement in their efficiency and durability. This study focuses on thin-film composite (TFC) membranes based on polyimide, utilizing a hydroxyl-containing polyimide selective layer for its processability and selectivity. A complete and smooth surface for the selective layer in the composite membranes was achieved by exclusively selecting a 10% concentration of the polyimide spin-coating solution. Subsequent thermal rearrangement at 430°C induced the formation of a rigid polybenzoxazole structure, enhancing gas permeance. Optimal separation performance was achieved through a two-hour rearrangement, resulting in a more than 30-fold increase in gas permeance while preserving selectivity. The study also delved into anti-physical aging performance. The thermally treated PI-10 wt%−2 h membrane exhibited a 10% reduction in CO2 and O2 permeance over 90 days, with O2/N2 and CO2/N2 selectivities nearly maintaining their initial levels. This research provides valuable insights for the development of efficient and durable gas separation membranes.

Supported Ionic Liquid Membrane with Highly-permeable Polyamide Armor by In Situ Interfacial Polymerization for Durable CO2 Separation.

Supported ionic liquid membranes (SILMs), owing to their capacities in harnessing physicochemical properties of ionic liquid for exceptional CO2 solubility, have emerged as a promising platform for CO2 extraction. Despite great achievements, existing SILMs suffer from poor structural and performance stability under high-pressure or long-term operations, significantly limiting their applications. Herein, a one-step and in situ interfacial polymerization strategy is proposed to elaborate a thin, mechanically-robust, and highly-permeable polyamide armor on the SILMs to effectively protect ionic liquid within porous supports, allowing for intensifying the overall stability of SILMs without compromising CO2 separation performance. The armored SILMs have a profound increase of breakthrough pressure by 105% compared to conventional counterparts without armor, and display high and stable operating pressure exceeding that of most SILMs previously reported. It is further demonstrated that the armored SILMs exhibit ultrahigh ideal CO2/N2 selectivity of about 200 and excellent CO2 permeation of 78 barrers upon over 150h operation, as opposed to the full failure of CO2 separation performance within 36h using conventional SILMs. The design concept of armor provides a flexible and additional dimension in developing high-performance and durable SILMs, pushing the practical application of ionic liquids in separation processes.

Low-Loading Mixed Matrix Materials: Fractal-Like Structure and Peculiarly Enhanced Gas Permeability.

Mixed matrix materials (MMMs) containing metal-organic framework (MOF) nanoparticles are attractive for membrane carbon capture. Particularly, adding <5 mass % MOFs in polymers dramatically increased gas permeability, far surpassing the Maxwell model's prediction. However, no sound mechanisms have been offered to explain this unusual low-loading phenomenon. Herein, we design an ideal series of MMMs containing polyethers (one of the leading polymers for CO2/N2 separation) and discrete metal-organic polyhedra (MOPs) with cage sizes of 2-5 nm. Adding 3 mass % MOP-3 in a polyether increases the CO2 permeability by 100% from 510 to 1000 Barrer at 35 °C because of the increased gas diffusivity. No discernible changes in typical physical properties governing gas transport properties are detected, such as glass transition temperature, fractional free volume, d-spacing, etc. We hypothesize that this behavior is attributed to fractal-like networks formed by highly porous MOPs, and for the first time, we validate this hypothesis using small-angle X-ray scattering analysis.

Exploring the separation properties of high-Si CHA membranes for the CO2 capturing technology: Impact of the selective layer thickness and growth mechanism

This study investigates the influence of the CHA selective layer's thickness, coated on a mono-channel alumina support using the secondary growth method, on the performance of CO2 separation from CH4. The membrane layer's thickness was explored by adjusting the concentration and duration of the seeding process, as well as the distribution of crystal size in the suspension. Single gas analysis results revealed that utilizing smaller crystal seeds, with an average crystallite size of approximately 600 nm, exhibited the highest performance. Furthermore, a membrane thickness of 2.1 μm demonstrated an ideal CO2/CH4 permselectivity of 205 with a CO2 permeance of 2.39 x 10-6 [mol/(m2sPa)]. For the equimolar CO2 - CH4 gas mixture, the high-Si membrane displayed a CO2/CH4 selectivity of 210 with a CO2 permeance of 8.06 x 10-7 [mol/(m2sPa)]. In both gas permeation measurement, CO2 permeance exhibited a slight linear decrease with increasing thickness, while CH4 permeance initially decreased and then increased non-linearly.

Linker-exchanged zeolitic imidazolate framework membranes for efficient CO2 separation

Metal organic framework (MOF) membranes have exhibited great potential for efficient gas separation owing to their excellent chemical and structural tunability. To realize CO2 separation from flue gas, linker modification or exchange is generally required to functionalize MOF membranes. However, the reported linker-exchanged polycrystalline MOF membranes have not yet achieved attractive CO2 separation performance due to the sacrificed permeance of the overly reduced pore apertures when employing bulky linkers. Herein, we report a 3-amino-1,2,4-triazole (Atz)-exchanged polycrystalline ZIF-8 membrane (Atz-ZIF-8) with Atz molar ratios ranging from 3% to 29% which shows an excellent CO2/N2 separation performance that significantly surpasses that of other linker-exchanged ZIF-8 membranes. The as-synthesized Atz-ZIF-8 membranes possess additional uncoordinated amines for CO2-preferential adsorption, resulting in an excellent CO2/N2 selectivity of 29. What's more, the size and configuration of Atz is similar to 2-methylimidazole (2-mIm), so the window size of Atz-ZIF-8 membranes is not significantly reduced, and thus high CO2 permeance (690 GPU) is achieved. Considering the superior performance and accessible manufacturing process, we conclude that our work could provide guidelines on designing advanced linker-exchanged ZIF-8 based polycrystalline membranes towards post-combustion carbon capture.

In-situ crosslinking of a novel conjugated dialkynyl-based polyimide for high performance carbon molecular sieve membrane

High-performance and reliable membranes for efficient gas separation and their design strategies are key points for membrane-based carbon capture. Here, we reported a high-performance carbon membrane design protocol from a microporous, highly rigid polyimide precursor (6FBD1-CR), which was produced by in-situ crosslinking a polyimide (6FBD1) containing conjugated dialkynyl group at 300 °C. The possible crosslinking mechanisms are the [2+2+2] trimerization of three ethynyl groups to form a benzene ring and the Diels-Alder reaction between the ethynyl bond and phenylacetylene to generate the fused naphthalene ring. After crosslinking, the membrane shows a ∼ 8-fold improved microporosity (37.1–298 m2 g−1). When pyrolyzed at 550 °C, the resulting 6FBD1-CMS demonstrates very high pure- and mixed-gas separation performance. E.g., the CO2 permeability reaches 7661 Barrer and CO2/CH4 selectivity of 44, additionally, even at the CO2/CH4 mixed-gas pressure of 30 bar, the PCO2 can still reach 4920 Barrer combined with a αCO2/CH4 of 28, which is well above the latest CO2/CH4 mixed-gas Upper Bound line. We consider this is due to the acetylene crosslinking induced a very stable microporous structure composed of rigid fused rings that alleviate the micropore collapse during the carbonization process, and thus, resulting in a relatively higher BET surface area, larger d-spacing, relatively higher concentration of continuous disordered phase (higher sp3/sp2) and small activation energy (Ep) for gas transport than the CMSM derived from polyimide without pre-crosslinking. This finding provides new opportunity for the structural design of high-performance CMSMs.

Boosting CO2 transport in mixed matrix membranes by chitosan-MOF networks

The chitosan (CS)-grafted UiO-66-NH2 (U6N) (CSGUN) was synthesized and incorporated into Pebax matrix to fabricate mixed matrix membranes (MMMs) for boosting CO2 transport. The excellent CO2 separation performance was exhibited for MMMs due to the following reasons: U6N with the suitable porosity and abundant CO2 transport carriers can provide a unique “expressway” for CO2 to boost CO2 transport through MMMs. The grafting CS in CSGUN can connect Pebax (regular roads) and U6N (expressway), acting as an “on-ramp” between them to enrich CO2 and guide CO2 through U6N in an orderly manner. Thus, the constructed “on-ramp” and “expressway” in MMMs can improve CO2 separation performance. Furthermore, the CS-MOF networks were formed in MMMs through using CS molecular chains to connect U6N for enhancing CO2 separation performance of MMMs. Therefore, Pebax/CSGUN MMMs exhibited the excellent CO2 separation performance with the CO2 permeability of 558 ± 7 Barrer and the CO2/CH4 selectivity of 37.5 ± 0.7. The study indicated that MMMs with CS-MOF bridged networks had great potential for boosting CO2 transport by combining “expressways” and “on-ramp”.

Dialdehyde polyethylene glycol cross‐linked poly(vinyl alcohol) membranes with enhanced CO2/N2 separation performance

AbstractThe development of carbon dioxide (CO2) separation technology is crucial for mitigating global climate change and promoting sustainable development. In this study, we successfully synthesized an array of cross‐linked poly(vinyl alcohol) (PVA) membranes, xALD‐PEG‐ALD‐c‐PVA, with enhanced CO2/N2 separation performance by employing dialdehyde polyethylene glycol (ALD‐PEG‐ALD) as a cross‐linker. The formation of the cross‐linked network structure not only inhibits the crystallization of PVA but also disrupts hydrogen bonding and thus increases fractional free volume of PVA chains. Under the synergistic effect of these multiple factors, the cross‐linked PVA membranes exhibit a significantly improved CO2 permeability. Moreover, they maintain high CO2/N2 selectivity, attributing to the CO2‐philic characteristic of ethylene oxide groups in the cross‐linked structure. At the ALD‐PEG‐ALD content of 1.6 mmol g−1, the xALD‐PEG‐ALD‐c‐PVA membrane demonstrates a CO2 permeability of 41.4 barrer and a CO2/N2 selectivity of 57.4 at 2 bar and 25°C. Furthermore, compared with the pristine PVA membrane, xALD‐PEG‐ALD‐c‐PVA membranes manifest superior mechanical properties and outstanding separation performance for a CO2/N2 (15/85, vol%) gas mixture. The excellent combination of permeability and selectivity makes xALD‐PEG‐ALD‐c‐PVA membranes highly promising for various CO2 separation applications.

Design Rationale for CO2 Separation Membranes with Micropatterned Surface Structures.

Here, we report the design rationale of CO2 separation membranes with micropatterned surface structures. Thin film composite (TFC) membranes with micropatterned surface structures were fabricated by spray coating amine-containing hydrogel particles on the top of micropatterned porous support membranes, which were synthesized by a polymerization-induced phase separation process in a micromold (PIPsμM). The pore size of the support membranes was optimized by tuning the proportion of good and poor solvents for the polymerization process so that the microgels would be assembled as a defect-free separation layer. The relationship between the size of the micropatterned structures on the surface of the support membrane and the thickness of the separation layer was optimized to maximize the surface area of the separation layer. The rationally designed micropatterned TFC membrane showed a CO2 permeability (835.8 GPU) proportional to the increase in surface area relative to the flat membrane with a high CO2/N2 selectivity of 58.7. The rational design for micropatterned TFC membranes will enable the development of inexpensive and high-performance functional membranes not only for CO2 separation but also for other applications such as water treatment and membrane reactors.

Mechanical and gas permeability properties of poly(L‐lactic acid)–based films and their application in fresh produce preservation—Review

AbstractPlastics have remained the best choice for fruit and vegetable packaging due to their exceptional transparency, flexibility, optional good barrier and cost‐effectiveness. However, a large number of plastics end up in the environment after serving their intended purpose and persist there for centuries. Since worldwide attention to environmental issues has become an irreversible historical trend, cost‐effective renewable biodegradable packaging material is highly desirable for the packaging industry to satisfy both industrial demands as well as consumer expectations. Poly (L‐lactic acid) (PLLA), a renewable biodegradable polymer derived from biomass, has been widely studied as a very attractive polymer for food packaging applications. However, its brittle nature and medium CO2/O2 permeability are not friendly for its utilisation in the aspect of fresh agricultural products packaging. In such a context, great efforts have been devoted to improvements in the mechanical and permeability of PLLA to meet the requirements of food packaging. The main purpose of the review is to summarise the mechanical and permeability properties of composite films developed by the incorporation of toughness reinforcing agents into the PLLA matrix. The elongation at break of PLA can be increased up to 250 times by modification, and the CO2 permeability of PLA can be increased by 2–3 times by modification. We also elaborate on studies of the preservation effect of PLLA composite films on fresh fruits and vegetables.

Modified cellulose nanocrystals enhanced polycaprolactone multifunctional films with barrier, UV-blocking and antimicrobial properties for food packaging

The packaging industry demands improved eco-friendly materials with new and enhanced properties. In this context, bio-nanocomposite films with antimicrobial and UV-shielding properties based on modified cellulose nanocrystals/polycaprolactone (MCNC/PCL) were fabricated via solution casting method, and then food packaging simulation was carried out. CNCs were obtained by acid hydrolysis followed by successful functionalization with Quaternary ammonium surfactant, confirmed by FTIR, XPS, XRD, TEM, and DLS analyses. Furthermore, the morphological, physical, antibacterial, and food packaging properties of all prepared films were investigated. Results showed that the mechanical, UV blocking, barrier properties, and antibacterial activity of all composite films were remarkably improved. Particularly, the addition of 3 wt% MCNC increased the tensile strength and elongation at break by 27.5 % and 20.0 %, respectively. Moreover, the permeability of O2, CO2, and water vapor dramatically reduced by 97.6 %, 96.7 %, and 49.8% compared to the Neat PCL. Further, the UV-blocking properties of the composite films were significantly improved. The antimicrobial properties of MCNC/PCL films showed good antimicrobial properties against S. aureus. Finally, cherry packaged with 1 and 3 wt% MCNC films exhibited satisfactory freshness after 22 days of preservation. Overall, the fabricated PCL nanocomposite films can be utilized in the food packaging industry.

Permeation of CO2 through the central pore of human AQP5

Permeation of CO2 through the central pore of human AQP5