Gas Pair Selectivity Research Articles

Exploring physical aging in PIM-1 using molecular dynamics

This comprehensive study explored the aging process of PIM-1, a ladder polymer of intrinsic microporosity (PIM), by applying molecular dynamics simulations for the first time. Through detailed analysis, our work illustrates the evolution of the polymer structure from a loosely packed, less dense state of the pristine polymer to a more tightly packed configuration due to physical aging. For this purpose, a novel molecular dynamics (MD) methodology was employed in the process toward equilibration of PIM-1. This structural transition was quantitatively captured by measuring key parameters such as density, fractional free volume (FFV), cohesive energy density (CED), d-spacing, surface area, and gas permeabilities. The simulations demonstrate a noticeable increase in density by approximately 7 % in aged PIM-1 compared to a fresh sample. This increase in density is accompanied by a corresponding decrease in FFV, suggesting a more compact molecular arrangement. The impact of these structural changes is evident in the gas transport properties. Permeabilities of all gases tested, He, H2, O2, N2, CO2 and CH4, decreased by 33 %–80 %. Moreover, the selectivity of gas pairs like CO2/CH4 and O2/N2 exhibited increasing trends due to aging, as previously reported in experimental work. Structural analysis performed on the fresh and aged structures indicated collapse of free volume over aging, by disappearance of pores larger than ∼6.5 Å. Furthermore, no intrachain rearrangement was observed during physical aging in the ladder PIM-1 structure; rather, the aging resulted in increased interchain packing efficiency. Our methodology can be employed to other PIM architectures, such as polyimides of intrinsic microporosity (PIM-PIs) as well as low-free volume glassy polymers.

Polynorbornenes with carbocyclic substituents: A perspective approach to highly permeable gas separation membranes

Gas transport properties of two sets of polynorbornenes bearing carbocyclic substituents of different nature (cyclohexyl, norbornyl, phenyl groups, and framed oligocyclic moieties) were systematically studied. The influence of side chain carbocyclic substituents on gas permeability and separation selectivity for CO2-containing gas pairs and gaseous hydrocarbons was evaluated. It was found that the presence of carbocyclic moieties in side chains of polynorbornenes promotes the increase in gas permeability similarly to that of SiMe3 groups, with this effect being enhanced by the increase in the number of cycles in the substituents. As a result, P(CO2) of the metathesis polymer with pentacene-based substituents is equal to 1200 Barrer. Studied polynorbornenes with carbocyclic substituents displayed promising gas separation selectivities: CO2/N2 separation selectivity up to 41 (above the 2008 upper bound in the Robeson diagram) and solubility-controlled hydrocarbon separation selectivity up to 16 for the n-butane/methane gas pair. For the polymer with pentacene-based substituents, the separation of the mixture of hydrocarbons and aging were studied. The studies revealed that the mixed gas C4/C1 separation selectivity of this polymer reaches 4, and a two-time reduction of permeability occurs in 8.5 months. Therefore, the incorporation of carbocyclic groups in the side chains of polynorbornenes can be considered as an efficient strategy for the design of polymeric gas separation membrane materials.

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.

Synthesis and gas transport properties of polynaphthoylenebenzimidazoles with keto- and sulfonic bridging groups

Polynaphthoylenebenzimidazoles (PNBI) with keto-(PNBI-CO) and sulfonic (PNBI-SO2) bridge groups were obtained by solid-state polycyclization of polyaminoimides (PANI) synthesized by polycondensation of 1,4,5,8-naphthalenetetracarboxylic acid dianhydride with 3,3`,4,4`-tetraaminobenzophenone and 3,3`,4,4`-tetraaminodiphenylsulfone in N-methylpyrrolidone, respectively. The polycondensation process and resulting chemical structure of PANI and PNBI were controlled by 1H NMR, 13C NMR and IR spectroscopy. It is shown that the temperature of solid-state polycyclization change makes it possible to obtain polymers of several of cyclization degrees. The experimental values of the gas permeability and diffusion coefficients for He, H2, N2, O2, CO2, CH4 were obtained. The gas solubility coefficients and the ideal selectivity for various gas pairs were calculated. It has been established that, in terms of the permeability-selectivity ratio, completely cyclized PNBIs occupy a more favorable position compared to incompletely cyclized ones. This result is important for polymer and a method selection to develop a selective layer of new composite membranes. The gas transport characteristics achieved for competely cyclized PNBI-SO2, as well as the film-forming properties, along with the very high thermal stability of polymers of this polymer class, are interest of further expanding the range of PNBI obtained, as well as the prospects for such new polymers using of in various gas separation processes.

Single and mixed gas permeability studies on mixed matrix membranes composed of MIL-101(Cr) or MIL-177(Ti) and highly permeable polymers of intrinsic microporosity

The gas transport properties of mixed matrix membranes (MMMs), prepared by dispersing nanoparticles of MOFs MIL-101(Cr) or MIL-177(Ti) into highly permeable Polymers of Intrinsic Microporosity PIM-EA-TB or PIM-TMN-Trip, were investigated. The homogeneity of the dispersion was confirmed by means of Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray (EDX) mapping analysis. Single gas time-lag measurements provided the permeability and ideal selectivity of different gas pairs, both after treatment of the MMMs with methanol and after natural aging over an extended period (up to 2000 days). This data demonstrated that the gas size-sieving pores of MIL-101(Cr) and MIL-177(Ti) and their good dispersion into the PIM matrix results in MMMs with enhanced gas separation performance, as compared to films composed solely of the polymer. The comparison of actual permeability with the Maxwell model for PIM-EA-TB with both MOFs confirmed the good dispersion and the absence of anomalies, whereas the inconsistency of permeability with prediction data for PIM-TMN-Trip suggests that the MOFs improved the polymer properties, stiffening or occupying the polymer free volume. In particular, the incorporation of MIL-101(Cr) into PIM-EA-TB significantly enhances the H2 permeability from ∼6000 to 13,000 Barrer, with a concurrent increase of the H2/N2 selectivity from 14 to 21. MIL-177(Ti) also enhances the H2/N2 selectivity to 20 due to a slight reduction of the N2 permeability. The addition of MIL-101(Cr) and MIL-177(Ti) into the ultra-permeable PIM-TMN-Trip showed more modest increases in H2 permeability and H2/N2 selectivity from 4.6 to ∼ 10. Hence the data for some of the MMMs surpass the 2008, and even approach the 2015/2019 Robeson’s upper bounds, particularly for gas pairs including H2.

Unusual 3,4-Oxidative Coupling Polymerization on 1,2,5-Trisubstituted Pyrroles for Novel Porous Organic Polymers.

Porous organic polymers (POPs) have demonstrated promising task-specific applications due to their structure designability and thus functionality. Herein, an unusual 3,4-polymerization on 1,2,5-trisubstituted pyrroles has been developed to give linear polypyrrole-3,4 in high efficiency, with Mn of 20000 and PDI of 1.7. This novel polymerization technique was applied to prepare a series of polypyrrole-based POPs (PY-POP-1-4), which exhibited high BET surface areas (up to 762 m2 g-1) with a meso-micro-supermicro hierarchically porous structure. Furthermore, PY-POPs were doped in the mixed matrix membranes based on the polysulfone matrix to enhance the gas permeability and gas pair selectivity, with H2/N2 selectivity up to 84.6 and CO2/CH4 and CO2/N2 selectivity up to 46.8 and 39.6.

Tailoring the CO2 permeation of Pebax1657/polyether imide thin film composite membrane via embedding Ag-based metal-organic framework

Thin-film composite/nanocomposite (TFC/TFN) membranes count as the latest version of carbon capture membranes characterized by their excellent gas permeance, great selectivity, cost-effectiveness, and the ability to separately manage and tune the constituents of the individual layer. In this regard, Pebax®1657 embedded with synthesized Ag-BTC metal-organic frameworks (MOFs) was coated on the polyether imide (PEI) support layer to produce TFNs. To assess the impacts of Ag-BTC particles (carboxyl-rich surface particles) on the features of the constructed TFC/TFN membranes and their gas permeation qualities, several CO2/N2 and CO2/CH4 separation experiments (mixed and pure gas testing) were carried out. Ag-BTC particles were distributed appropriately within the Pebax1657 matrix, improving the gas permeability (particularly for CO2) and enhancing CO2/N2 and CO2/CH4 separation factors. A simultaneous increase in CO2 permeability and gas pair selectivities was observed with rising feed pressure. A 394.97 Barrer CO2 permeability and 38.20 CO2/N2 and 21.25 CO2/CH4 selectivities were obtained for the TFN filled with 3 wt% Ag-BTC at 10 bar and 30 °C. A mixed gas test also showed a similar trend but lower values than a pure one. The results for the humidified conditions exhibited higher CO2 permselectivities in comparison with the dry gas given the water molecules facilitate the CO2 transportation through the Pebax®1657 matrix. Finally, enhancing Ag-BTC filler content and increasing the input pressure provided excellent conditions for approaching Robeson’s limit.

Structural evolution and gas separation properties of thermally rearranged polybenzoxazole (TR-PBO), polymer-carbon transition (PCT) and early-stage carbon (ESC) membranes derived from a 6FDA-hydroxyl-functionalized Tröger's base polyimide

The changes in physical, chemical, and gas permeation properties of a heat-treated hydroxyl-functionalized Tröger's base-derived intrinsically microporous polyimide, 6FDA-HTB, were systematically studied. Polyimide samples were heat-treated for 30 min at a fixed temperature to form (i) polybenzoxazole (PBO) by thermal rearrangement (TR) from ∼420 to ∼440 °C, (ii) polymer-carbon transition (PCT) from ∼460 to ∼500 °C, and (iii) early-stage carbon (ESC) membranes from >500 to 600 °C. As frequently observed in previous studies, TR-derived PBO membranes showed an increase in gas permeability with a concomitant decrease in gas-pair selectivity. The intermediate PCT region represents a transition state from a PBO to a partially carbonized polymer with superior properties by showing a simultaneous boost in gas permeability and gas-pair selectivity. The ESC membranes formed between 550 and 600 °C possessed further enhanced selectivity due to tightening of the amorphous CMS structure. A 135-day aged 6FDA-HTB-600 membrane showed very high hydrogen permeability of 3544 barrer coupled with an outstanding H2/CH4 selectivity of 939. The mixed-gas separation properties of selected aged samples, pristine 6FDA-HTB, PBO (6FDA-HTB-420), intermediate PCT (6FDA-HTB-480), and early stage CMS analogs (6FDA-HTB-550 and 600), were investigated using a 1:1 CO2/CH4 feed up to 30 bar. In this series, early-stage CMS membranes displayed the best mixed-gas CO2/CH4 separation properties with performance significantly surpassing the 2018 polymer mixed-gas upper bound.

The effect of filler loading and APTES treatment on the performance of PSf/SBA-15 mixed matrix membranes

Abstract The fabrication of mixed matrix membranes (MMMs) is of particular importance due to their advantages over neat membranes. The performance of MMMs is a function of the type and fraction of the nanoparticles in the membrane. Moreover, the proper interaction of the polymer and nanoparticles affects the MMMs performance. In this study polysulfone (PSf)/SBA-15 mesoporous silicas MMMs were prepared and their performance was evaluated for CO2/CH4 gas separation. SBA-15 mesoporous silicas were previously synthesized and functionalized with 3-aminopropyltriethoxysilane by post-synthesis treatment. Fourier transform infrared spectroscopy, field emission scanning electron microscopy, N2 adsorption–desorption, and Brunauer–Emmett–Teller analysis was applied to examine the functional groups, morphology, and textural properties of the unmodified and modified mesoporous silica in the prepared membrane, respectively. The effects of modified and unmodified SBA-15 particle loading were investigated for the gas separation performance of PSf/SBA-15 membranes. The experimental results illustrate that a higher modified mesoporous silica loading leads to an increase in gas permeance and gas pair selectivity. The highest increase in permeability and selectivity was related to the incorporation of S2 and AP-S2 into the PSf matrix, respectively.

Gas Separation Membranes with Atom-Thick Nanopores: The Potential of Nanoporous Single-Layer Graphene.

Gas separation is one of the most important industrial processes and is poised to take a larger role in the transition to renewable energy, e.g., carbon capture and hydrogen purification. Conventional gas separation processes involving cryogenic distillation, solvents, and sorbents are energy intensive, and as a result, the energy footprint of gas separations in the chemical industry is extraordinarily high. This has motivated fundamental research toward the development of novel materials for high-performance membranes to improve the energy efficiency of gas separation. These novel materials are expected to overcome the intrinsic limitations of the conventional membrane material, i.e., polymers, where a longstanding trade-off between the separation selectivity and the permeance has motivated research into nanoporous materials as the selective layer for the membranes. In this context, atom-thick materials such as nanoporous single-layer graphene constitute the ultimate limit for the selective layer. Gas transport from atom-thick nanopores is extremely fast, dependent primarily on the energy barrier that the gas molecule experiences in translocating the nanopore. Consequently, the difference in the energy barriers for two gas molecules determines the gas pair selectivity. In this Account, we summarize the development in the field of nanoporous single-layer graphene membranes for gas separation. We start by discussing the mechanism for gas transport across atom-thick nanopores, which then yields the crucial design elements needed to achieve high-performance membranes: (i) nanopores with an adequate electron-density gap to sieve the desired gas component (e.g., smaller than 0.289, 0.33, 0.346, 0.362, and 0.38 nm for H2, CO2, O2, N2, and CH4, respectively), (ii) narrow pore size distribution to limit the nonselective effusive transport from the tail end of the distribution, and (iii) high density of selective pores. We discuss and compare the state-of-the-art bottom-up and top-down routes for the synthesis of nanoporous graphene films. Mechanistic insights and parameters controlling the size, distribution, and density of nanopores are discussed. Fundamental insights are provided into the reaction of ozone with graphene, which has been successfully used by our group to develop membranes with record-high carbon capture performance. Postsynthetic modifications, which allow the tuning of the transport by (i) tailoring the relative contributions of adsorbed-phase and gas-phase transport, (ii) competitive adsorption, and (iii) molecular cutoff adjustment, are discussed. Finally, we discuss practical aspects that are crucial in successfully preparing practical membranes using atom-thick materials as the selective layer, allowing the eventual scale-up of these membranes. Crack- and tear-free preparation of membranes is discussed using the approach of mechanical reinforcement of graphene with nanoporous carbon and polymers, which led to the first reports of millimeter- and centimeter-scale gas-sieving membranes in the year 2018 and 2021, respectively. We conclude with insights and perspectives highlighting the key scientific and technological gaps that must be addressed in the future research.

Facile tailoring molecular sieving effect of PIM-1 by in-situ O3 treatment for high performance hydrogen separation

Highly efficient hydrogen separation membrane is critical in the future hydrogen powered zero carbon society. However, challenges left in searching for hydrogen separation membranes with high permeability, selectivity and easy accessibility. Here, this target was partially achieved by a series of membranes made from in-situ ozone oxidized intrinsic mciroporous polymer membranes (O3-PIMs) within minutes. The FT-IR spectra and XPS results suggested that CO and OH groups were formed after the oxidation reaction. The TGA and EDS mapping results indicated that the oxidation reaction happened only on the skin surface of PIM-1 membrane within 10 μm in depth. After O3 oxidation, the O3-PIMs showed a large drop in gas permeability whereas huge increases in gas pair selectivity, resulting in large improved overall performance for H2/N2, H2/CH4 and H2/CO2 that beat the latest trade-off lines. The O3-PIM-60 showed a H2 permeability of 1294 Barrer and H2/N2, H2/CH4 and H2/CO2 selectivity of 93.7, 121, and 2.92, respectively. The increased selectivity of O3-PIMs is due to their remarkably enhanced size sieving effect induced enhanced diffusion selectivity. Theoretical simulation showed that this is caused by the O3 reaction induced pore blocking effect that shifted the mciropore of PIM-1 to the ultra-micropore region, and thus, significantly improved the hydrogen sieving efficiency. In all, this facile and highly efficient ozone oxidation method provides a great perspective in searching for high performance hydrogen separation membranes.

The effect of chain rigidity and microstructure on gas separation performance of the cardo-based polyimides

A series of aromatic polyimides were synthesized from three Cardo-based diamines, BAF, DDA, BAP, with different building blocks (ether linkage, dioxane linkage and C–C linkage), and commercially available dianhydride 6FDA. All polyimides were readily soluble in conventional aprotic and protic solvents, which guaranted the fabrication of self-standing membranes for gas separation applications. The Cardo-based polyimides also exhibited excellent thermal stability, with weight loss temperatures (T 5 %) of over 538 °C, and good mechanical properties, with tensile strength in the range of 41–64 MPa. Compared to 6FDA-BAF, the substitution of dioxane linkage for ether linkage resulted in increased chain rigidity, looser polymer chain packing, and larger fractional free volume. Consequently, as displayed by single gas permeation tests, 6FDA-DDA showed higher permeability coefficients without significantly sacrificing selectivity. In addition, the CO 2 permeability of 6FDA-BAP was around 2 times as high as that of 6FDA-BAF, while the CO 2 /CH 4 selectivity increased by 11%. This can be explained by the lower rotational mobility and the relatively higher imide content caused by the absence of the flexible ether linkage and dioxane linkage in polymer backbone. In short, fundamental insights into structure-property relationships of the Cardo-based polyimides observed in this study provided guidance for the molecular structure design of the gas separation membranes. • Three Cardo-based polyimides containing different building blocks were successfully synthesized. • The effect of rigidity and microstructure on gas separation performance of the Cardo-based polyimides was elucidate. • The substitution of dioxane for ether linkage led to the increased chain rigidity and the looser chain packing. • The polyimide containing C–C linkage showed better combination of permeability and gas-pair selectivity.

Interfacial design of polysulfone/Cu-BTC membrane using [Bmim][Tf2N] and [Dmim][Cl] RTILs for CO2 separation: Performance assessment for single and mixed gas separation

An upsurge in searching membranes with improved perm-selectivity and long-term stability is crucial in membrane-based gas separation. Herein, we engineered the interfacial characteristics of Cu-BTC MOFs with the polysulfone (PSf) matrix using ionic liquids (ILs) at the polymer-filler interface. Two different ILs, [Bmim][Tf2N] and [Dmim][Cl], were used to prepare composite PSf/Cu-BTC/[Bmim][Tf2N] and PSf/Cu-BTC/[Dmim][Cl] membranes. The membranes micro and chemical structures were characterized using XRD, FESEM, and XPS techniques. Results indicated that the multifunctional properties of ILs and Cu-BTC particles influenced the membrane formation mechanism. The uniform dispersion of Cu-BTC filler particles with the inclusion of [Bmim][Tf2N] and [Dmim][Cl] ILs on the membrane surface was evident from the morphological studies. The absence of a leaky interface by the ILs integration in the PSf/Cu-BTC/[Bmim][Tf2N] and PSf/Cu-BTC/[Dmim][Cl] membranes results in a pronounced increase in selectivities. For instance, PSf/Cu-BTC/[Dmim][Cl] membranes, the CO2 permeance was noticed to be 29.35 ± 0.35 GPU, and their corresponding gas pair selectivities were noted to be significantly higher than the neat and PSf/Cu-BTC membranes. Similarly, in PSf/Cu-BTC/[Bmim][Tf2N] membrane exhibited optimum CO2 permeance of 30.44 ± 0.49 GPU with the CO2/N2 and CO2/CH4 selectivities of 61 and 59.70%, respectively, higher than neat PSf membrane. Furthermore, these membranes showed better anti-aging resistance and long-term stability than the neat PSf membranes. The merits of a simultaneous increase in permeance and selectivities and the good stability and scalable properties make the proposed PSf/Cu-BTC/IL membranes of great interest for the industrial CO2 separation from the gas exhausts.

ZIF-67 membranes supported on porous ZnO hollow fibers for hydrogen separation from gas mixtures

Co-based zeolitic imidazolate framework-67 (ZIF) membranes present impressive potential to be applied for efficient gas separation. However, the synthesis of ZIF-67 membrane is challenging because ZIF-67 crystals tend to nucleate and grow in the reaction solution rather than on the substrate surface. In this work, ZIF-67 membranes were developed on porous ZnO hollow fibers via in situ solvothermal growth method. The effects of different ZnO supports, surface activation, growth duration, and growing temperatures on the resultant membrane performance were carefully investigated. The optimized ZIF-67 composite hollow fiber membrane displayed desirable permeation performance, ideal selectivities for single gas, and separation factors for gas mixtures. The achieved H2 permeance was 8.98 × 10−8 mol m−2 s−1·Pa−1and ideal selectivities for H2 gas pairs with CO2, N2, and CH4 were 5.14, 11.75, and 15.89, respectively, which compared favorably with other reports in the literature. The developed ZIF-67 membrane exhibited good stability in terms of gas permeances between 30 and 200 °C. Furthermore, the membrane showed almost constant H2 permeance and H2/CH4 separation factor during the 50-h run at 150 °C, demonstrating the excellent thermal stability of membrane. The fabrication of the excellent ZIF-67 membrane can be achieved just in one cycle of growth with high reproducibility.

Tröger’s Base Polyimide Hybrid Membranes by Incorporating UiO-66-NH2 Nanoparticles for Gas Separation

Hybrid membranes based on the Tröger’s base polyimide (PI-TB) as a rigid microporous polymer matrix containing a kinked N-heterocycle and a kind of CO2-philic porous UiO-66-NH2 nano-filler were successfully fabricated for gas separation. This new hybrid membrane exhibits good interfacial compatibility through hydrogen bonds and charge transfer interactions between the PI-TB matrix and the UiO-66-NH2 nano-filler, thus enabling a relatively high loading and well dispersion of nano-filler. Meanwhile, the as-synthesized UiO-66-NH2 nanoparticles increase the average inter-chain spacing distance of PI-TB by interfering with the packing of polymer chains and thus providing additional transport pathways in the membranes. The pure gas permeabilities and ideal selectivities for CO2/CH4, CO2/N2, and O2/N2 systems were investigated at 35 °C and a feed pressure of 1 bar. Among them, the hybrid membrane containing 30 wt % UiO-66-NH2 exhibits the optimal gas separation performance with enhanced permeabilities of CO2 and O2 increased by ∼166% (415 and 92.3 Barrer, respectively) while maintaining comparable CO2/CH4, CO2/N2, and O2/N2 gas pair selectivities (25.0, 18.9, and 4.2, respectively) with the pristine PI-TB membrane. This work enriched the studies on the post-modification of PI-TB as a potential candidate membrane material for gas separation.

Unlocking the limits of diffusion and adsorption of metal-crosslinked reduced graphene oxide membranes for gas separation

Reduced graphene oxide (rGO) membranes have shown promising separation and purification in the fields of water treatment and gas capture due to their unique physicochemical characteristics. However, how to further reduce the transport resistance of rGO membranes and improve separation performance is still a thorny issue. In this work, the reported metal-crosslinked rGO membranes showed a reverse-selective separation behaviors for the adsorbed gas molecules at low pressure. The effect of metal ions on the membrane structure and the separation performance were investigated. Metal ions not only can improve the cross-linking structure and control interlamellar spacing of rGO membranes for the precise molecular sieve, but also can provide abundant metal sites to enhance the affinity, which led to a diffusion-controlled process for the targeted gas molecules. The resulting metal-crosslinked rGO membranes presented outstanding gas permeance and selectivity for gas pairs of the He/CO2, N2/CO2 and Kr/Xe, with the highest selectivity of 21, 11.4 and 4.1, respectively. The work will provide a new avenue towards specific molecule capture for the development of rGO membranes in the fields of natural gas purification and radioactive gas recovery.

Investigation on the effects of air gap distance on the formation of cellulose triacetate hollow fiber membrane for CO2 and CH4 gases permeation

CO2 removal is essential for the sweetening process of sour natural gas in order to meet the desired specifications for natural gas to be utilized and transported. CO2 separation from natural gas stream can be achieved through the application of membrane technology which will enable natural gas to be suitable for commercial use. Cellulose triacetate (CTA) is a well-known polymer material for CO2 removal due to its high affinity towards CO2. As compared to flat sheet membranes, hollow fiber configuration is preferable due to its large surface to unit volume ratio and self-supporting characteristics, therefore making it a superior choice compared to other configurations. In this work, fabrication of hollow fiber membranes by employing CTA as a polymer for the separation of CO2 and CH4 gases is focused. During fabrication, the air gap distance was varied from 1 cm to 7 cm. Following that, the morphology of the resulting fibers was investigated by using a scanning electron microscope (SEM). Next, the CO2 and CH4 gas permeation, and CO2/CH4 gas pair selectivity of the resultant HFMs were assessed using a custom-built gas permeation test rig. In addition, post-treatment of HFM with polydimethylsiloxane (PDMS) was also carried out. Then, the gas permeation and gas pair selectivity of the resultant hollow fiber membranes before and after the PDMS coating were compared. The optimum gas permeation performance of CTA HFM was obtained at an air gap distance of 3 cm and a feed pressure of 3 bar as it yielded the highest CO2 permeance of 78.77 GPU and CO2/CH4 gas pair selectivity of 2.80. Whereas for PDMS coated CTA HFM, fibers spun at air gap distance of 1 cm demonstrated the highest CO2 permeance of 14.85 GPU and CO2/CH4 selectivity of 2.54 at feed pressure of 9 bar.

Simultaneously enhanced CO2 permeability and CO2/N2 selectivity at sub-ambient temperature from two novel functionalized intrinsic microporous polymers

Most polymeric membranes are suffering from CO2 permeability and CO2/N2 selectivity trade-off effect. Here, a novel anti-trade-off effect by simultaneously increase CO2 permeability and CO2/N2 selectivity upon decreasing temperature was observed for two new functionalized fully carbon main chain PIMs (HSBI-4-CF3 and HSBI-3-CF3). They are synthesized by one-step Friedel-Crafts polycondensation reaction from cheap raw materials of 3,3,3’,3’-tetramethyl spirobisindane-6,6’-diol and 3(4)-trifluoromethylbenzaldehyde isomers. Both PIMs showed high molecular weight, good solubility, thermal and mechanical properties. HSBI-4-CF3 exhibited a higher BET surface area (318 vs 287 m2 g-1) and more open chain packing than HSBI-3-CF3. The two polymers showed modest permeability and selectivity as well as pressure and aging resistance. The HSBI-4-CF3 showed higher permeability but less gas pair selectivity than HSBI-3-CF3. Interestingly, upon decreasing temperature to −20 °C, both polymers showed improved CO2 permeability and CO2/N2 selectivity at the same time. The CO2 permeability of HSBI-3-CF3 increased from 292 to 453 Barrer and the CO2/N2 selectivity improved from 20.3 to 44.0. The higher permeability at low temperature was attributed to the increased solubility (SCO2 ∼ 3.2 fold) over diffusion coefficient drop (DCO2 ∼ 2.1 times) of CO2, which is opposite to that of N2, and thus enhanced the CO2/N2 selectivity. The origin of this effect comes from their minus activation energy of permeation (Ep) of CO2 (−0.78 and −1.1 kcal mol-1) while positive Ep for N2 (1.7 and 1.1 kcal mol-1), which is due to the diffusion activation energy (Ed) overweight the heat of sorption (-Hs) for N2 while inverse for CO2. This unique simultaneously increase CO2 permeability and CO2/N2 selectivity opened a new era in membrane-based sub-ambient temperature CO2 capture.

Highly Permeable Mixed Matrix Membranes of Thermally Rearranged Polymers and Porous Polymer Networks for Gas Separations

Mixed matrix membranes (MMMs) have been obtained by blending an aromatic ortho-hydroxypolyimide (PIOH) or an ortho-acetylpolyimide (PIOAc) with different loading amounts (15 and 30 wt %) of a microporous polymer network (PPN), which was obtained from the reaction of triptycene and isatin. The excellent thermal resistance of the PPN (above 500 °C) allowed it to be used as a filler to successfully prepare thermally rearranged polybenzoxazole (TR-PBO)-MMMs obtained from the thermal treatment of the above MMMs. Moreover, PPN showed relatively good compatibility with the polyimide matrix, which improved the TR-PBO formation. The gas separation performances of these MMMs before and after the thermal process were studied for five representative gases (He, O2, N2, CO2, and CH4). For the MMMs derived from ortho-functional polyimides, the gas permeability considerably increased for all of the gases, whereas the selectivity for gas pairs, such as CO2/N2 and CO2/CH4, remained similar. Thus, the selectivity-permeability performance of PIOH- and PIOAc-MMMs containing 30 wt % of PPN (PIOH30 and PIOAc30) surpassed the 1991 Robeson limit for the CO2/CH4 gas pair. In the case of TR-PBO-MMMs (TROH and TROAc-MMMs), the thermal rearrangement process led to an increase in the gas permeability, showing values much better than those observed for the TR-PBO material employed as a MMM matrix. The selectivity values were a little bit lower than the pristine TR-PBO membranes. The CO2 permeability of TROAc30 was 1036 barrer with a CO2/CH4 selectivity of 28, surpassing the 2008 Robeson limit.

Solvent-processable 0D covalent organic framework quantum dot engineered composite membranes for biogas upgrading

COF (covalent organic framework)-based composite membranes offer the opportunity for breakthroughs in separation performance through endowing COFs with easy film-forming properties. However, tough challenges of poor filler dispersity, limited functionality and interfacial compatibility severely restrict the development of COF-based composite membranes. Therefore, the careful design of physical and chemical structures for COFs is urgently needed. Here, a geometry transformation strategy, i.e. converting FCTF-1 (fluorinated covalent triazine frameworks) from 2D nanosheets to 0D quantum dots (QDs), is implemented to overcome some of the challenges. The greatly decreased filler size increases the solvent dispersibility and processability of FCTF-1 quantum dots (QD-FCTF-1), giving rise to thorough mixing between the polymer matrix and quantum dot filler. Apart from the changes in physical characteristics of filler, the geometry transformation using piranha solution introduces chemical functionality at the quantum dot edges, further strengthens the interfacial compatibility through the hydrogen bond interactions between polar hydroxy and amidine groups of QD-FCTF-1 and nitrile groups of PIM-1 (polymer of intrinsic microporosity). Moreover, these edge polar groups intensify favorable gas sorption due to their high affinity to CO2 molecules and thus significantly improve membrane gas pair selectivity. This work broadens the scope of framework-based materials and offers a new avenue for molecular separation.