H2 Permeance Research Articles

Hydrogen purification in nitrogen-doped two-dimensional conjugated microporous polymers

Conjugated microporous polymers (CMPs) have emerged as advanced membranes with unique pore structures for the separation of small kinetic diameter gases, including H2. In this work, we present a comprehensive investigation into the potential of N-doped pyridinyl functionalized conjugated microporous polymers, denoted as CHNX (X = 1, 2, 6), in H2 purification by using molecular dynamic simulations. Our findings demonstrate that CHNX membranes exhibit exceptional H2 purification performance with both high H2 selectivity and permeance. Notably, the CHN6 membrane shows the highest H2 permeance of 2.61 × 10−5 mol m−2 s−1 Pa−1, owing to its larger pore area relative to the other membranes. Moreover, both CHN and CHN2 membranes achieve 100 % gas selectivity based on the molar sieving mechanism. To gain deeper insights into the gas separation process, we analyze the gas-membrane interaction, 2D density distribution, center of mass, mean square displacement, etc. The elucidation of these dynamics enhances our understanding of the underlying gas separation mechanisms. Ultimately, this work provides crucial theoretical guidance for the future development of gas separation membranes in hydrogen purification applications, thereby advancing the field of sustainable energy production.

Analysis of vacuum operation on hydrogen separation from H2/H2O mixture via Pd membrane using Taguchi method, response surface methodology, and multivariate adaptive regression splines

The influence of vacuum pressure applied on H2 separation from a palladium membrane is explored in this study. Three factors with three levels are considered, including the membrane chamber temperature with levels 320 °C, 350 °C, and 380 °C; the retentate-side total pressure with levels 1, 2, and 3 atm; and the permeation-side vacuum pressure with levels 0, 25, and 50 kPa. The Taguchi, response surface methodology (RSM), and multivariate adaptive regression splines (MARS) methods are employed to analyze the effects of the three parameters on hydrogen separation and predict their optimal combination. The retentate-side total pressure exhibits the highest impact on H2 permeation, following the permeation-side vacuum pressure and then the membrane chamber temperature. The maximum H2 flux is 0.226 mol∙s−1∙m−2, with H2 recovery of 91 % obtained at the optimal conditions with a temperature of 380 °C, a total pressure of 3 atm, and a vacuum pressure of 50 kPa. The improvement in H2 flux reaches 21.6 % compared with the case without the imposed vacuum pressure at the same temperature and total pressure. This result shows the imposed vacuum pressure is an efficient way to enhance H2 permeation. The maximum relative errors between the experimental data and the predictions from the Taguchi, RSM, and MARS methods are 6.74 %, 3.37 %, and 8.08 %, respectively. The RSM method presents higher accuracy than Taguchi and MARS, perhaps due to a more precise analysis of the interaction terms. The smaller amount of input data and ignoring the temperature effect in MARS could be the reason for the lower accuracy. Nevertheless, the MARS method still demonstrates acceptable results. The cost of the Taguchi method is lower than that of the RSM method since it requires fewer experimental cases. In a word, the choice of the prediction method depends on the desired accuracy and the experimental cost.

High-Performance Anion Exchange Membrane Water Electrolyzers Enabled by Highly Gas Permeable and Dimensionally Stable Anion Exchange Ionomers.

Designing suitable anion exchange ionomers is critical to improving the performance and in situ durability of anion exchange membrane water electrolyzers (AEMWEs) as one of the promising devices for producing green hydrogen. Herein, highly gas-permeable and dimensionally stable anion exchange ionomers (QC6xBA and QC6xPA) are developed, in which bulky cyclohexyl (C6) groups are introduced into the polymer backbones. QC650BA-2.1 containing 50mol% C6 composition shows 16.6 times higher H2 permeability and 22.3 times higher O2 permeability than that of QC60BA-2.1 without C6 groups. Through-plane swelling of QC650BA-2.1 decreases to 12.5% from 31.1% (QC60BA-2.1)while OH- conductivity slightly decreases (64.9 and 56.2mScm-1 for QC60BA-2.1 and QC650BA-2.1, respectively, at 30 °C). The water electrolysis cell using the highly gas permeable QC650BA-2.1 ionomer and Ni0.8Co0.2O in the anode catalyst layer achieves two times higher performance (2.0Acm-2 at 1.69V, IR-included) than those of the previous cell using in-house ionomer (QPAF-4-2.0) (1.0Acm-2 at 1.69V, IR-included). During 1000h operation at 1.0Acm-2, the QC650BA-2.1 cell exhibits nearly constant cell voltage with a decay rate of 1.1µVh-1 after the initial increase of the cell voltage, proving the effectiveness of the highly gas permeable and dimensionally stable ionomer in AEMWEs.

Low-cost iron (Fe) hollow fiber membrane for hydrogen separation

Dense metal membranes are widely studied due to their promising performance in hydrogen separation. In this work, the low-cost dense iron (Fe) hollow fiber membranes (FeHFMs) were developed via the phase inversion and high-temperature sintering method. By tuning the preparation conditions, two typical gas-tight hollow fibers referred to as the sandwich- and honeycomb-structured membranes were successfully synthesized. H2 permeation behavior was well described by Sieverts equation based on the solution-diffusion mechanism. The sandwich-structured FeHFMs had an H2 flux of up to 7.51 mmol m−2 s−1 when operated at a temperature of 850 °C. In comparison, the honeycomb-structured FeHFMs further enhanced the H2 flux by a factor of 3.2 due to the optimized membrane morphology with only one single dense iron layer. The developed FeHFMs showed superior H2 permeation stability in thermal shock tests (40 h) and long-term stability tests (120 h). This work also expanded the potential application horizons of the FeHFMs, which can be easily tailored into porous or dense structures by tuning the sintering conditions.

Ionic conductivity and hydrogen crossover for IT-PEMFCs: Influence of pressure, temperature, relative humidity and reinforcement

Improved knowledge on Proton Exchange Membrane Fuel Cell (PEMFC) behaviour in the Intermediate Temperature (IT: 80–120 °C) is needed. Here, ionic conductivity and H2 permeability are analysed under H2/N2 using electrochemical impedance spectroscopy, linear sweep voltammetry for three catalyst-coated membranes (CCMs): Nafion HP (reinforced), Nafion 211 (non-reinforced) and a reinforced commercial membrane (RCM, membrane thickness 13 μm). Multiple relative humidity (RH) levels and pressure configurations are analysed at IT. Results show that ionic conductivity and H2 permeability increase with temperature and RH. However, lower crossover is measured above 100 °C and wet conditions due to low H2 partial pressure. The highest crossover is measured with an overpressure on the H2 side which, especially for RCM, suggests possible convection. The membrane reinforcement might reduce the permeability and it decreases the conductivity. Mass spectrometry confirmed that sprayed CCMs suffer from higher crossover than pristine membranes, although the membrane thickness is unchanged.

Hydrogen and NH3 co-adsorption on Pd–Ag membranes

Ammonia (NH3) represents a carbon-free hydrogen carrier that can be used as a zero-emission fuel in the maritime and heavy transport sector with hydrogen fuel cells. To use ammonia as feedstock, the hydrogen must be recovered through NH3 decomposition into H2 and N2. Ammonia decomposition by a membrane-enhanced reactor would inherently produce high-purity H2 through the membrane avoiding the need for a costly hydrogen separation/purification unit. Pd-based membrane reactors can obtain full ammonia decomposition and show significantly higher conversion than conventional reactors. However, the H2 permeability is found to be inhibited in the presence of NH3. A further fundamental understanding of the long-term stability and performance of the Pd-based membranes under exposure to NH3 is therefore required. In the current work, the adsorbate-adsorbate interactions during co-adsorption, the influence the presence of NH3 has on the hydrogen dissociation kinetics, and surface segregation effects in the presence of NH3 and/or hydrogen on the surface of Pd and Pd3Ag are investigated in detail using density functional theory calculations. We find that both the hydrogen surface coverage and dissociation kinetics are hindered by the presence of NH3 on the surface, and possible segregation of Ag towards the surface in the presence of NH3, which could explain the reduced hydrogen permeation in NH3.

Engineering interfacial interactions for hydrogen separation enhancement in Torlon®/mixed-linker ZIF-8 mixed matrix membranes

Polymer membranes traditionally used for gas separation can be easily fabricated via a solution process; however, they have limitations in gas separation performance. Mixed matrix membrane (MMM) with dispersed high-performance filler within a polymer matrix achieves both excellent separation efficiency and productivity. In this study, we employed Torlon® (polyamide-imide), a chemically stable and commercially available polymer, as the matrix of the membranes. To enhance the separation performance through the fabrication of MMM, we utilized 2abIm-mixed-linker ZIF-8 (ML-ZIF-8) as the filler particles to improve the interfacial interaction with the Torlon® matrix. FT-IR and XPS analyses on the Torlon®/ML-ZIF-8 MMM confirmed the occurrence of interactions between the Torlon® and ML-ZIF-8. As a result, the analysis of both the DSC and stress-strain curves confirmed the effectiveness of incorporating ML-ZIF-8 into the Torlon® for improving the stability of the polymer-filler interface. This enhancement resulted in an approximately 2.7 times higher H2 permeability for the Torlon®/ML-ZIF-8 MMM compared to the pure Torlon® membrane. Furthermore, the selectivity of H2/N2 and H2/CH4 separation increased by around 11 times and 9.6 times, respectively, leading to a significant overall improvement in separation performance.

Enhanced resistance of PdNiAu membranes under CO and H2S-containing streams

Pd-based membranes offer energy efficiency and economic advantages, providing a promising route for hydrogen separation and recovery from several streams. Alloying Pd with other elements increases its resistance to corrosive gases while improving H2 permeability. This study investigated the permselective properties of PdNiAu membranes with different Ni contents (17, 23, and 27 at.% Ni) in pure H2 and under CO/He/H2 and H2S/H2 mixtures. The permselective properties were evaluated at different temperatures (573–723 K) and pressure differences across the membrane (ΔP: 10–100 kPa). The membranes studied exhibited promising H2 permeation properties; the highest permeability at 673 K and 100 kPa was obtained with the membrane containing 23 at.% Ni (2.1 × 10−8 mol m−1 s−1 Pa−0.5). All membranes exhibited high H2/N2 selectivity (>10,000). Regarding chemical stability, the membranes with 23 and 27 at.% Ni, showed high tolerance to inhibition by CO and H2S. These results suggest that increasing Ni improves membrane performance in streams containing CO or S; however, the high chemical resistance of the membranes could be attributed to both Ni and Au.

Engineering robust porous/dense composite hollow fiber membranes for highly efficient hydrogen separation

BaZr0.7Ce0.2Y0.1O3-δ (BZCY), a perovskite-type mixed protonic and electronic conducting oxide, holds significant potential for H2 separation and purification as a membrane. The introduction of a porous modification layer as an exchange-active component offers a potential solution to enhance H2 permeability. However, achieving the desired porous/dense structure presents a notable challenge for practical implementation. In this study, various methods were developed to address sintering kinetics, encompassing both chemical aspects (e.g., interlayer reactions) and physical factors (e.g., thermal expansion), for fabricating a porous BZCY modification layer on a 4-channel BZCY hollow fiber membrane. The optimal composite hollow fiber membrane, resulting from a combination of readily sintered hollow fiber and powders for constructing the porous layers, exhibits an exceptional H2 permeation flux of 0.48 mL min−1 cm−2 at 900 °C using 50 % H2/N2 as the feed gas, which represents a threefold increase compared to the performance of the bare BZCY membrane. The H2 permeation flux stabilized at 0.52 mL min−1 cm−2 for 500 h under water vapor conditions. These findings enabled us to identify the most effective approach, unlocking the full potential of surface-modified hollow fiber membranes and advancing their capabilities in H2 permeation applications.

Polyvinyl Alcohol/Zr-based Metal Organic Framework Mixed-matrix Membranes Synthesis and Application for Hydrogen Separation

Membrane gas separation is an environmentally friendly and economical method used to separate valuable gases, industrial process gas wastes, and carbon dioxide from mixed gases. The most important part of this method is the membranes. Gas separation membranes are expected to have high separation and permeability performance, high mechanical strength, easy and fast production capability, and low prices. Polymer-based membranes are mostly preferred depending on the ease of modification capability. In this study, a zirconium-based metal organic framework (Zr-MOF, MIL-140 A) was synthesized and used as a filler within polyvinyl alcohol (PVA) matrix for the selective separation of hydrogen (H2) from carbon dioxide (CO2). The effect of MIL-140 A addition on the mechanical, structural, and morphological properties of PVA was evaluated. The MIL-140 A significantly improved the mechanical strength of the membrane. According to the gas separation results, the increasing concentration of MIL-140 A increased the selective separation performance of the nanocomposite membrane. The highest mechanical strength (43.1 MPa) and best film-forming ability were obtained with 3 wt% MIL-140 A loaded membrane. The ideal H2/CO2 selectivity and hydrogen permeability were obtained as 5.6 and 944 Barrer, respectively at 2 bar feed pressure and room temperature. The highest ideal H2/CO2 selectivity was obtained as 6.3 with the H2 permeability of 959 Barrer when the MIL-140 A ratio was 4 wt%.

Confined-Coordination Induced Intergrowth of Metal-Organic Frameworks into Precise Molecular Sieving Membranes.

Metal-organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect-free high-connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined-coordination induced intergrowth strategy to fabricate lattice-defect-free Zr-MOF membrane towards precise molecular separation. The confined-coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter-diffusion of MOF precursors (metal cluster and ligand), thereby inter-growing MOF crystals into integrated membrane. The resulting Zr-MOF membrane with angstrom-sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m-2 ⋅ h-1 ⋅ bar-1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state-of-the-art membranes. The molecular transport mechanism related to size-sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

Confined‐Coordination Induced Intergrowth of Metal–Organic Frameworks into Precise Molecular Sieving Membranes

AbstractMetal–organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect‐free high‐connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined‐coordination induced intergrowth strategy to fabricate lattice‐defect‐free Zr‐MOF membrane towards precise molecular separation. The confined‐coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter‐diffusion of MOF precursors (metal cluster and ligand), thereby inter‐growing MOF crystals into integrated membrane. The resulting Zr‐MOF membrane with angstrom‐sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m−2 ⋅ h−1 ⋅ bar−1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state‐of‐the‐art membranes. The molecular transport mechanism related to size‐sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

First-principles evaluation of Pd–Pt–Ag and Pd–Pt–Au ternary alloys as high performance membranes for hydrogen separation

Membrane technologies are considered as a promising candidate for high purity hydrogen separation. Ternary alloys membranes show high permeance, high purity quality and good stability for separation of hydrogen, while the relational separation mechanism at atomic scale is still unclear for the most of ternary alloys. In current work, molecular dynamics (MD) and first-principles simulation are applied to predict H2 separations performances of Pd–Pt–Au and Pd–Pt–Ag membranes from CH4, CO, N2, CO2, H2S and H2O mixed gases. The separation properties of the alloys are evaluated by permeability and selectivity of various gases, and the performance of Pd–Pt–Ag and Pd–Pt–Au three-phase membranes are compared to those of Pd–Pt alloy. Our calculations indicate that Pd–Pt–Au alloy present superior selectivities of 3.6 × 1047, 1.1 × 1028, 3.7 × 1011, 4.9 × 1018, 2.2 × 1010, 1.1 × 106 for H2/CO, H2/CO2, H2/CH4, H2/N2, H2/H2S, H2/H2O, respectively, and a permeability of 1.24 × 10−1mols−1m−2pa−1 toward separation of hydrogen from the gases mixture, which is accorded to experimental conclusions. H2 permeance of the alloys exceeds industrial production limit from 100 to 600 K, while other impurity gases do not reach the permeability limits of industrial production even at high temperatures of 600 K.

Insights into micro-structure and separation mechanism of benzimidazole-linked polymer membrane for H2/CO2 separation

Separation of hydrogen from carbon dioxide for sustainable H2 production and CO2 capture still faces great challenges due to the smaller size of H2 and higher condensability of CO2. Herein, a high-performance benzimidazole-linked polymer (BILP) membrane for hydrogen separation was directly prepared on α-Al2O3 substrate through a facile interfacial polymerization approach at room temperature. The separation performance of the BILP membrane were regulated by controlling the reaction time and the microstructure was systematically characterized. Molecular simulations were performed to deep understand the separation mechanism in the BILP membrane. The best performance membrane displays an extraordinary mixed gas selectivity of 40 for H2/CO2 together with outstanding H2 permeance of 250 gas permeation units (GPU) at 473 K, far exceeding the Robeson’s upper bound. Besides, the membrane can withstand high temperature and pressure, and also shows good H2/N2 and H2/CH4 selectivity. The excellent separation performance, coupled with high temperature and pressure resistance and easy preparation, render BILP membranes great potential for economic H2 purification, H2 recovery, and natural gas treatment.

Intrinsically Microporous Polyimides Derived from 2,2'-Dibromo-4,4',5,5'-bipohenyltetracarboxylic Dianhydride for Gas Separation Membranes.

This work aims to expand the structure-property relationships of bromo-containing polyimides and the influence of bromine atoms on the gas separation properties of such materials. A series of intrinsically microporous polyimides were synthesized from 2,2'-dibromo-4,4',5,5'-bipohenyltetracarboxylic dianhydride (Br-BPDA) and five bulky diamines, (7,7'-(mesitylmethylene)bis(8-methyldibenzo[b,e][1,4]dioxin-2-amine) (MMBMA), 7,7'-(Mesitylmethylene)bis(1,8-dimethyldibenzo[b,e][1,4] dioxin-2-amine) (MMBDA), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diamine (TBDA1), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-3,9-diamine (TBDA2), and (9R,10R)-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,6-diamine (DAT). The Br-BPDA-derived polyimides exhibited excellent solubility, high thermal stability, and good mechanical properties, with their tensile strength and modulus being 59.2-109.3 MPa and 1.8-2.2 GPa, respectively. The fractional free volumes (FFVs) and surface areas (SBET) of the Br-BPDA-derived polyimides were in the range of 0.169-0.216 and 211-342 m2 g-1, following the order of MMBDA > MMBMA > TBDA2 > DAT > TBDA1, wherein the Br-BPDA-MMBDA exhibited the highest SBET and FFV and thus highest CO2 permeability of 724.5 Barrer. Moreover, Br-BPDA-DAT displayed the best gas separation performance, with CO2, H2, O2, N2, and CH4 permeabilities of 349.8, 384.4, 69.8, 16.3, and 19.7 Barrer, and H2/N2 selectivity of 21.4. This can be ascribed to the ultra-micropores (<0.7 nm) caused by the high rigidity of Br-BPDA-DAT. In addition, all the bromo-containing polymers of intrinsic microporosity membranes exhibited excellent resistance to physical ageing.

Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior.

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N'-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2-3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility.

Membranes with Molecular Gatekeepers for Efficient CO2 Capture and H2 Purification.

The urgent need for CO2 capture and hydrogen energy has attracted great attention owing to greenhouse gas emissions and global warming problems. Efficient CO2 capture and H2 purification with membrane technology will reduce greenhouse gas emissions and help reach a carbon-neutral society. Here, 4-sulfocalix[4]arene (SC), which has an intrinsic cavity, was embedded into the Matrimid membrane as a molecular gatekeeper for CO2 capture and H2 purification. The interactions between SC and the Matrimid polymer chains immobilize SC molecules into the interchain gaps of the Matrimid membrane, and the strong hydrogen and ionic bondings were able to form homogeneous mixed-matrix membranes. The incorporation of the SC molecular gatekeeper with exceptional molecular-sieving properties improved the gas separation performance of the mixed-matrix membranes. Compared with that of the Matrimid membrane, the CO2 permeability of the Matrimid-SC-3% membrane increased from 16.75 to 119.78 Barrer, the CO2/N2 selectivity increased from 29.39 to 106.95, and the CO2/CH4 selectivity increased from 29.91 to 140.92. Furthermore, when the permeability of H2 was increased to 172.20 Barrer, the H2/N2 and H2/CH4 selectivities reached approximately 153.75 and 202.59, respectively, which are far superior to those of most existing Matrimid-based materials. The mixed-matrix membranes also exhibited excellent long-term operation stability, with separation performance for several important gas pairs still overtaking the Robeson upper limit after aging for 400 days.

Ti-substituted organosilica membranes for H2 sieving: Sol-gel and DFT insights

The fabrication of gas-sieving amorphous silica membranes exhibiting superior thermal/hydrothermal/chemical stability by the facile, scalable and controllable synthesis strategies is highly desired to bring the silica membranes a step closer to large scale deployment. Sol-gel modification of bis(triethoxylsilyl)ethane (BTESE) membrane pore network with Ti substitution resulted with a shift in the molecular level cut-off to smaller molecules establishing ideal selectivity improvement from 13 ± 2 to 20 for H2/N2 and 17 ± 4 to 28 for H2/CH4 at 200 °C. BTESE and analogous Ti-substituted BTESE membranes (BTESE-Ti) exhibited H2 permeances of (0.9–1.2)x10−6 mol/(m2sPa) and 5.8 × 10−7 mol/(m2sPa), respectively, reflecting a difference in their thickness. Decrease in the sol concentration by 2-fold led to an increase in the H2 permeance value to 8.9 × 10−7mol/(m2sPa) at 200 °C for BTESE-Ti membranes. We further theoretically designed amorphous BTESE and BTESE-Ti structures, and simulated their interactions with the gas molecules along with the pore diffusion of H2 as a model system to elucidate the membrane performance through the density functional theory (DFT) calculations. Theoretical findings suggest that the only transport mechanism is the direct transport from the gas phase to the pore entrance by excluding the surface diffusion possibility.

Organic template residues confined in silicalite-1 zeolite membranes for tuning pore structures and boosting H2/CO2 separation

Silicalite-1 zeolite membranes were successfully activated at 300–350 °C in a hydrogen atmosphere. FTIR, TGA, and N2 adsorption-desorption characterizations confirmed that the amount of the organic structure directing agent (OSDA) residue in silicalite-1 zeolite channels was tunable. The H2-calcined membrane showed a lower gas permeance than that of the air-calcined membrane due to the partial pore blockage by the OSDA residue. The permeance of H2 and CO2 increased with increasing temperature, showing an activated diffusion-like mechanism. The activation energies for the permeation of H2 and CO2 were in the range of 1.63–4.15 and 0.76–5.90 kJ mol−1, respectively, which confirmed the reduced average membrane pore size and different pore surface chemistry due to the presence of the OSDA residue in the membrane. Significant interactions between CO2 molecules and OSDA residues confined in the membrane resulted in a notably lower CO2 permeance compared to N2 and CH4, despite CO2 having a smaller kinetic diameter. Enhanced H2/CO2 selectivity in the silicalite-1 zeolite membrane, featuring confined OSDA residue, was attributed to the reduction in membrane pore size and the fine-tuning of pore surface chemistry.

Carbon–Carbon Composite Membranes Derived from Small-Molecule-Compatibilized Immiscible PBI/6FDA-DAM-DABA Polymer Blends

The use of immiscible polymer blends in gas separations is limited due to uncontrollable phase separation. In contrast, compatibilized immiscible polymer blends can be used as precursors with controlled morphologies that allow for a unique pore architecture. Herein, an immiscible polymer blend (1:1) comprising polybenzimidazole (PBI) and the copolyimide 6FDA-DAM:DABA [3:2], derived from reacting 4,4-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with 2,4,6-trimethyl-1,3-phenylenediamine (DAM) and 3,5-diaminobenzoic acid (DABA), were combined with durene diamine as a compatibilizer. The compatibilizer helped reduce the 6FDD domain sizes from 5.6 µm down to 0.77 µm and induced a more even 6FDA distribution and the formation of continuous thin-selective PBI layers. The carbon–carbon composite membranes derived from the compatibilized immiscible polymer blends showed a 3-fold increase in both H2 permeability and H2/CO2 selectivity compared to the membranes derived from non-compatibilized polymer blends. The H2 permeability of the compatibilized immiscible polymer blends increased from 3.6 to 27 Barrer, and their H2/CO2 selectivity increased from 7.2 to 20. The graphitic domain size of the carbon–carbon composite membranes derived from the polymer blends also increased from 6.3 nm for the non-compatibilized blend to 10.0 nm for the compatibilized blend.