Controlled synthesis of high performance carbon/zeolite T composite membrane materials for gas separation

A simple approach has been developed to synthesize the carbon/zeolite T composite membrane materials with the high gas separation performance. The precursors of the composite membrane are composed of polyimide matrix and dispersed zeolite T particles. The composite membranes prepared by pyrolysis at 973 K show excellent gas (H₂, CO₂, O₂, N₂, and CH₄) permeability and selectivity (O₂/N₂, CO₂/CH₄) for both single gas and mixed-gas. The gas separation performance of the composite membranes can be controlled in a wide range by only changing the zeolite T particle size. The maximum selectivity of O₂ over N₂ (21/79 mol%) for the composite membranes with the least zeolite T particle (0.5 μm) is 15 with an O₂ permeability of 347 Barrers (1 Barrer = 7.5 × 10⁻¹⁸ m² s⁻¹ Pa⁻¹) and the selectivity of CO₂ over CH₄ (50/50 mol%) reaches a value of 179 with a CO₂ permeability of 1532 Barrers. It is believed that the increase of gas permeability is attributed to the ordered microchannels in the zeolite and the interfacial gaps formed between zeolite and carbon matrix in the composite membranes. And the gas selectivity is tuned by the size of interfacial gaps which are varied with the zeolite particle size. This technique will provide a simple and convenient route to efficiently improve the trade-off relationship between the permeability and the selectivity and enable the construction of carbon-based composite materials with novel functionalities in membrane science.     

A high CO2 permselective mesoporous silica/carbon composite membrane for CO2 separation

Ordered mesoporous silica/carbon composite membranes with a high CO₂ permeability and selectivity were designed and prepared by incorporating SBA-15 or MCM-48 particles into polymeric precursors followed by heat treatment. The as-made composite membranes were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and N₂ adsorption, of which the gas separation performance in terms of gas permeability and selectivity were evaluated using the single gas (CO₂, N₂, CH₄) and gas mixtures (CO₂/N₂ and CO₂/CH₄, 50/50 mol.%). In comparison to the pure carbon membranes and microporous zeolite/C composite membranes, the as-made mesoporous silica/C composite membranes, and the MCM-48/C composite membrane in particular, exhibit an outstanding CO₂ gas permeability and selectivity for the separation of CO₂/CH₄ and CO₂/N₂ gas pairs owing to the smaller gas diffusive resistance through the membrane and additional gas permeation channels created by the incorporation of mesoporous silicas in carbon membrane matrix. The channel shape and dimension of mesoporous silicas are key parameters for governing the gas permeability of the as-made composite membranes. The gas separation mechanism and the functions of porous materials incorporated inside the composite membranes are addressed.     

Synthesis and characterization of poly (ethylene oxide) containing copolyimides for hydrogen purification

Reverse selective membranes comprising poly(ethylene oxide) (PEO) containing copolyimides (PEO-PI) with variations of acid dianhydrides and diamines have been synthesized for hydrogen purification. The reverse selectivity of the membranes decimate the energy required for hydrogen recompression process. Factors including PEO content, PEO molecular weight, and fractional free volume (FFV) that would affect the gas transport performance have been investigated and elucidated in terms of degree of crystallinity, phase separation in the PEO domain as well as inter-penetration between the hard and soft segments. In mixed gas tests of CO₂ and H₂ mixtures, a highly condensable CO₂ out compete H₂ for the sorption sites in hard segment and diminishes H₂ permeability. Thus the CO₂/H₂ selectivity in the mixed gas tests is much higher than that in pure gas tests. Mixed gas permeation tests at 35 °C and 2atm show that the best reverse selective membranes have a CO₂ permeability of 179.3 Barrers and a CO₂/H₂ permselectivity of 22.7. The physical properties of PEO-PIs have also been characterized by FTIR, DSC, GPC, WAXS, AFM and tensile strain tests.     

Preparation and characterization of polyvinylchloride based mixed matrix membrane filled with multi walled carbon nano tubes for carbon dioxide separation

Poly vinyl chloride/multi wall carbon nano tubes (PVC/MWCNTs) mixed matrix membranes (MMMs) were prepared for gas separation. Raw and functionalized MWCNTs (R-MWCNTs and C-MWCNTs) were utilized in membranes preparation. The C-MWCNT shows better performance compared to raw ones. Membrane (CO₂/CH₄) selectivity was increased from 39.21 to 52.18 at 2 bar pressure by MWCNT loading ratio. The modified membranes with styrene butadiene rubber (SBR-MMMs) showed 63.52 and 34.70 selectivity for (CO₂/CH₄) and (CO₂/N₂) at 2 bar pressure. Mechanical properties analysis exhibited tensile module improvement utilizing blending modification. Increase of feed pressure led to membrane gas permeability decreasing. But gas pair selectivity follows a nearly constant behavior for MMMs and increasing behavior for blend MMMs.     

Fabrication and modification of cellulose acetate based mixed matrix membrane: Gas separation and physical properties

[Cellulose acetate (CA)-blend-multi walled carbon nano tubes (MWCNTs)] mixed matrix membranes (MMMs), [CA/polyethylene glycol (PEG)/MWCNTs] and [CA/styrene butadiene rubber (SBR)/MWCNTs] blend MMMs were prepared by solution casting method for gas separation applications using Tetrahydrofuran (THF) as solvent. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxylic-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas separation performance of prepared membranes were investigated for pure He, N₂, CH₄ and CO₂ gases. Results indicated that utilizing C-MWCNT instead of R-MWCNTs in membrane fabrication has better performance and (CO₂/CH₄) and (CO₂/N₂) selectivity reached to 21.81 and 13.74 from 13.41 and 9.33 at 0.65 wt% of MWCNTs loading respectively. The effects of PEG and SBR on the gas transport performance and mechanical properties were also investigated. The highest CO₂/CH₄ selectivity at 2 bar pressure was reached to 53.98 for [CA/PEG/C-MWCNT] and 43.91 for [CA/SBR/C-MWCNT] blend MMMs at 0.5 wt% and 2 wt% MWCNTs loading ratio respectively. Moreover, increase of feed pressure led to membrane gas permeability and gas pair selectivity improvement for almost all prepared membranes. The mechanical properties analysis exhibited tensile modules improvement with increasing MWCNTs loading ratio and utilizing polymer blending.     

Hydrogen separation and purification in membranes of miscible polymer blends with interpenetration networks

This study demonstrates the successful implications of blending technique cum chemical modification for the fabrication of high performance polymeric membranes for gas separation applications. The effect of variation in composition on miscibility and microstructure, gas permeability and selectivity of blend membranes is investigated. It is found that augmentation in PBI composition results in enhancement in gas separation performance of membranes which is attributed mainly to the effect of diffusivity selectivity. Analysis of the microstructure of membranes confirms the variations in chain packing density, d-spacing and segmental mobility of polymer chains as a result of blending. Separation performance of membranes is further ameliorated through chemical modification of blend constituents. Modification of PBI phase with p-xylene dichloride brings about slight improvements in selectivity performance, especially for H₂/CO₂ and H₂/N₂. In contrast, the selectivity of membranes is improved significantly after cross-linking of Matrimid phase with p-xylene diamine. The results indicate that higher tendency of Matrimid toward cross-linking reaction contributes more in controlling the transport properties of membranes through diffusion coefficient by increase in chain packing density and diminishing the excess free volumes. Results obtained in this study reveal the promising features of developed membranes for gas separation applications with great potential for hydrogen separation and purification on industrial scale.     

Gas permeation and separation properties of polyimide/ZSM‐5 zeolite composite membranes containing liquid sulfolane

The preparation, characterization, and gas permeation properties of novel composite membranes containing polyimide (PI), liquid sulfolane (SF), and zeolite (ZSM‐5) were investigated to address the interface defects between the PI and the zeolite. The free‐standing composite membranes were prepared by the solvent casting method. The gas permeability of the PI+ZSM‐5 membrane was higher than that of PI, whereas its gas selectivity was significantly reduced, suggesting that these results are attributed to the interface defects. The CO₂ selectivity of PI+ZSM‐5+SF was higher than those of the PI+ZSM‐5 membranes because of the introduction of liquid SF into the interface defects. Furthermore, liquid SF enhanced the CO₂/H₂ selectivity near the recent upper bound. Therefore, the use of liquid SF could be an effective approach to preventing interface defects and increasing the CO₂ selectivity, particularly for CO₂/H₂. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Carbon molecular sieve gas separation membranes based on an intrinsically microporous polyimide precursor

We report the physical characteristics and gas transport properties for a series of pyrolyzed membranes derived from an intrinsically microporous polyimide containing spiro-centers (PIM-6FDA-OH) by step-wise heat treatment to 440, 530, 600, 630 and 800 °C, respectively. At 440 °C, the PIM-6FDA-OH was converted to a polybenzoxazole and exhibited a 3-fold increase in CO₂ permeability (from 251 to 683 Barrer) with a 50% reduction in selectivity over CH₄ (from 28 to 14). At 530 °C, a distinct intermediate amorphous carbon structure with superior gas separation properties was formed. A 56% increase in CO₂-probed surface area accompanied a 16-fold increase in CO₂ permeability (4110 Barrer) over the pristine polymer. The graphitic carbon membrane, obtained by heat treatment at 600 °C, exhibited excellent gas separation properties, including a remarkable CO₂ permeability of 5040 Barrer with a high selectivity over CH₄ of 38. Above 600 °C, the strong emergence of ultramicroporosity (<7 Å) as evidenced by WAXD and CO₂ adsorption studies elicits a prominent molecular sieving effect, yielding gas separation performance well above the permeability-selectivity trade-off curves of polymeric membranes.     

Uniformity control and ultra-micropore development of tubular carbon membrane for light gas separation

Tubular carbon molecular sieve (CMS) membranes have been recognized as a potential module for commercial application due to its high mechanical strength and large surface area. However, the carbon layer uniformity was restricted by substrate texture and dope fluidity when the dip-coating method was used. This study evaluated the influence of various parameters of dip-coating with an integrated vacuum-assisted system, including solvent vaporization rates, vertical immersion/withdrawal velocity, vacuum degree, dope composition, coating cycles on the microstructure, and gas separation performance of CMS membranes. Using vacuum assistance and a low-vaporization solvent minimized the influence of viscosity and gravity on dope fluidity as a result of fast phase inversion. The as-prepared tubular CMS membranes showed enhanced perm-selectivity according to a H₂/N₂ gas selectivity of 8.8, a CO₂/N₂ gas selectivity of 6.7, a H₂ permeability of 464 barrer, and a CO₂ permeability of 356 barrer.     

Role of critical concentration of PEI in NMP solutions on gas permeation characteristics of PEI gas separation membranes

In preparation of polymeric gas separation membranes by phase inversion method, polymer concentration is one of the most important variables which can change membrane morphology and behavior. In this research, critical concentration of the polyetherimide (PEI) solutions in N-methyl-2-pyrrolidone (NMP) as a solvent was determined by viscometric method. The influence of temperature on critical concentration was studied. Three asymmetric PDMS/PEI membranes with different concentrations of PEI were prepared and characterized for H₂/CH₄ separation. The results showed that the membranes with higher concentrations than critical concentration were more suitable for gas separation. In addition, the viscosity data were fitted by appropriate equations and the densities were satisfactorily correlated by a simple first-order polynomial with respect to temperature and the PEI mass fraction. The prepared membrane showed the selectivity of 26 for H₂/CH₄ separation at 1 bar and 25 °C for pure gas and 24.8 for mixed gas. The influence of the pressure on the H₂ and CH₄ permeance and the selectivity for a mixed binary gas showed that the permeance of both gases declined by pressure enhancement and the selectivity increased.     

Ultrathin carbon molecular sieve membrane for propylene/propane separation

Ultrathin (down to 300 nm), high quality carbon molecular sieve (CMS) membranes were synthesized on mesoporous γ‐alumina support by pyrolysis of defect free polymer films. The effect of membrane thickness on the micropore structure and gas transport properties of CMS membranes was studied with the feed of He/N₂ and C₃H₆/C₃H₈ mixtures. Gas permeance increases with constant selectivity as the membrane thickness decreases to 520 nm. The 520‐nm CMS membrane exhibits C₃H₆/C₃H₈ mixture selectivity of ～31 and C₃H₆ permeance of ～1.0 × 10^?8 mol m^?2 s^?1 Pa^?1. Both C₃H₈ permeance and He/N₂ selectivity increase, but the permeance of He, N₂, and C₃H₆ and the selectivity of C₃H₆/C₃H₈ decrease with further decrease in membrane thickness from 520 to 300 nm. These results can be explained by the thickness‐dependent chain mobility of the polymer film which yields thinner final CMS membranes with reduction in pore size and possible closure of C₃H₆‐accessible micropores. © 2015 American Institute of Chemical Engineers AIChE J, 62: 491–499, 2016     

Tubular MXene/SS membranes for highly efficient H2/CO2 separation

Accurately constructing membranes based on two-dimensional (2D) materials on commercial porous substrates remains a significant challenge for H₂ purification. In this work, a series of tubular 2D MXene membranes are prepared on commercial porous stainless steel substrates via fast electrophoretic deposition. Compared with other methods, such as filtration or drop coating, and so on. such preparation route shows the advantages of simple operation, high efficiency for membrane assembly (within 5 min) with attractive reproducibility, and ease for scale-up. The tubular MXene membranes present excellent gas separation performance with hydrogen permeance of 1290 GPU and H₂/CO₂ selectivity of 55. Furthermore, the membrane displays extremely stable performance during the long-term test for more than 1250 h, and about 93% of the membranes from one batch have exceeded the DOE target for CO₂ capture. Most importantly, this work provides valuable referential significance for other 2D materials-based membranes for future application development.     

Preparation and characterization of polysulfone mixed matrix membrane incorporated with palladium nanoparticles in the inversed microemulsion for hydrogen separation

Polysulfone (PSf) membrane shows acceptable gas separation performance, but its application is limited by the “trade-off” between selectivity and permeability. In this study, PSf mixed matrix membranes (MMMs) incorporated with palladium (Pd) nanoparticles in the inversed microemulsion were proposed for hydrogen (H₂) separation. Pd nanoparticles can be kinetically stabilized and dispersed using electrostatic and/or steric forces of a stabilizer which is typically introduced during the formation of Pd nanoparticles in the inversed microemulsion. Pd nanoparticles were synthesized by loading (PdCl₂) into the polymeric matrix, polyethylene glycol (PEG) which acts as reducing agent and stabilizer. The dry–wet phase inversion method was applied for the preparation of asymmetric PSf MMMs. The effects of Pd (0–4 wt%) on the membrane characteristics and separation performance were studied. Experimental findings verified that the MMMs are able to achieved a high H₂/N₂ selectivity of 21.69 and a satisfactory H₂ permeance of 46.24 GPU due to the changes in membrane structure from fully developed finger-like structure to closed cell structure besides the growth of dense layer. However, the selectivity of H₂/CO₂ decreased due to the addition of PEG.     

A systematic study on the effects of synthesis conditions of polyamide selective layer on the CO2/N2 separation of thin film composite polyamide membranes

Different top layer fabrication methods (amine-first, acid-first, spin coating), organic phase solvents (hexane, heptane, mixed hexane/heptane), acid acceptors (triethylamine, sodium carbonate, sodium hydroxide), and surfactant sodium dodecyl sulfate concentrations (0, 0.05, and 0.1 wt%) were utilized to fabricate thin film composite polyamide membranes for CO₂/N₂ separation. The results, according to an L9 orthogonal array of Taguchi approach, showed that employing acid-first method increases both CO₂ permeance and CO₂/N₂ selectivity of the membranes at a feed gas pressure of 3 bars. On the other hand, sodium hydroxide, and triethylamine should be used, as acid acceptors, to maximize CO₂ permeance and CO₂/N₂ selectivity, respectively. Moreover, the use of hexane solvent and 0 wt% surfactant led to maximum permeance, while, hexane solvent and 0.1 wt% surfactant were needed to reach the highest selectivity. The above level setting of synthesis parameters also resulted in the minimum sensitivity of the fabrication process to the noise factors effects. As shown by the analysis of variance, acid acceptor, and organic solvent types were the most influential parameters on CO₂ permeance and CO₂/N₂ selectivity, respectively. The effects of fabrication method and surfactant concentration, as single factors, on permeation/selectivity responses were also investigated.     

Vapor linker exchange of partially amorphous metal–organic framework membranes for ultra-selective gas separation

Metal–organic framework (MOF) membranes are promising for efficient separation applications. However, the uncontrollable pathways at atomic level impede the further development of these membranes for molecular separation. Herein we show that vapor linker exchange can induce partial amorphization of MOF membranes and then reduce their transport pathways for precisely molecular sieving. Through exchanging MOF linkers by incoming ones with similar topology but higher acidity, the resulted metal-linker bonds with lower strength cause the transformation of MOF membranes from order to disorder/amorphous. The linker exchange and partial amorphization can narrow intrinsic apertures and conglutinate grain boundary/crack defects of membranes. Because of the formation of ultra-microporous amorphous phase, the MOF composite membrane shows competitive H₂/CO₂ selectivity up to 2400, which is about two orders of magnitude higher than that of conventional MOF membranes, accompanied by high H₂ permeance of 13.4 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and good reproducibility and stability.     

Morphological, mechanical and gas-transport characteristics of crosslinked poly(propylene glycol): homopolymers, nanocomposites and blends

Linear polyethers possess unusually high CO₂ solubility and, hence, selectivity due to the presence of accessible ether linkages that can interact with the quadrupolar moment of CO₂ molecules. In this work, membranes derived from crosslinked poly(propylene glycol) diacrylate (PPGda) oligomers differing in molecular weight (M), as well as PPGda nanocomposites containing either an organically-modified montmorillonite clay or a methacrylate-terminated fumed silica are investigated and compared with highly CO₂-selective poly(ethylene glycol) diacrylate (PEGda) homopolymer and nanocomposite membranes previously reported. The rheological and permeation properties of PPGda depend sensitively on M, with the elastic modulus decreasing, but CO₂ permeability and CO₂/H₂ selectivity increasing, with increasing M. Incorporation of either nanofiller into PPGda enhances the elastic modulus and reduces the gas permeability in the resultant nanocomposites without strongly affecting CO₂/H₂ selectivity. Blending PPGda and PEGda prior to chemical crosslinking yields binary membranes that exhibit intermediate gas-transport properties accurately described by a linear rule of mixtures.     

Development of CO2-Philic Blended Membranes Using PIM–PI and PIM–PEG/PPG

Blending is a simple method through which one can effectively tailor new polymers exhibiting the properties of their parent ones. Because the original properties of polymers are maintained after blending, various studies have used these films as gas separation membranes. In this study, a new CO₂ separation membrane is developed by physically mixing a polymer of intrinsic microporosity (PIM) with high gas permeability, polyimide (PIM-PI), as the hard segment and CO₂-philic PIM-poly(ethylene glycol)/poly(propylene glycol), or PIM-PEG/PPG, as the soft segment. Prepared by adding 5 mol.% of PIM-PEG/PPG to PIM-PI, the blended membrane PPB-5, with a tensile strength of 54 MPa and 35.5% elongation at break, shows better mechanical properties than commercial high-performance polymer membranes developed for gas separation, PEG-based blended membranes, and corresponding copolymer membranes with similar compositions developed in a previous study. In addition, it shows high CO₂ permeability (1552.6 Barrer) and CO₂/N₂ selectivity (29.3) due to the well-developed microphase separation characteristics originating from the optimal two-component composition, and the gas separation performance is close to the Robeson (2008) upper bound.     

New high permeable polysulfone/ionic liquid membrane for gas separation

In this study, the effects of 1-Ethyl-3-methylimidazolium tetrafluoroborate ionic liquid on CO₂/CH₄ separation performance of symmetric polysulfone membranes are investigated. Pure polysulfone membrane and ionic liquid-containing membranes are characterized. Field emission scanning electron microscopy (FE-SEM) is used to analyze surface morphology and thickness of the fabricated membranes. Energy dispersive spectroscopy (EDS) and elemental mapping, Fourier transform infrared (FTIR), thermal gravimetric (TGA), X-ray diffraction (XRD) and Tensile strength analyses are also conducted to characterize the prepared membranes. CO₂/CH₄ separation performance of the membranes are measured twice at 0.3 MPa and room temperature (25 °C). Permeability measurements confirm that increasing ionic liquid content in polymer-ionic liquid membranes leads to a growth in CO₂ permeation and CO₂/CH₄ selectivity due to high affinity of the ionic liquid to carbon dioxide. CO₂ permeation significantly increases from 4.3 Barrer (1 Barrer=10^-10 cm³(STP)·cm·cm^-2·s^-1·cmHg^-1, 1cmHg=1.333kPa) for the pure polymer membrane to 601.9 Barrer for the 30 wt% ionic liquid membrane. Also, selectivity of this membrane is improved from 8.2 to 25.8. mixed gas tests are implemented to investigate gases interaction. The results showed, the disruptive effect of CH₄ molecules for CO₂ permeation lead to selectivity decrement compare to pure gas test. The fabricated membranes with high ionic liquid content in this study are promising materials for industrial CO₂/CH₄ separation membranes.     

Precise control on two-dimensional nanochannels at sub-nanometer level for customizable gas separation

Membranes with precise molecular sieving channels that break the permeability-selectivity trade-off are desirable for energy-efficient gas separation. Two-dimensional (2D) membranes sieve gas through their special interlayer channels between neighboring nanosheets. However, the regulation and precise control of the nanochannels that match well with the size of the gas molecules remains a big challenge. Herein, accurate tuning of the interlayer spacing of layered double hydroxide (LDH) membranes at sub-nanometer level was achieved by intercalation of Cl⁻, Br⁻, I⁻, and NO₃⁻ ions. Such high-precision control allows customizable gas separation by selecting specific LDH membranes with appropriate channels according to the size of the gas molecules. Two membranes were used for demonstration: Cl-LDH membrane shows high H₂ permeance of ∼1870 GPU and desirable selectivities for H₂/CO₂(81), H₂/N₂(197), H₂/CH₄(320), and H₂/C₃H₈(603); while I-LDH membrane displays CO₂ permeance of ∼1780 GPU and CO₂/N₂, CO₂/CH₄ selectivities of 182 and 297, respectively. The simultaneously high permeabilities and selectivities surpass the 2008 Robeson upper bounds. Molecular dynamics simulations quantitatively support the experiment results, further confirming the significant role of interlayer anions in the regulation of gas-sieving channels. Given the rich variability of layered spacing and interlayer microenvironment for LDH materials, this work provides a platform membrane for various molecular sieving, including gas separation, solvent purification, seawater desalination, and so on.     

Gas permeation and separation properties of sulfonated polyimide membranes

Permeability and selectivity of pure gas H₂, CO₂, O₂, N₂ and CH₄ as well as a mixture of CO₂/N₂ for sulfonated homopolyimides prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoro propane disulfonic acid (BAPHFDS) were measured and compared to those of the non-sulfonated homopolyimide having the same polymer backbone. The polyimide in a proton form (NTDA-BAPHFDS(H)) displayed higher selectivity of H₂ over CH₄ without loss of H₂ permeability. Strong intermolecular interaction induced by sulfonic acid groups decreased diffusivity of the larger molecules. The CO₂/N₂ (19/81) mixed gas permeation was investigated as a function of humidity. With increasing relative humidity from 0% RH to 90% RH, the CO₂ permeability for NTDA-BAPHFDS(H) polyimide increased by more than one order of magnitude, and the selectivity of CO₂/N₂ also increased twice or more. On the other hand, the gas permeability for the non-sulfonated polyimide slightly decreased with increasing humidity. NTDA-BAPHFDS(H) polyimide displayed a CO₂ permeability of 290×10⁻¹⁰ cm³ (STP) cm/(cm² s cmHg) and a separation factor of CO₂/N₂ of 51 at 96% RH, 50 °C and total pressure of 1 atm.