Gas Permselectivity Research Articles

Selective Gas Permeation in Mixed Matrix Membranes Accelerated by Hollow Ionic Covalent Organic Polymers

Mixed matrix membranes (MMMs) have gained great attention for the efficient CO2 removal from raw nature gas or biogas (CO2/CH4 separation) and flue gas (CO2/N2 separation). Nevertheless, the development of high-performance MMMs for industrial applications is largely limited by the lack of suitable porous fillers. Herein, a novel ionic covalent organic polymer (ICOP-1) consisting of gas selective pores and hollow cavities is facilely fabricated using a metal triflate catalyzed condensation reaction. Considering its unique structural properties, ICOP-1 is explored as a novel filler to enhance the gas separation properties of polysulfone (PSf) membranes. Defect-free MMMs are successfully prepared owing to the high polymer–filler affinity originating from the organic nature of these two phases. Besides, the large cavities and size-selective pores of ICOP-1 lead to a simultaneous increase in membrane CO2 permeability and CO2/CH4, CO2/N2 selectivities. With the addition of only 0.5 wt % of ICOP-1 fillers, the a...

Facilitating CO2 Transport Across Mixed Matrix Membranes Containing Multifunctional Nanocapsules.

Mixed matrix membranes (MMMs) have exhibited advantages in overcoming the trade-off effect, although it is still intensively demanded in the design of multifunctional fillers to improve CO2 separation performance. At present, MMMs with transport channels present an effective strategy to obtain ultrahigh CO2 permselectivity. In this work, Pebax-based MMMs was fabricated by incorporating nanocapsules (NCs), whose exterior, interior and transverse shell surfaces contained abundant carboxylic acid groups. NCs, similar to vesicles in cells, provide favorable physical and chemical microenvironments to the constructed CO2 transport channels, enhancing the CO2 permselectivity via both a facilitated transport mechanism and a solution-diffusion mechanism. CO2 permselectivity of MMMs doped with 20 wt % NCs surpassed the 2008 Robeson limit; an increase in CO2 permeability was up to 1431 ± 35 Barrer for pure gas, which was a 362% enhancement from the pure membrane, and an increase of the CO2/CH4 and CO2/N2 ideal selectivities to 46 ± 1.4 and 69 ± 2.7, corresponding to 44% and 23% enhancements from the pure membrane, respectively. This study provides an ingenious strategy to enhance the gas permselectivity of MMMs.

Poly(phenylene) synthesized using diels-alder chemistry and its sulfonation: Sulfonate group complexation with metal counter-ions, physical properties, and gas transport

Sulfonated poly(phenylene) (sPP) thermal stability, physical properties, and gas transport characteristics were evaluated as a function of ion-exchange capacity (x = IEC), and counter-ion type (M = H+, Cs+, Na+, Mg2+, and Al3+). Counter-ion substitution with its sulfonated groups produced Δv(SO3-) vibrational shifts that were related to ion type and valence observed using FTIR. Increasing IEC and counter ion complexation strength within sPP led to greater density and reduced fractional free volume (FFV). sPPx-M exhibited a single thermal degradation step greater than 550 °C that decreased with increasing IEC. Unsulfonated poly(phenylene) (PP) had a glass transition at 391 °C, which shifted to 420 °C upon sulfonation. Thermal stability and mechanical properties increased with counter-ion type in the following order: sPP2.5-Cs+ < sPP2.5-Na+ < sPP2.5-Mg2+. However, decreased thermal stability and physical properties were observed using Al3+ as a counter-ion (sPP2.5-Al3+). He, H2, CO2, N2, O2 and CH4 permeability decreased within sPP2.5-M based upon counter-ion type, size, and valence. Larger CO2 and H2 molecules were more permeable than He, which revealed a more complex transport process was occurring within sPP2.5. Metal-ion complexation with sPP2.5 significantly reduced CO2 and H2 permeability. PP possessed high CO2 permeability (152 Barrers) with moderate CO2/CH4 selectivity (12.6). sPP2.5-Na+ (IEC = 2.5 meq/g) had a CO2/CH4 selectivity of 71.3, and counter-ion substitution to create sPP2.5-Cs+ increased CO2/CH4 selectivity by 23%. Gas solubility and diffusivity decreased with increasing IEC and metal-ion complexation strength. sPPx-M had tunable ideal gas permselectivity that was largely a function of sulfonate group concentration and counter-ion type.

Polymeric membrane gas separation performance improvements through supercritical CO2 treatment

Supercritical carbon dioxide (sc-CO2) will plasticize and partially solubilise polymeric membranes, resulting in alteration to the polymer morphology, impacting the gas separation properties. Here, cellulose triacetate (CTA) and polyimides, Matrimid and 6FDA-TMPDA membranes were exposed to supercritical CO2 for 2 and 8 h, followed by two depressurization protocols; a rapid depressurization of 12 MPa/min and a slow depressurization of 0.17 MPa/min. The resulting impact on He, N2, CH4 and CO2 permeability as well as the corresponding selectivities were then quantified. Matrimid membranes undergo substantial plasticization in the presence of sc-CO2 resulting in significant increases in permeability and loss of selectivity, irrespective of the sc-CO2 exposure protocol. CTA and 6FDA-TMPDA membranes experience competing phenomenon under supercritical conditions, both demonstrate limited CO2 plasticization which is offset by sc-CO2 partly solubilising the polymers, enabling rearrangement to a more compact morphology. The depressurization protocol strongly impacted the underlying morphology for these two membranes. Rapid depressurization resulted in a higher fractional free volume and greater gas permeability, which is attributed to the sudden expansion of CO2 sorbed in the polymer opening up the morphology. Slower depressurization resulted in a lower fractional free volume and decreased gas permeabilities, because the CO2 gradually desorbs, leaving behind a more dense morphology. The resulting decrease in permeability also corresponded with a significant increase in selectivity. For both CTA and 6FDA-TMPDA membranes the sc-CO2 treatment improved the gas permselectivity relative to the original state. Therefore, exposure to sc-CO2 presents an advantageous process to improve the performance of common polymeric membranes for gas separation.

Effect of calcination atmosphere on microstructure and H2/CO2 separation of palladium-doped silica membranes

Abstract Palladium-doped silica materials with Si CH3 groups were fabricated by sol-gel method under various calcination atmospheres and membranes were made thereof by coating process. The results showed that air atmosphere can lead to the partial oxidation of metallic Pd0 to PdO while N2 and H2 atmospheres can effectively prevent metallic Pd0 from being oxidized. H2 atmosphere is proved to be a more prominent way to slow down the decomposition of organic Si CH3 group than N2 and air atmospheres. The surface area, micropore volume and porosity of palladium-doped silica membrane material calcined in H2 atmosphere are much higher than those calcined in N2 atmosphere. Compared with N2 atmosphere, the palladium-doped silica membranes calcined in H2 atmosphere showed higher H2 permeability and H2/CO2 selectivity before and after the steam exposure. The apparent activation energy of H2 permeation through the palladium-doped silica membrane calcined under H2 atmosphere (2.51 ± 0.05 kJ/mol) was slightly lower than that calcined under N2 atmosphere (2.84 ± 0.04 kJ/mol). Calcination atmosphere plays some role in membrane performance, which has greater influence on the permeance than on the gas permselectivity. Calcination under H2 atmosphere is well conducive to improve the gas permeance and H2 permselectivity of palladium-doped silica membrane.

Diethylenedioxane-bridged microporous organosilica membrane for gas and water separation

Abstract A new dioxane-bridged alkoxysilane, 2,5-bis[2-(triethoxysilyl)ethyl]-1,4-dioxane (BTES-ED), was synthesized as a precursor of organically bridged silica membrane to investigate the effects of the 1,4-dioxane ring as a rigid and polar spacer. The monomer was polymerized by the sol-gel process to give the gels in film, powder, and membrane forms. Nitrogen adsorption measurement of the gel powder showed the type-I adsorption/desorption isotherm, suggesting that the gel possessed a microporous structure with pore size distribution ranging from approximately 0.8 to 1.6 nm. Reflecting microporosity, the membrane exhibited selective gas permeation properties (H2/SF6 permeance ratio = ca. 1900) based on molecular sieving effects. Reverse osmosis experiments were conducted using a 2000 ppm NaCl aqueous solution at an operation pressure of 1.0 MPa, revealing water desalination properties with the water permeance of 1.84 × 10−13 m3/m2·s·Pa and the NaCl rejection of 98.5%. These results are discussed in comparison with the results of other bridged silica membranes reported previously. The 2,5-diethylene-1,4-dioxane bridging unit is rigid and polar, thereby contributing to improving gas and water separation performance.

Chemical Cross-Linking of 6FDA-6FPA Polyimides for Gas Permeation Membranes

Abstract In this work, 4,4ʹ-hexafluoroisopropiliden anhydride diftalic (6FDA) and 4,4ʹ-hexafluoropropiliden bis(p-phenilenoxi) dianiline (6FPA) polyimides were synthesized by two step polycondensation. The polyimides were utilized in the dense membrane formation by casting solution method. The membranes were cross–linked by immersion in a 1,5-pentadiamine in methanol solution prepared at different concentrations. The fraction of insoluble mass indicates the cross-linking degree, which varies from 0 to 1. In the other hand, the fractional free volume (FFV) calculated from Bondi theory decreases as the cross-linking degree increases in the membrane. The membranes were tested in the permeation of pure H2, CO2, N2 and CH4 gases. The results exhibited that permeability of the cross-linked membranes increases compared to the uncross-linked membrane. In the other hand, gas permselectivity data where located in the Robeson´s diagram being near to upper limit for the H2/CO2 pair of gases. The membranes are good candidates to carry out the low molecular weight’s gas separation.

Bis(triethoxysilyl)ethane (BTESE)-derived silica membranes: pore formation mechanism and gas permeation properties

Based on their high performance in gas and liquid-phase separations, 1,2-bis(triethoxysilyl)ethane (BTESE)-derived organosilica membranes have attracted much attention. To improve performance, we focused on the acid molar ratio (AR) in sol preparation and its effect on the pore formation mechanism during sol-gel processing. BTESE-derived sols with AR = 10−4–100 were prepared, and the effect of the AR on the gel structure was evaluated in detail via FT-IR, nuclear magnetic resonance (NMR), N2 adsorption, and positron annihilation lifetime (PAL) measurements. The chemical structure of the gels was confirmed by FT-IR and NMR and showed that sols with the largest number of silanol groups (AR = 10−2) experienced a significant increase in condensation during the firing process. The porous structures of fired gels characterized by N2 adsorption and PAL measurement showed that the AR = 10−2 fired gel consisted of a larger number of small pores that had formed during the firing process. Single-gas permeation experiments showed high H2 permeance (5–9 × 10−7 mol/(m2 Pa s)) and H2/CF4 selectivity (700–20,000). The gas permselectivity (He/H2, H2/N2, and H2/CF4) was highest for the intermediate AR (=10−2), which corresponded to the greatest amount of silanol groups in unfired gels and confirmed that small pores had formed from the condensation of silanol groups during firing.

Selective Gas Permeation in Graphene Oxide-Polymer Self-Assembled Multilayers.

The performance of polymer-based membranes for gas separation is currently limited by the Robeson limit, stating that it is impossible to have high gas permeability and high gas selectivity at the same time. We describe the production of membranes based on the ability of graphene oxide (GO) and poly(ethyleneimine) (PEI) multilayersto overcome such a limit. The PEI chains act as molecular spacers in between the GO sheets, yielding a highly reproducible, periodic multilayered structure with a constant spacing of 3.7 nm, giving a record combination of gas permeability and selectivity. The membranes feature a remarkable gas selectivity (up to 500 for He/CO2), allowing to overcome the Robeson limit. The permeability of these membranes to different gases depends exponentially on the diameter of the gas molecule, with a sieving mechanism never obtained in pure GO membranes, in which a size cutoff and a complex dependence on the chemical nature of the permeant is typically observed. The tunable permeability, the high selectivity, and the possibility to produce coatings on a wide range of polymers represent a new approach to produce gas separation membranes for large-scale applications.

Preparation and gas permeation of crown ether-containing co-polyimide with enhanced CO2 selectivity

In present work, crown ether-containing co-polyimide with improved gas permselectivity was developed for CO2/N2 and CO2/CH4 separation. Crown ether segment grants the co-polyimide material good affinity with CO2 as well as strong rigidity of polymer chain. This co-polyimide was prepared by the condensation polymerization and chemical imidization of 4, 4-hexafluoro-isopropylidene diphthalic anhydride, 4, 4′-diaminodiphenylmethane and trans-di(aminobenze)-18-C-6 (trans-DAB18C6). The synthesized materials were characterized by FT-IR, 1H NMR, XRD, TGA, DSC, GPC and electronic tensile machine. The fractional free volume (FFV) and fractional accessible volume (FAV) were calculated with molecular dynamics simulations. The mixed-gas permeation properties of the co-polyimide membranes were investigated to inspect the influence of trans-DAB18C6 content and feed gas pressure. Effects of testing temperature and humidity, casting solvent on membrane with 25mol% trans-DAB18C6 were investigated. The molecular dynamics simulations results showed that （FAV）CO2 decreased, while the ratios of （FAV）CO2/（FAV）N2 and （FAV）CO2/（FAV）CH4 increased progressively with trans-DAB18C6 content. The CO2 permeability increased firstly, and decreased subsequently with trans-DAB18C6 content. At the same time, both CO2/N2 and CO2/CH4 selectivities increased progressively. The co-polyimide membrane with 25mol% trans-DAB18C6 displayed the maximum CO2 permeability of 109.0 barrer and CO2/CH4 selectivity of 92.7, which surpassed the 2008 Robeson upper bound. The long term operation stability study demonstrated that the membrane had good stability.

Tuning the free volume of polynorbornene-based membranes by blending 6FDA-durene to improve H2 permselectivity

Homogeneous blend membranes of polynorbornene and 2,2′-bis(3,4′-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)-durene were prepared to improve H2 permselectivity. The thermal properties and gas permselectivity of membranes with different polynorbornene/6FDA-durene blending ratios were investigated. The structural changes due to blending result in increased permeability derived from an increase in the fractional free volume of polymer blends with increasing glass transition temperature. Moreover, an increase in the 6FDA-durene content in the blends improved gas permeation, diffusivity, and solubility. The prepared blend membranes showed excellent gas permeation properties, namely, high gas permeability for H2 (14–971 Barrer) and high H2/N2 (α=13–46) and H2/CO (α=10–29) permselectivities.

Synthesis and gas permselectivity of CuBTC–GO–PVDF mixed matrix membranes

A CuBTC (copper(ii) benzene-1,3,5-tricarboxylate) metal organic framework (MOF) and graphene oxide (GO) nanosheets were introduced into a semi-crystalline PVDF to produce mixed matrix membranes (MMMs) to promote gas separation performance.

High-performance perfluorodioxolane copolymer membranes for gas separation with tailored selectivity enhancement

A new family of amorphous perfluorodioxolane copolymers shows gas permselectivities above the current polymer upper bound and exhibits tunable transport properties.

Disk supported carbon membrane via spray coating method: Effect of carbonization temperature and atmosphere

This paper presents a method for the separation of CO2/CH4 and CO2/N2 by the use of disk supported carbon membranes derived from polyimide. The supported polyimide membranes were carbonized under controlled gas flow (three different atmospheres: N2, Ar and He) at different carbonization temperatures from 600 to 900 °C. The carbon membrane fabricated at various carbonization temperatures and atmospheres, showed improvement in gas permeation performance. The gas permeability increased with the increase of carbonization temperature due to densification of the micropores in the carbon membranes, leading to enhanced gas permselectivity. All carbon membrane samples carbonized under three different atmospheres at 800 °C surpassed the polymeric CO2/CH4 upper bound. Pure-gas permselectivity reached 98 for the carbon membrane carbonized at 800 °C under He gas flow, with CO2 permeability of about 120 Barrer, becoming the most selective carbon membrane for CO2/CH4 separation.

Mechanical and Gas Barrier Properties of Poly(L-Lactic Acid) by Plasma-Enhanced Chemical Vapor Deposition of SiOx

ABSTRACTIn this work, poly(L-lactic acid) film was coated with SiOx by the plasma-enhanced chemical vapor deposition with different deposition times. Compared with the neat poly(L-lactic acid) film, the oxygen (O2), carbon dioxide (CO2), nitrogen (N2), and water vapor permeability of the poly(L-lactic acid)/SiOx60 film (depositing for 60 min) decreased by 40.7, 30.6, 58.7, and 53.4% at 25°C, respectively. After treated by the SiOx deposition, the gas permselectivity of the poly(L-lactic acid)/SiOx60 film, such as α(CO2/O2), α(O2/N2), and α(CO2/N2), increased by 17.2, 43.9, and 67.5% at 25°C, respectively. In addition, Young’s modulus and tensile strength of poly(L-lactic acid)/SiOx60 film increased by 107.2 and 49.3%, respectively. Moreover, the poly(L-lactic acid)/SiOx60 films still kept good toughness with an elongation at break of 50.7%.

Molecular dynamics simulation and Monte Carlo study of transport and structural properties of PEBA 1657 and 2533 membranes modified by functionalized POSS-PEG material

In this study, structural and transport mechanism and properties of two kinds of polyether block amide (PEBA 1657, PEBA 2533) membrane in presence of POSS-PEG functionalized nanoparticles were investigated by molecular dynamic simulation (MD). Monte Carlo technique was used to investigate transport properties of the simulated membranes. The wide-angle XRD (WAXD), mean square displacement (MSD), glass transition temperature (Tg) and sorption isotherm were studied to investigate the aforementioned mixed matrix membranes. Also, solubility, diffusivity, permeability and permselectivity of light gases (N2, CO2, CH4, O2, and H2) were investigated carefully by Materials Studio simulator. The results showed higher permeability for membranes with PEBA 2533 rather than the ones with PEBA 1657; this conclusion is also confirmed with the experimental results which are reported in the literature. But, for the permselectivity of membranes, the results were opposite of the permeability results. Tg for all membranes decreased by increasing the POSS-PEG nanoparticle content. Additionally, by comparing the results of the simulations with the results of literature, it is revealed that simulation results were in acceptable agreement by experiment results.

Gas Permeability and Permselectivity of Poly(L-Lactic Acid) Film by Plasma Enhanced Chemical Vapor Deposition of SiOx

Gas Permeability and Permselectivity of Poly(L-Lactic Acid) Film by Plasma Enhanced Chemical Vapor Deposition of SiOx

Gas‐separation properties of amine‐crosslinked polyimide membranes modified by amine vapor

ABSTRACTThe modification of a polyimide (PI) membrane by aromatic amine vapor was performed in this work to increase the crosslinking of the membrane and to study the effect on gas permeability and the corresponding selectivity. The single‐gas permeability of the membranes at 35 °C was probed for H2, O2, N2, CO2, and CH4. From the relationship between the combinations of gases and ideal permselectivities, this study showed that amine‐crosslinked PI membranes tended to increase gas permselectivities exponentially with the increasing difference in gas kinetic diameter. Moreover, this study illustrated that the permeability of the membranes was influenced by the formation rate of amine‐crosslinked networks or chemical structures after the reaction. The membranes had the highest level of permselectivities among crosslinked PI membranes for O2/N2, and the H2/CH4 permselectivity increased 26 times after vapor modification. Furthermore, the modification method that used aromatic amine vapor produced thin and strongly modified layers. These findings indicate that modification is an advantageous technique for improving gas‐separation performance, even considering thinning. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44569.

Characterizations of PDMS-graft copolyimide membrane and the permselectivity of gases and aqueous organic mixtures

Abstract Polydimethylsiloxane (PDMS)-graft copolyimides were synthesized by polycondensation of 3,5-bis(4-aminophenoxy)benzyloxypropyl-terminated PDMS macromonomer with 4,4’-hexafluoroisopropylidene diphthalic anhydride (6FDA) followed by chemical imidization to investigate the effects of PDMS segment length of copolyimides on the membrane characteristics, the gas permeability and the pervaporation performance. The copolyimide membranes, which were prepared by solvent-casting method from the chloroform solutions, became insoluble in any solvents after the thermal treatment over 150 °C in vacuo. It was confirmed from TEM observation of these membranes that the phase separation of flexible PDMS domains and hard polyimide backbones. The gas permeability coefficients of copolyimide membranes were increased as the PDMS segment length increased, but decreased after thermal treatment at 200 °C. It would be due to the chain scission and the rearrangement of PDMS segments in the side chain to become a cross-linked structure, which make the membranes insoluble. From the results of pervaporation of dilute aqueous solutions of organics, it was found that the copolyimide membranes exhibited the excellent permselectivity toward several organics, such as ethanol, acetone, benzene, chloroform and dichloromethane with a high and stable permeation. Furthermore, it was confirmed that the removal of dichloromethane from its saturated aqueous solution was efficiently achieved by the pervaporation.

Development of a water-selective zeolite composite membrane by a new pore-plugging technique

A new membrane preparation method based on mechanical pore-plugging of the supports with appropriate sized seeds followed by secondary zeolite growth by hydrothermal synthesis was optimized for the production of hydroxy-sodalite (hydroxy-SOD) membranes. A series of parameters were investigated during membrane production, including seed loading, presence of Teflon on the coarse side of the support, top-layer orientation in the autoclave, autoclave agitation, synthesis time and temperature, membrane post-synthesis treatment, and nature and pore size of the support (TiO2/SS, SS, Al2O3 and ZrO2/TiO2). Zeolite seeds and synthesized membranes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas permeation. The gas permselective separation ability of these membranes was demonstrated from H2, N2, CO2 and H2O permeation tests at temperature up to 250 °C. The membranes fabricated with the optimized parameters achieved H2O/N2, H2O/CO2, and H2O/H2 ideal gas permselectivities of 5.1, 4.8 and 1.4 at 250 °C, while providing a high water permeance (ΠH2O of 1.26 × 10−7 mol Pa−1 m−2 s−1) at the same temperature.