CO2 Permselectivity Research Articles

Synthesis, gas permeability, and metal-induced gelation of poly(disubstituted acetylene)s having p,m-dimethoxyphenyl and p,m-dihydroxyphenyl groups

Diphenylacetylene having a trimethylsilyl group and two methoxy groups [p-(CH3)3SiC6H4CCC6H4-m,p-(OCH3)2 (1a)] polymerized with TaCl5/n-Bu4Sn catalyst to provide a high-molecular-weight polymer (2a). In contrast, monomers without trimethylsilyl group (1b) and having two t-butyldimethylsiloxy groups (1c) hardly polymerized because of poor solubility of the produced polymer 2b and steric hindrance of 1c. Polymer 3a with high molecular weight was obtained by desilylation of 2a using CF3COOH. Membranes having p,m-dihydroxyphenyl groups to certain degrees (4a: 40%; 4b 90%) were synthesized by cleavage of methoxy groups in the membrane of 3a using AlCl3. Membrane 4a showed high CO2 permselectivity, while membrane 5a was too brittle. Polymers 2a and 3a totally dissolved in CHCl3 and THF, and their solutions turned into gels upon addition of metal chlorides such as TaCl5, AlCl3, etc. The gels returned to solutions by adding a small amount of methanol. This indicates that 2a and 3a form non-covalent intermolecular bonds through metal–ligand interaction.

Thermal rearranged poly(benzoxazole)/polyimide blended membranes for CO2 separation

The exceptional CO2 permselectivity of thermal rearranged (TR) poly(benzoxazole) (PBO) membranes from α-hydroxy-polyimide precursors is well known. As a consequence of the TR process, the rigidity of the membrane increases, which can negatively impact the mechanical strength of the membrane. This is overcome here through blends of TR PBO with a polyimide that does not undergo TR. Within the blend membranes, the non-TR polyimide provides additional flexibility to the membrane and hence increases its mechanical strength. The TR polyimide precursor is based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB), while the non-TR polyimide is based on 6FDA and 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD). The diamine moiety HAB undergoes TR with 6FDA to form the benzoxazole within the blend membrane, while 4MPD does not undergo thermal rearrangement. Blend membranes of varying TR PBO/polyimide compositions were tested for gas separation of CO2 from CH4. The permeabilities of both gases were a function of blend concentration, and they decreased as 4MPD–6FDA content in the blend increased. For CO2 this was attributed mainly to decreasing diffusivity because of increasing membrane density, while CH4 permeability decrease followed the measured decrease in CH4 solubility. Importantly, the 75% TR PBO/polyimide blend membrane produced a CO2/CH4 permselectivity that was comparable with the current Robeson upper bound while maintained flexibility.

Improvements in the CO2 permeation selectivities of amino acid ionic liquid-based facilitated transport membranes by controlling their gas absorption properties

A series of amino acid ionic liquids (AAILs) composed of different cations of different sizes, including trihexyl(tetradecyl)phosphonium glycinate, tetrabutylphosphonium glycinate and triethyl(pentyl)phosphonium glycinate have been synthesized for the carrier of CO2 facilitated transport membrane. Their physical properties, including their viscosity, density, molar volume and N2 absorption amount were investigated. The amount of N2 absorption decreased as the size of the cation of the AAIL decreased. Facilitated transport membranes containing these AAILs were prepared, and their CO2 and N2 permeabilities were measured. The N2 permeabilities were systematically investigated both experimentally and theoretically with the aim of improving the selectivity of the CO2 permeation of the AAIL-based facilitated transport membrane. As expected, the CO2 permselectivity was improved using triethyl(pentyl)phosphonium glycinate, which contained the smallest cation of the AAILs investigated in this study. We have proposed a methodology for improving the CO2 selectivity of AAIL-based facilitated transport membranes based on reducing their N2 permeabilities.

Ionic Liquid Membranes Supported by Hydrophobic and Hydrophilic Metal–Organic Frameworks for CO2 Capture

Atomistic simulation is performed to investigate CO2 capture in ionic liquid (IL) membranes supported on metal–organic frameworks (MOFs). The IL considered is 1-n-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], while hydrophobic ZIF-71 and hydrophilic Na-rho-ZMOF with the same topology and similar pore size are used as supports. The [SCN]− anion prefers to locate near the metal cluster of ZIF-71 and the Na+ ion of Na-rho-ZMOF, in contrast to the bulky and chainlike [BMIM]+ cation that resides in the open cage. In both membranes, the [SCN]− interacts more strongly than the [BMIM]+ with the MOF supports. With regard to CO2 capture from CO2/N2 mixture, CO2 adsorption is greater than N2, while CO2 diffusion is slower in both membranes, particularly in [BMIM][SCN]/ZMOF because the Na+ ions in Na-rho-ZMOF act as strong binding sites for CO2. The permselectivity of CO2 over N2 is governed by adsorption selectivity, as diffusion selectivity remains a constant over the pressure range examined. Compared to many ...

Effect of coating method on gas separation by PDMS/PES membrane

AbstractIn this study, polydimethylsiloxane (PDMS)‐coated polyethersulfone (PES) composite membrane was prepared for gas separation. “Film casting” and “dip‐coating” techniques were used for producing selective PDMS layer on the surface of the PES support. The effects of coating technique and conditions including coating solution concentration and curing temperature on permselectivity of CO2, CH4, and N2 were investigated. The prepared PES support did not provide any selectivity to the gases. When the concentration of PDMS coating solution was increased, initially permeability of CO2 was rapidly dropped and then gradually reached to an almost constant value. The optimum concentration of coating solution was 5 wt%. Curing temperature showed no pronounced effect on the CO2 permeability and selectivity. In “film casting” method, double coating showed superior permeability and selectivity. However, triple “dip‐coating” was promising. The selectivity of composite membrane prepared by “dip‐coating” was higher than “film casting” method. CO2/N2 and CO2/CH4 selectivity of five sequential dip‐coated composite membranes was 45.5 and 9.3, respectively. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers

Preparation of CO2 Permselective Li4SiO4 Membranes by Using Mesoporous Silica as a Silica Source

High temperature CO2 separation is effective for the efficient CO2 reduction. Li4SiO4 is one of the high temperature CO2 adsorbents. In this study, a Li4SiO4 thin membrane was developed by using a novel solid state conversion method. Li4SiO4 membranes were successfully prepared on a porous alumina substrate at 600°C conversion from mesoporous silica as silica source. Mesoporous silica was better silica source to obtain Li4SiO4 at the low temperature conversion method compared with MFI zeolite, fumed silica and colloidal silica. CO2/N2 permselectivity was 0.85 at 600°C that is higher than the Knudsen diffusion difference. Thus, Li4SiO4 layer showed CO2 permselectivity at 600°C. It was important to use the mixtures of Li2CO3 and K2CO3 to fill the pinholes of the Li4SiO4 membrane. N2 permeance through the Li4SiO4 membrane decreased to 1.8 x 10-9mol m-2 s-1 Pa-1 from 3.9 x 10-7mol m-2 s-1 Pa-1 by the addition of the mixtures of Li2CO3 and K2CO3.

A series of poly(diphenylacetylene)s bearing sulfonic acids: Synthesis, characterization, and gas permeability

A series of diphenylacetylene derivatives [(CH3)3SiC6H4CCC6H4R; R=H (1a), CH3 (1b), n-C4H9 (1c), n-C8H17 (1d), PhC≡CC6H4R; R = CH3 (2b), n-C4H9 (2c), n-C8H17 (2d), t-C4H9 (2e)] were synthesized and then polymerized with TaCl5/n-Bu4Sn to afford the corresponding poly(diphenylacetylene)s (3a–d and 4b–e). These poly(diphenylacetylene)s were sulfonated using acetyl sulfate, which gave the sulfonated poly(diphenylacetylene)s (5a–d and 6b–e). Except 6b, the degrees of sulfonation were found to be around 1.0 from the titration of methanol-solutions of sulfonated polymers. Sulfonated polymer 5a exhibited the lowest gas permeability and the highest CO2 permselectivity among the present polymers. The gas permeability coefficients of 5b, 5c, and 5d became lager in this order. The presence of alkyl groups would dilute the effect of sulfonic acid and weak the intermolecular interaction by the steric hindrance. Sulfonated poly(diphenylacetylene)s having n-butyl and t-butyl groups (6c and 6e) showed almost the same CO2 permselectivity, while they showed quite different CO2 permeability. The spherical bulky t-butyl groups more effectively prevent the intermolecular interaction than the linear n-butyl groups in sulfonic acid-containing poly(diphenylacetylene)s.

Exploring the interplay of adsorption and diffusion for separation of CO2/CH4 in zeolite-like metal–organic frameworks by a molecular dynamics simulation

Atomically detailed simulations were performed to explore the coupled effects of adsorption and diffusion on the structural and transport properties of CO2, CH4, and their equimolar mixture in Na+ ion-exchanged rho zeolite-like metal–organic framework (Na- rho-ZMOF). It is observed that the adsorption of CO2 in rho-ZMOF leads to slight slowing-down and speeding-up of the drift motion of Na+ ions, depending on loading of CO2 molecules. The simulations of the diffusivities of single-component CO2 and CH4 indicate that a maximum diffusivity with varying loadings exists for both components, and for CO2 the maximum diffusivity arises mainly from the electrostatic interactions between the CO2 quadrupole moment and framework charges. According to our simulation results of the equimolar mixture, with an increase in loading, a diffusion selectivity reversal of CO2 over CH4 is observed; adsorption is the main factor for the separation process of CO2 and CH4 in Na- rho-ZMOF; and the permselectivity of CO2 with respect to CH4 as a function of total pressure also shows a maximum and finally approaches a plateau with an overall value of about 40.0, which is higher than many other commonly used MOF materials such as Cu-BTC and isoreticular MOFs at the same pressure.

Synthesis of poly(p-phenyleneethynylene)s bearing oligo-ethylene glycols and their gas permeation properties

1,4-Diiodobenzenes bearing oligo-ethylene glycols [IRC6H2IR, R = OCH2CH2OCH3 (1a), O(CH2CH2O)2CH3 (1b), O(CH2CH2O)3CH3 (1c)] were polymerized with 1,4-diethynylbenzene in the presence of Pd/Cu catalyst to afford poly(p-phenyleneethynylene)s bearing oligo-ethylene glycols (2a–c), respectively. Polymer 2a was insoluble in any solvents, but the other polymers (2b, 2c) were soluble in CHCl3. The weight-average molecular weights of 2b and 2c were 5.4 × 104 and 9.6 × 104, respectively, and they gave free-standing membranes by solution-casting method. The densities of membranes of 2b and 2c were 1.26 and 1.22 g/cm3, respectively, and their carbon dioxide permeability coefficients were 12.9 and 13.5 barrers, respectively. The CO2/N2 separation factor of membrane of 2b was as large as 33.7. Membrane of 3b, which contains triethylene glycols, exhibited higher CO2 permselectivity, and the CO2/N2 separation factor was 50.0.

Synthesis of sulfonated poly(diphenylacetylene)s with high CO2 permselectivity

AbstractHigh‐molecular‐weight poly[1‐phenyl‐2‐(4‐t‐butylphenyl)acetylene], poly[1‐phenyl‐2‐(4‐trimethylsilylphenyl) acetylene], and their copolymers were synthesized by the polymerization with TaCl5–n‐Bu4Sn. The obtained polymers were sulfonated by using acetyl sulfate to give sulfonated poly(diphenylacetylene)s with different degrees of substitution. The degrees of sulfonation of poly[1‐phenyl‐2‐(4‐t‐butylphenyl)acetylene] and copolymers were in the range of 0.57–0.85. When poly[1‐phenyl‐2‐(4‐trimethylsilylphenyl)acetylene] was sulfonated, the sulfonated poly(diphenylacetylene) with the highest degree of sulfonation was obtained among all the polymers in this study. Its degree of sulfonation was 1.55. All the sulfonated polymers exhibited high CO2 permselectivity, and their CO2/N2 separation factor were over 31. The sulfonated poly(diphenylacetylene) with the highest degree of sulfonation showed the highest CO2/N2 separation factor of 75. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6463–6471, 2009

Synthesis of PEG‐functionalized poly(diphenylacetylene)s and their gas permeation properties

AbstractThe polymerizations of 1‐(3‐methylphenyl)‐2‐(4‐trimethylsilyl)phenylacetylene (1a) and 1‐(4‐methylphenyl)‐2‐(4‐trimethylsilyl)phenylacetylene (1b) were carried out with TaCl5‐n‐Bu4Sn to give relatively high‐molecular‐weight polymers (2a and 2b) (Mn > 5 × 105). The obtained polymers were brominated by using benzoyl peroxide and N‐bromosuccinimide first, followed by substitution reaction of three types of polyethylene glycol. When diethylene glycol was used as a reagent on substitution reaction of meta‐substituted polymer, PEG‐functionalized poly(diphenylacetylene) with the highest content of oxyethylene unit [4a(2)] was obtained, and the degree of substitution was 0.60. The degrees of substitution decreased to 0.15 and 0.08 when the polyethylene glycols with higher molecular weights were used. PEG‐substitution reaction to the para‐substituted polymers was difficult to proceed, and hence the degree of substitution was 0.18 even when diethylene glycol was used. The CO2/N2 separation factor of PEG‐functionalized polymer [4a(2)] was as large as 28.8, although that of 2a was 7.41. The other PEG‐functionalized polymers also exhibited high CO2 permselectivity, and their CO2/N2 separation factors were over 20. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Microporous sol–gel derived aminosilicate membrane for enhanced carbon dioxide separation

A new aminosilicate, sol–gel derived microporous inorganic membrane has been developed for enhanced CO 2 separation in applications such as removal of metabolic CO 2 from the breathing loop of the NASA extravehicular mobility unit (EMU), natural gas purification, or CO 2 capture from coal-fired power plant emissions. This membrane consists of an inorganic, amorphous silica matrix of pore size 4–5 Å, containing randomly dispersed amine ( NH 2) functional groups in order to enhance its CO 2 selectivity, due to preferential adsorption of CO 2 in the membrane pore walls and simultaneous blocking of permeation of other gases (O 2, N 2 and CH 4). It is found that the gas feed condition during permeation (partial pressure of CO 2, relative humidity), post-synthetic treatments and aging, affect significantly the separation performance of the membranes. At this stage of development, with feeds of 1–20 vol.% CO 2 and 0–40% relative humidity at 22 °C, the highest CO 2:N 2 separation factor was in the range 100–200, while the CO 2 permeance was in the range 0.1–1.5 cm 3 (STP)/(cm 2 min atm). The results suggest that controlling the membrane pore size and method of activation of amine groups are the most critical factors for improving the CO 2-permselectivity of the membrane.

Permeation measurements in ethylene‐1‐hexene, ethylene‐1‐octene, and ethylene‐1‐dodecene copolymers synthesized with metallocene catalysts

AbstractThe permeation behavior of different ethylene‐1‐hexene, ethylene‐1‐octene, and ethylene‐1‐dodecene copolymers synthesized with metallocene catalysts has been analyzed. These copolymers cover a wide range of comonomer contents, so their crystallinities display rather considerable variations. The results for the permeability to oxygen of the different ethylene copolymers show that the main factor influencing the permeability is the noncrystalline fraction, although some influence of the kind of comonomer may also be present, which may be explained by the fact that when the alkyl branch of the α‐olefin is longer, there is an increase in the free volume in the amorphous and interfacial regions, causing slightly higher values of the permeability coefficient. From the results with different gases, it follows that, in general, an increase in the size of the penetrant (as expressed by its kinetic diameter or critical molar volume) leads to an increase in the solubility and a decrease in the diffusion coefficient. A wide range of permeability values is covered by these ethylene copolymers, depending basically on the crystallinity of the sample, but the permselectivity of CO2 with respect to oxygen (and probably between other pairs of gases) does not differ very much among the different copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2174–2184, 2003

Conditioning of Fluorine-Containing Polyimides. 2. Effect of Conditioning Protocol at 8 Volume Dilation on Gas-Transport Properties

The impact of conditioning on the permeability and permselectivity of CO2 and CH4 is discussed for two polyimide isomers that have been conditioned with carbon dioxide using several conditioning protocols. Partial depressurization conditioning at pressures that would induce approximately 8% volume dilation; resulted in up to 50% increases in the permeability, diffusivity, and solubility coefficients of the polyimide isomers. The para-connected isomer, 6FDA-6FpDA, exhibited a greater conditioning effect following partial depressurization conditioning with CO2 at 8% volume dilation than did the meta-connected isomer, 6FDA-6FmDA. Conditioning of 6FDA-6FpDA followed by an exchange with an equimolar mixture of CO2 and CH4 resulted in an increase in the methane permeability and a slight decrease in the permselectivity relative to the film prior to conditioning.

Novel Composite Membranes for Gas Separation: Preparation and Performance

High performance composite membranes based on molecular sieving silica (MSS) were synthesized using sols containing silicon co-polymers (methyltriethoxysilane and tetraethylorthosilicate). Alpha alumina supports were treated with hydrochloric acid prior to sol deposition. Permselectivity of CO2 over CH4 as high as 16.68 was achieved whilst permeability of CO2 up to 36.7 GPU (10−6 cm3 (STP) cm−2 · s−1 · cm Hg−1) was measured. The best membrane's permeability was finger printed during various stages of the synthesis process showing an increase in CO2/CH4 permselectivity by over 25 times from initial support condition (no membrane film) to the completion of pore structure tailoring. Transport measurement results indicate that the membrane pretreated with HCl has highest permselectivity and permeation rate. In particular, there is a definite cut-off pore size between 3.3 and 3.4 angstroms which is just below the kinetic diameters of Ar and CH4. This demonstrates that the mechanism for the separation in the prepared composite membrane is molecular sieving (activated diffusion), rather than Knudsen diffusion.

Formation of a Y-Type Zeolite Membrane on a Porous α-Alumina Tube for Gas Separation

A porous α-alumina support tube, polished with a finely powdered X-type zeolite for use as seeds, was placed vertically in an autoclave containing an aqueous mixture of water glass and sodium aluminate. Hydrothermal synthesis was carried out at 90 °C for 24 h. A polycrystalline layer of Y-type zeolite was thus formed on the outer surface of the support tube. After washing and drying in air, permeances of single components and mixtures of CO2 and N2, as well as CH4, C2H6, and SF6, were determined. The CO2 permeance was higher than that of N2 at temperatures of 30−130 °C. When an equimolar mixture of CO2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the single-component system and the N2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. This was due to selective adsorption of CO2 in subnanometer micropores of the membrane. At 30 °C, the permeance of CO2 was higher than 10-7 mol·m-2·s-1·Pa-1, and the permselectivity of CO2 to N2 was 20−100.

Preparation of polyethyleneglycol (PEG) and cellulose acetate (CA) blend membranes and their gas permeabilities

AbstractMiscible blend membranes containing 10 wt % PEG of low molecular weight 200, 600, 2000, and 6000, and 10 wt %, 20 wt %, 30 wt %, 40 wt %, 50 wt %, and 60 wt % of molecular weight 20,000 were prepared to investigate the effect of PEG on gas permeabilities and selectivities for CO2 over N2 and CH4. The permeabilities of CO2, H2, O2, CH4, and N2 were measured at temperatures from 30 to 80°C and pressures from 20 cmHg to 76 cmHg using a manometric permeation apparatus. It was determined that the blend membrane, which contained 10% PEG 20,000, exhibited higher permeability for CO2 and higher permselectivity for CO2 over N2 and CH4 than those of the membranes that contained 10% PEG of the molecular weight ranging from 200 to 6000. The high PEG 20,000 content blend membranes showed remarkable permeation properties such that the permeability coefficients of CO2 and the ideal separation factors for CO2 over N2 reached above 200 barrer and 22, respectively, at 70°C and 20 cmHg. Based on the data of gas permeability coefficients, time lags, and characterization of the membranes, it is proposed that the apparent solubility coefficients of all CA and PEG blend membranes for CO2 were lower than those of the CA membrane. However, almost all of the blend membranes containing PEG 20,000 showed higher apparent diffusivity coefficients for CO2, resulting in higher permeability coefficients of CO2 than those of the CA membrane. It is attributed to the high diffusivity selectivities of CA and PEG 20,000 blend membranes that their ideal separation factors for CO2 over N2 were higher than those of the CA membrane in the temperature range from 50 to 80°C, even though the ideal separation factors of all CA and PEG blend membranes for CO2 over CH4 became lower than those of the CA membrane over nearly the full temperature range from 30 to 80°C. © 1995 John Wiley & Sons, Inc.

Selective Permeation of Carbon Dioxide over Nitrogen through Polyethyleneoxide-Containing Polyimide Membranes

Abstract Polyethyleneoxide(PEO)-containing copolyimides having microphase separated structure have high permeability to CO2 and high permselectivity for CO2 over N2. The permeation occurs through soft segment phase of PEO and the high permselectivity is ascribed to high solubility selectivity.