Defect-free asymmetric hollow fiber membranes from Torlon, a polyamide–imide polymer, for high-pressure CO2 separations

Torlon^®, a polyamide–imide polymer, was used for high-pressure CO₂ separations, as it can form inter- and intra-chain hydrogen bonding that may provide stability against plasticization. Asymmetric hollow fiber membranes with a defect-free selective skin were successfully formed from Torlon^® using a dry–wet spinning process. Dope and spinning parameters were optimized to obtain these fibers, which had CO₂/CH₄ selectivity of 44 and O₂/N₂ selectivity of 7.7. These selectivities are about 85% of the intrinsic (dense film) value of 52 for CO₂/CH₄ and 90% of the intrinsic value of 8.3 for O₂/N₂, respectively. Based on analyses presented, the reduced selectivities are attributed to substructure resistance rather than actual skin layer defects. Macrovoids, which compromise the strength of the fiber, were reduced by increasing the polymer concentration. The resulting fiber could withstand up to 2000 psi of N₂, and a CO₂ permeation study indicates that this fiber can perform selective separations under supercritical conditions of 1100 psi of CO₂ at 35 °C.     

Bioinspired Design of a Giant [Mn86] Nanocage-Based Metal-Organic Framework with Specific CO2 Binding Pockets for Highly Selective CO2 Separation

Adsorption-based removal of carbon dioxide (CO₂) from gas mixtures has demonstrated great potential for solving energy security and environmental sustainability challenges. However, due to similar physicochemical properties between CO₂ and other gases as well as the co-adsorption behavior, the selectivity of CO₂ is severely limited in currently reported CO₂-selective sorbents. To address the challenge, we create a bioinspired design strategy and report a robust, microporous metal–organic framework (MOF) with unprecedented [Mn₈₆] nanocages. Attributed to the existence of unique enzyme-like confined pockets, strong coordination interactions and dipole-dipole interactions are generated for CO₂ molecules, resulting in only CO₂ molecules fitting in the pocket while other gas molecules are prohibited. Thus, this MOF can selectively remove CO₂ from various gas mixtures and show record-high selectivities of CO₂/CH₄ and CO₂/N₂ mixtures. Highly efficient CO₂/C₂H₂, CO₂/CH₄, and CO₂/N₂ separations are achieved, as verified by experimental breakthrough tests. This work paves a new avenue for the fabrication of adsorbents with high CO₂ selectivity and provides important guidance for designing highly effective adsorbents for gas separation.     

Investigation into the adsorption of CO2, N2 and CH4 on kaolinite clay

Investigating the adsorption characteristics of CO₂, N₂ and CH₄ on kaolinite clay is beneficial for enhanced shale gas recovery by gas injection. In this paper, the experiments of CO₂, N₂ and CH₄ adsorption at 288 K, 308 K and 328 K on kaolinite clay were conducted, and the thermodynamics analysis of adsorption of three gases was performed. The findings reveal that the order of the uptakes of three gases on kaolinite clay is as follows: N₂ < CH₄ < CO₂. Reducing temperature enlarges the separation coefficients of CO₂ over CH₄ (α_CO2/CH4), CO₂ over N₂ (α_CO2/N2), and CH₄ over N₂ (α_CH4/N2). The value of α_CO2/CH4 greater than one validates that CO₂ is capable to directly replace the pre-adsorbed CH₄. The spontaneity of CO₂ adsorption is the highest, while N₂ has the lowest adsorption spontaneity. Injecting N₂ into gas-bearing reservoir can cause CH₄ desorption by lowering the spontaneity of CH₄ adsorption. Adsorbed CO₂ molecules form a most ordered rearrangement on kaolinite surface. The decrease rate of entropy loss for N₂ adsorption is higher than those for CO₂ and CH₄ adsorption.     

Carbon Dioxide Capture: Prospects for New Materials

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO₂ emissions; however, currently the capture alone will increase the energy requirements of a plant by 25–40 %. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO₂ emissions to the atmosphere, namely postcombustion (predominantly CO₂/N₂ separation), precombustion (CO₂/H₂) capture, and natural gas sweetening (CO₂/CH₄). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO₂ separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal–organic frameworks.     

New polyimide-polyaniline hollow fibers: Synthesis, characterization and behavior in gas separation

New polyimide-polyaniline hollow fibers were produced by dissolution of the polymers in NMP and dry/wet spinning of the resulting solution in a non-solvent (H₂O). The morphology and thermal properties of the fibers, were examined by means of SEM and TGA, and FTIR spectroscopy was used for the study of their chemical structure. Permeability and selectivity measurements in different gases (He, H₂, CH₄, CO₂, O₂ and N₂) were performed in order to evaluate the performance of the membrane in gas separation applications. The results indicate that the novel membrane is a well structured hollow fiber, thermally stable up to 500°. The introduction of polyaniline into the polyimide matrix, results in a great enhancement in fiber permeability (60-600 times) possibly due to increase of the total free volume due to the introduction of shorter polyaniline molecules in the matrix, allowing larger quantities of gases to pass through the composite membrane. Perm-selectivity ratios for the composite membranes H₂/CH₄, He/N₂, H₂/N₂ and H₂/CO₂ were found lower by a factor of 6.4, 8.9, 7.7 and 1.47, respectively, compared to membranes produced using only polyimide. The opposite effect was observed for CH₄/CO₂ and N₂/CO₂ perm-selectivity ratios that showed an increase by a factor of 3.52 and 5.2, respectively. The ratio CH₄/CO₂ is of particular interest for natural gas purification purposes.     

A computational study of CO2, N2, and CH4 adsorption in zeolites

The adsorption properties of CO₂, N₂ and CH₄ in all-silica zeolites were studied using molecular simulations. Adsorption isotherms for single components in MFI were both measured and computed showing good agreement. In addition simulations in other all silica structures were performed for a wide range of pressures and temperatures and for single components as well as binary and ternary mixtures with varying bulk compositions. The adsorption selectivity was analyzed for mixtures with bulk composition of 50:50 CO₂/CH₄, 50:50 CO₂/N₂, 10:90 CO₂/N₂ and 5:90:5 CO₂/N₂/CH₄ in MFI, MOR, ISV, ITE, CHA and DDR showing high selectivity of adsorption of CO₂ over N₂ and CH₄ that varies with the type of crystal and with the mixture bulk composition.     

Gas permeability and permselectivity of polyimides with large aromatic rings

Polyimides with large aromatic rings were prepared from 3,6-diaminocarbazole (CDA), N-ethyl-3,6-diaminocarbazole (ECDA), 2,7-diaminofluorene (DAF), 2,7-diaminofluorenon (DAFO), and dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide (DDBT) with 2-bis(2,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). Their physical properties, including gas permeability and permselectivity, were investigated in comparison with those of the related polyimides from 1,3-phenylenediamine (mPD). Glass transition temperatures of the polyimides with large aromatic rings were much higher than those of the mPD-based polyimides as a result of increased rigidity of the former polymer chains. With changing diamine from mPD to the large aromatic diamines, charge transfer (CT) interaction between the moieties of acid anhydride and diamine seems to be enhanced, judging from the red shift of absorption edge of the polyimide films and the red shift of CT excitation band of the 6FDA-based polyimides in solution. Fraction of free space (V_F) was a little smaller for the polyimides with large aromatic rings except DDBT than for the mPD-based polyimides, probably because of enhancement in polymer chain-chain interactions as a result of the increased CT interaction. The DDBT-based polyimides had large V_F than the mPD-based polyimides because of the nonplanar structure of neighboring dibenzothiophene-5,5-dioxide and imide rings. For the 6FDA-based polyimides, permeability coefficients to H₂, O₂, N₂, CO₂, and CH₄ were in the order, DAFO < mPD ～ DAF < CDA < ECDA < DDBT. As for the membrane performance for H₂/CH₄, CO₂/CH₄, and O₂/N₂ systems, it is significant to change diamine from mPD to DDBT or CDA, but not to DAF or DAFO. The DDBT-based polyimides were excellent for H₂/CH₄ and CO₂/CH₄ separations. © 1995 John Wiley & Sons, Inc.     

Gas permeability,diffusivity, solubility,and aging characteristics of 6FDA-durene polyimide membranes

We have determined the intrinsic gas transport properties of He, H₂, O₂, N₂, CH₄, and CO₂ for a 6FDA-durene polyimide as a function of pressure, temperature and aging time. The permeability coefficients of O₂, N₂, CH₄, and CO₂ decrease slightly with increasing pressure. The pressure-dependent diffusion coefficients and solubility coefficients are consistent with the dual-sorption model and partial immobilization. All the gas permeabilities increase with temperature and their apparent activation energies for permeation increase with increasing gas molecular sizes in the order of CO₂, O₂, N₂, and CH₄.The percentages of permeability decay after 280 days of aging are 22, 32, 36, 40, 42, and 30% for He, H₂, O₂, N₂, CH₄, and CO₂, respectively. Interestingly, except for H₂ (kinetic diameter of 2.89 Å), the percentages of permeability decay increase exactly in the order of He (kinetic diameter of 2.6 Å), CO₂ (3.30 Å), O₂ (3.46 Å), N₂ (3.64 Å), and CH₄ (3.80 Å). The apparent activation energies of permeation for O₂, N₂, CH₄, and CO₂ increase with aging because of the increases in activation energies of diffusion and the decreases in solubility coefficients. The activation-energy increase for diffusion is probably due to the decrease in polymeric molar volume because of densification during aging. The reduction in solubility coefficient indicates the available sites for sorption decreasing with aging because of the reduction of microvoids and interstitial chain space.     

High Pressure Adsorption Data of Methane, Nitrogen, Carbon Dioxide and their Binary and Ternary Mixtures on Activated Carbon

Adsorption equilibria of the gases CH₄, N₂, and CO₂ and their binary and ternary mixtures on activated carbon Norit R1 Extra have been measured in the pressure range 0 P 6 MPa at T = 298 K. Pure gas adsorption equilibria were measured gravimetrically. Coadsorption data of the three binary mixtures CH₄/N₂, CH₄/CO₂, and CO₂/N₂ were obtained by the volume-gravimetric method. Isotherms of five ternary mixtures CH₄/CO₂/N₂ were measured using the volumetric-chromatographic method. First, we present in a short overview the method and procedure of measurement. In a second part, the measured data of pressures, surface excess amounts adsorbed and absolute amounts adsorbed are presented and analyzed. In the last part of the paper the resulting pure gas adsorption data are correlated using a generalized dual-site Langmuir isotherm. Mixture adsorption can be predicted by this model using only pure component parameters with fair accuracy. Results are presented and discussed in several tables and figures.     

High performance of PES-GNs MMMs for gas separation and selectivity

In this study, graphene nanosheets (GNs) were incorporated into polyethersulfone (PES) by phase inversion approach for preparing PES-GNs mixed matrix membranes (MMMs). To investigate the impact of filler content on membrane surface morphology, thermal stability, chemical composition, porosity and mechanical properties, MMMs were constructed with various GNs loadings (0.01, 0.02, 0.03, and 0.04 wt%). ?The performance of prepared MMMs was tested for separation and selectivity of CO₂, N₂, H₂ and CH₄ gases at various pressures from 1 to 6 bar and temperature varying from 20 to 60 °C. It was observed that, compared to the pristine PES membrane, the prepared MMMs significantly improved the gas separation and selectivity performance with adequate mechanical stability. The permeability of CO₂, N₂, H₂ and CH₄ for the PES + 0.04 wt% GNs increases from 9 to 2246, 11 to 2235, 9 to 7151, and 3 to 4176 Barrer respectively, as compared with pure PES membrane at 1 bar and 20 °C due to improving the membrane absorption and porosity. In addition, by increasing the pressure, the permeability and selectivity of CO₂, N₂, H₂ and CH₄ are increased due to the increased driving force for the transport of gas via membranes. Furthermore, the permeability of CO₂, N₂, H₂ and CH₄ increased by increasing the temperature from 20 to 60 °C due to the plasticization in the membranes and the improvement in polymer chain movement. This result proved that the prepared membranes can be used for gas separation applications.     

Theoretical simulation of CO2 capture in organic cage impregnated with polyoxometalates

To explore the adsorption and separation properties of CO₂ in a novel material consisting of a series of polyoxometalates (POMs) impregnated within supramolecular porous catenane (shorted as SPC), grand canonical Monte Carlo (GCMC) simulations and ab initio calculations were used. GCMC simulations showed this impregnation can enhance CO₂/CH₄ (or CO₂/N₂) selectivity almost 30 times compared to the bare SPC due to the strong interaction of CO₂ with the nPOMs@SPC structures. And, the loading of CO₂ inhibits the adsorption of CH₄ (or N₂) as CO₂ occupying the preferred adsorption sites. Furthermore, the effect of number, mass, and volume of POMs inserted in SPC on CO₂/CH₄ (or CO₂/N₂) selectivity over large pressure range was investigated in detail. Additionally, the accurate ab initio calculations further confirmed our GCMC simulations. As a result, the proposed nPOMs@SPC structures are promising candidates for CO₂/N₂ and CO₂/CH₄ separations. © 2017 Wiley Periodicals, Inc.     

Poly(amide-6-b-ethylene oxide)/SAPO-34 mixed matrix membrane for CO2 separation

In this paper, poly(amide-6-b-ethylene oxide) (Pebax1657)/SAPO-34 mixed matrix membranes (MMMs) were prepared by solvent-evaporation method with acetic acid as a novel solvent. CO₂, N₂, CH₄ and H₂ permeation properties were investigated, and the physical properties of Pebax/SAPO-34 MMMs were characterized by XRD and SEM. At low SAPO-34 content, it was homogeneously distributed in the Pebax matrix, and then precipitated and agglomerated at high SAPO-34 content. The crystallinity of Pebax phase in Pebax/SAPO-34 MMMs decreased initially and then rebounded as a result of phase separation. With the increase of transmembrane pressure difference, CO₂ permeability was enhanced due to the effect of pressure-induced plasticization. Owing to the happening of stratification, the CO₂ permeability of Pebax/SAPO-34 MMMs (50 wt% SAPO-34) increased to 338 Barrer from 111 Barrer of pristine Pebax, while the selectivities of CO₂/CH₄ and CO₂/N₂ were almost unchanged. Compared with the pristine Pebax, the gas separation performances of Pebax/SAPO-34 MMMs were remarkably enhanced.     

Electropolymerization of Molecular‐Sieving Polythiophene Membranes for H2 Separation

Membrane technologies that do not rely on heat for industrial gas separation would lower global energy cost. While polymeric, inorganic, and mixed‐matrix separation membranes have been rapidly developed, the bottleneck is balancing the processability, selectivity, and permeability. Reported here is a softness adjustment of rigid networks (SARs) strategy to produce flexible, stand‐alone, and molecular‐sieving membranes by electropolymerization. Here, 14 membranes were rationally designed and synthesized and their gas separation ability and mechanical performance were studied. The separation performance of the membranes for H₂/CO₂, H₂/N₂, and H₂/CH₄ can exceed the Robeson upper bound, among which, H₂/CO₂ separation selectivity reaches 50 with 626 Barrer of H₂ permeability. The long‐term and chemical stability tests demonstrate their potential for industrial applications. This simple, scalable, and cost‐effective strategy holds promise for the design other polymers for key energy‐intensive separations.     

PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation

In the present work, membranes from commercially available Pebax^® MH 1657 and its blends with low molecular weight poly(ethylene glycol) PEG were prepared by using a simple binary solvent (ethanol/water). Dense film membranes show excellent compatibility with PEG system up to 50 wt.% of content. Gas transport properties have been determined for four gases (H₂, N₂, CH₄, CO₂) and the obtained permeabilities were correlated with polymer properties and morphology of the membranes. The permeability of CO₂ in Pebax^®/PEG membrane (50 wt.% of PEG) was increased two fold regarding to the pristine Pebax^®. Although CO₂/N₂ and CO₂/CH₄ selectivity remained constant, an enhancement of CO₂/H₂ selectivity (∼11) was observed. These results were attributed to the presence of EO units which increases CO₂ permeability, and to a probable increase of fractional free-volume. Furthermore, for free-volume discussion and permeability of gases, additive and Maxwell models were used.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

Effect of protonation exothermicity on the chemical ionization mass spectra of some alkylbenzenes

The H₂, N₂/H₂, CO₂/H₂, N₂O/H₂, CO/H₂ and CH₄ chemical ionization mass spectra of thirteen C₈ to C₁₁ alkylbenzenes are reported. Characteristic hydride and alkide ion abstraction reactions are observed with all reagent gases. The major fragmentation reactions of [MH]⁺ are olefin elimination to form a protonated arene and arene elimination to form an alkyl ion. From the effect of structure and protonation exothermicity it is concluded that rearrangement of primary alkyl groups to the more stable secondary or tertiary structure occurs prior to alkyl ion formation. A detailed fragmentation mechanism for protonated arenes is proposed. The ‘effective’ proton affinity of the methane-derived reagent system is estimated to be ～556 kJ mol^?1.     

Adsorption separation of carbon dioxide,methane, and nitrogen on Hβ and Na-exchanged β-zeolite

Adsorption isotherms of carbon dioxide （CO2）, methane （CH4）, and nitrogen （N2） on Hβand sodium exchanged β-zeolite （Naβ） were volumetrically measured at 273 and 303 K. The results show that all isotherms were of Brunauer type I and well correlated with Langmuir-Freundlich model. After sodium ions exchange, the adsorption amounts of three adsorbates increased, while the increase magnitude of CO2 adsorption capacity was much higher than that of CH4 and N2. The selectivities of CO2 over CH4 and CO2 over N2 enhanced after sodium exchange. Also, the initial heat of adsorption data implied a stronger interaction of CO2 molecules with Na＋ ions in Naβ . These results can be attributed to the larger electrostatic interaction of CO2 with extraframework cations in zeolites. However, Naβ showed a decrease in the selectivity of CH4 over N2, which can be ascribed to the moderate affinity of N2 with Naβ. The variation of isosteric heats of adsorption as a function of loading indicates that the adsorption of CO2 in Naβ presents an energetically heterogeneous profile. On the contrary, the adsorption of CH4 was found to be essentially homogeneous, which suggests the dispersion interaction between CH4 and lattice oxygen atoms, and such interaction does not depend on the exchangeable cations of zeolite.     

Interstitial and lattice substituted models of an expanded ice-I hlattice

An ice-I _hlattice (O?O=0.286 nm) with small molecules placed interstitially and with some placed at lattice positions was investigated by the CNDO/2-MO technique. The interstitial molecules included H₂, N₂, O₂, HF, CO₂, H₂O, NH₃, CH₄ and CH₂O, whereas those involved in lattice substitution included HF, NH₃ and H₂O. From the calculations it is found that all interstitial and lattice substituted systems are stabler than the sum of the components, the enhanced stability depending on the system. Generally, lattice substituted systems are stabler than the corresponding interstitial models. Charges residing on the atomic positions reflect the amount of interaction with the matrix as well as indicating how the change in charge would facilitate other interactions with the solvent.     

A GC-system for the analysis of residual geothermal gases

Summary The gases evolved from geothermal fields, after condensation of H₂O, CO₂, H₂S and NH₃ in caustic solution, contain He, H₂, Ar, O₂, N₂, CH₄ and higher hydrocarbons. The analysis for the major components in these residual gas mixtures can be achieved by use of two simple gas chromatographs in parallel, and using 5Å molecular sieve. The separation of He and H₂ to baseline is achieved by using low temperatures (30°C) coupled with a relatively long column; and the difficult separation of Ar and O₂ is achieved by use of a cryogenically cooled column. The use of switching valves to backflush and bypass columns ensures that a minimum time for analysis can be achieved whilst retaining baseline separations of the He/H₂ and Ar/O₂ pairs.     

Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures

The temperature dependence of gas sorption and transport properties is examined for two polyimide isomers. The permeabilities and solubilities of five gases in these materials are reported over an extensive temperature range from 35 to 325°C. Also, the activation energies for permeation, the heats of sorption, and the activation energies for diffusion obtained for both polyimides are compared and correlated with physical properties of the polymers and penetrants. The influence of temperature on the selective properties of these membrane materials is discussed for three gas separations; He/N₂, CO₂/CH_4, and O₂/N_2. Thorough analysis of these data provides insight into the influence of the subtle difference in chain structure of the two isomers. The performance of the 6FDA-6F_p DA as a separation membrane at high temperatures suggests that it is an outstanding candidate for use in novel elevated temperature applications. ©1995 John Wiley & Sons, Inc.