Mixed Gas Selectivity Research Articles

Fabrication of Hofmann-type metal-organic framework based mixed-matrix membranes for efficient CH4/N2 separation

The analogous physical and chemical properties of methane (CH4) and nitrogen (N2) pose a great challenge for their separation. Mixed-matrix membranes (MMMs), combining the good separation performance of fillers and easy processability of polymers, are expected to address this challenging task. In this work, one Hofmann-type metal-organic framework (MOF), CoNi-DABCO, featuring abundant oppositely adjacent open metal sites, narrow pore size, and strong binding affinity toward CH4, is introduced into the polymer of polydimethylsiloxane to fabricate MMMs. The presence of CoNi-DABCO in the membranes could enhance the adsorption capacity for CH4, thus facilitating the transport of CH4 molecules through the membranes. Specially, the MMM containing 20 wt% CoNi-DABCO displays a high CH4 permeability of 1285 Barrer and maintains a good CH4/N2 mixed-gas selectivity of 3.7. Furthermore, the MMMs show good pressure resistance (up to 10 bar) and long-term stability for 30 days. This study suggests the potential of Hofmann-type MOF in the development of high-performance membranes for CH4/N2 separation.

Estimation of mixed-gas selectivity of membranes using the linear friction-based model

The development of analytical models for predicting the transport properties of membranes in the separation of multicomponent mixtures is highly relevant because (1) measurements of permeability and sorption of gas mixtures in membrane materials are laborious and time-consuming, and (2) estimation of transport properties based on pure gas measurements is desirable, however, rather unreliable and can lead to serious errors. In this paper, within the framework of the well-known frictional-coefficient formalism, we present nonlinear equations for the fluxes of gas mixture components through the membrane, which take into account the diffusional coupling of the fluxes compared to independent gas transport. As a result, an analytical equation for the mixed-gas selectivity was obtained. This equation has an implicit form, which is not quite convenient for practical application, since it has to be solved numerically. A simple explicit expression for mixed gas selectivity was derived using the so-called linear transport model, which is a linearized version of the original nonlinear flux equations. It is important to emphasize that a comparison of the linear model with the formally more rigorous nonlinear model showed only minor quantitative differences. The input parameters for the model are the single gas transport parameters. Testing the linear model using experimental data for the separation of hydrocarbons (n-butane/methane, n-butane/i-butane, propylene/propane) revealed an acceptable fit. A comparison of the model with recent experimental data on the separation of CO2-containing mixtures using microporous polymers is also presented. It is found that the calculated CO2/N2 mixed gas selectivity is in good agreement with the experimental data, while only a semi-quantitative agreement is observed for the CO2/CH4 separation. Besides, it is shown that the model developed for binary gas separation can be extended to ternary gas mixtures.

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

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

Large-Area Ultrathin Metal-Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations.

Ultrathin continuous metal-organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic-imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large-area fabrication. Here, we propose a ligand back diffusion-assisted bipolymer-directed metal ion distribution strategy for preparing large-area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross-linked two-dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large-area ultrathin MOF membranes. We report the practical fabrication of three representative defect-free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50-130 nm), including ZIFs and carboxylate-linker MOFs. Among these, the ZIF-8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale-up practicality, MOF membranes were prepared and incorporated into spiral-wound membrane modules with an active area of 4,800 cm2. The ZIF-8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Large‐Area Ultrathin Metal–Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations

AbstractUltrathin continuous metal–organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic‐imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large‐area fabrication. Here, we propose a ligand back diffusion‐assisted bipolymer‐directed metal ion distribution strategy for preparing large‐area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross‐linked two‐dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large‐area ultrathin MOF membranes. We report the practical fabrication of three representative defect‐free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50–130 nm), including ZIFs and carboxylate‐linker MOFs. Among these, the ZIF‐8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale‐up practicality, MOF membranes were prepared and incorporated into spiral‐wound membrane modules with an active area of 4,800 cm2. The ZIF‐8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Precise molecular sieving of ethylene from ethane using triptycene-derived submicroporous carbon membranes.

Replacement or debottlenecking of the extremely energy-intensive cryogenic distillation technology for the separation of ethylene from ethane has been a long-standing challenge. Membrane technology could be a desirable alternative with potentially lower energy consumption. However, the current key obstacle for industrial implementation of membrane technology is the low mixed-gas selectivity of polymeric, inorganic or hybrid membrane materials, arising from the similar sizes of ethylene (3.75 Å) and ethane (3.85 Å). Here we report precise molecular sieving and plasticization-resistant carbon membranes made by pyrolysing a shape-persistent three-dimensional triptycene-based ladder polymer of intrinsic microporosity with unparalleled mixed-gas performance for ethylene/ethane separation, with a selectivity of ~100 at 10 bar feed pressure, and with long-term continuous stability for 30 days demonstrated. These submicroporous carbon membranes offer opportunities for membrane technology in a wide range of notoriously difficult separation applications in the petrochemical and natural gas industry.

Two morphology distinct fillers with chemical similarity incorporated into Tröger’ base polymers towards enhanced gas separation

Mixed matrix membranes (MMMs) with one class of fillers always require high loadings to maximize gas permeability. The incompatibility between polymers and high content fillers causes non-selective interfacial voids in the MMMs and poor mechanical properties of MMMs. This work adopts two morphology distinct fillers with chemical similarity to achieve improved gas separation performance of Tröger’ base membranes without sacrificing their mechanical properties. The three-dimensional ZIF-8 nanoparticles are beneficial for membrane gas permeability. The addition of low content ZIF-8 nanosheets can greatly improve H2/CH4 and CO2/CH4 selectivities of the membranes. ZIF-8 particles and ZIF-8 nanosheets with similar chemical properties can mix more homogeneously and disperse well in the polymer matrix with expected hydrogen bonding interaction compared to mixed ZIF-8 and CuBDC nanosheets. Eventually, the resulting dual filler MMMs doped with 4 wt% ZIF-8 nanosheets and 10 wt% ZIF-8 nanoparticles have H2/CH4 ideal selectivity of 60 with H2 permeability of 426 Barrer, which is an increase of 18% and 102% over pure membranes, respectively. Moreover, the membrane has a significantly enhanced CO2/CH4 mixed gas selectivity of 68 at sub-ambient temperature, accompanied by temperature-dependent CO2-induced plasticization behavior. Mixing morphology distinct fillers with chemical similarity is proved to be an effective method to prepare a robust MMM at low filler amounts, and the synergistic effect on gas permeability and selectivity endows these dual filler MMMs with great H2/CH4 and CO2/CH4 separation performance.

Enhanced H2/CO2 separation of ZIF-8/NPBI mixed matrix membranes by forming cross-linking networks and crystal defects via acid doping

Fabricating a membrane with high H2/CO2 separation performance at high temperatures is highly desired. Herein, a cross-linked zeolitic imidazolate framework 8/poly[2,2′-(1,4-naphthalene)-5,5′-bibenzimidazoles] (ZIF-8/NPBI) mixed matrix membranes (MMMs) with tiny defects in ZIF-8 is fabricated via a phosphoric acid doping approach. The cross-linked NPBI chains and partially etched ZIF-8 by the acid result in significantly improved H2 permeability and H2/CO2 mixed gas selectivity. At 150 °C, compared to pure PBI membrane, the H2 permeability of the acid-treated 10% ZIF-8/NPBI membrane is enhanced from 39.97 Barrer to 98.36 Barrer, and the H2/CO2 mixed gas selectivity increases from 4.6 to 8.9, eventually exceeding the Robeson's upper bound predicted for 150 °C. When tested with the gas mixtures of 50% H2/50% CO2 continuously, the resulting membrane exhibits a stable H2 permeability of around 74 Barrer and H2/CO2 mixed gas selectivity of ≈8.69 at 150 °C for 24 h. This simple approach of improving H2 permeability and H2/CO2 mixed gas selectivity through crosslinking and etching provides a new approach for designing MMMs for H2/CO2 separation at high temperatures.

Efficient CO2/CH4 separation using [Bmim][Ac]/Pebax-1657 supported ionic liquid membranes and its prediction by density functional theory

The world's daily increase in population has become ravenous for natural resources for energy and potentially helps to become the main reason for causing global climate change. Carbon dioxide (CO2) is one of the critical elements of the greenhouse gas effect, as we all know. Last few decades, researchers have been much interested in CO2 capture and storage technologies. From such technologies, membrane separation has some good results in CO2 capture compared to others. In this investigation, ionic Liquid (IL) supported membranes were used to separate CO2 and methane (CH4) gas mixtures. The Pebax-1657 with 1-Butyl-3-methylimidazolium acetate concentrations 5%, 10%, 20% (wt.%, based on polymer) were prepared for gas separation study. Density functional theory (DFT) computations were also used to estimate CO2 and CH4 interactions with the IL. These membranes are examined for analytical and morphological research using various characterization methods such as FTIR and scanning electron microscopy. As expected, the gas separation results show that mixed gas selectivity (CO2/CH4) increases at high-pressure values due to the addition of IL. Membrane with 20 wt.% concentration (based on polymer) IL shows higher permeability and CO2/CH4 selectivity.

Tailoring the microporosity and gas separation property of soluble polybenzoxazole membranes derived from different regioisomer monomers

New polymers and the relationship between their molecular structure and gas separation performance are crucial in membrane based gas separation. In the study, a range of bisbenzoxazole containing diamines with spirobisindane as building block were synthesized, and consequently, their corresponding four soluble spirobisindane-bisbenzoxazole-polyimides (SPBO-PIs) (5a-6FDA to 5bb-6FDA) were obtained. These SPBO-PIs exhibit superior solubility, brilliant mechanical performance which shows high tensile strengths (about 80 MPa) and attractive elongation at break (7.6–12.8 %), and good thermal stability (Tg up to 491 °C). The SPBO-PIs from para-amino and symmetric linkages in the polymeric main chains has superior thermal and mechanical properties, slightly lower density (1.24, 1.25 vs 1.27, 1.29 cm3 g−1), higher surface area (561, 437 vs 364, 318 m2 g−1), bigger micropore (5.74, 5.60 vs 5.53, 5.49 Å), and larger free volume (0.302, 0.284 vs 0.282, 0.277) than the corresponding mata-amino and asymmetric linkages. The SPBO-PI from amine isomers at para-linked (5a-6FDA) exhibits better CO2/CH4 and CO2/N2 separation performance than the SPBO-PI from amine isomers at meta-linked (5aa-6FDA), meanwhile, the SPBO-PI from symmetric oxazole isomers (5aa-6FDA) shows higher permeability than the asymmetric oxazole isomers (5bb-6FDA). The 5b-6FDA exhibits reasonable anti-plasticization properties, and 97.3 Barrer for CO2 permeability and 24.1 of CO2/CH4 mixed-gas selectivity at 20 bar. The study about SPBO-PI membranes shed light on molecular design of high performance polyimide and polybenzoxazole-based membranes.

Hyperaging-induced H2-selective thin-film composite membranes with enhanced submicroporosity toward green hydrogen supply

Repurposing the existing natural gas infrastructure by blending hydrogen with methane (i.e., Hythane) is one feasible option to develop a low-carbon hydrogen supply chain, although this process requires extraction of the hydrogen from Hythane after distribution. Membrane technology is a potential solution to tackle this application given its many advantages over other separation methods. However, industrial use of developed membrane materials has been challenging due to several practical concerns; for example, insufficient separation abilities and accelerated physical aging of thin membranes in high-free-volume glassy polymers. Herein, we propose an integrated strategy to develop highly H2-selective thin-film composite (TFC) membranes by tuning the aging behavior of polymers of intrinsic microporosity (PIM) thin films. Detailed gas permeation and two-dimensional (2D) grazing incidence wide-angle x-ray scattering (GIWAXS) studies reveal that triptycene-based PIM TFC membranes can exploit beneficial aging effects resulting from aging-induced enhancement in submicroporosity. To directly deploy TFC membranes, a simple post-treatment step was introduced to increase the aging rate, termed “hyperaging.” The hyperaged TFC membranes exhibited high H2/CH4 mixed-gas selectivity (>100), moderate H2 permeance (∼100 GPU), and good long-term stability when tested using a binary mixture with dilute H2 concentration (20 mol%), demonstrating promise for downstream hydrogen extraction toward green hydrogen supply.

Optimal temperature range for the separation of a mixture of 96% protium and 4% deuterium with a palladium membrane

Hydrogen isotope separation can be achieved using a palladium (Pd) membrane because protium (H) and deuterium (D) exhibit different solubility, diffusivity and, hence, mixed-gas permeability in Pd. The permeability of H (kH2) and D (kD2) were evaluated using a 4 vol% D2 – 96 vol% H2 feed mixture at temperatures of 293 K–473 K and pressures of 239 kPa–446 kPa in a continuous Pd membrane separation unit. The Pd foil membrane (0.1 mm thick) is shown to be selective toward H2 over the entire temperature range, with both kH2 and kD2 increasing with temperature. As temperature increases, the mixed-gas selectivity (kH2/kD2) rises from 4.0 at 300 K to a maximum value of 9.6 in the 363 K–373 K temperature range (which represents the operating temperatures that would render the highest concentration of H2 in the permeate and the greatest concentration of D2 in the retentate), then decreases to 6.2 at 480 K. Competitive transport during co-permeation is a likely cause for the maximum separation factor kH2/kD2 observed in this temperature range, being larger than the values predicted using pure gases (2 for diffusion dominant separation). The membrane selectivity presumably decreases from the observed maximum because the reverse solubility isotope effect decreases as the temperature increases. Additionally, isotope diffusion begins to increasingly affect the selectivity at higher temperatures.

Interface engineering in MOF/crosslinked polyimide mixed matrix membranes for enhanced propylene/propane separation performance and plasticization resistance

The energy required for propylene/propane (C3H6/C3H8) separation is enormous: ∼0.3% of global energy consumption. Membrane separation is gaining much attention as an alternative or supplement for the conventional distillation processes, although the separation abilities and stabilities of current polymer membranes remain insufficient to meet industrial requirements. Herein, we propose a novel strategy to integrate the diamino crosslinking of polyimide (6FDA-DAM) membranes and the fabrication of their mixed matrix membranes (MMMs) to engineer the interfacial compatibility between filler and matrix. Crosslinking conditions for the 6FDA-DAM matrix are optimized for C3H6/C3H8 separation, and their MMMs were fabricated by incorporating UiO-66 (U) or UiO-66-NH2 (UN) fillers. After crosslinking the MMM precursors, detailed characterization results of the filler-matrix interfaces, which include morphological, spectroscopical, thermal, and mechanical analyses, indicate that the UN-incorporated MMMs display enhanced interfacial compatibility compared with their counterpart (i.e., the U-incorporated ones). Ultimately, the UN-incorporated MMMs showed significantly enhanced C3H6/C3H8 separation performances by surpassing both pure-gas and mixed-gas upper bounds. Besides, the MMM exhibited excellent plasticization resistance against the high feed pressure (5 bar) with high C3H6/C3H8 mixed-gas selectivity (23.7) and modest C3H6 permeability (6.3 Barrer), demonstrating its potential for use in industrial applications.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Ultrapermeable 2D-channeled graphene-wrapped zeolite molecular sieving membranes for hydrogen separation.

The efficient separation of hydrogen from methane and light hydrocarbons for clean energy applications remains a technical challenge in membrane science. To address this issue, we prepared a graphene-wrapped MFI (G-MFI) molecular-sieving membrane for the ultrafast separation of hydrogen from methane at a permeability reaching 5.8 × 106 barrers at a single gas selectivity of 245 and a mixed gas selectivity of 50. Our results set an upper bound for hydrogen separation. Efficient molecular sieving comes from the subnanoscale interfacial space between graphene and zeolite crystal faces according to molecular dynamic simulations. The hierarchical pore structure of the G-MFI membrane enabled rapid permeability, indicating a promising route for the ultrafast separation of hydrogen/methane and carbon dioxide/methane in view of energy-efficient industrial gas separation.

Performance of Multilayer Composite Hollow Membrane in Separation of CO2 from CH4 in Mixed Gas Conditions.

Composite membranes comprising NH2-MIL-125(Ti)/PEBAX coated on PDMS/PSf were prepared in this work, and their gas separation performance for high CO2 feed gas was investigated under various operating circumstances, such as pressure and CO2 concentration, in mixed gas conditions. The functional groups and morphology of the prepared membranes were characterized by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). CO2 concentration and feed gas pressure were demonstrated to have a considerable impact on the CO2 and CH4 permeance, as well as the CO2/CH4 mixed gas selectivity of the resultant membrane. As CO2 concentration was raised from 14.5 vol % to 70 vol %, a trade-off between permeance and selectivity was found, as CO2 permeance increased by 136% and CO2/CH4 selectivity reduced by 42.17%. The membrane produced in this work exhibited pressure durability up to 9 bar and adequate gas separation performance at feed gas conditions consisting of high CO2 content.

Selective separation of HFC-32 from R-410A using poly(dimethylsiloxane) and a copolymer of perfluoro(butenyl vinyl ether) and perfluoro(2,2-dimethyl-1,3-dioxole)

Currently, millions of kilograms of HFCs and HFC mixtures are in use with no method for efficiently separating the components. Membranes are an attractive method for separating HFC refrigerant mixtures due to both lower energy consumption and capital requirements compared with alternative separation methods such as distillation. This study investigates the use of poly(dimethylsiloxane) and CyclAFlor™, an amorphous copolymer of 5 mol% perfluoro(butenyl vinyl ether) and 95 mol% perfluoro(2,2-dimethyl-1,3-dioxole), for the separation of R-410A, an azeotropic mixture composed of 50 wt% difluoromethane (HFC-32, CH2F2) and 50 wt% pentafluoroethane (HFC-125, CHF2CF3). Pure gas permeability of HFC-32 and HFC-125 were measured using a static membrane apparatus and the pressure-rise method. Solubility and diffusivity were measured using a gravimetric microbalance. Permeability measurements indicate high selectivity of HFC-32 over HFC-125 with CyclAFlor™, and mixed gas selectivity measurements utilizing a mixed gas apparatus are in excellent agreement with ideal selectivity calculations based on solubility and diffusivity measurements. Stability studies indicate that plasticization of the fluorinated amorphous polymer membrane is minimal in the presence of HFC-32 and HFC-125.

Facile suppression of intensified plasticization in glassy polymer thin films towards scalable composite membranes for propylene/propane separation

Membrane-based propylene/propane (C3H6/C3H8) separation has the potential to significantly reduce the extremely high energy consumption in the conventional distillation process. However, no large-scale commercialization case currently exists despite decades of remarkable advancements in membrane materials. This challenge can potentially be attributed to a lack of understanding of the close relationship between material properties and membrane configurations, including confinement-driven transitions in polymer dynamics from the bulk to thin films (<1 μm). We first report design aspects of thin-film composite (TFC) membranes for C3H6/C3H8 separation based on a cost-effective, versatile, and scalable fabrication method. An unprecedented acceleration in C3 hydrocarbon-induced plasticization is observed in TFC membranes as the selective layer thickness decreases, causing anomalous gas transport properties and poor mixed-gas selectivities, which deviate from those of bulk membranes. To overcome this issue, a plasticization resistant (PR) layer is additionally coated onto the TFC membranes. Advanced thin-film characterization techniques, including quartz crystal microbalance (QCM) and nanomechanical analyses, demonstrate effective suppression of intensified plasticization in glassy polymer thin films by introducing a PR layer. Ultimately, the PR layer-coated TFC membranes exhibited excellent mixed-gas C3H6/C3H8 separation performances close to industrial requirements, which can be further extended to prepare large-area TFC membranes by roll-to-roll processes.

Mixed-Gas Selectivity Based on Pure Gas Permeation Measurements: An Approximate Model.

An approximate model based on friction-coefficient formalism is developed to predict the mixed-gas permeability and selectivity of polymeric membranes. More specifically, the model is a modification of Kedem’s approach to flux coupling. The crucial assumption of the developed model is the division of the inverse local permeability of the mixture component into two terms: the inverse local permeability of the corresponding pure gas and the term proportional to the friction between penetrants. Analytical expressions for permeability and selectivity of polymeric membranes in mixed-gas conditions were obtained within the model. The input parameters for the model are ideal selectivity and solubility coefficients for pure gases. Calculations have shown that, depending on the input parameters and the value of the membrane Peclét number (the measure of coupling), there can be both a reduction and an enhancement of selectivity compared to the ideal selectivity. The deviation between real and ideal selectivity increases at higher Peclét numbers; in the limit of large Peclét numbers, the mixed-gas selectivity tends to the value of the ideal solubility selectivity. The model has been validated using literature data on mixed-gas separation of n-butane/methane and propylene/propane through polymeric membranes.

Pure- and mixed-gas transport properties of a microporous Tröger's Base polymer (PIM-EA-TB)

Polymers of intrinsic microporosity (PIMs) offer tantalizing combinations of high selectivity and permeability in initial gas permeation measurements. Here, we report characterization of pure- and mixed-gas permeation properties of a thick (∼80 μm) PIM consisting of Tröger's Base (TB) and Ethanoanthracene (EA) films. The effects of feed pressure and temperature on pure-gas permeabilities of CH4, N2, O2, H2, and CO2 were investigated. The physical aging behavior of the thick film was tracked via pure-gas O2, N2, and CH4 permeability at 35 °C. Gas permeability decreased noticeably and selectivity increased as aging time increased. Particular attention was given to mixed-gas measurements of CO2 and CH4 (50/50) permeabilities at 35 °C and fugacities ranging from 2 to 18 atm to explore whether the rigid, bridged, bicyclic TB and EA units could resist CO2-induced plasticization. These results are presented along with pure-gas CO2 and CH4 results for membrane samples aged at different times. PIM-EA-TB aged for ∼24 h did not show signs of plasticization across the fugacity range considered. Dual-mode competitive sorption presumably caused the CO2/CH4 mixed-gas selectivity to be slightly higher than its corresponding pure-gas selectivity. However, as aging time increased, aged films underwent progressively more rapid and extensive CO2-induced plasticization with increasing fugacity, suggesting a systematic relationship between physical aging and plasticization in PIMs. Consequently, physical aging caused less improvement in mixed-gas CO2/CH4 selectivity than it did in pure-gas selectivity, due mainly to plasticization effects.