Gas Transport Properties Research Articles

A molecular simulation study on amine-functionalized silica/polysulfone mixed matrix membrane for mixed gas separation

Polysulfone (PSF) based mixed matrix membranes (MMMs) are one of the most broadly studied polymeric materials used for CO2/CH4 separation. The performance of existing PSF membranes encounters a bottleneck for widespread expansion in industrial applications due to the trade-off amongst permeability and selectivity. Membrane performance has been postulated to be enhanced via functionalization of filler at different weight percentages. Nonetheless, the preparation of functionalized MMMs without defects and its empirical study that exhibits improved CO2/CH4 separation performance is challenging at an experimental scale that needs prior knowledge of the compatibility between the filler and polymer. Molecular simulation approaches can be used to explore the effect of functionalization on MMM's gas transport properties at an atomic level without the challenges in the experimental study, however, they have received less scrutiny to date. In addition, most of the research has focused on pure gas studies while mixed gas transport properties that reflect real separation in functionalized silica/PSF MMMs are scarcely available. In this work, a molecular simulation computational framework has been developed to investigate the structural, physical properties and gas transport behavior of amine-functionalized silica/PSF-based MMMs. The effect of varying weight percentages (i.e., 15–30 wt.%) of amine-functionalized silica and gas concentrations (i.e., 30% CH4/CO2, 50% CH4/CO2, and 70% CH4/CO2) on physical and gas transport characteristics in amine-functionalized silica/PSF MMMs at 308.15 K and 1 atm has been investigated. Functionalization of silica nanoparticles was found to increase the diffusion and solubility coefficients, leading to an increase in the percentage enhancement of permeability and selectivity for amine-functionalized silica/PSF MMM by 566% and 56%, respectively, compared to silica/PSF-based MMMs at optimal weight percentage of 20 wt.%. The model's permeability differed by 7.1% under mixed gas conditions. The findings of this study could help to improve real CO2/CH4 separation in the future design and concept of functionalized MMMs using molecular simulation and empirical modeling strategies.

Gas permeameter for polymers and nanocomposites: a new equipment

Information on the gas transport coefficients in the permeation of homogeneous semipermeable materials such as semicrystalline polymers under extreme pressures and temperatures is rarely found in the literature. Therefore, the objective of this work is to showcase innovation regarding the accuracy and usability of the medium pressure and temperature gas permeameter suitable for polymeric and polymeric nanocomposite plates built by the authors, which was tested with nitrogen at 1 MPa and 69 °C (342.15 K) permeating pure high density polyethylene samples (HDPE) and HDPE samples with added nanoclay, redoing the tests published in a previous work on the subject. The results were compared against data obtained previously and information present in the literature, validating the permeameter presented in this work, which is capable of analyzing gas permeability under the described conditions with greater accuracy and ease of operation than the previous model for the values of the transport coefficients of nitrogen permeating HDPE, demonstrating that the implemented improvements and advances were adequate, allowing the measurement of the transport properties of gases permeating polymeric and nanocomposite plates, necessary information for the design of risers for the transportation of oil and natural gas, for example.

The Possibility of Intermediate–Temperature (120 oC)–Operated Polymer Electrolyte Fuel Cells

The technical possibility and possible issues of an intermediate–temperature–operated polymer electrolyte fuel cell (@120 °C, IT–PEFC) was investigated toward the launching of new fuel cell heavy–duty vehicles. Commercial perfluorosulfonic acid membrane (PSFA, NRE212) and a commercial Pt catalyst supported on carbon (Pt/C: TEC10E50E, TKK) were selected to clarify the possible issues in the fuel cells. The higher oxygen back–pressures, over 200 kPa(abs.), allowed us to obtain higher power densities than those for cells operated at 80 °C (Fig. 1), because the water vapor pressure at 120 °C exceeded 100 kPa(abs.) above 50% RH. The mass activity at 0.85 V increased monotonically with increasing back–pressure above the water vapor pressure of 160 kPa(abs.) at 120 °C, 80% RH. The open circuit voltage (OCV) exceeded 1.0 V for back–pressures above 200 kPa(abs) at the Pt loadings on the cathode catalyst layer above 0.20 mgPt cm–2 (Fig. 2), which would relate with the hydrogen crossover in the PFSA membrane. The mass activity (@0.85 V) decreased moderately with decreasing cathodic Pt loading amount at each back–pressures (Fig. 3), indicating that the limitation of Pt mass activity in the lower Pt loading cathode catalyst layers occurred at 120oC, 80 %RH. The load cycle durability test (0.60V-1.0 V, 6 sec.) of the Pt catalyst supported on carbon at 120 °C at low humidity (20% RH) was higher than that at 120 °C at high humidity (80% RH) and 80 °C at 80% RH (Fig. 4), indicating that effect of Ostwald ripening would mitigate. The degradation of PFSA membrane accelerated at open circuit voltage condition by increasing operating temperature despite of the operation under the high humidity condition of 80% RH. We propose that the further improvement of both the gas transport properties in gas diffusion layers and the durability of the PFSA membrane to realize the IT–PEFC operation. Acknowledgment This work was partially supported by funds for the project “Electrolytes, Catalysts and Catalyst layers with Extraordinary Efficiency, power and Durability for PEFCs-2040 (ECCEED’40) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Reference K. Kakinuma, H. Taniguchi, T. Asakawa, T. Miyao, M. Uchida, Y. Aoki, T. Akiyama, A. Masuda, N. Sato, A. Iiyama J. Electrochem. Soc. 169, 044522 (2022). Figure 1

Structure-Property-Transport Relationships of Miscible Ionomer Blend Systems

Systematically designing the spatial arrangement is a key to understanding the structure and property relationships of proton-exchange membranes (PEM)/ionomers. Miscible polymer blends provide a cost-effective method for developing new materials in polymer science industries instead of synthesizing new polymers. In this study, commercially available Pentablock Copolymer (PBC(1.0)) ionomers were blended with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) in varying blend ratios. The FTIR studies showed composition-dependent interaction between the methyl group of PPO and phenyl rings of Polystyrene blocks in PBC(1.0). However, the sulfonate peaks at PBC(1.0): PPO (50:50) blend ratios were significantly reduced . The reduction in the peak intensity at this blend ratio may be attributed to specific interactions within this blend composition. The miscibility of the blends was confirmed by the single glass transition temperature from the DSC studies. And the application of the Fox rule to the experimental glass transition confirmed the conformational rearrangements of the blends. At the same time, applying the Kwei equation to the glass transition temperature confirmed the presence of specific interactions in the PBC(1.0): PPO (50:50) blend ratio observed in FTIR studies. The thermal stability studies demonstrated morphology dependence behavior with the rule of mixtures with the highest stability of 454oC at PBC(1.0): PPO (50:50) blend ratio. Higher stability at the PBC(1.0): PPO (50:50) blend ratio may be attributed to the specific interactions within this composition. High water uptake correlated with enhanced conductivity for the blend membranes, which strongly depended upon the PBC composition in the blend. Gas permeation studies exhibited that the blending can significantly reduce the permeability of H2 without altering its selectivity compared to pure membranes and Nafion, indicating the desired low fuel cross-over during the fuel cell operation. In conclusion, the gas transport and physical properties of the miscible blends showed strong dependence on composition. The morphological transitions that occur due to the compositional changes will be investigated in future studies.

Modelling the effect of morphology on thermal aging of low-density closed-cell PU foams

Low-density closed-cell polyurethane (PU) foams are widely used in thermal insulation in diverse applications, therefore predicting their thermal aging has a distinct practical importance. Several approaches have been developed allowing forecasting the long-term changes of gas composition in foams, which manifest as the thermal aging. The accelerated aging test methods of foams need to be applied to each foam morphology of interest, while diffusion modelling-based methods, employing characterisation of the gas transport properties of PU polymer or monitoring the evolution of gas composition in foam cells, are relatively complicated. In this study, the experimentally determined evolution of PU foam conductivity is used for estimating the effective diffusivities of gases, based on which permeability of the PU polymer is subsequently evaluated by a structural model assuming Kelvin cell geometry of foam cells. The proposed approach enables prediction of aging dynamics of foams developed from the same PU polymer but possessing a markedly different morphology.

Thin Film Composite Membranes Based on the Polymer of Intrinsic Microporosity PIM-EA(Me2)-TB Blended with Matrimid®5218.

In this work, thin film composite (TFC) membranes were fabricated with the selective layer based on a blend of polyimide Matrimid®5218 and polymer of intrinsic microporosity (PIM) composed of Tröger’s base, TB, and dimethylethanoanthracene units, PIM-EA(Me2)-TB. The TFCs were prepared with different ratios of the two polymers and the effect of the PIM content in the blend of the gas transport properties was studied for pure He, H2, O2, N2, CH4, and CO2 using the well-known time lag method. The prepared TFC membranes were further characterized by IR spectroscopy and scanning electron microscopy (SEM). The role of the support properties for the TFC membrane preparation was analysed for four different commercial porous supports (Nanostone Water PV 350, Vladipor Fluoroplast 50, Synder PAN 30 kDa, and Sulzer PAN UF). The Sulzer PAN UF support with a relatively small pore size favoured the formation of a defect-free dense layer. All the TFC membranes supported on Sulzer PAN UF presented a synergistic enhancement in CO2 permeance, and CO2/CH4 and CO2/N2 ideal selectivity. The permeance increased about two orders of magnitude with respect to neat Matrimid, up to ca. 100 GPU, the ideal CO2/CH4 selectivity increased from approximately 10 to 14, and the CO2/N2 selectivity from approximately 20 to 26 compared to the thick dense reference membrane of PIM-EA(Me2)-TB. The TFC membranes exhibited lower CO2 permeances than expected on the basis of their thickness—most likely due to enhanced aging of thin films and to the low surface porosity of the support membrane, but a higher selectivity for the gas pairs CO2/N2, CO2/CH4, O2/N2, and H2/N2.

Gaseous transport properties of the ground and excited Cr, Co, and Ni cations in He: Ab initio study of electronic state chromatography.

The electronic state chromatography (ESC) effect allows the differentiation of ions in their ground and metastable states by their gaseous mobilities in the limit of low electrostatic fields. It is investigated here by means of accurate transport calculations with ab initio ion-atom potentials for the Cr, Co, and Ni cations in He buffer gas near room temperature. The values for the open-shell ions in degenerate states are shown to be well approximated by using the single isotropic interaction potential. Minimalistic implementation of the multireference configuration interaction (MRCI) method is enough to describe the zero-field transport properties of metastable ions in the 3dm-14s configuration, such as Cr+(a6D), Co+(a5F), and Ni+(4F), due to their weak and almost isotropic interaction with He atom and the low sensitivity of the measured mobilities to the potential well region. By contrast, interactions involving the ions in the ground 3dm states, such as Cr+(a6S), Co+(a3F), and Ni+(2D), are strong and anisotropic; the MRCI potentials poorly describe their transport coefficients. Even the coupled cluster with singles, doubles, and non-iterative triples approach taking into account vectorial spin-orbit coupling may not be accurate enough, as shown here for Ni+(2D). The sensitivity of ion mobility and the ESC effect to interaction potentials, similarities in ion-He interactions of the studied ions in distinct configurations, accuracy and possible improvements of the ab initio schemes, and control of the ESC effect by macroscopic parameters are discussed. Extensive sets of improved interaction potentials and transport data are generated.

Insights about biocrust effects on soil gas transport and aeration in drylands: Permeability, diffusivity, and their connection to hydraulic conductivity

Soil gas transport properties and aeration are essential influencing factors of soil chemical and biochemical reactions, especially the respiration processes of plant roots and microorganisms. As ecosystem engineers in drylands, biocrusts may greatly alter surface soil gas transport properties and aeration, but their influences remain unclear and understudied. In a semiarid dryland ecosystem on the Chinese Loess Plateau, the air permeability of bare soil and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) were measured in-situ with naturally changing soil water content. Undisturbed soil samples were also taken for each treatment, and their air permeability and relative gas diffusivity at air-dried and near-saturated conditions, as well as saturated hydraulic conductivity, were determined in the laboratory. We further analyzed the relationship between saturated hydraulic conductivity and air permeability or relative gas diffusivity and calculated the gas and permeability continuity (specific air permeability and specific gas diffusivity) and tortuosity indexes at air-dry and near-saturation. Our results showed that biocrusts were associated with a significantly higher air permeability in contrast to bare soil, and the in situ mean air permeability ranked in order of moss crusts > mixed crusts > cyanobacterial crusts > bare soil. At air-dried condition, biocrusts increased surface soil air permeability by 26% and relative gas diffusivity by 46% as compared with bare soil through decreased surface soil bulk density. In contrast, at near saturation biocrusts decreased air permeability and relative gas diffusivity by 33% and 20%, respectively, which was attributed to pore-clogging by extracellular polymeric substances of biocrusts (3.37 vs. 2.27 mg g−1). Accordingly, at air-dried condition biocrusts increased specific air permeability by 11% and specific gas diffusivity by 30% but decreased tortuosity by 7%. In contrast, they decreased specific air permeability by 76% and specific gas diffusivity by 39% but increased tortuosity by 28% at near-saturated condition. Additionally, the saturated hydraulic conductivity of cyanobacterial, cyanobacterial-moss mixed, and moss crusts were respectively 77%, 69%, and 61% lower than that of bare soil. We found that the relationship between saturated hydraulic conductivity and air permeability or relative gas diffusivity of biocrusts at air-dried condition can be well fitted with a positive linear log–log model (R2 > 0.95). In general, the improved gas transport properties of surface soil caused by different types of biocrusts leads to better soil aeration, which in turn would facilitate microbial activity, soil respiration, and root growth; and impact surface soil water vapor movement induced by evaporation or condensation processes in drylands. Therefore, biocrusts should be carefully considered in further assessments of global dryland hydrology and carbon or nitrogen cycling under changing climate.

Design of Microbial Methane Oxidation Systems for Landfills

Landfill methane currently represents the largest global source of greenhouse gas emissions from the solid waste sector. Emissions are expected to increase due to increasing waste generation, particularly in countries still landfilling biodegradable wastes. As a complementary measure to gas extraction with subsequent flaring or energy conversion, or for emissions reduction from old landfills or from landfills containing wastes with a low gas potential, microbial methane oxidation systems (MMOS) are considered a promising technology. Numerous studies relating to controlling factors and enhancement of microbial methane oxidation in biocovers, biowindows or biofilters, both in laboratory and in large scale field settings, have been published. The design of optimized MMOS requires thorough understanding of the involved processes, specifically the biological ones and of those related to the transport of gas and water in porous media, and of the impact of material properties and external environmental factors on these processes. Consequently, the selection of materials that are suitable from a biogeochemical and from a geotechnical point of view, meeting the required water and gas transport properties, are key aspects in the design process. This paper reviews the scientific background of the relevant concepts and processes dictating MMOS performance, and provides guidance on layout and design steps, including choice of materials and quality control. Further, a decision tree to support the choice of MMOS is proposed. This paper provides the scientific foundation for upcoming technical guidance documents.

Mixed-Matrix Membranes Based on Polyetherimide, Metal-Organic Framework and Ionic Liquid: Influence of the Composition and Morphology on Gas Transport Properties.

In this work, membranes based on polyetherimide (PEI), a ZIF-8 metal–organic framework and 1-ethyl-methylimidazolium tetrafluoroborate ionic liquid (IL) were prepared. IL and ZIF-8 contents amounting to 7 wt% and 25 wt%, respectively, were investigated. CO2, He and H2 transport properties of PEI/IL/ZIF-8 membranes were compared to those obtained for the respective PEI/ZIF-8 and PEI/IL systems. Membranes’ gas permeability and selectivity are discussed as a function of the membrane composition and morphology, and they were assessed in relation to existing experimental and theoretical data from the literature. Promising gas transport properties were obtained using the appropriate combination of ZIF-8 and IL amounts in the PEI matrix. Indeed, an increase in the CO2 permeability coefficient by a factor of around 7.5 and the He and H2 permeability coefficients by a factor of around 4 was achieved by adding 7 wt% IL and 10 wt% ZIF-8 to the PEI matrix. Moreover, diffusion was evidenced as a governing factor in the studied membrane series.

Novel organo-soluble poly(ether imide)s based on diethyltoluenediamine: Synthesis, characterization and gas transport properties

Novel dianhydrides containing bromine substituents were developed. Series of poly(ether imide)s (PEIs) using diethyltoluenediamine (DETDA) and several bis(ether anhydride)s containing bulky methyl, cyclohexylidene, isopropylidene, hexafluoroisopropylidene and bromine fragments was prepared by one-pot synthesis in benzoic acid. Chemical structures were confirmed by FT-IR and 1H NMR spectroscopy. All PEIs demonstrated great solubility in organic solvents. The weight-average molecular weight (Mw) of the polymers were in the range of 6.1–11.3 × 104 g/mol. PEIs showed glass transition temperature in the range of 285–315 °C according to DSC. Furthermore, PEIs demonstrated great thermal stability with degradation temperature (Td5%) higher 432 °C. Polymer films have good mechanical properties with tensile strength up to 91.6 MPa and elongation at break up to 89.4%. Correlation between the chemical structure and gas transport properties was investigated.

New polyimides containing methyl benzamidobenzoate or dimethyl benzamidoisophthalate as bulky pendant groups. Effects on solubility, thermal and gas transport properties

AbstractThis work reports the synthesis of six new polyimides obtained by the reaction of two novel diamine monomers: methyl 4‐(3,5‐diaminobenzamido)benzoate and dimethyl 5‐(3,5‐diaminobenzamido)isophthalate, bearing one or two methyl ester groups in their phenyl rings, respectively, with three known dianhydrides, 4,4′‐(hexafluoroisopropylidene) diphatalic anhydride (6FDA), 4,4′‐(dimethylsilanediyl) dipthalic anhydride (SiDA) and 4,4′‐oxydiphthalic anhydride (ODPA). The polyimides were synthesized using the “two‐step” method, which first involves forming a poly(amic acid) precursor that is subsequently cyclodehydrated to form the imide linkage. The yields were higher than 95% in all cases, and the structural characterization was performed by spectroscopic techniques like FT‐IR, 1H, 13C, and 29Si NMR. In addition, the effect on the solubility in common organic solvents and thermal and gas transport properties of these polyimides were studied as a function of the nature of the respective side group and the dianhydride used.

An approach for evaluating gas sorption in polymer matrix membranes based on balancing incoming and outgoing membrane mass fluxes

The transport of gases in polymeric membranes can be described by a solution–diffusion mechanism in which the individual transport parameters are determined from independent measurements of the gas permeability and diffusivity and further measurements of the gas solubility in the polymer of interest. Herein, we propose a new method for determining all three transport parameters from a single permeation experiment performed using a modified classical permeation cell with very small volumes above and below the membrane (1.192 and 0.658 mL, respectively). The new concept is based on the evaluation of gas sorption in the membrane determined from the balance of mass fluxes entering and leaving the membrane. The amount of gas sorbed in the membrane can then be calculated by integrating the area between the fluxes. This arrangement allows for the rapid and complex characterization of the gas transport and separation properties of polymeric materials using a single experiment. The novel approach was tested on a polydimethylsiloxane membrane for three gases having considerably different solubilities in the polymer, and the results were consistent with those obtained using classical procedures.

Synthesis and Gas Separation Properties of Aromatic Polyimides Containing Noncoplanar Rigid Sites

Four aromatic polyimides were prepared using a conventional two-stage polymerization reaction of diamine monomers containing different noncoplanar rigid sites with commercially available dianhydride 6FDA. All polymers displayed high relative molecular masses, excellent solubilities, and outstanding thermal properties with Tg values higher than 260 °C, which ensured that they were able to be casted as dense films for gas permeation measurement. The comprehensive gas transport properties of four prepared polyimides were higher than conventional aromatic polyimides (e.g., Matrimid 5218 and Kapton), which suggested that the incorporation of the noncoplanar rigid groups into the polymer main chain was beneficial to prevent segment packing as well as to enhance the gas separation properties. The pure CO2 permeability coefficient of the spirobichroman-based polyimide (6FDA-BDA) was 2.9 times as high as that of Matrimid 5218, without sacrificing selectivity. Moreover, the effect of different noncoplanar rigid sites on the gas separation performance of aromatic polyimides was systematically investigated. By measurements of FFV, density, and d-spacing, it has been observed that the incorporation of different noncoplanar rigid sites significantly affected the microstructure, thus affecting the gas transport. For example, because the polyimide containing pendent phenyl groups (6FDA-BDM) had the lowest FFV and the densest segment packing, it exhibited the lowest permeability. Hence, the basic insights to the structure/property relationships of the four polyimides obtained in this work offer a meaningful guide for next-generation gas separation membranes.

X-ray Diffraction and Molecular Simulations in the Study of Metal-Organic Frameworks for Membrane Gas Separation.

For more than a decade, researchers have been developing metal-organic frameworks (MOFs) in the form of pure MOF membranes as well as MOF-containing mixed-matrix membranes. MOF membranes have been used for H2/CO2 or C3H6/C3H8 separation, but relatively few MOF membranes enable the high-performance separation of CO2/N2, CO2/CH4, or N2/CH4. This article describes the use of in situ XRD analysis and molecular simulation to elucidate gas transport within MOFs and derivative membranes at the molecular level. In a review of recent studies by the authors and other research groups, this article examines the flexibility of MOFs initiated by activation, gas adsorption, and aging effects during gas permeation. This article also discusses the application of XRD analysis in conjunction with computational methods to investigate the CO2-MOF Coulombic interaction and its effects on CO2 separation. Note that this combined analysis approach is also useful in studying the effects of linker rotation on N2/CH4 separation. This article also examines the use of computational tools in identifying new MOFs for gas separation and, more importantly, in elaborating the relationship between the structure of MOFs and their corresponding gas transport properties.

Modeling of diffusion kinetics during gas adsorption in a coal seam with a dimensionless inversion method

Modeling gas diffusion kinetics in a coal seam plays an essential role in predicting performance of both coalbed methane production and its enhanced recovery. Throughout the diffusion period, not only does there exist a large deviation between the analytical solution of the classical Fickian unipore diffusion model (UDM) and experimental measurements, but also constant diffusion coefficients calculated with the inversion of the simplified empirical/semi-empirical correlations are still not accurate enough. In this paper, we proposed a free gas density gradient model (FGDGM) to characterize the gas adsorption and transport properties in a coal seam. Experimentally, isothermal experiments were performed using a high-temperature and high-pressure gas adsorption analyzer with methane gas. Theoretically, a Langmuir gas concentration was derived and improved as a function of free gas density, and then we correlated a numerical inversion model to determine the effective diffusion coefficients by combining dimensionless variables and empirical correlations. Finally, the developed models were validated with the measured gas adsorption isotherms. The numerical solution curves based on the FGDGM are found to be basically consistent with the measured data in the entire time scale, while the developed inversion method is accurate as well as efficient in computational expenses. On the basis of the ideal gas assumption, the actual density gradient and diffusion coefficient will be underestimated and overestimated, respectively. By incorporating real gas behaviour into the improved FGDGM, both gas mass flux and effective diffusion coefficient can be determined more accurately to model gas diffusion kinetics and gas flow patterns in a coal seam.

Polymers from organosilicon derivatives of 5-norbornene-2-methanol for membrane gas separation

In this work, we have proposed a new strategy to the synthesis of norbornene-type monomers with Si–O–C structural motifs via a straightforward and selective dehydrogenative addition reaction between 5-norbornene-2-methanol and trialkylsilanes. As a result, a set of monomers with various trilalkylsiloxy groups (alkyl – CH3, C2H5, and n-C3H7) was synthesized with the yields of 89–90%. These monomers exhibited a high reactivity in both metathesis and vinyl-addition polymerization, affording high-molecular-weight products in yields up to 90%. The metathesis polymers were rubbery with glass transition temperatures from −29 to +23 °C, while related isomeric vinyl-addition polynorbornenes were glassy polymers. Permeability (P) and diffusion (D) coefficients for these polymers were systematically evaluated for permanent gases (He, H2, O2, N2, and CO2) and C1–C4 alkanes. Studying their gas transport properties allowed to establish new structure-property relationships. Particularly, the length of alkyl fragments in trilalkylsiloxy group affected differently gas permeability of metathesis (rubbery) and vinyl-addition (glassy) polymers. Besides, it was shown that the method of linking the Si–O–C-containing group to the main chains of polymers played very important role: Alk3SiO-substituted polynorbornenes are more permeable than isomeric (AlkO)Alk2Si-substituted ones. For the metathesis polynorbornenes, P(CO2) were in the range of 152–226 Barrer, and ideal selectivity (α) of CO2/N2 – 13-16; for the vinyl-addition polynorbornenes, P(CO2) = 230–500 Barrer, and α(CO2/N2) = 14–17. Among studied polymers, promising results for ethane/methane separation was demonstrated by metathesis polynorbornene bearing tri(n-propyl)siloxymethyl substituents: P(C2H6) = 138 Barrer and α(C2H6/CH4) = 2.4. The obtained polymers were also characterized by using TGA, WAXD, and nitrogen adsorption-desorption methods.

Side-Chain Length and Dispersity in ROMP Polymers with Pore-Generating Side Chains for Gas Separations.

Bottlebrush polymers with flexible backbones and rigid side chains have shown ultrahigh CO2 permeability and plasticization resistance for membrane-based gas separations. To date, this class of polymers has only been studied with polydisperse side chains. Herein, we report gas transport properties of a methoxy (OMe) functionalized polymer synthesized via ring-opening metathesis polymerization (ROMP) with uniform side-chain lengths ranging from n = 2 to 5 repeat units to elucidate the role of both side-chain length and dispersity on gas transport properties and plasticization resistance. As side-chain length increased, both Brunauer–Emmett–Teller (BET) surface area and gas permeability increased with minimal losses in gas selectivity. Increased plasticization resistance was also observed with increasing side-chain length, which can be attributed to increased interchain rigidity from longer side chains. Controlling the side-chain length provides an effective strategy to rationally control and optimize the performance of ROMP polymers for CO2-based gas separations.

Finely tuning the microporosity in dual thermally crosslinked polyimide membranes for plasticization resistance gas separations

Thermally induced chemical crosslinking has attracted substantial attention for fabricating plasticization resistant membranes due to the facile structure tunability that enables the construction of robust and well-defined architecture for gas separation. In this study, we report a new series of dual thermally crosslinkable polyimides derived from 4,4′-diamino-2,2′-biphenyldicarboxylic acid (DCB) containing two carboxyl groups, and a systematic investigation of the thermal treatment above and below Tg demonstrated the decarboxylation-induced crosslinking. The dual thermally crosslinked membranes were insoluble in common organic solvents and maintained excellent mechanical properties. Due to the evolution of CO2 and collapse of chain segments during thermal treatment, the crosslinked membranes exhibited hierarchical microcavity size distribution featuring ultra-micropore size in the range of 2.0–6.0 Å and micropore size in the range of 6.5–10.0 Å. Gas transport properties of the crosslinked membranes were feasibly tuned through the chemical compositions and thermal treatment procedures. For instance, the CO2 permeability of crosslinked 6FDA-DAM0.7-TFMB0.1-DCB0.2 increased almost three-fold with only a slight decrease in CO2/CH4 selectivity. The crosslinked membranes also demonstrated superior plasticization resistance with mixed-gas feed pressure up to 40 bar and excellent low-temperature gas separation performance at −30 °C, making them attractive for aggressive gas separations.

CO2-Selective mixed matrix membranes of bimetallic Zn/Co-ZIF vs. ZIF-8 and ZIF-67

Bimetallic metal–organic frameworks (MOF) have received growing attention in several fields for gas separation/adsorption, energy storage and catalysis because of their synergic effect and enhanced properties compared to their parent structures. Here, we first report on a facile and low cost direct synthesis of solid solution of bimetallic Zn/Co-ZIF and its monometallic counterparts, ZIF-8 and ZIF-67 at room temperature. Zn/Co-ZIF showed higher pore volume and BET surface area than their parents, homogeneous distribution of metals within the framework and a slightly higher Zn-rich external surface. Next, mixed matrix membranes (MMM) of Zn/Co-ZIF, ZIF-8 and ZIF-67 based 6FDA-ODA polyimide were fabricated with different content (3–10 %wt) via a wet approach using undried MOF for CO2/CH4 single and mixed gas (10:90 vol%) separation application to determine the effect of the metal center on the gas transport properties. While Zn/Co-ZIF (10 wt%)/6FDA-ODA had the highest CO2/CH4 selectivity (39.8) and CO2 permeability (45.8 Barrer) compared to the MMM of ZIF-8 (PCO2 = 41.6 Barrer, SCO2/CH4 = 36.1) and ZIF-67 (PCO2 = 26.2 Barrer, SCO2/CH4 = 26.3) at the same concentration, the best performance was observed for Zn/Co (7.5 wt%)/6FDA-ODA MMM with 104% and 41% increase in CO2 permeability up to 44.2 Barrer and CO2/CH4 ideal selectivity to 45.1, compared to the neat polymer, respectively.