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The use of immiscible polymer blends in gas separations is limited due to uncontrollable phase separation. In contrast, compatibilized immiscible polymer blends can be used as precursors with controlled morphologies that allow for a unique pore architecture. Herein, an immiscible polymer blend (1:1) comprising polybenzimidazole (PBI) and the copolyimide 6FDA-DAM:DABA [3:2], derived from reacting 4,4-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with 2,4,6-trimethyl-1,3-phenylenediamine (DAM) and 3,5-diaminobenzoic acid (DABA), were combined with durene diamine as a compatibilizer. The compatibilizer helped reduce the 6FDD domain sizes from 5.6 µm down to 0.77 µm and induced a more even 6FDA distribution and the formation of continuous thin-selective PBI layers. The carbon–carbon composite membranes derived from the compatibilized immiscible polymer blends showed a 3-fold increase in both H2 permeability and H2/CO2 selectivity compared to the membranes derived from non-compatibilized polymer blends. The H2 permeability of the compatibilized immiscible polymer blends increased from 3.6 to 27 Barrer, and their H2/CO2 selectivity increased from 7.2 to 20. The graphitic domain size of the carbon–carbon composite membranes derived from the polymer blends also increased from 6.3 nm for the non-compatibilized blend to 10.0 nm for the compatibilized blend. [ABSTRACT FROM AUTHOR]

The effect of the annealing temperature of polybenzimidazole (PBI) membranes on H 2 /CO 2 gas separations was investigated. Membranes annealed from 250 °C to 400 °C were tested for gas permeation with pure H 2 , CO 2 , and N 2 gases and a H 2 :CO 2 (1:1) mixture at 35 °C, 100 °C, 200 °C, and 300 °C and at pressures up to 45 bar. Gas permeation data show that permeability and selectivity of the membranes is significantly impacted by the annealing temperature, the presence of adsorbed water, and remaining casting solvent (DMAc). At a testing temperature of 35 °C, ideal H 2 /CO 2 selectivities of 50, 49, and 66 with pure H 2 permeabilities of 1.5, 0.8, and 1.5 Barrer were obtained for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. At this temperature, high gas mixture H 2 /CO 2 selectivities of 41, 73, and 47 with H 2 permeabilities of 1.03, 0.26, and 0.50 Barrer were also obtained for these membranes. At testing temperatures of 300 °C, both the ideal and gas mixture H 2 /CO 2 selectivities dropped to 44, 28, and 30 (ideal, H 2 = 45, 45, 44 Barrer) and to 19, 22, and 23 (mixture, H 2 = 41, 43, and 44 Barrer) for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. As water was removed from the membranes at temperatures greater than 100 °C during permeation cycles, where the testing temperature was increased from 35 °C to 300 °C, the permselectivity properties of the membranes annealed at 400 °C became more reproducible. Permeabilities at 35 °C from a second permeability cycle increased, but H 2 /CO 2 selectivities decreased to 21 for gas mixtures (H 2 = 1.4 Barrer) and to 34 for pure gases (H 2 = 2.2 Barrer). The results suggest that high annealing temperatures may induce changes in the configuration and conformation of the polymer chains, imparting distinctive permselectivity properties to the membranes. Activation energies of permeability for H 2 , CO 2 , and N 2 from pure gases and H 2 :CO 2 mixtures correlated with these changes as well. [Display omitted] • Annealing PBI membranes at 300 °C–400 °C improves gas permeation properties. • Annealing at 300 °C–400 °C completes conversion of prepolymer into PBI. • Casting solvent and water released from polybenzimidazole membranes up to 400 °C. • Adsorbed water affects gas permeation at testing temperatures of up to 150 °C. • H 2 /CO 2 gas mixture separation performance retained up to 300 °C and 30 bar. [ABSTRACT FROM AUTHOR]

Gas separation for industrial, energy, and environmental applications requires low energy consumption and small footprint technology to minimize operating and capital costs for the processing of large volumes of gases. Among the separation methods currently being used, like distillation, amine scrubbing, and pressure and temperature swing adsorption, membrane-based gas separation has the potential to meet these demands. The key component, the membrane, must then be engineered to allow for high gas flux, high selectivity, and chemical and mechanical stability at the operating conditions of feed composition, pressure, and temperature. Among the new type of membranes studied that show promising results are the inorganic-based and the metal-organic framework-based mixed-matrix membranes (MOF-MMMs). A MOF is a unique material that offers the possibility of tuning the porosity of a membrane by introducing diffusional channels and forming a compatible interface with the polymer. This review details the origins of these membranes and their evolution since the first inorganic/polymer and MOF/polymer MMMs were reported in the open literature. The most significant advancements made in terms of materials, properties, and testing conditions are described in a chronological fashion. [ABSTRACT FROM AUTHOR]

Abstract: Zeolitic imidazolate frameworks (ZIFs), that have the potential for gas separation, were used as additives in mixed-matrix membranes (MMMs). ZIF-8, which exhibits the sodalite topology, is composed of zinc (II) ion clusters linked by imidazolate ligands. The ZIF-8 pore aperture is 3.4Å allowing it to readily absorb small molecules such as H2 and CO2. ZIF-8/Matrimid® MMMs were fabricated with loadings up to 80% (w/w), which are much higher than the typical loadings achieved with select zeolite materials. Only at the highest loading did the ZIF-8/Matrimid® MMM show a loss of mechanical strength, leading to a decrease in flexibility. The ZIF-8/Matrimid® MMMs permeability properties were tested for H2, CO2, O2, N2, CH4, C3H8, and gas mixtures of H2/CO2 and CO2/CH4. The permeability values increased as the ZIF-8 loading increased to 40% (w/w). However, at higher loadings of 50% and 60% (w/w), the permeability decreased for all gases, and the selectivities increased consistent with the influence of the ZIF-8 additive. The ideal selectivities of gas pairs containing small gases, such as H2/O2, H2/CO2, H2/CH4, CO2/CH4, CO2/C3H8, and H2/C3H8, showed improvement with the 50% (w/w) ZIF-8 loading, demonstrating a transition from a polymer-driven to a ZIF-8-controlled gas transport process. In control experiments using as-synthesized ZIF-8 with filled pores, there was no transition at 50% (w/w) loading. This may be the first example of an MMM wherein molecular sieving is evident and suggests that additive loadings >50% (w/w) may be required to observe this effect in MMMs. [Copyright &y& Elsevier]

Abstract: Metal–organic framework 5 (MOF-5) nanocrystals with a high surface area (3000m2/g) and high thermal stability (up to 400°C) were synthesized and added to Matrimid® to form mixed-matrix membranes for gas separations. Scanning electron microscopy (SEM) images of the membrane cross-sections revealed significant plastic deformation of the polymer matrix owing to the strong affinity between the MOF-5 and Matrimid®. At 30% MOF-5 loading, the permeabilities of the gases tested increased 120% while the ideal selectivities remained constant compared to Matrimid®. Residual gas analysis of permeates of gas blends with different mixture ratios revealed an increase in selectivity for CH4. [Copyright &y& Elsevier]

Thedemand for cost-efficient separations requires membranes withhigh gas flux and high selectivity which opens the path for furtherimprovements. Mixed matrix membranes (MMMs) made from 33.3 wt % ZIF-8in 6FDA-durene were tested at 35 °C and 3.5 atm. At 33.3 wt %loading of ZIF-8, H2, N2, O2, andCH4gas permeabilities increased approximately 400%. Cross-linkingthe surface of this MMM, by reacting with ethylenediamine vapor, yieldeda 10-fold increase in H2/CO2, H2/N2, and H2/CH4selectivities with respectto 6FDA-durene, preserving 55% of the H2permeability of6FDA-durene. The permselective properties of the cross-linked skinof the MMM fall above the most recent permeability–selectivitytrade-off lines (2008 Robeson upper bounds) for H2/CO2, H2/N2, and H2/CH4separations. To the best of our knowledge, this is the first exampleof a cross-linked ZIF/polymer MMM for gas separation. [ABSTRACT FROM AUTHOR]