Synergistic effects of zeolite imidazole framework@graphene oxide composites in humidified mixed matrix membranes on CO2 separation

In this study, composite nanosheets (ZIF-8@GO) were prepared via an in situ growth method and then incorporated into a polyimide (PI) matrix to fabricate mixed matrix membranes (MMMs) for CO₂ separation. The as-prepared MMMs were characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA) and water uptake measurements. Water uptake measurements establish the relationship between the gas permeability and water uptake of membranes and an increase in the water uptake contributes to the CO₂ permeability owing to an increase in the CO₂ transport channels. The MMMs exhibit excellent CO₂ permeability in when compared with an unfilled PI membrane in a humidified state. The ZIF-8@GO filled membranes can separate CO₂ efficiently due to the ZIF-8@GO nanocomposite materials combining the favorable attributes of GO and ZIF-8. First, the high-aspect ratio of the GO nanosheets enhances the diffusivity selectivity. Second, ZIF-8 with a high surface area and microporous structure is beneficial to the improvement of the CO₂ permeability. Third, ZIF-8@GO possesses synergistic effects for efficient CO₂ separation. The MMM with 20 wt% ZIF-8@GO exhibits the optimum gas separation performance with a CO₂ permeability of 238 barrer, CO₂/N₂ selectivity of 65, thus surpassing the 2008 Robeson upper bound line.

In this study, composite nanosheets (ZIF-8@GO) were prepared via an in situ growth method and then incorporated into a polyimide (PI) matrix to fabricate mixed matrix membranes (MMMs) for CO₂ separation.     

Effects of mesoporous silica particle size and pore structure on the performance of polymer-mesoporous silica mixed matrix membranes

The fabrication of mixed matrix membranes (MMMs) has been regarded as an effective and economic approach to enhance the gas permeability and selectivity properties of conventional polymeric membranes for gas separation applications. However, the poor compatibility between polymeric matrix and inorganic filler in MMMs could lead to the generation of interfacial defects resulting in reduced gas selectivity. In this work, with the aim of studying the effect of particle size and pore structure of the filler on the performance of the resultant MMMs, nano/micro sized spherical mesoporous silicas with 2D/3D pore structure (MCM-41 and MCM-48) were synthesized and selected as fillers for the preparation of polydimethylsiloxane (PDMS)-based MMMs. The separation properties of the membranes prepared were characterized by permeability measurements for nitrogen and organic vapors (C₃H₆ and n-C₄H₁₀). Compared with microsized particles, nanosized fillers have better dispersion in the polymer matrix which could minimize the formation of non-selective microvoids around the particles, leading to higher vapor/N₂ ideal selectivities of the MMMs, even at the high loading (15 wt%). Moreover, due to the conventional random packing orientation of the particles in the polymer, vapor permeation was severely hindered in the MMMs fabricated from mesoporous silica with 2D pore channels. The interface morphologies and gas diffusion paths in the MMMs have also been proposed. With an optimum loading of nanosized MCM-48 (3D pore structure), the vapor permeabilities and vapor/N₂ ideal selectivities of the MMMs were shown to increase simultaneously, compared with the neat polymer membrane.

The effects of filler particle size and pore structure on the gas separation performance of mixed matrix membranes were comprehensively investigated via elaborate synthesis of mesoporous silicas.     

Light-responsive metal–organic framework sheets constructed smart membranes with tunable transport channels for efficient gas separation

Exploring a new type of smart membrane with tunable separation performance is a promising area of research. In this study, new light-responsive metal–organic framework [Co(azpy)] sheets were prepared by a facile microwave method for the first time, and were then incorporated into a polymer matrix to fabricate smart mixed matrix membranes (MMMs) applied for flue gas desulfurization and decarburization. The smart MMMs exhibited significantly elevated SO₂(CO₂)/N₂ selectivity by 184(166)% in comparison with an unfilled polymer membrane. The light-responsive characteristic of the smart MMMs was investigated, and the permeability and selectivity of the Co(azpy) sheets-loaded smart MMMs were able to respond to external light stimuli. In particular, the selectivity of the smart MMM at the Co(azpy) content of 20% for the SO₂/N₂ system could be switched between 341 and 211 in situ irradiated with Vis and UV light, while the SO₂ permeability switched between 58 Barrer and 36 Barrer, respectively. This switching influence was mainly ascribed to the increased SO₂ adsorption capacity in the visible light condition, as verified by adsorption test. The CO₂ permeability and CO₂/N₂ selectivity of MMMs in the humidified state could achieve 248 Barrer and 103.2, surpassing the Robeson''s upper bound reported in 2019.

New light-responsive MOF sheets were synthesized to fabricate a smart membrane, which displays switchable gas separation performance stimulated with UV-Vis light.     

Ultrathin porous amine-based solid adsorbent incorporated zeolitic imidazolate framework-8 membrane for gas separation

A novel gas separation approach is proposed in this work by combining an amine-based solid adsorbent with a zeolitic imidazolate framework-8 (ZIF-8) membrane. This was achieved by incorporating the amine-based solid adsorbent during the fabrication of the ZIF-8 membrane on a macroporous substrate. An amine-based solid adsorbent was prepared using porous ZIF-8-3-isocyanatopropyltrimethoxysilane (IPTMS) and N-[(3-trimethoxysilyl)propyl]diethylenetriamine (3N-APS) amine compounds. The as-prepared porous amine-based solid adsorbent (denoted as ZIF-8-IPTMS-3N-APS) possessed excellent adsorptive CO₂/N₂ and CO₂/CH₄ separation performances. As the adsorbent needs to be regenerated, this could indicate that the CO₂ adsorption separation process cannot be continuously operated. In this work, an amine-based solid adsorbent was applied during the preparation of the ZIF-8 membranes owing to the following reasons: (i) gas separation by the membrane can be operated continuously; (ii) the amino group provides a heterogeneous nucleation site for ZIF-8 to grow; and (iii) the reparation of surface defects on the macroporous substrate can be performed prior to the growth of the ZIF-8 membrane. Herein, the ZIF-8 membrane was successfully fabricated, and it possessed excellent CO₂/CH₄, CO₂/N₂, and H₂/CH₄ separation performances. The 0.6 μm ultrathin ZIF-8 membrane demonstrated a high CO₂ permeance of 4.75 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 35 °C and 0.1 MPa, and ideal CO₂/N₂ and CO₂/CH₄ selectivities of 4.67 and 6.02, respectively. Furthermore, at 35 °C and 0.1 MPa, the ideal H₂/CH₄ selectivity of the ZIF-8 membrane reached 31.2, and a significantly high H₂ permeance of 2.45 × 10⁻⁵ mol m⁻² s⁻¹ Pa⁻¹.

The amine-based solid adsorbent was innovatively incorporated during the fabrication of the ZIF-8 membrane for gas separation.     

3D printed MOF-based mixed matrix thin-film composite membranes

MOF-based mixed-matrix membranes (MMMs) have attracted considerable attention due to their tremendous separation performance and facile processability. In large-scale applications such as CO₂ separation from flue gas, it is necessary to have high gas permeance, which can be achieved using thin membranes. However, there are only a handful of MOF MMMs that are fabricated in the form of thin-film composite (TFC) membranes. We propose herein the fabrication of robust thin-film composite mixed-matrix membranes (TFC MMMs) using a three dimensional (3D) printing technique with a thickness of 2–3 μm. We systematically studied the effect of casting concentration and number of electrospray cycles on membrane thickness and CO₂ separation performance. Using a low concentration of polymer of intrinsic microporosity (PIM-1) or PIM-1/HKUST-1 solution (0.1 wt%) leads to TFC membranes with a thickness of less than 500 nm, but the fabricated membranes showed poor CO₂/N₂ selectivity, which could be attributed to microscopic defects. To avoid these microscale defects, we increased the concentration of the casting solution to 0.5 wt% resulting in TFC MMMs with a thickness of 2–3 μm which showed three times higher CO₂ permeance than the neat PIM-1 membrane. These membranes represent the first examples of 3D printed TFC MMMs using the electrospray printing technique.

An electrospray 3D printing approach for fabricating thin-film composite mixed-matrix membranes (TFC MMM) with a thickness of 2–3 μm.     

Synthesis and gas transport properties of polyamide membranes containing PDMS groups

A series of gas-separation polyamide-poly(dimethylsiloxane) (PA-PDMS) membranes containing PDMS groups were synthesized through the polycondensation reaction. The structural characteristics of polymers were evaluated by ¹H-NMR spectroscopy (NMR), Fourier-transform infrared spectroscopy (FTIR) and UV-vis absorption spectroscopy. The permeability and selectivity behavior was studied at different temperatures (25–55 °C) and pressures (1.0–3.0 atm), using various gases, such as H₂, O₂, CO₂, CH₄, and N₂. The effect of chemical structure, PDMS content, operating pressure and temperature on gas permeability was explored and discussed. Gas-permeation measurements showed that polyamides containing PDMS groups exhibited different separation performance. The PA-PDMS-20 membrane with 20 wt% PDMS exhibited the highest selectivity (CO₂/N₂ = 41.84 and O₂/N₂ = 7.01) at 35 °C and 3.0 atm while CO₂ and O₂ permeability was 29.29 barrer and 4.91 barrer, respectively.

PA-PDMS membranes were synthesized by polycondensation reaction and the gas permeability was found to increase with an increase of PPG content, with the gas permeability of PA-PDMS-20 membrane reaching 29.29 at 35 °C and 3.0 atm.     

The use of a sucrose precursor to prepare a carbon membrane for the separation of hydrogen from methane

In this study, we present the use of sucrose (C₁₂H₂₂O₁₁), which exists in abundance in nature, to prepare a carbon membrane without any preceding treatments. The preparation procedure was conducted using a low pyrolysis temperature, i.e., in the range of 300–500 °C, followed by complete formation of the structure of the carbon membrane. The gas separation characteristics of the resulting membranes were assessed by evaluating both hydrogen and methane permeation. The highest selectivity obtained for H₂/CH₄ was 31.34 with H₂ permeability of 459.24 GPU. The entire fabrication procedure was designed to be economical in order to facilitate any future commercialization.

Development of an easier and less time-consuming technique to fabricate carbon membranes for the separation of H₂ and CH₄ using sucrose precursor.     

Molecular dynamics simulation of adsorption and separation of xylene isomers by Cu-HKUST-1

Metal–organic frameworks (MOFs) are widely used in the adsorption separation of various gases. A fundamental understanding of the effective separation of xylene isomers helps improve aromatic products'' separation efficiency and reduce industrial separation costs. Grand Canonical Monte Carlo (GCMC) simulations combined with Molecular Science is widely used to predict gas adsorption and diffusion in single crystals with metal–organic frameworks. We performed a GCMC + MD combined approach to study xylene isomers'' adsorption and separation in Cu-HKUST-1 to predict the permeability and selectivity of the ternary gas mixture in the MOF with the adsorption and diffusion usage data. Most current studies take into account the computational cost and difficulty. Most recent research models are limited to the adsorption of a single or specific molecule, such as hydrogen, methane, carbon dioxide, etc. For this reason, we report an attempt to study the adsorption separation of aromatic gases (p-xylene/o-xylene/m-xylene) based on Cu-HKUST-1 single-crystal materials based on some previous research methods with an appropriate increase in computational cost. To predict the adsorption selectivity and permeability of the ternary mixture of xylene isomers on the MOF surface, the model simulation calculates key parameters of gas adsorption, including gas adsorption volume (N), the heat of adsorption (Q_st), Henry coefficient (K), and diffusion coefficient (D).

Metal–organic frameworks (MOFs) are widely used in the adsorption separation of various gases.     

Preparation of PSF/FEP mixed matrix membrane with super hydrophobic surface for efficient water-in-oil emulsion separation

Polysulfone (PSF)/fluorinated ethylene propylene (FEP) mixed matrix membranes (MMMs) with super hydrophobic surface were successfully fabricated via non-solvent induced phase separation (NIPS) method. The effects of FEP content on the morphology, roughness, wettability, pore size, and mechanical property of PSF/FEP MMMs were characterized by scanning electron microscope, confocal microscopy, contact angle goniometer, mercury porosimetry, and tensile testing instrument, respectively. When the FEP content was 9 wt%, the average roughness of M-4 reached 0.712 μm. Meanwhile, the water contact angle (CA) and the water sliding angle (SA) was 153.3° and 6.1°, respectively. M-4 showed super hydrophobicity with a micro- and nanoscale structure surface. Then, M-4 was used for separating of water-in-oil emulsion, showing high separation efficiency for water-in-kerosene and water-in-diesel emulsions of 99.79% and 99.47%, respectively. The flux and separation efficiency changed slightly after 10 cycles. Therefore, this study indicated that the obtained PSF/FEP MMM with super hydrophobic surface could be used for efficient water-in-oil emulsion separation.

The PSF/FEP membrane with super hydrophobic and super oleophilic surface had an outstanding separation performance for water-in-oil emulsion.     

Introducing hydrophilic ultra-thin ZIF-L into mixed matrix membranes for CO2/CH4 separation

Mixed matrix membranes (MMMs) were developed by mixing hydrophilically modified two-dimensional (2D) imidazole framework (named as hZIF-L) flakes into a Pebax MH 1657 (Pebax) matrix, and designed to separate carbon dioxide/methane (CO₂/CH₄) mixtures. The hZIF-L flakes were important for increasing the effectiveness of the MMMs. First, the tannic acid (TA) etched hZIF-L flakes have a large number of microporous (1.8 nm) and two-dimensional anisotropic transport channels, which offered convenient gas transport channels and improved the permeability of CO₂. Second, the TA molecules provide the surface of the ZIF-L flakes with more hydrophilic functional groups such as carbonyl groups (C O) and hydroxyl groups (–OH), which could effectively prevent non-selective interfacial voids and filler agglomeration in the Pebax matrix, and also presented strong binding ability to water and CO₂ molecules. The satisfactory interface compatibility and affinity with the CO₂ molecule promoted its permeability, solubility, and selectivity. As a result, the MMMs exhibited the highest performance of gas separation with the hZIF-L flake weight content of 5%, at which the CO₂ permeability and CO₂/CH₄ selectivity were 502.44 barrer and 33.82 at 0.2 MPa and 25 °C, respectively.

Schematic diagram of CO₂ transfer in Pebax/hZIF-L mixed matrix membranes.     

Fabrication of a PVDF membrane with tailored morphology and properties via exploring and computing its ternary phase diagram for wastewater treatment and gas separation applications

We report a simple approach for tailoring the morphology of poly(vinylidene fluoride) (PVDF) membranes fabricated using a nonsolvent induced phase separation (NIPS) method that sustains both the hydrophilic and hydrophobic properties. Various membrane structures, i.e. skin layers and whole membrane structures as well, were obtained via an experimental method based on the obtained and computed ternary phase diagram. The nonsolvent interactions with polymer solution resulted in the different forms and properties of a surface layer of fabricated membranes that affected the overall transport of solvent and nonsolvent molecules inside and outside the bulk of the fabricated membranes. The resulting morphology and properties were confirmed using the 3D optical profiler, SEM, FT-IR and XRD methods. The effect of binary interaction parameters on the morphology of the fabricated membranes and on their separation performance was tested using water/oil mixture and gas separation. Both hydrophobic and hydrophilic properties of PVDF showed the excellent durable separation performance of the prepared membranes with 92% of oil separation and the maximum flux of 395 L h⁻¹ m⁻² along with 120 min of long-term stability. CO₂ separation from H₂, N₂, CH₄ and SF₆ gases was performed to further support the effect of tuned PVDF membranes with different micro/nanostructured morphologies. The gas performance demonstrated ultrahigh permeability and a several-fold greater than the Knudsen separation factor. The results demonstrate a facile and inexpensive approach can be successfully applied for the tailoring of the PVDF membranes to predict and design the resulting membrane structure.

We report a simple approach for tailoring the morphology of poly(vinylidene fluoride) (PVDF) membranes fabricated using a nonsolvent induced phase separation (NIPS) method that sustains both the hydrophilic and hydrophobic properties.     

Sonochemical synthesis of Co3O4 nanoparticles deposited on GO sheets and their potential application as a nanofiller in MMMs for O2/N2 separation

In this study we report an environmentally friendly, facile and straightforward sonochemical synthetic strategy for a Co₃O₄/GO nanocomposite using N,N′-bis(salicylidene)ethylenediaminocobalt(ii) as a precursor and graphene oxide sheets as an immobilization support for Co₃O₄ nanoparticles. The synthesis was facilitated by physical and chemical effects of cavitation bubbles. The synthesized nanocomposite was thoroughly characterized for its composition and morphology using Fourier transform infrared spectroscopy (FTIR), Energy dispersive X-ray spectroscopy (EDS), Scanning electron microscopy (SEM), UV-visible, Raman and X-ray diffraction spectroscopy (XRD), etc. The results show Co₃O₄ nanoparticles of 10 nm (SD 3 nm) were prepared on well exfoliated sheets of GO. The applicability of the synthesized Co₃O₄/GO nanocomposite was optimized as a nanofiller for mixed matrix membranes (MMMs) comprised of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and polyvinyl chloride. The affinity of the prepared MMMs was evaluated for the separation of O₂/N₂ gases by varying the concentration of nanofiller, i.e. 0.03%, 0.04%, 0.05% and 0.075% (w/v). The results display high separation performance for O₂/N₂ gases with excellent permeance (N₂ 167 GPU and O₂ 432 GPU at 1 bar) and O₂/N₂ selectivity of 2.58, when the MMMs were loaded with 0.05% (w/v) of Co₃O₄/GO nanocomposite.

Sonochemical synthesis of Co₃O₄/GO nanocomposite.     

Multicomponent gas separation and purification using advanced 2D carbonaceous nanomaterials

Multicomponent gas separation and purification is an important pre- or post-processing step in industry. Herein, we employed a multiscale computational approach to investigate the possibility of multicomponent low-weight gas (H₂, O₂, N₂, CO₂, CH₄) separation and purification using novel porous 2D carbonaceous nanomaterials, namely Graphdiyne (GD), Graphenylene (GN), and Rhombic-Graphyne (RG). The dispersion-corrected plane-wave density functional theory (DFT) calculation combined with the Climbing Image Nudged Elastic Band (CI-NEB) method was employed to study the gas/membrane interaction energy and diffusion barrier of different gases passing through the geometrically optimized membranes. The results from CI-NEB calculations were then fitted to the Morse potential function to construct a bridge between quantum mechanics calculations and non-equilibrium molecular dynamics (NEMD) simulation. The selectivity of each membrane for all binary mixtures was calculated using the estimated diffusion energy barriers based on the Arrhenius equation. Finally, a series of extensive NEMD simulations were carried out to evaluate the real word and time dependent separation process. According to the results, CH₄ molecules can be completely separated from the other gases using a GD membrane, O₂ molecules from CH₄, N₂, and CO₂ by a GN membrane, and H₂ molecules from all other gases using a RG membrane.

Multicomponent gas separation and purification is an important pre- or post-processing step in industry.     

Effect of relative humidity on the gas transport properties of zeolite A/PTMSP mixed matrix membranes

Increasing the knowledge of the influence of water vapor in new mixed matrix membranes (MMMs) could favor the integration of novel membrane materials in the recovery of CO₂ from wet industrial streams. In this work, the water vapor effect on the N₂, CH₄ and CO₂ permeability through MMMs comprised of 20 wt% hydrophilic zeolite 4A in hydrophobic PTMSP polymer were investigated in the relative humidity range 0–75%. While in the pure PTMSP membranes, the permeability of all gases decreases with water vapor activity, with almost unchanged CO₂/N₂ and CO₂/CH₄ selectivities, in zeolite A/PTMSP MMMs, the CO₂ permeability increases with increasing water content in the system up to 50% R.H., resulting in an increase in CO₂/N₂ and CO₂/CH₄ selectivities with respect to pure PTMSP. Gas sorption was studied so that the effect the residual humidity in the zeolite 4A has on the sorption of the different gases helped explaining the permeability observations. The sorption and humid permeation behavior were evaluated by a simple model equation based on the NELF theory, taking into account the multicomponent gas sorption and diffusion in the presence of humidity, as well as the counteracting effects of the hydrophobic PTMSP and hydrophilic zeolite A in a very accurate way.

CO₂ permeability of zeolite A/PTMSP MMM increases with water content in the system, enhancing CO₂/N₂ and CO₂/CH₄ selectivities of PTMSP.     

Preparation and characterization of an amphiphilic polyamide nanofiltration membrane with improved antifouling properties by two-step surface modification method

Membrane fouling is an urgent problem needing to be solved for practical application of nanofiltration membranes. In this study, an amphiphilic nanofiltration membrane with hydrophilic domains as well as low surface energy domains was developed, to integrate a fouling-resistant defense mechanism and a fouling-release defense mechanism. A simple and effective two-step surface modification of a polyamide NF membrane was applied. Firstly, triethanolamine (TEOA) with abundant hydrophilic functional groups was grafted to the membrane surface via reacting with the residual acyl chloride group of the nanofiltration membrane, making the nanofiltration membranes more hydrophilic; secondly, the 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), well-known as a low surface energy material, was covalently grafted on the hydroxyl functional groups through hydrogen bonding. Filtration experiments with model foulants (bovine serum albumin (BSA) protein solution, humic acid solution (HA) and sodium alginate solution (SA)) were performed to estimate the antifouling properties of the newly developed nanofiltration membranes. As a result of surface modification proposed in this study the antifouling properties of an amphiphilic modified F-PA/PSF membrane were enhanced more than 10% compared to the PA/PSF specimen in terms of flux recovery ratio.

Schematic diagram of amphiphilic NF membrane by a two-step modification.     

Ion transport through a nanoporous C2N membrane: the effect of electric field and layer number

Ion transport through a two-dimensional membrane with nanopores plays an important role in many scientific and technical applications (e.g., water desalination, ion separation and nanofiltration). Although there have been many two-dimensional membranes for these applications, the problem of how to controllably fabricate nanopores with proper shape and size still remains challenging. In the present work, the transport of ions through a C₂N membrane with intrinsically regular and uniformly distributed nanopores is investigated using all-atom molecular dynamic simulations. It was found that the monolayer C₂N membrane possesses higher ion permeability compared to the graphene membrane because of its higher density of nanopores. In addition, it exhibits excellent ion selectivity under a low electric field due to the distinct dehydration capabilities and interaction behaviors (with the pore edges) of the different ions. Furthermore, we found that multilayer C₂N membranes have weak ion selectivity, but show promising potential for desalination. The present study may provide some physical insights into the experimental design of C₂N-based nanodevices in nanofluids.

Using all-atom molecular dynamic simulations, we show that a monolayer C₂N membrane possesses higher permeability and excellent ion selectivity, and that multilayer C₂N membranes have promising potential for water desalination.     

Selection of a suitable ZIF-8/ionic liquid (IL) based composite for selective CO2 capture: the role of anions at the interface

The effective capture of CO₂ from the atmosphere is much needed to reduce its environmental impact. The design and development of CO₂ capturing materials is getting much attention. A zeolitic imidazolate framework (ZIF) can replace many of the conventional materials in gas separation due to its stability and high performance. Here, we analyzed the effect of encapsulation of ionic liquids (ILs) into the pores of ZIF-8 for selective CO₂ capture and separation. The [BMIM]⁺ cation with a series of anions was selected to study suitable carbon capture materials using density functional theory (DFT) approaches. Our calculations suggest that the nitrogen containing anions are not well adsorbed on the ZIF-8 surface but their gas separation performance is not affected by these interfacial interactions. This is confirmed from the CO₂/N₂ and CO₂/CH₄ selectivity of these composites, calculated using grand canonical Monte Carlo (GCMC) simulations. A suitable force field for the composites was identified by comparing the available force fields with the experiments. The IL@ZIF-8 composite shows better CO₂ selectivity compared to pristine ZIF-8. Fluorinated hydrophobic anions (such as [BF₄]⁻, [PF₆]⁻ and [Tf₂N]⁻) in the composites show better CO₂ adsorption and significant CO₂ selectivity than pristine ZIF-8, especially at low pressure. The nature of the anion plays an important role in CO₂ separation, rather than its stability at the pores of ZIF-8. Close scrutiny of the results reveal that the CO₂ selectivity of these composite materials depends on the anion of the IL and thus through the selection of a suitable anion we can significantly enhance the CO₂ selectivity for different flue gas mixtures. Our molecular level design shows that the selection of suitable anions in IL based composites is very important in identifying potential carbon capture materials for industrial applications.

The interfacial stability of hydrophilic/hydrophobic IL incorporated ZIF-8 is identified and the CO₂ selectivity depends on the fluorinated anions in the IL.     

The spirobichroman-based polyimides with different side groups: from structure–property relationships to chain packing and gas transport performance

Spirobichroman-based polymers with high gas permeability and selectivity are promising for their applications as membranes in gas separation. In this study, three spirobichroman-based polyimides (PIs; 6FDA-FH, 6FDA-DH, and 6FDA-MH) were synthesised by the polyreaction between diamines containing different substituents (benzene ring, pyridine ring, and methyl group) and 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA). The physical properties, gas transport behaviour, d-spacing, dihedral angle of molecules, and fractional free volume of the PIs were investigated through experiments and molecular simulations. The PIs exhibited excellent thermal stability and good solubility in common organic solvents. The gas permeability of the PIs was investigated; the results highlighted the critical role of the substituents in the enhancement of the gas separation performance of polymer membranes. Detailed analysis of the PIs showed that 6FDA-FH exhibits the highest gas permeability. This can be ascribed to the loose packing of the polymer chain owing to the increased dihedral angle between the two planes. However, the methyl substituent in 6FDA-MH disrupts the polymer chain packing rather than changing the dihedral angle between the two planes, thus enhancing the gas permeability of 6FDA-MH. Furthermore, 6FDA-DH exhibited the highest CO₂/CH₄ selectivity, which is attributed to the CO₂ affinity of the polymer containing the pyridine unit.

Effect of substituents on the dihedral angle and chain packing plays a critical role in the enhancement in the gas separation performance of polymer membranes.     

Superior performance of surface-treated NaX@Pebax-1657 membranes for O2/N2 separation

In this study, the performances of mixed matrix composite membranes (MMCMs) containing surface-treated NaX nanocrystals (ST-NaX-NCs) were experimentally and theoretically investigated for O₂/N₂ separation. For this purpose, the MMCMs were fabricated by the casting solution method and characterized by various analyses. The results reveal that there is a robust interaction between the polymer chains and the ST-NaX-NCs, and that the ST-NaX fillers are uniformly dispersed in the polymer matrix. The incorporation of ST-NaX-NCs alters the PEBAX polymer chain packing arrangement resulting in decreased membrane transport behavior for both O₂ and N₂ gases. The MMCM containing 16.7% wt ST-NaX-NCs has drastically enhanced air separation properties, with a selectivity that is increased to 204% of that of the neat membrane. Moreover, the Lewis–Nielsen model was modified by considering non-ideal effects in mixed matrix membranes, like the clogging of filler pores and polymer chain hardening around the nanocrystals, to predict the gas permeation behavior through the MMCMs. The comparison of the experimental and model results reveals that the modified model can accurately predict the gas permeability and selectivity through the MMCMs.

In this study, the performances of mixed matrix composite membranes (MMCMs) containing surface-treated NaX nanocrystals (ST-NaX-NCs) were experimentally and theoretically investigated for O₂/N₂ separation.     

Phthalide-containing poly(ether-imide)s based thermal rearrangement membranes for gas separation application

The diamine monomer 3,3-bis[4-(3-hydroxy-4-amino-phenoxy)phenyl]phthalide (BHAPPP) was firstly synthesized by the nucleophilic substitution of 5-fluoro-2-nitrophenol and phenolphthalein, followed by a reduction reaction. A series of phthalide-containing poly(ether imide)s (PEI) were then prepared through the polycondensation of BHAPPP with six kinds of dianhydrides, including 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), 4,4′-oxydiphthalic dianhydride (ODPA), 1,2,3,4-cyclobutane tetracarboxylic dianhydride (CBDA) and pyromellitic dianhydride (PMDA), as well as thermal imidization. After further thermal treatment, the corresponding thermal rearrangement (TR) membranes were obtained. Due to the existence of the phthalide lactone ring, the PEIs probably underwent TR and crosslinking simultaneously. With the increase of thermal treatment temperature, the mechanical properties of the TR membranes dramatically decreased, but the gas separation properties obviously increased. When the PEIs were treated at 450 °C for 1 h, the CO₂, H₂, O₂, N₂ and CH₄ permeability of TR(BHAPPP-6FDA) reached 258.5, 190.5, 38.35, 4.25 and 2.15 Barrers, respectively. Meanwhile, the CO₂/CH₄ selectivity of 120.2 sharply exceeded the 2008 Robeson limit, and O₂/N₂ selectivity was 9.02, close to the 2015 upper limit. Therefore, the TR membranes derived from phthalide-containing PEIs exhibit superior gas separation performance, andare expected to be applied in the field of gas separation.

The 3,3-bis[4-(3-hydroxy-4-amino-phenoxy)phenyl]phthalide was firstly synthesized and polymerized to prepare six kinds of phthalide-containing poly(ether imide)s. After further thermal rearrangement, the corresponding membranes exhibited excellent gas separation performance.