High-performance Gas Separation Research Articles

Blending of compatible polymer of intrinsic microporosity (PIM-1) with Tröger's Base polymer for gas separation membranes

The novel gas separation membranes have been prepared by blending highly gas permeable polymer of intrinsic microporosity (PIM-1) and with Tröger's Base polymer (TB). Interestingly, it was found firstly that the PIM-1 polymer was miscible with Tröger's Base polymer in any proportion. It was assumed that the excellent compatibility between two polymers could be attributed to the interaction between CN group in PIM-1 and N atoms in Tröger's Base moieties, as confirmed by ATR-IR results. Thus, flexible, tough and transparent blending membranes (PIM/TB) at different compositions between PIM-1 and TB were obtained by solution casting. The SEM results further confirmed the homogeneous morphology of blending membranes, indicating the excellent compatibility between two polymers. The gas separation performance testing by the volume-constant method suggested that the PIM/TB blending membrane showed lower pure gas permeability but higher ideal permselectivity than that of the pristine PIM-1 membrane, probably due to the lower d-spacing of TB polymers from commercial aromatic diamine which has a rotatable bond as confirmed by XRD results. The highest ideal permselectivity of CO2/CH4 was obtained for PIM/TB (6:4) membranes with CO2 permeability of 2585Barrer. Moreover, the excellent anti-plasticization ability of CO2 was observed for the blending membranes. The temperature dependence of gas separation and ideal permselectivity had also been observed for the membranes. The results suggested that the ideal permselectivity, particularly for the CO2/N2 and CO2/CH4 at lower temperature were much higher than that of the membrane at high temperature in spite of the decreasing permeability of CO2. The highest ideal permselectivity was achieved as high as 54.3 for CO2/N2 at − 25 ℃ for PIM/TB(8:2) membrane. Thus, the strategy of blending Tröger's Base with PIM-1 showed a promising approach for high performance gas separation membrane.

High Performance Gas Separation Mixed Matrix Membrane Fabricated by Incorporation of Functionalized Submicrometer-Sized Metal-Organic Framework.

Mixed matrix membranes (MMMs) attract great attention due to their outstanding gas separation performance. The compatibility between the fillers and the polymer matrix is one of the key points for the preparation of high-performance MMMs. In this work, MMMs consisting of metal-organic frameworks (MOFs) of amine-modified Cu-BTC (NH2-Cu-BTC; BTC = 1,3,5-benzenetricarboxylic acid) and submicrometer-sized amine-modified Cu-BTC (sub-NH2-Cu-BTC) incorporated into a Pebax-1657 polymer were fabricated for the gas separation. The SEM image and Fourier transform infrared spectroscopy (FTIR) spectra showed an increase in the surface roughness of MOFs and the presence of amino groups on the surface of Cu-BTC after the amination modification, and a decrease in the size of MOFs crystals after the submicrometer-sized aminated modification. Gas adsorption analysis indicated that NH2-Cu-BTC and sub-NH2-Cu-BTC had a higher gas adsorption capacity for CO2 compared to the unmodified Cu-BTC. The scanning electron microscopy (SEM) image showed that NH2-Cu-BTC and sub-NH2-Cu-BTC, especially sub-NH2-Cu-BTC, had a better compatibility with a polyether-block-amide (Pebax) matrix in the MMMs. The gas separation performance indicated that the Pebax/sub-NH2-Cu-BTC MMMs evidently improved the CO2/N2 and CO2/CH4 selectivity at the expense of a slight CO2 permeability. The results reveal that modified MOF-filled MMMs possess great potential for applications in the CO2 separation field.

Gas permeation through double-layer graphene oxide membranes: The role of interlayer distance and pore offset

Hierarchically stacked graphene oxide (GO) membranes have sparked considerable interest due to their prominent separation performance; however, the underlying separation mechanisms remain elusive. In this study, we conducted molecular dynamics (MD) simulations to explore the role of interlayer distance and pore offset in gas (H2, CH4, N2 and CO2) permeation through double-layer GO membranes. Gas permeance is found to increase with the interlayer distance and pore offset until the interlayer distance exceeds a critical value. With elongating the interlayer distance and pore offset, a sieving effect occurs to overcome preferential adsorption and dominates the transport in mixed H2/CO2, resulting in selective permeation shifting from CO2 to H2. This simulation study provides mechanistic insight into gas permeation through layered GO membranes, and would facilitate the design of new GO membranes for high-performance gas separation.

EFFECT OF DIFFERENT SOLVENT ON ASYMMETRIC POLYSULFONE (PSF) MEMBRANES FOR CO2/CH4 SEPARATION

The asymmetric membranes solution were developed for CO2/CH4 separation by mixing polysulfone (PSf) with N-Methyl-2-pyrrolidone (NMP). A volatile solvent, Tetrahydrofuran (THF) was introduced into the casting solution by various loading (0 to 75%) to form a high gas separation performance of asymmetric membranes via dry/wet phase inversion method. The produced membranes were characterized using Scanning Electron Microscopic (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and gas permeability test. SEM images of a membrane with NMP as a solvent has the thickest dense layer while a membrane with THF as a solvent has the thinnest thickness of the membrane. FTIR results indicating the present of sulfur dioxide (SO2) and methyl (CH3) group at wavelength 1170 cm-1 and 2969 cm-1 respectively verifying the use of PSf as a polymer. As for gas permeation test, the used of THF as a solvent shows the highest CO2/CH4 selectivity with the lowest permeability.

Poly(methyl methacrylate-co-methacrylic amide)-polyethylene glycol/polycarbonate and graphene nanoribbon-based nanocomposite membrane for gas separation

High-performance gas separation membranes were fabricated using 0.5–3wt.% graphene nanoribbon (GNR). Poly(methyl methacrylate-co-methacrylic amide)-polyethylene glycol (PMMA-co-MA-PEG) copolymer was prepared via condensation and blended with polycarbonate to form PMMA-co-MA-PEG/PC. The PMMA-co-MA-PEG/PC and GNR-based nanocomposite possess seamless micro-branched morphological pattern. Tensile strength and Young’s Modulus of PMMA-co-MA-PEG/PC/GNR0.5-3 increased from 64.3–74.7MPa and 76.7–99.9MPa, respectively. GNR loading increased the permselectivity αCO2/N2 (25.4–41.6) of nanocomposite membrane relative to blend membrane (20.1). However, permeability PCO2 was decreased from 163.9 to 139.7 Barrer than blend (174.3 Barrer). PMMA-co-MA-PEG/PC/GNR revealed 51.6% increase and 24.7% decrease in permselectivity and permeability owing to molecular sieving and barricade characteristics of graphene nanoribbon.

Polyetherimide-montmorillonite mixed matrix hollow fibre membranes: Effect of inorganic/organic montmorillonite on CO2/CH4 separation

Abstract Hollow fibre mixed matrix (HFMM) membranes with nano-filler embedded in polymer matrix offer an attractive route for the fabrication of high performance gas separation membranes. However, the quest to achieve high performance mixed matrix membranes remains a challenge without acquiring even filler distribution in polymer matrix. In this work, HFMM membranes comprising polyetherimide (PEI) with various inorganic and organic montmorillonite (I-MMT and O-MMT) loadings ranging from 1 to 4 wt%, were developed via phase inversion method and coated with PDMS for CO2/CH4 separation. Morphological, filler distribution, dispersion, surface topology and gas separation studies were carried out for developed hollow fibre (HF) membranes. Pure gases (CO2 and CH4) were used at varying pressure of 2–10 bars at ambient conditions. In addition, mixed gas test at CO2/CH4 composition of 50/50 v/v % was carried out for selected membranes. Upon incorporation of I-MMT, the developed mixed matrix membranes (MMMs) showed decrease in CO2/CH4 gas separation performance compared to neat PEI membrane. In contrast, the performance of asymmetric membrane was enhanced by incorporating O-MMT in PEI matrix to form MMMs. Uniform dispersion, void-free morphology and reduced surface roughness were observed for the aforementioned membranes. Furthermore, an increasing trend in ideal selectivity was observed up to 2 wt% O-MMT loading against all feed pressures. Thereafter, opposite trend was observed with increasing filler loading due to filler agglomeration. The maximum ideal selectivity achieved was 18.35 with 2 wt% loading at 4 bar pressure which is 52.2% higher than neat PEI hollow fibre membrane.

First Clear-Cut Experimental Evidence of a Glass Transition in a Polymer with Intrinsic Microporosity: PIM-1.

Polymers with intrinsic microporosity (PIMs) represent a novel, innovative class of materials with great potential in various applications from high-performance gas-separation membranes to electronic devices. Here, for the first time, for PIM-1, as the archetypal PIM, fast scanning calorimetry provides definitive evidence of a glass transition ( Tg = 715 K, heating rate 3 × 104 K/s) by decoupling the time scales responsible for glass transition and decomposition. Because the rigid molecular structure of PIM-1 prevents any conformational changes, small-scale bend and flex fluctuations must be considered the origin of its glass transition. This result has strong implications for the fundamental understanding of the glass transition and for the physical aging of PIMs and other complex polymers, both topical problems of materials science.

Triptycene-containing poly(benzoxazole-co-imide) membranes with enhanced mechanical strength for high-performance gas separation

Herein, a series of novel triptycene-containing thermally rearranged poly(benzoxazole-co-imide) membranes (TPI-PBOs) with enhanced mechanical properties and gas separation performance are reported. The effects of chemical structures and poly(benzoxazole-co-imide) composition on the fractional free volume, polymer chain packing, microcavity size and size distribution, mechanical properties, and gas transport properties have been comprehensively investigated. Due to the incorporation of triptycene-containing non-TR-able polyimide segments, the resulting poly(benzoxazole-co-imide) films exhibited good mechanical properties even after being treated at 450 °C. The incorporation of bulky and rigid triptycene units led to significantly improved fractional free volume and gas transport properties compared to previously reported poly(benzoxazole-co-imide)s. Several triptycene-containing poly(benzoxazole-co-imide) films displayed excellent gas separation performance for H2/CH4, H2/N2, and CO2/CH4 gas pairs that exceeded the 2008 upper bounds.

Controlled Dip-coating Times for Improving CO2 Selective of PI/NCCbased Supported Carbon Membrane

Carbon membrane can be regarded as the future separation media in gas separation process due to its high gas separation performance, easy processability, and moderate energy requirement compared to conventional separation process. In this study, the fabrication of tubular carbon membrane prepared from PI blends with nanocrystalline cellulose (NCC) was investigated. The influence of dip-coating duration (15, 30, 45, and 60 min) towards the physicochemical properties and CO2/CH4 and O2/N2 separation was explored. The process of carbonization was conducted at 800°C with rate of heating at 3°C/min under argon gas flow (200 mL/min). Pure gas permeation tests were performed to investigate the prepared carbon membrane’s mechanism of transport at ambient temperature and a feed pressure of 8 bar. The carbon membrane with CO2/CH4 and O2/N2 selectivity of 68.23 ± 3.27 and 9.29 ± 2.54 and CO2 and O2 permeance of 213.56 ± 2.17 and 29.92 ± 1.44 GPU, respectively, were obtained when applying 45 min dip-coating.

Two-dimensional nanosheet-based gas separation membranes

Two-dimensional nanosheets as building blocks for the preparation of high-performance gas separation membranes.

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheets

Two-dimensional metal-organic frameworks (MOFs) are promising candidates for high performance gas separation membranes. Currently, MOF nanosheets are mostly fabricated through delamination of layered MOFs, which often results in a low yield of intact free-standing nanosheets. In this work, we present a direct synthesis method for zinc(II)-benzimidazole-acetate (Zn(Bim)OAc) MOF nanosheets. The obtained nanosheets have a lateral dimension of 600 nm when synthesized at room temperature. By adjusting the synthesis temperature, the morphology of obtained nanosheets can be readily tuned from nanosheets to nanobelts. Using zeolite nanosheets of known thickness as internal standard, we introduce a method based on high-angle annular dark-field scanning transmission electron microscopy images to determine nanosheet thickness. The thickness of Zn(Bim)OAc nanosheets is determined to be 7 nm, which combined with their estimated pore opening makes them promising building blocks for gas separation membranes.

Facile Synthesis of a Hydroxyl-Functionalized Tröger’s Base Diamine: A New Building Block for High-Performance Polyimide Gas Separation Membranes

Two intrinsically microporous polyimides (PIM-PIs) were synthesized by the polycondensation reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3,3′,3′-tetramethylspirobisindane-6,7,6′,7′-tetracarboxylic dianhydride (SBI) with a newly designed o-hydroxyl-functionalized Troger’s base diamine, 1,7-diamino-6H,12H-5,11-methanodibenzo[1,5]diazocine-2,8-diol (HTB). Both amorphous PIM-PIs were soluble in aprotic solvents and showed excellent thermal stability with onset decomposition temperature of ∼380 °C. SBI-HTB displayed a higher CO2 permeability (466 vs 67 barrer) than 6FDA-HTB but a significantly lower selectivity for CO2/CH4 (29 vs 73), H2/CH4 (29 vs 181), O2/N2 (4.6 vs 6.0), and N2/CH4 (1 vs 2.5). 6FDA-HTB displayed the highest gas-pair permselectivity values of all reported OH-functionalized PIM-PIs to date. The high permselectivity of 6FDA-HTB resulted primarily from exceptional diffusion selectivity due to strong size-sieving properties caused by hydrogen bonding between the p...

Temperature-induced formation of cellulose nanofiber film with remarkably high gas separation performance

Free-standing cellulose nanofiber (CNF) films were prepared by a specially designed oven-drying approach. For the first time, the evaporation rate was carefully controlled to enhance the gas separation performance of the CNF film with the highest H2/N2 and H2/O2 selectivities of 159.4 and 218.2, respectively. The dense structure formed by the CNFs and the intra- and inter-fibrillar hydrogen bonds are believed to contribute to the separation property. Particularly, the specific role of floating-shape film precursor formed at the rapid evaporation stage (stage I) and the mechanism for film formation were investigated. This study demonstrated that the evaporation rate controlled by the heating temperature exhibited significant influence on surface, uniformity and gas separation of the CNF films.

Effective Acetylene/Ethylene Separation at Ambient Conditions by a Pigment-Based Covalent-Triazine Framework.

A novel covalent-triazine framework (CTF-PO71) is designed and prepared from an organic pigment molecule for high-performance gas separation. The functional sites with different electrostatic potentials on the pore surface of CTF-PO71 demonstrate a strong interaction between C2 H2 and CTF-PO71 to achieve preferential adsorption of C2 H2 over C2 H4 , thus enabling effective capture of a trace amount of C2 H2 from the gas mixture. This is the first organic porous polymer that is capable of separating C2 H2 and C2 H4 . The commercial availability and the low cost of the pigment as well as the high stability of the resultant framework endow CTF-PO71 with a significant potential for practical applications.

6FDA-DETDA: DABE polyimide-derived carbon molecular sieve hollow fiber membranes: Circumventing unusual aging phenomena

Transport properties are reported for asymmetric carbon molecular sieve (CMS) hollow fiber membranes based on polyimide precursors derived from a butanol esterified diamino benzoic acid (DABA) based polyimide, 6FDA-DETDA:DABE. Precursor fiber pretreatment with 10% Ethenyl(trimethoxy)silane [vinyltrimethoxysilane (VTMS)] solution in hexane followed by pyrolysis at 550°C in ultra-high purity argon created asymmetric CMS fibers with CO2 permeance above 1000 GPU and CO2/CH4 selectivities > 25. Storage of the as-made modules for 72 days in 7bar CO2 suppressed undesirable aging typically seen under vacuum or atmospheric pressure air and provided CO2 permeance and CO2/CH4 selectivity of 780 GPU and 48 respectively. These results are in contrast to significant losses in CO2 permeance and CO2/CH4 selectivity for CMS created under equivalent pyrolysis conditions from non-esterified 6FDA-DETDA: DABA variant, even under CO2 storage. The 6FDA-DETDA: DABA-derived CMS results were surprising, since dense film CMS samples from the same precursors did not show the undesirable permeance and selectivity changes. Moreover, other non-DABA containing precursors in dense film and asymmetric fiber forms can be effectively stabilized under CO2 storage. Such results suggest that high residual stress in the freshly formed thin selective layers of asymmetric structures in DABA-derived CMS causes undesirable micro-scale rearrangements. In dense films and even asymmetric skins of non-DABA derived CMS membranes, evidence of such undesirable rearrangements are not apparent. DABA-derived cross-links are presumably the cause of these effects. Mono-esterification of 6FDA-DETDA: DABA to form 6FDA-DETDA: DABE controls such crosslinking-derived stresses and resultant instabilities in the CMS fibers, thereby providing stable high performance gas separation properties under CO2 storage or use in mixed gas feeds.

Novel nanocomposite membranes prepared with PVC/ABS and silica nanoparticles for C2H6/CH4 separation

Composite membranes based on polyvinyl chloride and acrylonitrile butadiene styrene (ABS) copolymer have been prepared and then filled with 2–8 wt % of silica nanoparticles. Membranes were fabricated by solution casting method using dimethylacetamide. The performance of prepared membranes were studied for methane and ethane at the feed pressure of 1.0, 1.5, 2.0, and 3.0 bar at 35°C. By increasing the percentage of ABS, permeability of methane and ethane increased. In addition, by adding the silica nanoparticles in the membrane, permeability of gas increased in all cases. The highest gas pair selectivity for C2H6/CH4 could be obtained from PVC/BS (20 wt %) which loaded with 8 wt % of silica nanoparticles. The results of this study suggest that high performance gas separation nanocomposite membranes can be attained by adopting a judicious combination of blending technique for polymeric membrane, optimized loading percentage, and feed operating conditions.

Enhanced gas transport properties of mixed matrix membranes consisting of Matrimid and RHO type ZIF-12 particles

Zeolitic imidazolate frameworks (ZIF) have been showed as potential active inorganic fillers in high-performance gas separation membranes. Among them, zeolitic imidazolate framework-12 crystals were synthesized as a porous filler material by alcohol-based solution method using benzimidazole–toluene interactions and they were incorporated into a commercial glassy polyimide Matrimid 5218 with various ZIF-12 loadings (0, 10, 20, 30, 40wt.%), as the continuous phase. As-synthesized ZIF-12 was characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), multipoint Brunauer–Emmett–Teller (BET) surface analysis, Fourier Transform Infrared Spectroscopy (FT-IR) and Thermogravimetric Analysis (TGA). SEM images of ZIF-12 particles assert regular external surfaces with a rhombic dodecahedron structure. Matrimid mixed matrix membranes (MMMs) loaded with different amounts of ZIF-12 were prepared by solution casting method and characterized by XRD, FT-IR, TGA and SEM analyses. Our findings showed that there was a good dispersion and wetting of ZIF-12 particles in the continuous phase. Gas transport properties of the membranes were investigated by single gas permeation experiments of H2, CO2 and CH4 at 4bar feed pressure and 35°C. The permeability values increased as the ZIF-12 loading increased up to 20wt.%. However, at higher loadings, 30 and 40wt.%, the permeability decreased for all gases and the CO2/CH4 and H2/CH4 selectivities increased depending on the influence of the ZIF-12 filler. The maximum ideal separation factor for CO2/CH4 and H2/CH4 were found about 66.70 and 212.00, significantly higher than the pure polymer ideal separation factor.

CO2/CH4 separation using mixed matrix membrane-based polyurethane incorporated with ZIF-8 nanoparticles

This study presents using zeolitic imidazolate framework-8 (ZIF-8) as porous filler dispersed phase and polyurethane (PU) as continuous phase to synthesis mixed matrix membranes (MMMs). ZIF-8 nanoparticles were synthesized using centrifugal method. The synthesized nanoparticles were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). In order to investigate the effect of ZIF-8 loading on the membrane performance in CO2/CH4 separation, different membranes were prepared with various amounts of ZIF-8 (0–50 wt%). Membranes properties were characterized by SEM, XRD, TGA, differential scanning calorimetry (DSC), and tensile analysis. SEM images exhibit that the ZIF-8 is dispersed uniformly in cross section of membrane. Thermal stability of membranes increases with addition of the ZIF-8 nanoparticles into the polymer matrix. Both tensile strength and strain at break in the MMMs increase with the ZIF-8 loading. To study the effect of feed pressure on CO2 and CH4 transport properties of the membranes, single gas experiments were conducted at 4, 8, and 12 bar feed pressures. Incorporation of ZIF-8 crystals into continuous PU matrix resulted in high-performance gas separation membranes. Increasing feed pressure, significantly, increased separation performances in all the membranes.

Amine-Functionalized Covalent Organic Framework for Efficient SO2 Capture with High Reversibility

Removing sulfur dioxide (SO2) from exhaust flue gases of fossil fuel power plants is an important issue given the toxicity of SO2 and subsequent environmental problems. To address this issue, we successfully developed a new series of imide-linked covalent organic frameworks (COFs) that have high mesoporosity with large surface areas to support gas flowing through channels; furthermore, we incorporated 4-[(dimethylamino)methyl]aniline (DMMA) as the modulator to the imide-linked COF. We observed that the functionalized COFs serving as SO2 adsorbents exhibit outstanding molar SO2 sorption capacity, i.e., PI-COF-m10 record 6.30 mmol SO2 g−1 (40 wt%). To our knowledge, it is firstly reported COF as SO2 sorbent to date. We also observed that the adsorbed SO2 is completely desorbed in a short time period with remarkable reversibility. These results suggest that channel-wall functional engineering could be a facile and powerful strategy for developing mesoporous COFs for high-performance reproducible gas storage and separation.

Nanoscale MOF/organosilica membranes on tubular ceramic substrates for highly selective gas separation

An efficient strategy has been developed for fabricating novel tubular MOF/organosilica nano-composite membranes with high gas separation performance.