High-performance Gas Separation Research Articles

Structural engineering on copolyimide membranes for improved gas separation performance

The structural engineering and design of 6FDA/BPDA-based copolyimide membranes at a molecular level are developed to achieve advanced membrane materials with improved CO2 plasticization resistance as well as high gas separation performance. The commercially available diamine monomers with/without various electronegative groups (-CH3, –CF3 and -Cl) have been employed to prepare 6FDA/BPDA-based copolyimides via the one-step method. All of the copolyimides show excellent solubility in the common solvent, film-forming ability and thus mechanical properties. More interestingly, the intermolecular charge transfer complex (CTC) effects is finely tuned as confirmed by the fluorescence emission spectrum. The introducing of polar chloride groups in the copolyimides could significantly strengthen the CTC effects to prevent the intrusion of condensable CO2 and limit polymeric chain movements. Thus, the membrane based on the 2,2′,5,5′-tetrachlorobenzidine (e.g. 6FDA:BPDA-TSN:TCDB (1:1:1:1)) shows the best carbon dioxide (CO2) plasticization resistance and slowest physical aging without sacrificing gas separation performance versus the membrane having weak polar groups. The CO2 plasticization pressure as high as 350 psi has been observed for the 6FDA:BPDA-TSN:TCDB membrane with polar chloride groups. This value is much higher than that of the copolyimide membranes with –CH3 or –CF3 groups ranging from 100 psi to 200 psi. Moreover, this membrane shows a good gas separation performance, e.g. CO2 permeability of 45.7 barrer and CO2/CH4 ideal selectivity of 35.2 surpassing the commercial cellulose acetate and Matrimid. Such structurally defined polymer with good mechanical strength, plasticization resistance and solubility harnesses the full potential of promising gas separation membrane materials for hollow fiber spinning. This work offers structural engineering guidance for the preparation of high-performance polymers with good tolerance to CO2-induced plasticization.

MIL-101(Cr) Microporous Nanocrystals Intercalating Graphene Oxide Membrane for Efficient Hydrogen Purification.

Graphene oxide (GO) is a promising two-dimensional building block for fabricating high-performance gas separation membranes. Whereas the tortuous transport pathway may increase the transport distance and lead to a low gas permeation rate, introducing spacers into GO laminates is an effective strategy to enlarge the interlayer channel for enhanced gas permeance. Herein, we propose to intercalate CO2 -philic MIL-101(Cr) metal-organic framework nanocrystals into the GO laminates to construct a 2D/3D hybrid structure for gas separation. The interlayer channels were partially opened up to accelerate gas permeation. Meanwhile, the intrinsic pores of MIL-101 provided additional transport pathways, and the affinity of MIL-101 to CO2 molecules resulted in higher H2 /CO2 diffusion selectivity, leading to a simultaneous enhancement in gas permeance and separation selectivity. The MIL-101(Cr)/GO membrane with optimal structures exhibited outstanding and stable mixed-gas separation performance with H2 permeance of 67.5 GPU and H2 /CO2 selectivity of 30.3 during the 120-h continuous test, demonstrating its potential in H2 purification application.

Highly permeable and selective sepiolite hybrid mixed matrix carbon membranes supported on plate carbon substrates for gas separation

The development of promising dopants is essential for fabricating mixed matrix membranes with high gas separation performance. Here, mixed matrix carbon membranes (MMCMs) were produced by using polyimide as precursor and sepiolite as dopants on plate phenolic resin-based carbon supports via the processes of membrane formation and pyrolysis. The thermal stability, surface functional groups, microstructure and morphology of membrane samples were characterized by the thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscope, respectively. The effects of sepiolite amount on the microstructure and property of MMCMs were investigated. Besides, the change tendency of gas permeation with permeation temperature and permeation pressure was further examined. The results showed that the addition of sepiolite enhanced the thermal stability of the precursor membranes and the microstructural compactness of resultant MMCMs. Especially, it greatly improved the gas permeability and selectivity of MMCMs. The maximum separation performance was reached for MMCMs made by the sepiolite content of 0.2%, with the gas permeabilities of 8412.9Barrer (H2), 6711.5Barrer (CO2), 6062.0 Barrer (O2) and 220.9 Barrer (N2), and the selectivities of 38.1 (H2/N2), 30.4 (CO2/N2) and 27.4 (O2/N2). Moreover, the MMCMs can directly concentrate the mole fraction of O2 from compressed air to 93%.

Multipulsed Millisecond Ozone Gasification for Predictable Tuning of Nucleation and Nucleation-Decoupled Nanopore Expansion in Graphene for Carbon Capture.

Predictable and tunable etching of angstrom-scale nanopores in single-layer graphene (SLG) can allow one to realize high-performance gas separation even from similar-sized molecules. We advance toward this goal by developing two etching regimes for SLG where the incorporation of angstrom-scale vacancy defects can be controlled. We screen several exposure profiles for the etchant, controlled by a multipulse millisecond treatment, using a mathematical model predicting the nucleation and pore expansion rates. The screened profiles yield a narrow pore-size-distribution (PSD) with a majority of defects smaller than missing 16 carbon atoms, suitable for CO2/N2 separation, attributing to the reduced pore expansion rate at a high pore density. Resulting nanoporous SLG (N-SLG) membranes yield attractive CO2 permeance of 4400 ± 2070 GPU and CO2/N2 selectivity of 33.4 ± 7.9. In the second etching regime, by limiting the supply of the etchant, the nanopores are allowed to expand while suppressing the nucleation events. Extremely attractive carbon capture performance marked with CO2 permeance of 8730 GPU, and CO2/N2 selectivity of 33.4 is obtained when CO2-selective polymeric chains are functionalized on the expanded nanopores. We show that the etching strategy is uniform and scalable by successfully fabricating high-performance centimeter-scale membrane.

Self-crosslinking of bromomethylated 6FDA-DAM polyimide for gas separations

The CO2-induced plasticization effects impose detrimental effects on polymeric membranes separation performance. To address this issue, a high-performance crosslinked brominated 6FDA-based polyimide (BMPI) is developed in this work, exhibiting highly suppressed plasticizing effects. The permeability and selectivity of polyimide precursor membranes are tuned by introducing bromine atoms into the polyimide structure, creating a best performing membrane with bromination degree of 60%. The membrane CO2 permeability reaches 395 Barrer, and the selectivity of CO2/CH4 increases from 22.0 for a non-brominated analogue to 26.0. The debromination induced crosslinked BMPI membranes are achieved upon thermal treatment within the temperature range of 150 °C–350 °C with varied crosslinking degree, resulting in large differences for the membrane separation performance. The completely crosslinked BMPI treated at 350 °C for 10 h demonstrates high CO2 permeability of 483.6 Barrer and the CO2/CH4 selectivity of 26.0. Most importantly, the crosslinking reaction greatly stabilizes the membrane performance against plasticization under high CO2 feed pressure. Compared with the non-crosslinked membranes with a plasticizing pressure of 150 psia, the plasticization for the crosslinked membrane occurs at high CO2 pressure of 600 psia. This study provides a facile approach for the preparation of high-performance gas separation polymeric membranes with enhanced plasticization resistance.

High-performance Gas Separation System Using Gate-type Adsorbents

High-performance Gas Separation System Using Gate-type Adsorbents

"All Polyimide" Mixed Matrix Membranes for High Performance Gas Separation.

To improve the interfacial compatibility of mixed matrix membranes (MMMs) for gas separation, microporous polyimide particle (AP) was designed, synthesized, and introduced into intrinsic microporous polyimide matrix (6FDA-Durene) to form “all polyimide” MMMs. The AP fillers showed the feature of thermal stability, similar density with polyimide matrix, high porosity, high fractional free volume, large microporous dimension, and interpenetrating network architecture. As expected, the excellent interfacial compatibility between 6FDA-Durene and AP without obvious agglomeration even at a high AP loading of 10 wt.% was observed. As a result, the CO2 permeability coefficient of MMM with AP loading as low as 5 wt.% reaches up to 1291.13 Barrer, which is 2.58 times that of the pristine 6FDA-Durene membrane without the significant sacrificing of ideal selectivity of CO2/CH4. The improvement of permeability properties is much better than that of the previously reported MMMs, where high filler content is required to achieve a high permeability increase but usually leads to significant agglomeration or phase separation of fillers. It is believed that the excellent interfacial compatibility between the PI fillers and the PI matrix induce the effective utilization of porosity and free volume of AP fillers during gas transport. Thus, a higher diffusion coefficient of MMMs has been observed than that of the pristine PI membrane. Furthermore, the rigid polyimide fillers also result in the excellent anti-plasticization ability for CO2. The MMMs with a 10 wt.% AP loading shows a CO2 plasticization pressure of 300 psi.

Tailoring the Microporosity of Polymers of Intrinsic Microporosity for Advanced Gas Separation by Atomic Layer Deposition

Tailoring the microporosity of intrinsically microporous polymers at the atomic level is one of the biggest challenges in achieving high-performance polymeric gas separation membranes. In this study , for the first time, the Al 2 O 3 atomic layer deposition (ALD) technique was used to modify the microporosity of a typical polymer of intrinsic microporosity (PIM-1) at the atomic level. PIM-1 with six ALD cycles (PIM-1-Al 2 O 3 -6) exhibited simultaneous high thermal, mechanical, pure- and mixed-gas separation, and anti-aging properties. The O 2 /N 2 , H 2 /N 2 , and H 2 /CH 4 separation performances were adequate above the latest trade-off lines. PIM-1-Al 2 O 3 -6 showed CO 2 and O 2 permeabilities of 624 and 188 Barrer, combined with CO 2 /CH 4 and O 2 /N 2 selectivities of 56.2 and 8.8, respectively. This significantly enhanced performance was attributed to the strong size sieving effect induced by the Al 2 O 3 deposition.

Cross-Linked Mixed-Matrix Membranes Using Functionalized UiO-66-NH2 into PEG/PPG-PDMS-Based Rubbery Polymer for Efficient CO2 Separation.

Mixed-matrix membranes (MMMs) with an ideal polymer-filler interface and high gas separation performance are very challenging to fabricate because of incompatibility between the fillers and the polymer matrix. This work provides a simple technique to prepare a series of cross-linked MMMs (xMMM@n) by covalently attaching UiO-66-NB metal-organic frameworks (MOFs) within the PEG/PPG-PDMS copolymer matrix via ring-opening metathesis polymerization and in situ membrane casting. The norbornene-modified MOF (UiO-66-NB) is successfully copolymerized and dispersed homogeneously into a PEG/PPG-PDMS matrix because of very fast polymer formation and strong covalent interaction between MOFs and the rubbery polymer. A significant improvement in gas permeability is achieved in membranes up to a 5 wt % MOF loading compared to the pristine polymer membrane without affecting selectivity. The CO2/N2 separation performance of xMMM@1, xMMM@3, and xMMM@5 with 1, 3, and 5 wt % MOF loading, respectively, surpassed Robeson's 2008 upper bound. In addition, the best performing membrane, xMMM@3 (PCO2 = 585 Barrer and CO2/N2 ∼53), approaches the 2019 upper bound, indicating that the cross-linked MMMs (xMMM@n) are very promising for CO2 separation from flue gas. The experimental results of our study were evaluated and are supported by theoretical data obtained using the Maxwell model for MMMs. Moreover, the developed MMMs, xMMM@ns, displayed outstanding antiplasticization performance at pressures of up to 25 atm and very stable antiaging performance for up to 11 months with good temperature switching behaviors.

High-performance gas separation using mixed-matrix composite membranes containing graphene nanoplatelets

High-performance gas separation using mixed-matrix composite membranes containing graphene nanoplatelets

Isomer-Tailored Carbon Molecular Sieve Membranes with High Gas Separation Performance.

Partial substitution of the asymmetric 3,3',4,4'-biphenyl dianhydride monomer (aBPDA) into the backbone of a 6FDA-BPDA-DAM (6FDA=4,4'-hexafluoroisopropylidene, DAM=diaminomesitylene) diphthalic anhydride-based copolyimide based on symmetrical BPDA (sBPDA) was used to study membrane structure-processing-property relationships for gas separation. Properties of the polymer membrane as well as derived carbon molecular sieves (CMS) membranes were compared with copolyimides without the asymmetric monomer structure. CMS membranes derived from the copolyimides are very attractive for CO2 /CH4 separation. aBPDA provides the copolyimide with additional packing-inhibited structures compared with the symmetric ones and yields a corresponding CMS membrane with very high CO2 permeability and good CO2 /CH4 selectivity. This work, therefore, outlines a new strategy for tuning CMS membrane structure to meet separation performance needs.

Excellent separation performance in a mesoporous MOF induced by 1D rhombic channels and bare nitrogen-donor sites

Metal-organic frameworks with open active sites and appropriate pore size show great application promises in gas storage and separation due to their structural characteristics and functionality can be adjusted and controlled. Based on a benzimidazole and carboxylate decorated new MOF, [Cd(PCA)2]n, (YAU-1, HPCA=(2-(pyrid-4′-yl)-benzimidazole-5- carboxylic acid) has been gained under solvent-thermal conditions. Each dinuclear [Cd2] unit are linked through PCA− ligands to obtain a 2D layer structure, and the 2D structure is further connected by PCA− ligands to form the 3D framework of YAU-1. Remarkably, the un-coordinated benzimidazole and pyridine groups of PCA− ligand not only provide the 1D channels with exposed N atoms to create a mesoporous environment but also stabilizes 3D framework by π⋯π stacking. Taking advantage of these structural characteristics, YAU-1 shows different adsorption capacity for CO2, C2H2, C2H4 and CH4, and remarkable separation effect of C2H2/CH4, C2H4/CH4 and CO2/CH4. The research on the configuration-effect relationship of YAU-1 provides a promising methods to design and construction materials with high gas uptake and separation performance.

Seeded growth of high-performance ZIF-8 membranes in thick wall autoclaves assisted by modulator

The preparation of thin metal-organic framework (MOF) membranes by seeded secondary growth method is still a challenge. In this study, relatively thin ZIF-8 membranes with high gas separation performance on alumina hollow fibers are prepared in autoclaves with thick-walled Teflon-liners by the seeded secondary growth method under the modulation of sodium formate. The ZIF-8 membrane thickness can be greatly decreased and the cracks in the membrane layer can be avoided by using thick-walled Teflon-liner (e.g., 7.5 mm) during secondary growth mainly due to the slow heating and cooling of the synthesis solution, which can control the nucleation rate and growth rate of crystals, leading to an appropriate membrane formation rate. A certain amount of sodium formate can introduce ZnO particles in the seed layer to improve the formation of dense ZIF-8 membrane layer and control the growth rate of membrane layer. The suitable amount of sodium formate and thick-walled Teflon-liner are crucial in ensuring the well-intergrowth of relatively thin ZIF-8 membranes. Owing to reduced grain boundary defects, the prepared ZIF-8 membranes show an ideal H2/N2 separation selectivity of 30.9 ± 1.8, which is at least twice the highest value reported in the literature under similar separation conditions.

Rational Design of Halloysite Surface Chemistry for High Performance Nanotube-Thin Film Nanocomposite Gas Separation Membranes.

The interfacial region has a critical role in determining the gas separation properties of nanofiller-containing membranes. However, the effects of surface chemistry of nanofillers on gas separation performance of thin film nanocomposite (TFN) membranes, prepared by the interfacial polymerization method, have been rarely studied in depth. In this work, pristine and three differently surface-modified halloysite nanotubes (HNTs), by non- (SHNT), moderately (ASHNT), or highly CO2-philic (SFHNT) agents, are embedded in the polyamide top layer of thin film nanocomposite (TFN) membranes for CO2/N2 and CO2/CH4 separations. Trimethoxyoctyl silane, 3-(2-aminoethylaminopropyl)trimethoxysilane, and poly(styrenesulfonic acid) are used as modifying agents to quantitatively investigate the effects of interfacial interactions between the polyamide and HNTs on the gas permeation of TFNs. This allows us to provide an interfacial design strategy to fabricate high-performance gas separation membranes. Pure gas permeations conducted on the TFNs at the feed gas pressure of 10 bar showed that CO2 permeance and CO2/N2 and CO2/CH4 selectivities were increased by 145%, 130%, and 108%, respectively, after addition of 0.05 w/v% of sulfonated HNTs. The experimental gas permeations through all TFNs/HNTs, except TFNs/SFHNTs, agree well with predictions of a recently developed model, which suggests the importance of considering the neglected role of CO2 interactions with the HNT/polyamide interface in the model. These results unambiguously proved that designing the interfacial layer thickness in the nanotube-containing membranes is an effective approach to tuning the gas separation properties. The results show that the dispersion of HNTs in the polyamide top layer and the experimental CO2/gas selectivity was increased with increasing interfacial thickness, aint, upon surface modification. Moreover, it is quantitatively demonstrated that the thickness of the interfacial layer between the filler and polymer matrix is a function of gas pressure applied on the membrane.

Influence of dip-coating times towards oxygen separation performance

Several advantages possessed by carbon membrane such as can be easily processed, high gas separation performance and requires adequate amount of energy compared to the conventional separation process are the reasons it is deemed as the future media of separation. This study utilized PI blends and nanocrystalline cellulose (NCC) to prepare the tubular carbon membrane in order to investigate its fabrication. The effect of dip-coating duration (15, 30, 45, and 60 min) on physicochemical properties O2/N2 separation was determined. Carbonization was done at 800 °C with 3 °C/min rate of heating under argon gas flow (200 mL/min). Tests of pure gas permeation were conducted in order to study the transport mechanism of prepared carbon membrane at ambient temperature and 8 bar of feed pressure. Carbon membrane with O2 permeance of 29.92 ± 1.44 GPU and O2/N2 selectivity of 9.29 ± 2.54 were obtained after 45 min of dip coating.

Molecular-Sieving Membrane by Partitioning the Channels in Ultrafiltration Membrane by In Situ Polymerization.

Commercial ultrafiltration membranes have proliferated globally for water treatment. However, their pore sizes are too large to sieve gases. Conjugated microporous polymers (CMPs) feature well-developed microporosity yet are difficult to be fabricated into membranes. Herein, we report a strategy to prepare molecular-sieving membranes by partitioning the mesoscopic channels in water ultrafiltration membrane (PSU) into ultra-micropores by space-confined polymerization of multi-functionalized rigid building units. Nine CMP@PSU membranes were obtained, and their separation performance for H2 /CO2 , H2 /N2 , and H2 /CH4 pairs surpass the Robeson upper bound and rival against the best of those reported membranes. Furthermore, highly crosslinked skeletons inside the channels result in the structural robustness and transfer into the excellent aging resistance of the CMP@PSU. This strategy may shed light on the design and fabrication of high-performance polymeric gas separation membranes.

Macromolecular design strategies toward tailoring free volume in glassy polymers for high performance gas separation membranes

This review highlights recently reported novel macromolecular design strategies providing tailorable free volume for high performance gas separation membranes.

Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations

Finely tailoring the interfacial interaction to minimize the defective structure in the hybrid membranes is a key to yield a mixed matrix membrane with enhanced gas separation performance. Here, a highly selective mixed matrix membrane based on imidazole containing polyimides and ZIF-8 fillers is reported. The ZIF-8 with imidazole linker offers a more compatible interface with imidazole containing polyimides. At a MOF loading of 20 wt. %, the membranes have a 2.2-fold increase in the gas permeabilities over unfilled polymers with exceptional H2/CH4 and CO2/CH4 gas selectivities of 224 and 58, respectively. The H2 and CO2 gas permeabilities are 78.5 and 20.3 Barrer, respectively. The Zn2+ post-modification enables the formation of metal coordination crosslinking with enhanced polymer/ZIF-8 interaction, as indicated by increased glass transition temperature, improved thermal stability and insoluble properties in organic solvents. With the proper control of Zn2+ treatment conditions, gas permeabilities of H2 and CO2 can be increased to 110.1 and 27.4 Barrer, respectively, with the constant H2/CH4 and CO2/CH4 gas selectivity. For membranes treated at higher Zn2+ concentration, an enhancement in H2/CH4 gas selectivity was observed with the selectivity increasing from 223.9 to 318.3 and H2 permeability around 72.3 Barrer. The separation performance of H2/CH4 for all Zn2+ modified membranes exceeds the 2008 Robeson upper bound. This facile approach to tune polymer/MOF interaction via metal ion modification promotes the rational design of high-performance gas separation membranes.

A simple self-regulating permeability and selectivity of poly (arylene ether ketone) with amino groups for gas separation membrane

Poly (arylene ether ketone) containing different content of the amino groups (Am-PAEK) polymer membranes were prepared by nucleophilic polycondensation used as gas separation membranes. Am-PAEK was synthesized successfully confirmed by Fourier Transform Infrared (FT-IR). The glass transition temperature (Tg) of Am-PAEK membranes enhanced from 164.83 °C to 192.83 °C, and the XRD results showed that the crystallinity of the Am-PAEK membranes increased as the amino groups content increased. The tensile strength and young’s modulus of the Am-PAEK-10% achieved 44.10 MPa and 2053 MPa, respectively. All of the Am-PAEK membranes exhibited excellent thermal performance. The gas separation capability testing showed that the Am-PAEK-10% membrane showed higher pure gas permeability [P(CO2) = 12.38] as well as ideal permselectivity [α(CO2/CH4) = 21.34], the separation performance was closest the Robeson upper bound. It was due to that acid–base and hydrogen bond interactions between the amino groups and carbon dioxide exit in the membranes. Thus, the strategy of precise regulated the content of amino groups of Am-PAEK showed a prospective approach for high performance gas separation membranes.

Interfacial Property Modulation of PIM-1 through Polydopamine-Derived Submicrospheres for Enhanced CO2/N2 Separation Performance.

Polydopamine-modified additives have been thus far widely used in the mixed matrix membranes (MMMs) for gas separation. However, very few reports focus on the polydopamine alone and investigate its contribution to the gas separation performance. Herein, the polydopamine-derived submicrospheres (PDASS) were paired with polymers of intrinsic microporosity (PIM-1) to fabricate high-performance gas separation membranes, through which the effects of PDASS on gas permeability and CO2/N2 separation performance were systematically investigated. The addition of PDASS provides a 1.6-fold enhancement in CO2/N2 selectivity together with acceptable gas permeability as compared to the original polymeric membrane. Such enhanced separation behavior is supposed to stem from the densified membrane microstructure induced by the strong intermolecular interactions between PIM-1 and PDASS (i.e., charge transfer, π-π stacking, and hydrogen bonding). Importantly, the physical aging behavior, as judged by gas permeability, is retarded for PIM/PDASS membranes after 4 months of testing.