Enhanced Gas Separation Research Articles

Development of blend PEG-PES/NMP-DMF mixed matrix membrane for CO2/N2 separation

The carbon dioxide (CO2) separation technology has become a focus recently, and a developed example is the membrane technology. It is an alternative form of enhanced gas separation performance above the Robeson upper bound line resulting in the idea of mixed matrix membranes (MMMs). With attention given to membrane technologies, the MMMs were fabricated to have the most desirable gas separation performance. In this work, blend MMMs were synthesised by using two polymers, namely, poly(ether sulfone) (PES) and poly (ethylene glycol) (PEG). These polymers were dissolved in blend N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvents with the functionalised multi-walled carbon nanotubes (MWCNTs-F) fillers by using the mixing solution method. The embedding of the pristine MWCNTs and MWCNTs-F within the new synthesised MMM was then studied towards CO2/N2 separation. In addition, the optimisation of the loading of MWCNTs-F for blend MMM for CO2/N2 separation was also studied. The experimental results showed that the functionalised MWCNTs (MWCNTs-F) were a better choice at enhancing gas separation compared to the pristine MWCNTs (MWCNTs-P). Additionally, the effects of MWCNTs-F at loadings 0.01 to 0.05% were studied along with the polymer compositions for PES:PEG of 10:20, 20:20 and 30:10. Both these parameters of study affect the manner of gas separation performance in the blend MMMs. Overall, the best performing membrane showed a selectivity value of 1.01 + 0.05 for a blend MMM (MMM-0.03F) fabricated with 20 wt% of PES, 20 wt% of PEG and 0.03 wt% of MWCNTs-F. The MMM-0.03F was able to withstand a pressure of 2 bar, illustrating its mechanical strength and ability to be used in the post combustion carbon capture application industries where the flue gas pressure is at 1.01 bar.

High-performance gas separation membranes derived from thermal-oxidative block poly(benzoxazole-co-imide)

Thermally rearranged(TR) polymers are usually prepared at an insert atmosphere and display high gas permeability, but their low gas selectivity results in high energy consumption. Thermal oxidation is an effective method to enhance the gas selectivity of microporous polymers. In this work, the effect of oxygen in the purge environment on imide-to-benzoxazole conversion and membrane structures was investigated. 1% oxygen content of the purge environment could restrain imide-to-benzoxazole conversion of 6FDA-DETDA/bisAPAF hydroxyl block polyimide(bHPI) membranes effectively, and oxygen induced the oxygen bridge formation, improving the gas selectivities of the resulting thermal-oxidative membranes with the increased oxygen content. Thermal-oxidative PBOCI membranes (TO-PBOCI) based on block poly(benzoxazole-co-imide)(PBOCI) precursor membranes were prepared to enhance gas separation performance of microporous polymers. The stable polybenzoxazole structures and microphase separation of polybenzoxazole blocks maintained membrane structures and the gas transport channels in the thermal-oxidative process at high temperature. Significantly, the remaining hydroxyl groups in PBOCI precursor membranes were beneficial for oxygen bridge formations of TO-PBOCI membranes, enhancing H2/CH4 selectivity. As a result, the TO-PBOCI membranes treated at a high temperature kept high H2 permeability and H2/CH4 selectivity, surpassing the 2008 Robeson upper bound. The study about TO-PBOCI membranes promotes a better understanding of the polymeric materials and process development of microporous polymer membranes for hydrogen separation applications.

Fluorinated Polybenzimidazole as a Novel Precursor for Carbon Molecular Sieve Membranes with Enhanced Gas Separation Properties

Fluorinated Polybenzimidazole as a Novel Precursor for Carbon Molecular Sieve Membranes with Enhanced Gas Separation Properties

Enhanced gas separation by free volume tuning in a crown ether-containing polyimide membrane

Tuning coupled free volume and pore size distribution is of great significance for membrane-based gas separation. In the present work, a polyimide membrane containing bulky -CF3 group and crown ether ring structure was developed using 4, 4′ - (hexafluoroisopropylidene) diphthalic anhydride as dianhydride monomer, 2, 2′-bis [4-(4-aminophenoxy)phenyl] -hexafluoropropanane and di(aminobenze)-21-crown-7-ether as diamine monomers. Diamine monomer with large bulky -CF3 grant the polyimide membrane more free volume, which was helpful for gas permeability enhancement. Besides, crown ether could act as a pores size distribution tuning monomer, which endowed membranes with improved gas selectivity. Results showed that the introduction of crown ether into polyimide backbone exhibited a significant impact on free volume and pores size distribution. Typically, with the increasing crown ether content, the free volume decreased, whereas the relative available free volume ratio (FAVi/FAVj) achieved a remarkable increment. It’s also worth noting that the pores size distribution was changed by copolymerization with crown ether. The intensity of macro-pore gradually decreased accompanied with an increase in micro-pore structure. As a result, the gas selectivity significantly increased. Compared the membrane without crown ether, the maximum selectivity enhancement was achieved at 30 mol% di(aminobenze)-21-crown-7-ether content. It exhibited remarkable selectivity increments about 204.8%, 198.8%, 123.2%, and 115.4% for He/CH4, H2/CH4, CO2/CH4, and CO2/N2, respectively. These results took a huge step forward which nearly hit up to the 2008 Robeson’s upper bound, especially for CO2/CH4.

MOF/Polymer Mixed-Matrix Membranes Preparation: Effect of Main Synthesis Parameters on CO2/CH4 Separation Performance.

Design and preparation of mixed-matrix membranes (MMMs) with minimum defects and high performance for desired gas separations is still challenging as it depends on a variety of MMM synthesis parameters. In this study, 6FDA-DAM:DABA based MMMs using MOF-808 as filler were prepared to examine the impact of multiple variables on the preparation process of MMMs, including variation in polymer concentration, filler loading, volume of solution cast per membrane area, solvent type used and solvent evaporation rate, and to identify their impact on the CO2/CH4 separation performance of these membranes. Solvent evaporation rate proved to be the most critical synthesis parameter, directly influencing the performance and visual appearance of the membranes. Although less dominantly influencing the MMM performance, polymer concentration and solution volume also had an important role via control over the casting solution viscosity, particle agglomeration, and particle settling rate. Among all solvents studied, MMMs prepared with chloroform led to the best performance for this polymer-filler system. Chloroform-based MMMs containing 10 and 30 wt.% MOF-808 showed 73% and 62% increase in CO2 permeability, respectively, without a decrease in separation factor compared to unfilled membranes. The results indicate that enhanced gas separation performance of MMMs strongly depends on the cumulative effect of various synthesis parameters rather than individual impact, thus requiring a system-specific design and optimization.

Fabrication of polyarylate thin-film nanocomposite membrane based on graphene quantum dots interlayer for enhanced gas separation performance

Interlayered-thin film nanocomposite (i-TFN) membranes with nanomaterials as the interlayer has attracted more and more attention of researchers in recent years due to the effective regulation of the separation layer structure in the membrane separation field. In this work, the amino-functionalized graphene quantum dots (N-GQDs) were synthesized and applied as an interlayer to modify the polyethersulfone (PES) ultrafiltration substrate. Then interfacial polymerization (IP) between trimesoyl chloride (TMC) and β-cyclodextrin (β-CD) was performed to prepare novel i-TFN polyarylate membranes for CO2/N2 separation. The effect of the N-GQDs deposition amount on the structure and gas separation performance of the composite membrane was studied thoroughly. The results indicated that the i-TFN polyarylate membrane presented a particular sandwich structure and provided an effective transport channel for the diffusion of CO2. Moreover, the affinity of the N-containing groups (e. g. amino and amide groups) in the i-TFN membrane and the CO2 molecules helped to increase the CO2/N2 separation selectivity. At the optimal preparation condition, the obtained i-TFN membrane showed CO2/N2 selectivity of 23.3 with CO2 permeance of 174.5 GPU. This work proposes a novel nanomaterial interlayer to effectively improve the gas permselectivity by regulating the structure of i-TFN membranes, which has significant instruction for the preparation of high-performance i-TFN by exploring the interlayer of other nanomaterials.

Experimental investigation of natural polysaccharide-based mixed matrix membrane modified with graphene oxide and Pd-nanoparticles for enhanced gas separation performance

In this work, we proposed a mixed matrix membrane prepared by using a glycerol modified guar gum (GGP) polymer matrix incorporated with graphene oxide (GO). The influence of varying GO concentration on the gas separation performance was investigated and 2 wt% was found to be the optimum concentration for high performance. The 2 wt% GO mixed matrix membranes were further modified with Pd nanoparticles. When GO, and Pd nanoparticles were mixed, CO2 permeability increased by 49.94%, while the permeability of H2 gas molecules decreased by 98.11%, respectively, compared to the pristine GGP membrane. The selectivity of CO2/H2 was obtained as 18.27. The glass transition temperature of the membrane increased from 85 to 95.2 °C, tensile strength and elongation of the break were significantly improved by 29.09% and 84.37% through the addition of Pd and GO into the membrane. The scanning electron microscopy revealed a dense top surface after GO nanosheets incorporation. Further, the thermogravimetric analysis proposes that the modified membrane is thermally stable than GGP. Henceforth, the study suggests GO incorporation and Pd nanoparticles modification of guar gum membrane is a promising gas separation membrane with potentially high selectivity for CO2 gas.

A general strategy for fabricating polymer/nanofiller composite membranes with enhanced CO2/N2 separation performance

Aggregation of nanofillers, one of the critical challenges for developing high-performance composite membranes, induces non-selective gas transport paths, and thus reduces the gas separation selectivity. Here, we propose a facile and general filtration-spray strategy for fabricating polymer/nanofiller composite membranes to inhibit nanofiller aggregation. In this way, the nanofiller scaffold is first deposited on the surface of the porous substrate through filtration. Then, the polymer covers the scaffold via a spray method. Finally, a defect-free membrane is formed after penetration of polymer and thus filling of the inter-filler gaps during the solvent evaporation process. This strategy avoids redispersion and aggregation of nanofillers in polymer matrix, thus diminishing the non-selective defects of interparticles caused by nanofiller aggregation and forming dense polymer/nanofiller composite membranes with well-dispersed nanofillers. The as-prepared Pebax/PEI-ZIF-62 (polyethyleneimine functionalized ZIF-62) membrane exhibits an enhanced gas separation performance with a CO2 permeance of 58 GPU and a CO2/N2 selectivity of 83, which are 45% and 51% higher than those of the pure Pebax membrane, respectively, surpassing Robeson 2008 upper bound. However, the CO2 separation selectivity of PEI-ZIF-62@Pebax membrane prepared via spraying the mixed solution of Pebax and PEI-ZIF-62 decreases to 38, probably due to the formation of non-selective voids. To demonstrate the versatility of this facile strategy, various nanofillers, such as SiO2, ZIF-8, zeolite, and porous polymer sphere, have been used to prepare the composite membranes by this strategy, showing improved CO2 separation performance. Overall, these results demonstrate that the filtration-spray fabrication strategy is effective in preventing nanofiller aggregation in composite membranes.

Evaluation of CO2 separation performance with enhanced features of materials – Pebax® 2533 mixed matrix membranes containing ZIF-8-PEI@[P(3)HIm][Tf2N

Zeolitic imidazolate framework (ZIF-8) was functionalized by polyethyleneimine (PEI) and subsequently decorated with ionic liquid (IL) – [P(3)HIm][Tf2N]. ZIF-8-PEI@IL/Pebax® 2533 mixed matrix membranes (MMMs) were fabricated to enhance the CO2 capture performance of Pebax membranes. The amino groups and IL in ZIF-8-PEI improved the gas separation performance of the MMMs due to the additional CO2 transporting pathways by using the reversible reaction with amino groups and the high affinity between IL and CO2. The modified ZIF-8 particles demonstrated enhanced interfacial interactions and compatibility between fillers and polymer matrix due to the presence of amino groups and IL, confirmed by the enhanced gas separation performance, improved elongation at break, and tensile strength of MMMs. Comparing with the pristine membranes, CO2 permeability of MMMs containing 15 wt% ZIF-8-PEI@IL increased by 123% to 285 Barrer, and the CO2/N2 and CO2/CH4 ideal selectivity increased from 17 and 12 to 76 and 25, respectively.

Metal-Organic Frameworks (MOFs)-Based Mixed Matrix Membranes (MMMs) for Gas Separation: A Review on Advanced Materials in Harsh Environmental Applications.

The booming of global environmental awareness has driven the scientific community to search for alternative sustainable approaches. This is accentuated in the 13th sustainable development goal (SDG13), climate action, where urgent efforts are salient in combating the drastic effects of climate change. Membrane separation is one of the indispensable gas purification technologies that effectively reduces the carbon footprint and is energy-efficient for large-scale integration. Metal-organic frameworks (MOFs) are recognized as promising fillers embedded in mixed matrix membranes (MMMs) to enhance gas separation performance. Tremendous research studies on MOFs-based MMMs have been conducted. Herein, this review offers a critical summary of the MOFs-based MMMs developed in the past 3 years. The basic models to estimate gas transport, preparation methods, and challenges in developing MMMs are discussed. Subsequently, the application and separation performance of a variety of MOFs-based MMMs including those of advanced MOFs materials are summarized. To accommodate industrial needs and resolve commercialization hurdles, the latest exploration of MOF materials for a harsh operating condition is emphasized. Along with the contemplation on the outlook, future perspective, and opportunities of MMMs, it is anticipated that this review will serve as a stepping stone for the coming MMMs research on sustainable and benign environmental application.

Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

Induced polymer crystallinity in mixed matrix membranes by metal-organic framework nanosheets for gas separation

The induced crystallinity behavior of the polymer of intrinsic microporosity (PIM-1) matrix in mixed matrix membranes (MMMs) by a novel class of two-dimensional (2D) metal-organic framework nanosheets, i.e., NUS-8-COOH, is demonstrated. The proposed ideal of enhanced crystallinity induced by NUS-8-COOH nanosheets can be extended to other classes of 2D nanosheet-containing MMMs, thus opening up plentiful opportunities for constructing MMMs with superior interfacial compatibility toward enhanced gas separation performance.

Poly(benzyl ether)-type additive to engineer glassy polyimide membranes for enhanced gas separations.

This conceptual study demonstrates the reinforcement of glassy polyimide membranes by incorporating a poly(benzyl ether)-type additive. Traces of the sterically bulky additive alter the overall physical properties of the entire matrix and further enhance the separation properties of small gas molecules.

Fabrication of mixed matrix membranes with regulated MOF fillersviaincorporating guest molecules for optimizing light hydrocarbon separation performance

Guest molecules were incorporated into MOF fillers to construct MMMs withPIM-1for enhanced gas separation performance.

Significantly Enhanced Gas Separation Properties of Microporous Membranes by Precisely Tailoring Their Ultra-Microporosity Through Bromination/Debromination

Significantly Enhanced Gas Separation Properties of Microporous Membranes by Precisely Tailoring Their Ultra-Microporosity Through Bromination/Debromination

Molecular-level fabrication of highly selective composite ZIF-8-CNT-PDMS membranes for effective CO2/N2, CO2/H2 and olefin/paraffin separations

The molecular-scale fabrication method of unique CNT-ZIF-8-PDMS composite membranes with enhanced gas separation performance mainly focusing on CO2 capturing (CO2/N2, CO2/H2). Also olefin/paraffin separation is reported. For the first time, a successful synthesis of ZIF-8 gutter-layer on the free-standing chemically modified carbon nanotube (CNT) platforms was conducted via the ZnO linkages enabling the proper inter-grown and strong anchoring ZIF-8 crystals onto the CNT. The formation of aligned ZIF-8 arrays onto the CNT surface acted as an effective buffering layer that boosted the molecular gas transport through the membrane. In contrast, the conventional gutter layers which usually reduced the gas permeability. To diminish the interstitial pores at the surface/bulk of the CNT/ZIF-8 formations, thin polydimethylsiloxane (PDMS) layer was deposited via the spray-coating technique on the CNT/ZIF-8. The resulting non-defect morphology was confirmed by the SEM and the 3D optical profilometer observations. Furthermore, the PDMS deposition improved mechanical stability (stress vs. strain) of the composite with a twelvefold increased elongation and thermal stability led to a 46% mass reduction. Moreover, composite membranes exhibited outstanding gas separation performance with high CO2 and C3H6 permeabilities of 8705 and 4965 Barrer, respectively and selectivities overcoming the appropriate 2008 Robeson upper bounds for various gas pairs CO2/N2 (ideal selectivity α = 45.6) and CO2/H2 (α = 23.9) gas pairs. Similarly, achieved results of the other tested gas pairs, e.g. CO2/SF6 (α = 17.2), C3H6/N2 (α = 26.0) and C3H6/C3H8 (α = 4.9) were further remarkable in comparison with the literature. Hence, the CNT-ZIF-8-PDMS composite membrane concept has the excellent potential for the fabrication of efficient membranes suitable for targeted gas separation with high commercial and environmental relevance, such as the greenhouse gasses, hydrocarbons recovery for energy harvesting, CO2 capturing and olefin/paraffin separation.

Constructing MOF-doped two-dimensional composite material ZIF-90@C3N4 mixed matrix membranes for CO2/N2 separation

Two-dimensional nanomaterials have the characteristics of high aspect ratio and high space utilization, which has huge potential advantages in gas separation. In this work, g-C3N4 two-dimensional nanosheets with sieving pore structure were selected as templates and combined with ZIF-90 to construct a 0-dimensional/2-dimensional composite material ZIF-90@C3N4 (ZCN). The complete two-dimensional g-C3N4 nanosheets were stripped under acid etching conditions at 25 °C. ZIF-90 was distributed in an orderly manner on the two-dimensional nanosheets, and the nanosheets were spread out due to the growth of ZIF-90 to provide more free volume. The gas permeation experiment proved that the ZCN has a better CO2/N2 separation performance. The ZCN/Pebax mixed matrix membrane (MMM) with the optimized 8.0 wt% ZCN loading showed an improved CO2 permeability of 110.5 Barrer and a CO2 /N2 selectivity of 84.4, surpassing the Robeson upper bound (2008). The results demonstrated that the designed composite filler with 2D nanosheets is an effective strategy to enhance gas separation performance of MMMs and verified the application potential in gas purification industry.

Pentiptycene-based ladder polymers with configurational free volume for enhanced gas separation performance and physical aging resistance

Polymers of intrinsic microporosity (PIMs) have shown promise in pushing the limits of gas separation membranes, recently redefining upper bounds for a variety of gas pair separations. However, many of these membranes still suffer from reductions in permeability over time, removing the primary advantage of this class of polymer. In this work, a series of pentiptycene-based PIMs incorporated into copolymers with PIM-1 are examined to identify fundamental structure-property relationships between the configuration of the pentiptycene backbone and its accompanying linear or branched substituent group. The incorporation of pentiptycene provides a route to instill a more permanent, configuration-based free volume, resistant to physical aging via traditional collapse of conformation-based free volume. PPIM-ip-C and PPIM-np-S, copolymers with C- and S-shape backbones and branched isopropoxy and linear n-propoxy substituent groups, respectively, each exhibited initial separation performance enhancements relative to PIM-1. Additionally, aging-enhanced gas permeabilities were observed, a stark departure from the typical permeability losses pure PIM-1 experiences with aging. Mixed-gas separation data showed enhanced CO2/CH4 selectivity relative to the pure-gas permeation results, with only ∼20% decreases in selectivity when moving from a CO2 partial pressure of ∼2.4 to ∼7.1 atm (atmospheric pressure) when utilizing a mixed-gas CO2/CH4 feed stream. These results highlight the potential of pentiptycene's intrinsic, configurational free volume for simultaneously delivering size-sieving above the 2008 upper bound, along with exceptional resistance to physical aging that often plagues high free volume PIMs.

A Novel Composite Material UiO-66@HNT/Pebax Mixed Matrix Membranes for Enhanced CO2/N2 Separation.

Mixing a polymer matrix and nanofiller to prepare a mixed matrix membrane (MMM) is an effective method for enhancing gas separation performance. In this work, a unique UiO-66-decorated halloysite nanotubes composite material (UiO-66@HNT) was successfully synthesized via a solvothermal method and dispersed into the Pebax-1657 matrix to prepare MMMs for CO2/N2 separation. A remarkable characteristic of this MMM was that the HNT lumen provided the highway for CO2 diffusion due to the unique affinity of UiO-66 for CO2. Simultaneously, the close connection of the UiO-66 layer on the external surface of HNTs created relatively continuous pathways for gas permeation. A suite of microscopy, diffraction, and thermal techniques was used to characterize the morphology and structure of UiO-66@HNT and the membranes. As expected, the embedding UiO-66@HNT composite materials significantly improved the separation performances of the membranes. Impressively, the as-obtained membrane acquired a high CO2 permeability of 119.08 Barrer and CO2/N2 selectivity of 76.26. Additionally, the presence of UiO-66@HNT conferred good long-term stability and excellent interfacial compatibility on the MMMs. The results demonstrated that the composite filler with fast transport pathways designed in this study was an effective strategy to enhance gas separation performance of MMMs, verifying its application potential in the gas purification industry.

Ultrapermeable polymeric membranes based on particular ultra-rigid units for enhanced gas separation

To overcome the trade-off between permeability and selectivity of polymeric membranes and understand the influence of ultra-rigid units on gas separation performance, a series of PIMs (PIM-EN-x) based on spirodifluoranthene (EN) have been prepared. These polymers showed outstanding thermal stability. Their 5% weight loss temperature is higher than 503 °C. PIM-EN-x owned high BET surface area, ranging from 804 to 895 m2 g-1. With the increase of EN units, the gas permeability increased. The CO2 permeability of PIM-EN-40 is 11512 barrer, which is increased by 172% compared to PIM-1 with no loss in selectivity. The gas separation performance of PIM-EN-x surpassed the 2008 upper bound and near the latest upper bound. This work put forward a special structural unit to prepare polymers for CO2 separation and natural gas sweetening.