Defect-free Mixed-matrix Membranes Research Articles

Mixed matrix membranes incorporating amino-functionalized ZIF-8-NH2 in a carboxylic polyimide for molecularly selective gas separation

Mixed matrix membranes (MMMs) combine the advantages of polymers and inorganic materials and demonstrate higher gas permeability than polymeric membranes, which, however, face the challenge of incompatible interface ascribed to the intrinsically different nature of the polymer and inorganic phase. To overcome this hurdle, herein, co-polyimide (6FBDA) was designed with carboxylic side groups to enhance the interfacial compatibility with amino-functionalized MOF(ZIF-8-NH2) via intensive π-π stacking and hydrogen bonds. The imidazole units in 6FBDA resemble ZIF-8-NH2 ligands and promote π-π stacking of polymer and MOF while the carboxylic moieties establish hydrogen bonds with the amino ligands of ZIF-8-NH2, significantly improving the interfacial compatibility of polymer and MOF. The defect-free MMMs with 50 wt% MOF loading demonstrate a CO2 and H2 permeability of 80.8 Barrer and 127.5 Barrer, respectively, with a CO2/CH4 and H2/CH4 selectivity of 89.4 and 141.7, respectively, exceeding 2008 Robeson upper bounds. The experimental concept of this work opens a door for fabricating molecularly selective gas separation membranes with prospects for energy-efficient CO2 capture and hydrogen separations.

Rational design, synthesis, and characterization of facilitated transport membranes exhibiting enhanced permeability, selectivity and stability

Despite outperforming conventional polymer membranes in the short time frame, facilitated transport membranes (FTMs) are prone to photo/chemical aging, which causes a rapid decay of their performance. Moreover, when embedded in polymer materials to fabricate mixed matrix membranes, structural defects may form at the metal-polymer interface, causing a selectivity loss. To overcome these issues, a series of poly(amic acid)s (PAAs) have been synthesized using 4,4’-oxydiphathalic anhydride (ODPA) and Jeffamine monomers of varying molecular weight, and used to fabricate silver nanoparticles via the chelating reaction with silver ions, in attempts to simultaneously i) achieve defect-free mixed matrix membranes (MMMs), ii) target CO2 selective transport, and iii) enhance long-term stability. The occurrence of the chelating reaction between silver and the PAA was confirmed, and as a result, individual and un-aggregated PAA-coated silver nanoparticles were obtained and incorporated into a commercial polymer, Pebax 1657, to fabricate defect-free mixed matrix membranes. The effect of the PAA length and ether functional group concentration on the structure and performance of the resulting mixed matrix membranes was systematically investigated. Remarkably, inclusion of only 2 wt% nanoparticles in Pebax enhances the CO2 permeability by 50% and CO2 selectivity by 100% relative to neat polymer. A detailed analysis of the sorption and diffusion coefficients was performed to elucidate the molecular origin of the observed membrane performance. Finally, preliminary data show that the newly synthesized materials exhibit high chemical stability upon exposure to pure H2.

Pre-anchoring matrix onto zeolitic imidazolate frameworks towards defect-free mixed matrix membranes for efficient CO2 separation

Combined with the specific advantages of continuous polymer phase and dispersed filler phase, mixed matrix membranes (MMMs) possess the potential to serve as the next generation membrane materials. However, the further development of MMMs is hindered by a variety of challenges, such as the structural stability of fillers during processing, the homogeneity during the dispersion, the non-ideal interfacial morphology between matrix and filler. In this work, a strategy for pre-anchoring polymeric matrices to fillers was proposed, the amino group was introduced to ZIF-8 framework (ZIF-8-NH2) by mixed ligand method based on the principles of reticular chemistry, and the ZPX fillers were fabricated by pre-anchored Pebax onto the ZIF-8-NH2, the unique ZIF-8/pre-anchored Pebax/Pebax interfacial structure was fabricated. The as-prepared Pebax/ZPX-5(10) MMMs exhibited a competitive performance in CO2 capture: high CO2 permeability for 146.4 Barrer, excellent CO2/N2 selectivity for 88.3, which improved by 269.1% and 187.1% respectively, compared with Peabx membrane, the index of filler enhancement (Findex) was 2.79, and the Robeson upper bound (2008) was successfully surpassed. Our pre-anchoring strategy provided an alternative answer for the facile fabrication of voids-free MMMs.

Vacuum treated amorphous MOF mixed matrix membrane for methane/nitrogen separation

CH4/N2 separation is considered a feasible method for reducing the greenhouse effect, but its utility in industrial applications has limited by the low permselective performance. In this work, a novel method for enhancing CH4/N2 separation performance using amorphous UIO-66-NH2 with a carbonized structure (ACU) via high vacuum resistance calcination is proposed. Owing to the presence of carbonized framework that contains rich Zr4+ active sites, these vacuum-treated ACU nanoparticles showed high IAST CH4/N2 selectivity, good alkaline stability and satisfactory compatibility with polymers. Then, ACU were uniformly dispersed in polyvinyl alcohol (PVA) to obtain defect-free mixed matrix membranes (MMMs), the resulting ACU-based MMMs exhibited an enhanced CH4 separation performance. Remarkably, the PVA/ACU-120A/MPSf MMM that consisted of 80 ​wt% ACU-120A exhibited the outstanding CH4 permselectivity, achieving a CH4 permeance of 5584 GPU and CH4/N2 selectivity of 3.43.

Cellulose Triacetate-Based Mixed-Matrix Membranes with MXene 2D Filler-CO2/CH4 Separation Performance and Comparison with TiO2-Based 1D and 0D Fillers.

Mixed-matrix membranes (MMMs) possess the unique properties and inherent characteristics of their component polymer and inorganic fillers, or other possible types of additives. However, the successful fabrication of compact and defect-free MMMs with a homogeneous filler distribution poses a major challenge, due to poor filler/polymer compatibility. In this study, we use two-dimensional multi-layered Ti3C2Tx MXene nanofillers to improve the compatibility and CO2/CH4 separation performance of cellulose triacetate (CTA)-based MMMs. CTA-based MMMs with TiO2-based 1D (nanotubes) and 0D (nanofillers) additives were also fabricated and tested for comparison. The high thermal stability, compact homogeneous structure, and stable long-term CO2/CH4 separation performance of the CTA-2D samples suggest the potential application of the membrane in bio/natural gas separation. The best results were obtained for the CTA-2D sample with a loading of 3 wt.%, which exhibited a 5-fold increase in CO2 permeability and 2-fold increase in CO2/CH4 selectivity, compared with the pristine CTA membrane, approaching the state-of-the-art Robeson 2008 upper bound. The dimensional (shape) effect on separation performance was determined as 2D > 1D > 0D. The use of lamellar stacked MXene with abundant surface-terminating groups not only prevents the aggregation of particles but also enhances the CO2 adsorption properties and provides additional transport channels, resulting in improved CO2 permeability and CO2/CH4 selectivity.

Incorporating ZIF-71 into poly(ether-block-amide) (PEBA) to form mixed matrix membranes for enhanced separation of aromatic compounds from aqueous solutions by pervaporation

As important chemical intermediates, phenol and aniline are common aromatic contaminants in relevant industrial wastewater that have drawn increasing attention due to their wide existence and high toxicities. In this work, zeolitic imidazolate framework (ZIF-71) crystals were incorporated into poly(ether-block-amide) (PEBA) to form mixed matrix membranes (MMMs) for the pervaporative removal of phenol and aniline from their aqueous solutions. The physiochemical properties of the MMMs were characterized with FTIR, XRD, SEM, EDX, and contact angle measurements. The pervaporation performance of the prepared MMMs at different ZIF-71 contents and operating temperatures was investigated. The ZIF-71 crystals showed good compatibility with PEBA polymer, and defect-free MMMs were produced at a ZIF content of as high as 28.6 wt%. The membrane became more hydrophobic and less flexible with an increase in the ZIF-71 loading, and water permeability was reduced significantly, whereas the permeability of the aromatic components was largely unaffected (phenol) or slightly lowered (aniline) because of the preferential sorption/diffusion of the aromatic solutes within the inner ZIF-71 channels, resulting in increased solute enrichment (or selectivity). At a feed concentration of 6,000 ppm and temperature of 60 °C, the ZIF-71/PEBA MMM with a ZIF-71 content of 28.6 wt% displayed a separation factor of 50 and 70 for phenol/water and aniline/water separations, respectively, which demonstrated the excellent potential of ZIF-71/PEBA MMMs as organophilic membranes for pervaporative extraction where the concentrated permeate stream could readily undergo a phase separation to augment the overall separation when the permeate concentrations reached the solubility limit.

Functionalized MOF-Derived Nanoporous Carbon as Compatible Nanofiller to Fabricate Defect-Free PDMS-Based Mixed Matrix Pervaporation Membranes.

Metal–organic framework (MOF)-based polydimethylsiloxane mixed matrix membranes applied for alcohol recovery with high permeability and selectivity are drawing more and more attention. However, the design and fabrication of high-quality and stable MOF-based mixed matrix membrane for pervaporation are still a big challenge. Herein, PDMS functionalized MOF-derived nanoporous carbon (P-ZNC) was first explored as compatible nanofiller to mutually blend with polydimethylsiloxane on PVDF substrate to fabricate defect-free mixed matrix membranes via dip-coating and thermal cross-linkng. Induced by UV illumination, hydrophobic modification of MOF-derived nanoporous carbon was successfully realized under mild conditions within one step, simplifying the operation step. By using this facile strategy, we can not only solve the existing problem of agglomeration, but also covalently cross-link MOF derivative with polymeric matrix and effectively eliminate the interface defect between polymer and nanoparticles without any extra steps. The method also gives a good level of generality for the synthesis of versatile stable nanoporous MOF-derived carbon-based mixed matrix membranes on various supports. The prepared PDMS/P-ZNC with commendable structures possessed excellent separation performance in low concentration n-butanol recovery and had a good balance between permeance, selectivity, and stability.

Effective functionalization of porous polymer fillers to enhance CO2/N2 separation performance of mixed-matrix membranes

A series of porous organic polymers (denoted as PBP- x ) possessing various functional groups was synthesized via Friedel–Crafts alkylation followed by post-synthetic functionalization using amine and sulfonic groups. These functionalized porous polymers were incorporated into an in-house polyimide to fabricate mixed-matrix membranes for CO 2 /N 2 separation. Gas permeation testing revealed that these functionalized porous fillers substantially improved the CO 2 separation performance. Moreover, optimizing both the functionality and pore structure of the porous fillers was found to drastically improve the CO 2 /N 2 separation performance. In particular, the addition of 10 wt% PBP-menm (menm = 1,2-methylethylenediamine) induced an ultrahigh CO 2 permeability of 2988 barrers (158% higher than that of a pure polymer membrane), thus realizing excellent performance that surpasses the Robeson upper bound for CO 2 /N 2 separation. • Porous organic polymers possessing various functional groups were synthesized. • Defect-free mixed-matrix membranes comprising these functionalized porous fillers were successfully fabricated. • Optimizing both functionality and pore structure of porous fillers led to enhanced CO 2 transport. • The performance of the membrane with the optimum filler surpassed the Robeson upper bound for CO 2 /N 2 separation.

Sealing Tröger base/ZIF-8 mixed matrix membranes defects for improved gas separation performance

Manufacturing defect-free mixed matrix membranes (MMMs) especially with high nanoparticle concentrations exceeding 40 wt% is still an ongoing challenge. Desired performance improvements are often unfulfilled in MMMs with high filler loading because nanoparticles with increasing contents are prone to aggregating, forming non-selective cavities, and deteriorating membrane's mechanical properties. To address this key issue, our work develops MMMs using Tröger base (TB) polymer with more hydrogen bond acceptors, polydopamine (PDA), and ZIF-8 fillers. The PDA modified ZIF-8 to create TB/ZIF-8 MMMs especially with high ZIF-8 loading over 40 wt% improves membrane mechanical properties and completely seals structural defectives to demonstrate encouraging gas selectivities. Hydrogen bonding between CN:NH and CN:OH groups within TB polymers and PDA enhances ZIF-8 interactions and reduces particle agglomeration within the polymer matrix. A TB/ZIF-8@PDA MMM containing 50 wt% ZIF-8 has H2 and CO2 permeabilities of 664.1 and 277.3 Barrer. Its ideal H2/CH4 and CO2/CH4 selectivities are 37.7 and 15.5, respectively. Equal molar CO2/CH4 mixed gas tests at various pressures demonstrate strong CO2 plasticization resistance and enhanced CO2/CH4 gas selectivity over ideal selectivity within the membranes. The combination of PDA modification technique on filler surfaces and a polymer with many hydrogen bonds acceptors is not only a feasible route to fabricate high filler loading MMMs with sealed defective structures, but also demonstrates positive impacts on membrane mechanical properties.

Polymerizable metal-organic frameworks for the preparation of mixed matrix membranes with enhanced interfacial compatibility

SummaryThe preparation of flawless and defect-free mixed matrix membranes (MMMs) comprising metal-organic framework (MOF) and polymer is often difficult owing to the poor MOF/polymer interface compatibility. Herein, we present the synthesis of an important family of pillared-layered MOFs with polymerizable moieties based on the parent structure [Zn2L2P]n [L = vinyl containing benzenedicarboxylic acid linkers; P = 4,4′-bipyridine (bipy)]. The crystalline structures of polymerizable MOFs were analyzed using single-crystal X-ray crystallography. The presence of reactive double bonds in MOFs was verified by the successful thiol-ene click reaction with sulfhydryl compounds. The subsequent copolymerization of polymerizable MOFs with organic monomers produced mixed matrix membranes with enhanced MOF/polymer interfacial adhesion that enabled good separation efficiency of CO2 from flue gas. This strategy provides a stimulating platform to the preparation of highly efficient MMMs that are capable of mitigating energy consumption and environment issues.

Preparation of Amino-Functional UiO-66/PIMs Mixed Matrix Membranes with [bmim][Tf2N] as Regulator for Enhanced Gas Separation

Development of mixed matrix membranes (MMMs) with excellent permeance and selectivity applied for gas separation has been the focus of world attention. However, preparation of high-quality MMMs still remains a big challenge due to the lack of enough interfacial interaction. Herein, ionic liquid (IL)-modified UiO-66-NH2 filler was first incorporated into microporous organic polymer material (PIM-1) to prepare dense and defect-free mixed matrix membranes via a coating modification and priming technique. IL [bmim][Tf2N] not only improves the hydrophobicity of UiO-66-NH2 and facilitates better dispersion of UiO-66-NH2 nanoparticles into PIM-1 matrix, but also promotes the affinity between MOFs and polymer, sharply reducing interface non-selective defects of MMMs. By using this strategy, we can not only facilely synthesize high-quality MMMs ignoring non-selective interfacial voids, but also structurally regulate MOF nanoparticles in the polymer substrate and greatly improve interface compatibility and stability of MMMs. The method also gives suitable level of generality for fabrication of versatile defect-free MMMs based on different combination of MOFs and PIMs. The prepared UiO-66-NH2@IL/PIM-1 membrane exhibited outstanding gas separation behavior with large CO2 permeation of 8283.4 Barrer and high CO2/N2 selectivity of 22.5.

Defect-free mixed-matrix membranes consisting of anion-pillared metal-organic frameworks and poly(ionic liquid)s for separation of acetylene from ethylene

Membrane separation is an effective strategy to reduce the energy consumption during conventional processes in the petrochemical industry. Currently, membrane separations of light hydrocarbons are still underdeveloped and the general problem of the poor compatibility between the polymer and inorganic particles in mixed-matrix membranes (MMMs) must be addressed. Herein, the separation of acetylene/ethylene mixture employing a defect-free mixed-matrix membrane consisting of anion-pillared metal-organic frameworks (MOFs, SIFSIX-2-Cu-i, SIFSIX = hexafluorosilicate, 2 = 4,4′-dipyridylacetylene, i = interpenetrated) and poly(ionic liquid)s (PILs) is for the first time reported. The strategy of enhancing compatibility by strong electrostatic interactions between ionic polymer and MOFs is realized and defect-free MMMs can be prepared readily without extra treatment. With optimized ion density, an acetylene/ethylene separation selectivity of 112.0 is achieved on SIFSIX-2-Cu-i/P[ATMA][BF4]-g-PEG500 ([ATMA][BF4] = 2-(acryloyloxy)ethyl-trimethylammonium tetrafluoroborate, PEG = poly(ethylene glycol)) mixed membrane, indicating that our strategy is effective to prepare highly selective defect-free membranes. The strategy of strengthening electrostatic interactions between ionic polymer and anion-pillared MOFs provides a general approach for the fabrication of defect-free MMMs which has promise in gas separation applications.

Leveraging Nanocrystal HKUST-1 in Mixed-Matrix Membranes for Ethylene/Ethane Separation.

The energy-intensive ethylene/ethane separation process is a key challenge to the petrochemical industry. HKUST-1, a metal–organic framework (MOF) which possesses high accessible surface area and porosity, is utilized in mixed-matrix membrane fabrication to investigate its potential for improving the performance for C2H4/C2H6 separation. Prior to membrane fabrication and gas permeation analysis, nanocrystal HKUST-1 was first synthesized. This step is critical in order to ensure that defect-free mixed-matrix membranes can be formed. Then, polyimide-based polymers, ODPA-TMPDA and 6FDA-TMPDA, were chosen as the matrices. Our findings revealed that 20 wt% loading of HKUST-1 was capable of improving C2H4 permeability (155% for ODPA-TMPDA and 69% for 6FDA-TMPDA) without excessively sacrificing the C2H4/C2H6 selectivity. The C2H4 and C2H6 diffusivity, as well as solubility, were also improved substantially as compared to the pure polymeric membranes. Overall, our results edge near the upper bound, confirming the effectiveness of leveraging nanocrystal HKUST-1 filler for performance enhancements in mixed-matrix membranes for C2H4/C2H6 separation.

Mixed-matrix membranes based on 6FDA-ODA polyimide and silicalite-1 with homogeneous spatial distribution of particles

We report an experimental procedure leading to defect-free mixed-matrix membranes with a homogeneous spatial distribution of filler particles, volume fractions of which are within the interval [0.10, 0.35]. A polyimide matrix was synthetized from 4,4′-(hexafluoroisopropylidene) diphtalic anhydride and 4,4′-oxydianiline in the solution with N,N-dimethylformamide. (3-aminopropyl) triethoxysilane was used as a linker to improve the phase contact between silicalite-1 particles and the polyimide matrix. We found that a homogeneous spatial distribution of particles in the resulting membrane was achieved if kinematic viscosity of a polyimide precursor solution was greater than 9.5 cm2 s−1. To do so, an initial mass fraction of the solids in the solution, a reaction temperature and a concentration of water in the solvent had to carefully be selected and controlled. Besides these conditions, we fine-tuned the process of imidization, particularly the rate of evaporation of N,N-dimethylformamide. We arrived at the conclusion that its effect on reproducibility of the entire process was minor rather than major. In addition, we characterised microstructures of the resulting membranes by imaging in a scanning electron microscope. In this context we examined two ways of preparation of membrane specimens and showed that back-scattered electron imaging of polished sections obtained using a metallographic technique clearly outperformed the often-used imaging of fractured surfaces. Finally, we evaluated the homogeneity of spatial distributions of particles by means of morphological descriptors, namely a position-dependent volume fraction of the silicalite-1 phase.

Bottom-up Synthesis of Defect-free Mixed-matrix Membranes by Using Polymer-grafted Metal–Organic Polyhedra

Polyethylene glycol (PEG)-grafted metal–organic polyhedra (MOPs) provided defect-free mixed-matrix membranes via the solution-cast process. Compared to the neat PEG-based membranes, the membranes p...

Defect-Free MOF-Based Mixed-Matrix Membranes Obtained by Corona Cross-Linking.

Functionalized UiO-66 metal-organic frameworks (MOF) particles were covalently grafted with hydride-terminated poly(dimethylsiloxane) (PDMS) via postsynthetic modification. These PDMS-coated MOF particles (termed here "corona-MOF") were used in the preparation of mixed-matrix membranes (MMMs). Defect-free MMMs with weight loadings of 50% were achieved with corona-MOF particles, attributed to the improved dispersibility of the corona-MOF particles and covalent linkages between the corona-MOF particles and the polymer matrix. The PDMS MMMs showed distinct separation features in single gas permeation tests, displaying much higher CO2 gas permeation with no decrease in selectivity when compared to MMMs prepared with unmodified UiO-66 particles. Single gas separation tests with CO2, N2, and propane were performed to probe the separation mechanism of the corona-MOF MMMs, demonstrating that these MMMs avoid nonideal "sieve-in-a-cage" and "plugged sieves" scenarios. Additionally, due to covalent bond formation between both the MOF and the polymer matrix in corona-MOF MMMs, particle aggregation is negligible during film curing, allowing for the formation of flexible, self-standing MMMs of <1 μm in thickness. Low quantities of polymer covalently attached to the MOF surface (<5 wt %) are sufficient to fabricate thin, defect-free, high MOF-loading MMMs.

Selective Gas Permeation in Mixed Matrix Membranes Accelerated by Hollow Ionic Covalent Organic Polymers

Mixed matrix membranes (MMMs) have gained great attention for the efficient CO2 removal from raw nature gas or biogas (CO2/CH4 separation) and flue gas (CO2/N2 separation). Nevertheless, the development of high-performance MMMs for industrial applications is largely limited by the lack of suitable porous fillers. Herein, a novel ionic covalent organic polymer (ICOP-1) consisting of gas selective pores and hollow cavities is facilely fabricated using a metal triflate catalyzed condensation reaction. Considering its unique structural properties, ICOP-1 is explored as a novel filler to enhance the gas separation properties of polysulfone (PSf) membranes. Defect-free MMMs are successfully prepared owing to the high polymer–filler affinity originating from the organic nature of these two phases. Besides, the large cavities and size-selective pores of ICOP-1 lead to a simultaneous increase in membrane CO2 permeability and CO2/CH4, CO2/N2 selectivities. With the addition of only 0.5 wt % of ICOP-1 fillers, the a...

Metal-organic framework-based mixed matrix membranes: Synergetic effect of adsorption and diffusion for CO2/CH4 separation

ZIF-8 nanoparticles were doped into P84 polymer matrix to synthesize defect-free mixed matrix membranes (MMMs) for CO2/CH4 separation. A remarkable combination of gas permeability and separation factor appeared in MMMs with filler loading of 27 wt%. The permeability of CO2 achieved 10.92 Barrers along with CO2/CH4 separation factor of 92.6, separately corresponding to an increment of 339% and 35.7% compared with the pure P84 membrane, which breaks the trade-off between permeability and selectivity of polymer membranes. Adsorption and diffusion coefficient of CO2 and CH4 for ZIF-8/P84 MMMs as well as the apparent activation energy of the corresponding processes were quantitatively determined, indicative of a synergetic effect of the selective adsorption and molecular sieving property arising from ZIF-8 nanoparticle fillers on the significant improvement of separation performances.

Defect-Free Mixed-Matrix Membranes with Hydrophilic Metal-Organic Polyhedra for Efficient Carbon Dioxide Separation.

Defect-free mixed-matrix membranes (MMMs) were prepared by incorporating hydrophilic metal-organic polyhedra (MOPs) into cross-linked polyethylene oxide (XLPEO) for efficient CO2 separation. Hydrophilic MOPs with triethylene glycol pendant groups, which were assembled by 5-tri(ethylene glycol) monomethyl ether isophthalic acid and CuII ions, were uniformly dispersed in XLPEO without particle agglomeration. Compared to conventional neat XLPEO, the homogenous dispersion of EG3 -MOPs in XLPEO enhanced CO2 permeability of MMMs. Upon increasing the amount of EG3 -MOPs, the membrane performance such as CO2 /N2 selectivity was steadily improved because of unsaturated CuII sites at paddle-wheel units, which was confirmed by Cu K-edge XANES and TPD analysis. Therefore, such defect-free MMMs with unsaturated metal sites would contribute to enhance CO2 separation performance.

Interfacial control in perfluoropolymer mixed matrix membranes for natural gas sweetening

Defect-free mixed matrix membranes (MMMs) for natural gas separation obtained by adding SAPO-34 crystals to Hyflon® AD60X, a perfluoropolymer resistant to plasticization and tolerant to condensable hydrocarbons, are more permeable and selective (CO2/CH4 40 vs. 22) than the polymer. The experimental characterization of MMMs containing permeable and impermeable crystals of different size, shape and aspect ratio, and the macroscopic modeling of gas fluxes, allow to understand the mass transport phenomena at work. Polymer free volume increases at the interface, and barriers to the transport of gas are evidenced in the interior or on the surface of SAPO-34.