Zirconium-Based Nanoscale Metal-Organic Framework/Poly(ε-caprolactone) Mixed-Matrix Membranes as Effective Antimicrobials

Metal-organic framework (MOF)-polymer mixed-matrix membranes (MMMs) have shown their superior performance in gas separation. However, their biological application has not been well-explored yet. Herein, a series of zirconium-based MOF MMMs with high MOF loading and homogeneous composition have been prepared through a facile drawdown coating process. Poly(ε-caprolactone) (PCL) has been selected as a binder for its good biocompatibility and biodegradability. Zr-MOF nanoparticles, UiO-66, and MOF-525, have been utilized as "filler" because of their superior chemical stability, good biological safety, and versatile functions. Both UiO-66/PCL MMMs and MOF-525/PCL MMMs have a uniform appearance even at the highest loading of 50 wt % for UiO-66 and 30 wt % for MOF-525, respectively. The integrity of pore structures of UiO-66 within MMMs maintains well, which is evidenced by dye separation. All obtained MMMs possess good biocompatibility and mechanical property. Upon irradiation, MOF-525/PCL MMMs generate reactive oxygen species and serve as effective antibacterial photodynamic agents against Escherichia coli. This study offers an alternative system for forming homogeneous MOF/polymer MMMs and represents the first example of exploiting hybrid MMMs for biological applications.

Diamine-Appended Mg2(dobpdc) Nanorods as Phase-Change Fillers in Mixed-Matrix Membranes for Efficient CO2/N2 Separations

Despite the availability of chemistries to tailor the pore architectures of microporous polymer membranes for chemical separations, trade-offs in permeability and selectivity with functional group manipulations nevertheless persist, which ultimately places an upper bound on membrane performance. Here we introduce a new design strategy to uncouple these attributes of the membrane. Key to our success is the incorporation of phase-change metal-organic frameworks (MOFs) into the polymer matrix, which can be used to increase the solubility of a specific gas in the membrane, and thereby its permeability. We further show that it is necessary to scale the size of the phase-change MOF to nanoscopic dimensions, in order to take advantage of this effect in a gas separation. Our observation of an increase in solubility and permeability of only one of the gases during steady-state permeability measurements suggests fast exchange between free and chemisorbed gas molecules within the MOF pores. While the kinetics of this exchange in phase-change MOFs are not yet fully understood, their role in enhancing the efficacy and efficiency of the separation is clearly a compelling new direction for membrane technology.

Novel ZIF-300 Mixed-Matrix Membranes for Efficient CO2 Capture

Because of the high separation performance and easy preparation, mixed-matrix membranes (MMMs) consisting of metal-organic frameworks have received much attention. In this article, we report a novel ZIF-300/PEBA MMM consisting of zeolite imidazolate framework (ZIF-300) crystals and polyether block amide (PEBA) matrix. The ZIF-300 crystal size was effectively reduced by optimizing the hydrothermal reaction condition from ∼15 to ∼1 μm. The morphology and physicochemical and sorption properties of the synthesized ZIF-300 crystals and as-prepared ZIF-300/PEBA MMMs were systematically studied. The results showed that ZIF-300 crystals with a size of ∼1 μm maintained excellent preferential CO₂ sorption over N₂ without degradation of the crystal structure in the MMMs. As a result, uniformly incorporated ZIF-300 crystals highly enhanced both the CO₂ permeability and the CO₂/N₂ selectivity of pure PEBA membrane. The optimized ZIF-300-PEBA MMMs with a ZIF-300 loading of 30 wt % exhibited a high and stable CO₂ permeability of 83 Barrer and CO₂/N₂ selectivity of 84, which are 59.2% and 53.5% higher than pure PEBA membrane, respectively. The obtained performance surpassed the upper bound of state-of-the-art membranes for CO₂/N₂ separation. This work demonstrated that the proposed ZIF-300/PEBA MMM could be a potential candidate for an efficient CO₂ capture process.

Tuning the Interplay between Selectivity and Permeability of ZIF-7 Mixed Matrix Membranes

Nanoparticles of zeolitic imidazolate framework-7 (nZIF-7) were blended with poly(ether imide) (PEI) to fabricate a new mixed-matrix membrane (nZIF-7/PEI). nZIF-7 was chosen in order to demonstrate the power of postsynthetic modification (PSM) by linker exchange of benzimidazolate to benzotriazolate for tuning the permeability and selectivity properties of a resulting membrane (PSM-nZIF-7/PEI). These two new membranes were subjected to constant volume, variable pressure gas permeation measurements (H₂, N₂, O₂, CH₄, CO₂, C₂H₆, and C₃H₈), in which unique gas separation behavior was observed when compared to the pure PEI membrane. Specifically, the nZIF-7/PEI membrane exhibited the highest selectivities for CO₂/CH₄, CO₂/C₂H₆, and CO₂/C₃H₈ gas pairs. Furthermore, PSM-nZIF-7/PEI membrane displayed the highest permeabilities, which resulted in H₂/CH₄, N₂/CH₄, and H₂/CO₂ permselectivities that are remarkably well-positioned on the Robeson upper bound curves, thus, indicating its potential applicability for use in practical gas purifications.

Two-Dimensional Materials as Prospective Scaffolds for Mixed-Matrix Membrane-Based CO2 Separation

Membrane-based CO₂ separation technology plays a significant role in environmental remediation and clean energy. Two-dimensional (2D) materials with atomically precise structures have emerged as prospective scaffolds to develop mixed-matrix membranes (MMMs) for gas separation. Summarized in this perspective review are the latest breakthrough studies in the synthesis of 2D-material-based MMMs to separate CO₂ from gas mixtures. 2D materials including graphene oxide (GO), metal-organic framework (MOF)-derived nanosheets, covalent organic frameworks (COFs), and transition metal dichalcogenides (TMDs), as fascinating building blocks, have been comprehensively summarized, together with a focus on synthetic processes and gas separation properties. Challenges and the latest advances in the manufacture of novel synthetic 2D materials are briefly discussed to foresee emerging opportunities for the development of new generations of 2D-material-based MMMs.

Ultraselective Carbon Molecular Sieve Membranes with Tailored Synergistic Sorption Selective Properties

Membrane-based separations can reduce the energy consumption and the CO₂ footprint of large-scale fluid separations, which are traditionally practiced by energy-intensive thermally driven processes. Here, a new type of membrane structure based on nanoporous carbon is reported, which, according to this study, is best referred to as carbon/carbon mixed-matrix (CCMM) membranes. The CCMM membranes are formed by high-temperature (up to 900 °C) pyrolysis of polyimide precursor hollow-fiber membranes. Unprecedentedly high permselectivities are seen in CCMM membranes for CO₂ /CH₄ , N₂ /CH₄ , He/CH₄ , and H₂ /CH₄ separations. Analysis of permeation data suggests that the ultrahigh selectivities result from substantially increased sorption selectivities, which is hypothetically owing to the formation of ultraselective micropores that selectively exclude the bulkier CH₄ molecules. With tunable sorption selectivities, the CCMM membranes outperform flexible polymer membranes and traditional rigid molecular-sieve membranes. The capability to increase sorption selectivities is a powerful tool to leverage diffusion selectivities, and has opened the door to many challenging and economically important fluid separations that require ultrafine differentiation of closely sized molecules.

Mixed-Matrix Membranes

Research into extended porous materials such as metal-organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal-organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high-performance mixed-matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti-aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state-of-the-art in MOF-MMM fabrication, and the more recent use of POFs and molecular additives.

Membrane-Based Gas Separation Accelerated by Hollow Nanosphere Architectures

The coupling of hollow carbon nanospheres with triblock copolymers is a promising strategy to fabricate mixed-matrix membranes. This is because the symmetric microporous shells combine with the hollow space to promote gas transport, and the unique soft-rigid molecular structure of triblock copolymers can accommodate a high loading of fillers without a significant loss of mechanical strength.

Metal Organic Framework Crystals in Mixed-Matrix Membranes: Impact of the Filler Morphology on the Gas Separation Performance

Mixed-matrix membranes (MMMs) comprising NH2-MIL-53(Al) and Matrimid® or 6FDA-DAM have been investigated. The MOF loading has been varied between 5 and 20 wt%, while NH2-MIL-53(Al) with three different morphologies: nanoparticles, nanorods and microneedles have been dispersed in Matrimid®. The synthesized membranes have been tested in the separation of CO2 from CH4 in an equimolar mixture. At 3 bar and 298 K for 8 wt% MOF loading, incorporation of NH2-MIL-53(Al) nanoparticles leads to the largest improvement compared to nanorods and microneedles. The incorporation of the best performing filler, i.e. NH2-MIL-53(Al) nanoparticles, to the highly permeable 6FDA-DAM has a larger effect, and the CO2 permeability increased up to 85 % with slightly lower selectivities for 20 wt% MOF loading. Specifically, these membranes have a permeability of 660 Barrer with CO2/CH4 separation factor of 28, leading to a performance very close to the Robeson limit of 2008. Furthermore, a new non-destructive technique based on Raman spectroscopy mapping is introduced to assess the homogeneity of the filler dispersion in the polymer matrix. The MOF contribution can be calculated by modelling the spectra. The determined homogeneity of the MOF filler distribution in the polymer is confirmed by FIB-SEM analysis.

Hybrid and Mixed Matrix Membranes for Separations from Fermentations

Fermentations provide an alternative to fossil fuels for accessing a number of biofuel and chemical products from a variety of renewable and waste substrates. The recovery of these dilute fermentation products from the broth, however, can be incredibly energy intensive as a distillation process is generally involved and creates a barrier to commercialization. Membrane processes can provide a low energy aid/alternative for recovering these dilute fermentation products and reduce production costs. For these types of separations many current polymeric and inorganic membranes suffer from poor selectivity and high cost respectively. This paper reviews work in the production of novel mixed-matrix membranes (MMMs) for fermentative separations and those applicable to these separations. These membranes combine a trade-off of low-cost and processability of polymer membranes with the high selectivity of inorganic membranes. Work within the fields of nanofiltration, reverse osmosis and pervaporation has been discussed. The review shows that MMMs are currently providing some of the most high-performing membranes for these separations, with three areas for improvement identified: Further characterization and optimization of inorganic phase(s), Greater understanding of the compatibility between the polymer and inorganic phase(s), Improved methods for homogeneously dispersing the inorganic phase.

Microscopic Model of the Metal-Organic Framework/Polymer Interface: A First Step toward Understanding the Compatibility in Mixed Matrix Membranes

An innovative computational methodology integrating density functional theory calculations and force field-based molecular dynamics simulations was developed to provide a first microscopic model of the interactions at the metal-organic framework (MOF) surface/polymer interface. This was applied to the case of the composite formed by the polymer of intrinsic microporosity, PIM-1, and the zeolitic imidazolate framework, ZIF-8, as a model system. We found that the structure of the composite at the interface is the result of both the chemical affinity between PIM-1 and ZIF-8 and the rigidity of the polymer. Specifically, there is a preferential interaction between the -CN groups of PIM-1 and the NH terminal functions of the organic linker at the ZIF-8 surface. Additionally, the resulting conformation of the polymer gives rise to interfacial microvoids at the vicinity of the MOF surface. The porosity, rigidity, and density of the interfacial polymer were analyzed and compared to those for the bulk polymer. It was shown that the polymer still feels the impact of the MOF surface even at long distances above 15-20 Å. Further, both the polydispersity of the polymer and the flexibility of the MOF surface were revealed to only slightly affect the properties of the MOF/interface. This work, which delivers a microscopic picture of the MOF surface/polymer interactions at the interface, would lead, in turn, to the understanding of the compatibility in MOF-based mixed-matrix membranes.

Ultra-thin Solid-State Li-Ion Electrolyte Membrane Facilitated by a Self-Healing Polymer Matrix

Thin solid membranes are formed by a new strategy, whereby an in situ derived self-healing polymer matrix that penetrates the void space of an inorganic solid is created. The concept is applied as a separator in an all-solid-state battery with an FeS2 -based cathode and achieves tremendous performance for over 200 cycles. Processing in dry conditions represents a paradigm shift for incorporating high active-material mass loadings into mixed-matrix membranes.

Modulated UiO-66-Based Mixed-Matrix Membranes for CO2 Separation

Mixed-matrix membranes (MMMs) composed of polyimide (PI) and metal-organic frameworks (MOFs) were synthesized using Matrimid as the polymer and zirconium terephthalate UiO-66 as the filler. The modulation approach, combined with the use of amine-functionalized linkers, was used for synthesis of the MOF fillers in order to enhance the intrinsic separation performance of the MOF and improve the particle-PI compatibility. The presence of amine groups on the MOF outer surface introduced either through the linker, through the modulator, or through both led to covalent linking between the fillers and Matrimid, which resulted in very stable membranes. In addition, the presence of amine groups inside the pores of the MOFs and the presence of linker vacancies inside the MOFs positively influenced CO2 transport. MMMs with 30 wt % loading showed excellent separation performance for CO2/CH4 mixtures. A significant increase in the mixed-gas selectivity (47.7) and permeability (19.4 barrer) compared to the unfilled Matrimid membrane (i.e., 50% more selective and 540% more permeable) was thus achieved for the MMM containing the MOF prepared from 2-aminoterephthalic acid and 4-aminobenzoic acid, respectively used as the linker and as the modulator.

Mixed-Matrix Membranes with Metal-Organic Framework-Decorated CNT Fillers for Efficient CO2 Separation

Carbon nanotube (CNT) mixed-matrix membranes (MMMs) show great potential to achieve superior gas permeance because of the unique structure of CNTs. However, the challenges of CNT dispersion in polymer matrix and elimination of interfacial defects are still hindering MMMs to be prepared for high gas selectivity. A novel CNT/metal-organic framework (MOF) composite derived from the growth of NH2-MIL-101(Al) on the surface of CNTs have been synthesized and applied to fabricate polyimide-based MMMs. Extra amino groups and active sites were introduced to external surface of CNTs after MOF decoration. The good adhesion between the synthesized CNT-MIL fillers and polymer phase was observed, even at a high filler loadings up to 15%. Consequently, MMMs containing the synthesized MOF/CNT composite exhibit not only a large CO2 permeability but also a high CO2/CH4 selectivity; the combined performance of permeability and selectivity is even above the Robeson upper bound. The strategy of growing MOFs on CNTs can be further utilized to develop a more effective approach to further improve MMM performance through the decoration of MOFs on existing fillers that have high selectivity to specific gas.

Simultaneous spray self-assembly of highly loaded ZIF-8-PDMS nanohybrid membranes exhibiting exceptionally high biobutanol-permselective pervaporation

The ability to obtain a maximum loading of inorganic nanoparticles while maintaining uniform dispersion in the polymer is the key to the fabrication of mixed-matrix membranes with high pervaporation performance in bioalcohol recovery from aqueous solution. Herein, we report the simultaneous spray self-assembly of a zeolitic imidazolate framework (ZIF)-polymer suspension and a cross-linker/catalyst solution as a method for the fabrication of a well-dispersed ZIF-8-PDMS nanohybrid membrane with an extremely high loading. The ZIF-8-PDMS membrane showed excellent biobutanol-permselective pervaporation performance. When the ZIF-8 loading was increased to 40 wt%, the total flux and separation factor could reach 4846.2 g m(-2) h(-1) and 81.6, respectively, in the recovery of n-butanol from 1.0 wt% aqueous solution (80 °C). This new method is expected to have serious implications for the preparation of defect-free mixed-matrix membranes for many applications.