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

Cross-Linked Metathesis Polynorbornenes Based on Nadimides Bearing Hydrocarbon Substituents: Synthesis and Physicochemical Properties

Metathesis homo- and copolymerization of bifunctional monomers bearing two norbornene moieties was studied. The monomers were synthesized from cis-5-norbornene-exo-2,3-dicarboxylic anhydride and various diamines (hexamethylenediamine, decamethylenediamine, 1R,3S-isophoronediamine). The metathesis homopolymerization of these bis(nadimides) in the presence of the second-generation Grubbs catalyst afforded glassy cross-linked polymers in more than 90% yields. The metathesis copolymerization of the bis(nadimides) and a monofunctional norbornene derivative containing the β-pinene fragment also resulted in insoluble cross-linked polymers in nearly quantitative yields. The structures and purity of the synthesized polymers were confirmed via IR spectroscopy and CP/MAS NMR spectroscopy. Conditions for the fabrication of mechanically strong solution-cast thin films based on copolymers synthesized from the comonomers mentioned above were determined by varying the content of the cross-linking agent. It was shown that the films made in this way are stable in a range of organic solvents and could be useful as semipermeable or membrane materials for use in liquid organic media. The permeability of the polymer films in question to 1-phenylethanol and mandelic acid was studied. The results obtained are discussed along with the data from the DSC, TGA, and powder X-ray diffraction studies of the properties of the synthesized metathesis homo- and copolymers.

Supramolecular Complexation Reinforced Polymer Frustrated Packing: Controllable Dual Porosity for Improved Permselectivity of Coordination Nanocage Mixed Matrix Membranes

The developments of mixed matrix membranes (MMMs) are severely hindered by the complex inter-phase interaction and the resulting poor utilization of inorganics' microporosity. Herein, a dual porosity framework is constructed in MMMs to enhance the accessibility of inorganics' microporosity to external gas molecules for the effective application of microporosity for gas separation. Nanocomposite organogels are first prepared from the supramolecular complexation of rigid polymers and 2 nm microporous coordination nanocages (CNCs). The network structures can be maintained with microporous features after solvent removal originated from the rigid nature of polymers, and the strong coordination and hydrogen bond between the two components. Moreover, the strong supramolecular attraction reinforces the frustrated packing of the rigid polymers on CNC surface, leading to polymer networks' extrinsic pores and the interconnection of CNCs' micro-cavities for the fast gas transportation. The gas permeabilities of the MMMs are 869 times for H2 and 1099 times for CO2 higher than those of pure polymers. The open metal sites from nanocage also contribute to the enhanced gas selectivity and the overall performance surpasses 2008 H2/CO2 Robeson upper bound. The supramolecular complexation reinforced packing frustration strategy offers a simple and practical solution to achieve improved gas permselectivity in MMMs.

Tailoring the Structure-Property Relationship of Ring-Opened Metathesis Copolymers for CO2-Selective Membranes

In this work, we explore the use of ring-opening metathesis polymerization (ROMP) facilitated by a second-generation Grubbs catalyst (G2) for the development of advanced polymer membranes aimed at CO2 separation. By employing a novel copolymer blend incorporating 4,4'-oxidianiline (ODA), 1,6-hexanediamine (HDA), 1-adamantylamine (AA), and 3,6,9-trioxaundecylamine (TA), along with a CO2-selective poly(ethylene glycol)/poly(propylene glycol) copolymer (Jeffamine2003) and polydimethylsiloxane (PDMS) units, we have synthesized membranes under ambient conditions with exceptional CO2 separation capabilities. The strategic inclusion of PDMS, up to a 20% composition within the PEG/PPG matrix, has resulted in copolymer membranes that not only surpass the 2008 upper limit for CO2/N2 separation but also meet the commercial targets for CO2/H2 separation. Comprehensive analysis reveals that these membranes adhere to the mixing rule and exhibit percolation behavior across the entire range of compositions (0-100%), maintaining robust antiplasticization performance even under pressures up to 20 atm. Our findings underscore the potential of ROMP in creating precisely engineered membranes for efficient CO2 separation, paving the way for their application in large-scale environmental and industrial processes.

Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N'-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2-3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility.

Developing Mixed Matrix Membranes with Good CO2 Separation Performance Based on PEG-Modified UiO-66 MOF and 6FDA-Durene Polyimide

The use of mixed matrix membranes (MMMs) comprising metal-organic frameworks (MOFs) for the separation of CO2 from flue gas has gained recognition as an effective strategy for enhancing gas separation efficiency. When incorporating porous materials like MOFs into a polymeric matrix to create MMMs, the combined characteristics of each constituent typically manifest. Nevertheless, the inadequate dispersion of an inorganic MOF filler within an organic polymer matrix can compromise the compatibility between the filler and matrix. In this context, the aspiration is to develop an MMM that not only exhibits optimal interfacial compatibility between the polymer and filler but also delivers superior gas separation performance, specifically in the efficient extraction of CO2 from flue gas. In this study, we introduce a modification technique involving the grafting of poly(ethylene glycol) diglycidyl ether (PEGDE) onto a UiO-66-NH2 MOF filler (referred to as PEG-MOF), aimed at enhancing its compatibility with the 6FDA-durene matrix. Moreover, the inherent CO2-philic nature of PEGDE is anticipated to enhance the selectivity of CO2 over N2 and CH4. The resultant MMM, incorporating 10 wt% of PEG-MOF loading, exhibits a CO2 permeability of 1671.00 Barrer and a CO2/CH4 selectivity of 22.40. Notably, these values surpass the upper bound reported by Robeson in 2008.

Covalent connections between metal-organic frameworks and polymers including covalent organic frameworks

Hybrid composite materials combining metal-organic frameworks (MOFs) and polymers have emerged as a versatile platform for a broad range of applications. The crystalline, porous nature of MOFs and the flexibility and processability of polymers are synergistically integrated in MOF-polymer composite materials. Covalent bonds, which form between two distinct materials, have been extensively studied as a means of creating strong molecular connections to facilitate the dispersion of "hard" MOF particles in "soft" polymers. Numerous organic transformations have been applied to post-synthetically connect MOFs with polymeric species, resulting in a variety of covalently connected MOF-polymer systems with unique properties that are dependent on the characteristics of the MOFs, polymers, and connection modes. In this review, we provide a comprehensive overview of the development and strategies involved in preparing covalently connected MOFs and polymers, including recently developed MOF-covalent organic framework composites. The covalent bonds, grafting strategies, types of MOFs, and polymer backbones are summarized and categorized, along with their respective applications. We highlight how this knowledge can serve as a basis for preparing macromolecular composites with advanced functionality.

Chemically routed interpore molecular diffusion in metal-organic framework thin films

Transport diffusivity of molecules in a porous solid is constricted by the rate at which molecules move from one pore to the other, along the concentration gradient, i.e. by following Fickian diffusion. In heterogeneous porous materials, i.e. in the presence of pores of different sizes and chemical environments, diffusion rate and directionality remain tricky to estimate and adjust. In such a porous system, we have realized that molecular diffusion direction can be orthogonal to the concentration gradient. To experimentally determine this complex diffusion rate dependency and get insight of the microscopic diffusion pathway, we have designed a model nanoporous structure, metal-organic framework (MOF). In this model two chemically and geometrically distinct pore windows are spatially oriented by an epitaxial, layer-by-layer growth method. The specific design of the nanoporous channels and quantitative mass uptake rate measurements have indicated that the mass uptake is governed by the interpore diffusion along the direction orthogonal to the concentration gradient. This revelation allows chemically carving the nanopores, and accelerating the interpore diffusion and kinetic diffusion selectivity.

Uncertainty in Composite Membranes: From Defect Engineering to Film Processing

Composite membranes featuring metal-organic framework (MOF)-dispersed polymers have attracted tremendous attention in recent years. However, evaluating commercial viability is oftentimes obscured by the irreproducibility in both MOF synthesis and film manufacturing protocols. Variability in MOF property sets are typically ascribed to crystal defects resulting from subtle variations in synthesis, but quantitative studies investigating the role of defects on transport properties are exceedingly rare. Likewise, controlled film formation protocols are rarely reported in the open literature, making it difficult to provide substantial and informative structure-property correlations. This study aims to address these uncertainties. To this end, two samples of a prototypical MOF, UiO-66-NH₂, were synthesized to feature similar particle size, morphology, and colloidal stability. However, defect engineering protocols coupled with careful screening experiments were developed to synthesize the two MOFs with maximally different porosities. Composite membranes were prepared for each MOF and a high-performance polymer, 6FDA-Durene, and then tested for light gas permeation measurements, revealing a small and unexpected enhancement in CO₂/CH₄ performance for samples containing low-porosity UiO-66-NH₂. Mechanistic studies on sorption revealed a surprising 50% decrease in sorption capacity for high-porosity UiO-66-NH₂, completely offsetting enhancements from increased gas diffusion. By using multiple replicate experiments, the sample-to-sample variation was large enough to obscure any differences in permeability and selectivity between the two types of MOF composites at low volume fractions. Application of the Maxwell model to extrapolate pure-MOF performance led to significant variations in predicted values, demonstrating the importance of collecting and reporting replicate experiments for membrane preparation and testing.

Self-Assembled CMC/UiO-66-NH2 Membrane with Anti-Crude Oil Adhesion Property for Highly Efficient Oil-Water Separation

Developing the high-anti-fouling membrane has kept continuous attention in oil/water emulsion treatment. However, the majority of works on anti-fouling membranes mainly focused on low-viscosity oils, which greatly limited the development and application of a membrane to face the real crude oil wastewater. Inspired by the hydrophilicity of sodium carboxymethyl cellulose (CMC) and zirconium base metal-organic frame (Zr-MOF), an anti-oil-fouling CMC/UiO-66-NH₂ composite membrane was constructed by a self-assembly method. Profiting from the hydrophilicity and micro-nanostructure of the CMC/UiO-66-NH₂ layer, the obtained CMC/UiO-66-NH₂ membranes displayed underwater superoleophobicity and desired oil resistance. It could display the effective separation capability with 1282 ± 62 to 6160 ± 81 L/(m²·h·bar) and above 99.08% toward the different light oil emulsions. More importantly, the CMC/UiO-66-NH₂ membrane displayed ultralow crude oil adhesion behaviors toward the crude oil emulsions, which could achieve a considerably high flux (746 ± 60 to 5224 ± 87 L/(m²·h·bar)). Furthermore, electrostatic interaction and physical enwinding-wrapping between CMC and UiO-66-NH₂ also endowed the composite membranes with outstanding stability. After immersing the as-prepared membranes into the harsh environments for 24 h, the membranes still maintained high underwater-oil contact angles (UWOCA > 155°) and separation ability (oil rejection was above 99.0%). Therefore, CMC/UiO-66-NH₂ composite membranes could demonstrate promising prospects in real oily emulsion treatment.

Metal-organic frameworks and covalent organic frameworks as disruptive membrane materials for energy-efficient gas separation

In this Review we survey the molecular sieving behaviour of metal-organic framework (MOF) and covalent organic framework (COF) membranes, which is different from that of classical zeolite membranes. The nature of MOFs as inorganic-organic hybrid materials and COFs as purely organic materials is powerful and disruptive for the field of gas separation membranes. The possibility of growing neat MOFs and COFs on membrane supports, while also allowing successful blending into polymer-filler composites, has a huge advantage over classical zeolite molecular sieves. MOFs and COFs allow synthetic access to more than 100,000 different structures and tailor-made molecular gates. Additionally, soft evacuation below 100 °C is often enough to achieve pore activation. Therefore, a huge number of synthetic methods for supported MOF and COF membrane thin films, such as solvothermal synthesis, seed-mediated growth and counterdiffusion, exist. Among them, methods with high scale-up potential, for example, layer-by-layer dip- and spray-coating, chemical and physical vapour deposition, and electrochemical methods. Additionally, physical methods have been developed that involve external stimuli, such as electric fields and light. A particularly important point is their ability to react to stimuli, which has allowed the 'drawbacks' of the non-ideality of the molecular sieving properties to be exploited in a completely novel research direction. Controllable gas transport through membrane films is a next-level property of MOFs and COFs, leading towards adaptive process deviation. MOF and COF particles are highly compatible with polymers, which allows for mixed-matrix membranes. However, these membranes are not simple MOF-polymer blends, as they require improved polymer-filler interactions, such as cross-linking or surface functionalization.

n-Octyltrichlorosilane Modified SAPO-34/PDMS Mixed Matrix Membranes for Propane/Nitrogen Mixture Separation

In this study, zeolite molecular sieve SAPO-34/polydimethylsiloxane (PDMS) mixed matrix membranes (MMMs) were prepared to recover propane. n-Octyltrichlorosilane (OTCS) was introduced to improve compatibility between SAPO-34 and PDMS, and enhance the separation performance of the MMMs. Physicochemical properties of the MMMs were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and water contact angle (WCA). Results showed that, after modification, alkyl chains were successfully grafted onto SAPO-34 without changing its crystal structure, particles in the MMMs were evenly distributed in the base film, and the hydrophobicity of the MMMs was enhanced. Moreover, the effects of SAPO-34 filling content, operating pressure, and feed gas concentration on the separation performance was explored. This indicated that the modification with OTCS effectively enhanced the separation performance of SAPO-34/PDMS MMMs. When the filling content of modified SAPO-34 was 15%, the maximal separation factor of 22.1 was achieved, and the corresponding propane permeation rate was 101 GPU.

Mixed Matrix Membranes for Efficient CO2 Separation Using an Engineered UiO-66 MOF in a Pebax Polymer

Mixed matrix membranes (MMMs) have attracted significant attention for overcoming the limitations of traditional polymeric membranes for gas separation through the improvement of both permeability and selectivity. However, the development of defect-free MMMs remains challenging due to the poor compatibility of the metal–organic framework (MOF) with the polymer matrix. Thus, we report a surface-modification strategy for a MOF through grafting of a polymer with intrinsic microporosity onto the surface of UiO-66-NH₂. This method allows us to engineer the MOF–polymer interface in the MMMs using Pebax as a support. The insertion of a PIM structure onto the surface of UiO-66-NH₂ provides additional molecular transport channels and enhances the CO₂ transport by increasing the compatibility between the polymer and fillers for efficient gas separation. As a result, MMM with 1 wt% loading of PIM-grafted-MOF (PIM-g-MOF) exhibited very promising separation performance, with CO₂ permeability of 247 Barrer and CO₂/N₂ selectivity of 56.1, which lies on the 2008 Robeson upper bound. Moreover, this MMM has excellent anti-aging properties for up to 240 days and improved mechanical properties (yield stress of 16.08 MPa, Young’s modulus of 1.61 GPa, and 596.5% elongation at break).

PIM-PI-1 and Poly(ethylene glycol)/Poly(propylene glycol)-Based Mechanically Robust Copolyimide Membranes with High CO2-Selectivity and an Anti-aging Property: A Joint Experimental-Computational Exploration

Polymer membranes with excellent thermomechanical properties and good gas separation performance are desirable for efficient CO₂ separation. A series of copolyimide membranes are prepared for the first time using PIM-PI-1, a hard segment with high CO₂ permeability, and poly(ethylene glycol)/poly(propylene glycol) (PEG/PPG), a soft segment with high CO₂ selectivity. Two different unit polymers are combined to compensate the limitations of each polymer (e.g., the fast aging and moderate selectivity of PIM-PI-1 and the poor mechanical properties and lower permeability of PEG/PPG). The corresponding PIM-(durene-PEG/PPG) membranes exhibit an excellent combination of mechanical properties and gas separation performance compared to the typical PI-PEG-based copolymer membrane. The improved mechanical property is attributed to the unique chain threading and the reinforcement between the spiro unit of PIM and the flexible PEG/PPG at the molecular level, which has not previously been exploited for membranes. The PIM-(durene-PEG/PPG) membranes show a high CO₂ permeability of 350-669 Barrer and a high CO₂/N₂ selectivity of 33.5-40.3. The experimental results are further evaluated with theoretical results obtained from molecular simulation studies, and a very good agreement between the experimental results and simulation results is found. Moreover, the PIM-(durene-PEG/PPG) copolymer membranes display excellent anti-aging performance for up to 1 year.

Modified Graphene Oxide-Incorporated Thin-Film Composite Hollow Fiber Membranes through Interface Polymerization on Hydrophilic Substrate for CO2 Separation

Thin-film composite mixed matrix membranes (CMMMs) were fabricated using interfacial polymerization to achieve high permeance and selectivity for CO₂ separation. This study revealed the role of substrate properties on performance, which are not typically considered important. In order to enhance the affinity between the substrate and the coating solution during interfacial polymerization and increase the selectivity of CO₂, a mixture of polyethylene glycol (PEG) and dopamine (DOPA) was subjected to a spinning process. Then, the surface of the substrate was subjected to interfacial polymerization using polyethyleneimine (PEI), trimesoyl chloride (TMC), and sodium dodecyl sulfate (SDS). The effect of adding SDS as a surfactant on the structure and gas permeation properties of the fabricated membranes was examined. Thin-film composite hollow fiber membranes containing modified graphene oxide (mGO) were fabricated, and their characteristics were analyzed. The membranes exhibited very promising separation performance, with CO₂ permeance of 73 GPU and CO₂/N₂ selectivity of 60. From the design of a membrane substrate for separating CO₂, the CMMMs hollow fiber membrane was optimized using the active layer and mGO nanoparticles through interfacial polymerization.

Development of CO2-Selective Polyimide-Based Gas Separation Membranes Using Crown Ether and Polydimethylsiloxane

A series of CO2-selective polyimides (CE-PDMS-PI-x) was synthesized by copolymerizing crown ether diamine (trans-diamino-DB18C6) and PDMS-diamine with 4,4'-(hexafluoroisopropylidene) di-phthalic anhydride (6FDA) through the polycondensation reaction. The structural characteristics of the copolymers and corresponding membranes were characterized by nuclear magnetic resonance (NMR), infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and gel permeation chromatography (GPC). The effect of PDMS loading on the CE-PDMS-PI-x copolymers was further analyzed and a very good structure-property relationship was found. A well-distributed soft PDMS unit played a key role in the membrane's morphology, in which improved CO2-separation performance was observed at a low PDMS content (5 wt %). In contrast, the fine-grained phase separation adversely affected the separation behavior at a certain level of PDMS loading, and the PDMS was found to provide a flexible gas-diffusion path, affecting only the permeability without changing the selective gas-separation performance for the copolymers with a PDMS content of 20% or above.

Interfacial Design of Mixed Matrix Membranes via Grafting PVA on UiO-66-NH2 to Enhance the Gas Separation Performance

In this study, defect-free facilitated transport mixed matrix membrane (MMM) with high loading amount of UiO-66-NH2 nanoparticles as metal-organic frameworks (MOFs) was fabricated. The MOFs were covalently bonded with poly (vinyl alcohol) (PVA) to incorporate into a poly (vinyl amine) (PVAm) matrix solution. A uniform UiO-66-NH2 dispersion up to 55 wt.% was observed without precipitation and agglomeration after one month. This can be attributed to the high covalent interaction at interfaces of UiO-66-NH2 and PVAm, which was provided by PVA as a functionalized organic linker. The CO2 permeability and CO2/N2 and selectivity were significantly enhanced for the fabricated MMM by using optimal fabrication parameters. This improvement in gas performance is due to the strong impact of solubility and decreasing diffusion in obtained dense membrane to promote CO2 transport with a bicarbonate reversible reaction. Therefore, the highest amount of amine functional groups of PVAm among all polymers, plus the abundant amount of amines from UiO-66-NH2, facilitated the preferential CO2 permeation through the bicarbonate reversible reaction between CO2 and -NH2 in humidified conditions. XRD and FTIR were employed to study the MMM chemical structure and polymers-MOF particle interactions. Cross-sectional and surface morphology of the MMM was observed by SEM-EDX and 3D optical profilometer to detect the dispersion of MOFs into the polymer matrix and explore their interfacial morphology. This approach can be extended for a variety of polymer-filler interfacial designs for gas separation applications.

Substituted polynorbornene membranes: a modular template for targeted gas separations

This perspective focuses on substituted polynorbornenes as a promising modular platform to access advanced gas separation membranes, and highlights their synthetic versatility and robust performance.