MOF-based Mixed Matrix Membranes Research Articles

Thermally Annealed High-Aspect-Ratio ZIF-8 Nanoplates-Incorporated Mixed Matrix Membranes for Improved H2/CO2 Selectivity.

The main challenge in the preparation of MOF-based mixed matrix membranes is to construct a good interface morphology to improve the gas separation performance and stability of the membranes. Herein, high-aspect-ratio ZIF-8 nanoplates for H2/CO2 separation membranes were synthesized by direct template conversion. The ZIF-8 nanoplates were prepared with the commercial Matrimid polymer to form MMMs by the flat scraping method. The homogeneous dispersion of high-aspect-ratio nanoplates in the membrane and the good compatibility between the filler and the matrix caused by the thermal annealing operation improve the gas separation performance and mechanical properties of MMMs. The H2/CO2 selectivity of MMMs loaded with 30 wt % ZIF-8 nanoplates increased to 10.3, and the H2 permeability was 330.1 Barrer. This synthesis method can be extended to prepare various ZIF nanoplates with elevated aspect ratios to obtain excellent performance fillers for gas separation of MMMs. In addition, the thermal annealing operation allows more efficient gas separation in polymer membranes and is a feasible way to design excellent and stable MMMs.

A Zn-MOF-based mixed matrix membrane as an ultrastable luminescent sensor for selective and visual detection of antibiotics and pesticides in food samples

The detection of antibiotics and pesticides are of great significance since their residues threaten the health of human beings by accumulation. However, most traditional solid chemical sensors are suffer from the limitations of low sensitivity and economic practicability because of the aggregating nature and unstable of solid sensors. Herein, a new luminescent sensor 1@PMMA (1, [(ZnL)·H2O]n (H2L = 5-(4-(pyridin-4-yl)benzamido)benzene-1,3-dioic acid); PMMA = poly(methyl methacrylate)) was successfully prepared. Notably, the polymer matrix provided the chemical protection for MOF particles. The as fabricated 1@PMMA was stable in milk, honey and egg as well as exhibited strong blue emission under ultraviolet light irradiation, which can act as luminescent probe for detecting antibiotics and pesticides. More interestingly, 1@PMMA exhibited visual, real-time and recyclable detection of antibiotics ornidazole (ODZ) and pesticides 2,6-dichloro-4-nitrobenzenamine (DCN) in real food samples. This work shows that the luminescent MOF-based mixed matrix membranes could be applied as good candidates for sensing analytes in practical application.

Self-Sorting of Interfacial Compatibility in MOF-Based Mixed Matrix Membranes.

Metal-organic framework (MOF)-based mixed matrix membranes (MMMs) have shown great promises to overcome the performance upper limit of polymeric membranes for various gas separation processes. However, the gas separation properties of the MMMs largely depend on the MOF-polymer interfacial compatibility which is a metric difficult to quantify. In most cases, whether a MOF filler and a polymer matrix make a good pair is not revealed until the gas transport experiments are performed. This is because there is a lack of characterization techniques to directly probe the MOF-polymer interfacial compatibility. In this work, we demonstrate a self-sorting method to rank the interface compatibility among several MOF-polymer pairs. By mixing one MOF with two polymers in an MMM, the demixing of two polymers will form two polymer domains. The MOF particles will preferably partition into the "preferred" polymer domain due to their higher interfacial affinity. By scanning different polymer pairs, a rank of MOF-polymer interfacial compatibility from high to low can be obtained. Moreover, based on this ranking, it was also found that a highly compatible MOF-polymer pair suggested by this method also corresponds to a more predictable MMM gas separation performance.

Recent advances in the interfacial engineering of MOF-based mixed matrix membranes for gas separation.

The membrane process stands as a promising and transformative technology for efficient gas separation due to its high energy efficiency, operational simplicity, low environmental impact, and easy up-and-down scaling. Metal-organic framework (MOF)-polymer mixed matrix membranes (MMMs) combine MOFs' superior gas-separation performance with polymers' processing versatility, offering the opportunity to address the limitations of pure polymer or inorganic membranes for large-scale integration. However, the incompatibility between the rigid MOFs and flexible polymer chains poses a challenge in MOF MMM fabrication, which can cause issues such as MOF agglomeration, sedimentation, and interfacial defects, substantially weakening membrane separation efficiency and mechanical properties, particularly gas separation. This review focuses on engineering MMMs' interfaces, detailing recent strategies for reducing interfacial defects, improving MOF dispersion, and enhancing MOF loading. Advanced characterisation techniques for understanding membrane properties, specifically the MOF-polymer interface, are outlined. Lastly, it explores the remaining challenges in MMM research and outlines potential future research directions.

Accurately predicting the performance of MOF-based mixed matrix membranes for CO2 removal using a novel optimized extreme learning machine by BAT algorithm

The increasing number of mixed matrix membranes (MMMs) based on metal organic frameworks (MOFs) for carbon capture has created a demand for accurate and swift evaluation of their separation performance. Machine learning (ML) has emerged as a valuable tool for this purpose, providing an efficient approach for screening these materials and accelerating their practical application. In this study, we developed and optimized a novel and reliable hybrid machine learning paradigm based on the extreme learning machines (ELM) method using the BAT algorithm optimization. In order to predict the performance of MMMs, including gas permeability and selectivity parameters, nine machine learning models were developed by incorporating descriptors and fingerprints for polymer featurization, physical and structural features of MOFs as well as operating conditions. The impact of input features on MMMs performance was also explored using RReliefF analysis. The study found that the performance of the hybrid ELM-based algorithms was significantly improved by using the BAT algorithm. Furthermore, the RReliefF analysis revealed that the cage size of the MOF and the type of polymer matrix used are the most significant parameters in forecasting the permeability of MOF-based MMMs, while the loading amount and pressure were identified as critical determinants of selectivity. Overall, these findings contribute to the development of more efficient and accurate methods for evaluating MMMs, which are crucial for carbon capture applications.

Influence of polymer modification on intra-MOF self-diffusion in MOF-based mixed matrix membranes

Mixed-matrix membranes (MMMs) represent a promising membrane type for gas and liquid separations. Such membranes can be formed by dispersing crystals of metal–organic frameworks (MOFs) in polymers. For liquid separations, crosslinked polymers are desirable because crosslinking reduces polymer dilation and plasticization effects by liquid sorbates. However, polymer crosslinking processes can unfavorably change transport and related structural properties of MOF fillers. In this work, 13C pulsed field gradient (PFG) NMR was used to investigate possible changes in intra-MOF self-diffusion of p-xylene and o-xylene after MMM polymer crosslinking. The studied MMMs were formed by dispersing MOF crystals of the type ZIF-71 in Torlon (a poly(amide-imide)) or Matrimid (a polyimide) polymers. The reported PFG NMR data indicate that the polymer crosslinking process used does not influence the intra-ZIF diffusivities when all effects on these diffusivities that are related to diffusing molecules crossing over the crystal boundaries or reflected away from these boundaries are removed. In contrast, these data show that the crosslinking process resulted in a significant decrease of the effective size of ZIF-71 crystals inside the studied MMMs, especially in the ZIF-71/Matrimid MMM. This crystal size decrease is attributed to a partial degradation of the ZIF-71 crystals inside the MMMs due to the crosslinking process. This conclusion was found to be in agreement with the results of electron microscopy analysis. PFG NMR studies and data analysis similar to those presented here can be used for quantifying any types of MOF crystal degradation inside MOF-based MMMs.

Review of Synthesis and Separation Application of Metal-Organic Framework-Based Mixed-Matrix Membranes.

Metal-organic frameworks (MOFs) are porous crystalline materials assembled from organic ligands and metallic secondary building blocks. Their special structural composition gives them the advantages of high porosity, high specific surface area, adjustable pore size, and good stability. MOF membranes and MOF-based mixed-matrix membranes prepared from MOF crystals have ultra-high porosity, uniform pore size, excellent adsorption properties, high selectivity, and high throughput, which contribute to their being widely used in separation fields. This review summarizes the synthesis methods of MOF membranes, including in situ growth, secondary growth, and electrochemical methods. Mixed-matrix membranes composed of Zeolite Imidazolate Frameworks (ZIF), University of Oslo (UIO), and Materials of Institute Lavoisier (MIL) frameworks are introduced. In addition, the main applications of MOF membranes in lithium-sulfur battery separators, wastewater purification, seawater desalination, and gas separation are reviewed. Finally, we review the development prospects of MOF membranes for the large-scale application of MOF membranes in factories.

Establishing gas transport highways in MOF-based mixed matrix membranes.

Achieving percolation pathways in a metal-organic framework (MOF)-based mixed matrix membrane (MMM) without compromising its mechanical properties is challenging. We developed phase separated (PS)-MMMs with an interconnected MOF domain running across the whole membrane. Through demixing two immiscible polyimides, the MOF particles were selectively partitioned into one of the preferred polymer domains at over 50 volume % local packing density, leading to a percolated network at only 19 weight % MOF loading. The CO2 permeability of this PS-MMM is 6.6 times that of the pure polymer membrane, while the CO2/N2 and CO2/CH4 selectivity remain largely unchanged. Meanwhile, benefiting from its unique co-continuous morphology, the PS-MMM also exhibited markedly improved membrane ductility compared to the conventional MMM at similar MOF loading. PS-MMMs offer a practical solution to simultaneously achieve high membrane permeability and good mechanical properties.

Challenges in Developing MOF-Based Membranes for Gas Separation.

Metal-organic frameworks (MOFs) are promising candidates for membrane gas separation. MOF-based membranes include pure MOF membranes and MOF-based mixed matrix membranes (MMMs). This Perspective discusses the challenges for the next stage of the development of MOF-based membranes based on research conducted in the past decade. We focused on three major issues associated with pure MOF membranes. First, some MOF compounds have been overstudied, despite the availability of numerous MOFs. Second, gas adsorption and diffusion in MOFs are often independently investigated. The correlation between adsorption and diffusion has seldom been discussed. Third, we identify the importance of characterizing the gas distribution in MOFs to understand the structure-property relationships for gas adsorption and diffusion in MOF membranes. For MOF-based MMMs, engineering the MOF-polymer interface is essential for achieving the desired separation performance. Various approaches to modify the MOF surface or polymer molecular structure have been proposed to improve the MOF-polymer interface. Herein, we present defect engineering as a facile and efficient approach for engineering the MOF-polymer interfacial morphology and its extended application for various gas separations.

Regulating the pore engineering of MOFs by the confined dissolving of PSA template to improve CO2 capture

Pore size adjustment has been considered to be a successful strategy for enhancing the sieving of MOF-based mixed matrix membranes (MMMs). However, the increase in CO2/N2 selectivity usually comes at the expense of permeability. Herein, the regulating on the pore size of polystyrene-modified hollow NH2-MIL-101 (PHNM) is achieved by controlling the confined dissolving of PSA polymer chain. During the etching of PSA template, the dissolved PSA polymer chains diffused outward through the pores and intergranular space of NH2-MIL-101 under the concentration difference. As a consequence, the pores of NH2-MIL-101 were unavoidably constrained by the partly dissolved PSA segments, achieving the regulation on the pore size. Meanwhile, there are also some PSA segments adhering to the surface of NH2-MIL-101 to form a PSA coating layer. In this unique PHNM, the reduced pore size endows it with better CO2/N2 selectivity than NH2-MIL-101. Meanwhile, the high interface compatibility provided by surface-modified PSA segments and the enhanced CO2 affinity of PHNM resulting from amino groups also synergistically improve the CO2/N2 selectivity. Additionally, the hollow core offers gas molecules a low-resistance transfer channel, improving the permeability of CO2. The hollow size of PHNM was adjusted from 33 to 57 nm by controlling the etching time of PSA, and the pore size of PHNM was regulated from 0.58 to 0.83 nm by controlling the confined dissolving process of PSA. Thus, the PHNM-based MMMs exhibit high CO2 permeability as well as high CO2/N2 selectivity. Compared with NH2-MIL-10/Pebax MMM, the MMM with PHNM-2 loading of 10 wt% shows 30% and 21% enhancements in the CO2 permeability (226.3 Barrer) and CO2/N2 selectivity (71.3), respectively, far exceeding the Robeson upper bound (2008). These results reveal that the PHNM based MMMs express a great potential in CO2 separation.

Cobalt-Based Metal–Organic Frameworks and Its Mixed-Matrix Membranes for Discriminative Sensing of Amines and On-Site Detection of Ammonia

Developing suitable materials that can differentiate between chemically similar substances such as aliphatic and aromatic amines is challenging. Aliphatic and aromatic amines vary considerably in size and electronic properties despite possessing the same functional group. This makes the entire separation process more tedious. Metal–organic frameworks known for their inherent permanent porosity can be designed using appropriate building blocks that can lead to multifunctional materials. Here we utilize two Co-based multifunctional MOFs for discriminative sensing of amines and on-site detection of ammonia. Both the MOF materials display unique fluorescence behavior where aliphatic amines lead to “turn-off”, and aromatic amines show “turn-on” fluorescence intensities of the two MOFs. Real-time sensing experiments with MOF-based mixed matrix membranes show an instant color change when ammonia is liberated from a chemical reaction. Density functional theory calculations unravel that the aliphatic and aromatic amines interact with the MOF structures in different ways that lead to “turn-off” and “turn-on” fluorescence behavior, respectively.

MOF Membranes for CO2 Capture: Past, Present and Future

As the society is projected to continue relying on fossil fuels for energy needs in the next decades, the increasing level of CO2 emissions due to fossil fuel use can pose bigger threats to humans and environment, considering the associated amplified greenhouse effects. Membrane technology is a promising energy-efficient pathway to tackle the growing conundrum of CO2 emissions. Among various classes of porous materials, metal-organic frameworks (MOFs) deserve much interest regarding membrane applications due to their highly versatile structures that can be tailored for different gas separation needs. Computational modeling of MOFs for membrane-based separations can provide molecular-level information that may not be accessible via experiments and help establish the theoretical foundation for the performance trends observed in gas separation experiments. This perspective focuses on the investigation of pure MOF membranes and MOF-based composite membranes for CO2 capture applications such as post-combustion (CO2/N2) and pre-combustion CO2 separation (CO2/H2), and natural gas purification (CO2/CH4), with particular emphasis given to computational studies. We highlight the key advances and future directions in the development, modeling, and testing of early MOF membranes, ionic liquid/MOF membranes, ultrathin MOF membranes, MOF-COF (covalent organic framework) membranes, MOF glass membranes, and MOF-based mixed-matrix membranes (MMMs). Finally, an outlook on the potential opportunities and challenges associated with the computational modeling of MOF-based membranes and their use for carbon capture is provided.

Novel Eu-MOF-based mixed matrix membranes and 1D Eu-MOF-based ratiometric fluorescent sensor for the detection of metronidazole and PA in water

Metal organic framework-based mixed matrix membranes and ratiometric fluorescent sensors have received significant attention in application of sensing. Herein, a two-dimensional europium-based metal organic frameworks, [Eu(L-N2)2·(L-Cl4)1.5·H2O] (CUST-506). (L-Cl4 = 2,3,5,6-tetrachloroterephthalic acid, L-N2 = 1,10-phenanthroline) has been synthesized and characterized. Then, by immobilizing water-stable CUST-506 into agarose hydrogels, the Eu-MOF-based mixed matrix membranes (CUST-506-based MMMs) were successfully prepared. The CUST-506-based MMMs exhibit not only the integrity and the metronidazole, picric acid (PA) sensing nature of Eu-MOF powder, but also excellent processability. Moreover, the Eu-MOF-based MMMs show distinguished water-stability. More interestingly, during the hydrothermal synthesis process, through anchoring the formic acid into the framworks to break the symmetrical coordination environment of the CUST-506, a one dimensional (1D) compound {[Eu(L-N2)·(L-Cl4)0.5(COOH)·2H2O]·(L-Cl4)0.5}(CUST-507) with two emission centers from Eu3+-based centers and free L-Cl4 molecules is generated. CUST-507 displayes a unique ability of auto-calibration effect based on the dual emission mechanism and sensitive sensing of metronidazole and picric acid. This work highlights a simple fabrication strategy for MOF-based mixed matrix membranes and overcomes the environmental interference for ratiometric fluorescent sensor.

Insights into the Enhancement of MOF/Polymer Adhesion in Mixed-Matrix Membranes via Polymer Functionalization.

MOF-based mixed-matrix membranes (MMMs) prepared using standard routes often exhibit poor adhesion between polymers and MOFs. Herein, we report an unprecedented systematic exploration on polymer functionalization as the key to achieving defect-free MMMs. As a case study, we explored computationally MMMs based on the combination of the prototypical UiO-66(Zr) MOF with polymer of intrinsic porosity-1 (PIM-1) functionalized with various groups including amidoxime, tetrazole, and N-((2-ethanolamino)ethyl)carboxamide. Distinctly, the amidoxime-derivative PIM-1/UiO-66(Zr) MMM was predicted to express the desired enhanced MOF/polymer interfacial interactions and thus subsequently prepared and evaluated experimentally. Prominently, high-resolution transmission electron microscopy confirmed optimal adhesion between the two components in contrast to the nanometer-sized voids/defects shown by the pristine PIM-1/UiO-66(Zr) MMM. Notably, single-gas permeation measurements further corroborated the need of optimal MOF/polymer adhesion in order to effectively enable the MOF to play a role in the gas transport of the resulting MMM.

Vesicles-shaped MOF-based mixed matrix membranes with intensified interfacial affinity and CO2 transport freeway

The trade-off between gas permeability and selectivity represents a huge challenge for membrane separation. In this work, we design unique polystyrene-acrylate (PSA) modified hollow ZIF-8 (PHZ) nanospheres via a facile hard template-incomplete etching strategy as fillers for mixed matrix membranes (MMMs). In the vesicles shaped PHZ, the hollow core builds a low-resistance CO2 transport freeway, leading to improved CO2 permeability. Meanwhile, the outside PSA layer intensifies the interface affinity between ZIF-8 and Pebax through molecular chains entanglements and hydrogen bonding, enhancing membrane selectivity. Through controlling the PSA etching process, both the hollow core with tunable size and the outside PSA layer with adjustable thickness are achieved. The 10 wt% PHZ-2/Pebax MMMs present the best performance with a CO2 permeability of 172.4 Barrer and CO2/N2 selectivity of 87.9. The higher CO2/N2 selectivity proves that the PSA layer on the PHZ has better compatibility with Pebax. The higher CO2 permeability is attributed to the low-resistance hollow core. The gas separation performance of PHZ-based MMMs not only well exceeded the 2008 Robeson upper-bond line but also was superior to those of other published MOF-based MMMs. The concept of vesicle-shaped PHZ paves a way to concurrently enhance both the gas permeability and selectivity of MMMs.

Recent advances in simulating gas permeation through MOF membranes.

In the last two decades, metal organic frameworks (MOFs) have gained increasing attention in membrane-based gas separations due to their tunable structural properties. Computational methods play a critical role in providing molecular-level information about the membrane properties and identifying the most promising MOF membranes for various gas separations. In this review, we discuss the current state-of-the-art in molecular modeling methods to simulate gas permeation through MOF membranes and review the recent advancements. We finally address current opportunities and challenges of simulating gas permeation through MOF membranes to guide the development of high-performance MOF membranes in the future.

Rapid fabrication of MOF-based mixed matrix membranes through digital light processing

3D printing, also known as additive manufacturing technology, has greatly expanded across multiple sectors of technology replacing classical manufacturing methods by combining processing speed and high precision.

Novel MOF-based mixed-matrix membranes, N-CQDs@[Zn(HCOO)3][C2H8N]/PEG, as the effective antimicrobials

The purpose of this study was to prepare the affordable and novel materials as the efficient and effective antimicrobials. Therefore, the metal–organic framework (MOF) with the formula of [Zn(HCOO)3][C2H8N], polyethylene glycol (PEG), and nitrogen-doped carbon quantum dots (N-CQDs) was chosen to synthesize the two novel mixed-matrix membranes (MMMs). [Zn(HCOO)3][C2H8N]/PEG and N-CQDs@[Zn(HCOO)3][C2H8N]/PEG as the novel MMMs were simply synthesized in a few steps and their antimicrobial activity was investigated against the Gram-negative bacteria, Escherichia coli (E. coli). Furthermore, a novel method was used for the synthesis of [Zn(HCOO)3][C2H8N]. The particles were characterized by the Fourier-transform infrared spectroscopy, the X-ray powder diffraction, the CHNS elemental analysis, and the field-emission scanning electron microscopy. The cytotoxicity and antimicrobial activity of the particles were also studied. The result of MTT (methylthiazolyldiphenyl-tetrazolium bromide) assay showed that these novel MMMs have a good biocompatibility unlike the MOF ([Zn(HCOO)3][C2H8N]) alone. Moreover, the [Zn(HCOO)3][C2H8N]/PEG has a moderate antimicrobial activity against E. coli under the irradiation of UV or the ambient light. However, N-CQDs@[Zn(HCOO)3][C2H8N]/PEG as the novel mixed-matrix membrane (MMM) has excellent antimicrobial activity without the presence of antibiotics which is due to its high quantum yield under the irradiation of UV light. Therefore, this research enhances the biocompatibility of [Zn(HCOO)3][C2H8N] through producing MMM. Furthermore, the antimicrobial activity of MOF/PEG was increased in terms of cost-effectiveness by encapsulating the N-CQDs.

Engineering of the Filler/Polymer Interface in Metal-Organic Framework-Based Mixed-Matrix Membranes to Enhance Gas Separation.

Traditional films cannot fully adapt to industrial applications and to intensified processes. Advanced mixed-matrix membranes comprising metal-organic frameworks (MOF) embedded in a polymer matrix have been developed with the goal of breaking the trade-off effect of traditional polymer membranes and achieving separation performance beyond Robeson's upper limit. The key challenges in the fabrication of MOF-based mixed-matrix membranes are an enhancement in compatibility between the inorganic filler and the polymer matrix, elimination of the irregular morphology and non-selective interfacial defects, and further improvement in the gas-separation performance. This review summarizes the recent advances in protocols and strategies in terms of designing interfacial interactions to enhance the MOF/polymer interface compatibility. This review aims at providing some meaningful insights into preparing MOF-based mixed-matrix membranes targeting ideal interfacial morphology and leading to excellent gas-separation performance.

A Review on Computational Modeling Tools for MOF-Based Mixed Matrix Membranes

Computational modeling of membrane materials is a rapidly growing field to investigate the properties of membrane materials beyond the limits of experimental techniques and to complement the experimental membrane studies by providing insights at the atomic-level. In this study, we first reviewed the fundamental approaches employed to describe the gas permeability/selectivity trade-off of polymer membranes and then addressed the great promise of mixed matrix membranes (MMMs) to overcome this trade-off. We then reviewed the current approaches for predicting the gas permeation through MMMs and specifically focused on MMMs composed of metal organic frameworks (MOFs). Computational tools such as atomically-detailed molecular simulations that can predict the gas separation performances of MOF-based MMMs prior to experimental investigation have been reviewed and the new computational methods that can provide information about the compatibility between the MOF and the polymer of the MMM have been discussed. We finally addressed the opportunities and challenges of using computational studies to analyze the barriers that must be overcome to advance the application of MOF-based membranes.