Gas Separation Applications Research Articles

Metalation of metal-organic frameworks: fundamentals and applications.

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Predicting and screening high-performance polyimide membranes using negative correlation based deep ensemble methods.

Polyimide polymer membranes have become critical materials in gas separation and storage applications due to their high selectivity and excellent permeability. However, with over 107 known types of polyimides, relying solely on experimental research means potential high-performance candidates are likely to be overlooked. This study employs a deep learning method optimized by negative correlation ensemble techniques to predict the gas permeability and selectivity of polyimide structures, enabling rapid and efficient material screening. We propose a deep neural network model based on negative correlation deep ensemble methods (DNN-NCL), using Morgan molecular fingerprints as input. The DNN-NCL model achieves an R2 value of approximately 0.95 on the test set, which is a 4% improvement over recent model performance, and effectively mitigates overfitting with a maximum discrepancy of less than 0.03 between the training and test sets. High-throughput screening of over 8 million hypothetical polymers identified hundreds of promising candidates for gas separation membranes, with 14 structures exceeding the Robeson upper bound for CO2/N2 separation. Visualization of high-throughput predictions shows that although the Robeson upper bound was never explicitly used as a model constraint, the majority of predictions are compressed below this limit, demonstrating the deep learning model's ability to reflect real-world physical conditions. Reverse analysis of model predictions using SHAP analysis achieved interpretability of the deep learning model's predictions and identified three key functional groups deemed important by the deep neural network for gas permeability: carbonyl, thiophene, and ester groups. This established a bridge between the structure and properties of polyimide materials. Additionally, we confirmed that two polyimide structures predicted by the model to have excellent CO2/N2 selectivity, namely 6-methylpyrimidin-5-amine and 1,4,5,6-tetrahydropyrimidin-2-amine, have been experimentally validated in previous studies. This research demonstrates the feasibility of using deep learning methods to explore the vast chemical space of polyimides, providing a powerful tool for discovering high-performance gas separation membranes.

A microporous bismuth-based MOF for efficient separation of acetylene from carbon dioxide.

The separation of acetylene from carbon dioxide is challenging due to their almost identical molecular sizes and volatilities. Metal-organic frameworks (MOFs) in general are strong candidates for the separation of gas mixtures owing to the presence of functional pore surfaces that can selectively capture specific target molecules. Herein, we report a stable and easily synthesized bismuth-based MOF, Bi-BTC, which can achieve the separation of acetylene and carbon dioxide. We performed a detailed analysis of the sorption properties of the Bi-MOF. Bi-BTC shows good adsorption capacities for C2H2 with a capacity of 53.8 cm3 g-1 at 298 K and 1.0 bar, and C2H2/CO2 selectivity of 5.14/7.69 at 298 K and 1.0/0.1 bar. IAST selectivity calculations indicate that Bi-BTC possesses good separation capacity, and dynamic breakthrough experiments were performed to prove the separation of C2H2 and CO2. Bi-MOFs as a group of relatively less studied types of MOFs have interesting adsorption characteristics, and this study on Bi-based MOF will enrich three-dimensional Bi-MOF adsorbents for gas adsorption and separation applications.

A comprehensive study on transport behaviour and physicochemical characteristics of PU/based 3-phase mixed matrix membranes: Effect of [HNMP][HSO4] ionic liquid and ZnO nanoparticles

The gas separation industry has witnessed a significant focus on mixed matrix membranes (MMMs) due to their exceptional potential, surpassing the performance of conventional polymeric membranes. This study aimed to develop and evaluate MMMs using polyurethane (PU) as the polymer matrix and zinc oxide (ZnO) and ionic liquids (ILs) as fillers. PU/ZnO, PU/IL, and PU/ZnO/IL MMMs were prepared utilizing the dry-phase inversion method, and their efficacy in separating CO2, CH4, and N2 gases was investigated under various conditions. A comprehensive characterization protocol using FTIR, FESEM, TGA, AFM, and tensile testing was employed for their thorough characterization. The membranes’ transport properties were assessed at different pressures and filler concentrations. Results showed that IL incorporation increased gas permeability by enhancing solubility, diffusion, ionic interactions, swelling, and accessible volume. Incorporating ZnO and IL particles in the PU matrix simultaneously improved transport properties, particularly selectivity. Robeson’s diagram analysis shows that the PU/0.5%ZnO/2%IL MMM performed best. Furthermore, molecular simulation methods, including Monte Carlo and molecular dynamics simulations, assessed adsorption isotherms and physicochemical properties such as free volume, density, and X-ray diffraction. Simulation outcomes were highly reliable with experimental results, affirming the results’ efficiency and validation. Overall, this study demonstrated the efficiency of using ZnO and IL in the MMMs, highlighting their potential in gas separation applications.

Precise control over gas-transporting channels in zeolitic imidazolate framework glasses

Porous metal–organic frameworks have emerged to resolve important challenges of our modern society, such as CO2 sequestration. Zeolitic imidazolate frameworks (ZIFs) can undergo a glass transition to form ZIF glasses; they combine the liquid handling of classical glasses with the tremendous potential for gas separation applications of ZIFs. Using millimetre-sized ZIF-62 single crystals and centimetre-sized ZIF-62 glass, we demonstrate the scalability and processability of our materials. Further, following the evolution of gas penetration into ZIF crystals and ZIF glasses by infrared microimaging techniques, we determine the diffusion coefficients and changes to the pore architecture on the ångström scale. The evolution of the material on melting and processing is observed in situ on different length scales by using a microscope-coupled heating stage and analysed microstructurally by transmission electron microscopy. Pore collapse during glass processing is further tracked by changes in the volume and density of the glasses. Mass spectrometry was utilized to investigate the crystal-to-glass transition and thermal-processing ability. The controllable tuning of the pore diameter in ZIF glass may enable liquid-processable ZIF glass membranes for challenging gas separations.

Insights into adsorption and diffusion of CO2, CH4 and their mixture in MIL-101(Cr) via molecular simulation

An in-depth comprehension of the adsorption and diffusion properties of CH4, CO2, and their mixture in MIL-101(Cr) is crucial for its application in the highly selective separation of CO2 and CH4 mixtures in biogas. Existing studies often overlook the influence of gas molecular radius and MIL-101 topological structure on adsorption and lack quantitative analysis of the primary and secondary adsorption sites of MIL-101. Additionally, there is a significant gap in research on the adsorption mechanism of gas mixtures. To address these gaps and to provide new knowledge, this study employs Monte Carlo and molecular dynamics simulations to evaluate various factors such as CH4 and CO2 adsorption isotherms, density distribution profiles, self-diffusion coefficients, radial distribution functions (RDF), and CO2/CH4 selectivity within MIL-101 metal-organic framework (MOF). The insightful and comprehensive atomic-level study reveals several unique and new findings, include 1) the RDF analysis shows that the C = C double bond of the benzene ring within MIL-101 primarily governs the adsorption of CH4, while the open metal center (Cr) exhibits significant adsorption effects on CO2; 2) the density distribution profile analysis indicates that CH4 molecules tend to localize in the larger and medium-sized cages, whereas CO2 molecules exhibit relatively uniform distribution. 3) evaluation of self-diffusivity coefficients and diffusion activation energies demonstrated that CH4 diffuses more easily than CO2, and in the CO2 and CH4 two-component gas, the existence of CH4 can further reduce the diffusion coefficient of CO2. This investigation provides a microscopic understanding of adsorption and diffusion phenomena within MIL-101, contributing to the overall comprehension of its behavior in gas separation applications.

2D MOFs and Zeolites for Composite Membrane and Gas Separation Applications: A Brief Review.

Commercial membranes have predominantly been fabricated from polymers due to their economic viability and processability. This choice offers significant advantages in energy efficiency, cost-effectiveness, and operational simplicity compared to conventional separation techniques like distillation. However, polymeric membranes inherently exhibit a trade-off between their permeability and selectivity, which is summarized in the Robeson upper bound. To potentially surpass these limitations, mixed-matrix membranes (MMMs) can be an alternative solution, which can be constructed by combining polymers with inorganic additives such as metal-organic frameworks (MOFs) and zeolites. Incorporating high-aspect-ratio fillers like MOF nanosheets and zeolite nanosheets is of significant importance. This incorporation not only enhances the efficiency of separation processes but also reinforces the mechanical robustness of the membranes. We outline synthesis techniques for producing two-dimensional (2D) crystals (including nanocrystals with high aspect ratio) and provide examples of their integration into membranes to customize separation performances. Moreover, we propose a potential trajectory for research in the area of high-aspect-ratio materials-based MMMs, supported by a mathematical-model-based performance prediction.

Electro-thermal analysis of hybrid fluid through wavy divergent channel

In the bio-medical domains, the transportation of physiological fluids within the human body like chyme movement in the gastrointestinal tract, bolus through the esophagus, blood circulation in the small vessels and heart chambers etc., where the channels are nonuniform in nature. Due to diverse applications of nonuniform channel, in the current mathematical analysis the peristaltic flow of hybrid fluid (containing particular nanoparticles) under an applied axial electric field through the nonuniform channel is presented. To observe hybrid fluid performance, a set of modified hybrid fluid that comprise nanoparticles of titanium dioxide, aluminum trioxide and copper with water as a base fluid is considered in the analysis. Taking into consideration the long wavelength and low Reynolds number assumptions. Poisson-Boltzmann equations are linearized by the Debye-Hückel approximation. Solutions for the axial velocity, average flow rate, pressure gradient, pumping phenomena, heat transfer phenomena and volumetric fraction are obtained analytically. The physical impacts of numerous embedded parameters on the above-mentioned features are argued through graphs which are plotted by Mathematica 11.0 Software. Additionally, the influence of porous medium and nonuniform nature of flow geometry are also discussed in detail. The study could be useful in medical as well as the industrial domains since the electro-osmotic phenomenon is observed throughout the human body where through the osmosis, the body absorbs the nutrients from the food and toxins are removed from the blood stream. Also the nanofluids are widely used in medicines for drug delivery. Also in industries, electro-osmosis has many applications in microchips, biophysics, microfluidics, gas and oil manufacture and separations through the membranes etc.

MOF-GRU: A MOFid-Aided Deep Learning Model for Predicting the Gas Separation Performance of Metal-Organic Frameworks.

The remarkable versatility of metal-organic frameworks (MOFs) stems from their rich chemical information, leading to numerous successful applications. However, identifying optimal MOFs for specific tasks necessitates a thorough assessment of their chemical attributes. Conventional machine learning approaches for MOF prediction have relied on intricate chemical and structural details, hampering rapid evaluations. Drawing inspiration from recent advancements exemplified by Snurr et al., wherein a text string was used to represent a MOF (MOFid), we introduce a MOFid-aided deep learning model, named the MOF-GRU model. This model, founded on natural language processing principles and utilizing the gated recurrent unit architecture, leverages the serialized text string representation of metal-organic frameworks (MOFs) to forecast gas separation performance. Through a focused study on CH4/N2 separation, we substantiate the efficacy of this approach. Comparative assessments against traditional machine learning techniques underscore our model's superior predictive accuracy and its capacity to handle extensive data sets adeptly. The MOF-GRU model remarkably uncovers latent structure-performance relationships with only MOF sequences, obviating the necessity for intricate three-dimensional (3D) structural information. Overall, this model's judicious design empowers efficient data utilization, thereby hastening the discovery of high-performance materials tailored for gas separation applications.

Covalent Triazine Framework-Derived Membranes: Engineered Sol–Gel Construction and Gas Separation Application

Covalent Triazine Framework-Derived Membranes: Engineered Sol–Gel Construction and Gas Separation Application

Controlling Microstructure-Transport Interplay in Poly(ether-block-amide) Multiblock Copolymer Gas Separation Membranes.

In this study, we investigated the effect of morphology on the gas-transport properties of a poly(ether-block-amide) (PEBA) multiblock copolymer. We annealed the copolymer samples and varied the annealing temperature to evaluate the influence of changes in the microstructure on the gas transport properties of PEBA. In addition, we used time-resolved attenuated total reflection Fourier transform infrared spectroscopy to evaluate the diffusion coefficient of CO2 in PEBA based on the Fickian model. The effect of the annealing temperature on the microphase-separated structure of the multiblock copolymer is discussed in detail. Furthermore, the gas diffusivity was significantly affected by the purity of the soft domains. The annealed sample demonstrated a 38% increase in CO2 permeability while maintaining a high CO2/N2 permselectivity of approximately 53. The findings of this study provide valuable insight into the design and optimization of PEBA membranes for gas separation applications.

Plasticization resistant gas separation membranes derived from polyimides exhibiting polyethylene-oxide moieties

Despite exhibiting excellent thermal/chemical stability, mechanical robustness, and easy processability, which makes them attractive candidates for gas separation applications, aromatic polyimides are susceptible to plasticization. This study proposes an effective approach to suppress CO2-induced plasticization in a high fractional free volume (FFV) polyimide (PI*) derived from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride and 2,4,6-trimethylphenyldiamine. PI* was first modified by incorporating 3,5-diaminobenzoic acid (DABA) and polyethylene oxide segments (PEO) into the polymer backbone, with the goal of partially removing PEO through high temperature (450 °C) pyrolysis, thereby increasing the membrane FFV and, simultaneously, providing enhanced plasticization resistance via DABA cross-linking. To mitigate the effects of themomechanical history, the membranes were aged prior to the thermal treatment process. However, the great enhancement in plasticization resistance was not accompanied by a parallel improvement in the separation performance relative to PI*. To address this shortcoming, a microporous organic polymer (POP) was blended with the polyimides to fabricate mechanically-robust mixed matrix membranes (MMMs). Remarkably, MMM plasticization was suppressed even without the aid of thermal crosslinking for PEO-containing PI*, which was rationalized by molecular simulation invoking the formation of hydrogen bonds between the lactam groups of the POP and PEO moieties.

Self-limiting growth of thin dense LTA membranes boosts H2 gas separation performance

The preparation of dense LTA-type zeolite membranes with a thickness in the sub-micron range remains extremely difficult due to their high electronegativity. Herein, thin dense LTA membranes were prepared through the secondary growth of nano LTA seeds in a highly reactive gel. The uniformity of the nano seed layer contributed to the growth of continuous membranes. The self-limiting growth of the zeolite membrane layer was controlled by the synergistic effect of using a highly reactive gel and nano seeds, and resulted in the preparation of thin membranes. Optimized aging times of the gel further enhanced the packing density of the membranes via an efficient intergrowth of the zeolite crystals. This is the first time to obtain single-phase LTA membranes with a thickness of less than 1 μm. The ∼ 470 nm-thick membrane exhibited a H2 permeance of 1.5 × 10-6 mol m−2 s−1 Pa−1, which is tenfold higher than results for literature-reported membranes, a H2/CO2 selectivity of 20.9, and a H2/CH4 selectivity of 17.3. The approach presented offers an alternative pathway for the preparation of thin zeolite membranes with superior permeability and selectivity in gas separation applications.

Regulating thermal dynamics and gas transport of metal‐oraganic framework glass through silver composites

AbstractMetal–organic framework (MOF) glass is a versatile and processible material that retains its porous nature while transitioning between liquid and solid states. Incorporating composites into MOF glass can enhance its functionalities, but the effect of the added material on the solid–liquid phase transition of MOF glass remains understudied. This research explores the impact of silver ions on the properties of ZIF‐62 glass, revealing their influence on phase conversion at varying ligand concentrations. The presence of silver enhances gas separation processes, particularly improving CO2 and hydrocarbon separation selectivity compared to pure ZIF‐62 glass. These findings provide valuable insights for composite integration and highlight the potential of AgZIF‐62 glass in gas separation applications. Furthermore, this work paves the way for processing functional AgZIF‐62 glass films, enabling diverse applications such as gas separation, film catalysis, and antimicrobial glass.

Fabrication of high-flux defect-free hollow fiber membranes derived from a phenolphthalein-based copolyimide for gas separation

In the realm of gas purifications, hollow fiber membranes have gained significant prominence due to their unique advantages. However, achieving defect-free membranes poses a considerable challenge. In this study, we addressed this challenge with a novel copolymer, 6FDA-DAM:DAP(2:1), comprising 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,4.6-Trimethy-m-phenylenediamine (DAM), and a phenolphthalein-derived diamine of 3,3′-diaminophenolphthalein (DAP). We found that defect-free hollow fiber membranes could be prepared when 20 wt% or more ethanol was added in the polymer dope solution, but this led to the formation of oval hollow fibers that would be flattened under high pressure. We solved this limitation by lowering ethanol content to 15 wt%, adding 2 wt% LiNO3, and increasing the air–gap distance. These measures reduced the difference in phase inversion rates between the surface region and the bulk phase of the nascent hollow fiber. The CO2 permeance of this defect-free hollow fiber membrane reached 282 GPU with a CO2/CH4 selectivity of 50.2. Highly permeable defective hollow fiber membranes were also developed using a polymer dope without LiNO3 and a low THF content of 5 wt%. After coated by silicone rubber, CO2 permeance of the membrane reached 554 GPU with a CO2/CH4 selectivity of 45.0. In mixed gas tests, the two membranes exhibited O2 permeances of approximately 43.3 and 75.3 GPU, with O2/N2 selectivities of 5.5 and 5.4, respectively. Their CO2 permeances were 281 and 465 GPU, with CO2/CH4 selectivities of 45 and 41, CO2/N2 selectivities of 35.7 and 33.3, respectively. These separation performances were among the best of state-of-art polymeric hollow fiber membranes and demonstrated great potential for gas separation applications such as natural gas sweetening, flue gas treatment, and air separation.

Microwave-assisted synthesis of metal–organic frameworks UiO-66 and MOF-808 for enhanced CO2/CH4 separation in PIM-1 mixed matrix membranes

This study presents a sustainable microwave-assisted synthesis, based on the use of acetone and a water/acetic acid mixture instead of typical harmful DMF, to fabricate Zr-metal–organic frameworks (MOFs) UiO-66 and MOF-808 as fillers in a PIM-1 matrix for gas separation application. The mixed matrix membranes (MMMs) were prepared with varying loadings (2.5–10 wt%) of MOFs. The physicochemical properties (1H NMR, FTIR, XRD, N2 adsorption, TGA and SEM) of the resulting PIM-1, MOFs and MMMs were analyzed. The CO2/CH4 separation performance and membrane aging characteristics of the MMMs were evaluated. The incorporation of MOF fillers significantly improved CO2 permeability and CO2/CH4 selectivity, attributed to their CO2-philicity and narrow pore size (UiO-66 ≈ 0.6 nm and MOF-808 ≈ 1.8 nm). The MMMs with higher filler loadings (7.5 and 10 wt%) exhibited the most favorable separation performance. Due to the better crystallinity and textural properties, MOF-808 produced the best separation results at 10 wt% filler loading (a CO2/CH4 separation selectivity of 16.2 at 9090 Barrer of CO2 permeability). Aging led to a decrease in CO2 permeability but a slight increase in CO2/CH4 selectivity for all MMMs. Overall, the study highlights the potential of PIM-1/UiO-66 and PIM-1/MOF-808 MMMs as efficient materials for (CO2/CH4) separation comparing with the pristine PIM-1.

Recent Advancements in Metal-Organic Framework-Based Membranes for Hydrogen Separation: A Review.

Metal-organic frameworks (MOFs) are promising porous materials that have huge potential for gas separation when put in the membrane configuration. MOFs have huge potential due to certain salient features of the MOFs such as excellent pore size, ease of tuning the pore chemistry, higher surface area, and chemical and thermal stabilities. MOFs have been explored for various gas separation and storage applications. This review discusses various approaches for fabricating MOFs-based membranes for the separation of H2 gas from a variety of feeds having various gases CO2, CO, N2, and CH4 as impurities. The emphasis has been put on three types of membranes for H2 separation which include MOFs-based hollow fibrous/tubular/disk membranes, MOFs-based mixed matrix membranes (MMMs), and MOFs-based stand-alone membranes. In addition, various challenges such as reducing inhomogeneity between MOFs and polymeric matrices have also been discussed. Similarly, the approaches to successfully decorating MOFs on different supports in different configurations have been explained. The possible ways of improving the MOFs-based membranes for H2 have also been discussed.

Step-Nucleation In Situ Self-Repair to Prepare Rollable Large-Area Ultrathin MOF Membranes.

Ultrathin membranes with ultrahigh permeance and good gas selectivity have the potential to greatly decrease separation process costs, but it requires the practical preparation of large area membranes for implementation. Metal-organic frameworks (MOFs) are very attractive for membrane gas separation applications. However, to date, the largest MOF membrane area reported in the literature is only about 100 cm2 . In the present study, a new step-nucleation in situ self-repair strategy is proposed that enables the preparation of large-area (2400 cm2 ) ultrathin and rollable MOF membranes deposited on an inexpensive flexible polymer membrane support layer for the first time, combining a polyvinyl alcohol (PVA)-metal-ion layer and a pure metal-ion layer. The main role of the pure metal-ion layer is to act as the main nucleation sites for MOF membrane growth, while the PVA-metal-ion layer acts as a slow-release metal-ion source, which supplements MOF crystal nucleation to repair any defects occurring. Membrane modules are necessary components for membrane applications, and spiral-wound modules are among the most common module formats that are widely applied in gas separation. A 4800 cm2 spiral-wound membrane modulewas successfully prepared, demonstrating the practical implementation of large-area MOF membranes.

Water vapor sorption and transport in carbon molecular sieve membranes

Carbon Molecular Sieve (CMS) membranes have been extensively studied for gas separations. Many gas separation applications involve humidified streams, but reports on water vapor transport in CMS membranes are limited. In this study, water permeability, diffusivity, and solubility were determined as a function of water activity for CMS membranes. Water transport properties of membranes synthesized at different pyrolysis temperatures (550 °C and 800 °C) and with different polyimide precursors were examined. Water sorption followed Type V isotherms as previously observed for the adsorption of water in microporous carbons. Water permeability was much higher at all water activity values for CMS samples prepared at lower pyrolysis temperature. Water permeabilities as a function of water activity of the three different polyimides pyrolyzed at 800 °C were very similar. Water permeability of CMS membranes was high compared to many other polymeric materials, showing potential for dehydration applications. Finally, the steady-state water vapor diffusivity (Dss), estimated from steady state permeability and equilibrium sorption measurements, was compared to the experimental transient diffusivity (D), matching reasonably well.

Tunable nano-wrinked channels of reduced graphene oxide membranes for molecular sieving gas separation

Graphene oxide (GO) lamellar membranes have exhibited excellent performance in watertreatment, however there has been far less progress in the application of gas purification. Actually, exposure of the underlying graphitic framework of GO basically determines the gas permeableness of the GO lamellar membranes, which is usually neglected in the fabrication of GO lamellar membranes. Herein, we report nano-wrinkled lamellar reduced GO (rGO) sheets as gas separation membranes with oxygen-rich domains as face-to-face spacer for CO2 blockage and oxygen-poor domains for fast H2 permeation. Thanks to the excellent size-sieving mechanism of the nano-wrinkled channels, the rGO membranes exhibit high H2 permeance of 6.12 × 10−7 mol·m−2·s−1·pa−1 with exceptional selectivity of 529.9 for H2/CO2 gas mixture, surpassing the state-of-the-art ones. The membranes also show excellent stability in separation performance for H2/CO2 gas mixture under a long-term operation. Therefore, such lamellar membranes with well-tuned reduced GO nanosheets hold great potential in gas separation applications.