Metal-organic Framework Membranes Research Articles

Combining Machine Learning and Molecular Simulations to Unlock Gas Separation Potentials of MOF Membranes and MOF/Polymer MMMs.

Due to the enormous increase in the number of metal-organic frameworks (MOFs), combining molecular simulations with machine learning (ML) would be a very useful approach for the accurate and rapid assessment of the separation performances of thousands of materials. In this work, we combined these two powerful approaches, molecular simulations and ML, to evaluate MOF membranes and MOF/polymer mixed matrix membranes (MMMs) for six different gas separations: He/H2, He/N2, He/CH4, H2/N2, H2/CH4, and N2/CH4. Single-component gas uptakes and diffusivities were computed by grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, respectively, and these simulation results were used to assess gas permeabilities and selectivities of MOF membranes. Physical, chemical, and energetic features of MOFs were used as descriptors, and eight different ML models were developed to predict gas adsorption and diffusion properties of MOFs. Gas permeabilities and membrane selectivities of 5249 MOFs and 31,494 MOF/polymer MMMs were predicted using these ML models. To examine the transferability of the ML models, we also focused on computer-generated, hypothetical MOFs (hMOFs) and predicted the gas permeability and selectivity of 1000 hMOF/polymer MMMs. The ML models that we developed accurately predict the uptake and diffusion properties of He, H2, N2, and CH4 gases in MOFs and will significantly accelerate the assessment of separation performances of MOF membranes and MOF/polymer MMMs. These models will also be useful to direct the extensive experimental efforts and computationally demanding molecular simulations to the fabrication and analysis of membrane materials offering high performance for a target gas separation.

Large-Scale Screening and Machine Learning for Metal-Organic Framework Membranes to Capture CO2 from Flue Gas.

To combat global warming, as an energy-saving technology, membrane separation can be applied to capture CO2 from flue gas. Metal–organic frameworks (MOFs) with characteristics like high porosity have great potential as membrane materials for gas mixture separation. In this work, through a combination of grand canonical Monte Carlo and molecular dynamics simulations, the permeability of three gases (CO2, N2, and O2) was calculated and estimated in 6013 computation–ready experimental MOF membranes (CoRE–MOFMs). Then, the relationship between structural descriptors and permeance performance, and the importance of available permeance area to permeance performance of gas molecules with smaller kinetic diameters were found by univariate analysis. Furthermore, comparing the prediction accuracy of seven classification machine learning algorithms, XGBoost was selected to analyze the order of importance of six structural descriptors to permeance performance, through which the conclusion of the univariate analysis was demonstrated one more time. Finally, seven promising CoRE-MOFMs were selected, and their structural characteristics were analyzed. This work provides explicit directions and powerful guidelines to experimenters to accelerate the research on membrane separation for the purification of flue gas.

Electrical Detection of Gaseous Pollutants Using Nanoporous-Based Gas Sensors

The advanced in situ detection of gaseous pollutants, such as NOx or SOx, is of great interest in many applications, such as the automotive and coal industries. We will discuss the continued advancement of our low power sensors for these pollutants, leveraging the previous successful development of impedance spectroscopy-based sensors for the detection of gaseous I2.1-3 The sensors, composed of Pt interdigitated electrodes (IDEs) with a nanoporous adsorbent layer, can be tuned to selectively adsorb gases of interest through judicious material selection, and the electrical response directly correlated to gas concentration. The current work is focused on exploring these nanoporous phases (metal-organic frameworks (MOFs), zeolites, etc.) for the real-time detection of NOx. The sensors have been successfully demonstrated for detection of trace NO2 (0.5 – 5 ppm),4 and experimental results, collected at relatively low temperature (25-50°C), will be discussed. Other recent work exploring the direct growth of crystalline MOF membranes, through the chemical functionalization of the surface of interdigitated electrodes will also be discussed.5 Direct growth of thin MOF films on surface functionalized IDEs has been shown to result in increased sensor sensitivity and a faster response time. Lastly, we will present initial results for the use of the sensors in the selective detection of NO2 in complex environments (e.g. presence of H2O, CO2, etc.).6 Acknowledgements Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. References Small, L.J and Nenoff, T.M., ACS Appl. Mater. Interfaces, 2017, 9 (51), 44649.Small, L.J.; Krumhansl, J.L.; Rademacher, D.X.; Nenoff, T.M. Meso. Mater., 2019, 280, 82.Small, L.J.; Hill, R. C.; Krumhansl, J. L.; Schindelholz, M. E.; Chen, Z.; Chapman, K.W.; Zhang, X.; Yang, S.; Schroder, M.; Nenoff, T.M., ACS Applied Materials and Interfaces, 2019, 11 (31), 27982.Small, L.J.; Henkelis, S.E; Rademacher, D.X.; Schindelholz, M. E.; Krumhansl, J. L.; Vogel, D.J.; Nenoff, T.M, Advanced Functional Materials, 2020, 30 (50), 2006598.Henkelis, S.E; Percival, S.J.; Small, L.J; Rademacher, D.X.; Nenoff, T.M, Membranes, 2021, 11 (3), 176.Percival, S.J.; Henkelis, S.E; Li, M.; Schindelholz, M.E.; Krumhansl, J. L.; Small, L.J.; Lobo, R.F.; Nenoff, T.M., I Eng. Chem. Res., 2021, 60 (40), 14371.

2D CuBDC and IRMOF-1 as reverse osmosis membranes for seawater desalination: A molecular dynamics study

In this study, molecular dynamics simulation is conducted to evaluate the performance of CuBDC and IRMOF-1 metal–organic frameworks (MOFs) as 2D membranes in the reverse osmosis (RO) desalination process. Both 2D MOF membranes possess the same 1,4-benzenedicarboxylate linkers but different metal nodes, which correspond to different molecular structures. The performance of the 2D membranes is assessed in terms of their water flux and ion rejection rate. The effects of different metal nodes and membrane structures on the interactions between the 2D membranes and salt ions are investigated and explained according to the radial distribution function, interaction energy, and ion density distribution. Our results indicate that the pore entrance of both MOF membranes exhibit higher affinity towards Cl- ions than Na+ ions. Furthermore, complete ion rejection is achieved for both MOF membranes at half the thickness of their physical counterparts (CuBDC: ∼50 Å and IRMOF-1: 40 Å). The lower water flux in the CuBDC membrane is also determined to be caused by the low water density within it. Overall, MD simulation is particularly useful for studying 2D MOF membranes in RO since it is capable of accurately modeling nanoscale structures. Of the two 2D MOF membranes tested, the IRMOF-1 membrane displays the higher water flux due to its more porous structure.

Direct Synthesis of Ultrathin Two-Dimensional Co-Based Metal–Organic Framework Membranes by the Conversion of Co(OH)2 Sheets for Gas Separation

Direct Synthesis of Ultrathin Two-Dimensional Co-Based Metal–Organic Framework Membranes by the Conversion of Co(OH)₂ Sheets for Gas Separation

Asymmetric pore windows in MOF membranes for natural gas valorization

To use natural gas as a feedstock alternative to coal and oil, its main constituent, methane, needs to be isolated with high purity1. In particular, nitrogen dilutes the heating value of natural gas and is, therefore, of prime importance for removal2. However, the inertness of nitrogen and its similarities to methane in terms of kinetic size, polarizability and boiling point pose particular challenges for the development of energy-efficient nitrogen-removing processes3. Here we report a mixed-linker metal-organic framework (MOF) membrane based on fumarate (fum) and mesaconate (mes) linkers, Zr-fum67-mes33-fcu-MOF, with a pore aperture shape specific for effective nitrogen removal from natural gas. The deliberate introduction of asymmetry in the parent trefoil-shaped pore aperture induces a shape irregularity, blocking the transport of tetrahedral methane while allowing linear nitrogen to permeate. Zr-fum67-mes33-fcu-MOF membranes exhibit record-high nitrogen/methane selectivity and nitrogen permeance under practical pressures up to 50 bar, removing both carbon dioxide and nitrogen from natural gas. Techno-economic analysis shows that our membranes offer thepotential to reduce methane purification costs by about 66% for nitrogen rejection and about 73% for simultaneous removal of carbon dioxide and nitrogen, relative to cryogenic distillation and amine-based carbon dioxide capture.

Designing heterogeneous MOF-on-MOF membrane with hierarchical pores for effective water treatment

Rationally regulating the separation pores is an effective way to achieve efficient separation membrane. However, the developed metal-organic framework (MOF) membranes for water treatment are all formed by one kind of MOF, which can only provide rigid pores, makes it difficult to achieve high flux and excellent rejection ratio simultaneity. Here, we proposed a facile strategy for the synthesis of heterogeneous MOF-on-MOF membrane with hierarchical pores, by site-specific grown ZIF-8 layer on UiO-66-NH2 layer. Thus unique architecture of the obtained double-layered MOF membrane (UiO-66-NH2@ZIF-8) endows multidimensional separation pores: (i) the large pores of UiO-66-NH2 can facilitate water transportation, (ii) the molecular-sieving pores of ZIF-8 can intercept dyes, (iii) UiO-66-NH2 acting as active sites can promote the growth of ZIF-8. As a result, the optimized UiO-66-NH2@ZIF-8 membrane exhibits high flux (36.7 L m−2 h−1 bar−1) and excellent dye rejections (>97.0%). Through analysis and comparison, the regulated multi-type pores play an important role in membrane performance. Coupled with the facile membrane preparing method, this work can used as a potential approach of designing other advanced MOF membrane for water treatment.

Rational design of mixed-matrix metal-organic framework membranes for molecular separations.

Conventional separation technologies to separate valuable commodities are energy intensive, consuming 15% of the worldwide energy. Mixed-matrix membranes, combining processable polymers and selective adsorbents, offer the potential to deploy adsorbent distinct separation properties into processable matrix. We report the rational design and construction of a highly efficient, mixed-matrix metal-organic framework membrane based on three interlocked criteria: (i) a fluorinated metal-organic framework, AlFFIVE-1-Ni, as a molecular sieve adsorbent that selectively enhances hydrogen sulfide and carbon dioxide diffusion while excluding methane; (ii) tailoring crystal morphology into nanosheets with maximally exposed (001) facets; and (iii) in-plane alignment of (001) nanosheets in polymer matrix and attainment of [001]-oriented membrane. The membrane demonstrated exceptionally high hydrogen sulfide and carbon dioxide separation from natural gas under practical working conditions. This approach offers great potential to translate other key adsorbents into processable matrix.

ZIF-8 penetrating composite membrane for ion sieving

Ion sieving is essential for biological systems and membrane-based applications. At present, metal-organic framework (MOF) membranes are of great interest for use in ion separation, but it remains a great challenge to design high quality MOF membranes with a simple preparation process and desirable ion selectivity. Herein we fabricate a MOF composite membrane, containing highly penetrated ZIF-8 channels. The ZIF-8 channels are the dominant pathways for ion transport and display a good Li+/Mg2+ sieving performance (Li+/Mg2+ ​= ​6.1). The simulation reveals that the Li+/Mg2+ selectivity is associated with the significant disparity of transport energy barrier between them, which is caused by the different extent of ion dehydration in angstrom-scale ZIF-8 channels. This work provides an effective strategy to develop ionic sieving membranes based on penetrating MOF channels for lithium extraction.

Fabrication of the Metal-Organic Framework Membrane with Excellent Adsorption Properties for Paraben Based on Micro Fibrillated Cellulose

Fabrication of the Metal-Organic Framework Membrane with Excellent Adsorption Properties for Paraben Based on Micro Fibrillated Cellulose

Machine-Learning-Assisted High-Throughput computational screening of Metal–Organic framework membranes for hydrogen separation

The high-throughput computational screening (HTCS) and machine learning (ML) were applied to evaluate the H2 separation performances of computation-ready, experimental metal–organic framework membranes (CoRE-MOFMs). For the separation of H2/X (X = CH4, N2, H2S, O2, CO2, and He), based on the results of the structure-performance relationships by univariate analysis found that the performance of the top-performing candidates far exceeded Robeson’s upper bound. To evaluate the H2 permeability (PH2) and permselectivity, a new trade-off variable, the trade-off multiple selectivity and permeability (TMSP), was defined as the comprehensive performance of the CoRE-MOFMs. The TMSP could effectively evaluate the separation performance of CoRE-MOFMs for a variety of H2/X pairs at inifinite dilution. Eight ML methods based on supervised learning algorithms were employed to predict PH2 and TMSP, and the Gaussian process regression and random forest methods exhibited the first and second highest predictive abilities, respectively. Calculating the feature importance of each CoRE-MOFM to investigate the relationship between the number of features and the accuracy of the ML model. And a decision tree model was established following the principle of the minimum Gini coefficient, which would be accurately classified the CoRE-MOFMs based on the membrane structural descriptors; the optimal structural descriptors of MOF and the physical properties of the gas to be separated are used to establish a multivariate regression model which can predict the value of the optimal structural descriptors of MOFMs by knowing the physical properties of component X in the other H2/X separation. The quantitative structure-performance relationships on the microscopic scale could provide a theoretical basis for the screening of top-performing CoRE-MOFMs. The HTCS and ML methods could efficiently accelerate the design and the development of top-performing CoRE-MOFMs for H2 separation.

Facile synthesis disposable MOF membrane filter: Growth of NH2-MIL-125 (Ti) on filter paper for fast removal of organophosphorus pesticides in aqueous solution and vegetables

Metal-organic frameworks (MOFs) have great potential to remove pesticide residues. However, the lack of affinity between the materials and target and the process of trivial sample preparation resulted in limited removal efficiency. Here, we report a one-pot method for the fast preparation of NH2-MIL-125 (Ti)-based filter paper to synthesise NH2-MIL-125 (Ti)-based filter paper membranes. The NH2-MIL-125 (Ti)-based filter paper membrane takes advantage of π-π interactions between the organophosphorus pesticides (OPPs) and the benzene ring of MOFs. The affinity of amino groups and metal Ti for phosphorus atoms in the OPPs exhibits rapid removal efficiency for three OPPs, imidan, fenthion, and fenitrothion. The isothermal adsorption results for imidan, fenthion, and fenitrothion were consistent with the Langmuir, Freundlich, and Langmuir models, respectively. The kinetic results for imidan, fenthion, and fenitrothion agreed with the pseudo-second-order kinetic model, and the removal efficiency reached equilibrium within 1 min. There was no significant change in the adsorption capacity of OPPs in different pH solutions (pH = 2–10). Compared with that of MOFs, the NH2-MIL-125 (Ti)-based filter paper membrane removal efficiency of OPPs is the same, and it also has better removal efficiency in actual spinach samples. As a result, the sample pretreatment procedure was simplified using a low-cost and simple-to-synthesize disposable NH2-MIL-125 (Ti)-based filter paper membrane, samples’ quick separation and the simultaneous fast removal of OPPs.

Filter fabrication by constructing metal-organic frameworks membrane on waste maize straw for efficient phosphate removal from wastewater

Metal-organic frameworks (MOFs) have been demonstrated as effective adsorbents for removal of phosphate from wastewater, but the difficulty in processing and recycling of powdery MOFs limits their practical application. Shaping MOFs into application-oriented forms meanwhile maintaining their intrinsic property is important but challenging. Here, a novel UiO-66/MS filter is fabricated by constructing UiO-66 membrane on the cell wall of waste maize straw (MS) through a solvothermal process. The unique biological structure of MS provides well-developed channels for mass transfer, and the UiO-66 nanoparticles are uniformly anchored on the cell walls of MS to form a monolayer membrane, promoting the exposure of adsorption sites. Due to these structural advantages, the UiO-66/MS filter exhibits higher efficiency for phosphate removal from water than UiO-66 pellet. To realize continuous phosphate removal, an all-in-one device is fabricated by connecting UiO-66/MS filters in series, over which the residual P content (initial P = 3.0 mg·L−1) in the effluent still meets the primary (<0.5 mg·L−1 at 300 L·m−2·h−1) and secondary (<1.0 mg·L−1 at 800 L·m−2·h−1) standards of China’s integrated wastewater discharge standard. This study not only provides an efficient UiO-66/MS filter for phosphate remediation, but also provides a blueprint for value-added utilization of the waste MS.

Secondary-assembled defect-free MOF membrane via triple-needle electrostatic atomization for highly stable and selective organics permeation

Two-dimensional lamellar membranes featuring well-defined nanochannels hold great promise for efficient organics separation. However, the extensively existed non-selective microdefects within laminate architecture would impair separation efficiency and structure stability of membranes. Herein, defect-free metal-organic framework (MOF) lamellar membrane is facilely constructed by triple-needle electrostatic atomization technology. Specifically, one needle atomizes small-sized Cu-TCPP MOF nanosheets (TCPP = meso-tetra(4-carboxyphenyl)porphyrin) to build lamellar skeleton, and the other two needles meantime atomize Cu2+ and TCPP to secondary-assemble crystals among nanosheets. In this manner, these non-selective microdefects are subtly sutured by the regenerated crystals with identical chemical components, resulting in a defect-free MOF lamellar membrane with enhanced structural stability. Importantly, this novel secondary-assembly method highly enhances the separation efficiency and meanwhile keeps high organics permeation for MOF membrane, with rejection of 93.8% for Rose Bengal (1.2 nm) and acetone permeance of 86.9 L m−2 h−1 bar−1. In addition, the multipoint connection between nanosheets and crystals ensures excellent surface shearing and compaction resistance for MOF membrane, permitting high and steady operation performance for over 100 h. This secondary-assembly methodology by one-step process to construct defect-free and steady lamellar membrane provides a new avenue for the development of efficient separation membranes.

Light-Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal-Organic Framework Membranes.

High ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal-organic frameworks (MOFs) with ultra-small and high-density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack-free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m-2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m-2 upon light irradiation. This work provides a promising strategy for the design of high-performance osmotic energy harvesting systems.

Light‐Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal–Organic Framework Membranes

AbstractHigh ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal–organic frameworks (MOFs) with ultra‐small and high‐density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack‐free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m−2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m−2 upon light irradiation. This work provides a promising strategy for the design of high‐performance osmotic energy harvesting systems.

Modeling and optimization of metal-organic frameworks membranes for reverse osmosis with artificial neural networks

Metal-organic frameworks (MOFs) have recently attracted tremendous attention as membrane materials for desalination owing to their diversified structures, permselectivity, and tunable functionalities. However, the structure-performance relationship of MOF membranes still has not been elucidated. Herein, artificial neural networks (ANN) were developed to form prediction model of MOF thin film nanocomposite (TFN) membrane performances in reverse osmosis applications. Key parameters including MOF size, MOF pore diameter, MOF loading, selective layer thickness, salt concentration, and pressure were collected from literature to predict the water permeability and salt rejection. 5-fold cross-validation and hyperparameter tuning method were employed to acquire a better performing model. When the node structure was 6–9–9–8–2 with the learning rate of 0.001, the developed ANN model attained a remarkable prediction R2 of 90.62%. By introducing mean impact value algorithm into the model, the feature importance of each factor on the membrane performance was also calculated. Rather than the MOF loading and size, proper control of selective layer thickness and MOF pore diameter was key to breaking the permeability-selectivity tradeoff. Finally, ANN proved its ability to predict the membrane performance for water purification and further provide guidance for MOF-based membrane design, or even other TFN membrane design.

Gel-to-crystal route towards MOF-mixed MOF-matrix membranes

Metal-organic framework (MOF) membranes attract intensive attention for precisely molecular sieving. It is of greatly scientific interest to adjust transport property of MOF membranes. Inspired by formation and separation mechanisms of mixed matrix membranes (MMMs), in this study, we report a kind of MOF-mixed MOF-matrix membranes (M5), which consist of dispersed MOF fillers and continuous MOF matrixes and combine different features of two MOFs. A simple, controllable, and versatile gel-to-crystal methodology, similar to solvent-evaporation induced phase-inversion of polymer-based membranes and performed by precursor coating and thermally treating crystallization, is proposed to construct M5. The incorporated MOF fillers can improve membrane crystallization and have good compatibility toward MOF matrixes. As an example, because of UiO-66-NH2 filler has larger aperture than ZIF-8 matrix, the prepared M5 shows enhanced permeability while invariable selectivity compared with the pure one.

Interfacial nanoarchitectonics for ZIF-8 membranes with enhanced gas separation.

Metal-organic framework (MOF) membranes are potentially useful in gas separation applications. Conventional methods of MOF membrane preparation require multiple steps and high-pressure conditions. In this study, a reliable one-step interfacial synthesis method under atmospheric pressure has been developed to prepare zeolitic imidazolate framework-8 (ZIF-8) membranes supported on porous α-Al2O3 disks. To obtain optimal ZIF-8 membranes, three reaction parameters were investigated, namely, reaction temperature, reaction time, and concentration of the organic linker (i.e., 2-methylimidazole). The growth of ZIF-8 membranes under various parameters was evaluated by field-emission scanning electron microscopy, and the optimal synthesis conditions were determined (i.e., 80 °C for 12 h in 50 mM of 2-methylimidazole). The as-synthesized ZIF-8 membranes were then applied to CO2/N2 gas separation, which exhibited a maximum separation factor of 5.49 and CO2 gas permeance of 0.47 × 10−7 mol·m−2·s−1·Pa−1.

Metal–Organic Framework Membranes: Advances, Fabrication, and Applications

Metal–organic frameworks (MOFs) are emerging porous materials with the characteristics of uniform metal center distribution and adjustable pore size. They have important applications in the fields of energy, environment, and catalysis. However, the practical application of MOFs is limited due to insolubility, brittleness, and hard processing. Shaping MOFs into a useful structure is an urgent need. In this review, the research progress in the fabrication of MOF membranes is introduced with emphasis on two paths: the preparation of pure MOF membranes and the incorporation of MOFs into polymer matrix. The classic and emerging principles to prepare pure MOF membranes and MOF/polymer hybrid membranes are summarized. Through the analysis of classic cases, the advantages and disadvantages of different preparation strategies and products are compared, to provide general knowledge for the needs of different application scenarios. Finally, the important role that the MOF membrane plays in the applications of separation, adsorption, and catalysis is highlighted. The opportunities and challenges of MOF membranes in this new era are also proposed.