MOF-based Mixed Matrix Membranes Research Articles

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.

Rational matching between MOFs and polymers in mixed matrix membranes for propylene/propane separation

Mixed matrix membranes (MMMs) have attracted tremendous attention in the field of gas separation, which adopts the synergistic advantages of the polymer and inorganic phase. However, the MMMs rarely generate sufficient selectivity for energy-efficient gas separation, even if the dispersed molecular sieve is highly selective. Here, an unprecedented improvement (∼4.5-fold) of both C3H6/C3H8 permeability and selectivity on the crosslinked poly(ethylene oxide) (XLPEO) membranes containing ZIF-8 nanocrystals. When the ratio of polymer-to-sieve permeability (Rc/d) is in the range of 0.004–0.36, both C3H6/C3H8 permeability and selectivity increases with the ZIF-8 loading, and the separation performance on MMMs can approach and surpass the-state-of-the-art polymer upper bound. Besides C3H6/C3H8 separation, the universality of this proposed polymer-sieve matching law is suitable for other gas-pairs on MOF-based MMMs.

Defect-Free MOF-Based Mixed-Matrix Membranes Obtained by Corona Cross-Linking.

Functionalized UiO-66 metal-organic frameworks (MOF) particles were covalently grafted with hydride-terminated poly(dimethylsiloxane) (PDMS) via postsynthetic modification. These PDMS-coated MOF particles (termed here "corona-MOF") were used in the preparation of mixed-matrix membranes (MMMs). Defect-free MMMs with weight loadings of 50% were achieved with corona-MOF particles, attributed to the improved dispersibility of the corona-MOF particles and covalent linkages between the corona-MOF particles and the polymer matrix. The PDMS MMMs showed distinct separation features in single gas permeation tests, displaying much higher CO2 gas permeation with no decrease in selectivity when compared to MMMs prepared with unmodified UiO-66 particles. Single gas separation tests with CO2, N2, and propane were performed to probe the separation mechanism of the corona-MOF MMMs, demonstrating that these MMMs avoid nonideal "sieve-in-a-cage" and "plugged sieves" scenarios. Additionally, due to covalent bond formation between both the MOF and the polymer matrix in corona-MOF MMMs, particle aggregation is negligible during film curing, allowing for the formation of flexible, self-standing MMMs of <1 μm in thickness. Low quantities of polymer covalently attached to the MOF surface (<5 wt %) are sufficient to fabricate thin, defect-free, high MOF-loading MMMs.

Molecular Simulations of MOF Membranes and Performance Predictions of MOF/Polymer Mixed Matrix Membranes for CO2/CH4 Separations.

Efficient separation of CO2 from CO2/CH4 mixtures using membranes has economic, environmental and industrial importance. Membrane technologies are currently dominated by polymers due to their processing abilities and low manufacturing costs. However, polymeric membranes suffer from either low gas permeabilities or low selectivities. Metal organic frameworks (MOFs) are suggested as potential membrane candidates that offer both high selectivity and permeability for CO2/CH4 separation. Experimental testing of every single synthesized MOF material as membranes is not practical due to the availability of thousands of different MOF materials. A multilevel, high-throughput computational screening methodology was used to examine the MOF database for membrane-based CO2/CH4 separation. MOF membranes offering the best combination of CO2 permeability (>106 Barrer) and CO2/CH4 selectivity (>80) were identified by combining grand canonical Monte Carlo and molecular dynamics simulations. Results revealed that the best MOF membranes are located above the Robeson’s upper bound indicating that they outperform polymeric membranes for CO2/CH4 separation. The impact of framework flexibility on the membrane properties of the selected top MOFs was studied by comparing the results of rigid and flexible molecular simulations. Relations between structures and performances of MOFs were also investigated to provide atomic-level insights into the design of novel MOFs which will be useful for CO2/CH4 separation processes. We also predicted permeabilities and selectivities of the mixed matrix membranes (MMM) in which the best MOF candidates are incorporated as filler particles into polymers and found that MOF-based MMMs have significantly higher CO2 permeabilities and moderately higher selectivities than pure polymers.

Study of Separation Behavior of Activated and Non-Activated MOF-5 as Filler on MOF-based Mixed-Matrix Membranes in H2/CO2 Separation

In this study, cubic and tetragonal structures of MOF-5 (C-MOF-5 and T-MOF-5) were successfully synthesized, characterized and incorporated into cellulose acetate (CA) polymer matrix in the range of 6, 9 and 12 wt % to fabricate mixed matrix membranes (MMMs). The effects of smaller pore size of T-MOF-5 and more ZnO molecules in T-MOF-5, on the H2 and CO2 permeation properties of C-MOF-5/CA and T-MOF-5/CA MMMs were investigated. The all novel MMMs were prepared using the solution casting method and characterized by FTIR, TGA and SEM. SEM images as well as results of FTIR and TGA analyses confirmed good adhesion between both MOF-5s and CA matrix. Addition of both C-MOF-5 and T-MOF-5 into the CA improved the gas transport properties of the CA, especially in H2 separation. The H2/CO2 selectivity continued the increasing trend at 9 wt % and did not significantly reduce even at 12 wt % due to good adhesion between both MOF-5s and CA. The highest H2/CO2 selectivity was obtained at 12 and 9 wt % loading of C-MOF-5 and T-MOF-5, respectively. By changing the filler from C-MOF-5 to T-MOF-5, the increasing and reducing of adsorption site of H2 and CO2 (respectively), and also reducing in pore size, caused the appearance of H2 permeability to not change much but the CO2 permeability to reduce. Accordingly, the H2/CO2 selectivity in all T-MOF-5/CA MMMs is higher than that in all C-MOF-5/CA MMMs. According to obtained results, the activated MOFs (i.e., C-MOF-5 in this study) are not always the best choices for separation process.

Metal–organic frameworks for membrane-based separations

As research into metal–organic frameworks (MOFs) enters its third decade, efforts are naturally shifting from fundamental studies to applications, utilizing the unique features of these materials. Engineered forms of MOFs, such as membranes and films, are being investigated to transform laboratory-synthesized MOF powders to industrially viable products for separations, chemical sensors and catalysts. Following encouraging demonstrations of gas separations using MOF-based membranes, liquid-phase separations are now being explored in an effort to build effective membranes for these settings. In this Review, we highlight MOF applications that are in their nascent stages, specifically liquid-phase separations using MOF-based mixed-matrix membranes. We also highlight the analytical techniques that provide important insights into these materials, particularly at surfaces and interfaces, to better understand MOFs and their interactions with other materials, which will ultimately lead to their use in advanced technologies. The pores of metal–organic frameworks (MOFs) make them attractive materials for gas- and liquid-phase separations. In this Review, the fabrication of MOF-based membranes and analytical techniques used to characterize them are outlined with a focus on the surfaces and interfaces in these composite materials.

Origins and Evolution of Inorganic-Based and MOF-Based Mixed-Matrix Membranes for Gas Separations

Gas separation for industrial, energy, and environmental applications requires low energy consumption and small footprint technology to minimize operating and capital costs for the processing of large volumes of gases. Among the separation methods currently being used, like distillation, amine scrubbing, and pressure and temperature swing adsorption, membrane-based gas separation has the potential to meet these demands. The key component, the membrane, must then be engineered to allow for high gas flux, high selectivity, and chemical and mechanical stability at the operating conditions of feed composition, pressure, and temperature. Among the new type of membranes studied that show promising results are the inorganic-based and the metal-organic framework-based mixed-matrix membranes (MOF-MMMs). A MOF is a unique material that offers the possibility of tuning the porosity of a membrane by introducing diffusional channels and forming a compatible interface with the polymer. This review details the origins of these membranes and their evolution since the first inorganic/polymer and MOF/polymer MMMs were reported in the open literature. The most significant advancements made in terms of materials, properties, and testing conditions are described in a chronological fashion.

Computational Screening of MOF-Based Mixed Matrix Membranes for CO2/N2Separations

Atomically detailed simulations were used to examine CO2/N2separation potential of metal organic framework- (MOF-) based mixed matrix membranes (MMMs) in this study. Gas permeability and selectivity of 700 new MMMs composed of 70 different MOFs and 10 different polymers were calculated for CO2/N2separation. This is the largest number of MOF-based MMMs for which computational screening is done to date. Selecting the appropriate MOFs as filler particles in polymers resulted in MMMs that have higher CO2/N2selectivities and higher CO2permeabilities compared to pure polymer membranes. We showed that, for polymers that have low CO2permeabilities but high CO2selectivities, the identity of the MOF used as filler is not important. All MOFs enhanced the CO2permeabilities of this type of polymers without changing their selectivities. Several MOF-based MMMs were identified to exceed the upper bound established for polymers. The methods we introduced in this study will create many opportunities to select the MOF/polymer combinations with useful properties for CO2separation applications.

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

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

Metal−organic framework composite membranes: Synthesis and separation applications

Metal−organic framework (MOF) materials, which are built of metal ions or metal ion clusters bridged organic linkers, possess the well-defined pore structures, large surface area and extraordinary adsorption affinities. In recent years, MOF-based membranes have attracted large attention in separation applications due to their excellent performance. In respect to practical applications, the substrates and construction will greatly influence the industrial promotion and separation performance of the MOF-based membranes. Inorganic materials are the earliest substrates for continuous MOF membranes. Both the bare and modified substrates can be used for supporting the MOF membranes by changing the synthesis preparation process, such as solvothermal synthesis, interfacial synthesis and liquid phase epitaxy approach. In order to reduce the cost and enhance the processability of the MOF membrane, the polymer substrates can also be employed to obtain the continuous MOF membranes. Moreover, the hollow fiber substrates can provide the large membrane areas per volume. Since there is good compatibility between the MOFs and polymers, MOFs can also be employed as fillers to fabricate mixed matrix membranes. Those MOF-based membranes exhibit good performance in gas separation, pervaporation and nanofiltration. Therefore, this review gives an overview of the continuous MOF membranes supported by inorganic substrates and polymer substrates and the MOF-based mixed matrix membranes.

Molecular Modeling of MOF-based Mixed Matrix Membranes

Molecular Modeling of MOF-based Mixed Matrix Membranes

Metal‐Organic Frameworks: Visualizing MOF Mixed Matrix Membranes at the Nanoscale: Towards Structure‐Performance Relationships in CO2/CH4 Separation Over NH2‐MIL‐53(Al)@PI (Adv. Funct. Mater. 2/2014)

Focused ion beam-scanning electron microscopy (FIB-SEM) sheds light on the performance of MOFbased mixed matrix membranes. On page 249, 3D tomography is presented by T. Rodenas, J. Gascon, and co-workers as a powerful approach to assess the distribution of the MOF filler and its contact with the polymeric matrix. In addition, the influence of the MOF loading, its framework configuration in the membrane, and the influence of operation conditions on the CO2/CH4 separation performance are studied.

Recent Advances in Metal–Organic Framework‐Based Mixed Matrix Membranes

Mixed matrix membranes (MMMs) combine easy processability and low cost of polymers with the high selectivity of nanoporous filler particles. These membranes represent an important avenue for enhancing the performance of polymeric membranes in gas separations. Recently, a new group of nanoporous materials called metal-organic frameworks (MOFs) have been used as filler particles in MMM applications, and significant improvements in gas selectivity and permeability have been reported for these MOF-based membranes. We discuss the recent developments in MOF-based MMMs by reviewing both the experimental studies and computational modeling methods that can accelerate selection of MOF/polymer combinations for high-performance gas separations.