A new superior competitor for exceptional propylene/propane separations: ZIF-67 containing mixed matrix membranes

Highly Permeable Mixed Matrix Membranes for Gas Separation via Dual Defect-Engineered Zeolitic Imidazolate Framework-8

Defect engineering of metal-organic frameworks (MOFs) is a promising strategy for tailoring the interfacial characteristics between MOFs and polymers, aiming to create high-performance mixed matrix membranes (MMMs). This study introduces a new approach using dual defective alkylamine (AA)-modulated zeolitic imidazolate framework-8 (DAZIF-8), to develop high-flux MMMs. Tributylamine (TBA) and triethylamine (TEA) monodentate ligands coordinate with zinc ions in varying compositions. A mixture of Zn(CH3COO)2·2H2O:2-methylimidazole (Mim):AA in a 1:1.75:5 molar ratio facilitates high-yield coordination between Zn and multiple organic ligands, including Zn-Mim, Zn-TEA, and Zn-TBA (>80%). Remarkably, DAZIF-8 containing 3 mol% TBA and 2 mol% TEA exhibits exceptional characteristics, such as a Brunauer-Emmett-Teller surface area of 1745 m2 g-1 and enhanced framework rigidity. Furthermore, dual Zn-AA coordination sites on the framework's outer surface enhance compatibility with the polyimide (PI) matrix through electron donor-acceptor interactions, enabling the fabrication of high-loading MMMs with excellent mechanical durability. Importantly, the PI/DAZIF-8 (60/40 w/w) MMM demonstrates an unprecedented 759% enhancement in ethylene (C2H4) permeability (281 Barrer) with a moderate ethylene/ethane (C2H4/C2H6) selectivity of 2.95 compared to the PI, surpassing the polymeric upper limit for C2H4/C2H6 separation.

Advancing Membrane Technology: Ordered Macroporous ZIF-67 as a Filler in Mixed Matrix Membranes for Enhanced Propylene/Propane Separation

Conventional separation technologies for valuable commodities require substantial energy, accounting for 10%-15% of global consumption. Mixed-matrix membranes (MMMs) offer a promising solution by combining processable polymers with selective inorganic fillers. Here, the potential of using ordered microporous structured materials is demonstrated as MMM fillers. The use of ordered macroporous ZIF-67 in combination with the well-known 6FDA-DAM polymer leads to superior performance in the important separation of propylene from propane. The enhanced performance can be rationalized with the help of advanced microscopy, which demonstrates that the polymer is able to penetrate the macroporous network around which the MOF (Metal-Organic Framework) is synthesized, resulting in a much better interphase between the two components and the homogeneous distribution of the filler, even at high loadings.

Hydrogen-bonded organic frameworks in solution enables continuous and high-crystalline membranes

Hydrogen-Bonded organic frameworks (HOFs) are a type of emerging porous materials. At present, little research has been conducted on their solution state. This work demonstrates that HOFs fragment into small particles while maintaining their original assemblies upon dispersing in solvents, as confirmed by Cryo-electron microscopy coupled with 3D electron diffraction technology. 1D and 2D-Nuclear Magnetic Resonance (NMR) and zeta potential analyses indicate the HOF-based colloid solution and the isolated molecular solution have significant differences in intermolecular interactions and aggregation behavior. Such unique solution processibility allows for fabricating diverse continuous HOF membranes with high crystallinity and porosity through solution-casting approach on various substrates. Among them, HOF-BTB@AAO membranes show high C3H6 permeance (1.979 × 10-7 mol·s-1·m-2·Pa-1) and excellent separation performance toward C3H6 and C3H8 (SF = 14). This continuous membrane presents a green, low-cost, and efficient separation technology with potential applications in petroleum cracking and purification.

High-Performance FAU Zeolite Membranes Derived from Nano-Seeds for Gas Separation

In this study, high-performance FAU (NaY type) zeolite membranes were successfully synthesized using small-sized seeds of 50 nm, and their gas separation performance was systematically evaluated. Employing nano-sized NaY seeds and an ultra-dilute reaction solution with a molar composition of 80 Na2O: 1Al2O3: 19 SiO2: 5000H2O, the effects of synthesis temperature, crystallization time, and porous support (α-Al2O3 or mullite) on the formation of FAU membranes were investigated. The results illustrated that further extending the crystallization time or increasing the synthesis temperature led to the formation of a NaP impurity phase on the FAU membrane layer. The most promising FAU membrane with a thickness of 2.7 µm was synthesized on an α-Al2O3 support at 368 K for 8 h and had good reproducibility. The H2 permeance of the membrane was as high as 5.34 × 10-7 mol/(m2 s Pa), and the H2/C3H8 and H2/i-C4H10 selectivities were 183 and 315, respectively. The C3H6/C3H8 selectivity of the membrane was as high as 46, with a remarkably high C3H6 permeance of 1.35 × 10-7 mol/(m2 s Pa). The excellent separation performance of the membrane is mainly attributed to the thin, defect-free membrane layer and the relatively wide pore size (0.74 nm).

Mixed Matrix Membranes with Surface Functionalized Metal-Organic Framework Sieves for Efficient Propylene/Propane Separation

Membrane technology, regarded as an environmentally friendly and sustainable approach, offers great potential to address the large energy penalty associated with the energy-intensive propylene/propane separation. Quest for molecular sieving membranes for this important separation is of tremendous interest. Here, a fluorinated metal-organic framework (MOF) material, known as KAUST-7 (KAUST: King Abdullah University of Science and Technology) with well-defined narrow 1D channels that can effectively discriminate propylene from propane based on a size-sieving mechanism, is successfully incorporated into a polyimide matrix to fabricate molecular sieving mixed matrix membranes (MMMs). Markedly, the surface functionalization of KAUST-7 nanoparticles with carbene moieties affords the requisite interfacial compatibility, with minimal nonselective defects at polymer-filler interfaces, for the fabrication of a molecular sieving MMM. The optimal membrane with a high MOF loading (up to 45 wt.%) displays a propylene permeability of ≈95 barrer and a mixed propylene/propane selectivity of ≈20, far exceeding the state-of-the-art upper bound limits. Moreover, the resultant membrane exhibits robust structural stability under practical conditions, including high pressures (up to 8 bar) and temperatures (up to 100 °C). The observed outstanding performance attests to the importance of surface engineering for the preparation and plausible deployment of high-performance MMMs for industrial applications.

Cross-Linked Polyimide/ZIF-8 Mixed-Matrix Membranes by In Situ Formation of ZIF-8: Effect of Cross-Linking on Their Propylene/Propane Separation

Despite their potential for the scalable production of mixed-matrix membranes (MMMs), the MMMs prepared by the polymer-modification-enabled in situ metal-organic framework formation (PMMOF) process showed a considerable reduction in gas permeability as the filler loading increased. It was hypothesized that a correlation existed between the decrease in permeability and the change in the properties of the polymer, such as free volume and chain flexibility, upon in situ MOF formation. Herein, we aim to address the permeability reduction by using a cross-linked polyimide (6FDA-DAM:DABA (3:2)). It was found the degree of cross-linking affected not only the properties of the polymer, but also the in situ formation of the ZIF-8 filler particles in the cross-linked polymer. The proper degree of cross-linking resulted in suppressing C₃H₆ permeability reduction, suggesting a possible strategy to overcome the issue of PMMOF. The swelling of the polymer followed by chain rearrangement during the PMMOF, as well as the structural rigidity of the polymer, were found to be critical in mitigating permeability reduction.

Advances in metal-organic framework-based membranes

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

X-ray Diffraction and Molecular Simulations in the Study of Metal-Organic Frameworks for Membrane Gas Separation

For more than a decade, researchers have been developing metal-organic frameworks (MOFs) in the form of pure MOF membranes as well as MOF-containing mixed-matrix membranes. MOF membranes have been used for H₂/CO₂ or C₃H₆/C₃H₈ separation, but relatively few MOF membranes enable the high-performance separation of CO₂/N₂, CO₂/CH₄, or N₂/CH₄. This article describes the use of in situ XRD analysis and molecular simulation to elucidate gas transport within MOFs and derivative membranes at the molecular level. In a review of recent studies by the authors and other research groups, this article examines the flexibility of MOFs initiated by activation, gas adsorption, and aging effects during gas permeation. This article also discusses the application of XRD analysis in conjunction with computational methods to investigate the CO₂-MOF Coulombic interaction and its effects on CO₂ separation. Note that this combined analysis approach is also useful in studying the effects of linker rotation on N₂/CH₄ separation. This article also examines the use of computational tools in identifying new MOFs for gas separation and, more importantly, in elaborating the relationship between the structure of MOFs and their corresponding gas transport properties.

Recent Advances in Mixed-Matrix Membranes for Light Hydrocarbon (C1–C3) Separation

Light hydrocarbons, obtained through the petroleum refining process, are used in numerous applications. The separation of the various light hydrocarbons is challenging and expensive due to their similar melting and boiling points. Alternative methods have been investigated to supplement cryogenic distillation, which is energy intensive. Membrane technology, on the other hand, can be an attractive alternative in light hydrocarbon separation as a phase change that is known to be energy-intensive is not required during the separation. In this regard, this study focuses on recent advances in mixed-matrix membranes (MMMs) for light hydrocarbon (C1–C3) separation based on gas permeability and selectivity. Moreover, the future research and development direction of MMMs in light hydrocarbon separation is discussed, considering the low intrinsic gas permeability of polymeric membranes.

High-aspect ratio zeolitic imidazolate framework (ZIF) nanoplates for hydrocarbon separation membranes

Metal-organic frameworks with high aspect ratios have the potential to yield high-performance gas separation membranes. We demonstrate the scalable synthesis of high–aspect ratio zeolitic imidazolate framework (ZIF)–8 nanoplates via a direct template conversion method in which high aspect ratio–layered Zn hydroxide sheets [Zn₅(NO₃)₂(OH)₈] were used as the sacrificial precursor. Successful phase conversion occurs as a result of the collaboration of low template stability and delayed delivery of 2-methylimidazole in weakly interacting solvents, particularly using acetone. When the ZIF-8 nanoplates with an average aspect ratio of 20 were shear aligned in the 6FDA-DAM polymer matrix by bar coating, the separation performance for propylene/propane far surpassed that of the previously reported mixed matrix and polymeric membranes, showing a propylene permeability of 164 Barrer and selectivity of 33.4 at 40 weight % loadings.

Confined Ionic Liquid-Built Gas Transfer Pathways for Efficient Propylene/Propane Separation

The separation of light olefins from paraffins using membrane technology is highly desired; however, synthetic polymer membranes generally suffer a pernicious trade-off between permeability and selectivity. Herein, we show that this limitation can be overcome by constructing selective gas transfer pathways in a polymer matrix, as demonstrated by incorporating composites of ionic liquids and zeolitic imidazolate frameworks (ZIFs) to form mixed-matrix membranes. Using propylene/propane separation as a model system, dramatic improvements in the propylene permeability of 218.4 Barrer and propylene/propane separation factor of 45.7 were achieved compared to the values obtained using individual components as a filler. The synergy between the high solubility of the gas molecules in ionic liquids and the size screening ability of ZIF exacerbates the difference in the transmission of propylene and propane, thus leading to superior separation performance. This work presents a promising strategy for the design of membranes for efficient gas separation.

Recent Advances in Polymer-Inorganic Mixed Matrix Membranes for CO2 Separation

Since the second industrial revolution, the use of fossil fuels has been powering the advance of human society. However, the surge in carbon dioxide (CO2) emissions has raised unsettling concerns about global warming and its consequences. Membrane separation technologies have emerged as one of the major carbon reduction approaches because they are less energy-intensive and more environmentally friendly compared to other separation techniques. Compared to pure polymeric membranes, mixed matrix membranes (MMMs) that encompass both a polymeric matrix and molecular sieving fillers have received tremendous attention, as they have the potential to combine the advantages of both polymers and molecular sieves, while cancelling out each other's drawbacks. In this review, we will discuss recent advances in the development of MMMs for CO2 separation. We will discuss general mechanisms of CO2 separation in an MMM, and then compare the performances of MMMs that are based on zeolite, MOF, metal oxide nanoparticles and nanocarbons, with an emphasis on the materials' preparation methods and their chemistries. As the field is advancing fast, we will particularly focus on examples from the last 5 years, in order to provide the most up-to-date overview in this area.

Delayed Linker Addition (DLA) Synthesis for Hybrid SOD ZIFs with Unsubstituted Imidazolate Linkers for Propylene/Propane and n-Butane/i-Butane Separations

We present a novel synthesis strategy termed delayed linker addition (DLA) to synthesize hybrid zeolitic-imidazolate frameworks containing unsubstituted imidazolate linkers (Im) with SOD topology (hereafter termed Im/ZIF-8). Im linker incorporation can create larger voids and apertures, which are important properties for gas storage and separation. To date, there have been only a handful of reports of Im linkers incorporated into ZIF-8 frameworks, typically requiring arduous and complicated post synthesis approaches. DLA, as reported here, is a simple one-step synthesis strategy allowing high incorporation of Im linker into the ZIF-8 framework while still retaining its SOD topology. We fabricated mixed-matrix membranes (MMMs) with 6FDA-DAM polymer and Im/ZIF-8 obtained via DLA as a filler. The Im/ZIF-8-containing MMMs showed excellent performance for both propylene/propane and n-butane/i-butane separation, displaying permeability and ideal selectivity well above the polymer upper bound. Moreover, highly detailed molecular simulations shed light to the aperture size and flexibility response of Im/ZIF-8 and its improved diffusivity as compared to ZIF-8.

Defect Engineering in Metal-Organic Frameworks Towards Advanced Mixed Matrix Membranes for Efficient Propylene/Propane Separation

Highly permselective and durable membrane materials have been sought for energy-efficient C₃ H₆ /C₃ H₈ separation. Mixed-matrix membranes (MMMs) comprising a polymer matrix and metal-organic frameworks (MOFs) are promising candidates for this application; however, rational matching of filler-matrix is challenging and their separation performances need to be further improved. Here, we propose a novel strategy of "defect engineering" in MOFs as an additional degree of freedom to design advanced MMMs. MMMs incorporated with defect-engineered MOFs exhibit exceptionally high C₃ H₆ permeability and maintained C₃ H₆ /C₃ H₈ selectivity, especially with enhanced stability under industrial mixed-gas conditions. The gas transport, sorption, and material characterizations reveal that the defect sites in MOFs provide the resulting MMMs with not only ultrafast diffusion pathways but also favorable C₃ H₆ sorption by forming complexation with unsaturated open metal sites, confirmed by in situ FT-IR studies. Most importantly, the concept is also valid for different polymer matrices and gas pairs, demonstrating its versatile potential in other fields.

Solution processable metal-organic frameworks for mixed matrix membranes using porous liquids

The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that metal-organic frameworks, a type of highly crystalline porous solid, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known zeolitic imidazolate framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal-organic frameworks and other applications.

MOF-Based Membranes for Gas Separations

Metal-organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

All-Nanoporous Hybrid Membranes: Incorporating Zeolite Nanoparticles and Nanosheets with Zeolitic Imidazolate Framework Matrices

Metal-organic framework (MOF) membranes have attractive molecular separation properties but require challenging thin-film deposition techniques on expensive/specialty supports to obtain high performance relative to conventional polymer membranes. We demonstrate and analyze in detail the new concept of all-nanoporous hybrid membranes (ANHMs), which combines two or more nanoporous materials of different morphologies into a single membrane without the use of any polymeric materials. This allows access to a previously inaccessible region of very high permeability and selectivity properties, a feature that enables ANHMs to show high performance even when fabricated with simple coating and solvent evaporation methods on low-cost supports. We synthesize several types of ANHMs that combine the MOF material ZIF-8 with the high-silica zeolite MFI (the latter being employed in both nanoparticle and nanosheet forms). We show that continuous ANHMs can be obtained with high (∼50%) volume fractions of both MOF and zeolite components. Analysis of the multilayer microstructures of these ANHMs by multiple techniques allows estimation of the propylene/propane separation properties of individual ANHM layers, providing initial insight into the dramatically increased permeability and selectivity observed in ANHMs in relation to single-phase nanoporous membranes.

MOF-Polymer Hybrid Materials: From Simple Composites to Tailored Architectures

Metal-organic frameworks (MOFs) are inherently crystalline, brittle porous solids. Conversely, polymers are flexible, malleable, and processable solids that are used for a broad range of commonly used technologies. The stark differences between the nature of MOFs and polymers has motivated efforts to hybridize crystalline MOFs and flexible polymers to produce composites that retain the desired properties of these disparate materials. Importantly, studies have shown that MOFs can be used to influence polymer structure, and polymers can be used to modulate MOF growth and characteristics. In this Review, we highlight the development and recent advances in the synthesis of MOF-polymer mixed-matrix membranes (MMMs) and applications of these MMMs in gas and liquid separations and purifications, including aqueous applications such as dye removal, toxic heavy metal sequestration, and desalination. Other elegant ways of synthesizing MOF-polymer hybrid materials, such as grafting polymers to and from MOFs, polymerization of polymers within MOFs, using polymers to template MOFs, and the bottom-up synthesis of polyMOFs and polyMOPs are also discussed. This review highlights recent papers in the advancement of MOF-polymer hybrid materials, as well as seminal reports that significantly advanced the field.

Energy-efficient separation alternatives: metal–organic frameworks and membranes for hydrocarbon separation

The diversity of metal–organic frameworks enables the design of highly efficient adsorbents and membranes towards hydrocarbon separations for energy consumption mitigation.

Mixed matrix membranes for hydrocarbons separation and recovery: a critical review

The separation and purification of light hydrocarbons are significant challenges in the petrochemical and chemical industries. Because of the growing demand for light hydrocarbons and the environmental and economic issues of traditional separation technologies, much effort has been devoted to developing highly efficient separation techniques. Accordingly, polymeric membranes have gained increasing attention because of their low costs and energy requirements compared with other technologies; however, their industrial exploitation is often hampered because of the trade-off between selectivity and permeability. In this regard, high-performance mixed matrix membranes (MMMs) are prepared by embedding various organic and/or inorganic fillers into polymeric materials. MMMs exhibit the advantageous and disadvantageous properties of both polymer and filler materials. In this review, the influence of filler on polymer chain packing and membrane sieving properties are discussed. Furthermore, the influential parameters affecting MMMs affinity toward hydrocarbons separation are addressed. Selection criteria for a suitable combination of polymer and filler are discussed. Moreover, the challenges arising from polymer/filler interactions are analyzed to allow for the successful implementation of this promising class of membranes.