Polycrystalline Metal-organic Framework Research Articles

Hierarchically Porous Structured Adsorbents with Ultrahigh Metal-Organic Framework Loading for CO2 Capture.

Metal-organic frameworks (MOFs) have emerged as promising candidates for CO2 adsorption due to their ultrahigh-specific surface area and highly tunable pore-surface properties. However, their large-scale application is hindered by processing issues associated with their microcrystalline powder nature, such as dustiness, pressure drop, and poor mass transfer within packed beds. To address these challenges, shaping/structuring micron-sized polycrystalline MOF powders into millimeter-sized structured forms while preserving porosity and functionality represents an effective yet challenging approach. In this study, a facile and versatile strategy was employed to integrate moisture-stable and scalable microcrystalline MOFs (UiO-66 and ZIF-8) into a poly(acrylonitrile) matrix to fabricate readily processable, millimeter-sized hierarchically porous structured adsorbents with ultrahigh MOF loadings (∼90 wt %) for direct industrial carbon capture applications. These structured composite beads retained the physicochemical properties and separation performance of the pristine MOF crystal particles. Structured UiO-66 and ZIF-8 exhibited high specific surface areas of 1130 m2 g-1 and 1431 m2 g-1, respectively. The structured UiO-66 achieved a CO2 adsorption capacity of 2.0 mmol g-1 at 1 bar and a dynamic CO2/N2 selectivity of 17 for a CO2/N2 gas mixture with a 15/85 volume ratio at 25 °C. Furthermore, the structured adsorbents exhibited excellent cyclability in static and dynamic CO2 adsorption studies, making them promising candidates for practical application.

Crystal orientation control in angstrom-scale channel membranes for significantly enhanced blue energy harvesting

Achieving high-performance blue energy harvesting from an interface between seawater and river water remains challenging, because the current ion-selective membrane suffers from a trade-off between ion permeability and selectivity. Here, we tackle this issue by manipulating the crystal orientations within metal–organic framework (MOF) MIL-178 membranes, including one with the a-axis perpendicular to the substrate, and the other with the b-axis similarly configured. We show that the MIL-178-a-oriented membrane exhibited over one order of magnitude faster ion transport than the MIL-178-b-oriented membrane, and one-dimensional angstrom-scale (∼3.2 Å) channels endow the former with ultrahigh ion selectivity. Consequently, the MIL-178-a-oriented membrane achieves an ultrahigh power density of ∼9.64 W/m2, with an unprecedented energy conversion efficiency of ∼38.5 % approaching the theoretical upper limit of 50 %, when simulated seawater and river water are mixed, surpassing all the reported non-oriented MOF-based membranes. This study presents a method for finely tuning preferred orientations within polycrystalline MOF membranes, enabling precise engineering of ultrahigh-performance osmotic energy conversion.

The Key to MOF Membrane Fabrication and Application: the Trade-off between Crystallization and Film Formation.

Metal-organic frameworks (MOFs), owing the merits of ordered and tailored channel structures in the burgeoning crystalline porous materials, have demonstrated significant promise in construction of high-performance separation membranes. However, precisely because this crystal structure with strong molecular interaction in their lattice provides robust structural integrity and resistance to chemical and thermal degradation, crystalline MOFs typically exhibit insolubility, infusibility, stiffness and brittleness, and therefore their membrane-processing properties are far inferior to the flexible amorphous polymers and hinder their subsequent storage, transportation, and utilization. Hence, focusing on film-formation and crystallization is the foundation for exploring the fabrication and application of MOF membranes. In this review, the film-forming properties of crystalline MOFs are fundamentally analyzed from their inherent characteristics and compared with those of amorphous polymers, influencing factors of polycrystalline MOF membrane formation are summarized, the trade-off relationship between crystallization and membrane formation is discussed, and the strategy solving the film formation of crystalline MOFs in recent years are systematically reviewed, in anticipation of realizing the goal of preparing crystalline membranes with optimized processability and excellent performance.

Linker-exchanged zeolitic imidazolate framework membranes for efficient CO2 separation

Metal organic framework (MOF) membranes have exhibited great potential for efficient gas separation owing to their excellent chemical and structural tunability. To realize CO2 separation from flue gas, linker modification or exchange is generally required to functionalize MOF membranes. However, the reported linker-exchanged polycrystalline MOF membranes have not yet achieved attractive CO2 separation performance due to the sacrificed permeance of the overly reduced pore apertures when employing bulky linkers. Herein, we report a 3-amino-1,2,4-triazole (Atz)-exchanged polycrystalline ZIF-8 membrane (Atz-ZIF-8) with Atz molar ratios ranging from 3% to 29% which shows an excellent CO2/N2 separation performance that significantly surpasses that of other linker-exchanged ZIF-8 membranes. The as-synthesized Atz-ZIF-8 membranes possess additional uncoordinated amines for CO2-preferential adsorption, resulting in an excellent CO2/N2 selectivity of 29. What's more, the size and configuration of Atz is similar to 2-methylimidazole (2-mIm), so the window size of Atz-ZIF-8 membranes is not significantly reduced, and thus high CO2 permeance (690 GPU) is achieved. Considering the superior performance and accessible manufacturing process, we conclude that our work could provide guidelines on designing advanced linker-exchanged ZIF-8 based polycrystalline membranes towards post-combustion carbon capture.

The metal organic framework of UiO-66-NH2 reinforced nanofiltration membrane for highly efficient ion sieving

Metal organic framework (MOF) with fine pore structures, ultra-high porosity and large specific surface area can provide more water transport channels while achieving precise sieving of different sizes of ions, which is a promising material for high efficiency desalination. However, particle agglomeration, interface incompatibilities and interface defects severely restrict the performance of MOF-based membranes. It is still a challenge to construct MOF-based desalination membranes with both high water permeability and high ion sieving. Herein, the UiO-66-NH2 based nanofiltration membrane was fabricated by in-situ growth of UiO-66-NH2 on Nylon membrane and composed with polyamide (PA) layer by interfacial polymerization (IP). With 12 h solvothermal reaction, the UiO-66-NH2 nanoparticles are found uniformly loaded on the Nylon substrate. Therefore, the water molecule transport channels provided by UiO-66-NH2 nanoparticles reduce the transport resistance, which can improve the water permeability. More importantly, UiO-66-NH2 nanoparticles are found to be involved in the IP process, which can reduce the interfacial defects and thus enhance the bonding force between the PA layer and base film. The synergistic effect of the UiO-66-NH2 and PA layer improves the nanofiltration performance of the membrane with rejection rate of 92.97% for Na2SO4 and the pure water permeance (PWP) of 11.55 L m−2 h−1 bar−1. The N-UN12 @PA membrane also has excellent antifouling performance (flux recovery ratio (FRR) of 95.86%, reversible fouling ratio (Rr) of 33.93%, irreversible fouling ratio (Rir) of 4.14%). Unlike the mixed-matrix membrane (MMM) and polycrystalline MOF membrane, this strategy provides a promising method to prepare high-performance nanofiltration membrane for desalination.

Balancing the Crystallinity and Film Formation of Metal-Organic Framework Membranes through In Situ Modulation for Efficient Gas Separation.

Polycrystalline metal-organic framework (MOF) layers hold great promise as molecular sieve membranes for efficient gas separation. Nevertheless, the high crystallinity tends to cause inter-crystalline defects/cracks in the nearby crystals, which makes crystalline porous materials face a great challenge in the fabrication of defect-free membranes. Herein, for the first time, we demonstrate the balance between crystallinity and film formation of MOF membrane through a facile in situ modulation strategy. Monocarboxylic acid was introduced as a modulator to regulate the crystallinity via competitive complexation and thus concomitantly control the film-forming state during membrane growth. Through adjusting the ratio of modulator acid/linker acid, an appropriate balance between this structural "trade-off" was achieved. The resulting MOF membrane with moderate crystallinity and coherent morphology exhibits molecular sieving for H2 /CO2 separation with selectivity up to 82.5.

Metal-Organic Framework Membrane Hybridized with Graphitic Materials for Gas Separation.

Metal-organic frameworks (MOFs) are an exceptional class of crystalline materials that have been extensively used to fabricate membranes for various applications such as gas separation, ion transport, and desalination due to their well-defined pore structure, chemical features, and simple synthesis process. The incorporation of graphitic carbon materials in MOFs has garnered significant attention as it can provide abundant nucleation sites and modulate gas transport by influencing the orientation or rigidity of MOF crystals without changing their porous structure. This review insights of previous studies utilizing graphene, graphene oxide, carbon nanotubes, and graphene nanoribbons for MOF-based gas separation membranes, particularly focusing on polycrystalline MOF membrane hybridization with graphitic materials. We also briefly discuss the use of carbon/MOF hybrid materials for preparing mixed matrix membranes.

Polycrystalline Metal-Organic Framework Membranes for Separation of Light Hydrocarbons.

Due to facile designability and versatile nanospace, metal-organic frameworks (MOFs) have been considered as promising membrane materials. Compared to the mixed matrix membranes that incorporated with MOF particles, the polycrystalline MOF membranes demonstrates significant advantages in maximum utilizing the crystalline nanospace, and thus yielding a fruitful of achievements in the last twenty years. Although some reviews have summarized the development of MOF-based membranes, the theoretical framework for oriented design and preparation of polycrystalline MOF membranes for highly efficient separation of light hydrocarbons remains in infancy. Herein, in this review, the fabrication strategies of polycrystalline MOF membranes and the corresponding performance in the separation of light hydrocarbons were classified and summarized. Particularly, the MOF membranes with global and local dynamics have been proposed as an interesting topic promoted performance.

Postsynthetic modification strategies to improve polycrystalline metal-organic framework membranes

A wide variety of metal organic frameworks (MOFs) created by periodic combinations between organic ligands and metal ions or metal-oxo clusters have paved the way for the development of energy-efficient membrane-based separations that potentially can serve as feasible substitutes for thermal counterparts. Although significant progress has been made in the synthesis of polycrystalline MOF membranes over the last decade, only a limited number of MOFs have been utilized for relevant research. The lack of a clear solution for intercrystalline defects, non-selective diffusion pathways in polycrystalline membranes, is most likely a deciding factor behind the delay. Postsynthetic modifications (PSMs) are considered as newly emerging strategies for offering polycrystalline MOF membrane diversity by utilizing existing membranes as a platform and improving their separation functions via physical and/or chemical treatments and therefore, neither designing new MOFs nor customizing membrane synthesis techniques for targeted MOFs are required. In this minireview, eight subclasses ((1) covalent tethering, (2) intercrystalline defect plugging, (3) intracrystalline defect healing, (4) inclusion of functional materials in pores, (5) stiffening, (6) ligand exchange, (7) amorphization, and (8) MOF to MOF transformation) of PSM strategies, that have been applied to polycrystalline MOF membranes, are summarized, and challenges and future directions are discussed.

Low-cost in-situ luminescence: Opportunities for MOFs water vapor sensing

Luminescence is the most exploited phenomena for chemosensing. Real-time measurements must be accurate and reproducible along cycles for the selection of the best sensing materials. At the same time, sample degradation by excitation-light irradiation must be kept minimal. Last but not least, expensive equipment should be avoided, whenever possible. In this work, we present a low-cost in-situ luminescence measuring experimental array designed for solid-state sensing of vapors. It allows repeated activation and subsequent guest entrance along many measuring cycles. A common halogen lamp is used as heating element for the activation of the sample under vacuum in a sealed aluminium chamber. Then, vapor guest molecules are forced into the chamber by a laboratory vacuum pump. Luminescence is LED-excited and measured by a portable reflectance probe connected to a miniature spectrometer. The chosen case study is a luminescent polycrystalline metal-organic framework composed by europium(III), zinc(II) and oxydiacetate ligand. This compound has a honeycomb porous structure that can be repeatedly activated and rehydrated. The associated structural distortion is accompanied by a remarkable variation in the luminescence profile with minimal chemical degradation or crystallinity loss. Results are analyzed and critically compared to those obtained by conventional luminescence. They show that, while conventional luminescence struggles to provide reproducible absolute results, the presented method stands as a simple, robust and trustworthy approach, in which accurate and reproducible in- situ data can be obtained along many cycles. Furthermore, the presented experimental array comparatively minimizes the light-induced degradation of the tested materials. The effect of glue-immobilizing the sample is also addressed using the presented method. • A low-cost in-situ luminescence array for solid-state sensing is designed • Performance of the new array is compared to conventional luminescence • A water-sensitive luminescent Eu-Zn MOF is explored as case-study • Accurate and reproducible data can be obtained along multiple cycles • Light-induced degradation of the tested materials is reduced

Polycrystalline Iron(III) metal-organic framework membranes for organic solvent nanofiltration with high permeance

There is an urgent demand for separation membranes based on metal-organic frameworks (MOFs) with high efficiency, low waste generation, and low energy consumption. Herein, we report the fabrication of high-quality, continuous iron (III)-based PCN-250 polycrystalline MOF membranes for organic solvent nanofiltration. Due to the usage of rub seeding, the PCN-250 membrane has a small thickness of 3.7 μm and exhibits pure solvent permeance as high as 337.9 L m −2 h −1 bar −1 for methanol, 69.2 L m −2 h −1 bar −1 for ethanol, and 17.2 L m −2 h −1 bar −1 for N-methyl-2-pyrrolidone (NMP). The membrane has pore sizes ranging from 0.6 to 1.0 nm, with a molecular weight cut-off of ca. 1300 g/mol. In addition, it exhibits excellent molecular separation performance (methyl blue rejection of 93.7%) alongside a high NMP permeance of 2.6 L m −2 h −1 bar −1 . Moreover, our study confirms that the PCN-250 MOF membrane has good water, acid, and organic solvent resistance. This remarkable separation performance, combined with the outstanding stability in water, acid, and organic solvents, makes the PCN-250 membrane a promising candidate for organic solvent nanofiltration. • Fe(III)-based PCN-250 membranes were fabricated for organic solvent nanofiltration. • The thickness of PCN-250 membrane was controlled by rub seeding with secondary growth. • PCN-250 membrane has pore sizes ranging from 0.6 to 1.0 nm, with a molecular weight cut-off of ca. 1300 g/mol. • PCN-250 membrane has high solvent permeance with good solvent and acid stability. • PCN-250 membrane has methyl blue rejection of 93.7% with high NMP permeance of 2.6 L m −2 h −1 bar −1 .

Lithium Extraction by Emerging Metal–Organic Framework‐Based Membranes

AbstractLithium is mainly extracted from brine and ores; however, current lithium mining methods require large amounts of chemicals, discharge many wastes, and can have serious environmental repercussions. Metal–organic framework (MOF)‐based membranes have shown great potential in lithium extraction due to their uniform pore sizes, high porosities, and rich host–guest chemistry compared to other materials. In this review, the processes and disadvantages of current lithium extraction technologies are introduced. The structure features and corresponding design strategy of MOFs suitable for Li+ ion separations are presented. Following, recent advances of polycrystalline MOF membranes, mixed matrix membranes, and MOF channel membranes for lithium‐ion separation are discussed in detail. Finally, opportunities for future developments and challenges in this emerging research field are presented.

High Loading of Air-Sensitive Guest Molecules into Polycrystalline Metal-Organic Framework Hosts.

Loading air-sensitive guest molecules inside polycrystalline metal-organic framework (MOF) hosts is currently a challenging process. In this study, the air-sensitive guest molecule magnesocene (MgCp2) was loaded into two porous MOF hosts, polycrystalline Ni-MOF-74 and NH2-MIL-101(Al), using a gas-phase infiltration process. X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and scanning transmission electron microscopy-energy-dispersive X-ray mapping measurements demonstrated that MgCp2 was successfully loaded inside the three-dimensional pores of NH2-MIL-101(Al) with a maximum loading of 43.1 wt %. MgCp2 was found to cover the outside of Ni-MOF-74 owing to the small one-dimensional channels.

Design of metal-organic framework membranes towards ultimate gas separation

Metal-organic framework (MOF) membranes have shown unprecedented opportunities for energy efficient separation of diverse industrially important gas pairs (e.g. H2/CO2, CO2/N2, CO2/CH4, and C3H6/C3H8). Among the various factors, microstructure manipulation (including thickness, orientation, and grain boundary structure) and framework tuning (including pore size, framework rigidity/flexibility, and stimuli responsiveness) of MOF membranes have been found playing dominant roles in their separation performances. In this perspective, we highlighted some recent progress in polycrystalline MOF membranes with emphasis on the elucidation of the structure-performance relationship at different scales.

Polycrystalline rare-earth metal-organic framework membranes with in-situ healing ability for efficient alcohol dehydration

Because of their uniform yet tunable pore size and rich functionality, metal-organic frameworks (MOFs) have attracted huge interests as membrane materials. However, the relatively weak stability limits their applications in liquid separation. Herein, we report the fabrication of polycrystalline rare-earth MOF membranes (Sm-DOBDC) supported on alumina hollow fiber, exhibiting excellent water stability. Using these membranes for the dehydration of 5 wt% water/ethanol feed solution at 323 K, a total flux of 786.4 ± 33.7 g m−2 h−1 and a 99.8 ± 0.2 wt% water concentration in the permeate can be achieved, with a high separation factor of above 9481. In addition, in-situ healing ability of this membrane is demonstrated by treating with spent reaction solution, shedding light on the practical applications of polycrystalline MOF membranes in liquid separation.

A MOF Glass Membrane for Gas Separation.

Metal-organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF-62 MOF glass membrane and exploited its intrinsic gas-separation properties. The MOF glass membrane was fabricated by melt-quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF-62 MOF membrane on a porous ceramic alumina support. The molten ZIF-62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H2 /CH4 , CO2 /N2 and CO2 /CH4 mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.

Synthesis of high-performance polycrystalline metal–organic framework membranes at room temperature in a few minutes

Our method hinders the Ostwald ripening of polycrystalline MOF film during the solvothermal synthesis, allowing the growth of high-quality MOF films in just 8 min at room temperature.

Homochiral MOF–Polymer Mixed Matrix Membranes for Efficient Separation of Chiral Molecules

AbstractHomochiral metal–organic framework (MOF) membranes have been recently reported for chiral separations. However, only a few high‐quality homochiral polycrystalline MOF membranes have been fabricated due to the difficulty in crystallization of a chiral MOF layer without defects on porous substrates. Alternatively, mixed matrix membranes (MMMs), which combine potential advantages of MOFs and polymers, have been widely demonstrated for gas separation and water purification. Here we report novel homochiral MOF–polymer MMMs for efficient chiral separation. Homochirality was successfully incorporated into achiral MIL‐53‐NH2 nanocrystals by post‐synthetic modification with amino acids, such as l‐histidine (l‐His) and l‐glutamic acid (l‐Glu). The MIL‐53‐NH‐l‐His and MIL‐53‐NH‐l‐Glu nanocrystals were then embedded into polyethersulfone (PES) matrix to form homochiral MMMs, which exhibited excellent enantioselectivity for racemic 1‐phenylethanol with the highest enantiomeric excess value up to 100 %. This work, as an example, demonstrates the feasibility of fabricating diverse large‐scale homochiral MOF‐based MMMs for chiral separation.

Metal-Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy.

While metal-organic frameworks (MOFs) have been under thorough investigation over the pasttwo decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize time-resolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude-Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials.

Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membranes for Carbon Capture.

Although polycrystalline metal-organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO2 separation performance, crucial for energy-efficient carbon capture. ZIF-8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO2 /N2 and CO2 /CH4 separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF-8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH4 > CO2 > H2 ). Unprecedented CO2 /CH4 , CO2 /N2 , and H2 /CH4 selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C3 H6 , are achieved. Spectroscopic and X-ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF-8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas-separation performance of the MOF membranes.