MOF Membranes Research Articles

Oriented Titanium-MOF Membrane for Hydrogen Purification.

Precise hydrogen sorting from purge gas (H2/N2) and coke gas (H2/CH4), commonly carried out by cryogenic distillation, still suffers from low separation efficiency, high energy consumption, and considerable capital cost. Though still in its infancy, membrane technology offers a potential to achieve more efficient hydrogen purification. In this study, an optimum separation of hydrogen towards both methane and nitrogen via a kinetically-driven mechanism is realized through preferred orientation control of a MOF membrane. Relying on the 0.3 nm-sized window aligned vertical to the substrate, b-oriented Ti-MOF membrane exhibits ultra-high hydrogen selectivity, surpassing the upper bound limit of separating H2/N2 and H2/CH4 gas pairs attained so far by inorganic membranes. This spectacular selectivity is combined with a high H2 permeability owing to the synergistic effect of the 1 nm-sized MOF channel.

Oriented Titanium‐MOF Membrane for Hydrogen Purification

Precise hydrogen sorting from purge gas (H2/N2) and coke gas (H2/CH4), commonly carried out by cryogenic distillation, still suffers from low separation efficiency, high energy consumption, and considerable capital cost. Though still in its infancy, membrane technology offers a potential to achieve more efficient hydrogen purification. In this study, an optimum separation of hydrogen towards both methane and nitrogen via a kinetically‐driven mechanism is realized through preferred orientation control of a MOF membrane. Relying on the 0.3 nm‐sized window aligned vertical to the substrate, b‐oriented Ti‐MOF membrane exhibits ultra‐high hydrogen selectivity, surpassing the upper bound limit of separating H2/N2 and H2/CH4 gas pairs attained so far by inorganic membranes. This spectacular selectivity is combined with a high H2 permeability owing to the synergistic effect of the 1 nm‐sized MOF channel.

Low-crystallinity ZIF-8/dcIm membranes for CO2 sieving

Metal-organic framework membranes have shown great potential for efficient separation applications. However, unavoidable grain boundary defects and the inconstant pore size due to framework flexibility hinder the application of MOF membranes for gas separation. Herein, low-crystallinity ZIF-8/dcIm-x membranes are synthesized via fast current-driven synthesis. The competitive coordination of 4,5-dichloroimidazole (dcIm) linker leads to the formation of a low-crystallinity MOF with reduced grain boundary defects for better gas separation performance. Meanwhile, the in-situ substitution of the mIm linker by dcIm narrows the pore size and transforms the ZIF-8 structure in the stiffened Cm polymorph, allowing precise CO2 sieving. Consequently, the low-crystallinity ZIF-8/dcIm-14 membrane exhibits a promising CO2/CH4 selectivity of 32.4 with a CO2 permeance of 1.73 × 10−8 mol m−2 s−1 Pa−1, which surpasses other membranes. Furthermore, the membrane presents a remarkable pressure resistance up to 3 bar and stable temperature-swing operation between room temperature and 125 °C over 120 h.

Sustainable fabrication of highly (110)-oriented ZIF-8 membrane via supercritical fluid processing

Preferred orientation control represents a crucial step for performance enhancement of MOF membranes. Nevertheless, their sustainable preparation remains a significant challenge. In this work, we fabricated highly (110)-oriented ZIF-8 membranes through supercritical fluid (SCF)-assisted epitaxial growth of oriented seed layer. Benefiting from unique physicochemical properties and intrinsic chemical inertness of supercritical CO2, preferential orientation originated from the seed layer could be well maintained during epitaxial SCF processing; moreover, both unreacted ligands and discharged CO2 could be efficiently recovered, resulting in zero pollutant emission. Our ZIF-8 membrane exhibited ideal C3H6/C3H8 selectivity of 91.8. To the best of our knowledge, this represented the first report of the preparation of highly oriented ZIF-8 membrane with such high C3H6/C3H8 selectivity; more importantly, our study demonstrated that SCF processing represented an effective protocol for orientation modulation of MOF membranes towards superior separations.

Polymer-functionalized metal-organic framework nanosheet membranes for efficient CO2 capture

Two-dimensional metal-organic frameworks (2DMOFs) have emerged as highly appealing candidates as advanced separation membrane materials. However, constructing 2DMOF membranes with appropriate channel sizes to separate CO2 from N2 remains a great challenge. Here, we propose an in-situ crystallization approach to synthesize CO2-philic-polymer functionalized Zr-based MOF nanosheets, enabling the fabrication of efficient CO2 capture membranes. The in-situ MOF crystallization with polyethylenimine (PEI) polymers leads to a uniform distribution of amines, resulting in the formation of PEI-NUS hybrid nanosheet characterized by both high porosity and strong affinity to CO2. These nanosheets are assembled into ultrathin lamellar membranes with thicknesses ranging from 40 to 120 nm. The amines of PEI polymers significantly facilitate the CO2 transfer as reaction carriers, resulting in a substantially improved CO2/N2 separation performance with a CO2 permeance of 410 GPU and a CO2/N2 selectivity of 90, which are among the best for MOF membranes in CO2 separation. This in-situ functionalization strategy paves the way for the fabrication of ultrathin 2D membranes for various separations.

Dual‐Wing Ligand Constructed Metal–Organic Framework Membranes with Finely Tuned Apertures for Natural Gas Separation

AbstractMetal–organic framework (MOF) membranes with apparent molecular sieving effects have great potential for gas separation. However, their application in light‐gas separation (e.g., CO2/CH4 and H2/CH4) remains challenging due to their enlarged pore structures under transmembrane pressure. In this work, a series of MOF membranes constructed from dual‐wing (DW) ligands including 2‐chloromethylbenzimidazole, 2‐methylbenzimidazole, 2‐ethylbenzimidazole, and 2‐phenylbenzimidazole are reported, to finely tune apertures and enhance molecule sieving property for small‐size gases. These DW‐ligands provide steric resistance in two directions perpendicular to the coordination bond, leading to a much finer pore structure. The introduction of the DW‐ligands endows the DW/ZIF‐8 membranes with an adjustable bottleneck door for regulating gas diffusion, blocking the transport of large‐sized CH4 while allowing small‐sized H2 and CO2 to permeate. The membranes show significantly improved molecular sieving property with a CO2/CH4 mixed‐gas selectivity of 58.6 in the case of 2‐chloromethylbenzimidazole23/ZIF‐8, which represents the highest value among the reported ZIF membranes. This membrane also exhibits exceptional separation performance for H2/CH4 and H2/C3H8 with separation factors of 430 and 34341, which are the highest values among the reported MOF membranes. This study presents a facile and efficient strategy for regulating MOF aperture and constructing high‐performance membranes for natural gas separation.

Ultrathin and Self-Supporting MOF/COF-Based Composite Membranes for Hydrogen Separation and Purification from Coke Oven Gas.

Coke oven gas (COG) is considered to be one of the most likely raw materials for large-scale H2 production in the near or medium term, with membrane separation technologies standing out from traditional technologies due to their less energy-intensive structures as well as simple operation and occupation. Based on the "MOF-in/on-COF" pore modification strategy, the COF membrane (named the PBD membrane) and ZIF-67 were used as assembly elements to design advanced molecular sieving membranes for hydrogen separation. The composition and microstructure of membranes before and after ZIF-67 loading as well as ZIF-67-in-PBD membranes under different preparation conditions (metal ion concentration, metal-ligand ratio, and reaction time) were investigated by various characterizations to reveal the synthesis regularity and microstructure regulation. Furthermore, H2/CH4 separation performances and separation mechanisms were also analyzed and compared. Finally, a dense, continuous, ultrathin, and self-supporting ZIF-67-in-PBD membrane with a Co2+ concentration of 0.02 mol/L, a metal-ligand ratio of 1:4, and a reaction time of 6 h exhibited the largest specific surface area, micropore proportion, and the best H2/CH4 separation selectivity (α = 33.48), which was significantly higher than the Robeson upper limit and was in a leading position among reported MOF membranes. The separation mechanism was mainly size screening, and adsorption selectivity also contributed a little.

Room temperature aqueous synthesis of defect-engineered MOF-801 membrane towards efficient nanofiltration

Room temperature (RT) aqueous preparation of MOF membranes is of significant interest in meeting the demands of sustainability. Nevertheless, there remains no report on RT aqueous synthesis of water-stable high-valence MOF membranes to date. In this study, we developed a one-step RT protocol for fabricating defect-engineered MOF-801 membrane in aqueous media. Owing to enhanced structural hydrophilicity and enlarged micropore size, water was enabled to diffuse through the membrane with higher permeation flux. Obtained membrane exhibited exceptional dye rejection rate (CR:99.75 %, MB:99.69 % and MG:99.41 %) and unprecedented water flux (CR:77.33 L m−2 h−1·bar−1, MB:71.43 L m−2 h−1·bar−1 and MG:81.28 L m−2 h−1·bar−1), which were superior to state-of-the-art 3D pure MOF membranes. Our protocol paves a promising way towards sustainable production of water-stable MOF membranes under mild conditions.

Single-file diffusion and its influence on membrane gas separation: A case study on UTSA-280

Gas permeation in metal-organic framework-based (MOF-based) membranes is often elucidated through the solution-diffusion model, where the adsorption and diffusion of gases play equally vital roles. This study draws attention to a unique phenomenon known as single-file diffusion (SFD) occurring within MOF membranes that possess ultramicropores with dimensions comparable to the gas molecules themselves. When SFD takes place, the separation performance becomes solely reliant on the adsorption quantities on the feed side of the membrane. As a proof of concept, we fabricated a UTSA-280 membrane and subjected it to testing using gas mixtures of CO2/N2 and CO2/CH4. Two crucial observations substantiate the occurrence of single-file diffusion. Firstly, in the gas mixtures, the diffusion coefficients of N2 or CH4 were observed to align closely with that of CO2, despite the intrinsic diffusion coefficients of CO2, N2, and CH4, as determined from single-gas permeation experiments, exhibiting differences. Secondly, the CO2 mole fractions within the adsorption phase closely mirrored those in the gas phase on the permeate side. Given the high adsorption selectivity of UTSA-280 for CO2, the UTSA-280 membrane delivers outstanding permeation selectivity for CO2/N2 (611) and CO2/CH4 (80.7). Our experimental findings are supported by mesoscale kinetic Monte Carlo simulations, which confirmed the relation between the CO2 compositions in the adsorption phase on the feed side and in the gas phase on the permeate side. Furthermore, density functional theory calculations revealed high energy barriers associated with CO2 surpassing its counterparts, rendering such processes unfavorable, thereby reinforcing our experimental findings regarding single-file diffusion.

Robust Homochiral Polycrystalline Metal-Organic Framework Membranes for High-Performance Enantioselective Separation.

Homochiral MOF membranes offer a promising route to efficient chiral separation, but their fabrication remains challenging. Here, we report for the first time the design and preparation of homochiral polycrystalline MOF-808 membranes for the first time. The membrane exhibits a high integrity and thin membrane thickness. Achieving homochirality through chiral amino acid postsynthetic modification, MOF-808 membranes demonstrate remarkable solvent stability. Notably, they successfully separated racemic naproxen enantiomers, achieving enantiomeric excess (ee) values of up to ∼95.0%. This work paves the way for turning achiral polycrystalline MOF membranes into high-performance chiral membranes for enantioselective separation.

Removing mesopores from metal-organic framework MIL-100(Cr) membranes for superior CO2 separation

Membranes with high CO2 permeance and satisfactory CO2/N2 selectivity have been in demand for industrial applications, especially for the first stage of two-stage membrane-based process. MOF membranes with abundant pores have the potential to achieve high CO2 permeance, but most current MOF membranes fall short of the requirements (CO2 permeance >3000 GPU). Research on novel MOF membranes is urgently needed. Here, a new kind of non-crystalline (NC) MIL-100(Cr) membrane removed mesopores is fabricated by a non-fine assembly process. NC-MIL-100(Cr) possesses similar bonding structure and surface properties to crystalline MIL-100(Cr), but delivers more suitable pore size and high processibility for membrane formation. NC-MIL-100(Cr) membranes are prepared under different Cr3+ concentrations in reaction solution and secondary growth durations, among which NC-MIL-100(Cr) membranes prepared with Cr3+ concentration of 2 mmol/mol DMF and secondary growth duration of 36 h display CO2 permeance of about 4638 GPU and CO2/N2 selectivity of about 35 for mixed CO2/N2 (15/85, v/v) separation, as well as CO2 permeance of about 8670 GPU for single CO2 permeation.

Perfect confinement of crown ethers in MOF membrane for complete dehydration and fast transport of monovalent ions.

Fast transport of monovalent ions is imperative in selective monovalent ion separation based on membranes. Here, we report the in situ growth of crown ether@UiO-66 membranes at a mild condition, where dibenzo-18-crown-6 (DB18C6) or dibenzo-15-crown-5 is perfectly confined in the UiO-66 cavity. Crown ether@UiO-66 membranes exhibit enhanced monovalent ion transport rates and mono-/divalent ion selectivity, due to the combination of size sieving and interaction screening effects toward the complete monovalent ion dehydration. Specifically, the DB18C6@UiO-66 membrane shows a permeation rate (e.g., K+) of 1.2 mol per square meter per hour and a mono-/divalent ion selectivity (e.g., K+/Mg2+) of 57. Theoretical calculations and simulations illustrate that, presumably, ions are completely dehydrated while transporting through the DB18C6@UiO-66 cavity with a lower energy barrier than that of the UiO-66 cavity. This work provides a strategy to develop efficient ion separation membranes via integrating size sieving and interaction screening and to illuminate the effect of ion dehydration on fast ion transport.

Confined‐Coordination Induced Intergrowth of Metal–Organic Frameworks into Precise Molecular Sieving Membranes

AbstractMetal–organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect‐free high‐connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined‐coordination induced intergrowth strategy to fabricate lattice‐defect‐free Zr‐MOF membrane towards precise molecular separation. The confined‐coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter‐diffusion of MOF precursors (metal cluster and ligand), thereby inter‐growing MOF crystals into integrated membrane. The resulting Zr‐MOF membrane with angstrom‐sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m−2 ⋅ h−1 ⋅ bar−1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state‐of‐the‐art membranes. The molecular transport mechanism related to size‐sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

Highly flexible and controllable hierarchical MOF membrane for efficient drug release

Highly flexible and controllable hierarchical MOF membrane for efficient drug release

The effect of different solvents on the formation of large‐area MOF membranes

AbstractMetal–organic framework (MOF) membranes are widely used in gas separation processes. However, the effect of solvents on the formation of large‐area MOF membranes, which directly influences separation performance, has rarely been investigated. In this study, three solvents (methanol, ethanol, and water) were selected to systematically investigate their effects on the zeolite imidazolium frameworks (ZIF) membrane formation via experimental validation and simulations. The results indicated that the solvent selection influenced crystallization rate, crystal size, and ZIF‐8 membrane formation ability. Particularly, when water was used as a solvent, slow nucleation and crystallization of the ZIF‐8 crystals were observed, resulting in a more complete crystal structure and a thinner ZIF‐8 membrane. Therefore, the ZIF‐8 membrane exhibited a gas permeance of 10,000 gas permeation unit and enhanced CO2/N2 selectivity. Furthermore, ZIF‐8 membrane with a surface area of 2400 cm2 and spiral‐wound membrane modules were developed, paving the way for large‐scale industrial production.

Preparation of ZIF-62 polycrystalline and glass membranes for helium separation

Defects among the grains of MOF polycrystalline membranes lead to non-selective gas transport, thereby reducing their selectivity in gas separation. In this work, ZIF-62 polycrystalline membranes with a well-intergrown structure were prepared on MXene-modified supports. Subsequent thermal treatment transformed the membranes into glass membranes, effectively eliminating non-selective defects at grain boundaries. Due to the incorporation of the MXene film and the vertical positioning of the support during the solvothermal process, only a minimal portion of the glass melt infiltrated into the porous support. Across the temperature range of 303 K–423 K and pressure range of 1 bar–3 bar, the ZIF-62 glass membranes showcased superior helium separation property and long-term chemical stability (resistant to CO2 and H2O). The helium permeance reached approximately 51 GPU, with selectivities against N2 and CH4 being 17.4 and 13.9, respectively, outperforming current MOF membranes.

Co-gallate MOF Membrane for Efficient Pervaporation Separation of MeOH from MTBE

The pervaporation (PV) technology is attractive for azeotrope component separation of the MeOH/MTBE mixture due to the reduced energy and cost relative to the traditional ternary distillation process. However, the membrane separation performance is largely affected by its microstructure including pore size and shape as well as its surface properties such as hydrophilicity. Herein, we report a Co-gallate MOF membrane that features preferential adsorption and diffusion of methanol (MeOH) relative to the methyl tert-butyl ether (MTBE) molecules during the PV separation process. The high-quality Co-gallate MOF membrane has been prepared by introducing a regulator of KOH. The MeOH flux of the Co-gallate MOF membrane was 0.565 kg m−2 h−1 and the separation factor was up to 721 at 15 wt% MeOH/MTBE feed composition. It is worth noting that the separation performance exceeds most of the reported membrane materials for MeOH/MTBE separation in the PV process. Furthermore, the separation performance of the Co-gallate MOF membrane for MeOH/MTBE mixtures remained almost unchanged over 70 h, indicating the reliable long-term stability of the membrane. Thus, the Co-gallate MOF membrane demonstrated a potential practical application for MeOH/MTBE separation.

Ligand‐engineered Zr‐MOF membranes with fast water transport channels for alcohol–water separation

AbstractMetal–organic framework (MOF) membranes with rich functionality and a tunable pore system are promising for molecular separation. However, an inadequate pore microenvironment and low water stability may hamper the extension of their use in gas separation to liquid separation (e.g., organic solvent dehydration). In this study, we report a high‐valence Zr‐MOF membrane with high stability and tunable pore microenvironment, prepared using a mixed‐ligand strategy based on fumarate (fum) and 2‐methylfumarate (mes) ligands for the pervaporation dehydration of alcohols. The introduction of a methyl group in the ligand side chain narrowed the aperture size while balancing the hydrophilicity, thus facilitating the fast and selective permeation of water over alcohol molecules. The resulting mes/fum‐Zr‐MOF membrane with a mes content of 30% displayed excellent butanol/water separation performance with total flux of 5226 g−1·m−2·h−1, separation factor of 1281, and water content of 99.3% in the permeate, transcending the upper‐bound of state‐of‐the‐art MOF and polymer membranes, thereby exhibiting immense potential for alcohols dehydration.

SWCNTs-channeled MOF nanosheet membrane for high-efficient organic solvent nanofiltration

Organic solvent nanofiltration (OSN) has become increasingly important in petrochemical and pharmaceutical industries, demanding superior and robust membranes. 2D MOF nanosheets hold great promise in the preparation of OSN membrane. Nevertheless, the fabrication of defect-free 2D MOF membranes with robust mechanical strength is hampered due to the weak interlamellar interactions between MOF nanosheets. Herein, 2D MOF composite membrane for high-performance OSN application is fabricated through the assembly of copper 1,4-benzene dicarboxylate (CuBDC) nanosheets and NH2-SWCNTs. Within this composite configuration, NH2-SWCNTs can shelter the defects between the adjacent CuBDC nanosheets, rendering CuBDC/CNTs composite membranes precise molecular sieving ability. Meanwhile, the nanochannels created by NH2-SWCNTs with low frictional coefficients on the internal surface promote the permeation of organic solvents. Moreover, the multiple interactions between CuBDC and NH2-SWCNTs improve the mechanical strength of composite membranes. Accordingly, the composite membranes display excellent rejections of dye molecules on the basis of size-sieving mechanism with superior methanol permeance of 1260 L m−2h−1 bar−1, as well as steady congo red rejection at high trans-membrane pressure of 5 bar, demonstrating good pressure resistance of composite membranes. Moreover, the composite membranes exhibit outstanding stability against high-concentration feeds and long-term operation for 50 h, confirming excellent practicality in the purification of dyes from organic solvents. The design approach in this work may inspire the fabrication of other nanosheet-based membranes for high-efficient OSN.

Ultra-thin size-sieving gas separation membrane by precisely contra-diffusion-match growth of ZIF-67 among GO laminates

Ultrathin and defect-free MOF membranes are actively pursued to realize high-performance hydrogen purification and carbon capture, which are of great significance for energy conservation and environmental sustainability. Zeolitic imidazolate framework-67 (ZIF-67) possesses exceptional pore apertures for sieving hydrogen from other larger gases. However, the preferential heterogeneous nucleation and growth of ZIF-67 crystals greatly challenge the synthesis of ZIF-67 membranes. In this work, a tunable-reaction-zone contra-diffusion synthesis is proposed to effectively assemble ZIF-67 nanocrystals into ultrathin GO laminates. The contra-diffusion-match growth of ZIF-67 constructs a compact ZIF-67@GO hybrid structure with superior H2/CO2 separation factor of 140.8 and H2 permeance of 1.7 × 10−7 mol m−2 s−1 Pa−1, transcending the state-of-art upper bounds. The excellent separation performance is attributed to the collaborative effect of the size-sieving and the selective attraction of the ZIF-67@GO hybrid structure. Long-term operation also reveals the superior stability of the ZIF-67@GO hybrid membrane for H2/CO2 separation. The rational-design approach grants a novel way to fabricate ultrathin and defect-free ZIF-67 crystalline membranes for high-performance gas separation.