Rational Tuning of Zirconium Metal–Organic Framework Membranes for Hydrogen Purification

The effect of organic ligands on the separation performance of Zr based metal–organic framework (Zr‐MOF) membranes was investigated. A series of Zr‐MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 μm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr‐MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H₂/CO₂=26, H₂/N₂=13, H₂/CH₄=11.     

Modular Customization and Regulation of Metal–Organic Frameworks for Efficient Membrane Separations

Metal–organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H₂/CO₂ separation performance sequence is achieved (separation factor of 1656–5.4, H₂ permeance of 964–2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.     

Capture of toxic gases in MOFs: SO2, H2S,NH3 and NOx

MOFs are promising candidates for the capture of toxic gases since their adsorption properties can be tuned as a function of the topology and chemical composition of the pores. Although the main drawback of MOFs is their vulnerability to these highly corrosive gases which can compromise their chemical stability, remarkable examples have demonstrated high chemical stability to SO₂, H₂S, NH₃ and NO_x. Understanding the role of different chemical functionalities, within the pores of MOFs, is the key for accomplishing superior captures of these toxic gases. Thus, the interactions of such functional groups (coordinatively unsaturated metal sites, μ-OH groups, defective sites and halogen groups) with these toxic molecules, not only determines the capture properties of MOFs, but also can provide a guideline for the desigh of new multi-functionalised MOF materials. Thus, this perspective aims to provide valuable information on the significant progress on this environmental-remediation field, which could inspire more investigators to provide more and novel research on such challenging task.

MOFs are promising candidates for the capture of toxic gases such as SO₂, H₂S, NH₃ and NOx. Understanding the role of different chemical functionalities, within the pores of MOFs, is the key for accomplishing superior captures of these toxic gases.     

A window-space-directed assembly strategy for the construction of supertetrahedron-based zeolitic mesoporous metal–organic frameworks with ultramicroporous apertures for selective gas adsorption

Despite their scarcity due to synthetic challenges, supertetrahedron-based metal–organic frameworks (MOFs) possess intriguing architectures, diverse functionalities, and superb properties that make them in-demand materials. Employing a new window-space-directed assembly strategy, a family of mesoporous zeolitic MOFs have been constructed herein from corner-shared supertetrahedra based on homometallic or heterometallic trimers [M₃(OH/O)(COO)₆] (M₃ = Co₃, Ni₃ or Co₂Ti). These MOFs consisted of close-packed truncated octahedral cages possessing a sodalite topology and large β-cavity mesoporous cages (∼22 Å diameter) connected by ultramicroporous apertures (∼5.6 Å diameter). Notably, the supertetrahedron-based sodalite topology MOF combined with the Co₂Ti trimer exhibited high thermal and chemical stability as well as the ability to efficiently separate acetylene (C₂H₂) from carbon dioxide (CO₂).

A series of supertetrahedron (ST)-based sodalite (sod)-topology zeolitic MOFs specimens ST-sod-MOFs featuring ultramicroporous square windows and a mesoporous sodcage have been synthesized via a window-space-directed assembly approach.     

Controlling the alignment of 1D nanochannel arrays in oriented metal–organic framework films for host–guest materials design

Controlling the direction of molecular-scale pores enables the accommodation of guest molecular-scale species with alignment in the desired direction, allowing for the development of high-performance mechanical, thermal, electronic, photonic and biomedical organic devices (host–guest approach). Regularly ordered 1D nanochannels of metal–organic frameworks (MOFs) have been demonstrated as superior hosts for aligning functional molecules and polymers. However, controlling the orientation of MOF films with 1D nanochannels at commercially relevant scales remains a significant challenge. Here, we report the fabrication of macroscopically oriented films of Cu-based pillar-layered MOFs having regularly ordered 1D nanochannels. The direction of 1D nanochannels is controllable by optimizing the crystal growth process; 1D nanochannels align either perpendicular or parallel to substrates, offering molecular-scale pore arrays for a macroscopic alignment of functional guest molecules in the desired direction. Due to the fundamental interest and widespread technological importance of controlling the alignment of functional molecules and polymers in a particular direction, orientation-controllable MOF films will open up the possibility of realising the potential of MOFs in advanced technologies.

Orientation-controlled Cu₂(Linker)₂DABCO MOF films on macroscopic scales are fabricated for the development of high-performance devices; the direction of 1D nanochannels is controllable either perpendicular or parallel to substrates.     

Metal-organic framework membranes: From synthesis to electrocatalytic applications

Metal-organic frameworks (MOFs), as an emerging family of porous inorganic-organic crystal materials, exhibit widely applications in gas storage and separation, drug release, sensing, and catalysis, owing to easily adjustable pore sizes, uniformly distributed metal centers, high surface areas, and tunable functionalities. However, MOF crystal powders are usually difficult to be directly applied into specific devices because of their brittleness, insolubility and low compatibility. Therefore, to expand versatile MOF membranes with robustness and operational flexibility is urgent to satisfy practical applications. Although numerous reports have reviewed the synthesis and applications of MOF membranes, relatively few reports the electrocatalytic properties based on MOF membranes. Herein, this mini-review provides an overview of preparation of MOF membranes, including directed synthesis, secondary growth and electrochemical deposition method. Meanwhile, fabrication of ultrathin 2D MOF nanosheets those can be also defined as a kind of nanoscale MOF membranes is also mentioned. Electrocatalytic performance of oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR) for diverse MOF membranes/nanosheets and their derivatives are introduced.     

Hierarchical Mesoporous Metal–Organic Frameworks for Enhanced CO2 Capture

Hierarchical porous materials are promising for catalyst, separation and sorption applications. A ligand‐assisted etching process is developed for template‐free synthesis of hierarchical mesoporous MOFs as single crystals and well‐intergrown membranes at 40 °C. At 223 K, the hierarchical porous structures significantly improve the CO₂ capture capacity of HKUST‐1 by more than 44 % at pressures up to 20 kPa and 13 % at 100 kPa. Even at 323 K, the enhancement of CO₂ uptake is above 25 % at pressures up to 20 kPa and 7 % at 100 kPa. The mesoporous structures not only enhance the CO₂ uptake capacity but also improve the diffusion and mass transportation of CO₂. Similarly, well‐intergrown mesoporous HKUST‐1 membranes are synthesized, which hold the potential for film‐like porous devices. Mesoporous MOF‐5 crystals are also obtained by a similar ligand‐assisted etching process. This may provide a facile way to prepare hierarchical porous MOF single crystals and membranes for wide‐ranging applications.     

Water‐stable Adenine‐based MOFs with Polar Pores for Selective CO2 Capture

Here, we report two novel water‐stable amine‐functionalized MOFs, namely IISERP‐MOF26 ([NH₂(CH₃)₂][Cu₂O(Ad)(BDC)]?(H₂O)₂(DMA), 1 ) and IISERP‐MOF27 ([NH₂(CH₃)₂]_1/2[Zn₄O(Ad)₃(BDC)₂]?(H₂O)₂(DMF)_1/2, 2 ), which show selective CO₂ capture capabilities. They are made by combining inexpensive and readily available terephthalic acid and N‐rich adenine with Cu and Zn, respectively. They possess 1D channels decorated by the free amine group from the adenine and the polarizing oxygen atoms from the terephthalate units. Even more, there are dimethyl ammonium (DMA⁺) cations in the pore rendering an electrostatic environment within the channels. The activated Cu‐ and Zn‐MOFs physisorb about 2.7 and 2.2 mmol g^?1 of CO₂, respectively, with high CO₂/N₂ and moderate CO₂/CH₄ selectivity. The calculated heat of adsorption (HOA=21–23 kJ mol^?1) for the CO₂ in both MOFs suggest optimal physical interactions which corroborate well with their facile on‐off cycling of CO₂. Notably, both MOFs retain their crystallinity and porosity even after soaking in water for 24 hours as well as upon exposure to steam over 24 hours. The exceptional thermal and chemical stability, favorable CO₂ uptakes and selectivity and low HOA make these MOFs promising sorbents for selective CO₂ capture applications. However, the MOF′s low heat of adsorption despite having a highly CO₂‐loving groups lined walls is quite intriguing.     

Gram-Scale Synthesis of an Ultrastable Microporous Metal-Organic Framework for Efficient Adsorptive Separation of C2H2/CO2 and C2H2/CH4

A highly water and thermally stable metal-organic framework (MOF) Zn₂(Pydc)(Ata)₂ (1, H₂Pydc = 3,5-pyridinedicarboxylic acid; HAta = 3-amino-1,2,4-triazole) was synthesized on a large scale using inexpensive commercially available ligands for efficient separation of C₂H₂ from CH₄ and CO₂. Compound 1 could take up 47.2 mL/g of C₂H₂ under ambient conditions but only 33.0 mL/g of CO₂ and 19.1 mL/g of CH₄. The calculated ideal absorbed solution theory (IAST) selectivities for equimolar C₂H₂/CO₂ and C₂H₂/CH₄ were 5.1 and 21.5, respectively, comparable to those many popular MOFs. The Q_st values for C₂H₂, CO₂, and CH₄ at a near-zero loading in 1 were 43.1, 32.1, and 22.5 kJ mol⁻¹, respectively. The practical separation performance for C₂H₂/CO₂ mixtures was further confirmed by column breakthrough experiments.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10^?7 mol m^?2 s^?1 Pa^?1 and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

A Rigid Nested Metal–Organic Framework Featuring a Thermoresponsive Gating Effect Dominated by Counterions

We herein report a rigid nested metal–organic framework (MOF) featuring a unique thermoresponsive gating adsorption behavior, which, in contrast to any known flexibility modes for stimuli‐responsive MOFs, depends on the thermal motion of the extra‐framework counterions. In addition, this MOF also exhibits adsorption selectivity of CO₂ over N₂, H₂, and Ar at 273 K, thus enabling a strategic separation and encapsulation of CO₂.     

Tuning the Stacking Modes of Ultrathin Two-Dimensional Metal–Organic Framework Nanosheet Membranes for Highly Efficient Hydrogen Separation

Two-dimensional (2D) metal–organic framework (MOF) membranes are considered potential gas separation membranes of the next generation due to their structural diversity and geometrical functionality. However, achieving a rational structure design for a 2D MOF membrane and understanding the impact of MOF nanosheet stacking modes on membrane separation performance remain challenging tasks. Here, we report a novel kind of 2D MOF membrane based on [Cu₂Br(IN)₂]_n (IN=isonicotinato) nanosheets and propose that synergetic stacking modes of nanosheets have a significant influence on gas separation performance. The stacking of the 2D MOF nanosheets is controlled by solvent droplet dynamic behaviors at different temperatures of drop coating. Our 2D MOF nanosheet membranes exhibit high gas separation performances for H₂/CH₄ (selectivity >290 with H₂ permeance >520 GPU) and H₂/CO₂ (selectivity >190 with H₂ permeance >590 GPU) surpassing the Robeson upper bounds, paving a potential way for eco-friendly H₂ separation.     

Lithium‐Functionalized Metal–Organic Frameworks that Show >10 wt % H2 Uptake at Ambient Temperature

We have used grand canonical Monte Carlo simulations with a first‐principles‐based force field to show that metal–organic frameworks (MOFs) with Li functional groups (i.e. C? Li bonds) allow for exceptional H₂ uptake at ambient temperature. For example, at 298 K and 100 bar, IRMOF‐1‐4Li shows a total H₂ uptake of 5.54 wt % and MOF‐200‐27Li exhibits a total H₂ uptake of 10.30 wt %, which are much higher than the corresponding values with pristine MOFs. Li‐functionalized MOF‐200 (MOF‐200‐27Li) shows 11.84 wt % H₂ binding at 243 K and 100 bar. These hydrogen‐storage capacities exceed the 2015 DOE target of 5.5 wt % H₂. Moreover, the incorporation of Li functional groups into MOFs provides more benefits, such as higher delivery amount, for H₂ uptake than previously reported Li‐doped MOFs.     

Quantification of the mixed-valence and intervalence charge transfer properties of a cofacial metal–organic framework via single crystal electronic absorption spectroscopy

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal–Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials. These materials have potential in applications in porous conductors, electrocatalysts and energy storage devices; however the structure–property relationships pertaining to charge transfer and its quantification are relatively poorly understood. Here, the cofacial Cd(ii)-based MOF [Cd(BPPTzTz)(tdc)]·2DMF (where BPPTzTz = 2,5-bis(4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole, tdc²⁻ = 2,5-thiophene dicarboxylate) exhibits Intervalence Charge Transfer (IVCT) within its three-dimensional structure by virtue of the close, cofacial stacking of its redox-active BPPTzTz ligands. The mixed-valence and IVCT properties are characterised using a combined electrochemical, spectroelectrochemical and computational approach. Single crystal electronic absorption spectroscopy was employed to obtain the solid-state extinction coefficient, enabling the application of Marcus–Hush theory. The electronic coupling constant, H_ab, of 145 cm⁻¹ was consistent with the localised mixed-valence properties of both this framework and analogous systems that use alternative methods to obtain the H_ab parameter. This work demonstrates the first report of the successful characterisation of IVCT in a MOF material using single crystal electronic absorption spectroscopy and serves as an attractive alternative to more complex methods due to its simplicity and applicability.

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal–Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials.     

Role of ion-selective membranes in the carbon balance for CO2 electroreduction via gas diffusion electrode reactor designs

In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO₂ reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO₂ reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon balance paths for the captured CO₂ at the cathode/catholyte interface via reaction with OH⁻ when using different ion-selective membranes: (i) the captured CO₂ could be transported through an anion exchange membrane in the form of CO₃²⁻, subsequently releasing CO₂ along with O₂ in the anolyte, and (ii) with a cation exchange membrane, the captured CO₂ would be accumulated in the catholyte in the form of CO₃²⁻, while (iii) with the use of a bipolar membrane, the captured CO₂ could be released at the catholyte/membrane interface in the form of gaseous CO₂. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.

In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO₂ reduction in GDE-type flow electrolyzers.     

Free-standing metal–organic framework (MOF) monolayers by self-assembly of polymer-grafted nanoparticles

We report a general method for the synthesis of free-standing, self-assembled MOF monolayers (SAMMs) at an air–water interface using polymer-brush coated MOF nanoparticles. UiO-66, UiO-66-NH₂, and MIL-88B-NH₂ were functionalized with a catechol-bound chain-transfer agent (CTA) to graft poly(methyl methacrylate) (PMMA) from the surface of the MOF using reversible addition-fragmentation chain transfer polymerization (RAFT). The polymer-coated MOFs were self-assembled at the air–water interface into monolayer films ∼250 nm thick and capable of self-supporting at a total area of 40 mm². Mixed-particle films were prepared through the assembly of MOF mixtures, while multilayer films were achieved through sequential transfer of the monolayers to a glass slide substrate. This method offers a modular and generalizable route to fabricate thin-films with inherent porosity and sub-micron thickness composed of a variety of MOF particles and functionalities.

We report a general method for the synthesis of free-standing, self-assembled MOF monolayers (SAMMs) at an air–water interface using polymer-brush coated MOF nanoparticles.     

Mixed Metal–Organic Framework with Multiple Binding Sites for Efficient C2H2/CO2 Separation

The separation of C₂H₂/CO₂ is particularly challenging owing to their similarities in physical properties and molecular sizes. Reported here is a mixed metal–organic framework (M′MOF), [Fe(pyz)Ni(CN)₄] ( FeNi‐M′MOF , pyz=pyrazine), with multiple functional sites and compact one‐dimensional channels of about 4.0 Å for C₂H₂/CO₂ separation. This MOF shows not only a remarkable volumetric C₂H₂ uptake of 133 cm³ cm^?3, but also an excellent C₂H₂/CO₂ selectivity of 24 under ambient conditions, resulting in the second highest C₂H₂‐capture amount of 4.54 mol L^?1, thus outperforming most previous benchmark materials. The separation performance of this material is driven by π–π stacking and multiple intermolecular interactions between C₂H₂ molecules and the binding sites of FeNi‐M′MOF . This material can be facilely synthesized at room temperature and is water stable, highlighting FeNi‐M′MOF as a promising material for C₂H₂/CO₂ separation.     

Mixed Metal–Organic Framework with Multiple Binding Sites for Efficient C2H2/CO2 Separation

The separation of C₂H₂/CO₂ is particularly challenging owing to their similarities in physical properties and molecular sizes. Reported here is a mixed metal–organic framework (M′MOF), [Fe(pyz)Ni(CN)₄] ( FeNi-M′MOF , pyz=pyrazine), with multiple functional sites and compact one-dimensional channels of about 4.0 Å for C₂H₂/CO₂ separation. This MOF shows not only a remarkable volumetric C₂H₂ uptake of 133 cm³ cm⁻³, but also an excellent C₂H₂/CO₂ selectivity of 24 under ambient conditions, resulting in the second highest C₂H₂-capture amount of 4.54 mol L⁻¹, thus outperforming most previous benchmark materials. The separation performance of this material is driven by π–π stacking and multiple intermolecular interactions between C₂H₂ molecules and the binding sites of FeNi-M′MOF . This material can be facilely synthesized at room temperature and is water stable, highlighting FeNi-M′MOF as a promising material for C₂H₂/CO₂ separation.     

Inverse CO2/C2H2 Separation with MFU-4 and Selectivity Reversal via Postsynthetic Ligand Exchange

Although many porous materials, including metal–organic frameworks (MOFs), have been reported to selectively adsorb C₂H₂ in C₂H₂/CO₂ separation processes, CO₂-selective sorbents are much less common. Here, we report the remarkable performance of MFU-4 (Zn₅Cl₄(bbta)₃, bbta=benzo-1,2,4,5-bistriazolate) toward inverse CO₂/C₂H₂ separation. The MOF facilitates kinetic separation of CO₂ from C₂H₂, enabling the generation of high purity C₂H₂ (>98 %) with good productivity in dynamic breakthrough experiments. Adsorption kinetics measurements and computational studies show C₂H₂ is excluded from MFU-4 by narrow pore windows formed by Zn−Cl groups. Postsynthetic F⁻/Cl⁻ ligand exchange was used to synthesize an analogue ( MFU-4-F ) with expanded pore apertures, resulting in equilibrium C₂H₂/CO₂ separation with reversed selectivity compared to MFU-4 . MFU-4-F also exhibits a remarkably high C₂H₂ adsorption capacity (6.7 mmol g⁻¹), allowing fuel grade C₂H₂ (98 % purity) to be harvested from C₂H₂/CO₂ mixtures by room temperature desorption.