Ionic Metal-organic Frameworks Research Articles

Adsorption in Pyrene-Based Metal-Organic Frameworks: The Role of Pore Structure and Topology.

Pore topology and chemistry play crucial roles in the adsorption characteristics of metal-organic frameworks (MOFs). To deepen our understanding of the interactions between MOFs and CO2 during this process, we systematically investigate the adsorption properties of a group of pyrene-based MOFs. These MOFs feature Zn(II) as the metal ion and employ a pyrene-based ligand, specifically 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy). Including different additional ligands leads to frameworks with distinctive structural and chemical features. By comparing these structures, we could isolate the role that pore size, the presence of open-metal sites (OMS), metal-oxygen bridges, and framework charges play in the CO2 adsorption of these MOFs. Frameworks with constricted pore structures display a phenomenon known as the confinement effect, fostering stronger MOF-CO2 interactions and higher uptakes at low pressures. In contrast, entropic effects dominate at elevated pressures, and the MOF's pore volume becomes the driving factor. Through analysis of the CO2 uptakes of the benchmark materials ─some with narrower pores and others with larger pore volumes─it becomes evident that structures with narrower pores and high binding energies excel at low pressures. In contrast, those with larger volumes perform better at elevated pressures. Moreover, this research highlights that open-metal sites and inherent charges within the frameworks of ionic MOFs stand out as CO2-philic characteristics.

Tunable Gas Admission via a "Molecular Trapdoor" Mechanism in a Flexible Cationic Metal-Organic Framework Featuring 1D Channels.

Achieving high gas selectivity is challenging when dealing with gas pairs of similar size and physiochemical properties. The "molecular trapdoor" mechanism discovered in zeolites holds promise for highly selective gas adsorption separation but faces limitations like constrained pore volume and slow adsorption kinetics. To address these challenges, for the first time, a flexible metal-organic framework (MOF) featuring 1D channels and functioning as a "molecular trapdoor" material is intoduced. Extra-framework anions act as "gate-keeping" groups at the narrowest points of channels, permitting gas admissions via gate opening induced by thermal/pressure stimuli and guest interactions. Different guest molecules induce varied energy barriers for anion movement, enabling gas separation based on distinct threshold temperatures for gas admission. The flexible framework of Pytpy MOFs, featuring swelling structure with rotatable pyridine rings, facilitates faster gas adsorption than zeolite. Analyzing anion properties of Pytpy MOFs reveals a guiding principle for selecting anions to tailor threshold gas admission. This study not only overcomes the kinetic limitations related to gas admission in the "molecular trapdoor" zeolites but also underscores the potential of developing MOFs as molecular trapdoor adsorbents, providing valuable insights for designing ionic MOFs tailored to diverse gas separation applications.

Adjusting gate-opening behavior in a rigid cage-type “molecular trapdoor” metal–organic framework via anion modulation

Exclusive admission of targeted gas molecules by porous materials, i.e., molecular sieving, is highly sought-after to afford highly selective separation but is challenging when gas molecules have similar sizes. Although the “molecular trapdoor” mechanism discovered in zeolites allows for addressing this challenge, the limited adaptability of zeolites constrains the full harnessing of this mechanism. In this study, we have identified a cage-type of stimuli-responsive metal–organic framework (MOF) as a “molecular trapdoor” material through a combined experimental and simulation approach by adjusting its gate-opening behavior via anion modulation. The temperature-dependent gas admission observed does not merely result from the thermal motion of “gate-keeping” anions, but rather emerges from their interaction with guest molecules within the pore. By modifying the “gate-keeping” groups, we achieved the highest selectivity of CO2/N2 (697) and CO2/CH4 (22) at 273 K in ClO4-BTR MOF in this study. Additionally, we have identified a consistent pattern governing the diffusion rate during the gas adsorption process among NO3-BTR, BF4-BTR, and ClO4-BTR MOFs concerning variations in temperature and pressure. This work unveils the fundamental mechanism and enhances our understanding of gas admission in a MOF featuring pore-blocking anions, thus expanding the prospective applications of MOFs as “molecular trapdoor” adsorbents and offering valuable guidance for the strategic design of ionic MOFs with specific gas separation characteristics.

Ionic liquid-functionalized metal organic frameworks and their composite membranes for enhanced proton transport

Post-synthetic modification with ionic liquids is gaining prominence as a highly effective strategy for enhancing proton conductivity in metal–organic frameworks without significant structural changes.

A mesoporous ionic metal-organic framework decorated by flexible alkyl imidazolium bromide as heterogeneous catalyst for efficient conversion of CO2 to cyclic carbonates

A mesoporous ionic MOF-based catalyst (Br−)C3H7-Imidazolium-MOF-1 for the cycloaddition of CO2 to epoxides (CCE) was synthesized by template-assisted method, and its composite and structure were determined by FT-IR, PXRD, NMR, SEM, TEM, XPS and N2 adsorption. (Br−)C3H7-Imidazolium-MOF-1, is constructed by Zr6O4(OH)4 clusters and linear dicarboxylic acid ligands covalently modified by flexible alkyl imidazolium groups (1-methylene-3-propyl-imidazolium bromide). By the synergistic effect of Zr4+ Lewis acid sites and Br− nucleophiles, (Br−)C3H7-Imidazolium-MOF-1 shown superior catalytic performance for the cycloaddition of CO2 to epichlorohydrin in the absence of any co-catalyst under mild conditions. Small steric hindrance from the flexible imidazolium groups and rapid mass transport of reactants and products in the mesoporous catalyst acted synergistically to promote this process. A firm bond between the imidazolium bromide and the framework of (Br−)C3H7-Imidazolium-MOF-1 made it a recyclable heterogeneous catalyst. Furthermore, several additional coupling reactions were investigated with other epoxides as substrates.

Construction of hierarchical ZnS/SnO2 @rGO heterostructures as high-performance anode materials for lithium-ion batteries by mixed-precursors strategy

The rational design and construction of hierarchical heterogeneous nanostructures is effective for improving the electrical and ionic conductivity of anode material for lithium-ion batteries (LIBs). Herein, a hierarchical ZnS/SnO2 @rGO (rGO = reduced graphene oxide) composite has been designed and successfully synthesized via hydrothermal method by using metal-containing ionic liquid and ZIF-8 metal organic framework precursors. The synergistic effect of precursors hinders the agglomeration of nanoparticles, which makes SnO2 quantum dots (QDs) be anchored on the surface of polyhedral ZnS and further be uniformly dispersed on the rGO. The outer rGO matrix can improve the electrical conductivity and fully alleviate the volume expansion of ZnS/SnO2 during the cycling. Meanwhile, the ZnS/SnO2 heterostructure can endow the material with excellent reaction kinetics, accelerating the charge transfer and providing more active sites for electrochemical reactions. Benefiting from the compositional and structural features, the as-prepared ZnS/SnO2 @rGO as LIB anode provides the reversible specific capacity of 1030 and 630 mA h g−1 after 1150 cycles at the current density of 1.0 and 3.0 A g−1, respectively. This mixed-precursors strategy represents a novel and effective technique to in situ synthesize hierarchical metal sulfide/metal oxide/carbon anode materials for developing high-performance battery.

Confinement inside MOFs Enables Guest-Modulated Spin Crossover of Otherwise Low-Spin Coordination Cages.

Confinement of discrete coordination cages within nanoporous lattices is an intriguing strategy to gain unusual properties and functions. We demonstrate here that the confinement of coordination cages within metal-organic frameworks (MOFs) allows the spin state of the cages to be regulated through multilevel host-guest interactions. In particular, the confined in situ self-assembly of an anionic FeII4L6 nanocage within the mesoporous cationic framework of MIL-101 leads to the ionic MOF with an unusual hierarchical host-guest structure. While the nanocage in solution and in the solid state has been known to be invariantly diamagnetic with low-spin FeII, FeII4L6@MIL-101 exhibits spin-crossover (SCO) behavior in response to temperature and release/uptake of water guest within the MOF. The distinct color change concomitant with water-induced SCO enables the use of the material for highly selective colorimetric sensing of humidity. Moreover, the spin state and the SCO behavior can be modulated also by inclusion of a guest into the hydrophobic cavity of the confined cage. This is an essential demonstration of the phenomenon that the confinement within porous solids enables an SCO-inactive cage to show modulable SCO behaviors, opening perspectives for developing functional supramolecular materials through hierarchical host-guest structures.

In-situ construction of ionic ultramicroporous metal–organic frameworks for high-efficiency CO2/CH4 separation

The high-efficiency capture of CO2 from biogas by adsorption is highly desirable, which, yet faces the challenges regards of the unsatisfactory adsorption efficiency, selectivity, and regeneration performance. Herein, ionic ultramicroporous metal organic frameworks (IUM-MOFs) with high CO2 affinity are constructed by one-step in-situ coordination strategy and used for CO2/CH4 separation. The phenoxylate anion-functionalized ionic liquid (FIL) serves as competitive ligands in the formation of IUM-MOFs. The oxyl group exhibits high affinity with CO2 as the Lewis basic center, while the rigid counterion occupies the adsorption space of CH4 in the cavities of IUM-MOFs, thus bringing about superior CO2 adsorption capacity of 149 cm3 g−1 (150% higher than that of Cu-BTC) and high IAST predicted selectivity with CH4 of 22 under atmospheric condition. Mechanism analysis demonstrated that the formation of hydrogen bonds between CO2 with coordinated IL and H2O and IL-enhanced polarity of MOFs benefit to the CO2 adsorption. In addition, the IUM-MOFs could be facilely regenerated at 120 °C with favorable recycling stability. This work provides facile strategy for the rational design of IUM-MOFs and lays useful insights for structural tuning to promote their applications.

Confining organic cations in metal organic framework allows molecular level regulation of CO2 capture

AbstractHigh tunability of both ionic liquids (ILs) and metal organic frameworks (MOFs) enables great opportunity in the rational designation of IL/MOF composites for physical adsorption and separation. Traditionally, cations and anions of ILs as an entirety are combined with MOFs either inside or outside the microchannels. Herein, organic cations of ILs were confined into Cu‐BTC and the champion adsorbent is obtained by using 1‐propionic acid‐3‐vinylimidazole bromide as the precursor with a moderate loading amount, exhibiting higher CO2 uptakes of 8/5 mmol g−1 than Cu‐BTC (6.0/3.5 mmol g−1) at 273/298 K and 100 kPa, associating with significantly improved CO2/N2(CH4) selectivities. The organic cations are interacted with two adjacent CuII2(CRO2)2 paddle wheel units of Cu‐BTC, expanding the CuO bond to strengthen the CO2 affinity of open Cu sites and also serving as an additive CO2 adsorptive site. The promotion of CO2 capture ability is further reflected in the dynamic column breakthrough experiment.

A Protophilic MOF Enables Ni-Rich Lithium-Battery Stable Cycling in a High Water/Acid Content.

Trace protic impurities, such as water and hydrofluoric acid (HF), can severely degrade the stable and long cycling of lithium batteries. Therefore, the costly water removal process is inevitably needed throughout production of lithium batteries, leaving the paradox that energy-saving lithium-battery technology consumes non-negligible amounts of energy. Herein, a unique ionic metal-organic framework (MOF) is reported that enables highly destructive H2 O/HF-tolerant lithium batteries. The isolated ionic fluorine sites in the MOF exhibit unusual protophilicity and efficiently capture ppm-levels H2 O/HF from the highly polar electrolyte solvents. The resulting MOF-based LiNi0.6 Mn0.2 Co0.2 O2 │Li battery achieves over 300 cycles in the presence of 800ppm H2 O or 1107ppm acidic impurity. This tenfold longer battery lifespan relative to those for batteries with conventional standard separators demonstrates its excellent electrochemical cycling performance. The results reveal that the rational use of unique nanoporous features of MOFs can provide new possibilities for long-standing challenges in the lithium-battery industry.

Ionic liquid incorporated metal–organic framework as high conductivity solid conductor

As a new type of composite ionic conductor, the combination of ionic liquids (ILs) and metal organic framework (MOF) shows a good application prospect. In this study, two ionic liquids, 1,3-dimethylimidazolium tetrafluoroborate ([Dmim][BF4]) and 1-Allyl-3-methylimidazolium dicyanamide ([Amim][N(CN)2]) were successfully combined with MIL-101(Cr). The influence of the amount of ionic liquid and temperature on the electrical conductivity of composite IL@MIL-101(Cr) was explored. The optimal electrical conductivity reached 3.73 × 10-3S·cm−1 ([Dmim][BF4]) and 2.69 × 10-3S·cm−1 ([Amim][N(CN)2]) respectively when exposed to air at 100 °C. It is worth noting that the ionic conductivity of all composites exhibits linear Arrhenius behavior. Since the activation energies of composites are closely related to the type of ionic liquid and MOF types, these two composites exhibit different activation energies. The reason for the difference in activation energies is analyzed with the aid of quantum chemical calculations.

Cationic metal-organic framework with charge separation effect as a high output triboelectric nanogenerator material for self-powered anticorrosion.

New stable frictional materials based on metal-organic frameworks (MOFs) are greatly desired for applications in self-powered systems. This work reports an ionic MOF material with efficient charge separation mediated by charge induction. ZUT-iMOF-1(Cu) is chemically stable and its triboelectric output performance surpasses those of traditional MOF materials. The short-circuit current of the iMOF triboelectric nanogenerator is 73.79 μA at 5 Hz. The output performance remains stable over 50 000 cycles of continuous operation. The charge and power densities peak at 123.20 μC m-2 and 3133.23 mW m-2. Owing to its high output performance, ZUT-iMOF-1(Cu) effectively prevents metal corrosion in cathodic-protection systems. Theoretical calculations show that increasing the charge-separation effect promotes the frictional electricity generation behaviour. This study provides research suggestions for ionic MOF frictional materials and will promote their application in self-powered electrochemical cathodic-protection systems.

An ionic Fe-based metal-organic-framework with 4'-pyridyl-2,2':6',2''-terpyridine for catalytic hydroboration of alkynes.

An ionic metal-organic-framework (MOF) containing nanoscale channels was readily assembled from ditopic 4'-pyridyl-2,2':6',2''-terpyridine (pytpy) and a simple iron(ii) salt. X-ray structural analysis revealed a two-dimensional grid-like framework assembled by classic octahedral (pytpy)2FeII cations as linkers (with pytpy as a new ditopic pyridyl ligand) and octa-coordinate FeCl2 centers as nodes. The layer-by-layer assembly of the 2-D framework resulted in the formation of 3-D porous materials consisting of nano-scale channels. The charges of the cationic framework were balanced with anionic Cl3FeOFeCl3 in its void channels. The new Fe-based MOF material was employed as a precatalyst for syn-selective hydroboration of alkynes under mild, solvent-free conditions in the presence of an activator, leading to the synthesis of a range of trans-alkenylboronates in good yields. The larger scale applicability and recyclability of the new MOF catalyst was further explored. This represents a rare example of an ionic MOF material that can be utilized in hydroboration catalysis.

Highly Water-Stable Zn5 Cluster-Based Metal-Organic Framework for Efficient Gas Storage and Organic Dye Adsorption.

The development of multi-functional materials as physical adsorbents for gas storage and dye adsorption is of utmost importance for ecological environment management. Ionic metal-organic frameworks (MOFs) with exchangeable counterions would easily be modified to enhance their performance. Herein, we report a novel {Zn5}-based MOF (SDU-CP-2) with the dimethylamine cation [(CH3)2NH2]+ in its crystal structure, which can be readily exchanged with Li+ ions and cationic dyes. Consequently, it exhibits high CO2 and C2H2 adsorption capacities as well as charge/size-selective adsorption of dye molecules. The adsorption of SDU-CP-2 toward cationic dyes was confirmed to be efficient in terms of recyclability and ion-exchange mechanisms. The column filler experiment and high water stability decisively supported its potential application in waste water treatment.

Metal-Organic Framework Integrating Ionic Framework and Bimetallic Coupling Effect for Highly Efficient Oxygen Evolution Reaction.

Metal–organic frameworks (MOFs) are recognized as promising electrocatalysts for the oxygen evolution reaction (OER) because of their permanent porosity and rich architectural diversity; however, ionic MOFs enabling fast ions exchange during OER are rarely explored. Here, an ionic MOF (Ni‐btz) constructed with an azolate ligand is selected, and continuous 3D bimetallic MOF (NiFe‐btz) films deriving from high‐degree intergrowth of microsized MOFs particles are fabricated. The as‐prepared NiFe‐btz/NF‐OH electrode exhibits excellent OER performance with a low overpotential of 239 mV at 10 mA cm−2 under alkaline condition. The OER charge transfer process and bimetallic coupling effect in ionic NiFe‐btz are probed by density functional theory calculations and confirmed via X‐ray photoelectron spectroscopy and in situ Raman measurements. The partial density of states of NiFe‐btz indicates that the main contribution for electron density around the Fermi level is from Cl ions clarifying the profitable impact of ionic MOF framework. This work systematically demonstrates the relationship of electronic structure and OER activity in ionic, bimetallic MOFs and expands the scope of 3D MOF films for efficient OER.

Bifunctional metal–organic frameworks afforded by postsynthetic modification for efficient cycloaddition of CO2 and epoxides

AbstractBifunctional ionic metal–organic frameworks (MOFs) containing Lewis acid sites (unsaturated metal sites) and halide ions (Cl−, Br−, and I−) have attracted increasing attention due to their extra high activity for the cycloaddition of CO2 with epoxides. Herein, a novel microporous MOF (1‐Eu), namely, [Eu3(L)2(HCOO)(H2O)5]·14H2O (H4L = 2,6‐di(2,4‐dicarboxyphenyl)‐4‐(pyridine‐4‐yl)pyridine), has been synthesized by using a new bipyridyl‐based tetracarboxylate ligands (H4L). Structural analyses show that 1‐Eu is a 3D framework in which 1D chains with alternating triple and single carboxylate bridges are interlinked by the L ligands and contains microporous channels with uncoordinated pyridyl N atoms, which are easy to be modified by N‐methylation. Therefore, three bifunctional N‐methylation 1‐Eu MOFs, 1‐Eu‐MeX (X = Cl−, Br−, and I−), were successfully prepared from the 1‐Eu MOF by a postsynthetic modification (PSM) method. 1‐Eu‐MeX can efficiently catalyze the cycloaddition reaction without any cocatalyst and solvent. Among them, the 1‐Eu‐MeI catalyst displays the highest catalytic performance. Our work thus represents a rare demonstration of ionic MOFs as heterogeneous catalysts for efficient CO2 fixation with epoxides. More significantly, 1‐Eu‐MeX are the first reported Eu‐based ionic MOFs with the bipyridyl‐based tetracarboxylate ligand.

Three in one: Rational engineering of multifunctional MIL-101-based ionic catalysts for carbon dioxide-epoxide cycloaddition

CO2-epoxide cycloaddition represents one of most attractive approaches for chemical fixation and utilization of CO2, but highly efficient and fully heterogeneous green catalysis is still challenging. Ionic metal-organic frameworks (IMOFs) are nice porous matrices heterogenizing the essential free nucleophilic anions and meanwhile providing Lewis acids, but the catalytic efficiency has been limited for low anion content and lack of auxiliary catalytic sites. In this work, we report stepwise optimization of IMOF catalysts by integrating multiple catalytic sites into MIL-101. Various monocationic IMOFs with quaternary ammonium and bromide anions were prepared by facile alkyne-azide click chemistry. Different numbers of hydroxyl groups were introduced in this ionization step to optimize the activity. To introduce more anions, dicationic IMOFs were created by a second ionization step, N-quaternization of triazolyl to triazolium. By the stepwise ionization and functionalization, we demonstrated the synergic catalytic effects among different sites, the impacts of hydroxyl/bromide content, and the size effect of the cationic group. The optimal IMOFs are superior to previous ionic heterogeneous catalysts in absence of cocatalysts. In particular, the dicationic catalyst can quantitatively convert epichlorohydrin to the carbonate within 1 h at 1 MPa CO2 and 80 °C, the apparent turnover frequency being up to 478 h−1. The highly active and selective, fully heterogeneous and recyclable catalyst has the appeal for practical applications.

Anion-Afforded Functions of Ionic Metal-Organic Frameworks: Ionochromism, Anion Conduction, and Catalysis.

The exchangeable counterions in ionic metal-organic frameworks (IMOFs) provide facile and versatile handles to manipulate functions associated with the ionic guests themselves and host-guest interactions. However, anion-exchangeable stable IMOFs combining multiple anion-related functions are still undeveloped. In this work, a novel porous IMOF featuring unique self-penetration was constructed from an electron-deficient tris(pyridinium)-tricarboxylate zwitterionic ligand. The water-stable IMOF undergoes reversible and single-crystal-to-single-crystal anion exchange and shows selective and discriminative ionochromic behaviors toward electron-rich anions owing to donor-acceptor interactions. The IMOFs with different anions are good ionic conductors with low activation energy, the highest conductivity being observed with chloride. Furthermore, integrating Lewis acidic sites and nucleophilic guest anions in solid state, the IMOFs act as heterogeneous and recyclable catalysts to efficiently catalyze the cycloaddition of CO2 to epoxides without needing the use of halide cocatalysts. The catalytic activity is strongly dependent upon the guest anions, and the iodide shows the highest activity. The results demonstrate the great potential of developing IMOFs with various functions related to the guest ions included in the porous matrices.

Efficient ionic functionalization of metal-organic frameworks for efficient addition of carbon dioxide to epoxides

The use of homogeneous and polluting halides as nucleophilic cocatalysts lowers the economy and benignity of the CO 2 -epoxide cycloaddition over heterogeneous Lewis-acid catalysts like metal-organic frameworks (MOFs). Creating net positive charge at frameworks is a reasonable strategy to heterogenize halide anions within MOFs, but it is still challenging to achieve high halide loading while keeping adequate porosity. In this article, we report a facile approach to ionic MOFs with high halide content and improved catalytic activity. The catalysts were prepared by reacting azide-tagged MIL-101 with alkyne-tagged organic bromide salts. The efficient azide–alkyne click reaction within the mesoporous MOF allows quantitative cationization of the linkers, and the resultant bifunctional ionic MOFs contain one halide ion per metal center to the benefit of the cooperative catalysis. Catalytic studies suggest that the effects of varying the cationic groups and halide anions in the confined ionic space are different from those observed for homogeneous organic halides. The highest catalytic activity was demonstrated for the MOF having the smallest cationic group (quaternized trimethyl ammonium) and the intermediately nucleophilic halide (Br − ). The catalyst also outperforms the ionic MOF catalysts reported prior to this work. • A series of ionic MOFs were prepared by facile alkyne-azide click reactions. • The MOFs contain one halide per metal to the benefit of cooperative catalysis. • The confined ionic space changes the effects of cationic groups and halide anions. • The optimal catalyst, MIL-101-tz-TMA-Br, outperforms previous ionic MOF catalysts.

Ionic metal-organic frameworks (iMOFs): progress and prospects as ionic functional materials.

Metal-organic frameworks (MOFs) have been a research hotspot for the last two decades, witnessing an extraordinary upsurge across various domains in materials chemistry. Ionic MOFs (both anionic and cationic MOFs) have emerged as next-generation ionic functional materials and are an important subclass of MOFs owing to their ability to generate strong electrostatic interactions between their charged framework and guest molecules. Furthermore, the presence of extra-framework counter-ions in their confined nanospaces can serve as additional functionality in these materials, which endows them a significant advantage in specific host-guest interactions and ion-exchange-based applications. In the present review, we summarize the progress and future prospects of iMOFs both in terms of fundamental developments and potential applications. Furthermore, the design principles of ionic MOFs and their state-of-the-art ion exchange performances are discussed in detail and the future perspectives of these promising ionic materials are proposed.