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.

Construction of Cu(I)-Organic Frameworks with Effective Sorption Behavior for Iodine and Congo Red

The assembly of a tetradentate pyridine-derived ligand with CuX has afforded two isostructural Cu(I)-organic frameworks [Cu₂X₂(TBD)·DMF]_n (X = Cl for 1 and Br for 2) in this work. Structural analysis indicates that the compounds feature hybrid layered architectures, and the three-dimensional supramolecular frameworks are finally fabricated through the alternative stacking of adjacent layers wherein large open channels are simultaneously constructed. The chemical stability has been studied showing the excellent skeleton maintenance of the prepared solids in various solvents and even in water. Moreover, the iodine and dye sorption performance for compound 1 has been further tested. The Cu(I)-based metal-organic framework exhibits outstanding sorption and separation abilities on the targeted species, which could be considered as a promising adsorbent with high efficiency and selectivity.

Rational synthesis of isomorphic rare earth metal–organic framework materials for simultaneous adsorption and photocatalytic degradation of organic dyes in water

Two isomorphic rare earth metal–organic frameworks (MOFs) were synthesized by a solvothermal method. These MOFs have good removal effects on cationic and neutral dyes through simultaneous adsorption and photocatalysis.

Viologen-Based Cationic Metal-Organic Framework for Efficient Cr2O72- Adsorption and Dye Separation

A novel cationic metal-organic framework composed of {Cu₂(COO)₄} paddle-wheel units and a tetracarboxylic viologen derivative, namely, {[Cu₂(bdcbp)(H₂O)₂]·2NO₃·2H₂O}_n (Cu-CMOF, H₄bdcbpCl₂ = 1,1'-bis(3,5-dicarboxyphenyl)-4,4'-bipyridinium dichloride), has been successfully synthesized and structurally characterized. In Cu-CMOF, the {Cu₂(COO)₄} unit and viologen derivative both act as four-connected nodes forming an ssb-type cationic network with 4².8⁴ topology, in which the positive charges are distributed on the organic viologen moieties. Deeper insight of the structure indicates that the 3D architecture of Cu-CMOF can be seen as packing of a 26-faceted polyhedral cage and two cuboid cages. Notably, Cu-CMOF displays a highly efficient anion exchange ability for capture and removal of anionic pollutants. UV-vis absorption spectra and digital images demonstrate that Cu-CMOF is capable of adsorbing the dichromate anion and anionic dyes effectively, such as methyl orange (MO^-), Congo red (CR^2-), and New Coccine (NC^3-). Meaningfully, anionic dyes (MO^-, CR^2-, and NC^3-) can be efficiently and selectively removed by Cu-CMOF in the presence of cationic dye methylene blue (MLB⁺). Such behaviors of anionic pollutant adsorption and dye separation are mainly caused by an ion-exchange process facilitated by the large cavity and decentralized distribution of positive charge in Cu-CMOF.

Advanced applications of Zr-based MOFs in the removal of water pollutants

The global water pollution is caused by the increase of industrial and agricultural activities, which have produced various toxic pollutants. Pollutants in water generally consist of metal ions, pharmaceuticals and personal care products (PPCPs), oil spills, organic dyes, and other organic pollutants. Amongst the adsorbents that have been developed to deal with pollutants in water, Zr-based metal-organic frameworks (MOFs) have drawn scientists' great attention due to their excellent stability and adjustable functionalization. Herein, the present review article introduces the synthetic methods of functionalized Zr-based MOFs and summarizes their applications in water pollution treatment. It also clarifies the interactions and removal mechanisms between pollutants and Zr-based MOFs. The use of these MOFs with eminent adsorption ability and recycling performance have been discussed in detail. Zr-based MOFs also face some challenges such as high cost, lack of real water environment applications, selective removal of pollutants, and low ability to remove composite pollutants. Future research should focus on addressing these issues. Although there is still a blank of the practical utility of Zr-based MOFs on a commercial scale, the research reported to date clearly shows that they are very promising materials for the water treatment.

Recent advances in metal-organic frameworks as adsorbent materials for hazardous dye molecules

Water is vital for the sustenance of all forms of life. Hence, water pollution is a universal crisis for the survival for all forms of life and a hurdle in sustainable development. Textile industry is one of the anthropogenic activities that severely pollutes water bodies. Inefficient dyeing processes result in thousands of tons of synthetic dyes being dumped in water bodies every year. Therefore, the efficient removal of synthetic dyes from wastewater has become a challenging research field. Owing to their tuneable structure-property aspects, metal-organic frameworks (MOFs) have emerged as promising adsorbents for the adsorptive removal of dyes from wastewater and textile effluents. In this perspective, we highlight recent studies involving the application of MOFs for the adsorptive removal of hazardous dye molecules. We also classify the developed MOFs into cationic, anionic, and neutral framework categories to comprehend their suitability for the removal of a given class of dyes.

Aqueous Synthesis of a Mesoporous Zr-Based Coordination Polymer for Removal of Organic Dyes

Porous coordination polymers have received intensive attention for pollution abatement, such as dye removal, because of their high porosity and specific surface areas. However, the commonly used water-stable porous coordination polymers are microporous and synthesized within organic solvents, which deters seriously their widespread application. In this report, we developed a facile strategy for the synthesis of mesoporous Zr-based coordination polymer (Zr-BDC-CP) within aqueous solutions. The morphology and structure of Zr-BDC-CP were characterized with scanning electron microscopy, powder X-ray diffraction, and Fourier transform infrared spectroscopy. Pore size distribution analysis confirms that the as-synthesized material is mesoporous, which allows the efficient adsorption of methylene blue, 2.6 times higher than that of the microporous coordination polymer, UiO-66. The decolorization ratio can reach higher than 93.5% in the range of 10 and 400 mg/L for methylene blue solutions. This Zr-based coordination polymer shows wonderful pH stability, where no significant loss of adsorption capacities was observed between pH values of 3 and 11. The simulation of adsorption isotherm indicates that the Freundlich model can fit the adsorption isotherm very well, which reflects that the surface of adsorbents is inhomogeneous. Fitting of kinetic curves shows that the dye adsorption by Zr-BDC-CP follows the pseudo-second-order model, which confirms that the rate-determining step may be a chemisorption process involving valence forces because of the defects within the frameworks of the mesoporous coordination polymer. Zr-BDC-CP also shows desirable recyclability without significant capacity loss. This work presents a facile and sustainable method for the preparation of mesoporous Zr-based coordination polymer for dye removal with excellent stability and recyclability, which could further push the porous coordination polymers for application in the areas of pollution abatement.

A polythreaded MnII-MOF and its super-performances for dye adsorption and supercapacitors

One new polythreaded Mn^II-MOF was successfully prepared by employing a tridentate N-donor ligand with three long arms. Its excellent performances in dye adsorption and supercapacitor have been investigated in detail.

Novel 3D anionic heterometallic frameworks based on trinuclear CoII and trinuclear LnIII motifs displaying slow magnetic relaxation and selective adsorption of methylene blue

3D CoLn heterometallic frameworks have been synthesized. CoDy and CoHo show slow magnetic relaxation behavior. CoTb exhibits excellent adsorption capacity for methylene blue.

A Lead Carboxylate-Azolate Metal-Organic Framework Based on Hexanuclear Clusters: Luminescence and Accelerating the Thermal Decomposition of Ammonium Perchlorate

A lead carboxylate-azolate framework, [Pb₃ (μ₄ -O)(L)₂ (H₂ O)] ⋅ 2H₂ O (1), has been constructed from the reaction of 2-methyl-4-(1H-tetrazol-5-yl) benzoic acid (H₂ L) and Pb(NO₃ )₂ under solvothermal conditions. Compound 1 contains an unprecedented hexanuclear cluster [Pb₆ (μ₄ -O)₂ (COO)₄ (CN₄ )₂ ], which as an 8-connected node is connected by 3-connected L linkers to give rise to a three-dimensional framework with an uncommon (3,8)-connected tfz-d; UO₃ topology. Framework 1 possesses strong solid-state yellow luminescence (563 nm), which stems from ligand-to-metal charge transfer (LMCT), however, the removal of water molecules in 1 lead to the significant luminescent decrease of the sample. Importantly, in the presence of 1, the thermal decomposition of ammonium perchlorate is significantly accelerated.

Ab initio powder X-ray diffraction structural analysis of bispidine based 1D coordination polymers: insights into their guest responsive behaviour

The first ab initio synchrotron powder XRD structure solution of a desolvated 1D bispidine coordination polymer (CP) is reported, providing valuable insights into the stability, dynamic and guest responsive behavior of a new class of CPs.

Ultra-highly selective trapping of perrhenate/pertechnetate by a flexible cationic coordination framework

This work presents a 3D cationic coordination framework, showing ultra-highly selective trapping of perrhenate/pertechnetate in the presence of sulfate 20 000 times in excess.

Mechanisms of Solvent-Mediated Structural Transformations for Dynamic Crystals of Supramolecular Coordination Systems

Thus far, reports about the transformations for dynamic crystals of supramolecular coordination systems mainly include single-crystal-to-single-crystal reactions, and solvent-mediated processes. This Concept focuses on the mechanisms for solvent-mediated structural transformations of dynamic crystals, which can be classified into two categories, that is, core-on-shell and core-to-core. The core-on-shell mechanism means that the core of the new crystal is generated from the shell of original crystal, being concomitant with the dissolution of the mother crystal. In contrast, for the core-to-core case, the growth of new crystal core and the dissolution of original crystal core will proceed simultaneously, but they are physically separated. Herein, the two mechanisms are elucidated in detail, with some typical examples from recent advances.

Base-Resistant Ionic Metal-Organic Framework as a Porous Ion-Exchange Sorbent

A systematic approach has been employed to obtain a hydrolytically stable cationic metal-organic framework (MOF). The synthesized two-dimensional Ni(II)-centered cationic MOF, having its backbone built from purely neutral N-donor ligand, is found to exhibit uncommon resistance over wide pH range, particularly even under highly alkaline conditions. This report presents a rare case of a porous MOF retaining structural integrity under basic conditions, and an even rarer case of a porous cationic MOF. The features of stability and porosity in this ionic MOF have been harnessed for the function of charge- and size-selective capture of small organic dye through ion-exchange process across a wide pH range.

Design of two isoreticular Cd-biphenyltetracarboxylate frameworks for dye adsorption, separation and photocatalytic degradation

Two robust Cd-biphenyltetracarboxylate frameworks showing identical topological networks but different pore sizes exhibit not only high dye uptake capacity and selectivity, but also good photocatalytic degradation of RhB and MO.

An anionic single-walled metal–organic nanotube with an armchair (3,3) topology as an extremely smart adsorbent for the effective and selective adsorption of cationic carcinogenic dyes

A novel discrete single-walled MONT with a rare armchair (3,3) CNT topology and mesoporous channels is synthesized, which exhibits exclusive and remarkable ability to adsorb cationic carcinogens BR9 and BV14.