MOFs

Introduction

Metal-organic frameworks (MOFs) are an intriguing class of hybrid materials, which are crystalline materials possessing highly ordered structures consisting of networks formed by single metal ions or metal clusters connected by multidentate organic groups acting as linkers (see Fig.1)^[1]. Initial explorations into MOFs began with a report from Yaghi's research group in 1995, followed by the report of MOF-5 in 1999, one of the classic MOFs structures that contains Zn-based nodes, which demonstrated specific surface areas equal to 2900 m2/g and 60% porosity. Since then, numerous types of MOFs have been reported via judicious selection of suitable organic ligands (organic linkers) and metal ions. In addition to extensive combinations of ligands and metals, MOFs also exhibit diverse geometries, topologies, and functionalities, resulting in adjustable chemophysical characteristics, even when the same combination of metal nodes and organic linkers is used.

Fig.1 General scheme of MOF synthesis

The most interesting versions of MOFs display permanent nanoscale porosity, which makes these organic−inorganic hybrid materials similar to zeolites. Some of them have record surface areas exceeding those in zeolites. The surface area of 1 g of some MOFs is comparable with a square of football field (120×53.33 yards implies 6400 square yards or ca 5351 m²). The feature of porosity that can translate into large internal surface areas, ultralow densities, and the availability of uniformly structured cavities and portals of molecular dimensions. In addition, the MOFs also display the characteristics of tunable porous structure, structural flexibility, easy functionalization and substitutable components, which makes MOFs have been extensively studied and applied in diversified fields.

Classification

MOFs can be mainly divided into following several kinds according to their configuration including MIL series MOFs, PCN series MOFs, CPL series MOFs, UIO series MOFs, ZIF series MOFs and IRMOF series MOFs.

• MIL series MOFs: The MIL series (MIL stands for Materials Institute Lavoisier) MOFs are based mainly on carboxylate ligands (such as 1,3,5-benzene tricarboxylate, terephthalate, isophthalate etc.) and trivalent metal ions of V, Cr, Fe, Al, Ga and In and also lanthanides, but also includes with metal ions in different oxidation states, such as Ti(IV).
• PCN series MOFs: The PCN stands porous coordination network, the series MOFs contain cubic octahedral nano pore cages, and form a pore cage-pore channel-like topology in space, which has tremendous potential in gas storage.
• CPL series MOFs: CPL stands coordination pillared-layer, CPL series MOFs are structured by scaffolding 2D Cu(II) and pyrazine-2,3-dicarboxylate (pzdc) layers using dipyridyl ligands (such as pyrazine, 4,40-bipyridine and 1,2-di(4-pyridyl)ethylene, as the bridging pillars). CPL series MOFs can be facilely synthesised under mild conditions (e.g. at room temperature) and channel dimensions and surface functionalities can be systematically controlled by modification of the pillar ligands (L)^[2]. The series MOFs show good flexibility and unique guest-responsive nature.

Fig.2 The synthesis of CPL series MOFs

• UIO series MOFs: The UiO series MOFs are three-dimensional porous materials and constructed by Zr⁴⁺ and dicarboxylic acid ligands. The UiO series has more excellent chemical/thermal stability compared with other MOFs series. Although the ligands of UiO-MOFs have different lengths, UiO-64, UiO-66, UiO-67, UiO-68, UiO-69, and various derivatives have the same reticular structure, the change of the ligand does not affect the thermal stability of UiO-MOFs.
• ZIF series MOFs: The ZIF series MOFs are based on zeolitic imidazolate frameworks obtained by copolymerization of zinc and cobalt ions with imidazole-type linkers. The ZIF structures consist of nets of seven different aluminosilicate zeolites where the tetrahedral Si/Al clusters are replaced by Zn or Co and the bridging O by the bridging imidazolates.
• IRMOF series MOFs: The IRMOF series MOFs have the same net topology (isoreticular). In other words, they share a common topology as they are constructed from the same type of organic linkers, for instance, IRMOF-1, it is composed of tetrahedral Zn₄O clusters linked in a cubic unit by terephthalate linkers. Upon changing the terephthalate linker with other bidentate linear groups, a family of MOFs with similar morphology and cubic arrangement have been obtained called IRMOF-2, IRMOF-3 etc., or IRMOF-n.

Applications

Due to their high specific surface area, tunable porous structure, abundant coordination-unsaturated metal sites, and substitutable components, MOFs have been extensively studied and applied in gas storage and delivery, drug storage and delivery, catalysis, sensing, magnetic materials, electrochemical and separation membranes fields^[3].

• Gas storage and delivery: The main application described for MOFs is gas storage and delivery such as hydrogen and methane storage, CO₂ capture due to their higher surface areas which provide higher gas uptake capacity. Gas storage and delivery in MOFs is based on physisorption. It is worth mentioning that MOFs are ideal materials for hydrogen storage at low temperatures. The most promising are MOFs with high hydrogen storage densities exceeding 10 wt% and 58 g/L (77 K). The final target is to reach 7.5% of H₂ by mass, i.e. 70 g/L.

Fig.3 The diagrammatic drawing of gas storage and delivery by MOFs

• Drug storage and delivery: As hybrid organic-inorganic compounds, MOFs present themselves as optimal drug-delivery materials due to the adjustability of the framework's functional groups and the tunable pore size. With MOFs, the benefits of using organic materials (biocompatibility and the ability to uptake large amounts of drugs) and inorganic materials (controlled release) may both be utilized, which makes MOFs be a potential efficient drug storge and delivery materials.
• Catalysis: As porous materials, MOFs may prove to be very useful in catalysis. The pores of MOFs can be tailored in a systematic way allowing optimization for specifific catalytic applications. Besides the high metal content of MOFs, one of their greatest advantages is that the active sites are rarely different because of the highly crystalline nature of the material. All this makes catalysis become one of the most promising applications of MOFs materials.
• Luminescence and sensors: The luminescent MOF materials can be prepared by combining the luminescent metal ions (such as rare earths europium and terbium) or clusters and organic ligands, as well as special guest molecules. The MOFs that possess luminescent properties together with size-or shape-selective sorption properties can be used as sensing devices for selective ion monitoring, stress-induced chemical detection, anisotropic photoluminescence probes and so on^[4].

Fig.4 Structure and luminescence properties of luminescent MOFs

• Magnetic materials: Several MOFs exhibit specific magnetic and multiferroic properties such as ferromagnetic properties and ferrimagnetic properties. The use of chemical coordination or crystal engineering techniques allows for the systematic design of MOFs with adjustable magnetic properties. Furthermore, the porosity of MOFs provides additional convenience in regards to magnetic properties. The above magnetically active properties constitute an important opportunity for data storage, spin-frustrated catalysis and magnetosensing.
• Electrochemical: The high specific surface areas and tunable pores of MOFs can be favorable to interfacial charge transport, and the unparalleled synthetic flexibility, adjustable pore size are good for capturing and immobilizing guest molecules. Hence, MOFs have been identified as a promising platform in the field of electrochemistry in recent years. MOFs can be used as electrochemical sensing, electrocatalysis, and electrochemical energy storage devices (e.g., batteries and supercapacitors) and so on^[5].

Fig.5 The recent electrochemical application of MOFs

• Separation membranes: MOFs offer unprecedented opportunities for membrane-based gas and liquid separation and purification such as olefin/paraffin separations, CO₂/H₂, separations and CO₂/N₂ separations due to their facile control over pore size and functionality combined with tunable sorption behavior. Thereinto, thermally and chemically stable zeolitic imidazolate frameworks (ZIFs) are of particular interest as membrane materials.

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