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Meenu Kumar and Dr. Naveen V. Kulkarni
Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri 690525, Kerala, India
The chemistry of metal-organic frameworks is fascinating because it combines metal ions or metal clusters with specific organic molecules known as linkers to producesupramolecular assemblies meant toenthral scientists by offering solutions for a wide range of scientific and technological challenges.Even though synthetic zeolites, whichcontain polymeric inorganic materials like silicates and aluminates, have beenstudied thoroughly since the 1940s, MOFs, which are the hybrid inorganic-organic compounds rooted in coordination chemistryfirst came to the limelight in the early 1990s. The term metal-organic frameworks were introduced by Omar M Yaghiin 1995 for representing a new class of porous materials with high surface area, tunable properties, and potential applications in gas storage and separation, catalysis, drug delivery, and more . The field has since exploded, and MOFs have become one of the most attractive topics in materials chemistry research, with thousands of publications and patents generated every year.
The structure of MOFs comprises metal ions and organic ligands arranged in a crystalline network or framework (Fig:1). The central ions can be transition metals, rare earth metals, or non-metals, and the organic ligands can vary widely in composition and have functionalities like amine, imine, hydroxy, carboxy, etc. The metal ions serve as the nodes of the framework, while the organic ligands act as spacers, linking the metal ions to formthe extended network.The combination of metal ions and organic ligands primarily determines the structure and properties of MOFs. Precisely the length, the functional groups attached, and the geometry of the organic ligands rationalize the property of MOFs along with the influence by the inorganic nodes . MOF self-assembly process can lead to form a 1D, 2D, or 3D framework depending on the nature of the organic linker, metal ion as well as the reaction conditions. The 3D MOFs can beporous, with pore sizes ranging from several angstroms to several nanometers. Hence depending on the pore size, they can be classified as microporous (<2nm), mesoporous (2-50nm), and macroporous (50nm) materials.The porosity of MOFs makes them attractive for gas storage and separation applications, as gases can enter and exit the pores while larger molecules remain trapped inside the lattice. The key advantage is that, the pore size and shape of the MOFs can be easily tailored to fit the specific properties and dimensions of the target molecules . MOF synthesis has evolved significantly over the past few decades, from simple coordination chemistry reactions to more sophisticated approaches, such as biomimetic synthesis  and bottom-up assembly . Conventionally, MOFs were synthesized by combining metal salts with organic ligands in a solvent and heating the mixture (Fig:2). The metal ions and organic ligands formed coordination bonds in a self-assembly process, forming MOFs.However, the conventional approach has limitations, such as low yields, poor reproducibility, and limited control over the size and shape of the MOFs. Recently, several innovative techniques have been developed to overcome these limitations. For example, biomimetic synthesis mimics the process by which biological organisms form their shells or skeletons using inorganic and organic components. This method allows for the precise control of the size and shape of MOFs and has led to the development of novel MOFs with unique properties and applications .
The distinctive properties of MOFs have led to their application in a broad range of fields (Fig:3), including gas storage and separation, catalysis, sensing, drug delivery, and luminescence. One of the significant applications of MOFs is in storing and separating gases, such as hydrogen, carbon dioxide, and methane. The porosity of MOFs enables the selective uptake and release of these gases, making them attractive materials for gas storage and transport .MOFs are also widely used as catalysts in various chemical reactions, including organic transformation reactions of industrial importance. The metal ion-active sites of MOFs can effectively catalyze chemical reactions, leading to higher reaction rates and yields. Additionally, the specific porosity of MOFs can enhance the selectivity of catalytic reactions by allowing only the targeted molecules to enter and bind to the active sites .Another important application comes up with the ionic and electronic conductivity of MOFs, due to the high charge storage capacity and tunable conducting properties MOFs can be good candidates for super capacitor applications .Furthermore, MOFs have significant applications in the field of drug delivery. The porous structure and biocompatibility of MOFs make them ideal materials for encapsulating and delivering drugs to target sites in the body, thereby reducing the side effects of the drugs . MOFs can also be used in sensing applications, where they act as selective receptors for target molecules, resulting in a color change or luminescence .
Overall, MOFs are exciting materials that have captured the attention of scientists worldwide. Their unique properties and versatile applications have led to extensive research in the field of materials science. The structure, synthesis, and applications of MOFs are continuously evolving, leading to the development of novel materials with unique properties and functions. MOFs hold great promise for advancing several fields, including energy storage, medicine, and environmental protection. Further research into the synthesis and properties of MOFs is essential to unlock their full potential and realize their applications in various fields.
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