MOFs (metal-organic framework materials) refer to crystal materials with one-dimensional, two-dimensional or three-dimensional structures assembled by metal ions and multi-tooth organic ligands containing oxygen and nitrogen through coordination bonds, and various interesting topological structures will be formed in the assembly process. Compared with traditional microporous inorganic materials such as molecular sieves, MOFs have a wider range of choice of metal ions and organic ligands, so they can be designed and synthesized with a variety of structural forms and unique optical, electrical and magnetic properties of coordination compounds. MOFs have potential practical application value in gas storage, separation, catalysis, magnetism, nonlinearity, luminescence and other aspects, especially their excellent performance in low temperature hydrogen storage. Therefore, in recent years, MOFs have become the research focus of new hydrogen storage materials.
Figure 1. An example of MOF used in hydrogen storage.
In order to synthesize MOFs with certain properties and structures, it is necessary to select reasonable raw materials including central metal ions, organic ligands, anions, solvents and templates.
Figure 2. Schematic diagram for the synthesis of MOFs by the conventional solution method.
- Central metal ions: Central metal ions play a decisive role in the final structure of MOFs. Different metal ions can be selected to regulate the assembly process, and different metal ions with different coordination configurations can get different molecular network structures. Most of the central metal atoms are transition metal particles, including Zn2+, Cu2+, Ni2+, Pd2+, Pt2+, Ru2+, Co2+ and others.
- Organic ligands: Organic ligands also play a decisive role in the synthesis of MOFs. Different ligands affect both the synthesis of the complex and the spatial structure of the ligand network, controlling the distance between metal and metal and the dimension of crystal structure. Organic ligands require compounds containing two or more electron-donating atoms with multi-toothed functional groups such as -CO2H, -CS2H, -NO2, -SO3H, -PO3H and others. Among them, carboxylic acid ions and metal ions have strong coordination and various coordination modes, which are most widely used in the synthesis of MOFs. Common carboxylic acid ligands are aromatic carboxylic acid ligands such as terephthalic acid, trimesic acid, and others.
- Anions: The anions in the reaction system not only balance the charge, but also affect the formation of the complex topology. The final structure of the MOF is determined to some extent by the coordination capacity of anions.
- Solvents: Metal salts and most ligands are solid and need solvent to dissolve. At the same time, ligands should be deprotonated to promote the coordination of central ions. Currently, the most deprotonated bases are organic amines, such as triethylamine, N,N-dimethylformamide (DMF), N, N-diethylformamide (DEF) and others.
- Template agents: In order to make the synthesized MOFs with ideal porosity, it is necessary to select suitable template reagents. Organic amine ions, inorganic anions and neutral guest molecules can all play a template effect in the formation of coordination compounds, and different templates will produce completely different frames.
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