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A defining characteristic of Metal-Organic Frameworks is their exceptionally high internal surface area and porosity. The ordered, crystalline structure creates well-defined pores and channels, leading to Brunauer–Emmett–Teller (BET) surface areas that can exceed 7,000 m²/g. This value far surpasses traditional porous materials like zeolites or activated carbons, making the vast internal surface readily accessible for molecular 흡착.
The high surface area in MOFs is a direct consequence of their low-density crystalline structure. Unlike amorphous materials like activated carbon, which have a wide distribution of irregular pores, MOFs possess uniform, precisely engineered cavities. The surface area is calculated from gas adsorption isotherms, typically using nitrogen or argon at cryogenic temperatures, and applying the BET theory. The formula for the BET surface area is [latex]S_{BET} = \frac{v_m N_A s}{V_M}[/latex], where [latex]v_m[/latex] is the monolayer volume of the adsorbate gas, [latex]N_A[/latex] is Avogadro’s number, [latex]s[/latex] is the adsorption cross-section of the gas molecule, and [latex]V_M[/latex] is the molar volume of the gas.
The ability to design MOFs with very long organic linkers and low-density metal nodes allows for the creation of frameworks with extremely large void fractions, sometimes exceeding 90% of the crystal volume. For example, MOF-5, one of the archetypal MOFs, has a BET surface area of around 3,000 m²/g. Later developments, such as MOF-177 and the NU series (e.g., NU-110), pushed these values to over 7,000 m²/g and even 10,000 m²/g, respectively. This immense internal ‘real estate’ is crucial for applications that rely on surface interactions, such as gas storage, where molecules are physisorbed onto the framework’s interior surfaces, and catalysis, where active sites can be distributed throughout the material’s volume.
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