Nanoporous Transparent MOF Glasses with Accessible Internal Surface

While glassy materials can be made from virtually every class of liquid (metallic, molecular, covalent, and ionic), to date, formation of glasses in which structural units impart porosity on the nanoscopic level remains undeveloped. In view of the well-established porosity of metal-organic frameworks (MOFs) and the flexibility of their design, we have sought to combine their formation principles with the general versatility of glassy materials. Although the preparation of glassy MOFs can be achieved by amorphization of crystalline frameworks, transparent glassy MOFs exhibiting permanent porosity accessible to gases are yet to be reported. Here, we present a generalizable chemical strategy for making such MOF glasses by assembly from viscous solutions of metal node and organic strut and subsequent evaporation of a plasticizer-modulator solvent. This process yields glasses with 300 m²/g internal surface area (obtained from N₂ adsorption isotherms) and a 2 nm pore-pore separation. On a volumetric basis, this porosity (0.33 cm³/cm³) is 3 times that of the early MOFs (0.11 cm³/cm³ for MOF-2) and within range of the most porous MOFs known (0.60 cm³/cm³ for MOF-5). We believe the porosity originates from a 3D covalent network as evidenced by the disappearance of the glass transition signature as the solvent is removed and the highly cross-linked nanostructure builds up. Our work represents an important step forward in translating the versatility and porosity of MOFs to glassy materials.