High Pressure-Based Synthesis of Nanoporous Metal–Organic Framework ZIF-93 Giving Rise to a Phase for Proton Conduction

This study aims to develop a green, solvent-free synthesis of ZIF-93 (ZIF stands for zeolitic imidazolate framework) and to explore the formation of different phases. We report the solvent-free synthesis of a previously unreported nanoporous ZIF phase, ZIF-93_HP (HP referring to "high-pressure"), from zinc oxide using a dual high-pressure (150 MPa) and thermal (110 °C) method. The influence of key synthesis parameters, such as the amount of NH₄NO₃ promotor and reaction steps, was systematically investigated to maximize the conversion of ZnO into the intermediate ZIF-93_HP, while, in parallel, preventing its further conversion into nanoporous ZIF-93 phase. The material was extensively characterized by X-ray diffraction, thermogravimetry, electron microscopy and N₂ and CO₂ adsorption, which revealed insights into the structure, morphology and nanoporosity of ZIF-93_HP. ZIF-93_HP, with empirical formula of Zn-(C₅N₂OH₅)₂·1.2-(NH₄NO₃)·(H₂O), is related to the previously reported ZIF-93 (Zn-(C₅N₂OH₅)₂). Water washing of this phase led to the transformation into ZIF-93 and a significant increase in the BET specific surface area (from 4 to 181 m²/g). In addition, the presence of NH₄ ⁺ and NO₃ ^- ions into its structure makes ZIF-93_HP proton conductor at room temperature and moisture conditions (3.76 × 10^-3 S/cm), a property that decreases with increasing temperature due to dehydration. The discovery of ZIF-93_HP highlights the potential of the high-pressure, solvent-free synthesis as a powerful tool for the exploration of different ZIFs and reticular materials that are inaccessible through traditional solvothermal methods. As crystallization under solvent-free conditions is often influenced by nonthermodynamic equilibrium, this approach holds a great potential for expanding the material landscape by enabling the discovery of different phases and structures with unique properties, such as the promising proton conductivity demonstrated here.