Intrusion of Water in ZIF-8: Evidence of the Thermodynamic Instability under High Pressure
Résumé
High-pressure intrusion−extrusion experiments of water in hydrophobic zeolitic imidazolate framework (ZIF-8) were performed by two techniques: porosimetry and manometry coupled with calorimetry measurement. This study focuses on the variation of experimental duration (3−900 h) for both techniques in order to study the impacts of this parameter on the energetic performance of a “ZIF-8−water” system and the stability of ZIF-8. Prior to performing intrusion measurements, tests have been conducted in order to highlight that mechanical compression at 0.2 ton force to form pellets and outgassing at 473 K for 12 h under vacuum do not alter the structural and textural properties of starting ZIF-8 powder. The post-intruded samples have been characterized by X-ray diffraction, nitrogen sorption at 77 K, and scanning electron microscopy in order to evaluate the impact of experimental duration on the stability of the ZIF-8 framework. It has been observed that the duration of intrusion−extrusion tests affects system behavior. If the measurements’ duration is less than 4 days, the “ZIF-8−water” system acts as a molecular spring with fully reversible intrusion. In these conditions, the intrusion process is shown to be exothermic and the ZIF-8 framework is preserved. In contrast, for longer measurement durations, the system displays a combination of spring and bumper behaviors. This phenomenon is related to partial but gradual transformation of ZIF-8 into the nonporous ZIF-dia(Zn) phase. Complementary experiments have highlighted that both water and high pressure play crucial roles in this transformation of ZIF-8.
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