Intrusion of Water in ZIF-8: Evidence of the Thermodynamic Instability under High Pressure
Amir Astafan, Céline Dirand, Andrey Ryzhikov, Habiba Nouali, T. Jean Daou, Claire Marichal, Jean‐Pierre Bellat, Igor Bezverkhyy, Gérald Chaplais
Abstract
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.