Intrusion of Water in ZIF-8: Evidence of the Thermodynamic Instability under High Pressure

High-pressure intrusion-extrusionexperiments of water inhydrophobic zeolitic imidazolate framework (ZIF-8) were performedby two techniques: porosimetry and manometry coupled with calorimetrymeasurement. This study focuses on the variation of experimental duration(3-900 h) for both techniques in order to study the impactsof 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 compressionat 0.2 ton force to form pellets and outgassing at 473 K for 12 hunder vacuum do not alter the structural and textural properties ofstarting ZIF-8 powder. The post-intruded samples have been characterizedby X-ray diffraction, nitrogen sorption at 77 K, and scanning electronmicroscopy in order to evaluate the impact of experimental durationon the stability of the ZIF-8 framework. It has been observed thatthe 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 exothermicand the ZIF-8 framework is preserved. In contrast, for longer measurementdurations, the system displays a combination of spring and bumperbehaviors. This phenomenon is related to partial but gradual transformationof ZIF-8 into the nonporous ZIF-dia(Zn) phase. Complementary experimentshave highlighted that both water and high pressure play crucial rolesin this transformation of ZIF-8.