Phase Transformations of Metal–Organic Frameworks MAF‑6 and ZIF-71 during Intrusion–Extrusion Experiments

In this work, the energetic behaviors of two zeolitic imidazolate framework (ZIF) materials, ZIF-71 and the highly porous MAF-6, of RHO topology are studied in high-pressure intrusion–extrusion experiments using LiCl 20 M aqueous solution and/or water as nonwetting liquids. During the intrusion–extrusion experiments, both MAF-6 and ZIF-71 structures undergo phase transformations under the combined effects of high pressure and the nonwetting liquid. For the “MAF-6–H2O” system, the volume variation step observed on the intrusion curve of the first intrusion–extrusion cycle partially corresponds to the intrusion of water molecules inside the MAF-6 pores associated with a partial and irreversible phase transformation of MAF-6 into a nonporous and unidentified phase X. When the pressure is released, that is, when the experiment is stopped at the end of the volume variation step and the system returns to atmospheric pressure, the intruded water molecules are expelled from the porosity of the remaining MAF-6 material, and the “MAF-6–H2O” system, therefore, displays a mixture of shock-absorber and bumper behaviors. In the case of both “MAF-6–” and “ZIF-71–LiCl 20 M aqueous solution” systems, the volume variation step is shifted to a higher pressure range, indicating that the intrusion of the nonwetting liquid molecules occurs at a higher pressure because of the presence of electrolytes. For the “MAF-6–LiCl 20 M aqueous solution” system, a mixture of MAF-6 and phase X is obtained at the end of the volume variation step, which probably indicates energetic behavior similar to the one observed for the “MAF-6–H2O” system (i.e., a mixture of shock-absorber and bumper behaviors). In the case of the “ZIF-71–LiCl 20 M aqueous solution” system, the volume variation step observed on the intrusion curve of the first intrusion–extrusion cycle corresponds to a phase transformation. At the end of this step, the ZIF-71 structure is completely transformed into ZIF-72 (lcs). The “ZIF-71–LiCl 20 M aqueous solution” system, thus, displays bumper behavior, as the supplied mechanical energy is absorbed during the irreversible phase transformation phenomenon.