Reversibly Phase-Transformative Zeolitic Imidazolate Framework-108 and the Membrane Separation Utility

Reversible phase transformations (RPTs) of metal–organic frameworks not only create material diversity but also promise a self-restoration of crystals in a controllable manner. However, there are only limited examples because seeking for a convenient and effective trigger for RPTs, especially for RPTs with respect to spatiotemporal harmony in cleavage and reconstruction of metal–linker chemical bonds, is challenging. In this work, we found that zeolitic imidazolate framework (ZIF)-108 with Zn–N coordination bonds showing moderate strength was an ideal platform. We reported three crystal phases of ZIF-108, namely, sodalite (SOD), diamondoid (DIA), and large pore_sodalite (lp_SOD) topologies, and identified RPTs between phases: (1) when exposed to water or water vapor, the SOD structure could transform to a compact DIA version as a result of the decomposition of four-membered rings and synchronous reorganization of six-membered rings. Then, the DIA structure could also return back to SOD when soaked in dimethylformamide (DMF) or DMF vapor. (2) High-temperature treatment of SOD gives rise to lp_SOD, which then reverts to SOD by DMF. (3) lp_SOD could also be compressed into the DIA phase by water or water vapor and can then be restored via a two-step treatment, namely, soaking in DMF (DIA → SOD) right before a high-temperature therapy (SOD → lp_SOD). From the perspective of the separation utility, we found that the lp_SOD version of ZIF-108, relative to SOD-structured ZIF-108, can produce mixed matrix membranes having an interesting interfacial structure with the polymer chains, though both share the same chemical composition. We verified that the large pore of lp_SOD can allow being penetrated by polymer chains, which contributed to not only reinforcing the bi-phase interface but also sharpening the molecule sieve properties of fillers toward CO2 and CH4.