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

Abstract

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 sixmembered 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 CO 2 and CH 4 .

Understanding and controlling the nucleation and growth of metal–organic frameworks

“…76 The different strength of the Zn-N bond in different Zn-based imidazolate MOFs provides different applications for the materials; while stronger Zn-N bonds are desired for storage applications, weaker bonds can facilitate reversible phase transformation, which broadens the material's applicability. 77 Indeed, ZIF-108 undergoes metal substitution much easier than ZIF-8 does, the latter of which requires a base to prevent phase transformation during this process. 76,78 This suggests that the rate of growth should be higher for ZIF-8, as hydrolysis-resistant metal-ligand bonds lead to higher crystal growth rates.…”