Crystallographic Study of Water Distribution, Dehydration, Rehydration, Demethylation, and Decomposition Processes in Zeolitic Imidazolate Framework ZIF‑8

The structures of 19 single crystals of ZIF-8 that were subjected to different thermal treatments have been determined. Crystal 1 was as-synthesized from an in situ synthetic batch. Crystals 2–12 were thermally treated at various temperatures under dynamic vacuum for 48 h. A dehydrated ZIF-8 was rehydrated by flowing distilled water at 293 K for 1 h (crystal 13). Crystals 14–19 were thermally treated at 798 K under dynamic vacuum for 1, 2, 3, 4, 5, and 6 h, respectively. Their crystal structures were completely determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group I4̅3m at 100(1) K. In the structures of crystals 1 and 2, about 20 and 0.8 water molecules per unit cell occupied two crystallographic sites at O1 (opposite 4-ring) and O2 (center of 6-ring plane). Unlike the crystals 1 and 2, a trace amount of water molecules found only one crystallographic site at O2 in the structures of crystals 3–10. The crystals 11, 12, and 14 were completely dehydrated. In the structure of crystal 13, about 33 water molecules were readsorbed not only at O1 and O2 sites but also at the new site O3. In the structures of crystals 15–19, partial demethylation of 2-methylimidazolate ligands in ZIF-8 occurred, and the remaining methyl group of 2-methylimidazolate ligands were found with the occupancies of 22.4(3), 21.8(3), 20.7(5), 20.6(4), and 16.6(13), respectively.