Hydrolysis and condensation of ZIF-8 in water

Recent studies have shown that zeolitic imidazolate framework-8 (ZIF-8) crystallites are not stable in aqueous solution under ambient conditions. When immersed in water, ZIF-8 crystallites are hydrolyzed to zinc and imidazolate ions, which can be condensed to form a new solid substance upon removal of water by evaporation. The present work demonstrates such ZIF-8 phase shift substance is of crystalline structure but does not belong to any known (pseudo)polymorphs of ZIF-8, ZIF-L, dia(Zn), or decomposed product of ZIF-8: Zn(OH)2 and 2-methylimidazole. FTIR characterization shows the co-existence of Zn–O and Zn–N bond in the new substance, confirming the partial cleavage of Zn–N bond in ZIF-8 building unit in the hydrolysis step. EDS quantification for the ZIF-8 phase shift substance gives a C:N:Zn:O atomic ratio of 40:20:5:4. Based on EDS and FTIR results, the chemical formula of Zn5(HMIM)4(MIM)6(OH)4 for the building unit of such substance is proposed consisting of four partially hydrolyzed and one undisturbed secondary ZIF-8 building units. The ZIF-8 phase shift substance is crystalline with a nearly identical atomic composition as ZIF-8 (except for increased oxygen content), suggesting it might be a novel pseudo-polymorph of ZIF-8. The hydrolysis and condensation of ZIF-8 can be described by a mechanism that includes (1) hydrolysis of ZIF-8 through cleavage of Zn–N bond to form zinc and imidazolate ions and partially hydrolyzed ZIF-8 units and (2) condensation of these ions and clusters with the help of hydrogen bond to form the new structure of ZIF-8 phase shift substance.