Stability of ZIF-8 membranes and crystalline powders in water at room temperature

The perceived robust stability of zeolitic imidazolate frameworks (ZIFs) was regarded as an important feature of ZIFs that have intrigued intensive research efforts. However, hydrolysis of ZIF crystals was recently identified under hydrothermal conditions. In this work, the dynamic stability of α-alumina supported ZIF-8 membranes and static stability of both ZIF-8 membranes and powder in water were studied via pervaporation and batch durability tests at room temperature. The sharp increase in water pervaporation flux from 0.04kgm^-2h^-1 to 1.39kgm^-2h^-1 and the disappearance of ZIF-8 structure after pervaporation are consistent with morphology and phase structure data showing that ZIF-8 layer on the α-alumina support underwent degradation during water pervaporation. The gradual degradation of ZIF-8 layer was also experimentally observed in batch durability tests in pure water. To better understand the mechanism of stability of ZIF-8 structure, control experiments on hydrolysis of ZIF-8 powder in aqueous conditions were designed and performed. The continuous release of Zn²⁺ from ZIF-8 structure to the aqueous solution was detected along with the change in pH value of the aqueous system. The presence of 2-methylimidazolate in aqueous solution can stabilize the ZIF-8 structure of both membrane and crystals. These evidences confirm that ZIF-8 crystals can degrade in water due to the dissolution of ZIF-8. The rate and extent of the dissolution depends on the ZIF-8 crystal to water mass ratio.