Improvement of hydrothermal stability of zeolitic imidazolate frameworks

Abstract

The metal-organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation.Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are an emerging class of nanoporous materials showing a wide range of potential applications, e.g. gas storage, molecular separation, chromatography, heterogeneous catalysis and smart sensoring. 1 Yet for most of the MOFs, one of the major drawbacks is their poor hydrothermal stability, which is clearly a limitation to their practical applications. 2 Development of novel MOF structures with high stability is an important research topic. 3,4 To date, only a few MOFs are reported to possess satisfactory hydrothermal stability, e.g. the ZIF (zeolitic imidazolate framework) family, 4 MIL (Matérial Institut Lavoisier) analogues 3b and some zirconium 3c-e and pyrazolate 3a based MOFs. On the other hand, from a practical point of view, post-enhancement of hydrothermal stability of the already existing MOF materials is a more attractive option and one of the most active research domains nowadays. 5 However along with improved stability, the application properties of the parent MOFs were usually impaired, e.g. considerable decrease of the specific surface area (SSA) after different post-treatments. 5b-d

Thermal stability of ZIF-8 under oxidative and inert environments: A practical perspective on using ZIF-8 as a catalyst support

“…4(c). This loss of crystallinity is most likely caused by hydrolysis between the ZIF-8 ligands and the ambient moisture, which accelerates the decomposition of the ZIF-8 structure [32]. The samples were fully decomposed after exposure to steam at temperatures greater than 400°C for 5 h. Accordingly, we concluded that ZIF-8 was least stable in the steam atmosphere compared to the other two atmospheres.…”

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

“…This impurity should be a kind of crystals consisting of Hmim ligands and zinc ions. That is different from hydrolyzed product, ZnO, reported by Yang and co-workers [26]. More research on decoding the impurity crystal and how it is formed are still needed.…”

“…Caro and co-workers [5,8] used precursor solution with a deficient zinc to Hmim ratio (1:1.5) for ZIF-8 membrane preparation. However, in most studies synthesis of both ZIF-8 membrane and crystals was performed with excessive Hmim ligands in synthesis solution [3,6,7,[10][11][12]23,26,[29][30][31]. Though most studies did not report ZIF-8 yield in the synthesis, a ca.…”

“…In contrary, Yang and co-workers [26] recently revealed that ZIF crystals (ZIF-7, ZIF-8 and ZIF-93) underwent hydrolysis under hydrothermal conditions when contact with water at lower weight ratio of ZIF crystals to water (0.06 wt%). The hydrolysis of ZIF structures was attributed to the elimination of the inhibitive effect of imidazolate ligands.…”

“…Yaghi and coworkers did not report the presence of impurity in ZIF-8 powders suspended in water at room temperature, 50 1C and refluxing boiling water [22]. However, Yang and co-worker found ZIF-8 structure underwent hydrothermal decomposition at a lower ratio of ZIF-8 powder to water (0.06 wt%) [26]. The detection of impurity in this work might be ascribed to the lower weight ratio of ZIF-8 crystals to water (0.06 wt%).…”