Mechanistic Study On Hydrolysis Of ZIF-8 Structure (ZIF-8 Crystallites And Polycrystalline ZIF-8 Membrane) At Room Temperature And Its Hydrostability Improvement

ZIF-8 was regarded as a structure with exceptional water stability.Though a recent work reported ZIF-8 undergoes structural degradation in pure water under hydrothermal conditions,most adsorption studies using ZIF-8 as adsorbents under aqueous conditions still claimedthat ZIF-8 is water stable.These controversial even contrary reports on ZIF-8's hydrostability call for mechanistic study to clarify its hydrolysis behavior.Herein,this dissertation methodically examined the hydrolysis of ZIF-8 structure,both in the form of crystallites to mimick the adsorbents and intergrown polycrystalline thin film on supports as selective membrane,under static batch durability hydrolysis tests by suspending ZIF-8 crystallites or immersing ZIF-8membrane in pure water or dynamic water pervaporation tests on ZIF-8 membrane.For hydrolysis test of ZIF-8 powders,continuous release of Zn²⁺from ZIF-8 structure into the aqueous solution was detected along with the change in pH value of the aqueous system.These evidences confirm that ZIF-8 crystals degraded in water due to the hydrolysis of ZIF-8.The hydrolysis product from ZIF-8 with different crystallite sizes shares identical leaf-like morphology and an unknown dense,crystalline structure.Hydrolysis of ZIF-8 is general phenomenon in room temperature water,and hydrolysis extent increases with decreasing ZIF-8/water weight ratios,but irrespective of ZIF-8's sizes and synthesis methods.For ZIF-8 membrane the disappearance of ZIF-8 structure after pervaporation is consistent with morphology and phase structure data showing that ZIF-8 layer on the?-alumina support underwent degradation during pervaporation.The gradual degradation of ZIF-8 layer was also experimentally observed in batch durability tests in pure water,even in rather short time contact with water of just 10 min.Comprehensive characterizations were conducted on ZIF-8 hydrolysis product.XRD and FTIR characterizations confirm the partial cleavage of Zn-N bond and coordination of hydroxyl to Zn and protonation of imidazole ligands during hydrolysis.EDS quantification data for hydrolysis product show identical relative atomic composition with ideal ZIF-8 except for O.The formation of hydrolysis product from ZIF-8/water suspension system confirms the hydrolysis of ZIF-8 is irreversible.A postulation depicting hydrolysis of ZIF-8 and formation of hydrolysis product was developed based on EDS quantification and FTIR analysis,in which ZIF-8 is hydrolyzed into residuals through partial cleavage of Zn-N,and these residuals get reunited and crystallized to hydrolysis product under hydrogen bond effect between hydroxyl and repronated imidazole linkers during condensation of aqueous suspention system.Post-modification was conducted on pristine ZIF-8 crystallites and polycrystalline membranes to improve ZIF-8's hydrastability through utilization of a ligand exchange method which replaces 2-methylimidazole ligands on the outer surface of ZIF-8 crystals or membranes by the more hydrophobic,bulkier5,6-dimethylbenzimidazole.Ligand exchange modification does not change the crystal structure,morphology or gas permeance as shown ZIF-8 powder and membrane characterization experiments.And it was corroborated that ligand exchange occurs at the outermost surface of ZIF-8 crystals and membranes as evidenced by the increasing exchange ratios with decreasing sizes of ZIF-8 crystallites.The modified ZIF-8 powders and membranes retain identical morphologies and crystallinities after static and dynamic water immersion.The modified ZIF-8membrane exhibits stable water pervaporation flux controlled by the ZIF-8 layer while the unmodified ZIF-8 membrane experiences dissolution of the ZIF-8 layer during water pervaporation.