Propagation of Degradation-Induced Defects in Zeolitic Imidazolate Frameworks

Isolated defects induced by water and acid gases have been extensively characterized in ZIF-8, a prototypical metal organic framework material, but there is little understanding regarding how these single bond-breaking events lead to the structural amorphization observed after prolonged experimental exposure in acidic environments. We use density functional theory calculations to provide the first analysis of defect propagation in a zeolitic imidazolate framework (ZIF) material. Given a single bond breaking event (the first step in the formation of any defect state), we exhaustively explore the energetics of subsequent defect states and find strong preference for additional bond-breaking located adjacent to the previous defect in both two-defect and three-defect systems. This series of favorable reaction energies is more exothermic when we replace water with sulfuric acid as a protonating agent, in agreement with experimental observations that ZIF-8 degradation is accelerated in humid acid gas environments. To give initial insights into experimental signatures of defect propagation, we compare the simulated powder pattern in structures at varying levels of defect concentrations.