Aqueous Stability of Metal-Organic Frameworks Using ReaxFF-Based Metadynamics Simulations

Aqueous stability is a critical property for the application of metal-organic framework (MOF) materials in humid conditions. The sampling of the free energy surface for a water reaction is challenging due to a lack of a reactive force field. Here, we developed a ReaxFF force field for simulating the reaction of zeolitic imidazole frameworks (ZIFs) with water. We carried out metadynamics simulations based on ReaxFF to study the reaction of water with a few different types of MOFs. We also conducted an experimental water immersion test and characterized the XRD, TG, and gas adsorption properties of the MOFs before and after the immersion test. By considering the energy barrier for a hydrolysis reaction, the simulation results are in good agreement with the experiments. MOFs with open structures and large pores are found to be unstable in metadynamics simulations, where the water molecule can attack or bond with the metallic node relatively easily. In contrast, it is more difficult for water to attack the Zn atom in the ZnN₄ tetrahedral structure of ZIFs. We also found that ZIFs with the -NO₂ functional groups have higher water stability. Discrepancies between the metadynamics simulation and gas adsorption experiments have been accounted for from the phase/crystallinity change of the structure reflected in the X-ray diffraction and thermogravimetry analysis of the MOF samples.