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

Aqueousstability is a critical property for the application ofmetal-organic framework (MOF) materials in humid conditions.The sampling of the free energy surface for a water reaction is challengingdue to a lack of a reactive force field. Here, we developed a ReaxFFforce field for simulating the reaction of zeolitic imidazole frameworks(ZIFs) with water. We carried out metadynamics simulations based onReaxFF to study the reaction of water with a few different types ofMOFs. We also conducted an experimental water immersion test and characterizedthe XRD, TG, and gas adsorption properties of the MOFs before andafter the immersion test. By considering the energy barrier for ahydrolysis reaction, the simulation results are in good agreementwith the experiments. MOFs with open structures and large pores arefound to be unstable in metadynamics simulations, where the watermolecule can attack or bond with the metallic node relatively easily.In contrast, it is more difficult for water to attack the Zn atomin the ZnN4 tetrahedral structure of ZIFs. We also foundthat ZIFs with the -NO2 functional groups have higherwater stability. Discrepancies between the metadynamics simulationand gas adsorption experiments have been accounted for from the phase/crystallinitychange of the structure reflected in the X-ray diffraction and thermogravimetryanalysis of the MOF samples.

Automated summary

Aqueous stability is crucial for the application of MOF materials in humid conditions. We have developed a ReaxFF force field for simulating the reaction of ZIFs with water. By considering the energy barrier for a hydrolysis reaction, the simulation results are in agreement with the experiments. MOFs with open structures and large pores are found to be unstable, while ZIFs with the -NO2 functional groups exhibit higher water stability.