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.