Vapor-Like Water in the NU-1000 Zr-MOF: A Molecular Level Understanding of Balanced Hydrophobicity in Humid Conditions

While
the structural features and tunability of metal–organic
frameworks (MOFs) make them promising materials for chemical warfare
agent (CWA) hydrolysis, their stability and performance in conditions
of varying humidity is an unsolved challenge. Understanding what design
rules enable lasting hydrolytic functionality in evolving field conditions
is consequently essential to developing practical MOFs for such applications.
In this work, molecular dynamics simulations are carried out to examine
the behavior of water at various loadings in the Zr-based MOF NU-1000.
With its strong node-linker bonds, expansive pores, and balanced hydrophobicity,
pristine NU-1000 possesses the characteristic attributes for structural
stability and hydrolytic efficiency in the presence of environmental
water. Adsorption and residence time results reveal that while NU-1000
is hydrophilic enough to allow water to adsorb, internal hydrophobicity
discourages the distribution of H2O molecules to active
sites at the metal nodes. Water–water interactions take precedence
in NU-1000, forming a water cluster that grows with loading and distracts
individual molecules from diffusing throughout the framework. On the
other hand, self-diffusion coefficients and radial distribution function
patterns suggest a lack of hydrogen bonding, with the clustered molecules
having faster diffusion and less ordering than that of liquid-phase
water. The limited interactions between water and the metal nodes
indicate a lower likelihood of competition for sites impeding target
species hydrolysis in NU-1000. Additionally, the partially vapor structural
state of the aggregated water molecules in the expansive NU-1000 channels
indicates a lower likelihood of pore filling by water that interferes
with target species adsorption and diffusion. Such results evidence
a strong potential of the NU-1000 Zr-MOF for superior performance
in hydrolysis applications like toxic chemical decomposition.