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

Abstract

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 H 2 O 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.

A Nanocavitation Approach to Understanding Water Capture, Water Release, and Framework Physical Stability in Hierarchically Porous MOFs

“…Beyond ρ wat ≈ 0.3 g/g, the hexagonal mesopore becomes preferable to a triangular micropore (Figure S37). These sittings are consistent with a prior simulation study using a different force field …”

Advances in Metal–Organic Frameworks for the Removal of Chemical Warfare Agents: Insights into Hydrolysis and Oxidation Reaction Mechanisms

“…The more hydrophilic the material, the more water-framework interactions will precede interactions with target species, lowering active site accessibilities and target species reaction rates. These obstacles make hydrophilic MOFs a poor choice for chemical reaction applications in settings with large atmospheric moisture levels [15]. Hydrophobic MOFs show promise in this regard, as their resistance to chemical degradation by water [62] makes them chemically stable in humid conditions and unrestricted by competitive adsorption upon water exposure [63].…”

“…Establishing a perfect balance of stability, topology, and water affinity that diminishes these challenges would be a major stride in developing ideal MOFs for the given application. This was the focus behind our recent publication, where we investigated the behavior of environmental water at various loadings in NU-1000 [15], a Zr-MOF with combined features of strong node-linker bonds, large pore volumes, and internal hydrophobicity. Computational results demonstrated that water in NU-1000 exhibits a structural phase somewhere between liquid and vapor.…”

“…In recent years, MOFs have been identified as superior materials for detecting and breaking down target species, owing to their excellent adsorption, reactivity, and catalytic abilities [1]. Coupled with their exceptional porosities and large surface areas, many MOFs provide an ideal setting for the selective capture and detoxification of CWAs [1,8,9,11,[14][15][16][17].…”

A Miniaturized Method for Evaluating the Dynamic Gas-Phase Adsorption and Degradation of Sarin on Porous Adsorbents at Different Humidity Levels

“…The method’s development included measurements of the capacity of the Zr-MOF NU-1000 and nonimpregnated AC to adsorb and degrade GB under both dry and humid conditions. NU-1000 is a promising Zr-MOF with large pores, whose ability to adsorb and degrade OP-agents has been studied previously. ,,, …”