Water-Accelerated Transport:
Vapor-Phase Nerve Agent
Simulant Delivery within a Catalytic Zirconium Metal–Organic
Framework as a Function of Relative Humidity
posted on 2023-06-14, 16:04authored byRui Wang, Kaihang Shi, Jian Liu, Randall Q. Snurr, Joseph T. Hupp
Zirconium-based
metal–organic frameworks (MOFs)
are candidate
materials for effective nerve agent detoxification due to their thermo-
and water stability as well as high density of catalytic Zr sites.
However, as high-porosity materials, most of the active sites of Zr-MOFs
can only be accessed by diffusion into the crystal interior. Therefore,
the transport of nerve agents in nanopores is an important factor
in the catalytic performance of Zr-MOFs. Here, we investigated the
transport process and mechanism of a vapor-phase nerve agent simulant,
dimethyl methyl phosphonate (DMMP), through a representative Zr-MOF,
NU-1008, under practical conditions of varying humidity. Confocal
Raman microscopy was used to monitor the transport of DMMP vapor through
individual NU-1008 crystallites, where the relative humidity (RH)
of the environment was tuned to understand the impact of water. Counterintuitively,
water in the MOF channels, instead of blocking DMMP transport, assists
DMMP diffusion; indeed, the transport diffusivity (Dt) of DMMP in NU-1008 is one order of magnitude higher
at 70% than 0% RH. To understand the mechanism, magic angle spinning
NMR and molecular dynamics simulations were performed and suggested
that high water content in the channels prevents DMMP from hydrogen-bonding
with the nodes, allowing for faster diffusion of DMMP in the channels.
The simulated self-diffusivity (Ds) of
DMMP is observed to be concentration-dependent. At low loading of
DMMP, Ds is higher at 70% RH than 0% RH,
while at high loadings the trend reverses due to the DMMP aggregation
in water and the reduction of free volume in channels.