Modeling of Diffusion of Acetone in UiO-66

Highly
porous zirconium-based metal–organic frameworks (MOFs)
have been widely studied as materials for sorption and destruction
of chemical warfare agents (CWAs). It is important to understand the
diffusion of CWAs, their reaction products, and environmental molecules
through MOFs to utilize these materials for protection against CWA
threats. As a first step toward this goal, we study adsorption and
diffusion of acetone in pristine UiO-66. We have chosen to study UiO-66
because it has been demonstrated to be effective for destruction of
CWAs and simulants; we use acetone because it is a prototypical polar
organic molecule small enough to be expected to diffuse fairly rapidly
through nondefective UiO-66. We specifically examine the impact of
framework flexibility and hydrogen bonding between acetone and the
OH groups on the nodes of the framework on the diffusivity of acetone.
We find that inclusion of flexibility is essential for meaningful
predictions of diffusion of acetone. We have identified the dynamics
of the three linkers making up the triangular window between adjacent
pores as the critical factor in controlling diffusion of acetone.
We demonstrate from experiments and first-principles calculations
that acetone readily hydrogen bonds to UiO-66 framework OH groups.
We have modified the classical potential for UiO-66 to accurately
model the framework–acetone hydrogen bonds, which are not accounted
for in many MOF potentials. We find that hydrogen bonding decreases
the diffusivity by about 1 order of magnitude at low loading and about
a factor of 3 at high loading. Thus, proper accounting of hydrogen
bonding and framework flexibility are both critical for obtaining
physically realistic values of diffusivities for acetone and similar-sized
polar molecules in UiO-66.