Impact of Loading-Dependent Intrinsic Framework Flexibility on Adsorption in UiO-66

The vast majority of molecular simulations of adsorption
in porous
materials make use of the assumption that the framework of the porous
material may be held rigid without significantly impacting the accuracy
of the computed adsorption isotherm. One reason for this approximation
is that explicit inclusion of framework flexibility in adsorption
simulations dramatically increases the computational cost of the simulation.
Approximate methods for including framework flexibility have been
developed that are computationally efficient and have been applied
to adsorption of gases in metal–organic frameworks (MOFs) by
generating an ensemble of snapshots from a molecular dynamics calculation
of an empty MOF and employing these snapshots to generate an isotherm
averaged over all the snapshots, holding each snapshot rigid during
the adsorption calculation. This method assumes that the adsorbate
has negligible impact on the dynamic configurations of the MOF. We
demonstrate an efficient method for including adsorbate-induced changes
to the MOF framework and show that this assumption is not always valid.
Specifically, for acetone and isopropyl alcohol adsorbed in UiO-66,
the saturation loading and degree of hydrogen bonding with the framework
significantly increased through the inclusion of adsorbate-induced
framework flexibility. We show that adsorption simulations that include
adsorbate-induced flexibility match the experimental data for these
systems much better than simulations using either the rigid approximation
or including the framework flexibility of the empty MOF. We expect
that adsorbate-induced flexibility may be important for adsorption
in many similar systems.