Calculation and Measurement of Salt Loading in Metal–Organic Frameworks

The inclusion of salts within the pores of metal–organic
frameworks (MOFs) has been shown to increase the ionic conductivities
of the salt ions relative to that of the bulk salt, raising the possibility
that salt/MOF composites could be used as solid electrolytes. The
present study explores the mechanism by which the inclusion of ionic
liquids (ILs) occurs in MOFs and the mass loadings that can be achieved.
Simulated annealing simulations and cyclic differential scanning calorimetry
(DSC) experiments are used to investigate the loading of tetraethylammonium
bis(trifluoromethylsulfonyl)-imide ([NEt4][TFSI]) salt
in three different UiO-6x MOFs: UiO-66, UiO-67, and
a defective form of UiO-66 containing missing linkers. Good agreement
is achieved between the predicted loadings and those measured experimentally.
An analysis of the spatial distribution of salt ions within the MOF
pores as a function of loading shows that repulsive anion–anion
and cation–cation electrostatic interactions contribute to
the upper limit of loading. The surface area and void fraction of
the MOFs approach zero at the maximum loading, suggesting that the
ions completely fill the MOF pores. The absence of free volume at
the maximum loading is proposed as an explanation for the decrease
in conductivity observed in prior experiments; hence, conductivity
will be maximized for loadings that do not fill the MOF pores. Finally,
the surface area and pore volume of the empty MOFs are shown to be
good predictors of the maximum salt loading.