The Role of a Three Dimensionally Ordered Defect Sublattice on the Acidity of a Sulfonated Metal–Organic Framework

Understanding the
role that crystal imperfections or defects play
on the physical properties of a solid material is important for any
application. In this report, the highly unique crystal structure of
the metal–organic framework (MOF) zirconium 2-sulfoterephthalate
is presented. This MOF contains a large number of partially occupied
ligand and metal cluster sites which directly affect the physical
properties of the material. The partially occupied ligand positions
give rise to a continuum of pore sizes within this highly porous MOF,
supported by N2 gas sorption and micropore analysis. Furthermore,
this MOF is lined with sulfonic acid groups, implying a high proton
concentration in the pore, but defective zirconium clusters are found
to be effective proton trapping sites, which was investigated by a
combination of AC impedance analysis to measure the proton conductivity
and DFT calculations to determine the solvation energies of the protons
in the pore. Based on the calculations, methods to control the pKa of the clusters and improve the conductivity
by saturating the zirconium clusters with strong acids were utilized,
and a 5-fold increase in proton conductivity was achieved using these
methods. High proton conductivity of 5.62 × 10–3 S cm–1 at 95% relative humidity and 65 °C
could be achieved, with little change down to 40% relative humidity
at room temperature.