Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Brønsted Acidity: The Role of the Hemilabile Linker

The stability of
metal–organic frameworks (MOFs) typically
decreases with an increasing number of defects, limiting the number
of defects that can be created and limiting catalytic and other applications.
Herein, we use a hemilabile (Hl) linker to create up to a maximum
of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66
(Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate
(PSBA) as the hemilabile linker. The PSBA acts not only as a modulator
to create defects but also as a coligand that enhances the stability
of the resulting defective framework. Furthermore, upon a postsynthetic
treatment in H2SO4, the average number of defects
increases to the optimum of six missing BDC linkers per cluster (three
per formula unit), leaving the Zr-nodes on average sixfold coordinated.
Remarkably, the thermal stability of the materials further increases
upon this treatment. Periodic density functional theory calculations
confirm that the hemilabile ligands strengthen this highly defective
structure by several stabilizing interactions. Finally, the catalytic
activity of the obtained materials is evaluated in the acid-catalyzed
isomerization of α-pinene oxide. This reaction is particularly
sensitive to the Brønsted or Lewis acid sites in the catalyst.
In comparison to the pristine UiO-66, which mainly possesses Brønsted
acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66
structures exhibited a higher Lewis acidity and an enhanced activity
and selectivity. This is further explored by CD3CN spectroscopic
sorption experiments. We have shown that by tuning the number of defects
in UiO-66 using PSBA as the hemilabile linker, one can achieve highly
defective and stable MOFs and easily control the Brønsted to
Lewis acid ratio in the materials and thus their catalytic activity
and selectivity.