Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Brønsted Acidity

The Lewis/Brønsted acidity and catalytic properties of UiO-66-type metal-organic frameworks are studied in the context of tunable acid catalysts based on the presence of linker defects that create coordinatively unsaturated Zr⁴⁺ centers. Fourier transform infrared spectroscopy of adsorbed CO and direct pH measurements are employed to characterize hydrated and dehydrated UiO-66 containing different number of Zr⁴⁺ sites associated with defects. These sites can strongly polarize coordinated water molecules, which induces Brønsted acidity in the hydrated material. Upon dehydration of the solid, the coordinated water molecules are removed, and the underlying coordinatively unsaturated Zr⁴⁺ cations become exposed and available as Lewis acid sites. Herein we show, for various acid-catalyzed reactions, how it is possible to shift from a Brønsted acid to a Lewis acid catalyst by simply controlling the hydration degree of the solid. This control adds a new dimension to the design and engineering of MOFs for catalytic applications.