Probing Acidic and Defective Sites in Sulfated UiO-66 and ZrO₂ via Adsorptive FTIR Spectroscopy

Sulfation is a common strategy to enhance the acidity and modify the adsorption properties of metal-organic frameworks (MOFs), yet its impact on the coordination and accessibility of active sites remains unclear. In this study, we investigate two structurally related systems-sulfated UiO-66 (UiO-66-SO₄) and sulfated tetragonal zirconia (S-ZrO₂)-by FTIR spectroscopy with probe molecules. Isotope exchange experiments on S-ZrO₂ reveal that dehydration above 250 °C induces tridentate SO₄ coordination, while hydration leads to a reversible transition to a bidentate coordination mode. In UiO-66-SO₄, sulfates are coordinated in a bidentate fashion to Zr₆O₆ clusters, significantly affecting the accessibility of Zr sites in defective pores. This coordination prevents CO adsorption but allows acetonitrile adsorption even after room temperature activation. Unlike S-ZrO₂, due to its lower thermal stability, UiO-66-SO₄ cannot be evacuated at high temperatures and dehydration at 250 °C does not induce tridentate coordination. The presence of H-bonded hydroxyls in UiO-66-SO₄ after activation at 250 °C supports this coordination model, indicating the formation of OH-coordinated Zr sites that are inaccessible to CO but interact with stronger bases like acetonitrile. Overall, this study provides new insights into the coordination chemistry of sulfated UiO-66 and highlights that sulfation can tune acidity and adsorption in MOFs for potential catalytic and adsorption applications.