FTIR traced modification of defect-engineered UiO-66 for enhanced accessibility of zirconium sites

This research focuses on identifying the accessibility of active sites within the defect-engineered UiO-66 framework. The task is particularly challenging due to reversible changes in the framework during dehydroxylation: the loss of μ₃-OH groups with simultaneous reduction of the Zr coordination number and the possible creation of Zr⁴⁺ Lewis acid sites in defect MOFs. We used in-situ FTIR and XANES analyses, as well as interaction with probe molecules, to monitor the changes in Zr coordination and the host-guest interaction. The defects were introduced using benzoic acid as a modulator, which coordinated to Zr⁴⁺ in defective pores. Our results showed that the UiO-66 sample synthesized with benzoic acid contained defects, but these were concealed under benzoate residues and thus inaccessible. Standard washing and heating did not remove benzoate anions. Dehydroxylation of the sample leads to the development of “hidden” Lewis acidity: some Zr⁴⁺ sites were not able to form complexes with the weak base CO, but they interact with the stronger bases acetonitrile. Additionally, in-situ XANES analysis revealed that the effect of acetonitrile adsorption is similar to that of water rehydration. Treatment of the sample with HCl and DMF led to the replacement of benzoates with formate ions, exposing the bare Zr⁴⁺ sites within the defective pores. These cationic sites acted as true Lewis acids and were able to coordinate both CO and acetonitrile. Our findings emphasize that the active sites in UiO-66 highly depend on synthesis conditions and post-synthetic treatments. Comprehensive site-specific methods are crucial for accurately predicting and identifying these active sites.