The defect-modulated UiO-66(Ce) MOFs for enhancing photocatalytic selective organic oxidations

Defect engineering in metal organic frameworks (MOFs) has captured significant attention in the field of photocatalysis. A series of UiO-66(Ce) (UiO = University of Oslo) MOFs with different contents of missing-linker defects have been developed for the photocatalytic selective oxidation of benzylamine (BA) and thioanisole (TA) under visible light. The introduction of missing-linker defects promotes the formation of unsaturated Ce sites with a high Ce³⁺ content. It also generates a high concentration of oxygen vacancies. In situ Fourier transform infrared spectroscopy (FTIR) results revealed that BA and TA molecules were activated on coordinatively unsaturated Ce sites via the H–N···Ce and the C–S···Ce interactions, respectively. Simulated in situ electron paramagnetic resonance (EPR) data indicate that O₂ activation and reduction occur at coordinatively unsaturated Ce³⁺ sites to form ·O₂^–. This is accelerated by the Ce³⁺/Ce⁴⁺ redox cycle associated with the photogenerated electrons. The corresponding photogenerated holes are involved in the deprotonation of the activated BA and TA. The most active sample exhibits 98.4% and 95.5% conversion rates for BA and TA oxidation. Mechanisms for the molecular activation are proposed at the molecular level.