Photo-Induced Construction and Recovery of Cu⁺ Sites in Metal–Organic Frameworks

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu⁺ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu²⁺ doesn't have such ability. Meanwhile, Cu⁺ sites are easily oxidized to Cu²⁺, leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu⁺ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu⁺ sites in metal–organic frameworks (MOFs) and recover Cu⁺ after oxidation. The Cu²⁺ precursor was loaded on NH₂-MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti³⁺ from Ti⁴⁺ in framework, which reduces the supported Cu²⁺ in the absence of any additional reducing agent, thus simplifying the process for Cu⁺ generation significantly. Due to π-complexation interaction, the presence of Cu⁺ results in remarkably enhanced CO capture capacity (1.16 mmol g⁻¹) compared to NH₂-MIL-125 (0.49 mmol g⁻¹). More importantly, Cu⁺ can be recovered conveniently via re-irradiation when it is oxidized to Cu²⁺, and the oxidation-recovery process is reversible.