Controlled framework nickel exsolution in metal–organic frameworks creates confined active sites for chemoselective citral hydrogenation

Selective hydrogenation of citral to citronellal over non-noble-metal catalysts remains challenging because highly active metallic Ni simultaneously promotes efficient substrate activation and undesired over‑hydrogenation of the desired product. Herein, we develop a controlled exsolution strategy in waste polyethylene terephthalate (PET)-derived nickel metal–organic frameworks (Ni-MOFs) to transform framework Ni into confined metallic active sites while preserving the porous framework architecture. During reductive treatment, framework Ni 2+ species undergo gradual node-to-metal evolution, generating highly dispersed Ni 0 sites confined within the partially retained MOF framework. More importantly, the degree of framework Ni exsolution governs the balance between citral activation and citronellal over‑hydrogenation, thereby establishing a distinct chemoselective window. Within the optimal exsolution regime, the framework-confined Ni 0 sites enable efficient H 2 activation and selective hydrogenation of the C C bond while suppressing the subsequent hydrogenation of citronellal. Consequently, the optimized catalyst achieves ∼99% citral conversion and 100% citronellal selectivity at 90 °C and 2 MPa H 2 , together with excellent catalytic stability and recyclability. Beyond the sustainable valorization of waste PET, this work establishes controlled framework exsolution as an effective strategy for engineering confined active sites and regulating chemoselectivity in non-noble-metal hydrogenation catalysts. Figure. Controlled reduction of PET-derived Ni-BDC generates framework-confined Ni 0 species within the partially retained MOF structure, enabling selective hydrogenation of citral to citronellal. Download: Download high-res image (139KB) Download: Download full-size image