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

Abstract

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 Ni2+ species undergo gradual node-to-metal evolution, generating highly dispersed Ni0 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 Ni0 sites enable efficient H2 activation and selective hydrogenation of the CC 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 H2, 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.