Surface-Clean Au₂₅ Nanoclusters in Modulated Microenvironment Enabled by Metal–Organic Frameworks for Enhanced Catalysis

Metal nanoclusters (NCs) with atomically precise structures have sparked interest in catalysis. Unfortunately, their high aggregation tendency and the spatial resistance of surface ligands pose significant challenges. Herein, Au₂₅ NCs are encapsulated into isoreticular metal-organic frameworks (MOFs), namely UiO-66-X (X = H, NH₂, OH, and NO₂), followed by the removal of surface ligands on Au₂₅ NCs. The resulting surface-clean Au₂₅ NCs, protected by the MOF spatial confinement, exhibit much superior activity and stability with respect to pristine Au₂₅ NCs in the oxidative esterification of furfural. Remarkably, experimental and theoretical results jointly demonstrate that diverse functional groups on UiO-66-X modulate the Au₂₅ electronic state, giving rise to the discriminated substrate adsorption energy of Au₂₅@UiO-66-X. As a result, the high electron density and suitable substrate adsorption ability dominate the activity trend: Au₂₅@UiO-66-NH₂ > Au₂₅@UiO-66-OH > Au₂₅@UiO-66 > Au₂₅@UiO-66-NO₂. This work develops a new strategy for the stabilization of surface-clean metal NCs in pore wall-engineered MOFs for enhanced catalysis.