Encapsulation of ultrafine Pd nanoparticles within the shallow layers of UiO-67 for highly efficient hydrogenation reactions

Metal-organic frameworks (MOFs) have been used to encapsulate active metal nanoparticles (MNPs) to fabricate MNPs@MOFs composites with high catalytic efficiencies. However, the diffusion of reactants and the accessibility of MNPs located in the center of MOFs may be hindered due to the inherent microporous structures of MOFs, which would affect the catalytic activities of MNPs. Herein, we report a solvent assisted ligand exchange-hydrogen reduction (SALE-HR) strategy to selectively encapsulate ultrafine MNPs (Pd or Pt) within the shallow layers of a MOF, i.e., UiO-67. The particle sizes of the encapsulated MNPs and the thickness of the MNPs-embedded layers can be adjusted easily by controlling the SALE conditions (e.g. time and temperature). Crucially, the LE-Pd@UiO-80-0.5 composite with the thinnest Pd-embedded layers displays remarkable catalytic efficiency with a high turnover frequency (TOF) value of 600 h−1 towards hydrogenation of nitrobenzene under 1 atm H2 at room temperature. The results indicate that the catalytic efficiency and the utilization of MNPs can be enhanced by compactly encapsulating MNPs within the shallow layers of MOFs as close to their outer surfaces as possible, owing to the short masstransfer distance and enhanced accessibility of overall MNPs.