Tuning the hydrogenation activity of Pd NPs on Al-MIL-53 by linker modification

The hydrogenation activity of 3 wt.% Pd nanoparticles supported on various mono-group (H, OCH₃, NH₂, Cl, and NO₂) substituted Al-MIL-53 materials has been investigated. Substituents enhanced the dispersion of palladium nanoparticles on Al-MIL-53, leading to a narrow particle size distribution in the range of 2 to 4 nm. Pd nanoparticles on fresh catalysts were present as a mixture of Pd(ii) and Pd(0) with different ratios. These Pd species readily became metallic in a hydrogen flow even at room temperature. Their activities in hydrogenation of phenol and phenylacetylene are linked to the substituents on the aromatic ring of the framework. Catalysts with electron-donating groups (OCH₃ and NH₂) show much higher activity than those containing electron-withdrawing groups (Cl and NO₂). This behavior might be explained by the hydrogen dissociation abilities of metallic Pd nanoparticles affected by the organic linkers.