Uncoordinated carbonyl groups of MOFs as anchoring sites for the preparation of highly active Pd nano-catalysts

A novel Tm-containing metal-organic framework, [Tm(mu(2)-L)(mu(4)-L)(0.5)(H2O)(2)]center dot 3H(2)O (Tm-MOF), was prepared from the hydrothermal reaction of Tm(NO3)(3) with 5-methylpyrazine-2-carboxylic acid in water. The 2,5-pyrazinedicarboxylate ligand (L) in Tm-MOF was formed in situ from 5-methylpyrazine-2-carboxylic acid under the above conditions. Tm-MOF is a three-dimensional (3D) coordination network with 1D open channels running across the coordination layers. Each L in bridging mode mu(2) contains two uncoordinated carbonyl groups which point to the channels. By using the predesigned Tm-MOF as a host, we have successfully incorporated small palladium nanoparticles into the channels by a simple impregnation method. The Pd/Tm-MOF catalyst exhibited a high catalytic activity in the hydrogenation of styrene. Moreover, the catalyst could be reused at least three times without loss of any activity. In contrast, a low dispersion of metal and a low activity in styrene hydrogenation were observed over the Pd catalyst supported on ZIF-8, which has a similar surface area as the Tm-MOF, but no uncoordinated carbonyl groups. The results suggest that the uncoordinated carbonyl groups in the Tm-MOF may play an important role in facilitating the dispersion of Pd nanoparticles through an intermolecular interaction with the Pd cations in the course of immobilization.