Hydrogenation of levulinic acid to γ-valerolactone over Pd@UiO-66-NH2 with high metal dispersion and excellent reusability

Developing efficient heterogeneous catalysts with high metal dispersion and good reusability is crucial to the catalytic conversion of biomass. Herein, a metal-organic framework (MOF) UiO-66-NH2 was used as an ideal support to encapsulate Pd nanoparticles (Pd@UiO-66-NH2) via a simple double solvents method. This composite was characterized by XRD, N-2 adsorption-desorption, FT-IR, TEM, HAADF-STEM, EDS, TGA, XPS, and ICP-AES. Comprehensive characterization results reveal that metallic Pd particles are confined in the MOF cavities with good dispersion, which interact with the amine group in the framework. Pd@UiO-66-NH2 was applied to the hydrogenation of biomass-derived levulinic acid (LA) to gamma-valerolactone (GVL) for the first time. Owing to the high dispersion of Pd and the confinement effect of MOF cavities, the catalytic activity and reusability of Pd@UiO-66-NH2 were greatly enhanced in comparison with the reference catalysts. It gives the highest GVL productivity (1741.1 mmol g(-1)h(-1)) as compared to other reported Pd-based heterogeneous catalysts, and it also exhibits excellent reusability despite the loss of two organic linkers during the catalyst recycling experiment. The skeleton structure of the catalyst can be stabilized by its unique ability of structural rearrangement. A possible reaction mechanism was proposed, involving a hydrogenation-dehydration pathway.