Ru Clusters Confined in Hydrogen-bonded Organic Frameworks for Homogeneous Catalytic Hydrogenation of N-heterocyclic Compounds with Heterogeneous Recyclability

Abstract Combining the advantages of homogeneous catalysis and the recyclability of heterogeneous catalysts is highly desired for sustainable catalysis but remains a challenging goal. In the current work, hydrogen-bonded organic frameworks (HOFs) that consist of cage molecules with amino groups were prepared. The HOFs with heteroatom-containing porous cavities and stable backbone confine ultrafine Ru clusters with diameter of about 0.47 nm and high Ru loading of 30 wt%. The obtained Ru@HOF catalyst can well disperse in water in the form of discrete cage-confined Ru clusters similar to a homogeneous catalyst, and can be recycled easily through simple precipitation by the addition of acetone due to the formation of hydrogen bonds between discrete cages. The Ru@HOF was used as a homogenous catalyst for efficiently selective catalytic hydrogenation of N-heteroarene compounds, such as quinolines and indoles in aqueous solution. Especially quinoline compounds can be fully hydrogenated to decahydroquinoline with Ru@HOF catalyst, which is not easy to be implemented with other Ru nanoparticle-based catalysts. The high catalytic performance of Ru@HOF is attributed to the homogeneously dispersed and accessible active sites of the ultrafine Ru clusters. This study provides a novel strategy for fabricating ultrafine noble metal clusters-based catalysts with homogeneous catalytic performance and heterogeneous recyclability.