Metallic Ruthenium nanoparticles intercalated in hectorite for highly selective catalytic hydrogenations

The design of nanocomposites consisting of functional metals and proper matrices is a very active field of research for the development of recyclable catalysts. Highly active metallic nanoparticles must be stabilized by a suitable support in order to prevent aggregation to bulk metal. Hectorite, a representative smectite clay featured by its unique swelling properties and flexible intercalation capacity, provides an ideal platform for immobilizing metal nanoparticles. By intercalating organometallic benzene ruthenium complexes or Werner-type ruthenium(III) ions from RuCl3 ∙ xH2O as precursor, ruthenium nano¬particles intercalated in hectorite are successfully obtained via a reduction process with molecular hydrogen approach or sodium borohydride. Depending on the properties of solvents and the reduction conditions, a variety of ruthenium nanoparticles with different morphology are formed. In the catalytic hydrogenation of quinoline, hectorite-intercalated ruthenium nanoparticles show excellent reactivity and selectivity to the specific product. By using water or cyclohexane as reaction medium under a certain pressure of molecular H2, 1,2,3,4-tetrahydroquinoline and decahydroquinoline were exclusively obtained, respectively. Furthermore, by using sodium borohydride as reducing agent, the catalytic hydrogenation of quinoline proceeds in water under atmospheric pressure with the conversion and selectivity superior to 99%. Isotope labeling experiments combined with semi-empirical calculations reveal that both the sodium borohydride and water participate in the hydrogenation process by means of hydride transfer and proton transfer, respectively. Furthermore, hectorite-intercalated ruthenium nanoparticles can also be used for the hydrogenation of aromatic amino acids in aqueous media. By screening of the influencing factors, the pH of the solution was found to be critical for the complete conversion of aromatic amino acids. Critically, during the hydrogenation process, the chirality of the substrates remains unchanged.