Sandwiched Ruthenium/Carbon Nanostructures for Highly Active Heterogeneous Hydrogenation

Since the discovery of ruthenium as a catalyst for hydrogenation reactions, Ru catalysts have been widely used in the chemical, petrochemical, food, and pharmaceutical industries, and in energy-conversion technologies. The scope of homogeneous Ru catalysts has been well-illustrated recently. However, heterogeneous catalysts can be preferable from both industrial and environmental perspectives. Recently, Miao et al. described a method for preparing Ru catalysts by immobilizing Ru nanoparticles on montmorillonite (MMT) clay with the assistance of ionic liquids. While the Ru/ MMT catalyst exhibited excellent activity for the hydrogenation of benzene, the Ru nanoparticles were found to aggregate to form larger particles after several reaction runs. Sun et al. reported immobilization of Ru colloidal particles on carbon nanotubes via supercritical water. However, the use of carbon nanotubes as a catalyst support can be quite limited because of their low specific surface area. It should be mentioned that in both papers, the authors observed that the immobilized Ru catalysts tended to be easily oxidized upon exposure to air, thus, resulting in loss in catalytic activity. Additionally, Ru nanoparticles highly dispersed on the pore surfaces of mesoporous silicas, supported on alkali-exchanged zeolite Beta, on the surfaces of activated carbons, alumina, and titanium oxides prepared by using conventional methods were reported as well. However, these methods suffer from a number of problems, such as particle aggregation and catalyst leaching. Thus, searching for an alternative approach to preparing Ru catalysts with high stability and catalytic activity is receiving rapidly growing attention. The hard-template strategy for synthesizing templated porous carbon (TPC) materials has opened up opportunities for exploring novel heterogeneous catalysts. Lu et al. fabricated a Pd/Co–TPC catalyst with Co nanoparticles immobilized on the external surface of TPC to facilitate magnetic recovery in a reaction system. Chai et al. described a Pt–TPC electrocatalyst with Pt nanoparticles embedded in the carbon wall of TPC, which displayed good electrochemical catalytic performance. The encapsulation of Fe, Co, and Ni metal nanoparticles in TPCs as magnetically separable adsorbents was reported. It is to be noted that such metal particles embedded in TPCs are extremely stable against leaching while the pores of the carbon are highly accessible. More interestingly, the template method also offers chances for developing novel catalyst systems, which are rarely obtained with traditional preparation methods. For example, Lee et al. demonstrated the fabrication of a magnetically switchable bio-electrocatalytic system with immobilized enzymes for the oxidation of glucose based on templated mesocellular carbon. Ikeda et al. described novel Pt–TPC catalyst systems with Pt nanoparticles encapsulated in a hollow porous carbon shell as a highly active heterogeneous hydrogenation catalyst. The development of these novel heterogeneous catalyst systems is very