Solvent-Free Synthesis and Applications of Highly Dispersed Ruthenium Catalysts Prepared by Solid-Phase Mixing

Solvent-free catalyst synthesis provides significant environmental benefits and simplifies the synthesis process. In this study, we utilized the interaction between metal oxide and support to prepare highly dispersed and active Ru catalysts using a simple, solvent-free method. Bulk ruthenium dioxide was physically mixed with six kinds of solid oxide supports (Al₂O₃, SiO₂, TiO₂, tetragonal ZrO₂, monoclinic ZrO₂, and CeO₂), followed by thermal treatment in air at 773-1273 K. Ru K-edge X-ray absorption fine structure analysis of the treated samples showed that atomically dispersed Ru species were obtained at 973-1073 K when monoclinic ZrO₂ and CeO₂ were used as supports. In contrast, RuO₂ remained aggregated when Al₂O₃, SiO₂, and tetragonal ZrO₂ were used, and a Ru-TiO₂ solid solution was formed at 1173-1273 K when TiO₂ was employed. The catalysts were tested in benzyl alcohol oxidation, levulinic acid hydrodeoxygenation, and ammonia decomposition. Among the different supports examined, Ru supported on monoclinic ZrO₂ and CeO₂ demonstrated remarkably high activity in the reactions, leading to high yields of benzaldehyde, γ-valerolactone, and hydrogen; in contrast, Ru on Al₂O₃, SiO₂, TiO₂, and tetragonal ZrO₂ were either less active or completely inactive in the reactions.