Interplay of Product Selectivity and Isolated Ru-Oxo Sites in the Solvent-Free Oxidization of Benzyl Alcohol

Abstract

Understanding the structure–reactivity relationships of heterogeneous single-atom catalysts (SACs) is indispensable for advancing their potential in industrial applications. In this study, we assessed the catalytic performance of isolated Ru-oxo sites and RuO_x clusters supported on high-specific-surface-area TiO₂ (≥242 m² g^–1) in the solvent-free aerobic oxidation of benzyl alcohol, a key intermediate step in a wide range of chemical transformations. RuO_x/TiO₂ catalysts were characterized by a suite of techniques, including X-ray diffraction, H₂-temperature-programmed reduction, electron microscopy, diffuse reflectance UV–vis, and X-ray photoelectron spectroscopy, together with X-ray absorption spectroscopy at the near (XANES) and extended (EXAFS) Ru K-edge. The product selectivity was found to decisively depend on the nature of Ru species. While the catalysts rich in the surface-isolated Ru-oxo sites showed the highest selectivity toward benzaldehyde (83.6%), catalysts rich in high RuO_x nanostructures (dimers, subnanometer clusters, or particles) promoted lower benzaldehyde selectivity (25.6%) and a concomitant formation of other side products (dibenzyl ether and benzoic acid). Differences in the catalytic behavior could be correlated to the extent of interaction between TiO₂ and Ru species, which depends on the dispersion of Ru on the oxide surface. Cyclic stability measurements indicated stable catalytic performance after 4 consecutive runs, with postreaction structural characterizations indicating that RuO_x/TiO₂ catalysts do not suffer any measurable metal leaching or significant structural modulation during reaction, which together makes them excellent candidates for oxidation catalysis at liquid/solid interface.