Highly Dispersed Ruthenium Hydroxide Supported on Titanium Oxide Effective for Liquid‐Phase Hydrogen‐Transfer Reactions

Supported ruthenium hydroxide catalysts (Ru(OH)_x/support) were prepared with three different TiO₂ supports (anatase TiO₂ (TiO₂(A), BET surface area: 316 m² g^?1), anatase TiO₂ (TiO₂(B), 73 m² g^?1), and rutile TiO₂ (TiO₂(C), 3.2 m² g^?1)), as well as an Al₂O₃ support (160 m² g^?1). Characterizations with X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and X‐ray absorption fine structure (XAFS) showed the presence of monomeric ruthenium(III) hydroxide and polymeric ruthenium(III) hydroxide species. Judging from the coordination numbers of the nearest‐neighbor Ru atoms and the intensities of the ESR signals, the amount of monomeric hydroxide species increased in the order of Ru(OH)_x<Ru(OH)_x/TiO₂(C)<Ru(OH)_x/Al₂O₃<Ru(OH)_x/TiO₂(B)<Ru(OH)_x/TiO₂(A). These supported ruthenium hydroxide catalysts, especially Ru(OH)_x/TiO₂(A), showed high catalytic activities and selectivities for liquid‐phase hydrogen‐transfer reactions, such as racemization of chiral secondary alcohols and the reduction of carbonyl compounds and allylic alcohols. The catalytic activities of Ru(OH)_x/TiO₂(A) for these hydrogen‐transfer reactions were at least one order of magnitude higher than those of previously reported heterogeneous catalysts, such as Ru(OH)_x/Al₂O₃. These catalyses were truly heterogeneous, and the catalysts recovered after the reactions could be reused several times without loss of catalytic performance. The reaction rates monotonically increased with an increase in the amount of monomeric ruthenium hydroxide species, which suggests that the monomeric species are effective for these hydrogen‐transfer reactions.