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

Abstract

Supported ruthenium hydroxide catalysts (Ru(OH)(x)/support) were prepared with three different TiO(2) supports (anatase TiO(2) (TiO(2)(A), BET surface area: 316 m(2) g(-1)), anatase TiO(2) (TiO(2)(B), 73 m(2) g(-1)), and rutile TiO(2) (TiO(2)(C), 3.2 m(2) g(-1))), as well as an Al(2)O(3) support (160 m(2) 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)/TiO(2)(C)<Ru(OH)(x)/Al(2)O(3)<Ru(OH)(x)/TiO(2)(B)<Ru(OH)(x)/TiO(2)(A). These supported ruthenium hydroxide catalysts, especially Ru(OH)(x)/TiO(2)(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)/TiO(2)(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)/Al(2)O(3). 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.

Nanostructured RuO2 on MWCNTs: Efficient catalyst for transfer hydrogenation of carbonyl compounds and aerial oxidation of alcohols

“…The better yield is due to the higher surface area of RuO 2 /MWCNT (189.3 m 2 g -1 ) than the GNPs-RuO 2 NRs (65.17 m 2 g -1 ). Similarly, 2-octanone gave 2-octanol in an excellent yield of 98% (100% selectivity) just after 1 h (Table 3, entry 11) whereas Ru(OH) x /TiO 2 system exhibited 92% yield even after 2 h [12]. In contrast to Fe@SiO 2 Ru nanocatalyst [44], alicyclic alcohols with different ring size were produced from the alicyclic ketones with 100% selectivity, but they took longer reaction times.…”

“…Particularly, in the reduction of 2-methylacetopheone to 1-(o-tolyl)ethanol (Table 3, entry 5), the present catalytic system gave a better yield of 95% (100% selectivity) with an excellent TON (123) and TOF (148 h -1 ) values after only 50 min. Whereas Ru(OH) x /TiO 2 system gave 82% (with TON/TOF of 82/27 h -1 ) of the same product only after 3 h even at 90°C under Ar atmosphere [12]. Substituted benzophenone specifically 4-methoxybenzophenone was reduced to 4-methoxydiphenylmethanol (73%) after 45 min (Table 3, entry 7); moderate yield of the product may be due to the presence of the bulky substituent [43].…”

“…In contrast to Fe@SiO 2 Ru nanocatalyst [44], alicyclic alcohols with different ring size were produced from the alicyclic ketones with 100% selectivity, but they took longer reaction times. As the ring size increases, decreased reactivity was observed (Table 3, entries [12][13][14]. The reactivity order is cyclohexanone (92%) > cycloheptanone (88%) > cyclododecanone (77%).…”

“…Yamaguchi et al, [12] prepared highly dispersed Ru(OH) x /TiO 2 composite and used as a catalyst for the liquid-phase hydrogen transfer reactions. They found that the Ru(OH) x /TiO 2 catalyst is highly effective and selective.…”

Supported Metal Hydroxides as Efficient Heterogeneous Catalysts for Green Functional Group Transformations

“…Indeed, racemization of various optically active secondary alcohols efficiently proceeded in the presence of R u ( O H ) x / A l2O3 u n d e r " a n a e r o b i c " c o n d i t i o n s (Fig. 6) 32), 33) . For the reaction of the hydride species with O2 (step 3 in Fig.…”