Ir-ReOx/SiO2 acted as a highly active and selective heterogeneous catalyst for the hydrogenation of unsaturated aldehydes to unsaturated alcohols in water at low H2 pressure (0.8 MPa) and low temperature (303 K). The catalysis is derived from the synergy between Ir metal and ReOx.

The scope of metal oxide modified noble metal (M+M′O x ) catalysts was scrutinized in the hydrogenation of crotonaldehyde to crotyl alcohol as a model reaction under mild reaction conditions (303 K, 0.8 MPa, water solvent), demonstrating that MoO x , WO x , NbO x , FeO x and ReO x are effective metal oxides for Ir/SiO2 to enhance both the activity and selectivity, although the optimized (metal oxide)/(Ir metal) molar ratio depends on the metal oxide. MoO x modified Ir/SiO2 catalyst (Ir-MoO x /SiO2 (Mo/Ir = 1)) was the most efficient, providing a high yield of crotyl alcohol (90%) and a high TOF (217 h–1). The catalytic activity under such mild reaction conditions is the highest among the reported heterogeneous catalysts. These results showed that modification of active metals with an appropriate amount of metal oxides is an effective method for the development of efficient catalysts for selective hydrogenations. The reaction mechanism over the metal oxide modified Ir catalysts was investigated using Ir-ReO x /SiO2 (Re/Ir = 1) as a model catalyst by means of FTIR studies on H2/D2 adsorption, crotonaldehyde adsorption, and temperature-programmed desorption of crotonaldehyde, and kinetic studies on effects of H2 pressure and crotonaldehyde concentration, isotopic effect of hydrogen (V H2/V D2), and comparison of reactivities between the aldehyde group and olefin group using various substrates. The reaction proceeds via four steps: (i) adsorption of crotonaldehyde on ReO x species, (ii) generation of hydride species from H2 on Ir metal species, (iii) hydride attack to the crotonaldehyde adspecies, and (iv) desorption of the produced crotyl alcohol, and the third step is the rate-determining step. Ir metal plays a role in the generation of hydride (H–) species from H2, leading to the high selectivity to crotyl alcohol, and ReO x plays a role in promotion of crotonaldehyde adsorption, leading to the proximity of crotonaldehyde to the active site and activation of the aldehyde group, which results in high activity and further improvement in the selectivity.

NbO x -modified Ir/SiO 2 catalyst (Ir-NbO x /SiO 2 ) showed high activity and selectivity for gas-phase hydrogenation of crotonaldehyde, and Ir-NbO x /SiO 2 with Nb/Ir molar ratio of 0.5 (Ir-NbO x /SiO 2 (Nb/Ir = 0.5)) provided high crotyl alcohol yield of 87%. This is the highest crotyl alcohol yield among those in gas-phase over reported heterogeneous catalysts. The activities (TOF per surface Ir atom) at low conversion level (40%, W/F = 0.37 g cat •h•mol −1 ) and high conversion level (97%, W/F = 2.2 g cat •h•mol −1 ) were calculated to be 0.21 and 0.08 s −1 , which are more than 3 times higher than those over reported heterogeneous catalysts. High activity and high yield are derived from the combination of Ir metal with NbO x species. Based on the spectroscopic and kinetic studies, the high activity and high selectivity are attributed to strong adsorption of crotonaldehyde on NbO x species by the high Lewis acidity, and suppression of over-hydrogenation of the produced crotyl alcohol at high conversion level. It is suggested that the formation of hydride species at the interface between Ir metal and NbO x species can promote the selective hydrogenation considering the reaction order with respect to hydrogen pressure. High yield and high activity of Ir-NbO x /SiO 2 can contribute to energy saving, cost reduction, and waste reduction, which are directly connected to realization of sustainability.

Rapid Synthesis of Unsaturated Alcohols under Mild Conditions by Highly Selective Hydrogenation. -Ir/ReOx supported on silica shows high activity and selectivity in the hydrogenation of unsaturated aldehydes (I) to the corresponding unsaturated alcohols (II) at a low H2 pressure and a low temperature. -(TAMURA, M.; TOKONAMI, K.; NAKAGAWA, Y.; TOMISHIGE*, K.; Chem. Commun. (Cambridge) 49 (2013) 63, 7034-7036, http://dx.doi.org/10.1039/c3cc41526k ; Grad. Sch. Eng., Tohoku Univ., Aoba, Sendai 981, Japan; Eng.) -S. Karsten 49-049