Selective Hydrogenation of Crotonaldehyde to Crotyl Alcohol over Metal Oxide Modified Ir Catalysts and Mechanistic Insight

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.