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

Abstract

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.

Preparation of Highly Active Monometallic Rhenium Catalysts for Selective Synthesis of 1,4‐Butanediol from 1,4‐Anhydroerythritol

“…According to the report on the structure of Re/SiO 2 and Re‐Pd/SiO 2 , the surfaces of Re metal particles interact and are modified with cationic Re species. Moreover, noble metal particles modified with cationic Re species have been reported to be effective catalysts for the selective hydrogenation of unsaturated aldehydes to unsaturated alcohols, indicating that the presence of cationic Re species suppressed the hydrogenation of C=C bonds . In the present case, Re metal surfaces modified with Re cations can be effective for H 2 activation without the ability to hydrogenate C=C bonds.…”

“…Moreover,n oble metal particles modified with cationic Re species have been reportedt o be effective catalysts for the selective hydrogenation of unsaturateda ldehydes to unsaturated alcohols, indicatingt hat the presenceo fc ationicR es pecies suppressed the hydrogenation of C=Cb onds. [43][44][45] In the present case, Re metal surfaces modifiedw ith Re cations can be effective for H 2 activation withoutthe ability to hydrogenate C=Cbonds. The XRD patternso ft he used catalysts only showedt he peaks of CeO 2 and/orc arbon ( Figure S2 in the Supporting Information).I no ur previousr eport, [41] some of the Re speciesi n 14 wt %R eO x /SiO 2 were reduced to the metallic state after reaction and showedb road peaks in the range .N os ignal of Re species was detectedh ere, indicating that the Re species including metallico nes were highly dispersed and the aggregation of Re speciesd uring the reactiono f1 ,4-AHERY to 1,4-BuD was not significant.…”

The Catalytic Reduction of Carboxylic Acid Derivatives and CO2 by Metal Nanoparticles on Lewis‐Acidic Supports

“…A plausible mechanism could start with the dissociation of molecular H 2 on the Pt surface, followed by a reaction with the amide, in which the C=O bond is weakened by the Lewis‐acidic site of the support, resulting in the formation of the corresponding amine. These observations clearly indicate that in‐situ FT‐IR spectroscopy may serve as a powerful tool to rationalize and predict the catalytic performance in these reactions …”

Copper Supported on Hybrid C@SiO2 Hollow Submicron Spheres as Active Ethanol Dehydrogenation Catalyst

“…Furthermore, CH 3 CHO‐TPD was employed to understand the surface chemical properties of C@SiO 2 , as previous report revealed that the Si–OH groups on SiO 2 can adsorb and activate the carbonyl groups (‐CHO) of aldehydes . The desorption signals of CH 3 CHO at m/z=29 was compared in Figure .…”