Highly Selective Hydrogenation of Unsaturated Aldehydes in Aqueous Phase

Chemoselective hydrogenation of carbonyl in unsaturated aldehydes is a significant process in the chemical industry, in which the development of aqueous-phase reaction systems as a substitution to organic ones is challenging. Herein, we report Ir atomic cluster catalysts anchored onto WO 3– x nanorods via a reduction treatment at various temperatures (denoted as Ir/WO x -T, T = 200, 300, 400, and 500 °C), which accelerates the chemoselective hydrogenation of carbonyl groups in aqueous solutions. The optimal catalyst Ir/WO x -300 exhibits exceptional activity (TOF value: 1313.7 min –1 ) and chemoselectivity toward cinnamaldehyde (CAL) hydrogenation to cinnamyl alcohol (COL) (yield: ∼98.0%) in water medium, which is, to the best of our knowledge, the highest level compared with previously reported heterogeneous catalysts in liquid-phase reaction. Ac-HAADF-STEM, XAFS, and XPS verify the formation of interface structure (Ir δ + –O v –W 5+ (0 ≤ δ ≤ 4); O v denotes oxygen vacancy) induced by metal–support interaction and the largest concentration of interfacial Ir (Ir δ + ) in Ir/WO x -300. In situ studies (Raman, FT-IR), isotopic labeling measurements combined with DFT calculations substantiate that the hydrogenation of the C=O group consists of two pathways: water-mediated hydrogenation (predominant) and direct hydrogenation via H 2 dissociation (secondary). In the former case, W 5+ –O v site accelerates the activation adsorption of H 2 O, while Ir 0 site facilitates the H–H bond cleavage of H 2 and Ir δ + promotes the CAL adsorption. H 2 O molecule, as the source of hydrogen species, participates directly in the hydrogenation of the carbonyl group through a hydrogen-bonded network, with a largely reduced energy barrier relative to the H 2 dissociation path. This work demonstrates a green catalytic route that breaks the activity-selectivity trade-off toward the selective hydrogenation of unsaturated aldehydes, which shows great potential in heterogeneous catalysis.