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-W⁵⁺ (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 H₂ dissociation (secondary). In the former case, W⁵⁺-O_v site accelerates the activation adsorption of H₂O, while Ir⁰ site facilitates the H-H bond cleavage of H₂ and Ir^δ+ promotes the CAL adsorption. H₂O 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 H₂ 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.