Highly Selective Hydrogenation of Unsaturated Aldehydes in Aqueous Phase
Shijie Zhou, Yusen Yang, Tianyao Shen, Pan Yin, Lei Wang, Zhen Ren, Lirong Zheng, Bin Wang, Hong Yan, Min Wei
Abstract
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.