Selective Hydrogenation of trans-Cinnamaldehyde over SiO₂-Supported Co–Ir Bimetallic Catalysts

Abstract

Selective hydrogenation of trans-cinnamaldehyde was studied on SiO₂-supported Co–Ir bimetallic catalysts. Addition of Ir to Co/SiO₂ increased the hydrogenation selectivity and activity of cinnamaldehyde to the corresponding cinnamyl alcohol (UOL). A selectivity as higher as 93% to UOL at ambient temperature under H₂ pressure of 2.0 MPa was obtained over catalyst with loadings of 10 wt% Co and 0.5 wt% Ir (Co_10.0Ir_0.5/SiO₂). The XRD, Raman and TPR results showed that the higher dispersed Co₃O₄ particles were formed on SiO₂ due to the addition of Ir, which increased the reducibility of Co₃O₄ to Co⁰. The reduction of oxidized Co–Ir/SiO₂ samples occurred at the temperatures with about 200 K lower than that of the one without Ir species as evidenced by the observations of TPR and in-situ Raman characterizations. The XPS results indicated that the large parts of Co₃O₄ in the sample of Co–Ir/SiO₂ were reduced to Co⁰, but only small parts of that were reduced to Co⁰ in the sample of Co/SiO₂ under flowing 5%H₂/Ar at 673 K. The CO chemisorptions revealed that the irreversible uptakes of CO on the reduced Co–Ir/SiO₂ sample was much higher than those on the reduced Co/SiO₂ and Ir/SiO₂, and also higher than the combination of that on the reduced Co/SiO₂ and Ir/SiO₂, respectively. The experimental data suggested that the presence of Ir played a key role in the reduction of Co₃O₄ to Co⁰ through a strong interaction between them and that the amount of Co⁰ at the catalyst surfaces was correlated to the activity and more importantly to the UOL selectivity.