Selective Hydrogenation of Trans -Cinnamaldehyde over SiO 2 -Supported Co–Ir Bimetallic Catalysts

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 (Co10.0Ir0.5/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.