Kinetic study of liquid-phase hydrogenation of citral over Ir/TiO2 catalysts Original Research Article

The kinetics of liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) on Ir/TiO2 catalysts were studied in the temperature range of 303–363 K and a pressure range of 4.13–10.3 bar H2. Geraniol and nerol were the only reaction products, generated through the hydrogenation of the carbonyl group of citral. Negative near second-order dependence on citral concentration and near first-order dependence on hydrogen pressure were observed for the initial rate of citral hydrogenation over the Ir/TiO2-LTR and -HTR (Tred=473 and 773 K) catalysts. Furthermore, an increase in the catalytic activity as the reaction temperature, pressure and weight of catalysts was observed. Conversely, an increase in the citral concentration produces a drop in the reaction rate. Conventional Arrhenius behaviour was exhibited by all catalysts, Ir/TiO2-LTR and -HTR showed activation energies of 15.9 and 4.6 kJ/mol, respectively. An important enhancement in the catalytic activity was observed in the HTR catalyst compared with the LTR counterpart, which is attributed to a strong metal support interaction effect. The reaction kinetics with Ir/TiO2-LTR and -HTR catalysts under differential regimes was described by a conventional Langmuir–Hinshelwood treatment. It was found that under the conditions studied, the hydrogen adsorption was the rate-determining step.