Kinetic analysis of cinnamaldehyde hydrogenation over alumina-supported ruthenium catalysts

The kinetics of liquid-phase hydrogenation of cinnamaldehyde (CALD) were studied over Ru/Al2O3 catalysts of different metal dispersion in a slurry reactor, using ethanol as solvent, in the temperature range between 283 and 333 K, The reaction pathway has been described by using Langmuir-Hinshelwood type rate expressions. A two-site model was used to describe the kinetic experiments. The adsorption and hydrogenation of the C-C and C=O groups are suggested to occur on different sites with competitive adsorption of substrate and products. The kinetic parameters of the individual reaction steps have been determined by a nonlinear regression of the experimental data. The specific rate constants of the C=C double-bond hydrogenation are smaller at the lowest Ru dispersion as a consequence of the weaker adsorption of the olefinic bond on the flat surface of the larger Ru crystallites. The specific activity of C=O bond hydrogenation increases as the dispersion decreases. The adsorption strength of the carbonyl group remains constant regardless of the Ru particle size. The results have been explained by assuming that on the ruthenium metal particles having different size, the substrate is chemisorbed with a different geometry. Hydrogenation of CALD over a Ru-Sn/Al2O3 catalyst has also been investigated. The variation of the kinetic parameters on addition of tin is discussed. The high selectivity of the Sn-doped ruthenium catalysts has been attributed to an increase of the reactivity of the conjugated C=O group. The reactivity of the conjugated C=C double bond is not influenced by the presence of tin.