KINETIC-ANALYSIS OF CINNAMALDEHYDE HYDROGENATION OVER ALUMINA-SUPPORTED RUTHENIUM CATALYSTS

The kinetics of liquid-phase hydrogenation of cinnamaldehyde (CALD) we re studied over Ru/Al2O3 catalysts of different metal dispersion in a slurry reactor, using ethanol as solvent, in the temperature range bet ween 283 and 333 K, The reaction pathway has been described by using L angmuir-Hinshelwood type rate expressions. A two-site model was used t o describe the kinetic experiments. The adsorption and hydrogenation o f the C-C and C=O groups are suggested to occur on different sites wit h competitive adsorption of substrate and products. The kinetic parame ters of the individual reaction steps have been determined by a nonlin ear regression of the experimental data. The specific rate constants o f the C=C double-bond hydrogenation are smaller at the lowest Ru dispe rsion as a consequence of the weaker adsorption of the olefinic bond o n the flat surface of the larger Ru crystallites. The specific activit y 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 subst rate is chemisorbed with a different geometry. Hydrogenation of CALD o ver a Ru-Sn/Al2O3 catalyst has also been investigated. The variation o f the kinetic parameters on addition of tin is discussed. The high sel ectivity of the Sn-doped ruthenium catalysts has been attributed to an increase of the reactivity of the conjugated C=O group. The reactivit y of the conjugated C=C double bond is not influenced by the presence of tin.