Catalysis Today 66 (2001) 403–410 Kinetic and mass transfer in the hydrogenation of polyunsaturated organic compounds in the presence of supported Pd catalysts M. Di Serio, V. Balato, A. Dimiccoli, L. Maffucci, P. Iengo, E. Santacesaria ∗ Dipartimento di Chimica, Università degli Studi di Napoli “Federico II”, Via Cinthia, 80126 Naples, Italy Abstract The hydrogenation of 1,5-cyclo-octadiene has been studied on well-defined Pd catalysts. The reaction is affected by internal diffusion of hydrogen into the catalyst particles. Despite this limitation, the reaction orders of the reactants have been determined. All the performed kinetic runs have been simulated and the kinetic and mass-transfer parameters giving the best fit of experimental data have been evaluated. The reaction occurs in two steps, in the first, a conjugate diene (1,3-cyclo-octene) is formed via isomerization, then, hydrogenation occurs quickly forming the monoene. The hydrogenation of the obtained cyclo-octene is relatively slow and strongly inhibited by the presence of the cyclo-octadiene. This last reaction has, therefore, been used for comparing the activities of different palladium catalysts showing an exponential behaviour of the reaction rate with the metal dispersion. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogenation; Polyunsaturated organic compounds; Pd catalysts 1. Introduction In a previous work [1] on the partial hydrogenation of rapeseed oil, catalysed by supported palladium cat- alyst, we observed internal mass-transfer limitation despite the small mean size of the catalyst particles. For this reason and considering that at low tempera- ture at which chemical regime could be operative, the viscosity of the oil strongly increases undergoing so- lidification, kinetics of hydrogenation can be studied only indirectly, i.e., also by considering the influence of the mass transfer. Reaction order with respect to hydrogen is zero, while a Langmuir–Hinshelwood kinetic law [2] was observed for the unsaturated molecules. In the same work, we observed that hydro- genation rates for trienes and dienes were identical, on the contrary, the rate of hydrogenation for monoene ∗ Corresponding author. E-mail address: santacesaria@chemistry.unina.it (E. Santacesaria). was quite lower. We interpreted this behaviour by assuming that polyunsaturated molecules firstly give place to conjugated double bonds as a consequence of shifting reactions occurring before hydrogenation. In order to verify this aspect, in the present work, we have studied the hydrogenation of model molecules such as 1,5-cyclo-octadiene, cyclo-octene and 1,7-octadiene. In this way, we have had the possibility of comparing the hydrogenation activity of dienes conjugated or not and of monoenes. The effect of the distance among the two double bonds of dienes has been tested too. As expected, we observed that the conjugated dienes are hydrogenated very quickly. On the contrary, non-conjugated dienes are firstly isomerized to con- jugated ones and then hydrogenated faster than mo- noenes. By hydrogenating 1,5-cyclo-octadiene, e.g., 1,3-cyclo-octadiene is formed as an intermediate. The same behaviour occurs in the hydrogenation of 1,7- octadiene, but in this case, different dienic isomers are obtained and consequently many monoene isomers. 0920-5861/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0920-5861(00)00637-4