SELECTIVE HYDROGENATION OF ACETYLENE IN ETHYLENE FEEDSTOCKS ON PD CATALYSTS

Pumice supported palladium catalysts were compared with Pd/SiO2 and Pd /Al2O3 in the hydrogenation of acetylene using typical industrial ethy lene feedstocks: front-end and tail-end cuts. Pd/pumice catalysts exhi bit good activity and excellent selectivity and stability in the title reaction. Their activity/selectivity pattern is controlled by the com position of the reaction mixture. The turnover frequency (TOF) increas es, and the apparent activation energy (E(a)) decreases, with the H-2/ C2H2 ratio, but they are not affected by the C2H2/C2H4 ratio. The sele ctivity to ethane (S-E) does not change with acetylene conversion at l ow H-2/C2H2 ratio (tail-end cut) and increases at high H-2/C2H2 ratio (front-end cut). No dependence of S-E with acetylene conversion was fo und, at any H-2/C2H2 ratio, on varying the metal dispersion. TOF and S -E change with metal loading depending on the reaction mixtures; low m etal loaded catalysts gave the best activity/selectivity pattern. Pd/p umice catalysts do not show aging phenomena even when used in drastic conditions (high space velocities and front-end cuts). The catalytic b ehavior of Pd/pumice catalysts, as a function of the composition of re action mixture, elucidated by using the isokinetic relationship (IKR) approach shows that the relative amount of hydrogen in the feedstocks plays an important role in activation/deactivation processes which occ urs on the metal surface. By considering the presence of different cen ters on the catalyst and by performing data analysis with a suitable m athematical model, the microscopical mechanism and the role of surface deposits can be explained. A similar reaction mechanism is applicable to the other palladium catalysts, at least in the case of tail-end mi xtures.