Selective hydrogenation of acetylene in ethylene feedstocks on Pd catalysts Original Research Article

Pumice supported palladium catalysts were compared with PdSiO2 and PdAl2O3 in the hydrogenation of acetylene using typical industrial ethylene feedstocks: front-end and tail-end cuts. Pd/pumice catalysts exhibit good activity and excellent selectivity and stability in the title reaction. Their activity/selectivity pattern is controlled by the composition of the reaction mixture. The turnover frequency (TOF) increases, and the apparent activation energy (Ea) decreases, with the H2C2H2 ratio, but they are not affected by the C2H2C2H4 ratio. The selectivity to ethane (SE) does not change with acetylene conversion at low H2C2H2 ratio (tail-end cut) and increases at high H2C2H2 ratio (front-end cut). No dependence of SE with acetylene conversion was found, at any H2C2H2 ratio, on varying the metal dispersion. TOF and SE change with metal loading depending on the reaction mixtures; low metal loaded catalysts gave the best activity/selectivity pattern. Pd/pumice catalysts do not show aging phenomena even when used in drastic conditions (high space velocities and front-end cuts). The catalytic behavior of Pd/pumice catalysts, as a function of the composition of reaction 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 occurs on the metal surface. By considering the presence of different centers on the catalyst and by performing data analysis with a suitable mathematical 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 mixtures.