Palladium-silver sol-gel catalysts for selective hydrodechlorination of 1,2-dichloroethane into ethylene - IV. Deactivation mechanism and regeneration

The activity and selectivity of a 1.9% Pd-3.7% Ag/SiO2 catalyst during selective hydrodechlorination of 1,2-dichloroethane into ethylene have been followed in the course of time. The deactivation curves show two distinct periods: an initial period at the very beginning of catalyst operation, that is, during the first 20 operating hours, characterized by a sharp decrease of the rates of both reactions involved in the process (1,2-dichloroethane hydrodechlorination and undesired ethylene hydrogenation) and a second period, after the first 20 operating hours and investigated for more than 600 h, characterized by a clearly slower deactivation. During the initial period, the hydrodechlorination rate decreases less quickly than the hydrogenation rate and this results in a fast increase of ethylene selectivity which reaches a maximum at the end of this period. After the initial period, the hydrodechlorination rate decreases faster than the hydrogenation rate and this results in a slow decrease of ethylene selectivity. A comparison between the physico-chemical properties of the fresh catalyst and of the deactivated one suggests a deactivation mechanism by poisoning or coking of silver sites at the surface of the active Pd-Ag alloy particles whereas palladium sites would remain intact. The selectivity evolution during the second period, that is, after the very first operating hours, is in agreement with such a mechanism. However, the sharp ethylene selectivity increase during the initial operating hours suggests, in addition to the deactivation by silver sites disappearance, the presence of a further nonidentified phenomenon. The characterization of the catalyst after a regeneration treatment including an oxidation step followed by a reduction step shows that this treatment allows restoration near to the physico-chemical properties of the fresh catalyst. (C) 2003 Elsevier Inc. All rights reserved.