Three-stage deactivation of Pd/SiO2 and Pd-Ag/SiO2 catalysts during the selective hydrogenation of acetylene

The deactivation of Pd/SiO2 and Ag-promoted Pd/SiO2 catalysts was monitored during their use in the selective hydrogenation of acetylene. Based on the analysis of green oil accumulated on the deactivated Pd/SiO2 catalysts, it was proposed that the catalyst deactivation proceeded in three stages. In the initial stage (Stage 1), a large amount of relatively light green oil was deposited on, or in the vicinity of, the Pd surface, but the catalytic activity decreased only slightly. As the deactivation proceeded (Stage II), the 1,3-butadiene that had accumulated on the Pd surface was polymerized to relatively heavy green oil, a part of which moved from the Pd surface to the support. In the later stage of deactivation (Stage III), catalytic activity was drastically decreased because catalyst pores were blocked and hydrogen diffusion was limited in the thick film of the relatively heavy green oil. Although the initial activity of Ag-promoted Pd/SiO2 Was slightly lower than that of the unpromoted one, the amount of green oil deposited on the former catalyst was much smaller than that deposited on the latter. Consequently, the final stage of deactivation (Stage III) was not observed with Pd-Ag/SiO2 during the reaction period of this study. Pd-Ag/SiO2 showed Stage II, characterized by the transfer of green oil from the Pd to the support, at a period earlier than in the case of Pd/SiO2, which additionally contributed to the slow deactivation of the former catalyst. The green oil that formed on the Pd-Ag/SiO2 was more volatile and mobile than that formed on Pd/SiO2, because the added Ag geometrically blocked multi-coordinated large ensembles of the Pd surface and also modified the Pd electronically such that the adsorption of 1,3-butadiene on the Pd became weaker than in the absence of promotion. (C) 2009 Elsevier B.V. All rights reserved.