Three-stage deactivation of Pd/SiO 2 and Pd-Ag/SiO 2 catalysts during the selective hydrogenation of acetylene

Abstract

The deactivation of Pd/SiO 2 and Ag-promoted Pd/SiO 2 catalysts was monitored during their use in the selective hydrogenation of acetylene. Based on the analysis of green oil accumulated on the deactivated Pd/SiO 2 catalysts, it was proposed that the catalyst deactivation proceeded in three stages. In the initial stage (Stage I), 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/SiO 2 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/SiO 2 during the reaction period of this study. Pd-Ag/SiO 2 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/SiO 2, which additionally contributed to the slow deactivation of the former catalyst. The green oil that formed on the Pd-Ag/SiO 2 was more volatile and mobile than that formed on Pd/SiO 2, 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.