Can lanthanum doping enhance catalytic performance of silver in direct propylene epoxidation over NaMoAg/SiO₂?

Hany M.Abdel Dayem^1,2，Shar S.Al-Shihry²，Salah A.Hassan¹

1. Chemistry Department, Faculty of Science, Ain Shams University2. Chemistry Department, College of Science, King Faisal University

摘 要：In the attempt to reduce surface free energy of silica to improve interaction of silica with silver, silica was doped by different amounts of low surface energy lanthanum, La₂O₃, through impregnation. The doped and undoped silica were used as supports for preparation of Na/Ag/Mo/La₂O₃-SiO₂ catalysts. Catalytic performances of the catalysts were evaluated in direct epoxidation of propylene（DPO） using molecular oxygen under atmospheric pressure and without adding hydrogen. Adding 5 wt%La to the Na/Ag/Mo/SiO₂ catalyst improves both the catalysts electivity in DPO and its stability over 20h of time-on-stream.The characterization results show that La₂O₃ species interact strongly with silver particles on the silica surface which result in significant improvement in the dispersion profile of silver and marked decrease in the size of silver nanoparticles（AgNPs）. The estimated mean diameter of AgNPs is ca. 4.0 nm in Na/Ag/Mo/5 wt%La₂O₃-SiO₂, which is smaller than that（53.9 nm） found in Na/Ag/SiO₂. The presence of subnanometer AgNPs on Ag/La₂O₃-SiO₂ prior addition of MoO₃ and NaCl to the sample can enhance the mutual electronic synergism between Ag, MoO₃ and Na for selective production of propylene oxide.

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