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

Abstract

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/La2O3SiO2 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 20 h 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%La2O3SiO2, which is smaller than that (53.9 nm) found in Na/Ag/SiO2. The presence of subnanometer AgNPs on Ag/La2O3SiO2 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.