(683b) Tuning the Surface Active Sites By Surface Modification of Supported ReOx/(SiO2-Al2O3) Catalysts for Olefin Metathesis

Due to the global shortage of propylene, the chemical industry is turning to metathesis of ethylene and 2-butene to produce on purpose propylene. Heterogeneous supported ReO_x/Al₂O₃, MoO_x/Al₂O₃ and WO_x/SiO₂ catalysts have been found to be most efficient for metathesis of olefins. Although the supported ReO_x/Al₂O₃ catalyst has received much attention for olefin metathesis, the effect of the oxide support on olefin metathesis by supported ReO_x catalysts is still not well understood. For the most common Al₂O₃ support, not all rhenia anchoring sites lead to activation of rhenia for olefin metathesis. Modified Al₂O₃-SiO₂ mixed oxide supports are highly effective supports for promoting surface ReO_x sites for olefin metathesis. The improved metathesis performance of the Al₂O₃-SiO₂ mixed oxide supports has been attributed to the modified surface Lewis and Brønsted acidity, but supporting data were not provided in these studies. The nature of the surface AlO_x, SiO_x and ReO_x sites present in these mixed oxide supports are still unknown. The objective of the present study is to determine the origin of enhanced performance of the surface ReO_x sites on Al₂O₃-SiO₂ mixed oxide supports.

Modern in situ molecular spectroscopy (UV-Vis, Raman, IR and XAS) and chemical probe studies (ammonia chemisorption, C₂H₄/C₄H₈-titration, C₃H₆-TPSR and steady-state propylene self-metathesis) of the surface ReO_x sites on Al₂O₃ and SiO₂, and surface modified mixed oxide supports (SiO₂/Al₂O₃, Al₂O₃/SiO₂, and ZSM-5 (Si/Al=15)) were applied to address the origin of the promotional effect. The characterization studies of the supported rhenia catalysts demonstrated that the enhanced activity on Al₂O₃-SiO₂ mixed oxide supports is due to anchoring rhenia at Brønsted acidic Al-(OH)⁺-Si surface hydroxyls that results in a greater number of activated surface ReO_x sites.