Effects of TiO₂ in Low Temperature Propylene Epoxidation Using Gold Catalysts

Propylene epoxidation with molecular oxygen has been proposed as a green and alternative process to produce propylene oxide (PO). In order to develop catalysts with high selectivity, high conversion, and long stability for the direct propylene epoxidation with molecular oxygen, understanding of catalyst structure and reactivity relationships is needed. Here, we combined atomic layer deposition and deposition precipitation to synthesize series of well-defined Au-based catalysts to study the catalyst structure and reactivity relationships for propylene epoxidation at 373 K. We showed that by decorating TiO₂ on gold surface the inverse TiO₂/Au/SiO₂ catalysts maintained ∼90% selectivity to PO regardless of the weight loading of the TiO₂. The inverse TiO₂/Au/SiO₂ catalysts exhibited improved regeneration compared to Au/TiO₂/SiO₂. The inverse TiO₂/Au/SiO₂ catalysts can be regenerated in 10% oxygen at 373 K, while the Au/TiO₂/SiO₂ catalysts failed to regenerate at as high as 473 K. Combined characterizations of the Au-based catalysts by X-ray absorption spectroscopy, scanning transmission electron microscopy, and UV-vis spectroscopy suggested that the unique selectivity and regeneration of TiO₂/Au/SiO₂ are derived from the site-isolated Ti sites on Au surface and Au-SiO₂ interfaces which are critical to achieve high PO selectivity and generate only coke-like species with high oxygen content. The high oxygen content coke-like species can therefore be easily removed. These results indicate that inverse TiO₂/Au/SiO₂ catalyst represents a system capable of realizing sustainable gas phase propylene epoxidation with molecular oxygen at low temperature.