Gas-phase epoxidation of propylene over small gold ensembles on TS-1

A series of Au/TS-1 catalysts with varying gold and titanium contents was prepared by deposition-precipitation (DP) and examined at 140-200 °C in a 10/10/10/70 vol.% mixture of hydrogen, oxygen, propylene and helium at a space velocity of 7000 mL/h/gcat. Gold loading was found to be closely related to titanium loading, implying that low gold loadings using deposition-precipitation results in an inherently small number of very active sites. Forcing the gold loading to higher values resulted in poor activity and stability. A catalyst prepared with a Si/Ti = 36 and a gold loading of 0.05 wt% produced 116 gPO/h/kgcat at 200 °C, which is the highest rate thus reported for a TS-1-based catalyst, with no evidence of deactivation during the 40 h temperature program. Catalysts prepared with lower titanium and gold contents resulted in very active catalysts when rates were normalized to the total gold content, 350 gPO/h/gAu at 200 °C for 0.01 wt% Au/TS-1 (Si/Ti = 500), indicative of the more efficient use of gold and titanium for the epoxidation reaction. The low gold loadings coupled with the absence of gold particles in TEM micrographs make it likely that, in these materials, significant activity is attributable to gold entities much smaller than 2 nm.