Enhanced reaction rate for gas-phase epoxidation of propylene using H2 and O2 by Cs promotion of Au/TS-1

Gold clusters supported on titanium silicalite-1 (hereafter denoted as Au/TS-1) with high gold loading at ∼0.1–0.16 wt%, prepared by the deposition precipitation (DP) method, showed about two times enhancement in the PO rate (∼300versus∼150gPOh-1kgCat-1 at 200 °C), ∼10% increase in PO selectivity (∼80% versus ∼70%) and ∼5–10% increase in H2 selectivity (∼20% versus ∼10%) when Cs2CO3 instead of Na2CO3 was used as the precipitation agent. Using Cs2CO3 as the precipitation agent caused a fourfold increase in Au uptake efficiency, indicating a strong interaction between Cs and Au in the Au/TS-1 system. XPS/TEM analyses for two Au/TS-1 samples with the same gold loading at ∼0.16 wt% but different alkali metals (Cs versus Na) indicate that more Au was retained inside the TS-1 nanopores for the Cs sample. The presence of Cs is, therefore, proposed to help stabilize small gold clusters (<1 nm) inside the TS-1 nanoporous channels at the high gold loading (>0.1 wt%) due to the Cs/Au interaction, resulting in the promotion of PO rate per gram of catalyst. Furthermore, similar apparent activation energy at ∼30 kJ mole−1 observed for the Au/TS-1 catalysts with the presence of either Cs or Na suggests that the number, but not the nature of the active sites, is changed in the Cs-promoted samples. Finally, regardless of the type of alkali metal (Na or Cs) present in the catalysts, lower Ti content (Si/Ti molar ratio ∼100) for Au/TS-1 catalysts was found to favor PO catalytic performance.