Journal of Catalysis, Vol.308, 98-113, 2013
Enhanced reaction rate for gas-phase epoxidation of propylene using H-2 and O-2 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 (similar to 300 versus similar to 150 gpo h(-1) kg(cat)(-1), at 200 degrees C), similar to 10% increase in PO selectivity (similar to 80% versus similar to 70%) and similar to 5-10% increase in H-2 selectivity (-20% versus similar to 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 similar to 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. (C) 2013 Elsevier Inc. All rights reserved.