(278c) Cs Promotion of the Reaction Rate for Gas Phase Epoxidation of Propylene Using H2 and O2 Over Au/TS-1

A single-step, direct catalytic partial oxidation of propylene to propylene oxide (PO) using O₂ has long been desired as a greener and more economical process compared to the current PO production routes.  More than a decade ago, Haruta et al. [1] showed that by co-feeding H₂ with propylene and O₂ and using Au/TiO₂ as the catalyst, one can produce PO with high selectivity (~ 99%) at ambient pressure.  In our previous work, we have shown that the catalytic performance can be significantly improved by using titanium silicalite-1 (TS-1) with high Ti dispersion as the support [2], together with the proper preparation conditions (deposition-precipitation) for gold addition, such that the PO rate increased up to ~160 g_PO h^-1 kg_Cat^-1[3].  A PO rate per gram of Au as high as ~500 g_PO h^-1 g_Au^-1 was obtained on samples with gold loading <0.06 wt%, suggesting that the active Au entities catalyzing the PO reaction are size sensitive and should be <1 nm in diameter.

In this work, we examine the promotional effects of the use of Cs₂CO₃ as the Au precipitation agent.  Au/TS-1 with high gold loading ~0.1-0.16 wt%, prepared by the deposition precipitation (DP) method, showed a two times enhancement in the PO rate (~300 versus ~150 g_PO h^-1 kg_Cat^-1 and the highest yet observed at 200 °C), an approximately 10% increase in PO selectivity, and ~5-10% increase in H₂ selectivity when Cs₂CO₃ was used in place of Na₂CO₃ as the precipitation agent.  Furthermore, Cs₂CO₃ gave a fourfold increase in Au uptake efficiency, indicating a strong interaction between Cs and Au in the Au/TS-1 system.  XPS and TEM analyses for the two 0.16 wt% Au/TS-1 samples with Cs versus Na content suggest that Cs can retain more gold inside the TS-1 crystallites.  The presence of Cs is, therefore, proposed to help stablize small gold clusters (<1 nm) inside the TS-1 nanoporous channels at the higher gold loadings (>0.1 wt%) due to the Cs/Au interaction.  We conclude that this attraction increases the number of the gold active sites, resulting in the promotion of PO rate.  Regardless of the type of alkali metal (Na vs. Cs) present in the catalysts, lower Ti content in the TS-1 was found to favor PO catalytic performance. Kinteics of the PO reaction over these catalysts will also be discussed.

References:

(1)   T. Hayashi, K. Tanaka, and M. Haruta, J. Catal. 1998, 178, 566

(2)   E.E. Stangland, B. Taylor, R.P. Andres, and W.N. Delgass, J. Phys. Chem. B, 2005, 109, 2321

(3)   W.-S. Lee, M. C. Akatay, Eric Stach, F. H. Ribeiro and W. N. Delgass, J. Catal, 2012, 287, 178