(640b) Probing the Gold Active Sites In Au/TS-1 for Gas Phase Epoxidation of Propylene In the Presence of Hydrogen

Lee, W., Purdue University
Akatay, M. C., Purdue University
Stach, E. A., Purdue University
Ribeiro, F. H., Purdue University
Delgass, W. N., Purdue University

A single-step, direct catalytic partial oxidation of propylene to propylene oxide (PO) using molecular oxygen has long been desired as a greener and more economical process compared to the current PO production routes. By co-feeding hydrogen with propylene and oxygen and using Au/TiO2 as the catalyst, one can produce PO with high selectivity (~ 99%) at ambient pressure. However, the low PO rate (~10 gPO/h/kgCat), as well as poor catalyst stability, are short of the requirements for industrial application. The catalytic performance can be significantly improved by using TS-1(titanium silicalite-1) as the support for nano-gold clusters/particles such that the PO rate increases up to ~100 gPO/h/kgCat and the selectivity is maintained at ~ 80% PO. While gold particles with sizes in the range 2-5 nm in Au/TiO2 are assigned in the literature as the active sites in PO reaction, our previous work suggests that gold clusters smaller than 2 nm are much more active in propylene epoxidation. However, the issue of whether gold clusters inside the TS-1 micropores are active in the PO reaction has not been settled. This work addresses this question and attempts to further understand the gold active sites in Au/TS-1.

A TS-1 support coated with an inert layer of S-1 (silicalite-1) was designed and prepared for probing the activity of the gold clusters inside the TS-1 micropores for the PO reaction.  This coated material (S-1/TS-1) was characterized via TEM, XRD, XPS, DRUV-vis and nitrogen adsorption analysis at LN2 temperature to understand the result of the silicalite-1 coating and the corresponding influence on the spatial distribution of  the titanium.  Au/S-1/TS-1 was then prepared by using optimized deposition-precipitation (DP) conditions. Since neither Au nor Ti alone would give any significant PO activity, any significant PO activity observed from Au/S-1/TS-1 must come from the intimate Au-Ti contact inside the TS-1 core. The experimental results showed that this Au/S-1/TS-1 sample had at least 25 times higher PO production rate (gPO/h/kgCat), compared to that of an Au/S-1 sample.  This is an indication, as well as the first direct experimental evidence, showing that the gold clusters inside the TS-1 are the active ones for the PO reaction.