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

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.