Reactivity and stability of Au in and on TS-1 for epoxidation of propylene with H2 and O2

The direct vapor-phase epoxidation of propylene using hydrogen and oxygen over gold particles prepared by the deposition-precipitation (DP) method on various modified titanium silicalite-1 (TS-1) supports was studied over a reaction time of 24-36 h at a space velocity of 7000 ml g(cat)(-1) h(-1) and temperatures of 413, 443, and 473 K. Gold deposition at pH 9-10 allowed for a consistent amount of 1-3 wt% of the gold available in solution to be deposited, while still maintaining gold particle diameters in the 2-5 nm range, as observed by TEM. These Au/TS-1 catalysts achieved propylene conversions of 2.5-6.5% and PO selectivities of 60-85% at 443 K, with dilute Au and Ti catalysts exhibiting good stability. A key result of the work is that PO rates were not highly influenced by the TS-1 particle size and are thus not proportional to the specific external surface area of the support. The conclusion that activity may reside in the channels of the TS-1 is supported by the finding that the observable gold particles decorating the TS-1 particles only account for ca 30% of the total gold content of the catalyst. Increasing the gold loading up to 0.74 wt% did not increase the PO rates proportionally, suggesting that the active Au-Ti PO-forming centers are limited. In contrast to the prevailing interpretation of this catalyst that a critical Au particle diameter of 2-5 nm is essential for PO activity, our results are consistent with a molecular cluster model where extremely small gold clusters located near Ti sites inside the TS-1 pores or on the external surface are active for propylene epoxidation. (C) 2004 Elsevier Inc. All rights reserved.