266626 Gold Sites for Gas Phase Epoxidation of Propylene by H2 and O2 in the Pores of TS-1
A single-step, direct catalytic partial oxidation of propylene to propylene oxide (PO) using O2 has been on the short list of grand challenges in catalysis for some time. More than a decade ago, Haruta et al.  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 focused on titanium silicalite-1 (TS-1) with high Ti dispersion as the support , and have shown that, together with the proper preparation conditions (deposition-precipitation) for gold addition, Au/TS-1 catalysts can yield PO rates up to ~160 gPO h-1 kgCat-1. A PO rate per gram of Au as high as ~500 gPO h-1 gAu-1 was obtained on samples with gold loading <0.06 wt%. These data, together with other circumstantial evidence, suggest that the active Au entities catalyzing the PO reaction are size sensitive and should be <1 nm in diameter. A question we have been asking throughout this research is whether the active Au species can be created in the TS-1 channels. In this work we answer this question in the affirmative.
Specially prepared catalysts made by coating TS-1 seeds with silicalite-1 produced a material (S-1/TS-1) that was shown by XRD and selected area electron diffraction to possess a well defined MFI structure and by XPS to have no Ti is on the external surface even after addition of gold by deposition precipitation. This Au/S-1/TS-1 catalyst had at least 20 times higher PO rate (per gram of catalyst) at 200 °C compared to that of an Au/S-1 sample with a similar gold loading of ~0.05 wt%. Since neither Au nor Ti alone gives significant PO activity, and a variety of experiments show the need for close proximity of Au with Ti, this is the first direct experimental evidence showing that gold clusters inside the TS-1 are active for the PO reaction at 200 °C, H2/C3H6/O2/N2 = 3.5/3.5/3.5/24.5 mL min-1, and a space velocity of 14000 mL h-1 gcat-1. Further details of the kinetics of this reaction will also be discussed.
(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