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. [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 focused on titanium silicalite-1 (TS-1) with high Ti dispersion as the support [2], 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 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%.  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 g_cat^-1.  Further details of the kinetics of this reaction 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