Catalytic sites in Au/TS-1 and related epoxidation catalysts
A single-step, direct catalytic partial oxidation of propylene to propylene oxide (PO) via co-feeding O2 and H2 with propylene and using Au/titanium silicalite-1 (Au/TS-1) as the catalysts was studied in this work. The goal has been trying to understand the nature of the catalytic sites in the Au/TS-1 catalysts for the PO reaction and particularly to identify the location and the size of the gold active sites. This research started from investigating the effects of the gold deposition conditions on the PO catalytic performance (Chapter 1). This experimental observation together with a very high PO rate per gram of Au (~500 gPO h-1 gAu-1) obtained on the sample with very low gold loading (<0.06 wt%) suggest that the active gold sites for PO reaction are size sensitive and should be very small (<1 nm). Moreover, diluted systems (low Au, low Ti content) are recommended for preparing Au/TS-1 with high PO rate. On the other hand, the effects of the residual Cl and/or Na on the PO catalytic performance were found to be little as compared to the size of the nano gold particles/clusters. Furthermore, the catalytic sites in the Au/TS-1 for CO oxidation and the PO reaction were confirmed to be different by the observation of the opposite trends in rate as a function of the gold loading for those two reactions (Chapter 3). To probe the activity of Au clusters inside the TS-1 nanopores, a TS-1 support coated with an inert layer of silicalite-1 (S-1) was designed and prepared (Chapter 4). The experimental results showed that the Au/S-1/TS-1 sample had at least 20 times higher PO production rate (gPO h-1 kgCat-1) compared to that of an Au/S-1 sample, suggesting that the Au clusters inside the TS-1 are active for the PO reaction. An attempt of increasing the number of the Au active per gram of catalyst was achieved by preparing the Au/TS-1 catalyst via using Cs2CO 3 as the precipitation agent during the gold deposition step (Chapter 5). We found that Cs could further promote the initial PO rate of Au/TS-1 up to ~ 300-350 gPO h-1 kg Cat-1 at 200°C. The enhanced PO activity of the Cs promoted Au/TS-1 catalyst is attributed to increase the number of the active sites via the stabilization of the Au clusters inside the TS-1 nanoporous channels by the presences of the cesium. Finally, a totally different dynamic PO catalytic performance, as compared to that of the regular Au/TS-1 catalyst, was observed when an uncalcined TS-1 (with templates in, hereafter denoted as UTS-1) was used as the support. The observed increasing PO rate per gram of catalyst for Au/UTS-1 catalysts was found to be accompanied by the increasing BET surface area of the spent Au/UTS-1 catalysts. The preliminary characterization results suggest that the increasing PO rate per gram of catalyst in the Au/UTS-1 is attributed to the migration of the Au species to the in-situ generated nanopores at the reaction temperature (~200°C), which then creates and increases the number of the Au-Ti active sites for PO reaction.
Ribeiro, Purdue University.
Off-Campus Purdue Users:
To access this dissertation, please log in to our