(654g) Atomistic Design of Propylene Epoxidation Catalysts

Propylene oxide (PO) is a chemical intermediate of great value and in high demand. It is used to produce a number of polymeric materials. A direct catalytic epoxidation of propylene with hydrogen and oxygen to PO represents an environmentally friendly process and can be fulfilled over gold-titanium based catalysts. Au/TS-1 is one of the benchmark catalysts. The reaction is considered to be highly catalyst size dependent. In general, small (<5 nm) Au diameter is preferred. In addition, the gold-titanium synergic effect is desired for high turnovers and high hydrogen efficiency. Typically, gold nanocatalysts were deposited on TS-1 support via deposition precipitation (DP). This method utilizes zero charge of the titanium sites and deposit Au only on Ti sites. Incipient wetness impregnation (IWI) as a convenient, rapid, and in-expensive method is frequently used to prepare catalysts. However, Au nanocatalysts prepared by IWI was not as active as Au nanocatalysts prepared by DP, due to the large particle size, chlorine residues, and random distribution of Au on TS-1 support.

In this work, a modified IWI method has been developed. The catalysts prepared by this method show much better catalytic performance. The PO selectivity, propylene conversion, and hydrogen efficiency are comparable to catalyst Au/TS-1. The geometric and electronic effects of catalysts are studied in detail using a number of characterization techniques, including in situ X-ray absorption spectroscopy. Moreover, the effects of different preparation strategies including DP, IWI, and atomic layer deposition (ALD) will be discussed.