Characterization of gold-titania catalysts via oxidation of propylene to propylene oxide

Propylene oxidation to propylene oxide (PO) was performed with an H-2/O-2 m ixture in the temperature range of 373-473 K using Au/TiO2 catalysts synthe sized by a variety of techniques, including one which produced Au-Ti nanocl usters with controlled composition. The two most PO-active catalysts were o ne prepared by deposition-precipitation of gold onto titania-modified silic a and one consisting of silica-supported Au-Ti nanoclusters with a 200: 1 g old-to-titanium ratio. Most Au/TiO2 catalysts had a small spread in PO acti vity and selectivity at 373 It, but gave a much wider distribution in perfo rmance at temperatures above 413 K. With very few exceptions, these catalys ts exhibit a maximum in PO turnover frequency over the experimental tempera ture range, resulting from sequential reaction of PO to other oxidation pro ducts. The largest observed losses in PO selectivity with increasing temper ature and decreasing WHSV were to the oxidative cracking products ethanal a nd CO2, suggesting that maximizing the number of PO active sites is not the only solution to increase PO yields over these catalysts. The amount, phas e, and method of contacting titania with gold controlled both PO oligomeriz ation, as seen in the time on stream studies, and H-2-assisted oxidative cr acking rates. Catalysts that have isolated Ti atoms, such as the silica-sup ported Au-Ti nanoclusters, generally maintain a higher selectivity to PO wi th temperature. A D-2 kinetic isotope effect was observed for PO formation which strongly suggests that a hydroperoxy intermediate is involved in the rate-limiting step for PO formation. (C) 2000 Academic Press.