Characterization of Gold–Titania Catalysts via Oxidation of Propylene to Propylene Oxide

Abstract

Propylene oxidation to propylene oxide (PO) was performed with an H2/O2 mixture in the temperature range of 373–473 K using Au/TiO2 catalysts synthesized by a variety of techniques, including one which produced Au–Ti nanoclusters with controlled composition. The two most PO-active catalysts were one prepared by deposition–precipitation of gold onto titania-modified silica and one consisting of silica-supported Au–Ti nanoclusters with a 200:1 gold-to-titanium ratio. Most Au/TiO2 catalysts had a small spread in PO activity and selectivity at 373 K, but gave a much wider distribution in performance at temperatures above 413 K. With very few exceptions, these catalysts exhibit a maximum in PO turnover frequency over the experimental temperature range, resulting from sequential reaction of PO to other oxidation products. The largest observed losses in PO selectivity with increasing temperature and decreasing WHSV were to the oxidative cracking products ethanal and CO2, suggesting that maximizing the number of PO active sites is not the only solution to increase PO yields over these catalysts. The amount, phase, and method of contacting titania with gold controlled both PO oligomerization, as seen in the time on stream studies, and H2-assisted oxidative cracking rates. Catalysts that have isolated Ti atoms, such as the silica-supported Au–Ti nanoclusters, generally maintain a higher selectivity to PO with temperature. A D2 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.