The kinetics of the propene epoxidation and water formation over gold-titania catalysts

propene Introduction Propene oxide is a very important chemical intermediate, produced at about 6 million tons per year (2006) with demand still growing by 5 % annually. One of the important new developments for the production of propene oxide are the gold titania based epoxidation catalysts. Gold-titania catalysts can very selectively epoxidize propene at mild conditions using molecular oxygen in the presence of hydrogen as sacrificial reductant. Because of the use of sacrificial hydrogen, this reaction is best called a hydro-oxidation rather than an oxidation reaction. Gold-titania based catalyst systems have a number of disadvantages which need to be improved: the conversion levels remain low, often the catalyst stability is insufficient, and the hydrogen efficiency is low. The hydrogen efficiency (defined as the amount of propene oxide produced divided by the hydrogen consumed) is dominated by the water produced by the direct hydrogen oxidation. The low hydrogen efficiency is the most important problem that needs to be solved before this catalyst system can be applied commercially. This paper presents a kinetic study into the hydrogen oxidation. Both the behavior of gold on titania, the ‘standard’ catalysts of choice for the propene hydro-epoxidation, and the behavior of gold on silica are investigated. Gold on silica does not have any significant propene epoxidation activity and is studied to investigate the possible role of the support and as a way do de-couple the epoxidation from the water formation. For the hydro-oxidation of propene, it is commonly known that the support plays a crucial role in the reaction. Figure 2. Water production during hydrogen oxidation over 1 wt% Au/SiO2 catalyst. In one cycle propene is co-fed to the reaction mixture for 5 hours. (353 K, GHSV=9000 h) Figure 1. Propene oxide and water formation over a 1 wt% Au/TiO2 catalyst. Activity during epoxidation and hydrogen oxidation without propene. (325 K, GHSV=9000 h)