Insight into the free-radical chain mechanism of gold-catalyzed hydrocarbon oxidation reactions in the liquid phase

A free-radical mechanism has been evidenced in the liquid phase stereoselective epoxidation of trans-stilbene using methylcyclohexane (MCH) as solvent, limited amount of tert-butylhydroperoxide (TBHP), and supported gold catalysts. Trans-stilbene oxide is the major reaction product observed, with selectivities up to 88% when using the Au/TiO2 reference catalyst from the World Gold Council. However the selectivity decreases significantly when using Au/C instead of oxide-supported gold catalysts or H2O2 instead of TBHP. HPLC and GC-MS analyses indicate that a fraction of MCH is oxidized during the epoxidation process. It seems that TBHP is the radical source while MCH is propagating the active radical. On the other hand, hydroxyl radicals are responsible for the degradation of the molecule. XPS studies show the presence of Au-0 (90%) and Au+ (10%) on the Au/C catalyst and Au delta- (90%) and Au+ (10%) on the Au/TiO2 catalyst. Both gold and, to a minor extent, titania seem to be involved in the reaction cycle. (C) 2007 Elsevier B.V. All rights reserved.