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

• Published in: Catalysis today 2007-04, Vol.122 (3), p.284-291
• Main Authors: Lignier, Pascal, Morfin, Franck, Piccolo, Laurent, Rousset, Jean-Luc, Caps, Valérie
• Format: Article
• Language: English
• Subjects: Catalysis; Chemical Sciences; Chemistry; Environment and Society; Environmental Sciences; Epoxidation; Exact sciences and technology; Free-radical mechanism; General and physical chemistry; Gold catalysis; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Trans-stilbene
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2007.02.006

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/TiO 2 reference catalyst from the World Gold Council. However the selectivity decreases significantly when using Au/C instead of oxide-supported gold catalysts or H 2O 2 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 δ− (90%) and Au + (10%) on the Au/TiO 2 catalyst. Both gold and, to a minor extent, titania seem to be involved in the reaction cycle.