Aerobic methylcyclohexane-promoted epoxidation of stilbene over gold nanoparticles supported on Gd-doped titania
KAUST DepartmentKAUST Catalysis Center (KCC)
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AbstractAerobic partial oxidations of alkanes and alkenes are important processes of the petrochemical industry. The radical mechanisms involved can be catalyzed by soluble salts of transition metals (Co, Cu, Mn...). We show here that the model methylcyclohexane/stilbene co-oxidation reaction can be efficiently catalyzed at lower temperature by supported gold nanoparticles. The support has little influence on gold intrinsic activity but more on the apparent reaction rates which are a combination of catalytic activity and diffusion limitations. These are here minimized by using gadolinium-doped titania nanocrystallites as support for gold nanoparticles. This material is obtained by mild hydrolysis of a new Gd4TiO(OiPr)14 bimetallic oxoalkoxide. It leads to enhanced wettability of the < 3 nm gold particles in the tert-butyl hydroperoxide (TBHP)-initiated epoxidation of stilbene in methylcyclohexane; Au/TiO2:Gd3+ is in turn as active as the state-of-the-art hydrophobic Au/SiO2 catalyst. The rate-determining step of this reaction is identified as the gold-catalyzed homolytic decomposition of TBHP generating radicals and initiating the methylcyclohexane-mediated epoxidation of stilbene, yielding a methylcyclohexan-1-ol/trans-stilbene oxide mixture. Methylcyclohexan-1-ol can also be obtained in the absence of the alkene in the gold-catalyzed solvent-free autoxidation of methylcyclohexane, evidencing the catalytic potential of gold nanoparticles for low temperature C-H activation. © 2010 The Royal Society of Chemistry.
SponsorsDr V. M. and K. G. thank the French Ministry for Education and Research and the French National Research Agency (project ANR-08-JCJC-0090-01 ACTOGREEN), respectively, for PhD fellowships, as well as the IDECAT European Network of Excellence (6th FP) for sponsoring presentations of parts of this work in international conferences. The authors also thank F. Morfin for GC technical support, M. Daniel for TEM, P. Mascunan and N. Cristin for elemental analyzes and Dr A. Sorokin for fruitful discussions.
PublisherRoyal Society of Chemistry (RSC)