Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Different Solvents
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The effect of the size of gold particles deposited on CeO2 and TiO2 supports on their catalytic behavior in the aerobic oxidation of benzyl alcohol in different solvents (mesitylene, toluene, and supercritical carbon dioxide) has been investigated. The size of supported gold particles deposited via a colloidal route was in the range 1.3–11.3 nm, as determined by means of EXAFS and HAADF-STEM measurements. The catalytic performance of the supported gold catalysts in the different solvents revealed a significant effect of the gold particle size. Optimal activity was observed for catalysts with medium particle size (ca. 6.9 nm) whereas smaller and bigger particles showed inferior activity. Identical trends for the activity–particle size relationship were found using Au/CeO2 and Au/TiO2 for the reaction at atmospheric pressure in conventional solvents (mesitylene, toluene) as well as under supercritical conditions (scCO2). Selectivity to benzaldehyde was only weakly affected by the gold particle size and mainly depended on reaction conditions. In supercritical CO2 (scCO2) selectivity was higher than in the conventional solvents under atmospheric pressure. All catalysts tested with particle sizes ranging from 1.3 to 11.3 nm showed excellent selectivity of 99% or higher under supercritical conditions.
KeywordsGold catalyst Particle size effect Benzyl alcohol Aerobic oxidation Supercritical CO2 Gold colloids
Electron microscopy was performed at the EMEZ (ETH Zurich). ANKA (Forschungszentrum Karlsruhe, Germany) is acknowledged for providing beamtime for the fluorescence XAS investigations and S. Mangold (ANKA) and S. Marx (ETH Zurich) for help and support during beamtime. The work at the synchrotron radiation source was supported by the European Community—Research Infrastructure Action under the FP6 “Structuring the European Research Area” program (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”, Contract RII3-CT-2004-506008).
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