Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Differen
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Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Different Solvents Peter Haider Æ Bertram Kimmerle Æ Frank Krumeich Æ Wolfgang Kleist Æ Jan-Dierk Grunwaldt Æ Alfons Baiker
Received: 22 May 2008 / Accepted: 27 June 2008 / Published online: 16 July 2008 Ó Springer Science+Business Media, LLC 2008
Abstract 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.
P. Haider B. Kimmerle F. Krumeich W. Kleist J.-D. Grunwaldt A. Baiker (&) Department of Chemistry and Applied Biosciences, ETH Zu¨rich, Ho¨nggerberg, HCI, 8093 Zurich, Switzerland e-mail: [email protected] Present Address: J.-D. Grunwaldt Department of Chemical and Bioengineering, Technical University of Denmark, 2800 Lyngby, Denmark
Keywords Gold catalyst Particle size effect Benzyl alcohol Aerobic oxidation Supercritical CO2 Gold colloids
1 Introduction Gold (nano-) particles—larger colloids are often referred to as Purple of Cassius—are known and used for centuries [1] and various recipes have been described, e.g., in Libavius’ chemistry book ‘‘Alchemia’’ published in 1597 [2]. Faraday described in one of his lectures a dependence of the color of the Au particles on the concentration of the original solution [3] addressing a today well-known and welldescribed [4, 5] change in color for different particle diameters of Au [6]. Various recipes for the preparation of gold colloids are known [6–11] and Au colloids are applied in various fields [12]. Triggered by its unprecedented catalytic properties gold has also attracted tremendous interest in the past years in both heterogeneous and homogeneous catalysis [13]. Due to several complicating factors caused by the application of different catalyst precursors, preparation routes, and pretrea
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