Understanding Selective Hydrogenation of 伪,尾-Unsaturated Ketones to Unsaturated Alcohols on the Au25(SR)18 Cluster

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• 作者：Runhai Ouyang ; De-en Jiang
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：November 6, 2015
• 年：2015
• 卷：5
• 期：11
• 页码：6624-6629
• 全文大小：421K
• ISSN：2155-5435

文摘

The Au₂₅(SR)₁₈ cluster can catalyze 100% selective hydrogenation of 伪,尾-unsaturated ketones to unsaturated alcohols. However, the mechanism remains a mystery. Here we unravel the underlying mechanism by using first-principles density functional theory calculations with benzalacetone as a substrate. We find that the Au₂₅(SR)₁₈ cluster cannot directly activate either H₂ or benzalacetone separately. Instead, starting with coadsorption of H₂ and benzalacetone on Au₂₅(SR)₁₈, H₂ heterolytically cleaves to the substrate and to a surface Au atom of the cluster, followed by the facile transfer of H from the Au atom to the partially hydrogenated substrate. In this mechanism, C鈺怬 and C鈺怌 hydrogenations have barriers of 0.99 and 1.12 eV, respectively, in agreement with the experimentally observed selectivity toward unsaturated alcohol. In addition, we show that the ethanol solvent can further stabilize the partially hydrogenated intermediate of C鈺怬 hydrogenation via a hydrogen bond, leading to a smaller H₂ cleavage energy (0.90 eV). Hence, the heterolytic cleavage of H₂ on the Au₂₅ nanocluster favors the more polar C鈺怬 bond of benzalacetone, leading to selective formation of unsaturated alcohol. This work reveals that the weak interaction between H₂ and the Au cluster, the formation of a Au hydride, and the polar solvent are responsible for the high selectivity of the 伪,尾-unsaturated ketone hydrogenation to the corresponding unsaturated alcohol over the Au₂₅ nanocluster.