Critical Influence of Adsorption Geometry in the Heterogeneous Epoxidation of "Allylic" Alkenes: Structure and Reactivity of Three Phenylpropene Isomers on Cu(111)
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- 作者：Federico J. Williams ; Rachael L. Cropley ; Owain P. H. Vaughan ; Andrew J. Urquhart ; Mintcho S. Tikhov ; Christine Kolczewski ; Klaus Hermann ; and Richard M. Lambert
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：December 7, 2005
- 年：2005
- 卷：127
- 期：48
- 页码：17007 - 17011
- 全文大小：328K
- 年卷期：v.127,no.48(December 7, 2005)
- ISSN：1520-5126
文摘
It has long been conjectured that the difficulty of heterogeneously epoxidizing higher alkenessuch as propene is due to the presence in the molecule of "allylic" H atoms that are readily stripped off bythe oxygenated surface of the metal catalyst resulting in combustion. Here, taking advantage of theintrinsically higher epoxidation selectivity of Cu over Ag under vacuum conditions, we have used threephenylpropene structural isomers to examine the correlation between adsorption geometry and oxidationchemistry. It is found that under comparable conditions 
-methylstyrene, trans-methylstyrene, andallylbenzene behave very differently on the oxygenated Cu(111) surface: the first undergoes extensiveepoxidation accompanied by relatively little decomposition of the alkene; the second leads to some epoxideformation and extensive alkene decomposition; and the third is almost inert with respect to both reactionpathways. This reactive behavior is understandable in terms of the corresponding molecular conformationsdetermined by near-edge X-ray absorption fine structure spectroscopy and density functional theorycalculations. The proximity to the surface of the C=C function and of the allylic H atoms is critically importantin determining reaction selectivity. This demonstrates the importance of adsorption geometry and confirmsthat allylic H stripping is indeed a key process that limits epoxidation selectivity in such cases.
-methylstyrene, trans-methylstyrene, andallylbenzene behave very differently on the oxygenated Cu(111) surface: the first undergoes extensiveepoxidation accompanied by relatively little decomposition of the alkene; the second leads to some epoxideformation and extensive alkene decomposition; and the third is almost inert with respect to both reactionpathways. This reactive behavior is understandable in terms of the corresponding molecular conformationsdetermined by near-edge X-ray absorption fine structure spectroscopy and density functional theorycalculations. The proximity to the surface of the C=C function and of the allylic H atoms is critically importantin determining reaction selectivity. This demonstrates the importance of adsorption geometry and confirmsthat allylic H stripping is indeed a key process that limits epoxidation selectivity in such cases.