Conte Marco, Miyamura Hiroyuki, Kobayashi Shū, Chechik Victor
Department of Chemistry, University of York, Heslington, York, YO10 5DD, United Kingdom.
J Am Chem Soc. 2009 May 27;131(20):7189-96. doi: 10.1021/ja809883c.
Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.
利用电子顺磁共振（EPR）光谱和自旋捕集技术来探究金催化剂上醇氧化的机理。仲醇与负载型和非负载型金催化剂（例如Au/CeO₂、聚合物包裹的金纳米颗粒、三苯基膦保护的金纳米颗粒）在自旋捕集剂存在下反应会生成氢自旋加合物。通过同位素标记，我们证实了自旋加合物中的氢源自醇分子中C-H键的断裂。氢自旋加合物的形成很可能是由于自旋捕集剂从金表面夺取氢所致。因此，这些结果有力地表明在醇氧化过程中会形成Au-H物种中间体。该机理中氧气的作用是恢复催化活性而非氧化醇。在无氧条件下以氮氧化物作为氢夺取剂进行金催化的醇氧化反应，进一步证实了这一点。载体（例如金属氧化物）可活化氧气并作为从金表面夺取氢的试剂，从而使活性更快恢复。在醇氧化过程中还观察到了过氧自由基，这与自由基自氧化机理相符，但该机理可能是一个次要的副反应，不会导致生成大量氧化产物。
