CeO2 acted as a reusable and effective catalyst for the hydration of various nitriles to amides in water, under neutral conditions at low temperature (30-100 °C). To identify the active site, we examined the relationship between activity and the amount of the pair site of a low-coordinated Ce site (CeLC) (oxygen defect site) and adjacent Lewis base (exposed oxygen), determined by methanol adsorption on FTIR. It is revealed that the CeLC-O site is the active site for the reaction. To clarify the reaction mechanism, we carried out in situ FTIR studies on the reaction of acetonitrile with surface Ce-OH groups and kinetic studies such as H2O/D2O kinetic isotope effect and Hammett plot. The results give the following catalytic cycle: (1) dissociation of H2O on the CeLC-O site (oxygen defect site) to give OHδ- and Hδ+ species on the site, (2) formation of an adsorption complex between nitrile and CeO2, (3) addition of OH δ- to the carbon atom of the cyano group of the complex and (4) desorption of the amide from the CeO2 surface, accompanying a regeneration of the CeLC-O site. On the basis of the above fundamental information, we found a simple method for activity increase; preheating of CeO2 at 600 °C resulted in desorption of surface carbonate, and the exposed CeLC-O site thus prepared showed one order of magnitude higher activity for the hydration of various nitriles than untreated CeO2.
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