CeO₂-catalyzed nitrile hydration to amide: Reaction mechanism and active sites

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 (Ce_LC) (oxygen defect site) and adjacent Lewis base (exposed oxygen), determined by methanol adsorption on FTIR. It is revealed that the Ce_LC-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 Ce_LC-O site (oxygen defect site) to give OH^δ- and H^δ+ 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 Ce_LC-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 Ce_LC-O site thus prepared showed one order of magnitude higher activity for the hydration of various nitriles than untreated CeO₂.