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

Abstract

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 OHδ− 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.