Abstract:
Oxidative coupling of alcohols and amines represents a green synthetic method for preparing high value-added imines. Various heterogenous catalysts have been synthesized, among which CeO2 shows superior activity and selectivity towards the target product owing to the existence of oxygen vacancy. However, the influencing mechanism of oxygen vacancy on the catalytic performance is still controversial due to the complex surface physicochemical properties of CeO2. In this work, the molar ratio of Ce3+/Ce4+ was used to reflect the concentration of oxygen defects, and high temperature calcination and wet chemical reduction strategies were adopted to synthesize a series of ceria catalysts with the same morphology but different concentrations of surface oxygen defects, which provided an ideal scenario to systematically control the concentrations of surface oxygen defects on CeO2 nanorods. Employing the oxidative coupling of benzyl alcohol and aniline as a model reaction, the CeO2 nanorods with tailorable levels of surface oxygen defects showed a volcanic correlation between the concentrations of oxygen defects and the catalytic activities for the oxidative coupling of benzyl alcohol and aniline into imines. Mechanism analysis suggested that the oxygen activation capability of CeO2 was improved with the increase of the surface oxygen defects, resulting in the enhanced catalytic activity for the oxidative coupling reaction. However, the further increase of the surface oxygen defects weakened the C—H bond activation of benzyl alcohol on the catalyst surface, which inhibited the desorption of benzyl alcohol and weakened the subsequent coupling reaction. This work not only provides a systematic method for the regulation of oxygen defects on the ceria surface, but also provides new understanding about the crucial role of oxygen vacancy on CeO2 in the catalytic oxidation reaction.
