The scope and reaction mechanism of CeO2 catalysis in the imine formation from alcohols and amines were investigated. CeO2 catalyst was applicable to the synthesis of imines from various alcohols and amines, and also to the synthesis of imines by homo-coupling of aromatic methylamines, although aliphatic alcohols and sterically hindered alcohols such as secondary alcohols were not suitable substrates for the CeO2 catalyst system. Based on the kinetic and spectroscopic studies, the reaction mechanism of the imine formation from benzyl alcohol and aniline over CeO2 was proposed. The reaction proceeds by the five steps: (i) dissociative adsorption of alcohol on CeO2 to form the alkoxide adspecies and proton, (ii) oxidative dehydrogenation of the alkoxide adspecies to aldehyde adspecies by the lattice oxygen on CeO2 along with reduction of Ce4+ to Ce3+, (iii) nucleophilic addition of amine to the aldehyde adspeices to produce carbinolamine adspecies, and (iv) dehydration of carbinolamine adspecies to the corresponding imine and H2O. (v) Finally, oxidation of Ce3+ to Ce4+ by O2 from air to regenerate CeO2 catalyst. The rate-determining step is the step (ii), particularly the dissociation of benzylic C-H bond at the α-carbon of OH group in benzyl alcohol by the lattice oxygen of CeO2, and the oxidation follows Mars-van Krevelen mechanism. The reactive lattice oxygen species are mobile ones at the redox sites of CeO2, and the high mobility of the oxygen species is responsible for the high activity in the reaction. Based on the reaction mechanism, the low reactivity of the aliphatic alcohols and secondary alcohols can be explained by the steric hindrance, stability of the carbonyl adspecies and/or C-H bond dissociation energy.
- Cerium oxide
- Heterogeneous catalyst
- One-pot synthesis
ASJC Scopus subject areas
- Physical and Theoretical Chemistry