Catalytic Enhancement of Aldol Condensation by Oxygen Vacancy on CeO2 Catalysts

Aldol condensation is a very useful reaction for biomass upgrading by coupling small molecule platform compounds into high value-added products. Here we choose acetaldehyde (AcH) condensation, the rate-determining step in bioethanol transformations, as a targeted reaction, and prepared four CeO2 catalysts with different concentrations of surface oxygen vacancies to investigate the role of oxygen vacancies in this reaction. To the best of our knowledge, it is the first time to demonstrate that there is a linear correlation between the activity of aldol condensation and the concentration of oxygen vacancies. Based on in-situ FTIR studies, we conclude that oxygen vacancies are the Lewis acid sites for activation and stabilization of AcH, while lattice oxygen act as base sites for the formation of enolate species which are the key intermediates for AcH coupling reactions. Moreover, the proposed reaction mechanism indicates that Ce cations which are weak Lewis acid sites are not involved in the AcH condensation.