C-C coupling of small oxygenates to form molecules with longer carbon backbone is a pertinent step in the production of fuels and chemicals from renewable feedstock. Ceria, among other solid oxides, is known to catalyze the self-condensation of acetaldehyde (AcH) under mild conditions, primarily forming crotonaldehyde (CrH) but also other C3-C4 species. On the other hand, this reaction is not typically observed in AcH adsorption experiments carried out in UHV. Through extensive first-principles DFT calculations and simulated temperature programmed desorption and infrared spectra, we have now developed a comprehensive understanding of the primary reaction mechanisms of the self-condensation of AcH on both stoichiometric and partially reduced CeO2(111) surfaces. On stoichiometric sites, adjacent acid-base site pairs (Ce-O), like on other oxides such as TiO2, can rapidly catalyze the reaction to yield CrH, and the reaction is desorption-limited for CrH at ambient temperature. This pathway requires the availability and enolization of weakly bound AcH, which therefore renders it inoperative in UHV. On partially reduced CeO2(111), different pathways involving the oxygen vacancies-stabilized enolate of AcH become active depending on the reaction condition. For instance, the dynamic formation of oxygen vacancy dimers as active sites for C-C coupling is required under UHV conditions, whereas a point vacancy-promoted pathway is operative under a partial pressure of AcH. CrH desorption from an oxygen vacancy is more energetically hindered than from stoichiometric sites. With the insights into the catalytic self-condensation of AcH, we aim to advance the fundamental understanding of aldol condensation reactions on solid catalysts in general, and contribute to the design of earth-abundant catalytic materials for this important class of reaction.

Mechanisms of aldol condensation of acetaldehyde on CeO2(111)

ACS Spring Meeting 2021

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