Coupling of Acetaldehyde to Crotonaldehyde on CeO 2–x (111): Bifunctional Mechanism and Role of Oxygen Vacancies

• Published in: Journal of physical chemistry. C Vol. 123; no. 13
• Main Authors: Zhao, Chuanlin, Watt, Charles L., Kent, Paul R., Overbury, Steven H., Mullins, David R., Calaza, Florencia C., Savara, Aditya, Xu, Ye
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.10.2018
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 1932-7447; 1932-7455

Selective C–C coupling of oxygenates is pertinent to the manufacture of fuel and chemical products from biomass and from derivatives of C1 compounds (i.e., oxygenates produced from methane and CO2). Here we report a combined experimental and theoretical study on the temperature-programmed reaction (TPR) of acetaldehyde (AcH) on a partially reduced CeO2–x(111) thin film surface. The experiments have been carried out under ultra-high-vacuum conditions without continuous gas exposure, allowing better isolation of active sites and reactive intermediates than in flow reaction conditions. AcH does not undergo aldol condensation in a typical TPR procedure, even though the enolate form of AcH (CH2CHO) is readily produced on CeO2–x(111) with oxygen vacancies. We find however that a tailored “double-ramp” TPR procedure is able to successfully produce an aldol adduct, crotonaldehyde (CrA). Using density functional theory calculations and microkinetic modeling we explore several possible C–C coupling pathways. We conclude that the double-ramp procedure allows surface oxygen vacancy dimers, stabilized by adsorbate occupation, to form dynamically during the TPR. The vacancy dimers in turn enable C–C coupling to occur between an enolate and an adjacent AcH molecule via a bifunctional enolate–keto mechanism that is distinct from conventional acid- or base-catalyzed aldol condensation reactions. Here, the proposed mechanism indicates that CrA desorption is rate-limiting while C–C coupling is facile.